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dihydropyridine conjugates
Accepted Manuscript Synthesis and (spectro)electrochemical investigations of coordinatively-saturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates Alex McSkimming, Mohan M. Bhadbhade, Stephen B. Colbran PII: DOI: Reference:
S0020-1693(16)00030-X http://dx.doi.org/10.1016/j.ica.2016.01.019 ICA 16847
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
23 September 2015 8 January 2016 8 January 2016
Please cite this article as: A. McSkimming, M.M. Bhadbhade, S.B. Colbran, Synthesis and (spectro)electrochemical investigations of coordinatively-saturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.01.019
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Synthesis and (spectro)electrochemical investigations of coordinativelysaturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates Alex McSkimming,1 Mohan M. Bhadbhade2 and Stephen B. Colbran1,* 1
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
2
Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2052, Australia
* Email: [email protected] Abstract Complexes with the CpRu(PPh3) fragment bound by iminopyridine ligands functionalised by a Hantzsch dihydropyridine donor of hydride ion or by a Hantzsch pyridinium acceptor of hydride ion have been prepared, and their redox chemistry studied by cyclic voltammetry and EPR and UV-vis spectroelectrochemical investigations. These Ru(II) complexes have a coordinatively saturated, electronically precise (18-electron) ruthenium(II) centre with a non-labile ligand donor set, which suppresses complicating metal-centred reactivity and, thereby, allows the baseline physicochemical properties of the Hantzsch dihydropyridine/pyridinium-functionalised ligands to be investigated. In Ru(II) complexes, the iminopyridine chelate is linked to the Hantzsch pyridine groups by either an ortho-phenyl bridge (electronically delocalized) or by a meta-phenyl bridge (electronically isolated), which leads to notable differences in spectroscopic properties, even for ruthenium centre, and differences in redox reactions. Of note, the primary electrochemical reduction of the Ru(II) complexes with a Hantzsch pyridinium substituent is centred on this group, but did not afford the corresponding Ru(II) complexes with a 1,4-dihydropyridine substituent. Rather it was found that the reduction products were identical to the 1:1 hydroxide adducts formed upon addition of hydroxide ion to the starting Hantzsch pyridinium-substituted Ru(II) complexes. Based on these results and comparisions with data from the literature, the reduction products and hydroxide adducts are tentatively assigned as the corresponding hydroxy-dihydropyridine substituted Ru(II) complexes (during reduction, hydroxide ion was likely formed from the residual water present in the acetonitrile solvent). Implications for the electrochemical cycling of transition metal catalysts with Hantzsch pyridinium/dihydropyridine functional substituents are considered. Key words Ruthenium; Multifunctional ligand; Hantzsch dihydropyridine/pyridinium; Cyclic voltammetry; Spectroelectrochemistry 1
1. Introduction Arthur Hantzsch first reported his route to the dihydropyridine which bears his name, Hantzsch’s ester (Chart 1), over 130 years ago [1]. After languishing for well over a century, Hantzsch’s ester has advanced over the last decade to the forefront of the synthetic chemists’ armoury as an organic hydride reagent for asymmetric transfer hydrogenations of unsaturated organic substrates, reactions catalysed by chiral Brønsted and Lewis acids [2-9]. Although very useful, this reduction methodology suffers from the costly, atom inefficient, stoichiometric consumption of Hantzsch’s ester. Several very recent studies have revealed that Ru- or Fe-catalysed hydrogenation at high pressure may regenerate Hantzsch’s ester thereby allowing it to be employed in catalytic amounts [10-13].
O
H
H
O OEt
EtO Me
N
Me
H
Chart 1. Hantzsch’s ester
Seeking a versatile catalytic methodology for transfer hydrogenation using Hantzsch’s ester and other organic hydride donors under ambient conditions, we have followed a different line of attack. We recently introduced organic hydride-functionalised transition metal complexes as useful catalysts for the transfer hydrogenation of unsaturated substrates [14], and have reported several proof-of-concept demonstrations [15-18]. For example, Cp*Rh(diimine)Cl complexes were made where the diimines were the new pyridylimine (pi) ligands functionalised by Hantzsch dihydropyridine (heH) or Hantzsch pyridinium cation (he+) moieties [16]. The complex [Cp*Rh(pi2
he)Cl]2+ (Chart 2) proved to be an excellent catalyst for transfer hydrogenation of imines by formate
at room temperature in air. A catalytic cycle involving hydride transfer from a Rh-H species (formed by hydride transfer from coordinated formate) to the pyridinium ring to afford the dihydropyridine and open a site at the Rh(III) centre was proposed. In this cycle, the imine substrate binds the electrophilic Rh(III) centre and is thus activated to back-transfer of hydride from the tethered dihydropyridine. The steps in the catalytic cycle mimick those involving substrate polarisation by metal binding and hydride transfer to/from substrate from/to NAD(P)H/NAD(P)+ in the catalytic cycles of NAD(P)H/NAD(P)+-dependent metallo-(de)hydrogenases [14].
2
2 N Cl O
O
Rh N
O 1O
N 2
[Cp*Rh(pi-2he)Cl] 2+
Chart 2.
This paper is not about catalysis. Rather, in order to advance understanding of transition metal complexes functionalised by Hantzsch dihydropyridine/pyridinium substitutents in general, we have made the new complexes with a CpRu(PPh3) centre bound by dihydropyridine or pyridiniumfunctionalised pyridylimine ligands that are depicted in Chart 3. Detailed spectro-electrochemical studies have been undertaken and are reported herein. The non-labile ligand donor set combined with the coordinative and electronic (18-electron) saturation at the ruthenium(II) centre was deliberately targetted to suppress metal-centred reactivity (including all possiblity of catalytic reactions) thereby allowing the baseline electrochemical properties of the non-innocent Hantzsch dihydropyridine or pyridinium cation-substituted pyridylimine ligands to be examined. The work addresses the important question of whether direct electrochemical regeneration of a Hantzsch dihydropyridine substituent, as is proposed for the active species formed from catalyst [Cp*Rh(pi2
he)Cl]2+ (Chart 2) [16], is possible. Also, interesting effects resulting from the presence or absence
electronic communication between the metal and dihydropyridine/pyridinium cation centres are revealed.
2
O
Ph 3P O
N
H 2
Ru
O
N
O
N
N 3
O
Ph3P 2
Ru N N 3
O
O
1: [CpRu(PPh 3)(pi-2heH)] + 2: [CpRu(PPh 3)(pi-3heH)] +
O
3: [CpRu(PPh 3)(pi-2he)] 2+ 4: [CpRu(PPh 3)(pi-3he)] 2+
Chart 3
3
2. Experimental Section 2.1 General methods. NMR spectra were obtained on Bruker Avance DPX-300, 400, 500 and 600 spectrometers at 298 K operating at 300, 400, 500 and 600 MHz frequency for 1H NMR experiments and at 75.5, 100.6, 125.7 and 150.9 MHz for 13C NMR experiments, respectively. 1H and 13C chemical shifts were calibrated against solvent signals; spin-spin couplings are given in Hz. EPR spectra were recorded using a Bruker EMX EPR spectrometer at 298 K. FT-IR spectra were recorded on KBr discs using a Nicolet Avatar 360 FT-IR spectrometer at 2 cm-1 resolution. Mass spectra were run on a Thermo Fisher Scientific Orbitrap LTQ XL ion trap mass spectrometer using a nanospray ionisation source. UV-Vis spectra were recorded using a Varian Cary 50 Bio UV-Visible spectrophotometer. Elemental analyses were obtained at the Microanalytical Unit of the Research School of Chemistry, Australian National University.
2.2 Materials. Solvents were dried and obtained under dinitrogen from a PureSolv MD solvent purification system. The ligands, pi-2heH and pi-3heH, were available from a previous study [16]. [CpRuCl(PPh3)2] was synthesised according to a literature procedure [19]. 4-Methyl-N-(4-methylbenzylidene)aniline was synthesised heating equimolar amounts of p-tolylaldehyde and p-toluidine at reflux in ethanol for 30 min., cooling and collecting the resulting crystalline solid by filtration. Tetra(nbutyl)ammonium hexafluorophosphate was recrystallised from acetone and dried under vacuum before use. All other chemicals were commercial and were used as obtained.
2.3 Syntheses of complexes.
pi-2heH
pi-3heH
Chart 4. Atom labelling scheme adapted for describing the NMR spectra of the Hantzsch (dihydro)pyridine-substituted ligands.
4
[CpRuII(pi-2heH)(PPh3)][PF6] (1) Ligand pi-2heH (30 mg, 0.072 mmol), [CpRu(PPh3)2Cl] (52 mg, 0.072 mmol) and K[PF6] (100 mg, 0.54 mmol) were suspended in de-aerated methanol/THF (1:4, 30 mL) and stirred at reflux under dinitrogen overnight. The solvent was removed in vacuo, the resulting residue diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent was removed in vacuo. The crude mixture was purified using flash column chromatography on neutral alumina, eluting first with ether, then dichloromethane-acetone (4:1). The orange-red band was collected, the solvent removed in vacuo and the residue crystallised from acetone-toluene to give dark red crystals of the product, which were washed with ether and dried under vacuum (21 mg, 28%). 1H NMR (400 MHz, (CD3)2C(O)): δ 9.63 (d, J = 6, 1H, H-11), 8.14 (d, JPH = 2, 1H, H-i), 7.85 (t, J = 8, 1H, Ar-H), 7.54 (d, J = 8, 1H, Ar-H), 7.37 (t, J = 8, 3H, p-PPh3), 7.45 (t, J = 8, 6H, m-PPh3), 7.38 (t, J = 8, 1H, Ar-H), 7.20 (m, br, 6H, o-PPh3), 7.12−7.17, (m, 2H, 2 x Ar-H), 7.08 (d, J = 8, 1H, Ar-H), 6.82 (td, J = 8,2, 1H, Ar-H), 5.64 (s, 1H, H-g), 4.90 (s, 5H, Cp-H), 3.39 (s, 3H, H-f/f’), 3.37 (s, 3H, H-f/f’), 3.18 (s, 3H, H-a), 2.56 (s, 3H, H-b/b’), 2.52 (s, 3H, H-b/b’).
13
C {1H} NMR (100.6 MHz, (CD3)2C(O)): 168.61 (C-e/e’), 168.56 (C-e/e’), 166.10 (d, J =
2PH, Ci), 157.5 (Ar-C), 157.4 (Ar-C), 157.3 (Ar-C), 153.4 (Cc), 153.3 (Cc’), 152.2 (Ar-C), 136.9 (Ar-C), 135.1 (Ar-C), 134.6 (d, JCP = 11, o-PPh3), 132.4 (d, JCP = 40, ipso-PPh3), 131.9 (d, JCP = 2, p-PPh3), 130.1 (d, JCP = 10, m-PPh3), 129.3 (Ar-C), 127.6 (Ar-C), 127.2 (Ar-C), 127.1 (Ar-C), 126.5 (Ar-C), 105.3 (Cd/d’), 104.4 (C-d/d’), 80.9 (d, JCP = 1, Cp), 51.8 (C-f/f’), 51.6 (C-f/f’), 35.1 (C-a), 34.4 (C-g), 17.1 (Cb/b’), 17.0 (C-b/b’).
31
P{1H} NMR (161.9 MHz, (CD3)2C(O)): δ 45.9 (s, PPh3), -144.2 (sept, J = 700,
[PF6]−). FT-IR (KBr): νmax cm-1 1704 (s, C=O), 1691 (s, C=O), 1636 (s), 1571 (m), 1532 (m), 1571 (m), 1480 (m), 1449 (m), 1434 (s), 1381 (m), 1357 (m), 1289 (m), 1249 (m), 1232 (m), 1289 (s), 1212 (s), 1169 (s), 1126 (m), 1106 (m), 1090 (m), 1054 (m), 1006 (m), 955 (w), 939 (w), 919 (w), 838 (vs), 778 (m), 764 (m), 747 (m), 696 (s), 557 (s), 525 (s), 514 (m), 495 (m), 459 (m), 445 (m), 419 (w). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 225 (45), 270 (sh, 15), 323 (br, 13.3), 455 (br, 5.0). ESI HR-MS: Found: m/z 848.2181, Calcd. for (M+): m/z 848.2186. Anal. Calcd. for C47H45F6N3O4P2Ru: C, 56.85; H, 4.57; N, 4.23. Found: C, 57.10; H, 4.65; N 3.97 %.
[CpRuII(pi-2he)(PPh3)][PF6]2 (3) DDQ (2.8 mg, 0.012 mmol) was added to 1 (10.0 mg, 0.012 mmol) in acetonitrile (0.5 mL) and the solution left to stand for 10 mins. The solvent was removed in vacuo and the reaction mixture purified using flash column chromatography on silica gel (NaCl (sat.) in MeOH eluent). The red band was collected and the solvent removed in vacuo. The residue was dissolved in acetone and filtered to remove sodium chloride. K[PF6] (sat., aq.) was added to precipitate the crude product
5
which was collected by filtration, washed with water and recrystallised from acetone-cyclohexane to give 3 as crimson needles (9.3 mg, 80%). TEMPO[BF4] could be used in place of DDQ; the yield and work-up remained essentially unchanged. 1H NMR (400 MHz, (CD3)2C(O)): δ 9.36 (d, J = 5, 1H, H-11), 7.85 (d, J = 8, 1H, Ar-H), 7.99 (d, J = 3, 1H, H-i), 7.46-7.50 (m, 5H, p-PPh3,2 x Ar-H), 7.38−7.41 (t, J = 8, 6H, o-PPh3), 7.18−7.22 (m, 2H, 2 x Ar-H), 7.00−7.14 (m, 8H, m-PPh3, 2 x Ar-H), 4.94 (s, 5H, Cp-H), 4.10 (s, 3H, H-a), 3.80 (s, 3H, H-f/f’), 3.66 (s, 3H, H-f/f’), 2.89 (s, 3H, H-b/b’), 2.63 (s, 3H, H-b/b’). 13C {1H} NMR (100.6 MHz, CD3CN): δ 164.9 (C-e/e’), 163.7 (C-e/e’), 161.9 (d, JCP = 2, C-i), 156.0 (Ar-C), 155.9 (Ar-C), 155.8 (Ar-C), 155.8 (Ar-C), 154.7 (Ar-C), 150.5 (Ar-C), 147.8 (Ar-C), 134.8 (Ar-C), 134.4 (Ar-C), 132.7 (d, JCP = 11, o-PPh3), 131.5 (Ar-C), 131.1 (Ar-C), 130.7 (Ar-C), 150.6 (Ar-C), 150.5 (Ar-C), 130.1 (d, JCP = 2, p-PPh3), 130.4 (d, JCP = 40, ipso-PPh3), 129.5 (Ar-C), 129.3 (Ar-C), 128.9 (d, J = 10, mPPh3), 125.4 (Ar-C), 123.9 (Ar-C), 79.6 (d, JCP = 1, Cp-C), 54.0 (C-f/f’), 53.8 (C-f/f’), 42.2 (Ca), 20.6 (Cb/b’), 20.1 (C-b/b’). 31P{1H} NMR (161.9 MHz, CD3CN): δ 47.1 (s, PPh3), -144.3 (sept, J = 700, [PF6]−). FT-IR (KBr): νmax cm-1 1741 (s, C=O), 1607 (m), 1555 (m), 1527 (w), 1481 (m), 1469 (m), 1437 (m), 1383 (w), 1337 (m), 1301 (w), 1252 (s), 1202 (m), 1164 (m), 1091 (m), 1059 (m), 1015 (w), 999 (w), 917 (w), 841 (vs), 774 (m), 744 (m), 699 (m), 558 (s), 525 (s), 513 (m), 494 (m), 458 (w). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 290 (22.5), 310 (br, sh, 17), 480 (br, 5.0). ESI HR-MS: Found: m/z 992.1746, Calcd. for (M+): m/z 992.1749. Anal. Calcd. for C47H44F12N3O4P3Ru: C, 49.66; H, 3.90; N, 3.70. Found: C, 49.48; H, 3.97; N 3.63 %.
[CpRuII(pi-3heH)(PPh3)][PF6] (2) Ligand pi-3heH (50 mg, 0.119 mmol), [CpRu(PPh3)2Cl] (88 mg, 0.121 mmol) and K[PF6] (100 mg, 0.54 mmol) were suspended in de-aerated methanol and stirred at reflux under dinitrogen overnight. The solvent was removed in vacuo, the resulting residue diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent was removed in vacuo. The crude mixture was purified using flash column chromatography on neutral alumina, eluting first with ether, then dichloromethane-acetone (4:1). The orange-red band was collected, the solvent removed in vacuo and the residue crystallised from acetonitrile-ether to give large red prisms of 2 (80 mg, 68%). 1H NMR (400 MHz, CDCl3): δ 9.28 (d, J = 6, 1H, H-11), 8.14 (d, JPH = 2, 1H, H-i), 7.77 (d, J = 8, 1H, Ar-H), 7.68 (t, J = 8, 1H, Ar-H), 7.37 (t, J = 8, 4H, p-PPh3 + Ar-H), 7.29 (td, J = 8,2, 1H, Ar-H), 7.26 (td, J = 8,2, 6H, m-PPh3), 7.17 (d, J = 8, 1H, ArH) 6.99 (t, J = 8, 1H, Ar-H), 6.88−6.95 (m, br, 6H, o-PPh3), 6.10 (dd, J = 8, 2, 1H, Ar-H), 5.27 (s, 1H, Hg), 4.70 (s, 5H, Cp-H), 3.78 (s, 3H, H-f/f’), 3.73 (s, 3H, H-f/f’), 3.26 (s, 3H, H-a), 2.54 (s, 3H, H-b/b’), 2.53 (s, 3H, H-b/b’).
13
C {1H} NMR (100.6 MHz, CDCl3): δ 168.39 (C-e/e’), 168.27 (C-e/e’), 160.82 (d, J
= 2, C-i), 155.8 (Ar-C), 155.6 (Ar-C), 153.6 (Ar-C), 153.6 (Ar-C), 150.2 (C-c/c’), 150.0 (C-c/c’), 135.4 (Ar-
6
C), 132.9 (d, J = 11, o-PPh3), 130.8 (d, J = 40, ipso-PPh3) (Ar-C), 130.4 (d, JCP = 2, p-PPh3), 128.5 (Ar-C), 128.4 (Ar-C), 124.49 (d, J = 10, m-PPh3), 127.4 (Ar-C), 125.5 (Ar-C), 122.3 (Ar-C), 119.1 (Ar-C), 105.2 (C-d/d’), 79.56 (d, JCP = 1, Cp-C), 51.5 (C-f/f’), 51.4 (C-f/f’), 38.4 (C-a), 34.5 (C-g), 16.9 (C-b/b’), 16.7 (Cb/b’). 31P{1H} NMR (161.9 MHz, CDCl3): δ 47.4 (s, PPh3), -144.2 (sept, J = 700, [PF6]−). FT-IR (KBr): νmax cm-1 1694 (s, C=O), 1638 (m), 1572 (m), 1535 (w), 1478 (m), 1434 (m), 1383 (m), 1361 (m), 1328 (w), 1288 (m), 1249 (w), 1212 (s), 1188 (m), 1169 (m), 1136 (w), 1108 (m), 1092 (m), 1052 (m), 1003 (m), 933 (w), 838 (vs), 791 (w), 752 (w), 743 (w), 698 (m), 557 (m), 529 (m), 512 (m), 491 (w), 465 (w). UVVis (MeCN): λmax nm (εmax/103 M-1 cm-1) 230 (54.0), 330 (br, 17.1), 450 (5.3). ESI HR-MS: Found: m/z 848.2184, Calcd. for (M+): m/z 848.2186. Anal. Calcd. for C47H45F6N3O4P2Ru: C, 56.85; H, 4.57; N, 4.23. Found: C, 57.03; H, 4.39; N 4.35 %.
[CpRuII(pi-3he)(PPh3)][PF6]2 (4) Method (a). Chemical oxidation: TEMPO[BF4] (6 mg, 0.025 mmol) was added to 2 (20 mg, 0.020 mmol) in acetonitrile (5 mL) and the solution left to stand for 10 mins. The solvent was removed in vacuo and the reaction mixture purified using flash column chromatography on silica gel (NaCl (sat.) in MeOH). The red band was collected and the solvent removed in vacuo. The residue was dissolved in acetone and filtered to remove sodium chloride. K[PF6] (sat., aq.) was added to precipitate the crude product which was collected by filtration, washed with water and recrystallised from methanol to give 4 as maroon crystals (20 mg, 88%). DDQ could be substituted for TEMPO[BF4]; the preparation and yield remained essentially unchanged. 1H NMR (400 MHz, (CD3)2CO): δ 9.59 (d, J = 5, 1H, H-11), 8.66 (d, J = 3, 1H, H-i), 7.85 (t, J = 8, 1H, Ar-H), 7.80, (d, J = 8, 1H, Ar-H), 7.68 (t, J = 8, 1H, Ar-H), 7.56 (d, J = 8, 1H, Ar-H), 7.30−7.45 (m, 11H, p-PPh3, m-PPh3 + 2 x Ar-H), 7.10−7.20 (m, br, 7H, o-PPh3 + Ar-H), 5.00 (s, 5H, Cp-H), 4.67 (s, 3H, H-a), 3.83 (s, br, 3H, H-f/f’), 3.74 (s, br, 3H, H-f/f’), 3.19 (s, br, 3H, H-b/b’), 3.11 (s, br, 3H, H-b/b’). 13C {1H} NMR (100.6 MHz, (CD3)2CO): δ 164.2 (C-e/e’), 163.7 (C-e/e’), 163.4 (d, J = 2, C-i), 156.3 (Ar-C), 156.2 (Ar-C), 155.8 (Ar-C), 155.5 (Ar-C), 155.4 (Ar-C), 154.3 (Ar-C), 150.9 (Ar-C), 135.8 (Ar-C), 133.5 (Ar-C), 133.2 (Ar-C), 132.9 (d, JCP = 11, o-PPh3), 131.1 (d, JCP = 40, ipso-PPh3), 130.2 (Ar-C), 130.1 (d, JCP = 2, p-PPh3), 129.0 (Ar-C), 128.7 (d, JCP = 10, m-PPh3), 127.4 (Ar-C), 125.7 (Ar-C), 123.3 (Ar-C), 123.2 (Ar-C), 79.2 (d, JCP = 1, Cp-C), 53.6 (C-f/f’), 53.4 (C-f/f’), 42.2 (C-a), 19.71 (br, C-b + C-b’). 31P{ 1H} NMR (161.9 MHz, CD3CN): δ 48.2 (s, PPh3), -144.3 (sept, J = 700). FT-IR (KBr): νmax/cm-1 1741 (s, C=O), 1606 (m), 1557 (m), 1531 (w), 1481 (w), 1471 (w), 1436 (m), 1383 (w), 1340 (m), 1308 (w), 1251 (s), 1202 (m), 1161 (m), 1093 (m), 1058 (m), 1013 (w), 999 (w), 839 (vs), 770 (w), 745 (m), 698 (s), 558 (s), 525 (m), 514 (m), 495 (m). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 283 (24.5), 315 (sh, 15), 452 (4.8). ESI HR-MS: Found: m/z 992.1751, Calcd. for
7
(M+): m/z 992.1749. Anal. Calcd. for C47H44F12N3O4P3Ru.CH3OH: C, 49.28; H, 4.22; N, 3.59. Found: C, 49.22; H, 3.85; N 3.68 %.
Method (b). Electrochemical oxidation: Complex 2 (20 mg, 0.020 mmol) in MeCN-0.1 M K[PF6] under dinitrogen was exhaustively oxidised at +1.00 V. Upon completion of the electrolysis, the solvent was removed in vacuo, the crude reaction mixture diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent removed in vacuo. The crude residue was purified using chromatography as described above (12 mg, 54 % of 4 obtained). Over-oxidation of 2 to Ru(III) species perhaps accounts for the moderate yield.
2.4 Electrochemistry. CVs were recorded using a Pine Instrument Co. AFCBP1 bipotentiostat. A three-electrode system was used, consisting of a glassy carbon working electrode (1 mm diameter), a platinum wire counter electrode, and an Ag/AgCl reference electrode. Prior to recording each CV, the working electrode was polished with increasingly fine alumina, rinsed with water, then the solvent to be used in the experiment, and dried by rubbing with a clean tissue. The glassy carbon working electrode was then “conditioned” by repeatedly scanning the CV of the blank background for the solventelectrolyte solution between the onsets of anodic and cathodic solvent discharge limits until a constant current response was obtained (~ 10 sweeps). Ferrocene was employed as a reference compound, and gave E1/2 (Fc+/Fc) = 0.49 V vs. AgCl/Ag. UV-Vis spectroelectrochemical experiments were carried out using an airtight quartz cuvette (1 mm path-length) with a light-transparent platinum-gauze working electrode fitted with fritseparated platinum wire auxiliary and Ag/AgCl reference electrodes outside the light-path. EPR spectroelectrochemical experiments utilised a vacuum-tight 0.5 mm quartz cell fitted with spatially separated platinum wire working and auxiliary electrodes and a silver wire pseudoreference electrode. Quantitative bulk electrolysis was performed using a conventional glass ‘H-cell’ with the compartments containing the platinum gauze working and auxiliary electrodes separated by two porosity 4 glass frits; the same Ag/AgCl reference electrode as was used in the CV experiments.
3. Results and Discussion 3.1 Synthesis and Spectroscopic Properties
8
The targets for this investigation were complexes [CpRu(L)(PPh3)][PF6]n, 1: L = pi-2heH, n = 1; 2: L = pi-3heH, n = 1; 3: L = pi-2he+, n = 2; 4: L = pi-3he+, n = 2), Chart 3. Complexes 1 and 2 were synthesised in low (28 %) and good (68 %) yield, respectively, by heating equimolar amounts of the appropriate ligand precursor, pi-2heH or pi-3heH, and [CpRuCl(PPh3)2] together in the presence of excess K[PF6] in tetrahydrofuran-methanol (for 1; yield 28%) and methanol (for 2; yield 68%). In the synthesis of 1, the mixed solvent was necessary for solubility reasons and products from the hydrolysis of the pyridylimine were observed; the lower yields of 1 likely result from imine hydrolysis being competive with the complexation of the bulkier pi-2heH ligand to the ruthenium centre. Complexes 1 or 2 were selectively oxidised by TEMPO[BF4] [20, 21] or by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
to the corresponding Hantzsch
pyridinium-substituted
complexes,
[CpRu(L’)(PPh3)][PF6]2 (3: L’ = pi-2he+; 4: L’ = pi-3he+), Chart 1 . The use of TEMPO+ in these hydride transfer reactions is novel and proved highly effective affording 3 and 4 in over 80% yield. The reverse reductions of 3 and 4 to 1 and 2, respectively, were not attempted. All four ruthenium complexes 1–4 were air stable in both the solid state and in acetone or acetonitrile solutions. The 1H,
13
C{1H} and
31
P{1H} NMR, IR and UV-vis spectra of the Hantzsch dihydropyridine-
substituted complexes, 1 and 2, are similar, and distinct from those for the Hantzsch pyridiniumsubstituted complexes, 3 and 4, which are also similar. Representative 1H NMR spectra, of 2 and 4, are shown in Figures 1S and 2S (Supporting Information). Of note, the spectra of 1 and 2 show five singlets for the inequivalent methyl groups — rotation of the dihydropyridine ring can not exchange the methyl and methyl ester substituents. For 3 and 4, however, fast rotation of the Hantzsch pyridinium ring relative to the NMR timescale would lead to these pairs of methyl and methyl ester groups becoming equivalent. The 1H NMR spectrum of 3 shows five sharp methyl singlets revealing pyridinium ring rotation is relatively slow in this hindered complex. Five methyl singlets (at δ 4.67, 3.83, 3.74, 3.19 and 3.11) are also seen in the 1H NMR spectrum of 4, but the four downfield signals are heavily broadened consistent with the rotation of the pyridinium ring beginning to exchange the pairs of methyl groups on the timescale of the NMR experiment. Also of note, the singlet for the 4dihydropyridine proton appears at δ 5.64 for 1 and at δ 5.27 for 2, and is absent in the 1H NMR spectra of 3 and 4. The imine C-H appears as a doublet at ca. δ 8.14 (4JPH = 2 Hz) for 1 and 2 and at ca. δ 7.99 (4JPH = 3 Hz) for 3 and 4. FT-IR spectra of 1 and 2 exhibit strong ester carbonyl bands at 1704 and 1694 cm-1, respectively. The ester carbonyl bands appear ca. +45 cm-1 to higher energy at 1741 cm-1 for 3 and 4, which is consistent with polarisation of the C=O bond by the pyridinium ring. UV-vis spectra of 1 and 2 show bands at 225, 323 and 455 nm and at 220, 330 and 451 nm, respectively, whereas 3 shows bands at 290, 310 and 480 nm and 4 bands at 283, 318 and 452 nm. The broad bands at ca. 330 nm
9
(εmax ≈ 1.5 × 104 M-1 cm-1) are unique to complexes 1 and 2 and are also present with similar intensities in the UV-vis spectra of the Hantzsch dihydropyridine-substituted ligands alone. These bands are ascribed to the characteristic π → π* transitions(s) of a 1,4-dihydropyridine [22-25]. The lowest-energy bands (εmax ≈ 0.5 × 104 M-1 cm-1) at ca. 450 nm for 1, 2 and 4 and at 480 nm for 3 arise from dπ(Ru) → π*(pi-) MLCT transitions [26-31]. The difference in the MLCT band energies is negligible between complexes 2 and 4, but is appreciable between 1 and 3 (~1145 cm–1). πDelocalisation between the CpRu(pi-)(PPh3) centre and its pendant heH/he+ group is possible with the ortho-phenyl link in the 2-isomers 1 and 3, but not with the meta-phenyl link in the 3-isomers 2 and 4 (e.g., see Figure 4S, Supporting Information); thus electronic delocalisation between Ru and he+ centres likely underpins the appreciable red shift for the MLCT band of 3 compared to that of 1. Crystal structures of 2 and 4 were also obtained. Unfortunately, the quality of both structures is poor, and they are presented in the Supporting Information (see Fig. 11S, Table 1S and the accompanying text).
3.2 Cyclic voltammetry of Hantzsch dihydropyridine-substituted 1 and 2. The redox chemistry of the complexes was assayed by cyclic voltametry. Cyclic voltammograms (CVs) of the ligands , pi-2heH and pi-3heH, are presented in Figure 5S in the Supporting Information, and show an irreversible primary oxidation at ca. +1.0 V and an irreversible primary reduction process at –1.83 V for pi-2heH and –1.62 V for pi-3heH. The primary redox processes are dihydropyridine-centred. Figure 1 gives representative CVs of complexes 1 and 2, initiated to positive potential, and Table 1 provides a summary of the electrochemical potentials for the observed redox processes. The primary oxidation process at ca. +1.1 V arises from overlap of the irreversible heH-centred oxidation (which is seen in the CVs of the ligands alone; e.g., Figure 5S, Supporting Information), and the closely following Ru(III)/Ru(II) couple for the CpRu(pi-)(PPh3) centre, equation 2, which is electrochemically reversible {∆Ep ≈ 80 mV cf. ∆Ep = 75 mV for the ferrocenium (Fc+)/Fc couple at v = 100 mV s-1} but shows limited chemical reversibility (ip,c/ip,a ≈ 0.75). For comparison, CVs of [CpRuII(bpy)(PPh3)]+ show a reversible Ru(III)/Ru(II) couple at +1.03 V [32]. The small cathodic child peaks at ca. +0.37 V in the return negative sweeps after traversing the overlapping oxidation processes may arise from product(s) of ruthenium-centred oxidation as these peaks do not occur in CVs of the ligands alone (Figure 5S, Supporting Information). The product cathodic peak at –0.95 V in the return sweep for 2 has identical potential to the primary reduction of pyridinium cation-substituted 4 (see below), thus revealing the oxidation of 2 to produce 4, equation 1, within the (ms) timescale of the CV experiment. However, the product cathodic peak at –1.06 V in the
10
return sweep for 1 differs in potential from the primary reduction of 3, which appears at –0.76 V (see below). Therefore, oxidation of 1 does not produce pyridinium cation-substituted 3 on the CV timescale. The conjugation of the heH centre with, and/or the proximity to, the metal centre in 1 results in a change in the oxidative electrochemistry compared to 2. The nature of the oxidation product is considered below after presentation of all results.
1st oxid.
[CpRuII(pi-3heH)(PPh3)]+ [CpRuII(pi-3he+)(PPh3)]2+ + H+ + 2e–
(1)
2nd oxid.
[CpRuII(pi- )(PPh3)]z+ [CpRuIII(pi- )(PPh3)](z+1)+ + e–
(2)
* *
Figure 1. CVs of 1 (blue trace) and 2 (red trace) in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K using a 1.0 mm diameter minidisk glassy carbon working electrode and scan rate = 300 mV s–1. When the CVs were switched at +0.8 V, the peaks indicated by the asterisks disappeared. Table 1. Electrochemical potentials1 for peaks observed in cyclic voltammograms of 1 and 2.
1
2
Complex E1/2 (1st redn) E1/2 (2nd redn) Ep,a (1st oxidn) E1/2 (2nd oxidn) Ep,c (*)2 1 –1.27 –1.86 +0.98 +1.05 –0.95 2 –1.10 –1.70 +1.13 (sh) +1.17 –1.06 E1/2/V for couples and Ep/V for irreversible processes versus an Ag/AgCl reference electrode for which E1/2 (Fc+/Fc) = +0.49 V. Ep,c (*) is for the prominent cathodic child peak (marked by an asterisk in Figure 2) that arises in the reverse positive scan after the primary oxidation process is traversed. To negative potential in CVs of 1 and 2, the Nernstian primary reduction processes (ΔEp ≈ 70
mV) at –1.27 and –1.10 V, respectively, are attributed to 1e–-reduction of the pyridylimine group to the corresponding pyridylimino-radical, equation 3. The pyridylimino-centred radical product from 2 was observed by EPR spectroscopy (see below). That pyridylimine-centred reduction is observed for 1 and 2, but not for pi-2heH or pi-3heH is consistent with polarisation upon binding to Ru(II) ion raising the pi-/pi-•– reduction potential (i.e., reduction at more positive potentials). The second reduction process, at –1.86 V for 1 and at –1.67 V for 2, exhibits poor chemical reversibility and is
11
within 40 mV of that found for the respective isolated pi-heH ligand, and is typical for a 1e--reduction of the diester-substituted dihydropyridine ring [33].
[CpRuII(pi-heH)(PPh3)]2+ + e– [CpRuII(pi•–-heH)(PPh3)]+
(3)
3.3 Spectroelectrochemistry of 1 and 2 Further insights into the primary redox processes exhibited by 1 and 2 were provided by UVvis and EPR spectroelectrochemistry. UV-vis band data for the complexes and electrolysis products are given in Table 2. First, oxidation of 2 is considered, as the results are most easily understood. Figure 3(b) shows the changes in the the UV-spectra that accompany electrolysis of 2 at +1.0 V. The UV-vis bands for 2 (at 230 and 330 and 450 nm, see above) are smoothly replaced by those for 4 (at 283 and 452 nm, see above); isosbestic points indicative for a clean 1:1 transformation are observed at 267 and 307 nm. Notably, the dπ(Ru)–π*(pi-) MLCT band [26-31] at 450 nm does not shift between 2 and 4, which accords with isolation of the Ru(II) and heH/he+ centres. No radical intermediates were detected by EPR spectroscopy during an oxidative electrolysis of 2 in situ in an EPR spectroelectrochemical cell. Therefore, radical intermediate(s), if any, must be short-lived. That the primary oxidation product of 2 is 4 was confirmed by exhaustive electrolysis of 2 at +1.0 V in acetonitrile–K[PF6]; 4 was isolated in 54% yield after column chromatography (see the Experimental section). These results verify the conclusion from analysis of the CVs of 2 that the primary oxidation is dihydropyridine-centred and that pyridinium cation-substituted 4 is the product, equation 1.
Table 2. Wavelength and extinction coefficient data for 1 and 2 and their electrolysis products.
λmax/nm (εmax/103 M-1 cm-1) parent complex
… after 1st oxidation … after 2nd oxidation … after 1st reduction
1 225 (45), 270 sh (15), 323 (13), 455 (5.0) 312 (31), 360 sh (9), 475 (5.5) 267 (18), 310 (29), 360 sh (14) decomposes
2 230 (54), 330 (17), 451 (5.3)
283 (23), 452 (3.9) 280 (23), 435 sh (2.2) 340 (16), 400 sh (12), 460 (9), 530 sh (6)
The UV-vis spectrum of 1, with bands at 225, 323 and 455 nm, is similar to that of 2. However, UV-vis spectra acquired during oxidative electrolysis of 1 at +0.9 V, Figure 3(a), show that the oxidation of 1 follows a different course to oxidation of 2. This observation concurs with the deductions from the CV results, see above. The electronic bands of 1 were replaced by those for the product, which shows a sharp, intense peak at 312 nm (εmax ~ 3.1 × 104 M-1 cm-1) and a 12
dπ(Ru)–π*(pi-) MLCT band [26-31] that is red-shifted by 926 cm-1 to 475 nm. Isosbestic points at 270, 396, 460 and 525 nm suggest a clean transformation from 1 to its first oxidation product. The final spectrum clearly reveals that pyridinium-substituted 3 is not the oxidation product (e.g., the UV-vis spectrum of 3 exhibits a shoulder at 310 nm that is an order of magnitude weaker in extinction coefficient; compare Figure 2(a) with Figure 5(a) below). The orange solutions of 1 became intensely orange-red as the oxidation product formed. No radical intermediate(s) was detected during in situ oxidative electrolysis of 1 by EPR spectroscopy. The intensely orange-red primary oxidation product was extremely air-sensitive, which thwarted further characterisation of it.
(a)
(b)
Figure 2. UV-Vis spectra acquired using a transparent thin-layer Pt-gauze electrode cell during oxidative electrolysis of (a) 1 at +0.90 V and (b) 2 at +1.00 V in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K. In these and subsequent plots of UV-vis spectroelectrochemical data, the bold red line indicates the initial, pre-electrolysis spectrum and the bold deep blue line indicates the final, post-electrolysis spectrum; the arrows indicate the direction of change. Continued oxidative electrolysis of 1 at +1.2 V or of 2 at +1.1 V led to loss the bands at 475 and 452 nm, respectively, and only slight intensity changes in the other (Hantzsch group-centred) bands (Table 1 and Figure 8S, Supporting Information). The changes were accompanied by a colour change from orange-red to yellow-brown. In concurrence with the conclusions drawn from the CVs (see above), the bleaching of dπ(RuII)–π*(pi-) MLCT bands upon exhaustive oxidation is consistent with formation of a CpRuIII(pi-)(PPh3) centre, equation 2. Reduction of complexes 1 and 2 was also studied. Electrolysis of 1 at –1.3 V led to complete decomposition and deposition of insoluble black material on the Pt-mesh cathode. Upon reductive electrolysis of 2, the solution turned from orange to dark red and became highly air sensitive; upon exposure to atmospheric oxygen, the dark red solution rapidly returned to orange colour, and 2 could be recovered in >85% yield. Figure 3 shows UV-vis and EPR spectra acquired during electrolysis of 2 at –1.1 V. As the reduction proceeded, the characteristic heH-centred π–π* band shifted from 330 nm for 2 to 340 nm (a 890 cm–1 red-shift) and the dπ(Ru)–π*(pi-) MLCT band shifted from 452
13
nm for 2 to 460 nm (a 385 cm-1 red shift) concurrent with the rise of a prominent shoulder at 395 nm that may be attributed the π-π* transitions of a metal-bound pyridylimino radical [34]. The isosbestic points at 297 and 345 nm suggest the reduction process is a clean transformation. The EPR spectrum reveals a broad isotropic doublet at g = 1.995 with hyperfine coupling to the single imine proton (AH = 22 G), parameters indicative for a ligand-localised SOMO consistent with formation of the metal-bound pyridylimino-radical anion, equation 3 [35, 36].
(a)
(b)
Figure 3. (a) UV-vis and (b) X-band (9.781 GHz) EPR spectra acquired during reductive electrolysis of 2 at –1.10 V in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K.
3.4 Cyclic voltammetry of Hantzsch pyridinium-substituted 3 and 4. Figure 4 presents representative CVs of the pyridinium cation-substituted complexes 3 and 4. The primary reduction in CVs of 3 and 4 is a chemically irreversible process at –0.76 and –1.02 V, respectively. The peak currents for the primary reduction are double those of the other processes in the CVs, and are therefore overall 2e–-processes. Traversing the primary reduction peak for 3 and 4 lead in the return sweep to weak, chemically irreversible anodic process(es) at +0.85 V and at +0.63 and +0.93 V, respectively. These peak potentials are different from those for the Hantzsch dihydropyridine-centred oxidation in CVs of 1 and 2 at +0.99 and +1.20 V, respectively. The observation reveals that the primary reductions of 3 and 4 do not produce dihydropyridinesubstituted 1 and 2 on the (ms) CV timescale. That the primary reduction potential of 3 is 260 mV less negative than that of 4 is significant. As described above, the pyridylimine ligand is conjugated with the Hantzsch pyridinium ring for 3, but is electronically isolated for 4. Electronic delocalisation in 3 and its reduction product(s), but not in 4 and reduction product(s), may account for 3 being the easier to reduce. Simple pyridinium cations typically show a reversible one-electron reduction to afford the neutral pyridinyl radical, which is unstable to rapid non-stereospecific dimerisation [37-40].
14
However, ester-substituted pyridinium cations may undergo two-electron reduction to produce ester-stabilised anions [33, 41], which would lead to equation 4 describing the primary reduction process. Such anions are susceptible to protonation, e.g., by adventitious water, suggesting the corresponding dihydropyridine could be the ultimate product, equation 5. However, the spectroelectochemical results, to be discussed next, reveal that this is not the case.
[CpRuII(pi-he+)(PPh3)]2+ + 2e– [CpRuII(pi-he–)(PPh3 )]
(4)
[CpRuII(pi-he–)(PPh3)] + H+ (H2O) [CpRuII(pi-heH)(PPh3)]+ (+ OH–)
(5)
*
*
Figure 4: CVs of 3 (blue trace) and 4 (red trace) in MeCN−0.1 M [(n-Bu)4N][PF6]; other conditions as listed for Figure 1. The peaks marked by asterisks (*) disappeared when the initial scans were switched at –0.5 V, and the peaks marked by the diamonds () disappeared when the positive sweeps were switched at +1.0 V. Table 3. Electrochemical potentials1 for peaks observed in cyclic voltammograms of 3 and 4.
Complex Ep,c (1st redn) E1/2 (2nd redn) Ep,a (3rd redn) 3 –0.76 –1.18 –1.71 4 –1.02 –1.15 –1.71
1
2
E1/2 (oxidn) Ep (child)2 +1.12 +0.85*, –0.36 +1.11 +0.63*, +0.93*, –0.37 E1/2/V for couples and Ep/V for irreversible processes versus an Ag/AgCl reference electrode for which E1/2 (Fc+/Fc) = +0.49 V. * indicates the anodic child peak marked by an asterisk (*) in Figure 4 that arises in the reverse positive scan after the primary reduction process is traversed, and indicates the cathodic child peak marked by a diamond () in Figure 5 that arises in the reverse cathodic scan after traversing the primary oxidation process. Apart from the pyridinium-centred primary reduction, the CVs of 3 and 4 are similar to CVs
of 1 and 2. For 3 and 4, the pyridylimine-centred (pi/pi•−) couples appear at ca. –1.2 V, respectively, and are followed at ca. –1.7 V by the ester-centred, Hantzsch-pyridine reduction process [33]. The RuII/RuIII couples for 3 and 4 appear to positive potentials at ca. +1.1 V and are not fully chemically
15
reversible, giving rise to a cathodic peak at ca. +0.4 V in the reverse negative sweep for reduction of a product species.
3.5 Spectroelectrochemistry of 3 and 4. Results from UV-vis spectroelectrochemical experiments for 3 and 4 are presented in Table 4. UV-vis spectra recorded during reductive electrolysis 3 at –0.80 V and 4 at –1.00 V, Figure 5, reveal clean replacement of the Hantzsch pyridinium-centered π-π* band (at 290 and 283 nm, respectively) by a new π-π* band at 310 nm for the primary reduction products. During the reductions, the dπ(Ru)–π*(pi-) MLCT band of 3 blue-shifted (by 535 cm–1) from 480 to 468 nm whereas the dπ(Ru)–π*(pi-) MLCT band at 450 nm for 4 did not shift. The UV-vis spectra of the reduction products are different from those of 1 and 2. No radical intermediates were observed when these electrolyses performed in situ within the EPR spectrometer. A fortuitous result throws some light on the identities of the reduction products of 3 and 4. Titrations of 3 and 4 with hydroxide ion indicate the complexes form 1:1 adducts with hydroxide ion, 5 and 6; see Figure 9S in the Supporting Information. Addition of one equivalent of acetic acid or even dilution with water (to >10% v/v) completely reversed the reaction with hydroxide ion and quantitatively regenerated complexes 3 and 4. The ease of reversion of the hydroxide adducts, 5 and 6, back to the parent pyridinium-substituted complexes, 3 and 4, prevented their isolation. The hydroxide adducts, 5 and 6, display identical UV-vis spectra to the reduction products of 3 and 4, respectively; i.e. the reduction product and the hydroxide adduct of 3 or 4 is the same compound. Although hydroxide adducts of Hantzsch pyridinium cations (i.e. Hantzsch 4-hydroxy-1,4dihydropyridines or Hantzsch 2-hydroxy-1,2-dihydropyridine) are not previously reported [42], pyridinium cations do or are predicted to form weak adducts with basic anions including hydroxide ion [43, 44]. The UV-vis spectra for the hydroxide adducts of pyridinium ions that have been characterised also show a band centred at 300–330 nm [43, 44]. Based on these results, the reduction products and hydroxide adducts of 3 and 4 may be tentatively assigned as the corresponding hydroxy-dihydropyridine-substituted complexes, [CpRuII(pi-2heOH)(PPh3)]+ (5) and [CpRuII(pi-3heOH)(PPh3)]+ (6), such as the 4-hydroxy-1,4-dihydropyridine-substituted complexes depicted in Chart 5 (or as the corresponding 2-hydroxy-1,2-dihydropyidine adducts).
16
O N
Ph 3P O OH 2
Ru N N 3
O
O
5: [CpRu(PPh3)(pi-2heOH)]+ 6: [CpRu(PPh3)(pi-3heOH)]+
Chart 5
Table 4. Results from in situ UV-vis spectroelectrochemical studies of Hantzsch pyridiniumsubstituted complexes 3 and 4. λmax/nm (εmax/103 M-1 cm-1)
parent complex … after 1st reduction … after 2nd reduction
… after KOH titration … oxidation product
(a)
3 4 290 (23), 310 sh (17), 480 283 (25), 315 sh (15), 452 (4.8) (5) 310 (21), 468 (3) 310 (28), 450 (8) decomposition 262 sh (27), 305 (28), 400 br (18), 470 sh (10) 310 (25), 462 (5) 310 (27), 447 (6) 287 (20) 282 (21)
(b)
Figure 5. UV-vis spectra acquired during reductive electrolysis of: (a) 3 at –0.80 V and (b) 4 at –1.10 V in MeCN−0.1 M [Bu4N][PF6] at 295 K; also shown in (b) are the spectra of 2 (A) and 6 (B) and a calculated spectrum for a 1:1 mixture of 2 and 6 [((A) + (B))/2], all normalised to the same total concentration of CpRu species (as used in the electrolysis of 4). Some further comment on the electrochemical reduction of 3 or 4. The analysis of the UV-vis spectra shown in Figure 5(b) reveals the hydroxide-adduct 6 is cleanly produced upon reduction of 4, and not a mixture of 6 and dihydropyridine 2 as would result if equations 4 and 5 were followed. The
17
residual water always present in acetonitrile [45] is presumably the source of hydroxide ion. A Pt gauze working electrode was employed in the spectroelectrochemical experiments: Pt very efficiently catalyses reduction of water, equation 6. Thus it seems likely that the hydroxide ion necessary to form 5 and 6 arises from direct the reduction of the residual water at the Pt working electrode, and not from catalysis of water reduction by the complexes. Indeed, CVs of 4 at a glassy carbon working electrode in the presence of 10% v/v water, 10% v/v water and acetic acid (100 equiv.), and 5% v/v water plus p-toluene sulfonic acid were screened for overall catalysis of proton reduction. No catalysis was observed: the peak currents for the primary reduction process remained unchanged in the presence of the proton sources and no new peak was observed ahead of the onset hydrogen evolution observed without the complex (at ca. –1.6 V vs. Ag/AgCl).
2H2O + 2e– H2 + 2OH–
(6)
The second reduction process for 3 or 4 was also investigated. After exhaustive reduction of 3 to afford the primary reduction product, i.e. 5, switching the electrolysis potential to –1.20 V (i.e. negative of the pi/pi•– couple) resulted in decomposition and deposition of intractable precipitate on the working electrode (as was the case for 1, see above). Conversely, after exhaustive reduction of 4 to afford the primary reduction product, i.e. 6, switching the electrolysis potential to –1.20 V resulted the smooth formation of a second reduction product with bands at 305, 400 and 470 nm, Figure 6(a); isosbestic points were observed at 240, 306 and 338 nm. The spectroscopic changes were accompanied by a change in the colour of the solution from orange to dark red. As has already been noted, the 400 nm band is characteristic for a Ru(II)-bound pyridyliminyl radical. Figure 6(b) presents an X-band EPR spectrum recorded during in situ reductive electrolysis of 4 at the same potential (–1.20 V); i.e, complex 6 should first form and then be reduced. The spectrum features a slightly skewed doublet at g = 1.998 with AH = 24 G (imine CH), parameters that are similar to, but not the same as, those for the pyridylimine-centred radical observed upon reduction of 2 (see above). The slightly different parameters accord with a small difference in the tethered substituents to the CpRu(PPh3)-bound iminopyridinyl radical anion [35, 36], i.e. 1,4-dihydropyridine from 2 versus a hydroxy-dihydropyridine from 6.
18
(a)
(b)
Figure 6. (a) UV-vis and (b) X-band EPR (ν = 9.789 GHz) spectra acquired during reductive electrolysis of hydroxide adduct 6 (formed at the primary reduction of 3) at –1.20 V (the second reduction process in the CV for 3) in MeCN−0.1 M [Bu4N][PF6] at 295 K. Finally, oxidative electrolysis of 3 and 4 causes bleaching of the dπ(Ru)–π*(pi-) bands at 480 and 452 nm, respectively, and leaves the other bands slightly perturbed in energy and/or intensity (Figure 10S, Supporting Information). The result is consistent with oxidation to Ru(III)-species, CpRuIII(pi-)(PPh3)2+, equation 2 (see above).
4. Conclusion The redox chemistry of the (cyclopentadienyl)(triphenylphosphine)ruthenium–Hantzsch dihydropyridine conjugates has been studied by cyclic voltammetry allied with EPR and UV-vis spectroelectrochemical and some reactivity studies. Pronounced differences in redox potentials and the products of oxidation and reduction are observed that depend on whether the dihydropyridine/pyridinium substituent is π-delocalised with the pyridylimine chelate and, consequently, the ruthenium centre. The primary oxidation of the ‘free’ dihydropyridine ligands (pi-2heH and pi-3heH) and the dihydropyridine-substituted Ru(II) complex 2 (no conjugation) afforded the anticipated pyridinium cation-substituted products. In contrast, electrochemical oxidation of dihydropyridine-substituted 1 (with conjugated Ru(II) and dihydropyridine centres) cleanly afforded a different, as yet unidentified, product. The primary reduction products of the pyridinium-cation-substituted Ru(II) complexes, 3 and 4, are not the corresponding dihydropyridine-substituted Ru(II) complexes, 1 and 2. The reduction products have identical UV-vis spectra to the hydroxide-adducts 5 and 6 formed by the direct addition of hydroxide ion to 3 and 4. Formation of 5 and 6 is reversed by dilution with water or addition of weak acid (acetic acid). Cyclic voltammetric assays of 4 in the presence of water and aqueous acids revealed no evidence for catalysis of proton reduction.
19
These results will buttress future studies of Hantzsch pyridinium/dihydropyridinesubstituted metal complexes toward development of new, atom-efficient methodologies for Hantzsch ester-based asymmetric syntheses. So far only chemical regeneration of Hantzsch dihydropyridine- and other organohydride-substituted transition metal complexes using, for example, formate or molecular hydrogen as the reductant has lead to new methodology for transfer hydrogenation [14-16]. The present investigations highlight difficulties that remain to be surmounted in (photo)electrochemically-driven regeneration of Hantzsch dihydropyridinesubstituted complexes — such as the formation of the hydroxide adduct rather than the Hantzsch dihydropyridine (formally a hydride ion adduct) upon direct electrochemical reduction. These problems will have to be surmounted if light-driven and/or electricity-driven reductive catalyses based on Hantzsch ester-substituted transition metal catalysts are to be realised.
Acknowledgement We are thankful for the award of DP130103514 from the Australian Research Council, which funded this research.
Appendix A. Supplementary Material The Supplementary Material comprises additional figures and a table, plus cifs for the X-ray crystal structures of 2 and 4 (CCDC no. 1407649-1407650), and can be found in the online version of this article at http://dx.doi.org/10.1016/j.ica.2015.XX.YYY.
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Graphical Abstract (scale size to fit)
Syntheses and detailed cyclic voltammetry and EPR/UV-vis spectroelectrochemical studies of ruthenium complexes with tethered Hantzsch pyridinium/dihydropyridine centres are reported. Inter-centre electronic communication has a significant effect on physicochemical properties. Electrochemical regeneration of the dihydropyridine donor is hampered by formation of hydroxide– pyridinium adduct(s).
23
S0020-1693(16)00030-X http://dx.doi.org/10.1016/j.ica.2016.01.019 ICA 16847
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
23 September 2015 8 January 2016 8 January 2016
Please cite this article as: A. McSkimming, M.M. Bhadbhade, S.B. Colbran, Synthesis and (spectro)electrochemical investigations of coordinatively-saturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.01.019
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Synthesis and (spectro)electrochemical investigations of coordinativelysaturated (cyclopentadienyl)ruthenium–Hantzsch pyridinium/dihydropyridine conjugates Alex McSkimming,1 Mohan M. Bhadbhade2 and Stephen B. Colbran1,* 1
School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia
2
Mark Wainwright Analytical Centre, University of New South Wales, Sydney, NSW 2052, Australia
* Email: [email protected] Abstract Complexes with the CpRu(PPh3) fragment bound by iminopyridine ligands functionalised by a Hantzsch dihydropyridine donor of hydride ion or by a Hantzsch pyridinium acceptor of hydride ion have been prepared, and their redox chemistry studied by cyclic voltammetry and EPR and UV-vis spectroelectrochemical investigations. These Ru(II) complexes have a coordinatively saturated, electronically precise (18-electron) ruthenium(II) centre with a non-labile ligand donor set, which suppresses complicating metal-centred reactivity and, thereby, allows the baseline physicochemical properties of the Hantzsch dihydropyridine/pyridinium-functionalised ligands to be investigated. In Ru(II) complexes, the iminopyridine chelate is linked to the Hantzsch pyridine groups by either an ortho-phenyl bridge (electronically delocalized) or by a meta-phenyl bridge (electronically isolated), which leads to notable differences in spectroscopic properties, even for ruthenium centre, and differences in redox reactions. Of note, the primary electrochemical reduction of the Ru(II) complexes with a Hantzsch pyridinium substituent is centred on this group, but did not afford the corresponding Ru(II) complexes with a 1,4-dihydropyridine substituent. Rather it was found that the reduction products were identical to the 1:1 hydroxide adducts formed upon addition of hydroxide ion to the starting Hantzsch pyridinium-substituted Ru(II) complexes. Based on these results and comparisions with data from the literature, the reduction products and hydroxide adducts are tentatively assigned as the corresponding hydroxy-dihydropyridine substituted Ru(II) complexes (during reduction, hydroxide ion was likely formed from the residual water present in the acetonitrile solvent). Implications for the electrochemical cycling of transition metal catalysts with Hantzsch pyridinium/dihydropyridine functional substituents are considered. Key words Ruthenium; Multifunctional ligand; Hantzsch dihydropyridine/pyridinium; Cyclic voltammetry; Spectroelectrochemistry 1
1. Introduction Arthur Hantzsch first reported his route to the dihydropyridine which bears his name, Hantzsch’s ester (Chart 1), over 130 years ago [1]. After languishing for well over a century, Hantzsch’s ester has advanced over the last decade to the forefront of the synthetic chemists’ armoury as an organic hydride reagent for asymmetric transfer hydrogenations of unsaturated organic substrates, reactions catalysed by chiral Brønsted and Lewis acids [2-9]. Although very useful, this reduction methodology suffers from the costly, atom inefficient, stoichiometric consumption of Hantzsch’s ester. Several very recent studies have revealed that Ru- or Fe-catalysed hydrogenation at high pressure may regenerate Hantzsch’s ester thereby allowing it to be employed in catalytic amounts [10-13].
O
H
H
O OEt
EtO Me
N
Me
H
Chart 1. Hantzsch’s ester
Seeking a versatile catalytic methodology for transfer hydrogenation using Hantzsch’s ester and other organic hydride donors under ambient conditions, we have followed a different line of attack. We recently introduced organic hydride-functionalised transition metal complexes as useful catalysts for the transfer hydrogenation of unsaturated substrates [14], and have reported several proof-of-concept demonstrations [15-18]. For example, Cp*Rh(diimine)Cl complexes were made where the diimines were the new pyridylimine (pi) ligands functionalised by Hantzsch dihydropyridine (heH) or Hantzsch pyridinium cation (he+) moieties [16]. The complex [Cp*Rh(pi2
he)Cl]2+ (Chart 2) proved to be an excellent catalyst for transfer hydrogenation of imines by formate
at room temperature in air. A catalytic cycle involving hydride transfer from a Rh-H species (formed by hydride transfer from coordinated formate) to the pyridinium ring to afford the dihydropyridine and open a site at the Rh(III) centre was proposed. In this cycle, the imine substrate binds the electrophilic Rh(III) centre and is thus activated to back-transfer of hydride from the tethered dihydropyridine. The steps in the catalytic cycle mimick those involving substrate polarisation by metal binding and hydride transfer to/from substrate from/to NAD(P)H/NAD(P)+ in the catalytic cycles of NAD(P)H/NAD(P)+-dependent metallo-(de)hydrogenases [14].
2
2 N Cl O
O
Rh N
O 1O
N 2
[Cp*Rh(pi-2he)Cl] 2+
Chart 2.
This paper is not about catalysis. Rather, in order to advance understanding of transition metal complexes functionalised by Hantzsch dihydropyridine/pyridinium substitutents in general, we have made the new complexes with a CpRu(PPh3) centre bound by dihydropyridine or pyridiniumfunctionalised pyridylimine ligands that are depicted in Chart 3. Detailed spectro-electrochemical studies have been undertaken and are reported herein. The non-labile ligand donor set combined with the coordinative and electronic (18-electron) saturation at the ruthenium(II) centre was deliberately targetted to suppress metal-centred reactivity (including all possiblity of catalytic reactions) thereby allowing the baseline electrochemical properties of the non-innocent Hantzsch dihydropyridine or pyridinium cation-substituted pyridylimine ligands to be examined. The work addresses the important question of whether direct electrochemical regeneration of a Hantzsch dihydropyridine substituent, as is proposed for the active species formed from catalyst [Cp*Rh(pi2
he)Cl]2+ (Chart 2) [16], is possible. Also, interesting effects resulting from the presence or absence
electronic communication between the metal and dihydropyridine/pyridinium cation centres are revealed.
2
O
Ph 3P O
N
H 2
Ru
O
N
O
N
N 3
O
Ph3P 2
Ru N N 3
O
O
1: [CpRu(PPh 3)(pi-2heH)] + 2: [CpRu(PPh 3)(pi-3heH)] +
O
3: [CpRu(PPh 3)(pi-2he)] 2+ 4: [CpRu(PPh 3)(pi-3he)] 2+
Chart 3
3
2. Experimental Section 2.1 General methods. NMR spectra were obtained on Bruker Avance DPX-300, 400, 500 and 600 spectrometers at 298 K operating at 300, 400, 500 and 600 MHz frequency for 1H NMR experiments and at 75.5, 100.6, 125.7 and 150.9 MHz for 13C NMR experiments, respectively. 1H and 13C chemical shifts were calibrated against solvent signals; spin-spin couplings are given in Hz. EPR spectra were recorded using a Bruker EMX EPR spectrometer at 298 K. FT-IR spectra were recorded on KBr discs using a Nicolet Avatar 360 FT-IR spectrometer at 2 cm-1 resolution. Mass spectra were run on a Thermo Fisher Scientific Orbitrap LTQ XL ion trap mass spectrometer using a nanospray ionisation source. UV-Vis spectra were recorded using a Varian Cary 50 Bio UV-Visible spectrophotometer. Elemental analyses were obtained at the Microanalytical Unit of the Research School of Chemistry, Australian National University.
2.2 Materials. Solvents were dried and obtained under dinitrogen from a PureSolv MD solvent purification system. The ligands, pi-2heH and pi-3heH, were available from a previous study [16]. [CpRuCl(PPh3)2] was synthesised according to a literature procedure [19]. 4-Methyl-N-(4-methylbenzylidene)aniline was synthesised heating equimolar amounts of p-tolylaldehyde and p-toluidine at reflux in ethanol for 30 min., cooling and collecting the resulting crystalline solid by filtration. Tetra(nbutyl)ammonium hexafluorophosphate was recrystallised from acetone and dried under vacuum before use. All other chemicals were commercial and were used as obtained.
2.3 Syntheses of complexes.
pi-2heH
pi-3heH
Chart 4. Atom labelling scheme adapted for describing the NMR spectra of the Hantzsch (dihydro)pyridine-substituted ligands.
4
[CpRuII(pi-2heH)(PPh3)][PF6] (1) Ligand pi-2heH (30 mg, 0.072 mmol), [CpRu(PPh3)2Cl] (52 mg, 0.072 mmol) and K[PF6] (100 mg, 0.54 mmol) were suspended in de-aerated methanol/THF (1:4, 30 mL) and stirred at reflux under dinitrogen overnight. The solvent was removed in vacuo, the resulting residue diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent was removed in vacuo. The crude mixture was purified using flash column chromatography on neutral alumina, eluting first with ether, then dichloromethane-acetone (4:1). The orange-red band was collected, the solvent removed in vacuo and the residue crystallised from acetone-toluene to give dark red crystals of the product, which were washed with ether and dried under vacuum (21 mg, 28%). 1H NMR (400 MHz, (CD3)2C(O)): δ 9.63 (d, J = 6, 1H, H-11), 8.14 (d, JPH = 2, 1H, H-i), 7.85 (t, J = 8, 1H, Ar-H), 7.54 (d, J = 8, 1H, Ar-H), 7.37 (t, J = 8, 3H, p-PPh3), 7.45 (t, J = 8, 6H, m-PPh3), 7.38 (t, J = 8, 1H, Ar-H), 7.20 (m, br, 6H, o-PPh3), 7.12−7.17, (m, 2H, 2 x Ar-H), 7.08 (d, J = 8, 1H, Ar-H), 6.82 (td, J = 8,2, 1H, Ar-H), 5.64 (s, 1H, H-g), 4.90 (s, 5H, Cp-H), 3.39 (s, 3H, H-f/f’), 3.37 (s, 3H, H-f/f’), 3.18 (s, 3H, H-a), 2.56 (s, 3H, H-b/b’), 2.52 (s, 3H, H-b/b’).
13
C {1H} NMR (100.6 MHz, (CD3)2C(O)): 168.61 (C-e/e’), 168.56 (C-e/e’), 166.10 (d, J =
2PH, Ci), 157.5 (Ar-C), 157.4 (Ar-C), 157.3 (Ar-C), 153.4 (Cc), 153.3 (Cc’), 152.2 (Ar-C), 136.9 (Ar-C), 135.1 (Ar-C), 134.6 (d, JCP = 11, o-PPh3), 132.4 (d, JCP = 40, ipso-PPh3), 131.9 (d, JCP = 2, p-PPh3), 130.1 (d, JCP = 10, m-PPh3), 129.3 (Ar-C), 127.6 (Ar-C), 127.2 (Ar-C), 127.1 (Ar-C), 126.5 (Ar-C), 105.3 (Cd/d’), 104.4 (C-d/d’), 80.9 (d, JCP = 1, Cp), 51.8 (C-f/f’), 51.6 (C-f/f’), 35.1 (C-a), 34.4 (C-g), 17.1 (Cb/b’), 17.0 (C-b/b’).
31
P{1H} NMR (161.9 MHz, (CD3)2C(O)): δ 45.9 (s, PPh3), -144.2 (sept, J = 700,
[PF6]−). FT-IR (KBr): νmax cm-1 1704 (s, C=O), 1691 (s, C=O), 1636 (s), 1571 (m), 1532 (m), 1571 (m), 1480 (m), 1449 (m), 1434 (s), 1381 (m), 1357 (m), 1289 (m), 1249 (m), 1232 (m), 1289 (s), 1212 (s), 1169 (s), 1126 (m), 1106 (m), 1090 (m), 1054 (m), 1006 (m), 955 (w), 939 (w), 919 (w), 838 (vs), 778 (m), 764 (m), 747 (m), 696 (s), 557 (s), 525 (s), 514 (m), 495 (m), 459 (m), 445 (m), 419 (w). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 225 (45), 270 (sh, 15), 323 (br, 13.3), 455 (br, 5.0). ESI HR-MS: Found: m/z 848.2181, Calcd. for (M+): m/z 848.2186. Anal. Calcd. for C47H45F6N3O4P2Ru: C, 56.85; H, 4.57; N, 4.23. Found: C, 57.10; H, 4.65; N 3.97 %.
[CpRuII(pi-2he)(PPh3)][PF6]2 (3) DDQ (2.8 mg, 0.012 mmol) was added to 1 (10.0 mg, 0.012 mmol) in acetonitrile (0.5 mL) and the solution left to stand for 10 mins. The solvent was removed in vacuo and the reaction mixture purified using flash column chromatography on silica gel (NaCl (sat.) in MeOH eluent). The red band was collected and the solvent removed in vacuo. The residue was dissolved in acetone and filtered to remove sodium chloride. K[PF6] (sat., aq.) was added to precipitate the crude product
5
which was collected by filtration, washed with water and recrystallised from acetone-cyclohexane to give 3 as crimson needles (9.3 mg, 80%). TEMPO[BF4] could be used in place of DDQ; the yield and work-up remained essentially unchanged. 1H NMR (400 MHz, (CD3)2C(O)): δ 9.36 (d, J = 5, 1H, H-11), 7.85 (d, J = 8, 1H, Ar-H), 7.99 (d, J = 3, 1H, H-i), 7.46-7.50 (m, 5H, p-PPh3,2 x Ar-H), 7.38−7.41 (t, J = 8, 6H, o-PPh3), 7.18−7.22 (m, 2H, 2 x Ar-H), 7.00−7.14 (m, 8H, m-PPh3, 2 x Ar-H), 4.94 (s, 5H, Cp-H), 4.10 (s, 3H, H-a), 3.80 (s, 3H, H-f/f’), 3.66 (s, 3H, H-f/f’), 2.89 (s, 3H, H-b/b’), 2.63 (s, 3H, H-b/b’). 13C {1H} NMR (100.6 MHz, CD3CN): δ 164.9 (C-e/e’), 163.7 (C-e/e’), 161.9 (d, JCP = 2, C-i), 156.0 (Ar-C), 155.9 (Ar-C), 155.8 (Ar-C), 155.8 (Ar-C), 154.7 (Ar-C), 150.5 (Ar-C), 147.8 (Ar-C), 134.8 (Ar-C), 134.4 (Ar-C), 132.7 (d, JCP = 11, o-PPh3), 131.5 (Ar-C), 131.1 (Ar-C), 130.7 (Ar-C), 150.6 (Ar-C), 150.5 (Ar-C), 130.1 (d, JCP = 2, p-PPh3), 130.4 (d, JCP = 40, ipso-PPh3), 129.5 (Ar-C), 129.3 (Ar-C), 128.9 (d, J = 10, mPPh3), 125.4 (Ar-C), 123.9 (Ar-C), 79.6 (d, JCP = 1, Cp-C), 54.0 (C-f/f’), 53.8 (C-f/f’), 42.2 (Ca), 20.6 (Cb/b’), 20.1 (C-b/b’). 31P{1H} NMR (161.9 MHz, CD3CN): δ 47.1 (s, PPh3), -144.3 (sept, J = 700, [PF6]−). FT-IR (KBr): νmax cm-1 1741 (s, C=O), 1607 (m), 1555 (m), 1527 (w), 1481 (m), 1469 (m), 1437 (m), 1383 (w), 1337 (m), 1301 (w), 1252 (s), 1202 (m), 1164 (m), 1091 (m), 1059 (m), 1015 (w), 999 (w), 917 (w), 841 (vs), 774 (m), 744 (m), 699 (m), 558 (s), 525 (s), 513 (m), 494 (m), 458 (w). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 290 (22.5), 310 (br, sh, 17), 480 (br, 5.0). ESI HR-MS: Found: m/z 992.1746, Calcd. for (M+): m/z 992.1749. Anal. Calcd. for C47H44F12N3O4P3Ru: C, 49.66; H, 3.90; N, 3.70. Found: C, 49.48; H, 3.97; N 3.63 %.
[CpRuII(pi-3heH)(PPh3)][PF6] (2) Ligand pi-3heH (50 mg, 0.119 mmol), [CpRu(PPh3)2Cl] (88 mg, 0.121 mmol) and K[PF6] (100 mg, 0.54 mmol) were suspended in de-aerated methanol and stirred at reflux under dinitrogen overnight. The solvent was removed in vacuo, the resulting residue diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent was removed in vacuo. The crude mixture was purified using flash column chromatography on neutral alumina, eluting first with ether, then dichloromethane-acetone (4:1). The orange-red band was collected, the solvent removed in vacuo and the residue crystallised from acetonitrile-ether to give large red prisms of 2 (80 mg, 68%). 1H NMR (400 MHz, CDCl3): δ 9.28 (d, J = 6, 1H, H-11), 8.14 (d, JPH = 2, 1H, H-i), 7.77 (d, J = 8, 1H, Ar-H), 7.68 (t, J = 8, 1H, Ar-H), 7.37 (t, J = 8, 4H, p-PPh3 + Ar-H), 7.29 (td, J = 8,2, 1H, Ar-H), 7.26 (td, J = 8,2, 6H, m-PPh3), 7.17 (d, J = 8, 1H, ArH) 6.99 (t, J = 8, 1H, Ar-H), 6.88−6.95 (m, br, 6H, o-PPh3), 6.10 (dd, J = 8, 2, 1H, Ar-H), 5.27 (s, 1H, Hg), 4.70 (s, 5H, Cp-H), 3.78 (s, 3H, H-f/f’), 3.73 (s, 3H, H-f/f’), 3.26 (s, 3H, H-a), 2.54 (s, 3H, H-b/b’), 2.53 (s, 3H, H-b/b’).
13
C {1H} NMR (100.6 MHz, CDCl3): δ 168.39 (C-e/e’), 168.27 (C-e/e’), 160.82 (d, J
= 2, C-i), 155.8 (Ar-C), 155.6 (Ar-C), 153.6 (Ar-C), 153.6 (Ar-C), 150.2 (C-c/c’), 150.0 (C-c/c’), 135.4 (Ar-
6
C), 132.9 (d, J = 11, o-PPh3), 130.8 (d, J = 40, ipso-PPh3) (Ar-C), 130.4 (d, JCP = 2, p-PPh3), 128.5 (Ar-C), 128.4 (Ar-C), 124.49 (d, J = 10, m-PPh3), 127.4 (Ar-C), 125.5 (Ar-C), 122.3 (Ar-C), 119.1 (Ar-C), 105.2 (C-d/d’), 79.56 (d, JCP = 1, Cp-C), 51.5 (C-f/f’), 51.4 (C-f/f’), 38.4 (C-a), 34.5 (C-g), 16.9 (C-b/b’), 16.7 (Cb/b’). 31P{1H} NMR (161.9 MHz, CDCl3): δ 47.4 (s, PPh3), -144.2 (sept, J = 700, [PF6]−). FT-IR (KBr): νmax cm-1 1694 (s, C=O), 1638 (m), 1572 (m), 1535 (w), 1478 (m), 1434 (m), 1383 (m), 1361 (m), 1328 (w), 1288 (m), 1249 (w), 1212 (s), 1188 (m), 1169 (m), 1136 (w), 1108 (m), 1092 (m), 1052 (m), 1003 (m), 933 (w), 838 (vs), 791 (w), 752 (w), 743 (w), 698 (m), 557 (m), 529 (m), 512 (m), 491 (w), 465 (w). UVVis (MeCN): λmax nm (εmax/103 M-1 cm-1) 230 (54.0), 330 (br, 17.1), 450 (5.3). ESI HR-MS: Found: m/z 848.2184, Calcd. for (M+): m/z 848.2186. Anal. Calcd. for C47H45F6N3O4P2Ru: C, 56.85; H, 4.57; N, 4.23. Found: C, 57.03; H, 4.39; N 4.35 %.
[CpRuII(pi-3he)(PPh3)][PF6]2 (4) Method (a). Chemical oxidation: TEMPO[BF4] (6 mg, 0.025 mmol) was added to 2 (20 mg, 0.020 mmol) in acetonitrile (5 mL) and the solution left to stand for 10 mins. The solvent was removed in vacuo and the reaction mixture purified using flash column chromatography on silica gel (NaCl (sat.) in MeOH). The red band was collected and the solvent removed in vacuo. The residue was dissolved in acetone and filtered to remove sodium chloride. K[PF6] (sat., aq.) was added to precipitate the crude product which was collected by filtration, washed with water and recrystallised from methanol to give 4 as maroon crystals (20 mg, 88%). DDQ could be substituted for TEMPO[BF4]; the preparation and yield remained essentially unchanged. 1H NMR (400 MHz, (CD3)2CO): δ 9.59 (d, J = 5, 1H, H-11), 8.66 (d, J = 3, 1H, H-i), 7.85 (t, J = 8, 1H, Ar-H), 7.80, (d, J = 8, 1H, Ar-H), 7.68 (t, J = 8, 1H, Ar-H), 7.56 (d, J = 8, 1H, Ar-H), 7.30−7.45 (m, 11H, p-PPh3, m-PPh3 + 2 x Ar-H), 7.10−7.20 (m, br, 7H, o-PPh3 + Ar-H), 5.00 (s, 5H, Cp-H), 4.67 (s, 3H, H-a), 3.83 (s, br, 3H, H-f/f’), 3.74 (s, br, 3H, H-f/f’), 3.19 (s, br, 3H, H-b/b’), 3.11 (s, br, 3H, H-b/b’). 13C {1H} NMR (100.6 MHz, (CD3)2CO): δ 164.2 (C-e/e’), 163.7 (C-e/e’), 163.4 (d, J = 2, C-i), 156.3 (Ar-C), 156.2 (Ar-C), 155.8 (Ar-C), 155.5 (Ar-C), 155.4 (Ar-C), 154.3 (Ar-C), 150.9 (Ar-C), 135.8 (Ar-C), 133.5 (Ar-C), 133.2 (Ar-C), 132.9 (d, JCP = 11, o-PPh3), 131.1 (d, JCP = 40, ipso-PPh3), 130.2 (Ar-C), 130.1 (d, JCP = 2, p-PPh3), 129.0 (Ar-C), 128.7 (d, JCP = 10, m-PPh3), 127.4 (Ar-C), 125.7 (Ar-C), 123.3 (Ar-C), 123.2 (Ar-C), 79.2 (d, JCP = 1, Cp-C), 53.6 (C-f/f’), 53.4 (C-f/f’), 42.2 (C-a), 19.71 (br, C-b + C-b’). 31P{ 1H} NMR (161.9 MHz, CD3CN): δ 48.2 (s, PPh3), -144.3 (sept, J = 700). FT-IR (KBr): νmax/cm-1 1741 (s, C=O), 1606 (m), 1557 (m), 1531 (w), 1481 (w), 1471 (w), 1436 (m), 1383 (w), 1340 (m), 1308 (w), 1251 (s), 1202 (m), 1161 (m), 1093 (m), 1058 (m), 1013 (w), 999 (w), 839 (vs), 770 (w), 745 (m), 698 (s), 558 (s), 525 (m), 514 (m), 495 (m). UV-Vis (MeCN): λmax/nm (εmax/103 M-1 cm-1) 283 (24.5), 315 (sh, 15), 452 (4.8). ESI HR-MS: Found: m/z 992.1751, Calcd. for
7
(M+): m/z 992.1749. Anal. Calcd. for C47H44F12N3O4P3Ru.CH3OH: C, 49.28; H, 4.22; N, 3.59. Found: C, 49.22; H, 3.85; N 3.68 %.
Method (b). Electrochemical oxidation: Complex 2 (20 mg, 0.020 mmol) in MeCN-0.1 M K[PF6] under dinitrogen was exhaustively oxidised at +1.00 V. Upon completion of the electrolysis, the solvent was removed in vacuo, the crude reaction mixture diluted with water and extracted with several portions of dichloromethane. The combined organic extracts were dried over sodium sulfate, filtered and the solvent removed in vacuo. The crude residue was purified using chromatography as described above (12 mg, 54 % of 4 obtained). Over-oxidation of 2 to Ru(III) species perhaps accounts for the moderate yield.
2.4 Electrochemistry. CVs were recorded using a Pine Instrument Co. AFCBP1 bipotentiostat. A three-electrode system was used, consisting of a glassy carbon working electrode (1 mm diameter), a platinum wire counter electrode, and an Ag/AgCl reference electrode. Prior to recording each CV, the working electrode was polished with increasingly fine alumina, rinsed with water, then the solvent to be used in the experiment, and dried by rubbing with a clean tissue. The glassy carbon working electrode was then “conditioned” by repeatedly scanning the CV of the blank background for the solventelectrolyte solution between the onsets of anodic and cathodic solvent discharge limits until a constant current response was obtained (~ 10 sweeps). Ferrocene was employed as a reference compound, and gave E1/2 (Fc+/Fc) = 0.49 V vs. AgCl/Ag. UV-Vis spectroelectrochemical experiments were carried out using an airtight quartz cuvette (1 mm path-length) with a light-transparent platinum-gauze working electrode fitted with fritseparated platinum wire auxiliary and Ag/AgCl reference electrodes outside the light-path. EPR spectroelectrochemical experiments utilised a vacuum-tight 0.5 mm quartz cell fitted with spatially separated platinum wire working and auxiliary electrodes and a silver wire pseudoreference electrode. Quantitative bulk electrolysis was performed using a conventional glass ‘H-cell’ with the compartments containing the platinum gauze working and auxiliary electrodes separated by two porosity 4 glass frits; the same Ag/AgCl reference electrode as was used in the CV experiments.
3. Results and Discussion 3.1 Synthesis and Spectroscopic Properties
8
The targets for this investigation were complexes [CpRu(L)(PPh3)][PF6]n, 1: L = pi-2heH, n = 1; 2: L = pi-3heH, n = 1; 3: L = pi-2he+, n = 2; 4: L = pi-3he+, n = 2), Chart 3. Complexes 1 and 2 were synthesised in low (28 %) and good (68 %) yield, respectively, by heating equimolar amounts of the appropriate ligand precursor, pi-2heH or pi-3heH, and [CpRuCl(PPh3)2] together in the presence of excess K[PF6] in tetrahydrofuran-methanol (for 1; yield 28%) and methanol (for 2; yield 68%). In the synthesis of 1, the mixed solvent was necessary for solubility reasons and products from the hydrolysis of the pyridylimine were observed; the lower yields of 1 likely result from imine hydrolysis being competive with the complexation of the bulkier pi-2heH ligand to the ruthenium centre. Complexes 1 or 2 were selectively oxidised by TEMPO[BF4] [20, 21] or by 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)
to the corresponding Hantzsch
pyridinium-substituted
complexes,
[CpRu(L’)(PPh3)][PF6]2 (3: L’ = pi-2he+; 4: L’ = pi-3he+), Chart 1 . The use of TEMPO+ in these hydride transfer reactions is novel and proved highly effective affording 3 and 4 in over 80% yield. The reverse reductions of 3 and 4 to 1 and 2, respectively, were not attempted. All four ruthenium complexes 1–4 were air stable in both the solid state and in acetone or acetonitrile solutions. The 1H,
13
C{1H} and
31
P{1H} NMR, IR and UV-vis spectra of the Hantzsch dihydropyridine-
substituted complexes, 1 and 2, are similar, and distinct from those for the Hantzsch pyridiniumsubstituted complexes, 3 and 4, which are also similar. Representative 1H NMR spectra, of 2 and 4, are shown in Figures 1S and 2S (Supporting Information). Of note, the spectra of 1 and 2 show five singlets for the inequivalent methyl groups — rotation of the dihydropyridine ring can not exchange the methyl and methyl ester substituents. For 3 and 4, however, fast rotation of the Hantzsch pyridinium ring relative to the NMR timescale would lead to these pairs of methyl and methyl ester groups becoming equivalent. The 1H NMR spectrum of 3 shows five sharp methyl singlets revealing pyridinium ring rotation is relatively slow in this hindered complex. Five methyl singlets (at δ 4.67, 3.83, 3.74, 3.19 and 3.11) are also seen in the 1H NMR spectrum of 4, but the four downfield signals are heavily broadened consistent with the rotation of the pyridinium ring beginning to exchange the pairs of methyl groups on the timescale of the NMR experiment. Also of note, the singlet for the 4dihydropyridine proton appears at δ 5.64 for 1 and at δ 5.27 for 2, and is absent in the 1H NMR spectra of 3 and 4. The imine C-H appears as a doublet at ca. δ 8.14 (4JPH = 2 Hz) for 1 and 2 and at ca. δ 7.99 (4JPH = 3 Hz) for 3 and 4. FT-IR spectra of 1 and 2 exhibit strong ester carbonyl bands at 1704 and 1694 cm-1, respectively. The ester carbonyl bands appear ca. +45 cm-1 to higher energy at 1741 cm-1 for 3 and 4, which is consistent with polarisation of the C=O bond by the pyridinium ring. UV-vis spectra of 1 and 2 show bands at 225, 323 and 455 nm and at 220, 330 and 451 nm, respectively, whereas 3 shows bands at 290, 310 and 480 nm and 4 bands at 283, 318 and 452 nm. The broad bands at ca. 330 nm
9
(εmax ≈ 1.5 × 104 M-1 cm-1) are unique to complexes 1 and 2 and are also present with similar intensities in the UV-vis spectra of the Hantzsch dihydropyridine-substituted ligands alone. These bands are ascribed to the characteristic π → π* transitions(s) of a 1,4-dihydropyridine [22-25]. The lowest-energy bands (εmax ≈ 0.5 × 104 M-1 cm-1) at ca. 450 nm for 1, 2 and 4 and at 480 nm for 3 arise from dπ(Ru) → π*(pi-) MLCT transitions [26-31]. The difference in the MLCT band energies is negligible between complexes 2 and 4, but is appreciable between 1 and 3 (~1145 cm–1). πDelocalisation between the CpRu(pi-)(PPh3) centre and its pendant heH/he+ group is possible with the ortho-phenyl link in the 2-isomers 1 and 3, but not with the meta-phenyl link in the 3-isomers 2 and 4 (e.g., see Figure 4S, Supporting Information); thus electronic delocalisation between Ru and he+ centres likely underpins the appreciable red shift for the MLCT band of 3 compared to that of 1. Crystal structures of 2 and 4 were also obtained. Unfortunately, the quality of both structures is poor, and they are presented in the Supporting Information (see Fig. 11S, Table 1S and the accompanying text).
3.2 Cyclic voltammetry of Hantzsch dihydropyridine-substituted 1 and 2. The redox chemistry of the complexes was assayed by cyclic voltametry. Cyclic voltammograms (CVs) of the ligands , pi-2heH and pi-3heH, are presented in Figure 5S in the Supporting Information, and show an irreversible primary oxidation at ca. +1.0 V and an irreversible primary reduction process at –1.83 V for pi-2heH and –1.62 V for pi-3heH. The primary redox processes are dihydropyridine-centred. Figure 1 gives representative CVs of complexes 1 and 2, initiated to positive potential, and Table 1 provides a summary of the electrochemical potentials for the observed redox processes. The primary oxidation process at ca. +1.1 V arises from overlap of the irreversible heH-centred oxidation (which is seen in the CVs of the ligands alone; e.g., Figure 5S, Supporting Information), and the closely following Ru(III)/Ru(II) couple for the CpRu(pi-)(PPh3) centre, equation 2, which is electrochemically reversible {∆Ep ≈ 80 mV cf. ∆Ep = 75 mV for the ferrocenium (Fc+)/Fc couple at v = 100 mV s-1} but shows limited chemical reversibility (ip,c/ip,a ≈ 0.75). For comparison, CVs of [CpRuII(bpy)(PPh3)]+ show a reversible Ru(III)/Ru(II) couple at +1.03 V [32]. The small cathodic child peaks at ca. +0.37 V in the return negative sweeps after traversing the overlapping oxidation processes may arise from product(s) of ruthenium-centred oxidation as these peaks do not occur in CVs of the ligands alone (Figure 5S, Supporting Information). The product cathodic peak at –0.95 V in the return sweep for 2 has identical potential to the primary reduction of pyridinium cation-substituted 4 (see below), thus revealing the oxidation of 2 to produce 4, equation 1, within the (ms) timescale of the CV experiment. However, the product cathodic peak at –1.06 V in the
10
return sweep for 1 differs in potential from the primary reduction of 3, which appears at –0.76 V (see below). Therefore, oxidation of 1 does not produce pyridinium cation-substituted 3 on the CV timescale. The conjugation of the heH centre with, and/or the proximity to, the metal centre in 1 results in a change in the oxidative electrochemistry compared to 2. The nature of the oxidation product is considered below after presentation of all results.
1st oxid.
[CpRuII(pi-3heH)(PPh3)]+ [CpRuII(pi-3he+)(PPh3)]2+ + H+ + 2e–
(1)
2nd oxid.
[CpRuII(pi- )(PPh3)]z+ [CpRuIII(pi- )(PPh3)](z+1)+ + e–
(2)
* *
Figure 1. CVs of 1 (blue trace) and 2 (red trace) in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K using a 1.0 mm diameter minidisk glassy carbon working electrode and scan rate = 300 mV s–1. When the CVs were switched at +0.8 V, the peaks indicated by the asterisks disappeared. Table 1. Electrochemical potentials1 for peaks observed in cyclic voltammograms of 1 and 2.
1
2
Complex E1/2 (1st redn) E1/2 (2nd redn) Ep,a (1st oxidn) E1/2 (2nd oxidn) Ep,c (*)2 1 –1.27 –1.86 +0.98 +1.05 –0.95 2 –1.10 –1.70 +1.13 (sh) +1.17 –1.06 E1/2/V for couples and Ep/V for irreversible processes versus an Ag/AgCl reference electrode for which E1/2 (Fc+/Fc) = +0.49 V. Ep,c (*) is for the prominent cathodic child peak (marked by an asterisk in Figure 2) that arises in the reverse positive scan after the primary oxidation process is traversed. To negative potential in CVs of 1 and 2, the Nernstian primary reduction processes (ΔEp ≈ 70
mV) at –1.27 and –1.10 V, respectively, are attributed to 1e–-reduction of the pyridylimine group to the corresponding pyridylimino-radical, equation 3. The pyridylimino-centred radical product from 2 was observed by EPR spectroscopy (see below). That pyridylimine-centred reduction is observed for 1 and 2, but not for pi-2heH or pi-3heH is consistent with polarisation upon binding to Ru(II) ion raising the pi-/pi-•– reduction potential (i.e., reduction at more positive potentials). The second reduction process, at –1.86 V for 1 and at –1.67 V for 2, exhibits poor chemical reversibility and is
11
within 40 mV of that found for the respective isolated pi-heH ligand, and is typical for a 1e--reduction of the diester-substituted dihydropyridine ring [33].
[CpRuII(pi-heH)(PPh3)]2+ + e– [CpRuII(pi•–-heH)(PPh3)]+
(3)
3.3 Spectroelectrochemistry of 1 and 2 Further insights into the primary redox processes exhibited by 1 and 2 were provided by UVvis and EPR spectroelectrochemistry. UV-vis band data for the complexes and electrolysis products are given in Table 2. First, oxidation of 2 is considered, as the results are most easily understood. Figure 3(b) shows the changes in the the UV-spectra that accompany electrolysis of 2 at +1.0 V. The UV-vis bands for 2 (at 230 and 330 and 450 nm, see above) are smoothly replaced by those for 4 (at 283 and 452 nm, see above); isosbestic points indicative for a clean 1:1 transformation are observed at 267 and 307 nm. Notably, the dπ(Ru)–π*(pi-) MLCT band [26-31] at 450 nm does not shift between 2 and 4, which accords with isolation of the Ru(II) and heH/he+ centres. No radical intermediates were detected by EPR spectroscopy during an oxidative electrolysis of 2 in situ in an EPR spectroelectrochemical cell. Therefore, radical intermediate(s), if any, must be short-lived. That the primary oxidation product of 2 is 4 was confirmed by exhaustive electrolysis of 2 at +1.0 V in acetonitrile–K[PF6]; 4 was isolated in 54% yield after column chromatography (see the Experimental section). These results verify the conclusion from analysis of the CVs of 2 that the primary oxidation is dihydropyridine-centred and that pyridinium cation-substituted 4 is the product, equation 1.
Table 2. Wavelength and extinction coefficient data for 1 and 2 and their electrolysis products.
λmax/nm (εmax/103 M-1 cm-1) parent complex
… after 1st oxidation … after 2nd oxidation … after 1st reduction
1 225 (45), 270 sh (15), 323 (13), 455 (5.0) 312 (31), 360 sh (9), 475 (5.5) 267 (18), 310 (29), 360 sh (14) decomposes
2 230 (54), 330 (17), 451 (5.3)
283 (23), 452 (3.9) 280 (23), 435 sh (2.2) 340 (16), 400 sh (12), 460 (9), 530 sh (6)
The UV-vis spectrum of 1, with bands at 225, 323 and 455 nm, is similar to that of 2. However, UV-vis spectra acquired during oxidative electrolysis of 1 at +0.9 V, Figure 3(a), show that the oxidation of 1 follows a different course to oxidation of 2. This observation concurs with the deductions from the CV results, see above. The electronic bands of 1 were replaced by those for the product, which shows a sharp, intense peak at 312 nm (εmax ~ 3.1 × 104 M-1 cm-1) and a 12
dπ(Ru)–π*(pi-) MLCT band [26-31] that is red-shifted by 926 cm-1 to 475 nm. Isosbestic points at 270, 396, 460 and 525 nm suggest a clean transformation from 1 to its first oxidation product. The final spectrum clearly reveals that pyridinium-substituted 3 is not the oxidation product (e.g., the UV-vis spectrum of 3 exhibits a shoulder at 310 nm that is an order of magnitude weaker in extinction coefficient; compare Figure 2(a) with Figure 5(a) below). The orange solutions of 1 became intensely orange-red as the oxidation product formed. No radical intermediate(s) was detected during in situ oxidative electrolysis of 1 by EPR spectroscopy. The intensely orange-red primary oxidation product was extremely air-sensitive, which thwarted further characterisation of it.
(a)
(b)
Figure 2. UV-Vis spectra acquired using a transparent thin-layer Pt-gauze electrode cell during oxidative electrolysis of (a) 1 at +0.90 V and (b) 2 at +1.00 V in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K. In these and subsequent plots of UV-vis spectroelectrochemical data, the bold red line indicates the initial, pre-electrolysis spectrum and the bold deep blue line indicates the final, post-electrolysis spectrum; the arrows indicate the direction of change. Continued oxidative electrolysis of 1 at +1.2 V or of 2 at +1.1 V led to loss the bands at 475 and 452 nm, respectively, and only slight intensity changes in the other (Hantzsch group-centred) bands (Table 1 and Figure 8S, Supporting Information). The changes were accompanied by a colour change from orange-red to yellow-brown. In concurrence with the conclusions drawn from the CVs (see above), the bleaching of dπ(RuII)–π*(pi-) MLCT bands upon exhaustive oxidation is consistent with formation of a CpRuIII(pi-)(PPh3) centre, equation 2. Reduction of complexes 1 and 2 was also studied. Electrolysis of 1 at –1.3 V led to complete decomposition and deposition of insoluble black material on the Pt-mesh cathode. Upon reductive electrolysis of 2, the solution turned from orange to dark red and became highly air sensitive; upon exposure to atmospheric oxygen, the dark red solution rapidly returned to orange colour, and 2 could be recovered in >85% yield. Figure 3 shows UV-vis and EPR spectra acquired during electrolysis of 2 at –1.1 V. As the reduction proceeded, the characteristic heH-centred π–π* band shifted from 330 nm for 2 to 340 nm (a 890 cm–1 red-shift) and the dπ(Ru)–π*(pi-) MLCT band shifted from 452
13
nm for 2 to 460 nm (a 385 cm-1 red shift) concurrent with the rise of a prominent shoulder at 395 nm that may be attributed the π-π* transitions of a metal-bound pyridylimino radical [34]. The isosbestic points at 297 and 345 nm suggest the reduction process is a clean transformation. The EPR spectrum reveals a broad isotropic doublet at g = 1.995 with hyperfine coupling to the single imine proton (AH = 22 G), parameters indicative for a ligand-localised SOMO consistent with formation of the metal-bound pyridylimino-radical anion, equation 3 [35, 36].
(a)
(b)
Figure 3. (a) UV-vis and (b) X-band (9.781 GHz) EPR spectra acquired during reductive electrolysis of 2 at –1.10 V in MeCN−0.1 M [(n-Bu)4N][PF6] at 295 K.
3.4 Cyclic voltammetry of Hantzsch pyridinium-substituted 3 and 4. Figure 4 presents representative CVs of the pyridinium cation-substituted complexes 3 and 4. The primary reduction in CVs of 3 and 4 is a chemically irreversible process at –0.76 and –1.02 V, respectively. The peak currents for the primary reduction are double those of the other processes in the CVs, and are therefore overall 2e–-processes. Traversing the primary reduction peak for 3 and 4 lead in the return sweep to weak, chemically irreversible anodic process(es) at +0.85 V and at +0.63 and +0.93 V, respectively. These peak potentials are different from those for the Hantzsch dihydropyridine-centred oxidation in CVs of 1 and 2 at +0.99 and +1.20 V, respectively. The observation reveals that the primary reductions of 3 and 4 do not produce dihydropyridinesubstituted 1 and 2 on the (ms) CV timescale. That the primary reduction potential of 3 is 260 mV less negative than that of 4 is significant. As described above, the pyridylimine ligand is conjugated with the Hantzsch pyridinium ring for 3, but is electronically isolated for 4. Electronic delocalisation in 3 and its reduction product(s), but not in 4 and reduction product(s), may account for 3 being the easier to reduce. Simple pyridinium cations typically show a reversible one-electron reduction to afford the neutral pyridinyl radical, which is unstable to rapid non-stereospecific dimerisation [37-40].
14
However, ester-substituted pyridinium cations may undergo two-electron reduction to produce ester-stabilised anions [33, 41], which would lead to equation 4 describing the primary reduction process. Such anions are susceptible to protonation, e.g., by adventitious water, suggesting the corresponding dihydropyridine could be the ultimate product, equation 5. However, the spectroelectochemical results, to be discussed next, reveal that this is not the case.
[CpRuII(pi-he+)(PPh3)]2+ + 2e– [CpRuII(pi-he–)(PPh3 )]
(4)
[CpRuII(pi-he–)(PPh3)] + H+ (H2O) [CpRuII(pi-heH)(PPh3)]+ (+ OH–)
(5)
*
*
Figure 4: CVs of 3 (blue trace) and 4 (red trace) in MeCN−0.1 M [(n-Bu)4N][PF6]; other conditions as listed for Figure 1. The peaks marked by asterisks (*) disappeared when the initial scans were switched at –0.5 V, and the peaks marked by the diamonds () disappeared when the positive sweeps were switched at +1.0 V. Table 3. Electrochemical potentials1 for peaks observed in cyclic voltammograms of 3 and 4.
Complex Ep,c (1st redn) E1/2 (2nd redn) Ep,a (3rd redn) 3 –0.76 –1.18 –1.71 4 –1.02 –1.15 –1.71
1
2
E1/2 (oxidn) Ep (child)2 +1.12 +0.85*, –0.36 +1.11 +0.63*, +0.93*, –0.37 E1/2/V for couples and Ep/V for irreversible processes versus an Ag/AgCl reference electrode for which E1/2 (Fc+/Fc) = +0.49 V. * indicates the anodic child peak marked by an asterisk (*) in Figure 4 that arises in the reverse positive scan after the primary reduction process is traversed, and indicates the cathodic child peak marked by a diamond () in Figure 5 that arises in the reverse cathodic scan after traversing the primary oxidation process. Apart from the pyridinium-centred primary reduction, the CVs of 3 and 4 are similar to CVs
of 1 and 2. For 3 and 4, the pyridylimine-centred (pi/pi•−) couples appear at ca. –1.2 V, respectively, and are followed at ca. –1.7 V by the ester-centred, Hantzsch-pyridine reduction process [33]. The RuII/RuIII couples for 3 and 4 appear to positive potentials at ca. +1.1 V and are not fully chemically
15
reversible, giving rise to a cathodic peak at ca. +0.4 V in the reverse negative sweep for reduction of a product species.
3.5 Spectroelectrochemistry of 3 and 4. Results from UV-vis spectroelectrochemical experiments for 3 and 4 are presented in Table 4. UV-vis spectra recorded during reductive electrolysis 3 at –0.80 V and 4 at –1.00 V, Figure 5, reveal clean replacement of the Hantzsch pyridinium-centered π-π* band (at 290 and 283 nm, respectively) by a new π-π* band at 310 nm for the primary reduction products. During the reductions, the dπ(Ru)–π*(pi-) MLCT band of 3 blue-shifted (by 535 cm–1) from 480 to 468 nm whereas the dπ(Ru)–π*(pi-) MLCT band at 450 nm for 4 did not shift. The UV-vis spectra of the reduction products are different from those of 1 and 2. No radical intermediates were observed when these electrolyses performed in situ within the EPR spectrometer. A fortuitous result throws some light on the identities of the reduction products of 3 and 4. Titrations of 3 and 4 with hydroxide ion indicate the complexes form 1:1 adducts with hydroxide ion, 5 and 6; see Figure 9S in the Supporting Information. Addition of one equivalent of acetic acid or even dilution with water (to >10% v/v) completely reversed the reaction with hydroxide ion and quantitatively regenerated complexes 3 and 4. The ease of reversion of the hydroxide adducts, 5 and 6, back to the parent pyridinium-substituted complexes, 3 and 4, prevented their isolation. The hydroxide adducts, 5 and 6, display identical UV-vis spectra to the reduction products of 3 and 4, respectively; i.e. the reduction product and the hydroxide adduct of 3 or 4 is the same compound. Although hydroxide adducts of Hantzsch pyridinium cations (i.e. Hantzsch 4-hydroxy-1,4dihydropyridines or Hantzsch 2-hydroxy-1,2-dihydropyridine) are not previously reported [42], pyridinium cations do or are predicted to form weak adducts with basic anions including hydroxide ion [43, 44]. The UV-vis spectra for the hydroxide adducts of pyridinium ions that have been characterised also show a band centred at 300–330 nm [43, 44]. Based on these results, the reduction products and hydroxide adducts of 3 and 4 may be tentatively assigned as the corresponding hydroxy-dihydropyridine-substituted complexes, [CpRuII(pi-2heOH)(PPh3)]+ (5) and [CpRuII(pi-3heOH)(PPh3)]+ (6), such as the 4-hydroxy-1,4-dihydropyridine-substituted complexes depicted in Chart 5 (or as the corresponding 2-hydroxy-1,2-dihydropyidine adducts).
16
O N
Ph 3P O OH 2
Ru N N 3
O
O
5: [CpRu(PPh3)(pi-2heOH)]+ 6: [CpRu(PPh3)(pi-3heOH)]+
Chart 5
Table 4. Results from in situ UV-vis spectroelectrochemical studies of Hantzsch pyridiniumsubstituted complexes 3 and 4. λmax/nm (εmax/103 M-1 cm-1)
parent complex … after 1st reduction … after 2nd reduction
… after KOH titration … oxidation product
(a)
3 4 290 (23), 310 sh (17), 480 283 (25), 315 sh (15), 452 (4.8) (5) 310 (21), 468 (3) 310 (28), 450 (8) decomposition 262 sh (27), 305 (28), 400 br (18), 470 sh (10) 310 (25), 462 (5) 310 (27), 447 (6) 287 (20) 282 (21)
(b)
Figure 5. UV-vis spectra acquired during reductive electrolysis of: (a) 3 at –0.80 V and (b) 4 at –1.10 V in MeCN−0.1 M [Bu4N][PF6] at 295 K; also shown in (b) are the spectra of 2 (A) and 6 (B) and a calculated spectrum for a 1:1 mixture of 2 and 6 [((A) + (B))/2], all normalised to the same total concentration of CpRu species (as used in the electrolysis of 4). Some further comment on the electrochemical reduction of 3 or 4. The analysis of the UV-vis spectra shown in Figure 5(b) reveals the hydroxide-adduct 6 is cleanly produced upon reduction of 4, and not a mixture of 6 and dihydropyridine 2 as would result if equations 4 and 5 were followed. The
17
residual water always present in acetonitrile [45] is presumably the source of hydroxide ion. A Pt gauze working electrode was employed in the spectroelectrochemical experiments: Pt very efficiently catalyses reduction of water, equation 6. Thus it seems likely that the hydroxide ion necessary to form 5 and 6 arises from direct the reduction of the residual water at the Pt working electrode, and not from catalysis of water reduction by the complexes. Indeed, CVs of 4 at a glassy carbon working electrode in the presence of 10% v/v water, 10% v/v water and acetic acid (100 equiv.), and 5% v/v water plus p-toluene sulfonic acid were screened for overall catalysis of proton reduction. No catalysis was observed: the peak currents for the primary reduction process remained unchanged in the presence of the proton sources and no new peak was observed ahead of the onset hydrogen evolution observed without the complex (at ca. –1.6 V vs. Ag/AgCl).
2H2O + 2e– H2 + 2OH–
(6)
The second reduction process for 3 or 4 was also investigated. After exhaustive reduction of 3 to afford the primary reduction product, i.e. 5, switching the electrolysis potential to –1.20 V (i.e. negative of the pi/pi•– couple) resulted in decomposition and deposition of intractable precipitate on the working electrode (as was the case for 1, see above). Conversely, after exhaustive reduction of 4 to afford the primary reduction product, i.e. 6, switching the electrolysis potential to –1.20 V resulted the smooth formation of a second reduction product with bands at 305, 400 and 470 nm, Figure 6(a); isosbestic points were observed at 240, 306 and 338 nm. The spectroscopic changes were accompanied by a change in the colour of the solution from orange to dark red. As has already been noted, the 400 nm band is characteristic for a Ru(II)-bound pyridyliminyl radical. Figure 6(b) presents an X-band EPR spectrum recorded during in situ reductive electrolysis of 4 at the same potential (–1.20 V); i.e, complex 6 should first form and then be reduced. The spectrum features a slightly skewed doublet at g = 1.998 with AH = 24 G (imine CH), parameters that are similar to, but not the same as, those for the pyridylimine-centred radical observed upon reduction of 2 (see above). The slightly different parameters accord with a small difference in the tethered substituents to the CpRu(PPh3)-bound iminopyridinyl radical anion [35, 36], i.e. 1,4-dihydropyridine from 2 versus a hydroxy-dihydropyridine from 6.
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(a)
(b)
Figure 6. (a) UV-vis and (b) X-band EPR (ν = 9.789 GHz) spectra acquired during reductive electrolysis of hydroxide adduct 6 (formed at the primary reduction of 3) at –1.20 V (the second reduction process in the CV for 3) in MeCN−0.1 M [Bu4N][PF6] at 295 K. Finally, oxidative electrolysis of 3 and 4 causes bleaching of the dπ(Ru)–π*(pi-) bands at 480 and 452 nm, respectively, and leaves the other bands slightly perturbed in energy and/or intensity (Figure 10S, Supporting Information). The result is consistent with oxidation to Ru(III)-species, CpRuIII(pi-)(PPh3)2+, equation 2 (see above).
4. Conclusion The redox chemistry of the (cyclopentadienyl)(triphenylphosphine)ruthenium–Hantzsch dihydropyridine conjugates has been studied by cyclic voltammetry allied with EPR and UV-vis spectroelectrochemical and some reactivity studies. Pronounced differences in redox potentials and the products of oxidation and reduction are observed that depend on whether the dihydropyridine/pyridinium substituent is π-delocalised with the pyridylimine chelate and, consequently, the ruthenium centre. The primary oxidation of the ‘free’ dihydropyridine ligands (pi-2heH and pi-3heH) and the dihydropyridine-substituted Ru(II) complex 2 (no conjugation) afforded the anticipated pyridinium cation-substituted products. In contrast, electrochemical oxidation of dihydropyridine-substituted 1 (with conjugated Ru(II) and dihydropyridine centres) cleanly afforded a different, as yet unidentified, product. The primary reduction products of the pyridinium-cation-substituted Ru(II) complexes, 3 and 4, are not the corresponding dihydropyridine-substituted Ru(II) complexes, 1 and 2. The reduction products have identical UV-vis spectra to the hydroxide-adducts 5 and 6 formed by the direct addition of hydroxide ion to 3 and 4. Formation of 5 and 6 is reversed by dilution with water or addition of weak acid (acetic acid). Cyclic voltammetric assays of 4 in the presence of water and aqueous acids revealed no evidence for catalysis of proton reduction.
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These results will buttress future studies of Hantzsch pyridinium/dihydropyridinesubstituted metal complexes toward development of new, atom-efficient methodologies for Hantzsch ester-based asymmetric syntheses. So far only chemical regeneration of Hantzsch dihydropyridine- and other organohydride-substituted transition metal complexes using, for example, formate or molecular hydrogen as the reductant has lead to new methodology for transfer hydrogenation [14-16]. The present investigations highlight difficulties that remain to be surmounted in (photo)electrochemically-driven regeneration of Hantzsch dihydropyridinesubstituted complexes — such as the formation of the hydroxide adduct rather than the Hantzsch dihydropyridine (formally a hydride ion adduct) upon direct electrochemical reduction. These problems will have to be surmounted if light-driven and/or electricity-driven reductive catalyses based on Hantzsch ester-substituted transition metal catalysts are to be realised.
Acknowledgement We are thankful for the award of DP130103514 from the Australian Research Council, which funded this research.
Appendix A. Supplementary Material The Supplementary Material comprises additional figures and a table, plus cifs for the X-ray crystal structures of 2 and 4 (CCDC no. 1407649-1407650), and can be found in the online version of this article at http://dx.doi.org/10.1016/j.ica.2015.XX.YYY.
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Graphical Abstract (scale size to fit)
Syntheses and detailed cyclic voltammetry and EPR/UV-vis spectroelectrochemical studies of ruthenium complexes with tethered Hantzsch pyridinium/dihydropyridine centres are reported. Inter-centre electronic communication has a significant effect on physicochemical properties. Electrochemical regeneration of the dihydropyridine donor is hampered by formation of hydroxide– pyridinium adduct(s).
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