The facile synthesis of functionalised pyridine complexes using a ruthenium building block

www.elsevier.nl/locate/ica Inorganica Chimica Acta 313 (2001) 71 – 76

The facile synthesis of functionalised pyridine complexes using a ruthenium building block Anthony K. Burrell a,*1, David L. Officer a,*2, Sonya M. Scott a, Kirstie Y. Wild a, Keith C. Gordon b a

Department of Chemistry, IFS, Massey Uni6ersity, Pri6ate Bag 11222, Palmerston North, New Zealand b Department of Chemistry, Otago Uni6ersity, P.O. Box 56, Dunedin, New Zealand Received 24 June 2000; accepted 16 October 2000

Abstract A facile synthesis of metal complexes incorporating functionalised pyridines using a ruthenium(II) tetrakis(4-formylpyridine)dichloride building block is described. These complexes have been studied using electrochemistry, electronic absorbance and resonance Raman techniques. The molecules are able to be tuned electronically by varying the functionality attached to the pyridine in the complexes derived from ruthenium(II) tetrakis(4-formylpyridine)dichloride. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ruthenium; Supramolecular; Pyridine; Wittig reaction; Spectroscopy

1. Introduction The formation and photophysical properties of molecular assemblies employing transition metal complexes have been the focus of considerable attention for sometime [1,2]. More recently, we [3 – 5] and others [6,7] have been interested in extending the formation of these supramolecular arrays to include other chromophores, in particular porphyrins. During our investigations of the coordination chemistry of a pyridine functionalised porphyrin (1) we attempted to prepare coordination complexes where more than a single molecule of 1 was bound as a ligand. Our initial choice of precursor complex was Ru(DMSO)4Cl2 [8]. This complex is known to react with pyridine to form Ru(Py)4Cl2 [8]. Unfortunately, the direct reaction of Ru(DMSO)4Cl2 with 1 proved to be difficult, with inseparable mixtures containing variable numbers of coordinated porphyrins being obtained. In a attempt to overcome this difficulty we treated Ru(DMSO)4Cl2 with 4-pyridinecarboxalde1

Corresponding author. Tel.: +64-6-350 4358; fax: + 64-6-450 5682; e-mail: [email protected] 2 Corresponding author. Tel.: +64-6-350 4358; fax: + 64-6-450 5682

hyde and prepared trans-RuCl2(Py –CHO)4 (2). The tetrapyridyl complex 2 has proven to be a versatile starting material for the formation of supramolecular structures and herein we describe the synthesis of a number of such compounds and their spectroscopic properties.

2. Results and discussion

2.1. Synthesis RuCl2(Py –CHO)4 (2) was prepared by heating Ru(DMSO)4Cl2 [8] at reflux temperature in benzene with an excess of 4-pyridinecarboxaldehyde. The pure complex 2 precipitates from the solution and was isolated by simple filtration in good yield (83%). The 1H NMR spectrum is very simple and consistent with the formation of a single isomer where the chlorides are trans. Reactions of 2 under Wittig conditions enable the efficient conversion of 2 to new complexes with vinyl substituents. The reactions were carried out in a variety of conditions, but either refluxing dry dichloromethane or refluxing dry toluene under an inert atmosphere gave the best results. The reactions proceed quickly although

0020-1693/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 0 0 ) 0 0 3 6 3 - 7

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A.K. Burrell et al. / Inorganica Chimica Acta 313 (2001) 71–76

in some cases it was found to be advantageous to leave the reaction longer to optimise the amount of the all-trans isomer produced. High yields were achieved in most cases, although some of the products were found to be unstable under the reaction conditions. This may have been instrumental in bringing about the lower yields in these cases. The new complexes are shown in Fig. 1. Each of the four new vinylic bonds is potentially cis or trans, so many isomers are possible and a mixture can be observed during the reaction. The tetrameric porphyrin array (3) is an exception and remarkably gives a single product, that is the all-trans isomer. The vinyl resonances in the 1H NMR (CDCl3) spectrum both appear at 7.29 ppm, and appear as a singlet. However, in C6D6 the spectrum is slightly different with several of the proton signals shifting and resolving. In spectra run in C6D6 the vinyl resonances appear as the expected doublets at 6.83 and 6.89 with J =15 Hz. While all of the other complexes are initially formed as mixtures containing cis/trans vinyl linkages it has proven possible to isomerise them to the all trans configuration. For example, the examination of the 1H NMR of 4 indicates all of the double bonds have trans geometry as the vinyl protons (7.02 and 7.28 ppm) have a distinctive coupling constant of J= 16.5 Hz. The signals for the vinyl protons can be similarly identified in 1H NMR spectra of 5–7. The yield of the all-trans complex isolated for 7 was very low (6%). This appears to result in decomposition of the complex 7 under the conditions required for

the isomerisation. The relative stability observed may correlate with the relative electron donating characteristics of the substituents on the aryl group, with the more electron rich substituents decomposing more readily than those with electron withdrawing substituents.

2.2. Physical measurements The electrochemical data for the stable compounds 3–6 are presented in Table 1. All four compounds were examined under the same conditions using a glassy carbon microelectrode in dry CH2Cl2. For all compounds the reversible Ru(II)/Ru(III) oxidation and the Ru(II)/Ru(III) return reduction are observed. The potential for this couple shifts depending on the electron withdrawing ability of the ligand. Thus, the potential at which oxidation takes place for the Ru(II)/Ru(III) couple increases from 6 to 3 and 4 to 5. This effect has been noted in Ru polypyridyl chemistry with different substitutions effecting a tuning of the electronic and electrochemical properties of the complex [9]. For 6 a second irreversible oxidation can be observed, this is the oxidation based at the thiophene moiety [10]. The porphyrin system also shows redox characteristics in addition to the Ru(II)/Ru(III) couple. An oxidation and two reductions due to the porphyrin occur with the expected second porphyrin oxidation not accurately resolved due to the process occurring at the solvent redox limit. Each redox couple is a four-electron process with all four porphyrins undergoing the respective

Fig. 1. New complexes.

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Table 1 Electrochemical data for compounds in dichloromethane E°% (V) a Compound

RuII/RuIIIcouple

TXP 3 4 5 6

0.42 0.41 0.50 0.39

a b

Oxidation 1.00 1.13

Reduction 1.32

−0.99 −1.00

−1.19 −1.24

0.92 (i) b

Versus Fc/Fc+. (i) denotes an irreversible process.

Table 2 Electronic spectral data for compounds at room temperature Compound (solvent) 1 2 3 4 4 4 5 5 6 6 6 7

(CH2Cl2) (CH2Cl2) (CH2Cl2) (CH2Cl2) (CH3OH) (benzene) (CH2Cl2) (benzene) (CH2Cl2) (CH3OH) (benzene) (CH2Cl2)

u (nm) (m×10−3/M−1 cm−1) 429 (195) 275.5 (8) 277 (210) 310 (83) 311 (66) 310 (63) 333 (86) 334 (81) 314 (90) 315 (94) 315 (73) 331 (51)

431 (760)

oxidations or reductions at the same potentials [11]. For comparison the electrochemical data for 5,10,15,20-tetra(3,5-dimethylphenyl)porphyrin (TXP) is included in Table 1. The porphyrin reductions are at very similar potentials to the reductions observed for TXP. In contrast, the potential at which oxidation takes place are observed to be slightly shifted, with the first occurring at a potential more positive than that of TXP and the second unable to be detected at all. This is perhaps unsurprising given that once the Ru(II) centre has been oxidised to a Ru(III) centre, it could be expected to have a greater electron withdrawing effect on the porphyrin resulting in a system that is harder to oxidise. This shift shows that processes happening at the ruthenium centre can have some influence on the porphyrin [12]. The spectroscopy of these complexes is also interesting. Electronic absorption spectra were recorded of all species and the data presented in Table 2. All of the complexes prepared are highly coloured, ranging from violet for 2 to orange – red for 6. The UV – Vis spectra of all except 3 (which is dominated by porphyrin transitions) show only two bands. The lower energy band, assigned as an MLCT transition, is very solvatochromic and also shifts to lower energy when electron-withdrawing groups are substituted at the pyridines [13]. The

524 (15) 494.5(15) 523 (98) 488 (29) 460 (23) 509 (31) 539br (39) 556 (42) 482 (39) 458 (40) 502 (37) 485 (23)

564 (8) 525 (sh) 570 (sh)

602 (6)

657 (2.5)

604 (70)

656 (sh)

550 (sh)

higher energy transition at ca. 320 nm is not solvatochromic but does shift with substitution; this is assigned as a p* ’p LC transition [13]. Resonance Raman spectra of 3– 7 are presented in Fig. 2. Band positions are given in Table 3. These complexes are interesting due to the simplicity of the

Fig. 2. Resonance Raman spectra of 3 – 7 in dichloromethane (1 mM). Excitation wavelength was 514.5 nm (20 mW).

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Table 3 Observed wavenumbers (cm−1) for Raman bands of compounds in CH2Cl2 4

5

6

7

Assignment

1640 1605

1634 1607 1598

1634 1605 1594

1634 1607

1634 1609 1601

1218 1195

1218 1195

trans CC stretch w8a pyridyl w8a phenyl w(CbCb) porphyrin w(CbCb) porphyrin was(CaCm) porphyrin ws(CaN) porphyrin NO2 stretch w(CaCb) porphyrin Porphyrin mode w9a pyridyl (l(CH)) w9a pyridyl (w(C–Ph))

1342 1317 1267 1212 1195 a

1218 1195

1218 1195

4.1. Physical measurements

a

3

1553 1502 1456 1368

4. Experimental

Assignments taken from Refs. [14–18,22].

system. The spectrum of 3 is dominated by porphyrin modes [14], and the other complexes show similar spectral features. Bands at about 1598 and 1607 cm − 1 observed in 4, 5 and 7 correspond to the w8a bands of the phenyl and pyridine ring systems, respectively [15]. The 1195 and 1218 cm − 1 bands are also pyridine-based corresponding to the w9a (l(CH)) and w(C – Ph) modes [16]. The 1634 cm − 1 is assigned as the stretch of the trans-CC linkage as observed in allyl – pyridines and allyl –phenyl systems [14,15,17]. The complex 5 shows enhancement of the NO2 stretch at 1342 cm − 1 [14,18]. The enhancement of modes associated with each ring system suggests that the acceptor MO in the MLCT state is spread over both ring systems. Interestingly for 6, which has a thiophene ring, no modes associated with that ring are enhanced suggesting the MO may be less spread out in this case; thiophene has a strong band at 1491 cm − 1 [16a]. There has been increased interest in recent years in the photochemical and electrochemical properties of supramolecular assemblies composed of mononuclear metal polypyridyl complexes [19]. Such systems have applications in solar energy harvesting [20] and in molecular device technology [21]. With the relative simplicity of the spectra obtained for compounds 3–7, we believe this is an excellent system for probing the effect of substituents on charge transfer in ruthenium – pyridyl complexes and further work in this area is underway.

3. Conclusion We have demonstrated a straightforward and versatile synthesis of ruthenium centred arrays that provide a basis for further research on tuneable dyes in photochemical devices.

The instrumentation used in the measurement of UV –Vis [22], electrochemical [22] and resonance Raman properties [23] and the protocols used have been described elsewhere. 1H nuclear magnetic resonance spectra were obtained at 270 MHz using a JEOL GX270 FT-NMR spectrometer with Tecmag Libra upgrade. 1H nuclear magnetic resonance data are expressed in parts per million downfield shift from tetramethylsilane as an internal reference and are reported as position (lH), multiplicity (s=singlet, d =doublet, dd= doublet of doublet, t=triplet, and m=multiplet), relative integral, coupling constant (J Hz) and assignment. All spectra were recorded in deuterated chloroform. Mass spectra were recorded using a Varian VG70-250S double focusing magnetic sector mass spectrometer with an ionisation potential of 70 eV. Flash chromatography was carried out using Merck Kieselgel 60 (230 –400 mesh) with the indicated solvents. Thin-layer chromatography was performed using precoated silica gel plates (Merck Kieselgel 60F254). The synthesis of the porphyrin –phosphonium salt (10) has been reported previously [4], all others were prepared using a general literature procedure [24]. 4-Pyridine-carboxaldehyde was obtained from Merck and used as received. Ru(DMSO)4Cl2 was prepared following the literature procedure of Evans et al. [8].

4.2. 1 -Pyridyl-(trans-2 ¦-6inyl-5 ¦,10 ¦,15 ¦,20 ¦tetraxylylporphyrinyl) (1) A solution of phosphonium salt, 10 (50 mg, 0.048 mmol) and 4-pyridine-carboxaldehyde (5.3 mg, 0.05 mmol) in CH2Cl2 (30 ml) was stirred under nitrogen for 10 min at which point 1,8-diazabicyclo[5.4.0]undec-7ene (DBU) (0.32 ml) was added and stirring continued for a further 30 min. The solution was chromatographed (CH2Cl2) to remove the trace aldehyde remaining and the product was then removed from the column (5% EtOAc in CH2Cl2) and dried under high vacuum to yield a purple powder which was all-trans (33 mg, 83%). 1H NMR (CDCl3): l= − 2.59 (br s, 2H, NH), 2.55 (s, 6H, HMeXy), 2.62 (s, 12H, HMeXy), 2.65 (s, 6H, HMeXy), 7.18 (d, 2H, 3J= 6.0 Hz, Hpyridyl), 7.19 (d, 1H, 3J= 14.6 Hz, Htrans-ethenyl), 7.28 (d, 1H, 3J=14.6 Hz, Htrans-ethenyl), 7.42 (s, 2H, Hp-Ph), 7.46 (s, 2H, Hp-Ph), 7.88 –7.84 (m, 8H, Ho-Ph), 8.58 (d, 2H, 3J= 6.0 Hz, Hpyridyl), 8.88 –8.80 (m, 6H, Hb-pyrolix), 9.05 (s, 1H, Hb-pyrolix). FAB HRMS: m/z= 829.4192 (MH+). Calc. for C59H51N5 H+ = 829.4144.

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4.3. RuCl2(Py-CHO)4 (2) Ru(DMSO)4Cl2 (295 mg, 0.6mmol) and excess 4-pyridine-carboxaldehyde (495 mg, 4.5 mmol) were refluxed in benzene for 24 h forming a red solution. On cooling a ppt was observed in a plum coloured solution. Filtering isolated the product in good yield (299 mg, 83%). Only the trans isomer was observed. 1H NMR (CDCl3) l = 7.53 (d, 8H, Hpyridyl, 3J =7Hz), 8.81 (d, 8H, Hpyridyl, 3 J= 7Hz), 10.11 (s, 4H, CHO). FAB HRMS: m/z= 599.99051 (MH+). Calc. for C24H20N4O4RuCl2 H+ = 599.99171.

4.4. RuCl2(Py-TXP)4 (3) A solution of (5,10,15,20)-tetrakis(3%,5%-dimethylphenyl)porphyrin - 2 - yl)methyltriphenylphosphonium chloride, 10 (150 mg, 0.14 mmol) and 2 (22 mg, 0.036 mmol) was brought to reflux in dry CH2Cl2 under Ar. At reflux DBU (22 ml) was added and the reflux continued for a further 20 min before allowing the solution to cool to room temperature (r.t.) and removing the solvent under reduced pressure. The residue was chromatographed (2:1 CH2Cl2/hexane) to remove the faster running material and the product was then removed from the column (CH2Cl2) and dried under high vacuum to yield a purple powder which was all-trans (57 mg, 44%). 1H NMR (CDCl3) l = −2.59 (s, 8H, NH), 2.55 (s, 24H, HMe), 2.58(s, 24H, HMe), 2.61(s, 24H, HMe), 2.64 (s, 24H, HMe), 7.13 (d, 8H, Hpyridyl), 7.29 (s, 8H, Hethenyl), 7.38(s, 4H, Hp-Ph), 7.42 (s, 4H, Hp-Ph), 7.45 (s, 8H, Hp-Ph), 7.81 (s, 8H, Ho-Ph), 7.84 (s, 16H, Ho-Ph), 7.89 (s, 8H, Ho-Ph), 8.74 (d, 8H, Hpyridyl, J = 6Hz), 8.86 –8.78 (m, 24H, Hb-pyrolix), 9.11 (s, 4H, Hb-pyrolix). 1H NMR (C6D6) l = −1.63 (s, 8H, NH), 2.39 (s, 24H, HMe), 2.42 (s, 24H, HMe), 2.45 (s, 24H, HMe), 2.52 (s, 24H, HMe), 7.06 (d, 8H, Hpyridyl), 6.83 (d, 4H, Htrans-ethenyl, J= 15Hz), 6.89 (d, 4H, Htrans-ethenyl, J= 15Hz), 7.04 –8.02 (m, 54H, HPh, Hpyridyl), 9.06 – 9.14 (m, 24H, Hb-pyrolix), 9.55 (s, 4H, Hb-pyrolix). FAB MS: m/z= 3493 (MH+). Calc. for C236H204N20RuCl2 H+ =3493.

4.5. RuCl2(Py-Ph)4 (4) (Benzyl)triphenylphosphonium bromide, 11 (130 mg, 0.3 mmol) and 2 (30 mg, 0.05 mmol) were brought to reflux in dry CH2Cl2 under Ar. At reflux DBU (80 ml) was added and the reflux continued for a further 4 h to increase the amount of the trans isomer present before the solution was cooled to r.t. Reaction mixture was then filtered and solvent removed under reduced pressure. The residue was chromatographed (CH2Cl2) and dried under high vacuum to yield a purple powder which was a cis/trans mixture. The cis/trans mixture was refluxed for 48 h in CHCl3 to generate the all trans

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species (39 mg, 87%). 1H NMR (CDCl3) l= 7.02 (d, 4H, Hvinylic, J=16.5Hz) 7.17 (d, 8H, Hpyridyl, J=4Hz), 7.28 (d, 4H, Hvinylic, J=16.5Hz), 7.42 –7.32 (m, 12H, HPh), 7.55 –7.53 (m, 8H, HPh), 8.31 (br.d, 8H, Hpyridyl, J = 4Hz). FAB HRMS: m/z= 1002.2744 (MH+). Calc. for C59H51N5RuCl2 H+ = 1002.2643.

4.6. RuCl2(Py –NO2)4 (5) An excess of (4-nitrobenzyl)triphenylphosphonium bromide, 12 (64 mg, 0.13 mmol) was dissolved in dry CH2Cl2 under Ar. To this 2 (20 mg, 0.033 mmol) was added and DBU (30 ml) and the solution was stirred at r.t. After 1 min the reaction was observed to have gone to completion and the solvent was removed giving a purple solid. Column chromatography (5% EtOAc in CH2Cl2) allowed the isolation of a cis/trans mixture of 5. The mixture was refluxed in CHCl3 for 48 h, giving the pure trans product in good yield (33 mg, 93%). 1H NMR (CDCl3) l= 7.17 (d, 4H, Hvinylic, J=16Hz)7.23 (d, 8H, Hpyridyl, J= 6Hz), 7.35 (d, 4H, Hvinylic, J= 16Hz), 7.68 (d, 8H, HPh, J= 9Hz), 8.25 (d, 8H, HPh, J = 9Hz), 8.62 (d, 8H, Hpyridyl, J =6Hz). FAB HRMS: m/z= 1076.1359 (MH+). Calc. for C52H40N8O8RuCl2 H+ = 1076.1355.

4.7. RuCl2(Py –S)4 (6) A solution of (3-thienylmethylene)triphenylphosphonium bromide, 13 (120 mg, 0.27 mmol) and 2 (30 mg, 0.05 mmol) was brought to reflux in dry CH2Cl2 under Ar. At reflux DBU (50 ml) was added and the reflux continued for a further 2 h before allowing the solution to cool to r.t. The solution was then filtered and the solvent removed under reduced pressure. The residue was chromatographed (4:1 CH2Cl2/hexane) to remove the faster running material and the product was then removed from the column (CH2Cl2) and dried under high vacuum to yield a purple powder which was cis/trans. Refluxing in CHCl3 for 48 h resulted in the mixture being converted to all-trans (42 mg, 91%). 1H NMR (CDCl3) l= 6.84 (d, 4H, Hvinylic, J=16Hz), 7.12 (d, 8H, Hpyridyl, J=5Hz), 7.30 (d, 4H, Hvinylic, J= 16Hz), 7.38 –7.35 (m, 16H, Hthiophene), 8.46 (br.s, 8H, Hpyridyl). FAB HRMS: m/z= 922.0261 (MH+). Calc. for C44H36Cl2N4RuS4 H+ = 922.0263.

4.8. RuCl2(Py –OMe)4 (7) (4-Methoxylbenzyl)triphenylphosphonium bromide, 14 (100 mg, 0.24 mmol) was suspended in dry toluene under Ar and brought to reflux. To this was added DBU (100 ml) followed by 2 (30 mg, 0.05 mmol). After 10 min the reaction was deemed complete by TLC. The

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solution was filtered and the solvent removed under reduced pressure giving an orange residue. This was eluted through a short silica column giving 7 as the all-trans isomer (2 mg, 6%). 1H NMR (CDCl3) l =3.85 (s, 12H, OCH3), 6.88 (d, 4H; Hvinylic, J =16Hz), 6.92, (d, 8H, HPh, J =19Hz), 7.14 (br. s, 8H, Hpyridyl), 7.23 (d, 4H, Hvinylic, J=16Hz), 7.49 (d, 8H, HPh, J =19Hz), 8.50, (br.s, 8H, Hpyridyl). FAB HRMS: m/z =1016.2311 (MH+). Calc. for C56H52Cl2N4O4Ru H+ =1016.2349.

Acknowledgements We are grateful to The New Economy Research Fund (MAUX0014), the Massey University Research Fund and the Marsden Fund of New Zealand (MAU810) for support of this work.

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