Synthesis and electronic characterization of mixed diimine ligand rhodium(III) complexes using a versatile triflate precursor

Inorganica Chimica Acta 461 (2017) 239–247

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Research paper

Synthesis and electronic characterization of mixed diimine ligand rhodium(III) complexes using a versatile triflate precursor Daniel Amarante b, Cheryl Cherian a, Elise G. Megehee a,⇑ a b

Department of Chemistry, St. John’s University, Queens, NY 11439, USA Department of Chemistry & Biochemistry, Division of Natural Sciences, College of Mount Saint Vincent, Riverdale, NY 10471, USA

a r t i c l e

i n f o

Article history: Received 11 October 2016 Received in revised form 8 February 2017 Accepted 12 February 2017 Available online 20 February 2017

a b s t r a c t We report here the first general, high purity, high yield synthesis of thirteen mixed diimine ligand complexes of rhodium(III), nine of which are new and four which are known and were made as controls. We utilized a bis–triflate intermediate because the more labile triflate ligands were easily displaced by incoming diimine ligands and allowed these syntheses to proceed in high yields (84–97%) under relatively mild reaction conditions. The purity of the products was determined by elemental analysis and 1 H NMR. Electronic characterization was done on the compounds and we examined the effect of various electron donating and electron withdrawing substituents on the ground and excited state properties of the complexes. We observed that substitution on the 2,20 -bipyridine ligands causes only small changes in the redox potentials and the electronic spectra, while substitution on the 1,10-phenanthroline ligands causes more pronounced effects on the electronic properties, presumably due to the more rigid p system. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction In recent years there has been a great interest in mixed diimine {NN = various substituted 2,20 -bipyridines and 1,10-phenanthrolines, Scheme 1} ligand complexes of rhodium(III) because of their applications in such areas as photocleavage of DNA [1–9,29], inhibition of DNA transcription [10–13], photochemical reduction of H2O to H2 [14–16,30], and electrocatalytic reduction of CO to methanol [17,18]. However, published syntheses of mixed diimine ligand complexes have low yields due to the formation of multiple products, and extensive chromatography is necessary to isolate the purified complexes [19–22,31]. Clearly a general, high yield synthesis of these mixed diimine ligand rhodium(III) complexes is a highly desirable goal. As a part of our work to synthesize new luminescent complexes with interesting photophysical and electrochemical properties, we have developed a synthetic pathway to form a wide variety of mixed diimine ligand complexes of rhodium(III) using the trifluoromethanesulfonato (OTf) intermediate cis-[Rh(bpy)2(OTf)2] (OTf). This approach was inspired by the work of Meyer and coworkers [17,23] detailing the versatility of cis-[Ir(bpy)2(OTf)2] (OTf) as a precursor to mixed ligand iridium complexes. While the synthesis of cis-[Rh(bpy)2(OTf)2](OTf) was also reported, only

⇑ Corresponding author. E-mail addresses: [email protected] (D. Amarante), [email protected] (C. Cherian), [email protected] (E.G. Megehee). http://dx.doi.org/10.1016/j.ica.2017.02.011 0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

moderate yields were achieved and the compound was not used as a starting material for further reactions. Our modification of the synthesis of cis-[Rh(bpy)2(OTf)2](OTf) increases the yield to 98% over the previously reported 47% [17]. Further, we report here the synthesis and characterization of a series of thirteen [Rh(bpy)2(NN)](PF6)3, in 84–97% yields using cis-[Rh(bpy)2(OTf)2] (OTf) as the starting material. Nine of these compounds are new, and four are known [19–22] and were made as controls. Of those previously synthesized all yields were much higher using our method. The electronic properties of these complexes were examined to determine the effect of the various substituents.

2. Experimental section 2.1. Materials RhCl3xH2O was obtained from Pressure Chemical and was stored in a desiccator over anhydrous CaSO4 upon receipt. The following were purchased from Sigma-Aldrich: 4,40 -Dimethyl2,20 -bipyridine (4,40 -dmbpy), 5,50 -dimethyl-2,20 -bipyridine (5,50 -dmbpy), 4,40 -di-(t-butyl)-2,20 -bipyridine (4,40 -tBu2bpy), 4,40 -dimethoxy-2,20 -bipyridine (4,40 -dmobpy), and 4,7-diphenyl1,10-phenanthroline (4,7-dpphen). The 2,20 -bipyridine (bpy), 1,10-phenanthroline (phen), 4,7-dimethyl-1,10-phenanthroline (4,7-dmphen), 3,4,7,8-tetramethyl-1,10-phenanthroline (3,4,7,8tmphen), 4-methyl-1,10-phenanthroline (4-mphen), 5-chloro1,10-phenanthroline (5-Clphen), potassium hexafluorophosphate

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Cl Cl excess HOTf

N Cl

N

PF6

Rh

N N

N N

Rh

N

Cl

OTf

N OTf

OTf

N

N

N

Ethanol

Rh N

2 HCl

N

[PF6]3

N

N

N

N

N = CH3

H3C

4,7-dmphen

phen

bpy N

N

N

N

N

H3C H3C

4,4'-dmbpy N

N

N

H3C CH3 N

5,6-dmphen

4-mphen N

H3C

CH3

H3C

CH3

N

N

H3C

5,5'-dmbpy

5-mphen

N N

N

N

CH3

H3C

CH3 N

3,4,7,8-tmphen

N

Cl 4,4'-dtbubpy N

5-Clphen

N N

H3CO

4,7-dpphen

N N

OCH3

N

4,4'-dmobpy N

N Scheme 1. Synthesis of triflate intermediate and Rh(III) mixed ligand diimine complexes.

(KPF6), tetrabutylammonium hexafluorophosphate (tBu4NPF6), trifluoromethanesulfonic acid (triflic acid, HOTf), and Dry-Solv grade dimethylformamide (DMF) were purchased from Alfa-Aesar. The 5,6-dimethyl-1,10-phenanthroline (5,6-dmphen) and 5-methyl1,10-phenanthroline (5-mphen) were obtained from Lancaster Synthesis, Inc. Deuterated solvent, DMSO-d6 (0.01% tetramethylsilane (TMS) (v/v)), was purchased from Cambridge Isotope Laboratories. Acetonitrile (CH3CN) for spectroscopy was HPLC grade and dried on an MBraun solvent purification system prior to use. The DMF was kept sealed and removed under nitrogen to keep it dry. TBAPF6 was recrystallized from hot ethanol and dried under vacuum at 70 °C for at least 3 days prior to use. All other reagents were obtained from VWR and were used as received. cis-[Rh(bpy)2Cl2] PF6 was prepared as previously described [24], but on a 500 mg scale.

spectrophotometer and corrected for the solvent. Emission spectra were recorded with a SPEX Fluorolog-3 fluorometer equipped with a double excitation monochromator, a single emission monochromater (399 nm cutoff filters) and a thermoelectrically cooled, Hamamatsu R2685P photomultiplier tube. The 77 K glassy solutions were prepared by inserting a quartz tube (9 mm O.D., 7 mm I.D.) into a quartz tipped finger dewar. Emission spectra were corrected for instrumental response using manufacturer supplied data against an emission standard. Cyclic voltammograms (CVs) were recorded on a computer controlled BAS Epsilon potentiostat. CVs were obtained in dry dimethylformamide (DMF) solvent with 0.1 M tBu4PF6 as supporting electrolyte using a one compartment, three electrode cell with a Pt disk working electrode, Pt wire counter electrode, and a Ag/AgCl reference electrode. Ferrocene was added to the solution and all potentials are referenced to the ferrocene/ferrocenium couple.

2.2. Physical measurements 2.3. Syntheses All syntheses were carried out under a nitrogen atmosphere using standard Schlenk techniques. All diimine ligands were measured using Teflon spatulas to prevent contamination of the products by [Fe(NN)3]2+ ions. The 1H, 13C and 19F NMR spectra were recorded on a Bruker 400 MHz instrument. The 19F NMR spectra were referenced to trifluorotoluene (TFT) in deuterated benzene in a sealed tube placed inside the sample. Absorption spectra were recorded in acetonitrile using an HP diode array UV-visible

2.3.1. cis-[Rh(bpy)2(OTf)2](OTf) cis-[Rh(bpy)2Cl2](PF6) (501 mg, 0.794 mmol) and o-dichlorobenzene (50 mL) were placed in a 100 mL Schlenk flask, fitted with a condenser, then stirred under nitrogen and cooled on ice for 25 min. Using a syringe, triflic acid (2.11 mL, 23.8 mmol) was added to the reaction mixture, which was stirred on ice for 15 min. The solution was heated to 130 °C for 2.5 h. The flask

D. Amarante et al. / Inorganica Chimica Acta 461 (2017) 239–247

was then cooled on ice for 20 min, and another aliquot of triflic acid (2.11 mL, 23.8 mmol) was added to the reaction mixture. The solution was stirred for 15 min, followed by heating at 130 °C for another 2.5 h. The solution was cooled on ice again, and 80 mL of diethyl ether was added to the reaction mixture to induce precipitation. The solution was stirred on ice for 30 min, the analytically pure, off-white product was filtered under vacuum, washed with excess diethyl ether (3  30 mL) and dried under vacuum. Yield 675 mg, 0.783 mmol, 98%. Anal. Calcd for C22H16N4O6S2F6Rh: C, 32.03; H, 1.87; N, 6.50. Found: C, 31.80; H, 1.81; N, 6.37. 1H NMR (d6-DMSO, 400 MHz): d(ppm) aromatic protons 9.16 (d, J = 5.74 Hz, 2H), 9.08 (d, J = 8.16 Hz, 2H), 8.91 (d, J = 7.95 Hz, 2H), 8.81 (t, J = 7.83 Hz, 2H), 8.42 (multiplet, J = 6.24 Hz, 4H), 7.77 (d, J = 5.92 Hz, 2 H), 7.62 (t, J = 6.66 Hz, 2H). 19 F NMR (d6-DMSO, 376 MHz): d (ppm) 79.05 (s, free triflate), 79.27 (s, bound triflate). 2.3.2. [Rh(bpy)3](PF6)33H2O cis-[Rh(bpy)2(OTf)2](OTf) (105 mg, 0.122 mmol) and 2,20 -bipyridine (24 mg, 0.154 mmol) in ethanol (10 mL) were placed in a 50 mL Schlenk flask fitted with a condenser, then stirred under nitrogen for 25 min. The solution was heated at reflux for 30 min, and the color changed from colorless to pale pink after 5 min. The solution was cooled on ice for 5–10 min, aqueous KPF6 (25 mL) was added, and the resultant slurry was stirred on ice and under nitrogen for an addition 30 min. The solvent volume was reduced to 20 mL on a rotary evaporator. The analytically pure product was isolated by vacuum filtration, washed with a minimum amount of water, followed by diethyl ether (3  15 mL), and dried under vacuum. Yield of analytically pure, off-white powder (125 mg, 0.121 mmol) 94% yield. Anal. Calcd for C30H30N6RhO3P3F18: C, 33.98; H, 2.85; N, 7.93. Found: C, 33.80; H, 2.51; N, 7.84. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons 9.04 (d, J = 8.13 Hz, 4H), 8.57 (t, J = 7.48 Hz, 4H), 7.85 (d, J = 5.45 Hz, 4H), 7.79 (t, J = 6.57 Hz, 4H). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons 154.5, 150.9, 143.1, 130.4, 126.8. 2.3.3. [Rh(bpy)2(4,40 -dmbpy)](PF6)3H2O [Rh(bpy)2(4,40 -dmbpy)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (106 mg, 0.123 mmol) and 4,40 -dimethyl-2,20 -bipyridine (28.2 mg, 0.153 mmol) in 10 mL ethanol and refluxing for 30 min. Yield of analytically pure, off-white product (119 mg, 0.113 mmol), 92%. Anal. Calcd for C32H30N6RhOP3F18: C, 36.52; H, 2.87; N, 7.99. Found: C, 36.88; H, 2.76; N, 7.89. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.04 (d, J = 8.19 Hz, 4H), 8.93 (s, 2H), 8.56 (t, J = 7.48 Hz, 4H), 7.84 (m, 4H), 7.79 (quartet, J = 6.10 Hz, 4H), 7.69 (d, J = 5.96 Hz, 2H), 7.62(d, J = 5.82 Hz, 2H)], 2.62 (s, 6H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [155.7, 154.5, 154.4, 153.8, 150.9, 150.7, 149.8, 143.1, 130.7, 130.4, 127.3, 126.8], 21.1 (CH3). 2.3.4. [Rh(bpy)2(5,50 -dmbpy)](PF6)3 [Rh(bpy)2(5,50 -dmbpy)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (108 mg, 0.125 mmol) and 5,50 dimethyl-2,20 -bipyridine (29.1 mg, 0.158 mmol) in 10 mL ethanol and refluxing for 2.5 h. Yield of analytically pure, off-white product (122 mg, 0.118 mmol), 95%. Anal. Calcd for C32H28N6RhP3F18: C, 37.16; H, 2.73; N, 8.12. Found: C, 37.28; H, 2.74; N, 7.90. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.03 (t, J = 8.94 Hz, 4H), 8.89 (d, J = 8.33 Hz, 2H), 8.56 (quartet, J = 8.15 Hz, 4H), 8.40 (d, J = 8.59 Hz, 2H), 7.81 (m, 8H), 7.55 (s, 2H)], 2.29 (s, 6H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [154.7, 154.6, 152.0, 150.8, 150.4, 143.4, 143.0, 141.0, 130.3, 126.9, 126.7, 125.6], 18.5 (CH3).

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2.3.5. [Rh(bpy)2(4,40 -dtbubpy)](PF6)3 [Rh(bpy)2(4,40 -dtbubpy)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (112 mg, 0.130 mmol) and 4,40 -di-(tbutyl)-2,20 -bipyridine (42.0 mg, 0.156 mmol) in 20 mL ethanol and refluxing for 30 min. Yield of analytically pure, off-white product (135 mg, 0.121 mmol), 93%. Anal. Calcd for C38H40N6RhP3F18: C, 40.80; H, 3.60; N, 7.51. Found: C, 41.09; H, 3.77; N, 7.44. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.05 (dt, J = 7.52 Hz, J = 2.54 Hz, 6H), 8.57 (m, 4H), 7.82 (m, 8H), 7.68 (m, 4 H)], 1.43 (s, 18H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [167.2, 154.6, 154.5, 154.2, 150.9, 150.8, 150.3, 143.1, 130.5, 130.4, 126.9, 126.8, 124.5], 36.1 {C(CH3)3}, 29.8 {C (CH3)3}. 2.3.6. [Rh(bpy)2(4,40 -dmobpy)](PF6)3 [Rh(bpy)2(4,40 -dmobpy)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (102 mg, 0.118 mmol) and 4,40 -dimethoxy-2,20 -bipyridine (29.3 mg, 0.135 mmol) in 10 mL ethanol and refluxing for 30 min. Yield of analytically pure, off-white product (110 mg, 0.100 mmol), 84%. Anal. Calcd for C32H28N6RhO4P3F18: C, 34.99; H, 2.57; N, 7.65. Found: C, 35.00; H, 2.58; N, 7.55. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.04 (d, J = 8.23 Hz, 4H), 8.67 (d, J = 2.49 Hz, 2H), 8.57 (q, J = 7.95 Hz, 4H), 7.91 (d, J = 5.74 Hz, 2H), 7.84 (t, J = 8.18 Hz, 2H), 7.79 (q, J = 5.82 Hz, 4H), 7.57 (d, J = 6.83 Hz, 2H), 7.30 (dd, J = 6.34 Hz, 2H)], 4.08 (s, 6H, OCH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [169.8, 155.6, 154.6, 154.5, 151.2, 151.0, 150.7, 143.0, 130.4, 126.7, 120.6, 115.6, 113.7], 57.7 (OCH3). 2.3.7. [Rh(bpy)2(phen)](PF6)3 [Rh(bpy)2(phen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (112 mg, 0.130 mmol) and 1,10-phenanthroline (28.7 mg, 0.159 mmol) in 10 mL ethanol and refluxing for 30 min. Yield of analytically pure off-white product (123 mg, 0.119 mmol), 92%. Anal. Calcd for C32H24N6RhP3F18: C, 37.30; H, 2.35; N, 8.16. Found: C, 37.26; H, 2.24; N, 8.07. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons 9.19 (d, J = 8.40 Hz, 2H), 9.08 (d, J = 8.10 Hz, 2H), 9.03 (d, J = 8.11 Hz, 2H), 8.61 (t, J = 7.61 Hz, 2H), 8.57 (s, 2H), 8.49 (t, J = 7.40 Hz, 2H), 8.25 (d, J = 5.47 Hz, 2H), 8.12 (dd, J1 = 5.74 Hz, J2 = 7.88 Hz, 2H), 7.92 (d, J = 5.29 Hz, 2H), 7.85 (t, J = 6.42 Hz, 2H), 7.63 (t, J = 4.83 Hz, 2H), 7.60 (t, J = 5.65 Hz, 2H). 13 C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons 154.9, 154.6, 152.5, 151.3, 151.1, 144.3, 143.1, 142.9, 142.0, 132.0, 130.4 130.3, 128.8, 128.4, 126.8, 126.7. 2.3.8. [Rh(bpy)2(4-mphen)](PF6)3H2O [Rh(bpy)2(4-mphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (106 mg, 0.123 mmol) and 4-methyl1,10-phenanthroline (27.5 mg, 0.142 mmol) in 20 mL ethanol and refluxing for 30 min. Yield of analytically pure, pale peach product (126 mg, 0.119 mmol), 97%. Anal. Calcd for C33H28N6ORhP3F18: C, 37.31; H, 2.66; N, 7.91. Found: C, 37.11; H, 2.44; N, 7.77. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.19 (d, J = 8.10 Hz, 1H), 9.08 (d, J = 7.70 Hz, 4H), 9.03 (d, J = 8.33 Hz, 4H), 8.62 (quartet, J = 9.34 Hz, 4H), 8.49 (t, J = 7.45 Hz, 2H), 8.26 (d, J = 4.58 Hz, 1H), 8.11 (d, J = 4.76 Hz, 2H), 7.93 (m, 3H), 7.84 (t, J = 6.44 Hz, 2H), 7.62 (d, J = 4.27 Hz, 4H)], 3.02 (s, 3H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [154.9, 154.6, 153.1, 152.6, 151.4, 151.2, 150.9, 144.4, 143.8, 143.1, 142.9, 141.9, 131.8, 131.7, 130.4, 130.3, 128.6, 128.5, 128.3, 126.8, 126.7, 125.9, 116.1], 18.8 (CH3). 2.3.9. [Rh(bpy)2(5-mphen)](PF6)3H2O [Rh(bpy)2(5-mphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (106 mg, 0.123 mmol) and 5-methyl-

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1,10-phenanthroline (26.8 mg, 0.138 mmol) in 20 mL ethanol and refluxing for 30 min. Yield of analytically pure, pale peach product (117 mg, 0.110 mmol), 89%. Anal. Calcd for C33H28N6ORhP3F18: C, 37.31; H, 2.66; N, 7.91. Found: C, 36.99; H, 2.44; N, 7.74. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.21 (d, J = 8.34 Hz, 1H), 9.06 (quartet, J = 8.94 Hz, 5H), 8.61 (t, J = 7.66 Hz, 2H), 8.39 (s, 1H), 8.24 (d, J = 5.31 Hz, 1H), 8.18, (d, J = 5.14, 1H), 8.09 (m, 2H), 7.92 (t, J = 3.35 Hz, 2H), 7.85 (s, 2H), 7.61 (d, J = 6.31 Hz, 4H)], 2.94 (s, 3H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [154.9, 154.6, 152.1, 151.5, 151.2, 151.0, 144.6, 143.5, 143.1, 142.9, 141.1, 139.3, 137.2, 132.1, 131.7, 130.4, 130.3, 128.4, 128.0, 127.4, 126.8, 126.7, 116.3], 18.3 (CH3). 2.3.10. [Rh(bpy)2(5-Clphen)](PF6)3 [Rh(bpy)2(5-Clphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (109 mg, 0.126 mmol) and 5-chloro1,10-phenanthroline (37.4 mg, 0.174 mmol) in 10 mL ethanol and refluxing for 2 h. Yield of analytically pure, off-white product (122 mg, 0.114 mmol), 90%. Anal. Calcd for C32H23N6RhClP3F18: C, 36.10; H, 2.18; N, 7.89. Found: C, 35.82; H, 2.11; N, 7.64. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.26 (d, J = 8.49 Hz, 1H), 9.09 (t, J = 8.89 Hz, 3H), 9.03 (d, J = 8.41 Hz, 2H), 8.91 (s, 1H), 8.61 (t, J = 7.82 Hz, 2H), 8.50 (t, J = 7.40 Hz, 2H), 8.32 (d, J = 5.22 Hz, 1H), 8.26 (d, J = 5.28 Hz, 1H), 8.19 (dd, J1 = 5.42 Hz, J2 = 8.54 Hz, 1H), 8.13 (dd, J1 = 5.47 Hz, J2 = 8.31 Hz, 1H), 7.89 (t, J = 6.79 Hz, 2H), 7.84 (t, J = 6.68 Hz, 2H), 7.65 (t, J = 6.67 Hz, 2H), 7.60 (t, J = 6.51 Hz, 2H). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [155.0, 154.9, 154.5, 154.4, 151.4, 151.3, 151.2, 145.3, 143.8, 143.2, 143.0, 142.9, 141.2, 138.8, 131.4, 131.3, 130.4, 130.3, 130.2, 130.1, 129.1, 128.9, 128.0, 126.9, 126.7. 2.3.11. [Rh(bpy)2(4,7-dmphen)](PF6)3H2O [Rh(bpy)2(4,7-dmphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (109 mg, 0.127 mmol) and 4,7-dimethyl1,10-phenanthroline (30.9 mg, 0.148 mmol) in 20 mL ethanol and refluxing for 30 min. Yield of analytically pure, pale peach product (125 mg, 0.116 mmol), 92%. Anal. Calcd for C34H30N6ORhP3F18: C, 37.94; H, 2.81; N, 7.81. Found: C, 37.93; H, 2.72; N, 7.65. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.07 (d, J = 7.91 Hz, 2H), 9.03 (d, J = 8.05 Hz, 2H), 8.60 (s + t, J = 7.69 Hz, 4H), 8.49 (t, J = 6.71 Hz, 2H), 8.11 (d, J = 5.58 Hz, 2H), 7.93 (dd, J = 9.75 Hz, J = 5.69 Hz, 4H), 7.83 (t, J = 6.59 Hz, 2H), 7.60 (m, 4H)], 3.03 (s, 6H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic

carbons [154.8, 154.7, 153.1, 151.5, 151.1, 150.8, 143.9, 143.0, 142.9, 131.5, 130.4, 130.3, 128.5, 126.8, 126.7, 125.5], 18.8 (CH3). 2.3.12. [Rh(bpy)2(5,6-dmphen)](PF6)32H2O [Rh(bpy)2(5,6-dmphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (106 mg, 0.123 mmol) and 5,6-dimethyl1,10-phenanthroline (29.2 mg, 0.140 mmol) in 20 mL ethanol and refluxing for 30 min. Yield of analytically pure, pale peach product (118 mg, 0.112 mmol), 91%. Anal. Calcd for C34H32N6O2RhP3F18: C, 37.31; H, 2. 95; N, 7.68. Found: C, 37.42; H, 2.58; N, 7.62. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.28 (d, J = 8.55 Hz, 2H), 9.08 (d, J = 7.96, 2H), 9.04 (d, J = 8.18, 2H), 8.61 (t, J = 7.59 Hz, 2H), 8.50 (quintet, J = 4.30 Hz, 2H), 8.18 (d, J = 5.19 Hz, 2H), 8.07 (dd, J = 8.55 Hz, J = 5.40 Hz, 2H), 7.93 (d, J = 5.46 Hz, 2H), 7.85 (t, J = 6.75 Hz, 2H), 7.60 (d, J = 4.30 Hz, 4H)], 2.88 (s, 6H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [154.8, 154.6, 151.2, 151.1, 151.0, 143.4, 143.1, 142.9, 139.1, 133.7, 132.1, 130.4, 130.3, 128.0, 126.8, 126.7], 15.3 (CH3). 2.3.13. [Rh(bpy)2(3,4,7,8-tmphen)](PF6)3H2O [Rh(bpy)2(3,4,7,8-tmphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (107 mg, 0.125 mmol) and 3,4,7,8-tetramethyl-1,10-phenanthroline (36.2 mg, 0.153 mmol) in 20 mL ethanol and refluxing for 4 h. Yield of analytically pure, pale peach product (127 mg, 0.117 mmol), 94%. Anal. Calcd for C36H34N6ORhP3F18: C, 39.15; H, 3.10; N, 7.61. Found: C, 39.28; H, 3.10; N, 7.37. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons [9.03 (t, J = 9.04 Hz, 4H), 8.63 (s, 2H), 8.58 (t, J = 7.94 Hz, 2 H), 8.49 (t, J = 8.34 Hz, 2H), 7.91 (s, 2H), 7.88 (d, J = 5.32 Hz, 2H), 7.82 (t, J = 6.68 Hz, 2H), 7.59 (m, 4H)], 2.90 (s, 6H, CH3), 2.41 (s, 6H, CH3). 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons [154.8, 154.7, 151.6, 151.0, 150.7, 150.6, 142.9, 142.8, 137.1, 130.4, 130.3, 130.1, 126.7, 126.6, 125.2], 17.9 (CH3), 15.2 (CH3). 2.3.14. [Rh(bpy)2(4,7-dpphen)](PF6)3 [Rh(bpy)2(4,7-dpphen)](PF6)3 was prepared as above but using cis-[Rh(bpy)2(OTf)2](OTf) (105 mg, 0.122 mmol) and 4,7-diphenyl-1,10-phenanthroline (51.1 mg, 0.154 mmol) in 15 mL ethanol and refluxing for 5 h. Yield of analytically pure pale peach product (137 mg, 0.116 mmol), 96%. Anal. Calcd for C44H32N6RhP3F18: C, 44.69; H, 2.71; N, 7.11. Found: C, 44.64; H, 2.76; N, 6.91. 1H NMR (d6-DMSO, 400 MHz): d (ppm) aromatic protons 9.09 (dd, J1 = 8.12 Hz, J2 = 13.46 Hz, 4H), 8.63 (t, J = 7.84 Hz, 2H), 8.55 (t, J = 7.99 Hz, 2H), 8.36 (s, 2H), 8.31 (d, J = 5.61 Hz, 2H), 8.07 (d,

Table 1 Electronic Data for [Rh(bpy)2(NN)](PF6)3. NN=

Epc/V vs Fc a Rh(III)–Rh(I)

E½/V vs Fca NN, NN; NN, NN

kabs/nm (e/104 M1 cm1)b

bpy 4,40 -dmbpy 4,40 -dtbubpy 4,40 -dmobpy 5,50 -dmbpy phen 5-mphen 5,6-dmphen 4-mphen 4,7-dmphen 3,4,7,8-tmphen 4,7-dpphen 5-Clphen

1.28 1.31 1.31 1.36 1.32 1.30 1.32 1.31 1.33 1.32 1.33 1.28 1.27

1.72, 2.02 1.75, 2.05 1.75, 2.04 1.72, 2.02 1.75, 2.06 1.70, 2.00 1.71, 2.01 1.71, 2.03 1.74, 2.04 1.77, 2.07 1.80, 2.14 1.63(br), 1.96 1.62(br), 1.94

242 242 242 238 244 232 238 240 232 238 238 296 234

(3.84), (4.28), (4.57), (7.24), (4.04), (4.93), (4.71), (4.71), (4.37), (4.18), (4.65), (5.94), (4.81),

306 306 306 296 308 276 284 290 278 280 284 304 284

(3.66), (3.68), (3.79), (3.03), (3.44), (3.89), (3.78), (3.66), (4.04), (4.29), (4.67), (5.72), (3.85),

320 318 318 306 320 306 306 306 306 306 306 320 306

kem/nmc

(3.63) (3.50) (3.75) (3.43), 320 (2.63) (3.87), 332 (1.55) (3.17), 320 (3.02), 350 (0.11) (3.20), 320 (3.05), 344 (0.15), 360 (0.12) (3.03), 320 (2.95), 358 (0.14), 372sh (0.12) (3.12), 320 (2.80), 350 (0.14) (3.15), 320 (2.83), 350 (0.17) (3.20), 320 (2.95), 334sh (0.30), 352 (0.07) (4.19), 336sh (1.36), 368sh (0.43) (3.12), 320 (2.96), 342sh (0.12), 358 (0.09)

450, 450, 449, 449, 465, 450, 466, 484, 452, 456, 456, 486, 466,

484, 485, 482, 482, 501, 484, 490, 519, 484, 487, 489, 520, 500,

519, 522, 519, 520, 541, 520, 523 554 520, 524, 524, 551, 535

554 556 553 554 574 554

553 555 553 601sh

a Cyclic Voltammetry in dry DMF with 0.1 M nBu4NPF6 using Ag/AgCl (0.29 V vs. NHE) reference electrode, all signals are referenced to the ferrocene/ferrocenium redox potential; 200 mV/s scan rate. b UV–vis data in CH3CN. c Emission Spectra in DMF at 77 K, Excitation at 320 nm, 399 nm cut-on filter. Emission corrected for instrument and detector response.

D. Amarante et al. / Inorganica Chimica Acta 461 (2017) 239–247

3.1. Syntheses 3.1.1. Triflate intermediate The triflate intermediate was prepared from cis-[Rh(bpy)2Cl2] (PF6) by refluxing the metal starting material in the presence of excess triflic acid with o-dichlorobenzene as the solvent (Scheme 1). The driving force for this reaction is the protonation of the chloride ligands and their subsequent loss as HCl gas, which allows their replacement by two triflate groups. Several changes were made to the known literature procedure [17]. First, cis-[Rh (bpy)2Cl2](PF6) was used as the starting material rather than cis[Rh(bpy)2Cl2](Cl)2H2O. Second, triflic acid was added twice followed each time by heating for 2.5 h; this insures complete conversion of cis-[Rh(bpy)2Cl2](PF6) to cis-[Rh(bpy)2(OTf)2](OTf). Finally, we found that it is critical to keep the temperature well below reflux (130 °C) to prevent decomposition of the complexes. Higher temperatures (refluxing o-dichlorobenzene, 180 °C) causes lower yields. With these differences in procedure, the yield is raised from 47% [17] to 98%. 3.1.2. Mixed diimine ligand complexes The triflate groups are excellent leaving groups, making cis-[Rh (bpy)2(OTf)2](OTf) a reactive precursor in the synthesis of compounds with the general formula [Rh(bpy)2(NN)](PF6)3. We exploited the lability of the triflate ligands to develop a general, high yield synthesis requiring relatively mild reaction conditions (Scheme 1). These reactions were run in refluxing ethanol in the presence of a slight excess of the NN ligand (1.2 M excess). Reaction times were typically 30 min. However, four ligands required significantly longer reaction times. The longer addition times of the 5,50 -dmbpy and 3,4,7,8-tmphen ligands are most likely due to the presence of the methyl groups meta to the nitrogens. The longer addition times of the 5-Clphen and 4,7-dpphen ligands are consistent with the electron withdrawing effect of the substituents on the ligands. It was necessary to remove the ethanol and some excess water on the rotary evaporator so that the ethanol was not trapped in the crystal matrix. After precipitation and isolation as the PF 6 salt, no further purification was required. Purity of the compounds was based on the NMR, electronic absorbance, elemental analysis and emission spectroscopy, which showed no change upon further purification.

m

3. Results and discussion

aliphatic protons in the case of methyl and t-butyl substituted complexes. The lability of the triflate ligands makes it necessary to mix samples of [Rh(bpy)2(OTf)2](OTf) for 1H NMR immediately prior to collecting the spectrum. 19F NMR on the [Rh(bpy)2(OTf)2](OTf) in d6-DMSO showed two distinct signals, one at 79.05 ppm the signal for free triflate of NaOTf in d6-DMSO. The other signal at 79.27 ppm was assigned to rhodium bound OTf-. Monitoring the signal over one week showed that the steady loss of peak at 79.27 ppm, further supporting the conclusion that the bound triflate is being replaced by DMSO. At the end of one week only the

m

J = 5.69 Hz, 2H), 7.93 (d, J = 5.43 Hz, 2H), 7.88 (t, J = 6.81 Hz, 2H), 7.81 (d, J = 5.74 Hz, 2H), 7.70 (m, 10 H, C6H5), 7.63 (s, 2 H. 13C NMR (d6-DMSO, 100 MHz): d (ppm) aromatic carbons 154.9, 154.6, 152.9, 151.9, 151.2, 151.1, 145.1, 143.2, 143.0, 134.4, 130.5, 130.0, 129.7, 129.3, 129.0, 128.1, 127.0, 126.8.

243

The data from the 1H and 13C NMR spectra are all consistent with the given complexes. [Rh(bpy)3]3+ has D3 symmetry and the 1 H NMR shows the expected four aromatic signals expected from symmetry arguments. The [Rh(bpy)2(NN)]3+ complexes with disubstituted diimine ligands have C2 symmetry and the 1H NMR show at most eleven aromatic signals—eight for the bipyridine and three for the disubstituted diimine. The [Rh(bpy)2(NN)]3+ complexes containing monosubstituted phenanthroline ligands have C1 symmetry and the 1H NMR show at most fifteen aromatic signals—eight for the bipyridine and seven for the monosubstituted phenanthroline. In some cases there were fewer signals, due to superposition of similar protons. In addition, there are also

m

3.2. NMR spectroscopy

Fig. 1. Cyclic Voltammograms of [Rh(bpy)2(NN)]3+ in dry DMF with 0.1 M nBu4NPF6 using Ag/AgCl reference electrode, all signals are referenced to the ferrocene/ferrocenium redox potential; 200 mV/s scan rate.

244

D. Amarante et al. / Inorganica Chimica Acta 461 (2017) 239–247

free triflate peak at 79.05 ppm remained, indicating that all of the bound triflate had been replaced by solvent. 13C NMR could not be taken as approximately 30% of the material has been converted from [Rh(bpy)2(OTf)2][OTf] to the bis-solvated species by the end of the 12 h needed to collect the typical spectra for these complexes. 3.3. Electrochemistry Redox potentials for the thirteen [Rh(bpy)2(NN)](PF6)3 compounds are given in Table 1 and Fig. 1. Rhodium(III) diimine complexes are known to undergo irreversible two electron reduction from Rh(III) to Rh(I) with a concurrent change in coordination number and geometry from octahedral (CN = 6) to square planar (CN = 4) [25,26]. This reduction is followed by two reversible one-electron reductions of the diimine ligands [25,26]. All of our [Rh(bpy)2(NN)](PF6)3 compounds studied showed an irreversible two-electron wave between 1.27 V and 1.36 V followed by two reversible, one-electron diimine reductions. The first reduction corresponds to the reduction of Rh(III) to Rh(I) and the concomitant loss of one of the diimine ligands. However, with the mixed ligand systems, there are two possible products as shown in Scheme 2. The cyclic voltammogram of cis-[Rh(bpy)3](PF6)3 is in agreement with that of DeArmond et al. [25]. Comparison of the substituted 2,20 -bipyridine and substituted 1,10-phenanthroline ligand complexes shows that any substituent causes a slight shift of the irreversible, two-electron reduction to more negative potentials. [Rh(bpy)2(4,40 -dmobpy)]3+ shows the greatest shift due to the electron-donating effects of the methoxy groups, which makes the Rh(III) harder to reduce. The methyl substituted phenanthrolines all have slightly more negative irreversible two electron reduction potentials relative to [Rh (bpy)3]3+, while these values are small they are consistent from trial to trial. The [Rh(bpy)2(5-mphen)]3+ and [Rh(bpy)2(5,6-dmphen)]3+ exhibit two reversible, one-electron reductions which are essentially unchanged from those of [Rh(bpy)3]3+, suggesting that it is the 5-mphen or 5,6-dmphen ligand that is lost in these cases, see Fig. 1A. In the case of [Rh(bpy)2(4-mphen)]3+, [Rh(bpy)2(4,7dmphen)]3+ and [Rh(bpy)2(3,4,7,8-tmphen)]3+, increasing the number of methyl groups causes the E½ values for the two reversible diimine reductions to shift to progressively more negative potentials, see Fig. 1B. This continual shift supports the loss of one of the bipyridine ligands to form the [Rh(bpy)(NN)]+ complex

following the initial reduction of Rh(III) to Rh(I). The position of the alkyl groups has an effect on the redox potentials particularly where the electron donating alkyl group is para to the bound nitrogen. The two complexes containing electron withdrawing ligands, [Rh(bpy)2(4,7-dpphen)]3+ and [Rh(bpy)2(5-Clphen)]3+, show a broadening of the first reversible diimine reduction. This suggests that both [Rh(bpy)2]+ and [Rh(bpy)(NN)]+ are present in solution and that both four coordinate species undergo reduction, shown in Scheme 2. The two peaks within the broad band are relatively equal in height suggesting that the two four-coordinate species are present in roughly equal amounts. This implies that the bipyridine ligand is preferentially lost and the more electron withdrawing ligand remains. For [Rh(bpy)2(4,7-dpphen)]3+, the first one-electron reduction wave comes at 1.63 V, while the first one electron reduction wave comes at 1.62 V for [Rh(bpy)2(5Clphen)]3+, see Fig. 1C. In these two cases the first diimine reduction for the [Rh(bpy)(NN)]+ compound comes before the first diimine reduction for the [Rh(bpy)2]+ species. This is consistent with the presence of an electron-withdrawing group on one of the diimine ligands. Thus, the broad peak is the overlap of the redox potentials of the bipyridine and the electron-withdrawing substituted phenanthroline ligands. These data suggest that in most cases only one type of diimine ligands is lost. 3.4. Electronic spectroscopy 3.4.1. Absorbance The UV-vis absorbance spectra are given in Fig. 2 and the wavelength and extinction coefficient data are given in Table 1. The absorbance spectra of all thirteen compounds exhibit the characteristic bipyridine based pp⁄ transitions at 306 nm and 320 nm and a third band at 242 nm [27]. The energies and intensities of these bands are relatively insensitive to the nature of the third ligand. The [Rh(bpy)2(4,40 -dmbpy)]3+ and [Rh(bpy)2(4,40 -dtbubpy)]3+ absorbance spectra (Fig. 2A) show no shifts in the energies of the bands and only shifts in extinction coefficients relative to [Rh (bpy)3]3+. While both the methyl and t-butyl groups are electron donating, they appear to have little effect on the energy of the ligand based pp⁄ transitions. By contrast, the [Rh(bpy)2(5,50 dmbpy)]3+ absorbance spectrum shows a slight red shift of the higher energy band to 244 nm. There is also a new band at 332 nm on the low energy side of the bipyridine pp⁄ transitions.

Scheme 2. Possible reduction pathways of [Rh(bpy)2(NN)](PF6)3.

D. Amarante et al. / Inorganica Chimica Acta 461 (2017) 239–247

Molar Absorptivity/(1 x 104 cm–1M–1)

8 bpy 4,4'-dmbpy 5,5'-dmbpy 4,4'-dtbubpy 4,4'-dmobpy

A

7 6 5 4 3 2 1 0 220

270

Molar Absorptivity/(1 x 104 cm–1M–1)

7

370

0.4 0.3 0.2 0.1 0.0

B

6

320

Wavelength (nm)

5

330

4

380 phen

3

5-mphen 5,6-dmphen

2

5-Clphen 1 0 220

270

Molar Absorptivity/(1 x 104 cm–1M–1)

8

370

0.5 0.4 0.3 0.2 0.1 0.0

C

7

320 Wavelength (nm)

6 5

330 350 370 390

4 phen 4-mphen 4,7-dmphen 3,4,7,8-tmphen 4,7-dpphen

3 2 1 0

220

270

320 Wavelength (nm)

370

Fig. 2. UV–vis absorbance spectra of [Rh(bpy)2(NN)](PF6)3 in CH3CN at room temperature.

Both these changes arise from pp⁄ transitions of the 5,50 -dmbpy ligand. These bands are at lower energy than the 2,20 -bipyridine ligand and are consistent with the electron donating abilities of the methyl groups in the meta position relative to the nitrogens. The absorbance spectrum of [Rh(bpy)2(4,40 -dmobpy)]3+ shows a blue shift of the highest energy band to 238 nm and a splitting of this band into two peaks. In addition, a new band appears at 296 nm on the high energy side of the 2,20 -bipyridine pp⁄ transitions. These changes arise from the presence of the 4,40 -dmobpy ligand pp⁄ transitions. These bands are at higher energy due to the electron-withdrawing effect of the methoxy groups para to the nitrogens.

245

The [Rh(bpy)2(phen)]3+ absorption spectrum shows additional pp⁄ transitions at 232 nm and 276 nm due to the presence of the phenanthroline ligand and a ligand-to-metal charge transfer (LMCT) band (e = 1100 cm1 M1) at 350 nm that is not present in the 2,20 -bipyridine derived complexes (Fig. 2B and inset). Comparison of the eight 1,10-phenanthroline derivatives show that the positions of these phenanthroline pp⁄ bands are very dependent on the substituents. For all the 1,10-phenanthroline complexes there are one or more low energy transitions with molar absorptivity, <2000 cm1 M1, which would be consistent with an assignment of ligand-to-metal charge transfer bands as reported earlier as Crosby, et al. [20]. Comparison of substituents in the 5 and 6 position of 1,10phenanthroline (Fig. 2B and inset) shows that the presence of a methyl group or a chloro group in the 5 position both lead to a red shift in the phenanthroline bands to 284 nm, and a splitting and red shift of the MLCT band to  359 nm. The absorption spectra of [Rh(bpy)2(5-Clphen)]3+ and [Rh(bpy)2(5-mphen)]3+ differ in the high energy band, which is at 234 nm for [Rh(bpy)2(5Clphen)]3+ and 238 nm for the [Rh(bpy)2(5-mphen)]3+; both are red shifted relative to [Rh(bpy)2(phen)]3+, but the 5-mphen more so than the 5-Clphen. Placing two substituents on phenanthroline causes a more pronounced red shift in the 1,10-phenanthroline absorbances as seen in [Rh(bpy)2(5,6-dmphen)]3+, especially in the low energy absorbances. This indicates an additive effect of the electron-donating substituents. In presence of methyl groups in the 4 and 7 position of 1,10phenanthroline, [Rh(bpy)2(4-mphen)]3+ and [Rh(bpy)2(4,7dmphen)]3+, [Rh(bpy)2(3,4,7,8-tmphen)]3+ shows little change from 1,10-phenanthroline alone (Fig. 2C). Phenyl substituents on the 4 and 7 position cause major shifts of the phenanthroline absorbances to lower energy and higher intensities as the p electron density is extended out onto the phenyl rings. This is consistent with emission from the lower energy 4,7-dpphen p–p⁄ state as seen in the absorbance. 3.4.2. Emission Known rhodium(III) diimine complexes only exhibit emission at low temperatures in frozen solvent glasses or in the solid state. [Rh (NN)3]3+ exhibits highly structured emission from a ligand based 3 p–p⁄ state with a maximum at 450 nm [27,28]. By contrast [Rh(bpy)2X2]+, where bpy = 2,2-bipyridine and X = Cl, Br, or I, exhibit a broad featureless emission from a d–d⁄ state with a maximum at 700 nm [24,27,28]. All thirteen complexes studied gave a highly structured emission at 77 K in a frozen DMF glass characteristic of the ligand based 3pp⁄ emission (Table 1 and Fig. 3). With the exception of 5,50 -dmbpy, substitution of the bipyridine ligand does not appear to affect the energy of the excited state. Comparison of the bipyridine derivatives (Fig. 3A) shows only minor changes in the energies of the [Rh(bpy)3]3+ maxima at 450, 484, 519 and 554 nm for substituents in the 4 and 40 positions—[Rh(bpy)2(4,40 -dmbpy)]3+, [Rh(bpy)2(4,40 -dmobpy)]3+ 3+ 0 and [Rh(bpy)2(4,4 -dtbubpy)] . In these systems, the emission occurs from the 2,20 -bipyridine p–p⁄ state which is the lowest energy excited state based on Fig. 2A. There are shifts in the relative intensities, especially in the low energy shoulders which increase from bpy to 4,40 -dtbubpy to 4,40 -dmbpy to 4,40 -dmobpy. By contrast the [Rh(bpy)2(5,50 -dmbpy)]3+ exhibits a red shift in all its bands to 465, 501, 541 and 574 nm indicating that the emission is occurring from the lower energy 5,50 -dmbpy p–p⁄ state. The emission spectrum of [Rh(bpy)2(phen)]3+ is very similar to that of [Rh(bpy)3]3+ both in energies and intensities of emission (Fig. 3B and Table 1). The emission is occurring from the bipyridine p–p⁄ state. Substitution of phenanthroline in the 5 and 6 positions causes significant shifts in the emission maxima (Fig. 3B). Single

D. Amarante et al. / Inorganica Chimica Acta 461 (2017) 239–247

Normailzed Emission Intensity

246

the emission is from the bipyridine 3p–p⁄. By contrast, [Rh (bpy)2(4,7-dpphen)]3+ appears to have shifted the first vibrational band in the emission progression to significantly lower energy (485 nm) indicating the emission is from the 4,7-dpphen p–p⁄ excited state.

bpy 4,4'-dmbpy 5,5'-dmbpy 4,4'-dtbubpy 4,4'-dmobpy

A

10 8 6

4. Conclusions 4 2 0 430

480

530

580

630

680

730

Wavelength (nm)

Normalized Emission Intensity

12 phen 5-mphen 5,6-dmphen 5-Clphen

B

10 8 6 4 2 0 430

480

530

580

630

680

730

Wavelength (nm)

Normalized Emission Intensity

12 10

Funding sources

phen 4-mphen 4,7-dmphen 3,4,7,8-tmphen 4,7-dpphen

C

8

cis-[Rh(bpy)2(OTf)2](OTf) is a versatile intermediate for the formation of pure mixed ligand diimine complexes in high yield under relatively mild reaction conditions. The thirteen compounds we synthesized were pure enough to be used for photophysical measurements without further purification. The electrochemistry of these complexes shows an irreversible two-electron Rh(III) to Rh(I) reduction with concomitant ligand loss to form a square planar complex. Which ligand is lost depends on the substituents on the ligands; in three cases both possible intermediates appear to form. The choice of diimine ligand can also tune the potential of the one-electron bipyridine reductions over a range of 200 mV. The absorbance and emission spectra show that most of the absorbances and emissions are ligand based pp⁄ transitions. Substitution of the 2,20 -bipyridine ligand causes very little change in the absorbance and emission spectra. Substitution of the phenanthroline ligand causes more pronounced shifts in the electronic spectra due to the more rigid aromatic system. Inclusion of a 4-mphen, 4,7dmphen or 3,4,7,8-tmphen ligand causes very little change in the electronic spectra, while inclusion of a 5-mphen, 5,6-dmphen or 5-Clphen ligand shifts both the absorbance and emission spectra to lower energy. The biggest changes to the electronic spectra are seen with for [Rh(bpy)2(4,7-dpphen)]3+ which is shifted significantly to lower energy due to the extended p system.

This work was funded by the Clare Boothe Luce Program of the Henry Luce Foundation and St. John’s University. Acknowledgments

6

The authors want to thank Christopher Emmel and Dr. Vennesa O. Williams for their preliminary electrochemical studies on these compounds. We are grateful to Drs. Richard J. Rosso and Alison G. Hyslop of useful discussions.

4 2

0 430

480

530 580 630 Wavelength (nm)

680

730

Fig. 3. Emission spectra of [Rh(bpy)2(NN)](PF6)3 in DMF glass at 77 K in quartz tubes; kex = 320 nm; using a 399 nm cutoff filter.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2017.02.011. References

substitution in the 5 position, [Rh(bpy)2(5-mphen)]3+ and [Rh(bpy)2(5-Clphen)]3+, shifts the first maxima to 466 nm, but causes more pronounced shifts in the lower energy bands with the electron withdrawing 5-Clphen ligand. Double substitution in the 5 and 6 positions of [Rh(bpy)2(5,6-dmphen)]3+ leads to the greatest red shift. This suggests that the emission for these three complexes is coming from the substituted 1,10-phenanthroline ligand. Methyl substitution of 1,10-phenanthroline in the 4 position shows little change in the emission maxima for the [Rh(bpy)2(4-mphen)]3+ (Fig. 3C), but drops the intensities of the later emission bands. Multiple methyl substitution in the 3, 4, 7, and 8 positions, [Rh(bpy)2(4,7-dmphen)]3+ and [Rh(bpy)2(3,4,7,8tmphen)]3+, causes little to no change in the spectra indicating

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