Article pubs.acs.org/JPCA
Optoelectronic Properties and Structural Effects of the Incremental Addition of Pyridyl Moieties on a Rhodium Dimer Daniel Chartrand and Garry S. Hanan* Department of Chemistry, Université de Montréal, Montréal, QC H3T 1J4, Canada S Supporting Information *
ABSTRACT: The synthesis and characterization of five C−C coupling products obtained from the reaction of a paddlewheel tetrakis 4-bromo-N,N′-diphenylbenzamidinate dirhodium dimer with 4-pyridineboronic acid pinacol ester are reported. The coupling reactions occur on one to four amidinate ligands, leading to rhodium dimers containing [tetrakis, tris, cis-bis, trans-bis, or mono]-N,N′-diphenyl-4-(pyridin-4-yl)benzamidinate ligands, effectively creating new binding sites on the metal complexes. The new compounds were isolated by column chromatography, and the exact conformations were verified by X-ray crystallography. Redox processes showed only a small variation within the coupling products and included two oxidations (1.30 ± 0.02 V, 0.27 ± 0.01 V vs SCE) and one reduction (−1.55 ± 0.02 V vs SCE), all centered on the Rh−Rh core. Time-dependent density functional theory (TD-DFT) was used to analyze this series with four other fully characterized N,N′diphenyl-aryl-amidinate rhodium dimers that were found in the literature. The two main absorption bands of these nine rhodium dimers were compared to TD-DFT calculations, both giving excellent correlation. The first, a metal-to-metal (MM) transition around 11800 cm−1 (845 nm) was blue-shifted in the calculation, with an average difference of 1378 cm−1 but had only a 15 cm−1 standard deviation, showing a strong correlation despite the energy difference. The second, a metal-to-ligand charge transfer (MLCT) transition around 18900 cm−1 (530 nm) was a near perfect match with only a 64 cm−1 average difference and a 35 cm−1 standard deviation. The electronic transition, redox potentials, and HOMO and LUMO energies of all dimers were plotted versus the Hammett parameter (σ) of the aryl group and Taft’s model with 2 components: field effects (σF) and resonance (σR). The properties involving only the Rh−Rh core (MM band, all oxidation potentials, HOMO and LUMO) were fit with a single set of σF and σR contributions (73% and 27%), with a goodness-of-fit (R2) value ranging from 90% to 99.7%. The metal-dimer to ligand charge-transfer band, involving the amidinate ligand, displayed different values of contribution with 45% and 55% for the σF and σR, respectively, with a fit of 94.8%. The accuracy of these fits enables the designed modification of amidinate-based dirhodium complexes to achieve desirable redox and spectroscopic properties.
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form larger, often mixed-metal, assemblies.38−44 In this study, we look at a postmodification of a dirhodium dimer formed with four 4-bromo-N,N′-diphenylbenzamidine ligands via a Suzuki coupling to attach a pyridyl motif,45−50 thus creating new binding sites on the complex.51−53 Herein we present the synthesis and characterization of all five species formed by Suzuki coupling: the mono- (1), cis-bis(2a), trans-bis- (2b), tris- (3), and tetrakis- (4) N,N'-diphenyl4-(pyridin-4-yl)bezamidinates, as illustrated in Chart 1. The properties of the synthesized complexes are compared to other Rh2(N,N′-diphenylarylamidinate)4 complexes (Chart 1, 5 to 9), with an emphasis on the Hammett and Taft parameters, applied to an amidinate containing a central aryl group.54,55 These parameters have successfully been correlated to electrochemical and/or catalytic properties upon functionalization of the ligand
INTRODUCTION The paddle-wheel motif formed by many metal dimers offers a very modular and rigid scaffold to build up larger architectures. Many metals (e.g., copper, molybdenum, and rhodium) form neutral, dimetallic assemblies with carboxylate ligands.1−3 Among these complexes, rhodium tetracarboxylate dimers have been extensively developed for catalytic applications.4−11 These rhodium dimers have also been used for supramolecular assemblies, using axial or equatorial linkers to form discrete or infinite structures.12−24 Other families of ligands, such as amidines, can be used to form metal dimer assemblies. The amidine metal dimers show great stability and greater functionalization potential than the tetracarboxylate dimers, since moieties can be attached on both the central carbon and the nitrogen atoms.25−33 The drawback to this is their limited catalytic activity,34,35 although they have been shown to be photoactive to alkyl halide reduction36 and mixed amidinate and diimine dimers have also gathered interest as an all-in-one chromophore−catalyst system for hydrogen production.37 For their part, the tetraamidinate metal dimers have been used to © 2014 American Chemical Society
Special Issue: Current Topics in Photochemistry Received: March 4, 2014 Revised: July 19, 2014 Published: July 20, 2014 10340
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a step rate of 4 mV, a square-wave amplitude of 25 mV, and a frequency of 15 Hz. The criteria for reversibility were the separation of 60 mV between cathodic and anodic peaks, the close to unity ratio of the intensities of the cathodic and anodic currents, and the constancy of the peak potential on changing the scan rate. For irreversible processes, the peaks observed from square-wave voltammetry were used. Experimental uncertainties are as follows: absorption maxima, ± 0.5 nm; molar absorption coefficient, 10%; redox potentials, ± 30 mV. Synthetic Methods. S-PHOS (2-dicyclohexylphosphino2′,6′-dimethoxybiphenyl),59 4-pyridineboronic acid pinacol ester (4,4,5,5-tetramethyl-2-(4-pyridyl)-1−3-dioxaborolane),60 and Pd2(dba)3, (dba = dibenzylideneacetone)61 were prepared from literature procedures. The synthesis of the tetrakis(4bromo-N,N′-diphenylbenzamidinate) dirhodium(II) dimer (5) was previously reported,43,62 and it is done by a neat melt reaction at 180 °C of rhodium acetate and 4-bromo-N,N'diphenylbenzamidine (in 15-fold excess) under a flow of nitrogen for 10 min with a yield of up to 95% after removal of the excess ligand.43,62 Suzuki Coupling Reactions. The syntheses of species 1 to 4 were based on the same reaction conditions. However, synthesis of complex 4 could be optimized without any further attention to debromination, while the syntheses of 1 to 3 required optimization to lower debromination of the starting brominated benzamidinate rhodium dimer. Tetrakis(N,N'-diphenyl-(4-pyridin-4-yl)benzamidinate) dirhodium(II,II) (4). Rhodium dimer 5 (121.3 mg, 75.2 μmol), Pd2(dba)3 (0.68 mg, 0.7 μmol), S-PHOS (0.57 mg, 1.3 μmol), potassium carbonate (69 mg, 522 μmol), and 4-pyridine boronic ester (122 mg, 594 μmol) were dispersed in THF:water (15 mL, 3:1, v:v) solution under nitrogen then were sealed in a 20 mL microwave pressure tube and heated by microwave at 115 °C for 1 h. The red solution was reduced to 5 then 20 mL of water was added, leaving a red precipitate, which was collected by filtration and dissolved in DCM and MeOH (1:1, v:v) solution. This solution was evaporated to dryness to give 4. Purification: the bulk solid was dissolved in MeOH (5 mL) and concentrated HCl (3 drops) forming a brown solution. The solution was dried again to remove excess HCl, and the solid is dissolved in the eluent mixture [9 MeOH, 1 aqueous HCl (0.1M)) (v:v)] and applied to a Sephadex LH-20 column (1 cm diameter, 2 m length, flow rate of ∼10 mL/hour). The second fraction was isolated; complex 4 was obtained as a precipitate after neutralization with aqueous KOH (1 M, 5 mL) and reduction with hydrazine (0.2 mL). The isolated red solid was dissolved in DCM (10 mL), and all salts were extracted into the water phase (10 mL × 3). The organic phase was dried under vacuum, to give 115 mg (yield 95%) of complex 4. 1 H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 5 Hz, 8H: a), 7.27 (d, J = 6 Hz, 8H: b), 7.15 (d, J = 8 Hz, 8H: c), 7.04 (br, 16H: g), 6.98−6.93 (m, 8H: h), 6.93 (d, J = 6 Hz, 8H: d), 6.69 (br, 8H: e), 5.86 (br, 8H: f). 13C{1H} NMR (100 MHz, CDCl3): δ 170.2; 151.3; 150.1; 147.3; 136.36; 136.22; 132.4; 128.5; 127.8; 125.6; 123.0; 121.1. HRMS (ESI, CH2Cl2) (m/ z): [M + 4H]+4 [C96H72N12Rh2]4H+4 calcd, 400.6106; found, 400.6124. Anal. Calcd for C96H72N12Rh2+H2O: C, 71.28; H, 4.61; N, 10.39. Found: C, 71.03; H, 4.38; N, 10.40 (water confirmed by 1H NMR and crystal structure). Suzuki Coupling Conditions for Species 1 to 3. Rhodium dimer 5 (323.3 mg, 201 μmol), Pd2(dba)3 (1.74 mg, 1.9 μmol), S-PHOS (1.73 mg, 4.2 μmol), potassium carbonate (116 mg,
Chart 1. Rh2 Dimers 1 to 5 Prepared in This Study (Top); Other Dimers Used for Comparison (Bottom)
in both paddlewheel tetracarboxylate and tetraamidinate metal dimers in the past.35,56,57 In these past studies, the amidinate dimers were always formamidinate-based, as such, no data exist for functionalization at the central position. The present study focuses on the electrochemical and photophysical properties of the benzamidinate-functionalized dirhodium tetraamidinate dimers.
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EXPERIMENTAL SECTION General Considerations. All of the organic reagents were obtained from Sigma-Aldrich and rhodium(II) acetate from Pressure Chemical Co. Solvents from Fischer and Anachemia were used as received, except for acetone and dichloromethane (DCM) which were distilled. Nuclear magnetic resonance spectra were recorded using Bruker spectrometers at room temperature, with 1H (400 MHz) and 13C (100 MHz) chemical shifts referenced to residual solvent resonances (7.26 and 77.0 ppm, respectively, for CHCl3). Elemental analyses were performed on the desolvated bulk samples by the university departmental service. Room temperature photophysical measurements were done in air-equilibrated and distilled DCM, using a 1 cm quartz cell. Absorption and emission spectra were recorded using a Cary 500i UV−vis−NIR spectrophotometer and a Cary Eclipse 300 fluorimeter, respectively. Electrochemical measurements were carried out in argon-purged DCM at room temperature with a BAS CV50W potentiostat. The working electrode was a Pt electrode, the counter electrode was a Pt wire, and the pseudoreference electrode was a silver wire. The reference was set using an internal 1.0 mM ferrocene sample with its redox couple adjusted to 460 mV vs SCE in dichloromethane.58 The concentration of the compounds was around 1 mM. Tetrabutylammonium hexafluorophosphate (TBAP) was used as the supporting electrolyte at a concentration of 0.10 M. Cyclic voltammograms (CVs) were obtained at scan rates of 50, 100, and 200 mV/s. For reversible processes, half-wave potentials (vs SCE) were measured with square-wave voltammetry (SWV) experiments performed with 10341
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840 μmol), and 4-pyridine boronic ester (164.6 mg, 802 μmol) were dispersed in THF:water (45 mL, 3:1, v:v) solution under nitrogen then were sealed in a 100 mL pressure tube and heated at 115 °C for 15 min. The work up and purification is identical to 4, although the purification by Sephadex column separated the mixture of products into four pure fractions where complex 4 elutes first, followed by complexes 3, 2, and 1, respectively. Complex 2 was further purified into 2a and 2b by column chromatography (silica, hexane/AcOEt (v:v) (1:1): with a gradient to pure AcOEt). Rf of all species in AcOEt: 1, 0.94; 2a, 0.2; 2b, 0.57; 3, 0.05; 4, 0; 5, 1. Isolated yields are based on the removal of the starting material recovered mass (170 mg, 52%): 1, 16.3 mg (10%); 2a (cis), 26.5 mg (16%); 2b (trans), 13.1 mg (8%); 3, 17.1 mg (11%); 4, 3.9 mg (2%). Tris(N,N′-diphenyl-4-bromo-benzamidinate)(N,N′-diphenyl-(4-pyridin-4-yl)benzamidinate) dirhodium(II,II) (1). 1H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 6 Hz, 2H: a), 7.27 (d, J = 6 Hz, 2H: b), 7.14 (d, J = 8 Hz, 2H: c), 7.10−6.98 (m, 16H: g,g′), 6.96 (d, J = 8 Hz, 6H: c′), 6.95−6.90 (m (br), 8H: h,h′), 6.89 (d, J = 8 Hz, 2H: d), 6.65 (dd, J = 9, 1 Hz, 6H: d′), 6.58 (br, 8H: e,e′), 5.78 (br, 8H: f,f′). 13C{1H} NMR (100 MHz, CDCl3): δ 170.1; 169.7; 151.2; 151.09; 150.06; 147.3; 136.4; 136.1; 134.0; 133.1; 132.3; 130.4; 128.5 (br); 127.8 (br); 125.5; 123.04; 122.99; 121.7; 121.1. HRMS (ESI, CH2Cl2) (m/ z): [M]+ [C 81H 60 Br3N 9 Rh 2]+ calcd, 1601.0631; found, 1601.0613. Anal. Calcd for C81H60Br3N9Rh2: C, 60.62; H, 3.77; N, 7.85. Found: C, 60.60; H, 3.74; N, 7.86. cis-Bis(N,N′-diphenyl-4-bromo-benzamidinate)bis(N,N′-diphenyl-(4-pyridin-4-yl)benzamidinate) dirhodium(II,II) (2a). 1 H NMR (400 MHz, CDCl3): δ 8.52 (d, J = 5 Hz, 4H: a), 7.28 (d, J = 6 Hz, 4H: b), 7.14 (dd, J = 7, 5 Hz, 4H: c), 7.10−6.99 (m (br), 16H: g,g′), 6.99−6.96 (m, 4H: c′), 6.95−6.92 (m, 8H: h,h′), 6.91 (d, J = 8 Hz, 4H: d), 6.66 (d, J = 8 Hz, 4H: d′), 6.61 (br, 8H: e,e′), 5.81 (br, 8H: f,f′). 13C{1H} NMR (100 MHz, CDCl3): δ 170.1; 169.7; 151.26; 151.13; 150.1; 147.3; 136.37; 136.14; 134.1; 133.1; 132.3; 130.4; 128.5 (br); 127.8 (br); 125.6; 123.04; 122.98; 121.7; 121.1. HRMS (ESI, CH2Cl2) (m/ z): [M]+ [C86H64Br2N10Rh2]+ calcd, 1600.1792; found, 1600.1777. Anal. Calcd for C86H64Br2N10Rh2: C, 64.43; H, 4.02; N, 8.74. Found: C, 64.40; H, 4.01; N, 8.74. trans-Bis(N,N′-diphenyl-4-bromo-benzamidinate)bis(N,N′diphenyl-(4-pyridin-4-yl)benzamidinate) dirhodium(II,II) (2b). 1H NMR (400 MHz, CDCl3): δ 8.52 (dd, J = 5, 1 Hz, 4H: a), 7.28 (dd, J = 5, 2 Hz, 4H: b), 7.14 (d, J = 8 Hz, 4H: c), 7.11−6.99 (m (br), 16H: g,g′), 6.95 (d, J = 8 Hz, 4H: c′), 6.95−6.91 (m, 8H: h,h′), 6.90 (d, J = 8 Hz, 4H: d), 6.66 (d, J = 8 Hz, 4H: d′), 6.62 (br, 8H: e,e′), 5.80 (br, 8H: f,f′). 13C{1H} NMR (100 MHz, CDCl3): δ 170.1; 169.7; 151.26; 151.13; 150.0; 147.4; 136.35; 136.11; 134.1; 133.0; 132.3; 130.4; 128.5 (br); 127.7 (br); 125.6; 123.06; 123.01; 121.6; 121.1. HRMS (ESI, CH2Cl2) (m/z): [M]+ [C86H64Br2N10Rh2]+ calcd, 1600.1792; found, 1600.1761. Anal. Calcd for C86H64Br2N10Rh2: C, 64.43; H, 4.02; N, 8.74. Found: C, 64.40; H, 4.01; N, 8.74. (N,N′-Diphenyl-4-bromo-benzamidinate)tris(N,N′-diphenyl-(4-pyridin-4-yl)benzamidinate) dirhodium(II,II) (3). 1H NMR (400 MHz, CDCl3) δ 8.55−8.49 (m, 6H: a), 7.30− 7.25 (m, 6H: b), 7.17−7.11 (m, 6H: c), 7.04 (m (br), 16H: g,g′), 6.96 (d, J = 9 Hz, 2H: c′), 6.95−6.94 (m, 8H: h,h′), 6.92 (dd, J = 8, 1 Hz, 6H: d), 6.67 (d, J = 9 Hz, 2H: d′), 6.66 (br, 8H: e,e′), 5.83 (br, 8H: f,f′). 13C{1H} NMR (100 MHz, CDCl3): δ 170.1; 169.7; 151.29; 151.17; 150.1; 147.3; 136.37; 136.17; 134.1; 133.1; 132.3; 130.4; 128.5 (br); 127.8 (br);
125.5; 123.04; 122.99; 121.7; 121.1. HRMS (ESI, CH2Cl2) (m/ z): [M]+ [C91H6879BrN11Rh2]+ calcd, 1599.29471; found, 1599.2953. Anal. Calcd for C91H6879BrN11Rh2+H2O: C, 67.50; H, 4.36; N, 9.51. Found: C, 67.18; H, 4.18; N, 9.53 (water confirmed by 1H NMR). Computational Methods. All calculations were performed with Gaussian software (G03 and G09).63,64 G09 was used mainly for gas phase optimization and vibrational frequency determinations of the bigger systems and gave identical result to G03. All TD-DFT was performed with G03, and although G09 gave similar results, G03 was kept for better uniformity in the comparison with older calculated data. All models used crystallographic structure data as the starting point for groundstate geometry optimizations. The geometry optimization and IR frequency determination was carried out with the DFT method using the B3LYP functional in the gas phase.65,66 The 6-31G** basis set was used for C, H, N, and Br, while relativistic LANL2DZ with effective core potentials and one additional f-type polarization functional was applied for the Rh atom (αf = 1.350).67,68 No imaginary frequencies were observed for all optimized structures (Figure S9 of the Supporting Information). The absorption spectral properties and molecular orbital (MO) energies were calculated by the TD-DFT approach associated with the polarized continuum model (CPCM)69 using DCM as solvent and with a smaller basis set: 3−21 G was used for all atoms (N, C, H, and Br) except Rh atoms and the amidinate NCN core atoms, for which the previous basis set was left unchanged. Gaussview 3.09 was used to visualize MOs with an isodensity of 0.02; GaussSum 2.2 was employed to extract the absorption energies, oscillator strengths, and molecular orbital energies, and Chemissian program was used to sketch energies of MOs with their colorcoded atomic orbital (AO) contributions, using 0.008 eV as the threshold for degeneracy for the MO.70−72 Complex 6 was previously studied in our group using the same methods and was used for comparison; for completeness, its tabulated data are included in the Supporting Information.44 Crystal Structure Determination. X-ray quality crystals were obtained in various ways: for 1, a slow diffusion of acetone into a DCM solution; for 2a, slow diffusion of hexanes into a chloroform solution; for 2b, slow diffusion of hexanes into an ethyl acetate solution; and for 4, slow evaporation of an ethyl acetate, methanol, and water (50/40/10% volume) solution. Xray crystallographic data were collected from a single crystal sample, which was mounted on a loop fiber. For 1 and 2b, data were collected with a Bruker Microstar diffractometer equipped with a Platinum 135 charged-coupled device (CCD) detector at 150 (2) K. For 2a, data were collected with a Bruker Platform diffractometer, equipped with a Bruker SMART 4K CCD Area Detector and a Nonius FR591 rotating anode Cu Kα X-ray radiation source equipped with a Montel 200 optics at 200 (2) K. For 4, data were collected with a Bruker Platform diffractometer, equipped with a Bruker APEX II CCD Area Detector and a Mo Kα X-ray radiation source at 200 (2) K. The data was integrated with APEX2 and corrected for absorption using the SADABS package, except for 2b, where the crystal was nonmerohedrally twinned and was corrected for absorption using TWINABS, with the use of HKLF 5 files for final refinement. This resulted in considerable removal of data, leading to 80% completeness.73−75 Following analytical absorption corrections and solution by direct methods, the structures were refined against F2 with full-matrix least-squares using the program SHELXL-97.76 All nonwater H atoms were 10342
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Table 1. Details of X-ray Diffraction Studies for the Different Dimers 1aa formula color/form T (K); wavelength crystal system space group unit cell: a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3); Z R1(F); wR(F2) [I > σ(I)] R1(F); wR(F2) (all) GoF(F2) flack parameter
2ab
2ba
4c
5a
[C81H60Br3N9Rh2] · 1.5(CH2Cl2) red prism 150; 1.54178 tetragonal I4 14.7855(7) 14.7855(7) 17.6398(9) 90 90 90 3856.3(3); 2 0.0578;0.1594
[C86H64Br2N10Rh2]· 2.5(CH2Cl2) red plate 200; 1.54178 triclinic P1̅ 13.6685(4) 17.3026(5) 19.4769(6) 73.857(2) 74.732(1) 79.393(1) 4237.9(2); 2 0.0683; 0.1689
[C86H64Br2N10Rh2]· 2.5(C4H8O2) red plate 150; 1.54178 triclinic P1̅ 13.0797(3) 14.1503(4) 25.3837(7) 88.073(2) 75.8300(12) 72.6550(12) 4344.0(2); 2 0.0367; 0.1004
[C96H72N12Rh2]· 4(H2O) red spearhead 200; 0.71073 orthorhombic Fddd 15.689(4) 30.833(8) 33.866(8) 90 90 90 16382(7); 8 0.0375; 0.0659
[C76H56Br4N8Rh2]· 3(CH2Cl2) brown prism 150; 1.54178 tetragonal I4 14.8465(1) 14.8465(1) 17.3717(2) 90 90 90 3829.04(6) 0.0479;0.1231
0.0591;0.1614 1.076 0.057(18)
0.1238; 0.1889 0.980 n.a.
0.0440; 0.1048 1.039 n.a.
0.0855; 0.0739 0.962 n.a.
0.0513;0.1345 1.077 0.471(11)
a
Bruker Microstar diffractometer (Platinum 135 CCD Detector, Helios optics and, Kappa goniometer). bBruker/AXS Smart 6000 (4K) diffractometer (Mirror Montel 200-monochromated Cu Kα radiation, FR591 Rotating Anode). cBruker Smart APEX 2, graphite monochromator. a From ref 62 (CCDC 942878).
Scheme 1. Synthesis of the Rh Amidinate Dimer Assemblies; n is the Equivalents of Boronic Ester Added, m is the Number of Coupled Pyridyls on the Product
loss of mass is due to the chromatography which left mixed fractions. For complex 4, efficient synthesis and purification was possible, leading to an isolated yield of 95% (98% from 1H NMR analysis of the bulk material). NMR Spectroscopy. All of the proton NMR spectra of the dimers have two distinctive and broad signals (labeled e and f in Figure 1). These peaks are due to restricted rotation of the pendant phenyl rings and represent the splitting of the signal of the two protons ortho to the interannular bond. This rotation is known to be restricted and has a free-energy of activation of approximately 15 kcal/mol for various rhodium dimers with an aryl moiety in the amidinate backbone.42−44 Proton f is shifted upfield due to the proximity of a phenyl ring core above it from neighboring ligands, while e is facing in the opposite direction, away from aryl rings. This broadening effect is also observed for the protons in the meta position, but these protons are in an almost chemically equivalent environment, being less affected by the nearby aromatic rings, and subsequently, their signals do not split. By comparing 1H NMR spectra of complexes 5 and 4, a downfield shift of the protons e and f in complex 4 is observed as compared to e′ and f ′ in 5, suggesting that the phenylpyridine moiety is more electron withdrawing than the bromophenyl moiety on the center of the amidinate, which follows their Hammett parameter (0.44 and 0.23, respectively).54,55 For 1 to 3, a broader, averaged signal of the two positions is seen, confirmed by the very linear shift per number of pyridine coupled (Figure S2 of the Supporting Information). This
added at calculated positions and refined by use of the riding model with isotropic displacement parameters based on those of the parent atom, water H atoms were refined with constraints. Anisotropic displacement parameters were employed throughout for the nonhydrogen atoms, except where disorder was too severe to be meaningful, where it was kept isotropic (i.e., the phenyl-pyridine moiety in 1). For 2a, a positional disorder was found between a bromine and a phenyl moiety, trans to each other, and it was refined to be in an 80% to 20% ratio. A second positional disorder implicating the other bromine was not modeled, as it only showed 5% contribution, it is the cause of the highest remaining electron density (1.67 e) in the model. Images were generated using Mercury, Ortep III, and Pov-Ray.77−79 Specific parameters of each measurement are located in Table 1.
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RESULTS AND DISCUSSION The formation of complexes 1 to 4 by Suzuki coupling requires specific conditions (Scheme 1) due to the limited solubility of 5 (only chloroform, THF, and DMF). The synthesis of all of the species was quite effective using the optimized reaction and purification conditions outlined in the Supporting Information. Complexes 1, 2, and 3 were synthesized with four equivalents of boronic ester, and the reaction was stopped before completion to obtain a mixture of all three species, leading to 52% recovery of complex 5 that can be reused. In consideration of this recovery, the isolated yield was 47% for 1 to 4 (see Scheme 1 for distribution between species, for n = 4). The 53% 10343
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The geometry of the rhodium dimer core varies very little upon coupling of a pyridyl ring. This can be readily observed by looking at the two extremes, structures of 5 and 4, where the Rh−Rh and Rh−N bond lengths and the N−Rh−Rh−N′ torsion angle are not significantly different (
