Optoelectronic Properties and Structural Effects of the Incremental Addition of Pyridyl Moieties on a Rhodium Dimer

The synthesis and characterization of five C–C coupling products obtained from the reaction of a paddlewheel tetrakis 4-bromo-<i>N</i>,<i>N</i>′-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, <i>cis</i>-bis, <i>trans</i>-bis, or mono]-<i>N</i>,<i>N</i>′-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 <i>N</i>,<i>N</i>′-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 (σ_F) and resonance (σ_R). The properties involving only the Rh–Rh core (MM band, all oxidation potentials, HOMO and LUMO) were fit with a single set of σ_F and σ_R contributions (73% and 27%), with a goodness-of-fit (<i>R</i>²) 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 σ_F and σ_R, 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