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-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.