Structural, 103 Rh NMR and DFT Studies of a Bis(phosphane)Rh III –Porphyrin Derivative

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronicc i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronict r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.

Theoretical investigation on the interaction between RhIII octaethylporphyrin and a graphite basal surface: a comparison study of DFT, DFT-D, and AFM

Using density functional theory based calculations and atomic-force-microscopy observations, we investigated the interaction between [RhIII(OEP)(Cl)] (OEP = octaethylporphyrin) and a graphite basal surface, and the electronic structure of [RhIII(OEP)(Cl)]/graphite. The [RhIII(OEP)(Cl)] complex has an electronic structure effective for CO activation, possessing a closed singlet structure as its ground state; hence, both σ-donation from the CO molecule (anode-reaction reactant) to RhIII, and π-back-donation from RhIII to CO, occur, because the [RhIII(OEP)(Cl)] complex does not have a singlet occupied molecular orbital on the porphyrin ring, the π-π stacking interaction between porphyrin and graphite is not present and their interaction is dominated by dispersion forces. The [RhIII(OEP)(Cl)] complex easily diffused on the graphite basal surface, and an aggregated structure of [RhIII(OEP)(Cl)] was observed by atomic force microscopy. The difference of the electronic structures of [RhIII(OEP)(Cl)] before and after its adsorption is very small, the dispersion force being the dominant force for the adsorption. However, the lowest unoccupied molecular orbital of [RhIII(OEP)(Cl)]/graphite is a σ bonding orbital between RhIII and graphite that will cause fast electron transfer from [RhIII(OEP)(Cl)] to graphite during the CO electro-oxidation; this would be a reason why the carbon-supported [RhIII(OEP)(Cl)] has high catalytic activity for CO electro-oxidation.