Coordination Flexibility of the Rh(PXP) Complex to NH3, CO, and C2H4 (PXP = Diphosphine-Based Pincer Ligand; X = B, Al, and Ga): Theoretical Insight

Cooperative C-H activation of pyridine by PBP complexes of Rh and Ir can lead to bridging 2-pyridyls with different connectivity to the B-M unit

Pyridine and quinoline undergo selective C–H activation in the 2-position with Rh and Ir complexes of a boryl/bis(phosphine) PBP pincer ligand, resulting in a 2-pyridyl bridging the transition metal and the boron center. Examination of this reactivity with Rh and Ir complexes carrying different non-pincer ligands on the transition metal led to the realization of the possible isomerism derived from the 2-pyridyl fragment connecting either via B–N/C–M bonds or via B–C/N–M bonds. This M–C/M–N isomerism was systematically examined for four structural types. Each of these types has a defined set of ligands on Rh/Ir besides 2-pyridyl and PBP. A pair of M–C/M–N isomers for each type was computationally examined for Rh and for Ir, totaling 16 compounds. Several of these compounds were isolated or observed in solution by experimental methods, in addition to a few 2-quinolyl variants. The DFT predictions concerning the thermodynamic preference within each M–C/M–N isomeric match the experimental findings very well. In two cases where DFT predicts <2 kcal mol⁻¹ difference in free energy, both isomers were experimentally observed in solution. Analysis of the structural data, of the relevant Wiberg bond indices, and of the ETS-NOCV partitioning of the interaction of the 2-pyridyl fragment with the rest of the molecule points to the strength of the M–C(pyridyl) bond as the dominant parameter determining the relative M–C/M–N isomer favorability. This M–C bond is always stronger for the analogous Ir vs. Rh compounds, but the nature of the ligand trans to it has a significant influence, as well. DFT calculations were used to evaluate the mechanism of isomerization for one of the molecule types.

The thermodynamic preference between two isomeric products of C–H activation of pyridine, with 2-pyridyl bridging boron and iridium or rhodium, primarily depends on the M–C bond strength.

C2-Selective silylation of pyridines by a rhodium-aluminum complex

We have developed a C2-selective mono-silylation of a variety of pyridines using a Rh-Al complex. Both the site- and mono-selectivity are controlled via the pyridine coordination to the Lewis-acidic Al center prior to the activation of the pyridine C(2)-H bond at the proximal Rh center. A reaction mechanism is proposed based on several mechanistic studies, including the isolation of a (2-pyridyl)silylrhodium intermediate.

Selective C-O Bond Reduction and Borylation of Aryl Ethers Catalyzed by a Rhodium-Aluminum Heterobimetallic Complex

We report the catalytic reduction of a C-O bond and the borylation by a rhodium complex bearing an X-type PAlP pincer ligand. We have revealed the reaction mechanism based on the characterization of the reaction intermediate and deuterium-labeling experiments. Notably, this novel catalytic system shows steric-hindrance-dependent chemoselectivity that is distinct from conventional Ni-based catalysts and suggests a new strategy for selective C-O bond activation by heterobimetallic catalysis.