Stereoisomerism as an Origin of Different Reactivities of Ir(III) PC(sp3)P Pincer Catalysts.

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp3)P pincer ligand (1,8-bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV-visible) studies show the existence of two isomers of hexacoordinate complexes 1·MeCN, 2·MeCN, and 2·Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp3), whereas only the equatorial isomer is found for 1·Py. These complexes are stabilized by various intramolecular noncovalent C-H···Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1·L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

Coordination versatility of p-hydroquinone-functionalized dibenzobarrelene-based PC(sp3)P pincer ligands.

The manuscript describes the synthesis and coordination chemistry of a novel diphosphine pincer ligand based on a p-hydroquinone-functionalized dibenzobarrelene scaffold. The p-hydroquinone fragment of the ligand is oxidatively and coordinatively non-innocent and may render new reactivity to the metal center due to implied reversible redox behavior, tautomeric interconversion and metal-hydroxyl/alkoxide coordination switch of the pendant hydroxyl side-arm. Palladium, platinum and iridium complexes were prepared and characterized. Investigation of their coordination chemistry revealed that while tautomeric equilibrium exists in free ligands and in the chelate non-metalated complexes, it is essentially blocked in the corresponding C(sp3)-pincer compounds due to stabilizing hemilabile coordination of the hydroxyl group. However, its presence in close proximity to the metal center is essential for catalyzing acceptorless dehydrogenation of alcohols by the iridium complexes via the outer-sphere hydrogen transfer mechanism. Remarkably, we found a similar activity for the analogous palladium complexes, which is not characteristic of this metal. This unprecedented reactivity of palladium stresses the fact that besides the choice of an active metal, transformation-oriented design of the ligand is crucial for catalysis.