Reversible cyclometalation at Rh super(I) as a motif for metal-ligand bifunctional bond activation and base-free formic acid dehydrogenation

• Published in: Catalysis science & technology Vol. 6; no. 5; pp. 1320 - 1327
• Main Authors: Jongbloed, L S, de Bruin, B, Reek, JNH, Lutz, M, van der Vlugt, JI
• Format: Journal Article
• Language: English
• Published: 01.02.2016
• Subjects: Activation; Catalysis; Decarboxylation; Dehydrogenation; Formates; Formic acid; Mathematical analysis; Phenyls
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/c5cy01505g

Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined Rh super(I) complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (L super(H) = PNC super(H) = 2-di(tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh-C sub(Ph) bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal-ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H sub(2) evolution from putative Rh(CO)(H)(L super(H)) complex A is very facile, proposedly encompassing formal C-H oxidative addition at Rh to give Cviaagostic intermediate B and subsequent reductive elimination of H sub(2). Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H sub(2), supporting a cooperative pathway. Reversible C-H activation is thus a viable mechanism for metal-ligand bifunctional catalysis.