Mesoionic Carbene Cobalt Complexes as Multipurpose Catalyst Precursors for Hydrosilylation and Dihydropyrimidinone Synthesis

Abstract

Metalation of CH 2 OH-substituted triazolium salts with CoCl 2 under basic conditions affords C,O-bidentate chelating carbene Co(III) complexes (3a, 3b), while analogous phenyl-substituted triazolium salts produce monodentate carbene Co(II) complexes (3c, 3d). The distinct substituent-induced properties of the metal centers were demonstrated by electrochemical measurements and catalytic activities in two specific processes. The complexes showed appreciable activity in the reduction of C=O bonds through hydrosilylation, with methoxybenzene-functionalized triazolylidene Co(III) complex 3a achieving a high selectivity towards aldehydes vs. ketones with turnover frequencies (TOFs) up to 200 h À 1 . The C,O-chelate systems were also active catalysts in the Biginelli process, a one-step three-component reaction for efficient dihydropyrimidinone synthesis. Optimization of reaction conditions provides high activity with complex 3a, reaching TOFs of 800 h À 1 , the highest activity known for cobalt NHC complexes to date.

Application of first-row transition metal complexes bearing 1,2,3-triazolylidene ligands in catalysis and beyond

“…Tetrahedral cobalt( iii ) complexes Co7 containing C , O -bidentate chelating trz ligands are obtained upon deprotonation of the alkoxy-functionalized triazolium salt L19 and subsequent metalation with CoCl 2 ( Scheme 30 ). 115 The complexes have a magnetic moment of 3.1 μ B , indicative of an intermediate spin ( S = 1) Co III center, suggesting air-mediated one electron oxidation during coordination. Performing the same reaction with monodentate ligand precursor L1 (R = n Bu, Mes; R′ = Ph; X = OTf) affords complex Co8 akin to Co1 ( Scheme 30 ).…”