Synthesis, structures, and reactivities of six-membered C-N chelate protic pyrazole complexes of iridium

The reaction of the bis(acetato)iridium complex [Cp*Ir(OAc)(2)] with an equimolar amount of 3-benzyl-5-alkylpyrazoles followed by addition of an excess of potassium chloride afforded the six-membered, C-N chelate protic pyrazole complexes [Cp*IrCl(R-LH)] (2a, R = Me; 2b, R = tBu; R-LH = C6H4CH2-C3N2H2R-5-kappa C,kappa N). The reaction of 2a with carbon monoxide led to the formation of the carbonyl complex [Cp*Ir(CO) (Me-LH)]Cl (3a) featuring a CO stretching wavenumber comparable with that of the five-membered chelate complex. Meanwhile, dehydrochlorination of the methylpyrazole complex 2a with potassium tertbutoxide in toluene resulted in uptake of adventitious water to give the hydroxido-bridged dinuclear complex [(Cp*Ir)(2){mu-Me-LH...(MeL)}(mu-OH)] (4a). In 2-propanol, the reaction caused hydrogen transfer from the solvent to yield the hydrido-bridged dinuclear complex [(Cp*Ir)(2){mu-Me-LH...(Me-L)}(mu H)] (6a). These dinuclear products could not be obtained from the more sterically-demanding complex 2b, but the intermediary coordinatively unsaturated pyrazolato species can be trapped from both 2a and 2b by triphenylphosphine to give the pyrazolato-phosphine complexes [Cp*Ir(PPh3)(R-L)] (5a and 5b) The detailed structures of 2a, 2b, 3a, 4a, and 5b bearing a folded six-membered C-N chelate pyrazole ligand have been determined by X-ray analysis. The chlorido complexes 2a and 2b also catalyzed transfer hydrogenation of acetophenone with 2-propanol at 50 degrees C. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

A series of iridium complexes with pyrazole ligands were synthesized and their reaction characteristics were explored under different reaction conditions, with their structures characterized.