Irida-β-ketoimines derived from Hydrazines to Afford Metallapyrazoles or N-N Bond Cleavage: A Missing Metallacycle Disclosed by a Theoretical and Experimental Study

Abstract

Unprecedented metallapyrazoles [IrH2{Ph2P(o-C6H4)CNNHC(o-C6H4)PPh2}] (3) and [IrHCl{Ph2P(o-C6H4)CNNHC(o-C6H4)PPh2}] (4) were obtained by the reaction of the irida-β-ketoimine [IrHCl{(PPh2(o-C6H4CO))(PPh2(o-C6H4CNNH2))H}] (2) in MeOH heated at reflux in the presence and absence of KOH, respectively. In solution, iridapyrazole 3 undergoes a dynamic process due to prototropic tautomerism with an experimental barrier for the exchange of ΔGcoal⧧ = 53.7 kJ mol–1. DFT calculations agreed with an intrapyrazole proton transfer process assisted by two water molecules (ΔG = 63.1 kJ mol–1). An X-ray diffraction study on 4 indicated electron delocalization in the iridapyrazole ring. The reaction of the irida-β-diketone [IrHCl{(PPh2(o-C6H4CO))2H}] (1) with H2NNRR′ in aprotic solvents gave irida-β-ketoimines [IrHCl{(PPh2(o-C6H4CO))(PPh2(o-C6H4CNNRR′))H}] (R = R′ = Me (5); R = H, R′ = Ph (8)), which can undergo N–N bond cleavage to afford the acyl–amide complex [IrHCl(PPh2(o-C6H4CO))(PPh2(o-C6H4C(O)N(CH3)2))-κP,κO] (6) or [IrHCl(PPh2(o-C6H4CO))(PPh2(o-C6H4CN)-κP)(NH2NHPh-κNH2)] (9) containing o-(diphenylphosphine)benzonitrile and phenylhydrazine, respectively. From a CH2Cl2/CH3OH solution of 9 kept at −18 °C, single crystals of [IrHCl(PPh2(o-C6H4CO))(PPh2(o-C6H4CN)-κP))(HN═NPh-κNH)] (10) containing o-(diphenylphosphine)benzonitrile and phenyldiazene were formed, as shown by X-ray diffraction. The reaction of 1 with methylhydrazine in methanol gave the hydrazine complex [IrCl(PPh2(o-C6H4CO))2(NH2NH(CH3)-κNH2)] (7). Single-crystal X-ray diffraction analysis was performed on 6 and 7.