High-Valent Pyrazolate-Bridged Platinum Complexes: A Joint Experimental and Theoretical Study.

Abstract

Complexes [{Pt(C^C*)(μ-pz)}2] (HC^C*A = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene 1a, HC^C*B = 1-phenyl-3-methyl-1H-imidazol-2-ylidene 1b) react with methyl iodide (MeI) at room temperature in the dark to give compounds [{PtIV(C^C*)Me(μ-pz)}2(μ-I)]I (C^C*A2a, C^C*B2b). The reaction of 1a with benzyl bromide (BnBr) in the same conditions afforded [Br(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)Bn] (5a), which by heating in BnBr(l) became [{PtIV(C^C*A)Bn(μ-pz)}2(μ-Br)]Br (6a). Experimental investigations and density functional theory (DFT) calculations on the mechanisms of these reactions from 1a revealed that they follow a SN2 pathway in the two steps of the double oxidative addition (OA). Based on the DFT investigations, species such as [(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)R]X (RX = MeI Int-Me, BnBr Int-Bn) and [(C^C*A)PtII(μ-pz)2PtIV(C^C*A)(R)X] (RX = MeI Int′-Me, BnBr Int′-Bn) were proposed as intermediates for the first and the second OA reactions, respectively. In order to put the mechanisms on firmer grounds, Int-Me was prepared as [(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)Me]BF4 (3a′) and used to get [I(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)Me](4a), [(C^C*A)PtII(μ-pz)2PtIV(C^C*A)(Me)I](Int′-Me), and [{PtIV(C^C*)Me(μ-pz)}2(μ-I)]BF4 (2a′). The single-crystal X-ray structures of 2a, 2b, 3a′, and 5a along with the mono- and bi-dimensional 1H and 195Pt{1H} NMR spectra of all the named species allowed us to compare structural and spectroscopic data for high-valent complexes with the same core [{Pt(C^C*)(μ-pz)}2] but different oxidation states.