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

[Image: see text] 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 [{Pt(IV)(C^C*)Me(μ-pz)}(2)(μ-I)]I (C^C*(A)2a, C^C*(B)2b). The reaction of 1a with benzyl bromide (BnBr) in the same conditions afforded [Br(C^C*(A))Pt(III)(μ-pz)(2)Pt(III)(C^C*(A))Bn] (5a), which by heating in BnBr(l) became [{Pt(IV)(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 S(N)2 pathway in the two steps of the double oxidative addition (OA). Based on the DFT investigations, species such as [(C^C*(A))Pt(III)(μ-pz)(2)Pt(III)(C^C*(A))R]X (RX = MeI Int-Me, BnBr Int-Bn) and [(C^C*(A))Pt(II)(μ-pz)(2)Pt(IV)(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))Pt(III)(μ-pz)(2)Pt(III)(C^C*(A))Me]BF(4) (3a′) and used to get [I(C^C*(A))Pt(III)(μ-pz)(2)Pt(III)(C^C*(A))Me](4a), [(C^C*(A))Pt(II)(μ-pz)(2)Pt(IV)(C^C*(A))(Me)I](Int′-Me), and [{Pt(IV)(C^C*)Me(μ-pz)}(2)(μ-I)]BF(4) (2a′). The single-crystal X-ray structures of 2a, 2b, 3a′, and 5a along with the mono- and bi-dimensional (1)H and (195)Pt{(1)H} 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.