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

Complexes [{Pt(C^C*)(μ-pz)} 2 ] (HC^C* A = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1 H -imidazol-2-ylidene 1a , HC^C* B = 1-phenyl-3-methyl-1 H -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.