Oxidative Dehydrogenation of Tris(o-isopropylphenyl)phosphines by Platinum Complexes

Abstract

The binuclear cyclometalates [Pt2Cl2{2-CMe2C6H4P(C6H4(2-iPr))2}2] (1a) and [Pt2Cl2{2-CMe2C6H3(4-OMe)P(C6H3(2-iPr)(4-OMe))2}2] (1b) react with CHCl2CHCl2 to give the corresponding mononuclear phosphine-alkene chelates [PtCl2{2-CH2═CMeC6H4P(C6H4(2-iPr))2}] (2a) and [PtCl2{2-CH2═CMeC6H3(4-OMe)P(C6H3(2-iPr)(4-OMe))2}] (2b). The product 2a can also be formed directly from [PtCl2(NCtBu)2] and La in CHCl2CHCl2 or by addition of SO2Cl2 to 1a. Addition of an excess of SO2Cl2 to 1b gave [PtCl2{2-CH2═CMeC6H3(4-OMe)P(C6H2(2-iPr)(4-OMe)(5-Cl))2}] (3b), a derivative of 2b featuring meta-chlorine substituents on the terminal P groups as a result of electrophilic aromatic substitution. A mechanism for the conversion of 1a,b to 2a,b is proposed involving an electrophilic alkyl C−H activation by a coordinatively unsaturated platinum(IV) species. The mechanism is supported by the isolation of the diplatinum(IV) cyclometalate [Pt2Cl2{2-CH2C6H3(4-OMe)P(C6H3(2-Me)(4-OMe))2}] as a mixture of syn and anti isomers 5b and 5b′. The crystal structures of 2a and 3b have been determined.