Oxidative dehydrogenation of tris(o-isopropylphenyl)phosphines by platinum complexes

Article abstract of DOI:10.1021/om800382m

The binuclear cyclometalates [Pt₂Cl₂{2-CMe ₂C₆H₄P(C₆H₄(2- ⁱPr))₂}₂] (1a) and [Pt₂Cl ₂[2-CMe₂C₆H₃(4-OMe)P(C ₆H₃(2-ⁱPr)(4-OMe))₂}₂] (1b) react with CHCl₂CHCl₂ to give the corresponding mononuclear phosphine-alkene chelates [PtCl₂ {2-CH ₂=CMeC₆H₄P(C₆H₄(2- ⁱPr))₂}] (2a) and [PtCl₂ [2-CH ₂=CMeC₆H₃(4-OMe)P(C₆H ₃(2-ⁱPr)(4-OMe))₂}] (2b). The product 2a can also be formed directly from [PtCl₂(NC^tBu)₂] and L_a in CHCl₂CHCl₂ or by addition of SO ₂Cl₂ to 1a. Addition of an excess of SO₂Cl ₂ to 1b gave [PtCl₂ {2-CH₂=CMeC ₆H₃(4-OMe)P(C₆H₂(2- ⁱPr)(4-OMe)(5-Cl))₂}] (3b), a derivative of 2b featuring metachlorine 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-CH ₂C₆H₃(4-OMe)P(C₆H ₃(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.