Cationic Unsymmetrical 1,4-Diazabutadiene Complexes of Platinum(II)

The synthesis of the silyl-protected 1,4-diazabutadiene ligands glyoxal−bis((2-α-triisopropylsiloxymethyl)-6-methylphenyl)diimine (TIPS-6-MPD) and glyoxal−bis((2-α-triisopropylsiloxymethyl)-4-methylphenyl)diimine (TIPS-4-MPD) and their subsequent reactions with trans-Pt(SMe2)2(Me)Cl to generate the corresponding complexes (TIPS-6-MPD)Pt(Me)Cl (1a) and (TIPS-4-MPD)Pt(Me)Cl (1b) are described. Cationic complexes of the type [(N,N-chelate)Pt(Me)(L)]BF4 (where L = solvent/olefin and N,N-chelate = TIPS-6-MPD and TIPS-4-MPD) are generated by the reaction of chloro methyl complexes 1a and 1b with AgBF4 in the presence of L. Various exchange reactions were examined for [(TIPS-6-MPD)Pt(Me)(NCCH3)]BF4 (2a), in which it was determined that the coordinated solvent reversibly exchanges with acrylonitrile, ethylene, fumaronitrile, cis-pentenenitrile, benzonitrile, dimethyl sulfide, and carbon monoxide to generate the corresponding cationic complexes 3−9, respectively. Kinetics experiments under pseudo-first-order conditions using 10-, 20-, and 30-fold excesses of benzonitrile demonstrate that 2a undergoes ligand exchange via an associative pathway with a bimolecular rate constant k2 of (3.2 ± 2.0) × 10-4 M-1 s-1. Complex 2a was found to initiate the polymerization of various electron-rich monomers. A detailed analysis of the reaction demonstrates that the initiation is cationic in nature. The molecular structure of TIPS-6-MPD has been determined by a single-crystal X-ray diffraction analysis. The free ligand adopts an s-trans conformation with a planar NCCN backbone.