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(SMe 2 ) 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)]BF 4 (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 AgBF 4 in the presence of L. Various exchange reactions were examined for [(TIPS-6-MPD)Pt(Me)(NCCH 3 )]BF 4 ( 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 k 2 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 N C C N backbone.