Acid-Assisted Reductive Elimination as a Route to Platinum(II) Products from Platinum(IV) Tris(pyrazolyl)borate Reagents

A series of chiral cationic platinum(II) complexes of the type [κ 2 -((Hpz*)BHpz* 2 )Pt(R)(L)][BAr‘ 4 ] (R = Me, L = MeCN ( 3 ), t BuNC ( 4 ), py ( 5 ), CO ( 6 ), CH 2 CH 2 ( 7 ), PMe 2 Ph ( 8 ); R = Ph, L = MeCN ( 10 ); pz* = 3,5-dimethylpyrazolyl, BAr‘ 4 = tetrakis(3,5-trifluoromethylphenyl)borate) has been prepared. Protonation of Tp‘PtMe 2 H ( 1 ) or Tp‘PtPh 2 H ( 2 ) (Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) occurs selectively at a pyrazole nitrogen atom and induces reductive elimination of methane or benzene. Subsequent addition of ligand, L, traps the platinum(II) intermediate and yields 3 − 8 and 10 . Addition of 2 equiv of PMe 2 Ph led to the formation of trans -[κ 1 -((Hpz*)BHpz* 2 )Pt(Me)(PMe 2 Ph) 2 ][BAr‘ 4 ] ( 9 ), a square-planar complex containing a rare monodentate protonated tris(pyrazolyl)borate ligand. Deprotonation of the pyrazolium ring (present after protonation of 1 or 2 and trapping with added ligand) led to the synthesis of neutral platinum(II) complexes of the type Tp‘PtR(L) (R = Me, L = MeCN ( 11 ), SMe 2 ( 12 ), CO ( 13 ), CH 2 CH 2 ( 14 ); R = Ph, L = MeCN ( 15 )). Complexes 11, 12, and 15 exhibit square-planar coordination with a bidentate Tp‘ ligand, while complex 14 is trigonal bipyramidal. Tp‘PtMe(CO) ( 13 ) is present in solution in both isomeric forms. X-ray structure determinations for the cationic complexes 3 and 6 − 9 and the neutral square-planar complex 12 are reported.