CATIONIC CARBENE COMPLEXES OF PLATINUM(IV) - STRUCTURE OF A SECONDARY CARBENE COMPLEX

Treatment of the organoplatinum(II) precursors [PtMeCl((t)bu(2)bpy)] ((t)bu(2)bpy = 4,4'-di-tert-butyl-2,2'-bipyridine) and [PtMe(2)((t)bu(2)bpy)] with chloroiminium (Vilsmeier) salts affords the novel cationic platinum(IV)-carbene complexes [PtCl(2)Me(CHNMe(2))((t)bu(2)bpy)]Cl (1), [PtCl2-Me(CClNMe(2))((t)bu(2)bpy)]Cl (2), [PtClMe(2)(CHNMe(2))((t)bu(2)bpy)]Cl (3), and [PtClM(2)(CClNMe(2))((t)bu(2)bpy)]Cl (4) in good yield. For 1 and 2, the carbene moiety is located trans to one of the chloro ligands, whereas in 3 and 4 it is found trans to the N-donor. The complexes show good stability toward air and moisture, but 1 and 2 are thermally unstable and decompose to [PtCl2((t)bu(2)bpy)] after a few days at room temperature. Complex 4 can also be prepared as a mixture of two geometrical isomers which do not interconvert; it is proposed that the isomers are formed by two independent pathways. The major (4a) and minor (4b) isomers are formed by cis- and trans-oxidative addition, respectively, of [Cl2C=NMe(2)]Cl to [PtMe(2)((t)bu(2)bpy)], and likely mechanisms are discussed. Crystals of 1 are monoclinic, space group P2(1)/c, with a = 11.064(1) Angstrom, b = 21.310(2) Angstrom, c = 12.277(3) Angstrom, beta = 91.20(2)degrees, Z = 4, and R = 0.0718. The structure of 1 consists of discrete [PtCl(2)Me(CHNMe(2))((t)bu(2)bpy)](+) cations and chloride anions, with one CH2Cl2 molecule of crystallization per complex ion. The carbene ligand occupies a position trans to one of the chloro ligands, and the Pt-(Carbene) bond length (1.99(2) Angstrom) is significantly shorter than the Pt-Me distance (2.13(2) Angstrom). The C-N bond length of 1.31(2) Angstrom is indicative of substantial p(pi)-p(pi) bonding. Reaction of nucleophiles with 3 leads to rapid reductive elimination to give an iminium salt [H(X)C = NMe(2)]Cl (X = nucleophile) and [PtMe(2)((t)bu(2)bpy)]. This work demonstrates the versatility of chloroiminium salts in the preparation of aminocarbene complexes of metals in high oxidation states.