Synthesis of Trimethylplatinum(IV) Complexes withN,N- andN,O-Heterocyclic Carbene Ligands and Their Reductive C−C Elimination Reactions

Abstract

Reactions of the platinum(IV) complexes [PtMe(3)(OCMe(2))(3)](BF(4)) (1(BF(4))), [(PtMe(3)I)(4)] (2), and [PtMe(3)I(py)(2)] (3) with the N,N-heterocyclic carbene 1,3-dimethylimidazol-2-ylidene (N,N-hc, 4) resulted in a rapid reductive elimination of ethane, yielding platinum(II) complexes [PtMe(N,N-hc)(3)](+) (7), trans-[PtMeI(N,N-hc)(2)] (8), and cis-[PtMe(py)(N,N-hc)(2)](+) (10), respectively. Subsequent substitution of the iodo ligand in 8 by pyridine resulted in the formation of trans-[PtMe(py)(N,N-hc)(2)](CF(3)COO) (9(CF(3)COO)). 9 and 10 are stereoisomers. In contrast to this, the analogous reaction of [PtMe(3)(OCMe(2))(3)](BF(4)) (1(BF(4))) with the N,O-heterocyclic carbene 3-methyloxazol-2-ylidene (N,O-hc, 5) was found to yield the tris(carbene)trimethylplatinum(IV) compound [PtMe(3)(N,O-hc)(3)](BF(4)) (12(BF(4))), which is thermally stable up to 218 degrees C in the solid state. Furthermore, reactions of [{PtMe(3)(acac)}(2)] (15) with the N,X-heterocyclic carbenes (X = N, O, S; N,S-hc = 3-methylthiazol-2-ylidene, 6) resulted in the formation of monocarbenetrimethylplatinum(IV) complexes [PtMe(3)(acac)(N,X-hc)] (X = N, 16; X = O, 17; X = S, 18), which were found to be stable against reductive C-C elimination at ambient temperature. Reactions of 16 and 17 with 2,2'-bipyridine (bpy) in the presence of stoichiometric amounts of H(BF(4)) yielded cationic monocarbeneplatinum(IV) complexes, which were isolated as tetrafluoroborate salts [PtMe(3)(bpy)(N,X-hc)](BF(4)) (X = N, 19(BF(4)); X = O, 20(BF(4))). The compounds have been fully characterized analytically and NMR spectroscopically, and for the bis(carbene)platinum(II) compound 9(CF(3)COO) as well as the monocarbeneplatinum(IV) compounds 16 and 20(BF(4)) by single-crystal X-ray diffraction analyses. DFT calculations of mono-, bis-, and tris(carbene)trimethylplatinum(IV) complexes and their propensity to reductive eliminate ethane were performed. In accordance with the experimental findings, a much higher stability of the tris(N,O-hc)- compared with the tris(N,N-hc)trimethylplatinum(IV) complexes against reductive ethane elimination was found, which could be ascribed mainly to a higher steric demand of the N,N-hc ligand.