Synthesis of Trimethylplatinum(IV) Complexes with <i>N,N</i>- and <i>N,O</i>-Heterocyclic Carbene Ligands and Their Reductive C−C Elimination Reactions

Reactions of the platinum(IV) complexes [PtMe3(OCMe2)3](BF4) (1(BF4)), [(PtMe3I)4] (2), and [PtMe3I(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](CF3COO) (9(CF3COO)). 9 and 10 are stereoisomers. In contrast to this, the analogous reaction of [PtMe3(OCMe2)3](BF4) (1(BF4)) with the N,O-heterocyclic carbene 3-methyloxazol-2-ylidene (N,O-hc, 5) was found to yield the tris(carbene)trimethylplatinum(IV) compound [PtMe3(N,O-hc)3](BF4) (12(BF4)), which is thermally stable up to 218 °C in the solid state. Furthermore, reactions of [{PtMe3(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 [PtMe3(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(BF4) yielded cationic monocarbeneplatinum(IV) complexes, which were isolated as tetrafluoroborate salts [PtMe3(bpy)(N,X-hc)](BF4) (X = N, 19(BF4); X = O, 20(BF4)). The compounds have been fully characterized analytically and NMR spectroscopically, and for the bis(carbene)platinum(II) compound 9(CF3COO) as well as the monocarbeneplatinum(IV) compounds 16 and 20(BF4) 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.