Bis-hydrocarbyl Platinum(II) Ambiphilic Ligand Complexes: Alkyl–Aryl Exchange between Platinum and Boron

Reaction of TXPB (2,7-di-tert-butyl-5-diphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene)
with [PtMe2(COD)] (COD = 1,5-cyclooctadiene) yielded [PtMePh­(TXPB′)]
(1; TXPB′ = 2,7-di-tert-butyl-5-methylphenylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene),
presumably via undetected [PtMe2(TXPB)]. The ambiphilic
ligand in 1 is κ2PS-coordinated, and the trigonal plane of the borane is oriented approximately
parallel to the square plane at platinum. In solution, 1 is in equilibrium with zwitterionic [PtMe­(TXPB-Me)] (1′), in which the Pt-phenyl group in 1 has been abstracted by the borane; the ratio of 1 to 1′ is approximately 90:10 at room temperature
and 50:50 at 183 K. The presence of 1′ in solution
suggests that the pathway for phenyl–methyl exchange between
platinum and boron involves a zwitterionic platinum­(II) intermediate,
rather than a platinum­(IV) boryl complex resulting from B–C
bond oxidative addition. Heating compound 1 provided
an approximate 14:86 mixture of 1 and [PtPh2(TXPB″)] (2; TXPB″ = 2,7-di-tert-butyl-5-dimethylboryl-4-diphenylphosphino-9,9-dimethylthioxanthene); 1 and 2 are in equilibrium at elevated temperature,
and 2 is formed (ΔS⧧ = −45(12) J K–1 mol–1; ΔH⧧ = 94(4) kJ mol–1) via a second methyl–phenyl exchange between
platinum and boron. The solid-state structure of 2 is
analogous to that of 1, except with both phenyl groups
on platinum and both methyl groups on boron. Compound 1 reacted with PPh3 or P­(OPh)3 to afford neutral
[PtMePh­(L)­(TXPB′)] {L = PPh3 (3) or
P­(OPh)3 (4)}, in which the TXPB′ ligand
is κ1P-coordinated. However, 1 reacted with 2 equiv of CNXyl (Xyl = 2,6-dimethylphenyl)
to provide zwitterionic trans-[PtMe­(CNXyl)2(TXPB-Me)] (5; TXPB-Me is a κ1P-coordinated anionic phosphine). This divergent reactivity
is consistent with the accessibility of both neutral 1 and zwitterionic 1′ in solution.