Water-Soluble Mono- and Dimethyl N‑Heterocyclic Carbene Platinum(II) Complexes: Synthesis and Reactivity

A family
of water-soluble dimethyl complexes of formula cis-[PtMe2(dmso)­(NHC·Na)] (2), in which
NHC is an anionic N-heterocyclic carbene bearing a sulfonatopropyl
chain on one of the nitrogen atoms and a sulfonatopropyl (a), methyl (b), mesityl (c), or 2,6-diisopropylphenyl
group (d) on the other, have been prepared. The hydrolytic
stability of the Pt–C bonds in these complexes under different
neutral, alkaline, and acidic aqueous conditions has also been studied.
Complexes 2 were found to be quite stable at room temperature
in water under neutral or alkaline conditions. Degradation occurred
at higher temperatures but involved C sp3–H activation
and C–C reductive elimination processes in addition to Pt–Me
bond hydrolysis. Hydrolytic cleavage of the platinum–methyl
bonds was favored by good nucleophiles. Thus, the addition of KCN
to an aqueous solution of 2 resulted in formation of
the monomethyl complexes K­[PtMe­(CN)2(NHC·Na)] (9), whereas the dimethyl complexes K­[PtMe2(CNR)­(NHC·Na)]
(10) were formed with the isocyanide CNCH2COOK. The addition of stoichiometric amounts of protic acids to aqueous
solutions of 2 resulted in the clean cleavage of one
or both platinum­(II)–methyl bonds. Thus, the reaction of 2 with HCl afforded the complexes [PtClMe­(dmso)­(NHC·Na)]
(3) and [PtCl2(dmso)­(NHC·Na)] (4), whereas [PtMe­(OH2)­(dmso)­(NHC)] (5) and [Pt­(OH2)2(dmso)­(NHC)]­[BF4]
(7) were obtained upon treatment with HBF4. The crystal structure of 9a is remarkable in light
of the longitudinal channels around 6 Å in diameter internally
decorated with Pt–Me bonds.