Modulating the σ‑Accepting Properties of an Antimony Z‑type Ligand via Anion Abstraction: Remote-Controlled Reactivity of the Coordinated Platinum Atom

In
search of ligand platforms, which can be used to remotely control
the catalytic activity of a transition metal, we have investigated
the coordination noninnocence of ambiphilic L2/Z-type ligands
containing a trifluoro­stiborane unit as a Lewis acid. The known
dichloro­stiboranyl platinum complex ((o-(Ph2P)­C6H4)2SbCl2)­PtCl
(1) reacts with TlF in the presence of acetonitrile (MeCN)
and cyclohexyl isocyanide (CyNC) to afford the trifluoro­stiborane
platinum complexes 2 (((o-(Ph2P)­C6H4)2SbF3)­Pt-NCMe)
and 3 (((o-(Ph2P)­C6H4)2SbF3)­Pt-CNCy), respectively.
Formation of these complexes, which results from a redistribution
of the halide ligands about the dinuclear core, affects the nature
of the Pt–Sb bond. The latter switches from covalent in 1 to polar covalent (or dative) in 2 and 3 where the trifluoro­stiborane moiety engages the platinum
center in a Pt → Sb interaction. The polarity of the Pt–Sb
bond can be modulated further by abstraction of an antimony-bound
fluoride ligand using B­(C6F5)3. These
reactions afford the cationic complexes [((o-(Ph2P)­C6H4)2SbF2)­Pt-NCMe]+ ([5]+) and [((o-(Ph2P)­C6H4)2SbF2)­Pt-CNCy]+ ([6]+) which have been isolated as
[BF­(C6F5)3]− salts.
These complexes possess a highly Lewis acidic difluoro­stibonium
moiety, which exerts an intense draw on the electron density of the
platinum center. As a result, the latter becomes significantly more
electrophilic. In the case of [5]+, which
contains a labile acetonitrile ligand, this increased electrophilicity
translates into increased carbophilicity as reflected by the ability
of this complex to promote enyne cyclization reactions. These results
demonstrate that the coordination noninnocence of antimony Z-ligands
can be used to adjust the catalytic activity of an adjoining metal
center.