With the view of developing electrophilic late-transitionmetal catalysts, we have now synthesized [(o-(Ph 2 P)C 6 H 4 ) 2 Sb(OTf) 2 ]-Pt(OTf) (2) and [(o-( i Pr 2 P)C 6 H 4 ) 2 Sb(OTf) 2 ]Pt(OTf) (4) by treatment of the corresponding trichlorides ([(o-(R 2 P)C 6 H 4 ) 2 SbCl 2 ]PtCl (R = Ph, iPr)) with 3 equiv of AgOTf. The crystal structures of 2 and 4 confirmed that the chloride ligands have been fully substituted by more labile triflate ligands. Despite structural similarities in the dinuclear cores of 2 and 4, only 2 acts as a potent carbophilic catalyst in enyne cyclization reactions. The high activity of 2 is also reflected by its ability to promote the addition of pyrrole and thiophene derivatives to alkynes. Structural and computational analyses suggest that the superior reactivity of 2 results from both favorable steric and electronic effects. Finally, a comparison of 2 with the previously reported self-activating catalyst [(o-(Ph 2 P)C 6 H 4 ) 2 Sb(OTf) 2 ]PtCl underscores the benefits of triflate for chloride substitution.

As part of our interest in the chemistry of late transition metal complexes bearing cationic antimony ligands, we have investigated the reaction of ((o-(Ph 2 P)C 6 H 4 ) 3 )SbClPtCl (1) with H 2 O 2 and found that it affords [((o-(Ph 2 P)C 6 H 4 ) 2 (o-Ph 2 PO)C 6 H 4 )SbPtCl] + ([2−Cl] + ), a monocationic complex with the newly installed P=O moiety coordinated intramolecularly to the antimony atom via a P=O → Sb dative bond. The Pt−Cl bond of this complex is readily activated by addition of a ligand such as cyclohexyl isocyanide. When carried out in the presence of AgBF 4 , this reaction affords the dicationic complex [((o-(Ph 2 P)C 6 H 4 ) 2 (o-Ph 2 PO)-C 6 H 4 )SbPt(CNCy)] + ([2−CNCy] 2+). In addition to structurally characterizing [2−Cl] + and [2−CNCy] 2+ as their chloride and tetrafluoroborate salts, respectively, we have also studied the reaction of [2−Cl] + with AgBF 4 in the presence of PhCCH as a substrate surrogate. When monitored by 31 P NMR spectroscopy in CDCl 3 , this reaction shows the formation of a new species tentatively assigned to [((o-(Ph 2 P)C 6 H 4 ) 2 (o-Ph 2 PO)C 6 H 4 )SbPt] 2+ ([2] 2+ ) stabilized by coordination of the alkyne. This formulation is supported by the elevated carbophilic reactivity of [2] 2+ , which readily catalyzes the cyclization of 2-allyl-2-(2propynyl)malonate. Altogether, these results show that the accumulation of charge in such reactive complexes can be facilitated by the intramolecular base-stabilization of the dinuclear core.

As part of our investigations into the Lewis acidic behavior of antimony derivatives, we have decided to study the properties of 5-phenyl-5,5dichloro-λ 5 -dibenzostibole (1), a dichlorostiborane with an antimony atom confined to a five-membered heterocycle. Our work shows that the resulting geometrical constraints elevate the Lewis acidity of the antimony atom, as confirmed by the crystal structure of 1-THF and the solution study of the interaction of 1 with Ph 3 PO. The enhanced Lewis acidic properties of 1, which exceed those of simple dichlorostiboranes such as Ph 3 SbCl 2 , also become manifest in pnictogen bonding catalysis experiments involving the reductions of imines with Hantzsch ester. The influence of geometrical constraints in the chemistry of this compound is also supported by a computational activation strain analysis as well as by an energy decomposition analysis of a model Me 3 PO adduct.

No abstract

No abstract