A weakly coordinating anion as a tripodal “Br3”-ligand for platinum(IV) — Structure of [(closo-CB11H6Br6)PtMe3]

Abstract

Synthesis, structure, and NMR spectroscopic data for [(closo-CB11H6Br6)PtMe3] are reported. This neutral platinum(IV) complex contains the closo-CB11H6Br6– anion bonded to the trimethylplatinum(IV) cation via three boron-bound bromines. Closo-CB11H6Br6–, which often acts as weakly coordinating or even non-coordinating anion, adopts here a role still very rare for this anion: it acts as a tripodal capping ligand enabling a pseudo-octahedral geometry at a d6 metal center. Three bromines from the lower hemisphere of the hexahalogenated carboranate coordinate to Pt(IV), and distortions from ideal octahedral angles at Pt are marginal (<3°). Pt-Br bond lengths are 2.7279(18), 2.7129(17), and 2.7671(18) Å. Using the 2JPtH coupling constant of Pt-bonded methyl groups (79.0 Hz) as indicator of the donor strength of the tripodal cap, the prediction is obtained that closo-CB11H6Br6– is a relatively weak donor toward the trimethylplatinum(IV) cation. Ligand competition equilibria can be expected to depend on both the intrinsic donor strengths of competing ligands and on the effects of charge and geometry. We observe that closo-CB11H6Br6– is capable of replacing acetone from Me3Pt(acetone)3+, whereas BF4– counterion is unable to replace acetone under similar conditions.Key words: non-coordinating anion, platinum(IV), trimethyl, closo-carboranate, tripodal, trans-influence, NMR spectroscopy.