NMR Structure of an Unusual Pt2(μ-BINOL) Dimer: More Surprises from a Seemingly Simple Chiral Ligand

A novel [(dppe)2Pt2(μ-BINOL)]2+[OTf]-2 dimer (A) was cleanly produced by addition of 1 equiv of HOTf to (dppe)Pt(S-BINOL) (1) or via “comproportionation” of 1 with (dppe)Pt(OTf)2 (2). The structure of A was investigated by 1H, 31P, and 13C NMR spectroscopy, as well as 2D 1H/1H (COSY) and 13C/1H (HMQC, HMBC) correlation experiments, revealing that both halves of the bridging BINOL ligand have undergone “enol−keto” tautomerization in this complex. Two symmetry-equivalent (dppe)Pt fragments coordinate in an anti fashion via Pt−C σ bonds to the adjacent atoms C1 and C1‘ of the resulting “diketo” BINOL ligand. η1-Coordination of the carbonyl groups of “diketo” BINOL to Pt was proposed to fill the fourth coordination site of each Pt square plane in dimer A, generating favorable five-membered Pt-chelate rings; the P−Pt coupling constants observed for A are consistent with CO→Pt coordination. The driving force for dimer formation is likely electrostatic, as dimerization distributes the dicationic charge of Pt Lew...