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

A novel [(dppe) 2 Pt 2 (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 1 H, 31 P, and 13 C NMR spectroscopy, as well as 2D 1 H/ 1 H (COSY) and 13 C/ 1 H (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 C 1 and C 1 ‘ of the resulting “diketo” BINOL ligand. η 1 -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 C O→Pt coordination. The driving force for dimer formation is likely electrostatic, as dimerization distributes the dicationic charge of Pt Lewis acid 2 over two Pt centers, facilitated by the “diketo” BINOL bridge.