NMR structure of an unusual Pt2(μ-BINOL) dimer:: More surprises from a seemingly simple chiral ligand

A novel [(dppe)(2)Pt-2(mu-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 I with (dppe)Pt-(OTf)(2) (2). The structure of A was investigated by H-1, P-31, and C-13 NMR spectroscopy, as well as 2D H-1/H-1 (COSY) and C-13/H-1 (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 sigma bonds to the adjacent atoms C-1 and C-1' of the resulting "diketo" BINOL ligand. eta(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.