REDUCTIVE COUPLING OF CARBON-MONOXIDE WITH ALKYNES LEADING TO A NOVEL CASE OF CARBON-OXYGEN BOND FORMATION - THE 1ST EXAMPLES OF 3-METALLAFURAN COMPLEXES

The edge-sharing bioctahedral dirhenium complexes Re2(mu-X)(mu-CO)X3(L)(mu-dppm)2 (X = Cl when L = CO, xylNC; X = Br when L = CO; dppm = Ph2PCH2PPh2) react with terminal alkynes RCCH (R = H, n-Pr, n-Bu, Ph, p-tol) at room temperature in the presence of TlPF6 to afford the novel series of diamagnetic complexes [Re2(mu-X)(mu-COC(R)CH)X2(L)(mu-dppm)2]-PF6 in which the reductive coupling of a bridging CO ligand and the alkyne leads to carbon-oxygen bond formation and the generation of a 3-metallafuran ring. The reactions are regioselective to give the R substituent in the 5-position. These complexes were found to undergo anion-exchange reactions when L = xylNC to afford the analogous BF4- and BPh4- salts. They can be reduced to their neutral congeners Re2(mu-X)(mu-COC(R)CH)(L)(mu-dppm)2 by cobaltocene. In the case of the reaction between Re2(mu-Cl)(mu-CO)Cl3(CNxyl)(mu-dppm)2 and acetylene, the reaction intermediate [Re2Cl3(CO)(eta2-HCCH)(CNxyl)(mu-dppm)2]PF6 has been prepared and characterized spectroscopically. A combination of NMR spectroscopy and representative single-crystal X-ray structure determination has been used to confirm the structures of the coupled species and the connectivity within the 3-metallafuran rings. The identities of [Re2(mu-Cl)(mu-COCHCH)Cl2(CO)(mu-dppm)2]PF6 (3a) and (Re2(mu-Cl)(mu-COC(p-tol)CH)Cl2(CNxyl)(mu-dppm)2]BF4 (5h) have been established by X-ray crystallography. Crystal data for 3a: space group Cc (No. 9) with a = 19.396(2) angstrom, b = 15.495(2) angstrom, c = 20.001(7) angstrom, beta = 99.66(2)degrees, V = 5926(4) Angstrom3, and Z = 4. The structure was refined to R = 0.025 (R(w) = 0.031) for 3390 data with I > 3.0sigma(I). Crystal data for 5h: space group C2/c (No. 15) with a = 37.402(8) angstrom, b = 15.302(4) angstrom, c = 28.578(8) angstrom, beta = 119.06(3)degrees, V = 14 296(15) angstrom3, and Z = 8. The structure was refined to R = 0.066 (R(w) = 0.085) for 5158 data with I > 3.0sigma(I). In these structures the Re-Re distances are very similar (2.565(1) angstrom for 3a and 2.558(1) angstrom for 5h).