Synthesis, Reactivity, and ⁷⁷Se NMR Studies of the η²- and η¹(Se)-Selenophene Complexes Cp′(CO)₂Re(Sel)

Selenophenes coordinate in Cp′(CO)₂Re(Sel) (Cp′ = C₅H₅ or C₅Me₅; Sel = selenophenes) as a rapidly equilibrating mixture of η¹ and η² isomers. Increasing the electron density on the Re by replacing Cp with Cp* favors η-coordination, while addition of methyl groups to the selenophene favors the η¹(Se) isomer. These shifts in the η²⇄ η(Se) equilibrium can be readily explained by noting that electron-donating methyl groups in the selenophene enhance the donor ability of the Se and reduce the π-acceptor ability of the olefin which favors Se-coordination, while increasing the electron density on the Re strengthens π-back-bonding to the η² ligand. The uncoordinated selenophene Se atom in Cp*(CO)₂Re(η²-Sel) (1) is capable of binding to M(CO)₄(L)(THF) (M = Cr, Mo, W; L = CO or PPh₃) to give dinuclear selenophene-bridged complexes Cp*(CO)₂Re(µ₂-η²(Se)-Sel)[M(CO)₄(L)] in which the selenophene is η² bonded to the Re and Se-bonded to a second metal (M). The reaction of 1 with Fe₂(CO)₉ gives three different types of compounds. One of them (11) results from Fe(CO)₄ addition to the Se of the η² isomer; another presumably results from coordination of Fe(CO)₃ to the diene portion of the η¹(Se) isomer. The third product has a thiaferrole-type structure in which a C-Se bond has been cleaved. ⁷⁷Se NMR studies of these different types of selenophene complexes are also reported.