Synthesis, electrochemistry, MO properties, and X-ray diffraction structures of the new redox-active diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (tbpcd) and the rhenium compound fac-BrRe(CO)3(tbpcd)

Knoevenagel condensation of 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) with thiophene-2-carboxaldehyde furnishes the second-generation unsaturated diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (1, tbpcd) in high yield. The substitution chemistry of the rhenium compounds BrRe(CO)5 and BrRe(CO)3(THF)2 with tbpcd has been investigated and found to produce fac-BrRe(CO)3(tbpcd) (2). Compounds 1 and 2 have been isolated and fully characterized in solution by IR and NMR (1H and 31P) spectroscopies, in addition to mass spectrometry, and X-ray crystallography. The redox properties of 1 and 2 have been examined by cyclic voltammetry, and these data are discussed relative to the results obtained from extended Hückel MO calculations and emission spectroscopic studies, as well as related ligand derivatives previously prepared by us. Our data indicate that the lowest excited state in tbpcd and fac-BrRe(CO)3(tbpcd) arises from a π → π∗ intraligand (IL) transition confined exclusively to the tbpcd ligand.

Graphical abstractThe new diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (tbpcd) has been prepared from 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) and thiophene-2-carboxaldehyde. The reaction of the tbpcd ligand with BrRe(CO)5 or BrRe(CO)3(THF)2 yields fac-BrRe(CO)3(tbpcd) in high yield. The ligand and its rhenium complex have been fully characterized in solution and their solid-structures determined by X-ray crystallography. The cyclic voltammetric behavior, emission photophysics, and MO properties of these new compounds have also investigated and the results discussed relative to the parent diphosphine ligand bpcd and fac-BrRe(CO)3(bpcd).Figure optionsDownload full-size imageDownload as PowerPoint slide