4,4-p-Biphenyl bis-phosphinidene: generation of a bis-W(CO)5 complex and ab initio calculation of its electronic structure

The (bis-7-phosphanorbornadiene-7-yl)-biphenyl was prepared from the reaction of the 4,4′-biphenyl dianion with the readily available 1-cyano-3,4-dimethylphosphole. The resulting complex was then used as a precursor of the 4,4′-p-biphenyl bisphosphinidene (P–C6H4–C6H4–P), which is trapped either by insertion reaction with MeOH leading to the bisphosphine complex or by [1+2] cycloaddition with diphenylacetylene, leading to the 4,4′-p-biphenyl bisphosphirene complex. Ab initio calculations on biphosphinidene were carried out using the CASSCF/CASPT2 method in conjunction with the 6-31G(d) basis set and both (12 electron–12 orbital) and (14 electron–14 orbital) active spaces that include all four lone pair orbitals of phosphorus atoms and the higher-lying phenyl p-orbitals. The calculated results pointed out that, in the ligand-free form, the parent 4,4′-p-biphenyl bis-phosphinidene (BPBP) exhibits a quintet ground electronic state (5A) having a twisted geometry, but with small quintet-singlet and quintet-triplet (5A < 1Ag < 3B1u) energy gaps of only 6 and 16 kJ mol−1, respectively. While the singlet and triplet states need to be treated by multi-reference wavefunctions, the lowest-lying quintet state 5A can essentially be described by HF-based single reference methods. In this high-spin state, BPBP is formed by a loose connection between two phenyl phosphinidenes in keeping the spin state of the P-atoms intact, leading to a weak coupling between phosphorus non-bonding electrons through the biphenyl moiety.