[2 + 2] Photocycloadditions of [(η5-C5H5)Ru(DMPP) 2L]PF6 complexes

Article abstract of DOI:10.1016/S0022-328X(96)06817-9

A series of [(η⁵-C₅H₅)Ru(DMPP)₂L]PF₆ complexes, DMPP = 1-phenyl-3,4-dimethylphosphole, L = CH₃CN, Ph₃P, PhS(O)₂CH=CH₂, (CH₃)₂NC(O)CH=CH₂, PhN=C, CO and (CH₃O)₃P, was found to undergo sunlight initiated [2 + 2] photodimerization of the coordinated phospholes only when L is a good π-acceptor ligand. These [2 + 2] dimerizations are accompanied by [4 + 2] dimerizations. The ratio of the [2 + 2] to [4 + 2] cycloaddition products is a function of the steric bulk of L. The nature of the photoexcited state has been probed by electron absorption and emission spectroscopy. The electron absorption spectra show high ε bands in the near UV region that are attributed to DMPP π → π * transitions. The emission spectral lifetimes are a function of L and their magnitude at 77 K (about 0.2 to 2.0 μs), together with large Stokes shifts, is indicative of phosphorescence from a triplet excited state. It is postulated that this triplet excited state undergoes cycloaddition by way of a biradical intermediate. The complexes have been characterized by elemental analyses, infrared, electronic, and ¹H, ¹H{³¹P}, ¹³C{¹H} and ³¹P{¹H} NMR spectroscopy and cyclic voltammetry. The structure of {(η⁵-C₅H₅)Ru(DMPP)₂[2 + 2](CO)}PF₆ was confirmed by X-ray crystallography. The complex cation possesses a non-crystallographic mirror plane, the two Ru-P distances (2.282(1) and 2.281(2) A) are equivalent and the cyclobutane ring has a long (1.607(8) A) and a short (1.550(8) A) C-C bond.