[2+2] Photocycloadditions of [(eta(5)-C5H5)Ru(DMPP)(2)L]PF6 complexes

A series of [(eta(5)-C5H5)Ru(DMPP)(2)L]PF6 complexes, DMPP = 1-phenyl-3,4-dimethylphosphole, L = CH3CN, Ph3P, PhS(O)(2)CH=CH2, (CH3)(2)NC(O)CH=CH2, PhN=C, CO and (CH3O)(3)P, was found to undergo sunlight initiated [2 + 2] photodimerization of the coordinated phospholes only when L is a good pi-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 epsilon bands in the near UV region that are attributed to DMPP pi -->, pi* transitions. The emission spectral lifetimes are a function of L and their magnitude at 77 K (about 0.2 to 2.0 mu s), 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 H-1, H-1{P-31}, C-13{H-1} and P-31{H-1} NMR spectroscopy and cyclic voltammetry, The structure of {(eta(5)-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)Angstrom) are equivalent and the cyclobutane ring has a long(1.607(8)Angstrom) and a short (1.550(8)Angstrom) C-C bond.