Ru(ii) and Rh(iii) porphyrin complexes of primary phosphine-substituted porphyrinsDedicated to the memory of Bhaskar G. Maiya.

Abstract

Primary alkynyl phosphine porphyrins were prepared by AlHCl2 reduction of the corresponding alkynyl phosphonates. Dephosphorylation of the alkyne proved to be a major side reaction. Using LiAlH4 as reducing agent, the alkyne was found to be partially reduced to give the trans-alkenyl phosphine selectively. The primary phosphines coordinate to both ruthenium(II) and rhodium(III) porphyrins and readily form bis-phosphine complexes. The 1H and 31P NMR spectra for the ruthenium complexes show a pattern characteristic of an [AX2]2 spin system with an unusually large 2JPP coupling constant of 620.6 Hz. The IR spectrum of the complex (PAPH2)Ru(CO)(porphyrin) (PAPH2 = phenylacetylenephosphine) indicates weak σ-donor properties of the ligand. In contrast to the corresponding tertiary phosphine complexes, the bis-phosphine complexes with both ruthenium(II) and rhodium(III) porphyrins are more stable than the mono-phosphine complexes, as judged by NMR spectroscopy, and they can also be detected in the gas phase by LDI-TOF MS. In all cases the complexes could not be isolated and they degrade within hours at ambient temperatures when kept in solution. These compounds may therefore not be suitable for the construction of larger multiporphyrin systems, but their accessibility makes it possible to study their coordination behaviour with other transition metals.