Interaction of Rh(I) with meso-arylsapphyrins and -rubyrins: first structural characterization of bimetallic hetero-rubyrin complex

Abstract

The ligational behavior of meso-arylsapphyrins and rubyrins toward Rh(I) is investigated. Sapphyrins form monometallic complexes with coordination of one imine and amine type nitrogens of the bipyrrole unit in an η² fashion. The Rh(I) coordination is completed by the presence of two ancillary carbon monoxide ligands. Rubyrins form both monometallic and bimetallic complexes. Two types of bimetallic complexes have been isolated. In the first type, both rhodium atoms are projected above the mean rubyrin plane, while in the second type, one rhodium atom is projected above and the other below the mean plane. Detailed ¹H and 2D NMR spectral analyses along with IR and UV-visible spectra of the complexes confirm the proposed binding modes for the rhodium complexes. Furthermore, the single-crystal X-ray analysis of one of the bimetallic complexes of rubyrin shows a bowl-shaped symmetric structure where both Rh(I) atoms are projected above the mean rubyrin plane at an angle of 71.73°. The geometry around each rhodium center is approximately square planar [N1-Rh1-N2, 80.38(9)°; C15-Rh1-C16, 86.95(14)°; N1-Rh1-C15, 97.13(12)°; and N2-RH1-C16, 94.97(12)°]. The observed distance of 4.313 Å between the two rhodium centers reveals very little interaction between the two rhodium atoms. This type of metal binding is accompanied by a 180° ring flip of the heterocyclic ring connecting the two bipyrrole units. In dioxarubyrin, where one of the pyrrole rings of the bipyrrole unit is inverted, Rh(I) binds at the periphery to the pyrrole nitrogen, leaving the rubyrin cavity empty. The absence of one amino and one imino nitrogen on the dipyrromethene subunits in the sapphyrins and rubyrins described here forces Rh(I) to bind to bipyrrole nitrogens.