Structural features of the mixed η2:η4 coordination of two transition metal atoms in binuclear and trinuclear π-complexes with semiquinoid ligands. Crystal and molecular structure of (η2-ethylene)(4-methyl-4-exo-n-butyl-1-anti-η2-methylene-[(2,5-η4-cyclohexadiene)rhodium2,4-O,O′-pentanedionate])- rhodium-2,4-O,O′-pentanedionate

Add time:07/18/2019    Source:sciencedirect.com

The crystal and molecular structure of (η2-ethyelene)(4-methyl-4-exo-n-butyl-1-anti-η2-methylene-[(2,5-η4-cyclohexadiene)rhodium-2,4-O,O′-pentanedionate]- rhodium-2,4-O,O-pentanedionate has been determined by an X-ray diffraction study. Comparison of the data obtained with those of a previous study of a mononuclear η4-coordinated rhodium(I) complex with the 4-methyl-4-trichloromethyl-2,5-cyclohexadiene-1-one ligand reveals that the most prominent feature of the mixed η2:η4 coordination in this series consists in the greater deviation (up to 30 and 40°, respectively) of the exo-unsaturated and the saturated geminal fragments of the semiquinoid ligand from the central plane of its cyclohexadiene ring. The shielding of the HA proton of the n-butyl group (-CHAHBC3H7) by the η2-coordinated rhodium atom (as well as by other fragments of the molecule) is different from that of the HB proton of the same group, which accounts for the unusual features in the 1H NMR spectra of this complex. There is a pronounced transannular contact between the rhodium atom and the hydrogen atom of the peripheral CHA bond, practically at the face of the fragment being coordinated (the Rh…HA and Rh…CHA distances are 2.57 and 3.50 Å, respectively, and the Rh…XACHB angle is 160°). Use of molecular modelling based on the results of the X-ray diffraction study shows that for the recently synthesized trinuclear rhodium(I) complexes with two exo-methylene-2,5-cyclohexadiene ligands, the anti-trans configuration is the most likely.

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