Dirhodium(II) complexes with bi- and tri-dentate nitrogen-containing ligands

Abstract

The dirhodium(II) formamidinate complex [Rh₂{CH(NC₆H₄Me-p)₂}₂(O₂CCF₃)₂(H₂O)] reacted with 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen), molar ratio 1 : 1, giving [Rh₂{CH(NC₆H₄Me-p)₂}₂(O₂CCF₃)₂(L–L)](L–L = bipy 1 or phen 4). Both complexes in acetonitrile solution behave as 1 : 2 electrolytes showing that both trifluoroacetate groups are weakly bonded, so that the species in MeCN is very likely [Rh₂{CH(NC₆H₄Me-P)₂}₂(L–L)L_n][CF₃CO₂]₂(L = MeCN, n= 2 or 3). Proton NMR spectra show unambiguously that 1 and 4 exist in acetonitrile solution as a mixture (1 : 1) of two isomers, 1a and 4a, possessing an axial–equatorial chelated L–L group, and 1b and 4b with the chelating L–L in the equatorial plane. Isomers a and b interconvert on treating 1 and 4 with pyridine or cyclohexyl isocyanide (a→b) or by thermal reaction (b→a). When the reaction with L–L was carried out in the ratio 1 : 2 the complexes [Rh₂{CH(NC₆H₄Me-p)₂}(σ-O₂CCF₃)₂(L–L)₂](L–L = bipy 2 or phen 5) were obtained. In these complexes, which undergo dissociation of both trifluoroacetate groups in MeCN, the ligands are face-to-face co-ordinated in equatorial position. The parent complex reacted also with the new trinucleating compound 7-diphenylphosphino-2,4-dimethyl-1,8-naphthyridine (dpnapy) leading to the monoadduct [Rh₂{CH(NC₆H₄Me-p)₂}₂(O₂CCF₃)₂(dpnapy)]. Two isomers were obtained depending on the experimental conditions. As determined by ¹H and ³¹P NMR spectroscopy, mild conditions lead to an isomer containing the phosphorus atom in axial position and the naphthyridine fragment bridging the dirhodium core. Under more vigorous conditions the P-equatorial isomer with the bidentate L–L fragment axially–equatorially chelated was obtained.