Group 9 metal 1,1′-bis(phosphino)ferrocene complexes: synthesis, structures, solution conformation and unusual reactivity

Abstract

The crystal structure of [Ir(cod)(L–L)]⁺3[cod = cycloocta-1,5-diene, L–L = 1-(diisopropylphosphino)-1′-(diphenylphosphino)ferrocene] can be related to those of the analogous complexes with L–L = 1,1′-bis(diphenylphosphino)ferrocene (dppf)1 and 1,1′-bis(diisopropylphosphino)ferrocene (disppf), 2, all the complexes being readily synthesized from [Ir(cod)(py)₂]⁺(py = pyridine). An optimum diphosphine bite angle of approximately 99° is maintained in all three complexes by varying the twist of the ferrocene, which decreases with increasing steric profile of the phosphine, and by distortion of the geometry at the iridium atom away from square planar towards tetrahedral. The twist about the ferrocene moiety induces chirality at the iridium atom in all three complexes and the interchange of stereoisomers can be followed by variable-temperature ¹H NMR spectroscopy. Application of the Eyring equation gave approximate values of ΔG^‡ for this process of 36.1 ± 0.2, 39.3 ± 0.2 and 34.3 ± 0.2 kJ mol^–1 for 1–3 respectively. The ligand disoppf also induces considerable distortion away from square-planar geometry in the complex [Rh(nbd)(disoppf)][BF₄]4{nbd = norbornadiene (bicyclo[2.2.1]hepta-2,5-diene)}, as found in a crystal structure determination, which may account for the unusual lability of the chelating diphosphine. This is demonstrated by its reactions with Ph₂P(CH₂)_nPPh₂(n= 1 or 2) both of which give [Rh(L–L)₂]⁺. More surprisingly, considering its lability in [Rh(nbd)(dppf)]⁺, dppf also readily displaced disoppf from 4, to give [Rh(nbd)(dppf)][BF₄]5. The nbd ligand in this complex is not displaced by reaction with an excess of dppf.