A hydrogen-1 and carbon-13 nuclear magnetic resonance study of the bonding in some monosubstituted cyclopentadienyl complexes of rhodium(I) and the crystal and molecular structure of cyclo-octa-1,5-diene(η-methoxycarbonyl-cyclopentadienyl)rhodium(I)

Abstract

The ¹H and ¹³C n.m.r. spectra of a series of monosubstituted η-cyclopentadienylrhodium(I) complexes of the types [RhL₂(C₅H₄X)] and [Rh(LL)(C₅H₄X)] are described (L = ethylene or CO; LL = a 1,3-, 1,4-. or 1,5-diene; X = Me, CMe₃, CO₂Me, CO₂Et, CO₂Prⁱ, CHO, COCO₂Et, or CN). For complexes where the substituent X is electron-withdrawing in nature and where the neutral ligand is ethylene or a non-conjugated diene the ¹H spectra of the cyclopentadienyl ring protons show a novel temperature dependence (+40 to –60 °C) in CDCI₃ involving a chemical-shift cross-over without line broadening of the H(2), H(5) and H(3), H(4) resonances. In contrast, the n.m.r. behaviour of the complexes of conjugated dienes is normal, The unusual spectra observed for the non-conjugated alkene complexes are attributed to unequal barriers for the rotation of the cyclopentadienyl ring leading, at the lower temperatures, to preferential population of a particular rotamer state. A single-crystal X-ray structure determination of [Rh(cyclo-octa-1,5-diene)(C₅H₄CO₂Me)](1)suggests that the preferred rotamer in solution may be one in which the C₅ring is bonded to the metal asan η-allyl via C(2), C(1), and C(5)[2.230(8), 2.256(6), 2.226(5)Å] with a weaker interaction via an alkene function localized between C(3) and C(4)[2.303(8), 2.294(7)Å]. A simple bonding model is proposed which relates the n.m.r. spectra of solutions to the structure of the solid and which accounts for the differences in behaviour between complexes of conjugated dienes on the one hand and those containing isolated double bonds on the other.

Crystals of (1) are triclinic, space group P, Z= 2, with a= 7.051(8), b= 8.390(8), c= 12.660(10)Å, α= 100.16(9). β= 101.33(8), γ= 105.95(11)° The structure was solved by Patterson and Fourier methods from 2 023 independent reflections collected by counter methods and refined to R 0.039.