Multidentate ferrocenyl phosphine ligands are a fascinating class of compounds that offer a variety of coordination modes to stabilize and/or activate molecular complexes, small clusters, nanoparticles and even larger surfaces. In ferrocene, the η5 bond between the cyclopentadienyl (Cp) ligand and the Fe center allows for the rotational reorientation of the Cp rings around the vertical 5-fold symmetry axis of the molecule, giving rise to a series of symmetrical staggered and eclipsed conformations as they rotate freely around the metal center. The addition of substituents reduces the symmetry of the molecule and affects the rotation rate of the Cp rings around the Fe center. In ferrocenyl phosphines, this rotation can be evaluated using dynamic NMR techniques. Guided by DFT calculations, in this mixed theoretical/ experimental study, the dynamic behavior in solution of the tetraphosphine Fc(P)4tBu = (1,1′,2,2′-tetrakis(diphenylphosphino)-4,4′-di-tert-butylferrocene) and the related MX2Fc(P)4tBu (M = Ni, Pd; X = Cl, Br) complexes was evaluated. Contrary to initial studies suggesting that the presence of the tert-butyl groups prevented the rotation of the Cp rings, 31P-{1H} EXSY (EXSY = EXchange SpectroscopY) experiments showed that the Cp rings rotate around the Fe center. Moreover, in the case of the Ni and Pd complexes, shuttling of the metal (Ni, Pd) between the Cp rings was observed using 2D 1H-1H EXSY. Evaluation of this exchange over a range of temperatures allowed calculation of the thermodynamic parameters of the rotational barriers (ΔH#, ΔS# and, ΔG#). The effects of the steric bulk of the substituents on the Cp ring, as well as the solvent, on the rotational barriers were also studied.

“Eppur si muove! And yet it moves!”: The intermolecular dynamic behaviour of multidentate ferrocenyl phosphines and their metal complexes in solution

256th National Meeting (2018)

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