Carbon monoxide and isocyanide complexes of trivalent uranium metallocenes

Organic isocyanide complexes of trivalent uranium metallocenes of the type [Cp′3U(CNR)], where Cp′ = C5H5, MeC5H4, Me3CC5H4, and Me3-SiC5H4and R = Et; Cp′ = 1,3-(Me3-Si)2C5H3and R = tBu; Cp′ = Me4C5H and R = 4-(MeO)C6H4and 2,6-Me2C6-H3, have been isolated. When Cp′ = Me-C5H4and R = 2,6-Me2C6H3, both 1:1 and 1:2 adducts are obtained. The IR spectra show that ṽCNincreases slightly for the alkyl isocyanide complexes and decreases slightly for the aryl isocyanide complexes relative to VCN for the free ligands. The uranium metallocenes form carbon monoxide adducts in which ṽCOmoves to lower wavenumbers upon coordination (by 155 to 260 cm-1). In only one case, however, can an adduct be isolated in crystalline form; [(C5Me4H)3U(CO)]. The ṽCOstretching frequencies lie in the order 1,3-(Me3Si)2C5H3> Me3SiC5-H4> Me3CC5H4> C5Me4H; [(C5Me4-H)3U] is the best it donor in this series of metallocenes. Solution1H NMR spectra show that the adducts are fluxional and, in the case of [(C5Me4H)3U(L)] where L = CO or 4-(MeO)C6H4NC, two fluxional processes can be observed: dissociation of L at relatively high temperature and cessation of ring rotation at low temperature. The crystal structures of the base-free metallocenes [{1,3-(Me3-Si)2C5H3}3U] and [(C5Me4H)3U] show that these compounds are monomeric with idealized trigonal-planar geometry. The crystal structures of the 4-(MeO)-C6H4NC and CO adducts of the latter metallocene are also described. All of the experimental studies reported here-in are consistent with the view that the uranium metallocenes are better π donors than their cerium analogues, and the π-donating ability is dependent upon the ring substituents.