From Bent to Linear Uranium Metallocenes: Influence of Counterion, Solvent, and Metal Ion Oxidation State

Reactions of Cp*2UCl2 with a slight excess of Me3SiI or Me3SiOTf in acetonitrile provide convenient
routes to Cp*2UI2 (1) or Cp*2U(OTf)2 (2) (Cp* = η-C5Me5; OTf = OSO2CF3), which were isolated with
excellent yields. Crystals of the bent metallocenes Cp*2UI2(NCR) (R = Me (3), tBu (4)) have been
obtained by addition of excess RCN to a toluene solution of 1. The triflate analogue Cp*2U(OTf)2(NCMe) (5) contains monodentate OTf ligands. Treatment of Cp*2UMe2 with 2 equiv of HNEt3BPh4 in
acetonitrile led to the immediate formation of [Cp*2U(NCMe)5][BPh4]2 (6). The 1H NMR spectra of 1
and 2 in acetonitrile are similar to that of 6, indicating the presence of the dicationic species [Cp*2U(NCMe)5][X]2 [X = I (7), OTf (8)] in this solvent. Compounds 6−8 are the first linear metallocenes of
an f-element and exhibit an unprecedented structure with auxiliary donor ligands in the equatorial plane
parallel to the cyclopentadienyl rings. When treated with Me3SiOTf, the uranium(III) species Cp*2UCl2Na(thf)2 did not afford the trivalent complex Cp*2U(OTf) (9) but gave crystals of the dimeric uranium(IV) “ate” compound [Cp*2U(OTf)3Na(thf)2]2·2(CH2)4O (10·2(CH2)4O). The first organouranium(III)
triflate, 9, was isolated in 90% yield from the reaction of U(OTf)3 with 2 equiv of KCp* in thf. Dissolution
of Cp*2UI(py) in acetonitrile gave the adduct Cp*2UI(NCMe)2 (11), which, like its cerium analogue,
adopts a bent sandwich structure in its crystalline form. The 1H NMR spectra of 11 and Cp*2U(OTf)(NCMe)2 are similar in benzene and acetonitrile, suggesting that the trivalent linear metallocene [Cp*2U(NCMe)5][X] (X = I or OTf) is not present in pure acetonitrile. These observations are confirmed by the
addition of 3 equiv or excess MeCN to a solution of [Cp*2U(thf)2][BPh4] in thf, which afforded the
stable [Cp*2U(NCMe)3][BPh4] as the sole product.