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*U-2(OTf)(2) (2) (Cp* = eta-C5Me5; OTf = OSO2CF3), which were isolated with excellent yields. Crystals of the bent metallocenes Cp*2UI2(NCR) (R = Me (3), Bu-t (4)) have been obtained by addition of excess RCN to a toluene solution of 1. The triflate analogue Cp*U-2(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*U-2(NCMe)(5)][BPh4](2) (6). The H-1 NMR spectra of 1 and 2 in acetonitrile are similar to that of 6, indicating the presence of the dicationic species [Cp*U-2-(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*U-2(OTf) (9) but gave crystals of the dimeric uranium(IV) ate compound [Cp*U-2(OTf)(3)Na(thf)(2)](2)center dot 2(CH2)(4)O (10 center dot 2(CH2)(4)O). 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 H-1 NMR spectra of 11 and Cp*U-2(OTf)(NCMe)(2) are similar in benzene and acetonitrile, suggesting that the trivalent linear metallocene [Cp*U-2(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*U-2(thf)(2)][BPh4] in thf, which afforded the stable [Cp*U-2(NCMe)(3)][BPh4] as the sole product.