Cyanide Metallocenes of Trivalent f-Elements

Addition of N n Bu 4 CN to (C 5 Me 5 ) 2 UI(py), (C 5 Me 5 ) 2 CeI, or (C 5 Me 5 ) 2 M(OTf) (M = U, Ce) in acetonitrile led to the precipitation of the neutral monocyanide species [(C 5 Me 5 ) 2 M(μ-CN)] n [M = U ( 1 ), Ce ( 2 )], which likely have an oligomeric structure, as shown by the trimeric cyanide-bridged complex [(C 5 Me 5 ) 2 Ce(μ-CN)(CN t Bu)] 3 ( 3 ) obtained by addition of excess t BuNC into a suspension of 2 in acetonitrile. The structure of the U(III,IV) mixed valence compound [{(C 5 Me 5 ) 2 U} 2 (μ-CN){(μ-CN) 2 Na(thf)} 2 ] ∞ ( 4 ), which crystallized from a thf solution of (C 5 Me 5 ) 2 UI(py) in the presence of excess NaCN, reveals a unique example of an f-element−(μ-CN)−M interaction (M = main group or d transition metal). The anionic polycyanides [(C 5 Me 5 ) 2 M(CN) 3 ][N n Bu 4 ] 2 [M = U ( 5 ), Ce ( 6 )] were synthesized by treatment of 1 and 2 with 2 equiv or an excess of N n Bu 4 CN in acetonitrile; they were also prepared in a one-pot procedure by stepwise addition of 1 equiv of KCN and 2 equiv of N n Bu 4 CN to the parent iodides in acetonitrile. The bent metallocenes 5 and 6 are unique low-valent molecular polycyanide compounds of an f-element that have been structurally identified, while 5 is the first fully characterized actinide(III) cyanide. Comparison of the crystal structures of 5 and 6 shows that the M−C(C 5 Me 5 ) and M−C(CN) distances are 0.02−0.03 Å shorter for M = U than for M = Ce, while the ionic radius of uranium(III) is 0.02 Å larger than that of cerium(III).