Experimental Evidence for Magnetic Exchange in Di- and Trinuclear Uranium(IV) Ethynylbenzene Complexes

We report the preparation and magnetic property investigations of a structurally related family of mono-, di-, and trinuclear U(IV) aryl acetylide complexes. The reaction between [(NN'(3))UCl] and lithiated aryl acetylides leads to the formation of the hexacoordinate complexes [(NN'(3))U(CCPh)(2)(Li.THF)] (1) and [(NN'(3))(2)U-2(p.DEB)(THF)] (2) as red-brown and yellow-green crystalline solids, respectively. In contrast, combining the uranacycle [(bit-NN'(3))U] (bit-NN'(3) = [N(CH2CH2NSi'BuMe2)(2)(CH2CH2Si'BuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN'3)U(CCPh)] (3), [(NN'3)(2)U2-(m-DEB)] (4), [(NN'(3))(2)U-2(p-DEB)] (5), and [(NN'(3))(3)U-3(TEB)] (6), where NN'(3) = [N(CH2CH2NSi'BuMe2)(3)]. The measured magnetic susceptibilities for 1-6 trend toward non-magnetic ground states at low temperatures. Nevertheless, the di- and trinuclear pentacoordinate compounds 4-6 appear to display weak magnetic communication between the uranium centers. This communication is modeled by fitting of the direct current (DC) magnetic susceptibility data, using the spin Hamiltonian (H) over cap = -2J((S) over cap (i).(S) over cap (j)). These results are consistent with weak ferromagnetic coupling for complexes 4-6 (J = 4.76, 2.75, and 1.11 cm(-1), respectively), while the fit for 2 is consistent with a near-negligible exchange interaction (J = -0.05 cm(-1)). Geometry-optimized Stuttgart/6-31 g* B3LYP hybrid DFT calculations were carried out (spin-orbit coupling omitted) on model complexes of 3-5. The mononuclear complex shows a triplet ground state with singly occupied degenerate f orbitals. The meta- and para-bridged species are computed to show very weak ferro- and antiferromagnetic coupling, respectively. All three complexes show only small net spin density on the acetylide-containing ligands, The monomeric phenylacetylide complex 3 undergoes a reversible redox couple at -1.02 V versus [Cp2Fe](+/0) assignable to an oxidation of U(IV) to U(V).