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

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)2U2(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(CH2CH2NSitBuMe2)2(CH2CH2SitBuMeCH2]) with stoichiometric amounts of mono-, bis-, and tris(ethynyl) benzenes affords the yellow-green pentacoordinate arylacetylide complexes [(NN′3)U(CCPh)] (3), [(NN′3)2U2(m-DEB)] (4), [(NN′3)2U2(p-DEB)] (5), and [(NN′3)3U3(TEB)] (6), where NN′3 = [N(CH2CH2NSitBuMe2)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 Ĥ = −2J(Ŝi·Ŝj). 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).