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

Abstract

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(CH(2)CH(2)NSi(t)BuMe(2))(2)(CH(2)CH(2)Si(t)BuMeCH(2)]) 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)U(2)(m-DEB)] (4), [(NN'(3))(2)U(2)(p-DEB)] (5), and [(NN'(3))(3)U(3)(TEB)] (6), where NN'(3) = [N(CH(2)CH(2)NSi(t)BuMe(2))(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 = -2J(S(i) x S(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 [Cp(2)Fe](+/0), assignable to an oxidation of U(IV) to U(V).

A relativistic DFT study of magnetic exchange coupling in ketimide bimetallic uranium(IV) complexes

“…An important observation from the results of Table 2 is that, contrarily to B3LYP, the BP86 functional leads to very high energy differences between the HS and BS states. As stressed by previous authors [32-36, 41-46, 48-55], the resulting values of the exchange coupling constant J, in the GGA case, are systematically larger in absolute values, than those expected for such systems [62,63,77,78]. We noticed the same overestimation using another GGA functional.…”

“…In the present study, the metal electron configuration is 5f 2 for each U(IV) ion, whatever the HS or BS state, so that neglecting spinorbit corrections seems to be a valid assumption. Recent DFT calculations of magnetic exchange coupling for biuranium(IV) and (V) species [77,78,90] did not also include spin-orbit corrections.…”

“…More recently, a para-and meta-diethynylbenzene (DEB) bridging ligand have been used in binuclear isovalent U(IV)/U(IV) complexes with [(NN 0 3 ) 2 U 2 (DEB)] formula [78]. Experimental and theoretical DFT investigations of their magnetic properties have shown that both isomers exhibit a rather weak ferromagnetic and antiferromagnetic character for meta-and para-DEB, respectively [78].…”

“…Experimental and theoretical DFT investigations of their magnetic properties have shown that both isomers exhibit a rather weak ferromagnetic and antiferromagnetic character for meta-and para-DEB, respectively [78]. A new bis(imido) diuranium(V) complex [U(N t Bu) 2 (I) ( t Bu 2 bpy)] 2 exhibiting an moderate antiferromagnetic coupling between two metallic 5f 1 -5f 1 spin active centres has been synthesized and theoretically investigated [77].…”

Radical ligand-containing single-molecule magnets

“…This strategy led to the isolation of the first actinide-containing single-molecule magnets that possesses axial ligand fields [347,348]. Moreover, exchange coupling has been estimated in U IV and U V complexes [356][357][358][359][360][361] with coupling constants of up to |J| = 48 cm −1 , however, the presence of single-molecule magnet behavior has not been reported in these compounds. Another approach to the design of single-molecule magnets involves strong superexchange in actinyl systems via well-established cation-cation interactions, such as that observed in the mixed-valence neptunyl cluster compound (NpO 2 Cl 2 )[NpO 2 Cl(THF) 3 ] 2 [352].…”