Variations of heterometallic cores and magnetic properties in Co(iii)–Y/Ln isobutyrate clusters with N-butyldiethanolamine: from tetranuclear to octanuclear clusters

Abstract

Under similar synthetic conditions, i.e. by the interaction of Co(ib)₂ (Hib = isobutyric acid) and M(NO₃)₃·nH₂O with a tridentate organic ligand N-butyldiethanolamine (H₂bdea) in acetonitrile, a series of heterometallic {Co(III)–M(III)} complexes with different cores {Co₂M₂}, {Co₂M₄}, {Co₃M₂}, {Co₃M₃}, and {Co₄M₄} have been isolated. The series includes tetranuclear [Co₂M₂(OH)₂(ib)₄(bdea)₂(NO₃)₂]·2MeCN (M(III) = Y (1), Ho (2), Er (3)), pentanuclear [Co₃M₂(OH)(ib)₅(bdea)₃(NO₃)₃]·nMeCN (M(III) = Pr (n = 1.5, 4), Nd (n = 1.5, 5), Sm (n = 2, 6)), hexanuclear [Co₂Eu₄(OH)₂(ib)₈(bdea)₂(NO₃)₄(H₂O)₂]·(7) and [Co₃Yb₃(OH)₄(ib)₆(bdea)₃(H₂O)₃](NO₃)₂·3MeCN·1.5H₂O (8), and octanuclear [Co₄M₄(OH)₄(ib)₈(bdea)₄(NO₃)₄] (M(III) = Gd (9), Tb (10)) clusters. The structures of all clusters have been determined by single crystal X-ray diffraction and reveal novel structural features in the series. The tetranuclear (1–3), pentanuclear (4–6), and octanuclear (9–10) clusters are isostructural and isomorphic in their own group. Clusters 1–3 have a rhombic strictly planar tetranuclear central {Co₂M₂(μ₃-OH)₂} core resembling a common “butterfly” motif. Clusters 4–6 possess a robust and unusual topology, previously unseen in Co/Ln chemistry, of three {CoM₂(μ₃-OH)}, {CoM₂(μ₃-O_bdea)}, and {Co₂M(μ₃-O_bdea)} noncoplanar triangles sharing a common M(III) site. The arrangement of metal atoms in 9 and 10 is reminiscent of a smaller square defined by four M(III) sites inscribed in a larger square formed by four Co(III) sites, and can also be viewed as a core composed of four {CoM₂(μ₃-OH)} nearly coplanar triangles sharing two M(III) vertices with two neighbouring triangles. Cluster 7 can be viewed as a hexanuclear central core of two parallel {CoM₂(μ₃-OH)} triangles, while cluster 8 is composed of three {CoM₂(μ₃-OH)} triangles sharing the common M(III) vertices. DC magnetic data show non-interacting or very weakly interacting Ln(III) centers and low spin Co(III) centers. AC magnetic data show relevant out-of-phase signals only for 3 below 4 K and by application of static magnetic bias fields. First principles calculations have been performed to estimate the magnetic anisotropy energies of the prepared compounds and to calculate the exchange coupling between the lanthanides in them.