Synthetic diversity and change in nuclearity in [Co–Dy] coordination aggregates: bridge removal, solvent induced structural reorganization and AC susceptibility measurements

Abstract

Three new cobalt(II/III)-dysprosium(III) complexes, [Dy^III₃Co^II₂Co^III₂(L1)₂(O₂CCMe₃)₈(OH)₄(OMe)₂(H₂O)₄]·Dy(η¹-O₂CCMe₃)₂(η²-O₂CCMe₃)₂(MeOH)₂·4H₂O (1), [Dy^III₃Co^II₂Co^III₂(L2)₂(O₂CCMe₃)₈(OH)₄(OMe)₂(MeOH)₂(H₂O)₂]·Dy(η¹-O₂CCMe₃)₂(η²-O₂CCMe₃)₂(MeOH)₂·4MeOH (2) and Dy^III₂Co^II₂Co^III₂(L2)₂(O₂CCMe₃)₁₀(OH)₂ (3) have been reported. In the heptanuclear 3d–4f monocationic aggregates in 1 and 2 the three dysprosium and four cobalt sites are arranged into a vertex shared dicubane structure, induced by two structure-directing ligands. Interestingly, a unique and previously unknown dysprosium(III)-pivalate based counter anion, Dy(η¹-O₂CCMe₃)₂(η²-O₂CCMe₃)₂(MeOH)₂⁻, was trapped by the monocationic cores during crystallization. MeCN induced structural rearrangement of 2 through loss of OMe⁻ bridges and dysprosium(III) ions at the shared vertex resulted in the hexanuclear 3d–4f neutral aggregate 3, in which two dysprosium and four cobalt sites exhibit a near planar disposition. HRMS(+ve) of solutions of 1 and 2 revealed the pathway for aggregation processes and solvent induced structural transformation along with the importance of bridging OMe⁻ in directing the formation of these compounds. Magnetic studies show a non-zero out-of-phase component in the AC susceptibility measurements of 1 but not in 2 and 3, although 1 and 2 have a very similar {Co^III₂Co^II₂Dy^III₃} core and another Dy^III center. Ab initio single-ion calculations point to the different single-ion anisotropic behavior for Dy^III centers (energy in cm⁻¹ and g-tensors) as well as negative and positive D values for Co^II sites in 1 and 2 respectively reaffirming the experimental result. However, calculations envision that, zero-field out-of-phase signal and no out-of-phase signal in 1 and 2 respectively do not solely generate from the single-ion Dy/Co anisotropies and the overall relaxation mechanism can be understood by considering the exchange interactions between Dy^III–Dy^III and Dy^III–Co^II centres.