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

Three new cobalt( ii / iii )-dysprosium( iii ) complexes, [Dy III 3 Co II 2 Co III 2 (L1) 2 (O 2 CCMe 3 ) 8 (OH) 4 (OMe) 2 (H 2 O) 4 ]·Dy(η 1 -O 2 CCMe 3 ) 2 (η 2 -O 2 CCMe 3 ) 2 (MeOH) 2 ·4H 2 O ( 1 ), [Dy III 3 Co II 2 Co III 2 (L2) 2 (O 2 CCMe 3 ) 8 (OH) 4 (OMe) 2 (MeOH) 2 (H 2 O) 2 ]·Dy(η 1 -O 2 CCMe 3 ) 2 (η 2 -O 2 CCMe 3 ) 2 (MeOH) 2 ·4MeOH ( 2 ) and Dy III 2 Co II 2 Co III 2 (L2) 2 (O 2 CCMe 3 ) 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(η 1 -O 2 CCMe 3 ) 2 (η 2 -O 2 CCMe 3 ) 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 III 2 Co II 2 Dy III 3 } 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. Solvent induced structural reorganization, trapping of hitherto unknown Dy III -pivalate counter anion and different AC susceptibility behavior in three new Co II/III -Dy III aggregates has been discussed.