Shefali Vaidya

Role of Halide Ions in the Nature of the Magnetic Anisotropy in Tetrahedral CoII Complexes

A series of mononuclear tetrahedral CoII complexes with a general molecular formula [CoL2 X2 ] [L=thiourea and X=Cl (1), Br (2) and I (3)] were synthesized and their structures were characterized by single-crystal X-ray diffraction. Direct-current (dc) magnetic susceptibility [χM T(T) and M(H)] and its slow relaxation of magnetization were measured for all three complexes. The experimental dc magnetic data are excellently reproduced by fitting both χM T(T) and M(H) simultaneously with the parameters D=+10.8 cm-1 , g1 =2.2, g2 =2.2, and g3 =2.4 for 1; D=-18.7 cm-1 , giso =2.21 for 2; and D=-19.3 cm-1 , giso =2.3 for 3. The replacement of chloride in 1 by bromide or iodide (in 2 and 3, respectively) was accompanied by a change in both sign and magnitude of the magnetic anisotropy D. Field-induced out-of-phase susceptibility signals observed in 10 % diluted samples of 1-3 imply slow relaxation of magnetization of molecular origin. To better understand the magnetization relaxation dynamics of complexes 1-3, detailed ab initio CASSCF/NEVPT2 calculations were performed. The computed spin Hamiltonian parameters are in good agreement with experimental data. In particular, the calculations unveil the role of halide ions in switching the sign of D on moving from Cl- to I- . The large spin-orbit coupling constant associated with the heavier halide ion and weaker π donation reduces the ground state-excited state gap, which leads to a larger contribution to negative D for complex 3 compared to complex 1. Further magnetostructural D correlations were developed to understand the role of structural distortion in the sign and magnitude of D values in this family of complexes.

Substituted versus Naked Thiourea Ligand Containing Pseudotetrahedral Cobalt(II) Complexes: A Comparative Study on Its Magnetization Relaxation Dynamics Phenomenon

A series of mononuclear tetrahedral cobalt(II) complexes with the general molecular formula [Co(L1)2X2] [where L1 = tetramethylthiourea ([(CH3)2N]2C═S) and X = Cl (1), Br (2), and I (3)] were isolated, and their structures were characterized by single-crystal X-ray diffraction. The experimental direct-current magnetic data are excellently reproduced by fitting both χM T( T) and M( H) simultaneously using the spin Hamiltonian (SH) parameters D1 = -18.1 cm-1 and g1,iso = 2.26, D2 = -16.4 cm-1 and g2,iso = 2.33, and D3 = -22 cm-1 and g3,iso = 2.4 for 1-3, respectively, and the sign of D was unambiguously confirmed from X-band electron paramagnetic resonance measurements. The effective energy barrier extracted for the magnetically diluted complexes 1-3 (10%) is larger than the barrier observed for the pure samples and implies a nonzero contribution of dipolar interaction to the magnetization relaxation dynamics. The SH parameters extracted for the three complexes drastically differ from their respective parent complexes that possess the general molecular formula [Co(L)2X2] [where L = thiourea [(NH2)2C═S] and X = Cl (1a), Br (2a), and I (3a)], which is rationalized by detailed ab initio calculations. An exhaustive theoretical study reveals that both the ground and excited states are not pure but rather multideterminental in nature (1-3). Noticeably, the substitution of L by L1 induces structural distortion in 1-3 on the level of the secondary coordination sphere compared to 1a-3a. This distortion leads to an overall reduction in | E/ D| of 1-3 compared to 1a-3a. This may be one of the reasons for the origin of the slower relaxation times of 1-3 compared to 1a-3a.