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

Shefali Vaidya, Saurabh Kumar Singh, Pragya Shukla, Kamaluddin Ansari, Gopalan Rajaraman, Maheswaran Shanmugam

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A series of mononuclear tetrahedral CoII complexes with a general molecular formula [CoL2X2] [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 [χMT(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 χMT(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.

Original languageEnglish (US)
Pages (from-to)9546-9559
Number of pages14
JournalChemistry - A European Journal
Issue number40
StatePublished - Jul 18 2017

Bibliographical note

Funding Information:
M.S. thanks the funding agencies DST, DST Nanomission, INSA, and IIT Bombay for financial support. S.V. and P.S. thanks CSIR for financial support. K.A. thanks UGC for financial support. G.R. would like to thank SERB-DST, INSA for funding (EMR/2014/000247).

Publisher Copyright:
© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim


  • S ligands
  • ab initio calculations
  • cobalt
  • halides
  • magnetic properties


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