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Origin of the Magnetic Anisotropy in Heptacoordinate Ni II and Co II Complexes
Author(s) -
Ruamps Renaud,
Batchelor Luke J.,
Maurice Rémi,
Gogoi Nayanmoni,
JiménezLozano Pablo,
Guihéry Nathalie,
de Graaf Coen,
Barra AnneLaure,
Sutter JeanPascal,
Mallah Talal
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201202492
Subject(s) - magnetization , excited state , ground state , chemistry , ab initio , ab initio quantum chemistry methods , electron paramagnetic resonance , magnetic anisotropy , anisotropy , crystallography , atomic physics , condensed matter physics , nuclear magnetic resonance , magnetic field , physics , molecule , organic chemistry , quantum mechanics
Abstract The nature and magnitude of the magnetic anisotropy of heptacoordinate mononuclear Ni II and Co II complexes were investigated by a combination of experiment and ab initio calculations. The zero‐field splitting (ZFS) parameters D of [Ni(H 2 DAPBH)(H 2 O) 2 ](NO 3 ) 2 ⋅ 2 H 2 O ( 1 ) and [Co(H 2 DAPBH)(H 2 O)(NO 3 )](NO 3 ) [ 2 ; H 2 DAPBH=2,6‐diacetylpyridine bis‐ (benzoyl hydrazone)] were determined by means of magnetization measurements and high‐field high‐frequency EPR spectroscopy. The negative D value, and hence an easy axis of magnetization, found for the Ni II complex indicates stabilization of the highest M S value of the S =1 ground spin state, while a large and positive D value, and hence an easy plane of magnetization, found for Co II indicates stabilization of the M S =±1/2 sublevels of the S =3/2 spin state. Ab initio calculations were performed to rationalize the magnitude and the sign of D , by elucidating the chemical parameters that govern the magnitude of the anisotropy in these complexes. The negative D value for the Ni II complex is due largely to a first excited triplet state that is close in energy to the ground state. This relatively small energy gap between the ground and the first excited state is the result of a small energy difference between the d xy and ${{\rm{d}}_{x^2 - y^2 } }$ orbitals owing to the pseudo‐pentagonal‐bipyramidal symmetry of the complex. For Co II , all of the excited states contribute to a positive D value, which accounts for the large magnitude of the anisotropy for this complex.