Electronic Structures of Octahedral Ni(II) Complexes with “Click” Derived Triazole Ligands: A Combined Structural, Magnetometric, Spectroscopic, and Theoretical Study

The coordination
complexes of Ni­(II) with the tripodal ligands tpta (tris­[(1-phenyl-1H-1,2,3-triazol-4-yl)­methyl]­amine), tbta ([(1-benzyl-1H-1,2,3-triazol-4-yl)­methyl]­amine), and tdta (tris­[(1-(2,6-diisopropyl-phenyl)-1H-1,2,3-triazol-4-yl)­methyl]­amine) and the bidentate ligand
pyta (1-(2,6-diisopropylphenyl)-4-(2-pyridyl)-1,2,3-triazole), [Ni­(tpta)2]­(BF4)2 (1), [Ni­(tbta)2]­(BF4)2 (2), [Ni­(tdta)2]­(BF4)2 (3), and [Ni­(pyta)3]­(BF4)2 (4), were synthesized
from Ni­(BF4)2·6H2O and the corresponding
ligands. Complexes 2 and 4 were also characterized
structurally using X-ray diffraction and magnetically via susceptibility
measurements. Structural characterization of 2 that contains
the potentially tetradentate, tripodal tbta ligand revealed that the
Ni­(II) center in that complex is in a distorted octahedral environment,
being surrounded by two of the tripodal ligands. Each of those ligands
coordinate to the Ni­(II) center through the central amine nitrogen
atom and two of the triazole nitrogen donors; the Ni–N­(amine)
distances being longer than Ni–N­(triazole) distances. In case
of 4, three of the bidentate ligands pyta bind to the
Ni­(II) center with the binding of the triazole nitrogen atoms being
stronger than those of the pyridine. Temperature dependent susceptibility
measurements on 2 and 4 revealed a room
temperature χMT value of 1.18 and 1.20 cm3 K mol–1,
respectively, indicative of S = 1 systems. High-frequency
and -field EPR (HFEPR) measurements were performed on all the complexes
to accurately determine their g-tensors and the all-important
zero-field splitting (zfs) parameters D and E. Interpretation of the optical d–d absorption spectra
using ligand field theory revealed the B and Dq values for these complexes. Quantum chemical calculations
based on the X-ray and DFT optimized geometries and their ligand field
analysis have been used to characterize the metal–ligand bonding
and its influence on the magnitude and sign of the zfs parameters.
This is the first time that such extensive HFEPR, LFT, and advanced
computational studies are being reported on a series of mononuclear,
distorted octahedral Ni­(II) complexes containing different kinds of
nitrogen donating ligands in the same complex.