Tuning the Ising-type anisotropy in trigonal bipyramidal Co(II) complexes (original) (raw)
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Inorganic chemistry, 2017
This paper describes the correlation between Ising-type magnetic anisotropy and structure in trigonal bipyramidal Co(II) complexes. Three sulfur-containing trigonal bipyramidal Co(II) complexes were synthesized and characterized. It was shown that we can engineer the magnitude of the Ising anisotropy using ligand field theory arguments in conjunction with structural parameters. To prepare this series of compounds, we used, on the one hand, a tetradentate ligand containing three sulfur atoms and one amine (NS3(tBu)) and on the other hand three different axial ligands, namely, Cl(-), Br(-), and NCS(-). The organic ligand imposes a trigonal bipyramidal arrangement with the three sulfur atoms lying in the trigonal plane with long Co-S bond distances. The magnetic properties of the compounds were measured, and ab initio calculations were used to analyze the anisotropy parameters and perform magneto-structural correlations. We demonstrate that a smaller axial zero-field splitting paramete...
A synthetic strategy for switching the single ion anisotropy in tetrahedral Co( ii ) complexes
Chem. Commun., 2015
Four novel mononuclear tetrahedral cobalt(II) complexes containing exocyclic mesoionic ligands of molecular formulae [Co II (L 1 )(X) 2 (MeCN)] X = Cl (1) or Br (2) and [Co II (L 2 )(X) 2 (MeCN)], X = Cl (3) or Br (4) have been reported. It is found that simple substitution of L 1 (O donor in 1 and 2) by L 2 (S donor in 3 and 4) results in switching of the single ion magnetic anisotropy parameter (D) from positive to negative, with a significant change in magnitude.
Chemical Science, 2014
The magnetic anisotropy of two pentacoordinate trigonal bipyramidal (C 3v symmetry) Co(II) complexes, [Co(Me 6 tren)Cl]ClO 4 (1) and [Co(Me 6 tren)Br]Br (2), was investigated and analysed by magnetic studies, high field multifrequency electron paramagnetic resonance (EPR) and ab initio calculations. Negative D parameters expressing an Ising-type anisotropy (easy axis of magnetization) were found experimentally for both complexes. Calculations led to D values very close to the experimental ones, which allows a robust rationalisation of the magnetic anisotropy in these complexes. The wavefunctions of the ground and the first four excited states reveal that they are strongly multideterminantal i.e. linear combinations of several determinants. The most important contribution to the spin orbit coupling between the ground and lowest excited states stabilizes the largest M S ¼ AE3/2 components of the S ¼ 3/2 state and therefore brings a large negative contribution to D. The analysis of the difference between the magnitudes of the anisotropy of the two complexes led to the conclusion that a large Ising anisotropy is preferred when weak s-donating ligands are in the equatorial plane and strong p-donating ones are in axial positions; thus providing an efficient tool to chemists to predict the magnetic anisotropy in these types of complexes. The investigation of the magnetic behaviour of a single crystal of 1 by micro-SQUID shows, as expected, the presence of an easy axis of magnetization. The magnetic behaviour is consistent with quantum tunnelling of the magnetization mediated by intermolecular three-dimensional antiferromagnetic exchange interactions. Upon dilution of the Co(II) molecules in the isostructural Zn(II) compound, a blocking of the magnetization below 2 K is demonstrated; it results in an opening of the magnetization hysteresis loop in zero applied magnetic field. † Electronic supplementary information (ESI) available: Detailed information on the structures with distances and angles around the Co, c M T and c M ¼ f(T) and EPR spectra at different frequencies for 1 and 2, energy of the four rst excited states for 1 and 2, composition of the wavefunctions for the ground state and the rst four excited states for 1 and micro-SQUID data on a single crystal of 1. CCDC 956888, 956889 and 981004. For ESI and crystallographic data in CIF or other electronic format see
Inorganic chemistry, 2018
A rational approach of modulating the easy-plane magnetic anisotropy of mononuclear pentagonal bipyramidal Co single molecule magnets (SMMs) has been revealed in this paper. A class of three new pentagonal-bipyramidal complexes with formulas [Co(Hdaps)(MeOH)] (1), [Co(Hdaps)(NCS)(MeOH)]·(ClO)·(MeOH) (2), and [Co(Hdaps)(NCS)]·(MeOH) (3) (Hdaps = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine) were studied. In these complexes, the axial positions are successively replaced by different O and N donar ligands in a systematic way. Detailed magnetic measurements disclose the existence of large easy-plane magnetic anisotropy and field-induced slow magnetic relaxation behavior. Both experimental and ab initio theoretical calculations display that easy-plane magnetic anisotropy is maintained upon variation of coordination environments. Nevertheless, the magnitude of the D value was found to be increased in the case of weaker axially coordinated σ-donor ligands and a more symmetrical equatorial ...
Dalton Transactions, 2022
Large uniaxial magnetic anisotropy, expressed by a negative value of the axial zero-field splitting parameter D, has been achieved in a series of trigonal prismatic Co(II) complexes with the general formula [Co (L)X]Y, where L = 1,5,13,17,22-pentaazatricyclo[15.2.2.17,11]docosa-7,9,11(22)-triene, X = Cl − (1a,b), Br − (2), N 3 − (3), NCO − (4), NCS − (5), NCSe − (6), and Y = Cl − (1), Br − (2), NCS − (4), NCSe − (5), ClO 4 − (3,6). Complexes 1-6 are six-coordinate with the distorted trigonal prismatic geometry imparted by the pentadentate pyridine-/piperazine-based macrocyclic ligand L and by one monovalent coligand X −. Based on magnetic studies, all complexes 1-6 exhibit strong magnetic anisotropy with negative D-values ranging from about −20 to −41 cm −1. This variation in D (i.e. the increase of magnetic anisotropy) parallels the trend obtained by theoretical calculations and the lesser distortion of the coordination sphere with respect to the trigonal prismatic reference geometry. AC magnetic susceptibility investigations revealed field-induced singlemolecule magnet behaviour for all complexes except Cl − derivative 1. The series investigated represents a rare example of Co(II) complexes with a robust trigonal prismatic geometry.
Dedicated to Professor George Christou on the occasion of his 60th birthday One hundred years ago Alfred Werner won the Nobel Prize in chemistry for his pioneering work with inorganic coordination compounds, which was mainly attributed to his work on mononuclear cobalt complexes. Although this chemistry has been well-developed, in recent years it continues to reveal new and interesting magnetic properties derived from the molecular geometry of these complexes. [2] Indeed since the discovery of magnet-like behavior in a mononuclear transition-metal complex, [3] a sudden re-emergence of interest occurred in 3d molecules acting as molecular magnets. This is mainly due to the fact that these model mononuclear complexes can unravel the origin of magnetic anisotropy due to their unquenched first-order orbital angular momentum. When large uniaxial anisotropy (D) is coupled with the intrinsic spin (S) of a molecule, an anisotropic barrier (U = S 2 j D j) for the reversal of the magnetization can be seen. Such molecules are termed single-molecule magnets (SMMs) or, for mononuclear complexes, single-ion magnets (SIMs). 8] To compensate for low-spin values in 3d ions, highly anisotropic metal ions, such as Fe II or Co II , are used. [4, 6c] Moreover, 3d complexes can exhibit spin crossover (SCO) behavior. [9] For 3d 4 -3d 7 metal ions, high-spin (HS)-low-spin (LS) crossover can occur if the ligand field is tuned such that a balance between strong and weak field ligands is achieved. Strong-field terpyridine (terpy) ligands can be ideal for isolating such systems. With this in mind we have carefully studied three related compounds based on the Co II -terpy system where we fine-tune the ligand field by controlling the number of coordinated terpy ligands as well as the remaining terminal ligands, leading to unique magnetic properties of SIM and SCO behavior. Herein we unravel the inherent physical properties of [Co(terpy)Cl 2 ] (1), [Co(terpy)(NCS) 2 ] (2), and [Co(terpy) 2 ](NCS) 2 ·1.5 H 2 O (3) through structural, spectroscopic, computational, and magnetic studies.