Tuning the Ising-type anisotropy in trigonal bipyramidal Co(II) complexes (original) (raw)
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
Tuning Magnetic Anisotropy Through Ligand Substitution in Five-Coordinate Co(II) Complexes
Inorganic Chemistry, 2017
Understanding the origin of magnetic anisotropy and having the ability to tune it are essential needs of the rapidly developing field of molecular magnetism. Such attempts at determining the origin of magnetic anisotropy and its tuning are still relatively infrequent. One candidate for such attempts are mononuclear Co(II) complexes, some of which have recently been shown to possess slow relaxation of their magnetization. In this contribution we present four different five-coordinated Co(II) complexes, 1-4, that contain two different "click" derived tetradentate tripodal ligands and either Cl -or NCS -as an additional, axial ligand. The geometric structures of all four complexes are very similar. Despite this, major differences are observed in their electronic structures and hence in their magnetic properties as well. A combination of temperature dependent susceptibility measurements and high-frequency and -field EPR (HFEPR) spectroscopy was used to accurately determine the magnetic properties of these complexes, expressed through the spin Hamiltonian parameters: g-values and zero-field splitting (ZFS) parameters D and E. A combination of optical d-d absorption spectra together with ligand field theory was used to determine the B and Dq values of the complexes. Additionally, state of the art quantum chemical calculations were applied to obtain bonding parameters and to determine the origin of magnetic anisotropy in 1-4. This combined approach showed that the D values in these complexes are in the range from -9 to +9 cm -1 . Correlations have been drawn between the bonding nature of the ligands and the magnitude and sign of D. These results will thus have consequences for generating novel Co(II) complexes with tunable magnetic anisotropy and hence contribute to the field of molecular magnetism.
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.
Inorganic Chemistry, 1985
The crystal structure of (np,)CoBr, np, = tris(2-(diphenylphosphino)ethyl)amine, has been examined by X-ray methods with the aim of determining whether the triplet ground state of the molecule depends on a particular stereochemistry. Analogous complexes of Co(I), where H and C N replace the bromine anion, are diamagnetic with a trigonal-bipyramidal structure. The application of the 18-electron rule easily rationalizes the latter geometry, where the amine group of np, is apically coordinated to the d8 metal. On the other hand, the tripodal np, ligand is known to be flexible enough to coordinate only through its phosphorus atoms, while retaining C,, symmetry. This type of arrangement is found in the title compound, where the cobalt is tetrahedrally coordinated by the phosphorus and bromine atoms. The Co-N distance is 3.34 (1) A, and the P-Co-Br and P-Co-P angles average 115.2 (20) and 103.2 (3)O, respectively. The space group is monoclinic P2,; the unit cell dimensions are a = 20.578 (9) A, 6 = 8.979 (4) A, c = 10.186 (5) A, @ = 91.15 (6)O, V = 1181.69 A' , and Z = 2. Qualitative M O arguments, supported by extended Huckel calculations, indicate that transformation to the trigonal-bipyramidal geometry, attainable by translation of the amine along the threefold axis, may be in some cases symmetry forbidden as a result of a level crossing that switches the nature of HOMO-LUMO levels. 'I1) (12 pages)' Ordering information is given On any scomb, W. N.