Combined Experimental and Theoretical Investigation of the Origin of Magnetic Anisotropy in Pentagonal Bipyramidal Isothiocyanato Co(II), Ni(II), and Fe(III) Complexes with Quaternary-Ammonium-Functionalized 2,6-Diacetylpyridine Bisacylhydrazone

Abstract

The magnetic anisotropy in pentagonal bipyramidal complexes of Co(II) (1 and 2), Fe(III) (3 and 4) and Ni(II) (5) with 2,2′-[2,6-pyridinediylbis(ethylidyne-1-hydrazinyl-2ylidene)]bis[N,N,N-trimethyl-2-oxoethanaminium] equatorial ligand and isothiocyanato axial ligands has been investigated by magnetic susceptibility measurements, powder X-band EPR spectroscopy, Mössbauer spectroscopy, ab initio, and LFDFT calculations. The studied complexes display three distinct types of magnetic anisotropy. Co(II) complexes (1 and 2)show an easy plane anisotropy with large and positive D values and negligible rhombicity.Ni(II) complex ( 5) has uniaxial magnetic anisotropy with negative D value. Fe(III) complexes (3 and 4) have small ZFS parameters. Theoretical modeling was used to rationalize the magnetic anisotropy in these systems and to identify the most important excited states that are responsible for the zero-field splitting. These excitations are a consequence of the electronic structure of the central metal ion in ideal pentagonal bipyramidal coordination.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…Most of the seven‐coordinate Co II complexes are stabilised in pentagonal bipyramidal (PBP or D 5 h symmetry) geometry or distorted PBP geometries. To mention [Co‐(H 2 DAPBH)(H 2 O)(NO 3 )]NO 3 ( 1 ) [18] (H 2 DAPBH=2,6‐diacetylpyridine bis‐benzolylhydrazone), [Co(H 2 daps)(MeOH) 2 ] ( 2 ) [19] (H 2 daps=2,6‐bis(1‐salicyloylhydrazonoethyl) pyridine) and [Co(H 2 L)(NCS) 2 ](SCN) 2 ( 3 ) [20] (H 2 L=2,2′‐[2,6‐pyridinediylbis(ethylidyne‐1‐hydrazinyl‐2‐ylidene)]bis[ N , N , N ‐trimethyl‐2‐oxoethanaminium]) complexes possess D values of +35(31) cm −1 , +38(43) cm −1 and +41(30) cm −1 , respectively, as revealed from NEVPT2 calculations and SQUID data (see Figure 2 c–f). Ground state electronic configuration in these geometries can be represented by the d xz 2 d yz 2 d 1 d xy 1 d 1 electronic configuration.…”

“…Multi‐configurational CASSCF/NEVPT2 calculations reveal that the ZFS arises dominantly from the first four spin‐allowed excited ligand‐field states and all four consist of different M L valued electronic transition (see Figure 2 a). [18–21] It was found that the axial ligand donor strength and the equatorial ligand symmetry plays a very important role in determining the magnitude of the D value in these type of complexes. Particularly as the major contribution to D arises from different M L level excitations, D is expected to be positive, and as the gap between the d xz /d yz to d xy /d decides the magnitude of D , it is expected to be only moderate.…”

Synthesis, Characterization, Catalytic Activity, and DFT Calculations of Zn(II) Hydrazone Complexes

“…The τ value of 0.36 calculated for 1, indicates that the irregular coordination geometry about Zn1 is 36% trigonally distorted square-based pyramidal. The greatest basal angles O1−Zn1−N1 and N2−Zn1−N5 are 149.20 (7) • and 127.42(10) • , respectively. The Zn1 is lifted out of the plane of the four in-plane ligand atoms (O1, N1, N2 and N5) by a distance ρ of 0.6091( 3 coplanar nature of metal-ligand system.…”

“…Hydrazone ligands are one of the most important classes of flexible and versatile polydentate ligands which show very high efficiency in chelating various metal ions [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 ]. The coordination behavior of hydrazones is known to depend on the pH of the medium, the nature of the substituents and on the position of the hydrazone group relative to other moieties [ 2 , 3 , 4 ].…”

“…Moreover, deprotonation of the –NH group, which is readily achieved in the complexed ligand in particular, results in the formation of tautomeric anionic species (=N–N − –C=O or =N–N=C–O − ), having different coordination properties. Our interest in metal complexes with hydrazone-based ligands is partly due to their potential applications as catalysts [ 5 , 6 ] and molecular magnets [ 7 ]. Hydrazone ligands, obtained from Girard’s T reagent (trimethylammoniumacetohydrazide chloride) and 2-acetylpyridine or 2-quinolinecarboxaldehyde ( HL 3 Cl = ( E )- N , N , N -trimethyl-2-oxo-2-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)ethan-1-aminium chloride and HL 4 Cl = ( E )- N , N , N -trimethyl-2-oxo-2-(2-(quinolin-2-ylmethylene)hydrazinyl)ethan-1-aminium chloride) are tridentate ligands which show the potential to form mononuclear and binuclear structures with metal ions [ 3 , 4 , 6 , 8 , 9 , 10 ].…”