Effect of Coordination Geometry on Magnetic Properties in a Series of Cobalt(II) Complexes and Structural Transformation in Mother Liquor

Abstract

The three Co(II) complexes [Co(bbp) 2 ][Co(NCS) 4 ]•4DMF (1), [Co(bbp)(NCS) 2 (DMF)]•2DMF (2), and [Co(bbp)(NCS) 2 ](3) have been synthesized and characterized by single-crystal X-ray diffraction, magnetic, and various spectroscopic techniques. Complexes 1 and 3 are obtained by the reaction of Co(NCS) 2 with 2,6-bis(1H-benzo[d]imidazol-2yl)pyridine (bbp), and complex 1 undergoes a structural transformation to form complex 2. A single-crystal X-ray study revealed that complex 1 is comprised of two Co(II) centers, a cationic octahedral Co(II) unit and an anionic tetrahedral Co(II) unit, while the Co(II) ion is in a distorted-octahedral environment in 2. Moreover, in complex 3, the Co(II) ion is in a distorted-square-pyramidal geometry. The effect of coordination geometry on the magnetic properties was studied by both static and dynamic magnetic measurements. Direct current (dc) magnetic susceptibility measurements showed that all of the Co(II) ions are in high-spin state in these complexes. Alternating current (ac) magnetic susceptibility measurements indicated that complexes 2 and 3 display slow relaxation of magnetization in an external dc magnetic field, while complex 1 displayed no such property. EPR experiments and theoretical calculations were consistent with the above findings.

Pentacoordinate cobalt(ii) single ion magnets with pendant alkyl chains: shall we go for chloride or bromide?

“…Apart from rare-earth element complexes, Co( ii )-based SIMs present another outstanding class of complexes due to the presence of strong first-order spin–orbit coupling, non-integer ground spin state ( S = 3/2), and high magnetic anisotropy, which can be controlled by the coordination number and symmetry of the metal ion environment. 13–19 It is worth mentioning that pentacoordinate Co( ii ) complexes with one rigid tridentate N-donor ligand ( i.e. terpy ) and two terminal ligand anions present an exciting family of field-induced SIMs, 20–22 where the correlation between the geometry of coordination polyhedra and magnetic anisotropy might help us to understand the impact of molecular design on relaxation dynamics.…”

Reversible Spin-State Switching and Tuning of Nuclearity and Dimensionality via Nonlinear Pseudohalides in Cobalt(II) Complexes

“…In complex 1 at 296 K, the axial Co–N am bonds are significantly longer than the equatorial Co–N Py and Co–N dca bonds with average distances of 2.329, 2.043, and 2.083 Å, respectively (Figure , Table , and Table S2). The Co–N bond distances are very similar to those obtained in octahedral HS cobalt­(II) complexes. ,,,, The N am –Co–N am angle significantly deviates from linearity (180°) to 144.15(9)°, while N Py –Co–N Py and N dca –Co–N dca angles in the equatorial plane are slightly deviated from 90° to 87.3(1) and 92.7(1)°, respectively (Table and Table S2). The bridging arrangement Co–NC–N–CN–Co acquires a V-shaped conformation with a C(1)–N(7)–C(2) angle having a value of 125.3(3)°, while N–C–N angles are almost linear with values of 173.1(3) and 171.7(3)° (for N(5)–C(1)–N(7) and N(6)–C(2)–N(7), respectively).…”

“…Magnetic measurements were carried out on polycrystalline samples of 1 and 2 at magnetic fields of 1000 and 10000 Oe respectively, and in the temperature ranges of 2–300 K and 2–400 K for 1 and 2 , respectively in both cooling and heating modes. For complex 1 at 300 K, the measured χT (χ is the magnetic susceptibility equal to M / H per cobalt­(II) ion) value is 3.14 cm 3 mol –1 K (Figure ), lying in the expected range typically observed for octahedral cobalt­(II) ion in the HS state ( χT ≈ 2.1–3.4 cm 3 mol –1 K) with a significant orbital contribution. ,,,,, This value remains nearly constant up to 100 K. Below this temperature, the χT value decreases rapidly to 1.92 cm 3 mol –1 K at 1.85 K (Figure S11, log scale), which is probably due to the combination of magnetic anisotropy, spin–orbit coupling coming from a HS cobalt­(II) ion, and also weak antiferromagnetic intra- and intermolecular interactions between the HS cobalt­(II) centers.…”

“…The Co−N bond distances are very similar to those obtained in octahedral HS cobalt(II) complexes. 53,63,64,72,73 The N am −Co−N am angle 1 and Table S2). The average Co−N bond distance in 2 at 296 K is 2.080 Å with typical bond distance values of 2.320, 1.946, and 1.974 Å for Co−N am , Co−N Py , and Co−N tcm , respectively (Figure 6, Table 1, and Table S2).…”

“…The spectrum of 1 in DMF shows two broad bands centered at around 1030 and 550 nm and a shoulder at around 513 nm, which are ascribed to d−d transitions ( 4 T 1g → 4 T 2g ( 4 F), 4 T 1g → 4 T 1g ( 4 P), and 4 T 1g → 4 A 2g ( 4 F), respectively) in a HS cobalt(II) ion. 63,64,73 Complex 2 is in a LS/HS mixed state at room temperature (vide infra); the spectrum shows two broad bands centered at around 950 and 478 nm which are attributed to d−d transitions. Apart from the d−d transitions, the spectra of both complexes show a ligand to metal charge transfer (LMCT) transition, a metal to ligand charge transfer (MLCT) transition, and an interligand charge transfer (ILCT) transition at around 300−400 and 265 nm, respectively.…”