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

Abstract

The self-assembly of a macrocyclic tetradentate ligand, cobalt(II) tetrafluoroborate, and nonlinear pseudohalides (dicyanamide and tricyanomethanide) has led to two cobalt(II) N′-di-tert-butyl-2,11diaza[3,3](2,6)pyridinophane; dca − = dicyanamido; tcm − = tricyanomethanido). Both complexes were characterized by single-crystal X-ray diffraction, spectroscopic, magnetic, and electrochemical studies. Structural analyses revealed that 1 displays a one-dimensional (1D) coordination polymer containing [Co-(L)] 2+ repeating units bridged by μ 1,5 -dicyanamido groups in cis positions, while 2 represents a discreate dinuclear cobalt(II) molecule bridged by two μ 1,5 -tricyanomethanido groups in a cis conformation. Both complexes have a CoN 6 coordination environment around each cobalt center offered by the tetradentate ligand and cis coordinating bridging ligands. Complex 1 exhibits a high-spin (S = 3/2) state of cobalt(II) in the temperature range of 2−300 K with a weak ferromagnetic coupling between two dicyanamido-bridged cobalt(II) centers. Interestingly, complex 2 exhibits reversible spin-state switching associated with spin−spin coupling. Complexes 1 and 2 also exhibit interesting redox-stimuli-based reversible paramagnetic high-spin cobalt(II) to diamagnetic low-spin cobalt(III) conversion, offering an additional way to switch magnetic properties. A detailed theoretical calculation was consistent with the stated results.

Reversible Photo- and Thermo-Induced Spin-State Switching in a Heterometallic {5d-3d} W2Fe2 Molecular Square Complex

“…Switchable molecular magnetic materials have been representing an amazing class of systems offering a large range of desired physical properties with enormous potential application in quantum science and technologies, , mainly those exhibiting magnetic, optical, and/or electric bistabilities, which includes spin crossover (SCO), − single molecule magnets (SMMs), − single chain magnets (SCMs), − metal-to-metal electron transfer (MMET), − etc. Switching of the magnetic properties in iron­(II)-based SCO materials originates from their reversible spin-state switching between a low-spin (LS) and high-spin (HS) state upon application of an external stimulus, e.g., temperature, pressure, light irradiation, and magnetic field, with a drastic alteration in electric and optical behaviors. − Importantly, SCO occurring at around or above room temperature is desirable for developing SCO-based electronics and spintronics devices amiable for practical use. ,,− …”

Effect of Ligand Modulation on Metal-to-Metal Electron Transfer in a Series of [Fe2Co2] Molecular Square Complexes

“…All these structural parameters suggest the presence of an {Fe III LS –CN–Co II HS } 2 square unit in 1 at 275 K, in 2 at 240 K, and in 3 at 280 K. − When cooled to 100 K ( 1 ) and 120 K ( 3 ), a slight decrease in average Fe–C and Fe–N bond distances was observed, 1.901 and 2.010 Å (for 1 ), 1.892 and 2.005 Å (for 3 ), whereas the average Co–N bond distance changes significantly to 2.013 Å (Δ = 0.095 Å) for 1 and to 1.984 Å (Δ = 0.1 Å) for 3 (Tables and S3). The average Co–N bond distance lies in the range between Co II HS (∼2.1 Å) and Co III LS (∼1.9 Å). ,,, For complex 2 , while cooled to 120 K, a relatively slight change in the Fe–C and Fe–N bond distances were observed, 1.915 and 2.000 Å, respectively. A moderate change in the Co–N bond distances was also observed with an average Co–N NC bond distance change from 2.08 Å at 120 K to 2.09 Å at 240 K. According to a bond distance analysis for 1 – 3 , it confirms the presence of mixed states of {Fe II LS –CN–Co III LS } 2 and {Fe III LS –CN–Co II HS } 2 units at 100 K for 1 and at 120 K for both 2 and 3 .…”

“…The average Co−N bond distance lies in the range between Co II HS (∼2.1 Å) and Co III LS (∼1.9 Å). 8,17,39,40 For complex The modest change in the bond distance of 2 can be explained as the steric effect between methyl groups of the Tp* ligand and the ethyl group of the bik* ligand, which acts to prevent each group from approaching the other while the complex is cooled to 120 K. This can be justified from the intramolecular centroid distances between the pyrazole ring of Tp* and the closest imidazole ring of bik (1), bik* (2), or vbik (3) ligands. For complexes 1 and 3, while the temperature is decreased, the intramolecular centroid distances decrease, whereas for complex 2 this distance remains almost the same (Figure S6 and Table S7).…”

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

“…Secondly, the active space of theoretical calculations was limited to only five d-orbitals, thus the ligand-based orbitals are The energies of the six lowest Kramers doublets were used for the analysis of the parameters of the Hamiltonian in Equation (1). Such a procedure we applied for the first time in the investigation of the above mentioned [Co(neo)(PhCOO) 2 ] polymorphs [34] and then also for other Co II complexes [60][61][62]. This procedure resulted in the values of α•λ, ∆ ax , ∆ rh , which are listed in Table 2 and graphically presented in Figure S9.…”