Reversible Redox, Spin Crossover, and Superexchange Coupling in 3d Transition‐Metal Complexes of Bis‐azinyl Analogues of 2,2′:6′,2′′‐Terpyridine

Abstract

The terpyridine-inspired tridentate ligand, 2,6bis(5,6-dialkyl-1,2,4-triazin-3-yl)-pyridine (BTP), was synthesized and utilized to isolate five [M II (BTP) 2 ] 2+ (M = Fe and Co) transition-metal complexes of various anions (BF 4 -, ClO 4 -, and NCS -). Notably, when metal-halide precursors are employed, 1:1 M/L products [Co 2 II (μ-X) 2 (BTP) 2 X 2 ] (X = Cl or Br) are obtained, exhibiting μ-halide bridges. All complexes were structurally characterized through single-crystal X-ray diffraction and their electrochemical and magnetic properties were investigated. Electrochemical studies reveal that the free BTP ligand has a single irreversible reduction, however; upon coordination to a metal center, stabilization of the reduced BTP ligand occurs and up to four reductive processes associated with the molecule are observed within the solvent window. The metal-centered redox [a]

Macrocycle-Induced Modulation of Internuclear Interactions in Homobimetallic Complexes

“…While 2 [Fe] and 2 [Co] demonstrate modestly lower values for M at 7 T than their 3 [M] and 4 [M] counterparts, the magnetization of 2 [Ni] remains near 0 N A μ B at 7 T, in sharp contrast to the values of 4.4 and 4.5 N A μ B observed in 3 [Ni] and 4 [Ni] at the higher field strength. The antiferromagnetic coupling observed at the more oblique angles in 2 [M] supports the expectation from prior reports that the nature of the coupling would be related to the M–Cl–M angle in these complexes. These data appear to generally follow the predicted relationships defined by the Goodenough–Kanamori rules for superexchange.…”

“…While 2 [Fe] and 2 [Co] demonstrate modestly lower values for M at 7 T than their 3 [M] and 4 [M] counterparts, the magnetization of 2 [Ni] remains near 0 N A μ B at 7 T, in sharp contrast to the values of 4.4 and 4.5 N A μ B observed in 3 [Ni] and 4 [Ni] at the higher field strength. The antiferromagnetic coupling observed at the more oblique angles in 2 [M] supports the expectation from prior reports 129 that the nature of the Table 5. Experimentally Determined χT and Effective Magnetic Moment (μ eff ) Data Observed at 300 K, along with the Best-Fit Data to the Full SQUID Data Sets for Spin Orbit Coupling (λ), Orbital Reduction Factor (σ), Landég-Factor (g), Coupling Constant (J), and Temperature-Independent Paramagnetism (TIP) for the Given Metal Spin States (S 1 and S 2 ) and Orbital Angular Momenta (L 1 and L 2 )…”

“…Goodenough and Kanamori noted in solid-state materials that both the nature (ferromagnetic vs antiferromagnetic) and magnitude of a magnetic interaction between two metal centers is connected to their M–X–M bond angle. Subsequent empirical work with molecular complexes across late 3d metals has related the orbital arguments in the Goodenough–Kanamori rules to molecular structures. − While empirically derived magnetochemical relationships have been proposed for dimeric molecular 3d metals with halogen bridges, ,,− it has been noted that systematic studies of magnetic interactions in molecular complexes are challenging as changes in angles, bond lengths, and metal–metal distances often result in concurrent changes to the coordination geometry around the metal. ,, Multiple exceptions have thus been observed for these empirically developed predictors. ,,, While universal rules for magnetic behavior in coordination complexes remain elusive, the series explored here can offer internal consistency to probe general effects related to geometric changes.…”

Spin Crossover in 3D Metal Centers Binding Halide-Containing Ligands: Magnetism, Structure and Computational Studies

“…Murugesu et al [159] studied octahedral Co(II) complexes with a terpyridine-inspired tridentate ligand, 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-pyridine (L55). They obtained the mononuclear complexes [Co(L55) 2 ] 2+ or the binuclear [Co 2 (µ-X) 2 (L55) 2 X 2 ] (X = Cl or Br), depending on reaction conditions (Figure 17).…”

Ultra-high Seebeck coefficient of a thermal sensor through entropic optimisation of ligand length of Fe(ii) spin-crossover (SCO) materials

“…The SQUID magnetometry elucidated a low spin state for Fe( ii ) complexes at room temperature, revealing a diamagnetic electronic structure. 34 …”