Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

Abstract

The tetrapyridyl ligand bbpya (bbpya=N,N-bis(2,2'-bipyrid-6-yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)2 ] (1) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai's domain model, and powder X-ray diffraction measurements, 1 is low-spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T1/2 =418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong NH⋅⋅⋅S intermolecular H-bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1. DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N'-di(pyrid-2-yl)-2,2'-bipyridine-6,6'-diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)2 ] (2). Periodic DFT calculations suggest that crystal-packing effects are significant for compound 2, in which they destabilize the HS state by about 1500 cm(-1) . The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.

Spin State Crossover, Vibrational, Computational, and Structural Studies of FeII 1‐Isopropyl‐1H‐tetrazole Derivatives

“…Fe–N–C (NCS) angles vs. Fe–N (NCS) bond lengths for the trans ‐[Fe II (L) 2 (NCS) 2 ] or trans ‐[Fe II (L) 4 (NCS) 2 ] complexes found for the published X‐ray structures from the CCDC database. Additionally, the results obtained from heptacoordinated systems described in ref .…”

Replacing Pyridine with Pyrazine in Molecular Cobalt Catalysts: Effects on Electrochemical Properties and Aqueous H2 Generation

“…Catalysts 1-4 are derived from the structure of the parent molecule O-CAT that our group previously found to be highly active and stable for H 2 generation from aqueous solutions [23]. The synthesis of O-CAT was achieved by palladium-catalyzed coupling of 6-amino-2,2 -bipyridine with 6-bromo-2,2 -bipyridine followed by metalation using Co(II) salts [24]. The synthesis of 1-4 followed a similar strategy of macrocycle formation, followed by metalation with Co(II), although the introduction of pyrazine groups necessitated an additional coupling step to complete the macrocycle.…”

The Effect of Ligand Design on Metal Ion Spin State—Lessons from Spin Crossover Complexes

“…Although there have been few systematic studies, an example is provided by a series of iron(II) complexes of a series of hexadentate ligands Another example of the same effect occurs in some trans-[Fe(NCS) 2 L] complexes, where L represents a series of meridional tetrapyridyl ligands which adopt helically twisted conformations when coordinated to a metal ion. While not all were crystallographically characterized, examples that might be expected to adopt more twisted L ligands are high-spin, or exhibit significantly reduced SCO T ½ values, in the solid state [63,64]. Comparably distorted ligand conformations, imposed by intermolecular steric contacts rather than by ligand design, may also be responsible for stabilization of the high-spin state in some other crystalline metal complexes [65][66][67][68][69][70].…”