Strong Cooperative Spin Crossover in 2D and 3D Fe^II–M^I,II Hofmann-Like Coordination Polymers Based on 2-Fluoropyrazine

Abstract

Self-assembling iron(II), 2-fluoropyrazine (Fpz), and [M(CN)] (M = Ni, Pd, Pt) or [Au(CN)] building blocks have afforded a new series of two- (2D) and three-dimensional (3D) Hofmann-like spin crossover (SCO) coordination polymers with strong cooperative magnetic, calorimetric, and optical properties. The iron(II) ions, lying on inversion centers, define elongated octahedrons equatorially surrounded by four equivalent centrosymmetric μ-[M(CN)] groups. The axial positions are occupied by two terminal Fpz ligands affording significantly corrugated 2D layers {Fe(Fpz)([M(CN)]}. The Pt and Pd derivatives undergo thermal- and light-induced SCO characterized by T temperatures centered at 155.5 and 116 K and hysteresis loops 22 K wide, while the Ni derivative is high spin at all temperatures, even at pressures of 0.7 GPa. The great stability of the high-spin state in the Ni derivative has tentatively been ascribed to the tight packing of the layers, which contrasts with that of Pt and Pd derivatives in the high- and low-spin states. The synthesis and structure of the 3D frameworks formulated {Fe(Fpz)[Pt(CN)]}·1/2HO and {Fe(Fpz)[Au(CN)]}, where Fpz acts as bridging ligand, which is also discussed. The former is high spin at all temperatures, while the latter displays very strong cooperative SCO centered at 243 K accompanied by a hysteresis loop 42.5 K wide. The crystal structures and SCO properties are compared with those of related complexes derived from pyrazine, 3-fluoropyridine, and pyridine.

Increasing spin crossover cooperativity in 2D Hofmann-type materials with guest molecule removal

“…Notably, both of these dehydrated materials show ambient temperature spin state switching with thermal hysteresis loops that are by far the widest reported for any 2D framework material and which are commensurate with the widest achieved in 3D Hofmann-type materials. 13 , 20 , 21 Interestingly, despite the hydrated Pd and Pt phases showing near identical SCO behaviours with only marginal differences in their transition temperatures, their respective dehydrated phases show a clear distinction in both transition temperature and hysteresis loop width; the reason for this is unknown but may relate to variation in sample quality since 1(Pt) shows significantly reduced crystallinity compared to 1(Pd) (Fig. S17 and S21 † ).…”

Cyanido‐Bridged FeII–MI Dimetallic Hofmann‐Like Spin‐Crossover Coordination Polymers Based on 2,6‐Naphthyridine

“…Although {Fe(pz)[Ag I (CN) 2 ] 2 } displays a gradual and incomplete transition with hysteresis above 300 K, [8a] the homologous Au I derivative displays a cooperative SCO between 367 and 349 K. [8b] Even more cooperative SCO between 261 and 223 K has been observed for the corre-sponding Au I derivative with 2-fluoropyrazine (Fpz). [9] Notably, for longer bridging ligands such as 3,6-bis(4-pyridyl)-1,2,4,5tetrazine (bipytz), [10] 1,4-bis(pyridin-4-ylmethyl)piperazine (bpmp), [11] 2,5-bis(pyrid-4-yl)pyridine (bpp), [12] or 1,4-di(pyridin-4-yl)benzene (dpb, [13] Scheme 1) and M I = Au, interesting inclusion chemistry and cooperative SCO behavior have been described despite the interpenetration of the networks. In summary, the use of different organic ligands and cyanometallate anions has resulted in a Hofmann-type family, which exhibits customized SCO behavior coupled with other interesting properties.…”

Synthesis, structure, and magnetic properties of a dinuclear antiferromagnetically coupled iron(II) complex

“…The great interest in SCO complexes is associated with their potential applications as data storage media, molecular switches or guest dependent sensors [7][8][9]. Although much work has been published in this area there are still many problems to be solved including, for instance, those involving wide hysteresis loops at room temperature [10], multi-step hysteresis [11], multifunctional SCO [12] and how these properties correlate with structure [13][14][15].…”