Thermal and light-induced spin-transitions in iron(ii) complexes of 2,6-bis(4-halopyrazolyl)pyridines: the influence of polymorphism on a spin-crossover compound

Abstract

The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L1), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L2) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L3) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L1)2][BF4]2 crystallises in two different solvent-free polymorphs. The tetragonal (alpha) form crystallises in a known version of the "terpyridine embrace" structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (beta) form exhibits a modified form of the same packing motif, containing two unique iron sites in a 2 : 1 ratio. One-third of the complex molecules in that material undergo a very gradual thermal spin-crossover centred at 137 K. Comparison of the two structures implies that spin-crossover cooperativity in the alpha-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L2)2][BF4]2 is isostructural with alpha-[Fe(L1)2][BF4]2 and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L3)2][BF4]2 is low-spin as a powder at 360 K and below and can be crystallised as two different solvates from acetone solution. All three compounds exhibit the LIESST effect at 10 K, with photoconversions of 40-100%. Their LIESST relaxation temperatures obey the empirical T(LIESST) = T0- 0.3T(1/2) (T0 = 150 K) law that we have previously proposed for this class of compound.

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

“…The measured SCO enthalpy is ca. 50 % smaller than for other compounds of the [Fe(bpp)2] 2+ type, [20,23,27] which is consistent with the suggestion that the abrupt part of the spin-transition in 1 only involves a fraction of the iron centers in the sample (see above). Figure 6.…”

“…For all complexes, a drastic increase in the magnetic signal under green light irradiation was observed at 10 K. However, unusually for [Fe(bpp)2] 2+ derivatives, [7,20,[28][29][30] the photoconversion efficiency was not quantitative. The maximum MT values indicate photoconversion efficiencies of 70 % (for 1), 57 % (2) and 60 % (3).…”

“…[20,28] T(LIESST) = T0 -0.3T½ (1) However, although the differences are small, it is noteworthy that within this series 1 has both the lowest T1/2 and the lowest T(LIESST) values, in apparent violation of eq 1 and Hauser's inverse energy gap law.…”

“…[7,20,[28][29][30] The low spin high spin photoconversion was investigated on 1-3 in bulk condition using a SQUID magnetometer coupled to a CW optical source. The samples were irradiated at the following wavelengths: 405, 510, 640, 830 and 980 nm.…”

“…The complex cations pack in layers in the crystals, related to the "terpyridine embrace" lattice that is often adopted by salts of [Fe(bpp)2] 2+ derivatives. [18][19][20] The cation layers in the full embrace structure have strict or approximate four-fold symmetry, with each molecule interdigitating to four nearest neighbors via face-to-face and edge-to-face - interactions between their pyrazolyl rings.…”