A Survey of the Angular Distortion Landscape in the Coordination Geometries of High-Spin Iron(II) 2,6-Bis(pyrazolyl)pyridine Complexes

Abstract

Reaction of 2,4,6-trifluoropyridine with sodium 3,4dimethoxybenzenethiolate and 2 equiv of sodium pyrazolate in tetrahydrofuran at room temperature affords 4-(3,4-dimethoxyphenylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine (L), in 30% yield. The iron(II) complexes [FeL 2 ][BF 4 ] 2 (1a) and [FeL 2 ][ClO 4 ] 2 (1b) are high-spin with a highly distorted six-coordinate geometry. This structural deviation from ideal D 2d symmetry is common in highspin [Fe(bpp) 2 ] 2+ (bpp = di{pyrazol-1-yl}pyridine) derivatives, which are important in spin-crossover materials research. The magnitude of the distortion in 1a and 1b is the largest yet discovered for a mononuclear complex. Gas-phase DFT calculations at the ω-B97X-D/6-311G** level of theory identified four minimum or local minimum structural pathways across the distortion landscape, all of which are observed experimentally in different complexes. Small distortions from D 2d symmetry are energetically favorable in complexes with electron-donating ligand substituents, including sulfanyl groups, which also have smaller energy penalties associated with the lowest energy distortion pathway. Natural population analysis showed that these differences reflect greater changes to the Fe−N{pyridyl} σ-bonding as the distortion proceeds, in the presence of more electron-rich pyridyl donors. The results imply that [Fe(bpp) 2 ] 2+ derivatives with electron-donating pyridyl substituents are more likely to undergo cooperative spin transitions in the solid state. The high-spin salt [Fe(bpp) 2 ][CF 3 SO 3 ] 2 , which also has a strong angular distortion, is also briefly described and included in the analysis.

Activating a high-spin iron(ii) complex to thermal spin-crossover with an inert non-isomorphous molecular dopant

“…The Δ E (dist) values for [Fe(bpp) 2 ] 2+ by this protocol are <1 kcal mol −1 , 28 which is consistent with previous studies of this type using different functionals. 26,27,68–70…”

“…To identify a suitable dopant species, a gas phase DFT study was undertaken at the ω-B97X-D/6-311G** level (we used the same protocol in a recent survey of the structural preferences of the angular distortion). 28 Different [M(bpp) 2 ] 2+ complexes were minimised at stages along the angular distortion, by fixing the trans -N{pyridyl}-M-N{pyridyl} angle ( ϕ ) to values between 165 ≤ ϕ ≤ 155°; 26,63 the lower limit is the experimental value of ϕ in [Fe(bpp) 2 ][ClO 4 ] 2 . 35 The metal ions listed below were examined because salts of those [M(bpp) 2 ] 2+ complexes are known to be isomorphous with one of the iron complex phases shown in Fig.…”

“…Solid [Fe(bpp) 2 ][PF 6 ] 2 is high-spin, and crystallises in the C 2/ c crystal phase. 26 A crystal structure of [Fe(bpp) 2 ][CF 3 SO 3 ] 2 in the C 2/ c phase is also available, 28 although bulk samples of that salt contain a mixture of phases by powder diffraction (Fig. S16†).…”

“…2,6-Bis(pyrazol-1-yl)pyridine (bpp), 97 [Fe(bpp) 2 ][ClO 4 ] 2 , 35 [Fe(bpp) 2 ][PF 6 ] 2 26 and [Fe(bpp) 2 ][CF 3 SO 3 ] 2 28 were prepared by the published procedures.…”

“…16 The distortion is energetically facile, and a range of distorted geometries are computationally accessible to [Fe(bpp) 2 ] 2+ derivatives around room temperature. 26–28 Hence, the observation of distorted vs. undistorted geometries in a particular crystalline compound is dictated by packing considerations. 29 Materials containing distorted molecules are often inactive towards SCO, since conversion of distorted high-spin to undistorted low-spin molecular structures is kinetically inhibited by the solid lattice.…”