Intramolecular Spin State Locking in Iron(II) 2,6-Di(pyrazol-3-yl)pyridine Complexes by Phenyl Groups: An Experimental Study

Abstract

Here we report a series of 1-phenyl-5-substituted 2,6-di(pyrazol-3-yl)pyridine complexes with iron(II) ion found in a high spin state in solids (according to magnetochemistry) and in solution (according to NMR spectroscopy), providing experimental evidence for it being an intramolecular effect induced by the phenyl groups. According to X-ray diffraction, the high spin locking of the metal ion is a result of its highly distorted coordination environment (with a very low ‘twist’ angle atypical of 2,6-di(pyrazol-3-yl)pyridine complexes), which remains this way in complexes with different substituents and counterions, in a diamagnetic zinc(II) analogue and in their solutions. Three possible reasons behind it, including additional coordination with the phenyl group, energy penalty incurred by its rotation or intramolecular stacking interactions, are addressed experimentally.

Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…Although all previously known iron(II) complexes of N,N’‐disubstituted 3‐bpp (Scheme ) are HS, in the resulting crystallosolvates [Fe( L Et ) 2 ](ClO 4 ) 2 ⋅ 5.5 THF and [Fe( L Cl ) 2 ](BF 4 ) 2 ⋅ 8 THF, an LS state is adopted, as judged by their red colour and the bond lengths Fe−N at 120 K (Table ) common for LS iron(II) complexes with heterocyclic N ‐donor ligands. The complex with the smallest fluorine substituent in [Fe( L F ) 2 ](ClO 4 ) 2 ⋅ 2.42 THF ⋅ H 2 O (F<Cl<Me<Et) is still HS at this temperature, with the coordination environment around the iron(II) ion distorted (Figure a) towards an edge‐bicapped tetrahedron, as in earlier reported HS [Fe( L H ) 2 ](OTf) 2 . If quantified by continuous shape measures, (Table , Figure S2), its distortions in [Fe( L Et ) 2 ](ClO 4 ) 2 ⋅ 5.5 THF, [Fe( L Cl ) 2 ](BF 4 ) 2 ⋅ 8 THF and [Fe( L F ) 2 ](ClO 4 ) 2 ⋅ 2.42 THF ⋅ H 2 O are typical for 3‐bpp complexes of iron(II) in the LS and HS state, respectively.…”

“…The results of these measurements showed the fluorine derivative to remain HS in this entire temperature range, with a drop in χT below 50 K arising from zero‐field splitting . Above 50 K, the χT value is almost constant at 3.6 cm 3 mol −1 K, mirroring the magnetic susceptibility behaviour of the HS complex [Fe( L H ) 2 ](OTf) 2 with an even smaller R substituent, a hydrogen atom (H<F<Cl<Me<Et) . For [Fe( L R ) 2 ](ClO 4 ) 2 with larger substituents (Figure ), however, χT slowly decreases upon cooling, which is a clear sign of a gradual, nonhysteretic SCO often found in amorphous materials .…”

“… Variable‐temperature magnetic susceptibility data for fine‐crystalline samples of [Fe( L F ) 2 ](ClO 4 ) 2 (•) and [Fe( L Et ) 2 ](ClO 4 ) 2 (▪) according to the dc ‐magnetometry; the red line represents the fit with the following parameters: g iso =2.2, D=8.8 cm −1 . The arrows indicate the temperature change upon the measurements (cooling 1—heating 2—cooling 3).…”

“…A substitution by phenyl groups results in HS iron(II) complexes of both 1‐bpp and 3‐bpp and of a related tridentate ligand, 2,6‐bis(2‐pyridyl)pyridine (terpy) ([Fe(terpy) 2 ] 2+ itself is LS), although for a different reason . These groups ‘block’ an SCO to the LS state by a significant energy penalty of their rotation (and an associated loss of conjugation with the pyrazolyl moieties) to avoid clashing with the second ligand in a hypothetical LS state .…”

“… R‐Disubstituted tridentate ligands in the previously reported iron(II) complexes: 1‐bpp with R=methyl (Me), phenyl (Ph), iso ‐propyl ( i Pr), ethoxycarbonyl (CO 2 Et), hydroxymethyl (CH 2 OH), ferrocenyl (FeCp 2 ), and mesityl (Mes) groups; 3‐bpp with R=methyl (Me), phenyl (Ph), iso ‐propyl ( i Pr), benzyl (Bz) and allyl (All) groups; 1,3‐bpp and terpy with R=phenyl (Ph), 4‐methoxyphenyl (Ph−4‐OMe), p ‐tolyl (pTol) and methyl (Me) groups, bromine (Br), chlorine (Cl) and fluorine (F) atoms. An inset: ligands L R in this study with R=fluorine (F), chlorine (Cl), methyl (Me), ethyl (Et) and iso ‐propyl ( i Pr) substituents; L H was reported earlier …”