Iron(II) complexes of 2,6-di(1-alkylpyrazol-3-yl)pyridine derivatives – The influence of distal substituents on the spin state of the iron centre

Abstract

Dedicated to George Christou on the occasion of his 60 th birthday. 2 TOC EntryFour different complexes of the type shown, [Fe(L R ) 2 ] 2+ , have been prepared, with R = methyl, allyl, benzyl and isopropyl. All the compounds are high-spin in acetone solution and the solid state at room temperature and below, except for one salt of [Fe(L Me ) 2 ] 2+ which is predominantly low-spin at 150 K in the crystal. This contrasts with the parent complex (R = H), which exhibits thermal spin-crossover just below room temperature. KeywordsIron; N-donor ligands; Crystal Structure; Magnetic Measurements; Spin-Crossover

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

“…Unsubstituted [Fe(1-bpp)2] 2+ (Scheme 2) is SCO-active, with T½ = 248 K in (CD3)2CO [38], but [Fe(R2-1-bpp)2] 2+ are fully high-spin in the same solvent when R = Ph or iPr [39]. The same trend is shown by the isomeric 3-bpp complex series, in that [Fe(3-bpp)2] 2+ itself (Scheme 2) is SCO-active [5] but four [Fe(R2-3-bpp)2] 2+ derivatives with R ≠ H are all fully high-spin [40]. Like most derivatives of [Fe(terpy)2] 2+ , [Fe(terpy)2] 2+ itself is low-spin when R = H, but [Fe(Ph2terpy)2] 2+ undergoes SCO (T½ ≈ 300 K in EtCN solution) [41].…”

“…In each case, trends observed from solid state data on these compounds mirror the solution phase results [29][30][31][32]36,[38][39][40][41][44][45][46][47] [51,52] and other complexes in Scheme 4 [53,54] with sterically significant distal ligand substituents also exhibit stabilized high-spin states in solid phases. Interestingly, however, [Fe(Mes 2 -1-bpp) 2 ] 2+ presents an exception to the above discussion, being fully low-spin at room temperature despite the steric bulk of its distal mesityl substituents [39,51].…”

New Spin‐Crossover Complexes of Substituted 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…To achieve “true molecular” design with this family of ligands, more of them with different substituents are needed, especially those lacking the NH groups in the 1‐position that are behind the unwanted influence of the environment on the spin‐crossover. [7a] The search for such ligands, which is hampered by 1‐substituted 3‐bpp always producing SCO‐inactive iron(II) complexes in solutions,, [11e], is underway in our group.…”

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

“… 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 …”

“…Unfortunately, all the 3‐bpp ligands with substituted NH groups reported to date (Scheme ) gave HS iron(II) complexes . The only exception is a complex of methyl‐N,N’ ‐ disubstituted 3‐bpp that was suggested to undergo an SCO under certain conditions, but not in a solution. Various bulky groups at these positions of 1‐bpp also favour the HS state by forming a sterically crowded coordination environment around the metal ion, as is the common wisdom in the SCO research; salts with methyl‐, and hydroxymethyl‐disubstituted 1‐bpp featured a thermally‐induced SCO only in solids …”