Evidence of crystal packing effects in stabilizing high or low spin states of iron(II) complexes with functionalized 2,6-bis(pyrazol-1-yl)pyridine ligands (original) (raw)
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2021
The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH2) is described. The complex salts [Fe(bppCONH2)2](BF4)2 and [Fe(bppCONH2)2](ClO4)2 were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH2)2](BF4)2 indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ at 445 K with a 14 K hysteresis. For the BF4- salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF4 salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences ...
European Journal of Inorganic Chemistry, 2013
Keywords: Spin crossover / Iron / N ligands / Magnetic properties / Moessbauer spectroscopy A mononuclear iron(II) compound 1 of the general formula [Fe(L) 2 ](ClO 4 ) 2 {L = 4-[2,6-bis(pyrazol-1-yl)pyridin-4-yl]-benzaldehyde} was prepared and structurally characterised. Single-crystal X-ray structure analysis revealed the presence of a complex dication [Fe(L) 2 ] 2+ and two ClO 4 counteranions within the unit cell. The bond lengths and angles within the coordination polyhedron FeN 6 indicate the low-spin state of the central iron(II) metal ion at T = 180 K. Magnetic investigations elucidate spin crossover with T 1/2 = 285 K. The experimental magnetic susceptibility data could be satisfactorily [a]
Coordination of Pyridine-Substituted Pyrazoles and Their Influence on the Spin State of Iron(II)
Australian Journal of Chemistry, 1988
Iron(II) and nickel(II) [MN6]X2 type complexes have been prepared from 2-(pyrazol-1-yl]pyridine (1pp), 2-(pyrazol-1-yl) imidazoline (pi), 2- (pyrazol-3-yl)pyridine (3pp) and 2,6-bis(pyrazol-3-yl)pyridine ( bpp ). Variable-temperature magnetic and Mossbauer spectral studies establish that [Fe(1pp)3]X2 is low spin and [Fe(pi)3]X2 is high spin over an extended temperature range, while both [Fe(3pp)3]X2 and [Fe( bpp )2]X2 undergo temperature-induced low-spin ↔ high-spin transitions. The nature of the transition depends on the extent of hydration and for salts of both cations the singlet state is generally stabilized as the extent of hydration increases. Hydrogen bonding effects are believed to be responsible for this. For anhydrous [Fe( bpp )2] [BF4]2 the transition is discontinuous and associated with hysteresis with Tc ↓ 173 K for decreasing temperature and Tc ↑ 183 K for increasing temperatures. The transition to the singlet state species is complete at low temperatures provided that...
Spin Transition in a Chainlike Supramolecular Iron(II) Complex
Inorganic Chemistry, 2006
A one-dimensional supramolecular head-to-tail N + −H‚‚‚N-type hydrogen-bonded chain of the complex [Fe II (L) 2 H](ClO 4 ) 3 ‚MeOH [L ) 4′-(4′′′-pyridyl)-1,2′:6′1′′-bis(pyrazolyl)pyridine] exhibits a reversible, thermally driven spin transition at 286 K with a hysteresis loop of ca. 2 K.
Inorganic Chemistry Communications, 2005
The complex [Fe(tzimpy) 2 ](ClO 4 ) 2 AE 2H 2 O (tzimpy = 2,4,6-tris-(benzimidazol-2-yl)pyridine) shows an abrupt spin crossover (S = 0-2 transition) above room temperature centered at T c = 323 K with a hysteresis width of DT = 35 K. The neutral iron(II) complex with deprotonated bzimpy ligands (bzimpy = 2,6-bis(benzimidazol-2-yl)pyridine) exhibits a gradual spin transition on the first heating with T c = 424 K. There are irreversible changes between T = 503 and 523 K: the liberation of the crystal water, the color change (blue-green) followed by a structure change. Next thermal cycles are reproducible though, heating/cooling paths are different from the first heating.
Monatshefte f�r Chemie - Chemical Monthly, 1994
bis- (benzimidazol-2'-yl)pyridine and X = H, OH, C1] show thermally accessible spin-crossover behaviour in solution that depends on both the ligand and the solvent. 1H-NMR spectroscopy and UV-visible spectroscopy measurements suggest that ligand substituent effects, solvent donor-acceptor properties and hydrogen-bonding may be employed to "fine-tune" the ligand field strength and hence to affect the spin-crossover behaviour. The ligand substitution changes in solution are reflected by the magnetic data (X = H:l~exp = 2.50 gB; X = OH:/lexp = 4.20 ttB and X = Cl://exp = 4.30 ~l B at 294 K in MeOH), and by the shift of metal-to-ligand charge-transfer band (X = H, 2 = 557 rim; X = OH, 2 = 520 rim; X = C1, 2 = 500 rim). [Fe(bzimpy)2](C104) 2 exhibits a pronounced spin-crossover equilibrium (1A1 ~ ST2) in solution (Ksc = 0.26 at 293 K; l~exp = 1.30--, 3.40/1B for 213 ~ 328 K in MeOH).
Dalton Transactions, 2009
The syntheses of 2,6-bis(4-chloropyrazol-1-yl)pyridine (L 1 ), 2,6-bis(4-bromopyrazol-1-yl)pyridine (L 2 ) and 2,6-bis(4-iodopyrazol-1-yl)pyridine (L 3 ) by electrophilic halogenation of 2,6-bis(pyrazol-1-yl)pyridine are reported. The complex [Fe(L 1 ) 2 ][BF 4 ] 2 crystallises in two different solvent-free polymorphs. The tetragonal (a) form crystallises in a known version of the "terpyridine embrace" structure, and undergoes an abrupt spin-transition at 202 K. The orthorhombic (b) 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 a-polymorph is transmitted in two dimensions within the extended lattice. [Fe(L 2 ) 2 ][BF 4 ] 2 is isostructural with a-[Fe(L 1 ) 2 ][BF 4 ] 2 and exhibits a similarly abrupt spin-transition at 253 K. In contrast, [Fe(L 3 ) 2 ][BF 4 ] 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) = T 0 -0.3T 1/2 (T 0 = 150 K) law that we have previously proposed for this class of compound.
Spin-state splittings of iron(II) complexes with trispyrazolyl ligands
Polyhedron, 2010
We report a computational study at the OPBE/TZP level on the chemical bonding and spin ground-states of mono-nuclear iron(II) complexes with trispyrazolylborate and trispyrazolylmethane ligands. We are in particular interested in how substitution patterns on the pyrazolyl-rings influence the spin-state splittings, and how they can be rationalized in terms of electronic and steric effects. One of the main observations of this study is the large similarity of the covalent metal-ligand interactions for both the borate and methane ligands. Furthermore, we find that the spin-state preference of an individual transitionmetal (TM) complex does not always concur with that of an ensemble of TM-complexes in the solid-state. Finally, although the presence of methyl groups at the 3-position of the pyrazolyl groups leads to ligandligand repulsion, it is actually the loss of metal-ligand bonding interactions that is mainly responsible for shifts in spin-state preferences. (M. Swart). Scheme 1. General structure of [R n B(pz) 4Àn ] À , where n can be 0, 1 or 2, pz is a pyrazol-1-yl group and R can be H, an alkyl or aryl group.
Inorganic Chemistry, 2005
The new ligands Na[(p-IC 6 H 4 )B(3-Rpz) 3 ] (R ) H, Me) have been prepared by converting I 2 C 6 H 4 to IC 6 H 4 SiMe 3 with Li t Bu and SiMe 3 Cl, and then to IC 6 H 4 BBr 2 with BBr 3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaO t Bu. These new ligands react with FeBr 2 to give either purple, low-spin Fe[(p-IC 6 H 4 )B(pz) 3 ] 2 or colorless, high-spin Fe[(p-IC 6 H 4 )B(3-Mepz) 3 ] 2 . Depending upon the crystallization conditions, Fe[(p-IC 6 H 4 )B(3-Mepz) 3 ] 2 can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by variable-temperature X-ray crystallography, magnetic susceptibility, and Mo ¨ssbauer spectroscopy has revealed different electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing, independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC 6 H 4 )B-(3-Mepz) 3 ] 2 has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC 6 H 4 )B(3-Mepz) 3 ] 2 has a stacked two-dimensional supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure of the methylene chloride solvate of Fe[(p-IC 6 H 4 )B(3-Mepz) 3 ] 2 has a similar stacked two-dimensional supramolecular structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the longrange solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions, has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spincrossover temperatures and more abrupt spin-crossover behavior.
Ligand-Driven Spin-Crossover Behavior of FeII Molecules
MATERIALS TRANSACTIONS, 2016
In order to explore a way to tailor thermal hysteresis behavior of spin-crossover (SCO) complexes, a series of seven Fe II (LX 2) complexes with different ligand configurations has been designed or reconstructed. These Fe II (LX 2) complexes differ in axial ligands X = Py, CNPY, NC 5 H 4 CH 3 , NC 5 H 4 OCH 3 , NC 5 H 4 Cl, X = NC 5 H 4 Br, and Him. Geometric structure, electronic structure, and magnetic properties of Fe II (LX 2) complexes have been investigated using density-functional theory with full geometry optimization. Our calculated results show that the spinstate electrostatic-energy difference (¦U) of these Fe II SCO complexes can be tailored by adjusting the pK a constant of axial ligands X. The increase of ¦U of these Fe II SCO complexes from ¹12.18 eV to 6.64 eV results from the increase of pK a constant of axial ligands X from 1.10 to 7.00. The role of axial ligands X in determining SCO behavior of Fe II SCO complexes has been revealed. In addition, our previous study (N. A. Tuan: J. Appl. Phys. 111 (2012) 07D101) demonstrated that thermal hysteresis of spin-crossover increased with the ¦U of Fe II SCO complexes. These results would give some hints into how thermal hysteresis can be tailored in Fe II SCO complexes.