Structural, electronic, and magnetic properties of (Fe[HC(3,5-Me2pz)3]2)(BF4)2 (pz = pyrazolyl): observation of unusual spin-crossover behavior

Interplay of Spin Crossover and Coordination-Induced Spin State Switch for Iron Bis(pyrazolyl)methanes in Solution

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Mößbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N₅O} to {N₆} and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Mößbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.

Spin Crossover in New Iron(II) Coordination Compounds with Tris(pyrazol-1-yl)Methane

We review here new advances in the synthesis and investigation of iron(II) coordination compounds with tris(pyrazol-1-yl)methane and its derivatives as ligands. The complexes demonstrate thermally induced spin crossover accompanied by thermochromism. Factors that influence the nature and temperature of the spin crossover are discussed.

Selective Isomer Formation and Crystallization-Directed Magnetic Behavior in Nitrogen-Confused C-Scorpionate Complexes of Fe(O3SCF3)2

The complex [Fe(^HL*)₂](OTf)₂, 1, where ^HL* = bis(3,5-dimethylpyrazol-1-yl)(3-1H-pyrazole)methane, was prepared in order to compare its magnetic properties with those of the analogous parent complex, [Fe(^HL)₂](OTf)₂, that lacks methyl groups on pyrazolyl rings and that undergoes spin crossover (SCO) from the low spin (LS) to the high spin (HS) form above room temperature. It was anticipated that this new semibulky derivative should favor the HS state and undergo SCO at a lower temperature range. During this study, six crystalline forms of 1 were prepared by controlling the crystallization conditions. Thus, when reagents are combined in CH₃CN, an equilibrium mixture of cis and trans isomers is established that favors the latter below 311 K. The trans isomer can be isolated exclusively as a mixture of solvates, LS trans-1·2CH₃CN and HS trans-1·4CH₃CN, by cooling CH₃CN solutions to -20 °C with the former being favored at high concentrations and short crystallization times. Subsequently, vapor diffusion of Et₂O into CH₃CN solutions of pure trans-1·2CH₃CN gives solvate-free HS trans-1. Subjecting trans-1·2CH₃CN to vacuum at room temperature gives microcrystalline trans-1·CH₃CN, identified by elemental analysis and its distinct powder X-ray diffraction pattern. If an isomeric mixture of 1 is subject to room-temperature vapor diffusion, then a crystalline mixture of HS isomers cis-1 and trans-1 is obtained. Finally, slowly cooling hot acetonitrile solutions of isomeric mixtures of 1 to room temperature gives large prisms of HS co-1, a species with both cis and trans isomers in the unit cell. The complexes trans-1, trans-1·CH₃CN, cis-1, and co-1 undergo SCO below 250 K while trans-1·xCH₃CN (x = 2, 4) solvates do not undergo SCO before desolvation.

Trigonal antiprismatic Co(ii) single molecule magnets with large uniaxial anisotropies: importance of Raman and tunneling mechanisms

A trigonal antiprismatic Co(ii) molecule exhibits counterion-dependent relaxation of the magnetization due to the importance of the Raman relaxation process.

The air-stable mononuclear Co(ii) compounds [Co^II(Tpm)₂][ClO₄]₂ (1, Tpm = tris(pyrazol-1-yl)methane), [Co^II(Tpm)₂][BPh₄]₂·2MeCN (2) with trigonal antiprismatic geometry (trigonally elongated octahedral geometry) are reported. Magnetic and theoretical studies reveal that the complexes exhibit single-molecule magnet behavior with uniaxial anisotropy and a huge energy difference between ground and first excited Karmers' doublets (∼200 cm^–1). Under applied DC fields, compounds 1 and 2 exhibit frequency and temperature dependence of the imaginary susceptibility. The fit of the data to an Orbach relaxation process yields effective energy barriers of 30.6(1) and 44.7(6) cm^–1 for 1 and 2, respectively, but there is no real state at that energy. The inclusion of tunneling, direct and Raman relaxation processes leads to the conclusion that the inclusion of an Orbach process is not required to provide a good fit to the data. More interestingly, a detailed study of the dependence of the relaxation time with field shows that for these Kramers' ions, tunneling is the predominant process at low temperature and that differences in the counteranion allow for a tuning of the Raman process at higher temperatures. These findings underscore the fact that large uniaxial anisotropy can be achieved in hexacoordinate Co(ii) trigonal antiprismatic complexes which is an unexplored geometry in mononuclear single molecule magnets.

Distortion Pathways of Transition Metal Coordination Polyhedra Induced by Chelating Topology

A continuous shape measures analysis of the coordination polyhedra of a host of transition metal complexes with bi- and multidentate ligands discloses the distortion pathway associated with each particular topology of the chelate rings formed. The basic parameter that controls the degree of distortion is the metal-donor atom bond distance that induces nonideal bond angles due to the rigidity of the ligands. Thus, the degree of distortion within each family of complexes depends on the atomic size, on which the high- or low-spin state has a large effect. Special attention is therefore paid to several spin-crossover systems and to the enhanced distortions that go along with the transition from low- to high-spin state affected by temperature, light, or pressure. Several families of complexes show deviations from the expected distortion pathways in the high-spin state that can be associated to the onset of intermolecular interactions such as secondary coordination of counterions or solvent molecules. Also, significant displacement of counterions in an extended solid may result from the changes in metal-ligand bond distances when ligands are involved in intermolecular hydrogen bonding.

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).

Coordination, organometallic and related chemistry of tris(pyrazolyl)methane ligands

Tris(pyrazolyl)methanes are the neutral analogues of the widely exploited and highly useful tris(pyrazolyl)hydroborates, yet by comparison with their boron based counterparts their chemistry is underdeveloped. Recent breakthroughs in the synthesis of ring-substituted tris(pyrazolyl)methanes offer the opportunity for the development of this useful and promising class of ligand. This review summarises the current state of the coordination and organometallic chemistry of tris(pyrazolyl)methanes and highlights areas in which development is likely to occur.

Coordination chemistry of a terpyridine-tris(pyrazolyl) ditopic ligand

A new ditopic ligand, 4'-(4-(2,2,2-tris(1H-pyrazol-1-ido)ethoxymethyl)phenyl)-2,2':6',2''-terpyridine (pzt), has been prepared and its coordination chemistry studied. Metal ions with a preference for octahedral geometry form ML(2) complexes that are readily isolated and characterised, with the metal ion being bound to the terpyridine sites of both ligands. Other metal ions bind to the terpyridine site of just one ligand. In the case of silver(i), a dinuclear M(2)L(2) complex has been isolated in which each silver ion is coordinated to the terpyridine site of one ligand and to a single pyrazolyl donor group from the second ligand. Evidence for binding of metal ions to the tris(pyrazolyl) binding site was obtained by electrospray mass spectrometry and NMR techniques. The free ligand and three metal complexes, including the disilver complex, have been characterised by X-ray crystallographic techniques.

Low- and high-spin iron (II) complexes studied by effective crystal field method combined with molecular mechanics

A computational method targeted to Werner-type complexes is developed on the basis of quantum mechanical effective Hamiltonian crystal field (EHCF) methodology (previously proposed for describing electronic structure of transition metal complexes) combined with the Gillespie-Kepert version of molecular mechanics (MM). It is a special version of the hybrid quantum/MM approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving out the effects of the d-shell. The quantum mechanical EHCF part is limited to the metal ion d-shell. The method reproduces with reasonable accuracy geometry and spin states of the Fe(II) complexes with monodentate and polydentate aromatic ligands with nitrogen donor atoms. In this setting a single set of MM parameters set is shown to be sufficient for handling all spin states of the complexes under consideration.

Influences of changes in multitopic tris(pyrazolyl)methane ligand topology on silver(I) supramolecular structures

The reactions between silver tetrafluoroborate and the ligands 1,2,4,5-C(6)H(2)[CH(2)OCH(2)C(pz)(3)](4) (L1, pz = pyrazolyl ring), o-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2) (L2), and m-C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2) (L3) yield coordination polymers of the formula (C(6)H(6)(-)(n)[CH(2)OCH(2)C(pz)(3)](n)(AgBF4)(m))( infinity ) (n = 4, m = 2, 1; n = 2, ortho substitution, m = 1, 2; meta substitution, m = 2, 3). In the solid state, L2 molecules dimerize by a pair of C-H.pi interactions, forming an arrangement that resembles the tetratopic ligand L1. In the solid-state structure of 1, each silver atom is kappa(2)-bonded to two tris(pyrazolyl)methane units from different ligands with the overall structure a polymer made up from 32-atom macrocyclic rings formed by bonding tris(pyrazolyl)methane groups from nonadjacent positions on the central arene rings to the same two silver atoms. In 2, each silver is bonded to two tris(pyrazolyl)methane units in the same kappa(2)-kappa(2) fashion as with 1, forming a polymer chain. The chains are organized into dimeric units by strong face-to-face pi-pi stacking between the central arene rings making bitopic L2 act as half of tetratopic L1. The chains in both structures are organized by weak C-H.F hydrogen bonds and pi-pi stacking interactions into very similar 3D supramolecular architectures. The structure of 3 contains three types of silvers with the overall 3D supramolecular sinusoidal structure comprised of 32-atom macrocycles. Infrared studies confirm the importance of the noncovalent interactions. Calculations at the DFT (B3LYP/6-31G) level of theory have been carried out on L2 and also support C-H.pi interactions. Electrospray mass spectral data collected from acetone or acetonitrile show the presence of aggregated species such as [(L)Ag(2)(BF(4))](+) and [(L)Ag(2)](2+), despite the fact that (1)H NMR spectra of all compounds show that acetonitrile completely displaces the ligand whereas acetone does not.

A high-pressure iron K-edge x-ray absorption spectral study of the spin-state crossover in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2)

The room temperature iron K-edge X-ray absorption near edge structure spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) have been measured between ambient and 88 and 94 kbar, respectively, in an opposed diamond anvil cell. The iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2)undergoes the expected gradual spin-state crossover from the high-spin state to the low-spin state with increasing pressure. In contrast, the iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) remains high-spin between ambient and 78 kbar and is only transformed to the low-spin state at an applied pressure of between 78 and 94 kbar. No visible change is observed in the preedge peak in the spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) with increasing pressure, whereas the preedge peak in the spectra of ((e[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) changes as expected for a high-spin to low-spin crossover with increasing pressure. The difference in the spin-state crossover behavior of these two complexes is likely related to the unusual behavior of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) upon cooling.

Solid-state structural and magnetic investigations of [M[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (M = Fe, Co, Ni, Cu): observation of a thermally induced solid-state phase change controlling an iron(II) spin-state crossover

The reaction of M(BF(4))(2).xH(2)O (M = Co, Ni, and Cu) and HC(3,5-Me(2)pz)(3) in a 1:2 ratio yields [Co[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (2), [Ni[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (3), and [Cu[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (4). Over the temperature range from 5 to 350, 345, or 320 K, Curie law behavior is observed for microcrystalline samples of all three compounds showing them to have three, two, and one unpaired electrons, respectively, with no spin-crossover observed for 2. Crystalline samples of these compounds torque in the applied magnetic field the first time the sample is cooled to 5 K. The solid-state structures of all three are isomorphous at 220 K, monoclinic in the space group C2/c. The metal is located on a unique crystallographic site and has a trigonally distorted octahedral structure, with 4 showing the expected Jahn-Teller distortions. Cooling crystals of all three to low temperatures leads to the observation of the same phase change to triclinic in the new space group P(-)1 with nonmerohedral twinning. This change is reversible and yields two crystallographically unique metal sites at low temperature. The bond angles and distances for the two different metal sites for each compound in the low temperature structures are very similar to each other and to those in the 220 K structures. The same phase change, monoclinic to triclinic, has been observed previously for [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (1), except in this case, the phase change results in half of the cations changing over from the high-spin state to the low-spin state while the other half of the cations remain high-spin, with the low-spin form decreasing its Fe-N bond distances by 0.19 A. The new results with 2-4 show that it is the phase transition, which occurs in complexes of the type [M[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) with first row transition metals, that is driving the unusual spin-crossover behavior of [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2).

Lattice-dictated conformers in bis(pyrazolyl)pyridine-based iron(II) complexes: Mössbauer, NMR, and magnetic studies

Iron(II) complexes [FeL(2)](ClO(4))(2).CH(3)CN, [FeL(2)](BPh(4))(2).2CH(3)CN, and [FeL(2)](PF(6))(2) with an FeN(6) chromophore of the same ligand L (2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine) and differing counterions have been made and their crystal and molecular structures determined. The first two crystallized in triclinic space group P(-)1, and the third, with PF(6)(-) anion in Ibca space group. The FeL(2) complex ions in all lattices have similarly distorted octahedral geometry. Variable-temperature Mössbauer spectra of [FeL(2)](ClO(4))(2).CH(3)CN and [FeL(2)](PF(6))(2) measured in the temperature range 1.7-300 K reveal temperature-dependent populations of two different spin states with increased amount of low-spin form at high temperatures, a phenomenon unlike the normal spin crossover behavior; this abnormal behavior is interpreted here as due to the presence of two different conformations. It is very interesting to note that the two different compounds have similar spectra, Mössbauer parameters, and temperature dependence. But the variable-temperature Mössbauer spectra of [FeL(2)](BPh(4))(2).2CH(3)CN in the range 20-300 K do not show the presence of such different species but exhibit a clear phase transition at approximately 200 K. This phase transition is further supported by SQUID measurements. The results of variable-temperature (1)H NMR in CD(3)CN and the solution susceptibility measurement of all complexes also support the presence of high-spin and low-spin forms in solution. Hence, the complex ion [FeL(2)](2+) exhibits a thermally driven interconversion between low-spin and a high-spin structural forms-a phenomenon observed in the solid and solution states due to ligand dynamics. This is not due to the well-known spin crossover phenomenon. These results are compared with the case of normal spin crossover seen in [FeL'(2)](ClO(4))(2) (L' = 2,6-(bis(pyrazol-1-ylmethyl)pyridine)).

Supramolecular structures of cadmium(II) coordination polymers: a new class of ligands formed by linking tripodal tris(pyrazolyl)methane units

The new ligands C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2) (pz = pyrazolyl ring), 1 (ortho), 2 (meta), and 3 (para), that can potentially bond two or more metal centers were obtained in a single step from the reaction of the appropriate dibromoxylene, 2 equiv of tris-2,2,2-(1-pyrazolyl)ethanol, and excess NaH. Although the arrangement of the tris(pyrazolyl)methane units in the solid-state structures of 1 and 3 are similar, the orientation of these groups with respect to the phenyl ring are different, with 1 showing a twisted structure and 3 a stepped structure. The reaction of [Cd(2)(THF)(5)][BF(4)](4) with the appropriate ligand yields each of the three coordination polymers of the formula [C(6)H(4)[CH(2)OCH(2)C(pz)(3)](2)Cd](BF(4))(2)](n), 4 (ortho), 5 (meta), and 6 (para). In the solid-state structures of all three each tris(pyrazolyl)methane unit is tridentate, with each ligand bonded to two different cadmium(II) atoms, forming a coordination polymer containing 6-coordinate, pseudooctahedral cadmium(II) centers. Polymers 4 and 5 form wavelike chains whereas 6 is arranged in a stepped structure similar to the free ligand. The different connecting patterns of each ligand lead to very different supramolecular structures, structures organized by the BF(4)(-) groups through weak C-H...F hydrogen bonds. Crystallographic information: 1 is monoclinic, P2(1)/n, a = 16.3157(13) A, b = 8.5880(6) A, c = 21.9227(17) A, alpha = gamma = 90 degrees, beta = 100.134(2) degrees, Z = 4; 3 is monoclinic, P2(1)/n, a = 9.1965(9) A, b = 12.9185(12) A, c = 12.7306(12) A, alpha = gamma = 90 degrees, beta = 104.099(2) degrees, Z = 2; 4 is triclinic, P1, a = 10.7877(19) A, b = 12.582(2) A, c = 15.409(3) A, alpha =103.860(4) degrees, beta = 94.123(4) degrees, gamma = 92.573(4) degrees, Z = 2; 6 is monoclinic, C2/c, a = 23.023(3) A, b = 12.2588(14) A, c = 18.119(2) A, alpha = gamma = 90 degrees, beta = 121.281(2) degrees, Z = 4.

Variable-Temperature X-ray Structural Investigation of {Fe[HC(3,5-Me2pz)3]2}(BF4)2(pz = Pyrazolyl Ring): Observation of a Thermally Induced Spin State Change from All High Spin to an Equal High Spin-Low Spin Mixture, Concomitant with the Onset of Nonmerohedral Twinning

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.

A synthetic, structural, magnetic, and spectral study of several [Fe[tris(pyrazolyl)methane]2](BF4)2 complexes: observation of an unusual spin-state crossover

The complexes [Fe[HC(3,5-Me2pz)3]2](BF4)2 (1), [Fe[HC(pz)3]2](BF4)2 (2), and [Fe[PhC(pz)2(py)]2](BF4)2 (3) (pz = 1-pyrazolyl ring, py = pyridyl ring) have been synthesized by the reaction of the appropriate ligand with Fe(BF4)2.6H2O. Complex 1 is high-spin in the solid state and in solution at 298 K. In the solid phase, it undergoes a decrease in magnetic moment at lower temperatures, changing at ca. 206 K to a mixture of high-spin and low-spin forms, a spin-state mixture that does not change upon subsequent cooling to 5 K. Crystallographically, there is only one iron(II) site in the ambient-temperature solid-state structure, a structure that clearly shows the complex is high-spin. Mössbauer spectral studies show conclusively that the magnetic moment change observed at lower temperatures arises from the complex changing from a high-spin state at higher temperatures to a 50:50 mixture of high-spin and low-spin states at lower temperatures. Complexes 2 and 3 are low-spin in the solid phase at room temperature. Complex 2 in the solid phase gradually changes over to the high-spin state upon heating above 295 K and is completely high-spin at ca. 470 K. In solution, variable-temperature 1H NMR spectra of 2 show both high-spin and low-spin forms are present, with the percentage of the paramagnetic form increasing as the temperature increases. Complex 3 is low-spin at all temperatures studied in both the solid phase and solution. An X-ray absorption spectral study has been undertaken to investigate the electronic spin states of [Fe[HC(3,5-Me2pz)3]2](BF4)2 and [Fe[HC(pz)3]2](BF4)2. Crystallographic information: 2 is monoclinic, P2(1)/n, a = 10.1891(2) A, b = 7.6223(2) A, c = 17.2411(4) A, beta = 100.7733(12) degrees, Z = 2; 3 is triclinic, P1, a = 12.4769(2) A, b = 12.7449(2) A, c = 13.0215(2) A, alpha = 83.0105(8) degrees, beta = 84.5554(7) degrees, gamma = 62.5797(2) degrees, Z = 2.