Spin-crossover Cobalt Research Articles

How the spin state tunes the slow magnetic relaxation field dependence in spin crossover cobalt(II) complexes.

A novel family of cobalt(II) compounds with tridentate pyridine-2,6-diiminephenyl type ligands featuring electron-withdrawing substituents of general formula [Co(n-XPhPDI)2](ClO4)2·S [n-XPhPDI = 2,6-bis(N-n-halophenylformimidoyl)pyridine with n = 4 (1-3) and 3 (4); X = I (1), Br (2 and 4) and Cl (3); S = MeCN (1 and 2) and EtOAc (3)] has been synthesised and characterised by single-crystal X-ray diffraction, electron paramagnetic resonance, and static (dc) and dynamic (ac) magnetic measurements combined with theoretical calculations. The structures of 1-4 consist of mononuclear bis(chelating) cobalt(II) complex cations, [CoII(n-XPhPDI)2]2+, perchlorate anions, and acetonitrile (1 and 2) or ethyl acetate (3) molecules of crystallisation. This unique series of mononuclear six-coordinate octahedral cobalt(II) complexes displays both thermally-induced low-spin (LS)/high-spin (HS) transition and field-induced slow magnetic relaxation in both LS and HS states. A complete LS ↔ HS transition occurs for 1 and 2, while it is incomplete for 4, one-third of the complexes being HS at low temperatures. In contrast, 3 remains HS in all the temperature range. 1 and 2 show dual spin relaxation dynamics under the presence of an applied dc magnetic field (Hdc), with the occurrence of faster- (FR) and slower-relaxing (SR) processes at lower (Hdc = 1.0 kOe) and higher fields (Hdc = 2.5 kOe), respectively. On the contrary, 3 and 4 exhibit only SR and FR relaxations, regardless of Hdc. Overall, the distinct field-dependence of the single-molecule magnet (SMM) behaviour along with this family of spin-crossover (SCO) cobalt(II)-n-XPhPDI complexes is dominated by Raman mechanisms and, occasionally, with additional temperature-independent Intra-Kramer [LS or HS (D > 0)] or Quantum Tunneling of Magnetisation mechanisms [HS (D < 0)] also contributing.

Multielectron transfer and field-induced slow magnetic relaxation in opto-electroactive spin crossover cobalt(ii) complexes: structure–function correlations

A unique series of spin-crossover cobalt(ii)-pyridine-2,6-dimine molecular nanomagnets as prototypes of molecular spin quantum transitors and capacitors exhibiting a dual metal- and ligand-centred multielectron redox behaviour is reported.

Spin-crossover cobalt(II) complexes exhibiting temperature- and concentration-dependent optical changes in solution.

This work investigated the spin states of the cobalt(II) complexes [Co(L1)2](X)2 (1·X; L1 = 4'-(4-N,N'-diphenylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) and [Co(L2)2](X)2 (2·X; L2 = 4'-(4-N,N'-dimethylaminophenyl)-2,2':6',2''-terpyridine, X = PF6, BPh4) in the solid state and in solution. In the solid state, 1·PF6 and 2·PF6, both containing smaller PF6- counter anions, showed gradual spin-crossover. In contrast, 1·BPh4 and 2·BPh4 remained in the high-spin state over the temperature range of 5-400 K due to a lower degree of molecular cooperativity. Each of the cobalt(II) complexes exhibited effects of temperature and concentration on their absorption spectra that were related to the spin states in various organic solvents. This work provides new insights into the spectroscopic properties resulting from the spin states of cobalt(II) complexes in solution.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar-Ligand-Substituent Motion.

Ferroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh-terpy)2 ](BPh4 )2 ⋅3ac (1⋅3 ac; FPh-terpy=4'-((3-fluorophenyl)ethynyl)-2,2':6',2''-terpyridine) and is dependent on the degree of 180° flip-flop motion of the ligand's polar fluorophenyl ring. Single crystal X-ray structures at several temperatures confirmed the flip-flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid-state 19 F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip-flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin-state conversion being dependent on the molecular motion of a ligand-appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule-based magnetoelectric materials.

Ferroelectric and Spin Crossover Behavior in a Cobalt(II) Compound Induced by Polar‐Ligand‐Substituent Motion

AbstractFerroelectric spin crossover (SCO) behavior is demonstrated to occur in the cobalt(II) complex, [Co(FPh‐terpy)2](BPh4)2⋅3ac (1⋅3 ac; FPh‐terpy=4′‐((3‐fluorophenyl)ethynyl)‐2,2′:6′,2′′‐terpyridine) and is dependent on the degree of 180° flip–flop motion of the ligand's polar fluorophenyl ring. Single crystal X‐ray structures at several temperatures confirmed the flip–flop motion of fluorobenzene ring and also gave evidence for the SCO behavior with the latter behavior also confirmed by magnetic susceptibility measurements. The molecular motion of the fluorobenzene ring was also revealed using solid‐state 19F NMR spectroscopy. Thus the SCO behavior is accompanied by the flip–flop motion of the fluorobenzene ring, leading to destabilization of the low spin cobalt(II) state; with the magnitude of rotation able to be controlled by an electric field. This first example of spin‐state conversion being dependent on the molecular motion of a ligand‐appended fluorobenzene ring in a SCO cobalt(II) compound provides new insight for the design of a new category of molecule‐based magnetoelectric materials.

Incorporation of spin-crossover cobalt(II) complexes into conducting metallopolymers: Towards redox-controlled spin change

Metal complexes from cobalt(II/III) ions with two tridentate ligands and the corresponding conducting metallopolymers have been synthesized, characterized and subjected to magnetic studies which revealed a redox-controlled spin change. These complexes were designed to contain both a possible spin-crossover metal complex core and an electropolymerizable group for the production of the corresponding conducting metallopolymers. SQUID magnetic measurements revealed a gradual thermal spin crossover (T½ = 170 K) and a low spin state up to 400 K for the two complexes with terpyridine ligand cores chelating to Co(II) and Co(III) centers, respectively, supporting the viability of a spin change from paramagnetic (Co(II), S = 3/2) to diamagnetic (Co(III), S = 0) states via redox chemistry. We have also demonstrated that the reported cobalt(II) complexes can undergo a facile polymer growth process under electropolymerization conditions. Electrochemical studies revealed that the Co(II) and Co(III) metal centers in poly-CoIIL1 could be reversibly interconverted, which demonstrates that the magnetic properties of the conducting metallopolymers can be switched between paramagnetic and diamagnetic.

Bulky anion supported a five-coordinate spin-crossover cobalt(II) complex with slow magnetic relaxation

In this work, using a bulky anion, a mononuclear five-coordinate Co(II) complex [Co(12-TMC)(CH3CN)][B(C6F5)4]2·CH2Cl2 (12-TMC ​= ​1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) (1) has been constructed. X-ray single crystal diffraction shows that the central Co(II) ion of 1 binds with five N atoms, generating a distorted square pyramidal geometry. The direct-current (dc) magnetic data of 1 displayed a gradual and incomplete spin-crossover behaviour. Compared with reported complexes [Co(12-TMC)(CH3CN)](X)2 (X ​= ​BF4−, 2; and PF6−, 3), complex 1 exhibited much slower magnetic relaxation under an applied dc field of 1000 ​Oe, which originated from the low-spin state (S ​= ​1/2) of Co(II) ion.

Double-layered honeycomb architectures constructed via hierarchical self-assembly of hexagonal spin crossover cobalt(ii) metallacycles.

A hexagonal cobalt(ii) metallacycle and its "lipid packaged" derivatives, [Co6(R-bisterpy)6]X12 (R = H, OC16H33, OC27H55; X = BF4, C12-Glu), have been synthesized and characterized. The compounds incorporating BF4- anions formed sphere-like aggregates while the compounds with C12-Glu lipid anions gave double-layered honeycomb architectures composed of hexagonal stacked tubular structures, which exhibit spin crossover behaviour.

Semiconducting spin-crossover cobalt (II) compound with non-integer charge distribution among TCNQ radicals

A cobalt(II) compound, [CoII(terpy)2](TCNQ)4·3DMF·0.5H2O (1; terpy=2,2′;6′,2″-terpyridine, TCNQ=7,7′,8,8′-tetracyanoquinodimethane), was found to show incomplete spin-crossover properties in the cobalt(II) complex and semiconducting properties in the quasi one-dimensional packed TCNQδ− (0<δ<1) radical anions. Single-crystal structures characterized at different temperatures demonstrated non-integer charge distribution among TCNQ radicals. Moreover, electric conductivity measurements revealed that 1 act as semiconductor at 300K (3.3×10−4Scm−1). The coexistence of spin crossover and electric conductivity in one system will pave the way to study the interactions of such different properties and for the implementation of multifunctional materials.

Spin crossover in monoadducts of Co(Salen) with pyridine and imidazole: a quantum chemical study

Complexation of N,N′-ethylene- and N,N′-butylene-bis(salicylideneiminato)cobalt(II) (Co(salen)) with nitrogen-containing heterocyclic ligands (pyridine, imidazole) and investigation into the mechanism responsible for the spin crossover behavior of these complexes have been computationally studied using the DFT B3LYP*/6-311++G(d,p) method. The results of calculations of geometric characteristics and energy parameters are in good agreement with an available experimental data. An addition of a base molecule to Co(salen) has been shown to be accompanied by the narrowing of the energy gap between the high-spin and the low-spin electronic states up to the values typical for spin crossover cobalt complexes. According to the calculations, the energy barrier of spin-forbidden rearrangement of a monoadduct of N,N′-butylene-bis(salicylideneiminato) Co(II) with pyridine does not exceed of 6 kcal/mol. This finding allows one to consider the adducts of Co(salen) with nitrogen-centered ligands as the prospective spin crossover systems. The computational investigation into the spatial and electronic structure of N,N′-butylene-bis(salicylideneiminato) Co(II) dimer predicts the simultaneous existence in a crystal cell of two types of molecules with different metal spin states.

Stabilization of Long-Lived Metastable State in Long Alkylated Spin-Crossover Cobalt(II) Compound

The A mixed crystal compound [Co(0.8)Fe(0.2)(C16-terpy)(2)](BF(4))(2) (2) (C16-terpy is 4'-hexadecyloxy-2,2':6',2''-terpyridine) with long alkyl chains was prepared by mixing [Co(C16-terpy)(2)](BF(4))(2) (1), which exhibits unique magnetic behavior, and the diamagnetic iron(II) compound [Fe(C16-terpy)(2)](BF(4))(2) (3). The long-lived metastable state in the frozen-in effect was observed for the first time in the spin-crossover cobalt(II) compound 2. Furthermore, relaxation from metastable to stable states was very slow because of a large structural transition resulting from the long alkyl chains.

Systematic Study of Spin Crossover and Structure in [Co(terpyRX)2](Y)2 Systems (terpyRX = 4′-alkoxy-2,2′:6′,2′′-terpyridine, X = 4, 8, 12, Y = BF4−, ClO4−, PF6−, BPh4−)

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)(2)](Y)(2) x nH(2)O (X = 4, 8, 12 and Y = BF(4)(-), ClO(4)(-), PF(6)(-), BPh(4)(-)) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)(2)](ClO(4))(2), [Co(terpyR8)(2)](BF(4))(2) x H(2)O, and [Co(terpyR4)(2)](PF(6))(2) x 3 H(2)O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF(4)(-) family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)(2)](BF(4))(2) x H(2)O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn-Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).

Dynamic structural conversion in a spin-crossover cobalt(ii) compound with long alkyl chains

A cobalt(ii) compound [Co(C14-terpy)(2)](BF(4))(2).MeOH (1) with long alkyl chains was prepared, and exhibited unique SCO at T(1) = 50 K and spin transition at T(2) upward arrow = 206 K and T(2) downward arrow = 184 K with hysteresis (DeltaT = 22 K) accompanying the motion of the long alkyl chains.

Spin-Crossover Cobalt(II) Compound with Banana-Shaped Structure

A banana-shaped spin-crossover (SCO) cobalt(II) complex [Co(C16-terpy)2](BPh4)2 (1) with long alkyl chains, based on a terpyridine frame, was synthesized. Compound 1 exhibited very gradual SCO behavior and changes in the dielectric constant. This shows a way in which SCO materials can be used in electronic devices.

1-D cobalt(II) spin transition compound with strong interchain interaction: [Co(pyterpy)Cl(2)].X.

Cobalt(II) compounds [Co(pyterpy)Cl(2)].MeOH (1.(MeOH)) and [Co(pyterpy)Cl(2)].2H(2)O (1.(2H(2)O)) were synthesized. The compound 1.(MeOH) forms the quasi 3-D networks by making pi-pi stacking between the 1-D chains. The methanol molecules from 1.(MeOH) can be removed by heating, and substituted by absorption of water molecules. The MeOH molecules in 1.(MeOH) are removed by heating at 410 K, and they are substituted by water molecules to form 1.(2H(2)O). 1.(2H(2)O) exhibits a S = (3)/(2) (HS) left arrow over right arrow S = (1)/(2) (LS) spin transition with a thermal hysteresis. We have succeeded in constructing a guest dependent 1-D spin-crossover cobalt(II) compound.

Spectral and magnetic studies on normal cobalt(II) planar and cobalt(III) octahedral, spin-crossover cobalt(III) octahedral and planar-tetrahedral cobalt(II) carbodithioates

Five cobalt(II) complexes, a normal complex Co(4-PPipzcdt) 2 (4-PPipzcdt = 4-phenylpiperazine-1-carbodithioate), and four zwitterionic complexes, Co(4-PPipzcdtH) 2X 2 and Co(4-MPipzcdtH) 2X 2 (X = Cl, Br; 4-PPipzcdtH = 4-phenylpiperazine-1-carbodithioic acid, 4-MPipzcdtH = 4-methylpiperazine-1-carbodithioic acid), have been synthesized. Normal cobalt(III) complexes of the type Co(4-MPipzcdt) 3 and Co 2 {2-MPipz(cdt) 2} 3 (2-MPipz(cdt) 2 = 2-methylpiperazine-1,4-dicarbodithioate) and two zwitterionic cobalt(III) complexes of the type Co(4-MPipzcdtH) 3X 3 (X = Cl, Br) have also been obtained. In addition to the room temperature IR and electronic spectra and magnetic susceptibility studies, all the complexes, except the normal Co(4-MPipzcdt) 3 and Co(4-PPipzcdt) 2 and zwitterionic Co(4-MPipzcdtH) 3Cl 3, have been investigated by variable-temperature magnetic susceptibility measurements. The results of the variable-temperature magnetic susceptibility studies suggest that two cobalt(II) carbodithioates exhibit a square planar-tetrahedral equilibrium, while two cobalt(III) octahedral carbodithioates show a spin-crossover phenomenon.

Spin-crossover cobalt(III) complexes: steric and electronic control of spin state

The series of octahedral Co(III) spin-crossover complexes (III) is synthesized from the corresponding Co(II) complexes (Ia) by oxidation with the radical cation of (II).