CoII Centers Research Articles

Influence of the Coordination Environment on Easy-Plane Magnetic Anisotropy of Pentagonal Bipyramidal Cobalt(II) Complexes.

A rational approach of modulating the easy-plane magnetic anisotropy of mononuclear pentagonal bipyramidal CoII single molecule magnets (SMMs) has been revealed in this paper. A class of three new pentagonal-bipyramidal complexes with formulas [Co(H2daps)(MeOH)2] (1), [Co(H4daps)(NCS)(MeOH)]·(ClO4)·(MeOH) (2), and [Co(H4daps)(NCS)2]·(MeOH)2 (3) (H4daps = 2,6-bis(1-salicyloylhydrazonoethyl) pyridine) were studied. In these complexes, the axial positions are successively replaced by different O and N donar ligands in a systematic way. Detailed magnetic measurements disclose the existence of large easy-plane magnetic anisotropy and field-induced slow magnetic relaxation behavior. Both experimental and ab initio theoretical calculations display that easy-plane magnetic anisotropy is maintained upon variation of coordination environments. Nevertheless, the magnitude of the D value was found to be increased in the case of weaker axially coordinated σ-donor ligands and a more symmetrical equatorial ligand. Additionally, the detailed investigation of field and temperature dependence of relaxation time revealed that quantum tunnelling of magnetization is the predominant process for slow magnetic relaxation and the Raman process is significant which explicates the thermal dependence. Magnetic dilution experiments have been performed to eliminate the possible influence of intermolecular interactions on magnetic behaviors of adjacent CoII centers.

Two Interpenetrated Cobalt(II) Metal–Organic Frameworks with Guest-Dependent Structures and Field-Induced Single-Ion Magnet Behaviors

Two cobalt(II) metal–organic frameworks with 2-fold vertically interpenetrated (4,4) grids, namely, [Co(bpg)2(SCN)2]·3MeOH (1) and [Co(bpg)2(SCN)2]·2DMF (2) (bpg = meso-α,β-bi(4-pyridyl) glycol), were prepared under different conditions. The two complexes crystallize in different space groups (orthorhombic Pccn for 1, and tetragonal P̅421c for 2) with different guest molecules. Although their structures are very similar, different guest molecules subtly change the coordination environments of the CoII centers, the intergrid supramolecular interactions (hydrogen bond), and the dihedral angles of those two sets of interpenetrated grids. Magnetically, these modifications of the structures lead to the changes on their magnetism, especially the dynamic magnetic properties at low temperatures. Both compounds exhibit slow magnetic relaxation under an external field. For 1, we could not observe the peak maxima of the ac susceptibilities in our magnetometer at the lowest temperature and highest frequency because the relaxation time is never slow enough. In contrast for compound 2, the estimated energy barrier is significantly higher than that of 1, leading to much slower magnetic relaxation. The difference of magnetic properties between 1 and 2 illustrates the role of the metal–organic frameworks on the development of the tunable single-ion magnets and the prominent guest effect in the MOF based SIMs.

Syntheses, Crystal Structures, and Properties of Metal(II) Coordination Polymers based on Flexible Ligand 1,3‐Bis(4‐phenoxy)benzenedicarboxylic Acid and Bis(benzimidazole)

Three new coordination polymers, [Zn(PBDC)(bbbm)0.5]n (1), [Co(PBDC)(bbbm)]n (2), and [Cd(PBDC)(bbbm)]n (3) were prepared via hydrothermal reactions of different metal(II) nitrates with flexible 1,3‐bis(4‐phenoxy)benzenedicarboxylic acid (H2L) and 1,1‐(1,4‐butanediyl)bis(benzimidazole) ligand. All these complexes were fully characterized by elemental analysis, FT‐IR, thermogravimetric analysis (TGA), powder X‐ray diffraction, and single‐crystal X‐ray diffraction. Structure analyses revealed that complex 1 has a 2D→2D twofold interpenetrating framework simplified by a 4‐connected sql net with point symbol (44.62), whereas complexes 2 and 3 are isostructural and exhibit a 2D→2D twofold interpenetrating framework rationalized as a three‐connected hcb net with point symbol (63). Complexes 1–3 further expand to 3D supramolecular structures through non‐covalent C–H···O interactions. Additionally, the luminescent and magnetic properties of some of these complexes were studied. Complex 3 presents ideal photoluminescent behavior, whereas complex 2 shows antiferromagnetic coupling between the central CoII ions, suggesting its latent application in magnetic material.

A three-dimensional twofold interpenetrated cobalt(II) MOF containing a flexible carboxylate-based ligand: synthesis, structure and magnetic properties.

The design and synthesis of coordination polymers (CPs) have attracted much interest due to the intriguing diversity of their architectures and topologies. The functional solid catena-poly[μ2-aqua-triaqua{μ4-5-[4-carboxyphenoxy)methyl]benzene-1,3-dicarboxylato}{μ3-5-[4-carboxyphenoxy)methyl]benzene-1,3-dicarboxylato}dicobalt(II)], [Co2(C16H10O7)2(H2O)4]n or [Co2(HL)2(μ2-H2O)(H2O)3]n, was synthesized successfully by self-assembly of CoII ions with 5-[(4-carboxyphenoxy)methyl]isophthalic acid (H3L). The title compound was obtained under hydrothermal conditions and exhibits a twofold interpenetrated three-dimensional skeleton with hms 3,5-conn topology according to the cluster representation for valence-bonded metal-organic frameworks (MOFs). It has been characterized by single-crystal X-ray diffraction, IR spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis and susceptibility measurements. The antiferromagnetic coupling between adjacent CoII centres occurs via superexchange through the ligands.

A new series of tetrahedral Co(ii) complexes [CoLX2] (X = NCS, Cl, Br, I) manifesting single-ion magnet features.

A series of tetrahedral CoII complexes [CoLX2] (X = NCS (1), Cl (2), Br (3) and I (4); L = 9,9-dimethyl-4,5-bis(diphenylphosphino) xanthene) based on a P-donor ligand has been prepared to investigate the influence of terminal ligand field strength on the anisotropy of CoII single-ion magnets. It has been observed that heavier and softer terminal ligands are able to decrease the anisotropy of the tetrahedral CoII centers. Thorough analyses of experimental and theoretical studies show that all complexes have an easy-axis type magnetic anisotropy and slow relaxation behaviors of tetrahedral CoII centers. Detailed ab initio theory studies disclose that the changes in the ligand field strength imposed by the terminal ligands result in modifying the single ion anisotropy (D) of polyhedra 1-4. Furthermore, the isostructural ZnII analogue (5) has been prepared to examine the influence of dipolar interactions between adjacent CoII centres and magnetic dilution experiments were performed.

Reversible on-off switching of both spin crossover and single-molecule magnet behaviours via a crystal-to-crystal transformation.

The on-off switching of spin-crossover (SCO) and single-molecule magnetism (SMM) remains highly attractive, especially if it involves dynamic crystal-to-crystal transformation. Herein we report the first molecule, a mononuclear cobalt(ii) complex, that exhibits on-off switching between SCO and SMM reversibly during crystal-to-crystal transformation. Subtle structural transformation triggered by a simple dehydration-rehydration process induces significant geometrical changes of the CoII center and modification of the supramolecular interactions and switches its colour and magnetic properties (dark red/SCO-on/SMM-off ↔ orange/SCO-off/SMM-on). This work suggests that modification of the weak supramolecular interactions could be very effective in achieving switchable materials involving both SCO and SMM properties.

Slow magnetic relaxation in a {CoIICo} complex containing a high magnetic anisotropy trigonal bipyramidal CoII centre.

We report a trinuclear mixed-valence {CoIICoIII2} complex, where the CoII centre adopts a trigonal bipyramidal geometry, leading to a large, easy-plane magnetic anisotropy and field-induced slow magnetic relaxation with a Raman-like relaxation process.

Ligand Effect on the Single-Molecule Magnetism of Tetranuclear Co(II) Cubane.

A clear dependence on the ligand has been observed for the magnetic properties of a closely related series of Co(II) cubane structures, viz. [Co4(mbm or bm)4(ROH)4Br4] (1-MeOH, 1-EtOH, 2-MeOH, and 2-EtOH, where 1 = [Co4(mbm)4Br4], 2 = [Co4(bm)4Br4], bm = (1H-benzo[d]imidazol-2-yl)methanolate. and mbm = 1-Me-bm.) The [Co4(OR)4] cubane core consists of an octahedral CoII center chelated by the alkoxide oxygen and imidazole nitrogen atoms from monoanionic bm or mbm and coordinated by methanol/alcohol and bromine. Interestingly, electrospray ionization mass spectrometry (ESI-MS) indicates that 1-MeOH and 2-MeOH are unstable in methanol and transformed to the butterfly [Co4L6]2+ but that 1-EtOH and 2-EtOH are stable in ethanol. Their magnetic susceptibilities suggest ferromagnetic coupling between the nearest cobalt centers to give a theoretical S = 4 × 3/2 ground state with considerable magneto-crystalline behavior. The packing and intermolecular interactions appear to influence the geometry of the cubes and thus the anisotropy of cobalt, which leads to different blocking temperatures (TB). Consequently, the compounds with mbm, 1-MeOH and 1-EtOH, exhibit TB > 2 K as shown by the relaxation of magnetization in zero applied dc field where the barriers Ueff/kB are respectively 27 and 21 K and relaxation times are τ0 = 1.3 × 10-9 and 9.7 × 10-9 s. However, the compounds with bm, 2-MeOH and 2-EtOH, remain paramagnetic above 2 K and do not show nonlinear response of the ac susceptibilities. These findings reaffirm the subtle dependence of single-molecule magnetism on coordination geometry and intermolecular interaction.

Crystal structure of a looped-chain CoII coordination polymer: catena-poly[[bis-(nitrato-κO)cobalt(II)]bis-[μ-bis-(pyridin-3-ylmeth-yl)sulfane-κ2N:N']

The asymmetric unit of the title compound, [Co(NO3)2(C12H12N2S)2] n , contains a bis-(pyridin-3-ylmeth-yl)sulfane (L) ligand, an NO3- anion and half a CoII cation, which lies on an inversion centre. The CoII cation is six-coordinated, being bound to four pyridine N atoms from four symmetry-related L ligands. The remaining coordination sites are occupied by two O atoms from two symmetry-related nitrate anions in a monodentate manner. Thus, the CoII centre adopts a distorted octa-hedral geometry. Two symmetry-related L ligands are connected by two symmetry-related CoII cations, forming a 20-membered cyclic dimer, in which the CoII atoms are separated by 10.2922 (7) Å. The cyclic dimers are connected to each other by sharing CoII atoms, giving rise to the formation of an infinite looped chain propagating along the [101] direction. Inter-molecular C-H⋯π (H⋯ring centroid = 2.89 Å) inter-actions between one pair of corresponding L ligands and C-H⋯O hydrogen bonds between the L ligands and the nitrate anions occur in the looped chain. In the crystal, adjacent looped chains are connected by inter-molecular π-π stacking inter-actions [centroid-to-centroid distance = 3.8859 (14) Å] and C-H⋯π hydrogen bonds (H⋯ring centroid = 2.65 Å), leading to the formation of layers parallel to (101). These layers are further connected through C-H⋯O hydrogen bonds between the layers, resulting in the formation of a three-dimensional supra-molecular architecture.

Ultrathin Cobalt-Based Metal-Organic Framework Nanosheets with Both Metal and Ligand Redox Activities for Superior Lithium Storage.

The controllable synthesis and structural tailoring of nanostructured metal-organic frameworks (MOFs) is of huge significance in boosting their potential for rechargeable batteries. We herein present the facile synthesis of cobalt-based ultrathin metal-organic framework nanosheets (referred to as "u-CoTDA") by using 2,5-thiophenedicarboxylic (H2 TDA) as the organic building block through a one-pot ultrasonic method for the first time. The obtained u-CoTDA exhibits high reversible capacity (790 mAh g-1 after 400 cycles at 1 A g-1 ) and excellent rate capability (694 mAh g-1 at 2 A g-1 ), which outperforms its bulk counterpart. Moreover, the detailed lithiation/delithiation processes of u-CoTDA were studied by the combination of Co K-edge X-ray absorption near edge structure (XANES), O K-edge soft X-ray spectroscopy (sXAS) and electron paramagnetic resonance (EPR) techniques, which demonstrate that both the CoII centers and organic ligands of u-CoTDA are involved in the reduction/oxidation processes.

Quantitative Estimation of Ising-Type Magnetic Anisotropy in a Family of C3 -Symmetric CoII Complexes.

In this paper, the influence of the structural and chemical effects on the Ising-type magnetic anisotropy of pentacoordinate CoII complexes has been investigated by using a combined experimental and theoretical approach. For this, a deliberate design and synthesis of four pentacoordinate CoII complexes [Co(tpa)Cl]⋅ClO4 (1), [Co(tpa)Br]⋅ClO4 (2), [Co(tbta)Cl]⋅(ClO4 )⋅(MeCN)2 ⋅(H2 O) (3) and [Co(tbta)Br]⋅ClO4 (4) by using the tripodal ligands tris(2-methylpyridyl)amine (tpa) and tris[(1-benzyl-1 H-1,2,3-triazole-4-yl)methyl]amine) (tbta) have been carried out. Detailed dc and ac measurements show the existence of field-induced slow magnetic relaxation behavior of CoII centers with Ising-type magnetic anisotropy. A quantitative estimation of the zero-field splitting (ZFS) parameters has been effectively achieved by using detailed ab initio theory calculations. Computational studies reveal that the wavefunction of all the studied complexes has a very strong multiconfigurational character that stabilizes the largest ms =±3/2 components of the quartet state and hence produce a large negative contribution to the ZFS parameters. The difference in the magnitudes of the Ising-type anisotropy can be explained through ligand field theory considerations, that is, D is larger and negative in the case of weak equatorial σ-donating and strong apical π-donating ligands. To elucidate the role of intermolecular interactions in the magnetic relaxation behavior between adjacent CoII centers, a diamagnetic isostructural ZnII analog (5) was synthesized and the magnetic dilution experiment was performed.

Syntheses, structures and characterization of isomorphous CoII and NiII coordination polymers based on 2-[(1H-imidazol-1-yl)methyl]-6-methyl-1H-benzimidazole and benzene-1,4-dicarboxylate.

Careful choice of the organic ligands is one of the most important parameters in the rational design and synthesis of coordination polymers. Aromatic polycarboxylates have been widely used in the preparation of metal-organic polymers since they can utilize various coordination modes to form diverse structures and can act as hydrogen-bond acceptors and donors in the assembly of supramolecular structures. Nitrogen-heterocyclic organic compounds have also been used extensively as ligands for the construction of polymers with interesting structures. In the polymers catena-poly[[[diaquabis{2-[(1H-imidazol-1-yl)methyl]-6-methyl-1H-benzimidazole-κN3}cobalt(II)]-μ2-benzene-1,4-dicarboxylato-κ2O1:O4] dihydrate], {[Co(C8H4O4)(C12H11N4)2(H2O)2]·2H2O}n, (I), and catena-poly[[[diaquabis{2-[(1H-imidazol-1-yl)methyl]-6-methyl-1H-benzimidazole-κN3}nickel(II)]-μ2-benzene-1,4-dicarboxylato-κ2O1:O4] dihydrate], {[Ni(C8H4O4)(C12H11N4)2(H2O)2]·2H2O}n, (II), the CoII or NiII ion lies on an inversion centre and exhibits a slightly distorted octahedral coordination geometry, coordinated by two N atoms from two imidazole rings and four O atoms from two monodentate carboxylate groups and two water molecules. The dicarboxylate ligands bridge metal ions forming a polymeric chain. The 2-[(1H-imidazol-1-yl)methyl]-6-methyl-1H-benzimidazole ligands coordinate to the CoII or NiII centres in monodentate modes through an imidazole N atom and are pendant on opposite sides of the main chain. The two structures are isomorphous. In the crystal, the one-dimensional chains are further connected through O-H...O, O-H...N and N-H...O hydrogen bonds, leading to a three-dimensional supramolecular architecture. In addition, the IR spectroscopic properties, PXRD patterns, thermogravimetric behaviours and fluorescence properties of both polymers have been investigated.

Slow Magnetic Relaxation in One-Dimensional Azido-Bridged CoII Complexes.

Crystal structures and magnetic properties of three one-dimensional (1D) azido-bridged cobalt(II) chains with different amide ligands (L), [Co2(N3)4(DMF)3] (1), [Co4(N3)8(DEF)5] (2), and [Co2(N3)4(DIPF)2] (3) (DMF = N,N-dimethylformamide, DEF = N,N-diethylformamide, and DIPF = N,N-diisopropylformamide), are reported to investigate the influence of L on their structures and magnetic properties. Single-crystal X-ray crystallographic analysis revealed that, although 1-3 all consist of cobalt chains bridged by end-on (EO) azides, the coordination geometry of the CoII ions and the repeating units of the 1D structures are quite different. As the size of L increases, the ratio of L to CoII decreases from 6:4 in 1 to 5:4 and 4:4 in 2 and 3, respectively. In 1, two [CoN5O1] and two [CoN4O2] distorted octahedra form the {[CoN5O1][CoN4O2]2[CoN5O1]} tetramers (denoted as Co4A), which are linked to each other by sharing the N-N edge to form the chain. Similarly, the chain structure of 3 is constructed from a similar tetramer unit {[CoN5][CoN4O2]2[CoN5]} (denoted as Co4B), where half of the CoII centers are in the [CoN5] trigonal bipyramid because of the larger steric effect of the DIPF ligand, while, for compound 2 of the medium-sized amide, it has the transition structure between those of 1 and 3. The chain is composed of two different repeating units: Co4A unit similar to that in 1 and Co4B unit similar to that in 3. Because of their similar structures, compounds 1-3 exhibit analogical magnetic properties. Direct-current magnetic measurements demonstrated that all compounds show intrachain ferromagnetic coupling through the EO azides and interchain anti-ferromagnetic interactions. Alternating-current data revealed the slow magnetic relaxation in the anti-ferromagnetic ordered phases. While compound 1 exhibits spin glass behavior, compounds 2 and 3 behave as the single-chain magnets. This difference might come from the interference of the anti-ferromagnetic ordering on the magnetic dynamic of the magnetic chain.

Mixed valence trinuclear cobalt (II/III) complexes: Synthesis, structural characterization and phenoxazinone synthase activity

Three new mixed valence trinuclear Co(II/III) compounds [Co3L2(μ2-C6H5CO2−)2(CH3CN)2](ClO4−)2·(CH3CN)3 (1) [Co3LR2(μ2-C6H5CO2−)2(C6H5CO2−)2]·(CH3CN)2 (2) and [Co3LR2(μ2-C6H5CHCHCO2−)2(C6H5CHCHCO2−)2]·((CH3)2CO)2 (3) have been synthesized by reacting the di-Schiff base ligand [H2L] or its reduced analog [H2LR] (where H2L=N,N′-bis(salicylidene)-1,3-propanediamine) and (H2LR=N,N′-bis(2-hydroxybenzyl)-1,3-propanediamine) with cobalt perchlorate hexahydrate and sodium benzoate or sodium cinnamate. The complexes have been characterized by IR, UV–Vis and single crystal X-ray diffraction analyses. All the complexes are linear trinuclear species [CoIIICoIICoIII] in which two terminal octahedral CoIIIN2O2 cores coordinate to the central octahedral CoII ion through μ2-phenoxido oxygen atom and the bridging carboxylato ions. In addition, the complexes (2 and 3) derived from reduced Schiff base ligand have, rather unusually, a terminally coordinated carboxylato ion to CoIII, which is acetonitrile molecule in case of complex (1) with unreduced ligand. Complex 2 has been found to be an excellent functional model for the phenoxazinone synthase activity, in the aerial oxidation of 2-aminophenol to the corresponding 2-aminophenoxazine-3-one chromophore in acetonitrile solvent medium. Detailed kinetic data analysis of this oxidation reaction reveals a fairly high phenoxazinone synthase activity of 2 with kcat=153.60h−1.

Crystal structure of (5-{3-[(1,4,7,10,13-penta-oxa-16-aza-cyclo-octa-decan-16-yl)carbonyl-amino]-phen-yl}-10,15,20-tri-phenyl-porphyrinato)cobalt(II).

In the title compound, [Co(C57H52N6O6)], the central CoII atom is coordinated by four pyrrole N atoms of the porphyrin core and one O atom of the crown ether. The complex has a distorted porphyrin core, with mean absolute core-atom displacements of 0.14 (10) (Ca), 0.20 (10) (Cb), 0.24 (4) (Cm) and 0.18 (10) Å (Cav), respectively. The axial Co-O bond length is 2.3380 (15) and the average Co-Np bond length is 1.968 (5) Å. Intra-molecular N-H⋯O and inter-molecular C-H⋯π inter-actions are observed.

Probing the Effects of Ligand Field and Coordination Geometry on Magnetic Anisotropy of Pentacoordinate Cobalt(II) Single-Ion Magnets.

In this work, the effects of ligand field strength as well as the metal coordination geometry on magnetic anisotropy of pentacoordinated CoII complexes have been investigated using a combined experimental and theoretical approach. For that, a strategic design and synthesis of three pentacoordinate CoII complexes [Co(bbp)Cl2]·(MeOH) (1), [Co(bbp)Br2]·(MeOH) (2), and [Co(bbp)(NCS)2] (3) has been achieved by using the tridentate coordination environment of the ligand in conjunction with the accommodating terminal ligands (i.e., chloride, bromide, and thiocyanate). Detailed magnetic studies disclose the occurrence of slow magnetic relaxation behavior of CoII centers with an easy-plane magnetic anisotropy. A quantitative estimation of ZFS parameters has been successfully performed by density functional theory (DFT) calculations. Both the sign and magnitude of ZFS parameters are prophesied well by this DFT method. The theoretical results also reveal that the α → β (SOMO-SOMO) excitation contributes almost entirely to the total ZFS values for all complexes. It is worth noting that the excitation pertaining to the most positive contribution to the ZFS parameter is the dxy → dx2-y2 excitation for complexes 1 and 2, whereas for complex 3 it is the dz2 → dx2-y2 excitation.

Single‐Ion Magnetic Behavior in CoII–CoIII Mixed‐Valence Dinuclear and Pseudodinuclear Complexes

Two CoII–CoIII mixed‐valance complexes of molecular formulas [Co2(H2L)2(H2O)2][Co2(H2L)2(H2O)(m‐phth)]·8(H2O) {1; H2L2– = 2‐[(2‐hydroxy‐3‐methoxybenzylidene)amino]‐2‐(hydroxymethyl)propane‐1,3‐diolato, m‐phth = 1,3‐benzenedicarboxylate} and [Co4(H2L)4(H2O)2(ppda)]·2(dmf)·3.2(H2O) {2; ppda = 1,4‐phenylenediacrylate; dmf = N,N‐dimethylformamide} were synthesized and characterized by single‐crystal X‐ray diffraction and magnetic studies at low temperature. The structural determination reveals that 1 is composed of dinuclear ion pairs, namely, a cationic [Co2(H2L)2(H2O)2]+ (1+) and an anionic [Co2(H2L)2(H2O)(m‐phth)]– (1–) unit. In each of these ions, the CoII and CoIII centers present distorted octahedral geometries. Compound 2 is a centrosymmetric tetranuclear complex comprising two symmetry‐related dinuclear CoII–CoIII units bridged by ppda anions. Alternating current/direct current (ac/dc) magnetic studies revealed that the individual CoII–CoIII unit exhibits field‐induced slow magnetic relaxation consistent with single‐ion magnet (SIM) behavior. Ab initio NEVPT2 calculations confirm large zero‐field splitting (zfs) from a first‐order spin–orbit coupling (SOC) in both complexes (D = –62.4, –95.8, and –101.9 cm–1 and E/D = 0.219, 0.216, and 0.234 for 1+, 1–, and 2, respectively).

Linear, Trinuclear Cobalt Complexes with o-Phenylene-bis-Silylamido Ligands.

Transamination of [Co{N(SiMe3 )2 }2 ]2 with C6 H4 (NHSiiPr3 )2 gave the centrosymmetric trinuclear [{Coter N(SiMe3 )2 (μ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Coint ] (1) (Coter , Coint =terminal, internal Co, respectively), with 3-coordinate Coter , and Coint "sandwiched" between the o-phenylenes of the two ligands; experimental and computational data support CoII centres and ditopic o-amido-imino-cyclohexen-allyl ligands; magnetic studies reveal intermetallic ferromagnetic interactions and single-molecule magnet (SMM) character. One-electron reduction of 1 yielded the salt [K(18-crown-6)(THF)2 ][{Coter N(SiMe3 )2 (μ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Coint ] (4) with the anion isostructural to 1. The centrosymmetric Fe complex [{Feter N(SiMe3 )2 (μ-η-[o-C6 H4 (κNSiiPr3 )2 ])}2 Feint ] (5), analogous to 1, was also obtained.

Crystal Structure and Corrosion Inhibition Properties of Ferrocenyl- and Phenylendiamine-Iminomethylphenoxy Cobalt Complexes

The crystal structures of bis[2-(ferrocenyl-iminomethyl)phenoxy]cobalt(II) (Co II (L) 2 ) [monoclinic, a = 12.5466(3) A, b = 10.6782(3) A, c = 21.2695(6) A, α = γ = 90°, β = 92.944(2)°, V = 2845.82(13) A3, Z = 4, space group P21/c] and bis(2-[(4-dimethylamino-phenylimino)-methyl]-phenoxy) cobalt (II) (Co II (M) 2 ) [triclinic, a = 10.2916(4) A, b = 16.4867(10) A, c = 17.6782(11) A, α = 114.754(2)°, β = 96.614(4)°, γ = 97.736(4), Z = 4, space group P-1] dominated by extensive hydrogen bonding such as O–H···N, N–H···O and N–H···N interactions. In both structures the central CoII is displaying a slightly distorted tetrahedral coordination sphere involving two iminoethyl-phenoxy ligands. The inhibition efficiency of the Co complexes concerning the corrosion of mild steel in acidic solution has been investigated by electrochemical impedance spectroscopy. The crystal structures of two bis(iminomethylphenoxy) cobalt complexes containing respectively a ferrocenyl and a phenylendiamine group are described. First results on their inhibition properties concerning the corrosion of mild steel in hydrochloric acid are reported and discussed.

Modulating Magnetic Refrigeration through Structural Variation in CoII/III-GdIII Clusters.

Three heterometallic aggregates, [(CoII)2(GdIII)2(tBuPO3)2(O2CtBu)2(HO2CtBu)2(NO3)4]·NEt3 (1), [(CoII)2(CoIII)2(GdIII)3(μ3-OH)2(tBuPO3)2(O2CtBu)9(deaH)2(H2O)2] (2), and (CoIII)2(GdIII)5(μ2-OH)(μ3-OH)2(tBuPO3)2(O2CtBu)10(HO2CtBu)(deaH)2]·MeOH (3), were successfully isolated in reactions of [Co2(μ-OH2)(O2CtBu)4]·(HO2CtBu)4, Gd(NO3)3·6H2O, tBu-PO3H2, and diethanolamine (deaH3) by varying the stoichiometry of the reactants and/or changing the solvent. The structures of the final products were profoundly affected by these minor changes in stoichiometry or a change in solvent. The metal-oxo core of these complexes displays a hemicubane or a defective dicubane-like view. Bond valence sum calculations and bond lengths indicate the presence of CoII centers in compound 1, mixed valent Co centers (CoII/CoIII) in compound 2, and only CoIII centers in compound 3 as required for the charge balances and supported by the magnetic measurements. Magnetic studies reveal significant magnetic entropy changes for complexes 1-3 (-ΔSm values of 28.14, 25.06, and 29.19 J kg-1 K-1 for 3 K and 7 T, respectively). This study shows how magnetic refrigeration can be affected by anisotropy, magnetic interactions (ferro- or antiferromagnetic), the metal/ligand ratio, and the content of GdIII in the molecule.