Chiral Photomagnets Based on Copper(II) complexes of 1,2-Diaminocyclohexane and Octacyanidomolybdate(IV) Ions.

Chiral photomagnets compose a class of multifunctional molecule-based materials with light-induced alteration of magnetization and chiral properties. The rational design and synthesis of such assemblies is a challenge, and only few such systems are known. Herein, the remarkable octacyanide-bridged enantiomeric pair of 1-D chains [Cu((R,R)-chxn)2]2[Mo(CN)8]·H2O (1R) and [Cu((S,S)-chxn)2]2[Mo(CN)8]·H2O (1S) exhibiting enantiopure structural helicity, which results in optical activity in the 350-800 nm range as confirmed by natural circular dichroism (NCD) spectra, is reported. The photomagnetic effects of 1R, 1S, and 1rac result from the blue light excitation (436 nm) of the photomagnetically active octacyanidomolybdate(IV) ions. In the excited state MoIVHS centers with S = 1 couple antiferromagnetically with the neighboring CuII centers with JCuMo values of -1.3, -1.0, and -1.1 cm-1 for 1R, 1S, and 1rac, respectively. The values of thermal relaxation energy barriers have been estimated as 142 and 356 K for 1R and 1S, being comparable with the energy range of the thermal bath. The value for 1rac reveals a significantly lower value of 75 K. On the basis of these results the value of gMoHS has been estimated to be in the range 4.8-5.8.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions.

The chemistry of oxo/hydroxo-bridged diheme centers, connected covalently through bridges, has attracted much attention recently. Close approach of the two heme centers in the µ-hydroxo complex results in an unequal core deformation which leads to the unusual stabilization of two different spin states of iron in a single molecular framework. The spin states are also counter-anion specific and are reversibly interconvertable. An increased separation between the heme centers, however, leads to a weaker inter-ring interaction and, hence, renders the iron centers equivalent. The counter anion has been found to perturb the spin state ordering of iron via H-bonding interaction, switching positions between counter anion and axial ligand, ion-dipole interaction, charge polarization etc. A tightly associated counter anion with one of the heme centers generates significant steric effect in both the solid state and solution and induces significant change in the structure and properties, including the iron spin state, without affecting the overall topology of the complex or the metal oxidation state. A brief account of our systematic investigation on this subject is presented in the present Perspective article.

Syntheses, crystal structures and magnetic properties of a series of µ-phenoxo-µ1,1-carboxylato-µ1,3-carboxylato trinickel(II) compounds.

The work in this report describes the syntheses, characterization, crystal structures and magnetic properties of eight linear trinickel(ii) compounds of the composition [Ni(II)3(L(sal-pyr))2(propionate)4] (), [Ni(II)3(L(sal-pyr))2(benzoate)4]·CH3CN (), [Ni(II)3(L(sal-pip))2(acetate)4]·2CH3CN (), [Ni(II)3(L(sal-pip))2(propionate)4] (), [Ni(II)3(L(sal-pip))2(benzoate)4]·CH2Cl2 (), [Ni(II)3(L(sal-mor))2(propionate)4] (), [Ni(II)3(L(sal-mor))2(benzoate)4]·3CH2Cl2 () and [Ni(II)3(L(sal-mor))2(o-Cl-benzoate)4]·2CH3CN·2H2O (), where HL(sal-pyr), HL(sal-pip) and HL(sal-mor) are the 1 : 1 condensation products of salicylaldehyde and 1-(2-aminoethyl)-pyrrolidine, 1-(2-aminoethyl)-piperidine and 4-(2-aminoethyl)-morpholine, respectively. One-half of the trinuclear core in each complex is symmetry related to the second part due to the presence of an inversion centre on the central metal ion and so the terminal nickelcentral nickelterminal nickel angle is 180°. The terminal and central nickel(ii) ions are triply bridged by a phenoxo, a µ1,1-carboxylato and a µ1,3-carboxylato moiety. The µ1,1-carboxylato also acts as a chelating ligand for the terminal metal ion. Both the variable-temperature (2-300 K) susceptibilities at a fixed field strength of 0.1 T and variable-field (up to 7 T) magnetization at different fixed temperatures (2-10 K) were recorded. The magnetic data indicate the ferromagnetic interaction in all the cases with J (H = -2Jij∑SiSj) values ranging between 2.37 and 3.89 cm(-1) and the single-ion zero-field parameter (D) ranging between 7.21 and 8.94 cm(-1). Satisfactorily simulation of both the χMT vs. T and M vs. H data has been obtained. Comparison of the structures and magnetic properties of compounds with those of the previously published related systems reveals some interesting aspects.

Heterobridged dinuclear, tetranuclear, dinuclear-based 1-d, and heptanuclear-based 1-D complexes of copper(II) derived from a dinucleating ligand: syntheses, structures, magnetochemistry, spectroscopy, and catecholase activity.

The work in this paper presents syntheses, characterization, crystal structures, variable-temperature/field magnetic properties, catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) study of five copper(II) complexes of composition [Cu(II)(2)L(µ(1,1)-NO(3))(H(2)O)(NO(3))](NO(3)) (1), [{Cu(II)(2)L(µ-OH)(H(2)O)}(µ-ClO(4))](n)(ClO(4))(n) (2), [{Cu(II)(2)L(NCS)(2)}(µ(1,3)-NCS)](n) (3), [{Cu(II)(2)L(µ(1,1)-N(3))(ClO(4))}(2)(µ(1,3)-N(3))(2)] (4), and [{Cu(II)(2)L(µ-OH)}{Cu(II)(2)L(µ(1,1)-N(3))}{Cu(II)(µ(1,1)-N(3))(4)(dmf)}{Cu(II)(2)(µ(1,1)-N(3))(2)(N(3))(4)}](n)·ndmf (5), derived from a new compartmental ligand 2,6-bis[N-(2-pyridylethyl)formidoyl]-4-ethylphenol, which is the 1:2 condensation product of 4-ethyl-2,6-diformylphenol and 2-(2-aminoethyl)pyridine. The title compounds are either of the following nuclearities/topologies: dinuclear (1), dinuclear-based one-dimensional (2 and 3), tetranuclear (4), and heptanuclear-based one-dimensional (5). The bridging moieties in 1-5 are as follows: µ-phenoxo-µ(1,1)-nitrate (1), µ-phenoxo-µ-hydroxo and µ-perchlorate (2), µ-phenoxo and µ(1,3)-thiocyanate (3), µ-phenoxo-µ(1,1)-azide and µ(1,3)-azide (4), µ-phenoxo-µ-hydroxo, µ-phenoxo-µ(1,1)-azide, and µ(1,1)-azide (5). All the five compounds exhibit overall antiferromagnetic interaction. The J values in 1-4 have been determined (-135 cm(-1) for 1, -298 cm(-1) for 2, -105 cm(-1) for 3, -119.5 cm(-1) for 4). The pairwise interactions in 5 have been evaluated qualitatively to result in S(T) = 3/2 spin ground state, which has been verified by magnetization experiment. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH(2)) as the substrate, catecholase activity of all the five complexes have been checked. While 1 and 3 are inactive, complexes 2, 4, and 5 show catecholase activity with turn over numbers 39 h(-1) (for 2), 40 h(-1) (for 4), and 48 h(-1) (for 5) in dmf and 167 h(-1) (for 2) and 215 h(-1) (for 4) in acetonitrile. Conductance of the dmf solution of the complexes has been measured, revealing that bridging moieties and nuclearity have been almost retained in solution. Electrospray ionization mass (ESI-MS positive) spectra of complexes 1, 2, and 4 have been recorded in acetonitrile solutions and the positive ions have been well characterized. ESI-MS positive spectrum of complex 2 in presence of 3,5-DTBCH(2) have also been recorded and, interestingly, a positive ion [Cu(II)(2)L(µ-3,5-DTBC(2-))(3,5-DTBCH(-))Na(I)](+) has been identified.

Magnetic exchange interactions and magneto-structural correlations in heterobridged µ-phenoxo-µ(1,1)-azide dinickel(II) compounds: a combined experimental and theoretical exploration.

This investigation presents the syntheses, crystal structures, magnetic properties, and density functional theoretical modeling of magnetic behavior of two heterobridged µ-phenoxo-µ(1,1)-azido dinickel(II) compounds [Ni(II)(2)(L(1))(2)(µ(1,1)-N(3))(N(3))(H(2)O)]·CH(3)CH(2)OH (1) and [Ni(II)(2)(L(2))(2)(µ(1,1)-N(3))(CH(3)CN)(H(2)O)](ClO(4))·H(2)O·CH(3)CN (2), where HL(1) and HL(2) are the [1+1] condensation products of 3-methoxysalicylaldehyde and 1-(2-aminoethyl)-piperidine (for HL(1))/4-(2-aminoethyl)-morpholine (for HL(2)), along with density functional theoretical magneto-structural correlations of µ-phenoxo-µ(1,1)-azido dinickel(II) systems. Compounds 1 and 2 crystallize in orthorhombic (space group Pbca) and monoclinic (space group P2(1)/c) systems, respectively. The coordination environments of both metal centers are distorted octahedral. The variable-temperature (2-300 K) magnetic susceptibilities at 0.7 T of both compounds have been measured. The interaction between the metal centers is moderately ferromagnetic; J = 16.6 cm(-1), g = 2.2, and D = -7.3 cm(-1) for 1 and J = 16.92 cm(-1), g = 2.2, and D(Ni1) = D(Ni2) = -6.41 cm(-1) for 2. Broken symmetry density functional calculations of exchange interaction have been performed on complexes 1 and 2 and provide a good numerical estimate of J values (15.8 cm(-1) for 1 and 15.35 cm(-1) for 2) compared to experiments. The role of Ni-N bond length asymmetry on the magnetic coupling has been noted by comparing the structures and J values of complexes 1 and 2 together with previously published dimers 3 (Eur. J. Inorg. Chem. 2009, 4982), 4 (Inorg. Chem. 2004, 43, 2427), and 5 (Dalton Trans. 2008, 6539). Our extensive DFT calculations reveal an important clue to the mechanism of coupling where the orientation of the magnetic orbitals seems to differ with asymmetry in the Ni-N bond lengths. This difference in orientation leads to a large change in the overlap integral between the magnetic orbitals and thus the magnetic coupling. DFT calculations have also been extended to develop several magneto-structural correlations in this type of complexes and the correlation aim to focus on the asymmetry of the Ni-N bond lengths reveal that the asymmetry plays a proactive role in governing the magnitude of the coupling. From a completely symmetric Ni-N bond length, two behaviors have been noted: with a decrease in bond length there is an increase in the ferromagnetic coupling, while an increase in the bond lengths leads to a decrease in ferromagnetic interaction. The later correlation is supported by experiments. The magnetic properties of 1, 2, and three previously reported related compounds have been discussed in light of the structural parameters and also in light of the theoretical correlations determined here.

Syntheses, structures, and magnetic properties of three one-dimensional end-to-end azide/cyanate-bridged copper(II) compounds exhibiting ferromagnetic interaction: new type of solid state isomerism.

The work in this paper presents the syntheses, structures, and magnetic properties of three end-to-end (EE) azide/cyanate-bridged copper(II) compounds [Cu(II)L(1)(µ(1,3)-NCO)](n)·2nH(2)O (1), [Cu(II)L(1)(µ(1,3)-N(3))](n)·2nH(2)O (2), and [Cu(II)L(2)(µ(1,3)-N(3))](n) (3), where the ligands used to achieve these species, HL(1) and HL(2), are the tridentate Schiff base ligands obtained from [1 + 1] condensations of salicylaldehyde with 4-(2-aminoethyl)-morpholine and 3-methoxy salicylaldehyde with 1-(2-aminoethyl)-piperidine, respectively. Compounds 1 and 2 crystallize in the monoclinic P2(1)/c space group, while compound 3 crystallizes in the orthorhombic Pbca space group. The metal center in 1-3 is in all cases pentacoordinated. Three coordination positions of the metal center in 1, 2, or 3 are satisfied by the phenoxo oxygen atom, imine nitrogen atom, and morpholine (for 1 and 2) or piperidine (for 3) nitrogen atom of one deprotonated ligand, [L(1)](-) or [L(2)](-). The remaining two coordination positions are satisfied by two nitrogen atoms of two end-to-end bridging azide ligands for 2 and 3 and one nitrogen atom and one oxygen atom of two end-to-end bridging cyanate ligands for 1. The coordination geometry of the metal ion is distorted square pyramidal in which one EE azide/cyanate occupies the apical position. Variable-temperature (2-300 K) magnetic susceptibilities of 1-3 have been measured under magnetic fields of 0.05 (from 2 to 30 K) and 1.0 T (from 30 to 300 K). The simulation reveals a ferromagnetic interaction in all three compounds with J values of +0.19 ± 0.01, +0.79 ± 0.01, and +1.25 ± 0.007 cm(-1) for 1, 2, and 3, respectively. Compound 1 is the sole example of a ferromagnetically coupled EE cyanate-bridged 1-D copper(II) system. In addition, a rare example of supramolecular isomerism and a nice example of magnetic isomerism have been observed and most interestingly a new type of solid state isomerism has emerged as a result of the comparison of the structure and magnetic properties of 2 with a previously published compound (2A) having the same composition and even the same crystal system and space group (New J. Chem.2001, 25, 1203-1207).

Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II∕III) complex with two crystallographically inequivalent iron sites.

The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(µ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).

Magneto-structural correlation studies and theoretical calculations of a unique family of single end-to-end azide-bridged Ni(II)4 cyclic clusters.

The work in this paper aims to portray a complete structural, magnetic, and theoretical description of two original end-to-end (EE) µ(1,3)-azide-bridged, cyclic tetranuclear Ni(II) clusters, [{Ni(II)(L(1))(µ(1,3)-N(3))(H(2)O)}(4)] (1) and [{Ni(II)(L(2))(µ(1,3)-N(3))(H(2)O)}(4)] (2), where the ligands used to achieve these species, HL(1) and HL(2), are the tridentate Schiff base ligands obtained from [1 + 1] condensations of salicylaldehyde with 1-(2-aminoethyl)-piperidine and 4-(2-aminoethyl)-morpholine, respectively. The title compounds, 1 and 2, crystallize in a monoclinic P2(1) space group. Overall, both species can be described in a similar way; where all Ni(II) centers within each molecule are hexacoordinated and bound to [L(1)](-) or [L(2)](-) through the phenoxo oxygen, imine nitrogen, and piperidine/morpholine nitrogen atoms of the corresponding ligand. The remaining coordination sites are satisfied by one molecule of H(2)O and two nitrogen atoms from N(3)(-) anions. The latest act as bridges between Ni(II) ions, and eventually, only four azido groups are linked to the same number of Ni(II) centers resulting in the formation of cyclic Ni(II)(4) systems. Interestingly, compounds 1 and 2 are the two sole examples of tetranuclear clusters generated exclusively by EE azide-bridging ligands to date. All the N(azide)-Ni-N(azide) moieties are almost linear in 1 and 2 indicating trans arrangement of the azido ligand. Variable-temperature (2-300 K) magnetic susceptibilities of 1 and 2 have been measured under magnetic fields of 0.04 T (from 2 to 30 K) and 0.7 T (from 30 to 300 K), and magneto-structural correlations have been performed. Despite the presence of both ferromagnetic and antiferromagnetic interactions in both compounds, significant differences have been observed in their magnetic behaviors directly related to the arrangement of the bridging azido ligands. Hence, compound 1 has an overall moderate antiferromagnetic behavior due to the presence of an exchange pathway with an unprecedented Ni-N···N-Ni torsion angle close to 0°, meanwhile complex 2 exhibits a predominant ferromagnetic behavior, with torsion angles between 50 and 90°. Density functional theory calculations have been performed to provide more insight into the magnetic nature of this new family of Ni(II)-azido complexes and also to corroborate the fitting of the data.