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ABSTRACT: Three new trinuclear nickel (II) complexes with the general composition [Ni(3) L(3) (OH)(X)](ClO(4) ) have been prepared in which X=Cl(-) (1), OCN(-) (2), or N(3) (-) (3) and HL is the tridentate N,N,O donor Schiff base ligand 2-[(3-dimethylaminopropylimino)methyl]phenol. Single-crystal structural analyses revealed that all three complexes have a similar Ni(3) core motif with three different types of bridging, namely phenoxido (μ(2) and μ(3) ), hydroxido (μ(3) ), and μ(2) -Cl (1), μ(1,1) -NCO (2), or μ(1,1) -N(3) (3). The nickel(II) ions adopt a compressed octahedron geometry. Single-crystal magnetization measurements on complex 1 revealed that the pseudo-three-fold axis of Ni(3) corresponds to a magnetic easy axis, being consistent with the magnetic anisotropy expected from the coordination structure of each nickel ion. Temperature-dependent magnetic measurements indicated ferromagnetic coupling leading to an S=3 ground state with 2J/k=17, 17, and 28 K for 1, 2, and 3, respectively, with the nickel atoms in an approximate equilateral triangle. The high-frequency EPR spectra in combination with spin Hamiltonian simulations that include zero-field splitting parameters D(Ni) /k=-5, -4, and -4 K for 1, 2, and 3, respectively, reproduced the EPR spectra well after a anisotropic exchange term was introduced. Anisotropic exchange was identified as D(i,j) /k=-0.9, -0.8, and -0.8 K for 1, 2, and 3, respectively, whereas no evidence of single-ion rhombic anisotropy was observed spectroscopically. Slow relaxation of the magnetization at low temperatures is evident from the frequency-dependence of the out-of-phase ac susceptibilities. Pulsed-field magnetization recorded at 0.5 K shows clear steps in the hysteresis loop at 0.5-1 T, which has been assigned to quantum tunneling, and is characteristic of single-molecule magnets.
Chemistry 01/2013; · 5.93 Impact Factor
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ABSTRACT: The reaction of a tridentate Schiff base ligand HL (2-[(3-dimethylaminopropylimino)-methyl]-phenol) with Ni(II) acetate or perchlorate salts in the presence of azide as coligand has led to two new Ni(II) complexes of formulas [Ni 3 L 2 (OAc) 2 (μ 1,1 -N 3) 2 (H 2 O) 2 ]·2H 2 O (1) and [Ni 2 L 2 (μ 1,1 -N 3)(μ 1,3 -N 3)] n (2). Single crystal X-ray structures show that complex 1 is a linear trinuclear Ni(II) compound containing a μ 2 -phenoxido, an end-on (EO) azido and a syn-syn acetato bridge between the terminal and the central Ni(II) ions. Complex 2 can be viewed as a one-dimensional (1D) chain in which the triply bridged (di-μ 2 -phenoxido and EO azido) dimeric Ni 2 units are linked to each other in a zigzag pattern by a single end-to-end (EE) azido bridge. Variable-temperature magnetic susceptibility studies indicate the presence of moderate ferromagnetic exchange coupling in complex 1 with J value of 16.51(6) cm −1 . The magnetic behavior of 2 can be fitted in an alternating ferro-and antiferromagnetic model [J FM = +34.2(2.8) cm −1 and J AF = −21.6(1.1) cm −1 ] corresponding to the triple bridged dinuclear core and EE azido bridge respectively. Density functional theory (DFT) calculations were performed to corroborate the magnetic results of 1 and 2. The contributions of the different bridges toward magnetic interactions in both compounds have also been calculated. ■ INTRODUCTION Research on coordination polymers of transition-metals with novel topology has evoked great interest because of their impressive structural diversity and intriguing physical proper-ties. 1,2 Simple paramagnetic metal ions upon linking with appropriate bridging ligands may generate coordination polymers with interesting magnetic properties. Exchange interactions propagated by multiatom bridges between para-magnetic centers that are able to efficiently transmit the magnetic exchange interactions allow the preparation of interesting magnetic materials. 3−5 Among them metal-azido polynuclear complexes deserve special mention because of their fascinating structural diversities, their importance in under-standing magneto-structural correlations, and their promising potential applications in functional materials. 6 The main expression of the versatility of this ligand lies in the different coordination modes that it can offer; the most common ones are the end-to-end (μ 1,3 -N 3 , EE) and end-on (μ 1,1 -N 3 , EO) modes. In general, EE azido bridges propagate antiferromag-netic interactions, 7 whereas the EO coordination mode is associated with ferromagnetic coupling; 8 although exceptions to this general statement have been reported. 9 Furthermore, different bridging modes of the azido ions may simultaneously exist in the same species, leading to interesting topologies and magnetic behaviors. 10 The study of magnetic exchange mediated by azide in its different bridging modes is often complicated by the presence of additional bridging ligands as all these bridges may either add or counterbalance their effects. 11−13 Among the possibilities of innumerable combinations of different bridges, a combination of phenoxido and/or carboxylato groups with the azido ligand in one system is an interesting approach for constructing new materials and for modulating magnetic behaviors as both these ligand can transmit F or AF coupling between metal centers. For the phenoxido bridges, usually when the Ni−O-(phenoxido)−Ni bridging angle is less than 97° ferromagnetic coupling occurs whereas the coupling becomes antiferromag-netic at larger bridging angles. 14 On the other hand, the carboxylate functionality can offer a variety of magnetic interactions depending on its versatile bridging modes syn− syn, anti−anti, syn−anti (Scheme 1). In general, significant
Inorganic Chemistry 07/2012; · 4.60 Impact Factor
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ABSTRACT: Three new Mn(II) coordination compounds {[Mn(NCNCN)(2)(azpy)]·0.5azpy}(n) (1), {[Mn(NCS)(2)(azpy)(CH(3)OH)(2)]·azpy}(n) (2), and [Mn(azpy)(2)(H(2)O)(4)][Mn(azpy)(H(2)O)(5)]·4PF(6)·H(2)O·5.5azpy (3) (where azpy = 4,4'-azobis(pyridine)) have been synthesized by self-assembly of the primary ligands, dicyanamide, thiocyanate, and hexafluorophosphate, respectively, together with azpy as the secondary spacer. All three complexes were characterized by elemental analyses, IR spectroscopy, thermal analyses, and single crystal X-ray crystallography. The structural analyses reveal that complex 1 forms a two-dimensional (2D) grid sheet motif. These sheets assemble to form a microporous framework that incorporates coordination-free azpy by host-guest π···π and C-H···N hydrogen bonding interactions. Complex 2 features azpy bridged one-dimensional (1D) chains of centrosymmetric [Mn(NCS)(2)(CH (3)OH)(2)] units which form a 2D porous sheet via a CH(3)···π supramolecular interaction. A guest azpy molecule is incorporated within the pores by strong H-bonding interactions. Complex 3 affords a 0-D motif with two monomeric Mn(II) units in the asymmetric unit. There exist π···π, anion···π, and strong hydrogen bonding interactions between the azpy, water, and the anions. Density functional theory (DFT) calculations, at the M06/6-31+G* level of theory, are used to characterize a great variety of interactions that explicitly show the importance of host-guest supramolecular interactions for the stabilization of coordination compounds and creation of the fascinating three-dimensional (3D) architecture of the title compounds.
Inorganic Chemistry 02/2012; 51(3):1837-51. · 4.60 Impact Factor
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ABSTRACT: Four new Mn(II) coordination polymers [Mn(4,4′-bpy)(C6H5COO)2]n (1), [Mn(4,4′-bpy)(o-(NO2)C6H4COO)2]n (2), [Mn(4,4′-bpy)(m-(NO2)C6H4COO)2]n (3), and [Mn(4,4′-bpy)(p-(NO2)C6H4COO)2]n (4) (where 4,4′-bpy = 4,4′-bipyridine) have been synthesized by self-assembly of the primary ligands, benzoate and the o-, m-, and p-isomers of nitrobenzoates, respectively, together with 4,4′-bpy as the secondary spacer. All four complexes were characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction analyses, and variable-temperature magnetic measurements. The structural analyses reveal that complexes 1 and 3 are constructed by linear fused chains through double syn-syn (for 1) or syn-anti (for 3) carboxylate-bridged Mn(II), which are further linked to one another by trans coordinated 4,4′-bpy bridges, giving rise to a rectangular grid-like two-dimensional (2D) net. Complex 2 features one-dimensional (1D) molecular ladder formed by both syn-syn and syn-anti carboxylate-bridged dimeric Mn(II) units which are joined alternately by 4,4′-bpy. Complex 4 is formed by fused zigzag chains of double syn-syn carboxylate-bridged Mn(II) that are connected by cis oriented 4,4′-bpy to generate an unprecedented three-dimensional (3D) framework. The dimensionality of the complexes thus varies from 1D to 2D to 3D on changing the position of the nitro group from o- to m- to p- in the benzoate, showing explicitly the tuning ability of this apparently innocent substituent on the topology of the coordination polymer. Variable-temperature (1.8–300 K) magnetic susceptibility measurements showed the presence of antiferromagnetic coupling in all four complexes that have been fitted with an infinite classical-spin chain model derived by Fisher for 1, 3, and 4 (J = −0.779(2), −0.855(2), and −0.536(2) cm–1, respectively) and van Vleck equation for 2 (J = −0.354(2) cm–1).
10/2011;
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ABSTRACT: Two new nickel(II) complexes [Ni(2)L(2)(PhCOO)(2)(H(2)O)] (1), [Ni(2)L(2)(PhCH(2)COO)(2)(H(2)O)] (2) have been synthesized using a tridentate Schiff base ligand, HL (2-[(3-dimethylamino-propylimino)-methyl]-phenol) and the carboxylate monoanions, benzoate and phenylacetate, respectively. The complexes have been characterized by spectral analysis, variable temperature magnetic susceptibility measurement and crystal structure analysis. The structural analyses reveal that both complexes are dinuclear in which the distorted octahedral Ni(2+) ions share a face, bridged by one water molecule and two μ(2)-phenoxo oxygen atoms. A monodentate benzoate or phenylacetate anion and two nitrogen atoms of the chelating deprotonated Schiff base (L) complete the hexa-coordination around the metal ion. Variable-temperature magnetic susceptibility studies indicate the presence of dominant ferromagnetic exchange coupling in complexes 1 and 2 with J values of 11.1(2) and 10.9(2) cm(-1) respectively. An attempt has been made to rationalize the observed magneto-structural behavior considering the importance of the additional water bridge in the present two complexes and also in other similar species.
Dalton Transactions 05/2011; 40(19):5324-31. · 3.84 Impact Factor
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ABSTRACT: The preparation, crystal structures and magnetic properties of two new isoelectronic and isomorphous formate- and nitrite-bridged 1D chains of Mn(III)-salen complexes, [Mn(salen)(HCOO)](n) (1) and [Mn(salen)(NO(2))](n) (2), where salen is the dianion of N,N'-bis(salicylidene)-1,2-diaminoethane, are presented. The structures show that the salen ligand coordinates to the four equatorial sites of the metal ion and the formate or nitrite ions coordinate to the axial positions to bridge the Mn(III)-salen units through a syn-antiμ-1κO:2κO' coordination mode. Such a bridging mode is unprecedented in Mn(III) for formate and in any transition metal ion for nitrite. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 indicate the presence of ferromagnetic exchange interactions with J values of 0.0607 cm(-1) (for 1) and 0.0883 cm(-1) (for 2). The ac measurements indicate negligible frequency dependence for 1 whereas compound 2 exhibits a decrease of χ(ac)' and a concomitant increase of χ(ac)'' on elevating frequency around 2 K. This finding is an indication of slow magnetization reversal characteristic of single-chain magnets or spin-glasses. The μ-nitrito-1κO:2κO' bridge seems to be a potentially superior magnetic coupler to the formate bridge for the construction of single-molecule/-chain magnets as its coupling constant is greater and the χ(ac)' and χ(ac)'' show frequency dependence.
Dalton Transactions 02/2011; 40(13):3295-304. · 3.84 Impact Factor
