Methanol-Triggered Vapochromism Coupled with Solid-State Spin Switching in a Nickel(II)-Quinonoid Complex.

A highly methanol-selective vapochromic response has been realized in a NiII -quinonoid complex, [Ni(HLMe )2 ] (H2 LMe =4-methylamino-6-methyliminio-3-oxocyclohexa-1,4-dien-1-olate) which exhibits a reversible structural transformation including a coordination geometrical change between the square-planar and octahedral structure by the selective uptake of methanol vapor. This was accompanied by a remarkable color change between purple and orange, as well as temperature-robust spin-state switching in the solid state under ambient conditions. It is remarkable that the properties are derived by the fine structural modification of the quinonoid ligand such as methyl or ethyl analogues. Such a system has high potential for applications in memory devices as well as chemical sensors and smart responsive materials.

Colour tuning by the stepwise synthesis of mononuclear and homo- and hetero-dinuclear platinum(ii) complexes using a zwitterionic quinonoid ligand.

The stepwise reaction of a zwitterionic ligand, 4-methylamino-6-methyliminio-3-oxocyclohexa-1,4-dien-1-olate (QH2) with [Pt2(µ-Cl)2(ppy)2] (Hppy = 2-phenylpyridine) afforded a mononuclear complex, [Pt(ppy)(QH)] (1), and a dinuclear complex, [{Pt(ppy)}2(µ-Q)] (2). Using [Pd2(µ-Cl)2(ppy)2] in the second step resulted in the formation of a heterodinuclear complex, [{Pt(ppy)}(µ-Q){Pd(ppy)}] (3), which is the first heterodinuclear complex bridged by a quinonoid ligand. Single crystal X-ray diffraction analysis revealed that all three complexes adopted double-decker structures in the crystal. For 1, intermolecular N-HO interactions between uncoordinated N-H and O groups in two adjacent square-planar mononuclear units led to the formation of hydrogen-bonded dimers, which stacked to form a herringbone structure with a double-decker tetranuclear motif. For 2 and 3, dinuclear units bridged by Q(2-) formed a double-decker motif similar to that of 1, but a tetranuclear chain in the herringbone pattern was characteristic of the dinuclear complexes. PtPt (or PtPd) distances were more than 3.5 Å, twice the van der Waals radii of Pt, suggesting weak electronic metal-metal interactions in the crystal structures. Thus, the different colours observed (brown, purple, and dark green for 1, 2, and 3, respectively) mainly originated from the molecular structures. In fact, the three complexes exhibited colourful solutions of yellow, red, and green. UV-vis absorption spectroscopy and time-dependent density-functional theory (TD-DFT) calculations revealed that colour variations occurred depending on the electronic states composed of metal ions and the quinonoid ligand.

Synthesis of mixed-valence hexanuclear Mn(II/III) clusters from its Mn(II) precursor: variations of catecholase-like activity and magnetic coupling.

One Mn(II) coordination polymer, [Mn(o-(NO2)C6H4COO)2(pyz)(H2O)]n (1), has been synthesized and oxidized with n-Bu4NMnO4 in non-aqueous media to two mixed-valence hexanuclear Mn(II/III) complexes [MnIII2MnII4O2(pyz)0.61/(MeOH)0.39(o-(NO2)C6H4COO)10·(H2O)·{(CH3)2CO}2]·(CH3)2CO (2) and [MnIII2MnII4O2(pyz)0.28/(MeCN)3.72(o-(NO2)C6H4COO)10·(H2O)] (3) (where pyz = pyrazine). All three complexes were characterized by elemental analyses, IR spectroscopy, single-crystal X-ray diffraction analyses, and variable-temperature magnetic measurements. The structural analyses reveal that complex 1 is comprised of linear chains of pyz bridged Mn(II), which are further linked to one another by syn­anti carboxylate bridges, giving rise to a two-dimensional (2D) net. Complexes 2 and 3 feature mixed valence [MnIII2MnII4] units in which each of the six manganese centres reside in an octahedral environment. Apart from the variations in terminal ligands (acetone for 2 and acetonitrile for 3), the complexes are very similar. Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, the catecholase-like activity of the complexes has been studied and it is found that the mixed valent Mn6 complexes (2 and 3) are much more active towards aerial oxidation of catechol compared to the Mn(II) complex (1). Variable-temperature (1.8­300 K) magnetic susceptibility measurements showed the presence of antiferromagnetic coupling in all three complexes. The magnetic data have been fitted with a 2D quadratic model derived by Lines, giving the exchange constant J/kB = −0.0788(5) K for 1. For 2 and 3, antiferromagnetic interactions within the Mn6 cluster have been fitted with models containing three exchange constants: JA/kB = −70 K, JB/kB = −0.5 K, JC/kB = −2.9 K for 2 and JA/kB = −60 K, JB/kB = −0.3 K, JC/kB = −2.8 K for 3.

A new family of trinuclear nickel(II) complexes as single-molecule magnets.

Three new trinuclear nickel (II) complexes with the general composition [Ni3 L3 (OH)(X)](ClO4 ) have been prepared in which X=Cl(-) (1), OCN(-) (2), or N3(-) (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 Ni3 core motif with three different types of bridging, namely phenoxido (µ2 and µ3 ), hydroxido (µ3 ), and µ2 -Cl (1), µ1,1 -NCO (2), or µ1,1 -N3 (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 Ni3 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 DNi /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 Di,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.

Coordination polymers containing manganese(II)-azido layers connected by dipyridyl-tetrazine and 4,4'-azobis(pyridine) linkers.

Two new polynuclear manganese(II) complexes [Mn(dptz)(N(3))(2)](n) (1) and [Mn(azpy)(N(3))(2)](n) (2) (where dptz = dipyridyl-tetrazine and azpy = 4,4'-azobis(pyridine)) have been synthesized by self-assembly of the ligand azide, together with dptz and azpy as secondary spacers. The compounds are characterized by single-crystal X-ray diffraction analyses and variable-temperature magnetic measurements. The structural analyses reveal that in complex 1, which is the first reported Mn(II) complex with the ligand dptz, two µ(1,3) bridging azides connect neighboring manganese ions in a zigzag manner to generate a neutral two-dimensional (2D) sheet which is further connected by the dptz ligands to form a three-dimensional (3D) framework. By contrast, complex 2 contains dimeric [Mn(2)(µ(1,1)-N(3))(2)](2+) fragments linked to four identical motifs by means of four single µ(1,3)-N(3) bridges, that generates a neutral 2D Mn(II)-azide sheet which is further interconnected by azpy ligands to neighboring manganese ions forming an unprecedented 3D network. Variable-temperature (2-300 K) magnetic susceptibility measurements show the presence of predominantly antiferromagnetic coupling for both complexes that has been reproduced with a regular antiferromagnetic S = 5/2 chain (J) with interchain interactions (j) modeled with the molecular field approximation with J = -7.1 cm(-1) and j' = -0.8 cm(-1) for 1 and J = -4.2 cm(-1) and j' = 0.1 cm(-1) for 2.

A rare phenoxido/acetato/azido bridged trinuclear and an unprecedented phenoxido/azido bridged one-dimensional polynuclear nickel(II) complexes: synthesis, crystal structure, and magnetic properties with theoretical investigations on the exchange mechanism.

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.

Antiferromagnetic porous metal-organic framework containing mixed-valence [Mn(II)4Mn(III)2(µ4-O)2]10+ units with catecholase activity and selective gas adsorption.

A multifunctional porous metal organic framework based on mixed-valence hexa-nuclear [Mn(III)(2)Mn(II)(4)O(2)(pyz)(2)(C(6)H(5)CH(2)COO)(10)] (pyz = pyrazine) units has been synthesized. The complex has been characterized by elemental analysis, IR spectroscopy, single-crystal X-ray diffraction analysis, and variable-temperature magnetic measurements. The structural analysis reveals that the bidentate pyz molecules connect each [Mn(6)] unit to its four [Mn(6)] neighbors through the peripheral Mn(II) centers, giving rise to a three-dimensional (3D) distorted diamond-like porous framework. Variable-temperature (2-300 K) magnetic susceptibility measurements show the presence of dominant antiferromagnetic interactions within the discrete [Mn(6)] cluster that have been fitted with a model containing three exchange constants developed for the complex (J(1) = -8.6 cm(-1), J(2) = -3.9 cm(-1), and J(3) = -100.0 cm(-1)). Using 3,5-di-tert-butyl catechol (3,5-DTBC) as the substrate, catecholase activity of the complex has been studied; the turn over number is determined to be of 2547 h(-1) in acetonitrile. This porous compound shows remarkable selectivity for adsorption of CO(2) over N(2) that may be correlated with the effect of window flexibility of the pore to the corresponding adsorbate molecules.

Host-guest supramolecular interactions in the coordination compounds of 4,4'-azobis(pyridine) with MnX2 (X = NCS­, NCNCN­, and PF6(­)): structural analyses and theoretical study.

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.

The importance of an additional water bridge in making the exchange coupling of bis(µ-phenoxo) dinickel(II) complexes ferromagnetic.

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.

Structure and magnetic properties of an unprecedented syn-anti µ-nitrito-1κO:2κO' bridged Mn(III)-salen complex and its isoelectronic and isostructural formate analogue.

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.