Cubane Core Research Articles

A Family of Heterometallic Cubane‐Type Clusters with an exo‐Fe(CO)3 Fragment Anchored to the Cubane

Metal-rich metallaboranes: An exo-Fe(CO)3 moiety linked to a cubane core is the most prominent feature of complexes [(Cp*M)2(μ3-E)2B2H(μ-H){Fe(CO)2}2Fe(CO)3] (M=Mo, E=S, Se; M=Ru, E=CO; Cp*=η5-C5Me5), which are the first heterobimetallic cubane-type clusters with a boride unit (see structure).

A new tetranuclear copper(II) Schiff base complex containing Cu 4O 4 cubane core: Structural and spectral characterizations

A new tetra-nuclear coordination complex [Cu 4(HL) 4] ( 1) containing Cu 4O 4 cubane core has been synthesized by using Schiff base ligand [(OH)C 6H 4CH N C(CH 3)(CH 2OH) 2] (H 3L), obtained by the 1:1 condensation of 2-amino-2-methyl-1,3-propanediol with salicylaldehyde and thoroughly characterized by micro-analytical, FT-IR, UV–Vis, thermal and room temperature magnetic susceptibility measurements. Structural characterization of the complex has been done by single crystal X-ray diffraction analysis. Structural elucidation reveals versatile coordination modes for two identical alkoxo oxygen atoms of the Schiff base ligand; one in its deprotonated form exhibits μ 3-bridging to bind three similar copper(II) centers whilst the protonated one remains as monodentate or non-coordinating. Structural analysis also shows that the Cu 4O 4 cubane core in 1 consists of four μ 3-alkoxo oxygen bridged copper(II) atoms giving an approximately cubic array of alternating oxygen atoms and copper(II) atoms where the metal centers display both distorted square pyramidal and distorted octahedral geometries.

Structural similarities in enzymatic, homogeneous and heterogeneous catalysts of water oxidation

A virtually identical cubical or cubane core structure appears to be a key commonality in several active homogeneous, heterogeneous and enzymatic water oxidation catalysts. We show that X-ray crystal structures of the CaMn3O4 core of the water oxidizing complex of Photosystem II coincide remarkably closely with comparable structures in a range of non-biological Co or Mn-based homogeneous and heterogeneous water oxidation catalysts. Included amongst these are molecular Co4O4 and Mn4O4 cubanes, as well as Co and Mn spinels containing cubical arrangements. This observation supports the existence of an optimum structural element for water oxidation catalysis. It also offers a potential avenue for comparing and relating homogeneous, heterogeneous and enzymatic catalysis to each other.

Experimental and Theoretical Mössbauer Study of an Extended Family of [Fe8(μ4-O)4(μ-4-R-px)12X4] Clusters

Six [Fe(8)(μ(4)-O)(4)(μ-4-R-pyrazolato)(12)X(4)] complexes containing an identical Fe(8)(μ(4)-O)(4) core have been structurally characterized and studied by Mössbauer spectroscopy. In each case, an inner μ(4)-O bridged Fe(III) cubane core is surrounded by four trigonal bipyramidal iron centers, the two distinct sites occurring in a 1:1 ratio. The Mössbauer spectrum of each of the clusters consists of two quadrupole doublets, which, with one exception (X = NCS, R = H), overlap to give three absorption lines. The systematic variation of X and R causes significant changes in the Mössbauer spectra. A comparison with values for the same clusters computed using density functional theory allows us to establish an unequivocal assignment of these peaks in terms of a nested model for the overlapping doublets. The changes in Mössbauer parameters (both experimental and computed) for the 1-electron reduced species [Fe(8)(μ(4)-O)(4)(μ-4-Cl-pyrazolato)(12)Cl(4)](-) are consistent with a redox event that is localized within the cubane core.

A Tetranuclear Nickel(II) Cubane Complex with O-Vanillin Ligand

A novel cubane complex with formula of [Ni(μ3-OCH3)L(CH3OH)]4 (1) has been successfully assembled by using o-vanillin as multidentate ligand. Each Ni(II) ion is six coordinate, being bound to four oxygen atoms from four methanol molecules, one phenoxido oxygen atom and one aldehyde oxygen atom from the o-vanillin ligand. The cubane structure is characterized by a Ni4O4 core assembled from four nickel atoms and four deprotonated, bidentate o-vanillin ligands and the cubic array of alternating nickel and oxygen atoms give intracluster metal–metal separations of 3.054(1) and 3.076(1) A respectively. Preliminary magnetic studies suggest the existence of a weak antiferromagnetic coupling between the nickel ions. A novel tetranuclear nickel(II) complex with Ni4O4 cubane core has been assembled by using o-vanillin as multidentate ligand.

Magnetic Properties of Dysprosium Cubanes Dictated by the M−O−M Angles of the [Dy4(μ3-OH)4] Core

Two new dysprosium(III) coordination compounds, namely, [Dy(4)(HL)(4)(C(6)H(4)NH(2)COO)(2)(mu(3)-OH)(4)(mu-OH)(2)(H(2)O)(4)].4CH(3)CN.12H(2)O (1) and Dy(8)(HL)(10)(C(6)H(4)NH(2)COO)(2)(mu(3)-OH)(8)(OH)(2)(NO(3))(2)(H(2)O)(4)] (2), have been synthesized from the Schiff-base ligand 2-{[(2-hydroxy-3-methoxyphenyl)methylidene]amino}benzoic acid (H(2)L) and dysprosium chloride (1) or dysprosium nitrate (2). Single-crystal X-ray diffraction studies reveal that compound 1 exhibits a tetranuclear cubane-like structure and that 2 is an octanuclear, bis-cubane complex. The [Dy(4)(mu (3)-OH)(4)] cubane cores of 1 and 2 are structurally related; however, the magnetic properties of 1 and 2 are drastically different. Indeed, 2 shows slow relaxation of magnetization while no out-of-phase alternating current (ac) signal is noticed for 1. These significant disparities are most likely due to the different M-O-M angles observed for the respective cubane cores.

A dimeric polyoxometalate sandwich motif containing a truncated {Mn 3O 4} cubane core

A novel sandwich-type silicotungstate motif, K 18[Mn II 2{Mn II(H 2O) 5Mn III 3(H 2O)(B- β-SiW 9O 34)(B -β-SiW 6O 26)} 2]·20H 2O 1, has been isolated from the reaction of K 8[γ-SiW 10O 36]·12H 2O and manganese ions in aqueous acidic media. The transition metal-substituted polyoxometalate (TMSP) 1 has been fully characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis, and infrared spectroscopy. This dimeric polyanion consists structurally of the sandwich polyanion {Mn II(H 2O) 5Mn III 3(H 2O)(B- β-SiW 9O 34)(B- β-SiW 6O 26)}, dimerized via two manganese(II) linker ions. Each monomeric unit is composed of two non-equivalent Keggin fragments, (B- β-SiW 8O 31) and (B- β-SiW 6O 26), linked to each other via three manganese ions resulting in a truncated {Mn 3O 4} cubane core. Experimental, structural, and electrochemical aspects of the material are reported and discussed.

Novel 1D Cu(I) coordination polymers formed by the combination of Cu(I) halides and 4-(2-pyridyl)pyrimidine

Three novel 1D Cu(I) coordination polymers [Cu 4X 4(pprd) 2] n (X = Cl( 1), Br( 2) and I( 3); pprd = 4-(2-pyridyl)pyrimidine) were systematically synthesized by Cu(I) halides and the pprd ligand, and they have been characterized by X-ray, IR, and TG-DTA analyses. The molecular structure of complex 1 essentially resembles to that of complex 2. In complexes 1 and 2, four Cu(I) atoms are bridged by four Cl or Br anions to form an eight-membered Cu 4X 4 framework in the twist-chair form. Furthermore, the Cu 4X 4 frameworks are coordinated by the chelate and bridging sites of two pprd ligands to form a unique 1D two-stepped Cu(I) coordination polymer, in which two stairs are formed by the Cu 4X 4 core and two heteroaromatic planes of pprd. In the crystal packing structures, it is interesting that two heteroaromatic planes of pprd are stacking along the b-axis for complex 1 and the a-axis for complex 2. In contrast, four Cu(I) atoms in complex 3 are bridged by four I atoms to form a Cu 4I 4 stepped cubane tetramer. Additionally, the Cu 4I 4 stepped cubane cores are linked by the chelate and bridging sites of two pprd ligands to form an infinite 1D zigzag-chain Cu(I) coordination polymer. The thermal decomposition behaviors for Cu(I)–X/pprd complexes 1, 2 and 3 were determined by thermogravimetric analysis (TG-DTA). Although the thermal decomposition behaviors of complex 1 were unidentified, those of complexes 2 and 3 were assigned. The mass loss at the first stage of thermal decomposition for polymeric [Cu 4X 4(pprd) 2] n was identical to the formation of oligomeric [Cu 4X 4(pprd)] by the elimination of one pprd molecule. The mass loss at the next stage was decided to the formation of Cu 4X 4 by the elimination of another pprd molecule.

Carbonyl compound dependent hydrolysis of mono-condensed Schiff bases: A trinuclear Schiff base complex and a mononuclear mixed-ligand ternary complex of copper(II)

Two Schiff bases, HL 1 and HL 2 have been prepared by the condensation of N-methyl-1,3-propanediamine (mpn) with salicylaldehyde and 1-benzoylacetone (Hbn) respectively. HL 1 on reaction with Cu(ClO 4) 2·6H 2O in methanol produced a trinuclear Cu II complex, [(CuL 1) 3( μ 3-OH)](ClO 4) 2·H 2O·0.5CH 2Cl 2 ( 1) but HL 2 underwent hydrolysis under similar reaction conditions to result in a ternary Cu II complex, [Cu(bn)(mpn)ClO 4]. Both complexes have been characterised by single-crystal X-ray analyses, IR and UV–Vis spectroscopy and electrochemical studies. The partial cubane core [Cu 3O 4] of 1 consists of a central μ 3-OH and three peripheral phenoxo bridges from the Schiff base . All three copper atoms of the trinuclear unit are five-coordinate with a distorted square–pyramidal geometry. The ternary complex 2 is mononuclear with the square–pyramidal Cu II coordinated by a chelating bidentate diamine (mpn) and a benzoylacetonate (bn) moiety in the equatorial plane and one of the oxygen atoms of perchlorate in an axial position. The results show that the Schiff base (HL 2) derived from 1-benzoylacetone is more prone to hydrolysis than that from salicylaldehyde (HL 1). Magnetic measurements of 1 have been performed in the 1.8–300 K temperature range. The experimental data clearly indicate antiferromagnetism in the complex. The best-fit parameters for complex 1 are g = 2.18(1) and J = −15.4(2) cm −1.

Synthesis, Magnetic Properties, and STM Spectroscopy of Cobalt(II) Cubanes [CoII4(Cl)4(HL)4

Reaction of cobalt(II) chloride hexahydrate with N-substituted diethanolamines H(2)L(2-4) (3) in the presence of LiH in anhydrous THF leads under anaerobic conditions to the formation of three isostructural tetranuclear cobalt(II) complexes [Co(II) (4)(Cl)(4)(HL(2-4))(4)] (4) with a [Co(4)(mu(3)-O)(4)](4+) cubane core. According to X-ray structural analyses, the complexes 4 a,c crystallize in the tetragonal space group I4(1)/a, whereas for complex 4 b the tetragonal space group P$\bar 4$ was found. In the solid state the orientation of the cubane cores and the formation of a 3D framework were controlled by the ligand substituents of the cobalt(II) cubanes 4. This also allowed detailed magnetic investigations on single crystals. The analysis of the SQUID magnetic susceptibility data for 4 a gave intramolecular ferromagnetic couplings of the cobalt(II) ions (J(1) approximately 20.4 K, J(2) approximately 7.6 K), resulting in an S=6 ground-state multiplet. The anisotropy was found to be of the easy-axis type (D=-1.55 K) with a resulting anisotropy barrier of Delta approximately 55.8 K. Two-dimensional electron-gas (2DEG) Hall magnetization measurements revealed that complex 4 a is a single-molecule magnet and shows hysteretic magnetization characteristics with typical temperature and sweep-rate dependencies below a blocking temperature of about 4.4 K. The hysteresis loops collapse at zero field owing to fast quantum tunneling of the magnetization (QTM). The structural and electronic properties of cobalt(II) cubane 4 a, deposited on a highly oriented pyrolytic graphite (HOPG) surface, were investigated by means of STM and current imaging tunneling spectroscopy (CITS) at RT and standard atmospheric pressure. In CITS measurements the rather large contrast found at the expected locations of the metal centers of the molecules indicated the presence of a strongly localized LUMO.

Two-Dimensional Networks of Lanthanide Cubane-Shaped Dumbbells

The syntheses, structures, and magnetic properties are reported for three new lanthanide complexes, [Ln(III)(4)(mu(3)-OH)(2)(mu(3)-O)(2)(cpt)(6)(MeOH)(6)(H(2)O)](2) (Ln = Dy (1.15MeOH), Ho (2.14MeOH), and Tb (3.18MeOH)), based on 4-(4-carboxyphenyl)-1,2,4-triazole ligand (Hcpt). The three complexes were confirmed to be isomorphous by infrared spectroscopy and single-crystal X-ray diffraction. The crystal structure of 1 reveals that the eight-coordinate metal centers are organized in two cubane-shaped moieties composed of four Dy(III) ions each. All metal centers in the cubane core are bridged by two mu(3)-oxide and two mu(3)-hydroxide asymmetrical units. Moreover, each cubane is linked to its neighbor by two externally coordinating ligands, forming the dumbbell {Dy(III)(4)}(2) moiety. Electrostatic interactions between the ligands of the triazole-bridged dimers form an extended supramolecular two-dimensional arrangement analogous to a metal-organic framework with quadrilateral spaces occupied by ligands from axial sheets and by four solvent molecules. The magnetic properties of the three compounds have been investigated using dc and ac susceptibility measurements. For 1, the static and dynamic data corroborate the fact that the {Dy(III)(4)} cubane-shaped core exhibits slow relaxation of its magnetization below 5 K associated with a single-molecule magnet behavior.

Development of Bioinspired Mn4O4−Cubane Water Oxidation Catalysts: Lessons from Photosynthesis

Hydrogen is the most promising fuel of the future owing to its carbon-free, high-energy content and potential to be efficiently converted into either electrical or thermal energy. The greatest technical barrier to accessing this renewable resource remains the inability to create inexpensive catalysts for the solar-driven oxidation of water. To date, the most efficient system that uses solar energy to oxidize water is the photosystem II water-oxidizing complex (PSII-WOC), which is found within naturally occurring photosynthetic organisms. The catalytic core of this enzyme is a CaMn(4)O(x) cluster, which is present in all known species of oxygenic phototrophs and has been conserved since the emergence of this type of photosynthesis about 2.5 billion years ago. The key features that facilitate the catalytic success of the PSII-WOC offer important lessons for the design of abiological water oxidation catalysts. In this Account, we examine the chemical principles that may govern the PSII-WOC by comparing the water oxidation capabilities of structurally related synthetic manganese-oxo complexes, particularly those with a cubical Mn(4)O(4) core ("cubanes"). We summarize this research, from the self-assembly of the first such clusters, through the elucidation of their mechanism of photoinduced rearrangement to release O(2), to recent advances highlighting their capability to catalyze sustained light-activated electrolysis of water. The [Mn(4)O(4)](6+) cubane core assembles spontaneously in solution from monomeric precursors or from [Mn(2)O(2)](3+) core complexes in the presence of metrically appropriate bidentate chelates, for example, diarylphosphinates (ligands of Ph(2)PO(2)(-) and 4-phenyl-substituted derivatives), which bridge pairs of Mn atoms on each cube face (Mn(4)O(4)L(6)). The [Mn(4)O(4)](6+) core is enlarged relative to the [Mn(2)O(2)](3+) core, resulting in considerably weaker Mn-O bonds. Cubanes are ferocious oxidizing agents, stronger than analogous complexes with the [Mn(2)O(2)](3+) core, as demonstrated both by the range of substrates they dehydrogenate or oxygenate (unactivated alkanes, for example) and the 25% larger O-H bond enthalpy of the resulting mu(3)-OH bridge. The cubane core topology is structurally suited to releasing O(2), and it does so in high yield upon removal of one phosphinate by photoexcitation in the gas phase or thermal excitation in the solid state. This is quite unlike other Mn-oxo complexes and can be attributed to the elongated Mn-O bond lengths and low-energy transition state to the mu-peroxo precursor. The photoproduct, [Mn(4)O(2)L(5)](+), an intact nonplanar butterfly core complex, is poised for oxidative regeneration of the cubane core upon binding of two water molecules and coupling to an anode. Catalytic evolution of O(2) and protons from water exceeding 1000 turnovers can be readily achieved by suspending the oxidized cubane, [Mn(4)O(4)L(6)](+), into a proton-conducting membrane (Nafion) preadsorbed onto a conducting electrode and electroxidizing the photoreduced butterfly complexes by the application of an external bias. Catalytic water oxidation can be achieved using sunlight as the only source of energy by replacing the external electrical bias with redox coupling to a photoanode incorporating a Ru(bipyridyl) dye.

Ferromagnetic Coupling in a New Copper(II) Schiff Base Complex with Cubane Core: Structure, Magnetic Properties, DFT Study and Catalytic Activity

AbstractA new tetranuclear complex, [Cu4L4]·2H2O (1), has been synthesized from the self‐assembly of copper(II) perchlorate and the tridentate Schiff base ligand 4‐chloro‐2‐[(E)‐(2‐hydroxyethylimino)methyl]phenol (H2L). Single‐crystal X‐ray diffraction studies reveal that complex 1 consists of a Cu4O4 cubane core where the four copper(II) centers are linked by μ3‐alkoxo bridges. Each CuII ion assumes a distorted square‐pyramidal geometry with CuNO4 chromophore. Variable‐temperature magnetic susceptibility data exhibit distinct ferromagnetic exchange interaction, corroborating with the geometrical parameters of the structural unit. Theoretical calculations based on DFT method have been used to quantify the strength of the Cu···Cu magnetic interactions which help to evaluate the experimental coupling constants: J1 = +64.8, J2 = +24.4 and J3 = +3.0 cm–1. The EPR spectroscopy also gives some insight on the ferromagnetic S = 2 ground state of 1. In addition, detailed investigations have been carried out on the catalytic activity of the complex towards the oxidation of cycloalkanes using H2O2 as terminal oxidant.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Syntheses, Structures and Magnetic Properties of Heterobridged Dinuclear and Cubane‐Type Tetranuclear Complexes of Nickel(II) Derived from a Schiff Base Ligand

AbstractSyntheses, structures and magnetic properties of a heterobridged μ‐phenoxido‐μ1,1‐NCO dinickel(II) compound [NiII2(HL)3(μ‐NCO)]·2H2O (1) and a heterobridged bis(μ3‐phenoxido)bis(μ3‐alkoxido)tetranickel(II) system, [NiII4(L)2(HL)2(SeCN)2(H2O)2]·C3H7NO·4H2O (2), containing a cubane‐type NiII4O4 core, derived from the tetradentate Schiff base compartmental ligand N‐(2‐hydroxyethyl)‐3‐methoxysalicylaldimine (H2L) were described. Compounds 1 and 2 crystallize in triclinic P$\bar {1}$ and monoclinic P21/c systems, respectively. Compound 2, in which the Ni4O4 cubane core is composed of two μ3‐phenoxido and two μ3‐alkoxido moieties, represents the sole example of this type. The variable‐temperature (2–300 K) magnetic susceptibilities and magnetizations of these two compounds were measured. The interaction between the metal centres in dinuclear compound 1 is weak ferromagnetic, resulting in an ST = 2 ground state at 2 K. The overall interaction observed in 2 is weak ferromagnetic, leading to an unusual ST = 3 ground state. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A neutral cubane with a ZnII4O4core: tetrabenzoatotetrakis(μ3-hydroxydi-2-pyridylmethanolato)tetrazinc(II)–acetone–methanol (1/2/1)

In the title compound, [Zn4(C11H9N2O2)4(C7H5O2)4]·2(CH3)2CO·CH3OH, the tetra­nuclear mol­ecule lies on a fourfold inversion axis. ZnII ions and μ3-O atoms in the cubane core occupy alternating vertices, forming two inter­penetrating tetra­hedra. Each ZnII ion is further coordinated by two N atoms from two different (py)2C(OH)O ligands (py is pyrid­yl) and one O atom from a monodentate benzoate ligand, forming a distorted octa­hedral environment. The (py)2C(OH)O ligand acts in an η1:η3:η1:μ3 manner, forming two five-membered ZnNCCO chelating rings with two different ZnII atoms sharing a common C—O bond, and an alkoxide-type bond to a third ZnII ion. There are four symmetry-related intra­molecular O—H⋯O hydrogen bonds between the two types of ligands. In the asymmetric unit, there is a half-occupancy acetone solvent mol­ecule and a half-occupancy methanol solvent molecule that lies on a twofold rotation axis.

A novel tetra(μ 3-phenoxo) bridged copper(II) Schiff base complex containing a Cu 4O 4 cubane core: Synthesis, structural aspects and magneto-structural correlations

A novel tetranuclear complex, [Cu 4L 4] · Na · ClO 4 ( 1) has been prepared from an interesting multidentate Schiff base ligand H 2L resulting from the 1:1 condensation of 3-methoxysalicylaldehyde with benzhydrazide. The prepared complex has been characterized by elemental analysis, FT-IR, UV–Vis spectroscopy, electrochemical studies and single crystal X-ray diffraction analysis. The Cu 4O 4 cubane core consists of four μ 3-phenoxo-bridged copper(II) atoms giving an approximately cubic array of alternating copper(II) and oxygen atoms. Magneto-structural correlations have been drawn from cryomagnetic susceptibility measurements over a wide range of temperature (2–300 K) under 0.5 T magnetic field. The measurements reveal both ferromagnetic and antiferromagnetic interactions in a 2 J model [ J 11 = +13.6(4) cm −1 and J 12 = −34.9(4) cm −1] which in turn results in an overall antiferromagnetic behaviour of the magnetic system.

Hexa-, hepta- and dodeca-nuclear nickel(II) complexes of three Schiff-base ligands derived from 1,4-diformyl-2,3-dihydroxybenzene

Three new closely-related diimine ligands, H(4), H(4) and H(4), were prepared by the condensation of 1,4-diformyl-2,3-dihydroxybenzene () with two equivalents of 2-aminoethanol, 3-aminopropanol or 2-aminophenol, respectively. Three polynuclear complexes, [Ni(II)(7)(H(2))(4)(OAc)(6)(H(2)O)(2)].5H(2)O (.5H(2)O), [Ni(II)(6)(H(2))(2)(OAc)(6)(micro-OMe)(2)(H(2)O)(2)].5H(2)O (.5H(2)O) and [Ni(II)(12)()(6)(MeOH)(7)(H(2)O)(3)].4H(2)O (.4H(2)O), were prepared by reaction of the appropriate ligand with 1.50/1.75, 1.75/3.00 or 2.00 equivalents, respectively, of nickel(II) acetate tetrahydrate, in methanol. The crystal structures of these complexes are very different to one another, despite what initially appear to be relatively minor ligand modifications. While [Ni(II)(7)(H(2))(4)(OAc)(6)(MeOH)(2)].7MeOH.4H(2)O possesses C(2) symmetry and has a 4:7 L:M ratio, [Ni(II)(6)(H(2))(2)(OAc)(6)(micro-OMe)(2)(MeOH)(2)].0.5MeOH.0.5H(2)O.Et(2)O and [Ni(II)(12)()(6)(MeOH)(6.5)(H(2)O)(3.5)].solvent have no internal symmetry and have L:M ratios of 2:6 and 6:12, respectively. Two of the four ligands in [Ni(II)(7)(H(2))(4)(OAc)(6)(MeOH)(2)] are in similar planes, whilst the two remaining ligands are near perpendicular to these planes. The six ligands in [Ni(II)(12)()(6)(MeOH)(6.5)(H(2)O)(3.5)] adopt an offset grid-like arrangement with eight of the twelve metal ions at the intersections of the perpendicular, offset, ligands. In contrast, [Ni(II)(6)(H(2))(2)(OAc)(6)(micro-OMe)(2)(MeOH)(2)] features an [Ni(II)(4)(OR)(4)] cubane core with wing tips of the remaining two nickel(II) ions. All three complexes were subsequently prepared in stoichiometric syntheses, in yields of 50% (.5H(2)O), 48% (.5H(2)O) and 62% (.4H(2)O). The magnetic properties of .5H(2)O, .5H(2)O, .4H(2)O are consistent with intra-complex antiferromagnetic coupling dominating overall, and therefore all three complexes display low spin ground states.

Pivalates with the MII 4O4 cubane core (M = Co, Ni): synthesis, structure, magnetic properties, and solid-state thermolysis

Methods were developed for the controlled thermal synthesis of high-spin cubane-like pivalates {MII4(μ3−OR)4} (M = Co or Ni; R = H or Me) starting from mono-and polynuclear complexes. The solid-state thermal decomposition of the known pivalate clusters [MII4(μ3−OMe)4−(μ2−OOCBut)2(η2−OOCBut)2(MeOH)4] and the new clusters [M4II(μ3)−OH4(η1−OOCBut)3−(μ−(NH2)2C6H2Me2)3(η1−(NH2)2C6H2Me2)3]+(OOCBut)− (M = Co or Ni) was studied by differential scanning calorimetry and thermogravimetry. The thermolysis of cubane-like CoII and NiII pivalates is a destructive process. The phase composition of the decomposition products is determined by the nature of coordinated ligands and the structural features of the metal core.

Tetra- and Binuclear Complexes of Hydroxy-Rich Ligands: Supramolecular Structures, Stabilization of Unusual Water Clusters, and Magnetic Properties

Three tetranuclear complexes [Cu4(H2L1)4]·10H2O (1), [Ni4(H2O)3(H2L1)3(OH)]NO3·6H2O (2) and [Zn4(H2L1)4]·2MeOH·2H2O (3) and two binuclear complexes [Cu2(H3L1)2(NO3)2] (4) and [Cu2(H3L2)2Cl2](ClO4)2·2H2O (5) have been synthesized from two potentially pentadentate hydroxyl-rich ligands H4L1 and H3L2 (H4L1 = tris(hydroxymethyl)(2-hydroxybenzylamino)methane, and H3L2 = tris(hydroxymethyl)(2-pyridylamino) methane). X-ray analyses reveal that complexes 1, 2, and 3 have cubane core structures. For 1, ten lattice water molecules are linked by twelve hydrogen bonds, forming an extremely unusual adamantanoid (H2O)10 cluster, which is linked to two tetranuclear moieties by octadruple hydrogen bonds. For 2, four water dimers are bridged by two nitrates, giving a [(H2O)8(NO3)2]2- cluster. 4 has a bis(μ2-phenoxo)-bridged dicopper(II) structure with the binuclear species linked by multiple hydrogen bonds, affording a ladder-like structure. 5 has a bis(μ2-Cl)-bridged dicopper(II) structure with the binuclear moieties linked by π−π interactions and hydrogen bonds, affording a 2D network. In these complexes, the hydroxyl-rich ligands coordinate in dianionic, monoanionic or neutral forms, with one of the alkoxyl groups of each ligand left noncoordinated and open for intermolecular interactions, resulting in the stabilization of various water clusters and the formation of various supramolecular assemblies. Magnetic measurements reveal that, for 1 and 2, antiferromagnetic and ferromagnetic couplings coexist, thus resulting in interesting nonzero spin ground state. The ferromagnetic behavior of 1 can be ascribed to the nearly perpendicular arrangement of dx2−y2 magnetic orbitals of Cu(II) centers with the dihedral angle of 84.06(3)° between the Cu(II) equatorial planes. 4 shows a strong antiferromagnetic interaction. On the other hand, 5 shows a ferromagnetic interaction between copper(II) centers. Magneto-structural correlations have been discussed.

A Mixed-Valence Octanuclear Iron−Oxo Pyrazolate: Assessment of Electronic Delocalization by Structural and Spectroscopic Analysis

A formally Fe(III)(7)Fe(II) complex, containing an inner Fe(4)O(4)-cubane and four peripheral Fe centers, is derived from the one-electron reduction of its Fe(III)(8) precursor. Spectroscopic analysis of the former reveals that the redox activity of this Fe(8) system is confined within its cubane core. The resulting (Fe(4)O(4))(3+)-cubane, which is valence-delocalized in the NMR, Mössbauer, and IR spectroscopy time scales but valence-trapped in the X-ray photoelectron spectroscopy (XPS) time scale, is better described as a Robin-Day class-II system by the analysis of its near-infrared (NIR) intervalence charge transfer (IVCT) band profile.