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The neutral holmium(III) oxalate octadecahydrate {[Ho 2 (ox) 3 (H 2 O) 6 ]·12H 2 O} n of mixed hexagonal/decagonal (6 3 ·10 3 ) 3D net topology shows important changes in the magnetocaloric efficiency upon dehydration/rehydration by heating and water vapor exposition to give the holmium(III) oxalate decahydrate {[Ho 2 (ox) 3 (H 2 O) 6 ]·4H 2 O} n of hexagonal (6 3 ) 2D net topology through the intermediacy of the elusive amorphous anhydrous compound {Ho 2 (ox) 3 } n .

Two mononuclear nickel(II) complexes of the formula [Ni(terpyCOOH) 2 ](ClO 4 ) 2 ∙4H 2 O ( 1 ) and [Ni(terpyepy) 2 ](ClO 4 ) 2 MeOH ( 2 ) [terpyCOOH = 4'-carboxyl-2,2':6',2″-terpyridine and terpyepy = 4'-[(2-pyridin-4-yl)ethynyl]-2,2':6',2″-terpyridine] have been prepared and their structures determined by single-crystal X-ray diffraction. Complexes 1 and 2 are mononuclear compounds, where the nickel(II) ions are six-coordinate by the six nitrogen atoms from two tridentate terpy moieties. The mean values of the equatorial Ni-N bond distances [2.11(1) and 2.12(1) Å for Ni(1) at 1 and 2 , respectively, are somewhat longer than the axial ones [2.008(6) and 2.003(6) Å ( 1 )/2.000(1) and 1.999(1) Å ( 2 )]. The values of the shortest intermolecular nickel-nickel separation are 9.422(1) ( 1 ) and 8.901(1) Å ( 2 ). Variable-temperature (1.9-200 K) direct current (dc) magnetic susceptibility measurements on polycrystalline samples of 1 and 2 reveal a Curie law behavior in the high-temperature range, which corresponds to magnetically isolated spin triplets, the downturn of the χ M T product at lower temperatures being due to zero-field splitting effects ( D ). Values of D equal to -6.0 ( 1 ) and -4.7 cm -1 ( 2 ) were obtained through the joint analysis of the magnetic susceptibility data and the field dependence of the magnetization. These results from magnetometry were supported by theoretical calculations. Alternating current (ac) magnetic susceptibility measurements of 1 and 2 in the temperature range 2.0-5.5 K show the occurrence of incipient out-phase signals under applied dc fields, a phenomenon that is characteristic of field-induced Single-Molecule Magnet (SMM) behavior, which herein concerns the 2 mononuclear nickel(II) complexes. This slow relaxation of the magnetization in 1 and 2 has its origin in the axial compression of the octahedral surrounding at their nickel(II) ions that leads to negative values of D . A combination of an Orbach and a direct mechanism accounts for the field-dependent relation phenomena in 1 and 2 .

Self-assembly is the most powerful force for creating ordered supramolecular architectures from simple components under mild conditions. π···π stacking interactions have been widely explored in modern supramolecular chemistry as an attractive reversible noncovalent tool for the nondestructive fabrication of materials for different applications. Here, we report on the self-assembly of cytidine 5'-monophosphate (CMP) nucleotide and copper metal ions for the preparation of a rare nanoporous supramolecular metal-organic framework in water. π···π stacking interactions involving the aromatic groups of the ancillary 2,2'-bipyridine (bipy) ligands drive the self-assemblies of hexameric pseudo-amphiphilic [Cu6(bipy)6(CMP)2(µ-O)Br4] 2+ units. Owing to the supramolecular geometric matching between the aromatic tails, a nanoporous crystalline phase with hydrophobic and hydrophilic chiral pores of 1.2 and 0.8 nanometers, respectively, was successfully synthesized. The encoded chiral information, contained on the enantiopure building blocks, is transferred to the final supramolecular structure, assembled in the very unusual topology 8 T6 . These kinds of materials, owing to chiral channels with chiral active sites from ribose moieties, where the enantioselective recognition can occur, are, in principle, good candidates to carry out efficient separation of enantiomers, better than traditional inorganic and organic porous materials.

Two novel copper(II) complexes of formulas {[Cu(4-Hmpz) 4 ][Cu(4-Hmpz) 2 (µ 3 -ox- κ 2 O 1 , O 2 : κO 2' : κO 1' )(ClO 4 ) 2 ]} n ( 1 ) and {[Cu(3,4,5-Htmpz) 4 ] 2 [Cu(3,4,5-Htmpz) 2 (µ 3 -ox- κ 2 O 1 , O 2 : κO 2' : κO 1' )(H 2 O)(ClO 4 )] 2 [Cu 2 (3,4,5-Htmpz) 4 (µ-ox- κ 2 O 1 , O 2 : κ 2 O 2' , O 1' )]}(ClO 4 ) 4 ·6H 2 O ( 2 ) have been obtained by using 4-methyl-1 H -pyrazole (4-Hmpz) and 3,4,5-trimethyl-1 H -pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of 1 consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ 3 - κ 2 O 1 , O 2 : κO 2' : κO 1' -oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of 2 is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ- κ 2 O 1 , O 2 : κ 2 O 2' , O 1' -oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ 3 - κ 2 O 1 , O 2 : κO 2' : κO 1' -oxalato)copper(II) units. The magnetic properties of 1 and 2 were investigated in the temperature range 2.0-300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ 3 -ox- κ 2 O 1 , O 2 : κO 2' : κO 1' center occur in 1 [ J = -0.42(1) cm -1 , the spin Hamiltonian being defined as H = - J∑ S 1,i · S 2,i +1 ], whereas very weak intramolecular ferromagnetic [ J = +0.28(2) cm -1 ] and strong antiferromagnetic [ J' = -348(2) cm -1 ] couplings coexist in 2 which are mediated by the µ 3 -ox- κ 2 O 1 , O 2 : κO 2' : κO 1' and µ-ox- κ 2 O 1 , O 2 : κ 2 O 2' , O 1' centers, respectively. The variation in the nature and magnitude of the magnetic coupling for this pair of oxalato-centered inverse copper(II) complexes is discussed in the light of their different structural features, and a comparison with related oxalato-centered inverse copper(II)-pyrazole systems from the literature is carried out.

The coexistence of field-induced slow magnetic relaxation and moderately large magnetocaloric efficiency in the supra-Kelvin temperature region occurs in the 2D compound [Gd(ox) 3 (H 2 O) 6 ] n ·4nH 2 O (1), a feature that can be exploited in the proof-of-concept design of a new class of slow-relaxing magnetic materials for cryogenic magnetic refrigeration.

Two mononuclear cobalt(II) compounds of formula [Co(dmphen) 2 (OOCPh)]ClO 4 ·1/2H 2 O·1/2CH 3 OH ( 1 ) and [Co(dmbipy) 2 (OOCPh)]ClO 4 ( 2 ) (dmphen = 2,9-dimethyl-1,10-phenanthroline, dmbipy = 6,6'-dimethyl-2,2'-bipyridine and HOOCPh = benzoic acid) are prepared and magnetostructurally investigated. Each cobalt(II) ion is six-coordinate with a distorted octahedral CoN 4 O 2 environment. The complex cations are interlinked leading to supramolecular chains ( 1 ) and pairs ( 2 ) that grow along the crystallographic c -axis with racemic mixtures of (Δ,Λ)-Co units. FIRMS allowed us to directly measure the zero-field splitting between the two lowest Kramers doublets, which led to axial anisotropy values of 58.3 cm -1 ≤ D < 60.7 cm -1 ( 1 ) and 63.8 cm -1 ≤ D < 64.1 cm -1 ( 2 ). HFEPR spectra of polycrystalline samples of 1 and 2 at low temperatures confirm the positive sign of D and provide an estimate of the E / D quotient [0.147/0.187 ( 1 ) and 0.052 ( 2 )]. Detailed ac and dc magnetic studies reveal that 1 and 2 are new examples of field-induced single-ion magnets (SIMs) with small transversal anisotropy. CASSCF/NEVPT2 calculations support these results. Two Orbach processes or one Orbach plus a direct relaxation mechanism provide similar agreements with the nonlinear experimental Arrhenius plots at H = 500 and 2500 G for dc = 500 and 2500 G for 1 . Two independent relaxation processes occur in 2 , but in contrast to 1 , an observed linear dependence of ln(τ) vs 1/ T substantiates Orbach processes against the most widely proposed Raman and direct mechanisms. The analysis of each relaxation process in 2 provided values for E a and τ 0 that are very close to those found for 1 , validating the predominant role of the Orbach relaxations in both compounds and, probably, also in other cobalt(II) SIMs. A mechanism based on a spin-phonon coupling is proposed to account for the SIM behavior in 1 and 2 with any Raman or direct processes being discarded.

We report a new chiral coordination polymer, prepared from the cytidine 5'-monophosphate (CMP) nucleotide, capable of separating efficiently (enantiomeric excess of ca. 100%) racemic mixtures of l- and d-Asp in a temperature-dependent manner. The crystal structure of the host-guest adsorbate, with the d-Asp guest molecules loaded within its channels, could be solved allowing a direct visualization of the chiral recognition process.

A family of tetrahedral mononuclear Co II complexes with the cytosine nucleobase ligand is used as the playground for an in-depth study of the effects that the nature of the ligand, as well as their noninnocent distortions on the Co(II) environment, may have on the slow magnetic relaxation effects. Hence, those compounds with greater distortion from the ideal tetrahedral geometry showed a larger-magnitude axial magnetic anisotropy (D) together with a high rhombicity factor (E/D), and thus, slow magnetic relaxation effects also appear. In turn, the more symmetric compound possesses a much smaller value of the D parameter and, consequently, lacks single-ion magnet behavior.

The preparation and structural characterization of four novel oxalate-based iron(III) compounds of formulas {(MeNH 3 ) 2 [Fe 2 (ox) 2 Cl 4 ]·2.5H 2 O} n (1), K(MeNH 3 )[Fe(ox)Cl 3 (H 2 O)] (2), {MeNH 3 [Fe 2 (OH)(ox) 2 Cl 2 ]·2H 2 O} n (3), and {(H 3 O)(MeNH 3 )[Fe 2 O(ox) 2 Cl 2 ]·3H 2 O} n (4) (MeNH 3 + = methylammonium cation and H 2 ox = oxalic acid) are reported here. 1 is an anionic waving chain of oxalato-bridged iron(III) ions with peripheral chloro ligands, the charge balance being ensured by methylammonium cations. 2 is a mononuclear complex with a bidentate oxalate, three terminal chloro ligands, and a coordinated water molecule achieving the six-coordination around each iron(III) ion. Its negative charge is balanced by potassium(I) and methylammonium cations. 3 and 4 are made up of oxalate-bridged and either hydroxo (3)- or oxo-bridged (4) iron(III) chiral three-dimensional (3D) networks of formulas [Fe 2 (OH)(ox) 2 Cl 2 ] n n- (3) and [Fe 2 O(ox) 2 Cl 2 ] n 2n- (4) with methylammonium (3 and 4) and hydronium (4) as counterions. The common point these compounds share is related to their synthetic strategy, which consists of the use of mixed alkaline/alkylammonium cations as templating agents for the growth of the 1D or 3D iron(III) motifs. Interestingly, even in the presence of any given alkaline cation in the reaction solutions, the resulting coordination polymers (1, 3, and 4) exclusively contain the methylammonium cation, revealing the highly selective character of the 1D and 3D networks. Furthermore, the isolation of the very unstable compound 1 could be only achieved in the presence of the KCl salt, suggesting a probable templating effect of the potassium(I) cations. Finally, a study of the variable-temperature magnetic properties of the 3D compounds 3 and 4 showed the occurrence of weak ferromagnetic ordering due to a spin canting, the value of the critical temperature (T c ) being as high as 70 K.

The different natures of the weakly coordinating anions - triflate or perchlorate - in the Cu 2+ -mediated self-assembly of cytidine monophosphate nucleotide play a fundamental role in the homochiral resolution process, yielding one-dimensional copper(ii) coordination polymers of opposite helicity that can be easily inverted, in a reversible way, by changing the nature of the anion as revealed by circular dichroism experiments both in solution and in the solid state.

Two new two-dimensional homometallic compounds {[M2(bpm)(ox)2]n·5nH2O} with M = Co(II) (1) and Zn(II) (2) and the mononuclear nickel(II) complex [Ni(bpm)2(ox)]·2H2O (3) [bpm = 2,2'-bipyrimidine and ox = oxalate] have been prepared and structurally characterized. 1 and 2 are isostructural compounds whose structures are made up of oxalate-bridged M(II) cations cross-linked by bis-bidentate bpm molecules to afford a honeycomb layered network extending in the crystallographic ab plane. The layers are eclipsed along the crystallographic c axis and show graphitic-like interactions between the bpm rings. The three-dimensional supramolecular network deriving from such interactions is characterized by hexagonal-shaped channels extending in the same direction. Each M(II) ion in 1 and 2 is tris-chelated with four oxygen atoms from two oxalate groups and two bpm-nitrogen atoms building a distorted octahedral surrounding. The reduced values of the angles subtended by the bis-chelating bpm [77.69(8) (1) and 76.59(8)° (2)] and oxalate [79.69(6) (1) and 80.01(5)° (2)] are the main factors accounting for this distortion. The values of the metal-metal separation through bridging bpm are 5.6956(7) (1) and 5.7572(9) Å (2), whereas those across the bis-bidentate oxalate are 5.4306(4) (1) and 5.4058(5) Å (2). 3 is a neutral mononuclear nickel(II) complex where each metal ion is six-coordinate with four nitrogen atoms from two bpm ligands in a cis arrangement and two oxalate-oxygen atoms building a somewhat distorted octahedral surrounding. The values of the angles subtended at the nickel(II) ion by bpm and oxalate are 78.14(4) and 80.95(5)°, respectively. The magnetic properties of 1 have been investigated in the temperature range 1.9-295 K. They are typical of an overall antiferromagnetic coupling with a maximum of the magnetic susceptibility at 22.0 K. The analysis of the susceptibility data of 1 through an effective spin Hamiltonian allowed a satisfactory simulation in the temperature range 10-295 K with the best-fit parameters λ = -110 cm(-1), α = 1.1, |Δ| = 400 cm(-1), J(ox) = -11.1 cm(-1) and J(bpm) = -5.0 cm(-1). The values of the antiferromagnetic coupling through bpm and ox in 1 have also been supported by electronic structure calculations based on Density Functional Theory (DFT) and they compare well with those reported in the literature for bpm-bridged dicobalt(II) complexes and oxalate-bridged cobalt(II) chains.

A new heteroleptic 1D Cu(I)-Re(IV) coordination polymer of the formula {Cu(I)Re(IV)Cl4(μ-Cl)(μ-pyz)[Re(IV)Cl4(μ-bpym)]}n·nMeNO2 (1; pyz = pyrazine, bpym = 2,2'-bipyrimidine) has been prepared through the Cu(I)-mediated self-assembly of two different Re(IV) metalloligands, namely, [ReCl5(pyz)](-) and [ReCl4(bpym)]. 1 consists of chiral branched chains with an overall rack-type architecture displaying photoemission and magnetic ordering. These results constitute a first step toward making new multifunctional magnetic materials based on mixed 3d-5d molecular systems.

Two new dinuclear copper(II) metallacyclophanes with 1,4-disubstituted 9,10-anthraquinonebis(oxamate) bridging ligands are reported that can reversibly take and release electrons at the redox-active ligand and metal sites, respectively, to give the corresponding mono- and bis(semiquinonate and/or catecholate) Cu(II)2 species and mixed-valent Cu(II)/Cu(III) and high-valent Cu(III)2 ones. Density functional calculations allow us to give further insights on the dual ligand- and metal-based character of the redox processes in this novel family of antiferromagnetically coupled di- copper(II) anthraquinophanes. This unique ability for charge storage could be the basis for the development of new kinds of molecular spintronic devices, referred to as molecular magnetic capacitors (MMCs)., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A novel series of heteroleptic copper(II) compounds of formulas {[Cu2(μ-H2O)(μ-pz)2(μ-bpm)(ClO4)(H2O)]ClO4·2H2O}n (1), {[Cu2(μ-H2O)(μ-3-Mepz)2(μ-bpm)](ClO4)2·2H2O}n (2), and {[Cu2(μ-OH)(μ-3,5-Me2pz)(μ-bpm)(H-3,5-Me2pz)2](ClO4)2}n (3) [bpm = 2,2'-bipyrimidine, Hpz = pyrazole, H-3-Mepz = 3-methylpyrazole, and H-3,5-Me2pz = 3,5-dimethylpyrazole] have been synthesized and structurally characterized by X-ray diffraction methods. The crystal structures of 1 and 2 consist of copper(II) chains with regular alternating bpm and bis(pyrazolate)(aqua) bridges, whereas that of 3 is made up of copper(II) chains with regular alternating bpm and (pyrazolate)(hydroxo) bridges. The copper centers are six- (1) or five-coordinate (2) in axially elongated, octahedral (1) or square-pyramidal (2) environments in 1 and 2, whereas they are five-coordinate in distorted trigonal-bipyramidal surroundings in 3. The values of the copper-copper separations across the bpm/pyrazolate bridges are 5.5442(7)/3.3131(6) (1), 5.538(1)/3.235(1) (2), and 5.7673(7)/3.3220(6) Å (3). The magnetic properties of 1-3 have been investigated in the temperature range of 25-300 K. The analysis of their magnetic susceptibility data through the isotropic Hamiltonian for an alternating antiferromagnetic copper(II) chain model [H = -J∑i=1-n/2 (S2i·S2i-1 + αS2i·S2i+1), with α = J'/J and Si = SCu = 1/2] reveals the presence of a strong to moderate antiferromagnetic coupling through the bis(pyrazolate)(aqua) [-J = 217 (1) and 215 cm(-1) (2)] and (pyrazolate)(hydroxo) bridges [-J = 153 cm(-1) (3)], respectively, whereas a strong to weak antiferromagnetic coupling occurs through the bis-bidentate bpm [-J' = 211 (1), 213 (2), and 44 cm(-1) (3)]. A simple orbital analysis of the magnetic exchange interaction within the bpm- and pyrazolate-bridged dicopper(II) fragments of 1-3 visualizes the σ-type pathways involving the (dx(2)-y(2)) (1 and 2) or d(z(2)) (3) magnetic orbitals on each metal ion, which account for the variation of the magnetic properties in these three novel examples of one-dimensional copper(II) compounds with regular alternating intrachain antiferromagnetic interactions.

By using Vitamin B6 in its monodeprotonated pyridoxine form (PN-H) [PN = 3-hydroxy-4,5-bis(hydroxymethyl)-2-methylpyridine], two tetranuclear compounds of formula [Mn4(PN-H)4(CH3CO2)3Cl2]Cl·2CH3OH·2H2O (1) and [Cu4(PN-H)4Cl2(H2O)2]Cl2 (2) have been synthesized and magneto-structurally characterized. 1 crystallizes in the triclinic system with space group P1 whereas 2 crystallizes in the orthorhombic system with Fdd2 as space group. They exhibit Mn(II)2Mn(III)2 (1) and Cu(II)4 (2) cubane cores containing four monodeprotonated pyridoxine groups simultaneously acting as chelating and bridging ligands (1 and 2), three bridging acetate ligands in the syn-syn conformation (1), and two terminally bound chloride anions (1 and 2) plus two coordinated water molecules (2). The electroneutrality is achieved by the presence of chloride counterions in both compounds. Tri- [Mn(1) and Mn(3)] and divalent [Mn(2) and Mn(4)] manganese centers coexist in 1, all being six-coordinate with distorted Mn(1/3)O6 and Mn(2/4)O5Cl octahedral surroundings, respectively, the equatorial Mn-O bonds being about 0.2 Å shorter at the former ones. The two crystallographically independent copper(II) ions in 2 are five-coordinate in somewhat distorted CuO5 [Cu(1)] and CuO4Cl [Cu(2)] square pyramidal geometries. The values of the intracore metal-metal separation cover the ranges 3.144(1)-3.535(1) (1) and 2.922(6)-3.376(1) Å (2). The magnetic properties of 1 and 2 were investigated in the temperature range 1.9-300 K, and they correspond to an overall antiferromagnetic behavior with susceptibility maxima at 5.0 (1) and 65.0 K (2). The analysis of the magnetic susceptibility data showed the coexistence of intracore antiferro- and ferromagnetic interactions in the two compounds. Their values compare well with those existing in the literature for the parent systems.

The rhenium(IV) complex (NBu4)2[ReBr4(ox)] (1) (ox = oxalate and NBu4(+) = tetra-n-butylammonium cation) has been prepared and its crystal structure determined by X-ray diffraction. The structure is made up of discrete [ReBr4(ox)](2-) anions and bulky NBu4(+) cations. Each [ReBr4(ox)](2-) anion is surrounded by six NBu4(+) cations, which preclude any significant intermolecular contact between the anionic entities, the shortest rhenium···rhenium distance being 9.373(1) Å. Variable temperature dc and ac magnetic susceptibility measurements and field-dependent magnetization experiments on polycrystalline samples of 1 reveal the occurrence of highly anisotropic magnetically isolated Re(IV) centers (S(Re) = 3/2), which exhibit slow relaxation of the magnetization at very low temperatures in a dc field. Ac measurements conducted on a polycrystalline sample of the complex (NBu4)2[ReCl4(ox)] (2) [compound isostructural to 1 whose structure and dc magnetic susceptibility study were previously reported in Tomkiewicz, A.; Bartczak, T. J.; Kruszyński, R.; Mroziński, J. J. Mol. Struct. 2001, 595, 225] show a similar behavior, both complexes thus constituting new examples of mononuclear single-molecule magnets. High-frequency and -field electron paramagnetic resonance on polycrystalline samples of 1 and 2 and on single crystals of 2 allowed for the determination for the first time of the negative sign and confirmed a significant magnitude and rhombicity (E/D) of the zero-field splitting tensor of the [ReCl4(ox)](2-) and [ReBr4(ox)](2-) centers, originating from a combination of spin-orbit coupling and low molecular symmetry. D and E values of 1 and 2 were estimated through magnetization measurements and theoretically calculated through complete active space and density functional theory methodologies.

The Cu(2+)-mediated self-assembly of oxamato-based ligands derived from either the (S)- or (R)-enantiomers of the amino acid valine leads to the formation of two antiferromagnetically coupled homochiral anionic hexacopper(II) wheels in the presence of templating tetramethylammonium countercations.

Two novel double-stranded dicopper(II) metallacyclophanes of formula (nBu4N)4[Cu2(dpeba)2]·4MeOH·2Et2O (1) and (nBu4N)4[Cu2(tpeba)2]·12H2O (2) have been prepared by the Cu(II)-mediated self-assembly of the rigid ('rod-like') bridging ligands N,N'-4,4'-diphenylethynebis(oxamate) (dpeba) and N,N'-1,4-di(4-phenylethynyl)phenylenebis(oxamate) (tpeba), respectively. Single crystal X-ray diffraction analysis of 1 confirms the presence of a dicopper(II)tetraaza[3.3]4,4'-diphenylethynophane metallacyclic structure featuring a very long intermetallic distance between the two square planar Cu(II) ions [r = 14.95(1) Å]. The overall parallel-displaced π-stacked conformation of the two nearly planar para substituted diphenylethyne spacers [dihedral angle (ψ) of 7.8(1)°] leads to important deviations from the perpendicular orientation of the copper mean basal planes with respect to the facing benzene planes [dihedral angles (φ) of 56.4(1) and 58.4(1)°]. X-band EPR spectra together with variable-temperature magnetic susceptibility and variable-field magnetization measurements of 1 and 2, both in solution and in the solid state, show the occurrence of a non-negligible, moderate to weak intramolecular antiferromagnetic coupling [-J = 3.9-4.1 (1) and 0.5-0.9 cm(-1) (2); H = -JS1·S2 with S1 = S2 = SCu = 1/2]. Density functional calculations on the BS singlet (S = 0) and triplet (S = 1) spin states of the model complexes 1 and 2 with an ideal orthogonal molecular geometry (ψ = 0° and φ = 90°) support the occurrence of a spin polarization mechanism for the propagation of the exchange interaction between the two unpaired electrons occupying the dxy orbital of each square planar Cu(II) ion through the predominantly π-type orbital pathway of the double p-diphenylethyne (1) and di(phenylethynyl)phenylene spacers (2). Time-dependent density functional calculations reproduce the observed bathochromic shift of the main intraligand (IL) π-π* transition in the electronic absorption spectra of 1 and 2 [λ1 = 308 (1) and 316 nm (2)]. In the series of orthogonal model complexes 1-5 with linear oligo(p-phenylene-ethynylene) (OPE) spacers, -C6H4(C≡CC6H4)n- (n = 1-5), a linear increase of the IL π-π* transition energy with the reciprocal of the intermetallic distance is theoretically predicted [νmax = 1.99 × 10(4) + 2.15 × 10(5) (1/r) (S = 0) or ν = 2.01 × 10(4) + 2.18 × 10(5) (1/r) (S = 1)], which clearly indicates that the effective π-conjugation length increases with the number of phenylethyne repeating units. This is accompanied by an exponential decay of the antiferromagnetic coupling with the intermetallic distance [-J = 1.08 × 10(3) exp(-0.31r)], which supports the ability of the extended π-conjugated OPEs to mediate the exchange interaction between the unpaired electrons of the two Cu(II) centers with intermetallic distances in the range of 1.5-4.3 nm. Further developments may be then envisaged for this new family of oxamato-based dicopper(II) oligo-p-phenylethynophanes on the basis of the unique ligand capacity to act as a molecular antiferromagnetic wire.

The reaction of N-(2,6-dimethylphenyl)oxamic acid with dysprosium(III) ions in a controlled basic media afforded the first example of a mononuclear lanthanide oxamate complex exhibiting a field-induced slow magnetic relaxation behavior typical of single-ion magnets (SIMs). The hydrogen-bond-mediated self-assembly of this new bifunctional dysprosium(III) SIM in the solid state provides a unique example of 2D hydrogen-bonded polymer with a herringbone net topology.

The new dinuclear copper(II) metallacyclophane with the non-innocent N,N'-1,4-bis(oxamate)-9,10-anthraquinone bridging ligand possesses a dual multielectron redox behavior featuring stepwise one-electron oxidation of the antiferromagnetically coupled Cu(II) ions and two-electron reduction of the anthraquinone spacers in a π-stacked anti conformation.

Three novel heterobimetallic Re(IV) compounds of formulae [ReBr(4)(μ-ox)M(4,7-Cl(2)phen)(2)]·CH(3)CN·CH(3)NO(2) [M = Co(II) (1) and Ni(II) (2)] and [ReBr(4)(ox)](3)[Co(III)(5,6-dmphen)(3)](2)·CH(3)CN·2CH(3)NO(2)·4H(2)O (3) [ox = oxalate, 4,7-Cl(2)phen = 4,7-dichloro-1,10-phenanthroline and 5,6-dmphen = 5,6-dimethyl-1,10-phenanthroline] have been synthesised and the structures of 1 and 3 determined by single crystal X-ray diffraction. Compound 1 is an oxalato-bridged Re(IV)Co(II) heterodinuclear complex where the [ReBr(4)(ox)](2-) unit acts as a bidentate ligand towards the [Co(4,7-Cl(2)phen)(2)](2+) entity, the separation between Re(IV) and Co(II) across the oxalate being 5.482(1) Å. Compound 3 is an ionic salt whose structure is made up of [Re(IV)Br(4)(ox)](2-) anions and [Co(III)(5,6-dmphen)(3)](3+) cations plus acetonitrile, nitromethane and water as solvent molecules. The magnetic properties of 1-3 were investigated in the temperature range 1.9-300 K. Relatively large ferromagnetic interactions between Re(IV) and M(II) through the bis(bidentate) oxalato occur in 1 and 2 [J(ReM) = +11.0 (1) and +12.2 cm(-1) (2), the Hamiltonian being defined as Ĥ = -J(ReM)Ŝ(Re)·Ŝ(M)] which are explained on the basis of orbital symmetry considerations. A behaviour typical of a magnetically diluted Re(IV) complex with a large and positive value of zero-field splitting for the ground level (D(Re) = +43 cm(-1)) is observed for 3 in the high temperature range, whereas it exhibits spin canting in the low temperature domain as well as magnetic ordering below ca. 4.8 K.

The novel mononuclear Co(II) complex cis-[Co(II)(dmphen)(2)(NCS)(2)]·0.25EtOH (1) (dmphen = 2,9-dimethyl-1,10-phenanthroline) features a highly rhombically distorted octahedral environment that is responsible for the strong positive axial and rhombic magnetic anisotropy of the high-spin Co(II) ion (D = +98 cm(-1) and E = +8.4 cm(-1)). Slow magnetic relaxation effects were observed for 1 in the presence of a dc magnetic field, constituting the first example of field-induced single-molecule magnet behavior in a mononuclear six-coordinate Co(II) complex with a transverse anisotropy energy barrier.

The heterobimetallic complexes of formula [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)]·CH(3)CN [Me(2)phen = 2,9-dimethyl-1,10-phenanthroline and M = Ni (1), Co (2), Fe (3), and Mn (4)] have been prepared, and their crystal structures have been determined by X-ray diffraction on single crystals. Compounds 1-4 crystallize in the monoclinic C2/c space group, and their structure consists of neutral [(Me(2)phen)(2)M(μ-NCS)Re(NCS)(5)] heterodinuclear units with a Re-SCN-M bridge. Each Re(IV) ion in this series is six-coordinated with one sulfur and five nitrogen atoms from six thiocyanate groups building a somewhat distorted octahedral environment, whereas the M(II) metal ions are five-coordinated with four nitrogen atoms from two bidentate Me(2)phen molecules and a nitrogen atom from the bridging thiocyanate describing distorted trigonal bipyramidal surroundings. The values of the Re···M separation through the thiocyanate bridge in 1-4 vary in the range 5.903(1)-6.117(3) Å. The magnetic properties of 1-4 as well as those of the parent mononuclear Re(IV) compounds (NBu(4))(2)[Re(NCS)(6)] (A1) (NBu(4)(+) = tetra-n-butylammonium cation) and [Zn(NO(3))(Me(2)phen)(2)](2)[Re(NCS)(5)(SCN)] (A2) were investigated in the temperature range 1.9-300 K. Weak antiferromagnetic interactions between the Re(IV) and M(II) ions across the bridging thiocyanate were found in 1-4 [J = -4.3 (1), -2.4 (2), -1.8 (3), and -1.2 cm(-1) (4), the Hamiltonian being defined as Ĥ = -JŜ(Re)·Ŝ(M)]. The magnetic behavior of A2 is that of a magnetically diluted Re(IV) complex with a large and positive value of the zero-field splitting for the ground level (D(Re) = +37.0 cm(-1)). In the case of A1, although its magnetic behavior is similar to that of A2 in the high-temperature range (D(Re) being +19.0 cm(-1)), it exhibits a weak ferromagnetism below 3.0 K with a canting angle of 1.3°.

The preparation and X-ray crystal structure of four 2,2'-bipyrimidine (bpm)-containing copper(II) complexes of formula {[Cu(2)(μ-bpm)(H(2)O)(4)(μ-OH)(2)][Mn(H(2)O)(6)](SO(4))(2)}(n) (1), {[Cu(2)(μ-bpm)(H(2)O)(4)(μ-OH)(2)]SiF(6)}(n) (2), {Cu(2)(μ-bpm)(H(2)O)(2)(μ-F)(2)F(2)}(n) (3), and [Cu(bpm)(H(2)O)(2)F(NO(3))][Cu(bpm)(H(2)O)(3)F]NO(3)·2H(2)O (4) are reported. The structures of 1-3 consist of chains of copper(II) ions with regular alternation of bis-bidentate bpm and di-μ-hydroxo (1 and 2) or di-μ-fluoro (3) groups, the electroneutrality being achieved by either hexaaqua manganese(II) cations plus uncoordinated sulfate anions (1), uncoordinated hexafluorosilicate anions (2), or terminally bound fluoride ligands (3). Each copper(II) ion in 1-4 is six-coordinated in elongated octahedral surroundings. 1 and 2 show identical, linear chain motifs with two bpm-nitrogen atoms and two hydroxo groups building the equatorial plane at each copper(II) ion and the axial position being filled by water molecules. In the case of 3, the axial sites at the copper atom are occupied by a bpm-nitrogen atom and a bis-monodentate fluoride anion, producing a "step-like" chain motif. The values of the angle at the hydroxo and fluoro bridges are 94.11(6) (1), 94.75(4) (2), and 101.43(4)° (3). In each case, the copper-copper separation through the bis-bidentate bpm [5.428(1) (1), 5.449(1) (2), and 5.9250(4) Å (3)] is considerably longer than that through the di-μ-hydroxo [2.8320(4) (1) and 2.824(1) Å (2)] or di-μ-fluoro [3.3027(4) Å (3)] bridges. Compound 4 is a mononuclear species whose structure is made up of neutral [Cu(bpm)(H(2)O)(2)F(NO(3))] units, [Cu(bpm)(H(2)O)(3)F](+) cations, uncoordinated nitrate anions, and crystallization water molecules, giving rise to a pseudo-helical, one-dimensional (1D) supramolecular motif. The magnetic properties of 1-3 have been investigated in the temperature range 1.9-300 K. Relatively large, alternating antiferro- [J = -149 (1) and -141 cm(-1) (2) across bis-bidentate bpm] and ferromagnetic [αJ = +194 (1) and +176 cm(-1) (2) through the dihydroxo bridges] interactions occur in 1 and 2 [the Hamiltonian being defined as H = -J∑(i=1)(n/2) (S(2i)·S(2i-1) - αS(2i)·S(2i+1))]. These values compare well with those previously reported for parent examples. Two weak intrachain antiferromagnetic interactions [J = -0.30 and αJ = -8.1 cm(-1) across bpm and the di-μ-fluoro bridges, respectively] whose values were substantiated by density functional theory (DFT)-type calculations occur in 3.

The use of the [ReCl(4)(bpym)] precursor as a ligand toward the fully solvated nickel(II) metal ion affords the first example of a 2,2'-bipyrimidine-bridged Re(IV)-Ni(II) complex, [ReCl(4)(μ-bpym)NiBr(2)(H(2)O)(2)] (1), whose intramolecular ferromagnetic coupling has been substantiated from both experimental and theoretical studies., (© 2011 American Chemical Society)

Self-assembly of the rigid rodlike ligand N,N'-4,4'-diphenylethynebis(oxamate) (dpeba) and Cu(2+) ions affords a novel dinuclear copper(II) metallacyclophane (nBu(4)N)(4)[Cu(2)(dpeba)(2)]·4MeOH·2Et(2)O (1) featuring a very long intermetallic distance (r = 15.0 Å). Magnetic susceptibility measurements for 1 reveal a moderately weak but nonnegligible intramolecular antiferromagnetic coupling between the two metal centers across the double para-substituted diphenylethynediamidate bridge (J = -3.9 cm(-1); H = -JS(1)S(2), where S(1) = S(2) = S(Cu) = (1)/(2)). Density functional electronic structure calculations on 1 support the occurrence of a spin polarization mechanism.

Four heterotrinuclear Re(IV)(2)M(II) compounds of general formula (NBu(4))(2)[{Re(IV)Br(4)(μ-ox)}(2)M(II)(Him)(2)] [NBu(4)(+) = tetra-n-butylammonium cation, ox = oxalate, Him = imidazole; M = Mn (1), Co (2), Ni (3), and Cu (4)] have been synthesized by using the novel mononuclear complex [Re(IV)Br(4)(ox)](2-) as a ligand toward divalent first-row transition metal ions in the presence of imidazole. Compounds 1-4 are isostructural complexes whose structure contains discrete trinuclear [{Re(IV)Br(4)(μ-ox)}(2)M(II)(Him)(2)](2-) anions and bulky NBu(4)(+) cations. The Re and M atoms are six-coordinated: four peripheral bromo and two oxalate-oxygens (at Re), and two cis-coordinated imidazole molecules and four oxygen atoms from two oxalate ligands (at M), build distorted octahedral surroundings. Two peripheral [ReBr(4)(ox)](2-) units act as bidentate ligands through the oxalate group toward the central [M(II)(Him)(2)] fragment affording the trinuclear entities. The values of the intramolecular Re···M separation are 5.62(1) (1), 5.51(1) (2), 5.46(1) (3), and 5.55(1) Å (4). Magnetic susceptibility measurements on polycrystalline samples of 1-4 in the temperature range of 1.9-300 K show the occurrence of intramolecular antiferro- [J = -1.1 cm(-1) (1)] and ferromagnetic interactions [J = +3.9 (2), +19.7 (3), and +14.4 cm(-1) (4)], the Hamiltonian being defined as Ĥ = -J [Ŝ(M)(Ŝ(Re1) + Ŝ(Re2))]. The larger spin delocalization on the oxalato bridge in 1-4 when compared to the trinuclear Re(IV)(2)M(II) complexes with chloro instead of bromo as peripheral ligands (1'-4') accounts for the strengthening of the magnetic interactions in 1-4 [J = -0.35 (1'), +14.2 (3'), and +7.7 cm(-1) (4')]. An incipient frequency dependence of the out-of-phase ac signals of 3 at very low temperatures is reminiscent of a system with slow relaxation of the magnetization, a phenomenon characteristic of single-molecule magnet behavior., (© 2011 American Chemical Society)

The Re(IV)-Mo(VI) compound of formula (PPh(4))(2)[ReCl(4)(μ-ox)MoO(2)Cl(2)] (1) constitutes the first example of a heterodinuclear oxalato-bridged complex in the coordination chemistry of molybdenum.

The anticancer properties of a series of mononuclear Re(IV) compounds of formula ReCl(4)L (where L is bpy = 2,2'-bipyridine; bpym = 2,2'-bipyrimidine; dmbpy = 4,4'-dimethyl-2,2'-bipyridine; phen = 1,10-phenanthroline) were investigated for the first time. All compounds displayed potent in vitro antiproliferative activity against selected cancer cells., (© The Royal Society of Chemistry 2011)

The use of the mononuclear species (NBu(4))(2)[Re(IV)Cl(4)(ox)] (NBu(4)(+) = tetra-n-butylammonium cation; ox = oxalate dianion) as a ligand toward fully solvated divalent first-row transition-metal ions affords the tetranuclear complexes (NBu(4))(4)[{Re(IV)Cl(4)(mu-ox)}(3)M(II)] with M = Mn (1), Fe (2), Co (3), Ni (4), and Cu (5). Their structure is made up of discrete [{ReCl(4)(mu-ox)}(3)M](4-) anions and bulky NBu(4)(+) cations. The complexes 2-5 crystallize in the triclinic system with space group P1; 2 and 5 as well as 3 and 4 are isostructural. The Re and M atoms exhibit somewhat distorted ReCl(4)O(2) and MO(6) octahedral surroundings, with the oxalate groups adopting the bis-bidentate bridging mode. Magnetic susceptibility measurements on polycrystalline samples of 1-5 in the temperature range 1.9-300 K show the occurrence of intramolecular antiferromagnetic [J = -1.30 cm(-1) (1)] and ferromagnetic couplings [J = +1.62 (2), +3.0 (3), +16.3 (4), and +4.64 cm(-1) (5)], with the Hamiltonian being defined as H = -J[S(M)(S(Re1) + S(Re2) + S(Re3))]. Compound 4 is the first example of an oxalato-bridged heterometallic species that behaves as a single-molecule magnet with a ground-state spin S = (11)/(2) and D = -0.8(1) cm(-1), as shown by the study of its static and dynamic magnetic properties and a high-frequency electron paramagnetic resonance study on polycrystalline samples together with detailed micro-SQUID measurements on single crystals.

A dodecanuclear copper(II) globular-shaped structure has been obtained with the cytidine nucleoside and the templating carbonate anion. It shows receptor properties through anion-cation and multiple anion-pi interactions toward ClO 4 (-) as well as an overall antiferromagnetic coupling.

The three-dimensional cobalt(ii) compound of formula {[Co(2)(P(2)O(7))(bpym)(2)].12H(2)O}(n), where the pyrophosphate and 2,2'-bipyrimidine act as bridging ligands, is a new example of a spin-canted antiferromagnet with T(c) = 19 K.

A series of oxalate-bridged iron(III) complexes have been synthesized by the reaction of FeCl 3 with oxalic acid (H 2ox) and XCl, where X is a substituted univalent ammonium or an alkaline cation. We have obtained basically two different types of compounds by varying the nature and the shape of the counterion, with the dimensionality of the resulting product being strongly influenced by the counterion. Three-dimensional (3D) networks of oxo- and oxalato-bridged iron(III) ions of the general formula {X 2[Fe 2O(ox) 2Cl 2]. pH 2O} n have been obtained for X = Li (+) ( 1), Na (+) ( 2), and K (+) ( 3) with p = 4 and X = MeNH 3 (+) ( 4), Me 2NH 2 (+) ( 5), and EtNH 3 (+) ( 6) with p = 2. Similar 3D hydroxo- and oxalato-bridged iron(III) networks of the formula {X[Fe 2(OH)(ox) 2Cl 2].2H 2O} n resulted for X = EtNH 3 (+) ( 7a) and PrNH 3 (+) ( 8). Compound 7a undergoes a solid-to-solid transformation, leading to a new species of the formula {(H 3O)(EtNH 3)[Fe 2O(ox) 2Cl 2].H 2O} n ( 7b). Chainlike compounds of the formula {X 2[Fe 2(ox) 2Cl 4]. pH 2O} n [X = Me 2NH 2 (+)( 9, p = 1), Me 3NH (+) ( 10, p = 2), and Me 4N (+) ( 11, p = 0)] have been obtained for the bulkier alkylammonium cations. Magnetic susceptibility measurements in the temperature range 1.9-295 K show the occurrence of weak ferromagnetic ordering due to spin canting in the 3D networks 1- 8, with the value of the critical temperature ( T c) varying with the cation in the range 26 K ( 2) to 70 K ( 8) without significant structural modifications. The last three one-dimensional compounds exhibit the typical behavior of antiferromagnetically coupled chains of interacting spin sextets [ J = -8.3 ( 9), -6.9 ( 10), and -8.4 ( 11) cm (-1) with H = - J summation operator i S i S i+1 ].

Three new compounds of formula {[Cu(gua)(H(2)O)(3)](BF(4))(SiF(6))(1/2)}(n) (1), {[Cu(gua)(H(2)O)(3)](CF(3)SO(3))(2).H(2)O}(n) (2) and [Cu(gua)(2)(H(2)O)(HCOO)]ClO(4).H(2)O.1/2HCOOH] (3) [gua = 2-amino-1H-purin-6(9H)-one] showing the unprecedented coordination of neutral guanine, have been synthesised and structurally characterized. The structures of the compounds 1 and 2 contain uniform copper(II) chains of formula [Cu(gua)(H(2)O)(3)](n)(2n+), where the copper atoms are bridged by guanine ligands coordinated via N(3) and N(7). The electroneutrality is achieved by uncoordinated tetrafluoroborate and hexafluorosilicate (1) and triflate (2). Each copper atom in 1 and 2 is five-coordinated in a distorted square pyramidal environment: two water molecules in trans positions and the N(3) and N(7a) nitrogen atoms of two guanine ligands build the basal plane whereas a water molecule fills the axial position. The values of the copper-copper separation across the bridging guanine ligand are 7.183(1) (1) and 7.123(1) A (2). is an ionic salt whose structure is made up of mononuclear [Cu(gua)(2)(H(2)O)(HCOO)](+) cations and perchlorate anions plus water and formic acid as crystallization molecules. The two guanine ligands in the cation are coordinated to the copper centre through the N(9) atom. The copper atom in 3 is four-coordinated with two monodentate guanine molecules in the trans position, a water molecule and a monodenate formate ligand building a quasi square planar surrounding. Magnetic susceptibility measurements for 1 and 2 in the temperature range 1.9-300 K show the occurrence of significant intrachain antiferromagnetic interactions between the copper(ii) ions across the guanine bridge [J = -9.6(1) (1) and -10.3(1) cm(-1) (2) with H = -J summation operator(i)S(i).S(i+1)].

A new heterobimetallic Re(IV)Cu(II) compound has been prepared and its crystal structure determined by single-crystal X-ray diffraction; magnetic susceptibility measurements show that this compound behaves as a ferrimagnetic chain with significant antiferromagnetic interactions between Re(IV) and Cu(II) metal ions.

The preparation, crystal structure and magnetic properties of four heteroleptic copper(II) complexes with the tricyanomethanide (tcm(-)) and the heterocyclic nitrogen donors 3,6-bis(2-pyridyl)pyridazine (dppn), 2,5-bis(2-pyridyl)pyrazine (2,5-dpp), 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) and 2,3-bis(2-pyridyl)quinoxaline (2,3-dpq) are reported, {[Cu(2)(dppn)(OH)(tcm)(2)] x tcm}(n) (1), {[Cu(2,5-dpp)(tcm)] x tcm}(n) (2), {[Cu(2)(2,3-dpp)(2)(tcm)(3)(H(2)O)(0.5)] x tcm x 0.5H(2)O}(n) (3) and [Cu(2,3-dpq)(tcm)(2)](n) (4). 1 has a ladder-like structure with single mu-1,5-tcm ligands forming the sides and a bis-bidentate dppn and a single mu-hydroxo providing the rung. Each copper atom in 1 exhibits a distorted square pyramidal CuN(4)O surrounding: the basal plane is built by the hydroxo-oxygen, a nitrile-nitrogen atom from a tcm group and one pyrazine and a pyridyl nitrogen atoms from the dppn whereas the apical position is filled by a nitrile-nitrogen atom from a symmetry-related tcm ligand. The structures of 2-4 consists of zig-zag (2 and 3)/linear (4) chains of copper(II) ions which are bridged by either bis-bidentate 2,5-dpp (2) and 2,3-dpp (3) molecules or single mu-1,5-tcm (4) groups. The copper atoms in 2 and 4 are five coordinated with distorted trigonal bipyramidal (2) and square pyramidal (4) CuN(5) surroundings. The axial positions in 2 are occupied by two pyridyl-nitrogen atoms from two 2,5-dpp ligands whereas the trigonal plane is built by a nitrile-nitrogen from a terminally bound tcm group and two pyrazine nitrogen atoms from two 2,5-dpp molecules. The basal plane in 4 is defined by a pyridyl and a pyrazine nitrogen atoms from the bidentate 2,3-dpq ligand and two nitrile nitrogen atoms from two tcm groups (one terminal and the other bridging) whereas the apical position is filled by a nitrile nitrogen from another tcm ligand. The crystallographically independent copper atoms in 3 [Cu(1) and Cu(2)] exhibit elongated octahedral geometries being defined by four nitrogen atoms from two 2,3-dpp groups [Cu(1) and Cu(2)] either two terminally bound tcm ligands [Cu(1)] or a water molecule and a monodentate tcm ligand [Cu(2)] in cis positions. Magnetic susceptibility measurements for 1-4 in the temperature range 1.9-295 K reveal the occurrence of strong [J ca.-1000 cm(-1) (1); H = -JS(A) x S(B)] and weak [J = -0.13 (2), -0.67 (3) and -0.18 cm(-1) (4); H = -J Sigma(I)S(i) x S(i+1)] antiferromagnetic interactions in agreement with the different nature of the exchange pathways involved, diazine and single mu-hydroxo (1) and the extended 2,5-dpp (2), 2,3-dpp (3) and single mu-1,5-tcm (4) bridges with copper-copper separations of 3.363(8) (1), 7.111(1) (2), 6.823(1) and 7.056(1) (3) and 7.446(1) A (4).

Four novel rhenium complexes of formula [ReCl(4)(bpym)] (1), [ReBr(4)(bpym)] (2) PPh(4)[ReCl(4)(bpym)] (3) and NBu(4)[ReBr(4)(bpym)] (4) (bpym = 2,2'-bipyrimidine, PPh(4) = tetraphenylphosphonium cation and NBu(4) = tetrabutylammonium cation), have been synthesized and their crystal structures determined by single-crystal X-ray diffraction. The structures of 1 and 2 consist of [ReX(4)(bpym)] molecules held together by van der Waals forces. In both complexes the Re(iv) central atom is surrounded by four halide anions and two nitrogen atoms of a bpym bidentate ligand in a distorted octahedral environment. The structures of 3 and 4 consist of [ReX(4)(bpym)](-) anions and PPh(4)(+) () or NBu(4)(+) (4) cations. The coordination sphere of the Re(iii) metal ion is the same as in 1 and 2, respectively. However, whereas the Re-X bonds are longer the Re-N bonds are shorter than in 1 and 2. This fact reveals that the bpym ligand forms a stronger bond with Re(iii) than with Re(iv) resulting in a stabilisation of the lower oxidation state. [ReX(4)(bpym)] complexes are easily reduced, chemically and electrochemically, to the corresponding [ReX(4)(bpym)](-) anions. A voltammetric study shows that the electron transference is a reversible process characterized by formal redox potentials of +0.19 V (1) and +0.32 V (2) vs. NHE, in acetonitrile as solvent.

The reaction of [ReIVCl4(ox)]2- and fully solvated Ni2+ ions in a MeCN/i-PrOH mixture affords the heterotetranuclear complex (NBu4)4[Ni{ReCl4(ox)}3] where the rhenium precursor acts as a bidentate ligand toward the nicke(II) ion through the oxalate group. The mixed 3d-5d species exhibits intramolecular ferromagnetic coupling and it behaves like a single-molecule magnet.

The structural characterization of a series of complexes of formula [M(dca)2L]n, where dca = dicyanamide, L = 1,10-phenanthroline (phen) [1-4] and 2,9-dimethylphenanthroline (2,9-dmphen) [9-12], and M = Mn (1 and 9), Fe (2 and 10), Co (3 and 11), and Ni (4 and 12), has revealed the effect of the presence of the methyl substituents of L on the resulting network. The structure of [Mn(dca)2(phen)]n (1), which is identical to those of 2-4, together with the investigation of its magnetic properties in the temperature range of 77-300 K were reported elsewhere. The use of the 4,7-dimethylphenanthroline (4,7-dmphen) as the co-ligand yielded a series of compounds of formula [M(dca)2(4,7-dmphen)]n [M = Mn (5), Fe (6), Co (7), and Ni (8)], which are isostructural with 1-4. Compounds containing phen (1-4) and 4,7-dmphen (5-8) are made of two-dimensional grids of metal atoms, each metal atom being linked to three other metal centers through single (three metal atoms involved) and double (two metal atoms involved) dca bridges exhibiting the mu-1,5 coordination mode. The isostructural complexes [M(dca)2(2,9-dmphen)]n (9-12) also have a sheetlike structure, the metal atoms in each layer being linked by two single and one double mu-1,5-dca units, as in 1-8. However, the topology of the network in 9-12 is different from that in 1-8 because of the different arrangement of the two single mu-1,5 dca bridges: cis in 1-8 versus trans in 9-12. The magnetic study of compounds 1-12 in the temperature range of 1.9-290 K has revealed the occurrence of weak ferromagnetic (M = Ni) and antiferromagnetic interactions (M = Mn, Fe, and Co). The different magnetic behavior in 1-12 was analyzed in the light of their structures, and the values of the magnetic interactions were compared to those of related systems.

Intermolecular proton transfer in solid phase from the hydroxo bridge to a water molecule occurs in a new mu-hydroxo iron(III) compound of formula {EtNH3[Fe2(ox)2Cl2(mu-OH)].2H2O}n leading to a still crystalline compound in which the mu-oxo bridge replaces the mu-hydroxo one. Both three-dimensional compounds exhibit magnetic ordering at Tc ca. 70 K due to a spin canting.

The binuclear complex NiII2L(H2O)2(ClO4)2(1) and the neutral tetranuclear bimetallic compounds [{M(III)(phen)(CN)4}2{NiII2L(H2O)2}].2CH3CN with M=Fe (2) and Cr (3)[H2L=11,23-dimethyl-3,7,15,19-tetraazatricyclo[19.3.1.1(9,13)]hexacosa-2,7,9,11,13(26),14,19,21(25),22,24-decaene-25,26-diol] have been synthesized and the structures of and determined by single crystal X-ray diffraction. and are isostructural compounds whose structure is made up of centrosymmetric binuclear cations [Ni2(L)(H2O)2]2+ and two peripheral [M(phen)(CN)4]- anions [M=Fe (2) and Cr (3)] acting as monodentate ligands towards the nickel atoms through one of their four cyanide nitrogen atoms. The environment of the metal atoms in 2 and 3 is six-coordinated: two phen-nitrogen and four cyanide-carbon atoms at the iron and chromium atoms and a water molecule, one cyanide-nitrogen and two phenolate-oxygens and two imine-nitrogens from the binucleating ligand L2- at the nickel atom build distorted octahedral surroundings. The values of the FeNi and CrNi separations through the single cyanide bridge are 5.058(1) and 5.174(2)A respectively, whereas the Ni-Ni distances across the double phenolate bridge are 3.098(2)(2) and 3.101(1) A (3). The magnetic properties of have been investigated in the temperature range 1.9-290 K. The magnetic behaviour of corresponds to that of an antiferromagnetically coupled nickel(II) dimer with J=-61.0(1) cm-1, the Hamiltonian being defined as H=-J S(A).S(B). An overall antiferromagnetic behaviour is observed for and with a low-lying singlet spin state. The values of the intramolecular magnetic couplings are J(Fe-Ni)=+17.4(1) cm-1 and J(Ni-Ni(a))=-44.4(1) cm-1 for and J(Cr-Ni)=+11.8(1) cm-1 and J(Ni-Ni(a))=-44.6(1) cm-1 for [H=-J(M-Ni)(S(M).S(Ni)+S(Ma).S(Nia))-J(Ni-Nia)S(Ni)S(Nia)]. Theoretical calculations using methods based on density functional theory (DFT) have been employed on in order to analyze the efficiency of the exchange pathways involved and also to substantiate the exchange coupling parameters.

The Re(IV) complex [ReCl4(mal)]2-, in the form of two slightly different salts, (AsPh4)1.5(HNEt3)0.5[ReCl4(mal)] (1a) and (AsPh4)(HNEt3)[ReCl4(mal)] (1b), and the Re(IV)-Cu(II) bimetallic complexes [ReCl4(mu-mal)Cu(phen)2].CH3CN (2), [ReCl4(mu-mal)Cu(bpy)2] (3), and [ReCl4(mu-mal)Cu(terpy)] (4) (mal=malonate dianion, AsPh4=tetraphenylarsonium cation, HNEt3=triethylammonium cation, phen=1,10-phenanthroline, bpy=2,2'-bipyridine and terpy=2,2':6',2' '-terpyridine) have been synthesized and the structures of 1a, 1b, 2, and 3 determined by single-crystal X-ray diffraction. The structures of 1a and 1b are made up of discrete [ReCl4(mal)]2- anions and AsPh4+ and HNEt3+ cations, held together by electrostatic forces and hydrogen bonds. The Re(IV) atom is surrounded by four chloride anions and a bidentate malonate group, in a distorted octahedral environment. The structure of 2 consist of neutral dinuclear units [ReCl4(mu-mal)Cu(phen)2], with the metal ions united through a bridge carboxilato. The environment of Re(IV) is nearly identical to that in the mononuclear complex, and Cu(II) is five coordinate, being surrounded by four nitrogen atoms of two bidentate phen ligands and one oxygen atom of the malonato ligand. In 3, there are also dinuclear units, [ReCl4(mu-mal)Cu(bpy)2], but the Cu(II) ions complete a distorted octahedral coordination by binding with the free malonato oxygen atom of a neighbor unit, resulting in an infinite chain. The magnetic properties of 1-4 were also investigated in the temperature range 2.0-300 K. The magnetic behavior of 1a and 1b is as expected for a Re(IV) complex with a large value of the zero-field splitting (2D ca. 110 cm(-1)). For the bimetallic complexes, the magnetic coupling between Re(IV) and Cu(II) is antiferromagnetic in 2 (J=-0.39 cm(-1)), ferromagnetic in 4 (J=+1.51 cm(-1)), and nearly negligible in 3 (J=-0.09 cm(-1)).

Alkyl-substituted ammonium cations (X) allow the preparation of a series of spin-canted oxo- and oxalato-bridged three-dimensional iron(III) networks, exhibiting magnetic ordering at T(c) values ranging from 40 to 56 K. The value of T(c) varies with the cation despite the lack of significant structural modifications.

The hexahalorhenate(IV) salts of formula [Fe(C5H5)2]2[ReX6], with X = Cl (1), Br (2), and I (3), and [Fe(C5Me5)2]2[ReX6], with X = Cl (4), Br (5), and I (6) ([Fe(C5Me5)2]+ = decamethylferrocenium cation), have been synthesized and the structures of 1, 2, and 4 determined by single-crystal X-ray diffraction. 1, 2, and 4 crystallize in the orthorhombic system, space groups Pbca (1 and 2) and Ibam (4), with a = 14.099(2) A, b = 16.125(2) A, and c = 22.133(15) A, for 1, a = 14.317(3) A, b = 16.848(3) A, and c = 22.099(2) A for 2, and a = 15.8583(5) A, b = 15.9368(5) A, and c = 16.9816(6) A for 4. The three structures are made up of discrete [ReX6]2- anions and ferrocenium cations held together by electrostatic forces. There are anion-anion contacts in 1 and 2 but only through one direction. The [ReX6]2- octahedra are arranged along the y axis forming chains of Re and X atoms, -Re-X...X-Re-X...X-Re-, where the intermolecular X...X distances are shorter than the van der Waals distances. A somewhat greater separation between the anions occurs in 4. The magnetic properties of 1-6 were investigated in the temperature range 2.0-300 K. 1, 2, 4, and 5 exhibit an antiferromagnetic coupling between the anions, whereas a ferromagnetic coupling between anions and cations is the dominant interaction in 3. 6 behaves as a magnetically isolated compound, its susceptibility being the simple addition of the independent contributions of the uncoupled paramagnetic cations and anions.

Electrospray tandem mass spectrometry (ESI-MS/MS) is used to evaluate the assembling of cytosine and thymine nucleosides in the gas phase, through the formation of hydrogen bonded supermolecules. Mixtures of cytidine analogues and homologues deliver in the gas phase proton-bound heterodimers stabilized by multiple interactions, as proven by the kinetics of their dissociation into the corresponding protonated monomers. Theoretical calculations, performed on initial structures of methylcytosine homodimers available in the literature, converged to a minimized structure whereby the two pyrimidine rings interact through the formation of three hydrogen bonds of similar energy. The crystallographic data here reported show the equivalency of the two interacting pyrimidines which is attributable to the presence of an inversion center. Thymine and uracil pyrimidyl nucleosides form, by ESI, gaseous proton-bound dimers. The kinetic of their dissociation into the related protonated monomers shows that the nucleobases are weekly interacting through a single hydrogen bond. The minimized structure of the protonated heterodimer formed by thymine and N-1-methylthymine confirmed the existence of mainly one hydrogen bond which links the two nucleobases through the O4 oxygens. No crystallographic data exists on thymine proton-bound species, nor have we been able to obtain these aggregates in the solid phase. The gaseous phase, under high vacuum conditions, seems therefore a suitable environment where vanishing structures produced by ESI can be studied with a good degree of approximation.