Tuning the spin ground state in heterononanuclear nickel(II)-copper(II) cylinders with a triangular metallacyclophane core

Two new heterometallic Ni(II)(n)Cu(II)((9-n)) complexes [n = 1 (2) and 2 (3)] have been synthesized following a multicomponent self-assembly process from a n:(3 - n):2:6 stoichiometric mixture of Ni(2+), Cu(2+), L(6-), and [CuL'](2+), where L and L' are the bridging and blocking ligands 1,3,5-benzenetris(oxamate) and N,N,N',N',N'-pentamethyldiethylenetriamine, respectively. Complexes 2 and 3 possess a unique cyclindrical architecture formed by three oxamato-bridged trinuclear linear units connected through two 1,3,5-substituted benzenetris(amidate) bridges, giving a triangular metallacyclophane core. They behave as a ferromagnetically coupled trimer of two (2)/one (3) S = (1)/(2) Cu(II)(3) plus one (2)/two (3) S = 0 Ni(II)Cu(II)(2) linear units with overall S = 1 Ni(II)Cu(II)(8) (2) and S = (1)/(2) Ni(II)(2)Cu(II)(7) (3) ground states.     

Structural diversity in copper-sulfur chemistry: synthesis of novel Cu/S clusters through metathesis reactions

With the ultimate goal of understanding the Cu(4)S cluster in nitrous oxide reductase, studies of the fundamental chemistry of nitrogen-donor ligand-supported copper-sulfur species have been pursued. Reactions of Cu(II)X(2) (X = Cl(-) or CF(3)SO(3)(-)), N,N,N',N'-tetramethyl-trans-(1R,2R)-diaminocyclohexane, and Li(2)S or Na(2)S(2) yielded clusters that contain [Cu(2)(micro-S(2))(2)](2+), [Cu(3)(micro-S)(2)](3+), [Cu(4)(micro-S(2))(2)](4+), and/or [Cu(6)(micro-S(2))(4)](4+) cores, depending on the specific reaction conditions, notably the nature of X and the sulfur source used. Copper(II) and/or Copper(III) and variable sulfur oxidation levels, including S(2-), S(2)(2-), and S(2)(-*), were identified by X-ray crystallography and spectroscopy.     

X-ray absorption spectroscopic and computational investigation of a possible S···S interaction in the [Cu3S2]3+ core

The electronic structure of the [Cu(3)S(2)](3+) core of [(LCu)(3)(S)(2)](3+) (L = N,N,N',N'-tetramethyl-2R,3R-cyclohexanediamine) is investigated using a combination of Cu and S K-edge X-ray absorption spectroscopy and calculations at the density functional and multireference second-order perturbation levels of theory. The results show that the [Cu(3)S(2)](3+) core is best described as having all copper centers close to but more oxidized than Cu(2+), while the charge on the S(2) fragment is between that of a sulfide (S(2-)) and a subsulfide (S(2)(3-)) species. The [Cu(3)S(2)](3+) core thus is different from a previously described, analogous [Cu(3)O(2)](3+) core, which has a localized [(Cu(3+)Cu(2+)Cu(2+))(O(2-))(2)](3+) electronic structure. The difference in electronic structure between the two analogues is attributed to increased covalent overlap between the Cu 3d and S 3p orbitals and the increased radial distribution function of the S 3p orbital (relative to O 2p). These features result in donation of electron density from the S-S σ* to the Cu and result in some bonding interaction between the two S atoms at ~2.69 ? in [Cu(3)S(2)](3+), stabilizing a delocalized S = 1 ground state.     

Polymorphism and spin crossover in mononuclear Fe(II) species containing new dipyridylamino-substituted s-triazine ligands

Four new dipyridylamino-substituted s-triazine ligands DBB (N(2),N(2),N(4),N(4)-tetrabenzyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DDB (N(2),N(2),N(4),N(4)-tetrabutyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), DCCl (6-chloro-N(2),N(2)-dicyclohexyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine) and DDT (N(2),N(2),N(4),N(4)-tetraphenyl-N(6),N(6)-di(pyridin-2-yl)-1,3,5-triazine-2,4,6-triamine), have been incorporated into eight new, 0D Fe(II) compounds of type [Fe(II)(NCX)(2)(L)(2)]·Solvent (where NCX = NCS(-), NCSe(-) or N(CN)(2)(-)). The polymorphic compounds α-trans-[Fe(II)(NCS)(2)(DBB)(2)] (1) and β-trans-[Fe(II)(NCS)(2)(DBB)(2)] (2) display, respectively, a relatively abrupt, complete, one-step spin transition with T(?) ~ 170 K, and a more gradual, complete, one-step spin transition with T(?) ~ 300 K. Gradual, one-step spin transitions are observed for trans-[Fe(II)(N(CN)(2))(2)(DBB)(2)]·2CH(3)CH(2)OH (3) and trans-[Fe(II)(NCSe)(2)(DCCl)(2)]·2CH(3)OH (6) with T(?) ~ 280 K for both, while the one-step spin transition observed for a desolvated sample of trans-[Fe(II)(NCSe)(2)(DDB)(2)]·2CH(3)OH (4) is relatively abrupt, showing hysteresis with T(?↑) = 285 K and T(?↓) = 275 K. The compounds cis-[Fe(II)(NCS)(2)(DDB)(2)] (5) and trans-[Fe(II)(NCS)(2)(DDT)(2)]·4CH(2)Cl(2) (7) remain high spin, while structural data on trans-[Fe(II)(NCSe)(2)(DDT)(2)]·4CH(2)Cl(2) (8) suggests a spin transition at low temperatures. It is likely that distortion of the Fe(II)N(6) octahedron, intermolecular interactions and molecular conformation are crucial in deciding both the T(?) and abruptness of the spin transition for these species, although the nature of their influence varies. Variable temperature powder X-ray diffraction measurements on the polymorphs 1 and 2 reveal anisotropy in the unit cell parameters as the spin transition occurs.     

Can the hexamethylhydrazinium dication [Me(3)N-NMe(3)](2+) be prepared?

The long sought hexamethylhydrazinium(2+) dication, Me(3)N-NMe(3)(2+), calculationally unstable towards "coulombic explosion" because of formal positive charges on adjacent N atoms, can be synthesized and isolated as a CHB(11)Cl(11)(-) carborane salt.     

Binding modes in metal ion complexes of quinones and semiquinone radical anions: electron-transfer reactivity.

9,10-Phenanthrenequinone (PQ) and 1,10-phenanthroline-5,6-dione (PTQ) form 1:1 and 2:1 complexes with metal ions (M (n+)=Sc (3+), Y (3+), Mg (2+), and Ca (2+)) in acetonitrile (MeCN), respectively. The binding constants of PQ--M (n+) complexes vary depending on either the Lewis acidity or ion radius of metal ions. The one-electron reduced species (PTQ(-)) forms 1:1 complexes with M (n+), and PQ(-) also forms 1:1 complexes with Sc(3+), Mg(2+), and Ca(2+), whereas PQ(-) forms 1:2 complexes with Y(3+) and La(3+), as indicated by electron spin resonance (ESR) measurements. On the other hand, semiquinone radical anions (Q(-) and NQ(-)) derived from p-benzoquinone (Q) and 1,4-naphthoquinone (NQ) form Sc(3+)-bridged pi-dimer radical anion complexes, Q(-)--(Sc(3+))(n)--Q and NQ(-)--(Sc(3+))(n)-NQ (n=2 and 3), respectively. The one-electron reduction potentials of quinones (PQ, PTQ, and Q) are largely positively shifted in the presence of M (n+). The rate constant of electron transfer from CoTPP (TPP(2-)=dianion of tetraphenylporphyrin) to PQ increases with increasing the concentration of Sc(3+) to reach a constant value, when all PQ molecules form the 1:1 complex with Sc(3+). Rates of electron transfer from 10,10'-dimethyl-9,9'-biacridine [(AcrH)(2)] to PTQ are also accelerated significantly by the presence of Sc(3+), Y(3+), and Mg(2+), exhibiting a first-order dependence with respect to concentrations of metal ions. In contrast to the case of o-quinones, unusually high kinetic orders are observed for rates of Sc(3+)-promoted electron transfer from tris(2-phenylpyridine)iridium(III) [Ir(ppy)(3)] to p-quinones (Q): second-order dependence on concentration of Q, and second- and third-order dependence on concentration of Sc(3+) due to formation of highly ordered radical anion complexes, Q()--(Sc(3+))(n)--Q (n=2 and 3).     

Bis(μ-oxo) dicopper(III) species of the simplest peralkylated diamine: enhanced reactivity toward exogenous substrates

N,N,N',N'-tetramethylethylenediamine (TMED), the simplest and most extensively used peralkylated diamine ligand, is conspicuously absent from those known to form a bis(μ-oxo)dicopper(III) (O) species, [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+), upon oxygenation of its Cu(I) complex. Presented here is the characterization of this O species and its reactivity toward exogenous substrates. Its formation is complicated both by the instability of the [(TMED)Cu(I)](1+) precursor and by competitive formation of a presumed mixed-valent trinuclear [(TMED)(3)Cu(III)Cu(II)(2)(μ(3)-O)(2)](3+) (T) species. Under most reaction conditions, the T species dominates, yet, the O species can be formed preferentially (>80%) upon oxygenation of acetone solutions, if the copper concentration is low (<2 mM) and [(TMED)Cu(I)](1+) is prepared immediately before use. The experimental data of this simplest O species provide a benchmark by which to evaluate density functional theory (DFT) computational methods for geometry optimization and spectroscopic predictions. The enhanced thermal stability of [(TMED)(2)Cu(III)(2)(μ(2)-O)(2)](2+) and its limited steric demands compared to other O species allows more efficient oxidation of exogenous substrates, including benzyl alcohol to benzaldehyde (80% yield), highlighting the importance of ligand structure to not only enhance the oxidant stability but also maintain accessibility to the nascent metal/O(2) oxidant.     

Synthesis, structure, and electronic properties of RuN6 dinuclear Ru-Hbpp complexes

A series of RuN(6) dinuclear Ru-Hbpp complexes (Hbpp is the dinucleating tetraaza ligand 3,5-bis(pyridyl)pyrazole) of general formula {[Ru(II)(R(2)-trpy)(MeCN)](2)(μ-R(1)-bpp)}(3+), 10(3+)-14(3+), (R(1) = H, Me, or NO(2). and R(2) = H, Me, MeO; see Scheme 1) has been prepared from their Cl(-) or AcO(-) bridged precursors. The complexes have been characterized by UV-vis, NMR, CV, and some by X-ray. Complexes 10(3+)-14(3+), Ru(2)(II,II), were oxidized by 1 equiv in solution, leading to the mixed valence Ru(2)(II,III) complexes 10(4+)-14(4+) containing one unpaired electron and were characterized by EPR and UV-vis-near-IR, which showed metal-centered spin and the presence of low-energy IVCT bands. The H(ab) parameter indicates a relatively strong electronic coupling between the two ruthenium centers (class II). Further two electron oxidation in solution of the 10(3+)-14(3+) led to the formation of EPR silent Ru(2)(III,III) complexes 10(5+)-14(5+), that were further characterized by UV-vis-NIR. TD-DFT calculations are employed to assign the nature of the UV-vis transitions for the complexes in the various oxidation states, which are of metal to ligand charge transfer (MLCT) type for Ru(2)(II,II) and ligand to metal charge transfer (LMCT) type for Ru(2)(III,II) and Ru(2)(III,III).     

Structural criteria for the rational design of selective ligands. 3. Quantitative structure-stability relationship for iron(III) complexation by tris-catecholamide siderophores

We present an extended MM3 model for catecholamide ligands and their Fe(3+) complexes and the application of this model to understand how ligand architecture effects Fe(3+) binding affinity. Force field parameters were fit to geometries and energies from electronic structure calculations, and to crystal structure data. Optimized geometries are reported for phenol, acetamide, the phenol-phenol dimer, the acetamide-phenol dimer, and N-methylsalicylamide (HMSA) at the BLYP/DZVP2/A2 level of theory. Optimized geometries and relative energies are reported for the pseudo-octahedral ground state and the trigonal planar transition state of [Fe(CAT)(3)](3)(-) at the VWN/DZVP2/A1 level of theory. The MM3 model is validated by comparison of calculated structures with crystal structures containing 1,2-dihydroxybenzene (H(2)CAT) and 2,3-dihydroxy-N-methylbenzamide (H(2)MBA) fragments, crystal structures of [Fe(CAT)(3)](3)(-) and tris-catecholamide Fe(3+) complexes, and comparison of MM3 (6.8 kcal/mol) and VWN (5.9 kcal/mol) barriers for intramolecular octahedral inversion in [Fe(CAT)(3)](3)(-). The MM3 model also rationalizes the higher inversion barrier (14 to 18 kcal/mol) reported for [Ga(N,N-diisopropylterephthalamide)(3)](3)(-) ([Ga(DIPTA)(3)](3)(-)). Conformational searches were performed on enterobactin (H(6)ENT), 1,3,5-tris(2,3-dihydroxybenzamidomethyl)-2,4,6-triethylbenzene (H(6)EMECAM), 1,3,5-tris(2,3-dihydroxybenzamidomethyl)-2,4,6-trimethylbenzene (H(6)MMECAM), 1,3,5-tris(2,3-dihydroxybenzamidomethyl)benzene (H(6)MECAM), and 1,5,9-N,N',N' '-tris(2,3-dihydroxybenzoyl)cyclotriazatridecane (H(6)-3,3,4-CYCAM) and Fe(3+) complexes with each of these ligands. A conformational search also was done on the Fe(3+) complex with the 2,2',2' '-tris(2,3-dihydroxybenzamido)triethylammonium cation (H(7)TRENCAM(+)). The relationship between calculated steric energies and measured thermodynamic quantities is discussed, and linear correlations between formation constants and steric energy differences are reported. Extrapolation to zero strain predicts formation constants 8 +/- 5 orders of magnitude higher than that exhibited by ENT (10(49)) are possible. This prediction is supported by a formation constant of 10(63) estimated from the formation constant of [Fe(2,3-dihydroxy-N,N-dimethylbenzamide)(3)](3)(-) ([Fe(DMBA)(3)](3)(-)) by considering the entropic consequences of connecting three DMBA ligands to a rigid backbone. Structural criteria for the identification of improved tris-catecholate ligand architectures are presented.     

Relative energy of the high-(5T2g) and low-(1A1g) spin states of [Fe(H2O)6]2+, [Fe(NH3)6]2+, and [Fe(bpy)3]2+: CASPT2 versus density functional theory

High-level ab initio calculations using the CASPT2 method and extensive basis sets were performed on the energy differences of the high-[(5)T(2g):t(2g) (4)e(g) (2)] and low-[(1)A(1g):t(2g) (6)] spin states of the pseudo-octahedral Fe(II) complexes [Fe(H(2)O)(6)](2+), [Fe(NH(3))(6)](2+), and [Fe(bpy)(3)](2+). The results are compared to the results obtained from density functional theory calculations with the generalized gradient approximation functional BP86 and two hybrid functionals B3LYP and PBE0, and serve as a calibration for the latter methods. We find that large basis set CASPT2 calculations may provide results for the high-spin/low-spin splitting DeltaE(HL) that are accurate to within 1000 cm(-1), provided they are based on an adequately large CAS[10,12] reference wave function. The latter condition was found to be much more stringent for [Fe(bpy)(3)](2+) than for the other two complexes. Our "best" results for DeltaE(HL) (including a zero-point energy correction) are -17 690 cm(-1) for [Fe(H(2)O)(6)](2+), -8389 cm(-1) for [Fe(NH(3))(6)](2+), and 3820 cm(-1) for [Fe(bpy)(3)](2+).     

Metal ion-binding properties of 1-methyl-4-aminobenzimidazole (=9-methyl-1,3-dideazaadenine) and 1,4-dimethylbenzimidazole (=6,9-dimethyl-1,3-dideazapurine). quantification of the steric effect of the 6-amino group on metal ion binding at the N7 site of the adenine residue

The stability constants of the 1:1 complexes formed between Mg(2+), Ca(2+), Sr(2+), Ba(2+), Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), or Cd(2+) (=M(2+)) and 1-methyl-4-aminobenzimidazole (MABI) or 1,4-dimethylbenzimidazole (DMBI) were determined by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.5 M, NaNO(3)). Some of the stability constants were also measured by UV spectrophotometry. The acidity constants of the species H(2)(MABI)(2+) and H(DMBI)(+) were determined by the same methods, some twice. Comparison of the stability constants of the M(MABI)(2+) and M(DMBI)(2+) complexes with those calculated from log versus p straight-line plots, which were established previously for sterically unhindered benzimidazole-type ligands (=L), reveals that the stabilities of the M(MABI)(2+) and M(DMBI)(2+) complexes are significantly reduced due to steric effects of the C4 substituents on metal ion binding at N3. This effect is more pronounced in the M(DMBI)(2+) complexes. Considering the steric equivalence of methyl and (noncoordinating) amino groups (as they occur in adenines), it is concluded that the same extent of steric inhibition by the (C6)NH(2) group is to be expected on metal ion binding at N7 with adenine derivatives. The basicity of the amino group in MABI is significantly higher than in its corresponding adenine derivative. Indeed, it is concluded that in the M(MABI)(2+) complexes chelate formation involving the amino group occurs to some extent. The formation degrees of these "closed" species are calculated; they vary for the complexes of Mn(2+), Co(2+), Ni(2+), Cu(2+), Zn(2+), or Cd(2+) between about 50 and 90%. The stability of the M(MABI)(2+) and M(DMBI)(2+) complexes with the alkaline earth ions is very low but unaffected by the C4 substituent; this probably indicates that in these instances outersphere complexes (with a water molecule between N3 and the metal ion) are formed.     

ESI-MS and thermal melting studies of nanoscale platinum(II) metallomacrocycles with DNA

The hydrophilic, long-chain diamine PEGda (O,O'-bis(2-aminoethyl)octadeca(ethylene glycol)), when complexed with cis-protected Pt(II) ions afforded water-soluble complexes of the type [Pt(N,N)(PEGda)](NO(3))(2) (N,N = N,N,N',N'-tetramethyl-1,2-diaminoethane (tmeda), 1,2-diaminoethane (en), and 2,2'-bipyridine (2,2'-bipy)) featuring unusual 62-membered chelate rings. Equimolar mixtures containing either the 16-mer duplex DNA D2 or the single-stranded D2a and [Pt(N,N)(PEGda)](2+) were analyzed by negative-ion ESI-MS. Analysis of D2-Pt(II) mixtures showed the formation of 1 : 1 adducts of [Pt(en)(PEGda)](2+), [Pt(tmeda)(PEGda)](2+) and the previously-described metallomacrocycle [Pt(2)(2,2'-bipy)(2){4,4'-bipy(CH(2))(4)4,4'-bipy}(2)](8+) with D2; the dinuclear species bound to D2 most strongly, consistent with its greater charge and aromatic surface area. D2 formed 1 : 2 complexes with the acyclic species [Pt(2,2'-bipy)(Mebipy)(2)](4+) and [Pt(2,2'-bipy)(NH(3))(2)](2+). Analyses of D2a-Pt(II) mixtures gave results similar to those obtained with D2, although fragmentation was more pronounced, indicating that the nucleobases in D2a play more significant roles in mediating the decomposition of complexes than those in D2, in which they are paired in a complementary manner. Investigations were also conducted into the effects of selected platinum(II) complexes on the thermal denaturation of calf thymus DNA (CT-DNA) in buffered solution. Both [Pt(2)(2,2'-bipy)(2){4,4'-bipy(CH(2))(6)4,4'-bipy}(2)](8+) and [Pt(2,2'-bipy)(Mebipy)(2)](4+) stabilized CT-DNA. In contrast, [Pt(tmeda)(PEGda)](2+) and [Pt(en)(PEGda)](2+) (as well as free PEGda) caused negligible changes in melting temperature (ΔT(m)), suggesting that these species interact weakly with CT-DNA.     

Comparison of the surprising metal-ion-binding properties of 5- and 6-uracilmethylphosphonate (5Umpa2- and 6Umpa2-) in aqueous solution and crystal structures of the dimethyl and di(isopropyl) esters of H2(6Umpa)

5- and 6-Uracilmethylphosphonate (5Umpa(2-) and 6Umpa(2-)) as acyclic nucleotide analogues are in the focus of anticancer and antiviral research. Connected metabolic reactions involve metal ions; therefore, we determined the stability constants of M(Umpa) complexes (M(2+)=Mg(2+), Ca(2+), Mn(2+), Co(2+), Cu(2+), Zn(2+), or Cd(2+)). However, the coordination chemistry of these Umpa species is also of interest in its own right, for example, the phosphonate-coordinated M(2+) interacts with (C4)O to form seven-membered chelates with 5Umpa(2-), thus leading to intramolecular equilibria between open (op) and closed (cl) isomers. No such interaction occurs with 6Umpa(2-). In both M(Umpa) series deprotonation of the uracil residue leads to the formation of M(Umpa-H)(-) complexes at higher pH values. Their stability was evaluated by taking into account the fact that the uracilate residue can bind metal ions to give M(2)(Umpa-H)(+) species. This has led to two further important insights: 1) In M(6Umpa-H)-cl the H(+) is released from (N1)H, giving rise to six-membered chelates (degrees of formation of ca. 90 to 99.9 % with Mn(2+), Co(2+), Cu(2+), Zn(2+), or Cd(2+)). 2) In M(5Umpa-H)$-cl the (N3)H is deprotonated, leading to a higher stability of the seven-membered chelates involving (C4)O (even Mg(2+) and Ca(2+) chelates are formed up to approximately 50 %). In both instances the M(Umpa-H)-op species led to the formation of M(2)(Umpa-H)(+) complexes that have one M(2+) at the phosphonate and one at the (N3)(-) (plus carbonyl) site; this proves that nucleotides can bind metal ions independently at the phosphate and the nucleobase residues. X-ray structural analyses of 6Umpa derivatives show that in diesters the phosphonate group is turned away from the uracil residue, whereas in H(2)(6Umpa) the orientation is such that upon deprotonation in aqueous solution a strong hydrogen bond is formed between (N1)H and PO(3) (2-); replacement of the hydro gen with M(2+) gives the M(6Umpa-H)-cl chelates mentioned.     

Purely heterometallic lanthanide(III) macrocycles through controlled assembly of disulfide bonds for dual color emission

Lanthanide complexes based on bis(amides) of diethylenetriaminepentaacetic acid with thiol functionalities are modified with 2,2'-dipyridyl disulfide to give activated complexes that can selectively react with thiol-functionalized complexes to form heterometallic lanthanide macrocycles. The preparation and full characterization of the polyaminocarboxylate ligands N,N'-bis[p-thiophenyl(aminocarbonyl)]diethylenetriamine-N,N',N'-triacetic acid (H(3)L(x)) and the activated N,N'-bis[p-(pyridyldithio)[phenyl(aminocarbonyl)]]diethylenetriamine-N,N',N'-triacetic acid (H(3)L(y)) and the complexes LaL(x), NdL(x), SmL(x), EuL(x), GdL(x), DyL(x), TbL(x), ErL(x), and YbL(x) are reported. The luminescence properties of the LnL(x) complexes emitting in the visible (where Ln = Dy(3+), Tb(3+), Eu(3+), and Sm(3+)) are examined by steady-state and time-resolved photoluminescence, and the triplet state energy level of GdL(x) was estimated to be 24?100 cm(-1) from the 0-0 band of the 77 K phosphorescence spectrum. Near-infrared emission was detected for the NdL(x), YbL(x), and ErL(x) complexes, demonstrating the versatility of the thiophenol chromophore. The assembly of purely heterometallic EuTbL(x)(2) macrocycles by reaction of EuL(x) with TbL(y) was followed by UV-vis absorption spectroscopy, monitoring the characteristic absorption peak of pyridyl-2-thione at 353 nm. Analysis of the solution by mass spectrometry reveals the formation of purely heterometallic macrocycle EuTbL(x)(2). This is in contrast with the results obtained by dynamic self-assembly under oxidative conditions, where we observe a statistical mixture of macrocyclic complexes of Eu(2)L(x)(2), Tb(2)L(x)(2), and EuTbL(x)(2). The EuTbL(x)(2) macrocycle displays dual color emission, incorporating the characteristic f-f transitions of Eu(3+) and Tb(3+). Investigation into the time-resolved photophysical properties of EuTbL(x)(2) reveals energy transfer from Tb(3+) to Eu(3+), facilitated by the different conformations of the macrocycle in solution.     

Encapsulation of cyanometalates by a tris-macrocyclic ligand tricopper(II) complex: syntheses, structural variation, and magnetic exchange coupling pathways

The reaction of [M(CN)(6)](3-) (M = Cr(3+), Mn(3+), Fe(3+), Co(3+)) and [M(CN)(8)](4-/3-) (M = Mo(4+/5+), W(4+/5+)) with the trinuclear copper(II) complex of 1,3,5-triazine-2,4,6-triyltris[3-(1,3,5,8,12-pentaazacyclotetradecane)] ([Cu(3)(L)](6+)) leads to partially encapsulated cyanometalates. With hexacyanometalate(III) complexes, [Cu(3)(L)](6+) forms the isostructural host-guest complexes [[[Cu(3)(L)(OH(2))(2)][M(CN)(6)](2)][M(CN)(6)]][M(CN)(6)]30 H(2)O with one bridging, two partially encapsulated, and one isolated [M(CN)(6)](3-) unit. The octacyanometalates of Mo(4+/5+) and W(4+/5+) are encapsulated by two tris-macrocyclic host units. Due to the stability of the +IV oxidation state of Mo and W, only assemblies with [M(CN)(8)](4-) were obtained. The Mo(4+) and W(4+) complexes were crystallized in two different structural forms: [[Cu(3)(L)(OH(2))](2)[Mo(CN)(8)]](NO(3))(8)15 H(2)O with a structural motif that involves isolated spherical [[Cu(3)(L)(OH(2))](2)[M(CN)(8)]](8+) ions and a "string-of-pearls" type of structure [[[Cu(3)(L)](2)[M(CN)(8)]][M(CN)(8)]](NO(3))(4) 20 H(2)O, with [M(CN)(8)](4-) ions that bridge the encapsulated octacyanometalates in a two-dimensional network. The magnetic exchange coupling between the various paramagnetic centers is characterized by temperature-dependent magnetic susceptibility and field-dependent magnetization data. Exchange between the CuCu pairs in the [Cu(3)(L)](6+) "ligand" is weakly antiferromagnetic. Ferromagnetic interactions are observed in the cyanometalate assemblies with Cr(3+), exchange coupling of Mn(3+) and Fe(3+) is very small, and the octacoordinate Mo(4+) and W(4+) systems have a closed-shell ground state.     

Inosylyl(3'-->5')inosine (IpI-). Acid-base and metal ion-binding properties of a dinucleoside monophosphate in aqueous solution

The acidity constants of the (N7)H(+) sites of inosylyl(3'-->5')inosine (IpI(-)) were estimated and those of its (N1)H sites were measured by potentiometric pH titrations in aqueous solution (25 degrees C; I = 0.1 M, NaNO3). The same method was used for the determination of the stability constants of the 1:1 complexes formed between Mg(2+), Co(2+), Ni(2+), Zn(2+), or Cd(2+) (= M(2+)) and (IpI - H)(2-) and, in the case of Mg(2+), also of (IpI - 2H)(3-). The stability constants of the M(IpI)(+) complexes were estimated. The acidity constants of H(inosine)(+) and the stability constants of the M(Ino)(2+) and M(Ino - H)(+) complexes were taken from the literature. The comparison of these and related data allows the conclusion that, in the M(IpI - H) species, chelates are formed; most likely they are preferably of an N7/N7 type. For the metal ions Co(2+), Ni(2+), Zn(2+), or Cd(2+), the formation degrees of the chelates are on the order of 60-80%; no chelates could be detected for the Mg(IpI - H) complexes. It is noteworthy that the (N1)H deprotonation, which leads to the M(IpI - H) species, occurs in all M(IpI)(+) complexes in the physiological pH range of about 7.5 or even below.     

Structural and Quantum Chemical Study of Bi(5)(3+) and Isoelectronic Main-Group Metal Clusters. The Crystal Structure of Pentabismuth(3+) Tetrachlorogallate(III) Refined from X-ray Powder Diffraction Data and Synthetic Attempts on Its Antimony Analogue

Pentabismuth(3+) tetrachlorogallate(III), (Bi(5)(3+))(GaCl(4)(-))(3), has been synthesized by reducing a BiCl(3)-GaCl(3) melt with bismuth metal and the crystal structure refined from X-ray (Cu Kalpha(1)) powder diffraction data. The structure was found to belong to space group R-3c, with the lattice parameters a = 11.871(2) ? and c = 30.101(3) ? (Z = 6). It is isostructural with the previously characterized Bi(5)(AlCl(4))(3). An attempt to synthesise the antimony analogue Sb(5)(GaCl(4))(3) by reducing a SbCl(3)-GaCl(3) mixture with gallium metal produced a black solid phase. The gallium content of this phase is consistent with the stoichiometry Sb(5)(GaCl(4))(3), and the Raman spectrum of the phase dissolved in SbCl(3)-GaCl(3) comprises strong, low-frequency bands attributable to Sb-Sb stretch vibrations in Sb(5)(3+) or another reduced antimony species. Quantum chemical analyses have been performed for the isoelectronic, trigonal pyramidal closo-clusters Sn(5)(2-), Sb(5)(3+), Tl(5)(7-), Pb(5)(2-), and Bi(5)(3+), both with extended Hückel (eH) and Hartree-Fock (HF) methods. The HF calculations were performed with and without corrections for the local electron-electron correlation using second-order M?ller-Plesset perturbation theory (MP2). All theoretical results are compared and evaluted with respect to experimental cluster structures and vibrational frequencies. The results from the calculations agree well with available experimental data for the solid-state structures and vibrational spectra of these cluster ions, except for the Tl(5)(7-) ion. Isolated Tl(5)(7-) is suggested to be electronically unstable because of the high charge density. The Sb(5)(3+) cluster ion is indicated to be stable. According to the calculations, Sn(5)(2-) and Pb(5)(2-) may be described in terms of edge-localized bonds without substantial electron density between the equatorial atoms, whereas Sb(5)(3+) and Bi(5)(3+) have electron density evenly distributed over all M-M vectors. Furthermore, the theoretical results give no support for a D(3h) --> C(4v) fluxionality of these clusters.     

Giant metal-cyanide coordination clusters: tetracapped edge-bridged cubic Cr(12)Ni(12)(CN)(48) and double face-centered cubic Cr(14)Ni(13)(CN)(48) species

The crystal structures of two new metal-cyanide clusters with record high nuclearities are reported. A direct assembly reaction involving [(Me(3)tacn)Cr(CN)(3)] (Me(3)tacn = N,N',N"-trimethyl-1,4,7-triazacyclononane), NiI(2), and KCN in aqueous solution affords [(Me(3)tacn)(12)Cr(12)Ni(12)(CN)(48)](12+). The structure of this 24-metal cluster features a cube of eight Cr(III) centers linked along the edges by 12 trans-coordinated [Ni(CN)(4)](2)(-) units, and capped on four faces by [(Me(3)tacn)Cr](3+) moieties. Its metal-cyanide cage encloses a 900 A(3) cavity that is accessible through the two noncapped cube faces. A still larger cluster, [(Me(3)tacn)(14)Cr(14)Ni(13)(CN)(48)](20+), was obtained from a related reaction excluding the addition of KCN. This 27-metal species possesses a highly anisotropic geometry in which two face-centered cubic units are fused through a common Ni(II) vertex.     

How do nitriles compare with isoelectronic alkynyl groups in the electronic communication between iron centers bridged by phenylenebis- and -tris(nitrile) ligands? An electronic and crystal-structure study

Density functional theory (DFT) calculations on the model [{FeCp(dpe)}(2){1,4-C(6)H(4)(CN)(2)}](2+) (3(2+); dpe = diphosphinoethane) of salts of the cations [{FeCp(dppe)}(2){1,4-C(6)H(4)(CN)(2)}](2+) (1(2+); dppe = 1,2-bis[diphenyldiphosphino]ethane) and [{FeCp*(CO)(2)}(2){1,4-C(6)H(4)(CN)(2)}](2+) (2(2+)), for which the X-ray crystal structures have been determined, as well as on its isomer [{FeCp(dpe)}(2){1,3-C(6)H(4)(CN)(2)}](2+) (4(2+)) and on the related complex [{FeCp(dpe)}(3){1,3,5-C(6)H(3)(CN)(3)}](3+) (5(2+)), indicate that the highest occupied molecular orbitals (HOMOs) of these compounds are localized on the metal centers with negligible participation of the C(6) ring. Thus, the poly(nitrile)phenylene ligand efficiently quenches the electronic communication between the metal centers. This is at variance with the related isoelectronic polyacetylene phenylene complexes, in which the iron centers have been shown to be electronically coupled. Consistently, apart from the case of 3(3+), which shows some degree of delocalization, all of the oxidized forms of 3(2+), 4(2+), and 5(2+) can be described as class II, localized mixed-valent species, in agreement with the electrochemical data showing two close oxidation potentials around 1 V vs FeCp*(2). This is at variance with the p-phenylene-bridged biethynyldiiron analogue, for which extended electronic delocalization was earlier shown to provide greater degree of delocalization of the mixed valency. Time-dependent DFT calculations on 3(2+), 4(2+), and 5(2+) indicate that the lowest-energy absorption band is associated with metal-to-ligand charge-transfer transitions involving the metallic HOMOs and the two lowest unoccupied molecular orbitals that derive from the lowest π*(phenylene) orbitals with some π*(CN) bonding admixture.     

Formation and dissociation kinetics of triaza-crown-alkanoic acid complexes of transition metal(II) and lanthanide (III)

The formation and dissociation rates of some transition metal(II) and lanthanide(III) complexes of the 1,7,13-triaza-4,10,16-trioxacyclooctadecane N',N',N'-triacetic acid (1) and 1,7,13-triaza-4,10,16-trioxacyclooctadecane-N',N',N'- trimethylacetic acid (2) have been measured by the use of stopped-flow and conventional spectrophotometry. Experimental observations were made at 25.0 +/- 0.1 degrees C and at an ionic strength of 0.10 M KCl. The complexation of Zn(2+) and Cu(2+) ions with 1 and 2 proceeds through the formation of an intermediate complex (MH(3)L(+) *) in which the metal ion is incompletely coordinated. This may then lead to a final product in the rate-determining step. Between pH 4.68 and 5.55, the diprotonated (H(2)L(-)) form is revealed to be a kinetically active species despite its low concentration. The stability constants (log K (MH (3)L (+) *)) and specific base-catalyzed rate constants (k(OH)) of intermediate complexes have been determined from the kinetic data. The dissociation reactions of 1 and 2 complexes of Co(2+), Ni(2+), Zn(2+), Ce(3+), Eu(3+) and Yb(3+) were investigated with Cu(2+) ions as a scavenger in acetate buffer. All complexes exhibit acid-independent and acid-catalyzed contributions. The buffer and Cu(2+) concentration dependence on the dissociation rate has also been investigated. The metal and ligand effects on the dissociation rate of some transition metal(II) and lanthanide(III) complexes are discussed in terms of the ionic radius of the metal ions, the side-pendant arms and the rigidity of the ligands.