Synthesis,crystal structure,and magnetism of a binuclear Cu(II) complex with double azido as asymmetric basal–apical end-on bridged ligand

A new binuclear copper(II) complex, [Cu₂(μ_1,1-N₃)₂(PP)₂)] ? 2ClO₄ (PP = 2,6-dipyrazol-1-yl-pyridine), was synthesized with double azide as asymmetric end-on bridge ligand and 2,6-dipyrazol-1-yl-pyridine as the terminal ligand. The crystal structure was determined by X-ray crystallography. Cu(II) is located in a distorted square pyramidal geometry, and azide bridges the equatorial-axial linking two Cu(II) atoms with a separation of 3.3595(11) Å. The fitting for the data of the variable-temperature (2–300 K) magnetic susceptibilities by using the Curie–Weiss law gives the Weiss temperature θ = ?7.830 K, indicating a very weak anti-ferromagnetic interaction between the bridging Cu(II) complexes.     

Syntheses, crystal structures, and magnetic properties of novel manganese(II) complexes with flexible tripodal ligand 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene

Two novel metal-organic frameworks (MOFs)--[Mn(titmb)(N3)2] x 1.5H2O (1) and [Mn3(titmb)2(C2O4)3(H2O)] x 10H2O (2)--were obtained by reactions of the flexible tripodal ligand 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6-trimethylbenzene (titmb) with Mn(OAc)2 x 4H2O, together with NaN3 and K2C2O4, respectively. The structures of these MOFs were established by single-crystal X-ray diffraction analysis. The crystal data for 1 were as follows: monoclinic, C2/c, a = 20.956(13) A, b = 9.884(6) A, c = 24.318(14) A, beta = 95.87(5) degrees, Z = 8. The crystal data for 2 were as follows: triclinic, P1, a = 12.400(9) A, b = 16.827(12) A, c = 17.196(11) A, alpha = 66.35(5), beta = 95.87(5) degrees, gamma = 71.03(6), Z = 2. Complex 1 is a novel noninterpenetrating three-dimensional (3D) framework, in which the azide ligand connects Mn(II) atoms in an end-to-end (EE) mode to give [Mn-N-N-N-]n infinite one-dimensional (1D) chains, and complex 2 has a two-dimensional (2D) network structure in which the Mn(II) ions are linked by the oxalate anions to form 1D [Mn(C2O4)]n chains. Each titmb in these two complexes connects three metal atoms and serves as a three-connecting ligand. The magnetic properties of 1 and 2 were investigated. The results showed that the antiferromagnetic interactions occurred between the Mn(II) ions linked by the azide ligands in complex 1, and those linked by the oxalate anions and the carboxylate in syn-anti coordination mode in complex 2. The entirely different structures of complexes 1 and 2, on one hand, indicate that the azide and the oxalate ligands affected the structures of MOFs greatly, and on the other hand, reveals the potential applications of MOFs with the azide and oxalate ligands, which are efficient magnetic couplers.     

ParaCEST MRI contrast agents capable of derivatization via"click" chemistry

A comprehensive series of lanthanide chelates has been prepared with a tetrapropargyl DOTAM type ligand. The complexes have been characterized by a combination of (1)H NMR, single-crystal X-ray crystallography, CEST and relaxation studies and have also been evaluated for potential use as paramagnetic chemical exchange saturation transfer (ParaCEST) contrast agents in magnetic resonance imaging (MRI). We demonstrate the functionalization of several chelates by means of alkyne-azide "click" chemistry in which a glucosyl azide is used to produce a tetra-substituted carbohydrate-decorated lanthanide complex. The carbohydrate periphery of the chelates has a potent influence on the CEST properties as described herein.     

Second-sphere contributions to substrate-analogue binding in iron(III) superoxide dismutase

A combination of spectroscopic and computational methods has been employed to explore the nature of the yellow and pink low-temperature azide adducts of iron(III) superoxide dismutase (N(3)-FeSOD), which have been known for more than two decades. Variable-temperature variable-field magnetic circular dichroism (MCD) data suggest that both species possess similar ferric centers with a single azide ligand bound, contradicting previous proposals invoking two azide ligands in the pink form. Complementary data obtained on the azide complex of the Q69E FeSOD mutant reveal that relatively minor perturbations in the metal-center environment are sufficient to produce significant spectral changes; the Q69E N(3)-FeSOD species is red in color at all temperatures. Resonance Raman (RR) spectra of the wild-type and Q69E mutant N(3)-FeSOD complexes are consistent with similar Fe-N(3) units in all three species; however, variations in energies and relative intensities of the RR features associated with this unit reveal subtle differences in (N(3)(-))-Fe(3+) bonding. To understand these differences on a quantitative level, density functional theory and semiempirical INDO/S-CI calculations have been performed on N(3)-FeSOD models. These computations support our model that a single azide ligand is present in all three N(3)-FeSOD adducts and suggest that their different appearances reflect differences in the Fe-N-N bond angle. A 10 degrees increase in the Fe-N-N bond angle is sufficient to account for the spectral differences between the yellow and pink wild-type N(3)-FeSOD species. We show that this bond angle is strongly affected by the second coordination sphere, which therefore might also play an important role in orienting incoming substrate for reaction with the FeSOD active site.     

Synthesis,crystal structure,and magnetism of a binuclear Cu(II) complex with a single end-to-end bridged azido ligand

A binuclear copper(II) complex, [Cu₂(μ _1,3-N₃)(N₃)(pmp)₂(ClO₄)]ClO₄ (pmp = 2-((pyridin-2-yl) methoxy)-1,10-phenanthroline), was synthesized with a single azide as end-to-end bridge ligand, and pmp and perchlorate as ligands. In the crystal, Cu(II) is in a distorted square pyramidal geometry, and a single azide bridges equatorial-axial linking two Cu(II) ions with separation of 5.851 Å. There are π?π stacking interactions involving 1,10-phenanthroline rings. The variable-temperature (2–300 K) magnetic susceptibilities were analyzed using a binuclear Cu(II) magnetic formula and it indicates that there is a very weak ferromagnetic coupling with 2J = 2.82 cm^?1.     

Kinetics of 1,3-dipolar cycloaddition on the surfaces of Au nanoparticles

Triazole formation via 1,3-dipolar cycloaddition, or "click" chemistry, is a powerful synthetic method for incorporating chemical functionality onto the surfaces of Au nanoparticles. To investigate the factors that govern azide/alkyne reactivity at particle surfaces, we measured the general kinetic trends for the uncatalyzed reaction using FTIR spectroscopy. This study examines the roles of ligand length, electronic substitution of the alkyne species, and solvent on the reaction under pseudo-first-order conditions. The conversion of azide to triazole is found to depend more strongly on the relative surface coverage of azide terminated alkanethiol than on the ligand length and solvent.     

Mechanism of Redox‐Active Ligand‐Assisted Nitrene‐Group Transfer in a ZrIV Complex: Direct Ligand‐to‐Ligand Charge Transfer Preferred

The mechanism of the nitrene‐group transfer reaction from an organic azide to isonitrile catalyzed by a Zr^IV d⁰ complex carrying a redox‐active ligand was studied by using quantum chemical molecular‐modeling methods. The key step of the reaction involves the two‐electron reduction of the azide moiety to release dinitrogen and provide the nitrene fragment, which is subsequently transferred to the isonitrile substrate. The reducing equivalents are supplied by the redox‐active bis(2‐iso‐propylamido‐4‐methoxyphenyl)‐amide ligand. The main focus of this work is on the mechanism of this redox reaction, in particular, two plausible mechanistic scenarios are considered: 1) the metal center may actively participate in the electron‐transfer process by first recruiting the electrons from the redox‐active ligand and becoming formally reduced in the process, followed by a classical metal‐based reduction of the azide reactant. 2) Alternatively, a non‐classical, direct ligand‐to‐ligand charge‐transfer process can be envisioned, in which no appreciable amount of electron density is accumulated at the metal center during the course of the reaction. Our calculations indicate that the non‐classical ligand‐to‐ligand charge‐transfer mechanism is much more favorable energetically. Utilizing a series of carefully constructed putative intermediates, both mechanistic scenarios were compared and contrasted to rationalize the preference for ligand‐to‐ligand charge‐transfer mechanism.     

Rates of ligand substitution by bromide,thiocyanate, and azide lons at iron (III) ion in dimethyl sulfoxide as solvent

Rate constants for ligand substitution by bromide, thiocyanate and azide ions at iron(III) ion in dimethyl sulfoxide as solvent, determined by stopped-flow spectrophotometry, are similar and are consistent with a dissociative mechanism. In addition, azide ion gives a second much slower, reaction, attributed to formation of a binuclear complex. Results of similar measurements with thiocyanate ion in acetonitrile were more complicated, attributed to a marked influence of residual water on the reactivity of iron (III) ion.     

Manganese(II)-carboxylate-pseudohalide systems derived from 1,4-bis(4-carboxylatopyridinium-1-methylene)benzene: structures and magnetism

The reactions of manganese(II) acetate or perchlorate, sodium azide or sodium cyanate, and the zwitterionic dicarboxylate ligand 1,4-bis(4-carboxylatopyridinium-1-methylene)benzene (L) under different conditions yielded three different Mn(II) coordination polymers with mixed carboxylate and azide (or cyanate) bridges: {[Mn (L(1))(0.5)(N(3))(OAc)]·3H(2)O}(n) (1), {[Mn(4)(L(1))(N(3))(8)(H(2)O)(4)(CH(3)OH)(2)]·[L(1)]}(n) (2), and {[Mn(3)(L(1))(NCO)(6)(H(2)O)(4)]·[L(1)]·[H(2)O](2)}(n) (3). The compounds exhibit diverse structures and magnetic properties. In 1, the 1D uniform anionic [Mn(N(3))(COO)(2)](n) chains with the (μ-EO-N(3))(μ-COO)(2) triple bridges (EO = end-on) are interlinked by the dipyridinium L ligands into highly undulated 2D layers. Magnetic studies on 1 reveal that the mixed triple bridges induce antiferromagnetic coupling between Mn(II) ions. Compounds 2 and 3 consist of 1D neutral polymeric chains and co-crystallized zwitterions, and the chains are formed by the L ligands interlinking linear polynuclear units. The polynuclear unit in 2 is tetranuclear with (μ-EO-N(3))(2) as central bridges and (μ-EO-N(3))(2)(μ-COO) as peripheral bridges, while that in 3 is trinuclear with (μ-NCO)(2)(μ-COO) bridges. Magnetic studies demonstrate that the magnetic coupling through the mixed azide/isocyanate and carboxylate bridges in 2 and 3 is antiferromagnetic. An expression of magnetic susceptibility based on a 2-J model for linear tetranuclear systems of classical spins has been deduced and applied to 2.     

Copper(II) azide complexes of aliphatic and aromatic amine based tridentate ligands: novel structure, spectroscopy, and magnetic properties

Copper(II) azide complexes of three tridentate ligands namely 2,6-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L), 2,6-(pyrazol-1-ylmethyl)pyridine (L'), and dipropylenetriamine (dpt) yield three kinds of complexes with different azide-binding modes. The ligand L forms two end-on-end (mu-1,3) diazido-bridged binuclear complexes, [CuL(mu-N(3))](2)(ClO(4))(2) (1) and [CuL(mu-N(3))(ClO(4))](2).2CH(3)CN (2), and L' forms a perchlorato-bridged quasi-one-dimensional chain complex, [CuL'(N(3))(ClO(4))](n)() (3) with monodentate azide coordination. The ligation of dipropylenetriamine (dpt) gives a end-on (mu-1,1) diazido-bridged binuclear copper complex [Cu(dpt)(mu-N(3))](2)(ClO(4))(2) (4). The crystal and molecular structures of these complexes have been solved. Variable-temperature EPR results of 1 and 2 are identical and indicate the presence of both ferromagnetic and antiferromagnetic interactions within the dimer, the former dominating at low temperatures and the latter at high temperatures. The unusual temperature-dependent magnetic moment and EPR spectra of this dimer reveal the presence of temperature-dependent population of two triplet states, one being caused by antiferromagnetic and the other by ferromagnetic interaction, the former transforming to the latter on cooling. While the interaction of ground spin doublets of the two metal centers gives rise to a ferromagnetic coupling of J(g) = 90.73 cm(-1), the other coupling of J(e) = -185.64 cm(-1) is suggested to be caused by the interaction between an electron in one metal center and an electron from the azide of the other monomer by excitation of a d-electron to the empty ligand orbital. The ferromagnetic state is energetically favored by 104.39 cm(-1). Compound 3 exhibits axial spectra at room temperature and 77 K, and variable-temperature magnetic susceptibility data indicate that the copper centers form a weakly antiferromagnetic one-dimensional chain with J = -0.11 cm(-1). In the case of 4, the unique presence of two nonidentical dimeric units with different bond lengths and bond angles within the unit cell as inferred by crystal structure is proved by single-crystal EPR spectroscopy.     

Synthesis,Crystal Structure,and Magnetic Properties of a New Coordination Polymer Framework [CoII(4,4′‐bpy)(N3)2]n

A new cobalt(II) coordination polymer containing 4,4′‐bipyridine and azide as bridging ligand, [Co^II(4,4′‐bpy)(N₃)₂]_n ( 1 ) was synthesized under mild hydrothermal conditions and was characterized by single‐crystal X‐ray diffraction studies and magnetic susceptibility measurements. It exhibits an acentric structure, in which cobalt(II) ions are linked through end‐to‐end (EE) azido groups. The 4,4′‐bpy ligands are coordinated on the axial positions of the octahedral environment reinforcing the intermetallic connections and resulting in a network. Circular dichroism spectra of the compound exhibit a maximum negative Cotton effect at 260 nm, which indicates the chiral nature of 1 . Variable temperature magnetic susceptibility measurements in the temperature range 2–300 K reveal the existence of antiferromagnetic couplings in the framework.     

叠氮阴离子μ-1,1桥联双核Co(Ⅱ)配合物的合成及其晶体结构和磁性

用水热法合成了以邻菲啰啉(phen)为辅助配体叠氮桥联Co2+配位聚合物:[Co2(μ1,1-N3)2(phen)2(N3)2]。用IR和元素分析进行了表征,用单晶XRD测定晶体结构,属于三斜晶系,P1空间群,a=0.69272(14)nm,b=1.0318(2)nm,c=1.0381(2)nm,α=6.447(3)°,β=71.02(3)°,γ=85.79(3)°,Z=1,V=0.6312(2)nm3,D=1.701 mg/mm3,F(000)=326。确定N3-为μ-1,1桥联配位。测定了配合物固体的变温磁化率,证明配合物为亚铁磁性物质,其临界温度为15 K。     

X-Ray crystal structures of five-coordinate (salen)MnN3 derivatives and their binding abilities towards epoxides: chemistry relevant to the epoxide-CO2 copolymerization process

The synthesis of several (salen)MnN(3) complexes in good yields and purities were achieved by the reaction of manganese(iii) acetate and H(2)salen, followed by metathesis of the remaining acetate ligand with an aqueous solution of NaN(3). The X-ray structures of two derivatives, where salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediamine and N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine respectively, were determined. The complexes were shown to be monomeric 5-coordinate derivatives displaying a distorted square pyramidal geometry, and to be d(4) high-spin derivatives by solution magnetic moment measurements using the Evans method. Binding studies of the (salen)MnN(3) derivatives with added azide ions or cyclohexene oxide showed these complexes to have modest affinities for binding a sixth ligand. These observations are used to rationalize the low activity exhibited by manganese(iii) complexes relative to their chromium(iii) and cobalt(iii) analogs for serving as catalysts for the copolymerization of carbon dioxide and epoxides.     

Anion influence on the structure and magnetic properties of a series of multidimensional pyrimidine-2-carboxylato-bridged copper(II) complexes

Seven new polynuclear copper(II) complexes of formula [Cu(mu-pymca)2] (1) (pymca(-) = pyrimidine-2-carboxylato), [Cu(mu-pymca)Br] (2), [Cu(mu-pymca)Cl] (3), [Cu(mu-pymca)(SCN)(H2O)] x 4 H2O (4), [Cu(mu-pymca)N3] (5), [Cu2(mu1,5-dca)2(pymca)2] (6) (dca = dicyanamide), and K{[mu-Au(CN)2]2[(Cu(NH3)2)2(mu-pymca)]}[Au(CN)2]2 (7) have been synthesized by reactions of K-pymca with copper(II) ions in the presence of different counteranions. Compound 1 is a linear neutral chain with a carboxylato bridging ligand in a syn-anti coordination mode, whereas complexes 2 and 3 consist of cationic linear chains with cis and trans bis(chelating) pymca bridging ligands. Complex 4 adopts a helical pymca-bridged chain structure. In complex 5, zigzag pymca-bridged chains are connected by double end-on azide bridging ligands to afford a unique honeycomb layer structure. Complex 6 is a centrosymmetric dinuclear system with double mu 1,5-dicyanamide bridging ligands and pymca end-cap ligands. Complex 7 is made of pymca-bridged dinuclear [Cu(NH3)2(mu-pymca)Cu(NH3)2](3+) units connected by [Au(CN)2](-) anions to four other dinuclear units, giving rise to cationic (4,4) rectangular nets, which are linked by aurophilic interactions to afford a singular 3D network. Variable-temperature magnetic susceptibility measurements show that complex 1 exhibits a very weak antiferromagnetic coupling through the syn-anti (equatorial-axial) carboxylate bridge (J = -0.57 cm(-1)), whereas complexes 2-4 and 7 exhibit weak to strong antiferromagnetic couplings through the bis(chelating) pymca bridging ligand J = -17.5-276.1 cm(-1)). Quantum Monte Carlo methods have been used to analyze the experimental magnetic data for 5, leading to an antiferromagnetic coupling (J = -34 cm(-1)) through the pymca ligand and to a ferromagnetic coupling (J = 71 cm(-1)) through the azide bridging ligands. Complex 6 exhibits a very weak antiferromagnetic coupling through the dicyanamide bridging ligands (J = -5.1 cm(-1)). The magnitudes of the magnetic couplings in complexes 2-5 have been explained on the basis of the overlapping between magnetic orbitals and DFT theoretical calculations.     

Tricomponent azide, tetrazolate, and carboxylate cobridging magnetic systems: ferromagnetic coupling, metamagnetism, and single-chain magnetism

Three novel coordination polymers with azide and a bifunctional zwitterionic ligand bearing carboxylate and tetrazolate as bridging groups, [M(L)(N(3))]·xH(2)O [L=1-(carboxylatomethyl)-4-(5-tetrazolato)pyridinium, M=Cu (1, x=2), Ni (2, x=1), and Co (3, x=1)], have been synthesized and characterized by X-ray crystallography and magnetic measurements. The compounds consist of two-dimensional coordination layers in which uniform anionic chains with the unprecedented tricomponent (μ-azide)(μ-tetrazolate)(μ-carboxylate) bridges are cross-linked by cationic 1-methylenepyridinium spacers. The tricomponent bridges induce ferromagnetic interactions in all the compounds. Furthermore, this isostructural series of ferromagnetic-chain-based compounds has allowed us to observe distinct bulk properties that are dependent upon the natures of the different spin carriers: with the isotropic Cu(II) ion, 1 exhibits a paramagnetic phase of the ferromagnetic chains without long-range magnetic order above 2 K; with the weakly anisotropic Ni(II) ions, 2 displays antiferromagnetic ordering and field-induced metamagnetism without slow dynamic relaxation; and with Co(II), which has strong magnetic anisotropy due to first-order spin-orbital coupling, 3 exhibits magnetic hysteresis and slow magnetization dynamics typical of single-chain magnets.     

catena‐Poly[[{4‐bromo‐2‐[2‐(dimethyl­amino)ethyl­imino­methyl]­phenol­ato}­copper(II)]‐μ‐azido]

The title complex, [Cu(C₁₁H₁₄BrN₂O)(N₃)]_n, is an inter­esting azide‐bridged polynuclear copper(II) compound. The Cu^II atom is five‐coordinated in a square‐pyramidal configuration, with one O and two N atoms of one Schiff base and one terminal N atom of a bridging azide ligand defining the basal plane, and another terminal N atom of another bridging azide ligand occupying the axial position. The {4‐bromo‐2‐[2‐(dimethyl­amino)ethyl­imino­meth­yl]phenolato}copper(II) moieties are linked by the bridging azide ligands, forming polymeric chains running along the b axis. Adjacent chains are further linked by weak Br⋯Br inter­actions into a sheet.     

Enhancing reactivity via structural distortion

To examine how small structural changes influence the reactivity and magnetic properties of biologically relevant metal complexes, the reactivity and magnetic properties of two structurally related five-coordinate Fe(III) thiolate compounds are compared. (Et,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))]PF(6) (3) is synthesized via the abstraction of a sulfur from alkyl persulfide ligated [Fe(III)(S(2)(Me2)N(3)(Et,Pr))-S(pers)]PF(6) (2) using PEt(3). (Et,Pr)-3 is structurally related to (Pr,Pr)-ligated [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))]PF(6) (1), a nitrile hydratase model compound previously reported by our group, except it contains one fewer methylene unit in its ligand backbone. Removal of this methylene distorts the geometry, opens a S-Fe-N angle by approximately 10 degrees, alters the magnetic properties by stabilizing the S = 1/2 state relative to the S = 3/2 state, and increases reactivity. Reactivity differences between 3 and 1 were assessed by comparing the thermodynamics and kinetics of azide binding. Azide binds reversibly to both (Et,Pr)-3 and (Pr,Pr)-1 in MeOH solutions. The ambient temperature K(eq) describing the equilibrium between five-coordinate 1 or 3 and azide-bound 1-N(3) or 3-N(3) in MeOH is approximately 10 times larger for the (Et,Pr) system. In CH(2)Cl(2), azide binds approximately 3 times faster to 3 relative to 1, and in MeOH, azide dissociates 1 order of magnitude slower from 3-N(3) relative to 1-N(3). The increased on rates are most likely a consequence of the decreased structural rearrangement required to convert 3 to an approximately octahedral structure, or they reflect differences in the LUMO (vs SOMO) orbital population (i.e., spin-state differences). Dissociation rates from both 3-N(3) and 1-N(3) are much faster than one would expect for low-spin Fe(III). Most likely this is due to the labilizing effect of the thiolate sulfur that is trans to azide in these structures.     

Synthesis and Crystal Structure of a Three-dimensional Mn(Ⅱ) Coordination Polymer with 3-(Pyrazin-2-yloxy)-pyridine and Azide Anion as Mixed Bridge Ligand

A three-dimensional coordination polymer [Mn2(μ1,3-N3)4(μ-PP)2]n(PP = 3-(pyra-zin-2-yloxy)-pyridine) has been synthesized with 3-(pyrazin-2-yloxy)-pyridine and azide anion as mixed bridge ligand,and its crystal structure was determined by X-ray crystallography.The crystal data:triclinic system,space group P,with a = 6.794(4),b = 9.885(6),c = 9.947(6) ,α = 64.170(6),β = 84.190(8),γ = 85.319(8)°,V = 597.7(6) 3,Z = 1,C18H14Mn2N18O2,Mr = 624.35,Dc = 1.735 g/cm3,F(000) = 314 and μ = 1.117 mm-1.In the crystal,the azide anion acts as a bridge ligand and makes adjacent Mn(II) ions connect into a two-dimensional sheet on the ab plane,then 3-(pyrazin-2-yloxy)-pyridine serves as a bidentate bridge ligand to connect neighboring sheets along the c axis,and finally a three-dimensional structure is formed.     

Why is there an "inert" metal center in the active site of nitrile hydratase? Reactivity and ligand dissociation from a five-coordinate Co(III) nitrile hydratase model.

To determine how a substitutionally inert metal can play a catalytic role in the metalloenzyme nitrile hydratase (NHase), a reactive five-coordinate Co(III) thiolate complex ([Co(III)(S(2)(Me2)N(3)(Pr,Pr))](PF(6)) (1)) that resembles the active site of cobalt containing nitrile hydratase (Co NHase) was prepared. This was screened for reactivity, by using low-temperature electronic absorption spectroscopy, toward a number of biologically relevant "substrates". It was determined 1 will react with azide, thiocyanate, and ammonia, but is unreactive toward nitriles, NO, and butyrate. Substrate-bound 1 has similar spectroscopic and structural properties as [Co(III)(ADIT(2))](PF(6)) (2). Complex 2 is a six-coordinate Co(III) complex containing cis-thiolates and imine nitrogens, and has properties similar to the cobalt center of Co NHase. Substrate binding to 1 is reversible and temperature-dependent, allowing for the determination of the thermodynamic parameters of azide and thiocyanate binding and the rates of ligand dissociation. Azide and thiocyanate bind trans to a thiolate, and with similar entropies and enthalpies (thiocyanate: DeltaH = -7.5 +/- 1.1 kcal/mol, DeltaS = -17.2 +/- 3.2 eu; azide: DeltaH = -6.5 +/- 1.0 kcal/mol, DeltaS = -12.6 +/- 2.4 eu). The rates of azide and thiocyanate displacement from the metal center are also comparable to one another (k(d) = (7.22 +/- 0.04) x 10(-)(1) s(-)(1) for thiocyanate and k(d) = (2.14 +/- 0.50) x 10(-)(2) s(-)(1) for azide), and are considerably faster than one would expect for a low-spin d(6) six-coordinate Co(III) complex. These rates are comparable to those of an analogous Fe(III) complex, demonstrating that Co(III) and Fe(III) react at comparable rates when in this ligand environment. This study therefore indicates that ligand displacement from a low-spin Co(III) center in a ligand environment that resembles NHase is not prohibitively slow so as to disallow catalytic action in nonredox active cobalt metalloenzymes.     

Synthesis,crystal structures and spectral properties of cobalt (Ⅱ) complexes:[ CoL (N_3) ] ·ClO_4 and CoL (N_3)_2 [L=tris ((3,5-dimethylpyrazol-1-yl) methyl) amine

Two monomeric cobalt(Ⅱ)complexes,[CoL(N3)] ClO4(1)and CoL(N3)2(2),where L is tris((3,5-dimethylpyrazol-1-yl)methyl)amine,were synthesized and their crystal structures were determined by X-ray diffraction technique.Complex 1 is five coordinated with one azide nitrogen atom and four nitrogen atoms of the tris((3,5-dimethylpyrazol-l-yl)-methyl)amine ligand,and the metal center is in distorted trigonal bipyramidal environment.Complex 2 is six coordinated distorted octahedron with the two azide nitrogen atoms and four nitrogen donors of the tris((3,5-dimethylpyrazol-1-yl)-methyl)amine ligand.The solution behaviors of the title complexes have been further investigated by UV-Vis,and 1H NMR analysis.It is found that the formation of 1 and 2 depends on the molar ratio of the azide ion to metal salt and ligand Complex 1 attached with one azide group is more stable and easy to generate than complex 2 incorporated with two azide groups,and the reasons were well discussed.