Pentacoordinate nickel(II) complexes double bridged by phosphate ester or phosphinate ligands: spectroscopic, structural, kinetic, and magnetic studies

The bis(phosphatediester)-bridged complexes [[Ni([12]aneN(3))(mu-O(2)P(OR)(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene; R=Me (1), Bu (2), Ph (3), Ph-4-NO(2) (4); [12]aneN(3)=Me(4)[12]aneN(3), 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene; R=Me (5), Bu (6), Ph (7), Ph-4-NO(2) (8)] were prepared by hydrolysis of the phosphate triester with the hydroxo complex [[Ni([12]aneN(3))(mu-OH)](2)](PF(6))(2) or by acid-base reaction of the dialkyl or diaryl phosphoric acid and the above hydroxo complex. The acid-base reaction was also used to synthesise the phosphinate-bridged complexes [[Ni([12]aneN(3))(mu-O(2)PR(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), R=Me (9), Ph (10); [12]aneN(3)=Me(4)[12]aneN(3), R=Me (11), Ph (12)]. The molecular structures of complexes 2, 3 and 12 were established by single crystal X-ray diffraction studies. The eight-membered rings defined by the nickel atoms and the bridging ligands show distorted twist-boat, chair and boat-boat conformations in 2, 3 and 12, respectively. The experimental susceptibility data for compounds 2, 3 and 12 were fitted by least-squares methods to the analytical expression given by Ginsberg. The best fit was obtained with values of J=-0.11 cm(-1), D=-9.5 cm(-1) and g=2.20 for 2; J=-0.97 cm(-1), D=-9.3 cm(-1) and g=2.21 for 3; and J=-0.14 cm(-1), D=-11.9 cm(-1) and g=2.195 for 12. The magnetic-exchange pathways must involve the phosphate/phosphinate bridges, because these favour antiferromagnetic interactions. The observation of a higher exchange parameter for compound 3 is a consequence of a favourable disposition of the O-P-O bridges. The kinetics for the hydrolysis of TNP (tris(4-nitrophenyl)phosphate) with the dinuclear nickel(II) hydroxo complex [[Ni(Me(3)[12]aneN(3))(mu-OH)](2)](PF(6))(2) was studied by UV-visible spectroscopy. The proposed mechanism for TNP-promoted hydrolysis can be described as one-substrate/two-product, and can be fitted to a Michaelis-Menten equation.     

Reactivity of mu-hydroxodizinc(II) centers in enzymatic catalysis through model studies

The stable dinuclear complex [Zn2(BPAM)(mu-OH)(mu-O2PPh2)](ClO4)2, where BPAN = 2,7-bis[2-(2-pyridylethyl)-aminomethyl]-1,8-naphthyridine, was chosen as a model to investigate the reactivity of (mu-hydroxo)dizinc(II) centers in metallohydrolases. Two reactions, the hydrolysis of phosphodiesters and the hydrolysis of beta-lactams, were studied. These two processes are catalyzed in vivo by zinc(II)-containing enzymes: P1 nucleases and beta-lactamases, respectively. The former catalyzes the hydrolysis of single-stranded DNA and RNA. beta-Lactamases, expressed in many types of pathogenic bacteria, are responsible for the hydrolytic degradation of beta-lactam antibiotic drugs. In the first step of phosphodiester hydrolysis promoted by the dinuclear model complex, the substrate replaces the bridging diphenylphosphinate. The bridging hydroxide serves as a general base to deprotonate water, which acts as a nucleophile in the ensuing hydrolysis. The dinuclear model complex is only 1.8 times more reactive in hydrolyzing phosphodiesters than a mononuclear analogue, Zn(bpta)(OTf)2, where bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine. Hydrolysis of nitrocefin, a beta-lactam antibiotic analogue, catalyzed by [Zn2(BPAN)(mu-OH)(mu-O2PPh2)](ClO4)2 involves monodentate coordination of the substrate via its carboxylate group, followed by nucleophilic attack of the zinc(II)-bound terminal hydroxide at the beta-lactam carbonyl carbon atom. Collapse of the tetrahedral intermediate results in product formation. Mononuclear complexes Zn(cyclen)-(NO3)2 and Zn(bpta)(NO3)2, where cyclen = 1,4,7,10-tetraazacyclododecane, are as reactive in the beta-lactam hydrolysis as the dinuclear complex. Kinetic and mechanistic studies of the phosphodiester and beta-lactam hydrolyses indicate that the bridging hydroxide in [Zn2(BPAN)(mu-OH)(mu-O2PPh2)](ClO4)2 is not very reactive, despite its low pKa value. This low reactivity presumably arises from the two factors. First, the briding hydroxide and coordinated substrate in [Zn2(BPAN)(mu-OH)(substrate)]2+ are not aligned properly to favor nucleophilic attack. Second, the nucleophilicity of the bridging hydroxide is diminished because it is simultaneously bound to the two zinc(II) ions.     

Cyclometallated Pt(II) and Pd(II) complexes with a trithiacrown ligand

We report the synthesis and full characterization for a series of cyclometallated complexes of Pt(II) and Pd(II) incorporating the fluxional trithiacrown ligand 1,4,7-trithiacyclononane ([9]aneS3). Reaction of [M(C insertion mark N)(micro-Cl)]2 (M = Pt(II), Pd(II); C insertion mark N = 2-phenylpyridinate (ppy) or 7,8-benzoquinolinate (bzq)) with [9]aneS3 followed by metathesis with NH4PF6 yields [M(C insertion mark N)([9]aneS3)](PF6). The complexes [M(C insertion mark P)([9]aneS3)](PF6) (M = Pt(II), Pd(II); Cinsertion markP = [CH2C6H4P(o-tolyl)2-C,P]-) were synthesized from their respective [Pt(C insertion mark P)(micro-Cl)]2 or [Pd(C insertion mark P)(micro-O2CCH3)]2 (C insertion mark P) starting materials. All five new complexes have been fully characterized by multinuclear NMR, IR and UV-Vis spectroscopies in addition to elemental analysis, cyclic voltammetry, and single-crystal structural determinations. As expected, the coordinated [9]aneS3 ligand shows fluxional behavior in its NMR spectra, resulting in a single 13C NMR resonance despite the asymmetric coordination environment of the cyclometallating ligand. Electrochemical studies reveal irreversible one-electron metal-centered oxidations for all Pt(II) complexes, but unusual two-electron reversible oxidations for the Pd(II) complexes of ppy and bzq. The X-ray crystal structures of each complex indicate an axial M-S interaction formed by the endodentate conformation of the [9]aneS3 ligand. The structure of [Pd(bzq)([9]aneS3)](PF6) exhibits disorder in the [9]aneS3 conformation indicating a rare exodentate conformation as the major contributor in the solid-state structure. DFT calculations on [Pt([9]aneS3)(ppy)](PF6) and [Pd([9]aneS3)(ppy)](PF6) indicate the HOMO for both complexes is primarily dz2 in character with a significant contribution from the phenyl ring of the ppy ligand and p orbital of the axial sulfur donor. In contrast, the calculated LUMO is primarily ppy pi* in character for [Pt([9]aneS3)(ppy)](PF6), but dx2-y2 in character for [Pd([9]aneS3)(ppy)](PF6).     

Proton NMR spectroscopy and magnetic properties of a solution-stable dicopper(II) complex bearing a single mu-hydroxo bridge

The reaction of copper(II) perchlorate with the macrocyclic ligand [22]py4pz in the presence of base leads to formation of a dinuclear complex [Cu(2)([22]py4pz)(mu-OH)](ClO(4))(3)xH(2)O, in which two copper ions are bridged by a single mu-hydroxo bridge. Each copper ion is further surrounded by four nitrogen atoms of the ligand. The mu-hydroxo bridge mediates a strong antiferromagnetic coupling (2J = -691(35) cm(-1)) between the metal centers, leading to relatively sharp and well-resolved resonances in the (1)H NMR spectrum of the complex in solution. We herein report the crystal structure, the magnetic properties, and the full assignment of the hyperfine-shifted resonances in the NMR spectrum of the complex, as well as the determination of the exchange coupling constant in solution through temperature-dependent NMR studies.     

Dinuclear rhodium and iridium complexes with mixed amido/methoxo and amido/hydroxo bridges

The reactions of [[M(mu-OMe)(cod)](2)] (M = Rh, Ir; cod = 1,5- cyclooctadiene) with p-tolylamine, alpha-naphthylamine, and p-nitroaniline gave complexes with mixed-bridging ligands, [[M(cod)](2)(mu-NHAr)(mu-OMe)]. Similarly, the related complexes [[Rh(cod)](2)(mu-NHAr)(mu-OH)] were prepared from the reactions of [[Rh(mu-OH)(cod)](2)] with p-tolylamine, alpha-naphthylamine, and p-nitroaniline. The reactions of [[Rh(mu-OR)(cod)](2)] (R = H, Me) with o-nitroaniline gave the mononuclear complex [Rh(o-NO(2)C(6)H(4)NH)(cod)]. The syntheses of the amido complexes involve a proton exchange reaction from the amines to the methoxo or hydroxo ligands and the coordination of the amide ligand. These reactions were found to be reversible for the dinuclear complexes. The structure of [[Rh(cod)](2)(mu-NH[p-NO(2)C(6)H(4)])(mu-OMe)] shows two edge-shared square-planar rhodium centers folded at the edge with an anti configuration of the bridging ligands. The complex [[Rh(cod)](2)(mu-NH[alpha-naphthyl])(mu-OH)] cocrystallizes with [[Rh(mu-OH)(cod)](2)] and THF, forming a supramolecular aggregate supported by five hydrogen bridges in the solid state. In the mononuclear [Rh(o-NO(2)C(6)H(4)NH)(cod)] complex the o-nitroamido ligand chelates the rhodium center through the amido nitrogen and an oxygen of the nitro group.     

Synthetic, X-ray Structure, Electron Paramagnetic Resonance, and Magnetic Studies of the Manganese(II) Complex of 1-Thia-4,7-diazacyclononane ([9]aneN(2)S)

Reaction of manganese(II) perchlorate hexahydrate with a methanol solution of 1-thia-4,7-diazacyclononane ([9]aneN(2)S) resulted in the isolation of the manganese(II) complex [Mn([9]aneN(2)S)(2)](ClO(4))(2). The X-ray structure of this complex is reported: crystal system orthorhombic, space group Pbam, No. 55, a = 7.937(2) ?,b = 8.811(2) ?, c = 15.531(3) ?, Z = 2, R = 0.0579. The complex is high spin (S = (5)/(2)) with an effective magnetic moment (&mgr;(eff)) 5.82 &mgr;(B) at 298 K and 5.65 &mgr;(B) at 4.2 K. Computer simulation of the Q-band EPR spectrum of [Mn([9]aneN(2)S)(2)](ClO(4))(2) yields g = 1.99 +/- 0.01, |D| = 0.19 +/- 0.005 cm(-)(1), and E/D = 0.04 +/- 0.02. For the analogous hexaamine complex [Mn([9]aneN(3))(2)](ClO(4))(2) ([9]aneN(3) = 1,4,7-triazacyclononane) analysis of the EPR spectra produced the following values: g = 1.98 +/- 0.01, |D| = 0.09 +/- 0.003 cm(-)(1), and E/D = 0.1 +/- 0.01. The spin Hamiltonian parameters for [Mn([9]aneN(2)S)(2)](ClO(4))(2) derived from the EPR spectra produced a good fit to the magnetic susceptibility data.     

Synthesis of new tren-based tris-macrocycles. Anion cluster assembling inside the cavity generated by a bowl-shaped receptor

The synthesis of three new tris-macrocycles, containing three [12]aneN(4) (L1), [12]aneN(3)O (L2), or [14]aneN(4) (L3) moieties appended to a tren unit, is reported. The crystal structure of the [(Na(ClO(4))(6)) subset L1(2)H(13)]Na(6)Cl(2)(ClO(4))(12) compound shows the anionic cluster [Na(ClO(4))(6)](5)(-) assembled inside the cavity defined by two bowl-shaped polyammonium receptors, held by multiple charge-charge and hydrogen bond interactions.     

2:2 Fe(III):ligand and "adamantane core" 4:2 Fe(III):ligand (hydr)oxo complexes of an acyclic ditopic ligand

A bis-hydroxo-bridged diiron(III) complex and a bis-mu-oxo-bis-mu-hydroxo-bridged tetrairon(III) complex are isolated from the reaction of 2,6-bis((N,N'-bis-(2-picolyl)amino)methyl)-4-tert-butylphenol (Hbpbp) with iron perchlorate in acidic and neutral solutions respectively. The X-ray structure of the dinuclear complex [{(Hbpbp)Fe([mu-OH)}(2)](ClO(4))(4).2C(3)H(6)O (1.2C3H6O) shows that only one of the metal-binding cavities of each ligand is occupied by an iron(III) atom and two [Fe(Hbpbp)]3+ units are linked together by two hydroxo bridging groups to form a [Fe(III)-(mu-OH)](2) rhomb structure with Fe...Fe = 3.109(1)A. The non-coordinated tertiary amine of Hbpbp is protonated. Magnetic susceptibility measurements show a well-behaved weak antiferromagnetic coupling between the two Fe(III) atoms, J= -8 cm(-1). The tetranuclear complex [(bpbp)(2)Fe(4)(mu-O)(2)(mu-OH)(2)](ClO(4))(4)(2) was isolated as two different solvates .4CH(3)OH and .6H(2)O with markedly different crystal morphologies at pH ca. 6. Complex .4CH(3)OH forms red cubic crystals and .6H(2)O forms green crystalline platelets. The Fe(4)O(6) core of shows an adamantane-like structure: The six bridging oxygen atoms are provided by the two phenolato groups of the two bpbp(-) ligands, two bridging oxo groups and two bridging hydroxo groups. The hydroxo and oxo ligands could be distinguished on the basis of Fe-O bond lengths of the two different octahedral iron sites: Fe-mu-OH, 1.953(5), 2.013(5)A and Fe-mu-O, 1.803(5), 1.802(5)A. The difference in ligand environment is too small for allowing Mossbauer spectroscopy to distinguish between the two crystallographically independent Fe sites. The best fit to the magnetic susceptibility of .4CH(3)OH was achieved by using three coupling constants J(Fe-OPh-Fe)= 2.6 cm(-1), J(Fe-OH-Fe)=-0.9 cm(-1), J(Fe-O-Fe)=-101 cm(-1) and iron(III) single ion ZFS (|D|= 0.15 cm(-1)).     

The chemistry of dinuclear analogues of the anticancer drug cisplatin. A DFT/CDM study

The mechanism of the formation of dinuclear platinum(II) mu-hydroxo complexes from cisplatin hydrolysis products, their interconversion, decomposition, and reactions with biomolecules has been explored using a combined DFT/CDM approach. All activation barriers for the formation of [cis-{Pt(NH(3))(2)(X)}-(mu-OH)-cis-{Pt(NH(3))(2)(Y)}](n)()(+) (X, Y = Cl, OH(2), OH) via nucleophilic attack of a hydroxo complex on an aqua complex are lower than the activation barriers for cisplatin hydrolysis. Considering therapeutic Pt(II) concentrations in tumors, however, only the reaction between two molecules of cis-[Pt(NH(3))(2)(OH(2))(OH)](+) (E) yielding [cis-{Pt(NH(3))(2)(OH(2))}-(mu-OH)-cis-{Pt(NH(3))(2)(OH)}](2+) (5) remains kinetically superior to cisplatin hydrolysis. 5 is strongly stabilized by intramolecular hydrogen bonding between the terminal aqua and hydroxo ligands, resulting in an unusually high pK(a) of 5 and a low pK(a) of its conjugate acid. Unimolecular cyclization of 5 yields the dimers [cis-{Pt(NH(3))(2)}(mu-OH)](2)(2+) (7a with antiperiplanar OH groups and 7b with synperiplanar OH groups). The electronic structure of several diplatinum(II) complexes has been analyzed to clarify whether there are metal-metal interactions. The overall reactivity to guanine (Gua) and dimethyl sulfide (Met, representing the thioether functional group of methionine) increases in the order 5 < 7a approximately 7b < mononuclear complexes, whereas the kinetic selectivity to Gua relative to Met increases in the order 7a approximately 5 < 7b approximately monocationic mononuclear complexes < dicationic mononuclear complex. The results of this work (i) help assess whether dinuclear metabolites play a role in cisplatin chemotherapy, (ii) elucidate the toxicity and pharmacological inactivity of [cis-{Pt(NH(3))(2)}(mu-OH)](2)(2+), and (iii) suggest future investigations of dinuclear anticancer complexes that contain one mu-hydroxo ligand.     

Platinum and palladium imido and oxo complexes with small natural bite angle diphosphine ligands

Treatment of L(2)MCl(2) (M = Pt, Pd; L(2) = Ph(2)PCMe(2)PPh(2) (dppip), Ph(2)PNMePPh(2) (dppma)) with AgX (X = OTf, BF(4), NO(3)) in wet CH(2)Cl(2) yields the dinuclear dihydroxo complexes [L(2)M(mu-OH)](2)(X)(2), the mononuclear aqua complexes [L(2)M(OH(2))(2)](X)(2), the mononuclear anion complexes L(2)MX(2), or mixtures of complexes. Addition of aromatic amines to these complexes or mixtures gives the dinuclear diamido complexes [L(2)Pt(mu-NHAr)](2)(BF(4))(2), the mononuclear amine complexes [L(2)M(NH(2)Ar)(2)](X)(2), or the dinuclear amido-hydroxo complex [Pt(2)(mu-OH)(mu-NHAr)(dppip)(2)](BF(4))(2). Deprotonation of the Pd and Pt amine or diamido complexes with M'N(SiMe(3))(2) (M' = Li, Na, K) gives the diimido complexes [L(2)M(mu-NAr)](2) associated with M' salts. Structural studies of the Li derivatives indicate association through coordination of the imido nitrogen atoms to Li(+). Deprotonation of the amido-hydroxo complex gives the imido-oxo complex [Pt(2)(mu-O)(mu-NAr)(dppip)(2)].LiBF(4).LiN(SiMe(3))(2), and deprotonation of the dppip Pt hydroxo complex gives the dioxo complex [Pt(mu-O)(dppip)](2).LiN(SiMe(3))(2).2LiBF(4).     

Enantioselective/anion-selective incorporation of tris(ethylenediamine) complexes into 2D coordination spaces between tripalladium(II) supramolecular layers with D-penicillaminate

The formation of a cocrystallized coordination compound, [Pd(3)(D-pen)(3)](2)·[M(en)(3)](ClO(4))(3) (D-H(2)pen = D-penicillamine; M = Co(III) or Rh(III)), from [Pd(3)(D-pen)(3)] and [M(en)(3)](ClO(4))(3) is reported. In this compound, only the Δ-configurational [M(en)(3)](3+) cations were incorporated when its racemic (Δ/Λ) isomer was employed. Besides this enantioselective incorporation of complex cations, this compound was found to show the selective incorporation of ClO(4)(-) as the anion species.     

1,4,7,10-tetraazacyclododecane metal complexes as potent promoters of carboxyester hydrolysis under physiological conditions

New 1,4,7,10-tetrazacyclododecane ([12]aneN4 or cyclen) ligands with different heterocyclic spacers (triazine and pyridine) of various lengths (bi- and tripyridine) or an azacrown pendant and their mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes have been synthesized and characterized. The pKa values of water molecules coordinated to the complexed metal ions were determined by potentiometric pH titrations and vary from 7.7 to 11.2, depending on the metal-ion and ligand properties. The X-ray structure of [Zn2L2]mu-OH(ClO4)3.CH3CN.H2O shows each Zn(II) ion in a tetrahedral geometry, binding to three N atoms of cyclen (the average distance of Zn-N = 2.1 A) and having a mu-OH bridge at the apical site linking the two metal ions (the average distance of Zn-O- = 1.9 A). The distance between the Zn(II) ion and the fourth N atom is 2.6 A. All Zn(II) complexes promote the hydrolysis of 4-nitrophenyl acetate (NA) under physiological conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The kinetic studies in buffered solutions (0.05 M Tris, HEPES, or CHES, I = 0.1 M, NaCl) at 25 degrees C in the pH range of 6-11 under pseudo-first-order reaction conditions (excess of the metal complex) were analyzed by applying the method of initial rates. Comparison of the second-order pH-independent rate constants (kNA, M-1 s-1) for the mononuclear complexes ZnL1, ZnL3, and ZnL8, which are 0.39, 0.27, and 0.38, respectively, indicates that the heterocyclic moiety improves the rate of hydrolysis up to 4 times over the parent Zn([12]aneN4) complex (kNA = 0.09 M-1 s-1). The reactive species is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile, which attacks intermolecularly the carbonyl group of the acetate ester. For dinuclear complexes Zn2L2, Zn2L4, Zn2L5, Zn2L6, and Zn2L7, the mechanism of the reaction is defined by the degree of cooperation between the metal centers, determined by the spacer length. For Zn2L7, having the longest triaryl spacer, the two metal centers act independently in the hydrolysis; therefore, the reaction rate is twice as high as the rate of the mononuclear analogue (kNA = 0.78 M-1 s-1). The complexes with a monoaryl spacer show saturation kinetics with the formation of a Michaelis-Menten adduct. Their hydrolysis rates are 40 times higher than that of the Zn[12]aneN4 system (kNA approximately 4 M-1 s-1). Zn2L6 is a hybrid between these two mechanisms; a clear saturation curve is not visible nor are the metal cores completely independent from one another. Some of the Zn(II) complexes show a higher hydrolytic activity under physiological conditions compared to other previously reported complexes of this type.     

Thioether complexes of palladium(II) and platinum(II) as artificial peptidases. residue-selective peptide cleavage by a palladium(II) complex

We report the synthesis and characterization of perchlorate salts containing the following three novel complex cations each with a bidentate thioether ligand: binuclear cis-[Pt(CH3SCH2CH2CH2SCH3)(mu-OH)]22+, mononuclear cis-[Pt(CH3SCH2CH2CH2SCH3)(H2O)2]2+, and mononuclear cis-[Pd(CH3SCH2CH2CH2SCH3)(H2O)2]2+. Despite their analogous compositions, the mononuclear Pt(II) and Pd(II) complexes differ in the selectivity with which they promote the hydrolysis of polypeptides. The complex cis-[Pt(CH3SCH2CH2CH2SCH3)(H2O)2]2+ promotes slow but selective cleavage of Met-Pro peptide bonds at pH 2.0. The selectivity of the complex cis-[Pd(CH3SCH2CH2CH2SCH3)(H2O)2]2+ is pH-dependent. At pH 2.0, this Pd(II) complex promotes residue-selective hydrolysis of the X-Y bond in X-Y-Met and X-Y-His sequences; the rate is enhanced when residue Y is proline. At pH 7.0, this kinetic preference becomes sequence-selective in that the Pd(II) complex exclusively cleaves the X-Pro bond in X-Pro-Met and X-Pro-His sequences. The enhanced reactivity of the X-Pro amide group is attributed to the high basicity of its carbonyl oxygen atom. Binding of the metal(II) atom enhances the electrophilicity of the carbonyl carbon atom and promotes nucleophilic attack by a solvent water molecule. The bidentate thioether ligand disfavors the formation of hydrolytically unreactive complexes, allowing the Pd(II) complex to promote the cleavage reaction.     

Co-ordination chemistry of amino pendant arm derivatives of 1,4,7-triazacyclononane

The binding properties of 1,4,7-triazacyclononane ([9]aneN3) to metal cations can be adapted through sequential functionalisation of the secondary amines with aminoethyl or aminopropyl pendant arms to generate ligands with increasing numbers of donor atoms. The new amino functionalised pendant arm derivative of 1,4,7-triazacyclononane ([9]aneN3), L1, has been synthesised and its salt [H2L1]Cl2 characterised by X-ray diffraction. The protonation constants of the ligands L1-L4 having one, two or three aminoethyl or three aminopropyl pendant arms, respectively, on the [9]aneN3 framework, and the thermodynamic stabilities of their mononuclear complexes with CuII and ZnII have been investigated by potentiometric measurements in aqueous solutions. In order to discern the protonation sites of ligands L1-L4, 1H NMR spectroscopic studies were performed in D2O as a function of pH. While the stability constants of the CuII complexes increase on going from L1 to L2 and then decrease on going from L2 to L3 and L4, those for ZnII complexes increase from L1 to L3 and then decrease for L4. The X-ray crystal structures of the complexes [Cu(L1)(Br)]Br, [Zn(L1)(NO3)]NO3, [Cu(L2)](ClO4)2, [Ni(L2)(MeCN)](BF4)2, [Zn(L4)](BF4)2.MeCN and [Mn(L4)](NO3)2.1/2H2O have been determined. In both [Cu(L1)(Br)]Br and [Zn(L1)(NO3)]NO3 the metal ion is five co-ordinate and bound by four N-donors of the macrocyclic ligand and by one of the two counter-anions. The crystal structures of [Cu(L2)](ClO4)2 and [Ni(L2)(MeCN)](BF4)2 show the metal centre in slightly distorted square-based pyramidal and octahedral geometry, respectively, with a MeCN molecule completing the co-ordination sphere around NiII in the latter. In both [Zn(L4)](BF4)2.MeCN and [Mn(L4)](NO3)2.1/2H2O the metal ion is bound by all six N-donors of the macrocyclic ligand in a distorted octahedral geometry. Interestingly, and in agreement with the solution studies and with the marked preference of CuII to assume a square-based pyramidal geometry with these types of ligands, the reaction of L4 with one equivalent of Cu(BF4)2.4H2O in MeOH at room temperature yields a square-based pyramidal five co-ordinate CuII complex [Cu(L6)](BF4)2 where one of the three propylamino pendant arms of the starting ligand has been cleaved to give L6.     

Redox non-innocence of thioether crowns: elucidation of the electronic structure of the mononuclear Pd(III) complexes [Pd([9]aneS3)2]3+ and [Pd([18]aneS6)]3+

The Pd(II) complexes [Pd([9]aneS(3))(2)](PF(6))(2)·2MeCN (1) ([9]aneS(3) = 1,4,7-trithiacyclononane) and [Pd([18]aneS(6))](PF(6))(2) (2) ([18]aneS(6) = 1,4,7,10,13,16-hexathiacyclooctadecane) can be oxidized electrochemically or chemically oxidized with 70% HClO(4) to [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively. These centers have been characterized by single crystal X-ray diffraction, and by UV/vis and multifrequency electron paramagnetic resonance (EPR) spectroscopies. The single crystal X-ray structures of [Pd(III)([9]aneS(3))(2)](ClO(4))(6)·(H(3)O)(3)·(H(2)O)(4) (3) at 150 K and [Pd([18]aneS(6))](ClO(4))(6)·(H(5)O(2))(3) (4) at 90 K reveal distorted octahedral geometries with Pd-S distances of 2.3695(8), 2.3692(8), 2.5356(9) and 2.3490(6), 2.3454(5), 2.5474(6) ?, respectively, consistent with Jahn-Teller distortion at a low-spin d(7) Pd(III) center. The Pd(II) compound [Pd([9]aneS(3))(2)](PF(6))(2) shows a one-electron oxidation process in MeCN (0.2 M NBu(4)PF(6), 293 K) at E(1/2) = +0.57 V vs. Fc(+)/Fc assigned to a formal Pd(III)/Pd(II) couple. Multifrequency (Q-, X-, S-, and L-band) EPR spectroscopic analysis of [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+) gives g(iso) = 2.024, |A(iso(Pd))| = 18.9 × 10(-4) cm(-1); g(xx) = 2.046, g(yy) = 2.041, g(zz) = 2.004;?|A(xx(Pd))| = 24 × 10(-4) cm(-1), |A(yy(Pd))| = 22 × 10(-4) cm(-1), |A(zz(Pd))| = 14 × 10(-4) cm(-1), |a(xx(H))| = 4 × 10(-4) cm(-1), |a(yy(H))| = 5 × 10(-4) cm(-1), |a(zz(H))| = 5.5 × 10(-4) cm(-1) for [Pd([9]aneS(3))(2)](3+), and g(iso) = 2.015, |A(iso(Pd))| = 18.8× 10(-4) cm(-1); g(xx) = 2.048 g(yy) = 2.036, g(zz) = 1.998; |a(xx(H))| = 5, |a(yy(H))| = 5, |a(zz(H))| = 6 × 10(-4) cm(-1); |A(xx(Pd))| = 23× 10(-4) cm(-1), |A(yy(Pd))| = 22 × 10(-4) cm(-1), |A(zz(Pd))| = 4 × 10(-4) cm(-1) for [Pd([18]aneS(6))](3+). Both [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+) exhibit five-line superhyperfine splitting in the g(zz) region in their frozen solution EPR spectra. Double resonance spectroscopic measurements, supported by density functional theory (DFT) calculations, permit assignment of this superhyperfine to through-bond coupling involving four (1)H centers of the macrocyclic ring. Analysis of the spin Hamiltonian parameters for the singly occupied molecular orbital (SOMO) in these complexes gives about 20.4% and 25% Pd character in [Pd([9]aneS(3))(2)](3+) and [Pd([18]aneS(6))](3+), respectively, consistent with the compositions calculated from scalar relativistic DFT calculations.     

Five-coordinate nickel(II) complexes with carboxylate anions and derivatives of 1,5,9-triazacyclododec-1-ene: structural and 1H NMR spectroscopic studies

The hydroxo complexes [Ni2(mcN3)2(mu-OH)2]2(PF6)2[mcN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me3-mcN3) or 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene (Me4-mcN3)] react with the corresponding carboxylic acid [HA = benzoic (Hbz), salicylic (Hsal) or acetylsalicylic (Hacsal) acid] to give five-coordinate nickel(II) complexes of the type [Ni(mcN3)(A)](PF6). The complexes have been studied by spectroscopic methods (IR, UV-Vis and 1H NMR). In acetone solution they exhibit isotropically shifted 1H NMR resonances. The full assignment of these resonances has been made using one- and two-dimensional 1H NMR techniques. The single-crystal structures of [Ni(Me4-mcN3)(bz)](PF6), [Ni(Me4-mcN3)(sal)](PF6) and[Ni(Me4-mcN3)(acsal)](PF6) have been established by X-ray diffraction.     

Oxamidate-bridged dinuclear five-coordinate nickel(II) complexes: a magneto-structural study

The preparation, spectroscopic characterization, and magnetic study of three new oxamidate-bridged nickel(II) dinuclear complexes of formulas ([Ni(Me3[12]aneN3)]2(mu-oa))(PF6)2 (1), ([Ni(Me3[12]aneN3)]2(mu-dmoa))(PF6)2 (2), and ([Ni(Me3[12]aneN3)]2(mu-dpoa))(PF6)2 (3) (Me3[12]aneN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene, oa = oxamidate, dmoa = N,N'-dimethyloxamidate, and dpoa = N,N'-diphenyloxamidate) are reported. The crystal structures of two of them (1 and 3) have been determined. 1 and 3 crystallize in the monoclinic system, space group P2(1)/c, with Z = 2 and a = 7.901(4) A, b = 13.597(6) A, c = 17.565(10) A, and beta = 96.46(4) degrees for 1 and a = 13.854(3) A, b = 17.469(4) A, c = 12.543(3) A, and beta = 116.22(3) degrees for 3. The structures of 1 and 3 consist of dinuclear ([Ni(Me3[12]aneN3)]2(mu-oa))2+ and ([Ni(Me3[12]aneN3)]2(mu-dpoa))2+ cations and hexafluorophosphate anions. Each nickel in 1-3 is five-coordinate, and the substitution of the hydrogen atom of the amidate nitrogen of 1 by a methyl (2) or a phenyl (3) group causes a significant modification of the stereochemistry of the nickel(II) ions from square pyramidal toward trigonal bipyramidal (tau values of 0.12 and 0.48 for 1 and 3, respectively). The NOESY spectrum of 3 has allowed us to achieve the assignment of the phenyl protons of the N,N'-diphenyloxamidate. The value of magnetic coupling between the two nickel(II) ions across the oxamidate bridge [J = -57.0 (oa, 1), -38.0 (dmoa, 2) and -30.5 cm(-1) (dpoa, 3)] is very sensitive to this stereochemical change, and its variation is explained on the basis of orbital considerations. DFT type calculations have been performed to analyze and substantiate the trend of the magnetic coupling in 1-3.     

Synthesis and characterization of mononuclear zinc aryloxide complexes supported by nitrogen/sulfur ligands possessing an internal hydrogen bond donor

Treatment of a dinuclear zinc hydroxide complex ([(bmnpaZn)(2)(mu-OH)(2)](ClO(4))(2) (1) or [(benpaZn)(2)(mu-OH)(2)](ClO(4))(2) (2)) with excess equivalents of an aryl alcohol derivative (p-HOC(6)H(4)X; X = NO(2), CHO, CN, COCH(3), Br, H, OCH(3)) yielded the nitrogen/sulfur-ligated zinc aryloxide complexes [(bmnpa)Zn(p-OC(6)H(4)NO(2))](ClO(4)) (3), [(benpa)Zn(p-OC(6)H(4)NO(2))](ClO(4)) (4), [(benpa)Zn(p-OC(6)H(4)CHO)](ClO(4)) (5), [(benpa)Zn(p-OC(6)H(4)CN)](ClO(4)) (6), [(benpa)Zn(p-OC(6)H(4)COCH(3))](ClO(4)) x 0.5H(2)O (7), [(benpa)Zn(p-OC(6)H(4)Br)](ClO(4)) (8), [(benpa)Zn(p-OC(6)H(5))](ClO(4)) (9), and [(benpa)Zn(p-OC(6)H(5)OCH(3))](ClO(4)) (10). The solid state structures of 2, 3, 5, and 6 have been determined by X-ray crystallography. While 3 and 6 exhibit a mononuclear zinc ion possessing a distorted five-coordinate trigonal bipyramidal geometry, in 5 each zinc center exhibits a distorted six-coordinate octahedral geometry resulting from coordination of the aldehyde carbonyl oxygen of another zinc-bound aryloxide ligand, yielding a chain-type structure. Zinc coordination of the aldehyde carbonyl of 5 is indicated by a large shift (>40 cm(-)(1)) to lower energy of the carbonyl stretching vibration (nu(C[double bond]O) in solid state FTIR spectra of the complex. In the solid state structures of 3, 5, and 6, a hydrogen-bonding interaction is found between N(3)-H of the supporting bmnpa/benpa ligand and the zinc-bound oxygen atom of the aryloxide ligand (N(3)...O(1) approximately 2.78 A). Solution (1)H and (13)C NMR spectra of 3-10 in CD(3)CN and FTIR spectra in CH(3)CN are consistent with all of the aryloxide complexes having a similar solution structure, with retention of the hydrogen-bonding interaction involving N(3)-H and the oxygen atom of the zinc-coordinated aryloxide ligand. For this family of zinc aryloxide complexes, a correlation was discovered between the chemical shift position of the N(3)-H proton resonance and the pK(a) of the parent aryl alcohol. This correlation indicates that the strength of the hydrogen-bonding interaction involving the zinc-bound aryloxide oxygen is increasing as the aryloxide moiety increases in basicity.     

Thioester hydrolysis reactivity of zinc hydroxide complexes: investigating reactivity relevant to glyoxalase II enzymes

A recently reported binuclear zinc hydroxide complex [(L(1)Zn(2))(mu-OH)](ClO(4))(2) (, L(1) = 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenolate monoanion) containing a single bridging hydroxide was examined for thioester hydrolysis reactivity. Treatment of it with hydroxyphenylthioacetic acid S-methyl ester in dry CD(3)CN results in no reaction after approximately 65 h at 45(1) degrees C. Binuclear zinc hydroxide complexes of the N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine (L(2)) and N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)ethyl)amine (L(3)) chelate ligands were prepared by treatment of each ligand with molar equivalent amounts of Zn(ClO(4))(2).6H(2)O and KOH in methanol. These complexes, [(L(2)Zn)(2)(mu-OH)(2)](ClO(4))(2) and [(L(3)Zn)(2)(mu-OH)(2)](ClO(4))(2) (), which have been structurally characterized by X-ray crystallography, behave as 1 : 1 electrolytes in acetonitrile, indicating that the binuclear cations dissociate into monomeric zinc hydroxide species in solution. Treatment of them with one equivalent of hydroxyphenylthioacetic acid S-methyl ester per zinc center in acetonitrile results in the formation of a zinc alpha-hydroxycarboxylate complex, [(L(2))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O or [(L(3))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O, and CH(3)SH. These reactions, to our knowledge, are the first reported examples of thioester hydrolysis mediated by zinc hydroxide complexes. The results of this study suggest that a terminal Zn-OH moiety may be required for hydrolysis reactivity with a thioester substrate.     

Preparation and characterization of dinuclear Pd(II) complexes of binucleating tetraaza-thiophenolate ligands

The thioethers 4-tert-butyl-2,6-bis((2-(dimethylamino)ethylimino)methyl)phenyl(tert-butyl)sulfane (tBu-L3) and 4-tert-butyl-2,6-bis((2-(dimethylamino)ethylimino)methyl)phenyl(tert-butyl)sulfane (tBu-L4) react with PdCl2(NCMe)2 to give the dinuclear palladium thiophenolate complexes [(L3)Pd2Cl2]+ (2) and [(L4Pd2(mu-Cl)]2+ (3) (HL3= 2,6-bis((2-(dimethylamino)ethylimino)methyl)-4-tert-butylbenzenethiol, HL4 = 2,6-bis((2-(dimethylamino)ethylamino)methyl)-4-tert-butylbenzenethiol). The chloride ligands in could be replaced by neutral (NCMe) and anionic ligands (NCS-, N3-, CN-, OAc-) to give the diamagnetic Pd(II) complexes [(L3)Pd2(NCMe)2]3+ (4), [(L3)Pd2(NCS)2]+ (5), [(L3)Pd2(N3)2]+ (6), [{(L3)Pd2(mu-CN)}2]4+ (7) and [(L3)Pd2(OAc)]2+ (9). The nitrile ligands in and in [(L3)Pd2(NCCH2Cl)2]3+ are readily hydrated to give the corresponding amidato complexes [(L3)Pd2(CH3CONH)]2+ (8) and [(L3)Pd2(CH2ClCONH)]2+ (10). The reaction of [(L3)Pd2(NCMe)2]3+ with NaBPh4 gave the diphenyl complex [(L3)Pd2(Ph)2]+ (11). All complexes were either isolated as perchlorate or tetraphenylborate salts and studied by IR, 1H and 13C NMR spectroscopy. In addition, complexes 2[ClO4], 3[ClO4]2, 5[BPh4], 6[BPh4], 7[ClO4]4, 9[ClO4]2, 10[ClO4]2 and 11[BPh4] have been characterized by X-ray crystallography.