N4 Donor Research Articles

Flexible ligand synthesis, characterization and liquid phase hydroxylation of phenol by H2O2 with host (nanopores of zeolite-Y)/guest [VO([R]2–N2X2)]2+ (R = H, CH3; X = NH, O, S) nanocomposite materials

Encapsulation of Oxovanadium(IV) complexes of 12-membered macrocyclic ligands having N2O2, N2S2 and N4 donor atoms in the nanocavity of zeolite-Y by the Flexible-Ligand Method (FLM) have been described. Oxovanadium(IV) complexes with macrocyclic ligands were entrapped in the nanocavity of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of precursor ligand; 1,2-di(o-aminophenyl-, amino, oxo, thio)ethane, (N2X2); in the supercages of VO(IV)-Y; ([VO(N2X2)]2+-Y (X = NH, O, S); and (ii) in situ condensation of the oxovanadium(IV) precursor complex with glyoxal or biacetyl; [VO([R]2–N2X2)]2+-Y (R = H, CH3). The new Host–Guest Nanocomposite Materials (HGNM, [VO([R]2–N2X2)]2+-Y) have been characterized by FT-IR, DRS and UV–Vis spectroscopic techniques, XRD and elemental analysis, as well as nitrogen adsorption. Liquid-phase selective hydroxylation of phenol with H2O2 to a mixture of catechol and hydroquinone in CH3CN has been reported using [VO([R]2–N2X2)]2+-Y as catalysts. Reaction conditions have been optimized by considering the concentration of substrate and oxidant, amount of catalyst, effect of time, volume of solvent and temperature. Under the optimized reaction conditions, [VO([H]2–N4)]2+-Y gave 50.1% conversion of phenol after 6 h. All these catalysts are more selective toward catechol formation.

Synthesis, Structure, and Optical Properties of the Platinum(II) Complexes of Indaphyrin and Thiaindaphyrin

The novel free base meso-di(5'-methylthien-2'-yl)thiaindaphyrin, 10, was prepared from the corresponding meso-tetra(thien-2-yl)porphyrin using a methodology analogous to that for the preparation of known meso-diphenylindaphyrin, 5: beta,beta'-Dihydroxylation of the porphyrin is followed by oxidative diol cleavage. The resulting aldehyde moieties undergo an acid-catalyzed intramolecular Friedel-Crafts alkylation of the adjacent meso-thienyl groups with concomitant oxidation. Insertion of Pt(II) into either of the chromophores is facile, producing 5Pt and 10Pt. The crystal structure of 5Pt, the first for any indaphyrin, shows that the conformation of the indaphyrinato ligand is strongly ruffled, while the N(4) donor set that coordinates the central Pt(II) maintains a near-perfect square-planar coordination geometry around the central metal ion (crystal data for C(44)H(24)N(4)O(2)Pt: triclinic space group P1 with a = 8.8735(4) A, b = 12.9285(6) A, c = 14.3297(6) A, alpha = 88.785(1) degrees, beta = 82.248(1) degrees, gamma = 72.422(1) degrees; Z = 2). The UV-vis and emission spectra, triplet yields, and lifetimes of the Pt(II) complexes 5Pt and 10Pt were determined. Both complexes luminesce (in EtOH at 77 K) in the NIR (5Pt: lambda(max-emission) = 864, 974 nm, lifetime 2 micros; 10Pt: lambda(max-emission) = 990, 1112, 1276 nm) with modest to low quantum yields (Phi(p) approximately 1% and approximately 6 x 10(-3) %, respectively).

Synthesis, characterization and extraction studies of N,N″-bis[1-biphenyl-2-hydroxyimino-2-(4-acetylanilino)-1-ethylidene]-diamines and their homo-and heteronuclear copper(II) complexes

A new series of homo- and heteropolynuclear copper(II) complexes of N,N″-bis[1-biphenyl-2-hydroxyimino-2-(4-acetylanilino)-1-ethylidene]-diamines have been prepared and characterized by different physical techniques. The starting point of the research was the reaction of chloroacetyl chloride with biphenyl in the presence of aluminum chloride. 4-Biphenylhydroximoyl chloride was obtained by reacting synthesized 4-(chloroacetyl)biphenyl with alkyl nitrite. Substituted 4-(alkylaminoisonitrosoacetyl) biphenyl (ketooxime) was prepared by reacting 4-biphenylhydroximoyl chloride with 4-aminoacetophenone in EtOH. Homodi-, homotrinuclear and heterodinuclear copper(II) perchlorate complexes of tetradentate Schiff bases which possess N4 donor sets derived from the condensation of 4-(arylaminoisonitrosoacetyl)biphenyl and diamine derivatives were synthesized and characterized. Elemental analysis, FT-IR, ESR, molar conductivity, magnetic moment measurements and thermal analyses studies were utilized for the investigation of the complexes. The free ligands were also characterized by 1H- and 13C-NMR spectra. Elemental analyses, stoichiometric and spectroscopic data of the metal complexes indicated that the metal:ligand ratio of dinuclear copper(II) complexes were found to be 2: 1 while this ratio was 3: 2 in trinuclear copper(II) complexes and the metal complexes indicated that the metal ions are coordinated to the oxime and imine nitrogen atoms. The extraction abilities of the novel ligands were also evaluated in chloroform by using several transition metal picrates such as Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Hg2+. It has been observed that both ligands show a high affinity to Cu2+ ions.

Antifungal active tetraaza macrocyclic transition metal complexes: Designing, template synthesis, and spectral characterization

Eight novel macrocyclic complexes as candidates of antifungal agent were designed and synthesized by incorporating an N4 donor site via the template condensation of 4,4′-diaminodiphenylmethane, formaldehyde. p-anisidine, and metal salts. The structural features were determined on the basis of their elemental analyses, magnetic susceptibility, molar conductance, FAB Mass, UV-Vis, and IR spectral data. Electronic absorption spectral data of the complexes suggest a square-planar geometry around the central metal ion except the VO(IV) complex, which shows square-pyramidal geometry. The stoichiometry of the complexes had been found to be 1: 1 (metal: ligand). Electrolytic nature of the complexes is assessed from their high conductance data. Monomeric nature of the complexes is confirmed from magnetic susceptibility values. The X-band ESR spectra of the Cu(II) and VO(IV) complexes in DMSO at 300 and 77 K were recorded, and their salient features are reported. The antifungal activity of the macrocyclic metal complexes were screened in vitro against Aspergillus niger, Aspergillus fluvus, Trichoderma harizanum, Trichoderma viridae, and Rhizoctonia solani. The data showed that they possessed antifungal activity.

Synthesis, spectroscopic and redox properties of the mononuclear NiII, NiII(BPh2)2 containing (B-C) bond and trinuclear CuII-NiII-CuII type-metal complexes of N,N′-(4-amino-1-benzyl piperidine)-glyoxime

The novel vic-dioxime ligand containing the 4-amino-1-benzyl piperidine group, N,N′-(4-amino-1-benzyl piperidine)-glyoxime, (LH2) has been prepared from 4-amino-1-benzyl piperidine with anti-dichloroglyoxime at −15°C in absolute THF. Mononuclear NiII metal complex has been obtained with 1: 2 metal/ligand ratio. The NiII complex of this ligand is proposed to be square planar geometry. IR spectra show that the ligand acts in a tetradentate manner and coordinates N4 donor groups of LH2 to NiII ion. The detection of H-bonding (O-H···O) in the [Ni(LH)2] (1) metal complex by IR spectra supported the square-planar MN4 coordination of mononuclear complex. The disappereance of H-bonding (O-H···O) in the [Ni(L)2(BPh2)2] (2) complex shows that the BPh +2 -capped groups (BPh +2 cation formed BPh4 anion) attaches to the main oxime core. MN4 coordination of the [Ni(LH)2] (1) and [Ni(L)2(BPh2)2] (2) metal complexes were also determined by 1H-NMR spectroscopy. In the trinuclear CuII-NiII-CuII metal complexes, the NiII ion centered into the main oxime core by the coordination of the imino groups while the two CuII ions coordinate dianionic oxygen donors of the oxime groups and linked to the ligands of 1,10-phenanthroline, 2,2′-bipyridine, and 4,4′-bipyridine. The ligand and their mono and trinuclear metal complexes were characterized by elemental analyses, FT-IR, UV-Vis, 1H and 13C-NMR spectra, magnetic susceptibility measurements, molar conductivity, cyclic voltammetry, mass spectra and X-ray powder techniques. The cyclic voltammetric results show that the cathodic peak potential of [Ni(L)2(BPh2)2] shifted toward more negative value compared to that of [Ni(LH)2], probably due to a decreasing effect of back donation of metal-oxime moieties as a result of the BPh +2 -bridged complex formation. Also, the formation of the trinuclear CuII-NiII-CuII metal complexes caused considerable changes on the CV behaviour of mononuclear [Ni(LH)2] (1) complex. The spectroelectrochemical study of [Ni(L)2(BPh2)2] (2) showed distinctive spectral changes that the intensity of the band (λ = at 364 nm, assigned to n → π* transitions) decreased and a new broad band in low intensity about 460 nm appeared as a result of the reduction of the nickel centered in the oxime core.

Studies on Nickel(II) Complexes with Amide‐Based Ligands: Syntheses, Structures, Electrochemistry and Oxidation Chemistry

AbstractThe present work discusses the nickel chemistry in a set of amide‐based open‐chain ligands with subtle differences in the backbone or terminal amine substituents. The ligands coordinate to the Ni2+ ion through the Namide and Namine atoms maintaining a square‐planar geometry. Absorption spectra and NMR studies reveal that the solid‐state square‐planar geometry is retained in solution. The electrochemical results suggest that the NiIII/NiII redox couple primarily depends on the N4 donors, which is composed of two Namide and twoNamine atoms and not on the peripheral substituents. All four ligands with variable backbone and substituents are equally competent in stabilizing the NiIII state. On the basis of electrochemical findings, chemical oxidations were carried out, and they reveal generation of the NiIII state in two cases, whereas decomposition was observed in others. Preliminary alkene epoxidation reactions suggest that the present nickel complexes transiently stabilize the higher oxidation state of the nickel ion that possibly participates in the oxidation of the substrates.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Host (nanopores of zeolite-Y)/guest [manganese(II) with 12-membered tetradentate N2O2, N2S2 and N4 donor macrocyclic ligands] nanocatalysts: flexible ligand synthesis, characterization and catalytic activity

Mn(II) complexes of 12-membered macrocyclic ligands with three different donating atom sets (N2O2, N2S2 and N4) in the macrocyclic ring have been encapsulated in the nanopores of zeolite-Y by the Flexible-Ligand Method (FLM). The complexes were entrapped in the nanocavity of zeolite-Y by a two-step process in the liquid phase: (i) adsorption of 1,2-di(o-aminophenyl-, amino, oxo, thio)ethane in the supercages of the zeolite and (ii) in situ condensation of the Mn(II) precursor complex ([Mn(N2X2)]2+) with glyoxal or biacetyl. The new host–guest nanocatalysts, [Mn([R]2–N2X2)]2+–NaY (R = H, CH3; X = NH, O, S), have been characterized by various physico-chemical methods. These complexes, both in their free states and as host–guest nanocatalysts, were used for oxidation of cyclohexene with tert-butylhydroperoxide (TBHP) oxidant in different solvents. Di-2-cyclohexenylether was identified as the main product. 2-Cyclohexene-1-one, 2-cyclohexene-1-ol and 1-(tert-butylperoxy)-2-cyclohexene were obtained as minor products. [Mn([H]2–N4)]2+–NaY was found to give the best reactivity and selectivity.

Attachment of 4-methoxy benzyl units to a tripodal fluoroionophore shows reversal of output functionality with Cu(II) input

Syntheses of three tris-(2-aminoethyl)amine, tren based tripodal fluoroionophores ( L 2 , L 3 , and L 5 ), are reported. These fluoroionophores are designed based on the fluorophore–spacer–receptor format (choice of fluorophore in all three cases is anthryl unit). In L 2 , three anthracene moieties are attached to the three arms of tren via –CH 2-spacer whereas L 3 and L 5 have p-nitro benzyl and p-methoxy benzyl substitutions, respectively, on L 2 , which are in close proximity to the photoinduced electron transfer (PET) center. All three fluoroionophores show appreciably lower fluorescence compared to anthracene due to effective PET process in these systems but the quantum yield varies depending upon the nature of substitution at the PET center. In the cases of L 2 and L 5 different amounts of fluorescence recovery are observed in the presence of different cation inputs whereas L 3 is almost inactive toward cation sensing. Detailed fluorescence emission studies on L 2 and L 5 in the presence of different cation inputs showed that L 5 having N4 donor sets bearing three p-methoxy benzyl units attached to the three nitrogen centers involving photoinduced electron transfer process is a viable candidate for enhancement of fluorescence with Cu(II) input. In the absence of p-methoxy benzyl units at the nitrogen centers' resulting system, L 2 shows quenching of fluorescence with the Cu(II) under same experimental conditions.

Catena-Poly[[silver(I)-μ-1,4-di-3-pyridyl-2,3-diazabuta-1,3-diene] methanesulfonate

The asymmetric unit in the polymeric title compound, {[Ag(C12H10N4)]CH3SO3}n, comprises two Ag atoms, a 4-pyridine­aldazine mol­ecule in a general position, two 4-pyridine­aldazine mol­ecules each disposed about a centre of inversion, and two methane­sulfonate anions. The Ag atoms are in linear geometries within N2 donor sets. The topology of the polymer is linear and the strands are connected into double chains via weak argentophilic Ag⋯Ag [3.2088 (10) A] inter­actions that stack along the b axis inter­spersed by the methane­sulfonate anions. These layers are consolidated in the crystal structure primarily via weak Ag⋯O contacts (Ag⋯O > 2.69 A).

Catena-Poly[[silver(I)-μ-1,4-di-4-pyridyl-2,3-diazabuta-1,3-diene] methanesulfonate

The asymmetric unit in the polymeric title compound, {[Ag(C12H10N4)]CH3SO3}n, comprises two Ag atoms, a 4-pyridine­aldazine mol­ecule in a general position, two 4-pyridine­aldazine mol­ecules each disposed about a centre of inversion, and two methane­sulfonate anions. The Ag atoms are in linear geometries within N2 donor sets. The topology of the polymer is linear and the strands are connected into double chains via weak argentophilic Ag⋯Ag [3.2088 (10) Å] inter­actions that stack along the b axis inter­spersed by the methane­sulfonate anions. These layers are consolidated in the crystal structure primarily via weak Ag⋯O contacts (Ag⋯O > 2.69 Å).

Electrochemical Synthesis and Characterization of Zinc(II) and Cadmium(II) Complexes of Dianionic Tetradentate Schiff Base Ligand

AbstractThe electrochemical oxidation of anodic metal (zinc or cadmium) in acetonitrile solution of the potentially chelating Schiff base N,N‐(dithiodiethylenebis(aminylydenemethylydene)bis(1,2‐phenylene)ditosylamide (H2L) afforded stable complexes of empirical formula [ML]. The crystal and molecular structures of [ZnL]·CH3CN (1) and [CdL] (2) have been determined by X‐ray diffraction. In both complexes the metal atom is in a distorted tetrahedral environment with the Schiff base acting as a tetradentate N4 donor. Spectroscopic data for the complexes (IR, ES MS and 1H NMR spectra) are discussed and related to the structural information.

Synthesis, spectral characterization, catalytic and antibacterial activity of macrocyclic CuII compounds

Macrocyclic CuII compounds of the type Cu(L4)Cl2 (where L4 = N4 or N2O2 donor macrocyclic ligand) have been synthesized by treating the corresponding macrocycles with copper chloride in methanol. These compounds were characterized with the help of elemental analysis, i.r., mass, ESR, electronic spectra, conductance, magnetic and thermal studies. Distorted octahedral geometry has been proposed for all of these compounds. These compounds were found to be efficient in the catalytic oxidation of ascorbic acid and the percentage yields of oxidation products were determined spectrophotometrically. The biological activities of these compounds have been tested against gram +ve and gram −ve bacteria and found to be more active when compared with commercially available antibacterials like streptomycin and ampicillin.

Ruthenium Nitrosyls Derived from Polypyridine Ligands with Carboxamide or Imine Nitrogen Donor(s): Isoelectronic Complexes with Different NO Photolability

As part of our search for photoactive ruthenium nitrosyls, a set of {RuNO}6 nitrosyls has been synthesized and structurally characterized. In this set, the first nitrosyl [(SBPy3)Ru(NO)](BF4)3 (1) is derived from a polypyridine Schiff base ligand SBPy3, while the remaining three nitrosyls are derived from analogous polypyridine ligands containing either one ([(PaPy3)Ru(NO)](BF4)2 (2)) or two ([(Py3P)Ru(NO)]BF4 (3) and [(Py3P)Ru(NO)(Cl)] (4)) carboxamide group(s). The coordination structures of 1 and 2 are very similar except that in 2, a carboxamido nitrogen is coordinated to the ruthenium center in place of an imine nitrogen in case of 1. In 3 and 4, the ruthenium center is coordinated to two carboxamido nitrogens in the equatorial plane and the bound NO is trans to a pyridine nitrogen (in 3) and chloride (in 4), respectively. Complexes 1-3 contain N6 donor set, and the NO stretching frequencies (nuNO) correlate well with the N-O bond distances. All four diamagnetic {RuNO}(6) nitrosyls are photoactive and release NO rapidly upon illumination with low-intensity (5-10 mW) UV light. Interestingly, photolysis of 1 generates the diamagnetic Ru(II) photoproduct [(SBPy3)Ru(MeCN)](2+) while 2-4 afford paramagnetic Ru(III) species in MeCN solution. The quantum yield values of NO release under UV illumination (lambda(max) = 302 nm) lie in the range 0.06-0.17. Complexes 3 and 4 also exhibit considerable photoactivity under visible light. The efficiency of NO release increases in the order 2 < 3 < 4, indicating that photorelease of NO is facilitated by (a) the increase in the number of coordinated carboxamido nitrogen(s) and (b) the presence of negatively charged ligands (like chloride) trans to the bound NO.

New Cu(II) complexes with polydentate chelating Schiff base ligands: Synthesis, structures, characterisations and biochemical activity studies

We have reported herein the synthesis of three new Cu(II) complexes of tri- and tetradentate Schiff base ligands containing N3 or N4 donor set along with terminal NNN− or SCN− ligands: [L1Cu(NCS)]ClO4 (1), [L2Cu(NCS)2] (2) and [L3Cu(NNN)]ClO4 (3) [L1 = NC5H4C(CH3)=N(CH2)3N=C(CH3)C5H4N, L2= Me2N–(CH2)3–N=C(CH3)C5H4N and L3 = NC5H4CH=N–(CH2)4–N=CHC5H4N]. The complexes have been systematically characterised by elemental, spectroscopic and electrochemical techniques. Antimicrobial activities of the Schiff base ligands and their metal complexes have been studied using the disc diffusion method on the strains of Candida tropicalis and Bacillus megaterium. Structures of all the complexes have been unequivocally established from single crystal X-ray diffraction analyses that show the monomeric units containing a five-coordinated copper center in highly distorted square pyramidal geometry with thiocyanate or azide anion coordinated as terminal ligand. The complexes 1 and 3 crystallise in monoclinic (P21/c) and 2 in triclinic (P-1) space group, respectively.

A Cl? anion-responsive luminescent Eu3+ complex with a chiral tripod: ligand substituent effects on ternary complex stoichiometry and anion sensing selectivity

A new series of N3,O-mixed donor tripods was prepared for luminescent Eu3+ complexes, in which the soft quinoline nitrogen, tertiary amine nitrogen, and hard amide oxygen donors were cooperatively involved. The mixed donor tripods formed more stable 1 : 1 complexes with Eu(NO3)3, La(NO3)3 and Tb(NO3)3 than the corresponding N4 donor tripods, and their Eu3+ complexes particularly exhibited anion-responsive luminescence properties. NMR, UV, and luminescence spectroscopic characterizations revealed that -CH3 substitution on the tripod skeleton remarkably altered the preferred stoichiometry of the "tripod-Eu3+-anion" ternary complex and gave anion-dependent europium luminescence. Although the disubstituted tripod preferred to form non-luminescent 2 : 1 (tripod : Eu3+) complexes with Eu(NO3)3 and other salts, it formed a luminescent 1 : 1 complex with EuCl3. Thus, this type of tripod offered Cl- anion-selective luminescence enhancement that was easily observed by the naked eye.

Synthesis, crystal, and molecular structure of coordination polymers constructed by self-assembly of NiN4 cores with 2,2′-iminodibenzoate and nitroprusside ions

Two coordination compounds of compositions [Ni(L1)(idba)(H2O)]·1.5 H2O (1) and [Ni(L2)Fe(CN)5NO]·C2H5OH (2) where L1 is N, N′-bis(3-aminopropyl)ethylenediamine, L2 is 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2,11,13,15-pentaene, and idba2− is 2,2′-iminodibenzoate have been synthesized and characterized by elemental analysis, IR spectroscopy, thermal analysis, and single-crystal X-ray diffraction. Complex 1 crystallizes in the monoclinic space group P21/n (No. 14) with a=9.810(2) A, b=10.230(2) A, c=25.350(5) A, V=2543.6(9) A3, Z=4, and R=0.0727. The nickel atom is six-coordinated by four N atoms of amine and two O atoms of water and idba2−. The molecular packing of the complex comprises of an infinite one-dimensional layered network in which the molecules in the crystal are held together by a system of hydrogen bonding. Complex 2, however, crystallizes in the space group C2/c (No. 15) of the monoclinic system with a=19.7990(4) A, b=14.9440(3) A, c=19.8800(3) A, V=5115.90(17) A3, Z=4, and R=0.0540. The Ni ion in compound 2 has a slightly distorted octahedral arrangement of the N4 donor atoms of primary ligand L2 and two N-donor atoms of the secondary nitroprusside ligand. The structure of 2 displays an extended one-dimensional network formed by linear [—Ni—NC—Fe—CN—] units. A cyclic voltammetric study shows that compound 1 undergoes a quasireversible oxidation attributable to Ni2+ → Ni3+ in the range 300–420 mV vs SCE.

A new dioxime ligand and its trinuclear copper(II) complex: Synthesis, characterization and optical properties

The synthesis and characterization of a new dioxime ligand, (H2L), (III) and its trinuclear copper(II) complex, [Cu3L2(H2O)2](ClO4)2, (V) is described. IR spectra show that the ligand acts in a tetradentate manner and coordinates N4 donor groups of H2L to copper(II) ion. The structure of the metal chelate is clarified with the help of elemental analysis, magnetic moment, conductometric and spectroscopic measurements. The optical absorption studies reveal that the transition is direct with band gap energy values are calculated. The optical constants such as refractive index and dielectric constant of the compound were determined. The refractive index dispersion curves of the films obey the single-oscillator model and oscillator parameters. Optical dispersion parameters E0 and Ed developed by Wemple–DiDomenico were calculated.

Phosphate diester hydrolysis promoted by new Cu(II) alkoxide complexes

Three Cu(II) complexes of N3 donor pyridyl-amine ligands, bis-(2-pyridin-2-yl-ethyl)-amine (L1), which does not have an alcohol pendant, and 2-[bis-(2-pyridin-2-yl-ethyl)-amino]-ethanol (L2) and 3-[bis-(2-pyridin-2-yl-ethyl)-amino]-propan-1-ol (L3), both containing alcohol pendants, have been synthesized and structurally characterized. The reactivities of CuL1 (1), CuL2 (2), and CuL3 (3) for promoting the hydrolysis of bis(p-nitrophenyl)phosphate (BNPP) have been investigated, showing that 3 is much more active (observed second order rate constant k=6.1×10−1M−1s−1 at pH 8.4, 50°C) than 1 (k=9.4×10−3M−1s−1 at pH 8.4, 50°C) or 3 (observed second order rate constant k=5.2×10−3M−1s−1 at pH 8.4, 50°C) at promoting the hydrolysis of BNPP. The much higher reactivity of 3 compared to 2 and 1 may be explained by the higher flexibility of the alkoxide tether in 3, which may allow the nucleophilic alkoxide O-atom in 3 better access to attack a metal-bound BNPP substrate, whereas 2, which has a shorter alkoxide tether, may resort to hydrolyzing the BNPP substrate with a metal-bound hydroxide instead of utilizing its alkoxide moiety because of constraints in the ligand tether. The similar reactivities of 1 and 2 suggest that this is a possible scenario. Another possible explanation for the difference in reactivity between 3 and 2 may be attributed to the alcohol pendant in 3 being better oriented to serve as a proton acceptor from a Cu(II)-bound aqua ligand, which would facilitate its deprotonation and the subsequent nucleophilic attack of the Cu(II)-hydroxide moiety towards the substrate. The reactivities of 2 and 3 towards hydrolyzing BNPP are also compared with that of their Zn(II) analogues, showing that the Cu(II) complexes of the same ligand sets are more hydrolytically active than the corresponding Zn(II) complexes. Single crystal X-ray structures of 1, 2, and 3 are also reported, which show that complexes 2 and 3 form dimers with bridging alkoxide ligands in the solid state, while 1 forms a dimer with bridging chloride ligands in the solid state.

Mono- and Polynuclear Complexes of the Model Nucleobase 1-Methylcytosine. Synthesis and Characterization ofcis-[(PMe2Ph)2Pt{(1-MeCy(−H)}]3(NO3)3andcis-[(PPh3)2Pt{1-MeCy(−H)}(1-MeCy)]NO3

The hydroxo complex cis-[L2Pt(mu-OH)]2(NO3)2 (L = PMe2Ph), in various solvents, reacts with 1-methylcytosine (1-MeCy) to give as the final product the cyclic species cis-[L2Pt{1-MeCy(-H),N 3N 4}]3(NO3)3 (1) in high or quantitative yield. X-ray analysis of 1 evidences a trinuclear species with the NH(2)-deprotonated nucleobases bridging symmetrically the metal centers through the N3 and N4 donors. A multinuclear NMR study of the reaction in DMSO-d6 reveals the initial formation of the dinuclear species cis-[L2Pt{1-MeCy(-H),N 3N 4}]2(2+) (2), which quantitatively converts into 1 following a first-order kinetic law (at 50 degrees C, t(1/2) = 5 h). In chlorinated solvents, the deprotonation of the nucleobase affords as the major product (60-70%) the linkage isomer of 1, cis-[L2Pt{1-MeCy(-H)}]3(3+) (3), in which three cytosinate ligands bridge unsymmetrically three cis-L2Pt(2+) units. In solution, 3 slowly converts quantitatively into the thermodynamically more stable isomer 1. No polynuclear adducts were obtained with the hydroxo complex stabilized by PPh3. cis-[(PPh3)2Pt(mu-OH)]2(NO3)2 reacts with 1-MeCy, in DMSO or CH2Cl2, to give the mononuclear species cis-[(PPh3)2Pt{1-MeCy(-H)}(1-MeCy)](NO3) (4) containing one neutral and one NH2-deprotonated 1-MeCy molecule, coordinated to the same metal center at the N3 and N4 sites, respectively. X-ray analysis and NMR studies show an intramolecular H bond between the N4 amino group and the uncoordinated N3 atom of the two nucleobases.

Zn−OH2 and Zn−OH Complexes with Hydroborate-Derived Tripod Ligands: A Comprehensive Study

The complete array of those hydrotris(pyrazolyl/thioimidazolyl)borate ligands that were developed and used in the author's laboratories, with N3, N2S, NS2, and S3 donor sets, was scanned for their ability to form Zn-OH2 and Zn-OH complexes. The coordination motifs found were Zn-OH2, Zn-OH, Zn-OH-Zn, and Zn-O2H3-Zn. Of these, the well-established Zn-OH motif was complemented with novel species bearing N3, NS2, and S3 tripods. The Zn-OH2 motif was observed only with pyrazolylborate ligands and only in unusual situations with coordination numbers higher than 4 for zinc. The new Zn-OH-Zn motif was realized for three different pyrazolylborates, for one NS2 tripod, and for two S3 tripods. Finally, it was verified that the Zn-O2H3-Zn motif again occurs only with pyrazolylborate ligands. The new complexes were identified by a total of 11 structure determinations.