Ennio Zangrando

Trinuclear Metallacycles: Metallatriangles and Much More

In 1990, Fujita and co-workers described the first example of a rationally designed metallacycle, the molecular square [{Pd(en)(μ-4,4′-bipy)}4](NO3)8, prepared by self-assembly of the cis-protected square-planar Pd(II) precursor with two adjacent labile ligands [Pd(en)(ONO2)2] (en ) ethylenediamine) with the linear linker 4,4′-bipyridine (4,4′-bipy). Since then, supramolecular chemistry has produced an amazing number of fascinating 2D and 3D metal-mediated molecular architectures, including many macrocycles and cages. Several review articles have thoroughly covered this thriving field in recent years. Beside their shared structural beauty, some of these assemblies are finding applications as receptors and molecular vessels for trapping reactive intermediates, as well as for stoichiometric and catalytic reactions.

MOLECULAR TRIANGLE FROM ENPTII AND 2,2'-BIPYRAZINE

Twisting of the two pyrazine halves of the 2,2′-bipyrazine ligands appears to be responsible for the facile formation of a enPtII triangle (shown schematically on the right; en=ethylenediamine, omitted from the sketch for clarity).

Radical Pathway in Catecholase Activity with Zinc-Based Model Complexes of Compartmental Ligands

Four dinuclear and three mononuclear Zn(II) complexes of phenol-based compartmental ligands (HL(1)-HL(7)) have been synthesized with the aim to investigate the viability of a radical pathway in catecholase activity. The complexes have been characterized by routine physicochemical studies as well as X-ray single-crystal structure analysis: [Zn(2)(H(2)L(1))(OH)(H(2)O)(NO(3))](NO(3))(3) (1), [Zn(2)L(2)Cl(3)] (2), [Zn(2)L(3)Cl(3)] (3), [Zn(2)(L(4))(2)(CH(3)COO)(2)] (4), [Zn(HL(5))Cl(2)] (5), [Zn(HL(6))Cl(2)] (6), and [Zn(HL(7))Cl(2)] (7) [L(1)-L(3) and L(5)-L(7) = 2,6-bis(R-iminomethyl)-4-methylphenolato, where R= N-ethylpiperazine for L(1), R = 2-(N-ethyl)pyridine for L(2), R = N-ethylpyrrolidine for L(3), R = N-methylbenzene for L(5), R = 2-(N-methyl)thiophene for L(6), R = 2-(N-ethyl)thiophene for L(7), and L(4) = 2-formyl-4-methyl-6-N-methylbenzene-iminomethyl-phenolato]. Catecholase-like activity of the complexes has been investigated in methanol medium by UV-vis spectrophotometric study using 3,5-di-tert-butylcatechol as model substrate. All complexes are highly active in catalyzing the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ). Conversion of 3,5-DTBC to 3,5-DTBQ catalyzed by mononuclear complexes (5-7) is observed to proceed via formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically, a finding reported for the first time in any Zn(II) complex catalyzed oxidation of catechol. On the other hand, no such enzyme-substrate adduct has been identified, and 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by the dinuclear complexes (1-4) very smoothly. EPR experiment suggests generation of radicals in the presence of 3,5-DTBC, and that finding has been strengthened by cyclic voltammetric study. Thus, it may be proposed that the radical pathway is probably responsible for conversion of 3,5-DTBC to 3,5-DTBQ promoted by complexes of redox-innocent Zn(II) ion. The ligand-centered radical generation has further been verified by density functional theory calculation.

Synthesis, crystal structure and magnetic behavior of three polynuclear complexes: [Co(pyo)2(dca)2]n, [Co3(ac)4(bpe)3(dca)2]n and [{Co(male)(H2O)2}(H2O)]n [pyo, pyridine-N-oxide; dca, dicyanamide; ac, acetate; bpe, 1,2-bis-(4-pyridyl)ethane and male, maleate]

Three polymeric cobalt(II) complexes of formulae [Co(pyo)2(dca)2]n (1), [Co3(ac)4(bpe)3 (dca)2]n (2) and [{Co(male)(H2O)2(H2O)]n (3) [pyo, pyridine-N-oxide; dca, dicyanamide; ac, acetate; bpe, 1,2-bis-(4-pyridyl)ethane and male, maleate] have been synthesized and characterized structurally as well as magnetically. The structure determination of complex 1 shows that each octahedral Co(II) in the 1D coordination chain is attached with four μ-1,5-dicyanamide and two pendant pyridine-N-oxide ligands, which form mutual relationships with other 1D chains through non-covalent π-π interactions, giving rise to a 2D infinite sheet-like structure. The molecular structure reveals that complex 2 adopts an infinite three-leg ladder-like structure in which three parallel 1D Co(bpe) chains are connected by syn-syn and oxo bridging acetate ligands. The dicyanamide ligands are pendant to the terminal Co(II) centers. Complex 3 is an infinite 3D network in which carboxylate groups of the maleate ligand are linked to Co(II) centers in syn-anti fashion. The structure of complex 3 has already been reported. The variable temperature (300–2 K) magnetic measurements have been performed for all three complexes. In the case of 2, the full structure can be rationalized as quasi-isolated trimers; the exchange Hamiltonian that describes magnetic interactions between the effective S′ = 1/2 spins, at low temperature is . Fixing g⊥ = 6.01 and g∣∣ = 2.25, according to the EPR measurements at 4 K, 2J = −3.3 cm−1 is the best-fit parameter. For 1 and 3, complete fits are not possible for calculating the corresponding J parameters for the one- and three-dimensional structures, respectively. Only an approximate J value has been calculated for 1.

1-(2-Picolyl)-substituted 1,2,3-triazole as novel chelating ligand for the preparation of ruthenium complexes with potential anticancer activity

The 1,4-disubstituted 1,2,3-triazole ligand prepared by click chemistry 1-(2-picolyl)-4-phenyl-1H-1,2,3-triazole (ppt) was investigated as novel chelating ligand for Ru(II) complexes with potential antitumor activity. The preparation and structural characterization, mainly by NMR spectroscopy in solution and by X-ray crystallography in the solid state, of four new Ru(II) complexes is reported: two isomeric Ru-dmso compounds, trans,cis-[RuCl(2)(dmso-S)(2)(ppt)] (1) and cis,cis-[RuCl(2)(dmso-S)(2)(ppt)] (2), and two half-sandwich Ru-[9]aneS(3) coordination compounds, [Ru([9]aneS(3))(dmso-S)(ppt)][CF(3)SO(3)](2) (3) and [Ru([9]aneS(3))Cl(ppt)][CF(3)SO(3)] (4). In all compounds ppt firmly binds to ruthenium in a bidentate fashion through the pyridyl nitrogen atom and the triazole N2, thus forming a puckered six-membered ring. The chemical behavior in aqueous solution of the water-soluble complexes 3 and 4 was studied by UV-Vis and NMR spectroscopy and compared to that of the previously described organometallic analogue [Ru(η(6)-p-cymene)Cl(ppt)][Cl] (5) in view of their potential antitumor activity. Compounds 3-5 were tested also in vitro for cytotoxic activity against two human cancer cell lines, one sensitive and one resistant to cisplatin, in comparison with cisplatin. Compound 4, the one that aquates faster, was found to be more cytotoxic than cisplatin against human lung squamose carcinoma cell line (A-549).

Polymeric networks of copper(II) using succinate and aromatic N–N donor ligands: synthesis, crystal structure, magnetic behaviour and the effect of weak interactions on their crystal packing

Four succinato-bridged complexes of copper(II) have been synthesized. Complex 1, [Cu(2)(mu-OH(2))(2)L(bpy)(2)(NO(3))(2)](n) and 2, [Cu(2)(mu-OH(2))(2)L(phen)(2)(NO(3))(2)](n)(bpy = 2,2[prime or minute]-bipyridine; phen = 1,10-phenanthroline and LH(2)= succinic acid) exhibit 1D coordination polymer structures where both the nitrate ions are directly linked to the copper(ii) producing synthons in a 2D sheet. A novel 2D grid-like network, ([Cu(4)L(2)(bpy)(4)(H(2)O)(2)](ClO(4))(4)(H(2)O))n3, is obtained upon changing the nitrate by perchlorate anion in complex 1, where the channels are occupied by the anions. On changing the nitrate by tetrafluoroborate anion in complex 2, a novel octanuclear complex, [Cu(8)L(4)(phen)(12)](BF(4))(8).8H(2)O 4, is isolated. The coligand bpy and phen in these complexes show face-to-face (in 1,2,3,4) or edge-to-face (in 4 )pi-pi interactions forming the multidimensional supramolecular architectures. Interestingly, the appearance of edge-to-face pi-pi interactions in complex facilitates the formation of discrete octanuclear entities. Variable-temperature (300-2 K) magnetic measurements of complexes have been done. Complexes 1 and 2 show very weak antiferromagnetic (OOC-CH(2)-CH(2)-COO) and ferromagnetic coupling (mu-H(2)O). Complex 3 also shows antiferromagnetic (syn-syn mu-OCO), and ferromagnetic coupling (mu-O of the -COO group). Complex 4 with two types (syn-syn and syn-anti) of binding modes of the carboxylate group shows strong antiferromagnetic interaction.

Coordination and release of NO by ruthenium–dimethylsulfoxide complexes—implications for antimetastases activity

Abstract Several Ru(II) and Ru(III)–dimethylsulfoxide (DMSO) complexes, that are not cytotoxic in vitro, are endowed with anticancer and, in particular, antimetastatic activity against animal tumor models. One possibility for explaining the activity of such compounds against disseminated tumors is that they interfere with NO metabolism in vivo. Thus, we investigated the reactivity of ruthenium–chloride–DMSO complexes towards NO with the aim of producing well-characterized models to be used as reference compounds in subsequent biomimetic studies. In this contribution, we report on the synthesis, spectroscopic, structural and electrochemical characterization of anionic (e.g. [(DMSO)2H][trans-RuCl4(DMSO-O)(NO)] (1)), neutral (e.g. mer,cis-RuCl3(DMSO-O)2(NO) (2)) and new cationic (e.g. [cis,fac-RuCl2(DMSO-O)3(NO)][BF4] (9)) Ru–DMSO nitrosyls, derived from both Ru(II) and Ru(III)–chloride–DMSO precursors. Coordination of the strong π-acceptor NO favors coordination of DMSO ligands through oxygen (DMSO-O) to avoid competition for π electrons. The reactivity of some Ru–DMSO–NO complexes towards heterocyclic N-ligands, leading to compounds such as [(Im)2H][trans-RuCl4(Im)(NO)] (Im=imidazole, 6) and [cis,mer-RuCl2(py)3(NO)][BF4] (py=pyridine, 10), is also described. The spectroscopic and X-ray structural features for all these complexes are consistent with the {Ru(NO)}6 formulation, that is a diamagnetic Ru(II) nucleus bound to NO+. Electrochemical measurements on the Ru–NO complexes showed that they are all redox active in DMF solutions and the site of reduction is the NO+ moiety. With the exception of 10, the reduced complexes are not stable and rapidly release the NO radical.

STRUCTURAL ASPECTS OF PT COMPLEXES CONTAINING MODEL NUCLEOBASES

Abstract Compounds of Pt(II) and Pt(IV)-containing nucleobases as ligands (also including a few related ligands) are summarized and described, according to the nuclearity of the complexes. The large amount of available crystallographic data allows geometrical parameters such as bond lengths and angles, angular distortions, torsional angles related to the nucleobase plane orientations etc., to be derived with relatively high accuracy. Simple relationships between some of these parameters are reported and discussed. On mononuclear Pt complexes containing one or more nucleobase ligands a simple descriptive statistical analysis has been performed. The structural properties of polynuclear, often heteronuclear species have also been reviewed where the nucleobases, particularly pyrimidines, act as polydentate-often bridging-ligands. A simple MO theoretical analysis of the metal-metal interaction in homo- and heterodinuclear species allows the degree of the intermetallic bond formation to be rationalized and the correlation of the qualitative results with the experimental metal-metal distances. Cyclic polynuclear species, which appear to be a new expanding field in the chemistry of Pt-nucleobase complexes, are also described.

A novel bimetallic alternating chain: synthesis, crystal structure and magnetic study

A novel one dimensional (1D) bimetallic polymeric complex [ catena -bis(μ- N -(aminoethyl)-3-aminopropanolato)-di-copper(II)-tetracyanonickelate(II)dihydrate] has been synthesized by the reaction of 1:1 mixture of CuCl 2 ·2H 2 O and the hydoxyamine ligand, [ N -(3-hydroxypropyl)ethane-1,2-diamine] with half equivalent of [Ni(CN) 4 ] −2 in aqueous solution. X-ray quality single crystals were obtained by slow evaporation of the ammoniacal solution of the complex. The single crystal structure reveals that it is an alkoxo bridged Cu(II) dimer linked through [Ni(CN) 4 ] 2− in trans -fashion resulting in a 1D chain. The properties of the complex have been studied by variable temperature magnetic susceptibility measurements. The susceptibility data fit well with the Cu(lI) dimer equation to give 2 J =−622 cm −1 , g =2.05. The very high coupling parameter value clearly indicates the presence of very strong antiferromagnetic interaction between the Cu(II) ions.

Isolation of four new CoII/CoIII and NiII complexes with a pentadentate Schiff base ligand: syntheses, structural descriptions and magnetic studies

In this paper we report the temperature and pH dependent syntheses and systematic characterization of four new Co(II)/Co(III) and Ni(II) complexes with a pentadentate Schiff base ligand H(3)L obtained by condensing 1,3,-diaminopropan-2-ol with 2-hydroxyacetophenone in 1:2 molar ratio. The room temperature syntheses involving Co(II) and Ni(II) nitrates and the ligand H(3)L lead to the isolation of the dinuclear species [Co(2)L(2)(H(2)O)] (1), and the mononuclear complex [Ni(LH)] (3), respectively, whereas refluxing at basic pH leads to the tetranuclear complexes, [Co(II)(2)Co(III)(2)L(2)(μ(3)-OMe)(2)(NO(3))(H(2)O)(2)]NO(3)·2(H(2)O) (2), and [Ni(4)L(2)(μ(3)-OMe)(2)(H(2)O)(2)]·2H(2)O (4). 1 is found to be a simple mono alkoxo-bridged Co(III) dinuclear species, whereas 2 and 4 are both rhomb-like tetrameric complexes with double oxo bridges and μ(3)-methoxo bridges, derived from the methanol solvent, in an open dicubane arrangement. Moreover 2 shows six coordinate ordered Co(II) and Co(III) ions and 4 has both six- and five-coordinate Ni(II) centers. Compound 3 is assigned a tentative mononuclear structure based on IR, UV-Vis spectroscopic, (1)H-NMR and ESI mass study results and is supposed to have one Ni(II) center coordinated with a ligand fragment in square planar geometry. The variable temperature magnetic susceptibility study for 2 and 4 is performed which indicate for both 2 and 4 the presence of intracluster dominant ferromagnetic interactions.