Giampaolo Marangoni

The role of the non-participating groups in substitution reactions at cationic Pt(II) complexes containing tridentate chelating nitrogen donors. Crystal structure of {Pt[bis(2-pyridylmethyl)amine](py)}(CF3SO3)2

Abstract Kinetic measurements on the displacement of chloride with the nucleophiles Br−, I− and N (N=a number of isosteric pyridines and morpholine) from the substrates [Pt(NNN)Cl]+ [NNN=bis(2-pyridylmethyl)amine (bpma); 2,6-bis(aminomethyl)pyridine (dap); diethylenetriamine (dien)] have been carried out in methanol at 25°C. The results, compared with those previously obtained on the complex [Pt(terpy)Cl]+ (terpy=2,2′:6′,2′′-terpyridine), are discussed in terms of reactivity and discrimination ability of the reaction centre. The significant differences in kinetic behaviour along the series are particularly related to the presence of pyridine rings in the non-participating chelate ligand and steric effects. The study of the reverse process, i.e. the displacement of N with a chloride ion from the complexes [Pt(NNN)(N)]2+, allows the determination of the equilibrium constants from the ratio of the rate constants. The crystal structure of [Pt(bpma)(py)](CF3SO3)2 has been determined by the X-ray diffraction technique. It consists of essentially SP (square-planar) [Pt(bpma)(py)]2+ cations. The plane through the pyridine ring makes an angle of 86.1(3)° with that of Pt and the three nitrogen atoms of bpma. The packing is characterised by a hydrogen bond between the NH of the ligand and one oxygen of a triflate anion.

The reactivity of neutral nitrogen donors in planar d8 metal complexes. Part 3. The system chloro(2,2′ : 6′,2″-terpyridine)platinum(II) cation with pyridines and ammonia in methanol. Effect of basicity, π-acceptor capacity and steric hindrance

Abstract The kinetics of the forward and reverse steps of the process [Pt(NNN)Cl]+ + am ⇌ [Pt(NNN)(am)]2+ + Cl− (NNN = 2,2′ : 6′,2″-terpyridine; am = one of a number of pyridines and NH3 covering a wide range of basicity) have been studied in methanol at 25°C. Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution. The reactivity and the ability of the chloro-complex to discriminate among the nucleophiles, as well as the sensitivity of the rate of the chloro entry upon the nature of the displaced base and the steric factors in both the forward and reverse processes are discussed in terms of intimate mechanism and compared with data for a number of different PtII systems. The equilibrium constants for the reactions have been determined from the ratio of the rate constants.

Synthesis and characterization of square-planar platinum(II) and palladium(II) complexes with pyridine-2,6-dicarboxylic acid (H2dipic). X-Ray crystal structure of trans-Na2[Pt(dipic)2]6H2O and K[Pt(dipic)I]1/2H2O

Abstract The tetra-coordinate complexes trans-[Pt(Hdipic)2]·2H2O (1) and trans-[Pd(Hdipic)2]·2H2O (2) as well as their potassium and sodium derivatives trans-K2[Pt(dipic)2] (3), trans-Na2[Pt(dipic)2]·6H2O (4) and trans-Na2[Pd(dipic)2]·2H2O (5) have been obtained in high yield by reacting aqueous solutions of K2[PtCl4] and K2[PdCl4] with pyridine-2,6-dicarboxylic acid (H2dipic) or its potassium or sodium salts, respectively, in 1:2 molar ratio. 1 and 2 are also formed on reacting equimolar amounts of reagents in water (pH 1.4) leaving in solution unreacted K2[PtCl4] and K2[PdCl4]. The resulting mixtures brought to pH 5 with KOH undergo a redistribution reaction with formation of the monoanionic species K[Pt(dipic)Cl]·H2O (6) and K[Pd(dipic)Cl]·H2O (7), respectively. The iodo derivative K[Pt(dipic)l]·1/2H2O (8) was obtained from the chloro species by replacement of the coordinated chloride with iodide in aqueous solution. All the complexes have been characterized by a number of physico-chemical measurements including the X-ray analysis of trans-Na2[Pt(dipic)2]·6H2O (4) and K[Pt(dipic)I]·1/2H2O (8). Crystals of 4 are monoclinic, space group P21/c, Z=2, with unit-cell dimensions a=11.912(1), b=6.544(1), c=12.699(1) A, β=91.56(1)°. Crystals of 8 are triclinic, space group P 1 , Z=4, with unit- cell dimensions a=11.018(1), b=11.585(2), c=9.946(1) A, a=97.99(1), β=115.07(1), γ=73.14(1)°. The structures were solved from three dimensional counter data by Patterson and Fourier methods and refined by full-matrix least-squares to R=0.019 and 0.030 for 1556 and 4118 observed reflections, respectively. The crystal of 4 is built up by centrosymmetric Pt(dipic)22− anions and Na+ cations hexacoordinated by four water molecules and two carboxylate oxygens belonging to the ligands. Each dipicolinate ion behaves as a bidentate ligand, and the resulting coordination around the platinum is square planar. The asymmetric unit of compound 8 contains two independent square planar Pt(dipic)l− anions, in each of them the dipicolinate ion acts as a tridentate ligand and the fourth position is occupied by an iodine atom. The planar complex anions Pt(dipic)l− are stacked but not parallel and Pt atoms form infinite zigzag chains.

Binding ability of 2,6-bis(methylthiomethyl)pyridine with proton, palladium(II) and copper(II) in aqueous solutions

Abstract The acidity constant of the tridentate ligand 2,6-bis(methylthiomethyl)pyridine (L) and formation constants of its Pd(II) and Cu(II) complexes [PdLTu]2+ and [CuL(H2O)]2+ have been determined in aqueous solutions by potentiometric and spectrophotometric techniques. The acidity constant as determined by potentiometry is log K=4.04±0.04 (4.01±0.02 by spectrophotometry), whereas the formation constants for the Pd(II) and Cu(II) species are log K=28.92±0.09 and 4.6±0.1 (4.41±0.04), respectively. Some preliminary results on the high selectivity for Pd(II) over Cu(II) of a macroporous polystyrene-divinylbenzene resin bearing the same chelating group are also reported.

The reactivity of cationic platinum (II) complexes containing the tridentate ligand bis(2-pyridylmethyl)sulphide

Abstract The kinetics of the displacement of coordinated nitrogen donor bases (L) by chloride from complexes of the type [Pt(NSN)(L)]2+ [NSN = bis(2-pyridylmethyl) sulphide; L = a series of pyridines, isoquinoline and NH3] as well as chloride substitution from the substrate [Pt(NSN)Cl]+ by Br− and I− have been studied in methanol at 25°C and constant ionic strength and compared with those of the corresponding platinum(II) complexes containing the 2,6-bis(methylsulphanylmethyl)pyridine tridentate ligand. The two-term rate law usually found in substitutions at square-planar platinum(II) complexes is obeyed. Both the first and second-order rate constants for the displacement of L decrease as the basicity of the leaving group increases. π interactions between coordinated pyridines and the metal centre are suggested by comparison of their lability with that of ammonia as well as with that of para-substituted pyridines with enhanced π system. Steric hindrance on the leaving base also significantly decreases the reactivity.

Reactivity of uranyl ion with quinquedentate chelating hydrazine derivatives. Part 1. 2,6-Diacetylpyridine bis(2′-pyridylhydrazone)

Uranyl nitrate reacts with 2,6-diacetylpyridine bis(2′-pyridylhydrazone)(H2dapp) to give [UO2(H2dapp)(NO3)]2–[UO2(NO3)4](1) or a mixture (2) of ionic mononuclear species, in each of which H2dapp acts as a quinquedentate chelating ligand, depending on the experimental conditions. The complexes have been characterized by a number of physicochemical measurements including the X-ray analysis of (1). Crystals of (1) are triclinic with a= 14.071(9), b= 10.801(7), c= 10.122(6)A, α= 63.86(7), β= 75.65(9), γ= 78.86(9)°, space group P[graphic ommitted], and Z= 1. The structure has been solved by conventional techniques, and refined to a final R of 0.07. The unit cell contains two very highly distorted eight-co-ordinated cations [UO2(H2dapp)(NO3)]+, related by a centre of symmetry and with the N5O donor set in a symmetrically twisted ‘equatorial plane’[U–N (mean) 2.62 A, N–U–N (mean) 59.5°, and U–O 2.48 A for the unidentate nitrate group], and an uranyl tetranitrate counter anion [UO2(NO3)4]2–[two bidentate nitrates (U–O 2.53 A) and two unidentate (U–O 2.45 A)], at the centre of symmetry, in a hexagonal bipyramidal arrangement. The nature of the solvent(s), the high stabilizing effect of the 5,5,5,5 chelation mode, and reactivity tests, which allow the isolation of [UO2(H2dapp)(NO3)][BPh4](3) and [UO2(H2dapp)][ClO4]2(4), are discussed. Owing to the deprotonation of the diazapropenic sequences CN–NH–, the neutral complex [UO2(dapp)](6), containing U–N covalent bonds, can be obtained by the action of non-hydroxylated bases on (1)–(4). Like uranyl amides, compound (6) reacts with alcohols to give alkoxy-derivatives. Polymeric [{(UO2)2(H2dapp)(OCH3)4·xCH3OH}n](7) has been isolated.

Crystal structure of square-planar [1,2-bis(phenylsulfanyl)ethane]dihalogenoplatinum(II) complexes: cis-chelate effect and mutual trans influence

Abstract The crystal structures of six isomorphous [1,2-bis(phenylsulfany)ethane]dihalogenoplatinum(II) sqaure-planar neutral complexes, i.e. dichloro ( 1 ), dibromo ( 2 ), diiodo ( 3 ), chlorobromo ( 4 ), chloroiodo ( 5 ) and bromoiodo ( 6 ) species, have been determined by the single-crystal X-ray diffraction technique. Crystals are monoclinic, space group P 2 1 / c , Z =4, and their structures have been solved from three-dimensional counter data by Patterson and Fourier methods. Mixed dihalogeno species are disordered as far as the positions of the two halides are concerned and refinements have been carried out with occupancies of 0.5. The platinum atom is coordinated by means of the two sulfur atoms of the bidentate ligand and two halogen atoms in cis position. Comparison with other classes of platinum(II) complexes, carrying either chelating or monodentate thiotheric ligands and halides, allows to estimate a cis -chelate effect as well as the mutual trans influence of the different ligands.

Crystal structure of cationic square planar platinum(II) complexes containing the tridentate chelate ligand 2,6-bis(methylthiomethyl)pyridine

Abstract The crystal structures of four platinum(II) complex cations containing the organic ligand 2,6-bis(methylthiomethyl)pyridine (SNS), [Pt(SNS)Cl]+ (1), [Pt(SNS) (SCN)]+ (2), [Pt(SNS)Br]+ (3) and [Pt(SNS)Py)]2+ (4) have been determined by X-ray diffraction. In all complexes SNS behaves as a tridentate neutral ligand with the two sulphur atoms in trans positions to each other. The fourth coordination position, trans to the nitrogen atom of SNS, is occupied either by a monodentate chloride, sulphur-bonded thiocyanate, bromide or nitrogen of a pyridine molecule, respectively, in a square planar arrangement. No PtPt interactions in the linear chain are present. PtN(pyridine) bond distances are related to the charge of the complexes.

Nucleophilic reactivity in substitution reactions at planar tetra-coordinate monocationic platinum(II) complexes. Kinetics of displacement of chloride from chloro[2,6-bis(methylthiomethyl)pyridine]platinum(II) cation

Abstract The kinetics of replacement of chloride in the [Pt(SNS)Cl]+ cation [SNS = 2,6-bis(methylthiomethyl)pyridine] by a wide variety of nucleophiles [H2O, OH−, Me2SO, NO2−, NH3, pyridine (py), C5H10NH (pip), N3−, Br−, (C2H5)2S, (CH2)4S, I−, N,N,N′,N′-tetramethylthiourea (tmtu), SCN−, Ph3As, SO32−, thiourea (tu), SeCN− and CN−] have been studied in water at 25°C, μ = 0.50 mol dm−3 (LiClO4). The log k2° values (k2° is the second-order rate constant at μ = 0) do not follow the usual dependence upon the nPt° scale, while there is a good linear relationship between log k2° for the present mono-cationic substrate and log k2° for the substitution reactions of a smaller number of nucleophiles with other mono-cationic platinum(II) substrates previously studied. A new more extensive nucleophilicity scale, based on [Pt(SNS)Cl]+ as the standard substrate, that is appropriate to mono-cationic platinum(II) substrates, is proposed.

Cross-linked polystyrene resins with 2,6-diformylpyridine bis(2′-pyridylhydrazone) groups. syntheses and metal ion chelation properties

Abstract Gel-type and macroporous chloromethylated poly(styrene-divinylbenzene) have been functionalized with 2,6-diformylpyridine bis(2′-pyridylhydrazone) groups. The synthetic route involved stepwise chemical modification of the polymer support. Polymer-bound 2,6-dimethoxycarbonylpyridine was converted to the diformyl derivative via a dihydroxymethyl intermediate, and condensation of the resin containing formyl groups with 2-hydrazinopyridine yielded the required chelating functionality. The entire sequence of reactions was also carried out in solution using 4-benzyloxy-2,6-dimethoxycarbonylpyridine as a model compound. The macroporous polystyrene-based resin exhibited a higher loading capacity for Cu(11) than the gel-type resin, the maximum capacity of the former being in the range 0.54–0.57 mmol/g. The resin proved to be stable under acid stripping conditions and showed a higher affinity for Cu(II) than for Co(II), Ni(II) or Zn(II).