Matteo Cusumano

Photochemistry of planar four-co-ordinate palladium(II) complexes. Part 2. trans–cis Isomerization of dihalogenobis(tri-n-propylphosphine)palladium(II) in solution

The photoisomerization of trans-dihalogenobis (tri-n-propylphosphine)palladium(II) complexes has been studied in various solvents. The quantum yields depend strongly both on the nature of the co-ordinating halide and on the excitation wavelength. The photochemical results are discussed in terms of the possible mechanisms for the trans–cis photoisomerization of square-planar complexes. Photolysis of cis-[Pd(PPrn3)2Cl2] yields the chloro-bridged complex, [Pd2(PPrn3)2Cl4].

Kinetics and thermodynamics of fast square-planar–octahedral inter-conversion. Part 1. The reaction of [biacetyl bis-α-hydroxybenzylidene-hydrazone(2–)-N1N1′OO′]nickel(II) with some pyridines in chlorobenzene

A kinetic study has been made of the equilibrium (i)[bbh = biacetyl bis-α-hydroxybenzylidenehydrazone(2–)-N1N1′OO′ L = pyridine, 3-methylpyridine, and 4-methylpyridine] in chlorobenzene by using a microwave temperature-jump apparatus. [graphic omitted] + 2L ⇌ [graphic omitted] (i) The experimental results are consistent with the mechanism in (ii) and (iii) where (ii) is the rate-determining step. [Ni(bbh)L2] [graphic omitted] [Ni(bbh)L]+ L (ii), [Ni(bbh)L]⇌[Ni(bbh)]+ L (iii) Activation Parameters associated with the rate constant k1, are in agreement with the dissociative nature of the reaction.

Kinetics and thermodynamics of fast square-planar–octahedral inter-conversion. Part 3. The reaction of [biacetyl bis-α-hydroxybenzyl-idenehydrazone(2–)-N1N1′OO′] nickel(II) with bidentate heterocyclic amines in chlorobenzene

The square-planar complex [Ni(bbh)][bbh = biacetyl bis-α-hydroxybenzylidenehydrazone (2–)-N1N1′OO′] reacts with 2,2′-bipyridine and substituted 1,10-phenanthrolines to give octahedral adducts in which, presumably, the ligand bbh behaves as a tridentate ligand and a hydroxy-group of the water present in the solvent occupies one of the positions around the nickel atom. A comparative thermodynamic and kinetic study of these reactions has been made. The mechanism proposed implies a nickel-to-oxygen bond breaking as the rate-determining step. Thermodynamic and activation parameters associated with the reactions are discussed.

Kinetics and thermodynamics of fast square-planar–octahedral inter-conversion. Part II. Influence of the basicity and steric hindrance of heterocyclic amines in the reaction with [biacetyl bis-α-hydroxybenzyl-idenehydrazone(2–)-N1N1′OO′]nickel(II) in chlorobenzene

A comparative thermodynamic and kinetic study has been made of reaction (i)[bbh = biacetyl bis-α-hydroxy- [graphic omitted] (i) benzylidenehydrazone(2–)-N1N1′OO′ L = 4-cyano-, 3-chloro-, 2-methyl-, 2,4-dimethyl-, and 3,5-dimethyl-pyridine, morpholine, or piperidine] in chlorobenzene. Apart from a drastic steric-hindrance effect, variations of the structure and basicity of the amine have little effect on the values of the rate constant k1, which refers to the release of one molecule of amine from the octahedral complexes as in (ii). The values of k1, are correlated with the [Ni(bbh)L2] [graphic omitted] [Ni(bbh)L]+ L (ii) equilibrium constants K for reaction (i). Rate and activation parameters associated with reactions (ii) are discussed. Thermodynamic parameters ΔH⊖, ΔS⊖, and δG⊖ obtained for reaction (i) in chlorobenzene are also discussed and compared with the analogous values for reaction in benzene.

Fast reactions at planar four-co-ordinate complexes. Part I. Nucleophilicity towards palladium(II) complexes

Rates of nucleophilic substitution reactions (i)[L = PPrn3, AsEt3, or piperidine, C5H11N; Y–= Cl–, N3–, Br–, I–, SCN–, or (NH2)2CS] have been measured in methanol at 25 °C. A set of nucleophilic reactivity constants, trans-[PdL2(NO2)2]+ Y–→trans-[PdL2(NO2)Y]+ NO2–(i)nPd0= log (k2[MeOH]/k1), have been calculated, k1 and k2 referring to rate constants for the solvolytic path and for direct nucleophilic attack on the complex trans-[Pd(PPrn3)2(NO2)2]. All the complexes studied obey the linear free-energy relation log k2=snPd0+ log k1, where s is a nucleophilic discrimination factor. The nPd0 sequence resembles that already found for platinum(II) complexes.

Fast reactions at planar four-co-ordinate complexes. Part 4. The reaction of chelate dichloropalladium(II) complexes with some bidentate ligands

The reaction between the square-planar complexes [Pd(L–L)Cl2][L–L = 1,2-bis(diphenylphosphino) ethane, o-phenylenebis (dimethylarsine), ethylenediamine, 1,10-phenanthroline, 2,2′-bipyridine, or 1,2-bis(phenylthio)ethane] and ethylenediamine proceeds in two consecutive stages yielding [Pd(en)2]Cl2 as a final product. The intermediate species is [Pd(L–L)(en)]Cl2 or [Pd(en)Cl2] when L–L is 1,2-bis(phenylthio)ethane. The rate of replacement of the first chloride co-ordinated to palladium(II) in the substrates [Pd(L–L)Cl2] at 25 °C in dimethylformamide is strongly dependent on the nature of the ligand L–L, covering four orders of magnitude. The reaction between the substrates [M(bipy)Cl2](M = Pd or Pt, bipy = 2,2′-bipyridine) and the bidentate nucleophiles ethylenediamine and dithio-oxamide proceeds according to an analoguous two-stage reaction scheme when M is palladium; in the platinum derivatives, however, the chelate bipyridine is inert towards substitution. In the solvent dimethylformamide the entering group ethylenediamine behaves as a very effective nucleophile.

Kinetics of the displacement of chloride in 5-substituted-1,10-phenanthrolinedichloropalladium(II) complexes

The rate constants for the displacement of the first chloride by the nucleophiles Br–, I–, SCN–, and SC(NH2)2 in the complexes [Pd(L–L)Cl2](L–L = 1,10-phenanthroline, 5-chloro-1,10-phenanthroline, 5-nitro-1,10-phenanthroline, or 5-methyl-1,10-phenanthroline) have been determined in dimethylformamide at 25 °C. The rate constants, as well as the discriminating ability of the complexes, increase on decreasing pKa of the unreplaceable ligand L–L.

Thermally reversible photoisomerization of a square-planar trans-diphosphinedichloropalladium(II) complex to yield its elusive cis-isomer

U.v. irradiation of trans-bis(tri-n-propylphosphine)dichloropalladium(II) in solution yields a photostationary trans–cis mixture which reverts quantitatively to the trans-form in the dark with an exceptionally high activation energy; the hitherto elusive cis-form has been characterised by u.v., i.r., and 31P n.m.r. spectroscopy and is relatively stable in solution (t½ 103 h) at 273 K.

Displacement of organic sulphides by amines in trans-dichlorobis(organic sulphide)palladium(II) complexes

The kinetics of nucleophilic displacement of a variety of organic sulphides RSR′ from complexes of the type trans-[Pd(RSR′)2Cl2] by amines of different basicity and steric hindrance have been measured in 1,2-dimethoxyethane at 25 °C. The overall reaction occurs in two separate stages, the first being much faster than the second. The investigated processes correspond to the reaction: trans-[Pd(RSR′)2Cl2]+ amine →trans-[Pd(RSR′)(amine)-Cl2]+ RSR′(amine = pyridine or substituted pyridine). The nucleophilic substitutions obey the usual two-term rate equation. The rate of the reactions, the discrimination of one complex against the individual entering amines, and the lability of the individual sulphides when they are displaced by a given amine are compared and discussed.

Fast reactions at planar four-co-ordinate complexes. Part 3. Kinetics and mechanism of substitution reactions of sterically hindered aniono-(mesityl)bis(triethylphosphine)nickel(II) complexes

The kinetics of displacement of ligand X from the complexes trans-[Ni(PEt3)2(C6H2Me3-2,4.6)X] by a variety of nucleophiles Y–(X = Cl, Br, I, N3, or NO2; Y = Cl, Br, I, NO2, SCN, or N3) has been studied in ethanol at 25 °C. The results suggest that the substitution proceeds by means of two competing paths, one associative and the other solvolytic. The kinetic parameters are compared with those relating to the reactions of the analogous complexes trans-[Ni(PEt3)2(C6H4Me-o)X] and trans-[Pt(PEt3)2R(Cl)](R = mesityl or o-tolyl) under the same experimental conditions.