Lucio Cattalini

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.

Five-coordinate ethylene complexes of platinum(II) containing different types of n-donor bidentate ligands

Abstract Diimines, aliphatic and aromatic diamines react with Zeises salt to give stable five-coordinate complexes of general formula [Pt(C 2 H 4 )Cl 2 (NN)], which have trigonal bipyramidal structure with the chlorine atoms in the apical positions and the bidentate ligand in the equatorial plane. The five-coordinate species decompose releasing ethylene and giving square-planar complexes of formula [PtCl 2 (NN)].

The infrared spectra (450–70 cm−1) of square planar pyridine an substituted pyridine complexes of gold(III)

Abstract A series of gold(III) complexes of the type AuX 3 . L, where L = pyridine, pyridine-d 5 , 2-, 3- or 4-methyl pyridine, 2,6-, 3,5- or 2,4-lutidine, or 4-cyanopyridine and X = Cl or Br, have been prepared, the bromo-complexes for the first time, and their infrared spectra have been recorded in the range 450–70 cm −1 as nujol mulls. The metal-halogen stretching vibrations in the complexes are assigned, and compared with the corresponding vibrations in the parent AuX 4 − ions. The metal-base stretching vibrations are also assigned where possible, and an attempt is made to correlate these with the strenghts of the bases.

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.

NUCLEOPHILICITY OF THIOLS TOWARDS PLANAR TETRACOORDINATED PLATINUM(II) COMPLEXES

The nucleophilicity of the undissociated thiols is compared to that of thioethers in the displacement of chloride ion from the PtII complexes [Pt(bipy)(NO2)Cl] (bipy=2,2′-bipyridine) and [Pt(terpy)Cl]+(terpy= 2,2′-6,2′′- terpyridine). As far as the neutral substrate is concerned the second order rate constants for RSH and RSR nucleophiles are equally related to the inductive and steric effects of the groups linked to the sulphur donor atom. By contrast, only thiols are reactive (under standard experimental conditions) towards the cationic substrate, and the difference can be explained, in this case, by the immediate deprotonation of the substituted product forcing the reaction to completion. The reverse reaction has been examined in one case (PhSH) and the complete protonation of the coordinated thiolate species requires [H+]>2 mol·dm−3. In both series of reactions, thiol molecules also containing a second acidic group (–OH, –CO2H, –NH3+) capable of facile interaction via hydrogen bonding with the developing chloride in the transition state, exhibit a reactivity larger than that expected on the basis of electronic and steric effects alone.

Uranyl complexes containing polydentate schiff bases

Abstract A number of uranyl complexes containing polydentate Schiff bases and neutral ligands have been synthesised in three different ways and characterized by physico-chemical measurements. The geometry of coordination of UO22+ and the relative tendencies of the ligands to bond to uranyl are discussed.

Kinetics of the displacement of cyclobutane-1,1-dicarboxylate from diammine(cyclobutane-1,1-dicarboxylato)platinum(II) in aqueous solution

The displacement of 1,1-cyclobutanedicarboxylate (cbdca2–) from [Pt(NH3)2(cbdca)] has been studied in aqueous solution. In the presence of acid the process resembles the successive displacement of two monodentate carboxylates. The first (ring-opening) stage follows the rate law kobs.=(k0+k1k0[H+])(1 +k0[H+])–1, k0= 8 × 10–5 s–1, K0= 0.6 dm3 mol–1, k1= 8.0 × 10–4 s–1 at 25 °C, while the second follows the simple relationship kobs.=k[H+], k= 1.61 × 10–4 dm3 mol–1 s–1 at 25 °C. In the absence of acid and other nucleophiles the complex is inert and in the presence of chloride the displacement of ligand follows a first-order dependence on [Cl–], kobs.=kCl[Cl–]. At 80 °C, kCl= 1.32 × 10–4 dm3 mol–1 s–1. The chelate differs from the bis-monodentate carboxylate species in the great importance of the reverse, ring-closing process, which can be prevented in the presence of acid.

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.

Decomposition of [Pt(C2H4)Cl2(bipyridyl)] to [PtCl2(bipyridyl)] and ethylene in different solvents

Abstract The decomposition of the 5-coordinate complex [Pt(C2H4)Cl2(bipy)] to the 4-coordinate [PtCl2(bipy)] and ethylene has been studied in 1,2-dichloroethane and in aqueous methanol (95% v/v). The reaction proceeds with different mechanisms in the two solvents. In 1,2-dichloroethane the 5-coordinate substrate decomposes in a single step with ΔH‡ = 18.6 ± 0.6 Kcal mol−1 and ΔS‡ = −8 ± 2 cal K−1 mol−1. In aqueous methanol [Pt(C2H4)Cl2(bipy)] gives the cationic ethylene complex [Pt(C2H4)Cl(bipy)]+ which then reacts with chloride ion in a slow bimolecular process to form [PtCl2(bipy)] and ethylene.

Reactivity of neutral nitrogen donors in planar d8 metal complexes. Part 2. The system [2,6-bis(methylsulfanyl-methyl)pyridine]chloroplatinum(II) with pyridines and amines in methanol. Effect of basicity, π-acceptor capacity and steric hindrance

The kinetics of the forward and reverse steps of the process [Pt(C5H3N(CH2SMe)2}Cl]++ am ⇌[Pt{C5H3N(CH2SMe)2}(am)]2++ Cl–[C5H3N(CH2SMe)2= 2,6-bis(methylsulfanylmethyl)pyridine; am = one of a number of pyridines and some heterocyclic and aromatic amines covering a wide range of basicity] has 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 second-order rate constants for the forward reactions, k2f, show only a slight dependence upon the basicity of the entering am, and steric hindrance markedly decreases the reactivity. The second-order rate constants for the reverse reactions, k2r, are very sensitive to the nature of the leaving group and plots of log k2r against the pKa, of the conjugate acids of unhindered pyridines with different π systems are linear with a slope of –0.45. A comparison among the different pyridines and sp3 nitrogen donor bases indicates an appreciable π contribution to the stability of the Pt–N(sp2) bond. The equilibrium constants for the reactions have been determined in a number of cases from the ratio of the rate constants and a plot of log K against the pKa is linear with a slope of 0.66. The results are compared with data from the literature.