Raffaello Romeo

Dissociative Pathways in Platinum(ll) Chemistry

Abstract Steric and electronic factors favor the addition of a fifth ligand to a square planar d8 complex to form five coordinate species either as discrete compounds or reaction intermediates. The search for factors promoting the conversion of the normal associative mode of reaction into a dissociative process has attracted much attention. The attempt to achieve this goal by using bulky ancillary ligands has been unsuccessful. More convincing evidence in favor of a dissociative mechanism involving a Pt(II) 3-coordinate intermediate comes not from classical Werner compounds but from reactions involving organometallic substrates. It is now reasonably clear that the formation of such 3-coordinate 14-electron compounds offers a favorable reaction route to a number of fundamental processes as an alternative to the intermediacy of 4- and 5-coordinate species. Among them, the uncatalyzed cis to trans isomerization of complexes of the type cis-[Pt(PE3)2(R)X] (R = alkyl or aryl groups; X = halide ion), β-hydride ...

Platinum(II) complexes containing dimethylsulphoxide and linear aliphatic diamines formation of a seven-membered chelate ring

Abstract The synthesis and properties of complexes of the type [Pt(diamine)(dmso)Cl]Cl containing chelate rings of increasing size are reported (dmso = dimethylsulphoxide; diamine = 1,2-diamminoethane (en), 1,3-diaminopropane (tn), and 1,4-diaminobutane (bn)). The 1,4-diaminobutane complex contains the rate seven-membered ring and is formed together with a binuclear complex, trans-[Pt(dmso)Cl2]2(NH2(CH2)4NH2), in which the amine bridges two platinum atoms. The compounds have been characterised by analysis, conductance, spectroscopic and nuclear magnetic resonance measurements. On heating the en and the tn complexes under reduced pressure dimethylsulphoxide is lost and the corresponding uncharged [Pt(diamine)Cl2] species is formed.

Micellar aggregates of platinum(II) complexes containing porphyrins

Abstract The complex [Pt(Cy2dim)Me]4(TpyP)(CF3SO3)4 (1) (Cy2dim=dicyclohexyldiimine; TpyP=5,10,15,20-tetrakis(4-pyridyl)-21H,23H-porphine) has been synthesized and characterized with a number of spectroscopic techniques. The solubilization of 1 in anionic (sodium dodecyl sulfate, SDS) and neutral (Triton X-100, TX-100) aqueous surfactant solutions has revealed two different modes of interaction: (i) aggregation in the hydrophobic region of SDS micelles; and (ii) aggregation in the hydrophobic region with partial intercalation in the solvent accessible area of TX-100 micelles. In the case of SDS, the formation of premicellar aggregates has been evidenced by a surfactant concentration range two orders of magnitude lower than the critical micellar concentration (cmc). The metalated Mn(III)TpyP derivative of complex 1 exhibits a moderate peroxidase activity in agreement with the aggregation models proposed for the micellar solutions.

Role of halide ion in the mechanism of protonolysis of the PtC bond in Pt(II) alkyl and aryl complexes

Abstract A mechanistic study is described for the electrophilic cleavage of the Pt C σ bond in complexes cis -[PtPh 2 (PEtPh 2 ) 2 ] and trans -[PtXR(PEt 3 ) 2 ] (X = Cl and Br, R = Me, Et, n-Pr, n-Bu, CH 2 Ph) by the proton in the presence of halide ions in aqueous methanol (MeOH/H 2 O, 91v/v) which yields cis -[PtClPh(PEtPh 2 ) 2 ] and trans -[PtX 2 (PEt 3 ) 2 ], respectively. The reactions are first-order in substrate for both systems and the general bivariate rate law k obs = [H + ]{ k H + k x K [[X − ]}/(1 + K [[X − ]) is obeyed. The proposed mechanism involves a fast pre-equilibrium formation (K) of a platinum(II) anionic intermediate via interaction of the halide with the square-planar substrate, combined with slow, parallel protonation of both the substrate ( k H ) and the intermediate ( k x ), causing the cleavage of the metal carbon σ bond. The intermediate should be conceived of as weak association product, since all attempts at measuring the constant K by UV, visible and 31 P{ 1 H} NMR spectroscopy were unsuccessful. This mechanism will rationalize the previously reported diverse kinetic results for the protonolysis of Pt C σ bonds, within a unified picture which takes into account the electronic and steric properties of the group to be cleaved and of ancillary ligands. The role of the halide ion can be put into evidence by the use of substrates that will promote association through a reduced electron density on the central metal.

Structural properties of the metallointercalator cationic complex (2,2′:6′,2″-terpyridine)methylplatinum(II) ion

Abstract The complex [Pt(tpy)(Me)]+(BPh4)− (tpy=2,2′:6′,2′′-terpyridine) crystallizes in the triclinic space group P1 with a=13.463(2) A, b=13.618(3) A, c=20.151(4) A, α=73.59(2), β=74.56(3), γ=65.82(2)° and Z=4. The final conventional R factor is 0.035. In the unit cell a couple of weakly interacting dimers are formed by stack of two [Pt(tpy)(Me)]+ cations in a head-to-tail fashion with intermolecular Pt⋯Pt distances of 4.437(1) and 4.931(1) A, respectively. The absorption spectra of [Pt(tpy)(Me)]+(BPh4)− in acetonitrile show bands assigned to π–π* and to MLCT transitions. The analysis of the dependence of the spectra on complex concentration gives a fairly low value for the dimerization equilibrium constant (Kd=180±52 M−1 at 298 K). These fluid solutions are not emissive. The room-temperature solid-state emission spectra of the salts [Pt(tpy)(Me)]X are strongly dependent on the counterion (X=BPh4−, Cl−, PF6−, ClO4−, CF3SO3−). The cationic complex shows a considerable stability upon acidification and carbonilation in water.

Geometrical configuration of monomethyl–platinum(II) complexes driven by the size of entering nitrogen ligands

The reaction of the monoalkyl complex trans-[Pt(DMSO)2Cl(CH3)] with a large variety of heterocyclic nitrogen bases L, in chloroform solution, leads to the formation of uncharged complexes of the type [Pt(DMSO)(L)Cl(CH3)], containing four different groups coordinated to the metal center. Only two out of the three different possible isomers were detected in solution. These two trans(C,N) and cis(C,N) species can be unambiguously identified through 1H NMR spectroscopy. For the trans(C,N) isomers, average values of 2JPtH=75±4 Hz and 3JPtH=36±4 Hz have been observed for the coordinated methyl and DMSO ligands, respectively. In the case of the cis(C,N) isomers, these values increase to 2JPtH=83±2 Hz, and decrease to 3JPtH=26±3 Hz due to the mutual exchange of ligands in trans position to CH3 and DMSO. In the case of bulky asymmetric ligands, such as quinoline, 2-quinolinecarboxaldehyde, 2-methylquinoline, 5-aminoquinoline, 2-phenylpyridine and 2-chloropyridine, slow rotation of the hindered group around the PtN bond makes the coordinated DMSO ligand prochiral. NMR experiments have shown that the first reaction product is the trans(C,N) isomer as a consequence of the very fast removal of one DMSO ligand by the nitrogen bases from the starting complex trans-[Pt(DMSO)2Cl(CH3)]. This trans kinetic product undergoes a geometrical conversion into the more stable cis(C,N) isomer through the intermediacy of fast exchanging aqua-species. The rate of isomerization and the relative stability of the two isomers depends essentially on the rate of aquation and on the steric congestion imposed by the new L ligand on the metal.

Ion-pair mechanism in square planar substitution. Reactivity of cationic platinum (II) complexes with negatively charged nucleophiles in solvents of high, medium and low polarity☆

The rates of displacement of dimethyl sulfoxide from the cation [Pt(phen) (CH3) (Me2SO)]+ by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) mixture. The starting complex and the reaction products were characterized either as solids or in solution by their IR and 1H NMR spectra. The substitution reactions take place by way of a direct bimolecular attack of the ligand on the substrate. The sequence of reactivity observed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en) (NH3)Cl]+ used as standard). The difference of reactivity between the first (t-BuNH2) and the last (SeCN−) members of the series spans five orders of magnitude. The value measured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series of protic and dipolar aprotic solvents, using chloride ion as nucleophile, the rate of formation of [Pt (phen) (CH3)Cl] is dominated by the extent of solvation of Cl−, as measured by its values of the Gibbs molar energy of transfer ΔtG0. Conductivity measurements at 25°C in dichloromethane were fitted to the Fuoss equation and the values of the dissociation constants Kd for the ion pairs were calculated as follows: 2.27 × 10−5 M for Bu4NCl, 2.75 × 10−5 M for Bu4NSCN and 17.05 × 10−5 M for [Pt(phen) (CH3) (Me2SO)]PF6. The pseudo-first-order rate constants kobs for the reactions with Bu4NCl, Bu4NBr, Bu4NSCN and Bu4NI showed a curvilinear dependence on the concentration of the salt which levels off very soon (at concentrations higher than 0.005 M the kinetics are zero order in [Bu4NX]). On addition of the inert electrolyte Bu4NPF6 the rates slow down and the kinetics follow the rate law kobs = kKip[Bu4NX]/[Bu4NPF6] + Kip[Bu4NX]). These findings fit well with a reaction scheme which involves a pre-equilibrium Kip between ion pairs, followed by unimolecular substitution within the contact ion pair [Pt(phen) (CH3) (Me2SO)X]ip. Values of the equilibrium constants Kip for ion-pair exchange and of the internal substitution rates k were derived. The latter showed that the discrimination in reactivity between Cl−, Br−, SCN− and I− is greatly reduced with respect to aqueous solutions. The reason behind this may be desolvation of the ions coupled to the fact that a contact ion pair is already at a certain distance along the reaction coordinate in the direction of the transition state. Applications of the special salt effect and of ion pairing to synthesis are discussed.

Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a Platinum(II) Complex

Complexes of the type cis-[PtPh2(CO)(η1-P−N)] and cis-[PtPh2(CO)(η1-P−S)], where bidentate phosphorus−nitrogen and phosphorus−sulfur ligands are bound to the metal centre in a monodentate fashion [P−N = Ph2PC5H4N (Ph2PPy), Ph2P(CH2)2C5H4N (ppye), Ph2P(o-C6H4)NMe2 (PNMe2), Ph2P(CH2)nNMe2 (n = 2, 3, i.e., peNMe2 and ppNMe2) and P−S = Ph2P(CH2)2SEt (P−SEt), Ph2P(CH2)nSPh (n = 1, 2, i.e., P−CH2SPh and P−SPh)], were prepared in situ by reaction of the hybrid ligands with cis-[PtPh2(CO)(SEt2)]. In each case, the first observed process was the fast substitution of diethyl sulfide by the phosphanyl group leading to the monosubstituted ring-open cis-[PtPh2(CO)(η1-P−X)] (X = N or S) complexes, which were characterised in solution by 1H and 31P{1H} NMR spectroscopy. These initially formed species undergo a slow ring closure process with extrusion of carbon monoxide and formation of the chelate [PtPh2(P−X)] products, except in the case of the short-bite Ph2PPy and P−CH2SPh ligands and of ppNMe2, where ring closure was not observed. The chelate complexes were isolated as solids from the reaction of the ligands with cis-[PtPh2(Me2SO)2]. A single-crystal X-ray diffractometric study of cis-[PtPh2(P−SEt)] (18) was performed. The crystal packing showed linear chains originated by weak intermolecular Pt···H−C hydrogen bonding interactions. The chelation kinetics of P−X in the cis-[PtPh2(CO)(η1-P−X)] complexes have been monitored in [D]chloroform by 1H and 31P{1H} NMR. The rates of ring closure were found to be strongly dependent on the nature (S or N) and steric hindrance of the chelating end of the monocoordinated bidentate P−X ligand, and on the size of the ring formed. In contrast, ring size plays a negligible role, if any, in the dechelation reactions of cis-[PtPh2(S−S)] [S−S = 1,2-bis(phenylthio)ethane, dpte and 1,3-bis(phenylthio)propane, dptp] using diphosphanes (dppm and dppp) as reagents. These kinetic data, together with those of previous work, give useful insight into the factors controlling cyclisation reactions and the stability of the rings in square planar platinum(II) complexes.

Ligand exchange and substitution at platinum(II) complexes: evidence for a dissociative mechanism

Abstract Square-planar complexes of the type cis -[Pt(Me) 2 (Me 2 SO)(PR 3 )] ( 1 – 6 ) where PR 3 represents a series of isosteric tertiary phosphanes [P( 4 -MeOC 6 H 4 ) 3 , P( 4 -MeC 6 H 4 ) 3 , P(C 6 H 5 ) 3, P( 4 -FC 6 H 4 ) 3 , P( 4 -ClC 6 H 4 ) 3 , P( 4 -CF 3 C 6 H 4 ) 3 ] have been synthesised and fully characterised through elemental analysis, 1 H and 31 P{ 1 H} NMR. The coupling constants 1 J PtP with the isotopically abundant 195 Pt (33%, I =1/2) of 1 – 6 , as those of the pyridine cis -[Pt(Me) 2 (py)(PR 3 )] derivatives ( 7 – 12 ), show linear dependencies on the basicity of the coordinated phosphane. The rates of dimethyl sulfoxide exchange for all the complexes have been measured at relatively low temperatures by 1 H NMR isotopic labelling experiments with deuterated chloroform as the solvent. Pyridine for dimethyl sulfoxide substitution has been studied at higher temperatures through conventional spectrophotometric techniques. The rates of both processes show no dependence on ligand concentration, for each complex the value of the rate of ligand substitution is in reasonable agreement with the value of the rate of ligand exchange at the same temperature, and the kinetics are characterised by largely positive entropies of activation. There is a compensation-effect between Δ H ‡ and Δ S ‡ , i.e., a greater Δ H ‡ is accompanied by a larger positive Δ S ‡ , indicating that all complexes react via the same mechanism. The basicity of the phosphane does not affect significantly the reaction rates. The general pattern of behaviour indicates that the rate determining step for substitution is the dissociation of the sulfoxide ligand and the formation of a three-coordinated [Pt(Me) 2 (PR 3 )] uncharged intermediate.

Displacement of chloride under the trans-effect of strong σ-donor groups in Pt(II) complexes

Abstract The kinetics of the reactions: trans -[PtL 2 (R)Cl] + Y − → trans [PtL 2 (R)Y] + Cl − (R = H, C 2 H 5 , m -CF 3 C 6 H 4 F 5 ; L = P(C 2 H 5 ) 3 ; Y − = Br − , I − , N − 3 , NO − 2 , CN − , SCN − , thiourea) in CH 3 OH at 25 °C are described. The pseudo-first-order rate constants fit the usual two-term rate law, k obs = k 1 + k 2 [ Y − ], with the k 2 term strongly depending on the entering group Y − . To the extent that the term k 1 can be taken as a measure of trans effect, this is found to parallel the trans -influence order of R, H > C 2 H 5 > m -CF 3 C 6 H 4 > C 6 F 5- . This suggests a mechanism wherein the weakening of the PtCI bond trans to strong σ donors R in the ground state is the driving force of the bimolecular reaction with a weak nucleophile which is devoid of π acceptor ability, such as methanol (ground state σ- trans -effect). When Y − has good π acceptor abilities, contribution from the k 2 term becomes predominant, indicating that the driving force of the reaction is now the stabilization of the 5-coordinate transition state (transition state π - trans -effect).