The Antimicrobial Properties of PdII - and RuII -pyta Complexes.

Infections associated with antimicrobial resistance (AMR) are poised to become the leading cause of death in the next few decades, a scenario that can be ascribed to two phenomena: antibiotic over-prescription and a lack of antibiotic drug development. The crowd-sourced initiative Community for Open Antimicrobial Drug Discovery (CO-ADD) has been testing research compounds contributed by researchers around the world to find new antimicrobials to combat AMR, and during this campaign has found that metallodrugs might be a promising, yet untapped source. To this end, we submitted 18 PdII - and RuII -pyridyl-1,2,3-triazolyl complexes that were developed as catalysts to assess their antimicrobial properties. It was found that the Pd complexes, especially Pd1, possessed potent antifungal activity with MICs between 0.06 and 0.125 µg mL-1 against Candida glabrata. The in-vitro studies were extended to in-vivo studies in Galleria mellonella larvae, where it was established that the compounds were nontoxic. Here, we effectively demonstrate the potential of PdII -pyta complexes as antifungal agents.

Acid and base strength variations: rationalization for cyclic amine bases and acidic aqua cations.

This perspective highlights and evaluates recent key developments in the thermodynamic approach used to analyze trends in acid and base strength variation. According to this approach, acid and base strength ranking can be interpreted by using thermodynamic or thermochemical cycles. Each cycle generally consists of three independent but well-defined steps. The modus operandi described here entails the identification of the dominant step and the rationalization of its free energy/enthalpy/energy change along a selected series in terms of known structural chemical concepts. Developments in this approach are described by focusing on two related series of bases and two series of acids. In the case of the former the protonation of a series of N-heterocyclic amine bases together with their methyl-substituted analogs receives particular attention while in the case of acids, the acidic properties of aqua dications of elements in period 4 and group 2 are probed. It is illustrated how significant progress in computational chemistry and mass spectrometric techniques can be employed to compare 'inherent' basicity or acidity in the selected families of compounds by using simple gas-phase energy cycles. Unique, dual functions for both electronegativity (element and orbital) and charge density (for aqua cations) indicators are identified and used to evaluate these cycles. Solvent effects (in aqueous solution) are accommodated by including dehydration and hydration changes in appropriately-extended, three-step free energy cycles. It is further suggested that the dominant step in the extended thermodynamic cycle for monomeric aqua cations is the transfer of M(H2O)n2+ complex hydrates from the gas-phase to bulk water. Charge density of the aqua cations again features prominently in proposed rationalizations. Finally, this article also sheds light on salient relationships that exist between empirically and quantum-chemically estimated enthalpy and entropy changes for the aforementioned transfer process.

Phenylacetylene polymerisation mediated by cationic cyclometallated palladium(ii) complexes bearing benzylidene 2,6-diisopropylphenylamine and its derivatives as ligands.

A series of novel cationic palladacycle complexes bearing benzylidene-2,6-diisopropylphenylamine derivatives as ligands and with the general formula [Pd(MeCN)(L)(R-C6H3)CH[double bond, length as m-dash]N{2,6-iPr2-C6H3}][B(3,5-(CF3)2-C6H3)] (R = H, Cl, Br, F, OMe and L = 1,3,5-triaza-7-phosphaadamantane (PTA), tricyclohexylphosphine (PCy3) and triphenylphosphine (PPh3)) were prepared and characterized by a range of analytical techniques. These cationic palladacycle complexes were found to be active precatalysts for the polymerisation of phenylacetylene. The meta-substituent on the cyclometallated ring was found to have a marked effect on the catalyst performance in that complexes, which contained electron-withdrawing substituents, were found to be the most active in the polymerisation process. Furthermore, increasing the steric bulk of the phosphine ligand led to the production of higher molecular weight polyphenylacetylenes (PPA). Polymerisation reactions performed at 25 °C gave a mixture of both cis-transoidal and trans-cisoidal PPA while trans-cisoidal PPA was selectively produced at elevated temperatures (60 °C). Preliminary mechanistic studies demonstrated that polymerisation proceeds via dissociation of the C,N-chelating ligand, and involves the formation of an iminium cationic fragment.

Synthesis and hydroformylation evaluation of Fréchet-type organometallic dendrons with N,O-salicylaldimine Rh(i) complexes at the focal point.

A series of organometallic dendrons containing N,O-salicylaldimine entities at the focal point were synthesised by reacting the N,O-salicylaldimine-functionalised Fréchet dendrons (G0, G1 and G2) with a [Rh(µ-Cl)(η2:η2-COD)]2 dimer to yield the corresponding Rh(COD) [COD = cyclooctadiene] complexes. These Rh(COD) complexes were exposed to an atmosphere of CO to yield a new class of rhodium carbonyl organometallic dendrons with Rh(CO)2 units at the focal point. All the compounds were characterised using standard spectroscopic and analytical techniques, these include nuclear magnetic resonance, infrared spectroscopy, mass spectrometry and single-crystal X-ray diffraction for compounds 1, 4 and 7. All of the complexes were evaluated in the hydroformylation of 1-octene, with excellent conversion and chemoselectivity towards aldehydes. The G0-(CO)2 catalyst precursor (7) was active in the hydroformylation of 1-octene, styrene, 7-tetradecene, methyl oleate, triolein, d-limonene and R-citronellal. The conversion and chemoselectivity towards aldehydes for 7-tetradecene, methyl oleate, triolein and d-limonene were promising. Across a particular dendron series, an increase in chemoselectivity was observed due to the dendritic effect. Mercury drop tests were performed for the G0-analogues and these confirm that the hydroformylation can be attributed to a combination of homogeneous and heterogeneous catalysis.

Recoverable and recyclable water-soluble sulphonated salicylaldimine Rh(I) complexes for 1-octene hydroformylation in aqueous biphasic media.

A series of water-soluble Rh(i) mononuclear complexes of general formula: [Rh(sulphsal-X-R)(COD)] [sulphsal = sulphonated salicylaldimine, COD = cyclooctadiene; where R = H, Cl, CH3 and X = H, (t)Bu] have been synthesized. All the compounds were characterised using various spectroscopic and analytical techniques such as nuclear magnetic resonance spectroscopy, infrared spectroscopy, single crystal X-ray diffraction (for complex ) and mass spectrometry. All the compounds display excellent water-solubility at room temperature and were tested as catalyst precursors in the aqueous biphasic hydroformylation of 1-octene. The catalysts could be easily recovered by phase separation and were used up to 5 times without any significant loss in activity and 1-octene conversion. Very high yields of the expected aldehydes were obtained without addition of any phase transfer agents, co-solvents or hydrophobic ligands. Excellent aldehyde chemoselectivity is observed for all the catalysts but this varied each time the catalysts were recycled, with the formation of a small amount of internal olefins. ICP-OES and mercury poisoning experiments show that a combination of homogeneous catalysis and catalysis mediated by nanoparticles is taking place in these systems.

Cyclometallated platinum(II) complexes of benzylidene-2,6-di-isopropylphenylamine containing bidentate phosphines: synthesis, structural properties and reactivity studies.

The reaction of the cyclometallated complex [PtCl(N^C)(dmso)], 1 (N^C represents the cyclometallated Schiff base, benzylidene-2,6-diisopropylphenylamine), with 1,1'-bis(diphenylphosphino)ferrocene, dppf, bis(diphenylphosphino)methane, dppm, or 1,2-bis(diphenylphosphino)ethane, dppe, in a 2 : 1 ratio or an equimolar ratio using acetone as the solvent produced the corresponding binuclear or mononuclear diphosphine platinum complexes. In the case of the mononuclear complexes, the diphosphines act as either a bidentate ligand or a monodentate ligand depending on the size of the bite angle of the diphosphines, while in the case of the binuclear complexes, the diphosphines act as a bridging ligand between the two metal centres. The solid state structures of some of the binuclear as well as mononuclear species are reported. The mononuclear derivatives were found to show different behaviour in solution and in the solid state when compared to the binuclear analogues. This behaviour is also influenced by the nature of the diphosphine ligands employed.

The synthesis and application of novel Ni(II) N-alkyl dipyridylaldiminato complexes as selective ethylene oligomerisation catalysts.

A series of N-alkyl 2,2'-dipyridylamine ligands of general formula (2-C5H3NR)2NR', (a): R = H, R' = Me; (b): R = H, R' = benzyl; (c): R = H, R' = methylcyclohexyl; (d): R = H, R' = neopentyl; (e): R = Me, R' = Me) were prepared by a modified method involving base-mediated N-alkylation with the respective alkyl halide. Reaction of the ligands, a-e, with NiCl2(DME) allowed for the isolation of µ-Cl Ni(ii) complexes: [Ni(µ-Cl){a}Cl]2 (1a); [Ni(µ-Cl){b}Cl]2 (1b); [Ni(µ-Cl){c}Cl]2 (1c); [Ni(µ-Cl){d}Cl]2 (1d) and [Ni(µ-Cl){e}Cl]2 (1e). The complexes were characterised by FT-IR spectroscopy, magnetic susceptibility measurements, mass spectrometry, elemental analyses and in the case of 1a, SCD analysis. In the case of complex 1e, an acid-mediated hydrolysis process was identified. The product of hydrolysis, the protonated ligand and a tetrachloronickelate salt (1e-A), was characterised by SCD analysis. Activation of 1a-1e with alkyl aluminium reagents generated highly active catalysts for the oligomerisation of ethylene, with activities of up to 864 kgoligomers molNi(-1) h(-1) and high selectivity toward the formation of butenes. In general, trans 2-butene was observed as the major isomer, with the exception of 1e. In the case of 1e, the selectivity for 1-butene was 98%, thereby demonstrating the significant effect that the introduction of a low degree of steric pressure in the coordination sphere of the catalyst has on selectivity.

Platinacycloalkane complexes containing [P,N] bidentate ligands: synthesis and decomposition studies.

A new series of platinacyclopentanes (2a-2f) and platinacycloheptanes (3a-3f) of the type [Pt(N^P)(CH2)n] (n = 4, 6) were obtained by the reaction of [Pt(COD)(CH2)n] with the appropriate iminophosphine ligand (1a-1f). These complexes were characterised using a variety of spectroscopic and analytical techniques. X-ray structure analysis of complex 2a revealed a slightly distorted square planar geometry around platinum as a consequence of the ring strain imposed by the [P,N] chelate ring formed and the metallocycloalkane. Thermal decomposition analyses of the platinacycloalkanes revealed that the platinacyclopentanes are markedly stable, with the decomposition reaction requiring temperatures higher than 100 °C to occur. The major products obtained from the thermal decomposition reactions were 1-butene (for platinacyclopentanes) and 1-hexene (for platinacycloheptanes).

Synthesis, structural characterization and cis-trans isomerization of novel (salicylaldiminato)platinum(II) complexes.

The reaction of cis-[PtCl2(dmso)] with the salicylaldimine ligand, N-(2-hydroxybenzylidene)-2,6-di-isopropylaniline, LA in the presence of sodium acetate in methanol produced both cis- and trans-[PtClLA(dmso)], 1a and 1b. An analogous reaction for the less bulky ligand, N-(2-hydroxybenzylidene)aniline LB produced only cis-[PtClLB(dmso)], 2. The reactions of these dmso complexes with triphenylphosphine also yielded complexes with different geometries depending on the nature of the salicylaldiminato ligand. Thus the cis-trans isomerization of cis-[PtClLA(PPh3)] 3a was investigated both experimentally and computationally, and a tetrahedral transition state was detected in this process. A good agreement of the experimental activation parameters with those determined theoretically using DFT was obtained. LA was also reacted with [PtClMe(cod)] in methanol to yield the corresponding salicylaldiminato complex 6 in which the methyl group is cis to the imine nitrogen. X-ray crystal structures of some compounds obtained are reported.