Electronic and ring size effects of N-heterocyclic carbenes on the kinetics of ligand substitution reactions and DNA/protein interactions of their palladium(II) complexes.

The synthesis, substitution kinetics and DNA/BSA interactions of four cationic Pd(II) complexes [Pd(1)Cl]BF4 (Pd1), [Pd(2)Cl]BF4 (Pd2), [Pd(3)Cl]BF4 (Pd3) and [Pd(4)Cl]BF4 (Pd4), derived from the reaction of [PdCl2(NCCH3)2] with ligands 2,6-bis(3-methylimidazolium-1-yl)pyridine dibromide (1), 2,6-bis(3-ethylimidazolium-1-yl)pyridine dibromide (2), 2,6-bis(1-methylimidazole-2-thione)pyridine (3), and 2,6-bis(1-ethylimidazole-2-thione)pyridine (4), respectively are reported. The complexes were characterised by various spectroscopic techniques and single crystal X-ray diffraction for compound Pd2. Kinetic reactivity of the complexes with the biologically relevant nucleophiles thiourea (Tu), L-methionine (L-Met) and guanosine 5'-monophosphate sodium salt (5'-GMP) was in the order: Pd1 > Pd2 > Pd3 > Pd4, which was largely dependent on the electronic and ring size of the chelate ligands, consistent with Density functional theory (DFT) simulations. The interactions of the complexes with calf thymus DNA (CT-DNA) and bovine serum albumin (BSA) binding titrations showed strong binding. Both the experimental and in silico data reveal CT-DNA intercalative binding mode.

Synthesis, substitution kinetics, DNA/BSA binding and cytotoxicity of tridentate N

The pincer complexes, [Pd(L1)Cl]BF4 (PdL1), [Pd(L2)Cl]BF4 (PdL2), [Pd(L3)Cl]BF4 (PdL3), [Pd(L4)Cl]BF4 (PdL4) were prepared by reacting the corresponding ligands, 2,6-bis[(1H-pyrazol-1-yl)methyl]pyridine (L1), bis[2-(1H-pyrazol-1-yl)ethyl]amine (L2), bis[2-(1H-pyrazol-1-yl)ethyl]ether (L3), and bis[2-(1H-prazol-1-yl)ethyl]sulphide (L4) with [PdCl2(NCMe)]2 in the presence NaBF4. The solid-state structures of complexes PdL1-PdL4 confirmed a tridentate coordination mode, with one chloro ligand completing the coordination sphere to afford square-planar complexes. Chemical behaviour of the complexes in solution confirms their stability in both aqueous and DMSO stock media. The electrochemical properties of the compounds showed irreversible two-electron reduction process. Kinetic reactivity of Pd complexes with the biological nucleophiles viz, thiourea (Tu), L-methionine (L-Met) and guanosine 5'-diphosphate disodium salt (5'-GMP) followed the order: PdL2 < PdL3 < PdL4, and PdL2 < PdL1. The kinetic reactivity is subject to the electronic effects of the spectator ligand(s), and the trend was supported by the DFT computed results. The palladium complexes PdL1-PdL4 bind to calf thymus (CT-DNA) via intercalation mode. In addition, the bovine serum albumin (BSA) showed good binding affinity to the complexes. The mode of quenching mechanism of the intrinsic fluorescence of CT-DNA and BSA by the complexes was found to be static. The order of interactions of the complexes with DNA and BSA was in tandem with the rate of substitution kinetics. The complexes, however, displayed relatively low cytotoxicity (IC50 > 100 µM) when tested against the human cervical adenocarcinoma (HeLa) cell line and the transformed human lung fibroblast cell line (MRC-5 SV2).

N,N-Bis(pyridin-2-ylmeth-yl)cyclo-hexa-namine.

The pyridine rings of the title compound, C(18)H(23)N(3), are in a nearly perpendicular orientation relative to the plane defined by the three amino-bonded C atoms, making dihedral angles of 87.4 (1) ° and 84.2 (1) °. One of the pyridine N atoms acts as an hydrogen-bond acceptor for two pyridine C-H groups. By means of these intermolecular hydrogen bonds, the mol-ecules form a two-dimensional network parallel to the ab plane.

Heterodinuclear Ru-Pt Complexes Bridged with 2,3-Bis(pyridyl)pyrazinyl Ligands: Studies on Kinetics, Deoxyribonucleic Acid/Bovine Serum Albumin Binding and Cleavage, In Vitro Cytotoxicity, and In Vivo Toxicity on Zebrafish Embryo Activities.

Di- and poly-homo/heteronuclear complexes have great potential as anticancer drugs. Here, we report their reactivity, deoxyribonucleic acid (DNA)/bovine serum albumin (BSA) binding and cleavage interactions, in vitro cytotoxicity, and in vivo zebrafish embryo toxicity of [(phen)2Ru(µ-L)PtCl2]2+ (phen = 1,10-phenanthroline and L = 2,3-bis(2-pyridyl)pyrazine, bpp, C1 ; 2,3-bis(2-pyridyl)quinoxaline, bpq, C2ial ; 2,3-bis(2-pyridyl)benzo[g]quinoxaline, bbq, C3 ) anticancer prodrugs. The substitution reactivity increases from C1 to C3 owing to an increase in the π-conjugation on the bridging chelate which facilitates π-back bonding. As a result, the electrophilicity index on the C3 complex increases than that on the complex C2 followed by C1 which leads to higher rates of substitution and thus the reactivity order follows C1 < C2 < C3 . The coordination of Ru at one end of each of the complexes enhances water solubility. Moreover, the charge addition of the two metal ions increases their reactivity toward substitution in addition to ensuring electrostatic interactions at target sites such as the DNA/BSA. Spectroscopic (UV-vis absorption and fluorescence quenching) titration and viscosity measurement results of the interactions of C1/2/3 with CT-DNA established the formation of stable, nonconvent C1/2/3 -DNA adducts with DNA most likely via the intercalative binding mode. Furthermore, studies with BSA showed a good binding affinity of these complexes owing to hydrophobic interactions with the coordinated ligands. The interactions of these complexes with DNA/BSA are in line with the reactivity trend, and all these experimental findings were further supported by molecular docking analysis. In vitro MTT cytotoxic activities on human breast cancer cell line MCF-7 revealed that all the complexes have high cytotoxicity activity (IC50 > 9 µM); furthermore, the selectivity index and SI values were higher (>3). Complex C3 showed the highest cytotoxicity with IC50 = 3.1 µM and SI value (5.55) against MCF7 cell lines and these values were comparable to those of the cisplatin (IC50 and SI values are 5.0 µM and 4.02, respectively). In vivo toxicological assessments on zebrafish embryos revealed that all the Ru-Pt complexes (CI/2/3 ) have poor embryo acute toxic effects over 96 h postfertilization, hpf with LC50 > 65.2 µM. The complex C3 has shown the lowest embryo toxicity (LC50 = 148.8 µM), which is comparable to that of commercial cisplatin (LC50 = 181.1 µM). Based on the cytotoxicity results, complexes C2 and C3 could be considered for further development as chemotherapeutic agents against MCF breast cancer cells.

(1H-pyrazole-κN2)(2,2':6',2''-terpyridine-κ3N,N',N'')platinum(II) bis(perchlorate) nitromethane monosolvate.

The reaction between [PtCl(terpy)]·2H(2)O (terpy is 2,2':6',2''-terpyridine) and pyrazole in the presence of two equivalents of AgClO(4) in nitromethane yields the title compound, [Pt(C(3)H(4)N(2))(C(15)H(11)N(3))](ClO(4))(2)·CH(3)NO(2), as a yellow crystalline solid. Single-crystal X-ray diffraction shows that the dicationic platinum(II) chelate is square planar with the terpyridine ligand occupying three sites and the pyrazole ligand occupying the fourth. The torsion angle subtended by the pyrazole ring relative to the terpyridine chelate is 62.4 (6)°. Density functional theory calculations at the LANL2DZ/PBE1PBE level of theory show that in vacuo the lowest-energy conformation has the pyrazole ligand in an orientation perpendicular to the terpyridine ligand (i.e. 90°). Seemingly, the stability gained by the formation of hydrogen bonds between the pyrazole NH group and the perchlorate anion in the solid-state structure is sufficient for the chelate to adopt a higher-energy conformation.

Electrostatic repulsion between the cations of (1-methyl-1H-imidazole-κN)(2,2':6',2''-terpyridine-κN,N',N'')platinum(II) perchlorate nitro-methane monosolvate prevents Pt⋯Pt inter-actions.

The reaction between [Pt(terpy)Cl]·2H(2)O (terpy = 2',2'':6',2''-terpyridine) and 1-methyl-imidazole (MIm) in the presence of two equivalents of AgClO(4) in nitro-methane yields the title compound, [Pt(C(15)H(11)N(3))(C(4)H(6)N(2))](ClO(4))(2)·CH(3)NO(2). The dicationic complexes are arranged in a staggered configuration. The torsion angle subtended by the 1-methyl-imidazole ring relative to the terpyridine ring is 114.9 (5)°. Inter-molecular C-H⋯O inter-actions between the perchlorate anions and the H atoms of the terpy ligand are observed. Consideration of related phenyl-bipyridyl complexes of platinum(II), which are monocationic, leads to the conclusion that the electrostatic repulsion between the dicationic chelates prevents the formation of Pt⋯Pt inter-actions. These inter-actions are a common feature associated with the monocationic species.

Role of π-conjugation on the coordination behaviour, substitution kinetics, DNA/BSA interactions, and in vitro cytotoxicity of carboxamide palladium(II) complexes.

Treatments of N-(pyridin-2-ylmethyl)pyrazine-2-carboxamide (L1), N-(quinolin-8-yl)pyrazine-2-carboxamide (L2), N-(quinolin-8-yl)picolinamide (L3) and N-(quinolin-8-yl)quinoline-2-carboxamide (L4) with [PdCl2(NCMe)]2 afforded the corresponding Pd(ii) complexes, [Pd(L1)Cl] (PdL1); [Pd(L2)Cl] (PdL2); [Pd(L3)Cl] (PdL3); and [Pd(L4)Cl] (PdL4) in moderate yields. Structural characterisation of the compounds was achieved by NMR and FT-IR spectroscopies, elemental analyses and single crystal X-ray crystallography. The solid-state structures of complexes PdL2-PdL4 established the presence of one tridentate carboxamide and Cl ligands around the Pd(ii) coordination sphere, to give distorted square planar complexes. Electrochemical investigations of PdL1-PdL4 showed irreversible one-electron oxidation reactions. Kinetics reactivity of the complexes towards bio-molecules, thiourea (Tu), l-methionine (L-Met) and guanosine 5'-diphosphate disodium salt (5'-GMP) decreased in the order: PdL1 > PdL2 > PdL3 > PdL4, in tandem with the density functional theory (DFT) data. The complexes bind favourably to calf thymus (CT-DNA), and bovine serum albumin (BSA), and the order of their interactions agrees with the substitution kinetics trends. The in vitro cytotoxic activities of PdL1-PdL4 were examined in cancer cell lines A549, PC-3, HT-29, Caco-2, and HeLa, and a normal cell line, KMST-6. Overall, PdL1 and PdL3 displayed potent cytotoxic effects on A549, PC-3 HT-29 and Caco-2 comparable to cisplatin. All the investigated complexes exhibited lower toxicity on normal cells than cisplatin.

Palladium(II) complexes of tridentate bis(benzazole) ligands: Structural, substitution kinetics, DNA interactions and cytotoxicity studies.

Reactions of 2,6-bis(benzimidazol-2-yl)pyridine (L1), 2,6-bis(benzoxazol-2-yl)pyridine (L2), and 2,6-bis(benzothiazol-2-yl)pyridine (L3) with [Pd(NCMe)2Cl2] in the presence of NaBF4 afforded the corresponding Pd(II) complexes, [Pd(L1)Cl]BF4, PdL1; [Pd(L2)Cl]BF4, PdL2; [Pd(L3)Cl]BF4, PdL3; respectively, while reaction of bis[(1H-benzimidazol-2-yl)methyl]amine (L4) with [Pd(NCMe)2Cl2] afforded complex [Pd(L4)Cl]Cl, PdL4. Characterisation of the complexes was accomplished using NMR, IR, MS, elemental analyses and single crystal X-ray crystallography. Ligand substitution kinetics of these complexes by biological nucleophiles thiourea (Tu), L-methionine (L-Met) and guanosine 5'-diphosphate disodium salt (5-GMP) were examined under pseudo-first order conditions. The reactivity of the complexes decreased in the order: PdL1 > PdL2 > PdL3 > PdL4, ascribed to electronic effects. Density functional theory (DFT) supported this trend. Studies of interaction of the Pd(II) complexes with calf thymus DNA (CT-DNA) revealed strong binding affinities via intercalative binding mode. Molecular docking studies established associative non-covalent interactions between the Pd complexes and DNA. The in vitro cytotoxic activities of PdL1-PdL4 were assessed in cancer cell lines HeLa and MRC5-SV2 and a normal cell line MRC-5, using the 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay. PdL1 exhibited cytotoxic potency and selectivity against HeLa cell that was comparable to cisplatin's. Complex PdL1, unlike cisplatin, did not significantly induce caspase-dependent apoptosis.

Controlling the reactivity of [Pd(II)(N

Four [(N^N^N)Pd(II)Cl]+ complexes [chloride-(2,2':6',2''-terpyridine)Pd(II)]Cl (PdL1), [chlorido(2,6-bis(N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL2), [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-yl)pyridine)Pd(II)]Cl (PdL3) and [chlorido(2,6-bis(3,5-dimethyl-N-pyrazol-2-ylmethyl)pyridine)Pd(II)]BF4 (PdL4) were synthesized and characterized. The rates of substitution of these Pd(II) complexes with thiourea nucleophiles viz; thiourea (Tu), N,N'-dimethylthiourea (Dmtu) and N,N,N',N'-tetramethylthiourea (Tmtu) was investigated under pseudo first-order conditions as a function of nucleophile concentration [Nu] and temperature using the stopped-flow technique. The observed rate constants vary linearly with [Nu]; kobs = k2[Nu] and decreased in the order: PdL1 > PdL2 > PdL3 â‰« PdL4. The lower π-acceptability of the cis-coordinated N-pyrazol-2-yl groups (which coordinates via pyrazollic-N π-donor atoms) of the PdL2-4 significantly decelerates the reactivity relative to PdL1. Furthermore, the six-membered chelates having methylene bridge in PdL4 do not allow π-extension in the ligand and introduces steric hindrance further lowering the reactivity. Trends in DFT calculated data supported the observed reactivity trend. Spectrophotometric titration data of complexes with calf thymus DNA (CT-DNA) and viscosity measurements of the resultant mixtures suggested that associative interactions occur between the complexes and CT-DNA, likely through groove binding with high binding constants (Kb = 104 M-1). In vitro MTT [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide] cytotoxic activity data showed that PdL1 was the most potent complex against MCF7 breast cancer cells; its IC50 value is lower than that of cisplatin. The results demonstrate how modification of a spectator ligand can be used to slow down the reactivity of Pd(II) complexes. This is of special importance in controlling drug toxicity in both pharmaceutical and biomedical applications.

N,N-Bis(2-pyridylmeth-yl)-tert-butyl-amine.

In the title compound, C(16)H(21)N(3), the dihedral angle between the two pyridine rings is 88.11 (9)°. In the crystal, mol-ecules are linked through inter-molecular C-H⋯π inter-actions, forming a layer expanding parallel to the (10) plane.

Seven membered chelate Pt(ii) complexes with 2,3-di(2-pyridyl)quinoxaline ligands: studies of substitution kinetics by sulfur donor nucleophiles, interactions with CT-DNA, BSA and in vitro cytotoxicity activities.

Dichloro platinum(ii) complexes coordinated with 2,3-di(2-pyridyl)quinoxaline ligands which form seven-membered chelates namely, bpqPtCl2, dmbpqPtCl2 and bbqPtCl2 (where bpq, dmbpq and bbq are 2,3-di(2-pyridyl)quinoxaline, 6,7-dimethyl-2,3-di(2-pyridyl)quinoxaline and 2,3-bis(2'pyriyl)benzo[g]quinoxaline, respectively) were synthesized, characterised and their respective hydrated product complexes namely, bpqPt(OH2)2 2+, dmbpqPt(OH2)2 2+ and bbqPt(OH2)2 2+ were prepared by chloride metathesis. The substitution kinetics of the aquated cations by thiourea nucleophiles indicated that the two aqua ligands are substituted simultaneously according to the rate law: k obs = k 2[Nu]. This is followed by a forced dechelation of the ligands from the Pt (II) to form Pt(Nu)4 2+ species. The dechelation step is considerably slow to be monitored reliably. The rate of substitution is marginally enhanced by introducing two methyl groups and by extending the π-conjugation on the bpq core ligand. The reactivity order increased as bpqPt(OH2)2 2+ < dmbpqPt(OH2)2 2+ < bbqPt(OH2)2 2+. Reactivity trends were well supported by theoretical computed DFT electronic descriptors. The interactions of the Pt(ii) complexes with CT-DNA and BSA were also examined spectroscopically in tris buffers at pH 7.2. Spectroscopic and viscosity measurements suggested strong associative interactions between the Pt(ii) complexes and CT-DNA, most likely through groove binding. In silico theoretical binding studies showed energetically stable poses through associative non-covalent interactions. In vitro MTT cytotoxicity IC50 values of the Pt(ii) complexes on human liver carcinoma cells (HepG2) cancer cell lines revealed bbqPtCl2 as the least active. The fluorescence staining assays revealed the morphological changes suggested early apoptotic induction as well as non-specific necrosis.

One-dimensional C-H...N hydrogen-bonded polymers in flexible tetrapyridyl systems.

In N,N,N',N'-tetrakis(2-pyridylmethyl)propane-1,3-diamine, C(27)H(30)N(6), (I), and N,N,N',N'-tetrakis(2-pyridylmethyl)butane-1,4-diamine, C(28)H(32)N(6), (II), the twofold rotational symmetry of (I) favours the formation of a one-dimensional hydrogen-bonded polymer with two columns of C-H...N hydrogen bonds, while the inversion symmetry of (II) allows the formation of a one-dimensional hydrogen-bonded polymer stabilized by four columns of C-H...N hydrogen bonds. The possible role played by the chain length of the linking alkanediamine in determining the type of supramolecular architecture in this series of compounds is discussed.

Three new asymmetric trans-amine(azole)dichloridoplatinum complexes that overcome cisplatin resistance and their reactions with 5'-GMP.

Three new asymmetric platinum(II) complexes comprising an isopropylamine ligand trans to an azole ligand were synthesized and fully characterized by (1)H NMR, (195)Pt NMR, IR and elemental analysis. In addition the X-ray crystal structure of all three complexes was determined. The reaction kinetics of the complexes with DNA model base guanosine-5'-monophosphate (GMP) was studied, revealing reaction kinetics comparable to cisplatin. To gain insight in the complexes as potential antitumor agents, cytotoxicity assays were performed on a variety of human tumor cell lines. These assays showed the complexes all to possess cytotoxicity profiles comparable to cisplatin. Furthermore, the complexes largely retain their activity in a human ovarian carcinoma cell line resistant to cisplatin, A2780R, compared to the cisplatin sensitive parent cell line A2780. These results are of fundamental importance, illustrating how platinum complexes of trans geometry can show improved activity compared to cisplatin in both cisplatin sensitive and cisplatin resistant cell lines.

Kinetic and mechanistic studies of 1,3-bis(2-pyridylimino)isoindolate Pt(II) derivatives. Experimental and new computational approach.

The rate of substitution of the chloride ligand by three bio-relevant nucleophiles, thiourea (Tu), N,N-dimethylthiourea (Dmtu) and N,N,N,N-tetramethylthiourea (Tmtu), in the complexes: 1,3-bis(2-pyridylimino)isoindoline platinum(II) chloride (Pt2), 1,3-bis(2-pyridylimino)benz(f)isoindoline platinum(II) chloride (Pt3) and 1,3-bis(1-isoquinolylimino)isoindoline platinum(II) complex (Pt4) was investigated under pseudo first-order conditions as a function of concentration and temperature using stopped-flow and UV-Visible spectrophotometry. Computational modeled data of bis(pyridylimino)3,4-pyrrolate platinum(II) chloride (Pt1) were incorporated in the study for comparison. The observed pseudo first-order rate constants for substitution reactions obey the rate law kobs = k2[Nu]. High negative activation entropies and second-order kinetics for the displacement reactions all support an associative mode of activation. The reactivity is dependent on stabilization of the LUMO energy and inversely proportional to the number of phenyl rings added irrespective of the site of attachment. The electron density on the ligand moiety plays a significant role in the substitution behavior of the platinum(II) complexes, as supported by DFT descriptors {electrophilicity index (ω) and chemical hardness (η)}.

Mechanistic elucidation of linker and ancillary ligand substitution reactions in Pt(II) dinuclear complexes.

The rate of substitution of aqua ligands by three nucleophiles, thiourea (TU), N,N-dimethylthiourea (DMTU) and N,N,N,N-tetramethylthiourea (TMTU), for the complexes [cis-{PtOH2(NH3)2}2-µ-pyrazine](ClO4)2 (pzn), [cis-{PtOH2(NH3)2}2-µ-2,3-dimethylpyrazine](ClO4)2 (2,3pzn), [cis-{PtOH2(NH3)2}2-µ-2,5-pyrazine](ClO4)2 (2,5pzn) and [cis-{PtOH2(NH3)2}2-µ-2,6-dimethylpyrazine](ClO4)2 (2,6pzn) was investigated under pseudo first-order conditions as a function of concentration and temperature by stopped-flow and UV-Visible spectrophotometry. The reaction proceeded in three consecutive steps; each step follows first order kinetics with respect to each complex and nucleophile. The pseudo first-order rate constants, k(obs(1/2/3)), for sequential substitution of the aqua ligands and subsequent displacement of the linker obeyed the rate law: k(obs(1/2/3)) = k((1/2/3))[nucleophile]. The steric hindrance properties of the pyrazine-bridging ligand control the overall reaction pattern. The order of reactivity of the complexes is 2,3pzn ≈ 2,5pzn < 2,6pzn < pzn. The difference in reactivity attributed to the steric crowding at the Pt(II) centre imposed by the methyl groups reduces the lability of the aqua complexes. The order of reactivity of the nucleophiles decreases with the increase in steric demand TU > DMTU > TMTU. 1H and 195Pt NMR spectroscopic results confirmed the observed dissociation of the bridging ligand from the metal centre of the cis-dinuclear complexes and its derivatives in the third step. The dissociation process is accelerated by the introduction of the steric effect on the linker in conjunction with the increased ligand field strength imparted by additional thiourea ligands at each metal centre. The large negative entropy of activation ΔS(≠) values in all cases support an associative substitution mechanism.

The π-acceptor effect in the substitution reactions of tridentate N-donor ligand complexes of platinum(II): a detailed kinetic and mechanistic study.

The nucleophilic substitution reactions of complexes [Pt{4'-(2'''-CH(3)-phenyl)-2,2':6',2''-terpyridine}Cl]CF(3)SO(3), [CH(3)PhPtCl], [Pt{4'-(2'''-CH(3)-phenyl)-6-(3''-isoquinoyl)-2,2'bipyridine}Cl]SbF(6), [CH(3)PhisoqPtCl], [Pt{2-(2'-pyridyl)-1,10-phenanthroline}Cl]Cl, [pyPhenPtCl], and [Pt(terpyridine)Cl](+), [PtCl] with a series of nucleophiles: thiourea (TU), N,N-dimethylthiourea (DMTU), N,N,N,N-tetramethylthiourea (TMTU), I(-), Br(-), and SCN(-) were studied in 0.1 M LiCF(3)SO(3) in methanol (in the presence of 10 mM LiCl). The reactivity of the investigated complexes follows the order pyPhenPtCl > PtCl > CH(3)PhPtCl > CH(3)PhisoqPtCl. The lability of the chloride ligand is dependent on the strength of π-backbonding properties of the spectator ligands around the platinum centre. The experimental data is strongly supported by DFT calculations. The dependence of the second-order rate constants on concentration of the nucleophiles as well as the large negative values reported for the activation entropy (ΔS(‡)) confirmed an associative mechanism of substitution.

A kinetics and mechanistic study on the role of the structural rigidity of the linker on the substitution reactions of chelated dinuclear Pt(II) complexes.

Substitution reactions of platinum complexes bearing cyclohexylamine/diamine moieties viz., [Pt(H(2)O)(N,N-bis(2-pyridylmethyl)cyclohexylamine)](CF(3)SO(3))(2), bpcHna; [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-trans-1,4-cyclohexyldiamine)](CF(3)SO(3))(4), cHn and [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-4,4'-dicyclohexylmethanediamine)](CF(3)SO(3))(4), dcHnm and phenylamine/diamine moieties viz., ([Pt(H(2)O)N,N-bis(2-pyridylmethyl)phenylamine)](CF(3)SO(3))(2), bpPha; [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-1,3-phenyldiamine)](CF(3)SO(3))(4), mPh; [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-1,4-phenyldiamine)](CF(3)SO(3))(4), pPh and [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-4,4'-diphenylmethanediamine)](CF(3)SO(3))(4)), dPhm with thiourea nucleophiles were studied in acidified 0.01 M LiCF(3)SO(3) aqueous medium under pseudo-first-order conditions using stopped-flow and UV-visible spectrophotometric techniques. The rate of substitution follows a similar trend in the two sets of complexes and decreases in the order: bpcHna > dcHnm > cHn and bpPha > dPhm ≈ pPh ≈ mPh), respectively. The result of this study has shown that the rigidity and/or the planarity of a diamine bridge linking the two (2-pyridylmethyl)amine-chelated Pt(II) centres, influences the reactivity of the metal centres by protracting similar symmetry elements within the complexes, which determines the amount of steric influences felt on the coordination square-plane. Hence, the order of reactivity is controlled by both the steric hindrance and the magnitude of the trans σ-inductive effect originating from the linker towards the metal centre. These two factors also impact on the acidity of the complexes. The high negative entropies and low positive enthalpies support an associative mode of activation.

Understanding the role of the flexible bridging linker through kinetics and mechanistic study of Pt(II) amphiphiles derived from a bis(2-pyridylmethyl)amine chelate head group.

The substitution of aqua ligands of mononuclear Pt(II) complexes of the general form [Pt(H(2)O)(N,N-bis(2-pyridylmethyl)-N(CH(2))(n)-CH(3); -NC(CH(3))(3); -NH](CF(3)SO(3))(2), n = 1 (bpea); 2 (bppa); 3 (bpba); 5 (bpha), 9 (bpda) -NC(CH(3))(3) (bpbta) and -NH (bpma) by thiourea nucleophiles was investigated under pseudo first-order conditions as a function of concentration and temperature using the stopped-flow technique and UV-vis spectroscopy. The substitution reactions occur via two separate reaction steps, each fitting to a single exponential curve. In the two reaction steps, the thiourea nucleophiles first substitute the coordinated aqua ligand followed by ring opening via dechelation of one of the pyridyl units. The mode of activation for both steps remains associative in nature and the observed rate constants can be fitted to the equation k(obs(1st/2nd)) = k(2(1st/2nd))[Nu]. Appending a primary alkyl hydrocarbon group on the trans-N donor atom of the chelate head group marginally increases the rate of substitution of the aqua leaving group due to the weaker trans-influence of its alkyl amine donor group. However, when a tert-butyl group is the pendant group, reactivity increases by a factor of about two, reiterating the inductive nature of the flow of electron density from the tailing groups towards the Pt(II) metal centres. A comparison of the reactivities of the studied complexes with their dinuclear analogues bridged by alkyl diamines has demonstrated that the electronic effect of the alkyl diamine bridge on the overall reactivity of the multinuclear Pt(II) complexes is weak and insignificant when compared to steric effects due to the constraining bridge.

Tuning the reactivity of chelated dinuclear Pt(II) complexes through a flexible diamine linker. A detailed kinetic and mechanistic study.

The rate of displacement of the aqua ligands by three neutral nucleophiles (Nu) of different steric demands, namely thiourea (tu), N,N'-dimethylthiourea (dmtu) and N,N,N',N'-tetramethylthiourea (tmtu) and an anionic nucleophile (I(-)) in complexes of the form [{Pt(H(2)O)}(2)(N,N,N',N'-tetrakis(2-pyridylmethyl)-N(CH(2))(n)N](CF(3)SO(3))(4), n = 2 (En); 3 (Prop); 4 (But); 6 (Hex); 8 (Oct) and 10 (Dec), was studied under pseudo first-order conditions as a function of concentration, temperature and pressure using stopped-flow techniques and UV-visible spectrophotometry. The pseudo first-order rate constants, k(obs(1(st)/2(nd))), for the simultaneous substitution of the aqua ligands and the proposed subsequent dechelation of the pyridyl units, respectively, agreed well to the rate law: k(obs(1(st)/2(nd))) = k(2(1(st)/2(nd)))[Nu]. High negative activation entropies, negative volumes of activation and second-order kinetics for the displacement reactions all support an associative mode of activation. Except for Prop, the rate of the simultaneous substitution of the aqua ligands in the complexes was found to increase as the chain length of the linker increases from En to Hex, beyond which any further increase in chain length is not accompanied by a further increase in reactivity. The reactivity trend of the even-bridged complexes with C(2h) symmetry is ascribed to a concomitant decrease in axial steric influences imposed on one side of the square-planar picolyl chelates by the other as the chain length increases. Based on the model structures of the complexes, this kind of steric imposition occurs only in complexes with an even number of CH(2) groups within the linker. The Prop complex, having a C(2v) symmetry showed exceptional high reactivity towards the nucleophiles. A cage effect, evolving from its bowl-shaped molecular structure, is proposed to explain this high reactivity. The order of reactivity of the nucleophiles increased in the order I(-) >> tu approximately dmtu > tmtu, in line with the strong electrostatic interactions between the highly polarizable iodide nucleophile and the Pt centers, steric retardation effects in the case of tmtu and dominating positive inductive effects for the dmtu nucleophile.