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.     

Influence of the bridging ligand on the substitution behaviour of dinuclear Pt(II) complexes. An experimental and theoretical approach

A series of dinuclear Pt(II) complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(H2O)2]4+ were synthesized. Acid-base titrations, and concentration and temperature dependent stopped-flow measurements of the reaction with chloride were performed to study the thermodynamic and kinetic behaviour of the dinuclear bridged complexes. The results indicate that there is a clear interaction between the two Pt(II) centres, which becomes weaker as the aliphatic chain increases in length. From a certain chain length onwards, the Pt(II) centres become independent of each other and exhibit identical thermodynamic and kinetic properties. The experimental results are discussed in reference to structures obtained by DFT (BP86/LACVP*) calculations.     

Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

The substitution kinetics of the complexes [Pt{4′‐(o‐CH₃‐Ph)‐terpy} Cl]SbF₆ (CH₃PhPtCl(Sb)), [Pt{4′‐(o‐CH₃‐Ph)‐terpy}Cl]CF₃SO₃ (CH₃PhPtCl(CF)), [Pt(4′‐Ph‐terpy)Cl]SbF₆ (PhPtCl), [Pt(terpy)Cl]Cl·2H₂O (PtCl), [Pt{4′‐(o‐Cl‐Ph)‐terpy}Cl]SbF₆ (ClPhPtCl), and [Pt{4′‐(o‐CF₃‐Ph)‐terpy}Cl]SbF₆ (CF₃PhPtCl), where terpy is 2,2′:6′,2″‐terpyridine, with the nucleophiles thiourea (TU), N,N′‐dimethylthiourea (DMTU), and N,N,N′,N′‐tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo‐first‐order conditions using the stopped‐flow technique. The reactions followed the simple rate law k_obs = k₂[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of π‐backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro‐leaving group. The strength of the electron‐donating or ‐withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron‐donating substituents decrease the rate of substitution, whereas electron‐withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron‐withdrawing substituent in CF₃PhPtCl. The opposite was found to be true with the strongest electron‐donating substituent in CH₃PhPtCl. Thiourea was found to be the best nucleophile with N,N,N′,N′‐tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808–818, 2008     

Substitution behaviour of novel dinuclear Pt(II) complexes with bio-relevant nucleophiles

The novel dinuclear Pt(II) complexes [{trans-Pt(NH(3))(2)Cl}(2)(μ-pyrazine)](ClO(4))(2) (Pt1), [{trans-Pt(NH(3))(2)Cl}(2)(μ-4,4'-bipyridyl)](ClO(4))(2)·DMF (Pt2), and [{trans-Pt(NH(3))(2)Cl}(2)(μ-1,2-bis(4-pyridyl)ethane)](ClO(4))(2) (Pt3), were synthesized. Acid-base titrations, and temperature and concentration dependent kinetic measurements of the reactions with biologically relevant ligands such as thiourea (Tu), glutathione (GSH) and guanosine-5'-monophosphate (5'-GMP) were studied at pH 2.5 and 7.2. The reactions were followed under pseudo-first-order conditions by stopped-flow and UV-vis spectrophotometry. (1)H NMR spectroscopy was used to follow the substitution of chloride in the complex [{trans-Pt(NH(3))(2)Cl}(2)(μ-4,4'-bipyridyl)](ClO(4))(2)·DMF by guanosine-5'-monophosphate (5'-GMP) under second-order conditions. The results indicate that the bridging ligand has an influence on the reactivity of the complexes towards nucleophiles. The order of reactivity of the investigated complexes is Pt1 > Pt2 > Pt3.     

Possible biotransformation reactions of polynuclear Pt(II) complexes

The reactions of the two complexes BBR3464 [{trans-PtCl(NH3)2}2{mu-trans-Pt(NH3)2(NH2(CH2)6NH2)2}](4+) and BBR3610 [{trans-PtCl(NH3)2}2{mu-C2H4(NH2(CH2)6NH2)2}](4+) and the corresponding diaqua complexes with the nucleophiles thiourea (tu) and l-methionine (l-Met), were investigated under pseudo-first-order conditions as a function of concentration and temperature, using UV-vis spectrophotometric and stopped-flow techniques. 1H NMR spectroscopy was used to follow the stepwise substitution of the chloro ligands by guanosine-5'-monophosphate under second-order conditions. For the sulfur donor containing nucleophiles (tu and l-Met), a second reaction step, the displacement of the labilized amine chain linker, as a result of the strong trans-effect of tu and l-Met, was found. The activation parameters for all reactions studied suggest an associative substitution mechanism. The displacement of the chain linker by S-donor nucleophiles illustrates the limit of application of polynuclear complexes with monodentate aliphatic amine bridges and primary ammines, in agreement with previous studies reported in the literature.     

A kinetics study of ligand substitution reaction on dinuclear platinum complexes: Stochastic versus deterministic approach

The kinetics on a basic ligand substitution reaction on dinuclear platinum complexes [Pt(PEt₃)₂PhPt(PEt₃)₂]²⁺ and [Pt(PEt₃)₂PhCOPhPt(PEt₃)₂]²⁺ , with the ligands pyridine and 3-chloropyridine, is studied. This is a fundamental step in a self-assembly, and the time evolution has been observed with a new experimental technique, QASAP (quantitative analysis of self-assembly process), which is recently developed by Hiraoka's group. As a result of numerical calculations based on master equation, we succeed in specifying the reaction rate constants with a simple reaction model. In addition, the time evolutions of all the intermediate components produced and consumed in chemical reaction are revealed, including those unobserved in the experiments. The convergence behavior of the existence ratios of specific chemical species calculated with the stochastic algorithm method is compared with those obtained from deterministic formalism based on rate equations, revealing a clear dependence on the number of constituent molecules. © 2018 Wiley Periodicals, Inc.     

Substitution behaviour of amine-bridged dinuclear Pt(II) complexes with bio-relevant nucleophiles

Complexes of the type [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(HO)2]4+ and [Pt2(N,N,N',N'-tetrakis(2-pyridylmethyl)diamine(Cl)2]2+ were used to study their reactions with a series of bio-relevant nucleophiles, viz. thiourea, L-methionine and guanosine-5'-monophosphate (5'-GMP2-) as a function of nucleophile concentration and temperature. The reactions with the sulfur containing nucleophiles (thiourea and L-methionine) were followed under pseudo-first-order conditions by stopped-flow and UV-Vis spectrophotometry. The reaction with 5'-GMP2- was carried out under second order conditions and studied by NMR spectroscopy. The results indicate that the bridged dinuclear complexes remain intact after coordination of the studied nucleophiles for an extended period of time, which differs significantly from that reported for other multinuclear platinum complexes in the literature.     

Cytotoxic performances of new anionic cyclometalated Pt(II) complexes bearing chelated O^O ligands

The in vitro biological activity towards the MDA-MB-231 triple-negative breast cancer cell line of two different series of anionic Pt(II) organometallic complexes was tested. For the first time, cytotoxic activity of anionic Pt(II) complexes has been observed. The anionic compounds of general formula NBu₄[(C^N)Pt(O^O)], where (C^N) represents the cyclometalated form of 2-phenylpyridine (H(PhPy)), 2-thienylpyridine (H(Thpy)) or 2-benzo[h]quinoline (H(Bzq)), feature two different (O^O) chelated ligands: tetrabromocatechol [BrCat]²⁻ ( 1 – 3 ) or alizarine [Aliz]²⁻ ( 4 – 6 ). Complexes 1 – 6 displayed a significant cytotoxic effect against the studied cell line (IC₅₀ range of 1.9–52.8 μM). For BrCat-containing complexes 1 – 3 , the biological activity was independent of the nature of the coordinated (C^N) ligand. In contrast, in the case of 4 – 6 , the cytotoxicity (significantly high for 4 ) was concomitantly induced by the presence of either the PhPy or the [Aliz]²⁻ ligand. Since complexes 1–6 are emissive in solution, the potential use of 4 as a theranostic agent was investigated using confocal analysis. The fluorescence signal from MDA-MB-231 cells incubated with 4 indicated the localization of the compound into the cytosol region.     

ESR study of the kinetics and mechanism of ligand substitution reactions in Cu(II) bis-salicylaldehydate

The kinetics and mechanism of successive substitution of chelate ligands in Cu(II) bissalicyladehydate have been studied by ESR method.

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- (II).

     

Mono- and dinuclear metallacyclic complexes of Pt(II) synthesized from some eugenol derivatives

The complexes K[PtCl₃(Meug)] (1; Meug = methyleugenol), K[PtCl₃(Meteug)] (2; Meteug = methyl eugenoxyacetate), and K[PtCl₃(Eteug)] (3; Eteug = ethyl eugenoxyacetate) reacted with AgNO₃, SnCl₂, KOH, or ethanol–water solutions to lose one aryl proton and form dinuclear metallacyclic complexes Pt₂Cl₂(Meug-1H)₂ (4), Pt₂Cl₂(Meteug-1H)₂ (5), and Pt₂Cl₂(Eteug-1H)₂ (6), respectively. Complexes 4–6 reacted with aliphatic, aromatic, and heterocyclic amines to give various mononuclear metallacyclic platinum complexes 7–15. ¹H NMR spectra showed that in 4–15 Meug, Meteug, and Eteug are bound with Pt(II) both at the benzene carbon and at the ethylenic double bond of the side chain. NOESY spectra and single-crystal X-ray diffraction indicated that in 7–15 the amines are in cis-position with respect to the ethylenic double bond.     

Efficient structural modification of electron-withdrawing substituents on Pt(II) complexes for red emitters: A theoretical study

In this study, the electronic structures and optical properties of a cyclometalated Pt(II) complex (M1) and a series of derivatives (M1–F, M1–CF₃, and M1–CN) with electron-withdrawing substituents (–F, –CF₃, and –CN) at the carbazole moiety were theoretically investigated by density functional theory and time-dependent density functional theory. The calculation results reveal that these Pt complexes display deep red phosphorescence emission above Λ = 640 nm. When the ³MLCT/π → π* to triplet metal-centered ³MC/d–d state decay mechanism is taken into consideration, the nonradiative decay rate constant (k_nr) decreased in the order M1 > M1–CF₃ > M1–F > M1–CN. The <T₁|H_SOC|S_m> and kr values of M1-F are similar with those of M1, however the Knr rate ofM1-F is larger than that of M1. M1–F is expected to have improved quantum yields. Moreover, through the analyses of the HOMO/LUMO level and triplet energy, it is found that the introduction of –F and –CN substituents in M1 results in efficient energy transfer from the host material 4,4′-N,N′-dicarbazole-biphenyl to these complexes. In view of the electroluminescent applications in organic light-emitting diodes, M1–F can serve as efficient deep-red guest materials with improved electron injection and transport ability.     

Kinetic and mechanistic study of the Pt(II) versus Pt(IV) effect in the platinum-mediated nitrile-hydroxylamine coupling

The metal-mediated coupling between coordinated EtCN in the platinum(II) and platinum(IV) complexes cis- and trans-[PtCl(2)(EtCN)(2)], trans-[PtCl(4)(EtCN)(2)], a mixture of cis/trans-[PtCl(4)(EtCN)(2)] or [Ph(3)PCH(2)Ph][PtCl(n)(EtCN)] (n = 3, 5), and dialkyl- and dibenzylhydroxylamines R(2)NOH (R = Me, Et, CH(2)Ph, CH(2)C(6)H(4)Cl-p) proceeds smoothly in CH(2)Cl(2) at 20-25 degrees C and the subsequent workup allowed the isolation of new imino species [PtCl(n){NH=C(Et)ONR(2)}(2)] (n = 2, R = Me, cis-1 and trans-1; Et, cis-2 and trans-2; CH(2)Ph, cis-3 and trans-3; CH(2)C(6)H(4)Cl-p, cis-4 and trans-4; n = 4, R = Me, trans-9; Et, trans-10; CH(2)Ph, trans-11; CH(2)C(6)H(4)Cl-p, trans-12) or [Ph(3)PCH(2)Ph][PtCl(n){NH=C(Et)ONR(2)}] (n = 3, R = Me, 5; Et, 6; CH(2)Ph, 7; CH(2)C(6)H(4)Cl-p, 8; n = 5, R = Me, 13; Et, 14; CH(2)Ph, 15; CH(2)C(6)H(4)Cl-p, 16) in excellent to good (95-80%) isolated yields. The reduction of the Pt(IV) complexes 9-16 with the ylide Ph(3)P=CHCO(2)Me allows the synthesis of Pt(II) species 1-8. The compounds 1-16 were characterized by elemental analyses (C, H, N), FAB-MS, IR, (1)H, (13)C{(1)H}, and (31)P{(1)H} NMR (the latter for the anionic type complexes 5-8 and 13-16) and by X-ray crystallography for the Pt(II) (cis-1, cis-2, and trans-4) and Pt(IV) (15) species. Kinetic studies of addition of R(2)NOH (R = CH(2)C(6)H(4)Cl-p) to complexes [Ph(3)PCH(2)Ph][Pt(II)Cl(3)(EtCN)] and [Ph(3)PCH(2)Ph][Pt(IV)Cl(5)(EtCN)] by the (1)H NMR technique revealed that both reactions are first order in (p-ClC(6)H(4)CH(2))(2)NOH and Pt(II) or Pt(IV) complex, the second-order rate constant k(2) being three orders of magnitude larger for the Pt(IV) complex. The reactions are intermolecular in nature as proved by the independence of k(2) on the concentrations of added EtC triple bond N and Cl(-). These data and the calculated values of Delta H++ and Delta S++ are consistent with the mechanism involving the rate-limiting nucleophilic attack of the oxygen of (p-ClC(6)H(4)CH(2))(2)NOH at the sp-carbon of the C triple bond N bond followed by a fast proton migration.     

Synthesis and structural characterization of three dinuclear Copper(II) complexes incorporating pyrazolyl-derived ligands

Three new binuclear copper complexes of formulae $ \left[ {{\text{Cu}}_{2}^{\text{II}} {\text{Pz}}_{2}^{\text{Me3}} {\text{Br}}_{ 2} \left( {{\text{PPh}}_{ 3} } \right)_{ 2} } \right] $ (1), $ \left[ {{\text{Cu}}_{ 2}^{\text{II}} {\text{Pz}}_{2}^{\text{Ph2Me}} {\text{Cl}}_{ 2} \left( {{\text{PPh}}_{ 3} } \right)_{ 2} } \right] $ (2) and $ \left[ {{\text{Cu}}_{2}^{\text{II}} \left( {{\text{Pz}}^{\text{PhMe}} } \right)_{ 4} {\text{Cl}}_{ 4} } \right] $ (3) (Pz^Me3?=?3,4,5-trimethylpyrazole, Pz^Ph2Me?=?4-methyl-3,5-diphenylpyrazole and Pz^PhMe?=?3-methyl-5-phenylpyrazole) have been synthesized and characterized by chemical analysis, FTIR and ³¹P NMR spectroscopy and single-crystal X-ray diffraction. Complex 1 is a doubly bromo-bridged dimer, while complexes 2 and 3 are chloro-bridged dimers. The Cu(II) centers are in a distorted tetrahedral geometry for 1 and 2 and a distorted square pyramidal N₂Cl₃ environment for 3.     

Kinetic and mechanistic study with optically active, four-coordinate nickel(II) complexes: stereoselectivity in ligand substitution

Conventional and rapid scan stopped-flow spectrophotometry as well as polarimetry was used to study the kinetics of ligand substitution in six chiral bis N-alkylsalicylaldiminato nickel(II) complexes NiA2 by different chiral salen-type ligands H2B, according to NiA2 + H2B --> NiB + 2HA, in acetone at 298 K and, partly, at variable temperature. In most cases ligand substitution was found to follow monophasic second-order kinetics, rate = k x [NiA2] x [H2B]. Second-order rate constant k, lying in the range 10(-2)-400 M(-1) s(-1) at 298 K, was determined for the various combinations of enantiomers in a given system NiA2/H2B, namely, R-NiA2/R-H2B, S-NiA2/R-H2B, R-NiA2/S-H2B, and S-NiA2/S-H2B. It was found that ligand substitution is subject to chiral discrimination. The ratio of second-order rate constants, kfast/kslow, with kfast being rate constant k for the faster reacting pair of enantiomers and vice versa, lies in the range 1.0-3.0, depending on the nature of the N-alkyl groups in NiA2 and organic groups attached to the ethylene bridge in the salen ligands H2B. The rate discrimination factor of 3.0, as obtained for NiA2 = bis[N-dehydroabietylsalicylaldiminato]nickel(II) reacting with the R- and with the S-enantiomer of H2B = N,N'-disalicylidene-1,2-diamino-4-methylpentane, appears to be the highest stereoselectivity reported so far for ligand substitution in nickel(II) complexes. With NiA2 = R- and S-bis[N-(1-phenylethyl)-5-nitrosalicylaldiminato]nickel(II) and H2B = R- and S-N,N-disalicylidene-1,2-diamino-4-methylpentane, the kinetics of ligand substitution are biphasic, describing initial adduct formation between NiA2 and H2B (equilibrium constant K) and stepwise loss of the two bidentate ligands HA (first-order rate constants k1 and k2). The data for K, k1, and k2 for one of the combinations of enantiomers were determined at variable temperature, and the corresponding activation parameters are presented.     

Spectral and photophysical properties of mono and dinuclear Pt(II) and Pd(II) complexes: An unusual emission behaviour

Eight mononuclear complexes of the formula [M(N-N)(DHB)] and two binuclear complexes of the formula [M₂(BPY)₂(THB)] where M = Pd(II) or Pt(II), N-N = 2,2′-bipyridine (BPY), 2,2′-biquinoline (BIQ), 4,7-diphenyl-1,10-phenanthroline (DPP), 1,10-phenanthroline (PHEN); DHB = dianion of 3,4-dihydroxybenzaldehyde and THB = tetraanion of 3,3′,4,4′-tetrahydroxy benzaldazine were prepared and their electrochemical, spectral and photophysical properties were examined. These complexes were characterized by chemical analysis, IR and proton NMR spectroscopy. A detailed study on the absorption spectroscopy of these complexes was made. These complexes were found to show a low-energy solvatochromic ligand-to-ligand charge-transfer (LLCT) band. The electronic energies of these bands have been analyzed and compared with electrochemical data. Emission behaviour of the complexes of the series, [Pt(N-N)(DHB)], [Pt(N-N)(DHBA)] where DHBA is the dianion of 3,4-dihydroxybenzoic acid and [Pt₂(BPY)₂(THB)] was also investigated. These platinum complexes were found to emit from a low-energy state at low temperature and a high-energy state at room temperature. Photophysics of these complexes is also discussed.     

Synthesis and structural features of 1,3,2,5-dioxaboraphosphorinane complexes with Pt(II) and Pd(II) salts

Complexes of composition L₂MCl₂ [M=Pt, R=H (I), Me (II), Ph (III)], and LMC1₂ [M=Pd, R=H (IV)] are prepared by reaction of 4,6-R₂-2,5-diphenyl-1,3,2,5-dioxaboraphosphorinanes (L) with MCl₂. Far-IR and³¹P NMR spectroscopy are used to demonstrate that I is cis whereas II and III are trans complexes in the solid. The conformational behavior of I is studied by³¹P and¹H NMR. The asymmetric form of I exhibits anomalous stability.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2309–2312, October, 1991.