Substitution of [Pt(terpy)H2O]2+ and [Pt(bpma)H2O]2+ with thiols in acidic aqueous solution. terpy = 2,2′:6′2″-terpyridine; bpma = bis(2-pyridylmethyl)amine

The substitution behaviour of [Pt(terpy)H₂O]²⁺ and [Pt(bpma)H₂O]²⁺, where terpy is 2,2:62-terpyridine and bpma is bis(2-pyridylmethyl)amine, was studied as a function of entering thiol concentration and temperature. The reactions between the Pt-complexes and DL-penicillamine, L-cysteine and glutathione were carried out in a 0.10 mol dm^–3 aqueous HClO₄ medium using stopped-flow and conventional u.v.–vis spectrophotometry. The observed pseudo-first-order rate constants for the substitutions are given by k _obs = k ₂[thiol] + k _–2. The k _–2 term represents the reverse solvolysis. This was found to be zero for Pt^II(terpy) which was the most reactive complex. The second-order rate constants, k ₂, for the three thiols varied between 0.107 ± 0.001 and 0.517 ± 0.025 M^–1 s^–1 for Pt^II(bpma) and 10.7 ± 0.7–711.9 ± 18.3 M^–1 S^–1 for Pt^II(terpy), whereas glutathione was found to be the strongest nucleophile. An analysis of the activation parameters, H and S , clearly shows that the substitution process is associative in nature.     

Kinetic and mechanistic study on the reactions of [Pt(bpma)(H2O)]2+ and [Pd(bpma)(H2O)]2+ with some nucleophiles. Crystal structure of [Pd(bpma)(py)](ClO4)2

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+ and [Pt(bpma)(H2O)]2+, where bpma = bis(2-pyridylmethyl)amine, with TU, DMTU and TMTU for both complexes and Cl-, Br-, I- and SCN- for the platinum complex, were studied in aqueous 0.10 M NaClO4 at pH 2.5 using a variable-temperature stopped-flow spectrophotometer. The pKa value for the coordinated water molecule in [Pd(bpma)(H2O)]2+ (6.67) is a unit higher than that of [Pt(bpma)(H2O)]2+. The observed pseudo-first-order rate constants k(obs) (s(-1)) obeyed the equation k(obs) = k2[Nu] (Nu = nucleophile). The second-order rate constants indicate that the Pd(II) complex is a factor of 10(3) more reactive than Pt(II) complex. The nucleophile reactivity attributed to the steric hindrance in case of TMTU and the inductive effect for DMTU was found to be DMTU > TU > TMTU for [Pt(bpma)(H2O)]2+ and DMTU approximately TU > TMTU for [Pd(bpma)(H2O)]2+. The trend for ionic nucleophile was I- > SCN- > Br- > Cl-, an order linked to their polarizability and the softness or hardness of the metal. Activation parameters were determined for all reactions and the negative entropies of activation (Delta S++) support an associative ligand substitution mechanism. The X-ray crystal structure of [Pd(bpma)(py)](ClO4)2 was determined; it belongs to the triclinic space group P1 and has one formula unit in the unit cell. The unit cell dimensions are a = 8.522(2), b = 8.627(2), c = 16.730(4) A; alpha = 89.20(2), beta = 81.03(2), gamma = 60.61(2) degrees ; V = 1055.7(5) A3. The structure was solved using direct methods in WinGX's implementation of SHELXS-97 and refined to R = 0.054. The coordination geometry of [Pd(bpma)(py)]2+ is distorted square-planar. The Pd-N(central) bond distance, 1.996(3) A, is shorter than the other two Pd-N distances, 2.017(3) and 2.019(3) A. The Pd-N(pyridine) distance is 2.037(3) A.     

The role of substituents in a bidentate <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-chelating ligand on the substitution of aqua ligands from mononuclear Pt(II) complexes

The rate of substitution of aqua ligands from three mononuclear platinum(II) complexes, namely [Pt{2-(pyrazol-1-ylmethyl)pyridine}(H₂O)₂](ClO₄)₂, [Pt(H ₂ Py)]; [Pt{2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine}(H₂O)₂](ClO₄)₂, [Pt(dCH ₃ Py)] and [Pt{2-[(3,5-bis(trifluoromethyl)pyrazoly-1-ylmethyl]pyridine}(H₂O)₂](ClO₄)₂, [Pt(dCF ₃ Py)] by thiourea, N,N-dimethylthiourea and N,N,N′,N′-tetramethylthiourea, was studied in aqueous perchloric acid medium of constant ionic strength. The substitution reactions were investigated under pseudo-first-order conditions as a function of nucleophile concentration and temperature using UV/Visible and stopped-flow spectrophotometries. The observed pseudo-first-order rate constants, \( k_{{{\text{obs }}\left( {1/2} \right)}} \), for the stepwise substitution of the first and second aqua ligands obeyed the rate law: \( k_{{{\text{obs}}\left( {1/2} \right)}} = k_{{2 \left( { 1 {\text{st/2nd}}} \right)}} \left[ {\text{Nu}} \right] \). The first substitution reaction takes place trans to the pyrazole ligand, while the second entering nucleophile is stabilised at the reaction site trans to the pyridine ligand. The rate of substitution of the first aqua ligand from the complexes followed the order: Pt(dCF ₃ Py) > Pt(H ₂ Py) > Pt(dCH ₃ Py), while that of the second was Pt(H ₂ Py) ≈ Pt(dCF ₃ Py) > Pt(dCH ₃ Py). Lower pK _a values were found for the deprotonation of the aqua ligand cis to the pyrazole ring. Density functional theory calculations were performed to support the interpretation of the experimental results.     

Aqua substitution from mononuclear Pt(II) complexes with 2-(pyrazol-1-ylmethyl)quinoline non-leaving ligands

The rates of aqua substitution from [Pt{2-(pyrazol-1-ylmethyl)quinoline}(H₂O)₂](ClO₄)₂, [Pt(H₂Qn)], [Pt{2-(3,5-dimethylpyrazol-1-ylmethyl)quinoline}(H₂O)₂](ClO₄)₂, [Pt(dCH₃Qn)], [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]quinoline}(H₂O)₂](ClO₄)₂, [Pt(dCF₃Qn)], and [Pt{2-[(3,5-bis(trifluoromethyl)pyrazol-1-ylmethyl]pyridine}(H₂O)₂](ClO₄)₂, [Pt(dCF₃Py)], with three sulfur donor nucleophiles were studied. The reactions were followed under pseudo-first-order conditions as a function of nucleophile concentration and temperature using a stopped-flow analyzer and UV/visible spectrophotometry. The substitution reactions proceeded sequentially. The second-order rate constants for substituting the aqua ligands in the first substitution step increased in the order Pt(dCH₃Qn) < Pt(dCF₃Qn) < Pt(H₂Qn) < Pt(dCF₃Py), while that of the second substitution step was Pt(dCH₃Qn) < Pt(dCF₃Qn) < Pt(dCF₃Py) < Pt(H₂Qn). The reactivity trends confirm that the quinoline substructure in the (pyrazolylmethyl)quinoline ligands acts as an apparent donor of electron density toward the metal center rather than being a π-acceptor. Measured pK_a values from spectrophotometric acid–base titrations were Pt(H₂Qn) (pK_a1 = 4.56; pK_a2 = 6.32), Pt(dCH₃Qn) (pK_a1 = 4.88; pK_a2 = 6.31), Pt(dCF₃Qn) (pK_a1 = 4.07; pK_a2 = 6.35), and Pt(dCF₃Py) (pK_a1 = 4.76; pK_a2 = 6.27). The activation parameters from the temperature dependence of the second-order rate constants support an associative mechanism of substitution.     

Electronic tuning of the lability of Pt(II) complexes through pi-acceptor effects. Correlations between thermodynamic,kinetic, and theoretical parameters

pi-Acceptor effects are often used to account for the unusual high lability of [Pt(terpy)L]((2)(-)(n)+) (terpy = 2,2':6',2' '-terpyridine) complexes. To gain further insight into this phenomenon, the pi-acceptor effect was varied systematically by studying the lability of [Pt(diethylenetriamine)OH(2)](2+) (aaa), [Pt(2,6-bis-aminomethylpyridine)OH(2)](2+) (apa), [Pt(N-(pyridyl-2-methyl)-1,2-diamino-ethane)OH(2)](2+) (aap), [Pt(bis(2-pyridylmethyl)amine)OH(2)](2+) (pap), [Pt(2,2'-bipyridine)(NH(3))(OH(2))](2+) (app), and [Pt(terpy)OH(2)](2+) (ppp). The crystal structure of the apa precursor [Pt(2,6-bis-aminomethylpyridine)Cl]Cl.H(2)O was determined. The substitution of water by a series of nucleophiles, viz. thiourea, N,N-dimethylthiourea, N,N,N',N'-tetramethylthiourea, I(-), and SCN(-), was studied under pseudo-first-order conditions as a function of concentration, pH, temperature, and pressure, using stopped-flow techniques. The data enable an overall comparison of the substitution behavior of these complexes, emphasizing the role played by the kinetic cis and trans pi-acceptor effects. The results indicate that the cis pi-acceptor effect is larger than the trans pi-acceptor effect, and that the pi-acceptor effects are multiplicative. DFT calculations at the B3LYP/LACVP level of theory show that, by the addition of pi-acceptor ligands to the metal, the positive charge on the metal center increases, and the energy separation of the frontier molecular orbitals (E(LUMO) - E(HOMO)) of the ground state Pt(II) complexes decreases. The calculations collectively support the experimentally observed additional increase in reactivity when two pi-accepting rings are adjacent to each other (app and ppp), which is ascribed to "electronic communication" between the pyridine rings. The results furthermore indicate that the pK(a) value of the platinum bound water molecule is controlled by the pi-accepting nature of the chelate system and reflects the electron density around the metal center. This in turn controls the rate of the associative substitution reaction and was analyzed using the Hammett equation.     

Role of bridging diamine linkers on the rate of ligand substitution in a series of dinuclear PtII complexes

A series of dinuclear platinum^II complexes of the type [{trans‐Pt(H₂O)(NH₃)₂}₂‐NH₂(CH₂)_nH₂N]⁴⁺ (where n = 2, 3, 4, and 6) were synthesized to investigate the influence of the bridging diamine linker on the reactivity of the platinum centers. The pK_a values were determined, and the rates of substitution of the aqua moieties by a series of neutral nucleophiles viz. thiourea, 1,3‐dimethyl‐2‐thiourea, and 1,1,3,3‐tetramethyl‐2‐thiourea were studied as a function of concentration and temperature. All reactions studied gave excellent fits to a single exponential and obeyed the simple rate law, k_obs=k₂[Nu]. Negative activation entropies support an associative mode of substitution. The results obtained suggest that the rate of substitution is definitely influenced by the length of the diamine chain, with the rate of substitution decreasing as the length of the diamine chain increases. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 202–210, 2006     

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     

The role of the double bond position in hydroboration using HBBr2 · SMe2, HBCl2 · SMe2 and H2BBr · SMe2: A detailed kinetic and mechanistic study

Hydroboration reactions of 4-octene with HBBr₂ · SMe₂, HBCl₂ · SMe₂ and H₂BBr · SMe₂ in CH₂Cl₂ were studied as function of concentration and temperature and compared with those of 1-octene. On average, hydroboration with dihaloborane proceeded 16 times slower for 4-octene than for 1-octene. In the case of the reactions with the monohaloborane, this factor is halved. This can be explained by the difference in the relative rates of dissociates of Me₂S from the dihaloborane and a monohaloborane complex, respectively. The reactions involving H₂BBr · SMe₂ also exhibited a k_?2 value, an indication of the presence of a parallel reaction, most likely a rearrangement process facilitating isomerization by way of a π-complex. The moderate ΔH^≠ values accompanied by small ΔS^≠ values (94 ± 4 kJ mol^?1, ?3 ± 13 J K^?1 mol^?1 for HBBr₂ · SMe₂; 93 ± 1 kJ mol^?1, ?17 ± 4 J K^?1 mol^?1 for HBCl₂ · SMe₂ and in the case of H₂BBr · SMe₂, 90 ± 13 kJ mol^?1, +12 ± 44 J K^?1 mol^?1 and 83 ± 13 kJ mol^?1, ?24 ± 45 J K^?1 mol^?1, respectively, for the k₂ and k_?2 processes) imply a process that is dissociatively dominated, with the overall mode of activation being interchange dissociative (I_d).     

(1H‐Pyrazole‐κN2)(2,2′:6′,2′′‐terpyridine‐κ3N,N′,N′′)platinum(II) bis(perchlorate) nitromethane monosolvate

The reaction between [PtCl(terpy)]·2H₂O (terpy is 2,2′:6′,2′′‐terpyridine) and pyrazole in the presence of two equivalents of AgClO₄ in nitromethane yields the title compound, [Pt(C₃H₄N₂)(C₁₅H₁₁N₃)](ClO₄)₂·CH₃NO₂, 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.     

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.