Ligand substitution reaction at a binuclear organoplatinum(II) complex

The ligand substitution reactions of the N-donor ligand in the binuclear dimethylplatinum(II) complex of formula cis,cis-[Me₂Pt(μ-NN)(μ-dppm)PtMe₂], 1, in which dppm = bis(diphenylphosphino)methane and NN = phthalazine, by different nucleophilic phosphorous-donors L, L = P(O-ⁱPr)₃ or PPh₃ and L₂ = dppm, to form the dinuclear complexes 2, cis,cis-[Me₂LPt(μ-dppm)PtLMe₂] and cis,cis-[Me₂Pt(μ-dppm)₂PtMe₂], respectively, are studied. Complex 1 has a MLCT band in the visible region which was used to easily follow the kinetics of its ligand substitution reactions. These reactions which involve diplatinum(II) complex 1 containing cis Pt-C bonds, proceeded by the normal associative mechanism. In associative reactions of the present work, as expected, the rate of the reactions was depended on the concentration and the nature of the entering group. The nucleophilicity of PPh₃ is stronger than P(O-ⁱPr)₃ on the basis of its stronger σ-donor ability and its lower solvation and is responsible for the observed 3-fold increase of its rate as compared to that of P(O-ⁱPr)₃. Also, the solvation energy involved is suggested to be responsible for the observation of higher rates in benzene than in acetone. The ΔH^‡/ΔS^‡ compensation plot gives a straight line which suggests the operation of the same mechanism for all entering nucleophiles.     

Photoacoustic calorimetry and quantum yields of Mo(CO)(6) ligand exchange in linear alkanes: determination of volume of reaction, energetics, and kinetics of nucleophile displacement of alkane from Mo(CO)(5)(alkane).

Photoacoustic calorimetry (PAC) and actinometry studies were used to determine the enthalpies and volumes of reaction for the production of a transient intermediate, Mo(CO)(5)-alkane, and for its subsequent reaction with tetrahydrofuran (THF). Both the enthalpy and the volume of reaction contribute to the photoacoustic signal and have been resolved by changing the solvent thermal expansion properties with a series of linear alkanes. The enthalpies for substitution of CO on Mo(CO)(6) by an alkane and of coordinated alkane on Mo(CO)(5)(alkane) by THF are 30 and -14 kcal/mol, respectively. Likewise, the volumes of reaction are 18 and -1 mL/mol. From available data for the Mo--CO bond energy, these results allow the calculation of the Mo-alkane and Mo-THF bond energies (11 and 25 kcal/mol, respectively). The Mo-alkane result is 7 kcal/mol less than that from our previous PAC study, which ignored the volume of reaction, and is in better agreement with the results of kinetic studies. The large absolute difference in the reaction volumes for each step is partially attributed to a void volume created during the formation of the Mo--THF bond. In general, the volume of reaction cannot be neglected in the calculation of enthalpies of ligand substitution from PAC studies. The quantum yields for photosubstitution of Mo(CO)(6), in contrast to Cr(CO)(6), were found to be insensitive to the chain length of the alkane solvent.     

Chelate ring closure kinetics in the reaction of a platinum(II) complex with diphosphines

The reactions of PPh2(CH2)nPPh2 (n=1–4, P-P) and PPh3 with cis- [PtPh2(CO)(SEt2)] have been studied in chloroform-d by 1H- and 31P{1H}-n.m.r. When n=2 or3 the first product observed is [PtPh2(P-P)], where the diphosphine is acting as a chelate, and ring closure is fast compared to the rate of entry of the phosphine into the complex. When n=1 or 4 the first observed product is [PtPh2(CO)(P-P)], with P-P acting as monodentate, and the second observable stage of reaction is ring closure. The rate constants and activation parameters kc at 298K (s–1), H (kJmol–1), S (JK–1mol–1) for the dppm and dppb complexes are 0.0198, 88±1, +17±3; and 0.00273, 38±2, –169±6, respectively. The formation of the large seven-membered ring is a strainless process, comparable to the intermolecular process. The increase in the enthalpy of activation as ring size decreases is due to ring strain and inter-atomic repulsions associated to the conformation of the four-membered chelate ring.     

Activation parameters for cyclohexene oxygenation by an oxoiron(IV) porphyrin pi-cation radical complex: entropy control of an allylic hydroxylation reaction

Activation parameters for epoxidation and allylic hydroxylation reactions of cyclohexene with FeIVO(TMP)*+Cl (1) were determined. Within the experimental temperature range, the epoxidation reaction was enthalpy-controlled (i.e., DeltaH > -TDeltaS), while the allylic hydroxylation reaction was entropy-controlled (i.e., -TDeltaS > DeltaH). An unexpectedly large contribution of the entropy term for the allylic hydroxylation reaction indicated that the free energy of activation, DeltaG, rather than the activation energy, Ea, should be used to discuss the reaction mechanism and chemoselectivity. The results of this study bring caution to previous density functional theory studies, in which the reaction mechanism and chemoselectivity are evaluated from calculated Ea.     

A binuclear tantalum(III) complex with a bridging carboxylato ligand

Ta₂Cl₆(SMe₂)₃ reacts with one equivalent of C₄H₉CO₂Li to give a complex with a bridging carboxylate ligand, Ta₂Cl₅(O₂CC₄H₉)(SMe₂)(THF)₂. The product was isolated in two crystalline forms, 1 and 2, from a THF/hexane and benzene/hexane solvent mixture, respectively. The following are the unit cell parameters, for 1: monoclinic (P2₁/n), a = 10.537(5) Å, b = 22.015(4) Å, c = 11.663(4) Å, β = 107.80(3)°, V = 2576(3) ų, and Z = 4; for 2: monoclinic (P2₁/c), a = 15.584(4) Å, b = 15.647(4) Å, c = 11.275(3) Å, β = 106.04(5)°, V = 2642(3) ų and Z = 4. The complex is a dimer with a distorted octahedra-sharing-an-edge geometry. The TaTa distances in 1 and 2 were 2.766(1) Å and 2.779(1) Å, respectively, which is somewhat longer than in previously reported Nb(III) and Ta(III) dinuclear compounds. Diamagnetism of the complex is shown by NMR. Fenske—Hall calculations, which correctly predict an electronic transition at about 16,000 cm^?1, indicate a double TaTa bond. The observed elongation of the metalmetal bond is attributed mainly to steric crowding. The complex is the first proven low-valent Ta species with a coordinated carboxylate ion.     

Ni(III) vs. Ni(II)-thiyl radical: charge-delocalisation in a binuclear Ni(III)Ni(II)-dithiolate complex

Multi-frequency EPR spectroscopy on 61Ni-labelled samples of [Ni2(L)]3+ confirms extensive charge-delocalisation between the Ni(III) centre and thiolate donors in the Ni(II)Ni(III) complex.     

Monitoring the DNA binding kinetics of a binuclear ruthenium complex by energy transfer: evidence for slow shuffling

The semirigid binuclear ruthenium complex Delta,Delta-[mu-(11,11'-bidppz)(phen)(4)Ru(2)](4+) has been shown to rearrange slowly from an initial groove-bound nonluminescent state to a final intercalated emissive state by threading one of its bulky Ru(phen)(2) moieties through the DNA base stack. When this complex binds to poly[d(A-T)(2)], a further increase in emission from the complex is observed after completion of the intercalation, assigned to reorganization of the intercalated complex. We here report a study of the threading process in poly[d(A-T)(2)], in which the minor groove binding dye DAPI is used as an energy transfer probe molecule to assess the distribution of ruthenium complex during and also after the actual threading phase. The emission from DAPI is found to change with the same rate as the emission from the ruthenium complex, and furthermore, DAPI does not disturb the binding kinetics of the latter, justifying it as a good probe of both the threading and the reorganization processes. We conclude from the change in the emission from both DAPI and the ruthenium complex with time that DAPI-ruthenium interactions are most pronounced during the process of threading of the complex, suggesting that the complexes are initially threaded slightly anticooperatively and thereafter redistribute along the DNA to reach their thermodynamically most favorable distribution. The final distribution is characterized by a small but significant binding cooperativity, probably as a result of hydrophobic interactions between the complex ions despite their tetravalent positive charges. The mechanism of "shuffling" the complex along the DNA chain is discussed, i.e., whether the ruthenium complex remains threaded (requiring sequential base-pair openings) or if unthreading followed by lateral diffusion within the ionic atmosphere of the DNA and rethreading occurs.     

Synthesis and structural characterization of a binuclear zirconium complex of tetraanionic p-tert-butylthiacalix[4]arene bridged by methanol

A zirconium complex with the p-tert-butylthiacalix[4]arene anion was synthesized and its crystal structure was determined by single-crystal X-ray analysis. The complex [Zr(μ₂-CH₃OH)(p-tert-butylthiacalix[4]arene)]₂·9H₂O (1) belongs to the orthorhombic system, space group Pnnm, with a?=?20.436(16), b?=?12.160(8), c?=?20.305(12)?Å, V?=?6774(7)?ų and Z?=?2. In Complex 1 zirconium coordinates to four phenolic anions of the deprotonated p-tert-butylthiacalix[4]arene and is bridged by two methanol molecules; the p-tert-butylthiacalix[4]arene adopts a cone conformation.     

Enantioselective ring opening of meso-epoxides with thiols catalyzed by a chiral (salen)Ti(IV) complex

A chiral (salen)Ti(IV) complex was an efficient catalyst in the asymmetric ring opening of meso-epoxides 1 with thiols 2 in high yields and good ee.     

Specific features of radical generation in the reaction of thiols with hydrogen peroxide

Russian Chemical Bulletin - Using the method of inhibitors it was found that the reduction of H2O2 with natural thiols in aqueous solutions is accompanied by the formation of radicals. The reaction...     

Methyl abstraction kinetics of CpFe(CO)2Me using the benzyl radical clock

The rate constant for the methyl abstraction reaction of CpFe(CO)₂Me has been measured with the benzyl radical clock as (1.1 ± 0.2) × 10⁵ M⁻¹ s⁻¹ at room temperature. Time-resolved Fourier-transform Infrared (FTIR) absorption spectroscopy pointed towards the formation of the CpFe(CO)₂ radical upon benzyl abstraction. The main stable product has been established by a linear scan of the reaction mixture as Cp₂Fe₂(CO)₄ produced by the dimerization of the CpFe(CO)₂ radicals. The transition state structure for the abstraction process was also found at UB3LYP/6-311+G* level of theory to contain a planar CH₃ group.     

Reactions of a ruthenium(II) arene antitumor complex with cysteine and methionine

The Ru(II) organometallic antitumor complex [(eta(6)-biphenyl)RuCl(en)][PF(6)] (1) reacts slowly with the amino acid L-cysteine (L-CysH(2)) in aqueous solution at 310 K. Reactions were followed over periods of up to 48 h using HPLC, electronic absorption spectroscopy, LC-ESI-MS, and 1D or 2D (1)H and (15)N NMR spectroscopy. Reactions at a 1 mM/2 mM (Ru/L-CysH(2)) ratio were multiphasic in acidic solutions (pH 5.1) and appeared to involve aquation as the first step. Initially, 1:1 adducts involving substitution of Cl by S-bound or O-bound L-CysH(2), [(eta(6)-biphenyl)Ru(S-L-CysH)(en)](+) (4a) and [(eta(6)-biphenyl)Ru(O-L-CysH(2))(en)](2+) (4b) formed, followed by the cystine adduct [(eta(6)-biphenyl)Ru(O-Cys(2)H(2))(en)](2+) (3), and two dinuclear complexes from which half or all of the chelated ethylenediamine had been displaced, [(eta(6)-biphenyl)Ru(H(2)O)(microS,N-L-Cys)Ru(eta(6)-biphenyl)(en)](2+) (5) containing one bridging cysteine, and [(eta(6)-biphenyl)Ru(O,N-L-Cys-S)(S-L-Cys-N)Ru(eta(6)-biphenyl)(H(2)O)] (6) containing two bridging cysteines. The unusual cluster species [(biphenyl)Ru](8) (7a) was also detected by MS and was more prevalent in reactions at higher L-CysH(2) concentrations. Complex 5 was the dominant product at pH 2-5, but overall, only ca. 50% of 1 reacted with L-CysH(2) in these conditions. The reaction between 1 and L-CysH(2) was suppressed in 50 mM triethylammonium acetate solution at pH > 5 or in 100 mM NaCl. Only 27% of complex 1 reacted with L-methionine (L-MetH) at an initial pH of 5.7 after 48 h at 310 K and gave rise to only one adduct [(eta(6)-biphenyl)Ru(S-L-MetH)(en)](2+) (8).     

Homogeneous and heterogeneous olefin epoxidation catalyzed by a binuclear Mn(II)Mn(III) complex

The synthesis and characterization of a binuclear carboxylated bridged manganese complex containing the heptadentate ligand N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)-2-ol-1,3-propanediamine (H₃bbppnol) is reported. This complex was characterized by elemental analysis; infrared, electronic (UV–vis) and EPR spectroscopy; and conductivity measurements. The complex was immobilized on silica by either adsorption or entrapment via a sol–gel route. The obtained solids were characterized by thermogravimetric analyses (TG and DSC), UV–vis and infrared spectroscopy, and X-ray diffraction. The catalytic performance of the binuclear manganese complex in epoxidation reactions was evaluated for both homogeneous and heterogeneous systems. The catalytic investigation revealed that the complex performs well as an epoxidation catalyst for the substrates cyclohexene (26–39%) and cyclooctene (29–74%). The solids containing the immobilized complex can be recovered from the reaction medium and reused, maintaining good catalytic activity.     

Theoretical investigation of the kinetics for the hydrogen abstraction reaction of 1,1,1,2-tetrafluoroethane (HFC-134a) by chlorine radical

The hydrogen abstraction reaction of 1,1,1,2-tetrafluoroethane (HFC-134a) by chlorine radical is investigated by theoretical calculations. Equilibrium geometries and harmonic vibrational frequencies of the reactants, transition state, and products are calculated using high-level ab initio methods. Rate constants of forward and backward reactions for the temperatures from 200 to 1000 K are calculated using classical transition state theory with Eckart tunneling correction, fitted in the expressions k_f (T) = 1.19 × 10⁻²³T^3.93exp (−1110/T), and k_b (T) = 8.86 × 10⁻²⁴T^3.32exp (−959/T) cm³ molecule⁻¹ s⁻¹ for forward and backward reactions, respectively, and are in reasonable agreement with the available experimental values.