Competitive aryl-iodide vs aryl-aryl reductive elimination reactions in Pt(IV) complexes: experimental and theoretical studies

The platinum(IV) complex trans-(dmpe)Pt(IV)(Ar)2I2 (2, dmpe = 1,2-dimethylphosphinoethane, Ar = 4-FC6H4) rapidly reacts, upon moderate heating in solution under ambient light, via two distinct pathways: isomerization to the corresponding cis-isomer (3) and Ar-I reductive elimination to give (dmpe)Pt(II)(Ar)I (4). Complex 3 undergoes, upon prolonged heating at high temperatures, an exclusive Ar-Ar reductive elimination reaction to give (dmpe)Pt(II)I2. Experimental and DFT studies showed that the 2-to-3 isomerization proceeds via three pathways: photochemical or thermal phosphine chelate opening and a mechanism involving cleavage of the Pt-I bond. The isomerization reaction is significantly slowed down but not stopped in the absence of light or in the presence of an excess of tetra-n-butylammonium iodide. On the other hand, the Ar-I reductive elimination from 2 proceeds via the Pt(delta+)-I(delta-) ion pairlike transition state. Use of the rigid dmpe analogue 1,2-dimethylphosphinobenzene (dmpbz) as the ligand shuts down the chelate ring-opening isomerization pathway and enables faster Ar-I reductive elimination thus making the latter reaction the major reaction route for the dmpbz supported trans-diiodo Pt(IV) complex 8.     

Photochemical and electronic properties of conjugated bis(azo) compounds: an experimental and computational study

We have investigated the photophysical, photochemical and electrochemical properties of two bis(azo) derivatives, (E,E)-m-1 and (E,E)-p-1. The two compounds, which can be viewed as being composed of a pair of azobenzene units sharing one of their phenyl rings, differ only for the relative position of the two azo groups on the central phenyl ring-meta and para for m-1 and p-1, respectively. The UV-visible absorption spectra and photoisomerisation properties are noticeably different for the two structural isomers; (E,E)-m-1 behaves similarly to (E)-azobenzene, while (E,E)-p-1 exhibits a substantial red shift in the absorption bands and a decreased photoreactivity. The three geometric isomers of m-1, namely the E,E, E,Z and Z,Z isomers, cannot be resolved in a mixture by absorption spectroscopy, while the presence of three distinct species can be revealed by analysis of the absorption changes observed upon photoisomerisation of (E,E)-p-1. Quantum chemical ZINDO/1 calculations of vertical excitation energies nicely reproduce the observed absorption changes and support the idea that, while the absorption spectra of the geometrical isomers of m-1 are approximately given by the sum of the spectra of the constituting azobenzene units in their relevant isomeric form, this is not the case for p-1. From a detailed study on the E-->Z photoisomerisation reaction it was observed that the photoreactivity of an azo unit in m-1 is influenced by the isomeric state of the other one. Such observations indicate a different degree of electronic coupling and communication between the two azo units in m-1 and p-1, as confirmed by electrochemical experiments and quantum chemical calculations. The decreased photoisomerisation efficiency of (E,E)-p-1 compared to (E,E)-m-1 is rationalised by modelling the geometry relaxation of the lowest pi-pi* state. These results are expected to be important for the design of novel oligomers and polymers, based on the azobenzene unit, with predetermined photoreactivity.     

Structural studies of bis(histidinato)nickel(II): Combined experimental and computational studies

A distorted octahedral nickel(II) complex, [Ni(2-amino-3-(1H-imidazol-4-yl)propanoic acid)₂] (1), has been synthesized by a solvothermal method and characterized by single-crystal X-ray diffraction. Geometry optimization in the gas phase and pyridine together with Hirshfeld surface and reduced density gradient analyses reveal that this complex shows different distortions from octahedral in the gas, liquid, and solid phases. The reason seems to be because of the presence of two intramolecular NH⋯O weak interactions in the gas phase and two sets of rather strong intermolecular NH⋯O and CH⋯O interactions in the solid phase. Time-dependent density functional theory (TD-DFT) calculations suggest that these different distortions result in different electronic absorption spectra.     

Voltammetric surface dealloying of Pt bimetallic nanoparticles: an experimental and DFT computational analysis

Voltammetric dealloying of bimetallic platinum-copper (Pt-Cu) alloys has been shown to be an effective strategy to modify the surface electrocatalytic reactivity of Pt bimetallic nanoparticles (S. Koh and P. Strasser, J. Am. Chem. Soc., 2007, 129, 12624). Using cyclic voltammetry and structural XRD studies, we systematically characterize the Pt-Cu precursor compounds as well as the early stages of the selective Cu surface dissolution (dealloying) process for Pt(25)Cu(75), Pt(50)Cu(50), and Pt(75)Cu(25) alloy nanoparticles annealed at both low and high temperature. We also assess the impact of the synthesis conditions on the electrocatalytic reactivity for the oxygen reduction reaction (ORR). To gain atomistic insight into the observed voltammetric profiles, we compare our experimental results with periodic DFT calculations of trends in the thermodynamics of surface Cu dissolution potentials from highly stepped and kinked Pt(854) single crystal surfaces. The modeling suggests a dependence of the electrochemical Cu dissolution potentials on the detailed atomic environment (coordination number, nature of coordinating atoms) of the bimetallic Pt-Cu surfaces. The DFT-predicted shifts in electrochemical Cu dissolution potentials are shown to qualitatively account for the observed voltammetric profiles during Cu dealloying. Our study suggests that metal-specific energetics have to be taken into account to explain the detailed dealloying behavior of bimetallic surfaces.     

General relations holding in variation of physical properties of organic compounds within homologous series

The patterns of variation of different parameters (A) of organic compounds within a homologous series (such as boiling point under atmospheric pressure, critical temperature, critical pressure, refractive index, relative density, viscosity, surface tension, saturated vapor pressure, dielectric constant, first adiabatic ionization energies, etc.) are identical, and they can be described in terms of a single linear recurrent equation, A(n+1)=aA(n)+b. This equation relates a property of any member of a homologous series to the corresponding parameters of preceding homologs. The largest deviations from the proposed relation were revealed only in some homologous series for a few (1 or 2) simplest representatives which are characterized by deviations of all parameters.     

Human serum albumin binding studies of a new platinum(IV) complex containing the drug pregabalin: experimental and computational methods

A new platinum(IV) complex, [Pt(en)(Cl)₂(Pregabalin)₂], containing the drug pregabalin was synthesized and characterized by elemental analysis, FT-IR, ¹H NMR, mass spectrum, thermogravimetric analysis (TGA), molecular docking and RHF/PM6 method. Also, the interaction of Pt(IV) complex with human serum albumin (HSA) was studied by using UV–vis, fluorometric, circular dichroism (CD) spectroscopies and molecular docking techniques. The results demonstrated that the binding of the complex to HSA caused strong fluorescence quenching of HSA through static quenching mechanism. Hydrogen bonds and van der Waals contacts are the major forces in the stability of protein-Pt(IV) complex and the process of the binding of complex with HSA was enthalpy driven (ΔH = –105.8?kJ·mol^?1). The results of CD and UV–vis spectroscopy indicated that the binding of the complex to HSA caused conformational changes in HSA. In addition, the study of molecular docking and RHF/PM6 method confirm the experimental results with respect to the mechanism of binding.     

Dissociation of carbanions from acyl iridium compounds: an experimental and computational investigation

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp(PMe(3))Ir(H)[C(O)CF(3)] (1) leads to the quantitative formation of Cp(PMe(3))Ir(CO) (2) and CF(3)H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp(PMe(3))Ir(H)(CO)](+)[CF(3)](-). Further evidence for the ability of CF(3)(-) to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp(PMe(3))Ir(Me)[C(O)CF(3)] and Cp(PMe(3))Ir(Cl)[C(O)CF(3)]. Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF(3)D in CD(3)OD. Three additional acyl hydrides, Cp(PMe(3))Ir(H)[C(O)R(F)] (where R(F) = CF(2)CF(3), CF(2)CF(2)CF(3), or CF(2)(CF(2))(6)CF(3)) undergo R(F)-H elimination to give 2 at a faster rate than CF(3)H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the beta-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium.     

Preparation, properties and structures of the first series of organometallic Pt(II) and Pt(IV) complexes with stibine co-ligands

The planar Pt(II) monomers [PtMe2(L-L)] and [(PtMe2)2(L'-L')2] dimers (L-L = R2Sb(CH2)3SbR2, o-C6H4(CH2SbMe2)2; L'-L' = R2SbCH2SbR2; R = Me or Ph) are obtained in good yield via reaction of [PtMe2(SMe2)2] with L-L or L'-L' in benzene. The Pt(iv) stibines, [PtMe3(L-L)I] (L-L = R2Sb(CH2)3SbR2, o-C6H4(CH2SbMe2)2 or 2 x SbPh3, SbMePh2 or SbMe2Ph) are obtained by treatment of [PtMe3I] with L-L in chloroform. These represent the first series of stable Pt(IV) stibine complexes. All of the products have been characterised by 1H, 13C{1H}, 195Pt NMR spectroscopy, electrospray mass spectrometry and analysis. Crystal structure determinations on [PtMe3{R2Sb(CH2)3SbR2}I], [PtMe3{o-C6H4(CH2SbMe2)2}I] and [PtMe3(SbPh3)2I] confirm the distorted octahedral environment at Pt, with fac Me groups and mutually cis Sb donor atoms. The Sb-Pt-Sb angle in the seven-membered chelate ring of the o-C6H4(CH2SbMe2)2 complex is ca. 96 degrees , compared to <90 degrees in the complexes with six-membered chelates. The C1-distibines R2SbCH2SbR2 afford only the dinuclear [(PtMe3)2(mu-R2SbCH2SbR2)(mu-I)2] in which the stibine ligand and two I atoms bridge two Pt atoms giving an edge sharing bioctahedral geometry which has been confirmed by a crystal structure analysis. The Pt(II) species undergo oxidative addition with MeI to give the corresponding Pt(IV) species, while the Pt(IV) species reductively eliminate ethane upon thermolysis.     

Complexes of Pt(II) and Pt(IV) with disubstituted purines

Mixed-ligand complexes of Pt(II) and Pt(IV) with 2,6-diaminopurine and 6-thioguanine were synthesized and characterised. The complexes were prepared in acidic and basic media. The binding of the ligands to the metal ion varies according to the pH of the medium. Thus, in the complexes of 6-thioguanine, the ligand acts as a monodentate ligand coordinating through the neutral C₆-SH group in the acidic medium and in the basic medium as a bidentate ligand binding to the metal ion through C₆S^? and N₇, forming a five-membered chelate ring. In an acidic medium 2,6-diaminopurine forms mononuclear complexes with Pt(II) and Pt(IV) binding through N₇. In a basic medium binuclear hydroxobridged complexes are formed with Pt(IV) and the ligand is monodentate, coordinating through N₇.     

Reaction of sterically hindered arylmercury compounds with Pt(IV), Pt(II), and Pd(II) chloride complexes

Conclusion Heating of a solution of the PtCl₆ ^2– ion and an arylmercury compound containing a substituent in the ortho position to mercury leads to the formation of a diaryl (in the case of -naphthylmercury) or arene (in the case of the mesityl or pentamethylphenyl mercury derivatives).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2374–2376, October, 1986.The authors express their gratitude to A. E. Shilov and A. K. Yatsimirskii for a useful discussion of these results.     

Temperature behavior of the electric field-induced entropy increment within a homologous series of nematogenic compounds

Based on the temperature behavior of the entropy increment induced by the probing electric field in the isotropic phase of mesogenic compounds belonging to the homologous series C(n)H(2n+1)PhCOOPhCN ( n = 4-10), it is found that an ability to the dipolar aggregation of the molecules depends on the alkyl tail length, and, in particular, the ability is strongly reinforced when the number n changes from 7 to 8. The role of the molecular structure in the self-assembling process is discussed.     

Reduction of an asymmetric Pt(IV) prodrug fac-[Pt(dach)Cl3(OC(=O)CH3)] by biological thiol compounds: kinetic and mechanistic characterizations

Transition Metal Chemistry - An asymmetric Pt(IV) prodrug fac-[Pt (dach)Cl3(OC(=O)CH3)] (dach?=?1,2-diaminocyclohexane) was synthesized, and the reduction of the Pt(IV) prodrug by three...     

Some new organotin(IV) schiff base adducts: Synthesis, spectroscopic characterization and thermal studies

Three new Schiff base adducts, [SnMe₂Cl₂(H₂cdnaphen)] (1), [SnPh₂Cl₂(H₂cdnaphen)₂].C₆H₆ (2) and [SnBu₂Cl₂(H₂cdnaphen)₂] (3) were synthesized by the reaction of SnR₂Cl₂ (R = Me, Bu and Ph) with a Schiff base ligand, Methyl 2-[2-(2-hydroxynaphthaldimino)ethylamino]-1-cyclopentene-1-dithiocarboxylate (H₂cdnaphen). The new products were characterized by elemental analysis, IR, ¹H NMR and ¹¹⁹Sn NMR spectroscopies. Spectroscopic data suggest that H₂cdnaphen exists predominately in keto-amine tautomeric form and in all complexes acts as a monodentate neutral ligand coordinating with the metal through oxygen atom, while the sulfur atom and imine nitrogen are not involved in coordination with the tin. Thermal decomposition of the complexes was studied through thermogravimetry and the thermodynamic activation parameters were determined by the Coats-Redfern method.     

Carbon-13 NMR studies of some organotin(IV) compounds

The ¹³C chemical shifts and ¹³C−¹¹⁹Sn, ¹¹⁷Sn coupling constants for several organotin(IV) compounds R_xSnCl_4−x (R = Me, Buⁿ, Ph; x = 1−4) have been measured in both inert (CDCl₃) and donor (DMSO-d₆) solvents, as have ¹³C data for the compounds R_xSnR′_4−x (R = Me, Ph; R′ = Buⁿ and R = Me; R′ = Ph; x = 1−3) and the compounds Me₃SnX (X = pseudo halide). The δ and ¹J(C-Sn) values appear to depend mainly on the type and number of substituents on tin and the donor ability of the solvent. There are linear relationships between the number of substituents (x) and both δ and ¹J(C-¹¹⁹Sn) for almost the R_xSnX_4−x series (R = Me, Buⁿ, Ph; X = Cl and R = Me, Buⁿ; X = Ph; x = 1−4), when measured in a single solvent, e.g. CDCl₃. There is an excellent linear relationship between ¹J(C-¹¹⁹Sn) and ²J(¹HC-¹¹⁹Sn) for the compounds Me_xSnCl_4−x. Determination of ¹³C data for Me₃SnCl and Ph₃SnCl in a range of solvents reveals that the value of ¹J(C-Sn) increases with the donor ability of the solvent.The marked increase in the values of ¹J(C-¹¹⁹Sn) in DMSO-d₆ for the compounds R_xSnCl_4−x(R = Me, Buⁿ,Ph) on going progressively from x = 4 to x = suggest tin coordination numbers of 4, 5, 6 and 6, respectively. Some additional physical data are presented for the isolated complexes from DMSO and the compounds Ph_xSnCl_4−x(x = 1−3) and Me₃SnX with X = N₃ or OCOMe.     

Completion of the series of M2(hpp)4Cl2 compounds from W to Pt: the W, Os, and Pt compounds

The series of M2(hpp)4Cl2 complexes (hpp is the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) from M = W to M = Pt has been completed by the preparation and characterization of those with M = W, Os, and Pt. W(hpp)4Cl2 (1) has a W-W distance of 2.250(2) A, is diamagnetic, and can be assigned a W-W triple bond based on a sigma 2 pi 4 electron configuration. Os2(hpp)4Cl2 (2) has an Os-Os distance of 2.379(2) A and displays a temperature-independent paramagnetism. It can be assigned a sigma 2 pi 4 delta 2 delta*2 configuration. Pt2(hpp)4Cl2 has a Pt-Pt distance of 2.440(1) A and is diamagnetic. A bond order of 1, based on a configuration in which only the sigma* orbital is empty, is consistent with these data.     

Biological activity studies on organotin(IV) complexes and parent compounds

This review summarized the literature and own data on the parent organotin(IV) compounds and complexes formed with biologically active ligands.     

Combined experimental and computational study of W(II), Ru(II), Pt(IV) and Cu(I) amine and amido complexes using 15N NMR spectroscopy

Using 2D proton-coupled gHSQC pulse sequences in addition to 1D ¹⁵N NMR experiments of ¹⁵N labeled systems, ¹⁵N NMR chemical shifts of a range of transition metal amido and amine complexes were determined. Tungsten(II), ruthenium(II), platinum(IV) and copper(I) complexes with aniline and their anilido variants were studied and compared to free aniline, lithium anilido and anilinium tetrafluoroborate. Upon coordination of aniline to transition metals, upfield chemical shifts of 20–60 ppm were observed. Deprotonation of the amine complexes to form amido complexes resulted in downfield chemical shifts of 40–60 ppm for all of the complexes except for the tungsten d⁴ system. For the tungsten(II) complexes, the cationic aniline complex displayed a downfield shift of approximately 56 ppm relative to the neutral anilido complex. The change in chemical shift for amine to amido conversion is proposed to depend on the ability of the amido ligand to π-bond with the metal center, which influences the magnitude of the paramagnetic screening term.     

Some new butylthiobenzoatotin(IV) compounds: spectral studies and analysis of ligand's bonding

Five new monorganotin(IV) compounds with thiobenzoate ligand, [BuSn(SOCPh)2]2O (1), [BuSn(O)(SOCPh)]2 (2), BuSn(Cl)(SOCPh)2 (3), BuSn(Cl)2(SOCPh) (4) and [BuSn(OH)(Cl)(SOCPh)]2 (5) were synthesized and characterized by elemental analyses, IR, 1H and 13C NMR spectroscopy. 119Sn NMR spectroscopy was used to determine the coordination geometry around Sn(IV) in the cases of 2 and 4. 1, 2 and 5 are dimeric while 3 and 4 are monomeric. In all these molecules the thiobenzoate ion is coordinated only through its sulfur atom. Molecular structures of the compounds have been optimized by MM2 calculations. Semi-empirical quantum mechanical calculations (PM3 method) were performed to explain the monodentate-bonding pattern of thiobenzoate ligand.     

Concerted reductive coupling of an alkyl chloride at Pt(IV)

Oxidation of a doubly cyclometallated platinum(II) complex results in two isomeric platinum(IV) complexes. Whereas the trans isomer is robust, being manipulable in air at room temperature, the cis isomer decomposes at -20 °C and above. Reductive coupling of an alkyl chloride at the cis isomer gives a new species which can be reoxidised. The independence of this coupling on additional halide rules out the reverse of an S(N)2 reaction, leaving a concerted process as the only sensible reaction pathway.