Preparation and electronic spectra of some alkyl- and aryl(2,2′-bipyridine)platinum(II) complexes

Donor Ability of Bisphosphinemonoxide Ligands Relevant to Late-Metal Olefin Polymerization Catalysis

Late-transition-metal catalysts for polymerization of olefins have drawn a significant amount of attention owing to their ability to tolerate and incorporate polar comonomers. However, a systematic way to experimentally quantify the electronic properties of the ligands used in these systems has not been developed. Quantified ligand parameters will allow for the rational design of tailored polymerization catalysts, which would target specific polymer properties. We report a series of platinum complexes bearing bisphosphinemonoxide ligands, which resemble those used in the polymerization catalysts of Nozaki and Chen. Their electronic properties are investigated experimentally, and trends are rationalized by using computed spectral properties. Benchmarking computational data with known experimental parameters further enhances the utility of both methods for determining optimal ligands for catalytic application.

Platinum Cyclooctadiene Complexes with Activity against Gram-positive Bacteria

Antimicrobial resistance is a looming health crisis, and it is becoming increasingly clear that organic chemistry alone is not sufficient to continue to provide the world with novel and effective antibiotics. Recently there has been an increased number of reports describing promising antimicrobial properties of metal-containing compounds. Platinum complexes are well known in the field of inorganic medicinal chemistry for their tremendous success as anticancer agents. Here we report on the promising antibacterial properties of platinum cyclooctadiene (COD) complexes. Amongst the 15 compounds studied, the simplest compounds Pt(COD)X2 (X=Cl, I, Pt1 and Pt2) showed excellent activity against a panel of Gram-positive bacteria including vancomycin and methicillin resistant Staphylococcus aureus. Additionally, the lead compounds show no toxicity against mammalian cells or haemolytic properties at the highest tested concentrations, indicating that the observed activity is specific against bacteria. Finally, these compounds showed no toxicity against Galleria mellonella at the highest measured concentrations. However, preliminary efficacy studies in the same animal model found no decrease in bacterial load upon treatment with Pt1 and Pt2. Serum exchange studies suggest that these compounds exhibit high serum binding which reduces their bioavailability in vivo, mandating alternative administration routes such as e. g. topical application.

Reactions of organoplatinum complexes with dimethylamine-borane

Reactions of organoplatinum complexes with dimethylamineborane are reported.

Dynamics of the efficient cyclometalation of the undercoordinated organoplatinum complex [Pt(COD)(neoPh)]+ (neoPh = 2-methyl-2-phenylpropyl)

The cyclometalation reaction of [Pt(COD)(κ¹-neoPh)]⁺ (neoPh = 2-methyl-2-phenylpropyl) to [Pt(COD)(κ²-neoPh)] was studied experimentally and mechanistically using DFT and MD simulations.

Doltsinis N, Klein A. Reactions of the organoplatinum complex [Pt(cod) (neoSi)Cl] (neoSi = trimethylsilylmethyl) with the non-coordinating anions SbF6– and BPh4–

Reactions of the organoplatinum complex [Pt(cod)(neoSi)Cl] (neoSi = (trimethylsilylmethyl) with the Ag(I) salts of oxo or fluoride containing anions A– = NO3–, ClO4–, OTf – (trifluoromethanesulfonate) and SbF6– lead to the desired abstraction of the chlorido ligand and precipitation of AgCl. However, further reaction of the resulting Pt complexes [Pt(cod)(neoSi) (solvent)]+ with diverse N-heterocyclic ligands L such as pyridines, caffeine, and guanine did not yield the targeted complexes [Pt(cod)(neoSi)(L)](A) in most of the cases, but to extensive decomposition yielding [Pt(cod)(Me) (solvent)]+, thus transforming the neoSi into a methyl ligand. A detailed study on the reaction with SbF6– combining DFT calculations with NMR and MS revealed that Pt catalysed decomposition of SbF6‒ and fluorination of the neoSi silicon atom leading to FSiMe3. When reacting the parent complex with Ag(BPh4), the arylated derivative [Pt(cod)(neoSi)(Ph)] was obtained and characterised by multinuclear NMR, MS and single crystal XRD.

Mild and selective Pd-Ar protonolysis and C-H activation promoted by a ligand aryloxide group

A bidentate nitrogen-donor ligand with an appended phenol group, C₅H₄NCH[double bond, length as m-dash]N-2-C₆H₄OH, H(L1) was treated with a palladium cycloneophyl complex [Pd(CH₂CMe₂C₆H₄)(COD)], with both Pd-aryl and Pd-alkyl bonds, to give a Pd-alkyl complex, [Pd(CH₂CMe₂C₆H₅)(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 1. The cleavage of the Pd-aryl bond and the deprotonation of the ligand phenol to afford a bound aryloxide, indicates facile Pd-aryl bond protonolysis. Deuterium labelling experiments confirmed that the ligand phenol promotes protonolysis and that the reverse, aryl C-H activation, occurs under very mild reaction conditions (within 10 min at room temperature). An unusual isomerization of the Pd-alkyl complex 1 to a Pd-aryl complex, [Pd(C₆H₄(2-t-Bu))(κ³-N,N',O-OC₆H₄N[double bond, length as m-dash]CH(2-C₅H₄N))], 2, was observed to give an equilibrium with [2]/[1] = 9 after 5 days in methanol. The isomerization requires that both aryl C-H activation and Pd-alkyl protonolysis steps occur. The very large KIE value (k_H/k_D = ca. 40) for isomerization of 1 to 2, suggests a concerted S_E2-type mechanism for the Pd-alkyl protonolysis step.

Photoswitchable and pH responsive organoplatinum(ii) complexes with azopyridine ligands

Several platinum(ii) complexes with ligands containing azo groups have been prepared and structurally characterised, and their photoswitching between trans and cis azo group isomers has been studied. The azo groups in the cationic complexes [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄X)][PF₆], X = H, OH or NMe₂, and in the dicationic complex [Pt(bipy)(4-H₂NC₆H₄-N[double bond, length as m-dash]N-C₆H₅)₂][OTf]₂ undergo trans to cis photoswitching on irradiation at 365 nm. The complex [PtMe(bipy)(4-NC₅H₄-N[double bond, length as m-dash]N-4-C₆H₄NMe)₂][PF₆] also exhibits a reversible halochromic effect on protonation to give the dicationic complex [PtMe(bipy)(4-NC₅H₄-NH[double bond, length as m-dash]N-4-C₆H₄NMe₂]²⁺. The nature of the frontier orbitals in the platinum(ii) complexes depends on the charge on the complex and on the degree of metal-ligand π-bonding.

Intramolecular cyclization assisted oxidative addition: synthesis of octahedral cycloplatinated methyl complexes

Cycloplatination of XMeC₆H₄CCC-(R_f) (N-4-OCH₃C₆H₄) [X = S, Se and R_f = perfluoroalkyl groups] with PtCl₂ gave a monomeric octahedral Pt(iv) complex. Emission and reactivity studies of the Pt(iv) complexes are discussed.

Photoswitchable organoplatinum complexes containing an azobenzene derivative of 3,6-di(2-pyridyl)pyridazine

The new, unsymmetrical azobenzene-tagged ligand 4-(4-azobenzene)-3,6-di(2-pyridyl)pyridazine, adpp, forms complexes with platinum(II) and platinum(IV), which exist as a mixture of geometrical isomers. The complexes are characterized primarily by their NMR spectra, while the structures of the ligand and its complexes [PtMe2(adpp)], [PtBrMe2(CH2C6H4-4-t-Bu)(adpp)], and [PtBrMe2(CH2C6H3-3,5-t-Bu2)(adpp)] have been structurally characterized. In solution, the compounds undergo easy photochemical trans−cis switching of the azobenzene group, with subsequent slow thermal isomerization back to the more stable trans-azobenzene isomer.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene]bis-(4-methyl-phen-yl)platinum(II)

In the mononuclear title complex, [Pt(C₇H₇)₂(C₈H₁₂)], the Pt^II ion exhibits a square-planar coordination geometry defined by two methyl­phenyl ligands and the mid-points of the two π-coordinated double bonds of cyclo­octa-1,5-diene. The two methyl­phenyl groups have a cis relationship with a C—Pt—C bond angle of 88.54 (18)° and a dihedral angle between the mean planes of the benzene rings of 83.87 (1)°. Each complex mol­ecule links to four symmetry-related ones through inter­molecular C—H⋯π inter­actions, forming a layer almost parallel to the bc plane.

Mechanistic Studies of Platinum(II)-Catalyzed Ethylene Dimerization: Determination of Barriers to Migratory Insertion in Diimine Pt(II) Hydrido Ethylene and Ethyl Ethylene Intermediates

The diimine platinum(II) ethylene hydride complex [(N/\N)Pt(H)(ethylene)][BAr'4] (1, N/\N = [(2,6-Me2C6H3)N=C(An)-C(An)=N(2,6-Me2C6H3)], An = 1,8-naphthalenediyl, Ar' = 3,5-(CF3)2C6H3) was prepared by protonation of the diethyl complex (N/\N)PtEt2 with [H(OEt2)2][BAr'4]. The energy barrier to interchange of the platinum hydride with the olefinic hydrogens in 1 was determined to be 19.2 kcal/mol by spin saturation transfer experiments. Complex 1 initiates ethylene dimerization; the ethyl ethylene complex (N/\N)Pt(Et)(ethylene)+ (2) has been identified as the catalyst resting state. Trapping of 1 by ethylene to yield 2 is a second-order process; kinetic studies suggest this occurs via trapping of a reversibly formed beta-agostic ethyl complex. Complex 2 has been isolated and characterized by X-ray crystallography. The barrier to migratory insertion of 2, the turnover-limiting step in catalysis, was determined to be 29.8 kcal/mol. The 1-butene hydride complex, (N/\N)Pt(H)(1-butene)+ (3), is a key intermediate in the dimerization cycle and has also been isolated and characterized. Surprisingly rapid rates of degenerate associative exchange of free ethylene with bound ethylene in complexes 1 and 2 as well as the rate of degenerate exchange of free nitrile with bound nitrile in (N/\N)Pt(Et)(CH3CN)+ are reported.

Participation of co-ligands in electronic transitions of platinum(II) diazabutadiene complexes

The low-lying electronic transitions and photochemical reactions of a series of [((i)Pr-DAB)Pt(R)(2)] (where the co-ligand R = CH(3), CD(3), adme, neop, neoSi, C(triple bond)C(t)Bu, C(triple bond)CPh, Ph, Mes) compounds were studied using both experimental (electronic absorption and resonance Raman spectroscopy) and theoretical (density functional theory, DFT) techniques. The high-lying filled orbitals were revealed to have a significant co-ligand contribution in the case of alkyl complexes, while this contribution is predominant for the complexes with unsaturated co-ligands. Because the electronic transition removes electron density from the sigma(Pt-C) bond in the former complexes, it is best described as a metal-to-ligand charge transfer transition (MLCT) with partial sigma-bond-to-ligand charge transfer (SBLCT) character. Because the sigma(Pt-C) orbital is not involved in the HOMOs of the latter complexes, the low-lying transitions were characterized as mixed MLCT/L'LCT, where L'LCT stands for ligand-to-ligand charge transfer from the pi system of the unsaturated co-ligand to the pi((i)Pr-DAB) orbital. The alkyl complexes are photoreactive on visible light irradiation with Pt-C bond homolysis as the primary step. The efficiency of the photoreaction increases with increasing sigma donor strength of the alkyl ligand. The absolute quantum yield is quite low. The other complexes are virtually photostable, except when irradiated at relatively high energies.

To what extent can cyclometalation promote associative or dissociative ligand substitution at platinum(II) complexes? A combined kinetic and theoretical approach

The ligand exchange rate constants for the reactions [Pt(bph)(SR2)2] + 2*SR2 --> [Pt(bph)(*SR2)2] + 2SR2 (bph = 2,2'-biphenyl dianion; R = Me and Et) and cis-[PtPh2(SMe2)2] + 2*SMe2 --> cis-[PtPh2(*SMe2)2] + 2SMe2 have been determined in CDCl3 as a function of ligand concentration and temperature, by 1H NMR isotopic labeling and magnetization transfer experiments. The rates of exchange show no dependence on ligand concentration and the kinetics are characterized by largely positive entropies of activation. The kinetics of displacement of the thioethers from [Pt(bph)(SR2)2] with the dinitrogen ligands 2,2'-bipyridine and 1,10-phenanthroline (N-N) to yield [Pt(bph)(N-N)], carried out in the presence of sufficient excess of thioether and N-N to ensure pseudo-first-order conditions, follow a nonlinear rate law k(obsd) = a[N-N]/(b[SR2] + [N-N]). The general pattern of behavior indicates that the rate-determining step for substitution is the dissociation of a thioether ligand and the formation of a three-coordinated [Pt(bph)(SR2)] intermediate. The value of the parameter a, which measures the rate of ligand dissociation, is constant and independent of the nature of N-N, and it is in reasonable agreement with the value of the rate of ligand exchange at the same temperature. Theoretical ab initio calculations were performed for both [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], and for their three-coordinated derivatives upon the loss of one SMe2 ligand. The latter optimize in a T-shaped structure. Calculations were performed in the HF approximation (LANL2DZ basis set) and refined by introducing the correlation terms (Becke3LYP model). The activation enthalpies from the optimized vacuum-phase geometries are 52.3 and 72.2 kJ moll compared to the experimental values in CDCl3 solution, 80 +/- 1 and 93 +/- 1 kJ mol(-1) for [Pt(bph)(SMe2)2] and cis-[PtPh2(SMe2)2], respectively. The electrostatic potential maps of both parent compounds show a remarkable concentration of negative charge over the platinum atom which exerts a repulsion force on an axially incoming nucleophile. On the other hand, the strength of the organic carbanions trans to the leaving group and the stabilization of the T-shaped intermediate in the singlet ground state may also rationalize the preference for the dissociative mechanism. All of the kinetic and theoretical data support the latter hypothesis and indicate, in particular, that dissociation from the complex containing the planar 2,2'-biphenyl dianion is easier than from its analogue with single aryl ligands. Electron back-donation from filled d orbitals of the metal to empty pi* of the in-plane cyclometalated rings is weak or absent and is not operative in promoting an associative mode of activation.

Molecular Structure, Acidic Properties, and Kinetic Behavior of the Cationic Complex (Methyl)(dimethyl sulfoxide)(bis-2-pyridylamine)platinum(II) Ion

The complex [PtMe(dpa)(Me(2)SO)](+)(CF(3)SO(3))(-) (dpa = bis(2-pyridyl)amine) crystallizes in the monoclinic space group P2(1)/c with a = 11.010(2) Å, b = 18.366(2) Å, c = 10.333(5) Å, beta = 111.62(2) degrees, and Z = 4. Least-squares refinement of the structure led to an R factor of 2.41%. To avoid steric repulsions, the chelate six-membered ring assumes a boat configuration in which the two pyridyl rings are folded with a dihedral angle of 46.4(1) degrees. There is a strong hydrogen-bonding interaction involving the amine hydrogen (N3) and a triflate anion oxygen O3, 2.898(5) Å. The tendency by the NH group of the ligand moiety to attract anions is maintained in a solution of nonpolar solvents. Tight ion-pairs of structure similar to that in the solid state are formed with PF(6)(-), BF(4)(-), CF(3)SO(3)(-), and Cl(-) in chloroform, as shown by the strong dependence of the chemical shifts of the NH, H(3), and H(3') protons of the dpa ligand on the nature of the counterion. (19)F{(1)H} HOESY experiments on [PtMe(dpa)(Me(2)SO)](+)(PF(6))(-) in CD(2)Cl(2) confirmed that the preferential position of the counterion is close to the NH proton. The absorption spectra are also strongly affected by the nature of the counterion. This allowed for a stopped-flow measure of the PF(6)(-) for Cl(-) exchange rate at the NH site, which is a bimolecular process with k(2) = 96.4 +/- 4 M(-)(1) s(-)(1). The cation [PtMe(dpa)(Me(2)SO)](+) shows acidic properties in water (pK(a) = 12.1 +/- 0.2, at 25 degrees C, &mgr; = 0.1 M, NaNO(3)), and the corresponding amido species [PtMe(dpa-H)(Me(2)SO)] can be isolated on basification. Ion-pairing and full deprotonation of the amine ligand have remarkably little effect on the reactivity, as shown by the comparison of the rates of isotopic exchange of dimethyl sulfoxide of these species followed by (1)H NMR in chloroform. The rates of substitution of dimethyl sulfoxide from [PtMe(dpa)(Me(2)SO)](+) by various charged nucleophiles were measured in methanol, where ion-pairing effects are absent, and compared with those of the parent [PtMe(phen)(Me(2)SO)](+) (phen = 1,10-phenanthroline) complex. Because of the reduced capacity of electron withdrawal from the metal of the ancillary ligand, the dpa complex is less reactive and possesses a minor nucleophilic discrimination ability compared with the phen complex.

Chiral Cyclometalated Platinum(II) and Palladium(II) Complexes with Derivatives of Thienylpyridine as Ligands: Helical Distortion of the Square Planar (SP-4) Geometry

Four cis-bis-cyclometalated complexes of platinum(II) and palladium(II) have been synthesized with chiral substituted thienylpyridine ligands. These ligands are (5aR,6aS)-5,5a,6a,7-tetrahydro-6,6-dimethyl-3-(2'-thienyl)cyclopropa[g]isoquinoline (th4,5capy) (1), (6aR,7aS)-5,6,6a,7a-tetrahydro-7,7-dimethyl-2-(2'-thienyl)cyclopropa[h]quinoline (th5,6capy) (2), (6R,8R)-5,6,7,8-tetrahydro-9,9-dimethyl-2-(2'-thienyl)-6,8-methanoquinoline (th5,6betappy) (3), and (6R,8R)-5,6,7,8-tetrahydro-9,9-dimethyl-3-(2'-thienyl)-6,8-methanoisoquinoline (th4,5ppy) (5). Two complexes have a well-defined chirality at the metal center, due to the sterical interaction of the ligands. These complexes are Delta-Pt(th5,6capy)(2) (8) and Lambda-Pt(th5,6betappy)(thpy) (9). The crystal structure of 8 shows a strong distortion from the ideal square planar geometry. Crystals of Delta-Pt(th5,6capy)(2) (8) are tetragonal, space group P4(3)2(1)2, a = 13.407(1) Å, b = 13.407(1) Å, c = 14.860(2) Å, alpha = beta = gamma = 90 degrees, and Z = 4. Final R = 0.0237 and R(w) = 0.0450 for 2353 observed reflections. Compound 8 possesses a crystallographic C(2) symmetry.

Synthesis, Characterization, and Electrochemistry of Heterometallic Dendrimers

A method for the synthesis of new heterometallic dendritic molecules containing organoplatinum centers is reported. Reaction of Pt(2)Me(4)(&mgr;-SMe(2))(2) with bpy-Fc(2) (1) (bpy-Fc(2) = 4,4'-bis(ferrocenylvinyl)-2,2'-bipyridine) gave the platinum(II) complex PtMe(2)(bpy-Fc(2)) (2). Complex 2 undergoes oxidative addition with iodomethane to yield PtMe(3)I(bpy-Fc(2)) (3) and with bis(bromomethyl)benzenes to yield complexes of the type [PtMe(2)Br(bpy-Fc(2))CH(2)-](2)R (4, R = 1,2-C(6)H(4); 5, R = 1,3-C(6)H(4); 6, R = 1,4-C(6)H(4)). 1, 3, and 4 were characterized by X-ray structure determinations. For 1, a = 11.780(3) Å, b = 10.662(3) Å, c = 11.642(3) Å, beta = 118.61(2) degrees, and V = 1283.7(6) Å(3) while, for 2, a = 11.497(2) Å, b = 22.796(6) Å, c = 15.801(3) Å, beta = 106.08(1) degrees, and V = 3979(2) Å(3). Only poor quality crystals of 4 could be obtained resulting in a partial structure refinement with a = 14.46(1) Å, b = 15.14(1) Å, c = 21.32(2) Å, alpha = 69.45(4) degrees, beta = 71.14(5) degrees, gamma = 81.86(5) degrees and V = 4140(6) Å(3). The preferred conformation for 1 places the the bipyridine groups anti with respect to each other. However, as a consequence of the coordination to the metal centers in 3 and 4, they are located cis to each other, and the cyclopentadienyl rings attached directly to the vinyl groups lie on the same plane. In compound 4 the planes containing the platinum bipyridine units lie above and below the plane of the benzene ring, presumably to minimize steric hindrance. 2 also reacts with 1,3,5-tris(bromomethyl)mesitylene, 1,2,4,5-tetrakis(bromomethyl)benzene, and 1,2,3,4,5,6-hexakis(bromomethyl)benzene in 3:1, 4:1, and 6:1 molar ratios to yield the dendritic molecules [PtMe(2)Br(bpy-Fc(2))CH(2)-](n)()R [7, n = 3, R = 1,3,5-C(6)(CH(3))(3); 8, n = 4, R = 1,2,4,5-C(6)H(2); 9, n = 6, R = 1,2,3,4,5,6-C(6)], respectively. The largest dendrimer 9 contains six platinum and 12 ferrocene centers. Each complex undergoes a single 2n-electron electrochemical oxidation (corresponding to the ferrocenium/ferrocene couple of the bpy-Fc(2) ligands), confirming that the pendant ferrocenyl units are noninteracting. The number of ferrocene centers for each complex was determined both by exhaustive electrolysis and by a chronocoulometric/voltammetric assay.

High-pressure methods as a tool in organometallic syntheses: facilitation of oxidative addition to platinum(II)

High-pressure (2 GPa) batch reactors now commercially available may offer substantial accelerations of organometallic syntheses, without resort to heating, when the activation process is multicentered or involves the generation and solvation of ions. As an example of the latter class of reactions, the kinetics of the oxidative additions of methyl and ethyl iodides (RI) to dimethyl(2,2′-bipyridine)platinum(II) in acetone have been studied over the pressure range 0–200 MPa. The volumes of activation ΔV1≠, if assumed to be constant over this range, are −11.7 ± 0.3 and −9.7 ± 0.7 cm3 mol−1, respectively, implying an acceleration of ca. 3000-fold for a batch synthesis of this sort at 2 GPa. However, a possible slight pressure dependence of ΔV1≠ may reduce this acceleration to ca. 1 000-fold. The ΔV1≠ data and the 500-fold retardation on going from R = Me to R = Et are consistent with an SN2 attack of Pt11 on the α-carbon in the alkyl iodides, forming I− and [RMe2Pt(bpy)]+. Key words: volumes of activation, high pressure, oxidative addition, platinum(II), organometallic syntheses.

Organoplatinum(IV) derivatized vinyl monomers and polymers prepared by oxidative addition

Organoplatinum(IV) complexes of general formula [PtXMe2R(NN)], containing vinyl substituents, have been prepared by oxidative addition of RX, (RX = methyl 2-(bromomethyl)acrylate, 2-(bromomethyl)acrylic acid, 2-bromoethyl methacrylate, acryloyl chloride, and chloromethylstyrene), to [PtMe2(NN)], NN = 2,2′-bipyridine or 4,4′-di-tert-butyl-2,2′-bipyridine. Polymers with organoplatinum(IV) substituents have been prepared either by free radical polymerization of the organoplatinum(IV) derivatized monomers or by free radical polymerization of the organic monomers, followed by the oxidative addition of the C-X substituents of these polymers to [PtMe2(NN)]. In the latter method, it is generally not possible to metallate all the C—X bonds, but a high degree of platinum incorporation can be achieved. Key words: organoplatinum, polymer, oxidative addition, vinyl.