The synthesis of the tripodal phosphine CH3C{CH2P(m-CF3C6H4)2}3 and its rhodium coordination chemistry

The new tripodal phosphine CH₃C{CH₂P(m-CF₃C₆H₄)₂}₃, CF₃PPP, was prepared by reacting CH₃C(CH₂Br)₃ with Li⁺P(m-CF₃C₆H₄)₂⁻, the latter being best obtained by adding Li⁺NⁱPr₂⁻ to PH(m-CF₃C₆H₄)₂. The rhodium complexes [RhCl(CO)(CF₃PPP)], [Rh(LL)(CF₃PPP)](CF₃SO₃) (LL = 2 CO or NBD), [RhX₃(CF₃PPP)], [RhX(MeCN)₃(CF₃PPP)](CF₃SO₃)₂ (X = H and Cl), [RhCl₂(MeCN)(CF₃PPP)](CF₃SO₃) and [Rh(MeCN)₃(CF₃PPP)](CF₃SO₃)₃ were prepared and characterized. The X-ray crystal structure of [Rh(NBD)(CF₃PPP)](CF₃SO₃) is reported. The lower oxygen sensitivity of the CF₃PPP rhodium(I) complexes, relative to the corresponding species with the parent ligand CH₃C(CH₂PPh₂)₃, is attributed to the higher effective nuclear charge on the metal centers caused by the presence of the six CF₃ substituents on the terdentate phosphine. A similar effect may be responsible for the easier hydrolysis of the CF₃PPP-containing, cationic rhodium(III) complexes relative to the corresponding compounds of the parent ligand.     

The synthesis and reactivity of electrophilic iridium (III) complexes containing bis (diphenylphosphino) ethane

The complex Ir(CH₃) (CO) (CF₃SO₃)₂ (dppe) (1) has been synthesized from the reaction of Ir(CH₃)I₂(CO) (dppe) and silver triflate. Methane and IrH(CO) (CF₃SO₃)₂ (dppe) (2) are formed when a methylene chloride solution of 1 is placed under 760 torr dihydrogen. Conductivity studies indicate that methylene chloride solutions of complexes 1 and 2 are weak electrolytes and only partially ionized at concentrations above 1 mM. Complex 2 is an effective hydrogenation catalyst for ethylene and 1-hexene while acetone hydrogenation is inhibited by the formation of [IrH₂(HOCH(CH₃)₂) (CO) (dppe)] (OTf) (3). Linear dimerization and polymerization of styrene occurs via a carbocationic mechanism initiated by triflic acid elimination from 2. Treatment of an acetonitrile solution of Ir(CH₃)I₂(CO) (dppe) with silver hexafluorophosphate produces the solvent promoted carbonyl insertion product [Ir(C(O)CH₃) (NCCH₃)₃ (dppe)] [PF₆]₂ (7) which readily undergoes deinsertion in methylene chloride to form [Ir(CH₃) (CO) (NCCH₃)₂ (dppe)] [PF₆]₂ (8) and acetonitrile.     

Electrophilic abstractions on Os(H)2X(NO)L2: selective access to new unsaturated cationic Os(II) hydrides

Nitrosylation of Os(H)₃ClL₂ (L = P¹Pr₃) affords the known Os(H)₂Cl(NO)L₂ (2). Soft electrophiles (Ag⁻, Na⁻) react with complex 2 by chloride abstraction to ultimately yield truly 16-electron dihydride Os(H)₂(NO)(P¹Pr₃)₂⁻ (4a), characterized by variable-temperature NMR. Complex 4a reversibly binds H₂, forming Os(H)₂(H₂)(NO)(P¹Pr₃)⁻ with an unusually high barrier for intramolecular hydride exchange. Under kinetic conditions, protonation of 2 with strong acids follows the selectivity for chloride abstraction. Thermodynamically, protonation at the hydride is preferred, quantitatively producing cationic OsHCl(NO)L₂⁺, isolated and characterized by X-ray diffraction as the BAr₄^F− salt (7) (Ar^F=3,5−(CF₃)₂C₆H₃). Structures of isoelectronic OsHCl(NO)(PH₃)₂ and OsHCl(CO)(PH₃)₂ were optimized with ab initio DFT (Becke3LYP) methods and compared to show the greater unsaturation of the metal in the cationic species. Both complexes, 4a and 7, are highly electrophilic and reversibly coordinate dichloromethane in solution. The observed reactivity patterns of the synthesized unsaturated hydrides are rationalized in terms of the determining influence of the ‘push-pull’ π-stabilization of the metal center.     

Facile and stereoselective condensation of acetone with ammonia ligands on cobalt(III): structure of a N-bonded cyanate complex containing the 2-methyl-2-amino-4-imino-pentane ligand

The complex [(NH₃)₅CoO₃SCF₃](CF₃SO₃)₂ reacts with excess NaNCO in warm acetone solution to give, stereoselectively, a Schiff base complex (40%) which has been characterized by standard NMR techniques as one of the six isomers of [Co{NH₂C(CH₃)₂CH₂C(CH₃)=NH}₂(NH₃)NCO](ClO₄)₂ · H₂O, confirmed by a single crystal X-ray structural analysis. Schiff base formation in non-basic conditions for kinetically inert Co(III) complexes is unprecedented. Also, this is only the second cyanate complex of pentaaminecobalt(III) to be structurally characterized (CoNCO: Co–N, 1.908 Å; N–C, 1.152 Å; C–O, 1.206 Å; Co–N–C, 170°; N–C–O, 177°).     

Intermolecular Re---H·H---X hydrogen bonding (X = N, C) involving ReH5(PPh3)3

Ab initio (B3LYP) calculations show that PD·H---ReH₄(PH₃)₃ (PD = Proton donor) interactions follow the order PD = pyrrole > NH₃ > HCCH > C₂H₄ > CH₃---H 0 and decrease with the pK_a of the PD. For equivalent pK_a's, NH interacts more strongly than CH. However, intermolecular hydrogen-bonding of the M---H·H---C type is too weak to be detected experimentally in FTIR or UV-vis studies between ReH₅(PPh₃)₃ and PhCCH, C₆F₅H or PhCHCl₂.     

Hetero-polynuclear complexes containing bridging diphenylphosphino-alkenes and -alkynes. The crystal structure of an Rh2{μ−ν:ν−P(Ph2) (CH2)3C≡CR}Co2 complex

The phosphinoalkenes Ph₂P(CH₂)_nCH=CH₂ (n= 1, 2, 3) and phosphinoalkynes Ph₂P(CH₂)_n C≡CR (R = H, N = 2, 3; R = CH₃, N = 1) have been prepared and reacted with the dirhodium complex (η−C₅H₅)₂Rh₂(μ−CO) (μ−η²−CF₃C₂CF₃). Six new complexes of the type (ν−C₅H₅)₂(Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃)L, where L is a P-coordinated phosphinoalkene, or phosphinoalkyne have been isolated and fully characterized; the carbonyl and phosphine ligands are predominantly trans on the Rh---Rh bond, but there is spectroscopic evidence that a small amount of the cis-isomer is formed also. Treatment of the dirhodium-phosphinoalkene complexes with (η−CH₃C₅H₄)Mn(CO)₂thf resulted in coordination of the manganese to the alkene function. The Rh₂---Mn complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂P(CH₂)₃CH=CH₂} (η−CH₃C₅H₄)Mn(CO)₂] was fully characterized. Simi treatment of the dirhodium-phosphinoalkyne complexes with Co₂(CO)₈ resulted in the coordination of Co₂(CO)₆ to the alkyne function. The Rh₂---Co₂ complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂PCH₂C≡CCH₃}Co₂(CO)₂], C₃₇H₂₅Co₂F₆O₇PRh₂, was fully characteriz spectroscopically, and the molecular structure of this complex was determined by a single crystal X-ray diffraction study. It is triclinic, space group (C_i¹, No. 2) with a = 18.454(6), B = 11.418(3), C = 10.124(3) Å, = 112.16(2), β = 102.34(3), γ = 91.62(3)°, Z = 2. Conventional R on |F| was 0.052 fo observed (I > 3σ(I)) reflections. The Rh₂ and Co₂ parts of the molecule are distinct, the carbonyl and phosphine are mutually trans on the Rh---Rh bond, and the orientations of the alkynes are parallel for Rh₂ and perpendicular for Co₂. Attempts to induce Rh₂Co₂ cluster formation were unsuccessful.     

Synthesis and biological activity of cis-dichloro mono- and bis(platinum) complexes with N-alkyl-ethylenediamine ligands

Mono- and bis(platinum) complexes containing N-alkyl-ethylenediamine units of the type {cis-PtCl₂[H₂NCH₂CH₂NH(CH₂)_nCH₃]} (n=8, 9, 11, 15) and [{cis-PtCl₂(H₂NCH₂CH₂NH)}₂(CH₂)_n] (n=6, 8, 10, 12) and their corresponding dihydroxo-platinum(IV) complexes were synthesized. The structures of the metal chelates were derived from elemental analyses and their ¹H, ¹³C, IR spectra. The length of the aliphatic chains has been varied systematically, in order to increase the lipophilicity. Enlargement of the linker could also lead to more flexibility of one platinum sphere in reference to the attached DNA species. Using in vitro cytotoxicity tests it is shown that the biological activity of the bis(platinum) complexes increased, up to n=12, with the length of the linker. The longest linker in the ligands resulted in the most effective bis(platinum) complexes against L1210 murine leukemia cells.     

Synthesis, characterization, and reactivity of organometallic Zr(IV) carboxylate complexes

A series of zirconium(IV) complexes, [ZrX₂(XDK)], where XDK is the constrained carboxylate ligand m-xylylenediamine bis(Kemp's triacid imide), were prepared and structurally characterized. The solid state structure of the mononuclear carboxylate alkyl complex [Zr(CH₂Ph)₂(XDK)] reveals that one benzyl group is bonded in an η²-fashion to the metal center. The reactivity of [Zr(CH₂Ph)₂(XDK)] displays its electrophilic character toward nucleophiles strong enough to displace the η²-benzyl group. Thus, weak sigma donor ligands such as CO, alkynes and anilines do not react, whereas strong sigma donors, such as pyridines and isocyanides, rapidly form the monoadduct [Zr(CH₂Ph)₂(4-tert-butylpyridine)(XDK)] and [Zr{η²-2,6-Me₂PhNCCH₂Ph}₂(XDK)], an η²-iminoacyl derivative, respectively. Attempts to prepare zirconium amido complexes with H₂XDK generally afforded the eight-coordinate [Zr(XDK)₂] complex but use of the small amido ligand precursorZr(NMe₂)₄ allowed [Zr(NMe₂)₂(4-tert-butylpyridine)(XDK)] to be isolated in good yield.     

Dioxo-molybdenum(VI) and -tungsten(VI) BINOL and alkoxide complexes: Synthesis and catalysis in sulfoxidation, olefin epoxidation and hydrosilylation of carbonyl groups

The reaction of MoO₂(mes)₂ with S-H₂BINOL (mes = 2,4,6-Me₃C₆H₂; H₂BINOL = 1,1′-bi-2-naphthol) and (CF₃)₂MeCOH in THF yielded the novel dioxo-molybdenum(VI) complexes MoO₂(S-BINOL)(THF)₂ and MoO₂[OCMe(CF₃)₂]₂(THF), respectively. Similar tungsten derivatives WO₂(S-BINOL)(THF) and WO₂[OCMe(CF₃)₂]₂(THF) have been prepared by the reaction of WO₂Cl₂(DME) with the corresponding lithium salts of BINOL and 1,1,1,3,3,3-hexafluoro-2-methyl-2-propanol, respectively. Catalytic experiments have shown that MoO₂(S-BINOL)(THF)₂ is an active catalyst in the sulfoxidation of methyl phenyl sulfide and in the epoxidation of cis-cyclooctene with tert-butylhydroperoxide, under mild conditions. The BINOL complex was, however, not found to be enantioselective. In addition, the catalytic activity of the molybdenum species MoO₂(S-BINOL)(THF)₂ and MoO₂[OCMe(CF₃)₂]₂(THF) in the hydrosilylation of carbonyl groups has been explored.     

A reversible NO complex of Fe(TIM): an S = 1/2FeNO nitrosyl

[Fe(TIM)(CH₃CN)₂](PF₆)₂ (1) (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclodeca-1,3,8,10-tetraene) forms a complex with NO reversibly in CH₃CN (53±1% converted to the NO complex) or 60% CH₃OH/40% CH₃CN (81±1% conversion). Quantitative NO complexation occurs in H₂O or CH₃OH solvents. The EPR spectrum of [Fe(TIM)(solvent)NO]²⁺ in frozen 60/40 CH₃OH/CH₃CN at 77 K shows a three line feature at g=2.01, 1.99 and 1.97 of an S=1/2FeNO⁷ ground state. The middle line exhibits a three-line N-shf coupling of 24 G indicating a six-coordinate complex with either CH₃OH or CH₃CN as a ligand trans to NO. In H₂O [Fe(TIM)(H₂O)₂]²⁺ undergoes a slow decomposition, liberating 2,3-butanedione, as detected by ¹H NMR in D₂O, unless a π-acceptor axial ligand, L=CO, CH₃CN or NO is present. An equilibrium of 1 in water containing CH₃CN forms [Fe(TIM)(CH₃CN)(H₂O)]²⁺ which has a formation constant K_CH3CN=320 M⁻¹. In water K_NOK_CH3CN since NO completely displaces CH₃CN. [Fe(TIM)(CH₃CN)₂]²⁺ binds either CO or NO in CH₃CN with K_NO/K_CO=0.46, sigificantly lower than the ratio for [Fe^II(hemes)] of 1100 in various media. A steric influence due to bumping of β-CH₂ protons of the TIM macrocycle with a bent S=1/2 nitrosyl as opposed to much lessened steric factors for the linear Fe---CO unit is proposed to explain the lower K_NO/K_CO ratio for the [Fe(TIM)(CH₃CN)]²⁺ adducts of NO or CO. Estimates for formation constants with [Fe(TIM)]²⁺ in CH₃CN of K_NO=80.1 M⁻¹ and K_CO=173 M⁻ are much lower than to hemoglobin (where K_NO=2.5×10¹⁰ M⁻¹ and K_CO=2.3×10⁷) due to a reversal of steric factors and stronger π-backdonation from [Fe^II(heme)] than from [Fe^II(TIM)(CH₃CN)]²⁺.     

Synthesis, spectroscopic properties and X-ray crystal structures of two dinuclear alkoxo-bridged copper(II) compounds with the ligand bis(1-methyl-2-benzimidazolyl) propane. A unique alkoxo-bridged Cu(II) dinuclear compound with an additional bidentate bridging triflate anion

Based on the new ligand bis(1-methyl-2-benzimidazolyl) propane (abbreviated as mtbz) several new copper(II) coordination compounds have been prepared and characterized structurally and spectroscopically. Two representative compounds, i.e. [Cu₂(mtbz)₂(CH₃)₂- (CF₃SO₃)](CF₃SO₃) (1) and [Cu₂(mtbz)₂(CH₃O)₂](ClO₄)₂ (4) were characterized structurally by X-ray diffraction. Crystal data for 1: monoclinic, space group P2₁/c, a=13.6585(5), B=39.981(3), C=20.919(1) Å, β=125.98(1)°, Z=8. Crystal data for 4: monoclinic, space group P2₁/c, a=13.115(2), B=9.523(2), C=17.908(4) Å, β=111.71(1)°, Z=2. Structures 1 and 4 each consist of a dinuclear unit with bridging methoxo groups and one ligand linked to each copper via an N atom. Structure 1 (which consists of two dinuclear, crystallographically independent, but chemically identical units) has the two copper atoms bridged by a triflate anion, providing each copper atom a square-pyramidal coordination, while the copper atoms in structure 4 have an almost a square-planar geometry. The Cu---Cu distances (Å) within the dinuclear units are: 1, 2.9775(13), 2.9751(13); 4, 2.9872(16); the Cu---O---Cu bridging angles (°) are: 1, 101.7(3), 101.7(3), 100.9(3), 102.1(3); 4, 103.2(2). The mid-IR section focused on the vibrations of the triflate anion reveals interesting results concerning the assignments of that anion related to the v_as(S---O) band. Characteristic Cu---O vibrations in the far-IR section were found at 386 and 230 cm⁻¹ for the methoxo-bridged and 454 and 332 cm⁻¹ for the ethoxo-bridged compounds. These dinuclear species are EPR silent, and only a weak signal of monomeric impurities is observed. They also show a diamagnetic behavior below room temperature.     

Intramolecular and intermolecular C-H activation at a cationic Pt center

Syntheses and C-H bond activation reactions of the novel electrophilic Pt^II complexes [(tmeda)Pt(CH₃)(OEt₂)][BAr₁], [(tmeda)Pt(CH₃)(THF)][BAr_f], and [(tmeda)Pt(CH₃)(NC₅F₅)][BAr_f] are described {[BAr_f]⁻ = [(3,5-C₆H₃(CF₃)₂)₄B]⁻} (tmeda is N,N,N′,N′-tetramethylethylenediamine), [(tmeda)Pt(CH₃)(OEt₂)][BAr_f] and [(tmeda)Pt(CH₃)(THF)][BAr_f] are unstable at room temperature, yielding methane and the Fischer carbene Pt^II hydrides, [(tmeda)Pt(=C(CH₃)(OCH₂CH₃))(H)][BAr_f] and . The methane liberated from [(tmeda)Pt(CH₃)(OEt₂-d₁₀)][BAr_f] consists of an isotopomeric mixture, (CH₄, CH₃D, CH₂D₂ and CHD₃), indicating a multiple H/D exchange reaction following the C-D activation and prior to methane loss. [(tmeda)Pt(CH₃)(THF-d₈)][BAr] liberates CH₄ and CH₃D. Methane-¹³C, cyclohexane, toluene, and benzene react with [(tmeda)Pt(CH₃)(NC₅F₅)][BAr_f] to yield methane and new organoplatinum complexes. Deuterated alkanes and arenes react with [(tmeda)Pt(CH₃)(NC₅F₅] [BAr_f] to give a mixture of methane isotopomers. The relevance of these results to the oxidation of alkanes by aqueous platinum complexes is discussed.     

Nitrogen, sulfur and oxygen donor adducts with copper(II) complexes of antitumor 2-formylpyridinethiosemicarbazone analogs: Physicochemical and cytotoxic studies

The preparation of N-, S- and O-donor ligand adducts with CuX⁺(HX=6-methyl-2-formylpyridinethiosemicarbazone (6HL); 2-formylpyridine-2^′-methylthiosemicarbazone (2′L); 2-formylpyridine-4′-methylthiosemicarbazone (4′HL)) is described. The N-donors, 2,2′-bipyridyl (bipy), 4-dimethylaminopyridine (dmap) give the complexes [Cu(6L)(bipy)]PF₆, [Cu(6L)(bipy)]Cl·5H₂O, [Cu(4′L)(bipy)]PF₆, [Cu(6L)(dmap)₂]PF₆·2.5 H₂O and [Cu(4′L)(dmap)₂]PF₆·H₂O which have been characterized by physical and spectroscopic techniques. Pentafluorothiophenolate (pftp) gives S-donor complexes [CuX(pftp)] (X=6L and 4′L) and thiolato co-ordination is proposed on the basis of spectroscopic evidence. Paratritylphenolate (ptp) and HPO²⁻₄ give O-donor complexes [Cu(6L)(ptp)], [Cu(4′L)(ptp)], [{Cu(6L)}₂HPO₄]·4H₂O, and [{Cu(4^′L)}₂HPO₄]·5H₂O which have been characterized by physical and spectroscopic techniques, as have the precursor complexes [Cu(6L)(CH₃COO)]·H₂O, [Cu(4′L)(CH₃COO)], Cu(6HL)(CF₃COO)](CF₃COO)·0.5H₂O, [Cu(4′HL)(CF₃COO)](CF₃COO), [Cu(2′L)Cl₂] and [Cu(2′L)(NO₃)₂]. Protonation constants for the ligands and some of their complexes have been determined. 2-Formylpyridinethiosemicarbazone (HL) complexes of silver, gold, zinc, mercury, cadmium and lead are also discussed. Cytotoxicity against the human tumor cell line HCT-8 and antiviral data for selected compounds are presented.     

The kinetics and energetics of formation of a molecular hydrogen complex involving a dinuclear ruthenium(II) centre

The reversible equilibrium conversion under H₂ of [RuCl(dppb) (μ-Cl)]₂ (1) to generate (η²-H₂) (dppb) (μ-Cl)₃RuCl(dppb) in CH₂Cl₂ (dppb = Ph₂P(CH₂)₄PPh₂) has been studied at 0–25 °C by UV-Vis and ³¹P{¹H} NMR spectroscopy, and by stoppe kinetics; the equilibrium constant and corresponding thermodynamic parameters, and the forward and reverse rate constants at 25 °C have been determined. A measured ΔH° value of 0 kJ mol⁻¹ allows for an estimation of an exothermicity of 60 kJ mol⁻¹ for binding an η²-H₂ at an Ru(II) centre; a ΔS° value of 60 J mol⁻¹ K⁻¹ indicates that in solution 1 contain s coordinated CH₂Cl₂. The kinetic and thermodynamic data are compared to those obtained from a previously studied hydrogenation of styrene catalyzed by 1. Preliminary findings on related systems containing Ph₂P(CH₂)₃PPh₂ and (C₆H₁₁)₂P(C₆H₁₁)₂ are also noted.     

Mixed anion lanthanide(III) crown ether complexes: crystal structures of [LaCl2(NO3)(12-crown-4)]2, [La(NO3)(OH2)4-(12-crown-4)]Cl2·CH3CN and [LaCl2(NO3)(18-crown-6)]

Reaction of LaCl₃·7H₂O containing small amounts of La(NO₃)₃·7H₂O as an impurity with 12-crown-4 or 18-crown-6 in 3:1 CH₃CN:CH₃OH resulted in the isolation of the mixed anion complexes [LaCl₂(NO₃)(12-crown-4)]₂, [La(NO₃)(OH₂)₄(12-crown-4)]Cl₂·CH₃CN and [LaCl₂(NO₃)(18-crown-6)]. The nine-coordinate dimer, [LaCl₂(NO₃)(12-crown-4)]₂, has all of the anions in the inner coordination sphere and La³⁺ has a capped square antiprismatic geometry. It crystallizes in the orthorhombic space group Pbca with (at −150 °C) a = 12.938(6), B = 15.704(3), C = 13.962(2) Å, and D_calc = 2.08 g cm⁻³ for Z = 4. The second complex isolated from the same reaction, [La(NO₃)(OH₂)₄(12-crown-4)]Cl₂·CH₃CN, has the bidentate nitrate anion in the inner coordination sphere but the two chloride anions are in a hydrogen bonded outer sphere. This complex is ten-coordinate 4A,6B-expanded dodecahedral and crystallizes in the monoclinic space group P2₁ with (at 20 °C) A = 7.651(2), B = 11.704(7), C = 11.608(4) Å, β = 95.11(2)°, and D_calc = 1.80 g cm⁻³ for Z = 2. The 18-crown-6 complex, [LaCl₂(NO₃)(18-crown-6)], has all inner sphere anions and has ten-coordinate 4A,6B-expanded dodecahedral La³⁺ centers. It crystallizes in the orthorhombic space group Pbca with (at 20 °C) a = 14.122(7), B = 13.563(5), C = 19.311(9) Å, and D_calc = 1.89 g cm⁻³ for Z = 8.     

Labeling studies of some carbonylation reactions of styrene with cationic palladium complexes

The dicarbonylation reaction of E-β-deuteriostyrene to syndiotactic poly(1-oxo-2-phenyltrimethylene) as well as to dimethyl-2-phenylbutanedioate and dimethyl-2,5-diphenyl-4-oxoheptanedioate using Pd(CF₃COO)₂/2,2′-bipyridine as the catalyst precursor in the presence of 1,4-benzoquinone in methanol takes place stereospecifically in a syn-fashion with complete retention of the label. The same result was found for the dicarbonylation to dimethyl 2-phenylbutanedioate catalyzed by [Pd(CF₃COO)₂(Diop)]. In the absence of the oxidant the latter catalytic system produces methyl 2- and 3-phenylpropionates for which some scrambling of deuterium is observed when using either -deuteriostyrene or CH₃OD as the labeled substrate. [Pd(CH₃CN)₄][BF₄]₂ modified with different ligands catalyses the formation of E-1,5-diphenylpent-1-en-3-one or of E-1,4-diphenylpent-1-en-3-one in tetrahydrofuran as the solvent. The label distribution using E-β-deuteriostyrene as the substrate (or styrene in the presence of dideuterium) suggests that in the synthesis of ketones catalyzed by [Pd(p-CH₃C₆H₄SO₃)₂(Dppp)]·2H₂O the regioselectivity of the first inserted olefin unit does not determine the ketone regioisomer; rather which regioisomeric product preferentially forms depends on the rate of carbon monoxide insertion in either the branched or linear metal-hydrocarbyl intermediate. β-Hydrogen elimination is very rapid both after the first and the second olefin insertion.     

Thermal and photochemical substitution reactions of CpRe(PPh3)2H2 and CpRe(PPh3)H4. Catalytic insertion of ethylene into the C-H bond of benzene

The thermal and photochemical reactions of CpRe(PPh₃)₂H₄ and CpRe(PPh₃)H₄ (Cp = η⁵-C₅H₅) with PMe₃, P(p-tolyl)₃, PMe₂Ph, DMPE, DPPE, DPPM, CO, 2,6-xylylisocyanide and ethylene have been examined. While CpRe(PPh₃)₂H₂ is thermally inert, it will undergo photochemical substitution of one or two PPh₃ ligands. With ethylene, substitution is followed by insertion of the olefin into the C-H bond of benzene, giving ethylbenzene. CpRe(PPh₃)H₄ undergoes thermal loss of PPh₃, which leads to substituted products of the type CpRe(L) H₄. Photochemically, reductive elimination of dihydrogen occurs preferentially. The complex trans-CpRe(DMPE)H₂ was structurally characterized, crystallizing in the monoclinic space group P2₁/n (No. 14) with a = 6.249(6), b = 16.671(8), c = 13.867(7) Å, β = 92.11(6)°, V = 1443.7(2.9) Å and Z = 4. The complex trans-CpRe(PMe₂Ph)₂H₂ was structurally characterized, crystallizing in the monoclinic space group P2₁/n (No. 14) with a = 7.467(3), b = 23.874(14), c = 11.798(6) Å, β = 100.16(4)°, V = 2070.2(3.4) ų and Z = 4.     

Solution and crystal structure of the dihydrogen complex [Ru(H2)(H)(PMe2Ph)4]PF6, an active alkyne hydrogenation catalyst

The complex [Ru(H₂)(H)(PMe₂Ph)₄]PF₆ (1) has been prepared by reaction of [Ru(H)(PMe₂Ph)₅] FP₆ (2) in THF with 1 atm H₂ and characterised by variable temperature ³¹P and ¹H NMR. It undergoes four distinct fluxional processes listed in order of decreasing activation energy: (i) exchange of H₂ in solution with the dihydrogen ligand above 273 K; (ii) isomerisation of cis and trans isomers of 1 above 230 K; (iii) exchange of H atoms between H₂ and hydride in trans-1 above 180 K; (iv) rapid H₂/hydride exchange in cis-1 to below 180 K. A single crystal X-ray diffraction study of 1 at 173 K shows that the complex has the cis geometry in the solid state but does not clearly reveal the positions of the hydrogen ligands. Complex 1 starts out as a catalyst of high activity for the selective hydrogenation of 1-alkynes to 1-alkenes (RC≡CH; R=¹¹Bu, Ph) but it is rapidly deactivated, possibly because of formation of the enynyl complex [Ru(η³RC₃CHR)(PMe₂Ph)₄]⁺. Complex 1 efficiently catalyzes the hydrogenation of internal alkynes (3-hexyne, 2-pentyne) to internal cis-alkenes with little deactivation, although some isomerisation of the alkene produced is observed. These observations are consistent with those of Nkosi, Coville, Albers and Singleton who reported that complex 2 must dissociate one PMe₂Ph ligand to produce the species active for alkyne hydrogenation. Complex 2 catalyses these hydrogenations with slower initial rates than complex 1 but deactivates less readily. In contrast to 1, complex 2 does not appear to cause the isomerisation of internal alkenes.     

Photochemistry of platinum phosphine complexes. Perspective in C---H bond activation

The photochemistry of the diphosphino Pt(II) hydrides [LPtH₂] (L=(t-Bu)₂P(CH₂)₂P(t-Bu)₂ (7); L=(t-Bu)₂P(CH₂)₃P(t-Bu)₂ (8);L=(t-Bu)(Ph)P(CH₂)₂P(Ph)(t-Bu) (9)) is reported. The primary photoevent is the dissociation of H₂ and formation of the 14-e [LPt] species. These coordinatively unsaturated intermediates provide a versatile entry point into the C---H bond activation of hydrocarbons. [LPt] reacts with benzene in an oxidative addition reaction to yield [LPt(H)(C₆H₅)] complexes. The importance of the metal centre and ancillary ligation in the C---H bond activation is discussed.     

Structural studies on products of reduction of a nitro-Ni1C3N2-metallocene

The reduction of 12-nitro-(1,1,2,8,9,9-hexamethyl-3,7,10-14-tetraaza-4,6-oxa-5-hydra-tetradeca-2,7,10-12-tetrene)nickel(II) (Nioyl-NO₂), with Zn(s) and NaOH or HCl solution or utilizing Pd-H₂ under most conditions produces an intensely purplee complex ion ε_(max) at 552 nm which is not the expected amine. This product was found to be a conjugated dimer ion with two Nioyls multiply bonded to a single nitrogen atom. It was shown that the initial reduction produces the amine or amine hydrochloride which oxidizes rapidly in the presence of traces of O₂ under low acidity conditions to the dimer. Under high acidity conditions the amine salt is isolated. The X-ray crystal structures of three complexes are described: [(Nioyl)₂NH](ClO₄)₂·2.5CCl₄, [(Nioyl-NH₃)₂H]ZnCl₄Cl·3H₂O, [Nioyl-NH₃]H_0.5(ClO₄)_1.5·2CH₃CN·2H₂O and structural differences are discussed. The 2e⁻ reduction of [(Nioyl)₂N]⁺ with dithionite ion reversibly gives the yellow [(Nioyl)₂NH]⁺ which is extremely sensitive to air oxidation. A postulated reaction sequence is presented and discussed to explain the formation of the highly stable conjugated dimeric purple product.