Inorganic Compounds with Unusual Properties—II

The selective hydrogenation of soybean methyl ester, short chain dienes, and cyclooctadiene to the monoene stage under the catalytic influence of [Pt(PO₃)₂(SnCl₃)Cl] is described. In this catalyst, the platinum can be replaced by palladium, the phosphorus by arsenic, antimony, sulfur, or selenium, the phenyl groups by other aromatic, aliphatic, or ester groups, the tin by lead or germanium, and the chlorine by bromine, iodine, or cyanogen. Terminal double bonds, even in monoenes, are hydrogenated. Isomerization to conjugation apparently precedes hydrogenation. Under mild conditions only isomerization is observed. The isomerized products are largely in the trans form. The catalyst, when made heterogeneous by attaching it to cross-linked polystyrene, still retains its ability to hydrogenate polyunsaturated molecules selectively.

The chemistry of cationic rhodium complexes containing chelating diphosphine ligands was found to differ significantly from that of corresponding complexes of monodentate phosphine ligands. The characterization of [Rh(DIPHOS)]⁺ and of its alkene and arene adducts is described, together with studies on the kinetics of the catalytic hydrogenation of alkenes in which such adducts are intermediates. The results of these studies are pertinent to the role of such complexes as catalysts for the asymmetric hydrogenation of prochiral olefins.

Addition of triethyl amine to Rh(PPh₃)₃Cl or to complexes formed from [Rh(1,5-hexadiene)Cl]₂ and phosphines under hydrogen yields very active catalysts for the hydrogenation of aromatic nitro compounds to amines. The dark brown homogeneous catalyst solutions show highest activity at molar ratios of Rh/PR₃/Et₃N = 1:1.2:3. Turnovers above 1 mol H₂/mol Rh min are achieved.

A new series of hydrogenation catalysts based on pentamethylcyclopentadienyl-rhodium and -iridium complexes are described. These are derived from [M(C₅Me₅)Cl₂]₂ (1a, M = Rh; 1b, M = Ir) which are obtained from hexamethyl Dewar benzene in a two-step reaction. These complexes homogeneously hydrogenate simple olefins at 1 atm and 20°C in the presence of base; polar noncoordinating solvents give the best rates. Benzene and substituted benzenes are also hydrogenated to cyclohexanes by 1a under more vigorous conditions. The mechanism of the olefin hydrogenation reaction has been investigated and shown to involve mono- nuclear species such as [M(C₅Me₅)H₂(solv)]. The binuclear μ-hydrido complexes [HM₂(C₅Me₅)₂Cl₃], [H₂Ir₂(C₅Me₅)^2- Cl₂], and [H₃Ir₂(C₅Me₅)₂]⁺ are much poorer olefin hydrogenation catalysts than 1 and the reasons for this are discussed.

Product distributions can be altered by the addition of neutral micelle-forming surfactants to the K₃[Co(CN)₅H]- catalyzed hydrogenation of 2-methyl-1,3-butadiene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Although rate accelerations are modest, the micelles significantly prevent decomposition of the catalyst and solubilize the organic substrates in the aqueous medium. Phase transfer reaction conditions have proved to be even more successful for these metal-catalyzed reactions. The reactions are inexpensive, easy to set up, generally regioselective and, because of substantial rate accelerations and stabilization of the catalyst, large amounts of substrate are rapidly converted to product.

A catalyst for asymmetric hydrosilylation of ketones was prepared from [Rh(COD)Cl]₂ and DIOP. The hydrosilylation of acetophenone yields 1-phenylethanol after hydrolysis. The optical yield strongly depends on the nature of the silane RR'SiH₂ that was used. Several types of ketones were asymmetrically reduced into chiral alcohols, the highest asymmetric induction being observed from some α-chloro ketones or α-ketoesters. It was remarkable that prochiral benzophenones such as p-OMeC₆H₄COC₆H₅ could be reduced with up to 26% e.e. Mechanism of asymmetric hydrosilylation was discussed in relation with some spin trap experiments. Studies were made on supported rhodium—DIOP catalysts. Some rhodium leaching from support was demonstrated by a three-phase test.

Iron complexes containing bidentate alkyl and aryl phosphorus ligands cleave a variety of C-H bonds under mild conditions. Hydrido acetylide complexes were prepared by oxidative addition of primary acetylenes in the Fe(DPPE)₂ and the Fe(DMPE)₂ systems [DPPE = bis(diphenylphosphino) ethane, DMPE = bis(dimethylphosphino)ethane]. The Fe(DMPE)₂ system also cleaves C-H bonds of activated methyl groups, aromatic compounds, and certain other sp² hybridized molecules. The C-H cleavage reactions are reversible, resulting in equilibrium mixtures of isomeric products in many cases. Studies of substituted benzenes show that while product stability is favored by electron withdrawing substituents, steric effects play a predominant role in the determination of product distribution.

Summarized are our recent studies of the homogeneous catalysis of the water gas shift reaction. Characterization of the previously reported catalyst based on Ru₃(CO)₁₂ in alkaline, aqueous ethoxyethanol solution indicates that the principal ruthenium components are tetraruthenium carbonyl hydride anions. A mechanism is proposed involving the attack of OH- or H₂O on coordinated CO to give, after loss of C0₂, a dihydride metal species MH₂. Reductive elimination of hydrogen and coordination of another CO regenerates the original metal carbonyl MCO. A number of other metal carbonyls proved to form active catalysts under analogous conditions. Active catalysts are also formed from H₄Ru₄(CO)₁₂ in acidic aqueous diglyme solution and from H₄Ru₄CO)₁₂ or H₄Ru₄(CO)₁₂/Fe(CO)₅ mixtures in organic amine solutions.

Methanol or 1-butanol solutions of the mononuclear metal carbonyls M(CO)₆ (M = Cr, Mo, and W) and Fe(CO)₅ in the presence of aqueous sodium or potassium hydroxide are active homogeneous catalysts for the water gas shift reaction (CO + H₂O ⇄ CO₂ + H₂). The effects of temperature, pressure, and base concentration on the rate of hydrogen production from CO and H₂O in the presence of Fe(CO)₅ and NaOH have been investigated. The observation by IR spectroscopy that HFe(CO)₄^- reacts with CO under pressure in 1-butanol or THF to give Fe(CO)₅ suggests the following catalytic cycle for the water gas shift reaction catalyzed by basic solutions of Fe(CO)₅: (1) HFe(CO)₄^- + CO → Fe- (CO)₅ + H^-; (2) H^- + H₂O → OH^- + H₂ ; (3) Fe(CO)₅ + OH^- → Fe(CO)₄C(O)OH^-; (4) Fe(CO)₄C(O)OH^- → HFe- (CO)₄^- + CO₂.

An essential step in the homogeneous catalyzed water gas shift reaction involves nucleophilic attack of hydroxide ion at the carbon atom of a metal-bound CO, with a subsequent process leading to the extrusion of CO₂. These two processes have been studied for the reaction of oxygen-18 enriched NaOH with Cr(CO)₆ in a biphasic medium in the presence of a phase-transfer catalyst. Oxygen exchange was observed to occur at a faster rate than metal hydride, μ-H[Cr(CO)₅]₂^-, formation with concomitant production of CO₂. The kinetic parameters for the reaction of μ H[Cr(CO)₅]₂^- with CO in alcoholic solvent to afford Cr(CO)₆ and H₂ have been determined.

Carbonyl complexes of rhodium, ruthenium, osmium, iridium, and platinum, in the presence of H₂O and a weak base (e.g., trimethylamine), act as catalysts for the conversion of propene to a mixture of butanal and methylpropanal; with the exception of the platinum system, these catalysts are considerably more active than Fe(CO)₅ as reported by Reppe. Under the same conditions, but in the absence of olefin, the carbonyls act as catalysts for the conversion of CO and H₂O to CO₂ and H₂. The metal carbonyls, together with Fe(CO)₅, in the presence of H₂O, CO, and a weak base such as Me₃N, serve as catalysts for the conversion of nitrobenzene, dinitrobenzene, and 2,4- and 2,6-dinitrotoluene to the corresponding aminobenzene derivatives.

Anionic metal formyl complexes are produced upon reaction of K⁺ HB[OCH(CH₃)₂]₃^- with metal carbonyls. The formyl compounds can be observed in solution by ¹H NMR (14-168) and have been isolated as stable solids in several cases: (CH₃CH₂)₄N⁺{[3,5-(CH₃)₂-C₆H₃O]sP}(CO)₃FeCHO- is kinetically quite stable but decomposes upon heating to 70°C to (CO)₄FeH- (E_a = 29.7 ± 2 kcal mol^-1). No metal formyl compound can be observed in equilibrium with the corresponding metal hydride. In contrast, the equilibrium between L(CO)₃FeCOCH₃^- and (CO)₄FeCH₃^- lies entirely on the side of the acetyl iron compound. Metal formyl compounds can act as hydride donors to ketones, alkyl halides, and metal carbonyls. Metal carbene complexes react with molecular hydrogen, leading to reductive cleavage of the carbene ligand.

Electron transfer from metal oxide surfaces to CO can be quite facile, occurring at room temperature. This process can be important as an initial CO activation step in metal oxide catalyzed reduction schemes. We have attempted to clarify what types of metal oxides interact (MO + CO → MO⁺ . . . CO^-.) with CO in this way, and what surface features these active metal oxides possess. Only MgO, CaO, SrO, BaO, and ThO₂ were electron transfer active. These oxides have in common the possession of both Lewis basic sites and one electron reducing site. It appears that CO is first adsorbed on Lewis base sites followed by slow migration to electron transfer reducing sites. The studies leading to this conclusion are discussed.

Nickel(II) phosphine complexes have been reported to be efficient catalysts in carbonylation reactions. To investigate this reaction mechanism, we have studied the reaction of CO on the related Ni(II) complexes: NiX₂(PMe₃)n (n = 2,3) and [NiX(PMe₃)m]BFt (m = 3,4). Pentacoordinate carbonyl nickel(II) species (without reduction of Ni(II) to Ni(O)) were isolated (1) by direct substitution of PMe₃ by CO in the pentacoordinate complex and (2) by addition of CO on the trans square-planar tetracoordinate complex. These compounds are trigonal-bipyramidal complexes with CO in equatorial position. The Ni-CO distance (1.73 Å) is the shortest reported Ni-CO distance. Since these carbonylation reactions can be viewed as substitution of an equatorial PMe₃ by CO in a d⁸ TBP, they can be related to the substitution reactions in square-planar d8 metal complexes.

A group of novel binuclear metal ligands composed of two alkyl porphyrins covalently linked in a cofacial configuration has been synthesized. The interplanar distance of the diporphyrins can be varied from 6.4 to 4.2 Å by changing the length of the linkage. The presence of exciton interaction in these dimers was evidenced by a substantial blue shift (10-30 nm) of the Soret peak. Both homo- and hetero-dimetalloporphyrins have been prepared, e.g., Fe-Fe, Cu-Cu, Mg-Mg, Co-Co, and Fe-Cu. Dioxygen was found to form both 1:1 and 2:1 complexes with Co(II) and Fe(II) diporphyrins. The ring separation played a major role in determining the metal-to-oxygen ratio. Implications of the dimer studies on biological oxygen reduction and the "special-pair" chlorophylls are discussed.

The products of oxidation of Cu(I) chloride by oxygen in aprotic ligand/solvent systems are useful catalysts for a variety of oxidative coupling reactions of molecular oxygen. The ligand/solvent environment determines the nature of the products. The copper-reduced oxygen products obtained in pyridine are growing polymers with a pyridinestabilized CuO core, which can be separated from the py₂-CuCl₂ co-product, and are the initiators for the oxidative coupling of phenols. Saturated, methylated amines are ineffective stabilizers of the copper-reduced oxygen interaction while amide and lactam ligands stabilize distinct clustered primary oxidation products. Cryoscopic measurements and a determination of the structure of a catalyst derivative with coordinated N-methyl-2-pyrrolidinone ligands are valuable in furthering our understanding of the structure of the products formed in other ligand/solvent systems.

The ease of interaction of Ni(0) complexes with organic substrates has been shown to depend upon both the ligands on nickel and the solvent. The presence of α,α'-bipyridyl with the Ni(0) complex and the alkyne led to the isolation of a nickelacyclopropene, an observation in accord with the recently proposed metallocyclic pathway for the Ni(0)-catalyzed trimerization of alkynes. Allylic and benzylic ethers and epoxides have been observed to undergo oxidative insertion of Ni(0) into their C -O bonds with solvent (TMEDA > THF > Et₂O > C₆H₆) and ligand (Et₃P > Ph₃P; α,α'-bipy > COD) effects consistent with an electron-transfer attack by Ni(0). With such sulfur heterocycles as dibenzothiophene, phenoxathiin, phenothiazine, and thianthrene, a 1:1 admixture of (COD)₂Ni with α,α'-bipyridyl gave as the principal product the desulfurized, ring-contracted cyclic product.

The proposal that (Chl a · ₂H₂O)₂ is the photoreaction center Chl a aggregate for the water splitting reaction in plant photosynthesis led us to the belief that chlorophyll dihydrate polycrystals can be used in efficient water photolysis in vitro. In this chapter, the observation of gaseous evolution under white light illumination of platinized chlorophyll dihydrate polycrystals is described. The photoelectrolytic products were determined to be molecular hydrogen and oxygen by diffusion pyrolysis and mass spectrometry. The photochemical activity of (Chl a·₂H₂o)_n is attributed to the presence of water in Chl a aggregation via the C9 keto C = O · · · H(H)O · · · Mg bonding interaction. The demonstration of the decomposition of water by (Chl a·₂H₂O)_n lends support to the suggestion that a single photosystem in plant photosynthesis may be capable of splitting water in vivo.

The position of the lowest allowed electronic transition (1a_2u → 2a_1g in D_4h) in Rh₂L₄²⁺ (L = binucleating isocyanide) depends on the rotameric configuration of the complex [553 nm in Rh₂(bridge)₄²⁺ (eclipsed); 515 nm in Rh₂(TM4-bridge)₄²⁺ (30° staggered)]; the Rh(I)-Rh(I) distances are 3.26 and 3.25 Å, respectively. Reaction of Rh₂(bridge)₄²⁺ (or Rh₂²⁺) with H₂ in dilute aqueous acidic solutions yields Rh₄H₂⁴⁺ (λmax = 780 nm). The Rh₄H₂⁴⁺ species also is produced by low-temperature (-60°C) irradiation (λ > 520 nm) of 6M HCl solution of HRh₂Cl²⁺ (λmax = 578 nm). The possible role of Rh₄H₂⁴⁺ as an intermediate in the H₂-producing photoreaction of HRh₂Cl²⁺ (HRh₂Cl²⁺ Rh₂Cl₂²⁺ + H₂) is discussed.

Excited states of transition metal complexes are often quenched by electron donors or electron acceptors in redox processes that can involve efficient conversion of light energy into high energy products. We have examined ways to circumvent the energy-wasting back reactions by modification of the complex, quencher, and medium or by adding reactive substrates to intercept products generated by electron transfer. For oxidative quenching, a variety of substrates are rapidly oxidized by certain RuL₃³⁺ complexes such that RuL₃²⁺ is regenerated but Oxrcd survives. For reductive quenching processes a combination of relatively slow rates of back reaction and rapid reaction of Redox enables the isolation of the high energy RuL₃⁺ as a stable but highly reactive product.

Inert eight-coordinate W(IV) chelates have been synthesized in our laboratories and have properties analogous to the ruthenium complexes that are currently an energy-transfer focus. Substituted tetrakis(8-quinolinolato)tungsten(IV) complexes (1), substituted tetrakis(picolinato)tungsten(IV) complexes (2), tetrakis(pyrazinecarboxylato)tungsten(IV) (3), tetrakis(isoquinoline-1-carboxylato)tungsten(IV) (4), and bis(N,N'-disalicylidene-1,2-phenylenediaminato)tungsten(IV) (5) have been synthesized. The ML₄ chelates are substitution-inert d² chelates with low energy metal-to-ligand charge-transfer transitions based on relative energies and photooxidation to d¹-W(V) analogs, which possess ligand-to-metal charge-transfer transitions in the visible region. We have initiated the synthesis of analogous polymeric complexes as well.

Tricarbonylbis(N,N-dimethyldithiocarbamato)tungsten, (W(CO)₃(dmtc)₂), has been synthesized, and two distinct intramolecular dynamic processes have been identified by variable temperature ¹³C NMR studies of this seven-coordinate molecule. The sodium salt of dimethyldithiocarbamate reacts with tetracarbonyldiiodotungsten to form the above product, W(CO)₃(dmtc)₂. Analytical, IR and NMR data confirm this formulation. The ¹³C NMR spectrum at -110°C has three distinct resonances, two of which initially coalesce independently of the third (ΔG╪‚ = 8.1 kcal mol^-1) as the temperature is increased. All three carbon monoxide signals are averaged at higher temperatures (ΔG╪‚ = 9.0 kcal mol^-1). Reversible loss of one CO occurs upon heating to form an insoluble blue W(CO)₂(dmtc)₂ compound.

Desorption isotherms for the hydrides of LaNi₄.₆Al₀.₄ and LaNi₄.₅Al₀.₅ are presented and values for the enthalpy and entropy changes of the hydriding reactions are calculated from the van't Hoff plots of log P vs. 1/T. A crystallographic model of LaNi₄Al is shown and consideration of the nearest neighbor atom distribution leads to a rationalization of the observed linear relationship between the enthalpy change, ΔH, and the aluminum composition. Brief discussions of methods to predict dissociation pressures or interstitial site occupation are included. The cubic and hexagonal AB₅ phases are compared and, finally, the application of these alloys in chemical heat pump systems is noted.

Absorption spectra and redox potentials in acid solution limit sunlight engineering efficiency (S.E.E.) of unsensitized iron-thiazine photogalvanic cells to ~ 2%. The highest S.E.E. value obtained with totally illuminated single thin-layer (TI-TL) iron-thionine cells with SnO₂ anodes and Pt cathodes, .036%, corresponds to V_{power point} ~ 35% of theoretical limit. Potentials at the selective anode are dominated by the dye-leucodye couple. Potentials at the poorly selective cathode are dominated by the iron couple. I_sc varies linearly with photostationary concentration of leucothionine and, with electrode spacing. ≤50μm, is not limited by solution lifetime of charge carriers. Inefficient electron transfer at the electrodes is believed to reduce S.E.E. by a factor of ~ 5, possibly because of surface-promoted back reaction on SnO₂.

In connection with the activation of saturated hydrocarbons via homogeneous catalysis, we have examined transition metal catalyzed reactions of various strained hydrocarbon systems that have unique steric and electronic properties. Strained carbon-to-carbon single bonds have considerable π-bonding character. The chemistry of these substrates should be intermediate between well-documented transition metal chemistry of alkenes and rather unclarified alkane chemistry (1, 2, 3). Our attention has been focused particularly on the stereoselectivity, regioselectivity, and periselectivity of the Ni(0)-catalyzed reactions (4-14).

Copper(I) compounds accelerate the rates of a diverse assortment of olefin photoreactions, including rearrangement, oligomerization, and molecular fragmentation. This chapter seeks to provide a basis for understanding this sensitization effect in terms of the ground- and excited-state properties of Cu(I). Thus the ability to generate vacant coordination sites and form stable olefin complexes, as well as the absence of low-lying vacant d orbitals, are two characteristics of the metal which have potentially significant consequences for sensitization. A review of Cu(I)-sensitized olefin photoreactions is presented and a general classification of sensitization processes is proposed. The potential sensitization behavior of some previously unstudied Cu(I) systems also is explored.

The applications of transition metal complexes as catalysts for the conversion of quadricyclane to norbornadiene in the energy release step of a solar energy storage system are discussed. Co(II) tetraarylporphyrins anchored to macroreticular polystyrene through sulfonamide and carboxamide linkages can be prepared which are active catalysts for the conversion of quadricyclane to norbornadiene. Certain polystyrene-supported phosphine Pd(II) chloride derivatives are also active catalysts for this reaction. New homogeneous catalysts for the conversion of quadricyclane to norbornadiene include the triphenylcyclopropenylnickel complexes [(C₆H₅)₃C₃Ni(CO)X]₂ (X = Cl and Br) and the metal dithiolenes [(CF₃)₂C₂S₂]₃MO and [(CF₃)₂C₂S₂]₂Ni.

The molecular structure of [Co(en)₂(SCH₂CH₂NH₂) · Cu-(CH₃CN)₂]₂(ClO₄)₆ · ₂H₂O (en = ethylenediamine) contains a unique mercaptide-bridged Cu(I)₂(SR)₂ planar unit which is structurally related to proposed models for (reduced) Type 3 (EPR-nondetectable) copper in multicopper oxidases. The Cu-Cu distance of 3.146(2) Å is well within the 5-6 Å estimated as a maximum separation for Type 3 coppers. The S-S distance of 3.557(6) Å rules out a disulfide bond. The S-Cu-S bond angle (97°) indicates that the planar ring imparts, or at least allows, a geometry about the copper atoms intermediate between that preferred by Cu(I) (tetrahedral) and Cu(II) (tetragonal), a fact which may be of importance in the catalysis of biological redox processes by Type 3 copper.

A class of organochalcogen compounds containing one or two chalcogen-chalcogen bonds was chosen as ligands for organometallic synthesis. New discrete molecular complexes containing two, four, and six metal atoms were prepared and characterized. These cluster systems exhibit rich electrochemistry as established by cyclic voltammetry and unusual stereochemistry as revealed by x-ray crystallography. A new class of organometallic polymers based on these ligands was synthesized and characterized. Temperature-dependent electrical conductivity measurements revealed semiconductivity consistent with pseudo-one-dimensionality. Electrical conductivity can be correlated with the oxidation potential of the free ligands. These new semiconducting organometallic polymers can be used as reversible anode materials for a rechargeable battery system.

Crystallization of zeolites is often a nucleation-controlled process occurring from molecularly inhomogeneous, aqueous gels. The product is strongly dependent on the cation distribution in these mixtures. A methodology is developed for interpreting crystallization data and for identifying template effects by added quaternary ammonium cations. These techniques are then examined, by way of example, with a series of quaternary ammonium polymers. It is shown that such polymers can force crystallization of large-pore zeolites (where small-pore structures would otherwise result), and that they can preserve the integrity of a pore system during crystallization of fault-prone zeolite structures.

The following generalized reactions are proposed for compounds containing metal-metal triple bonds (M = Mo, W). (1) Carbene-like addition across the M-M bond; (2) oxidative addition with reduction in M-M bond order (three to two); (3) reductive elimination with an increase in M-M bond order (three to four); (4) reversible Lewis base association which may or may not change the bond order, depending upon the electronic configuration of the M₂ moiety; (5) metal-metal triple bond cleavage by a carbyne-like reagent; and (6) oligomerization of the M ≡ M unit to form a cluster or polynuclear complex. These generalized reactions are discussed in the light of recent experimental observations in the reactivity patterns of M₂(OR)₆ and Cp₂M₂(CO)₄ compounds.