Piero Stoppioni

CO2 absorption by aqueous NH3 solutions : speciation of ammonium carbamate, bicarbonate and carbonate by a 13C NMR study

The absorption of CO2 in aqueous NH3 solutions occurs with high efficiency and loading capacity at room temperature and atmospheric pressure producing the ammonium salts of bicarbonate (HCO3−), carbonate (CO32−), and carbamate (NH2CO2−) anions. 13C NMR spectroscopy at room temperature has been proven to be a simple and reliable method to investigate the speciation in solution of these three ionic species. Fast equilibration of HCO3−/CO32− anions results in a single NMR peak whose chemical shift depends on the relative concentration of the two species. A method has been developed to correlate the chemical shift of this carbon resonance to the ratio of the two anionic species. Integration of the carbamate carbon peak provided the relative amount of this species with respect to HCO3−/CO32− pair. No other species was detected in solution by 13C NMR, and no solid compounds separated out under our experimental conditions. Finally, the relative amount of HCO3−, CO32−, and NH2CO2− in solution have been correlated to the molar ratio between free ammonia in solution and absorbed CO2.

Structural correlations and NMR properties of mononuclear cyclic triphosphorus complexes

Abstract The [(triphos)Ni(η-P 3 )BF 4 · C 2 H 5 OH compound has been synthesized by treating Ni(BF 4 ) 2 · 6H 2 O with P 4 S 3 , in the presence of the triphos ligand (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane). The crystal structure of the complex and that of the previously obtained platinum isomorph have been investigated by X-ray diffraction. Correlations are drawn within the series of the monocationic [(triphos)M(η 3 -P 3 )] + (M = Ni, Pd, Pt) complexes and that of the neutral [(triphos)M(η 3 -P 3 )] (M = Co, Rh, Ir) compounds, as well as between the two series. Phosphorus NMR data are reported for the compounds of the two sets and trends are analyzed in terms of simple models of the bonding.

The P4Se3 cage molecule as a ligand. Crystal structure of [(N(CH2CH2PPh2)3) Ni(P4Se3]·2C6H6

Abstract The structure of the compound [(np 3 )Ni(P 4 Se 3 )]·2C 6 H 6 , obtained from reaction of the nickel(0) complex (np 3 )Ni (np 3  tris(2-diphenylphosphinoethyl)amine) with tetraphosphorus triselenide, P 4 Se 3 , has been determined by X-ray diffraction studies. Crystal data: cubic, space group P 2 1 3, a 17.413(7) A, Z  4; final R  0.050. The intact P 4 Se 3 entity is coordinated to the metal through the apical phosphorus atom. The small changes occurring in the geometry of the cage molecule upon coordination are analyzed by a comparison with the structure of uncoordinated P 4 Se 3 , which has been refined to R  0.045.

Hydridodinitrogen and hydrido complexes of iron(II) with the linear tetratertiary phosphine hexaphenyl-1,4,7,10-tetraphosphadecane. Crystal structure of the complex [FeH(N2)((C6H5)2PC2H4P(C6H5)C2H4P(C6H5)C2H4P(C6H5)2]Br · C2H5OH

Abstract Treatment of the chromophore [FeX(P4)]+, where X = Br, I and P4 is the tetradentate open-chain ligand hexaphenyl-1,4,7,10-tetraphoshadecane, with sodium tetrahydroborate under helium or nitrogen gave cationic hydrido- or hydrido-dinitrogen-iron(II) complexes of formula [FeH(P4]X and [FeH(N2)(P4)]X. The structure of the [FeH(N2)(P4)]Br · C2H5OH complex was determined from three dimensional X-ray data collected by counter methods. The crystals are triclinic, space group P1- with cell dimensions a = 15.392(8), b = 16.109(10), c = 9.914(5) ]A, α = 69.57(8), β = 78.43(9), γ = 68.46(9)°. The structure was solved the heavy atom technique to a final conventional R factor of 0.053 over 1631 independent observed reflections. The structure consists of [FeH(N2)(P4)]+ cations, bromide anions, and solvating C2H5OH molecules. The metal atom is octahedrally coordinated with the four phosphorus atoms of the ligand in the equatorial plane and weith the hydrogen atom and one atom of the dinitrogen molecule in the axial positions.

Halo-, hydrido- and dinitrogen-complexes of iron(II) with tritertiary phosphines

Abstract From the five-coordinate iron(II) complexes [FeXL] BPh 4 , (L = tris(2-diphenylphosphinoethyl)amine, np 3 , and tris(2-diphenylphosphinoethyl)phosphine, pp 3 ) hydrido and hydrido dinitrogen complexes have been obtained; these have formula [FeH(pp 3 )]BPh 4 and [FeHN 2 L]BPh 4 (L = np 3 , PP 3 . In the latter complexes the dinitrogen molecule may be replaced by other neutral ligands such as CO, CH 3 CN, C 6 H 5 CN.

Experimental Evidence of Phosphine Oxide Generation in Solution and Trapping by Ruthenium Complexes

Phosphorus oxides, oxyacids, and their esters are important chemicals for industry. Apart from playing a role in most organisms in ruling their energy transformations, they find wide and diverse applications, such as fertilizers, pesticides, herbicides, lubricants, flame retardants, additives for special plastics and materials, and drugs for different diseases. Little attention has however been paid to lower-oxidation-state species, such as PO, HPO, and P2O, for which synthetic isolation procedures and even direct evidence of their existence are scarce. One of the most elusive species in this regard is phosphine oxide, H3PO (I ; Scheme 1). This molecule was first observed by reacting atomic oxygen with PH3 using a discharge–flow system equipped with molecularbeam sampling mass spectrometry. Alternatively, red-light photolysis of co-deposited PH3/O3 onto an argon matrix at 12–18 K was used to generate and trap I in a very diluted concentration together with its tautomer phosphinous acid H2P(OH) (II), which was identified by FTIR. [4] Finally, Ault and Kayser observed the formation of H3PO in argon matrices after photochemical irradiation of a mixture of VOCl3, CrO2Cl2, and PH3. [5] Both molecules have been studied by theoretical methods. Application of an adequate phosphorus basis set has recently shown that, in contrast with previous ab initio studies, I is more stable than II by only about 1 kcalmol 1 in the gas phase. In contrast, computational analysis in aqueous solution showed that upon solvation I is largely preferred by about 10 kcal mol 1 owing to stronger hydrogen bonding with the highly polar P!O bond. The possible involvement of H3PO in the oxidative polymerization of phosphine to give polyhydride phosphorus PxHy polymers has been also proposed. Herein we show that the previously unknown P I species H3PO (I) can be easily generated in solution by electrochemical methods, and we provide evidence of its solution stability, its characterization by conventional NMR spectroscopy, and its trapping as a ligand in the coordination sphere of hydrosoluble ruthenium complexes after tautomerization to II. The electrochemical generation of H3PO was performed in a single electrochemical cell with a lead cathode and a sacrificial zinc anode using P4 melted in a slightly acidic water/ ethanol solution (2:1 volume ratio, water acidified with HCl, 2m) at 60 8C (Supporting Information, Figures S1, S2). The overall electrochemical process may be divided in two parts. In the first step, the electrochemical generation of PH3 on the lead cathode takes place as previously described, 11] while in the second step, mild oxidation of PH3 to H3PO occurs at the anodic surface of the zinc electrode. In agreement with cyclic voltammetry experiments showing an irreversible oxidation wave, PH3 is electrochemically active in the anodic potential range + 0.80–1.25 V (vs. Ag/AgNO3, 0.01m in CHCN3) and can be therefore oxidized in acidic water/ethanol 2:1 solution to H3PO (Supporting Information, Figure S3). Scheme 2 shows the overall electrochemical process resulting in the cathodic reduction of P4 to PH3 and anodic oxidation of PH3 to H3PO (E = + 1.24 V vs. Ag/AgNO3, 0.01m in CHCN3). Different working conditions were investigated to optimize the production of H3PO. The best performance was Scheme 1. Phosphine oxide (I) and its tautomer, phosphinous acid (II).

Stabilization of the tautomers HP(OH)2 and P(OH)3 of hypophosphorous and phosphorous acids as ligands

Treatment of [CpRu(PPh(3))(2)Cl] 1 with the stoichiometric amount of H(3)PO(2) or H(3)PO(3) in the presence of chloride scavengers (AgCF(3)SO(3) or TlPF(6)) yields compounds of formula [CpRu(PPh(3))(2)(HP(OH)(2))]Y (Y = CF(3)SO(3) 2a or PF(6) 2b) and [CpRu(PPh(3))(2)(P(OH)(3))]Y (Y = CF(3)SO(3) 3aor PF(6) 3b) which contain, respectively, the HP(OH)(2) and P(OH)(3) tautomers of hypophosphorous and phosphorous acids bound to ruthenium through the phosphorus atom. The triflate derivatives 2a and 3a react further with hypophosphorous or phosphorous acids to yield, respectively, the complexes [CpRu(PPh(3))(HP(OH)(2))(2)]CF(3)SO(3) 4 and [CpRu(PPh(3))(P(OH)(3))(2)]CF(3)SO(3) 5 which are formed by substitution of one molecule of the acid for a coordinated triphenylphosphine molecule. The compounds 2 and 3 are quite stable in the solid state and in solutions of common organic solvents, but the hexafluorophosphate derivatives undergo easy transformations in CH(2)Cl(2): the hypophosphorous acid complex 2b yields the compound [CpRu(PPh(3))(2)(HP(OH)(2))]PF(2)O(2) 6, whose difluorophosphate anion originates from hydrolysis of PF(6)(-); the phosphorous acid complex 3b yields the compound [CpRu(PPh(3))(2)(PF(OH)(2))]PF(2)O(2) 7, which is produced by hydrolysis of hexafluorophosphate and substitution of a fluorine for an OH group of the coordinated acid molecule. All the compounds have been characterized by elemental analyses and NMR measurements. The crystal structures of 2a, 3a and 7 have been determined by X-ray diffraction methods.

Synthesis, x-ray crystal structure and bonding in [(PPh3)2PtSe]2

Abstract The compound [(PPh 3 ) 2 PtSe] 2 has been obtained by reacting a selenide solution with (PPh 3 ) 2 PtCl 2 and its structure has been determined by X-ray diffraction analysis. The molecular geometry of the complex consists of a four membered Pt 2 Se 2 ring with two triphenylphosphine ligands coordinated to each platinum, giving a planar (P 2 Pt) 2 Se 2 arrangement. MO calculations have shown that the planar geometry is indeed slightly favoured for the neutral compound but anticipate a bent geometry for a hypothetical cationic species, in agreement with what has been found for related tellurium derivatives.

Controlling the Activation of White Phosphorus: Formation of Phosphorous Acid and Ruthenium‐Coordinated 1‐Hydroxytriphosphane by Hydrolysis of Doubly Metalated P4

The reactivity of white phosphorus with transition-metal compounds is a mature field of inorganic and organometallic chemistry that has been extensively investigated in the past few decades. Research in this field has led to the synthesis of an amazing variety of transition-metal complexes containing Pn units originating from either the coupling or the degradation of the cage molecule(s) as well as from the recombination of smaller fragments into polyatomic aggregates. These compounds often contain species with unique geometric and electronic properties which, apart from exhibiting a rich and intriguing chemistry, have found interest either as building blocks for the construction of networks of monoand polydimensional inorganic structures or as phosphorustransfer agents towards inorganic and organic molecules. The recent activation of white phosphorus with either heterocyclic carbenes or highly nucleophilic main group compounds has led to new opportunities in this area, especially by allowing the partial degradation of the molecule and its functionalization by insertion of organic fragments into the assembled polyphosphorus units without the involvement of a transition metal. Previous work from our group has highlighted the utility of {CpRuL2} moieties (Cp R =C5H5, C5Me5; L= phosphane) for coordinating the intact P4 molecule in reactions that yield stable monoor dinuclear cationic complexes [{CpRuL2}n(h -P4)] n+ (n= 1, 2). Furthermore, these monoor bimetallic compounds, which are easily obtained in gram amounts, have proved to be useful for investigating the reactivity of coordinated P4 under mild conditions. For example, we have found that the reactivity of the coordinated P4 molecule in the cyclopentadienyl derivatives is spectacularly modified with respect to that of the free molecule as it readily undergoes quantitative disproportionation with water at room temperature. Thus, addition of excess water (100 equivalents) to one equivalent of [CpRuL2(h -P4)](CF3SO3) (1) or [{CpRuL2}2(m,h -P4)](CF3SO3)2 (2) in THF hydrolyzes the coordinated P4 ligand in a few hours to yield a mixture of phosphine (PH3), diphosphane (P2H4), and the phosphorus oxyacids H3PO2 and H3PO3 in ratios that depend strongly on the hapticity of the P4 molecule (h 1 vs. m,h). The hydrogenated molecules are stabilized by coordination to {CpRu(PPh3)2} fragment(s), [8b,c,9] whereas the oxo derivatives are obtained as either free molecules or coordinate to ruthenium after tautomerization to the pyramidal species PH(OH)2 and P(OH)3, respectively. [10] The above products, which contain one or two phosphorus atoms from the parent P4 molecule, are clearly the thermodynamic sinks in the degradation of P4, which appears to follow aspecific pathways. Herein we report further breakthroughs in this reaction and show that the reactivity of coordinated P4 is a modular process which, surprisingly, is strongly dependent on the amount of water used for the hydrolysis reaction. In particular, we demonstrate that rapid quenching of the hydrolysis of the dinuclear derivative 2 with a large excess of water affords only phosphorous acid (H3PO3) and a new bimetallic compound containing the previously unknown 1hydroxytriphosphane molecule, which is stabilized as a bridging ligand between two {CpRu(PPh3)2} fragments (Scheme 1). The formation of this molecule, besides its intrinsic interest due to the fact that it has never been observed previously either in the free state or as a ligand, gives important hints regarding the initial step of the hydrolytic degradation of coordinated P4 and pinpoints the existence of a selective disproportionation of P4 that differs from its well-known alkaline hydrolysis, which gives only PH3 and hypophosphorous acid (H3PO2). The addition of 500 equivalents of water to one equivalent of 2 in THF is a simple process that leads to the formation of one equivalent of H3PO3 and one equivalent of the new complex [{CpRu(PPh3)2}2{m ,h-PH(OH)PHPH2}](CF3SO3)2 (3) within a few minutes ( P NMR monitoring). Work-up of this solution gave 3 in excellent yield, and recrystallization from CHCl3/n-hexane provided yellow crystals suitable for X-ray analysis. The diruthenium cation in 3 contains the previously unknown molecule PH(OH)PHPH2, which bridges two {CpRu(PPh3)2} moieties through the phosphorus atoms of the PH(OH) and PH2 end-groups. These two groups are affected by twofold positional disorder in the solid-state [*] Prof. Dr. M. Di Vaira, Dr. S. Seniori Costantini, Prof. Dr. P. Stoppioni Dipartimento di Chimica, Universit& di Firenze via della Lastruccia,3, 50019 Sesto Fiorentino, Firenze, (Italy) Fax: (+39)0554573385 E-mail: piero.stoppioni@unifi.it

Synthesis and structural study of a cobalt complex with the diarsenic-sulfur cyclic unit as a trihapto ligand

Abstract The reaction of Co(BF4)2·6H2O with tetraarsenic trisulfide in the presence of triphos (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane), yields the compound [(triphos)Co(As2S)]BF4·C6H6 in which the diarsenic-sulfur cyclic unit acts as a trihapto ligand. The structure of the compound has been determined by single-crystal X-ray diffraction methods. The compound crystallizes in the monoclinic P21/n space group with Z = 4 and unit cell dimensions: a = 16.987(7), b = 20.608(7), c = 13.098(5) A, β = 104.83(5)°. The metal atom is in a distorted six-coordinate environment formed by the triphos P atoms and by the atoms of the heterocyclic As2S unit.