Hartmut Schönberg

A Biologically Inspired Organometallic Fuel Cell (OMFC) That Converts Renewable Alcohols into Energy and Chemicals

The simultaneous conversion of alcohols and sugars into energy and chemicals is a target of primary importance in sustainable chemistry. The realization of such a process provides renewable energy with no CO2 emission and, at the same time, leads to the production of industrially relevant feedstocks, such as aldehydes, ketones, and carboxylic acids, from biomasses. Two established types of fuel cells operating in alkaline media can convert the free energy of alcohols (RCH2OH) into electrical energy and the corresponding carboxylate product: the direct alcohol fuel cell (DAFC), and the enzymatic biofuel cell (EBFC). 8] In a DAFC, an alcohol such as ethanol (CH3CH2OH) is selectively converted into acetate (CH3COO ) and the electrolyte is an anionexchange membrane. On the anode, ethanol is oxidized, releasing four electrons [Eq. (1)] that are utilized to reduce one oxygen molecule to four hydroxide ions on the cathode [Eq. (2)]. Efficient devices of this type have been recently developed for a variety of renewable alcohols and polyalcohols, such as ethylene glycol, glycerol, 1,2-propandiol, and C6 and C5 sugars. [3–6] (For drawings of a DAFC, a EBFC, and typical power density curves, see the Supporting Information, Figure S1 a–d).

High-resolution EPR spectroscopic investigations of a homologous set of d9-cobalt(0), d9-rhodium(0), and d9-iridium(0) complexes.

The 17-electron complexes [M(tropp(ph))2] (M=Co0, Rh0, Ir0) were prepared and isolated (tropp = tropylidene phosphane). A structural analysis of [Co(tropp(ph))2] revealed this complex to be almost tetrahedral, while the heavier homologues have more planar structures. Partially deuterated tropp complexes [D6][M(tropp(ph))2] were synthesised for M = Rh and Ir in order to enhance the resolution in the EPR spectra. This synthesis involves a four-fold intramolecular C-H activation reaction, whereby alkyl groups are transformed into olefins. Dihydrides were observed as intermediates for M = Ir. The electronic and geometric structures of all complexes [M(tropp(ph))2] (M = Co, Rh, Ir) and [D6][M(tropp(ph))2] (M = Rh, Ir) were investigated by continuous wave (CW) and echo-detected EPR in combination with pulse ENDOR and ESEEM techniques. In accord with their planar structures, cis and trans isomers were detected for [M(tropp(ph))2] (M = Rh0, Ir0) for which a dynamic equilibrium was established. The thermodynamic data show that the cis isomer is slightly preferred by deltaH(o) = -4.7 +/- 0.3 kJ mol(-1) (M = Rh) and delta H(o) = -5.1 +/- 0.5 kJ mol(-1); (M = Ir). The entropies for the process trans-[M(tropp(ph))2] <==> cis-[M(tropp(ph))2] are also negative [deltaS(o) = -5 +/- 1.5 J mol(-1) (M = Rh); deltaS(o) = -17 +/- 3.7 J mol(-1) (M = Rh)], indicating higher steric congestion in the cis isomers. The cobalt(0) and irdium(0) complexes show rather large g anisotropies, while that of the rhodium(0) complex is small (Co: g(parallel) = 2.320, g(perpendicular) = 2.080; cis-Rh: g(parallel) = 2.030, g(perpendicular) = 2.0135; trans-Rh: g(parallel) = 2.050, g(perpendicular) = 2.030; cis-Ir: g(parallel) = 2.030, g(perpendicular) = 2.060; trans-Ir: g(parallel) = 1.980, g(perpendicular) = 2.150). The g matrices of [M(tropp(ph))2] (M = Co, Rh) are axially symmetric with g(parallel) > g(perpendicular), indicating either a distorted square planar structure (SOMO essentially d(x2 - y2) or a compressed tetrahedron (SOMO essentially d(xy)). Interestingly, for [Ir(tropp(ph))2] the inverse ordering, g(perpendicular) > g(parallel) is found; this cannot be explained by simple ligand field arguments and must await a more sophisticated analysis. The hyperfine interactions of the unpaired electron with the metal nuclei, phosphorus nuclei, protons, deuterons and carbon nuclei were determined. By comparison with atomic constants, the spin densities on these centres were estimated and found to be small. However, the good agreement of the distance between the olefinic protons and the metal centres determined from the dipolar coupling parameter indicates that the unpaired electron is primarily located at the metal centre.

Phosphorus lone pairs stabilization of carbocations: the synthesis and dynamics of unsymmetrical methylene phosphonium ions

Halide abstractions from P-chlorinated phosphorus ylides 9a–c either by AlCl3 or SnCl2 yield unsymmetrical methylene phosphonium ions 10a–c in which energy barriers > 83 kJ mol–1 for the rotation around the PC have been estimated by NMR techniques. The salts with AlCl4– counteranions are stable, but compounds with SnCl3– anions decompose stereoselectively to unsymmetrical methylene phosphanes, SnCl2, and ButCl.

Dibenzotropylidene phosphanes (TROPPs): synthesis and coinage metal complexes

Dibenzocycloheptatrienyl phosphanes (dibenzotropylidene phosphanes=TROPPR) 11a–c may be easily prepared from dibenzocycloheptatrienyl chloride 8 and the secondary phosphanes R2PH [9a: R=Ph; 9b: R=4-Me-C6H4; 9c: R=cyclohexyl (Cyc)] in good yields. Alternatively, the di(tert-butyl)phosphanyl substituted TROPPBut derivative 4 was obtained along with the phosphane 5 by a mechanistically still unknown rearrangement of a strained phosphorus ylide I. The conformations of these new phosphanes were established by X-ray analyses performed for the compounds TROPPBut4 and TROPPPh11a. The R2P moiety is bonded to an axial position of the central seven-membered ring, which adopts a boat conformation. Thereby a rigid concave binding site containing a phosphane and an olefin function is formed, which should allow the synthesis of a wide range of transition metal complexes. In order to test how far the particular shape of the TROPP-type ligands enforces metal–olefin interactions, the coinage metal complexes [(TROPPPh)Cu(µ2-Cl)]2, 13, [(TROPPPh)Ag(µ2-O2SOCF3)]2, 16, [(TROPPPh)2Ag][O3SCF3], 17 and [(TROPPPh)AuCl], 19 were prepared. These were completely characterized by multinuclear NMR experiments in solution and the solid state, as well as by X-ray analyses. The structural and NMR data show that the interaction between the metal center M and the olefin moiety of the TROPP ligand is weak and decreases in the order Cu>Ag>Au. Indeed, for 19 (M=Au) no interaction could be observed. In the silver complex 17, coupling between an Ag nucleus and a proton of a bonded olefin was determined for the first time [J(109/107AgH)=0.4 Hz]. In solution the complexes derived from TROPP-type ligands seem to have an enhanced (kinetic) stability.

A Monomeric d9-Rhodium(0) Complex

A pocket suitable for bonding transition metals is formed by the 5-phosphanyl group and the olefinic unit of the central seven-membered ring, which has a rigid boat conformation, of the ligand troppPh (1). This new ligand system allows the synthesis and isolation of stable d9 and d10 rhodium complexes 2 and 3, respectively.

Protonation and Hydrogenation Experiments with Iridium(0) and Iridium(−1) tropp Complexes: Formation of Hydrides

The reactions of three different tetracoordinated Ir complexes, [Ir(troppph)2]n (n=+1, 0, −1), which differ in the formal oxidation state of the metal from +1 to −1, with proton sources and dihydrogen were investigated (tropp=5-(diphenylphosphanyl)dibenzo[a,d]cycloheptene). It was found that the cationic 16-electron complex [Ir(troppph)2]+ (2) cannot be protonated but reacts with NaBH4 to the very stable 18-electron IrI hydride [IrH(troppph)2] (5), which is further protonated with medium strong acids to give the 18-electron IrIII dihydride [IrH2(troppph)2]+ (6; pKs in CH2Cl2/THF/H2O 1 : 1 : 2 ca. 2.2). Both, the neutral 17-electron Ir0 complex [Ir(troppph)2] (3) and the anionic 18-electron complex [Ir(troppph)2]− (4) react rapidly with H2O to give the monohydride 5. In reactions of 3 with H2O, the terminal IrI hydroxide [Ir(OH)(troppph)2] (8) is formed in equal amounts. All these complexes, apart from 5, which is inert, do react rapidly with dihydrogen. The complex 2 gives the dihydride 6 in an oxidative addition reaction, while 3, 4, and 8 give the monohydride 5. Interestingly, a salt-type hydride (i.e., LiH) is formed as further product in the unexpected reaction with [Li(thf)x]+[Ir(troppph)2]− (4). Because 3 undergoes disproportionation into 2 and 4 according to 2 3⇄2+4 (Kdisp=2.7⋅10−5), it is likely that actually the diamagnetic species and not the odd-electron complex 3 is involved in the reactions studied here, and possible mechanisms for these are discussed.

EIN MONOMERER D9-RHODIUM(0)-KOMPLEX

In einer Tasche gebunden, die aus der 5-Phosphanylgruppe und der Olefineinheit des zentralen, in starrer Wannenkonformation vorliegenden siebengliedrigen Ringes gebildet wird, ist das Rh-Zentrum in den mit dem troppPh-Liganden 1 einfach zuganglichen d9- und d10-Rh-Komplexen 2 bzw. 3.

Strain in organometallics: synthesis of rhodium and iridium complexes with a novel rigid tetrachelating phosphine olefin ligand and their redox properties

Abstract The new tetrachelating diphosphanes 1,2-[(5 H -dibenzo[a,d]cyclohepten-5-yl)phenylphosphano]ethane, R , S - 11 [ R , S -bis(tropp Ph )], and the corresponding enantiomers R , R -bis(tropp Ph ), R , R - 12 , and S , S -bis(tropp Ph ), S , S - 12 , were synthesised from 1,2-bis(phenylphosphano)ethane ( 9 ), and 5-chloro-5 H -dibenzo[a,d]cycloheptene ( 10 ). With the meso form R , S - 11 , the rhodium(I) complex [Rh{ R , S -bis(tropp Ph )}]PF 6 ( 15 ), and the iridium(I) complex [Ir(cod){ R , S -bis(tropp Ph )}]O 3 SCF 3 ( 16 ), were prepared. The cation of the 16-electron rhodium complex 15 has a square planar structure, which is markedly distorted towards a square pyramid with the rhodium centre in the apex. The structure of the cation of the 18-electron complex of 16 is trigonal bipyramidal and unprecedented a phosphorus and an olefin unit occupy the axial positions. Cyclic voltammetry data of 15 indicate, that considerable strain energy (ca. 30 kJ mol −1 ) is build up when the [Rh{ R , S -bis(tropp Ph )}] + cation is reversibly reduced in THF by one electron to the paramagnetic neutral complex [Rh{ R , S -bis(tropp Ph )}] 0 .

Coordination chemistry of phosphanyl amino acids: solid state and solution structures of neutral and cationic rhodium complexes

Copper phosphide or arsenide complexes, [Cu(EPh(2))(neo)] (E = P, As, neo = 2,9-dimethyl-1,10-phenanthroline; trivial name: neocuprine) react selectively with the N-protected brominated serine derivatives, 2-(S)-(alkoxycarbonylamino)-3-bromomethylpropionates ((ROCO)SerBr, : R = PhCH(2), : tBu, : Me) to give the corresponding phosphanylated or arsanylated amino acids, (ROCO)SerPhos (: Phos = PPh(2)) and (Z)SerArs (Ars = AsPh(2), Z = PhCH(2)OCO). The dipeptide (Z)AlaSerPhos was likewise prepared. The phosphanes , and the arsane reacted cleanly with [Rh(2)(micro-Cl)(2)(cod)(2)] to give the rhodium(I) complexes [RhCl(cod)((Z)SerPhos)] , [RhCl(cod)((Boc)SerPhos)] (Boc = tBuOCO), [RhCl(cod)((Z)AlaSerPhos)] , and [RhCl(cod)((Z)SerArs)] which were characterized by X-ray diffraction studies. A common structural feature is an intramolecular (N)H[dot dot dot]Cl(Rh)-hydrogen bridge which according to NMR investigations remains intact in solution. The abstraction of chloride from the coordination sphere of Rh(I) in or has a profound structural impact. While in and , the ligands bind in a monodentate fashion, via the phosphorus atom only, they serve as bidentate ligands via the phosphorus centre and the peptidic C=O group in [Rh(cod)(kappa(2)-(Z)SerPhos)]PF(6) and [Rh(cod)(kappa(2)-(Z)AlaSerPhos)]PF(6). This causes also the amino acid residue structures to change from alpha-helix type in and to a beta-sheet type in both. Addition of chloride to and fully re-establishes the structures of both. The complexes [RhCl(cod)((Z)SerPhos)] and [RhCl(cod)((Boc)SerPhos)] show good activities in homogeneously catalyzed hydrogenations of olefins while the dipeptide complex is less active. Phosphane addition to greatly diminishes the catalytic activity. The cationic complex [Rh(cod)(kappa(2)-(Z)AlaSerPhos)]PF(6) shows low activity which, however, is greatly increased by addition of one equivalent of phosphane.

Ligands Make the Catalyst: Synthesis of Novel Functionalized Phosphines

Many catalytic active transition metal complexes contain phosphines as ligands which control decisively the activity and selectivity of the catalyst. Therefore synthetic methods which allow the preparation of highly functionalized phosphines are needed to match the increasing demands for higher activity and selectivity in catalytic processes. In this report we introduce a simple general new method for the synthesis of phosphines, arsines, and stibines. This allowed for example the preparations of phosphines based on serine and thymine derivatives (SerPhos and ThymPhos, respectively).