Inorganic chemistry:A reducing role for boron

Carbon monoxide molecules are typically coupled together using metal catalysts. The discovery that boron, a non-metal, mediates such a reaction is startling, and raises the prospect of potentially useful carbon–carbon bond-forming processes

Microbial carbon use efficiency: accounting for population, community, and ecosystem-scale controls over the fate of metabolized organic matter

Microbial carbon use efficiency (CUE) is a critical regulator of soil organic matter dynamics and terrestrial carbon fluxes, with strong implications for soil biogeochemistry models. While ecologists increasingly appreciate the importance of CUE, its core concepts remain ambiguous: terminology is inconsistent and confusing, methods capture variable temporal and spatial scales, and the significance of many fundamental drivers remains inconclusive. Here we outline the processes underlying microbial efficiency and propose a conceptual framework that structures the definition of CUE according to increasingly broad temporal and spatial drivers where (1) CUEP reflects population-scale carbon use efficiency of microbes governed by species-specific metabolic and thermodynamic constraints, (2) CUEC defines community-scale microbial efficiency as gross biomass production per unit substrate taken up over short time scales, largely excluding recycling of microbial necromass and exudates, and (3) CUEE reflects the ecosystem-scale efficiency of net microbial biomass production (growth) per unit substrate taken up as iterative breakdown and recycling of microbial products occurs. CUEE integrates all internal and extracellular constraints on CUE and hence embodies an ecosystem perspective that fully captures all drivers of microbial biomass synthesis and decay. These three definitions are distinct yet complementary, capturing the capacity for carbon storage in microbial biomass across different ecological scales. By unifying the existing concepts and terminology underlying microbial efficiency, our framework enhances data interpretation and theoretical advances

Structural and reductive chemistry of low-valent lanthanide complexes featuring modified porphyrinogens

This thesis describes studies into the synthesis, characterisation and reactivity of samarium(II) and samarium(III), europium(II) and ytterbium(II) complexes derived from the modified porphyrinogens \trans-N,N'\-dimethyl-meso-octaethylporphyrinogen, Et\_8\N\_4\Me\_2\H\_2\, and \trans\-calix[2]benzene[2]pyrrole, Me\_8\N\_2\Ph\_2\H\_2\. Chapter 2 is concerned with the synthesis of a new modified porphyrinogen \N,N'\-dimethyl-\meso\-octamethylporphyrinogen, Me\_8\N\_4\Me\_2\H\_2\, via a convergent \3+1\" procedure from the condensation of 1-methy1-25-bis(11'-dimethylhydroxymethyl)pyrrole with 1-methy1-25-bis {(2'-pyrroly)dimethylmethyl}pyrrole in acetonitrile in the presence of scandium trifluoromethanesulfonate. The stepwise nature of the synthesis potentially allows independent functionalisation of various parts of the molecule. The unique electronic and steric properties of complexes derived from doubly deprotonated \NN'\-dimethyl-\meso\-octaethylporphyrinogen were exploited to force unusual reactivity and/or structural features in a range of lanthanide(II) and lanthanide(III) complexes. Chapter 3 details the synthesis of samarium(II) europium(II) and ytterbium(II) complexes of this macrocycle. Subsequent reaction with a range of 14-diazabuta-13-dienes (R-N=C(H)-C(H)=N-R R = \t\-Bu \i\-Pr and \n\-Bu) gave complexes featuring 14-diazabuta-13-diene binding to the lanthanide centres as neutral Lewis base donors chelating radical anions and bridging reduced dianions. Steric limitations were found to alter these structural outcomes and the complexes were characterised by X-ray crystal structure determination and NMR spectroscopy. Steric factors were also implicated in the observation of an unusual solvent mediated Sm(II)/Sm(III) reversibility in which a Sm(III) centre reverted to Sm(II) upon addition of coordinating solvent (R = \t\-Bu). Steric competition in the \NN'\-dimethyl-\meso\-octaethylporphyrinogen system was further examined in Chapter 4 by synthesis of a highly strained cyclopentadienyl Sm(III) complex [(Et\_8\N\_4\Me\_2\)Sm(C\_5\H\_5\)] featuring a major conformational deformation in the macrocycle. Also synthesised was a centrosymmetric bimetallic cyclooctatetraenediyl bound Sm(III) complex [{(Et\_8\N\_4\Me\_2\)Sm}\_2\(¬¨¬µ:‚àÜvª\^2\:‚àÜvª\^2\-COT)] in which the cyclooctatetraenediyl dianion is forced to adopt an ‚àÜvª\^2\ -binding mode to each Sm centre. Solid state molecular structures of these strained molecules were complemented by \^1\H \^{13}\C 2D and variable temperature NMR studies of the cyclopentadienyl complex to examine fluctional processes in solution. Chapter 5 describes ligand substitution reactions in which Sm(III) complexes of 14-diazabuta-13-dienes were reacted with reducible substrates. Samarium(III) complexes of \t\-butyl- 14-diaza-1 3-diene and \i\-propyl-14-diaza-13-diene were found to reduce benzil to give the binuclear complex [{(Et\_8\N\_4\Me\_2\)Sm}\_2\{¬¨¬µ-OC(Ph)C(Ph)O}] with the concomitant formation of the free 14-diazabuta-13-diene. Also investigated was the highly strained [(Et\_8\N\_4\Me\_2\)Sm(C\_5\H\_5\)] which was found to react with 14-benzoquinone to give the binuclear complex [{(Et\_8\N\_4\Me\_2\)Sm}\_2\{¬¨¬µ-O(C\_6\H\_4\)O}]. Chapter 6 describes the reductive chemistry of the Sm(II) complex [(Et\_8\N\_4\Me\_2\)Sm(THF)\_2\]. It was found to reduce CO\_2\ in a disproportionation reaction to give carbon monoxide and a bridging CO\_3\\^{2-}\ moiety. The resulting binuclear samarium(III) complex was characterised by X-ray crystal structure determination and NMR spectroscopy. The Sm(II) complex was also used in redox transmetallation reactions with mercury thallium and silver salts. The reaction with silver tetrafluoroborate gave a Sm(III) tetrafluoroborate intermediate which underwent subsequent salt metathesis reactions with sodium cyclopentadienide and lithium iodide to give the respective samarium(III) derivatives. Chapter 7 details the synthesis of \trans\-calix[2]benzene[2]pyrrole by condensation of pyrrole with 13-bis(1'1'- dimethylhydroxymethyl)benzene in acetonitrile. The literature procedure for the synthesis of this macrocycle was improved by the use of a catalytic amount of scandium trifluoromethanesulfonate in place of stoichiometric boron trifluoride as Lewis acid. As a counterpoint to the conformationally restricted \NN'\-dimethyl-\meso\-octaethylporphyrinogen the less rigid doubly deprotonated \trans\-calix[2]benzene[2]pyrrole was investigated as a ligand for lanthanide metals. The potassium salt was synthesised by deprotonation of the neutral porphyrinogen with potassium metal. The lanthanide chemistry was investigated by reaction of the dipotassium salt with SmI\_2\. The reaction was sensitive to conditions and resulted in mixtures from which a number of Sm(II) complexes featuring varying degrees of solvation and an unsolvated \"\N\-confused\" dimer were isolated. Molecular structures of the dipotassium salt mono- and bis-THF Sm(II) adducts and Sm(II) \N\-confused binuclear dimer were obtained. As derivatives a cationic Sm(III) cyclooctatetraenediyl complex and a potassium containing Sm(III) cyclooctatetraendiyl complex were obtained and characterised by X-ray crystal structure determination. Macrocyclic binding modes fell between the extremes of the samarium(II) mono-THF adduct (featuring a bis(‚àÜvª\^3\-arene) structural motif with only a slight metallocene bend angle ‚àÜvª\^5\ -bound pyrrolide rings) and the cyclooctatetraenediyl Sm(III) complexes in which the macrocycle splays back to allow the large planar COT full access to the Sm coordination sphere resulting in an ‚àÜvª\^8\ Sm-COT interaction and concomitant reduction in arene hapticity to a slipped ‚àÜvª\^1\-arrangement with pyrrolide rings ‚àÜvª\^1\ -bound through the nitrogen. The Sm(II) complexes were also characterised by \^1\H NMR and/or variable temperature \^1\H NMR spectroscopy.

Carbon dioxide activation by a uranium(III) complex derived from a chelating bis(aryloxide) ligand

The new dianionic ligand C6H4{p-C(CH3)2C6H2Me2O–}2 (=p-Me2bp), featuring two aryloxide donors and a central arene ring, has been synthesized and used to prepare the mixed-ligand U(III) compound [U(Cp*)(p-Me2bp)], which exhibits an η6 interaction with the uranium center. Reductive activation of CO2 was investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave a dinuclear uranium carbonate complex, {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO2 showed the formation of a rare U(IV) oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the last two products is temperature dependent: at low temperatures (−78 °C) oxalate formation is favored, while at room temperature the carbonate is the dominant product. The U(IV) iodide [U(Cp*)(p-Me2bp)I] was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIIIredox couple of −2.18 V vs FeCp2+/0 as well as a possible electrochemically accessible UIII/UIIreduction process at −2.56 V vs FeCp2+/0

Carbon dioxide activation by a uranium(III) complex derived from a chelating bis(aryloxide) ligand

The new dianionic ligand C6H4{p-C(CH3)2C6H2Me2O–}2 (=p-Me2bp), featuring two aryloxide donors and a central arene ring, has been synthesized and used to prepare the mixed-ligand U(III) compound [U(Cp*)(p-Me2bp)], which exhibits an η6 interaction with the uranium center. Reductive activation of CO2 was investigated using [U(Cp*)(p-Me2bp)] in supercritical CO2, which gave a dinuclear uranium carbonate complex, {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3), cleanly and selectively. Reactivity studies in conventional solvents using lower pressures of CO2 showed the formation of a rare U(IV) oxalate complex, {U(Cp*)(p-Me2bp)}2(μ-η2:η2-C2O2), alongside {U(Cp*)(p-Me2bp)}2(μ-η1:η2-CO3). The relative ratio of the last two products is temperature dependent: at low temperatures (−78 °C) oxalate formation is favored, while at room temperature the carbonate is the dominant product. The U(IV) iodide [U(Cp*)(p-Me2bp)I] was also synthesized and used as part of an electrochemical study, the results of which showed that [U(Cp*)(p-Me2bp)] has a UIV/UIIIredox couple of −2.18 V vs FeCp2+/0 as well as a possible electrochemically accessible UIII/UIIreduction process at −2.56 V vs FeCp2+/0.</p

Synthesis and reactivity of trans-N,N'-dimethyl-meso-octaalkylporphyrinogen Sm(II), Eu(II) and Yb(II) complexes: Metal-based influences on the reduction of t-butyl-1,4-diazabuta-1,3-diene

Sm(II), Eu(II) and Yb(II) complexes of doubly deprotonated trans-N,N¡ä-dimethyl-meso-octaethylporphyrinogen were synthesised as tetrahydrofuran adducts (Sm and Eu, bis; Yb, mono) by metathetical exchange reactions of the dipotassium macrocyclic precursor complexes with the corresponding metal diiodides in tetrahydrofuran. The Sm and Eu complexes partially desolvate in non-coordinating solvents to give mono-tetrahydrofuran adducts. Subsequent reactions of the initial Eu(II) and Yb(II) complexes with 1,4-di-t-butyl-1,4-diazabuta-1,3-diene failed to yield complexes featuring the 1,4-diazabuta-1,3-diene binding to the lanthanide centres either as neutral Lewis base donors or reduced ligands, which contrasts with previous findings in the case of the analogous Sm(II) reaction. These outcomes are discussed in relation to the variety of Ln(III)¨CLn(II) reduction potentials, coordination number and oxidation state dependent ionic radii of the metals and macrocycle¨Cancillary ligand steric interactions. The complexes were characterised by X-ray crystal structure determination, satisfactory microanalysis and NMR spectroscopy, where possible