The catabolism of lignin-derived p-methoxylated aromatic compounds by Rhodococcus jostii RHA1.

Emergent strategies to valorize lignin, an abundant but underutilized aromatic biopolymer, include tandem processes that integrate chemical depolymerization and biological catalysis. To date, aromatic monomers from C-O bond cleavage of lignin have been converted to bioproducts, but the presence of recalcitrant C-C bonds in lignin limits the product yield. A promising chemocatalytic strategy that overcomes this limitation involves phenol methyl protection and autoxidation. Incorporating this into a tandem process requires microbial cell factories able to transform the p-methoxylated products in the resulting methylated lignin stream. In this study, we assessed the ability of Rhodococcus jostii RHA1 to catabolize the major aromatic products in a methylated lignin stream and elucidated the pathways responsible for this catabolism. RHA1 grew on a methylated pine lignin stream, catabolizing the major aromatic monomers: p-methoxybenzoate (p-MBA), veratrate, and veratraldehyde. Bioinformatic analyses suggested that a cytochrome P450, PbdA, and its cognate reductase, PbdB, are involved in p-MBA catabolism. Gene deletion studies established that both pbdA and pbdB are essential for growth on p-MBA and several derivatives. Furthermore, a deletion mutant of a candidate p-hydroxybenzoate (p-HBA) hydroxylase, ΔpobA, did not grow on p-HBA. Veratraldehyde and veratrate catabolism required both vanillin dehydrogenase (Vdh) and vanillate O-demethylase (VanAB), revealing previously unknown roles of these enzymes. Finally, a ΔpcaL strain grew on neither p-MBA nor veratrate, indicating they are catabolized through the ß-ketoadipate pathway. This study expands our understanding of the bacterial catabolism of aromatic compounds and facilitates the development of biocatalysts for lignin valorization.IMPORTANCELignin, an abundant aromatic polymer found in plant biomass, is a promising renewable replacement for fossil fuels as a feedstock for the chemical industry. Strategies for upgrading lignin include processes that couple the catalytic fractionation of biomass and biocatalytic transformation of the resulting aromatic compounds with a microbial cell factory. Engineering microbial cell factories for this biocatalysis requires characterization of bacterial pathways involved in catabolizing lignin-derived aromatic compounds. This study identifies new pathways for lignin-derived aromatic degradation in Rhodococcus, a genus of bacteria well suited for biocatalysis. Additionally, we describe previously unknown activities of characterized enzymes on lignin-derived compounds, expanding their utility. This work advances the development of strategies to replace fossil fuel-based feedstocks with sustainable alternatives.

Catalytic carbon-carbon bond cleavage in lignin via manganese-zirconium-mediated autoxidation.

Efforts to produce aromatic monomers through catalytic lignin depolymerization have historically focused on aryl-ether bond cleavage. A large fraction of aromatic monomers in lignin, however, are linked by various carbon-carbon (C-C) bonds that are more challenging to cleave and limit the yields of aromatic monomers from lignin depolymerization. Here, we report a catalytic autoxidation method to cleave C-C bonds in lignin-derived dimers and oligomers from pine and poplar. The method uses manganese and zirconium salts as catalysts in acetic acid and produces aromatic carboxylic acids as primary products. The mixtures of the oxygenated monomers are efficiently converted to cis,cis-muconic acid in an engineered strain of Pseudomonas putida KT2440 that conducts aromatic O-demethylation reactions at the 4-position. This work demonstrates that autoxidation of lignin with Mn and Zr offers a catalytic strategy to increase the yield of valuable aromatic monomers from lignin.

Autoxidation Catalysis for Carbon-Carbon Bond Cleavage in Lignin.

Selective lignin depolymerization is a key step in lignin valorization to value-added products, and there are multiple catalytic methods to cleave labile aryl-ether bonds in lignin. However, the overall aromatic monomer yield is inherently limited by refractory carbon-carbon linkages, which are abundant in lignin and remain intact during most selective lignin deconstruction processes. In this work, we demonstrate that a Co/Mn/Br-based catalytic autoxidation method promotes carbon-carbon bond cleavage in acetylated lignin oligomers produced from reductive catalytic fractionation. The oxidation products include acetyl vanillic acid and acetyl vanillin, which are ideal substrates for bioconversion. Using an engineered strain of Pseudomonas putida, we demonstrate the conversion of these aromatic monomers to cis,cis-muconic acid. Overall, this study demonstrates that autoxidation enables higher yields of bioavailable aromatic monomers, exceeding the limits set by ether-bond cleavage alone.

Design Principles for the Development of Gd(III) Polarizing Agents for Magic Angle Spinning Dynamic Nuclear Polarization.

Nuclear magnetic resonance suffers from an intrinsically low sensitivity, which can be overcome by dynamic nuclear polarization (DNP). Gd(III) complexes are attractive exogenous polarizing agents for magic angle spinning (MAS) DNP due to their high chemical stability in contrast to nitroxide-based radicals. However, even the state-of-the-art Gd(III) complexes have so far provided relatively low DNP signal enhancements of ca. 36 in comparison to standard DNP biradicals, which show enhancements of over 200. Here, we report a series of new Gd(III) complexes for DNP and show that the observed DNP enhancements of the new and existing Gd(III) complexes are inversely proportional to the square of the zero-field splitting (ZFS) parameter D, which is in turn determined by the ligand-type and the local coordination environment. The experimental DNP enhancements at 9.4 T and the ZFS parameters measured with pulsed electron paramagnetic resonance (EPR) spectroscopy agree with the above model, paving the way for the development of more efficient Gd(III) polarizing agents.

Single metal four-electron reduction by U(ii) and masked "U(ii)" compounds.

The redox chemistry of uranium is dominated by single electron transfer reactions while single metal four-electron transfers remain unknown in f-element chemistry. Here we show that the oxo bridged diuranium(iii) complex [K(2.2.2-cryptand)]2[{((Me3Si)2N)3U}2(µ-O)], 1, effects the two-electron reduction of diphenylacetylene and the four-electron reduction of azobenzene through a masked U(ii) intermediate affording a stable metallacyclopropene complex of uranium(iv), [K(2.2.2-cryptand)][U(η 2-C2Ph2){N(SiMe3)2}3], 3, and a bis(imido)uranium(vi) complex [K(2.2.2-cryptand)][U(NPh)2{N(SiMe3)2}3], 4, respectively. The same reactivity is observed for the previously reported U(ii) complex [K(2.2.2-cryptand)][U{N(SiMe3)2}3], 2. Computational studies indicate that the four-electron reduction of azobenzene occurs at a single U(ii) centre via two consecutive two-electron transfers and involves the formation of a U(iv) hydrazide intermediate. The isolation of the cis-hydrazide intermediate [K(2.2.2-cryptand)][U(N2Ph2){N(SiMe3)2}3], 5, corroborated the mechanism proposed for the formation of the U(vi) bis(imido) complex. The reduction of azobenzene by U(ii) provided the first example of a "clear-cut" single metal four-electron transfer in f-element chemistry.

Delivery of a Masked Uranium(II) by an Oxide-Bridged Diuranium(III) Complex.

Oxide is an attractive linker for building polymetallic complexes that provide molecular models for metal oxide activity, but studies of these systems are limited to metals in high oxidation states. Herein, we synthesized and characterized the molecular and electronic structure of diuranium bridged UIII /UIV and UIII /UIII complexes. Reactivity studies of these complexes revealed that the U-O bond is easily broken upon addition of N-heterocycles resulting in the delivery of a formal equivalent of UIII and UII , respectively, along with the uranium(IV) terminal-oxo coproduct. In particular, the UIII /UIII oxide complex effects the reductive coupling of pyridine and two-electron reduction of 4,4'-bipyridine affording unique examples of diuranium(III) complexes bridged by N-heterocyclic redox-active ligands. These results provide insight into the chemistry of low oxidation state metal oxides and demonstrate the use of oxo-bridged UIII /UIII complexes as a strategy to explore UII reactivity.

Synthesis and crystallographic characterization of di-phenyl-amide rare-earth metal complexes Ln(NPh2)3(THF)2 and [(Ph2N)2 Ln(µ-NPh2)]2.

Studies of the coordination chemistry between the di-phenyl-amide ligand, NPh2, and the smaller rare-earth Ln III ions, Ln = Y, Dy, and Er, led to the structural characterization by single-crystal X-ray diffraction crystallography of both solvated and unsolvated complexes, namely, tris-(di-phenyl-amido-κN)bis-(tetrahydro-furan-κO)yttrium(III), Y(NPh2)3(THF)2 or [Y(C12H10N)3(C4H8O)2], 1-Y, and the erbium(III) (Er), 1-Er, analogue, and bis-[µ-1κN:2(η6)-di-phenyl-amido]-bis-[bis-(di-phenyl-amido-κN)yttrium(III)], [(Ph2N)2Y(µ-NPh2)]2 or [Y2(C12H10N)6], 2-Y, and the dysprosium(III) (Dy), 2-Dy, analogue. The THF ligands of 1-Er are modeled with disorder across two positions with occupancies of 0.627 (12):0.323 (12) and 0.633 (7):0.367 (7). Also structurally characterized was the tetra-metallic ErIII bridging oxide hydrolysis product, bis-(µ-di-phenyl-amido-κ2 N:N)bis-[µ-1κN:2(η6)-di-phenyl-amido]-tetra-kis-(di-phenyl-amido-κN)di-µ3-oxido-tetra-erbium(III) benzene disolvate, {[(Ph2N)Er(µ-NPh2)]4(µ-O)2}·(C6H6)2 or [Er4(C12H10N)8O2]·2C6H6, 3-Er. The 3-Er structure was refined as a three-component twin with occupancies 0.7375:0.2010:0.0615.

Accessing the +IV Oxidation State in Molecular Complexes of Praseodymium.

Out of the 14 lanthanide (Ln) ions, molecular complexes of Ln(IV) were known only for cerium and more recently terbium. Here we demonstrate that the +IV oxidation state is also accessible for the large praseodymium (Pr) cation. The oxidation of the tetrakis(triphenysiloxide) Pr(III) ate complex, [KPr(OSiPh3)4(THF)3], 1-PrPh, with [N(C6H4Br)3][SbCl6], affords the Pr(IV) complex [Pr(OSiPh3)4(MeCN)2], 2-PrPh, which is stable once isolated. The solid state structure, UV-visible spectroscopy, magnetometry, and cyclic voltammetry data along with the DFT computations of the 2-PrPh complex unambiguously confirm the presence of Pr(IV).

C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride.

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe3)2 ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H2. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)3][((Me3Si)2N)2U(µ-N)(µ-κ2:CN̵-CH2SiMe2NSiMe3)U(N(SiMe3)2)2] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt3BPh3 yielded the cationic U(IV)/U(IV) nitride complex, [{((Me3Si)2N)2U(THF)}2(µ-N)][BPh4] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe3)2 ligand and the uranium-methylene bond. Reduction of 3 with KC8 afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me3Si)2N)2U(THF)}2(µ-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe3)2 ligand across the uranium-nitride moiety to give the U(III)/U(IV) imide cyclometalate complex, [((Me3Si)2N)2(THF)U(µ-NH)(µ-κ2:C,N̵-CH2SiMe2NSiMe3)U(N(SiMe3)2))(THF)], 5. Complex 4 also reacts with toluene at -80 °C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me3Si)2N)2U(THF)}2(µ-N)][{((Me3Si)2N)3U(µ-NH)U(N(SiMe3)2)}2(C7H8)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me3Si)2N)2U(THF)}2(µ-N)][((Me3Si)2N)2U(η1-CCPh)(µ2-NH)(µ2-η2:η1-CCPh)U(N(SiMe3)2)2], 7. Complex 4 also reacts with H2 to produce an imide hydride U(III)/U(IV) complex, [{((Me3Si)2N)2U(THF)}2(µ-NH)(µ-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

Stabilization of the Oxidation State +IV in Siloxide-Supported Terbium Compounds.

The synthesis of lanthanides other than cerium in the oxidation state +IV has remained a desirable but unmet target until recently, when two examples of TbIV with saturated coordination spheres were isolated. Here we report the third example of an isolated molecular complex of terbium(IV), where the supporting siloxide ligands do not saturate the coordination sphere. The fully characterized six-coordinate complex [TbIV (OSiPh3 )4 (MeCN)2 ], 2-TbPh , shows high stability and the labile MeCN ligands can be replaced by phosphinoxide ligands. Computational studies suggest that the stability is due to a strong π(O-Tb) interaction which is stronger than in the previously reported TbIV complexes. Cyclic-voltammetry experiments demonstrate that non-binding counterions contribute to the stability of TbIV in solution by destabilizing the +III oxidation state, while alkali ions promote TbIV /TbIII electron transfer.

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands.

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U[double bond, length as m-dash]N[double bond, length as m-dash]U bonding imposed by the supporting ligand. The U4+-U4+ nitride complexes, [NBu4][{(( t BuO)3SiO)3U}2(µ-N)], [NBu4]-1, and [NBu4][((Me3Si)2N)3U}2(µ-N)], 2, were synthesised by reacting NBu4N3 with the U3+ complexes, [U(OSi(O t Bu)3)2(µ-OSi(O t Bu)3)]2 and [U(N(SiMe3)2)3], respectively. Oxidation of 2 with AgBPh4 gave the U4+-U5+ analogue, [((Me3Si)2N)3U}2(µ-N)], 4. The previously reported methylene-bridged U4+-U4+ nitride [Na(dme)3][((Me3Si)2)2U(µ-N)(µ-κ2-C,N-CH2SiMe2NSiMe3)U(N(SiMe3)2)2] (dme = 1,2-dimethoxyethane), [Na(dme)3]-3, provided a versatile precursor for the synthesis of the mixed-ligand U4+-U4+ nitride complex, [Na(dme)3][((Me3Si)2N)3U(µ-N)U(N(SiMe3)2)(OSi(O t Bu)3)], 5. The reactivity of the 1-5 complexes was assessed with CO2, CO, and H2. Complex [NBu4]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{(( t BuO)3SiO)3U}2(µ-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO2 but not H2. The nitride complexes [NBu4]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu4]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(µ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

Molecular Complex of Tb in the +4 Oxidation State.

Lanthanides (Ln) usually occur in the +3, or more recently the +2, oxidation states. The only example of an isolated molecular Ln4+ so far remains Ce4+. Here we show that the +4 oxidation state is also accessible in a molecular compound of terbium as demonstrated by oxidation of the tetrakis(siloxide)terbium(III) ate complex, [KTb(OSi(O tBu)3)4], 1-Tb, with the tris(4-bromophenyl)amminium oxidant, [N(C6H4Br)3][SbCl6], to afford the Tb4+ complex [Tb(OSi(O tBu)3)4], 2-Tb. The solid state structures of 1-Tb and 2-Tb were determined by X-ray crystallography, and the presence of Tb4+ was unambiguously confirmed by electron paramagnetic resonance and magnetometry. Complex 2-Tb displays a similar voltammogram to the Ce4+ analogue but with redox events that are about 1 V more positive.

Using Diamagnetic Yttrium and Lanthanum Complexes to Explore Ligand Reduction and C-H Bond Activation in a Tris(aryloxide)mesitylene Ligand System.

[Y(N(SiMe3)2)3] reacts with (Ad,MeArOH)3mes to form the Y3+ complex [((Ad,MeArO)3mes)Y], 1-Y. This complex reacts with potassium metal in the presence of 2.2.2-cryptand to give a cocrystallized mixture of [K(2.2.2-cryptand)][((Ad,MeArO)3mes)Y], 2-Y, and [K(2.2.2-cryptand)][((Ad,MeArO)3mes)YH], 3-Y. The electron paramagnetic resonance spectrum of this crystalline mixture exhibits an isotropic signal at 77 K ( giso = 2.000, Wiso = 1.8 mT), suggesting that 2-Y is best described as a Y3+ complex of the tris(aryloxide)mesitylene radical ((Ad,MeArO)3mes)4-. Evidence of the hydride ligand in 3-Y was obtained by 89Y-1H heteronuclear multiple quantum coherence NMR spectroscopy, and a coupling constant of JYH = 93 Hz was observed. A single crystal of 3-Y was also obtained in pure form and structurally characterized for comparison with the crystal data on the mixed component 2-Ln/3-Ln crystals. The origin of the hydride in 3-Ln is unknown, but further studies of the reduction of 1-La, previously found to form 2-La, revealed a possible source. Ligand-based C-H bond activation and loss of hydrogen can occur under reducing conditions to form a tetraanionic ligand derived from ((Ad,MeArO)3mes)3-, as observed in [K(2.2.2-cryptand)][((Ad,MeArO)3(C6Me3(CH2)2CH)La], 4-La.

Metal versus Ligand Reduction in Ln3+ Complexes of a Mesitylene-Anchored Tris(Aryloxide) Ligand.

The synthesis of 4f n Ln3+ complexes of the tris(aryloxide) mesitylene ligand, ((Ad,MeArO)3mes)3-, with Ln = La, Ce, Pr, Sm, and Yb, and their reduction with potassium have revealed that this ligand system can be redox active with some metals. Protonolysis of [Ln(N(SiMe3)2)3] (Ln = La, Ce, Pr, Sm, Yb) with the tris(phenol) (Ad,MeArOH)3mes yielded the Ln3+ complexes [((Ad,MeArO)3mes)Ln] (Ln = La, Ce, Pr, Sm, Yb), 1-Ln. Single electron reduction of each 4f n complex, 1-Ln, using potassium yielded the reduced products, [K(2.2.2-cryptand)][((Ad,MeArO)3mes)Ln] (Ln = La, Ce, Pr, Sm, Yb), 2-Ln. The Sm and Yb complexes have properties consistent with the presence of Ln2+ ions with traditional 4f n+1 electron configurations. However, the La, Ce, and Pr complexes appear to formally contain Ln3+ ions and ((Ad,MeArO)3mes)4- ligands. Structural comparisons of the [((Ad,MeArO)3mes)Ln] and [((Ad,MeOAr)3mes)Ln]1- complexes along with UV-vis absorption and EPR spectroscopy as well as density functional theory calculations support these ground state assignments.

Electrocatalytic H2O Reduction with f-Elements: Mechanistic Insight and Overpotential Tuning in a Series of Lanthanide Complexes.

Electrocatalytic energy conversion with molecular f-element catalysts is still in an early phase of its development. We here report detailed electrochemical investigations on the recently reported trivalent lanthanide coordination complexes [((Ad,MeArO)3mes)Ln] (1-Ln), with Ln = La, Ce, Pr, Nd, Sm, Gd, Dy, Er, and Yb, which were now found to perform as active electrocatalysts for the reduction of water to dihydrogen. Reactivity studies involving complexes 1-Ln and the Ln(II) analogues [K(2.2.2-crypt)][((Ad,MeArO)3mes)Ln] (2-Ln) suggest a reaction mechanism that differs significantly from the reaction pathway found for the corresponding uranium catalyst [((Ad,MeArO)3mes)U] (1-U). While complexes 1-Ln activate water via a radical pathway, only upon a 1 e- reduction to yield the reduced species 2-Ln, the 5f analogue 1-U directly reduces H2O via a 2 e- pathway. The electrocatalytic H2O reduction by complexes 1-Ln is initiated by the respective Ln(III)/Ln(II) redox couples, which gradually turn to more positive values across the Ln series. This correlation has been exploited to tune the catalytic overpotential of water reduction by choice of the lanthanide ion. Kinetic studies of the 1-Ln series were performed to elucidate correlations between overpotential and turnover frequencies of the 4f-based electrocatalysts.

Comparisons of lanthanide/actinide +2 ions in a tris(aryloxide)arene coordination environment.

A new series of Ln3+ and Ln2+ complexes has been synthesized using the tris(aryloxide)arene ligand system, ((Ad,MeArO)3mes)3-, recently used to isolate a complex of U2+. The triphenol precursor, (Ad,MeArOH)3mes, reacts with the Ln3+ amides, Ln(NR2)3 (R = SiMe3), to form a series of [((Ad,MeArO)3mes)Ln] complexes, 1-Ln. Crystallographic characterization was achieved for Ln = Nd, Gd, Dy, and Er. The complexes 1-Ln can be reduced with potassium graphite in the presence of 2.2.2-cryptand (crypt) to form highly absorbing solutions with properties consistent with Ln2+ complexes, [K(crypt)][((Ad,MeArO)3mes)Ln], 2-Ln. The synthesis of the Nd2+ complex [K(crypt)][((Ad,MeArO)3mes)Nd], 2-Nd, was unambiguously confirmed by X-ray crystallography. In the case of the other lanthanides, crystals were found to contain mixtures of 2-Ln co-crystallized with either a Ln3+ hydride complex, [K(crypt)][((Ad,MeArO)3mes)LnH], 3-Ln, for Ln = Gd, Dy, and Er, or a hydroxide complex, [K(crypt)][((Ad,MeArO)3mes)Ln(OH)], 4-Ln, for Ln = Dy. A Dy2+ complex with 18-crown-6 as the potassium chelator, [K(18-crown-6)(THF)2][((Ad,MeArO)3mes)Dy], 5-Dy, was isolated as a co-crystallized mixture with the Dy3+ hydride complex, [K(18-crown-6)(THF)2][((Ad,MeArO)3mes)DyH], 6-Dy. Structural comparisons of 1-Ln and 2-Ln are presented with respect to their uranium analogs and correlated with density functional theory calculations on their electronic structures.

Isolation of +2 rare earth metal ions with three anionic carbocyclic rings: bimetallic bis(cyclopentadienyl) reduced arene complexes of La2+ and Ce2+ are four electron reductants.

A new option for stabilizing unusual Ln2+ ions has been identified in the reaction of Cp'3Ln, 1-Ln (Ln = La, Ce; Cp' = C5H4SiMe3), with potassium graphite (KC8) in benzene in the presence of 2.2.2-cryptand. This generates [K(2.2.2-cryptand)]2[(Cp'2Ln)2(µ-η6:η6-C6H6)], 2-Ln, complexes that contain La and Ce in the formal +2 oxidation state. These complexes expand the range of coordination environments known for these ions beyond the previously established examples, (Cp''3Ln)1- and (Cp'3Ln)1- (Cp'' = C5H3(SiMe3)2-1,3), and generalize the viability of using three anionic carbocyclic rings to stabilize highly reactive Ln2+ ions. In 2-Ln, a non-planar bridging (C6H6)2- ligand shared between two metals takes the place of a cyclopentadienyl ligand in (Cp'3Ln)1-. The intensely colored (ε = ∼8000 M-1 cm-1) 2-Ln complexes react as four electron reductants with two equiv. of naphthalene to produce two equiv. of the reduced naphthalenide complex, [K(2.2.2-cryptand)][Cp'2Ln(η4-C10H8)].