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Structural Analysis and Separation of Lanthanides With Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202002653 & CoordinationChemistry The Earlier the Better:Structural Analysis and Separation of Lanthanides with Pyrroloquinoline Quinone HenningLumpe+,[a] Annika Menke+,[a] Christoph Haisch,[b] Peter Mayer,[a] Anke Kabelitz,[c] Kirill V. Yusenko,[c] Ana Guilherme Buzanich,[c] Theresa Block,[d] Rainer Pçttgen,[d] Franziska Emmerling,[c] and Lena J. Daumann*[a] Abstract: Lanthanides (Ln) are critical raw materials,howev- ronment.The complex crystallizes as an inversion symmetric er,their mining and purification have aconsiderable nega- dimer,Eu2PQQ2,with binding of Eu in the biologically rele- tive environmental impact andsustainable recycling and vant pocket of PQQ. LnPQQ and Ln1Ln2PQQ complexes separation strategies for these elements are needed. In this were characterized by using inductively coupled plasma study,the precipitationand solubility behavior of Ln com- mass spectrometry (ICP-MS), infrared (IR) spectroscopy, 151Eu- plexes with pyrroloquinoline quinone (PQQ), the cofactor of Mçssbauer spectroscopy,X-ray total scattering, andextend- recently discovered lanthanide (Ln) dependent methanol de- ed X-ray absorption fine structure (EXAFS). It is shown that a hydrogenase (MDH)enzymes, is presented. In this context, natural enzymatic cofactor is capable to achieveseparation the molecular structure of abiorelevant europium PQQ com- by precipitationofthe notoriously similar,and thusdifficult plex was for the first time elucidated outsideaprotein envi- to separate, lanthanides to some extent. Introduction also called “vitamins, or seeds of technology” and the global demand of rare earth oxides is growing steadily.[1] Unlike their Rare earth elements (REE) include the elements 21Sc, 39Yand name suggests, REE are not particularly rare and the occur- 57La, in addition to the 14 lanthanides (Ln) from 58Ce to 71Lu. rence of the two least abundant ones, Tm and Lu, is even Due to their extensive usage in modern technologies, they are higher than the one of silver.[2] Miningofthose elements is, however,achallenge, due to their dispersion and low concen- trations in REE containing ores. In addition, extraction methods [a] Dr.H.Lumpe,+ A. Menke,+ Dr.P.Mayer, Prof. Dr.L.J.Daumann DepartmentofChemistry include strong acids or bases and produce large scales of radi- Ludwig-Maximilians-University Munich oactiveand heavy metal contaminated waste.[3] Separations of Butenandtstraße5–13,81377 München (Germany) the chemically similarREE are energy-intensive and challeng- E-mail:[email protected] ing.[4] However,severalexciting new directionsfor REE separa- [b] Prof. Dr.C.Haisch Chair of Analytical Chemistry and Water Chemistry tion have been presented recently.The group of Schelter used 3 Technical UniversityofMunich the size-sensitive ligand TriNOx À (Figure 1B), which is able to Marchioninistraße 17,81377 München (Germany) form aself-associative equilibrium out of REE mixtures and can [c] Dr.A.Kabelitz, Dr.K.V.Yusenko,Dr. A. GuilhermeBuzanich, be used for REE separation by leaching.[5] Sun,Bünzli and co- Dr.F.Emmerling workers used asupramolecular approach with atris-tridentate Division Structure Analysis Federal Institute for Materials Research and Testing(BAM) ligand,which forms 4-nuclear cages preferentially with the [6] Richard-Willstätter-Straße11, 12489 Berlin (Germany) smaller,late REE (Figure 1A). [d] T. Block,Prof. Dr.R.Pçttgen With amodificationofthe ligand, using long alkyl chains, Institut fürAnorganischeund Analytische Chemie the group was able to perform aliquid-liquid extraction of late UniversitätMünster (WWU) REE, while the early ones remained in the aqueousphase.[8] Re- Corrensstraße 30,48149 Münster (Germany) cently,also magnetic field driven REE separations have been [+] These authors contributed equally to this work. reported.[9] Among the REE, the earlylanthanides (La–Eu)are Supporting information and the ORCID identification number(s) for the au- thor(s) of this article can be found under: now recognized as biorelevant for methylotrophic bacteria ha- https://doi.org/10.1002/chem.202002653. bituating anumber of different ecosystems (plant phyllo- 2020 The Authors. Published by Wiley-VCH Verlag GmbH&Co. KGaA. spheres,volcanic mudpots, soil andaquatic environments).[10] This is an open access article under the terms of Creative Commons Attri- Those bacteria use Ln containing enzymes(methanol dehydro- bution NonCommercialLicense, whichpermitsuse, distributionand repro- genases, MDH, activesite shown in Figure 2A)intheir C1 me- duction in any medium, provided the original work is properlycited and is not used for commercial purposes. tabolism. The active site of the Ln-containing enzymes includes Part of aSpecial Collection to commemorate young and emergingscien- redox cofactorPQQ (Figure 2B)that coordinates the central tists. To view the completecollection, visit:Young Chemists 2020. metal in atridentate fashion.[11] Chem. Eur.J.2020, 26,10133 –10139 10133 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Full Paper Chemistry—A European Journal doi.org/10.1002/chem.202002653 be exploited for Ln-separation. In addition, we report the first crystal structure of PQQwith abiorelevant metal ion (Eu) out- side of aMDH protein environment and withoutsynthetic co- coordinating ligandsormodified PQQ precursors. Results and Discussion Ln1PQQcomplexes:characterization In MDH, PQQ acts as atridentate ligand for lanthanides, using 7’-O;6-N and 5-O (see numbering in Figure 2B). Without the protein environment, all carboxylgroups, aswell as 4-O can participate in metal coordination, often complicating analy- sis.[17] From aqueous solutions of the sodium salt of the cofac- tor (Na2PQQ, 2,isolated from vitamincapsules) complexes with Figure 1. A) Tris-tridentate ligand and supramolecularLncomplexreported lanthanides rapidly precipitate after mixing, showing1:1 stoi- [6] [17a] by Sun, Bünzli et al. and B) H3TriNOx ligand and Ln TriNOx THF complex chiometry,even with lanthanides added in excess (6 equiv). from Schelter et al.[7] Single crystals from the first Eu2PQQ2 complex outside the MDH protein environment wereobtained for X-ray structural analysis(Figure 3). To this end, very few crystal structures of PQQ complexes have been reported[18] andnone of them con- tained metals relevant for MDH activity (Ca, Ln). Astructure of a1:1 complexwith aPQQ derivative(here the trimethylester [18a] PQQMe3 and copper(I) were used) was reported by Kaim. 2+ Kojimareported astructure of PQQMe3 with Ru bearing a terpyridine coligand. Astructure of PQQ (1)with Cu2+ and the same coligand was reported by Suzuki.[18b, c] Crystals of a Eu2PQQ2 complex were derived after several days from amix- ture of aqueous Na2PQQ and EuCl3 solutionsat808C, which Figure 2. A) Active site of aLn-dependent MDH (PDB 6FKW). B) Structureof was allowed to slowly cool down to room temperature (see PQQ and related species.Water adduct 3 forms readily in aqueoussolution. Numbering scheme according to Unkefer et al.[12] Supporting Information). The Eu-structure is consistentwith a 1:1stoichiometry but surprisingly reveals adimer with headto tail coordination of PQQ. While no other co-coordinating li- Remarkably,early lanthanides are taken up more quicklyand gands other than water werenecessary to crystallize the com- preferentially by bacteria than the later ones.Itwas shown plex, acarboxylic acid of asecond PQQ molecule is neededto that Methylorubrum extorquens AM1 can even grow with Nd- complete the coordination sphere.Modified PQQ derivatives, containing hard-drive magnets as the only source of Ln, where the carboxylicacid moiety at the pyrrole ring is either making those bacteria interesting for bioleachingorbiomin- blockedbyalkylationtoyield an ester or replaced entirely by ing.[13] With early Ln, bacteria grow faster and their respective MDH enzymesare more efficient in turning over methanol. Seemingly,naturalsystemshave been tuned specifically by evolution to work best with the earlier,larger, and more abun- Lena J. Daumannobtained her Diploma in dant lanthanides. The reasonsfor the preference of natural sys- Chemistryin2010 from the University of Hei- delberg workingwith Peter Comba. After an tems for early lanthanides remainsomewhat elusive. However, internship at BASF she completed her PhD in factors such as changing coordination numbersacross the Ln- 2013 with Lawrie Gahan at the University of series, lack of efficient activation and negative impact on redox Queensland in Australia workingonpesticide- cycling of PQQ by certain Ln in the activesite have been pro- degrading enzymes and biomimetic com- plexes. Her postdoctoral work as aFeodor- posed.[14] PQQ is one of the few pincer ligandsexisting in Lynenfellow with Ken RaymondatUCBerke- nature and coordinates via acarboxylic acid moiety,apyridyl ley involved luminescent lanthanide com- nitrogen and aquinone oxygen atom.[15] Similar binding motifs plexeswith siderophore-inspired ligands.In have been used in ligands employed in the separation of lan- 2016, she took up apositionasW2Professor for bioinorganicand coordinationchemistry thanides and actinides. 2,6-pyridine dicarboxylic acid (PDCA) at the Ludwig-Maximilians-Universität and derivatives have been widely used for solventextraction Munich,where she is exploringthe bioinorganic chemistry of lanthanides and and ion chromatography.[16] The tris-tridentateligand shown in the role of high valentiron speciesinepigenetic processes. She has won nu- Figure 1A also features an ONO binding
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