Angewandte Research Articles Chemie How to cite: Angew.Chem. Int. Ed. 2020, 59,13109–13115 Actinide Complexes International Edition: doi.org/10.1002/anie.202004501 German Edition: doi.org/10.1002/ange.202004501 Stable Actinide p Complexes of aNeutral 1,4-Diborabenzene Valerie Paprocki, Peter Hrobµrik, Katie L. M. Harriman, Martin S. Luff,Thomas Kupfer, Martin Kaupp,* Muralee Murugesu, and Holger Braunschweig* [5] [6] Abstract: The p coordination of arene and anionic heteroarene numerous actinide p complexes including anionic C4–C8 [7] ligands is aubiquitous bonding motif in the organometallic and neutral C6 rings, as well as anionic fused aromatics such chemistry of d-block and f-block elements.Bycontrast, related as naphthalene[8] or pentalene.[9] When it comes to related p interactions of neutral heteroarenes including neutral bora- heteroarene complexes,the diversity becomes significantly p-aromatics are less prevalent particularly for the f-block, due smaller, and astrong imbalance in favor of the d-block to less effective metal-to-ligand backbonding. In fact, p transition metals is encountered. Thus, p-ligated heteroarene complexes with neutral heteroarene ligands are essentially complexes of the d-elements have been realized for alarge unknown for the actinides.Wehave now overcome these number of aromatic heterocycles all across the periodic table limitations by exploiting the exceptionally strong p donor including B-based systems (BNC2,B2N2,BC4,BNC3,BC5, [10] capabilities of aneutral 1,4-diborabenzene.Aseries of B2C4,BNC4,BC6) and benzene analogs EC5 (E = B-Ga, Si- remarkably robust, p-coordinated thorium(IV) and uranium- Sn, N-Sb),[11] to name only afew.For the f-elements, p [12] [13] [14] [5h,7i,15] (IV) half-sandwichcomplexes were synthesized by simply complexation of anionic BNC3, BC5, AlC5, NC4, [16] [6a] [17] [18] [19] [20] [21] combining the bora-p-aromatic with ThCl4(dme)2 or UCl4, N2C3, C2P2, PC4, P3C2, PN2C2, P4/As4, and P5 representing the first examples of actinide complexes with has been verified for selected lanthanide and actinide aneutral boracycle as sandwich-type ligand. Experimental and molecules.Bycontrast, f-block complexes bearing neutral computational studies showed that the strong actinide–hetero- heteroarenes as sandwich-type ligands are exceedingly rare, [22] arene interactions are predominately electrostatic in nature and limited to [(tBu3-C5H2P)2Ho] (I). Pyridine(diamine) with distinct ligand-to-metal p donation and without significant uranium species of the type II formally also contain aneutral p/d backbonding contributions. nitrogen heterocycle,however, p coordination to uranium involves reduced anionic pyridine rings (Figure 1).[23] We note Introduction that p complexation of boracycles is still unknown for the actinides in general. The p-type complexation of aromatic carbocycles by d-block and f-block metal centers takes aunique position in the history of organometallic chemistry with landmark moments such as the discoveries of ferrocene,[1] bis- (benzene)chromium[2] and uranocene.[3] In fact, this concept was one of the first that has been successfully transferred from transition metal to actinide chemistry,[3b, 4] thus such species have always been of high value for studying f-element-ligand bonding and determining critical parameters such as the extent of 5f-orbital participation and metal-ligand covalency. Nowadays,most prototypic aromatic carbocycles have been Figure 1. Stable f-block element p complexeswith (formally) neutral incorporated as unsupported sandwich-type ligands into heteroarene ligands (Ar= mesityl;An= Th, U). [*] Dr.V.Paprocki,M.S.Luff, Dr.T.Kupfer,Prof. Dr.H.Braunschweig Prof. Dr.P.Hrobµrik Institut fürAnorganische Chemie Department of Inorganic Chemistry Julius-Maximilians-UniversitätWürzburg Faculty of Natural Sciences, Comenius University Am Hubland, 97074 Würzburg (Germany) 84215 Bratislava (Slovakia) E-mail:[email protected] K. L. M. Harriman, Prof. Dr.M.Murugesu Dr.V.Paprocki,M.S.Luff, Dr.T.Kupfer,Prof. Dr.H.Braunschweig Department of Chemistry and Biomolecular Sciences Institute for Sustainable Chemistry & Catalysis with Boron UniversityofOttawa Julius-Maximilians-UniversitätWürzburg 10 Marie Curie, Ottawa, Ontario, K1N 6N5 (Canada) Am Hubland, 97074 Würzburg (Germany) Supportinginformation and the ORCID identification number(s) for Prof. Dr.P.Hrobµrik, Prof. Dr.M.Kaupp the author(s) of this article can be found under https://doi.org/10. Institut fürChemie, TheoretischeChemie/Quantenchemie, Sekr.C7 1002/anie.202004501. Technische UniversitätBerlin 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. Straße des 17. Juni 135, 10623 Berlin (Germany) KGaA. This is an open access article under the terms of the Creative E-mail:[email protected] Commons AttributionLicense, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Angew.Chem. Int.Ed. 2020, 59,13109 –13115 2020 The Authors. Published by Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim 13109 Angewandte Research Articles Chemie At this point, we wondered what requirements had to be nation of the dbb ligand is remarkable,and clearly emphasizes met by the actinide metal center and aneutral heteroaromatic the unique strength of these p interactions. ligand to create more stable p interactions.Ingeneral, the By contrast, complexes 2a/b and 3a/b proved highly strength of metal-arene p bonding is dictated by two factors: sensitive under redox conditions.Inour hands,chemical (i)Electrostatics,which explains the preference of “hard” oxidation or reduction consistently led to decomposition of actinide cations for p complexation of “hard” anionic 2a/b and 3a/b to afford free dbb 1 and unknown actinide (hetero)arene ligands.(ii)Metal-to-ligand backdonation, species.Itshould be noted that 2a/b and 3a/b were also which is the dominant part of bonding interactions in p formed when the reactions were carried out in chlorinated [24] complexes of neutral (hetero)arenes. We reasoned that the (CH2Cl2)oraromatic solvents (benzene/toluene) in the electrostatic term is maximized by employing high-oxidation- presence of 1equivalent of thf/MeCN,although yields were state metal precursors (ThIV,UIV–VI), which, at the same time, lower in these cases.Inthe absence of donor solvents, will limit ligand reduction processes,thus allowing the however, no reaction occurred for UCl4 (presumably because generation of species with truly neutral heteroaromatic of its low solubility), and ThCl4(dme)2 is partly converted to ligands.This,however,will significantly lower the backdona- the dme-bridged dimer [{(dbb)ThCl4}2-k-dme] (4)(optimized tion capabilities of the actinide metal center, thus electron- conditions:fluorobenzene, DT,20h,10% isolated yield; rich heteroarenes with very strong p donor strengths will be Figure S23). Thus,our initial experiments indicated that required as antidote.Recent studies in our group have actinide p complexes of neutral diborabenzene 1 are readily highlighted the exceptional p donor strength of the bora-p- accessible simply by combining the ligand with standard aromatic 1,4-bis(cAAC)2-1,4-diborabenzene [1;dbb;cAAC = actinide reactants.Wenote that the simplicity of this cyclic (alkyl)(amino)carbene][25] in remarkably stable Group approach is very uncommon in condensed phase keeping in 6half-sandwich complexes [(dbb)M(CO)3](M= Cr,Mo, mind that the p complexation process usually requires pre- W).[10s] We were thus confident that the dbb ligand might be activation of the metal center under ligand abstracting asuitable choice for generating the first stable actinide p conditions such as reduction, oxidation, photolysis,orhalide complexes with neutral heteroarene ligands. abstraction. We next turned our attention to the electronic structure of the actinide metal centers of 2a/b and 3a/b.Formally, Results and Discussion oxidation states of + IV are required to exclude the occur- rence of ligand reduction processes upon dbb coordination, When ThCl4(dme)2 and UCl4 were allowed to react with and to ascertain the neutral nature of the dbb p ligand. For 2a/ 1.1 equivalents of the neutral bora-p-aromatic 1 in adonor b,their chemical composition and solution NMR spectra in solvent (thf,MeCN) at refluxing conditions (12 h), either the normal diamagnetic range strongly indicate an oxidation purple suspensions (Th) or deep-red solutions (U) formed, state of + IV for the thorium centers,even though acoupled from which p complexes [(dbb)(L)AnCl4](2a:An= Th,L= biradical character due to non-innocence of the dbb ligand thf; 2b:An= Th,L= MeCN; 3a:An= U, L = thf; 3b:An= cannot be ruled out completely.The 1HNMR spectra of 2a/b U, L = MeCN) were isolated as red solids in moderate to good confirm the presence of a1:1 ratio of coordinated dbb and yields (Scheme 1). Compounds 2a/b and 3a/b are thermally Lewis base with their expected signal patterns.Noteworthy robust, even in the presence of an excess of the respective are the chemical shifts for the aromatic dbb ring protons (2a: donor solvent, which strongly contrasts with the labile p dH = 7.18; 2b: dH = 7.78), which almost remain unaltered from 11 coordination of benzene and its methylated analogs in related that of the free ligand 1 (dH = 7.31). Similarly,the BNMR 6 [7a,d] species such as [(h -C6HnMe6 n)UX3](X= BH4,AlCl4), resonances of the boron nuclei (2a: dB = 27.5; 2b: dB = 27.8) 6 À[7b] and [(h -C6Me6)2U2Cl7][AlCl4]. However,when dissolved are only slightly shifted to higher frequencies upon