Stable Actinide π Complexes of a Neutral 1,4‐Diborabenzene

Abstract The π coordination of arene and anionic heteroarene ligands is a ubiquitous bonding motif in the organometallic chemistry of d‐block and f‐block elements. By contrast, related π interactions of neutral heteroarenes including neutral bora‐π‐aromatics are less prevalent particularly for the f‐block, due to less effective metal‐to‐ligand backbonding. In fact, π complexes with neutral heteroarene ligands are essentially unknown for the actinides. We have now overcome these limitations by exploiting the exceptionally strong π donor capabilities of a neutral 1,4‐diborabenzene. A series of remarkably robust, π‐coordinated thorium(IV) and uranium(IV) half‐sandwich complexes were synthesized by simply combining the bora‐π‐aromatic with ThCl4(dme)2 or UCl4, representing the first examples of actinide complexes with a neutral boracycle as sandwich‐type ligand. Experimental and computational studies showed that the strong actinide–heteroarene interactions are predominately electrostatic in nature with distinct ligand‐to‐metal π donation and without significant π/δ backbonding contributions.


Introduction
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,m ost prototypic aromatic carbocycles have been incorporated as unsupported sandwich-type ligands into numerous actinide p complexes [5] including anionic C 4 -C 8 [6] and neutral C 6 rings, [7] as well as anionic fused aromatics such as naphthalene [8] or pentalene. [9] When it comes to related heteroarene complexes,t he diversity becomes significantly smaller, and as trong imbalance in favor of the d-block transition metals is encountered. Thus, p-ligated heteroarene complexes of the d-elements have been realized for al arge number of aromatic heterocycles all across the periodic table including B-based systems (BNC 2 ,B 2 N 2 ,B C 4 ,B NC 3 ,B C 5 , B 2 C 4 ,BNC 4 ,BC 6 ) [10] and benzene analogs EC 5 (E = B-Ga, Si-Sn, N-Sb), [11] to name only af ew.F or the f-elements, p complexation of anionic BNC 3 , [12] BC 5 , [13] AlC 5 , [14] NC 4 ,[5h,7i,15] N 2 C 3 , [16] C 2 P 2 , [6a] PC 4 , [17] P 3 C 2 , [18] PN 2 C 2 , [19] P 4 /As 4 , [20] and P 5 [21] has been verified for selected lanthanide and actinide molecules.B yc ontrast, f-block complexes bearing neutral heteroarenes as sandwich-type ligands are exceedingly rare, and limited to [(tBu 3 -C 5 H 2 P) 2 Ho] (I). [22] Pyridine(diamine) uranium species of the type II formally also contain aneutral nitrogen heterocycle,h owever, p coordination to uranium involves reduced anionic pyridine rings ( Figure 1). [23] We note that p complexation of boracycles is still unknown for the actinides in general. At this point, we wondered what requirements had to be met by the actinide metal center and aneutral heteroaromatic ligand to create more stable p interactions.I ng eneral, the strength of metal-arene p bonding is dictated by two factors: (i)Electrostatics,w hich explains the preference of "hard" actinide cations for p complexation of "hard" anionic (hetero)arene ligands.( ii)Metal-to-ligand backdonation, which is the dominant part of bonding interactions in p complexes of neutral (hetero)arenes. [24] We reasoned that the electrostatic term is maximized by employing high-oxidationstate metal precursors (Th IV ,U IV-VI ), which, at the same time, will limit ligand reduction processes,t hus allowing the generation of species with truly neutral heteroaromatic ligands.T his,however,will significantly lower the backdonation capabilities of the actinide metal center, thus electronrich heteroarenes with very strong p donor strengths will be required as antidote.R ecent studies in our group have highlighted the exceptional p donor strength of the bora-paromatic 1,4-bis(cAAC) 2 -1,4-diborabenzene [1;dbb;cAAC = cyclic (alkyl)(amino)carbene] [25] in remarkably stable Group 6h alf-sandwich complexes [(dbb)M(CO) 3 ]( M = Cr,M o, W). [10s] We were thus confident that the dbb ligand might be as uitable choice for generating the first stable actinide p complexes with neutral heteroarene ligands.

Results and Discussion
When ThCl 4 (dme) 2 and UCl 4 were allowed to react with 1.1 equivalents of the neutral bora-p-aromatic 1 in ad onor solvent (thf,M eCN) at refluxing conditions (12 h), either purple suspensions (Th) or deep-red solutions (U) formed, from which p complexes [(dbb)(L)AnCl 4 ](2a:An= Th,L= thf; 2b:An= Th,L= MeCN; 3a:An= U, L = thf; 3b:An= U, L = MeCN) were isolated as red solids in moderate to good yields (Scheme 1). Compounds 2a/b and 3a/b are thermally robust, even in the presence of an excess of the respective donor solvent, which strongly contrasts with the labile p coordination of benzene and its methylated analogs in related species such as [(h 6 -C 6 H n Me 6Àn )UX 3 ]( X= BH 4 ,A lCl 4 ), [7a,d] and [(h 6 -C 6 Me 6 ) 2 U 2 Cl 7 ][AlCl 4 ]. [7b] However,w hen dissolved in thf,the acetonitrile ligand of 2b and 3b is readily displaced quantitatively to afford thf complexes 2a and 3a.Nochanges were observed upon dissolving 2a and 3a in MeCN.T his reactivity is not surprising given the better s donor properties of thf,a nd the oxophilicity of the actinides.T he fact that ligand displacement reactions preferably occur at the Lewis base site of [(dbb)(L)AnCl 4 ]w ithout affecting the p coordi-nation of the dbb ligand is remarkable,and clearly emphasizes the unique strength of these p interactions.
By contrast, complexes 2a/b and 3a/b proved highly sensitive under redox conditions.I no ur hands,c hemical oxidation or reduction consistently led to decomposition of 2a/b and 3a/b to afford free dbb 1 and unknown actinide species.I ts hould be noted that 2a/b and 3a/b were also formed when the reactions were carried out in chlorinated (CH 2 Cl 2 )o ra romatic solvents (benzene/toluene) in the presence of 1equivalent of thf/MeCN,a lthough yields were lower in these cases.I nt he absence of donor solvents, however, no reaction occurred for UCl 4 (presumably because of its low solubility), and ThCl 4 (dme) 2 is partly converted to the dme-bridged dimer [{(dbb)ThCl 4 } 2 -k-dme] (4)(optimized conditions:f luorobenzene, DT,2 0h,1 0% isolated yield; Figure S23). Thus,o ur initial experiments indicated that actinide p complexes of neutral diborabenzene 1 are readily accessible simply by combining the ligand with standard actinide reactants.W en ote that the simplicity of this approach is very uncommon in condensed phase keeping in mind that the p complexation process usually requires preactivation of the metal center under ligand abstracting conditions such as reduction, oxidation, photolysis,o rh alide abstraction.
We next turned our attention to the electronic structure of the actinide metal centers of 2a/b and 3a/b.F ormally, oxidation states of + IV are required to exclude the occurrence of ligand reduction processes upon dbb coordination, and to ascertain the neutral nature of the dbb p ligand. For 2a/ b,t heir chemical composition and solution NMR spectra in the normal diamagnetic range strongly indicate an oxidation state of + IV for the thorium centers,even though acoupled biradical character due to non-innocence of the dbb ligand cannot be ruled out completely.The 1 HNMR spectra of 2a/b confirm the presence of a1 :1 ratio of coordinated dbb and Lewis base with their expected signal patterns.N oteworthy are the chemical shifts for the aromatic dbb ring protons (2a: d H = 7.18; 2b: d H = 7.78), which almost remain unaltered from that of the free ligand 1 (d H = 7.31). Similarly,t he 11 BNMR resonances of the boron nuclei (2a: d B = 27.5; 2b: d B = 27.8) are only slightly shifted to higher frequencies upon complexation (1: d B = 24.8). [25] By contrast, the related Group 6h alfsandwich complexes [(dbb)M(CO) 3 ]( M = Cr,M o, W) exhibited asignificant high-field shift of both the 1 H(d H = 4.74-4.97) and 11 BNMR (d B = 6.0-7.0) resonances of the diborabenzene ligand. [10s] This behavior was interpreted in terms of strong metal-to-ligand backbonding contributions from the electron-rich Group 6m etal centers to the empty dbb ligand orbitals,t hus creating highly covalent bonding interactions. Consequently,t he present findings indicate af undamentally different bonding picture for 2a/b with larger electrostatic and rather small metal-to-ligand backbonding contributions, which is in line with the higher oxidation state of Th IV and its lack of fe lectrons.
For 3a/b,m agnetic susceptibility measurements also account for an oxidation state of + IV of the uranium centers, thus verifying the presence of neutral dbb p ligands in 3a/b as well. In solution, 3a/b show paramagnetic behavior at room temperature with paramagnetically shifted and broadened Scheme 1. Reactivity of dbb 1 with ThCl 4 (dme) 2 and UCl 4 to afford actinide half-sandwich complexes 2 and 3.
Thee xact nature of the An-dbb p interaction in complexes 2a/b and 3a/b was assessed by DFT calculations. To this end, we studied the electronic structures of 1, 2a, 3a, the hypothetical benzene analogues [(h 6 -C 6 H 6 )(thf)AnCl 4 ] (An = Th,U ), and some literature-known [(h 6 -C 6 H n Me 6Àn )UX 3 ]( X = BH 4 ,A lCl 4 )s pecies,a pplying 5f 0 d 0 , 5f 2 d 0 and 5f 3 d 0 electron configurations for the Th IV ,U IV ,a nd U III centers,r espectively.T he computed structural and spectroscopic parameters of 2a and 3a agree very well with experimentally determined values (Supporting Information). Thec alculations suggest that the An-dbb interactions in 2a and 3ashould be viewed as largely electrostatic in nature with small, but distinct orbital contributions,which coincides with only marginal changes in NMR shifts after complexation of dbb by Th IV .T hus,d elocalization indices (QTAIM DIs), which serve as am easure of the bond covalencyf or ag iven pair of atoms, [27] show rather small values for the An À Cbonds ,a nd their shape resembles that of the frontier molecular orbitals HOMO and HOMOÀ1offree dbb 1. [25] While HOMO of 2a illustrates the p donor interaction of the delocalized aromatic p system of 1 (HOMO) to thoriumsv acant 6d orbitals,H OMOÀ1a nd HOMOÀ16 are reflective of ligand-to-metal p bonding emanating from C=C-centered ligand p orbitals (HOMOÀ1) to empty 5f and 6d orbitals of thorium. It should be emphasized here that MOs associated with metal-to-ligand p/d backbonding could not be located by our calculations.S imilar interactions were also derived for uranium complex 3a ( Figure S27), while the presence of two fe lectrons in principle allows for metal-toligand backbonding interactions.S pin-density calculations, however, have shown that the two unpaired fe lectrons predominately reside at the U IV center with small negative spin densities at chlorine atoms (Figure 4), making such metal-to-ligand p/d backbonding contributions rather weak in nature.
Thei solation of crystalline 2a/b and 3a/b allowed us to elucidate their solid-state structures by X-ray diffraction analyses (Figures 5a/b,S 18, S22). All complexes exhibit pseudo-octahedral geometries with four chloride ligands in equatorial positions,a nd one molecule each of Lewis base L (thf,M eCN) and dbb mutually trans in axial positions. Unexpectedly,t he dbb heteroarene is not perfectly planar, instead complexation results in minor deviations of the dbb ligand from planarity in all cases,that is,the two boron atoms are slightly bent out of the ring plane (6.1 to 8.88 8)away from the metal center. Thus,t he hapticity of the An-dbb p coordination seems to be best described as h 4 with close An-C dbb contacts.H owever,t heoretical evidence of weak covalent An-B interactions suggests that the bonding picture is not that simple and that h 6 -typec ontributions have to be considered as well.Hence,the true bonding situation most likely lies within the h 4 -h 6 -continuum, but definitely on the h 4 -side.
Notwithstanding its hapticity,t heoretical and experimental considerations clearly show that the diborabenzene ligand is bound to the actinide centers of 2a/b and 3a/b via its fully conjugated p system, and not via interaction of the actinide metal centers with two isolated C = Cd ouble bonds of the heteroarene,a sm ight be reasoned from strong h 4 -contributions.First of all, our computations emphasize the significance of ligand-type orbitals for An-dbb bonding,w hich mainly involve HOMO and HOMOÀ1o ff ree dbb 1,o rbitals of p symmetry spanning the whole B 2 C 4 heterocyclic backbone (resonance structure 1, Figure 5c). More importantly,the type of p coordination active in molecules 2a/b and 3a/b is expected to directly affect their spectroscopic and structural properties.Hence,interaction of the actinide centers with two isolated C=Cd ouble bonds would require the unfavorable breakup of aromatic p conjugation within dbb,r esulting in unfavorable biradical or charge separated resonance structures 1' ' and 1' '' ' (Figure 5c). In our hands,the presence of such resonance structures can be excluded for 2a/b and 3a/b. While any biradical character (1' ')c an be ruled out on the basis of EPR spectroscopic studies,c harge separation (1' '' ') appears very unlikely when closely inspecting the solid-state structures of 2a/b and 3a/b.T hus, p coordination of dbb via resonance structures 1' ' and 1' '' ' most likely causes significant elongation of the endocyclic BÀC dbb of the dbb ligand, while, as aconsequence,exocyclic B À C cAAC and C cAAC À N cAAC bonds will become shorter and longer, respectively.For 2a/b and 3a/ b however,t he opposite is true,a nd B À C dbb (1.507(7)-1.526 (3) )and C cAAC ÀN cAAC distances (1.313(6)-1.322 (5) ) are smaller than in 1,w hile C dbb =C dbb (1.395(4)-1.403 (6) ) and BÀC cAAC (1.584(6)-1.597 (5) (3) ). [25] Consequently,X -ray diffraction data clearly support our theoretical findings that An-dbb p coordination involves the whole aromatic B 2 C 4 framework.
IR spectroscopic studies on 1, 2a/b and 3a/b in the solid state also support this p bonding picture (Figures S12-S17).
Here,IRbands associated with the endocyclic C = Cbonds are shifted to lower energies upon complexation of dbb,t hat is, from 1412-1472 cm À1 in 1 to 1365-1423 cm À1 in 2a/b and 3a/b. At the same time,t he strong IR absorption of the C cAAC À N cAAC bond is shifted to higher wavenumbers (cf. 1:1423 cm À1 ; 2a/b, 3a/b:1454-1458 cm À1 ), which is consistent with stronger  C cAAC ÀN cAAC bonds in p complexes 2a/b and 3a/b (assignment of IR bands supported by by frequencies calculations).
An À Cbond lengths were determined to be in the range of 2.831(2) to 2.948(4) (2a:Th-dbb cent 2.586 ; 2b:Th-dbb cent 2.556 ; 3a:U -dbb cent 2.585 ; 3b:U -dbb cent 2.490 ). We note that these contacts are quite short, which illustrates the strong actinide-heteroarene interaction in 2a/b and 3a/b.F or 2a/b,aCSD search on Th complexes featuring neutral p arene ligands provided reasonably longer Th-C cent distances (2.706-2.950 ). [7g,h,l,m] TheU -C distances of 3a/b,h owever, strongly resemble those in [(h 6 -C 6 Me 6 )UX 3 ](X= BH 4 ,AlCl 4 ; av.U-C 2.92 ), [7a,d] (3) )are in agreement with theory and rather weak An À Bb onding interactions.Overall, experimental and theoretical data suggest that neutral dbb is tightly bound to Th IV and U IV in a h 4 -type coordination mode via p interactions involving the whole aromatic p system (mediated primarily by electrostatics in combination with distinct covalent bonding contributions; ligand-to-metal donation;non otable p/d backbonding).
Finally,weset out to overcome the well-known tendency of actinide ions to preferably bind "hard" donor ligands and tried to incorporate the "soft" Lewis base PMe 3 in dbb complexes of the type [(dbb)(L)AnCl 4 ]. Thus,the reactions of ThCl 4 (dme) 2 and UCl 4 with 1.1 equivalent of dbb 1 in the presence of PMe 3 in benzene under refluxing conditions resulted in the generation of PMe 3 -substituted species [(dbb)-(PMe 3 )AnCl 4 ]i ns olution. Due to labile An À PMe 3 bonds, however, only [(dbb)(PMe 3 )ThCl 4 ]( 2c)e xhibited sufficient stability to allow isolation (in low yields of 11 %) (Scheme 1, Supporting Information), while its uranium analog eluded isolation and could only be observed spectroscopically.
Red crystalline 2c represents the most sensitive and least stable species in the [(dbb)(L)AnCl 4 ]s eries,r eadily reacting in polar/coordinating solvents,a nd decomposing under vacuum conditions,m aking its purification extremely difficult. Nevertheless,i ts identity was clearly verified by NMR spectroscopy and X-ray diffraction studies ( Figure 6). In solution, diamagnetic 2c shows a 11 BNMR resonance at d B = 27.7 for the p ligated diborabenzene ligand (cf. 2a: d B = 27.5; 2b: d B = 27.8). TheT h IV -PMe 3 interaction of 2c is characterized by a 31 PNMR signal with ac hemical shift of d P = À30.6 in solution, and by aTh ÀPbond distance of 3.053 (1) in its solid-state structure.O ther structural parameters are roughly the same as those of compounds 2a and 2b.W ewere surprised to find that dative Th-P interactions are still rare, and only one paper has been published reporting related stable ThÀPd ative bonding interactions involving nonchelating tertiary phosphine ligands,that is,[(BH 4 ) 4 Th(PR 3 ) 2 ] (R = Me,Et). [29] In addition, only afew species containing the bidentate 1,2-bis(dimethylphosphino)ethane ligand (dmpe) are known that are suitable for comparison. [30] Here, 31 PNMR chemical shifts range from d P = À33.3 to À4. 5 (cf.[(BH 4 ) 4 Th-(PMe 3 ) 2 ]: d P = À22.2), and Th-P bond lengths range from 3.096 (3) in [(BH 4 ) 4 Th(PEt 3 ) 2 ]t o3 .237 (2) in [Cp 2 -(CH 2 Ph) 4 Th(dmpe) 2 ]. Nevertheless,t he Th-P interaction of 2c must still be considered rather weak, and the PMe 3 ligand is prone to dissociation in the presence of "hard" Lewis bases. When dissolved in either thf or MeCN,the "soft" PMe 3 ligand is replaced instantaneously,a nd 2c converts quantitatively into its analogs 2a and 2b,r espectively,w hich is consistent with the preferred coordination of "hard" donor ligands to thorium.

Conclusion
In summary,w eh ave succeeded in the realization of the first actinide-based molecules with an aromatic boracycle as sandwich-type p ligand, [(dbb)(L)AnCl 4 ]. Complexes 2a-c and 3a/b are remarkably stable even in the presence of coordinating solvents,w hich contrasts with the labile p coordination often observed for related species with unsupported benzene ligands.T hus,l igand displacement reactions proceeded at the Lewis basic site in trans-position to the dbb ligand without affecting actinide-heteroarene bonding.A combination of experimental and theoretical techniques was used to verify the neutral nature of the diborabenzene ligand and its p-type coordination to the Th IV and U IV metal centers. Theunique strength of the actinide-heteroarene interaction is closely related to the outstanding p donor capabilities of the aromatic dbb heterocycle,t hus enabling (i)strong electrostatic interactions with the electron-poor actinide centers,and (ii)distinct covalent orbital interactions primarily via ligandto-metal electron donation and without notable backbonding contributions.