Solid‐State Structure of Tris‐Cyclopentadienide Uranium(III) and Plutonium(III)

Abstract The organometallic tris‐cyclopentadienide actinide(III) (AnCp3) complexes were first reported about 50 years ago. However, up until now, only the NpCp3 solid state structure has been studied. Here we report on the solid state structures of UCp3 and PuCp3 which are isostructural to the Np analogue. The structural models are supported by theoretical calculations and compared to their lanthanide analogues. The observed trends in changes of bond lengths might be indicator for an increased covalency in the bonding in the tris‐cyclopentadienide actinide(III) complexes (AnCp3) compared to their lanthanide homologues.

The organometallic actinidec hemistryw ith cyclopentadienyl ligands was developed in Karlsruhea nd Munich by the pioneering work of E. O. Fischer, F. Baumgärtner,a nd B. Kanellakopulos together with P. Laubereau, then of the National Laboratories at Oak Ridge.
Theo xidation state + IIIi sn ot the most stable fora ll actinides. Nevertheless thes olvent free non-stabilized tris-cyclopentadienidea ctinide(III) complexesA nCp 3 were reported 50 years ago, [1] af ew yearsa fter thef irst reportso nt he AnCp 4 complexes. [2] Type LnCp 3 (Ln: lanthanide)c omplexes nots tabilized by Lewisb asea dductf ormationh avebeenpreviouslys tudied. [3] However, as thef irst exampleo fa nn on-stabilisedA nCp 3 complex,t he synthesisa nd solid-states tructure of NpCp 3 haso nly recently been published. [4] This wasf ollowedb yt he firstr eport on as tructurally characterizedo rganometallic Pu III complex Pu(Cp(TMS) 2 ) 3 and its reduced Pu II analogue [5] and then the first report on aP u IV organometallic plutonocene derivative. [6] Fifty years after the first reports, the structures of the UCp 3 or PuCp 3 complexes are still unknown. This is because even in the case of the adduct free LnCp 3 complexes,h igh quality single crystalsa re not easily obtained.I ndeed different forms are sometimeso bserved depending on the crystallization conditions. [3c] In case of the actinides, additionally,a ging of solids is observed:a fter some weeks of storaget hey show drastically decreased solubility. [1c] This effect is however lessn oticeable when pure single crystalline materialiss tored.
Here, we close the knowledge gap on the solid-state structures of AnCp 3 (An:U ,P u). Comparing them to the structures of NpCp 3 and relatedL nCp 3 complexes offers the opportunity to gain am ore detailed insight in the bonding. Thisi si mportant for the understanding of 4f or 5f electron behaviour and differences therein. UCp 3 was prepared by reductive elimination of chloride from UCp 3 Cl with sodium amalgam in diethylether.P uCp 3 was obtained from the direct reaction of PuCl 3 with as light excess of KCp. Both werep urified by filtration and evaporation of the solventf ollowed by extraction with pentane or pentane/Et 2 O mixtures. The IR spectroscopic data revealafingerprint consistent with that previously reported for UCp 3 and PuCp 3 . [1b,c] The 1 HNMR spectra of UCp 3 show one singler esonance at d H = À15.60 ppm ([D 8 ]THF) or À13.62 ppm ([D 3 ]MeCN) for the formed adducts under these conditions, which are in agreement with literature-known values. [7] The cross-peakf or the CH C-atom is observed at low fielda t2 72.4 ppm in the 13 Cf requencyr esulting in an overall comparable situation as observed in the bis-TMSs ubstituted uranocene derivative in [7b] ( Figure S1). The NMRs pectroscopici nvestigationso nP uCp 3 are the 4 th example of aP uo rganometallic complex for which ap roton resonancei sr eported and the 2 nd complex on which multi-dimensional NMR spectroscopy was performed. [5,6] In [D 6 ]benzene there is one resonance observed for [PuCp 3 (thf)] at 11.59 ppm (in good agreement with the values reported in Ref. [5]) giving rise to ac ross-peak in the CH correlated spectrum at 81.4 ppm ( Figure S2). This is as ign that the Cp rings are in equilibrium duet of ast chemical exchangei nt he sample. It seems that in all Pu organometallic complexes reported up to now the chemical shifts observed for the proton as wella sf or the 13 Cr esonances appear in the same range independently on the oxidation state of the metal being + II, + III, or + IV. [5, 6, 7b] [a] Dr.C . Apostolidis By extractions ingle crystals are obtained suitable for X-ray diffraction analyses (Figure 1, all experimental details see the Supporting Information). Both compounds, UCp 3 and PuCp 3 , form crystalst hat are isomorphic to the NpCp 3 analogue. [4] For Cm and Bk, the cell parameters have been identifiedb y Debye-Scherrer analyses together with as eries of LnCp 3 complexes [1e,f] all containingo ne axis doubled. Also discussed are some structures of LnCp 3 complexes with comparable cell parameters, maybe containing one axis doubled but also with an identicalr educed cell. [3] Most of these structuress how disorder of the Cp rings, and data collection was performeda tr oom temperature. Both these factors prevent ag ood determination of the atom positions concerned, whichl eads to high standard deviations in distances and angles and makes any discussion on as ignificant level more difficult (see Baisch et al. [3c] ). Therefore we performed our diffraction analyses at at emperature of 100 Ki no rder to collect datasetso fg ood quality.W ed escribe the systemsa so rthorhombic Cmc2 1 with a % 14.15, b % 8.70, and c % 9.60 ,w hich corresponds to am onoclinicr educed cell of a % 8.30, b % 9.60, and c % 8.30 with b % 116.58 (rounded valuesf rom all three data sets). The monoclinic cell has been used before to describeL aCp 3 [3a] and PrCp 3 [3b] whereas the orthorhombic cell was appliedinthe case for one PrCp 3 structure which has been deposited at the CCDC [3f] but the space group reportedi sw ith Pbnm differentfrom our findings.
We are now convinced that at least in the cases for the three actinide complexes AnCp 3 (An:U ,N p, Pu) the description in the orthorhombic space group Cmc2 1 is best, as in the monoclinic reduced cell for the refinement ad isorder must be introduced which is not the case in the orthorhombic cell. This leads for the monoclinic case in the refinementw ith identical crystallographic independentc ell volume to nearly double the refined parameters but higherRvalues. As the two com-poundsU Cp 3 and PuCp 3 form the same structure, only PuCp 3 is depicted representatively in Figure 1.
In the sphere of the metal all Cp rings show h 5 -coordination. The Lewis acidity of the actinidesc auses the formation of one additional h 1 -coordination to one Cp ring of an eighboured AnCp 3 residue;t his Cp ring is m-h 5 ,h 1 -coordinated (bridging atom C11, Figure 1). This results in the polymeric zig-zags tructure motif which is known from the complexes LnCp 3 . [3] We can exclude an interaction on the base of a m-h 5 ,h 2 -coordinated bridging cyclopentadienyl group as described earlier [3b] for the AnCp 3 complexes also for all LnCp 3 complexes whose solid state structures we have determined in the past years resulting in low temperature high qualityd atasets. [8] Ac oordination environment of four Cp rings three establishing h 5 -a nd one h 1 -coordination is also established in K[NpCp 4 ] the KCp adduct to NpCp 3 . [4] As ymmetrical bonding of the h 5coordinated Cp rings is produced (mean Np-Ct Cp 251 pm, see footnote Table 1) together with ac loser interaction to the h 1coordinated C-atom of the fourth Cp ring (NpÀC2 75.2(7) pm) showingt hat Cp in KCp is ab etter Lewis base than in NpCp 3 . Lewis base adduct formation like in [UCp 3 (thf)] or in [UCp' 3 (quinuclidine)] produces as imilar situation with symmetrical h 5 -coordination of the Cp rings with ac loser interaction to the donor atom of the Lewis base involvedt han observed here for the m-h 1 -coordinatedC -atom. [9] The bonding of the three Cp rings in h 5 -coordination in AnCp 3 (An:U ,N p, Pu)i sn ot symmetrical:o ne of the rings (not the one involved in the bridging mode) in all the three structures, is localized closer to the central An III ion than the other two (Table 1). This is also the case forthe recently studied complex Pu(CpTMS 2 ) 3 . [5] This behaviour supports the high coordinative flexibility of both the Cp rings and the actinide ions.
In agreement with the asymmetrical bonding of the Cp rings the UÀCb ond lengths for the Cp ring closer to the coordinated metal are 265.8 to 274.7 pm, for the othert wo Cp rings 279.4 to 293.7 pm.T he corresponding values for the PuCp 3 are 264.4 to 272.0 and 276.9 to 291.5 pm, respectively. Accordingly the distances between metal ions and the centres of the Cp rings (Ct Cp in Ta ble 1) are found to 241.6,2 60.4, 260.8 pm (U) and 239.2, 256.5, 257.4 pm for PuCp 3 .F or the series U, Np, Pu one can see, that the Cp rings approachtot he metal about 3pm ( Table 1). This is reflected as welli nt he mean AnÀCb ondl engths (Table 1). The effect is comparable to the one observedf or the lanthanide complexes LnCp 3 [see Figure S3, right] and might be attributed to actinidec ontrac-  265.8-274.7;2 70.1 [d] 266. 8-273.6; 270.3 [d] 264.4-272.0; 267.9 [d] MÀC [c] 279.4-293.7;2 87.2 [d] 278.9-292.2; 284.3 [d] 276.9-291.5; 283.9 [d] Standard deviations in parentheses only for dedicatedb onds not for calculated ideal positions or ranges. tion. As the h 5 -p-coordinatedC pr ing approaches the An III ion centres the h 1 -interactiont ot he m-h 5 ,h 1 -coordinated Ca tom decreases. This results in an elongation of the bond length M-C(m-h 1 )f rom 278(2) for UCp 3 over 281(2) for NpCp 3 to 283(1) pm for PuCp 3 (Table 1). This increase of % 5pmd escribes at rend;t he high standard deviationsd isable to make a clear statementb ased only on experimental data. However, over the series of the three complexes the elongation of the h 1 -interaction to the m-h 5 ,h 1 -coordinated Ca tom of % 5pm seems to be aboutt wice as much as that observed for the corresponding lanthanide complexes[ see Figure S3, left].S oi n the case of the complexes MCp 3 ,t his bond might possibly be regardeda sa ni ndicator for changes in the metal electronic environment. This is because the outer orbitals of the actinide ions in AnCp 3 reach out far enough to establish ag ood interaction to the p-coordinated Cp rings at the given distance demonstrating again the high coordinative flexibility of both the Cp rings and the actinide ions. This hypothesis is supported by the results from DFT calculations we performed using ad imeric molecular modelo fs elected Ln andA nc omplexes reducing the structural motif to an egatively charged unit (Cp 3 -M-Cp-M-Cp 3 ) À with the central Cp ring in the bridging position (details see Supporting Information and FigureS3). The geometry optimisations reproduced the h 5 ,h 1 -coordinationo ft he bridging Cp ring, confirming that this unique interaction belongs to the basic bondingp roperties of the complexesa nd is not enforced by the packing effects. Similarly,t he competitive nature of h 5 ,h 1 -interactions are confirmed by the calculations, the results reflecting the already described changes in the MÀCd istances. During the geometry optimisationsw eo bserved that the system is very flexible;i te xhibits af lat potentiale nergy hyperface. Hence slight changes in force can cause significant changes in the structurei nt he h 1 -MÀCd istances. Another significant clue on the bondingw as the verifiedi mportance of the 4f subshell for the LnÀCp donor-acceptor interactions, calculations using the 4f-in-core Ln pseudopotentials failed to reproduce the characteristic change of the h 1 -LnÀCp distances. On the other hand, the experimentally suggested gradual change in the MÀCb ond lengths for h 5 -a nd h 1 -coordinated Cp rings across the 4f/5fr ows were only partially reproduced by the calculations. The probable reason lies in the already mentioned flatp otential energy surfacea nd the dimericm odel structure (size limited by technical problems in the calculations) being unable to account forl ong-range cooperative or solid-state effects.
Our experimentalr esultsd escribed here close the knowledge gap on the solid state structure of the long known complexes PuCp 3 and UCp 3 .T hey indicatet hat covalency in AnCp 3 is higher than in LnCp 3 (at least for the herereported minor actinide complexes), whichi si na greementw ith theoretical considerations. [10] Series comparing experimental data of transition metal or lanthanide complexes to their actinidea nalogues together with theoretical calculation showed in other cases as well:5fand or 6d orbitalcontributionc ontributes to covalency in the bondingo fa ctinidec omplexes. It is influenced by the interplay between the metal ions and the ligands. [11] With this background it seems promising to compare as well the cyclohexylisonitrile adducts AnCp 3 (CNC 6 H 11 )t ot hose of the corresponding lanthanides. The IR CN-stretching vibration of the isonitrile ligand is an excellent sensor on its binding mode and forces which enables the detection of differences between the lanthanides and actinides in their complexes MCp 3 (CNC 6 H 11 ). [1c, 12] Crystallographic data CCDC 570389( PuCp 3 )a nd 1570390 (UCp 3 ), contain the supplementaryc rystallographic dataf or this paper.T hese data are provided free of charge by The Cambridge Crystallographic Data Centre. For furtherinformation, please see the Supporting Information.