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The first cis-dioxido uranyl compound under scrutiny.

The discovery of a cis-dioxido uranyl compound, an exciting event and a milestone in the history of actinide chemistry, was recently reported in this journal by P. B. Duval et al. However, several peculiar points in the synthesis and structure of this compound, [UO2(fcdc)(thf)·fc]n (fcdc = 1,2-ferrocenedicarboxylate, fc = ferrocene), appeared questionable to us. The particularly easy and reversible formation of this polymeric product from uranyl acetate dihydrate and ferrocenecarboxylic acid (fccH), occurring at room temperature and involving for each uranyl unit the activation, breaking, and building of two C H and two C C bonds, seemed to us extraordinary and, it must be said, quite difficult to believe. The H NMR spectra in CD3OD, and their interpretation concerning the fluxional behavior of the compound in solution, were not understandable to us. The crystal structure was refined in the Ama2 space group to a R1 factor of 4.99%. However, examination of the Crystallographic Information File deposited at the Cambridge Crystallographic Data Centre revealed several anomalies. The THF moiety is affected by very large disorder effects and some displacement ellipsoids are clearly anomalous. This last point is particularly apparent in the coordinated fcdc molecule, in which some atoms of the aromatic ring bearing the acid groups are affected with extremely anisotropic displacement parameters, whereas the acid groups themselves have large, but quite isotropic parameters. The C4 C7 bond, which connects these two parts, is out of the usual range, as indicated by Duval et al. It is also surprising for the O-U-O angle of 708, which is considerably smaller than the O-M-O angles in cis-dioxido transitionmetal complexes, to be associated with U O distances equal to those in trans uranyl complexes. All these observations indicated that a closer look at the structure, which is an essential part of the paper, was warranted, considering the importance of the claim at stake. To settle these doubts, we decided to reproduce the synthesis of this compound. By following the same procedure as described by Duval et al, starting from a 1:2 mixture of [UO2(OAc)2(H2O)2], where OAc is acetate, and fccH in CH2Cl2/THF, we did not obtain the 80–83% yield of red crystals as reported in ref. [1]. We repeatedly observed instead the formation of a few small red microcrystals, not suitable for X-ray diffraction, along with a reddishorange powder and other dark material. Dissolution of this mixture in pyridine or methanol gave red solutions which, upon addition of pentane, deposited red crystals of [UO2(fcc)2(py)2]·py (1) and [UO2(fcc)2(MeOH)]2·2MeOH (2), respectively, in approximately 80% yield. The crystal structures of 1 and 2 are shown in Figure 1 and Figure 2, respectively, together with selected bond lengths and angles; the dimeric structure of 2 is quite similar to those of [{UO2(OAc)2(L)}2] (L = Ph3AsO, [2a] Ph3PO, [2b] and AcOH), the trans uranyl units being bound to one terminal (chelating) and two bridging carboxylate ligands. It can be noted that compound 2 was obtained in the very solvent used for the NMR spectroscopic study by Duval et al. A large quantity of red crystals, also of sufficient quality for an accurate and unambiguous crystal-structure determination, was obtained when an excess of fccH (3 or 4 molar equivalents) was used in its reaction with [UO2(OAc)2(H2O)2] in THF; indeed, this excess should shift the equilibrium postulated by Duval et al towards the formation of the cis-dioxido uranyl compound. The one-dimensional polymeric crystal structure of the compound obtained from this reaction, [UO2(fcc)2(H2O)·thf]n (3 ; Figure 3), is quite similar