Activation of carbon dioxide by new mixed sandwich uranium(iii) complexes incorporating cyclooctatetraenyl and pyrrolide, phospholide, or arsolide ligands

A series of uranium(III) mixed-sandwich complexes of the type [U(COT)(Cp)] (Cp = EC4Me4, E is N, P or As, and COT = C8H6{1,4-Si Pr3}), featuring a heterocyclic five membered ring, have been synthesised and their X-ray crystal structures determined. The redox properties of these complexes have been assessed using cyclic voltammetry and the results compared to the purely carbocyclic mixedsandwich analogues. The reactions of [U(COT)(Cp)] and [U(COT)(Cp)] with CO2 afford the structurally characterised carbamate and phosphacarbonate complexes [U(COT)]2(m-O)(m-Z :ZO2CEC4Me4)2 (E = N and P respectively), arising from CO2 reduction and insertion.


Introduction
The use of carbocyclic aromatic ligands in organouranium chemistry has been prominent ever since the synthesis of [Cp 3 UCl] in 1956, 1 and subsequent expansion of this area to include 6, 7 and 8 membered rings illustrates the versatility of aromatic ligands in this field. The cyclopentadienyl (Cp) ligand and its substituted derivatives are ubiquitous in organouranium chemistry, 2 however aromatic heterocyclic analogues have received comparatively little attention. Of the few reported uranium complexes featuring heterocyclic 5-membered rings, we reported the only example of a homoleptic uranium complex, featuring the 1,3-di-tert-butyl-1,2,4-triphospholyl ligand, 3 and more extensive studies by Ephritikhine et al. have employed the tetramethylphospholyl (Cp PMe4 ) ligand, as this bears the most resemblance to Cp*. 4,5 The latter results also demonstrated that these ligands can also bond through the pnictogen lone pair, allowing dimerisation of the complexes via Z 1 :Z 5 -coordination. However the Cp PMe4 ligand was also observed to be more labile than its cyclopentadienyl analogues, illustrated by the ready protonation of the ligand in a mixed-sandwich complex to generate a uranium(IV) cation. 6,7 In recent years, the use of organometallic uranium complexes for small molecule activation, has attracted significant interest; 8 in the specific case of CO 2 , reduction to afford uranium oxo complexes and CO has been achieved previously using U(III) complexes incorporating tripodal tris(aryloxide) 9 or siloxide ligands, 10 and disproportionation to CO and uranium carbonate derivatives has been described for neutral and anionic U(III) siloxide, 10 and tris(aryloxide) systems. 11 In recent years, we have employed uranium(III) mixed-sandwich complexes featuring substituted COT and Cp ligands for the reductive activation of CO and CO 2 , and comprehensive studies have determined that the steric properties of the mixed-sandwich complexes dictate the outcome of these reactions. 12,13 Hence we decided to investigate the effect of changing the electronic properties of these mixed sandwich complexes, and herein we report the results obtained from incorporation of a heterocyclic ring in to the U(III) mixed sandwich motif and subsequent reactivity towards CO 2 .

Synthesis of mixed-sandwich complexes
The three mixed-sandwich complexes [U(COT TIPS2 )(Cp EMe4 )] (E = N (1), P (2), As (3)) were prepared by successive salt metathesis reactions of UI 3 with K[Cp EMe4 ] and K 2 [COT TIPS2 ] in low to moderate yield (Scheme 1). This 'one-pot' methodology is an adaptation of the synthetic route employed for the synthesis of [U(COT TIPS2 )(Cp*)(THF)] and other substituted cyclopentadienyl analogues, although 1-3 are formed less cleanly and in lower yields (16-40%) than their purely carbocyclic counterparts. 12 The phospholyl and arsolyl mixed-sandwich complexes (2 and 3) displayed comparable, paramagnetically shifted 1 H and 29 Si{ 1 H} NMR spectra, whereas the pyrrolyl mixed-sandwich complex 1 displayed a different pattern of proton resonances, indicative of a more complex structure in solution (vide infra). All three complexes form stable adducts with THF, 1ÁTHF, 2ÁTHF, and 3ÁTHF, respectively. Mass spectrometry and microanalysis supported the formulation of 1-3, and the molecular structures were confirmed by single crystal X-ray diffraction studies on the THF complexes, and the structures are shown in Fig. 1 with selected data in Table 1.
High resolution data could not be obtained for 1ÁTHF and the molecular structure of this complex therefore only illustrates connectivity. The molecular structure of 2ÁTHF features a phospholyl ring disordered over two positions, which has been modelled accordingly (see ESI † for full details). The three complexes are isostructural, and only small differences are observed between 2ÁTHF and 3ÁTHF, due to the lengthening of the U-E bond on descending the pnictogen group. These structures are similar to their carbocyclic analogue [U(COT TIPS2 )(Cp Me4 )(THF)], demonstrating that incorporation of a pnictogen has not significantly altered the overall structural properties of the complexes. Comparison of 2ÁTHF to the only other mixed-sandwich complex featuring a heterocyclic ligand, the U(IV) complex[U(COT)(Cp PMe4 )-(BH 4 )(THF)], illustrates a similar U-Ct 2 bond length (2.610(8) Å). 6 However, the Ct 1 -U-Ct 2 angle is more acute (135.6(3)1) and the U-Ct 1 distance is longer (2.013(9) Å) presumably due to the presence of the BH 4 group.
The molecular structure of base-free 1 was also determined by single crystal X-ray diffraction, and shows that this complex is dimeric in the solid-state (see Fig. 2). As a consequence of the dimeric structure, the Ct 1 -U-Ct 2 angle is more acute than those in 2ÁTHF and 3ÁTHF, however the U-Ct 1 , U-Ct 2 and U-O bond lengths are similar. Other heterocyclic complexes have also been reported featuring Z 5 :Z 1 coordination, however only [{U(Z 5 -Cp PMe4 )(m-Z 5 :Z 1 -Cp PMe4 )(BH 4 )} 2 ] is comparable to 1. 4 The latter features similar U-Ct 2 distances (2.56(1) and 2.54(1) Å) to 2ÁTHF and similar U-P bond lengths (2.945(3) and 2.995(3) Å), demonstrating that Z 1 -coordination does not affect the Z 5 -bonding. The dimeric structure of 1 presumably persists in solution since it would account for the more complex NMR spectra observed for 1 as opposed to those for monomeric 2 and 3; unfortunately DOSY experiments on 1 were only suggestive of a dimeric structure and its low solubility in suitable solvents precluded cryoscopy. Scheme 1 Synthetic route to uranium(III) mixed-sandwich complexes.

Cyclic voltammetry
In order to compare their U IV /U III redox couples with the carbocyclic analogues, cyclic voltammetry was performed on 1-3. Complex 1 exhibits a distorted quasi-reversible wave at À1.88 V vs. FeCp 2 , which is within the expected range for the U IV /U III redox couple. Complexes 2 and 3 also exhibit an electrochemical event at this approximate potential. However the degree of distortion of the voltammograms becomes more pronounced descending the pnictogen group, precluding accurate determination of E 1/2 . Two other electrochemical events were observed for the three complexes and an additional two events were observed for 1 (see ESI †). These events could however not be unambiguously assigned and demonstrate the complex behaviour of the heteroatom containing mixedsandwich system in the cyclic voltammetry experiment, as opposed to the more straightforward behaviour of the purely carbocyclic complexes. 13 The assumed E 1/2 value of the U IV /U III redox couple for 1 is slightly less negative than that for [U(COT TIPS2 )(Cp Me4 )(THF)] (À2.08 V), demonstrating the increased thermodynamic stability of the U III oxidation state relative to the U IV oxidation state in 1. This is in agreement with other published studies, which found the U IV /U III redox couple is ca. 0.2 V anodically shifted for complexes featuring phospholyl ligands. 14 This arises from loss of degeneracy of the five-membered ring e-symmetry orbitals, causing a decrease in the HOMO-LUMO gap, an effect which has also been observed in transition metal complexes; 15 the low energy vacant orbital in the phospholyl complex 1 (and indeed the N and As analogues) thus likely stabilises the U(III) centre. Hence, whilst complexes 1-3 can still be regarded as potent reducing agents, they are somewhat less powerful than their purely carbocyclic analogues.

Reactivity with CO 2
Addition of excess carbon dioxide to 1 and 2 afforded the complexes [U(COT TIPS2 )] 2 (m-O)(m-O 2 CEC 4 Me 4 ) 2 (E = N (4), P (5)), which are formed by reduction of 0.5 equivalents CO 2 per uranium centre to give the oxo unit. A further equivalent CO 2 is inserted into the U-E bond, giving rise to the carbamate and phosphacarbonate units respectively, so that a total of 1.5 equivalents carbon dioxide are required for the transformation (Scheme 2). The reaction can be conveniently monitored by 13 C NMR using 13 CO 2 , and shows the formation of 4 and 5 by the appearance of resonances at À7.1 and À46.6 ppm corresponding to the carbamate and phosphacarbonate groups, respectively; free 13 CO formed from the reduction of CO 2 to form the bridging oxo unit was also observed in both cases.
Monitoring of the formation of 4 in C 7 D 8 by 1 H NMR spectroscopy revealed its formation to be quantitative; however the thermal instability of this complex resulted in consistently low values of carbon by microanalysis, but 4 did display a parent ion in the mass spectrum (EI). The formation of 5 was found to proceed less cleanly and in lower yield. The 1 H NMR spectrum of 5 in C 7 D 8 at 303 K was broad and with few clearly defined resonances. The spectrum sharpened at 363 K, (possibly due to a fluxional process, the nature of which however could not be established), allowing the assignment of all but the COT ring protons. However, microanalysis and mass spectral data (EI) agreed with the proposed formulation of 5. Attempts to react 3 with carbon dioxide were unsuccessful and resulted in decomposition of the complex to form intractable products.
The proposed structures of 4 and 5 were confirmed by single crystal X-ray diffraction (see Fig. 3 and Table 2), and to the best of our knowledge, 5 represents the first example of a phosphacarbonate ligand bound to a uranium centre. Both complexes are structurally similar, and exhibit slightly shorter U-Ct 1 distances than the parent mixed-sandwich complexes. The oxo unit is   , and asymmetrical bridging carbonate moieties. 16 However, some structural differences are observed between the carbamate and phosphacarbonate units. In 4, the nitrogen lone pair overlaps with the CO 2 unit, evidenced by the short N-CO 2 bonds, and with the pyrrolyl diene unit, which gives rise to near linear Ct 2 -N-C angles (169.1(13) and 179.6(12)1), and a delocalised carbamate moiety with an aromatic pyrrolyl ring. The phosphacarbonate fragment in 5 does not exhibit this feature, and has discrete diene and P-CO 2 moieties and bent Ct 2 -P-C angles (116.6(2) and 116.2(3)1), with trigonal pyramidal geometry around the phosphorus atoms.

Conclusion
Three new mixed-sandwich complexes of the type [U(COT TIPS2 )-(Cp EMe4 )] (where E is N, P or As and COT TIPS2 = C 8 H 6 {1,4-Si i Pr 3 }) have been synthesised featuring a heterocyclic alternative to the cyclopentadienyl ligand. These complexes are structurally comparable to their purely carbocyclic analogues, but feature slightly less negative U IV /U III redox potentials as a result of the heteroatom incorporation in to the 5-membered ring. However, they are still capable of reducing CO 2 , but the presence of the heteroatom also results in CO 2 insertion chemistry and the formation of the first uranium phosphacarbonate complex.

General considerations
All manipulations were carried out under an inert atmosphere of argon using standard Schlenk techniques or under an argon atmosphere in an MBraun glovebox. Solvents were dried over appropriate drying agents (NaK 3 , pentane; K, THF) prior to distillation under N 2 . Solvents were stored over K mirrors or 4 Å molecular sieves. Deuterated solvents were dried over K, vacuum distilled and stored over 4 Å molecular sieves under Ar. NMR spectra were recorded on a Varian VNMR spectrometer operating at 400 MHz ( 1 H). 1 H and 13 C spectra were referenced internally to residual solvent signals, 28 Si spectra were referenced externally to SiMe 4 and 31 P spectra were referenced externally to 85% H 3

X-ray crystallographic studies
Data for 1, 2, 3 and 5 were collected on a Enraf-Nonius CAD4 diffractometer with graphite-monochromated Mo Ka radiation (l = 0.71073) source, and data for 1ÁTHF were collected using a Agilent Technologies Xcalibur Gemini ultra diffractometer with a Cu Ka radiation (l = 1.54184) source at 173 K using an Oxford Cryosystems Cobra low temperature device, operating in o scanning mode with C and o scans to fill the Ewald sphere. The programs used for control and integration were Collect,22 Scalepack and Denzo. 23 Absorption corrections were based on equivalent reflections using SADABS. 24 Data for 4 were collected and processed by the UK National Crystallography Service at the University of Southampton. 25 The crystals were mounted on a glass fibre with silicon grease, from dried vacuum oil kept over 4 Å molecular sieves in an MBraun glovebox under Ar. All solutions and refinements were performed using the WinGX or Olex2 packages and software therein. All non-hydrogen atoms were refined with anisotropic displacement parameters and all hydrogen atoms were refined using a riding model. Disordered solvent molecules were modelled using the SQUEEZE 26 function in PLATON. 27 Crystal structure and refinement data are given in Table S1 of the ESI. † CCDC 1051779-1051784.