Synthesis and Characterization of Cerium(iv) Metallocenes

By applying a salt metathesis approach between Ce(O t Bu3)2(NO3)2(THF)2 and the potassium salts of mono-and ditrimethylsilyl substituted cyclopentadienes, we were able to isolate two new Ce(IV) metallocenes, including to the best of our knowledge, the first structurally characterized bis-cyclopentadiene Ce(IV) compound.


Introduction
Cerium(IV) is a strong one-electron oxidant and its use in areas such as organic synthesis [1][2][3], bioinorganic chemistry [4,5], materials science [6] and industrial catalysts such as automotive catalytic converters [7][8][9][10][11][12][13] is widespread.Soluble cerium(IV) complexes are also important precursors for the preparation of ceria nanoparticles [14] and this has increased the demand for well-characterized precursors for the development of this area.Ce(IV) alkoxides are well suited for this purpose and reports of Ce(IV) alkoxides exist where these have exhibited utility as precursors in CVD applications [15] and in ring-opening polymerization of lactide [16].The synthesis of Ce(IV) organometallic complexes, however, is limited by the lack of available starting materials and this has constrained the growth and development of traditional organometallic chemistry and reactivity using this high-oxidation state lanthanide [17].Two main approaches exist as entry points into Ce(IV) chemistry; the oxidation of Ce(III) compounds and the use of materials such as Ce(OPr i )4, Ce(OTf)4, cerium ammonium nitrate (CAN) derived [Ce(O t Bu)2(NO3)2(THF)2] [18] and most recently, the amide complex, Ce[(N i Pr2)4] [19].The oxidation of Ce(III) is challenging and the results are often unpredictable due to ligand redistribution reactions.For example, CeX[N(SiMe3)2]3 can be prepared from Ce[N(SiMe3)2]3 using TeCl4 or PBr2Ph3 as oxidants for X = Cl, Br respectively in low yields (Cl: 24%, Br: 30%) [20,21] but these are unstable is solvents other than THF.In contrast, TeCl4 was ineffective in the oxidation of Ce[(OCMe2CH2(1-C{NCHCHNPr i ])], while benzoquinone readily oxidized it to the tetravalent cerium carbene Ce[(OCMe2CH2(1-C{NCHCHNPr i ])]4 [22].Benzoquinone was also used effectively to oxidize [Ce{N(SiMe3)2}3] to form the new cerium(IV) silylamide complex [Ce{N(SiMe3)2}3(bda)0.5]2(bda = 1,4-benzenediolato) [23].While cyclopentadienyl Ce(IV) complexes have been characterized spectroscopically [24,25] or even claimed [26] and refuted [27], to the best of our knowledge and with the aid of the Cambridge Crystallographic Database (CCDC), we are only aware of two reports of structurally characterized Ce(IV) metallocene complexes.The first was reported by Evans et al. in 1989 and was prepared from Ce(O t Bu3)n(NO3)m(THF)2 (m = 2 or 3; n + m = 4) and mNaCp to give Cp2Ce(O t Bu3)2 and Cp3Ce(O t Bu3), the latter being crystallographically characterized [28].These complexes utilized a soluble Ce(IV) alkoxide nitrate species which is readily prepared from cerium ammonium nitrate (CAN) [18].Following this in 2010, Anwander and Edelmann reported the oxidation of CeCp3 with PhICl2 to form Cp3CeCl [29].Herein we present further expansion of known cyclopentadiene Ce(IV) complexes, with the first structurally characterized biscyclopentadiene Ce(IV) compound, and we will briefly discuss attempts to perform subsequent transformations of these relatively rare compounds.

Preparation of Cerium(IV) Metallocenes
Our initial goal was to access a system containing sterically demanding ligands that would direct chemistry to the ancillary ligand sites.To this end we decided to utilize -SiMe3-substituted cyclopentadienyl ligands.We added two equivalents of KCp′ (Cp′ = C5H4SiMe3) to a solution of Ce(O t Bu3)2(NO3)2(THF)2 in THF and observed an immediate color change from orange to deep red.The solution was allowed to stir overnight and the solvent was removed.The solid was extracted with HMDSO, filtered and a microcrystalline purple solid was obtained after storage at −20 °C for several days.This was analyzed by 1 H-and 13 C{ 1 H}-NMR spectroscopy which permitted the characterization of 1 as (Cp′)2Ce(OCMe3)2, produced in reasonable yield (Scheme 1).Despite numerous attempts using various solvents and solvent combinations, single crystals for X-ray crystallography could not be obtained.
The same methodology was applied to the 1,3-substituted cyclopentadiene, KCp″, (Cp″ = C5H4(SiMe3)2-1,3) (Scheme 1) and when extracted with hexamethyldisiloxane (HMDSO) this afforded red single crystals which enabled us to crystallographically characterize 2 as (Cp″)2Ce(O t Bu3)2 (Figure 1).The structure of 2 was solved in monoclinic P21/c and exhibits a pseudo-tetrahedral geometry with two coordinated Cp″ ligands and two coordinated O t Bu ligands.Selected bond lengths and angles are provided in Table 1.Due to the steric bulk associated with the TMS groups, the angle between the rings is more obtuse causing a deviation from true Td geometry with respect to the O-Ce-O angle between the two O t Bu groups (103.9°).The Ce-centroid distances in 2 (2.511 Å average) are comparable to those in Cp3Ce(O t Bu3) (2.509 Å average) [28] but significantly longer than those reported for Cp3CeCl (2.460 Å average) [29] presumably due to the added steric congestion closer to the metal center resulting from the O t Bu groups.The Ce-O bond lengths in 2 (2.077(5) and 2.097(6) Å) are slightly elongated compared to that in Cp3Ce(O t Bu3) (2.045(6) Å) [28] which is most likely a steric effect rather than an electronic effect due to the comparable Ce-centroid distance between substituted and unsubstituted ring systems.
or or Scheme 1. Reaction scheme for the preparation of 1 and 2.
Figure 1.Thermal ellipsoid plot of the structure of 2 (hydrogen atoms omitted ellipsoids at 30% probability).

Attempted Reactivity of 1 and 2
In order to probe the reactivity of 1 and 2, and synthesize additional cyclopentadienyl compounds of Ce(IV), each was reacted in tetrahydrofuran or toluene with one and two equivalents of KOTf, TMS-Cl, Ph3SiOH, MeNH2, Et3N, MeMgBr, Bu2Mg with no reaction observed by NMR spectroscopy.Reaction with dilute HCl as a solution in Et2O led to reduction of both 1 and 2 as evidence by paramagnetic NMR spectra although no tractable product could be isolated and characterized.

Experimental Section
Unless otherwise noted, all reactions were performed using standard Schlenk-line techniques or in a Vacuum Atmosphere glove box.All glassware and Celite were stored in an oven at ca. 425 K. Hexamethyldisiloxane was distilled and degassed before use.Hexane, toluene, and THF were freshly distilled from sodium and degassed with nitrogen prior to use.C6D6 was vacuum transferred from sodium/benzophenone. NMR spectra were recorded at ambient temperature on Bruker AV-400. 1 H and 13 C{ 1 H} chemical shifts are given relative to residual protic solvent peaks and coupling constants (J) are given in Hz.Infra-red samples were prepared as Nujol mulls and taken between KBr plates.Melting points were determined using sealed capillaries prepared under nitrogen and are uncorrected.Unless otherwise noted, all reagents were acquired from commercial sources and used as received.
A crystal of 2 was mounted in a nylon cryoloop from Paratone-N oil.The data were collected on a Bruker D8 diffractometer, with an APEX II charge-coupled-device (CCD) detector, and a Bruker Kryoflex liquid nitrogen low temperature device (140 K).The instrument was equipped with a graphite monochromatized MoKα X-ray source (λ = 0.71073 Å), and a 0.5 mm monocapillary.A hemisphere of data was collected using ω scans, with 10-s frame exposures and 0.5° frame widths.Data collection and initial indexing and cell refinement were handled using APEX II [30] software.Frame integration, including Lorentz-polarization corrections, and final cell parameter calculations were carried out using SAINT+ [31] software.The data were corrected for absorption using redundant reflections and the SADABS [32] program.Decay of reflection intensity was not observed as monitored via analysis of redundant frames.The structure was solved using Direct methods and difference Fourier techniques.One t-butyl group was disordered, and the methyl groups were refined in two positions (C28/C28′, C29/C29′, and C30/C30′).Site occupancy factors for disordered pairs were tied to 1.0.A total of 31 bond distance and temperature factor restraints were used to force convergence of the disordered group.All hydrogen atom positions were idealized, and rode on the atom they were attached to.The final refinement included anisotropic temperature factors on all non-hydrogen atoms.Structure solution, refinement, graphics, and creation of publication materials were performed using SHELXTL [33] software.Additional details can be found in crystallographic information file (CCDC No. 1433004) and in the Supplementary Information.

Preparation of (Cp′)2Ce(O t Bu)2 (1)
KCp′ (0.352 g, 2.0 mmol) was dissolved in THF (2.5 mL) and added to a suspension of Ce(O t Bu)2(NO3)2(THF)2 (0.555 g, 1.0 mmol) in THF (2.5 mL), resulting in an immediate color change to deep red.The reaction was allowed to stir overnight and the solvent removed in vacuo.The resultant