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Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
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Silyl Transfer Pathway to a Ce(IV) Imido Complex Thibault Cheisson,† Lukman A. Solola,† Michael R. Gau, Patrick J. Carroll, and Eric J. Schelter* P. Roy and Diana T. Vagelos Laboratories, Department of Chemistry, University of Pennsylvania, 231 South 34th Street, Philadelphia, Pennsylvania 19104, United States
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S Supporting Information *
ABSTRACT: The formation and stabilization of multiple bonds between a lanthanide cation and an anionic p-block element fragment are challenging, and only a few examples of such terminal complexes have been reported to date. Notably, all reported synthetic routes to lanthanide-imido moieties employed deprotonation as the key formation step. In the present report, we describe the generation of a Ce(IV) imido complex by a trimethylsilyl group transfer pathway. Experimental and computational evidence supports an equilibrium proceeding by a bimolecular nucleophilic substitution mechanism. Scheme 2. Synthesis of a Ce(IV) Imido Species (2F4) by (i) Silyl Transfer or (ii) Deprotonationa
Complexes containing rare earth (RE) metal−ligand multiple bonds are rare by virtue of their extreme polarization and limitedor nonexistentcovalent character. In particular, there are only a handful of RE-imido complexes (terminal or bridging).1 Taking advantage of the accessible +IV oxidation state of cerium, we recently synthesized a family of alkali-metal capped and terminal cerium(IV)-imido complexes.2 Notably, all known RE-imido complexes, including ours, have been synthesized by deprotonation routes.1,2 Due to the potential of such moieties to access new manifolds of reactivity as a result of their unique electronic properties,1a it was of interest to explore new synthetic strategies for generating RE-imido complexes. In particular, silyl elimination pathways have been widely employed for the synthesis of group IV imido complexes.3 We and others have also observed, among f elements, such reactivity to yield uranium-imido complexes (Scheme 1).4 In this context, we set out to explore the feasibility of this approach for generating CeIV-imido fragments. As an entry into this chemistry, we synthesized [(TriNOx)Ce(N(Ar F4)(SiMe3))] (TriNOx3− = [{(2-tBuNO)C6H4CH2}3N]3−, ArF4 = 2,3,5,6-tetrafluorophenyl) (1F4; Scheme 2) in 64% yield by salt metathesis from the chloride precursor.2a The 19F{1H}
a Legend: (i) KN(ArF4)(SiMe3)·(S) (5 equiv), C6D6, DME, room temperature, 1 h; (ii) KN(SiMe3)2 (1 equiv), THF, DME, room temperature, 1 h.
NMR spectrum of 1F4 showed four coupled 19F signals (δF −143.47 to −145.78 ppm) in accord with hindered rotation of the aryl moiety about the Cipso−N bond (Figure S6). Other spectroscopic data were closely related to those of the reported complex [(TriNOx)Ce(N(ArCF3)(SiMe3))] (1CF3; ArCF3 = 3,5-bis(trifluoromethyl)phenyl).2a X-ray-quality crystals of 1F4 were grown by slowly cooling a hot n-hexane solution to −25 °C (Figure 1a). For 1F4, bond distances and angles were similar to those of 1CF3.2a Inorganic and organic fluoride salts (e.g., KF, NBu4F, [NBu4][Ph3SiF2]) are commonly employed as desilylation agents.5 However, mixing KF with 1F4 led to the displacement of the amide ligand to yield the previously characterized [(TriNOx)CeF] complex.6 We reasoned that employing the corresponding potassium amide salt (KN(ArF4)(SiMe3)) as the
Scheme 1. Examples of Imido Syntheses by Silyl Transfer/ Elimination Reactions for f-4b and d-Block3b Metals
Special Issue: Organometallic Complexes of Electropositive Elements for Selective Synthesis Received: May 30, 2018
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.8b00366 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 1. Thermal ellipsoid plots (50% probability) of 1F4 (a) and 2F4 (c) and depiction of DFT-optimized TS1− (b). Most hydrogen atoms were omitted; some fragments are depicted in wireframe for clarity. Red arrows indicate the main displacement vectors associated with the transition state. Selected bond lengths (Å) and angles (deg): 1F4, Ce(1)−O(1) 2.189(2), Ce(1)−N(5) 2.392(3), N(5)−Si(1) 1.756(3), Ce(1)−N(5)− C(34) 112.0(2); TS1−, Ce(1)−O(1) 2.213, Ce(1)−N(5) 2.841, N(5)−Si(1) 2.113, Si(1)−N(6) 2.269, Ce(1)−N(5)−C(34) 104.7, N(5)− Si(1)−N(6) 176.9; 2F4, Ce(1)−O(1) 2.272(3), Ce(1)−N(5) 2.090(4), N(5)−K(1) 2.887(4), O(1)−K(1) 2.619(3), F(1)−K(1) 2.773(3), Ce(1)−N(5)−C(34) 157.2(3).
a THF solution of 3 led to a characteristic dark purple mixture. Recrystallization from DME afforded 2F4 in 35% crystalline yield (Scheme 2). X-ray diffraction studies on single crystals of 2F4 revealed a solid-state structure related to that of 2CF3 (Figure 1c). As already observed, the potassium cation was coordinated to the TriNOx3− framework and the imido fragment forming a 4-membered ring. This cycle was distorted in comparison with 2CF3: the K(1)−N(5) distance was elongated by ca. 0.15 Å, whereas the imido Ce(1)−N(5) bond decreased by ∼0.03 Å. The Ce(1)−N(5) bond in 2F4 is the second shortest cerium−imido bond crystallographically recorded and the shortest alkali-metal-capped bond.2b Although the geometric variations between 2CF3 and 2F4 were small, possible explanations for the observed differences included (i) the differing electronic properties of the ArF4 versus ArCF3 groups7 and (ii) the presence of a strong F→K interaction (Figure 1c, K(1)−F(1) 2.773(3) Å)8 that likely decreased the effective charge of K(1) and consequently reduced the interaction with N(5), to enforce the Ce(1)− N(5) interaction. In order to address any differences in the bonding and the electronic structure of 2F4 (in comparison to 2CF3), we turned our attention to computation and performed an NBO 6.0 analysis of its DFT-optimized computational surrogate IIF4 (see Figures S15 and S16). Results for the N(5)−Ce(1) interaction were, in all points, similar to those obtained for 2CF3.2a Namely, two bonding interactions of respective σ and π symmetry polarized toward N(5) (87 and 84% respectively) were evident (Figures S15 and S16). The cerium 5d/4f orbital contributions to these interactions (57/31 and 47/53, respectively) were consistent with our previous observations.2 The Mayer bond order (MBO) for Ce(1)−N(5) was computed at 1.58, in agreement with the computed MBO of 1.52 for 2CF3. Finally, the negative charge was largely delocalized on the para position (qC37 = −0.42 e, Figure S17), a likely reason for the shielding of the resonance for H(37) in the 1H NMR spectrum of 2F4 (δH (ppm, THF-d8): 4.73 (2F4), 5.73 (3), and 6.46 (ArF4NH2)). A comparison of the 19F{1H} and 1H NMR spectra of the crude reaction mixture obtained by treatment of 1F4 with
nucleophile would prevent deleterious substitution reactions. Moreover, the electron-withdrawing aryl moiety was expected to increase the electrophilicity of the trimethylsilyl group, as we observed with the analogous salt KN(C6F5)(SiMe3) in related chemistry.4b Treating a red C6D6 solution of 1F4 with 5 equiv of KN(ArF4)(SiMe3) and 5 drops of DME (Scheme 2) led to gradual formation of a new CeIV species, which was later identified as the expected imido complex [K(DME)2][(TriNOx)Ce=NArF4] (2F4). After 1 h, the conversion of 1F4 to 2F4 stabilized at ∼25% and did not increase further. Notably, an excess of the potassium salt (KN(ArF4)(SiMe3)) was necessary to achieve conversion. Increasing the reaction time or temperature did not improve the yield of 2F4 but, rather, increased the formation of degradation products such as the anilido complex 3 (vide infra). The disilazane byproduct N(ArF4)(SiMe3)2 was also observed by both 1H and 19F{1H} NMR spectroscopy and its identity confirmed by an independent synthesis (Figures S12 and S14). However, the stoichiometry of 2F4 and N(ArF4)(SiMe3)2 was not perfectly equimolar; ∼0.3 equiv of the latter was observed (see NMR data in the Supporting Information). The fate of the remainder of N(ArF4)(SiMe3)2 is, as yet, unclear. However, the appearance of the anilide complex 3 indicates that further chemistry occurs in the reaction mixture that we expect accounts for the inconsistent stoichiometries of the products. Similar results were obtained when 1CF3 was treated with excess KN(ArCF3)(SiMe3).2a The reaction between KN(ArCF3)(SiMe3) and 1F4 induced simple amide metathesis. Using the more common ArF5 (C6F5) substitution pattern resulted in the formation of a complex tetrameric perfluorinated compound through an as yet unclear mechanism. In order to confirm the formation of 2F4, we performed an independent synthesis employing our previously described protonolysis route.2 The tetrafluoro-anilide complex 3 was first isolated in 81% yield and characterized by multinuclear NMR and X-ray crystallography (Figure S1). Notably, the 19F{1H} spectrum of 3 consisted of two coupled signals, agreeing with postulation of free rotation about the NH−ArF4 bond. Subsequently, addition of a THF solution of KN(SiMe3)2 to B
DOI: 10.1021/acs.organomet.8b00366 Organometallics XXXX, XXX, XXX−XXX
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excess KN(ArF4)(SiMe3) indicated resonances that were identical with those of isolated 2F4, thus confirming the existence of a desilylation pathway (Figures S12−S14). However, some questions remained: in particular, regarding the mechanism and the low conversion associated with that transformation. A DFT investigation of a putative reaction coordinate afforded a transition state (TS1−, Figure 1b) for the reaction between IF4 and (ArF4)(SiMe3)N− located 22.0 kcal mol−1 (ΔG⧧) above the reactants.9 The geometry and the imaginary frequency associated with TS1− demonstrated a SN2-type transition state through a trigonal-bipyramidal silicon atom (τ5 = 0.88).10 Notably, the λ5-Si geometry is only associated with the transition state and is not a stable intermediate.11 Following the intrinsic reaction coordinate confirmed TS1− to link the reactants and the products (IF4− and (ArF4)(SiMe3)2N; see Figure S18). Computing the relative energy of the products revealed an overall endergonic reaction by ∼5.7 kcal mol−1. However, including the potassium cation, with the reactants and products, returned a slightly favorable process (ΔG = −1.1 kcal mol−1; see Figure S19). Including the potassium atom in TS1− was attempted but was not achievable at reasonable computational cost. Other effects, such as the solvation of K(1), undoubtedly also contribute to the overall reaction profile but were not further investigated. Qualitatively, the DFT study affords the conclusion that an equilibrium reaction that is accessible at room temperature (ΔG⧧ ≈ 22 kcal mol−1) and is nearly thermoneutral (ΔG° ≈ 0 kcal mol−1). These findings were in good agreement with the experimental observations (vide supra). The existence of this path is not surprising, since it fundamentally constitutes the microscopically reverse reaction of the (irreversible) silylation of 2CF3 by SiMe3Cl.2a In conclusion, we demonstrated thatas in d-block metals and actinidesa cerium(IV)-imido complex can be generated by a desylilation or silyl transfer pathway. DFT studies helped illuminate the probable mechanism, which is best described as a SN2-type reaction. Despite the poor efficiency of the described silyl transfer route in comparison to the seminal deprotonation method, the present work demonstrates that the chemistry of Ce(IV)-imido moieties is not only limited to highly basic environments. The existence of silyl transfer equilibria further motivates investigating the reactivity of such systems, work that is currently underway in our laboratory.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for E.J.S.:
[email protected]. ORCID
Thibault Cheisson: 0000-0003-4359-5115 Eric J. Schelter: 0000-0002-8143-6206 Author Contributions †
T.C. and L.A.S. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge and thank Dr. George T. Furst (1946−2018) for his assistance in recording NMR data presented in this paper and for his tireless support of the UPenn NMR facility. We thank the National Science Foundation (CHE-1664928) and the University of Pennsylvania for financial support to this work. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by NSF (ACI-1548563). L.A.S. thanks the NSF Graduate Research Fellowship program for support. T.C. acknowledges the Camille and Henry Dreyfus Foundation for a postdoctoral fellowship.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00366. Experimental, crystallographic, and computational details (PDF) Cartesian coordinates for DFT-optimized structures (XYZ) Accession Codes
CCDC 1829905−1829907 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. C
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DOI: 10.1021/acs.organomet.8b00366 Organometallics XXXX, XXX, XXX−XXX
