Structure and Reactivity of Chromium (VI) Alkylidenes

Np3Ta=CHtBu,1 transition metal alkylidenes have be- come an iconic class of organometallic compounds. Their utility as catalysts for the olefin metath...
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Structure and Reactivity of Chromium(VI) Alkylidenes. Pengcheng Wu, Glenn P. A. Yap, and Klaus H. Theopold J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04882 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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Structure and Reactivity of Chromium(VI) Alkylidenes. Pengcheng Wu, Glenn P. A. Yap, and Klaus H. Theopold* Department of Chemistry and Biochemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, DE, 19716, USA.

Supporting Information Placeholder ABSTRACT: Bis(arylimido)Cr(VI) dialkyls lacking -

hydrogen decompose by -hydrogen abstraction and, upon trapping with triphenylphosphine, yield isolable alkylidene complexes. Two such complexes, namely (ArN)2Cr=CHR(PPh3) (R= tBu, SiMe3) have been structurally characterized. The coordinatively unsaturated alkylidene intermediates are highly reactive; they effect CH activation of saturated hydrocarbons and they react with olefins to produce metallacyclobutanes.

In the decades since Schrock’s seminal discovery of Np3Ta=CHtBu,1 transition metal alkylidenes have become an iconic class of organometallic compounds. Their utility as catalysts for the olefin metathesis reaction has been widely recognized.2-4 While many d-block elements support the formation of metal-carbon multiple bonds not stabilized by heteroatom substitution at the carbon, the majority of such compounds contains the heavier group 6 elements (Mo and W), predominantly in their +VI oxidation state.5 The absence of any structurally characterized Cr(VI) alkylidene may thus be surprising, even though the relative instability of the highest formal oxidation state and the weaker metal-carbon bonds of the first-row metal generally make for sparse organometallic chemistry of hexavalent chromium.6-22 Gibson et al. have communicated the synthesis of the neopentylidene complexes (ArN)2Cr=CHtBu(L) (Ar = 2,6-iPr2C6H3, L = THF, PMe3);10 however, structural characterization of these species “proved elusive” and their chemistry seemed abandoned.11 As part of our extended survey of organochromium chemistry, we have now revisited them. Herein we report the first molecular structures of Cr(VI) alkylidenes and their reactions with alkanes and alkenes. The simplest synthetic approach to alkylidenes involves the -hydrogen abstraction from metal dialkyls that are not subject to the more facile -hydrogen abstraction. Accordingly, we have prepared chromium(VI) alkyls of the type (ArN)2Cr(CH2R)2 (R = H, tBu, Ph, SiMe3, see Scheme 1). The syntheses of 1-3 had previously been reported,10,22 but 4 is a new compound; the molecular

structures of 3 and 4 have been determined by X-ray diffraction and are included in the ESI. They are pseudotetrahedral molecules with short Cr-N bonds (1.64 – 1.65 Å) and Cr-C distances consistent with single bonds (2.01 – 2.08 Å). Scheme 1. Chromium(VI) Alkyls and Alkylidenes (Ar = 2,6-iPr2C6H3).

As described by Gibson et al., 2 slowly (over 3 days) eliminates neopentane at room temperature, and the resulting neopentylidene can be trapped by Lewis bases (THF, PMe3).10 We have now found that the reaction with PPh3 in THF yielded the compound (ArN)2Cr=CHtBu(PPh3) (5), which gave red crystals suitable for a structure determination by X-ray diffraction. The molecular structure of 5 is shown in Figure 1. The complex is pseudo-tetrahedral, with the largest bond angle – i.e., N1-Cr-N2, 124.26(18)o – being significantly opened up compared to dialkyl 2 (111.71(8)o). Only one hydrocarbyl ligand remains bonded to chromium and its Cr-C bond distance of 1.848(5) Å is considerably shorter

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than the single bond distances in 2 (2.030(2) and 2.042(2) Å), consistent with the assignment of a double bond. The lone -hydrogen atom of the neopentylidene ligand was located on a difference map and its location was refined. The resulting sum of the bond angles about the metalbound carbon (357.1o) suggests planar sp2-hybridization, while the Cr-C1-C2 angle of 137.4(4)o does not intimate an agostic Cr…H…C1 interaction. The relevant spectroscopic data of 5 – 1H NMR (THF-d8):  14.97 ppm (1JCH = 126.1 Hz, 3JPH = 5.6 Hz); 13C NMR:  341.7 ppm (2JPC = 21.4 Hz) – are similar to those reported by Gibson et al. and entirely consistent with an alkylidene complex.

Figure 1. The molecular structure of neopentylidene complex (ArN)2Cr=CHtBu(PPh3) (5); selected interatomic distances (Å) and angles (o): Cr-N1 1.675(3), Cr-N2 1.656(4), Cr-C1 1.848(5), Cr-P1 2.3647(13); N1-Cr-N2 124.26(18), N1-Cr-C1 107.2(2), N1-Cr-P1 107.81(13), N2-Cr-C1 107.4(2), P1-Cr-C1 100.91(16), Cr-C1-C2 137.4(4).

It was then of obvious interest to explore the formation of analogous alkylidenes from the other dialkyls. 4 showed no evidence of elimination of SiMe4 at ambient temperature. However, upon heating a THF solution in the presence of one equivalent of PPh3 to 80oC for 10 days, alkylidene complex (ArN)2Cr=CHSiMe3(PPh3) (6) was eventually obtained. The NMR chemical shifts of 6 – 1H NMR (C6D6):  17.60 ppm (1JCH = 119.9 Hz, 3JPH = 7.6 Hz); 13C NMR:  340.6 ppm (2JPC = 21.0 Hz); 29Si NMR:  - 4.59 ppm (3JSiP = 3.24 Hz) – were very similar to those of 5, save for the rather more downfield 1H NMR shift of the -proton. Being only the second example of an isolable Cr(VI) alkylidene, the molecular structure of 6 was also determined by X-ray diffraction (see Figure S19 in the ESI). Much like 5, 6 adopts pseudo-tetrahedral

coordination of Cr, with N1-Cr-N2, 122.60(14)o once again being the largest bond angle. The Cr=C bond (1.843(4) Å) is short and the Cr-C-Si angle of 133.1(2) is consistent with sp2-hybridization of the alkylidene carbon. We attribute the more facile formation of 5 to greater steric repulsions in 2; we note that this echoes the relative stability of TaNp5 and Ta(CH2SiMe3)5.23 In our hands neither 1 nor 3 have yielded isolable alkylidenes. Upon prolonged heating in the presence of PPh3, solutions of these compounds monitored by 1H NMR spectroscopy produced some methane or toluene, respectively, but the reactions gave mixtures of organometallic reaction products, which proved intractable. We believe that 5 and 6 are formed by rate-determining -abstractions, followed by rapid trapping of the three-coordinate alkylidene intermediates (ArN)2Cr=CHR with PPh3. There is some reason to believe that the initial -abstraction step may be reversible. Thus, when 2 is dissolved in C6D6, it slowly converts into (ArN)2CrCHDtBu(C6D5), i.e. the likely product of a C-D bond activation by the 3-coordinate neopentylidene intermediate.10 This observation raises the question whether the coordinatively unsaturated alkylidenes might also activate the C-H bonds of saturated hydrocarbons. To answer this question, we have studied the reaction of 2 with SiMe4. After 3 days at room temperature a solution of 2 in pure SiMe4 was quantitatively transformed into the mixed alkyl (ArN)2Cr(CH2tBu)(CH2SiMe3) (7), as shown by NMR spectroscopy and LIFDI-MS. Exchange of the second neopentyl group was slower – presumably due to lesser steric pressure in 7, but after 17 days at 45oC all starting material had been cleanly converted into 4 (see Scheme 2). While a mechanism involving a -bond metathesis cannot presently be ruled out, the observation that the various reactions of 2 all proceed at qualitatively the same rate suggests a common rate-determining step and intermediate. Accordingly, we suggest that the alkyl group exchange goes via -elimination of neopentane, followed by C-H activation of SiMe4 by the neopentylidene intermediate, yielding 7. A second round of neopentane elimination – to form (ArN)2Cr=CHSiMe3 – and CH activation of SiMe4 then yields 4. Metal alkylidenes have previously been shown to react with unactivated hydrocarbons.24-32 Our results suggest that this mode of reactivity extends to Cr(VI) alkylidenes. Scheme 2. Alkyl Exchange via C-H Activation

The most prominent application of metal alkylidenes is the catalysis of the olefin metathesis reaction.33 Novel types of alkylidenes may offer improved reactivity profiles, and therefore an exploration of the utility of

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Cr(VI) alkylidenes in this context is of interest. Neither 5 nor 6 reacts with alkenes, suggesting that tight binding of the phosphine prevents coordination of the potential substrate. However, a green solution of 2 in cyclopentene turned red after 3 days at ambient temperature. Standard workup yielded a new organometallic product, which was characterized spectroscopically and by X-ray diffraction. The molecular structure of the new compound is shown in Figure 2; it is the bicyclic metallacyclobutane 8 resulting from a [2+2] cycloaddition of the neopentylidene (ArN)2Cr=CHtBu and one molecule of cyclopentene.34 The four-membered metallacycle is essentially planar and the two Cr-C bonds (1.932(4) and 1.965(4) Å) are remarkably short, despite involving secondary carbons. The cyclopentyl ring and the tert-butyl substituent are in trans positions to minimize steric interactions. 8 did not catalyze the polymerization of cyclopentene, nor did a solution of 2 in norbornene, when heated to 60oC, show any evidence of ROMP catalysis.35 Thus it appears that the ready transformation of the neopentylidene into chromacyclobutanes is effectively irreversible, presumably due to substantially greater thermodynamic stability of the latter. Future experiments will be directed at shifting this equilibrium, so as to facilitate metathesis catalysis.36-37

compounds and their applications in homogeneous catalysis. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Experimental procedures, spectroscopic information for 4-8, and pictures of the molecular structures of 3, 4, and 6 (PDF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF) Crystallographic data for 5 (CIF) Crystallographic data for 6 (CIF) Crystallographic data for 8 (CIF)

AUTHOR INFORMATION Corresponding Author

[email protected] ORCID Klaus H. Theopold: 0000-0001-5168-1625 Glenn P. A. Yap: 0000-0003-0385-387X Notes

The authors declare no competing financial interest. The supplementary crystallographic data for all structure determination as available free of charge from the Cambridge Crystallographic Data Centre (https://www.ccdc.cam.ac.uk/) under the reference numbers CCDC 1831800 (3), 1831801 (4), 1831804 (5), 1831802 (6), and 1831803 (8).

ACKNOWLEDGMENT We gratefully acknowledge NSF (CHE-1565955) for funding. Support for shared instrumentation was provided by NSF (CHE-1048367 and CHE-0840401) and NIH (P20GM104316, P30GM110758, S10RR026962-01).

REFERENCES

Figure 2. The molecular structure of metallacyclobutane 8; selected interatomic distances (Å) and angles (o): Cr-N1 1.656(4), Cr-N2 1.662(3), Cr-C25 1.932(4), Cr-C30 1.965(4), C25-C26 1.582(6), C26-C30 1.595(6); N1-Cr-N2 117.2(2), N1-Cr-C25 109.42(18), N1-Cr-C30 111.4(2), N2Cr-C25 109.69(17), N2-Cr-C30 113.0(9), C25-Cr-C30 87.61(18).

We have structurally characterized the first alkylidenes of hexavalent chromium. Coordinatively unsaturated species of the composition (ArN)2Cr=CHR are highly reactive, engaging in the C-H activation of hydrocarbons and adding to simple olefins to produce metallacycles. We are now exploring the preparation of related

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