Decreasing the Bond Order Between Vanadium and Oxo Ligand to Form 3d Schrock Carbynes

Abstract

Preserving vanadium in a high oxidation state during chemical transformations can be challenging due to the oxidizing nature of V(+5) species. Oxo and similar isoelectronic ligands have been utilized to stabilize V(+5) by extensive π-donation. However, decreasing the bond order between V and the oxo ligand often results in reducing the metal center. Herein, we report a unique transformation involving anionic V(+5) alkylidene that converts a V(+5) oxo complex to a V(+5) alkylidyne in three steps without altering the oxidation state of the metal center. This method has been used to obtain rare 3d Schrock carbynes, which provides easy and scalable access to V(+5) alkylidynes.

Graphical Abstract

High oxidation state vanadium(V) complexes play an essential role in numerous catalytic transformations, primarily oxidation reactions,^1-8 both in industry⁹ and nature.^{10, 11} The fundamental property that is responsible for this reactivity is an easily attainable variety of lower oxidation states for V. Other catalytic transformations, including olefin and alkyne polymerizations, catalyzed by V(+5) complexes are also thought to proceed via reduced V species.^{12, 13} In contrast, the preservation of V in a high oxidation state during chemical transformations can present a challenge due to the oxidizing nature of V(+5). Thus, studies on V-based olefin metathesis showed that the reduction of V is a primary degradation pathway for d⁰ V alkylidene catalysts.^14-19

One strategy to stabilize a metal center in a high oxidation state is introducing a ligand that can form multiple bonds with a metal and participate in extensive π-donation.²⁰ Indeed, V(+5) complexes that do not contain multiple bonds are scarce.^{21, 22} VCl₅ decomposes above −10 °C.²³ VF₅ is a stable compound²⁴ that readily reacts with F₃P=O to give F₃V=O and PF₅ highlighting the strength of the V=O bond.^{25, 26} Oxo ligand (V=O)²⁷ is the most common moiety for V(+5) complexes, many of those compounds are commercially available. Other isoelectronic ligands have also been utilized (V=X, where X = NR, S, Se, Te), and usually introduced in reactions with corresponding V oxo starting materials.^{12, 28}

V(+5) complexes that do not contain oxo and related ligands are rare and usually prepared via oxidation of low valent counterparts.^29-32 Thus, all reported V(+5) alkylidynes are based on β-diketiminate (Nacnac) ligand and synthesized from VCl₃ utilizing the method discovered by the Mindiola group in 2004.^{13, 30, 31} The key transformation in a six-steps synthesis of (Nacnac)V≡C^tBu(OTf) is α-hydrogen abstraction initiated by oxidation of V(+4) alkylidene complex (Scheme 1). All intermediates are paramagnetic, which hinders the development of alternative pathways and optimized conditions.

Scheme 1.

Previous strategy and this work.

V(+5) alkylidynes present the significantly underdeveloped class of 3d Schrock carbynes that can be utilized in numerous transformations,^{13, 33} including alkyne metathesis.^34-40 However, no alternative pathways to V(+5) alkylidynes have been proposed in the last 20 years, highlighting the challenges of 3d carbyne chemistry. In this study, we have developed a simple, scalable, and reproducible method for converting a V(+5) oxo complex to a V(+5) alkylidyne in three steps without altering the oxidation state of the metal center. This method involves a unique transformation using the anionic V alkylidene intermediate (Scheme 1), which ultimately leads to a reduction in the bond order between V and the oxo ligand. This transformation is rare, considering the high strength of the V=O bond.

Recently, we developed a series of V oxo alkylidenes, a new class of compounds that exhibit unique reactivity in olefin metathesis.^{18, 41} The key starting material for the synthesis of those catalysts is VO(CH₂TMS)₃ (1). Complex 1 was made from the oxidation reaction between V(CH₂TMS)₃•THF and styrene oxide. The reaction was challenging and suffered from scalability and reproducibility. To improve the synthesis of 1 we screened several reaction conditions and reagents, including commercially available V(+5) oxo complexes and alkylating agents (TMSCH₂Li, TMSCH₂MgCl, (TMSCH₂)₂Zn, (TMSCH₂)₃Al). Scheme 2 shows the optimal conditions for scalable synthesis of 1.

Scheme 2.

Optimized scalable synthesis of 1 from commercially available starting materials.

The key finding is the use of 2.3 equiv. of TMSCH₂MgCl (instead of 3.0 equiv.) and VO(OⁱPr)₃ at −78 °C to avoid the reduction of V species, which can be easily seen by the appearance of green color during the reaction.

Serendipitously, during the screening of optimal conditions, we observed a new peak in the alkylidene region by ¹H NMR in the reactions where the excess of alkylating agents was used. In separate experiments, we found that 1 reacts with strong bases, including MN(TMS)₂ (M = Li or K), or organolithium reagents (RLi), to form V anionic alkylidene 2 (Scheme 3A, Figure S1). The reaction is reversible. Thus, the addition of 1 equiv. of Ph₃SiOH to 2 leads to the formation of 1 (Figure S2). Base screening showed that t-BuOK can deprotonate 1 (with the following decomposition of 2 in the presence of t-BuOH), but Ph₃SiOK does not lead to alkylidene formation. Therefore, the estimated pKa of 1 is between 16–18 (H₂O).⁴²

Scheme 3.

A: Reversible intermolecular α-hydrogen abstraction to form anionic V alkylidene (cation is not shown). B: Synthesis and X-ray structure of 2•dtbbpy. Thermal ellipsoids are shown at 30% probability, and hydrogen atoms are omitted for clarity. C: Comparison of structural parameters of 2•dtbbpy with reported four-coordinate V oxo alkylidene 3.⁴¹

The intermolecular α-hydrogen abstraction to form alkylidenes from metal alkyl complexes is unusual. To our knowledge, there are no reported instances of anionic alkylidenes forming from metal oxo alkyl species. This transformation is exciting by itself from a fundamental standpoint and can provide some crucial insights about the classical intramolecular α-hydrogen abstraction reaction.

Complexes 2 (as a Li or K salt) are stable and isolable but not crystalline, which hinders their purification and characterization. DOSY NMR experiment suggests that 2 (Li salt) forms oligomers in benzene solution (Figure S6). Fortunately, we could isolate crystalline Li salt of 2 as a 4,4′-bis(tert-butyl)-2,2′-bipyridine complex (2•dtbbpy) in 88% isolated yield (Scheme 3B).

The X-ray structure of 2•dtbbpy revealed a dimer with bridged Li cations with a slightly distorted tetrahedral geometry at the V center. The comparison of structure details of 2•dtbbpy to the only known four-coordinate tetrahedral V oxo alkylidene 3 prepared by our group⁴¹ confirmed the presence of one alkylidene and two alkyl groups (Scheme 3C). The V─O bond in 2•dtbbpy is elongated compared to the V=O bond in complex 3 (1.6684(9) Å vs. 1.588(5) Å) but still shorter compared to the V─O bond in alkoxide (1.845(5) Å). It is worth noting that the V=C─Si angle (143.57(8)°) is significantly higher than the V=C─Si angle in 3 (130.5(5)°). This can be explained by a greater extent of α-hydrogen agostic interaction in 2•dtbbpy,^{17, 18, 41} but unfortunately, due to the broadened alkylidene peak, we were unable to determine the J_CH constant.

NBO calculations further support the structure of 2•dtbbpy. Thus, the computed V─O bond order is 0.99, with the charges on the vanadium and oxygen atoms of +1.15 and −0.88, respectively (Figure S7). Bond orders of V=C and V─C are 1.91 and 0.97, respectively (Figure S8).

We assumed that the resulting complex resembles a simple carbon-based enolate anion and might react with electrophiles, for example, with silyl chlorides, to provide siloxide complexes in analogy to the formation of enol silyl ethers, taking advantage of the strength of Si─O bond. Indeed, complexes 2 (as a Li or K salt) slowly react with silyl chlorides to form new alkylidenes by ¹H NMR that we assign as V alkylidene siloxide complexes (Figure S3). Similar to 2, the resulting complexes are not crystalline, and, unfortunately, we could not study their structural parameters.

Surprisingly, the reaction between 2•dtbbpy and silyl chlorides provides V alkylidyne complexes 6a-c (Scheme 4). We speculate the mechanism includes coordination of dtbbpy to V siloxide complex 4 that leads to 5 followed by intramolecular α-hydrogen abstraction to form alkylidynes 6a-c.

Scheme 4.

Synthesis of V alkylidyne complexes 6a-c and proposed intermediates.

Complexes 6b (Figure S6) and 6c were successfully crystallized and studied by X-ray crystallography. The X-ray structure of 6c revealed that it has a distorted square pyramidal geometry (τ = 0.18) with alkylidyne in the apical position (Figure 1). The V≡C bond (1.707(6)Å) in five-coordinate 6c is slightly longer compared to the V≡C bond in reported four-coordinate V alkylidyne (1.674Å),³⁰ and significantly shorter than the typical V=C bond (Scheme 4). The V≡C─Si angle is 167.6(4)°, smaller than in the reported V alkylidyne (177.6°),³⁰ but significantly larger than V=C─Si in 2•dtbbpy (143.57(8)°). The SiMe₃ alkylidyne group is tilted away from the OSiPh₃ group, presumably for steric reasons. Noteworthy, the V─O bond in 6c (1.853(4)Å) is comparable to that in 3 (1.845(5)Å), supporting that 6c is indeed a V alkylidyne siloxide complex. The large V─O─Si (147.3(2)°) suggests the significant π-donation from the oxygen atom to V.^{40, 43} The alkylidyne carbon in complex 6c has a chemical shift of 421 ppm in ¹³C NMR, which is more deshielded compared to known V alkylidynes (343 – 375 ppm).³¹ For comparison, the alkylidene carbon in complex 2•dtbbpy has a chemical shift of 343 ppm in ¹³C NMR. Therefore, the NMR and X-ray studies unambiguously confirm that 6c is a V alkylidyne siloxide complex. The resulting complexes resemble well-known Schrock Mo alkylidynes supported by siloxide and bipyridine (or phenanthroline) ligands.^{44, 45}

Figure 1.

The perspective view of the crystal structure of complex 6c with thermal ellipsoids are shown at 30% probability, and hydrogen atoms are omitted for clarity.

The presented method is a rare example of converting oxo ligand at V to vanadium oxygen single bond (V=O to V─OR). From a fundamental standpoint, the illustrated transformation is an elegant strategy to convert early transition metal oxo alkyl complexes to siloxide alkylidynes. In addition, this transformation resembles a reverse reaction of the formation of metal oxo alkylidenes from metal alkylidynes, which was utilized to make the first catalytically active Mo oxo alkylidene.⁴⁶

In summary, we have proposed a new synthetic pathway to produce d⁰ V alkylidynes, an underdeveloped class of 3d Schrock carbynes. The main process involves creating an anionic V alkylidene complex that contains a nucleophilic oxygen atom, making it reactive towards electrophiles such as silyl chlorides. Subsequent α-hydrogen abstraction from the alkylidene results in the formation of alkylidynes. We are eager to investigate the reactivity of the obtained complexes, including their potential for alkyne metathesis, and expand the scope of this transformation to other metal oxo alkyl complexes.

Supplementary Material

The Supporting Information is available free of charge on the ACS Publications website.

Synthesis details, NMR spectra, NBO calculations, and details of X-ray studies (PDF).

Cartesian coordinates of the DFT structure (XYZ).

CCDC 2359502 (6b), 2359503 (2•dtbbpy), and 2359505 (6c) 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 data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

ABBREVIATIONS

NBO

natural bond orbital

NHC

N-heterocyclic carbene

ⁱPr

isopropyl

THF

tetrahydrofuran

TMS

trimethylsilyl group