Communication pubs.acs.org/Organometallics
Synthesis and Characterization of Tungsten Alkylidene and Alkylidyne Complexes Supported by a New Pyrrolide-Centered Trianionic ONO3− Pincer-Type Ligand Matthew E. O’Reilly, Soufiane S. Nadif, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Department of Chemistry, Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States S Supporting Information *
ABSTRACT: Synthetic protocols for a pyrrolide-centered ONO3− trianionic pincer-type ligand are presented. Treating (tBuO)3WCtBu with the proligand [pyr-ONO]H3 (2) results in the formation of the trianionic pincer alkylidene complex [pyr-ONO]WCHtBu(OtBu) (3). Addition of a mild base to complex 3 provides the trianionic pincer alkylidyne complex {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4). All new compounds were characterized by NMR spectroscopy, combustion analysis, and, in the case of complex 4, single-crystal X-ray crystallography. DFT calculations performed on 4 provide insight into its electronic structure and indicate that the HOMO is ligand-based and localized on the pyrrolide π orbitals.
T
rianionic pincer and pincer-type ligands are an emerging ligand class for high-valent metal catalysis, which now includes aerobic oxidation,1−3 nitrene and carbene transfer,4−6 ethylene and alkyne polymerization,7−9 and alkene isomerization.10 One potential application for trianionic pincer and pincer-type ligands is to improve tungsten- and molybdenumcatalyzed alkyne cross metathesis.11−15 Theoretical calculations by Jia and Lin16 show that a locked T-shaped geometry of the ancillary ligands should mitigate the relative transition state barrier by ∼24 kcal/mol. Trianionic pincer ligands, with their rigid tridentate framework, lock the supporting anionic donors in a T-shaped geometry, thereby minimizing the cycloaddition energy barrier and providing an open coordination site for the alkyne to access.17 Our previous attempts to realize an alkyne metathesis catalyst supported by a [CF3-ONO] pincer-type ligand yielded unusually stable tungstenacyclobutadiene complexes (Figure 1; B).18 Factors contributing to their stability are poor steric pressure by the pendant alkoxide arms toward the WC3 ring and the “inorganic enamine”18,19 effect arising from an N atom lone pair in the ligand backbone that aligns with the WC bond. The inorganic enamine effect significantly destabilizes the ground state of the W−alkylidyne (A), thus rendering the metallacyclobutadiene intermediate (B) too thermodynamically stable to be competent for retrocycloaddition. An ideal trianionic ONO3− pincer-type ligand suitable for alkyne metathesis should enlarge the steric groups on the pendant arms while effectively removing the influence of the N atom lone pair. A pyrrole-based trianionic ONO3− pincer-type ligand offers the potential to remove the nitrogen lone pair by sequestering it within π bonds of the aromatic ring and to provide more electronically deficient complexes. Additionally, ample evidence indicates pyrrolideligated molybdenum and tungsten alkylidene complexes are © 2013 American Chemical Society
Figure 1. (A) Destabilized ONO3− pincer alkylidyne as a consequence of inorganic enamine effect. (B) Stable metallacyclobutadiene illustrating poor steric congestion between ligand pendant arms and the metallacycle.
among the most highly active alkene metathesis catalysts and provide remarkable selectivity for Z-isomer alkenes.20−25 Also, rapid conversion between η1-N and η5 coordination modes imparts stability to the catalysts.26−28 Herein, we introduce the first pyrrolide-centered trianionic pincer-type ligand and its corresponding tungsten alkylidene and alkylidyne complexes. Scheme 1 depicts the synthetic steps to achieve the new ligand precursor incorporating a pyrrole moiety within the central ring. Suzuki coupling of (3-(tert-butyl)-2-methoxyphenyl)boronic acid29 (2.3 equiv) with 2,5-dibromo-1H-pyrrole30 (1 equiv) appends the Received: September 20, 2013 Published: November 1, 2013 836
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Communication
benzene solution of 3 removes the liberated tBuOH, yielding a purple powder. Cooling a concentrated pentane solution of 3 to −35 °C precipitates an analytically pure sample as a microcrystalline solid. 1H NMR spectroscopy (C6D6) provides good evidence for the assignment of 3 as an alkylidene. The alkylidene proton appears at 7.67 ppm, and the corresponding Cα resonates at 268.6 ppm. The pyrrolide CH protons appear as a singlet well downfield at 7.77 ppm. Using indirect detection, the pyrrolide N atom resonates at 191.4 ppm, which is a shift of 38.1 ppm downfield from proligand 2, a clear indication of coordination to tungsten. The 13C{1H} NMR resonances for the pyrrolide C atoms appear at 112.9 and 137.6 ppm and are similar to those for proligand 2. Observation of similar pyrrolide 13C resonances between 3 and proligand 2 suggests that η1-N coordination of the pyrrolide dominates in solution. For instance, in a series of isoelectronic Ti4+ complexes, η5pyrrolide 13C resonances experience a ∼15 ppm downfield shift relative to the free ligand value, while the 13C resonances are minimally affected by η1-N coordination.31,32 The coordination mode within 3 is cautiously assigned. In tungsten alkylidene complexes prepared by Schrock and co-workers that feature two pyrrolide ligands, the solid-state structure contained both η1-N and η5-pyrrolide coordination modes. However, in solution at ambient temperature the ligands rapidly interconvert to yield a single set of pyrrolide resonances that are very close to the chemical shift of free pyrrole.26−28 Treating complex 3 with the mild base H2CPPh3 in pentane rapidly deprotonates the alkylidene to afford the anionic alkylidyne {MePPh 3 }{[pyr-ONO]WC t Bu(O tBu)} (4), which precipitates as a yellow pastelike solid. Filtration of the solid and removal of all volatiles eventually afford a fine powder. Complex 4 is sparingly soluble in benzene and diethyl ether but readily dissolves in chloroform. The initial 1H NMR spectrum of complex 4 in CDCl3 exhibits broad resonances, but after 0.5 h in solution, the spectrum resolves into sharp resonances. Supporting the assignment of a WCα bond, the 13 C{1H} NMR spectrum of 4 in CDCl3 reveals a resonance at 301.1 ppm. Interestingly, the aromatic signals also shift significantly upfield from those in 3; the pyrrole CH’s resonate at 6.56 ppm, a total shift of 1.21 ppm. Moving in the opposite direction, the 15N resonance of complex 4 appears downfield from that of 3 at 212.2 ppm. Again, the 13C resonances for the pyrrole ring carbons appear at 106.5 and 137.6 ppm; similar to the resonances for both 2 and 3. Single crystals deposit via slow evaporation from a concentrated diethyl ether solution of 4. Figure 2 depicts the
Scheme 1. Synthesis of [pyr-ONO]Me2 (1) and [pyr-ONO]H3 (2)a
a
Legend: (i) 10% Pd(PPh3)4, 8 Na2CO3, 3 KCl, toluene, EtOH, and H2O, HCl/dioxane; (ii) KOtBu, [HSCH2CH2NHMe2]Cl, and DMF, HCl.
anisole fragments to the central pyrrole ring. The reaction proceeds at 95 °C for 18 h. After several purification steps, treating the product with hydrochloric acid in dioxane at 50 °C removes the BOC protecting group, and recrystallization from cold isopropyl alcohol provides [pyr-ONO]Me2 (1). 1H and 13 C{1H} NMR spectroscopy confirm the identity of 1. The pyrrole NH proton appears as a broad singlet at 9.90 ppm, and the pyrrole aromatic protons resonate as a doublet (4J = 2.68 Hz) at 6.55 ppm. Refluxing a DMF solution of 1 with KOtBu and 2-(dimethylamino)ethanethiol hydrochloride removes the methyl groups to provide [pyr-ONO]H3 (2) in modest yield (36%) after recrystallization from pentane. The 1H NMR spectrum of 2 exhibits a resonance attributable to the −OH protons at 6.12 ppm, the pyrrole NH resonance appears at 8.96 ppm, and indirect 15 N detection indicates that the N atom resonates at 153.5 ppm (see the Supporting Information). The 13C{1H} NMR spectrum of 2 exhibits resonances for the pyrrole C atoms at 108.6 and 126.0 ppm. Combining benzene solutions of proligand 2 and (tBuO)3W CtBu results in a color change to deep violet over 0.5 h. The color change results from expulsion of 2 equiv of tert-butyl alcohol and coordination of the trianionic pincer ligand (Scheme 2). During metalation, a proton transfer to the Scheme 2. Synthesis of [pyr-ONO]WCHtBu(OtBu) (3) and {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4)
Figure 2. Molecular structure of 4 with hydrogen atoms, diethyl ether lattice molecule, and methyltriphenylphosphonium group removed for clarity.
alkylidyne bond gives the trianionic pincer tungsten alkylidene [pyr-ONO]WCHtBu(OtBu) (3). Applying vacuum to the
molecular structure of complex 4 obtained from a single-crystal X-ray diffraction experiment. This is an important structure, 837
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and the initial broadness in the 1H spectrum of 4 in CDCl3. Presumably time is required for the solvent to break apart the lattice interactions. More importantly, the close C−H···π contacts suggest significant electron density exists on the pyrrolide portion of the [pyr-ONO]3− ligand. This physical observation is consistent with both computational and experimental findings (vide infra). DFT geometry optimization of the anionic tungsten alkylidyne matches very well the crystallographic structure of 4 (see the Supporting Information). Figure 4 depicts selected molecular
since previous attempts to grow single crystals of another anionic alkylidyne, {MePPh3}{[CF3−ONO]WCtBu(OtBu)} (5), were unsuccessful.18 The anion of complex 4 is pseudo-Cs symmetric, and the tungsten ion adopts a distorted (τ = 0.09)33 square-pyramidal geometry, with the WC bond occupying the axial position. The [pyr-ONO]3− trianionic pincer and −OtBu ligands coordinate in the basal plane. The pyrrolide binds through the σ-N coordination bond, supporting the 13C NMR assignment. The W1−C29 bond length is 1.741(3) Å, which is shorter by 0.013(3) Å than the corresponding bond found in the only other ONO3− tungsten alkylidyne ([CF3ONO]WCtBu(OEt2)) to be crystallographically characterized.18 The W1−N1 bond length of 2.161(3) Å is consistent with other complexes featuring an η1-pyrrolide coordinated to a W(VI) ion.20−28 The W1−N1 bond length is longer, as expected, than the Namido−W bond of 2.008(2) Å found in [CF3ONO]WCtBu(OEt2). Examining the close contact distances surrounding the anionic fragment of complex 4 reveals some interesting features within the crystal lattice. The [pyr-ONO]3− ligand is encapsulated by three nearby Ph3PCH3+ cations (Figure 3). Notably,
Figure 4. Single-point calculation of the [pyr-ONO]WCtBu(OtBu)− fragment 4′ (isovalue 0.051687).
orbitals obtained from single-point calculations. An important observation is that an N atom lone pair is not within the HOMO and results in two energetically similar tungsten alkylidyne π bonds (HOMO-1 and HOMO-2). These results are in contrast with the electronic structure of the alkylidyne anion {MePPh3}{[CF3-ONO]WCtBu(OtBu)} (5), featuring an ONO ligand with a prominent N atom lone pair that aligns with the M−C multiple bond to produce an inorganic enamine.18,19 Interestingly, the HOMO-1 contains a slightly antibonding interaction to the π combination within the pyrrolide ring, though it is a significantly weaker interaction in comparison to that observed within 5.18,19 To access a neutral species, the tert-butoxide ligand must be eliminated. An established protocol9,18 is to add methyl triflate to produce methyl tert-butyl ether (MeOtBu) and the salt [MePPh3][OTf]. Adding methyl triflate to 4 in Et2O, with the intention of removing the −OtBu ligand, produces copious amounts of [MePPh3][OTf] precipitate, but a 1H NMR spectrum of the supernatant reveals the presence of multiple unidentifiable products. Within the complicated NMR mixture, resonances attributable to pyrrole-Me protons were observed, which provides some evidence that alkylation occurs at the [pyr-ONO]3− ligand. In addition, alkylation of the pyrrolide is consistent with the single-point calculations, indicating the π orbitals of the central pyrrolide are the HOMO and are relatively unhindered toward electrophilic attack. These results are not surprising, considering literature precedent for electrophilic aromatic substitution of pyrroles.35,36 A comparative example is the protonation of a pyrrolide-ligated tungsten alkylidene to afford pyrrolenine (eq 1).27
Figure 3. Close C−H···π contact between MePPh3+ and [pyrONO]WCtBu(OtBu)− in the solid-state structure of complex 4.
the distances between the calculated hydrogen atom positions of the Ph3PCH3+ are too close to the anion to be regular van der Waals contacts (2.94 Å), suggesting an interaction between the phosphonium protons and the π bonds of the [pyr-ONO]3− ligand. The closest contacts to each Ph3PCH3+ are depicted in Figure 3, and their distances are 2.612(3), 2.658(3), and 2.716(2) Å. These distances are well within reported Ph3PCH3+ C−H···π contact distances with a related furan.34 This interaction also explains the poor solubility of complex 4 in C6D6 838
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Communication
In conclusion, presented is the synthesis of 2, the first pyrrolide-centered trianionic pincer-type ligand. Considering that the first step of the ligand synthesis employs a boronic acid reagent, it is conceivable that a wide array of derivatives would be amenable to the coupling reaction, therefore providing access to numerous pyrrole-based trianionic pincers. Facile metalation of 2 with (tBuO)3WCtBu is a particularly exciting result and presages a rich transition-metal coordination chemistry for this ligand. The solid-state structure of the anionic alkylidyne {MePPh3}{[pyr-ONO]WCtBu(OtBu)} (4) is consistent with the NMR findings that the pyrrolide coordinates in an κ-N fashion. The DFT single-point calculations confirm that the pyrrolide provides negligible inorganic enamine bonding. However, the electron-rich pyrrolide moiety is susceptible to electrophilic aromatic substitution by alkylating agents. Thus, a new synthetic approach must be employed to achieve a neutral tungsten alkylidyne ligated with a [pyr-ONO]3− pincer-type ligand, and this is the subject of ongoing work in our laboratories.
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ASSOCIATED CONTENT
S Supporting Information *
Text, figures, tables, and a CIF file giving NMR spectra, X-ray crystallographic data, computational details, and experimental procedures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail for A.S.V.:
[email protected]fl.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS A.S.V. acknowledges the UF and the NSF (CHE-1265993), for providing funding for this research. K.A.A. acknowledges the NSF (CHE-0821346) for the purchase of X-ray equipment. Computational resources were provided by the University of Florida High-Performance Computing Center.
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REFERENCES
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