Thorium Metallacycle Facilitates Catalytic Alkyne Hydrophosphination

Sep 11, 2017 - The bis(NHC)borate-supported thorium-bis(mesitylphosphido) complex (1) undergoes reversible intramolecular C–H bond activation enabli...
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Thorium Metallacycle Facilitates Catalytic Alkyne Hydrophosphination Mary E. Garner, Bernard F. Parker, Stephan Hohloch, Robert G. Bergman, and John Arnold* Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States S Supporting Information *

340°) and the orientation of the P atom lone pair toward the C atom equip 1 for this spin−spin interaction. The dynamic nature of this phenomenon is unveiled through an isotopic labeling experiment (Scheme 1). Treatment of 1 with

ABSTRACT: The bis(NHC)borate-supported thoriumbis(mesitylphosphido) complex (1) undergoes reversible intramolecular C−H bond activation enabling the catalytic hydrophosphination of unactivated internal alkynes. Catalytic and stoichiometric experiments support a mechanism involving reactive Th−NHC metallacycle intermediates (Int and 2).

Scheme 1. Proposed Mechanism for Deuterium Exchange

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igand cyclometalation via intramolecular C−H bond activation1−6 not only gives insight into the corresponding, synthetically valuable, intermolecular C−H activation process,7−11 but also provides metallacycles that are often useful for further reactivity.12−18 In fact, a rich field of late transition metalmediated catalysis relies on such metallacycles as reactive intermediates.19−21 The formation of f-block metallacycles is often favorable due to the use of sterically bulky supporting ligands combined with electron poor metal centers; however, the subsequent reactivity of these complexes has been dominated by ring-opening in the presence of acidic substrates.22−25 Here we present an alternative mode of reactivity that capitalizes on the drive for cyclometalation but then favors transformations that leave the chelate ring intact. We have recently described the synthesis and ligand-based reductive elimination reactivity of a bis(phosphido) Th−NHC complex (1).26 Key to this transformation is the fact that oxidation state changes do not take place at the thorium center, but instead occur exclusively at the bipyridine and phosphido ligands. Throughout this and previously reported redox chemistry,27,28,36 the bis(NHC)borate supporting ligand remains inert. We now demonstrate a different reactivity manifold, in which the NHC ligand scaffold assumes an alternative role beyond that of a simple spectator. Examining the molecular structure of 1 reveals the complex is poised for intramolecular C−H bond activation. The 13C NMR spectrum of 1 in C6D6 shows through-space coupling (TSJC−P = 4.5 Hz) between a methyl carbon located on an NHC-mesityl substituent and a nearby phosphorus atom on a phosphido ligand. Together with structural information provided by X-ray diffraction studies, a suite of 2D NMR experiments verify the identity and location of the atoms involved in this coupling (Figures S1−S4). Through-space 13C−31P coupling is rarely observed;29−31 however, the relatively close proximity of the phosphido ligand P atom to the NHC ligand mesityl methyl C atom (∼3.8 Å) combined with the pyramidalization at P (∑∠ ≈ © 2017 American Chemical Society

excess D2P(Mes) leads to deuterium incorporation into both the phosphido P−H/D bond and the methyl C−H/D bond engaged in through-space coupling. Free H2P(Mes) and HDP(Mes) are also produced during this exchange (confirmed by 2H and 31P NMR spectroscopy, Figure S5). This study implicates the existence of a reversible intramolecular C−H bond activation process wherein 1 is in an equilibrium with a mono(phosphido) thorium metallacycle (Int). Treating 1 with 2 equiv of diphenylacetylene capitalizes on the reversible C−H bond activation behavior in a unique way, affording a vinyl phosphine product (VP) and a thorium−NHC metallacycle (2) in a 1:1 ratio (Scheme 2). Scheme 2. Reactivity of 1 with Diphenylacetylene

Received: August 6, 2017 Published: September 11, 2017 12935

DOI: 10.1021/jacs.7b08323 J. Am. Chem. Soc. 2017, 139, 12935−12938

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Journal of the American Chemical Society The 1H NMR spectrum of 2 is highly asymmetric and broadened at room temperature (296 K); nonetheless, cooling a toluene-d8 solution of 2 to 233 K permits full assignment (Figures S6 and S11). Despite the broadness, diagnostic spectroscopic features that are critical to structural elucidation can be observed at room temperature. Specifically, the hydrogen atoms of the methylene group bound to the thorium center appear as two diastereotopic doublets at surprisingly distinct chemical shifts (δH = 3.04 and 0.80 ppm); they split one another with a characteristic coupling constant of 2JH−H = 9.2 Hz. The 31P NMR spectrum of 2 contains a single peak at +171.0 ppm that shows coupling to the olefinic hydrogen atom on the vinylphosphido ligand, with a coupling constant of 3JP−H = 16.2 Hz. Pink-orange crystals of 2 deposit from a concentrated hexanes solution stored at room temperature for 24 h. Single crystal X-ray diffraction studies confirm that 2 is a six-coordinate mono(phosphido) thorium metallacycle with a CCH2−−Th bond length of 2.563(4) Å (Figure 1).

phosphido ligands of 1, releasing free H2P(Mes) and forming stable C2 symmetric bis(acetylide) thorium complexes. Further evidence for this alternative mode of reactivity with terminal alkynes is provided by the independent synthesis of complex (3) by treating 1 with 5 equiv of p-tolylacetylene (Scheme 3). Scheme 3. Synthesis of 3 via Protonolysis of 1

Single crystals of 3 grow from a room temperature vapor diffusion of hexanes into a 10:1 benzene:pentane solution over 24 h. X-ray diffraction studies confirm the C2 symmetry of 3 is maintained in the solid state and, to the best of our knowledge, distinguish it as the first crystallographically characterized thorium-acetylide complex (Figure S38). The catalysis is sensitive to the relative concentrations of phosphine (H2PR) and alkyne (Alk). That is, utilizing >1:1

[H2P]0:[Alk]0 nearly shuts down the reactivity. Exploring this behavior through a series of catalytic kinetics investigations under pseudo-first-order conditions (where [Alk]0 ≫ [H2P(Mes)]0, such that [Alk]0 = [Alk], and [1]0 = [1]) informs our understanding of the mechanism. Not only is the transformation sensitive to [H2P(Mes)]:[Alk], but the observed rate of VP formation displays an inverse first order dependence on [H2P(Mes)] (Figure S29a). Varying the initial concentrations of either 1 or Alk indicate first-order dependence on [1] and on [Alk] (Figures S29b,c, respectively). Taken together, these observations give the overall experimental rate law summarized in eq 1. These results, in conjunction with the high reactivity of 2, support a mechanism involving thorium metallacycle intermediates, wherein reversible C−H bond activation and H2P(Mes) elimination provide entrance to the catalytic cycle (Figure 2). Int and 2 are not observed throughout the course of our catalytic investigations; however, a number of stoichiometric experiments substantiate their intermediacy. For example, the

Figure 1. Molecular structure of 2 (thermal ellipsoids drawn at the 50% probability level). Most hydrogen atoms omitted for clarity. Selected bond distances (Å): Th1−P1, 2.913(5); Th1−C39, 2.563(4); C67− C68, 1.343(6).

Treatment of 2 with excess H2P(Mes) rapidly liberates an equivalent of VP and reforms complex 1. The ability to cycle back to 1 is promising for extending this reactivity to catalysis. Moreover, the fact that 1 performs this chemistry cleanly under mild conditions is significant because catalytic hydrophosphination of unactivated alkynes without the impetus of light, radical initiators, or elevated temperature is uncommon.17,32,33 Treating 1 with 10 equiv of H2P(Mes) and 10 equiv of diphenylacetylene (Alk) at 50 °C affords a highly stereoselective (9:1 E:Z) mixture of VP isomers in >95% conversion as judged by 31P NMR spectroscopy. The steric bulk of the phosphine, rather than the alkyne, appears to enforce the E-selectivity, as hydrophosphination of Alk with H2P(Ph) and H2P(Cy) also occurs selectively (4:1 and 16:1 E:Z, respectively). Compound 1 also selectively hydrophosphinates the aliphatic acetylene, 3-hexyne, with H2PR (R = Mes, Ph, Cy; 11:1, 5.8:1, and 16:1 E:Z, respectively). In all cases the catalysis is exceptionally clean with no overphosphinated or dehydrocoupled species produced, which would arise from double hydrophosphination, or radical phosphido combination, respectively. Moreover, no product inhibition is observed, likely due to the steric encumbrance afforded by the NHC ligands as well as the hard−soft mismatch of the thorium(IV) center in 1 and phosphorus(III) donor atom of VP. Different reactivity is observed with terminal alkynes. Instead of hydrophosphination, reactions with tert-butylacetylene, ptolylacetylene, and phenylacetylene result in protonolysis of the

Figure 2. Proposed catalytic cycle for the hydrophosphination of diphenylacetylene (Alk). 12936

DOI: 10.1021/jacs.7b08323 J. Am. Chem. Soc. 2017, 139, 12935−12938

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Journal of the American Chemical Society

an effective way to spectroscopically observe Int, the highly transient nature of this species precludes its isolation. In fact, even in the stoichiometric reaction between 2 and H2P(Mes), the very reactive Int formed in situ is readily trapped by remaining H2P(Mes) and converted back to 1. A 31P NMR experiment modeling step 3 but utilizing >1 equiv of H2P(Mes) capitalizes on this unavoidable reversion of Int to 1 as a means to observe the inverse of step 1. Monitoring the loss of Int (formed in situ) after all of 2 is consumed verifies the similar rates of 1 regeneration and Int and H2P(Mes) loss (Figure S31). In contrast, simulating step 3 with the bulky secondary diphenylphosphine (HPPh2) cleanly affords a stable mono(phosphido) thorium metallacycle (5) concomitant with the release of VP (Scheme 5).

initial isotopic labeling study involving D2P(Mes) and 1 already implicates the existence of an equilibrium between the bis(phosphido) thorium complex 1 and the mono(phosphido) thorium metallacycle Int (step 1). Next, introducing 1 atm of H2 to a C6H6 solution of 2 induces metallacycle ring-opening and ligand hydrogenation resulting in 4 (Scheme 4). Scheme 4. Synthesis of 4 via H2 Addition to 2

Scheme 5. Formation of 5 from 2 and HPPh2 Single crystal X-ray diffraction studies confirm that 4 contains a dianionic, multidentate phosphido ligand (Figure 3). This ligand is stable toward a variety of protic substrates, including H2P(Mes).

Again, the 1H NMR spectrum of 5 in C6D6 contains two diastereotopic doublets corresponding to the hydrogen atoms of the thorium-bound methylene group (δH = 3.08 and 0.44 ppm, 2 JH−H = 9.1 Hz). The persistent nature of 5 makes isolation and crystallographic characterization feasible. Red-orange blocks suitable for single crystal X-ray diffraction studies grow from a room temperature vapor diffusion of hexanes into a concentrated benzene solution over 48 h (Figure 4). Unambiguous structural determination of this stable analog further supports the identity of the proposed intermediate Int as a C−H activated mono(phosphido) thorium complex.

Figure 3. Molecular structure of 4 (thermal ellipsoids drawn at the 50% probability level). Most hydrogen atoms omitted for clarity. Selected bond distances (Å): Th1−P1, 2.849(2); Th1−C67, 2.674(8); C67− C68, 1.525(11).

Performing the catalysis in the presence of H2 is a useful probe for the presence of 2 during the catalytic hydrophosphination. That is, whereas the interaction of 2 with H2P(Mes) should form Int and VP (Figure 2, step 3), which facilitates the catalysis, the reaction of 2 with H2 should form 4, which is not catalytically active and should buildup. Indeed, conducting the hydrophosphination under 1 atm of H2 affords a mixture of VP and 4 products and indicates the existence of 2 during the catalysis. Independently synthesized 2 (Scheme 2) provides a way to interrogate step 3. Treating 2 with 1 equiv of H2P(Mes) immediately generates signals for two new species in the 31P NMR spectrum. The first is easily identified as VP (E isomer, dd, δP = −56.1 ppm, 1JP−H = 223.8 Hz, 3JP−H = 9.6 Hz) and the second is assigned as Int on the basis of its large 1JP−H coupling constant of 183.0 Hz and chemical shift of −29.0 ppm (C6D6, 296 K). Additional support for the identity of Int is provided by diagnostic features in the 1H NMR spectrum of the crude reaction mixture (Figure S30), specifically, two diastereotopic doublets corresponding to the hydrogen atoms of the thoriumbound methylene group (δH = 2.95 and 0.25 ppm, 2JH−H = 9.0 Hz). Comparing these peaks to those in fully characterized 2 suggests that such doublets are a hallmark of Th−NHC metallacycles that can provide a powerful spectroscopic probe of this family of compounds. Although acquiring NMR spectra soon after H2P(Mes) addition to 2 during the step 3 simulation is

Figure 4. Molecular structure of 5 (thermal ellipsoids drawn at the 50% probability level). Most hydrogen atoms omitted for clarity. Selected bond distances (Å): Th1−P1, 2.950(1); Th1−C48, 2.590(3).

Our ability to establish step 2 via the appropriate stochiometric reaction is hampered by the highly transient nature of Int. At this time, it is unclear whether step 2 proceeds by direct alkyne insertion into the P−H bond of Int, or if alkyne inserts into the Th−P bond and undergoes rapid isomerization in a concerted fashion to form 2. Although direct P−H insertion is rare,34,35 experimental results suggest that it cannot be entirely ruled out. For example, internal alkynes do not react with the isolable secondary phosphido thorium complex 5 (which lacks a P−H bond for alkyne to insert into) even under forcing conditions, i.e., 100 °C, 2 days. Accordingly, the attempted catalytic reaction 12937

DOI: 10.1021/jacs.7b08323 J. Am. Chem. Soc. 2017, 139, 12935−12938

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Journal of the American Chemical Society

Geosciences, and Biosciences Heavy Element Chemistry Program of the U.S. Department of Energy (DOE) at LBNL under Contract No. DE-AC02-05CH11231. M.E.G. acknowledges the NSF-GRFP for a graduate research fellowship (DGE 1106400) and S.H. acknowledges the German Academic Exchange Service (DAAD) for a postdoctoral scholarship. The authors gratefully acknowledge Mr. Peter J. Waller for his valuable discussion and advice, Dr. Hasan Celik for his assistance and consultation regarding NMR experiments, and Dr. Olayinka Olatunji-Ojo for her helpful discussion.

between 1 and excess HPPh2 and diphenylacetylene is not productive, and the only new product formed is 5. One possible explanation for this could be the steric crowding enforced by the bulky dipheylphosphido ligand of 5, which blocks access to the thorium center needed for alkyne insertion. However, it is worth mentioning that despite the congested coordination sphere of 5, the bis(diphenylphosphido) thorium−NHC complex26 is accessible by treating 5 with excess HPPh2 and heating mildly (55 °C), thus demonstrating enough space for alternative reactivity. Applying the steady-state approximation to the concentrations of Int and 2 is valid in this case because neither intermediate builds up during the course of the catalytic investigations. The resulting rate law reflects the experimentally observed dependencies on 1, Alk, and H2P(Mes) (eq 2):

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In summary, we have provided evidence for the unusual existence of metallacycle intermediates in the catalytic hydrophosphination of alkynes. These results showcase the utility and competency of C−H activated f-element complexes as reactive species in catalysis. Kinetic and stoichiometric experiments support a mechanism initiated by a reversible C−H bond activation phosphine elimination step that then proceeds through reactive Th−NHC metallacycles. Diagnostic NMR features of this class of compounds assist in the characterization of isolable products and are critical to establishing the intermediacy of highly transient species. Preliminary substrate screening indicates that the catalysis is exceptionally clean, produces exclusively secondary vinyl phosphine products, does not require light, radical initiators or elevated temperature, and is relatively selective for the E isomer. Moreover, this work adds to the burgeoning reaction chemistry accessible by bis(NHC)borate-supported thorium complexes.26−28,36 Beyond functioning as inert spectators, the NHC ligands of 1 have now been shown to participate via reversible intramolecular C−H bond activation, which can be leveraged toward the catalytic hydrophosphination of unactivated internal alkynes. We are currently exploring additional ways to capitalize on this behavior to engage new substrates in reactivity.

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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/jacs.7b08323. Synthetic procedures and characterization information (PDF) Corresponding CIF files (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Mary E. Garner: 0000-0002-4630-0661 John Arnold: 0000-0001-9671-227X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, 12938

DOI: 10.1021/jacs.7b08323 J. Am. Chem. Soc. 2017, 139, 12935−12938