Impact of Ligands and Metals on the Formation of Metallacyclic Intermediates and a Nontraditional Mechanism for Group VI Alkyne Metathesis Catalysts

Abstract

The intermediacy of metallacyclobutadienes as part of a [2 + 2]/retro-[2 + 2] cycloaddition-based mechanism is a well-established paradigm in alkyne metathesis with alternative species viewed as off-cycle decomposition products that interfere with efficient product formation. Recent work has shown that the exclusive intermediate isolated from a siloxide podand-supported molybdenum-based catalyst was not the expected metallacyclobutadiene but instead a dynamic metallatetrahedrane. Despite their paucity in the chemical literature, theoretical work has shown these species to be thermodynamically more stable as well as having modest barriers for cycloaddition. Consequentially, we report the synthesis of a library of group VI alkylidynes as well as the roles metal identity, ligand flexibility, secondary coordination sphere, and substrate identity all have on isolable intermediates. Furthermore, we report the disparities in catalyst competency as a function of ligand sterics and metal choice. Dispersion-corrected DFT calculations are used to shed light on the mechanism and role of ligand and metal on the intermediacy of metallacyclobutadiene and metallatetrahedrane as well as their implications to alkyne metathesis.

Scheme 1. Commonly Accepted Mechanism for Alkyne Metathesis by Way of a MCBD Intermediate

MT_d formation is considered as an off-cycle intermediate.

Scheme 2. Precedence of Mo(VI)/W(VI)-Based Metallatetrahedranes (MT_d) Complexes

Scheme 3. (a) Synthesis of SiP^R (R = Ph, Et) Ligands SiP^Ph and SiP^Et; (b) Synthesis of Catalysts Cat2–5 via Protonolysis of Catalyst Precursors Pre1 and Pre2

Figure 1

Molecular structures of compounds Cat2–5 at 90 K showing thermal ellipsoids at the 50% probability level with H atoms, solvent, disordered groups, and peripheral phenyl groups omitted for clarity. Mo: light blue, W: bright green, C: gray, O: red, Si: yellow.

Scheme 4. Formation of Mo(OSiPh₃)₃(C₃Et₃), MCBD1

Confirmed by NMR at −70 °C.

Figure 2

(a) ¹H NMR of MT_d1 highlighting the upfield shifted methylene proton as a result of CH···π interaction. CH···phenyl_centroid distance highlighted in (b) X-ray crystal structure and (c) energy-minimized structure and the corresponding (d) NCI plot of MT_d1. Green color represents van der Waals interaction.

Figure 3

(a) Formation of a pentylidyne (Cat7) and a metallatetrahedrane (MT_d2) using 5-decyne. (b) Molecular structure of compound MT_d2 at 90 K showing thermal ellipsoids at the 50% probability level with H atoms and peripheral phenyl groups omitted for clarity. Mo: light blue, C: gray, O: red, Si: yellow. (c) Local geometry about Mo1 in molecular structure of MT_d2. Bond distances are colored in blue and bond angles in red.

Figure 4

(a) Synthesis of MCBD2–5 from Cat3 and Cat5. (b) Molecular structures of compounds MCBD2–5 at 90 K showing thermal ellipsoids at the 50% probability level with H atoms, solvent and peripheral phenyl groups omitted for clarity. W: bright green, C: gray, O: red, Si: yellow.

Figure 5

(a) Synthesis of MCBD6 from Cat3. (b) Molecular structure of compound MCBD6 at 90 K showing thermal ellipsoids at the 50% probability level with H atoms, solvent and peripheral phenyl groups omitted for clarity. W: bright green, C: gray, O: red, Si: yellow.

Figure 6

Dynamic scrambling of 1-methoxy-4-(phenylethynyl)benzene (0.1 mM in C₆D₆) catalyzed by 1 mol % of Cat3–5 at rt monitored by ¹H NMR.

Figure 7

(a) Synthesis of Nitride1 via nitrile metathesis with Cat5. (b) Molecular structure of Cat5·PhCN at 90 K showing thermal ellipsoids at the 50% probability level with H atoms omitted for clarity. W: bright green, C: gray, N: blue, O: red, Si: yellow. (c) Molecular structure of Nitride1 at 90 K showing thermal ellipsoids at the 50% probability level with H atoms and solvent omitted for clarity. W: bright green, C: gray, N: blue, O: red, Si: yellow.

Figure 8

Energetics of MCBD and MT_d formation via [2 + 2] cycloaddition for tungsten with SiP^Me (outside parentheses) and SiP^Ph (inside parentheses) ligands. Free energies (kcal/mol) are computed at the B3LYP-D3/def2TZVP-SDD(W)-CPCM(benzene)//B3LYP-D3/def2SVP-LANL2DZ(W)-CPCM(benzene) level of theory. For enthalpy and electronic energies, refer to Figure S60 and S61. The red pathway shows the [2 + 2] cycloaddition followed by pseudorotation and finally retro-[2 + 2] to yield product. Alternatively, the red pathway from the [2 + 2] cycloaddition can lead to the blue pathway in which the metallatetrahedrane is involved in the product formation. The structures highlighted in blue are accessible via the metallatetrahedrane pathway (blue lines). Notably, the green pathway in which the [2 + 2]/retro-[2 + 2] occurs with no pseudorotation is energetically inaccessible.

Figure 9

Energetics of MCBD and MT_d formation via [2 + 2] cycloaddition for molybdenum with SiP^Me (outside parentheses) and SiP^Ph (inside parentheses) ligands. Free energies (kcal/mol) are computed at the B3LYP-D3/def2TZVP-SDD(Mo)-CPCM(benzene)//B3LYP-D3/def2SVP-LANL2DZ(Mo)-CPCM(benzene) level of theory. For enthalpy and electronic energies, refer to Figure S62 and S63. The red pathway shows the [2 + 2] cycloaddition followed by pseudorotation and finally retro-[2 + 2] to yield product. Alternatively, the red pathway from the [2 + 2] cycloaddition can lead to the blue pathway in which the metallatetrahedrane is involved in the product formation. The structures highlighted in blue are accessible via the metallatetrahedrane pathway (blue lines). Notably, the green pathway in which the [2 + 2]/retro-[2 + 2] occurs with no pseudorotation is energetically inaccessible.