Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap

Abstract

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo≡CR entities in a stoichiometric triple-bond metathesis event to furnish RC≡CR and the corresponding dinuclear complex, 8, with a Mo≡Mo core. In addition to the regular analytical techniques, ⁹⁵ Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.

Keywords: alkyne metathesis; metal alkylidynes; metal-metal bonding; molybdenum; tungsten.

Scheme 1

a) ArC≡Mo(OtBu)₃ (4), toluene; Ar=2,6‐(Me)₂C₆H₃‐ or p‐(MeO)C₆H₄‐.

Figure 1

Tungsten alkylidynes with very different catalytic activity.

Scheme 2

Canopy complexes such as 1 d with a benzylidyne group and lateral aryl substituents on the silicon linkers form supramolecular dimers (oligomers) as a result of numerous intra‐ and intermolecular C−H/π‐ and π/π interactions shown schematically in red; these aggregates disassemble in solution upon gentle warming or upon coordination of MeCN.

Scheme 3

Formation and fate of catalyst 1 a.

Figure 2

Relevant regions of the ¹³C NMR (left) and ⁹⁵Mo NMR (right) spectra ([D₈]toluene) of the product formed upon reaction of the trisilanol ligand 3 a with the molybdenum alkylidyne 4 a, comprising the expected canopy complex 1 a and a second—likely oligomeric—alkylidyne complex.

Scheme 4

a) i) tBuLi, Et₂O, −125 °C; ii) R₂SiXH, −125 °C to RT, 91 % (R=Et), 89 % (R=nBu), 77 % (R=iso‐Bu), 51 % (R=n‐octyl); b) mCPBA, CH₂Cl₂, 0 °C to RT, 99 % (3 f, R=Et), 71 % (3 g, R=nBu), 97 % (3 h, R=iso‐Bu), 86 % (3 i, R=n‐octyl); c) 4 a, toluene, 99 % (1 f), 58 % (1 g), 86 % (1 h) 30 % (1 i); mCBPA=meta‐chloroperbenzoic acid.

Figure 3

Structure of complex 1 f in the solid state.

Figure 4

Benchmarking of the catalytic activity of the new complexes: the consumption of 1‐methoxy‐4‐(prop‐1‐yn‐1‐yl)benzene was monitored by ¹H NMR ([D₈]‐toluene, 27 °C, 5 mol% catalyst loading).

Scheme 5

Formation of a cyclo‐tetrameric array.

Figure 5

Top: Truncated view of the cyclo‐tetramer 10 containing four molybdenum benzylidyne units and four coordinated pyridine ligands (the two additional phenyl groups at each silicon atom as well as all hydrogen atoms are omitted for the sake of clarity). Bottom: one of the four alkylidyne units in the cyclo‐tetramer with a partly opened ligand framework; the full structure and additional crystallographic information are found in the Supporting Information. Color code: C=black, Mo=yellow, N=blue, O=red, Si=green; averaged values of selected bond lengths [Å] and angles [°]: Mo1−C1 1.746(7), Mo1−N1 2.271(6), Mo1−O1 1.935(5), Mo1−O2 1.931(4), Mo1−O3 1.924(4), Mo1−C1−C2 177.5(66), Mo1−O1−Si1 162.3(3), Mo1−O2−Si2 152.7(3), Mo1−O3−Si3 158.6(3).

Figure 6

The inner cores of the canopy complex [1 d ⋅ MeCN] (top) and one of the four alkylidyne units comprised in 10 (bottom) have notably different curvatures. Dihedral angles C1−Mo1−O−Si: 142.8, 115.6, 11.9 ([1 d ⋅ MeCN]); 12.4, 10.3, 15.5, 14.2, 10.9, 15.5 (10).

Scheme 6

Preparation of the tungsten complex 6 and concentration‐dependent equilibrium with the cyclo‐tetrameric array 12.

Figure 7

Top: Structure of the cyclo‐tetrameric tungsten alkylidyne complex 12 in the solid state; a capped‐sticks presentation is chosen and H‐atoms are omitted for clarity. Bottom: One of the four constituent tungsten alkylidyne units, showing the partly opened chelate ligand sphere; for the full structure and additional crystallographic information, see the Supporting Information. Color code: W=cyan, O=red; averaged values of selected bond lengths [Å] and dihedral angles [°]: W1‐C1 1.778(14), W1−O1 1.867(8), W1−O2 1.866(8), W1−O3 1.872(8), C1−W1−O1−C75 17.4(14), C1−W1−O2−C81 20.5(15), WC1−W1−O3−C13 40.5(19).