Molecular and Silica-Supported Molybdenum Alkyne Metathesis Catalysts: Influence of Electronics and Dynamics on Activity Revealed by Kinetics, Solid-State NMR, and Chemical Shift Analysis

Abstract

Molybdenum-based molecular alkylidyne complexes of the type [MesC≡Mo{OC(CH)(CF)}] (MoF, x = 0; MoF, x = 1; MoF, x = 2; MoF, x = 3; Mes = 2,4,6-trimethylphenyl) and their silica-supported analogues are prepared and characterized at the molecular level, in particular by solid-state NMR, and their alkyne metathesis catalytic activity is evaluated. The C NMR chemical shift of the alkylidyne carbon increases with increasing number of fluorine atoms on the alkoxide ligands for both molecular and supported catalysts but with more shielded values for the supported complexes. The activity of these catalysts increases in the order MoF < MoF < MoF before sharply decreasing for MoF, with a similar effect for the supported systems (MoF ≈ MoF < MoF < MoF). This is consistent with the different kinetic behavior (zeroth order in alkyne for MoF derivatives instead of first order for the others) and the isolation of stable metallacyclobutadiene intermediates of MoF for both molecular and supported species. Detailed solid-state NMR analysis of molecular and silica-supported metal alkylidyne catalysts coupled with DFT/ZORA calculations rationalize the NMR spectroscopic signatures and discernible activity trends at the frontier orbital level: (1) increasing the number of fluorine atoms lowers the energy of the π*(M≡C) orbital, explaining the more deshielded chemical shift values; it also leads to an increased electrophilicity and higher reactivity for catalysts up to MoF, prior to a sharp decrease in reactivity for MoF due to the formation of stable metallacyclobutadiene intermediates; (2) the silica-supported catalysts are less active than their molecular analogues because they are less electrophilic and dynamic, as revealed by their C NMR chemical shift tensors.

Well‐Defined Alkyne Metathesis Catalysts: Developments and Recent Applications

“…However,t he calculations also indicated energetically stabilized MCBD complexes for MoF9, WF6 and WF9 and their [2+ +2] cycloelimination could become the rate-determining step for these catalysts. [64,65] As for actual catalytic performance, the activity increased in the order MoF0 < MoF3 < MoF6,w hichw as in line with ad ecreased activation barrier.C orrelating witha no verly stabilized MCBD, the catalytic activity was diminished for MoF9 andn ot erminal alkyne metathesis could be detected. For the tested tungsten complex WF3,h igh yields even in terminal alkynem etathesis were accomplished.…”

“…[63] For an extended study on the structure-activity relationship, as eries of fluoroalkoxy-supported benzylidyne complexes of the type [MesCM{OC(CF 3 ) n Me 3-n } 3 ]( Mes = mesityl, n = 0, 1, 2, 3) was synthesized and ac ombinedt heoretical and experimental investigation on their catalytic activity was performed (Scheme 5). [64,65] As for tungsten, only WF3 was synthesized and analyzed because WF6 already showedn oc atalytic activity.D FT calculations on the rate-determining step of alkyne metathesis as af unctiono ft he fluorination degree suggested ac lear trend. With increasing degree of fluorination, the inter-Scheme4.Synthesis of tungsten and molybdenum benzylidyne complexes 11 as precursors for active alkyne metathesis catalysts.…”

“…In reactions of MoF9 with excesso f 1-phenyl-1-propyneo r3 -hexyne, stable metallacycles MoF9-MCBD Me and MoF9-MCBD Et formed upon cooling the saturated reaction mixtures (Scheme 6). [64,65] These molybdenacyclobutadienes represent rare structurally characterized MCBD examples. [32,68,69] During the early 2000s, M. J.…”

“…[89] More recent results in the field of heterogeneousa lkyne metathesis include the graftingo ft he series of catalyst MoF0-MoF9 onto dehydroxylated silica (Figure 4). [62,65] Silica-supported www.chemeurj.org alkylidynec omplexesw ith different fluoroalkoxy ligands [MesCMo{OC(CF 3 ) n Me 3-n } 2 (SiO)] (n = 0, 1, 2, 3) were prepared and their structure-activity relationship was investigated. [62,64,65] In general,t he immobilized alkylidynec omplexese xhibit slightly decreased catalytic activityc ompared to their molecular analogues MoF0-MoF9 (Scheme 5, section 2.1), which was attributedt ot he decreased electrophilicity at the metal and the increased rigidity of the system.…”

Unraveling the Mechanism of 1,3‐Diyne Cross‐Metathesis Catalyzed by Silanolate‐Supported Tungsten Alkylidyne Complexes

“…In contrast, both the transition state (TS) and the α,α‐MCBD contain one silanolate ligand bound in a chelating OSi{OSi(O t Bu) 3 } 3 ‐κ 2 O fashion. Surprisingly, the two W‐O distances are almost identical in the TS structure (2.212 and 2.199 Å), whereas the MCBD features a shorter W−O1 (2.199 Å) and a longer W−O2 bond (2.578 Å), which resembles the situation found in the complex [MesC≡W{OSi(O t Bu) 3 }{OC(CF 3 ) 3 } 2 ] . These findings indicate that secondary interactions with the silanolate ligand play a crucial role in the stabilization of transition states and intermediates and are potentially a key factor for the understanding of the observed high diyne metathesis selectivity of catalyst 1 .…”