Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes: Structure–Productivity Correlations and Mechanistic Insights

Abstract

The syntheses and single‐crystal X‐ray structures of a series of Mo–imido alkylidene N‐heterocyclic carbene (NHC) complexes (1–15) and of the first complexes containing bidentate NHC‐phenolate ligands (16–18) are reported. Mo(N‐2,6‐Me2‐C6H3)((1‐R‐phenethyl)‐3‐mesitylimidazolidin‐2‐ylidene)(CHR)(OTf)2 (R=CMe2Ph, 1) is the first enantiomerically pure Mo–imido alkylidene NHC catalyst. With [Mo(N‐2,6‐Me2‐C6H3)(IMes)(CHR)(CH3CN)(OTf)(CH3CN)+ B(ArF)4−] (7), turnover numbers up to 545 000 were achieved in the homometathesis (HM) of 1‐octene and 1‐nonene (≤95 % E). With 7 and 1‐nonene, a turnover frequency (TOF4 min) of 8860 min−1 was determined. Productivity and E/Z‐selectivity were correlated with catalyst structure. For 1, Mo(N‐3,5‐Me2‐C6H3)(IMesH2)(CHR)(OTf)2 (9) and Mo(N‐3,5‐Me2‐C6H3)(IMes)(CHR)(OTf)2 (10), productivity was correlated with the coalescence temperature of the two triflates, determined by variable‐temperature 19F NMR spectroscopy. The square‐planar conformer is postulated to be the most relevant for the catalyst activation.

Latent and Air‐Stable Pre‐Catalysts for the Polymerization of Dicyclopentadiene: From Penta‐ to Hexacoordination in Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes

“…Also, as anticipated, I5 , with the electron‐withdrawing chloro‐substituents at the aromatic imido ligand, displays the highest T exo,max =186 °C and T onset =115 °C of all screened bistriflate catalysts. In addition, T onset for I2 – I4 also correlates with their previously published coalescence temperature T c =60 °C ( I4 ), T c =79 °C ( I2 ) and T c =85 °C ( I3 ) (previously measured in deuterated 1,2‐dichlorobenzene, Table ). Poly(DCPD) derived from the polymerization with catalysts I1 ( T= 110 °C), I3 ( T= 110 °C) and I4 ( T= 80 °C) showed swelling in toluene of 25, 45 and 50 %, whereas poly(DCPD) polymerized by I2 ( T= 80 °C) and I5 ( T= 150 °C) showed no swelling at all, indicating high crosslinking (Table S13, S.I.).…”

“…I2 adopts a distorted square pyramidal (SP) structure ( τ =0.325) in which one triflate ligand is coordinated trans to the NHC (Mo−O4, 216.97(17) pm, Figure ). I2 and I3 , I4 as well as I5 , which also show a distorted SP structure, display comparable bond lengths, angles and geometry in the solid state. For I2 , I3 and I5 the longest Mo‐OTf bond is coordinated trans to the NHC, whereas for I4 the longest Mo‐OTf bond is situated trans to the imido ligand (Table ).…”

“…T c is the temperature at which the two triflate ligands in the complexes begin to coalesce in 19 F NMR spectroscopy. This was postulated to be a consequence of the formation of a cationic complex in which the dissociated triflate and the one bound to molybdenum exchange rapidly . For I2 and I3 , T c increases with decreasing Mo‐OTf bond length ( trans to the NHC, Table ).…”

Origin and Use of Hydroxyl Group Tolerance in Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Catalysts

“…We already have shown that cationic molybdenum imido alkylidene NHC complexes are highly active catalysts in various olefin metathesis reactions . Here we prepared a wide range of complexes with the general formula [Mo( N R′)(CHCMe 2 R)(NHC)(X)][B(Ar F ) 4 ] (Figure , X=anionic ligand).…”

Molybdenum and Tungsten Imido Alkylidene N‐Heterocyclic Carbene Catalysts Bearing Cationic Ligands for Use in Biphasic Olefin Metathesis

“…Addition of one equivalent of lithium pentafluorophenoxide to 3 resulted in the quantitative exchange of the betaine but not the triflate ligand by pentafluorophenoxide yielding [Mo{ N ‐2,6‐(Me 2 )C 6 H 3 }(CHCMe 2 Ph)(IMesH 2 )(OTf)(OC 6 F 5 )] ( 4 ) ).…”