Cationic Tungsten-Oxo-Alkylidene-N-Heterocyclic Carbene Complexes: Highly Active Olefin Metathesis Catalysts Roman Schowner, § Wolfgang Frey, ‡ and Michael R. Buchmeiser* ,§ § Institute of Polymer Chemistry and ‡ Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany * S Supporting Information ABSTRACT: The synthesis, structure, and olefin meta- thesis activity of the first neutral and cationic W-oxo- alkylidene-N-heterocyclic carbene (NHC) catalysts are reported. Neutral W-oxo-alkylidene-NHC catalysts can be prepared in up to 90% isolated yield. Depending on the ligands used, they possess either an octahedral (O h ) or trigonal bipyramidal ligand sphere. They can be activated with excess AlCl 3 to form cationic olefin metathesis-active W-complexes; however, these readily convert into neutral chloro-complexes. Well-defined, stable cationic species can be prepared by stoichiometric substitution of one chloro ligand in the parent, neutral W-oxo-alkylidene-NHC complexes with Ag(MeCN) 2 B(Ar F ) 4 or NaB(Ar F ) 4 ; B- (Ar F ) 4 = B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 . They are highly active olefin metathesis catalysts, allowing for turnover numbers up to 10,000 in various olefin metathesis reactions including alkenes bearing nitrile, sec-amine, and thioether groups. C ompared to molybdenum or ruthenium alkylidenes, reports on tungsten-oxo-alkylidenes are scarce. They date back to the 1980’s when Schrock et al. 1-6 described the first complexes of this kind. Mayer reported on the synthesis of W(O)Cl 2 (PMe 2 Ph) 2 (CR 2 ) via reaction of W(Cl 2 )(PMe 2 Ph) 4 with 2 equiv of an aldehyde or ketone. 7,8 A similar approach to W-oxo-alkylidenes starting from W 2 (OCH 2 -tBu) 6 (pyridine) 2 was used by Chisholm et al. 9 Boncella et al. described the complex W(O)Cl(CH-tBu)(Tp′) (Tp′ = hydrotris(3,5-dimeth- yl-1-pyrazolyl)borate). 10 Upon activation with AlCl 3 , the precatalyst was active in the ring-opening metathesis polymer- ization (ROMP) of cis-cyclooctene and norborn-2-ene under ambient conditions. After a period in which only scattered reports on W-oxo-alkylidenes appeared, 11-13 the Schrock group 13,14 very recently created metathesis-active catalysts with the aid of pyrrolides and sterically demanding phenoxides, which display high Z-selectivity. 15,16 Furthermore, highly active silica-immobilized versions, 17,18 activation with boranes 19 and the use in stereospecific ROMP 20,21 have been reported. In view of the reactivity and, compared to other Mo-imido alkylidenes, high functional group tolerance of NHC-Mo- imidoalkylidene bistriflate complexes, 22 we were interested whether tungsten-oxo-alkylidene-NHC complexes as a novel class of metathesis catalysts could be prepared at all, and if yes, which reactivity in olefin metathesis they would display. Here we report our results. Catalysts 2-8 were prepared as outlined in Scheme 1. Notably, while W-NHC-complexes are known, 23-28 these are the first tungsten-oxo-alkylidene NHC complexes. The dichlorotungsten-oxo-alkylidene-NHC complex 2 was prepared from W(O)Cl 2 (PMe 2 Ph) 2 (CHCMe 2 Ph) (1) 14 via reaction with 1,3-dimesitylimidazol-2-ylidene (IMes). Attempts to remove the labile phosphine with CuCl resulted in decom- position. Compound 2 (Figure 1) crystallizes in the triclinic space group, P̅ -1, a = 1126.91(6) pm, b = 1302.24(7) pm, c = 1656.58(9) pm, α = 103.326(3)°, β = 96.653(3)°, γ = 102.402(3) °, Z = 2. Relevant bond lengths and angles are summarized in Figure 1. Received: April 12, 2015 Published: May 4, 2015 Scheme 1. Synthesis of Catalysts 2-8 Communication pubs.acs.org/JACS © 2015 American Chemical Society 6188 DOI: 10.1021/jacs.5b03788 J. Am. Chem. Soc. 2015, 137, 6188-6191