Application of Imidazolinium Salts and N-Heterocyclic Olefins for the Synthesis of Anionic and Neutral Tungsten Imido Alkylidene Complexes

General. All manipulations of moisture and air sensitive materials were performed under a nitrogen atmosphere with standard Schlenk techniques or in a N2-filled glove box (Lab Master 130, MBraun, Garching, Germany). Methylene chloride, diethyl ether, toluene, pentane and tetrahydrofuran were dried by a solvent purification system (SPS, MBraun). 1,2Dimethoxyethane (DME) and benzene were vacuum distilled from a dark purple solution of sodium benzophenone ketyl, and degassed three times by freeze-pump-thaw technique. Starting materials and all reagents were purchased from Sigma-Aldrich (Munich, Germany), Alfa Aesar (Karlsruhe, Germany), ABCR (Karlsruhe, Germany) and Acros Chemicals (Geel, Belgium), dried and, where appropriate, distilled prior to use. NMR measurements were recorded on a Bruker Avance III 400. Chemical shifts are reported in ppm relative to the solvent signal. GC-MS data were recorded on an Agilent Technologies device consisting of a 7693 autosampler, a 7890 A GC and a 5975C quadrupole MS. Dodecane was used as internal standard. An SPB-5 fused silica column (34.13 m x 0.25 mm x 0.25 mm film thickness) was used. The injection temperature was set to 150°C. The column temperature ramped from 45°C to 250°C within eight minutes and was then held for further five minutes. The column flow was 1.05 mL per minute. All substrates for HM, CM, RCM, and SM were either distilled from CaH2 or recrystallized and stored under nitrogen.

or [Cp*Mo(NO)(CH 2 SiMe 3 )-(CHSiMe 3 )] 2 [Li 2 (THF) 3 ] 13 as described by Legzdins.So far, all these complexes have been solely used as intermediates for the synthesis of non-ionic compounds and no activity in olefin metathesis reactions of these complexes was reported.To exploit the above-described reaction in a systematic manner, complexes 2-4 were prepared as outlined in Scheme 1. Starting from the literature-known precursor complex 1, 5,7 compound 2 was conveniently obtained in high yield by adding a suspension of 1,3-bis (2,4,6-trimethylphenyl)imidazolinium chloride to a stirred solution of complex 1 in benzene.Analytically pure samples could be crystallized from CH 2 Cl 2 by adding small amounts of pentane.Complex 2 crystallizes in the monoclinic space group P2 1 /c, a = 2168.65(19) pm, b = 1887.37(16) pm, c = 1610.05(13) pm, a = 901, b = 106.667(3)1,g = 901, Z = 4. Relevant bond lengths and angles are summarized in Fig. 1.
In the solid state, complex 2 adopts a slightly distorted square pyramidal (SP) configuration with the alkylidene forming the apex.The tungsten alkylidene bond is 190.2(5) pm and the pyrrolides have almost the same distance to the metal centre (213.6(5) and 215.0(5) pm), respectively.Both, bond angles and bond lengths, indicate an Z 1 -coordination of both pyrrolides.In the solid state, the alkylidene is observed in a syn configuration, which correlates with solution NMR data (J C-H = 121.63Hz (CD 2 Cl 2 )) at À20 1C.
Nonetheless, broad signals observed in 1 H NMR for the alkylidene, pyrrolides and the 2-propyl groups of the imido ligand (Fig. S1 and S2, ESI †) suggest a dynamic structure in solution at room temperature.
Importantly, further functionalization of 2 could be easily achieved.Reaction with one equivalent of lithium pentafluorophenoxide leads to the formation of 3 in high yield (Scheme 1), analytically pure samples of which were again crystallized from CH 2 Cl 2 by adding small amounts of pentane.Complex 3 crystallizes in the monoclinic space group P2 1 /c, a = 1124.44(3)pm, b = 2612.67(5) pm, c = 2061.54(4)pm, a = 901, b = 94.9910(10), g = 901, Z = 4. Relevant bond lengths and angles are summarized in Fig. 2. Similar to 2, in the solid state complex 3 adopts a distorted square pyramidal configuration again with the alkylidene forming the apex.The tungsten alkylidene bond is 188.9(2)pm, which is comparable to the bond in 2. Again, the pyrrolides have almost the same distance to the metal center (212.74 (18) and 213.48(18) pm), and Z 1 -bonding of the pyrrolide can be deducted from bond angles and bond lengths.In the solid state the alkylidene is observed in a syn configuration, which correlates with solution NMR data (J C-H = 123.0Hz (CD 2 Cl 2 )).The shorter bond lengths of the other ligands are believed to be a result of the more pronounced electron-withdrawing character of the pentafluorophenoxide compared to the chloro-ligand.Unlike 2, compound 3 shows a sharp alkylidene and pyrrolide signal in proton NMR (S3), accompanied by a splitting of the 2-propyl groups at the imido ligand, which is indicative of the hindered rotation of the latter.
Notably, the reaction of the dipyrrolyl precursor [W(N-2,6-iPr 2 C 6 H 3 )(CHCMe 2 Ph)(2,5-Me 2 Pyr) 2 ] (1) with two equivalents of 1,3-(2,6-diisopropylphenyl)imidazolin-2-ium chloride leads to the formation of the anionic W-imido dichloro alkylidene pyrrolide complex 4, underlining that this way to generate anionic alkylidenes is not limited to a ''magic pair'', but might be achieved with a number of educts.It should be noted that analogous reactions with 1,3dimethylimidazolium iodide or [1,3-bis-(2,4,6-trimethylphenyl)imidazolinium] tetrafluoroborate did not result in the formation of the corresponding anionic complexes, most probably as a consequence of the decreased nucleophilicity of iodide and tetrafluoroborate, respectively.Use of iodide resulted in no reaction at all, while application of the tetrafluoroborate salt resulted in complete decomposition of the metal complex.
The anionic tungsten alkylidene complexes 2, 3 and 4 were subjected to a set of simple olefin metathesis reactions to test for their activity and functional group tolerance in ring-closing metathesis (RCM), homometathesis (HM), self-metathesis (SM) and cross-metathesis (CM) reactions.Generally, the activity of group 6 metal alkylidenes in olefin metathesis reactions strongly depends on the polarization of the metal-alkylidene bond and the electrophilic character of the metal center. 7In this regard, no metathetical activity of anionic compounds 2-4 can be expected.Surprisingly, 2-4 show a low but significant propensity to catalyze olefin metathesis reactions (Table S1, ESI †): in the RCM of diallydiphenylsilane and the HM of terminal alkenes such as 1-hexene and 1-octene turnover numbers (TONs) in the range of 50 to 410 could be achieved.Potential ''activation'' of complexes 2 and 4 by abstraction of chloride (using AlCl 3 ) has failed so far, but remains under investigation since this represents an attractive access to latent polymerization systems. 14,15 view of the above-described findings, attention was then shifted to NHOs.This class of very electron-rich and polarized olefins is currently receiving increasing interest. 6The charge separation characterizing the double bond in NHOs can be strong enough to allow for the formation of CO 2 -adducts; 16 likewise, their considerable nucleophilicity and basicity were recently put to use in the first NHO-mediated organopolymerizations. 17,18A limited number of NHO-metal complexes are also known, [19][20][21][22][23] and remarkably, it was found that NHOs can confer more electron density onto the metal centre than the traditionally strongly donating NHCs. 24Furthermore, Rivard and co-workers have demonstrated that NHOs can be considered ''soft'' donors, with higher s-donor propensity but less bonding strength than NHCs. 25n summary, this recommends NHOs for deeper investigations in organometallic chemistry, particularly if considered what penetrating impact NHCs have had in this field.NHO alkylidenes have not been described before, and we were curious as to if it was possible to generate them by direct reaction with a precursor complex.Selecting 2-methylene-1,3,4,5-tetramethylimidazoline (Scheme 2) as a sterically non-hindered and synthetically well accessible, strongly polar NHO 20,26 and W(CHCMe 2 Ph)(N-2,6-iPr 2 C 6 H 3 )(OTf) 2 (dme) 5 (5) as precursor alkylidene, it was found that direct reaction in diethyl ether using two equivalents of the NHO cleanly afforded complex 6.This compound bears two NHO-derived ligands; reactions using only one equivalent of NHO resulted in 50 : 50 mixtures of educt 5 and compound 6.Analytically pure samples were crystallized from CH 2 Cl 2 by adding small amounts of pentane.Complex 6 crystallizes in the triclinic space group P% 1, a = 117.112(7)  NHO alkylidene 6 adopts a slightly distorted octahedral configuration with one triflate and the imido ligand forming the apices.The tungsten alkylidene bond is 192.02(2)pm, which is comparable to the reported bistriflate NHC complexes. 1The NHO ligands are in trans-position (153.42(9)1) to each other and sp 3 hybridized at the methylene moiety, which indicates full charge separation and formation of a covalent bond to the metal centre.In the solid state the alkylidene is syn to the imido ligand (J C-H = 114.06ppm).With 227.04( 16) pm and 240.05( 16) pm the triflate-metal bonds are very dissimilar and weak.This is compounded by 19 F NMR, which displays a signal at À78.4 ppm, suggesting a weakly bound triflate in solution that is close in chemical shift to free triflate (see Table S2 for a comparison of This journal is © The Royal Society of Chemistry 2016 different alkylidene triflate complexes with regard to the chemical shift and bond length, ESI †). 1 Complex 6 was also subjected to a set of basic olefin metathesis reactions (Table S1, ESI †).The RCM of diallydiphenylsilane and 1-hexene led to conversion with TONs in the range of 140 to 250, but the reactivity was generally low, most probably as a result of the steric (and electronic) saturation of complex 6.Since NHOs are easily modulated, this situation is expected to improve soon and investigations are ongoing in our group.
In conclusion, anionic tungsten imido alkylidene complexes have been reported and fully characterized, accessible via a very simple route using imidazolinium chlorides.Additionally, an alkylidene NHO complex could be synthesized, which represents the first of a class of potentially very promising catalysts for metathesis reactions.