Stable Mesoionic N‐Heterocyclic Olefins (mNHOs)

Abstract We report a new class of stable mesoionic N‐heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground‐state structure cannot be described through an uncharged mesomeric Lewis‐structure, thereby structurally distinguishing them from traditional N‐heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N′‐diaryl‐1,2,3‐triazolium and N,N′‐diaryl‐imidazolium salts, respectively. In their reactivity, they represent strong σ‐donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.


Introduction
Novel bonding modes of carbon, especially strong carbonbased donor ligands,s uch as carbenes,h ave received great interest throughout the last decades and continue to have large impact on the chemical community and beyond. Nheterocyclic olefins (NHOs, III), cyclic derivatives of ene-1,1diamines, [1] featuring formally an alkylidene moiety appended to aN -heterocyclic carbene,w ere first described by Kuhn et al. in 1993, [2] and popularized by the acronym NHOs by Rivard et al. in 2011. [3] As initially reported, they represent electron rich "ylidic olefins" which can be described through an eutral (III)a nd zwitterionic mesomeric structure (IV) electronically reminiscent to methylene phosphoranes (I/II) (Scheme 1). As ar esult of the good stabilization of the positive charge by the aromatic imidazolium moiety,N HOs are highly polarized towards the exocyclicc arbon atom leading to strong s-donor properties,even exceeding those of NHCs. [4] So far NHOs and their derivatives,w hich can be considered as deoxy-Breslow type intermediates, [5] have found broad application in several fields such as transition metal and organo catalysis, [6] main-group [7] and coordination chemistry, [8] or polymer chemistry. [9,10] Besides Arduengotype N-heterocyclic carbenes, [11] abnormal N-heterocyclic carbenes (aNHC), [12] and mesoionic carbenes (MICs), 1H-1,2,3-triazol-5-ylidenes [13] have been shown to be isolable, stable and bottable carbenes with strong s-donor properties exceeding traditional NHCs. [14] While formal methylene extension of phosphines and Nheterocyclic carbenes leads to methylene phosphoranes (I/II) and NHOs (III/IV)r espectively,w ew ere curious if formal methylene extension of mesoionic and abnormal carbenes would result in stable compounds (Scheme 1). Note,i n contrast to I-IV,s uch "olefins" would be mesoionic, [15] in analogy to MICs,asthey cannot be represented by acanonical Lewis resonance structure without charge separation (V-VIII). Such structural motive is rare,asin general, mesoionic compounds typically feature chalcogen atoms and not carbon in the exocyclicp osition. [15] Such an overlooked class of mesoionic N-heterocyclic olefins (mNHOs) should exhibit strong donor abilities,c onsidering ac orrelation with the parent carbene entities (see below).

Results and Discussion
In order to access V/VI we envisioned deprotonation of the corresponding methylated 1,2,3-triazolium salts. N,N'-Diaryl-1,2,3-triazolium salts such as 1 or 3 are easily accessible in two steps starting from commercial starting materials Scheme 1. Comparison between methylene phosphoranes (I/II), Nheterocyclic olefins (NHOs) (III/IV), and mNHOs (V-VIII)( this work). through formal cycloaddition of 1,3-diaza-2-azoniaallene salts with alkynes. [16] Upon addition of one equivalent potassium bis(trimethylsilyl)amide (KHMDS) to as olution of 1 an instant color change from colorless to deep purple is observed. Upon extraction with pentane, 2 is isolated as deep purple solid in 67 %y ield (Scheme 2). 2 is highly sensitive towards oxygen, however, stable as solid or in solution under inert atmosphere over several days. 1 HNMR spectroscopy of 2 in [D 8 ]THF at room temperature shows two signals at d = 3.03 and 2.19 ppm for the exocyclico lefinic (MIC) = CH 2 protons.T he unusual chemical shift range is in agreement with olefin signals observed for NHOs,b eing typically in the range of 2.4-3.2 ppm (for as ummary of typical NMR shifts,s ee the Supporting Information). Thee xocyclic 13 Cs ignal for 2 is obtained at pronounced high field (d = 44.3 ppm), clearly deviating from the normal olefin range,b ut in line with NHOs (see the Supporting Information). [5a] Intrigued by the broad shape of the proton signals 1 HEXSY NMR spectroscopy indicated, at first sight, exchange of the two exocyclic protons indicating rotation of the CÀCb ond. However,u pon addition of small quantities of KHMDS to the NMR sample we observe sharp 1 HNMR signals for 2 at room temperature (for an NMR discussion see the Supporting Information). We reason that asmall amount of H + catalyzes the proton exchange reaction (fast protonation, rotation, deprotonation). Clearly,t he unusual chemical 1 Ha nd 13 CNMR shifts indicate ah igh charge density at the exocyclic CH 2 moiety and ah ighly polarized zwitterionic structure (VI;S cheme 1). Next, we investigated the structurally simple symmetrical diaryl-1,2,3triazolium salt featuring two methyl groups (3). In analogy, addition of one equivalent KHMDS afforded, after extraction, 4 as red solid in 69 %y ield. From a À35 8 8Cp entane solution we were able to obtain single crystals of 2 suitable for X-ray diffraction ( Figure 1). [17] Most strikingly,t he CH 2 fragment is positioned coplanar to the five-membered ring. TheC1 À C2 distance [1.361 (1) ] is significantly shortened compared to the cationic starting material MICÀCH 3 (1) [ 1.478(5) ,s ee the Supporting Information] and slightly longer compared to Kuhnsreported tetramethyl substituted N-heterocyclic olefin [1.357-(3) ] [2a] (for ac omparison of bond distances,s ee the Supporting Information). We also obtained single crystals of 4,h owever,X -ray analysis cannot discriminate between the two À CH 3 / = CH 2 sites as in the solid-state both molecule orientations overlay,causing averaged bond distances (see the Supporting Information).
Next, we investigated mNHOs derived from abnormal NHCs.D eprotonation of 5,e asily accessible by methylation of the corresponding stable abnormal carbene,w ith one equivalent of KHMDS leads to an intense deep green solution (Scheme 3). Extraction with toluene affords 6 in 59 %y ield. 1 HNMR spectroscopy in [D 8 ]THF shows for the exocyclic moiety signals at 3.21 ppm and 2.04 ppm and a 13 Cs ignal at 46.6 ppm. We were able to obtain single crystals of 6 suitable for X-ray diffraction ( Figure 2). TheC 1 À C2 bond distance [1.363(2) ]i sc lose to 2 but longer than traditional NHOs (see Figure S2, Supporting Information) and is positioned in the range of recently reported lithiated NHOs. [7a,b] Interestingly, 6 is stable in the solid-state,b ut in contrast to 2 and 4, as low (t 1/2 % 5-6 days) rearrangement process takes place at room temperature to give 7.This species is reminiscent of the decomposition product of abnormal carbenes, [12] formed through deprotonation of the isopropyl group and intramolecular attack of the resulting anion onto the imidazolium C2-position. DFT calculations indicate the rearrangement from 6 to 7 being exergonic by DG %À3.2 kcal mol À1 ,w hile the analogous hypothetical rearrangement of 2 would be endergonic by circa 7.2 kcal mol À1 (see the Supporting Information). This result hints towards the extraordinarily high basicity of the new mNHO class.
Computational analysis at the TD-DFT [B3LYP/def2-TZVPP//B3LYP/def2-TZVP] level of theory is in good agreement with the observed visible transitions (2:c alc. 527 nm, 337 nm;for 4 and 6 see the Supporting Information). Them ain bathochromic shifted absorptions are due to HOMO!LUMO charge transfer transitions from the negatively polarized -CH 2 moiety to the cationic heterocyclic moiety (Figure 4).
In order to rationalize our experimental results,w e performed DFT calculations of the parent olefins A, B,a nd C as well as the full systems D, 2, 4,and 6 (Scheme 4). While A and B are isomeric structures,similar to NHC and aNHC,the deprotonation to generate A over B is thermodynamically favored by 21.8 kcal mol À1 (B3LYP/def2-TZVPP). Natural resonance theory (NRT) calculations implemented into the NBO 6.0 program were performed to evaluate the relative contributions of resonance structures.I nl ine with the definition of the word mesoionic, B and C cannot be satisfactory described by as ingle covalent or polar structure but expressed as ar esonance hybrid of as eries of dipolar canonical structures (for af ull list see the Supporting Information). [15] While A is described by at otal relative ratio of 64 %C = CH 2 and 32 %C ÀCH 2 À contribution, this ratio increases for B and C (both 57 %C=CH 2 to 43 %CÀCH 2 À )indicating alarger ylidic polarization of mNHOs compared to traditional NHOs (see the Supporting Information). In line,c alculated proton affinities [18] at the BLYP/def2-TZVPP level reflect the basicity trend A (274.      (1)  NHO D is the most basic NHO with ac alculated PA of 282.1 kcal mol À1 . [18] Clearly,b oth new mNHO classes exceed PA so ft raditional NHCs (262-275 kcal mol À1 ), [19] regular NHOs [18] and are at the limit of the strongest known carbon based PA s. [19,20] Interestingly,w ea lso calculated at the same level of theory the PA so ft he parent carbene entities,w hich feature an analogous trend (NHC:283.7;MIC:284.2;aNHC: 296.8).
Competition experiments between mNHOs and protonated species were performed in [D 8 ]THF to test the calculated PA trend (Scheme 5). Upon mixing E with mNHO 2,t he clean and quantitative formation of NHO F and triazolium salt 1 was observed (see the Supporting Information). Furthermore,a ddition of mNHO 6 to 1 leads quantitatively and instantaneously to mNHO 2 and salt 5.Both competition experiments clearly establish the experimental basicity trend NHO (F) < mNHO (2/4) < mNHO (6).
In order to study the application of the mNHOs as ligands and to analyze their donor strengths,w ep repared the corresponding rhodium and boron complexes (Scheme 6).
IR stretching frequencies of [RhCl(CO) 2 L] complexes are generally used to derive the Tolmann electronic parameter (TEP) which relates to the overall donor properties of al igand L. [21] As reference we prepared the corresponding IPr and IPr = CH 2 complexes and measured the IR carbonyl stretching frequencies in CH 2 Cl 2 ( Table 1; for ATRmeasurements see the Supporting Information). As pointed out previously, [4] NHOs are overall stronger donors than NHCs (n av 2038 vs.2 014 cm À1 ;e ntry 1a nd 2), due to negligible p-  backbonding.Importantly,inline with the basicity trends,the IR frequencies follow the trend n av (NHO) > n av (8) % n av (9) > n av (10). Note, 10 (TEP:2023 cm À1 )isanexceptional strong donor exceeding common ligand classes such as phosphines, NHCs,NHOs,a nd even MICs and aNHOs.
Interestingly,the donor properties of the new mNHOs can be correlated with the overall donor properties of the parent carbene entities.I Rs tretching frequencies of RhÀNHC (n av : 2038 cm À1 ;T EP:2 051 cm À1 ) > MIC (n av :2 032 cm À1 ;T EP: 2046 cm À1 ) > aNHC (n av :2 023 cm À1 ;T EP:2 039 cm À1 )c omplexes (see the Supporting Information), follow the same trend as observed for the corresponding olefins (shifted by D % 20 cm À1 ), but being able to participate in p-back donation. In the extreme picture,t his can be rationalized as at ransfer of the electronic properties through am ethylene spacer ( Figure 6). Furthermore,asT olman electronic parameter relate to the overall donor abilities (combined s and pcontributions), but not necessary to the metal-ligand bond strength, we also investigated through competition experiments which ligand class would form the thermodynamically most robust metal-ligand bond (Scheme 7).
Upon addition of mNHO 2 to (IPrCH 2 )RhCl(CO) 2 the clean liberation of free NHO F and the formation of 8 was observed. Additionally,mNHO 6 can displace mNHO 2 in the coordination sphere of rhodium (see the Supporting Information). These results are in line with the formation of the thermodynamically most stable Rh-complexes with the more donating ligands,a si nt hese cases p-backbonding can be neglected. However,the reaction of free IPr carbene with 8 or 10 leads to (IPr)RhCl(CO) 2 (Scheme 7 ii), indicating that the overall less donating free carbene forms the stronger metalligand bond as previously observed for regular NHOs. [4a] In this case the interplay of both s-donation and p-backbonding results in astronger metal-ligand bond.

Conclusion
In summary,w er eport as of ar overlooked class of mesoionic N-heterocyclic olefins (mNHOs), which are best described through an array of dipolar canonical resonance structures.T heir basicity and donor abilities outreach traditional NHOs as shown by experiment and theory.The simple synthetic access should allow straightforward tune-ability of their electronic and steric properties.W ec ould prepare the first rhodium and boron coordination complexes of mNHOs and expect al arger exploration in transition metal and main group chemistry,which is under current investigation.