Synergic Deprotonation Generates Alkali‐Metal Salts of Tethered Fluorenide‐NHC Ligands Co‐Complexed to Alkali‐Metal Amides

Abstract Synergic combinations of alkali‐metal hydrocarbyl/amide reagents were used to synthesise saturated N‐heterocyclic carbene (NHC) ligands tethered to a fluorenide anion through deprotonation of a spirocyclic precursor, whereas conventional bases were not successful. The Li2 derivatives displayed a bridging amide between two Li atoms within the fluorenide‐NHC pocket, whereas the Na2 and K2 analogues displayed extended solid‐state structures with the fluorenide‐NHC ligand chelating one alkali metal centre.

forded the desired NHC-fluorenide salt. [11] Potassium salts of alkoxy-carbenes with an unsaturated backbone could also be readily formed from the parent imidazolium salt with an excess of KH. [5b] In contrast, the analogous saturated alkoxycarbene system (Scheme 1b)d isplayed different behaviour in which spirocyclic III was formed with one equivalent of base, and it was then ring-opened with strongly basic rare-earth salts. [12] Previously,w eh ave used diamino-fluorene precursors to generate saturated N-heterocyclics tannylenes (NHSns) with a fluorenyl tether, [13] and we recognised that the corresponding saturated NHCs could be accessible from the same precursor. Herein, we describe that synergic bimetallic bases are required to generate fluorenide-tethered saturated NHCs, forming bimetallicc omplexes with the alkali-metal amide in the process (Scheme 1c).
Imidazoliniums alt 1 was synthesised by orthoformatec yclisation using NH 4 BF 4 from ap reviously reported diamine [13] (Scheme 2, see the Supporting Information for the molecular structure). [14] In contrast to the reactivity found for the unsaturated analogues, [10a] reactiono fi midazoliniums alt 1 with nBuLi yielded spirocycle 2 as ac olourless solid. [12] Compound 2 was found to be highly soluble in organic solvents andw as recrystallisedf rom as aturated petroleum-ether solution (see the Supporting Information for the moleculars tructure). 1 HNMR spectroscopy reveals ad istinctive singlet at 5.45 ppm fort he imidazoline Ha tom, and all four CH 3 groups on the 2,6-diisopropylphenyl (Dipp) substituent are inequivalent in 2.T he cyclisation is similar to that observed for saturated alkoxy-carbenes. [12a] This reactivity is relatedt ot he differing electronics between saturated and unsaturated NHCs, suggesting that saturated NHCs act as better nucleophiles and electrophiles. [15] Althought he alkoxy-carbene adduct III could be ring-opened with strong bases, attempts to do so with 2 using nBuLi or KCH 2 Ph were unsuccessful. Even the use of nBuLi/TMEDA gave minimal conversion to the desired product, whereas Schlosser's base (nBuLi/KOtBu) [16] led to am ixture of products.
Moving to the heavierG roup 1a nalogues, reaction of 2 with either 1:1N aCH 2 Ph/NaN(SiMe 3 ) 2 or 1:1K CH 2 Ph/KN(SiMe 3 ) 2 yielded very poorly soluble complexes 6 and 7 as polymeric disodiuma nd dipotassium species, respectively ( Figure 3). In 6, one Na fully occupies the fluorenyl-carbene pocket, while the other interacts with one of the six-membered rings of the tethered fluorenyl and an eighbouring fluorenyl group. The pocket of 7 is occupied with as ingle Ka nd the second Ki nteracts with the flanking Dipp aryl ring and the fluorenyl ring on a neighbouring unit. This creates ap rogression in which, as the size of the cation increases, the more it favourst he single oc-cupation of the fluorenyl-carbene pocket. The NaÀcarbene distance of 2.578(3) closely matches previously reported distances. [5d,g, 6a] The KÀcarbene bond length of 3.011(5) also fits within the range of previously reported K-carbene complexes, [5a,b,g, 6a, 7c, 10a] whereas the angle of 23.38 between Ka nd the NCN plane (Table 1) is similar to those in previouss tructures. The angle between the NCN plane and the metal also fits the trend previously mentioned:a st he metal cation increasesi ns ize, so does the distortion from planarity.T he yaw angle is lessa ffected by the change in metal cation. The MÀN bond length between the metal bound to the NHC and the amide increases with the metal ion size and remains longer than the MÀNd istance to the other metal ion. Av ariety of coordination modes to fluorenide anions have been previously observed. With sodium,i nteractions are dominated by h 5 -a nd slipped h 5 -geometries, [22] whereas potassium interactions are more varied. [23] Preliminary studies have shown that 3 acts as au seful ligand-transfer reagent in the reaction with [Rh(CO) 2 Cl] 2 ,i n which coordination of both the NHC and fluorenide donors to aR h(CO) fragment is observed. The presence of LiN(SiMe 3 ) 2 does not interfere with this reaction,a nd work in this direction is currently ongoing.
In summary,s ynergic combinationso fa lkali-metal reagents have been used to accesst ethered saturated-NHC complexes forming lithium,s odium and potassium homobimetallic complexes. The lithium complexes feature ab ridging amide between the two Li atoms within the fluorenyl-NHC pocket and are monomeric in nature. This contrasts with the sodium and potassium complexes that are polymeric because the fluorenyl-NHCp ocket is filled by only one metal cation leadingt ot he otherm etal cation forming interactions between molecules. The structure of the homobimetallic species LiPh/LiN(SiMe 3 ) 2 revealed aL i 2 (m 2 -X) 2 core forming ap olymerics tructure through additional Li-h 3 -Ph interactions. [24] (3) 2.361(2) 2.727 (3) [a] Deviation from planarity of Mtot he NCN plane.
[b] Deviation from linearity of the M-NHC angle. Figure 3. Molecular structures of 6 and 7 (thermale llipsoidsat5 0%). All H atoms have been removed for clarity,except on the saturated NHC backbone. The fluorenyl ring system of additional molecules have been displayed to demonstrate their extended nature.