Lithium Complexes with Bridging and Terminal NHC Ligands: The Decisive Influence of an Anionic Tether

: Deprotonation of the fluorenyl-tethered imidazolin-ium salt [9-(C 13 H 9 )C 2 H 4 N(CH)C 2 H 4 N(2,4,6-Me 3 C 6 H 2 )][BF 4 ] gave a spirocyclic compound that reacted with a synergic mixture of LiPh/LiN(SiMe 3 ) 2 or Li n Bu/LiN(SiMe 3 ) 2 to give a dilithium complex incorporating a bridging N(SiMe 3 ) 2 ligand. In contrast, de-protonation of the imidazolium salt [9-(C 13 H 9 )C 2 H 4 N(CH)- C 2 H 2 N(Me)][Br] instead yielded the free NHC, which reacted with n BuLi to form a dimeric, NHC-bridged dilithium complex. Addition of LiN(SiMe 3 ) 2 led to coordination and the formation


Introduction
N-heterocyclic carbenes (NHCs) are well established as strong σ-donor ligands in coordination chemistry and homogeneous catalysis. [1]The N-substituents can be changed to predominantly alter the steric profile of the ligand, [2] and the carbon backbone can be unsaturated, benzannulated or saturated to change the influence of aromaticity on the NHC's properties. [3]t has been established that saturated NHCs, whilst giving Rh carbonyl or Ni carbonyl complexes with very similar CO stretching frequencies to unsaturated NHCs, [1a,4] can have substantially different donor properties to unsaturated NHCs [1a,5] including: i) increased activity in catalytic reactions, [6] ii) the potential for enhanced π-backdonation [4c,6c,7] and iii) enhanced stability against reactions of the backbone. [8]NHCs, and other stabilised carbenes, have also been utilised extensively for supporting unusual bonding and oxidation states in the p-block, with the strong σ-donor properties of these ligands proving essential in stabilising many molecules of fundamental interest. [9]These include the first examples of a molecule with a boron-boron triple bond and a disilicon(0) compound with a Si-Si double bond, both achieved using the stabilisation provided by the coordination of two NHCs. [10]For the s-block, [11] coordination chemistry has mainly focused on unsaturated NHCs, [8c,12] with of a dilithium complex with a bridging N(SiMe 3 ) 2 ligand, which was characterised in the solid state as a 1D coordination polymer.The reaction of 1,3-bis (2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with lithium indenide and lithium fluorenide gave soluble species with terminal binding of the NHC to the lithium cation and η 5 coordination of indenyl or fluorenyl.A symmetrical bridging mode for an NHC donor was therefore observed only if a tethered fluorenyl anion was present with no additional amide ligand.far fewer complexes of saturated NHCs [13] and CAACs (cyclic alkyl amino carbenes) [14] described.
Tethered NHC ligands feature an NHC attached to another donor group, such as neutral P, S or N donors, [15] or anionic donors [16] such as amide, [17] alkoxide/aryloxide [15g,18] and Cp, including benzannulated analogues such as indenyl (Ind) or fluorenyl (Flu). [19]With the anionic donors, a hybrid ligand is realised [20] that features very different bonding from the two donors, with the "soft" NHC donor featuring a large component of covalency in bonding to metals, whereas the anionic donors, particular with "hard" O atoms, feature a substantial ionic component to the bonding.This can lead to interesting hemilability effects of the NHC in early transition metal and lanthanide complexes, [21] or the potential for lability/reactivity of the O donor in late transition metal complexes. [22]With Cp, Ind and Flu donors, the situation is more nuanced with strong donation expected from both donors to late transition metals.This will lead to enhanced overall stability of the complex through the chelate effect, whilst constraining the bite angle between the two donors.This has a knock-on effect on the energies of the various metal orbitals involved in ligand bonding, as well as the frontier molecular orbitals, as seen in small bite-angle ligand systems. [23]lthough NHCs are predominantly observed as terminal ligands, there are situations where bridging behaviour is seen, [24] a situation more widely encountered in the tin analogues of saturated NHCs, N-heterocyclic stannylenes (NHSns). [25]Bridging behaviour is most often observed with tethered NHCs when coordinated to Cu and Ag. [26]Here, the geometric constraints of the tether lead to many complexes that feature bridging NHCs, however, there are few investigations into the factors that produce terminal and bridging complexes with alkali metals. [11]In this work, we explore how the coordination to lithium cations is affected by tethering the NHC to a fluorenide anion, how modifying the NHC can produce structural differences and what effect a bridging amide ligand has.This is contrasted with terminal 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) coordination to lithium indenide or fluorenide.
Comparing 3, 6 and 7, both 6 and 7 feature η 5 interactions to the central fluorenyl rings, whereas 3, and the Dipp analogue, [27] both feature η 6 interactions to flanking 6-membered rings of the fluorenyl system.25d] It is likely that the smaller Me substituent in 7 leads to additional intermolecular interactions forming a coordination polymer, whereas the larger Mes and Dipp substituents sterically protect the 2-coordinate Li atom.Asymmetric bridging has been seen in other Li structures with bridging NHC ligands [C-Li = 2.169( 5) and 2.339( 5) Å; C-Li = 2.181(3) and 2.335(3) Å]. [24c,30] In non-coordinating solvents, 7 Li NMR spectroscopy was useful in providing information about the coordination environment around the Li atoms in solution.For 3, two 7 Li resonances were observed at -1.45 ppm and -5.62 ppm, in accordance with the solid-state structure that has a two-coordinate Li atom and a Li atom bound η 6 -to an aromatic benzene ring in the fluorenyl ligand observed at very low chemical shift.6 could not be dissolved in non-coordinating solvents, while the use of [D 8 ]thf resulted in the complete loss of the bridging structure as 7 Li NMR spectroscopy revealed 4-coordinate tetrahedral [Li(thf ) 4 ] + ions (δ = 0.0 ppm).7 showed 7 Li resonances that are very similar to 3 (-1.58and -5.52 ppm). 13C{ 1 H} NMR spectroscopy showed a broad resonance for 7 at 195.8 ppm for the carbenic carbon due to coupling to the quadrupolar 7 Li nucleus.Very broad and weak resonances were observed for 3 and the Dipp analogue as well (see the Supporting Information), whereas 6 in thf showed a free, unbound carbene (sharp resonance at 197.5 ppm).Thus, it has been demonstrated that fluorenyl-tethered NHC systems bind LiN(SiMe 3 ) 2 to form homobimetallic structures with ease for a variety of N-substituents.The NMR spectroscopic properties and structures of these species are distinctive, and they are stable, soluble species in non-coordinating solvents.Without LiN(SiMe 3 ) 2 , an NHCbridged dimer is formed that is very poorly soluble (6), with the bridging structure lost in thf solution.

Lithium Complexes with SIPr
12h] Adding SIPr to insoluble LiInd and LiFlu in benzene and heating to 80 °C caused the dissolution of these species forming either a pale-yellow (8) or orange solution (9, Scheme 2). 7Li NMR spectroscopic studies showed a single resonance for each species at very low chemical shift; -9.75 ppm for 8 and-8.9525d] For unsaturated-NHC adducts of the Cp derivative C 5 H 2 (SiMe 3 ) 3 , 7Li NMR chemical shifts of δ = -7.78,-7.63 and -9.01 ppm (NHC substituents = tert-butyl, 1-adamantyl and Mes, respectively) were observed [12h] at lower chemical shift than LiCp (-6.9 ppm). [32]Clearly, the additional NHC donor has led to substantially lower chemical shifts for the 7 Li atoms for a variety of NHCs and Cp, Ind and Flu.The 13 C NMR spectroscopic resonances for the carbene C atoms of both 8 and 9 were extremely broad and weak, presumably due to coupling to the quadrupolar 7 Li nucleus (see the Supporting Information).Upon standing at room temperature, colourless crystals of 8 (24 %) and yellow crystals of 9 (54 %) were formed that were suitable for X-ray diffraction experiments.Solving and refining this data demonstrated that the NHC ligands were bound in a terminal fashion to the lithium cations [Li-C carbene = 2.103( 2 2) Å], but the Ind centroid -Li-C carbene angle is not linear (169.0°).12h] Complexes 8 and 9 demonstrate the solubilising properties of the SIPr ligand through conventional terminal binding to the Li cation.

Conclusions
This study has revealed a variety of coordination modes for NHCs with lithium cations.The tethered NHC systems, whether they have a large substituent (Mes) or small substituent (Me), are all primed to coordinate one equivalent of lithium hexamethyldisilylazide, incorporating it into the ligand pocket with a bridging amide between the two lithium atoms.Without the bridging amide, which could only be achieved synthetically for the unsaturated NHC ligand because the saturated analogue required a synergic mixture of LiPh/Li amide, a dilithium species was produced with symmetrically-bridging NHC donors.The monodentate NHC SIPr ligand was found to coordinate in a conventional terminal fashion to lithium fluorenide and lithium indenide, greatly increasing the solubility of these organometallic reagents.
Crystallographic Details: Single crystals suitable for X-ray diffraction were covered in inert oil and placed under the cold stream of a Bruker D8 Venture at 100 K (7-9) or an Oxford Diffraction fourcircle Supernova diffractometer (University of Edinburgh) at 120 K (3 and 6).Exposures were collected using Mo-K α radiation (λ = 0.71073).Indexing, data collection and absorption corrections were performed.The structures were then solved using SHELXT [37] and refined by full-matrix least-squares refinement (SHELXL) [38] interfaced with the programme OLEX2 [39] (Table S1).The fluorenyl ligand in 9 was found to be disordered over several positions and the C atoms could only be satisfactorily modelled isotropically.
CCDC 1946066 (for 3), 1946067 (for 6), 1946068 (for 7), 1946069 (for 8), and 1946070 (for 9) contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre.
The molecular structure of 6 shows two Li atoms complexed to two NHC-tethered fluorenyl ligands bridged through the NHC C atoms with the whole molecule positioned on a two-fold rotation axis.The Li cations are η 5 -coordinated to the fluorenide anions [Li-C distances from 2.269(4) to 2.343(4) Å], and the Li-NHC distances are very similar [2.226(4) and 2.246(4) Å], although slightly longer than the terminal interaction in 3 [2.109(6)Å].The solid-state structure of 7 shows an extended 1D coordination polymer of tethered NHC ligands, with Li atoms η 5 -coordinated to the central fluorenyl rings and connected to a μ 2 -N(SiMe 3 ) 2 ligand.Connections between molecules are formed with the other 3-coordinate Li atoms that are bound to the NHC, μ 2 -N(SiMe 3 ) 2 and a neighbouring fluorenyl ring.These intermolecular interactions could be considered to be η 1 or η 2 , as the closest Li-C distance is between 2.431(3) to 2.514(3) Å, whilst the interaction to the neighbouring C atom ranges between 2.542(3) and 2.728(3) Å.The angles between the centroids of the η 5 -fluorenyl interactions, Li atoms and bridging N atoms are 174.