N-Heterocyclic Carbenes and Charge Separation in Heterometallic s-Block Silylamides

Abstract

Addition of the N-heterocyclic carbene (NHC), 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene (IPr), to equimolar quantities of group 1 and group 2 bis(trimethylsilyl)amides results in the isolation of charge separated species, [M(IPr)(2)](+)[M'{N(SiMe(3))(2)}(3)](-) (M = Li, Na, K; M' = Mg, Ca, Sr, Ba). Although these systems were found to be prone to the separation of oily, most likely liquid clathrate, materials, either slow cooling or careful diffusion of the less polar solvent hexane into toluene solutions yielded nine crystalline heterobimetallic complexes in which the coordination sphere of the cationic group 1 center was found by X-ray diffraction analysis to be provided by two IPr ligands. These derivatives are the first examples of any compounds in which coordination at the central alkali metal cation is provided exclusively by NHC ligands and, for the cases where M = Na, are the first instances of any type in which an NHC ligand is bound to sodium. The anionic group 2-containing component of each compound was found to comprise three bis(trimethylsilyl)amido ligands coordinated in an approximately trigonal array about the divalent metal center. The bonding within the unusual cationic components of the compounds has been investigated by density functional theoretical (DFT) methods. Natural Bond Orbital (NBO) analyses have revealed that the coordination is provided by donation of the sp-hydridized IPr lone pair into the valence s-orbital of the alkali metal cation and are consistent with weaker binding, and consequently more labile solution behavior, as group 1 is descended.

Group 1 and 2 cyclic (alkyl)(amino)carbene complexes

“…20 It is noted that the C carbene resonance of free Me2 CAAC was reported to be 304.2 ppm (thf-d 8 ). 27 Complex 6 was structurally characterised in a single crystal X-ray diffraction study (Fig. 24 In order to further investigate the nature of the s-block M-CAAC bond, density functional theory (DFT) calculations were performed at the BP86 or B3LYP level for complexes 1, 2 and 4.…”

Molecular Manipulations of a Utility Nitrogen–Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

“…The elongation of the latter can be rationalized considering the ancillary nature of these NaÀCb onds when compared with the MgÀCb onds that anchor the magnesiate framework, to which the {Na(IPr)} + fragments are affixed. [14,15,22,23] While redistributionp rocesses on magnesiate chemistry have been previously noted, [24] this is as far as we can ascertain the first one induced by an N-heterocyclic carbene. The persistence of IPr coordination to Na is best indicated by an informative resonance at 202.9 ppm in the 13 CNMR spectrum at as imilar chemical shift to those previously found in other Na-NHCc omplexes.…”

“…[17,18] Performing this reactiona tr oom temperature in ahexane/benzene non-Lewis basic solvent mixture, using equimolar amounts of IPr and NaMgR 3 led to the isolation of sodium tetra(alkyl) magnesiate [IPr 2 Na 2 MgR 4 ]( 1)a s colourless crystals in a1 3% yield (note that the maximum possible yield for this reactioni s5 0%,S cheme 1a). At etracoordinated distorted tetrathedral Mga tom occupies the centreo ft he trinuclearb imetallic unit with four R-groups forming bridges with outer IPr-capped sodium atoms in an almostl inear arrangement along C1···Na1···Mg···Na2···C44 plane, evidenced by C1···Na1···Mg = 175.09 (15)8,N a1···Mg···Na2 = 176.32 (9)8 and Mg···Na2···C44 = 171.17 (16)8 angles. [19] X-ray crystallographic studies established the molecular structureo f1 to be ad iscrete contacted ion pair( CIP) structure ( Figure 1).…”

“…The persistence of IPr coordination to Na is best indicated by an informative resonance at 202.9 ppm in the 13 CNMR spectrum at as imilar chemical shift to those previously found in other Na-NHCc omplexes. [15] Scheme1.Synthesis of 1 and 2. Furthermore,p revious attempts trying to preparea nd isolate solvent free [Na 2 MgR 4 ]h ad provedu nsuccessful leadingi nstead to the isolation of the relevant triorganomagnesiate [NaMgR 3 ]a nd NaR.…”

“…[10] Furthermore, there are several literaturee xamples of structurally defined NHC complexes containing Mg amides, Grignard reagents and other dialkylmagnesium compounds. [15] Intriguedb yt hese contrasting reactivities, and aiming to expand the scope and applications of anionic NHCs in s-block metal chemistry, here we reportanovel series of magnesium compounds containing neutral, anionic and abnormal NHC ligands using am ixed-sodium-magnesium approach. [14] Interestingly,t hese studies using heteroleptic magnesiates which combine the highly basic amido group TMP (TMP = 2,2,6,6-tetramethylpiperidide) with butyl ligands, have revealed that the regioselectivi-ty of these reactions is finely controlled by the structure of the bimetallic base employed, enabling IPr to be metallated at its backbone, but also at the para positiono fo ne of its substituent aromatic arms.…”