Controlling Al–M Interactions in Group 1 Metal Aluminyls (M = Li, Na, and K). Facile Conversion of Dimers to Monomeric and Separated Ion Pairs

Abstract

The aluminyl compounds [M{Al(NON Dipp )}] 2 (NON Dipp = [O(SiMe 2 NDipp) 2 ] 2− , Dipp = 2,6-iPr 2 C 6 H 3 ), which exist as contacted dimeric pairs in both the solution and solid states, have been converted to monomeric ion pairs and separated ion pairs for each of the group 1 metals, M = Li, Na, and K. The monomeric ion pairs contain discrete, highly polarized Al−M bonds between the aluminum and the group 1 metal and have been isolated with monodentate (THF, M = Li and Na) or bidentate (TMEDA, M = Li, Na, and K) ligands at M. The separated ion pairs comprise group 1 cations that are encapsulated by polydentate ligands, rendering the aluminyl anion, [Al(NON Dipp )] − "naked". For M = Li, this structure type was isolated as the [Li(TMEDA) 2 ] + salt directly from a solution of the corresponding contacted dimeric pair in neat TMEDA, while the polydentate [2.2.2]cryptand ligand was used to generate the separated ion pairs for the heavier group 1 metals M = Na and K. This work shows that starting from the corresponding contacted dimeric pairs, the extent of the Al−M interaction in these aluminyl systems can be readily controlled with appropriate chelating reagents.

Alkali metal influences in aluminyl complexes

“…9 In contrast, state-of-the-art calculations on the reaction of I with H 2 did not reveal any assisting role of the K + ion. 10 Although the first series of aluminyl anions have been made exclusively with K + counter cations, it is meanwhile clear that these aluminyls are also stable as free anions 11 or as contact ion-pairs with other alkali metal cations like Li + or Na + . 11 b ,12 Recently, complex II has been prepared with the full series of alkali metal counter cations Li–Cs.…”

Trapping an Unusual Ring-Opened Product of THF within a Lithium Hydrido Aluminate

“…However, the chemical shift is inconsistent with VI , where a strong ring-current effect causes a shift to high field (δ Li −2.93) . Furthermore, the chemical shift confirms that the expected MIP (NON Dipp )­Al–Li­(THF) 3 is not formed (δ Li 3.25), although the 7 Li NMR resonance for 1 is in the region associated with other (NON Dipp )­Al–Li­(L) n MIPs (δ Li 2.68 (C 7 D 8 , 233 K) for L = Et 2 O, n = 2; δ Li 3.66 (C 7 D 8 ) for L = TMEDA, n = 1).…”

“…This compound exists as a contacted dimeric pair (CDP) in the solid state and in solution, with the two “Li­[Al­(NON Dipp )]” fragments linked by reciprocal Li···π­(arene) interactions to a Dipp group of a neighboring unit. Upon the addition of Et 2 O, THF, or TMEDA, the CDP is cleaved to afford the corresponding monomeric ionic pairs (MIPs) (NON Dipp )­Al–Li­(Et 2 O) 2 , (NON Dipp )­Al–Li­(THF) 3 ( VII ), and (NON Dipp )­Al–Li­(TMEDA) . X-ray structures show that these MIPs contain rare examples of unsupported Al–Li bonds, with computational analyses indicating stabilization from strong donor–acceptor interactions between the [Al­(NON Dipp )] − aluminyl and the cationic [Li­( L ) x ] + components.…”

“…We therefore examined the stability of THF solutions of Al­(NON Dipp )­I and the contact dimeric pair [Li­{Al­(NON Dipp )}] 2 (which dissociates to form the monomeric ion pair (NON Dipp )­Al–Li­(THF) 3 ( VII ) when dissolved in THF) by NMR spectroscopy (Scheme ). In all cases, analysis of the crude products did not reveal any of the key signatures for the formation of 1 (Figures S10 and S11), suggesting that the ring opening occurs during the reduction process.…”

Extending chain growth beyond C1 → C4 in CO homologation: aluminyl promoted formation of the [C5O5]5− ligand

“…27 Our research interests have focused on the synthesis and reactivity of anionic aluminyl anions. 28 We have shown that the aluminyl anion [Al(NON Dipp )] − is reactive towards gases such as CO 2 and N 2 O, 29 and reported the reaction of CO with an alumoxane (to form the ethenetetraolate ligand), 30 and as a reagent for the carbonylation of ethene. 31 Although the activation of CO has been observed by an anionic aluminium imide, 32 it is surprising that the reactivity between the parent aluminyl systems and CO has yet to be reported.…”