Seven‐Membered Cyclic Potassium Diamidoalumanyls

Abstract

The seven-membered cyclic potassium alumanyl species, [{SiN Mes }AlK] 2 [{SiN Mes } = {CH 2 SiMe 2 N(Mes)} 2 ; Mes = 2,4,6-Me 3 C 6 H 2 ], which adopts a dimeric structure supported by flanking K-aryl interactions, has been isolated either by direct reduction of the iodide precursor, [{SiN Mes }AlI], or in a stepwise manner via the intermediate dialumane, [{SiN Mes }Al] 2 . Although the intermediate dialumane has not been observed by reduction of a Dipp-substituted analogue (Dipp = 2,6-i-Pr 2 C 6 H 3 ), partial oxidation of the potassium alumanyl species, [{SiN Dipp }AlK] 2 , where {SiN Dipp } = {CH 2 SiMe 2 N(Dipp)} 2 , provided the extremely encumbered dialumane [{SiN Dipp }Al] 2 . [{SiN Dipp } AlK] 2 reacts with toluene by reductive activation of a methyl C(sp 3 )-H bond to provide the benzyl hydridoaluminate, [{SiN Dipp }AlH(CH 2 Ph)]K, and as a nucleophile with BPh 3 and RN=C=NR (R = i-Pr, Cy) to yield the respective Al-B-and Al-Cbonded potassium aluminaborate and alumina-amidinate products. The dimeric structure of [{SiN Dipp }AlK] 2 can be disrupted by partial or complete sequestration of potassium. Equimolar reactions with 18-crown-6 result in the corresponding monomeric potassium alumanyl, [{SiN Dipp }AlÀ K(18-cr-6)], which provides a rare example of a direct AlÀ K contact. In contrast, complete encapsulation of the potassium cation of [{SiN Dipp }AlK] 2 , either by an excess of 18-cr-6 or 2,2,2-cryptand, allows the respective isolation of bright orange chargeseparated species comprising the 'free' [{SiN Dipp }Al] À alumanyl anion. Density functional theory (DFT) calculations performed on this moiety indicate HOMO-LUMO energy gaps in the of order 200-250 kJ mol À 1 .

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

“…Compound 2 -K contains two molecules of TMEDA that are both bidentate at potassium. The Al–K bond length is intermediate between the other structurally characterized examples of discrete Al–K bonds, III Dipp -K­(18-crown-6) (3.9133(6) Å), and V -K­(toluene) 2 (3.4549(5) Å) …”

“…We therefore adopted an established method for sequestering the heavier group 1 metals by the addition of a polydentate cryptand ligand. In the case of potassium, this has been successful in the isolation of SIPs 31 The reaction between [2. gave bright yellow crystals of 4-K in high yields. In contrast, crystallization of 4-Na could only be achieved from THF.…”

“…The CDPs provide a convenient source of the aluminyl anions from which to access the corresponding MIPs and SIPs, provided that the donor ligands that are introduced support interactions that are sufficient to overcome the K•••π(arene) contacts. This has been demonstrated by the isolation of the MIP III Dipp -K(18-crown-6) 31 31 The motivation for accessing these different structural classes is more than just a curiosity, with the "naked" aluminyl anion component of the SIPs showing increased and in some cases divergent reactivity from the corresponding CDPs. A prime example of this is observed in the cleavage of either a C−H or C−C bond of benzene by CDP [I-K] 2 25 and SIP 32 respectively, demonstrating the impact that the group 1 metal cation exerts in mediating the reactivity of aluminyl anion [I] − .…”

“…The CDP structural motif is currently exclusive to the N -heterocyclic aluminyl anions that contain aryl substituents on the nitrogen atoms and is observed in the potassium aluminyls [ I -K] 2 , [ II -K] 2 , [ III Dipp -K] 2 , [ III Mes -K] 2 , and [ IV -K] 2 . In contrast, the dialkyl aluminyl V -K­(toluene) 2 was isolated as the MIP, containing a highly polarized Al–K bond and K···π­(arene) interactions supplied by two molecules of toluene.…”

“…The CDPs provide a convenient source of the aluminyl anions from which to access the corresponding MIPs and SIPs, provided that the donor ligands that are introduced support interactions that are sufficient to overcome the K···π­(arene) contacts. This has been demonstrated by the isolation of the MIP III Dipp -K­(18-crown-6) and the SIPs [ I ]­[K­(2.2.2)­crypt], [ III Dipp ]­[K­(2.2.2)­crypt], and [ III Dipp ]­[K­(18-crown-6) 1.5 ] . The motivation for accessing these different structural classes is more than just a curiosity, with the “naked” aluminyl anion component of the SIPs showing increased and in some cases divergent reactivity from the corresponding CDPs.…”