Hydrocarbon Soluble Alkali‐Metal‐Aluminium Hydride Surrog[ATES]

Abstract

A series of group 1 hydrocarbon-soluble donor free aluminates [AM( t BuDHP)(TMP)Al( i Bu) 2 ] (AM=Li, Na, K, Rb) have been synthesised by combining an alkali metal dihydropyridyl unit [(2-t BuC 5 H 5 N)AM)] containing a surrogate hydride (sp 3 CÀ H) with [( i Bu) 2 Al(TMP)]. These aluminates have been characterised by X-ray crystallography and NMR spectroscopy. While the lithium aluminate forms a monomer, the heavier alkali metal aluminates exist as polymeric chains propagated by non-covalent interactions between the alkali metal cations and the alkyldihydropyridyl units. Solvates [(THF)Li-( t BuDHP)(TMP)Al( i Bu) 2 ] and [(TMEDA)Na( t BuDHP)(TMP)Al( i Bu) 2 ] have also been crystallographically characterised. Theoretical calculations show how the dispersion forces tend to increase on moving from Li to Rb, as opposed to the electrostatic forces of stabilization, which are orders of magnitude more significant. Having unique structural features, these bimetallic compounds can be considered as starting points for exploring unique reactivity trends as alkali-metal-aluminium hydride surrog [ATES].

Incorporating Trimethylaluminium into the Structures of Alkali Metal (Li, Na, K, Cs) dihydropyridines

“…These Al-N distances are longer than those seen in Nikonov's recent series of neutral Al-DHP complexes [1.815(5)-1.839( 2) Å] [16] on account of having four anionic ligands rather than three, and are more in line with the related AM(tBuDHP)Al(TMP)iBu2 complexes [1.937(2)-1.986( 2) Å] which have a similar arrangement. [8] The Al-DHP relationship also differs from that in AM(tBuDHP)AlMe3 on The AM-NDHP distances are longer than the dative AM-Ndonor distances in each complex 1-4, suggesting that there is little interaction between the alkali-metal and the formally amido (DHP) anion. The position of the alkali-metal barely changes across the series 1-4, as demonstrated by the AM-N/AM-C and AM-C/AM-C' ratios which lie in a tight range (0.918-1.057) and demonstrate the central location of the alkali-metal above the C5N ring in all cases.…”

“…Specifically, the lithium complex was prepared by the stoichiometric 1,2-addition of tBuLi to pyridine in hexane solution, [9a] with the heavier complexes formed by transmetallation of this lithium complex using an appropriate alkali-metal alkoxide (scheme 1). [8,10] The resulting homometallic complexes were then co-complexed with AlMe3 in hexane followed by the addition of a polydentate N-donor until a homogeneous solution was obtained, with crystals being obtained for four complexes (specifically containing the alkali-metals Li, Na, K and Cs; 1-4 respectively) upon standing the solutions at -26 o C. Determined by X-ray diffraction studies, the crystal structures of these complexes are displayed in figures 1-4 with pertinent bond parameters listed collectively in table 1. 20(5) 53.55(6) N1-Al1-C10 108.86(6) 108.44(17) 109.38(18) 109.44(9) 107.24(9) 106.92(11) N1-Al1-C11 103.96(5) 103.61(18) 104.95(21) 108.05(8) 107.76(9) 108.67(11) N1-Al1-C12 109.00(5) 110.25(17) 110.41(17) 109.24(8) 111.13(9) 110.76(11) Al1-C12-AM1A ---176.69 (10) 174.41(11) 158.38( 14)…”

“…In practice, that reaction only works efficiently with lithium, [9] as the lower stability of the heavier AMtBu complexes require that the AM(tBuDHP) (AM = Na, K, Rb, Cs) complexes are best prepared by transmetallation from the lithium congener and heavier alkali tert-alkoxides. [8,10] These monometallic alkali metal complexes, though formally a special category of alkali metal amide, have shown promise in several catalytic applications as hydride surrogates including hydroboration of aldehydes and ketones, dehydrogenation of diamine boranes and dehydrocoupling of amine boranes. [11] The hydride Incorporating trimethylaluminium into the structures of alkali metal (Li, Na, K, Cs) dihydropyridines comes from in situ elimination of an alkali metal hydride unit facilitated by aromatisation of the metal dihydropyridine to a neutral pyridine.…”

“…[7] In our latest contribution to expanding the variety of aluminium hydride species, we reported the synthesis and crystal structures of the alkali metal aluminate complexes [AM(tBuDHP)(TMP)Al(iBu) 2 ] (AM = Li, Na, K, Rb), where tBuDHP is the 2-tert-butyl-1,2-dihydropyridyl anion (TMP is 2,2,6,6tetramethylpiperidide). [8] Formally, the AM(tBuDHP) moieties represent the product of the addition of the corresponding metal alkyls AMtBu across the azomethine bond of pyridine, an action which destroys the aromaticity of pyridine in forming a dihydropyridyl anion that contains an aza-dienyl unit. In practice, that reaction only works efficiently with lithium, [9] as the lower stability of the heavier AMtBu complexes require that the AM(tBuDHP) (AM = Na, K, Rb, Cs) complexes are best prepared by transmetallation from the lithium congener and heavier alkali metal tert-alkoxides.…”

“…Co-complexation reactions of the set of AM(tBuDHP) complexes with the bulky dialkylaluminium amide iBu 2 Al(TMP) generated the [AM(tBuDHP)(TMP)Al(iBu) 2 ] aluminate complexes (AM = Li, Na, K, Rb) though the caesium congener could not be obtained in a pure solid form. [8] The aim of this present study was to attempt to prepare and fully characterize a complementary series of alkali metal dihydropyridyl aluminates using the commercial alkylaluminium compound Me 3 Al in place of non-commercial iBu 2 Al(TMP), which needs to be synthesised from iBu 2 AlCl and LiTMP. [12] As now outlined, this study has been successful with the characterization of four alkali-metal aluminate complexes.…”