Hexameric lithium phenolate: crystal structures and lithium-7 quadrupole coupling in solid and solution phases

Abstract

Whereas lithium phenolate in dioxolane coexists as a mixture of comparable amounts of a hexamer and tetramer, together with minor amounts of less aggregated species, in THF the predominant species is the tetramer. Two crystalline forms of lithium phenolate have been obtained from THF and THF/hexane solutions, and X-ray crystallographic studies show that they consist of very similar hexagonal prismatic hexamers of lithium phenolate with one THF molecule bound to each lithium cation. The 7Li nuclear quadrupole coupling constant (QCC = e2Qqzz/h = 67 f 1 kHz) and the asymmetry parameter (7 = 0.77 f 0.05) have been obtained from line shape analysis of the powder spectrum of one of the crystalline samples. Proton decoupling was used to remove 1H/7Li dipoledipole interactions, and samples diluted with 6Li were employed to estimate the contribution of the 7Li/7Li interaction to the line shape.These data give a value (73 kHz) for the quadrupole splitting constant [QSC = (1 + ~2/3)112(eZQqzz/h)] in agreement with that (72 f 10 kHz) determined for hexameric lithium phenolate in dioxolane from spin-lattice relaxation data. Thevalues of QSC for the tetrameric species in THF and dioxolane are 46 f 0.5 and 59 f 6 kHz, respectively. CP-MAS 13C NMR spectroscopy is of limited value for characterizing solid samples of the aggregates.The reactions of organic lithium compounds with electrophiles in weakly polar aprotic solvents are known to be influenced by thestateof aggregation of thelithiumreagent. Thosecompounds in which lithium is bound to oxygen most commonly exist as dimers and/or tetramers. More recently, however, the formation of hexamers has been established and, in one case at least, a hexamer has been shown to be the primary reactant.' In this paper, we present a full characterization of the hexameric lithium phenolate (LiOPh*SOlV)6 in the solid state (Solv = THF) and in solution (Solv = dioxolane), which we regard as archetypal hexamers. The study includes X-ray structures as well as characterization of the species in both the solid state and solution by 13C and 'Li NMR spectroscopy.Several hexameric lithium enolates have previously been characterized in the crystalline state.2 The unsolvated hexamer of lithium enolate derived from pinacolone generated with LDA in heptane has a hexagonal prismatic structure.2a.b A similar structure has been found for the lithium enolate of ethyl N,Ndiethylglycinate, in which the five-membered chelate rings stabilize the hexamer.2c Lithium N-isopropylbenzamide forms the same type of hexameric crystals in THF/hexane when 1 equiv of TMEDA is added."r This aggregate, which contains three THF molecules but no TMEDA, is also stabilized by chelation. Hexamers of (hexamethy1enimido)lithium have tricyclic laddertype structures.3Our earlier studies of solution species revealed the existence of hexameric lithium phenolates in the weakly polar aprotic solvent dioxolane. For example, in dioxolane at low temperatures, both lithium phenolate and its 3,s-dimethyl homologue coexist as the Hodgson, S. M.;...

Lithium and Ytterbium Aminotroponate Complexes

“…Since no solvent molecules are bound to the lithium atoms, they are -as a result of the hexamerization -tetracoordinated. Similar hexameric structures were observed before in some lithium alkoxides such as lithium phenolate, [25] [LiCϵCSiMe 2 C 6 H 4 OMe] 6 , [26] and the thiophene derivative [LiOSiMe 2 (2-C 4 H 3 S)] 6 . [27] The 1 H and 13 C{ 1 H} NMR spectra of compound 1 show the expected set of signals for the (iPrAT) -ligand.…”

Synthesis and Structural Characterization of Lithium and Iron Complexes Containing a Chelating Phenolate Phosphane Ligand

“…The 1 H NMR spectra of these solutions are indicative of the presence of C 1 -symmetric molecules, due possibly to the formation of higher aggregates. [26][27][28] In agreement with this hypothesis, these precipitates re-dissolve in solution upon addition of an excess of diethyl ether. Similar phenomena were also found for {[L tBu ]Li 2 (DME)} 2 .…”

Die Kristallstruktur von Lithiummesitolat [{LiOC6H2-2,4,6-(CH3)3}4(THF)3]

“…Die Koordinationszahl des Li-Kations betra È gt meist vier, seltener drei [9] oder gar zwei [8]. Lithiumphenolat-Tetrahydrofuran bildet im Festzustand sowohl ein Hexamer [4] als auch ein Tetramer [10], wa È hrend 2,6-substituierte Phenolate dimer auftreten ko È nnen (s. Ûbersicht in [7]). LiOC 6 H 3 -2,6-Me 2 und LiOC 6 H 2 ±2,4,6-Me 3 bilden in Lo È sung vorwiegend dimere Einheiten [13,14].…”