Developing a Hetero‐Alkali‐Metal Chemistry of 2,2,6,6‐Tetramethylpiperidide (TMP): Stoichiometric and Structural Diversity within a Series of Lithium/Sodium, Lithium/Potassium and Sodium/Potassium TMP Compounds

Abstract

Studied extensively in solution and in the solid state, Li(TMP) (TMP=2,2,6,6-tetramethylpiperidide) is an important utility reagent popular as a strongly basic, weakly nucleophilic tool for C–H metallation. Recently, there has been a surge in interest in mixed metal derivatives containing the bulky TMP anion. Herein, we start to develop hetero (alkali metal) TMP chemistry by reporting the N,N,N′,N′-tetramethylethylenediamine (TMEDA)-hemisolvated sodium–lithium cycloheterodimer [(tmeda)Na(μ-tmp)2Li], and its TMEDA-free variant [{Na(μ-tmp)Li(μ-tmp)}∞], which provides a rare example of a crystallographically authenticated polymeric alkali metal amide. Experimental observations suggest that the former is a kinetic intermediate en route to the latter thermodynamic product. Furthermore, a third modification, the mixed potassium–lithium-rich cycloheterotrimer [(tmeda)K(μ-tmp)Li(μ-tmp)Li(μ-tmp)], has also been synthesised and crystallographically characterised. On moving to the bulkier tridentate donor N,N,N′,N′′,N′′-pentamethyldiethylenediamine (PMDETA), the additional ligation forces the sodium–lithium and potassium–dilithium ring species to open giving the acyclic arc-shaped complexes [(pmdeta)Na(μ-tmp)Li(tmp)] and [(pmdeta)K(μ-tmp)Li(μ-tmp)Li(tmp)], respectively. Completing the series, the potassium–lithium and potassium–sodium derivatives [(pmdeta)K(μ-tmp)2M] (M=Li, Na) have also been isolated as closed structures with a distinctly asymmetric central MN2K ring. Collectively, these seven new bimetallic compounds display five distinct structural motifs, four of which have never hitherto been witnessed in TMP chemistry and three of which are unprecedented in the vast structural library of alkali metal amide chemistry.

Mixed Lithium Complexes: Structure and Applicationin Synthesis

“…More recently, the chemistry of mixed alkali metal TMP complexes has started to emerge [14]. During this work, the structures of three lithium-sodium TMP [11][12][13].…”

“…At this point in time, the only mixed Li/K homoleptic compounds that have been reported contain HMDS − , TMP − , or t BuO − anions. The amide [(THF) 3 • LiK [14]. Turning to heteroleptic mixed lithium-potassium species, two complexes have been crystallographically characterized.…”

“…Examples of homoleptic lithium-sodium TMP complexes 6-8[14]. Breastplate motif of the mixed lithium sodium tert-butoxide[Li 4 Na 4 (μ-O t Bu) 8 ] 9, and the trimetallic butoxides [(donor).…”

Nützliche Alkalimetallamide für die Synthese: Lithium‐, Natrium‐ und Kaliumhexamethyldisilazide, ‐diisopropylamide und ‐tetramethylpiperidide

“…TMP-Basen (Li/Na: 14, Li/K: 15, Na/K: 16) Eine bemerkenswerte Serie aus sieben TMP-Heteroalkalimetallkomplexen wurde vor kurzem von Mulvey und Robertson eingeführt. [77] Der TMEDA-solvatisierte Li/NaKomplex (14·TMEDA) offenbarte einen Li-N-Na-N-Ring mit einem einzelnen Molekül an TMEDA, das auf zweizäh-nige Weise an das schwerere Alkalimetallatom bindet, und einem Lithiumatom, das in einer gespannten zweifach koordinierten Umgebung vorlag [N-Li-N: 132.5(4)8]. Das Verhalten in Lçsung von 14·TMEDA konnte nicht ausreichend untersucht werden, da das TMEDA rasch vollständig dekoordiniert, woraufhin der resultierende lçsungsmittelfreie Stammkomplex 14 aus der Lçsung als Kettenpolymer mit alternierenden Lithium-und Natriumatomen ausfällt.…”

Lithium‐Free Synthesis of Sodium 2,2,6,6‐Tetramethylpiperidide and Its Synthetic Applications

“…Although several synthetic methods for the preparation of NaTMP have been reported, such as the deprotonation of TMPH with n BuNa prepared from n BuLi and NaO t Bu (Scheme 1A), [5] and the transmetallation between LiTMP and NaO t Bu (Scheme 1B), [5] the methods require lithium compounds and hence additional purification steps to obtain lithium-free NaTMP, which may exhibit different reactivity from that of lithium-containing NaTMP. [6] Furthermore, the use of the latter may be hampered by recent concerns about the scarcity of lithium as well as its rapidly increasing price because of competing consumption for the production of lithium-ion batteries. In continuation of our research interest in sustainable organic synthesis with organosodium compounds, [7] we report here novel direct syntheses of NaTMP using sodium dispersion (SD) without the need for lithium compounds.…”