A Structural and Computational Study of Synthetically Important Alkali‐Metal/Tetramethylpiperidide (TMP) Amine Solvates

Abstract

Two heavy alkali-metal salts of the sterically demanding amine, 2,2,6,6-tetramethylpiperidine (TMPH), have been prepared using different methodologies. Complex 1, [((tmeda)Na(tmp))2] (TMEDA=N,N, N',N'-tetramethylethylenediamine), can be synthesized by a deprotonative route. This is achieved by reacting butylsodium with TMPH in the presence of excess TMEDA in hexane. The potassium congener [((tmeda)K(tmp))2] (2), can be prepared by treating KTMP (made using a metathesis reaction between LiTMP and potassium tert-butoxide) with an excess of TMEDA in hexane. In the solid state, 1 and 2 are essentially isostructural. They are discretely dimeric and their framework consists of a four-membered M-N-M-N ring (M=Na or K, N=TMP). Due to the high steric demand of the TMP ligand, the TMEDA molecules bind to the metal centers in an asymmetric manner. In 2, each of the coordination spheres of the metals is completed by an agostic K...CH3(TMP) interaction. DFT calculations at the B3 LYP/6-311G** level give an insight into why 1 and 2 adopt dramatically different structures from their previously reported, "open-dimeric", lithium counterpart. The theoretical work also focuses on the TMEDA-free parent amide complexes and reveals that the energy difference for the formation of [(M(tmp))x] (in which, M=Li or Na, x=3 or 4; and M=K, x=2, 3 or 4) are small.

Structural Elucidation of tmeda‐Solvated Alkali Metal Diphenylamide Complexes

“…A similar scenario was encountered during the synthesis of the tmeda adduct of K(tmp). [66] X-ray quality crystals of 1-3 precipitated from the respective solutions at ambient temperature in moderate yields (typically 36-48 %). The crystals were isolated in vacuo, and in the case of 3, this resulted in a loss of crystallinity.…”

Lithium and aluminium carbamato derivatives of the utility amide 2,2,6,6-tetramethylpiperidide

“…2 In the presence of the popular bidentate donor N, N, N', N'-tetramethylethylenediamine (TMEDA) this crystalline form changes to a hemisolvated 'open dimer'. 3 These alkali-metal amide structures are highly dependent on the identity of the alkali-metal, since the unsolvated sodium derivative is a smaller cyclotrimer 4 and the TMEDA-solvated sodium and potassium derivatives 5 are more conventional cyclic (MN) 2 dimers with a donor molecule capping each metal. The donor-deficient open dimer structure is presumably preferred so as to alleviate the strain which a four-membered Li 2 N 2 ring would experience; the closed dimer would also bring the bulky diamine and amide moieties into too close proximity as a consequence of shorter Li-N bonds.…”

Synthesis and Structures of [(Trimethylsilyl)methyl]sodium and ‐potassium with Bi‐ and Tridentate N‐Donor Ligands

“…[20] Again, the most examined (trimethylsilyl)methyl compound to date is the lithium congener, [(trimethylsilyl)methyl]lithium [Me 3 SiCH 2 Li] n , which is known to form a hexamer (n = 6) in the solid state, [21] whereas both N,N,NЈ,NЈ-tetramethylethylenediamine (TMEDA) and the (-)-sparteine adducts form dimers (n = 2), and the higher-denticity ligand N,N,NЈ,NЈЈ,NЈЈ-pentamethyldiethylenetriamine (PMDETA) forms a monomer (n = 1). [22] [(Trimethylsilyl)methyl]potassium has been used in C-metallation reactions of cyclohexene, [23] toluene, [24] and tetrahydrofuran, [25] and in an N-metallation reaction of a secondary amine to form potassium 2,2,6,6-tetramethylpiperidide [26] in situ, [27] which has recently been employed as a convenient precursor in synergic mixed-metal chemistry. [28] The primary aim of the present work is to fully characterise the [(trimethylsilyl)methyl] compounds of sodium and potassium as a prelude to developing them further in synthetic applications.…”