Structural Variations within Group 1 (Li−Cs)⁺(2,2,6,6-Tetramethyl-1-piperidinyloxy)⁻ Complexes Made via Metallic Reduction of the Nitroxyl Radical

Abstract

Treatment of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) with a group 1 metal (Li, Na, K, Rb, or Cs), resulted in the reduction of this important radical to the TEMPO(-) anion--the first examples of elemental-metal single electron reduction of the radical to its anionic form. The synthesis and characterization of seven alkali metal TEMPO(-) complexes are reported. A variety of structural motifs are encountered depending on the choice of metal and/or solvent. (THF)(2) x [Li(+)(TEMPO(-))](4) 1 crystallized from THF as a cyclic (Li(4)O(4)) molecule. Two Li centers are stabilized by coordination to a THF molecule; the others by intramolecular coordination to N(TEMPO) atoms. [(THF) x Na(+)(TEMPO(-))](4) 2 exists as a distorted cubane where each Na center is coordinated to a THF molecule. No appreciable Na-N(TEMPO) coordination is observed. [(THF)(2) x Na(+)(3)(TEMPO(-))(2)(OH)](2) 3 was serendipitously prepared and exists as a distorted bis(cubane). It is envisaged that 3 is formed from 2 by insertion of a (Na-OH)(2) double bridge into its framework. [Na(+)(4)(mu(3)-TEMPO(-))(2)(mu(2)-TEMPO(-))(2)(TMEDA)(2)] 4, adopts a four-runged ladder structure, whereby the two outer Na centers are coordinated to TMEDA, in addition to two mu(2)-O and a N atom. The inner metal atoms are bound to three mu(3)-O atoms and a N atom. [(THF) x K(+)(TEMPO(-))](4) 5 resembles the motif found for 2; however, presumably because of the larger size of the metal, K-N(TEMPO) interactions are present in 5. The asymmetric unit of [(TMEDA) x Rb(+)(2)(TEMPO(-))(2)](2) 6 comprises a Rb(4)O(4) cubane with half a molecule of TMEDA coordinated to each metal. From a supramolecular perspective, 6 exists as a polymeric array of cubane units connected by TMEDA bridges. Completing the series, [Cs(+)(TEMPO)](infinity) 7 crystallizes from hexane to form a donor-free polymeric complex. Complexes 1, 2, and 4-7 are soluble in D(8)-THF solution, and their NMR spectra are reported. The solution structures in donor solvent appear virtually identical.

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

“…Three (trimethylsilyl)methyl groups [including the methanide carbon atoms C(1), C(2), and C(3)] are directed away from the plane formed by the TMEDA-coordinated potas-sium atoms K(1), K(2) and K(3). The free electron pairs of the (trimethylsilyl)methyl groups are directed towards the midpoints X n of the K(3)···K(4) and K(2)···K (4) Though unusual for alkyl compounds of the heavier alkali metals sodium and potassium, many examples of potassium heterocubanes are known for more complex heteroatom anionic systems, including alkoxides, [33] silanolates, [34] enolates, [35] hydroxylamides, [36] phosphoraneiminates [37] and silanides. [38] www.eurjic.org Recently, Stalke et al [39] and Mitzel et al [40] could identify very similar structural motifs in solvated alkyllithium compounds.…”

Accessing the Single‐Electron Manifold: Magnesium‐mediated Hydrogen Release from Silanes

“…Mulvey and co‐workers reported the first Group 1 and 2 complexes of TEMPO in 2001,7a noting not only its coordination as a neutral two electron donor but also its ability to adopt an anionic electron configuration when ligating Group 2 cations in complexes of the form [{(Me 3 Si) 2 N}Mg{μ‐TEMPO}] 2 . A subsequent, elegant report from this group of single‐electron transfer (SET) from elemental Group 1 metals to TEMPO yielded a structurally diverse series of [M{TEMPO}] compounds (M=Li, Na, K, Rb, Cs) once again containing TEMPO anions (hereafter referred to as TEMPOxide) 7b. A demonstration of TEMPO reactivity with Group 2 reagents has also been described by Fedushkin et al.…”

A comparison of MOP-phosphonite ligands and their applications in Rh(i)- and Pd(ii)-catalysed asymmetric transformations

“…18 Interestingly, reflecting the ability of TEMPO to act as a radical trapping reagent, 36 when Qx is treated with sodium magnesiate 1 in the presence of one molar equivalent of this nitroxyl radical, the formation of 2 is totally inhibited, affording instead 4 in almost quantitative yield (as evidenced by monitoring the reaction using 1 H NMR spectroscopy). The molecular structure of 4 revealed a contacted ion pair (CIP) bimetallic motif where the two metals are connected by a TEMPO anion, which coordinates in an asymmetric fashion, using its N and O atoms ( Na-O1 and N3-O1 respectively] with those reported in related complexes of these metals containing µ 2 -TEMPO anions, 32,33,35 it is clear that the ligand present in 4 is anionic as it must be for valency considerations. 37 Scheme 4 (i) Reaction of sodium magnesiate 5 with Quinoxaline (Qx), (ii) hydrolysis under aerobic conditions, and (iii) molecular structure of reduced product 7b with displacement ellipsoids shown at the 30% probability level.…”