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

2009; American Chemical Society; Volume: 48; Issue: 14 Linguagem: Inglês

10.1021/ic900609e

1520-510X

L. Balloch, A.M. Drummond, Pablo García‐Álvarez, David V. Graham, Alan R. Kennedy, J. Klett, Robert E. Mulvey, Charles T. O’Hara, P.J.A. Rodger, I.D. Rushworth,

Radical Photochemical Reactions

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.