Macrobicyclic Chromium(III) Hexaamine Complexes

Peter Comba, Inge I. Creaser, Lisandra L. Martin, Alan M. Sargeson, Lawrence R. Gahan, Geoffrey A. Lawrance, Jack Mac B. Harrowfield, Albert W.H. Mau, Wolfgang H.F. Sasse, Michael R. Snow

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Abstract

The syntheses and characterization of the macrobicyclic hexaamine chromium(III) complexes of diamsar (1,8-diamino-3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) and sar (3,6,10,13,16,19-hexaazabicyclo[6.6.6]eicosane) ligands (1–3) are reported. The complexes were isolated as stable, paramagnetic, and monomeric salts. crystallized in the space group P21/c, (Z = 4, a = 9.474 (2) A, b = 13.331 (3) A, c = 18.221 (5) A, β = 92.756 (0)°) with the octahedral cation in the C3-lel conformation and an average Cr-N distance of 2.070 (3) A. The complexes are high-spin d3 with room-temperature magnetic moments near the spin-only value of 3.87 μB• The aqueous electrochemistry exhibits a quasi-reversible Cr(III/II) couple below -1 V for each complex and a further irreversible Cr(II/0) reduction near -1.6 V; the chromium(II) complexes are not stable in the coulometric time scales (minutes). The Cr(III) electronic spectra show the expected spin-allowed absorption bands of 4T2g4A2gand 4T1g4A2g origin as well as weak spin-forbidden bands of 2Eg, 2Tlg4Agorigin in the visible region. The first of these (4T2gtransition) is split by -400 cm-1and is relatively high in intensity, and the splitting is probably due to the lowered symmetry (D3). Cr(sar)3+ and Cr(diamsar)3+ emit rather strongly at 77 K 0.02), but give no detectable emission above 273 K. Photodecomposition of Cr(diamsar)3+ is ~ 104-fold smaller than that of the Cr(en)33+ ion, and possible reasons for this are discussed.

Original languageEnglish
Pages (from-to)384-389
Number of pages6
JournalInorganic Chemistry
Volume25
Issue number3
DOIs
Publication statusPublished - 1 Jan 1986
Externally publishedYes

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