Tetrabutylammonium Tetracyanocobaltate(II) Dioxygen Carriers

The second example of a low-spin, square-planar
Co(II)L4 species (L = unidentate ligand),
(Bu4N)2Co(CN)4
(1),
and a new family of tetrabutylammonium tetracyanocobaltate(II)
complexes have been isolated and characterized.
All of the solid compounds react reversibly with dioxygen to form
1:1 cobalt(III)−superoxo complexes. Since
a variety of neutral bases react readily with 1 and the
complexes can be obtained as solids, the effects of
different
axial bases on the dioxygen-binding behavior of the solids could also
be studied. Interestingly, reversible phase
changes occur on oxygenation/deoxygenation for
(Bu4N)2[Co(CN)4·C5H5N]
(2) and
[(Bu4N)2Co(CN)4]2·(C10H8N2)
(3). Presumably this happens because the crystal
lattices of the unoxygenated materials do not contain
enough
room for the dioxygen. Complex 2 crystallizes in the
orthorhombic space group Pbcm with the following
cell
parameters at 25 °C:  a = 9.024(3) Å,
b = 21.886(8) Å, c = 23.184(6) Å,
V = 4578.6(27) Å3, and Z =
4. From
the molecular packing, it is apparent that the
[Co(CN)4·C5H5N]2-
anions are aligned parallel to the a axis, with
the pyridine of each anionic unit pointing toward the vacant site on
the cobalt of the adjacent anion. The
tetrabutylammonium cations completely surround the anions, forming
hydrophobic casings that also run parallel
to the a axis. Thus, it is clear that the bulky
tetrabutylammonium cations are capable of separating the
cobalt
centers so that μ-peroxo dimers cannot form.