Synthesis, Structures, and Properties of a Series of Four-, Five-, and Six-Coordinate Cobalt(III) Triazacorrole Complexes: The First Examples of Transition Metal Corrolazines

The syntheses of the first transition metal corrolazine complexes,
in which the meso carbon atoms of a corrole framework have
been replaced by N atoms, are reported. Metalation of the
corrolazine [(TBP)8CzH3] (TBP = 4-tert-butylphenyl) (1) with Co(acac)2 gives [(TBP)8CzCoIII] (2) in good yield. Addition of PPh3 to
2 in pyridine results in the formation of [(TBP)8CzCoIII(PPh3)] (3),
which was characterized by X-ray crystallography. Likewise,
addition of an excess of pyridine to 2 in CH2Cl2 followed by slow
diffusion of MeOH gives [(TBP)8CzCoIII(py)2] (4) as a crystalline
solid, which was also characterized by X-ray crystallography. The
crystal structures of 3 and 4 reveal that the corrolazine cavity is
significantly smaller (∼0.1 Å) than their regular corrole analogues.
Characterization of 2−4 by UV−vis spectroscopy reveals some
interesting features in the absorption spectra of these compounds,
including a dramatic red-shift of the Soret band. In addition, binding
of pyridine to 2 was evaluated quantitatively by UV−vis titration,
revealing a formation constant of β2 = 9.0 × 107 M-2 , which is
larger than any of the regular CoIII corrole analogues.