Pitfalls in Bromination Reactions of Zinc Porphyrins: Two-Sided Ring Opening of the Porphyrin Macrocycle

Reaction
of [ZnII(TTP)] (1) (TTP = dianion of 5,10,15,20-meso-tetrakis­(p-tolyl)­porphyrin)
with 16 equiv of N-bromosuccinamide (NBS) in methanol
at reflux led to the unexpected two-sidedd open-ring brominated product
[ZnII(C26H20­N2O2­Br5)2] (2). Similar observations have been
made with other meso-substituted zinc porphyrins
as well [ZnII(por)] {por = dianion of 5,10,15,20-meso-tetrakis­(aryl)­porphyrin; aryl = phenyl
(TPP), p-tBu-phenyl (TBPP), m-Cl-phenyl (TClPP)}. The respective products [ZnII(C24H16­N2O2­Br5)2] (3), [ZnII(C32H32­N2O2­Br5)2] (4), and [ZnII(C24H14­N2O2Cl2­Br5)2] (5) have been isolated in good
to moderate yields and characterized by elemental analysis and UV–vis, 1H NMR, and mass spectrometry. Additional bromination reaction
of 1 with 8 equiv of NBS in a chloroform/methanol mixture
led (after the two-sided ring opening) to nonmetalated brominated
bi­(pyrrole) product, C36H34­N2O4­Br4 (6). The detailed
structures of complexes 1, 2, 3, and 6, available in a single crystal form, have been
confirmed by X-ray diffraction analysis.