The aromatic PO2 complexes of meso-triaryl-25-oxasmaragdyrins were synthesized by treating the free base 25-oxasmaragdyrins with POCl3 in toluene/triethylamine at refluxing temperature. The complexes are stable and characterized by X-ray and different spectroscopic techniques. In these complexes, the phosphorus(V) ion was bound to two pyrrolic nitrogen atoms of the smaragdyrin macrocycle and two oxygen atoms in tetrahedral geometry. The X-ray structure revealed that the smaragdyrin macrocycle showed significant distortion upon insertion of a PO2 unit, and the phosphorus atom lies 1.339 Å above the mean plane defined by three meso-carbon atoms of the macrocycle. These complexes absorb strongly in the visible region and are 2.5 times more strongly fluorescent than free base 25-oxasmaragdyrins. The smaragdyrin macrocycle becomes electron-deficient upon complexation with a PO2 unit because these complexes are easier to reduce but difficult to oxidize compared to free base smaragdyrins. We designed and synthesized a covalently linked BODIPY-PO2-smaragdyrin dyad and demonstrated efficient energy transfer from the BODIPY unit to the PO2-smaragdyrin unit.
BF2-complex of meso-free 25-oxasmaragdyrin is synthesized under simple reaction conditions in high yield, and the reactivity of meso-free carbon atom was demonstrated by carrying out functionalization followed by coupling reactions.
Unsymmetrical 22-oxacorrole containing two aryl groups and one pyrrole group at the meso position was synthesized by condensing one equivalent of 16-oxatripyrrane with one equivalent of meso aryl dipyromethane under mild acid-catalyzed conditions followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). This [3+2] condensation approach was expected to yield meso-free 25-oxasmaragdyrin but unexpectedly afforded unsymmetrical meso-pyrrole-substituted 22-oxacorrole. We demonstrated the versatility of the reaction by synthesizing four new meso-pyrrole-substituted 22-oxacorroles. The reactivity of α-position of meso-pyrrole was tested by carrying out various functionalization reactions such as bromination, formylation, and nitration and obtained the functionalized meso-pyrrole-substituted 22-oxacorroles in decent yields. The X-ray structure obtained for one of the functionalized meso-pyrrole substituted 22-oxacorrole revealed that the macrocycle was nearly planar and the meso-pyrrole was in the perpendicular orientation with respect to the macrocyclic plane. The meso-pyrrole-substituted 22-oxacorroles absorb strongly in 400-700 nm region with one strong Soret band and four weak Q bands. The 22-oxacorroles are strongly fluorescent and showed emission maxima at ≈650 nm with decent quantum yields and singlet-state lifetimes. The 22-oxacorroles are redox-active and exhibited three irreversible oxidations and one or two reversible reduction(s). A preliminary biological study indicated that meso-pyrrole corroles are biocompatible.
Unprecedented examples of smaragdyrin macrocycles containing seven membered heterocyclic rings were synthesized under simple reaction conditions in high yields. The heterocycle formed inside smaragdyrin macrocycle is rare example of heterocycle containing five different atoms, such as B, C, N, O, and P atoms. The mixed B(III) and P(V) complexes of smaragdyrin macrocycles showed new structural, spectral, and electrochemical properties.
Novel boron-dipyrromethene (BODIPY)-bridged 22-oxacorrole dyads, using meso-pyrrolyl 22-oxacorrole as a key synthon, have been synthesized. The reactivity of the meso-pyrrolyl group of 22-oxacorrole was exploited to synthesize the first examples of BODIPY-bridged 22-oxacorrole dyads in ≈40 % yield. The dyads are stable and exhibited interesting spectral and electrochemical properties.
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