Covalent-Organic Frameworks Composed of Rhenium Bipyridine and Metal Porphyrins: Designing Heterobimetallic Frameworks with Two Distinct Metal Sites

The incorporation
of homogeneous catalysts for CO2 reduction
into extended frameworks has been a successful strategy for increasing
catalyst lifetime and activity, but the effects of the linkers on
catalysis are underexplored. In this work, a novel rhenium bipyridine
complex was synthesized for the purpose of designing a covalent-organic
framework (COF) with both metalloporphyrin and metal bipyridine moieties.
Investigation of the rhenium complex as a homogeneous catalyst shows
a faradaic efficiency of 81(8)% for the electrocatalytic conversion
of CO2 to CO upon the addition of methanol as the proton
source. Treatment of the rhenium complex with tetra­(4-aminophenyl)­porphyrin
under Schiff base conditions produces the desired COF, as indicated
by powder X-ray diffraction (PXRD) studies. Metalation of the porphyrins
was accomplished through postsynthetic modification with CoCl2 and FeCl3 metal precursors. The retention of the
PXRD peaks and appearance of new Co and Fe peaks in the corresponding
X-ray photoelectron spectroscopy spectra suggest the successful incorporation
of a secondary metal site into the framework. Cyclic voltammetry measurements
display increases in current densities when the atmosphere is changed
from N2 to CO2. Controlled potential electrolyses
show that the cobalt-postmetalated COF has the highest activity toward
CO2 reduction, reaching a faradaic efficiency of 18(2)%.