The conversion of carbon dioxide to formate is of great
importance
for hydrogen storage as well as being a step to access an array of
olefins. Herein, we have prepared a JMS-5 metal–organic framework
(MOF) using a bipyridyl dicarboxylate linker, with the molecular formula
[La2(bpdc)3/2(dmf)2(OAc)3]·dmf. The MOF was functionalized by cyclometalation using Pd(II),
Pt(II), Ru(II), Rh(III), and Ir(III) complexes. All metal catalysts
supported on JMS-5 showed activity for CO2 hydrogenation
to formate, with Rh(III)@JMS-5a and Ir(III)@JMS-5a yielding 4319 and
5473 TON, respectively. X-ray photoelectron spectroscopy of the most
active catalyst Ir(III)@JMS-5a revealed that the iridium binding energies
shifted to lower values, consistent with formation of Ir–H
active species during catalysis. The transmission electron microscopy
images of the recovered catalysts of Ir(III)@JMS-5a and Rh(III)@JMS-5a
did not show any nanoparticles. This suggests that the catalytic activity
observed was due to Ir(III) and Rh(III). The high activity displayed
by Ir(III)@JMS-5a and Rh(III)@JMS-5a compared to using the Ir(III)
and Rh(III) complexes on their own is attributed to the stabilization
of the Ir(III) and Rh(III) on the nitrogen and carbon atom of the
MOF backbone.