# Synthesis and Electrochemistry of Formazan(ate) Re(I) Complexes: Ligand-Based Reactivity toward CO2

## TL;DR

This paper studies how certain rhenium complexes with redox-active ligands react with CO2, forming stable adducts and offering insights into catalyst design for small molecule conversion.

## Contribution

The study reveals ligand-based reactivity of formazan(ate) Re(I) complexes toward CO2, forming carbamate adducts and limiting catalytic turnover.

## Key findings

- The redox-active ligand acts as a two-electron sink, enabling two consecutive one-electron reductions.
- CO2 reacts with reduced formazanate Re(I) complexes to form carbamate-type adducts at low overpotentials.
- Ligand-based reactivity hinders catalytic turnover via metal-centered CO2 activation.

## Abstract

Re­(I) tricarbonyl
complexes of the type fac-[ReX­(CO)3(L)]
n
(n = −1,
0, +1) (X = Br–, MeCN), furnished with the redox-active
formazan (L = Ph–N­(R)–NCH–NN–Ph;
R = H, (H5

Br

); R = Me, (Me5

X

)) or formazanate (L = [Ph–NN–C­(−Ph-4-R1)N–N–Ph]−; R1 = H [1

Br

]

–
, Me [2

Br

]

–
, MeO [3

Br

]

–
, F [4

X

]

–
; L = [Ph–NN–C­(−H)N–N–Ph]−, Py = pyridine 5

Py

) ligands were prepared and characterized by spectroscopy
and electrochemistry. In situ characterization of
the reduced species by (spectroelectro)­chemical and computational
methods revealed that the redox-active scaffold behaves as a two-electron
sink, allowing two consecutive one-electron reductions to take place
at the ligand. The reactivity of the reduced formazan­(ate) rhenium
complexes toward CO2 was explored. (Spectroelectro)­chemical
experiments along with DFT calculations suggested that CO2 reacts with the reduced formazanate Re­(I) complexes at low overpotentials
forming a carbamate-type adduct. This ligand-based reactivity provides
a thermodynamic sink for CO2 binding and hinders catalytic
turnover via metal-centered CO2 activation. These findings
provide new insights into the advantages and limitations of using
catalysts with redox-active ligands to activate and convert small
molecules.