Optical Response of a Cyclometalated Iridium(III) Hydrazino Complex to Carbon Dioxide: Generation of a Strongly Luminescent Iridium(III) Carbazate

A system pairing the luminescent core of [Ir­(ppy)2L2]+ (ppy = 2-phenylpyridine) with simple
hydrazino
ancillary ligands (L = N2H4) has been prepared
for the direct optical detection of carbon dioxide (CO2). Silver-assisted and silver-free techniques were used for the successful
introduction of N2H4 into the [Ir­(ppy)2Cl]2 coordination sphere at room temperature to give the
corresponding biscyclometalated iridium­(III) hydrazino species as
either a CF3SO3– (OTf–, 2a) or Cl– (2b) salt. The silver-free route was accomplished by the direct
replacement of the ligated Cl– using a slight excess
of hydrazine. The luminescence profile of the cationic iridium­(III)
hydrazino complex 2a (λmax = 501 nm)
undergoes a red shift (λmax = 524 nm), accompanied
by a change in the peak shape during exposure to CO2 in
solution. The spectral changes observed are attributed to the formation
of the corresponding neutral carbazate species Ir­(ppy)2(H2NNHCOO) (3) and are not consistent with
protonation of the ligated hydrazine. Conversion of the hydrazino
species to the carbazate species is solvent-dependent and irreversible.
The hydrazino and carbazate species have been structurally characterized
by single-crystal X-ray diffraction; both compounds exhibit long-lived
and intense room temperature luminescence in solution with τ
= 1.56 and 1.80 μs and ϕem = 0.42 and 0.45,
respectively.