Synthesis, Structure, and Optical Property of Tris(biaryldiyl)metal Complexes Consisting of Group 9 Elements

We
report the synthesis and characterization of tris(biphenyl-2,2′-diyl)metal
complexes of trivalent group 9 elements (1M, M = Co, Rh, Ir) and their nonplanarly π-extended
analogs, tris(1,1′-binaphthyl-2,2′-diyl)metal complexes
(2M, M = Rh, Ir). Single crystal
X-ray crystallography reveals the distorted octahedral geometry with
an approximate C3 symmetry of trianionic
complexes 1M (M = Co, Rh, Ir)
and 2M (M = Rh, Ir), which are
contacted by three Li+ ions in the crystal. Complex 1Ir exhibits yellow luminescence in
THF with a photoluminescence quantum yield (ΦPL) of up to 0.73, along with a distinctive photophysical
property, namely, a concentration dependence of the emission wavelength
from 530 to 580 nm. This is a characteristic property of 1Ir and has not been observed in its isoelectronic
analog, tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3).
The concentration-dependent optical properties originate from the
dissociation equilibria of Li+ ions from the anionic chromophore.
Complex 2Ir also exhibits luminescence
at 715 nm in THF, with a notable bathochromic shift from 1Ir through the π-extension. The findings
offer insights into the photophysical properties of homoleptic organo-transition
metal complexes, providing the foundation for the design of related
transition metal complexes.