Orange Luminescence and Structural Properties of Three Isostructural Halocyclohexylisonitrilegold(I) Complexes

The preparation of three isonitrile complexes (CyNC)AuICl, (CyNC)AuIBr, and (CyNC)AuII, along with their structural
and spectral characterization, are reported. X-ray crystal structures reveal that these crystallize in the same space
group and have closely related structures. The structures involve pleated chains of linear, two-coordinate monomers
that are arranged in a head−tail fashion. However, these chains vary significantly in the degree of aurophilic
interactions among the individual molecules. Thus, (CyNC)AuICl forms infinite chains with alternating Au···Au distances
of 3.3894(7) and 3.5816(7) Å. Within the chains of (CyNC)AuIBr, however, the alternation of Au···Au distances is
more pronounced so that there are dimers, with an Au···Au distance of 3.4864(9) Å, and neighboring gold centers
at 3.7036(9) Å. In (CyNC)AuII, the gold−gold contacts do not lie within the range of significant aurophilic bonding.
The closest Au···Au distance is 3.7182(11) Å while every other Au···Au distance is 3.9304(12) Å. The steric factor
of the X ligand and dipole−dipole interactions between the antiparallel complexes is much more significant than
aurophilic interactions in governing the self-association of the complexes in this series. The colorless crystals of
each solid display an orange luminescence band with a strikingly large Stokes' shift (∼21 000 cm-1, 2.6 eV).
However, considerable care had to be taken to ensure that the crystals used for the study of the luminescence
were free of a surface impurity that produced a turquoise-green luminescence in (CyNC)AuICl. The diffuse reflectance
spectra for the solids show a similar three-band pattern in the 200−330 nm range.