Angewandte
Chemie
from that of the mono-gold compound. Luminescence spectra
of both compounds show vibronic structure, with peak-to-
peak spacings near 1200 cmꢁ1. Structured emission is typical
for gold(I) aryls; the peak separations suggest that ring
deformation modes are activated in the emitting state. The
vibronic emission and the large Stokes shift suggest phos-
phorescence from a triplet excited state, as observed in other
gold(I) organometallics.[25–29]
geminally diaurated species. These results suggest some gold
participation in the excited states of geminally diaurated aryls,
whereas phosphorescence of normal gold(I) aryls and alkyn-
yls is ligand-centered.[25–29,31–33]
The available crystal structures suggest disrupted aroma-
ticity, as do the optimized geometries. Perturbations of
carbon–carbon bond lengths decrease with distance from
the dimetallation site. The same trend prevails for natural
ꢁ
Density functional theory calculations have been applied
to models of 1, 6, and 11, where PMe3 ligands replace PPh3 for
computational simplicity. These model complexes are 1’, 6’
and 11’, respectively. Geometries were optimized with gold–
gold distances constrained to those in the crystal structures.
These constraints are imposed because of the dispersive
nature of aurophilic interactions,[30] which DFT does not
entirely capture. Converged geometries of 1’ and 6’ agree well
with the crystal structures. Harmonic vibrational frequency
calculations find all three to be energy minima. Full computa-
tional details appear in the Supporting Information.
For the three model compounds, the highest-occupied
Kohn–Sham orbital (HOMO) resides on the aryl ligand. The
lowest unoccupied Kohn–Sham orbital (LUMO) has mixed
aryl–gold(I) character. Figure 3 depicts a frontier orbital
energy diagram of 6’; that for 11’ appears in the Supporting
Information. Plots of selected orbitals appear at right.
Methylene chloride solvation is included using a polarizable
continuum model. In these compounds, the HOMO lies
mainly on the aryl ligand, whereas the LUMO, the
HOMOꢁ1, and other near-frontier orbitals have sizable
(> 10%) contributions from the (phosphine)gold(I) frag-
ments. Percentages are of electron density, from Mulliken
population analysis. A de facto separability of gold- and aryl-
based orbitals in normal gold(I) aryls does not extend to
bond orders calculated for aromatic C C bonds. The mean
bond length between the metalated and adjacent carbons is
1.413 ꢂ for 1’, 6’, and 11’ (range: 1.391–1.437 ꢂ); the
corresponding value calculated for [Me3PAuPh] (Cs symme-
try) is 1.403 ꢂ. For the three model compounds, the mean
gold–carbon bond length is 2.169 ꢂ (range: 2.162–2.174 ꢂ);
that computed for [Me3PAuPh] is 2.065 ꢂ. The average,
calculated C-Cmetalated-C bond angle is 118.08; for mono-gold
[Me3PAuPh], it is 116.48. A single metalation at carbon does
not much perturb sp2 hybridization, and the aurated carbon
does not approach a tetrahedral geometry. The mean
calculated Au-C-Au angle of 1’, 6’ and 11’ is 78.08; the
aurophilic interaction precludes sp3 hybridization at C.
Wiberg bond orders in the Lowdin basis[34] also suggest
diminished aromaticity. Table 2 collects bond orders calcu-
lated for 1’, 6’, and 11’; for their mono-gold analogues, and for
the corresponding Wheland complexes. Both calculated geo-
metries and bond orders suggest that geminal diauration
degrades aromaticity, but less so than outright protonation
(Wheland complex formation). In agreement with Reed and
co-workers,[35] we find that positive charge does not collect at
carbon. Natural atomic charges are positive and roughly equal
(ca. 0.25) for gold and hydrogen, and largest (> 0.95) for
phosphorus. Carbon atoms have negative charges, be they
protonated, geminally diaurated, or aromatic. The mean
natural charge of dimetalated car-
bons across 1’, 6’, and 11’ is ꢁ0.50.
In diaurated compounds, positive
charge buildup at ortho and para
sites is muted compared to that in
Wheland complexes.
Time-dependent DFT calcula-
tions indicate that the aryl ligand
dominates the emission and low-
energy absorption features. Naph-
thyl 11’ is typical. The lowest
excited singlet and triplet states
!
are mainly LUMO HOMO
transitions; the triplet has small
contributions from other triplet
states with which it undergoes
configuration interaction. The
frontier orbitals of geminally dia-
urated aryls have greater gold
character than in mono-gold ana-
logues. Hence, emission from 11’
is not cleanly ligand-centered, but
the naphthyl moiety dominates.
Calculations on 1’, 6’, and other di-
Figure 3. Partial Kohn–Sham orbital energy diagram of 6’. Plots of selected orbitals and percentage
compositions in terms of fragments appear at right. Implicit methylene chloride solvation is included;
see Supporting Information for details.
gold aryls herein suggest that their
excited states are similar.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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