Gold(I) complexes of brominated azadipyrromethene ligands

Article abstract of DOI:10.1021/ic300709n

Azadipyrromethenes are luminescent, red-light absorbing dyes that readily bind BF₂⁺ and metals. Their framework allows for structural modification at the phenyl arms and the two pyrrolic carbon positions. Here we report five new gold

Full text of DOI:10.1021/ic300709n

Article
pubs.acs.org/IC
Gold(I) Complexes of Brominated Azadipyrromethene Ligands
Lei Gao, Nihal Deligonul, and Thomas G. Gray*
Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland Ohio 44106, United States
S
* Supporting Information
ABSTRACT: Azadipyrromethenes are luminescent, red-light absorbing dyes that
readily bind BF2+ and metals. Their framework allows for structural modification at the
phenyl arms and the two pyrrolic carbon positions. Here we report five new gold(I)
complexes with azadipyrromethene ligands brominated at the pyrrolic carbons and/or
the four phenyl substituents. New complexes are characterized by multinuclear NMR
spectroscopy, X-ray crystallography, optical absorption and emission spectroscopy,
and elemental analysis. The new compounds have a perturbed two-coordinate
geometry in the crystalline state, with gold(I) binding one dimethylphenylphosphine
ancillary ligand and one pyrrole nitrogen of the azadipyrromethene. The second
azadipyrromethene pyrrole nitrogen perturbs the linear coordination. These
complexes maintain the absorption features of the free ligands. Excitation in the
near-ultraviolet generates emission in the near-UV and visible regions. Density-
functional theory calculations indicate that the photoproperties of the new compounds
arise almost entirely from the conjugated ligands and not from the (phosphine)gold(I) fragments.
azadipyrromethene ligands. An intense visible absorption near
600 nm results from a linear combination of one-electron
transitions involving the lowest unoccupied molecular orbital
(LUMO), highest occupied molecular orbital (HOMO), and
HOMO − 1. In zinc bis(azadipyrromethene) complexes, visible
light absorption is broader. As many as four transitions are
involved.6
Boron azadipyrromethenes are visible (red) fluorophores,
even when substituted with heavy atoms such as bromine. An
ongoing need is to design emitters of near-infrared light for use
in solar cells and light-emitting diodes, and as heat-absorbers
and bioimaging dyes.12,13 Results from this laboratory14−21 and
elsewhere22−24 show that binding even a single gold atom to
organic fluorophores generates phosphorescence while quench-
ing fluorescence. Our earlier study8 of a gold(I) complex of an
unsubstituted tetraarylazadipyrromethene found only fluores-
cence, with a small Stokes shift and no evidence of triplet
emission. Here we report a series of (phosphine)gold(I)
complexes of brominated azadipyrromethene ligands. Three
new complexes are characterized crystallographically. Several
show dual emission with the usual azadipyrromethene
fluorescence and a new, broad emission at longer wavelengths.
INTRODUCTION
Azadipyrromethenes are nitrogenous pigments that bind BF2
■
+
and chelate metal ions. In recent years, they have emerged as
photodynamic therapy mediators, luminescent probes, and light
harvesters.1−3 Most azadipyrromethenes are synthesized from
chalcone precursors that themselves derive from reactions of
aldehydes with acetophenones. Thus, the typical azadipyrro-
methene has four aryl substituents, denoted proximal and distal
below. The proximal aryls derive from an acetophenone; the
distal aryls, from an aldehyde. The aryl arms can carry
substituents, and aryl modification is a ready way of modulating
the optical properties of azadipyrromethene complexes.
Substitution also occurs at the pyrrolic carbon with suitable
electrophiles. If the distal arms carry Lewis basic sites, then
azadipyrromethenes potentially become tridentate ligands.4 If
the proximal arms do so, then tetradentate ligands result.5
RESULTS AND DISCUSSION
■
The coordination chemistry of azadipyrromethene ligands is
steadily emerging. Bidentate azadipyrromethene complexes
have been characterized for zinc and mercury,6 rhenium(I),7
the coinage metals,8,9 and nickel(II), cobalt(II), and iron(II).10
When bound to metals, azadipyrromethenes retain their
chromophoric character. INDO/S6 and density-functional
theory calculations8,9 find that the frontier orbitals, at least of
d10-complexes, reside on the ligand π-system. As with
porphyrins,11 configuration interaction is prominent for
Syntheses. Scheme 1 enumerates azadipyrromethene
ligands and complexes, and summarizes their syntheses. The
free ligands were prepared by the methods of O’Shea and co-
workers.1 Ligands La−c are known; Ld is new. Bromination at
the pyrrolic carbon atoms (La−dBr2 series) was effected with N-
bromosuccinimide (NBS).25 The isolated azadipyrromethene
Received: April 5, 2012
Published: July 3, 2012
© 2012 American Chemical Society
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Scheme 1
complexes were stirred with 2 equiv of N-bromosuccinimide
(NBS) in dichloromethane. Dark precipitates of La−dBr2
formed quickly. The products were purified by washing with
dichloromethane. They are sparingly soluble in organic solvents
and can be used in the synthesis of gold(I) complexes without
further purification.
upfield; resonances for 1 and 5 are further upfield than those of
2−4. The electronic effect of the methoxy substituents is less,
based on 31P{1H} NMR. Compounds 2, 3, and 4 have similar
31P{1H} chemical shifts. Upon binding to the Me2PhPAu+
fragment, a 0.1−0.4 ppm downfield shift of the meta and ortho
protons of the proximal azadipyrromethene phenyl substituents
occurs. The doublet assigned to the methyl groups on PMe2Ph
shifts upfield by 0.1−0.3 ppm.
Crystallography. Compounds 1, 2, and 4 have been
characterized crystallographically. No aurophilic contacts are
present. In all structures gold binds the azadipyrromethene
ligand asymmetrically. In no case does gold have a regular,
three-coordinate geometry. Coordination geometries are nearer
two-coordinate, although P−Au−N angles deviate from strict
linearity. Thermal ellipsoid projections of 1, 2, and 4 appear in
Figures 1−3, respectively. Table S2, Supporting Information,
assembles crystallographic data for the new complexes.
The interatomic distances of Table 1 show asymmetric
gold−nitrogen binding. Atom numbers are assigned so that N1
The free ligands were deprotonated with 2 equiv of KOtBu
or NaOtBu in tetrahydrofuran (THF). The ligands solubilize
upon adding base, and a homogeneous turquoise solution
results. The reaction mixture was deoxygenated and stirred
under argon for a period of hours. The gold(I) starting
material, Me2PhPAuCl, was added to the reaction flask under
air. The use of PMe2Ph as ancillary ligand is motivated by its
smaller size compared to PPh3 (Θ = 122° vs 145°,
respectively)26 which we have employed on earlier work8 on
group 11 azadipyrromethene complexes. The solution was
again purged with argon, and the resulting turquoise-colored
mixture was stirred for 48 h. The solvent was stripped under
vacuum to leave a lustrous red solid. This residue was
redissolved in benzene, and the benzene solution was filtered
through Celite. The filtrate was evaporated to dryness.
Triturating the residue with pentane led to the isolation of
gold(I) azadipyrromethene complexes as iridescent red
powders. Analytically pure materials resulted from vapor
diffusion of n-pentane into benzene solutions or layering n-
pentane onto benzene solutions. Crystals of compounds 1, 2,
and 4 take on a lustrous red color, and 3 has a green tint. A
greenish powder was collected for 5 after crystallization.
31P{1H} NMR confirms the binding of gold to the
azadipyrromethene ligand. The 31P{1H} resonance of free
PMe2Ph occurs at −46.0 ppm, and that of Me2PhPAuCl
appears at 4.2 ppm. Coordination of the AuCl group induces a
50-ppm downfield shift. Collected in Table S1, Supporting
Information, are 31P{1H} chemical shifts of the new gold(I)
complexes. Compared with Me2PhPAuCl, an upfield shift of 5−
6 ppm occurs in the new complexes. Successive bromination of
the azadipyrromethene ligand shifts the 31P{1H} resonance
Figure 1. Thermal ellipsoid depiction of 1 (50% probability).
Unlabeled atoms are carbon; hydrogen atoms are omitted for clarity.
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nitrogen bond lengths are 2.166 and 2.406 Å.33 Gold(I) in
this cation might be described as perturbed, linear two-
coordinate. The geometries about gold in 1, 2, and 4 are unlike
those in our earlier gold(I) azadipyrromethene8 or in the
aminotroponiminate complexes of Roesky and co-workers.34 In
both sets of compounds, gold(I) is clearly three-coordinate, and
binding is approximately trigonal.
Optical Spectroscopy. The absorption spectra of gold(I)
azadipyrromethenes share the essential features of the free
ligands. Figure 4 shows the absorption and emission spectra of
Figure 2. Thermal ellipsoid depiction of 2 (50% probability).
Unlabeled atoms are carbon; hydrogen atoms are omitted for clarity.
Figure 3. Thermal ellipsoid depiction of 4 (50% probability).
Unlabeled atoms are carbon; hydrogen atoms are omitted for clarity.
Table 1. Selected Interatomic Distances (Å) and Angles
(deg) in Crystal Structures of 1, 2, and 4
1
2
4
Au−N1
Au−N2
Au−P
∠N1−Au−P
∠P−Au−N2
2.140(3)
2.420(4)
2.2093(12)
155.05(9)
125.29(9)
77.3 (1)
2.119(2)
2.490(2)
2.2071(8)
158.48(7)
121.57(6)
79.78(8)
2.113(2)
2.560(2)
2.2179(8)
160.00(7)
117.44(6)
82.44(8)
a
a
a
∠N1−Au−N2
a
The atom in boldface type lies at the vertex of the angle.
is the nitrogen atom nearest gold and N2 is the more distant.
The backbone nitrogen N3 does not bind gold. Values of the
Au−N1 bond length are not significantly different over the
three complexes.27 The backbone C−Nmeso−C angles are
distended from an ideal 120° for sp2-hybridized nitrogen.
The angles range from 125.8(4)° (1) to 117.44(6)° (4). This
bending back of the azadipyrromethene spine is a result of
auration. The backside C−Nmeso−C angle of LaBr2BF2, a typical
boron chelate, is 119.5(2)°.1
Figure 4. Absorption (black) and emission spectra (red) for (a) Ld,
(b) compound 1, and (c) compound 5 in 2-MeTHF at room
temperature.
The ligand bite angle is defined as ∠N1−Au−N2; these
appear in Table 1. The bite angles of Table 1 reflect the
lopsided coordination of gold(I). In 1, 2, and 4, the geometry
about gold accords with its tendency toward linear two-
coordination.28−32 A short bond forms to one nitrogen, and the
second disturbs the N1−Au−P moiety so that it is nonlinear.
The mean ∠N1−Au−P is near 158°. This arrangement is
Ld, compound 1 and compound 5; Table 2 collects absorption
and emission maxima in 2-methyltetrahydrofuran (2-MeTHF).
The higher-energy absorption band is largely unshifted upon
metalation. The lower energy absorption peaks are sensitive to
aryl substituents. The methoxy or bromo groups on the
proximal phenyl arms red-shift the lower-energy peak. Methoxy
substituents also cause a red shift of the higher-energy emission
+
similar to that in (2,2′-bipyridine)AuPPh3 , where gold−
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The visible-light absorption of azadipyrromethene complexes
results from an isolated, empty low-lying orbital. Figure 5
depicts a partial Kohn−Sham orbital energy level diagram of 1,
which is representative; diagrams for 2−5 appear as Figures
S4−S7, Supporting Information. At right in Figure 5 are plots
of the highest-occupied and lowest-unoccupied Kohn−Sham
orbitals (HOMO and LUMO, respectively). Also shown are
fractional contributions from fragment orbitals calculated
according to Mulliken.35 The HOMO and LUMO reside
almost wholly on the azadipyrromethene ligand. The sizable
energy gaps between HOMO and LUMO and LUMO and
LUMO + 1 are consistent with the compounds’ absorption
profiles and with the reductive electrochemistry of azadipyrro-
methenes.7 The pendant bromines of Ld make small, but not
insignificant, contributions to the frontier orbitals of 1. They
modulate the orbital energies and, indirectly, the energies of
excited electronic states.
Table 2. Absorption and Emission Maxima of Gold(I)
Azadipyrromethene Complexes and Ligands La‑d Recorded
at Room-Temperature in 2-MeTHF
compound
absorption maxima (nm)
emission maxima (nm)a
La
Lb
Lc
Ld
1
308, 595
320, 619
308, 606
318, 655
316, 610
309, 585
315, 400, 610
312, 595
315, 605
362, 652
359, 667
369, 672
375, 477, 668
385, 671
2
359
3
372, 435−500 (sh), 670
372
4
5
355, 435−500 (sh)
peak. When comparing 1 with 5, it is seen that brominating the
pyrrolic positions scarcely affects the absorption profile, but the
higher-energy emission peak undergoes a 30-nm blue shift.
The free ligand Ld shows one emission peak at 373 nm and
another broad peak around 770 nm. Both emissions are weak.
Gold complexes 1−5 show similar emission spectra, except that
two compounds emit weakly in the red. The near-ultraviolet
emission of 1 is red-shifted from its value in the free ligand; it
appears from 355−385 nm for 1−5 and strongly intensifies.
The lower energy emission band appears around 700 nm with
very low intensity. Emission near 700 nm, when it occurs, is
feeble. It is not observable for compounds 2, 4, or 5.
Compound 3 shows a new absorption at 400 nm. An emission
shoulder in compounds 3 and 5 appears around 435−500 nm.
Optical spectra of 2−4 appear as Figures S1−S3, Supporting
Information.
Calculations. Density-functional theory calculations were
performed on compounds 1−5. All calculations were spin-
restricted. Geometries were optimized without restraint or
imposed symmetry; converged metrics are acceptably near
crystallographic values. Optimized structures reproduce the
asymmetric chelation seen crystallographically, with rough
linearity at gold. Intraligand metrics for the phosphine and the
azadipyrromethenes are unexceptional.
A time-dependent density functional theory (DFT) calcu-
lation on 1 was performed on the ground-state (singlet)
geometry. The calculation included a continuum treatment of
THF solvation. The computation predicts that the first singlet
excited state derives from a LUMO←HOMO transition, and
the vertical excitation energy is calculated to be 2.12 eV (585
nm). The corresponding LUMO←HOMO triplet is the lowest-
lying triplet-state. The configuration interaction that occurs in
9
+
the first excited states of Cu(I), Ag(I), and BF2 complexes is
muted in 1.
CONCLUSIONS
■
Brominated azadipyrromethene ligands have been prepared
with N-bromosuccinimide as the brominating agent, or from
pre-brominated aldehyde and acetophenone precursors. Among
these is a ligand, Ld, brominated in four places, in both proximal
and distal arms. These azadipyrromethene ligands bind gold in
rapid reactions of PhMe2PAuCl with base in THF. Five such
gold complexes were prepared; three are crystallographically
characterized. The structures show that gold binds asymmetri-
cally to these potentially chelating ligands. Coordination
geometries at gold can be described as perturbed two-
Figure 5. Kohn−Sham orbital energy level diagram of 1. Percentages are of electron density. Plots of selected orbitals appear at right (contour level
0.03 au).
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reaction mixture was kept under stirring overnight. Vacuum filtration
led to the isolation of a black solid. This solid was washed several times
with CH2Cl2, and dried under vacuum. Because of their limited
solubility, 1H NMR was only performed with LbBr2. These compouds
were directly used for the synthesis of gold(I) complexes. Yield: LaBr2:
67%, LbBr2: 77%, LcBr2: 78%, LdBr2: 79%. 1H (LbBr2, CDCl3) δ
(ppm) 8.04 (d, 4H, J = 8.8 Hz); 7.77−7.82 (m, 4H); 7.35−7.49 (m,
6H); 7.05 (d, 4H, J = 9.2 Hz), 3.91 (s, 6H).
coordinate. The new complexes show gold(I)’s propensity to
adopt a linear two-coordinate geometry, whereas gold(I) in our
earlier azadipyrromethene complex is three-coordinate.
Gold(I) azadipyrromethenes reprise the absorption features
of the free ligands. Excitation in the near-ultraviolet generates
dual fluorescence in the near-UV and visible wavelengths. Two
compounds show weak red emission that may be phosphor-
escence. Such an assignment is consistent with Castellano’s
work, which shows phosphorescence from BODIPY dyes
brought about by an intramolecular triplet−triplet energy
transfer from a tethered, cyclometalated iridium(III).36
Density-functional theory calculations indicate that the
photoproperties of the new compounds arise from the
azadipyrromethene ligands. Gold(I) is a perturbation. Both
the HOMO and LUMO rest on the conjugated ligand, and
transitions between these orbitals account for the first singlet
and triplet excited states.
The bromine-substituted ligands herein commend azadipyr-
romethenes for incorporation into polymeric materials, such as
the light-harvesting components of solar cells. Azadipyrrome-
thenes easily bind gold(I) and baser coinage metals, and their
colors and emission spectra are preserved. This ligand class also
binds the fac-[Re(CO)3]+ fragment and 3d transition metals.
Metalla-azadipyrromethenes have many exciting prospects.
LdAuPMe2Ph (1). Ld (0.2186 g, 0.28 mmol) and NaOtBu (0.0550 g,
0.57 mmol) were combined in 10 mL of dry THF. The mixture was
purged with argon for 5 min and allowed to stir at room temperature
under argon overnight. Me2PhPAuCl (0.1086 g, 0.29 mmol) was then
transferred to the reaction mixture, and the resulting mixture was
purged with argon for 5 min and allowed to stir at room temperature
under argon for 48 h. THF was removed under vacuum, leaving a
shiny residue. Benzene was added, and the solution was filtered
through Celite. The filtrate was heated to 40 °C, and benzene was
removed under vacuum. Trituration of the residue with pentane led to
the isolation of a lustrous powder. This powder was then dissolved in
benzene, and vapor diffusion of pentane into the benzene solution led
to the isolation of dark red crystals. Yield: 0.1870 g, 58%. 31P{1H}
1
(C6D6) δ (ppm): −1.61. H (C6D6) δ (ppm): 8.10−8.13 (m, 4H),
7.78 (dd, 8H, J = 8.4 Hz, 1.6 Hz), 7.42 (d, 4H, J = 8.4 Hz), 7.23 (d,
4H, J = 8.4 Hz), 7.12 (s, 2H), 6.80−7.00 (m, 5H), 0.53 (bs, 6H). Anal.
Calcd. for C40H29AuBr4N3P: C, 43.71; H, 2.66; N, 3.82. Found: C,
43.83; H, 2.43; N, 3.63. UV−vis (2-MeTHF): λ (ε) 316 nm (6.16 ×
104 M−1 cm−1), 610 nm (5.36 × 104 M−1 cm−1); Emission (2-MeTHF,
ex. 312 nm): 385 nm, 671 nm.
EXPERIMENTAL SECTION
LaBr2AuPMe2Ph (2). LaBr2 (0.0251 g, 0.04 mmol) and KOtBu
(0.0093 g, 0.08 mmol) were mixed in 10 mL of dry THF. The mixture
was purged with argon for 5 min and allowed to stir at room
temperature under argon overnight. Me2PhPAuCl (0.0130 g, 0.04
mmol) was then transferred to the reaction mixture. The resulting
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon for 48 h. THF was removed under vacuum,
leaving a lustrous residue. Benzene was added, and the solution was
filtered through Celite. The filtrate was heated to 40 °C, and benzene
was removed under vacuum. Trituration of the residue with pentane
led to the isolation of a lustrous powder. This powder was then
dissolved in benzene, and vapor diffusion of pentane into the benzene
solution led to the isolation of dark red crystals. Yield: 0.0210 g, 54%.
31P{1H} (C6D6) δ (ppm): −0.82. 1H (C6D6) δ (ppm): 8.07−8.10 (m,
4H), 7.94−7.97 (m, 4H), 7.17−7.26 (m, 5H), 7.01−7.05 (m, 4H),
6.95−7.00 (m, 4H), 6.88−6.92 (m, 2H), 6.72−6.77 (m, 2H), 0.45 (d,
6H, J = 10.4 Hz). Anal. Calcd. for C40H31AuBr2N3P: C, 51.03; H, 3.32;
N, 4.46. Found: C, 51.30; H, 3.38; N, 4.20. UV−vis (2-MeTHF): λ (ε)
309 nm (3.40 × 104 M−1 cm−1), 585 nm (8.45 × 104 M−1 cm−1);
Emission (2-MeTHF, ex. 309 nm): 359 nm.
LbBr2AuPMe2Ph (3). LbBr2 (0.1084 g, 0.16 mmol) and KOtBu
(0.0360 g, 0.32 mmol) were combined in 10 mL of dry THF. The
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon overnight. Me2PhPAuCl (0.0500 g, 0.16
mmol) was then transferred to the reaction mixture. The resulting
mixture was purged with argon for 5 min and allowed to stir at room
tempertaure (RT) under argon for 48 h. THF was removed under
vacuum, leaving a lustrous residue. Benzene was added, and the
solution was filtered through Celite. The filtrate was heated to 40 °C,
and benzene was removed under vacuum. Trituration of the residue
with pentane led to the isolation of a lustrous powder. This powder
was then dissolved in benzene, and vapor diffusion of pentane into the
benzene solution led to the isolation of dark red crystals. Yield: 0.0660
g, 39%. 31P{1H} (C6D6) δ (ppm): −0.85; 1H (C6D6) δ (ppm): 8.11−
8.13 (m, 4H), 8.00−8.02 (m, 4H), 7.25−7.29 (m, 5H), 6.89−6.99 (m,
3H), 6.78−6.86 (m, 2H), 6.65−6.67 (m, 5H), 3.21 (s, 6H), 0.54 (d,
6H, J = 10.0 Hz). Anal. Calcd. for C42H35AuBr2N3O2P: C, 50.37; H,
3.52; N, 4.20. Found: C, 50.10; H, 3.31; N, 4.18. HR-MS: Calcd. m/z
= 1002.05561 (M+H)+, Found m/z = 1002.05508. UV−vis (2-
MeTHF): λ (ε) 315 nm (3.06 × 104 M−1 cm−1), 400 nm (1.04 × 104
M−1 cm−1), 610 nm (7.60 × 104 M−1 cm−1); Emission (2-MeTHF, ex.
315 nm): 372 nm, 467 nm, 670 nm.
■
Materials. Solvents and reagents were purchased from commercial
suppliers and used without further purification unless indicated.
Me2PhPAuCl and Me2PhPAuBr were synthesized following literature
procedures.37,38
1H and 31P{1H} NMR spectra were recorded with a Varian AS-400
1
spectrometer operating at 399.7 and 161.8 MHz, respectively. For H
NMR spectra, chemical shifts (δ) were determined in parts per million
(ppm) relative to the solvent residual peaks. For 31P{1H} NMR,
chemical shifts were determined relative to concentrated H3PO4.
Microanalyses (C, H, and N) were performed by Robertson Microlit
Laboratories. Mass spectrometry was performed at the University of
Cincinnati Mass Spectrometry facility. UV−vis spectra were collected
on a Cary 500 spectrophotometer in HPLC grade solvents.
Fluorescence measurements were carried out with a Cary Eclipse
Spectrophotometer at room temperature. All samples were purged
with argon for at least 15 min before the luminescence measurement.
Synthesis. 1,3-Bis(4-bromophenyl)-4-nitrobutan-1-one (Inter-
mediate in the Synthesis of Ld). (E)-1,3-bis(4-bromophenyl)prop-2-
en-1-one (8.67 g, 23.7 mmol), nitromethane (7.22 g, 0.12 mol), and
diethylamine (15.35 g, 0.21 mol) were combined in 250 mL of
methanol and heated to reflux for 24 h. The resultant solution was
evaporated to dryness, and solvents and volatile materials were
removed. A brown oily solid was isolated. This material was used
1
without further purification. Yield: 3.30 g, 33%. H (CDCl3) δ (ppm)
7.76−7.78 (m, 2H); 7.60−7.62 (m, 2H); 7.46−7.48 (m, 2H); 7.15−
7.17 (m, 2H); 4.79 (dd, 1H, J = 12.8 Hz, 6.8 Hz), 4.66 (dd, 1H, J =
12.4 Hz, 8.0 Hz); 4.19 (quintet, 1H, J = 6.8 Hz); 3.42 (d, 2H, J = 6.8
Hz).
Ld. 1,3-Bis(4-bromophenyl)-4-nitrobutan-1-one (3.30 g, 77.8
mmol) and ammonium acetate (30 g, 0.39 mol) were combined in
200 mL of 1-butanol and heated to reflux for 24 h. The solvent was
stripped off and a dark residue was isolated. This residue was washed
with hot methanol and vacuum filtration led to the isolation of a black
solid. This material was used without further purification. Yield: 2.20 g,
74%. 1H (CDCl3) δ (ppm) 7.88 (d, 4H, J = 8.8 Hz); 7.77 (d, 4H, J =
8.4 Hz); 7.68 (d, 4H, J = 8.4 Hz); 7.56(d, 4H, J = 8.8 Hz); 7.52 (s,
2H).
LaBr2-LdBr2. L (1 equiv) was dissolved in 20 mL of dry CH2Cl2, and
N-bromosuccinimide (2.2 equiv) was added. The solution was
degassed under argon, and stirred vigorously under argon at room
temperature. After several minutes, a precipitate was generated. The
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LcBr2AuPMe2Ph (4). LcBr2 (0.0704 g, 0.11 mmol) and KOtBu
(0.0240 g, 0.22 mmol) were combined in 10 mL of dry THF. The
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon overnight. Me2PhPAuCl (0.0330 g, 0.11
mmol) was then transferred to the reaction mixture, and the resulting
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon for 48 h. THF was removed under vacuum,
leaving a lustrous residue. Benzene was added, and the solution was
filtered through Celite. The filtrate was heated to 40 °C, and benzene
was removed under vacuum. Trituration of the residue with pentane
led to the isolation of a lustrous powder. This powder was then
dissolved in benzene, and vapor diffusion of pentane into the benzene
solution led to the isolation of dark red crystals. Yield: 0.0300 g, 28%.
ASSOCIATED CONTENT
■
S
* Supporting Information
Absorption and emission of 2, 3, and 4; Kohn−Sham orbital
energy level diagrams of 2−5, and optimized Cartesian
coordinates of 1−5 calculated by density-functional theory as
described in the Experimental Section. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tgray@case.edu.
Notes
1
31P{1H} (C6D6) δ (ppm), −0.79; H (C6D6) δ (ppm) 8.10−8.13 (m,
The authors declare no competing financial interest.
4H), 7.94−7.96 (m, 4H), 7.03−7.07 (m, 5H), 6.95−7.00 (m, 3H),
6.89−6.92 (m, 2H), 6.83−6.85 (m, 5H), 3.38 (s, 6 h), 0.49 (d, 6H, J =
10.4 Hz). Anal. Calcd. for C42H35AuBr2N3O2P: C, 50.37; H, 3.52; N,
4.20. Found: C, 50.65; H, 3.27; N, 4.01. UV−vis (2-MeTHF): λ (ε)
312 nm (8.6 × 103 M−1 cm−1), 595 nm (5.47 × 104 M−1 cm−1);
Emission (2-MeTHF, ex. 312 nm): 372 nm.
ACKNOWLEDGMENTS
■
The authors thank the National Science Foundation (Grant
CHE-1057659 to T.G.G.) for support. The diffractometer at
Case Western Reserve was funded by NSF Grant CHE-
0541766. N.D. thanks the Republic of Turkey for a fellowship.
We thank Dr. T. S. Teets for experimental assistance and
Professor D. G. Nocera (both then at the Massachusetts
Institute of Technology) for access to instrumentation. We
thank also Dr. Matthias Zeller, Youngstown State University,
for assistance with the crystallography.
LdBr2AuPMe2Ph (5). LdBr2 (0.1040 g, 0.11 mmol) and KOtBu
(0.0240 g, 0.22 mmol) were combined in 10 mL of dry THF. The
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon overnight. Me2PhPAuCl (0.0330 g, 0.11
mmol) was then transferred to the reaction mixture. The resulting
mixture was purged with argon for 5 min and allowed to stir at room
temperature under argon for 48 h. THF was removed under vacuum,
leaving a lustrous residue. Benzene was added, and the solution was
filtered through Celite. The filtrate was heated to 40 °C, and benzene
was removed under vacuum. Trituration of the residue with pentane
led to the isolation of a lustrous powder. This powder was then
dissolved in benzene, and vapor diffusion of pentane into the benzene
solution led to the isolation of a dark greenish powder. Yield: 0.0638 g,
47%. 31P{1H} (C6D6) δ (ppm), −1.81; 1H (C6D6) δ (ppm) 7.68−7.72
(m, 4H), 7.46−7.49 (m, 4H), 7.31−7.34 (m, 4H), 7.11−7.12 (m, 4H),
7.00−7.02 (m, 3H), 6.71−6.76 (m, 2H), 0.42 (d, 6H, J = 10.4 Hz).
Anal. Calcd. for C40H27AuBr6N3P: C, 38.22; H, 2.16; N, 3.34. Found:
C, 38.15; H, 2.06; N, 3.14. UV−vis (2-MeTHF): λ (ε) 315 nm (3.54
× 103 M−1 cm−1), 605 nm (7.24 × 103 M−1 cm−1); Emission (2-
MeTHF, ex. 312 nm): 355 nm, 467 nm.
X-ray Single Crystal Structure Analysis. Single crystal X-ray
data were collected on a Bruker AXS SMART APEX CCD
diffractometer using monochromatic Mo Kα radiation with the ω
scan technique. The unit cells were determined using SMART39 and
SAINT+.40 Data collection for all crystals was conducted at 100 K
(−173.5 °C). All structures were solved by direct methods and refined
by full matrix least-squares against F2 with all reflections using
SHELXTL.41 Refinement of extinction coefficients was found to be
insignificant. All non-hydrogen atoms were refined anisotropically. All
hydrogen atoms were placed in standard calculated positions, and all
hydrogen atoms were refined with an isotropic displacement
parameter 1.2 times that of the adjacent carbon.
Calculations. Spin-restricted density-functional theory calculations
were performed within Gaussian 09 rev. A.02.42 Geometries were
optimized without imposed symmetry. Calculations employed the
exchange and correlation functionals of Perdew, Burke, and
Ernzerhof,43 and the TZVP basis set of Godbelt, Andzelm, and co-
workers for carbon, hydrogen, oxygen, and phosphorus.44 For gold,
the Stuttgart 97 effective core potential and basis set were used;45
scalar relativistic effects are included implicitly. Harmonic frequency
calculations returned all real vibrational frequencies. The calculations,
including geometry optimizations, impose continuum solvation in
THF, using the integral equation formalism of Tomasi’s polarizable
continuum model.46−49 Population analyses were performed with the
AOMIX-CDA software of Gorelsky.50,51
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