Halogen photoreductive elimination from metal-metal bonded iridium(II)-gold(II) heterobimetallic complexes

Article abstract of DOI:10.1021/ic902590u

Halogen oxidation of [Ir^IAu^I(dcpm) ₂(CO)X](PF₆) (dcpm = bis(dicyclohexylphosphino)methane, X = Cl, Br) and [Ir^IAu^I (dppm)₂(CN ^tBu)₂](PF₆)₂ (dppm = bis(diphenylphosphino)methane) furnishes the heretofore unknown class of d ⁷-d⁹ compounds comprising an Ir^IIAu ^II heterobimetallic core. A direct metal-metal bond is evident from a 0.2 A contraction in the intermetallic distance, as determined by X-ray crystallography. The photophysical consequence of iridium-gold bond formation, as elucidated by experimental and computational investigations, is an electronic structure dominated by a σ → σ* transition that possesses significant ligand-to-metal charge transfer (LMCT) character. Accordingly, these compounds are non-emissive but photoreactive. Excitation of Ir^IIAu^II complexes in the presence of a halogen trap prompts a net photoreductive elimination of halogen and the production of the two-electron reduced Ir^IAu^I species with about 10% quantum efficiency. The Ir^IIAu^II complexes add to a growing library of d⁷-d⁹ heterobimetallic species from which halogen elimination may be driven by a photon.

Full text of DOI:10.1021/ic902590u

Inorg. Chem. 2010, 49, 3035–3043 3035
DOI: 10.1021/ic902590u
Halogen Photoreductive Elimination from Metal-Metal Bonded
Iridium(II)-Gold(II) Heterobimetallic Complexes
Thomas S. Teets, Daniel A. Lutterman, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139-4307
Received December 29, 2009
Halogen oxidation of [IrIAuI(dcpm)2(CO)X](PF6) (dcpm = bis(dicyclohexylphosphino)methane, X = Cl, Br) and
[IrIAuI(dppm)2(CNtBu)2](PF6)2 (dppm = bis(diphenylphosphino)methane) furnishes the heretofore unknown class of
7
9
II II
˚
d ;d compounds comprising an Ir Au heterobimetallic core. A direct metal-metal bond is evident from a 0.2 A
contraction in the intermetallic distance, as determined by X-ray crystallography. The photophysical consequence of
iridium-gold bond formation, as elucidated by experimental and computational investigations, is an electronic
structure dominated by a σ f σ* transition that possesses significant ligand-to-metal charge transfer (LMCT)
character. Accordingly, these compounds are non-emissive but photoreactive. Excitation of IrIIAuII complexes in the
presence of a halogen trap prompts a net photoreductive elimination of halogen and the production of the two-electron
reduced IrIAuI species with about 10% quantum efficiency. The IrIIAuII complexes add to a growing library of d7;d9
heterobimetallic species from which halogen elimination may be driven by a photon.
Introduction
reduction to its PtIIAuI precursor with a quantum yield of
5.7% when the halogen is trapped,5 nearly a 10-fold increase
over the efficiency of halogen elimination from the d7;d9
RhII;Rh0 bimetallic core. Even more efficient halogen
photoelimination is achieved from more highly oxidized Pt2
cores. Pt2III,III(tfepma)2Cl6 undergoes efficient two-electron
photoreduction (Φ = 38%) at high trap concentrations, and
is able to eliminate Cl2 when irradiated in the solid state,
providing what is the first example of authentic, trap-free X2
reductive photoelimination from a transition-metal center.6
Ligand-to-metal charge transfer (LMCT) excitation of phos-
phine-ligated gold(III) halide complexes that lack a metal-
metal bond can also promote efficient halogen elimina-
tion from solution in the presence of trap (Φ = 26%) and
from the solid state in the absence of trap.7
Our interest in heterobimetallic complexes stems from a
desire to strike a balance in designing molecular photocata-
lysts that can manage the formal reduction of protons to
hydrogen but at the same time are oxidizing enough to
efficiently eliminate halogen to complete HX-splitting cycles.
Armed with the knowledge (i) that metal-metal bonded
diiridium complexes display a rich reaction chemistry with
hydrohalic acids and reversibly add hydrogen8 and (ii) that
Metal photocatalyzed hydrogen production from HX
(X = Cl, Br) hinges upon halogen elimination via M;X
bond activation.1 This lesson is learned from the hydrogen
photocycle established by bimetallic Rh complexes bridg-
ed by three bis(difluorophosphino)methylamine (dfpma)
ligands.2 A hydrido-halide Rh2II,II(H)2(X)2 species is gener-
ated from the addition of two HX molecules to a parent
Rh2 complex.3 Whereas H2 photoelimination is facile to
0,0
furnish a two-electron Rh20,II(X)2 mixed valence complex,
photoelimination of halogen from this d7;d9 complex is
sluggish.3,4 The overall efficiency for H2 photogeneration
has little to do with hydrogen production but rather is limited
by the efficiency of halogen photoelimination to regenerate
the initial photoreactant. Accordingly, recent efforts in our
group have sought to develop a multielectron halogen-
elimination photochemistry predicated on dissociative dσ*
excited states that are heavily admixed with contributions
from coordinated halide ligands. We have found that the
d7;d9 electron count of late transition metals is particularly
predisposed to halogen photoelimination. The d7;d9 com-
plex [PtIIIAuII(dppm)2PhCl3]PF6 undergoes photochemical
*To whom correspondence should be addressed. E-mail: nocera@mit.
edu.
(5) Cook, T. R.; Esswein, A. J.; Nocera, D. G. J. Am. Chem. Soc. 2007,
(1) Esswein, A. J.; Nocera, D. G. Chem. Rev. 2007, 107, 4022–4047.
(2) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639–1641.
(3) Esswein, A. J.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc. 2005,
127, 16641–16651.
(4) Odom, A. L.; Heyduk, A. F.; Nocera, D. G. Inorg. Chim. Acta 2000,
297, 330–337.
129, 10094–10095.
(6) Cook, T. R.; Surendranath, Y.; Nocera, D. G. J. Am. Chem. Soc. 2009,
131, 28–29.
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9426.
r
2010 American Chemical Society
Published on Web 02/18/2010
pubs.acs.org/IC

3036 Inorganic Chemistry, Vol. 49, No. 6, 2010
Teets et al.
complexes containing more oxidizing transition metals effi-
ciently eliminate halogen (vide supra), we turned our atten-
tion to iridium-gold heterobimetallic complexes as promis-
ing candidates as next-generation HX-splitting catalysts.
Iridium(I)-gold(I) heterobimetallic complexes were first
reported by Shaw and co-workers, who detailed the synthesis
of [IrIAuI(dppm)2COCl]Cl,9 and later described synthetic
methods for substituting methyl isocyanide (MeNC) onto the
iridium center.10 Since these initial reports, photophysical11-14
and computational studies15 of IrIAuI complexes have
emerged, though the reactivity of these complexes remains
underexplored. Reactions with select Lewis acids such as HgCl2
and BF3 have been characterized for [IrIAuI(dppm)2COCl]-
(PF6)16 and a trimetallic IrIAuIIrI complex has been oxidized.17
But oxidative-addition to bimetallic iridium-gold species has
not been detailed, and photochemical transformations invol-
ving iridium-gold heterobimetallics are likewise unknown.
In this work, we describe the synthesis of d8;d10 IrIAuI
complexes spanned by two bridging diphosphines. Oxidation
with halogen furnishes the d7;d9 IrIIAuII bimetallic com-
plex; this electron count has heretofore not been observed for
Ir bonded to Au. The complexes possess a formal metal-
metal σ-bond, which is validated by structural metrics from
X-ray crystallography. Experimental and computational
studies establish an electronic structure that is dominated
by dσ f dσ* character with significant halide admixture.
Consistent with this bonding formulation, irradiation of the
IrIIAuII complexes activates M;X bonds, and halogen
photoreductive elimination is achieved.
Varian Mercury 300 NMR Spectrometer or a Varian Inova-500
NMR Spectrometer. 31P{1H} NMR spectra were referenced to
an external standard of 85% D3PO4, and 1H spectra were
referenced to tetramethylsilane (TMS) using the residual proteo
solvent resonances. UV-vis spectra were recorded at 293 K in
CH2Cl2 solutions in quartz cuvettes on a Varian Cary 5000
UV-vis-NIR spectrophotometer. Extinction coefficients were
determined over a concentration range of ∼2-50 μM, for which
all compounds obeyed Beer’s Law. Steady state emission spectra
were recorded on an automated Photon Technology Interna-
tional (PTI) QM 4 fluorimeter equipped with a 150-W Xe arc
lamp and a Hamamatsu R928 photomultiplier tube. Excitation
light was excluded with appropriate glass filters. Samples were
housed in custom quartz EPR tubes with a ground-glass joint
and Teflon plug. Solution samples were prepared in 1:1 CH2Cl2/
toluene and freeze pump thaw degassed (4 cycles, 1 ꢀ 10-5 torr).
Solid samples were prepared by evaporation of a CH2Cl2
solution under vacuum; the resulting solid film was evacuated
to <1 ꢀ 10-5 torr. Spectra were recorded at 77 K by immersion
of the sealed EPR tubes into a liquid nitrogen filled quartz dewar
flask. Time resolved phosphorescence lifetimes were recorded
on a nanosecond laser system described previously.22 IR spectra
of powdered samples were recorded on a PerkinElmer Spectrum
400 FT-IR/FT-FIR Spectrometer outfitted with a Pike Tech-
nologies GladiATR attenuated total reflectance accessory with
a monolithic diamond crystal stage and pressure clamp.
Photochemistry. Photochemical reactions were performed
using a 1000 W high-pressure Hg/Xe arc lamp (Oriel). The beam
was passed through a water-jacketed filter holder containing
appropriate long pass filters, an iris and collimating lens. Samples
for all photolysis experiments were prepared in a nitrogen-filled
glovebox in quartz cuvettes equipped with a magnetic stir bar and a
threaded screw-cap and kept rigorously excluded from light until
the start of irradiation. Monochromatic light was generated by
combining the output from a long-pass filter with a Hg line filter of
the appropriate wavelength. For quantum yield measurements,
potassium ferrioxalate was synthesized via a published procedure
and used as a chemical actinometer.23 The photon flux was
determined from the average of the actinometric measurements
collected before and after irradiation of a set of six samples.
UV-vis spectra for steady-state photolysis experiments and quan-
tum yield measurements were recorded on a Spectral Instruments
400 diode array spectrophotometer and were blanked to the
appropriate solvent.
Experimental Section
General Considerations. All reactions involving air-sensitive
materials were executed in a nitrogen-filled glovebox using
solvents previously dried by passage through an alumina col-
umn under argon. The starting materials bis(dicyclohexylphos-
phino)methane (dcpm) and [IrI(COD)Cl]2 (COD = 1,5-cyclo-
octadiene) were obtained from Strem, and bromine and
tert-butylisocyanide (CNtBu) were obtained from Sigma-
Aldrich. Chlorine was delivered as the iodobenzene adduct
PhICl2.18 [IrI(dcpm)2CO]Cl and [IrI(dcpm)2CO]Br were pre-
pared in an analogous fashion to that described for [IrI(dppm)2-
CO]Cl.19,20 Gold(I) starting materials AuI(tht)Cl (tht = tetra-
hydrothiophene) and AuI(tht)Br were synthesized as previously
described.21 For additions of Br2 to reactions, a stock solution
was prepared using a weighed amount of bromine, and then an
appropriate aliquot was added using an autopipet.
Computational Methods. All calculations were performed
using the Gaussian 03 program suite.24 Where possible, atomic
coordinates for geometry optimizations originated from X-ray
diffraction data. Cyclohexyl, phenyl, and tert-butyl groups were
replaced with methyl groups, and counterions were omitted.
Density functional theory (DFT) calculations were carried out
using the three hybrid functionals of Becke25 in conjunction with
the correlation functional of Lee, Yang, and Parr (B3LYP).26,27
The 6-31G(p,d) basis set was applied to all non-metal atoms,28,29
and the Stuttgart effective core potential and associated basis set
was used for Ir and Au.30 All calculations were spin-restricted on
Physical Methods. All NMR spectra were recorded at the
MIT Department of Chemistry Instrumentation Facility on a
(9) Hutton, A. T.; Pringle, P. G.; Shaw, B. L. Organometallics 1983, 2,
1889–1891.
(10) Langrick, C. R.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1985, 511–
516.
(11) Balch, A. L.; Catalano, V. J.; Olmstead, M. M. Inorg. Chem. 1990, 29,
585–586.
(12) Balch, A. L.; Catalano, V. J. Inorg. Chem. 1991, 30, 1302–1308.
(13) Striplin, D. R.; Brozik, J. A.; Crosby, G. A. Chem. Phys. Lett. 1994,
231, 159–163.
(14) Striplin, D. R.; Crosby, J. A. J. Phys. Chem. 1995, 99, 7977–7984.
(15) Liu, T.; Zhou, X.; Zhang, H.-X.; Xia, B.-H. Dalton. Trans. 2008,
1065–1072.
(22) Loh, Z.-H.; Miller, S. E.; Chang, C. J.; Carpenter, S. D.; Nocera, D.
G. J. Phys. Chem. A 2002, 106, 11700–11708.
(23) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; Taylor and Francis: Boca Raton, FL, 2006.
(24) Gaussian 03, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2004; Full
citation in the Supporting Information.
(25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(26) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(27) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989,
157, 200–206.
(28) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213–222.
(29) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M.
S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654–3655.
(30) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86,
866–872.
(16) Balch, A. L.; Catalano, V. J. Inorg. Chem. 1992, 31, 2730–2734.
(17) Balch, A. L.; Nagle, J. K.; Oram, D. E.; Reedy, P. E., Jr. J. Am.
Chem. Soc. 1988, 110, 454–462.
(18) Zielinska, A.; Skulski, L. Tetrahedron Lett. 2004, 45, 1087–1089.
(19) Miller, J. S.; Caulton, K. G. J. Am. Chem. Soc. 1975, 97, 1067–1073.
(20) Vaska, L.; Catone, D. L. J. Am. Chem. Soc. 1966, 88, 5324–5325.
ꢀ
(21) Uson, R.; Laguna, A.; Laguna, M. Inorg. Synth. 1989, 26, 85–91.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3037
gas-phase molecules, with no imposed symmetry. Time depen-
dent calculations were initiated from the optimized geometry
using the B3LYP exchange and correlation functionals. Molec-
ular orbitals were imaged in the program GaussView 3.0 with an
isodensity of 0.04 applied. Optimized Cartesian coordinates for
the model compounds considered here are deposited in the
Supporting Information.
PhICl2 (23.3 mg, 0.0848 mmol, 1.20 equiv) in 0.7 mL of CH2Cl2
were both chilled with liquid N2 in the glovebox cold well. Once
frozen, the two samples were removed, and upon melting the
PhICl2 solution was added dropwise to the solution of 1. The
color immediately faded to a lighter orange, and the solution
was stirred at room temperature for 3 h. Dropwise addition of 15
mL of pentane to the stirred solution yielded a yellow-orange
solid, which was separated from the supernatant by decantation
and dried in vacuo. Yield: 104 mg (99.0%). 1H NMR (500 MHz,
CD2Cl2) δ/ppm: 3.62-3.73 (m, 2H), 3.08-3.22 (br, m, 4H),
2.81-2.90 (br, m, 2H), 2.68-2.77 (br, m, 2H), 2.54-2.64 (br, m,
2H), 2.44-2.51 (br, m, 2H), 2.08-2.32 (br, m, 6H), 1.20-2.05
(br, m, 72H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm: 31.6
(pseudotriplet, 2P), -11.0 (pseudotriplet, 2P), -144.1 (sept.,
1JP-F = 714 Hz, 1P). UV-vis (CH2Cl2): λ/nm {ε/(M-1 cm-1)}
266 {22000}, 331 {23000}, 396 {5800} 455 (sh) {1200}. IR (solid):
ν~CO = 2009 cm-1. Anal. Calcd. for C51H92AuCl3F6IrOP5: C,
41.23; H, 6.24. Found: C, 40.96; H, 6.15.
Preparation of [IrIIAuII(dcpm)2COBr3](PF6) (5). A sample of
2 (100 mg, 0.0685 mmol) was dissolved in 4 mL of acetonitrile. A
solution of Br2 (11.5 mg, 0.0720 mmol, 1.05 equiv) in 0.3 mL of ace-
tonitrile was added dropwise. The resulting suspension was stirred at
room temperature for 30 min. All volatiles were removed by rotary
evaporation to afford an orange solid, which was suspended in
15 mL of diethyl ether and collected by filtration. The product was
washed with 10 mL of diethyl ether and dried in vacuo. Yield: 97 mg
(87%). 1H NMR (500 MHz, CD2Cl2) δ/ppm: 3.84-3.94 (m, 2H),
3.35-3.49 (br, m, 4H), 2.88-2.96 (br, m, 2H), 2.73-2.81 (br, m,
2H), 2.33(br, d,2H) 2.24(br, d, 2H), 2.08(br, d, 2H), 1.21-2.05 (br,
m, 76H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm: 25.7 (pseu-
dotriplet, 2P), -18.6 (pseudotriplet, 2P), -144.2 (sept., 1JP-F =711
Hz, 1P). UV-vis (CH2Cl2): λ/nm {ε/(M-1 cm-1)} 283 (sh)
{16000}, 305 {23000}, 369 {15000}, 416 (sh) {9400}, 496 (sh)
{1500}. IR (solid): ν~CO = 2007 cm-1. Anal. Calcd. for C51H92Au-
Br3F6IrOP5: C, 37.83; H, 5.73. Found: C, 37.59; H, 5.64.
Preparation of [IrIAuI(dcpm)2COCl](PF6) (1). To a solution
of [IrI(dcpm)2CO]Cl (1.00 g, 0.932 mmol) in 6 mL of CH2Cl2 was
added a solution of Au(tht)Cl (299 mg, 0.932 mmol) in 8 mL of
CH2Cl2. The orange solution was stirred at room temperature
for 1 h, at which time a solution of NH4PF6 (228 mg, 1.40 mmol)
in 15 mL of methanol was added. The solution was concentrated
to <10 mL via rotary evaporation, during which time and
orange solid precipitated. The solid was collected by filtration,
washed with 5 mL of methanol and dried in vacuo. Yield: 1.04 g
(78.8%). 1H NMR (500 MHz, CD2Cl2) δ/ppm: 2.50-2.69 (br,
m, 6H), 2.33 (br, d, 2H), 2.17-2.29 (br, m, 4H), 1.14-2.09 (br,
m, 80H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm: 51.0 (m,
2P), 25.3 (m, 2P), -144.1 (sept., 1JP-F = 711 Hz, 1P). UV-vis
(CH2Cl2): λ/nm {ε/(M-1 cm-1)} 300 (sh) {4300}, 316 {5700},
355 {2500}, 431 {17000}, 517 {300}. IR (solid): ν~CO = 1944
cm-1. Anal. Calcd. for C51H92AuClF6IrOP5: C, 43.30; H, 6.55.
Found: C, 43.37; H, 6.51.
Preparation of [IrIAuI(dcpm)2COBr](PF6) (2). A solution of
Au(tht)Br (327 mg, 0.895 mmol) in 8 mL of CH2Cl2 was added
to a solution of [IrI(dcpm)2CO]Br (1.00 g, 0.0895 mmol) in 6 mL
of CH2Cl2. The resulting dark orange solution was stirred for 90
min at room temperature and then filtered through a plug of
glass wool. A solution of NH4PF6 (220 mg, 1.35 mmol) in 15 mL
of methanol was added, and the resulting bright orange solution
was stirred for 1 h. After concentrating to <10 mL by rotary
evaporation, an orange solid formed, which was collected by
filtration, washed with 10 mL of methanol, and dried in vacuo.
1
Yield: 1.14 g (87.0%). H NMR (500 MHz, CD2Cl2) δ/ppm:
2.50-2.82 (br, m, 6H), 2.15-2.40 (br, m, 6H), 1.00-2.10 (br, m,
80H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm: 50.4 (m, 2P),
23.2 (m, 2P), -144.2 (sept., 1JP-F = 711 Hz, 1P). UV-vis (CH2-
Cl2): λ/nm {ε/(M-1 cm-1)} 307 (sh) {5800}, 319 {6400}, 360
Preparation of [IrIIAuII(dppm)2(CNtBu)2Cl2](PF6)2 (6). A so-
lution 3 (100 mg, 0.0620 mmol) in 1.5 mL of CH2Cl2 and a
solution of PhICl2 (18 mg, 0.065 mmol, 1.2 equiv) in 1 mL of
CH2Cl2 were prepared and frozen in the glovebox cold well.
After removing from the cold well and allowing the solution to
melt, the PhICl2 solution was added dropwise, prompting an
immediate color change from dark green to bright yellow. The
reaction solution was stirred in the dark for 1 h. The yellow
solution was filtered through a plug of glass wool into 18 mL of
pentane, which resulted in the formation of a pale yellow
powder. The supernatant was decanted, and the remaining solid
was washed with pentane and dried in vacuo. Yield: 94 mg
(90%). 1H NMR (500 MHz, CD2Cl2) δ/ppm: 7.61-7.70 (br, m,
16H), 7.46-7.56 (br, m, 24H), 5.24 (br, pseudoquintet, 4H), 1.01
(s, 18H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm: 7.7
(pseudotriplet, 2P), -17.6 (pseudotriplet, 2P), -143.9 (sept.,
1JP-F = 711 Hz, 2P). UV-vis (CH2Cl2): λ/nm {ε/(M-1 cm-1)}
{2600}, 437 {18000}, 526 {320}. IR (solid): ν~CO = 1945 cm-1
.
Anal. Calcd. for C51H92AuBrF6IrOP5: C, 41.98; H, 6.35.
Found: C, 41.90; H, 6.36.
Preparation of [IrIAuI(dppm)2(CNtBu)2](PF6)2 (3). To a solu-
tion of [Ir(COD)Cl]2 (149 mg, 0.222 mmol) in 5 mL of CH2Cl2
was added a solution of dppm (341 mg, 0.887 mmol) in 2 mL of
CH2Cl2, giving a yellow solution. A solution of CNtBu (73.7 mg,
0.887 mmol) was added immediately, initially causing a change to a
lighter yellow color, which darkened to yellow-orange as the
solution was stirred for 20 min. At this time, a solution of Au(tht)Cl
(142 mg, 0.443 mmol) in 2 mL of CH2Cl2 was added, giving a deep
red solution which was stirred for 45 min. Addition of a solution of
NH4PF6 (180 mg, 1.10 mmol) in 20 mL of MeOH prompted an
instantaneous color change to dark green. The solution was con-
centrated to 5 mL, and the resulting green solid was filtered and
washed with 5 mL of methanol. The crude solid was dissolved in 2
mL of CH2Cl2 and recrystallized by layering with 1 mL of methanol
followed by 17 mL of diethyl ether. After several hours, green
microcrystals formed, which were separated from the supernatant
257 {34000}, 320 {33000}, 388 (sh) {2600}. IR (solid): ν~CN
=
2192 cm-1. Anal. Calcd. for C60H62AuCl2F12IrN2P6: C, 42.77;
H, 3.71; N, 1.66. Found: C, 42.43; H, 3.87; N, 1.60.
Preparation of [IrIIAuII(dppm)2(CNtBu)2Br2](PF6)2 (7). A
10 mL Schlenk tube was charged with a solution of 3 (100 mg,
0.0620 mmol) in 2 mL of CH2Cl2 and chilled to -78 °C under an
argon atmosphere. A solution of Br2 (10.4 mg, 0.0650 mmol, 1.05
equiv) in 228 μL of CH2Cl2 was added dropwise, causing an
immediate color change to pale orange. The solution was allowed
to warm to room temperature and stirred for 15 min. Addition of 17
mL of pentane afforded a yellow-orange solid, which was separated
by decanting the supernatant and dried in vacuo. Yield: 106 mg
(96.4%). 1H NMR (500 MHz, CD2Cl2) δ/ppm: 7.61-7.69 (br, m,
16H), 7.35-7.60 (br, m, 24H), 5.33-5.37 (br, m, 4H), 1.06 (s, 18H).
31P{1H} NMR (202.5 MHz, CD2Cl2) δ/ppm: 3.1 (pseudotriplet,
2P), -22.2 (pseudotriplet, 2P), -143.9 (sept., 1JP-F = 712 Hz, 2P).
UV-vis (CH2Cl2): λ/nm {ε/(M-1 cm-1)} 255 (sh) {34000}, 344
1
by decantation and dried in vacuo. Yield: 382 mg (53.4%). H
NMR (500 MHz, CD2Cl2) δ/ppm: 7.84-7.90 (m, 8H), 7.68-7.74
(m, 8H), 7.52-7.63 (m, 12H), 7.44-7.50 (m, 4H), 7.38-7.43 (m,
8H), 4.10 (br, pseudoquintet, 4H), 0.63 (s, 18H). 31P{1H} NMR
(202.5 MHz, CD2Cl2) δ/ppm: 30.0 (m, 2P), 10.4 (m, 2P), -143.9
1
(sept., JP-F = 712 Hz, 2P). UV-vis (CH2Cl2): λ/nm {ε/(M-1
cm-1)} 345 {11000}, 402 {4400}, 490 {26000}, 606 {520}. IR
(solid): ν~CN= 2138 cm-1. Anal. Calcd. for C60H62AuF12IrN2P6:
C, 44.65; H, 3.87; N, 1.74. Found: C, 44.39; H, 3.82; N, 1.59.
Preparation of [IrIIAuII(dcpm)2COCl3](PF6) (4). A solution of
1 (100 mg, 0.0707 mmol) in 1.5 mL of CH2Cl2 and a solution of

3038 Inorganic Chemistry, Vol. 49, No. 6, 2010
Teets et al.
Table 1. Crystallographic Summary for Complexes 1, 2, 4, 5, 6, 7
1
2 Et2O
3
4 CH2Cl2
3
5 CH2Cl2
3
6 2CH2Cl2
3
7 2CH2Cl2
3
formula
C51H92Au-
ClF6IrOP5
1414.71
100(2) K
monoclinic
P21/c
orange
13.5909(13)
22.404(2)
18.9079(17)
90
94.232(2)
90
5741.6(9)
4
0.0416
C55H102Au-
BrF6IrO2P5
1533.29
100(2) K
monoclinic
P21/n
C52H94Au-
Cl5F6IrOP5
1570.54
100(2) K
monoclinic
P21/c
orange
14.5925(16)
20.500(2)
20.651(2)
90
95.279(2)
90
6151.7(11)
4
0.0474
C52H94
-
AuBr3Cl2F6IrOP5
C62H66Au-
Cl6F12IrN2P6
1854.85
100(2) K
monoclinic
P21/c
C62H66AuBr2-
Cl4F12IrN2P6
1943.77
100(2) K
monoclinic
P21/c
fw, g/mol
temperature
cryst. syst.
space group
color
1703.92
100(2) K
monoclinic
P21/c
orange
14.6114(14)
20.703(2)
20.627(2)
90
95.402(2)
90
6212.0(10)
4
0.0534
0.0839
0.0351
0.0759
1.012
orange
yellow
orange
˚
a (A)
15.0304(16)
21.199(2)
20.366(2)
90
102.708(2)
90
6330.3(11)
4
0.0356
0.0669
0.0278
16.551(2)
20.888(3)
21.963(3)
90
106.100(2)
90
7295.4(17)
4
0.0582
0.1000
0.0399
16.4460(18)
20.891(2)
22.145(2)
90
105.181(2)°.
90
7343.1(14)
4
0.0690
0.1070
0.0417
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
3
˚
V (A )
Z
R1a (all data)
wR2b (all data)
R1 [(I > 2σ)]
wR2 [(I > 2σ)]
GOFc
0.0702
0.0305
0.0657
1.070
0.0826
0.0348
0.0765
1.051
0.0636
1.049
0.0894
1.038
0.0931
1.023
P
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|. b wR2 = [ w(Fo2 - Fc2)2/ w(Fo2)2]1/2. c GOF = [ w(Fo2 - Fc2)2/(n - p)]1/2 where n is the number of data and p is the
number of parameters refined.
{38000}. IR (solid): ν~CN = 2189 cm-1. Anal. Calcd. for C60H62-
AuBr2F12IrN2P6: C, 40.62; H, 3.52; N, 1.58. Found: C, 40.63; H,
3.62; N, 1.45.
Scheme 1
X-ray Crystallographic Details. Single crystals of 1 and 2 were
obtained by vapor diffusion of diethyl ether into an acetonitrile
solution, crystals of 4 were grown by layering a CH2Cl2 solution
with methanol/diethyl ether, and crystals of 5, 6, and 7 were
obtained from CH2Cl2 solutions layered with diethyl ether. The
crystals were mounted on a Bruker three circle goniometer
platform equipped with an APEX detector. A graphic mono-
chromator was employed for wavelength selection of the Mo KR
˚
radiation (λ = 0.71073 A). The data were processed and refined
using the program SAINT supplied by Siemens Industrial
Automation. Structures were solved by direct methods in
SHELXS and refined by standard difference Fourier techniques
in the SHELXTL program suite (6.10 v., Sheldrick G. M., and
Siemens Industrial Automation, 2000). Hydrogen atoms were
placed in calculated positions using the standard riding model
Scheme 1. The complexes [IrIAuI(dcpm)2(CO)X](PF6)
[X = Cl (1), Br (2)] are prepared by a method similar to
that used for the synthesis of the dppm-bridged complex.9
and refined isotropically; all other atoms were refined aniso-
Reaction of [IrI(dcpm)2CO]X with Au(tht)X in CH2Cl2,
tropically. A positional disorder of the CO ligand and the halide
trans to it was present in the structures of 1, 2, 4, and 5. The
followed by anion exchange with methanolic NH4PF6,
affords 1 and 2 as bright orange solids. The H NMR
spectra of 1 and 2 are not particularly informative,
thermal displacement parameters of the CO ligand in the minor
1
disordered component were constrained to be identical to those
of the CO ligand in the major component. The structures of 5, 6,
consisting mainly of broad, overlapping cyclohexyl reso-
nances. The identity and purity of these compounds are
most readily established by 31P NMR. In the 31P{1H}
spectrum of both 1 and 2, 14 of the 20 possible lines
resulting from the AA0XX0 spin system can be resolved,
and the position of the two frequencies is responsive to
halide substitution. The two peaks in the spectrum of 1,
centered at 51.0 and 25.3 ppm, shift to 50.4 and 23.2 ppm
in 2. The larger shift of the upfield resonance suggests
that it originates from the phosphorus atoms bound to
iridium, which is the site of the halide substitution.
Complexes 1 and 2 also possess distinctive CtO IR
stretching frequencies at 1944 cm-1 (1) and 1945 cm-1
(2), which are invariant to halide substitution.
and 7 all contained one dichloromethane solvate molecule that
was modeled as a two-part disorder. In 6, one of the two tert-
butyl groups was found to be disordered over two sites. The
checkCIF reports for isostructural 6 and 7 each indicated
analogous level A alerts; one can be attributed to thermal
motion in tert-butyl groups which were not able to be satisfac-
torily modeled as two-part disorders, and the other is a close
intermolecular contact caused by substantial disorder in the
dichloromethane solvate molecules. The (1,2) and (1,3) dis-
tances of all disordered parts were restrained to be similar using
the SADI command; the rigid-bond restraints SIMU and
DELU were also used on disordered parts. Unit cell parameters,
morphology, and solution statistics for the structures of 1, 2, 4,
5, 6, and 7 are summarized in Table 1 below. All thermal
ellipsoid plots are drawn at 50% probability level, with hydro-
gen atoms, counterions, and solvent molecules omitted.
An IrIAuI complex, with isocyanide ligands on the
iridium, was also prepared. Though the methyl iso-
cyanide complex [IrIAuI(dppm)2(CNMe2)](PF6)2 was
reported by Shaw and co-workers,10 we opted to prepare
the tert-butyl isocyanide (CNtBu) complex owing to the
Results
Synthesis and Characterization. IrIAuI compounds
were synthesized by following the route depicted in

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3039
Figure 1. Thermal ellipsoid plots for 1 and 2. Ellipsoids are shown at 50% probability level with counterions, solvent molecules, and hydrogen atoms
˚
˚
omitted for clarity. Data was collected at 100 ( 2 K. The metal-metal distance is d(Ir Au) = 2.9411(3) A for 1 and d(Ir Au) = 2.9174(2) A for 2.
3 3 3
3 3 3
commercial availability of this isocyanide. The desired
[IrIAuI(dppm)2(CNtBu)2]2þ product, which can be iso-
lated as its green PF6 salt (3), is obtained in moderate
Scheme 2
-
yields in a stepwise fashion starting with [Ir(COD)Cl]2,
which reacts sequentially with stoichiometric amounts of
dppm, CNtBu, and Au(tht)Cl. The 31P{1H} NMR spec-
trum of 3 shows a complex splitting pattern, with two
multiplets centered at 30.0 and 10.4 ppm. As with 1 and 2,
14 of the 20 lines can be discerned. The splitting pattern is
distinct from that of 1 and 2, however, because the two
outermost lines of each 7-line multiplet are the most
intense. Complex 3 shows a characteristic CtN stretch
complexes. Resonances at 31.6 and -11.0 ppm in the 31P
NMR spectrum of 4 shift to 25.7 and -18.6 ppm in bromide-
substituted 5. Similarly, peaks at 7.7 and -17.6 ppm in the
spectrum of 6 shift upfield to 3.1 and -22.2 ppm for bromide
complex 7. This substantial perturbation of both 31P reso-
nances is consistent with halogen ligation of both metal
centers. Complexes 4-7 also possess distinct CtO or
CtN stretching frequencies, which appear at 2009 (4),
2007 (5), 2192 (6), and 2189 cm-1 (7). These IR stretches
are independent of the identity of the halogen ligands.
Structural characterization of 4-7 confirms the assign-
ment of these products as metal-metal bonded IrIIAuII
complexes. Thermal ellipsoid plots for 4-7 are depicted in
Figure 2. Clearly evident are the octahedral and square
planar coordination environments of the d7 IrII and the d9
AuII centers, respectively. Furthermore, a significant con-
traction in the Ir-Au bond distance is observed upon oxi-
dation. For the [IrIIAuII(dcpm)2(CO)X3](PF6) series, the
in the IR spectrum at 2138 cm-1
.
Compounds 1 and 2 were structurally characterized by
X-ray crystallography. Table 1 summarizes crystallo-
graphic data for 1, 2, and 4-7. The thermal ellipsoid
plots of the structures of 1 and 2, shown in Figure 1,
exhibit the expected square planar coordination environ-
ment for d8 IrI and linear geometry for d10 AuI. The
Ir Au distance changes minimally with halide substitu-
3 3 3
˚
tion, contracting by <0.03 A when the chloride (in 1) is
replaced with a bromide (in 2). The Ir Au distances in 1
3 3 3
˚
and 2 are slightly shorter than the distance of 2.986(1) A
reported for the dppm-bridged analogue [IrIAuI(dppm)2-
COCl](PF6).11 Complex 3 eluded structural characteriza-
tion, but the related complex [IrIAuI(dppm)2(CNMe2)]-
12
˚
(PF6)2 has a reported Ir Au distance of 2.944(1) A.
3 3 3
Whereas treatment of the known complex [IrIAuI-
(dppm)2COCl](PF6)9 with PhICl2 leads to a mixture of
two products that decompose in a matter of hours to an
intractable white solid, treatment of complexes 1-3 with
PhICl2 or Br2 leads to a clean two-electron oxidation that
quantitatively furnishes d7;d9 IrIIAuII complexes. The com-
plexes [IrIIAuII(dcpm)2(CO)X3](PF6) (X = Cl (4); Br (5))
and [IrIIAuII(dppm)2(CNtBu)2X2](PF6)2 (X=Cl (6); Br (7))
were prepared in good isolated yields by halogen oxidation of
the appropriate IrIAuI starting material. Scheme 2 sum-
marizes the syntheses of these complexes where X2 refers to
the halogen oxidant, either PhICl2 or Br2. Oxidation of the
IrIAuI complexes results in a sizable upfield shift of the
31P{1H} spectrum, which for the IrIIAuII complexes consists
of two deceptively simple AA0XX0 1:2:1 triplets where only 6
of the 20 lines of the AA0XX0 splitting pattern are resolved.
The AA0XX0 triplets undergo a significant upfield shift upon
substituting chloride for bromide in otherwise identical
˚
˚
Ir-Au bond lengths of 2.6954(3) A (4) and 2.7096(4) A
(5) are >0.2 A shorter than those of their precursors 1 and 2.
˚
The metal-metal bond lengths in isocyanide complexes 6
˚
˚
and 7at 2.6729(4) A and 2.6840(4) A, respectively, are slight-
ly shorter than their CO analogues 4 and 5, respectively.
Photophysical Properties. Figure 3 displays electronic
spectra for IrIAuI complexes 1 and 3; the spectrum of 2 is
nearly identical to 1and is shown in Supporting Information,
Figure S1. The absorption profiles are dominated by the
intense dσ* f pσ transition, as has been assigned for other
complexes of this type.11,12 The transition is fairly metal loca-
lized as indicated by a minimal shift in the band maxi-
mum from 431 nm (ε=17000 M-1 cm-1) for 1 to 437 nm
(ε= 18000 M-1 cm-1) for 2. Complex 3, [IrIAuI(dppm)2-
(CNtBu)2](PF6)2, shows an intense low-energy maximum
at 490 nm (ε = 26000 M-1 cm-1). Time-dependent DFT

3040 Inorganic Chemistry, Vol. 49, No. 6, 2010
Teets et al.
Figure 2. Thermalellipsoid plots for 4-7. Ellipsoidsare shownat50%probabilitylevelwith counterions, solvent molecules, methylgroups, andhydrogen
˚
˚
atoms omitted for clarity. Data was collected at 100 ( 2 K. The metal-metal distance is d(Ir-Au) = 2.6954(3) A for 4, d(Ir-Au) = 2.7096(4) A for 5,
˚
˚
d(Ir-Au) = 2.6729(4) A for 6, and d(Ir-Au) = 2.6840(4) A for 7.
Table 2. Summary of 77 K Emission Data for Complexes 1-3a
solutionb
solid state
cmpd
λem/nm
τ/μs
λem/nm
τ1/μs
τ2/μs
1
2
3
575
582
640
9.8
9.8
6.8
605
620
670
0.81 (0.49)
0.55 (0.55)
0.67 (0.42)
6.7 (0.51)
4.0 (0.45)
3.6 (0.68)
a Parenthetical numbers represent the relative contributions of the
two biexponential fitting parameters. b 4ꢀ10-5 M in 1:1 CH2Cl2/toluene.
minimal halide contributions to the computed frontier orbitals
of 10 and 20 are observed. Our results are consistent with re-
cent computational findings on a series of IrIAuI complexes.15
The dσ*pσ excited states of these complexes are emis-
sive at 77 K, both in rigid solvent glass (1:1 CH2Cl2/
toluene) and in the solid state (Figure 3 and Supporting
Information, Figure S1). Emission data for all complexes
are summarized in Table 2. The 77 K emission spectra in
1:1 CH2Cl2/toluene display maxima of 575 nm (1), 582
nm (2), and 640 nm (3), corresponding to Stokes shifts of
5800 cm-1 (1), 5700 cm-1 (2), and 4800 cm-1 (3), respec-
tively. The emission is further red-shifted by about 1000
cm-1 in the solid state. Emission of samples in solvent
glass decay monoexponentially (τ = 9.8 μs (1), 9.8 μs (2),
and 6.8 μs (3)) whereas the emission decays of solid state
samples are better approximated by a biexponential fit.
The lifetimes and excited state characteristics of 1-3 are
in line with previous studies on triplet state emission of
IrIAuI heterobimetallic complexes.11-14
Figure 3. Overlaid absorption (red), glass emission (green), and solid-state
emission (blue) spectra. The spectra of 1 are shown on top, and 3 on the
bottom set of axes. The absorption spectrum was recorded at 293 K in
CH2Cl2 solution. Emission spectra (λexc = 431 nm (1); 490 nm (3)) were re-
corded at 77 K; the frozen glass was of a 1:1 CH2Cl2/toluene solvent mixture.
calculations on the truncated phosphine model complexes
[IrIAuI(dmpm)2(CO)X]þ (X = Cl (10); X = Br (20)) and
[IrIAuI(dmpm)2(CNMe)2]2þ (30), where dmpm = bis(di-
methylphosphino)methane, confirm the dσ* f pσ nature
of the intense low-energy transitions in 1-3. Moreover,

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3041
Figure 4. Overlaidelectronic absorptionspectra, measuredinCH2Cl2 at
293 K. The spectra of 4 (red) and 5 (green) are shown on top, 6 (red) and 7
(green) on the bottom set of axes.
The spectral properties of the IrIAuI core change mark-
edly upon oxidation to IrIIAuII. Figure 4 displays overlaid
absorption spectra for complexes 4-7, measured at 293 K in
CH2Cl2 solution. The most intense absorption features
occur at significantly higher energies for the IrIIAuII com-
plexes. In all cases, these absorption maxima shift bath-
ochromically upon substitution of bromide for chloride in
otherwise isostructural complexes. The IrIIAuII complexes
are non-emissive at room temperature and 77 K, both in
solution and in the solid state.
Figure 5. Plots for the predominant orbitals involvedin the mostintense
electronic transitions of50 and 70, with contour levels of0.04 au. Geometry
optimizations were initiated from coordinates obtained from the crystal
structures of 5 and 7. Excitation energies (E = 3.69 eV for 50 and E = 3.53
for 70) and oscillator strengths (f336 nm = 0.38 for 50 and f351 nm = 0.53 for
70) were determined from time-dependent DFT calculations.
chloride, the HOMO and other nearby filled orbitals for
50 and 70 are destabilized by ∼0.3 eV (Supporting Infor-
mation, Figures S5-S6). These higher-lying occupied
orbitals involve M;X σ* or π* interactions, which are
responsible for the destabilization that occurs when
chloride is replaced with bromide. In contrast, the LU-
MOs and other nearby virtual orbitals are very minimally
perturbed.
The electronic structure of the IrIIAuII complexes was
examined by DFT calculations. Model complexes were
truncated by replacing all phenyl, cyclohexyl, and tert-
butyl groups with methyl groups, and counterions were
omitted ([IrIIAuII(dmpm)2(CO)X3]þ, X = Cl (40); Br (50)
II
II
2þ
0
and [Ir Au (dmpm)2(CNMe)2 X2] , X = Cl (60); Br
(70). Frequency calculations ensured that all optimized
geometries represent minima on the potential energy
surface. Metal-metal distances in the geometry-opti-
Time-dependent DFT calculations on the geometry
optimized models provide insight into the nature of
IrIIAuII excited states. Calculations show that the most
intense electronic transitions are primarily metal-metal
σ f σ* in character. Figure 5 shows the orbitals that are
involved in this transition for bromide complexes 50 and
70. The transitions involved in the analogous chloride
complexes are essentially identical and are indicated
pictorially in Supporting Information, Figure S2. The
most intense electronic transition in 40 and 50 is between
the HOMO-7, a distorted σ-bonding orbital of mixed d
and p parentage, and the dσ* LUMO. This one-electron
promotion comprises 74% (40) and 81% (50) of the excited
state of the highest oscillator strength. The calculated
transitions at 314 nm for 40 and 336 nm for 50 are reason-
ably close to the observed wavelengths of 331 and 369 nm
for 4 and 5. The transitions in 60 and 70 are also best
described as (d,p)σ f dσ* and are reminiscent of the
electronic transition calculated for an isostructural
RhIIAuII complex.31 The most intense excitation in 60 is
calculated to occur at 328 nm, whereas that for 70 at 351
nm. Again, these are reasonably close to the observed
˚
mized models are consistently 0.06-0.10 A longer than
the observed distances from X-ray crystallography, but in
general there is good agreement between calculated and
observed structures. The orbital manifolds of 40 and 50
each contain two distorted σ-bonding orbitals responsible
for the net metal-metal bond. The highest occupied
molecular orbital (HOMO) is primarily derived from d
orbitals with nearly equal contribution from the two
metal atoms. The HOMO-7 also shows σ-orbital density
between the metal atoms; this orbital is polarized toward
the iridium and is of mixed d and p parentage, with
considerable Au s contribution as well. In 60 and 70, a
single cylindrically symmetric σ-bonding orbital of mixed
d and p parentage at the HOMO-2 level is responsible for
the metal-metal bond. The HOMO in these complexes is
a primarily iridium-centered dπ* orbital. The lowest
unoccupied molecular orbital (LUMO) in 40-70 is the
dσ* orbital, which is also σ-antibonding with respect to
the axial halide ligands. Consistent with the sizable bath-
ochromic shift that was observed in the absorption
spectrum of the IrIIAuII complexes upon substitution of
(31) Esswein, A. J.; Dempsey, J. L.; Nocera, D. G. Inorg. Chem. 2007, 46,
2362–2364.

3042 Inorganic Chemistry, Vol. 49, No. 6, 2010
Teets et al.
Figure 7. Spectral evolution during the photolysis of 5 with monochro-
mic 370 nm light in CH2Cl2 with 2.2 M DMBD at 283 K. The open circles
depict the values for the product appearance quantum yields, Φp,
measured in 2.2 M DMBD as a function of excitation wavelength. The
inset graph shows the dependence of the quantum yield on DMBD
concentration with excitation provided at 370 nm.
Figure 6. Spectral evolution during the photolysis of 4 with monochro-
mic 320 nm light in CH2Cl2 with 2.2 M DMBD at 283 K. The open circles
depict the values for the product appearance quantum yields, Φp,
measured in 2.2 M DMBD as a function of excitation wavelength.
transitions of 320 and 344 nm for 6 and 7. The calcula-
tions also demonstrate that there is significant LMCT
character in the low-energy transitions of 40-70. In all
cases, the Ir-Au σ-bonding orbital possesses substantial
halide character, whereas the dσ* LUMO has greater
metal character as a percentage of the total orbital
composition. As a representative example, the HOMO-7
(d,p)σ orbital for 50 has a 66.8% contribution from
bromine atomic orbitals, but the dσ* LUMO is only
18.1% derived from bromine atomic orbitals.
quantitative formation of IrIAuI complex 2. Clean photo-
chemistry is reproduced at other excitation wavelengths
that fall within the absorption manifold of 5 (Figure 4).
The product appearance quantum yield of 5 decreases
substantially as the excitation wavelength is increased,
with values of 9.4(6)% at 320 nm, 3.9(3)% at 370 nm,
3.2(1)% at 410 nm, and 0.7(1)% at 465 nm, when the
DMBD concentration is held at 2.2 M. In contrast to 4,
however, the photochemical quantum yield of 5 is sensi-
tive to the DMBD concentration. As shown in the inset of
Figure 7, the photochemical quantum yield increases
linearly over the range of 0.055 to 4.4 M DMBD.
Photochemistry. Irradiation of the IrIIAuII complexes
4-7 in the presence of the halogen trap, 2,3-dimethyl-1,3-
butadiene (DMBD), and with the exclusion of O2, results
in the prompt conversion of the photoreactants to the
corresponding IrIAuI complexes.
Photochemistry of [IrIIAuII(dppm)2(CNtBu)2X2](PF6)2,
6 and 7, results in the formation of 3, as judged by the
changes to the UV-vis absorption spectra (see Supporting
Information, Figures S3-S4). Photoproduct 3 is unstable
to photolytic reaction conditions; the absorption bands of
3 bleach over extended irradiation times, and a colorless
UV-absorbing species forms. Consistent with these obser-
vations, isosbestic points are not maintained over the
course of photolysis of either 6 or 7.
Figure 6 depicts the evolution of the absorption spec-
trum when a CH2Cl2 solution of [IrIIAuII(dcpm)2COCl3]-
(PF6), 4, at 45 μM, is irradiated with monochromic 320
nm light in the presence of 2.2 M DMBD. Pristine
isosbestic points are maintained at 408, 462, and 506
nm, indicating the absence of photochemical intermedi-
ates on a steady-state time scale. The final spectrum
indicates quantitative production of the IrIAuI complex
1. Clean formation of 1 is also observed when 4 is
irradiated at other wavelengths within the photoreac-
tant’s absorption manifold. The dependence of the pro-
duct-formation quantum yield, Φp, on the wavelength of
excitation, with 2.2 M DMBD present, is also presented in
Figure 6. The observed photochemical quantum yield
decreases dramatically as the excitation energy is lowered.
At 320 nm excitation, the observed quantum yield is
6.0(2)%, which decreases to 2.3(3)% at 370 nm and
0.30(3)% at 410 nm. At 465 nm excitation, negligible
photoconversion is observed. The dependence of the
quantum yield on DMBD concentration was investigated
for λexc = 320 and 370 nm. In both cases, the quantum
yields were found to be independent of trap concentra-
tion, over the range of 0.055 to 2.2 M for λexc = 320 nm32
and 0.055 to 4.4 M for λexc = 370 nm.
Discussion
The stable IrIIAuII heterobimetallic complexes provide
heretofore unknown examples of a direct iridium-gold bond
in a bimetallic complex. The compounds are obtained with
facility by halogen oxidation of parent IrIAuI complexes,
which are furnished by following methods similar to that
employed by Shaw and co-workers in their preparation of
[IrIAuI(dppm)2COCl]Cl by treating [IrI(dppm)2CO]Cl with
AuI(PPh3)Cl in boiling acetone.9 We elected to use the AuI
starting material AuI(tht)X, which is more labile and thus
allows for milder reaction conditions. Considerable π back-
donation from the IrI center is evident from the low energy of
the CtO stretching vibrations (ν~CO = 1944 and 1945 cm-1
for 1 and 2 vs, 2143 cm-1 for 12C16O).33 The CNMe analogue
to 3 had been reported by ligand substitution on a pre-assem-
bled IrIAuI complex.12 The IrIAuI complex [IrIAuI(dppm)2-
(CNtBu)2](PF6)2, 3, was prepared as previously described for
isostructural RhIAuI complexes,31 but initial attempts to
prepare 3 by treatment of [IrIAuI(dppm)2COCl](PF6) with
2 equiv of CNtBu resulted in a mixture of starting material,
bis-CNtBu and tris-CNtBu complexes. The CtN IR stretch
The photochemistry of 5 largely parallels that observed
for 4. Figure 7 shows the changes in the absorption
spectrum during the photolysis (λexc = 370 nm) of a
CH2Cl2 solution containing [IrIIAuII(dcpm)2COBr3]-
(PF6), 5, at 60 μM and 2.2 M DMBD. Well-anchored
isosbestic points are maintained but in this case at 421 and
461 nm, and the final photolysis spectrum reveals the
(33) Harris, D. C.; Bertolucci, M. D. Symmetry and Spectroscopy: An
Introduction to Vibrational and Electronic Spectroscopy; Dover Publications:
New York, 1978.
(32) UV absorption by DMBD precluded accurate quantum yield deter-
mination beyond 2.2 M when excited at 320 nm.

Article
Inorganic Chemistry, Vol. 49, No. 6, 2010 3043
for 3 is observed at 2138 cm-1, which is smaller than the value
of 2306 cm-1 for free CNtBu34 and again indicates significant
π back-donation.
establish that thesame photochemistry is retained for IrIIAuII
cores. Complexes 4 and 5 cleanly reduce to 1 and 2, respec-
tively, when illuminated in the presence of a halogen trap. In
both cases, the quantum yields for this photoreaction de-
crease substantially as the excitation wavelength is increased,
as has been observed previously for d7;d9 bimetallic cores
ligated by halide.38 However, the dependence of quantum
yield on the trap concentration for chloride-bound 4 and
bromide-bound 5 is different. The photochemical quantum
yield of 4 is invariant with respect to trap concentration over
the range of 55 mM to 4.4 M DMBD, whereas the observed
quantum yield for 5 increases linearly over the same range of
concentrations. This divergent behavior suggests possible
mechanistic differences in the photochemical transforma-
tions of the two complexes, which is currently under inves-
tigation with time-resolved spectroscopy.
Metal-metal bonding in the IrIIAuII species is supported
by all physical and spectroscopic characterization methods
employed in this study. The CtO and CtN IR stretches shift
by ∼50-60 cm-1 to higher energy, consistent with dimin-
ished π-back-donation resulting from oxidation of the bime-
tallic core. The crystal structures of these complexes show a
contraction of the Ir-Au bond length that is indicative of
direct metal-metal σ-bonding. The metal-metal bond
length of 4-7 compare favorably with other d7;d9 third-
row bimetallic complexes; they are slightly longer than
structurally characterized PtIIIAuII 5 and PtIIIPtI 6 complexes,
and slightly shorter than isoelectronic IrIIIr0 complexes.8 The
electronic structure of the IrIIAuII core is also dominated by
the metal-metal bond interaction. The most intense absorp-
tions in 4-7 arise from metal-metal σ f σ* transitions; the
transition is Laporteallowed and consistentwiththe presence
of an intense UV absorption. Moreover, the metal-metal
bond shows characteristic mixing with the terminal halide
ligands, especially from the halide axially coordinated along
the metal-metal bond. The pronounced bathochromic shift
in the absorption features upon substitution of chloride for
bromide is indicative of substantial LMCT character in the
excited state. The IrIAuI complexes show intense HOMO f
LUMO absorptions that are primarily metal-centered. But in
the reduced bimetallic core, the dσ* orbital is filled and the
metal based transition is of dσ* f pσ character. As is typical
of this type of excited state, 35-37 a pronounced emission is
observed for both solution and solid-state samples of the
IrIAuI complexes. With axial ligands removed from the
metal-metal axis together with a reduced metal core, halide
mixing into the metal based transitions of the IrIAuI com-
plexes is greatly diminished. The absorption maximum of
chloride-substituted 1 shifts by only ∼300 cm-1 in bromide-
substituted 2, consistent with a primarily metal-based transi-
tion devoid of significant halide character, an observation
which is corroborated by time-dependent DFT calculations.
The presence of LMCT character in the lowest energy
excited state of the IrIIAuII complexes gives rise to a sig-
nificant photochemistry. We have previously noted5,6,38 that
dσ* excited states with heavily admixed LMCT character can
promote the photoreductive elimination of halogen with the
concomitant two-electron reduction of the metal core. The
UV-vis spectroscopic features of the IrIIAuII complexes
In summary, we show that d7;d9 metal-metal bonded
IrIIAuII complexes are prepared by halogen oxidation of
d8 d10 IrIAuI complexes. In contrast to thermally unstable
3 3 3
tfepma-bridged RhIIAuII complexes,31 which are prepared by
halogen transfer from tetrachloroaurate, the IrIIAuII com-
plexes described here are robust and their synthesis is general
and straightforward. The IrIIAuII complexes add to a grow-
ing library of d7;d9 species that photoeliminate X2 by
reductive elimination. Previous to the d7;d9 class of com-
pounds, X2 reductive elimination had not been observed
from coordination or organometallic compounds. As de-
monstrated here for the IrIIAuII complexes, the electronic
structure of d7;d9 compounds, together with the low re-
organization energy of the primary coordination sphere,39,40
predisposes late transition metal heterobimetallic d7;d9
complexes to X2 photoelimination, which is a key finding
in overcoming the major impediment to achieving HX
splitting photocatalysis.
Acknowledgment. This work was supported by NSF
Grant CHE-0750239. Grants from the NSF also sup-
ported the MIT Department of Chemistry Instrumenta-
tion Facility (CHE-9808061, DBI-9729592). T.S.T. is
grateful to the Fannie and John Hertz Foundation for a
predoctoral fellowship. D.A.L. thankfully acknowledges
the Jane Coffin Childs Memorial Fund for Medicinal
Research for a postdoctoral fellowship.
Supporting Information Available: Electronic absorption and
emission spectra of 2, electronic transitions for 40 and 60 from
time-dependent DFT, UV-vis traces for the photolysis of 6 and
7, partial orbital energy level diagrams for 40-70, Cartesian
coordinates for the optimized geometries of 10-70, the full
citation for reference 23, and the crystallographic information
file (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
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