Studies of the structures and bonding of gold-bridged dirhenium carbonyl cluster complexes

Article abstract of DOI:10.1021/om4010127

The compounds Re₂(CO)₈(μ-AuPPh₃) ₂, 1, a dimer of Re(CO)₄(μ-AuPPh₃), and ax,ax-Re₂(CO)₈(PPh₃)₂ were obtained from UV-vis radiation-induced decarbonylation of the compound Re(CO) ₅[Au(PPh₃)]. Compound 1 contains two rhenium atoms bridged by two AuPPh₃ groups. The complex has 32 valence electrons and is formally unsaturated by the amount of two electrons. The Re-Re bond distance in 1 is unusually short (Re-Re = 2.9070(3) A), as found by a single-crystal structural analysis. The nature of the metal-metal bonding in 1 was investigated by DFT computational analyses, which have provided evidence not only for σ-bonding but also significant complementary π-bonding directly between the two rhenium atoms. The electronic structure of Re₂(CO) ₈(μ-H)₂, 2, was similarly analyzed and is compared with that of 1. Compound 1 is intensely colored due to low-energy, metal-based electronic transitions between the HOMO and HOMO-2 and the LUMO. Compound 1 reacts with I₂ to yield Re₂(CO)₈(μ- AuPPh₃)(μ-I), 3, and the known compound Re₂(CO) ₈(μ-I)₂, 4, by substitution of the bridging AuPPh ₃ groups with bridging iodide ligands. Compound 3 is electronically saturated, 34 valence electrons, and contains a formal Re-Re single bond: Re-Re = 3.2067(5) A. Compound 3 was also in a high yield (83%) from the reaction of Re₂(CO)₈(μ-H)(μ-CH=CHC₄H₉) with Au(PPh₃)I. The Re-Re bonding in compounds 3, 4, and Re ₂(CO)₁₀ was also analyzed computationally, and this bonding was compared with their bonding in 1 and 2.

Full text of DOI:10.1021/om4010127

Article
pubs.acs.org/Organometallics
Studies of the Structures and Bonding of Gold-Bridged Dirhenium
Carbonyl Cluster Complexes
Richard D. Adams,* Yuen Onn Wong, and Qiang Zhang
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
S
* Supporting Information
ABSTRACT: The compounds Re2(CO)8(μ-AuPPh3)2, 1, a dimer of Re(CO)4-
(μ-AuPPh3), and ax,ax-Re2(CO)8(PPh3)2 were obtained from UV−vis radiation-
induced decarbonylation of the compound Re(CO)5[Au(PPh3)]. Compound 1
contains two rhenium atoms bridged by two AuPPh3 groups. The complex has 32
valence electrons and is formally unsaturated by the amount of two electrons. The
Re−Re bond distance in 1 is unusually short (Re−Re = 2.9070(3) Å), as found by a
single-crystal structural analysis. The nature of the metal−metal bonding in 1 was
investigated by DFT computational analyses, which have provided evidence not only
for σ-bonding but also significant complementary π-bonding directly between the
two rhenium atoms. The electronic structure of Re2(CO)8(μ-H)2, 2, was similarly
analyzed and is compared with that of 1. Compound 1 is intensely colored due to
low-energy, metal-based electronic transitions between the HOMO and HOMO-2
and the LUMO. Compound 1 reacts with I2 to yield Re2(CO)8(μ-AuPPh3)(μ-I), 3,
and the known compound Re2(CO)8(μ-I)2, 4, by substitution of the bridging AuPPh3 groups with bridging iodide ligands.
Compound 3 is electronically saturated, 34 valence electrons, and contains a formal Re−Re single bond: Re−Re = 3.2067(5) Å.
Compound 3 was also in a high yield (83%) from the reaction of Re2(CO)8(μ-H)(μ-CHCHC4H9) with Au(PPh3)I. The Re−
Re bonding in compounds 3, 4, and Re2(CO)10 was also analyzed computationally, and this bonding was compared with their
bonding in 1 and 2.
Au(PPh3) groups. The two rhenium atoms contain a total of 32
valence electrons, leaving the metal atoms formally unsaturated
by the amount of two electrons. In accord with this, the Re−Re
distance in 1 is unusually short. The metal−metal bonding in 1
was investigated by DFT computational analyses, which have
provided evidence for strong bonding directly between the two
rhenium atoms. For comparisons, the electronic structure of
Re2(CO)8(μ-H)2, 2, was also investigated. Reactions of 1 with
I2 were investigated and were found to provide the new elec-
tronically saturated compound Re2(CO)8(μ-AuPPh3)(μ-I), 3,
and the previously reported complex Re2(CO)8(μ-I)2, 4.14 Com-
pound 3 was prepared independently in a high yield from the re-
action of Re2(CO)8(μ-H)(μ-CHCHC4H9) with Au(PPh3)I. The
Re−Re bonding in 3 and 4 was also investigated by computational
methods. Details of these studies are provided in this report.
INTRODUCTION
■
The similarities between the hydrogen atom and the Au(PPh3)
group are well-known.1 The two species are effectively isolobal and
both contain only one valence electron. H and the Au(PPh3) group
are well-known to bridge metal−metal bonds effectively in poly-
nuclear metal complexes.2,3 There are a number of hydride-bridged
metal carbonyl cluster complexes, such as Re2(CO)8(μ-H)2, A,4
Os3(CO)10(μ-H)2,5 B,5 Re4(CO)12(μ-H)4,6 C, and higher nuclearity
cluster complexes, such as Pt2Re3(CO)9(P-t-Bu3)3(μ-H)6,7 and
[Ru3(CO)8(μ3-CMe)(μ-H)2(μ3-H)]2,8 that have attracted interest
because they are formally electronically unsaturated. Unsaturated
metal cluster complexes are of interest because they exhibit higher
reactivity than their electronically saturated counterparts.7−10
EXPERIMENTAL SECTION
■
General Data. Reagent grade solvents were dried by the standard
procedures and were freshly distilled prior to use. Infrared spectra were
recorded on a Thermo Nicolet Avatar 360 FT-IR spectrophotometer.
1
Room-temperature H NMR and 31P{1H} NMR were recorded on
A few unsaturated triosmium carbonyl cluster complexes con-
taining the bridging Au(PR3) group(s) have also been prepared
(e.g., Os3(CO)10(μ-AuPEt3)2,11 Os3(CO)10(μ-AuPPh3)(μ-H),12
and Os3(CO)10(μ-AuPPh3)(μ-Ph)13) that are related to Os3(CO)10-
(μ-H)2.
a Bruker Avance/DRX 400 NMR spectrometer operating at 400.3
and 162.0 MHz, respectively. Positive/negative ion mass spectra were
recorded on a Micromass Q-TOF instrument by using electrospray
(ES) ionization or electron impact (EI) ionization. UV−vis spectra
We have now prepared and characterized the new dirhenium
complex Re2(CO)8(μ-AuPPh3)2, 1, that contains two bridging
Received: October 15, 2013
© XXXX American Chemical Society
A
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were recorded on a (Agilent/Varian Cary 50) UV−vis spectrometer in
methylene chloride solvent at a concentration of 0.46 × 10−4 M.
Re(CO)5(AuPPh3),15 Re2(CO)8(μ-H)(μ-CHCHC4H9),16 and
Au(PPh3)I17 were prepared by the previously reported procedures.
Product separations were performed by TLC in air on Analtech
0.25 mm silica gel 60 Å F254 glass plates.
Synthesis of Re2(CO)8(μ-AuPPh3)2, 1. A 45.0 mg (0.0572 mmol)
portion of Re(CO)5Au(PPh3) was dissolved in 25 mL of benzene and
irradiated for 30 min, and the solution turned to brown. The solvent
was removed in vacuo. The residue was separated by TLC by using a
4:1 hexane/methylene chloride (v/v) solvent mixture to yield in order
of elution: (1) a colorless band of Re2(CO)10, 0.7 mg (4% yield), (2) a
colorless band identified as Re2(CO)8(PPh3)2,16 6.7 mg (21% yield),
and (3) a red band of Re2(CO)8(μ-AuPPh3)2, 1, 7.6 mg (18% yield).
Spectral data for 1: IR νCO (cm−1 in methylene chloride): 2065(w),
1
2028(s), 1976(vs), 1942(m), 1917(s). H NMR (CD2Cl2, in ppm):
δ = 7.20−7.36 (m, 30H, Ph). 31P NMR (CD2Cl2, in ppm): δ = 76.17
(s). Mass Spec. EI/MS m/z: 1514, M+, 1486, M+ − CO. The isotope
distribution pattern is consistent with the presence of two gold atoms
and two rhenium atoms. The UV−vis absorption spectrum in CH2Cl2
solvent: λ max = 327 nm, ε = 6692 cm−1 M−1, λ max = 420 nm, ε = 2820
cm−1 M−1, and λ
= 493 nm, ε = 5836 cm−1 M−1.
max
Synthesis of Re2(CO)8(μ-AuPPh3)(μ-I), 3. A 3.2 mg (0.0126
mmol) portion of I2 was added to 19.0 mg (0.0125 mmol) of 1 in
10 mL of benzene. The solution was stirred at room temperature for
15 min. During this time, the solution turned from orange to yellow.
The solvent was removed in vacuo, and the products were separated
by TLC by using a 4:1 (v/v) hexane/methylene chloride solvent mix-
ture to yield in order of elution: (1) a yellow band of 1.9 mg of
Re2(CO)8(μ-AuPPh3)(μ-I), 3, (13% yield), and (2) a colorless band of
Au(PPh3)I, 4.9 mg (33% yield). Spectral data for 3: IR νCO (cm−1 in
methylene chloride): 2094(w), 2064(m), 2005(s), 1972(m), 1935(m).
31P NMR (CD2Cl2, in ppm): δ = 76.79 (s). Mass Spec. EI/MS m/z:
1182, M+, 1154, M+ − CO. The isotope distribution pattern is con-
sistent with the presence of one gold atom and two rhenium atoms.
Alternative Synthesis of 3. A 45.0 mg (0.0661 mmol) portion of
Re2(CO)8(μ-H)(μ-CHCHC4H9) was added to 38.8 mg (0.0661
mmol) of Au(PPh3)I in 20 mL of hexane. The solution was refluxed
for 30 min. After cooling, the solvent was removed in vacuo and the
product was isolated by TLC by using hexane solvent to give a yellow
band of 3, 65.0 mg (83% yield).
Figure 1. ORTEP diagram of the molecular structure of Re2(CO)8(μ-
AuPPh3)2, 1, showing 40% thermal ellipsoid probability. The hydrogen
atoms are omitted for clarity. Selected interatomic bond distances (Å)
and angles (deg) are as follows: Re1−Au1 = 2.7914(2), Re1−Au1* =
2.7977(2), Re1−Re1* = 2.9070(3), Au1−P1 = 2.3308(11); Au1−
Re1−Au1* = 117.320(6), Re1−Au1−Re1* = 62.681(6), P1−Au1−
Re1 = 145.57(3), P1−Au1−Re1 = 147.68(3).
Synthesis of Re2(CO)8(μ-I)2, 4, from the reaction of 1 with I2.
A 6.6 mg (0.026 mmol) portion of I2 was added to a solution of
20.0 mg (0.0125 mmol) of 1 in 10 mL of benzene. The solution was
stirred at room temperature for 15 min. During this time, the solution
turned from orange to colorless. The solvent was removed in vacuo, and
the residual was separated by TLC by using a 4:1 (v/v) hexane/
methylene chloride solvent mixture to give two products in order of
polarization effects were also applied using SAINT+. An empirical
absorption correction based on the multiple measurement of equivalent
reflections was applied using the program SADABS. All structures were
solved by a combination of direct methods and difference Fourier syn-
theses, and were refined by full-matrix least-squares on F2 by using the
SHELXTL software package.19 Crystal data, data collection parameters, and
results of the analyses are available in the Supporting Information.
Computational Details. Density functional theory (DFT) cal-
culations were performed with the Amsterdam Density Functional (ADF)
suite of programs20 by using the PBEsol functional21 with the valence
quadruple-ζ + 4 polarization function, relativistically optimized (QZ4P)
basis sets for rhenium and gold, the valence triple-ζ + 2 polarization
function (TZ2P) basis set for iodide, and double-ζ function (DZ) basis sets
for the phosphorus, carbon, oxygen, and hydrogen atoms with nonfrozen
cores. The molecular orbitals for 1−4 and Re2(CO)10 and their energies
were determined by geometry-optimized calculations, with scalar relativistic
corrections, that were initiated by using the atom positional parameters as
determined from the crystal structure analyses. Electron densities at the
bond critical points and Mayer bond orders were calculated by using the
Bader quantum theory of atoms in molecules (QTAIM) model.22,23
Natural bond orbital (NBO) analyses were performed using the GENNBO
6.0 package embedded in ADF 2013.24 Time-dependent DFT (TDDFT)
calculations were performed for models in the gas phase by using the
PBEsol functional with the same basis sets.
elution: (1) a colorless band containing 3.6 mg of Re2(CO)8(μ-I)2,14
4
(32% yield), and (2) a colorless band of Au(PPh3)I, 4.0 mg (26% yield).
Spectral data for 4: IR νCO (cm−1 in methylene chloride): 2107(m),
2030(s), 1997(m), 1960(m). Mass Spec. EI/MS m/z: 850, M+.
Reaction of 3 with I2. A 3.2 mg (0.0126 mmol) portion of I2 was
added to 30.0 mg (0.0125 mmol) of 3 that was dissolved in 10 mL of
benzene. The solution was then stirred for 15 min at room tem-
perature. During this time, the color of the solution turned from
yellow to colorless. The solvent was removed in vacuo. The products
were isolated by TLC by using a 4:1 hexane/methylene chloride (v/v)
solvent mixture to give two products in order of elution: (1) a
colorless band that contained 5.0 mg of 4 (23% yield) and (2) a
colorless band of Au(PPh3)I, 9.8 mg (66% yield).
Crystallographic Analyses. Red crystals of 1 suitable for X-ray
diffraction analyses were obtained from a methylene chloride/hexane
solution by slow evaporation of a solvent at 25 °C. Yellow crystals of 3
suitable for X-ray diffraction analyses were obtained from a benzene/
octane solution by slow evaporation of a solvent at 15 °C. X-ray
intensity data were measured by using a Bruker SMART APEX CCD-
based diffractometer by using Mo Kα radiation (λ = 0.71073 Å). The
raw data frames were integrated with the SAINT+ program by using
a narrow-frame integration algorithm.18 Corrections for Lorentz and
B
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Figure 2. Selected molecular orbital diagrams of the LUMO+1, LUMO, HOMO, HOMO-1, HOMO-2, HOMO-5, HOMO-7, and HOMO-25
(isovalue = 0.04) with calculated energies showing the nature of the bonding in the Re2Au2 core of the structure of 1.
direct bonding between the two Re atoms,28 A, a Re−Re single
RESULTS AND DISCUSSION
■
bond, D, or a Re−Re double bond, E.4b
Photolytic decarbonylation of the compound Re(CO)5[Au-
(PPh3)] led to formation of the new complex dirhenium com-
plex Re2(CO)8(μ-AuPPh3)2, 1, in 18% yield. A coproduct in
this reaction was the dirhenium complex ax,ax-Re2(CO)8-
15
(PPh3)2 (21% yield). Both compounds were characterized
Jeiowska-Trzebiatowska et al. analyzed the electronic structure
crystallographically. See the Supporting Information for details
of 2 by using Fenske−Hall (FH) and extended Huckel (EH)
̈
on the structural analysis of Re2(CO)8(PPh3)2. An ORTEP
diagram of the molecular structure of 1 is shown in Figure 1. In
the solid state, the complex sits on a crystallographic center of
symmetry. The molecule contains two Re(CO)4 groups linked
by two bridging AuPPh3 groups. The two independent Re−Au
bond distances, Re1−Au1 = 2.7914(2) Å and Re1−Au1* =
2.7977(2) Å, are similar to those found in the compounds
Re2(CO)8[μ-Au(PPh3)](μ-C2Ph),25 2.744(1) and 2.844(2) Å,
and Re2(CO)8[μ-Au(PPh3)](μ-C4Fc)26 (Fc = ferrocenyl),
2.7369(3) and 2.8268(3) Å. The Re−Re distance in 1 is
2.9070(3) Å, which is significantly shorter than the Re−Re single
bond distance found in Re2(CO)10: Re−Re = 3.041(1) Å.27 The
dirhenium center in 1 contains only 32 valence electrons and is
thus formally unsaturated. Compound 1 is structurally similar to
Re2(CO)8(μ-H)2, 2, which has two bridging hydrido ligands in
the locations of the bridging Au(PPh3) groups of 1. Compound
2 is also formally unsaturated and has a similarly short Re−Re
distance, 2.876(1) Å.4 Compound 2 also contains only 32
valence electrons and has been variously described as having no
methods.28 King reported MPW1PW91 and BP86 geometry-
optimized DFT calculations for 2.29 To investigate the nature of
the Re−Re bonding in 1, we have performed geometry-optimized
PBEsol DFT molecular orbital calculations. For the purposes of
comparison, we have also performed similar DFT calculations
upon 2; see below.
Selected DFT MOs that show the bonding interactions between
the Re and Au atoms in 1 are shown in Figure 2. There is evidence
for direct Re−Re bonding in the HOMO, the HOMO-5, and the
HOMO-25. The HOMO-5 is a totally symmetric A1g orbital that is
dominated by a 4-center bonding interaction that lies principally in
the plane of the Re2Au2 core of the molecule. The HOMO-2 is
dominated by a delocalized 4-center interaction about the Re2Au2
core of the molecule with a node along the Re−Re vector. The
HOMO-7 provides evidence for a significant d−d π-bonding
interaction that lies perpendicular to the Re2Au2 plane of the
molecule. The low-lying HOMO-25 shows evidence of direct
σ-bonding interaction between the two Re atoms.
C
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Figure 3. Selected electron densities at bond critical points shown in
red calculated by the QTAIM method by using the DFT-optimized
structure of 1.
Figure 4. (a) UV−vis absorption spectrum of 1 in CH2Cl2 solvent. (b)
TDDFT calculated UV−vis absorption spectrum of 1.
Figure 3 shows the QTAIM analyzed bond paths about
the Re2Au2 core and lists the electron densities at selected
bond critical points (BCPs) that were obtained from the
DFT-optimized structure of 1. Interestingly, there is a significant
electron density of 0.051 e/bohr3 at the Re−Re BCP. It is
almost as large as the electron density at the Re−Au BCPs,
0.055 and 0.056 e/bohr3. This supports the idea of a significant
direct Re−Re bonding interaction in 1. Within the framework
of the isolobal analogy, Re2(CO)10 probably provides the best
example of a Re−Re σ-type single bond.30,31 For comparison,
we have similarly calculated the QTAIM electron density at
the Re−Re BCP of Re2(CO)10 by using our PBEsol DFT-
optimized analysis. The HOMO of Re2(CO)10 is consistent
with the presence of a strong Re−Re σ-bond; however, the mo-
lecular orbital calculations also show that there are two orbitals,
HOMO-3 and HOMO-4, that exhibit some supplementary Re−Re
π-type bonding interactions; see the Supporting Information.
The QTAIM electron density for Re2(CO)10 at the Re−Re
BCP obtained by this calculation was 0.038 e/bohr3. A
comparison with that of 1 would lead one to the conclusion
that the Re−Re bonding interaction in 1 is significantly
greater than that in Re2(CO)10. Indeed, the π-interaction
represented in the HOMO-7 of 1 supports the idea of a
partial multiple bonding character directly between the two
rhenium atoms. To pursue the Re−Re bonding analysis
further, we have also performed NBO analyses of 1. These
calculations provided Mayer indices, natural atomic orbitals
(NAOs), natural localized molecular orbitals (NLMOs),
and a natural population analysis (NPA). These results are
presented in Table 1 together with the corresponding results
for Re2(CO)10 and for compounds 2, 3 and 4; see below. The
Mayer, NAO, NLMO/NPA, and Wiberg indices for 1 are
0.777, 0.192, 0.624, and 0.224, respectively. The corresponding
Re−Re values for Re2(CO)10, 0.264, 0.0324, 0.284, and 0.0922,
are all considerably smaller than those of 1. If we assume that
the Re−Re bond order in Re2(CO)10 is formally 1.0, then it is
reasonably concluded that the Re−Re bond order in 1 is
greater than 1.0 by all of these measures.
Compound 1 is intensely red in color. Accordingly, a UV−vis
spectrum of 1 was obtained. This spectrum is shown in Figure 4a.
It exhibits three absorptions in the visible region of the
spectrum, λ max = 327 nm, ε = 6692 cm−1 M−1, λmax = 420 nm,
Table 1. NBO Analyses of the Re−Re Bond Orders Calculated for 1−4 and for Re2(CO)10
a
b
b
c
compound
Mayer bond order
NAO bond order
NLMO/NPA bond orders
Wiberg bond index
1
0.777
0.522
0.457
0.124
0.264
0.192
0.624
0.224
2
0.134
0.480
0.180
3
0.0888
0.319
0.118
4
−8.8 × 10−3
0.0324
1.32 × 10−2
0.284
6.4 × 10−3
0.0922
Re2(CO)10
a
b
c
See ref 34. NAO = natural atomic orbital, NLMO = natural localized molecular orbitals, NPA = natural population analysis. See ref 35.
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Figure 5. Molecular orbital diagrams of the HOMO-3, HOMO-5, HOMO-6, HOMO-23, and HOMO-28 (isovalue = 0.04) with calculated energies
showing the bonding in the Re2H2 core of the structure of 2.
ε = 2820 cm−1 M−1, and λmax = 493 nm, ε = 5836 cm−1 M−1,
that correlate to electronic transitions between the orbitals
in the Re2Au2 core of the molecule. A time-dependent DFT
(TDDFT) analysis of the electronic transitions has provided the
spectrum shown in Figure 4b. The major visible transition at
493 nm (calculated at 525 nm) is attributed to two closely
spaced allowed transitions: (1) from the HOMO (74%) and
the HOMO-2 (22%) to LUMO (f = 0.074) and (2) from the
HOMO-2 (75%) and the HOMO (22%) to LUMO (f = 0.032).
The observed shoulder at 420 nm, calculated to be at 476 nm, is
attributed to the HOMO-4 (98%, f = 0.0043) to LUMO. The
absorption at 327 nm, calculated to be at 372 nm, is attributed
to the HOMO-2 (94%, f = 0.155) to LUMO+1 transition; see
Figure 2 for the MOs.
Figure 6. Selected electron densities at bond critical points shown in
red and the Re2H2 ring point (in green) calculated by the QTAIM
method by using the DFT-optimized structure of 2.
To compare the influence of the Au(PPh3) group with H, we
have also performed a geometry-optimized DFT analysis of 2.
Selected MOs that focus on the bonding in the Re2H2 core of
the molecule are shown in Figure 5. The HOMO-3 and HOMO-7
show evidence for direct Re−Re interactions, although they
also include substantial overlaps with the two hydrogen atoms.
The HOMO-28 is a low-lying symmetric orbital with a signif-
icant component delocalized about the Re2H2 core, but this
orbital also contains significant contributions from the σ-bonding
from the four in-plane CO ligands that are largely responsible
for its low energy. The HOMO-5 shows that there is significant
out-of-plane Re−Re π-bonding as also found in the HOMO-7
in 1. The HOMO-6 and HOMO-23 show bonding that is pre-
dominantly between hydride ligands and the metal atoms.
Figure 6 shows the QTAIM analyzed bond paths about the
Re2H2 core and includes selected electron densities at the BCPs
obtained from the optimized structure of 2. Most interestingly,
the electron density along the Re−Re vector (calculated at the
ring point of the Re2H2 core) is even larger, 0.058 e/bohr3, than
that in 1. The Mayer, NAO, NLMO/NPA, and Wiberg indices
for the Re−Re interaction in 2 are similar to those for 1 (see
Table 1) and lead to the conclusion that the Re−Re bonding
in 2 is also partially multiple in character and very similar to
that in 1. Note: this result differs from the FH and EH analyses
performed some years ago, where it was concluded that there is
practically no direct M−M bond.28
compound 1 was allowed to react with I2, the new compound
Re2(CO)8(μ-AuPPh3)(μ-I), 3, was obtained in 13% yield.
Compound 3 was obtained independently in a much better
yield (83%) from the reaction of Re2(CO)8(μ-H)(μ-CH
CHC4H9) with Au(PPh3)I. Compound 3 was found to react
with I2 to yield Re2(CO)8(μ-I)2, 4, in 23% yield. Compound 4
can be obtained directly from 1 in 32% yield by using an
excess of I2.
Compound 3 was characterized by a single-crystal X-ray
diffraction analysis, and an ORTEP diagram of the molecular
structure is shown in Figure 7. The two rhenium atoms are held
together by one bridging AuPPh3 group and one bridging
iodide ligand. The Re−Re bond distance in 3 at 3.1890(12) Å
is approximately 0.3 Å longer than that in 1 and 2 and approx-
imately 0.15 Å longer than that in Re2(CO)10. Because the
iodide ligand serves formally as a three-electron donor, the two
rhenium atoms in 3 contain a total of 34 valence electrons, and
a formal Re−Re single bond is anticipated. This is consistent
with the increased length of the Re−Re distance relative to that
of 1 and 2. The Re−Au distances are similar to those found
in 1: Re2−Au1 = 2.7871(9) Å and Re1−Au1 = 2.7935(9) Å.
The Re−Re and Re−Au distances in 3 are similar to those
observed in the following compounds: Re2(CO)8(μ-AuPPh3)-
(μ-PPh2),32 Re−Re = 3.225(2) Å, Re−Au = 2.810(2) and
̀
To investigate the reactivity of 1, vis-a-vis, its electronic un-
saturation, the reaction of 1 with I2 was performed. When
E
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Figure 7. ORTEP diagram of the molecular structure of Re2(CO)8(μ-
AuPPh3)(μ-I), 3, showing 30% thermal ellipsoid probability. The hydrogen
atoms are omitted for clarity. Selected interatomic bond distances (Å)
and angles (deg) are as follows: Re1−Re2 = 3.1890(12), Re2−Au1 =
2.7871(9), Re1−Au1 = 2.7935(9), Re1−I1 = 2.7566(14), Re2−I1 =
2.7845(14), Au1−P1 = 2.322(4); I1−Re1−Au1 = 110.31(4), Au1−Re2−
I1 = 109.68(4), Re2−Au1−Re1 = 69.70(3), Re1−I1−Re2 = 70.27(3),
Re1−Au1−P1 = 146.92(11), Re2−Au1−P1 = 143.27(11).
Figure 8. Molecular orbital diagrams of the HOMO-1, HOMO-14,
HOMO-15, and HOMO-16 (isovalue = 0.04) with calculated energies
showing the bonding in the Re2AuI core of the molecule of 3.
2.764(2) Å, and Re2(CO)8(μ-AuPPh3)(μ-S-2-naphthyl),33 Re−
Re = 3.1356(7) Å, Re−Au = 22.7853(8) and 2.8110(7) Å.33
The Re−I distances are Re1−I1 = 2.7566(14) Å and
Re2−I1 = 2.7845(14) Å, but both are significantly shorter than
the distances found in the diiodide complex 4, 2.827(2), 2.826(2),
2.813(2), and 2.814 (2) Å, which contains no formal Re−Re bond.14
NBO analyses of 3 and 4 were also performed; see Table 1.
Assuming that the iodide ligand is a three-electron donor and
all bonds are of a 2 center−2 electron type, the Re−Re bond
order in 3 should be 1. The NBO bond orders for 3 are slightly
larger than those for Re2(CO)10 but smaller than those for 1
and 2. The QTAIM electron density at the Re−Re BCP in 3 at
0.0357 e/bohr3 is only slightly larger than that in Re2(CO)10.
Perhaps, the notion of the iodide ligand as a three-electron
donor having two 2 center−2 electron σ-bonds to the Re atoms
is too simple. Indeed, the HOMO-14, HOMO-15, and
HOMO-16 obtained for 3 in our DFT calculations show
both delocalized σ- and π-bonding interactions between the
iodide ligand and the two rhenium atoms; see Figure 8. The
effect of this bonding on the formal Re−Re bond order is
difficult to evaluate, but the 3-center σ-bonding of the Re2I
group shown in the HOMO-16 could lead to a slight increase
in the electron density at the Re−Re BCP as observed.
Formally, there is no need for the formation of a Re−Re
bond in compound 4. With two bridging iodide ligands serving
as three-electron donors, both Re atoms have 18 electron
configurations. The observed Re−Re distance in 4 is long at
4.218(2) Å14 and is consistent with the absence of a Re−Re
bond. Consistent with this, the Mayer, NAO, NLMO/NPA,
and Wiberg indices that were obtained after a geometry-
optimized PBEsol MO refinement of the structure of 4 were all
close to zero as expected.
SUMMARY AND CONCLUSIONS
■
The doubly-Au(PPh3) bridged unsaturated complex 1 has been
prepared by the photodecarbonylation of Re(CO)5[Au(PPh3)].
It is assumed that 1 was subsequently formed by the
dimerization of two unobserved 16-electron “Re(CO)4[Au-
(PPh3)]” groups. Concurrently, the dirhenium complex ax,ax-
Re2(CO)8(PPh3)2 was formed. This could have been formed by
the loss of Au from the incipient “Re(CO)4[Au(PPh3)]”,
followed by the dimerization of two Re(CO)4(PPh3) radicals.
Compound 1 is analogous to the unsaturated, doubly hydride
bridged complex 2. Both compounds possess short Re−Re bond
distances. Computational analyses suggest the existence of partial
multiple Re−Re bonds in both 1 and 2. It can be concluded that
Au(PPh3) and H have similar effects on the Re−Re bonding in
these two compounds. Compound 1 reacts with I2 to yield the
compounds 3 and 4. This reaction appears to follow the sequence
of two steps shown in eq 1. Indeed, we have confirmed
independently that 3 does reacts with I2 to yield 4.
The sequential replacement of the bridging one-electron
Au(PPh3) groups with three electron-donating bridging iodide
F
dx.doi.org/10.1021/om4010127 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(16) Nubel, P. O.; Brown, T. L. J. Am. Chem. Soc. 1984, 106, 644−
ligands reduces the direct Re−Re bonding present in 1 until it
is virtually eliminated in compound 4.
652.
(17) Westland, A. D. Can. J. Chem. 1969, 47, 4135−4140.
(18) SAINT+, Version 6.2a; Bruker Analytical X-ray System, Inc.:
Madison, WI, 2001.
(19) Sheldrick, G. M. SHELXTL, Version 6.1; Bruker Analytical X-
ray Systems, Inc.: Madison, WI, 1997.
(20) ADF2013; SCM Theoretical Chemistry,Vrije Universiteit:
Amsterdam, The Netherlands. http://www.scm.com.
(21) Perdew, J. P.; Ruzsinszky, A.; Csonka, G. I.; Vydrov, O. A.;
Scuseria, G. E. Phys. Rev. Lett. 2008, 100, 136406.
ASSOCIATED CONTENT
■
S
* Supporting Information
Details for the computational analyses of Re2(CO)10, 1, 2, 3, and 4
and for the structural analyses of 1, 3, and ax,ax-Re2(CO)8(PPh3)2,
including their CIF files, are available. This material is available free
of charge via the Internet at http://pubs.acs.org.
(22) (a) Bader, R. F. W. Atoms in Molecules: A Quantum Theory;
AUTHOR INFORMATION
Clarendon: Oxford, U.K., 1990. (b) Cortes
W. Coord. Chem. Rev. 2005, 249, 633−662.
(23) Keith, T. A. AIMAll, Version 12.11.09; TK Gristmill Software:
Overland Park, KS, 2012. aim.tkgristmill.com.
́ ́
-Guzman, F.; Bader, R. F.
■
Corresponding Author
*E-mail: Adamsrd@mailbox.sc.edu.
Notes
(24) (a) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.; Carpenter,
J. E.; Bohmann, J. A.; Morales, C. M.; Weinhold, F. NBO 5.0;
Theoretical Chemistry Institute, University of Wisconsin: Madison,
WI, 2001. (b) Weinhold, F.; Landis, C. R. Valency and Bonding: A
Natural Bond Order Donor-Acceptor Perspective; Cambridge University
Press: Cambridge, U.K., 2005; pp 32−36.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the following grants from the
National Science Foundation: CHE-1111496 and CHE-
1048629.
(25) Bruce, M. I.; Paul, J.; Low, P. J.; Skelton, B. W.; White, A. H. J.
Organomet. Chem. 1996, 515, 65−79.
(26) Chedia, R. V.; Dolgushin, F. M.; Smolyakov, A. F.; Lekashvili, O.
I.; Kakulia, T. V.; Janiashvili, L. K.; Sheloumov, A. M.; Ezernitskaya, M.
G.; Peregudova, S. M.; Petrovskii, P. V.; Koridze, A. A. Inorg. Chim.
Acta 2011, 378, 264−268.
REFERENCES
■
(1) (a) Raubenheimer, H. G.; Schmidbaur, H. Organometallics 2012,
31, 2507−2522. (b) Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1984,
23, 89−99. (c) Evans, D. G.; Mingos, D. M. P. J. Organomet. Chem.
1982, 232, 171−191.
(2) (a) Bau, R.; Drabnis, M. H. Inorg. Chim. Acta 1997, 259, 27−50.
(b) Teller, R. G.; Bau, R. Struct. Bonding 1981, 41, 1−82.
(c) Humphries, A. P.; Kaesz, H. D. Prog. Inorg. Chem. 1979, 25,
145−222.
(3) (a) Salter, I. D. In Comprehensive Organometallic Chemistry II;
Abel, E. W.; Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford,
U.K., 1995; Vol. 10, Chapter 5, pp 255−322. (b) Salter, I. D. In Metal
Clusters in Chemistry; Braunstein, P., Oro, L. A., Raithby, P. R., Eds.;
Wiley-VCH: Weinheim, Germany, 1999; Vol. 1, Chapter 1.27, pp
509−534. (c) Salter, I. D. Adv. Organomet. Chem. 1989, 29, 249−343.
(4) (a) Bennett, M. J.; Graham, W. A. G.; Hoyano, J. K.; Hutcheon,
W. L. J. Am. Chem. Soc. 1972, 94, 6232−6233. (b) Masciocchi, N.;
Sironi, A.; D’Alfonso, D. J. Am. Chem. Soc. 1990, 112, 9395−9397.
(5) (a) Broach, R. W.; Williams, J. M. Inorg. Chem. 1979, 18, 314−
319. (b) Churchill, M. R.; Hollander, F. J.; Hutchinson, J. P. Inorg.
Chem. 1977, 16, 2697−2700. (c) Allen, V. F.; Mason, R.; Hitchcock, P.
B. J. Organomet. Chem. 1977, 140, 297−307.
(27) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J. Inorg. Chem.
1981, 20, 1609−1611.
(28) Jeiowska-Trzebiatowska, B.; Nissen-Sobocihska, B. J. Organomet.
Chem. 1988, 342, 215−233.
(29) Li, N.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Phys. Chem. A
2010, 114, 11670−11680.
(30) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711−724.
(31) Xu, B.; Li, Q.-S.; Xie, Y.; King, R. B.; Schaefer, H. F., III J. Chem.
Soc., Dalton Trans. 2008, 2495−2502.
(32) Haupt, H.-J.; Heinekamp, C.; Florke, U. Inorg. Chem. 1990, 29,
2955−2963.
(33) Egold, H.; Schwarze, D.; Florke, U. J. Chem. Soc., Dalton Trans.
̈
1999, 3203−3207.
(34) Mayer, I. J. Comput. Chem. 2007, 28, 204−221.
(35) Wiberg, K. B. Tetrahedron 1968, 24, 1083−1096.
(6) Wilson, R. D.; Bau, R. J. Am. Chem. Soc. 1976, 98, 4687−4689.
(7) Adams, R. D; Captain, B.; Beddie, C.; Hall, M. B. J. Am. Chem.
Soc. 2007, 129, 986−1000.
(8) Adams, R. D; Kan, Y.; Zhang, Q.; Hall, M. B.; Trufan, E.
Organometallics 2012, 31, 50−53.
(9) (a) Deeming, A. J. Adv. Organomet. Chem. 1986, 26, 1−96.
(b) Keister, J. B.; Shapley, J. R. J. Am. Chem. Soc. 1976, 98, 1056−
1057. (c) Hudson, R. H. E.; Poe, A. J. Organometallics 1995, 14, 3238−
3248.
(10) Wang, S. R.; Wang, S.-L.; Cheng, C. P. J. Organomet. Chem.
1992, 431, 215−226.
(11) Burgess, K.; Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby,
P. R.; Syed,-Mustaffa, S. N. A. B. J. Chem. Soc., Chem. Commun. 1983,
455−457.
(12) Johnson, B. F. G.; Kaner, D. A.; Lewis, J.; Raithby, P. R. J.
Organomet. Chem. 1981, 215, C33−C37.
(13) Adams, R. D; Rassolov, V.; Zhang, Q. Organometallics 2013, 32,
6368−6378.
(14) Darst, K. P.; Lenhert, P. G.; Lukehart, C. M.; Warfield, L. T. J.
Organomet. Chem. 1980, 195, 317−324.
(15) Nicholson, B. K.; Bruce, M. I.; bin Shawkataly, O.; Tiekink, E. R.
T. J. Organomet. Chem. 1992, 440, 411−418.
G
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