Bimetallic N-Heterocyclic Carbene Rh(I) Complexes: Probing the Cooperative Effect for the Catalyzed Hydroelementation of Alkynes

Article abstract of DOI:10.1021/acs.organomet.5b00594

A series of bimetallic N-heterocyclic carbene Rh(I) complexes (13, 14, 16, 18, 19, and 21) were prepared, in which two metal fragments are linked to a central organic scaffold. These complexes were investigated as catalysts for the hydroelementation of al

Full text of DOI:10.1021/acs.organomet.5b00594

Article
pubs.acs.org/Organometallics
Bimetallic N‑Heterocyclic Carbene Rh(I) Complexes: Probing the
Cooperative Effect for the Catalyzed Hydroelementation of Alkynes
Vera Diachenko,† Michael J. Page,† Mark R. D. Gatus,† Mohan Bhadbhade,‡ and Barbara A. Messerle*,†,§
‡
†School of Chemistry and Mark Wainwright Analytical Centre, University of New South Wales, Sydney, Kensington, New South
Wales 2051, Australia
S
* Supporting Information
ABSTRACT: A series of bimetallic N-heterocyclic carbene
Rh(I) complexes (13, 14, 16, 18, 19, and 21) were prepared,
in which two metal fragments are linked to a central organic
scaffold. These complexes were investigated as catalysts for the
hydroelementation of alkynes, and their efficiency as catalysts
was compared to that of a series of monometallic analogues
(12, 15, 17, and 20). To determine the catalyst properties that
result in a cooperative, bimetallic enhancement of the reaction
rate, systematic variation of the intermetallic distance and the
ligand donor properties of the bimetallic complexes was
explored. Two related catalyzed reactions were investigated: the dihydroalkoxylation and hydrosilylation of alkynes. Nanoparticle
formation was observed during the hydrosilylation reaction using complexes 17−19.
(Figure 1a), which was found to be a poor catalyst for the ring-
opening polymerization metathesis of alkenes,10b and the Rh2
INTRODUCTION
■
The use of bimetallic complexes as catalysts to enhance the rate
and selectivity of catalyzed reactions is of significant interest. A
bimetallic catalyst is often substantially more efficient than a
monometallic catalyst of similar structure, with the increase in
catalyst performance attributed to “cooperative” interactions
between the two metals and the substrate of the reaction.1 How
the two metals of a bimetallic catalyst interact with each other
and/or the reaction substrate during the catalytic cycle is
difficult to determine from direct methods, and whether or not
a new bimetallic complex will promote a cooperative increase in
catalyst performance is hard to predict. One of the more
versatile approaches to bimetallic catalyst design is to link two
discrete metal complexes of proven catalytic efficiency by a
covalently bound tether.2 Such an approach has been used with
great effect for the synthesis of bimetallic Co2, Cu2, and Zn2
catalysts for the hydrolysis of phosphate esters;3−5 Cr2, Al2, and
Co2 catalysts for the asymmetric ring opening of epoxides;6 and
various Ni2, Ti2, and Zr2 polymerization catalysts.7,8
A number of bimetallic complexes have been reported that
contain a di-N-heterocyclic carbene ligand bridging the two
metal fragments.9,10 N-Heterocyclic carbenes (NHCs) form
exceptionally strong metal−ligand bonds of high thermody-
namic stability and low kinetic lability.11 They are therefore
ideally suited for studying bimetallic-catalyzed reaction
mechanisms by ensuring the structural integrity of the
bimetallic unit. NHC ligands have also proven to be
spectacularly successful in yielding metal complexes of high
catalytic efficiency for a variety of reactions.12 Despite these
advantages, few bimetallic complexes containing two NHC
ligands bridged by a phenylene scaffold have been investigated
as catalysts.10 Selected examples include the Ru2 complex
Figure 1. Bimetallic catalysts containing NHC ligands.
complex (Figure 1b), which was found to be an efficient
catalyst for the hydrosilylation of selected unsaturated func-
tional groups, including alkenes.10f,g The Pd2 complex (Figure
1c), however, was found to decompose to metallic nano-
particles during the catalyzed Heck reaction.10e Of particular
note was that the Heck reaction was not catalyzed by the bis-Pd
complex but was catalyzed by palladium nanoparticles which
were a result of the complex decomposing under the reaction
conditions. It has been shown that carbene ligands can assist in
the stabilization of nanoparticles,13 which may have led to the
Pd2 (Figure 1c) complex decomposing. To date, however, there
have been no reports of rhodium complexes with NHC ligands
Received: July 9, 2015
© XXXX American Chemical Society
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decomposing into nanoparticles under the reaction conditions
for catalysis.
We have previously reported the use of mono- and bimetallic
Rh complexes as catalysts for the intramolecular dihydroalkox-
ylation of alkynediols (Figure 2, previous work14,15). The
arylimidazoles 1−4 were first prepared by a copper-catalyzed
Ullmann coupling of the corresponding aryl bromide with
imidazole, following modified literature procedures.17 Alkyla-
tion of the imidazole nitrogen of 1−4 with methyl iodide in
MeCN afforded the imidazolium salts 5−8, which have been
reported previously.18 Alkylation of the imidazole nitrogen of
1−3 with 1-(2-bromoethyl)pyrazole in refluxing DMF,
followed by anion exchange of the bromide with NaBPh4,
afforded the imidazolium salts 9−11.
The monodentate NHC ligands derived from 5−8 are
denoted κ1-C-mono, κ1-C-ortho, κ1-C-meta, and κ1-C-para,
respectively, and the bidentate NHC-pyrazole chelates derived
from 9−11 are denoted κ2-C,N-mono, κ2-C,N-ortho, and κ2-
C,N-meta, respectively.
The synthesis of Rh(I) 1,5-cyclooctadiene (COD) complexes
was attempted by reaction of the imidazolium salts 5−11 with
NaOEt and [Rh(COD)(μ-Cl)]2 in methanol (Scheme 1). This
procedure worked well for the synthesis of the monometallic
complexes 12 and 15 and the bimetallic complexes 13, 14, and
16, where the two Rh fragments are disposed in a meta or para
position on the phenylene ring. The complexes 12−16 were
isolated in good yields (40−80%) as air-stable yellow powders.
For the neutral complexes 12−14 the halide ligand was
unequivocally identified to be iodide through HRMS and/or
elemental analysis of the compound, indicating complete
metathesis of the chloride ligand initially present in [Rh-
(COD)(μ-Cl)]2. In each of the 1H and 13C NMR spectra of the
bimetallic complexes 13, 14, and 16, two separate products of
similar composition were identified. This is characteristic of the
formation of two atropisomers resulting from restricted
rotation of the Ph−N bonds. The formation of such isomeric
mixtures has been reported previously by Hollis and co-workers
with similar bimetallic Rh(I) complexes.10a
Reaction of the ortho-substituted imidazolium salts 6 and 10
with NaOEt and [Rh(COD)(μ-Cl)]2 in methanol gave a
complex mixture of products. To improve this outcome, a
variety of base, solvent, and temperature regimes were
explored: however, with no benefit. The difficulty in
coordinating two metals to the ortho-disposed ligands 6 and
10 is perhaps unsurprising considering the steric hindrance an
ortho disposition would impose on such a bimetallic structure.
The reaction of analogous ortho-bis-carbene ligands with
[Rh(COD)(μ-Cl)]2 has previously been shown to yield
complexes where the two NHCs chelate to the one metal
Figure 2. Previous work: mono-/bimetallic complexes containing
N,N-bidentate donor ligands on a phenylene scaffold. This work:
mono-/bimetallic complexes containing a monodentate NHC ligand
or a bidentate NHC-pyrazole donor ligand on a phenylene scaffold.
efficiency of these catalysts was found to be strongly dependent
on the intermetallic Rh−Rh distance, the conformational
freedom of the bimetallic structure and the relative alignment
of the two metal centers.14a−c,15
The aim of this work was to synthesize a series of analogous
bimetallic Rh NHC complexes bridged by a phenylene scaffold
(Figure 2) and develop an understanding of which structural
parameters affect the efficiency of the bimetallic catalysts. We
investigated two types of metal binding pockets: a monodentate
NHC and a bidentate NHC-pyrazole group and, by changing
the relative positions of complex pairs on the phenylene ring,
varied the distance between the two metal centers. This
provided significant scope in the structure of the bimetallic
catalysts. To explore which reaction properties benefited most
from the particular nature of the bimetallic catalyst structure,
we investigated two reaction types for catalysis: the
dihydroalkoxylation and hydrosilylation of alkynes. As a basis
for comparison, the substrates under consideration, in particular
the alkynediol used for the dihydroalkoxylation reaction, have
been examined extensively as substrates for the two reaction
types.14d−f,16
RESULTS AND DISCUSSION
Synthesis. The imidazolium ligand precursors 5−11 were
synthesized in two steps, according to Scheme 1. The
■
Scheme 1. Synthesis of κ1 and κ2 Complexes
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center.19 An alternative process involving dimerization of the
NHCs has also been reported.20
The carbon monoxide containing complexes 17−21 were
synthesized by exposing a solution of the respective COD
containing complexes 12−16 to an atmosphere of CO (Scheme
1). Disappearance of coordinated COD resonances was
1
immediately evident in the H NMR spectra of the products
17−21, consistent with displacement of the COD ligand with
1
CO. No atropisomerism was observed in the H NMR spectra
of the bimetallic complexes 19 and 21; however, complex 18
contained highly broadened 1H resonances, suggesting
conformational exchange processes were occurring on a time
Figure 3. X-ray structure displaying the cationic fragment of 15.
Ellipsoids are shown at the 50% probability level.
1
frame similar to that of the signal acquisition. The H NMR
spectrum of 18 at 250 K showed two atropisomers present in a
2:3 ratio. These observations indicate that rotation of the Ph−
N bond is much more facile in complexes 18, 19, and 21 in
comparison to complexes 13, 14, and 16 containing the bulkier
COD coligand.
The IR spectra of complexes 17−21 contained strong
carbonyl stretching bands in the region 1960−2090 cm−1
(Table 1) characteristic of coordinated CO. The IR spectrum
Table 1. IR Stretching Frequencies for the CO Ligands of
Complexes 17−21
complex
ν(CO) stretching frequency/cm−1
17 (κ1-mono-CO)
18 (κ1-meta-CO)
19 (κ1-para-CO)
20 (κ2-mono-CO)
21 (κ2-meta-CO)
2006, 2071
1989, 2002, 2069, 2084
1968, 1997, 2016, 2070
2025, 2087
2027, 2090
Figure 4. X-ray structure displaying the cationic fragment of 16,
viewed parallel (a) and perpendicular (b) to the axis of the phenylene
ring. Ellipsoids are shown at the 50% probability level.
of the monometallic Rh(I) complex 17 contained two CO
stretching frequencies corresponding to symmetric and
asymmetric CO stretching modes typical of such dicarbonyl
complexes. However, in the IR spectra of the bimetallic
complexes 18 and 19, four CO stretching frequencies are
observed. This would indicate a strong coupling of the CO
vibrational modes from each fragment, a result that is not
typical of previously reported bimetallic carbonyl containing
complexes.14a−c,15 In contrast, the IR spectrum of the bimetallic
complex 21 contains only two CO stretching frequencies,
indicating the loss of vibrational coupling between Rh(CO)2
fragments. The CO stretching frequencies of the monometallic
complex 20 are nearly identical with those observed for the
analogous bimetallic complex 21. This would suggest that the
Rh−CO bonding in both complexes is very similar and that for
complex 21 the electronics of each Rh(CO)2 fragment are not
significantly perturbed by the neighboring Rh center.
X-ray Crystallography. X-ray-quality single crystals of the
monometallic complex 15 (κ2-C,N-mono-Rh(COD)) and the
bimetallic complex 16 (κ2-C,N-meta-Rh(COD)) were grown
by recrystallization of the complex from dichloromethane/
diethyl ether or from saturated acetone-d6, respectively. The
structures of the complex cations are shown in Figure 3 (for
complex 15) and Figure 4 (for complex 16) with selected bond
lengths and angles given in Table 2. The structure of the
bimetallic complex 16 contains the two Rh fragments disposed
on opposite sides of the phenylene plane. This sterically
favorable orientation results in a long Rh−Rh separation of
8.567(1) Å. The Rh−ligand bond lengths of each Rh fragment
are similar but not quite equivalent and are either within error
or are slightly larger than the corresponding bonds of the
Table 2. Selected Bond Lengths (Å) and Angles (deg) of 15
and 16
a
16
15
Rh−CNHC
Rh−N
Rh−CODtransN
Rh−CODcisN
N−Rh−CNHC
2.027(5)
2.090(4)
2.009
2.028(6)
2.100(5)
2.019
2.046(6)
2.113(5)
2.068
2.080
2.078
2.102
86.7(2)
86.9(2)
86.3(2)
a
Rh−COD distances are measured to the centroid of the CC bond.
monometallic complex 15. This similarity in coordination
geometry between monometallic and bimetallic structures
would suggest that the individual Rh fragments of 16 are also
chemically very similar to the monometallic analogue 15.
Catalysis. The monometallic and bimetallic complexes
containing CO coligands 17−21 were investigated as catalysts
for two different reactions. It has previously been demonstrated
that Rh(I) complexes containing NHC and/or N-donor ligands
in combination with CO or COD coligands can be highly
efficient catalysts for the addition of O−H and Si−H bonds
across unsaturated alkyne CC bonds.10f,14a This hydro-
elementation of alkynes provides an atom-economical route
toward the synthesis of new O−C and Si−C bonds. The two
reactions we chose to investigate were the intramolecular
dihydroalkoxylation of alkynediols to yield spiroketals and the
intermolecular hydrosilylation of alkynes to yield vinylsilanes.
The two reactions proceed through different mechanisms, and
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it was our aim to discover what reaction properties would
benefit from the use of a bimetallic catalyst structure.
Dihydroalkoxylation Reaction. The dihydroalkoxylation of
the alkynediol 22 to yield spiroketals 23a,b is a two-step
tandem process (Scheme 2). The Rh(I) complex catalyzes the
principle is governing the rate enhancement observed in this
reaction and is not confined/unique to one particular type of
catalyst design. The turnover frequency of complex 18 (52 h−1)
is approximately 5 times faster than that of 17 (11 h−1), which
compares favorably with the rate enhancements achieved by
catalysts of related structure (Figure 2, previous work).14
To investigate catalysts 17−19 further, we performed the
reaction in the presence of NaBArF (BArF = tetrakis[3,5-
Scheme 2. Dihydroalkoxylation Reaction Pathways of 22 To
Form either the 6,5-Spiroketal 23a or the 5,6-Spiroketal 23b
4
4
bis(trifluoromethyl)phenyl]borate (1 equiv per Rh, Figure 5b).
It was anticipated that the substitution of the iodide ligand for
the weakly coordinating BArF anion would produce a
4
significant improvement on the reaction rate. The catalytic
efficiency for complexes 17−19 improved dramatically with
TOF values 2.1−2.5 times greater than the catalyzed reactions
that did not contain NaBArF4 and significantly reduced reaction
times for the complexes (90% vs 50% conversion at 5 h) for
complexes 17 and 19. The turnover frequency of complex 18
(κ1-C-meta-(Rh(CO)2)2; 135 h−1) is still approximately 4 times
faster than that of 17 (κ1-C-mono-Rh(CO)2; 35 h−1), which
would indicate that the cooperative rate enhancement achieved
with the bimetallic catalyst 18 is independent of the chemistry
of the iodide ligand.
reaction via coordination of the alkyne to the Lewis acidic metal
center.14d,21 This facilitates the addition of the first OH group
to the alkyne triple bond which can occur with either alcohol to
yield either a five- or six-membered heterocycle. The addition
of the second OH group to the vinyl ether intermediate is rapid
(Scheme 2), and the mono-hydroalkoxylated product is
typically not observed.14d,21
The cationic complexes 20 (κ2-C,N-mono-Rh(CO)2) and 21
(κ2-C,N-meta-(Rh(CO)2)2) were far less effective catalysts for
the dihydroalkoxylation of 22 in comparison to catalysts 17−19
(Figure 5c). The bidentate ligand coordination appears to
inhibit the activity of these complexes, suggesting dissociation
of the iodide coligand in complexes 17−19 allows the substrate
to bind to the metal center more effectively. A notable increase
in the catalytic efficiency of the bimetallic complex 21 in
comparison to the monometallic analogue 20 is still apparent.
The mechanism of the hydroalkoxylation reaction using
monometallic complexes has been described previously and
studied by a number of research teams using a variety of metal
centers.14d,21,22 In order to obtain a mechanistic understanding
of the nature of the bimetallic cooperativity in enhancing the
reaction rates of the dihydroalkoxylation reaction, a significant
investigation of the mechanism incorporating DFT calculations
is currently underway in our research group.
The results obtained using complexes 17−21 to catalyze the
dihydroalkoxylation of 22 are shown in Figure 5. The reactions
were performed in an NMR tube in tetrachloroethane-d2
(TCE-d2) at 100 °C using 1 mol % Rh concentration (i.e.,
0.5 mol % of the bimetallic catalyst). The reaction progress was
monitored in situ using 1H NMR spectroscopy, with
conversions determined by comparison of the substrate and
product integrals. Isomer 23a was found to be the major
product in each case (23a:23b ≈ 2:1). Figure 5a clearly shows
that the bimetallic catalyst 18 (κ1-C-meta-(Rh(CO)2)2) is a
superior catalyst in comparison to either the monometallic
analogue 17 (κ1-C-mono-Rh(CO)2) or the bimetallic complex
19 (κ1-C-para-(Rh(CO)2)2), with both 17 and 19 showing very
similar reaction profiles. This confirms conclusions drawn from
our previous studies, which showed a short spatial separation
between metals is necessary to achieve effective cooperative rate
enhancements for this reaction.14,15 This work provides another
example where the dihydroalkoxylation of 22 is cooperatively
enhanced by use of a bimetallic catalyst with optimum Rh−Rh
separation. This in turn suggests a more general cooperativity
Hydrosilylation Reaction. The mechanism for the hydro-
silylation of alkynes is well established.23 The hydrosilylation of
diphenylacetylene with Et3SiH to yield the vinylsilane products
Figure 5. Time course plots of the catalyzed dihydroalkoxylation reaction at 100 °C in TCE-d2: (a) κ1 catalysts 17−19; (b) κ1 catalysts 17−19 with
the addition of 1 mol % NaBArF , and (c) κ2 catalysts 20 and 21.
4
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Scheme 3. Proposed Hydrosilylation Reaction Pathway of Diphenylacetylene with Triethylsilane using a Rh(I) Complex
25-Z and 25-E proceeds via the initial oxidative addition of the
silane Si−H bond to Rh(I) (Scheme 3). This is followed by
insertion of the alkyne into the Rh−Si bond and reductive
elimination of the vinyl silane product 25-Z, which slowly
isomerizes to 25-E.
The reaction profiles obtained using complexes 17−19 as
catalysts for the hydrosilylation of diphenylacetylene with
Et3SiH are shown in Figure 6a. The reactions were performed
nanoparticles are present, the mercury will form an amalgam
with the nanoparticles and the reaction will be inhibited. Under
these conditions, less than 1% conversion was obtained after 1
h at 50 °C, suggesting that Rh nanoparticles were indeed
responsible for the catalytic activity originally observed. A TEM
image of the reaction mixture using catalyst 18 was also
obtained (Figure 6b), providing clear evidence for the
formation of nanoparticles during the reaction. It is curious
to note that complexes 17−19 display varying induction times
for this reaction. This would indicate that the bimetallic
structure has an effect on the rate at which the complexes
decompose to form nanoparticles. Most notably, for complex
19 (κ1-C-para-(Rh(CO)2)2), the p-phenylene ligand scaffold
appears to inhibit the degradation of the complex in
comparison to both 17 (κ1-C-mono-Rh(CO)2) and 18 (κ1-C-
meta-(Rh(CO)2)2). The mercury poisoning test was sub-
sequently performed for the dihydroalkoxylation of 22 using
catalyst 17. The reaction was not affected by the addition of
mercury, suggesting that the reducing potential of Et3SiH is
responsible for nanoparticle formation in the hydrosilylation
reaction. The hydrosilylation of diphenylacetylene with Et3SiH
was also performed using catalysts 20 and 21, containing the
chelating (κ2) NHC-pyrazole ligands. For catalyst 20 (κ2-C,N-
mono-Rh(CO)2) the reaction was complete within 3 min at 50
°C. The rapid reaction rate at 50 °C did not allow a clear
understanding of the reaction profile; thus, the temperature of
the catalyzed reaction was reduced to 25 °C. Both catalysts
promote the reaction at an extremely rapid rate at this
temperature, affording complete conversion of the substrate
within 25 min (Figure 7a). Very little difference in reaction rate
was observed between the catalysts, showing no cooperative
effect of the bimetallic catalyst 21. A mercury poisoning test
was also performed, but no effect on the reaction rates was
observed, which suggests that the reactions are catalyzed by a
homogeneous metal complex in this case. The chelating NHC-
pyrazole ligand group in 20 and 21 therefore appears to
stabilize the complexes against degradation to metallic
nanoparticles compared to complexes 17−19, which contain
monodentate NHC ligands.
Rhodium(I) complexes containing COD coligands have also
been used to effectively catalyze the hydrosilylation of alkynes
previously.10a,24 We therefore investigated the catalytic activity
of complexes 12, 13, 15, and 16 for the hydrosilylation of
diphenylacetylene at 50 °C over 1.5 h (Figures 7b,c). A
surprising difference in reactivity was observed between the
monometallic catalysts 12 and 15 in comparison to their
bimetallic counterparts 13 and 16, respectively. For example,
for the complexes containing the bidentate (κ2) ligand group
(Figure 7c), a large increase in the reaction rate was observed
with the bimetallic catalyst 16 (κ2-C,N-meta-(Rh(COD))2, 68%
conversion) in comparison to the monometallic catalyst 15 (κ2-
C,N-mono-Rh(COD), 0% conversion). Curiously, the opposite
trend was true for the complexes containing the monodentate
(κ1) ligand group (Figure 7b), where the monometallic catalyst
12 (κ1-C-mono-Rh(COD)) achieved 20% conversion of the
Figure 6. (a) Time course plots of the catalyzed hydrosilylation
reaction of diphenylacetylene with Et3SiH using the κ1 complexes 17−
19 in THF-d8 at 50 °C and (b) TEM image of the resultant mixture
for the hydrosilylation reaction using complex 18.
in an NMR tube at 50 °C in THF-d8, using 4 mol equiv of
Et3SiH and 2 mol % of catalyst relative to substrate. Isomer 25-
Z was found to be the major product in each case (Z:E ≈ 97:3).
The most notable feature of this catalysis is the sigmoidal
reaction profile, which is due to the significant induction period
before conversion of the substrate proceeds. A comparison of
the slope of the graphs between 10% and 90% conversion
reveals that all three catalysts have very similar activities once
the reaction is initiated. These features are indicative of
complexes 17−19 decomposing to yield Rh nanoparticles as
the active catalytic species. To confirm this hypothesis, a
mercury poisoning test was performed by exposing the reaction
solution to a large excess of elemental mercury at time 0. If Rh
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Figure 7. Time course plots of the catalyzed hydrosilylation reaction in THF-d8: (a) κ2 catalysts 20 and 21 at 25 °C; (b) κ1 catalysts 12 and 13 at 50
°C; (c) κ2 catalysts 15 and 16 at 50 °C.
substrate and the bimetallic catalyst 13 (κ1-C,N-meta-(Rh-
(COD))2) was entirely unreactive. These results highlight how
the design of cooperative bimetallic catalysts can be
substantially affected by small changes in the coordination
sphere of the two complex fragments, often in ways that cannot
be predicted from the reactivity of the corresponding
monometallic complexes.
The two active catalysts 12 and 15 were both unaffected by
the presence of mercury for the hydrosilylation reaction of 24,
consistent with a homogeneous complex structure being
retained during the reaction. The resistance of complex 12
(κ1-C-mono-Rh(COD)) to degradation indicates that the COD
coligand plays an important role in stabilizing the complex
during the hydrosilylation reaction. As described above, the
analogous CO containing complex 17 (κ1-C-mono-Rh(CO)2)
rapidly decomposes to metallic nanoparticles under the same
reaction conditions.
with these nanoparticles being responsible for the catalytic
activity observed. The integrity of the homogeneous catalyst
structure was retained in the case that a bidentate (κ2) NHC-
pyrazole ligand group was present (i.e., for 20 and 21) or if a
COD coligand was used in place of CO (i.e., for 12, 13, 15, and
16). The formation of nanoparticles from the Rh-NHC
complexes 17−19 is unusual and was not observed with similar
NHC complexes reported by Huckaba et al.10f for catalyzed
hydrosilylation reactions. A cooperative increase in the
hydrosilylation reaction rate was observed in one case, with
the bimetallic complex 16 (κ2-C,N-meta-(Rh(COD))2) being a
far superior catalyst to its monometallic analogue 15 (κ2-C,N-
mono-Rh(COD)). This highlights the sensitivity of the
cooperative phenomenon to small changes in the coordination
environment of the metal center.
EXPERIMENTAL SECTION
■
All manipulations of metal complexes and air-sensitive reagents were
carried out using standard Schlenk techniques or in a nitrogen-filled
glovebox. Reagents were purchased from Aldrich Chemicals or Alfa
Aesar and were used without further purification unless otherwise
stated. Rhodium(III) chloride hydrate was obtained from Precious
Metals Online, and all deuterated solvents were obtained from
Cambridge Isotopes Laboratories. Diethyl ether, n-pentane, and
dichloromethane were obtained dry under nitrogen from a PuraSolv
solvent purification system. Methanol was distilled under nitrogen
from dimethoxymagnesium. The compounds 1−4,16 5−8,17 and
CONCLUSIONS
■
A series of structurally related monometallic and bimetallic Rh
NHC complexes 12−21 were prepared that vary according to
the degree of separation between metals (meta vs para), the
donor properties of the ligand (κ1-NHC donor vs an κ2-
N,NHC donor), and the coligand identity (COD vs CO).
On promotion of the dihydroalkoxylation of 22 using
catalysts 17−21, a cooperative rate enhancement was observed
on using both meta-disposed bimetallic complexes 18 (κ1-C-
meta-(Rh(CO)2)2) and 21 (κ2-C,N-meta-(Rh(CO)2)2) as
catalysts; however, the para-disposed bimetallic complex 19
(κ1-C-para-(Rh(CO)2)2) had a reactivity identical with that of
the monometallic analogue 17 (κ1-C-mono-Rh(CO)2). These
results indicate that a cooperative rate enhancement is achieved
most effectively by a short spatial separation between the pairs
of metal centers and any electronic influence of the neighboring
metal center has a minimal impact on this reaction. The impact
of spatial configuration on the bimetallic cooperativity mirrors
our previous observations with N,N-donor bimetallic cata-
lysts.14a This suggests that the reaction mechanisms are similar
for both Rh complexes containing NHC donors or N,N-
donors.
25
1
[Rh(μ-Cl)(COD)]2 were synthesized using reported methods. H
and 13C{1H} NMR spectra were recorded on Bruker DPX300,
DMX400, and DMX600 spectrometers at 298 K unless otherwise
specified. Chemical shifts (δ) are quoted in ppm and are referenced to
internal solvent resonances. The abbreviations pz (pyrazole) and im
(imidazole) are used to identify NMR signals. Infrared spectra were
measured using a Nicolet 380 Avatar FTIR spectrometer. Elemental
analyses were carried out at the Elemental Analysis Unit, Australian
National University, Canberra, Australia. Single-crystal X-ray analyses
were carried out at the Mark Wainwright Analytical Centre, University
of New South Wales, Sydney. X-ray diffraction measurements were
carried out on a Bruker Kappa APEXII CCD diffractometer using
graphite-monochromated Mo Kα radiation (λ = 0.710723 Å). All
structures were solved by direct methods, and the full-matrix least-
squares refinements were carried out using SHELXL.26 Absorption
correction was performed using Multiscan SADABS, and H atom
parameters were treated as constrained. The structure of 16 showed
disorder in two moieties: one of the COD groups coordinated to
Complexes 17−19 led to the formation of metallic
nanoparticles during the catalyzed hydrosilylation reaction,
F
DOI: 10.1021/acs.organomet.5b00594
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Rh1B is disordered over two sites with occupancies of 0.75 and 0.25.
Two of the phenyl rings attached to B1F also exhibited orientational
disorder over two sites. TEM samples were prepared by applying a
droplet of the reaction solution onto a carbon plus Formvar coated
copper grid. The droplet was allowed to sit on the grid for 1 min, and
then the excess fluid was wicked off using filter paper. The grids were
examined in a JEOL 1400 TEM instrument operating at 100 kV.
Synthesis of the Ligand Precursors 9−11. The ligand
precursors 9−11 were all synthesized following the same procedure.
A solution of the appropriate mono- or bis(imidazolyl)benzene 1−3
(2.60 mmol) and 1-(2-bromoethyl)pyrazole (470 mg, 2.69 mmol for 1
or 950 mg, 5.43 mmol for 2 and 3) in N,N-dimethylformamide (20
mL) was refluxed for 48 h. The solvent was then removed by
distillation under vacuum and the residue dissolved in methanol (10
mL). Sodium tetraphenylborate (890 mg, 2.6 mmol for 1 or 1.78 g, 5.2
mmol for 2 and 3) was added to the solution, resulting in the
immediate formation of a pale brown precipitate. The precipitate was
filtered and recrystallized from dichloromethane/methanol for ligand
precursor 9 or from acetone/methanol for ligand precursors 10 and
11.
precipitate of the desired product. Complex 16 appeared to degrade
during recrystallization from dichloromethane, and the poor solubility
of 14 prevented recrystallization; therefore, both 14 and 16 were
washed with methanol (3 × 10 mL) and used without further
purification.
κ1-C-mono-Rh(COD) (12). Yield: 50%. 1H NMR (CDCl3, 400
MHz): δ 8.21 (d, 2H, J = 7.6 Hz, PhH), 7.52 (t, 2H, J = 7.6 Hz, PhH),
7.43 (t, 1H, J = 7.6 Hz, PhH), 7.16 (d, 1H, J = 1.7 Hz, imH), 7.00 (d,
1H, J = 1.7 Hz, imH), 5.27 (m, 1H, COD-CH), 5.14 (m, 1H, COD-
CH), 4.12 (s, 3H, CH3), 3.43 (m, 1H, COD-CH), 2.73 (m, 1H, COD-
CH), 2.26 (m, 2H, COD-CH2), 2.06 (m, 1H, COD-CH2), 1.77 (m,
3H, COD-CH2), 1.43 (m, 2H, COD-CH2) ppm. 13C{1H} NMR
(CDCl3, 100 MHz): δ 183.1 (d, JRhC = 49.3 Hz, NCN), 140.3 (PhC),
128.9 (PhC), 128.0 (PhC), 124.6 (PhC), 123.0 (imC), 121.6 (imC),
96.1 (d, JRhC = 7.5 Hz, COD-CH), 96.0 (d, JRhC = 7.5 Hz, COD-CH),
71.6 (d, JRhC = 14.3 Hz, COD-CH), 71.3 (d, JRhC = 14.3 Hz, COD-
CH), 39.4 (CH3), 33.4 (COD-CH2), 30.9 (COD-CH2), 30.1 (COD-
CH2), 28.9 (COD-CH2) ppm. Anal. Found (calcd for C18H22IN2Rh):
C, 43.63 (43.57); H, 4.55 (4.47); N, 5.67 (5.65).
κ1-C-meta-(Rh(COD))2 (13). Yield: 77%. 1H NMR (CDCl3, 300
MHz), two atropisomers identified as Ha and Hb (a/b = 0.4/0.6): δ
9.95 (t, 0.4H, J = 2.1 Hz, PhHa), 9.02 (t, 0.6H, J = 2.1 Hz, PhHb), 8.72
(dd, 0.8H, J = 8.1 and 2.1 Hz, PhHa), 8.56 (dd, 1.2H, J = 8.1 and 2.1
Hz, PhHb), 7.70 (t, 1H, J = 8.1 Hz, PhHa,b), 7.61 (d, 1.2H, J = 2.0 Hz,
imHb), 7.52 (d, 0.8H, J = 2.0 Hz, imHa), 7.08 (d, 0.8H, J = 2.0 Hz,
imHa), 7.06 (d, 1.2H, J = 2.0 Hz, imHb), 5.21 (m, 4H, COD-CHa,b),
1
κ2-C,N-mono (9). Yield: 62%. H NMR (DMSO-d6, 300 MHz): δ
9.56 (t, 1H, J = 1.6 Hz, NCHN), 8.25 (t, 1H, J = 1.9 Hz, imH), 7.72 (t,
1H, J = 1.9 Hz, imH), 7.70−7.55 (m, 6H, PhH and pzH), 7.49 (dd,
1H, J = 1.9 and 0.7 Hz, pzH), 7.17 (m, 8H, o-BPh4), 6.92 (t, 8H, J =
7.3 Hz, m-BPh4), 6.78 (t, 4H, J = 7.3 Hz, p-BPh4), 6.26 (t, 1H, J = 2.1
Hz, pzH), 4.68 (s, 4H, CH2) ppm. 13C{1H} NMR (DMSO-d6, 75
MHz): δ 163.4 (q, JBC = 49.5 Hz, BC), 139.8 (pzC), 135.8 (NCN),
135.6 (m, o-BPh4), 134.6 (PhC), 130.9 (pzC), 130.4 (PhC), 130.1
(PhC), 125.4 (q, JBC = 2.8 Hz, m-BPh4), 123.5 (imC), 121.8 (PhC),
121.6 (p-BPh4), 121.2 (imC), 105.8 (pzC), 50.3 (CH2), 49.7(CH2)
ppm. Anal. Found (calcd for C38H35BN4·0.5CH3OH): C, 80.77
(80.48); H, 6.31 (6.49); N, 10.06 (9.75). ESI-HRMS, found
(calculated): m/z 239.1289 (239.1291, [M − BPh4]+).
a
b
4.21 (s, 2.4H, CH3 ), 4.16 (s, 3.2H, CH3 ), 3.49−3.40 (m, 2H, COD-
CHa,b), 2.96−2.82 (m, 2H, COD-CHa,b), 2.52−1.35 (m, 16H, COD-
CH2a,b) ppm. 13C{1H} NMR (CDCl3, 75 MHz), atropisomers a and b
could not be distinguished between the closely spaced resonance pairs:
δ 183.4 (d, J = 49.2 Hz, NCN), 183.2 (d, J = 49.2 Hz, NCN), 140.2
(PhC), 140.0 (PhC), 129.1 (2 × PhC), 123.6 (imC), 123.5 (imC),
122.8 (PhC), 121.8 (imC), 121.4 (imC), 121.2 (PhC), 120.5 (PhC),
116.9 (PhC), 96.8 (d, J = 7.0 Hz, COD-CH), 96.2 (d, J = 7.0 Hz,
COD-CH), 95.8 (d, J = 7.0 Hz, COD-CH), 95.6 (d, J = 7.0 Hz, COD-
CH), 72.5 (d, J = 14.0 Hz, COD-CH), 72.2 (d, J = 14.0 Hz, COD-
CH), 72.1 (d, J = 14.0 Hz, COD-CH), 71.6 (d, J = 14.0 Hz, COD-
κ2-C,N-ortho (10). Yield: 66%. 1H NMR (acetone-d6, 300 MHz): δ
8.97 (t, 2H, J = 1.5 Hz, NCHN), 7.91 (m, 2H, PhH), 7.77 (m, 2H,
PhH), 7.66 (m, 4H, imH and pzH), 7.57 (t, 2H, J = 1.9 Hz, imH), 7.51
(d, 2H, J = 1.9 Hz, pzH), 7.33 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3
Hz, m-BPh4), 6.77 (t, 8H, J = 7.3 Hz, p-BPh4), 6.32 (t, 2H, J = 2.1 Hz,
pzH), 4.82 (m, 4H, CH2), 4.70 (m, 4H, CH2) ppm. 13C{1H} NMR
(acetone-d6, 75 MHz): δ 164.9 (q, JBC = 49.6 Hz, BC), 141.1 (pzC),
138.8 (NCN), 137.0 (m, o-BPh4), 133.9 (PhC), 131.6 (pzC), 130.9
(PhC), 129.3 (PhC), 126.1 (q, JBC = 2.8 Hz, m-BPh4), 125.1 (imC),
124.5 (imC), 122.3 (p-BPh4), 107.0 (pzC), 51.5 (2 × CH2) ppm. ESI-
HRMS, found (calculated): m/z 200.1053 (200.1057, [M −
(BPh4)2]2+), 719.3771 (719.3777, [M − BPh4]+),
a
b
CH), 39.9 (CH3 ), 39.4 (CH3 ), 33.5 (COD-CH2), 32.7 (COD-CH2),
31.7 (COD-CH2), 30.8 (COD-CH2), 30.7 (COD-CH2), 29.7 (COD-
CH2), 29.6 (COD-CH2), 28.8 (COD-CH2) ppm. Anal. Found (calcd
for C30H38I2N4Rh2·H2O): C, 38.50 (38.65); H, 4.17 (4.32); N, 5.96
(6.01).
κ1-C-para-(Rh(COD))2 (14). Yield: 80%. 1H NMR (CDCl3, 400
MHz), two atropisomers identified as Ha and Hb (a/b = 0.45/0.55): δ
8.61 (s, 2.2H, PhHb), 8.46 (s, 1.8H, PhHa), 7.28 (d, 1.1H, J = 2.0 Hz,
imHb), 2.24 (d, 0.9H, J = 2.0 Hz, imHa), 7.08 (d, 1.1H, J = 2.0 Hz,
imHb), 7.07 (d, 0.9H, J = 2.0 Hz, imHa), 5.26 (m, 4H, COD-CHa,b),
1
κ2-C,N-meta (11). Yield: 52%. H NMR (acetone-d6, 300 MHz): δ
8.93 (t, 2H, J = 1.6 Hz, NCHN), 7.96 (m, 3H, imH and PhH), 7.91
(m, 1H, PhH), 7.83 (m, 2H, PhH), 7.65 (t, 2H, J = 1.9 Hz, imH), 7.61
(dd, 2H, J = 2.3 and 0.6 Hz, pzH), 7.47 (dd, 2H, J = 1.9 and 0.6 Hz,
pzH), 7.32 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3 Hz, m-BPh4), 6.76
(m, 8H, p-BPh4), 6.26 (dd, 2H, J = 2.3 and1.9 Hz, pzH), 4.84 (m, 4H,
CH2), 4.73 (m, 4H, CH2) ppm. 13C{1H} NMR (acetone-d6, 75 MHz):
δ 164.9 (q, JBC = 49.6 Hz, BC), 140.9 (pzC), 137.0 (m, o-BPh4), 136.8
(PhC), 136.7 (NCN), 133.4 (PhC), 131.5 (pzC), 126.1 (q, JBC = 2.6
Hz, m-BPh4), 125.2 (PhC), 125.0 (imC), 122.9 (imC), 122.3 (p-BPh4),
118.1 (PhC), 106.9 (pzC), 51.4 (CH2), 51.3 (CH2) ppm. ESI-HRMS,
found (calculated): m/z 200.1054 (200.1057, [M − (BPh4)2]2+),
719.3779 (719.3777, [M − BPh4]+). Anal. Found (calcd for
C70H64B2N8): C, 80.61 (80.93); H, 6.33 (6.21); N, 10.89 (10.79).
Synthesis of COD-Containing Complexes 12−16. The
rhodium COD complexes 12−16 were synthesized by following the
same procedure as above. The appropriate ligand precursor 5−11
(0.88 mmol), [Rh(COD)Cl]2 (217 mg, 0.44 mmol for 12 and 15 or
434 mg, 0.88 mmol for 13, 14, and 16) and sodium ethoxide (112 mg,
1.64 mmol for 12 and 15 or 224 mg, 3.28 mmol for 13, 14, and 16)
were suspended in methanol (25 mL) and refluxed for 1 h (for
complex 16, refluxing the solution for only 30 min afforded a cleaner
product). Upon cooling to room temperature a yellow precipitate of
the product formed. Complexes 12, 13, and 15 were filtered and
recrystallized from dichloromethane/pentane to form a clean
b
a
4.20 (s, 3.3H, CH3 ), 4.17 (s, 2.7H, CH3 ), 3.49 (m, 2H, COD-CHa,b),
2.87 (m, 2H, COD-CHa,b), 2.44−1.37 (m, 16H, COD-CH2a,b) ppm.
The low solubility of 14 prevented acquisition of the 13C{1H} NMR
spectrum. Anal. Found (calcd for C30H38I2N4Rh2): C, 40.18 (39.87);
H, 4.02 (4.01); N, 6.23 (6.18).
κ2-C,N-mono-Rh(COD) (15). Yield: 80%. 1H NMR (acetone-d6, 400
MHz): δ 8.12 (d, 2H, J = 7.3 Hz, PhH), 8.04 (d, 1H, J = 2.3 Hz, pzH),
7.87 (d, 1H, J = 2.3 Hz, pzH), 7.71 (t, 2H, J = 7.3 Hz, PhH), 7.62 (t,
1H, J = 7.3 Hz, PhH), 7.52 (d, 1H, J = 2.0 Hz, imH), 7.35 (m, 9H,
imH and o-BPh4), 6.94 (m, 9H, NCH2 and m-BPh4), 6.78 (t, 4H, J =
7.3 Hz, p-BPh4), 6.47 (t, 1H, J = 2.3 Hz, pzH), 5.21 (m, 2H, NCH2
and COD-CH), 5.11 (ddd, 1H, J = 14.6, 4.6, and 3.2 Hz, NCH2), 4.68
(ddd, 1H, J = 14.6, 11.2, and 5.2 Hz, NCH2), 4.44 (q, 1H, J = 7.4 Hz,
COD-CH), 4.36 (m, 1H, COD-CH), 2.80 (m, 1H, COD-CH), 2.57
(m, 2H, COD-CH2), 2.25 (m, 1H, COD-CH2), 2.11 (m, 2H, COD-
CH2), 1.80 (m, 1H, COD-CH2), 1.56 (m, 2H, COD-CH2) ppm.
13C{1H} NMR (acetone-d6, 100 MHz): δ 176.4 (d, JRhC = 50.8 Hz,
NCN), 165.2 (q, JBC = 49.3 Hz, BC), 141.9 (pzC), 141.5 (PhC), 137.1
(m, o-BPh4), 134.9 (pzC), 130.3 (PhC), 129.8 (PhC), 126.0 (q, JBC
=
2.6 Hz, m-BPh4), 125.6 (PhC), 125.3 (imC), 123.8 (imC), 122.3 (p-
BPh4), 109.2 (pzC), 97.2 (d, JRhC = 7.9 Hz, COD-CH), 95.1 (d, JRhC
7.9 Hz, COD-CH), 80.3 (d, JRhC = 13.0 Hz, COD-CH), 74.7 (d, JRhC
=
=
13.0 Hz, COD-CH), 51.4 (NCH2), 50.3 (NCH2), 34.6 (COD-CH2),
G
DOI: 10.1021/acs.organomet.5b00594
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
30.8 (COD-CH2), 30.3 (under acetone-CD3, COD-CH2), 28.4 (COD-
CH2) ppm. Anal. Found (calcd for C46H46BN4Rh): C, 71.98 (71.88);
H, 5.95 (6.03); N, 7.35 (7.29).
(PhC), 39.9 (2 × CH3) ppm. FTIR (Nujol): ν 2084 (m, CO), 2069 (s,
CO), 2002 (s, CO), 1989 (m, CO) cm−1. Anal. Found (calcd for
C18H14I2N4O4Rh2): C, 27.15 (26.69); H, 1.63 (1.74); N, 7.05 (6.92).
κ1-C-para-(Rh(CO)2)2 (19). Yield: 48%. 1H NMR (CDCl3, 300
MHz): δ 7.89 (s, 4H, PhH), 7.39 (br s, 2H, imH), 7.22 (d, 2H, J = 2.0
Hz, imH), 3.97 (s, 6H, CH3) ppm. 13C{1H} (CDCl3, 75 MHz): δ
187.0 (d, JRhC = 53.4 Hz, CO), 180.9 (d, JRhC = 77.0 Hz, CO), 173.9
(d, JRhC = 42.5 Hz, NCN), 139.7 (PhC), 126.1 (PhC), 124.3 (imC),
122.7 (imC), 39.9 (CH3) ppm. FTIR (Nujol): ν 2070 (s, CO), 2016
(s, CO), 1997 (s, CO), 1968 (m, CO) cm−1. Anal. Found (calcd for
C18H14I2N4O4Rh2): C, 27.15 (26.69); H, 1.60 (1.74); N, 6.99 (6.92).
κ2-C,N-mono-Rh(CO)2 (20). Yield: 65%. 1H NMR (acetone-d6, 400
MHz): δ 7.98 (d, 1H, J = 2.5 Hz, pzH), 7.95 (d, 1H, J = 2.3 Hz, pzH),
7.80 (m, 2H, PhH), 7.60 (m, 4H, imH and PhH), 7.39 (m, 8H, o-
BPh4), 7.28 (d, 1H, J = 2.0 Hz, imH), 6.95 (t, 8H, J = 7.3 Hz, m-BPh4),
6.80 (t, 4H, J = 7.3 Hz, p-BPh4), 6.56 (t, 1H, J = 2.4 Hz, pzH), 5.28 (br
t, 2H, J = 4.9 Hz, CH2), 4.50 (t, 2H, J = 5.6 Hz, CH2) ppm. 13C{1H}
NMR (acetone-d6, 100 MHz): δ 185.4 (d, JRhC = 55.8 Hz, CO), 185.3
(d, JRhC = 71.4 Hz, CO), 167.7 (d, JRhC = 45.5 Hz, NCN), 164.9 (q, JBC
= 49.3 Hz, BC), 144.3 (pzC), 141.1 (PhC), 137.0 (m, o-BPh4), 135.3
(pzC), 130.6 (PhC), 130.3 (PhC), 127.0 (PhC), 126.1 (q, JBC = 2.6
Hz, m-BPh4), 125.9 (imC), 124.5 (imC), 122.3 (p-BPh4), 110.0 (pzC),
50.9 (CH2), 50.6 (CH2) ppm. FTIR (Nujol): ν 2087 (s, CO), 2025 (s,
CO) cm−1. Anal. Found (calcd for C40H34BN4O2Rh): C, 67.24
(67.06); H, 4.90 (4.78); N, 8.09 (7.82).
1
κ2-C,N-meta-(Rh(COD))2 (16). Yield: 40%. H NMR (acetone-d6,
400 MHz), two atropisomers identified as Ha and Hb (a/b = 0.5/0.5):
δ 9.21 (d, 1H, J = 7.2 Hz, PhHb), 9.10 (d, 1H, J = 7.2 Hz, PhHa), 8.24
(br s, 0.5H, PhHb), 8.21 (t, 0.5H, J = 8.0 Hz, PhHb), 8.17 (t, 0.5H, J =
8.0 Hz, PhHa), 8.14 (d, 1H, J = 2.5 Hz, pzHb), 8.10 (d, 1H, J = 2.5 Hz,
pzHa), 8.00 (br s, 0.5H, PhHa), 7.90 (d, 2H, J = 2.0 Hz, pzHa,b), 7.82
(d, 1H, J = 1.8 Hz, imHa), 7.71 (d, 1H, J = 1.8 Hz, imHb), 7.49 (d, 1H,
J = 1.8 Hz, imHa), 7.44 (d, 1H, J = 1.8 Hz, imHb), 7.31 (m, 16H, o-
BPh4), 7.10−6.72 (m, 2H, NCH2a,b), 6.91 (t, 16H, J = 7.3 Hz, m-
BPh4), 6.77 (t, 8H, J = 7.3 Hz, p-BPh4), 6.52 (t, 1H, J= 2.3 Hz, pzHb),
a,b
6.51 (t, 1H, J = 2.3 Hz, pzHa), 5.40−5.05 (m, 4H, NCH2 and 2H,
a,b
COD-CHa,b), 4.85−4.35 (m, 2H, NCH2 and 4H, COD-CHa,b),
a,b
3.10−2.75 (m, 2H, COD-CHa,b), 2.70−1.50 (m, 16H, COD-CH2
)
ppm. 13C{1H} NMR (acetone-d6, 100 MHz), atropisomers a and b
could not be distinguished between the closely spaced resonance pairs,
quaternary resonances PhC and NCN were not defined due to
insufficient signal strength: δ 164.9 (q, JBC = 49.1 Hz, BC), 142.1
(pzC), 141.8 (pzC), 137.0 (m, o-BPh4), 135.5 (pzC), 135.2 (pzC),
131.2 (PhC), 130.9 (PhC), 126.0 (q, JBC = 2.6 Hz, m-BPh4), 125.8
(imC), 125.7 (2 × PhC), 125.6 (imC), 123.9 (imC), 123.7 (imC),
122.3 (p-BPh4), 120.1 (PhC), 119.5 (PhC), 109.2 (pzC), 109.1 (pzC),
97.6 (d, JRhC = 7.8 Hz, COD-CH), 97.0 (d, JRhC = 7.8 Hz, COD-CH),
96.6 (d, JRhC = 7.8 Hz, COD-CH), 95.8 (d, JRhC = 7.8 Hz, COD-CH),
80.8 (d, JRhC = 13.2 Hz, COD-CH), 80.5 (d, JRhC = 13.2 Hz, COD-
CH), 75.2 (d, JRhC = 13.2 Hz, COD-CH), 74.8 (d, JRhC = 13.2 Hz,
COD-CH), 51.5 (NCH2), 51.4 (NCH2), 50.3 (NCH2), 50.1 (NCH2),
34.6 (COD-CH2), 33.7 (COD-CH2), 31.4 (COD-CH2), 31.1 (COD-
CH2), 30.5 (under acetone-CD3, COD-CH2), 30.3 (under acetone-
CD3, COD-CH2), 29.1 (COD-CH2), 28.3 (COD-CH2) ppm. ESI-
HRMS found (calculated): m/z 410.0977 (410.0972, [M −
(BPh4)2]2+), 1139.3607 (1139.3608, [M − BPh4]+).
κ2-C,N-meta-(Rh(CO)2)2 (21). Yield: 41%. 1H NMR (acetone-d6,
400 MHz): δ 8.33 (dd, 2H, J = 8.0 and 2.1 Hz, PhH), 8.25 (m, 3H,
pzH and PhH), 7.98 (d, 2H, J = 2.3 Hz, pzH), 7.93 (t, 1H, J = 8.0 Hz,
PhH), 7.86 (d, 2H, J = 2.0 Hz, imH), 7.62 (d, 2H, J = 2.0 Hz, imH),
7.34 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3 Hz, m-BPh4), 6.77 (t, 8H,
J = 7.3 Hz, p-BPh4), 6.66 (t, 2H, J = 2.4 Hz, pzH), 5.65 (br s, 4H,
CH2), 4.89 (t, 4H, J = 5.7 Hz, CH2) ppm. 13C{1H} NMR (acetone-d6,
100 MHz): δ 185.6 (d, JRhC = 71.3 Hz, CO), 185.3 (d, JRhC = 55.8 Hz,
CO), 168.3 (d, JRhC = 44.8 Hz, NCN), 165.0 (q, JBC = 49.1 Hz, BC),
144.5 (pzC), 141.9 (PhC), 137.1 (m, o-BPh4), 135.7 (pzC), 132.4
(PhC), 128.7 (PhC), 126.5 (imC), 126.0 (q, JBC = 2.6 Hz, m-BPh4),
125.0 (imC), 123.8 (PhC), 122.3 (p-BPh4), 110.2 (pzC), 51.3 (CH2),
50.9 (CH2) ppm. FTIR (Nujol): ν 2090 (s, CO), 2027 (s, CO) cm−1.
ESI-HRMS, found (calculated): m/z 1035.1537 (1035.1527, [M −
BPh4]+).
Synthesis of CO-Containing Complexes 17−21. The Rh(I)
CO complexes 17−21 were synthesized by following the same
procedure. CO gas was bubbled through a solution of the appropriate
COD containing complex 12−16 (0.26 mmol) in dichloromethane
(15 mL) for 10 min, resulting in a color change to pale yellow. In the
case of complexes 19 and 21, the reactions were performed in
chloroform and acetone, respectively. The resulting solution was
added to a large volume of pentane (125 mL), presaturated with a CO
atmosphere, resulting in precipitation of the desired product. For
complex 17 concentration of the dcm/pentane solution to 20 mL
under reduced pressure was necessary to precipitate the complex from
solution. The precipitates were filtered, washed with pentane, and
dried under vacuum.
Catalyzed Dihydroalkoxylation of Alkynediol (22). The
dihydroalkoxylation reaction was performed under nitrogen in NMR
tubes fitted with a Young’s concentric Teflon valve. The alkynediol 22
(38 mg, 200 μmol), the catalyst (2 μmol of the monometallic
complexes or 1 μmol of the bimetallic complexes), and NaBArF if
4
desired (2.0 mg, 2.3 μmol) were dissolved in TCE-d2 (0.6 mL). The
tube was inserted into the NMR spectrometer preheated to 100 °C.
The reaction progress was monitored by 1H NMR spectroscopy, with
conversions determined by comparison of characteristic substrate and
product integrals (see Figure S27 in the Supporting Information).
Catalyzed Hydrosilylation of Diphenylacetylene (24). The
hydrosilylation reaction was performed under nitrogen in NMR tubes
fitted with a Young’s concentric Teflon valve. Diphenylacetylene (36
mg, 200 μmol), the catalyst (4 μmol of the monometallic complexes or
2 μmol of the bimetallic complexes), and NaBArF4 if desired (4.0 mg,
4.5 μmol) were dissolved in THF-d8 (0.6 mL). Triethylsilane (93 mg,
800 μmol) was then injected into the solution and the tube inserted
into the NMR spectrometer preheated to the desired temperature (50
or 25 °C). The reaction progress was monitored by 1H NMR
spectroscopy with conversions determined by comparison of
characteristic substrate and product integrals (see Figure S28 in the
Supporting Information).
κ1-C-mono-Rh(CO)2 (17). Yield: 47%. 1H NMR (CDCl3, 600
MHz): δ 7.71 (d, 2H, J = 7.6 Hz, PhH), 7.49 (t, 2H, J = 7.6 Hz, PhH),
7.45 (t, 1H, J = 7.6 Hz, PhH), 7.27 (br s, 1H, imH), 7.16 (br s, 1H,
imH), 3.95 (s, 3H, CH3) ppm. 13C{1H} (CDCl3, 150 MHz): δ 187.2
(d, JRhC = 53.9 Hz, CO), 181.1 (d, JRhC = 77.8 Hz, CO), 173.3 (d, JRhC
= 41.7 Hz, NCN), 139.6 (PhC), 129.3 (PhC), 129.0 (PhC), 125.4
(PhC), 123.9 (imC), 122.8 (imC), 39.8 (CH3) ppm. FTIR (Nujol): ν
2071 (s, CO), 2006 (s, CO) cm−1. Anal. Found (calcd for
C12H10IN2O2Rh): C, 32.65 (32.46); H, 2.41 (2.27); N, 6.31 (6.31).
κ1-C-meta-(Rh(CO)2)2 (18). Yield: 52%. 1H NMR (CDCl3, 400
MHz, 250 K), two atropisomers identified as Ha and Hb (a/b = 0.4/
0.6): δ 8.32 (t, 0.4H, J = 2.0 Hz, PhHa), 8.22 (t, 0.6H, J = 2.0 Hz,
PhHb), 8.02 (dd, 1.2H, J = 8.0 and 2.0 Hz, PhHb), 7.93 (dd, 0.8H, J =
8.0 and 2.0 Hz, PhHa), 7.65 (t, 0.6H, J = 8.0 Hz, PhHb), 7.64 (t, 0.4H,
J = 8.0 Hz, PhHa), 7.55 (d, 1.2H, J = 2.0 Hz, imHb), 7.41 (d, 0.8H, J =
2.0 Hz, imHa), 7.25 (d, 1.2H, J = 2.0 Hz, imHb), 7.23 (d, 0.8H, J = 2.0
a
b
Hz, imHa), 3.98 (s, 2.4H, CH3 ), 3.96 (s, 3.6H, CH3 ) ppm. 13C{1H}
NMR (CDCl3, 100 MHz, 250 K), atropisomers a and b could not be
distinguished between the closely spaced resonance pairs: δ 186.8 (d,
JRhC = 53.4 Hz, 2 × CO), 180.5 (d, JRhC = 77.8 Hz, 2 × CO), 172.5 (d,
JRhC = 41.6 Hz, NCN), 172.4 (d, JRhC = 41.6 Hz, NCN), 139.7 (PhC),
139.5 (PhC), 130.1 (PhC), 130.0 (PhC), 124.8 (PhC), 124.6 (PhC and
imC), 124.5 (imC), 122.7 (imC), 122.5 (imC), 121.9 (PhC), 121.3
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00594. Crystallographic data (without structure factors)
for the structures of 15 and 16 reported in this paper have also
H
DOI: 10.1021/acs.organomet.5b00594
Organometallics XXXX, XXX, XXX−XXX

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mail, deposit@ccdc.cam.ac.uk.
NMR spectra of ligands and complexes 9−21 and
catalyzed reaction mixtures (PDF)
Crystallographic data for 15 and 16 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for B.A.M.: barbara.messerle@mq.edu.au.
■
Present Address
§Department of Chemistry and Biomolecular Sciences,
Macquarie University, North Ryde 2109, Australia.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported under Australian Research
Council’s Discovery Projects funding scheme (project number
DP110101611). Financial support from the University of New
South Wales and Macquarie University is gratefully acknowl-
edged, and the Australian Government is acknowledged for a
Ph.D. stipend (V.D.). We thank Katie Levick for the TEM
image and Professor Andrew Weller for helpful discussions.
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