Enhanced Electrophilicity of Heterobimetallic Bi-Rh Paddlewheel Carbene Complexes: A Combined Experimental, Spectroscopic, and Computational Study

Article abstract of DOI:10.1021/jacs.8b08384

Dirhodium paddlewheel complexes are indispensable tools in modern organometallic catalysis for the controlled decomposition of diazo-compounds. Tuning the reactivity of the thus-formed transient carbenes remains an active and dynamic field of research. Herein, we present our findings that the distal metal center plays an as yet underappreciated role in modulating this reactivity. Replacement of one rhodium atom in the bimetallic core for bismuth results in the formation of a significantly more electrophilic carbene complex. Bismuth-rhodium catalysts thereby facilitate previously unknown modes of reactivity for α-diazoester compounds, including the cyclopropanation of alkenes as electron deficient as trichloroethylene. While dirhodium paddlewheel complexes remain the catalysts of choice for many carbene-mediated transformations, their bismuth-rhodium analogues exhibit complementary reactivity and show great potential for small molecule and solvent activation chemistry. DFT calculations highlight the importance of metal-metal bonding interactions in controlling carbene electrophilicity. The paucity of these interactions between the 4d orbitals of rhodium and the 6p orbitals of bismuth results in weaker π-back-bonding interactions for bismuth-rhodium carbene complexes compared to dirhodium carbene complexes. This leads to weakening of the rhodium-carbene bond and to a more carbene-centered LUMO, accounting for the observed enhancement in bismuth-rhodium carbene electrophilicity. These findings are supported by a detailed spectroscopic study of the "donor-donor" carbene complexes Rh₂(esp)₂C(p-MeOPh)₂ (19) and BiRh(esp)₂C(p-MeOPh)₂ (20), employing a combination of UV-vis and resonance Raman spectroscopy. The results reveal that carbene chemoselectivity in MRh(L)₄ catalysis can be modulated to a previously unrecognized extent by the distal metalloligand.

Full text of DOI:10.1021/jacs.8b08384

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Article
Enhanced Electrophilicity of Heterobimetallic Bi-
Rh Paddlewheel Carbene Complexes: A Combined
Experimental, Spectroscopic and Computational Study
Lee R. Collins, Maurice van Gastel, Frank Neese, and Alois Fürstner
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08384 • Publication Date (Web): 14 Sep 2018
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Enhanced Electrophilicity of Heterobimetallic BiꢀRh Paddlewheel Carbene Complexes: A
Combined Experimental, Spectroscopic and Computational Study
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Lee R. Collins, Maurice van Gastel, Frank Neese,* and Alois Fürstner*
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MaxꢀPlanckꢀInstitut für Kohlenforschung, Mülheim/Ruhr 45470, Germany
ABSTRACT
Dirhodium paddlewheel complexes are indispensable tools in modern organometallic catalysis
for the controlled decomposition of diazoꢀcompounds. Tuning the reactivity of the thusꢀformed
transient carbenes remains an active and dynamic field of research. Herein, we present our
findings that the distal metal center plays an as yet underappreciated role in modulating this
reactivity. Replacement of one rhodium atom in the bimetallic core for bismuth results in the
formation of a significantly more electrophilic carbene complex. Bismuthꢀrhodium catalysts
thereby facilitate previously unknown modes of reactivity for αꢀdiazoester compounds, including
the cyclopropanation of alkenes as electron deficient as trichloroethylene. While dirhodium
paddlewheel complexes remain the catalysts of choice for many carbeneꢀmediated
transformations, their bismuthꢀrhodium analogues exhibit complimentary reactivity and show
great potential for small molecule and solvent activation chemistry. DFT calculations highlight
the importance of metalꢀmetal bonding interactions in controlling carbene electrophilicity. The
paucity of these interactions between the 4d orbitals of rhodium and the 6p orbitals of bismuth
results in weaker πꢀbackꢀbonding interactions for bismuthꢀrhodium carbene complexes compared
to dirhodium carbene complexes. This leads to weakening of the rhodiumꢀcarbene bond and to a
more carbeneꢀcentered LUMO, accounting for the observed enhancement in bismuthꢀrhodium
carbene electrophilicity. These findings are supported by a detailed spectroscopic study of the
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‘donorꢀdonor’ carbene complexes Rh2(esp)2C(pꢀMeOPh)2 (19) and BiRh(esp)2C(pꢀMeOPh)2
(20), employing a combination of UVꢀVis and resonance Raman spectroscopy. The results reveal
that carbene chemoselectivity in MRh(L)4 catalysis can be modulated to be previously
unrecognized extent by the distal metalloligand.
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INTRODUCTION
The discovery nearly fifty years ago that Rh2(OAc)4 (A, R = Me) catalytically decomposes
diazo compounds opened a broad and exciting field of research.1 The electrophilic character of
the thus formed carbene intermediates has since been exploited in a wide range of catalytic
reactions, including cyclopropanation, CꢀH functionalization, cycloadditions, carbonꢀheteroatom
insertions and ylideꢀtype chemistry.2 Replacement of acetate with more functionalized bridging
ligands has led to the development of a plethora of (chiral) catalysts whose properties and
selectivities provide the modernꢀday synthetic chemist with a diverse toolꢀbox from which to
choose.2,3 Decades later, research continues toward understanding the nature of these
electrophilic carbene intermediates and the rationalization of their reactivity and selectivity
patterns.
Before direct spectroscopic evidence became available for these highly reactive intermediates,
numerous experimental and computational studies were conducted.4 These studies converge on a
frontier orbital description of dirhodium carbenes as consisting of a singlet carbene interacting in
a σꢀdonating and πꢀaccepting manner with the dirhodium core, in analogy to Fischer carbenes.
However, in contrast to Fischer carbenes, dirhodium carbenes display enhanced electrophilic
reactivity, being able to activate nucleophiles as weak as CꢀH bonds. While this is part due to the
absence of heteroatom stabilization,5 work by the groups of Nakamura and Berry has identified
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the importance of σ and π 3c4e bonding interactions in making dirhodium carbenes
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‘superelectrophilic’.4c,6
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Figure 1. Prototype paddlewheel complexes to be discussed herein; L = kinetically labile twoꢀelectron
donor ligand
Recently, dirhodium carbene complexes bearing one (B) or two electron donating groups (C)
were characterized spectroscopically and crystallographically for the first time.7,8 These data
provide clarity to the role of the carboxylate framework and carbene substituents in determining
the trajectory of an incoming nucleophile and thus help to rationalize the stereoselectivity profiles
observed in catalysis. While variation of the bridging ligands remains the dominant method by
which Rh(II) carbene reactivity and selectivity are tuned, increasing attention is being paid to the
possible role of axial ligands.9 Incorporation of a Lewis base (either as solvent, coꢀsolvent or
stoichiometric additive) can have dramatic consequences on chemoꢀ, diastereoꢀ and
enantioselectivity.10 For example, the Jessop and Davies groups reported that coordinating
solvents or addition of methyl benzoate increase enantioselectivity in donorꢀacceptor
cyclopropanation reactions of styrene.10d,e These effects may be rationalized by invoking axial
coordination. A conclusive explanation, however, is difficult to reach due to the lability of axial
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ligands and the implicit uncertainty over what is actually coordinated inꢀsitu to the distal metal at
the instant of carbene formation and/or its encounter with the reaction partner (Figure 2).11
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Figure 2. Possible equilibria involved in Rh(II) carbene formation and reactivity. L = neutral, two electron
donor ligand.
Alternative modifications to the dirhodium core include oxidation to mixed valence Rh2(II,III)
complexes,9,12 or to even replace the distal rhodium atom itself with a different metal. The only
such heterobimetallic MRh(L)4 complexes reported in the literature are those where M = Bi (D),
first prepared by Dikarev in 2005.13 These complexes were found to display Lewis acidic
behavior only at the rhodium face, thus providing a system with no ambiguity over the nature of
the distal metal center and its state of ligation.13a,13b,14 Despite the arguably rather dramatic
change that the incorporation of a main group element in lieu of the second Rh atom might entail,
such bismuthꢀrhodium complexes were described to exhibit qualitatively similar reactivity to
their dirhodium counterparts in a series of cyclopropanation and CꢀH activation reactions with
donorꢀacceptor type carbenes; the most significant difference was the much reduced rate of diazo
decomposition (by a factor of ~103).15 Seeming to offer no advantage over more readily available
dirhodium complexes, no follow up studies of bismuthꢀrhodium carbene chemistry have been
reported, to date.
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Figure 3. Complexes employed as catalysts in this study. M = Rh or Bi.
During our research into controlling carbene mediated chemistry,8,16 we have revisited these
heterobimetallic complexes and their reactivity with acceptorꢀtype αꢀdiazoesters (Figure 3). In
addition to the kinetic differences observed before,15 our results very clearly show that changing
the bimetallic core actually results in strikingly divergent reactivity profiles. Bismuthꢀrhodium
based acceptor carbene complexes of type E (Figure 4, M = Bi) are even more electrophilic than
their already ‘superelectrophilic’ dirhodium analogues.6 This extreme character opens new vistas
in solvent activation and the functionalization of small molecules with very little precedent and
therefore offers a complimentary reactivity profile to established dirhodium complexes
(E, M = Rh). Furthermore, BiRh(esp)2 is found to be a remarkably robust catalyst for these
purposes, providing high isolated yields with <1 mol % catalyst loadings.
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Figure 4. Bimetallic (left) ‘acceptor’ and (right) ‘donorꢀdonor’ carbene complexes. M = Rh or Bi.
Struck by the unexpected reactivity profile of these bismuthꢀrhodium carbenes and the
disparity between our experimental findings and those of Davies,15 we sought to understand the
origins of this behavior. To this end, we present a combined experimental and theoretical
investigation into the electronic structure of more stabilized donorꢀdonor carbene complexes (F)
to support our computational studies on pure acceptor type carbene complexes (E) which have so
far proved too labile for direct inspection. In contrast to previous studies, our computational
analyses are conducted on complete molecular systems with no truncations of the ligand set and −
crucially − with spectroscopic calibration. Together, these data provide detailed insight into the
electronic structure of catalytically important homoꢀ and heterobimetallic carbene intermediates
and, in part, correct previous conclusions as to the origins of their “superelectrophilicity”. Most
notably, our results shed light on the as yet underappreciated importance of the distal metal in
modulating πꢀback donation and consequently controlling the stability as well as reactivity of the
carbene.
RESULTS AND DISCUSSION
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Reactivity of αꢀDiazoesters with MRh(L)4 Complexes. During our studies into chiral
dirhodium complexes,8c a catalyst screening was performed on prenyl diazoacetate (1) as a
simple substrate with the potential for enantioselective intramolecular reactivity. We were
surprised to observe that heterobimetallic BiRh(L)4 paddlewheel complexes gave very poor
conversions to the cyclic lactone (2), especially given that 1 undergoes cyclopropanation so
readily with dirhodium catalysts such as Rh2(5RꢀMEPY)4.17 Remarkably, these bismuthꢀrhodium
complexes were instead found to catalyze insertion of the derived acceptor carbene into a CꢀCl
bond of the CH2Cl2 solvent (Scheme 1). To the best of our knowledge, this is the first
documented example of a rhodium catalyzed insertion of a carbene into a CꢀCl bond. Such
carbene CꢀCl insertion reactions, where both carbon and chlorine form bonds to the carbene
center, have thus far been restricted to certain silver and zinc catalysts.18,19 Interestingly, no such
solvent activation was reported in the study of BiRh(L)4 catalyzed intermolecular
cyclopropanations by the Davies group, which had been focused on the resonance stabilized and
hence less electrophilic donorꢀacceptor carbenes but also contained an example of a pure acceptor
type.15,20
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Scheme 1. Divergent Reactivity of Prenyl Diazoacetate (1) Catalyzed by Dirhodium and
BismuthꢀRhodium Complexes
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Scheme 2. Decomposition of 4 Catalyzed by Dirhodium and BismuthꢀRhodium Complexes
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Spurred on by this unexpected finding, we turned our attention to the less functionalized and
commercially available ethyl diazoacetate (EDA, 4). Screening a range of dirhodium and
bismuthꢀrhodium catalysts confirmed that only heterobimetallic complexes catalyze the CꢀCl
insertion reaction (Table 1, entries 1 ꢀ 2). Dirhodium complexes decompose 4 more quickly, in
accordance with the findings of Davies,15 but lead only to the fumarate and maleate products (5)
of carbene dimerization (entries 4 ꢀ 6). That no CꢀCl insertion was observed when using
BiRh(TFA)4 (entry 3) can be explained by noting that electron deficient dirhodium complexes
have been shown to not support organometallic carbene intermediates derived from ‘acceptor’
type diazo compounds; instead they are thought to release metalꢀfree, organic carbenes.10i,21
Direct comparison of the isosteric BiRh(esp)2 and Rh2(esp)2 complexes (entries 1 and 4) makes
clear that this divergent reactivity is an electronic effect and that the distal metal center indeed
plays a crucial role in controlling the outcome. Isolation of all identifiable components of the
reaction mixtures revealed that material is ‘lost’ in the reactions with bismuthꢀrhodium catalysts
to a greater extent than is the case with dirhodium complexes. Reactivity with other substrates
(vide infra) did not suffer from this low mass balance, perhaps indicating that the highly reactive
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chloroꢀylide intermediates being produced during CꢀCl insertion can decompose in a range of
unforeseen ways.22
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a
Table 1. Catalyst Screening for the Reactivity of 4 with CH2Cl2
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Unreacted
Entry
Catalyst
6 (%)
5 (%)
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Total (%)
4 (%)
BiRh(esp)
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Bi(TFA)
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Reaction conditions: EDA (0.775 mmol) added in one portion, catalyst (0.6 mol %), CH2Cl2 (5 mL),
40°C, 2 h. Yields determined by NMR relative to an internal standard (mesitylene). Brackets denote
b
isolated yields. For the sake of comparison, reaction times were limited to 2 h. Catalyst decomposition
was observed.
We next sought to explore the reactivity profile of the bismuthꢀrhodium carbene intermediate
in more detail and in direct comparison to its dirhodium analogue (Scheme 3). Cyclopropanation
of styrene with EDA is definitely best performed with Rh2(esp)2; the reaction catalyzed by
BiRh(esp)2 gave only a low yield of cyclopropane 7, comparable to that obtained by the Davies
group using BiRh(Piv)2(TFA)2;15 in our case, a nonꢀnegligible amount of CꢀCl insertion product
6 was formed, not reported previously. Both catalysts performed similarly well in the Buchner
reaction with toluene leading to the formation of the tropene derivative 8.
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Scheme 3. Reactivity Profile of 4 Catalyzed by BiRh(esp)2 and Rh2(esp)2
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a Reagents and conditions, unless otherwise stated: solvent (5 mL), BiRh(esp)2 or Rh2(esp)2 (0.6 mol %),
EDA (0.775 mmol), 40°C, 2 h, isolated yields unless otherwise specified; (a) CH2Cl2 (40 mL), styrene
(5 eq), catalyst (2.0 mol %), EDA added over 2 h, 40°C, 3 h;15 (b) toluene; (c) cyclohexane, 50°C, 3 h;
(d) MeCN, 70°C, 90 h; (e) CH2Cl2, catalyst (2.0 mol %), EDA added over 2 h, 60°C, 3 h; (f) CHCl3;
(g) tBuCl; isobutene was formed concomitantly; (h) (E)ꢀdichloroethylene (20 mL), EDA added over 16 h,
45°C, 18 h; (i) trichloroethylene (20 mL), EDA added over 16 h, 60°C, 18 h. Analysis of crude reaction
mixtures revealed diethyl maleate/fumarate (5) as the only other products formed.
In all other cases of solvent activation, however, the heterobimetallic catalyst showed a largely
superior performance. This includes the CꢀH activation of cyclohexene (9) and the conversion of
acetonitrile into oxazole 10. Most notable is the haloꢀylide reactivity by CꢀCl insertion with
CH2Cl2 (6) or CHCl3 (11) and, owing to the availability of βꢀhydrogens, HꢀCl elimination with
tBuCl (12).18a,d The yield of the CH2Cl2 insertion product 6 could be improved to 51% by slow
addition of 4 to compare favorably with the most successful Ag catalysts known to date (37%).18d
Although the CꢀCl bonds of alkenyl halides such as (E)ꢀdichloroethylene could not be activated,
this substrate was efficiently cyclopropanated (13). Such electron poor alkenes have not, to date,
been successfully cyclopropanated by any dirhodium catalyst; the control experiment with
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Rh2(esp)2 confirmed that this catalyst is indeed inadequate.3a,23 Remarkably, the even more
electron deficient and sterically encumbered trichloroethylene reacted well with BiRh(esp)2 under
mild conditions (14); to the best of our knowledge, this substrate has not been successfully
cyclopropanated by any metal carbene species.3a,23 Optimization of the reaction conditions
(Table 2) allowed for isolation of both isomers of trichlorocyclopropane 14 in a combined 72%
yield and an appreciable diastereoselectivity (d.r. = 8 : 1).24 The reaction is tolerant of air, does
not require purification of solvents and works with <1 mol % of BiRh(esp)2, which is even robust
enough to be reꢀisolated by flash chromatography. Recovery of the unreacted solvent from the
reaction mixture further increases the synthetic value of this process.
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Table 2. Optimization of Reaction Conditions for the Synthesis of Trichlorocyclopropane
14a
EDA Addition
Time (h)
Reaction
Time (h)
Solvent
Volume (mL)
d.r.
T (°C)
40
14 (%)b
26
(trans : cis)c
Instant
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3
1 : 7.8
1 : 8.8
1 : 9.0
1 : 7.9
1 : 8.0
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Reagents and conditions, unless otherwise stated: EDA (0.775 mmol) added by syringe pump,
BiRh(esp)2 (0.6 mol %). b Combined isolated yield of both isomers. d.r. determined by NMR analysis of
c
the crude reaction mixture.
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Scheme 4. Reactivity of 4 with Cyclohexene
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Table 3. Product Distributions in the BismuthꢀRhodium Catalyzed Decomposition of 4 in
Cyclohexenea
Catalyst
BiRh(esp)2
BiRh(Piv)4
BiRh(TPA)4
15 (%)
16 (%)
38
17 (%)
18
15 : 16 : 17b
1 : 4 : 3
13
5
74
17
1 : 18 : 6
1 : 3 : 2
28
42
29
a
Reagents and conditions: cyclohexene (5 mL), EDA (0.775 mmol) added in one portion, BiRh(L)4
b
(2.0 mol %), 50°C, 4 h. Isolated yields. Product ratios were determined by GCMS analysis of the crude
reaction mixture.
To understand more about the origins of this unexpected reactivity profile, we sought to
clarify whether the reactive intermediates are truly organometallic in nature or whether ‘free’
carbenes might be formed at some point along the reaction coordinate. Decomposition of 4 in
cyclohexene (Scheme 4) indeed produced significantly different ratios of CꢀH activation (15) and
cyclopropanation products (16 and 17) depending on the nature of the carboxylate ligands of
three different bismuthꢀrhodium catalysts (Table 3). This strong ligand dependence suggests that
the carbene is metalꢀbound at the time of reactive encounter with cyclohexene as the substrate.25
Interestingly, however, the situation is less clearꢀcut for trichloroethylene as the reaction
partner: a control experiment, in which EDA was photochemically decomposed to the free
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carbene in trichloroethylene as the solvent, showed formation of small amounts of the
corresponding cyclopropanes 14; compared with the reaction catalyzed by BiRh(esp)2, which
provided up to 72% of 14 in analytically pure form (see Table 2), the yield was low (< 20%, GC)
and numerous unidentified side products made an isolation of clean material impossible (see the
Supporting Information). Nevertheless, the result might imply that BiRh(esp)2 generates a metalꢀ
bound carbene in the first place, which then releases a free carbene at the elevated temperature at
which the cyclopropanation of trichloroethylene is best carried out. We therefore set out to
understand and explain the role of the distal metal in determining the strikingly divergent and
somehow complementary reactivity profiles of bismuthꢀrhodium and dirhodium carbenes
manifest in the examples shown in Scheme 3; at the same time, we wanted to determine the
relevant thermodynamic parameters of such bimetallic carbene complexes in order to assess if
release of a free carbene is feasible under reaction conditions.
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Inꢀsitu Generation of DonorꢀDonor Carbene Complexes. Based on our experience with
dirhodium and gold,8a we anticipated that donorꢀdonor carbenes with electron releasing
substituents (F, Figure 4) would give us the best chance of observing the reactive intermediate.
Solutions of the donorꢀdonor carbene complexes Rh2(esp)2C(pꢀMeOPh)2 (19) and
BiRh(esp)2C(pꢀMeOPh)2 (20) were generated at low temperatures upon addition of a purple
solution of diazo derivative 18 to CH2Cl2 solutions of the green (Rh2(esp)2)26 or yellow
(BiRh(esp)2) paddlewheel complexes (Scheme 5). Formation of the carbene was apparent by the
distinctive deep green solution formed in both cases. In accordance with the findings of Davies
and coꢀworkers, we observed a significantly slower reaction in the case of the bismuthꢀrhodium
complex.15 Solutions of 20 also decayed much more rapidly than the corresponding dirhodium
complex 19. NMR studies showed that 19 is stable for 18 h at ꢀ10°C, whereas 20 decays even
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at ꢀ50°C with a halfꢀlife of approximately 2 h. These spectra furthermore showed that a
stoichiometric reaction of 18 with either paddlewheel complex produces a mixture of starting
complex, carbene complex and diazine 21, whose exact composition is highly sensitive to the
reaction conditions (temperature, speed of addition of 18, concentration). As even these stabilized
‘donorꢀdonor’ carbene complexes proved unisolable, we took a combined experimental and
theoretical approach to their study.
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Scheme 5. Reaction of 18 with MRh(esp)2 (M = Rh or Bi)
Spectroscopic and Theoretical Study. The only previous theoretical investigation of
bismuthꢀrhodium carbenes focused on the reaction profile of donorꢀacceptor carbene
cyclopropanation for a model system with formate or trifluoroacetate bridging ligands.15 Higher
activation barriers were calculated for rate determining carbene formation in the case of bismuthꢀ
rhodium catalysis (BiRh: 19.2 kcal/mol; Rh2: 11.3 kcal/mol), in accordance with the experimental
findings that bismuthꢀrhodium catalysis is significantly slower. The activation barriers for
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cyclopropane formation were found to be similar (BiRh: 3.6 kcal/mol; Rh2: 4.5 kcal/mol). These
calculations led to the conclusion that “the selectivity profile of BiꢀRh donor/acceptor carbenoids
is similar to that of analogous dirhodium carbenoids.” No detailed MO analysis was conducted to
probe the electronic structures; rather, a charge distribution analysis suggested that ‘bismuth can
also act as a redox pool’.15
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Our experimental results with acceptorꢀtype diazo compounds show that the presence of
bismuth does fundamentally change the reactivity profile of the carbene complex and that the
nature of the supporting ligands plays an important role in determining selectivity. We therefore
saw the need to relaunch a comprehensive theoretical study with no truncations of the supporting
ligand set, and, crucially, with calibration to spectroscopic data.27
In order to understand the electronic structure relationship between 19 and 20, it is highly
instructive to also study the precursors Rh2(esp)2 and BiRh(esp)2. All DFT allꢀelectron
calculations were carried out using the BP8628 and B3LYP29 functionals and the def2ꢀTZVP basis
set30 together with the scalar relativistic 0thꢀorder regular approximation (ZORA Hamiltonian)31
as implemented in the ORCA program suite.32 Below, a summary is reported. For the detailed
arguments that led us to our conclusions, the reader is kindly referred to the SI.
Geometric Structure. Our theoretical investigations began with a geometry optimization of
the donorꢀdonor carbene complexes 19 and 20 as well as their paddlewheel precursors. The
optimized structure of Rh2(esp)2 (Figure 5) is in excellent agreement with the crystallographically
determined structure, although the crystal structure features two additional axially coordinated
acetone molecules.26 The optimized RhꢀRh distance of 2.385 Å is in near perfect agreement with
the experimental value of 2.382 Å26 and for BiRh(esp)2, the DFT calculated bond length
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(2.538 Å) is consistent with the experimentally determined value (2.53(1) Å, see SI for the solid
state structure of BiRh(esp)2·H2O). The RhꢀO distance optimizes to an average of 2.06 Å which
is slightly longer than the experimental 1.97ꢀ2.06 Å, while the optimized BiꢀO distances (average
of 2.41 Å) fall within the experimentally determined range of 2.36ꢀ2.42 Å (Table 4).
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b
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Figure 5. Geometry optimized structures of (a) Rh2(esp)2, (b) BiRh(esp)2, (c) Rh2(esp)2C(pꢀMeOPh)2 (19)
and (d) BiRh(esp)2C(pꢀMeOPh)2 (20).
For the Rh2(esp)2C(pꢀMeOPh)2 complex 19, similarly good agreement is obtained between
theory and the solid state structure of the closely related Rh2(esp)2C(pꢀMe2NPh)2 complex.8b In
particular, the optimized RhꢀC bond (2.022 Å) is in perfect agreement with the experimentally
determined 2.025 Å. A direct comparison to experimental data is not possible for 20 due to the
lack of a comparable crystal structure, although it is interesting to note that the computed RhꢀC
distance of 2.139 Å is distinctly longer than for the dirhodium analogue 19.15 With excellent
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agreement between the theoretical and crystallographically determined structures, we next sought
to investigate the origins of the observed variations in reactivity in terms of electronic structure.
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Table 4. Selected Structural Parameters (Å) of Geometry Optimized and
Crystallographically Determined Structures
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Rh2(esp)2
DFT
BiRh(esp)2
DFT
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20
DFT
Expa
Expb
DFT
Expc
2.385
2.058
2.064
2.382
2.538
2.051
2.408
2.53(1)
1.97ꢀ2.06
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2.477
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MꢀRh
RhꢀO
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RhꢀC
a Rh2(esp)2·2(OCMe2).26 b BiRh(esp)2·H2O (see SI for data). c Rh2(esp)2C(pꢀMe2NPh)2.8b
Electronic Structure. The electronic structures of dirhodium and bismuthꢀrhodium
paddlewheels are well studied, but only in systems containing simple bridging ligands.6,13a,14,33
Since the nature of the bridging ligands will have an effect on the molecular orbitals and
reactivity of the complexes in question, we present here a complete analysis of the Rh2(esp)2 and
BiRh(esp)2 complexes with no truncation of the ligand sphere. This discussion also allows for a
direct comparison to the corresponding carbene complexes which were also computed in their
entirety without truncations (vide infra) as well as offering the reader contextualization of the
bonding descriptions.
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Rh2(esp)2
BiRh(esp)2
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Figure 6. Molecular orbital scheme for Rh2(esp)2 (left) and BiRh(esp)2 (right) with symmetry labels under
approximate D4h and C4v symmetry, respectively. The structures have been truncated for purposes of
clarity and for all e sets, only one orbital is displayed.
Rh2(esp)2 features two 4d7 Rh2+ ions that form a classic metalꢀmetal bond.3b Hence, the
resulting species is wellꢀdescribed as a closedꢀshell complex in which 14 electrons distribute over
the 10 metalꢀbased molecular orbitals. As shown in Figure 6, the coordination geometry around
each Rh ion is essentially square planar. In the approximate D4h point group, the dx ꢀy
orbitals are highest in energy and form a pair of unoccupied b2u and b1g molecular orbitals. The
strongest RhꢀRh interaction occurs between the two dz orbitals that form a σꢀbonding a1g (doubly
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occupied) orbital and a σꢀantibonding a2u (unoccupied) orbital. The dxz and dyz orbitals form πꢀ
bonding and πꢀantibonding combinations of, respectively, eu and eg symmetry. These four orbitals
are all fully occupied, as are the δꢀbonding (b2g) and δꢀantibonding (b1u) combinations of the dxy
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orbitals. The system is therefore best described as having a single σꢀbond. While the HOMO and
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LUMO are formed by b1u (δ*) and a2u (σ*) orbitals, it is rather the orbitals of e symmetry which
are most important for the modulation of the carbene reactivity (vide infra).33d,eA completely
different electronic structure is obtained for the BiRh(esp)2 precursor complex (Figure 6,
right).13a,14 While this compound also has a diamagnetic closedꢀshell singlet ground state, the
bonding involves Biꢀ6p as well as Rhꢀ4d orbitals, of which, the Biꢀ6p orbitals are considerably
higher in energy. Thus, the bonding situation is described by lower energy molecular orbitals of
mainly Rhꢀ4d character and higher energy orbitals of mainly Biꢀ6p character which remain
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empty. The strongest BiꢀRh interaction is between the Rhꢀ4dz and the Biꢀ6pz orbitals that leads
2
to a σꢀbonding MO of a1 symmetry and a σꢀantibonding MO, also of a1 symmetry. In analogy to
Rh2(esp)2, this antibonding orbital is the LUMO of this complex while the HOMO is the Rhꢀdxy
based molecular orbital. The Biꢀ6px and 6py orbitals form a set of eꢀsymmetry orbitals that is high
in energy due to strong σꢀantibonding interactions with the orbitals of the carboxylate ligands.
Although there would be the potential for these to interact with the Rh based orbitals of e
symmetry, our calculations show that there is only 2.6% Bi character mixed into the Rhꢀ4dxz and
4dyz MOs. Although BiꢀRh πꢀinteractions are possible, they remain very weak for energetic
reasons.34
Taken together, these results show that while Rh2(esp)2 and BiRh(esp)2 both feature a single
metalꢀmetal σꢀbond and have analogous HOMOs and LUMOs, their electronic structures are
indeed quite different.35 In Rh2(esp)2, the two dxz and dyz sets have a strong πꢀinteraction that
significantly destabilizes the antibonding eg component whereas in BiRh(esp)2, the Rh 4dxz and
4dyz are essentially nonꢀbonding. Consequently, these orbitals are significantly higher in energy
for Rh2(esp)2 (e1g = −4.85 eV) than for BiRh(esp)2 (E = −5.65 eV). The difference in energy of
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the πꢀorbitals (HOMOꢀ1 in both compounds) plays a keyꢀrole for the interaction of the both
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Rh2(esp)2C(pꢀMeOPh)2 (19)
BiRh(esp)2C(pꢀMeOPh)2 (20)
Figure 7. Molecular orbital scheme for Rh2(esp)2C(pꢀMeOPh)2 (19, left) and BiRh(esp)2C(pꢀMeOPh)2
(20, right) with symmetry labels under C2v symmetry. The ligand structures have been truncated for
purposes of clarity.
It is wellꢀknown that carbenes bind to metals by means of σꢀdonor and πꢀacceptor interactions.36
The latter interaction is of keyꢀimportance for modulating the electrophilicity of the bound
carbene. In fact, extended backꢀdonation decreases the electrophilicity of the carbene. In the
present case, symmetry dictates that the carbene π∗−LUMO interacts with the Rhꢀ4dxz and dyz
orbitals. Based on the discussion above, it is evident that this interaction must be much larger in
the dirhodium carbene complex 19 than in the bismuthꢀrhodium analogue 20. These expectations
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are fully confirmed by the DFT calculations (Figure 7). Upon interaction with the carbene, the
degenerate Rhꢀ4dxz and dyz orbitals rehybridize and lose their degeneracy. The extent of this
energy splitting correlates with the strength of the backꢀbonding interaction. Our calculations
indicate that for 19, the metalꢀbased dxz and dyz orbitals (2b1, 2b2) split by 0.22 eV and take on
3.4% and 0% carbene character, respectively. In the BiꢀRh analogue (20), the corresponding
orbitals constituting the (1b1, 1b2) set split by only 0.18 eV and have 0.5% and 0.2% carbene
character, respectively.
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The stronger Rh to carbene backꢀbonding in 19 has a number of consequences. First of all, the
strength of the Rhꢀcarbene bond in 19 is higher than in the BiꢀRh analogue 20. This is reflected
both in the shorter RhꢀC distance (19: 2.022 Å; 20: 2.139 Å), in the much larger calculated force
constant of the RhꢀC bond (19: 1.35 mDynÅꢀ1; 20: 0.89 mDynÅꢀ1) and in the larger Rhꢀcarbene
bond dissociation energy (19: 33.1 kcal/mol ; 20: 7.8 kcal/mol; both values are corrected for the
basis set superposition error (BSSE)). Secondly, the antibonding component of the Rhꢀcarbene πꢀ
bond (LUMO) that is mostly carbene in character will be more destabilized in the dirhodium
complex 19 compared to the bismuthꢀrhodium analogue 20.
Connection to Spectroscopy. The electronic structure discussion above is based on a
molecular orbital picture and is therefore qualitative in nature. It is illuminating to investigate
how these electronic structure relationships are reflected in the spectroscopic properties of the
respective compounds. Spectroscopy both corroborates the accuracy of the calculated electronic
structures and provides experimental information about the nature of the metalꢀcarbene bonds.
These insights are readily transferable to a range of metalꢀcarbene systems and will help to
develop a semiꢀquantitative relationship between spectroscopic properties and chemical
reactivity. In this work, we have studied the optical absorption (UVꢀVis) and resonance Raman
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signatures of the precursors Rh2(esp)2 and BiRh(esp)2 as well as the carbene complexes 19 and
20. Since complexes 19 and 20 could not be isolated in pure form, they were generated inꢀsitu
(vide supra) and an approximate deconvolution was applied, utilizing further information from
NMR studies (see SI for full details). The resulting spectra (Figure 8c and 8f) were subjected to a
Gaussian deconvolution.
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Figure 8. Overview of the absorption spectra recorded at −45°C of (a) a mixture of 19, Rh2(esp)2 and 21
(starting concentration of Rh2(esp)2: 20 ꢀM in CH2Cl2), (b) Rh2(esp)2 (in CH2Cl2). (c) Estimated spectrum
of complex 19 along with a Gaussian deconvolution of all structural features; (d) mixture of 20,
BiRh(esp)2 and 21 (starting concentration of BiRh(esp)2: 20 ꢀM in CH2Cl2), (e) BiRh(esp)2 (in CH2Cl2),
(f) estimated spectrum of complex 20 along with a Gaussian deconvolution of all structural features.
Both Rh2(esp)2 and BiRh(esp)2 feature a number of weak absorption features in the visible
region between 10,000 cmꢀ1 and 30,000 cmꢀ1 and moderately strong bands in the nearꢀUV region
between 30,000 cmꢀ1 and 50,000 cmꢀ1 (Figure 8b and 8e). Detailed assignment of these bands in
terms of manyꢀparticle states and elementary oneꢀelectron excitations are contained in the SI.
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Here, we only mention in passing that the strongest bands (bands 3 and 5) for Rh2(esp)2 are
assigned to a carboxylate to rhodium (LMCT) transition. The analogous bands (3 and 4) for
BiRh(esp)2 are best described as RhꢀtoꢀBi charge transfer transitions. It is striking that upon
binding of the carbene ligand, the absorption features intensify significantly and must therefore
be connected to the formation of the Rhꢀcarbene bond. In fact, our assignments suggest that all of
the intense features in both spectra originate from Rh to carbene metalꢀtoꢀligand (MLCT) charge
transfer transitions. Of particular interest is band 3 in both compounds, which, according to our
assignment, arises from the bondingꢀtoꢀantibonding Rhꢀ4dxz to carbene π* excitation. Both the
position and the intensity of this transition carry chemically relevant information. In fact, a
simple model37 shows that the transition dipole moment of such a bondingꢀtoꢀantibonding
transition is proportional to
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(1)
In this equation, RML is proportional to the metalꢀcarbene distance and α2 is a measure of bond
covalency, for which a perfectly covalent bond has α2 = 1/2 and maximizes fosc. The fact that the
absorption band 3 is more intense for complex 19 than for complex 20 is an experimental
reflection of the higher covalency and greater degree of metalꢀligand backꢀbonding in the
dirhodium carbene 19 than in the bismuthꢀrhodium carbene 20. Secondly, the shift of the band
from 19 to higher energies in 20 is a reflection of the energetic differences in the b1 symmetry
orbitals which are most relevant for π backꢀbonding interactions.
The correlations can be further probed by resonance Raman (rR) spectroscopy. The rR effect
arises from laser excitation into an absorption band. Analysis of the inelastically scattered
radiation provides a selective vibrational signature of the chromophore in the sense that
vibrations that are localized at the absorbing unit are selectively enhanced by several orders of
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magnitude relative to the standard nonꢀresonance Raman effect. In fact, the dominant intensity
mechanism involves the enhancement of vibrations along normal modes which distort in the
excited state relative to the ground state. This happens most prominently if a bond is stretched in
the excited state. This is the case if bonding orbitals are depopulated and antibonding orbitals are
populated as a consequence of photon absorption. Experimental rR spectra of 19 and 20 are
shown in Figure 9. They show a rich spectrum of features that are analyzed in detail in the SI.
Our assignment relies on detailed calculation of the rR spectra by TDꢀDFT calculation according
to the methods developed in reference 37. We focus here on the region between 500 cmꢀ1 and
1000 cmꢀ1 since the metalꢀcarbene vibrations are found in this area (Figure 10). The assignments
indicate that for 20, the BiꢀRh stretching frequency contributes to modes calculated at 177 cm−1
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and 251 cm−1 (experimentally found at 193 cm−1 and 255 cmꢀ1, see SI) while RhꢀC frequencies
are found at 759 cm−1 and 853 cm−1 (experimentally found at 795 and 898 cmꢀ1, Figure 10). For
complex 19, the experimental spectrum is less rich, which is indeed well reproduced by the
calculations. The RhꢀRh stretching vibration delocalizes over multiple modes and contributes to
numerous bands between 283 cm−1 and 305 cm−1. More importantly, the calculated vibrations
with RhꢀC character occur at 585 cm−1, 771 cm−1 and 873 cm−1 and these bands are indeed
clearly visible in the experimental spectrum at 597 cm−1, 790 cm−1 and 892 cm−1. The
delocalization of the metalꢀcarbene vibrations makes it difficult to obtain a clearꢀcut experimental
conclusion about the stiffness of the Rhꢀcarbene bond. However, the good agreement between
experiment and theory provides a clear indication that the calculated electronic structure is well
calibrated and gives us confidence in our interrogation of the carbene bonding situation and
evaluation of bond strengths (vide supra).
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Figure 9. Normalized resonance Raman spectra of (a) BiRh(esp)2C(pꢀMeOPh)2 (20) and (b)
Rh2(esp)2C(pꢀMeOPh)2 (19) recorded at different wavelengths.
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Figure 10. Above: Zooms of the normalized resonance Raman spectra of (left) Rh2(esp)2C(pꢀMeOPh)2
(19) (20) and BiRh(esp)2C(pꢀMeOPh)2 (right) recorded at 407 nm (24570 cmꢀ1). Below: TDꢀDFT
calculated rR spectra based on a simulated excitation at the indicated energies. Note that the DFT
excitation energy was chosen such that, according to the assignment, the same excited electronic state is
probed as in the experiment.
Electronic Structure: Summary. The results of our combined experimental and theoretical
investigation can be summarized as follows: In the RhꢀRh precursor complex Rh2(esp)2, the
relevant Rhꢀdonor orbitals are destabilized by a strong metalꢀmetal interaction relative to
BiRh(esp)2. Upon binding of the carbene, this leads to an enhanced metalꢀcarbene backꢀbonding
interaction in 19. The consequence of this backꢀbonding is that the electron density at the carbene
ligand is increased and the carbene π*ꢀorbital is destabilized. Both effects will act in concert to
reduce the electrophilicity of the carbene in the dirhodium core relative to the bismuthꢀrhodium
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analogue. Experimental evidence for the correctness of this bonding picture is obtained by
studying the optical and vibrational spectra of both compounds where the trends in metalꢀcarbene
backꢀbonding are related to the position and intensity of the MLCT absorption bands as well as
the vibrational signature of the metalꢀcarbene stretching vibrations.
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Our MO diagram for complex 19 is qualitatively similar to Berry’s model system
(Rh2(formate)4=CH2)6 but differs in the orbital surface topologies. Both corroborate the
description of a dirhodium carbene complex as having both σ and π threeꢀcenter fourꢀelectron
(3c4e) bonding manifolds. In contrast, bismuthꢀrhodium carbene complexes such as 20 have only
3c4e σꢀbonding interactions, with an essentially localized 2c2e πꢀinteraction between Rh and C
due to the poor overlap of the Bi 6p orbitals. Since 20 is clearly the more electrophilic of the two,
“superelectrophilicity” of M−Rh carbene complexes is therefore largely a result of 3c4e
interactions in the σꢀframework and is diminished by 3c4e πꢀbonding. We believe that the
important distinction that these two effects actually oppose each other has not been made
before.38
Electronic Structure of αꢀDiazoester Derived Carbene Complexes. As DFT reproduced
our experimental spectra of the donorꢀdonor complexes 19 and 20 so accurately, we were
confident to apply the methodology to the EDA derived acceptor carbene complexes
Rh2(esp)2C(H)(COOEt) (22) and BiRh(esp)2C(H)(COOEt) (23) for which strikingly divergent
reactivity was observed (vide supra). Similar physical and electronic structures were calculated as
for the systems with donorꢀdonor substituents described above, with the major differences being
that the acceptorꢀtype carbenes adopt an eclipsed geometry and feature an even more
carbeneꢀcentered LUMO. The conformation is straightforwardly understood, as the carbene
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substituents have no propensity to interact with the carboxylates of the esp ligands and
reorientation to an eclipsed geometry allows for optimized π overlap of the Rh 4d orbitals with
the carboxylate and carbenic p orbitals, in line with previous findings.4c,f This eclipsed geometry
results in the dyz orbitals being orthogonal to the carbene π* orbital and only the dxz orbital is of
appropriate symmetry to provide π backꢀbonding interactions.
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Rh2(esp)2C(H)(COOEt) (22)
BiRh(esp)2C(H)(COOEt) (23)
Figure 11. Frontier orbital structure of (left) Rh2(esp)2C(H)(COOEt) (22) and (right)
BiRh(esp)2C(H)(COOEt) (23). For complex 23, a set of phenylꢀcentered orbitals (HOMO−3 to HOMO−6)
has been omitted for clarity. Accidental degeneracy results in phenyl orbitals appearing to mix with the
Rhꢀdyz (HOMOꢀ2) orbital. A more complete list of orbitals is given in the supporting information.
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As found for the donorꢀdonor carbene systems 19 and 20, the higher energy antibonding dxz
orbital of Rh2(esp)2 interacts much more strongly with the carbene fragment than the
corresponding orbital of BiRh(esp)2. Our calculations show that for complex 22, contributions
from the carbene constitute 19.1% of the rhodium based dxz orbital, whereas for complex 23, this
is only 6.1%. Bismuthꢀrhodium carbene 23 exhibits not only weaker π back bonding than
complex 22, but also a weaker RhꢀC bond, as shown by comparison of the calculated force
constants and Mayer BOs (22: k = 2.19 mDynÅꢀ1, BO = 1.31; 23: k = 1.26 mDynÅꢀ1, BO = 0.80),
and a less delocalized LUMO (Figure 11: 22 (left): 47% C character, 31% RhRh character;
23 (right): 52% C character, 21% BiRh character). Furthermore, QTAIM analysis of the two
systems supports the RhꢀC bond of 22 having a greater degree of π character on account of its
greater ellipticity and density (ε = 0.0839 and ρ = 0.1525) at the RhꢀC bond critical point
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compared to that of 23 (ε = 0.0468 and ρ = 0.1322).39 The calculated RhꢀC bond dissociation
energies for 22 and 23 amount to 38 kcal/mol and 16 kcal/mol, respectively.
The drastically reduced computed force constants and bond dissociation energies for the BiRhꢀ
carbene complexes 20 and 23 suggest that free carbenes might form and intervene, especially in
reactions that require higher temperatures. Therefore, we conducted control experiments in which
EDA was photolytically decomposed and the resulting free carbene intercepted with
trichloroethylene. As outlined above, the expected cyclopropane 14 does form even though the
reaction was plagued by the formation of numerous byꢀproducts and is therefore of little
preparative value. Yet, the observed formation of 14 from the free carbene is consistent with our
calculations, which show that the reaction enthalpy for the cycloaddition of trichloroethylene
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