Experimental and Computational Characterization of the Transition State for C-X Bimetallic Oxidative Addition at a Cu-Fe Reaction Center

Article abstract of DOI:10.1021/acs.organomet.5b00476

The heterobimetallic complex (IPr)Cu-Fp (IPr = N,N′-bis(2,6-diisopropylimidazol-2-ylidene, Fp = FeCp(CO)₂) was identified previously as a nonprecious metal catalyst for C-H borylation. To better understand the nature of the bimetallic reaction pathways operative in this system, we have conducted a thorough mechanistic study of alkyl halide activation by the Cu-Fe heterobimetallic reaction center. Use of cyclopropylmethyl halide substrates as radical clocks established that alkyl halide activation occurs by a two-electron mechanism for alkyl bromides and chlorides but not iodides. Eyring analysis of the activation of benzyl chloride allowed for experimental determination of activation parameters, including a large and negative entropy of activation (ΔS^? = -36 eu). A Hammett study with para-substituted benzyl chlorides revealed a reaction constant of ρ = 1.6, indicating accumulation of negative charge in the transition state on the alkyl halide carbon. The Ru analogue, (IPr)Cu-Rp (Rp = RuCp(CO)₂), was found to react approximately 17-25 times more slowly with selected benzyl chlorides than (IPr)Cu-Fp, indicating that the relative nucleophilicities of the free metal carbonyl anions are predictive of the relative reactivities of their heterobimetallic counterparts. Synthesis and characterization of the new Ag and Au analogues, (IPr)Ag-Fp and (IPr)Au-Fp, are reported along with the observation that these more covalent congeners are significantly less reactive toward alkyl halides. DFT calculations were used to model a transition state for the Cu-Fe reaction, which was identified as stereoinvertive at the alkyl halide carbon. NBO calculations indicate crucial roles played by the CO ligands within the Fp group: they both act as redox noninnocent ligands and also provide structural templating to stabilize the transition state as the metal-metal bond breaks. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00476

Article
pubs.acs.org/Organometallics
Experimental and Computational Characterization of the Transition
State for C−X Bimetallic Oxidative Addition at a Cu−Fe Reaction
Center
Malkanthi K. Karunananda,† Sean R. Parmelee,† Greyson W. Waldhart, and Neal P. Mankad*
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Chicago, Illinois 60607, United States
S
* Supporting Information
ABSTRACT: The heterobimetallic complex (IPr)Cu-Fp (IPr =
N,N′-bis(2,6-diisopropylimidazol-2-ylidene, Fp = FeCp(CO)2)
was identified previously as a nonprecious metal catalyst for C−
H borylation. To better understand the nature of the bimetallic
reaction pathways operative in this system, we have conducted a
thorough mechanistic study of alkyl halide activation by the Cu−
Fe heterobimetallic reaction center. Use of cyclopropylmethyl
halide substrates as radical clocks established that alkyl halide activation occurs by a two-electron mechanism for alkyl bromides
and chlorides but not iodides. Eyring analysis of the activation of benzyl chloride allowed for experimental determination of
activation parameters, including a large and negative entropy of activation (ΔS⧧ = −36 eu). A Hammett study with para-
substituted benzyl chlorides revealed a reaction constant of ρ = 1.6, indicating accumulation of negative charge in the transition
state on the alkyl halide carbon. The Ru analogue, (IPr)Cu-Rp (Rp = RuCp(CO)2), was found to react approximately 17−25
times more slowly with selected benzyl chlorides than (IPr)Cu-Fp, indicating that the relative nucleophilicities of the free metal
carbonyl anions are predictive of the relative reactivities of their heterobimetallic counterparts. Synthesis and characterization of
the new Ag and Au analogues, (IPr)Ag-Fp and (IPr)Au-Fp, are reported along with the observation that these more covalent
congeners are significantly less reactive toward alkyl halides. DFT calculations were used to model a transition state for the Cu−
Fe reaction, which was identified as stereoinvertive at the alkyl halide carbon. NBO calculations indicate crucial roles played by
the CO ligands within the Fp group: they both act as redox noninnocent ligands and also provide structural templating to
stabilize the transition state as the metal−metal bond breaks.
difficult for us to probe experimentally with regard to
mechanistic detail, and so we have been restricted to a
predominantly computational mechanistic study.66 Motivated
to better understand the nature of the bimetallic reaction
pathways available to (IPr)Cu-Fp through experimental data,
we have chosen to examine carefully the mechanism of
carbon−halogen activation by this heterobimetallic species.67
We previously reported a spectroscopic study that elucidated
the redistribution of electron density during methyl iodide
BOA by (IPr)Cu-Fp and related complexes.68 In the current
report, we describe a combined experimental/computational
study of the mechanism for alkyl halide BOA by (IPr)Cu-Fp
and provide insight into its proposed bimetallic transition state.
INTRODUCTION
■
The mechanisms of oxidative addition (OA) and reductive
elimination (RE) at single metal sites have been subject to
intense study, in part due to their versatility in a wide range of
important catalytic transformations. Analogous reactions of
higher nuclearity, for example bimetallic oxidative addition
(BOA) and bimetallic reductive elimination (BRE) at binuclear
sites, are comparatively underexplored mechanistically, as well
as for catalytic applications.1−4 While many studies on BOA
and BRE mechanisms have been reported with precious
metals,5−48 which are also typically capable of conducting
single-site OA/RE chemistry, BOA and BRE pathways become
particularly intriguing when they feature earth-abundant first-
row transition metals not usually associated with single-site
OA/RE independently.49 Mechanistic studies on such bimet-
allic pathways are more limited in number.50−62
Our group recently reported that the heterobimetallic
complex (IPr)Cu-Fp (IPr = N,N′-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene, Fp = FeCp(CO)2) is a catalyst for the C−H
borylation of arenes,63 a reaction more typically conducted
using single-site OA/RE cycling by Ir catalysts.64 The proposed
mechanism for this reaction involves B−H BOA and H−H
BRE processes that are unavailable to the Cu-only and Fe-only
fragments that compose the heterobimetallic catalyst.65
However, these specific BOA and BRE processes have proven
RESULTS AND DISCUSSION
■
Single-Electron versus Two-Electron Pathways. We
previously reported that the complexes (IPr)Cu-Fp and
(IMes)Cu-Fp (IMes = N,N′-bis(2,4,6-trimethylphenyl)-
imidazol-2-ylidene) activate methyl iodide slowly at room
temperature, producing the corresponding (NHC)Cu-I and
Fp-CH3 products cleanly (NHC = N-heterocyclic carbene).67
Received: June 5, 2015
© XXXX American Chemical Society
A
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Article
However, no other alkyl halide substrates were tested.
Subsequent spectroscopic and computational analyses of the
starting point and end point of this methyl iodide reaction
revealed that the Cu and Fe centers participate to a surprisingly
limited extent in the redox changes in the formally BOA
process, whereas the redox noninnocent CO ligands within the
Fp group bear a large portion of the redox changes.68 However,
no information about the pathway for alkyl halide activation
was probed in that study. Before embarking on reaction kinetics
analysis, we first sought to establish that the BOA process
occurs by a two-electron pathway rather than by a pathway
involving radical intermediates. To do this, we examined the
behavior of cyclopropylmethyl halide substrates (Table 1).
1 (52%), 2 (30%), and Fp2 (19%). Clearly, while both Na[Fp]
and (IPr)Cu-Fp activate alkyl iodides using single-electron
pathways, these pathways are distinct. This observation is
important because it indicates that, in general, it is unlikely that
the reactivity of (IPr)Cu-Fp proceeds through unmasking of
the bare [Fp]− anion, but rather it occurs through distinct
bimetallic pathways. Further discussion of this divergence will
be included below.
Reaction Kinetics. Having established that alkyl bromide
and chloride substrates participate in two-electron reaction
pathways, we proceeded to analyze representative reaction
kinetics using benzyl chlorides as substrates. Using the method
of initial rates, a pseudo-first-order rate constant of 2.3(2) ×
10−5 s−1 was measured for the reaction in toluene-d8 at 298 K
between (IPr)Cu-Fp and benzyl chloride, which yielded
(IPr)Cu-Cl and PhCH2Fp as products. An Eyring analysis
over the temperature range 273−373 K gave a linear plot
(Figure 1), indicating activation parameters of ΔH⧧ = 13(2)
Table 1. Reaction Profiles of (IPr)Cu-Fp and Na[Fp] with
a
Radical Clock Substrates
M
X
1
2
Fp2
(IPr)Cu
(IPr)Cu
Na
Br
I
>97%
0%
0%
18%
<3%
30%
0%
52%
0%
Br
I
>97%
52%
Na
19%
a
Conditions: THF, room temperature, overnight (∼18 h).
The reaction between (IPr)Cu-Fp and cyclopropylmethyl
bromide in THF at room temperature produced (IPr)Cu-Br
and the Fp-R species 1, with no evidence for the ring-opened
form 2. This observation indicates that the activation of alkyl
bromide substrates by (IPr)Cu-Fp does not involve any radical
intermediates with lifetimes of 10−8 s or more69 and, likely,
represents a bona fide two-electron reaction (or one with two
single electron transfer events in rapid succession). On the
other hand, the reaction between (IPr)Cu-Fp and cyclo-
propylmethyl iodide produced (IPr)Cu-I along with predom-
inantly Fp2, a signature of Fp• formation, as the major Fe-
containing product (52%). Minor products included a small
(18%) amount of ring-opened 2, a signature of the cyclo-
propylmethyl radical, as well as a complicated mixture of
unidentified species. No evidence of 1 was found. These
observations indicate that the activation of alkyl iodide
substrates by (IPr)Cu-Fp involves radical intermediates,
presumably formed by initial reduction of the alkyl iodide
substrate by the electron-rich Fe center. While alkyl chlorides
were not probed in this manner, we assume that they behave
like alkyl bromides in their reactions with (IPr)Cu-Fp.
It is useful to compare these results with those for the bare
[Fp]− anion, which also is known to activate alkyl halide
substrates readily by an SN2 mechanism.70 Krusic et al. reported
in 1977 that the reaction of Na[Fp] with cyclopropylmethyl
bromide in THF at 0 °C produces a >97:<3 mixture of 1 and 2,
while the same reaction with cyclopropylmethyl iodide
produces a 70:30 mixture of 1 and 2.71 In order to compare
the chemistry of Na[Fp] directly with the chemistry of
(IPr)Cu-Fp, and because the reactivity of (IPr)Cu-Fp at 0 °C
is quite sluggish, we chose to repeat these seminal Na[Fp]
experiments at room temperature (Table 1). In our hands, the
reaction between Na[Fp] and cyclopropylmethyl bromide in
THF at room temperature produced a >97:<3 mixture of 1 and
2. The same reaction with cyclopropylmethyl iodide produced
Figure 1. Eyring plot for the reaction between (IPr)Cu-Fp and benzyl
chloride in toluene-d8 from 273 to 373 K (R2 = 0.821). Each data point
results from a pseudo-first-order rate constant measured by the
method of initial rates.
kcal/mol and ΔS⧧ = −36(7) eu. The resulting ΔG⧧ = 24(3)
kcal/mol calculated at 298 K is consistent with the qualitative
observation that the reaction between (IPr)Cu-Fp and benzyl
chloride is relatively sluggish at room temperature. The large
and negative ΔS⧧ value is consistent with a highly organized
transition state, as indicated by computational studies discussed
below.
A Hammett analysis also was conducted at 298 K using para-
substituted benzyl chloride substrates with various electron-
donating (OMe, Me) and -withdrawing (Cl, CO2Me, CF3, CN)
substituents. A linear free energy correlation emerged (Figure
2), indicating a reaction constant of ρ = 1.6(2) when using
standard σpara substitutent values. This reaction constant
indicates that the reaction between (IPr)Cu-Fp and para-
substituted benzyl chlorides is moderately accelerated by
electron-withdrawing groups, consistent with a buildup of
negative charge on the carbon atom of the C−X bond being
activated.
Lastly, no experimentally determined Eyring activation
parameters or Hammett reaction constants are available for
B
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Table 2. Pseudo-First-Order Rate Constants for Benzyl
a
Chloride Activation
X
M = Fe
M = Ru
CF3
CN
1.6(7) × 10−4 s−1
6(3) × 10−4 s−1
7(2) × 10−6 s−1
3.6(3) × 10−5 s−1
a
Conditions: toluene-d8, 298 K, 10-fold excess of benzyl chloride.
the rate constants as a function of the nucleophilic metal (Fe vs
Ru), the Fp derivative exhibited rate constants 17−25 times
larger than the Rp derivative for both the trifluoromethyl- and
cyano-substituted benzyl chloride substrates. Given the wide
range of nucleophilicities exhibited by metal carbonyl anions
generally (ranging over 8 orders of magnitude),74 the observed
trends in Table 2 are quite similar to the established trends for
[Fp]− versus [Rp]−. We consider this to be an indication that
the known reaction rates of the bare [M]− metal carbonyl
anions can be used to predict relative reactivities of
heterobimetallic (NHC)Cu-[M] complexes with the same
NHC ligand, to within an order of magnitude. The (IPr)Cu-
WCp(CO)3 derivative, whose metal carbonyl anion is 7 × 10−6
times less nucleophilic than [Fp]−, did not react even with p-
cyanobenzyl chloride under these conditions.
Figure 2. Hammett plot from reactions between (IPr)Cu-Fp and para-
substituted benzyl chlorides in toluene-d8 at 298 K (para substituents
as indicated for each data point, ρ = 1.6(2), R2 = 0.905). Each data
point results from a pseudo-first-order rate constant measured by the
method of initial rates.
reactions between [Fp]− and alkyl halides for comparison. This
absence is due in large part to the rapid rates of these reactions,
precluding accurate rate constant measurements. Nonetheless,
we note that these Fe-only reactions with alkyl chlorides and
bromides are thought to occur by an SN2 pathway due to the
results of Krusic et al.’s radical clock experiments71 as well as
Whitesides’s determination of stereochemical course.72,73 This
SN2 reaction, with [Fp]− as the nucleophile and an alkyl halide
as the electrophile, would also be expected to exhibit a large
and negative ΔS⧧ as well as a reaction constant of ρ > 1. The
Whitesides stereochemical probe did not react with either
(IPr)Cu-Fp or (IMes)Cu-Fp, even after long reaction times or
under forcing conditions. While this prevented us from
experimentally determining the stereochemical course of alkyl
halide activation by the (NHC)Cu-Fp complexes, it nonethe-
less further indicates that the (NHC)Cu-Fp complexes are not
simply masked sources of [Fp]− but rather react by distinct
bimetallic pathways. Nonetheless, from the perspective of the
alkyl halide electrophile, the reaction resembles an SN2
displacement, as indicated by both Eyring analysis and
Hammett correlation.
We also were interested in periodic trends with regard to the
group 11 metal in the heterobimetallic pair. To understand this
effect, we synthesized (IPr)Ag-Fp and (IPr)Au-Fp by reaction
of [Fp]− with the corresponding metal chloride precursors. The
solid-state structures (Figure 3) of both species are
Effects of Metal Substitutions. The [Fp]− anion is one of
many metal carbonyl anions whose relative rates of nucleophilic
reaction are well established.74 We are tempted to use these
reported values as predictors for the relative reactivity of
different (NHC)Cu-[M] complexes toward electrophilic
substrates ([M] = metal carbonyl anion), but this approach
has not been validated experimentally. As a test case, we sought
to examine the relative reactivity of (IPr)Cu-Fp versus (IPr)Cu-
Rp (Rp = RuCp(CO)2). Specifically, [Fp]− is known to be ∼9
times more nucleophilic than [Rp]−.74 Does this mean
(IPr)Cu-Fp is ∼9 times more reactive toward alkyl halides
than (IPr)Cu-Rp?
No reaction was observed between (IPr)Cu-Rp and benzyl
chloride at room temperature over several days. However,
benzyl chloride substrates with strong electron-withdrawing
groups in the para position did react rapidly enough to obtain
pseudo-first-order rate constants (Table 2). When comparing
Figure 3. Solid-state structures of (IPr)Ag-Fp (left) and (IPr)Au-Fp
(right) determined by X-ray crystallography. Atoms are depicted as
50% probability thermal ellipsoids. Hydrogen atoms and cocrystallized
toluene molecules have been omitted. Selected bond distances (Å):
Ag(1)−Fe(1), 2.5215(11); Ag(1)−C(33), 2.80(1); Ag(1)−C(34),
2.768(8); Au(1)−Fe(1), 2.5059(7); Au(1)−C(33), 2.808(7); Au(1)−
C(34), 2.780(7).
topologically similar to the known structure of (IPr)Cu-Fp.67
The M−Fe distances in the (IPr)M-Fp series follow the
expected trend down the triad (Cu−Fe, 2.3462(5) Å; Ag−Fe,
2.5215(11) Å; Au−Fe, 2.5059(7) Å) and, when correcting for
the size of the metals, have essentially identical formal shortness
ratios75 (Cu−Fe, 1.004; Ag−Fe, 1.007; Au−Fe, 1.002). While
(IPr)Cu-Fp contains two semibridging carbonyl ligands
C
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Article
according to calculations of their asymmetry parameters,76
(IPr)Ag-Fp has two CO ligands that are borderline cases
between semibridging and terminal, and (IPr)Au-Fp has one
borderline CO and one terminal CO.77
Neither (IPr)Ag-Fp nor (IPr)Au-Fp was observed to react
with benzyl chloride at room temperature, indicating that the
effect of moving down the group 11 triad is to significantly
decrease the rate of alkyl halide BOA. This may be related to
the increasing covalency of M−Fe bonding down the triad that
is expected because of the increasing radial extent of the d-
orbitals and the increasing electronegativity of the group 11
metals. Evidence for this increasing M−Fe covalency can be
found both experimentally and computationally (Table 3).
consistent with the experimental observation of the reaction
occurring at ambient conditions. For comparison, an alternative
mechanism would involve ionization of the Cu−Fe bond to
reveal [Fp]−, which could then react with the alkyl halide by the
known SN2 pathway.70 We have previously calculated that the
[(IMe)Cu]+[Fp]− ion pair is higher in energy by >80 kcal/mol
than neutral (IMe)Cu-Fp in benzene solution, i.e., too high in
energy to be accessible under ambient conditions.66 This fact,
combined with the phenomenological observations detailed
above regarding product profiles, causes us to rule out this
alternative pathway, as it is inconsistent with experimental
observations.
The structure of TS1 (Figure 4) results from approach of
CH3Cl toward a point between Cu and Fe along a trajectory
Table 3. Selected Experimental and Computational
a
Parameters for (NHC)M-Fp (M = Cu, Ag, Au)
experimental νCO
(cm−1
calculated νCO
(cm−1
M−Fe
b
M−Fe
c
complex
)
)
WBI
Δq
(NHC)Cu-
1914, 1849
1916, 1853
1933, 1872
1926, 1875
1936, 1891
1953, 1909
0.39
0.42
0.48
1.58
1.53
1.45
d
Fp
(NHC)Ag-
Fp
(NHC)Au-
Fp
a
NHC = IPr for experimental data and IMe = N,N′-dimethylimidazol-
b
2-ylidene for computational data. Wiberg bond index from NBO
calculations. Calculated difference in natural charge between M and
Fe. Data from ref 67.
c
d
Experimentally, we observe that the νCO values for the [Fp]−
fragment measured by FT-IR increase from (IPr)Cu-Fp to
(IPr)Ag-Fp to (IPr)Au-Fp. The νCO values calculated by DFT
reproduce this trend. The increasing νCO values likely are
indicative of a less electron-rich Fe center as the M−Fe
bonding becomes less ionic and more covalent down the triad.
The Wiberg bond indices from NBO calculations also indicate
an increasing degree of M−Fe covalency down the triad. Lastly,
the M−Fe bond polarity from natural charge calculations also
indicates decreasing ionic character down the triad. We should
also note that the decrease in BOA reaction rate when moving
from Cu to Ag and Au may relate to the less efficient
semibridging M···CO interactions for the heavier congeners, as
the computational studies described below indicate a possible
role for bridging carbonyls in the BOA transition state.
Figure 4. (a) Calculated reaction coordinate diagram for reactants
(IMe)Cu-Fp + CH3Cl to products (IMe)Cu-Cl + Fp-CH3, with
relative free energies given in kcal/mol at 298 K. (b) Structure of TS1.
that bisects one of the Cp(centroid)−Fe−CO angles. The C−
Cl bond is almost fully broken (2.45 Å) relative to the
calculated C−Cl distance in free CH3Cl (1.84 Å). The [CH3]
unit is planar, resembling the stereoinvertive transition-state
structure of a typical SN2 reaction. The Cu−Fe bond is intact
but slightly elongated (2.41 Å) relative to the reactant structure
(2.33 Å). The Cu···CO interaction nearest to the incoming
CH3Cl molecule is elongated (2.94 Å) relative to the reactant
structure (2.57 Å), while the Cu···CO interaction on the face
opposite the incoming CH3Cl molecule is contracted (2.22 Å)
relative to the reactant structure (2.49 Å). Because no new Cu−
CO bond ultimately forms in the products, this latter Cu···CO
contraction in TS1 highlights the templating role played by that
CO ligand, holding the bimetallic transition-state structure
together as the Cu−Fe bond breaks. The Fe−CH3 bond is only
slightly formed (2.85 Å) relative to the calculated distance in
the Fp-CH3 product (2.05 Å). The [CH3] carbon also engages
in a close Cu···C contact (2.19 Å) despite the fact that no new
Cu−C bond forms in the products, indicating that the Cu
center plays an important role in the transition state, as well.
NBO calculations on TS1 were carried out to analyze how
both natural charge distribution and bonding interactions
change during the reaction pathway. For comparative purposes
(see Figure 4a), we also present these data for the reactants and
products.
Computational Transition-State Characterization. In
order to examine the heterobimetallic reaction computationally,
we chose to begin with gas-phase DFT calculations at the
BVP86/LANL2TZ(f) level of theory on the model reaction
between the reactants (IMe)Cu-Fp + CH3Cl to yield products
(IMe)Cu-Cl + Fp-CH3 (IMe = N,N′-dimethylimidazol-2-
ylidene). The overall reaction was calculated to be exergonic
by ΔG298 K = −20.0 kcal/mol. A transition state (TS1) for this
reaction was identified computationally as lying higher in
⧧
energy by ΔG298 K = +26.2 kcal/mol than the individual
reactants. A surprisingly good match was found between the
calculated activation parameters for the (IMe)Cu-Fp + CH3Cl
reaction (ΔH⧧ = +14.6 kcal/mol; ΔS⧧ = −39.0 eu) and the
experimental ones for the (IPr)Cu-Fp + benzyl chloride
reaction (ΔH⧧ = +13(2) kcal/mol; ΔS⧧ = −36(7) eu).
When benzene solvation was included in the calculations, these
energetics differed by only ∼2 kcal/mol compared to the gas-
phase values, and so gas-phase calculations were utilized
subsequently. The calculated energetics of this pathway are
Selected natural charge data are compiled in Table 4.
Consistent with the formally BOA process, the [Fp] fragment is
oxidized during the reaction. As we have noted before,66,68
within the [Fp] fragment the redox chemistry is dominated by
D
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+28.6 kcal/mol), indicating a Hammett constant close to zero.
When conducting the calculations with a different DFT
functional (B3LYP instead of BVP86) or when including
implicit benzene solvation, the absolute values of the energies
changed, but the collective trend remained unchanged. While
this inability to reproduce a kinetic parameter in the BOA
reaction is likely reflective of the various sources of error
associated with quantitatively correlating computational mech-
anisms to experimental rate constants (especially regarding
entropic contributions to highly polarized systems such as this
one),78,79 we are nonetheless confident in the ability of our
model to qualitatively reveal the detailed nature of the BOA
transition state highlighted in the discussion of TS1 above.
Table 4. Calculated Natural Charges along the Reaction
Coordinate
complex
q(IMeCu)
q(Fp)
q(2CO)
q(CH3)
q(Cl)
reactants
TS1
0.62
0.28
0.52
−0.62
−0.28
0.01
0.39
0.48
0.72
0.08
0.10
−0.08
−0.50
−0.52
products
−0.01
the CO ligands (Table 4) and Cp ligand, and the natural charge
on Fe is relatively constant (between q = −1.17 and −1.00)
throughout the reaction pathway. Regarding the CH3Cl
molecule undergoing BOA, there is essentially no change in
natural charge residing on the [CH3] unit. The chloride,
however, experiences dramatic ionization at TS1 and remains
largely anionic in character in the (IMe)Cu-Cl product.
CONCLUSIONS
■
Selected Wiberg and Mayer bond index data are compiled in
Table 5. As expected, the CH3Cl carbon experiences a smooth
In conclusion, the transition state for alkyl halide BOA by
(IPr)Cu-Fp and related complexes has been thoroughly
characterized by both experimental and computational
methods. Cyclopropylmethyl halide substrates were used to
indicate the limitations of assuming a two-electron BOA
pathway without long-lived radical intermediates. Eyring
analysis was used to measure activation parameters for the
reaction between (IPr)Cu-Fp and benzyl chloride, indicating a
large and negative entropy of activation consistent with a highly
organized transition state. Hammett analysis was used to
determine the electronic bias of the reaction pathway,
indicating a buildup of negative charge in the transition state
on the alkyl halide carbon. Substituting Fe for Ru results in an
order of magnitude decrease in reaction rate, consistent with
previous observations for the free [Fp]− and [Rp]− anions
themselves. Substituting Cu for Ag or Au results in a dramatic
decrease in reaction rate, possibly indicating that more covalent
M−Fe bonds are less reactive toward alkyl halide BOA.
Computational analysis revealed important features of the
transition state unavailable from experimental data, including
the dual role of the carbonyl ligands in the [Fp] group both of
acting as redox noninnocent ligands during the BOA reaction
and also of providing structural stability to the transition state
as the metal−metal bond breaks.
Table 5. Selected Wiberg and Mayer Bond Index Values
a
along the Reaction Coordinate
Cu−
Fe−
Cu−C(proximal
Cu−C(distal
b
b
complex
C(methyl) C(methyl)
CO)
CO)
reactants
TS1
0.13/0.13
0.07/0.07
0.15/0.15
0.26/0.26
0.25/0.25
0.31/0.31
0.71/0.71
products
a
b
Values are given as Wiberg/Mayer. Proximal and distal refer to the
position of a CO ligand relative to the incoming CH3Cl molecule.
increase in bonding interaction with the Fe center during the
reaction. Of particular note is that while a Cu−CH3 bond does
not form in the products, there is a significant Cu−CH3
interaction in TS1 of equal magnitude to the Fe−CH3
interaction. This observation indicates that TS1 is a true
bimetallic transition state, where both Cu and Fe interact with
the BOA substrate equally. As expected because the [(IMe)Cu]
and [Fp] fragments are separating in this reaction, the Cu···CO
interaction with the carbonyl group proximal to the incoming
CH3Cl substrate experiences a smooth decrease during the
reaction. Of particular note is the Cu···CO interaction distal to
the incoming CH3Cl molecule. Whereas this semibridging Cu···
CO interaction in (IMe)Cu-Fp ultimately breaks, the
interaction is actually amplified in TS1. In other words, the
distal semibridging CO ligand bridges the Cu−Fe bond more
significantly in the transition state than in either the reactants or
the products. This observation indicates that, in addition to
playing an important role in the redox changes during BOA and
BRE (see above),66,68 the semibridging CO ligands also play a
key structural role, essentially templating the bimetallic
transition state to stabilize it as the metal−metal bond is
breaking.
Lastly, we attempted to study transition states related to TS1
with various para-substituted benzyl chloride substrates in place
of CH3Cl. A similar transition state, TS2-H, was located for the
parent benzyl chloride, with a similar barrier height to that for
CH3Cl (ΔG298 K⧧ = +28.6 kcal/mol) and with similar activation
parameters (ΔH⧧ = +15.8 kcal/mol; ΔS⧧ = −43.0 eu).
Transition states also were located for benzyl chloride
substrates with cyano, methyl, and methoxy substituents in
the para position (TS2-CN, TS2-Me, TS2-OMe). Unlike the
experimentally determined Hammett reaction constant of ρ =
1.6(2), the barrier heights corresponding to TS2-H, TS2-CN,
TS2-Me, and TS2-OMe were very similar (between +27.1 and
EXPERIMENTAL SECTION
■
General Considerations. All reactions and manipulations were
conducted under purified N2 using standard Schlenk line techniques or
in a glovebox. Glassware was oven-dried prior to use. Bulk reaction
solvents were purified using a Glass Contours Solvent System built by
Pure Process Technology, LLC. Deuterated solvents were degassed by
repeated freeze−pump−thaw cycles and stored over activated 3 Å
molecular sieves prior to use. 1H and 13C NMR spectra were recorded
using Bruker Avance 400 MHz or 500 MHz NMR spectrometers.
NMR spectra were recorded at room temperature unless otherwise
indicated, and chemical shifts were referenced to residual solvent
peaks. FT-IR spectra were recorded in a glovebox on powder samples
using a Bruker ALPHA spectrometer fitted with a diamond-ATR
detection unit. Elemental analyses were performed by Midwest
Microlab, LLC, in Indianapolis, IN, USA. Single-crystal X-ray
diffraction studies were performed using a Bruker SMART X2S
benchtop diffractometer fitted with an Oxford Cryostreams desktop
cooler. Solution and refinement were accomplished with the
SHELXTL suite of programs,80 using standard methods,81 and CIF
files are included as Supporting Information. Literature methods were
used to synthesize KFp,82 (IPr)Cu-Fp,67 (IPr)Cu-Rp,77 (IPr)Ag-Cl,83
and (IPr)Au-Cl.84 All other reagents were purchased from commercial
vendors.
Kinetics Measurements. Stock solutions of (IPr)Cu-Fp (16 mM)
and mesitylene (79 mM) were made in toluene-d8. Prior to each
E
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Article
individual experiment, the (IPr)Cu-Fp solution (500 μL) and the
mesitylene solution (100 μL) were combined, and the appropriate
benzyl chloride (10 equiv relative to IPrCuFp) was added. The
resulting solution was mixed, filtered, and transferred to a J. Young
NMR tube. The tube was removed from the glovebox and placed in a
−78 °C bath until ready for use. In order to obtain rate data, the tube
was placed in an NMR probe preset to the appropriate temperature
and allowed to equilibrate. The initial disappearance (over
approximately 2800−5800 s) of (IPr)Cu-Fp was monitored by
following the more downfield isopropyl doublet, the isopropyl septet,
and the cyclopentadienyl singlet (unless peak overlap precluded
monitoring of one or more of these resonances). Early data points
reflective of temperature equilibration or other instrumental artifacts
were deleted manually (typically ∼2% of all data points). The resulting
data were fit to the kinetic model in eq 1, where At is the integrated
area of the (IPr)Cu-Fp resonance of interest (normalized to the
mesitylene standard) at time t, A0 is the initial integrated area of that
resonance, and k is the pseudo-first-order rate constant. Each reported
rate constant results from averaging over each of the resonances listed
above across multiple experimental runs, and error bars were
calculated using standard error propagation. For cases where a large
variance was observed between duplicate runs, we consider
contamination by trace amounts of air and small deviations in sample
concentrations to be the likeliest sources of error.
Computational Methods. All calculations were performed using
Gaussian09, Revision B.01.85 Density functional theory (DFT)
calculations were carried out using a hybrid functional, BVP86,
consisting of Becke’s 1988 gradient-corrected Slater exchange
functional86 combined with the VWNS local electron correlation
functional and Perdew’s 1986 nonlocal electron correlation func-
tional.87 Mixed basis sets were employed for the smaller MeCl system:
the LANL2TZ(f) triple-ζ basis set88−90 with effective core
potential88,91,92 was used for Cu, Ag, Au, and Fe; the LANL2DZ
double-ζ basis set with effective core potential was used for Cl; and the
Gaussian09 internal 6-311+G(d) basis set was used for C, H, N, and
O. All geometry optimizations were performed using LANL2DZ
effective core potentials (ECP) for the larger systems with BnCl and
BnCl derivatives. Single-point electronic energy calculations were then
carried out using larger mixed basis sets: the LANL2TZ(f) triple-ζ
basis set with effective core potential for Cu and Fe, the LANL2DZ
double-ζ basis set with effective core potential for Cl, and the
Gaussian09 internal 6-311+G(d) basis set for C, H, N, and O. In some
cases, effects of solvation by benzene were calculated with the
polarizable continuum model (PCM) using the integral equation
formalism variant (IEFPCM)93 with default settings as implemented in
Gaussian09. The 2,6-diisopropylphenyl groups on the NHC ligands
were truncated to methyl groups in order to minimize computational
time. Vibrational frequency analysis confirmed that all stationary
points were correctly identified either as stable intermediates with zero
imaginary frequencies or transition states with only one imaginary
frequency. Intrinsic reaction coordinate calculations confirmed that
each transition state was situated between its corresponding reactant
and product states on the Born−Oppenheimer potential energy
surface. Natural population analysis was used to determine atomic and
fragment charges, and Wiberg and Mayer bond indices were used to
determine bond orders: both were obtained from NBO v. 3.194
calculations within Gaussian09.
At = A0e−kt
(1)
Preparation of (IPr)Ag-Fp. (IPr)AgCl (0.3670 g, 0.670 mmol, 1
equiv) was dissolved in toluene (10 mL), and KFp (0.2981 g, 1.38
mmol, 2 equiv) was added to the solution. The brown solution was
stirred for 16 h at room temperature. The solution was filtered through
Celite, and the filtrate was evaporated to dryness. The remaining solid
was suspended in pentane (10 mL) and stirred vigorously for 30 min.
The suspension was filtered through a fritted glass filter, and the yellow
solid was washed with pentane (3 × 5 mL) and then dried in vacuo.
Yield: 0.2705 g, 0.402 mmol, 58%. The solid was stored in a glovebox
freezer at −36 °C. X-ray quality crystals were grown by vapor diffusion
ASSOCIATED CONTENT
* Supporting Information
■
S
1
Spectral data, computational output, crystallographic CIF files.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00476.
of pentane into a concentrated toluene solution at −36 °C. H NMR
(500 MHz, C6D6): δ 7.20 (t, 3JHH = 7.8 Hz, 2H, p-CH), 7.08 (d, 3JHH
4
= 7.7 Hz, 4H, m-CH), 6.31 (d, JAgH = 1.1 Hz, 2H, NCH), 4.30 (s,
3
3
Cp), 2.60 (sept., JHH = 6.8 Hz, 4H, CH(CH3)2), 1.45 (d, JHH = 6.7
3
Hz, 12H, CH(CH3)2), 1.09 (d, JHH = 6.9 Hz, 12H, CH(CH3)2).
13C{1H} NMR (125 MHz, C6D6): δ 221.7 (CO), 145.7, 135.0, 130.4,
124.0, 121.9, 76.5 (Cp), 28.7, 24.5, 23.6. Note: Even after 10 000 scans
on a near-saturated solution, the 13C signal for the carbene carbon was
not detected. IR (solid, cm−1): 3148, 3098, 3061, 2961, 2928, 2866,
1915 (νCO), 1852 (νCO), 1457, 1406, 1326, 1106, 1058, 802, 757, 654,
587, 502. Anal. Calcd for C34H41AgFeN2O2: C, 60.64; H, 6.14; N,
4.16. Found: C, 60.74; H, 6.25; N, 4.25.
AUTHOR INFORMATION
Corresponding Author
*E-mail: npm@uic.edu.
■
Author Contributions
†M. K. Karunananda and S. R. Parmelee contributed equally.
Notes
Preparation of (IPr)Au-Fp. (IPr)AuCl (0.0526 g, 0.085 mmol, 1
equiv) was dissolved in toluene (10 mL), and KFp (0.0366 g, 0.169
mmol, 2 equiv) was added to the solution. The brown solution was
stirred at room temperature for 16 h. The solution was filtered through
Celite, and the filtrate was evaporated to dryness. The remaining solid
was suspeneded in pentane (10 mL) and stirred vigorously for 30 min.
The suspension was filtered through a fritted glass filter, and the yellow
solid was washed with pentane (3 × 5 mL) and then dried in vacuo.
Yield: 0.0505 g, 0.0662 mmol, 82%. The solid was stored in a glovebox
freezer at −36 °C. X-ray quality crystals were grown by vapor diffusion
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Funding was provided by the National Science Foundation
(CHE-1362294) and the UIC Department of Chemistry. Dr.
Dan McElheny assisted with NMR spectroscopy. Dr. Frank
́
Vazquez assisted with DFT calculations. Computational
resources were provided by the Extreme Computing cluster
at UIC. Dr. Bernard Santarsiero assisted with X-ray
crystallography analysis. The authors would like to thank the
reviewers of the manuscript for insightful suggestions.
1
of pentane into a concentrated toluene solution at −36 °C. H NMR
(500 MHz, C6D6): δ 7.22 (t, 3JHH = 7.7 Hz, 2H, p-CH), 7.10 (d, 3JHH
= 7.7 Hz, 4H, m-CH), 6.29 (s, 2H, NCH), 4.26 (s, Cp), 2.68 (sept.,
3
3JHH = 6.7 Hz, 4H, CH(CH3)2), 1.57 (d, JHH = 6.9 Hz, 12H,
CH(CH3)2), 1.12 (d, 3JHH = 6.9 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(125 MHz, C6D6): δ 218.4 (CO), 195.4 (NCCu), 146.0, 135.0, 130.6,
124.2, 121.4, 78.7 (Cp), 29.1, 24.5, 24.1. IR (solid, cm−1): 3150, 3102,
3068, 2960, 2929, 2865, 1933 (νCO), 1872 (νCO), 1593, 1468, 1456,
1412, 1326, 1102, 1058, 1012, 816, 802, 758, 747, 650, 601, 582, 547.
Anal. Calcd for C34H41AuFeN2O2: C, 53.56; H, 5.42; N, 3.67. Found:
C, 53.57; H, 5.52; N, 3.68.
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DOI: 10.1021/acs.organomet.5b00476
Organometallics XXXX, XXX, XXX−XXX