Chemistry surrounding monomeric copper(I) methyl, phenyl, anilido, ethoxide, and phenoxide complexes supported by N-heterocyclic carbene ligands: Reactivity consistent with both early and late transition metal systems

Article abstract of DOI:10.1021/ic0611995

Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes [IPr = 1,3-bis(2,6-diisopropylphenyl) imidazol-2-ylidene, SIPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene, IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions of (NHC)Cu^I methyl complexes at temperatures between 100 and 130C produce methane, ethane, and ethylene. The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe = 1,2-bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes is localized primarily on the amido nitrogen with some pπ_anilido- dπ_Cu π*-character. [(IPr)Cu(μ-H)]₂ and (IPr)Cu(Ph) react with aniline to quantitatively produce (IPr)Cu(NHPh)/ dihydrogen and (IPr)Cu(NHPh)/benzene, respectively. Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu(μ-H)]₂ and aniline to (IPr)Cu(NHPh) and dihydrogen is favorable with ΔH ≈ -7 kcal/mol and ΔG ≈ -9 kcal/mol.

Full text of DOI:10.1021/ic0611995

Inorg. Chem. 2006, 45, 9032−9045
Chemistry Surrounding Monomeric Copper(I) Methyl, Phenyl, Anilido,
Ethoxide, and Phenoxide Complexes Supported by N-Heterocyclic
Carbene Ligands: Reactivity Consistent with Both Early and Late
Transition Metal Systems
Laurel A. Goj,† Elizabeth D. Blue,† Samuel A. Delp,† T. Brent Gunnoe,*,† Thomas R. Cundari,§
Aaron W. Pierpont,§ Jeffrey L. Petersen,‡ and Paul D. Boyle†
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
C. Eugene Bennett Department of Chemistry, West Virginia UniVersity, Morgantown, West
Virginia 26506-6045, and Center for AdVanced Scientific Computing and Modeling (CASCaM),
Department of Chemistry, UniVersity of North Texas, Box 305070, Denton, Texas 76203-5070
Received June 30, 2006
Monomeric copper(I) alkyl complexes that possess the N-heterocyclic carbene (NHC) ligands IPr, SIPr, and IMes
[IPr 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, SIPr 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene,
IMes 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] react with amines or alcohols to release alkane and form
the corresponding monomeric copper(I) amido, alkoxide, or aryloxide complexes. Thermal decomposition reactions
of (NHC)CuI methyl complexes at temperatures between 100 and 130
C produce methane, ethane, and ethylene.
The reactions of (NHC)Cu(NHPh) complexes with bromoethane reveal increasing nucleophilic reactivity at the
anilido ligand in the order (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh) < (dtbpe)Cu(NHPh) [dtbpe 1,2-
bis(di-tert-butylphosphino)ethane]. DFT calculations suggest that the HOMO for the series of Cu anilido complexes
is localized primarily on the amido nitrogen with some pπanilido *-character. [(IPr)Cu( -H)]2 and (IPr)Cu(Ph)
react with aniline to quantitatively produce (IPr)Cu(NHPh)/dihydrogen and (IPr)Cu(NHPh)/benzene, respectively.
Analysis of the DFT calculations reveals that the conversion of [(IPr)Cu( -H)]2 and aniline to (IPr)Cu(NHPh) and
)
)
)
°
)
−dπCu
π
µ
µ
dihydrogen is favorable with
∆H
≈ −7 kcal/mol and
∆G
≈ −9 kcal/mol.
Introduction
eroatomic ligands to transition metals in low oxidation states
disrupts ligand-to-metal π-donation and often imparts highly
nucleophilic and basic character to the nondative ligand.3-13
For example, late transition metal systems with nondative
heteroaromatic ligands have been used for catalytic aryl
The study of transition metal complexes with high d-
electron counts that possess nondative heteroaromatic ligands
(e.g., amido, alkoxide, imido, oxo, etc.) has substantially
increased in the past decade.1,2 Interest in such systems is
derived, in part, from the disruption of ligand-to-metal
π-donation that results from filled dπ-manifolds. Nondative
heteroatomic ligands coordinated to transition metals in high
oxidation states can π-donate and form metal-ligand
multiple bonds, which can render the nondative ligands
relatively inert. In contrast, coordination of nondative het-
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brent_gunnoe@ncsu.edu.
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‡ West Virginia University.
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9032 Inorganic Chemistry, Vol. 45, No. 22, 2006
10.1021/ic0611995 CCC: $33.50
© 2006 American Chemical Society
Published on Web 09/27/2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
Chart 1. N-Heterocyclic Carbene Ligands Used to Form CuI Systems
and Generic Depiction of the NHC Ligands
amination,14,15 metal-mediated H-H and C-H bond activa-
tion,5,16-20 heterolytic cleavage of C-H bonds,3,7,21-25 po-
lymerization of anilines, â-lactams, and carbodiimides,8,26,27
catalytic hydration of nitriles,28,29 stoichiometric N-C and
O-C bond formation,4,13,24,25,30,31 and access to low-coordina-
tion numbers and high-spin states (in some cases) with imido
or oxo ligands.32-38
On the basis of a d10 electron count (i.e., filled dπ-orbital
set) and metal-based Lewis acidity, it might be anticipated
that CuI complexes with amido, alkoxide, and related ligands
would be highly reactive; however, the potential for increased
covalent character for the Cu-X bonds (compared with
earlier transition metals) may serve to attenuate reactivity at
the nondative ligand. According to several scales, elemental
Cu possesses substantial electronegativity relative to other
transition metals;39 however, although elemental Cu is more
electronegative than elemental Ru by most scales, CuI is
reported to be less electronegative than RuII using the Pauling
scale for electronegativity (1.9 vs 2.2).40 With the latter
electronegativities, a more polar metal-ligand bond is
expected for CuI compared with that of RuII, a prediction
that is counter to general trends in electronegativity. Hence,
the effects of formal oxidation states complicate any
comparisons between transition metal complexes that are
based on electronegativity differences.
Examples of monomeric and well-defined copper com-
plexes with alkyl, aryl, amido, alkoxide, and aryloxide
ligands are relatively uncommon. Sadighi et al. have recently
reported the isolation and reactivity of a unique two-
coordinate CuI methyl complex with a N-heterocyclic carbene
(NHC) supporting ligation.41 Although more common than
alkyl complexes, structurally characterized monomeric CuI
alkoxides are also quite limited.42-44 To our knowledge, the
isolation of monomeric CuI amido complexes is extremely
rare.45,46
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Cundari, T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982-
7994.
We recently communicated that the reactions of substrates
that possess N-H, O-H, and acidic C-H bonds with CuI
alkyl complexes with the NHC ligand IPr {IPr ) 1,3-bis-
(2,6-diisopropylphenyl)imidazol-2-ylidene} produce mono-
meric CuI anilido, ethoxide, phenoxide, phenylacetylide, and
N-pyrrolyl complexes, which serve as catalysts for the anti-
Markovnikov addition of amines and alcohols to electron-
deficient olefins.46-47 Herein, we report the synthesis and
characterization of a series of CuI methyl, phenyl, anilido,
ethoxide, and phenoxide complexes with the NHC ligands
IPr, IMes [IMes ) 1,3-bis(2,4,6-trimethylphenyl)imidazol-
2-ylidene], and SIPr [SIPr ) 1,3-bis(2,6-diisopropylphenyl)-
imidazolin-2-ylidene], including details of characterization,
solid-state structures, comparative reactivity studies, and DFT
calculations relevant to the observed reactivity.
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Results and Discussion
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Synthesis and Characterization of CuI Methyl and
Phenyl Complexes. Herein, we report a series of monomeric
copper (I) methyl, phenyl, anilido, ethoxide, and phenoxide
complexes that have been formed using three different NHC
ligands (Chart 1). Sadighi et al. previously reported the
synthesis of (IPr)Cu(Me) (1) from (IPr)Cu(OAc) and tri-
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Inorganic Chemistry, Vol. 45, No. 22, 2006 9033

Goj et al.
Scheme 1. Reaction of (NHC)Cu(Me) Complexes with Aniline,
Ethanol, or Phenol to Produce the Corresponding (NHC)Cu(X) (X )
anilido, ethoxide, or phenoxide) Complexes
Figure 1. ORTEP of (SIPr)Cu(NHPh) (9) at 30% probability (selected
hydrogen atoms omitted). Selected bond distances (Å): C2-C3 ) 1.517-
(3), N3-C1 ) 1.334(2), N2-C1 ) 1.347(2), Cu1-C1 ) 1.876(2), Cu1-
N1 ) 1.846(2), N1-C28 ) 1.359(3). Selected bond angles (deg): N2-
C1-Cu1 ) 121.9(1), N3-C1-Cu1 ) 130.8(1), C1-Cu1-N1 ) 175.21(7),
Cu1-N1-C28 ) 127.9(1), N2-C1-N3 ) 107.2(2).
methylaluminum.42 We used the same methodology to
synthesize the analogous monomeric CuI complexes (SIPr)-
Cu(Me) (2) and (IMes)Cu(Me) (3) (eq 1).
synthesized by metathesis of (NHC)Cu(Cl) with LiNHPh.46
Complex (IMes)Cu(OEt) (13) could not be isolated in the
solid-state and is only stable in solution in the presence of a
slight excess of free EtOH (see Experimental Section for
details).
Solid-State Structures of CuI Anilido Complexes. The
solid-state structures of 6 and (dtbpe)Cu(NHPh) [dtbpe )
1,2-bis(di-tert-butylphosphino)ethane] have been reported.45,46
Single-crystals of 9 suitable for X-ray diffraction (XRD)
study were obtained to compare the structures of the copper
anilido complexes. Figure 1 depicts an ORTEP of 9, and
Table 1 provides crystallographic data and collection pa-
rameters for all structures reported herein. Both (NHC)Cu-
(NHPh) complexes 6 and 9 exhibit a linear geometry with
C1-Cu1-N1 bond angles of approximately 175°. The plane
of the anilido phenyl is approximately coplanar with the
H-N-Cphenyl plane, which optimizes delocalization of the
amido lone pair into the phenyl π*-system (see discussion
below). This geometric feature is consistent for all three CuI
anilido complexes (Figure 2). The Cu1-N1 bond distances
of complexes 6 and 9 are statistically identical at 1.841(2)
and 1.846(2) Å, respectively. Thus, the saturated backbone
of the SIPr ligand, compared with the unsaturated IPr
backbone, does not appear to substantially impact Cu-Namido
bonding. The Cu-Namido bond distances are also shorter than,
but comparable to, the Cu-Namido bond distance of 1.890-
(6) Å for (dtbpe)Cu(NHPh).45,46 The Namido-Cipso bond
distances of complexes 6, 9, and (dtbpe)Cu(NHPh) are
statistically identical at 1.351(4), 1.359(3), and 1.354(9) Å,
respectively. These bond distances are shorter than a typical
N-C single bond (∼1.47 Å) and the corresponding bond
distance of the amine complex [(dtbpe)Cu(NH2Ph)][PF6],
which is 1.444(4) Å.45 Thus, the solid-state Namido-Cipso bond
distances of the CuI anilido complexes are consistent with
some N-C multiple bonding.
In addition, complex 3 is cleanly produced upon reaction of
(IMes)Cu(Cl) and MeLi. The phenyl complexes, (IPr)Cu-
(Ph) (4) and (SIPr)Cu(Ph) (5), are formed from the reaction
of the corresponding (NHC)Cu(Cl) complex and Ph2Mg (eq
2).
1H NMR spectroscopy reveals that 5 is formed with a minor
intractable impurity in an approximately 4:1 ratio. Although
the NMR spectra reveal pure material, we were not able to
obtain satisfactory elemental analysis of the phenyl complex
4. Similar difficulties have been reported for the (NHC)Cu-
(Me) systems.42,48
Synthesis and Characterization of CuI Anilido, Ethox-
ide, and Phenoxide Complexes. Recently, we reported the
syntheses and characterization of (IPr)Cu(NHPh) (6), (IPr)-
Cu(OEt) (7), and (IPr)Cu(OPh) (8) upon reaction of complex
1 with aniline, ethanol, and phenol, respectively.46 Similar
procedures were used to prepare and isolate the series of
complexes (SIPr)Cu(X) [X ) NHPh (9), OEt (10), and OPh
(11)] and (IMes)Cu(X) [X ) NHPh (12), OEt (13), and OPh
(14)] (Scheme 1). In addition, the anilido complexes can be
Variable-Temperature NMR of (IPr)Cu(NHPh) (6).
The ability of the aryl π*-system to delocalize electron
density from the amido lone pair is likely a contributing
factor for the relatively high abundance of aryl amido
complexes, compared with that of the parent and alkyl amido
systems. Hindered bond rotations about the Namido-Cipso
(48) Goj, L. A.; Blue, E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.;
Petersen, J. L. Organometallics 2006, 25, 4097-4104.
9034 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
Figure 2. Depiction of orientation of phenyl substituents of anilido ligands of (SIPr)Cu(NHPh) (9) (left), (IPr)Cu(NHPh) (6) (middle), and (dtbpe)Cu-
(NHPh) (right) with most atoms removed (carbon and hydrogen are gray, nitrogen is blue, copper is orange, and phosphorus is pink). The structures of the
latter two complexes have been previously reported.45,46
Table 1. Selected Crystallographic Data and Collection Parameters for Complexes (SIPr)Cu(NHPh) (9), (SIPr)Cu(OEt) (10), (SIPr)Cu(OPh) (11), and
(IMes)Cu(OPh) (14)
9
10
11
14
empirical formula
fw
C33H44CuN3
546.25
C29H43CuN2O
499.19
C38.25H49CuN2O
616.33
C30H25CuN2O
500.14
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
orthorhombic
P212121
11.932(1)
12.545(1)
20.706(2)
monoclinic
Cc
17.434(3)
12.791(2)
13.284(2)
triclinic
P1h
monoclinic
C2/c
16.8260(9)
23.1927(1)
15.4484(1)
12.115(2)
14.279(2)
22.971(4)
77.627(3)
86.060(3)
71.445(3)
3680(1)
90.0
93.200(4)
120.164(2)
3099.3(4)
2957.6(9)
5212.3(6)
Z
D
4
4
4
8
calcd (g cm-3
)
1.171
1.121
1.113
1.275
cryst size (mm)
R1, wR2 {I>2σ(I)}a
GOFa
0.15 × 0.20 × 0.50
0.0408, 0.0931
1.019
0.16 × 0.22 × 0.30
0.0649, 0.1311
0.924
0.14 × 0.20 × 0.40
0.0677, 0.1455
0.716
0.30 × 0.27 × 0.07
0.044, 0.056
1.75
a For definitions of wR2 and GOF see Supporting Information.
bonds of the aryl amido complexes have been observed,22,49-51
and partial Namido-Cipso multiple bonding caused by electron
delocalization may contribute in some cases to the activation
barriers for N-C bond rotation. Support for this delocaliza-
tion can also be found in the DFT-optimized geometries of
NHC-Cu-anilido (NHC ) IPr, SIPr, and IMes), which
revealed calculated Namido-Cipso bond distances of 1.38 Å,
or approximately 0.04 Å shorter than aniline-optimized at
Table 2. Rate Constants and Calculated Gibbs Free Energy of
Activation for Rotation about NAmido-CIpso Bond of (IPr)Cu(NHPh) (6)
temp (°C)
k (s-1
)
∆Gq (kcal/mol)
0
-10
-20
-40
8.7 × 103
2.9 × 103
1.5 × 103
6.4 × 102
11.0
11.2
11.0
10.5
Solid-State Structures of CuI Ethoxide and Phenoxide
Complexes. We have previously reported details of the solid-
state structure of (IPr)Cu(OEt) (7).46 Single-crystals of 10
suitable for an XRD study were obtained (Figure 4). Both
(NHC)Cu(OEt) complexes 7 and 10 exhibit a nearly linear
geometry with C1-Cu1-O1 bond angles of approximately
176°. The Cu1-O1 bond distances of complexes 7 and 10
are statistically identical at 1.799(3) and 1.793(7) Å, respec-
tively.46 The solid-state structures of two monomeric CuI
alkoxide complexes have been reported. The Cu-O bond
lengths for Cu{OCH(CF3)2}(PPh3) and (IPr)Cu(OtBu) are
2.087(6) and 1.8104(13) Å, respectively.42,44 The longer bond
distance of Cu{OCH(CF3)2}(PPh3) is likely a result of the
electron-withdrawing perfluoromethyl groups.
The solid-state structure of (IPr)Cu(OPh) (8) has been
previously reported,46 and herein, we report the structures
of complexes 11 and 14 (Figures 5 and 6). Complex 11
contained two independent molecules in the crystallographic
asymmetric cell. All three copper phenoxide complexes
exhibit a linear geometry with C1-Cu1-O1 bond angles
ranging from 173° to 178°. The Cu1-O1 bond distances of
complexes 8, 11, and 14 are similar at 1.839(2), 1.831(3),
and 1.833(1) Å, respectively.46 These bond distances are
shorter than the Cu-O bond length of 1.917(3) Å reported
for the complex [(p-MeC6H4NC)2Cu(2,6-tBu2C6H3O)].52
the same level of theory, as well as the experimental Namido
-
Cipso bond distances of ∼1.35 Å (see above). Variable-
temperature 1H NMR spectroscopy in toluene-d8 (22 to -80
°C) was used to study the dynamics of complex 6. At room
temperature, a single time-averaged resonance is observed
because of the ortho position of the amido phenyl. At reduced
temperatures, this resonance broadens and decoalesces into
two doublets, which resonate at 6.26 and 5.33 ppm in the
slow-exchange regime, which is accessed at -70 °C. The
observed changes are consistent with slow rotation (at lower
temperatures) about the Namido-Cipso bond relative to the
NMR time scale. No evidence for slow rotation (on the NMR
time scale) about the Cu-Namido bond is observed at
temperatures down to -80 °C. The rate of Namido-Cipso bond
rotation has been calculated at the coalescence temperature
(-40 °C, k ) 6.4 × 102 s-1) and in the fast-exchange regime
using line broadening. Table 2 depicts rate constants and
calculated ∆Gq values. An Eyring plot of these data reveals
∆Ηq ) 7 kcal/mol and ∆Sq ) -14 eu (Figure 3).
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1864.
(50) Albe´niz, A. C.; Calle, V.; Espinet, P.; Go´mez, S. Inorg. Chem. 2001,
40, 4211-4216 and references therein.
(51) Jayaprakash, K. N.; Gunnoe, T. B.; Boyle, P. D. Inorg. Chem. 2001,
40, 6481-6486.
Inorganic Chemistry, Vol. 45, No. 22, 2006 9035

Goj et al.
Figure 3. Eyring plot for Namido-Cipso bond rotation of (IPr)Cu(NHPh)
(6) (R2 ) 0.92).
Figure 6. ORTEP of (IMes)Cu(OPh) (14) at 30% probability (hydrogen
atoms omitted). Selected bond distances (Å): C2-C3 ) 1.352(2), N1-C1
) 1.354(2), N2-C1 ) 1.366(2), Cu1-C1 ) 1.864(2), Cu1-O1 ) 1.833-
(1), O1-C22 ) 1.344(2). Selected bond angles (deg): N1-C1-Cu1 )
130.2(1), N2-C1-Cu1 ) 125.5(1), C1-Cu1-O1 ) 175.53(6), Cu1-O1-
C22 ) 117.36(6).
Table 3. Cu-CNHC Bond Distances (Å) of (NHC)Cu(X) (NHC ) IPr
or SIPr; X ) NHPh, OEt, or OPh) as a Function of the Identity of Xa
X
Cu-CIPr (Å)
Cu-CSIPr (Å)
Me
NHPh
OEt
1.887(5)
1.875(3)
1.863(5)
1.868(2)
1.876(2)
1.861(8)
1.881(4)
Figure 4. ORTEP of (SIPr)Cu(OEt) (10) at 30% probability (hydrogen
atoms omitted). Selected bond distances (Å): C2-C3 ) 1.531(9), N1-C1
) 1.321(8), N2-C1 ) 1.353(8), Cu1-C1 ) 1.861(8), Cu1-O1 ) 1.793-
(7), O1-C28 ) 1.31(1), C28-C29 ) 1.49(1). Selected bond angles (deg):
N1-C1-Cu1 ) 128.0(5), N2-C1-Cu1 ) 124.6(5), C1-Cu1-O1 )
175.9(4), Cu1-O1-C28 ) 123.3(6), N1-C1-N2 ) 107.4(6).
OPh
a Solid-state structures of (IPr)Cu(Me) and (IPr)Cu(X) have been
previously reported.42,46
Scheme 2. Thermal Decomposition of (NHC)Cu(Me) Complexes
1-3
NMR spectroscopy (Scheme 2). Visual inspection of the
NMR tubes indicated decomposition of the copper complexes
in the form of a black precipitate or pink plating and a
concomitant change in solution from colorless to pale yellow.
The 1H NMR spectra reveal the formation of several
uncharacterized (NHC)Cu systems without the formation of
free or protonated NHC ligands. In addition, three new
singlets at 0.16, 0.80, and 5.26 ppm are observed, which are
consistent with the resonances for methane, ethane, and
ethylene, respectively. The resonance from methane is a
singlet, which is consistent with the formation of CH4 rather
than CH3D. Mass spectrometry was used to analyze the
headspace of the reaction mixture from the thermolysis of
complex 2, and the data confirm the formation of methane,
ethane, and ethylene. Similar to the methyl complexes 1-3,
heating of the C6D6 solutions of the phenyl complexes 4 and
5 to 120 and 130 °C, respectively, results in decomposition;
however, unambiguous identification of the products (organic
or organometallic) was not possible.
Figure 5. ORTEP of one of two independent molecules of (SIPr)Cu-
(OPh) (11) at 30% probability (hydrogen atoms omitted). Selected bond
distances (Å): C2-C3 ) 1.520(5), N1-C1 ) 1.341(5), N2-C1 ) 1.322-
(5), Cu1-C1 ) 1.881(4), Cu1-O1 ) 1.831(3), O1-C28 ) 1.302(5).
Selected bond angles (deg): N1-C1-Cu1 ) 122.6(3), N2-C1-Cu1 )
128.1(3), C1-Cu1-O1 ) 178.3(2), Cu1-O1-C28 ) 121.5(3), N1-C1-
N2 ) 109.3(4).
The effect of the identity of the nondative ligand on the
trans Cu-CNHC bond length of (NHC)Cu(X) (X ) Me,
NHPh, OEt, or OPh) systems was examined (Table 3). The
Cu-CIPr bond distance decreases in the order Me > NHPh
> OEt ≈ OPh. As the electronegativity of the nondative
ligand increases and its σ-donor ability presumably decreases,
the trans Cu-CNHC bond distance decreases.
Thermal Decomposition of CuI Methyl and Phenyl
Complexes. Solutions of 1, 2, and 3 in C6D6 were heated to
temperatures between 100 and 130 °C and monitored by 1H
(52) Fiaschi, P.; Floriani, C.; Pasquali, M.; Chiesa-Villa, A.; Guastini, C.
J. Chem. Soc., Chem. Commun. 1984, 888-890.
9036 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
Scheme 3. Calculated Free Energy Changes (kcal/mol) for C-H
Activation of Benzene by (SIPr)Cu(Me) (2) and (SIPr)Cu(Ph) (5)
One potential pathway for the formation of the organic
products from (NHC)Cu(Me) is the initial bond homolysis
of the Cu-Me linkage leading to the transient formation of
a methyl radical. The heating of solutions of 2 in C6D6 in
the presence of TEMPO (2,2,6,6-tetramethylpiperidinyloxy)
or 1,4-cyclohexadiene leads to the formation of methane,
ethane, and ethylene without additional organic products that
would result from trapping of the methyl radical by TEMPO
or 1,4-cyclohexadiene. Furthermore, heating of solutions of
1 or 2 in toluene-d8, which possesses a weak benzylic
C-H(D) bond and would be anticipated to yield CH3D if
methyl radical were formed, results in no change from
reactions in C6D6. Thus, the thermal decompositions of 1-3
do not likely involve Cu-CMe bond homolysis. The lack of
Cu-R bond homolysis, even at elevated temperatures, is
consistent with calculated Cu-CMe bond dissociation ener-
gies for (NHC)Cu(Me) systems of ∼80 kcal/mol (see below).
Figure 7. B3LYP/6-31G(d) calculated transition states for benzene C-H
activation by (SIPr)Cu(Ph) (5) (top) and (SIPr)Cu(Me) (2) (bottom).
Hydrogen atoms are omitted for clarity except for the active hydrogen
participating in the metathesis reaction; atoms of the NHC and the active
site are shown as spheres with the remainder in wire frame. Views are
along the Cu-CNHC axis (right) and perpendicular to this axis (left).
distributions that are consistent with Cu-C bond homolysis.54
The (NHC)Cu(Me) systems do not undergo Cu-C bond
homolysis nor do they possess â-hydrogen atoms for an
initial â-hydride elimination step. Thus, the thermal decom-
position of (NHC)Cu(Me) complexes likely proceeds by a
mechanism that is distinct from those observed for (Bu3P)-
Cu(R) systems.
The heating of complexes 1-5 in C6D6 (100 °C to 130
°C) does not lead to activation of benzene C-H(D) bonds
to form (NHC)Cu(Ph-d5) and either CH3D or C6H5D. DFT
calculations on full SIPr models were performed to ascertain
the energetics of benzene C-H bond activation by (SIPr)-
Cu(Me) (2) and (SIPr)Cu(Ph) (5) (Scheme 3). We first
assessed the potential radical reactivity of (SIPr)Cu(Me) (2)
and (SIPr)Cu(Ph) (5) through calculation of the Cu-C
homolytic bond dissociation enthalpies (BDEs) to produce
methyl and phenyl radicals, respectively. The B3LYP/6-31G-
(d) computations indicate that the Cu-C bonds possess
substantial bond strength with BDECu-Me ) 80 kcal/mol for
2 and BDECu-Ph ) 95 kcal/mol for 5. The large Cu-C BDEs
are consistent with experimental results that suggest that
Cu-C bond homolysis does not occur, even at elevated
temperatures.
We have recently reported evidence that single-electron
oxidation of (NHC)Cu(R) (R ) Me or Et) complexes to
unobserved CuII cations, [(NHC)Cu(R)]+, results in rapid
reductive elimination of alkane “R2” and formation of
(NHC)Cu(X).48 Mechanistic studies suggest that the elimina-
tion of R2 from CuII occurs via a bimolecular pathway that
does not involve Cu-C bond homolysis; however, in contrast
to the thermal decomposition of CuI complexes, such as
(NHC)Cu(Me), decomposition of the CuII alkyl complexes
does not result in formation of olefins (by 1H NMR
spectroscopy), and methane formation is negligible from
CuII.48 Thus, the decomposition pathways of (NHC)Cu(R)
and [(NHC)Cu(R)]+ appear to be mechanistically distinct,
although neither appears to occur through Cu-Calkyl bond
homolysis. Unfortunately, attempts to follow the kinetics of
the thermal decomposition of (NHC)Cu(Me) systems gave
highly varied results. Whitesides et al. have reported that
the decomposition of (Bu3P)Cu(n-butyl) at 0 °C involves
initial â-hydride elimination, followed by reduction of
starting material by the resulting Cu-hydride complex,53
while [{PhC(Me)2CH2}Cu(PBu3)]n, which lacks a â-hydro-
gen atom, undergoes decomposition at 30 °C with product
The thermodynamics and kinetics for benzene C-H bond
activation via four-centered σ-bond metathesis transition
states (TS) were also calculated at the B3LYP/6-31G(d) level
of theory for full SIPr models. The calculated TSs are shown
in Figure 7. Pertinent calculated thermodynamics (in kcal/
mol) at STP are ∆Hrxn ) -9.1 (Me), 0.0 (Ph) and ∆Grxn
)
-7.9 (Me), 0.0 (Ph). The favorable driving force for Me/Ph
ligand exchange for complex 2 is consistent with the relative
BDEs of Cu-Ph > Cu-Me (by ∼15 kcal/mol), while the
difference in the C-H BDE between CH4 and C6H6 is
(53) Whitesides, G. M.; Stedronsky, E. R.; Casey, C. P.; San Filippo, J.,
(54) Whitesides, G. M.; Panek, E. J.; Stedronsky, E. R. J. Am. Chem. Soc.
1972, 94, 232-239.
Jr. J. Am. Chem. Soc. 1970, 92, 1426-1427.
Inorganic Chemistry, Vol. 45, No. 22, 2006 9037

Goj et al.
bromoethane. The combination of 6 and tBuNC reveals new
resonances (1H NMR spectroscopy) consistent with the
formation of (IPr)Cu(NHPh)(tBuNC) in equilibrium with 6
Scheme 4. Nucleophilic Substitution Reaction of Amido Complexes
with Bromoethane
t
and free BuNC (eq 3).
Table 4. Kinetic Data for the Reaction of Bromoethane with
(IPr)Cu(NHPh) (6), (SIPr)Cu(NHPh) (9), (IMes)Cu(NHPh) (12),
(dtbpe)Cu(NHPh), and TpRu(PMe3)2(NHPh)a
c
k
obs (s-1
)
t1/2
∆Gqd
(dtbpe)Cu(NHPh)
5.5(2) × 10-4
3.3(3) × 10-4
2.4(2) × 10-4
1.1(1) × 10-4
8.6(3) × 10-6
21.0(6)
35(3)
49(4)
109(8)
22e
21.7
22.0
22.2
22.6
29.0
(IMes)Cu(NHPh) (12)
(IPr)Cu(NHPh) (6)
(SIPr)Cu(NHPh) (9)
TpRu(PMe3)2(NHPh)b
The equilibrium constant (Keq) is 0.4 at room temperature.
Even though the three-coordinate complex (IPr)Cu(NHPh)-
(CNtBu) is not isolable, the rate of the reaction with
bromoethane was determined to discern if the coordination
of the isonitrile influences the rate of reaction. The kinetic
studies of 6 and bromoethane in the presence of 2 equiv of
tBuNC (relative to [6]) reveal kobs ) 2.36(9) × 10-4 s-1,
which is within the standard deviation of the rate observed
for 6 and bromoethane in the absence of tBuNC. Therefore,
weak binding of tBuNC does not result in enhanced reactivity
of 6 with bromoethane.
a All reactions at room temperature except TpRu(PMe3)2NHPh, which
was acquired at 80 °C. b Reported in ref 45. c In minutes. d In kilocalories
per mole. e In hours.
approximately 6 kcal/mol {B3LYP/6-31G(d)-calculated BDE-
(Me-H) ) 104.8 kcal/mol, BDE(Ph-H) ) 110.7 kcal/mol}.
The calculated activation barriers for benzene C-H activa-
tion by 2 and 5 are substantial [transition states relative to
separated reactants (SIPr)Cu(R) and benzene] with similar
enthalpies, ∆Hq ) 35.3 (Me/Ph) and 35.1 (Ph/Ph) kcal/mol.
In terms of free energy, there is a more discernible difference
between Me/Ph and Ph/Ph metathesis: ∆Gq ) 45.3 (Me/
Ph) and 46.6 (Ph/Ph) kcal/mol.
Reactivity of Copper(I) Anilido Complexes with Bro-
moethane. We have reported that the anilido complexes
TpRu(PMe3)2NHPh and (dtbpe)Cu(NHPh) undergo apparent
SN2 reactions with bromoethane to produce TpRu(PMe3)2-
Br and (dtbpe)Cu(Br), respectively, as well as ethylaniline.45
The (NHC)Cu(NHPh) [NHC ) IPr (6), SIPr (9), or IMes
(12)] systems undergo analogous reactions. To compare the
impact of the ancillary ligand on nucleophilic reactivity,
kinetic studies were undertaken for the reaction of (IPr)Cu-
(NHPh) (6), (SIPr)Cu(NHPh) (9), and (IMes)Cu(NHPh) (12)
with bromoethane. All three reactions cleanly produce
ethylaniline and a (NHC)Cu product with resonances con-
sistent with (NHC)Cu(Br) (Scheme 4). The rates and half-
lives of these reactions and those of previously reported
complexes are summarized in Table 4.
Comparison of Calculated Electronic Structures of CuI
Anilido Complexes. Geometry optimizations of full experi-
mental models were carried out at the B3LYP/6-31G(d) level
of theory for the CuI anilido complexes (dtbpe)Cu(NHPh),
(IPr)Cu(NHPh) (6), (SIPr)Cu(NHPh) (9), and (IMes)Cu-
(NHPh) (12). For the dtbpe (two molecules in the asymmetric
unit), IPr, and SIPr complexes, direct comparison of theory
and experiment are possible (calculated values are given first
with experimental values in parentheses). (IPr)Cu(NHPh)
(6): Cu-N ) 1.822 Å (1.841 Å), Cu-CNHC ) 1.835 Å
(1.875 Å), N-Cu-CNHC ) 179.2° (174.8°), Cu-N-Cipso
) 126.8° (127.5°). (SIPr)Cu(NHPh) (12): Cu-N ) 1.818
Å (1.846 Å), Cu-CNHC ) 1.833 Å (1.876 Å), N-Cu-CNHC
) 178.5° (175.2°), Cu-N-Cipso ) 124.6° (127.9°). (dtbpe)-
Cu(NHPh): Cu-N ) 1.872 Å (1.890 Å, 1.898 Å), Cu-P )
2.224 and 2.249 Å (2.249-2.298 Å), P-Cu-P ) 95.8° (93.2
and 93.6°), P-Cu-N ) 140.9 and 122.9° (141.0 and 125.6°,
143.3 and 123.0°), Cu-N-Cipso ) 135.4° (134.6 and 133.3°).
The comparative data reveal good agreement between the
calculated and experimental bond lengths and bond angles.
At the obtained minima, pertinent molecular and electronic
structural parameters were calculated at the same B3LYP/
6-31G(d) level of theory, and these data are organized in
Table 5. As anticipated, it is apparent that the N-heterocyclic
carbene complexes are more similar to each other and thus
distinct from the bulky, chelating bisphosphine complex
(dtbpe)Cu(NHPh). The Cu-N bond is longer by ∼0.05 Å
for the dtbpe complex than for the NHC derivatives,
commensurate with the experimentally measured bond length
differences. The calculated Cu-Namido bond energies are ∼7
kcal/mol stronger for the NHC complexes than for (dtbpe)-
Cu(NHPh), regardless of the geometries of the bond-
dissociation products (anilido radical and LCu) being allowed
to relax (the bond-dissociation enthalpies) or not (the so-
called “snap” bond energies). The calculated BDEs (Cu-
The copper(I) anilido complexes undergo significantly
more rapid reaction with bromoethane than the TpRu anilido
complex. For the (NHC)Cu(NHPh) complexes, the reactivity
decreases in the order IMes > IPr > SIPr (Chart 2). The
difference in rate between 6 and 12 is likely the result of
steric effects. For complex 9, the saturated backbone of the
SIPr ligand decreases the rate of the nucleophilic reaction
by approximately 2-fold relative to that of the IPr analogue
6.
We postulated that the increased electron density of the
three-coordinate (dtbpe)Cu(NHPh) system might accelerate
the nucleophilic reactivity compared with the two-coordinate
(NHC)Cu(NHPh) systems. To test this possibility, we
t
attempted to coordinate the Lewis base BuNC to complex
6 and probe the impact on the rate of reaction with
9038 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
Chart 2. Order of Reaction Rate for the Conversion of Anilido Complexes and Bromoethane to the Metal-Bromide and Ethylaniline
Table 5. Calculated Molecular and Electronic Structural Properties of
Cu(NHPh). The copper charges suggest that the metal will
be less acidic/electrophilic for the dtbpe complex versus the
CuI-Anilido Complexesa
b
c
c
d
d
Cu-N Cu‚‚‚HR Cu-N-Cipso ꢀHOKSO ꢀLUKSO
qN
qCu
NHC derivatives, a conjecture supported by the calculation
of the energy of the lowest-unoccupied Kohn-Sham orbital
(KS-LUMO), which is higher by ∼0.4 eV for the dtbpe
complex. However, caution is appropriate for the latter point
as the KS-LUMO is a Cu-based orbital for the dtbpe
complex, while the KS-LUMOs possess primarily ligand-
based character for the NHC complexes (see Figure 8). There
is more uniformity in the orbital composition of the KS-
HOMOs, with the DFT calculations indicating that most of
the orbital composition is on the anilido ligand. There is
minimal copper character to the KS-HOMOs, and that which
is found appears to be Cu 3d rather than Cu 4p. Thus,
analysis of DFT calculations reveal the KS-HOMOs to be
primarily an anilido pπ-orbital with slight π*-character from
a Cu dπ-orbital. These results are consistent with the lack
of evidence of hindered Cu-Nanilido bond rotation at tem-
peratures down to -80 °C experimental, which may also
suggest negligible anilido to Cu-4p π-donation for these
complexes. The artificial construction of a Cs symmetric
structure in which the anilido ligand is perpendicular to the
NHC ring to mimic a plausible transition state for Cu-N
rotation yields a stationary point that is only 1 kcal/mol
higher in enthalpy, suggesting a relatively low barrier for
torsion about this bond. In all cases, the KS-HOMOs have
π-delocalization among the anilido nitrogen and the carbons
that compose the phenyl ring. This interaction is presumably
sufficient in strength to keep the anilido nitrogen planar. The
calculation for rotation about the Nanilido-Cipso bond reveals
an enthalpy of activation of 11 kcal/mol, which is similar to
the experimentally determined value of 7 kcal/mol (see
above).
(Å)
(Å)
(deg)
(eV)
(eV)
(au) (au)
IPr
1.822
2.509
2.519
2.522
2.485
126.8
124.8
123.2
135.4
-3.62 -0.55 -0.82 0.25
-3.61 -0.60 -0.84 0.25
-3.68 -0.45 -0.83 0.25
-3.64 -0.14 -0.80 0.05
IMes 1.815
SIPr 1.818
dtbpe 1.872
a Properties calculated at optimized geometries for full ligand models at
the B3LYP/6-31G(d) level of theory. b Distance (Å) from copper to proton
attached to anilido nitrogen. c Energies (eV) of the highest-occupied and
lowest-unoccupied Kohn-Sham orbitals, KS-HOMO and KS-LUMO,
respectively. d Mulliken atomic charges at the anilido nitrogen and copper.
Figure 8. Highest-occupied (left) and lowest-unoccupied (right) Kohn-
Sham orbitals for (dtbpe)Cu(NHPh) (top pair) and (IMes)Cu(NHPh) (bottom
pair). The orbitals for other NHC CuI anilido complexes are similar.
Experimental and Computational Study of Reactions
of (IPr)Cu(NHPh) (6) and Benzene or Dihydrogen. (IPr)-
Cu(NHPh) (6) was separately heated (80 °C) in C6H6 and
C6D6 for 24 h with no reaction. Conversely, (IPr)Cu(Ph) (4)
reacts with aniline in C6D6 to form 6 and benzene (eq 4).
Namido) are substantial at 87.3 (IPr), 88.3 (IMes), 86.7 (SIPr),
and 81.8 kcal/mol (dtbpe).
While the calculated atomic charges always possess some
degree of uncertainty, particularly for transition metal
complexes, the similarity of the systems being studied
provides confidence that the values are reasonable, if not in
magnitude then in direction. All CuI complexes possess a
negative charge at the anilido nitrogen, which implies
nucleophilic character at this site, with the dtbpe complex
being slightly less negative than the NHC counterparts.
Additionally, the Cu is calculated to have a positive charge
of +0.25 for the NHC complexes and +0.05 for (dtbpe)-
Phenyl complex 4 is completely consumed at room temper-
Inorganic Chemistry, Vol. 45, No. 22, 2006 9039

Goj et al.
Scheme 5. DFT Calculated Changes in Enthalpy for Conversion of Transition Metal Hydride Complexes and Aniline to Transition Metal Anilido
Complexes and Dihydrogen
ature after 24 h. If it is assumed that 5% of 4 (relative to 6)
can be detected by 1H NMR spectroscopy, the Keq is greater
than 1000 for the equilibrium between 4/aniline and 6/ben-
zene.
is often disrupted (see above).12,55 Examples of systems that
follow these trends include the conversion of Cp*Ru(PMe3)2-
(NPh2) (Cp* ) pentamethylcyclopentadienyl) and dihydro-
gen to produce free amine and Ru hydride complexes,56
reaction of (PCP)Ru(CO)(NH2) (PCP ) η3-C6H3-2,6-(CH2Pt-
Bu2)2) and dihydrogen to produce free ammonia and Ru
hydride complexes,5 the release of aniline and formation of
a Pt dihydride upon combination of trans-(PEt3)2Pt(H)NHPh
and dihydrogen,57 and the reaction of Cp*2M(H)2 (M ) Zr
or Hf) with ammonia to produce dihydrogen and the
corresponding parent amido complexes.56 Interestingly,
despite evidence that the amido-to-Cu π-interaction is
negligible or weak (see above), the conversion of [(IPr)Cu-
(µ-H)]2 and aniline to (IPr)Cu(NHPh) (6) and dihydrogen is
consistent with the reactivity of early transition metal
complexes.58 The conversion of (IPr)Cu(Ph) (4) and aniline
to (IPr)Cu(NHPh) (6) and benzene also contrasts to reactivity
observed with another late transition metal anilido complex.
(PCP)Ir(H)(Ph) and free NH2Ph are thermally favored over
(PCP)Ir(H)(NHPh) and free C6H6 with Keq ) 105.59 Thus,
the (IPr)CuI fragment demonstrates a thermodynamic pro-
pensity for the formation of a Cu-NHPh bond that is atypical
of late transition metal systems, which seems to suggest a
shift in the relative magnitude of the M-H/M-Nanilido and
M-Cphenyl/M-Nanilido BDEs from established norms. To
probe these results in more details, we used computational
studies for equilibria between metal-hydride/aniline and
metal-anilido/dihydrogen for four systems (Scheme 5).
When complex 6 is placed under 60 psi of dihydrogen in
benzene at ambient temperature, no reaction occurs after 18
h. The anilido complex persists, and neither aniline nor the
previously reported CuI hydride complex [(IPr)Cu(µ-H)]2 is
produced.42 To determine if the kinetics or thermodynamics
is responsible for the lack of reaction between 6 and
dihydrogen, [(IPr)Cu(µ-H)]2 was synthesized according to
the procedure reported by Sadighi et al.6 The reaction of
[(IPr)Cu(µ-H)]2 with aniline rapidly produces gas (presum-
ably H2) and complex 6 at room temperature, as confirmed
1
by H NMR spectroscopy (eq 5).
It has been noted that the thermodynamics of reactions
between transition metal amido complexes and dihydrogen
to form metal hydride complexes and free amine are
dependent upon the identity of the metal center.12 Early
transition metal systems tend to favor transition metal amido/
free dihydrogen, while late transition metal complexes exhibit
a predilection toward transition metal hydride/free amine
formation. This trend has been rationalized by increased
metal-amido bond energies resulting from the efficient
amido-to-metal π-bonding for early transition metal systems
versus late transition metal complexes, for which π-bonding
(55) Connor, J. A. Topics Curr. Chem. 1977, 71, 71-110.
(56) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E.
J. Am. Chem. Soc. 1987, 109, 1444-1456.
(57) Cowan, R. L.; Trogler, W. C. J. Am. Chem. Soc. 1989, 111, 4750-
4761.
(58) Hillhouse, G. L.; Bercaw, J. E. J. Am. Chem. Soc. 1984, 106, 5472-
5478.
(59) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao, J.;
Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644-
13645.
9040 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
The experimental BDE of dihydrogen is 104 kcal/mol and
the N-H BDE of aniline is 92 kcal/mol,60 and thus, ignoring
entropic contributions, if the resulting M-H bond is > 12
kcal/mol stronger than the M-Namido bond, then the equi-
librium will favor the transition metal hydride/amine side
of the reaction. Calculations at the B3LYP/6-31G(d) level
of theory reveal a difference in the BDE values of dihydrogen
and aniline (N-H) of ∼20 kcal/mol. For the equilibrium
between putative monomeric complex (IPr)Cu(H) and free
aniline with complex 6 and free dihydrogen, these calcula-
tions reveal that ∆H ) -19.3 kcal/mol with ∆G ) -17.9
kcal/mol (Scheme 5). If the dimerization energy for [(IPr)-
Cu(µ-H)]2 (i.e., net reaction is 0.5 equiv of the Cu dimer
and aniline converted to 1 equiv of 6 and dihydrogen) is
taken into account, the reaction is less exothermic and
exergonic but still favorable: ∆H ) -6.8 kcal/mol and ∆G
) -9.3 kcal/mol.
A series of B3LYP/CEP-31G(d) calculations of M-H/
free aniline and M-NHPh/free dihydrogen equilibria for
representative early transition metal, late transition metal,
and (NHC)Cu models was performed (Scheme 5). The choice
of an effective core potential approach [CEP-31G(d)] is
necessitated by the heavy metals Zr, Ru, and Pt. Calculations
reproduce the experimental observations [i.e., (NHC)Cu
hydrides (monomer and dimer) favor the anilido complex
as does the early high-valent metal complex Cp2ZrH2]. The
later low-valent Ru and Pt complexes favor the hydride side
of the equilibrium.
Analysis of the metal-anilido and metal-hydride BDEs
is instructive, although these values should be viewed as
approximate given the small hydrogen basis sets being used.
For the Pt and Ru complexes, the calculated BDEs(M-
anilido) are substantially weaker than the corresponding
BDEs(M-H): BDE(Pt-H) ) 72 kcal/mol, BDE(Pt-
anilido) ) 43 kcal/mol, BDE(Ru-H) ) 71 kcal/mol, and
BDE(Ru-anilido) ) 35 kcal/mol. Hence, the considerably
stronger M-H bond versus the M-Nanilido bond counter-
mands the thermodynamic preference for H2 versus aniline.
For Zr and Cu, the M-H and M-Nanilido bond strengths are
comparable, thus pushing the equilibrium to the right given
the 12 kcal/mol (experimental) advantage for BDE(H-H)
over BDE(aniline N-H): BDE(Zr-H) ) 68 kcal/mol, BDE-
(Zr-anilido) ) 67 kcal/mol, and BDE{monomeric (IPr)Cu-
H} ) BDE{(IPr)Cu-Nanilido} ) 87 kcal/mol. The calculated
BDE(M-H) values show much less variance than the BDE-
(M-anilido) values, implying that it is primarily differences
in the latter that dictate the course of the aniline/H2
equilibrium. The difference in M-Nanilido BDEs between Ru,
Pt, and Zr can be rationalized by relatively strong anilido-
to-Zr π-bonding with such a π-bonding interaction nonexist-
ent or negligible for Ru and Pt. Octahedral RuII has no vacant
orbital of π-symmetry and PtII only offers a relatively high-
energy pπ-orbital. Consistent with the differences in π-bond-
ing interactions between ZrIV, PtII, and RuII, calculations
reveal that the Ru and Pt anilido complexes display
significant pyramidal character at the anilido nitrogen, while
the Zr complex is planar at the anilido nitrogen. The
calculated Cu complexes also display a planar anilido ligand;
however, similar to PtII, the only vacant Cu-based atomic
orbital of π-symmetry is a relatively high-energy 4p orbital,
and both computational results suggest negligible amido-to-
Cu π-donation. Thus, the strong Cu-anilido bond is difficult
to rationalize using traditional considerations.
Natural bond-orbital (NBO) calculations show the anilido
nitrogen in the Pt and Ru complexes to be sp3 hybridized,
consistent with the pyramidal geometry at this atom.61 The
calculations reveal that the anilido nitrogen atoms in the Zr
and Cu complexes are planar, which may arise, in part, from
the π-interaction between the anilido N and the phenyl
substituent rather than anilido-to-metal π-donation, as ex-
pected for earlier high-valent metal complexes such as ZrIV.
Although the arguments about hybridization based on planar-
ity versus pyramidal character for amido ligands are tenuous
(e.g., Grotjahn et al. have reported a gas-phase structural
analysis of LiNH2 that suggests a planar nitrogen),62 the fact
that geometry optimization of an (NHC)Cu(NH2) model
yields a pyramidal amido nitrogen suggests that the phenyl
ring may play a role in the planarity of the anilido nitrogen
and, hence, suggest sp2 hybridization. While the Zr orbital
used for the bond to the anilido nitrogen is primarily d in
nature, Cu uses an orbital that is predominantly 4s (89% s
and 10% d by the NBO analysis). Combining the various
pieces of evidence, we suggest the possibility that the
combination of an sp2-hybridized anilido nitrogen with the
larger 4s orbital used for bonding by Cu may result in better
metal-anilido σ-overlap, which would result in relatively
strong Cu-Nanilido bonds. It is also likely that the low-
coordination number enforced by the bulky NHC ligands
also enhances copper-anilido bonding.
Conclusions
Monomeric copper(I) methyl complexes that possess the
N-heterocyclic carbene ligands IPr, SIPr, and IMes react with
aniline, ethanol, or phenol to release methane and form the
corresponding copper(I) anilido, ethoxide, and phenoxide
complexes, and the solid-state structures have confirmed the
monomeric nature of these complexes. The (NHC)CuI
methyl, phenyl, and anilido complexes are quite stable under
an inert atmosphere. The methyl complexes do not decom-
pose until they are heated (in C6D6) to temperatures between
100 and 130 °C. The CuI phenyl systems decompose between
120 and 130 °C, and (IPr)Cu(NHPh) is stable for prolonged
periods of time in C6D6 at temperatures up to 130 °C. The
stability of these systems is likely attributable, at least in
part, to the substantial BDEs (calculated) of ∼80 kcal/mol
for the Cu-Me bond, ∼95 kcal/mol for the Cu-Ph bond,
and ∼87 kcal/mol for the Cu-Nanilido bond.
(61) Weinhold, F.; Landis, C. R. Chem. Educ.: Res. Pract. Eur. 2001, 2,
91-104.
(62) Grotjahn, D. B.; Sheridan, P. M.; Al Jihad, I.; Ziurys, L. M. J. Am.
Chem. Soc. 2001, 123, 5489-5494.
(63) Cope, A. C. J. Am. Chem. Soc. 1935, 57, 2238-2240.
(64) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999,
55, 14523-14534.
(60) Luo, Y.-R. Handbook of Bond Dissociation Energies in Organic
Compounds; CRC Press: Boca Raton, FL, 2003.
Inorganic Chemistry, Vol. 45, No. 22, 2006 9041

Goj et al.
tylammonium hexafluorophosphate as the electrolyte. Tetrabutyl-
ammonium hexafluorophosphate was dried under dynamic vacuum
at 140 °C for 48 h prior to use. All potentials are reported versus
NHE (normal hydrogen electrode) using cobaltocenium hexafluo-
rophosphate as an internal standard. Copper(I) chloride, TEMPO,
MeLi, and 1,4-cyclohexadiene were obtained from commercial
sources and used as received. Copper(I) acetate was purchased from
Strem Chemical and used as received. Diphenylmagnesium was
prepared according to a reported procedure.63 The ligands SIPr and
IMes were prepared according to reported procedures.64 The
synthesis and characterization of (IPr)Cu(NHPh) (6), (IPr)Cu(OEt)
(7), and (IPr)Cu(OPh) (8) have been reported in a preliminary
communication.46 The copper complexes (IPr)Cu(Cl), (IPr)Cu-
(OAc), (IPr)Cu(Me) (1), [(IPr)Cu(µ-H)]2, (SIPr)Cu(Me) (2), and
(IMes)Cu(Me) (3) were prepared according to reported proce-
dures.41,42,46,48 Synthetic and characterization details of (SIPr)Cu-
(Cl), (IMes)Cu(Cl), (IMes)Cu(Me), and (IPr)Cu(Br) are available
in the Supporting Information.
For reaction with bromoethane, which resembles a tradi-
tional SN2 transformation, the CuI anilido complexes have
been shown to undergo reaction more rapidly than a related
RuII anilido complex, and relative kinetics are consistent with
increasing nucleophilicity in the order TpRu(PMe3)2(NHPh)
, (SIPr)Cu(NHPh) < (IPr)Cu(NHPh) < (IMes)Cu(NHPh)
< (dtbpe)Cu(NHPh). Given that the RuII amido complexes
are among the most reactive (i.e., nucleophilic and basic)
transition metal amido complexes, the enhanced reactivity
of the Cu systems compared to Ru suggests that the Cu
complexes might be exploited for a variety of metal-mediated
synthetic processes. Highly nucleophilic reactivity is a
common feature of amido ligands coordinated to late
transition metals in low oxidation states.
(IPr)Cu(NHPh)/free dihydrogen and (IPr)Cu(NHPh)/free
benzene are thermodynamically favored over (IPr)Cu(µ-H)]2/
free NH2Ph and (IPr)Cu(Ph)/free NH2Ph, respectively, which
is in contrast to previously observed reactivity patterns of
late transition metal amido systems and is consistent with
early transition metal systems. Thus, the CuI-anilido com-
plexes exhibit an interesting mixture of features consistent
with both early (likely based on the homolytically strong
Cu-X bonds) and late (likely based on weak Cu-X
π-interaction) transition metal complexes. Calculations sug-
gest that the origin of the thermodynamic preferences is an
unusually strong M-Nanilido BDE. It should be noted that
the (dtbpe)Cu(NHPh) and (NHC)Cu(NHPh) are, to our
knowledge, the only examples of isolable monomeric CuI
amido complexes, and perhaps the calculated large Cu-
Nanilido BDEs are a hallmark of this class of complexes.
Although the source of the apparently large Cu-Nanilido BDE
is not definitively known, the involvement of the Cu 4s
orbital in Cu-N σ-bonding, possible sp2 hybridization of
the anilido nitrogen, and low coordination number of the
Cu systems may contribute.
(IPr)Cu(Ph) (4). Five mL of benzene was added to a round-
bottom flask charged with (IPr)Cu(Cl) (0.100 g, 0.19 mmol) and
diphenylmagnesium (0.056 g, 0.31 mmol). The pale yellow solution
was stirred for 1 h, after which it was filtered through a plug of
Celite. The solvent volume was reduced by approximately half in
vacuo, and hexanes were added to yield a cream precipitate. The
solid was collected by vacuum filtration and dried (0.072 g, 64%).
NMR (C6D6): δ 7.65 (d, J ) 6 Hz, 2H, o-Ph), 7.23 (br t, J ) 7
Hz, 3H, overlap of p-aryl of IPr and Ph), 7.08 (br d, J ) 7 Hz, 3H,
overlap of m-aryl of IPr and Ph), 6.30 (s, 2H, NCH), 2.66 (sept, J
) 6 Hz, 4H, CH(CH3)2), 1.43 (d, J ) 6 Hz, 12H, CH(CH3)2), 1.12
(d, J ) 6 Hz, 12H, CH(CH3)2). 13C NMR (C6D6): δ 186.2 (NCCu),
166.0, 146.2, 141.1, 135.8, 128.9, 126.6, 124.7, 124.6, 122.7 (aryl
of IPr, phenyl and NCH), 29.6 (CH(CH3)2), 25.7 (CH(CH3)2), 24.2
(CH(CH3)2). We were unable to obtain satisfactory elemental
1
analysis of this complex. The H NMR spectrum is provided in
the Supporting Information.
(SIPr)Cu(Ph) (5). Five mL of THF was added to a round-bottom
flask charged with (SIPr)Cu(Cl) (0.130 g, 0.31 mmol) and diphe-
nylmagnesium (0.100 g, 0.40 mmol). The pale yellow solution was
stirred for 18 h and was then filtered through Celite. The solvent
volume was reduced by approximately half in vacuo, and hexanes
were added to yield a tan solid. The solid was collected by vacuum
filtration and dried (0.080 g, 49%). The 1H NMR spectrum reveals
resonances consistent with 5 plus ∼20% of an intractable impurity.
1H NMR (C6D6): δ 7.55 (d, J ) 7 Hz, 2H, o-Ph), 7.20 (t, J ) 8
Hz, 2H, p-aryl of SIPr), 7.08 (d, J ) 7 Hz, 4H, m-aryl of SIPr),
3.17 (s, 4H, NCH), 3.04 (sept, J ) 7 Hz, 4H, CH(CH3)2), 1.49 (d,
J ) 7 Hz, 12H, CH(CH3)2), 1.20 (d, J ) 7 Hz, 12H, CH(CH3)2).
The meta and para protons on the phenyl group are coincidental
with the solvent and SIPr aryl peaks. 13C NMR (C6D6): δ 207.7
(NCCu), 165.4, 147.0, 140.7, 135.4, 130.1, 129.8, 126.3, 116.5,
124.7, 124.6, 124.4 (phenyl on SIPr ligand and phenyl), 53.4 (NCH),
29.2 (CH(CH3)2), 25.7 (CH(CH3)2), 23.8 (CH(CH3)2). The 1H NMR
spectrum is provided in the Supporting Information.
Experimental Section
General Methods. All procedures were performed in a glovebox
under an inert atmosphere of dinitrogen or using standard Schlenk
techniques. The glovebox atmosphere was maintained by periodic
nitrogen purges and monitored by an oxygen analyzer {O2 (g) <
15 ppm for all reactions}. Benzene, toluene, THF, and hexanes
were purified by reflux over sodium, followed by distillation.
Benzene-d6 and toluene-d8 were distilled over sodium, degassed
by three freeze-pump-thaw cycles, and stored over 4 Å molecular
sieves. CDCl3 was distilled over calcium hydride, degassed by three
freeze-pump-thaw cycles, and stored over 4 Å molecular sieves.
All reactions performed on an NMR scale used J-Young NMR tubes
1
that were loaded in a glovebox. H and 13C NMR measurements
were performed on either a Varian Mercury 400 MHz or a Varian
Mercury 300 MHz spectrometer (operating frequencies for 13C
NMR spectra were 100 and 75 MHz, respectively) and referenced
to TMS using resonances from residual protons in the deuterated
solvents or the 13C resonances of the deuterated solvents. IR spectra
were obtained on a Mattson Genesis II spectrometer either as thin
films on a KBr plate or in solution using a KBr solution cell.
Electrochemical experiments were performed under a nitrogen
atmosphere using a BAS Epsilon potentiostat. Cyclic voltammo-
grams were recorded in a standard three-electrode cell from -2.00
to +2.00 V with a glassy carbon working electrode and tetrabu-
(SIPr)Cu(NHPh) (9). Method A. (SIPr)Cu(Me) (0.080 g, 0.17
mmol), benzene (5 mL), and aniline (16 µL, 0.17 mmol) were added
sequentially to a thick-walled glass pressure tube. The solution was
heated for 48 h at 60 °C. The solution was cooled to room
temperature, the volume reduced approximately by half in vacuo,
and hexanes were added to yield a white solid. The solid was
collected and dried (0.060 g, 65%). Method B. (SIPr)Cu(Cl) (0.144
g, 0.29 mmol) was dissolved in benzene (10 mL), and LiNHPh
(0.0276 g, 0.28 mmol) was added. The pale yellow solution was
stirred for 2 h and then filtered through Celite. The benzene volume
9042 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
was reduced by approximately half in vacuo, and hexanes were
added to yield a pale yellow solid (0.0934 g, 61%). Crystals suitable
for a solid-state X-ray diffraction study were grown at room
13C NMR (C6D6): δ 181.8 (NCCu), 161.2 (CuNC), 139.3, 135.9,
134.9, 129.6, 129.0, 121.4, 116.3, 110.4 (aryl of IMes and anilido
ligands and NCH), 21.3 (p-CH3), 17.9 (o-CH3). We were unable
to obtain a satisfactory elemental analysis of this complex. The 1H
NMR spectrum is provided in the Supporting Information.
1
temperature by layering a toluene solution of 9 with pentane. H
NMR (C6D6): δ 7.27 (t, J ) 8 Hz, 2H, p-aryl of SIPr), 7.11 (d, J
) 8 Hz, 4H, m-aryl of SIPr), 6.97 (t, J ) 7 Hz, 2H, m-phenyl of
NHPh), 6.49 (t, J ) 7 Hz, 1H, p-phenyl of NHPh), 5.88 (d, J ) 7
Hz, 2H, o-phenyl of NHPh), 3.17 (s, 4H, NCH), 3.12 (br s, 1H,
NH), 2.98 (sept, J ) 7 Hz, 4H, CH(CH3)2), 1.42 (d, J ) 7 Hz,
12H, CH(CH3)2), 1.18 (d, J ) 7 Hz, 12H, CH(CH3)2). 13C NMR
(C6D6): δ 204.7 (NCCu), 161.4, 147.9, 135.9, 130.6, 129.7, 125.4,
116.8, 111.1, 100.9 (SIPr aryl, anilido phenyl), 54.0 (NCH), 29.8
(CH(CH3)2), 26.2 (CH(CH3)2), 24.6 (CH(CH3)2). We were unable
to obtain a satisfactory elemental analysis of this complex. The 1H
spectrum is provided in the Supporting Information.
(IMes)Cu(OEt) (13). Ethanol (16 µL, 0.28 mmol) was added
to a pressure tube charged with (IMes)Cu(Me) (3) (0.075 g, 0.20
mmol) and 10 mL of benzene. The solution was heated for 1 h at
60 °C. A 0.5 mL aliquot was removed from the pressure tube, and
the nonvolatiles were evaporated with a stream of dinitrogen. The
residue was taken up in C6D6, and a 1H NMR spectrum was
acquired that was consistent with the formation of (IMes)Cu(OEt)
in the presence of free EtOH. At room temperature, resonances
from the Cu-OEt moiety are broad, consistent with a rapid
exchange (on the NMR time scale) between residual free EtOH
and the Cu-OEt ligand. In contrast to the other (NHC)Cu(OEt)
systems, complex 13 is apparently only stable in the presence of
excess ethanol. Complete removal of EtOH, via repeated removal
of nonvolatile material under reduced pressure, results in the
isolation of a solid whose NMR spectra reveal multiple and
intractable (NHC)Cu systems.
(SIPr)Cu(OEt) (10). Ethanol (9 µL, 0.16 mmol) was added to
a pressure tube charged with (SIPr)Cu(Me) (0.075 g, 0.16 mmol)
and 4 mL of benzene. The solution was heated for 1 h at 60 °C.
The solution volume was reduced approximately by half in vacuo,
and hexanes were added to yield a white solid. The solid was
collected by vacuum filtration and dried (0.076 g, 72%). Crystals
were grown by layering a concentrated benzene solution of 10 with
pentane. 1H NMR (C6D6): δ 7.21 (t, J ) 8 Hz, 2H, p-aryl of SIPr),
7.07 (d, J ) 8 Hz, 4H, m-aryl of SIPr), 4.24 (q, J ) 7 Hz, 2H,
OCH2CH3), 3.20 (s, 4H, NCH), 2.99 (sept, J ) 7 Hz, 4H,
CH(CH3)2), 1.47 (d, J ) 6 Hz, 12H, CH(CH3)2), 1.19 (d, J ) 7
Hz, 12H, CH(CH3)2), 1.08 (t, J ) 7 Hz, 3H, OCH2CH3). 13C NMR
(C6D6): δ 205.0 (NCCu), 147.3, 135.9, 130.1, 126.9, 125.0 (aryl
of SIPr), 53.7 (NCH), 29.4 (CH(CH3)2), 25.9 (CH(CH3)2), 24.3 (CH-
(CH3)2). Resonances from Cu-OCH2CH3 were not observed. We
were unable to obtain a satisfactory elemental analysis of this
(IMes)Cu(OPh) (14). Phenol (0.021 g, 0.23 mmol) was added
to a round-bottom flask charged with (IMes)Cu(Me) (0.085 g, 0.22
mmol) and 4 mL of benzene. Evolution of a gas (presumably
methane) was immediately observed. After the mixture was stirred
for approximately 10 min, the solvent volume was reduced by
approximately half in vacuo, and hexanes were added to yield a
white solid. The solid was collected by vacuum filtration and dried
(0.083 g, 81%). Crystals were grown by layering a concentrated
1
benzene solution of 14 with pentane. H NMR (CDCl3): δ 6.90
(s, 2H, NCH), 6.85 (s, 4H, m-aryl of IMes), 6.64 (t, J ) 7.8 Hz,
2H, m-OPh), 6.22 (t, J ) 7.1 Hz, 1H, p-OPh), 5.90 (d, J ) 8.1 Hz,
2H, o-OPh), 2.21 (s, 6H, p-CH3), 1.95 (s, 12H, o-CH3). 13C NMR
(C6D6): δ 181.1 (NCCu), 170.5 (ipso-OPh), 140.1, 136.3, 135.5,
130.3, 129.9, 122.4, 121.1 (aryl of IMes and OPh), 114.4 (NCH),
21.9 (p-CH3), 18.4 (o-CH3). Anal. Calcd for C27H29CuN2O: C,
70.33: H, 6.34: N, 6.08. Found: C, 68.92: H, 6.44: N, 5.61. The
1H NMR spectrum is provided in the Supporting Information.
1
complex. The H NMR spectrum is provided in the Supporting
Information.
(SIPr)Cu(OPh) (11). Phenol (0.015 g, 0.16 mmol) was added
to a round-bottom flask charged with (SIPr)Cu(Me) (0.075 g, 0.16
mmol) and 4 mL of benzene. Evolution of a gas (presumably
methane) was immediately observed. The benzene volume was
reduced by approximately half in vacuo, and hexanes were added
to yield a white solid. The solid was collected by vacuum filtration
and dried (0.075 g, 86%). Crystals were grown by layering a
concentrated toluene solution of 11 with pentane. 1H NMR
(C6D6): δ 7.25 (t, J ) 8 Hz, 2H, p-aryl of SIPr), 7.12-7.07
(overlapping multiplets, 6H, m-OPh and m-aryl of SIPr), 6.68 (t, J
) 7 Hz, 1H, p-OPh), 6.38 (d, J ) 8 Hz, 2H, o-OPh), 3.15 (s, 4H,
NCH), 2.93 (sept, J ) 7 Hz, 4H, CH(CH3)2), 1.35 (d, J ) 7 Hz,
12H, CH(CH3)2), 1.16 (d, J ) 7 Hz, 12H, CH(CH3)2). 13C NMR
(C6D6): δ 205.2 (NCCu), 147.6, 135.7, 130.8, 129.8, 125.5, 120.9
114.3, 112.5 (aryl of SIPr and OPh), 54.0 (NCH), 29.8 (CH(CH3)2),
26.2 (CH(CH3)2), 24.5 (CH(CH3)2). One resonance from the aryl
rings was not observed presumably because of coincidental overlap.
Anal. Calcd for C33H43CuN2O: C, 72.43; H, 7.92; N, 5.12. Found:
C, 72.18; H, 7.81; N, 5.05.
Thermolysis of (NHC)Cu(Me) (2) in C6D6. A representative
procedure is given. A screwcap NMR tube was charged with a
solution of 2 (0.015 g, 0.032 mmol) in C6D6 (0.6 mL) and heated.
The temperature ranged between 60 and 110 °C with the temper-
ature increased in 15-20 °C intervals after 24 h of no observable
transformation. After 70 h at 110 °C, complex 2 was completely
consumed to form new (NHC)Cu complexes and the free organic
substrates methane, ethane, and ethylene. A JEOL HX110 magnetic
sector mass spectrometer (Tokyo, Japan) operating in the EI+
(electron ionization) positive-ion detection mode was used to detect
methane, ethane, and ethylene in the headspace of screwcap NMR
tubes. The resolving power was 1000 R (90% Valley) with a source
temperature of 150 °C, an ionizing voltage of 70 eV, and an
accelerating voltage of 10 kV. The data were calibrated externally
using PFK (perfluorokerosene) ions, and 20 µL of gas was injected
for each experiment.
(IMes)Cu(NHPh) (12). A round-bottom flask was charged with
(IMes)Cu(Cl) (0.050 g, 0.12 mmol) and 5 mL of benzene, and
LiNHPh (0.012 g, 0.12 mmol) was added to the solution. The pale
yellow solution was stirred for 4 h and then filtered through Celite.
The solvent volume was reduced by approximately half in vacuo,
and hexanes were added to yield a pale yellow precipitate. The
solid was collected by vacuum filtration and dried (0.027 g, 47%).
1H NMR (C6D6): δ 7.05 (t, J ) 7 Hz, 2H, m-phenyl of NHPh),
6.72 (s, 4H, m-aryl of IMes), 6.55 (t, J ) 7 Hz, 1H, p-phenyl of
NHPh), 6.25 (d, J ) 7 Hz, 2H, o-phenyl of NHPh) 5.97 (s, 2H,
NCH), 3.38 (bs, 1H, NH), 2.13 (s, 6H, p-CH3), 1.95 (s, 12H, o-CH3).
Thermolysis of (SIPr)Cu(Me) (2) in the Presence of TEMPO
in C6D6. A solution of TEMPO (2,2,6,6-tetramethylpiperidinyloxy)
(0.006 g, 0.038 mmol) and C6D6 (0.2 mL) was added to a J-Young
tube charged with a solution of 2 (0.015 g, 0.032 mmol) in C6D6
(0.4 mL). The solution turned dark immediately. After 24 h at 130
°C, complex 2 was completely consumed to form predominantly
one new copper complex and the free organics methane, ethane,
and ethylene.
Inorganic Chemistry, Vol. 45, No. 22, 2006 9043

Goj et al.
Thermolysis of (SIPr)Cu(Me) (2) and 1,4-Cyclohexadiene in
C6D6. 1,4-Cyclohexadiene (6 µL, 0.064 mmol) was injected into a
J-Young tube charged with a solution of 2 (0.015 g, 0.032 mmol)
in C6D6 (0.6 mL). After 23 h at 130 °C, complex 2 was completely
consumed to form new (NHC)Cu complexes and the free organic
substrates methane, ethane, and ethylene.
was monitored by 1H NMR periodically, and after 48 h, the NMR
remained the same with no reaction and no decomposition of 6.
Reaction of (IPr)Cu(Ph) (4) and Aniline. A solution of 4
(0.0165 g, 0.031 mmol) was dissolved in C6D6 (0.774 g, 9.19 mmol)
with hexamethyldisiloxane (5 µL, 0.024 mmol) as an internal
standard and added to a J-Young NMR tube. Aniline (3.1 µL, 0.034
mmol) was added to the tube, which was then sealed. The solution
was monitored by H NMR, and complete reaction to form 6 and
disappearance of 4 occurred within 24 h; Keq, calculated on the
basis of a >90% conversion, is >1000.
Thermolysis of (NHC)Cu(Me) (1) in Toluene-d8. A representa-
tive procedure is given. A J-Young tube was charged with a solution
of 1 (0.015 g, 0.032 mmol) in toluene-d8 (0.6 mL) and heated. The
temperature ranged between 60 and 110 °C with the temperature
increased in 15-20 °C intervals after 24 h of no observable
transformation. After 70 h at 110 °C, 1 was completely consumed
to form new (NHC)Cu complexes and the free organic substrates
methane, ethane, and ethylene.
1
Reaction of (IPr)Cu(NHPh) (6) and H2. Complex 6 (0.043 g,
0.079 mmol) was dissolved in benzene (15 mL) in a glass high-
pressure tube. The tube was pressurized with H2 to 60 psi for 18 h.
The solution turned slightly brown. The solvent was removed from
1
the solution in vacuo, and a H NMR spectrum (C6D6) was taken
Thermolysis of (NHC)Cu(Ph) (4) in C6D6. The procedure and
observations were identical for 4 and 5. A J-Young tube was
charged with a solution of 4 (0.010 g, 0.018 mmol) in C6D6 (0.6
mL) and heated at 120 °C. After 24 h, complex 4 was >95%
consumed to form new (NHC)Cu complexes. Definitive identifica-
tion of organic products was not possible.
of the residue. The resonances were consistent with the starting
material and a small amount of decomposition, and no resonances
for [IPrCu(µ-H)]2 were observed.
Reaction of [(IPr)Cu(µ-H)]2 and Aniline. The bright yellow
solid [(IPr)Cu(µ-H)]2 (∼0.01 g, 0.022 mmol) was dissolved in C6D6
and added to a J-Young NMR tube. Aniline (∼0.1 mL) was added
to the tube, which was then sealed. Gas evolution was observed,
and the solution was colorless after 10 min. Complete reaction to
form 6 and H2 (4.47 ppm) and disappearance of [(IPr)Cu(µ-H)]2
Kinetics of (NHC)Cu(NHPh) (6) and Bromoethane. A rep-
resentative procedure is given. Bromoethane (73 µL, 0.99 mmol)
was added to a solution of 6 (0.045 g, 0.082 mmol) in C6D6 (2.0
mL) with hexamethylbenzene (0.0031 g, 0.019 mmol) as an internal
standard. The solution was immediately divided between 3 screw-
cap NMR tubes and frozen in an acetone/dry ice slush bath. The
reaction mixtures were each allowed to warm to room temperature,
1
within 10 min was confirmed by H NMR by comparison with
data for 6 and observations of H2 in C6D6.
Computational Methods. All calculations were carried out with
the Gaussian03 package.65 The B3LYP functional (Becke’s three-
parameter hybrid functional66 using the LYP correlation functional
containing both the local and nonlocal terms of Lee, Yang, and
Parr)67 and VWN (Slater local-exchange functional68 and the local
correlation functional of Vosko, Wilk, and Nusair)69 were employed
in conjunction with the 6-31G(d) all-electron basis set, unless
specified otherwise. Closed-shell (diamagnetic) and open-shell
(paramagnetic) species were modeled within the restricted and
unrestricted Kohn-Sham formalisms, respectively. All systems
were fully optimized without symmetry constraint, and analytic
calculations of the energy Hessian were performed to confirm
species as minima and to obtain enthalpies in the gas phase at 1
atm and 298.15 K.
1
and the conversions were followed by H NMR spectroscopy at
room temperature. The integration of the peaks because of the
disappearance of 6 and the appearance of ethyl aniline versus the
internal standard were used to calculate the rate of the reaction
using first-order plots.
Reaction of (IPr)Cu(NHPh) (6) with tert-Butylisocyanide.
Complex 6 (9.4 mg, 0.017 mmol) was dissolved in toluene-d8 (0.7
mL) in a J. Young NMR tube with 2 equiv of tert-butylisocyanide
(4 µL, 0.035 mmol) and hexamethyldisiloxane (2 µL, 0.009 mmol)
1
as an internal standard. A H NMR spectrum was taken at room
temperature. Distinct new resonances appear for the alkyl peaks of
the NHC ligand. The downfield methyl peak for the 2-coordinate
complex has a chemical shift of 1.34 ppm,, and the new peak of
the 3-coordinate complex appears at 1.44 ppm. The solution was
allowed to equilibrate, and Keq ) 0.4 was determined at room
temperature using the integration of the methyl peaks. Variable-
temperature NMR of the solution between 25 and -75 °C
demonstrated that the equilibrium is surprisingly independent of
temperature.
Acknowledgment. The Office of Basic Energy Sciences,
U.S. Department of Energy (DE-FG02-03ER15490), the NSF
(CAREER Award CHE 0238167), and the Alfred P. Sloan
Foundation (Research Fellowship) are acknowledged for
(65) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(66) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377.
Kinetics of (IPr)Cu(NHPh) and Bromoethane with Added
tBuNC. Complex 6 (16.1 mg, 0.030 mmol) was dissolved in C6D6
(0.73 mL) in a J-Young NMR tube with hexamethyldisiloxane (6.2
µL, 0.030 mmol) as an internal standard and 2 equiv of tert-
butylisocyanide (6.7 µL, 0.059 mmol). Bromoethane (12 equiv, 26.3
µL, 0.35 mmol) was added by syringe, and immediately the reaction
was frozen in an acetone/dry ice slush bath. The reaction was then
allowed to warm to room-temperature just before the reaction was
1
monitored at regular intervals by H NMR. The integrals of the
formation of ethyl aniline and disappearance of the amido complex
were recorded versus the internal standard to calculate kobs ) 2.36-
(9) × 10-4 s-1, corresponding to a t1/2 of 49.1 min.
Reaction of (IPr)Cu(NHPh) (6) and Benzene. A solution of 6
(0.0123 g, 0.022 mmol), C6H6 (6 µL, 0.067 mmol), and hexa-
methylbenzene (0.0002 g, 0.017 mmol) in C6D6 (0.7 mL) was sealed
in a J-Young NMR tube and heated to 80 °C for 24 h. The reaction
(67) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785-789.
(68) Kohn, W.; Sham, L. Phys. ReV. 1965, 140, A1133-A1138.
(69) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Chem. 1980, 58, 1200.
9044 Inorganic Chemistry, Vol. 45, No. 22, 2006

Cu(I) Complexes with N-Heterocyclic Carbene Ligands
support of this research. E.D.B. acknowledges support from
the American Association of University Women for an
American Fellowship (Dissertation). The authors would also
like to thank Professor David C. Muddiman and Mr. Matthew
M. Lyndon for assistance with the mass spectrometry
experiments. T.R.C. acknowledges the U.S. Department of
Education for its support of the CASCaM facility. This
research was supported in part by a grant from the Offices
of Basic Energy Sciences, U.S. Department of Energy (Grant
DEFG02-03ER15387). Calculations employed the UNT
computational chemistry resource, for which T.R.C. ac-
knowledges the NSF for support through Grant CHE-
0342824.
Supporting Information Available: Complete data from XRD
studies of complexes 9, 10, 11, and 14, kinetic plots and 1H NMR
spectra of complexes 4, 5, 9, 10, 12, and 14, Cartesian coordinates
for calculated structures, and experimental details for (SIPr)Cu-
(Cl), (IMes)Cu(Cl), (IMes)Cu(Me), and (IPr)Cu(Br). This material
is available free of charge via the Internet at http://pubs.acs.org.
IC0611995
Inorganic Chemistry, Vol. 45, No. 22, 2006 9045