Single-electron oxidation of monomeric copper(I) alkyl complexes: Evidence for reductive elimination through bimolecular formation of alkanes

Article abstract of DOI:10.1021/om060409i

Monomeric Cu(I) alkyl complexes (NHC)Cu(R) (NHC = N-heterocyclic carbene; R = Me or Et) and (dtbpe)Cu(Me) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane) have been prepared, isolated, and characterized. Single-electron oxidation of the Cu(I) alkyl complexes upon reaction with AgOTf to form putative Cu(II) intermediates of the type [(L)Cu(R)]⁺ (L = NHC or dtbpe, R = Me or Et) results in the rapid production of (L)Cu(X) (X = OTf) and R₂. Experimental studies suggest that the reductive elimination of R₂ from Cu(II) occurs through a nonradical bimolecular mechanism. Computational studies of the Cu-C_methyl yield bond dissociation enthalpies of [(SIPr)Cu-CH₃]ⁿ⁺ (80 kcal/mol for n = 0 {Cu(I)} and 38 kcal/mol for n = 1 (Cu(II)}).

Full text of DOI:10.1021/om060409i

Organometallics 2006, 25, 4097-4104
4097
Single-Electron Oxidation of Monomeric Copper(I) Alkyl
Complexes: Evidence for Reductive Elimination through
Bimolecular Formation of Alkanes
Laurel A. Goj,† Elizabeth D. Blue,† Samuel A. Delp,† T. Brent Gunnoe,*,†
Thomas R. Cundari,‡ and Jeffrey L. Petersen§
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
Center for AdVanced Scientific Computing and Modeling (CASCaM), Department of Chemistry,
UniVersity of North Texas, Box 305070, Denton, Texas 76203-5070, and C. Eugene Bennett Department of
Chemistry, West Virginia UniVersity, Morgantown, West Virginia 26506-6045
ReceiVed May 12, 2006
Monomeric Cu(I) alkyl complexes (NHC)Cu(R) (NHC ) N-heterocyclic carbene; R ) Me or Et) and
(dtbpe)Cu(Me) (dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane) have been prepared, isolated, and
characterized. Single-electron oxidation of the Cu(I) alkyl complexes upon reaction with AgOTf to form
putative Cu(II) intermediates of the type [(L)Cu(R)]+ (L ) NHC or dtbpe, R ) Me or Et) results in the
rapid production of (L)Cu(X) (X ) OTf) and R2. Experimental studies suggest that the reductive elimination
of R2 from Cu(II) occurs through a nonradical bimolecular mechanism. Computational studies of the
Cu-Cmethyl yield bond dissociation enthalpies of [(SIPr)Cu-CH3]n+ (80 kcal/mol for n ) 0 {Cu(I)} and
38 kcal/mol for n ) 1 {Cu(II)}).
Scheme 1. Possible Pathways for Elimination of
Introduction
Metal-Carbon Bonds
In the area of homogeneous catalysis, the development of
well-defined and selective catalytic cycles often requires access
to even-electron transformations in preference to odd-electron
radical processes. Therefore, relatively strong M-C bonds can
be desirable for organometallic systems, and understanding the
factors that control M-C bond dissociation enthalpies (BDEs)
as well as other metal-ligand BDEs is of fundamental
importance.1-7 In addition to the propensity of a system to
initiate M-C bond homolysis and radical chemistry, the
mechanism of C-C reductive elimination processes depends
on M-C bond energies. For example, the net reductive
elimination of M-C bonds can proceed by direct C-C bond
formation from a single metal center (formal two-electron
reduction of the metal), C-C elimination from two metal centers
(formal single-electron reduction of each metal), or initial
metal-carbon bond homolysis (formal single-electron reduction
of the metal; Scheme 1).5,8-24
Copper complexes have played a prominent role in homo-
geneous catalysis and metal-mediated synthesis of organic
molecules.25-28 Despite the substantial utility of copper com-
(12) Nappa, M. J.; Santi, R.; Diefenbach, S. P.; Halpern, J. J. Am. Chem.
Soc. 1982, 104, 619-621.
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41.
* To whom correspondence should be addressed. E-mail: brent_gunnoe@
ncsu.edu.
(14) Wegman, R. W.; Brown, T. L. J. Am. Chem. Soc. 1980, 102, 2494-
2495.
† North Carolina State University.
‡ University of North Texas.
(15) Whitesides, G. M.; Casey, C. P.; Krieger, J. K. J. Am. Chem. Soc.
1971, 93, 1379-1389.
§ West Virginia University.
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Publication on Web 07/21/2006

4098 Organometallics, Vol. 25, No. 17, 2006
Goj et al.
plexes and the integral role of transition metal complexes that
possess metal-alkyl or metal-aryl ligands in the field of
homogeneous catalysis, the number of copper hydride, alkyl,
or aryl complexes that are isolable and well-defined are few,
and this is especially true of monomeric systems.29-32 Copper
systems that possess Cu-R (R ) alkyl or aryl) or Cu-H bonds
are often highly reactive substrates. The reduction of Cu(II)
halides with tetraalkyllead has been studied, which may involve
Cu(II) alkyl systems.33 Whitesides et al. have studied the
mechanism of decomposition of Cu(I) alkyl complexes in the
presence of phosphine ligands,34,35 and, most germane here, the
elimination of alkanes upon reaction of dialkylcuprates with
molecular oxygen has been explored.36
Recently, Sadighi et al. have reported the synthesis, isolation,
and full characterization of the two-coordinate Cu(I) methyl
complex (IPr)Cu(Me) (IPr ) 1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene).30 Given the diversity of reactivity available
for the Lewis acidic Cu(II) oxidation state, we became interested
in exploring the possible preparation of Cu(II) alkyl complexes.
To our knowledge, no isolable Cu(II) alkyl complexes have been
reported. Herein, we report that single-electron oxidation of a
series of two-coordinate Cu(I) complexes of the type (NHC)-
Cu(R) (NHC ) N-heterocyclic carbene, R ) methyl or ethyl)
and the three-coordinate complex (dtbpe)Cu(Me) {dtbpe ) 1,2-
bis(di-tert-butylphosphino)ethane} results in the formation of
unstable systems that rapidly undergo reductive elimination of
the alkyl ligand to return to the Cu(I) oxidation state.
Figure 1. Partially labeled ORTEP of (SIPr)Cu(OAc) (1) at 30%
probability (hydrogen atoms omitted). Selected bond distances
(Å): Cu1-O1 1.838(2), Cu1-C1 1.886(3), O1-C28 1.276(4),
O2-C28 1.222(6), C1-N1 1.322(6), C1-N2 1.324(6), C2-C3
1.512(8). Selected bond angles (deg): C1-Cu1-O1 172.5(2),
Cu1-O1-C28 125.3(3), O1-C28-O2 125.2(4), O1-C28-C29
120.2(4), Cu1-C1-N2 126.5(3), Cu1-C1-N1 124.4(4), N1-C1-
N2 109.1(3).
Results and Discussion
Synthesis of Copper(I) Complexes. The complexes (SIPr)-
Cu(OAc) (1) and (IMes)Cu(OAc) (2) (SIPr ) 1,3-bis(2,6-
diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene; IMes ) 1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) can be prepared
in reactions analogous to the synthesis of (IPr)Cu(OAc).30
Complexes 1 and 2 have been characterized by 1H and 13C NMR
spectroscopy and elemental analysis. In addition, single-crystal
X-ray diffraction studies of 1 and 2 have confirmed their identity
(Figures 1 and 2 and Table 1). Other examples of structural
characterization of Cu-carboxylate complexes, including both
Cu(I) and Cu(II) oxidation states, have been reported.30,37-44
Figure 2. ORTEP of (IMes)Cu(OAc) (2) at 30% probability
(hydrogen atoms omitted). Selected bond distances (Å): Cu1-O1
1.867(3), Cu1-C1 1.859(3), O1-C22 1.233(5), O2-C22 1.223-
(6), C1-N1 1.354(3), C1-N2 1.356(3), C2-C3 1.349(4). Selected
bond angles (deg): C1-Cu1-O1 171.1(1), Cu1-O1-C22 116.3-
(3), O1-C22-O2 124.4(4), O1-C22-C23 117.0(4), Cu1-C1-
N2 127.6(2), Cu1-C1-N1 128.8(2), N1-C1-N2 103.5(2).
(26) Ley, S. V.; Thomas, A. W. Angew. Chem., Int. Ed. 2003, 42, 5400-
5449.
(27) Krause, N. Modern Organocopper Chemistry; Wiley-VCH: New
York, 2002.
(28) Johnson, J. S.; Evans, D. A. Acc. Chem. Res. 2000, 33, 325-335.
(29) Dempsey, D. F.; Girolami, G. S. Organometallics 1988, 7, 1208-
1213.
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metallics 2004, 23, 1191-1193.
(31) Coan, P. S.; Folting, K.; Huffman, J. C.; Caulton, K. G. Organo-
metallics 1989, 8, 2724-2728.
The structures of 1 and 2 reveal nearly linear C1-Cu1-O1
linkages with bond angles of 172.5(2)° and 171.1(1)°, respec-
tively. The Cu-O bond distance of 1 is 1.838(2) Å, while that
of 2 is 1.867(3) Å. Thus, the Cu-O bond distances differ by
0.029(4) Å, with the bond length for the SIPr complex 1 being
shorter. As previously reported,30 (IPr)Cu(OAc) exhibits a
Cu-O bond distance of 1.850(3) Å, which is intermediate
between 1 and 2. Similar to (IPr)Cu(OAc), the acetate ligands
(32) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics 2004,
23, 3369-3371.
(33) Clinton, N. A.; Kochi, J. K. J. Organomet. Chem. 1973, 56, 243-
254.
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Soc. 1972, 94, 232-239.
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P. J. Am. Chem. Soc. 1969, 91, 6542-6544.
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Am. Chem. Soc. 1967, 89, 5302-5303.
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8840.
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5476.
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Tyagi, S.; Hathaway, B. Chem. Commun. 1982, 196-198.
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99, 805-812.
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Downey, C. W. J. Am. Chem. Soc. 2003, 125, 12692-12693.
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J. H. Inorg. Chem. 1994, 33, 531-537.
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Single-Electron Oxidation of Monomeric Cu(I) Alkyl Complexes
Organometallics, Vol. 25, No. 17, 2006 4099
Table 1. Selected Crystallographic Data and Collection Parameters for (SIPr)Cu(OAc)-C6H6 (1-C6H6), (IMes)Cu(OAc) (2),
(dtbpe)Cu(OAc) (5), and (IPr)Cu(OTf) (7)
1-C6H6
2
5
7
empirical formula
fw
C35H47CuN2O2
591.29
C23H27CuN2O2
427.01
C20H43CuO2P2
441.02
C28H36CuF3N2O3S
601.19
cryst syst
space group
a, Å
b, Å
c, Å
monoclinic
P21
10.1166(9)
16.599(2)
10.980(1)
monoclinic
P21/c
9.4280(6)
20.037(1)
11.9928(8)
triclinic
P1h
orthorhombic
P212121
10.5522(5)
14.2330(7)
20.448(1)
15.1868(9)
15.6168(9)
16.027(1)
98.396(1)
98.581(1)
99.305(1)
3652.4(4)
6
R, deg
â, deg
γ, deg
V (Å3)
Z
110.600(2)
104.074(1)
1725.9(3)
2
1.138
2197.6(2)
4
1.291
3071.1(3)
4
1.300
D
calcd, g cm-3
1.203
cryst size (mm)
R1, wR2 {I > 2σ(I)}
GOF
0.12 × 0.14 × 0.50
0.0593, 0.1361
1.013
0.32 × 0.40 × 0.58
0.0565, 0.1591
1.031
0.36 × 0.38 × 0.40
0.0434, 0.1122
1.035
0.25 × 0.38 × 0.40
0.0453, 0.1050
1.009
of 1 and 2 both exhibit a κ1-coordination mode, and Sadighi et
al. have suggested the possibility that the preferred κ1-coordina-
tion mode (in preference to the κ2-coordination mode) is the
result of an intermolecular interaction between the distal oxygen
and a hydrogen of a second carbene ligand.30 The distance
between Cu and the distal oxygen is shorter for the IMes
complex 2 (2.831 Å) than for the SIPr complex 1 (3.082 Å)
but is similar to that of (IPr)Cu(OAc) {2.868(4) Å}. The similar
bond distances of 2 and (IPr)Cu(OAc) suggest that electronic
effects of the carbene ligand likely govern the interaction, a
result supported by density functional calculations on (NHC)-
Cu(acetate), which show conversion of the acetate ligand from
κ2 to κ1 upon energy minimization, presumably driven by the
stability of linear, two-coordinate geometries for d10-Cu(I). If
steric effects were dominant, it is anticipated that (IPr)Cu(OAc)
and (SIPr)Cu(OAc) (1) would exhibit more structural similarity.
Furthermore, space-filling diagrams for 1 and 2 (taken from
the X-ray structures) suggest more facile access to the Cu center
for complex 2 relative to complex 1 (Figure 3).
a single crystal of 5 revealed three independent molecules in
the crystallographic asymmetric unit. An ORTEP of one
molecule is depicted in Figure 4, and data collection and
structure solution parameters are given in Table 1. In contrast
to the (NHC)Cu(OAc) complexes (see above), in all three
molecules, the acetate ligand of 5 is coordinated in a κ2-mode
with varying degrees of asymmetry. The Cu-O bond distances
of 5 are substantially elongated (average of 2.146 Å) compared
with the Cu-O bond distances of complexes 1 {1.838(2) Å}
and 2 {1.867(3) Å}. For the three independent molecules in
the solid-state structure of 5, the Cu-O bond distances are
The reaction of 1 or 2 with AlMe3 produces (SIPr)Cu(Me)
(3) and (IMes)Cu(Me) (4), respectively (eq 1). The Cu(I) methyl
complexes have been characterized by 1H and 13C NMR
spectroscopy. The methyl groups resonate as singlets at -0.61
ppm for 3 and -0.28 ppm for 4 in the 1H NMR spectrum, which
are similar to the resonance at -0.49 ppm for the previously
reported complex (IPr)Cu(Me) {IPr ) 1,3-bis(2,6)-diisopropy-
lphenyl)imidazol-2-ylidene}.
Figure 3. Space-filling diagrams for (SIPr)Cu(OAc) (1) (depicted
on the left) and (IMes)Cu(OAc) (2) (depicted on the right) with
view from above the NHC plane.
Figure 4. Labeled ORTEP of (dtbpe)Cu(OAc) (5) at 30%
probability (hydrogen atoms omitted). Selected bond distances
(Å): Cu2-O3 2.121(2), Cu2-O4 2.172(2), Cu2-P3 2.2358(7),
Cu2-P4 2.2529(8), C21-O3 1.254(4), C21-O4 1.240(3), C21-
C22 1.515(4). Selected bond angles (deg): Cu2-O4-C21 88.0-
(2), Cu2-O3-C21 90.0(2), O3-C21-O4 121.2(2), O3-C21-
C22 119.5(3), O4-C21-C22 119.4(3), P3-Cu2-P4 95.87(3),
O3-Cu2-O4 60.78(8), P3-Cu2-O3 131.26(7), P4-Cu2-O3
119.32(7), P4-Cu2-O4 122.49(7).
The combination of dtbpe and Cu(OAc) yields (dtbpe)Cu-
(OAc) (5) in 66% isolated yield. An X-ray diffraction study of

4100 Organometallics, Vol. 25, No. 17, 2006
Goj et al.
1
After 10 min of reaction time, a H NMR spectrum of a C6D6
solution of (IPr)Cu(Me) and 0.5 equiv of AgOTf reveals the
formation of ethane and an approximately 1:1 molar ratio of
(IPr)Cu(Me) and complex 7. The combination of the previously
reported ethyl complex (IPr)Cu(Et) and AgOTf in C6D6
produces (IPr)Cu(OTf) and butane as the primary products.
Identical observations are made for reactions of (SIPr)Cu(Me)
(3) and (IMes)Cu(Me) (4) with AgOTf in either C6D6 or toluene-
d8. The 1H NMR spectrum of a C6D6 solution of (IPr)Cu(Me),
(IPr)Cu(Et), and AgOTf (1:1:2 molar ratio) reveals resonances
consistent with the production of ethane, propane, and butane
as well as (IPr)Cu(OTf) (eq 4).
Figure 5. Partially labeled ORTEP of (IPr)Cu(OTf) (7) at 30%
probability (hydrogen atoms omitted). Selected bond distances
(Å): Cu1-O1 1.875(2), Cu1-C1 1.863(3), O1-S1 1.469(3), O2-
S1 1.423(3), O3-S1 1.405(3), C1-N1 1.353(4), C1-N2 1.346-
(4). Selected bond angles (deg): C1-Cu1-O1 175.6(1), Cu1-
O1-S1 121.9(1), O1-S1-C28 100.2(2), O1-S1-O3 112.6(2),
O1-S1-O2 113.8(2).
2.078(2)/2.270(2) Å (∆bond length ) 0.192 Å), 2.121(2)/2.172-
(2) Å (∆bond length ) 0.051 Å), and 2.086(2)/2.246(2) Å
(∆bond length ) 0.16 Å).
In a fashion similar to the NHC complexes, the reaction of 5
with AlMe3 produces (dtbpe)Cu(Me) (6) (eq 2). The methyl
1
ligand of 6 resonates as a broad singlet at 0.35 ppm in the H
The observations upon reaction of Cu-methyl complexes
with single-electron oxidants are consistent with a reaction
pathway that involves initial oxidation of Cu(I) to the unob-
served (even at low temperatures) Cu(II) systems [(NHC)Cu-
(R)][OTf] followed by net reductive elimination of R to reduce
Cu(II) to Cu(I). Consistent with a reaction pathway that involves
initial single-electron transfer to form Cu(II) and elemental Ag,
the combination of (IPr)Cu(Me) and [Cp2Fe][PF6] in C6D6
produces Cp2Fe (singlet at ∼4.0 ppm in the 1H NMR spectrum),
ethane, and a single IPr-Cu complex that is likely (IPr)Cu-
(FPF5) or [(IPr)Cu(η2-benzene)][PF6]. A 19F NMR spectrum of
NMR spectrum, while the 13C NMR spectrum reveals a broad
singlet at -7.1 ppm.
Oxidation of Copper(I) Methyl Complexes. Reaction of the
previously reported (IPr)Cu(Me) with AgOTf should provide
access to single-electron transfer to produce the Cu(II) species
[(IPr)Cu(Me)][OTf] and Ag(s). The combination of these two
reagents in sealed NMR tubes in C6D6 cleanly produces the
the latter product reveals only a doublet at -68 ppm (1JFP
)
709 Hz). Thus, the PF6 anion remains intact and is not ruptured
to form a Cu-fluoride complex and PF5. Although the reaction
of (IPr)Cu(Me) with ferrocenium demonstrates that outer-sphere
electron transfer to form [(IPr)Cu(Me)]+ followed by C-C
reductive elimination is a viable pathway, it does not definitely
eliminate the possibility of forming Ag(I) alkyl complexes upon
reaction of (NHC)Cu(R) systems with AgOTf. However, the
decomposition of Ag(I) methyl and ethyl systems has been
reported to occur via Ag-C bond homolysis and to produce
olefins as well as alkanes,48 observations that are inconsistent
with the transformations involving the (NHC)Cu(R) systems
(see below). Thus, it is likely that reaction of AgOTf with
(NHC)Cu(R) complexes is proceeding through outer-sphere
single-electron transfer to form unobserved [(NHC)Cu(R)]+
intermediates. The subsequent reductive elimination of alkane
to re-form Cu(I) likely involves either an initial Cu-C bond
homolysis or bimetallic reductive elimination from two metal
centers.
1
complex (IPr)Cu(OTf) (7) and ethane (as determined by H
NMR spectroscopy) within 5 min at room temperature (eq 3).
Jordan et al. have reported that reactions of Zr(IV) alkyl
complexes with Ag(I) salts result in “redox cleavage” of Zr-
alkyl bonds.45,46 Although the preparation of complex 7 has been
previously reported,47 single crystals were grown for a solid-
state X-ray diffraction study (Figure 5, Table 1). The triflate
ligand is coordinated through a single oxygen atom with a Cu1-
O1 bond distance of 1.875(2) Å. As anticipated for two-
coordinate Cu(I), the C1-Cu1-O1 linkage is nearly linear, with
a bond angle of 175.6(1)°. For the reaction of (IPr)Cu(Me) and
AgOTf, the singlet due to ethane appears at 0.82 ppm. A minor
resonance slightly above the noise threshold consistent with CH4
is also detected. The source of the hydrogen atom to yield CH4
is unknown. Neither performing the reaction in the presence of
the radical trap 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)
nor using toluene-d8 as solvent substantially alters the reaction.
We have previously demonstrated that oxidation of Ru(II)
complexes of the type TpRu(L)(L′)Me (L, L′ ) neutral, two-
electron-donating ligands) to Ru(III) in deuterated solvents
results in rapid reduction to the Ru(II) systems TpRu(L)(L′)-
(45) Jordan, R. F.; Dasher, W. E.; Echols, S. F. J. Am. Chem. Soc. 1986,
108, 1718-1719.
(46) Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S. F.; Willett,
R. J. Am. Chem. Soc. 1987, 109, 4111-4113.
(47) Munro-Leighton, C.; Blue, E. D.; Gunnoe, T. B. J. Am. Chem. Soc.
2006, 128, 1446-1447.
(48) Beverwijk, C. D. M.; Van der Kerk, G. J. M.; Leusink, A. J.; Noltes,
J. G. Organometallic Chem. ReV. Sect. A 1970, 5, 215-280.

Single-Electron Oxidation of Monomeric Cu(I) Alkyl Complexes
Organometallics, Vol. 25, No. 17, 2006 4101
Scheme 2. Proposed Pathway for Conversion of
(NHC)Cu(R) and AgOTf to R2 and (NHC)Cu(OTf) Involves
Initial Oxidation to Cu(II) and Bimetallic Elimination of R2
(NHC ) N-Heterocyclic Carbene)
Scheme 3. B3LYP/6-31G(d)-Calculated Cu-Cmethyl Bond
Dissociation Enthalpies and Ionization Potentials for
(SIPr)CuI(Me) (3) (bottom) and [(SIPr)CuII(Me)]+ (top)
OTf with production of CH3D and perprotio ethane.49 These
transformations likely proceed through a facile Ru-C bond
homolysis, as indicated by substantial production of CH3D in
toluene-d8 {weak benzylic C-H(D) bond increases methane
production} and the lack of production of methane/ethane in
the presence of radical traps such as TEMPO. In contrast,
regardless of whether the reaction is performed in C6D6 or
toluene-d8, oxidation of the copper systems and return to Cu(I)
produces predominantly ethane (or butane in the case of R )
ethyl) with no eVidence for formation of CH3D. Also, the
addition of TEMPO does not alter the reactiVity. Thus, a
bimetallic pathway for alkane elimination seems likely (Scheme
2). We currently have no evidence that suggests the nature of
the putative bimetallic system that likely precedes alkane
elimination.
Similar to the (NHC)Cu(Me) complexes, at room temperature
the reaction of AgOTf and (dtbpe)Cu(Me) (6) in C6D6 produces
ethane and a diamagnetic Cu complex consistent with (dtbpe)-
Cu(OTf) (eq 5). Nearly identical observations are made for the
analogous reaction in toluene-d8. In contrast to oxidation of
(NHC)Cu(Me) systems, no evidence for formation of methane
is observed for the oxidation of complex 6. Thus, the results
are consistent with initial oxidation of complex 6 to the Cu(II)
complex [(dtbpe)Cu(Me)][OTf] followed by elimination of
ethane to produce (dtbpe)Cu(OTf) in a transformation that does
not likely involve Cu-C bond homolysis.
complex (SIPr)Cu(Me) (3) to the Cu(II) (d9, 13-electron
complex) system [(SIPr)Cu(Me)]+ significantly decreases the
homolytic Cu-Cmethyl BDE from 80 kcal/mol to 38 kcal/mol.
This represents a 53% reduction in BDE upon single-electron
oxidation. In comparison, we have recently reported that
oxidation of the octahedral Ru(II) (d6, 18-electron complex)
TpRu(CO)(NCMe)(Me) (Tp ) hydridotris(pyrazolyl)borate) to
the Ru(III) (d5, 17-electron complex) is calculated to reduce
the Ru-Cmethyl BDE from 49 kcal/mol to 23 kcal/mol, which
is an approximately 52% reduction in BDE.49 Thus, for two
seemingly disparate systems, single-electron oxidation decreases
the BDE by a remarkably similar magnitude. In addition to the
calculated decrease in BDE upon conversion of (SIPr)CuI(Me)
(3) to [(SIPr)CuII(Me)]+ of 42 kcal/mol, the ionization potential
of (SIPr)CuI(Me) (3) (5.97 eV) is calculated to be approximately
1.83 eV (42 kcal/mol) higher than that of (SIPr)Cu (4.14 eV).
The calculated Cu-CMe vibrational frequencies for [(SIPr)CuII-
(Me)]+ and (SIPr)CuI(Me) are nearly identical, ca. 572 cm-1
.
The calculated Cu-CMe bond lengths are also roughly equiva-
lent, 1.896 Å for the Cu(II) complex and 1.909 Å for the Cu(I)
complex 3, a variation commensurate with literature values for
ionic radii differences for both tetrahedral and octahedral Cu-
(II) and Cu(I) ions [∆rion{Cu(I/II)} ≈ 0.03-0.04 Å].50 Hence,
the computations suggest the possibility that both copper-
methyl complexes have intrinsically similar copper-methyl
bond thermodynamics and that the lower BDE for the Cu(II)-
methyl complex primarily reflects formation of a stable Cu(I)
complex upon scission of [(SIPr)CuII(Me)]+.
Summary and Conclusions. Transition metal alkyl linkages
are often unstable for paramagnetic complexes, and this is
largely attributable to an increased predilection toward M-C
bond homolysis that results from decreased M-C BDEs. As
an example, we recently reported that single-electron oxidation
of thermally stable Ru(II) complexes of the type TpRu(L)(L′)R
to Ru(III) complexes [TpRu(L)(L′)R]+ results in reactivity that
is attributable to rapid Ru-C homolysis.49 These results are
consistent with the scarcity of isolable Ru(III) methyl complexes,
examples of which are primarily limited to systems with por-
phyrin ligands that are strongly donating and potentially capable
of delocalizing radical spin density. In addition, a recent report
by Caulton et al. details the incorporation of a chelating amido
ligand to stabilize a Ru(III) dialkyl complex, and similar to the
Computational Results. We have studied the Cu-C BDEs
of (SIPr)Cu(Me) and [(SIPr)Cu(Me)]+ using B3LYP/6-31G(d)
calculations on the full SIPr models (Scheme 3). At the Cu(I)
oxidation state, the Cu-Me bond possesses substantial strength
with a calculated BDE of 80 kcal/mol. This value represents a
relatively strong metal-carbon (alkyl) bond and may reflect
substantial covalent nature for the relatively electronegative
transition metal coordinated to a methyl ligand. The calculations
reveal that oxidation of the Cu(I) (d10, 14-electron complex)
(49) Lail, M.; Gunnoe, T. B.; Barakat, K. A.; Cundari, T. R. Organo-
metallics 2005, 24, 1301-1305.
(50) Huheey, J. E. Inorganic Chemistry: Principles of Structure and
ReactiVity, 3rd ed.; Harper & Row: New York, 1983.

4102 Organometallics, Vol. 25, No. 17, 2006
Goj et al.
(SIPr)Cu(OAc) (1). A round-bottom flask was charged with SIPr
(0.177 g, 0.500 mmol), Cu(OAc) (0.059 g, 0.50 mmol), and toluene
(15 mL), and the resulting solution was stirred for 2 h at room
temperature. The solution was filtered through Celite, approximately
half of the solvent was removed in vacuo, and hexanes were added
to form a white precipitate. The solid was collected by vacuum
filtration and dried in vacuo (0.165 g, 64%). Crystals suitable for
a solid-state X-ray diffraction study were grown at room temperature
by slow diffusion of pentane into a benzene solution of 1. 1H NMR
(C6D6, δ): 7.16-7.12 (overlapping with solvent peak, 2H, para-
Ru(III) porphyrin complexes, the persistence of the Ru(III)
dialkyl system is likely attributable to a combination of amido-
to-metal donation and delocalization of radical character, steric
protection from the ligand set, and low-coordination num-
ber.7,23,51
Similar to our previous observations with TpRu(L)(L′)R
systems, calculations suggest that single-electron oxidation of
the even-electron Cu(I) methyl complexes to odd-electron Cu(II)
complexes renders the Cu-C bonds easier to break. However,
in contrast to observations made for [TpRuIII(L)(L′)R]+ sys-
tems,49 the Cu(II) methyl complexes appear to undergo decom-
position through a pathway that does not involve metal-carbon
bond homolysis to generate a radical species. Rather, we propose
a bimolecular pathway that results in the elimination of ethane
(or butane for Cu-Et bonds) and reduction of two metal centers,
each by a single electron. In each case in deuterated solvents,
a small amount of CH4 is potentially formed with no evidence
for the formation of CH3D, which likely indicates that C-H
reductive elimination from the putative bimolecular copper
species is also possible. Although oxidation from Cu(I) to Cu(II)
apparently decreases the Cu-C BDEs by a similar magnitude
compared to the TpRu systems, the initial metal-carbon BDE
for CuI-C is calculated to be approximately 31 kcal/mol more
substantial (or, ∼64% greater) than the RuII-C BDE. Thus, even
though oxidation of Cu(I) to Cu(II) results in less stable
complexes, the corresponding Cu(II)-C bonds apparently
remain strong enough to kinetically suppress Cu-C bond
homolysis and are, in fact, calculated to be only 11 kcal/mol
weaker (CuII-C BDE ≈ 38 kcal/mol) than the BDE of the stable
(at room temperature) TpRu(CO)(NCMe)Me (RuII-C BDE ≈
49 kcal/mol) complex. Similar to Ru(III) systems (see above),
the incorporation of donating ancillary ligands capable of spin
delocalization may allow isolation of relatively stable Cu(II)
alkyl or aryl complexes and access to reactivity from these
species.
3
CH of aryl group), 7.04 (d, JHH ) 7 Hz, 4H, meta-CH), 3.16 (s,
3
4H, NCH), 2.99 (sept, JHH ) 6 Hz, 4H, CH(CH3)2), 1.90 (s, 3H,
CO2CH3), 1.53 (d, 3JHH ) 7 Hz, 12H, CH(CH3)2), 1.18 (d, 3JHH
)
7 Hz, 12H, CH(CH3)2). 13C NMR (C6D6, δ): 204.8 (NCCu), 147.2,
135.4, 130.4, 125.0, 112.1 (aryl of SIPr ligand and NCH), 29.5
{CH(CH3)2}, 25.9 {CH(CH3)2}, 24.3 (CH(CH3)2). (Note: the
resonances due to the carbonyl and methyl groups of the acetate
ligand were not observed. For (IPr)Cu(OAc), these resonances are
observed at 128.9 and 24.2 ppm, respectively, and they could be
coincidental with other observed resonances for complex 1.) Anal.
Calcd for C29H41CuN2O: C, 67.87; H, 8.05; N, 5.46. Found: C,
67.85; H, 8.02; N, 5.45.
(IMes)Cu(OAc) (2). A round-bottom flask was charged with
IMes (0.152 g, 0.500 mmol), Cu(OAc) (0.059 g, 0.50 mmol), and
toluene (15 mL), and the resulting solution was stirred overnight
at room temperature. The solution was filtered through Celite, and
approximately half of the solvent was removed in vacuo. Hexanes
were added to form a white precipitate. The solid was collected by
vacuum filtration and dried in vacuo (0.162 g, 76%). Crystals
suitable for a solid-state X-ray diffraction study were grown at room
1
temperature by layering a benzene solution of 2 with pentane. H
NMR (C6D6, δ): 6.67 (s, 4H, meta-CH), 5.97 (s, 4H, NCH), 2.06
(s, 6H, para-CH3), 2.05 (s, 3H, CO2CH3), 2.02 (s, 12H, ortho-
CH3). 13C NMR (C6D6, δ): 180.9 (NCCu), 139.5, 136.0, 135.2,
130.0, 122.1 (aryl of IMes ligand and NCH), 24.1 (CO2CH3), 21.7
(para-CH3), 18.4 (ortho-CH3). (Note: similar to complex 1, the
resonance due to the carbonyl of the acetate ligand for complex 2
was not observed and is presumed to be coincident with a resonance
in the range of 120 to 130 ppm.) Anal. Calcd for C23H27CuN2O:
C, 64.69; H, 6.37; N, 6.56. Found: C, 65.26; H, 6.36; N, 6.29.
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. Diethyl
ether was used as received. Benzene-d6 and toluene-d8 were distilled
over sodium, degassed by three freeze-pump-thaw cycles, and
stored over 4 Å molecular sieves. All reactions performed on an
NMR scale utilized J-Young NMR tubes with Teflon screw caps
or in NMR tubes sealed with rubber septa. 1H 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 due to residual protons in the
deuterated solvents (1H NMR) or the 13C resonances of the
deuterated solvents. 19F spectra were recorded on a Varian Mercury
instrument operating at a frequency of 376.5 MHz with C6F6 as
external standard. Elemental analyses were performed by Atlantic
Microlabs, Inc. Trimethylaluminum in hexanes, TEMPO, silver
triflate, ferrocenium hexafluorophosphate, and anhydrous ethanol
were obtained from Sigma Aldrich, Cu(I) acetate was obtained from
Strem Chemical, and these reagents were used as received. The
ligands IMes, SIPr, and dtbpe as well as the complexes (IPr)Cu-
(OAc), (IPr)Cu(Me), (IPr)Cu(Et), and (IPr)Cu(OTf) were prepared
according to literature procedures.30,47,52-54
(SIPr)Cu(Me) (3). To a precooled Schlenk flask (-60 °C)
charged with (SIPr)Cu(OAc) (1) (0.200 g, 0.393 mmol) and diethyl
ether (4.0 mL) was added a solution of ether (0.5 mL), AlMe3 (0.5
mL of a 1.0 M solution in hexanes, 1 mmol), and ethanol (60 µL,
1 mmol). The solution was stirred at -60 °C for 1 h followed by
an additional hour of stirring at room temperature. Approximately
half of the solvent was removed in vacuo, and hexanes were added
to form a white precipitate. The solid was collected by vacuum
filtration and dried (0.156 g, 84%). To prevent slow decomposition,
complex 3 was stored in an inert atmosphere at -20 °C. 1H NMR
(C6D6, δ): 7.18 (t, 3JHH ) 7 Hz, 2H, para-CH), 7.08 (d, 3JHH ) 7
3
Hz, 4H, meta-CH), 3.17 (s, 4H, NCH), 3.07 (sept, JHH ) 7 Hz,
3
4H, CH(CH3)2), 1.52 (d, JHH ) 7 Hz, 12H, CH(CH3)2), 1.21 (d,
3JHH ) 7 Hz, 12H, CH(CH3)2), -0.61 (s, 3H, Cu-CH3). 13C NMR
(C6D6, δ): 182.8 (NCCu), 147.3, 136.0, 130.0, 124.8 (phenyl of
IPr ligand), 54.0 (NCH), 29.5 (CH(CH3)2), 25.9 (CH(CH3)2), 24.2
(CH(CH3)2), -11.6 (CuCH3). Complex 3 decomposes over a period
of days under inert atmosphere, and its instability precludes
satisfactory elemental analysis.
(IMes)Cu(Me) (4). To a precooled Schlenk flask (-60 °C)
charged with (IMes)Cu(OAc) (2) (0.168 g, 0.393 mmol) and diethyl
ether (4.0 mL) was added a solution of ether (0.5 mL), AlMe3 (0.5
mL of a 1.0 M solution in hexanes, 1 mmol), and ethanol (60 µL,
1 mmol) . The solution was stirred at -60 °C for 1 h, removed
from the cold bath, and stirred an additional hour at room
temperature. Approximately half of the solvent was removed in
(51) Alexander, C. S.; Rettig, S. J.; James, B. R. Organometallics 1994,
13, 2542-2544.

Single-Electron Oxidation of Monomeric Cu(I) Alkyl Complexes
Organometallics, Vol. 25, No. 17, 2006 4103
vacuo, and hexanes were added to form a white precipitate. The
solid was collected by vacuum filtration and dried (0.115 g, 77%).
To prevent slow decomposition, complex 4 was stored in the solid
of one copper complex, identified as (SIPr)Cu(OTf), and ethane.
(dtbpe)Cu(Me) (6): To a colorless solution of (dtbpe)Cu(Me) (6)
(0.007 g, 0.017 mmol) and C6D6 (0.5 mL) in an NMR tube sealed
with a rubber septum was injected a solution of AgOTf (0.005 g,
0.024 mmol) and C6D6 (0.1 mL). The solution immediately turned
dark brown. 1H NMR revealed clean formation of one copper
complex, identified as (dtbpe)Cu(OTf), and the organic product
ethane. (IPr)Cu(Et): To a colorless solution of (IPr)Cu(Et) (0.010
g, 0.021 mmol) and C6D6 (0.5 mL) in an NMR tube sealed with a
rubber septum was injected a solution of AgOTf (0.006 g, 0.029
mmol) and C6D6 (0.1 mL). The solution immediately turned dark
brown. 1H NMR revealed clean formation of one copper complex,
identified as (IPr)Cu(OTf), and the organic product butane. (IPr)-
Cu(Me) with [Cp2Fe][PF6]: To a colorless solution of (IPr)Cu-
(Me) (0.010 g, 0.021 mmol) and C6D6 (0.5 mL) in an NMR tube
sealed with a rubber septum was injected a solution of [Cp2Fe]-
[PF6] (0.008 g, 0.023 mmol) and C6D6 (0.1 mL). The solution
immediately turned yellow-orange. 1H NMR revealed clean forma-
tion of a single copper complex, likely either (IPr)Cu(PF6) or (IPr)-
Cu(F), and the organic product ethane. (IPr)Cu(Me) with TEMPO:
To an orange-brown solution of (IPr)Cu(Me) (0.010 g, 0.021 mmol),
TEMPO (0.017 g, 0.11 mmol), and C6D6 (0.5 mL) in an NMR
tube sealed with a rubber septum was injected a solution of AgOTf
(0.006 g, 0.023 mmol) and C6D6 (0.1 mL). The solution im-
1
state under an inert atmosphere at -20 °C. H NMR (C6D6, δ):
6.69 (s, 4H, meta-CH), 6.00 (s, 4H, NCH), 2.08 (s, 6H, para-CH3),
2.06 (s, 12H, ortho-CH3), -0.28 (s, 3H, Cu-CH3). 13C NMR (C6D6,
δ): 184.7 (NCCu), 139.1, 136.5, 135.1, 129.8, 121.2 (aryl of IMes
ligand and NCH), 21.4, 18.3 (CH3 of IMes ligand), -11.6 (Cu-
CH3). Complex 4 decomposes over a period of days under inert
atmosphere, and its instability precludes satisfactory elemental
analysis.
(dtbpe)Cu(OAc) (5). Cu(OAc) (0.293 g, 2.40 mmol) was added
to a colorless solution of dtbpe (0.690 g, 2.2 mmol) in THF (20
mL). The solution immediately turned dark brown and was stirred
for 4 h, at which time an orange slurry was present. The solution
was filtered through Celite to remove the orange solids, and the
filtrate was reduced in vacuo to approximately 2 mL. Hexanes (∼10
mL) were added to form a white precipitate, which was collected
1
by vacuum filtration and dried (0.707 g, 76% yield). H NMR
(CDCl3, δ): 2.02 (s, 3H, OCOCH3), 1.77 (vt, N ) 5 Hz, 4H,
methylene CH2), 1.20 (vt, N ) 13 Hz, 36H, C(CH3)3). 13C NMR
(CDCl3, δ): 178.6 (s, OCOCH3), 33.4 (vt, N ) 9 Hz, C(CH3)3),
30.3 (vt, N ) 9 Hz, C(CH3)3), 23.2 (s, OCOCH3), 20.6 (vt, N ) 26
Hz, methylene CH2). 31P NMR (CDCl3, δ): 26.4 (s). Anal. Calcd
for C20H43Cu1O2P2: C 54.46; H 9.83; O 7.26. Found: C 54.39; H
9.97; O 7.20.
1
mediately turned dark brown. H NMR revealed clean formation
of one copper complex, identified as (IPr)Cu(OTf), and ethane.
Computational Methods. All calculations were carried out
utilizing the Gaussian03 package.55 The B3LYP functional (Becke’s
three-parameter hybrid functional56 using the LYP correlation
functional containing both local and nonlocal terms of Lee, Yang,
and Parr)57 and VWN (Slater local exchange functional58 plus the
local correlation functional of Vosko, Wilk, and Nusair59) were
employed in conjunction with the 6-31G(d) all-electron basis set.
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 conform species as minima and to obtain
enthalpies in the gas phase at 1 atm and 298.15 K.
(dtbpe)Cu(Me) (6). A slurry of complex 5 (0.578 g, 1.3 mmol)
in diethyl ether (10 mL) was stirred in a bath at -60 °C for 30
min. Ethanol (0.2 mL, 3.4 mmol) was added to a solution of 2.0 M
AlMe3 (1.7 mL, 3.4 mmol) in ether (5 mL), which bubbled
vigorously upon mixing. This solution was slowly added to the
solution of 5, and the resulting solution was stirred at -60 °C for
1 h. After allowing to warm to room temperature, the volatiles were
removed in vacuo. A minimal amount of hexanes (approximately
3 mL) was added, resulting in the formation of a precipitate after
storage overnight at -20 °C under N2. The white product was
1
collected by vacuum filtration and dried (0.136 g, 25% yield). H
NMR (C6D6, δ): 1.46 (br s, 4H, CH2), 1.09 (vt, N ) 13 Hz, 36H,
C(CH3)3), 0.35 (br s, 3H, Cu-CH3). 13C NMR (CDCl3, δ): 34.5
(vt, N ) 6 Hz, C(CH3)3), 31.3 (vt, N ) 10 Hz, C(CH3)3), 22.1 (vt,
N ) 23 Hz, methylene CH2), -7.1 (br s, CuCH3). 31P NMR (C6D6,
δ): 33.1 (s). Complex 6 decomposes over a period of days under
inert atmosphere, and its instability precludes satisfactory elemental
analysis.
Acknowledgment. T.B.G. acknowledges The Office of
Basic Energy Sciences, U.S. Department of Energy (DE-FG02-
03ER15490), and the Alfred P. Sloan Foundation (Research
Fellowship to T.B.G.) for financial support. E.D.B. acknowl-
edges the American Association of University Women Educa-
tional Foundation for an American Dissertation Fellowship.
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 (Grants No. DEFG02-03ER15387).
Calculations employed the UNT computational chemistry
resource, for which T.R.C. acknowledges the NSF for support
through grant CHE-0342824. We thank the group of Prof.
Joseph Sadighi (Massachusetts Institute of Technology) for
useful suggestions.
(IPr)Cu(OTf) (7). The synthesis and characterization of this
complex have been previously reported.47 X-ray-quality crystals
of this complex were grown from a solution of CH2Cl2 layered
with pentane.
Oxidation of Cu(I) Methyl Complexes. Some representative
experiments are given. (IPr)Cu(Me): To a colorless solution of
(IPr)Cu(Me) (0.010 g, 0.021 mmol) and C6D6 (0.5 mL) in an NMR
tube sealed with a rubber septum was injected a solution of AgOTf
(0.006 g, 0.023 mmol) and C6D6 (0.1 mL). The solution im-
1
mediately turned dark brown. H NMR revealed clean formation
of one copper complex, identified as (IPr)Cu(OTf), and the organic
product ethane. (SIPr)Cu(Me): To a colorless solution of (SIPr)-
Cu(Me) (3) (0.010 g, 0.021 mmol) and C6D6 (0.5 mL) in an NMR
tube sealed with a rubber septum was injected a solution of AgOTf
(0.006 g, 0.023 mmol) and C6D6 (0.1 mL). The solution im-
(52) Poerschke, K. R.; Pluta, C.; Proft, B.; Lutz, F.; Kru¨ger, C. Z.
Naturforsch., B: Chem. Sci. 1993, 48, 608-626.
(53) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R. Tetrahedron 1999,
55, 14523-14534.
(54) Goj, L. A.; Blue, E. D.; Munro-Leighton, C.; Gunnoe, T. B.;
Petersen, J. L. Inorg. Chem. 2005, 44, 8647-8649.
(55) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian Inc.:
Wallingford, CT, 2004.
(56) Becke, A. D. J. Chem. Phys. 1993, 98, 1372-1377.
(57) Lee, C. T.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785-789.
(58) Kohn, W.; Sham, L. J. Phys. ReV. 1965, A140, 1133-1138.
(59) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-
1211.
1
mediately turned dark brown. H NMR revealed clean formation
of one copper complex, identified as (SIPr)Cu(OTf), and ethane.
(SIPr)Cu(Me) (3) in toluene-d8: To a colorless solution of (SIPr)-
Cu(Me) (3) (0.010 g, 0.021 mmol) and toluene-d8 (0.5 mL) in an
NMR tube sealed with a rubber septum was injected a solution of
AgOTf (0.006 g, 0.023 mmol) and toluene-d8 (0.1 mL). The solution
immediately turned dark brown. 1H NMR revealed clean formation

4104 Organometallics, Vol. 25, No. 17, 2006
Goj et al.
Supporting Information Available: Complete tables of crystal
data, collection and refinement data, atomic coordinates, bond
distances and angles, and anisotropic displacement coefficients for
(SIPr)Cu(OAc) (1), (IMes)Cu(OAc) (2), (dtbpe)Cu(OAc) (5), and
complexes. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
(IPr)Cu(OTf) (7). H and 13C NMR spectra of all new Cu methyl
OM060409I