Cleavage of X-H bonds (X = N, o, or C) by copper(I) alkyl complexes to form monomeric two-coordinate copper(I) systems.

Abstract

The monomeric copper(I) alkyl complexes (IPr)Cu(R) [R = Me or Et; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] react with substrates that possess N-H, O-H, and acidic C-H bonds to form monomeric systems of the type (IPr)Cu(X) (X = anilido, phenoxide, ethoxide, phenylacetylide, or N-pyrrolyl) and methane or ethane. Solid-state X-ray crystal structures of the anilido, ethoxide, and phenoxide complexes confirm that they are monomeric systems. Experimental studies on the reaction of (IPr)Cu(Me) and aniline to produce (IPr)Cu(NHPh) suggest that a likely reaction pathway is coordination of aniline to Cu(I) followed by proton transfer to produce methane and the copper(I) anilido complex.

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Cleavage of X−H Bonds (X ) N, O, or C) by Copper(I) Alkyl Complexes
To Form Monomeric Two-Coordinate Copper(I) Systems
Laurel A. Goj,†Elizabeth D. Blue,†Colleen Munro-Leighton,†T. Brent Gunnoe,*,†and
Jeffrey L. Petersen‡
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204,
and C. Eugene Bennett Department of Chemistry, West Virginia UniVersity,
Morgantown, West Virginia 26506-6045
Received October 11, 2005
The monomeric copper(I) alkyl complexes (IPr)Cu(R) [R ) Me or
Et; IPr ) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] react with
substrates that possess N−H, O−H, and acidic C−H bonds to form
monomeric systems of the type (IPr)Cu(X) (X ) anilido, phenoxide,
ethoxide, phenylacetylide, or N-pyrrolyl) and methane or ethane.
Solid-state X-ray crystal structures of the anilido, ethoxide, and
phenoxide complexes confirm that they are monomeric systems.
Experimental studies on the reaction of (IPr)Cu(Me) and aniline
to produce (IPr)Cu(NHPh) suggest that a likely reaction pathway
is coordination of aniline to Cu(I) followed by proton transfer to
produce methane and the copper(I) anilido complex.
Controlled activation of small molecules by transition
metals provides the foundation for homogeneous catalytic
cycles. Among the transition series, copper is an attractive
metal because of its relatively low expense and utility in a
variety of important synthetic methods.1-6However, the use
of well-defined and tunable [i.e., by variation of ancillary
ligand(s)] copper-based systems for transformations that
involve metal-mediated bond cleavage is relatively limited.7-9
Two challenges for the development of such systems are
avoiding bond homolysis reactions that generate free radicals,
which are relatively common for first-row transition metals,
and accessing copper systems that remain monomeric
throughout a series of transformations. The latter issue is
especially problematic for nondative heteroaromatic ligands
(e.g., amido, alkoxide, etc.). For example, despite increased
interest in late-transition-metal complexes with nondative
ligands in the past decade,10-19the isolation of monomeric
copper(I) amido complexes is extremely rare.20Although
more common than amido systems, structurally characterized
monomeric copper(I) alkoxides are also limited.21-24Herein,
we report on the cleavage of N-H, O-H, and acidic C-H
bonds by copper(I) alkyl complexes to produce monomeric
copper(I) anilido, ethoxide, phenoxide, phenylacetylide, and
N-pyrrolyl systems.
Copper alkyl reagents have been utilized extensively in
organic synthesis; however, the preparation of well-defined
systems is limited.25Sadighi et al. have previously reported
* Towhomcorrespondence shouldbe addressed. E-mail:
brent_gunnoe@ncsu.edu.
†North Carolina State University.
‡West Virginia University.
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P. S. J. Am. Chem. Soc. 2003, 125, 9435-9441.
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Inorg. Chem. 2005, 44, 8647−8649
10.1021/ic0517624 CCC: $30.25
Published on Web 11/02/2005
© 2005 American Chemical Society
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and fully characterized the copper(I) methyl complex (IPr)-
Cu(Me) [1; IPr ) 1,3-bis(2,6-diisopropylphenyl)imidazol-
2-ylidene].26Heating (60 °C) 1 in C6D6 with 1 equiv of
aniline for 22 h leads to the quantitative (1H NMR) formation
of (IPr)Cu(NHPh) (2, 70% isolated yield; Scheme 1).
Consistent with an independent1H NMR spectrum of CH4
in C6D6, the observation of a singlet at 0.16 ppm (1H NMR)
from the reaction of 1 with NH2Ph is likely due to the
formation of CH4. To confirm that CH4is formed from the
methyl ligand of 1, we prepared (IPr)Cu(Et) (3; Scheme 2).
The reaction of 3 with aniline in a sealed NMR tube produces
2 and a resonance (0.8 ppm,1H NMR) that is consistent with
formation of ethane without observation of the resonance
due to CH4. The monomeric nature of 2 has been confirmed
by a single-crystal X-ray diffraction (XRD) study (Figure
1). To our knowledge, the formally 14-electron complex 2
is the second example of a structurally characterized mon-
omeric copper(I) amido system, with the other example being
the 16-electron, three-coordinate complex (dtbpe)Cu(NHPh)
[dtbpe ) 1,2-bis(di-tert-butylphosphino)ethane].20The Cu-N
bond length of 2 is 1.841(2) Å, consistent with the anticipated
bond length for a Cu-N single bond (see the Supporting
Information), and is similar to the Cu-N bond distance
[1.890(6) Å] of (dtbpe)Cu(NHPh). As expected for a two-
coordinate d10system, the C-Cu-N linkage is nearly linear
at 174.8(1)°.
We have considered two likely pathways for the formation
of 2. Bond homolysis of the Cu-Me linkage of 1 could lead
to the formation of a methyl radical with subsequent
hydrogen atom abstraction from aniline to produce methane
and complex 2. Alternatively, 1 could coordinate aniline and
activate it toward proton transfer to the methyl ligand of 1
to form 2 and CH4. The latter pathway provides greater
opportunity for incorporation into selective catalytic se-
quences. In the presence of either 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) or 1,4-cyclohexadiene, the reaction
of 1 with aniline forms 2 and CH4without observation of
additional organic compounds that would result from trapping
of the methyl radical by TEMPO or 1,4-cyclohexadiene. The
conversion of 1 and aniline to 2 in toluene-d8, which
possesses a relatively weak benzylic C-H(D) bond, yields
observations identical with those of reactions performed in
C6D6. These experiments suggest that Cu-C bond homolysis
is an unlikely reaction pathway. In addition to this point,
complex 1 has only been found to react with relatively acidic
bonds and has failed to produce clean transformations with
substrates that possess pKa’s (in DMSO) > ∼31 (e.g.,
ammonia, furan, and benzene). Finally, the rate of conversion
of 1 and aniline to 2 increases with increasing concentration
of aniline (Figure 2), a result which is inconsistent with a
rate-limiting Cu-C bond homolysis.
(26) Mankad, N. P.; Gray, T. G.; Laitar, D. S.; Sadighi, J. P. Organome-
tallics 2004, 23, 1191-1193.
Scheme 1.
Reactions of (IPr)Cu(Me) (1) {[Cu] ) (IPr)Cu}
Scheme 2.
and Ethane
Preparation of 3 and Reaction with Aniline To Produce 2
Figure 1. ORTEP of (IPr)Cu(NHPh) (2; 20% probability). Selected bond
distances (Å): N3-C1, 1.353(3); N2-C1, 1.359(3); Cu1-C1, 1.875(3);
Cu1-N1, 1.841(2); N1-C28, 1.351(4). Selected bond angles (deg): N2-
C1-Cu1, 124.0(2); C1-Cu1-N1, 174.8(1); Cu1-N1-C28, 127.4(2).
Figure 2. Plot of kobsversus concentration of aniline for the conversion
of complex 1 and aniline to complex 2.
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The reaction of 1 in benzene at room temperature with
ethanol or phenol immediately releases CH4and forms the
copper complexes (IPr)Cu(OEt) (4) and (IPr)Cu(OPh) (5)
in 72% and 75% isolated yields, respectively (Scheme 1).
The ethoxide complex 4 shows no evidence of ?-hydride
elimination after several hours at room temperature in C6D6.
XRD studies reveal that 4 and 5 crystallize as monomeric
copper systems (Figures 3 and 4). The Cu-O bond length
of 5 is 1.839(2) Å and is consistent with a Cu-O single
bond. The corresponding bond length of 4 is shorter at 1.799-
(3) Å and is intermediate between the anticipated bond length
for a single and double bond. Sadighi et al. have reported
the solid-state structure of (IPr)Cu(OtBu), for which the
Cu-O bond distance is 1.8104(13) Å.21The Cu1-O1-C28
[128.1(4)°] and C1-Cu1-O1 [176.9(2)°] bond angles of 4
are similar to the analogous angles [122.85(12)° and 179.05-
(7)°] of (IPr)Cu(OtBu). Crystals of complex 5 were grown
from a reaction with excess PhOH, and 5 crystallized with
1 equiv of PhOH, which forms a hydrogen bond with the
phenoxide ligand of 5 (Figure 4). The reaction of 1 with 1
equiv of PhOH also forms 5 with no evidence of free PhOH
in the final purified product.
The combination of 1 and phenylacetylene in C6D6 for
22 h at 60 °C leads to the formation of (IPr)Cu(CtCPh) (6)
in quantitative yield (as determined by1H NMR spectros-
copy) as well as the production of CH4(Scheme 1). Complex
6 is characterized by resonances at 105.5 and 101.0 ppm in
the13C NMR spectrum that are assigned as the acetylide
carbons. Thus, apparently complex 1 can mediate the
cleavage of weakly acidic C-H bonds. The reaction of 6
with HCl in C6D6cleanly produces the previously reported
complex (IPr)Cu(Cl) and free phenylacetylene (eq 1).8
Complex 1 can also initiate the cleavage of the N-H bond
of pyrrole. After 4 h at room temperature, a solution of 1
and pyrrole converts to (IPr)Cu(N-pyrrolyl) (7). The1H NMR
spectrum of 7 reveals resonances due to symmetry-equivalent
R- and ?-hydrogens of the pyrrolyl fragment at 6.88 and
6.71 ppm, respectively.
Two-coordinate copper(I) alkyl complexes of the type
(IPr)Cu(R) can be used to mediate the cleavage of X-H (X
) N, O, or C) bonds. Such transformations could be integral
to the development of catalytic cycles if metal-mediated
bond-forming reactions are accessible with these and related
Cu systems. On the basis of preliminary results, the acidity
of the X-H bond appears to be a key factor, albeit not likely
the sole factor, for kinetic accessibility to these reactions.
Thus, it may be feasible to break less activated bonds (e.g.,
arenes, alkanes) with closely related Cu systems that are more
electrophilic. We are currently exploring the incorporation
of these systems into catalytic cycles as well as the
preparation of Cu complexes with increased reactivity.
Acknowledgment. The Office of Basic Energy Sciences,
United States Department of Energy (Grant DE-FG02-
03ER15490), the NSF (CAREER Award, Grant CHE
0238167), and the Alfred P. Sloan Foundation (Research
Fellowship to T.B.G.) are acknowledged for support of this
research. E.D.B. acknowledges support from the National
Science Foundation via a Graduate Fellowship and from the
American Association of University Women Educational
Foundation for an American Fellowship. We also thank Prof.
J. P. Sadighi (MIT) for a donation of (IPr)Cu(OAc) and
helpful discussions.
Supporting Information Available: Complete data from XRD
studies of complexes 2, 4, and 5, details of synthesis and
characterization, kinetic plots, and bond distance estimates. This
material is available free of charge via the Internet at http://
pubs.acs.org.
IC0517624
Figure 3. ORTEP of (IPr)Cu(OEt) (4; 20% probability). Selected bond
distances (Å): N1-C1, 1.363(6); N2-C1, 1.347(6); Cu1-C1, 1.863(5);
Cu1-O1, 1.799(3); O1-C28, 1.366(8). Selected bond angles (deg): N1-
C1-Cu1, 128.1(4); N2-C1-Cu1, 129.1(4); C1-Cu1-O1, 176.9(2); Cu1-
O1-C28, 128.1(4).
Figure 4. ORTEP of (IPr)Cu(OPh)-(HOPh) (5-HOPh; 20% probability).
Selected bond distances (Å): N1-C1, 1.356(2); N2-C1, 1.357(2); Cu1-
C1, 1.868(2); Cu1-O1, 1.839(2); O1-C28, 1.339(3). Selected bond angles
(deg): N1-C1-Cu1, 122.7(1); N2-C1-Cu1, 133.5(1); C1-Cu1-O1,
173.0(1); Cu1-O1-C28, 124.0(1).
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Inorganic Chemistry, Vol. 44, No. 24, 2005 8649