Synthesis and Reactivity of New Copper(I) Hydride Dimers

Article abstract of DOI:10.1021/acs.organomet.6b00025

The expanded-ring N-heterocyclic carbenes 6Dipp and 7Dipp (6Dipp = 1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene and 7Dipp = 1,3-bis(2,6-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene) support isolable neutral copper hydride dimers. [(6Dipp)CuH]₂ reacts with 1-hexene to give (6Dipp)copper(I) hexyl by 1,2-insertion and with benzyl isonitrile to afford an η¹-formimidoyl by 1,1-insertion.

Full text of DOI:10.1021/acs.organomet.6b00025

Communication
pubs.acs.org/Organometallics
Synthesis and Reactivity of New Copper(I) Hydride Dimers
Abraham J. Jordan,† Chelsea M. Wyss,† John Bacsa,‡ and Joseph P. Sadighi*,†
†School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States
‡X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United
States
S
* Supporting Information
ABSTRACT: The expanded-ring N-heterocyclic carbenes
6Dipp and 7Dipp (6Dipp = 1,3-bis(2,6-diisopropylphenyl)-
3,4,5,6-tetrahydropyrimidin-2-ylidene and 7Dipp = 1,3-bis(2,6-
diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene)
support isolable neutral copper hydride dimers. [(6Dipp)CuH]2
reacts with 1-hexene to give (6Dipp)copper(I) hexyl by 1,2-
insertion and with benzyl isonitrile to afford an η1-formimidoyl
by 1,1-insertion.
opper hydrides are key intermediates in transformations
of a wide range of unsaturated organic subsrates.1 The
Schomaker et al. by changing the isopropyl groups of IDipp
to 3-pentyl or 4-heptyl.15 Diffusion NMR experiments (DOSY)
revealed that the (NHC)copper(I) hydrides favor a dimeric
form in solution as well as in the solid state.15
C
hexameric cluster [(Ph3P)CuH]6, first reported by Osborn et
al.2 and subsequently known as Stryker’s reagent,3−5 is among
the most widely studied copper hydride complexes.6 Various
phosphine donors have provided excellent scaffolds for copper
hydrides in numerous reduction processes,1 with new
applications appearing rapidly.7−11 Exploring lower-nuclearity
copper hydrides, Caulton and co-workers showed the
formation of a hydride bridge in a dimeric {Cu2(μ-H2)} core
to be stronger than a potential third P−Cu bond.12 N-
heterocyclic carbenes (NHCs) also serve as excellent
supporting ligands for copper-catalyzed reduction pro-
cesses.13−17 While the active (NHC)CuH species is typically
generated in situ, the solid-state structure of [(IDipp)CuH]2
(1a; Figure 1) also reveals a dimeric {Cu2(μ-H2)} core.18
Expanded-ring NHCs, derived from six- and seven-
membered cyclic amidinium salts, have been shown to increase
the stability of other reactive metal species.20−22 Recently,
Whittlesey et al. reported that (6Mes)copper(I) hydride (6Mes
= 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-
ylidene) proved too unstable to isolate but could be trapped
efficiently with alkynes or ketones.23 We now report the
synthesis of [(6Dipp)CuH]2 (3a) and [(7Dipp)CuH]2 (3b)
(6Dipp = 1,3-bis(2,6-diisopropylphenyl)-3,4,5,6-tetrahydropyr-
imidin-2-ylidene; 7Dipp =1,3-bis(2,6-diisopropylphenyl)-
4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene). These complexes
are stable at ambient temperature, both in the solid state and
in solution. Complex 3a undergoes clean 1,2-insertion of an
alkene substrate and 1,1-insertion of an isonitrile.
The generation of 3a or 3b by addition of pinacolborane
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin) to a solution
of (6Dipp)Cu(O-t-Bu) or (7Dipp)Cu(O-t-Bu) is evidenced by
the generation of an intense yellow or orange color (Scheme
1
1). The H NMR spectra of these hydrides in C6D6 solution
Scheme 1. Synthesis and Carboxylation of (NHC)CuH
Dimers
Figure 1. Isolable (carbene)copper hydrides: [(IDipp)CuH]2 (1a), ref
17; [(CAAC)CuH]2 (2), ref 18; 3a,b, this work.
Recently, Bertrand et al. showed that cyclic alkylaminocar-
bene (CAAC) ligands greatly improve the stability of the
{Cu2(μ-H2)} core (2; Figure 1).19 The authors attribute this
stability to the increased steric bulk about the metal hindering
the formation of larger oligomers. A slight increase in the
stability of (NHC)copper(I) hydrides was achieved by
Received: January 13, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00025
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
show characteristic hydride singlets at δ 0.77 ppm (3a) and at δ
0.47 ppm (3b). For comparison, the spectrum for the
analogous [(5Dipp)CuH]2 (1b; 5Dipp = 1,3-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene)24 shows a hydride
resonance at δ 1.93 ppm25 and that of [(IDipp)CuH]2 at δ
2.67 ppm.18 This upfield shift in the hydride resonances may be
attributed not only to increased electron donation by the
NHCs but also to anisotropic shielding by the aryl rings as they
are brought closer to the hydrides by the wider N−C−N
angle.26,27
The solid-state infrared spectra of 3a,b show absorptions
assigned to bridging hydride−copper stretches at 909 and 912
cm−1, respectively. These values fall between those of 881 cm−1
for 1b18 and 981 cm−1 for 2.19 Although terminal copper
hydride stretching frequencies have not been reported, to the
best of our knowledge, the IR spectrum of monomeric
(IDipp)AuH displays a sharp Au−H stretch at 1976 cm−1.28
We therefore wondered whether dissociation in solution to
form a terminal hydride might be observable by IR spectros-
copy, but the solution IR spectrum of 3a (Figure S5 in the
Supporting Information) displayed no absorption we would
attribute to a terminal copper hydride. Moreover, the bridging
hydride absorption remains, suggesting that the dimeric form is
favored in solution, as it is for hydrides bearing five-membered
NHCs.15 The intense yellow color of copper hydride dimers,
unusual for copper(I) complexes, is well documented.18,19,29
The UV−vis absorption spectra of 3a,b show strong
absorptions at 453 and 476 nm, respectively (Figure 2).
recently, the 1,2-insertion of unactivated alkenes into copper
hydrides remained elusive. Buchwald et al. reported the
insertion of copper hydrides into unactivated alkenes supported
by bidentate phosphine ligands in the hydroamination of
olefins.11,38 Although (NHC)copper(I) alkyl systems are
known, they are typically prepared via transmetalation.39−42
1
After treatment of 3a or 3b with 1-hexene, the H NMR
spectrum shows the growth of a triplet resonance at high field,
assigned to a newly formed copper-bound methylene. In the
reaction of 1-hexene with 3a in THF solution, the yellow color
disappears within 24 h to yield (6Dipp)copper(I) 1-hexyl (5a).
Under 1 atm of CO2, 5a cleanly forms (6Dipp)copper(I)
heptanoate (6a) within 40 min (Scheme 2). This reaction is
Scheme 2. Formation and Carboxylation of a Copper Alkyl
consistent with the insertion of CO2 into other (NHC)-
copper−carbon bonds.39,42 With 1 equiv of 1-hexene per Cu,
insertion into [(5Dipp)CuH]2 (generated in situ) was also
1
evident by H NMR spectroscopy but proceeded to the extent
of only 20% after 48 h. Decomposition became apparent after 2
h and continued throughout the reaction period (see Figure
S20 in the Supporting Information).
Discrete 1,1-insertion reactions of isonitriles with copper
hydrides have not been reported, but isonitriles have been
shown to insert into Cu−B bonds in the synthesis of 2-
borylated indoles,43 and the insertion of isonitriles into other
M−H bonds is well documented.44 Benzyl isonitrile readily
inserts into [(6Dipp)CuH]2, yielding a new copper formimi-
doyl (7a; Scheme 3). Interestingly, Bertrand et al. treated 2
with t-BuNC and observed the net insertion of the supporting
carbene, rather than the isonitrile, into the copper−hydride
bond.19
Scheme 3. Isonitrile Insertion into [(6Dipp)CuH]2
Figure 2. UV−vis absorption spectra of 3a (yellow) and 3b (orange)
in toluene solution.
1
These electronic transitions have not been definitively assigned,
but the close Cu···Cu contact18,19 may allow a dσ* → pσ
transition analogous to those identified by Gray et al. for d8−d8
and d10−d10 complexes of Pd and Pt.30−33
The H NMR spectrum of 7a in C6D6 solution shows a
characteristic singlet resonance at δ 9.37 ppm, consistent with
the formation of a formimidoyl moiety. Furthermore, the
benzyl protons shift from δ 3.61 ppm for benzyl isonitrile to δ
4.53 ppm in 7a. The 13C NMR spectrum shows a new
resonance at δ 209.9 ppm, characteristic of the metal-bound
carbon of an η1-formimidoyl.45,46 This resonance was
confirmed to be that of a CH moiety by a DEPT 90
experiment (see Figure S18 in the Supporting Information).
Slow diffusion of pentane into a toluene solution of 7a afforded
yellow crystals suitable for X-ray diffraction (Figure 3). The
structure shows a nearly linear geometry about copper, with an
angle of 173.26(8)°, while the Cu−C−N angle of the
formimidoyl is 120.4(2)°. The copper−carbon distances to
both the NHC and formimidoyl are quite similar at 1.933(2)
and 1.916(2) Å, respectively. The formimidoyl C−N distance is
1.291(3) Å, consistent with a CN double bond. Benzyl
isonitrile also reacts with 3b, as judged by 1H NMR
Whereas decomposition of 1a is evident after 1 h in solution
at ambient temperature, the ligands 6Dipp and 7Dipp confer
greatly improved stability. Complexes 3a,b are stable for
multiple days in solution and in the solid state. We attribute this
increase in stability to the steric encumbrance enforced by the
expanded NHC backbone through the N-aryl groups. Despite
this increased stability, the hydride remains quite reactive: the
rapid insertion of CO2 into 3a,b is discernible by an immediate
loss of color when their solutions are exposed to an atmosphere
of CO2 (Scheme 1). The products are formates 4a,b, analogous
to those formed from copper hydrides supported by other
NHCs.34−37
Hydrocupration of alkynes is well documented and is a key
catalytic step in many reductive processes.17,18,23,25 Until
B
DOI: 10.1021/acs.organomet.6b00025
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00025.
■
S
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Experimental procedures and spectral and crystallo-
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Crystallographic data for 3e (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.P.S.: joseph.sadighi@chemistry.gatech.edu.
■
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation (CHE-
1300659 to J.P.S) and the Georgia Institute of Technology
for generous support of this research. Professor Jake D. Soper
allowed us the use of his group’s FTIR and UV−vis
spectrometers.
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