Electron transfer from hexameric copper hydrides

Article abstract of DOI:10.1021/ja408925m

The octahedral core of 84-electron LCuH hexamers does not dissociate appreciably in solution, although their hydride ligands undergo rapid intramolecular rearrangement. The single-electron transfer proposed as an initial step in the reaction of these hexamers with certain substrates has been observed by stopped-flow techniques when [(Ph₃P)CuH]₆ is treated with a pyridinium cation. The same radical cation has been prepared by the oxidation of [(Ph₃P)CuH]₆ with Cp* ₂Fe⁺ and its reversible formation observed by cyclic voltammetry; its UV-vis spectrum has been confirmed by spectroelectrochemistry. The 48-electron trimer [(dppbz)CuH]₃ has been prepared by use of the chelating ligand 1,2-bis(diphenylphosphino)benzene (dppbz).

Full text of DOI:10.1021/ja408925m

Communication
pubs.acs.org/JACS
Electron Transfer from Hexameric Copper Hydrides
†
Michael S. Eberhart, Jack R. Norton,* Ashley Zuzek, Wesley Sattler, and Serge Ruccolo
Department of Chemistry, Columbia University, New York, New York 10027, United States
*
S Supporting Information
19
have been said to be “concentration dependent”. However,
Caulton, the first to observe their hydride ligands by H NMR,
1
ABSTRACT: The octahedral core of 84-electron LCuH
hexamers does not dissociate appreciably in solution,
although their hydride ligands undergo rapid intra-
molecular rearrangement. The single-electron transfer
proposed as an initial step in the reaction of these
hexamers with certain substrates has been observed by
stopped-flow techniques when [(Ph P)CuH] is treated
reported for 1b a “structured multiplet...five components of a
31
20
septet” that collapsed with P decoupling, suggesting that the
hexamer remains intact in solution.
The reactivity of these LCuH complexes varies with the
nature of their phosphine ligand. For example, the Ph3P
hexamer (1a) is selective for the 1,4 reduction of enones,
3
6
with a pyridinium cation. The same radical cation has been
whereas the Me PhP complex (whose extent of aggregation has
2
+
21
prepared by the oxidation of [(Ph P)CuH] with Cp* Fe
not been established) gives only 1,2 reduction. We have
3
6
2
and its reversible formation observed by cyclic voltamme-
try; its UV−vis spectrum has been confirmed by
spectroelectrochemistry. The 48-electron trimer [(dppbz)-
therefore prepared a number of LCuH complexes and
examined their structures, spectra, and reactivityparticularly
their ability to transfer an electron.
i
CuH] has been prepared by use of the chelating ligand
3
Clusters [(p-anisyl) PCuH] (1c) and [( PrPh P)CuH]
3
6
2
6
22
1,2-bis(diphenylphosphino)benzene (dppbz).
(
1d) were prepared by established methods, from CuCl, 1
t
equiv of KO Bu, and 2 equiv of the phosphine ligand under an
i
H atmosphere in benzene or toluene. The PrPh P complex 1d
2
2
opper hydrides are well-known catalysts for the hydro-
genation of α,β-unsaturated carbonyl compounds, of
and the (p-anisyl)
3
P complex 1c (structures in Figure 1) have
1
C
other CC bonds conjugated to electron-withdrawing
2
3
groups, and of alkynes. Although electron transfer has been
2
b,4
suggested as a possible first step for these reactions,
Stryker
has shown that electron transfer is not involved in the
5
hydrogenation of α,β-unsaturated carbonyls, noting that “the
reaction [affords] major products deriving from hydride
1
delivery to the least hindered face of the substrate”. The
absence of electrochemical data for copper hydrides, however,
has left open the possibility that electron transfer does occur in
6
other reactions. We now demonstrate its occurrence in the
transfer of hydride from [Ph PCuH] (1a) to pyridinium
3
6
cations.
Copper hydrides have been known for many years. Wu
prepared an insoluble CuH in 1844, the first reported
7
̈
rtz
Figure 1. Core ellipsoid plots (50% probability) for 1c [(p-
8
i
anisyl) PCuH] and 1d [ PrPh PCuH] . The short Cu−Cu bonds
3
6
2
6
transition-metal hydride, while Lutsenko et al. reported soluble
average 2.49 Å (1c), 2.47 Å (1d); the long Cu−Cu bonds average 2.70
Å (1c), 2.76 Å (1d).
9
LCuH without establishing the extent of aggregation. Several
1
0
11
(
μ-H)2 dimers are known, as well as an octamer,
a
1
2
13
pentamer, a trimer, and a number of clusters with interstitial
hydrides. However, hexameric [LCuH] clusters form the
1
4
proven isostructural with the previously reported 1a and 1b,
with the six smaller octahedral faces capped by hydrides. (The
Cu−Cu distances around these faces are slightly shorter than
6
7
most widespread and important class of copper hydrides.
Osborn, Churchill, and co-workers established the stoichiom-
the distances in metallic copper, 2.5562 Å.) [BnPh PCuH]
etry of the Ph P derivative 1a and established its hexameric
2
6
3
2
3
15
(
1e) was also prepared, but its crystals were twinned.
structure crystallographically in 1971, although they did not
locate the hydride ligands. The hydrides in [P(NMe ) CuH]
6
One equivalent of a chelating ligand, 1,2-bis-
2
3
(
diphenylphosphino)benzene (dppbz), gives the related trimer
were shown by Caulton in 1985 to cap six of the eight faces of
1
11
2 (eq 1), with the H NMR of its hydride ligands at δ 0.60
an octahedral core of Cu atoms, and the structure was
(
C D ). Its X-ray structure (Figure 2) shows that the hydride
confirmed for [p-tol PCuH] (1b) by a subsequent neutron
6 6
3
6
16
ligands are μ , in the plane of the three Cu atoms; the Cu−Cu
diffraction study by Bau and Koetzle. (The structure is similar
2
17
to that of [n-BuLi] . )
6
The solution structure of these complexes has been the
subject of debate. The extinction coefficients of 1a and 1b
Received: August 29, 2013
Published: October 30, 2013
18
©
2013 American Chemical Society
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dx.doi.org/10.1021/ja408925m | J. Am. Chem. Soc. 2013, 135, 17262−17265

Journal of the American Chemical Society
Communication
Figure 2. Core ellipsoid plot of 2 [(dppbz)CuH] . The Cu−Cu bonds
3
average 2.58 Å.
distances average 2.58 Å, about the same as the short Cu−Cu
distances (on the capped faces) in the hexamers 1a−d.
The same ligand (dppbz) has been used with excess
Cu(OAc) ·H O and a silane to prepare an apparent (dppbz)-
2
2
24
CuH adduct 3 in solution, although the hydride chemical shift
reported for 3 (δ 1.49 in C D ) differs substantially from that of
1
Figure 3. H NMR spectra of 1a hydride at (top) 263 and (bottom)
303 K. J(P−H) = 5.4 Hz.
6
6
2
4a,b,25
1
2.
We see (by H NMR) 3 as an initial product when
dppbz is added to a C D solution of 1a, although the final
6
6
Intramolecular hydride exchange within [LCuH] probably
6
product is 2.
Molecular Orbital Calculations. The 84 valence electrons
in L Cu (μ -H) are consistent with the 12 Cu−Cu bonds
involves the two uncapped faces of the Cu octahedra.
6
Electrochemistry of [LCuH] . The hexamers 1a−c have
6
6
6
3
6
been examined by cyclic voltammetry (CV) in CH Cl .
2
2
(
each a shared electron pair) in its octahedral core, as suggested
0/+1
Reversible waves for [LCuH]6
(E1/2) have been observed
at −1.01 (1a), −1.20 (1b), and −1.15 V (1c) vs Fc/Fc .
Reversible cyclic voltammograms for 1a−c (see Figure 4 for
26
by the 18-electron rule. DFT calculations in D symmetry on
+
3
d
a simplified model, with PH in place of the more computa-
3
tionally demanding ligands actually present, confirm that the
bonding and nonbonding molecular orbitals are exactly filled,
and that the antibonding ones are empty.
The HOMO−LUMO gap is large (3.2 eV). The results of
our calculations are consistent with those reported in 2007 for
the octahedral cores of the rhodium clusters
0
27
[
(H P) Rh H ] ,1+,2+. The Cu−Cu distances predicted by
3
6
6
12
our DFT calculations (2.54 Å on the hydride-capped faces, 2.71
Å on the uncapped ones) correspond well to the crystallo-
graphically determined ones.
Similarly, the 48 valence electrons in the trimer 2 are
consistent with the three Cu−Cu bonds suggested by the 18-
electron rule.
Figure 4. Reversible cyclic voltammogram for 1c in CH Cl .
2
2
Solution Structure of [LCuH] . We have found the
6
19
absorbance for 1a at λmax = 524 nm to be linear in CH Cl2
2
(
2
Figure S-1) over a substantial range of [1a], implying that ε is
790(50) L·mol ·cm . This result, and Caulton’s early
1
CV for 1c) suggest that an electron is removed from 1a−c,
giving a radical cation intermediate without significant
structural rearrangement. (The peak separations are 67, 63,
−
1
−1
observation by H NMR spectroscopy of intramolecular hydride
exchange within 1b, suggest that LCuH hexamers remain
28
29
and 66 mV, respectively. The fact that little current flows at
intact in solution. We have found excellent agreement between
the beginning of the scan reflects the fact that it starts near the
rest potential. ) All three hexamers are thus excellent single-
1
30
the H NMR spectrum of 1a in toluene-d at 263 K and that
8
calculated with JH−P = 5.4 Hz to six equivalent phosphorus
atoms (Figure 3). The spectrum broadens as the temperature is
raised to 303 K, presumably from residual coupling to 6 Cu
electron reductants, with potentials only about 130 mV less
0/+
31
reducing than that of Cp Co in the same solvent.
2
3,65
+•
+•
The formation of intensely green [Ph PCuH] (1a , λ
3
6
max
(
quadrupolar nuclei that are relaxed more slowly as the
655 nm) from red 1a can be observed by spectroelectrochem-
+
viscosity of the solvent decreases). The spectrum also broadens
when the temperature is decreased substantially (e.g., to 200
K), presumably because intramolecular exchange becomes
slower.
istry when a potential of −800 mV vs Fc/Fc is applied (Figure
5), although the lifetime of the radical cation is limited (it
persists at least 30 min). The radical cation 1a+ can be reduced
cleanly back to 1a (and the sequence of spectra in Figure 5
•
1
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Journal of the American Chemical Society
Communication
ASSOCIATED CONTENT
Supporting Information
Experimental procedures, NMR chemical shifts for H, P, and
■
*
S
1
31
13
C, X-ray crystallographic data, and additional CV data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
jrn11@columbia edu
Present Address
†
+
W.S.: Division of Chemistry and Chemical Engineering,
Figure 5. Oxidation of 1a over 30 s at −0.8 V vs Fc/Fc .
California Institute of Technology, Pasadena, CA 91125
+
Notes
reversed) if the potential is changed to −1.3 V vs Fc/Fc after
00 s of oxidation. The radical cation can also be obtained by
chemical oxidation of 1a with [Cp* Fe][PF ] (for Cp* Fe/
1
The authors declare no competing financial interest.
2
6
2
+
+
31
Cp* Fe , E = −590 mV vs Fc/Fc ).
ACKNOWLEDGMENTS
2
1/2
■
Additional oxidation waves, at less negative potentials, can be
observed for some hexamers. For example, a second,
irreversible, oxidation is seen with 1a and 1b, and four
additional oxidations (also irreversible) are seen with 1c (see
Supporting Information).
The National Science Foundation (CHE-0619638) is thanked
for acquisition of an X-ray diffractometer. Preliminary work was
funded by NSF grant CHE-0749537, and Boulder Scientific
and OFS Fitel are thanked for continuing support. We thank
Yan Yang (Brown), Prof. Chip Nataro (Lafayette), Dr.
Anthony P. Shaw (Picatinny Arsenal), and Prof. James Camara
(Yeshiva) for useful discussions.
Electron Transfer during Hydride Transfer from
[
LCuH] . These CV data show that the hexamers 1 are better
6
reductants than Cp*Ru(dppf)H, a hydride that is known to
undergo electron transfer during its reaction with the
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■
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(
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(
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(
(
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