Ni(bpy)(cod): A Convenient Entryway into the Efficient Hydroboration of Ketones, Aldehydes, and Imines

Article abstract of DOI:10.1002/ejic.201600143

The catalytic hydroboration of ketones, aldehydes, and imines with pinacol borane and Ni(bpy)(cod) has been demonstrated in benzene at room temperature and low catalyst loadings (0.03-0.3 mol-%). Spectroscopic and structural evidence support the formulati

Full text of DOI:10.1002/ejic.201600143

DOI: 10.1002/ejic.201600143
Communication
Hydroboration
Ni(bpy)(cod): A Convenient Entryway into the Efficient
Hydroboration of Ketones, Aldehydes, and Imines
Amanda E. King,*[a] S. Chantal E. Stieber,[a,b] Neil J. Henson,[c] Stosh A. Kozimor,[a]
Brian L. Scott,[d] Nathan C. Smythe,*[a] Andrew D. Sutton,*[a] and John C. Gordon*[a]
Abstract: The catalytic hydroboration of ketones, aldehydes, dence support the formulation of Ni(bpy)(cod) as containing a
and imines with pinacol borane and Ni(bpy)(cod) has been NiI cation and a bpy·– ligand. The Ni(bpy)(cod) complex reacts
demonstrated in benzene at room temperature and low catalyst quickly with ketonic substrates to form an adduct that appears
loadings (0.03–0.3 mol-%). Spectroscopic and structural evi- to function as an entryway into catalytic activity.
Introduction
and alkene hydrogenation.[6] We have initiated studies aimed
towards the reduction of highly oxidized substrates of relevance
The development of new catalysts based on earth-abundant
elements, especially first-row transition metals, is an area of
great interest, both with regard to the potential for lower cata-
lyst cost as well as harnessing new modes of reactivity. Despite
these advantages, the development of catalysts based on these
elements has lagged behind the development of catalysts
based on precious metal analogs for several reasons. Often,
these classes of catalysts are less efficient than their second-
and third-row congeners and require higher catalyst loadings
for comparable product yields.[1] In contrast to platinum group
elements, first-row transition metals exhibit a tendency to un-
dergo one-electron redox changes, often resulting in different
reactivities.
One strategy used to mitigate against the propensity for
one-electron redox events involving first-row metals is the use
of redox-active ligands that can act as electron reservoirs to
enable multi-electron transformations. This has resulted in sev-
eral examples of first-row transition metals that mimic catalytic
behavior typically exhibited by platinum group metal com-
plexes, such as proton[2] and O2 reduction,[3] Negishi-type cross
coupling,[4] hydrazine disproportionation via nitrene transfer[5]
[a] Chemistry Division, Los Alamos National Laboratory,
MS K558, Los Alamos, NM 875, USA
E-mail: aeking@lanl.gov
nsmythe@lanl.gov
adsutton@lanl.gov
jgordon@lanl.gov
pearl1.lanl.gov/external/c-iiac/chemical-energy/
[b] Department of Chemistry and Biochemistry, California State Polytechnic
University,
Pomona, California 91768, USA
[c] Material Physics Applications Division,
Los Alamos National Laboratory
Los Alamos, NM, USA
[d] Theoretical Division, Los Alamos National Laboratory,
Los Alamos, NM, USA
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600143.
Scheme 1. Reported catalytic and stoichiometric reactivity of 1.[9,10]
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to biomass-to-fuel conversion strategies, including the reduc- tion of 1 with one equivalent of benzophenone in benzene
tion of ketones via hydrodeoxygenation,[7] using first-row transi- results in a rapid color change from dark purple to dark blue
tion metal catalysts supported by redox-active ligands. During with concomitant precipitation of Ni(bpy)(Ph2CO) (2, vide infra),
our efforts, we became interested in the potential reductive which had been previously postulated as the likely reaction
reactivity of Ni(bpy)(cod) (1).
product.[10e,10f,11] An equilibrium constant of Keq = 1.06 can be
derived from the free benzophenone and free 1,5-cyclooctadi-
ene resonances, resulting in a free energy difference of
–0.04 kcal/mol. Subsequent addition of one equivalent of pin-
acol borane to this solution results in an instantaneous color
change back to a homogeneous dark purple. We identified the
resulting formation of the hydroborated benzophenone prod-
uct in quantitative yield and the regeneration of compound 1
in 88 % yield based on the amount of added pinacol borane,
as established by 1H NMR spectroscopy (Figure 1). The addition
of pinacol borane to compound 1 resulted in no observable
spectroscopic change (Figures S1 and S2), suggesting that com-
pound 2 is likely formed prior to borane activation in the hydro-
boration of benzophenone.
First reported by Mori et al. in 1970[8a] and subsequently
isolated and studied more thoroughly by Dinjus and Schind-
ler,[8b,8c] 1 is known to exhibit both catalytic and stoichiometric
reactivity towards a number of substrates (Scheme 1). Com-
pound 1 will catalytically oligomerize unsaturated hydrocarb-
ons,[9a] cyclodimerize substituted cyclopropenes and norborna-
diene,[9a] dimerize propionaldehyde,[9b] and alkylate anhydrides
with alkylzinc reagents.[9c] Stoichiometric reactivity of 1 in-
cludes the ring-opening of aziridines[10a] and α-amino acid N-
carboxy anhydrides,[10b] the dimerization of 1,4-substituted
allenes,[10c] coupling of CO with 1,2-diphenylacetylene,[10d] and
the reduction of CO2 with both 1,4-dienes and aldehydes.[10e,10f]
The stoichiometric reactions shown in Scheme 1 result in formal
oxidation of the nickel center. The addition of an external re-
ductant such as pinacol borane to these reactions could pro-
mote release of the reduced substrate from the nickel center
and regenerate the starting [Ni(bpy)] moiety, thus potentially
creating a catalytic cycle. In particular, we postulated that the
reported coordination of ketones and aldehydes to compound
1[10e] could be exploited in a catalytic fashion.
Scheme 2. Stoichiometric hydroboration of benzophenone with pinacol bor-
ane using 1.
Results and Discussion
The regeneration of 1 upon stoichiometric hydroboration of
benzophenone suggested that 1 could be a competent precat-
alyst for the hydroboration of a variety of ketones and alde-
hydes. Compound 1 proved to be a highly active precatalyst,
with 94 % conversion of benzophenone and pinacol borane
into the hydroborated product within 30 min at room tempera-
ture. The process proceeds at 0.03 mol-% catalyst loading (turn-
Herein we present results that establish Ni(bpy)(cod) as an effi-
cient precatalyst for the hydroboration of ketones, aldehydes,
and imines; that suggest an alternative electronic structure de-
scription of 1; and that also suggest a different mechanistic
pathway for the observed hydroboration chemistry may be in
operation.
Our initial efforts focused on the stoichiometric hydrobor- over number ≈ 3100 and turnover frequency ≈ 6200 h–1). Fur-
ation of benzophenone with pinacol borane (Scheme 2). Reac- thermore, the hydroboration of benzophenone was unaffected
Figure 1. 1H NMR spectra of compound 1 (top, red), a 1:1 mixture of 1 and benzophenone (middle, green), and a 1:1:1 mixture of 1, benzophenone, and
pinacolborane (bottom, blue). Tri-tert-butylbenzene was used as an internal standard. Resonances are denoted as follows: 1 (●), 1,5-cyclooctadiene (‡),
benzophenone (■), hydroborated product (▲), tri-tert-butylbenzene (♦).
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by the addition of excess Hg metal, consistent with a homoge-
neous species effecting catalysis.[12] Encouraged by the obser-
vation of such efficient reactivity under mild conditions, we ex-
amined the substrate scope of hydroboration and the effect of
catalyst loading using compound 1 under the conditions shown
in Scheme 3. Hydroboration of benzophenone, benzaldehyde
and acetophenone substrates proceeds readily in good to excel-
lent yields (79–96 %) within 3 hours (Table 1, entries 1–8). In
general, the inclusion of electron-donating groups such as
methyl and methoxy substituents has no effect on substrate
conversion or product yield for the benzophenone, acetophen-
one, and benzaldehyde substrates. Aryl-substituted imines are
also reduced in excellent yields at room temperature, albeit at
higher catalyst loadings (0.3 mol-%; Table 1, entries 9–11). In
contrast, alkenes are unreactive towards hydroboration; accord-
ingly, the hydroboration of neither 1-octene nor styrene occurs
(Table 1, entries 12 and 13). Thus, compound 1 displays reactiv-
ity competitive with previously reported magnesium-,[13a] tita-
nium-,[13b,13c] gallium-,[13d] aluminum-,[13e] molybdenum-,[13f]
ruthenium,[13g] germanium and tin,[13h] and phosphine-contain-
ing[13i] ketone and aldehyde hydroboration catalysts, and is one
of the most efficient pre-catalysts reported to date for the cata-
lytic hydroboration of imines.[14]
Table 1. Catalytic hydroboration of ketones, aldehydes, and imines with 1.[a]
Scheme 3. Catalytic hydroboration of ketones, aldehydes, and imines with
pinacol borane using 1.
The ability of bipyridine to display redox non-innocence dur-
ing catalysis is established with respect to CO2 reductions,[15]
and we questioned whether similar behavior might be in opera-
tion under the conditions shown in Scheme 3. Compound 1
has traditionally been assigned as a nickel(0) complex with a
1
neutral bipyridine ligand on the basis of its H NMR spectrum,
whose features are in the range typical for diamagnetic com-
pounds, as well as inferences made from the oxidation state of
the Ni(cod)2 precursor.[10e,10f] An alternative formulation for 1 is
a nickel(I) complex supported by a bipyridine anion radical. The
first indication that compound 1 contains a reduced bipyridine
ligand comes from its UV/Visible spectrum (Figure 2). The UV/
Visible spectra of both Ni(cod)2 and bipyridine are featureless
between 400 and 1100 nm (Figure S14), while the spectrum of
1 reveals intense bands at 298 nm and 568 nm and broad,
overlapping bands between 700 and 1000 nm. Furthermore,
the observed transitions are similar to those observed for other
metal complexes supported by a bpy·– ligand (Figure 2 and
Table S4).[16]
[a] General conditions: [Pinacol borane] = 100 m
M, [substrate] = 100 mM, [1] =
0.03 or 0.3 m
M
, C6D6 solvent, room temperature. [b] 1H NMR yields using tri-
tert-butylbenzene as an internal standard.
The internal C=C alkene bond lengths of the bound cycloocta-
The structural parameters of compound 1 were also investi- diene ligand are statistically identical to those found in
gated (Figure 3).[17] The crystal structure of 1 reveals a nickel Ni(cod)2;[18] however, the mean Ni–C bond length of
center located in a pseudo-tetrahedral environment, with Ni–N 2.0528(12) Å is contracted in comparison to Ni(cod)2 [2.117(3)–
bond lengths measuring 1.9397(10) Å and 1.9319(9) Å, and Ni- 2.130(9) Å], perhaps suggesting that the Ni center in 1 is oxi-
C distances measuring between 2.0438(12) Å and 2.0640(12) Å. dized.[19] The contraction of the inter-pyridine C1–C6 bond
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ized orbitals.[25] This results in two frontier orbitals that are Ni-
and bipyridine-based (Figure 4). The latter orbital has a
weighted occupation of 94 % singly occupied, which supports
the bipyridine ligand being best considered as monoanionic
in 1.
Figure 2. UV/Visible spectra of 1 (black) and 1:1 mixture of Na/bipyridine
(blue). Inset show spectra plotted on same scale. The spectra were recorded
in benzene solvent.
length to 1.4578(15)
Å {from 1.530(2) Å in Ni(bpy)-
(H2O)2(SO4)[20a]} is consistent with reduction of the bipyridine
ligand, as are the observed contractions of the C1–C2, C3–C4,
and C1–N1 bond lengths (see Table S2).[16,20] However, as several
groups have demonstrated, metrical parameters alone cannot
rule out the possibility of extensive backbonding from a Ni0
center, thus necessitating the use of electronic structure calcu-
lations.[21]
Figure 4. Ruedenberg-localized frontier molecular orbitals showing Ni (left)
and bipyridine (right) character.
The characteristic spectral features of the bpy·– species in
Figure 2 disappear upon the addition of benzophenone, benz-
aldehyde, or benzophenone imine to solutions of 1 (Figure
S15).[10e] These UV/Vis results suggest that the reaction be-
tween 1 and R2CO results in oxidation of the bipyridine ligand,
loss of cod, and reduction of the NiI ion. Hence, 2 is best de-
scribed as containing a neutral bipyridine ligand. DFT calcula-
tions suggest that 2 contains a Ni0 center with substantial inter-
action with the benzophenone ligand, with both RKS, UKS, and
broken symmetry approaches converging to a closed-shell, Ni0
species (Figure S18).[26] In support of this interpretation, the
formal reduction of the nickel center, in this case to a Ni0 oxid-
ation state, upon the binding of a π-acid such as benzophen-
one has experimental precedent.[27] The computed electronic
structure of 2 implies that the bipyridine redox activity likely
does not contribute to the catalytic activity seen when 1 is used
as a precatalyst.
A generally accepted mechanism for the hydroboration of
ketones, aldehydes, and imines is shown in Scheme 4.[28] A key
feature of previously proposed mechanisms is the direct inter-
Figure 3. Molecular structure of 1. Atoms in the bipyridine ring have been
labelled for clarity. Ellipsoids shown at 50 % probability.
Attempting to model the electronic state of bpyNi(COD) action of the catalyst with the borane substrate to form a metal
computationally using the B3LYP functional, as has been dem- hydride species, prior to activation of the ketonic substrate. In
onstrated for a variety of Ni[22] and Fe[20d,23] complexes, results contrast, the experimental data presented herein highlights the
in a stable broken symmetry BS(1,1) solution (Figure S18) that possibility for another catalytic pathway that proceeds via the
is energetically (–1.5 kcal/mol) and structurally indistinguish- nickel-ketone adduct. In this alternative mechanism, compound
able from the closed-shell state. Given the potential multi-refer- 2 could be the active catalytic species under the reaction condi-
ence character of 1 and because DFT can misidentify BS solu- tions. Hydroboration of benzophenone proceeds smoothly to
tions[24] we turned to the CASSCF method. The potential redox completion with a catalytic amount of pre-formed and crystal-
activity of bipyridine-type ligands has recently been studied us- lographically characterized Ni(tBubpy)(benzophenone)[29] within
ing multi-reference methods, with the authors using the soft 30 min. Thermodynamic calculations support the essentially
criterion of significant electron occupation of the metal-ligand thermoneutral reaction between benzophenone and 1 with a
anti-bonding orbital as the indicator of ligand reduction.[24b] As calculated free energy of 2.6 kcal/mol (vs. an experimental value
the CSF weights of 1 fall between what is attributed to ligand of –0.04 kcal/mol, vide supra). While the experimental data
reduction vs. π-back-donation, the CASSCF wavefunction of 1 point to the likelihood of 2 being a catalytically active species,
was further analyzed using localization followed by calculation the possibility of a nickel hydride intermediate cannot be con-
of the configurational interaction (CI) coefficients for the local- clusively dismissed on the basis of thermodynamic arguments.
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DFT calculations indicate that the net dissociation of benzo- ated by Los Alamos National Security, LLC, for the National Nu-
phenone from 2 and subsequent oxidative addition of pinacol clear Security Administration of U.S. Department of Energy
borane[30] is only 8.3 kcal/mol uphill in energy (Scheme 5).
(contract DE-AC52-06NA25396).
Keywords: Homogeneous catalysis · Nickel · Hydroboration ·
Redox chemistry · Electronic structure
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Scheme 5. Calculated free energies for the reaction of benzophenone and
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Conclusion
Compound 1 provides an extremely efficient entryway into the
hydroboration of ketones, aldehydes, and imines. In contrast to
previously reported assignments, 1 is best described as contain-
ing a NiI(bpy·–) fragment. However, the likely catalytic species,
compound 2, contains a neutral bipyridine ligand. Both the ob-
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Acknowledgments
The authors thank the Laboratory Directed Research and Devel-
opment (LDRD) program for a Director's Postdoctoral Fellow-
ship to A. E. K. S. C. E. S. was supported by a LANL Glenn T.
Seaborg Institute Postdoctoral Fellowship and by startup fund-
ing from the College of Science at the California State Polytech-
nic University, Pomona. Additional funding was provided by the
under the Heavy Element Chemistry Program at LANL by the
Division of Chemical Sciences, Geosciences, and Biosciences, Of-
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Received: February 12, 2016
Published Online: March 18, 2016
Eur. J. Inorg. Chem. 2016, 1635–1640
www.eurjic.org
1640
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