CO2 reduction with protons and electrons at a boron-based reaction center

Article abstract of DOI:10.1039/c9sc02792k

Borohydrides are widely used reducing agents in chemical synthesis and have emerging energy applications as hydrogen storage materials and reagents for the reduction of CO₂. Unfortunately, the high energy cost associated with the multistep preparation of borohydrides starting from alkali metals precludes large scale implementation of these latter uses. One potential solution to this issue is the direct synthesis of borohydrides from the protonation of reduced boron compounds. We herein report reactions of the redox series [Au(B₂P₂)]ⁿ (n = +1, 0, -1) (B₂P₂, 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) and their conversion into corresponding mono- and diborohydride complexes. Crucially, the monoborohydride can be accessed via protonation of [Au(B₂P₂)]^-, a masked borane dianion equivalent accessible at relatively mild potentials (-2.05 V vs. Fc/Fc⁺). This species reduces CO₂ to produce the corresponding formate complex. Cleavage of the formate complex can be achieved by reduction (ca. -1.7 V vs. Fc/Fc⁺) or by the addition of electrophiles including H⁺. Additionally, direct reaction of [Au(B₂P₂)]^- with CO₂ results in reductive disproportion to release CO and generate a carbonate complex. Together, these reactions constitute a synthetic cycle for CO₂ reduction at a boron-based reaction center that proceeds through a B-H unit generated via protonation of a reduced borane with weak organic acids.

Full text of DOI:10.1039/c9sc02792k

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ARTICLE
CO2 Reduction with Protons and Electrons at a Boron-Based
Reaction Center
Jordan W. Taylor, Alex McSkimming, Laura A. Essex and W. Hill Harman*a
Received 00th January 20xx,
Accepted 00th January 20xx
Borohydrides are widely used reducing agents in chemical synthesis and have emerging energy applications as hydrogen
storage materials and reagents for the reduction of CO2. Unfortunately, the high energy cost associated with the multistep
preparation of borohydrides starting from alkali metals precludes large scale implementation of these latter uses. One
potential solution to this issue is the direct synthesis of borohydrides from the protonation of reduced boron compounds.
We herein report reactions of the redox series [Au(B2P2)]n (n = +1, 0, –1) (B2P2, 9,10-bis(2-(diisopropylphosphino)phenyl)-
9,10-dihydroboranthrene) and their conversion into corresponding mono- and diborohydride complexes. Crucially, the
monoborohydride can be accessed via protonation of [Au(B2P2)]–, a masked borane dianion equivalent accessible at
relatively mild potentials (–2.05 V vs. Fc/Fc+). This species reduces CO2 to produce the corresponding formate complex.
Cleavage of the formate complex can be achieved by reduction (ca. –1.7 V vs. Fc/Fc+) or by the addition of electrophiles
including H+. Additionally, direct reaction of [Au(B2P2)]– with CO2 results in reductive disproportion to release CO and
generate a carbonate complex. Together, these reactions constitute a synthetic cycle for CO2 reduction at a boron-based
reaction center that proceeds through a B–H unit generated via protonation of a reduced borane with weak organic acids.
DOI: 10.1039/x0xx00000x
monoxide10 and formaldehyde,11 formic acid,12 and methanol
equivalents.13
Despite their broad utility, one drawback to these reagents
is the substantial energy required for their production. For
Introduction
Since their discovery by Schlesinger and Brown in the
1940s,1 borohydrides have become ubiquitous reducing agents
in the synthesis of fine and commodity chemicals.2 More
recently, borohydrides have attracted interest in energy storage
applications,3 both as a dense and readily handled source of H2
and in the reduction of CO2 to fuels such as formic acid and
methanol.4 The first report of the reaction of NaBH4 with CO2
dates back to 1955 when Wartick and Pearson described the
solution (dimethyl ether) and solid-state reactions with mass-
balance and hydrolytic analysis of the products.5 In 1967, the
reaction of NaBH4 with CO2 in aqueous conditions was shown to
produce sodium formate.6 The intrinsic role of the borohydride
ion in CO2 reduction was later investigated by Mizuta in 2014
who showed that, in the presence of a catalytic amount of
example, millions of kg of NaBH4 are produced each year by the
NaH reduction of B(OMe)3 in the Brown-Schlesinger process.14
B(OCH3)3 + 4 NaH → NaBH4 + 3 NaOCH3 (Eq 1)
NaH is prepared from the reaction of metallic Na with H2 gas,14
and the reduction of NaCl to metallic Na in the Downs Process.
The process requires high temperature (>600 °C) and operating
potentials of 8 V or more for practical reaction rates.15 As a
result, the stoichiometric reduction of CO2 with NaBH4, or
similarly produced B–H species,16 entails a significant energy
cost.
NaBH4, BH3
・
THF effectively reduces CO2 to
trimethylboroxine.7 A year later, Cummins reported the direct
reaction of NaBH4 with CO2 yields the trisformatohydroborate
that could be hydrolyzed to yield formic acid.8 Additional
studies over the last decade have found that in the presence of
a suitable catalyst, pinacol borane (HBpin) and catechol borane
(HBcat) can reduce CO2 to a range of products9 including carbon
Figure 1. Redox-coupled hydride formation at a transition metal complex (top) and
borane (bottom) as a strategy for CO2 electroreduction to formate.
One potential solution to this issue would involve the direct
construction of B–H bonds from the protonation of reduced
boron compounds accessed at potentials tolerable to common
solvents. Such a process would be analogous to the generation
of transition metal hydrides via the reduction and protonation
a. Department of Chemistry, University of California, Riverside, Riverside, California,
92521, United States. Email: hill.harman@ucr.edu
† Electronic Supplementary Information (ESI) available: Synthetic and experimental
details and characterization data. X-ray crystallographic data is available from the
CCDC under accession numbers 1921501–1921506 . See DOI: 10.1039/x0xx00000x
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of a transition metal complex (Figure 1). Despite the difficulty in thought of as a masked borane dianion, we wondered if
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accessing doubly reduced boranes,17 there has been recent protonation of this complex could provide direct access to a
DOI: 10.1039/C9SC02792K
progress in the protonation of low valent boron compounds to hydridic B–H unit (Figure 2). We herein report the synthesis and
generate B–H bonds (Figure 2). In 2006, Nozaki et al. reported characterization of this borohydride complex, Au(B2P2)H (2),
the isolation of the boryllithium compound LiB(2,6- which can be generated via direct hydride reduction of
iPrC6H4)NCH2)2, which reacts as a boryl anion equivalent, and Au(B2P2)Cl (3), FLP-type dihydrogen activation, H-atom addition
showed that it could be protonated with H2O to give the to the boron-centered radical complex Au(B2P2) (4) or, most
corresponding hydroborane.18 Bertrand et al. have prepared notably, by protonation of
1 with mild organic acids.
the carbene-stabilized borylene (CAAC)2BH (CAAC = cyclic Borohydride 2 is sufficiently reactive to reduce CO2 to formate,
(alkyl)(amino)carbene) that could be protonated with and cleavage of the resulting B–OCHO bond can be achieved
trifluoromethanesulfonic acid (TfOH) to yield the corresponding either by addition of an electrophile (including H+) or by one
boronium
salt.19
A
related
bis(oxazol-2-ylidene)– electron reduction. These results collectively represent a
phenylborylene system reported by Kinjo also produced a B–H synthetic cycle for CO2 reduction to formate with protons and
moeity via protonation with TfOH.20 Furthermore, Wagner electrons, with the key finding being the generation of a
synthesized a diborylmethane dianion and a doubly boron- hydridic borohydride unit via protonation of a reduced borane
doped dibenzo[g,p]chrysene dianion that could be protonated at modest potentials. This reaction sequence, along with
to yield the corresponding hydride bridged B–H–B species.21 Wagner’s recent reports of CO2 hydrogenation25 and the
26
Additionally, Szymczak et al. reported a conceptually relevant reductive disproportionation of CO2 catalyzed by DBA
series of Mn/Cr-borazine complexes that could be reduced and derivatives, illustrates the promise of redox-active boranes in
then protonated to yield a bridging metalloborohydride.22 The the reduction of CO2, an area dominated by transition metal
reactivities (in particular, towards hydride transfer) of these B– chemistry.27 Furthermore, given the recent application of
H bonds formed via protonation have not been reported.
boron-doped graphene to CO2 electroreduction,28 these
molecules offer well-defined molecular models29 of
fundamental chemical processes that may underlie small
molecule reactivity at doped graphitic carbon surfaces.
Results and discussion
Our initial synthetic approaches to a B–H containing
derivative of Au(B2P2) began with direct hydride for halide
substitution. Addition of 1 equiv. K[sec-Bu3BH] to 3 gave
borohydride 2 in 89% yield. This complex could also be prepared
in good yield by exposing 3 to an excess of Et3SiH. Additionally,
2 could be synthesized via frustrated Lewis pair (FLP) H2
activation in the presence of DBU (DBU
=
1,8-
diazabicyclo[5.4.0]undec-7-ene): exposure of a THF solution of
equimolar 3 and DBU to 1 atm H2 resulted in the immediate
formation of 2 as judged by 1H, 11B, and 31P NMR spectroscopies.
This reaction is presumably mediated by the pseudo-three-
coordinate boron and is consistent with a relatively weak Au–B
interaction in 3. Single-crystal X-ray diffraction (XRD) studies of
2 show it to be a zwitterion in the solid state with an intact B–H
bond on the DBA face opposite the Au center, analogous to the
previously reported structure of zwitterion 3 (Figure 3). The H
atom was located in the electron difference map and is bound
to a quasi-tetrahedral B atom (C-B-C = 340.5°). On the
opposite side of the DBA linker, the Au ion is bound by both P
donors in a roughly linear fashion (P-Au-P = 156.8°) with a
single Au–B interaction (dAuB = 2.644(1) Å) to one nearly-planar
B atom (C-B-C = 359.8°). The 1H NMR spectrum of 2 features
a signal at 5.09 ppm for a B–H proton resonance with the
expected four-line pattern for coupling to a single 11B atom (JBH
= 80 Hz). The 11B NMR shows a corresponding doublet at −10.01
ppm (JBH = 80 Hz) while the 31P NMR spectrum features two
doublets at 58.4 and 55.9 ppm (JPP = 255.0 Hz), consistent with
inequivalent phosphine ligands on the NMR timescale. Given
the precedent for H-atom transfer reactivity at boron,30 we also
Figure 2. Protonation of reduced boron compounds to give tetracoordinate
B–H moieties.
We recently reported the synthesis of the disphosphine
tethered diboraanthracene ligand B2P2 (9,10-bis(2-
(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) and
a series of its gold complexes in three states of charge. The
anionic complex, [Au(B2P2)][K(18-c-6)] (1), which features an
unprecedented boroauride moiety, could be accessed at
remarkably mild potentials (–2.05 V vs. Fc/Fc+, MeCN)23 owing
to the formation of a strong 3-centered, 2-electron bond
between Au and the two B atoms.24 Considering that 1 can be
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targeted the synthesis of 2 from the neutral radical species 4 via broad singlet at 26.9 ppm. Similar to 3, the observed C2v symmetry in
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11
H-atom addition. Reaction of Au(B2P2) with one equivalent of solution along with a B chemical shift betDwOeIe:n10t.1h0a3t9o/Cf9bSoCn0a27f9id2Ke
Bu3SnH produces 2 and Bu6Sn2 over 12 hours via apparent H- three- and four-coordinate B centers in DBA containing molecules17h,f
atom transfer as judged by NMR of reaction mixtures.
implies rapid exchange of the formate ion between each boron site
of the DBA ring, a phenomenon also observed in 3.22 The 13C{1H} NMR
spectrum of 5 contains a resonance at 168.2 ppm corresponding to
the formate carbon atom, and FTIR spectroscopy revealed a strong
band at 1672 cm–1 for the formate C=O stretch.
Scheme 1. Synthesis of Au(B2P2)H from H–, H•, and H+
Scheme 2. CO2 Reduction to Formate by Au(B2P2)H
To probe the synthesis of 2 via protonation of 1, we exposed 1 to
a series of organic acids. Addition of one equivalent of solid
[Ph2NH2][Cl] or [2,6-Me2pyH][Cl] to CD3CN solutions of 1 resulted in
immediate effervescence and formation of dark purple reaction
mixtures which were revealed by NMR analysis to contain a mixture
of H2, 3, and 4. Fortunately, addition of one equivalent of DBU•HCl
to a solution 1 in CD3CN gave rise to a pale-yellow solution exhibiting
1H, 11B, and 31P NMR resonances that matched authentic samples of
2 (Figures S5–7.) with no H2 observed. Furthermore, 2 is stable to
treatment with excess DBU•HCl.
Next, we assessed the ability of electrophiles such as TMSCl
and HCl to effect B–O bond scission. Treatment of 5 with excess
TMSCl resulted in the immediate formation of 3 as judged by 1H
and 31P NMR along with new resonances for
trimethylsilylformate (Figure S13), accomplishing the net
hydrosilylation of CO2.31 While silyl electrophiles have strong
precedent to mediate E–O bond cleavage in main group
systems,32 we wondered if a suitable acid would be able to
protonate the B–OCHO bond to yield formic acid. To this end,
addition of [Ph2NH2][Cl] or DBU•HCl to CDCl3 solutions of 5 saw
no observable reaction over the course of days at room
temperature. However, addition of one equivalent of HCl•Et2O
to 5 resulted in the immediate formation of the chloride
complex 3 by 1H and 31P NMR spectroscopy with resonances for
HCO2H observed in the 1H NMR spectrum.
Au
P
P
Au
P
P
B
B
B
B
O
H
H
O
Figure 3. Thermal ellipsoid plots (50%) of Au(B2P2)H (2, left) and
Au(B2P2)(O2CH) (5, right). Unlabeled ellipsoids correspond to carbon. Solvent
molecules and most hydrogen atoms have been omitted for clarity.
With borohydride 2 in hand, we examined its reactivity with CO2.
Exposure of a C6D6 solution of 2 to 1 atm of CO2 resulted in
quantitative formation of the corresponding formate complex
Au(B2P2)(O2CH) (5) within seconds. The solid-state structure of 5
confirms the insertion of CO2 into the B–H bond, with the resulting
formate ion bound through a single O to one boron atom (Figure 3).
The zwitterionic structure of 5 is analogous to 2 and 3, with a
tetrahedral boron center bound to the formate oxygen (C-B-C =
339.1°) and an intermediate length contact between Au and the non-
formate bound B atom (dAuB = 2.632 Å). The 1H NMR spectrum of 5 in
CDCl3 is consistent with C2v symmetry in solution, with a lone singlet
in the 31P NMR spectrum at 56.1 ppm and 11B{1H} NMR displaying a
Figure 4. Cyclic voltammogram of Au(B2P2)(O2CH) performed in 0.1 M
[nBu4N][PF6] in CH3CN at a scan rate of 100 mV/s.
Following successful electrophilic B–O bond cleavage in 5 to yield
3, the reductive cleavage of the B–OCHO unit was explored. Cyclic
voltammetry (CV) performed on 5 revealed a quasireversible process
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at E1/2 = –1.74 V vs. Fc/Fc+ which gave rise to an oxidative daughter doublet, respectively, for the B–H moieties in 6, VaienwaAlortgicoleuOsnlitnoe
wave at ca. –1.5 V (Figure 4). This daughter wave occurs at roughly the monoborohydride is
(Figure S14D)O. I: C10o.1m03p9o/Cu9nSdC02792K
2
6
the same potential as the oxidation of [Au(B2P2)]0/+1, consistent with structurally related to the diborohydride species resulting from
the hypothesis that one electron reduction of 5 forms [5]− which then H2 activation by reduced DBA compounds reported by
undergoes formate dissociation on the CV timescale to afford neutral Wagner.34
4. To unambiguously confirm the liberation of formate ion, 5 was
Diborohydride 6 is also reactive towards CO2 (1 atm), cleanly
treated with 1 equivalent of Na(C10H8) in THF. A turbid, dark-purple generating the corresponding diformate complex, which was
solution immediately formed, indicating formation of 4. The isolated as its 18-crown-6 adduct, [Au(B2P2)(O2CH)2)][K(18-c-6)]
precipitated solids were collected by filtration and dissolved in D2O (7). Single-crystal XRD confirmed the insertion of CO2 into both
with 1H NMR analysis revealing formation of NaCO2H (Figure S12). To B–H bonds, with B atoms puckered out of planarity to
our knowledge, this is the only example of reductive cleavage of a B– accommodate each OCHO unit. Additionally, the formate
1
OCHO bond to yield formate. This process is presumably driven by moieties in 7 were identified by H NMR (8.34 ppm), 13C{1H}
the accessible one-electron reduction chemistry of 5 (–1.74 V vs. NMR (168.2 ppm) and FTIR spectroscopy (C=O, 1672 cm–1).
Fc/Fc+) enabled by the DBA core and speaks to the utility of frontier
orbital modulation by incorporation of boron into polycyclic aromatic
hydrocarbon frameworks.33 Collectively, these results outline a
synthetic cycle for CO2 reduction with protons and electrons
mediated exclusively at boron (Figure 5).
Scheme 3. Synthesis of Diborohydride 6 and its Reactivity with CO2
P
P
Au
Au
P
P
B
B
H
B
H
B
O
K
H
H
K
O
O
O
H
H
B
O
B
O
O
O
O
Au
K
O
P
P
Figure 5. A closed synthetic cycle for the reduction of CO2 to formic acid with
O
protons and electrons mediated by
diboraanthracene platform.
a redox active, metal-stabilized
O
In addition to the monoborohydride chemistry described Figure 6. Thermal ellipsoid plots (50%) of the dimer of [Au(B2P2)H2][K(Et2O)]
(6, left) and [Au(B2P2)(O2CH)2)][K(18-c-6)] (7, right). Unlabeled ellipsoids
above, we were able to generate a diborohydride species via
correspond to carbon. Solvent molecules,
a disordered crown ether
formal sequential hydride addition to 3. Two equivalents of
K[sec-Bu3BH] reacted with 3 in Et2O to give the anionic
diborohydride complex [Au(B2P2)H2][K(Et2O)] (6). Single-crystal
XRD reveals 6 to be a dimer in the solid state, (Figure 6) with
two [Au(B2P2)H2]– units bridged by two [K(Et2O)]+ cations via
contacts between the B–H and K atoms. The [(Au(B2P2)H2]–
component features a linear P–Au–P moiety (P-Au-P = 161.9°)
that sits above the DBA core with no appreciable interaction
between Au and B (dAuB > 3.25 Å). The H atoms bound to both B
atoms were located on the electron difference map and both B
centers exhibit a pseudo-tetrahedral geometry (C-B-C =
component in 6, and most hydrogen atoms have been omitted for clarity.
Having probed the [Au(B2P2)] system for CO2 reduction chemistry
from its mono- and dihydrides, we additionally explored the direct
reaction of CO2 with 1. Wagner et al. recently reported that 9,10-
dilithio-9,10-diborataanthracene reacts with CO2 to yield CO3 and CO
via reductive disproportionation.25a Reaction of 1 with 1 atm of CO2
in C6D6 resulted in the immediate formation of a colorless solution
with no detectable precipitate. A single crystal suitable for XRD
studies was obtained from CO2 saturated reaction mixtures and
revealed the product to be the carbonate adduct,
[Au(B2P2)CO3][K(18-c-6)] (8, Figure 7). The asymmetric unit of 8
1
337.5°). H and 11B NMR spectra feature a 4-line pattern and
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contains two crystallographically distinct molecules which are at 51.5 and
ARTICLE
a
sharper singlet at 2.99 ppm. Two
View Article Online
DOI: 10.1039/C9SC02792K
chemically equivalent in gross terms but exhibit minor differences in crystallographically distinct molecules of 9 are present in the
their geometries, most notably in their B–O distances, which range asymmetric unit although their geometries are essentially
from 1.550(6) Å to 1.578(8) Å. This spread of B–O distances is identical (Figure S44). The formation of 9 occurs with CO loss as
consistent with a soft [CO3]2– binding potential, in agreement with judged by 13CO2 labeling experiments. Exposure of 9 to
Wagner’s results.25a Solution NMR spectroscopy of 8 is consistent additional CO2 (1 atm) did not lead to any further reaction,
with C2v symmetry and the [CO3]2– unit was identified by 13C{1H} NMR ruling out additional reaction of the -OTIPS moiety with CO2 as
(168.0 ppm) and FTIR (1592 cm–1) spectroscopies with 13CO2 has been previously observed for metal-siloxide units.35
experiments confirming these assignments (see SI).
Au
hypothesized intermediate
P
P
P
P
iPr
iPr
Au
iPr
iPr
iPr
iPr
iPr
iPr
P
Au
P
B
P
P
+ CO2
CO2
B
B
B
Au
B
B
O
O
–
O
B
B
[K(18-c-6)]
Si
O
C
[K(18-c-6)]
O
O
1-CO2
1
iPr3SiCl
iPr
iPr
CO2
CO
iPr
–
–
–
CO
KCl
18-c-6
Figure 7. Thermal ellipsoid plots (50%) of the anion [Au(B2P2)(CO3)][K(18-c-6)]
(8, left) and Au(B2P2)(OSiiPr3) (9, right). Unlabeled ellipsoids correspond to
carbon. Solvent molecules, disordered components, and hydrogen atoms
have been omitted for clarity.
iPr
P
Au
P
–
iPr
iPr
P
iPr
i
Pr
B
B
P
Au
O
O
C
O
B
B
Conclusions
OSiiPr3
K(18-c-6)
8
9
In conclusion, we have synthesized a reactive borohydride
complex, 2, via formal hydride addition, dihydrogen activation,
radical H-atom addition, and, most crucially, reduction/protonation.
The borohydride unit rapidly inserts CO2 to afford the formate
complex 5. Both CV and synthetic studies confirm that 5 can be
reduced at E1/2 = –1.79 V vs. Fc/Fc+ and undergo formate loss to give
the neutral radical 4 and formate via reductive B–O cleavage. The
cleavage of the B–O bond in 5 can also be achieved via addition of
electrophiles including TMSCl or HCl•Et2O, to give 3 and TMSOCHO
or formic acid, respectively. Additionally, direct reaction of the anion
1 with 1 atm of CO2 results in the extrusion of CO and the formation
of carbonate complex 8 via the reductive disproportionation of CO2.
Collectively, these reactions outline a strategy for the reduction of
CO2 with protons and electrons via reactive borohydride species
generated at redox-active, conjugated boranes, and efforts to extend
this chemistry to electrocatalytic systems are underway.
Scheme 4. Reduction of CO2 to CO by [Au(B2P2)]–
In order to probe the mechanism of the formation of 8 from
1 and CO2, we explored the reaction by NMR spectroscopy at
low temperature. Recent reports on reduced boron-containing
heterocycles,21 offer extensive precedent for the cycloaddition
of CO2 across the boron atoms, and we sought to probe whether
a similar CO2 adduct, [Au(B2P2)CO2]– (1-CO2), might be an
intermediate in the formation of 8. Addition of a single
equivalent of CO2 to a frozen solution of the anion 1 in d8-
toluene followed by insertion into a precooled (–50 °C) NMR
spectrometer and subsequent variable temperature NMR
monitoring resulted in a 1:1 mixture of starting material 1 and
the carbonate adduct 8. When the same experiment was
carried out with 13CO2 enhanced intensity was observed
corresponding to the Au(B2P2)13CO3 resonance at 168.2 ppm
along with free 13CO at 184.5 ppm. These results are consistent
with endergonic binding of CO2 to the DBA core followed by
rapid reaction with a second equivalent of CO2 to release CO
Conflicts of interest
and form 8 (Scheme 4), analogous to the mechanism There are no conflicts to declare.
established by Wagner in a related system.25a As we were
unable to observe the putative CO2 adduct [Au(B2P2)CO2]–, we
Acknowledgements
This research was supported by the National Science
attempted to trap it by other chemical means during the
reaction of 1 with CO2. Solutions of 1 are stable in the presence
of triisopropylsilylchloride (TIPSCl); however, addition of 1 atm
CO2 to a thawing C6D6 solution of 1 and TIPSiCl resulted in the
immediate formation of a pale-yellow solution. Single-crystal
XRD studies revealed the product to be the zwitterion
Au(B2P2)(OSiiPr3) (9), which contains a triisopropylsiloxide anion
bound to one B atom (Figure 7). The 31P NMR spectrum contains
a set of doublets at 56.0 (JP-P = 240 Hz) and 52.5 (JP-P = 240 Hz)
ppm while the 11B NMR spectrum featured a broad resonance
Foundation (CHE-1752876) and the American Chemical Society
Petroleum Research Fund (57314). NMR spectra were collected
on instruments funded by an NSF MRI award (CHE-162673) and
an Army Research Office instrumentation grant (W911NF-16-1-
0523). W.H.H. is a member of the University of California,
Riverside Center for Catalysis. Dr. Fook Tham and Dr. Charlene
Tsay are acknowledged for X-ray crystallographic analysis
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Notes and references
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DOI: 10.1039/C9SC02792K
1 H. I. Schlesinger, H. C. Brown J. Am. Chem. Soc. 1940, 62, 3429.
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Au
H
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Au
B
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P
B
B
O
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