Reversible dehydrogenation of a primary aryl borane

Article abstract of DOI:10.1039/d0cc05503d

The consecutive activation of B-H bonds in mesitylborane (H2BMes; Mes = 2,4,6-(CH3)3C6H2) by a 16-electron rhodium(i) monocarbonyl complex, (iPrNNN)Rh(CO) (1-CO; iPrNNN = 2,5-[iPr2PN(4-iPrC6H4)]2N(C4H2)-) is described. Dehydrogenative extrusion of the {BMes} fragment led to the isolation of (iPrNNN)(CO)RhBMes (1-BMes). Addition of H2 gas to 1-BMes regenerated 1-CO and H2BMes, highlighting the ability of 1-CO to facilitate interconversion of {BMes} with dihydrogen. Reactivity studies revealed that 1-BMes promotes formal group transfer and that {BAr} fragments accessed by dehydrogenation are reactive entities. This journal is

Full text of DOI:10.1039/d0cc05503d

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DOI: 10.1039/D0CC05503D
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Reversible dehydrogenation of a primary aryl borane†‡
Connor S. MacNeil, Shou-Jen Hsiang and Paul G. Hayes*
Received 00th June 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The consecutive activation of B–H bonds in mesitylborane (H2BMes; complexes is the potential for hydride ligands to interact with
Mes = 2,4,6-(CH3)3C6H2) by a 16-electron rhodium(I) monocarbonyl boron, thereby stabilizing the resulting borylene fragment.4e
complex,
(
iPrNNN)Rh(CO) (1-CO; iPrNNN
=
2,5-[iPr2P=N(4- Borylene extrusion by reversible and consecutive B–H activation
iPrC6H4)]2N(C4H2)–) is described. Dehydrogenative extrusion of the is therefore complicated by the fidelity of B…H interactions in
{BMes} fragment led to the isolation of (iPrNNN)(CO)RhBMes (1- transition metal systems that contain hydride ligands.6 The
BMes). Addition of H2 gas to 1-BMes regenerated 1-CO and H2BMes, development of thermal- and photo-driven intermetallic
highlighting the ability of 1-CO to faciliate interconversion of borylene transfer from [(CO)5M=B=N(SiMe3)2] (M = Cr, Mo, W)
{BMes} with dihydrogen. Reactivity studies revealed that 1-BMes is to metal carbonyl complexes (e.g. CpM(CO)n; M = V, Re, Co, Rh,
capable of promoting formal group transfer and that {BAr} and Ir) overcomes the issues involved in consecutive B–H
fragments accessed by dehydrogenation are reactive entities.
activation of dihydroboranes.7 Thus, complementary routes
that leverage reversible dehydrogenation in the formation of
unsupported metal–boron bonds are of interest, as such species
provide access to reactive alkyl and aryl {BR} fragments sourced
directly from H2BR.
Metal-catalyzed hydroboration and C–H borylation are
powerful methods for the conversion of feedstock
hydrocarbons into value-added chemicals.1 Oxidative addition
of a borane B–H bond to an electron-rich metal centre is well-
precedented for a variety of secondary boron electrophiles (e.g.
9-borabicyclo[3.3.1]nonane,
pinacol-,
catechol-,
and
dialkylboranes).2 Conversely, reactions between transition
metal complexes and dihydroboranes (H2BR) wherein
consecutive B–H activation removes both hydrogen atoms from
boron are exceedingly rare.3 To this end, aryl dihydroboranes
(H2BAr) have been leveraged in the isolation of terminal
borylene complexes following a dehydrogenative pathway
described by Alcaraz and Sabo-Etienne.4 Aldridge and co-
workers have shown that amino-boryl complexes (e.g.
LnM{B(H)NR2}; R = iPr, Cy), formed by B–H oxidative addition of
aminodihydroboranes (H2BNR2), can undergo boron-to-metal

-hydride migration to produce metal borylenes.5
An inherent challenge involved in removing dihydrogen
from dihydroboranes using well-defined transition metal
a.Department of Chemistry and Biochemistry and Canadian Centre for Research in
Advanced Fluorine Technologies, University of Lethbridge, 4401 University Dr.,
Lethbridge, AB T1K 3M4 (Canada)
Scheme 1 (A) Formation of 1-BMes. (B) Solid state molecular structure of 1-
BMes with thermal ellipsoids at 30% probability. Hydrogen atoms are
omitted for clarity; selected metrical data shown.
†Dedicated to the memory of Prof. Suning Wang.
†Electronic Supplementary Information (ESI) available: Experimental details and
NMR spectra, computational and crystallographic details and example inputs. CCDC
1985378 and 1985379. For ESI see DOI: 10.1039/x0xx00000x
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Using strongly-donating phosphinimine ligands, our group interaction. Infrared (IR) spectroscopic analysis oVfie1w-ACrOticle(1O9nl3in0e
–1
has demonstrated straightforward dehydrogenation of primary cm–1) and 1-BMes (1909 cm ) revealed a redshift of the CO
DOI: 10.1039/D0CC05503D
and secondary silanes at a mononuclear rhodium(I) centre stretch as a result of borylene formation, emphasizing the
-
leading to the isolation of stable rhodium(I) silylenes donor ability of borylene relative to phosphinimine. The B–Rh
(iPrNNN)(CO)RhSiRPh (R = H, Ph).8 Encouraged by these results, Wiberg bond index (WBI) of 0.93 falls slightly below the range
we sought to extend this reactivity in a conceptually-diagonal established for terminal neutral borylene complexes of Co, Rh,
approach that combined an electron-rich rhodium(I) complex and Ir [(
(iPrNNN)Rh(CO) (1-CO) with primary aryl boranes to promote (0.97-1.33).12 Second order perturbation analysis indicated

5-C5H5)(L)M=BNX2); L = CO, PMe3, X = Me, SiH3, SiMe3]
similar dehydrogenative reactivity.
strong delocalization of the Lewis-type nitrogen lone pair donor
Analysis by 1H and 31P NMR spectra revealed that treatment orbital, N(lp), into a vacant acceptor orbital on boron, B(p)
of 1-CO with 1 equivalent of mesitylborane (H2BMes) in (Figure 1B). Notably, the N(lp)→B(p) interaction [E(2): 35.2 kcal
benzene-d6 gave a mixture of unreacted 1-CO, hydrogen gas, mol–1] is greater than the donor-acceptor

-interaction
and a new, unsymmetrical product (Scheme 1A). When the between rhodium and boron [E(2): 10.3 kcal mol–1]. These data
reaction was carried out in toluene, recrystallization from Et2O imply that both phosphinimine -donation, along with Rh-

at –35 °C provided a bright yellow crystalline solid backdonation, are important in stabilizing the acceptor p-type
((iPrNNN)(CO)RhBMes, 1-BMes) in 22% yield. X-ray diffraction orbital on boron, and that 1-BMes has characteristics of both a
studies on single crystals of 1-BMes revealed a 4-coordinate rhodium borylene and iminoboryl, with the former
rhodium centre bearing a {BMes} fragment stabilized by a representation drawn for clarity (Figure 1).
phosphinimine nitrogen (Scheme 1B). The geometry at the 3-
Recognizing that the synthesis of 1-BMes via
coordinate boron centre ( angles = 359.8 °) is trigonal planar with dehydrogenation resulted in poor yields, we reasoned that the

mesityl o-CH3 groups occupying the axial space above and product could react with evolved hydrogen gas to establish an
below the equatorial plane defined by the phosphinimine equilibrium mixture with 1-CO and H2BMes. To better
nitrogen, mesityl ipso-carbon, and rhodium centre. The understand the reversibility of this process, a 1:1 mixture of
rhodium-boron distance in 1-BMes is 2.024(5) Å]. In benzene- H2BMes and D2BMes (1 total equivalent of borane) was
d6, 1-BMes features a broad 11B resonance at 32.6 (
Hz), upfield compared to known rhodium aminoborylene speciation by 31P NMR spectroscopy after short reaction times
1/2 = 457 combined with 1-CO in benzene-d6. Monitoring product
complexes (75 to 120),9,10 suggesting that

-donation from revealed 1-CO
,
1-BMes, and an unsymmetrical intermediate
1
nitrogen to boron in 1-BMes effectively shields the 11B nucleus. (vide infra). Both H2 and HD gas were identified in the H NMR
Moreover, the chemical shift in 1-BMes closely resembles those spectrum (Figure S18). Isotope mixing supports the presence of
reported for platinum iminoboryl complexes.11
a reactive boryl hydride species,5,13 capable of rapid Rh–H(D)
The gas-phase structure of 1-BMes was optimized at the and B–D(H) exchange at a rate that is competitive with the
B3LYP/aug-cc-pVDZ level of theory with associated formation of 1-BMes. The reaction between H2BMes and
pseudopotentials for Rh (see SI for details). Metrical parameters D2BMes in the absence of 1-CO showed no sign of H/D
were reproduced in good agreement with experiment (see SI for scrambling or evolution of HD gas.
details). Natural bond orbital (NBO) analysis was performed on
the optimized structure of 1-BMes to better guide our temperature NMR spectroscopy in toluene-d8 at –40 °C led to
understanding of the orbital interactions between rhodium, the observation of 31P resonances at 50.4 and
47.8,
boron, and nitrogen. The B–Rh -bonding NBO (Figure S26) is resembling the intermediate observed in reactions between 1-
slightly polarized toward boron, suggesting a bonding model in CO and H2BMes/D2BMes (vide supra). The accompanying H
which boron-to-rhodium -donation is the predominant NMR spectrum revealed a broad resonance centered at –2.7.
Monitoring the reaction of 1-CO and H2BMes by variable


1


Fig. 1 Orbital interactions in 1-BMes illustrating the extent of delocalization from N lone-pair and Rh-based donors toward the boron p-type acceptor NBO. Orbital surfaces plotted
with an Isovalue of 0.08.
2 | J. Name., 2012, 00, 1-3
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Upon warming to 22 °C, 1H and 31P peaks corresponding to the
intermediate species were consumed while those attributed to
1-BMes grew in intensity. Notably, H2 gas (4.56) was visible in
solution throughout the reaction. We propose that the
intermediate species forms by dissociation of phosphinimine
from rhodium, followed by coordination to the aryl
dihydroborane to afford B–H agostic complex 1-CO·H2BMes. A
B–H bond then undergoes oxidative addition, producing the
Rh(III) boryl hydride species (iPrNNN)(CO)Rh(H)(BHMes). Finally,
H2 loss presumably occurs via either a 4-centred transition state
involving Rh–H (+) and hydridic B–H (–), or a Brønsted
pathway mediated by a free phosphinimine group.8
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DOI: 10.1039/D0CC05503D
In an effort to corroborate the reversibility of 1-BMes
formation, a J. Young NMR tube containing 1-BMes in benzene-
d6 was pressurized with 4 atmospheres of H2 and monitored by
1H and 31P NMR spectroscopy. Rapid regeneration of
mesitylborane and formation of 1-CO (>80% in 10 minutes) was
confirmed by comparing the product of the reaction to
authentic samples (Scheme 2). Examination of the 31P NMR
spectrum after 1 week showed that the equilibrium of 1-CO
H2BMes and 1-BMes + H2 was maintained under H2 pressure (1-
BMes 1-CO = 9:1). Hence, it was concluded that the poor yields
+
:
Fig. 2 (A) Molecular structure of 1-CO·H2BArF at 30% probability ellipsoids with H-atoms
(except for H2BArF) omitted for clarity. Principle metrical parameters shown. (B) Orbital
interactions (NBOs) derived from second-order perturbation theory analysis. Orbital
surfaces plotted with an isovalue of 0.08.
obtained for reactions conducted at ambient temperature were
likely the result of rapid hydrogen uptake by 1-BMes, even
under reduced pressure. Altering the original procedure by
carrying out the reaction at low temperature (see SI for details)
allowed the isolation of analytically pure 1-BMes in
substantially improved (53%) yield.14 Rapid back-reactivity of 1-
BMes with dihydrogen encouraged us to explore other means
of {BMes} group transfer. Addition of pinacol to a benzene-d6
solution of 1-BMes resulted in gradual formation of the
boronate ester MesBpin and 1-CO, along with H2 gas. This proof-
of-principle reaction demonstrates that a {BMes} fragment
obtained through dehydrogenation of H2BMes can undergo
formal borylene transfer.7
The generality of aryl borane dehydrogenation was studied
by reducing the steric profile about boron and altering the
electronics of the aryl group. The trifluoromethylated borane
H2BArF (ArF = 3,5-(CF3)2C6H3) was combined with 1-CO in
benzene-d6 leading to new 31P NMR resonances at
 51.2 and 
48.8, which closely resemble the intermediate observed in the
low temperature reactions between 1-CO and mesitylborane (
50.4 and 47.8, vide supra), as well as those in the H/D
scrambling experiments. In benzene-d6, the 11B NMR spectrum
exhibits a single peak at –5.92 ( 1/2 = 11.1 Hz). In anticipation




of unproductive back-reaction with H2, the experiment was
repeated at –30 °C in diethylether. A gradual colour change
from orange to dark-yellow was accompanied by formation of a
crystalline yellow solid which was isolated in 80% yield. Single
crystals suitable for X-ray diffraction analysis were grown from
a concentrated pentane/benzene (5:1) solution. The solid-state
structure of the isolated product 1-CO·H2BArF was confirmed as
a B–H agostic complex between 1-CO and H2BArF, stabilized by
phosphinimine coordination to boron [N1–B1 = 1.558(2) Å] and
a B–H agostic interaction [H1–Rh1 = 1.61(2) Å] with the Rh(I)
centre (Figure 2A). Notably, 1-CO·H2BArF is a model complex for
initiating dehydrogenation of aryl dihydroboranes at 1-CO
Second-order perturbation analysis of 1-CO·H2BArF show
delocalization interactions between Rh 4dxy/4dxz and (B–H)
.

*
orbitals (Figure 2B). Back-donation from Rh to B through the B–
H agostic bridge is consistent with the IR spectroscopic data that
shows a strong CO stretching frequency (1950 cm–1), blue-
shifted in the order 1-CO·H2BArF
> 1-CO > 1-BMes. These data
are consistent with Nakazawa’s report that strong
delocalization of electron density from Rh to B is facilitated by a
Rh…H–B linkage.15 At 23 °C, the 1H NMR spectrum of 1-
CO·H2BArF contains a broad resonance at
 –3.8, tentatively
assigned to the two rapidly exchanging BH2. Upon cooling a
toluene-d8 solution of 1-CO·H2BArF to –80 °C distinct resonances
at
signals coalesce at –25 °C (
as the original broad resonance (

3.9 (B–H) and

–9.8 (Rh–H–B) were observed. The two

G‡ = 9.9(4) kcal mol–1), reappearing

–3.8) near 0 °C (Figure S21).
Scheme 2 Reactivity of 1-BMes with H2 and pinacol.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Despite the attenuated steric profile of H2BArF, no evidence
for borylene formation was observed. The product of oxidative
addition, a Rh(III) boryl hydride, is likely unstable to reductive
Notes and references
View Article Online
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elimination and reformation of 1-CO·H2BArF
.
These
observations are in line with a recent contribution from
Braunschweig detailing the steric and electronic parameters
associated with dehydrogenative borylene formation from
Ru(PCy3)2HCl(H2) and various dihydroboranes.16 For
spontaneous dehydrogenation to occur, aryl groups bearing
ortho-substituents were required.
Without a viable pathway for release of H2, and
consequently, borylene formation, reaction progress appears to
stall at 1-CO·H2BArF. When a sample of 1-CO·H2BArF was left at
23 °C in benzene-d6 for 48 h, a gradual change in the 31P NMR
spectrum revealed a new unsymmetrical product with peaks
2
3
4
located at 52.0 and
of a dissociated phosphinimine,8 implying that the product
features a
2-bound iPrNNN ligand scaffold. Examination of the
 10.6. The latter resonance is diagnostic

corresponding IR spectrum revealed no distinguishable CO
stretching frequencies. Accordingly, the 13C-labelled complex 1-
13CO·H2BArF was prepared using 1-13CO. The 13C NMR spectrum
of 1-13CO·H2BArF in benzene-d6 exhibited a doublet (1JCRh = 73.4
Hz) centered at 190.6 which disappeared after 12 h at 23 °C.
The 13C label was not identified in any by-products, so it is
reasonable to conclude that direct 13CO elimination from 1-
13CO·H2BArF occurred. While spontaneous CO elimination from
square-planar Rh(I) is rare, Nakazawa showed that a B–H–Rh
5
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linkage can destabilize 
-back-donation from Rh(I) to CO.15
For the first time the reversible dehydrogenation of an aryl
dihydroborane has been observed at rhodium. Complexation of
H2BMes by a hemilabile phosphinimine led to spontaneous H2
loss, highlighting the importance of the iPrNNN ligand in
promoting cooperative reactivity. Group transfer of the {BMes}
fragment was observed in reactions with H2 and pinacol, where
the latter generated the boronate ester MesBpin, along with H2
gas. Entrapment of the electron-deficient borane H2B(3,5-
(CF3)2C6H3) by 1-CO highlights that subtle steric and electronic
factors are involved in spontaneous dehydrogenation.
Ultimately, with the demonstration of formal borylene transfer,
these results provide a method for interconversion of boranes
and borylenes with hydrogen, from which new and more
efficient routes to organoboranes are being explored.
Financial support was provided by the NSERC of Canada
(Discovery Grant to P.G.H and CGS-D to C.S.M). Dr. Eric G. Bowes
is thanked for insightful discussions. Mr. Tony Montina and Mr.
Michael Opyr are acknowledged for expert technical assistance
with NMR experiments. Mr. Dylan J. Webb is acknowledged for
collection of combustion analysis data. P.G.H. thanks the
University of Lethbridge for a Tier I Board of Governors
Research Chair in Organometallic Chemistry.
8
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Conflicts of interest
There are no conflicts to declare.
Chem. Eur. J
.
2019, 25, 13566.
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DOI: 10.1039/D0CC05503D
A rhodium carbonyl complex facilitates the reversible dehydrogenation of a primary aryl borane leading
to a reactive rhodium borylene capable of engaging in group transfer reactivity.