B-H bond cleavage via metal-ligand cooperation by dearomatized ruthenium pincer complexes

Article abstract of DOI:10.1021/om500311a

Organic derivatives of boronic acid are widely used reagents useful in various synthetic applications. A fundamental understanding and the exploration of new reaction pathways of boronic reagents with organometallic systems hold promise for useful advancement in chemical catalysis. Herein we present the reactions of simple boranes with dearomatized ruthenium pincer complexes based on PNP (2,6-bis(di-tert-butylphosphinomethyl)pyridine) or PNN (2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine) ligands. NMR studies revealed dehydrogenative addition of the borane B-H bond across the metal center and the ligand. Remarkably, new complexes were observed, which contain the boryl moiety at the benzylic carbon of the pincer ligand arm. X-ray crystal structures of new dearomatized boryl pincer complexes were obtained, and DFT calculations revealed mechanistic details of the adduct formation process through a dehydrogenative pathway. In addition, catalytic aryl-boron coupling reactions were explored. The new boryl pincer systems may possibly be useful in future postmodification techniques for ruthenium pincer complexes, as well as in catalytic B-B and B-C coupling reactions.

Full text of DOI:10.1021/om500311a

Article
pubs.acs.org/Organometallics
B−H Bond Cleavage via Metal−Ligand Cooperation by Dearomatized
Ruthenium Pincer Complexes
Aviel Anaby,† Burkhard Butschke,† Yehoshoa Ben-David,† Linda J. W. Shimon,‡ Gregory Leitus,‡
Moran Feller,*,† and David Milstein*,†
‡
†Department of Organic Chemistry and Department of Chemical Research Support, The Weizmann Institute of Science, Rehovot
76100, Israel
S
* Supporting Information
ABSTRACT: Organic derivatives of boronic acid are widely used reagents
useful in various synthetic applications. A fundamental understanding and the
exploration of new reaction pathways of boronic reagents with organometallic
systems hold promise for useful advancement in chemical catalysis. Herein we
present the reactions of simple boranes with dearomatized ruthenium pincer
complexes based on PNP (2,6-bis(di-tert-butylphosphinomethyl)pyridine) or
PNN (2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine) li-
gands. NMR studies revealed dehydrogenative addition of the borane B−H
bond across the metal center and the ligand. Remarkably, new complexes were
observed, which contain the boryl moiety at the benzylic carbon of the pincer ligand arm. X-ray crystal structures of new
dearomatized boryl pincer complexes were obtained, and DFT calculations revealed mechanistic details of the adduct formation
process through a dehydrogenative pathway. In addition, catalytic aryl−boron coupling reactions were explored. The new boryl
pincer systems may possibly be useful in future postmodification techniques for ruthenium pincer complexes, as well as in
catalytic B−B and B−C coupling reactions.
the reversible interaction of boranes and diborons with
palladium complexes bearing noninnocent pincer ligands,
whereby a boron atom adds to the basic amine group of the
ligand and may be detached by hydrolysis.41 More recently,
Leitner et al. disclosed the Z-selective hydroboration of
terminal alkenes and alkynes using a nonclassical hydride
PNP ruthenium pincer complex and were able to isolate a
hydrogen-bridged ruthenium−boryl adduct complex.42 Alkene
hydroboration has also been reported using pincer-type iron43
and cobalt44 complexes. Chirik et al. demonstrated very
recently dehydrogenative B−Caryl coupling by cobalt pincer
complexes and isolated a proposed intermediate bearing a σ-
boryl ligand attached to the metal center.45
Ruthenium PNP (2,6-bis(di-tert-butylphosphinomethyl)-
pyridine) and PNN (2-(di-tert-butylphosphinomethyl)-6-
(diethylaminomethyl)pyridine) pincer complexes, previously
disclosed by our group,46−48 are prone to deprotonation at the
benzylic position of the ligand, resulting in loss of aromaticity in
the ligand backbone (Scheme 1a). The aromatization−
dearomatization processes, with no change in the formal
oxidation state of the metal, were found to mediate several
fundamental catalytic hydrogenation and dehydrogenation
reactions.49−52 It is worth noting that a key intermediate
believed to take part in such transformations is a trans-
dihydride complex, which may also be formed independently
by exposure of the dearomatized ruthenium pincer complexes 1
INTRODUCTION
■
Organic derivatives of boronic acid have become highly useful
reagents in modern synthetic organic chemistry.1 The unique
and sometimes peculiar chemistry of aryl- and alkylborane
reagents has been the focus of research for over a century.2−4
Brown et al. pioneered the field with studies on applicative
borane chemistry.5−7 Ever since, these versatile reagents have
found wide use in polymerization reactions,8−11 medicinal
chemistry,12 organometallic ligand design,13−16 and various
organic synthetic protocols.17−21 Most outstanding is the use of
organo-boronic acids or esters in the catalytic Suzuki−Miyaura
carbon−carbon cross-coupling reaction.22−24 Dehydrogenative
borylation of aryls, catalyzed by rhodium(I)25−30 and iridium-
(I)28−32 complexes, is a recent synthetic method to access these
instrumental reagents. To our knowledge, efficient borylation of
unactivated arenes using homogeneous ruthenium catalysis has
not yet been reported.30,33,34 Research focus should still be
directed toward improved selectivity, substrate scope, and atom
efficiency of these reactions. Boron−boron dehydrogenative
coupling has also spurred interest as diborons, the products of
such coupling reactions, may serve as borylation agents, with
the added advantage of being air and moisture stable.35−38
A
fundamental understanding of boron−boron bond formation is
in itself of interest.25,26,39
The search for key intermediates in catalytic hydroboration
reactions has shed light on the nature of borane B−H addition
to late-transition-metal complexes and on the dynamics of the
metal−boron bond.40 Presenting a new mode of borane
addition to organometallic complexes, Ozerov et al. reported
Received: March 25, 2014
© XXXX American Chemical Society
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Scheme 1
Figure 1. 1H NMR spectrum (300 MHz, C6D6) of complex 5 formed
by the reaction of 3 equiv of PinBH with complex 1 at 80 °C for 2 h.
Excess PinBH present in the sample is denoted by asterisks. Inset:
enlarged aliphatic region.
and 2 to molecular hydrogen (Scheme 1b).46−48 trans-
Dihydride complexes 3 and 4 can readily lose H2 (e.g., by
heating) to regenerate the dearomatized starting complexes.
Herein we describe the reactions of 4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (pinacol borane, PinBH) and 1,3,2-benzodiox-
aborole (catechol borane, CatBH) with ruthenium pincer
complexes 1 and 2. Mechanistic details for the reaction of 1
with CatBH are proposed by DFT calculations. Moreover,
catalytic B−Caryl coupling reactions are described.
ppm), which is consistent with the expected chemical shift of a
hydride in the apical position of a square-pyramidal ruthenium
pincer complex.46,47 Integration of the signal corresponding to
the benzylic arm position (2.8 ppm) reveals that only one
protonated methylene unit is present in the ligand. Perhaps the
1
most outstanding feature in the H NMR spectrum of 5 is the
chemical shift of the pyridine H3 proton, which is significantly
shifted downfield to 8.5 ppm.
As shown in Figure 1, complex 5 was identified as a
dearomatized complex, formed by borane dehydrogenative
addition to the benzylic arm position of the PNP ligand,
generating a new boron−carbon bond.
RESULTS AND DISCUSSION
■
Boronic ester derivatives of the general formula R2B−H (R2 =
diol) possess duality, since the boron atom is Lewis acidic and
hence susceptible to react with the deprotonated ligand of
complexes 1 and 2 but is also a strong σ donor, meaning it may
tend to directly coordinate to the metal atom.37,38 We were
thus curious to learn which manner would prevail when
boranes bearing a B−H bond were added to dearomatized
ruthenium pincer complexes.
Reactions of PNP Complex 1. Addition of excess PinBH
(3 equiv) to a solution of the dearomatized ruthenium pincer
complex 1, at room temperature, results in a gradual color
change from dark green to intense red within 30 min. Analysis
of the red product solution by NMR spectroscopy confirmed
the formation of two new species: the previously reported trans-
dihydride PNP complex 3, known to form upon exposure of
complex 1 to hydrogen,46 and a new species, complex 5
(Scheme 2). Heating the mixture above at 60 °C, preferably in
an open system to allow evolution of hydrogen, results in
almost full conversion to complex 5. The 31P{1H} NMR
spectrum of 5 consists of an AB quartet (84.4, 82.0 ppm),
Analogous to the formation of complex 5 by reaction of
complex 1 with PinBH, the addition of CatBH to complex 1 at
room temperature results in the rapid formation of complex 6
(Scheme 2). The pyridine H3 proton of complex 6 is found at
8.5 ppm in the 1H NMR spectrum, similar to that of complex 5.
This downfield shift of the H3 pyridine proton in complexes 5
and 6 may be explained by through-space electronic deshielding
of this proton by the pinacolato or catecholato oxygen atom of
the boryl unit, respectively. A similar effect was observed for a
CO2-bearing anionic pincer complex of nickel, previously
disclosed by our group.53 The addition reactions of PinBH and
CatBH to complex 1, and the formation of complexes 5 and 6,
respectively, were performed with an internal standard (1,4-
dioxane), confirming quantitative conversion to the latter
complexes (see the Supporting Information).
The structure of the new borylated PNP pincer complexes
was confirmed by X-ray crystallography of single crystals of
complexes 5 and 6 (Figure 2). The square-pyramidal geometry
of complexes 5 and 6 resembles that of the parent complex
1.46−48 Examination of the bond lengths in the crystal
structures of 5 and 6 suggests π delocalization of the C1−C2
formal double bond over the C1−P1 bond (phosphor-ylide
resonance donation)48 and, possibly to some extent, over the
C1−B1 bond (alkylidene-borane resonance contribution),1
with both of these bonds being shorter than the conventional
carbon−heteroatom bond lengths.48,54 Resonance stabilization
may account for the ease with which the initial borane adduct
loses hydrogen to form the metastable dearomatized product
complex. This mode of addition, that is, the boron atom
binding to the nucleophilic ligand arm and the borane
hydrogen atom residing on the metal, may be expected for
the electrophilic boranes.1,41,53 Our premise is that complexes 5
and 6 are the result of molecular hydrogen loss from aromatic
species, bearing a boryl moiety attached to the benzylic arm
position of the pincer ligand and two hydrides, trans to each
1
indicative of an asymmetric pincer ligand. In the H NMR
spectrum (Figure 1) a high-field hydride signal appears (−24.6
Scheme 2
B
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Having identified a new mode of addition of boranes to
complex 1, we were compelled to deepen our understanding of
these reactions. DFT calculations, performed on the reaction of
1 with CatBH, may explain why the boron atom binds
preferentially to the arm and not to the metal center and
indicate that a trans-dihydride species with a boryl ligand bound
to the arm is indeed a low-concentration intermediate. The
corresponding potential-energy surfaces for two possible
reaction pathways are given in Figure 3. Initially, the borane
has two options to form an encounter complex with 1: while
coordination of the borane B−H bond to the ruthenium center
to generate A′ is endergonic by 40 kJ mol−1, bond formation of
the boron center with the dearomatized ligand arm gives rise to
complex A, which is located 20 kJ mol−1 below the separated
starting compounds 1 and CatBH. Note that in A the boron
center is covalently bound rather than just coordinated to the
carbon center, as indicated by the tetrahedral environment (see
the Supporting Information), and that the boron-bound
hydrogen atom coordinates weakly to the ruthenium center.
As a consequence, the B−H bond is elongated by 0.19 Å
relative to the free CatBH, while in A′ the borane B−H bond is
only elongated by 0.11 Å. As a result, starting from A and A′,
the barrier for the addition of the B−H bond across the
ruthenium center and the ligand arm to give rise to B and B′,
respectively, is 58 kJ mol−1 lower for A than for A′. Thus, under
the given experimental conditions, it can be excluded that B′ is
formed from adduct complex A′, although B′ is more stable
than B. Finally, H2 elimination from B gives rise to the
formation of dearomatized 6 via the H2 intermediate C (note
that H2 elimination from B, involving the CH2 arm instead of
the boron-containing arm, has a barrier that is higher by 18 kJ
mol−1 and that the resulting dearomatized complex is 37 kJ
mol−1 higher in energy; see the Supporting Information). While
the barrier TS B/C is higher than expected for a process that
already occurs at room temperature, the involvement of traces
of H2O might drastically diminish this barrier, as shown
previously by our group.56
Figure 2. (a) X-ray crystal structure of complex 5 with thermal
ellipsoids set at 50% probability. tert-Butyl methyl groups and pinacol
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): B1−C1 1.512(5), C1−C2 1.428(5), P1−C1 1.795(3),
P2−C7 1.836(5); C2−C1−B1 121.2(3), B1−C1−P1 124.4(3). (b) X-
ray crystal structure of complex 6 with thermal ellipsoids set at 50%
probability. tert-Butyl methyl groups and catechol hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (deg): B1−
C1 1.503(6), C1−C2 1.422(6), P1−C1 1.803(4), P2−C7 1.833(4);
C2−C1−B1 123.1(4), B1−C1−P1 122.4(3).
other, at the metal center (see depiction in Scheme 2).
However, no direct evidence has been found for the existence
of such saturated intermediates. Nevertheless, coinciding with
the formation of the dearomatized adducts, the respective trans-
dihydride complex 3 (Scheme 2) is consistently observed,
indicating the evolution of molecular hydrogen, which is, most
probably, trapped by residual starting complex 1 to form
complex 3. Alternatively, one may envision precoordination of
the boron atom to the metal center and subsequent transfer of
the boryl moiety to the benzylic arm position, in a pseudo boryl
migration step.55 This scenario, however, may be ruled out on
the basis of our DFT calculations (vide infra).
It is worth noting that complexes 5 and 6 are highly unstable.
Complex 6 undergoes degradation fairly rapidly upon most
workup procedures (vide infra). Complex 5 undergoes what
seems to be hydrolysis by traces of water in the system.41 The
product of this hydrolysis is the pinacol boronic acid (2-
hydroxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane, PinBOH),
which can then add to the dearomatized complex 1 to form
the aromatic boryloxo complex 7 (Scheme 3). PinBOH was
independently synthesized and added to complex 1, forming
exclusively complex 7 (see the Supporting Information).
Activation of the borane B−H bond through metal−ligand
cooperation distinguishes the ruthenium−carbonyl pincer
complexes from the nonclassical hydride ruthenium PNP
complex reported by Leitner et al.42 Whereas for Leitner’s
complex a hydrogen-bridged boron−ruthenium bond is
formed, the square-pyramidal ruthenium−carbonyl pincer
complexes 1 and 2 force cleavage of the B−H bond across
the metal center and the ligand backbone.
Scheme 3
Reactions of PNN Complex 2. The addition pathway in
the reactions of PNN pincer complex 2 with PinBH was found
to follow the same principles as described above for PNP
complex 1. However, a more dynamic and versatile behavior is
observed according to NMR spectroscopy. Upon addition of 3
equiv of PinBH to complex 2 at room temperature, we
observed the formation of dearomatized complex 8 (Scheme
4a), analogous to PNP complexes 5 and 6. Conversion to
complex 8 is only partial, with concomitant detection of the
trans-dihydride complex 4. The 1H NMR spectrum of complex
8 shows a hydride signal at −26.4 ppm and a downfield-shifted
pyridine H3 proton signal at 8.5 ppm. With time and upon
heating, several new species appear in solution. Heating is
needed to promote conversion of complex 2 and enables
hydrogen loss of the trans-dihydride complex 4. After the
mixture is heated to 75 °C for 2 h, an additional proton signal
1
at 8.5 ppm is observed in the H NMR spectrum, which
overlaps with the pyridine H3 proton signal of complex 8. This
C
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Figure 3. Simplified potential energy surfaces for the reaction of 1 and CatBH. Gibbs free energies (ΔG) are given in kJ mol−1 at 298 K. See the
Supporting Information for further details.
Scheme 4
borylated species, possessing one boryl unit attached at the
ligand benzylic position and a second boryl ligand attached at
the metal center. This hypothesis could not be verified
explicitly. As is the case with the reaction of PinBH with
complex 1, unwarranted hydrolysis of the PinBH (or the
borylated complex 8) results in the formation of the aromatic
1
boryloxo complex 9 (Scheme 4a), evidenced in the H NMR
spectrum by a hydride signal at −15.6 ppm and verified by
independent synthesis from complex 2 and PinBOH (see the
Supporting Information). Unfortunately, high solubility and
poor stability prevented the isolation of the minor products in
the reaction between 2 and PinBH; hence, their structures
could not be unequivocally elucidated. Complex 8 could only
be fully characterized by NMR in situ.
The reaction of 2 with the sterically less demanding CatBH
proceeds readily at room temperature. The main species
identified in solution is complex 10 (Scheme 4b). Several other
minor species are observed, which could not be identified. In
the 1H NMR spectrum of complex 10, the methylene porotons
of the amine pincer arm appear as a wide AB quartet (4.0, 2.8
ppm), which is indicative of a bulky ligand at the apical position
of the complex, inducing disparate chemical environments for
each of the geminal methylene protons. The catecholato
aromatic protons appear as two multiplets (at 7.0 and 6.7 ppm,
two protons each), and a corresponding hydride signal is not
observed. A doublet at 3.5 ppm (2JPH = 3.0 Hz), corresponding
to a single proton, implies a double bond at the phosphine
pincer arm pointing to a dearomatized species with no
borylation at the ligand backbone. The 11B{1H} NMR signal
assigned to complex 10 is found at 47.4 ppm, in comparison to
30.7, 33.3, and 30.1 ppm for the benzylic boryl of complexes 5,
6, and 8, respectively, and is well in the range for a metal-bound
boryl.38a Thus, we conclude that complex 10 is, surprisingly, a
dearomatized σ-boryl complex. Such a species, comparable to
species D′ in the DFT study (Figure 3), has not been identified
additional downfield pyridine H3 proton, with no correspond-
ing hydride signal, may suggest the formation of a doubly
D
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as the major component in any of the aforementioned
experiments.
Scheme 5
Although the minor species in solution could not be
identified in the stoichiometric addition of CatBH to complex
2, further addition of excess CatBH to complex 10 induces
formation of a new double-borylated complex, 11 (Scheme 4c).
Complex 11 gives rise to the characteristic pyridine H3 proton
1H NMR signal at 8.5 ppm, and a corresponding hydride signal
is not observed. In the aromatic region, four sets of multiplets,
pertaining to two catecholato moieties, are observed. Similar to
the spectrum of complex 10, the methylene protons on the
amine pincer arm give rise to a wide AB quartet (3.8, 2.9 ppm)
1
in the H NMR spectrum, but no signal for a proton on the
phosphine ligand arm is observed. Complexes 10 and 11 were
characterized by NMR in situ but could not be isolated
independently. Several other unidentified complexes and
related boron species are formed with time in the reaction of
complex 2 with CatBH, as is the case with PinBH. Therefore,
the ligand-bound boron atom signal could not be resolved by
11B NMR, restricting the unequivocal spectroscopic assignment
of complex 11.
It is worth noting that the major addition complexes
identified after addition of PinBH or CatBH to complex 2
(complexes 8, 10, and 11) appear to be deprotonated at the
methylene unit on the phosphine arm of the pincer ligand
rather than at the amine arm. Complex 2 was reported to
possess a thermodynamic tendency to form addition complexes
through the “tautomeric” form of the deprotonated amine
pincer arm.57 In contrast to this, it appears that for the borane
addition complexes deprotonation of the phosphine ligand arm
is favored, most likely owing to stabilization of the
dearomatized form through π delocalization. We cannot,
however, preclude the possibility that some of the minor
addition complexes formed in solution are, in fact, deproto-
nated at the amine arm.
Probing for the reversibility of the boron−carbon bond
formation, spin-saturation transfer (SST) 1H NMR experiments
were conducted for complex 5 in the presence of excess PinBH
(see the Supporting Information). Despite our expectations, no
chemical exchange signal was observed between bound and free
pinacol borane even at elevated temperatures (up to 70 °C).
Further, we investigated the reaction of boryl complexes 5 and
6 with hydrogen. Thus, applying 1.5 bar of H2 pressure
(corresponds to ca. 2 equiv) to an NMR tube containing a
benzene solution of complex 5 results in gradual formation of
the corresponding trans-dihydride complex 3 and formation of
free PinBH (Scheme 6 and the Supporting Information). In
Scheme 6
Chemical Exchange Studies. Having established the
reactivity pattern of CatBH and PinBH with dearomatized
complexes 1 and 2, we were intrigued to learn more about the
mechanism of these reactions. First, the reversibility of the
addition was examined. When excess CatBH is added to a
solution of pinacol boryl complex 5 in benzene at room
temperature, within 2 h quantitative conversion to the
corresponding CatBH addition complex 6 becomes evident
by NMR. Concomitant formation of free PinBH is also
observed (see the Supporting Information). The reverse
reaction, that of complex 6 with PinBH to generate 5 and
CatBH, was not detected at all, even upon heating. According
to DFT calculations, the reaction of complex 5 with free CatBH
to yield complex 6 and free PinBH is exergonic by 9 kJ mol−1
(Scheme 5). Assuming that 5 + CatBH and 6 + PinBH are in
rapid equilibrium (without further calculations of possible
intermediates and barriers), the relative ratio of 5 + CatBH to 6
+ PinBH corresponds to ca. 1:38. According to this value,
complex 5 may be expected to be undetectable by NMR
spectroscopy, all the more in the presence of excess CatBH in
solution. As is shown in Figure 2, the boron−carbon bond in
catechol boryl complex 6 is slightly shorter than that of pinacol
boryl complex 5. Although the difference is negligible, it may
stand testament to the formation of a stronger B−C bond in
the case of complex 6, making it less prone to substitution with
PinBH. Moreover, the outcome of the exchange experiments is
in line with the fact that CatBH is slightly more Lewis acidic1
and less sterically hindered than PinBH.
contrast, the PNP catechol-boryl complex 6 is reluctant to
undergo hydrogenation. The reaction of 6 with hydrogen
results in only partial conversion to complex 3, accompanied by
rapid formation of an unidentified precipitate. Generally, the
catechol boryl complexes 6, 10, and 11 seem to be unstable in
solution and tend to gradually undergo degradation. We
speculate that the catechol boryl complexes form ionic borate
complexes through disproportionation of catechol borane or
the corresponding boronic acid.58 The product of this
presumable process gives rise to very broad signals in the
NMR spectra (see Supporting Information) and is insoluble in
nonpolar solvents. Elucidation of this observed degradation
process and its products are beyond the scope of this article.59
The dynamic behavior observed in the reactions of complex
2 with PinBH and CatBH and the possibility of boron to
coordinate directly to the ruthenium atom suggest complex 2 as
a potential catalyst for dehydrogenative coupling reactions of
boranes (vide infra).
The observed trends and differences in complex formation
when switching from PinBH to CatBH and between pincer
complexes 1 and 2 suggest a largely steric control over the
outcome of these reactions between the bifunctional pincer
complexes and the dual-natured boranes. It is also quite likely
that steric hindrance prevents the formation of a ruthenium−
boron bond in the case of borane addition to the bulky PNP
complex 1. Clearly, when steric bulk is not detrimental, a σ-
E
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boryl complex is the kinetic product, as seen in the case of
CatBH addition to complex 2. σ-Boryl ligands are known to
exert a strong trans influence, which is on the order of that of a
hydride ligand.26,38a,60 The formation of 10 is most likely the
result of the strong trans influence of the catechol boryl ligand,
promoting loss of the trans hydride ligand in cooperation with
the benzylic arm proton, as molecular hydrogen. Thus, while
hydrogen loss to yield dearomatized complexes 5, 6, and 8 may
be thermodynamically favored by the possible π delocalization
around the boron−carbon bond on the ligand, complex 10 may
be thermodynamically favored in the dearomatized form due to
the strong trans influence of the ruthenium-bound boryl ligand.
Catalytic Cross-Coupling Reactions. The facile activation
of boranes by ruthenium pincer complexes, its observed
fluctuation, and the ease with which dehydrogenation seems
to occur led us to probe catalytic dehydrogenative coupling
reactions.25−36,46,47,61 Initially, 0.5 M PinBH was refluxed in
THF for 18 h with 1 mol % of complex 1 or 2 (Scheme 7a).
Table 1. Cross-Coupling Reactions of PinBBPin Catalyzed
by Complex 2
a
a
Conditions: 0.5 M PinBBPin (2 equiv) in respective arene solution, 1
mol % catalyst loading, reactions under reflux conditions (oil bath
temperature 90 °C for benzene, 120 °C for toluene), under an argon
b
atmosphere. Isolated yields, obtained by elution of the evaporated
Scheme 7
mixture through SiO2 with pentane. The purity was determined to be
>95% by H NMR.
1
system may contribute to this difference in reactivity.36 Higher
yields were obtained for toluene, rather than benzene, as a
substrate for aryl borylation. This is most probably due to the
higher boiling point (and thus reaction temperature) of
toluene. The products of toluene borylation, catalyzed by 1
mol % of 2, were isolated in 56% yield and found to consist of
the meta- and para-borylated isomers in a statistical 2:1 ratio,
respectively. Negligible amounts of the ortho- and benzyl-
borylated isomers could also be detected by GC-MS. Catalytic
borylations of other arenes with PinBBPin were attempted as
well, albeit with poor isolated yields (see the Supporting
Information). Nevertheless, in all cases a statistical isomeric
outcome was obtained, befitting steric control in aryl
borylation. Similar steric selection was reported with the
more efficient iridium and rhodium borylation cata-
lysts,25,27,29,30−32 as well as with cobalt pincer complexes.45
In order to assess whether borylation by PinBBPin is
analogous in mechanism to PinBH, we examined the adduct
formation of PinBBPin with the pincer complexes 1 and 2.
Although heating is needed to promote reactivity, B−B bond
activation seems to follow the same pathways as observed for
B−H activation of the substrate PinBH. Thus, the predominant
products of heating complex 1 with PinBBPin are complex 5
and PinBH (Scheme 8a). The trans-dihydride complex 3 and
hydrogen are also formed (see the Supporting Information),
indicating an indirect pathway from the starting mixture to the
product complexes. Heating complex 2 with excess PinBBPin
results in only partial conversion to form a mixture of
composition similar to that observed for stoichiometric
reactions with PinBH (Scheme 8b). Prolonged heating of
complex 2 with PinBBPin (18 h at 80 °C) does not result in
Complex 2 showed some reactivity, albeit very poor: less than
10% conversion (by GC-MS) to the corresponding diboron
species bis(pinacolato)diboron (PinBBPin) was observed under
these conditions. Complex 1 was even less active under the
same conditions. Unexpectedly, when the same reaction was
conducted in benzene, after 18 h two products were observed
by GC-MS (Scheme 7b): the diboron species PinBBPin and
the B−Caryl cross-coupling product with the solvent phenyl-
(pinacol)borane (PhBPin). Although B−C coupling products
are expected to be thermodynamically favored over B−B
coupling products,26,30,61a C−H activation by this type of
ruthenium pincer complex has not yet been observed. Reports
of B−Caryl cross coupling of nonactivated arenes catalyzed by
homogeneous ruthenium catalysts are also scarce.33,34 Con-
ducting the reactions in higher-boiling solvents (1,4-dioxane,
toluene) or for longer times (up to 72 h) only marginally
improved the yields. Nevertheless, since PinBBPin was
observed to be formed in situ in reactions leading to aryl
borylation and since this reagent is relatively air and moisture
stable,35−38 we attempted B−Caryl cross coupling catalysis with
PinBBPin as the borylation agent (Scheme 7c).
1
substantial conversion to new boryl complexes, yet H NMR
and GC-MS monitoring of the mixture heated in C6D6 reveals
the formation of the B−C cross-coupling product: phenyl-
(pinacol)borane-d5 (see the Supporting Information). Thus, we
reason that reactions of the diboron species PinBBPin and
borane species PinBH follow a similar mechanism. The reaction
of the diboron serves as an efficient stepping stone toward B−C
coupling reactions, while the borane PinBH is prone to
hydrolysis or oxidation and is a less viable precursor for open
system reactions.
Table 1 summarizes the reactions of complex 2 with
PinBBPin in benzene (entry 1) and toluene (entry 2). The
use of PinBBPin results in the same cross-coupling products as
expected for PinBH but with superior yields. Poor thermal
stability as well as evaporation of PinBH in the open reaction
F
dx.doi.org/10.1021/om500311a | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
AMX-500 NMR spectrometers. H and 13C{1H} NMR spectra are
reported in ppm downfield from tetramethylsilane and referenced to
residual protonated solvent shifts.63 31P NMR chemical shifts are
Scheme 8
referenced to an external 85% solution of phosphoric acid in D2O. 11
B
NMR chemical shifts are referenced to an external 15% solution of
BF3−diethyl etherate (ca. 50% in ether) in CDCl3. Abbreviations used
in the NMR spectral assignments: br, broad; s, singlet; d, doublet; dd,
doublet of doublets; t, triplet; q, quartet; m, multiplet; v, virtual.
Computational Methods. All calculations were performed using
Gaussian 09 (Revision C.01).64 The Perdew−Burke−Ernzerhof (PBE)
generalized-gradient approximation (GGA) functional was used for
geometry optimizations,65,66 and the PBE0 hybride version of Adamo
and Barone was used for energy calculations.65−67 For geometry
optimizations and frequency calculations, the split-valence basis set
Def2-SVP of Ahlrichs and co-workers was used for all atoms,68 and for
ruthenium the corresponding ECP was added.69 In the energy
calculations, the Dunning cc-pVTZ basis set was used for H, B, C, N,
and O,70 Wilson’s cc-pV(D+d)Z was used for P,71 and Peterson’s cc-
pVTZ-PP basis set−RECP combination was used for Ru.72 When the
PBE functional was used, density fitting basis sets, specifically the
fitting sets generated using the automatic generation algorithm
implemented in Gaussian 09, were employed in order to speed up
the calculations.73,74 The accuracy of the DFT method was improved
by adding the empirical dispersion correction as recommended by
Grimme.75,76 The older version (DFTD2) is available in Gaussian 09
(with analytical gradients and Hessians) and was used during geometry
optimizations and frequency calculations;75 our version of Gaussian 09
was locally modified to allow its use for any DFT functional rather
than just for the limited set of functionals as included in the
commercially available version. The newer and more accurate DFTD3
version was used as an a posteriori correction to the PBE0 energies
obtained from Gaussian 09;76 a locally modified version of the stand-
alone program written by Grimme was used.77 Only the singlet states
of the various ruthenium-containing species were considered in the
calculations.
SUMMARY AND CONCLUSIONS
■
Facile addition of boranes to ruthenium−carbonyl pincer
complexes bearing dearomatized PNP or PNN ligands was
shown to involve metal−ligand cooperation. The Lewis acidic
boron atom is inclined to reside on the benzylic arm position of
the ligand, whereas the hydride adds to the metal center.
Notwithstanding, complexes bearing a boryl ligand attached to
the ruthenium atom were also identified. Both benzyl-boryl and
σ-boryl complexes readily lose dihydrogen to form new
dearomatized pincer complexes. In addition to these new
dearomatized pincer complexes, several other boryl complexes
were observed in solution. These minor, dynamic species may
be the catalytically active species for the boron−boron as well
as aryl−boron dehydrogenative coupling reactions observed.
Further catalysis experimentation is currently being pursued in
our group, both with ruthenium and with other late-transition-
metal pincer complexes.
Detailed Characterization of Borylated Pincer Complexes.
[Ru(PNPtBu*-BPin)](H)(CO)] (5). To a vigorously stirred solution of
PinBH (9.0 mg, 0.07 mmol) in benzene or THF (0.3 mL) in a dry
nitrogen glovebox was added slowly a solution of complex 1 (12.3 mg,
0.023 mmol) in the same solvent (0.3 mL). The dark red solution was
left to stir vigorously in an open vial at room temperature for 30 min.
NMR spectroscopy showed formation of complex 5 alongside the
trans-dihydride complex 3 in a 10:1 ratio. Alternatively, PinBH (9.0
mg, 0.07 mmol) was added to a solution of complex 1 (12.3 mg, 0.23
mmol) in C6D6 in a J. Young NMR tube equipped with a condenser.
The system was connected to a Schlenk line, and the mixture was
heated to 80 °C for 2 h under an argon atmosphere. Attempts to
obtain complex 5 in higher purity resulted in formation of complex 7
(see below). Single crystals suitable for X-ray diffraction were obtained
from a concentrated pentane solution of the obtained mixture stored
1
at −20 °C under a dry nitrogen atmosphere. Spectroscopic data: H
NMR (300 MHz, C6D6, δ) 8.54 (d, 3JHH = 9.1 Hz, 1H, Py-H3), 6.85−
6.77 (m, 1H, Py-H4), 5.86 (d, 3JHH = 6.5 Hz, 1H, Py-H5), 2.86 (d, 2JPH
EXPERIMENTAL SECTION
■
3
= 8.0 Hz, 2H, Py-CH2P), 1.71 (d, JPH = 13.0 Hz, 9H, PC(CH3)3),
General Considerations. Dearomatized ruthenium complexes 1
and 2 and the trans-dihydride complex 3 were synthesized according to
previously reported methods.46,47 Pinacol borane (PinBH), catechol
borane (CatBH), and bis(pinacolato)diboron (PinBBPin) were
purchased from commercial sources and used without further
purification. Pinacol boronic acid (PinBOH) was synthesized by
hydrolysis of PinBH on the basis of a previously reported method.62
All solvents used were dried and distilled according to known
procedures to ensure their purity and the absence of water. All
reactions were carried out inside a nitrogen atmosphere glovebox or
using Schlenk techniques to ensure oxygen- and water-free environ-
ments. Catalytic cross-coupling reactions were carried out under an
argon atmosphere. Gas chromatography was carried out on an HP
5973 (MS detector) instrument equipped with a 30 m column (Restek
5 MS, 0.32 mm internal diameter) with a 5% phenylmethylsilicone
3
1.30 (d, JPH = 11.6 Hz, 9H, PC(CH3)3), 1.20 (s, 6H, BPin-(CH3)2),
3
1.19 (s, 6H, BPin-(CH3)2), 1.06 (d, JPH = 12.0 Hz, 9H, PC(CH3)3),
1.00 (d, 3JPH = 11.6 Hz, 9H, PC(CH3)3), −24.61 (dd, 2JPH = 16.5, 14.7
Hz, 1H, Ru-H) ppm; 31P{1H} NMR (121 MHz, C6D6, δ) 84.4, 82.0
2
(ABq, JPP = 211.4 Hz) ppm; 13C{1H} NMR (126 MHz, C6D6, δ)
210.32 (d, 2JPC = 10.3 Hz, Ru-CO), 180.52 (dd, 2JPC = 15.3, 3JPC = 5.4
2
3
Hz, PyC2), 159.94 (dd, JPC = 7.5, JPC = 5.0 Hz, PyC6), 132.97
3
4
(PyC4), 119.12 (dd, JPC = 16.70, JPC = 1.3 Hz, PyC3), 104.28 (d,
3JPC = 10.67 Hz, PyC5), 81.08 (BPin(C(CH3)2) 63 (br, m, Py
1
1
C(BPin)), 44.77 (d, JPC = 19.11, P-C(CH3)3), 37.19 (d, JPC = 14.0
1
3
Hz, Py-CH2-P), 35.52 (dd, JPC = 20.9, JPC = 2.9 Hz, P-C(CH3)3),
35.28 (dd, JPC = 12.4, JPC = 2.9 Hz, P-C(CH3)3), 34.18 (dd, JPC
15.6, JPC = 2.7 Hz, P-C(CH3)3), 31.97 (dd, JPC = 6.0, JPC = 1.2 Hz,
PC(CH3)3), 29.50 (dd, JPC = 5.2, JPC = 1.1 Hz, PC(CH3)3), 29.17
1
3
1
=
3
2
4
2
4
1
2
4
2
coating (0.25 mm) and helium as carrier gas. H, 13C, 11B, and 31P
NMR spectra were recorded using Bruker AMX-300, AMX-400, or
(dd, JPC= 4.5, JPC = 0.8 Hz, PC(CH3)3), 25.10 (d, JPC = 5.4 Hz
PC(CH3)3), 24.64 (BPin(CH3)4) ppm; 11B{1H} NMR (128 MHz,
G
dx.doi.org/10.1021/om500311a | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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C6D6, δ) 30.7 (br) ppm. Crystal data: C30H54BNO3P2Ru, brown, 0.12
× 0.10 × 0.02 mm3, monoclinic, P21/n (No. 14), a = 7.6760(15) Å, b
= 30.854(6) Å, c = 13.989(3) Å, β = 98.87(3)° from 20 degrees of
data, T = 120(2) K, V = 3273.5(11) Å3, Z = 4, formula weight 650.56,
Dc = 1.320 Mg m−3, μ = 0.606 mm−1. Data collection and processing:
Nonius KappaCCD diffractometer, Mo Kα (λ = 0.71073 Å), graphite
monochromator, 26518 reflections collected, −9 ≤ h ≤ 9, −39 ≤ k ≤
39, −17 ≤ l ≤ 17, frame scan width 0.5°, scan speed 1.0° per 120 s,
3.02 (ABq of vt, 2JHH = 15.9 Hz, JPH = 3.2 Hz, 4H, Py-(CH2-P)2), 1.49
(vt, JPH = 6.5 Hz, 18H, P-C(CH3)3), 1.26 (s, 12H, Ru−BPin-(CH3)4),
1.14 (vt, JPH = 6.14 Hz, 18H, P-C(CH3)3), −16.11 (t, 2JPH = 19.9 Hz,
1H, Ru-H) ppm; 31P{1H} NMR (160 MHz, C6D6, δ) 91.56 (s) ppm;
2
13C{1H} NMR (100 MHz, C6D6, δ) 209.81 (t, JPC = 11.8 Hz, Ru-
CO), 163.58 (vt, JPC = 4.7 Hz, PyC2,6), 136.49 (PyC4), 119.58 (vt, JPC
= 4.5 Hz, PyC3,5), 78.14 (Ru-O−B(OC(CH3)2)2), 37.31 (vt, J PC= 5.8
Hz, Py-(CH2-P)2), 36.96 (vt, JPC = 5.3 Hz, P-C(CH3)3), 35.05 (vt, JPC
= 9.9 Hz, P-C(CH3)3), 30.47 (vt, JPC = 3.0 Hz, P-C(CH3)3), 29.81 (vt,
JPC = 3.0 Hz, P-C(CH3)3), 26.00 (Ru-O-B(OC(CH3)2)2) ppm;
11B{1H} NMR (128 MHz, C6D6, δ) 23 (br, overlaps that of PinBOH)
ppm.
typical peak mosaicity 0.97°, 7191 independent reflections (Rint
=
0.0610). The data were processed with Denzo-Scalepack. Solution and
refinement: structure solved by direct methods with SHELXS-97, full-
matrix least-squares refinement based on F2 with SHELXL-97, 373
parameters with 0 restraints, final R1 = 0.0499 (based on F2) for data
with I > 2σ(I) and R1 = 0.0649 on 7191 reflections, goodness of fit on
F2 = 1.064, largest electron density peak 1.327 e Å−3, deepest hole
−0.802 e Å−3.
[Ru(PNN*-BPin)(H)(CO)] (8). In a dry nitrogen glovebox, PinBH
(10.2 mg, 0.08 mmol) was added to a solution of 2 (18 mg, 0.04
mmol) in 0.5 mL of C6D6 in an NMR tube. The mixture was
mechanically shaken for 30 min, after which 1H and 31P NMR spectra
were recorded, showing a mixture comprising ca. 24% complex 8,
alongside the trans-dihydride complex 4 (2%) and the starting complex
2 (74%). The solution was heated and intermittently shaken for 2 h at
75 °C, upon which the solution turned dark red. NMR spectroscopy
showed formation of complex 8 in ca. 95% yield (1% starting complex
2 and the rest unidentified complexes). Traces of molecular hydrogen
[Ru(PNPtBu*-BCat)(H)(CO)] (6). In a dry nitrogen glovebox, to a
vigorously stirred solution of CatBH (2.7 mg, 0.023 mmol) in benzene
or THF (0.3 mL) was added slowly a solution of complex 1 (11.7 mg,
0.022 mmol) in the same solvent (0.3 mL). The bright red solution
was stirred vigorously in an open vial at room temperature for 30 min.
NMR spectroscopy showed formation of complex 6 in ca. 95% yield
alongside the trans-dihydride complex 3. Further attempts at
purification resulted in the formation of bis(catecholato)borate ionic
species.59 Single crystals suitable for X-ray diffraction were obtained by
slow evaporation of a benzene/pentane solution of the obtained
mixture at room temperature under a dry nitrogen atmosphere.
Spectroscopic data: 1H NMR (300 MHz, C6D6, δ) 8.46 (d, 3JHH = 9.0
1
were also observed in solution (4.46 ppm, s). Spectroscopic data: H
3
NMR (300 MHz, C6D6, δ) 8.51 (d, JHH = 9.2 Hz, 1H, Py-H3), 6.83
3
(m, 1H, Py-H4), 5.63 (d, JHH = 6.5 Hz, 1H, Py-H5), 3.3, 2.9 (ABq,
2JHH = 13.9 Hz, 2H, Py-CH2-N), 2.89−2.58 (m, 1H, N-CHH-CH3),
2.60−2.46 (1H, m, N-CHH-CH3), 2.22−2.09 (m, 1H, N-CHH-CH3),
1.95 (m, 1H, N-CHH-CH3), 1.68 (d, 3JPH = 14.2 Hz, 9H, PC(CH3)3),
1.44 (d, 3JPH = 12.7 Hz, 9H, PC(CH3)3), 1.19 (br, 12H, PyC-BPin-
3
4
Hz, 1H, Py-H3), 7.12 (dd, JHH = 5.6, JHH = 3.4 Hz, 2H, BCat-H3,6),
3
6.89 (overlaps catechol signals, m, 1H, Py-H4), 6.84 (dd, JHH = 5.7,
3
3
(CH3)4), 0.86−0.76 (t, JHH = 7.1 Hz, 3H, N-CH2CH3), 0.70 (t, JHH
4JHH = 3.3 Hz, 2H, BCat-H4,5), 5.94 (d, JHH = 6.7 Hz, 1H, Py-H5),
3
2
= 7.4 Hz, 3H, N-CH2CH3), −26.39 (d, JPH = 24.5 Hz, 1H, Ru-H)
2
3
5
2.86 (d, JPH = 6.8 Hz, 2H, Py-CH2-P), 1.71 (dd, JPH = 11.0, JPH
=
ppm; 31P{1H} NMR (121 MHz, C6D6, δ) 105.42 (s) ppm; 13C{1H}
NMR (126 MHz, C6D6, δ) 208.13 (d, 2JPC = 10.0 Hz, Ru-CO), 176.98
(PyC2, d, 2JPC = 12.0 Hz), 155.61 (PyC6), 132.81 (PyC4), 119.24 (d,
3JPC = 12.08 Hz, PyC3), 102.74 (br, PyC5), 81.05 (BPin(C(CH3)2)),
68.07 (br, Py=C−BPin), 64.64 (Py-CH2-N), 54.53 (N-CH2CH3),
3
5
3.2 Hz, 9H, PC(CH3)3), 1.25 (dd, JPH = 10.2, JPH = 2.5 Hz, 9H,
PC(CH3)3), 1.03 (dd, 3JPH = 10.6, 5JPH = 2.7 Hz, 9H, PC(CH3)3), 0.97
(dd, 3JPH = 9.9, 5JPH = 3.0 Hz, 9H, PC(CH3)3), −24.71 (t, 2JPH = 15.7
Hz, 1H, Ru-H) ppm; 31P{1H} NMR (121 MHz, C6D6, δ) 83.8, 82.6
(ABq, JPP = 214.5 Hz) ppm; 13C{1H} NMR (126 MHz, C6D6, δ)
2
1
50.04 (N-CH2CH3), 41.69 (d, JPC = 24.6 Hz, P-C(CH3)3), 35.72 (d,
209.94 (d, 2JPC = 8.9 Hz, 5Ru-CO), 180.02 (dd, 2JPC = 14.0, 3JPC = 6.4
1JPC = 24.2 Hz, P-C(CH3)3), 31.54 (d, JPC = 5.2 Hz, PC(CH3)3),
2
2
3
Hz, PyC2), 160.36 (dd, JPC = 6.7, JPC = 5.5 Hz, PyC6), 149.49
(BCatC1,6), 134.06 (PyC4), 123.23 (BCatC2,5), 118.64 (dd, JPC
14.4, JPC = 1.3 Hz, PyC3), 112.94 (BCatC3,4), 105.99 (d, JPC = 9.7
2
29.85 (d, JPC = 3.6 Hz, P-C(CH3)3), 25.06 (BPin-(CH3)2), 25.11
3
=
4
3
(BPin-(CH3)2), 10.73 (NCH2CH3), 10.72 (NCH2CH3) ppm; 11B{1H}
NMR (128 MHz, C6D6, δ) 31.0 (br) ppm.
1
Hz, PyC5), 65.87−64.70 (br, m, PyC(BCat)-P), 44.34 (dd, JPC
=
17.2, 3JPC = 2.2 Hz, P-C(CH3)3), 37.16 (d, 1JPC = 13.1 Hz, Py-CH2-P),
[Ru(PNN)(H)(OBPin)(CO)] (9). In a dry nitrogen glovebox, a solution
of complex 2 (10.4 mg, 0.023 mmol) and PinBOH (3.3 mg, 0.023
mmol) in 0.6 mL of C6D6 was mixed in an NMR tube for 30 min.
NMR spectroscopy of the yellow solution showed full conversion to
complex 9. Spectroscopic data: 1H NMR (400 MHz, C6D6, δ) 6.90 (t,
1
3
1
35.45 (dd, JPC = 18.83, JPC = 4.5 Hz, P-C(CH3)3), 35.38 (dd, JPC
=
11.2, 3JPC = 3.4 Hz, P-C(CH3)3), 34.14 (dd, 1JPC = 14.9, 3JPC = 3.7 Hz,
P-C(CH3)3), 31.78 (d, 2JPC = 4.5 Hz, PC(CH3)3), 29.53 (d, 2JPC = 4.3
Hz, PC(CH3)3), 29.41 (d, 2JPC = 3.5 Hz, PC(CH3)3), 29.13 (d, 2JPC
=
3
3JHH = 7.7 Hz, 1H, Py-H4), 6.56 (d, JHH = 7.8 Hz, 1H, Py-H5), 6.47
3.5 Hz, PC(CH3)3) ppm; 11B{1H} NMR (128 MHz, THF, δ) 33.3
(br) ppm. Crystal data: C30H46BNO3P2Ru, red plate, 0.28 × 0.20 ×
0.05 mm3, monoclinic, P21/c, a = 8.2384(2) Å, b = 17.5293(3) Å, c =
21.3522(4) Å, β = 92.9126(9)° from 7032 reflections, T = 120(2) K, V
= 3079.56(11) Å3, Z = 4, formula weight 642.50, Dc = 1.386 Mg m−3,
μ = 0.644 mm−1. Data collection and processing: Nonius KappaCCD
diffractometer, Mo Kα (λ = 0.71073 Å), MiraCol optics, graphite
(d, 3JHH = 7.7 Hz, 1H, Py-H3), 5.1, 3.1 (ABq, 2JHH = 15.9 Hz, 2H, Py-
CH2-N), 3.55 (m, 1H, N-CHHCH3), 3.44 (td, 2JHH = 13.6, 3JHH = 6.7
Hz, 1H, N-CHHCH3), 3.04, 2.80 (ABq, 2JHH = 16.71 Hz, 2H, Py-CH2-
P), 2.60 (dq, 2JHH = 14.2, 3JHH = 7.2 Hz, 1H, N-CHHCH3), 2.42 (ddd,
3
4
2JHH = 12.8, JHH = 6.9, JPH = 2.5 Hz, 1H, N-CHHCH3), 1.38
(overlap) (d, 3JPH = 13.0 Hz, 9H, PC(CH3)3), 1.36 (overlap) (s, 12H,
3
O-BOPin(CH3)4), 1.23 (d, JPH = 12.9 Hz, 9H, P-C(CH3)3), 1.05−
monochromator, 0 ≤ h ≤ 10, 0 ≤ k ≤ 21, −26 ≤ l ≤ 26, 2θmax
=
1.00 (m (overlapping with excess PinBOH methyls, 6H, N-
54.42°, frame scan width 1.0°, scan speed 1.0° per 120 s, typical peak
mosaicity 1.178, 43396 reflections collected, 7270 independent
reflections (Rint = 0.054). The data were processed with HKL-
scalepack. Solution and refinement: structure solved with Shelxs-97,
full-matrix least-squares refinement based on F2 with SHELXL-97, 359
parameters with 0 restraints, final R1 = 0.0469 (based on F2) for data
with I > 2σ(I), R1 = 0.0548 on 5825 reflections, goodness of fit on F2
1.244, largest electron density peak 0.601 e Å−3, largest hole −0.735 e
Å−3.
(CH2CH3)2), −15.72 (d, JPH = 27.6 Hz, 1H, Ru-H); 31P{1H} NMR
2
(160 MHz, C6D6, δ) 112.02 (s) ppm; 13C{1H} NMR (100 MHz,
2
2
C6D6, δ) 208.82 (dd, JPC = 16.5 Hz, Ru-CO), 161.34 (d, JPC = 4.2
Hz, PyC2), 161.27 (d, JPC = 2.1 Hz, PyC6), 136.01 (PyC4), 119.38
(d, JPC = 9.2 Hz), 119.01 (PyC5), 78.46 (Ru-O-B(OC(CH3)2)2),
3
3
64.04 (Py-CH2-N), 53.73 (N-CH2CH3), 49.46 (N-CH2CH3), 37.53
(d, 1JPC = 11.7 Hz, P-C(CH3)3), 37.43 (d, 1JPC = 19.3 Hz, Py-CH2-P),
1
2
34.86 (d, JPC = 23.9 Hz, P-C(CH3)3), 30.37 (d, JPC = 2.7 Hz, P-
2
[Ru(PNPtBu)(H)(OBPin)(CO)] (7). In a dry nitrogen glovebox, a
solution of complex 1 (10.5 mg, 0.020 mmol) and PinBOH (2.9 mg,
0.020 mmol) in 0.6 mL of C6D6 was mixed in an NMR tube for 20 h.
NMR spectroscopy of the yellow solution showed full conversion to
complex 7. Spectroscopic data: 1H NMR (400 MHz, C6D6, δ) 6.87 (t,
3JHH = 7.7 Hz, 1H, Py-H3), 6.66 (d, 3JHH = 7.7 Hz, 2H, Py-H2,4), 3.73,
C(CH3)3), 29.77 (d, JPC = 4.7 Hz, P-C(CH3)3), 25.72 (N-CH2CH3),
25.53 (Ru-O-B(OC(CH3)2)2), 24.64 (N-CH2CH3) ppm; 11B{1H}
NMR (128 MHz, C6D6, δ) 23 (br, overlaps that of PinBOH) ppm.
[Ru(PNN*)(BCat)(CO)] (10). In a dry nitrogen glovebox, a C6D6
solution (0.3 mL) of complex 2 (15.0 mg, 0.033 mmol) was added
slowly to a vigorously stirred solution of CatBH (4.5 mg, 0.037 mmol)
H
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Organometallics
Article
in C6D6 (0.3 mL). The bright red solution was stirred vigorously in an
open vial at room temperature for 30 min. NMR spectroscopy showed
formation of complex 10 in ca. 90% conversion, with other
unidentified complexes and unreacted excess CatBH. When the
addition was performed in a closed NMR tube that was vigorously
shaken, molecular hydrogen was also observed in solution (4.47 ppm,
s, 1:3 to complex 10; see the Supporting Information). Spectroscopic
condenser. The red solution was refluxed under an argon atmosphere.
The conversion was monitored by GC-MS analysis. The solution after
the reaction was fully evaporated under high vacuum, removing
solvent and cyclooctane (and unreacted borane). The residue after
evaporation was weighed, and 0.1 mmol of 1,4-dioxane was added as
an internal standard. CDCl3 (0.5 mL) was added, and the yield was
calculated from the appropriate signals in the H NMR spectra. For
isolation, the residue after evaporation was eluted through a silica
column with n-pentane or 1/9 ethyl acetate/n-pentane.
1
1
3
4
data: H NMR (500 MHz, C6D6, δ) 7.04 (dd, JHH = 5.9, JHH =3.5
3
4
Hz, 2H, BCat-H4,5), 6.71 (dd, JHH = 5.7, JHH = 3.3 Hz, 2H, BCat-
H2,5), 6.52 (ddd, 3JHH = 8.6, 6.4, 5JHH3 = 1.9 Hz, 1H, Py-H4), 6.40 (d,
3JHH = 9.0 Hz, 1H, Py-H3), 5.40 (d, JHH = 6.4 Hz, 1H, Py-H5), 4.05
(d, 2JHH = 13.7 Hz, 1H, Py-CHHN), 3.45 (d, 2JPH = 3.0 Hz, 1H, Py
CH-P), 2.82 (dd, 2JHH = 13.8, 5JHH = 1.8 Hz, 1H, Py-CHH−N), 2.67−
2.58 (m, 1H, N-CHHCH3), 2.43 (dq, 2JHH = 13.8, 3JHH = 7.0 Hz, 1H,
Reaction of 5 with CatBH. In a dry nitrogen glovebox, CatBH
(3.2 mg, 0.027 mmol) was added to a solution of complex 5 (9.7 mg,
0.015 mmol, containing 20% complex 7) in C6D6 (0.6 mL) in an
1
NMR tube. The H NMR spectrum taken immediately after mixing
showed complete disappearance of residual complex 7. Further
shaking of the sample at room temperature resulted in quantitative
conversion to catechol boryl complex 6 within 2 h (see the Supporting
Information).
2
3
N-CHHCH3), 2.27 (dq, JHH = 14.4, JHH = 7.2 Hz, 1H, N-
2
3
4
CHHCH3), 1.97 (ddq, JHH = 14.4, JHH = 7.2, JPH 2.7 Hz, 1H, N-
CHHCH3), 1.33 (d, JPH = 12.5 Hz, 9H, P-C(CH3)3), 1.15 (d, 3JPH
=
3
3
Reaction of 5 with H2. Warning! H2 is an explosive gas, and
proper measures should be taken for its safe handling. In a dry
nitrogen glovebox, a solution of complex 5 containing 20% complex 7
(21 mg, ca. 0.032 mmol) in C6D6 (0.6 mL) in a J. Young NMR tube
was pressurized three times to 1.5 bar of H2 (ca. 2 equiv in the gas
phase) and then shaken overnight at room temperature. Changes were
monitored by NMR. Within minutes, the trans-dihydride complex 3
appeared, with disappearance of complex 7 and subsequently
disappearance of complex 5. Free PinBH could also be observed
(see the Supporting Information).
14.1 Hz, 9H, P-C(CH3)3), 0.68 (t, JHH = 7.2 Hz, 3H, N-CH2CH3),
0.50 (t, JHH = 7.2 Hz, 3H, N-CH2CH3) ppm; 31P{1H} NMR (162
3
MHz, C6D6, δ) 84.88 (s) ppm; 13C{1H} NMR (125 MHz, C6D6, δ)
2
2
206.40 (d, JPC = 9.4 Hz, Ru-CO), 169.05 (d, JPC = 15.5 Hz, PyC2),
156.18 (PyC6), 150.75 (BCat-C1,6), 132.34 (PyC4), 121.25 (BCat-
C2,5), 115.04 (d, JPC = 17.5 Hz, PyC3), 111.23 (BCat-C3,4), 97.30
3
1
(PyC5), 64.80 (Py-CH2-N), 64.16 (d, JPC = 56.0 Hz, PyCH-P),
54.49 (N-CH2CH3), 47.58 (N-CH2CH3), 40.85 (d, 1JPC = 25.3 Hz, P-
C(CH3)3), 35.07 (d, 1JPC = 28.2 Hz, P-C(CH3)3), 29.21 (PC(CH3)3),
28.85 (N-CH2CH3), 9.94 (PC(CH3)3), 9.57 (N-CH2CH3) ppm;
11B{1H} NMR (128 MHz, C6D6, δ) 47.4 (br) ppm.
Substoichiometric Reactions of 1 with PinBBPin. In a dry
nitrogen glovebox, PinBBPin (8.6 mg, 0.036 mmol, 2.1 mol equiv) was
added to a solution of 1 (17.7 mg, 0.033 mmol) in C6D6 (0.6 mL) in
an NMR tube. 1H and 31P NMR spectra were recorded at room
temperature and during heating. Complexes 5 and the trans-dihydride
3 formed in a ca. 1:1 ratio at room temperature; however, with heating,
all species converted to complex 5. Free hydrogen as well as free
[Ru(PNN*-BCat)(BCat)(CO) (11). In a dry nitrogen glovebox, to a
solution of complex 10 (90%, prepared in situ, see above) in 0.6 mL of
C6D6 was added 1.8 equiv of CatBH (7.0 mg, 0.058 mmol), and the
solution was stirred under nitrogen overnight. NMR spectroscopy
showed 25% conversion to complex 11. Further attempts to purify
complex 11 out of the mixture failed. Characterization was possible in
situ, with the aid of 2D NMR techniques. Spectroscopic data: 1H
NMR (500 MHz, C6D6, δ) 8.50 (d, 3JHH = 9.1 Hz, 1H, Py-H3), 7.16−
7.13 (m, 2H, Ru-BCat-H3,6), 7.08 (dd, 3JHH = 5.6, 4JHH = 3.5 Hz, 2H,
1
PinBH (detected by 11B NMR) could also be detected by H NMR
(see the Supporting Information).
Substoichiometric Reactions of 2 with PinBBPin. In a dry
nitrogen glovebox PinBBPin (4.7 mg, 0.02 mmol, 2 mol equiv) was
added to a solution 2 of (8.0 mg, 0.018 mmol) in C6D6 (0.5 mL) in an
NMR tube. The mixture showed no reaction at room temperature by
1H NMR. After the solution was heated to 80 °C for 18 h, a 1H NMR
spectrum showed that only a small fraction of the starting complex 2
had reacted. The product complexes were not identified, except for
1.5% of complex 8 that was clearly formed. A signal at 1.10 ppm
indicated the formation of deuterated phenyl(pinacol)borane
(PhBPin-d5) in 37% conversion relative to PinBBPin (see the
Supporting Information). GC-MS confirmed the formation of the
deuterated PhBPin.
3
PyC-BCat-H3,6), 6.95−6.91 (m, 2H, Py-H4), 6.82 (dd, JHH = 5.8,
4JHH = 3.2 Hz, 2H, Ru-BCat-H4,5), 6.78 (dd, 3JHH = 5.8, 4JHH = 3.2 Hz,
2H, PyC-BCat-H3,6), 5.84 (d, 3JHH = 6.8 Hz, 1H, Py-H5), 3.84 (d,
2JHH = 14.0 Hz, 1H, Py-CHH-N), 2.92 (m, 1H, N-CHHCH3), 2.91
(m, 1H, Py-CHH-N), 2.62 (m, 1H, N-CHHCH3), 2.14 (m, 1H, N-
3
CHHCH3), 1.82 (m, 1H, N-CHHCH3), 1.56 (d, JPH = 14.9 Hz, 9H,
P-C(CH3)3), 1.36 (d, 3JPH = 11.4 Hz, 9H, P-C(CH3)3), 0.70 (t, 3JHH
=
3
6.9 Hz, 3H, N-CH2CH3), 0.44 (t, JHH = 7.1 Hz, 3H, N-CH2CH3)
ppm; 31P{1H} NMR (162 MHz, C6D6, δ) 97.53 (s) ppm; 13C{1H}
NMR (126 MHz, C6D6, δ) 207.10 (d, 2JPC = 10.0 Hz, Ru-CO), 176.05
(PyC2), 155.16 (PyC6), 150.63 (Ru-BCatC1,6), 149.31 (PyC-
BCatC1,6), 134.12 (PyC4), 121.73 (Ru-BCatC2,5), 121.33 (PyC-
3
ASSOCIATED CONTENT
* Supporting Information
BCatC2,5), 119.51 (d, JPC = 14.23 Hz, PyC3), 111.43 (PyC-
■
BCatC3,4), 111.10 (Ru-BCatC3,4), 105.06 (PyC5), 65.86 (Py-CH2-
N), 55.38 (N-CH2CH3), 48.18 (N-CH2CH3), 43.3 (br, PyC-BCat),
S
Text, tables, figures, and CIF files giving crystallographic data
for 5 and 6, NMR spectra of boryl addition complexes, detailed
exchange experiments, catalytic borylation of substituted
arenes, and computational details. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
1
42.57 (d, JPC = 23.4 Hz, P-C(CH3)3), 36.33 (d, JPC = 24.8 Hz, P-
C(CH3)3), 30.91 (PC(CH3)3), 29.97 (PC(CH3)3), 10.53 (N-
CH2CH3), 8.64 (N-CH2CH3) ppm; 11B NMR could not be resolved
in the mixture.
General Procedure for Catalytic Boron−Boron Coupling
Reactions, Demonstrated for 1 with PinBH in THF. In a dry
nitrogen glovebox, complex 1 (6.0 mg, 0.011 mmol), cyclooctane
(internal standard, 0.1 mmol), and PinBH (145.0 mg, 1.13 mmol)
were dissolved in THF (2.5 mL) in a Schlenk flask equipped with a
condenser. The red solution was refluxed under an argon atmosphere
(oil bath temperature 75 °C). The reaction was monitored by GC-MS
analysis until no increase in conversion was observed. The product was
not isolated due to low yields.
General Procedure for Catalytic Aryl Borylation Reactions,
Demonstrated for 2 with PinBBPin in Benzene. In a dry nitrogen
glovebox, complex 2 (14.3 mg, 0.031 mmol), cyclooctane (internal
standard, 0.1 mmol), and PinBBPin (380 mg, 1.6 mmol) were
dissolved in benzene (3 mL) in a Schlenk flask equipped with a
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.F.: moran.feller@weizmann.ac.il.
*E-mail for D.M.: david.milstein@weizmann.ac.il.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was supported by the European Research Council
under the FP7 framework (ERC No. 246837) and by the Israel
■
I
dx.doi.org/10.1021/om500311a | Organometallics XXXX, XXX, XXX−XXX

Organometallics
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dx.doi.org/10.1021/om500311a | Organometallics XXXX, XXX, XXX−XXX