Rapid reversible borane to boryl hydride exchange by metal shuttling on the carborane cluster surface

Article abstract of DOI:10.1039/c7sc01846k

In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh₃) pincer complex (POBOP = 1,7-OP(i-Pr)₂-m-2-carboranyl) features extreme distortion of the two-center-two-electron Ru-B bond due to the presence of a strong three-center-two-electron B-H?Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh₃) pincer complex, which possesses B-Ru, B-H?Ru, and Ru-H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal-boron and metal-hydrogen bonds, is unexpectedly facile at temperatures above -50 °C corresponding to an activation barrier of 12.2 kcal mol^-1. Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru(iv) intermediates, respectively. The presence of this hemilabile vicinal B-H?Ru interaction in (POBOP)Ru(H)(PPh₃) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C.

Full text of DOI:10.1039/c7sc01846k

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Rapid Reversible Borane to Boryl Hydride Exchange by Metal Shuttling on the
Carborane Cluster Surface
a
a
b
a
Bennett J. Eleazer , Mark D. Smith , Alexey A. Popov *, and Dmitry V. Peryshkov *.
a). Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter St.,
Columbia, South Carolina 29208, United States.
b). Leibniz Institute for Solid State and Materials Research, Helmholtzstrasse 20, 01069 Dresden,
Germany.
Abstract.
In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond
acting as a supporting hemilabile ligand in exohedrally metalated threeꢀdimensional carborane
clusters. The (POBOP)Ru(Cl)(PPh ) pincer complex (POBOP = 1,7ꢀOP(iꢀPr) ꢀmꢀ2ꢀcarboranyl)
3
2
features extreme distortion of the twoꢀcenterꢀtwoꢀelectron Ru–B bond due to the presence of a
strong threeꢀcenterꢀtwoꢀelectron B–HꢁꢁꢁRu vicinal interaction. Replacement of the chloride ligand
for a hydride afforded the (POBOP)Ru(H)(PPh ) pincer complex, which possesses B–Ru, B–
3
HꢁꢁꢁRu, and Ru–H bonds. This complex was found to exhibit a rapid exchange between hydrogen
atoms of the borane and the terminal hydride through metal center shuttling between two boron
atoms of the carborane cage. This exchange process, which involves sequential cleavage and
formation of strong covalent metalꢀboron and metalꢀhydrogen bonds, is unexpectedly facile at
temperatures above –50 ºC corresponding to an activation barrier of 12.2 kcal/mol. Theoretical
calculations suggested two equally probable pathways for the exchange process through formally
Ru(0) or Ru(IV) intermediates, respectively. The presence of this hemilabile vicinal B–HꢁꢁꢁRu
interaction in (POBOP)Ru(H)(PPh ) was found to stabilize a latent coordination site at the metal
3
center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and
air at 170 ºC.
Introduction
Multidentate ligands containing a heteroatom backbone have attracted significant recent
attention in ligand design and catalysis. Boronꢀbased ligand systems demonstrate versatility in the
possible bonding interactions to a transition metal center as a neutral borane as a Zꢀ or an Lꢀtype
ligand, an anionic borate as an Lꢀtype ligand, or a central anionic boryl moiety as an Xꢀtype
1–4
ligand. Interconversion between these coordination modes has been demonstrated to provide an
additional reactivity manifold through metalꢀligand cooperation. Pincer boryl ligands with
diazaborole, diarylborane, and carborane fragments have been recently introduced incorporating
strongly electronꢀdonating boryl donors in tridentate meridional coordination frameworks (Chart
5–19
1
).
1

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Chart 1. Examples of borylꢀbased pincer complexes.
Polyhedral boron clusters, such as icosahedral C B H carborane cages, are chemically robust
2
10 12
molecular species possessing unique electronic properties and increased steric bulk and often are
2
0–23
considered as inorganic threeꢀdimensional “pseudoaromatic” analogs of arenes.
Carboranes
have been shown to be promising molecular building blocks for potential application in metalꢀ
organic frameworks, organomimetic architectures, luminescent materials, batteries, liquid
2
4–36
crystals, coordination chemistry and catalysis.
Icosahedral carborane clusters represent an unusual 3ꢀD ligand framework where one metalated
boron vertex of the cage is surrounded by multiple vicinal B–H bonds. These B–H bonds can
serve as hemilabile neutral ligands to the metal center and, in principle, can themselves be
activated. Recently, we demonstrated that utilization of the carborane cage backbone in the
pincerꢀtype chelating ligand POBOP (POBOP = 1,7ꢀOP(iꢀPr) ꢀmꢀ2ꢀcarboranyl) led to the close
2
simultaneous contact of the coordinated metal center and two cluster vertices. The double B–H
bond activation at adjacent boron vertices by a single ruthenium center resulted in the formation
of the first example of a (BB)ꢀcarboryne complex with a highly strained threeꢀmembered BB>Ru
37
metallacycle.
The functionalization of boron clusters by B–H bond activation represents an attractive
3
8–41
synthetic strategy.
The use of directing groups on a boron cage has been demonstrated to lead
4
2–44
to metalꢀpromoted derivatization of neighboring boron vertices.
Regioselectivity of B–H
bond activation is highly desired given the high number of potential isomers for an exoꢀ
substituted icosahedral cage. In this work, we probed the B–H bond activation process in detail
and experimentally assessed the possibility of intramolecular interconversion between the
coordinated borane B–HꢁꢁꢁRu and the metal boryl hydride B–Ru–H interactions or, in other
words, reversibility of B–H bond activation and the possibility of migration of the metal center on
the carborane cluster surface (Chart 2). The use of the strained threeꢀdimensional POBOP
carboranyl pincer framework allowed us to enforce an unprecedented simultaneous coordination
of the ruthenium metal center with a terminal hydride ligand to the boron cluster through the
metalꢀboryl bond and vicinal metalꢀborane bond. Furthermore, the reactivity of this unusual
(
POBOP)Ru(H)(PPh ) boryl hydride complex featuring a latent open coordination site was
3
probed in reactions with H , D , and in catalytic dehydrogenation of cyclooctane.
2
2
2

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Chart 2. The pendulum clockꢀtype fluxional behavior of the (POBOP)Ru(H)(PPh ) complex
3
resulting from the rapid interchange between the B–Ru–H boryl hydride and the B–HꢁꢁꢁRu
coordinated borane.
Results and Discussion
Synthesis and Structure of (POBOP)Ru(Cl)(PPh3)
The reaction of the ligand precursor (POBOP)H (1) and Ru(PPh ) Cl in THF at reflux
3
3
2
temperature led to the B–H bond activation of the ligand and the formation of the Bꢀcarboranyl
pincer complex (POBOP)Ru(Cl)(PPh ) (2, Scheme 1). The product was isolated by
3
recrystallization from an ether/hexanes mixture at –30 ºC in high yield (86%) as an orange
powder.
Scheme 1. Synthesis of (POBOP)Ru(Cl)(PPh ) (2).
3
The single crystal Xꢀray structure of 2 revealed a surprisingly high degree of bond strain in the
boronꢀruthenium bond imposed by the stabilizing effect of the vicinal B–HꢁꢁꢁRu interaction and
ligand geometry (Fig. 1a,b). The Ru1–B1 bond length is 2.086(2) Å, which is comparable to a
twoꢀcenterꢀtwoꢀelectron (2cꢀ2e) Ru–B bond length in previously reported ruthenium diazaborolyl
2
,7–9
PBP pincer complexes (2.022(4) – 2.080(14) Å) . The short Ru1ꢁꢁꢁB2 (2.417(2) Å) and RuꢁꢁꢁH2
1.96(2) Å) distances are indicative of a strong bridging B–HꢁꢁꢁRu interaction. This apparently
(
strong 3cꢀ2e bond attracts the metal center to the B2 vertex, and, as a consequence, results in a
significant distortion of the vicinal 2cꢀ2e Ru1–B1 bond. The B2–B1–Ru1 angle is 77.4(1)º, which
deviates drastically from an unstrained exohedral bond angle of 120º for an idealized icosahedral
cluster. For comparison, the corresponding exohedral B2–B1–H1 angle in the ligand precursor
(
POBOP)H is 116.1(9)°. Previously reported 2ꢀBꢀmetalated mꢀcarborane complexes exhibit
significantly larger values of the analogous B2–B1–M angle in the range from 104.5(1)º to
1
20(1)º, with the only exception being the ruthenium BBꢀcarboryne complex previously
1
3–16,37,45
synthesized in our group featuring two adjacent 2cꢀ2e metalꢀboron bonds.
Thus, the
presence of the strong B–HꢁꢁꢁRu interaction in 2 led to the unprecedented distortion of the vicinal
3

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metal boryl bond. The coordination geometry of the ruthenium center is, therefore, octahedral,
with the borane B–H bond serving as one of the ligands. Triphenylphosphine is located transꢀ to
the coordinated B–H bond with the H2ꢁꢁꢁRu1–P3 angle of 176.5(5)º.
The strong bridging B–HꢁꢁꢁRu interaction is persistent in solution as manifested by the
1
characteristic broadened 1:1:1:1 quartet at –5.4 ppm in the H NMR spectrum of 2 in C D (Fig
1
6
6
1
1
c). The 1:1:1:1 splitting pattern is indicative of the coupling of a proton to a B nucleus. This
signal is shifted considerably upfield in comparison to the remaining hydrogen atoms of the
carborane cluster which are represented by a set of overlapping signals in the range from +4 to +1
1
11
1
ppm. In the boronꢀdecoupled H{ B} NMR spectrum, this quartet signal ( J = 116 Hz)
BH
3
1
2
converted into a doublet due to coupling to the P nucleus of the triphenylphosphine ligand ( J
PH
=
28 Hz) coordinated to the metal center, which is another indication of the strong bridging B–
HꢁꢁꢁRu interaction.
Fig. 1. (a, b) Displacement ellipsoid plots (50% probability) of (POBOP)Ru(Cl)(PPh ) (POBOP =
3
1
,7ꢀOP(iꢀPr) ꢀ2ꢀdehydroꢀmꢀcarboranyl) (2). (a): a general view (b): a view perpendicular to the
2
(
B2–B1–Ru1) plane. Atoms belonging to isopropyl groups of the ligand arms and phenyl rings of
triphenylphosphine have been omitted for clarity. Hydrogen atoms of the boron cluster, except for
H2 are not shown. Selected bond distances (Å) and angles (°): Ru1–B1 = 2.086(2), Ru1ꢁꢁꢁB2 =
2
1
.417(2), RuꢁꢁꢁH2 = 1.96(2), Ru1–Cl1 = 2.501(1), B2–B1–Ru1 = 77.4(1), B1–Ru1–Cl1 =
59.6(1), and H2ꢁꢁꢁRu1–P3 = 176.5(5). (c) Fragments of H and H{ B} NMR spectra of 2
1
1
11
featuring a signal from the bridging hydride B2–H1ꢁꢁꢁRu1 and its coupling to B2 and P3 atom
nuclei, respectively.
Persistent bridging 3cꢀ2e B–HꢁꢁꢁM interactions have been often observed between cationic metal
4
6–49
complexes and anionic heteroboranes that can be considered as cationꢀanion pairs.
For
example, the nidoꢀcarborane anions with pendant donor groups have been shown to exhibit strong
1
binding to a ruthenium center with signals corresponding to B–HꢁꢁꢁRu coordination in H NMR
spectra ranging from –2 ppm (a larger contribution from a B–H extreme form) to –18 ppm (a
5
0–52
larger contribution from a Ru–H extreme form).
The intramolecular combination of
interactions observed in the complex 2 where the metal center is simultaneously bound to one
4

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boron atom of a neutral carborane cluster with a 2cꢀ2e B–Ru bond and to the adjacent boronꢀ
hydrogen bond of the same cluster with a 3cꢀ2e B–HꢁꢁꢁRu bond has not been observed prior to
this work. Notably, formation of intermediates of this type may account for rare cases of
isomerization of Bꢀmetalated boron clusters at high temperatures, where a metal center changes
5
3
its position on the heteroborane cage. Furthermore, the H ꢀmediated Ru−C to Ru−B bond
2
conversion has been reported for cyclopentadienylcarboranyl complexes, which likely proceeds
5
4
through intermediates similar to 2. To probe the possibility of such “cage walking” by the metal
center, we synthesized the ruthenium carboranyl hydride complex (POBOP)Ru(H)(PPh ) in the
3
reaction of the carboranyl chloride complex 2 and a hydride source.
Scheme 2. Synthesis of (POBOP)Ru(H)(PPh ) (3).
3
Synthesis and Structure of (POBOP)Ru(H)(PPh3)
Reaction of (POBOP)Ru(Cl)(PPh ) and NaH in THF at reflux temperature for 36 h resulted in
3
3
1
the clean formation of a single product according to P NMR spectral data (Scheme 2). The
product was crystallized from diethyl ether as a pale yellow solid in 98% yield. The crystal
structure determination revealed an unusual coordination geometry of the ruthenium hydride
complex (POBOP)Ru(H)(PPh ) (3) (Fig. 2). Two crystallographically independent but chemically
3
identical molecules were found. The most striking feature of the molecular structure of 3 is the
extreme ligandꢀinduced strain of the B–Ru bond as demonstrated by the acute exohedral B2–B1–
Ru1 angle of 69.4(2)º. This extreme bond strain results in the remarkable closeness of the
covalent 2cꢀ2e boryl B1–Ru1 bond length (2.208(3) Å) and the vicinal 3cꢀ2e coordinated borane
(
3
H2)B2ꢁꢁꢁRu1 distance (2.276(3) Å). Importantly, the value of the exohedral B2–B1–Ru1 angle in
is the smallest for 2cꢀ2e M–B bonds in any icosahedral carborane complex reported to date with
the only exception the ruthenium BBꢀcarboryne complex (B2–B1–Ru angle values 65.5(1)º and
37
6
1
8.4(1)º). The PPh ligand is located transꢀ to B–Ru bond with the B1–Ru1–P3 angle of
3
72.5(1)º. The hydride ligand H1 and the bridging borohydride H2 are located transꢀ to each
other with a H1–Ru1ꢁꢁꢁH2 angle of 177(1)º. The Ru1–H1 bond length in the crystal structure of 3
is 1.70(3) Å and the Ru1ꢁꢁꢁH2 distance is 1.77(3) Å. These hydrogen atoms were clearly located
using the electron density difference map. The metal center is, therefore, in the distorted
octahedral environment.
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Fig. 2. Displacement ellipsoid plots (50% probability) of (POBOP)Ru(H)(PPh ) (3). (a): a general
3
view (b): a view perpendicular to the (B2–B1–Ru1) plane. Atoms belonging to isopropyl groups
of the ligand arms and phenyl rings of triphenylphosphine have been omitted for clarity.
Hydrogen atoms of the boron cluster, except for H2 are not shown. Selected bond distances (Å)
and angles (°): Ru1–B1 = 2.208(3), Ru1ꢁꢁꢁB2 = 2.276(3), Ru1–H1 = 1.70(3) Ru1ꢁꢁꢁH2 = 1.77(3),
Ru1–P3 = 2.343(1), B2–B1–Ru1 = 69.4(2), B1–B2–Ru1 = 65.5(1), B1–Ru1–P3 = 172.5(1), and
H1–Ru1ꢁꢁꢁH2 = 177(1).
Dynamic Behavior of (POBOP)Ru(H)(PPh ) and its variable temperature NMR spectra.
3
1
The most prominent feature of the H NMR spectrum of 3 at room temperature was the
presence of a broad signal at –8.8 ppm with an integral intensity corresponding to two hydrogen
atoms in the complex (Fig. 3). At the same time, no other signals were found in the range from 0
ppm to –15 ppm. These observations suggested that the Ru–H hydride and the RuꢁꢁꢁH–B bridging
borohydride group in (POBOP)Ru(H)(PPh ) undergo a rapid exchange at room temperature. This
3
exchange process is likely intramolecular due to significant steric hindrance of the POBOP pincer
ligand with diisopropylphosphinite arms that preclude bimolecular interactions between
complexes. To probe whether dissociation of the PPh ligand is responsible for the observed
3
dynamic behavior, an excess of a smaller cone angle, more electronꢀrich phosphine, PEt was
3
,
added to the solution of (POBOP)Ru(H)(PPh ) in C D . No replacement of the coordinated PPh
3
3
6
6
in the complex by PEt was observed at room temperature for 16 h. suggesting that the
3
dissociation of PPh is slow and unlikely to be responsible for the observed rapid borane/boryl
3
hydride exchange (see ESI for details).
1
The series of the H NMR spectra of 3 recorded at the temperature range from +22 ºC to –90 ºC
shed light on the dynamics of the exchange process. Instead of one broad signal at –8.8 ppm at
room temperature, two signals at –6.1 ppm and –11.8 ppm were observed at –90 ºC. The signal at
1
11
–
6.1 ppm sharpened in the H{ B} NMR spectrum indicating its correspondence to the
coordinated B–HꢁꢁꢁRu borane while the signal at –11.8 ppm corresponded to the Ru–H hydride.
The coalescence temperature for the exchange process was estimated to be close to –50 ºC. Based
6

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‡
on this value of the coalescence temperature (223 K), the activation energy
transformation was determined to be 12.2 kcal/mol.
ꢂG for the exchange
1
Fig. 3. The fragments of H NMR spectra of (POBOP)Ru(H)(PPh ) in CD Cl recorded in the
3
2
2
temperature range from 25 ºC to –90 ºC.
The (POBOP)Ru(H)(PPh ) complex reacted with 1 atm D gas at room temperature in benzene
3
2
solution leading to the formation of 3-d featuring selective deuteration and the formation of the
2
2
B–DꢁꢁꢁRu borane group and the Ru–D hydride. The H NMR spectrum of 3-d exhibited one
2
signal at –8.9 ppm at room temperature. Analogously to 3, cooling the solution of 3-d in
2
2
dichloromethane to –90 ºC led to the appearance of two distinct signals in the H NMR spectrum:
one at –6.3 ppm corresponding to the coordinated borane moiety and another at –12.1 ppm
‡
corresponding to the metal hydride (see ESI for details). The activation energy
Δ
G of the
exchange process was determined to be 13.3 kcal/mol for 3-d based on the observed coalescence
2
temperature of 223 K. The value of kinetic isotope effect for the B–HꢁꢁꢁRu/Ru–H exchange was
estimated to be 6.53 at 223 K.
The rapid intramolecular hydrogen atom exchange along with observation of the H/D exchange
upon conversion of 3 to 3-d prompted us to explore the reaction of 3 with dihydrogen. Exposure
2
of a degassed solution sample of 3 in C D to 1 atm of H led to a partial (ca. 30%) conversion of
6
6
2
31
3
to new species 5 according to P NMR spectral data with the new set of signals at 241.5 ppm
(
2P, pincer ligand arms) and 42.8 ppm (1P, coordinated PPh ligand). Replacement of 1 atm of H
3
2
1
with 1 atm of N led to the complete conversion of 5 back to 3. The H NMR spectrum of the
2
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mixture of 3 and 5 under dihydrogen atmosphere exhibited a new signal at –10.5 ppm (a sharp
doublet of triplets, 1H, Ru–H), and a broadened signal at –4.8 ppm with an integral intensity
corresponding to two hydrogen atoms that was assigned to the coordinated dihydrogen (Ru–H )
2
1
1
(
Fig. 4). No correlations between these signals were observed in the Hꢀ H NOESY NMR
spectrum of the mixture of 3 and 5 suggesting the probable transꢀ mutual orientation of the
hydride and dihydrogen ligands. Furthermore, no signals corresponding to bridging B–HꢁꢁꢁRu
interactions were observed, suggesting the replacement of the coordinated borane in 3 by the
dihydrogen molecule in 5.
Fig. 4. The fragment of the 1H NMR spectrum of the sample of (POBOP)Ru(H)(PPh ) (3) under
3
H (1 atm) at room temperature. Signals from the proposed dihydrogen complex 5 (2H, br, –4.8
2
ppm and 1H, dt, –10.5 ppm) are highlighted.
Proposed Exchange Mechanisms. Metal Center “Shuttling” on the Cluster Surface.
One plausible sequence of steps responsible for the exchange between the hydride and
borohydride groups in 3 (Scheme 3) may be the reductive elimination of the hydride ligand from
the metal center with the formation of the B1–H1 bond and a formally Ru(0) diborane complex 4-
1
followed by the oxidative addition of the B2–H2 bond with the formation of Ru1–H2 hydride
and Ru1–B2 boryl, and the “pendulum”ꢀlike swing of the triphenylphosphine ligand. Another
possible mechanism involves the oxidative addition of the B2–H2 bond with the formation of a
formally Ru(IV) diboryl dihydride intermediate 4-2 followed by the reductive elimination of one
hydride ligand from the metal center and the formation of the B–H bond coordinated to
ruthenium ending with the “pendulum”ꢀlike swing of the triphenylphosphine ligand. Both these
processes produce the same starting complex with the metal center moving from one boron atom
11
11
1
to another on the carborane cage. The B and B{ H} NMR spectra at room temperature
exhibited a characteristic singlet at –2 ppm corresponding to an integral intensity of two boron
11
atoms while all other signals appeared as doublets in the B spectrum corresponding to the
presence of B–H bonds thus suggesting the chemical exchange of the metalated boron atom and
the coordinated vicinal borane B–Ru/B–HꢁꢁꢁRu.
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Scheme 3. Possible reaction sequences responsible for the rapid exchange of the ruthenium
hydride and coordinated borane groups in (POBOP)Ru(H)(PPh ). Chelating ligand arms are not
3
shown for clarity. (a) The pathway involving the Ru(0) diborane complex 4-1 as an intermediate.
(
b) The pathway involving the Ru(IV) BBꢀcarboryne dihydride complex 4-2 as an intermediate.
Note “walking” of the metal center on the carborane cage in both cases.
A relevant fluxional behavior has been observed by Heinekey and coꢀworkers for the putative
5
5
iridium(III) dihydride POCOP complex featuring the σꢀcoordinated borane HBPin. The
exchange of hydrogen atoms of the metal hydride and the borane has been determined to occur
1
with an activation barrier of 14 kcal/mol based on the coalescence temperature of 31 ºC in H
NMR spectra. Notably, this transformation does not involve the formation of a 2cꢀ2e metalꢀboryl
bond. Baker, Marder, and coꢀworkers reported an example of an intramolecular hydrogen atom
exchange between a metal hydride and a coordinated borane likely also proceeding through a
hydroborate intermediate with an activation barrier of 15 kcal/mol for the ruthenium complex and
5
6
1
3 kcal/mol for the osmium congener. SaboꢀEtienne and coꢀworkers have extensively studied
structural motifs of metal hydride borane complexes with the particular focus on the distinction
5
7–59
between boraneꢀhydride and dihydroborate coordination modes.
The complex
(
POBOP)Ru(H)(PPh ) reported herein, featuring metal boryl, metal hydride, and metalꢀborane
3
moieties, may be the first example of the rapid transformation between these configurations
through the exchange of not only hydrogen atoms but also boron atoms at the metal center.
Importantly, the exchange reported herein is likely to occur through (borane)ꢀ(boryl/hydride)
conversion, as the anionic borate configuration is not normally attainable for boron clusters due to
the lack of accessible pꢀorbitals on boron vertices.
Theoretical Calculations.
Theoretical calculations at the PBE/TZ2P level were employed in order to obtain better
understanding of the possible exchange mechanism in 3 (see ESI for details). Two reaction
pathways with comparable barriers were identified during the 2D scan of a potential energy
surface vs. B2–Ru1–H2 and B1–Ru1–H1 angle coordinates (Fig. 5a). The first pathway with an
activation barrier of 12.9 kcal/mol proceeded through an intermediate complex 4-1 that can be
best described as a fiveꢀcoordinate Ru(0) complex with two B–HꢁꢁꢁRu coordinated borane groups,
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two phosphinite pincer arms, and the triphenylphosphine ligand. As discussed above, the
formation of 4-1 from 3 can be represented as the reductive elimination of the boryl and hydride
ligands from the divalent ruthenium center. The second pathway with an activation barrier of 13.5
kcal/mol proceeded through an intermediate complex 4-2 that can be described as a sixꢀ
coordinate (BB)ꢀcarboryne complex of Ru(IV) with two B–Ru bonds, two Ru–H hydrides, two
phosphinite pincer arms, and the triphenylphosphine ligand. Close values of energy barriers
corresponding to these two pathways suggest similar probability of these two processes leading to
the exchange transformation in 3.
Bonding situation in the complex 3, and the proposed intermediates 4-1 and 4-2 was further
analyzed using the analysis of the electron density in the framework of the quantum theory of
6
0,61
atomsꢀinꢀmolecules (QTAIM)
for the electron density computed at the PBE0/defꢀTZVP level.
Fig. 5bꢀd shows Laplacian maps and bond paths for the three structures plotted in the B1–Ru1–
B2 plane. In accordance with the description of 4-2 as a Ru(IV) diboryl dihydride complex, its
optimized structure contained two Ru–H and two Ru–B bond paths with delocalization indices
(
0
DI, the number of electron pairs shared between two atoms, QTAIM analogs of bond orders) of
.78 and 0.79 (Ru–H bonds) and 0.69 and 0.71 (Ru–B bonds). The bonding between the
ruthenium center and the boron atoms B1 and B2 with outwardꢀbent density concentration is
3
7
similar to that in the related BBꢀcarboryne complex (POBOP)Ru(CO)2. In 4-1, direct Ru–B
bond paths are absent. Instead, hydrogen atoms are connected by curved bond paths to both
B1/B2 and Ru, representing neutral borane ligand coordination to the formally Ru(0) metal
center. Delocalization indices are 0.52 and 0.54 for the Ru–H bonds and 0.53 and 0.54 for the B–
H bonds, thus demonstrating that bridging hydrogen atoms have the same bond order with both
boron atoms and the metal center. The Ru–B bond delocalization indices of 0.34 and 0.37 in 4-1
are considerably smaller than in 4-2, but not negligible despite the absence of direct bond paths.
In the complex 3, there is only one Ru–B bond path with a delocalization index value 0.71
while the second metalꢀboron interaction does not have a bond path but has the DI value 0.23.
The terminal Ru–H bond has the DI value 0.90 while bridging B–HꢁꢁꢁRu interaction possesses DI
=
0.48 for the Ru–B bond and DI = 0.52 for the B–H bond. These values of delocalization indices
are consistent with the complex 3 formulation as the Ru(II) boryl hydride with a coordinated
borane ligand, thus exhibiting bonding features of both 4-1 and 4-2 intermediates.
1
0

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Fig. 5. (a) 2D potential energy surface scan along B–Ru–H angle coordinates (α : B1–Ru–H1, α :
1
2
B2–Ru–H2). Transition states TS1/TS1’ and a shallow minimum between them correspond to 4-
, TS2 corresponds to 4-2. (bꢀd) Laplacian maps and bond path in the B1–B2–Ru1 plane in 4-1
b), 4-2 (c), and 3 (d). QTAIM delocalization indices are shown for the selected B–H, Ru–H, and
Ru–B bonds.
1
(
1
1

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Fluxional behavior related to dissociation/coordination of neutral ligand arms in pincer
complexes have been documented while reports of room temperature fluxionality in anionic
11,62–68
backbone/arms are rare.
The POBOP pincer framework features the threeꢀdimensional
carborane backbone that allows the metal center to be in proximity of two cage vertices
simultaneously. This geometric arrangement accompanied by the flexibility of boryl/borane
coordination modes predisposes the metal center to the rapid reductive elimination/oxidative
addition sequence resulting in unique rapid metal center shuttling between two boron atoms of the
cage and exchange of hydrogen atoms belonging to B–H and Ru–H bonds. Notably, this process
involves breaking and formation of strong covalent metalꢀboron and metalꢀhydrogen bonds.
Catalytic Transfer Dehydrogenation of Cyclooctane Promoted by (POBOP)Ru(H)(PPh3)
The rapid shuttling of the metal center between boron atoms on the surface of the carborane
cage in (POBOP)Ru(H)(PPh ), which, according to theoretical calculations, may proceed through
3
thermally accessible Ru(0) fiveꢀcoordinate intermediate (see above), and the extreme thermal
stability of icosahedral carborane clusters, prompted us to investigate the reactivity of this
complex in alkane dehydrogenation. Dehydrogenation of alkanes is an attractive strategy for
conversion of readily available but inert saturated hydrocarbons into alkenes, which can serve as
versatile building blocks for further transformations. Highꢀtemperature (400–600 ºC)
dehydrogenation of alkanes proceeds with the use of heterogeneous catalysts, thus, an active
69
wellꢀdefined homogeneous catalyst operating at lower temperatures would be desirable.
Transfer dehydrogenation of alkanes promoted by iridium pincer complexes receives continuing
12,70–76
attention.
Phosphiniteꢀcontaining POCOP iridium pincers have been reported as effective
catalysts with high longevity in the reaction mixture leading to turnover numbers as high as 6000.
Many iridium(I)ꢀbased systems for transfer hydrogenation operate under strictly inert conditions,
including absence of oxygen, water, and dinitrogen. Recently, examples of ruthenium(II) pincer
complexes that are competent in alkane dehydrogenation emerged, starting with the prominent
7
6,77
example of Roddick’s πꢀaccepting pincer system.
A subsequent report by Huang indicated that
optimization of reaction conditions and the use of POCOPꢀbased ruthenium(II) hydride
7
8
complexes can lead to turnover numbers as high as 370 under Ar and 294 under N2.
We found that (POBOP)Ru(H)(PPh ) efficiently promotes catalytic dehydrogenation of
3
cyclooctane (COA) to cyclooctene (COE) with the use of tertꢀbutylethylene (TBE) as a hydrogen
acceptor under dinitrogen atmosphere. The results are summarized in Table 1. A turnover number
(
TON) of 77±5 was achieved with the initial COA:TBE:catalyst molar ratio of 3000:3000:1 (0.03
mol% of catalyst loading) at 170 ºC within 2 h in a sealed glass vessel. Longer reaction times (>2
h) did not result in an increased conversion. It has been previously noted that large excess of TBE
can impede the reaction, possibly through the formation of a relatively stable complex with the
7
8
catalyst. Lowering the amount of TBE in the reaction mixture to COA:TBE:catalyst molar ratio
of 5700:1000:1 with the effective catalyst loading of 0.1 mol% relative to the amount of TBE led
to the increase of the observed TON to 400±8 under N . Transfer dehydrogenation experiments
2
were also carried out under air at 170 ºC leading to the appreciable TON of 288±8 after 2 h with
0
.1 mol% catalyst loading relative to TBE and 5700:1000:1 COA:TBE:catalyst molar ratio. The
dehydrogenation of nꢀoctane (OA) under analogous conditions (5700:1000:1 OA:TBE:catalyst,
70 °C, 1 h) resulted in the TON of 85±10. Regioselectivity of formation of 1ꢀoctene was low
1
1
2

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(
less than 5% according to GCꢀMS analysis). In the light of these results, we conclude that the
catalytic activity of the (POBOP)Ru(H)(PPh ) complex in the dehydrogenation of cyclooctane is
3
comparable, if not higher, to the most active rutheniumꢀbased pincer systems reported to date.
Table 1. Catalytic Dehydrogenation of Cyclooctane (COA) to Cyclooctene (COE) with tertꢀ
butylethylene (TBE) as a hydrogen acceptor promoted by (POBOP)Ru(H)(PPh ) (3).
3
a
COA, equiv.
TBE, equiv.
3000
3, equiv.
Atmosphere
TON
77±5
3
5
5
000
700
700
1
1
1
N2
N2
1000
400±8
288±8
1000
Air
a
1
Experiments were carried out at 170 ºC for 2 h in sealed tubes. TONs were determined from H
NMR spectra of reaction mixtures with the use of naphthalene as an internal standard. Each entry
corresponds to three independent experiments.
Transfer dehydrogenation of deuterated nꢀoctaneꢀd was studied to probe the mechanism of the
1
8
transformation. Cycling between Ir(III) and Ir(I) species has been proposed to be an operating
7
0
mechanism for iridium pincer catalysts. In the case of the (POBOP)Ru(H)(PPh ) complex, both
3
transient lowꢀvalent Ru(0) and highꢀvalent Ru(IV) species may be accessible during the exchange
process according to the theoretical calculations above. Transfer dehydrogenation of nꢀoctaneꢀd18
(
1
dꢀOA) with TBE as a hydrogen acceptor at 170 ºC for 1 h and dꢀOA:TBE:catalyst molar ratio of
4:10:1 led to the complete consumption of TBE. The P NMR spectrum of the reaction mixture
3
1
indicated the presence of the starting complex 3 as well as a new unidentified pincer complex in a
1
3
0
:1 ratio. The H NMR spectrum of the reaction mixture did not contain signals in the range from
2
ppm to –15 ppm. On the other hand, the H NMR spectrum of the mixture after evaporation of
volatiles contained a broadened signal at –8.9 ppm corresponding to the partially deuterated
complex 3-d (see above). These results suggest that the metal hydride exchanges with hydrogen
2
atoms of an alkane substrate either upon dehydrogenation through σꢀbond metathesis or during
possible isomerization of an alkene product.
Conclusions
In summary, the carboranyl pincer POBOP framework serves as both an anionic (boryl) and a
neutral (borane) ligand at the same time causing significant distortion of the metalꢀboron covalent
bond rendering it more reactive. This unique coordination environment results in the rapid metal
center “cageꢀwalking” between two adjacent boron atoms of the carborane cage at room
temperature representing the “pendulum clock”ꢀtype fluxional behavior. This facile B–H bond
activation and reꢀformation observed in the (POBOP)Ru(H)(PPh ) complex highlights an
3
importance of the bridging vicinal B–HꢁꢁꢁM interactions and provides an insight to the possible
1
3

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mechanism of the isomerization during metalꢀpromoted coupling reactions of boron clusters
involving initial activation of boronꢀhydrogen or boronꢀhalogen bonds. Furthermore, the B–HꢁꢁꢁM
interaction served as a hemilabile ligand protecting a latent coordination site in the
(
POBOP)Ru(H)(PPh ) complex, which is a competent catalyst of the transfer dehydrogenation of
3
cyclooctane.
Acknowledgments
Acknowledgment is made to the donors of the American Chemical Society Petroleum Research
Fund (Award 54504ꢀDNI3 to D.V.P.) and to the National Science Foundation (Award CHEꢀ
1
654301 to D.V.P.). We are grateful to Dr. Perry. J. Pelechia (Director of NMR Services,
University of South Carolina) for immense help with variable temperature NMR spectroscopy
studies. A.A.P. acknowledges funding by the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation programme (grant agreement No
6
48295 “GraM3”). Computational resources were provided by the Center for Information
Services and High Performance Computing (ZIH) in TU Dresden. The authors thank Ulrike
Nitzsche for technical assistance with computational resources in IFW Dresden and Ekaterina
Dolgopolova (University of South Carolina) for help with the production of the TOC graphic.
AUTHOR INFORMATION
Corresponding Authors
Dmitry V. Peryshkov Eꢀmail: peryshkov@sc.edu
Alexey A. Popov Eꢀmail: A.Popov@ifwꢀdresden.de
1
4

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