A half-sandwich 1,2-azaborolyl ruthenium complex: Synthesis, characterization, and evaluation of its catalytic activities

Article abstract of DOI:10.1039/c3dt51177d

A half-sandwich 1,2-azaborolyl (Ab) ruthenium complex, (Ab-CCPh) RuCl(PPh₃)₂ (1), has been synthesized by treating RuCl₂(PPh₃)₃ with lithium 1,2-azaborolide L-1, or by treating either RuCl₂(PPh₃)₃ or RuHCl(PPh₃)₃ directly with 1,2-azaborole LH-1. It is evaluated as a suitable precatalyst in [2 + 2] cycloadditions of norbornene derivatives with DMAD and in atom transfer radical additions of halogenated compounds with olefins. The Royal Society of Chemistry.

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A half-sandwich 1,2-azaborolyl ruthenium complex:
synthesis, characterization, and evaluation of its
catalytic activities†
Cite this: DOI: 10.1039/c3dt51177d
Received 5th May 2013,
Accepted 24th June 2013
DOI: 10.1039/c3dt51177d
Zicheng Liu, Junjie Xu, Wenqing Ruan, Chen Fu, Hui-Jun Zhang and Ting-Bin Wen*
www.rsc.org/dalton
A
half-sandwich 1,2-azaborolyl (Ab) ruthenium complex,
(Ab–CuCPh)RuCl(PPh3)2 (1), has been synthesized by treating
RuCl2(PPh3)3 with lithium 1,2-azaborolide L-1, or by treating
either RuCl2(PPh3)3 or RuHCl(PPh3)3 directly with 1,2-azaborole
LH-1. It is evaluated as a suitable precatalyst in [2 + 2] cycloaddi-
tions of norbornene derivatives with DMAD and in atom transfer
radical additions of halogenated compounds with olefins.
Fig. 1 Cyclopentadienyl and 1,2-azaborolyl ligand.
Cyclopentadienyl (Cp) ligands are among the most important
and widely used ligands in organometallic chemistry.1 Since
simple substituent modifications on the Cp ring may lead to
significant changes of the reactivity and selectivity of Cp-based
half-sandwich and metallocene catalysts, considerable effort
has been made towards the synthesis of various substituted Cp
ligands in the pursuit of new efficient catalysts.2 Besides, the
development of heterocyclic Cp analogues, especially aromatic
borataheterocycles, which are expected to display a comparable
coordination ability and different electronic properties relative
to Cp ligands, has recently attracted much attention.3
1,2-Azaborolyl (Ab) ligand is one of the most investigated
isoelectronic and isolobal surrogates of the Cp ligand,3a–e,4–7
which is derived from the Cp ring by formal replacement of
two adjacent carbon atoms with boron and nitrogen (Fig. 1).
In the 1980s, Schmid reported the first synthesis of Ab
ligands4 and pioneered 1,2-azaborolyl/transition-metal chemis-
try.3a,b An alternative synthetic route for Ab ligands was then
developed by Ashe and coworkers taking advantage of ring
closing metathesis.5 Impressive progress in this area was made
in the past decade by Fu, who has developed a general syn-
thetic method for the Ab ligands with diverse substituents on
boron and has further investigated the properties of a series of
transition-metal complexes,6 showing that the Ab ligands were
more electron-rich than Cp ligands and that the exocyclic
substituent at boron had a great electronic effect on the metal
center. More recently, Fang’s group provided an easy access to
tri- and tetrasubstituted Ab rings for the preparation of a wide
array of B-, N- and C-substituted Ab ligands and related rare-
earth-metal complexes with good catalytic activities for olefin
polymerization.7 Fang’s work also suggested that Ab ligands
could offer a particular steric environment around the metal
center due to their different ring geometry compared with Cp
counterparts.7a
On the other hand, cyclopentadienyl half-sandwich
ruthenium complexes are versatile catalyst precursors in a
large number of C–C bond forming reactions.8 However, while
plenty of 1,2-azaborolyl transition-metal complexes have been
reported, only two Ab–Ru complexes are known, both of
which are sandwich complexes.5,9 To the best of our knowl-
edge, half-sandwich ruthenium complexes based on Ab ligands
have never been reported so far. In this context, it is of interest
to synthesize half-sandwich AbRu complexes and investigate
their catalytic activities in organic reactions. Herein, we
describe the preparation and characterization of the first half-
sandwich AbRu complex (Ab–CuCPh)RuCl(PPh3)2 (1) (Ab–Cu
CPh = l-tert-butyl-2-phenylethynyl-1,2-azaborolyl), which acts
as an efficient catalyst precursor for [2 + 2] cycloaddition of
norbornene derivatives with DMAD and also for the radical
addition of olefins with polyhalogenated compounds.
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen, 361005, People’s Republic of China. E-mail: chwtb@xmu.edu.cn;
Fax: (+86) 592-218-6295; Tel: (+86) 592-218-6085
†Electronic supplementary information (ESI) available: Experimental details,
Synthesis and identification of half-
sandwich Ab ruthenium complexes
According to the procedure described by Fu and coworkers,6b
we readily prepared lithium 1,2-azaborolide L-1. The reaction
characterization data, X-ray data, and NMR spectral charts. CCDC 908726 (1),
908727 (2), 908728 (5c) and 908729 (5i). For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt51177d
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1,2-azaborolyl ring. Notably, the tert-butyl group on L-1 is
oriented away from the PPh3 ligands, probably to minimize
the steric interaction.
Consistent with its X-ray structure, the 31P NMR spectrum
of 1 showed two doublet signals at 36.4 ppm and 49.7 ppm
respectively, demonstrating two diastereotopic Ru-bound PPh3.
In the 1H NMR spectrum of 1, the resonances for the 3-CH and
4-CH of the Ab-ring are both shifted upfield to 1.56 and
4.88 ppm respectively, relative to the free ligand L-1 (4.62 and
5.75 ppm, respectively). However, the resonance of 5-CH is
shifted downfield to 6.31 ppm in comparison with the free Ab
ligand (6.16 ppm). In the 13C NMR, the signal belonging to the
intra-ring C adjacent to the B atom is shifted much more
upfield (61.0 ppm) relative to the two other intra-ring C atoms
(89.0 ppm (4-C) and 90.7 ppm (5-C)), suggesting a major nega-
tive charge resting on the 3-C.
Scheme 1 Synthesis of half-sandwich complexes 1 and 2.
The reaction of lithium 1,2-azaborolide L-1 with RuHCl-
(PPh3)3 was also investigated, which gave rise to a half-sand-
wich ruthenium hydride complex (Ab–CuCPh)RuH(PPh3)2 (2)
as a yellow powder in 80% yield (Scheme 1). Furthermore,
complex 2 could be completely converted into the corres-
ponding chloride derivative 1 smoothly at room temperature
in CHCl3 within 30 min. An analogous rapid hydride/chloride
metathesis between CpRuH(PPh3)2 and CCl4 has been
reported.12
The structure of complex 2 has also been confirmed by a
single crystal X-ray diffraction study (Fig. 3), which is similar to
that of complex 1 with the Cl ligand in the latter replaced by a
hydride. The hydride was located and refined, with the orien-
tation away from the tert-butyl group of the Ab-ring.
Fig. 2 ORTEP drawing of
1 showing 30% probability thermal ellipsoids.
Selected bond lengths [Å] and angles [°]: Ru1–N1 2.280(2), Ru1–B1 2.357(3),
Ru1–C11 2.244(2), Ru1–C12 2.202(2), Ru1–C13 2.157(3), C18–C19 1.201(3),
Ru1–Cl1 2.4514(6), Ru1–P1 2.2834(7), Ru1–P2 2.3542(7); P1–Ru1–P2 100.84(2),
P1–Ru1–Cl1 92.25(2), P2–Ru1–Cl1 91.35(2). CCDC 908726.
Complex 2 is fluxional in solution. At room temperature
(RT), the 31P NMR spectrum of 2 exhibited a broad signal
of RuCl2(PPh3)3 with 1.1 equiv. of L-1 in toluene at room temp- at 59.5 ppm and an even broader signal at 61.8 ppm with a
erature furnished the desired half-sandwich Ab-ligated ruthe- similar integrated intensity. When the temperature was raised,
nium complex (Ab–CuCPh)RuCl(PPh3)2 (1) as a yellow powder the two broad signals sharpened gradually, which, at 348 K,
in 78% yield (Scheme 1). Complex 1 is soluble in several gave rise to an unresolved doublet at 62.3 ppm ( J(PP) =
common organic solvents, such as toluene, THF, and CH2Cl2, 15.1 Hz) and a broad singlet at 65.6 ppm (see ESI†). On the
and is substantially air-sensitive in solution.
other hand, when the temperature was lowered to 228 K, the
The structure of complex 1 was established by an X-ray
diffraction analysis. The asymmetric unit of the crystal struc-
ture contains a pair of planar-chiral enantiomers with similar
bond distances and angles, indicating that the isolated
complex 1 is a racemic mixture. The crystal structure of one
enantiomer is depicted in Fig. 2. Complex 1 displays the
typical three-legged piano-stool coordination geometry around
the Ru center. The Ab ligand coordinates to the Ru atom in
such an unsymmetrical way that the Ru atom is pretty far from
the B atom (Ru–B: 2.357 Å) and is therefore more tightly con-
nected with intra-ring C atoms (Ru–C: 2.157–2.244 Å) and
N atom (Ru–N: 2.280 Å). The Ru–Cl bond distance (2.451 Å) is
10
similar to that reported for CpRuCl(PPh3)2
(2.453 Å) and
11
slightly shorter than that of Cp*RuCl(PPh3)2 (2.483 Å). The
two Ru–P bonds (2.283 and 2.354 Å) are slightly shorter and
longer, respectively, relative to CpRuCl(PPh3)2 (2.335 and
2.337 Å, respectively) and Cp*RuCl(PPh3)2 (2.338 and 2.346 Å,
Fig. 3 ORTEP drawing of
2 showing 30% probability thermal ellipsoids.
Selected bond lengths [Å] and angles [°]: Ru1–N1 2.384(2), Ru1–B1 2.402(3),
Ru1–C1 2.256(3), Ru1–C2 2.217(2), Ru1–C3 2.194(3), C8–C9 1.204(4), Ru1–H1
1.463(18), Ru1–P1 2.2897(8), Ru1–P2 2.2893(7); P1–Ru1–P2 99.32(3), P1–Ru1–
respectively), presumably due to steric hindrance of the H1 79.1(8), P2–Ru1–H1 79.5(8). CCDC 908727.
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cycloaddition of strained alkenes with various disubstituted
alkynes. Recently, it was found that electron-rich CpRuCl-
(PPh3)2 activated with methyl iodide can also catalyze the
[2 + 2] cycloaddition of norbornene (or norbornadiene) with
disubstituted alkynes.18
To test the catalytic activities of 1 in [2 + 2] cycloaddition
reactions, we selected norbornene derivatives 3 and DMAD 4
as the substrates. Initially, several solvents including dioxane,
THF, DCE, toluene, and n-hexane were screened (see ESI,
Table S1†). In the presence of 5 mol% 1, the reaction of
norbornene 3a with 4 in dioxane at 90 °C for 12 h proved to be
most effective, affording the desired product 5a in 86% yield,
which can compete with the result obtained with CpRuCl-
(PPh3)2/MeI.18
The scope of norbornene derivatives was then investigated
(Table 1). Gratifyingly, all the norbornene derivatives 3 were
converted stereoselectively into exo cycloadducts 519 in good
yields. Among the norbornene derivatives that were examined,
cyclopentadiene dimer 3b was employed in the reaction to
provide the product 5b regioselectively in 94% yield which
was derived from the selective reaction of the strained
double bond. Additionally, the reaction of 5,6-disubstituted
norbornene derivatives 3c–e and 3g–i gave the expected pro-
ducts 5c–e and 5g–i in good yields. The reaction time for 3e is
much shorter (8 h) relative to 3d (12 h), possibly due to the
chelation of a bridging O atom with the Ru center. 5-Sub-
stituted norbornene derivative 3f reacted with 4 for 5 h
affording the corresponding product 5f in 95% yield. We have
also investigated the scope of alkyne substrates. Unfortunately,
no reactions of 3 with less electron-deficient alkynes such as
but-2-yne-1,4-diyl diacetate and tolane, and terminal alkynes
such as phenylacetylene were observed under the same
reaction conditions.
Scheme 2 Two alternative routes to complex 1.
two original broad signals split into two sets of 31P signals
with a ratio of about 10 : 1 for the integrated intensity, which
comprise the doublets at 60.6 and 63.1 ppm ( J(PP) = 15.1 Hz)
(major) and the doublets at 65.8 and 76.4 ppm ( J(PP) =
22.0 Hz) (minor). The 1H NMR spectrum showed similar
features, which displayed one set of broadened signals for the
Ab ring protons, the tert-butyl group, and the hydride proton
at RT and broadened upon lowering the temperature and then
split into two sets of signals in an integration ratio of about
10 : 1. In particular, the hydride proton showed a broad reson-
ance at −16.90 ppm at RT and split into a doublet of doublets
at −17.05 ppm ( J(PH) = 36.5, 28.0 Hz) (major) and a triplet at
−11.18 ppm ( J(PH) = 32.8 Hz) (minor) when the temperature
was lowered to 228 K. Notably, when the temperature was
1
raised to 348 K, the H signals for the 3-CH and 4-CH of the
Ab-ring appeared as unresolved doublets at 3.88 and 5.16 ppm
( J(HH) = 4.0 Hz), respectively. The reason for the observation
of two sets of NMR data of 2 at low temperature is not clear,
but could presumably be attributed to the two isomers derived
from the different orientations of the hydride and PPh3
ligands relative to the Ab-ring substituents, which furnish an
equilibrium in solution. It can be assumed that the major
isomer of complex 2 is the one for which single crystals have
been obtained from the solution (vide supra).
In order to avoid the use of organolithium reagent, we
also explored the direct reaction of 1,2-azaborole LH-1 with
RuX2(PPh3)3 (X = Cl, H) (Scheme 2). Treatment of RuCl2(PPh3)3
with 1.3 equiv. of LH-1 in toluene at 130 °C for 12 h proved to
be successful, as monitored using 31P NMR spectroscopy.13
Moreover, in the presence of 1.8 equiv. of NEt3, the reaction
Table 1 [2 + 2] Cycloaddition of norbornene derivatives 3 with DMAD 4 in
the presence of precatalyst 1a
time was substantially reduced to
8 h. In contrast to
RuCl2(PPh3)3, the direct reaction of the hydride complex
RuHCl(PPh)3 with 1.3 equiv. of LH-1 works well at 90 °C in
toluene for 6 h. These two alternative routes via direct C–H
activation of LH-1 seem more attractive than the previous
synthesis, and help to avoid the preparation of extremely
moisture- and air-sensitive lithium azaborolide.14
[2 + 2] Cycloadditions catalyzed by
complex 1
[2 + 2] Cycloaddition of olefins with alkynes is a valuable
pathway for the synthesis of cyclobutene derivatives.15
Half-sandwich ruthenium complexes such as Cp*RuCl(COD)16
and (η5-C9H7)RuCl(COD)17 are effective catalysts for the
a All reactions were performed with 5 mol% complex 1 (0.02 mmol);
norbornene derivatives 3 (1.20 mmol), DMAD 4 (0.40 mmol) in
dioxane (3 mL) at 90 °C. bIsolated yields based on DMAD 4.
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longer reaction times (see ESI†). Using 2 mol% precatalyst 1,
the reactions of less active organochlorides such as CHCl3 7b,
ATRA reactions catalyzed by complex 1
Transition-metal catalyzed atom transfer radical addition CH3CCl3 7c, and CCl3COOEt 7d with various alkenes were also
(ATRA) of organohalides to olefins, named the Kharasch reac- realized, although in several cases both higher reaction temp-
tion, is an effective method for carbon–carbon and carbon– eratures and relatively longer reaction times are necessary
halogen bond formation.20,21 The half-sandwich ruthenium (see ESI†). Notably, in these examples, the catalytic reactivities
complex Cp*RuCl(PPh3)2 proved to be one of the most active of complex 1 are slightly higher relative to Cp*RuCl(PPh3)2,
and efficient precatalysts for ATRA reactions.21b
probably resulting from the specific steric and electronic pro-
As a preliminary study, we also checked the catalytic pro- perties of the ancillary Ab ligand. Surprisingly, the reactions
perties of 1 in Kharasch processes in the absence of any addi- of α-methylstyrene with CCl4 and CHCl3 directly led to the
tives, and the detailed results are summarized in Table 2. The formation of α-substituted styrene derivatives 9ha and 9hb
reactions were carried out at rt–85 °C in toluene in the pres- derived from the dehydrochlorination of the corresponding
ence of 0.33–2 mol% precatalyst and various alkenes 6 and addition products.
halogenated compounds 7 were involved (for more details see
ESI†).
In summary, we have developed several methods for the
synthesis of the first examples of half-sandwich l,2-azaborolyl
In the presence of 0.33 mol% precatalyst 1, the reaction of ruthenium complexes (Ab–CuCPh)RuX(PPh3)2 (X = Cl, 1;
styrene 6a with CCl4 7a proceeded smoothly even at room X = H, 2). Whereas the reaction of RuXCl(PPh3)3 (X = Cl, H)
temperature to afford the adduct 8aa in 93% yield after 3 h. with anionic 1,2-azaborolyl L-1 is an efficient protocol for the
A high initial turnover frequency (TOF) value (470 h−1) was synthesis of complexes 1 and 2, direct C–H activation of
observed in the initial 24 min which is slightly superior to the the neutral 1,2-azaborole LH-1 by RuXCl(PPh3)3 (X = Cl, H) pro-
TOF value of Cp*RuCl(PPh3)2 (around 400 h−1).21b By reducing vides a more attractive approach for the preparation of 1. The
the catalyst loading of 1 from 0.33 mol% to 0.04 mol%, we involvement of complex 1 as a catalyst precursor without
observed a total turnover number (TON) of 1875 which is com- any additives for [2 + 2] cycloadditions and ATRA reactions
parable with Cp*RuCl(PPh3)2 (1600–1700).21b The reaction of competes favorably with respect to CpRuCl(PPh3)2 and
CCl4 7a with p-chlorostyrene 6b, methyl acrylate 6c, and Cp*RuCl(PPh3)2. Further studies toward the preparation and
methyl methacrylate 6g gave the corresponding products 8ba, application of various half-sandwich l,2-azaborolyl ruthenium
8ca, and 8ga in 95%, 80%, and 72% yields respectively. The complexes are underway.
ATRA of CCl4 7a to less reactive alkene substrates 6d–f required
This work was financially supported by the Natural Science
higher catalyst loadings with higher reaction temperatures and Foundation of China (no. 20772100 and 21072161), the
National Basic Research Program of China (973 Program, no.
2012CB821600) and the Program for Changjiang Scholars and
Innovative Research Team in University (PCSIRT).
Table 2 ATRA of polychlorinated compounds to various olefins catalyzed by 1a
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