Migratory insertion and reductive coupling of tetraarylruthenium(IV) complexes

Article abstract of DOI:10.1021/om500707e

Tetraarylruthenium(IV) complexes have been synthesized and their migratory insertion, reductive coupling, and bromination reactions investigated. Treatment of [Ru(acac)₃] (acac^- = acetylacetonate) with RMgBr, followed by column chromatography in air afforded the tetraaryl complexes [RuR₄] (R = 2,5-dimethylphenyl (R¹); 4-methoxy-2-methylphenyl (R²)). Oxidation of [RuR²₄] with AgBF₄ gave the Ru(V) complex [Ru^VR²₄](BF₄), which has a measured μ_eff of 1.8 μ_B. [RuR₄] and can catalyze the oxidation of methyl p-tolyl sulfide with PhIO. Reaction of [RuR²₄] with PhICl₂ led to C-C reductive coupling and the formation of the Ru(II) η⁶-biaryl complex [(η⁶-R²-R²)RuCl₂]₂ (1). Treatment of [RuR¹₄] with excess NO gave the tetranuclear Ru(II) aryl-N-nitrosohydroxylaminato complex [RuR¹(N){κ₂-O,O′-ON(R¹)NO}(μ-NO₂)]₄ (2). Bromination of [RuR¹₄] with N-bromosuccinimide resulted in formation of [RuR′₄] (R′ = 4-bromo-2,5-dimethylphenyl) (3). The crystal structures of [RuR²₄], [RuR²₄](BF₄), and 1-3 have been determined.

Full text of DOI:10.1021/om500707e

Article
pubs.acs.org/Organometallics
Migratory Insertion and Reductive Coupling of
Tetraarylruthenium(IV) Complexes
Shiu-Chun So,† Wai-Man Cheung,† Guo-Cang Wang,† Enrique Kwan Huang,† Man-Kit Lau,†
Qian-Feng Zhang,‡ Herman H.-Y. Sung,† Ian D. Williams,† and Wa-Hung Leung*,†
†Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R.
China
‡Institute of Molecular Engineering and Applied Chemistry, Anhui University of Technology, Ma’anshan, Anhui 243002, P. R. China
S
* Supporting Information
ABSTRACT: Tetraarylruthenium(IV) complexes have been synthesized
and their migratory insertion, reductive coupling, and bromination
reactions investigated. Treatment of [Ru(acac)3] (acac− = acetylaceto-
nate) with RMgBr, followed by column chromatography in air afforded
the tetraaryl complexes [RuR4] (R = 2,5-dimethylphenyl (R1); 4-
methoxy-2-methylphenyl (R2)). Oxidation of [RuR2 ] with AgBF4 gave
4
the Ru(V) complex [RuVR2 ](BF4), which has a measured μeff of 1.8 μB.
4
[RuR4] and can catalyze the oxidation of methyl p-tolyl sulfide with PhIO.
Reaction of [RuR2 ] with PhICl2 led to C−C reductive coupling and the
4
formation of the Ru(II) η6-biaryl complex [(η6-R2-R2)RuCl2]2 (1).
Treatment of [RuR1 ] with excess NO gave the tetranuclear Ru(II) aryl-
4
N-nitrosohydroxylaminato complex [RuR1(N){κ2-O,O′-ON(R1)NO}(μ-
NO2)]4 (2). Bromination of [RuR1 ] with N-bromosuccinimide resulted
4
in formation of [RuR′4] (R′ = 4-bromo-2,5-dimethylphenyl) (3). The
crystal structures of [RuR2 ], [RuR2 ](BF4), and 1−3 have been determined.
4
4
and formation of [{η6-2(2-MeC6H4)MeC6H4}OsII(2-
MeC6H4)2L], whereas [OsII(2-MeC6H4)2(CNR)4] and cyclo-
metalated [OsII{C,N-3-Me(2-C(2-MeC6H4)NCMe3)C6H3}(2-
MeC6H4)(CNR)3] were obtained from the reactions with RNC
(R = tert-butyl-2,6-dimethylphenyl).3 Oxidation of
tetraarylosmium(IV) with AgBF4 and PhIO afforded Os(V)11
and oxo-Os(VI)12 complexes, respectively. Also, we found that
INTRODUCTION
■
One pathway for decomposition of transition-metal σ-aryl
complexes is reductive coupling of the aryl ligands to give
biaryls (Scheme 1).1−3 It has been shown that the reductive
Scheme 1. Reductive Coupling of Metal σ-Aryl Complexes
[OsR1 ] (R1 = 2,5-dimethylphenyl) can be brominated with
4
pyridinium tribromide to [OsR′4] (R′ = 4-bromo-2,5-
dimethylphenyl), which underwent Suzuki coupling with
arylboronic acids to give homoleptic Os(IV) σ-biaryl
complexes.13
Compared with the osmium analogues, the reaction
chemistry of tetraarylruthenium(IV) has not been well
explored. This may be due, in part, to lack of general methods
to synthesize [RuR4] complexes in high yield. In an effort to
study the electronic factors affecting the stability/reactivity of
[RuR4] complexes, we sought to synthesize homoleptic 2,5-
dimethylphenyl (R1) and 4-methoxy-2-methylphenyl (R2)
complexes of Ru and investigate their organometallic chemistry.
Herein, we report the synthesis of [RuR4] (R = R1, R2) by
alkylation of [Ru(acac)3] with Grignard reagents. We found
coupling of some σ-aryl complexes can be inhibited by ortho
substituents of the aryl ligands. Thus, transition-metal
complexes with ortho-substituted aryl ligands such as 2,4,6-
trimethylphenyl (mes) and 2-methylphenyl (2-MeC6H4) are
generally more stable than the phenyl analogues.4
Previously, Wilkinson and co-workers have synthesized a
series of homoleptic 2-methylphenyl complexes [M(2-
MeC6H4)4] (M = Re, Ru, Os, Mo, Cr)5−8 and studied their
reactivity. [M(2-MeC6H4)4] (M = Re, Cr, Mo, W) reacted with
phosphines to give η2-aryne complexes via ortho hydrogen
abstraction.7−9 Analogous Nb and W η2-aryne complexes have
been synthesized similarly from the corresponding tetraaryl
precursors.10 By contrast, the treatment of [Os(2-MeC6H4)4]
with L (L = CO, PMe3) led to reductive coupling of the aryls
that [RuR2 ] undergoes reductive coupling with PhICl2 to give
4
a dinuclear Ru(II) η6-biaryl complex. Furthermore, reaction of
Received: July 10, 2014
© XXXX American Chemical Society
A
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a
Scheme 2. Synthesis and Reactivity of [RuR4] (R = R1, R2)
a
Reagents and conditions: (i) 7 equiv of RMgBr, THF, silica chromatography in air; (ii) AgBF4, CH2Cl2; (iii) PhICl2, CH2Cl2; (iv) NO(g), CH2Cl2;
(v) CH2Cl2, NBS, 1 M H2SO4.
[RuR1 ] with NO resulted in insertion of NO into the Ru−C
Ph), 133.96 (s, Ph), 140.06 (s, Ph), 149.47 (s, Ph), 158.96 (s, Cipso
4
Ph). Anal. Calcd for C32H36O4Ru: C, 65.62; H, 6.20. Found: C, 65.51;
bond and the formation of a tetranuclear aryl-N-nitro-
sohydroxylaminato complex. The bromination of [RuR1 ]
H, 6.24.
4
[RuR2 ](BF4). To a solution of [RuR2 ] (30 mg, 0.051 mmol) in
4
4
with N-bromosuccinimide (NBS) has also been studied.
CH2Cl2 (10 mL) was added 1 equiv of AgBF4 (10 mg, 0.051 mmol).
The green solution was stirred at room temperature for 1 h, and the
volatiles were removed in vacuo. The residue was washed with hexanes
and then extracted with CH2Cl2. Recrystallization from CH2Cl2/
hexanes afforded dark green crystals which were suitable for X-ray
analysis. Yield: 22 mg, 67%. μeff (Evans method,12 chloroform) = 1.8
μB. Anal. Calcd for C32H36BF4O4Ru: C, 57.15; H, 5.40. Found: C,
56.63; H, 5.17.
EXPERIMENTAL SECTION
General Considerations. All manipulations were carried out
under nitrogen by standard Schlenk techniques. Solvents were dried,
distilled, and degassed before use. H NMR spectra were recorded at
■
1
room temperature on a Bruker AV 400 MHz NMR spectrometer
operating at 400 (for 1H) and 100 (for 13C) MHz. Chemical shifts (δ,
ppm) were reported with reference to SiMe4. Cyclic voltammetry was
performed using a CH Instrument model 600D potentiostat. The
working and reference electrodes were glassy carbon and Ag/AgNO3
(0.1 M in MeCN), respectively. Gas chromatography was performed
with a Bruker 430-GC instrument equipped with a FID detector.
Elemental analyses were performed by Medac Ltd., Surrey, U.K. The
compound [Ru(acac)3] (acac− = acetylacetonate) was prepared
according to a literature method.14
[Ru(η6-R2-R2)Cl2]2 (1). To a solution of [RuR2 ] (30 mg, 0.05 mmol)
4
in CH2Cl2 (10 mL) was added 4 equiv of PhICl2. The yellowish
orange solution was stirred at room temperature for 1 h, and the
volatiles were removed in vacuo. The residue was washed with hexanes
and then extracted with CH2Cl2. Recrystallization from CH2Cl2/Et2O/
hexanes afforded dark yellow crystals which were suitable for X-ray
analysis. Yield: 10 mg, 48% (with respect to Ru). 1H NMR (400 MHz,
CDCl3): δ 1.93 (s, 6H, CH3), 1.96 (s, 6H, CH3), 3.80 (s, 6H, OCH3),
4.10 (s, 6H, OCH3), 5.12−5.22 (m, 2H, Ph), 5.40−5.52 (m, 2H, Ph),
5.59−5.61 (m, 2H, Ph), 6.70 (s, 2H, Ph), 6.78−6.81 (m, 2H, Ph),
7.95−7.98 (m, 2H, Ph) Anal. Calcd for C32H36Cl4O4Ru2·0.5CH2Cl2:
C, 45.16; H, 3.92. Found: C, 44.81; H, 4.28.
Syntheses. [RuR1 ] (R1 = 2,5-Dimethylphenyl). To a solution of
4
[Ru(acac)3] (400 mg, 1.01 mmol) in tetrahydrofuran (THF) (10 mL)
was added 7 equiv of R1MgBr (7 mL of 1 M solution in THF, 7
mmol). The resulting brown mixture was stirred at room temperature
overnight, and the volatiles were removed in vacuo. The residue was
extracted into diethyl ether and subjected to column chromatography
in air (silica gel, CH2Cl2/hexanes, 1:1, v/v). Recrystallization from
[RuR1(NO){κ2-O,O′-ON(R1)NO}(μ-NO2)]4 (2). To a solution of
[RuR1 ] (30 mg, 0.058 mmol) in CH2Cl2 (10 mL) was bubbled
4
NO(g) for 5 min at 0 °C, during which the dark solution turned
reddish orange. The resulting mixture was stirred at room temperature
overnight, and the volatiles were removed in vacuo. Recrystallization
from CH2Cl2/Et2O/hexanes afforded orange-red crystals which were
1
hexanes afforded brown crystals. Yield: 15 mg, 28%. H NMR (400
MHz, CDCl3): δ 2.09 (s, 3H, CH3), 2.24 (s, 3H, CH3), 6.75−6.82 (m,
2H, Ph), 6.99 (s, 1H, Ph). 13C {1H} NMR (100, MHz, CDCl3): δ
20.55 (s, CH3), 24.34 (s, CH3), 127.18 (s, Ph), 127.58 (s, Ph), 132.92
(s, Ph), 133.41 (s, Ph), 134.90 (s, Ph), 160.62 (s, Cipso Ph). Anal. Calcd
for C32H36Ru: C, 73.72; H, 7.03. Found: C, 73.36; H, 7.12.
1
suitable for X-ray analysis. Yield: 8 mg, 35% (based on Ru). H NMR
(400 MHz, C6D6): δ 1.84 (s, 12H, CH3), 2.08 (s, 12H, CH3), 2.43 (s,
12H, CH3), 2.64 (s, 12H, CH3), 6.56−6.90 (m, 16H, Ph), 7.68 (m,
8H, Ph). IR (KBr, cm−1): 1879 [v(N−O)], 1509 [νs(NO2)], 1261
[νas(NO2)]. Anal. Calcd for C64H72N16O20Ru4: C, 42.95; H, 4.06; N,
12.52. Found: C, 43.08; H, 4.32; N, 12.32.
[RuR2 ] (R2 = 4-Methoxy-2-methylphenyl). This compound was
4
synthesized similarly as for [RuR1 ] using R2 MgBr (1 M in THF) as
4
the alkylating agent. The product was eluted with Et2O/CH2Cl2 (1:1,
[RuR′4] (R′ = 4-Bromo-2,5-dimethylphenyl) (3). To a solution of
v/v). Recrystallization from hexanes afforded purple crystals which
were suitable for X-ray diffraction analysis. Yield: 17 mg, 29%. H
[RuR1 ] (80 mg, 0.15 mmol) in CH2Cl2 (10 mL) was added 6 equiv of
1
4
NMR (400 MHz, CDCl3): δ 2.09 (s, 3H, CH3), 3.78 (s, 3H, OCH3),
6.56−6.62 (m, 2H, Ph), 7.07 (m 1H, Ph). 13C {1H} NMR (100 MHz,
CDCl3): δ 24.96 (s, CH3), 54.47 (s, OCH3), 109.27 (s, Ph), 114.51 (s,
N-bromosuccinimide (160 mg, 0.90 mmol) in H2O/1 M H2SO4 (1:1,
v/v) (10 mL). The resulting mixture was then stirred at room
temperature for 1 h and water (10 mL) was added. The purple organic
B
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layer was separated, washed with brine solution, and dried with
MgSO4. The filtrate was concentrated and subjected to column
chromatography (silica gel, hexanes). Recrystallization from hexanes
afforded purple crystals which were suitable for X-ray analysis. Yield:
52 mg, 42%. 1H NMR (400 MHz, CDCl3): δ 2.03 (s, 3H, CH3), 2.30
(s, 3H, CH3), 6.94 (s, 1H, Ph), 7.10 (s, 1H, Ph). 13C {1H} NMR (100
MHz, CDCl3): δ 21.97 (s, CH3), 23.96 (s, CH3), 124.20 (s, Ph),
131.45 (s, Ph), 133.60 (s, Ph), 133.76 (s, Ph), 137.07 (s, Ph), 158.42
(s, Cipso Ph). Anal. Calcd for C32H32Br4Ru: C, 45.90; H, 3.85. Found:
C, 45.41; H, 3.91.
Ru-Catalyzed Decomposition of PhIO. To a solution of [RuR4]
(R = R1, R2; 2 mg) in CH2Cl2 (5 mL) was added 100 equiv of PhIO
(85 mg, 0.384 mmol). The resulting mixture was stirred at room
temperature for 1 h. The yield of PhI formed was determined by GLC.
Ru-Catalyzed Sulfide Oxidation. A mixture of methyl p-tolyl
sulfide (1.49 mmol), PhIO (0.03 mmol), and Ru catalyst (0.0015
mmol) in 1 mL of CH2Cl2 was stirred under nitrogen at room
temperature for 2 h. The yield of the organic product was determined
by GLC and based on PhI formed.
X-ray Crystallography. Intensity data were collected on a Bruker
APEX 1000 CCD diffractometer using graphite-monochromated Mo
Kα radiation (λ = 0.71073 Å). The collected frames were processed
with the software SAINT. Structures were solved by the direct
methods and refined by full-matrix least-squares on F2 using the
SHELXTL software package.13,14 Atomic positions of non-hydrogen
atoms were refined with anisotropic parameters and with suitable
restraints. However, disordered atoms were refined isotropically.
Hydrogen atoms were generated geometrically and allowed to ride on
their respective parent carbon atoms before the final cycle of least-
squares refinement.
Figure 1. Molecular structure of [RuR2 ]. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level. Selected bond length (Å) and angles (deg): Ru−C 1.986(3); C−
Ru−C′ 105.99(15), 111.24(8).
4
Treatment of [RuR2 ] with 1 equiv of AgBF4 afforded
RESULTS AND DISCUSSION
4
■
[RuVR2 ](BF4), which was isolated as green crystals. The
4
Synthesis of [RuR4]. The synthesis and reactivity of
tetraarylruthenium(IV) complexes are summarized in Scheme
2. Previously, homoleptic alkyls and aryls of Ru(IV) have been
prepared by alkylation of (Et4N)[RuCl5(THF)], [RuO4]−,
[Ru2(CH3CO2)4Cl], or [RuCl3(THF)3] (THT = tetrahydro-
thiophene) with Grignard reagents.5,18,19 However, attempts to
synthesize [RuR4] (R = 2,5-dimethylphenyl (R1), 4-methoxy-2-
methylphenyl (R2)) by alkylation of these Ru starting materials
led to isolation of the desired products in very low yield (<5%).
On the other hand, we found that [RuR4] could be obtained in
ca. 28% yield (unoptimized) from the reaction of [Ru(acac)3]
(acac− = acetylacetonate) with RMgBr, followed by column
chromatography in air. It seems likely that the alkylation of
[Ru(acac)3] initially gave an Ru(III) intermediate, possibly
[RuR4]−, which was subsequently air oxidized to [RuR4] during
the workup. Attempts to isolate the Ru(III) intermediates by
carrying out the workup under nitrogen failed.
measured magnetic moment of ca. 1.8 μB is consistent with the
S = 1/2 spin state. A similar Ru(V) complex, [RuV(mes)4]+
(E1/2 = 0.32 V), has been isolated previously, but its structure
has not been determined.19 While [RuR2 ](BF4) is air-stable in
4
solution, it is reduced to [RuR2 ] readily when reacted with
4
−
PPh3 and nucleophiles such as N3 . Figure 2 shows the
molecular structure of [RuR2 ](BF4). Similar to isoelectronic
4
[Os(2-MeC6H4)4]+, and [Re(2-MeC6H4)4],5b the complex
6b
Like the 2-methylphenyl analogue, [RuR1 ] and [RuR2 ] are
4
4
air-stable and diamagnetic. X-ray quality single crystals of
[RuR2 ] were obtained by slow evaporation of a saturated
4
CH2Cl2/hexanes solution in air. Figure 1 shows the molecular
structure of [RuR2 ]. Similar to [Ru(2-MeC6H4)4],5b the
structure of [RuR2 4] exhibits a 4-fold symmetry, and the
4
geometry around Ru is roughly tetrahedral [C−Ru−C′ angles =
105.99(15) and 111.24(8)°]. The Ru−C distance [1.986(3) Å]
and C−Ru−C′ angles [105.99(15) and 111.24(8)°] compare
well with those in [Ru(2-MeC6H4)4].5b
Oxidation of [RuR4]. The cyclic voltammograms of
[RuR1 ] and [RuR2 ] in acetonitrile displayed reversible
4
4
couples at 0.5 and 0.2 V vs Cp2Fe+/0, respectively, which are
assigned to the metal-centered Ru(V/IV) couple. The Ru(V/
Figure 2. Molecular structure of [RuR2 ](BF4). Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at 30% probability.
Selected bond lengths (Å) and angles (deg): Ru−C 1.970(3)−
1.979(3); C−Ru−C′ 99.21(10)−116.54(11).
4
IV) potential for [RuR2 ] is lower than those for [Ru(2-
4
MeC6H4)4] (0.51 V vs Cp2Fe+/0, CH2Cl2),7 indicating the
ability of the methoxy substituent in stabilizing the Ru(V) state.
C
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cation [RuR2 ]+ features a pseudo 4-fold symmetry. The range
PhI(OAc)2 (OAc = acetate), whereas [RuR1 ] reacted with
4
4
1
of C−Ru−C′ angles [99.21(10)−116.54(11)°] in [RuR2 ]+ is
PhICl2 to produce a brown species that exhibited H NMR
4
obviously larger than that for [RuR2 ], due to the Jahn−Teller
signals at δ 5−6 ppm, which can be attributed to an η6-arene
ligand. We have not been able to crystallize this brown product
for further characterization. On the other hand, reaction of
4
effect associated with the low-spin d3 electron configuration.7,19
The Ru−C bond lengths in [RuR2 ]+ [1.970(3)−1.979(3) Å]
4
are slightly shorter than those in [RuR2 ].
[RuR2 ] with PhICl2 afforded a brown crystalline solid, which
4
4
Reaction of [RuR4] with PhIO. The reaction between
was identified as the Ru(II) η6-biaryl complex [(η6-R2-
1
tetraarylruthenium(IV) complexes and PhIO was studied
R2)RuCl2]2 (1) by X-ray diffraction. The H NMR spectrum
because the Os(VI) mono-oxo complex [OsVI(O)R1 ] has
of 1 displayed resonances at δ 5.12−5.61 ppm due to the
phenyl protons of the η6-bonded aryl ring along with multiplets
at δ 6.70−6.98 ppm corresponding to the uncoordinated aryl
ring of the biaryl ligand. Figure 3 shows the molecular structure
of 1. The structure consists of two symmetry-related [(η6-
biaryl)RuCl2] fragments linked together by two bridging
chlorides. The Ru−C [2.153(4)−2.231(3) Å] and Ru−Cl
[2.4404(9) and 2.4311(8) Å] distances are typical for [(η6-
arene)RuCl2]-type complexes, e.g. [(η6-p-cymene)RuCl2]2.21
The two aryl rings in the biaryl ligand are roughly perpendicular
to each other (dihedral angle = 96.63°).
4
been synthesized by oxidation of [OsR1 ].12 Treatment of
4
[RuR1 ] in CDCl3 with 1 equiv of PhIO at room temperature
4
resulted in a purple solution. GLC analysis showed that PhI was
produced in ca. 90% yield. 1H NMR spectroscopy indicated the
reaction mixture contained [RuR1 ] along with some other
4
diamagnetic species. We have not been able to separate the
product mixture by recrystallization. [RuR1 ] was found to be a
4
catalyst for decomposition of PhIO. Thus, in the presence of 1
mol % of [RuR1 ], PhIO was converted to PhI in over 95%
4
yield in 1 h. The byproduct oxygen has been identified by GC
analysis. It seems likely that the reaction of [RuR1 ] with PhIO
The C−C and C−Cl reductive elimination of high-valent
transition-metal aryl halide complexes is well documented.
Recently, Whitfield and Sanford reported that upon heating cis-
[PdIV(ppy)2Cl2] (ppy = 2-(2-pyridyl)phenyl), which was
prepared by the chlorination of cis-[PdII(ppy)2] with PhICl2,
underwent C−C and C−Cl reductive elimination to afford 4,4′-
bi(2-phenylpyridine) and 2-(2-chlorophenyl)pyridine, respec-
tively.22 Therefore, we believe that a similar pathway is involved
4
afforded a reactive intermediate, possibly a high-valent oxo or
PhIO complex,20 which decomposed rapidly to [RuR1 ] with
4
elimination of oxygen. In the presence of an organic substrate,
[RuR1 ] can catalyze the oxo transfer of PhIO (presumably via
4
the intermediacy of an oxo or PhIO complex). For example,
treatment of methyl p-tolyl sulfide with PhIO in the presence of
1 mol % of [RuR1 ] afforded methyl p-tolyl sulfoxide in 29%
4
in the formation of 1. Reaction of [RuR2 ] with PhICl2 gives a
yield (based on PhI formed) (eq 1); no methyl p-tolyl sulfone
4
high-valent Ru chloride complex (possibly a RuVI species
was detected. A similar yield (28%) was found when [RuR2 ]
4
because RuV aryl complexes such as [RuR1 ]+ and [RuR2 ]+ are
was used as catalyst.
4
4
stable with respect to reducing elimination), which undergoes
C−C and C−Cl reductive elimination to afford [(η6-R2-
R2)RuCl2], which dimerizes to 1. Consistent with this proposal,
the aryl chloride 4-chloro-3-methylanisole (R2Cl) has been
detected in the reaction mixture by GLC analysis. It may also
be noted that Os(II) η6-biaryl complexes have been obtained
from the ligand-induced reductive coupling of [Os(2-
MeC6H4)4].3 Attempts to characterize the reactive Ru chloride
Reductive Coupling of [RuR2 ]. Reactions of [RuR1 ]
4
4
with other hypervalent iodine reagents have also been
examined. [RuR1 ] reacted readily with PhINTs (Ts = tosyl)
4
to produce a purple species along with PhI and TsNH2. We
have not been able to crystallize the purple species for
intermediate(s) by monitoring the reaction of [RuR2 ] with 1
4
equiv of PhICl2 in CDCl3 by using NMR spectroscopy failed.
characterization. No reaction was found between [RuR1 ] and
Only a complex 1H NMR spectrum showing peaks
4
Figure 3. Molecular structure of 1. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are drawn at 30% probability. Selected bond lengths
(Å): Ru1−C 2.153(4)−2.231(3), Ru1−Cl1, 2.4404(9), Ru1−Cl1A 2.4311(8), Ru1−Cl2 2.4017(9).
D
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corresponding to 1 and unreacted [RuR2 ], along with some
Scheme 3. Proposed Pathway for the Formation of Alky-N-
4
nitrosohydroxylaminato Complexes25
unidentified species, was observed.
Migratory Insertion of [RuR1 ] with NO. Treatment of
4
[RuR1 ] with excess NO led to formation of the tetranuclear
4
aryl-N-nitrosohydroxylaminato complex [Ru(NO)R1{κ2-O,O′-
ON(R1)NO}(μ-NO2)]4 (2). Complex 2 is diamagnetic and
exhibits sharp 1H NMR signals, consistent with the {Ru(NO)}6
electron configuration. The IR N−O stretching frequency of
1879 cm−1 is typical for the linear nitrosyl ligand.23 Figure 4
Scheme 4. Plausible Mechanism for the Formation of 2
converted to the tetramer 2. It should be noted that the
formation of Ru(II) nitrosyl nitrito complexes by reactions of
Ru porphyrin and Schiff base complexes with excess nitric oxide
is well precedented.27,28
Figure 4. Molecular structure of 2. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at 30% probability. Selected bond
lengths (Å) and angles (deg): Ru−C 2.067(4)−2.080(3), Ru−
N(nitrosyl) 1.731(3)−1.738(3), Ru−O(nitrosohydroxylaminato)
1.997(3)−2.055(3), Ru−O(nitrito) 2.029(3)−2.045(3), Ru−N-
(nitrito) 2.230(3)−2.255(2); Ru−N−O (nitrosyl) avg 177.7°.
Bromination of [RuR1 ]. In previous work, we reported
4
that the aryl ligands in [OsR1 ] could be brominated selectively
4
by pyridinium tribromide.13 In this work, we found that under
the same conditions the bromination of [RuR1 ] with excess
4
pyridinium tribromide led to formation of a mixture of mono-
and dibrominated products, characterized by mass spectrom-
etry. On the other hand, reaction of [RuR1 ] with NBS yielded
shows the crystal structure of 2. The structure consists of four
[Ru(NO)(R1){κ2-O,O′-ON(R1)NO}]+ units that are bridged
by four bidentate-N,O nitrite ligands. The geometry around
each Ru center is pseudo octahedral with the nitrosyl opposite
to an oxygen of the nitrosohydroxylaminate chelate and the aryl
ligand trans to an oxygen of the nitrite. The nitrosyl binds to Ru
in a linear fashion (average Ru−N−O angle = 177.7°). The
Ru−C distances [2.067(4)−2.080(3) Å] are similar to those in
4
the tetrabrominated complex [RuR′4] (R′ = 4-bromo-2,5-
dimethylphenyl) (3) as the sole product. Similar to the Os
analogue, the aryl ligands in [RuR1 ] were brominated
4
selectively at the para position with respect to the Ru−C
bond. The crystal structure of 3 has been determined (Figure
5). The Ru−C distance [1.984(5) Å] and C−Ru−C′ angles
[106.3(3) and 111.06(14)°] in 3 are similar to those in
[RuR1 ], indicating that the bromo substituents have very little
[RuR1 ]. The O−N (O−N 1.330(4)−1.352(3) Å, ON
4
4
influence on the Ru−C bonds. The synthesis of Ru complexes
with functionalized aryl ligands using 3 as the starting material
is underway.
1.294(4)−1.323(4) Å) and N−N (1.263(5)−1.276(4) Å) bond
lengths in the Ru[ON(R1)NO] metallacycles compare well
with those in reported alky-N-nitrosohydroxylaminato com-
plexes.24
CONCLUSIONS
Reactions of metal alkyls with NO leading to the formation
of alkyl-N-nitrosohydroxylaminato complexes are well docu-
mented. It is believed that the insertion of NO into the metal−
alkyl bond gives a radical intermediate, which reacts rapidly
with another molecule of NO to yield an alkyl-N-nitro-
sohydroxylaminato group (Scheme 3).24,25 Therefore, a
plausible mechanism for the formation of 2 is proposed in
■
In summary, we have synthesized the Ru(IV) tetraaryl
complexes [RuR1 ] and [RuR2 ] by alkylation of [Ru(acac)3]
4
4
with Grignard reagents. [RuR1 ] can catalyze the oxo transfer of
4
PhIO, possibly via the intermediacy of an oxo species or PhIO
complex. Reaction of [RuR2 ] with PhICl2 afforded a Ru(II) η6-
4
biaryl complex, presumably via the C−C and C−Cl reductive
Scheme 4. Reaction of [RuR1 ] with NO results in reductive
elimination. Migratory insertion of [RuR1 ] with NO led to
4
4
coupling of aryl groups and formation of a Ru(II) η2-biaryl
intermediate. Migratory insertion of the Ru aryl with NO gives
the Ru[ON(R1)NO] metallacycle. Dissociation of the labile η6-
formation of the aryl-N-nitrosohydroxylaminato complex
[Ru(NO)R1{κ2-O,O′-ON(R1)NO}(μ-NO2)]4. [RuR1 ] can be
4
selectively brominated with NBS to give a homoleptic Ru(IV)
bromoaryl complex. The results of this work demonstrate that
both the Ru center and aryl ligands in tetraarylruthenium(IV)
complexes display rich reaction chemistry.
biaryl ligand26 and coordination of NO2 , which is presumably
−
derived from disproportionation of NO,27 affords five-
coordinated Ru[ON(R1)NO](R1)(NO)(NO2), which is finally
E
dx.doi.org/10.1021/om500707e | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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(20) Reactive metal iodosylarene complexes that can undergo oxo
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L.; Kurahashi, T.; Fujii, H. Angew. Chem., Int. Ed. 2012, 51, 7809.
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Figure 5. Molecular structure of 3. Hydrogen atoms are omitted for
clarity. Thermal ellipsoids are drawn at the 30% probability level.
Selected bond length (Å) and angles (deg): Ru−C 1.984(5); C−Ru−
C′ 106.3(3), 111.06(14).
ASSOCIATED CONTENT
* Supporting Information
■
S
Crystallographic data and experimental details, crystal struc-
tures and selected bond lengths and angles, and CIF data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chleung@ust.hk.
■
(25) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 5th
ed.; Wiley: New York, p 1210.
(26) The biaryl ligand in [{η6-2(2-MeC6H4)MeC6H4}Os(2-
MeC6H4)2L] (L = RNC) is labile. Reaction of [{η6-2(2-MeC6H4)
MeC6H4}Os(2-MeC6H4)2L] with excess L gave cis-[Os(2-
MeC6H4)2L4] (ref 3a).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(27) Miranda, K. M.; Bu, X.; Lorkovic,
1997, 36, 4838.
́
I.; Ford, P. C. Inorg. Chem.
■
This work was supported by the Hong Kong Research Grants
Council (Project 602913) and the Hong Kong University of
Science and Technology.
(28) (a) Carrondo, M. A. A. F. de C. T.; Rudolf, P. R.; Skapski, A. C.;
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Addo, G. B. Inorg. Chem. 1996, 35, 1343. (c) Bohle, D. S.; Hung, C.-
H.; Smith, B. D. Inorg. Chem. 1998, 37, 5798.
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dx.doi.org/10.1021/om500707e | Organometallics XXXX, XXX, XXX−XXX