Organometallics
Note
C23H41Cl2CoN2O11P3Ru: C, 32.68; H, 4.89; N, 3.31. Found: C, 32.38;
H, 4.55, N, 3.34.
Data for 4 are as follows. Yield: 50 mg (41% with respect to Ru). μeff
= 1.9 μB. IR (KBr, cm−1): 3285, 3232 (ν(N−H)). MS (ESI): m/z 858
(M+). Anal. Calcd for C27H52Cl2CoNP9P3Ru: C, 37.77; H, 6.10; N,
1.63. Found: C, 37.62; H, 6.10, N, 1.92.
Ru-Catalyzed Reduction of 4-Nitrophenyl Azide with Cyclo-
hexene. A mixture of 4-nitrophenyl azide (105 mg, 0.64 mmol) and 1
(18 mg, 0.013 mmol) was heated in neat cyclohexene (5 mL) at 60 °C
overnight. The organic volatiles were pumped off, and the residue was
subjected to silica column chromatography. The organic products, N-
(2-cyclohexenyl)-4-nitroaniline (7% yield with respect to 4-nitro-
phenyl azide) and 4-nitroaniline (70%), were eluted with hexane and
CH2Cl2, respectively, and characterized by NMR and IR spectrosco-
py.10b
Preparation of [LOEtRu(κ2N,C-AdNCHC6H4)Cl] (5). A mixture
of 1 (100 mg, 0.07 mmol) and 1-azidoadamantane (25 mg, 0.141
mmol) in toluene (5 mL) was heated at reflux overnight. The solvent
was removed in vacuo, and the residue was washed with cold hexane.
Recrystallization from Et2O afforded red crystals that were suitable for
X-ray diffraction. Yield: 58 mg (45% with respect to Ru). μeff = 2.0 μB.
Anal. Calcd for C34H56Cl2CoNO9P3Ru: C, 44.82; H, 6.19; N, 1.54.
Found: C, 44.63; H, 6.10, N, 1.55.
Figure 1. Molecular structure of 3. Hydrogen atoms are omitted for
clarity. The thermal ellipsoids are drawn at the 30% probability level.
Selected bond lengths (Å): Ru1−O8 = 2.045(3), Ru1−O7 = 2.058(4),
Ru1−O9 = 2.073(4), Ru1−N1 = 2.106(5), Ru1−Cl1 = 2.3387(14),
Ru−Cl2 = 2.3387(14).
The possibility of using 1 as a catalyst for the allylic
amination of cyclohexene10 has been explored. Treatment of 4-
nitrophenyl azide with 2 mol % of 1 in neat cyclohexene at
room temperature resulted in a red solution (possibly
containing 3); no allylic amination product was found. Heating
the reaction mixture at 60 °C overnight led to isolation of 4-
nitroaniline (70%) along with a small amount of N-(2-
cyclohexenyl)-4-nitroaniline (ca. 7%) (Scheme 2). In the
X-ray Crystallography. Intensity data were collected on a Bruker
SMART APEX 1000 CCD diffractometer using graphite-monochro-
mated Cu Kα radiation (λ = 1.541 78 Å). The data were corrected for
absorption using the program SADABS.23 Structures were solved by
direct methods and refined by full-matrix least squares on F2 using the
SHELXTL software package.24
RESULTS AND DISCUSSION
■
Scheme 2. Ru-Catalyzed Reduction of 4-Nitrophenyl Azide
with Cyclohexene
Reactions of 1 with Aryl Azides. Treatment of
[LOEtRuCl2]2 (1) with phenyl azide in CH2Cl2 at room
temperature resulted in a color change from orange to green in
2 h. Evaporation of the solvent afforded a paramagnetic green
oily material that did not crystallize. No reaction was found
between this green oil and PPh3. This green species is probably
a phenyl azide adduct rather than an imido complex.
Transition-metal complexes containing organic azide ligands
are well-known.25 The IR spectrum of the reaction mixture
showed a band at 2126 cm−1 (cf. 2129 cm−1 for free phenyl
azide) assignable to the azido group. A similar N−N stretching
frequency (2106 cm−1) has been found for a recently reported
Ru aryl azide complex, in which the azide ligand binds to Ru in
an η1 mode.15 While this green species is stable in CH2Cl2
solution for hours, recrystallization from Et2O overnight
afforded air-stable red crystals characterized as the known
compound [LOEtRuCl2(NH2Ph)] (2).21 Similarly, the reaction
of 1 with 4-nitrophenyl azide, followed by recrystallization from
E t 2 O , a ff o r d e d t h e 4 - n i t r o a n i l i n e c o m p l e x
[LOEtRuCl2(NH2C6H4NO2-4)] (3) , which has been charac-
terized by X-ray diffraction26 (Figure 1). The Ru−O, Ru−Cl,
and Ru−N distances in 3 (average 2.059, 2.3387(14) and
2.106(5) Å, respectively) are similar to those in
[LOEtRuCl2(NH2Ph)].21 It seems unlikely that the phenyl
azide was initially reduced to aniline that subsequently binds to
Ru, because no free aniline was detected during the course of
the reaction. Also, the reaction of 1 with aniline resulted in a
color change from orange to dark red only; no green
intermediate was observed. Reduction of Ru-bound aryl azide
to aniline by organic solvents to give an amine complex has
been reported previously.10a Therefore, we believe that the
aniline ligands in 2 and 3 were formed by Ru-mediated
reduction of the nitrene intermediates (derived from the
corresponding aryl azide precursors) with Et2O.
absence of the Ru catalyst, the reaction between 4-nitrophenyl
azide and cyclohexene at 60 °C afforded 4-nitroaniline in very
low yield (<5%). Thus, while 1 is a poor catalyst for the allylic
amination, it can catalyze the reduction of 4-nitrophenyl azide
with cyclohexene at 60 °C with a turnover number of 35 (with
respect to 1). Gallo and co-workers reported that a Ru(VI)
imido porphyrin complex can abstract hydrogen atoms from
cyclohexene.10a Therefore, the 1-mediated reduction of aryl
azide possibly involves the formation of a nitrene intermediate
that abstracts hydrogen atoms from cyclohexene to give aniline.
Reaction of 1 with 1-Azidoadamantane. Treatment of 1
with 1-azidoadamantane (AdN3) in organic solvents such as
CH2Cl2 and toluene at room temperature, followed by
recrystallization from Et2O, afforded the amine complex
[LOEtRuCl2(NH2Ad)] (4). On the other hand, refluxing 1
with AdN3 in toluene led to isolation of the cyclometalated
Ru(III) complex [LOEtRu(κ2N,C-AdNCHC6H4)Cl] (5) in
45% yield (with respect to Ru). In contrast with AdN3,
refluxing 1 with phenyl azide in toluene gave 2 as the sole
isolated product. 1 also reacted with AdN3 in other
alkylbenzenes such as ethylbenzene and isopropylbenzene
under the same conditions to give dark red materials. However,
we were not able to characterize and crystallize the para-
magnetic red products in these reactions. The crystal structure
of 5 has been determined.27 Figure 2 shows the molecular
structure of 5. The geometry around Ru is pseudo-octahedral.
The C47−N1 bond distance of the cyclometalated ligand
(1.296(4) Å) is consistent with an imine CN double bond.
7021
dx.doi.org/10.1021/om300499x | Organometallics 2012, 31, 7020−7023