Inorganic Chemistry
Article
m m o l ) . Y i e l d : 0 . 1 0 0
C70H62Cl4Ru2N3O2P2.H2O: C, 60.00%; H, 4.60%; N, 3.00%;
g
( 9 9 % ) . A n a l . c a l c d f o r
proton NMR spectral studies in selected cases. These
ruthenium complexes were utilized for the borohydride
mediated reduction of assorted nitro-substrates. The substrate
range asserted wider catalysis scope including an excellent
control over the chemo-selectivity. The mechanistic inves-
tigations not only confirmed the participation of [Ru−H]
species in the catalysis but also substantiated the involvement
of several organic intermediates during the course of catalysis.
Utilization of a similar Ru(II) complex but without having a
pincer cavity substantiated the unique role played by the
pyridine-2,6-dicarboxamide based pincer cavity in substrate
binding and catalysis. The involvement of H-bonding assisted
catalysis draws parallels to the elegant biological systems.
Found: C, 59.42%; H, 4.53%; N, 2.75%. FTIR spectrum (Zn−Se
ATR, selected peaks): 3267 (νNH), 1687 (νC=O asym) cm−1. H NMR
1
spectrum (400 MHz, CDCl3): δ 10.13 (s, 2H), 8.42 (d, J = 7.7 Hz,
2H), 8.05 (t, J = 7.6 Hz, 1H), 7.86−7.62 (m, 16H), 7.29 (dt, J = 26.6,
8.1 Hz, 8H), 7.16 (d, J = 7.0 Hz, 4H), 5.11 (d, J = 5.5 Hz, 4H), 4.87
(d, J = 5.4 Hz, 4H), 2.75 (s, 2H), 1.62 (s, 6H), 1.05 (d, J = 6.7 Hz,
12H) ppm. 13C NMR spectrum (100 MHz, CDCl3): δ 162.11,
149.14, 135.53, 134.21, 134.12, 133.90, 130.30, 129.12, 128.31,
128.11, 128.02, 125.82, 125.38, 120.25, 111.07, 96.29, 88.80, 87.21,
30.37, 21.97, 17.81 ppm. 31P NMR spectrum (162 MHz, CDCl3): δ
25.01 ppm. ESI-MS+ spectrum (CH3CN) (m/z): 1322.10 [2 + Na+]+
(calcd: 1322.14).
[(HL3)Ru(p-Cymene)(Cl)2] (3). Ligand HL3 (0.100 g, 0.261 mmol)
and [Ru(p-cymene)(μ-Cl)(Cl)]2 (0.080 g, 0.130 mmol) were
dissolved in 10 mL of toluene and stirred for 4 h at room
temperature. The reaction mixture was filtered through a pad of Celite
in a medium porosity frit and the filtrate was concentrated under the
reduced pressure. This afforded a solid that was isolated after washing
with diethyl ether. Recrystallization was achieved by layering hexane
over a saturated solution of the crude product in toluene that afforded
a highly crystalline product within a weak. This product was isolated,
washed with diethyl ether, and dried under vacuum. Yield: 0.150 g
(83%). FTIR spectrum (Zn−Se ATR, selected peaks): 3330 (νNH),
EXPERIMENTAL SECTION
■
Materials. All reagents were purchased from the commercial
sources and used without further purification. The solvents were dried
or purified using the standard literature procedures.60 The ligands
H2L1 and H2L2 were synthesized as per our earlier reports,17,61 while
precursors [Ru(p-cymene)(μ-Cl)(Cl)]2 and [Ru(p-cymene)-
(Cl)2PPh3] were prepared according to the literature methods.62
Synthesis. HL3. 3-Diphenylphosphinoaniline (1.000 g, 3.610
mmol) and 2-picolinic acid (0.443 g, 3.598 mmol) were dissolved
in 5 mL of pyridine and stirred over an oil bath maintained at 80 °C
for 30 min. To the said reaction mixture, triphenylphosphite (1.118 g,
4.332 mmol) was added dropwise, and the reaction mixture was
stirred for 12 h at 80 °C. After the reaction mixture was cooled to
room temperature, excess pyridine was removed under the reduced
pressure to afford an oily product. The product thus obtained was
washed with ice-cold water followed by diethyl ether. The solid thus
obtained was further washed with acetone, filtered, and dried under
vacuum. Yield: 1.000 g (72%). FTIR spectrum (Zn−Se ATR, selected
1
1683 (νC=O asym) cm−1. H NMR spectrum (400 MHz, CDCl3) δ
10.09 (s, 1H), 8.61 (d, J = 4.4 Hz, 1H), 8.32 (d, J = 11.7 Hz, 1H),
8.26 (d, J = 7.8 Hz, 1H), 7.97 (d, J = 5.7 Hz, 1H), 7.92−7.77 (m,
3H), 7.56−7.46 (m, 1H), 7.38 (d, J = 6.5 Hz, 10H), 5.31 (d, J = 6.1
Hz, 2H), 5.12 (d, J = 5.6 Hz, 2H), 2.85 (m, J = 13.8, 7.0 Hz, 1H),
1.88 (s, 3H), 1.10 (d, J = 6.9 Hz, 6H) ppm. 13C NMR spectrum (100
MHz, CDCl3) δ 162.35, 149.47, 148.11, 137.66, 137.43, 134.57,
134.44, 133.42, 130.52, 129.91, 128.74, 128.05, 127.95, 122.40,
121.79, 111.02, 96.23, 89.07, 89.04, 87.36, 87.30, 30.28, 21.86, 17.63
ppm. 31P NMR spectrum (162 MHz, CDCl3) δ 25.31 ppm.
1
peaks): 3290 (νNH), 1685 (νC=O asym) cm−1. H NMR spectrum (400
MHz, CDCl3): δ 9.95 (s, 1H), 8.58 (d, J = 4.4 Hz, 1H), 8.27 (d, J =
7.8 Hz, 1H), 8.04 (d, J = 7.2 Hz, 1H), 7.89 (td, J = 7.7, 1.5 Hz, 1H),
7.51 (d, J = 8.3 Hz, 1H), 7.49−7.41 (t, 1H), 7.35 (m, 11H), 7.06 (t, J
= 7.4 Hz, 1H) ppm. 13C NMR spectrum (100 MHz, CDCl3) δ
162.06, 149.70, 147.96, 138.50, 138.39, 137.98, 137.90, 137.69,
136.98, 136.87, 133.92, 133.72, 129.63, 129.46, 129.39, 129.32,
128.84, 128.61, 128.54, 126.50, 124.76, 124.53, 122.45, 120.29 ppm.
31P NMR spectrum (162 MHz, CDCl3) δ −4.98 ppm.
Typical Procedure for Reduction of Nitro Substrates. In a
round-bottom flask, nitro-substrate (1.0 mmol), ruthenium complex
(1 mol %), and sodium borohydride (1.2 mmol) were taken in EtOH
(2 mL). This reaction mixture was stirred under ambient conditions,
while progress of the reaction was monitored by thin layer
chromatography (TLC) or gas chromatography (GC). After
completion, reaction mixture was passed through a plug of alumina
and concentrated to afford the organic product(s). The product(s)
were identified or characterized by the combination of GC and NMR
techniques. In a few representative cases, organic products were
[(H2L1)Ru2(p-Cymene)2(Cl)4] (1). Ligand H2L1 (0.100 g, 0.145
mmol) and [Ru(p-cymene)(μ-Cl)(Cl)]2 (0.089 g, 0.145 mmol) were
dissolved in 15 mL of toluene and stirred for 4 h at room temperature.
During this time, an orange colored product was precipitated. The
reaction mixture was filtered, precipitate was washed with diethyl
ether, and the product was dried under vacuum. Recrystallization was
achieved by layering MeOH over a saturated solution of 1 in CH2Cl2
that afforded a highly crystalline product within a weak that was
isolated, washed with diethyl ether, and dried under vacuum. Yield:
0.098 g (97%). Anal. calcd for C70H62Cl4Ru2N3O2P2. H2O. CH3OH:
C, 59.50%; H, 4.78%; N, 2.93%; Found: C, 59.75%; H, 4.53%; N,
2.89%. FTIR spectrum (Zn−Se ATR, selected peaks): 3286 (νNH),
1
most of the control and optimization experiments were done using 4-
iodonitrobenzene as a substrate that produces 4-iodoaniline as the
product, the calibration plot was studied for this substrate/product
Characterization Data for a Few Representative Organic
1
Products. Aniline. H NMR spectrum (400 MHz, CDCl3): δ 7.23−
1
1683 (νC=O asym) cm−1. H NMR spectrum (400 MHz, CDCl3): δ
7.10 (m, 2H), 6.79 (t, J = 7.4 Hz, 1H), 6.70 (d, J = 7.5 Hz, 1H), 3.52
(s, 2H) ppm. 13C NMR spectrum (100 MHz, CDCl3): δ 146.53,
129.64, 118.79, 115.17 ppm.
10.69 (s, 2H), 8.57 (d, J = 12.1 Hz, 2H), 8.42 (d, J = 7.8 Hz, 2H),
8.05 (t, J = 5.4 Hz, 3H), 7.88−7.83 (m, 8H), 7.40 (d, J = 5.0 Hz,
12H), 7.33 (d, J = 7.9 Hz,2H), 7.18 (t, J = 9.0 Hz,2H), 5.31 (d, J =
6.0 Hz, 4H), 5.08 (d, J = 5.6 Hz, 4H), 2.82 (dt, J = 13.7, 6.8 Hz, 2H),
1.85 (s, 6H), 1.08 (d, J = 6.9 Hz, 12H) ppm. 13C NMR spectrum
(100 MHz, CDCl3): δ 162.06, 149.20, 138.98, 137.89, 137.29, 137.15,
134.43, 134.34, 133.99, 133.65, 130.45, 130.43, 129.05, 128.18,
128.08, 127.62, 127.50, 125.57, 125.31, 123.56, 123.54, 111.14, 96.32,
88.94, 88.91, 87.59, 87.54, 30.35, 21.87, 21.50, 17.68 ppm. 31P NMR
spectrum (162 MHz, CDCl3): δ 25.85 ppm. ESI-MS+ spectrum
(CH3CN) (m/z): 1264.14 [1 − Cl]+ (calcd: 1264.14).
4-Iodoaniline. 1H NMR spectrum (400 MHz, CDCl3): δ 7.39 (d, J
= 8.7 Hz, 2H), 6.46 (d, J = 8.7 Hz, 2H), 3.67 (s, 2H) ppm.13C NMR
spectrum (100 MHz, CDCl3): δ 139.07, 129.42, 120.07, 114.80 ppm.
1,5-Diaminonaphthalene. 1H NMR spectrum (400 MHz,
CD3OD): δ 7.26 (d, J = 8.5 Hz, 2H), 7.09 (t, J = 7.9 Hz, 2H),
6.70 (d, J = 7.3 Hz, 2H) ppm. 13C NMR spectrum (100 MHz,
CD3OD): δ 143.14, 124.85, 124.59, 111.55, 109.54 ppm.
Ethyl-4-amino-benzoate. 1H NMR spectrum (400 MHz, CDCl3):
δ 7.84 (d, J = 8.7 Hz, 2H), 6.62 (d, J = 8.8 Hz, 2H), 4.30 (q, J = 7.1
Hz, 2H), 4.04 (s, 2H), 1.34 (t, J = 7.1 Hz, 3H) ppm. 13C NMR
spectrum (100 MHz, CDCl3): δ 166.86, 150.79, 131.54, 120.26,
113.51, 60.57, 14.61 ppm.
[(H2L1)Ru2(p-Cymene)2(Cl)4] (2). A similar procedure as outlined
for complex 1 was adopted utilizing the following reagents: H2L2
(0.100 g, 0.145 mmol), [Ru(p-cymene)(μ-Cl)(Cl)]2 (0.089 g, 0.145
2017
Inorg. Chem. 2021, 60, 2009−2022