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SABOUNCHEI ET AL.
water and then with EtOH and dried under vacuum. The
purity was checked using TLC with eluent of
CH3COOC2H5–C2H3OH (2:1). The complex was weakly
soluble in DMSO. Yield 0.1244 g (96.3%); m.p. > 300 °C.
Anal. Found (%): C, 32.39; H, 2.64; N, 12.74.
K[C18H18Cl4N6O4Rh] requires (%): C, 32.45; H, 2.72; N,
3
|
RESULTS AND DISCUSSION
3.1 | Synthesis
Reaction of K3[RhCl6] with HL1, HL2 and HL3 (1:2 molar
ratio) in aqueous H2O–C2H5OH gave the complexes 1, 2a
and 2b, and 3a and 3b (Scheme 2). Complex 1 is insoluble
in most organic solvents such as chloroform, acetone and
toluene. Dissolution of this complex in DMSO leads to the
formation of complexes 1a and 1b. The formation of heavy
metal complexes with solvent molecules is natural for sol-
vents such as DMSO, which behave as strong ligands (L)
and form M─L bonds. Recently, the synthesis and character-
ization of a mononuclear Pt(II) complex containing one
DMSO as ligand were reported by our group.[14] As evident
from the literature, the trans isomers 2b and 3b are more sta-
ble than the cis ones. This is due to the fact that the metal
complexes with hydantoin ligands in cis positions have more
steric hindrance than that of complexes with ligands in trans
positions.[14]
1
12.62). H NMR (250 MHz, DMSO‐d6, δ, ppm): 11.03 (br
s, 2H, N2─H2cis and trans), 9.15 (br s, 2H, N1─H1cis and trans),
8.76–8.96 (m, 4H, Phcis and trans), 7.61–7.63 (m, 4H, Phcis and
trans), 1.67 (s, 3H, CH3trans), 1.21 (s, 3H, CH3cis). HRMS
(m/z): [M− − K] calcd for [C18H18Cl4N6O4Rh]; found: 629.7.
2.4 | General experimental procedure for
hydrogenation of ketones
Ketones (2 mmol, 0.2 M) and KOH (0.12 mmol) were added
to a solution of the Rh complex (0.005 mmol) in 10 ml of
anhydrous 2‐propanol under N2 atmosphere. Reaction mix-
ture was stirred for 4 h at 80 °C until the substrates vanished.
Reaction progress was monitored by TLC. After completion
of the reaction, the solvent was removed under reduced pres-
sure and the residue was purified using chromatography
(ethyl acetate–petroleum ether, 1:15 to 1:10). Progress of
3.2 | Spectroscopy
1
reaction was monitored using IR, H NMR and 13C NMR
The structure of complex 1 was characterized successfully
using IR and mass spectroscopies and other conventional
techniques such as elemental analysis and cyclic voltamme-
try. The elemental analysis of Rh(III) complexes 1, 2a, 2b,
3a and 3b indicates a 1:2 stoichiometry between the Rh(III)
salt and ligand. Structures of 1a, 1b, 2a, 2b, 3a and 3b were
determined using IR, 1H NMR and 13C NMR spectroscopies
and unequivocal structure of complex 1a was obtained using
the single‐crystal X‐ray diffraction technique.
Table 1 summarizes the most important vibrational
modes of the ligands and complexes in IR spectra. Compara-
tive analysis of IR spectra of the complexes and free ligands
reveals that stretching vibrations of ν(C═N) in pyridine ring
shift to higher frequencies (Table 1). This is due to the coor-
dination of ligand through nitrogen atom of pyridine ring
(chelating mode) to the metal ion that causes a significant
increase in the ν(C═N) frequency. Identifying of the
wavenumbers of Rh─N vibrations is rather difficult since
the M─N stretching modes (where M is a heavy atom) are
usually of low intensity.[16]
spectroscopies.
2.5 | Characterization of hydrogenation products
Data for 1‐(4‐nitrophenyl)ethanol (4). IR (KBr disc, ν, cm−1):
1
3374 (OH). M.p. 112–114 °C (decomposition). H NMR
(250 MHz, CDCl3, δ, ppm): 7.50–7.54 (d, 2H, phenyl),
8.16–8.19 (d, 2H, phenyl), 4.98–5.02 (m, 1H, OH), 3.44–
3.46 (br, 1H, CH), 1.48–1.51 (s, 3H, Me). 13C NMR
(62.90 MHz, CDCl3, δ, ppm): 153.14, 147.12, 125.71,
123.71, 69.45, 50.78, 24.46.
Data for 1‐(4‐methoxyphenyl)ethanol (5). IR (Nujol, ν,
1
cm−1): 3343 (OH). H NMR (250 MHz, CDCl3, δ, ppm):
6.58–6.2 (d, 2H, phenyl), 7.26–7.30 (d, 2H, phenyl), 4.80–
4.87 (m, 1H, OH), 3.73–3.79 (s, 3H, OMe), 3.45–3.52 (br,
1H, CH), 1.45–1.48 (d, 3H, Me). 13C NMR (62.90 MHz,
CDCl3, δ, ppm): 158.94, 137.99, 126.65, 113.81, 69.82,
55.27, 24.54.
Data for diphenylmethanol (6). IR (KBr disc, ν, cm−1):
1
3387 (OH). M.p. 66–68 °C. H NMR (89.60 MHz, CDCl3,
Bands related to stretching vibrations of carbonyl groups
and more acidic NH groups ν(N2─H2) in the spectra of
HL1–HL3 remain almost unchanged for all complexes. This
δ, ppm): 7.20–7.35 (m, 10H, phenyl), 5.97 (br, 1H, OH),
5.73 (br, 1H, CH).
Data for 1‐(4‐chlorophenyl)ethanol (7). IR (Nujol, ν,
1
cm−1): 3350 (OH). H NMR (250 MHz, CDCl3, δ, ppm):
7.44–7.54 (m, 2H, phenyl), 8.16–8.25 (m, 2H, phenyl),
4.98–5.04 (m, H, OH) 2.29–2.33 (br, 1H, CH), 1.43–1.51
(d, 3H, Me).
Data for 1‐(naphthalenyl)ethanol (8). IR (KBr disc, ν,
1
cm−1): 3262 (OH). M.p. 63–65 °C. H NMR (89.60 MHz,
CDCl3, δ, ppm): 7.51–8.10 (m, 7H, phenyl), 5.46 (br, H,
OH), 5.32 (br, 1H, CH), 1.47 (br, 3H, Me).
SCHEME 2 Synthesis of complexes 1, 1a, 1b, 2a, 2b, 3a and 3b