A R T I C L E S
Cotton et al.
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18.0 g (91%). H NMR (δ, ppm in DMSO-d6): 10.70 (s, 2H, NH),
7.70 (d, 4H, aromatic CH), 6.94 (d, 4H, aromatic CH), 3.73 (s, 6H,
OCH3).
corresponding compounds are designated as 4 and 5. We also
present molecular mechanics and density functional theory
calculations that help explain the conformation and electronic
structures of the R and â isomers.
Preparation of r Isomers 2 and 3. The two compounds were
synthesized similarly. To a stirred mixture of Mo2(DAniF)3(O2CCH3)
(0.410 g, 0.400 mmol) and N,N′-diphenyloxamide (0.050 g, 0.208
mmol) for 2, or N,N′-di-p-anisyloxamide (0.062 g, 0.206 mmol) for 3,
in 25 mL of THF was added 1.5 mL of a 0.5 M solution of NaOCH3
in methanol. An orange color developed immediately. The reaction
mixture was stirred at room temperature for 3 h, and then the solvent
was removed under vacuum. The orange residue was extracted with
15 mL of dichloromethane, and the mixture was passed through a filter
frit packed with Celite to remove insoluble materials. The solvent from
the filtered extract was removed under vacuum. Acetonitrile (20 mL)
was added to the residue, producing a free-flowing, yellow solid. The
mixture was stirred for 0.5 h; then the supernatant solution was decanted
and the solid was washed with 15 mL of ethanol followed by 15 mL
of hexanes and dried under vacuum. The crude product was dissolved
in ca. 15 mL of dichloromethane; after filtration through a frit packed
with Celite, hexanes (40 mL) were added to the filtrate to generate a
yellow precipitate. Finally, the solid was collected by filtration and
dried under vacuum. Yield of 2: 0.320 g (75%). Yield of 3: 0.340 g
(78%). Crystals were obtained from CH2Cl2/hexane solutions. 1H NMR
(δ in CDCl3) for 2: 8.33 (s, 4H, -NCHN-), 8.25 (s, 2H, -NCHN-),
6.91 (t, 2H, aromatic C-H), 6.69 (d, 8H, aromatic C-H), 6.66 (d, 8H,
aromatic C-H), 6.56 (d, 4H, aromatic C-H), 6.48 (d, 4H, aromatic
C-H), 6.40 (d, 8H, aromatic C-H), 6.33 (d, 8H, aromatic C-H), 6.12
(d, 4H, aromatic C-H), 6.01 (d, 4H, aromatic C-H), 5.66 (d, 8H,
aromatic C-H), 3.71 (s, 12H, -OCH3), 3.66 (s, 12H, -OCH3), 3.62
Experimental Section
Materials and Methods. Solvents were freshly distilled under N2
by employing standard procedures or dried and degassed using a Glass
Contour solvent purification system. All synthetic operations were
conducted under N2 using Schlenk line techniques. Mo2(O2CCH3)4 was
prepared by following a published method;19 commercially available
chemicals were used as received.
Physical Measurements. Elemental analyses were performed by
Canadian Microanalytical Service, Delta, British Columbia, Canada.
Electronic spectra in CH2Cl2 were measured in a range of 300 to 800
nm on a SHIMADZU UV-2501PC spectrophotometer and in the range
of 800 to 3000 nm on a Cary 17 spectrophotometer. 1H NMR spectra
were recorded on a Mercury-300 NMR spectrometer with chemical
shifts (δ ppm) referenced to CDCl3 or DMSO-d6. The electrochemical
measurements were recorded at room temperature on a BAS 100
electrochemical analyzer with Pt working and auxiliary electrodes, Ag/
AgCl reference electrode, scan rate of 100 mV/s, and 0.1 M Bu4NPF6
(in CH2Cl2) as electrolyte. Under these experimental conditions, the
E1/2 (Fc+/Fc) was measured at 440 mV.
Preparation of Mo2(DAniF)3(O2CCH3), 1. A mixture of yellow
Mo2(O2CCH3)4 (1.43 g, 3.33 mmol) and N,N′-di-p-anisylformamidine
(2.56 g, 10.00 mmol) was suspended in 100 mL of THF. While stirring,
20 mL of a 0.5 M solution of NaOCH3 in methanol was added slowly.
The color turned first to red and then to brown. The reaction was stirred
for 5 h at room temperature, producing a deposit of white crystalline
sodium acetate. After removal of the solvent under reduced pressure,
the residue was extracted with ca. 40 mL of dichloromethane; filtration
removed NaO2CCH3. The volume of the filtrate was then reduced to
about 15 mL by vacuum evaporation. Ethanol (50 mL) was added to
the residue with vigorous stirring. A yellow solid and a dark brown
supernatant solution were obtained. After decanting, the solid was
washed with ethanol (2 × 20 mL), followed by 20 mL of hexanes,
and dried under vacuum. Yield: 2.83 g (84%). 1H NMR (δ in CDCl3):
8.50 (s, 2H, -NCHN-), 8.40 (s, 1H, -NCHN-), 6.67 (d, 8H, aromatic
C-H), 6.50 (d, 8H, aromatic C-H), 6.45 (d, 4H, aromatic C-H), 6.24
(d, 4H, aromatic C-H), 3.72 (s, 12H, -OCH3), 3.50 (s, 6H, -OCH3),
2.61 (s, 3H, CH3). UV-vis λmax (nm) (ꢀ, M-1 cm-1): 439 (1.0 × 103).
Anal. Calcd for C47H48Mo2N6O8: C, 55.52; H, 4.76; N, 8.27. Found:
C, 55.62; H, 4.74; N, 7.73.
Preparation of N,N′-Diphenyloxamide. A 500-mL flask containing
a solution of oxalyl chloride, 8.00 g (63.0 mmol) in 100 mL of THF,
was equipped with a dropping funnel. The flask was connected to a
flask containing a saturated NaOH solution and cooled in an ice bath.
With vigorous stirring, aniline (12.0 g, 129 mmol) was added dropwise
to the solution of oxalyl chloride through the dropping funnel. A white
solid was produced immediately, accompanied by a release of HCl
which was absorbed by the NaOH solution. After the addition, the
mixture was stirred for an additional half hour until heat release ceased.
Then, 10 mL of H2O was slowly added through the funnel, followed
by addition of another 100 mL of H2O. The precipitated white solid
was collected by filtration, washed with H2O, and dried overnight in
air and then in a vacuum. Yield: 14.45 g (95%). 1H NMR (δ, ppm in
DMSO-d6): 10.82 (s, 2H, NH), 7.86 (d, 4H, aromatic CH), 7.36 (t,
4H, aromatic CH), 7.14 (t, 2H, aromatic CH).
1
(s, 6H, -OCH3), 3.60 (s, 6H, -OCH3). H NMR (δ in CDCl3) for 3:
8.38 (s, 4H, -NCHN-), 8.27 (s, 2H, -NCHN-), 6.71 (d, 8H, aromatic
C-H), 6.69 (d, 8H, aromatic C-H), 6.42 (d, 16H, aromatic C-H),
6.34 (d, 4H, aromatic C-H), 6.18 (d, 4H, aromatic C-H), 6.10 (d,
4H, aromatic C-H), 6.04 (d, 4H, aromatic C-H), 5.76 (d, 8H, aromatic
C-H), 3.74 (s, 12H, -OCH3), 3.70 (s, 12H, -OCH3), 3.65 (s, 6H,
-OCH3), 3.63 (s, 6H, -OCH3), 3.60 (s, 6H, -OCH3). UV-vis λmax
(nm) (ꢀ, M-1 cm-1) for 2: 436 (2.0 × 103). UV-vis λmax (nm) (ꢀ, M-1
cm-1) for 3: 436 (1.8 × 103) Anal. Calcd for C104H100Mo4N14O14 (2):
C, 58.00; H, 4.68; N, 9.10. Found: C, 57.81; H, 4.58; N, 9.07. Anal.
Calcd for C106H104Mo4N14O14 (3): C, 58.35; H, 4.80; N, 8.99. Found:
C, 58.14; H, 4.84; N, 8.68.
Preparation of â Isomers 4 and 5. A scale and procedure similar
to the one described above was followed, but after removal of solvent
in the first step, the residue was treated with hot ethanol (ca. 50 °C)
instead of acetonitrile. Upon addition of the hot ethanol, a dark red
solid formed immediately; the mixture was stirred at this temperature
for half an hour and then cooled to room temperature. After the
supernatant solution was decanted, the solid was washed with 15 mL
of ethanol, followed by 15 mL of hexanes, and dried under vacuum.
The red solid was dissolved in 15 mL of dicloromethane, and the
solution was layered with 40 mL of hexanes. After diffusion over a
period of 1 week, large dark-red crystals of the â isomer, along with
yellow crystals of the corresponding R isomer, were formed. Manual
separation from the mixture yielded 0.103 g (24%) of 4 and 0.113 g
(26%) of 5. 1H NMR at 25 °C (δ in CDCl3) for 4: 6.62 (t, 2H, aromatic
C-H), 6.57 (m, 4H, aromatic C-H), 6.41(d, 8H, aromatic C-H),
6.35-6.23 (m, 24H, aromatic C-H), 6.14 (d, 16H, aromatic C-H)
5.85 (d, 4H, aromatic C-H) 3.74 (s, 6H, -OCH3), 3.69 (s, 12H,
-OCH3), 3.58 (s, 12H, -OCH3), 3.51 (s, 6H, -OCH3). At -50 °C:
8.57 (s, 4H, -NCHN-), 8.32 (s, 2H, -NCHN-), 6.60 (d, 4H, aromatic
C-H), 6.52 (t, 2H, aromatic C-H), 6.42-6.27 (m, 32H, aromatic
C-H), 6.12-6.03 (m, 16H, aromatic C-H), 5.86 (d, 4H, aromatic
C-H), 3.75 (s, 6H, -OCH3), 3.68 (s, 12H, -OCH3), 3.57 (s, 12H,
-OCH3), 3.45 (s, 6H, -OCH3). UV-vis λmax (ꢀ, M-1 cm-1) for 4:
Preparation of N,N′-Di-p-anisyloxamide. A procedure similar to
that just described was followed. With vigorous stirring, a solution of
p-anisidine, 16.5 g (134 mmol) in 50 mL of THF, was added dropwise
to a solution of oxalyl chloride, 8.33 g (65.7 mmol) in 100 mL of THF.
This reaction produced a white solid of N,N′-dianisyloxamide. Yield:
λ
max(1), 461 nm (2.0 × 103); λmax(2), 541 nm (1.2 × 104). 1H NMR at
(19) Brignole, A. B.; Cotton, F. A. Inorg. Synth. 1972, 13, 87.
25 °C (δ in CDCl3) for 5: 6.60 (d, 4H, aromatic C-H), 6.41 (d, 8H,
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13566 J. AM. CHEM. SOC. VOL. 125, NO. 44, 2003