Organometallics
Article
the resultant precipitate was washed with benzene (3 × 10 mL) until
the washings became colorless. The filtrate was evaporated, and the
product was crystallized from toluene/hexane and then dried under
vacuum to afford compound 1-PPh2(C6F5) as a dark purple solid (41
mg, 77%). 1H NMR (500 MHz, CDCl3): δH 8.12 (t, J = 7.5 Hz, 2H),
7.78 (m, 9H), 7.66 (d, J = 7.8 Hz, 3H), 7.63−7.51 (m, 8H), 7.51−
7.39 (m, 10H), 7.35 (t, J = 7.4 Hz, 7H), 7.26 (dt, J = 19.2, 6.1 Hz,
4H), 7.01 (t, J = 6.6 Hz, 4H), 6.83 (br s, 3H). 13C NMR (126 MHz,
CDCl3): δC 274.19 (dd, J = 80.5, 42.2 Hz), 163.87 (t, J = 20.5 Hz),
161.75 (q, J = 99.7, 49.8 Hz), 147.12 (q, J = 10.0 Hz), 145.38−144.92
(m), 144.85−144.20 (m), 143.88, 142.24−141.15 (m), 138.95−
138.18 (m), 136.50 (dd, J = 18.0, 13.4 Hz), 134.96−134.64 (m),
134.05, 133.83 (br s), 133.31, 132.25 (d, J = 14.0 Hz), 131.17, 130.97
(d, J = 1.9 Hz), 130.50 (t, J = 22.3 Hz), 128.88 (t, J = 5.2 Hz), 128.88
(qq, J = 31.4, 2.9 Hz), 128.56 (d, J = 10.0 Hz), 124.59 (q, J = 272.4
Hz), 117.45, 107.83−106.90 (m). 31P NMR (202 MHz, CDCl3): δP
47.38 (dd, J = 160.8, 30.2 Hz), 5.22−3.78 (m). HRMS (ESI-TOF)
m/z: [M]+ calcd for C55H38F5P3Rh, 989.1162; found, 989.1101. Anal.
Calcd for C87H50BF29P3Rh: C, 56.39; H: 2.72. Found: C, 56.25; H,
3.06.
providing an oxidized product in 78% yield, while 4-
bromobenzyl alcohol only provided the corresponding
aldehyde in 42% yield (Table 4, entries 3−5).
In comparison to many base-metal pincer systems, catalyst
1-PPh3 proved to be inefficient at alcohol oxidation.10
However, these reactions do provide an important proof of
concept for the ability of PCcarbeneP pincer complexes to
partake in other catalytic reactions.
In summary, pincer complexes 1-L (L = PPh3, PPh2(C6F5),
PCy3) have been shown to be capable of deoxygenating
trimethylamine oxide. Subsequent reduction of compound 2-
PPh3 to 1-PPh3 with isopropyl alcohol provides the basis for
the catalytic deoxygenation of N-oxides with complexes 1-L.
Employing optimized conditions for the deoxygenation of
trimethylamine N-oxide with isopropyl alcohol reductant,
catalysts 1-L enabled the deoxygenation of alkyl- and arylamine
N-oxides and of pyridine N-oxides. The catalysts could also be
employed to oxidize a range of alcohols in the presence of
trimethylamine oxide. These reactions provide a proof of
concept for the ability of PCcarbeneP complexes to enable
catalysis through ligand cooperativity.
General Procedure 2 for the Formation of Complexes 2-L.
The corresponding Rh precursor [RhCl(L)(COD)], the keto ligand
A, and Na[BArF ] were charged in a Schlenk followed by addition of
4
DCM. The resulting solution was stirred overnight at rt. The reaction
mixture was filtered, and the solvent was removed. After washing with
hexane (3 × 10 mL) and drying under vacuum the product was
obtained. If necessary, the product was further purified by
crystallization from DCM/n-hexanes.
EXPERIMENTAL SECTION
■
General Information. All manipulations were carried out under
nitrogen using a glovebox and/or Schlenk techniques. All reactions
were performed in glassware that was oven-dried for at least 12 h.
Benzene was distilled over sodium and benzophenone under a
nitrogen atmosphere and stored over 4 Å molecular sieves prior to
use. Diethyl ether and n-hexane were dried over activated alumina
using an LC Technology Solution Inc. SP-1 Solvent Purification
System and deoxygenated prior to use. The C6D6 used was stirred
over CaH2 at room temperature under a nitrogen atmosphere
overnight prior to distillation under reduced pressure and stored over
4 Å molecular sieves. Ligands A9a and B,11 metal complexes
[RhCl(COD)PPh3],12 [RhCl(COD)PCy3]13 (COD = 1,4-cyclo-
octadiene), 1-PPh3,5b and 1-PCy3,5b and Na[BArF ]14 ([BArF ]−=
Compound 2-PPh3. Prepared from [RhCl(COD)PPh3] (92.4 mg,
0.018 mmol), POP ligand A (100 mg, 0.18 mmol, 1 equiv), and
Na[BArF ] (177.1 mg, 0.18 mmol, 1 equiv) following general
4
procedure 2. Brown solid (238 mg, 74%). 1H NMR (500 MHz,
CD2Cl2): δH 8.14−8.09 (m, 2H), 7.80 (dtd, J = 7.4, 5.5, 1.7 Hz, 4H),
7.74 (dt, J = 5.0, 2.2 Hz, 8H), 7.64−7.48 (m, 14H), 7.30−7.22 (m,
9H), 7.11 (ddt, J = 7.4, 4.7, 2.3 Hz, 4H), 7.01 (td, J = 7.8, 2.4 Hz,
6H), 6.80−6.73 (m, 4H), 6.45 (dt, J = 7.5, 5.7 Hz, 4H). 13C NMR
(126 MHz, benzene-d6): δC 161.7 (d, J = 49.8 Hz), 146.9−146.6 (m),
146.1 (t, J = 7.2 Hz), 135.8 (t, J = 20.3 Hz), 135.3, 134.8 (tq, J = 3.4,
1.8 Hz), 133.9 (t, J = 6.9 Hz), 133.7 (d, J = 12.3 Hz), 132.7 (t, J = 5.8
Hz), 131.7−131.3 (m), 131.2−131.0 (m), 131.0 (t, J = 3.6 Hz),
130.94 (d, J = 1.1 Hz), 130.92 (d, J = 2.5 Hz), 130.8−130.6 (m),
130.1, 129.3 (t, J = 24.1 Hz), 129.2 (t, J = 5.4 Hz), 128.9 (dd, J =
31.6, 2.9 Hz), 128.3 (d, J = 10.9 Hz), 128.2 (t, J = 4.8 Hz), 128.0 (t, J
= 5.2 Hz), 124.6 (q, J = 272.4 Hz), 117.4 (p, J = 4.1 Hz). 31P NMR
(202 MHz, CD2Cl2): δP 35.40 (dt, J = 191.1, 33.0 Hz), 27.41 (dd, J =
123.1, 33.3 Hz). HRMS (ESI-TOF) m/z: [M]+ calcd for
C55H43OP3Rh, 915.1576; found, 915.1571. Anal. Calcd for
C87H55BF24OP3Rh: C, 58.74; H: 3.12. Found: C, 58.46; H, 3.04.
Compound 2-PCy3. Prepared from [RhCl(COD)PCy3] (95.6 mg,
0.018 mmol), POP ligand A (100 mg, 0.18 mmol, 1 equiv), and
4
4
[B(3,5-(CF3)2-C6H3)4]−) were prepared according to the reported
methods. All other reagents were purchased from commercial sources
and used as received. NMR spectroscopy data were obtained using
Bruker AV-300, AV-400, and AV-500 spectrometers. GC-MS studies
were performed on a Shimadzu QP-2010-SE GC-MS system. HRMS
(ESI-TOF) spectra were obtained using an Agilent Technologies
6230 TOF LC/MS instrument. IR spectroscopy data were obtained
using Bruker ALPHA FTIR spectrometers. 1H and 13C chemical shifts
are given in ppm relative to TMS, using the solvent signals as
references. 31P and 19F chemical shifts are given in ppm relative to
H3PO4 and CFCl3, respectively (external standards). X-ray diffraction
analysis was performed by the XRAY department at NUS. The X-ray
intensity data were measured on a Bruker D8 Venture dual-source
diffractometer. The crystal structures were solved by direct methods
using SHELXS-97 and refined with SHELXL-2014 using Olex3. The
crystals suitable for X-ray analysis were grown by slow diffusion of
different solvent mixtures of the metal complexes or by slow solvent
evaporation/cooling of saturated solutions of the corresponding
compounds. Where accurate elemental analysis could not be obtained,
a the compound purity was ascertained by 1H and 31P NMR
spectroscopies.
Na[BArF ] (177.1 mg, 0.18 mmol, 1 equiv) following general
4
procedure 2. Brown solid (253 mg, 78%). 1H NMR (500 MHz,
CDCl3): δH 7.96 (d, J = 7.9 Hz, 2H), 7.86 (q, J = 6.4 Hz, 4H), 7.77−
7.73 (m, 8H), 7.65−7.48 (m, 14H), 7.48−7.39 (m, 6H), 7.35 (q, J =
5.9 Hz, 4H), 7.21 (dt, J = 8.6, 5.0 Hz, 2H), 2.01−1.85 (m, 1H), 1.84−
1.72 (m, 6H), 1.65 (s, 3H), 1.56−1.21 (m, 15H), 1.03−0.86 (m, 3H),
0.43 (s, 5H). 13C NMR (126 MHz, CDCl3): δC 161.7 (q, J = 49.9
Hz), 148.0 (dd, J = 28.2, 17.8 Hz), 147.5 (t, J = 7.1 Hz), 135.0, 134.8,
134.4 (t, J = 5.5 Hz), 133.8, 133.6, 133.5 (t, J = 6.7 Hz), 132.9−132.5
(m), 131.5 (d, J = 20.7 Hz), 131.2, 131.1, 130.5 (t, J = 25.1 Hz),
130.1 (t, J = 3.5 Hz), 129.3 (t, J = 5.4 Hz), 129.2 (d, J = 4.6 Hz),
128.9 (dd, J = 31.5, 2.8 Hz), 128.2, 127.2 (t, J = 5.3 Hz), 124.6 (q, J =
272.5 Hz), 117.4 (t, J = 3.7 Hz), 37.8 (d, J = 23.4 Hz), 30.5, 26.6 (d, J
= 10.7 Hz), 25.3. 31P NMR (202 MHz, CDCl3): δP 45.96 (dt, J =
174.7, 27.9 Hz), 28.80 (dd, J = 129.6, 27.8 Hz). HRMS (ESI-TOF)
m/z: [M]+ calcd for C55H61OP3Rh, 933.2985; found, 933.3019. Anal.
Calcd for C87H73BF24OP3Rh: C, 58.15; H: 4.09. Found: C, 58.01; H,
4.20.
General Procedure 1 for the Formation of Complexes 1-L.
Compound 1-PPh2(C6F5). Toluene (20 mL) was added to a mixture
of proligand B (600 mg, 1.09 mmol) and [RhCl(COD)]2 (268 mg,
0.54 mmol, 0.5 equiv), and the resultant solution was stirred at room
temperature for 2 h. A dark orange precipitate was filtered and washed
with toluene (3 × 10 mL). To the obtained solid were added
diphenyl(pentafluorophenyl)phosphine (383 mg, 1.09 mmol, 1 equiv)
and Na[BArF ] (963 mg, 1.09 mmol, 1 equiv). The components were
4
dissolved in benzene (20 mL) to form an orange solution, which
turned dark purple overnight. The reaction mixture was filtered, and
Compound 3-PPh3. Prepared from [RhCl(COD)PPh3] (92.4 mg,
0.018 mmol) and POP ligand A (100 mg, 0.18 mmol, 1 equiv)
E
Organometallics XXXX, XXX, XXX−XXX