Organometallics
Article
out using Macherey-Nagel silica gel 60 (0.04−0.063 mm particle
size).
However, it should be noted that lower yields (59−63%) were
obtained in the case of aliphatic aldehydes and aromatic
ketones. Recently, we have found that simple reductive
amination between benzylacetone and piperidine can be
problematic for classical reducing systems.19 Therefore, we
tried to make some additional optimization. We managed to
increase the yield of 8 (from 60 to 74%) by reducing the
pressure and increasing the reaction time (see the Supporting
Information). Despite the additional optimization of con-
ditions for the synthesis of 7 and 9, we did not get a significant
improvement. Also, it is important to use a freshly prepared
catalyst; otherwise, there are significant discrepancies in the
results. The complex [2b](SbF6)2 is the most active for the
reductive amination with CO as a reducing agent among all
iridium catalysts.20 The highest turnover number (TON) is 82
per iridium, whereas the previously achieved value was 48 for
[CpIrI2]n.20b
[(η5-C5R5)M(η6-Fluorene)](SbF6)2 ([1](SbF6)2 and [2a,b]-
(SbF6)2). MeNO2 (4 mL) was added to a mixture of the iodide
complex [(η5-C5R5)MI2]n (0.149 mmol), fluorene (27 mg, 0.163
mmol), and AgSbF6 (107 mg, 0.311 mmol). The reaction mixture was
vigorously stirred for 1 h, and the precipitate of AgI was centrifuged
off. Then, an excess of ether was added. The precipitate that formed
was reprecipitated twice from nitromethane by ether and dried in
vacuo. Compounds [1](SbF6)2 and [2a,b](SbF6)2 were obtained as
colored solids.
1
[1](SbF6)2. M = Co, R = Me, yellow solid, yield 67 mg (50%). H
3
NMR (CD3NO2): δ 8.29 (d, J = 8.0 Hz, 1H, C5 or C8, fluorene),
8.00 (d, 3J = 8.0 Hz, 1H, C5 or C8, fluorene), 7.90 (m, 1H, C6 or C7,
fluorene), 7.83 (m, 1H, C6 or C7, fluorene), 7.63 (br. s, 1H, C1 or
C4, fluorene), 7.50 (br. s, 1H, C1 or C4, fluorene), 7.27 (br. s, 2H, C2
2
and C3, fluorene), 4.43 (d, J = 24.0 Hz, 2H, C9, fluorene), 4.34 (s,
3H, MeNO2), 1.97 (s, 15H, Cp*). 13C NMR (CD3NO2): δ 146.7 (s,
C4b or C8a, fluorene), 134.6 (s, C5−C8, fluorene), 130.1 (s, C4b or
C8a, fluorene), 129.6 (s, C5−C8, fluorene), 126.7 (s, C5−C8,
fluorene), 125.3 (s, C5−C8, fluorene), 123.0 (br. s, C4a and C9a,
fluorene), 110.8 (br. s, C5Me5), 104.0 (br. s, C1−C4, fluorene), 35.2
(s, C9, fluorene), 8.1 (s, C5Me5). Anal. Calcd for C23H25CoF12Sb2·
MeNO2: C, 32.28; H, 3.16. Found: C, 32.33; H, 3.31.
CONCLUSIONS
■
We developed a general approach to the η6-fluorene complexes
[(L)M(η6-fluorene)](SbF6)2 (L = Cp, Cp*, indenyl; M = Co,
Rh, Ir) based on the reaction of iodides [(L)MI2]n with
fluorene in the presence of AgSbF6. The fluorene ligand in the
rhodium complex [CpRh(η6-fluorene)](SbF6)2 proved to be
considerably more labile than benzene in [CpRh(η6-C6H6)]-
(SbF6)2. The facilitation of the fluorene replacement is reached
as a result of its spontaneous deprotonation and subsequent η6
→ η5 → η1 haptotropic rearrangements. The observed
“fluorene effect” makes fluorene a good leaving group, the
removal of which opens the way to a highly active system for
homogeneous catalysis. At the same time, this effect is of a
different nature than the classical indenyl and fluorenyl effects,
where generation of vacant coordination sites on the catalyst
results from a reversible change of the ligand hapticity. The
fluorene complexes proved to be active catalysts in the reaction
of reductive amination with carbon monoxide in water. The
activity of the synthesized (fluorene)rhodium catalysts is less
than that of the previously described (indenyl)rhodium ones,
whereas the (fluorene)iridium catalysts have shown the highest
activity among all iridium catalysts known.
1
[2a](SbF6)2. M = Rh, R = H, green solid, yield 60 mg (46%). H
NMR (CD3NO2): δ 8.30 (d, 3J = 8.0 Hz, 1H, fluorene), 8.24 (d, 3J =
3
8.0 Hz, 1H, fluorene), 8.11 (d, J = 6.0 Hz, 1H, fluorene), 7.83 (m,
2H, fluorene), 7.69 (m, 1H, fluorene), 7.63 (m, 1H, fluorene), 7.54
(m, 1H, fluorene), 6.61 (s, 5H, Cp), 4.50 (s, 2H, C9, fluorene), 4.34
(s, 3H, MeNO2). 13C NMR (CD3NO2): δ 146.8 (s, C4b or C8a,
fluorene), 135.0 (s, C5−C8, fluorene), 131.2 (s, C4b or C8a,
fluorene), 128.9 (s, C5−C8, fluorene), 126.7 (d, 1JRh−C = 4.0 Hz, C4a
1
or C9a, fluorene), 126.5 (d, JRh−C = 4.0 Hz, C4a or C9a, fluorene),
126.4 (s, C5−C8, fluorene), 125.0 (s, C5−C8, fluorene), 102.9 (d,
1
1JRh−C = 4.0 Hz, C1−C4, fluorene), 102.8 (d, JRh−C = 4.0 Hz, C1−
1
C4, fluorene), 100.7 (d, JRh−C = 4.0 Hz, C1−C4, fluorene), 96.2 (d,
1
1JRh−C = 7.0 Hz, Cp), 96.1 (d, JRh−C = 4.0 Hz, C1−C4, fluorene),
37.7 (s, C9, fluorene). Anal. Calcd for C18H15F12RhSb2·MeNO2: C,
26.33; H, 2.09. Found: C, 26.27; H, 2.37.
1
[2b](SbF6)2. M = Ir, R = H, cream solid, yield 101 mg (71%). H
NMR (CD3NO2): δ 8.41 (d, 3J = 8.0 Hz, 1H, fluorene), 8.20 (m, 2H,
fluorene), 7.83 (m, 2H, fluorene), 7.72 (m, 2H, fluorene), 7.59 (m,
2
1H, fluorene), 6.64 (s, 5H, Cp), 4.39 (s, 3H, MeNO2), 4.25 (d, J =
24.0 Hz, 2H, C9, fluorene). 13C NMR (CD3NO2): δ 146.2 (s, C4b or
C8a, fluorene), 135.0 (s, C5−C8, fluorene), 130.6 (s, C4b or C8a,
fluorene), 128.7 (s, C5−C8, fluorene), 126.1 (s, C5−C8, fluorene),
125.1 (s, C5−C8, fluorene), 120.6 (s, C4a or C9a, fluorene), 118.3 (s,
C4a or C9a, fluorene), 94.6 (s, C1−C4, fluorene), 94.5 (s, C1−C4,
fluorene), 93.1 (s, C1−C4, fluorene), 89.2 (s, Cp), 87.9 (s, C1−C4,
fluorene), 37.5 (s, C9, fluorene). Anal. Calcd for C18H15F12IrSb2·
MeNO2: C, 23.87; H, 1.89. Found: C, 24.26; H, 1.90.
EXPERIMENTAL SECTION
General. All reactions were carried out under an argon atmosphere
in anhydrous solvents, which were purified and dried using standard
■
1
procedures. Isolation of all products was carried out in air. H and
13C{1H} NMR spectra were recorded on a Varian Inova 400
spectrometer operating at 400.13 and 100.61 MHz, respectively.
Chemical shifts are reported in ppm using the residual signals of the
solvents as internal standards. The signals of indenyl and fluorene
systems in the NMR spectra and X-ray diffraction data were assigned
according to the IUPAC recommendation (Scheme 4).21 Starting
materials [Cp*CoI2]2,22 [CpRhI2]n,23 [CpIrI2]n,24 [(η5-indenyl)-
[(η5-Indenyl)M(η6-Fluorene)](SbF6)2 ([3a,b](SbF6)2). MeNO2
(4 mL) was added to a mixture of the iodide complex [(η5-
indenyl)MI2]n (0.148 mmol), fluorene (27 mg, 0.163 mmol), and
AgSbF6 (107 mg, 0.311 mmol). The reaction mixture was vigorously
stirred for 1 h, and the precipitate of AgI was centrifuged off. Then, an
excess of ether was added. The precipitate that formed was
reprecipitated twice from nitromethane by ether and dried in vacuo.
Compounds [3a,b](SbF6)2 were obtained as yellow solids.
25
RhI2]n,2b and [(η5-indenyl)IrI2]n were prepared as described in
the literature. All other reagents were purchased from Acros or
Aldrich and used as received. Column chromatography was carried
1
[3a](SbF6)2. M = Rh, yield 75 mg (57%). H NMR (CD3NO2): δ
7.91 (m, 5H, fluorene and indenyl), 7.78 (m, 2H, C4−C7, indenyl),
7.65 (m, 1H, fluorene), 7.49 (m, 3H, fluorene), 7.31 (m, 1H,
fluorene), 7.25 (br. s, 1H, C1 or C3, indenyl), 7.19 (m, 1H, fluorene),
7.14 (br. s, 1H, C1 or C3, indenyl), 6.60 (br. s, 1H, C2, indenyl), 4.39
(s, 3H, MeNO2), 4.13 (d, 2J = 24.0 Hz, 2H, C9, fluorene). 13C NMR
(CD3NO2): δ 146.5 (s, C4b or C8a, fluorene), 140.4 (s, C4−C7,
indenyl), 139.7 (s, C4−C7, indenyl), 134.7 (s, C5−C8, fluorene),
128.9 (s, C5−C8, fluorene), 127.9 (s, C4b or C8a, fluorene), 126.0 (s,
Scheme 4. Numbering Scheme of the Carbon Atoms in the
Indenyl and Fluorene Ligands
1
C4−C7, indenyl), 125.9 (s, C4−C7, indenyl), 125.1 (d, JRh−C = 4.0
1
Hz, C4a or C9a, fluorene), 123.6 (d, JRh−C = 4.0 Hz, C4a or C9a,
E
Organometallics XXXX, XXX, XXX−XXX