Rhodium(I) Ferrocenylcarbene Complexes: Synthesis, Structural Determination, Electrochemistry, and Application as Hydroformylation Catalyst Precursors

Article abstract of DOI:10.1021/acs.organomet.5b00843

New examples of the rare class of rhodium(I) ferrocenyl Fischer carbene complexes 1-8, [Rh(LL)Cl{C(XR)Fc}] [LL = cod, (CO)₂, (CO, PR₃) (R = Ph, Cy or OPh), and (CO, AsPh₃); XR = OEt or NHⁿPr] were prepared, and the electronic effects of coligands and alkoxy vs aminocarbene substituents were investigated by spectroscopic and electrochemical methods. The molecular structures of complexes 1, 2, and 4-6 were confirmed by single-crystal X-ray diffraction. The use of the complexes 1-8 as homogeneous catalysts for the hydroformylation of 1-octene was demonstrated, and the influence of the carbene substituents and coligands on the activity and regioselectivity of the catalysts evaluated. Finally, the stability of the Rh-C_carbene bond of complex 1 under hydroformylation conditions was confirmed with ¹³C NMR experiments.

Full text of DOI:10.1021/acs.organomet.5b00843

Article
pubs.acs.org/Organometallics
Rhodium(I) Ferrocenylcarbene Complexes: Synthesis, Structural
Determination, Electrochemistry, and Application as
Hydroformylation Catalyst Precursors
†
†
†
‡
‡
́ ́
G. Kabelo Ramollo, María J. Lopez-Gomez, David C. Liles, Leah C. Matsinha, Gregory S. Smith,
,†
and Daniela I. Bezuidenhout*
†
Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield 0028, South Africa
‡
Department of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa
*
S Supporting Information
ABSTRACT: New examples of the rare class of rhodium(I)
ferrocenyl Fischer carbene complexes 1−8, [Rh(LL)Cl{C-
(
XR)Fc}] [LL = cod, (CO) , (CO, PR ) (R = Ph, Cy or
2 3
n
OPh), and (CO, AsPh ); XR = OEt or NH Pr] were prepared,
3
and the electronic effects of coligands and alkoxy vs
aminocarbene substituents were investigated by spectroscopic
and electrochemical methods. The molecular structures of
complexes 1, 2, and 4−6 were confirmed by single-crystal X-
ray diffraction. The use of the complexes 1−8 as homogeneous
catalysts for the hydroformylation of 1-octene was demonstrated, and the influence of the carbene substituents and coligands on
the activity and regioselectivity of the catalysts evaluated. Finally, the stability of the Rh−C
bond of complex 1 under
carbene
hydroformylation conditions was confirmed with 13C NMR experiments.
INTRODUCTION
electron-withdrawing phosphane or phosphite as coligand.
This conclusion prompted the investigation into the use of
■
The importance of optimizing the ligand structure of
homogeneous rhodium(I) complexes for the industrially
important hydroformylation reaction is well-known, where
sufficient electron density on the rhodium(I) center is required
for maximum conversion of terminal olefins to the more
preferred linear aldehydes as the desired hydroformylation
electrophilic Fischer carbene complexes of rhodium(I) as5
catalyst precursors for the hydroformylation of 1-octene.
Examples of isolated rhodium(I) Fischer carbene complexes are
6
rare, although they have been implicated as the catalytically
7
8
active intermediates in the cyclization of allenes and alkynes
1
with group 6 Fischer alkenylcarbene complexes.
product for most applications. As the catalytic mechanism for
The use of rhodium(I) Fischer carbene complexes in the
catalytic hydroformylation of alkenes is unexplored in the
literature. Herein we report the synthesis of a range of
ferrocenyl-substituted rhodium(I) Fischer carbene complexes
and the investigation of their application as catalysts in the
hydroformylation reaction of 1-octene. This constitutes one of
the few examples of the catalytic use of Fischer carbene
this conversion is well-understood, the modulation of the steric
and electronic properties of the commonly used phosphane
ligands has been directed toward the optimization of both the
activity and the selectivity of these rhodium-based catalysts.
Specifically, an increase in the n/iso ratio of the aldehydes has
been found due to a high steric demand around the rhodium
center when excess phosphanes were employed or when
phosphanes with stronger π-acceptor properties were used.
With the advent of the versatile N-heterocyclic carbenes
NHCs), a new class of carbon-based ligands with similar
binding properties to phosphanes was introduced. NHCs, with
9
complexes. Ferrocene was included as a carbene substituent in
1
an effort to stabilize the electrophilic Fischer carbene ligand, in
order to prevent carbene ligand dissociation. This approach of
including a second metal also confers possible advantages such
as improved activity and increased reaction rates, specifically
involving examples where one metal acts as the main catalytic
(
2
their strong σ-donor and poor π-accepting properties, yield
stable metal−NHC complexes, and a number of rhodium(I)−
NHC complexes have been reported for application in the
3
10
center whereas the other metal serves as an electron reservoir.
4
hydroformylation of olefins. Significant improvement in the
RESULTS AND DISCUSSION
■
alkene conversion, activity, and selectivity to the linear
aldehydes was observed when increasing the electron-with-
drawing capacity of the modifying ligand. This can be achieved
by the reduction of the strong electron-donor properties of the
NHC ligand via inclusion of electron-withdrawing N-
substituents on the NHC or by combination with an
Synthesis and Characterization of Rhodium(I) Ferro-
cenylcarbene Complexes. Until the independent reports of
Received: October 6, 2015
Published: December 14, 2015
©
2015 American Chemical Society
5745
DOI: 10.1021/acs.organomet.5b00843
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Organometallics
Article
Scheme 1. Synthesis of Rhodium(I) Ferrocenyl Fischer Carbene Complexes 1−8
Figure 1. Molecular structures of (a) 1, (b) 2, (c) 4, (d) 5, and (e) 6, showing 50% probability ellipsoids and partial atom-numbering schemes. Two
CH Cl solvent molecules were omitted from the structure of 5 for clarity.
2
2
the catalytic transfer of a carbene ligand from a group 6 Fischer
carbene complex to a palladium reagent by Sierra and co-
with [Rh(cod)Cl] (cod = 1,5-cyclooctadiene), in a modified
2
procedure from that described by Barluenga et al. for the
1
1
12
6a
workers and Narasaka and co-workers, the use of Fischer
carbene ligands to adjust the reactivity of late transition metal
complexes was mostly undeveloped, and the first examples of
cationic rhodium(I) Fischer carbene complexes obtained via a
transmetalation reaction from the pentacarbonyl group 6
preparation of neutral rhodium(I) alkenylcarbene complexes;
however only the self-dimerization products could be isolated
after column chromatography. The decomposition self-
dimerization products include a carbene−carbene-coupled
olefin as well as the regeneration of the rhodium precursor,
analogously to the carbene dimerization decomposition
6b,13
transition metal precursor were only reported thereafter.
15
This investigation was initiated by first attempting the
observed for other late transition metal Fischer carbenes.
stoichiometric transmetalation of the chromium(0) heteroaryl
ethoxycarbene complexes (heteroaryl = 2-thienyl or 2-furyl)
It was reasoned that the electrophilic Fischer carbene ligand
in Barluenga’s system was stabilized by the stronger donating
14
5
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Organometallics
Article
NHC coligand, and hence it was decided to similarly
circumvent self-dimerization by modifying the group 6 Fischer
carbene precursor to contain the more donating, redox-active
Table 1. Spectroscopic Data for Complexes 1−8
b
IR
13
a
13
1
a
b
Cδ(Ccarbene),
C δ(CO),
IR
ν (CO),
av
16
1
e
ferrocenyl (Fc) substituent.
( J(RhC) and
J(PC) (Hz))
( J(RhC) and
J(PC) (Hz))
ν(CO)
TEP
2
2
−1
−1
complex
(cm
)
(cm
)
The precursor chromium(0) ferrocenyl ethoxycarbene
17
complex was prepared according to literature procedures,
1
2
3
302 (d, 43)
258 (d, 40)
289 (d, 38)
dissolved in a dichloromethane solution with an equimolar
amount of rhodium precursor at room temperature, and stirred
for 14 days. The desired product [Rh(cod)Cl{C(OEt)Fc}] (1)
was isolated in high yield (95%) after column chromatography,
whereafter it was employed as starting material for the
preparation of complexes 2−8 (see Scheme 1). To compare
with a more donating carbene ligand, the ethoxycarbene
substituent was replaced with an n-propylamino substituent by
direct aminolysis of the alkoxycarbene with the primary amine
c
d
187 (d, 50)
2001
2084
1995
2077
1965
1946
1986
1966
2042
2054
2036
2049
d
c
e
183 (d, 77)
c
d
c
4
238 (d, 35)
187 (d, 51)
d
e
184 (d, 78)
d
d
d
d
d
d
d
5
6
7
8
300 (dd, 110, 40) 188 (dd, 81, 16)
303 (dd, 103, 39) 189 (dd, 83, 17)
298 (dd, 174, 39) 185 (dd, 79, 21)
d
295 (d, 45)
187 (d, 80)
1
8
a
b
c
in diethyl ether. The availability of the nitrogen lone pair for
donation toward the carbene carbon results in an increased
Recorded in CDCl . Recorded in CH Cl . CO ligand trans to
3 2 2
d e
carbene. CO ligand trans to Cl. Calculated using the linear
regression model TEP = 0.8001ν (CO)Rh + 420 cm .
−1
3a
19
Ccarbene−N bond order. Surprisingly, both NMR and single-
av
crystal X-ray diffraction studies confirm that only the anti-
n
aminocarbene isomer [Rh(cod)Cl{C(NH Pr)}] (2, 75%) is
are relatively insensitive to the changes in the electronic
properties of the metal. The infrared carbonyl stretching
frequencies of the monocarbonyl carbene complexes 5−8
obtained due to restricted rotation around the C
and the steric bulk of the ferrocenyl substituent.
−N bond
carbene
2
0
The substitution of the cyclooctadiene ligand was also
effected, in an effort to investigate the effect of other π-acceptor
ligands in the metal coordination sphere with regard to the
activity and selectivity of the prepared complexes as hydro-
formylation catalysts. To this end, carbon monoxide gas was
bubbled through dichloromethane solutions of 1 and 2 at −10
(
Table 1) give a better reflection of the donating ability of the
electronic environment around the Rh(I) center and correlate
with the expected donor ability of the ligands in the order PCy
3
21
>
PPh , AsPh > P(OPh) . In the case of the two dicarbonyl
3 3 3
carbene complexes 3 and 4, the ν (CO) could be used to
av
estimate and compare the stereoelectronic properties of the
ferrocenyl ethoxycarbene ligand and the ferrocenyl amino-
carbene ligand with each other and known imidazolylidene-
based NHCs. This was done by calculating the TEP (Tolman
electronic parameter) using the simple linear regression model
°
C or at room temperature, respectively. The dicarbonyl
carbene complex 3, [Rh(CO) Cl{C(OEt)Fc}] (65%), and 4,
2
n
[
Rh(CO) Cl{C(NH Pr)Fc}] (84%), were isolated after
2
recrystallization by layering the dichloromethane reaction
mixture with hexanes. The anti-conformation around the
Ccarbene−N bond of the aminocarbene ligand of 4 is retained,
with the H atom on the amino moiety orientated toward the Fc
substituent (see Figure 1). Finally, one of the carbonyl ligands
of 3 could be substituted by a phosphane, phosphite, or arsane
3a
reported by Glorius to correlate ν (CO) of [RhCl(carbene)-
av
(CO) ] with the TEPs for the [LNi(CO) ] system originally
2
3
2
1
described by Tolman and expanded for [IrCl(carbene)-
(CO)2] and [RhCl(carbene)(CO) ]. The TEPs calculated
22
23
2
−1
−1
for 3 (2054 cm ) and 4 (2049 cm ) demonstrate, to the best
of our knowledge, for the first time the comparable donor
strength of both ferrocenylcarbene ligands with known
saturated and unsaturated NHCs with TEPs ranging between
ligand, by reaction of one equivalent of PR (R = Ph, Cy, or
3
OPh) or AsPh in dichloromethane at room temperature to
3
yield complexes [Rh(CO)(L)Cl{C(OEt)Fc}] 5 (L = PPh ), 6
3
2055 and 2049 cm−
1
22b
and also indicate the stronger donor
(
L = PCy ), 7 {L = P(OPh) }, and 8 (L = AsPh ) in yields
3
3
3
ranging from 63% to 83%.
character of the aminocarbene vs the alkoxycarbene ligands.
The molecular structures of 1, 2, and 4−6 were confirmed by
single-crystal X-ray structure analyses (Figure 1), and selected
bond distances and angles are presented in Table 2. The
complexes display pseudo-square-planar (1, 2) or square-planar
Compounds 1−8 were characterized by NMR and FT-IR
spectroscopy and mass spectrometry, and single crystals for X-
ray diffraction could be obtained for 1, 2, and 4−6. The
spectroscopic data clearly verify the presence of the carbene
and the CO ligands, with Rh−C
doublet resonances for
the ethoxycarbene complexes 1, 3, and 5−8 in the range 289−
geometry (4−6) at the rhodium(I) center, with Rh−C
carbene
carbene
bond distances ranging from 1.958(2) to 2.061(2) Å. The Rh−
303 ppm, and for the aminocarbene complexes 2 and 4 the
C
bond lengths are comparable to those reported for
carbene
values are 258 and 238 ppm, respectively. These values are
slightly upfield from those reported by Barluenga and co-
workers (307−314 ppm for the alkoxycarbene complexes and
previously isolated Rh(I) Fischer carbene complexes (1.930−
6
2.113 Å). The increased Rh−C carbene distance of the
aminocarbene complex 2 (2.0178(13) Å) compared to the
ethoxy analogue 1 (1.958(2) Å) is indicative of the greater
carbene carbon stabilization from the N-heteroatom compared
to the O-carbene substituent and resultant decreased π-back-
bonding required from the rhodium metal toward the carbene
carbon atom. Likewise, shorter Ccarbene−N bond distances
(1.3083(18) Å for 2; 1.305(5) Å for 4) compared to the
Ccarbene−O bond lengths (1.3116(16)−1.322(3) Å for 1, 5, and
6a
2
43−245 ppm for the aminocarbene complexes) (see Table
1). Presumably the upfield shift is due to the increased donating
ability of the ferrocenylcarbene ligand compared to the
1
13
alkenylcarbenes. In both the H and C NMR spectra of
compounds 1−8, four different proton and carbon resonances,
respectively, are observed for the substituted cyclopentadienyl
ring of the ferrocenyl moiety. This clearly indicates the
hindered rotation of the ferrocenyl group in contrast to the
symmetric substitution pattern observed (only two proton and
carbon resonances, respectively) for the precursor chromium
6) and less acute N−C
−C bond angles (115.56(12)° for
carbene
Fc
2, 116.1(2)° for 4) compared to the O−C
−C bond
carbene
Fc
angles (109.3(2)−111.5(4) Å for 1, 5, and 6) also attest to the
14,19
13
ferrocenylcarbene complex.
The carbonyl C resonances
increased C
−N bond order. Additionally, the effect of the
carbene
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Table 2. Selected Bond Lengths (Å) and Angles (deg) for the Complexes 1, 2, and 4−6
1
2
4
5
6
Bond Lengths
2.018(1)
Rh−Ccarbene
Ccarbene-O/N
Ccarbene−Cipso‑Fc
Rh−Cl
1.958(2)
1.322(3)
1.448(3)
2.375(1)
2.005(1)
2.166(1)
2.06(1)
2.017(4)
1.312 (7)
1.448(6)
2.381(1)
1.832(6)
2.027(1)
1.312(2)
1.441(2)
2.374(1)
1.799(2)
2.357(1)f
1.308(2)
1.305(3)
1.470(3)
2.374(1)
1.826(2)
1.932(3)
1.467(2)
2.391(1)
ab
a
c
b
Rh−Y or Rh−CO
1.988(1)
trans
e
c
,
d
f
d
e
f
Rh−Y or Rh−COcis or Rh−P
Bond Angles
2.104(1)
2.329(1)
Ccarbene−Rh−Cl
O/N−Ccarbene−Cipso‑Fc
Torsion Angles
94.90(7)
109.3(2)
87.61(4)
115.6(1)
88.12(6)
116.1(2)
86.4(1)
86.28(4)
110.5(1)
111.5(4)
Cα‑Fc−Cipso‑Fc−Ccarbene−O/N
1.8(3)
3.4(2)
11.5(3)
d
3.4(2)
1.9(2)
a
b
c
Y = midpoint of C(5)−C(6). Y = midpoint of C(1)−C(6). Y= midpoint of C(1)−C(2). Y = midpoint of C(9)−C(10).
π-acidic carbonyl ligands in 4 (Rh−Ccarbene distance = 2.061(2)
Å) on the π-back-donation of the metal to the carbene carbon
Table 3. Potentials (V) for the Three Redox Processes
+
Observed for Complexes 1−8 vs the Ag/Ag Couple Using
+1/0
in 2 (Rh−C
distance = 2.0178(13) Å) results in a
significant increase of the carbene carbon−rhodium bond
the Redox Couple [Fe(η-C Me ) ]
as Internal Standard
carbene
5
5 2
in the Test Solutions
length, similar to the effect of the coligands CO and PR (5, R
3
Epox (V) [Rh(I/II)],
E°′ (V)
=
Ph, Rh−C
2.018(4) Å; 6, R = Cy, Rh−C
complex Epred (V) [RhC/ Rh−C ] [Fe(II/III)]
−
•
carbene
carbene
[Rh(II/III)]
2
1
.0267(3) Å) compared to the more donating cod ligand in
(Rh−C 1.958(2) Å).
1
2
3
4
5
6
7
8
−2.42
−2.60
−1.95
−2.30
−2.30
−2.34
−2.16
−2.20
0.25
0.13
0.36
0.24
0.28
0.28
0.31
0.29
0.80, 1.02
carbene
0.52, 0.69
b
In like manner, the Rh−CO bond lengths of the carbonyl
ligands trans to the chlorido ligand are all significantly shorter
a
0.87
(
1.799(2)−1.832(6) Å) than that of the Rh−CO bond length
0.54, 0.59
0.66, 0.79
of the CO trans to the carbene ligand in 4 (1.932(3) Å). The
steric bulk of the coligand trans to the carbene influences the
a
0.92
rotational freedom of the ferrocenyl moiety. The Cα‑Fc
−
0.61, 0.71
Cipso‑Fc−C
−O/N torsion angles for the bulky cod- (1,
carbene
a
b
Overlapping waves of [Rh(I/II)] and [Rh(II/III)]. Not observed in
1
.8(3)°; 2, 3.4(2)°) and the phosphane-substituted complexes
the solvent window employed.
(5, 3.4(2)°; 6, 1.9(2)°) are significantly smaller than the torsion
angle of 11.5(3)° observed for the dicarbonyl complex 4 (see
Table 2).
ox
peak potentials at E°′ = 0.25 V and E = 0.80 and 1.02 V
p
Cyclic Voltammetry. The approach to monitor changes in
the carbonyl stretching frequencies of carbonyl complexes via
infrared spectroscopy is the classic method to evaluate the
electron-donating properties of ligands. However, the electro-
chemical approach, where the redox potentials are determined
by cyclic voltammetry, is a more sensitive tool for determining
the electronic environment surrounding the central rhodium(I)
metal. The redox potentials of Ru(II/III) metal complexes have
been used to establish the Lever electronic parameters (LEP),
which reflect the donor capacity of the ligands bound to the Ru
(
Figure 2a (2)). When the scan was curtailed at 0.4 V (Figure
a (1)), the first oxidation wave was reversible. Thus, the one-
electron oxidation product of 1, [Rh(cod)Cl{C(OEt)Fc}] , is
stable on the CV time scale.
2
+
This result is consistent with that shown for 7 in Figure 2c.
The CV of 2 from −2.71 to 0.79 V (Figure 2b) shows one
red
reduction wave with peak potential E = −2.60 V and three
p
ox
oxidation waves with peak potentials at E°′ = 0.13 V and E
=
p
0
.52 and 0.69 V. The CVs also show no overlap between the
wave corresponding to Fe(II/III) and that of the redox
potentials of the Rh(I/II) and Rh(II/III) processes. The
similarity of the Fe(II/III) redox potential values of the
ferrocenylcarbene substituents (the only waves that meet the
criteria for a fully reversible system in our series) allows only in
certain cases the data to be useful as a comparative tool to
discriminate between the overall electronic effect of the more
(aminocarbene) or less (ethoxycarbene) donating carbene
ligands and the coligands (cod; (CO) ; CO, PR (R = Ph, Cy,
2
4
metal. Although correlations between LEPs and TEPs are
2
5
rare, correlations between ν(CO) in [Rh(CO) Cl(carbene)]
2
23
and the Rh(I/II) redox potentials have been reported. Also,
the redox potentials do not directly provide information on the
electron-donating capacity of a given ligand, but represent the
energy difference between the reduced metal complex and the
oxidized metal complex.
The cyclic voltammograms (CVs) of 1−8 at a glassy carbon
2
3
electrode in CH Cl show irreversible reduction of the carbene
or OPh), and CO, AsPh ). For example, there is a significant
2
2
3
at ca. −2.3 V, reversible oxidation of the ferrocenyl moiety at ca.
.2 V, and ill-defined peaks related to the Rh(I/II) and Rh(II/
difference of 0.11 V between E°′ observed for the Fe(II/III)
redox potential values of 4 and its cod analogue 2 (which can
be oxidized at lower potentials because it has a more donating
ligand). However, the irreversible nature of the Rh(I/II) and
0
III) couples at higher potentials, ranging between 0.52 V for 2
+
and 1.02 V for 1 (vs Ag/Ag , E°′ = −0.54 V for the couple
5
+1/0
ox
[
Fe(η -C Me ) ]
as an internal standard, referenced to the
Rh(II/III) processes (tabulated as Ep in Table 3) means that a
5
5 2
ferrocene/ferrocenium couple at 0 V).
The values obtained for 1−8 are summarized in Table 3. The
CV of 1, from −1.05 to 1.3 V, shows three oxidation waves with
correlation between the LEPs and TEPs is impossible.
The irreversible reduction wave at negative potential
corresponds to the one-electron reduction of the RhC
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n
Figure 2. Cyclic voltammograms of (a) [Rh(cod)Cl{C(OEt)Fc}] (1), (b) [Rh(cod)Cl{C(NH Pr)Fc}] (2), and (c) [Rh(CO){P(OPh) }Cl{C-
3
−1
(
OEt)Fc}] (7), respectively, at a glassy carbon electrode, scan rate 0.1 V s in CH Cl , with the internal standard used (marked as Fc*).
2 2
a
Table 4. Hydroformylation of 1-Octene with Catalyst Precursors 1−8
b
catalyst
% conversion
% total aldehydes
% internal octenes
% n-aldehydes
n/iso ratio
TOF
1
100
100
0
44 (4.0)
62(0.1)
70(2.1)
62(8.7)
55(3.0)
50(1.2)
57(3.8)
49(2.7)
55(1.1)
51(0.5)
57(2.7)
0.79 (0.130)
1.63(0.01)
418 (14.2)
256(51.7)
233(29.0)
204(40.0)
366(9.0)
379(14.1)
343(8.3)
407(0.9)
380(3.9)
409(27.2)
346(10.0)
c
1
91(1.0)
98(0.6)
90(10.1)
100
63(11.1)
58(7.9)
55(4.0)
90(2.3)
100
37(11.1)
42(7.9)
45(4.0)
10(2.3)
0
d
1
e
2.37(0.240)
1.69(0.610)
1.25(0.160)
0.98(0.047)
1.33(0.210)
0.98(0.100)
1.20(0.050)
1.06(0.020)
1.32(0.140)
1
2
3
4
5
6
7
8
100
99(0.1)
100
85(0.5)
100
15(0.5)
0
100
94(1.2)
100
6(1.2)
0
100
100
85(2.4)
15(2.4)
a
Reactions were carried out with CO/H (1:1) at 40 bar, 80 °C in toluene (5 mL) with 6.37 mmol of 1-octene and 0.0039 mmol of Rh catalyst.
2
After 4 h, the GC conversions were obtained using n-decane as an internal standard in relation to authentic standard internal octenes and aldehydes.
b
−1
Reactions were performed in triplicate, and the standard deviations are given in parentheses for all results. TOF = (mol aldehydes/mol cat.) h .
c
d
e
Reaction conditions 40 bar, 70 °C, 4 h. Reaction conditions 30 bar, 80 °C, 4 h. Reaction conditions 20 bar, 80 °C, 4 h.
−
•
−1
bond to Rh−C , similarly to the reduction of carbene ligands
The TOF values ranged from 343 to 418 h , where the
14b,16a
of group 6 Fischer carbene complexes.
In this case, a clear
ethoxycarbene complexes consistently display higher activities
than the analogous aminocarbene complexes 2 and 4. The
effect of the donating capacity of the coligands also results in
the general trend for activity (TOF) of the ferrocenylcarbene
complexes, arranged in order of decreasing activity: (CO) >
CO, L (PR , AsPh ) > cod. In contrast, the n/iso selectivity
qualitative trend of the overall electron-withdrawing ability of
the carbene ligand and the coligands can be established,
corresponding to the trend observed for the IR carbonyl
stretching frequencies of the complexes 1−8. The reduction
potentials can be arranged in order of decreasing negative
2
3
3
n
values for 2 (cod, NH Pr; −2.60 V) > 1 (cod, OEt; −2.42 V) >
(regioselectivity) displays the reversed trend, with n/iso
(aminocarbene 2, 4) > n/iso (ethoxycarbene 1, 3), contrary
to the expected improved selectivity for more electron-
withdrawing carbene ligands. Again this trend is also reflected
by the results obtained for complexes 5−8, where the most
6
(
(
(PCy , CO, OEt; −2.34 V) > 5 (PPh , CO, OEt; −2.30 V); 4
3
3
n
(CO) , NH Pr; −2.30 V) > 8 (AsPh , CO, OEt; −2.20 V) > 7
2
3
P(OPh) , CO, OEt; −2.16 V) > 3 ((CO ), OEt; −1.95 V),
3
2
where the more electron-withdrawing carbene and coligands
display greater ease of reduction.
donating phosphane coligand (PCy , 6) displays a higher n/iso
3
Hydroformylation of 1-Octene. Complexes 1−8 were
evaluated as catalyst precursors in the hydroformylation of 1-
octene. The reaction conditions for 1 were optimized by
variation of the syngas pressure (20−40 bar), temperature
ratio than for example 5 (PPh ) or 7 (P(OPh) ). However, this
3
3
observation should take into account the overall chemo-
selectivity, as the n/iso ratio generally decreases as the total
percentage of aldehyde formation increases.
(
70−90 °C), and reaction time (4−8 h). Under the optimized
Examples of very active [Rh(cod)X(NHC)] complexes as 1-
octene hydroformylation catalyst precursors, with TOFs
conditions of 40 bar and 80 °C, the catalyst precursors
displayed excellent conversion (>99%) of 1-octene after 4 h, as
well as good chemoselectivity (85−100%) toward aldehydes
−1
ranging from 480 to 3540 h , were reported by Weberskirch
27
and co-workers; however the selectivity decreased to n/iso
ratios of less than 0.50 close to full conversion due to olefin
isomerization. In contrast, Trzeciak et al. reported excellent
selectivities with n/iso ratios in the range 16−27 with the
addition of phosphorus ligands to the [Rh(cod)Cl(NHC)]
catalysts, but this occurred at the expense of low aldehyde
(Table 4). Only moderate regioselectivity was observed with n/
iso-aldehyde ratios ranging from 0.79 to 1.33, generally favoring
the formation of linear aldehydes. A mercury drop-test was
performed on catalyst 1 with no resulting significant change in
either the conversion or chemo/regioselectivity of the catalyst,
thereby indicating that a heterogeneous catalytic mode of
28
yields of 18−26%. Examples of dinuclear Rh-NHC complexes
26
action can be excluded.
featuring a bridging bisNHC-pyridyl ligand showed full
5
749
DOI: 10.1021/acs.organomet.5b00843
Organometallics 2015, 34, 5745−5753

Organometallics
Article
conversion to the aldehydes, but mostly branched aldehydes
−1
29
dichloromethane was distilled over CaH
commercially available and were used as received.
NMR spectra were recorded on Bruker Ultrashield Plus 400
AVANCE 3 and Bruker Ultrashield 300 AVANCE 3 spectrometers
2
. All other reagents are
(86−100%) were obtained with TOF values of 3.2−15.7 h .
To address the issue of the Rh−C bond stability, a
carbene
reaction similar to those proposed to test Rh−NHC bond
4d,26,29
using CDCl and C D as solvents at 25 °C. The NMR spectra were
3 6 6
13
stability was performed.
charged with 1 (0.03 g, 0.06 mmol) and substrate 1-hexene
0.50 mmol, 0.6 mL) dissolved in C D (0.70 mL). The
A high pressure NMR tube was
1
recorded for H at 300.13 MHz, C at 100.163 and 75.468 MHz, and
31
P at 161.976 and 121.495 MHz. The chemical shifts were recorded in
(
6
6
ppm, using deuterated solvent signals for internal references: for
reaction vessel was sequentially pressurized with CO(g) and
CDCl and C D respectively δH at 7.2600 and 7.1500 ppm and δC at
3
6
6
31
1
H (g) and heated at 80 °C for 8 h. The immediate formation of
77.360 and 128.000 ppm. The P{ H} NMR spectra were referenced
to the deuterated lock solvent, which had been referenced to 85%
2
1
3
1
3
from 1 was observed (see Figure S20(a), SI: C{ H} NMR δ
2
90.8 (d, J = 39.0 Hz, Rh−C
), 188.2 (d, J = 49.8 Hz, Rh−
H
3
PO
4
. Infrared spectroscopy was performed on a PerkinElmer
carbene
Spectrum RXI FT-IR spectrophotometer over the range 3400 to 1600
CO) and 184.0 (d, J = 75.7 Hz, Rh−CO)). After 8 h of heating,
a new carbene chemical shift and a new broadened carbonyl
−
1
cm . Solution IR spectra were recorded in CH Cl using a NaCl cell
2
2
ligand resonance were observed (Figure S20(b), 13C NMR δ
with a path length of ca. 1.0 mm. Melting points were measured with a
Stuart SMP10 melting point apparatus.
2
85.3 (d, J = 48.7 Hz, Rh−Ccarbene), 185.2 (d, J = 92.2 Hz, Rh-
Most of the crystals were grown by slow diffusion of n-hexane into a
concentrated CH Cl solution of the metal complexes at 4 °C, except
CO)). This result clearly evidences the retention of the Rh−
C
Presumably, a dimeric Rh−carbene carbonyl complex is
formed, as only one new carbonyl and carbene carbon doublet
is observed.
2
2
bond, albeit a modified Rh−carbene carbonyl complex.
6a
carbene
for those of [Rh(CO) Cl{C(OEt)Fc}] (3), which required −30 °C.
2
X-ray single-crystal intensity data for 1, 2, 5, and 6 were collected at
120 K (1) or 150 K (2, 5, and 6) on a Bruker D8 Venture
diffractometer with a kappa geometry goniometer and a Photon 100
CMOS detector. Data for 4 were collected at 173 K on a Nonius
diffractometer with a kappa geometry goniometer and CCD detector
No upfield hydride resonances (up to −20 ppm) were
1
observed in the H NMR spectrum to support the formation of
3
2
and were scaled and reduced using SAINT (1, 2, 5, and 6) or
a catalytically active Rh(carbene)−hydrido species (analogous
3
3
30
DENZO-SMN (4). Absorption corrections were performed using
to the Wilkinson catalyst), according to the underlying
32
SADABS. The structures were solved by a novel dual-space
27
mechanism of hydroformylation.
34
algorithm using SHELXT (1, 2, 5, and 6) or by direct methods
2
(
4) and were refined by full-matrix least-squares methods based on F
35
CONCLUSIONS
using SHELXL version 2014/7. All non-hydrogen atoms were
refined anisotropically. All hydrogen atoms except amine-H atoms
were placed in idealized positions and refined using riding models. The
amine-H atom positions in 2 and 4 were located and were refined. The
N−H bond distance in 4 was constrained to 0.970(5) Å. In 6, the
chlorine and trans carbonyl ligands are disordered with the chlorine
and carbonyl position interchanged. The site occupation factors
refined to 0.8077(15) for the main orientation and 0.1923(15) for the
flipped orientation. Mass spectral analyses were performed on a Synapt
G2 HDMS by direct infusion at 5 μL/min with positive electron spray
as the ionization technique. The m/z values were measured in the
range 400−1500 with acetonitrile as solvent. Elemental analyses were
carried out using a Thermo Flash 1112 Series CHNS-O analyzer.
Following extensive drying, analyses of complexes 2, 5, 7, and 8 are
outside acceptable limits and are ascribed to the presence of solvent
■
In this study, we achieved the synthesis and isolation of the
novel ferrocenylcarbene complexes of rhodium(I) (1−8).
Spectroscopic characterization to determine the carbonyl
stretching frequencies of complexes 3−8 and calculated TEP
values for 3 and 4 indicate the strongly donating effect of the
ferrocenylcarbene substituent and to a greater extent for the
aminocarbene ligand of 4 compared to the alkoxy analogue 3.
The expected trend for the coligands cod > CO, PCy > CO,
3
PPh > CO, AsPh > CO, P(OPh) > (CO) toward decreasing
3
3
3
2
donor ability was also confirmed by NMR and FT-IR
spectroscopic results and cyclic voltammetry. The complexes
1−8 were screened as catalyst precursors for the hydro-
1
formylation of 1-octene. Excellent conversion of the substrate
olefins was observed, with turnover frequencies and chemo-
and regioselectivity toward the linear aldehydes comparable to
results reported for rhodium NHC complexes. Although the
ferrocenyl substituent results in Fischer carbene ligands with
electron-donating abilities similar to NHCs, the Fischer carbene
ligand is dissymmetric, in contrast to typical NHCs and
phosphines. Undoubtedly this steric effect also plays a role with
regard to the selectivity of the precursor catalysts. Finally, the
retention of the rhodium−carbene bond under hydroformyla-
molecules (CH Cl , n-hexane) and/or silicon grease. The full H and
2
2
13
31
C (and, where applicable, P) NMR spectra are therefore included
in the SI to attest to the purity of the compounds.
Electrochemical studies were carried out using a Metrohm μAutolab
type III potentiostat linked to a computer using GPES Electro-
chemistry software, in conjunction with a three-electrode cell. The
working electrode was a glassy carbon disc (3.0 mm diameter), and the
counter electrode was a platinum wire. The reference was a
+
nonaqueous Ag/Ag electrode separated from the test solution by a
−3
−3
fine-porosity frit. Solutions in CH Cl were 1.0 × 10 mol dm in
2
2
−3
n
the test compound and 0.1 mol dm in [N Bu ][PF ] as the
4
6
1
3
1
tion conditions was confirmed in a C{ H} NMR spectro-
supporting electrolyte. Under these conditions, E°′ for the redox
0/1+
scopic study.
couple [Fe(η-C
5
Me
5
)
2
]
, added to the test solutions as internal
ox
red
standard, is −0.54 V. All E , Ep , and E°′ values are at scan rates of
p
−1
1
00 mV s .
EXPERIMENTAL SECTION
■
The new metal complexes prepared are stable under nitrogen and
dissolve in solvents such as dichloromethane, chloroform, or benzene
to give air-sensitive solutions.
General Procedures. The preparation, purification, and reactions
of the complexes described were carried out under an atmosphere of
dry, oxygen-free dinitrogen or argon, using standard Schlenk
techniques. All reaction mixtures were mechanically stirred, and,
where appropriate, the progress of a reaction was monitored by IR
General Procedure for the Hydroformylation Experiments.
Hydroformylation reactions were conducted (in triplicate) in a 90 mL
stainless steel pipe reactor. In a typical experiment, the catalyst
precursor (1−8) (0.0039 mmol), substrate 1-octene (721.0 mg, 6.37
mmol), and the internal standard, n-decane (180.0 mg, 1.26 mmol),
were dissolved in toluene (5.0 mL) and transferred into a stainless
steel pipe reactor (90 mL). The airtight reactor was then deaerated by
flushing three times with N2 gas and twice with syngas, then
17
spectroscopy. The precursors [Cr(CO) {C(OEt)Fc}] and [Rh-
5
31
(
cod)Cl]2 were prepared according to literature procedures. Silica
gel 60 (particle size 0.0063−0.200 mm) was used as resin for all
separation in column chromatography. Anhydrous tetrahydrofuran,
diethyl ether, and n-hexane were distilled over sodium metal, and
5
750
DOI: 10.1021/acs.organomet.5b00843
Organometallics 2015, 34, 5745−5753

Organometallics
Article
1
1
pressurized with syngas (1:1, CO/H ratio) and heated to the desired
J(RhC) = 77.4 Hz, CO), 85.6 (d, J(RhC) = 2.1 Hz FeCp-Cipso), 80.6
(OCH CH ), 77.2 (FeCp′), 77.0 (FeCp′), 74.7 (FeCp′), 71.2
2
temperature and pressure. After the reaction time, the reactor was
depressurized and the reaction mixture transferred for cooling. The
samples were analyzed by gas chromatography, and the products were
confirmed in relation to authentic iso-octenes and aldehydes.
2
3
(FeCp), 70.5 (FeCp′), 14.8 (OCH CH ). IR (CH Cl , ν(CO),
cm ): 2001, 2084. Anal. Calcd for C H O ClFeRh: C 41.28, H 3.23.
Found: C 41.39, H 2.88. ESI-MS (15 V, positive mode, m/z): calcd for
[M − Cl − CO] 372.9398; found 372.9282.
2
3
2
2
−
1
15
14
3
+
Syntheses of Complexes 1−8. [Rh(cod)Cl{C(OEt)Fc}], 1. A
n
mixture of [Cr(CO) {C(OEt)Fc}] (1.67 g, 3.84 mmol) and
[Rh(CO) Cl{C(NH Pr)Fc}], 4. This complex was prepared similarly to
5
2
[
Rh(cod)Cl] (0.946 g, 1.92 mmol) in CH Cl (30 mL) was stirred
2 (0.105 g, 0.21 mmol), but at room temperature. In this case, no
visible color change was observed upon CO bubbling. Slow addition of
n-hexane to the reaction mixture allowed a dark orange powder, which
2
2
2
for 14 days at room temperature. The formation of unknown
byproducts can be observed by TLC if the temperature is increased.
The resulting dark red solution was reduced in volume in vacuo and
then added to a silica chromatography column. Elution with CH Cl2
gave a deep red band, which was collected and evaporated to dryness.
The product, a dark red oil, was dissolved in 3 mL of CH Cl and
treated with n-hexane (5 mL) to precipitate a crystalline red solid.
Yield = 0.89 g, 95%. Mp: 110−111 °C. H NMR (300 MHz, CDCl ):
δ 5.97 (dq, J(HH) = 10.4, J(HH) = 7.2 Hz, 1H, OCH CH ), 5.51
was washed with n-hexane (10 mL) and dried in vacuo. Yield = 0.08 g,
1
84%. Mp (dec): 138−139 °C. H NMR (300 MHz, CDCl ): δ 8.78 (s,
2
3
br, 1H, NHCH CH CH ), 5.09 (s, br, 1H, FeCp′), 4.91 (s, br, 1H,
2
2
3
FeCp′), 4.62 (s, br, 2H, FeCp′), 4.29 (s, 5H, FeCp), 4.13 (s, br, 1H,
2
2
NHCH
2H, NHCH
NHCH CH CH ). C{ H} NMR (75 MHz, CDCl ): δ 237.8 (d,
3 3
CH
CH CH
CH CH ), 1.01 (t, J(HH) = 7.4 Hz, 3H,
2 3
), 3.54 (s, br, 1H, NHCH
CH ), 1.81−1.68 (m,
2 3
2
2
3
2
1
3
3
2
2
3
13
1
2
3
2
2
2
3
1
1
(
dq, J(HH) = 10.4 Hz, J(HH) = 7.1 Hz, 1H, OCH CH ), 5.41 (dd,
J(RhC) = 34.7 Hz, Ccarbene), 186.9 (d, J(RhC) = 51.3 Hz, CO), 184.2
2
3
3
4
1
J(HH) = 1.3 Hz, J(HH) = 1.3 Hz, 1H, FeCp′), 5.38−5.32 (m, 1H,
(d, J(RhC) = 78.2 Hz, CO), 83.0 (FeCp′-C ), 73.6 (FeCp′), 70.5
ipso
3
cod-CH), 5.21−5.14 (m, 1H, cod-CH), 4.83 (dd, J(HH) = 1.3 Hz,
(FeCp), 67.5 (FeCp′), 56.9 (NHCH
2
CH
). IR (CH
2
CH
3
), 22.9
, ν(CO)
2
4
J(HH) = 1.3 Hz, 1H, FeCp′), 4.79−4.76 (m, 1H, FeCp′), 4.66−4.64
(NHCH
CH
2
2
CH
3
), 11.5 (NHCH
2
CH
2
CH
3
2
Cl
−
1
and ν(NH), cm ): 1995, 2077, 3315. Anal. Calcd for
(
m, 1H, FeCp′), 4.39 (s, 5H, FeCp), 3.38−3.32 (m, 1H, cod-CH),
C H NO ClFeRh: C 42.66, H 4.03, N 3.11. Found: C 42.55, H
3
.24−3.17 (m, 1H, cod-CH), 2.60−1.90 (m, 8H, cod-CH ), 1.65 (t,
16 18
2
2
3
13
1
3.83, N 3.41. ESI-HRMS (15 V, positive mode, m/z): calcd for [M −
J(HH) = 7.1 Hz, 3H, OCH CH ). C{ H} NMR (75 MHz, CDCl ):
2
3
3
+
1
1
Cl − CO] 385.9714; found 385.9647.
δ 309.2 (d, J(RhC) = 43.2 Hz, C
cod-CH), 107.1 (d, J(RhC) = 4.5 Hz, cod-CH), 86.7 (d, J(RhC) =
2
7
6
3
Anal. Calcd For C H OClFeRh: C 51.62, H 5.36. Found: C 51.99, H
4
4
), 107.5 (d, J(RhC) = 4.6 Hz,
carbene
1
2
[Rh(CO)(PPh )Cl{C(OEt)Fc}], 5. Carbon monoxide gas was bubbled
3
for 5 min through a stirred solution of 1 (0.215 g, 0.44 mmol) in
CH Cl (10 mL) in the absence of light at −10 °C. The flow of CO
.1 Hz, FeCp′-C ), 78.3 (OCH CH ), 75.4 (FeCp′), 74.8 (FeCp′),
ipso
2
3
1
2
2
3.8 (d, J(RhC) = 14.9 Hz, cod-CH), 73.7 (FeCp′), 70.6 (FeCp),
1
was stopped, and the solution allowed to reach room temperature.
9.7 (FeCp′), 68.0 (d, J (RhC) = 14.9 Hz, cod-CH), 33.7 (cod-CH ),
2
Solid PPh (0.116 g, 0.44 mmol) was then added, and the mixture
3
2.4 (cod-CH ), 29.0 (cod-CH ), 27.9 (cod-CH ), 15.8 (OCH CH ).
2
2
2
2
3
stirred for 5 min. Slow concentration of the mixture of the filtrate and
n-hexane under reduced pressure gave a purple-red solid. Yield = 0.265
2
1
26
+
.94. ESI-HRMS (15 V, positive mode, m/z): calcd for [M − Cl]
53.0388; found 453.0357.
Rh(cod)Cl{C(NH Pr)Fc}], 2. To a dark red solution of 1 (0.149 g,
.30 mmol) in 10 mL of Et O was added dropwise PrNH (0.05 mL,
1
g, 83%. Mp: 100−102 °C. H NMR (300 MHz, CDCl ): δ 7.74−7.68
3
2
n
(m, 6H, PPh
3
), 7.42−7.39 (m, 9H, PPh
Hz, J(HH) = 7.2 Hz, 1H, OCH CH ), 5.52 (dq, J(HH) = 10.5 Hz,
3
), 5.73 (dq, J(HH) = 10.5
[
3
2
n
2 3
0
0
2
2
3
J(HH) = 7.2 Hz, 1H, OCH CH ), 5.40 (s, br, 1H, FeCp′), 5.16 (s, br,
2
3
.60 mmol) at room temperature. The mixture was stirred for 2 h,
1
2
1
H, FeCp′), 4.87 (dd, J(HH) = 3.9, J(HH) = 2.4 Hz, 2H, FeCp′),
during which period the yellow-orange product 2 slowly precipitated
out of solution. The solvent was decanted, and the solid obtained
washed with n-hexane (20 mL) and dried in vacuo. Yield = 0.120 g,
7
3
4
.39 (s, 5H, FeCp), 1.65 (t, J(HH) = 7.2 Hz, 3H, OCH CH ).
2
3
1
1
3
1
2
C{ H} NMR (75 MHz, CDCl ): δ 299.7 (dd, J(PC) = 110.2 Hz,
3
1
2
1
J(RhC) 40.1 Hz, Ccarbene), 187.5 (dd, J(RhC) = 81.4 Hz, J(PC) =
5%. Mp (dec): 164−167 °C. H NMR (300 MHz, CDCl ): δ 8.08 (s,
3
1
1
6.2 Hz, CO), 134.9 (d, J(PC) = 11.9 Hz, PPh -Cipso), 134.0 (d,
br, 1H, NHCH CH CH ), 5.18 (s, br, 1H, FeCp′), 5.15−5.08 (m, 1H,
3
2
2
3
2
3
J(PC)= 38.3 Hz, PPh ), 130.2 (PPh ), 128.48 (d, J(PC) = 9.7 Hz,
cod-CH), 5.02−4.95 (m, 1H, cod-CH), 4.74 (s, br, 1H, FeCp′), 4.56
s, br, 1H, FeCp′), 4.49 (s, br, 1H, FeCp′), 4.52−4.36 (m, 3H,
NHCH CH CH ), 4.26 (s, 5H, FeCp), 3.41−3.35 (m, 1H, cod-CH),
3
3
2
3
(
PPh
(OCH
(OCH
3
), 87.3 (dd, J(RhC) = 8.2, J(PC) = 2.3 Hz, FeCp′-Cipso), 79.9
CH ), 75.9 (FeCp′), 75.3 (FeCp′), 70.8 (FeCp), 15.23
CH
J(RhP) = 99.9 Hz). IR (CH
for C32 PClFeRh: C 57.30, H 4.36. Found: C 56.47, H 4.27. ESI-
HRMS (15 V, positive mode, m/z): calcd for [M − Cl − PPh ]
2
2
3
2
3
3
1
1
3
.28−3.22 (m, 1H, cod-CH), 2.56−1.77 (m, 10H, cod-CH ,
2
3
). P{ H} NMR (121 MHz, CDCl
3
): δ 27.4 (d,
Cl , ν(CO), cm ): 1965. Anal. Calcd
2 2
2
3
1
−1
NHCH CH CH ), 1.12 (t, J(HH) = 7.4 Hz, 3H, NHCH CH CH ).
2
2
3
2
2
3
1
3
1
1
C{ H} NMR (75 MHz, CDCl ): δ 257.9 (d, J(RhC) = 40.1 Hz,
H O
29 2
3
+
1
1
Ccarbene), 100.9 (d, J(RhC) = 6.1 Hz, cod-CH), 100.8 (d, J(RhC) =
7
(
(
(
CH ), 28.4 (cod-CH ), 23.5 (NHCH CH CH ), 11.9
(
for C H NClFeRh + 0.06 equiv of CH Cl : C 50.53, H 5.49, N 2.64.
Found: C 51.18, H 5.23, N 2.19. ESI-HRMS (15 V, positive mode, m/
3
3
72.9398; found 372.9416. The analogues [Rh(LL)Cl{C(XR)Fc}]
.3 Hz, cod-CH), 83.6 (FeCp′-C ), 72.4 (FeCp′), 72.1 (FeCp′), 71.6
ipso
1
[
8
LL = (CO, PR ) (R = Cy or OPh) and (CO, AsPh ); XR = OEt] 6−
FeCp′), 71.0 (d, J(RhC) = 15.2 Hz, cod-CH), 70.4 (FeCp), 68.7
3
3
1
were prepared similarly as purple-red powders (6) or as oily solids
FeCp′), 68.4 (d, J(RhC) = 15.0 Hz, cod-CH), 56.1
(
7, 8).
Rh(CO)(PCy )Cl{C(OEt)Fc}], 6. Yield = 0.102 g, 65%. Mp: 167−169
NHCH CH CH ), 33.9 (cod-CH ), 32.3 (cod-CH ), 29.5 (cod-
2
2
3
2
2
[
3
2
2
2
2
3
1
2
−1
°C. H NMR (300 MHz, CDCl ): δ 5.64 (dq, J(HH) = 10.5 Hz,
NHCH CH CH . IR (CH Cl , ν(NH), cm ): 3320. Anal. Calcd
3
2
2
3
2
2
3
3
2
J(HH) = 7.2 Hz, 1H, OCH CH ), 5.42 (dq, J(HH) = 10.6 Hz,
J(HH) = 7.2 Hz, 1H, OCH
2
3
22
29
2
2
CH
), 5.31 (s, br, 1H, FeCp′), 5.10 (s, br
2
3
+
2
2
z): calcd for [M − Cl] 466.0704; found 466.0718.
1H, FeCp′), 4.81 (dd, J(HH) = 2.6 Hz, J(HH) = 2.6 Hz, 2H,
[
Rh(CO) Cl{C(OEt)Fc}], 3. Carbon monoxide gas was bubbled for 5
FeCp′), 4.38 (s, 5H, FeCp), 2.26−2.16 (m, 3H, PCy
3
), 2.07−2.03 (m,
), 1.62 (t, J(HH) = 7.2 Hz, 3H,
3
2
3
min through a stirred solution of 1 (0.139 g, 0.28 mmol) in CH Cl (5
6H, PCy
OCH CH
302.5 (dd, J(PC) = 103.2 Hz, J(RhC) = 38.9 Hz. Ccarbene), 188.0 (dd,
3
), 1.83−1.60 (m, 15H, PCy
2
2
13
1
mL) in the absence of light and at −10 °C. Immediate color change
from deep red to dark purple was observed. The flow of CO was
stopped, and purple needle-like crystals of the product were grown by
slow diffusion of n-hexane (5 mL) into the concentrated CH Cl2
reaction mixture at −30 °C. Yield = 0.079 g, 65%. Mp (dec): 120−122
°
2
), 1.29 (s, 9H, PCy ). C{ H} NMR (75 MHz, CDCl ): δ
3 3 3
2
1
1
2
2
J(RhC) = 82.3 Hz, J(PC) = 16.7 Hz, CO), 87.6 (dd, J(RhC) = 7.5
3
Hz, J(PC) = 2.5 Hz, FeCp′-Cipso), 78.7 (OCH
CH
74.5 (FeCp′), 70.3 (FeCp), 33.4 (d, J(PC) = 17.7 Hz, PCy
30.3 (PCy ), 27.8 (PCy ), 27.7 (PCy ), 26.7 (PCy ), 14.9
(OCH CH ). P{ H} NMR (121 MHz, CDCl ): δ 34.7 (d,
J(RhP) = 98.4 Hz). IR (CH , ν(CO), cm ): 1946. Anal. Calcd
for C H O PClFeRh + 0.33 equiv of C H : C 56.91, H 7.26. Found:
), 75.1 (FeCp′),
2
2
3
1
-Cipso),
3
1
3
C. H NMR (300 MHz, CDCl ): δ 5.35 (q, br, J(HH) = 6.8 Hz, 2H,
3
3
3
3
3
3
1
1
OCH CH ), 5.20 (s, br, 1H, FeCp′), 5.14 (s, br, 1H, FeCp′), 5.01 (s,
2
3
3
2
3
3
1
−1
br, 2H, FeCp′), 4.44 (s, 5H, FeCp), 1.59 (t, J(HH) = 7.1 Hz, 3H,
Cl
2 2
1
3
1
1
OCH CH ). C{ H} NMR (75 MHz, CDCl ): δ 289.3 (d, J(RhC) =
2
3
3
32 47
2
6
14
1
3
7.9 Hz, Ccarbene), 186.5 (d, J(RhC) = 49.5 Hz, CO), 182.8 (d,
C 56.41, H 6.57. ESI-HRMS (15 V, positive mode, m/z): calcd for [M
5
751
DOI: 10.1021/acs.organomet.5b00843
Organometallics 2015, 34, 5745−5753

Organometallics
Article
+
+
−
Cl − CO] 625.1769; found 625.1733; calcd for [M − Cl − PCy ]
Africa (D.I.B.), and the NRF-DST Centre of Excellence in
Catalysis (c* change) (G.S.S.). A generous loan of rhodium
trichloride hydrate from Johnson Matthey/AngloAmerican
Platinum Ltd. Corporation is gratefully acknowledged.
3
3
72.9398; found 372.9416.
Rh(CO)(P(OPh) )Cl{C(OEt)Fc}], 7. Yield = 0.163 g, 63%. Mp: 74−
[
3
1
2
7
6 °C. H NMR (300 MHz, CDCl ): δ 7.41 (dd, J(HH) = 7.8 Hz,
3
2 2
3
J(HH) = 7.8 Hz, 6H, OPh ), 7.37 (dd, J(HH) = 7.9 Hz, J(HH) =
3
2
3
7
.9 Hz, 6H, OPh ), 7.19 (dd, J(HH) = 8.1 Hz, J(HH) = 8.1 Hz, 3H,
3
3
OPh ), 5.06 (q, J(HH) = 7.1 Hz, 2H, OCH CH ), 4.88−4.87 (m,
REFERENCES
3
2
3
■
2
H, FeCp′), 4.83−4.80 (m, 2H, FeCp′), 4.38 (s, 5H, FeCp), 1.41 (t,
(
1) For reviews on Rh-catalyzed hydroformylation, see: (a) Whiteker,
3
13
1
J(HH) = 7.2 Hz, 3H, OCH CH ). C{ H} NMR (75 MHz, CDCl ):
2
3
3
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2
1
δ 298.4 (dd, J(PC) = 174.1 Hz, J(RhC) = 38.9 Hz, C
), 184.8
carbene
1
2
2
(
4
5
dd, J(RhC) = 79.1 Hz, J(PC) = 21.0 Hz, CO), 151.9 (d, J(PC) =
3
.8 Hz, OPh -C ), 129.8 (OPh ), 125.0 (OPh ), 122.2 (d, J(PC) =
3 ipso 3 3
2
3
.5 Hz, OPh ), 86.5 (dd, J(RhC) = 12.6 Hz, J(PC) = 2.5 Hz, FeCp′-
3
C
ipso
), 79.9 (OCH CH ), 76.3 (FeCp′), 75.9 (FeCp′), 71.0 (FeCp),
2
3
31 1
1
5.0 (OCH CH ). P{ H} NMR (121 MHz, CDCl ): δ 127.0 (d,
2
3
3
1
−1
J(RhP) = 177.4 Hz). IR (CH Cl , ν(CO), cm ): 1986. Anal. Calcd
2
2
for C H O PClFeRh: C 53.47, H 4.07. Found: C 48.19, H 3.77. ESI-
32
29
5
+
HRMS (15 V, positive mode, m/z): calcd for [M − Cl − CO]
+
6
55.0208; found 655.0435; calcd for [M − Cl − P(OPh) ] 372.9398;
3
found 372.9282.
Rh(CO)(AsPh )Cl{C(OEt)Fc}], 8. Yield = 0.096 g, 63%. Mp: 98−100
2
31−269. (f) van Leeuwen, P. W. N. M.; Claver, C., Eds. Rhodium
[
3
Catalyzed Hydroformylation; Kluwer Academic Publishers: Dordrecht,
1
3
°
C. H NMR (300 MHz, CDCl ): δ 7.65 (dd, J(HH) = 3.6 Hz,
3
2
1
000. (g) Trzeciak, A. M.; Ziol
90−192, 883−900.
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́
kowski, J. J. Coord. Chem. Rev. 1999,
3
J(HH) = 3.6 Hz, 6H, AsPh ), 7.41−7.40 (m, 9H, AsPh ), 5.75 (dq,
J(HH) = 10.4 Hz, J(HH) = 7.2 Hz, 1H, OCH CH ), 5.57 (dq,
J(HH) = 10.2 Hz, J(HH) = 7.2 Hz, 1H, OCH CH ), 5.40 (s, br, 1H,
3
3
2
2
3
2
3
3
2
3
(a) Jahnke, M. C.; Hahn, F. E. Chem. Lett. 2015, 44, 226−237.
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3
13
1
FeCp), 1.66 (t, J(HH) = 7.1 Hz, 3H, OCH CH ). C{ H} NMR (75
2
3
1
MHz, CDCl ): δ 295.3 (d, J(RhC) = 45.1 Hz, Ccarbene), 186.5 (d,
3
1
J(RhC) = 80.3 Hz, CO), 135.5 (AsPh ), 134.4 (AsPh ), 129.9
3
3
2
(
(
(
AsPh ), 129.0 (AsPh ), 87.4(d, J(RhC) = 3.5 Hz, FeCp′-Cipso), 80.1
3
3
OCH CH ), 75.8 (FeCp′), 75.4 (FeCp′), 70.9 (FeCp), 15.2
2
3
−1
OCH CH ). IR (CH Cl , ν(CO), cm ): 1966. Anal. Calcd for
2
3
2
2
C H O AsClFeRh: C 53.78, H 4.09. Found: C 51.71, H 3.52. ESI-
HRMS (15 V, positive mode, m/z): calcd for [M − Cl − AsPh) ]
72.9398; found 372.9282.
3
2
29
2
+
3
3
(
6
3) (a) Dro
952. (b) Díez-Gonzal
874−883.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00843.
■
(
S
2
(
́
3
4
Figures giving the NMR and IR spectra of complexes 1−
and a table giving the X-ray crystallographic collection
data and parameters for 1, 2, and 4−6 (PDF)
X-ray crystallographic collection data and parameters for
8
(
f) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239−2246.
5) Selected recent reviews on Fischer carbene complexes, and
references therein: (a) Raubenheimer, H. G. Dalton Trans. 2014, 43,
(
1
, 2, and 4−6 (CIF)
16959−16973. (b) Herndon, J. W. Coord. Chem. Rev. 2013, 286, 30−
150. (c) Bezuidenhout, D. I.; Lotz, S.; Liles, D. C.; van der
Westhuizen, B. Coord. Chem. Rev. 2012, 256, 479−524. (d) Strassner,
AUTHOR INFORMATION
Corresponding Author
E-mail: daniela.bezuidenhout@up.ac.za.
■
T. Metal Carbenes in Organic Synthesis. In Topics in Organometallic
Chemistry; Do
6) (a) Barluenga, J.; Vicente, R.; Lο
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̈
tz, K. H., Ed.; Springer-Verlag: Berlin, 2004; Vol. 13,.
*
(
́
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Author Contributions
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, M.; Alvarez-Rua, C. J. Am. Chem.
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Radhakrishnan, University of Pretoria, for valuable discussions
with regard to the cyclic voltammetry experiments. This work
was supported by funding from the National Research
Foundation, South Africa (D.I.B., grant numbers 87890,
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DOI: 10.1021/acs.organomet.5b00843
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