C 2-symmetric iron(II) diphosphine-dialkoxide dicarbonyl and related complexes

Article abstract of DOI:10.1021/om300631a

Reaction of Fe(bda)(CO)₃ (bda = benzylideneacetone) and Ph ₂P-2-C₆H₄CHO (PCHO) affords the bisphosphine bisalkoxide complex Fe[(Ph₂PC₆H₄) ₂C₂H₂O₂](CO)₂ (1) arising from the head-to-head coupling of two formyl groups concomitant with oxidation of Fe(0) to Fe(II). Crystallographic studies show that 1 features cis alkoxide ligands that are trans to CO; the two phosphine groups are mutually trans with a P-Fe-P angle of 167.44(4)°. The pathway leading to 1 was examined, starting with the adduct Fe(PCHO)(CO)₄ (2), which was obtained by addition of PCHO to Fe₂(CO)₉. Compound 2 decarbonylates to give tricarbonyl Fe(κ¹,η²-PCHO)(CO)₃ (3), which features a π-bonded aldehyde. Photolysis of 2 gives a mixture of 3 and isomeric hydride HFe(κ²-PCO)(CO)₃. Complex 3 reacts with an additional equivalent of PCHO to afford 1, whereas treatment with PPh₃ afforded the substituted product Fe(κ¹, η²-PCHO)(PPh₃)(CO)₂ (4). In 4, the phosphine ligands are trans and the aldehyde is π-bonded. The geometry around Fe is pseudo trigonal bipyramidal. To gain insights into the mechanism and scope of the C-C coupling reaction, complexes were prepared with the imine Ph₂PC₆H₄CH=NC₆H₄Cl (abbreviated as PCHNAr), derived by condensation of 4-chloroaniline and PCHO. PCHNAr reacts with Fe₂(CO)₉ and with Fe(bda)(CO) ₃ to afford the tetra- and tricarbonyl compounds Fe(PCHNAr)(CO) ₄ (5) and Fe(PCHNAr)(CO)₃ (6), respectively. Treatment of 6 with PCHO gave the unsymmetrical C-C coupling complex Fe[(Ph ₂PC₆H₄)₂CH(O)CH(NAr)](CO) ₂ (7). Compound 7 was also prepared by the reaction of 3 and PCHNAr. The solid-state structure of 7, as established by X-ray crystallography, is similar to that of 1 but with an amido group in place of one alkoxide. The deuterium-labeled phosphine aldehyde PCDO was prepared by the reaction of ortho-lithiated phosphine Ph₂PC₆H₄-2-Li with DMF-d₇. Reaction of 6 with PCDO gave 7-d₁ with no scrambling of the deuterium label. Attempted oxidation of 1 with FcBF ₄ (Fc⁺ = ferrocenium) gave the adduct Fe[(Ph ₂PC₆H₄)₂C₂H ₂O₂(BF₃)₂](CO)₂ (8). The structures of 1 and 8 are almost identical. Compound 8 was independently synthesized by treating 1 with BF₃OEt₂ via the intermediacy of the 1:1 adduct, which was detected spectroscopically. Qualitative tests showed that 1 also reversibly protonates with HOSO ₂CF₃ and binds TiCl₄.

Full text of DOI:10.1021/om300631a

Article
pubs.acs.org/Organometallics
C2‑Symmetric Iron(II) Diphosphine−Dialkoxide Dicarbonyl and
Related Complexes
Hao Lei, Aaron M. Royer, Thomas B. Rauchfuss,* and Danielle Gray
School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States
S
* Supporting Information
ABSTRACT: Reaction of Fe(bda)(CO)3 (bda = benzylidenea-
cetone) and Ph2P-2-C6H4CHO (PCHO) affords the bis-
phosphine bisalkoxide complex Fe[(Ph2PC6H4)2C2H2O2](CO)2
(1) arising from the head-to-head coupling of two formyl groups
concomitant with oxidation of Fe(0) to Fe(II). Crystallographic
studies show that 1 features cis alkoxide ligands that are trans to
CO; the two phosphine groups are mutually trans with a P−Fe−
P angle of 167.44(4)°. The pathway leading to 1 was examined,
starting with the adduct Fe(PCHO)(CO)4 (2), which was obtained by addition of PCHO to Fe2(CO)9. Compound 2
decarbonylates to give tricarbonyl Fe(κ1,η2-PCHO)(CO)3 (3), which features a π-bonded aldehyde. Photolysis of 2 gives a
mixture of 3 and isomeric hydride HFe(κ2-PCO)(CO)3. Complex 3 reacts with an additional equivalent of PCHO to afford 1,
whereas treatment with PPh3 afforded the substituted product Fe(κ1,η2-PCHO)(PPh3)(CO)2 (4). In 4, the phosphine ligands
are trans and the aldehyde is π-bonded. The geometry around Fe is pseudo trigonal bipyramidal. To gain insights into the
mechanism and scope of the C−C coupling reaction, complexes were prepared with the imine Ph2PC6H4CHNC6H4Cl
(abbreviated as PCHNAr), derived by condensation of 4-chloroaniline and PCHO. PCHNAr reacts with Fe2(CO)9 and with
Fe(bda)(CO)3 to afford the tetra- and tricarbonyl compounds Fe(PCHNAr)(CO)4 (5) and Fe(PCHNAr)(CO)3 (6),
respectively. Treatment of 6 with PCHO gave the unsymmetrical C−C coupling complex Fe[(Ph2PC6H4)2CH(O)CH(NAr)]-
(CO)2 (7). Compound 7 was also prepared by the reaction of 3 and PCHNAr. The solid-state structure of 7, as established by X-
ray crystallography, is similar to that of 1 but with an amido group in place of one alkoxide. The deuterium-labeled phosphine
aldehyde PCDO was prepared by the reaction of ortho-lithiated phosphine Ph2PC6H4-2-Li with DMF-d7. Reaction of 6 with
PCDO gave 7-d1 with no scrambling of the deuterium label. Attempted oxidation of 1 with FcBF4 (Fc+ = ferrocenium) gave the
adduct Fe[(Ph2PC6H4)2C2H2O2(BF3)2](CO)2 (8). The structures of 1 and 8 are almost identical. Compound 8 was
independently synthesized by treating 1 with BF3OEt2 via the intermediacy of the 1:1 adduct, which was detected
spectroscopically. Qualitative tests showed that 1 also reversibly protonates with HOSO2CF3 and binds TiCl4.
important reactive intermediate in the process.8 Combinations
INTRODUCTION
■
of FeCl3/nBuLi or (Bu4N)2[Fe4S4(SPh)4]/nBuLi have been
C2-Symmetric tetradentate ligands represent an important class
of reagents.1 Their complexes are useful catalysts in various
shown to mediate the couplings of aromatic ketones and
aldehydes in moderate yields.9 Recently, an Fe(I) dinitrogen
asymmetric catalytic transformations. For example, tetradentate
bis(phenolate) ligands have been considered as feasible
complex LFeNNFeL (L = β-diketiminato ligand) was shown to
reduce acetophenone to afford the rac-diastereomer of the
alternatives to the C2-symmetric ansa-metallocene found in
pinacolate.10
olefin polymerization catalysts.2 Group 4 metal catalysts bearing
such bis(phenolate) ligands with various additional neutral
The discoveries reported in this paper grew from our work
on phosphine-acyl chelates of Fe(II),11 which represent
donors (such as N,3 S,4 or P5) have been heavily investigated.
structural models for the pyridinol-acyl chelates found in the
The binding mode of the bis(phenolate) ligands usually
hydrogenase Hmd.12 This work led us to examine the addition
depends on the nature of the additional donors, the size of
of Ph2PC6H4-2-CHO (abbreviated PCHO) to Fe(0) carbonyls.
the metal ion, and the ligand framework.
The coordination chemistry of PCHO with other metals is
Discovered more than 150 years ago, pinacol coupling
remains a versatile tool for chemists to construct C−C bonds.6
Numerous reagents have been developed for both stoichio-
metric and catalytic pinacol coupling reactions. More recently,
stereoselection has been achieved using specifically designed
metal complexes.7 Among these metal reagents, reports of Fe-
promoted pinacol coupling are rather limited. Pinacol coupling
reactions have been studied in the presence of Fe(0) carbonyls
and pyridine, and it was proposed that [Fe2(CO)8]2− is an
already well established, although mainly with second- and
third-row metals.13 Aside from forming simple κ1-P adducts,
PCHO has been shown to give κ2-P,O chelate rings involving
σ-bonding from the carbonyl oxygen, e.g., Re(CO)3Cl-
(PCHO)14 and RuCl2(PCHO)2.15 In some complexes, the
Received: July 6, 2012
Published: August 15, 2012
© 2012 American Chemical Society
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formyl group forms π-bonds to the metal, as illustrated by
Cp*Co(PCHO)16 and some W17 and Mo18 complexes. The
addition of PCHO to electron-rich late transition metals, such
as Rh(I),19 Ir(I),20 Pt(0),21 and Co(I),22 frequently results in
chelate-assisted C−H bond activation to give phosphine acyl
η2-P-acyl derivatives. In two examples, pairs of PCHO ligands
undergo C−C coupling to afford complexes of the glycolate
ligand [(Ph2PC6H4)2C2H2O2]2−. The formation of the same
P2O2 ligand can be templated on a Tc center.23 Insights into
the coupling process were obtained when starting with W(0)
precursors,17 which gives both κ3- and κ4-derivatives (Scheme
1), albeit in low yields.
Scheme 1. C−C Coupling Reactions of Ph2P-2-C6H4CHO
on W and Tc Templates
Figure 1. Molecular structure of Fe[(Ph2PC6H4)2C2H2O2](CO)2 (1)
with hydrogen atoms omitted for clarity. The thermal ellipsoids are
shown at 50% probability with Fe, orange; P, purple; C, gray; O, red.
Selected bond lengths (Å) and angles (deg): Fe(1)−P(1) 2.2496(9),
Fe(1)−P(2) 2.2395(10), Fe(1)−O(1) 1.945(2), Fe(1)−O(2)
1.954(2), Fe(1)−C(39) 1.774(3), Fe(1)−C(40) 1.754(3), C(19)−
C(26) 1.549(4), C(40)−Fe(1)−C(39) 92.64(14), C(40)−Fe(1)−
O(1) 96.01(12), C(39)−Fe(1)−O(2) 86.26(12), O(1)−Fe(1)−O(2)
85.07(8), P(2)−Fe(1)−P(1) 167.44(4).
Scheme 2. Pathway for Formation of 1 and Related
Reactions and Selected NMR Data
RESULTS AND DISCUSSION
■
Synthesis and Structure of Fe[(Ph2PC6H4)2C2H2O2]-
(CO)2. Heating a THF solution of Fe(bda)(CO)3 (bda =
benzylideneacetone) in the presence of 2 equiv of PCHO gave
an 89% yield of the yellow compound Fe-
[(Ph2PC6H4)2C2H2O2](CO)2 (1) (eq 1).
Fe(bda)(CO)3 + 2Ph2PC6H4CHO
→ Fe[(Ph2PC6H4)2C2H2O2](CO)2 + bda + CO
(1)
1
The same product was also obtained in slightly lower yield by
the reaction of PCHO with Fe2(CO)9 at 60 °C in THF. The
solution IR spectrum of 1 featured a pair of comparably intense
CO bands at 1965 and 2025 cm−1. The 31P NMR spectrum
consisted of a singlet at δ 21.1. Aside from the phenyl signals,
the 1H NMR spectrum features a singlet at δ 4.49 that
integrated in a 1:14 ratio relative to the phenyl signals.
Single-crystal X-ray diffraction confirmed that 1 is a ferrous
complex of a bisphosphine glycolate ligand. The ligand results
from the apparent head-to-head coupling of two formyl groups,
which was confirmed by the distance between the two aliphatic
carbon atoms (C(19)−C(26) = 1.549(4) Å). The cis alkoxide
ligands are trans to CO, and the two phosphine groups are
mutually trans with a P−Fe−P angle of 167.44(4)°. Few ferrous
alkoxides are known, and fewer still with CO ligands.24
Studies on the C−C Coupling Pathway. We investigated
the pathway for the formation of 1 starting with the adduct
Fe(PCHO)(CO)4 (2) (Scheme 2). This tetracarbonyl was
obtained in good yield by addition of PCHO to Fe2(CO)9 at
room temperature, with the concomitant formation of a small
portion of tricarbonyl complex Fe(PCHO)(CO)3 (3).
Complex 2 displays an IR spectrum typical of phosphine
adducts of the type Fe(PR3)(CO)4.25 Photolysis of a toluene
solution of 2 at room temperature resulted in decarbonylation
to give a mixture of mainly two isomeric tricarbonyls as well as
some 1, which could be removed by hexane extraction of the
two tricarbonyls. The initially formed tricarbonyls, assigned the
formula Fe(κ1,η2-PCHO)(CO)3 (3), feature the π-bonded
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1
aldehyde, as evidenced by an H NMR signal of δ 5.90. A
similar chemical shift of δ 6.28 was observed for the π-bonded
aldehyde group in W(CO)3(η1-PCHO)(κ1,η2-PCHO).17 Re-
action of Fe(bda)(CO)3 with 1 equiv of PCHO at room
temperature afforded a mixture of complexes 3 and 1. Samples
of 3 were always contaminated by small amounts of the
isomeric acyl hydride HFe(κ2-PCO)(CO)3. This species is
characterized by doublets at δ −7.7 and at δ 89.5 in the 1H and
31P NMR spectra, respectively.
Crystallographic analysis of 4 revealed that the Fe center is
coordinated to two P atoms from the phosphine ligands, two
carbonyl groups, and a π-bonded formyl group. The two
phosphine groups are mutually trans, with an almost linear
P(1)−Fe(1)−P(2) angle (175.27(3)o). The cis-CO groups and
the π-bonded formyl group are almost planar (sum of
interligand angles equals 359.9°). The geometry around Fe is
thus pseudo trigonal bipyramidal. Most of the geometric
parameters in 4 resemble those of Fe(PR3)2(R′CHO)(CO)2;
however, the CO bond (1.365(4) Å) in 4 is much longer
than the corresponding values (1.29(2) Å, R = Et, R′ = Ph;
1.32(2) Å, R = OMe, R′ = H).26 Indeed, this value is much
closer to the value of a C−O single bond (∼1.45 Å) than that
of a CO bond (∼1.20 Å). This indicates strong π-donation
toward the Fe center and significant weakening of the CO
bond, which might be crucial for the subsequent formation of
the Fe−O and C−C bonds. The coordination geometry around
the Fe atoms in 4 and 1 are quite similar, reinforcing the view
that 4 is a good model for an intermediate in the formation of
1. One interesting feature is the short distance (1.974(2) Å)
between the Fe(1) atom and O(3) of the formyl group. This
distance is almost the same as the Fe−O distances in 1.
Imine-Formyl Cross-Coupling. Analogous to the pinacol
coupling that gives 1 is the corresponding imine-formyl cross-
coupling. This process was investigated using the imine
Ph2PC6H4CHNC6H4Cl (abbreviated as PCHNAr), derived
by condensation of 4-chloroaniline and PCHO.27 PCHNAr
reacts with Fe2(CO)9 at room temperature to afford the
monophospine adduct Fe(PCHNAr)(CO)4 (5). In addition,
treatment of Fe(bda)(CO)3 with PCHNAr affords the expected
chelate complex Fe(PCHNAr)(CO)3 (6), which is green.
Alternatively, compound 6 could be generated by photolysis of
5. Treatment of complex 6 with PCHO gave good yields of the
unsymmetrical C−C coupling complex Fe[(Ph2PC6H4)2CH-
(O)CH(NAr)](CO)2 (7). The IR spectrum in the CO region
shows a pair of CO bands at 1959 and 2022 cm−1. In contrast
to 1, the methine groups give rise to two 1H NMR signals, at δ
4.69 and 4.14, corresponding to the OCH and NCH groups,
respectively. The 31P NMR spectrum of 7 features an AB
quartet with a large 31P−31P coupling constant (301 Hz).
Compound 7 could also be prepared by the reaction of 3 and
PCHNAr, although traces of compound 1 also formed.
Attempted coupling of PCHNAr and 6 failed to afford the
diamido complex.
Although 3 was not obtained in analytical purity, the crude
product can be used to test steps in the formation of 1.
Complex 3 reacts with an additional equiv of PCHO to afford 1
in good yield over the course of a week. When this conversion
was monitored by 31P NMR spectroscopy, no intermediates
were detected. Slightly more quickly than the reaction with
PCHO, 3 underwent substitution with PPh3 to give Fe-
(PCHO)(PPh3)(CO)2 (4), analogous to the intermediate
proposed for the reaction of 3 and PCHO. The 31P NMR
spectrum of this heterodisubstituted compound shows an AB
quartet with large 31P−31P coupling constant (169 Hz),
1
suggesting that the phosphines are mutually trans. The H
NMR spectrum exhibits a doublet at δ 4.98 indicative of a π-
bonded formyl group. The solid-state structure of 4 was
confirmed by X-ray crystallography (Figure 2). Although many
examples of π-bonded aldehyde metal complexes are known,
only a few feature first-row metals.13 The compounds
Fe(PR3)2(R′CHO)(CO)2 (R = Et, R′ = Ph; R = OMe, R′ =
H) represent the only structurally characterized iron-aldehyde
complexes.26
The solid-state structure of compound 7 was established by
X-ray crystallography (Figure 3). The molecular structure of 7
is similar to that of 1: the Fe center is pseudo-octahedral. The
coordination sphere is completed by two mutually cis carbonyl
ligands, two mutually trans phosphine groups, and the amido-
alkoxo ligand. The large difference between the C(1)−Fe(1)−
N(1) (100.38(8)o) angle and the C(2)−Fe(1)−O(3)
(87.85(8)o) angle may be the consequence of the bulky aryl
substituent on the amido nitrogen atom.
Isotopic Labeling Experiment. The imine-formyl cross-
coupling reaction was further examined by studies on the
reaction of 6 with deuterium-labeled phosphinealdehyde Ph2P-
2-C6H4CDO (abbreviated as PCDO). A toluene solution of 6
Figure 2. Molecular structure of Fe(κ1,η2-PCHO)(PPh3)(CO)2 (4)
with hydrogen atoms omitted for clarity. The thermal ellipsoids are
shown at 50% probability with Fe, orange; P, purple; C, gray; O, red.
Selected bond lengths (Å) and angles (deg): Fe(1)−P(1) 2.2314(10),
Fe(1)−P(2) 2.2613(10), Fe(1)−C(1) 1.791(3), Fe(1)−C(2)
1.762(4), Fe(1)−C(3) 2.026(3), Fe(1)−O(3) 1.974(2), C(1)−
Fe(1)−C(2) 104.49(15), C(2)−Fe(1)−C(3) 105.25(15), C(3)−
Fe(1)−O(3) 39.88(13), O(3)−Fe(1)−C(1) 110.28(12), P(1)−
Fe(1)−P(2) 175.27(3).
1
and PCDO was heated at 50 °C, and the H and 31P NMR
spectra were monitored (Scheme 3). After 2 h, the AB quartet
shown in the 31P NMR spectrum clearly indicated the
formation of the cross-coupled product. In the 1H NMR
spectrum, however, only the CHNR signal was detected (δ
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Figure 3. Molecular structure of Fe[(Ph2PC6H4)2CH(O)CH(NAr)]-
(CO)2 (7) with hydrogen atoms omitted for clarity. The thermal
ellipsoids are shown at 50% probability with Fe, orange; P, purple; N,
pink; Cl, green; C, gray; O, red. Selected bond lengths (Å) and angles
(deg): Fe(1)−P(1) 2.2651(6), Fe(1)−P(2) 2.2656(6), Fe(1)−O(3)
1.9622(13), Fe(1)−N(1) 2.0184(15), Fe(1)−C(1) 1.781(2), Fe(1)−
C(2) 1.781(2), C(1)−Fe(1)−C(2) 87.92(9), C(1)−Fe(1)−N(1)
100.38(8), C(2)−Fe(1)−O(3) 87.85(8), N(1)−Fe(1)−O(3)
83.85(6), P(1)−Fe(1)−P(2) 166.55(2).
Figure 4. Molecular structure of Fe[(Ph2PC6H4)2C2H2O2(BF3)2]-
(CO)2 (8) with hydrogen atoms omitted for clarity. The thermal
ellipsoids are shown at 50% probability with Fe, orange; P, purple; C,
gray; O, red; B, pink; F, green. Selected bond lengths (Å) and angles
(deg): Fe(1)−P(1) 2.3136(11), Fe(1)−P(2) 2.3119(11), Fe(1)−
O(3) 2.011(3), Fe(1)−O(4) 2.016(3), Fe(1)−C(1) 1.782(4), Fe(1)−
C(2) 1.777(4), C(1)−Fe(1)−C(2) 85.59(18), C(1)−Fe(1)−O(4)
96.95(15), C(2)−Fe(1)−O(3) 98.20(15), O(3)−Fe(1)−O(4)
80.84(10), P(2)−Fe(1)−P(1) 171.81(4).
Scheme 3. Reaction of Ph2P-2-C6H4CDO with 6
equivalent phosphines (δ 28.1, 25.4, J = 245 Hz). The
difference in νCO of the two adducts (28 and 18 cm−1) is just
lower than half of the overall νCO change from 1 to 8 (58 and
41 cm−1).
Fe[(Ph2PC6H4)2C2H2O2](CO)2 + 2BF
3
1
→ Fe[(Ph2PC6H4)2C2H2O2(BF) ](CO)2
3 2
(2)
8
The generality of the binding of Lewis acids to the alkoxide
ligands in 1 was further tested qualitatively with 1 and 2 equiv
of HOTf (OTf = CF3SO3), MeOTf, and TiCl4. Formation of
1:1 or 1:2 derivatives with these electrophiles was confirmed by
the IR spectra (Table 1). Protonation and methylation induce
4.26). No scrambling was observed. Only one signal (δ 4.91)
was observed in the H NMR spectrum.
2
Reactions of 1 with Lewis Acids. Expecting that the
alkoxide ligands would stabilize ferric derivatives, we attempted
to oxidize 1 with FcBF4 (Fc+ = ferrocenium). Instead of
oxidation, this reaction resulted in about 50% yield of
Fe[(Ph2PC6H4)2C2H2O2(BF3)2](CO)2 (8). The fate of the
Table 1. IR Measurements (CH2Cl2 solutions, νCO, cm−1) of
the Derivatives Formed from 1 and Various Electrophiles
−
fluoride lost from the BF4 was not determined, but the
Lewis acids
1:1 adduct
1:2 adduct
stoichiometry of this reaction would be consistent with the
formation of a ferrous fluoride species. Compound 8 was
characterized by single-crystal X-ray diffraction (Figure 4). The
product contains BF3 bound to the two alkoxide centers. The
coordination at Fe is relatively unchanged compared with 1. A
small expansion of the P−Fe−P angle from 167.44(4)o to
171.81(4)o was observed.
Compound 8 was independently synthesized by the reaction
of 1 and 2 equiv of BF3OEt2 in CH2Cl2 (eq 2). The product
features a pair of CO bands at 2023 and 2066 cm−1 in the IR
spectrum. When instead 1 equiv of BF3OEt2 was used in the
reaction, the 1:1 adduct could be detected (νCO = 1995, 2048
cm−1), the 31P NMR spectrum of which indicates non-
BF3
H+
1995, 2048
2001, 2050
2002, 2051
2000, 2050
2023, 2066
2036, 2075
2030, 2070
2025, 2068
Me+
TiCl4
the most significant change in νCO, indicating the stronger
Lewis acidity of H+ and Me+ compared to BF3 and TiCl4.28
However, double methylation was slow.
CONCLUSIONS
The ferrous complex of an unusual diphosphine dialkoxide
ligand was prepared in high yields by the pinacol-like coupling
■
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133.5, 132.0, 131.8, 130.5, 129.5. 31P NMR (202 MHz, CD2Cl2): δ
67.8 (s). IR spectrum (CH2Cl2): νCO 1944, 1978, 2052 cm−1. Anal.
Calcd for C23H15FeO5P (found): C, 60.29 (60.03); H, 3.30 (3.18); N,
0.00 (0.28).
of 2 equiv of a phosphine aldehyde. Routes to this ligand
involved the use of an exotic metal (Tc)23 or proceeded in low
yields (W).17 Our studies point to the intermediacy of a π-
aldehyde complex, which undergoes substitution by a second
phosphine aldehyde followed rapidly by C−C coupling. The
two phosphine ligands in the proposed Fe(0) intermediate
would be trans, as seen for Fe(κ1,η2-PCHO)(PPh3)(CO)2 and
as observed in the coupled product. Isotopic labeling studies
using PCDO are consistent with this mechanism.
The new diphosphineglycolate platform offers scope for
further work, especially in view of their efficient formation of
the iron complex. The discovery of the imine-formyl cross-
coupling expands the range of these tetradentate ligands
because so many iminophosphines are known.29 The new iron
dialkoxide was shown to bind a variety of Lewis acids through
the alkoxide centers to afford 1:1 or 1:2 adducts. Investigations
aimed at liberating the diphosphine-glycol and its application to
other metal systems are currently under way.
Fe(κ1,η2-PCHO)(CO)3 (3). A solution of 1.022 g (2.23 mmol) of 2 in
20 mL of CH2Cl2 was irradiated, while monitoring the progress of the
reaction by 31P NMR spectroscopy. After the 31P NMR signal for 2
was no longer apparent (ca. 30 h), the solution was concentrated to 10
mL and then diluted with 40 mL of hexanes. The red precipitate was
collected. The filtrate was further concentrated and then stored at −20
°C for several days to afford additional product, and the sequence of
concentrating followed by cooling was repeated. Several recrystalliza-
tions removed most of the less soluble components, affording a sample
of 3 with >70% purity. The sample was used for the preparation of 4
1
without further purification. H NMR (500 MHz, toluene-d8): δ 5.90
(s, CHO). 13C NMR (126 MHz, CD2Cl2): δ 90.2 (s, CHO). 31P NMR
(202 MHz, toluene-d8): δ 58.2 (s). IR spectrum (CH2Cl2): νCO 1957,
1989, 2051 cm−1.
Fe(κ1,η2-PCHO)(PPh3)(CO)2 (4). A solution of 0.198 g (0.46 mmol)
of 3 and 0.127 g (0.48 mmol) of PPh3 in 20 mL of toluene was heated
at 50 °C for 6 h. The orange solution was evacuated to dryness under
vacuum. The residue was washed with 3 × 10 mL of hexanes, and an
orange solid was obtained. The solid was dissolved in CH2Cl2 (5 mL),
and this solution was layered with pentane (15 mL). Slow diffusion
overnight at room temperature yielded orange crystals. Yield: 0.099 g
(32%). 1H NMR (500 MHz, toluene-d8): δ 4.98 (d, 1H, CHO), 6.76−
8.20 (m, 29H, phenyl-H). 13C NMR (126 MHz, CD2Cl2): δ 218.8
(dd, CO, JCP = 24, 26 Hz), 213.9 (dd, CO, JCP = 24, 28 Hz), 159.1,
134.2, 134.0, 133.8, 132.0, 131.6, 130.8, 130.5, 130.4, 128.8, 128.7,
128.5, 126.8, 126.1, 91.6 (d, CHO, JCP = 4 Hz). 31P NMR (202 MHz,
CD2Cl2): δ 62.7, 64.7 (AB quartet, JPP = 169 Hz). IR spectrum
(CH2Cl2): νCO 1900, 1967 cm−1. Anal. Calcd for C39H30FeO3P2
(found): C, 70.50 (69.70); H, 4.55 (4.32); N, 0.00 (0.36).
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise indicated, reactions
were conducted using standard Schlenk techniques under an Ar
atmosphere at room temperature with stirring. Ph2PC6H4-2-CHO,30
31
32
Ph2PC6H4-2-Li(OEt2)0.74
,
and Fe(bda)(CO)3 were synthesized
according to literature preparations. Fe2(CO)9 (Strem Chemicals), 1
M TiCl4 in CH2Cl2 (Aldrich), and BF3OEt2 (Aldrich) were used as
received. FcBF4 was obtained from Aldrich and recrystallized prior to
1
use. H, 13C, and 31P NMR spectra were acquired on Varian UNITY
2
INOVA 500NB and UNITY 500 NB instruments. H NMR spectra
were acquired on a Varian UNITY INOVA 600 instrument. Elemental
analyses were performed by the School of Chemical Sciences
Microanalysis Laboratory utilizing a model CE 440 CHN analyzer.
A Spectroline model MB 100 lamp (λ = 365 nm) was used for
experiments requiring UV irradiation.
Ph2PC6H4CHNC6H4Cl (PCHNAr). This ligand was prepared following
a modification of the literature procedure.27 A solution of 0.679 g
(2.34 mmol) of PCHO and 0.304 g (2.38 mmol) of 4-chloroaniline in
20 mL of toluene was refluxed for 16 h in the presence of molecular
sieves. The solution was cooled to room temperature and filtered.
Solvent was removed under vacuum, and the residue was washed with
2 × 2 mL of Et2O. The yellow powder was dried under vacuum. Yield:
Fe[(Ph2PC6H4)2C2H2O2](CO)2 (1). A solution of 0.991 g (3.47
mmol) of Fe(bda)(CO)3 and 2.012 g (6.93 mmol) of PCHO in 30
mL of THF was heated to 60 °C for 4 h. The reaction was followed by
solution IR spectroscopy, and after 2 h, the signals for Fe(bda)(CO)3
were no longer apparent. A large amount of bright yellow precipitate
formed. The slurry was concentrated to 15 mL under reduced pressure
and filtered, leaving bda in the filtrate. The solid was washed with an
additional 5 mL of THF followed by 3 × 10 mL of hexanes and then
1
0.763 g (82%). H NMR (500 MHz, CDCl3): δ 6.81 (d, 2H, phenyl-
H), 6.93 (m, 1H, phenyl-H), 7.22−7.36 (m, 13H, phenyl-H), 7.46 (t,
1H, phenyl-H), 8.17 (m, 1H, phenyl-H), 9.01 (d, 1H, CHN). 31P
NMR (202 MHz, CDCl3): δ −12.4 (s).
Fe(PCHNAr)(CO)4 (5). A solution of 0.897 g (2.46 mmol) of
Fe2(CO)9 and 0.986 g (2.46 mmol) of PCHNAr in 30 mL of THF was
stirred at room temperature for 2 h. All solvents were evaporated
under vacuum, and the residue was extracted with 60 mL of hexanes.
The solution was filtered and concentrated to 10 mL. After the
solution was stored at −20 °C overnight, a yellow powder was
1
left under reduced pressure overnight. H NMR analysis confirmed
that the crystalline product contains 0.5 equiv of THF. The crystalline
yellow solid was collected in a drybox. Yield: 2.26 g (89%).
Redissolving the solid in CH2Cl2 followed by removal of solvent
under reduced pressure afforded a yellow powder containing 1 equiv
of CH2Cl2, which was confirmed by 1H NMR spectroscopy and
elemental analysis. 1H NMR (500 MHz, CD2Cl2): δ 4.49 (s, 2H,
OCH), 7.10−7.64 (m, 24H, phenyl-H), 8.06 (s, 4H, phenyl-H). 13C
NMR (126 MHz, CD2Cl2): δ 211.7 (t, CO, JCP = 21 Hz), 155.2, 136.1,
135.3, 134.5, 133.5, 132.9, 130.9, 130.8, 130.3, 128.8, 128.6, 127.4,
127.3, 123.9, 87.0 (s, OCH). 31P NMR (202 MHz, CD2Cl2): δ 21.1
(s). IR spectrum (CH2Cl2): νCO 1965, 2025 cm−1. Anal. Calcd for
C41H32Cl2FeO4P2 (found): C, 63.35 (63.21); H, 4.15 (4.06); N, 0.00
(0.33).
1
obtained and dried under vacuum. Yield: 0.657 g (47%). H NMR
(500 MHz, CDCl3): δ 6.48 (dt, 2H, phenyl-H), 7.12 (dt, 2H, phenyl-
H), 7.30 (ddd, 1H, phenyl-H), 7.44−7.52 (m, 7H, phenyl-H), 7.59−
7.66 (m, 5H, phenyl-H), 8.40 (ddd, 1H, phenyl-H), 8.51 (s, 1H,
CHN). 13C NMR (126 MHz, CD2Cl2): δ 213.6 (d, CO, JCP = 18 Hz),
158.6 (d, CHN, JCP = 6 Hz), 149.9, 138.1, 135.1, 134.9, 134.2, 133.8,
131.9, 131.8, 131.6, 131.1, 129.5, 129.4, 129.2, 122.8. 31P NMR (202
MHz, CDCl3): δ 68.5 (s). IR spectrum (hexanes): νCO 1948, 1981,
2053 cm−1. Anal. Calcd for C29H19ClFeNO4P (found): C, 61.35
(61.50); H, 3.37 (3.35); N, 2.47 (2.59).
Fe(PCHNAr)(CO)3 (6). A solution of 0.224 g (0.78 mmol) of
Fe(bda)(CO)3 and 0.296 g (0.74 mmol) of PCHNAr in 30 mL of
toluene was heated at 45 °C overnight. The reaction was followed by
31P NMR spectroscopy. After ca. 24 h, the 31P NMR signals for
PCHNAr fully disappeared, indicating the completion of the reaction.
Solvents were removed under reduced pressure, and the obtained solid
contains the desired product with bda as impurity. The mixture can be
used for the preparation of 7 without further purification. Alternatively,
compound 6 could be generated by photolysis of 5 in a NMR tube.
However, the photolysis is not efficient enough for large-scale
Fe(PCHO)(CO)4 (2). A solution of 1.00 g (3.44 mmol) of PCHO in
50 mL of THF was transferred via cannula onto 1.25 g (3.44 mmol) of
solid Fe2(CO)9. The mixture was stirred at 23 °C until all solids
dissolved (40 min). 31P NMR spectroscopy confirmed the
consumption of the PCHO. The solvent and most Fe(CO)5 were
removed under reduced pressure. The yellow residue was extracted
into 2 × 50 mL of hexanes, and the solvent was removed under
1
reduced pressure to give a yellow powder. Yield: 0.768 g (49%). H
NMR (500 MHz, CD2Cl2): δ 7.17−8.07 (m, 14H, phenyl-H), 10.18
(s, 1H, CHO). 13C NMR (126 MHz, CD2Cl2): δ 213.6 (d, CO, JCP
=
18 Hz), 189.8 (d, CHO, JCP = 8 Hz), 138.0, 137.2, 134.9, 134.2, 133.7,
6412
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Organometallics
Article
1
preparation. H NMR (500 MHz, toluene-d8): δ 6.65−7.86 (m, 18H,
was allowed to stir at 23 °C over 24 h with no noticeable change in the
solution IR spectrum.
HOTf. A solution of 10.6 mg of 1 (0.015 mmol) in CH2Cl2 was
phenyl-H), 4.94 (s, br, 1H, CHN). 31P NMR (202 MHz, toluene-d8):
δ 62.8 (s). IR spectrum (hexanes): νCO = 1962, 1989, 2046 cm−1.
Fe[(Ph2PC6H4)2CH(O)CH(NAr)](CO)2 (7). A mixture of crude
compound 6 (prepared from 0.224 g (0.78 mmol) of Fe(bda)(CO)3
and 0.296 g (0.74 mmol) of PCHNAr) and 0.218 g (0.75 mmol) of
Ph2PC6H4CHO was dissolved in 20 mL of toluene. The solution was
heated at 50 °C for 2 h. The greenish-brown solution was filtered off
from the cold reaction mixture, and the dark purple precipitate was
washed with about 10 mL of hexanes. The solid was dried under
reduced pressure. Yield: 0.433 g (72%). 1H NMR (500 MHz,
CD2Cl2): δ 7.99 (t, 2H, phenyl-H), 7.83 (m, 2H, phenyl-H), 7.59 (m,
3H, phenyl-H), 7.53 (t, 1H, phenyl-H), 7.35−7.42 (m, 9H, phenyl-H),
7.28 (t, 2H, phenyl-H), 7.22 (t, 1H, phenyl-H), 7.14 (m, 3H, phenyl-
H), 7.05 (t, 2H, phenyl-H), 6.93 (t, 1H, phenyl-H), 6.87 (t, 2H,
phenyl-H), 6.21 (d, 2H, phenyl-H), 5.90 (d, 2H, phenyl-H), 4.69 (d,
1H, OCH), 4.14 (dd, 1H, NCH). 13C NMR (126 MHz, CD2Cl2): δ
212.6 (CO), 211.3 (CO), 157.7, 154.4, 154.0, 136.4, 136.2, 134.3,
134.2, 133.7, 133.6, 133.0, 132.9, 132.6, 132.5, 131.6, 130.8, 130.6,
130.5, 129.8, 129.6, 129.5, 128.9, 128.8, 128.7, 128.6, 128.1, 128.0,
127.7, 127.6, 127.4, 127.1, 125.3, 124.4, 119.4, 115.8, 86.7 (s, OCH),
74.7 (s, NCH). 31P NMR (202 MHz, CD2Cl2): δ 26.6, 25.1, 22.2, 20.7
(AB quartet, JPP = 301 Hz). IR spectrum (CH2Cl2): νCO 1959, 2022
cm−1. Anal. Calcd for C46H34ClFeNO3P2 (found): C, 68.89 (68.40);
H, 4.27 (4.26); N, 1.75 (2.01).
treated with 0.15 mL of CH2Cl2 solution of HOTf (vCH2Cl2:vHOTf
=
100:1), and the IR spectrum was obtained. Two CO bands at 2001
and 2050 cm−1 were observed. Another 0.15 mL of HOTf solution
was added, and the mixture was stirred for 20 min. The IR spectrum
was taken, and CO bands at 2036 and 2075 cm−1 were obtained.
MeOTf. A solution of 18.7 mg of 1 (0.027 mmol) in CH2Cl2 was
treated with 0.3 mL of a CH2Cl2 solution of MeOTf (vCH2Cl2:vMeOTf
=
100:1), and the IR spectrum was obtained. Two CO bands at 2002
and 2051 cm−1 were observed. With additional MeOTf, we detected
bands at 2030 and 2070 cm−1 assigned to the dication.
Detection of Intermediates in Formation of 1. A solution of
9.5 mg (0.02 mmol) of 2 in 0.8 mL of toluene-d8 was prepared in a J.
Young NMR tube. The tube was sealed, and 1H and 31P NMR spectra
1
were obtained. The tube was then irradiated, and H and 31P NMR
spectra were collected periodically over the course of 2 h. After 2 h, a
large quantity of the hydride product was detected with 1 also formed.
1
In Situ Detection of Hydride HFe(κ2-PCO)(CO)3. H NMR (500
MHz, CD2Cl2): δ −7.7 (d, 47.5 Hz, 1 H, Fe−H). 31P{1H} NMR (202
MHz, CD2Cl2): δ 89.5 (s, P−Fe−H). 31P NMR (202 MHz, CD2Cl2):
δ 89.5 (d, 47.5 Hz, P−Fe−H).
1
In Situ Detection of 3. H NMR (500 MHz, CD2Cl2): δ 5.91 (s 1
H, Fe-(CHO)). 31P NMR (202 MHz, CD2Cl2): δ 58.2 (s, P−Fe).
Ph2PC6H4CDO (PCDO). Our procedure was adapted from the
literature reaction of DMF and the lithiated phosphine.33 A solution of
1.02 g (3.16 mmol) of Ph2PC6H4Li(OEt2)0.74 and 1.0 mL (12.8 mmol)
of DMF-d7 in 20 mL of ether was stirred overnight. The reaction
mixture was treated with 10 mL of 3 M HCl and then extracted with
CH2Cl2 (20 mL × 3). The organic phase was dried with Na2SO4 and
evaporated under vacuum. The crude product was washed with 10 mL
of methanol and 10 mL of pentane and then dried under reduced
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, 31P NMR, and IR spectra for 1−8 and CIF files
including X-ray crystallographic data for the structures of 1, 4,
7, and 8. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
pressure. Yield: 0.448 g (49%). H NMR (500 MHz, CDCl3): δ 7.98
AUTHOR INFORMATION
Corresponding Author
*E-mail: rauchfuz@illinois.edu.
Notes
■
(m, 1H, phenyl-H), 7.45−7.52 (m, 2H, phenyl-H), 7.33−7.37 (m, 6H,
phenyl-H), 7.27−7.31 (m, 4H, phenyl-H), 6.98 (m, 1H, phenyl-H).
2H NMR (92 MHz, CHCl3/CDCl3): δ 10.53 (s, CDO). 31P NMR
(202 MHz, CDCl3): δ −11.0 (s).
Reaction of PCDO with 6 to give 7-d1. A NMR tube was charged
with 5.1 mg (0.018 mmol) of Fe(bda)(CO)3, 7.9 mg (0.020 mmol) of
PCHNAr, and 0.5 mL of toluene-d8. The mixture was heated at 45 °C
overnight and then treated with 5.4 mg (0.018 mmol) of PCDO.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
1
This research was conducted under contract DEFG02-
90ER14146 with the U.S. Department of Energy by its
Division of Chemical Sciences, Office of Basic Energy Sciences.
NMR spectra were recorded after heating at 50 °C for 2 h. H NMR
(500 MHz, toluene-d8): δ 8.23 (m, 2H, phenyl-H), 7.97 (t, 2H,
phenyl-H), 7.60 (t, 2H, phenyl-H), 6.78−7.24 (m, 22H, phenyl-H),
6.46 (m, 2H, phenyl-H), 6.12 (d, 2H, phenyl-H), 4.26 (s, 1H, NCH).
2H NMR (92 MHz, C7H8/C7D8): δ 4.91 (s, OCD). 31P NMR (202
MHz, toluene-d8): δ 26.4, 24.9, 21.6, 20.1 (AB quartet, JPP = 305 Hz).
Fe[(Ph2PC6H4)2C2H2O2(BF3)2](CO)2 (8). A solution of 0.563 g (0.813
mmol) of 1 in 20 mL of CH2Cl2 was treated with 0.20 mL (1.63
mmol) of BF3OEt2 added dropwise via syringe. The solution was
stirred for 1 h, before the solvent was removed under reduced
pressure. The yellow powder was recrystallized from CH2Cl2/hexanes.
Residual solvent was removed under reduced pressure overnight. The
product was obtained as a microcrystalline yellow solid. Yield: 0.673 g
(95%). 1H NMR (500 MHz, CD2Cl2): δ 5.05 (s, 2H, OCH), 7.02 (q,
2H, phenyl-H), 7.36 (m, 6H, phenyl-H), 7.47 (m, 9H, phenyl-H), 7.57
(m, 4H, phenyl-H), 7.66 (t, 3H, phenyl-H), 7.92 (q, 4H, phenyl-H).
19F NMR (470 MHz, CD2Cl2): δ −146 (s). 31P NMR (202 MHz,
CD2Cl2): δ 31.5 (s). IR spectrum (CH2Cl2): νCO 2023, 2066 cm−1.
Anal. Calcd for C40H30B2F6FeO4P2 (found): C, 58.02 (57.62); H, 3.65
(3.58); N, 0.00 (0.27).
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