Imine-Centered Reactions in Imino-Phosphine Complexes of Iron Carbonyls

Article abstract of DOI:10.1021/acs.organomet.6b00318

Fundamental reactions of imino-phosphine ligands were elucidated through studies on Ph₂PC₆H₄CH=NC₆H₄-4-Cl (PCHNAr^Cl) complexes of iron(0), iron(I), and iron(II). The reaction of PCHNAr^Cl with Fe(bda)(CO)₃ gives Fe(PCHNAr^Cl)(CO)₃ (1), featuring an η²-imine. DNMR studies, its optical properties, and DFT calculations suggest that 1 racemizes on the NMR time scale via an achiral N-bonded imine intermediate. The N-imine isomer is more stable in Fe(PCHNAr^OMe)(CO)₃ (1^OMe), which crystallized despite being the minor isomer in solution. Protonation of 1 by HBF₄·Et₂O gave the iminium complex [1H]BF₄. The related diphosphine complex Fe(PCHNAr^Cl)(PMe₃)(CO)₂ (2), which features an η²-imine, was shown to also undergo N protonation. Oxidation of 1 and 2 with FcBF₄ gave the Fe(I) compounds [1]BF₄ and [2]BF₄. The oxidation-induced change in hapticity of the imine from η² in [1]⁰ to κ¹ in [1]⁺ was verified crystallographically. Substitution of a CO ligand in 1 with PCHNAr^Cl gave Fe[P₂(NAr^Cl)₂](CO)₂ (3), which contains the tetradentate diamidodiphosphine ligand. This C-C coupling is reversed by chemical oxidation of 3 with FcOTf. The oxidized product of [Fe(PCHNAr^Cl)₂(CO)₂]²⁺ ([4]²⁺) was prepared independently by the reaction of [1]⁺, PCHNAr^Cl, and Fc⁺. The C-C scission is proposed to proceed concomitantly with the reduction of Fe(II) via an intermediate related to [2]⁺.

Full text of DOI:10.1021/acs.organomet.6b00318

Article
pubs.acs.org/Organometallics
Imine-Centered Reactions in Imino-Phosphine Complexes of Iron
Carbonyls
Wan-Yi Chu, Casseday P. Richers, Elizabeth R. Kahle, and Thomas B. Rauchfuss*
School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
Federica Arrigoni and Giuseppe Zampella
Department of Biotechnology and Biosciences, University of Milano-Bicocca, Piazza della Scienza 2, 20126 Milan, Italy
S
* Supporting Information
ABSTRACT: Fundamental reactions of imino-phosphine ligands were
elucidated through studies on Ph2PC6H4CHNC6H4-4-Cl
(PCHNArCl) complexes of iron(0), iron(I), and iron(II). The reaction
of PCHNArCl with Fe(bda)(CO)3 gives Fe(PCHNArCl)(CO)3 (1),
featuring an η2-imine. DNMR studies, its optical properties, and DFT
calculations suggest that 1 racemizes on the NMR time scale via an
achiral N-bonded imine intermediate. The N-imine isomer is more
stable in Fe(PCHNArOMe)(CO)3 (1OMe), which crystallized despite
being the minor isomer in solution. Protonation of 1 by HBF4·Et2O gave
the iminium complex [1H]BF4. The related diphosphine complex
Fe(PCHNArCl)(PMe3)(CO)2 (2), which features an η2-imine, was
shown to also undergo N protonation. Oxidation of 1 and 2 with FcBF4 gave the Fe(I) compounds [1]BF4 and [2]BF4. The
oxidation-induced change in hapticity of the imine from η2 in [1]0 to κ1 in [1]+ was verified crystallographically. Substitution of a
CO ligand in 1 with PCHNArCl gave Fe[P2(NArCl)2](CO)2 (3), which contains the tetradentate diamidodiphosphine ligand.
This C−C coupling is reversed by chemical oxidation of 3 with FcOTf. The oxidized product of [Fe(PCHNArCl)2(CO)2]2+
([4]2+) was prepared independently by the reaction of [1]+, PCHNArCl, and Fc+. The C−C scission is proposed to proceed
concomitantly with the reduction of Fe(II) via an intermediate related to [2]+.
tetradentate 6-5-6 diiminodiphosphines,12,13 tetradentate 5-5-5
INTRODUCTION
■
diiminodiphosphines,14 tetradentate and macrocyclic 6-5-6-5
Although imines are common ligands in classical coordination
chemistry, they also serve as platforms for exciting develop-
diminodiphosphines,15 and 5-5 diphosphine-imines,16 where
ments in organometallic chemistry. Applications, manifested or
the hyphenated numbers refer to the size of the resulting
promising, include post-metallocene catalysis of alkene
polymerization,1 a theme that exploits the modular synthesis
chelate rings. Other phosphine-imine complexes of iron have
been described.17,18
of imines and convenient introduction of chiral centers by
reduction to amines.2 Imine ligands also support Fe- and Co-
In contrast to the extensive work on tri- and tetradentate
based alkene hydrofunctionalization catalysts.3 Excellent
electrocatalysts for hydrogen evolution often are imine
phosphine-imines, relatively little effort has focused on the
properties of iron complexes of simple bidentate phosphine-
complexes.4 Thus, although imine complexes have been
imines. To probe this area, the present report examines the iron
derivatives of Fe(0), Fe(I), and Fe(II) oxidation states. These
studies uncovered an unrecognized and reversible responsive-
ness of the phosphine-imine platform: redox-induced N
complexation vs π complexation and C−C coupling vs scission
of the imine center. As is shown below, even the simple Fe(0)
complexes exhibit dynamic behavior involving equilibria
between N- and π-bonded imine ligands.
exhaustively studied in classical coordination chemistry, they
remain fertile in an organometallic context. In many cases, the
intriguing reactivity is attributable to the redox-noninnocent
character of α-diimines.5
In view of the successes with diimine ligands, hybrids of
imines and phosphines represent attractive ligand platforms
that combine the modularity of the imine ligands and the high
affinity of soft metal centers for phosphine ligands.6 Iron(II)
complexes of phosphine-imines have been extensively devel-
oped.7 Chelating phosphine-imine ligands are usually generated
by the condensation of diamines with diphenylphosphino-2-
benzaldehyde (PCHO)8 or the phosphine-substituted acetalde-
hydes R2PCH2CHO.9−11 Ligand platforms include, inter alia,
Received: April 21, 2016
© XXXX American Chemical Society
A
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Although π-bonded aldehyde complexes have been charac-
terized,22 η2-imines are more typical for early metals:23 e.g.,
metallaaziridines.24 Iron phosphine-imine complexes typically
are N-bonded,11,12,17,25 although a ferraaziridine has been
recently reported.12 Examples of Fe(0) containing π-bound
aldimines have been reported:26 e.g., [Fe(DIP)]2 (DIP =
NC5H3-2,6-(CHNAr)2, Ar = 2,6-C6H3(iPr)2).27
RESULTS AND DISCUSSION
■
Studies focused on iron carbonyl derivatives of the phosphine-
imine derived from PCHO and 4-chloroaniline, abbreviated
PCHNArCl. The new complexes are depicted in Scheme 1.
Scheme 1. Conversions of PCHNArCl Complexes Discussed
in This Report
With bands at 2044, 1983, and 1955 cm−1, the IR bands of 1
in the νCO region are similar to those in styryl phosphine
complex Fe(CO)3(Ph2PC6H4CHCH2), which is yellow:
2039, 1972, 1945 cm−1. In contrast, derivatives with chelating
donors exhibit lower frequency modes: 1985, 1913, and 1897
cm−1 (Fe(dppbz)(CO)3, which is yellow) and 1968, 1897, and
1862 cm−1 (Fe(phen)(CO)3, which is blue).28 These
1
comparisons are consistent with an η2-imine ligand in 1. H
NMR spectra for both yellow d12-cyclohexane solutions and
green CD2Cl2 solutions of 1 are characteristic of η2-imine
species as indicated by the resonances at δ ∼5 for the CHN
group. In the −100 °C 13C NMR spectrum, three 31P-coupled
signals are observed for Fe-CO groups. At room temperature,
these signals coalesce.
Solutions of 1 exhibit a number of properties that indicate
dynamics and/or isomerism. Solutions are solvatochromic,
being yellow in alkanes and green in chlorocarbons and
aromatic solvents. The UV−vis spectrum of 1 exhibited a
maximum at 395 nm in pentane that shifts in CH2Cl2 to a
shoulder ∼380 nm (see Figure S6 in the Supporting
Information). Pentane solutions of 1 are also thermochromic,
changing from yellow to green upon cooling to −78 °C. IR
spectra in the νCO region were very similar for both yellow and
green solutions, consisting of the expected three bands (also see
below). 1H NMR and IR spectra for both yellow d12-
cyclohexane solutions and green CD2Cl2 solutions of 1 are
Fe(PCHNArCl)(CO)3. The complex Fe(PCHNArCl)(CO)3
(1) has been observed spectroscopically as an intermediate in
recent work.20 Treatment of Fe(bda)(CO)3 (bda = benzylide-
neacetone) with 1 equiv of PCHNArCl gave 1 in high yield.
Crude samples of 1 were always contaminated with a small
amount of a phosphine-containing complex (31P NMR: δ 25.6)
later identified as the C−C bond coupled product Fe-
[P2(NArCl)2](CO)2, (3). Recrystallization afforded analytically
pure samples of 1 as yellow crystals.
1
similar. The H NMR signals of 1 shift significantly in the aryl
region as a function of temperature (see Figures S3 and S5 in
the Supporting Information). The signal assigned to the π-
CHNAr ligand is broad near 0 °C, but sharper at higher and
lower temperatures. The signal is only slightly shifted from its
low-temperature position. The δ 62.8 signal in the 31P NMR
spectra of 1 remained invariant between −70 and 20 °C. These
observations can be explained by first recognizing that 1 is
chiral, as are all complexes containing one η2-aldimine ligand.
On the NMR time scale, the rate of racemization is slow below
room temperature. The deep colors arise from traces of the N-
bonded isomer, which is an intermediate in the racemization.
This hypothesis was verified by experiments on an analogue of
1 and further examined by DFT calculations.
Crystallographic analysis of 1 revealed a chelated complex
with a π-bound imine (Figure 1). The complex features Fe(1)−
Studies on Fe(PCHNHArOMe)(CO)3. The analogue of 1
with 4-methoxyphenyl in place of a chlorophenyl group, i.e.
1OMe, was prepared straightforwardly. As substituents on aryl
rings, Cl and OMe differ strongly electronically, as reflected by
their Hammett constants (σp), which are 0.23 and 0.27,
respectively.29 The spectroscopy of 1OMe is consistent with the
presence of two isomers. The FT-IR spectrum of the product
consisted of two sets of bands in the νCO region, three in the
region expected for the η2-imine, and a partially overlapping
envelope at ∼80 cm−1 lower frequency assigned to the N-
bonded isomer (Figure 2). The 31P NMR spectrum of 1OMe in
CD2Cl2 solutions consists of a broad singlet at δ 64.2 at room
temperature. At −100 °C, the spectrum decoalesced into
singlets at δ 70.4 and 65.2 in a 1:4.5 ratio. The corresponding
1H NMR spectrum also showed two sets of signals also in a 1:4
ratio at −100 °C. Signals at δ 8.27 and 4.10 are assigned to
Figure 1. Structure (50% thermal ellipsoids) of 1. Hydrogen atoms are
omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)−
P(1), 2.2510(5); (Fe−C)avg, 1.887(2); (C−O)avg, 1.1367; Fe(1)−
N(1), 1.981(2); Fe(1)−C(4), 2.066(2); C(4)−N(1), 1.380(3); P(1)−
Fe(1)−C(1), 171.54(8).
C(4) and Fe(1)−N(1) bond distances of 2.066(2) and
1.981(2) Å, respectively. The imine carbon−nitrogen distance
of 1.380(3) Å is longer than that of a typical κ1-imine (1.25−1.3
Å),10−12 suggesting significant π back-donation from the metal
center. Related complexes adopting similar structures, but not
crystallographically characterized, include the styryl and formyl
derivatives Fe(Ph2PC6H4CHX)(CO)3 for X = CH2,21 O.
B
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from cold CH2Cl2/pentane solutions. Crystallographic analysis
revealed a trigonal-bipyramidal complex with a κ1-imine ligand
bound to a trigonal-bipyramidal Fe(0) center (Figure 4).
Figure 4. Two views of the structure (50% thermal ellipsoids) of 1OMe
.
Hydrogen atoms are omitted for clarity. Selected distances (Å) and
angles (deg): Fe(1)−P(1), 2.2027(5); (Fe−C)avg, 1.770; Fe(1)−N(1),
2.033(1); C(10)−N(1), 1.295(2); (C−O)avg, 1.1545; P(1)−Fe(1)−
N(1), 83.83; C1−Fe1−C2, 86.31(6); C1−Fe1−C3, 117.09(6); C2−
Fe1−C3, 95.42(6).
Figure 2. FT-IR spectra of 1 and 1OMe in CH2Cl2 solution. Both
isomers contribute to the band at 1980 cm−1. Scrutiny of the spectrum
of 1 reveals very weak bands in the region expected for the N-bonded
isomer.
CHN for the N-and π-bonded isomers, respectively, in a
1
1:4.4 ratio. The H NMR signal expected for an N-bonded
Metrical aspects of the CO ligands are consistent with 1OMe
being Fe(0): (C−O)avg = 1.154 Å and (Fe−C)avg = 1.770 Å.
These distances are longer and shorter by 0.02 and 0.02 Å,
respectively, with respect to the same parameters in 1 (1.137,
1.80 Å).
aldimine is δ ∼9.19 The room-temperature H NMR spectrum
1
features a signal at δ 6.25, corresponding to a time average of
the signals for the N- and π-bonded isomers.
The 13C NMR spectrum of 1OMe at −100 °C not only
confirms the presence of two isomers but also provides insights
into their stereochemistry (Figure 3). A signal near δ 165 was
Certain structural features of 1OMe are reminiscent of the
complex (PNPBu‑t)Fe(CO)2, where PNP is the bulky pincer
ligand C5H3N-2,5-(PBut2)2.31 In contrast to the related red
complex containing PPri2 substituents, the PBut2 derivative is
deep blue and adopts an unusual geometry with N−Fe−C
angles of 152.52(5) and 101.57(4)° and an C−Fe−C angle of
105°. In 1OMe, these angles are 134, 108, and 117°,
respectively.32 In (PNPBu‑t)Fe(CO)2 the Fe−C−O bond angles
are 176.68(10) and 171.87(10)°, comparable to the angles for
the CO ligands in the trigonal plane: 175.1(1), 173.6(1),
175.1(1)°. However, the complexes Fe(bipy)(CO)3 and
Fe(phen)(CO)3 also are deep blue and feature comparably
distorted Fe-CO linkages, with angles between 175 and 176°.28
DFT Calculations on Fe(PCHNArR)(CO)3. Using the
BP86/TZVP setup, the structures of 1 and 1OMe were examined
by DFT. The crystallographically observed π- and N-bonded
structures were reproduced with good accuracy. The greatest
deviation is with regard to the Fe−P bond in 1 (0.05 Å longer
in DFT model vs experiment). With respect to the energetics,
the calculations suggest that the N- and π-bonded imine
complexes have similar energies but that the N-bonding is
favored by 1.8−2.9 kcal/mol for 1OMe. The calculations indicate
that that barriers for the N- to π-bonded isomerism range from
8.6 to 14 kcal/mol, depending on the functional. These barriers
are compatible with the results of the DNMR studies, which
indicate fast exchange between π- and N-bonded structures. For
the N-bonded isomers, the calculations do not favor N vs P as
the apical ligand on the trigonal-bipyramidal Fe(0) center. The
νCO bands for these two possibilities differ by less than 3 cm−1.
When π-bonded, aldimines can exist as cis and trans isomers,
but the calculations indicate that the trans isomer is stabilized
by 6.8−8.3 kcal/mol, depending on the functional (Scheme 2).
IR spectra are predicted to differ strongly for the N- vs π-
bonded isomers. For both 1 and 1OMe, DFT-calculated νCO
bands occur on average at 38 cm−1 higher energy for the η2-
imine isomers in comparison to the N-bonded isomers. The
Figure 3. Low-field portion of the 13C NMR spectrum of 1OMe at
−100 °C (left) and at room temperature (right). The low-field singlet
in the −100 °C spectrum is assigned to the three rapidly
interconverting CO ligands of the N-bonded isomer. The multiplets
near δ 210 are assigned to the three diastereotopic CO ligands, each
exhibiting 31P coupling.
1
assigned to CHNAr by the H−13C HSQC method. In the
CO region, two sets of signals are observed: three doublets and
a singlet. The doublets are assigned to 31P-coupled signals for
the π-bonded isomer, a quasi-octahedral and hence rigidified
species relative to the pentacoordinate N-bonded isomer, which
gives rise to a singlet owing to Berry pseudorotation. No
31P−13CO coupling is observed for this signal, although
typically for Fe(CO)5−x(PR3)x complexes J(31P,13C) is ∼20−
40 Hz.30 Near room temperature, all four CO signals coalesce.
Comparable analysis was applied to the ipso aryl carbon centers
(Figure S12 in the Supporting Information).
Qualitative experiments were conducted to test other imine
substituents. The complexes Fe(PCHNBu-t)(CO)3 and Fe-
(PCHNArNMe2)(CO)3 were prepared using the phosphine-
imines derived from t-BuNH2 and 4-dimethylaminoaniline,
respectively. These complexes and the ligands were charac-
1
terized by H and 31P NMR spectroscopy. The IR spectra in
the νCO region showed that the Fe(CO)3 derivatives consist of
both N- and π-bonded isomers in a ratio comparable to that
observed for 1OMe
.
Crystal Structure of Fe(PCHNArOMe)(CO)3. Despite being
the minor species in solution, the N-bonded isomer crystallized
values are (νCO
)
1993 vs 1955 cm−1 for 1 and 1988 vs 1950
avg
C
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Scheme 2. Gas-Phase DFT-Calculated (BP86/TZVP Level) Energies for Isomers of 1, Including Activation Barriers in Two
Cases
cm−1 for 1OMe. Calculated IR bands are in satisfying quantitative
agreement with experimental observations for the η2-imine in
both 1 and 1OMe. For the N-bonded isomer of 1OMe, the
deviation is greater for low-frequency modes. This last issue is
probably due to the necessity of calculating an analytical
Hessian (and thus bond force constants) of the systems under
investigation in the gas phase: i.e., not including solvation
effects.
carbons. An analogous pinacol-type coupling has been observed
for iron(0) complexes of Ph2PC6H4CHO.20 The diamido
complex 3 was also obtained by heating Fe(bda)(CO)3 with 2
equiv of PCHNArCl in a toluene solution. Logically, the
formation of 3 follows the steps in Scheme 3.
Scheme 3. Proposed Pathway for Conversion of 1 to 3
Substitution Reactions of Fe(PCHNArCl)(CO)3 by PMe3.
Treatment of 1 with excess PMe3 gave the dicarbonyl
Fe(PCHNArCl)(PMe3)(CO)2 (2). No evidence of double
substitution was observed. Unlike the case for 1, the bright
orange complex 2 does not exhibit solvatochromism or
thermochromism, consistent with the nearly exclusive existence
of the π-bonded isomer. The 31P NMR spectrum of 2 was
indicative of a bis(phosphine) complex, which exhibited
doublets at δ 64.1 and 21.4. The large coupling constant of
J
P−P = 166 Hz suggests that the phosphines are mutually trans,
as is typical.33 The H NMR spectrum is similar to that of 1,
with an additional set of doublet of doublets at δ 1.04 for the
PMe3 resonances. At 20 °C, NC6H4Cl resonances at δ 6.89−
6.60 were broad, and a dynamic process analyzed for 1 and
1OMe is assumed to apply to 2. The structure of 2 was
confirmed crystallographically. It indeed features mutually trans
phosphines (Figure 5). The CN distance of 1.391(2) Å is
elongated by only ∼0.01 Å in comparison to that in 1, reflecting
increased π back-bonding.
1
The 31P NMR chemical shifts for this (and related) ferrous
dicarbonyl occurs at relatively high fields, near δ 25 (Table 1).
The relative donor properties of the ligands [P2O2]2−,
[P2O(NArCl)]2−, and [P2(NArCl)2]2− are indicated by the
Reaction of Fe(PCHNHArCl)(CO)3 with PCHNHArCl.
When it was heated in solution in the presence of additional
PCHNArCl, 1 converted to the dark blue diamido complex
Fe[P2(NArCl)2]CO)2 (3), where [P2(NArCl)2]2− is the diamide
energies of (νCO
)
in the corresponding ferrous dicarbonyl
avg
complexes. These values indicate that the amido substituents
are stronger donors than alkoxo ligands.
[Ph2PC6H4CH(NArCl)]2 arising from coupling of the imine
2−
The structure of 3 was confirmed crystallographically (Figure
6). Reduction of the imine functionality is evident from the
long N(1)−C(16) and N(2)−C(15) bonds, with an average
distance of 1.457 Å. The complex has idealized C2 symmetry.
The average Fe−Namide distance of 2.039 Å for the 18e species
3 is also long in comparison with the Fe−Namide distance of
1.857 Å observed for the 16e phosphine-amido iron complex
FeH(PNP)CO (PNP = N(CH2CH2PiPr2)2).34 At 1.779(3) and
1.774(3) Å, the Fe−CO distances are shorter than in 1 and
even 2, consistent with strong π back-bonding, enhanced by the
presence of the two strongly π donating amido ligands. The
length of the unique C(15)−C(16) bond, 1.530(4) Å, is not
elongated for a C−C single bond.
Protonation of Fe(PCHNArCl)(CO)3. The protonation of 1
was investigated to evaluate the relative basicity of the Fe(0)35
vs the imine centers. Treatment of 1 with H(Et2O)2BArF4 (ArF
= 3,5-(CF3)2C6H3) gave the corresponding η2-iminium
complex [Fe(PCHNHArCl)(CO)3]BArF ([1H]BArF ). Proto-
Figure 5. Structure (50% thermal ellipsoids) of Fe(PCHNArCl)-
(PMe3)(CO)2 (2). Hydrogen atoms have been omitted for clarity.
Selected distances (Å) and angles (deg): Fe(1)−P(1), 2.2465(5);
Fe(1)−C(1), 1.847(1); Fe(1)−C(2), 1.844(1); Fe(1)−C(3),
1.801(1); Fe(1)−N(1), 2.021(1); Fe(1)−C(4), 2.975(1); C(6)−
N(1), 1.391(2); P(1)−Fe(1)−C(1), 173.29(5).
4
4
1
nation at nitrogen was indicated by H−13C HSQC method
1
(Figure S20 in the Supporting Information). Specifically, H
NMR signals for the CHNHArCl group manifested at δ 5.56
and 4.70 (1:1 intensity), but only the δ 5.56 signal correlated to
the 13C signal at δ 63.4, which is thus assigned to CH. Having
D
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The iminium cation [H1]+ was unreactive toward excess
Table 1. Spectroscopic Properties for New Complexes and
Selected Reference Compounds
H(OEt2)2BArF .
4
Although [1H]BArF could not be obtained as X-ray-quality
4
δ(31P)
(CD2Cl2),
ppm
δ(1H) NCH
(CD2Cl2),
ppm
crystals, single crystals of [1H]BF4·THF were obtained by
pentane vapor diffusion into a THF/CH2Cl2 solution. The
crystallographic results were consistent with the structure
νCO (CH2Cl2),
compound
cm−1
20
PCHNArCl
−12.4
68.5
9.01
8.51
5.16
deduced by NMR spectroscopy for the related BArF − salt
20
Fe(κ1-PCHNArCl)(CO)4
Fe(PCHNArCl)(CO)3 (1), 2044, 1983, 1955
2053, 1981, 1948
4
(Figure 7). The η2-iminium ligand exhibited trans geometry,
62.9
π isomer
(DFT: 2035,
1981, 1963)
a
a
Fe(PCHNArOMe)(CO)3
2039, 1980, 1949
(DFT: 2030,
1977, 1958)
65.1
5.0
(1OMe), π isomer
a
a
Fe(PCHNArOMe)(CO)3
1980, 1902, 1870
(DFT: 1995,
70.4
8.27
(1OMe), N isomer
1937, 1919)
b
Fe(PCHNArCl)(PMe3)
(CO)2 (2)
1965, 1900
64.1, 21.4 4.27
64.8 4.70
72.3, 16.9 4.39
[Fe(PCHNHArCl)(CO)3] 2089, 2039, 2017
BArF ([1H]BArF )
4
4
[Fe(PCHNHArCl)(PMe3) 2015, 1956
(CO)2]BArF ([2H]
4
BArF )
4
[Fe(PCHNArCl)(CO)3]
2080, 2022, 1991
1990, 1920
BF4 ([1]BF4)
[Fe(PCHNArCl)(PMe3)
(CO)2]BF4 ([2]BF4)
Fe[P2(NArCl)2](CO)2 (3) 2010, 1954
25.9
21.5
4.37
4.49
(OCH)
20
Fe(P2O2)(CO)2
2025, 1965
2022, 1959
2059, 2023
Figure 7. Structure (50% thermal ellipsoids) of [1H]BF4·THF.
Selected hydrogen atoms, solvent, and anion have been omitted for
clarity. Selected distances (Å) and angles (deg): Fe(1)−P(1),
2.2705(8); Fe(1)−C(1), 1.834(3); Fe(1)−C(2), 1.819(3); Fe(1)−
C(3), 1.786(3); Fe(1)−N(1), 2.020(3); Fe(1)−C(4), 2.013(3);
C(4)−N(1), 1.427(4); N(1)−H(1), 0.89(5); P(1)−Fe(1)−C(1),
176.8(1).
20
Fe[P2O(NArCl)](CO)2
25.9, 21.4 4.14, 4.69
(OCH)
[Fe(PCHNArCl)2(CO)2]
(OTf)2 ([4](OTf)2)
40.5
8.26
a
b
Recorded at −100 °C in CD2Cl2 solution. Recorded on THF
solution.
and the C(4)−N(1) distance (1.427(4) Å) is elongated by 0.04
Å in comparison to that in 1. A similarly elongated C−N
distance, 1.434(5) Å, has been observed for an iron carbonyl
complex of a chelating iminium ligand.12
In contrast to the case for [1H]BArF , [1H]BF4 exists as
4
isomers in CD2Cl2, as indicated by two sets of resonances in the
1H and 31P NMR spectra. The second species is proposed to be
the cis isomer of [1H]+. The equilibrium ratio trans-
[1H]BF4:cis-[1H]BF4 is 93:7 (eq 1 and Figures S23−S25 in
the Supporting Information). The isomerization is fast at room
temperature but not faster than the NMR time scale: NMR
spectra recorded within minutes after dissolution of single
crystals of [1H]BF4 in CD2Cl2 always showed a mixture of
isomers. The FT-IR spectrum of [1H]BF4 showed two sets of
partially resolved νCO signals (see Figure S31 in the Supporting
Information).
Figure 6. Structure (50% thermal ellipsoids) of 3. Selected hydrogen
atoms have been omitted for clarity. Selected distances (Å) and angles
(deg): Fe(1)−P(1), 2.2691(8); Fe(1)−P(2), 2.2680(8); Fe(1)−C(1),
1.779(3); Fe(1)−C(2), 1.774(3); Fe(1)−N(1), 2.033(2); Fe(1)−
N(2), 2.044(2); N(1)−C(16), 1.456(3); N(2)−C(15), 1.458(3);
C(15)−C(16), 1.530 (4); P(1)−Fe(1)−P(2), 168.72(4).
Qualitative experiments monitored by IR and 31P{1H} NMR
spectroscopy showed that 2 also N-protonates to give [2H]+.
With BArF − as the anion, only one isomer was observed in the
4
no 13C correlation, the δ 4.7 signal is assigned to NH. Although
IR spectrum (νCO 2016, 1956 cm−1). Its 31P NMR spectrum
1
−
both cis- and trans-iminium ligands are possible, only one H
was characterized by two doublets (δ 72.1, 16.7). For the BF4
NMR signal was observed for CH in the BArF − salt. N-
salt, two isomers (νCO = 2014, 1952 cm−1 and 2004, 1941
4
protonation induced a ∼50 cm−1 increase in νCO vs that in 1.
cm−1) were observed.
E
DOI: 10.1021/acs.organomet.6b00318
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Article
Redox Properties of Fe(PCHNArCl)(CO)3. The addition of
FcBF4 (Fc = [Fe(C5H5)2]+) to CH2Cl2 solutions of 1 gave the
green salt [1]BF4. This solution was unstable at room
temperature, as indicated by decomposition to unidentified
carbonyl species after a few minutes. Dark green single crystals
of [1]BF4 were obtained when the synthesis was conducted at
−35 °C. X-ray crystallographic analysis revealed that oxidation
induced a change in the hapticity of the imine, which is N-
bound in the cation (Figure 8). In comparison to 1OMe, the Fe−
Figure 9. X-band EPR spectrum of [1]BF4 in CH2Cl2/toluene at 0 °C.
process is assigned to the [1]0/+ couple. This redox couple is
stable in the electrochemical environment at this scan rate, and
repeated cycling of the reversible couple up to three cycles did
not give observable changes. When the switching potential was
extended to 1 V, a second more complicated event was
detected at ∼0.6 V (Figure 10). The second oxidation is
attributed to the [1]+/2+ couple, but the doubly oxidized species
is unstable.
Figure 8. Structure (50% thermal ellipsoids) of [Fe(PCHNArCl)-
(CO)3]BF4 ([1]BF4). Hydrogen atoms, solvent, and anion have been
omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)−
P(1), 2.2465(5); Fe(1)−C(1), 1.847(1); Fe(1)−C(2), 1.844(1);
Fe(1)−C(3), 1.801(1); Fe(1)−N(1), 2.021(1); Fe(1)−C(4),
2.975(1); C(4)−N(1), 1.289(2); P(1)−Fe(1)−C(1), 173.29(5).
N distance of 2.021(1) Å is slightly contracted by 0.01 Å, and
the Fe−CO distances are elongated by 0.05 Å.26,36 Both
changes are consistent with a more Lewis acidic Fe center. In
contrast to the trigonal-bipyramidal structure exhibited by 1OMe
,
the [1]+ adopts a distorted-square-pyramidal structure, with the
CO ligand occupying the apical position.
Oxidation of 1 induces a shift in νCO of ∼40 cm−1 to 2044,
1983, 1955 cm−1 vs 2080, 2022, 1991 cm−1. For comparison, a
ΔνCO value of ∼124 cm−1 is observed for the [Fe-
(CO)3(PR3)2]+/0 pair (R = Ph, Cy).37,38 The observation of a
single band for [Fe(CO)3(PR3)2]+ has been explained in two
distinct ways: retention of the D3h structure of its Fe(0)
precursor37 or a distorted-square-pyramidal species that is
fluxional on the IR time scale.39 The smaller value of ΔνCO for
the [1]+/0 couple is attributed to a compensating structural
change that accompanies the oxidation: 1 features an η2-imine π
acceptor, whereas [1]+ features a κ1-imine σ donor.
Figure 10. Cyclic voltammogram of 1 in CH2Cl2 solution at a scan
rate of 0.02 V s−1. Conditions: 0.001 M 1 with 0.01 M [Bu4N]PF6;
glassy-carbon working electrode. Potentials are referenced to Fc+/0
.
The oxidation of 2 by FcBF4 was also examined briefly. A
change in the hapticity of the imine is supported by the
relatively small shift of the CO frequencies from 2 to [2]+
(Δν(CO
)
of 22.5 cm−1). In the FT-IR spectrum, the νCN
avg
band shifted from 1583 cm−1 for 2 to a broad signal at 1600
cm−1 for [2]BF4.
For CH2Cl2 solutions of 1, νCN was observed at 1584 cm−1;
for CH2Cl2 solutions of [1]BF4, νCN was observed at 1599
cm−1. The 15 cm−1 shift to higher energy for νCN upon
oxidation of 1 supports the change from the π-accepting η2-
imine to the σ-donating κ1-imine.
Oxidation of Fe[P2(NArCl)2](CO)2. The diamido complex 3
is a 2e− reductant. Upon treatment with 2 equiv of FcBArF , 3
4
converted to the yellow [4](BArF )2. The FT-IR spectrum of
4
this salt exhibited νCO bands at 2065 and 2030 cm−1,
corresponding to a shift of 65 cm−1 vs the neutral precursor
3. ESI-MS revealed strong signals at m/z 455.05 and 427.3,
corresponding respectively to [Fe(PCHNArCl)2(CO)2]2+ and
[Fe(PCHNArCl)2]2+ (loss of 2 × CO corresponds to m/z 56/
2). The 31P NMR spectrum showed a singlet at δ 40.5.
Oxidation of 3 to [4]2+ can also be accomplished using FcBF4
The X-band EPR spectrum of [1]BF4 in fluid solution
exhibited a doublet pattern centered at g = 2.047, which arises
from coupling to the 31P center (Figure 9). The hyperfine
coupling constant A(31P) is 53.0 MHz. For comparison, the
isotropic EPR spectrum of [Fe(CO)3(PPh3)2]+ exhibits a
doublet centered at g = 2.053 with A(31P) = 18.7 MHz,37−39
the smaller hyperfine coupling reflecting the delocalization over
two phosphorus centers.
and FcOTf (OTf− = CF3SO3 ), although [4](BF4)2 was
−
unstable in solution, even at −35 °C. The decomposition is
proposed to be related to the reaction of the dialkoxo analogue
of 3, Fe(P2O2)(CO)2, with FcBF4 to give the BF3 adduct
Fe[P2(OBF3)2](CO)2.20
The cyclic voltammogram of a CH2Cl2 solution of 1 showed
a reversible event at E1/2 = −0.41 V with ipa/ipc = 0.915. This
F
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Cyclic voltammetry results are consistent with the Fc+-
induced fragmentation of the [P2(NArCl)2]2− ligand in 3.
Specifically, the cyclic voltammogram of a CH2Cl2 solution of 3
exhibits an irreversible oxidation event at −0.20 V when scan
rates were varied between 0.02 and 1 V s−1. This process is
assigned to the 3/[4]2+ couple. Following the irreversible
oxidation, the product exhibits reductions at −0.43, −0.84, and
−1.37 V. These reductive events only appeared after scanning
through the oxidation at −0.20 V.
Scheme 4. Proposed Pathway for Oxidative C−C Cleavage
in the Conversion of 3 to [4]2+
The 1H NMR spectrum of [4]2+ exhibits an anomalous signal
at δ 4.83 integrating for 4H. Through a combination of 2D
1
NMR techniques (1H−13C HMQC and H,1H-COSY, Figures
S32 and S33 in the Supporting Information), the signal at δ
4.83 was determined to arise from the C6H4Cl centers. The
imine proton (CHNAr) was assigned to a signal at δ 8.3. The
crystal structure of [4]2+ indeed reveals close contacts between
2,6-protons on each C6H4-4-Cl with two arene rings (Figure
S35 in the Supporting Information). Ring current effects have
been observed in other phosphine complexes.40
CONCLUSIONS
■
Iron complexes derived from Ph2PC6H4CHNAr ligands
exhibit rich imine-centered reactivity: σ−π isomerism, N-
protonation, and reductive coupling. One of the simplest
transformations possible for an imine ligand is demonstrated
the isomerization of N-bonded to π-bonded imines. The system
Fe(Ph2PC6H4CHNAr)(CO)3 exists as an equilibrium
mixture of both σ- and π-bonded isomers, and the minor
isomer fortuitously crystallizes. This dynamic process illustrates
the pathway for racemization of π-bonded imines (Scheme 5).
X-ray crystallographic analysis of [4](OTf)2 confirmed the
proposed structure (Figure 11). Complex [4]2+ is a rare
Scheme 5. Racemization Pathway for the π-Imine Isomer of
Fe(PCHNR)(CO)3 via the Intermediacy of the N-Bonded
Isomer
Figure 11. Two views of the dication in [4](OTf)2 with hydrogen
atoms omitted for clarity (50% thermal ellipsoids). Selected distances
(Å) and angles (deg): Fe(1)−P(1), 2.321(2); Fe(1)−P(2), 2.321(2),
Fe(1)−N(1), 2.092(3); Fe(1)−N(2), 2.061(4); Fe(1)−C(26),
1.797(5); Fe(1)−C(27), 1.787(4); N(1)−C(7), 1.295(5); N(2)−
C(34), 1.298(7); P(1)−Fe(1)−P(2), 176.77(6).
Although the system Fe(Ph2PC6H4CHNAr)(CO)3 is
exquisitely amenable to analysis, related dynamics have been
implicated previously. In W(acac)2(η2-imine)(CO), the imine
rapidly diastereomerizes, although the implicated N-bonded
intermediate was not detected.42 The unsymmetrical bis-
(ketimine) Ni(0)-NHC complex Ni(κ1-HNCPh2)(η2-
HNCPh2)(IPr) has been examined crystallographically.43
Aldehyde complexes, e.g. [Cp*Re(PPh3)(NO)(ArCHO)]+,
participate in very rapid κ1-η2 equilibria.44
example of a dicationic iron(II) dicarbonyl complex.41 The C2
symmetry was retained. The C−N distances of 1.297 Å are
significantly shortened in comparison to its diamido precursor
3. The imine carbon centers C(7) and C(34) are well separated
at 3.269(5) Å.
As verified crystallographically, the [1]0/+ couple is an
example of a redox-induced hapticity change at an imine ligand.
This transformation is proposed to proceed via the sequence π-
[1]0 → N-[1]0 → N-[1]+, i.e. the more electron-rich N-bonded
Fe(0) complex is oxidized. A related N- vs η2-linkage isomerism
has been observed for the [Ru(NH3)5(Me2CO)]2+/3+ couple.45
The new results provide insights into the proposed pathway
for the coupling reactions that give rise to (P2O2)2−,20,46
(P2ONArCl)2−,20 and [P2(NArCl)2]2− ligands from the
respective phosphine-aldehydes and phosphine-imines. The
intermediacy of Fe(κ1,η2-PCHNArCl)(κ1-PCHNArCl)(CO)2 is
strongly implicated. This hypothesis is supported by the
characterization of the analogous PMe3 complex 2. The C−C
coupling is proposed to proceed via attack of uncoordinated
imine electrophile with a η2-imine nucleophile (Scheme 3).
Imine coupling reactions have been observed for early-
transition-metal complexes containing η2-imine ligands47,48 as
well as uranium(IV) complexes.49
To illuminate the mechanism of the oxidation of 3, we
examined its partial oxidation. Upon treatment with 1 equiv of
FcBArF , 3 (νCO 2010, 1954 cm−1) was completely consumed.
4
IR analysis revealed a mixture characterized by νCO bands at
2039, 1988, and 1913 cm−1. A similar mixture can be obtained
by addition of 1 equiv of PCHNArCl to [1]BArF . For this
4
mixture the band at 1913 cm−1 is more intense than the 2039
and 1988 cm−1 bands. These partially oxidized solutions react
with another 1 equiv of FcBArF to give [4](BArF )2 (see
4
4
Figures S43 and S44 in the Supporting Information). It is
telling that 1e oxidation shifts two νCO bands to the region
exhibited by [Fe(PCHNArCl)(PMe3)(CO)2]+ (1990, 1920
cm−1). These results underpin a tentative mechanism involving
oxidative-induced reduction of the Fe(II) concomitant with C−
C scission (Scheme 4).
G
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Article
(3 × 20 mL) and dried under vacuum. Yield: 171 mg, 0.32 mmol
(91%). Crystals of 1OMe were grown by slow diffusion of pentane into
CH2Cl2 solutions. H NMR (500 MHz, CD2Cl2): δ 7.95−7.19 (br,
Reversible bond formation/breakage of the imine carbon
bonds is a rare case of mild oxidative cleavage of C−C bonds.
The first step in the oxidative scission of the diamide is
proposed to be oxidation at iron. The resulting d5 center would
have a hole delocalized over Fe and the two amide nitrogen
centers. In the d5 amido complexes [Ru(NH3)5(NHCH2R)]2+,
α-CH centers are susceptible to deprotonation.50 In contrast,
Fe[P2(NArCl)2](CO)2 lacks α−C-H groups and thus is
susceptible to oxidatively induced C−C heterolysis. The C−
C scission leads to formal reduction of Fe(III) to Fe(I).
1
overlapping signals, 14H), 6.98 (br, 2H), 6.75 (br, 2H), 6.14 (br, 1H),
3.75 (s, 3H). 13C{1H} NMR (151 MHz, CD2Cl2, −100 °C, π-bonded
isomer): δ 212 (d, J = 62 Hz, CO), 211 (d, J = 59 Hz, CO), 208.40 (d,
J = 49 Hz, CO), 157 (s), 153.40 (s), 153 (s), 133 (s), 131.14 (s), 131
(s), 131 (s), 131 (s), 130 (s), 130 (s), 129.76 (s), 128.31 (s), 126.64
(s), 125.88 (s), 125.49 (s), 119 (s), 114 (s), 111 (s), 73.8 (s, CH
NAr), 54.9(s, OMe). 31P NMR (202 MHz, CD2Cl2): δ 64.2 (s). IR
spectrum (CH2Cl2): νCO 2039, 1980, 1949, 1902, 1870 cm−1. Anal.
Calcd for C28H19NClFeO3P (found): C, 65.07 (64.81); H, 4.14
(4.09); N, 2.62 (2.76).
EXPERIMENTAL SECTION
Fe(PCHNBu-t)(CO)3. The ligand, PCHNBu-t, was prepared by the
reaction of t-BuNH2 with 2-(diphenylphosphino)benzaldehyde. A 50
mL glass bomb was charged with toluene (20 mL), PCHO (600 mg,
2.07 mmol, 1 equiv), t-BuNH2 (2.2 mL, 1.51 g, 20.7 mmol, 10 equiv),
and approximately 500 mg of 4 Å molecular sieves. The bomb was
sealed and heated at 110 °C. After 18 h the reaction mixture was
cooled to room temperature, and the bomb opened. The molecular
sieves were removed, and the solvent was removed by rotary
evaporation. A pinkish solid was obtained and used without further
■
Materials and Methods. Unless otherwise indicated, reactions
were conducted using standard Schlenk techniques under an N2
atmosphere at room temperature with stirring. Solvents were high-
performance liquid chromatography grade and were dried and
deoxygenated on a Glass Contour System or MBraun solvent
purification system and stored over 4 Å molecular sieves. The
compounds Fe(bda)(CO)3,51 FcBF4,52 FcOTf,53 HBArF ·2Et2O,54
4
19
55
PCHNArOMe
,
and PCHNArCl
were synthesized according to
1
purification. Yield: 523 mg (73%). H NMR (500 MHz, CD2Cl2): δ
literature procedures. HBF4·Et2O was used as received from Sigma-
Aldrich. PMe3 was used as received from Strem. 1H and 31P{1H} NMR
spectra were acquired on Varian Unity Inova 500NB and Unity 500
8.77 (d, J = 5 Hz, 1H), 7.93 (m, 1H), 7.59−7.17 (m, 12H), 6.82 (m,
1H), 1.05 (s, 9H). 31P{1H} NMR (202 MHz, CD2Cl2): δ −11.6 (s).
Inside a nitrogen-filled glovebox, a THF solution of Fe(bda)(CO)3
(428 mg, 1.50 mmol, 1 equiv) was treated with a THF solution of
PCHNBu-t (517 mg, 1.50 mmol, 1 equiv). The reaction mixture was
stirred at room temperature for 18 h, during which time the solution
changed color from orange to dark blue. The solvent was removed
under vacuum, and the residual solid was washed with pentane (3 × 20
mL) and then dried under vacuum. Yield: 395 mg, 0.81 mmol (54%).
1
NB instruments at 20 °C unless otherwise noted. H NMR spectra
were referenced to residual solvent relative to tetramethylsilane, and
31P{1H} NMR spectra were referenced to an external 85% H3PO4
standard. Crystallographic data were collected using either a Siemens
SMART diffractometer equipped with a Mo Kα source (λ = 0.71073
Å) and an Apex II detector or a Bruker D8 Venture diffractometer
equipped with a Mo Kα microfocus source and a Photon 100 detector.
EPR spectra were recorded on 1−5 mM solutions in toluene/CH2Cl2
(1/1) at 10 °C. The instrument was a Varian E-line E-12 Century
Series X-band continuous-wave spectrometer. FT-IR spectra were
recorded on a PerkinElmer 100 spectrometer. Cyclic voltammetry
experiments were conducted using a 10 mL one-compartment glass
cell with a tight-fitting Teflon top inside a N2-filled glovebox. The
working electrode was a glassy-carbon disk (diameter 3.00 mm). The
counter electrode was a Pt wire, and a Ag wire was used as a quasi-
reference electrode. Each cyclic voltammogram was referenced to the
Fc0/+ couple by adding ferrocene to the sample and collecting 1 scan.
The School of Chemical Sciences Microanalysis Laboratory utilizing a
model CE 440 CHN analyzer performed elemental analyses.
1
IR spectrum (CH2Cl2): νCO 2023, 1971, 1936, 1887, 1858 cm−1. H
NMR (500 MHz, CD2Cl2): δ 7.74−7.25 (overlapping signals, 11H),
7.29 (m, 2H), 7.08 (m, 1H), 1.04 (s, 9H). 31P{1H} NMR (202 MHz,
CD2Cl2): δ 64.4 (s).
Fe(PCHNArNMe2)(CO)3. The ligand, PCHNArNMe2, was prepared
by the reaction of 4-Me2NC6H4NH2 with 2-(diphenylphosphino)-
benzaldehyde. A 50 mL glass bomb was charged with toluene (20
mL), 2-(diphenylphosphino)benzaldehyde (600 mg, 2.07 mmol, 1
equiv), Me2NC6H4NH2 (281 mg, 20.7 mmol, 1 equiv), and
approximately 500 mg of 4 Å molecular sieves. The bomb was sealed
from the surrounding atmosphere and heated to 110 °C for 18 h. The
reaction mixture was then cooled to room temperature and the bomb
opened to ambient atmosphere. The molecular sieves were removed
by filtration, and the solvent was removed by rotary evaporation. A
brown solid was obtained and used without further purification. Yield:
774 mg, 1.90 mmol (92%). 1H NMR (500 MHz, CDCl3): δ 9.09 (d, J
= 6 Hz, 1H), 8.17 (m, 1H), 7.63−7.15 (m, 11H), 6.98 (d, J = 9 Hz,
1H), 6.87 (m, 1H), 6.62 (d, J = 9 Hz, 2H), 2.90 (s, 6H). 31P{1H}
NMR (202 MHz, CDCl3): δ −12.9 (s).
Fe(PCHNArCl)(CO)3 (1). This compound was synthesized using a
procedure modified from a previous report.20 A THF solution (15
mL) of Fe(bda)(CO)3 (212 mg, 0.74 mmol) was treated with
PCHNArCl (297 mg, 0.74 mmol). This orange mixture was stirred in a
55 °C oil bath for 16 h to give a dark green solution. After the solution
was cooled, solvents were removed under reduced pressure to yield a
dark green powder. The crude material was dissolved in a minimal
amount of CH2Cl2 (ca. 3 mL). The solution was layered with pentane
(15 mL) and then maintained at −35 °C. After 48 h, yellow crystals
appeared of the target compound. Yield: 272 mg (68%). Single crystals
suitable for X-ray diffraction were obtained by storing a pentane
Inside a nitrogen-filled glovebox, a THF solution of Fe(bda)(CO)3
(428 mg, 1.50 mmol, 1 equiv) was treated with a THF solution of
PCHNBu-t (517 mg, 1.50 mmol, 1 equiv). The reaction mixture was
stirred at room temperature for 18 h, during which time the solution
changed colors from orange to dark blue. The solvent was removed
under vacuum, and the residual solid was washed with pentane (3 × 20
mL) and then dried under vacuum. Yield: 916 mg, 1.67 mmol, (90%).
IR spectrum (CH2Cl2): νCO 2037, 1980, 1945, 1990, 1887, 1868 cm−1.
1H NMR (500 MHz, CD2Cl2): δ 7.74−7.25 (overlapping signals,
14H), 6.99 (d, J = 8 Hz, 2H), 6.60 (d, J = 8 Hz, 2H), 2.92 (s, 6H).
31P{1H} NMR (202 MHz, CD2Cl2): δ 64.0 (s).
1
solution of 1 at −35 °C for 24 h. H NMR (500 MHz, CD2Cl2): δ
7.94 (br, 2H), 7.73−7.67 (m, 1H), 7.55−7.34 (m, 10H), 7.31−7.21
1
(m, 1H), 7.13−7.04 (m, 2H), 7.02−6.87 (m, 2H), 5.12 (br, 1H). H
NMR (500 MHz, C6D12): δ 7.94 (br, 2H), 7.57 (m, 1H), 7.44−7.15
(m, 11H), 7.08−7.01 (m, 1H), 6.98−6.88 (m, 2H), 6.85−6.80 (m,
2H), 5.16 (br, 1H). 31P NMR (202 MHz, CD2Cl2): δ 62.9 (s). IR
spectrum (CH2Cl2): νCO 2044, 1983, 1955 cm−1. Anal. Calcd for
C28H19NClFeO3P (found): C, 62.31 (61.91); H, 3.55 (3.43); N, 2.60
(2.90).
Fe(PCHNArCl)(PMe3)(CO)2 (2). A THF solution (5 mL) of 1 (21.2
mg, 0.039 mmol) was treated with PMe3 (0.2 mL, 1.9 mmol). This
mixture was stirred at room temperature for 16 h to give an orange
solution. Solvents were then removed under reduced pressure, and the
solids were washed with pentane to give the target compound as
orange microcrystals. Longer reaction times were required when less
PMe3 was used. Single crystals suitable for X-ray diffraction were
Fe(PCHNArOMe)(CO)3 (1OMe). This compound was synthesized
analogously to 1. A THF solution of Fe(bda)(CO)3 (100 mg, 0.35
mmol) was treated with a THF solution of PCHNArOMe (138 mg, 0.35
mmol). The resulting dark green solution was stirred at room
temperature for 16 h, followed by cooling and removal of the THF
solvent under vacuum. The solid residue was washed with dry pentane
1
obtained by slow diffusion of pentane into a CH2Cl2 solution. H
H
DOI: 10.1021/acs.organomet.6b00318
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Article
NMR (500 MHz, CD2Cl2): δ 7.95−7.86 (m, 2H), 7.64−7.58 (m, 1H),
7.44−7.27 (m, 11H), 7.13−7.07 (m, 1H), 6.88 (d, J = 8.4 Hz, 1H),
6.83−6.54 (br, 2H), 4.27 (dd, J = 5.6, 1.9 Hz, 1H), 1.07 (dd, J = 9.3,
1.6 Hz, 9H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 64.10 (d, J = 165
Hz), 21.4 (d, J = 165 Hz). IR spectrum (THF): νCO 1965, 1900 cm−1.
Anal. Calcd for C30H28NClFeO2P2 (found): C, 61.3 (61.31); H, 4.8
(4.64); N, 2.38 (2.65).
and washed with Et2O. Yield: 38 mg (64%). IR (CH2Cl2): νCO 2059,
2023 cm−1. Anal. Calcd for C54H38Cl2F6FeN2O8P2S2·2.5CH2Cl2
(found): C, 47.72 (47.33); H, 3.05 (2.85); N, 1.97 (2.13). The
analogous [Fe(PCHNArCl)2(CO)2](BArF )2 was prepared in a similar
4
manner using FcBArF , and NMR spectra were collected on
4
[Fe(PCHNArCl)2(CO)2](BArF )2 due to its favorable solubility in
4
CD2Cl2 in comparison to the poorly soluble [Fe(PCHNArCl)2(CO)2]-
(OTf)2. 1H NMR (500 MHz, CD2Cl2): δ 8.30 (t, J = 2.9 Hz, 2H), 8.07
(t, J = 7.6 Hz, 2H), 7.93−7.84 (m, 4H), 7.81−7.65 (m, 26H), 7.62 (t, J
= 7.8 Hz, 4H), 7.53 (s, 8H), 7.37 (d, J = 7.3 Hz, 4H), 7.30−7.24 (m,
4H), 7.14 (d, J = 8.5 Hz, 4H), 6.83−6.74 (m, 2H), 4.80 (br, 2H). 31P
NMR (202 MHz, CD2Cl2): δ 37.2 (s). 13C{1H} NMR (151 MHz,
CD2Cl2, 25 °C): δ 207 (t, J = 22 Hz, CO), 182 (s, CHNAr), 162
[Fe(PCHNHArCl)(CO)3]X ([1H]X; X− = BF4−, BArF −). To an Et2O
4
solution (30 mL) of 1 (75 mg, 0.139 mmol) was added 5 drops of
HBF4·Et2O. A bright yellow solid precipitated immediately. After it
was stirred for 1 h, the slurry was concentrated to ca. 5 mL. Pentane
(10 mL) was added, and the mixture was filtered to give the target
compound as a pale yellow powder. Yield: 75 mg (86%). Single
crystals suitable for X-ray diffraction were obtained by vapor diffusion
(dd, J = 100, 50 Hz, BArF ), 154 (s), 138 (s), 137 (s), 138 (s), 137 (s),
4
136 (s), 136 (s, BArF ), 135 (s), 133 (s), 132 (s), 131 (s). 131 (s), 131
1
of pentane into a THF/CH2Cl2 solution of this salt. H NMR (500
4
(s), 130 (q, J = 32 Hz, BArF ), 126 (t, J = 26 Hz), 125 (q, J = 272 Hz,
MHz, CD2Cl2), trans-[1H]BF4: δ 8.20 (dd, J = 7.5, 3.1 Hz, 1H), 7.89−
7.76 (m, 3H), 7.75−7.59 (m, 6H), 7.52 (td, J = 7.9, 3.1 Hz, 2H), 7.45
(q, J = 8.8 Hz, 4H), 7.33 (dd, J = 12.8, 7.9 Hz, 2H), 5.86 (br, 1H), 5.76
(br, 1H). 1H NMR (500 MHz, CD2Cl2), cis-[1H]BF4: δ 8.30 (dd, J =
8.0, 3.4 Hz, 1H), 7.87−7.09 (m), 6.58−6.46 (m, 4H). Resonances at δ
7.87−7.09 were not integrated due to overlapping trans-[1H]BF4
resonances. The iminium resonances were not observed due to
overlapping resonances from trans-[1H]BF4. 31P{1H} NMR (202
MHz, CD2Cl2): δ 65.7 (trans), 56.5 (cis). IR (CH2Cl2), trans-[1H]BF4
and cis-[1H]BF4: 2087, 2084, 2036, 2032, 2010 cm−1. Anal. Calcd for
C28H20BClF4FeNO3P·0.5CH2Cl2 (found): C, 51.09 (51.08); H, 3.16
4
BArF ), 123 (s), 121 (t, J = 18 Hz), 120 (t, J = 26 Hz), 118 (s, BArF ).
4
4
DFT Methods. Computations have been carried out with the
TURBOMOLE suite of programs.56 A valence triple-ζ basis set with
polarization functions on all atoms (TZVP)57 has been adopted. Both
pure (BP86)58 and hybrid (B3LYP)59 density functionals have been
used, in order to test the effect of the level of theory on computed
structures and energies. The resolution-of-identity technique (RI)60
has been used to speed up BP86 computations, by reducing all four-
center two-electron Coulomb integrals to precomputed three- and
two-center integrals.
Harmonic vibrational frequencies were calculated (at the same level
of theory) in order to define the nature of each stationary point of the
energy hypersurface and to verify whether computed structures
correspond to genuine minima (absence of imaginary frequencies).
The search and optimization of transition states have been carried out
with a pseudo-Newton−Raphson method. First, a guess transition-
state structure is optimized, by constraining the degrees of freedom
related to the reaction coordinate (RC). Then, a vibrational analysis on
the constrained structure is performed, in order to pinpoint the single
negative eigenmode associated with the RC of interest. The negative
eigenmode is thus followed, in order to locate the first-order saddle
point representing the pure transition-state structure.
Free energy (G) values can be obtained by including enthalpy, zero-
point energy (ZPE), and entropy contributions to the self-consistent-
field SCF electronic energy. To do so, three different contributions
(qtranslational, qvibrational, and qrotational) have been considered to evaluate
the overall partition function.61 The T and P values have been set to
298.15 K and 1 bar, respectively. The scaling factor for SCF
wavenumbers has been set to 0.9914 for BP86 (default value
implemented in TURBOMOLE56) and to 0.9679 for the B3LYP
functional.62
(3.16); N, 2.09 (2.36). The salt [H2]BArF was prepared similarly:
4
combining a CH2Cl2 (3 mL) solution of 2 (4.4 mg, 0.0094 mmol)
with a CH2Cl2 (2 mL) solution of HBArF ·2Et2O (9 mg, 0.088 mmol)
4
converted the green solution immediately to bright orange-yellow.
Solvent was then removed under reduced pressure to yield a bright
1
yellow solid. H NMR (500 MHz, CD2Cl2): δ 8.06 (dd, J = 7.8, 3.5
Hz, 1H), 7.85−7.51 (m, 23H), 7.49 (d, J = 8.8 Hz, 2H), 7.40−7.32
(m, 2H), 7.28 (d, J = 8.7 Hz, 2H), 5.56 (dd, J = 7.1, 4.4 Hz, 1H),
4.74−4.64 (m, 1H). 31P{1H} NMR (202 MHz, CD2Cl2): δ 64.8 (s).
IR (CH2Cl2): νCO 2089, 2039, 2017 cm−1.
[Fe(PCHNArCl)(CO)3]BF4 ([1]BF4). A solution of FcBF4 (24.9 mg,
mmol) in CH2Cl2 (7 mL), cooled to −35 °C, was added to a CH2Cl2
solution (2 mL) of 1 (50 mg, mmol), also cooled to −35 °C. The
resulting dark green solution was immediately layered with pentane
(ca. 15 mL), and the mixture was maintained at −35 °C. After 1 week,
dark green crystals were produced that were suitable for X-ray
crystallography. IR (CH2Cl2): νCO 2084, 2022, 1991 cm−1. Anal. Calcd
for C28H19NBClF4FeO3P·CH2Cl2 (found): C, 48.96 (48.92); H, 2.97
(2.91); N, 1.97 (2.14).
Fe[P2(NArCl)2)](CO)2 (3). In a 250 mL thick-walled round-bottom
flask, a solution of Fe(bda)(CO)3 (344 mg, 1.2 mmol) in toluene (30
mL) was treated with PCHNArCl (847 mg, 2.1 mmol). The flask was
sealed, and the orange solution was stirred in a 80 °C oil bath for 16 h,
resulting in a dark blue solution. After the solution was cooled,
solvents were removed under reduced pressure. The blue residue was
extracted into THF (20 mL) and filtered through a short plug of
Celite. The filtrate was concentrated to ca. 5 mL and layered with
pentane (15 mL). Upon standing at −35 °C for 48 h, the solution
yielded the target compound as dark blue crystals. Yield: 476 mg
(49%). Single crystals suitable for X-ray diffraction were obtained by
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00318.
NMR and IR spectra (PDF)
Cartesian coordinates (PDF)
X-ray crystallographic data (CIF)
1
diffusion of pentane into a concentrated CH2Cl2 solution. H NMR
(500 MHz, CD2Cl2): δ 7.76−7.68 (m, 2H), 7.57 (t, J = 7.5 Hz, 2H),
7.53−7.41 (m, 6H), 7.32−7.23 (m, 6H), 7.16 (t, J = 7.5 Hz, 2H), 7.07
(q, J = 3.2 Hz, 8H), 6.97−6.93 (m, 2H), 6.37 (d, J = 8.9 Hz, 2H), 6.07
(d, J = 8.8 Hz, 2H), 4.38 (t, J = 2.4 Hz, 2H). 31P NMR (202 MHz,
CD2Cl2): δ 25.9 (s). IR (CH2Cl2): νCO 2011, 1954 cm−1. Anal. Calcd
for C52H38N2Cl2FeN2O2P2·0.5C5H10 (found): C, 69.15 (69.22); H,
4.58 (4.7); N, 2.96 (3.09).
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.B.R.: rauchfuz@illinois.edu.
■
Notes
[Fe(PCHNArCl)2(CO)2](OTf)2 ([4](OTf)2). To a CH2Cl2 (10 mL)
solution of 3 (45 mg, 0.049 mmol) was added FcOTf (30 mg, 0.090
mmol), and a color change from dark blue to green was immediately
observed. This solution was stirred for 30 min, followed by filtration
through a short plug of Celite. The filtrate was diluted with 15 mL of
pentane to precipitate a yellow solid, which was collected by filtration
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research was conducted under contract DEFG02-
90ER14146 with the U.S. Department of Energy by its
■
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DOI: 10.1021/acs.organomet.6b00318
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Division of Chemical Sciences, Geosciences, and Biosciences
Division, Office of Basic Energy Sciences. We thank Danielle
Gray for collecting the crystallographic data and advice on
structure solutions. We thank Professor Joseph Templeton
(UNC) for advice.
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