Reduction of an Fe(I) mesityl complex induced by π-acid ligands

Article abstract of DOI:10.1039/c4dt00170b

Treatment of the Fe(i) mesityl complex [Fe(Mes)(BPEP-Ph)] (BPEP-Ph = 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine) with π-acid ligands (L = CO, RNC) leads to one-electron reduction via Mes group migration from Fe to P, followed by homolytic elimination of the 2,4,6-tBu ₃C₆H₂ group, to afford Fe(0) complexes of the formula [Fe(L)₂(BPEP-Ph*)] (BPEP-Ph* = 2-[1-phenyl-2-mesityl-2-phosphaethenyl]-6-[1-phenyl-2-(2,4,6-tri-tert- butylphenyl)-2-phosphaethenyl]pyridine). This reduction process is supported by radical trapping experiments and theoretical studies. The 2,4,6-tBu ₃C₆H₂ radical is captured by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in high yield. DFT calculations reveal the mechanism of Mes group migration with a reasonable energy profile. the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00170b

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Reduction of an Fe(I) mesityl complex induced by
π-acid ligands†
Cite this: Dalton Trans., 2014, 43,
9032
Ya-Fan Lin, Yumiko Nakajima*‡ and Fumiyuki Ozawa*
Treatment of the Fe(I) mesityl complex [Fe(Mes)(BPEP-Ph)] (BPEP-Ph = 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-
butylphenyl)-2-phosphaethenyl]pyridine) with π-acid ligands (L = CO, RNC) leads to one-electron
reduction via Mes group migration from Fe to P, followed by homolytic elimination of the 2,4,6-tBu3C6H2
group, to afford Fe(0) complexes of the formula [Fe(L)2(BPEP-Ph*)] (BPEP-Ph* = 2-[1-phenyl-2-mesityl-
2-phosphaethenyl]-6-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). This reduction
Received 17th January 2014,
Accepted 11th February 2014
•
process is supported by radical trapping experiments and theoretical studies. The 2,4,6-tBu3C6H2 radical
DOI: 10.1039/c4dt00170b
is captured by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in high yield. DFT calculations reveal the
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mechanism of Mes group migration with a reasonable energy profile.
Introduction
Low-coordinate Fe(I) complexes have been recognized to play
an important role in catalytic organic transformations and
enzymatic reaction systems.1,2 However, their chemical pro-
perties including redox behaviour and substrate reactivity have
been poorly explored owing to the lack of well-defined Fe(I)
complexes.3,4 Recently, we have reported that four-coordinate
Fe(I) complexes of the formula [Fe(X)(BPEP-Ph)] (X = Br, Mes)
are successfully prepared by using a PNP-pincer type phos-
phaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphe-
nyl)-2-phosphaethenyl]pyridine (BPEP-Ph).5 Unlike PNP-pincer
ligands with phosphine arms,6 BPEP-Ph having PvC bonds at
the 2,6-positions of pyridine possesses an extremely low-lying
π* orbital, thus serving as a strong π-acceptor towards tran-
sition metals.7 We have demonstrated that this particular
ligand property effectively stabilizes coordinatively unsaturated
Fe(I) complexes with a 15e configuration.
Scheme 1 Me3SiNC was employed as the isomeric form Me3SiCN.
DFT calculations indicate that the highly flexible nature of the
π orbital system of BPEP-Ph, which is characteristic of phos-
phaalkene compounds, facilitates the Mes group migration.
Results and discussion
Reactions of [Fe(Mes)(BPEP-Ph)] (1) with π-acid ligands
This paper reports a novel reduction process of the Fe(I)
mesityl complex [Fe(Mes)(BPEP-Ph)] (1). We found that
complex 1 readily undergoes one-electron reduction in the
presence of isocyanides and CO (Scheme 1). Interestingly, the
reaction involves Mes group migration from iron to phos-
phorus, followed by homolytic elimination of the Mes* group.
Complex 1 was stable in neat benzene, but was rapidly con-
verted to Fe(0) complexes of the formula [Fe(L)2(BPEP-Ph*)]
(2a–d) in the presence of π-acid ligands (L = CO, RNC). For
example, treatment of 1 with tBuNC (2 equiv.) in C6D6 at room
temperature led to an instant colour change of the reaction
solution from brown to green. The 31P{1H} NMR spectrum
exhibited two sets of AX signals at δ 263.0 and 239.3 (2JPP = 131 Hz)
and δ 165.7 and 58.3 (2JPP = 93 Hz), which were assignable
to 2c and unidentified complex 3c, respectively. Complex 2c
was formed in 50% yield based on 1, and remained unchanged
for several days in the reaction solution. Complex 3c was also
detected in considerable quantity (45%/1), but readily changed
into a paramagnetic species. Reactions of 1 with other π-acid
International Research Center for Elements Science (IRCELS), Institute for
Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan.
E-mail: ozawa@scl.kyoto-u.ac.jp
†Electronic supplementary information (ESI) available: Details of crystal
structure determination and DFT calculations. CCDC 973730 and 973731.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00170b
‡Present address: National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba 305-8565, Japan. E-mail: yumiko-nakajima@aist.go.jp
9032 | Dalton Trans., 2014, 43, 9032–9037
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Scheme 2 Formation of 2 in the presence of TEMPO.
Fig. 1 Crystal structure of 2c·0.5Et2O with 50% probability ellipsoids.
Hydrogen atoms and a crystal solvent (Et2O) are omitted for clarity.
Selected bond distances (Å) and angles (°): Fe–C1 1.820(6), Fe–C2
1.869(7), Fe–P1 2.151(2), Fe–P2 2.126(2), Fe–N1 2.011(5), P1–C3 1.747(6),
P2–C4 1.717(6), C1–N2 1.179(8), C2–N3 1.184(8), C1–Fe–N1 169.2(2),
P1–Fe–P2 146.14(8), C1–Fe–C2 88.8(3), C2–Fe–P1 106.1(2), C2–Fe–P2
105.6(2), C2–Fe–N1 101.3(2).
ligands similarly proceeded (see the Experimental section). In
contrast, treatment of 1 with phosphine ligands such as PMe3
and PPh3 resulted in dissociation of BPEP-Ph from iron.
Complexes 2a–d were isolated in 35–52% yields/1 by
column chromatography, and characterized by IR and NMR
spectroscopy. The crystal structures of 2c and 2d were deter-
mined by X-ray diffraction analysis. Fig. 1 shows the structure
of 2c, which adopts a square pyramidal geometry around iron
(τ = 0.38).8 The tBuNC ligands are accommodated at the apical
and equatorial positions, respectively. Complex 2d has a
similar structure (τ = 0.39, see Fig. S1†). The most striking
feature of 2c is the replacement of one of the Mes* groups on
phosphorus by the Mes group which was originally bonded to
iron. Thus, BPEP-Ph was converted to the unsymmetrical
ligand BPEP-Ph* with Mes and Mes* groups on phosphorus
atoms, respectively. The P–C bond lengths (1.747(6), 1.717(6)
Å) are comparable to those of 1 (1.735(3) Å).5b The conversion
of BPEP-Ph into BPEP-Ph* was also confirmed for 2a, 2b
and 2d by NMR spectroscopy and/or by X-ray structural
analysis (2d).
Scheme 3 Proposed mechanism for the formation of 2 and 4.
(89 and 84%). Scheme 3 presents a plausible reaction process
affording 2 and 4. The first step should be coordination of a
π-acid ligand (L) to 1, because the addition of L is essential for
the reaction to be initiated. Then, five-coordinate intermediate
A undergoes migration of the Mes ligand to one of the phos-
phorus atoms to form B.10 Finally, homolytic cleavage of the
P–Mes* bond in B, along with the coordination of L, provides
•
2. At the same time, the Mes* radical generated in the system
combines with TEMPO to yield 4 via the known intramolecular
radical rearrangement.11 Although P–C bond cleavage in a
homolytic manner under non-photochemical conditions has
been scarcely documented, it seems reasonable to assume that
the highly sterically demanding phosphine moiety in B with
Mes and Mes* groups undergoes homolytic elimination of the
Mes* group to relieve the steric congestion,12 where the stabi-
lity of the resulting phosphaalkene radical C with an extended
π-conjugated system13 should also facilitate the reaction.
Radical trapping experiments with TEMPO
The conversion of 1 into 2a–d involved replacement of one of
the P-Mes* groups with the Fe-Mes group. The reaction also
involved one-electron reduction from Fe(I) to Fe(0), where one
of the Mes* groups was missing from the reaction products,
probably due to the occurrence of homolytic elimination of
DFT calculations on Mes group migration
the Mes* group. Because we could not observe the expected Next, we examined the formation process of B by DFT calcu-
organic products such as 1,3,5-tri-tert-butylbenzene and 1,4-bis- lations using [Fe(Mes)(BPEP′)] (1′; BPEP′ = 2,6-bis(2-mesityl-2-
(3,5-di-tert-butylphenyl)-1,1,4,4-tetramethylbutane in the reac- phosphaethenyl)pyridine) and MeNC as computational
•
tion solution, we supposed that the Mes* radical was com- models of 1 and L, respectively. Because we already confirmed
bined with an Fe(I) species to form 3a–d, although they were that the real complex 1 is in a low-spin state (S = 1/2),5b and
too unstable to be identified. Therefore, we next tried to isocyanides commonly act as a strong field ligand, all calcu-
capture the •Mes* radical by 2,2,6,6-tetramethylpiperidine-1- lations were conducted postulating the spin multiplicity of 2.
oxyl (TEMPO) (Scheme 2).9
Fig. 2(a) presents the energy diagram thus evaluated. In
Complex 1 was stable towards TEMPO itself (1 equiv.) at accord with the crystal structure of 1,5b complex 1′ adopts a
room temperature, but instantly converted to 2c (100%) and 4 distorted trigonal monopyramidal configuration with the basal
(98%) in the presence of tBuNC. Similarly, treatment of 1 with plane consisting of P1, P2, C(Mes) and Fe atoms; the sum of
2,6-Me2C6H3NC and TMSCN in the presence of TEMPO three bond angles in the basal plane is 360.0°. The pyridine
formed 2b (92%) and 2d (71%), respectively, along with 4 ring is located at the apical position, and its opposite side is
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phosphaalkene unit (P1–C3). In other words, the highly
flexible nature of the π orbital system of BPEP-Ph causes the
facile migration of the Mes group from iron to phosphorus.
Conclusions
We have found a novel reduction process of the Fe(I) mesityl
complex [Fe(Mes)(BPEP-Ph)] (1) induced by π-acid ligands (L =
CO, RNC). Coordination of L to 1 causes Mes group migration
from iron to phosphorus, and homolytic elimination of Mes*
from a highly congested phosphorus atom results in one-elec-
tron reduction to form [Fe(L)2(BPEP-Ph*)] (2a–d). DFT calcu-
lations have indicated that the strong π-accepting ability of
phosphaalkene effectively promotes the Mes group migration.
Fig. 2 (a) Schematic geometrical changes and energy changes in the
reaction of 1’ with MeNC evaluated by DFT calculations. (b) HOMO of B’.
Experimental
All manipulations were performed under an inert atmosphere
using Schlenk techniques and a glove box. Toluene (Kanto, de-
hydrated), hexane, and Et2O (Wako, dehydrated) were purified
by a solvent purification system (MBRAUN SPS-800) prior to
use. tBuNC and TMSCN were dried over CaH2 and distilled.
CO gas was passed through a P2O5 column (Merck, SICAPENT).
C6D6 was dried over sodium benzophenone ketyl and distilled.
Al2O3 was dried overnight at 180 °C under vacuum prior to
use. The complex [Fe(Mes)(BPEP-Ph)] (1) was prepared as
previously reported.5b
widely opened. Accordingly, MeNC approaches iron from this
side to form A′. This step is a highly exothermic process (ΔE =
−20.9 kcal mol−1) with small activation energy (Ea = 7.1 kcal
mol−1). The subsequent migration of the Mes group from iron
to phosphorus (A′ → B′) also proceeds with a small activation
barrier (5.8 kcal mol−1).
As depicted in Scheme 3, the Mes group migration (A → B)
is considered to involve a large structural variation in the
BPEP-Ph ligand.14 This situation was clearly reflected in the
bond orders of A′, TS2 and B′ as listed in Table 1. In accord
with the presumed structures of A and B in Scheme 3, the
bond order of P2–C4 decreases whereas that of C4–C6
increases when the Mes group shifts from Fe to P2. To our sur-
prise, the bond order of P1–C3 is also reduced to a consider-
able extent upon the migration, showing the occurrence of
strong π-back-donation from Fe to P1. It is reasonable that the
phosphine moiety (P2) with two Mes groups in B′ possesses a
strong σ-donating ability, thereby increasing the electron
density of iron. As a result, the P1–C3 bond undergoes an
effective π-back-donation from iron, and reduces its bond
order.
NMR spectra were recorded on a Bruker Avance 400 spectro-
meter (1H NMR, 400.13 MHz; 13C NMR, 100.62 MHz; 31P NMR,
161.98 MHz) or a Bruker Avance III 800US Plus spectrometer
(1H NMR, 800.15 MHz; 13C NMR, 201.20 MHz). Chemical
1
shifts are reported in δ (ppm), referenced to H (residual) and
13C signals of deuterated solvents as internal standards or to
31P signal of 85% H3PO4 as an external standard. Assignments
of 1H and 13C NMR resonances are based on HH COSY and
CH HSQC and HMBC experiments. IR spectra were recorded
on a JASCO FT/IR-4100 spectrometer. HRMS was carried out
on a Bruker micrOTOF II spectrometer using an ESI technique.
Elemental analysis was performed by the ICR Analytical
Laboratory, Kyoto University.
Fig. 2(b) shows the HOMO of B′, which clearly demonstrates
the occurrence of π-bonding interaction between Fe and
P1. Accordingly, we may conclude that the Mes group
migration is promoted by strong π-accepting properties of the
Reactions of 1 with π-acid ligands
Reaction with CO (1 atm). An NMR sample tube equipped
with a Teflon cock was charged with a solution of 1 (15.0 mg,
0.015 mmol) in C6D6 (0.5 mL). After the solution was degassed
by freeze–pump–thaw cycles, CO (1 atm) was introduced to the
solution using a balloon at −78 °C. The colour of the solution
instantly turned dark green. The reaction solution was filtered
through a Celite pad and concentrated to dryness under
vacuum. The resulting green solid was dissolved in toluene,
and subjected to a column chromatography (Al2O3, hexane–
toluene = 5/1), to give a dark green solid of 2a (5.5 mg,
0.0069 mmol, 46%).
Table 1 Changes in Mayer bond orders upon Mes group migrationa
Description
A′
TS2
B′
P1–C3
C3–C5
P2–C4
C4–C6
1.40
1.24
1.40
1.24
1.19
1.31
1.35
1.24
1.06
1.35
1.11
1.44
a The atomic numbering scheme is given in Fig. 2.
9034 | Dalton Trans., 2014, 43, 9032–9037
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2a: 1H NMR (C6D6): δ 1.32 (s, 9H, p-tBu), 1.67 (s, 18H,
Reaction with tBuNC. A solution of 1 (15.0 mg, 0.015 mmol)
o-tBu), 1.97 (s, 3H, p-Me), 2.68 (6H, o-Me), 6.69 (s, 2H, m-Mes), in C6D6 (0.5 mL) was charged to an NMR sample tube. tBuNC
6.99 (m, 4H, o-Ph), 7.07 (m, 4H, m-Ph), 7.29 (m, 3H, p-Ph, (3.5 μL, 0.031 mmol, 2.1 equiv.) was added at room tempera-
4-Py), 7.46 (s, 2H, m-Mes*), 7.90 (1H, 3-Py), 8.03 (1H, 3-Py). 13C ture, and the reaction was monitored by 31P{1H} NMR spec-
{1H} NMR (C6D6): δ 21.6 (s, p-Me), 24.1 (d, o-Me, JCP = 8 Hz), troscopy using a C6D6 solution of PPh3 (0.13 M) sealed in a
32.0 (s, p-tBu), 34.4 (s, o-tBu), 35.1 (s, p-tBu), 39.6 (s, o-tBu), capillary tube as an internal standard. A set of doublets assign-
116.2 (d, JCP = 12 Hz, Ar), 118.0 (d, JCP = 16 Hz, Ar), 122.6 (d, able to 2c and a set of doublets at δ 165.7 and 58.3 (JPP
=
JCP = 8 Hz, Ar), 126.1 (s, Ar), 126.3 (d, JCP = 11 Hz, Ar), 126.6 (d, 93 Hz) of an unidentified compound 3c instantly appeared in an
JCP = 7 Hz, Ar), 126.9 (s, Ar), 127.8 (s, Ar), 129.5 (d, JCP = 7 Hz, intensity ratio of 10 : 9. After 2 h, the signals of 3c disappeared,
Ar), 130.3 (d, JCP = 15 Hz, Ar), 131.3 (d, JCP = 7 Hz, Ar), 131.5 (d, whereas the signal intensity of 2c remained unchanged. The
JCP = 16 Hz, Ar), 131.8 (d, JCP = 5 Hz, Ar), 135.1 (d, JCP = 11 Hz, solution was filtered through a Celite pad, and concentrated to
Ar), 137.5 (d, JCP = 11 Hz, Ar), 141.7 (s, Ar), 143.0 (d, JCP = 9 Hz, dryness under vacuum. The residue was subjected to a column
Ar), 153.1 (s, Ar), 158.5 (s, Ar), 203.8 (s, CO). The chemical shift chromatography (Al2O3, Et2O), to give 2c as a dark green solid
of one of the Ar carbons was obscured due to the overlap with (7.0 mg, 0.0077 mmol, 51%). Single crystals suitable for X-ray
the signal of C6D6. The signals of two CvP groups and two of diffraction analysis were grown from a mixed solvent of Et2O
the ipso-Ar carbons were not observed due to low signal inten- and isooctane (3 : 2) at −35 °C.
sities. 31P{1H} NMR (C6D6): δ 278.7 (d, JPP = 86 Hz), 250.8 (d,
2c: 1H NMR (C6D6): δ 0.78 (s, 18H, tBuNC), 1.38 (s, 9H,
JPP = 86 Hz). IR (ATR): 1978, 1931 cm−1 (νCO). Complex 2a is a p-tBu), 1.81 (br, 18H, o-tBu), 2.10 (s, 3H, p-Me), 2.77 (6H, o-Me),
highly air sensitive solid, and did not give a satisfactory 6.84 (2H, m-Mes), 7.02 (m, 2H, p-Ph), 7.12 (m, 2H, m-Ph), 7.19
elemental analysis.
(m, 2H, m-Ph), 7.28 (br, 2H, o-Ph,), 7.37 (dd, 1H, 4-Py), 7.55 (s,
Reaction with 2,6-Me2C6H3NC (DMPI). A solution of 1 2H, m-Mes*), 7.56 (d, JHH = 10.0 Hz, 2H, o-Ph), 8.08 (1H, 3-Py),
(20.5 mg, 0.021 mmol) in C6D6 (0.6 mL) was charged to an 8.20 (1H, 3-Py). 13C{1H} NMR (C6D6): δ 21.7 (s, p-Me), 24.4 (s,
NMR sample tube. DMPI (5.7 mg, 0.044 mmol, 2.1 equiv.) was o-Me), 24.5 (s, o-Me), 31.3 (s, tBuNC), 32.0 (s, p-tBu), 34.5 (s,
added at room temperature, and the mixture was examined by o-tBu), 35.5 (s, p-tBu), 39.6 (s, o-tBu), 56.3 (s, tBuNC), 114.4 (d,
31P{1H} NMR spectroscopy using a C6D6 solution of PPh3 (0.13 M) JCP = 22 Hz, Ar), 115.5 (d, JCP = 24 Hz, Ar), 121.8 (d, JCP = 4 Hz,
sealed in a capillary tube as an internal standard. A set of Ar), 124.5 (br, Ar), 124.8 (br, Ar), 125.2 (br, Ar), 127.5 (br, Ar),
doublets assignable to 2b and a set of doublets at δ 183.7 and 129.1 (br, Ar), 131.9 (br, Ar), 132.5 (d, JCP = 13 Hz, Ar), 134.7 (d,
50.2 (JPP = 78 Hz) arising from an unidentified compound 3b JCP = 12 Hz, Ar), 134.8 (d, JCP = 12 Hz, Ar), 135.6 (d, JCP = 10 Hz,
instantly appeared in an intensity ratio of 1 : 1. After 4 h, the Ar), 136.2 (d, JCP = 10 Hz, Ar), 137.9 (d, JCP = 9 Hz, Ar), 139.6 (s,
signals of 3b disappeared, whereas the signals of 2b remained Ar), 142.8 (br, tBuNC), 151.6 (s, Ar), 157.5 (s, Ar), 157.9 (s, Ar),
unchanged. The solution was filtered through a Celite pad, 160.8 (d, JCP = 46 Hz, CvP), 161.0 (d, JCP = 47 Hz, CvP). The
and concentrated to dryness under vacuum. The residue was chemical shifts of two Ar carbons were obscured due to the
subjected to a column chromatography (Al2O3, Et2O–toluene = overlap with the signal of C6D6. 31P{1H} NMR (C6D6): δ 263.0
1/1), to give a dark green solid of 2b (8.7 mg, 0.0087 mmol, (d, JPP = 131 Hz), 239.3 (d, JPP = 131 Hz). IR (ATR): 2086,
41%).
2017 cm−1 (νCN). Anal. calcd for C56H71N3P2Fe: C, 74.40; H,
2b: H NMR (C6D6): δ 1.35 (s, 9H, p-tBu), 1.71 (18H, o-tBu), 7.92; N, 4.65. Found: C, 74.56; H, 8.12; N, 4.38.
1.88 (s, 3H, p-Me), 1.93 (12H, 2,6-Me2C6H3NC), 2.67 (6H, Reaction with Me3SiCN. A solution of
1
1
(18.3 mg,
o-Me), 6.56 (s, 2H, m-Mes), 6.64 (br, 6H, m, p-2,6-Me2C6H3NC), 0.019 mmol) in C6D6 (0.5 mL) was charged to an NMR sample
7.00 (m, 4H, o-Ph), 7.07 (m, 4H, m-Ph), 7.28 (m, 2H, p-Ph,), tube, and Me3SiCN (5.5 μL, 0.044 mmol, 2.4 equiv.) was added
1
7.44 (dd, JHH = 7.6 and 7.6 Hz, 1H, 4-Py), 7.48 (s, 2H, m-Mes*), at room temperature. The reaction was monitored by H NMR
8.10 (1H, 3-Py), 8.15 (1H, 3-Py). 13C{1H} NMR (C6D6): δ 19.1 spectroscopy to reveal disappearance of 1 after 2 h. The reac-
(2,6-Me2C6H3NC), 21.5 (s, p-Me), 24.2 (s, o-Me), 24.3 (s, o-Me), tion solution was filtered through a Celite pad, and evaporated
32.0 (s, p-tBu), 34.6 (s, o-tBu), 35.6 (s, p-tBu), 39.6 (s, o-tBu), under vacuum. The resulting green solid was extracted with
115.6 (d, JCP = 12 Hz, Ar), 116.3 (d, JCP = 11 Hz, Ar), 122.3 (d, hexane (ca. 1.5 mL × 3), and evaporated under vacuum to give
JCP = 6 Hz, Ar), 125.1 (br, Ar), 125.7 (m, Ar), 127.3 (d, JCP
=
a dark green solid of 2d (7.3 mg, 0.0077 mmol, 41%). Single
8 Hz, Ar), 127.6 (s, Ar), 127.9 (br, Ar), 131.6 (d, JCP = 4 Hz, Ar), crystals suitable for X-ray diffraction analysis were obtained
131.9 (s, Ar), 132.1 (br, Ar), 133.7 (d, JCP = 11 Hz, Ar), 133.9 (s, from a 2 : 1 mixture of Et2O and hexane at −35 °C.
Ar), 136.1 (d, JCP = 18 Hz, Ar), 138.2 (d, JCP = 15 Hz, Ar), 140.3
2d: 1H NMR (C6D6): δ −0.20 (s, 18H, TMSNC) 1.35 (s, 9H,
(s, Ar), 141.9 (d, JCP = 8 Hz, Ar), 143.1 (d, JCP = 9 Hz, Ar), 152.0 p-tBu), 1.82 (s, 18H, o-tBu), 2.10 (s, 3H, p-Me), 2.76 (6H, o-Me),
(s, p-Mes*), 157.7 (s, o-Mes*), 160.7 (br, 2,6-Me2C6H3NC), 167.2 6.82 (2H, m-Mes), 7.01 (m, 2H, p-Ph), 7.10 (m, 2H, m-Ph), 7.18
(d, JCP = 42 Hz, CvP). The chemical shifts of two Ar carbons (m, 2H, m-Ph), 7.27 (m, 2H, o-Ph,) , 7.37 (br, 1H, 4-Py), 7.54 (s,
were obscured due to the overlap with the signal of C6D6. 31P 2H, m-Mes*), 7.55 (d, JHH = 8.8 Hz, 2H, o-Ph), 8.06 (1H, 3-Py),
{1H} NMR (C6D6): δ 267.0 (d, JPP = 117 Hz), 241.5 (d, J = 8.20 (1H, 3-Py). 13C{1H} NMR (C6D6): δ 0.59 (s, TMSNC), 21.6
117 Hz). IR (ATR): 2051, 1992 cm−1 (νCN). Complex 2b is a (s, p-Me), 24.2 (s, o-Me), 24.3 (s, o-Me), 32.1 (s, p-tBu), 34.4 (s,
highly air sensitive solid, did not give a satisfactory elemental o-tBu), 35.6 (s, p-tBu), 39.5 (s, o-tBu), 115.1 (d, JCP = 12 Hz, Ar),
analysis.
116.4 (d, JCP = 12 Hz, Ar), 121.8 (d, JCP = 6 Hz, Ar), 124.8 (s, Ar),
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125.0 (s, Ar), 125.5 (s, Ar), 127.5 (s, Ar), 131.4 (d, JCP = 5 Hz, Ar), and the summary of solution and refinement are listed in
132.0 (d, JCP = 4 Hz, Ar), 133.8 (d, JCP = 12 Hz, Ar), 135.0 (d, JCP Table S1.†
= 15 Hz, Ar), 136.1 (d, JCP = 10 Hz, Ar), 136.8 (d, JCP = 10 Hz,
DFT calculations
Ar), 140.0 (s, Ar), 143.3 (br, TMSNC), 151.9 (s, Ar), 157.7 (s, Ar).
The chemical shift of an Ar carbon was obscured due to the
overlap with the signal of C6D6. The carbon signals of two
CvP groups were not observed due to low signal intensities.
Intermediates and transition structures on potential energy
surfaces were searched by DFT calculations using the Gaussian
09 program17 and the UB3LYP density functional. The Fe atom
was described with the SDD basis set including a triple-ζ basis
set and effective core potentials (ECPs up to 2p).18 The 6-31G(d)
basis set was employed for the atoms bonded to Fe in the
Mes, pyridyl, phosphaalkene, and isocyanide ligands.19 The
6-31G basis set was applied to other atoms.19 All structures were
optimized in the gas phase. Systematic vibrational analyses
were carried out for all reaction species to characterize station-
ary-point structures. Complexes 1′ and TS1 exhibit small ima-
ginary frequencies due to methyl group rotations even after
applying a tight convergence. An appropriate connection
between a reactant and a product for each reaction step was
confirmed by IRC calculations.20,21 The orbital plot in Fig. 2
was generated with Molekel.22
31P{1H} NMR (C6D6): δ 266.5 (d, JPP = 117 Hz), 241.2 (d, JPP
=
117 Hz). IR (ATR): 1998, 1921 cm−1 (νCN). Anal. calcd for
C54H71N3P2Si2Fe·C4H10O: C, 69.02; H, 7.99; N, 4.16. Found: C,
68.54; H, 8.21; N, 3.97.
Radical trapping experiments with TEMPO
Reaction with tBuNC. An NMR sample tube equipped with
a Teflon cock (J-Young) was charged with a 1 : 1 mixture of 1
(12.0 mg, 0.012 mmol) and TEMPO (1.9 mg, 0.012 mmol) in
C6D6. tBuNC (2.8 μL, 0.025 mmol) was added, and the mixture
was examined by 1H NMR spectroscopy using mesitylene
(0.012 mmol) as an internal standard. The reaction was com-
pleted instantly at room temperature to form 2c and 4 in 100
and 98% yields, respectively.
1
Compound 4 was identified by NMR and HRMS: H NMR
Acknowledgements
(C6D6): δ 1.07 (s, 6H, Me), 1.12 (s, 6H, Me), 1.26 (br, 2H, CH2)
1.37 (s, 18H, tBu), 1.42 (br, 4H, CH2) 1.49 (s, 6H, Me), 3.96 (s,
2H, OCH2), 7.43 (s, H, Ar), 7.45 (s, 2H, Ar). 13C{1H} NMR
(C6D6): δ 17.8, 20.6, 26.6, 32.2, 33.7, 35.4, 40.1, 40.4, 60.5, 86.5,
120.2, 121.1, 147.5, 150.5. HRMS (m/z): calcd for C27H47NO
402.3730 [M + H]+; obsd 402.3737.
Reaction with DMPI. Similarly, 1 was treated with DMPI (2
equiv.) in C6D6 in the presence of TEMPO (1 equiv.). The reac-
tion was completed instantly at room temperature to afford 2b
and 4 in 92 and 89% yields, respectively.
This work was supported by a Grant-in-Aid for Scientific
Research from Mext Japan (no. 24109010) and a JST PRESTO
program. Y.L. is grateful to the MEXT Project “Integrated
Research on Chemical Synthesis” for the provision of a post-
doctoral fellowship.
Notes and references
Reaction with TMSCN. The reaction with TMSCN (2.5
equiv.) under otherwise the same conditions formed 2d and 4
in 26 and 36% yields, respectively, along with the formation of
Z,Z-BPEP-Ph in 16% yield. The yields of 2d and 4 increased to
71 and 84%, respectively, when excess TMSCN (13 equiv.) was
employed.
Reaction with CO. Treatment of 1 with CO (1 atm) in the
presence of TEMPO (1 equiv.) resulted in decomposition of the
complex; neither 2a nor 4 could be observed by 1H NMR
spectroscopy.
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The intensity data were collected on Rigaku Mercury CCD
diffractometers with monochromated Mo Kα radiation (λ =
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