Selective activation of C-F and C-H bonds with iron complexes, the relevant mechanism study by DFT calculations and study on the chemical properties of hydrido iron complex

Article abstract of DOI:10.1039/c2dt31795h

The reactions of (2,6-difluorophenyl)phenylmethanone (2,6-F ₂C₆H₃-C(O)-C₆H₅) (1) and (2,6-difluorophenyl)phenylmethanimine (2,6-F₂C₆H ₃-C(NH)-C₆H₅) (3) with Fe(PMe₃) ₄ afforded different selective C-F/C-H bond activation products. The reaction of 1 with Fe(PMe₃)₄ gave rise to bis-chelate iron(ii) complex [C₆H₅-C(O)-3-FC₆H ₃)Fe(PMe₃)]₂ (2) via C-F bond activation. The reaction of 3 with Fe(PMe₃)₄ delivered chelate hydrido iron(ii) complex 2,6-F₂C₆H₃-C(NH)-C ₆H₄)Fe(H)(PMe₃)₃ (4) through C-H bond activation. The DFT calculations show the detailed elementary steps of the mechanism of formation of hydrido complex 4 and indicate 4 is the kinetically preferred product. Complex 4 reacted with HCl, CH₃Br and CH ₃I delivered the chelate iron halides (2,6-F₂C ₆H₃-C(NH)-C₆H₄)Fe(PMe ₃)₃X (X = Cl (5); Br (6); I (7)). A ligand (PMe ₃) replacement by CO of 4 was observed giving (2,6-F ₂C₆H₃-C(NH)-C₆H₄)Fe(H) (CO)(PMe₃)₂ (8). The chelate ligand exchange occurred through the reaction of 4 with salicylaldehydes. The reaction of 4 with Me ₃SiCCH afforded (2,6-F₂C₆H₃-C(N)- C₆H₅)Fe(CC-SiMe₃)(PMe₃)₃ (11). A reaction mechanism from 4 to 11 was discussed with the support of IR monitoring. The molecular structures of complexes 2, 4, 6, 7, 10 and 11 were determined by X-ray diffraction.

Full text of DOI:10.1039/c2dt31795h

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Selective activation of C–F and C–H bonds with iron
complexes, the relevant mechanism study by DFT
calculations and study on the chemical properties
of hydrido iron complex†
Cite this: Dalton Trans., 2013, 42, 3417
Xiaofeng Xu,‡ Jiong Jia,‡ Hongjian Sun, Yuxia Liu, Wengang Xu, Yujie Shi,
Dongju Zhang* and Xiaoyan Li*
The reactions of (2,6-difluorophenyl)phenylmethanone (2,6-F2C6H3–C(vO)–C6H5) (1) and (2,6-difluoro-
phenyl)phenylmethanimine (2,6-F2C6H3–C(vNH)–C6H5) (3) with Fe(PMe3)4 afforded different selective
C–F/C–H bond activation products. The reaction of 1 with Fe(PMe3)4 gave rise to bis-chelate iron(II)
complex [C6H5–C(vO)–3-FC6H3)Fe(PMe3)]2 (2) via C–F bond activation. The reaction of 3 with Fe(PMe3)4
delivered chelate hydrido iron(II) complex 2,6-F2C6H3–C(vNH)–C6H4)Fe(H)(PMe3)3 (4) through C–H bond
activation. The DFT calculations show the detailed elementary steps of the mechanism of formation of
hydrido complex 4 and indicate 4 is the kinetically preferred product. Complex 4 reacted with HCl, CH3Br
and CH3I delivered the chelate iron halides (2,6-F2C6H3–C(vNH)–C6H4)Fe(PMe3)3X (X = Cl (5); Br (6); I (7)).
A ligand (PMe3) replacement by CO of 4 was observed giving (2,6-F2C6H3–C(vNH)–C6H4)Fe(H)(CO)-
(PMe3)2 (8). The chelate ligand exchange occurred through the reaction of 4 with salicylaldehydes. The
reaction of 4 with Me3SiCuCH afforded (2,6-F2C6H3–C(vN)–C6H5)Fe(CuC–SiMe3)(PMe3)3 (11). A reaction
mechanism from 4 to 11 was discussed with the support of IR monitoring. The molecular structures of
complexes 2, 4, 6, 7, 10 and 11 were determined by X-ray diffraction.
Received 6th August 2012,
Accepted 20th November 2012
DOI: 10.1039/c2dt31795h
www.rsc.org/dalton
on the selective process. There are at least two factors that
dominate the activation of C–F bonds over C–H bonds. They
1 Introduction
The activation and functionalization of C–F and C–H bonds are the dπ–pπ repulsions in fluoride complexes2 and the
have now been observed with a wide variety of transition degree of ionicity in the M–F bond.3a,b However, any acti-
metals.1 Potential applications of these activation products for vations assemble both kinetic and thermodynamic effects.
pharmaceuticals and agrochemicals attract increasing interest Holland realized hydrogen transfer from hydrocarbon (C–H
from chemists. The difficulty associated with these processes bond activation of 1,4-cyclohexadiene) to three-coordinate iron
is the high inertness of C–F and C–H bonds. If activators, imides.3c With a “masked” two-coordinate cobalt(I) complex,
e.g. transition metal complexes, for these inert bonds are to the C–F bonds in fluorobenzene could be cleaved.3d,e
find application for synthesis of fluoro-substituted aromatics,
Iron complexes are good activators for C–H4 and C–F
they have to exhibit selectivity for C–F bonds over C–H bonds. bonds.5 Iron-based catalysts with comparable activity benefit
The selectivity of these activations has been extensively docu- from their potentially lower cost and lack of toxicity and
mented and rationalized using some fragments. The kinetic environmental impact. In recent years, iron complexes as cata-
barrier for C–F bond activation produces a significant impact lysts have been investigated more widely for the formation of
new C–C bonds from aromatic C–H bonds. However, C–F bond
activation is relatively underrepresented, especially in the com-
petition of C–F bond activation over C–H bond activation.
School of Chemistry and Chemical Engineering, Shandong University, Shanda Nanlu
27, 250100 Jinan, People’s Republic of China. E-mail: xli63@sdu.edu.cn,
Hydrido iron complexes undergo interesting reactions
because of the anionic nature of the hydrogen ligand. The
hydrido ligands react with Brønsted acid or other acidic hydro-
gen via the elimination of dihydrogen.
In this paper, the selective activation of C–F and C–H bonds
with iron complexes supported by trimethylphosphine is
zhangdj@sdu.edu.cn; Fax: +86 531 88361350
†Electronic supplementary information (ESI) available: Cartesian coordinates
and free energies for all structures given in Fig. 3 as well as the imaginary fre-
quencies for transition states. CCDC 847492, 847494, 847495, 847496, 847490
and 847491. For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c2dt31795h
‡The first two authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2013
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2
2
presented. The influence of anchoring groups upon the selec- (dt, JP,H = 20.5 Hz, JP,H = 81.0 Hz, 1H, Fe–H), 1.07, 1.34
tivity are investigated and discussed with the results of density (m, 27H, P(CH3)3), 6.63–8.29 (m, 7H, Ar–H), 9.41 (s, 1H,
functional theory (DFT) calculations. The chemical properties CvNH). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 23.5 (t,
2
2
of hydrido iron complexes are studied with alkylhalide, salicyl- 2JP,P = 82.8 Hz, 1P, P(CH3)3), 18.5 (m, JP,H = 81.0 Hz, JP,P
=
aldehyde, alkyne and carbon monoxide. The reaction mechan- 82.8 Hz, 2P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K,
ism of hydrido iron complexes with trimethylsilylacetylene was ppm): δ −109.0 (s).
monitored by IR spectroscopy.
Synthesis of 5. Compound 4 (1.0 g, 2.0 mmol) in 50 ml
diethyl ether was added to hydrogen chloride solutions in
diethyl ether (4.4 ml, 0.5 M) at −80 °C. The reaction mixture
was allowed to warm to room temperature and stirred for 24 h.
During this period the reaction mixture turned from violet to
red. After filtration, the solid residue was extracted with 30 ml
2 Experimental
General procedures and materials
Standard vacuum techniques were used in manipulations of of diethyl ether. The combined diethyl ether solutions crystal-
volatile and air-sensitive materials. Solvents were dried by lized at −20 °C. Complex 5 as red crystals was obtained from
known procedures and distilled under nitrogen before use. diethyl ether. Yield: 0.80 g, 75%; m.p. (dec) > 120 °C. Anal.
Literature methods were used in the preparation of tetrakis- Calcd for C22H35ClF2FeNP3 (535.72 g mol−1): C, 49.32; H, 6.58;
(trimethylphosphine)iron(0).6,7 (2,6-Difluorophenyl)(phenyl)- N, 2.61. Found: C, 49.41; H, 6.74; N, 2.69. IR (Nujol, cm−1):
methanimine and (2,6-difluorophenyl)(phenyl)methanone 3300 ν(N–H), 1622 ν(CvN), 1585, 1573, 1548, 1518 ν(CvC),
1
were obtained by addition reaction of 2,6-difluorobenzonitrile 938 ρ1(PCH3). H NMR (300 MHz, CDCl3, 298 K, ppm): δ 1.53
and phenylmagnesiumbromide subsequently quenching with (m, 27H, P(CH3)3), 5.15–6.43 (m, 7H, Ar–H), 11.01 (s, 1H,
methanol and dilute hydrochloride in diethyl ether solutions.8 CvNH). 31P NMR (121.5 MHz, CDCl3, 298 K, ppm): δ 38.6
Infrared spectra (4000–400 cm−1), as obtained from Nujol (s, 1P, P(CH3)3), 33.4 (s, 1P, P(CH3)3), 10.1 (s, 1P, P(CH3)3).
mulls between KBr disks, were recorded on a Bruker ALPHA 19F NMR (282.4 MHz, CDCl3, 298 K, ppm): δ −108.6 (s).
FT-IR. NMR spectra were recorded using a Bruker Avance 300
Synthesis of 6. Compound 4 (1.0 g, 2.0 mmol) in 50 ml
or 400 MHz spectrometer. X-Ray crystallography was per- diethyl ether was added to methylbromide (0.20 g, 2.1 mmol)
formed with a Bruker Smart 1000 diffractometer. Melting in 30 ml diethyl ether at −80 °C. The reaction mixture was
points were measured in capillaries sealed under nitrogen and allowed to warm to room temperature and stirred for 24 h.
were uncorrected. Elemental analyses were carried out on an During this period the reaction mixture turned from violet to
Elementar Vario EL III.
red. After filtration, the solid residue was extracted with 30 ml
Synthesis of 2. (2,6-Difluorophenyl)phenylmethanone
1
of diethyl ether. Complex 6 as red crystals suitable for X-ray
(0.60 g, 2.75 mmol) in 30 ml diethyl ether was added to a single crystal diffraction was obtained from diethyl ether
sample of Fe(PMe3)4 (0.50 g, 1.39 mmol) in 20 ml diethyl ether solution at −20 °C. Yield: 0.74 g, 64%; m.p. (dec) > 120 °C.
at −80 °C. The mixture was kept stirring at ambient tem- Anal. Calcd for C22H35BrF2FeNP3 (580.19 g mol−1): C, 45.54;
perature for 16 h. During this period, the color of the mixture H, 6.08; N, 2.41. Found: C, 46.07; H, 6.14; N, 2.33. IR (Nujol,
turned blue slowly. After filtration, crystallization at −20 °C cm−1): 3300 ν(N–H), 1622 ν(CvN), 1573, 1548, 1518 ν(CvC),
afforded blue crystals of 2. Yield: 0.62 g, 75%; m.p. (dec) 938 ρ1(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm):
> 120 °C. Anal. Calcd for C32H34F2FeO2P2 (606.38 g mol−1): δ 1.45 (m, 27H, P(CH3)3), 6.54–7.72 (m, 7H, Ar–H), 9.81
C, 63.38; H, 5.65. Found: C, 63.77; H, 6.02. IR (Nujol, cm−1): (s, 1H, CvNH). 31P NMR (121.5 MHz, C6D6, 298 K, ppm):
2
2
1623 ν(CvO), 1580, 1541, 1519 ν(CvC), 941 ρ1(PCH3). δ 23.8 (t, JP,P = 60.6 Hz, 1P, P(CH3)3), 13.9 (d, JP,P = 60.6 Hz,
1H NMR (300 MHz, C6D6, 298 K, ppm): δ 0.99 (m, 18H, 2P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K, ppm):
P(CH3)3), 6.63–8.30 (m, 16H, Ar–H). 31P NMR (121.5 MHz, δ −108.9 (s).
C6D6, 298 K, ppm): δ 16.3 (s, P, P(CH3)3). 19F NMR (282.4 MHz,
C6D6, 298 K, ppm): δ −102.0 (s).
Synthesis of 7. Compound 4 (1.0 g, 2.0 mmol) in 50 ml
diethyl ether was added to methyliodide (0.30 g, 2.1 mmol) in
Synthesis of 4. (2,6-Difluorophenyl)(phenyl)methanimine 3 30 ml diethyl ether at −80 °C. The reaction mixture was
(0.60 g, 2.76 mmol) in 30 ml diethyl ether was added to a allowed to warm to room temperature and stirred for 24 h.
sample of Fe(PMe3)4 (1.0 g, 2.78 mmol) in 20 ml diethyl ether During this period the reaction mixture turned from violet to
at −80 °C. The mixture was kept stirring at ambient tempera- red. After filtration, the solid residue was extracted with 30 ml
ture for 16 h. During this period the color of the mixture of diethyl ether. Complex 7, suitable for X-ray diffraction as red
turned violet slowly. After removal of the solvent at reduced crystals, was obtained from the diethyl ether solution at
pressure the solid residue was extracted with two portions of −20 °C. Yield: 0.74 g, 59%; m.p. (dec) > 120 °C. Anal. Calcd
pentane (50 ml × 2). Crystallization at −20 °C afforded violet for C22H35F2FeINP3 (627.17 g mol−1): C, 42.13; H, 5.62; N,
crystals of 4. Yield: 1.11 g, 80%; m.p. (dec) > 85 °C. Anal. Calcd 2.23. Found: C, 42.16; H, 6.03; N, 2.29. IR (Nujol, cm−1): 3300
for C22H36F2FeNP3 (501.28 g mol−1): C, 52.71; H, 7.24; N, 2.79. ν(N–H), 1622 ν(CvN), 1573, 1550, 1518 ν(CvC), 938 ρ1(PCH3).
Found: C, 52.46; H, 7.59; N, 2.72. IR (Nujol, cm−1): 3296 1H NMR (300 MHz, C6D6, 298 K, ppm): δ 1.23 (m, 27H,
ν(N–H), 1733 ν(Fe–H), 1622 ν(CvN), 1584, 1543 ν(CvC), 939 P(CH3)3), 6.50–7.76 (m, 7H, Ar–H), 9.88 (s, 1H, CvNH).
ρ1(PCH3). 1H NMR (300 MHz, C6D6, 298 K, ppm): δ −15.2 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 20.7 (t, 2JP,P = 56.8 Hz,
3418 | Dalton Trans., 2013, 42, 3417–3428
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2
1P, P(CH3)3), 10.8 (d, JP,P = 56.8 Hz, 2P, P(CH3)3). 19F NMR 0.89 g, 75%; m.p. (dec)
>
100 °C. Anal. Calcd for
(282.4 MHz, C6D6, 298 K, ppm): δ −108.8 (s).
C27H44F2FeNP3Si (597.48 g mol−1): C, 54.27; H, 7.42; N, 2.34.
Synthesis of 8. Compound 4 (1.0 g, 2.0 mmol) in 50 ml Found: C, 54.62; H, 7.39. N, 2.37. IR (Nujol, cm−1): 1964
diethyl ether was stirred under 1 bar of CO to provide violet ν(CuC), 1615 ν(CvN), 1582, 1557 ν(CvC), 939 ρ1(PCH3). 1H
crystals of 8, which crystallized at −20 °C. Yield: 0.73 g, 80%; NMR (300 MHz, C6D6, 298 K, ppm): δ 0.45 (s, 9H, Si(CH3)3),
m.p. (dec) > 85 °C. Anal. Calcd for C20H27F2FeNOP2 (453.22 g 1.01 (m, 18H, P(CH3)3), 1.45 (m, 9H, P(CH3)3), 6.33–7.60 (m,
mol−1): C, 53.00; H, 6.00; N, 3.09. Found: C, 53.41; H, 6.13; N, 8H, Ar–H). 31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 30.7 (m,
3.01. IR (Nujol, cm−1): 3296 ν(N–H), 1877 ν(CuO), 1731 ν(Fe– 2P, P(CH3)3), 19.4 (s, 1P, P(CH3)3). 19F NMR (282.4 MHz, C6D6,
1
H), 1624 ν(CvN), 1588, 1543 ν(CvC), 944 ρ1(PCH3). H NMR 298 K, ppm): δ −112.1 (s).
(300 MHz, C6D6, 298 K, ppm): δ −15.0 (m, 1H, Fe–H), 0.95 (m,
18H, P(CH3)3), 6.53–8.03 (m, 7H, Ar–H), 9.98 (s, 1H, CvNH).
31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 30.4 (s, P(CH3)3).
19F NMR (282.4 MHz, C6D6, 298 K, ppm): δ −112.1 (s).
Synthesis of 9. Compound 4 (1.0 g, 2.0 mmol) in 50 ml
diethyl ether was combined with 2-hydroxybenzaldehyde
(0.26 g, 2.1 mmol) in 30 ml diethyl ether and trimethylphos-
Confirmation of F2PMe3
The formation of F2PMe3 was confirmed by the in situ 31P and
19F NMR spectra of the reaction solutions.9
IR monitoring
phine (0.16 g, 2.1 mmol) at −80 °C. The reaction mixture was To explore the reaction mechanism, the reaction of compound
allowed to warm to room temperature and stirred for 24 h. 4 with trimethylsilylethyne (please see Synthesis of 11) was
During this period the reaction mixture turned from violet to carried out in benzene instead of diethyl ether. Each time one
red. After filtration, the solid residue was extracted with 30 ml portion (ca. 5 mL) of the reaction mother solution was
of diethyl ether. Complex 9, as red crystals, was obtained from sampled over varying intervals of time and the volatiles of this
diethyl ether solution at −20 °C. Yield: 0.53 g, 55%; m.p. (dec) solution were removed via vacuum. The solid residue was used
> 110 °C. Anal. Calcd for C19H40FeO2P4 (480.26 g mol−1): in the FTIR analysis.
C, 47.52; H, 8.39. Found: C, 47.74; H, 8.61. IR (Nujol, cm−1): 1618
ν(CvO), 1583, 1526 ν(CvC), 946 ρ1(PCH3). 1H NMR (300 MHz,
X-Ray structure determinations
C6D6, 298 K, ppm): δ 1.02 (m, 18H, P(CH3)3), 1.06 (m, 9H,
Intensity data were collected on a Bruker SMART diffract-
P(CH3)3), 1.24 (m, 9H, P(CH3)3), 6.74–8.04 (m, 4H, Ar–H).
ometer with graphite-monochromated Mo Kα radiation (λ =
31P NMR (121.5 MHz, C6D6, 298 K, ppm): δ 26.9 (t, 2JP,P = 47.6 Hz,
0.71073 Å). Crystallographic data for complexes 2, 4, 6, 7, 10
2
2
1P, P(CH3)3), 15.7 (dd, JP,P = 48.1 Hz, JP,P = 19.4 Hz, 2P,
P(CH3)3), −1.08 (s, 1P, P(CH3)3).
and 11 are summarized in Table 1. The structures were solved
by direct methods and refined with full-matrix least-
squares on all F2 (SHELXL-97) with non-hydrogen atoms aniso-
tropic. CCDC-847492 (2), CCDC-847494 (4), CCDC-847495 (6),
CCDC-847496 (7), CCDC-847490 (10) and CCDC-847491 (11)
contain the supplementary crystallographic data for this
paper.†
Synthesis of 10. Compound 4 (1.0 g, 2.0 mmol) in 50 ml
diethyl ether was combined with 2-hydroxy-3-methoxybenzal-
dehyde (0.32 g, 2.1 mmol) in 30 ml diethyl ether and trimethyl-
phosphine (0.16 g, 2.1 mmol) at −80 °C. The reaction mixture
was allowed to warm to room temperature and stirred for 24 h.
During this period the reaction mixture turned from violet to
red. After filtration, the solid residue was extracted with 30 ml
of diethyl ether. Complex 10, as red crystals, was obtained
DFT study
from the diethyl ether solution at −20 °C. Yield: 0.61 g, 60%; Theoretical calculations presented in this work were carried
m.p. (dec) > 110 °C. Anal. Calcd for C20H42FeO3P4 (510.27 g out in the framework of density functional theory (DFT)
mol−1): C, 47.07; H, 8.30. Found: C, 47.55; H, 8.36. IR (Nujol, employing the Becke three-parameter hybrid density func-
cm−1): 1628 ν(CvO), 1581, 1532 ν(CvC), 942 ρ1(PCH3). 1H tional combined with Lee–Yang–Parr correlation functional
NMR (300 MHz, C6D6, 298 K, ppm): δ 1.06 (m, 18H, P(CH3)3), (B3LYP) level.10,11 The relativistic effective core potentials
1.12 (m, 9H, P(CH3)3), 1.31 (m, 9H, P(CH3)3), 3.79 (s, 3H, Ar– (ECPS) of Hay and Wadt12,13 with double-ζ valence basis sets
OCH3), 6.68–7.85 (m, 3H, Ar–H). 31P NMR (121.5 MHz, C6D6, (LanL2DZ) were used to describe Fe atom, while the standard
2
298 K, ppm): δ 27.7 (t, JP,P = 50.6 Hz, 1P, P(CH3)3), 16.4 (dd, 6-31G(d,p) basis set was chosen for C, N, F, and P atoms. Mole-
2
2JP,P = 55.3 Hz, JP,P = 17.9 Hz, 2P, P(CH3)3), –0.2 (s, 1P, cular geometries of minima and transition states were comple-
P(CH3)3).
tely optimized by total energy minimization with the use of
Synthesis of 11. Compound 4 (1.0 g, 2.0 mmol) in 50 ml analytic gradient techniques. Harmonic vibrational frequency
diethyl ether was combined with trimethylsilylethyne (0.20 g, calculations have also been conducted to verify all stationary
2.1 mmol) in 30 ml diethyl ether at −80 °C. The reaction points as minima (zero imaginary frequency) or first-order
mixture was allowed to warm to room temperature and stirred saddle points (one imaginary frequency). Intrinsic reaction
for 24 h. During this period the reaction mixture remained coordinates (IRC)14,15 were calculated for the transition states
violet. After filtration the solid residue was extracted with to verify that such structures indeed connect two relevant
30 ml of diethyl ether. Complex 11 as violet crystals was minima. All calculations were implemented using the
obtained from the diethyl ether solution at −20 °C. Yield: Gaussian-03 software package.16
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Table 1 Crystallographic data for complexes 2, 4, 6, 7, 10 and 11
2
4
6
7
10
11
Empirical formula
Mw
Cryst. syst.
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
32H34F2FeO2P2
C22H36F2FeNP3
501.28
Triclinic
ˉ
P1
9.300(7)
11.375(9)
13.62(1)
66.927(11)
79.162(13)
74.851(13)
1273.5(17)
2
C22H35BrF2FeNP3
580.18
Orthorhombic
Cmca
13.432(3)
12.134(3)
33.252(7)
90
90
90
5420(2)
8
1.422
C22H35F2FeINP3
627.17
Orthorhombic
Cmca
13.492(7)
12.097(6)
34.022(17)
90
90
90
5553(5)
8
1.500
C20H42FeO3P4
510.27
Monoclinic
P2(1)/n
10.094(3)
18.751(6)
14.086(5)
90
100.510(6)
90
2621.2(15)
4
C27H44F2FeNP3Si
597.48
Monoclinic
P2(1)/n
9.661(2)
21.657(5)
15.892(4)
90
102.072(4)
90
3251.5(12)
4
606.38
Monoclinic
P2(1)/c
9.6343(8)
9.3988(8)
33.449(3)
90
92.814(2)
90
3025.2(5)
4
γ (°)
V (Å3)
Z
Dc (g cm−3
)
1.331
1.307
1.293
1.220
No. of rflns collected
No. of unique data
Rint
17 439
6858
0.0558
27.55
6552
4435
0.0808
25.00
14 857
3201
0.0547
27.42
17 416
3628
0.1621
28.50
14 531
5153
0.0673
26.00
18 087
6383
0.0412
26.00
θmax (°)
R1 (I > 2σ(I))
wR2 (all data)
0.0513
0.1479
0.0466
0.1298
0.0625
0.1952
0.0532
0.1639
0.0393
0.0959
0.0436
0.1039
3 Results and discussion
3.1 Reactions of Fe(PMe3)4 with fluorinated methanone
The reaction of Fe(PMe3)4 with (2,6-difluorophenyl)phenyl-
methanone 1 in diethyl ether at room temperature for 16 h
affords the C–F bond activation product 2 (eqn (1)). During
this period the color of the reaction solution turned blue
slowly. With the assistance of the ketone group, C–F bond acti-
vation proceeds. A cis-bis-chelate diorgano iron(II) complex 2
was produced.
Fig. 1 Molecular structure of complex
2
with 30% probability ellipsoids.
Selected bond distances (Å) and angles (°) (all the hydrogen atoms were
omitted for clarity): Fe–O1 1.929(2), Fe–C1 1.957(3), Fe–C14 1.962(3), Fe–O2
1.965(2), O1–C7 1.258(4), O2–C20 1.253(3); O1–Fe–C1 81.63(13), O1–Fe–C14
166.51(12), C1–Fe–C14 111.71(15), O1–Fe–O2 84.86(9), C1–Fe–O2 166.43(13),
C14–Fe–O2 81.84(12), O1–Fe–P1 92.48(7), C1–Fe–P1 87.08(10), C14–Fe–P1
86.45(10), O2–Fe–P1 94.72(7), O1–Fe–P2 96.08(7), C1–Fe–P2 86.43(10), C14–
Fe–P2 87.04(10), O2–Fe–P2 93.85(7), P1–Fe–P2 168.39(4).
ð1Þ
Complex 2 crystallizes from its diethyl ether solution as
blue crystals at −20 °C. In the IR spectra the ν(CvO) band
displays a significant bathochromic shift from 1678 cm−1 in
compound 1 to 1623 cm−1 in complex 2 due to the coordi-
equatorial plane. The cis-orientation of the two chelate ligands
is caused by the strong trans-effect of the cyclometalated
phenyl-C atoms. The two bite angles of C1–Fe–O1 and C14–Fe–
O2 are 81.64(12) and 81.88(12)° respectively. The sum of bond
angles of C1–Fe1–O1 (81.64(12)°), C14–Fe–O2 (81.88(12)°),
O1–Fe1–O2 (84.842(9)°), and C1–Fe1–C14 (111.69(5)°) angles is
360.0°. This implies that the five atoms [C1, C14, O1, O2 and
Fe] are in one plane.
It is suggested that the reaction mechanism (Scheme 1) is
similar to that of the reaction of Fe(PMe3)4 with fluorinated
benzalimines.17 The replacement of PMe3 by the carbonyl
group of 1 is the first step to form intermediate 1a. Oxidative
addition of a C–F bond affords the iron(II) intermediate 1b
with the support of the anchoring group (CvO). The for-
mation of F2PMe3 from the unstable 1b delivers iron(I)
1
nation of carbonyl-O atom to the iron center. In the H NMR
and 31P NMR spectra, one signal of trimethylphosphine ligand
shows the chemical shifts at 0.99 ppm and at 16.3 ppm
respectively. The 19F NMR spectra show one resonance at
−102.0 ppm. All the spectroscopic information indicates a
cis-bis-chelate octahedral iron(II) complex with two trimethyl-
phosphine ligands in the trans-positions. The formation of
byproduct F2PMe3 was verified via 31P and 19F NMR. The
spectroscopic data match the literature values.9
The molecular structure (Fig. 1) of complex 2 shows a
slightly distorted octahedral frame of an iron(II) complex
bearing two trimethylphosphine ligands in axial positions
with the bond angle P1–Fe–P2 of 168.38(4)°. The two cyclome-
talated-C atoms and the two ketone-O atoms occupy the
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Scheme 1 Proposed reaction mechanism of 1 with Fe(PMe3)4.
complex 1c, which transforms iron(III) fluoride 1d through oxi-
dative addition of the C–F bond of the second ketone ligand.
The bis-chelate end product 2 is obtained by the formation of
F2PMe3. The in situ 31P and 19F NMR spectra confirmed that
the ratio of complex 2 to F2PMe3 is 1 : 1. The formation of
F2PMe3 supports this putative mechanism of formation of
complex 2.
ð2Þ
Complex 4 crystallizes as violet crystals from its pentane
solutions at −20 °C. The IR spectra show a typical ν(Fe–H)
signal at 1733 cm−1. In the 1H NMR spectra the significant
hydrido signal appears at −15.2 ppm as a doublets of triplets
3.2 Reactions of Fe(PMe3)4 with fluorinated methanimine
due to the three meridional-positioned trimethylphosphine
2
In contrast to the formation of C–F activation product 2 from ligands with coupling constants J(P,H)trans of 20.5 Hz and
the reaction of Fe(PMe3)4 with (2,6-difluorophenyl)phenyl- 2J(P,H)cis of 81.0 Hz. This coupling pattern of trimethylphosphine
methanone 1 (eqn (1)), the reaction of Fe(PMe3)4 with (2,6- ligands indicates that the iron center has an octahedral coordi-
difluorophenyl)phenylmethanimine 3 in diethyl ether at room nation geometry made up of three trimethylphosphine
temperature for 16 h afforded C–H bond activation product 4 ligands. In the 31P NMR spectra two types of PMe3 groups
(eqn (2)). During this period, the color of the mixture turned appear at 23.5 and 18.5 ppm with relative integral ratio of 1 to
violet slowly. This is a simple ortho-metalation reaction via 2. The 19F NMR spectra show the fluorine signal at
C–H bond activation.
−109.0 ppm.
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X-Ray diffraction analysis of complex 4 indicates that the chelate ligand and the small hydrido ligand. The Fe–H dis-
cyclometalated imine ligand coordinates to the iron center tance of 1.42(3) Å is shorter than the average length of a term-
through the imine nitrogen N1 and phenyl carbon C13 atom inal Fe–H bond (1.575(17) Å).18
forming a five-membered chelated ring with the bite angle
Fig. 3 shows the calculated energy profiles and mechanism
N1–Fe1–C13 of 79.79(12)° (Fig. 2). The sum (539.99°) of the details on the reaction of (2,6-difluorophenyl)phenylmethan-
internal angles (N1–Fe1–C13: 79.79(12)°; Fe1–C13–C8: 114.0 imine 3 with Fe(PMe3)4 along both the C–H and C–F bond acti-
(2)°; C13–C8–C7: 112.2(3)°; C8–C7–N1: 113.5(2)°; C7–N1–Fe1: vation branches, resulting in the products PC–H and PC–F. For
120.5(2)°) of the chelate ring shows a planarity of the five- imine 3, there are two configuration isomers, RC–H and RC–F
,
membered metalacycle. P2 and P3 atoms are at the opposite which are located as the preferred geometries for the C–H
positions with the bond angle P2–Fe1–P3 of 152.07(4)° remark- bond and C–F bond activations, respectively. As shown in
ably deviating from 180° owing to the influence of bulky Fig. 3, in the case of C–H bond activation, the reaction involves
two main elementary steps: the oxidative addition of the C–H
bond to the metal center via TS1 with a barrier of 3.8 kcal
mol−1 followed by the rearrangement of the C–H bond inser-
tion species via TS2 with a barrier of 6.2 kcal mol−1 to form
the observed product PC–H (4), which is more stable than RC–H
by 36.6 kcal mol−1. In contrast, the C–F bond activation starts
from RC–F, which is energetically less favorable by 1.8 kcal
mol−1 than RC–H, and furthermore the conformational trans-
ition from RC–H to RC–F via TS3 needs to overcome a barrier as
high as 26.7 kcal mol−1, implying that the C–F bond activation
should be much more difficult than the C–H bond activation.
The large barrier between RC–H and RC–F might be attributed
to the strong intramolecular hydrogen bond F⋯N–H in RC–H
.
As seen in Fig. 3, the isomerization from RC–H to RC–F is the
bottleneck of the C–F bond activation branch, which needs
much more energy than the following oxidative addition of the
C–F bond via TS4. Thus the reaction of 3 with Fe(PMe3)4
prefers the C–H bond activation to the C–F bond activation,
although the latter can result in the more stable product than
the former (36.6 vs. 64.7 kcal mol−1 below the reactants). The
calculated results, which not only give a good support to the
Fig. 2 Molecular structure of complex
4 with 30% probability ellipsoids.
Selected bond distances (Å) and angles (°): Fe1–H1 1.42(3), N1–H2 0.8600, Fe1–
N1 1.928(2), Fe1–P1 2.2150(15), C13–Fe1 1.977(3), C7–N1 1.315(4); N1–Fe1–H1
175.5(13), C13–Fe1–H1 97.0(13), P1–Fe1–H1 95.4(13), P2–Fe1–H1 79.4(13), P3–
Fe1–H1 74.0(13), P2–Fe1–P3 152.07(4), C13–Fe1–P1 167.54(9), N1–Fe1–C13
79.79(12).
Fig. 3 Calculated energy profiles for C–H activation vs. C–F activation. The relative free energies are given in kcal mol−1
.
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experimental observation but also show the detailed elemen-
tary-step mechanism of the reaction, indicate that the reaction
proceeds along the C–H bond activation branch to result in
the kinetically preferred product PC–H (4). From this result we
consider that in eqn (1), complex 2 is the thermodynamically
preferred product because no cis–trans isomer exists for
ketone.
3.3 Substitution reactions of hydrido iron complex 4 with
HCl, CH3X (X = Br and I), CO and salicylaldehydes
The reaction of hydrido iron complex 4 with hydrochloride
afforded complex 5 with releasing one molecule of dihydrogen
(eqn (3)). The reaction indicates that complex 4 is hydridic.
Fig. 4 Molecular structure of 6 with 30% probability ellipsoids. Selected bond
distances (Å) and angles (°): Fe1–N1 1.952(5), Fe1–C1 1.967(6), Fe1–P2 2.247(2),
Fe1–P1 2.270(1), Fe1–Br1 2.577(1), N1–C7 1.279(9), C1–C6 1.44(1), C6–C7 1.47
(1); N1–Fe1–C1 80.1(3), N1–Fe1–P2 175.05(18), C1–Fe1–Br1 167.7(2), C7–N1–
Fe1 120.8(5), C6–C1–Fe1 113.7(5), C1–C6–C7 112.6(6), N1–C7–C6 112.8(6).
ð3Þ
1
In the H NMR spectra of complex 5, three trimethylphos-
phine ligands were found at 1.53 ppm as a multiplet while the
NH resonance was registered as a singlet at 11.01 ppm. The
corresponding resonances of the three trimethylphos-
phine ligands in the 31P NMR spectra were found at 38.6, 33.4
and 10.1 ppm. In the IR spectra the N–H vibration was
recorded at 3300 cm−1
.
The reaction of hydrido iron complex 4 with methylhalide
(CH3Br and CH3I) afforded complexes 6 and 7 with the elimin-
ation of one molecule of methane (eqn (4)). Iron(II) bromide 6
was obtained from the diethyl ether solution as red crystals at
−20 °C. The resonances of trimethylphosphine ligands and
the N–H proton are shifted lower field in comparison with
those in complex 4. The reasonable explanation for the shift-
ing is that the electron density at the iron nucleus is reduced
due to the σ electron-withdrawing capability of the halides.22
The 31P NMR spectra show typical resonances of the meridional
arrangement of the three trimethylphosphine ligands.
Fig. 5 Molecular structure of 7 with 30% probability ellipsoids. Selected bond
distances (Å) and angles (°): Fe1–N1 1.945(9), Fe1–C1 1.973(10), Fe1–P2 2.238
(3), Fe1–P1 2.268(2), Fe1–I1 2.769(2), N1–C7 1.287(13); N1–Fe1–C1 79.1(4), N1–
Fe1–P2 174.5(3), C1–Fe1–I1 167.1(3), C7–N1–Fe1 121.2(8), C6–C1–Fe1 113.6(8),
C7–C6–C1 113.4(10), N1–C7–C6 112.8(11).
This is close to (79.79(12)°) in complex 4. The sum of the
internal angles for the five-membered chelate ring (Fe1–C1–
C6–C7–N1) is 540.0°. This indicates the planarity of the chelate
ring. The coordination Fe–P and Fe–N distances are in the
typical ranges.19–21 The CvN bond (1.279(9) Å) is lengthened
due to the coordination of the imine-N atom. Because of the
strong trans-effect the hydrido ligand is located in the trans-
position to the imine-N atom in complex 4, while the bromine
atom is situated in the trans-position to the phenyl-C atom in
complex 6. For the same reason, the Fe–Br bond (2.577(1) Å) is
longer than the typical value of this bond.21 This reaction is
not only a formal replacement of the hydrido ligand by
the bromine atom, but also a position exchange between the
ð4Þ
Red crystals of complex 6 suitable for X-ray single crystal Br atom and one PMe3 ligand.
Complex 7 was isolated as red crystals from the diethyl
ether solution at −20 °C. Suitable crystals of complex 7 for
diffraction were obtained from the diethyl ether solution at
−20 °C. The molecular structure of complex 6 shows a dis-
torted octahedral geometry (Fig. 4). Three meridional PMe3 X-ray diffraction analysis were obtained. The molecular struc-
ture of complex 7 is depicted in Fig. 5. In the octahedral con-
figuration three trimethylphosphine ligands occupy three
ligands attach to the iron center, while the Br and C, N atoms
of the metalacycle occupy three other coordination positions.
The bite angle (C1–Fe1–N1) of the metalacycle ring is 80.1(3)°. meridional coordination sites. The remaining three positions
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are occupied by an iodide and [C,N]-chelating ligands. The
The reaction of hydrido iron complex 4 with the substituted
Fe1, C1, N1, P2, and I atoms are in the equatorial plane, while salicylaldehydes afforded phenoxyl acyl iron(II) complexes 9
P1 and P1A are in the axial direction with the bond angle of and 10 (Scheme 2) in the presence of trimethylphosphine. The
P1–Fe2–P1A (169.79(16)°) deviating from 180°. The bite angle reactions likely begin with the evolution of dihydrogen pro-
of C1–Fe2–N1 of the five-membered chelate ring is 79.1(4)°. duced by the combination of the basic hydrido hydrogen and
The chelated metalacycle and the metalated phenyl ring are the acidic hydrogen of the phenol-OH group of the substituted
coplanar. The iodide group orientates at the trans-position of salicylaldehyde to form intermediate 4a. Subsequently C–H
phenyl-C1 atom as the Br atom in complex 6 (Fig. 4). The bond activation of the aldehyde group takes place affording
markedly long Fe–I distance (2.769(2) Å) is also attributable to another hydrido iron intermediate 4b with the support of the
the strong trans-effect of the phenyl-C atom.21
coordination of the phenoxy group. The reductive elimination
The reaction of hydrido complex 4 with carbon monoxide of hydrido hydrogen with the phenyl-C atom of the imine
was carried out under one bar of CO. In this reaction one ligand gives final acyl iron(II) complex 9 or 10 and one mole-
trimethylphosphine ligand is replaced by one carbon mono- cule of the “free” imine ligand. This reaction is a ligand trans-
xide molecule. This selective substitution provides only fer process (chelate ligand replacement) of the both chelate
complex 8 with the two phosphine ligands in the axial pos- ligands.
itions (eqn (5)).4a The IR spectra show a strong terminal CO
In the IR spectra the stretching vibrations of CvO of the
stretching band at 1877 cm−1. The significant bathochromic acyl groups in complexes 9 and 10 are recorded at 1618 (9)
shift for the carbonyl ligand indicates that strong back- and 1628 (10) cm−1. In the 1H NMR spectra three signals of
bonding between iron and CO exits and therefore the CuO trimethylphosphine ligands with the integral ratio of
bond is weakened. The typical stretching of Fe–H bond is 2 : 1 : 1 give the first clue of an octahedral frame.
observed at 1731 cm−1, which is comparable with that
The molecular structure of complex 10 displays an octa-
(1733 cm−1) in hydrido iron complex 4. In the 1H NMR spectra hedral coordination geometry of the iron center chelated with
the resonance of Fe–H bond at −15.0 ppm is close to that acyl-phenoxy-[C,O]-coordinated salicylaldehyde group bearing
(−15.2 ppm) of complex 4.
four trimethylphosphine ligands (Fig. 6). P1 and P4 atoms are
located in the axial directions with a bond angle P1–Fe–P4 of
167.23(3)°. The other two phosphorus atoms (P2 and P3) are
coplanar with the salicylaldehyde group. The sum of bond
angles around the central iron atom is 360.10°. The dihedral
angle between the five-membered chelate ring and the phenyl
ring is 3.2°. The distance of C8–O2 (1.249(3) Å) is elongated
ð5Þ
Scheme 2 Proposed reaction mechanism of 4 with salicylaldehydes.
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comparing with normal CvO distance (∼1.22 Å) due to the
coordination of acyl-C to the iron center.23
A direct synthesis of complexes 9 and 10 was realized by the
reaction of FeMe2(PMe3)4 with the substituted salicylaldehydes
(eqn (6)).24
ð6Þ
3.4 Reactions of hydrido iron complex 4 with
trimethylsilylethyne and the experimental evidence for the
mechanism of formation of alkynyl iron(II) complex 11
Transition metal hydrido complexes can react with alkenes or
alkynes giving rise to σ-alkyl or σ-alkenyl metal complexes. For
cobalt hydrides and nickel hydrides the insertion of the un-
saturated bonds into the metal–hydrido bonds has been demon-
strated.25 The reaction of iron hydride with terminal alkyne
usually eliminates dihydrogen affording terminal alkynyl iron
complexes.26
When trimethylsilylethyne was added to the solution of
hydrido iron complex 4 at −70 °C, the evolution of gas was
observed during warm-up. After work-up, a new compound
was isolated. In the IR spectra the strong stretching vibration
of CuC bond is observed at 1964 cm−1. Interestingly, not only
the (Fe–H) band, but also the (N–H) vibration disappeared.
The 1H NMR spectra show shift of trimethylsilyl group at
Fig. 6 Molecular structure of 10 with 30% probability ellipsoids. Selected
bond distances (Å) and angles (°): Fe1–C8 1.935(3), Fe1–O1 2.0048(17), Fe1–P2
2.2137(10), Fe1–P1 2.2651(10), Fe1–P4 2.2766(10), Fe1–P3 3498(10), O1–C1
308(3), O2–C8 1.249(3), O3–C2 1.385(3), O3–C7 1.397(3), C6–C8 1.494(4); C8– 0.45 ppm. In the 31P NMR spectra two signals were recorded
Fe1–O1 84.42(10), C8–Fe1–P2 91.31(8), O1–Fe1–P2 175.30(6), C8–Fe1–P1 86.49
at 30.7 and 19.4 ppm with the integral intensity ratio of 2 : 1.
All of the aforementioned evidences imply the coordination
geometry of trigonal bipyramid at the iron center with the
(9), O1–Fe1–P1 84.78(6), P2–Fe1–P1 93.03(3), C8–Fe1–P4 84.76(9), O1–Fe1–P4
85.13(6), P2–Fe1–P4 96.46(3), P1–Fe1–P4 167.23(3), C8–Fe1–P3 168.29(8), O1–
Fe1–P3 83.90(6), P2–Fe1–P3 100.39(3), P1–Fe1–P3 93.38(4), P4–Fe1–P3 93.30
terminal alkynyl and imino ligand in complex 11 bearing three
(4), C1–O1–Fe1 111.74(16), O1–C1–C6 120.1(2), C6–C8–Fe1 110.25(19), C1–C6–
C8 113.4(2).
trimethylphosphines (Scheme 3).
Scheme 3 Proposed reaction mechanism of 4 with trimethylsilylethyne.
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The X-ray diffraction analysis of 11 verified this conjection angle N1–Fe1–C23 (139.22(12)°) is significantly larger than the
(Fig. 7). The axial bond angle of P1–Fe1–P2 (163.28(3)°) shows bond angle N1–Fe1–P3 (125.98(8)°) and much larger than the
a large deviation from 180°. The imino-N atom and trimethyl- bond angle P3–Fe1–C23 (94.65(9)°). The dihedral angle
silylethynyl-C atom together with the third phosphorus atom between the fluoro-substituted phenyl ring and trigonal plane
are nearly in one plane with the bond angles (N1–Fe1–C23 = [C23N1P1] is 79.9°. The distance of C7–N1 (1.281(3) Å) is much
139.22(12)°, N–Fe1–P3 = 125.98(8)° and P3–Fe1–C23 = 94.65(9)°). shorter than that (1.315(4) Å) in hydrido iron complex 4. The
Because of the steric crowding of the fluoro-substituted distance of Fe1–N1 (1.740(1) Å) in complex 11 is also shorter
phenyl ring and the trimethylsilylethynyl group the bond than the distance of Fe1–N1 (1.928(2) Å) in complex 4. This
unusual shortening of the Fe–N bond distance (literature
value: 1.95 Å27) might be attributed to the strong π-backbond-
ing between the iron atom and the imino group. The bond
angle of C7–N1–Fe1 (172.3(2)°) closer to 180° proves that
there is no hydrogen atom binding to the iminato-N atom. The
Fe1–C23 distance (1.904(3) Å) is a little bit shorter than that
reported in the literature.26
The formation of complex 11 demonstrates a novel mechan-
ism compared to the above reaction mechanisms because it
involves the N–H bond cleavage (Scheme 3). The first step
might be the replacement of one phosphine ligand by one
molecule of trimethylsilylethyne to form η2-(CuC)-coordinate
intermediate 4c. The reductive elimination of the phenyl-C
and the hydrido hydrogen in iron(II) complex 4c delivers
iron(0) intermediate 4d with the imine 3 as a mono-dentate
ligand to form N–Fe σ-coordination. A “new” hydrido iron(II)
complex 4e is formed through oxidative addition of termi-
nal C–H of trimethylsilylethyne at the iron(0) center. The
Fig. 7 Molecular structure of 11 with 30% probability ellipsoids. Selected
bond distances (Å) and angles (°) (all the hydrogen atoms were omitted for
clarity): C7–N1 1.281(3), Fe1–N1 1.740(2), Fe1–P1 2.2229(9), C23–C24 1.219(4),
C23–Fe1 1.904(3), C24–Si1 1.799(3), P2–Fe1–P1 163.28(3), N1–Fe1–C23 139.22
combination of basic hydrido hydrogen and the acidic hydro-
gen of the imine hydrogen makes the formation of final
(12), C7–N1–Fe1 172.3(2), N1–Fe1–P3 125.98(8), N1–Fe1–P2 87.89(8),
C24–C23–Fe1 175.5(3), C23–C24–Si1 169.8(3), C23–Fe1–P3 94.65(9).
Scheme 4 IR monitoring.
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Scheme 5 IR monitoring with D–CuCSiMe3.
ethynyl iron(II) complex 11 possible with the evolution of
dihydrogen.
In order to explore the mechanism of formation of 11, the
reaction was monitored by IR (Scheme 4). Hydrido iron
complex 4 reacted with trimethylsilylethyne in benzene at
30 °C to obtain more details on the reaction mechanism
because the reaction in benzene is slower than that in
diethyl ether. At the beginning (the first 10 min) of the reac-
ð7Þ
tion, ν(Fe–H) (1730 cm−1
) for hydrido complex 4 was
observed. With the increasing of the reaction time, the
vibration of the hydrido bond gradually disappeared. At the
4 Conclusion
time of 500 min a new hydrido bond vibration (1810 cm−1
)
In conclusion, with the keto-O atom as an anchoring group,
the reaction of (2,6-difluorophenyl)phenylmethanone (1) with
Fe(PMe3)4 afforded bis-chelate iron(II) complex 2 via C–F bond
activation. Instead of 1, (2,6-difluorophenyl)phenylmethani-
mine (3) was used for the reaction with the iron(0) complex
giving rise to chelate hydrido iron(II) complex 4. DFT calcu-
lations indicate that 4 is a kinetically preferred product. The
reaction properties of 4 were investigated via reaction with
HCl, CH3Br, CH3I, CO, salicylaldehydes and trimethylsilyl-
ethyne. The reaction mechanism of 4 with trimethylsilyl-
ethyne was proposed with the results from IR monitoring. The
reductive elimination of the hydrido ligand and the phenyl-C
atom precedes the oxidative addition of the terminal C–H
bond of trimethylsilylethyne. The driving force of the escape of
H2 is the deprotonation of the imine hydrogen.
appeared. This implies the formation of hydrido intermediate
4e. At the end of the reaction the vibration at 1810 cm−1 dis-
appeared too. The transmittance of ν(CuC) (1964 cm−1
)
increased during the whole reaction shows that the final
product 11 was generated from the beginning. After the reac-
tion finished the signal of N–H (3000 cm−1) disappeared.
This is also an evidence of the proposed mechanism in
Scheme 3.
However, it is not sure if the hydrogen eliminated to the
phenyl ring through reductive elimination really comes from
the hydrido hydrogen of complex 4 or the terminal hydrogen
of trimethylsilylethyne. A reaction of hydrido iron complex 4
with deuterated trimethylsilylethyne (D–CuCSiMe3) was
carried out (eqn (7)). The result (Scheme 5) shows that the
stretching vibration of Fe–H intermediate 4e at 1810 cm−1 in
the IR spectra was not observed. The expected Fe–D vibration
at 1280 cm−1 might be covered by other bands. These evi-
dences reveal that the hydrogen eliminated to the phenyl
carbon does come from the hydrido hydrogen of complex 4.
The liberated dihydrogen comes from the terminal
hydrogen of trimethylsilylethyne and the hydrogen of the
imine group.
Acknowledgements
We gratefully acknowledge the financial support by NSF China
No. 20972087/21172132 and the support from Prof. Dr Dieter
Fenske and Dr Olaf Fuhr (Karlsruhe Nano-Micro Facility) on
the determination of the crystal structures.
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7 H. H. Karsch, Chem. Ber., 1977, 110, 2699.
8 P. L. Pikard and T. L. Tolebert, J. Org. Chem., 1961, 26,
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