Imine-assisted C-F bond activation by electron-rich iron complexes supported by trimethylphosphine

Article abstract of DOI:10.1039/c1dt10843c

C-F bond activation of ortho-fluorinated benzalimines 2,6-F ₂C₆R1R2R3-CHN-R (1-3) using the electron-rich complex Fe(PMe₃)₄ is reported. With the assistance of the imine group as the anchoring group, bis-chelated iron(ii) complexes (C ₆FR1R2R3-CHN-R)₂Fe(PMe₃)₂ (4-6) were formed. The reaction of 2,6-difluorobenzylidenenaphthalen-1-amine 2,6-F ₂C₆H₃-CHN-C₁₀H₇ (9) with Fe(PMe₃)₄ affords [CNC]-pincer iron(ii) complex (C ₆H₃F-CHN-C₁₀H₆)Fe(PMe ₃)₃ (10) through both C-F and C-H bond activation and π-(CN) coordinate iron(0) complex (C₆H₃F-CHN-C ₁₀H₇)₂Fe(PMe₃)₂ (11) with C,C-coupling, while a similar reaction with perfluorobenzylidenenaphthalen-1- amine C₆F₅-CHN-C₁₀H₇ (14) gave rise to only [CNC]-pincer iron(ii) complex (C₆F₄-CHN-C ₁₀H₆)Fe(PMe₃)₃ (15). The proposed formation mechanisms of these complexes are discussed. The structures of complexes 5, 6, 10 and 11 were confirmed by X-ray single crystal diffraction.

Full text of DOI:10.1039/c1dt10843c

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PAPER
Imine-assisted C–F bond activation by electron-rich iron complexes supported
by trimethylphosphine†
Xiaofeng Xu, Hongjian Sun, Yujie Shi, Jiong Jia* and Xiaoyan Li*
Received 5th May 2011, Accepted 25th May 2011
DOI: 10.1039/c1dt10843c
C–F bond activation of ortho-fluorinated benzalimines 2,6-F2C6R1R2R3–CH N–R (1–3) using the
electron-rich complex Fe(PMe3)4 is reported. With the assistance of the imine group as the anchoring
group, bis-chelated iron(II) complexes (C6FR1R2R3–CH N–R)2Fe(PMe3)2 (4–6) were formed. The
reaction of 2,6-difluorobenzylidenenaphthalen-1-amine 2,6-F2C6H3–CH N–C10H7 (9) with Fe(PMe3)4
affords [CNC]-pincer iron(II) complex (C6H3F–CH N–C10H6)Fe(PMe3)3 (10) through both C–F and
C–H bond activation and p-(C N) coordinate iron(0) complex (C6H3F–CH N–C10H7)2Fe(PMe3)2
(11) with C,C-coupling, while a similar reaction with perfluorobenzylidenenaphthalen-1-amine
C6F5–CH N–C10H7 (14) gave rise to only [CNC]-pincer iron(II) complex
(C6F4–CH N–C10H6)Fe(PMe3)3 (15). The proposed formation mechanisms of these complexes are
discussed. The structures of complexes 5, 6, 10 and 11 were confirmed by X-ray single crystal
diffraction.
It has been confirmed that the perfluorovinyldiiron complex
reacts readily with nucleophiles to give novel organometallic
1. Introduction
complexes through an unusual C–F bond cleavage process.15
Holland reported the synthesis and reactivity of low-coordinate
iron(II) fluoride with diketiminate iron(II) ligands.16 It was found
that direct reaction of fluorinated aromatics with iron(II) hydride
complexes did not occur, but C–F bond activation became feasible
in the presence of silane as a fluoride trap. Therefore, hydrodeflu-
orination of aromatic perfluorocarbons can be achieved with iron
fluoride as the catalyst when triethylsilane is added as a hydrogen
source.
We disclosed a cyclometalation reaction involving C–F bond
activation at a cobalt(I) center with an aldazine N atom as
an anchoring group to obtain the first ortho-chelated cobalt(III)
complexes containing a [C–Co–F] fragment.17 Recently, we pub-
lished our new progress in this field.18 With an imine N atom
as the anchoring group, an ortho-metalation reaction involving
C–F bond activation at a cobalt center(I) was studied to afford
ortho-chelated cobalt(III) complexes containing a [C–Co–F] frag-
ment. Carbonylation of theses cobalt(III) fluorides delivered novel
organic fluorides via demetalation. The formation mechanism
of the organic fluorides was, in the most part, experimentally
verified.
C–F bond activation mediated by transition-metal complexes has
attracted more attention with regard to exploring the effective
synthesis of new organic fluorides and to provide novel method-
ology for the defluorination of organic fluorides.1 Most of these
publications involved investigations using noble metals, such as
Ru,2 Pd,3 Os,4 Ir,5 Pt6 and Rh.7 There are few reports on C–
F bond activation induced by Fe, Co and Ni8 complexes. In
2005, Braun reported a catalytic C,C-coupling reaction at nickel
center by C–F bond activation of pyrimidine.9 Radius realized
catalytic C–C bond formation by selective C–F bond activation
of perfluorinated arenes with N-heterocyclic carbene NHC-
stabilized nickel complexes.10 Johnson completed the selective C–
F bond activation of tetrafluorobenzene by nickel(0) complexes.11
Recently, Perutz and Mcgrady used DFT to explain the origin of
the chemoselectivity and regioselectivity of C–F bond activation
in pentafluoropyridine with phosphine-supported nickel species.12
Examples of C–F bond activation by cobalt and iron complexes
are very rare. The earlier work on C–F bond activation by
iron was published in 1997.13 Petillon described the reactions
of perfluorovinyldiiron complex with amines. In these reactions,
the C–F bonds of the coordinated ligands undergo activation.14
Continuing our study of C–F bond activation of fluori-
nated imines, we extended this investigation to electron-rich
low-valent iron complexes. The reaction of tetrakis(trimethyl-
phosphine)iron(0) with fluorinated benzalimines afforded novel
ortho-metalated bis-chelated diorganoiron(II) complexes through
intermolecular double C–F bond activation. The influence of the
aryl groups of imine N atom on the reaction products and reaction
pathway are discussed.
School of Chemistry and Chemical Engineering, Shandong University,
Shanda Nanlu 27, 250100, Jinan, People’s Republic of China. E-mail:
xli63@sdu.edu.cn, jiongjia@sdu.edu.cn; Fax: +86 531 88361350
† CCDC reference numbers 805472–804575. Copies of the data can be ob-
tained free of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax:(+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/c1dt10843c
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stirring at -80 ◦C. The reaction mixture was allowed to warm
to ambient temperature and stirred for 24 h. During this period,
the reaction mixture turned green in color. After removal of the
solvent at reduced pressure the solid residue was extracted with
50 mL of pentane. Crystallization at -20 ◦C afforded green crystals
of 10. Yield: 0.35 g, 44%. mp (dec) > 115 ◦C. Anal. calcd for
C26H37FFeNP3 (531.33 g mol-1): C, 58.77; H, 7.02. Found: C, 58.92;
H, 7.35. IR (Nujol): 1562, 944 r1(PCH3) cm-1. 1H NMR (300 MHz,
C6D6, 294 K, ppm): d 1.10 (m, 27H, P(CH3)3), 7.46–8.12 (m,
9H, Ar-H), 9.10 (s, 1H, CH N). 31P NMR (121.5 MHz, C6D6,
2. Experimental section
2.1 General procedures and materials
Standard vacuum techniques were used in the manipula-
tions of volatile and air-sensitive materials. Solvents were
dried by known procedures and distilled under nitrogen be-
fore use. Literature methods were used in the preparation of
tetrakis(trimethylphosphine)iron(0).19 Corresponding Schiff bases
were obtained by condensation of 2,6-difluorobenzaldehyde and
perfluorobenzaldehyde with amines in alcohol solutions.18,20 In-
frared spectra (4000–400 cm-1), as obtained from Nujol mulls
between KBr disks, were recorded on a Nicolet 5700. NMR
spectra were recorded using a Bruker Avance 300 or 400 MHz
spectrometer. X-ray crystallography was performed with a Bruker
Smart 1000 diffractometer. Melting points were measured in
capillaries sealed under N2 and were uncorrected. Elemental
analyses were carried out on an Elementar Vario ELIII.
2
298 K, ppm): d 24.4 (t, JP,P = 62.3 Hz, 1P, P(CH3)3), 18.4 (d,
2JP,P = 62.2 Hz, 2P, P(CH3)3). 19F NMR (282.4 MHz, C6D6, 298 K,
ppm): d -115.2 (s). 13C NMR (75.5 MHz, C6D6, 298 K, ppm):
3
1
d 16.5 (t¢, |1JP,C + JP,C | = 21.2 Hz, P(CH3)3), 23.2 (d, JP,C
=
15.8 Hz, P(CH3)3), 100.2 s, 104.6 s, 106.1 s, 106.5 s, 117.3 s, 117.6 s,
120.0 s, 124.4 s, 125.6 s, 126.8 s, 130.0 s, 134.9 s, 140.6 s, 145.4 s,
150.6 s, 153.7 s, 159.9 s. Continuing crystallization at -20 ◦C after
separating 10 from the soluti◦on afforded 11 as red crystals. Yield:
0.11 g, 20%. mp (dec) > 95 C. Anal. calcd for C40H40F2FeN2P2
(704.53 g mol-1): C, 68.20; H, 5.72. Found: C, 68.54; H, 5.66.
IR (Nujol): 1594, 1554, 1536, 1503, 940 r1(PCH3) cm-1. 31P NMR
(121.5 MHz, C6D6, 298 K, ppm): d 41.1 (s). 19F NMR (282.4 MHz,
C6D6, 298 K, ppm): d -111.9 (s).
2.2 Syntheses
Synthesis of 4. 2,6-difluorobenzylidenebenzenamine 1 (0.32 g,
1.48 mmol) in30 mL ofdiethylether was combined withFe(PMe3)4
(0.54 g, 1.50 mmol) in 30 mL of diethyl ether with stirring at
-80 ◦C. The reaction mixture was allowed to warm to ambient
temperature and stirred for 24 h. During this period, the reaction
mixture turned green in color. After removal of the solvent at
reduced pressure, the solid residue was extracted with 50 mL
of pentane. Crystallization at -20 ◦C afforded green crystals
of 4. Yield: 0.18 g, 40%. mp (dec) > 120 ◦C. Anal. calcd for
C32H36F2FeN2P2 (604.44 g mol-1): C, 63.59; H, 6.00. Found: C,
According to the method given for 10, only complex 15 was
obtained.
Synthesis of 15. Yield: 0.45 g, 51%. mp (dec) >130 ◦C. Anal.
calcd for C26H34F4FeNP3 (585.31 g mol-1): C, 53.35; H, 5.85.
Found: C, 53.54; H, 5.91. IR (Nujol): 1628, 1602, 1576, 942
r1(PCH3) cm-1. 1H NMR (300 MHz, C6D6, 298 K, ppm): d 1.05–
1.24 (s, 27H, P(CH3)3), 7.52–8.17 (m, 6H, Ar-H), 9.44 (s, 1H,
CH N). 31P NMR (121.5 MHz, CDCl3, 298 K, ppm): d 7.5 (m,
1P, P(CH3)3), 16.2 (m, 2P, P(CH3)3). 19F NMR (282.4 MHz, C6D6,
298 K, ppm): d -166.8 (m, 1F), -156.4 (m, 1F), -140.9 (m, 1F),
-112.1 (m, 1F).
63.65; H, 6.05. IR (Nujol): 1625, 945 r1(PCH3) cm-1. H NMR
1
(300 MHz, C6D6, 298 K, ppm): d 0.95 (s, 18H, P(CH3)3), 6.62–
7.80 (m, 16H, Ar-H), 8.40 (s, 2H, CH N). 31P NMR (121.5 MHz,
CDCl3, 298 K, ppm): d 33.4 (s, P(CH3)3). 19F NMR (282.4 MHz,
C6D6, 298 K, ppm): d -114.3 (s).
Complexes 5 and 6 were synthesized according to the method
given above for 4.
2.3 X-ray structure determination
Intensity data were collected on a Bruker SMART diffractometer
◦
Synthesis of 5. Yield: 0.30 g, 57%. mp (dec) > 134 C. Anal.
˚
with graphite-monochromated Mo-Ka radiation (l) 0.71073 A).
calcd for C32H30F8FeN2P2 (712.37 g mol-1): C, 53.95; H, 4.24.
Found: C, 53.76; H, 4.63. IR (Nujol): 1624, 1595, 1528, 949
r1(PCH3) cm-1. 1H NMR (300 MHz, C6D6, 298 K, ppm): d 0.89 (s,
18H, P(CH3)3), 6.95–7.41 (m, 10H, Ar-H), 8.23 (s, 2H, CH N).
31P NMR (121.5 MHz, CDCl3, 298 K, ppm): d 7.7 (s, P(CH3)3).
19F NMR (282.4 MHz, C6D6, 298 K, ppm): d -154.7 (m, 2F),
-154.2 (m, 2F), -142.9 (m, 2F), -136.7 (m, 2F).
The crystallographic data for complexes 5, 6, 10, and 11 are
summarized in Table 1. The structures were solved by direct
methods and refined with the full-matrix least-squares method on
all F2 (SHELXL-97) with non-hydrogen atoms anisotropic. The
SQUEEZE was used for complex 6. CCDC-805474(5), CCDC-
805475 (6), CCDC-805472 (10) and CCDC-805473 (11) contain
the supplementary crystallographic data for this paper†.
Synthesis of 6. Yield: 0.22 g, 48%. m.p. (dec) > 115 ◦C. Anal.
calcd for C24H34F6FeN4P2 (610.34 g mol-1): C, 47.23; H, 5.61.
Found: C, 47.37; H, 5.89. IR (Nujol): 1616, 1550, 1510, 944
r1(PCH3) cm-1. 1H NMR (300 MHz, C6D6, 298 K, ppm): d 0.97
(s, 18H, P(CH3)3), 2.49 (s, 6H, -N-CH3), 2.89 (s, 6H, N-CH3),
3.64 (m, br, 2H, Ar-NH), 8.31 (s, br, 2H, CH N). 31P NMR
(121.5 MHz, C6D6, 298 K, ppm): d 22.0 (s). 19F NMR (282.4 MHz,
C6D6, 298 K, ppm): d -108.2 (m, 2F), -146.1 (m, 2F), -165.4 (m,
2F).
3. Results and discussion
3.1. Reaction of Fe(PMe3)4 with fluorinated benzalimines
Reactions of tetrakis(trimethylphosphine)iron(0) complex and
ortho-fluorinated benzalimines 1–3 with an imine-N donor as an
anchoring group resulted in the ortho-metalated bis-chelated prod-
ucts, diorganoiron(II) complexes 4–6, via intermolecular double
C–F bond activation (eqn (1)). Complexes 4–6 were isolated as
green crystals in yields of 40%, 57% and 48% respectively.
Synthesis of 10 and 11. 2,6-difluorobenzylidenenaphthalen-1-
amine 9 (0.40 g, 1.50 mmol) in 30 mL of diethyl ether was combined
with Fe(PMe3)4 (0.54 g, 1.50 mmol) in 30 mL of diethyl ether with
Complexes 4–6 were characterized by elemental analysis, IR
1
and NMR spectroscopy. In H NMR spectra of complexes 4–6
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Table 1 Crystallographic data for complexes 5, 6, 10 and 11
5
6
10
11
Empirical formula
Fw
Cryst. syst.
C32H30F8FeN2P2
712.37
C24H34F6FeN4P2
610.34
C26H37FFeNP3
531.33
C40H40F2FeN2P2
704.53
Monoclinic
P21/c
10.564(2)
23.134(5)
14.585(3)
90
90.696(4)
90
3563.9(13)
4
1.313
13533
6038
0.0709
25.00
Triclinic
Triclinic
Triclinic
¯
¯
¯
Space group
P1
P1
P1
˚
a/A
9.6956(14)
9.9219(14)
16.955(2)
90.243(2)
91.242(2)
106.088(2)
1566.7(4)
2
10.4588(1)
11.1141(1)
14.4844(13)
96.8050(1)
93.3370(1)
110.7310(1)
1554.4(2)
2
9.6349(11)
10.3933(12)
14.1806(17)
76.894(2)
81.832(2)
75.707(2)
1334.6(3)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
1.510
8955
6679
0.0264
27.48
0.0481
0.1259
1.304
7756
5426
0.0128
25.00
0.0315
0.0941
1.322
6648
4665
0.0219
24.99
0.0457
0.1426
No. of rflns collected
No. of unique data
Rint
qmax (◦)
R1 (I > 2s(I))
wR2 (all data)
0.0434
0.0851
(1)
two imine-CH protons were recorded at 8.40 (4), 8.23 (5) and 8.31
(6) ppm as singlet signals. In the IR spectra of complex 6 (N–H)
vibration was found at 3445 cm-1. The N–H proton was registered
at 3.64 ppm.
are positioned in the cis-conformation, while two coordinated
phenyl-C atoms are in the trans-orientation. Two chelate planes
are mutually perpendicular. Because of the repulsion between the
cyclometallated phenyl groups and cis-positioned trimethylphos-
phine ligands bond angles (C14–Fe1–P1 = 98.65(6)◦ and C7–Fe1–
P2 = 100.29(6)◦) are much larger than the ideal value (90◦). Fe–P
The molecular structures of complexes 5 and 6 were identi-
fied through X-ray diffraction (Figs 1 and 2). Complex 5 has
an octahedral coordination geometry with two cis-orientated
trimethylphosphine ligands. Two imine N atoms are located in
the axial positions. Both of the five-membered chelate rings
(Fe1C32C12C23N2 and Fe1C31C20C27N1) are perpendicular to
each other. The bond angle P2–Fe–P3 (103.88(4)◦) is larger than
90◦, which indicates stronger repulsion between the two phosphine
ligands and the orientation direction of the two phenyl groups on
the imine N atoms. Due to the strong trans-influence of the two
perfluorinated phenyl groups, two Fe–P bond distances (Fe1–P2 =
˚
˚
distances (Fe1–P1 = 2.2455(6) A and Fe1–P2 = 2.2424(6) A) are
shorter than those in complex 5.
The putative formation mechanism of complexes 4–6 is sug-
gested in Scheme 1. The first step is the p-coordination of the
imine group to the iron atom to give rise to the intermediate 7a.
With the chelate effect, the C–F bond is cleaved through ortho-
metalation to afford intermediate organo iron fluoride 7b. The
isoelectronic stable cobalt(III) complexes containing the [C–Co–
F] moiety was obtained through the reaction of CoMe(PMe3)4
with fluorinated benzalimines.18 The iron(I) intermediate 8a and
difluorotrimethylphosphorane (F2PMe3) were formed through the
redox reaction of 7b with PMe3. The C–F bond activation of
second imine ligand generates intermediate iron(III) complex 8b,
which results in the formation of the end products 4–6 and the
second F2PMe3 molecule. The formation of F2PMe3 in solution
˚
˚
2.3297(10) A and Fe1–P3 = 2.3175(11) A) are remarkably longer
˚
than the normal Fe–P distances. The N1–C27 (1.312(4) A) and
˚
N2–C23 (1.305(4) A) distances are closer to those in the similar
iron complexes.21
In the octahedral coordination of complex 6, the angle between
two trimethylphosphine ligands is 92.88(2)◦. This indicates a
weaker repulsion between these two phosphine ligands. This is
also attributed to the smaller methyl groups on imine-N atom
in complex 6 comparing with the phenyl groups in complex 5.
In contrast with the situation in complex 5, two imine N atoms
22
was verified via 31P NMR and 19F NMR. The formation of
F2PMe3 provides strong evidence for the above putative formation
mechanism of complexes 4–6. A similar reaction mechanism
was reported in our early work on the reaction of Co(PMe3)4
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Scheme 1
Scheme 2
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Fig. 2 The molecular structure of 6 with 30% probability ellipsoids
(all of the hydrogen atoms are omitted for clarity), selected bond
˚
distances (A) and angles (deg): Fe1–N2 1.9959(16), Fe1–N1 2.0160(16),
Fe1–C7 2.0437(19), Fe1–C14 2.0366(19), Fe1–P1 2.2455(6), Fe1–P2
2.2424(6), N2–C8 1.280(3), N1–C1 1.280(3); N2–Fe1–N1 86.73(7),
N1–Fe1–C7 80.45(7), N1–Fe1–C14 89.15(7), N2–Fe1–C14 81.00(7),
C14–Fe1–C7 167.54(8), N1–Fe1–P1 90.89(5), N2–Fe1–P1 177.61(5),
C7–Fe1–P1 88.42(5), C14–Fe1–P1 98.65(6), N1–Fe1–P2 176.18(5),
N2–Fe1–P2 89.49(5), C7–Fe1–P2 100.29(6), C14–Fe1–P2 89.66(6),
P2–Fe1–P1 92.88(2).
on the reaction products under similar reaction conditions; no
expected complex, such as complex 4, was isolated. The reaction
of Fe(PMe3)4 with compound 9 gave rise to two products: bis-
chelate iron(II) complex 10 with a [CNC]-pincer ligand in a yield
of 44% as green crystals and double p-(C N)-coordinate iron(0)
complex 11 in a yield of 20% as red crystals (eqn (2)). Complex
10 was formed through a combination of C–F bond activation
and C–H bond activation, while complex 11 was produced with
a C,C-coupling reaction via intermolecular double C–F bond
activation. The reaction of CoMe(PMe3)4 with 2,6-difluorinated
phenylmethylidene naphthalenamine delivered only the C–F bond
activation product.18
Fig. 1 The molecular structure of 5 with 30% probability ellip-
soids (all of the hydrogen atoms are omitted for clarity), selected
˚
bond distances (A) and angles (deg): Fe1–C32 2.007(3), Fe1–C31
2.011(3), Fe1–N1 2.021(3), Fe1–N2 2.035(3), Fe1–P3 2.3175(11), Fe1–P2
2.3297(10), N2–C23 1.305(4), N1–C27 1.312(4); C32–Fe1–C31 86.34(13),
C32–Fe1–N1 93.09(12), C31–Fe1–N1 80.67(13), C32–Fe1–N2 80.44(13),
C31–Fe1–N2 92.54(12), N1–Fe1–N2 170.95(12), C32–Fe1–P3 86.12(10),
C31–Fe1–P3 171.61(10), N1–Fe1–P3 96.08(9), N2–Fe1–P3 89.82(8),
C32–Fe1–P2 169.48(10), C31–Fe1–P2 83.87(10), N1–Fe1–P2 89.14(8),
N2–Fe1–P2 96.11(8), P3–Fe1–P2 103.88(4).
with perfluorinated toluene.23 In Scheme 1, we can also conclude
that a synergistic effect of an electron-rich iron(0) center and a
trimethylphosphine ligand is responsible for the C–F activation of
imine ligand and formation of complexes 4–6.
It can be concluded that increasing the number of fluorine
atoms on the phenyl ring enhances the reactivity of the fluorinated
benzalimines 1–3 with Fe(PMe3)4 and improves the yields of C–F
cleavage products 4–6.
According to this result, the formation mechanism of complexes
10 and 11 are proposed in Scheme 2. The first step is the
formation of intermediate 12 with C–F bond activation through
oxidative addition, because the existence of any hydrido iron
intermediate was not observed by NMR monitoring or in situ
IR. Complex 10 was produced according to path a, through the
elimination of HF with C–H bond activation of the naphthyl
group due to its spatially advantageous position. The formation
of the second five-membered chelate ring benefits the C–H bond
activation. Alternatively, the transformation of intermediate 12
into intermediate 13 (path b) is the same as the formation of
complexes 4–6 in Scheme 1. Complex 11 was created through C,C-
coupling via reductive elimination, which is caused by the bulky
naphthyl groups. Iron(II) center in 13 is reduced to iron(0) in 11.
This process was not observed in eqn (1) because the C–H bond
3.2. Reaction of Fe(PMe3)4 with 2,6-difluorophenylmethylidene
naphthalenamine 9
Instead of 2,6-difluorophenylmethylidene phenylamine 1, 2,6-
difluorophenylmethylidene naphthalenamine 9 was used to in-
vestigate the influence of the aryl groups on the imine N atom
(2)
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activation of the phenyl group in imine-N atom in complexes 4
and 5 is not favorable due to the formation of the four-membered
chelate ring. In addition, F2PMe3 was confirmed by 31P NMR
monitoring in solution.
Complex 10 has a molecular structure of octahedral coordinate
geometry (Fig. 3). The trans-influence of imine N atom (N1) leads
˚
to shortening of Fe–P3 bond length (2.2279(10) A) compared to
˚
the other two Fe–P bonds (Fe–P1 = 2.2465(11) A and Fe–P2 =
˚
˚
2.2427(11) A). Fe–C bond lengths (Fe–C1 = 2.007(4) A and Fe–
˚
C16 = 2.031(4) A) are in the normal region. The axial P1–Fe–
P2 angle (171.21(4)◦) deviates from 180◦. The sum (360◦) of the
four bond angles around the Fe atom (N1–Fe–C16 = 81.35(13)◦,
N1–Fe–C1 = 80.56(13)◦, C16–Fe–P3 = 104.48(11)◦ and C1–Fe–
P3 = 93.61(11)◦) indicates that the five atoms (FeC1N1C16P3) are
coplanar, while the sum (360◦) of the three bond angles around
N1 (C7–N1–C8 = 124.2(3)◦, C7–N1–Fe = 117.8(3)◦ and C8–N1–
Fe = 118.0(2)◦) shows that two five-membered chelate rings and
the phenyl ring, as well as the naphthyl ring, are in one plane.
Fig. 4 The molecular structure of 11 with 30% probability ellipsoids (all
of the hydrogen atoms are omitted for clarity), selected bond distances (A)
˚
and angles (deg): Fe1–N1 2.037(3), Fe1–N2 2.070(3), Fe1–C24 2.123(4),
Fe1–C13 2.122(3), Fe1–P1 2.369(1), Fe1–P2 2.344(1), N2–C24 1.362(4),
N1–C13 1.360(4), C1–C7 1.503(5); N1–Fe1–N2 113.05(12), N2–Fe1–C24
37.88(11), N1–Fe1–C24 100.25(13), N2–Fe1–C13 96.87(12), N1–Fe1–C13
38.11(11), C24–Fe1–C13 109.11(14), N2–Fe1–P1 100.81(9) N1–Fe1–P1
119.45(9), C24–Fe1–P1 133.90(10), C13–Fe1–P1 91.44(10), N2–Fe1–P2
122.87(8), N1–Fe1–P2 101.23(9), C24–Fe1–P2 93.73(10), C13–Fe1–P2
134.96(10), P1–Fe1–P2 100.01(5).
˚
˚
(C24–N2 = 1.362(4) A) and C13–N1 = 1.360(4) A) in complex
11 are even longer than the aforementioned C N distances as
a result of p-coodination of the C N bond, which significantly
weakens the C N bond. Complex 11 has a C2 symmetry axis
through the central iron atom. Compared to the green bis-chelate
iron(II) complex 10, the red iron(0) complex 11 is more sensitive in
the air.
3.3. Reaction of Fe(PMe3)4 with perfluorophenylmethylidene
naphthalenamine 14
When treating perfluorophenylmethylidene naphthalenamine 14
with Fe(PMe3)4 in diethyl ether at -80 ◦C, only [CNC]-pincer
complex 15 was afforded in a yield of 51%. Comparing 14 with 9,
the C–F bond is more easily activated in 14 due to the strong
electron-withdrawing effect caused by the increasing fluorine
atoms on the aromatic ring. The elimination of hydrogen fluoride
releases enough energy to activate the C–H bond of the naphthyl
ring to favor the formation of [CNC]-pincer iron(II) complex 15
(eqn (3)). No C,C-coupling product, like complex 11 in eqn (3),
was observed.
Fig. 3 The molecular structure of 10 with 30% probability ellipsoids (all
of the hydrogen atoms are omitted for clarity), selected bond distances
˚
(A) and angles (deg): Fe–N1 1.970(3), Fe–C1 2.007(4), Fe–C16 2.031(4),
Fe–P3 2.2279(10), Fe–P2 2.2427(11), Fe–P1 2.2465(11), N1–C7 1.303(5),
C7–C6 1.415(5); N1–Fe–C1 80.56(13), N1–Fe–C16 81.35(13), C1–Fe–C16
161.91(15), N1–Fe–P3 174.17(9), C1–Fe–P3 93.61(11), C16–Fe–P3
104.48(11), C16–Fe–P2 85.32(10), C1–Fe–P1 93.39(10), C16–Fe–P1
86.39(10), P2–Fe–P1 171.21(4), C7–N1–C8 124.2(3), C7–N1–Fe 117.8(3),
C8–N1–Fe 118.0(2).
Fig. 4 describes the molecular structure of iron(0) com-
plex 11. The central iron atom has a tetrahedral coordina-
tion geometry with two p-coordinate C N groups and two
trimethylphosphine ligands. Both imine moieties combine with
each other to form a non-coplanar seven-membered chelate ring
(Fe1C24C8C7C1C2C13) through C,C-coupling via double C–F
4. Conclusion
In conclusion, with the assistance of an aldazine N atom
as the anchoring group, the reactions of tetrakis(trimethyl-
phosphine)iron(0) with fluorinated benzalimines afford novel
ortho-metalated bis-chelated diorganoiron(II) complexes 4–
6 through intermolecular double C–F bond activation. A
synergistic effect of an electron-rich iron(0) center and a
trimethylphosphine ligand is responsible for the C–F activation of
the imine ligand and formation of 4–6. It was found that increasing
the number of fluorine atoms on the aromatic rings benefits the
C–F bond cleavage and improves the yields of the reactions. When
2,6-difluorophenylmethylidene phenylamine 1 was replaced by
2,6-difluorophenylmethylidene naphthalenamine 9 under similar
˚
bond cleavage. Two Fe–P bond distances (Fe1–P1 = 2.369(1) A
˚
and Fe1–P2 = 2.344(1) A) are longer than the Fe–P bond distances
in iron complexes 5, 6 and 10. Though the C N distances in
˚
complexes 5, 6 and 10 (5: C23–N2 = 1.305(4) A, C27–N1 =
˚
˚
˚
1.312(4) A; 6: C8–N2 = 1.280(3) A, C1–N1 = 1.280(3) A and 10:
˚
C7–N1 = 1.303(5) A) are a little bit longer than the average value
(1.27 A) of the C N bond length in free imine molecules because
˚
of the coordination of imine N atoms, the C N bond lengths
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7866–7872 | 7871
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(3)
reaction conditions, bis-chelate iron(II) complex 10 with a
[CNC]-pincer ligand was formed through a combination of
C–F bond activation and C–H bond activation of the naphthyl
group. Additionally, double p-(C N)-coordinate iron(0)
complex 11 was obtained through double C–F bond activation
accompanying the C,C-coupling reaction caused by the bulky
naphthyl ring. In the case of perfluorobenzylidenenaphthalen-
1-amine 14, only [CNC]-pincer complex 15 was isolated.
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Acknowledgements
We gratefully acknowledge the support by NSF China No.
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