CO bond activation and subsequent decarbonylation of ester promoted by Fe(PMe3)4

Article abstract of DOI:10.1016/j.inoche.2013.12.026

The reactions of 8-quinolinyl esters with iron(0) complex supported by trimethylphosphine ligands afforded four hexa-coordinate chelate-[C,N] iron(II) carbonyl complexes 5-8 via C_acylO bond activation and subsequent decarbonylation. Complexes 5

Full text of DOI:10.1016/j.inoche.2013.12.026

Inorganic Chemistry Communications 41 (2014) 51–53
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
C\O bond activation and subsequent decarbonylation of ester promoted
by Fe(PMe3)4
⁎
Yunqiang Lu, Xiaoyan Li, Lin Wang, Hongjian Sun
School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu 27, 250199 Jinan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of 8-quinolinyl esters with iron(0) complex supported by trimethylphosphine ligands afforded four
hexa-coordinate chelate-[C,N] iron(II) carbonyl complexes 5–8 via Cacyl\O bond activation and subsequent
decarbonylation. Complexes 5–8 were characterized through IR, 1H NMR, 31P NMR and elemental analysis. The
crystal structures of complexes 5–8 were determined by X-ray diffraction.
Received 20 November 2013
Accepted 20 December 2013
Available online 30 December 2013
© 2014 Elsevier B.V. All rights reserved.
Keywords:
C\O activation
Decarbonylation
Iron(0) complex
Trimethylphosphine
Organic halides are commonly employed as electrophilic coupling
partners in transition-metal-catalyzed cross-coupling reactions for the
construction of carbon–carbon and carbon–heteroatom bonds [1].
Recently, phenolic derivatives as electrophiles attract much attention
[2] since they are cheap and easily available. Iron-catalyzed Cphenol\O
bond activation for the synthesis of trizoles was reported by Shi [2b].
The C\O bond activation of benzyl alcohols with Grignard reagents
was studied with Fe, Co and Ni complexes in direct arylation and alkyl-
ation [2c]. Phenol-derived electrophiles, such as aryl sulfonates, have
also been investigated [3]. Esters, as one kind of phenol derivatives,
could be used to construct C\C [4] and C\N [5] bonds. Shi reported
the cross-coupling of aryl/alkenyl pivalates with organozinc reagents
through nickel-catalyzed C\O bond activation under mild reaction
conditions [6]. In general, there are two possible forms of the C\O
bond activation for aryl esters: Cacyl\O or Caryloxy\O bond activation.
The reaction of electron-rich hydrido iron complex supported by
diphosphine ligands with methyl benzoate delivered Cmethoxy\O bond
activation product [7], but the related mechanism was not fully
discussed [8]. Liu published a theoretical analysis of the Ni-catalyzed
cross-coupling reaction of aryl esters and explained the excellent selec-
tivity of C\O bond activation [9]. These results indicate that the Cacyl\O
bond cleavage is involved in the oxidative addition of carboxylate es-
ters and this process is reversible. These calculated results are in good
agreement with the experimental results.
bond activation with nitrogen atom as an anchoring group. To our
knowledge, this could be the first example of iron(0)-promoted C\O
bond activation.
The reactions of ester 1–4 [10] with Fe(PMe3)4 [11] afforded the
hexa-coordinate Fe(II) carbonyl complexes 5–8 via C\O bond activa-
tion and subsequent decarbonylation (Eq. (1)) [12].
ð1Þ
The iron(II) complexes 5–8 were obtained as red crystals in the
yields of 62–87% by crystallization from pentane at −30 °C. Complexes
5–8 in the solid state are stable for several days at room temperature. In
the IR spectra of complexes 5–8, the stretching vibrations of the termi-
nal carbonyl group are situated at 1892 (5), 1888 (6) 1889 (7) and 1876
(8) cm−1 respectively. In the 1H NMR spectra of the four complexes, the
resonances of PMe3 ligands were recorded as a triplet at 0.73 (5), 0.53
(6), 0.79 (7) and 0.89 (8) ppm respectively. Only one 31P NMR signal
was observed at 20.4 (5), 33.2 (6), 36.0 (7) and 36.4 (8) ppm. These
data imply that two trimethylphosphine ligands are in trans-
orientation. All of the spectroscopic information indicates that com-
plexes 5–8 have a hexa-coordination geometry.
In this work, it is found that the Cacyl\O bond cleavage occurred,
when aryl ester is treated with electron-rich iron(0) complex,
Fe(PMe3)4, but the final product is a carbonyl organoiron(II) phenolate
complex. This implies that a decarbonylation ensued after the C\O
bond activation. The present work provides a new strategy on C\O
The molecular structures of complexes 5–8 confirm a hexa-
coordination octahedral geometry in the crystals (Figs. 1–4) [13]. In
⁎
Corresponding author. Tel.: +86 531 88361350; fax: +86 531 88564464.
E-mail address: hjsun@sdu.edu.cn (H. Sun).
1387-7003/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.12.026

52
Y. Lu et al. / Inorganic Chemistry Communications 41 (2014) 51–53
Fig. 1. Molecular structure of 5, selected bond distances (Å) and angles (°): N1\Fe1
2.017(2), Fe1\C7 1.715(3), Fe1\O2 2.012(2), Fe1\C17 2.013(3), Fe1\P2 2.2352(8),
Fe1\P1 2.2368(8), O1\C7 1.165(4); C7\Fe1\O2 176.6(1), P2\Fe1\P1 173.85(3),
N1\Fe1\P1 92.71(7), C10\Fe1\P2 174.71(5), C17\Fe1\P1 86.86(8), C17\Fe1\P2
87.26(8), N1\Fe1\P2 93.38(7), O2\Fe1\P1 91.43(6), O2\Fe1\N1 80.94(9),
C7\Fe1\P1 90.47(9).
Fig. 3. Molecular structure of 7, selected bond distances (Å) and angles (°): N1\Fe1
2.018(2), Fe1\C7 2.014(2), Fe1\O2 2.004(1), Fe1\C8 1.722(2), Fe1\P2 2.2524(5),
Fe1\P1 2.2415(5), O1\C8 1.166(4); C8\Fe1\O2 172.45(7), P2\Fe1\P1 175.35(2),
N1\Fe1\C7 169.43(7), O2\Fe1\N1 80.75(6), N1\Fe1\C8 91.78(7), C8\Fe1\C7
98.70(8), O2\Fe1\C7 88.74(6), N1\Fe1\P1 92.62(5), C7\Fe1\P2 88.82(5),
O2\Fe1\P2 87.23(4).
the molecular structure, the iron atom is centered in an octahedron with
one N, one O, two C and two P atom(s) at the apical positions. A five-
membered metallacycle is formed through the [C,N]-chelate
coordination. Owing to the trans-influence, the CO ligand is located
opposite to the phenolate-O atom in all of the four structures. Unlike
the trans-orientation of the two phosphine ligands in complexes 5, 7
and 8, the two phosphine ligands in complex 6 are in the cis-positions.
It could be conjectured that this difference is related to the ortho-
methyl group in the quinoline ring because this is the only variation
between complexes 5 and 6. It might also be caused by packing effect
during the crystallization since the two phosphine ligands in complex
6 are also in the trans-positions in the solution. This was verified by
the NMR data in the solution. The C`O bond distances of complexes
5–8 (ca. 1.16 Å) are in the normal range. For Fe\Ophenolate bonds,
complex 6 has the shortest distance (1.983(1) Å), while complex 5
has the largest distance (2.012(2) Å) among the four complexes.
However, for Fe\N bonds, complex 6 has the largest distance
(2.047(2) Å) among them. This also may be due to the strong trans-
influence of the trimethylphosphine ligand in complex 6. Because of
the coordination of n-pentyl group, Fe1\C7(sp3) bond length with
2.062(5) Å in complex 8 is a little bit longer than the other three
Fe1\C(sp2) bonds. Complex 8 is very stable in the air. No expected β-H
elimination product with regard to the n-pentyl group was observed.
This confirms that the formally negatively-charged n-pentyl group well
bonds with the central Fe(II) atom at room temperature, especially
with the support of the strong backbonding between the iron center
and the carbonyl ligand.
In summary, we reported the first Fe(PMe3)4 induced Cacyl\O
activation and subsequent decarbonylation of 8-quinolinyl esters. In
this process, the quinoline-N atom as an anchoring group plays an im-
portant role, therefore a chelate ring is formed. The chelation effect
thermodynamically facilitates the C\O bond cleavage. It has been
experimentally confirmed that no similar Cacyl\O bond activation
occurred in the reaction of 1-naphthyl ester with Fe(PMe3)4. The crystal
structures of complexes 5–8 show an octahedral coordination.
Fig. 2. Molecular structure of 6, selected bond distances (Å) and angles (°): N1\Fe1
2.047(2), Fe1\C15 1.722(2), Fe1\O2 1.983(1), Fe1\C10 2.029(2), Fe1\P2 2.2920(7),
Fe1\P1 2.2179(9), O1\C15 1.162(3); C15\Fe1\O2 172.44(8), N1\Fe1\P1 165.14(5),
C10\Fe1\P2 174.71(5), C15\Fe1\N1 104.78(8), C15\Fe1\P1 88.11(7), N1\Fe1\P2
89.16(5), C10\Fe1\P1 86.67(5), O2\Fe1\P1 85.62(4), O2\Fe1\N1 82.01(6),
C15\Fe1\C10 91.84(9).
Fig. 4. Molecular structure of 8, selected bond distances (Å) and angles (°): N1\Fe1
2.010(4), Fe1\C7 1.714(6), Fe1\O2 2.008(3), Fe1\C17 2.062(5), Fe1\P2 2.246(2),
Fe1\P1 2.235(2), O1\C7 1.166(6); C7\Fe1\O2 178.1(2), C10\Fe1\P2 174.71(5),
P2\Fe1\P1 177.54(7), C7\Fe1\C17 93.0(2), O2\Fe1\N1 81.3(2), N1\Fe1\C7
100.6(2), O2\Fe1\C17 85.1(2), N1\Fe1\P1 90.6(1).

Y. Lu et al. / Inorganic Chemistry Communications 41 (2014) 51–53
53
(dd, J = 4.8 Hz, 1H, Hquinoline-6), 6.72 (d, J = 7.8 Hz, 1H, Hquinoline-7), 7.34 (d,
J = 7.8 Hz, 1H, Hquinoline-5), 7.34 (m, J = 7.8Hz, 5H, C6H5), 7.95 (d, J = 7.2 Hz, 1H,
Hquinoline-4), 8.68 (dd, J = 1.5 Hz, 1H, Hquinoline-3), 9.01 (d, J = 7.8 Hz, 1H, H-
quinoline-2) ppm. 31P NMR (121 MHz, C6D6, 297 K): δ 20.4 (s, PCH3) ppm. 13C NMR
(75 MHz, CDCl3, 297 K): δ 169.1, 168.9, 168.5, 147.7, 146.7, 144.4, 139.3, 134.1,
Acknowledgment
We gratefully acknowledge the financial support by NSF China no.
21072113, GIIFSDU No. 2011GGX1023 and the support from Prof. Dr.
Dieter Fenske and Dr. Olaf Fuhr (Karlsruhe Nano-Micro Facility) on
the determination of the crystal structures.
130.2, 129.8, 125.7, 125.1, 121.4, 120.7, 113.7, 107.4, 12.9 (t′, 1J(PC) + 3J(PC)
=
23 Hz, PCH3) ppm.Complexes 6–8 were synthesized according to the similar meth-
od as that for complex 5.Synthesis of 6: Complex 6 as yellow crystals was obtained
in the yield of 62%. Anal. Calc. for 6, C23H31FeNO2P2 (471.28 g mol−1): C, 58.62; H,
6.63; N, 2.97. Found: C, 59.02; H, 6.70; N, 3.12. IR (Nujol mull, cm−1): 3063
ν(CAr\H), 1888 ν(CO), 1600 ν(C_N), 1564 ν(C_C), 947 ρ(PMe3). 1H NMR
(300 MHz, C6D6, 297 K): δ 0.53 (t′, 2J(PH) + 4J(PH) = 6.0 Hz, 18H, PCH3), 2.68
(s, 3H, Ar–CH3), 6.41 (d, J = 8.4 Hz, 1H, Hquinoline-7), 6.53 (dd, J = 6 Hz, 1H,
Appendix A. Supplementary material
CCDC 951489 (5), 951490 (6), 951491 (7) and 951494 (8) contain
the supplementary crystallographic data. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk. Supplementary data to this article can be found online
at http://dx.doi.org/10.1016/j.inoche.2013.12.026.
H
quinoline-5), 7.01 (m, 1H, C6H5), 7.10 (m, 1H, Hquinoline-6), 7.24 (m, 4H, C6H5),
7.87(d, J = 9 Hz, 1H, Hquinoline-3), 9.02 (d, J = 9 Hz, 1H, Hquinoline-4) ppm. 31P
NMR (121 MHz, C6D6, 297 K): δ 33.2 (s, PCH3) ppm. 13C NMR (75 MHz, CDCl3,
297 K): δ 169.1, 166.4, 166.0, 165.6, 157.9, 147.0, 142.8, 139.6, 135.4, 128.9, 128.1,
125.7, 125.1, 123.5, 120.7, 114.3, 107.5, 29.1, 13.4 (t′, 1J(PC) + 3J(PC) = 23 Hz,
PCH3) ppm.Synthesis of 7: Complex 7 as yellow crystals was obtained in the yield
of 72%. Anal. Calcd for 7, C22H28ClFeNO2P2 (491.69 g mol−1): C, 53.74; H, 5.74; N,
2. 85. Found: C, 53.87; H, 5.69; N, 2. 92. IR (Nujol mull, cm−1): 3086 ν(CAr\H),
1889 ν(CO), 1599 ν(C_N), 1562 ν(C_C), 940 ρ(PMe3). 1H NMR (300 MHz, C6D6,
297 K): δ 0.79 (t′, 2J(PH) + 4J(PH) = 7.8 Hz, 18H, PCH3), 6.63 (dd, J = 4.8 Hz,
1H, Hquinoline-6), 6.70 (d, J = 7.8 Hz, 1H, Hquinoline-7), 6.99 (m, J = 2.7 Hz, 1H,
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943 ρ(PMe3). 1H NMR (300 MHz, C6D6, 297 K): δ 0.89 (t′, 2J(PH) + 4J(PH)
=
7.2 Hz, 18H, PCH3), 1.19 (t, J = 6.9 Hz,3H, CH3), 1.30 (m, 4H, CH2CH2), 1.77 (qd, 4H,
CH2CH2), 6.59 (dd, J = 8.4 Hz, 1H, Hquinoline-6), 6.70 (d, J = 7.8 Hz, 1H, Hquinoline-7),
7.30 (d, J = 7.8 Hz, 1H, Hquinoline-5), 7.45 (t, J = 7.8 Hz, 1H, Hquinoline-3), 7.51 (d,
J = 9 Hz, 1H, Hquinoline-4), 8.57 (d, J = 2.7 Hz, 1H, Hquinoline-2) ppm. 31P NMR
(121 MHz, C6D6, 297 K): δ 36.4 (s, PCH3) ppm. 13C NMR (75 MHz, CDCl3, 297 K): δ
169.4, 147.3, 146.4, 133.7, 130.0, 129.6, 121.2, 113.8, 107.8, 38.3, 35.8, 22.6, 14.5, 12.9,
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V = 2198.8(1) Å3, T = 298 K, Z = 4, Dc = 1.381 g/cm3, μ = 0.849 mm−1. A total
of 14,958 reflections were collected, 4380 unique (Rint = 0.0534), R1 = 0.0417
(for 4380 reflections with I N 2 sigma(I)), wR2 = 0.1018 (all data).Crystallographic
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data for 6. C23H31FeNO2P2, Mr = 471.28, monoclinic, space group P2(1)/c,
a = 8.1811(16) Å, b = 11.200(2) Å, c = 26.008(5) Å, β = 95.60(3)° V =
2371.8(8) Å3, T = 298 K, Z = 4, Dc = 1.320 g/cm3, μ = 0.789 mm−1. A total of
13,349 reflections were collected, 5232 unique (Rint = 0.0245), R1 = 0.0341 (for
5232 reflections with I N 2 sigma(I)), wR2 = 0.0848 (all data).Crystallographic data
[11] H.H. Karsch, H.-F. Klein, H. Schmidbaur, Chem. Ber. 110 (1977) 2200–2212.
[12] Synthesis of 5: A solution of compound 1 (0.50 g, 2.0 mmol) in THF (50 ml) was
added to the solution of Fe(PMe3)4 (0.72 g, 2.0 mmol) in THF (50 ml) at −80 °C.
This mixture was allowed to warm to 20 °C and stirred for 6 h to form a brown so-
lution. The volatiles were transferred under vacuum, and the residue was extracted
with pentane (40 ml) and diethyl ether (30 ml) respectively. Crystallization from
pentane at −30 °C afforded brown single crystals of 5 (0.74 g, 81%) suitable for
X-ray analysis. Anal. Calcd for 5, C21H29FeNO2P2 (457.25 g mol−1): C, 55.16; H,
6.39; N, 3.06. Found: C, 55.42; H, 6.63; N, 2.97. IR (Nujol mull, cm−1): 3049
ν(CAr\H), 1892 ν(CO), 1591ν(C_N), 1562 ν(C_C), 942 ρ(PMe3). 1H NMR
(300 MHz, C6D6, 297 K): δ 0.73 (t′, 2J(PH) + 4J(PH) = 7.8 Hz, 18H, PCH3), 6.61
for 7. C22H28ClFeNO2P2, Mr = 491.69, monoclinic, space group P2(1)/n,
a = 8.9167(8) Å, b = 26.1560(19) Å, c = 9.9759(8) Å, β = 97.551(7)°
V = 2306.5(3) Å3, T = 298 K, Z = 4, Dc = 1.416 g/cm3, μ = 0.927 mm−1. A
total of 14,409 reflections were collected, 4701 unique (Rint = 0.0534),
R1 = 0.0338 (for 4701 reflections with I N 2 sigma(I)), wR2 = 0.0972 (all
data).Crystallographic data for 8. C21H35FeNO2P2, Mr = 451.29, monoclinic, space
group P2(1)/c, a = 15.0485(13) Å, b = 10.3042(6) Å, c = 15.6769 (6) Å,
β = 110.559(6)° V = 2276.1(3) Å3, T = 298 K, Z = 4, Dc = 1.317 g/cm3,
μ = 0.819 mm−1. A total of 9576 reflections were collected, 4545 unique (Rint =
0.0953), R1 = 0.0485 (for 4545 reflections with I N 2 sigma(I)), wR2 = 0.1100
(all data).