C(sp3)N(sp) bond cleavage of isocyanides at a cationic [CCH] subunit in a bisdisphenoidal eight-atom tetrairontetracarbon cluster

Article abstract of DOI:10.1246/cl.130337

Reaction of [(η⁵- C₅H₄Me) ₄Fe₄ (HCCH)(HCCNC₄H₄N)]-(PF ₆)₂ (1) with tert-butyl isocyanide led to C(sp ³)N(sp) bond activation to give [(η⁵-C ₅H₄Me)₄Fe₄(HCCH)(HCCCN)](PF ₆)(2). The resulting tert-butyl cation was trapped as the adduct with pyrazine. Reactions of 1 with 2,6-dimethylphenyl isocyanide and cyclohexyl isocyanide led to the coordination of the isocyanides to the cationic [CCH] subunit. Thermolysis of [(η⁵-C₅H₄Me) ₄Fe₄(HCCH)(HCCCNCy)](PF₆)₂ (6) yielded 2.

Full text of DOI:10.1246/cl.130337

doi:10.1246/cl.130337
Published on the web May 31, 2013
807
C(sp3)-N(sp) Bond Cleavage of Isocyanides at a Cationic [CCH] Subunit
in a Bisdisphenoidal Eight-atom Tetrairon-Tetracarbon Cluster
Wataru Taniwaki and Masaaki Okazaki*
Department of Frontier Materials Chemistry, Graduate School of Science and Technology,
Hirosaki University, Hirosaki 036-8561
(Received April 12, 2013; CL-130337; E-mail: mokazaki@cc.hirosaki-u.ac.jp)
Reaction of [(η5-C5H4Me)4Fe4(HCCH)(HCC-NC4H4N)]-
(PF6)2 (1) with tert-butyl isocyanide led to C(sp3)-N(sp) bond
activation to give [(η5-C5H4Me)4Fe4(HCCH)(HCC-CN)](PF6)
(2). The resulting tert-butyl cation was trapped as the adduct
with pyrazine. Reactions of 1 with 2,6-dimethylphenyl iso-
cyanide and cyclohexyl isocyanide led to the coordination of the
isocyanides to the cationic [CCH] subunit. Thermolysis of [(η5-
C5H4Me)4Fe4(HCCH)(HCC-CNCy)](PF6)2 (6) yielded 2.
Scheme 1. Reaction of 1 with tert-butyl isocyanide.
A carbenium ion is stabilized through delocalization of
adjacent ³ electrons over the vacant orbital of the cationic
carbon center. Dolby and Robinson reported the reaction
between [Co3(CO)9(¯3-CCl)] and arenes (Ar-H) in the presence
of AlCl3 to give [Co3(CO)9(¯3-CAr)].1 This result implies the
possibility for the transient formation of an unprecedented
pyramidal carbenium ion, [Co3(CO)9(¯3-C+)].2 Back donation
of the ³ electron from the tricobalt core to the electron-deficient
carbon atom would enhance its stability. The carbonyl adduct,
[Co3(CO)9(¯3-CCO)], was studied from experimental and
theoretical viewpoints.3 Our recent effort has focused on the
nature of the cationic [CCH] subunit on the polymetallic core,4
generation of which has been assumed in the heterogeneous
catalytic reactions.5 We have reported the Lewis acid chemistry
of the cationic [CCH] subunit in a bisdisphenoidal eight-atom
tetrairon-tetracarbon cluster.6 Treatment of [(η5-C5H4Me)4-
Fe4(HCCH)(HCC-Br)](PF6) with AgPF6 in the presence of
pyrazine gave [(η5-C5H4Me)4Fe4(HCCH)(HCC-NC4H4N)]-
(PF6)2 (1). Coordination of the pyrazine moiety is labile, and
the resulting cationic carbon atom activates an acetonitrile
molecule to allow deprotonation under very mild conditions
(triethylamine or diisopropylethylamine, room temperature,
30 min). We herein report the reaction of 1 with tert-butyl
isocyanide, leading to cleavage of the C(sp3)-N(sp) bond to give
[(η5-C5H4Me)4Fe4(HCCH)(HCC-CN)](PF6) (2). The tert-butyl
cation was trapped as the adduct with pyrazine. To obtain insight
into the reaction mechanism, the reactions of 1 with cyclohexyl
isocyanide and 2,6-dimethylphenyl isocyanide were also exam-
ined.
C3
Fe3
Fe2
C4
Fe1
C2
C1
Fe4
C5
N
Figure 1. ORTEP drawing of 4. Thermal ellipsoids are drawn
at the 30% probability level. The methyl groups on the cyclo-
pentadienyl ligands and hydrogen atoms, except for acetylenic
hydrogens, are omitted for clarity. Selected bond lengths ( ) and
¡
angles (degree): Fe1-Fe3 2.4871(4), Fe1-Fe4 2.4994(4), Fe2-
Fe3 2.4973(4), Fe2-Fe4 2.5032(4), C1-C2 1.519(3), C3-C4
1.497(3), C1-C5 1.437(3), C5-N 1.161(3), C1-C5-N 179.0(3).
CN)] (4), was synthesized by the reaction of 2 with [Cp2Co]
as dark brown crystals in 92% yield. In the downfield region
of the 1H NMR spectrum of 4, two singlet signals were observed
at ¤ 9.92 (2H) and 10.31 (1H), which are assigned to the
acetylenic protons of HCCH and HCC-CN, respectively. The
13C{1H} NMR spectrum shows a resonance at ¤ 130.9 assign-
able to the cyano group. The infrared (IR) spectrum shows a
strong-intensity band at 2160 cm¹1, assignable to the C-N
stretching vibration mode.
The reaction of 1 with tert-butyl isocyanide in acetonitrile
gave [(η5-C5H4Me)4Fe4(HCCH)(HCC-CN)](PF6) (2) as dark
green crystals in 86% yield (Scheme 1).7 From the reaction
mixture, a white solid of 1-tert-butylpyrazinium salt 3 was
isolated in 32% yield. Monitoring of the reaction at room
temperature by NMR spectroscopy indicated the instantaneous
formation of 2 (quant.), 3 (37%), and isobutene (33%), in which
no intermediate was detected.
1
The H NMR spectrum of 3 contains three kinds of signals
at ¤ 1.78 (9H), 8.92 (2H), and 9.39 (2H). The first is assignable
to the tert-butyl group, and the latter to two aromatic protons.
The ESI-MS spectrum shows a peak of tert-butylpyrazinium ion
at m/z 137.
As expected from the cluster electron count (59 electrons),
complex 2 is paramagnetic. For full characterization, the
diamagnetic neutral form, [(η5-C5H4Me)4Fe4(HCCH)(HCC-
The single-crystal X-ray diffraction analysis of 4 clearly
revealed the introduction of the cyano group at one of the
acetylenic carbons (Figure 1).7,8 The C1-C5 distance is
Chem. Lett. 2013, 42, 807-809
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

808
Fe4
C2
C4
Fe1
C6
C5
N
Fe2
C3
C1
Fe3
Scheme 2. A plausible mechanism for formation of 2 and 3.
Figure 2. ORTEP drawing of 5. Thermal ellipsoids are drawn
at the 30% probability level. The methyl groups on the cyclo-
pentadienyl ligands and hydrogen atoms, except for acetylenic
hydrogens, are omitted for clarity. Selected bond lengths (¡) and
angels (degree): Fe1-Fe3 2.5042(9), Fe1-Fe4 2.5124(10), Fe2-
Fe3 2.4861(10), Fe2-Fe4 2.4741(10), C1-C2 1.538(7), C3-C4
1.484(7), C1-C5 1.400(7), C5-N 1.167(7), C6-N 1.396(7), C1-
C5-N 177.6(5). C5-N-C6 176.8(6).
Scheme 3. Reactions of 1 with 2,6-dimethylphenyl isocyanide
and cyclohexyl isocyanide.
1.437(3) ¡, which is typical for carbon-carbon single bonds next
to a nitrile group (1.466(5) ¡).9 The C5-N bond distance
(1.161(3) ¡) and the C1-C5-N bond angle (179.0(3)°) are
consistent with the existence of an sp-hybridized cyano group.7
The structure of the [4Fe-4C] core is quite similar to that in
[(η5-C5H4Me)4Fe4(HCCH)2].10
Scheme 4. Thermolysis of 6 in acetonitrile-d3.
A plausible mechanism for the formation of 2 and 3
involves the substitution of pyrazine by tert-butyl isocyanide
to generate [(η5-C5H4Me)4Fe4(HCCH)(HCC-CNtBu)](PF6)2 (A)
(Scheme 2). The related nitrilium complexes have been reported
by Casey and his co-workers.11 The reaction of [(η5-C5H5)2Fe2-
C6-N (1.396(7) ¡) lie in the range expected for the typical
single bonds next to the carbon-nitrogen triple bonds.9 The bond
angles of C1-C5-N and C5-N-C6 are 177.6(5) and 176.8(6)°,
respectively.
The thermal stability of 5 and 6 was examined by NMR
spectroscopy. Compound 5 in acetonitrile-d3 was stable at 70 °C.
Further heating of 5 in dimethyl sulfoxide-d6 at 150 °C resulted
in decomposition, in which formation of 2 was not confirmed.
Heating of 6 in acetonitrile-d3 at 70 °C allowed the complete
transformation to 2, accompanied by the formation of cyclo-
hexene (13%) (Scheme 4). Thermal reaction in the presence of
pyrazine enhanced the yield of cyclohexene (57%), although the
reaction rate was independent of the concentration of pyrazine.
Thus, thermolysis of 6 enhances the heterolytic cleavage of
the carbon-nitrogen bond. Pyrazine deprotonates the resulting
cyclohexyl cation to form cyclohexene. The tendency in
reactivity of the carbon-nitrogen bond cleavage can be
explained based on the stability of the resulting carbenium
ion. Stability of the carbenium ion, estimated by the heterolytic
¹
(CO)2(¯-CO)(¯-CR)]+PF6 (R = H, Me) with tert-butyl iso-
cyanide led to the formation of the thermally stable [(η5-
¹
C5H5)2Fe2(CO)2(¯-CO)(¯-C(R)CNC(CH3)3)]+PF6 . The hetero-
lytic cleavage of the carbon-nitrogen bond gives 2 and tert-butyl
cation. The latter is trapped by pyrazine to give 3. The driving
force for the heterolytic cleavage of the carbon-nitrogen bond is
probably related to the high Lewis acidity of the cationic [CCH]
subunit compared to the tert-butyl cation.
The reactions of 1 with 2,6-dimethylphenyl isocyanide and
cyclohexyl isocyanide at room temperature for 3 h afforded
the corresponding isocyanide-coordinated compounds 5 and 6
(Scheme 3). These compounds were unequivocally character-
ized by the results of elemental analysis and spectroscopic data.
The IR spectra of 5 and 6 show strong-intensity bands at 2245
and 2274 cm¹1, respectively, which are assigned to the C-N
stretching vibration mode of the nitrilium CN bond.12
¹
bond dissociation energies of D(R+-H ), is in the following
order, tert-butyl cation (233 kcal mol¹1) > cyclohexyl cation
(243 kcal mol¹1) > phenyl cation (294 kcal mol¹1).14
The structure of the cationic part of 5 is depicted in
Figure 2.7,8 The bonding parameter around the nitrilium moiety
indicates sp hybridization of the CN unit:13 The bond distance of
C5-N (1.167(7) ¡) is close to that of a typical carbon-nitrogen
triple bond. The bond distances of C1-C5 (1.400(7) ¡) and
Dealkylation of the coordinated tert-butyl isocyanide has
been reported in some transition-metal complexes, although
examples are limited.15 Jones and his co-workers reported that
thermolysis of [Fe(PMe3)2(t-BuNC)3] at 60 °C for 24 h resulted
Chem. Lett. 2013, 42, 807-809
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809
in the formation of [Fe(PMe3)2(t-BuNC)2(H)(CN)] and isobu-
tene. They proposed a mechanism involving homolysis of the
C-N bond to generate a tert-butyl radical.16 Activation of
secondary alkyl isocyanides was very limited and was reported
in the decamethylvanadocene system.15b Tokitoh and his co-
workers reported the thermal reaction of Tbt(Mes)Si=Si(Mes)-
Tbt {Tbt: 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes: mes-
ityl} with tert-butyl isocyanide at 70 °C to form [Tbt(Mes)-
SiH(CN)] and isobutene. They proposed a mechanism involving
the initial formation of a silylene-isocyanide complex, followed
by the concerted proton migration of the tert-butyl group to the
silicon atom and elimination of isobutene.17 Power and his co-
workers reported that the isolable germylene GeAr2 {Ar =
C6H3-2,6-[C6H2-2,4,6-(CH3)3]2} reacted with tert-butyl isocya-
nide to give a germylene-isocyanide complex, which was
further converted to GeAr2H(CN) and isobutene. DFT calcu-
lations indicated a concerted pathway similar to the mechanism
proposed by Tokitoh.18 To the best of our knowledge, activation
of the carbon-nitrogen bond of isocyanides by the carbocations
is unprecedented. In our reaction, the extremely high Lewis
acidity of the cationic [CCH] subunit plays a crucial role in the
activation of secondary and tertiary alkyl isocyanides under
extremely mild conditions.
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Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre as supplementary publication
No. CCDC-933591 (4) and CCDC-933592 (5). Copies
of the data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Center, 12, Union Road,
Cambridge, CB2 1EZ, U.K.; fax +44 1223 336033; or
deposit@ccdc.cam.ac.uk).
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This research was supported by the funding program for
Next Generation World-Leading Researchers and Grant for
Hirosaki University Institution Research.
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Chem. Lett. 2013, 42, 807-809
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/