Structural Interconversion of Bimetallic Carbonyl Clusters Induced by a NHC Ligand and Its Selectivity on Fragments

Article abstract of DOI:10.1002/zaac.201400108

The reactions of IMes-HCl with bimetallic iron-copper clusters were investigated and it showed that interconversions among bimetallic Fe _xCu_y(CO)_z^n- clusters were induced by IMes-HCl. The reaction priority of IMes-HCl with fragments in Fe_xCu_y(CO)_z^n- was also investigated and the reaction behaviors of zero valent Fe(CO)₄ and divalent Fe(CO)₄ with IMes-HCl were discussed. Copyright

Full text of DOI:10.1002/zaac.201400108

ARTICLE
DOI: 10.1002/zaac.201400108
Structural Interconversion of Bimetallic Carbonyl Clusters Induced by a
NHC Ligand and Its Selectivity on Fragments
Xiao-Ming Liu,[a] Shan-Fang Hu,[a] Guang Zeng,[b] Zhan Shi,[b] and Lei Zhu*[a]
Keywords: NHC ligand; Iron; Copper; Bimetallic carbonyl cluster; Structural transformation
Abstract. The reactions of IMes-HCl with bimetallic iron-copper clus-
tion priority of IMes-HCl with fragments in FexCuy(CO)zn– was also
ters were investigated and it showed that interconversions among bi- investigated and the reaction behaviors of zero valent Fe(CO)4 and
metallic FexCuy(CO)zn– clusters were induced by IMes-HCl. The reac-
divalent Fe(CO)4 with IMes-HCl were discussed.
[o-triars = bis(o-dimethylarsinophenyl) methylarsine] from the
reaction of (o-triars)CuCl with Fe(CO)4 ,
2– [10] but neither struc-
Introduction
N-Heterocyclic carbine (NHC) is an important ligand in or-
ganometallic chemistry for its strong electron-donating prop-
erty and the ability to afford a variable steric environments,[1]
due to its abundant electron density and bulky volume.[2] Ow-
ing to their unique properties, NHCs have been used as build-
ing blocks in the synthesis of mono- and high-nuclear organo-
metallic complexes. On the other hand, the development of
transition-metal catalyzed organic transformations based on the
first row transition metals such as Fe, Co, Ni, and Cu are of
importance because of their low cost and environmental benig-
nity compared with precious metals.[3] Iron carbonyl clusters
containing NHCs are potential catalysts, which can be applied
to the catalysis of areneborylation,[4,5] hydrosilylation,[4,5] re-
duction of sulfoxides,[6] and aziridination of alkenes.[7] Hence,
preparation of NHC-containing cluster complexes of cheap
metals is of practical importance and has been the focus of
intense synthetic attention. A large number of mono-metallic
NHC complexes have been reported recently.[8] However, most
of the reports focus on the catalytic properties of the metal-
NHC complexes, and reactions between NHCs and non-pre-
cious bimetallic clusters are rarely reported. In this paper, we
investigated the reaction of NHC ligand induced structural
transformation of FexCuy(CO)zn– clusters and the reaction pri-
ority of IMes-HCl with fragments in carbonyl cluster com-
plexes.
ture could be identified. Afterwards, LCuFe(CO)4CuL (L =
phosphine ligands) was synthesized by Doyle et al.[11] Doyle
3–
et al. also reported a triangular cluster Cu3[Fe(CO)4]3 (1), a
3–
butterfly cluster Cu5[Fe(CO)4]4 (2), and a tetrahedral cluster
Cu6[Fe(CO)4]42– (3), which is a large ionic compound contain-
ing four face-capping iron carbonyl groups on a copper trigo-
nal anti-prism.[11,12] Compounds 1–3 can transform to each
other by the addition of Na2Fe(CO)4 or CuCl (see Figure 1).
Compound 1 converts to the copper-rich cluster 2 by reacting
with copper(I), and similarly the copper-rich clusters react with
2–
Fe(CO)4 to furnish the iron-rich clusters, i.e., compound 3
reacts with Fe(CO)42– to give 2 and 1.[12] Herein, we use IMes-
2–
HCl to achieve this transformation rather than Fe(CO)4
.
A bimetallic Fe-Cu cluster of composition [(NH3)2Cu]2-
Fe(CO)4 was first reported by Hieber.[9] Following reports
were made by Nyholm on the compound [(o-triars)Cu]2Fe(CO)4
* Dr. L. Zhu
Fax: +86-731-88713642
E-Mail: lzhu@hnu.edu.cn
[a] College of Chemistry and Chemical Engineering
Hunan University
Figure 1. Reaction scheme showing the formation and interconversion
3–
of Cu6[Fe(CO)4]42–, Cu5[Fe(CO)4]43–, and Cu3[Fe(CO)4]3
.
Changsha, 410082, P.R. China
[b] State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry
Results and Discussion
Compound 3 was prepared from the reaction of Na2Fe(CO)4·
1.5C4H8O2 with CuBr at room temperature with an unusual
College of Chemistry
Jilin University
Changchun 130012, P.R. China
Z. Anorg. Allg. Chem. 2014, 640, (10), 1983–1986
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1983

X.-M. Liu, S.-F. Hu, G. Zeng, Z. Shi, L. Zhu
ARTICLE
close-packed planar array of six atoms, which has previously charge on compound 3, a Fe(CO)4 fragment has to bear a for-
2–
been reported by G. Doyle et al.[11,12] A point of interest about mal –2 charge, namely Fe(CO)4 rather than Fe(CO)4. NHCs,
3 is the possibility to serve as a building block for the forma- as strong electron-donating ligands, have a stronger affinity to
tion of other clusters, for example, compound 1 and 2. To ex- the electron-poor copper over electron-rich Fe. With the leave
plore the reaction of NHC ligands with heteronuclear mixed of Cu+ the structure cluster 3 gradually changed, as shown in
metal clusters, bimetallic iron-copper clusters 1–3 were treated Figure 2, when 3 lost one Cu+, the triangle cluster switched to
with varying amounts of IMes-HCl. In a typical run, com- compound 2 with a butterfly structure. Successive cleaving off
pound 3 was treated with equal molar amount of IMes-HCl, Cu+ lead to the formation of 1 and 1 eventually fragmented
and compound 2 was furnished, in the meanwhile CuCl-IMes into CuCl-IMes and Fe3(CO)12. Cluster fragmentation is pos-
was also isolated from the reaction mixture, showing that one sibly due to NHC’s bulky volume and electronic repulsion with
of the copper core atoms was cleaved off from 3 and when the adjacent Fe(CO)4 after combination with core Cu atoms. Re-
2–
tetrahedral structure lost one edge, it changed into a butterfly maining Fe(CO)4 may be unstable and is readily oxidized in
structure (see Figure 2). As we gradually increased the amount air to give 4.
of IMes-HCl, compound 1 began to form and when the ratio
of IMes-HCl to compound 3 reached 2:1, compound 1 and
CuCl-IMes were furnished in 13.0 mg (0.013 mmol) and
12.0 mg (0.027 mmol) isolated yields, respectively, which
indicated that two of the copper core atoms were cleaved off
from cluster 3. When IMes-HCl was in excess, instead of fur-
nishing NHC derivatives, the reaction merely gave products of
CuCl-IMes and Fe3(CO)12, 4, indicating that IMes-HCl
cleaved off all the copper atoms and the remaining Fe(CO)4
fragments combined to form Fe3(CO)12. As a result, with the
addition of IMes-HCl, compound 3 was converted to 2, 1, and
4 gradually.
Scheme 1.
Scheme 2.
Fe(CO)4 unit in carbonyl clusters could be either in the di-
valent Fe(CO)42– form or in the zero valent Fe(CO)4 form. The
latter may be subject to nucleophilic attack to form NHC ad-
2–
duct while divalent Fe(CO)4 is quite resistant to nucleo-
philes. The different behaviors of the two are illustrated in
Figure 3. Further research on the electronic properties of those
two types of Fe(CO)4 units is being conducted based on pre-
vious reports.[17,18]
Figure 2. Reaction scheme showing the formation and interconversion
of compounds 1–3 by IMes-HCl.
Conclusions
It is interesting to note that the IMes ligand has an exclusive
IMes-HCl induces structural transformation on bimetallic
affinity to copper atoms, whereas Fe(CO)4-NHC type com- iron-copper clusters in high yields. This process is reversible
pounds are commonplace. For example, the reaction of in the presence of CuCl. Discussion about the reaction priority
Fe3(CO)12 with IMes-HCl was report by Warratz et al.[13] of IMes-HCl with FexCuy(CO)zn– carbonyl clusters shows that
(Scheme 1). Other reports with structures like LFe(CO)x-NHC the NHC ligand tends to attack the electron-poor copper atoms.
(L = CO, I, Cl, Br, x = 1–4) have been made by da Costa,[14] This protocol helps to give predictive directions on other reac-
Cardoso,[6] Buchgraber,[15] and Mercs etc.[16] (Scheme 2). tions of NHCs with mixed-metal clusters.
However, herein Fe(CO)4-NHC was not found even in trace
The ligand IMes has a strong affinity on copper atoms over
amount. Since copper atoms are actually cleaved off as Cu+ iron due to its electron richness. The different behaviors of the
cations, it is reasonable to assume that in the cluster all the ligand IMes the reactions with iron carbonyl clusters can be
copper atoms carry a formal +1 charge and thus to balance the explained by the electronic properties of Fe(CO)4 units.
1984
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 1983–1986

Structural Interconversion of Bimetallic Carbonyl Clusters
ther TLC nor infrared spectra showed the existence of compound 2.
The yield of compound 1 was 5.0 mg (68%) and 10.3 mg (81%) for
CuCl-IMes.
Reaction of 3 with IMes-HCl in Excess: Compound 3 (0.010 mmol)
was dissolved in THF (6 mL) in a 3-neck reaction flask equipped with
a rubber septum and gas outlet and IMes-HCl (0.12 mmol) was added,
the mixture was stirred for 16 h at room temperature. The reaction
mixture was separated by TLC. A deep green band was collected with
4.7 mg (65%) yield and characterized by infrared and by single-crystal
X-ray diffraction as Fe3(CO)12. A light purple band was obtained with
a 22 mg (82%) yield and characterized as CuCl-IMes by single-crystal
X-ray diffraction.
Reaction of 2 with IMes-HCl in Equal Molar Ratio: Compound 2
(0.010 mmol) was dissolved in THF (6 mL), and IMes-HCl
(0.010 mmol) was added. The mixture was stirred for 16 h at room
temperature, and the solid product was collected after removal of THF.
The solid product was washed by using a mixture of CH2Cl2 and hex-
ane at a 1:5 ratio, then crystallized from a mixture of THF and hexane
at –25 °C as yellow crystals with a yield of 7.8 mg (64%). The crys-
tal’s infrared spectra in THF (1923 cm–1, 1798 cm–1) were consistent
with the previous report on compound 1 (1925 cm–1, 1800 cm–1). A
3.3 mg amount of CuCl-IMes was also isolated (75% yield).
Figure 3. Zero valent Fe(CO)4 tendency in the reaction with NHC-Cl
and divalent Fe(CO)4 tendency in the reaction with IMes-HCl.
2–
Experimental Section
General Data: All reactions were performed in an argon atmosphere
using Schlenk techniques. Reagent grade solvents were dried by the
standard procedures and were freshly distilled prior to use. Compounds
1–3 were prepared according to the reported procedures.[12] IMes-HCl
was purchased from J&K, Na2Fe(CO)4·1.5C4H8O2 was synthesized ac-
cording to a patent procedure.[19] Infrared spectra were recorded with
a FI-IR 650 spectrophotometer. Product separations were performed
by TLC on 0.25 mm silica gel 60 Å F254 glass plates.
Reaction of 2 with IMes-HCl in Excess: Compound 2 (0.010 mmol)
was dissolved in THF (6 mL) and IMes-HCl (0.075 mmol) was added.
The mixture was stirred for 16 h at room temperature. The volume of
solvent was reduced by vacuum and products were separated by TLC.
A deep green band was collected and identified to be Fe3(CO)12 and
a pale purple band was collected and characterized as CuCl-IMes by
single-crystal X-ray diffraction with 5.7 mg (85%) and 18.4 mg (83%)
yields, respectively.
Reaction of 3 with IMes-HCl in Equal Molar Ratio: Compound 3
(0.010 mmol) was dissolved in THF (10 mL) in a 3-neck reaction flask
equipped with a rubber septum and gas outlet and IMes-HCl
(0.010 mmol) was added. The mixture was stirred for 16 h at room
temperature, and the solvent was removed by evacuation. The slush
residue was washed by using a mixture of CH2Cl2 and hexane at a 1:5
ratio, and then crystallized at –25 °C as yellow crystalline solid from
a mixture of THF and hexane with a yield of 5.6 mg (53%). The
IR spectrum of the solid product in THF showed the characteristic
absorptions at 1949 cm–1 and 1851 cm–1, which are in agreement with
the previous report on compound 2 (1950 cm–1, 1850 cm–1). A 3.2 mg
amount of CuCl-IMes was also separated from the reaction mixture
with a yield of 73%.
Reaction of 1 with IMes-HCl in Excess: Compound 1 (0.020 mmol)
was dissolved in THF (6 mL) and IMes-HCl (0.090 mmol) THF solu-
tion was added. The mixture was stirred for 16 h at room temperature.
The volume of solvent was reduced by vacuum and products were
separated by TLC. A deep green band appeared, which was charac-
terized as Fe3(CO)12 and a pale purple band was characterized as
CuCl-IMes by single-crystal X-ray diffraction.
Acknowledgements
This work was supported by NSFC (Grant No. 21271070), the State
Key Laboratory of Inorganic Synthesis and Preparative Chemistry at
Jilin University (Grant No. 2014–23), and the Training Program for
Young Teachers at Hunan University.
Reaction of 3 with Variant Amounts of IMes-HCl: Compound 3
(0.010 mmol) was dissolved in THF (6 mL) in a 3-neck reaction flask
equipped with a rubber septum and gas outlet and a varying amount
of IMes-HCl (0.013 mmol, 0.015 mmol, 0.018 mmol) was added. The
mixture was stirred for 16 h at room temperature, and the solvent was
removed by evacuation. TLC and infrared spectra showed the solid
product was a mixture of compound 1 and 2.
References
[1] a) W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41, 1290; b)
R. H. Crabtree, J. Organomet. Chem 2005, 690, 5451.
[2] C. E. Cooke, T. Ramnial, M. C. Jennings, R. K. Pomeroy, J. A. C.
Clyburne, Chem. Commun. 2007, 1755.
[3] S. Enthaler, K. Junge, M. Beller, Angew. Chem. 2008, 120, 3363.
[4] L. C. M. Castro, J. B. Sortais, C. Dareel, Chem. Commun. 2012,
48, 151.
[5] V. V. K. M. Kandepi, J. M. S. Cardoso, E. Peris, B. Royo, Orga-
nometallics 2010, 29, 2777.
[6] J. M. S. Cardoso, B. Royo, Chem. Commun. 2012, 48, 4944.
[7] S. A. Carmer, D. M. Jenkines, J. Am. Chem. Soc. 2011, 133,
19342.
Reaction of 3 with IMes-HCl in 1:2 Molar Ratios: Compound 3
(0.014 mmol) was dissolved in of THF (6 mL) in a 3-neck reaction
flask equipped with a rubber septum and gas outlet and IMes-HCl
(0.028 mmol) was added. The mixture was stirred for 16 h at room
temperature and solvent removed by vacuum. The residue was washed
by using a mixture of CH2Cl2 and hexane at a 1:5 ratio, then crys-
tallized at –25 °C as yellow crystalline solid from a mixture of THF/
hexane. Infrared spectra of the solid product in THF showed the char-
acteristic absorption at 1923 cm–1 and 1798 cm–1, which is consistent
with the previous report on compound 1 (1925 cm–1, 1800 cm–1). Nei-
[8] B. Liu, Q. Xia, W. Chen, Angew. Chem. Int. Ed. 2009, 48, 5513.
Z. Anorg. Allg. Chem. 2014, 1983–1986
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1985

X.-M. Liu, S.-F. Hu, G. Zeng, Z. Shi, L. Zhu
ARTICLE
[9] W. Hieber, E. Fack, Z. Anorg. Allg. Chem. 1938, 236, 86.
[10] A. S. Kasenally, R. S. Nyholm, M. H. B. Stiddard, J. Chem. Soc.
1965, 5343.
[11] G. Doyle, K. A. Eriksen, D. Van Engn, J. Am. Chem. Soc. 1985,
107, 7914.
[12] G. Doyle, K. A. Eriksen, D. Van Engn, J. Am. Chem. Soc. 1986,
108, 445.
[13] S. Warratz, L. Postigo, B. Royo, Organometallics 2013, 32, 893.
[14] A. P. da Costa, J. A. Mata, B. Royo, E. Peris, Organometallics
2010, 29, 1832.
[15] P. Buchgraber, L. Toupet, V. Guerchais, Organometallics 2003,
22, 5144.
[16] L. Mercs, G. Labat, A. Neels, A. Ehlers, M. Albrecht, Organome-
tallics 2006, 25, 5648.
[17] H. B. Chin, R. Bau, J. Am. Chem. Soc. 1976, 98, 2434.
[18] R. B. King, Inorg. Chim. Acta 1994, 227, 207.
[19] J. P.Collman, R. G. Komoto, United States Patent 3985770,
Oct.12, 1976.
Received: March 10, 2014
Published Online: May 14, 2014
1986
www.zaac.wiley-vch.de
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 1983–1986