Formal Co(0), Fe(0), and Mn(0) complexes with NHC and styrene ligation

Article abstract of DOI:10.1016/j.cclet.2019.11.019

The limited knowledge on low-coordinate zero-valent transition-metal species has intrigued great synthetic efforts in developing ligand sets for their stabilization. While the combined ligand set of N-heterocyclic carbene (NHC) with vinylsilanes was the only known ligand system amenable to the stabilization of three-coordinate formal zero-valent cobalt, iron, and manganese complexes, the exploration on other ligands has proved that the ligand set of NHCs with styrene is equally effective in stabilizing three-coordinate formal zero-valent metal complexes in the form of (NHC)M(η²-CH₂CHPh)₂ (NHC = IPr, IMes; M = Co, Fe, Mn). These styrene complexes can be prepared by the one-pot reactions of MCl₂ with styrene, NHC and KC₈, and have been characterized by various spectroscopic methods. Preliminary reactivity study indicated that the interaction of [(IMes)Fe(η²-CH₂CHPh)₂] with DippN₃ produces the iron(IV) bisimido complex [(IMes)Fe(NDipp)₂] and styrene, which hints at the utility of these zero-valent metal styene complexes as synthons of the mono-coordinate species (NHC)M(0).

Full text of DOI:10.1016/j.cclet.2019.11.019

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Formal Co(0), Fe(0), and Mn(0) complexes with NHC and styrene ligation
Wenwei Chen, Qi Chen, Yingjie Ma, Xuebing Leng, Sheng-Di Bai,
Liang Deng
PII:
S1001-8417(19)30666-7
https://doi.org/10.1016/j.cclet.2019.11.019
CCLET 5326
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Chinese Chemical Letters
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5 November 2019
12 November 2019
Please cite this article as: Chen W, Chen Q, Ma Y, Leng X, Bai S-Di, Deng L, Formal Co(0),
Fe(0), and Mn(0) complexes with NHC and styrene ligation, Chinese Chemical Letters (2019),
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Formal Co(0), Fe(0), and Mn(0) complexes with NHC and styrene ligation
Wenwei Chena, b, Qi Chenb, Yingjie Mab, Xuebing Lengb, Sheng-Di Baia, * sdbai@sxu.edu.cn, Liang
Dengb,* deng@sioc.ac.cn
a Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, China
b State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, Shanghai 200032, China
*Corresponding authors. (S.-D. Bai), (L. Deng)
ABSTRACT
The limited knowledge on low-coordinate zero-valent transition-metal species has intrigued great synthetic efforts in
developing ligand sets for their stabilization. While the combined ligand set of N-heterocyclic carbene (NHC) with vinylsilanes
was the only known ligand system amenable to the stabilization of three-coordinate formal zero-valent cobalt, iron, and
manganese complexes, the exploration on other ligands has proved that the ligand set of NHCs with styrene is equally effective
2
in stabilizing three-coordinate formal zero-valent metal complexes in the form of (NHC)M(η -CH2CHPh)2 (NHC = IPr, IMes;
M = Co, Fe, Mn). These styrene complexes can be prepared by the one-pot reactions of MCl2 with styrene, NHC and KC8, and
have been characterized by various spectroscopic methods. Preliminary reactivity study indicated that the interaction of
2
[(IMes)Fe(η -CH2CHPh)2] with DippN3 produces the iron(IV) bisimido complex [(IMes)Fe(NDipp)2] and styrene, which hints
at the utility of these zero-valent metal styene complexes as synthons of the mono-coordinate species (NHC)M(0).
Keywords: Alkene, Cobalt, Iron, N-Heterocyclic carbene , Manganese
Coordinatively unsaturated zero-valent transition-metal species are putative intermediates in many transition-metal-catalyzed
reactions and have thus intrigued great synthetic interests on analogous isolable complexes [1-5]. So far, the most well-studied
coordinatively unsaturated zero-valent metal complexes are the three- and two-coordinate group 10 metal (Ni, Pd, Pt) complexes [6-8],
and coordinatively unsaturated zero-valent metal complexes of the other transition-metals are mainly known for the ones with
coordination numbers of five and four [9-12]. As for the three- and two-coordinate zero-valent metal complexes of group 4-9 metals,
the two-coordinate complexes with cyclic (alkyl)(amino)carbene (cAAC) ligation M(cAAC)2 (M = Co, Fe, Mn) [13], the inorganic
Grignard reagent-type complex [(Mesnacnac)MgMn(N(C6H2-2,6-(CHPh2)2-4-Pri)(SiPri3))] [14], and the three-coordinate complexes with
persistent carbene and vinylsilane ligation (L)M(η2-CH2CHSiR3)2 (L = N-heterocyclic carbene (NHC), cAAC; M = Co, Fe, Mn) [15-19]
are among the rarely known examples. In these cases, the π-accepting nature of the ligand environment plays a key role for the
stabilization of the low-coordinate low-valent metal complexes. The success in the preparation of three-coordinate formal cobalt(0),
iron(0), and manganese(0) complexes with the combined ligand set of persistent carbenes and vinylsilanes prompted investigations on
the applicability of other alkene ligands in combination with NHCs in stabilizing analogous low-coordinate metal complexes. Herein,
we wish to communicate the finding that the readily available alkene, styrene, in combination with NHC fulfills this task.
Scheme 1. Synthetic route to [(NHC)M(η2-CH2CHPh)2].
The synthesis of the three-coordinate formal zero-valent metal styrene complexes can be achieved by the one-pot reaction protocol
similar to that used for the preparation of (NHC)M(η2-CH2CHSiMe3)2. Stirring the mixture of MCl2 (M = Co, Fe, Mn) with 1 equiv. of
NHC in THF at room temperature for hours led to the formation of clear solutions, to which styrene (2 equiv.) and potassium graphite
(2 equiv.) were added subsequently. The reaction mixtures were then kept stirring at room temperature for hours. Further workup on the
resultant mixtures led to the isolation of the styrene complexes [(IMes)Co(η2-CH2CHPh)2] (1, IMes = 1,3-bis(2',4',6'-
trimethylphenyl)imidazol-2-ylidene), [(IPr)Fe(η2-CH2CHPh)2] (2, IPr = 1,3-bis(2',6'-diisopropylphenyl)imidazol-2-ylidene),
[(IMes)Fe(η2-CH2CHPh)2] (3), and [(IPr)Mn(η2-CH2CHPh)2] (4) in 40%-62% yields (Scheme 1). These isolated yields are comparable
to those of the corresponding vinylsilanes complexes [15-19].

Fig. 1. Molecular structures of [(IMes)Co(η2-CH2CHPh)2] (1, left) [(IPr)Fe(η2-CH2CHPh)2] (2, middle), and [(IPr)Mn(η2-CH2CHPh)2] (4,
right), showing 30% probability ellipsoids and the partial atom numbering scheme. Hydrogen atoms are omitted for clarity. For simplicity, only
one of the three crystallographically independent molecules in the unit cells of 2 and 4 are shown.
Complexes 1-4 are air- and moisture-sensitive, quite soluble in THF, and slightly soluble in diethyl ether and toluene. Their
absorption spectra measured in THF exhibit two intensive bands in the UV-visible region with the absorption maxima around 300 and
330 nm. The large absorption coefficients of these bands (more than 1000 dm-3 cm-1) indicate their charge-transfer nature. Being similar
to the vinylsilane complexes (NHC)M(η2:η2-dvtms) and (NHC)M(η2-CH2CHSiMe3)2 (M = Co, Fe, Mn) [15-19], the styrene complexes
1-4 also show two characteristic absorption bands in region of 600-900 nm (670 and 830 nm for 1, 640 and 780 nm for 2 and 3, and 650
and 770 nm for 4). Complexes 1-4 are paramagnetic. Their solution magnetic moments measured by Evans’ method are comparable to
those of the corresponding vinylsilane complexes (NHC)M(η2:η2-dvtms) (M = Co, Fe, Mn) [15-19].
Single-crystal X-ray diffraction studies unambiguously confirmed the three-coordinate nature of 1-4 (CCDC: 1957350-1957353). Fig.
1 shows the structures of 1, 2, and 4. Their key interatomic distances and angles are compiled in Table S2 (Supporting information).
The structure of 3 can be find in Fig. S1 (Supporting information). In these formal zero-valent metal complexes, the styrene ligands are
coordinating to the metal centers via their vinyl moiety in an η2-fashion, and the vinyl groups are nearly coplanar with their phenyl rings.
Being typical of three-coordinate formal zero-valent metal complexes with alkene and NHC ligation, the Calkene-Calkene bonds in 1-4 with
the distances around 1.42 Å locate between typical C-C single bond and C=C double bond, and their M-Ccarbene bonds are on the short
end of the M-Ccarbene bonds of three-coordinate metal-NHC complexes [20, 21]. Probably due to the different steric nature of Ph versus
SiMe3, the dihedral angle between the imidazole plane of the NHC ligands and the idealized plane defined by the metal center and the
four Calkene atoms in 1-4 are slightly smaller than those of their corresponding vinylsilane complexes (Table S2). Despite this difference,
the M-Ccarbene, M-Calkene, and Calkene-Calkene distances in 1-4 are comparable to corresponding bond distances of the formal zero-valent
metal vinylsilane complexes [15-19].
Calculations on [(IPr)Fe(η2-CH2CHPh)2] (2) at the B3LYP/TZVP level of theory [22-24] indicate that the complex with an S = 1
2
ground spin-state has an electronic configuration of (dxy+πC=C*)2(dx -y2+π’C=C*)2(dz2)2(dxz)1(dyz)1 and that the π-backdonation in 2 when
gauged by the contribution of the 3d orbitals (69% and 64% Fe 3d in UNOs 172 and 173, Fig. S8 in Supporting information) is
comparable to that in the three-coordinate iron(0) vinylsilane complex [(IPr)Fe(η2-CH2CHSiMe3)2] (61% and 72% Fe 3d in its π-
bonding orbitals) [19]. Being consistent with their comparable π-accepting nature, the addition of two equiv. of CH2CHPh into the C6D6
solution of [(IPr)Fe(η2-CH2CHSiMe3)2] gave a mixture of [(IPr)Fe(η2-CH2CHPh)2] and [(IPr)Fe(η2-CH2CHSiMe3)2].
One intriguing reactivity of three-coordinate formal cobalt(0) and iron(0) vinylsilane complexes is the redox reactions with substrates
[15-19]. This reactivity feature seems to be retained in the styrene complexes (NHC)M(η2-CH2CHPh)2 as the preliminary reactivity
study indicated that the reaction of [(IMes)Fe(η2-CH2CHPh)2] with two equiv. of the organic azide DippN3 (Dipp: 2,6-
diisopropylphenyl) can produce the three-coordinate metal imido complexes [(IMes)Fe(NDipp)2] [25] and styrene in quantitative yield
(Eq. 1 and Fig. S13 in Supporting information).
[(IMes)Fe(CH2CHPh)2] + 2 DippN3 
[(IMes)Fe(NDipp)2] + 2 N2 + 2 CH2CHPh (eq. 1)
In summary, we found that styrene with monodentate NHC is an effective ligand set for the stabilization of three-coordinate formal
cobalt(0), iron(0), and manganese(0) complexes in the form of [(NHC)M(η2-CH2CHPh)2]. These styrene complexes have their structure
and electronic features similar to the vinylsilane complexes [(NHC)M(η2-CH2CHSiMe3)2], and can also perform redox reactions with
DippN3 to form three-coordinate formal M(IV) imido complexes (NHC)M(NDipp)2. The similarity hints at the synthetic utility of the
NHC-metal-styrene complexes as new synthons of the mono-coordinate species (NHC)M(0). In addition, these formal zero-valent
metal complexes might have potential application as new chemical vapor deposition precursors [26], and their volatility is now under
investigation.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that
could have appeared to influence the work reported in this paper.
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests
Acknowledgments
We sincerely thank the financial support from the National Natural Science Foundation of China (Nos. 21725104, 21690062,
21432001, and 21821002), the National Key Research and Development Program (No. 2016YFA0202900), the Strategic Priority

Research Program of the Chinese Academy of Sciences (No. XDB20000000), and the Program of Shanghai Academic Research Leader
(No. 19XD1424800).

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