Nucleophilic addition of carborane anion to Ir, Rh-coordinated Cp* ring: C-C bond formation accompanied by reduction of metal center

Article abstract of DOI:10.1039/c3dt51906f

Reaction of carboranyl anion or its thiolate derivative with a fully-substituted Cp*-based group 9 (Ir, Rh) metal complex affords respectively a C-C bond formation complex and a salt metathesis product. This example represents the first nucleophilic addition attempt in the area of M-Cp* chemistry.

Full text of DOI:10.1039/c3dt51906f

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Nucleophilic addition of carborane anion to Ir,
Rh-coordinated Cp* ring: C–C bond formation
accompanied by reduction of metal center†
Cite this: Dalton Trans., 2014, 43,
4938
Received 14th July 2013,
Accepted 23rd July 2013
Zi-Jian Yao,a Yue-Jian Lin,a Bin Xua and Guo-Xin Jin*a,b
DOI: 10.1039/c3dt51906f
www.rsc.org/dalton
Reaction of carboranyl anion or its thiolate derivative with a fully-
substituted Cp*-based group 9 (Ir, Rh) metal complex affords
respectively a C–C bond formation complex and a salt metathesis
product. This example represents the first nucleophilic addition
attempt in the area of M-Cp* chemistry.
Nucleophilic (Nu−) addition of carbanions to the arenes in 18-
electron transition-metal complexes (such as (η6-arene)Co(CO)3
and (η6-arene)Fe+Cp) has been used successfully as a specific
synthetic application because electron withdrawal by the tran-
sition-metal unit of these complexes renders the arene ligand
susceptible to nucleophilic attack.1 In some Co2 and Fe3 di-
cationic series, the successive addition of two distinct dianionic
carbanions should provide heterobifunctional cyclohexadienyl
complexes. These types of reactions are easily influenced and
restricted by the nature of the nucleophile, arene and solvent.
Carbon-based and other strong nucleophiles such as H− and
CN− have attracted the most intensive study and addition reac-
tions of Nu− often proceed in the unsubstituted arene site.4
The research on half-sandwich transition metal complexes
based on various substituted-carboranyl ligands is of great
interest because of the excellent electronic and steric effects of
the bulky cage.5 We previously reported the reactivity of mono-
phosphine o-carborane sulfide towards half-sandwich iridium
and rhodium complexes. During that study a small amount of
byproduct 1 was obtained occasionally in the isolation of the
C,S-coordination mode iridium complex A (Scheme 1).6 The
obtained complex 1 is unexpected: in fact, it is known that
the cyclopentadienyl group not only is strongly coordinated to the
metal, but it is generally also rather inert. The novel structure
Scheme 1 Synthesis of C,S-chelated complex A.
of this byproduct has attracted our attention. We speculated
that this complex was generated by the direct nucleophilic
addition of carboranyl anion to complex A. The analogous
reaction was reported by Basato et al.4j In that example, the Cp
ring in [RuCl(Cp)(PPh3)2] underwent an apparent nucleophilic
attack by the carbanion carb− (Hcarb = 2-Me-1,2-dicarba-closo-
dodecaborane), to give an H−/carb− exchange process, which is
favored by coordination of the hydride to the ruthenium
center. Although reactions of electron-rich metal centers in
cyclopentadienyl ruthenium complexes with nucleophilic
reagent carboranyl anions have been reported,4k,l no examples
have shown the addition reactions of group 9 metal complexes
with carboranyl anions. Moreover, activation of a fully-substi-
tuted aromatic Cp* fragment has been scarcely reported,
because of its high electron density and crowded steric effects.
Therefore we believe that this paper represents a successful
addition attempt in the area of M-Cp* (M = Ir, Rh) chemistry.
As mentioned above, the unexpected complex 1 was isolated
as the byproduct during the synthesis of complex A. In order
to confirm our speculation of the nucleophilic addition reac-
tion, we have investigated reactions of the C,S-coordination
mode complex A or the analogous rhodium complex B with
nucleophilic reagent carboranyl lithium. As shown in
Scheme 2, carboranyl lithium was added slowly to the solution
of A or B at low temperature, the corresponding addition pro-
ducts 1 and 2 were generated via an intramolecular redox in
moderate yields after purification by silica chromatography.
They are soluble in organic solvents, such as toluene, CH2Cl2,
aShanghai Key Laboratory of Molecular Catalysis and Innovative Material,
Department of Chemistry, Fudan University, Shanghai, 200433, P. R. China.
E-mail: gxjin@fudan.edu.cn; Fax: (+86)-21-65641740
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Science, Shanghai 200032, P. R. China
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, characterization and crystallographic data. CCDC 932867 (1), 932868
(2), 932869 (3) and 932870 (4). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c3dt51906f
4938 | Dalton Trans., 2014, 43, 4938–4940
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Scheme 2 Nucleophilic reactions of carboranyl lithium with C, S-che-
lated half-sandwich complexes.
Fig. 1 Molecular structures of 1 (left) and 2 (right) with 30% probability
ellipsoids. (H atoms were omitted.)
and CHCl3, and mostly insoluble in n-hexane or diethyl ether.
The two complexes are very stable in their solid state, but
showed slow decomposition in solution. Because of the for-
1
mation of the C–C bond, the H NMR spectrum of complex 1
steric hindrance may force the attack on the Cp* ring, instead
of the metal center. Moreover, another two carbanions LiPh or
nBuLi were also used as nucleophiles; unfortunately, only
some unidentified products were obtained, which indicates
the stabilization effect of the bulky carborane cage.
showed that the resonances of the methyl groups of Cp* was
split into three single peaks at δ 1.53, 1.81, 2.00 ppm. One reso-
nance at about δ 59.7 ppm was observed in the 31P NMR spec-
trum of 1, which is very similar to that of the starting material
A. The IR spectrum displays a typical strong and broad charac-
teristic B–H absorption at approximately 2565 cm−1, and the
absorption at 625 cm−1 is ascribed to PvS bond vibrations.
The NMR and IR data of 2 is similar to that of 1.
Besides the C,S-mode complexes A and B, the addition reac-
tion of the S,S-mode rhodium complex C with carboranyl
lithium also occurred smoothly (Scheme 3). Similar to 1 and 2,
1
the H NMR spectrum of 3 showed that the resonances of the
X-ray structure analysis for 1 and 2 confirmed that they are
isomorphous. For complex 1, the lengths of Ir(1)–C(1) (2.087(5) Å)
and S(1)–Ir(1) (2.334(15) Å) bonds are slightly shorter than the
corresponding lengths of A, and the C(1)–Ir(1)–S(1) angle
increased accordingly (from 89.0° to 91.2°). The P–S bond
length (2.007(19) Å) is almost identical to that of A. The conju-
gated system of the Cp* ring was destroyed to generate the six-
substituted cyclopentadienyl group due to the new C–C bond
formation. The longer bond length of C(15)–C(16) (1.555(7) Å)
than that of C(16)–C(17) (1.421(8) Å) also indicates the five
carbon atoms of this ring are not equivalent. Correspondingly,
the absolutely planar five-membered ring was folded with the
dihedral angle of 31.3° between the planes of C(15)–C(16)–
C(19) and C(16)–C(17)–C(18)–C(19), and the C(25)–C(15)–C(20)
angle of 110.4° agrees well with the tetrahedral geometry of
the sp3 hybrid C(15) atom (Fig. 1, inset). A similar structural
feature was also observed in complex 2. Interestingly, we found
that the Ir(III) center of A was reduced to Ir(I) in complex 1 via
an intramolecular redox, which is scarcely observed in pre-
vious studies. According to the reaction mechanism,4l the last
step of a H-shift does not occur because there is no H atom in
the Cp* fragment, and this may be the reason for the metal
reduction. The distorted octahedral geometry of the iridium
center was transformed to a tetrahedral geometry because
of the removal of the chloride ion. To our knowledge, nucleo-
philic additions generally occurred on the unsubstituted site of
the aromatic group, and this is the first example of a fully-sub-
stituted Cp* complex employed in this type of reaction. This
result may be ascribed to the unique carboranyl anion: its
strong basicity7 lets it serve as a carbanion-bearing electron-
withdrawing group like in nitriles; on the other hand, the high
methyl groups of Cp* were split. The structure of complex 3 is
shown in Fig. 2. The three-legged piano stool geometry of
rhodium no longer exists because of the removal of the chlor-
ide ion. The metal center was also reduced by the carboranyl
anion via an intramolecular redox reaction, and the six-coordi-
nate Rh(III) complex was transformed to the four-coordinate
Rh(I) complex. The two Rh–S bond lengths are almost the
same as each other. We explored the reactivity of the B,S,S-
coordinated Ir complex in this reaction,6 however no positive
result was obtained, which may indicate the removed group
plays an important role in this reaction.
Scheme 3 Reactivity of rhodium complex
C
with different
nucleophiles.
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basicity carboranyl thiolate was employed in such a reaction.
A reactivity study of other Ir and Rh complexes is currently
underway in our laboratory.
This work was supported by the National Science Foun-
dation of China (91122017), the Program for Changjiang Scho-
lars and Innovative Research Team in University (IRT1117), the
National Basic Research Program of China (2010DFA41160,
2011CB808505) and the Shanghai Science and Technology
Committee (12DZ2275100).
Notes and references
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Fig. 2 Molecular structure of 3 with 30% probability ellipsoids. (H
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Fig. 3 Molecular structure of 4 with 30% probability ellipsoids. (H
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With complexes 1–3 in hand, we extended our study to
other nucleophiles in the reaction. We wondered if the C–X
bond could be obtained when other nucleophiles were used in
this reaction. Hence, the reaction of carboranyl mono-thiolate
derivative [1-S-o-C2B10H11]− with rhodium complex C was per-
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exchange product 4 (Scheme 3), which is a similar result to
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Rh(III) is a soft acid and the thiolate anion is a softer base com-
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Fig. 3. Density functional theory calculations (DFT) were used
to further confirm the experimental results (see ESI†).
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complexes via an intramolecular redox reaction. The basicity
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4940 | Dalton Trans., 2014, 43, 4938–4940
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