Stacking reactions of the borole complex Cp*Rh(η5- C4H4BPh) with the dicationic fragments [Cp*M] 2+ (M = Rh or Ir)

Article abstract of DOI:10.1007/s11172-006-0457-y

The reaction of the (borole)rhodium iodide complex [(η-C ₄H₄BPh)RhI]₄ with Cp*Li afforded the sandwich compound Cp*Rh(η-C₄H₄BPh) (4). The reactions of compound 4 with the solvated complexes [Cp*M(MeNO

Full text of DOI:10.1007/s11172-006-0457-y

Russian Chemical Bulletin, International Edition, Vol. 55, No. 9, pp. 1581—1584, September, 2006
1581
Stacking reactions of the borole complex Cp*Rh(η5ꢀC4H4BPh)
with the dicationic fragments [Cp*M]2+ (M = Rh or Ir)
D. A. Loginov, D. V. Muratov, Z. A. Starikova, P. V. Petrovskii, and A. R. Kudinovꢀ
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (495) 135 5085. Eꢀmail: arkudinov@ineos.ac.ru
The reaction of the (borole)rhodium iodide complex [(ηꢀC4H4BPh)RhI]4 with Cp*Li
afforded the sandwich compound Cp*Rh(ηꢀC4H4BPh) (4). The reactions of compound 4 with
–
the solvated complexes [Cp*M(MeNO2)3]2+(BF4
) gave tripleꢀdecker cationic complexes
2
5
5
–
with the central borole ligand [Cp*Rh(µꢀη :η ꢀC4H4BPh)MCp*]2+(BF4
) (M = Rh (5)
2
or Ir (7)). The structure of complex 4 was established by Xꢀray diffraction.
Key words: sandwich compounds, tripleꢀdecker complexes, rhodium, iridium.
Electrophilic stacking reactions of sandwich comꢀ
pounds with the [(ring)M]n+ fragments are used as the
main method for the synthesis of cationic tripleꢀdecker
complexes.1 In particular, this method was used for the
preparation of compounds with bridging boronꢀconꢀ
taining heterocycles, such as boratabenzene C5H5BR
(see Refs 2—4) and borole C4H4BR.5—7 Recently, we
have demonstrated8,9 that in the case of the Bꢀphenylꢀ
borole rhodium complexes CpRh(ηꢀC4H4BPh) (1) and
(ηꢀ9ꢀSMe2ꢀ7,8ꢀC2B9H10)Rh(ηꢀC4H4BPh) (2), the elecꢀ
trophilic attack occurs on the borole and/or benzene ring.
For example, the reaction of 1 with the [Cp*Ir]2+ fragꢀ
ment (Cp* is pentamethylcyclopentadienyl) affords the
tripleꢀdecker cation [CpRh(µꢀη5:η5ꢀC4H4BPh)IrCp*]2+
containing the areneꢀtype complex [CpRh(µꢀη5:η6ꢀ
C4H4BPh)IrCp*]2+ as an impurity. To the contrary, the
analogous reaction with [Cp*Rh]2+ resulted only in coorꢀ
dination at the benzene ring giving rise to [CpRh(µꢀη5:η6ꢀ
C4H4BPh)RhCp*]2+ (3). Unlike complex 1, the
Cp*Rh(ηꢀC4H4BPh) derivative (4) contains five methyl
groups, whose electronꢀdonating effect leads to an inꢀ
crease in the electron density at the borole ring. Hence,
the electrophilic attack on the borole ring would be exꢀ
pected to be more favorable. In the present study, we
synthesized sandwich compound 4 and investigated its
reactions with the [Cp*M]2+ fragments (M = Rh or Ir).
has been used earlier9 for the synthesis of rhodacarboꢀ
rane 2.
Scheme 1
The stacking reaction of 4 with the [Cp*Rh]2+ fragꢀ
ment afforded the tripleꢀdecker complex [Cp*Rh(µꢀη5:η5ꢀ
C4H4BPh)RhCp*]2+ (5) as the only product (Scheme 2).*
It should be noted that the pentamethylated analog
[CpRh(µꢀη5:η5ꢀC4H4BPh)RhCp*]2+ (6) is unstable at
room temperature.8 To the contrary, complex 5 proved to
be very stable. It does not decompose even upon refluxing
in nitromethane (100 °C) due apparently to the favorable
effect of five additional methyl groups. The analogous
stacking reaction of 4 with the [Cp*Ir]2+ fragment gave
the [Cp*Rh(µꢀη5:η5ꢀC4H4BPh)IrCp*]2+ complex (7).
The tripleꢀdecker structures of cations 5 and 7 were
1
confirmed by H NMR spectroscopy. Earlier, we have
demonstrated that the tripleꢀdecker µꢀborole complexes
[CpRh(µꢀη5:η5ꢀC4H4BPh)M(ring)]2+ are characterized
Results and Discussion
* The [Cp*M]2+ fragments (M = Rh or Ir) were generated as the
solvated complexes [Cp*M(MeNO2)3]2+ by elimination of chloꢀ
ride ions from [Cp*MCl2]2 with AgBF4. All resulting cationic
complexes were isolated as salts with the BF4– anion (the anions
are omitted in the schemes).
The reaction of the borole iodide complex
[(ηꢀC4H4BPh)RhI]4 with Cp*Li in tetrahydrofuran proꢀ
duced sandwich complex 4 (Scheme 1). The analogous reꢀ
action with the carborane anion [9ꢀSMe2ꢀ7,8ꢀC2B9H10]–
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1525—1527, September, 2006.
1066ꢀ5285/06/5509ꢀ1581 © 2006 Springer Science+Business Media, Inc.

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Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
Loginov et al.
Scheme 2
C(4)
C(6)
C(7)
C(8)
B(1)
C(3)
C(2)
C(5)
C(1)
C(10)
C(9)
Rh(1)
C(11)
C(17)
C(16)
C(13)
C(18)
C(20)
C(14)
C(19)
Fig. 1. Structure of complex 4.
C(15)
M = Rh (5), Ir (7)
ceeds through the intermediate formation of tripleꢀdecker
cation 6. This cation decomposes to form sandwich comꢀ
pound 1 and the [Cp*Rh]2+ fragment. The attack of the
latter on the benzene ring of complex 1 gives rise to comꢀ
plex 3. The stacking reaction of [Cp*Rh]2+ with the startꢀ
ing compound 4 gives cation 5.
by a strong shift of the signals for the protons of the borole
ring to lower field (∆δ = 0.7—1.3 ppm) compared to the
corresponding signals in the spectrum of sandwich comꢀ
pound 1, whereas these signals in the spectra of areneꢀ
type complexes are shifted in the same direction but to a
smaller extent (∆δ = 0.3—0.4 ppm).8 In the spectra of
cations 5 and 7, the shifts of the signals for the protons of
the borole ring relative to the analogous signals for 4 are
1.2—1.3 ppm. In addition, the positions of the signal
for the protons of the methyl groups of the Cp* ring
in tripleꢀdecker (1.8—2.0 ppm) and arene complexes
(2.2—2.3 ppm) are also substantially different.8,9 For catꢀ
ions 5 and 7, these shifts are 1.9—2.0 ppm. Finally, the
1H NMR spectrum of complex 7 is very similar to
that of the pentamethylated analog [CpRh(µꢀη5:η5ꢀ
C4H4BPh)IrCp*]2+, whose structure was established by
Xꢀray diffraction.
Recently, we have developed a procedure for the genꢀ
eration of the unsubstituted [CpRh]2+ fragment and synꢀ
thesized arene and tripleꢀdecker complexes based on this
fragment.4,10 The reaction of 4 with this fragment unexꢀ
pectedly produced a mixture of tripleꢀdecker cation 5 and
areneꢀtype complex 3, which were identified by 1H NMR
spectroscopy (Scheme 3). Apparently, this reaction proꢀ
The structure of compound 4 was established by Xꢀray
diffraction. Complex 4 has the expected sandwich strucꢀ
ture (Fig. 1). Selected bond lengths and bond angles are
given in Table 1. These parameters are very similar to
those for nonmethylated analog 1.8 The electronꢀdonatꢀ
ing effect of five methyl groups in compound 4 would be
expected to increase the strength of the bond between the
rhodium atom and the borole ligand due to an increase
in back donation. However, the distance between the
rhodium atom and the borole ring (1.801 Å) is somewhat
longer than the analogous distance in compound 1
(1.792 Å). Presumably, this is associated with steric reꢀ
pulsion between the substituents of the borole and pentaꢀ
methylcyclopentadienyl ligands in the eclipsed conforꢀ
mation.
Therefore, we demonstrated that the electrophilic
attack of compound 4 by the dicationic fragments
[(ring)M]2+ proceeds exclusively on the borole ring to
form tripleꢀdecker µꢀborole complexes in accordance with
Scheme 3

Reactions of Cp*Rh(η5ꢀC4H4BPh) with [Cp*M]2+
Russ.Chem.Bull., Int.Ed., Vol. 55, No. 9, September, 2006
1583
1
Table 1. Selected bond lengths (d) and bond angles (ω) in comꢀ
plex 4
lated (%): C, 45.62; H, 4.98. H NMR (acetoneꢀd6), δ: 1.90 (s,
30 H, Cp*); 5.19 (m, 2 H, αꢀC4H4B); 6.19 (m, 2 H, βꢀC4H4B);
7.46 (m, 1 H, pꢀPh); 7.54 (m, 2 H, mꢀPh); 7.75 (m, 2 H, oꢀPh).
11B NMR (acetoneꢀd6), δ: –0.1 (s, BF4), 12.4 (br.s, C4H4B).
Compound 7, the yield was 55 mg (95%). Found (%): C, 40.96;
H, 4.46. C30H39B3F8RhIr. Calculated (%): C, 40.98; H, 4.47.
1H NMR (acetoneꢀd6), δ: 1.81 (s, 15 H, RhCp*); 1.98 (s, 15 H,
IrCp*); 5.21 (m, 2 H, αꢀC4H4B); 6.19 (m, 2 H, βꢀC4H4B); 7.43
(m, 1 H, pꢀPh); 7.53 (m, 2 H, mꢀPh); 7.72 (m, 2 H, oꢀPh).
11B NMR (acetoneꢀd6), δ: –0.9 (s, BF4), 7.5 (br.s, C4H4B).
Reaction of complex 4 with the [CpRh]2+ fragment. Nitroꢀ
methane (1 mL) was added to a mixture of the [CpRhCl2]2
complex (16 mg, 0.03 mmol) and AgBF4•3dioxane (62 mg,
0.14 mmol), and the reaction mixture was stirred for ∼0.5 h. The
precipitate of AgCl that formed was removed by centrifugation,
and the resulting solution was added to complex 4 (25 mg,
0.07 mmol). The reaction mixture was stirred for 2 h. The addiꢀ
tion of diethyl ether (∼10 mL) afforded a yellow precipitate
(34 mg). According to the 1H NMR spectroscopic data, the
precipitate contained complexes 3 (see Ref. 8) and 5 in a raꢀ
tio of 2 : 1.
Parameter
Value
Parameter
Value
Bond
d/Å
Bond
d/Å
Rh(1)—B(1)
Rh(1)—C(1)
Rh(1)—C(2)
Rh(1)—C(3)
Rh(1)—C(4)
Rh(1)—C(11)
Rh(1)—C(12)
Rh(1)—C(13)
2.275(6)
2.193(4)
2.145(4)
2.157(4)
2.199(5)
2.215(5)
2.180(5)
2.190(5)
Rh(1)—C(14)
Rh(1)—C(15)
C(1)—C(2)
C(2)—C(3)
C(3)—C(4)
B(1)—C(1)
B(1)—C(4)
B(1)—C(5)
2.167(5)
2.196(5)
1.443(7)
1.414(8)
1.423(7)
1.533(8)
1.545(6)
1.577(8)
Angle
ω/deg
Angle
ω/deg
C(1)—C(2)—C(3) 110.1(4)
C(2)—C(3)—C(4) 110.5(4)
B(1)—C(1)—C(2) 108.0(4)
C(3)—C(4)—B(1) 108.3(5)
C(1)—B(1)—C(4) 103.0(5)
an increase in the electron density at the borole ring due
to the electronꢀdonating effect of five methyl groups.
Xꢀray diffraction study of complex 4. Yellow plateꢀlike crysꢀ
tals of composition C20H28BRh, which were grown by slow
evaporation of a solution in petroleum ether, are orthorhombic.
The unit cell parameters are a = 8.845(2) Å, b = 13.763(3) Å,
c = 14.155(3) Å, V = 1723.2(6) Å3, space group P212121, Z = 4,
dcalc = 1.457 g cm–3. A total of 16323 reflections were collected
on a Bruker SMART 1000 CCD diffractometer at 140 К
Experimental
The reactions were carried out under argon with the use of
anhydrous solvents prepared according to standard proceꢀ
dures. Operations associated with isolation of the reaction
products were carried out in air. The starting comꢀ
pounds [(ηꢀC4H4BPh)RhI]4,11 [Cp*RhCl2]2, [Cp*IrCl2]2,12
[CpRhCl2]2,13 and AgBF4·3dioxane14 were synthesized accordꢀ
ing to known procedures. The H and 11B NMR spectra were
recorded on a Bruker AMXꢀ400 instrument (400.13 MHz for 1H
and 128.38 MHz for 11B).
(MoꢀKα radiation, 2θ
= 54.00) from a single crystal of diꢀ
max
mensions 0.45×0.30×0.05 mm. After merging of equivalent reꢀ
flections, the data set consisted of 3680 independent reflections
(Rint = 0.0495), which were used for the structure solution and
refinement. An absorption correction (µ = 0.985 mm–1) was
applied with the use of the SADABS program (Tmax and Tmin are
0.802 and 0.481, respectively). The structure was solved by diꢀ
rect methods. All nonhydrogen atoms were located in difference
electron density maps and refined against F 2hkl with anisotropic
displacement parameters. All hydrogen atoms were placed in
geometrically calculated positions and refined using a riding
model with U(H) = nU(C), where U(C) is the equivalent therꢀ
mal parameter of the corresponding pivot carbon atom, n = 1.2
and 1.5 for the CH and Me groups, respectively. The final
R factors were as follows: R1 = 0.0421 (calculated based on Fhkl
for 3332 reflections with I > 2σ(I )), wR2 = 0.1017 (calculated
1
(ηꢀPentamethylcyclopentadienyl)(ηꢀBꢀphenylborole)rhodium,
Cp*Rh(ηꢀC4H4BPh) (4). A mixture of [(ηꢀC4H4BPh)RhI]4
(130 mg, 0.09 mmol) and Cp*Li (61 mg, 0.43 mmol) in tetraꢀ
hydrofuran (5 mL) was stirred for 24 h. The solvent was removed
in vacuo and the residue was chromatographed with petroleum
ether on a silica gel column (1×10 cm). The yellow fraction was
collected, and the solvent was removed in vacuo. Complex 4 was
obtained as a yellow solid in a yield of 58 mg (44%). Found (%):
C, 63.89; H, 6.56. C20H24BRh. Calculated (%): C, 63.53;
1
H, 6.40. H NMR (CDCl3), δ: 1.78 (s, 15 H, Cp*); 3.96 (m,
based on F 2 for all 3680 reflections), 199 parameters were
2 H, αꢀC4H4B); 4.90 (m, 2 H, βꢀC4H4B); 7.19 (m, 1 H, pꢀPh);
7.28 (m, 2 H, mꢀPh), 7.55 (m, 2 H, oꢀPh). 11B NMR (CDCl3),
δ: 14.2 (br.s, C4H4B).
[(ηꢀPentamethylcyclopentadienyl)rhodium]ꢀ
[(ηꢀpentamethylcyclopentadienyl)iridium](µꢀη:ηꢀBꢀphenylꢀ
borole)[(ηꢀpentamethylcyclopentadienyl)rhodium] bis(tetraꢀ
hkl
refined, GOOF = 1.008. All calculations were carried out using
the SHELXTL PLUS 5 program package.15 The atomic coordiꢀ
nates, displacement parameters, and complete data on geometꢀ
ric parameters were deposited with the Cambridge Structural
Database.
(5)
and
5
5
fluoroborate) (7), [Cp*M(µꢀη :η ꢀC4H4BPh)RhCp*](BF4)2.
Nitromethane (1 mL) was added to a mixture of the [Cp*MCl2]2
complex (0.03 mmol) and AgBF4•3dioxane (63 mg, 0.14 mmol),
and the reaction mixture was stirred for ∼0.5 h. The precipitate
of AgCl that formed was removed by centrifugation and the
resulting solution was added to complex 4 (25 mg, 0.07 mmol).
The reaction mixture was stirred for 2 h, and then diethyl ether
(∼10 mL) was added. The yellow precipitate that formed was
filtered off and twice reprecipitated with diethyl ether from acꢀ
etone or dichloromethane. Compound 5, the yield was 45 mg
(86%). Found (%): C, 44.24; H, 5.07. C30H39B3F8Rh2. Calcuꢀ
This study was financially supported by the Division
of Chemistry and Materials Science of the Russian Acadꢀ
emy of Sciences (Grant 05ꢀ07) and the Russian Foundaꢀ
tion for Basic Research (Project No. 03ꢀ03ꢀ32214).
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Received March 23, 2006;
in revised form June 13, 2006