μ-Borole triple-decker complexes as catalysts for oxidative coupling of benzoic acid with alkynes. Structure of a hybrid rhodacyclopentadienyl/borole triple-decker complex

Article abstract of DOI:10.1016/j.molcata.2016.07.004

Reaction of dimethylamine adduct of 1-methyl-3-borolene with [(C₂H₄)₂RhCl]₂ gives the triple-decker complex (η-C₄H₄BMe)Rh(μ-η:η-C₄H₄BMe)Rh(η-C₄H₄

Full text of DOI:10.1016/j.molcata.2016.07.004

Accepted Manuscript
Title: ␮-Borole triple-decker complexes as catalysts for
oxidative coupling of benzoic acid with alkynes. Structure of
hybrid rhodacyclopentadienyl/borole triple-decker complex
Author: Dmitry A. Loginov Dmitry V. Muratov Yulia V.
Nelyubina Julia Laskova Alexander R. Kudinov
PII:
S1381-1169(16)30254-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.07.004
MOLCAA 9938
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
18-4-2016
28-6-2016
2-7-2016
Please cite this article as: Dmitry A.Loginov, Dmitry V.Muratov, Yulia V.Nelyubina,
Julia Laskova, Alexander R.Kudinov, ␮-Borole triple-decker complexes as
catalysts for oxidative coupling of benzoic acid with alkynes.Structure of hybrid
rhodacyclopentadienyl/borole triple-decker complex, Journal of Molecular Catalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.07.004
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Highlights
- Triple-decker complex with a bridging rhodacyclopentadienyl ligand was described.
- Triple-decker complexes effectively catalyze coupling of benzoic acid with alkynes.
- Catalytic selectivity is determined by terminal ligands in triple-decker structure.

Graphical abstract

-Borole triple-decker complexes as catalysts for oxidative coupling of benzoic acid with
alkynes. Structure of hybrid rhodacyclopentadienyl/borole triple-decker complex
Dmitry A. Loginov,* Dmitry V. Muratov, Yulia V. Nelyubina, Julia Laskova, Alexander R.
Kudinov*
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow GSP-1, Russian Federation
Dedicated to 70th anniversary of Prof. Georgiy B. Shul’pin in recognition of his great
contribution to the development of the metal-complex catalyzed CH activation.
Abstract
Reaction of dimethylamine adduct of 1-methyl-3-borolene with [(C2H4)2RhCl]2 gives the
triple-decker complex (η-C4H4BMe)Rh(-η:η-C4H4BMe)Rh(η-C4H4BMe) (1a) in 62% yield and
trace amount (<1%) of the hybrid rhodacyclopentadienyl/borole triple-decker complex (η-
C4H4BMe)Rh(-η:η-C4H4Rh{(-η:η-C4H4BMe)Rh(η-C4H4BMe)})Rh(η-C4H4BMe) (2). The
structure of 2 was determined by X-ray diffraction. In the presence of Cu(OAc)2, 1a and (η-
C4H4BPh)Rh(-η:η-C4H4BPh)Rh(η-C4H4BPh) (1b) catalyze the oxidative coupling of benzoic
acid with diphenylacetylene selectively giving 1,2,3,4-tetraphenylnaphtalene in 50–90% yields.
Analogous reactions of benzoic acid with 1-phenyl-1-butyne catalyzed by 1a and [CpRhI2]2
regioselectively give 1,4-diethyl-2,3-diphenylnaphthalene. The related dicationic triple-decker
complexes [(9-SMe2-7,8-C2B9H10)Rh(-η:η-C4H4BPh)Rh(9-SMe2-7,8-C2B9H10)]2+ (3) and
[Cp*Rh(-η:η-C4H4BPh)IrCp*]2+ (4) were also tested as catalysts.
Keywords: CH activation; Rhodium; Metal complex catalysis; Triple-decker complexes
* Corresponding authors. E-mail: dloginov@ineos.ac.ru (D.A. Loginov), arkudinov@ineos.ac.ru
(A.R. Kudinov)

1. Introduction
The metal-catalyzed C–H functionalization of organic compounds is of great significance
in synthetic chemistry.1 In particular, the catalytic coupling reactions of aromatic compounds
with alkynes give a number of oxygen- and nitrogen-containing heterocycles. Extensive
investigations in this area have revealed that the cyclopentadienyl complexes [Cp*MCl2]2 (M =
Rh, Ir) can effect regioselective arene C–H bond activation under chelation-assistance of an
appropriate directing group (e.g., COOR, OH, C(O)NR2 substituents).2 Satoh and Miura have
shown that these complexes catalyze the oxidative coupling of benzoic acids with internal
alkynes giving isocoumarins (M = Rh) and naphthalenes (M = Ir; Scheme 1).3 Recently, we have
demonstrated that the selectivity depends not only on the nature of the metal, but also on the
ligand. Thus, use of carborane-containing complexes [(C2B9H11)M(C6H6)]+ instead of
cyclopentadienyl derivatives dramatically changes the selectivity of oxidative coupling giving
naphthalenes for Rh and isocoumarins for Ir.4
Scheme 1. Reaction of oxidative coupling of benzoic acids with internal alkynes.
Triple-decker complexes of transition metals are well known.5 They have unique
structure in which two metal atoms are located between three cyclic frames. In 1972, Werner and
Salzer have synthesized the first representative of such complexes, viz., the nickel derivative
[CpNi(-η:η-Cp)NiCp]+ containing cyclopentadienyl ligand in a bridging position.6 Boron-
containing heterocycles possess the highest ability for the bifacial bonding with two metal
atoms.7 In particular, borole ligand C4H4BR was used for the synthesis of numerous triple-decker
complexes.8 However, the complexes of this class have not yet been applied in catalysis.
Herein we report unexpected formation of the hybrid rhodacyclopentadienyl/borole
triple-decker
complex
(η-C4H4BMe)Rh(-η:η-C4H4Rh{(-η:η-C4H4BMe)Rh(η-
C4H4BMe)})Rh(η-C4H4BMe) (2) as byproduct in reaction of dimethylamine adduct of 1-methyl-
3-borolene C4H6BMe·NMe2H with bis(ethylene)rhodium complex [(C2H4)2RhCl]2 as well as
study of the catalytic activity of -borole triple-decker complexes (η-C4H4BR)Rh(-η:η-
C4H4BR)Rh(η-C4H4BR) (1a: R = Me, 1b: R = Ph),9 [(9-SMe2-7,8-C2B9H10)Rh(-η:η-

C4H4BPh)Rh(9-SMe2-7,8-C2B9H10)]2+ (3)10 and [Cp*Rh(-η:η-C4H4BPh)IrCp*]2+ (4)11
in
oxidative coupling of benzoic acid with internal alkynes.
2. Results and discussion
2.1. Reaction of dimethylamine adduct of 1-methyl-3-borolene with [(C2H4)2RhCl]2
Herberich et al. have previously shown that reaction of borolenes C4H6BR with
[(C2H4)2RhCl]2 is an effective method for the synthesis of triple-decker complexes with a
bridging borole ligand (η-C4H4BR)Rh(-η:η-C4H4BR)Rh(η-C4H4BR) (1a: R = Me, 1b: R = Ph).9
In present work we found that the analogous reaction of complex C4H6BMe·NMe2H with
[(C2H4)2RhCl]2 results in the formation of the hybrid rhodacyclopentadienyl/borole triple-decker
complex (η-C4H4BMe)Rh(-η:η-C4H4Rh{(-η:η-C4H4BMe)Rh(η-C4H4BMe)})Rh(η-C4H4BMe)
(2) as a minor product (<1%) along with the main product 1a (Scheme 2).
Scheme 2. Reaction of C4H6BMe·NMe2H with [(C2H4)2RhCl]2.
1
The H spectral data for 2 are consistent with the expected structure. The proton signals
related to the Rh-cycle are observed at  = 5.19 (-Н) and 6.84 (-Н) ppm; they are very close to
the signals previously reported for rhodium complexes with the C4H4Rh ring (5.58−6.86
ppm).12,13 The signals of the bridging borole ring protons are upfield-shifted (Δδ = 0.15–0.31
ppm) as compared to the corresponding signals for the terminal borole ring.
The structure of 2 was investigated by X-ray diffraction study (Fig. 1). It is the first
crystallographically characterized triple-decker complex with bridging rhodacyclopentadienyl

ligand. Earlier, Müller and co-workers12 have described complexes (η4-diene)(-H)Rh(-η:η-
C4R4Rh{η4-diene})Rh(η4-diene) containing diene ligands in terminal positions, however, their
structure was not confirmed by X-ray diffraction.
Figure 1. Structure of complex 2. Atoms are represented by 50% thermal ellipsoids. Hydrogen
atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Rh2Rh3 2.6500(5),
Rh2Rh4 2.6299(5), Rh1C1 2.183(5), Rh1C2 2.150(5), Rh1C3 2.122(5), Rh1C4 2.170(5),
Rh1B1 2.267(5), Rh1C6 2.245(5), Rh1C7 2.198(4), Rh1C8 2.219(4), Rh1C9 2.221(4),
Rh1B2 2.287(5), Rh2C6 2.223(4), Rh2C7 2.229(4), Rh2C8 2.200(4), Rh2C9 2.255(5),
Rh2B2 2.322(5), Rh2C21 2.024(4), Rh2C24 2.020(4), Rh3C16 2.191(5), Rh3C17
2.131(5), Rh3C18 2.131(5), Rh3C19 2.166(5), Rh3B3 2.288(5), Rh3C21 2.233(4),
Rh3C22 2.205(4), Rh3C23 2.199(4), Rh3C24 2.270(4), Rh4C11 2.177(5), Rh4C12
2.136(4), Rh4C13 2.159(4), Rh4C14 2.180(5), Rh4B4 2.282(5), Rh4C21 2.290(4),
Rh4C22 2.234(4), Rh4C23 2.188(4), Rh4C24 2.227(4), C21C22 1.434(6), C22C23
1.467(6), C23C24 1.449(6), C21Rh2C24 78.39(18), C1B1C4 102.7(4), C6B2C9
100.6(4), C16B3C19 100.9(4), C11B4C14 100.9(4).

Complex 2 consists of two triple-decker fragments: (η-C4H4B)Rh(-η:η-C4H4Rh)Rh(η-
C4H4B) and (η-C4H4B)Rh(-η:η-C4H4B)Rh. In both cases the planes of the cyclic ligands are
almost parallel (dihedral angles C4H4Rh/C4H4B = 3.9 and 5.1°; C4H4B/C4H4B = 3.8°).
Commonly, the heterocyclic ligands in sandwich compounds are folded along the axis Cα···Cα',
with the heteroatom being deviated from the C4 plane away from the metal atom. For example,
the folding angle† and the deviation of the Rh atom from the C4 plane for (η4-nbd)Rh(η-
C4Me2H2Rh{η4-nbd}PMe3) (nbd – norbornadiene) are 17.1° and 0.47 Å, respectively.12 In
contrast, the bridging rings C4H4Rh and C4H4B in 2 are almost planar (folding angles are 0.7 and
1.2°). The C4H4Rh ring is strongly distorted pentagon owing to great difference of the C–C and
Rh–C bond lengths (av. 1.450 and 2.022 Å, respectively); the angle C21Rh2C24 (78.39(18)°)
is considerably smaller than 108° (typical of a regular pentagon).
The distance Rh3···Rh4 (3.654 Å) in 2 exceeds the sum of the covalent radii Rh–Rh (av.
2.92 Å)14 suggesting the absence of the direct Rh–Rh bond in this case. At the same time the
Rh2–Rh3 and Rh2–Rh4 bonds (2.6500(5) and 2.6299(5) Å) are shorter than the corresponding
bond in sandwich complex (η4-nbd)Rh(η-C4Me2H2Rh{η4-nbd}PMe3) (2.704 Å).12 Although
these bonds are considerably longer than the Rh1–B2 bond (2.267(5) Å), the metal-to-ring
distances for bridging rings Rh3···C4H4Rh2, Rh4···C4H4Rh2 and Rh1···C4H4B2 are practically
the same (1.825, 1.828 and 1.830 Å, respectively). It is noteworthy that these values are very
close to the corresponding distances in the related -borole triple-decker complexes (η-
C4H4BPh)Rh(-η:η-C4H4BPh)Rh(η-C4H4BPh) and (η-C4H4BPh)Rh(-η:η-C4H4BPh)Ru(η-C5R5)
(1.8261.845 Å, av. 1.835 Å).15 The Rh···C4H4B distances in 2 for the terminal borole ligands
(1.7801.788 Å, av. 1.784 Å) are shorter than for the bridging one (1.830 Å) that is in
accordance with general tendency for triple-decker complexes.16
A supposed pathway of the reaction of dimethylamine adduct of 1-methyl-3-borolene
with [(C2H4)2RhCl]2 is illustrated in Scheme 3. Probably, dehydrogenation of the borolene and
its subsequent coordination mainly produce the chloride complex [(η-C4H4BMe)RhCl]4 (A).
Similar iodides [(η-C4H4BR)RhI]4 (R = Me, Ph) have been described and structurally
characterized earlier.17 Further transformation of A leads to the formation of the triple-decker
complex 1a (the main product of the reaction). Side reaction takes place in such a way that at the
first stage, the removal of boron atom from the starting borolene occurs. The subsequent reaction
with [(C2H4)2RhCl]2 affords the rhodacyclopentadienyl derivative (C2H4)2Rh(η-
C4H4Rh(H)2(C2H4)2) (B). Related intermediates have been suggested earlier by Müller and co-
workers12 in reactions of [(η4-nbd)MCl]2 with magnesium butadiene (C4H6Mg). Possibly the
† Folding angle is dihedral angle between C–X–C and C4 planes, where X is heteroatom.

coordinated amine, NMe2H, acts as the basic agent to facilitate removal of the boron atom.
Subsequent reaction of B with A and C4H6BMe·NMe2H finally gives the minor product 2.
Scheme 3. A supposed pathway for the formation of 2.
2.2. Oxidative coupling of benzoic acid with alkynes
It is known that the triple-decker complexes easily undergo nucleophilic degradation.18
For example, Herberich has shown that complexes 1a,b react with CpNa giving two sandwich
compounds CpRh(η-C4H4BR) and [Rh(η-C4H4BR)2]−.9b Similar reaction of 1a,b with I2 affords
iodine derivatives [(η-C4H4BR)RhI]4 and Rh(η-C4H4BR)2I.17 Taking into account this reactivity,
we examined 1a,b as well as the related -borole triple-decker complexes [(9-SMe2-7,8-
C2B9H10)Rh(-η:η-C4H4BPh)Rh(9-SMe2-7,8-C2B9H10)]2+
(3)10
and
[Cp*Rh(-η:η-
C4H4BPh)IrCp*]2+ (4)11 as catalysts for the oxidative coupling of benzoic acid with
diphenylacetylene in refluxing o-xylene (Scheme 4). This reaction has two possible pathways
giving either isocumarin 5a or naphthalene 5b. Copper acetate and silver carbonate were used as
co-catalysts (necessary for oxidation of MI to MIII). We hypothesized that the triple-decker
complexes would be cleaved by benzoic acid, giving the catalytically active species (η-
C4H4BR)Rh, (9-SMe2-7,8-C2B9H10)Rh or Cp*M (M = Rh, Ir), which may act as catalysts of C–
H activation in aromatic compounds.

Scheme 4. Oxidative coupling of benzoic acid with diphenylacetylene.
As can be seen from Table 1, complexes 1a,b selectively catalyze the oxidative coupling
to give naphthalene 5b as a sole product in 50–90% yields. The turnover number (TON) is ca.
20–45. The phenyl-substituted complex 1b reveals higher catalytic activity than the methyl
derivative 1a, which may be explained by better stabilization of catalytic intermediates by
phenyl substituent. It is interesting to note that the rhodium complex [Cp*RhCl2]2 predominantly
gives isocumarin 5a instead of 5b.3
The triple-decker complex 3 having two carborane ligands in terminal positions catalyzes
the oxidative coupling reaction giving the naphthalene 5b as the major product in moderate yield
(40%). Earlier we have shown that chloride (η-9-SMe2-7,8-Me2-C2B9H8)RhCl2 reacts in the
same manner,4a and suggested that the key catalytic species is (η-9-SMe2-7,8-C2B9H8R2)Rh (R =
H, Me). This leads to the conclusion that the selectivity of the oxidative coupling is mainly
determined by the nature of the terminal ligands in the triple-decker structure.
Taking into account the different selectivity of rhodium and iridium cyclopentadienyl
complexes3 we have assumed that the use of the heterometallic RhIr triple-decker complex 4
with different co-catalysts (Cu(OAc)2 or Ag2CO3) may lead to different products. However, in
both cases the reaction predominantly gives isocoumarin 5a, suggesting that the selectivity is
determined by direction of nucleophilic degradation of 4. Usually, the bonding in triple-decker
complexes between bridging ligand and rhodium atom is weaker compared to iridium. Therefore
the most probable catalytic particle in these reactions is Cp*Rh.
Table 1. Catalytic activity of triple-decker complexes 1a,b, 3 and 4 as well as halides [CpRhI2]2,
[Cp*RhCl2]2 and (η-9-SMe2-7,8-Me2-C2B9H8)RhCl2 in the oxidative coupling of benzoic acid
with diphenylacetylene.
Catalyst
Co-catalyst
Yield, % a
5a
–
5b
51
91
40
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
1a
1b
3
–
3

Cu(OAc)2
Ag2CO3
54
60
–
17
29
91
12
32
4
4
b
[CpRhI2]2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
c
[Cp*RhCl2]2
82
b
(η-9-SMe2-7,8-Me2-C2B9H8)RhCl2
–
aYield is based on the amount of benzoic acid used; the ratio of 5a and 5b in product mixture
was determined by 1H NMR. bRef.4a cRef.3
The reaction of benzoic acid with 1-phenyl-1-butyne catalyzed by 1a regioselectively gives 1,4-
diethyl-2,3-diphenylnaphthalene (6) in 54% yield (Scheme 5). The (cyclopentadienyl)rhodium
complex [CpRhI2]2 catalyzes this reaction in a similar way to give 6 in 81% yield. In accordance
with the mechanism proposed by Satoh and Miura,3 the formation of naphthalenes in oxidative
coupling reactions proceeds via rhodacycle intermediate C. The observed regioselectivity is
explained by better stabilization of α-carbon atoms in C by phenyl substituent.19
Scheme 5. Oxidative coupling of benzoic acid with 1-phenyl-1-butyne.
3. Conclusion
The formation of a minor product, the hybrid rhodacyclopentadienyl/borole triple-decker
complex 2, caused by the coordinated amine, was observed in the reaction of C4H6BMe·NMe2H
with [(C2H4)2RhCl]2. To our knowledge, this is the first example of a crystallographically
characterized triple-decker complex with a bridging rhodacyclopentadienyl ligand. The main
product of this reaction is the triple-decker complex 1a. X-ray data for 2 suggest that the bonding
of rhodium with the rhodacyclopentadienyl ligand C4H4Rh is very close to that with borole
C4H4B.
The triple-decker complexes with a bridging borole ligand were shown to be effective
catalysts for the oxidative coupling of benzoic acid with alkynes. For instance, complexes 1a,b
exhibit notable catalytic activity in reactions of benzoic acid with alkynes in the presence of
Cu(OAc)2, giving exclusively naphthalenes. The observed catalytic selectivity is determined by

the direction of the nucleophilic degradation of triple-decker structure and the nature of terminal
ligands.
4. Experimental
General
The reactions were carried out under an inert atmosphere in dry degassed solvents. The
isolation of products was conducted in air. Starting complex [Rh(C2H4)2Cl]2 was prepared as
described.20 Complex C4H6BMe·NMe2H was synthesized from C4H6BNMe2 and LiMe in a
1
similar manner as C4H6BPh·NMe2H.21 The H, 11B and 13C NMR spectra ( in ppm) were
recorded with a Bruker Avance-400 spectrometer operating at 400.13, 128.38 and 100.61 MHz,
respectively.
(-C4H4BMe)Rh(µ-:-C4H4BMe)Rh(-C4H4BMe) (1a)
The 50 ml of THF was added to the mixture of [Rh(C2H4)2Cl]2 (1.94 g, 5 mmol) and
C4H6BMe·NMe2H (3.13 g, 25 mmol). The reaction mixture was stirred for 48 h at ambient
temperature. The solvent was removed in a high vacuo and the residue was subjected to column
chromatography (aluminium oxide activity grade II; 3 × 10 cm). Elution first with hexane led to
a brown band, which was discarded. The further elution with hexane/CH2Cl2 mixture (1/1) gave
the second dark-orange band, which was evaporated till 20 ml, crystallized at –78 °C and held
for 4 h in a high vacuo. Yield of 1a is 1.05 g (3.12 mmol; 62%). Orange solid, which has 99%
1
purity according to NMR. H NMR (CDCl3) : 5.21 (m, 4Н, -Н, η-С4Н4В); 4.77 (m, 2Н, -Н,
µ-η:η-С4Н4В); 4.05 (m, 4Н, -Н, η-С4Н4В); 3.86 (m, 2Н, -Н, µ-η:η-С4Н4В); 0.63 (s, 3Н, µ-
η:η-С4Н4ВMe); 0.39 (s, 6Н, η-С4Н4ВMe). 11В {1Н} NMR (CDCl3) : 21.4 (broad, 2B, η-
С4Н4В), 10.1 (broad, 1B, µ-η:η-С4Н4В). 13C {1Н} NMR (CDCl3) : 88.55 (d, J(C-Rh) = 9 Hz, -
C, η-С4Н4В); 80.20 (broad, -C, η-С4Н4В); 69.60 (broad, -C, µ-η:η-С4Н4В), 62.98 (t, J(C-Rh)
= 8 Hz, -C, µ-η:η-С4Н4В), –3.71 (broad, Me) (cf.9b). Found (%): C, 40.80; H, 4.64; B, 7.37; Rh,
46.46. Calc. for C15H21B3Rh2 (%): C, 40.99; H, 4.82; B, 7.38; Rh, 46.82.
After recrystallization of 1a by slow evaporation of its hexane solution, orange admixture
crystals (<1%) of (η-C4H4BMe)Rh(-η:η-C4H4Rh{(-η:η-C4H4BMe)Rh(η-C4H4BMe)})Rh(η-
C4H4BMe) (2) were selected. 1H NMR (CDCl3) for 2, : 6.84 (m, 2Н, -Н, µ-η:η-С4Н4Rh), 5.26

(m, 2Н, -Н, η-С4Н4В), 5.19 (m, 2Н, -Н, µ-η:η-С4Н4Rh), 5.18 (m, 4Н, -Н, η-С4Н4В), 4.95
(m, 2Н, -Н, µ-η:η-С4Н4В), 4.21 (m, 4Н, -Н, η-С4Н4В), 4.10 (m, 2Н, -Н, η-С4Н4В), 3.95 (m,
2Н, -Н, µ-η:η-С4Н4В), 0.67 (s, 3Н, µ-η:η-С4Н4ВMe), 0.44 (s, 3Н, η-С4Н4ВMe), 0.27 (s, 6Н, η-
С4Н4ВMe).
Oxidative coupling of benzoic acid with alkynes (general procedure)
A mixture of benzoic acid (31 mg, 0.25 mmol), alkyne (0.5 mmol), catalyst (0.005
mmol), o-xylene (2 ml), and Cu(OAc)2 (182 mg, 1.00 mmol) or Ag2CO3 (138 mg, 0.5 mmol)
was refluxed with vigorous stirring for 6 h. The solvent was removed in vacuo, and the residue
was extracted with diethyl ether. The extract was chromatographed on a silica column (1 × 15
cm). Unreacted alkyne was washed off with petroleum ether. Then the yellow band was
collected using diethyl ether as the eluent. After the removal of the solvent in vacuo, isocumarin
5a or naphthalenes 5b and 6 were obtained as yellow oils.
5a, 1H NMR (CDCl3) : 8.44 (d, 1H, J = 8.0 Hz); 7.67 (m, 1H); 7.55 (m, 1H); 7.44 (m, 3H), 7.35
(m, 2H), 7.20−7.30 (m, 6H) (cf.3).
5b, 1H NMR (CDCl3) : 7.76−7.78 (m, 2H); 7.47−7.49 (m, 2H); 7.32−7.34 (m, 10H); 6.95−6.97
(m, 10H) (cf.22).
1
6, H NMR (CDCl3) : 8.30 (m, 2H); 7.68 (m, 2H); 7.11−7.24 (m, 10H); 2.98 (q, 4H, J = 10.0
Hz), 1.30 (t, 6H, J = 10.0 Hz) (cf.23).
X-ray crystallography
Crystals of 2 were grown by slow evaporation of hexane solution of crude 1a. Crystals of 2
(C24H32B4Rh4, M = 775.37) are triclinic, space group P-1, at 120 K: a = 10.6545(9), b =
10.9399(9), c = 11.9022(10) Å,  = 89.571(2),  = 85.811(2),  = 61.680(2)°, V = 1217.42(18)
Å3, Z = 2, dcalc = 2.115 gcm–3, (MoK) = 26.75 сm-1, F(000) = 752. Intensities of 15185
reflections were measured with a Bruker APEX2 DUO diffractometer using graphite
monochromated Mo-K radiation (λ = 0.71073 Å, -scans, 2  58°), and 6460 independent
reflections [Rint 0.0536] were used in further refinement. The structure was solved by direct
method and refined by the full-matrix least-squares against F2 in anisotropic approximation for
non-hydrogen atoms. The positions of hydrogen atoms were calculated, and they were refined in
isotropic approximation in riding model. The refinement converged to wR2 = 0.0855 and GOF =
1.006 for all the independent reflections (R1 = 0.0359 was calculated against F for 5224 observed

reflections with I>2(I)). All calculations were performed using the SHELXTL PLUS 5.0
software. 24
Acknowledgements
This work was supported by Russian Foundation for Basic Research (grant # 15-03-04057).
Appendix A. Supplementary material
CCDC 1473095 contains the supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.…

Graphical Abstract
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