Metallaboranes from metal carbonyl compounds and their utilization as catalysts for alkyne cyclotrimerization

Article abstract of DOI:10.1002/cplu.201400013

The photolysis of [M₂(CO)₁₀] (M=Re or Mn) with BH₃·thf at room temperature yields arachno-1 and 2, [(CO) ₈M₂B₂H₆] (1: M=Re, 2: M=Mn). Both the compounds show a butterfly structure with seven skeletal electron pairs and 42 valence electrons. This result presents a new method for general access to low-boron-content metal-boron compounds without the cyclopentadienyl ligand at the metal centers. This new synthetic route is superior to the existing procedures because it avoids the use of [LiBH₄] and metal polychlorides, for which the synthesis is very tedious. Compound 1 catalyzes the cyclotrimerization of a series of internal and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The reactivity of 1 with alkynes demonstrates for the first time that the introduction of the [B ₂H₆] moiety into the [Re₂(CO)₁₀] framework significantly enhances the catalytic activity. Note that [Re ₂(CO)₁₀] catalyzes the same set of alkynes under harsh conditions over a prolonged period of time. Quantum-chemical calculations using DFT methods are applied to afford further insight into the electronic structure, stability, and bonding of 1 and 2. All the compounds are characterized by IR and ¹H, ¹¹B, and ¹³C NMR spectroscopy, and the geometry of 1 is established unambiguously through crystallographic analysis. Arachno compounds: Room-temperature UV photolysis of [Re₂(CO) ₁₀] with BH₃·thf yielded arachno-[(CO) ₈Re₂B₂H₆], which catalyzed the cyclotrimerization of various internal and terminal alkynes to yield mixtures of 1,3,5- and 1,2,4-substituted benzenes (see figure). Copyright

Full text of DOI:10.1002/cplu.201400013

CHEMPLUSCHEM
FULL PAPERS
DOI: 10.1002/cplu.201400013
Metallaboranes from Metal Carbonyl Compounds and
Their Utilization as Catalysts for Alkyne Cyclotrimerization
V. P. Anju, Subrat Kumar Barik, Bijnaneswar Mondal, V. Ramkumar, and
Sundargopal Ghosh*[a]
The photolysis of [M2(CO)10] (M=Re or Mn) with BH3·thf at
room temperature yields arachno-1 and 2, [(CO)8M2B2H6] (1:
M=Re, 2: M=Mn). Both the compounds show a butterfly
structure with seven skeletal electron pairs and 42 valence
electrons. This result presents a new method for general
access to low-boron-content metal–boron compounds without
the cyclopentadienyl ligand at the metal centers. This new syn-
thetic route is superior to the existing procedures because it
avoids the use of [LiBH4] and metal polychlorides, for which
the synthesis is very tedious. Compound 1 catalyzes the cyclo-
trimerization of a series of internal and terminal alkynes to
yield mixtures of 1,3,5- and 1,2,4-substituted benzenes. The re-
activity of 1 with alkynes demonstrates for the first time that
the introduction of the [B2H6] moiety into the [Re2(CO)10]
framework significantly enhances the catalytic activity. Note
that [Re2(CO)10] catalyzes the same set of alkynes under harsh
conditions over a prolonged period of time. Quantum-chemi-
cal calculations using DFT methods are applied to afford fur-
ther insight into the electronic structure, stability, and bonding
1
of 1 and 2. All the compounds are characterized by IR and H,
11B, and 13C NMR spectroscopy, and the geometry of 1 is estab-
lished unambiguously through crystallographic analysis.
Introduction
Organometallic chemistry can be regarded as the confluence
of coordination chemistry and organic chemistry. The mutually
synergistic interactions of metals with organic ligands or frag-
ments and vice versa created new chemistries of both funda-
mental and practical importance.[1–3] On the other hand, some-
what earlier, metallaborane chemistry provided a close link
with organometallic systems, and hence, was rationalized with
the 18 electron rule.[4–9] Indeed, many of these clusters, gener-
ated from organometallic compounds by replacing C with B
plus BÀHÀB bridging hydrogen atoms, could be related to sev-
eral classic organometallic complexes that define basic struc-
tural and bonding paradigms, for example, [(CO)4Fe(h2-C2H4)]
versus
[{(CO)4Fe}B2H5]À,[10]
[(PR3)ClPd(h3-C3H5)]
versus
[{(h5-
[{(h5-
[{(PR3)2Pd}B3H7],[11]
C5H5)Co}B4H8],[12]
[(h5-C5H5)Co(h4-C4H4)]
[(h5-C5H5)Fe(h5-C5H5)]
versus
versus
C5H5)Fe}B5H10],[13] and [(Cp*Ru)B8H14(RuCp*)] versus [Cp*Ru-
(C8H6)RuCp*][14] (Scheme 1). In addition, although the transition
metal fragments can be isolobal to borane fragments, they
possess bonding capabilities that extend beyond this isolobal
analogy.[15–17]
Scheme 1. Metallaborane analogues to p ligands in organometallic com-
plexes. Cp=h5-C5H5. [(CO)4Fe(h2-C2H4)] (I), [{(CO)4Fe}B2H5]À (II), [(PR3)ClPd(h3-
C3H5)] (III), [{(PR3)2Pd}B3H7] (IV), [(h5-C5H5)Co(h4-C4H4)] (V), [{(h5-C5H5)Co}B4H8]
(VI), [(h5-C5H5)Fe(h5-C5H5)] (VII), [{(h5-C5H5)Fe}B5H10] (VIII), [(Cp*Ru)B8H14-
(RuCp*)] (IX), [Cp*Ru(C8H6)RuCp*] (X).
The pioneering studies of Hawthorne and Grimes on the
synthesis of metallacarboranes has paved the way for develop-
ment in the field of metallaborane chemistry. Successively, the
[a] V. P. Anju, S. K. Barik, B. Mondal, V. Ramkumar, Prof. S. Ghosh
Department of Chemistry
contributions of Fehlner,[4,6] Green,[18,19] Kennedy,[7,20] and
others[9,21,22] have made significant impacts in this field, which
has attracted significant attention owing to the uncommon
structures involved as well as their unique bonding, uneven
degrees of reactivity, and catalytic activity.[6,18–27] The earliest
synthesis methods for metallaboranes were thermolysis or
Indian Institute of Technology Madras
Chennai 600036 (India)
Fax:(+91)4422574202
E-mail: sghosh@iitm.ac.in
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201400013.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 546

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
a simple metathesis reactions between preformed polyborane
anions and transition-metal halides.[28] However, Fehlner et al.
later showed that the addition of monoboranes (LiBH4·thf,
BH3·thf, or BHCl2·SMe2) to monocyclopentadienyl metal chlor-
ides, [Cp*MCl]n, is an excellent method for the generation of
metallaboranes containing transition metals from Group 5 to
Group 9.[29]
Although the methods described above were found to be
suitable, and in a few instances, metal hydrides were discov-
ered to be acceptable precursors for metallaboranes,[30–35] the
reactions often led to various metallaboranes with low yields.
Thus, the greatest challenge for chemists is to find new precur-
sors for metallaboranes. Using the monocyclopentadienyl
metal chloride and monoborane reagents, we have formed
metallaboranes from Group 4 to Group 9.[36–42] Further, in the
search for suitable metal precursors, we found that
[Cp*MCl(CO)3] (M=Mo, W, Fe) are alternative precursors con-
taining the Cp*M fragment.[43] Metal carbonyl compounds have
been found to play a key role in organometallic as well as met-
allaborane chemistry. Here, we report the synthesis of arachno-
[(CO)8M2B2H6] obtained from the UV photolysis of a metal car-
bonyl [M2(CO)10] (M=Mn or Re) and BH3·thf. In addition, we
have succeeded in utilizing arachno-[(CO)8Re2B2H6]) as an
active catalyst for the cyclotrimerization of various terminal
and internal alkynes under mild conditions.
Figure 1. Molecular structure and labeling diagram for 1. Selected bond
lengths [ꢁ] and angles [8] are as follows: Re1ÀRe2 3.1104(9), B1ÀB2
1.7600(3), B1ÀRe1 2.3700(2), B1ÀRe2 2.500(19), B2ÀRe1 2.4200(2), B1-Re1-B2
43.1(7), B2-B1-Re2 120.5(14), Re1-B1-Re2 79.4(6), B1-B2-Re1 66.8(9).
The molecular structure of 1, shown in Figure 1, is very simi-
lar to [B4H10], and can best be described as a butterfly geome-
try in which one of the wing tips and hinges are replaced by
Re(CO)4 fragments. Although the ReÀRe bond length of 3.11 ꢁ
is longer than those in other rhenaborane clusters,[44–46] it is
analogous with the ReÀRe bond length of 3.04 ꢁ in
[Re2(CO)10].[47] It has been observed that the axial ReÀC dis-
tance of 1.92 ꢁ in [Re2(CO)10] is 0.058 ꢁ shorter than the aver-
age equatorial bond length of 1.98 ꢁ. In contrast, the axial ReÀ
C distance in 1 is longer than the equatorial bond length. The
Re1ÀB1 bond length of 2.3700(2) ꢁ is markedly shorter than
the Re1ÀB2 bond length of 2.500(19) ꢁ, as also evident in
most arachno structures.[48] Note that the UV photolysis of
[M2(CO)10] (M=Mn or Re) in the presence of phosphine or
phosphite ligands results in the formation of the disubstituted
[1,2-M2(CO)8L2] (L=PPh3). Similarly, the room-temperature pho-
tolysis of a n-hexane or toluene solution of [Re2(CO)10] in the
presence of excess 1-alkene yielded [(m-H)Re2(CO)8 (m-(h2-CH=
CHR))] (R=H, CH3, C2H5 or n-C4H7).[49] The alkenyl ligand
bridges the two metals, forming a s bond to one Re and a p
bond to the other. Compounds 1 and 2 may be considered as
direct structural analogues of [(m-H)Re2(CO)8(m-(h2-CH=CH2)].
The existence of compound 1 permits structural comparison
with other similar compounds of Ru, Ir, and Co without the dis-
ruption caused by additional metal fragments or ligands
(Table 1).
Results and Discussion
As shown in Scheme 2, the photolysis of [M2(CO)10] (M=Re,
Mn) and BH3·thf in n-hexane leads to the formation of arach-
no-[(CO)8M2B2H6] (1: M=Re; 2: M=Mn). The compositions and
structures of these compounds were established through X-ray
diffraction studies and further confirmed by mass spectral anal-
ysis and multinuclear NMR spectroscopy.
The spectroscopic data of 1 are consistent with its solid-
state X-ray structure. The mass spectroscopic data suggest
a molecular formula of [(CO)8Re2B2H6]. The 11B{1H} NMR spec-
Scheme 2. Synthesis of 1 and 2.
Table 1. Comparison of structural parameters of arachno-[(CO)8Re2B2H6], 1, and known dimetallatetraborane complexes.
Metallaborane
spe[a]
d[MÀM] [ꢁ]
davg[MÀB] [ꢁ]
d[BÀB] [ꢁ]
dihedral angle [8]
d [ppm] M-H-B, B-H-B
d [ppm] B1, B2
1
7
7
7
7
6
7
7
3.11
2.92
–
2.92
3.00
2.82
2.42
2.43
2.25
2.24
2.28
2.22
2.19
2.09
1.76
1.82
1.77
1.7
1.73
1.83
1.79
128.0
117.1
122.7
119.3
165.9
115.7
–
À4.41, À10.63
1.50, À12.71
6.46, À1.12
11.6, 13.5
[(Cp*RuCO)2B2H5(C6H4O2H)]
[Cp*Fe(CO)B3H8]
[(Cp*RuCO)2B2H6]
[(Cp*Ru)2(PMe3)B2H6]
[(Cp*IrH)2B2H6]
À1.87, À15.4
À0.5, À41.3
21.3, À3.0
41.5, À4.8
14.2, À14.6
24.2, 7.35
À3.73, À13.15
À2.82, À10.93, À8.45[b]
À2.75, À13.99
À5.51, À21.15
[(CpCo)2(PPh2)B2H5]
[a] Skeletal pair electron. [b] À2.82(B-H-B),À10.93(M-H-B), À8.45(M-H-B).
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 547

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
trum of 1 rationalizes the presence of two types of boron envi-
ronments in a 1:1 ratio at d=6.46 and À1.1 ppm. The chemical
shift at lower field is assigned to the boron that bridges two
basis sets (6–31 g*) for all the main-group elements and
LANL2DZ for the Re atom (Table S1). The DFT study shows that
there is a considerable bonding interaction in the HOMOÀ2 of
compound 1 (Figure 2). The Wiberg bond index (WBI)[55] of
0.36 (NBO analysis)[56] further confirms that the bonding inter-
action corresponds to the coupling of two Re centers.
1
rhenium atoms. In addition, the H{11B} NMR spectrum reveals
the presence of BÀHÀB, ReÀHÀB, and ReÀHÀRe protons, which
appear at d=À4.41, À10.63, and À17.20, respectively, in
1
a 1:1:1 ratio. The 11B and H NMR chemical shifts of 1 and 2 cal-
culated on the grounds of DFT by using the GIAO-DFT method
(Table S2 in the Supporting Information) corroborates the ex-
perimental values. The IR spectrum of compound 1 shows an
intense band at 1957 cmÀ1, which is characteristic of the termi-
nal carbonyl groups, and a band at 2403 cmÀ1 attributed to
the terminal BÀH stretches. The Mn analogue of 1 is highly
sensitive to air and moisture, and all our attempts to isolate di-
manganaborane selectively failed. As a result, the characteriza-
tion of 2 is based solely on its limited spectroscopic data. The
11B NMR spectrum of the reaction mixture displays two reso-
nances at d=31 and 11.42 ppm in a 1:1 ratio. The 1H NMR
chemical shifts at dÀ3.6, À10.2, and À20.55 ppm are assigned
to BÀHÀB, MnÀHÀB, and MnÀHÀMn protons, respectively. The
1H and 11B NMR chemical shifts show acceptable agreement
with the calculated values (Table S2).
Figure 2. Kohn–Sham orbitals describing ReÀRe bonding interactions in
HOMOÀ2 of 1.
The relative thermodynamic stabilities of 1, 1a, and 1b were
calculated to find out if the presence of CO ligands confers
any extra stability to 1. The HOMO–LUMO gap (Egap, 3.17 eV) of
compound 1 is significantly larger than that of the model com-
pounds 1a and 1b (Figure S1 and Table S3). Further, the first
vertical ionization potential (VIP) above 8 eV is consistent with
the observed high stability of 1 (Table S3). The high stability of
1 may be caused by the unique features of both s donation
and strong p acceptance of the carbonyl ligands present in the
rhenium centers.
In addition to the formation of 1, the UV photolysis of
[Re2(CO)10] with BH3·thf also yielded the known trirhenium hy-
dride [Re3(CO)14H], 3. According to earlier reports, compound 3
is accessible in good yield through the UV photolysis of
[Re(CO)5H]. It can also be prepared through the photochemical
reaction of dirhenium-docacarbonyl with thiophene and tri-
phenyl silane,[50–52] or the photolysis of [Re2(CO)10] in THF. Con-
firmation of the structure of 3 was obtained through single-
crystal X-ray structure determination and the reported spectro-
scopic data.
[2+2+2] Cycloaddition reactions of alkynes with arachno-
According to the skeletal electron-counting formalism, clus-
ter 1 is analogous to B4H10 (A) or [(m-H)Re2(CO)8(m-(h2-CH=
CHR))] having seven skeletal electron pairs ([3{Re(CO)4}ꢂ2+
2(BH)ꢂ1+3(BH2)ꢂ1+1(bridging H)ꢂ3/2=14 electrons]). The
geometries of [(Cp*RuCO)2B2H6] (C), [Cp*Fe(CO)B3H8] (D), and A
are very similar to that of 1. Interestingly, in C, the
[(Cp*RuCO)2(m-H)] fragment is bridged asymmetrically by
a B2H5 ligand, whereas in D, the BH2 group is replaced by
a [Cp*Fe(CO)] fragment. Iridaborane [(CpIr)2B2H8] also exhibits
a butterfly structure with two triangular wings sharing an
edge, that is, the body of the butterfly[53,54] (Scheme 3).
1 as catalyst
The classic development of a subarea of chemistry begins with
the discovery of a new synthetic method and proceeds
through structural improvement to a systematic examination
of the reactivity. Rhenium complexes are not usually very pop-
ular catalysts for organic synthesis except in a few cases such
as catalytic reactions with rhenium oxo and rhenium carbonyl
complexes.[57] A few accounts of metallaborane reactivity with
alkynes have been put forward. For example, 1) a facile reac-
tion occurs between compound [(Cp*RuH)2B3H7] and a variety
of substituted alkynes, leading a range of metallacarboranes,[58]
and 2) the reaction of [(Cp*Rh)2B3H7] or C with internal and ter-
minal alkynes leads to catalytic cyclotrimerization.[58,59] Com-
pound 1 is isoelectronic and isostructural with C, so we have
begun a systematic examination of the reaction chemistry of
this compound with alkynes.
A comprehensive computational study was performed to
probe the bonding and stability of 1 and the hypothetically
modeled
compounds
[(Cp*Re)2(CO)2B2H8]
1a
and
[(Cp*Re)2(CO)3B2H6] 1b (Figures S4 and S5). The geometry opti-
mizations were made with the BP86 functional and mixed
During our investigation, we rec-
ognized that compound 1 plays
an important role in the cyclotri-
merization of various terminal
and internal alkynes. The activi-
ties and selectivities for the cy-
clotrimerization of a variety of
Scheme 3. Arachno-B4H10 analogues [CpIrB3H9], [Cp*Fe(CO)B3H8], and [(Cp*RuCO)2B2H6].
terminal and internal alkynes are
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 548

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
the cyclotrimerization of alkynes by a homogenous organome-
tallic catalyst,[65] in which a metallacycle intermediate is pro-
posed by the consecutive addition and coupling of three al-
kynes into the metal coordination sphere. Recently, we report-
ed a detailed mechanism for the cyclotrimerization of alkynes
with ruthenaborane C as catalyst.[59] Compound C is isoelec-
tronic and isostructural with 1 and has the same number of
hydrogen atoms. Therefore, we believe that all the cyclotrime-
rization reactions reported here involve the initial formation of
a metallacycle intermediate and subsequent alkyne insertion.
Table 2. Cyclotrimerization of terminal and internal alkynes catalyzed by
compound 1.
Serial No. R1
R2
Cat./substr.[a] t [h] T [oC] Yield [%][b] Ratio[c]
1
2
3
4
5
6
7
8
9
Ph
Me
Ph
Ph
Me
H
1:60
1:50
1:70
5
30
21 60
30
25 65
60
Hpb[d]
–
1:3
–
n/c[h]
70
3
SiMe3 SiMe3 1:20
n/c[h]
85
40
45
35
n/c[h]
CO2Me
SiMe3
Ph
[g]
CH3
H
H
1:80
1:40
2
3
9
5
30
30
30
30
1:6
1:3
SiMe3 1:60
H
Ph
Tmsb[e]
1:2
1:10
1:15
22 60
Tmpb[f]
Conclusion
[a] Catalyst to substrate ratio, reaction time t, reaction temperature T,
total yield, ratio of 1,3,5- to 1,2,4-trisubstituted benzene. [b] Yields are
based on product formation as determined by 1H NMR spectroscopy.
[c] Single isomer was isolated. [d] Hexaphenylbenzene. [e] 1,2,4-Trimethyl-
The reaction of cyclopentadienyl metal polyhalides with mono-
borane reagents provides a route to a limited class of metalla-
boranes. In this report, we describe the synthesis of arachno-
[(CO)8Re2B2H6] 1 and arachno-[(CO)8Mn2B2H6] 2 through the UV
photolysis of [M2(CO)10] (M=Mn or Re) with BH3·thf. The use of
these alternative metal carbonyl precursors can overcome the
limitation of the monocyclopentadienyl metal-halide route for
the synthesis of metallaboranes. Studies of their reactivities
demonstrate that, just as organometallic chemistry provides
a lucid approach to the chemistry of the MÀC bond, so too
does metallaborane chemistry. In addition, we have shown
that rhenaborane 1 catalyzes the cyclotrimerization of both in-
ternal and terminal alkynes under mild conditions. In turn, this
fascinating structural and reaction chemistry will fuel interest
and lead to rapid progress in this field.
silyl-3,5,6-triphenyl-benzene.
[g] CH3CH2CH(SiMe3). [h] No cyclotrimerization.
[f] 1,2,4-Trimethyl-3,5,6-triphenyl-benzene.
listed in Table 2. It is evident that the catalytic activity is higher
for terminal than for internal alkynes, and that the presence of
electron-withdrawing groups enhances the reactivity of an
alkyne. The decreasing activities of 1 for RCꢀCSiMe3 (R=H
and Ph) imply the presence of a significant steric factor in the
reaction pathway. The isomer ratios were determined through
1H NMR measurements taken directly in the reaction mixture
after removal of unreacted alkyne and solvent. In all the above
the cases, the 1,2,4-isomer was found to be the major product.
Note that Takai and co-workers recently reported the regio-
selective synthesis of multisubstituted benzene derivatives
with the use of rhenium carbonyl as a catalyst.[60,61] Regardless
of the excellent regioselectivity, this method has some limita-
tions. It requires a high temperature and exceptionally long re-
action time, which narrows the variety of substrates. However,
the facile preparation of the four-vertex arachno-1 and its ther-
mal stability makes this system very promising for the catalytic
cyclotrimerization of alkynes. In contrast with other transition-
metal cyclotrimerization catalysts developed in our laborato-
ry,[59,62] compound 1 has been found to be very efficient, as the
reaction is very fast at room temperature.
Experimental Section
General procedures and instrumentation
All syntheses were performed under an argon atmosphere by
using standard Schlenk and glove-box techniques. Solvents were
dried through common methods and distilled under N2 before use.
Chemicals were obtained from Sigma Aldrich and used as received.
The external reference for 11B NMR, [Bu4N(B3H8)], was synthesized
according to the literature method.[66] Thin-layer chromatography
was performed on 250 mm diameter aluminum supported silica
gel TLC plates (MERCK TLC Plates). The NMR spectra were recorded
on a 400 MHz FT-NMR spectrometer. Residual solvent protons were
used as reference (d, ppm, [D6]benzene, 7.16), and a sealed tube
containing [Bu4N(B3H8)] in [D6]benzene (dB, ppm, À30.07) was used
as an external reference for the 11B NMR spectroscopy. Infrared
spectra were recorded on a Nicolet iS10 spectrometer. MALDI-TOF
mass spectra of the compounds were obtained on a Bruker Ultra-
flextreme with 1,2-dihydroxybenzene as a matrix and a ground
steel target plate.
It was demonstrated earlier by Decker and Klobukowski that
the dissociation of CO is the rate-determining step for alkyne
cyclotrimerization with a carbonyl-based catalyst.[63,64] There-
fore, DFT computational methods were used to understand the
catalytic activity of 1. We calculated the bond dissociation en-
ergies (BDEs) for both 1 and the parent [Re2(CO)10]. The first
CO BDE value for 1 was found to be 14.7 kcalmolÀ1 lower than
that of [Re2(CO)10], which might mean 1 shows better catalytic
activity than [Re2(CO)10]. This was confirmed by the reaction of
1 with various internal and terminal alkynes in the presence of
CO gas. No traces of cyclotrimerized products were observed
in any of these cases. Further, no cyclotrimerization was ob-
served upon using [Re2(CO)10] and other labile species such as
[Re2(CO)8(PPh3)2], [Cp*Re(CO)3], and [Re3(CO)14H] at room tem-
perature, so we believe that the active species in the cyclotri-
merization process may be a metallaborane species. Note that
Schore and others have proposed a common mechanism for
Synthesis of arachno-1
In a flame-dried Schlenk tube, [Re2(CO)10] (0.03 g, 0.045 mmol) and
BH3·thf (1 mL, 1 mmol) in n-hexane (10 mL) were irradiated for
1.5 h. The volatile components were removed under vacuum, and
the remaining residue was extracted into n-hexane and passed
through celite. After removal of the solvent, the residue was sub-
jected to chromatographic workup using silica gel TLC plates. Elu-
tion with n-hexane/CH2Cl2 (95:5 v/v) yielded very pale yellow
1 (0.010 g, 0.01 mmol, 10%). Mass calcd for 12C8 H611B216O8186Re2:
1
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 549

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
624.16; found: 625.18; 11B NMR (228C, 128 MHz, CDCl3): d=6.46 (s,
Theoretical calculations
1
1B), À1.12 ppm (s 1B); H NMR (228C, 400 MHz, CDCl3): d=7.81 (B-
Quantum chemical calculations were performed on compound 1
and model compounds 1a and 1b by using density functional
theory (DFT) as implemented in the Gaussian09[68] program pack-
age. All the compounds were fully optimized in the gaseous state
(no solvent effect) without any symmetry constraints with the
BP86[69] functional paired with the LANL2DZ effective core poten-
tial (ECP)[70] basis set for the Re atom and 6–31 g* for C, B, O, and
H atoms. The optimized geometries are in true minima in the PES
diagram. Stationary points were confirmed through analytic com-
putation of harmonic force constants at the aforementioned DFT
level. The NMR chemical shifts were calculated by using the BP86/
(LANL2DZ,6-31g*) optimized geometries. Computation of the NMR
shielding tensors employed gauge-including atomic orbitals
(GIAOs),[71,72] using the implementation of Schreckenbach, Wolff,
Ziegler, and co-workers.[71–77] The projected 11B chemical shielding
values determined were referenced to B2H6 (BP86/6–31 g* B shield-
ing constant 93.5 ppm) as the primary reference point, and these
chemical shift values (d) were then converted to the standard
BF3·OEt2 scale by using the experimental value of +16.6 ppm for
B2H6.
Ht), À4.41 (s, 1H, B-H-B), À10.63 (s, 1H, Re-H-B), À17.20 ppm (s,
1H, Re-H-Re); 13C NMR (228C, 100 MHz, CDCl3): d=188.3, 183 ppm
(CO); IR: n˜ =2403 (B-Ht), 1957, 2002 cmÀ1 (CO); elemental analysis
calcd for C8O8Re2B2H6: C 15.37, H 0.97; found: C 14.90, H 1.2.
Synthesis of arachno-2
In a flame-dried Schlenk tube, [Mn2(CO)10] (0.03 g, 0.079 mmol) and
BH3·thf (1 mL, 1 mmol) in n-hexane (10 mL) were irradiated for
1.5 h. The volatile components were removed under vacuum and
the remaining residue was extracted into n-hexane and passed
through celite. 11B NMR (228C, 128 MHz, CDCl3): d=31 (s, 1B),
1
11.42 ppm (s, 1B); H NMR (228C, 400 MHz, CDCl3): d=7.62 (B-Ht),
À3.6 (s, 1H, B-H-B), À10.2 (s, 1H, Mn-H-B), À20.55 ppm (s, 1H, Mn-
H-Mn); 13C NMR (228C, 100 MHz, CDCl3): d=205.6, 199.3 ppm (CO);
IR: n˜ =2434 (B-Ht), 1980, 2014 cmÀ1 (CO).
General procedure for catalytic process
Acknowledgements
Catalytic data for the alkyne cyclotrimerization are shown in
Table 2. All the alkynes were purchased from Sigma Aldrich and
used as received. In a typical reaction, 1 (0.015 g) was dissolved in
n-hexane (12 mL), and alkynes were added to the resulting solu-
tion. The reaction mixture was stirred at room temperature for 2–
5 h (Table 2). The solvent was evaporated in vacuo. The residue
was extracted into n-hexane and passed through celite. After re-
moval of the solvent from the filtrate, the residue was subjected to
chromatography by using silica gel TLC plates. Elution with n-
hexane/CH2Cl2 yielded the corresponding benzene derivatives.
This study was supported by the Department of Atomic Energy,
Board of Research in Nuclear Sciences, BRNS (Project No. 2011/
37C/54/BRNS), BARC, Trombay, Mumbai, India.
Keywords: boron
· cyclotrimerization · density functional
calculations · metallaborane · rhenium
[1] C. Elschenbroich, A. Salzer, Organometallics VCH, New York, 1989.
[2] R. A. Love, T. F. Koetzle, G. J. B. Williams, L. C. Andrews, R. Bau, Inorg.
Chem. 1975, 14, 2653–2657.
[3] S. T. Frey, M. L. Gong, W. D. Horrocks, Jr., Inorg. Chem. 1994, 33, 3229–
3234.
Note that the formation of the cyclotrimerized products and the
ratios of the isomers were confirmed by 1 H and 13C NMR spectros-
copy and mass spectrometry. The yield of the cyclotrimerized prod-
1
ucts was determined by H NMR spectroscopy by employing a ref-
[4] T. P. Fehlner, Organometallics 2000, 19, 2643–2651.
[5] T. P. Fehlner, Pure Appl. Chem. 2006, 78, 1323–1331.
[6] a) T. P. Fehlner, J.-F. Halet, J.-Y. Saillard, Molecular Clusters. A Bridge to
Solid-State Chemistry, Cambridge University Press, Cambridge, 2007;
b) T. P. Fehlner, Electron Deficient Boron and Carbon Clusters, Wiley, New
York, 1991.
erence compound of known weight. The concentration of the ana-
lyte in the reaction mixture was determined by comparing the
areas of the analyte and reference peaks (ignoring the decomposi-
tion of the reference).
[7] a) J. D. Kennedy, Prog. Inorg. Chem. 1984, 32, 519–679; b) C. E. House-
croft in Organometallic Chemistry (Ed.: T. P. Fehlner), Plenum, New York,
1992, pp. 73; c) H. Yan, A. M. Beatty, T. P. Fehlner, Organometallics 2002,
21, 5029–5037; d) A. ꢃlvarez, R. Macꢄas, M. J. Fabra, F. J. Lahoz, L. A. Oro,
J. Am. Chem. Soc. 2008, 130, 2148–2149.
[8] a) C. E. Housecroft, Boranes and Metalloboranes: Structure, Bonding and
Reactivity, Ellis Horwood, Chichester, 1990, pp. 119; b) C. E. Housecroft,
Adv. Organomet. Chem. 1991, 33, 1–50; c) D. F. Gaines in Recent Advan-
ces in the Chemistry of Pentaborane (Eds.: R. W. Parry, G. Kodama), Perga-
mon, Oxford, 1980; d) L. Barton, D. K. Srivastava in Comprehensive Or-
ganometallic Chemistry II, Vol. 1 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkin-
son), Pergamon, Oxford, 1995, pp. 275–372.
[9] S. K. Bose, K. Geetharani, V. Ramkumar, S. M. Mobin, S. Ghosh, Chem.
Eur. J. 2009, 15, 13483–13490.
[10] T. J. Coffy, G. Medford, J. Plotkin, G. J. Long, J. C. Huffman, S. G. Shore,
Organometallics 1989, 8, 2404–2409.
[11] L. J. Guggenberger, A. R. Kane, E. L. Muetterties, J. Am. Chem. Soc. 1972,
94, 5665–5673.
[12] T. L. Venable, E. Sinn, R. N. Grimes, J. Chem. Soc. Dalton Trans. 1984,
2275–2279.
X-ray structure determination
The crystal data for 1 were collected and integrated by using
a Bruker Axs kappa apex2 CCD diffractometer, with graphite mono-
chromated MoKa (l=0.71073 ꢁ) radiation at 173 K. The structures
were solved by heavy-atom methods by using SHELXS-97 or SIR92
and refined with SHELXL-97.[67]
CCDC 952638 (1) 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.
Crystal data for 1: C8H6B2O8Re2; Mr =624.15; monoclinic; P21/n; a=
6.5766(4), b=27.5492(16), c=8.5288(4) ꢁ; b=109.889(3)8; V=
1453.08(14) ꢁ3; Z=4; 1calcd =2.853 cmÀ3; m=16.673 mmÀ1; F(000)=
1112 ; R1 =0.0552; wR2 =0.1162; independent reflections 2558 [2qꢁ
50.008]; 181 parameters.
[13] R. Weiss, R. N. Grimes, J. Am. Chem. Soc. 1977, 99, 8087–8088.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 550

CHEMPLUSCHEM
FULL PAPERS
www.chempluschem.org
[14] S. Ghosh, B. C. Noll, T. P. Fehlner, Angew. Chem. 2005, 117, 6726–6729;
Angew. Chem. Int. Ed. 2005, 44, 6568–6571.
[45] S. Ghosh, M. Shang, Y. Li, T. P. Fehlner, Angew. Chem. 2001, 113, 1159–
1162; Angew. Chem. Int. Ed. 2001, 40, 1125–1128.
[15] R. Hoffmann, Angew. Chem. 1982, 94, 725–739; Angew. Chem. Int. Ed.
Engl. 1982, 21, 711–724.
[46] S. Ghosh, A. L. Rheingold, T. P. Fehlner, Chem. Commun. 2001, 895–896.
[47] M. R. Churchill, K. N. Amoh, H. J. Wasserman, Inorg. Chem. 1981, 20,
1609–1611.
[48] R. Ahmad, J. E. Crook, N. N. Greenwood, J. D. Kennedy, W. S. McDonald,
J. Chem. Soc. Chem. Commun. 1982, 1019–1020.
[49] P. O. Nubel, T. L. Brown, J. Am. Chem. Soc. 1982, 104, 4955–4957.
[50] a) W. Fellmann, H. D. Kaesz, Inorg. Nucl. Chem. Lett. 1966, 2, 63–67;
b) J. K. Hoyano, W. A. G. Graham, Inorg. Chem. 1972, 11, 1265–1269.
[51] C. S. Yang, C. P. Cheng, L. W. Guo, Y. Wang, J. Chin. Chem. Soc. 1985, 32,
17–22.
[16] R. G. Woolley, Inorg. Chem. 1985, 24, 3525–3532.
[17] T. P. Fehlner, J. Organomet. Chem. 2009, 694, 1671–1677.
[18] H. J. Bullick, P. D. Grebenik, M. L. H. Green, A. K. Hughes, J. B. Leach, P.
Mountford, J. Chem. Soc. Dalton Trans. 1994, 3337–3342.
[19] H. J. Bullick, P. D. Grebenik, M. L. H. Green, A. K. Hughes, J. B. Leach, P. C.
McGowan, J. Chem. Soc. Dalton Trans. 1995, 67–75.
[20] J. D. Kennedy, Prog. Inorg. Chem. 1986, 34, 211–434.
[21] N. N. Greenwood, I. M. Ward, Chem. Soc. Rev. 1974, 3, 231–271.
[22] D. M. P. Mingos, D. J. Wales, Introduction to Cluster Chemistry, Prentice
Hall, Englewood, 1990.
[52] S. Lotz, M. Landman, A. J. Olivier, D. I. Bezuidenhout, D. C. Liles, E. R.
Palmer, Dalton Trans. 2011, 40, 9394–9403.
[23] a) R. N. Grimes, Acc. Chem. Res. 1978, 11, 420–427; b) R. N. Grimes in
Metal Interactions with Boron clusters (Ed.: R. N. Grimes), Plenum, New
York, 1982, pp. 269–319; c) R. N. Grimes, Pure Appl. Chem. 1982, 54,
43–58.
[53] X. Lei, A. K. Bandyopadhyay, M. Shang, T. P. Fehlner, Organometallics
1999, 18, 2294–2296.
[54] X. Lei, M. Shang, T. P. Fehlner, Chem. Eur. J. 2000, 6, 2653–2664.
[55] K. Wiberg, Tetrahedron 1968, 24, 1083–1096.
[24] C. E. Housecroft, Coord. Chem. Rev. 1995, 143, 297–330.
[25] J. D. Kennedy in Disobedient Skeletons (Ed.: J. Casanova), Wiley, New
York, 1998.
[26] D. F. Gaines in Recent Advances in the Chemistry of Pentaborane, Vol. 9
(Eds.: R. W. Parry, G. Kodama), Pergamon, Oxford, 1980.
[27] R. N. Grimes in Comprehensive Organometallic Chemistry II, Vol. 1 (Eds.:
E. Abel, F. G. A. Stone, G. Wilkinson), Pergamon, New York, 1995, Chap-
ter 9.
[28] C. E. Housecroft, T. P. Fehlner, Adv. Organomet. Chem. 1982, 21, 57–112.
[29] a) T. P. Fehlner, J. Chem. Soc. Dalton Trans. 1998, 1525–1532.
[30] a) M. A. Peldo, A. M. Beatty, T. P. Fehlner, Organometallics 2002, 21,
2821–2823; b) M. A. Peldo, A. M. Beatty, T. P. Fehlner, Organometallics
2003, 22, 3698–3702.
[56] A. E. Reed, L. A. Curtiss, F. Weinhold, Chem. Rev. 1988, 88, 899–926.
[57] a) W. A. Herrmann, J. Organomet. Chem. 1995, 500, 149–173; b) W. A.
Herrmann, F. E. Kꢅhn, Acc. Chem. Res. 1997, 30, 169–180; c) J. H. Espen-
son, Adv. Inorg. Chem. 2003, 54, 157–202; d) F. E. Kꢅhn, A. M. Santos,
W. A. Herrmann, Dalton Trans. 2005, 2483–2491; e) H. Kusama, K. J. Nar-
asaka, J. Synth. Org. Chem. Jpn. 1996, 54, 644–653; f) R. Hua, J.-L. Jiang,
Curr. Org. Synth. 2007, 4, 151–174.
[58] H. Yan, A. M. Beatty, T. P. Fehlner, Angew. Chem. 2001, 113, 4630–4633;
Angew. Chem. Int. Ed. 2001, 40, 4498–4501.
[59] K. Geetharani, S. Tussupbayev, J. Borowka, M. C. Holthausen, S. Ghosh,
Chem. Eur. J. 2012, 18, 8482–8489.
[60] Y. Kuninobu, T. Iwanaga, M. Nishi, K. Takai, Chem. Lett. 2010, 39, 894–
895.
[31] P. D. Grebenik, J. B. Leach, M. L. H. Green, N. M. Walker, J. Organomet.
Chem. 1988, 345, C31–C34.
[32] P. D. Grebenik, M. L. H. Green, M. A. Kelland, J. B. Leach, P. Mountford, G.
Stringer, N. M. Walker, L. L. Wong, J. Chem. Soc. Chem. Commun. 1988,
799–801.
[61] a) A. Kawata, Y. Kuninobu, K. Takai, Chem. Lett. 2009, 38, 836–837; b) Y.
Kuninobu, M. Nishi, A. Kawata, H. Takata, Y. Hanatani, S. Yudha, A. Iwai,
K. Takai, J. Org. Chem. 2010, 75, 334–341.
[62] V. P. Anju, D. K. Roy, R. S. Anju, S. Ghosh, J. Organomet. Chem. 2013, 733,
79–81.
[33] J. F. Hartwig, S. R. De Gala, J. Am. Chem. Soc. 1994, 116, 3661–3662.
[34] S. Ghosh, A. M. Beatty, T. P. Fehlner, Czech. Collect. Chem. Commun.
2002, 67, 808–812.
[35] M. A. Peldo, M. Shang, T. P. Fehlner, J. Organomet. Chem. 2000, 609, 77–
82.
[36] a) S. K. Bose, K. Geetharani, B. Varghese, S. M. Mobin, S. Ghosh, Chem.
Eur. J. 2008, 14, 9058–9064; b) D. K. Roy, S. K. Bose, K. Geetharani,
K. K. V. Chakrahari, S. M. Mobin, S. Ghosh, Chem. Eur. J. 2012, 18, 9983–
9991.
[63] J. Pearson, J. Cooke, J. Takats, R. B. Jordan, J. Am. Chem. Soc. 1998, 120,
1434–1440.
[64] S. A. Decker, M. Klobukowski, J. Am. Chem. Soc. 1998, 120, 9342–9355.
[65] N. E. Schore, Chem. Rev. 1988, 88, 1081–1119.
[66] G. E. Ryschkewitsch, K. C. Nainan, Inorg. Synth. 1974, 15, 113–114.
[67] a) G. M. Sheldrick, SHELXS-97, University of Gçttingen, Gçttingen (Ger-
many), 1997; b) G. M. Sheldrick, SHELXL-97, University of Gçttingen,
Gçttingen (Germany), 1997; c) A. Altomare, G. Cascarano, C. Giacovazzo,
A. Guagliardi, J. Appl. Crystallogr. 1993, 26, 343–350.
[37] a) S. K. Bose, K. Geetharani, V. Ramkumar, B. Varghese, S. Ghosh, Inorg.
Chem. 2010, 49, 2881–2888; b) S. K. Bose, K. Geetharani, B. Varghese, S.
Ghosh, Inorg. Chem. 2010, 49, 6375–6377.
[38] R. S. Dhayal, K. K. V. Chakrahari, B. Varghese, S. M. Mobin, S. Ghosh,
Inorg. Chem. 2010, 49, 7741–7747.
[68] Gaussian 09, Revision B.01, M. J. Frisch, et al. Gaussian, Inc., Wallingford
CT, 2010. See the Supporting Information of the full citation.
[69] a) H. L. Schmider, A. D. Becke, J. Chem. Phys. 1998, 108, 9624–9631;
b) J. P. Perdew, Phys. Rev. B 1986, 33, 8822–8824.
[70] D. Andrae, U. Hꢆußermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim.
Acta 1990, 77, 123–141.
[39] S. Sahoo, S. M. Mobin, S. Ghosh, J. Organomet. Chem. 2010, 695, 945–
949.
[71] F. London, J. Phys. Radium. 1937, 8, 397–409.
[40] K. Geetharani, S. K. Bose, B. Varghese, S. Ghosh, Chem. Eur. J. 2010, 16,
11357–11366.
[72] K. Wolinski, J. F. Hinton, P. Pulay, J. Am. Chem. Soc. 1990, 112, 8251–
8260.
[41] a) S. Ghosh, T. P. Fehlner, B. C. Noll, Chem. Commun. 2005, 3080–3082;
b) S. Ghosh, X. Lei, M. Shang, T. P. Fehlner, Inorg. Chem. 2000, 39, 5373–
5382.
[42] a) S. K. Bose, S. Ghosh, Organometallics 2007, 26, 5377–5385; b) K. Gee-
tharani, S. K. Bose, S. Sahoo, S. Ghosh, Angew. Chem. 2011, 123, 3994–
3997; Angew. Chem. Int. Ed. 2011, 50, 3908–3911.
[73] G. Schreckenbach, T. Ziegler, J. Phys. Chem. 1995, 99, 606–611.
[74] G. Schreckenbach, T. Ziegler, Int. J. Quantum Chem. 1997, 61, 899–918.
[75] G. Schreckenbach, T. Ziegler, Int. J. Quantum Chem. 1996, 60, 753–766.
[76] S. K. Wolff, T. Ziegler, J. Chem. Phys. 1998, 109, 895–905.
[77] S. K. Wolff, T. Ziegler, E. Van Lenthe, E. J. Baerends, J. Chem. Phys. 1999,
110, 7689–7698.
[43] K. Geetharani, S. K. Bose, G. Pramanik, T. K. Saha, V. Ramkumar, S. Ghosh,
Eur. J. Inorg. Chem. 2009, 1483–1487.
Received: January 8, 2014
[44] S. Ghosh, X. Lie, C. L. Cahill, T. P. Fehlner, Angew. Chem. 2000, 112,
3022–3024; Angew. Chem. Int. Ed. 2000, 39, 2900–2902.
Published online on February 6, 2014
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 546 – 551 551