Synthesis, characterization, and preliminary fluorescence study of a mixed-ligand bis(dicarbollyl)nickel complex bearing a tryptophan-BODIPY FRET couple

Article abstract of DOI:10.1016/j.jorganchem.2015.04.035

In continuation of our work on nickelacarborane-based nanomolecular devices, the design and synthesis of a bis(dicarbollyl)nickel complex, in both formal Ni(III) and Ni(IV) oxidation states, bearing two fluorophore molecules capable of fluorescence resona

Full text of DOI:10.1016/j.jorganchem.2015.04.035

Accepted Manuscript
Synthesis, Characterization, and Preliminary Fluorescence Study of a Mixed-ligand
Bis(dicarbollyl)nickel Complex Bearing a Tryptophan-BODIPY FRET Couple
Natalia I. Shlyakhtina, Alexander V. Safronov, Yulia V. Sevryugina, Satish S. Jalisatgi,
M. Frederick Hawthorne
PII:
S0022-328X(15)00247-8
10.1016/j.jorganchem.2015.04.035
JOM 19021
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 27 March 2015
Revised Date: 17 April 2015
Accepted Date: 18 April 2015
Please cite this article as: N.I. Shlyakhtina, A.V. Safronov, Y.V. Sevryugina, S.S. Jalisatgi, M.F.
Hawthorne, Synthesis, Characterization, and Preliminary Fluorescence Study of a Mixed-ligand
Bis(dicarbollyl)nickel Complex Bearing a Tryptophan-BODIPY FRET Couple, Journal of Organometallic
Chemistry (2015), doi: 10.1016/j.jorganchem.2015.04.035.
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Graphical Abstract Synopsis
The design and synthesis of bis(dicarbollyl)nickel complexes bearing two fluorophore molecules
capable of fluorescence resonance energy transfer was performed. The FRET couple with R0 of
approximately 26 Å consisted of tryptophan and BODIPY. The target compounds were prepared
via a multistep organic/organometallic synthesis and their fluorescence spectra were studied.

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Synthesis, Characterization, and Preliminary Fluorescence Study of
Bis(dicarbollyl)nickel Complex Bearing a Tryptophan-BODIPY FRET Couple
a
Mixed-ligand
Natalia I. Shlyakhtina,a Alexander V. Safronov,a Yulia V. Sevryugina,b Satish S. Jalisatgi,a M. Frederick
Hawthornea,*
aInternational Institute of Nano and Molecular Medicine, School of Medicine, University of Missouri-
Columbia, 1514 Research Park Drive, Columbia, MO 65211, USA.
bDepartment of Chemistry, Texas Christian University, Fort Worth, TX 76129, USA.
*Corresponding author. Fax 1-573-884-6900. E-mail: hawthornem@missouri.edu
Abstract
In continuation of our work on nickelacarborane-based nanomolecular devices, the design and
synthesis of a bis(dicarbollyl)nickel complex, in both formal Ni(III) and Ni(IV) oxidation states, bearing
two fluorophore molecules capable of fluorescence resonance energy transfer (FRET) was performed.
The FRET couple with a small Förster radius (R0 of 26 Å) consisted of tryptophan and 4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene (BODIPY). Each fluorophore was connected to the nickelacarborane core by
a rigid linker containing two pairs of alternating ethynyl and para-phenylene groups, which created a
specific distance (l) between the fluorophore molecules depending on the conformation of the
nickelacarborane core. The presence (13 Å < l < 39 Å) or absence (l > 39 Å) of energy transfer in the
designed system provides insight into the conformational changes of nickelacarboranes in solution. The
target nickelacarboranes were prepared via a multistep organic/organometallic synthesis. The structures
and compositions of the intermediates and final products were established by a combination of
spectroscopic methods and X-ray structure analysis. A preliminary fluorescence study of the prepared
nickelacarboranes was performed.
Keywords: Nickelacarborane; Molecular motor; Synthesis; Fluorescence; BODIPY; FRET.
1. Introduction
Transition metal metallacarborane π-complexes were first discovered in the mid-1960s [1]. Since
that time, most of the research in the area has been concentrated on the synthesis of new types of
complexes [2] and their application in homogeneous catalysis [3]. However, when the general interest in
homogeneous catalysis research began to decline at the end of the 1990s, medicinal applications of
transition metal metallacarboranes started to emerge. A number of potent HIV inhibitors [4], nitric oxide

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synthase inhibitors [5], new boron neutron capture therapy reagents [6], PET-CT [7] and Raman
spectroscopy [8] imaging agents containing metallacarborane complexes of various transition metals were
discovered. During the explosive development of nanoscience and nanotechnology in the 2000s, it was
postulated that their unique capability of producing complexes with redox-controlled rotational motion
might allow some transition metals metallacarboranes to find applications in nanoelectronics and
nanomachine engineering [9].
A key step toward the application of metallacarboranes in nanotechnology is the visualization of
their controllable conformational interconversion in solutions and, eventually, on a surface. Fluorescence
spectroscopy is a very powerful tool permitting such conformational assignments in solution. Among
fluorescence spectroscopy techniques, fluorescence resonance energy transfer (FRET) is extensively used
in biology for structural and dynamical studies of proteins [10]. FRET occurs between two fluorescent
molecules in solution, and its efficiency depends on many parameters. The most important parameter is
the degree of overlap of the emission spectrum of the donor and the absorption spectrum of the acceptor.
For the energy transfer to occur, the distance l between the donor and acceptor molecules must obey the
Förster equation, 0.5 R0 < l < 1.5 R0, where R0 is the Förster radius, the distance at which fluorescence
energy transfer occurs with 50% probability. In practice, FRET does not occur at distances of l > 1.5 R0.
Besides for spectral overlap and the appropriate distance, the relative orientations of the fluorophores in
space is also very important.
Recently, it was demonstrated that tryptophan and a BODIPY dye derivative, N-(4,4-difluoro-5,7-
dimethyl-4-bora-3a,4adiaza-s-indacene-3-yl)methyl iodoacetamide, form a FRET couple with an R0 value
of approximately 26 Å [11]. During the folding study of the two-state protein S6 (from Thermus
thermophilus), this small value of R0 allowed the investigators to probe intramolecular distances ranging
between 17 and 34 Å with an error of less than 10%. The tryptophan/BODIPY couple appeared to be very
attractive for small-molecule FRET measurements, such as conformational assignments in
nickelacarboranes. The distances determined for the tryptophan/BODIPY couple within an effective
FRET range are easily achievable in metallacarborane chemistry. The conformational changes of the
nickelacarborane core during oxidation-reduction processes would provide the necessary distance
changes between the dye molecules, and the presence or absence of the FRET effect would theoretically
allow for the conformational assignment of the metallacarborane moiety.
Herein, the design, synthesis, characterization, and preliminary fluorescence study of Ni(III) and
Ni(IV) nickelacarborane complexes bearing tryptophan/BODIPY FRET couple connected to the
metallacarborane core by rigid linkers are reported.

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2. Results and discussion
2.1. Design of the target compounds
To facilitate the synthesis of the target products, N-boc-tryptophan and 8-(4′-bromophenyl)-4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene were chosen as the FRET donor and acceptor, respectively. It was
decided that the introduction of the tryptophan fluorophore into the molecule could be achieved by an
esterification reaction such the amino-group of tryptophan was protected to avoid the formation of any
by-products. The introduction of a brominated phenyl ring into the BODIPY molecule was necessary to
facilitate attachment of the molecule to the complex via cross-coupling.
To ensure that the fluorescent molecules form a FRET couple with a Förster radius of
approximately 26 Å, their absorption and fluorescence behavior was studied. As seen from Figure 1, the
spectroscopic behavior of both dye molecules is as expected. N-boc-tryptophan showed absorption and
emission maxima at 280 and 326 nm, respectively. The BODIPY dye molecule revealed two absorption
maxima at 354 and 505 nm and one emission maximum at 531 nm. The fluorescence band of N-boc-
tryptophan shows a significant overlap with the lower wavelength absorption band of 8-(4′-
bromophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene. Using the recorded spectra of the compounds
and Photochemcad 2.1 software [12], the Förster radius R0 of this FRET couple in CH2Cl2 was calculated
to be 25.5 Å, which is very close to the published data [11].
Figure 1. Normalized absorption (solid black line) and fluorescence (dashed black line) spectra of N-boc-
tryptophan and absorption (solid grey line) and fluorescence (dashed grey line) spectra of 8-(4′-
bromophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (10-4 M, CH2Cl2, 293 K).

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During the design of the complex, four main requirements were formulated for the linkers
connecting the dye molecules to the nickelacarborane core:
1) Rigidity. The linkers must provide a constant distance between each carborane cage of the
nickelacarborane complex and the fluorescent dye molecule.
2) Length. The length of the linkers must provide a distance of 12 Å < l < 38 Å (0.5 R0 < l < 1.5 R0)
between the dye molecules in the cis conformation of the nickelacarborane [Ni(IV)] and a distance of at
least 38 Å, or more than 1.5 R0, in the trans conformation [Ni(III)].
3) Synthesis. The complexes containing the linkers satisfying the two previous conditions must not be
extremely difficult to prepare.
4) Spectroscopic properties. The absorption and fluorescence of the linkers must not interfere with the
absorption and fluorescence of the attached dye molecules.
The first two requirements were relatively easy to satisfy. Indeed, alternating para-phenylene and
ethynyl fragments could provide the linkers with the necessary rigidity. Geometry optimization [13] of
the suggested structure with two alternating ethynyl and two para-phenylene linkers (Figure 2) showed
that the distance between the dye molecules would be approximately 45 Å and 22 Å in the trans and cis
conformations of nickelacarborane, respectively, which lie within the required distance range.
Synthetically, the attachment of such linkers to the carborane cage could be achieved via Pd-
catalyzed cross-coupling reactions. In Figure 2, the linkers are positioned on atoms B(6) and B(6′) of the
nickelacarborane core because the starting material for this attachment (i.e., 3-iodo-1,2-dicarba-closo-
dodecaborane) is easily accessible and the symmetry of the products avoids any stereoisomerism-related
problems.
Figure 2. Schematic representation of the target nickelacarborane complexes.

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Although the synthesis and characterization of oligo- and poly(phenylacetylenes) has been the
subject of multiple studies [14], spectroscopic information of lower-molecular-weight species is not
readily available. Thus, the decision regarding the applicability of the conjugated (phenylethynyl) linkers
in the system under consideration was based on the information available for 1,2-diphenylacetylene. It
was demonstrated that this compound’s absorption and fluorescence bands appear at approximately 300
nm; however, the fluorescence quantum yield was determined to be 0.003 [15]. Theoretically, if the
linkers absorb some of the excitation energy, their contribution to the fluorescence will be minimal.
The results of the preliminary spectroscopic studies and calculations were encouraging, and based
on these results, the synthesis of the target nickelacarboranes was initiated.
2.2. Synthesis of the target nickelacarboranes
Initially, the preparation of two nido-carboranes was envisioned, each bearing a dye molecule
attached to the cage through the linker at position B(3). Next, an equimolar mixture of these two
compounds would be used in the reaction with Ni(acac)2 in the presence of an appropriate base. This
reaction would yield a mixture of three complexes: a mixed-ligand target compound and two symmetrical
nickelacarboranes. The separation of the mixed-ligand complex from the by-products was to be achieved
chromatographically.
The synthesis of the closo-carborane complexes began from the preparation of the mono-protected
1,4-diethynylbenzene 2 (Scheme 1). The organozinc derivative of 2 prepared by deprotonation of the free
ethynyl group was cross-coupled with 3-iodo-1,2-dicarba-closo-dodecaborane (3) in the presence of the
Pd(PPh3)4 catalyst. Compound 4 was isolated in very good yield and characterized by a variety of
spectroscopic techniques (see Experimental).
Scheme 1. Synthesis of protected closo-carborane 4.

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The protective group was removed using TBAF at -70 °C to avoid deboronation of the carborane
cage. The product 5 (Scheme 2) was introduced into a cross-coupling reaction with 8-(4′-bromophenyl)-
4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 6 in the presence of N,N-diisopropylethylamine (DIPEA) and
Pd(PPh3)4 catalyst according to the Sonogashira protocol. Compound 7 was isolated chromatographically
1
and characterized by multinuclear NMR spectroscopy. The H NMR spectrum of 7 showed the presence
of the ortho-carborane fragment (characteristic broad singlet at 3.78 ppm and a broad multiplet in the
range 2.9 – 1.6 ppm, which correspond to the cage C–H and B–H resonances, respectively), two para-
phenylene groups of the linker and the BODIPY fragment. In the 11B NMR spectrum of 7, a 2:1:1:2:4
pattern in the 0 – (-20) ppm range characteristic of 3-substituted closo-ortho-carboranes was observed,
along with an additional triplet signal at 0.1 ppm (JB–F 28 Hz) from the BF2-group of the BODIPY. In the
19F{1H} NMR spectrum of 7, a characteristic quartet (JF–11B 28 Hz) overlapping with a lower intensity
septet (JF–10B 7.5 Hz) was observed at -145 ppm. HRMS data confirmed the composition of the
compound.
Scheme 2. Synthesis of closo-carborane 7.
We were able to obtain single crystals of 7 and study its structure by X-ray diffraction (Figure 3).
The structure of 7 was found to contain a closo ortho-carborane fragment substituted at position B(3). The
substituent contained two alternating ethynyl and two para-phenylene fragments. The para-position of
the terminal phenyl ring was connected to a BODIPY dye consisting of two pyrrole rings connected
through a carbon atom in the α-positions and coordinated to a BF2-fragment via the nitrogen atoms of the
rings. The interatomic distances within the carborane, the linker and the dye were in the normal range.
The distance between the substituted cage boron atom B(3) and the boron atom B(13) of the BF2 fragment

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of the dye was determined to range from 18.092 to 18.390 Å for different crystallographically unique
units (molecules) of 7.
Figure 3. ORTEP representation of 7, drawn at the 40% probability level. Only one of three crystallographically
unique molecules is shown.
Unfortunately, attempts to synthesize nido-carborane 8 from closo-carborane 7 in satisfactory
yields were unsuccessful (Scheme 3). Standard methods [16] such as the use of NaOH/EtOH or
TBAF/THF led to the decomposition of the BODIPY fragment. Attempts to use other systems, such as
NaCN/EtOH, KF(dry)/18-crown-6/DCM, KF(wet)/18-crown-6/DCM, CsF/TBABr/DCM, or Deoxo-
Fluor/DCM, were unsuccessful and resulted in only trace amounts of the target nido-carborane 8.
Scheme 3. Synthesis of nido-carborane 8.

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Based on the obtained results, the synthetic approach was modified slightly. Instead of
constructing the target nickelacarborane from the nido-carborane compounds bearing fluorescent dyes
with subsequent separation of the nickelacarborane mixture, the revised approach involved first preparing
and isolating the mixed ligand nickelacarborane complex bearing modified linkers and then attaching the
dye molecules at the end of the process.
The first nido-carborane was prepared by deboronation of the closo-carborane 4 in mild conditions
[17] to avoid TIPS-deprotection (Scheme 4). Similar to the spectra of 4, the 1H NMR and 13C{1H} spectra
of compound 9 contained characteristic signals of the TIPS group (see Experimental). The 11B NMR
spectrum of 9 contained characteristic nido-carborane 2:2:1:2:1:1 intensity pattern in the range 0 – (-40)
ppm as well as a unique non-splitting resonance at -18.2 ppm, which corresponds to the substituted boron
atom B(3). HRMS data confirmed the suggested composition of 9.
Scheme 4. Synthesis of nido-carborane 9.
For the preparation of the second nido-carborane, the closo-carborane 5 containing a terminal
acetylene functionality was introduced into a cross-coupling reaction with ethyl 4-bromobenzoate (10) to
produce closo-carborane 11 (Scheme 5). LiAlH4 in THF reduced the ester group in the latter compound
1
with the formation of the corresponding benzyl alcohol in the target closo-carborane 12. In the H NMR
spectrum of 12, three groups of signals were observed. The ortho-carborane fragment revealed a
characteristic broad singlet at 3.76 ppm and a broad multiplet in the range 2.9 – 1.7 ppm corresponding to
the cage C–H and B–H resonances, respectively. The signals of the linker were observed in the aromatic
region as two pairs of doublets of the two para-phenylene groups. The benzyl alcohol showed two signals
11
at 4.73 and 1.69 ppm from the benzyl CH2-group and OH-group, respectively. The B NMR spectrum
revealed a standard 2:1:1:3:3 signal pattern for 3-substituted ortho-carboranes with a non-splitting
resonance of the substituted boron atom B(3) at -11.2 ppm. HRMS data confirmed the suggested
composition of 12.
Scheme 5. Synthesis of closo-carborane 12.

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Closo-carborane 12 was deboronated under standard conditions using TBAF in THF at reflux
temperature, and the product 13 was isolated in excellent yield by water treatment of the evaporated
reaction mixture and filtration of the precipitate (Scheme 6). Product 13 was characterized based on
multinuclear NMR spectroscopy and HRMS data (see Experimental).
Scheme 6. Synthesis of nido-carborane 13.
The synthesis of the mixed-ligand Ni(III) nickelacarborane 14 was achieved by the reaction of the
equimolar mixture of the tetrabutylammonium salts of nido-carboranes 9 and 13 with excess butyllithium
and then with Ni(acac)2 in THF (Scheme 7). The separation of nickelacarborane mixtures can often be
very difficult and generally results in poor yields of the individual compounds. However, the presence of
the hydroxyl group facilitated the separation process for the mixture of complexes 14–16. After addition
of butyllithium, the hydroxyl group is deprotonated along with nido-carborane anions. After the addition
of Ni(acac)2, all three complexes initially form dianionic Ni(II) compounds. Therefore, complex 14 exists
in the reaction mixture as a trianion and complexes 15 and 16 as a di- and tetra-anions, respectively. The
charge of these species determines their solubility in THF: complex 15 appeared to be the most soluble

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and complex 16 the least soluble, whereas complex 14 had limited solubility. Removing the supernatant
from the heterogeneous reaction mixture and then washing the precipitate with a small amount of
anhydrous THF achieved the initial separation. The supernatant and the THF wash contained complex 15
and a small amount of complex 14, whereas the precipitate contained only complexes 14 and 16. After air
oxidation and methanol quenching, pairs of complexes were separated chromatographically without
complications. In the next step, compound 14 was deprotected by TBAF in THF to form the paramagnetic
Ni(III) complex 17 in excellent yield (Scheme 7).
Scheme 7. Synthesis of mixed-ligand nickelacarboranes.
In the final step, the heterodisubstituted nickelacarborane 17 was functionalized by the two
components of the FRET couple. First, N-boc-tryptophan (18) was esterified by the benzyl alcohol group
of 17 under mild conditions using BOP (Castro’s reagent) in the presence of triethylamine in
dichloromethane in 76% yield (Scheme 8). Second, the isolated and purified complex 19 was introduced
into the cross-coupling reaction with 8-(4′-bromophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 6 in
the presence of Pd(PPh3)4 catalyst according to the Sonogashira protocol. Although compound 20 is
1
paramagnetic, it was characterized by multinuclear NMR spectroscopy and HRMS. In the H NMR
spectrum of 20, all signals from linkers and dyes were observed except for the two pairs of protons of the
phenyl rings closest to the metallacarborane core. Assignment of the signals in the 1H and 13C{1H} NMR
1
1
11
spectra of 20 was based on the H-1H COSY and H-13C HMQC spectra (see Experimental). In the B
NMR spectrum of 20, along with a set of very broad resonances of nido-carboranes in the range 20 – (-
60) ppm, a characteristic triplet resonance of the BF2-group was observed at 0.1 ppm. A quartet resonance

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at -145.2 ppm (JF–B 28 Hz) was also observed in the 19F{1H} spectrum of 20. The composition of the
compounds was also confirmed by the HRMS data.
Scheme 8. Synthesis of the target Ni(III) nickelacarborane 20.
Oxidation of nickelacarboranes from formal Ni(III) to Ni(IV) oxidation states is usually achieved
by the treatment of the former with ferric chloride in polar organic solvents, such as acetonitrile or
methanol [18]. However, all attempts to quantitatively oxidize complex 20 by ferric chloride led to its
decomposition. Good yields of 21 were achieved by air oxidation of solutions of 20 in dichloromethane
(Scheme 9) with subsequent chromatographic purification of the product 21 (see Experimental).
Scheme 9. Synthesis of the target Ni(IV) nickelacarborane 21.

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The NMR spectra of 20 and 21 differ only slightly. For example, all resonances from the para-
phenylene fragments of the linkers were observed in the normal range for aromatic protons (7 – 8 ppm) in
the 1H and 13C{1H} NMR spectra of 21. Additionally, both the 1H and 13C{1H} NMR spectra of 21 lacked
signals corresponding to the TBA counter ion. In the 11B NMR spectrum, resonances of 21 were observed
in a more compact manner; for the Ni(IV) compounds, these resonances were found in the characteristic
range 20 – (-20) ppm. The chemical shifts and 11B–19F spin coupling constants of the signals
corresponding to the BF2-fragment persisted both in the 11B and 19F{1H} NMR spectra of 21 (see
Experimental). HRMS data for 21 clearly confirmed the suggested composition of the compound.
2.3. Preliminary fluorescence study of the nickelacarboranes 20 and 21
The absorption and fluorescence spectra of complexes 20 and 21 were studied. Figure 4 shows the
absorption spectra of complexes 20 and 21 in CH2Cl2 at room temperature. The spectra reveal
characteristic absorption bands for all parts of the molecules. The broad band centered at approximately
300 nm corresponds to the absorption of tryptophan and the linker in the case of compound 20 and
tryptophan, the linker, and the Ni(IV) nickelacarborane core in case of compound 21. A broad band in the
350 – 400 nm region corresponds to the first absorption of BODIPY in the case of 21 and BODIPY and
the Ni(III) nickelacarborane core in the case of 20. Both spectra show a band at approximately 500 nm,
which corresponds to the second absorption of BODIPY.

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Figure 4. Absorption spectra of complexes 20 (solid line) and 21 (dashed line) in CH2Cl2 (1×10-5 M, 293
K).
Although the absorption band of tryptophan cannot be clearly distinguished in the spectra of 20
and 21, the absorption wavelength of free tryptophan at 280 nm was chosen for the fluorescence study.
The fluorescence spectra of complexes 20 and 21 are shown in Figure 5. In the emission spectrum of the
formal Ni(III) complex 20, the intense BODIPY fluorescence band was centered at 532 nm. Two
additional bands were present in the spectrum: a lower intensity band at 345 nm, which was interpreted as
the residual fluorescence of tryptophan, and a broad emission in the range 450 – 500 nm, which was
difficult to assign. The emission spectrum of the Ni(IV) complex 21 showed only two bands. The more
intense band centered at 359 nm can also be assigned as the residual fluorescence of tryptophan, and the
band at 532 nm is assigned to BODIPY. The presence of the BODIPY emission band at 532 nm in the
spectra of both complexes can only be explained by fluorescent resonance energy transfer in solution for
both the Ni(III) and Ni(IV) compounds of the studied system.

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Figure 5. Fluorescence spectra of complexes 20 (solid line) and 21 (dashed line) in CH2Cl2 (7.5×10-5 M,
293 K, λex 280 nm).
Two possible explanations for the observed emission spectra can be suggested. First, the Ni(III)
compound 20 may exist in solution mostly as an intermediate gauche conformation between the cis and
trans conformations. Recently, it was demonstrated [19] that bis(dicarbollyl) complexes of Ni(III) bearing
massive substituents in positions B(6) and B(6′) exist in acetonitrile solutions at room temperature as
almost equimolar mixtures of trans and gauche conformers. If the same is true for the solution of 20, then
the distance between the components of the FRET couple could be less than 38 Å, which would result in
fluorescent resonance energy transfer between the fluorophores. Second, energy transfer between the
fluorophores in the Ni(III) compound may occur through the linkers and the nickelacarborane core. A
similar energy transfer was previously observed in a bichromophoric complex containing two different
iridium-based fluorescent dyes connected to the cage carbon atoms of para-carborane through 1,4-
diethynylbenzene linkers [20]. This type of energy transfer could theoretically occur for the Ni(IV)
complex 21 if it competes with the FRET process.
Although conclusions regarding the conformations of the nickelacarborane core cannot be drawn
directly from the preliminary spectroscopic study of complexes 20 and 21, the prepared nickelacarborane

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complexes appear to be very attractive for further fluorescence studies using various solvents and
temperatures and as a subject of molecular orbital calculations. The system design can also be varied to
study the effect of the nature of the linkers and metal on the spectroscopic behavior of the compounds.
3. Conclusions
Two bichromophoric nickelacarborane complexes containing a tryptophan/BODIPY FRET couple
attached to a metallacarborane core by rigid linkers were modeled. The compounds were successfully
prepared via a multistep organic/organometallic synthesis and characterized by various analytical
techniques. The initial spectroscopic study showed that fluorescent resonance energy transfer occurs for
both Ni(III) and Ni(IV) compounds, hindering their unambiguous conformational assignment.
4. Experimental
4.1. Materials
All reactions were performed in an argon atmosphere using standard Schlenk-line and glove-box
techniques. Triisopropylsilyl chloride (TIPSCl), 1,4-diethynylbenzene, anhydrous ZnBr2, n-butyllithium
(BuLi, 2.5 M solution in hexane), copper(I) iodide (CuI), nickel acetylacetonate [Ni(acac)2], di-tert-butyl
dicarbonate (Boc2O), sodium cyanide (NaCN), tetrabutylammonium bromide (TBABr), lithium
aluminum hydride (LiAlH4), triethylamine (Et3N), N,N-diisopropylethylamine (DIPEA) were purchased
from Aldrich and used as received. (Benzotriazol-1-yloxy)tris(dimethylamino)phosphonium
hexafluorophosphate (BOP) was purchased from Fluka. Potassium carbonate (K2CO3) and sodium
hydroxide (NaOH) were purchased from Fisher. Ethyl 4-bromobenzoate, L(-)-tryptophan, boron
trifluoride etherate (BF3·Et2O) were purchased from Acros Organics and used as received.
Tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] was purchased from Strem Chemicals.
Tetrabutylammonium fluoride (TBAF) was purchased from TCI America as a 1 M solution in THF.
Tetrahydrofuran (THF) was distilled in an argon atmosphere from sodium benzophenone ketyl
before use. Dichloromethane (CH2Cl2), toluene, triethylamine, and DIPEA were distilled in an argon
atmosphere over CaH2 prior to use. Column chromatography was performed in air using Sorbtech silica
gel (60 Å, 63–200 ꢀm). Thin-layer chromatography was performed on Merck pre-coated glass plates
(silica 60 F254) using a palladium stain solution for spot development.
((4-Ethynylphenyl)ethynyl)triisopropylsilane (2) was prepared according to a previously
published procedure [21]. 3-Iodo-1,2-dicarba-closo-dodecaborane (3) was prepared according to a
previously published procedure [22] and azeotropically dried with benzene prior to use. 8-(4′-
Bromophenyl)-4,4-difluoro-4-bora-3a,4a-diaza-s-indacene 6 was prepared according to a previously

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published procedure [23]. N-boc-Tryptophan was also prepared according to a previously published
procedure [24].
4.2. Physical measurements
1
11
13
The H, B, 11B{1H}, and C NMR spectra and all 2D NMR spectra were recorded on Bruker
1
13
Avance-400 and Avance-500 NMR spectrometers. H NMR and C{1H} NMR spectra were referenced
to the residual solvent peak. Boron NMR spectra were referenced to 15% BF3ꢁEt2O in CDCl3 set as 0
ppm. Fluorine NMR spectra were referenced to CFCl3 set as 0 ppm. Chemical shifts are reported in ppm
and coupling constants in Hz. Mass spectra were obtained on an ABI QSTAR and Mariner
Biospectrometry Workstation by PerSeptive Biosystems. IR spectra, UV-Vis spectra, and fluorescence
spectra were recorded on a Nicolet Nexus 470 FT-IR spectrometer, Varian Cary 50 UV-Vis
spectrophotometer, and Varian Cary Eclipse fluorescence spectrophotometer, respectively. Melting points
were measured in sealed capillaries using an SRS OptiMelt apparatus.
4.3. Descriptions of syntheses and compound characterization
4.3.1. Synthesis of closo-carborane 4
To a cooled (-70 °C) solution of 2 (0.355 g, 1.25 mmol) in 5 mL of THF was added 0.5 mL (0.2 mmol) of
2.5 M BuLi in hexane, and the reaction mixture was stirred at -70 °C for 30 min. A solution of anhydrous
ZnBr2 (0.324 g, 1.44 mmol) in 2 mL of THF was added to the reaction mixture, and the mixture was then
stirred at room temperature for 15 min. Next, a solution of 3 (0.260 g, 0.96 mmol) and Pd(PPh3)4 (55.0
mg, 4.8×10-5mol) in 3 mL of THF was added to the reaction mixture, and the resulting solution was kept
at reflux temperature for 20 h. The reaction mixture was quenched with water and extracted with EtOAc
(3×5 mL). The combined organic layers were dried over Na2SO4 and evaporated. Purification of the
product was achieved by flash chromatography in hexane to give 4 (0.278 g, 79%) as a pale-yellow glassy
1
solid. H NMR (400 MHz, CDCl3): δ 7.41 (m, 4H, C6H4), 3.76 (br s, 2H, Ccb–H), 2.94–1.53 (m, 9H, B–
11
H), 1.13 (br s+m, 21H, TIPS). B NMR (128 MHz, CDCl3): δ -2.6 (d, J 151, 2B), -8.7 (d, J 159, 1B), -
13
11.4 (1B), -12.7 (d, J 159, 3B), -13.5 (3B). C{1H} (100 MHz, CDCl3): δ 132.35, 132.32, 124.8, 121.8,
106.6, 94.1, 57.6 (Ccb–H), 19.0 (TIPS), 11.7 (TIPS). HRMS (TIS–): m/z 424.4505 [M]– (calcd for
C21H36B10Si 424.3589). Rf = 0.25 (hexanes–EtOAc, 5:1). IR (KBr, cm–1): 3071 (Ccb–H), 2938, 2890, 2863
(C–H), 2604 (br. BH), 2197, 2154 (C≡C).
4.3.2. Synthesis of closo-carborane 5

ACCEPTED MANUSCRIPT
17
To a solution of 4 (0.500 g, 1.17 mmol) in 10 mL of THF was added 1 M TBAF in THF (1.17 mL, 1.17
mmol) at -70 °C. The reaction mixture was allowed to warm to room temperature for 30 min, and then 20
mL of water was added. The reaction mixture was extracted with ether (3×10 mL). Combined ethereal
layers were dried over Na2SO4 and evaporated. The residue was treated by column chromatography using
an EtOAc/hexane mixture (10:90) as the eluent. After evaporation and drying in vacuum, compound 5
1
was obtained as a light-brown solid (0.301 g, 96%). H NMR (500 MHz, CDCl3): δ 7.46–7.42 (m, 4H,
C6H4), 3.76 (br s, 2H, Ccb–H), 3.19 (s, 1H, ≡C–H), 2.80–1.75 (m, 9H, B–H). 11B NMR (160 MHz,
CDCl3): δ -2.4 (d, J 149, 2B), -8.5 (d, J 152, 1B), -11.1 [s, 1B, B(3)], -12.5 (d, J 167, 2B), -13.5 (d, 4B).
13C{1H} (125 MHz, CDCl3): δ 132.45, 132.44, 123.4, 122.5, 83.2, 79.9, 57.5 (Ccb–H). HRMS (APCI–):
m/z 268.2034 [M]– (calcd for C12H16B10 268.2256). m.p. 126–128 °C. Rf = 0.50 (hexanes–EtOAc, 10:1).
IR (KBr, cm–1): 3285 (≡C–H), 3065 (Ccb–H), 2922 (C–H), 2652, 2611, 2581, 2572, 2558 (B–H), 2194,
2101 (C≡C).
4.3.3. Synthesis of closo-carborane 7
A mixture of 5 (0.200 g, 0.746 mmol), 6 (0.258 g, 0.746 mmol), CuI (0.007 g, 0.037 mmol), Pd(PPh3)4
(0.043 g, 0.037 mmol), and DIPEA (0.195 mL, 1.12 mmol) in 10 mL of anhydrous toluene was stirred at
40 °C for 20 h. Next, the reaction mixture was partitioned between CH2Cl2 (50 mL) and 1 M NaHSO4 (15
mL). The dichloromethane layer was dried over Na2SO4 and co-evaporated with 10 mL of silica.
Treatment of the reaction mixture by column chromatography using an EtOAc/hexane mixture (15:85) as
the eluent gave after evaporation and drying in vacuum compound 7 (0.271 g, 68%) as a deep-red solid.
1H NMR (400 MHz, CDCl3): δ 7.95 (br s, 2H, BODIPY-pyrrole), 7.66 (d, 2H, J 8.2, linker-C6H4), 7.58
(d, 2H, J 8.4, linker-C6H4), 7.52 (d, 2H, J 8.6, linker-C6H4), 7.50 (d, 2H, J 8.6, linker-C6H4), 6.93 (d, 2H,
J 4.1, BODIPY-pyrrole), 6.56 (dd, 2H, J1 4.3, J2 1.4, BODIPY-pyrrole), 3.78 (s, 2H, Ccb–H), 2.94-1.61
(m, 9H, B–H). 11B NMR (128 MHz, CDCl3): δ 0.12 (t, 1B, JB–F 28, BF2), -2.6 (d, 2B, J 152), -8.5 (d, 1B,
J 152), -11.1 [s, 1B, B(3)], -12.7 (d, J 163, 2B), -13.1 (d, 4B). {1H} (100 MHz, CDCl3): δ 146.4
(BODIPY-pyrrole), 144.5 (linker-C6H4), 134.8 (BODIPY-pyrrole), 133.9, 132.4, 131.8, 131.7 (linker-
C6H4), 131.5 (BODIPY-pyrrole), 130.7, 125.8, 123.7, 122.2 (linker-C6H4), 118.8 (BODIPY-pyrrole),
91.6, 90.8 (–C≡C–), 57.4 (Ccb–H). 19F{1H} NMR (376 MHz, CDCl3): δ -145.0 (q, JF–B 28). HRMS (ES–):
m/z 533.3600 [M-H]– (calcd for C27H25B11F2N2 534.3082). m.p. 260–262 °C (decomp.). Rf = 0.45
(hexanes–EtOAc, 7:3). IR (KBr, cm–1): 3070 (Ccb–H), 2959, 2923, 2852 (C–H), 2596 (B–H), 2197
(C≡C), 1557 (B–F).
4.3.4. Synthesis of nido-carborane 9

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18
To a solution of 4 (2.3 g, 5.42 mmol) in 50 mL of EtOH was added NaCN (1.3 g, 27.12 mmol), and the
reaction mixture was kept at reflux temperature for 20 h. The solvent was removed under reduced
pressure, and the residue was dissolved in 5 mL of water, after which TBABr (2.27 g, 7.05 mmol) was
added. The precipitate formed was filtered on a fine-porosity glass filter and washed with water (3×5 mL)
and Et2O (3×5 mL) to give after drying under vacuum over P2O5 the product 9 (3.26 g, 92%) as a white
1
solid. H NMR (400 MHz, acetone-d6): δ 7.46–7.43 (m, 2H, C6H4), 7.39–7.37 (m, 2H, C6H4), 3.49 (m,
8H, TBA), 2.59–(-0.29) (m, 8H, B–H), 1.88 (br m, 10H, Ccb–H+TBA), 1.48 (m, 8H, TBA), 1.18 (s+m,
3
11
21H, TIPS), 1.02 (t, 12H, J 7.2, TBA), -2.62 (br m, 1H, B–H–B). B NMR (128 MHz, acetone-d6): δ -
10.6 (d, J 142, 2B), -16.3 (d, J 139, 2B), -18.2 [s, 1B, B(3)], -21.3 (d, J 151, 2B), -33.7 (br d, J 135, 1B), -
36.9 (d, J 144, 1B). 13C{1H} (100 MHz, acetone-d6): δ 133.3, 133.2, 127.2, 123.5, 108.7, 92.9, 60.2
(TBA), 47.6 (Ccb–H), 25.2 (TBA), 21.1 (TBA), 19.7 (TIPS), 14.6 (TBA), 12.8 (TIPS). HRMS (TIS–): m/z
414.3400 [M+H]– (calcd for C21H35B9Si 413.3381). IR (KBr, cm–1): 3034 (Ccb–H), 2961, 2942, 2866 (C–
H), 2523 (B–H), 2153 (C≡C).
4.3.5. Synthesis of closo-carborane 11
To a solution of 5 (2.45 g, 9.17 mmol) in 50 mL of THF at -70 °C was added 3.7 mL (9.25 mmol) of 2.5
M BuLi in hexane, and the reaction mixture was stirred at -70 °C for 30 min. Anhydrous ZnBr2 (2.48 g,
11.0 mmol) was dissolved in 20 mL of THF and added dropwise to the reaction mixture at -70 °C. The
reaction mixture was stirred at -70 °C for 15 min and then allowed to warm to room temperature. A
solution of ethyl 4-bromobenzoate (10, 2.73 g, 11.9 mmol) and Pd(PPh3)4 (0.529 g, 0.450 mmol) in 25
mL of THF was added to the reaction mixture, and the resulting solution was kept at reflux temperature
for 24 h. The reaction mixture was quenched with water and extracted with EtOAc (3×20 mL). The
combined organic layers were dried over Na2SO4 and evaporated. The compound was purified by column
chromatography on silica gel using an EtOAc gradient (0→7%) in hexane. After drying in vacuum,
compound 11 (2.93 g, 77%) was isolated as a pale-yellow crystalline solid. 1H NMR (400 MHz, CDCl3):
δ 8.03 (m, 2H, C6H4), 7.58 (m, 2H, C6H4), 7.51-7.46 (m, 4H, C6H4), 4.39 (q, 2H, 3J 7.2, CH2-O), 3.76 (br
s, 2H, Ccb–H), 2.93–1.78 (m, 9H, B–H), 1.40 (t, 3H, 3J 6.8, CH3). 11B NMR (128 MHz, CDCl3): δ -2.4 (d,
13
J 157, 2B), -8.6 (d, J 155, 1B), -11.3 [s, 1B, B(3)], -12.6 (d, J 168, 3B), -13.7 (3B). C{1H} (100 MHz,
CDCl3): δ 166.4 (O–C=O), 132.5, 132.0, 131.8, 130.5, 129.9, 127.7, 124.0, 122.3, 91.8, 91.4, 61.5 (CH2-
O), 57.6 (Ccb–H), 14.72 (CH3). HRMS (APCI-): m/z 416.2234 [M]– (calcd for C21H24B10O2 416.2779).
m.p. 188–190 °C. Rf = 0.50 (hexanes–EtOAc, 5:1). IR (KBr, cm–1): 3043 (Ccb–H), 2980 (C–H), 2591 (B–
H), 2195 (C≡C), 1703 (C=O).
4.3.6. Synthesis of closo-carborane 12

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To a suspension of LiAlH4 (0.32 g, 8.45 mmol) in 100 mL of THF at 0 °C was added a solution of 11
(2.93 g. 7.04 mmol) in 50 mL of THF, and the suspension formed was stirred at room temperature for 1.5
h. The reaction mixture was quenched by water (dropwise, slowly) at 0 °C until gas evolution ceased, and
the mixture was then passed through a SiO2/Na2SO4/celite plug. After evaporation, the compound was
isolated by column chromatography on silica gel using an EtOAc/hexane mixture (20:80) as the eluent.
After evaporation and drying in vacuum, compound 12 (2.51 g, 95%) was obtained as a pale-yellow solid.
3
1H NMR (400 MHz, CDCl3): δ 7.53–7.44 (m, 6H, C6H4), 7.36 (m, 2H, C6H4), 4.73 (d, 2H, J 6.0,
CH2Ph), 3.76 (br s, 2H, Ccb–H), 2.95–1.70 (m, 9H, B–H), 1.69 (t, 1H, 3J 6.0, –OH). 11B NMR (128 MHz,
CDCl3): δ -2.5 (d, J 149, 2B), -8.6 (d, J 149, 1B), -11.2 (s, 1B), -12.6 (d, J 159, 3B), -13.5 (3B). 13C{1H}
(100 MHz, CDCl3): δ 141.7, 132.5, 132.1, 131.8, 127.2, 124.6, 122.4, 121.8, 92.1, 89.1, 65.2, 57.6 (Ccb–
H). HRMS (APCI–): m/z 374.1841 [M]– (calcd for C19H22B10O 374.2673). m.p. 188–190 °C. Rf = 0.20
(hexanes–EtOAc, 5:1). IR (KBr, cm–1): 3446 (OH), 3069 (Ccb–H), 2960, 2931, 2872 (C–H), 2528 (B–H),
2211 (C≡C).
4.3.7. Synthesis of nido-carborane 13
To a solution of 12 (0.500 g, 1.33 mmol) in 4 mL of THF was added 8 mL of a 1 M TBAF in THF (8.00
mmol), and the reaction mixture was kept at reflux temperature for 2 h. The solvent was removed under
reduced pressure, and 5 mL of water was added to the residue. The precipitate formed was filtered on a
fine-porosity glass filter and washed with water (3×5 mL) and Et2O (3×5 mL) to give after drying under
1
vacuum over P2O5 the product 13 (0.790 g, 97%) as a white solid. H NMR (400 MHz, acetone-d6): δ
7.54 (m, 2H, C6H4), 7.50 (m, 2H, C6H4), 7.45–7.41 (m, 4H, C6H4), 4.69 (s, 2H, Ph–CH2), 3.49 (m, 8H,
3
TBA), 2.15–0.33 (m, 8H, B–H), 1.93 (br m, 10H, Ccb–H+TBA), 1.47 (m, 8H, TBA), 1.02 (t, 12H, J 7.6,
TBA), -2.62 (br m, 1H, B–H–B). 11B NMR (128 MHz, acetone-d6): δ -10.6 (d, J 133, 2B), -16.3 (d, J 143,
13
2B), -18.2 [s, 1B, B(3)], -21.3 (d, J 154, 2B), -33.7 (br d, 1B), -36.9 (d, J 143, 1B). C{1H} (100 MHz,
acetone-d6): δ 145.0, 133.3, 132.94, 132.90, 128.2, 126.9, 123.6, 123.0, 92.3, 90.2, 64.9, 60.2 (TBA), 47.5
(Ccb–H), 25.2 (TBA), 21.1 (TBA), 14.6 (TBA). HRMS (TIS–): m/z 364.2235 [M]– (calcd for C19H22B9O
364.2544). IR (KBr, cm–1): 3446 (OH), 2960, 2931, 2872 (C–H), 2548 (B–H), 2211 (C≡C).
4.3.8. Synthesis of mixed-ligand nickelacarborane 14
A mixture of compounds 9 (0.156 g, 0.230 mmol) and 13 (0.145 g, 0.230 mmol) was azeotropically dried
with 50 mL of benzene. After removing the remaining benzene under vacuum, the resulting mixture was
dissolved in 50 mL of THF, and 0.29 mL (0.725 mmol) of 2.5 M BuLi in hexane was added at 0°C. After
2 h, a solution of Ni(acac)2 (0.073 g, 0.280 mmol) in 20 mL of THF was added, and the reaction mixture

ACCEPTED MANUSCRIPT
20
was stirred at room temperature for 16 h. The supernatant, which contained compound 15 (by MS) almost
exclusively, was separated from the reaction mixture via a syringe. The precipitate containing (by MS) a
mixture of complexes 14 and 16 was re-dissolved in methanol and co-evaporated with 5 mL of silica. The
mixture was treated by column chromatography using a CHCl3 gradient (0→80%) in hexane. After
evaporation and drying in vacuum, compound 14 (0.085 g, 33%) was isolated as an orange-brown
paramagnetic solid. HRMS (TIS–): m/z 833.4525 [M]– (calcd for C40H56B18NiOSi 833.5237). m.p. 228–
230 °C. Rf = 0.55 (CHCl3–MeOH, 10:1). IR (KBr, cm–1): 3422 (OH), 2957, 2924, 2864 (C–H), 2553 (B–
H), 2260, 2154 (C≡C).
4.3.9. Synthesis of nickelacarborane 17
To a solution of 14 (0.121 g, 0.112 mmol) in 2.5 mL of THF was added 0.5 mL of a 1 M TBAF in THF
(0.45 mmol), and the reaction mixture was stirred at room temperature for 4 h. The reaction mixture was
quenched with water (5 mL) and extracted with CH2Cl2 (3×5 mL). Combined organic extracts were dried
over Na2SO4 and evaporated to dryness. The residue was washed with hexane (3×3 mL) to remove TIPS
fluoride. After drying under vacuum, compound 17 (0.095 g, 92%) was isolated as a brown paramagnetic
solid. HRMS (TIS–): m/z 678.3210 [M+H]– (calcd for C31H36B18NiO 677.3903). m.p. 215–216 °C (with
decomposition). Rf = 0.5 (CHCl3–MeOH, 10:1). IR (KBr, cm–1): 3430 (OH), 3281 (≡C–H), 3037 (Ccb–H),
2959, 2924, 2853 (C–H), 2562 (B–H), 2187 (C≡C).
4.3.10. Synthesis of nickelacarborane 19
To a mixture of 17 (0.091 g, 0.099 mmol), 18 (0.033 g, 0.108 mmol), and Et3N (0.015 g, 0.148 mmol) in
4 mL of CH2Cl2 was added BOP (0.066 g, 0.148 mmol) at 0 °C. The reaction mixture was then stirred at
room temperature for 16 h. The reaction mixture was washed with water (6×3 mL), dried over Na2SO4,
and co-evaporated with 2 mL of silica before being subjected to column chromatography using a CHCl3
gradient (0→100%) in hexane followed by a MeOH gradient (0→5%) in CHCl3. After evaporation and
drying in vacuum, compound 19 (0.090 g, 76%) was isolated as a brown paramagnetic solid. HRMS
(ESI–): m/z 964.4032 [M]– (calcd for C47H54B18N2NiO4 964.5224). m.p. 247–249 °C (with
decomposition). Rf = 0.35 (CHCl3–MeOH, 10:1). IR (KBr, cm–1): 3425 (NH), 3121 (≡C–H), 3043 (Ccb–
H), 2961, 2930, 2871 (C–H), 2563 (B–H), 2185 (C≡C).
4.3.11.Synthesis of nickelacarborane 20
A mixture of 19 (61.0 mg, 0.050 mmol), 6 (17.5 mg, 0.050 mmol), CuI (0.2 mg, 0.0025 mmol), Pd(PPh3)4
(28.0 mg, 0.0025 mmol), and DIPEA (0.013 mL, 0.075 mmol) in 3 mL of anhydrous toluene was stirred

ACCEPTED MANUSCRIPT
21
at 40 °C for 20 h. Next, the reaction mixture was partitioned between CH2Cl2 (15 mL) and 1 M NaHSO4
(3 mL). The dichloromethane layer was dried over Na2SO4 and co-evaporated with 1 mL of silica.
Treatment of the reaction mixture by column chromatography using a CHCl3 gradient (0→100%) in
hexane followed by a MeOH gradient (0→10%) in CHCl3 gave, after evaporation and drying in vacuum,
compound 20 (11.4 mg, 15%) as a red solid. 1H NMR (400 MHz, CD2Cl2): δ 8.48 (br s, 2H, linker-C6H4),
8.44 (br s, 2H, linker-C6H4), 8.18 (br s, 1H, Trp-indole-NH), 7.93 (br s, 2H, BODIPY-pyrrole), 7.67 (d,
2H, J 8.4, linker-C6H4), 7.61 (d, 2H, J 7.9, linker-C6H4), 7.55 (d, 1H, J 8.0, Trp-C6H4), 7.38 (d, 3H, J 8.0,
linker-C6H4+Trp-C6H4), 7.28 (d, 2H, J 8.0, linker-C6H4), 7.20 (t, 1H, J 7.5, Trp-C6H4), 7.11 (t, 1H, J 7.5,
Trp-C6H4), 7.01 (d, 2H, J 3.9, BODIPY-pyrrole), 6.92 (s, 1H, Trp-indole-CH), 6.61 (d, 2H, J 3.3,
BODIPY-pyrrole), 5.81 (br s, 4H, Ccb–H), 5.18 (s, 2H, OCH2), 5.10 (br s, 1H, Trp-NH-boc), 4.65 (m, 1H,
Trp-CH–C=O), 3.29 (d, 2H, J 5.7, Trp-CH2), 3.15 (br m, 8H, TBA), 1.6–0.7 (m, 18H, B-H),1.68 (br m,
8H, TBA), 1.49 (br m, 8H, TBA), 1.42 (br s, 9H, t-Bu), 1.03 (br s, 12H, TBA). 11B NMR (128 MHz,
CD2Cl2): δ 11.7, 0.1 (t, JB–F 28, BF2), -19.3, -51.3. 13C{1H} (100 MHz, CD2Cl2): δ 173.2(C=O), 172.8
(C=O), 147.1 (BODIPY-pyrrole), 145.0, 137.4, 137.0, 135.8, 135.1, 134.0, 133.9, 132.3 (BODIPY-
pyrrole), 131.0, 128.5, 126.5 (linker-C6H4), 123.8, 122.9, 120.3 (Trp-C6H4), 119.4 (Trp-C6H4+BODIPY-
pyrrole), 112.0 (Trp-C6H4), 78.4, 78.1, 77.8, 67.0 (OCH2), 60.6 (TBA), 55.2, 30.50, 28.8 (t-Bu), 27.9
19
(Trp-CH2), 25.0 (TBA), 20.8 (TBA), 14.4 (TBA). F{1H} NMR (376 MHz, CD2Cl2): δ -145.19 (q, JF–B
28, BF2). HRMS (TIS–): m/z 1230.4674 [M]– (calcd for C62H64B19F2N4NiO4 1230.6141). m.p. 245–246
°C (with decomposition). Rf = 0.6 (CHCl3–MeOH, 10:1). IR (KBr, cm–1): 3421 (NH), 3044 (Ccb–H),
2960, 2925, 2873, 2853 (C–H), 2563 (B–H), 2213, 2187 (C≡C), 1749, 1739 (C=O), 1558 (B–F).
4.3.12. Synthesis of nickelacarborane 21
Compound 21 was isolated with 5.5% yield during the chromatographic separation of compound 20.
Compound 21 was prepared in quantitative yield by air oxidation of a solution of 20 in CHCl3 over 3 days
with subsequent isolation of 21 by column chromatography in chloroform. 1H NMR (400 MHz, CD2Cl2):
δ 8.18 (br s, 1H, Trp-indole-NH), 7.94 (br s, 1H, BODIPY-pyrrole), 7.70 (d, 2H, J 7.7, linker-C6H4),
7.65-7.49 (m, 13H), 7.37 (d, 1H, J 8.3, Trp-C6H4), 7.24 (d, 1H, J 8.3, Trp-C6H4), 7.19 (t, 1H, J 7.6, Trp-
C6H4), 7.10 (t, 1H, J 7.3, Trp-C6H4), 6.98 (d, 2H, J 4.0, BODIPY-pyrrole), 6.94 (s, 1H, Trp-indole-CH),
6.60 (d, 2H, J 3.7, BODIPY-pyrrole), 5.12 (s, 3H, NH-boc, OCH2), 4.88 (br s, 4H, Ccb–H), 4.67 (br m,
11
1H, CH–C=O), 4.5–0.5 (m, 18H, B-H), 3.30 (br d, 2H, Trp-CH2), 1.41 (br s, 9H, t-Bu). B NMR (128
MHz, CD2Cl2): δ 18.0, 3.4, 0.1 (t, JB–F 28, BF2), -6.5, -14.4. 13C{1H} (100 MHz, CD2Cl2): δ 172.8, 167.5,
147.1, 145.0, 137.4, 137.0, 135.8, 135.1, 134.0, 133.9, 132.3, 131.0, 128.5, 126.5, 123.8, 122.9, 120.3,
119.5, 119.4, 118.2, 116.5, 112.0, 67.0, 55.2, 55.1, 30.5, 27.9. 19F{1H} NMR (376 MHz, CD2Cl2): δ -

ACCEPTED MANUSCRIPT
22
145.1 (q, JF–B 28). HRMS-TIS: m/z 1231.4337 [M]– (calcd for C62H64B19F2N4NiO4 1231.6141). m.p. 270–
271 °C (with decomposition). Rf = 0.95 (CHCl3–MeOH, 10:1). IR (KBr, cm–1): 3422 (NH), 3043 (Ccb–H),
2958, 2927, 2874, 2860 (C–H), 2560 (B–H), 2212, 2189 (C≡C), 1749, 1739 (C=O), 1557 (B–F).
4.4. Crystal structure determination
X-ray quality crystals of compound 7 were obtained by slow evaporation of its CH2Cl2 solution. Data
collection for 7 was performed at –100 °C on a Bruker SMART 1000 CCD area detector system using the
ω
scan technique with Mo Kα radiation (λ = 0.71073 Å) from a graphite monochromator. The APEX II
[25] and SAINT [26] software packages were used for data collection and data integration. The data were
corrected for absorption effects using the SADABS empirical method [27]. The structures were solved
and refined using the Bruker SHELXTL [28] software package. All of the non-hydrogen atoms were
refined with anisotropic thermal parameters except those in the disordered diazaindacene moiety and one
of the phenyl groups. All of the hydrogen atoms in 7 were included at geometrically idealized positions.
The crystallographic data and the details of the data collection and structure refinement are provided in
Table 1.
Table 1. Crystallographic details for 7.
compound
empirical formula
Fw
7
C27H25B11F2N2
534.40
crystal size (mm3)
crystal system
space group
a (Å)
0.14 × 0.08 × 0.06
triclinic
P1
10.132(2)
15.079(3)
29.558(6)
76.433(3)
85.249(3)
81.123(3)
4331.9(15)
6
b (Å)
c (Å)
α
β
γ
(deg)
(deg)
(deg)
V (Å3)
Z
T (K)
173(2)

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23
0.71073
λ
(Å)
dcalc (g·cm−3)
(mm−1)
max (deg)
1.229
0.075
µ
23.26
θ
unique data
36940
4351
observed data [I > 2
parameters
σ(I)]
1114
GOFa on F2
0.951
R1b, wR2c [I > 2
R1b, wR2c (all data)
max,min (e A−3)
Tmin/Tmax
σ(I)]
0.0796, 0.1620
0.2439, 0.2326
0.360, −0.322
0.990 /0.996
∆
ρ
a GOF = [Σ[w(Fo – Fc2)2]/(Nobs – Nparams)]1/2.
2
2
2 2
b R1 = Σ||Fo| − |Fc||/Σ|Fo|. c wR2 = [Σ[w(Fo – Fc2)2]/Σ[w(Fo ) ]]1/2.
5. Acknowledgments
The authors would like to thank Dr. Mark Lee, Mr. Brett Meers and Mr. James Woody for
recording the mass spectra of the prepared compounds.
6. Appendix A. Supplementary data
CCDC 1051561 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.
7. References
[1]
[2]
M. F. Hawthorne, T. Andrews, Carborane analogs of cobalticinium ion, Chem. Commun. (1965)
443-444.
See, for example: (a) C. Vinas, J. Llop, F. Teixidor, R. Kivekäs, R. Sillanpää, Restricted rotation
in unbridged sandwich complexes: rotational behavior of closo-[Co(η5-NC4H4)(C2B9H11)]
derivatives, Chem. Eur. J. 11 (2005) 1933-1941; (b) Rosario Núñez, O. Tutusaus, F. Teixidor, C.
Viñas, R. Sillanpää, R. Kivekäs, Highly stable neutral and positively charged dicarbollide
sandwich complexes, Chem. Eur. J. 11 (2005) 5637-5647, and references therein.

ACCEPTED MANUSCRIPT
24
[3]
[4]
O. Tutusaus, S. Delfosse, A. Demonceau, A. F. Noels, C. Viñas, F. Teixidor, Kharasch addition
catalyzed by half-sandwich ruthenium complexes. Enhanced activity of ruthenacarboranes Tet.
Lett. 2003, 44, 8421-8425 and references therein.
P. Cígler, M. Kožíšek, P. Řezáčová, J. Brynda, Z. Otwinowski, J. Pokorná, J. Plešek, B. Grüner,
L. Dolečková-Marešová, M. Máša, J. Sedláček, J. Bodem, H.-G. Kräusslich, V. Král, J.
Konvalinka, From nonpeptide toward noncarbon protease inhibitors: Metallacarboranes as
specific and potent inhibitors of HIV protease, Proc. Natl. Acad. Sci. USA 102 (2005) 15395-
15399.
[5]
[6]
R. Kaplánek, P. Martásek, B. Grüner, S. Panda, J. Rak, B. S. S. Masters, V. Král, L. J. Roman,
Nitric oxide synthases activation and inhibition by metallacarborane-cluster-based isoform-
specific affectors, J. Med. Chem. 55 (2012) 9541-9548.
M. Białek-Pietras, A. B. Olejniczak, S. Tachikawa, H. Nakamura, Z. J. Leśnikowski, Towards
new boron carriers for boron neutron capture therapy: Metallacarboranes bearing cobalt, iron
and chromium and their cholesterol conjugates, Bioorg. Med. Chem. 21 (2013) 1136-1142.
K. B. Gona, A. Zaulet, V. Gómez-Vallejo, F. Teixidor, J. Llop, C. Viñas, COSAN as a molecular
imaging platform: synthesis and “in vivo” imaging, Chem. Commun. 50 (2014) 11415-11417.
M. Tarrés, E. Canetta, C. Viñas, F. Teixidor, A. J. Harwood, Imaging in living cells using νB–H
Raman spectroscopy: monitoring COSAN uptake, Chem. Commun. 50 (2014) 3370-3372
(a) M. F. Hawthorne, J. I. Zink, J. M. Skelton, M. J. Bayer, C. Liu, E. Livshits, R. Baer, D.
Neuhauser, Electrical or photocontrol of the rotary motion of a metallacarborane, Science 303
(2004) 1849-1851. (b) M. F. Hawthorne, B. M. Ramachandran, R. D. Kennedy, C. B. Knobler,
Approaches to rotary molecular motors, Pure. Appl. Chem. 78 (2006) 1299-1304.
[7]
[8]
[9]
[10] See for example: (a) J. R. Lakowicz, Principles of fluorescence spectroscopy, third ed.,
Springer, 2006. (b) P. Wu, L. Brand, Resonance energy transfer: methods and applications,
Anal. Biochem. 218 (1994) 1-13.
[11] M. Olofsson, S. Kalinin, J. Zdunek, M. Oliveberd, L. B.-A. Johansson, Tryptophan–BODIPY: a
versatile donor–acceptor pair for probing generic changes of intraprotein distances, Phys. Chem.
Chem. Phys. 8 (2006) 3130-3140.
[12] J. M. Dixon, M. Taniguchi, J. S. Lindsey, PhotochemCAD 2: A refined program with
accompanying spectral data bases for photochemical calculations, Photochem. Photobiol. 81
(2005) 212-213.
[13] HyperChem Pro 8 (www.hyper.com) was used for building the molecules and performing
geometry optimization (MM+ force field with Polak-Ribiere geometry optimization algorithm).

ACCEPTED MANUSCRIPT
25
[14] For example: (a) S. Misumi, Alternant Conjugate Systems. I. Syntheses and Properties of
Polytolans, Bulletin of the Chemical Society of Japan 34 (1961) 1827-1832. (b) C.-C. Han, A.
Balasubramanian, Highly fluorescent mono-substituted poly(arylacetylene)s containing tolane
chromophores, J. Polym. Sci., Part A: Polym. Chem. 46 (2008) 5483-5498, and references
therein.
[15] Camilla Fenante, Uwe Kensy, Bernhard Dick, Does Diphenylacetylene (Tolan) Fluoresce
from Its Second Excited Singlet State? Semiempirical MO calculations and fluorescence quantum
yield measurements, J. Phys. Chem. 97 (1993) 13457-13463.
[16] (a) M. F. Hawthorne, D. C. Young, P. M. Garrett, D. A. Owen, S. G. Schwerin, F. N. Tebbe, P. A.
Wegner, The preparation and characterization of the (3)-1,2- and (3)-1,7-
dicarbadodecahydroundecaborate (-1) ions, J. Am. Chem. Soc. 90 (1968) 862-868. (b) M. A.
Fox, W. R. Gill, P. L. Herbertson, J. A. H. MacBride, K. Wade, H. M. Colquhoun,
Deboronation of C-substituted ortho- and meta-closo-carboranes using "wet" fluoride ion
solutions, Polyhedron 15 (1996) 565-571. (c) J. Yoo, J.-W. Hwang, Y. Do, Facile and mild
deboronation of o-carboranes using cesium fluoride, Inorg. Chem. 40 (2001) 568-570.
[17] L. Ma, J. Hamdi, F. Wong, M. F. Hawthorne, Closomers of high boron content: synthesis,
characterization, and potential application as unimolecular nanoparticle delivery vehicles for
boron neutron capture therapy, Inorg. Chem. 45 (2006) 278-285.
[18] M. F. Hawthorne, D. C. Young, T. D. Andrews, D. V. Howe, R. L. Pilling, A. D. Pitts, M.
Reintjes, L. F. Warren, Jr., P. A. Wegner, π-Dicarbollyl derivatives of the transition metals.
Metallocene analogs, J. Am. Chem. Soc. 90 (1968) 879-896.
[19] A. V. Safronov, N. I. Shlyakhtina, T. A. Everett, M. R. VanGordon, Y. V. Sevryugina, S. S.
Jalisatgi, M. F. Hawthorne, Direct observation of bis(dicarbollyl)nickel conformers in solution by
fluorescence spectroscopy: an approach to redox-controlled metallacarborane molecular motors,
Inorg. Chem. 53 (2014) 10045-10053.
[20] M. T. Indelli, T. Bura, R. Ziessel, p-Carborane-bridged bipyridine ligands for energy transfer
between two iridium centers, Inorg. Chem. 52 (2013) 2918-2926.
[21] K. Onitsuka, M. Fujimoto, H. Kitajima, H. Ohshiro, F. Takei, S. Takahashi, Convergent
synthesis of platinum-acetylide dendrimers, Chem. Eur. J. 10 (2004) 6433-6446.
[22] A. V. Safronov, M.F. Hawthorne, Novel synthesis of 3-iodo-ortho-carborane, Inorg. Chim.
Acta 375 (2011) 308-310.
[23] R. Hu, C. F. A. Gómez-Durán, J. W. Y. Lam, J. L. Belmonte-Vázquez, C. Deng, S. Chen,
R. Ye, E. Peña-Cabrera, Y. Zhong, K. S. Wong, B. Z. Tang, Synthesis, solvatochromism,

ACCEPTED MANUSCRIPT
26
aggregation-induced emission and cell imaging of tetraphenylethene-containing BODIPY
derivatives with large Stokes shifts, Chem. Commun. 48 (2012) 10099–10101.
[24] T. W.Greene, P. G. M. Wuts, Protective groups in organic synthesis, third ed., John Wiley &
Sons, 1999.
[25] Bruker, Apex 2 v APEX 2010.7; Bruker AXS Inc.: Madison, WI, 2010.
[26] Bruker, SAINT v 7.68A; Bruker AXS Inc.: Madison, WI, 2009.
[27] Sheldrick, G. M. SADABS 2008/1; Gottingen, 2008.
[28] Sheldrick, G. M. SHELXTL, v 2008/4; Bruker AXS, Inc.: Madison, WI, 2008.

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Highlights
•
•
•
Design of a bichromophoric nickelacarborane for the conformational assignment of the
nickelacarborane core by means of fluorescence spectroscopy.
Multistep organic/organometallic synthesis and characterization of both Ni(III) and
Ni(IV) nickelacarboranes bearing tryptophan/BODIPY FRET couple.
Possible energy transfer through the nickelacarborane core revealed during the
preliminary fluorescence study.