Boryl-functionalized s-alkynyl and vinylidene rhodium complexes: Synthesis and electronic properties

Article abstract of DOI:10.1002/chem.201303681

The synthesis, reactivity, and properties of borylfunctionalized s-alkynyl and vinylidene rhodium complexes such as trans-[RhCl(=C=CHBMes2)(PiPr ₃)₂] and trans-[Rh(C CBMes2)(IMe)(PiPr₃) ₂] are reported. An equil

Full text of DOI:10.1002/chem.201303681

DOI: 10.1002/chem.201303681
Full Paper
&
Borylated Complexes
Boryl-Functionalized s-Alkynyl and Vinylidene Rhodium
Complexes: Synthesis and Electronic Properties
Holger Braunschweig,*[a] Christopher K. L. Brown,[a] Rian D. Dewhurst,[a] J. Oscar C. Jimenez-
Halla,[a, b] Thomas Kramer,[a] Ivo Krummenacher,[a] and Bernd Pfaffinger[a]
Abstract: The synthesis, reactivity, and properties of boryl-
functionalized s-alkynyl and vinylidene rhodium complexes
thylimidazol-2-ylidene) was found to exhibit solvatochrom-
ism and can be quasireversibly oxidized and reduced elec-
such as trans-[RhCl(=C=CHBMes2)(PiPr3)2] and trans-[Rh(Cꢀ trochemically. Density functional calculations were per-
CBMes2)(IMe)(PiPr3)2] are reported. An equilibrium was found
to exist between rhodium vinylidene complexes and the cor-
responding hydrido s-alkynyl complexes in solution. The
complex trans-[Rh(CꢀCBMes2)(IMe)(PiPr3)2] (IMe=1,3-dime-
formed to determine the reaction mechanism and to help
rationalize the photophysical properties of trans-[Rh(Cꢀ
CBMes2)(IMe)(PiPr3)2].
unisolable RhIII sC-ethynylborane counterpart. We present an
experimental and computational study on the participating re-
action mechanisms, as well as the UV/Vis and electrochemical
properties of the products. Overall, the results reveal a system
in which the oxidation states and bonding modes can be pre-
dictably switched by standard synthetic and electrochemical
methods. Given the electronic communication between the
electrochemically active Rh and B centers in these molecules,
the interconversion between the RhI vinylidene, RhIII alkynyl,
and RhI alkynyl species may be useful for the future develop-
ment of functional boron-containing organometallic complexes
with excellent metal–boron communication.
Introduction
Borylated molecular and polymeric species have become of
great importance due to their broad applications in organic
and inorganic synthesis and materials science.[1–10] In particular,
the design of p-conjugated organic compounds displaying
three-coordinate boron centers is a highly topical area of re-
search, since the functional materials obtained therefrom are
promising for use in light-emitting devices, sensors, and elec-
tronic circuits.[1–3,5] Metallacumulene complexes have been in-
tensively studied for more than 30 years due to their ability to
act as catalysts for olefin metathesis reactions,[11] stereoselec-
tive formation of E alkenes,[12] and CꢁN[13] and CꢁC coupling re-
actions.[14,15] In the course of this research, metallacumulenes
have been functionalized with a broad variety of functional
groups to promote specific reactions. In addition, their nonlin-
ear optical properties and physical properties have attracted
interest.[16,17]
Results and Discussion
Warming a mixture of dimesityl(ethynyl)borane and cis-[{Rh(m-
Cl)(PiPr3)2}2] from ꢁ40 to ꢁ208C resulted in a yellow suspen-
sion which turned red at about ꢁ158C (Scheme 1). After
Nevertheless, the number of boryl-functionalized vinylidene
complexes remains limited,[18,19] and no related sC-ethynylbor-
ane complex has been reported. Herein, we report a simple
protocol for the preparation of both RhI borylvinylidene and
RhI sC-ethynylborane complexes, as well as the observation of
an equilibrium between an RhI borylvinylidene complex and its
[a] Prof. Dr. H. Braunschweig, C. K. L. Brown, Dr. R. D. Dewhurst,
Dr. J. O. C. Jimenez-Halla, T. Kramer, Dr. I. Krummenacher, Dr. B. Pfaffinger
Institut fꢀr Anorganische Chemie
Julius-Maximilians-Universitꢁt Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@mail.uni-wuerzburg.de
[b] Dr. J. O. C. Jimenez-Halla
Department of Chemistry (DCNE), Universidad de Guanajuato
Noria Alta s/n 36050 Guanajuato (Mexico)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303681.
Scheme 1. Formation of 1 and 2.
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workup at room temperature a red solid (2) was isolated
which showed a broad 11B{1H} NMR signal at 61 ppm and
a doublet in the 31P NMR spectrum at 43.9 ppm (1JPRh
=
138.0 Hz) with a coupling constant typical for rhodium vinyli-
dene complexes (1JPRh ꢂ140 Hz).[12] Formation of the vinylidene
complex was confirmed by the 1H NMR spectrum, which
showed a singlet at 2.36 ppm for the vinylidene proton. The a-
carbon atom of the vinylidene chain was detected in the
Scheme 2. Equilibrium between 1 and 2.
13C NMR spectrum at 300.7 ppm as a doublet of triplets (1JCRh
=
tempted to shift the equilibrium to favor the s-alkynyl hydrido
rhodium(III) complex by adding pyridine to generate the octa-
hedral pyridine complex trans-[RhHCl(CꢀCBMes2)(PiPr3)2(py)] (3;
Scheme 3). However, even use of neat pyridine as solvent pro-
vided a mixture of 2 and 3. The octahedral complex 3 is char-
2
60.0, JCP =12.1 Hz). The b-carbon atom appeared at 109.4 ppm
as a doublet (2JCRh =18.0 Hz).
To assess unequivocally the connectivity of 2, a single-crystal
X-ray diffraction study was carried out (Figure 1). The RhꢁC1
(1.775(3) ꢁ) and C1ꢁC2 (1.324(4) ꢁ) distances are comparable
to those in the related complex trans-[RhCl(C=CHMe)(PiPr3)2]
(RhꢁC1 1.775(6), C1ꢁC2 1.32(1) ꢁ) and seem not to be affected
by the presence of the boryl group. The C1ꢁC2 (1.324(4) ꢁ)
and C2ꢁB (1.544(4) ꢁ) distances of 2 are also longer than those
of the parent dimesityl(ethynyl)borane. The boryl group sits at
an angle of 48.5(2)8 to the C2ꢁC1 axis, and thus conjugation
between the boryl group and the vinylidene moiety can be ex-
cluded.
1
acterized by a broad singlet at ꢁ17.8 ppm in the H NMR spec-
trum, while its 31P NMR spectrum showed a broad doublet
around 38 ppm (1JPRh =97 Hz). Both the H and 31P NMR data
1
were consistent with those reported for trans-[RhHCl(CꢀCPh)-
(PiPr3)2(py)].[21] Solid-state IR spectroscopy confirmed the exis-
ꢁ1
~
tence of a CꢀC bond (n=1944 cm ).
Scheme 3. Reaction of 2 with pyridine to form 3 and subsequent reaction
with LDA with formation of 4.
Figure 1. Molecular structure of 2. Hydrogen atoms have been removed for
clarity. Thermal ellipsoids are drawn at 50% probability. Selected bond
lengths [ꢁ] and angles [8]: RhꢁCl 2.3837(8), RhꢁC1 1.775(3), C1ꢁC2 1.324(4),
BꢁC2 1.544(4); C1-Rh-Cl 176.70(8), C2-C1-Rh 177.5(2), C1-C2-B 131.5(2).
Elimination of HCl from the rhodium center could be effect-
ed by addition of lithium diisopropylamide (LDA) to the reac-
tion mixture, yielding trans-[Rh(CꢀCBMes2)(PiPr3)2(py)] (4;
Scheme 3). This complex was found to decompose slowly
when stored in THF solution, and much faster in contact with
air, water, or CH2Cl2. A singlet at 50.9 ppm was detected in the
11B NMR spectrum of 4 and a doublet at 40.5 ppm in its 31P
spectrum (1JRhP =149 Hz). Compound 4 showed a characteristic
During the synthesis of 2 at low temperatures, we detected
intermediate 1 which transforms into 2 on warming to room
temperature. In a separate experiment, 1 was isolated and
characterized as a yellow solid at low temperature. The yellow
temperature-sensitive reaction intermediate 1 was found to be
not the respective p-alkyne rhodium(I) complex but the s-al-
kynyl hydrido rhodium(III) complex, which shows a doublet of
2
doublet of triplets at 195.4 ppm (1JCRh =45.4, JPC =22.0 Hz) for
the rhodium-bound alkynyl carbon atom in its 13C NMR spec-
trum, while the boron-bound alkynyl carbon nucleus was not
detected. Solid-state IR spectroscopy revealed a CꢀC stretching
band at 1934 cmꢁ1.
2
1
triplets at ꢁ27.41 ppm (1JHRh =43.7, JHP =12.0 Hz) in its H NMR
spectrum, and a doublet at 49.5 ppm (1JPRh =97 Hz) in its
31P NMR spectrum. Solid-state IR spectroscopy revealed a CꢀC
stretching band at 2015 cmꢁ1 and thus confirmed the presence
of a CꢀC bond. The spectroscopic data are comparable with
those of trans-[RhHCl(CꢀCFc)(PiPr3)2] (Fc=ferrocenyl).[20]
In contrast to pyridine, the N-heterocyclic carbene 1,3-dime-
thylimidazol-2-ylidene (IMe) is strong enough to eliminate HCl
from 2, yielding trans-[Rh(CꢀCBMes2)(IMe)(PiPr3)2] (5;
Scheme 4). Compound 5 is stable towards dichloromethane
and can be isolated by column chromatography on alumina. In
its 13C{1H} NMR spectrum, the signal of the rhodium-bound al-
An equilibrium between 1 and 2 was observed in benzene
at room temperature (Scheme 2). As we were able to monitor
the position of the equilibrium by NMR spectroscopy, we at-
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Scheme 4. Reaction of 2 with IMe.
kynyl carbon atom was detected as a doublet of triplets (d=
202.5 ppm; 1JRhC =36.7, 2JPC =22.4 Hz), and the signal for the
NHC carbene carbon nucleus was found at 196.6 ppm (1JRhC
=
41.30, JPC =4.3 Hz). A doublet at 47.1 ppm (1JRhP =149 Hz) was
detected in its 31P{1H} NMR spectrum, and a broad singlet at
52 ppm in its 11B NMR spectrum. IR spectroscopy showed a Cꢀ
C stretching band at 1944 cmꢁ1. Overall, the spectroscopic pa-
rameters of 4 and 5 are very similar, whereas their stabilities
are very different.
2
To gain more detailed insight into the structural properties
of 4 and 5, both compounds were subjected to single-crystal
X-ray diffraction analysis (Figure 2). Both rhodium centers
adopt a square-planar geometry. The RhꢁC1 distances of the
two complexes (4: 1.932(2) ꢁ; 5: 1.983(2) ꢁ) are significantly
different, presumably owing to the much stronger s-donation
properties and trans influence of the IMe ligand relative to pyr-
idine. The C1ꢁC2 (4: 1.242(3) ꢁ; 5: 1.233(3) ꢁ) distances in both
complexes are equivalent within experimental uncertainty. The
BMes2 moiety is distorted to a lesser extent from the C1ꢁC2
axis (4: 13.1(2)8; 5: 11.9(2)8) than in 2. This deviation from ex-
pected linearity is most probably caused by the steric bulk of
the Mes groups.
As the complexes 2 and 5 turned out to be unexpectedly
stable, we subjected them to a more detailed analysis involv-
ing both spectroscopic and computational methods. Initially
we attempted to explain the observation of hydrido complex
1, which was isolated as an intermediate in the formation of 2
but also forms an equilibrium with 2 in solution. Thereby, DFT
calculations at the M06-2X/’BS2’//M06-2X/’BS1’ level of theory
were performed (Figure 3; see Supporting Information for fur-
ther details of the computational methodology). The observa-
tion of 1 instead of the respective side-on p-alkyne complex
(Reactant) can be explained by the small conversion barrier
(TS1, 3 kcalmolꢁ1) and by 1 being 6.6 kcalmolꢁ1 lower in
energy than the side-on complex. The observation of 1 forming
an equilibrium with 2 is also attributed to a small energy differ-
ence of 2.5 kcalmolꢁ1 between the two isomers, in agreement
with the observed magnitude of the equilibrium constant and
position of the equilibrium. The mechanism of formation of 5
was also clarified. Formation of 5 does not proceed via an oc-
tahedral intermediate analogous to 3 followed by elimination
of HCl·IMe. Instead, IMe attacks the vinylidene proton, and sub-
sequent elimination of HCl·IMe (energy barrier, TS2=2.6 kcal
molꢁ1) generates a highly unsaturated rhodium(I) center (Int3),
which immediately reacts with a second equivalent of IMe
with formation of 5. This process (Int2!Product) is highly en-
ergetically favored, as it releases 23.4 kcalmolꢁ1 of energy.
Since both 2 and 5 give intensely colored solutions, UV/Vis
spectroscopy was performed on solutions of the complexes in
Figure 2. Molecular structures of 4 (top) and 5 (bottom). Hydrogen atoms
have been removed for clarity. Thermal ellipsoids are drawn at 50% proba-
bility. Selected bond lengths [ꢁ] and angles [8] for 4: RhꢁN 2.126(2), RhꢁC1
1.932(2), C1ꢁC2 1.242(3), B1ꢁC2 1.494(4); C1-Rh1-N 175.10(9), C2-C1-Rh1
174.8(2), C1-C2-B1 166.9(2). Selected bond lengths [ꢁ] and angles [8] for 5:
RhꢁC1 1.983(2), RhꢁC3 2.058(2), C1ꢁC2 1.233(3), BꢁC2 1.501(3); C1-Rh-C3
176.23(8), C2-C1-Rh 175.62(17), C1-C2-B 168.1(2).
hexane (Figure 4). The spectrum of 2 shows absorptions bands
at 358 (e=21000), 292 (e=19000), and 204 nm (lmax, e=
75000 Lmolꢁ1 cmꢁ1). The absorption spectrum of 5 is similar to
that of 2 with lmax at 205 nm (e=54000 Lmolꢁ1 cmꢁ1) and
bands at 291 (e=11000) and 374 nm (e=14000 Lmolꢁ1 cmꢁ1).
Compound 5 was subjected to a time-dependent DFT (TD-
DFT) study in order to assign the absorption bands to the re-
spective transitions (Figure 5). The weak band between 400
and 500 nm is assigned to the HOMO!LUMO transition, and
that at 374 nm to the HOMOꢁ1!LUMO transition. With the
HOMO being located mostly at the metal center and the
LUMO mostly at boron, this can be considered a typical metal-
to-ligand charge-transfer transition. The absorption band at
292 nm was found to comprise multiple transitions including
HOMOꢁ3!LUMO+1/LUMO
LUMO+10.
and
HOMOꢁ2!LUMO+12/
Compound 5 was also investigated by cyclic voltammetry,
which revealed a quasireversible reduction at ꢁ3.2 V and a qua-
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Figure 3. Calculated mechanism of the formation of 5 with Gibbs free energies at 298 K.
sireversible oxidation at ꢁ0.5 V versus ferrocene/ferrocenium
(Fc/Fc+). These two processes are most likely due to the for-
mation of a boron radical and oxidation of the electron-rich
RhI center, respectively (Figure 6). The reduction potential of 5
is slightly more negative than those of similar neutral boranes
such as Mes3B,[22] which is reduced at E1/2 =ꢁ2.9 V (vs. Fc/Fc+)
Cꢁ
in THF to radical anion Mes3B . This difference can be ex-
plained by effective p overlap of the metal alkynyl substituent
with the empty boron pz orbital. The assignment of the redox
process at ꢁ0.5 V (vs. Fc/Fc+) to a metal-centered oxidation is
based on the voltammetric behavior of various other Rh sys-
tems, but remains speculative at this point.[23]
To verify the assumed oxidation and reduction processes,
Fukui functions were calculated, which showed nucleophilic
attack at the metal center and electrophilic attack at the Cꢀ
CBMes2 unit, lending support to our descriptions of the elec-
trochemical processes (Figure 7).
Figure 4. UV/Vis absorption spectra of 2 and 5 in hexane solution.
Conclusion
We have reported a convenient synthesis of boryl-functional-
ized vinylidene complex 2 and were also able to isolate and
spectroscopically characterize intermediate hydrido s-alkynyl
complex 1, which provides insight into the reaction mecha-
nism. Furthermore the reaction of 2 with IMe results in the for-
mation of unusually stable s-alkynyl complex 5 in an unprece-
dented manner. Complex 5 shows solvatochromism and can
be quasireversibly reduced and oxidized. The experimental
work was supported by theoretical calculations, which help to
elucidate the mechanism of product formation and the nature
of the observed physical phenomena.
Figure 5. Frontier orbitals of 5, as derived from DFT calculations.
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3JPC for 13C NMR or 3JPH +5JPH for 1H NMR. Infrared data were ac-
quired on a JASCO FT/IR-6200 type A instrument. Melting points
were measured with a Mettler Toledo DSC 823. Microanalysis was
performed on an Elementar Vario MICRO cube elemental analyzer.
Elemental analyses were acquired on an Elementar Vario MICRO
cube instrument or a CHNS-932 (Leco). UV/Vis spectra were mea-
sured with a JASCO-V660 UV/Vis spectrometer in a quartz glass cell
(10 mm). All cyclic voltammetry experiments were conducted in an
argon-filled glovebox by using a Gamry Instruments Reference 600
potentiostat. A standard three-electrode cell configuration was em-
ployed with a platinum disk working electrode, a platinum wire
counter electrode, and a silver wire, separated by a Vycor tip, serv-
ing as the reference electrode. Formal redox potentials are report-
ed versus the Fc0/+ redox couple, obtained by using ferrocene as
internal standard.
Synthesis of 2
A
solution of dimesityl(ethynyl)borane in pentane (5.00 mL,
0.828 mmol, c=45.0 mgmLꢁ1) was added dropwise to a solution
of [{RhCl(PiPr3)2}2] (380 mg, 0.414 mmol) in pentane (5.00 mL) in
a Schlenk tube at room temperature. The orange reaction mixture
was stirred for 30 min at room temperature, and all volatile sub-
stances were removed under high vacuum. The yellow residue was
washed with pentane (2ꢂ10.0 mL) at low temperature and subse-
quently dried under high vacuum at room temperature. Com-
pound 2 was isolated as a red solid (375 mg, 0.511 mmol, 62%).
Single crystals suitable for X-ray analysis were obtained by storing
a solution in THF at ꢁ708C for one week. M.p. 165.88C; decomp
ꢂ228.88C; 1H NMR (500.1 MHz, C6D6, 296 K): d=6.71 (s, 4H, m-
Figure 6. Cyclic voltammogram and presumed electrochemical reduction
and oxidation steps for 5. The CV measurement was performed in THF with
C
MesH), 2.61–2.56 (m, 6H, PCHCH3), 2.47 (s, 12H, o-CMesH3), 2.36 (s,
0.1m nBu4NPF6 as electrolyte at a scan rate of 300 mVsꢁ1
.
1H, HCBMes2), 2.14 (s, 6H, p-CMesH3), 1.25 ppm (dvt, 36H, JHH
=
2
6.7 Hz, N=13.7 Hz, PCHCH3); 13C{1H} NMR (125.7 MHz, C6D6, 296 K):
d=300.7 (dt, JRhC =60.0, JPC =12.1 Hz, Rh=C), 139.8 (brs, BCq-Mes),
1
2
137.52 (s, Cq-Mes), 128.86 (s, CMesH), 109.36 (d, JRhC =18.0 Hz, C=CH),
2
30.11 (s, p-CMesH3), 24.90 (vt, N=20.3 Hz, PCHCH3), 24.02 (s, o-
C
MesH3), 20.39 ppm (s, PCHCH3); 11B{1H} NMR (160.4 MHz, C6D6,
296 K): d=61 ppm (s); 31P{1H} NMR (202.4 MHz, C6D6, 296 K): d=
43.9 ppm (d, 1JPRh =138 Hz); IR (hexane): n=2956 (CꢁH stretch),
~
1608 (C=C stretch), 1539 cmꢁ1 (C=C stretch); UV/Vis (hexane):
lmax =203.5 nm (e=75032 Lmolꢁ1 cmꢁ1); elemental analysis calcd
(%) for C38H65BClP2Rh: C 62.26, H 8.94; found: C 62.36, H 9.00.
Figure 7. Calculated nucleophilic (f+, left) and electrophilic (fꢁ, right) Fukui
functions.
Low-temperature isolation of 1
Experimental Section
A solid sample of 1 was prepared analogously to the synthesis of
2, but the reaction was performed at ꢁ408C. The temperature of
the reaction mixture or solid was not allowed to exceed ꢁ208C, as
the compound is converted to 2 at room temperature. The reac-
tion mixture turned yellow at ꢁ308C. While warming to ꢁ208C, all
volatile substances were removed under high vacuum. The result-
ing yellow residue was washed with pentane (5 mL) at ꢁ408C and
subsequently dried under high vacuum at ꢁ408C. Accordingly, the
NMR experiments were performed at low temperatures. 1H NMR
(400.1 MHz CD2Cl2, 276 K): d=6.69 (s, 4H, m-CMesH), 2.78 (m, 6H,
PCHCH3), 2.32–2.22 (m, 12H, o-CMesH3), 2.21 (s, 6H, p-CMesH3), 1.22
All manipulations were performed under an inert atmosphere of
dry argon by using standard Schlenk-line or glovebox techniques.
Deuterated solvents were dried over molecular sieves and de-
gassed by three freeze–pump–thaw cycles prior to use. All other
solvents were distilled from appropriate drying agents.[24] Solvents
were stored under argon over activated molecular sieves. IMe[25]
[26]
and [RhCl(PiPr3)2]2 were prepared according to published proce-
dures. NMR spectra were recorded on a Bruker Avance 500 NMR
1
spectrometer (500.1 MHz for H, 160.4 MHz for 11B, 125.76 MHz for
13C{1H}) or Bruker Avance 400 NMR spectrometer (128.38 MHz for
11B). Chemical shifts d are given in parts per million (ppm) and are
referenced against external Me4Si (1H, 13C), H3PO4 (31P), and
(dvt, 36H, JHH =7.8 Hz, N=30.7 Hz, PCHCH3), ꢁ27.41 ppm (dt, 1H,
2
1JRhH =43.7, 2JPH =12.0 Hz, RhH); 11B{1H} NMR (128.38 MHz, CD2Cl2,
276 K): d=59 ppm (s); 31P{1H} NMR (161.98 MHz, CD2Cl2, 276 K): d=
[BF3·Et2O] (11B). Cq refers to quaternary carbon nuclei. jNj =1JPC
+
49.5 ppm (d, 1JRhP =97 Hz); IR (solid): n=2015 cmꢁ1 (CꢀC stretch).
~
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1604 cmꢁ1 (C=C stretch); UV/Vis (hexane): lmax =205 nm (e=
Synthesis of 3
53670 Lmolꢁ1 cmꢁ1);
elemental
analysis
calcd
(%)
for
Compound 2 (47.4 mg, 0.065 mmol) was treated with pyridine
(2.00 mL) in a Schlenk tube. The pale yellow solution was stirred
for 1 h at room temperature. After slowly evaporating the solvent
C43H72BN2P2Rh: C 65.15, H 9.15, N 3.53; found: C 64.83, H 9.26, N
3.51.
in
a glovebox, the residue was washed with cold pentane
X-ray crystallography
(2.00 mL). NMR spectroscopy revealed a mixture of 2 and 3.
1H NMR (400.1 MHz, C6D6, 296 K): d=7.00–6.70 (m, 5H, CpyH), 6.84
(s, 4H, CMesH), 2.91–2.81 (m, 6H, PCHCH3), 2.65 (s, 12H, o-CMesH3),
Compound 2:
2
2.21 (s, 6H, p-CMesH3), 1.16–1.02 (vt, 36H, JHH =6.5 Hz, N=20.3 Hz,
The XRD data of 2 were collected on a Bruker SMART-APEX diffrac-
tometer with a CCD area detector and graphite-monochromated
MoKa radiation. The structure was solved by direct methods, re-
fined with the SHELX software package, and expanded by Fourier
techniques. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were included in structure-factor calculations. All
hydrogen atoms were assigned to idealized geometric positions.
Crystal data for 2: C42H73BClOP2Rh, Mr =805.11; red block, 0.25ꢂ
PCHCH3), ꢁ17.75 ppm (brs, 1H, RhH); 31P{1H} NMR (161.98 MHz,
1
~
C6D6, 296 K): d=38.5 ppm (d, JRhP =97.0 Hz); IR (hexane): n=1944
(CꢀC stretch), 1604 cmꢁ1 (C=C stretch).
Synthesis of 4
Compound 2 (47.4 mg, 0.065 mmol) and LDA (6.90 mg,
0.065 mmol) were dissolved in pyridine (3.00 mL) in a Schlenk
tube. While stirring the red reaction mixture for 48 h at room tem-
perature, darkening was observed. After removing all volatile sub-
stances under high vacuum, the residue was extracted with THF
(2.00 mL). After removing all volatile substances under high
vacuum, compound 4 was obtained as a red solid, which still con-
tained impurities. Single crystals suitable for X-ray analysis were
obtained by slowly evaporating a solution in benzene in a glove-
box. M.p. 202.58C; decomp >3208C; 1H NMR (500.1 MHz, C6D6,
297 K): d=8.67–8.65 (m, 2H, CpyH), 6.88 (s, 4H, CMesH), 6.59–6.55
(m, 1H, p-CpyH), 6.22–6.19 (m, 2H, CPyH), 2.79 (s, 12H, o-CMesH3),
2.25 (s, 6H, p-CMesH3), 2.17–2.07 (m, 6H, PCHCH3), 1.23 ppm (dvt,
36H, 2JHH =7.2 Hz, N=19.8 Hz, PCHCH3); 13C{1H} NMR (125.7 MHz,
C6D6, 297 K): d=195.4 (dt, 1JRhC =45.4, 2JPC =22.0 Hz), 155.31 (s,
CpyH), 145.95 (s, Cq-Mes), 140.17 (s, o-Cq-Mes), 135.78 (s, m-Cq-Mes),
134.38 (s, p-CpyH), 128.33 (s, CMesH), 122.96 (s, CpyH), 25.17 (vt, N=
16.2 Hz, PCHCH3), 23.64 (s, o-CMesH3), 21.16 (s, p-CMesH3), 20.92 ppm
ꢀ
0.23ꢂ0.16 mm; triclinic, space group P1; a=9.8330(16), b=
14.350(2), c=16.409(3) ꢁ; a=104.649(2), b=96.611(2), g=
101.133(2)8; V=2164.9(6) ꢁ3; Z=2; 1calcd =1.235 gcmꢁ3
;
m=
0.559 mmꢁ1; F(000)=860; T=173(2) K, R1 =0.0487, wR2 =0.1151;
8506 independent reflections (2qꢃ52.38) and 451 parameters.
Compound 4:
The crystal data of 4 were collected on a Bruker X8-Apex II diffrac-
tometer with a CCD area detector and multilayer-mirror-monochro-
mated MoKa radiation. The structure was solved by direct methods,
refined with the SHELX software package, and expanded by Fouri-
er techniques. All non-hydrogen atoms were refined anisotropical-
ly. Hydrogen atoms were included in structure-factor calculations.
All hydrogen atoms were assigned to idealized geometric posi-
tions. The 54 reported least-squares restraints, as shown by the
refine ls number restraints key, were attributed to the DELU key-
word in ShelXL input (“rigid bond” restraint for all bonds in the
connectivity list; standard values of 0.01 for both parameters s1
and s2 were used). The displacement parameters of atoms P2 and
P3 of residue 1 and of atoms C1 and C3 of residues 8, 9, 10, 18, 19,
and 20 were constrained to the same value. The displacement of
atoms P2 and P3 of the residue 1 and of atoms C1 and C3 of the
residues 8, 9, 10, 18, 19 and 20 were restrained to the same value
with similarity restraint SIMU. The displacement parameters Uii of
atoms P2 and P3 of residue 1 and atoms C1 and C3 of residues 8
and 18 were restrained with the ISOR keyword to approximate iso-
tropic behavior. Crystal data for 4: C43H69BNP2Rh, Mr =775.65; red
block, 0.384ꢂ0.246ꢂ0.153 mm; monoclinic space group P21/c; a=
12.871(9), b=20.354(11), c=16.624(8) ꢁ; b=103.10(2)8; V=
(s, PCHCH3); 11B{1H} NMR (160.4 MHz, C6D6, 297 K): d=50.9 ppm (s);
1
31P{1H} NMR (202.4 MHz, C6D6, 297 K): d=40.5 ppm (d, JRhP
=
ꢁ1
~
150 Hz); IR (solid): n=1934 cm (CꢀC stretch).
Synthesis of 5
Compound 2 (90.0 mg, 0.125 mmol) was dissolved in toluene
(2.00 mL) in a Schlenk tube. While stirring, a solution of IMe in tolu-
ene (2.45 mL, 0.250 mmol, c=10.0 mgmLꢁ1) was added by syringe.
The red reaction mixture was stirred for 10 min. After removing all
volatile substances under high vacuum, the residue was extracted
with diethyl ether (2.00 mL) and subjected to column chromatog-
raphy (Al2O3, neutral, activity V, 10 cm) with diethyl ether/pentane
(9/1) as eluent. After removing the solvent under vacuum, the resi-
due was washed with cold pentane (3ꢂ2.00 mL) and dried under
high vacuum. Compound 5 was isolated as a red solid (84.0 mg,
0.110 mmol, 85%). Single crystals suitable for X-ray analysis were
obtained by slowly evaporating a solution in benzene in a glove-
box. M.p. 247.68C; decomp 271.58C; 1H NMR (500.1 MHz, C6D6,
296 K): d=6.89–6.87 (m, 4H, m-CMesH), 6.05 (s, 2H, CIMeH), 3.50 (s,
6H, CIMeH3), 2.79 (s, 12H, o-CMesH3), 2.36–2.26 (m, 6H, PCHCH3), 2.25
(s, 6H, p-CMesH3), 1.18 ppm (dvt, 36H, 2JHH =6.3 Hz, N=13.1 Hz,
4242(4) ꢁ3; Z=4; 1calcd =1.215 gcmꢁ3
; ; F(000)=
m=0.507 mmꢁ1
1656; T=100(2) K; R1 =0.0536, wR2 =0.0735; 8338 independent re-
flections (2qꢃ52.048) and 530 parameters.
Compound 5:
The crystal data of 5 were collected on a Bruker X8-Apex II diffrac-
tometer with a CCD area detector and multilayer-mirror-monochro-
mated MoKa radiation. The structure was solved by direct methods,
refined with the SHELX software package, and expanded by Fouri-
er techniques. All non-hydrogen atoms were refined anisotropical-
ly. Hydrogen atoms were included in structure-factor calculations.
All hydrogen atoms were assigned to idealized geometric posi-
tions. The 348 reported least-squares restraints, as shown by the
refine ls number restraints key, were attributed to the DELU key-
word in ShelXL input (“rigid bond” restraint for all bonds in the
connectivity list; standard values of 0.01 for both parameters s1
PCHCH3); 13C{1H} NMR (125.7 MHz, C6D6, 296 K): d=202.5 (dt,
2
1JRhC =36. 7, 2JPC =22.4 Hz, RhCꢀ), 196.6 (dt, 1JRhC =41.3, JPC
=
14.3 Hz, RhCq-IMe), 145.57 (s, Cq-Mes), 140.21 (s, Cq-Mes), 136.0 (s, Cq-Mes),
130.08 (s, Cq-Mes), 126.07 (s, m-CMesH), 120.55 (s, CIMeH), 38.48 (s,
C
IMeH3), 27.33 (vt, N=18.2 Hz, PCHCH3), 23.50 (s, o-CMesH3), 21.34 (s,
p-CMesH3), 21.20 ppm (s, PCHCH3); 11B{1H} NMR (160.4 MHz, C6D6,
296 K): d=52 ppm (s); 31P{1H} NMR (202.4 MHz, C6D6, 296 K): d=
47.1 ppm (d, 1JRhP =149.0 Hz); IR (solid): n=1944 (CꢀC stretch),
~
Chem. Eur. J. 2014, 20, 1427 – 1433
www.chemeurj.org
1432
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
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and s2 were used). The displacement parameters of atoms P51 to
C70 were restrained to the same value with similarity restraint
SIMU. The displacement parameters Uii of atoms P51 to C70 were
restrained with ISOR keyword to approximate isotropic behavior.
Crystal data for 5: C43H72BN2P2Rh, Mr =792.69; red block, 0.23ꢂ
0.14ꢂ0.12 mm; monoclinic, space group P21/c; a=12.8252(4), b=
20.1969(6), c=16.9479(5) ꢁ; b=101.0450(10)8; V=4308.7(2) ꢁ3; Z=
4, 1calcd =1.222 gcmꢁ3; m=0.501 mmꢁ1; F(000)=1696; T=100(2) K;
R1 =0.0369; wR2 =0.0705; 8498 independent reflections (2qꢃ
52.048) and 559 parameters.
CCDC 961895 (2), 961896(4) and 961897(5) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
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Acknowledgements
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Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
Keywords: alkynyl ligands
·
boron
·
rhodium
·
solvatochromism · vinylidene ligands
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Received: September 19, 2013
Published online on December 20, 2013
Chem. Eur. J. 2014, 20, 1427 – 1433
www.chemeurj.org
1433
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim