Dibenzo[b,f]phosphepines: Novel phosphane-olefin ligands for transition metals

Article abstract of DOI:10.1021/om301012z

New, stable heterobidentate phosphane-olefin ligands based on the dibenzo[b,f]phosphepine backbone are reported together with their redox properties and coordination chemistry to rhodium(I). The X-ray crystal structures and DFT calculations show different conformations for the P-phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltammetry (vs Fc/Fc⁺) of 6a supported by UV-vis spectroelectrochemistry showed two cathodic waves, a reversible one at E_1/2 = -2.62 V (I_f/I_b = 1.0) and a quasi-reversible (I_f/I_b ≈ 1.2) one at E _1/2 = -3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a^?]^-, characterized by EPR spectroscopy, and dianion [6a]^2-, for which an X-ray crystal structure was obtained. Both 6a and 6b bind to Rh^I centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two dibenzo[b,f]phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b showed two close-lying waves, a one-electron reversible wave at -1.45 V and a two-electron quasi-reversible wave at -1.80 V (I_f/I_b ≈ 1.3). 8a showed a reversible wave at -1.71 V and irreversible waves at lower potentials.

Full text of DOI:10.1021/om301012z

Article
pubs.acs.org/Organometallics
Dibenzo[b,f]phosphepines: Novel Phosphane−Olefin Ligands for
Transition Metals
Volodymyr Lyaskovskyy,†,‡ Relinde J. A. van Dijk-Moes,† Sebastian Burck,† Wojciech I. Dzik,‡
Martin Lutz,§ Andreas W. Ehlers,† J. Chris Slootweg,† Bas de Bruin,*,‡ and Koop Lammertsma*,†
†Department of Chemistry and Pharmaceutical Sciences, VU University Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The
Netherlands
‡Van ‘t Hoff Institute for Molecular Sciences (HIMS), Homogeneous and Supramolecular Catalysis, University of Amsterdam (UvA),
Science Park 904, 1098 XH Amsterdam, The Netherlands
§Crystal and Structural Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Padualaan 8, 3584 CH Utrecht, The
Netherlands
S
* Supporting Information
ABSTRACT: New, stable heterobidentate phosphane−olefin
ligands based on the dibenzo[b,f ]phosphepine backbone are
reported together with their redox properties and coordination
chemistry to rhodium(I). The X-ray crystal structures and
DFT calculations show different conformations for the P-
phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltamme-
try (vs Fc/Fc+) of 6a supported by UV−vis spectroelec-
trochemistry showed two cathodic waves, a reversible one at
E
1/2 = −2.62 V (If/Ib = 1.0) and a quasi-reversible (If/Ib ≈ 1.2)
one at E1/2 = −3.03 V. Reduction with sodium afforded a
mixture of the radical anion [6a•]−, characterized by EPR spectroscopy, and dianion [6a]2−, for which an X-ray crystal structure
was obtained. Both 6a and 6b bind to RhI centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two
dibenzo[b,f ]phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic
double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b
showed two close-lying waves, a one-electron reversible wave at −1.45 V and a two-electron quasi-reversible wave at −1.80 V (If/
Ib ≈ 1.3). 8a showed a reversible wave at −1.71 V and irreversible waves at lower potentials.
Scheme 1. Electromeric Phosphane−Olefin Complexes
INTRODUCTION
■
Olefin complexes of transition metals, while known for almost
200 years, remain of immense importance.1 They are crucial
intermediates in alkene modifications, where coordination of
the double bond to the metal center is the first step in the
catalytic transformation, as exemplified in the oxidation at
palladium (Wacker oxidation), the reduction at rhodium
(Wilkinson’s catalyst), and various hydrofunctionalizations
(hydroamination, hydroformylation, hydrocyanation, etc.).2 As
auxiliary ligands, olefins steer transition-metal-catalyzed 1,4-
additions and (transfer) hydrogenations.3,4 Especially valuable
are heterobidentate ligands that contain, in addition to the
olefin, an amine or phosphane donor group.5 Recently, a
tridentate PhP(trop)2 ligand (Scheme 1) was shown to form
two unprecedented electromeric complexes with rhodium that
differ substantially in their electronic structure (one isomer is
the metal-centered radical [Rh0(L0)PPh3] (1b) while the other
is the ligand-centered radical [RhI(L•−)PPh3] (1a)) but only
slightly in their conformation.6 Phosphane−olefin ligands have
further shown potential as “hydrogen reservoirs” in hydro-
genation catalysis, wherein the olefin part of the ligand gets
reversibly hydrogenated under the catalytic conditions, which
allows hydrogen transfer to the substrate via the “cooperative
ligand”. This behavior potentially leads to faster reactions and
special selectivities.4,7
The noted examples point to intriguing properties associated
with heterobidentate phosphane−olefin ligands and thereby call
for new ones to apply in homogeneous catalysis and potentially
expand the scope of electromers. Hence, we targeted the
synthesis of phosphepines 2, which differ from the known
PhP(trop)2 framework in that the phosphorus atom is
embedded in the seven-membered ring (Scheme 2). The
coordination chemistry of 2 is virtually unknown. We are aware
Received: October 30, 2012
© XXXX American Chemical Society
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Scheme 2. Decomposition Pathway of Phosphepines
of only a single bidentate complex with iridium8 and a few
monodentate P-coordination complexes.9 The underlying
reason is the thermal lability of the phosphepine. So far, only
the parent di-tert-butyl phosphepine 2a has been isolated and
characterized.10 Less crowded phosphepines (2b) are elusive,
since they cyclize to unstable phosphanorcaradienes (3b) that
eliminate phosphinidenes (4b).11 The stability of phosphepines
improves significantly on benzannulation,12 as is the case for
dibenzophosphepines 6 (Scheme 3).13 Here we report on their
synthesis, conformational analysis, (spectro)electrochemical
behavior, and coordination chemistry to rhodium(I).
Figure 1. Molecular structures of 6a (left) and 6b (right) in the crystal
form. Displacement ellipsoids are drawn at the 50% probability level.
Selected bond lengths (Å), bond angles (deg), and torsion angles
(deg) are as follows. 6a: P1−C1 = 1.8247(13), P1−C15 = 1.8373(13),
C1−C6 = 1.4089(18), C7−C8 = 1.338(2), C6−C7 = 1.4669(19);
C1−P1−C15 = 102.42(6), C1−P1−C14 = 102.84(6), C15−P1−C14
= 103.70(6); C16−C15−P1−C14 = 38.01(12). 6b (only one of two
independent molecules is shown; hydrogen atoms are omitted for
clarity): P1−C11 = 1.8348(19), P1−C151 = 1.840(2), C11−C61 =
1.411(2), C71−C81 = 1.332(3), C61−C71 = 1.468(3); C11−P1−
C151 = 102.60(9), C11−P1−C141 = 105.16(8), C151−P1−C141 =
104.45(9); C161−C151−P1−C141 = −52.94(18).
RESULTS AND DISCUSSION
■
Synthesis. Dibenzo[b,f ]phosphepines 6a (R = Ph) and 6b
(R = Mes) were synthesized quantitatively, as monitored by 31
P
group almost perpendicular to the phosphepine ring and the
phosphorus lone pair oriented in the direction to the vinylic
bond instead of the benzannulated rings (Figure 1, right).
Hence, 6b is preorganized to bind transition metals in a
heterobidentate manner, while 6a has to undergo pyramidal
inversion at phosphorus to bind metals in a similar manner.
The flexibility of the two dibenzo[b,f ]phosphepines was
investigated computationally at the B3PW91/6-311G(d,p) level
of theory.16 Three different conformations I−III were
identified for 6a and 6b′ each (Figure 2; 6b′ lacks the mesityl’s
NMR spectroscopy, from (Z)-dibromostilbene 5 by lithiation
with tert-butyllithium and addition of the corresponding
aryldichlorophosphane at −78 °C (Scheme 3) but proved
difficult to isolate due to oxidation of the phosphorus atom
during chromatography.14 Therefore, BH3−SMe2 was added to
obtain the readily purifiable borane complexes (yields 82%
(7a), 32% (7b); 31P NMR 13.4 ppm (7a), 10.4 ppm (7b)),
which on deprotection with DABCO (16 h, room temperature,
DCM or toluene as solvent) afforded the desired dibenzo-
[b,f]phosphepines 6 (yields 97% (6a), 96% (6b); 31P NMR
−8.1 ppm (6a), −30.5 ppm (6b)).
Crystals suitable for X-ray diffraction were obtained by slow
vapor diffusion of pentane into dichloromethane solutions of
6a,b. Both molecular structures (Figure 1) have P−C bond
lengths similar to those in triarylphosphanes (1.82−1.84 Å for
6a and 6b vs 1.83 Å for PPh3).15 The sum of the bond angles
around phosphorus reveals comparable pyramidalities for 6a
(308.96(10)°) and 6b (312.21(15)°), as for PPh3 (309°), but
the conformations differ. The phenyl group of 6a is oriented
parallel to the heterocyclic double bond and has an acute
dihedral angle of 60.98(9)° with the phosphepine P1−C1−C14
plane, thereby rendering the phosphorus lone pair in the same
spatial direction as the benzannulated rings (Figure 1, left). The
molecular structure of 6b is quite different, with its mesityl
Figure 2. Conformations of P-aryl dibenzo[b,f ]phosphepines and
their relative B3PW91/6-311G(d,p) energies (in kcal mol−1, ZPE
corrected).
Scheme 3. Synthesis of the Dibenzo[b,f]phosphepines 6a and 6b from (Z)-Dibromostilbene 5
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p-CH3 group for simplicity). I resembles the molecular
structure of 6a and II that of 6b, while the aryl group in III
is rotated by 90° around the P−C bond as compared to the
case for II. In agreement with the experimentally obtained
crystal structures, I is energetically favored for the phenyl
derivative 6a over II and III by 1.4 and 6.6 kcal mol−1,
respectively, and II is favored for 6b′ over I and III by +3.4 and
+11.1 kcal mol−1, respectively. It is evident that the o-methyl
groups of the P-aryl substituent have a significant impact on the
conformational stability of the dibenzo[b,f ]phosphepine.
While the experimentally observed conformational difference
between 6a and 6b in the solid state are in agreement with the
above DFT conformational analysis, we cannot exclude an
additional influence of the crystal packing. In the crystal
structure of 6b π−π interactions between the mesityl ring of
one molecule and the aryl ring of the dibenzophosphepine
moiety of another molecule are apparent (see Supporting
Information, Figure S9).
Scheme 4. One- and Two-Electron Reduction of
Dibenzo[b,f ]phosphepine 6
even with traces of water in the solvent/electrolyte, leading to
the redox active followup product observed with CV. More
rigorous exclusion of water resulted in a fully reversible wave at
−2.62 V.
We conclude that the accessibility and reversibility of both
reduction waves are indicative of the electron-accepting ability
of 6a, suggesting that dibenzo[b,f ]phosphepines can indeed be
suitable π-acceptors and/or redox-noninnocent ligands in
coordination chemistry.17,18
The electrochemical reduction of benzophosphepines 6a in
THF was also monitored by UV−vis spectroelectrochemistry
using an optically transparent thin-layer electrochemical
(OTTLE) cell. The spectrum shows an absorption at 304 nm
(in addition to strong ones below 280 nm), which upon one-
electron reduction gradually disappears with new absorptions
emerging at 322, 350, 508, and 831 nm (Figure 4, left). On the
basis of TD-B3LYP/def2-TZVP calculations,19 most of these
bands can be assigned to stem from transitions from the
SOMO to higher lying orbitals (see Table 1).
1
The NMR (both 31P and H) spectra of 6a and 6b do not
reveal the presence of different conformers, indicating a rapid
equilibrium between the two major conformers I and II on the
NMR time scale for both 6a and 6b (the third isomer III seems
to be too high in energy to contribute significantly), which is in
agreement with the low calculated barrier for I → II
interconversion (∼6 kcal mol−1) in structurally related
phosphepine systems.11a
Cyclic Voltammetry and UV−Vis Spectroelectrochem-
istry. To investigate the electron-accepting properties of the
dibenzo[b,f ]phosphepine ligands, we investigated the redox
behavior of 6a with cyclic voltammetry (CV). In THF 6a shows
no redox activity in the potential range between −2.45 and
+0.50 V (vs Fc/Fc+, Figure 3). At +0.55 V there is an onset of
The three characteristic absorptions of [6a•]− disappear in
the second reduction step when a single new absorption
emerges at 552 nm. This corresponds to filling the SOMO of
[6a•]− with an additional electron and limits the possible
optical transitions for the diamagnetic, closed-shell [6a]2−. Its
TD-DFT calculated spectrum gives three absorptions at 416,
505, and 628 nm, of which the blue-shifted absorption is of very
low intensity. A similar weak absorption at 405 nm is observed
in the experimental spectrum. The calculated strong absorption
at 505 nm and the weak absorption at 628 nm are interpreted
to jointly correlate with the observed broad absorption at 552
nm (see Table 1). Additional weak absorptions are predicted by
the TD-DFT calculations at 1051 nm for [6a•]− and 889 nm
for [6a]2−, but these are not observed in the experimental
spectra (probably due to their weakness and broadening). The
TD-DFT calculations do not reproduce the experimental
electronic spectra very precisely, and some deviations in the
range between 400 and 700 nm are apparent. The calculated
band at 423 nm for [6a•]− is predicted at an overly high energy
(experimental 508 nm), while for [6a]2− the predicted energy
separation between the two calculated bands at 628 and 505
nm is larger than that observed (one broad peak at 508 nm).
Nonetheless, overall the calculations provide a useful guideline
to assign the bands.
Figure 3. Cyclic voltammograms of 6a: full (blue) and separate (red)
scans over the first reduction wave and full (green) scan over the
second one. Scan rate: 200 mV s−1. Signals marked with * stem from
the irreversible oxidation at potentials >+0.55 V. Signals marked with #
are attributed to a reaction product of [6a]2− with traces of water (see
text).
an irreversible oxidation. The CV shows a reversible cathodic
wave at E1/2 = −2.62 V that forms the dibenzophosphepine
radical anion [6a•]− (Scheme 4). A second wave, correspond-
ing to the transfer of a second electron to form the
phosphepine dianion [6a]2−, appears at E1/2 = −3.03 V.
Measurements in commonly purified solvents and electrolyte
first indicated that this wave is not fully reversible; scanning
over it resulted in the appearance of a followup product
showing an anodic wave at −1.76 V (marked with # in Figure
3). The two-electron-reduced phosphepine apparently reacts
Chemical Reduction of the Dibenzophosphepines
with Elemental Sodium. Encouraged by the CV and UV−
vis spectroelectrochemical results, we set out to synthesize and
isolate the singly (1e) and hopefully the doubly (2e) reduced
dibenzo[b,f ]phosphepines. On the basis of the CV data we
chose elemental sodium as a reducing agent (E°(Na) = −3.04
V vs Fc/Fc+).20 When 10 equiv was added at room temperature
to a solution of 6a in THF, the initial yellow mixture turned
deeply purple over 4 h. The EPR spectrum confirmed that the
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Figure 4. (top) Evolution of the UV−vis spectra during the first (left) and second (right) reductions of 6a. (bottom) Calculated (Orca) TD-DFT,
B3LYP/def2-TZVP spectra (lowest 10 transitions in the visible range) of [6a•]− (A) and [6a]2− (B).
Table 1. TD-DFT Parameters of the UV−Vis Spectra of
[6a•]− and [6a]2−
a
λcalcd
,
rel intensity,
a
compd
λexptl, nm
nm
%
main excitation
[6a·]−
1051
783
618
572
485
473
429
423
889
628
2
5
SOMO → LUMO+3
SOMO → LUMO+4
SOMO → LUMO+5
SOMO → LUMO+6
SOMO → LUMO+7
HOMO → SOMO
831
tail
2
508
20
2
∼480 (sh)
466
4
tail
18
100
1
SOMO → LUMO+9
HOMO → SOMO
350, 322
[6a]2−
SOMO → LUMO+3
HOMO → LUMO+5
HOMO → LUMO+6
HOMO → LUMO+5
HOMO → LUMO+6
HOMO → LUMO+10
552
30
Figure 5. Experimental (in THF) and simulated EPR spectra of (a)
Na+[6a•]− (frequency 9.357723 GHz; microwave power 2 mW; T =
298 K; modulation amplitude 1 G) and (b) Na+[6b•]− (frequency
9.359971 GHz; microwave power 2 mW; T = 298 K; modulation
amplitude 0.2 G). (c) Atom numbering used for the hyperfine
couplings in Table 2.
505
100
405
416
10
a
Orca, TD-DFT, B3LYP/def2-TZVP, COSMO ε = 2.28.
radical anion Na+[6a•]− had formed (Figure 5a, Scheme 4); the
solution was NMR silent.
Table 2. Experimental and DFT g Values and Hyperfine
Couplings (MHz) of Na+[6a•]− and Na+[6b•]−
[6a•]−
[6b•]−
A satisfactory EPR spectrum could be simulated with
parameters that reflect delocalization of the spin density over
the olefinic and aromatic carbons (Table 2, Figure 5). Resolved
hyperfine couplings with 13 nuclei in 7 independent sets were
obtained: i.e., with the phosphorus atom, the olefinic hydrogens
(H1), four sets of aromatic hydrogen nuclei (H2−5), and a
small, poorly resolved coupling with the meta hydrogens (H7)
of the phenyl substituent (Figure 5a). The assignment of the
hyperfine couplings is based on supporting DFT calculations
(ORCA, B3LYP/TZVP,19 geometry optimized with Turbo-
mole at the BP86/SV(P) level21) of Na+[6a•]− (Table 2,
Supporting Information). Evidently, reducing 6a to [6a•]−
places an electron in the fully delocalized π* SOMO with a
similarly shaped spin density distribution (Figure 6) in support
of the observed EPR spectrum.
a
b
a
b
exptl
DFT
exptl
2.0030
DFT
giso
2.0034
2.0027
2.0024
c
hyperfine coupling
P
−11.34
−10.74
−6.10
3.01
−12.93
−12.41
−7.40
2.56
−16.3
−11.1
−4.0
3.8
−13.5
−12.3
−6.6
2.1
d
H1
d
H2
d
H3
d
H4
−8.20
4.49
−11.83
4.42
−9.4
−11.1
4.7
d
e
H5
2.0
d
e
H7
−0.36
−0.54
not resolved
−0.02, 0.04
a
Derived from least-squares curve fitting of the spectrum recorded in
THF at T = 298 K. The signs were assigned on the basis of the DFT
b
c
d
calculations. B3LYP/TZVP. See Figure 5 for atom numbering. Two
e
equivalent hydrogens. Poorly resolved.
The EPR spectrum for the mesityl-derivative Na+[6b•]− was
obtained likewise (Figure 5b), but it is broader than that of
Na+[6a•]− even at low concentrations. Consequently, the
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metallic sodium consists of a mixture containing both the singly
and doubly reduced products [Na+][6a•]− and [Na+]2[6a]2−.
The molecular structure of the dianion adopts conformation
I just like the neutral precursor, but charge delocalization
profoundly effects the C−C bonds, lengthening the “aromatic”
C1−C6 ([6a]2− 1.462(3) Å vs 6a 1.4089(18) Å) and C7−C8
“double” bonds ([6a]2− 1.462(3) Å vs 6a 1.338(2) Å) and
shortening the C6−C7 ‘”single” bonds ([6a]2− 1.396(3) Å vs
6a 1.4669(19) Å), while the P-pyramidality remains essentially
unchanged (sum of the bond angles around phosphorus: [6a]2−
310.81(14)° vs 6a 308.96(10)°).
Magnetic susceptibility measurements at room temperature
on crystals of [6a]2− (μeff ≈ 0) support a closed-shell
configuration, which is favored by 14.9 kcal mol−1 over the
triplet exited state according to B3LYP/6-311(d,p) calculations.
Nonetheless, no 31P NMR spectrum of diamagnetic [6a]2−
could be recorded, likely due to the presence of (trace amounts
of) the radical anion, as rapid electron transfer between the two
species is expected to cause substantial NMR signal broadening.
Two mechanistic pathways can lead to formation of dianion
[6a]2−. First, reduction of [6a•]− with excess sodium may
occur. The redox potential corresponding to the second
reduction of 6a (E1/2 = −3.03 V vs Fc/Fc+) is very close to
the reduction potential of sodium (E° = −3.03 V vs Fc/Fc+),
which does in fact explain the presence of both [6a•]− and
[6a]2− in the reaction mixture.20 Second, disproportionation
(2[6a•]− ⇌ [6a]2− + 6a) may be followed by selective
crystallization of [Na+]2[6a]2−. On the basis of the CV redox
potentials, this disproportionation equilibrium lies strongly to
the left (ΔGo = 9.4 kcal mol−1), but crystallization of [6a]2−
may be feasible given the equilibrium constant of Kdisp ≈ 1 ×
10−7.
Figure 6. SOMO (left) and spin density (right) distribution of [6a•]−.
smallest hyperfine couplings with the H5 hydrogens are poorly
resolved and those with the H7 hydrogens of the P-Mes group
are not resolved at all (Figure 5b, Table 2). The spectrum could
be simulated in a satisfactory manner using the DFT-calculated
values for [6b•]− as a starting point (Table 2, Supporting
Information). The DFT optimized structures of the two radical
anions adopt the same conformations as their neutral
precursors, that is I for [6a•]− and II for [6b′•]− (Figure 2),
but with longer C7−C8 double (1.39−1.40 Å vs 1.35 Å) and
shorter C6−C8 single bonds (1.42 vs 1.46 Å) for the seven-
membered ring, reflecting its delocalized nature (see Figure 1
for numbering).
Single crystals suitable for X-ray diffraction were obtained by
slow vapor diffusion of pentane into the THF solution obtained
from the reaction of 6a with sodium. Surprisingly, these proved
to be of dianionic [Na+]2[6a]2− (Figure 7) instead of the
expected radical anion [Na+][6a•]−, which was observed by
EPR spectroscopy. Dianion [6a]2− crystallizes as a dimer with
two [6a]2− species bridged by three THF molecules
([Na+]4[6a2−]2·7THF) (Figure 7). Apparently, the purple
solution that is obtained by mixing 6a with 10 equiv of
Unfortunately, we were unable to grow crystals of
[Na+]2[6b]2−, possibly due to its higher solubility. However,
like its neutral precursor, B3PW91/6-311G(d,p) calculations
show a preference for conformation II and, like [6a]2−,
elongated “aromatic” and “double” C−C bonds (dC7−C8 = 1.35
(6b′), 1.42 Å ([6b′]2−); dC1−C6 = 1.41 (6b′), 1.46 Å ([6b′]2−))
and shortened “single” bonds (dC6−C7 = 1.46 (6b′), 1.40 Å
([6b′]2−)) for the phosphepine ring.
In summary, the collective UV−vis, structural, and computa-
tional data show that upon 1e reduction an electron is placed in
the π* LUMO of 6, being the SOMO of [6•]−, and two on 2e
reduction, forming the HOMO of [6]2− without changing the
conformation, which is I for the P-phenyl and II for the P-
mesityl derivative (Figures 6 and Figures S6 and S8
(Supporting Information)). Electronically, the dibenzo[b,f ]-
phosphepines in [6•]− and [6]2− are then best viewed as a
combination of two isolated fragments, namely a 1e- or 2e-
reduced π-conjugated stilbene part and a noninteracting
phosphane lone pair, making them ideal ligands for
coordination chemistry.
Complexation to Rhodium(I). To probe the ability of 6a
and 6b to act as heterobidentate phosphane−olefin ligands, we
investigated their affinity for RhI precursors as a necessary first
step toward exploring their “redox noninnocence” features and
behavior in catalysis. First, we show that both are potent
chelating ligands, despite their conformational differences.
We start with the coordination chemistry of ligand 6a.
Despite the fact that the phosphorus lone pair and the olefinic
double bond of the free ligand are not preorganized for metal
chelation (Figure 2), pyramidal inversion at P1 occurs readily,
allowing the ligand to act as a heterobidentate phosphane−
Figure 7. Molecular structure of [Na+]4[6a2−]2·7THF (two different
viewpoints are shown for clarity) from an X-ray crystal structure
determination (hydrogen atoms and disordered noncoordinated
solvent molecules are omitted for clarity, only major disorder
components of the THF molecules are shown). Displacement
ellipsoids are drawn at the 50% probability level. Selected bond
lengths (Å), bond angles (deg), and torsion angles (deg): P1−C11 =
1.811(2), P1−C151 = 1.8524(18), C11−C61 = 1.462(3), C71−C81 =
1.462(3), C61−C71 = 1.396(3), Na1−C71 = 2.7515(19), Na1−C81 =
2.592(2), Na1−C151 = 2.7494(18), Na1−C161 = 2.8596(19), Na2−
C71 = 2.6818(19), Na2−C81 = 2.6707(19); C11−P1−C151 =
100.60(8), C11−P1−C141 = 110.27(9), C151−P1−C141 = 99.94(8);
C161−C151−P1−C141 = 45.46(15).
E
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olefin ligand. The reaction of 6a with [Rh(cod)2][OTf] in a 3:1
ratio in DCM (room temperature, 16 h) led to the quantitative
formation of the yellow tris-coordinated complex 8a (Scheme
5). A lower ligand: Rh ratio (2:1) also leads to formation of 8a
(based on 31P NMR), but with incomplete conversion of the
RhI precursor.
ppm and a very broad signal at 36.0 ppm (Figure 9a). Lowering
the temperature leads to sharpening of the low-field signal to
three doublet of doublets of doublets at 215 K (31P NMR 19.7
ppm (1JPR1h = 119.0, 2JPP = 38.1, 2JPP = 21.8 Hz), 31.4 ppm (2JPP
2
= 395.0, JPRh = 95.2, JPP = 21.8 Hz), 39.2 ppm (2JPP = 395.0,
2
1JPRh = 95.2, JPP = 38.1 Hz)) (Figure 9f). The signals at 39.2
and 31.4 ppm show a very large trans 2JPP coupling constant of
395.0 Hz and are therefore assigned to stem from the two trans-
positioned phosphorus atoms in the bipyramid.
Scheme 5. Coordination of 6a with RhI Species
VT-31P NMR (Figure 9) reveals a dynamic process due to
Rh−P3 bond rotation of the monodentate ligand in 8a
(Scheme 6). The magnetic anisotropy of the aryl rings of this
ligand affects the chemical shifts of the two trans-positioned
axial phosphorus atoms, which explains their inequivalence
(39.2 and 31.4 ppm) below the coalescence temperature and
their dynamic interconversion above. Rotation of the
monodentate ligand does not affect its own chemical shift in
The molecular structure of 8a, resolved by X-ray crystal
structure determination (Figure 8), reveals a strongly distorted
2
the 215−298 K temperature range but averages the JPP
couplings with the axial P atoms (2JPP = 38.1 and 21.8 Hz)
2
to JPP = 29.5 Hz at the coalescence temperature, where these
atoms become equivalent on the NMR time scale. The
experimental ΔG⧧ value for Rh−P3 bond rotation of the
equatorial ligand is estimated at 12.6 kcal mol−1.23
In sharp contrast to the RhI coordination of 6a, the
analogous reaction with 6b yielded (91%) by 31P NMR a
10:1 mixture of the square-planar [Rh(6b)2][OTf] complexes
cis-8b (δ(31P) 34.6 ppm, 1JPRh = 149.0 Hz) and trans-8b (δ(31P)
1
43.1 ppm, JPRh = 127.3 Hz), which carry two instead of three
ligands (Scheme 7). It is noteworthy that 8b, in contrast to 8a,
does not reveal any dynamic behavior on the NMR time scale.
Deep red crystals of the major isomer cis-8b were obtained in
55% yield by slow evaporation of pentane into the dichloro-
methane solution of the mixture. The molecular structure of 8b
(Figure 10) reveals a cis coordination mode for the two
dibenzophosphepine rings. In the crystal structure we find an
intramolecular π-stacking interaction between the two mesityl
substituents. These substituents are nearly parallel (deviation
2.62(11)°), and the centers of the rings have perpendicular
distances to the neighboring ring plane of 3.5945(9) and
3.5807(11) Å, respectively. This interaction may explain the
relative stability of cis-8b over the trans-8b isomer.
Dibenzophosphepine in cis-8b adopts conformation III, which
is higher in energy than conformation II for the free ligand.
Coordination of this higher energy conformation is perhaps less
surprising than is the case for 8a. Obviously, II would lead to
substantial steric repulsion between two mesityl rings in cis-8b.
The phosphorus atoms of 8b (320.74(16)° for P1 and
320.14(17)° for P2) are slightly less pyramidal than in 6b
(312.21(15)°) and dianion [6a]2− (310.81(14)°) according to
the sums of their bond angles. The P−Rh bonds have lengths
similar to the equatorial bonds of 8a (2.2823(5)−2.3079(5) vs
2.2973(11)−2.3667(11) Å), and the C7−C8 double bonds are
likewise elongated compared to those of the free ligand
(1.400(3)−1.405(3) vs 1.332(3)−1.339(3) Å), indicating
substantial metal-to-ligand π-back-bonding. The observed
difference in the ligand to Rh coordination stoichiometry
between 8a (3:1) and 8b (2:1) is presumably caused by the
steric crowding of the methyl groups in 6b.
Figure 8. Molecular structure of the cationic Rh complex 8a in the
crystal (hydrogen atoms, disordered solvent molecules, and the triflate
counterion are omitted for clarity). Only one of the two independent
molecules is shown. Displacement ellipsoids are drawn at the 50%
probability level. Selected bond lengths (Å) and angles (deg): P1−Rh1
= 2.3667(11), P2−Rh1 = 2.2973(11), P3−Rh1 = 2.4232(11), C71−
C81 = 1.432(6), C72−C82 = 1.422(6), C73−C83 = 1.334(6), C61−
C71 = 1.492(6), Rh1−1/2(C71−C81) = 2.076(3), Rh1−1/2(C72−
C82) = 2.110(3); P1−Rh1−P3 = 95.29(4), P1−Rh1−P2 = 164.47(4),
1/2(C71−C81)−Rh1−1/2(C72−C82) = 127.72(12).
trigonal bipyramidal complex (τ = 0.61)22 carrying two
heterobidentate benzophosphepine ligands with axial phospho-
rus atoms and equatorial double bonds and a third (equatorial)
benzophosphepine that coordinates to the rhodium by its
phosphorus atom only. This Rh−P3 bond is longer than the
other Rh−P1 and Rh−P2 bonds (2.4232(11) vs 2.2973(11)−
2.3667(11) Å). Rh coordination substantially elongates the
olefinic bonds (1.422(6)−1.432(6) vs 1.334(6) Å), which
points to substantial π-back bonding and hence a strong
metallo-cyclopropane resonance contribution. Remarkably,
complex 8a adopts conformations II and III for the bidentate
ligand and II for the monodentate ligand, in contrast to the
preferred conformation I for the free ligand (Figure 2). This
may be of steric origin, although chelating effects may also
contribute.
Complex cis-8b[OTf] is sparsely soluble in ethers, which
hampers a study of its redox properties. To increase the
solubility in THF, we exchanged the triflate ion by stirring cis-
−
The room-temperature 31P NMR spectrum of 8a shows a
sharp doublet of triplets (1JPRh = 119.0, 2JPP = 29.5 Hz) at 20.5
8b[OTf] with NaBArF (BArF− = B(C6H3(CF3)2)4 ) in DCM
to obtain deep red crystalline cis-8b[BArF] (80%). Its cyclic
F
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Figure 9. 31P NMR spectra of the complex 8a at different temperatures.
Scheme 6. Dynamic Behavior of Complex 8a
Scheme 7. Coordination Chemistry of Ligand 6b toward
Rhodium(I)
Figure 10. Molecular structure of cis-8b from X-ray crystal structure
determination (hydrogen atoms, noncoordinated triflate counterion,
and CH2Cl2 solvent molecules are omitted for clarity). Displacement
ellipsoids are drawn at the 50% probability level. Selected bond lengths
(Å), bond angles (deg), and torsion angles (deg): P1−C11 =
1.822(2), P1−C151 = 1.819(2), C71−C81 = 1.400(3) (C72−C82 =
1.405(3) in the second ligand), C61−C71 = 1.475(3), P1−Rh1 =
2.3079(5), C71−Rh1 = 2.219(2), C81−Rh1 = 2.201(2); C11−P1−
C151 = 112.79(9), C11−P1−C141 = 94.53(9), C151−P1−C141 =
113.42(9); C161−C151−P1−C11 = −42.62(19).
voltammogram (Figure 11) shows two cathodic waves with E1/2
= −1.45 and −1.80 V (vs Fc/Fc+). The first corresponds to a
reversible one-electron reduction (If/Ib = 1.0) and the second
to a nonfully reversible two-electron transfer (If/Ib ≈ 1.3).
Unfortunately, the closeness of these potentials and the
unpreventable disproportionation hamper the isolation of the
rhodium(0) radical anion,6,24,25 which agrees with our inability
to isolate reduced species from the reaction of cis-8b[BArF]
with either sodium or cobaltocene as reducing agent. Cyclic
voltammetry of 8a (see Supporting Information, Figure S3)
revealed one reversible cathodic wave at −1.71 V and several
nonreversible waves at lower potentials.
which are electronically best described as a combination of two
isolated fragments, that is a 1e (or 2e) reduced stilbene
fragment and a phosphane lone pair. Dibenzophosphepines
deserve attention as new heterobidentate phosphane−olefin
ligands in the coordination chemistry to late transition metals,
as demonstrated for RhI. The structure of the rhodium
complexes can be tuned by changing the substituents at the
phosphorus atom. Thus, whereas dibenzophenyl phosphepine
6a gives the 3:1 (ligand:Rh) complex 8a, the more bulky
mesityl analogue 6b leads to the 2:1 complex 8b. In the
distorted-trigonal-bipyramidal 8a two of the dibenzophosphe-
pines act as bidentate ligands, while the third ligand binds as a
monodentate P donor. In the square-planar complex cis-8b
both dibenzophosphepine ligands 6b coordinate in a bidentate
fashion. Interestingly, the ligands in these two complexes adopt
conformations that are less favorable for the noncoordinated
CONCLUSIONS
■
In this paper we describe the synthesis, redox properties, and
initial coordination studies of new heterobidentate phosphane−
olefin ligands based on the dibenzophosphepine scaffold. The
P-phenyl and P-mesityl derivatives 6a,b are readily synthesiz-
able from commercially available precursors. They show
interesting redox activity as free ligands and can be reduced
by sodium to the radical anion [6a•]− and dianion [6a]2−,
G
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Experimental X-band EPR spectra were recorded on a Bruker EMX
spectrometer. The spectra were simulated by iteration of the
anisotropic g values, (super)hyperfine coupling constants, and line
widths using the W95EPR program (available upon request from Prof.
Frank Neese, Max Planck Institute for Bioinorganic Chemistry).
Flash chromatography: silica gel SiliaFlash P60 (0.040−0.063 mm)
with an overpressure of about 0.5 bar. Melting points were measured
on samples in unsealed capillaries on a Stuart Scientific SMP3 melting
point apparatus and are uncorrected. (Z)-1,2-Bis(2-bromophenyl)-
ethene (5)27 and mesityldichlorophosphine28 were prepared according
to the literature procedures.
DFT Calculations. DFT Conformational Analysis. The conforma-
tional analysis of [6a] and [6b′•]−, as well as the spin state energies of
[6a]2−, were evaluated with Gaussian03 at the B3PW91 level of theory.
In this functional, the exchange energy is described with contributions
from local and nonlocal (B3-parameter Becke29 and Hartree−Fock
(HF)) exchange terms, and the correlation energy is given by the
Perdew and Wang 9130 nonlocal functional within the generalized
gradient approximation (GGA).31 The 6-311G(d,p) basis set for atoms
C, H, N, O, and P has been used. The nature of each stationary point
was confirmed by a frequency calculation. Relative energies were
corrected with zero-point energies.
DFT Property Calculations. The geometries of the full atom models
of [6a], [6a•]−, [6a•]2−, [6b•]−, and [6b]2− were optimized with the
Turbomole program21a coupled to the PQS Baker optimizer32 at the
BP86 DFT level33 using the Turbomole SV(P) basis.21c,f The EPR
parameters34 of [6a•]− and [6b′•]− were subsequently calculated with
ORCA,19 using the B3LYP33 functional and TZVP21c,f basis set
supplied with the program (Ahlrichs polarization functions on all
atoms). The calculations were performed according to the spin
unrestricted coupled-perturbed Kohn−Sham formalism, which ex-
presses the g tensor as a second-order response property with respect
to homogeneous external magnetic field and spin−orbit coupling
(SOC) perturbations.35 Calculation of the hyperfine interactions takes
the SOC effects into account.36 The UV−vis parameters of [6a•]− and
[6a]2− were calculated with ORCA at the TD-DFT b3-lyp, def2-TZVP
level employing COSMO37 dielectric solvent corrections (ε = 2.28).
The coordinates from the structure optimized in Turbomole were
used as inputs for the ORCA calculations. Orbital and spin density
plots were generated with Molden.38
Figure 11. Cyclic voltammogram of cis-8b [BArF]. Scan rate: 100 mV
s−1.
dibenzophosphepines. These initial investigations on the
dibenzophosphepine ligand system bode well for the future,
where we will explore their “redox noninnocent” behavior and
their potential to generate new classes of transition-metal-based
catalysts.
EXPERIMENTAL SECTION
■
General Considerations. All syntheses were performed with the
use of Schlenk techniques under an atmosphere of dry nitrogen.
Pentane, THF, toluene, and diethyl ether were distilled under nitrogen
from sodium; dichloromethane was distilled under nitrogen from
calcium hydride. Bu4NPF6 was dried under vacuum.
1
NMR spectra were recorded at 293 K. H NMR: Bruker Advance
250 (250 MHz), Bruker Advance 400 (400 MHz), Bruker UltraSchield
500 (500 MHz), referenced internally to residual solvent resonance of
CHCl3 (δ 7.26 ppm) and CH2Cl2 (δ 5.32 ppm). 13C{1H} NMR:
Bruker Advance 400 (101 MHz), Bruker UltraSchield 500 (126 MHz)
referenced internally to residual solvent resonance of CDCl3 (δ 77.2
ppm) and CD2Cl2 (δ 53.8 ppm). 31P{1H} NMR: Bruker Advance 250
(101 MHz), Bruker Advance 400 (162 MHz) using 85% H3PO4 as an
external standard (δ 0.0 ppm). 11B NMR: Bruker Advance 400 (128
MHz) using BF3·OEt2 as an external standard (δ 0.0 ppm). 19F{1H}
NMR: Bruker Advance 250 (235 MHz) using BF3·OEt2 as an external
standard (δ 0.0 ppm). All NMR signals are given in ppm. For
numbering of the assignments see Figure 5c.
IR spectra were recorded on a Bruker Alpha-P FT-IR spectrometer.
Peak intensities are marked as follows: s, strong; m, medium; w, weak.
All IR bands are given in cm−1. High-resolution fast atom
bombardment mass spectra (HR-FAB MS) were recorded on a
JEOL JMS SX/SX 102A four-sector mass spectrometer, coupled to a
JEOL MS-MP9021D/UPD system program. High-resolution electro-
spray ionization mass spectra (HR-ESI MS) were recorded on Bruker
MicroTOFQ, ESI positive mode, capillary voltage 4.5 kV.
For electrochemical measurements a 2 mM phosphepine/0.1 M
Bu4NPF6 THF solution was used. Cyclic voltammograms were
recorded in a gastight single-compartment three-electrode cell
equipped with platinum working (apparent surface of 0.42 mm2),
coiled platinum wire auxiliary, and silver wire pseudoreference
electrodes. The cell was connected to a computer-controlled PAR
Model 283 potentiostat. All redox potentials are reported against the
ferrocene/ferrocenium (Fc/Fc+) redox couple. Ferrocene was used as
internal standard. Spectroelectrochemical measurements were per-
formed in an optically transparent thin-layer electrochemical cell26
with minigrid platinum working and auxiliary electrodes and silver
microwire pseudoreference electrode. The cell was connected to a
computer-controlled PAR Model 283 potentiostat. UV−vis spectra
were recorded with a Hewlett-Packard HP 8453 diode-array
spectrophotometer.
Synthetic Procedures. 5-Phenyl-5H-dibenzo[b,f ]phosphepine−
Borane (7a). A solution of (Z)-1,2-bis(2-bromophenyl)ethene (5;27
1.83 mmol, 618 mg) in 9 mL of diethyl ether was added dropwise to a
solution of t-BuLi (7.32 mmol, 4.58 mL of 1.6 M solution in pentane)
in 9 mL of diethyl ether at −78 °C, and the mixture was stirred for 1 h
at this temperature. Then, phenyldichlorophosphine (1.83 mmol, 328
mg, 0.25 mL) was added, and the reaction mixture was warmed to
room temperature. After the mixture was stirred at room temperature
for 1.5 h, a 2.0 M solution of BH3−SMe2 in diethyl ether (1.92 mmol,
0.96 mL) was added dropwise, and the resulting mixture was stirred
overnight followed by the addition of water (10 mL) and diethyl ether
(20 mL), filtration through Celite, and column chromatography
purification on silica (Et2O/pentane 1/1). This gave 448 mg (1.49
mmol, 82% yield) of 7a as a colorless solid. It is recommended to
monitor the reaction by 31P NMR before isolation, since in some cases
we observed slow formation of the BH3 adduct (this, probably,
depends on the quality of BH3−SMe2). If conversion is not completed,
more BH3−SMe2 solution should be added. Mp: 182−184 °C
(colorless crystals). 1H NMR (400 MHz, CDCl3): 1.35 (q, 3H, 1JHB
78.8 Hz, BH3), 6.81 (s, 2H, H-1), 6.90−6.92 (m, 2H, o-Ph), 7.18 (td,
=
2
4
2H, JHH = 7.9, JHP = 2.3 Hz, m-Ph), 7.30 (m, 1H, p-Ph), 7.46−7.49
(m, 2H, H-2), 7.59−7.64 (m, 4H, H-3, H-5), 8.42−8.47 (m, 2H, H-4).
31P{1H} NMR (162 MHz, CDCl3): 13.4 (q, 1JPB = 71.8 Hz). 11B NMR
(128 MHz, CDCl3): −37.9 (m). 13C{1H} NMR (101 MHz, CDCl3):
126.8 (d, 1JCP = 55.8 Hz, C-5a), 128.0 (d, 3JCP = 10.6 Hz, m-Ph), 128.9
(d, 2JCP = 12.6 Hz, C-5), 129.6 (d, 1JCP = 61.5 Hz, i-Ph), 130.1 (d, 4JCP
3
2
= 2.6 Hz, p-Ph), 130.2 (d, JCP = 6.5 Hz, C-2), 130.9 (d, JCP = 10.1
4
3
Hz, o-Ph), 131.7 (d, JCP = 1.8 Hz, C-2), 132.5 (d, JCP = 1.4 Hz, C-
10), 135.1 (d, JCP = 17.8 Hz, C-4), 139.3 (C-1a). IR: ν 3058 (w),
̃
3016 (w), 2379 (s), 2343 (m), 1476 (m), 1437 (m), 1140 (m), 1062
3
H
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(s), 807 (s), 733 (s), 697 (s), 616 (m), 460 (s). HR-FAB MS: calcd for
C20H17BP (M − H) 299.1165, found 299.1169; m/z (%) 299 (100)
[M − H], 209 (15) [M − BH3 − Ph], 178 (25) [M − BH3 − Ph − P],
154 (80), 136 (60), 107 (20) [C6H4P].
FAB MS: calcd for C23H23BP (M − H) 341.1635, found 341.1639; m/
z (%) 341 (100) [M − H], 328 (55) [M − BH3], 209 (20) [M − BH3
− Mes], 183 (20) [M − BH3 − Mes − CHCH], 150 (15).
5-Mesityl-5H-dibenzo[b,f ]phosphepine (6b). A 1.47 g portion
(13.10 mmol) of DABCO was added to a solution of 447 mg (1.31
mmol) of 7b in 20 mL of dichloromethane at room temperature, and
the mixture was stirred overnight. The next day the mixture was
filtered over a silica pad and then through Celite, washed with a 5%
aqueous solution of HCl, and extracted with diethyl ether. The
combined organic extracts were dried with MgSO4, and all volatiles
were evaporated under reduced pressure. This gave 398 mg (1.31
mmol, 96%) of phosphine 6b as a yellowish solid. It is recommended
to monitor the reaction by 31P NMR before isolation, since in some
cases we observed slow deborylation of the BH3 adduct. If conversion
is not completed, more DABCO should be added. Mp: 183−186 °C
(yellowish solid). 1H NMR (400 MHz, CDCl3): 2.42 (s, 6H, o-
CH3(Mes)), 2.43 (s, 3H, p-CH3(Mes)), 6.81 (t, 2H, JHH = 6.8 Hz),
6.96 (s, 2H, H-1), 7.10−7.14 (m, 4H), 7.17−7.24 (m, 4H). 31P{1H}
NMR (162 MHz, CDCl3): −30.5. 13C{1H} NMR (101 MHz): 21.4
5-Phenyl-5H-dibenzo[b,f ]phosphepine (6a). A 1.14 g portion
(10.2 mmol) of DABCO was added to a solution of 1.53 g (5.1 mmol)
of 7a in 40 mL of toluene at room temperature, and the mixture was
stirred overnight. The next day the mixture was quenched with a 5%
aqueous solution of HCl and extracted with diethyl ether. The
combined organic extracts were dried with MgSO4, and all volatiles
were removed under reduced pressure. This gave 1.42 g (4.97 mmol,
97%) of phosphine 6a as a colorless solid. It is recommended to
monitor the reaction by 31P NMR before isolation, since in some cases
we observed slow deborylation of the BH3 adduct. If conversion is not
completed, more DABCO should be added. Mp: 132−134 °C
(colorless solid). 1H NMR (500 MHz, CDCl3): 6.78 (s, 2H, H-1), 6.90
2
4
(dd, 2H, JHH = 7.7, JHP = 1.7 Hz, m-Ph), 7.13−7.20 (m, 3H, o-, p-
Ph), 7.39−7.46 (m, 6H, H-2, H-3, H-4/5), 7.83−7.88 (m, 2H, H-5/4).
31P{1H} NMR (162 MHz, CDCl3): −8.1. 13C{1H} NMR (126 MHz,
CDCl3): 127.4 (p-Ph), 127.7 (d, 2JCP = 4.8 Hz, o-Ph), 128.2 (d, JCP
=
(p-CH3(Mes)), 24.6 (d, 2JCP = 18.9 Hz, o-CH3(Mes)), 125.5 (d, 3JCP
8.4 Hz), 127.4, 128.5, 129.7 (d, JCP = 6.3 Hz), 129.8 (d, JCP = 4.7 Hz),
130.4 (d, JCP = 4.7 Hz), 134.7 (d, 3JCP = 6.3 Hz, C-1), 137.8 (d, JCP
16.5 Hz), 138.6 (d, JCP = 21.0 Hz), 140.6 (d, JCP = 1.4 Hz), 147.0 (d,
JCP = 17.2). IR: ν 3051 (w), 3012 (w), 2963 (w), 2917 (w), 2853 (w),
=
15.2, C-4/5), 129.7 (d, 4JCP = 0.7 Hz, C-3), 130.5 (d, 3JCP = 1.6 Hz, C-
3
3
2), 131.4 (d, JCP = 16.3 Hz, m-Ph), 132.6 (d, JCP = 1.3 Hz, C-1),
=
135.8 (d, 1JCP = 9.3 Hz, C-5a), 137.0 (d, JCP = 45.8 Hz, C-5/4), 137.4
(d, 1JCP = 9.5 Hz, i-Ph), 139.9 (C-1a). IR: ν
̃
3048 (m), 3009 (m), 1476
̃
(m), 1427 (m), 1264 (m), 804 (s), 775 (s), 740 (s), 694 (s), 496 (m),
460 (s). HR-ESI MS: calcd for C20H16P (M + H) 287.0984, found
297.0979; m/z (%) 287 (100) [M + H], 219 (10), 165 (80).
Reduction of 6a with Sodium. Compound 6a (143 mg, 0.5 mmol)
was dissolved in 1.5 mL of THF, and 115 mg (5.0 mmol) of sodium
was added to this solution at room temperature. Upon stirring, the
reaction mixture changed color from reddish to deep purple. After 4 h
of stirring at room temperature the solution was decanted from
unreacted sodium. The mixture was 31P NMR silent. Crystals of the
dianionic species [6a]2− were obtained by slow evaporation of pentane
into the THF solution obtained after decantation from sodium; these
were also 31P NMR silent.
1469 (m), 1423 (m), 1094 (m), 1027 (m), 800 (s), 747 (s). HR-FAB
MS: calcd for C23H22P (M + H) 329.1459, found 329.1459; m/z (%)
328 (60) [M], 281 (20), 207 (25), 147 (40), 73 (100).
Reduction of 5-Mesityl-5H-dibenzo[b,f ]phosphepine (6b) with
Sodium. Reduction was carried out similarly to the reduction of the
phosphepine 6a. The mixture was 31P NMR silent.
Tris(5-phenyl-5H-dibenzo[b,f ]phosphepine)rhodium(I) Triflate
(8a[OTf]). A solution of dibenzophosphepine 6a (0.34 mmol, 97
mg) in 5 mL of dichloromethane was added to a solution of
dicyclooctadiene rhodium(I) triflate (0.11 mmol, 51 mg) in 5 mL of
dichloromethane at room temperature. The mixture was stirred for 6
h, all volatiles were removed by vacuum evaporation, and the solid was
washed with pentane in order to get rid of COD. This gave 8a as a
yellow solid in quantitative yield. Crystals suitable for an X-ray study
can be obtained by dissolution of the complex in DCM and slow
evaporation of pentane into this solution. Mp: >270 °C (dec without
5-Mesityl-5H-dibenzo[b,f ]phosphepine−Borane (7b). A solution
of (Z)-1,2-bis(2-bromophenyl)ethene (5;27 4.70 mmol, 1.59 g) in 30
mL of diethyl ether was added dropwise to a solution of t-BuLi (18.82
mmol, 11.7 mL of a 1.7 M solution in pentane) in 30 mL of diethyl
ether at −78 °C, and the mixture was stirred for 1 h at this
temperature. Then, mesityldichlorophosphine28 (mixture of MesPCl2
and MesPClBr, approximately 4/1, 4.70 mmol, 1.08 g) in 15 mL of
diethyl ether was added, and the reaction mixture was warmed to room
temperature. The mixture was stirred at room temperature overnight.
Full conversion was observed with 31P NMR. After that a 2.0 M
solution of BH3−SMe2 in diethyl ether (4.94 mmol, 2.47 mL) was
added dropwise to the reaction mixture at room temperature. The
mixture was stirred overnight, followed by the addition of water (20
mL) and diethyl ether (40 mL), filtration through Celite, and column
chromatography purification on silica (MTBE/pentane 10/0.1, then
10/1). This gave 520 mg (1.52 mmol, 32% yield) of 7b as a colorless
solid. It is recommended to monitor the reaction by 31P NMR before
isolation, since in some cases we observed slow formation of the BH3
adduct (this, probably, depends on the quality of BH3−SMe2). If
conversion is not completed, more BH3−SMe2 solution should be
1
melting). H NMR (500 MHz, CD2Cl2, 215 K): 4.31 (s, CHCH),
4.85 (s, CHCH), 5.41−5.64 (m, Ar), 6.06 (s, CHCH), 6.58−7.91
(m, Ar). 31P{1H} NMR (202 MHz, CD2Cl2, 215 K): 19.7 (ddd, 1JPRh
=
=
=
119.0, 2JPP = 38.1, 2JPP = 21.8 Hz, 1P), 31.4 (ddd, 2JPP = 395.0, 1JPRh
2
2
1
2
95.2, JPP = 21.8 Hz, 1P), 39.2 (ddd, JPP = 395.0, JPRh = 95.2, JPP
38.1 Hz, 1P). 19F{1H} NMR (235 MHz, CD2Cl2, 298 K): −153.7. IR:
̃
ν 2966 (w), 2920 (w), 2850 (w), 1455 (w), 1356 (w), 1278 (m), 1260
(s), 1083 (m), 1027 (s), 793 (s). HR-FAB MS: calcd for C60H45P3Rh
(M − OTf) 961.1789, found 961.1786; m/z (%) 961 (12) [M −
OTf], 675 (100) [M − OTf − 6a], 497 (10), 389 (25) [M − OTf −
26a], 154 (48), 136 (33).
Bis(5-mesityl-5H-dibenzo[b,f ]phosphepine)rhodium(I) Triflate
(8b[OTf]). A solution of dibenzophosphepine 6b (0.30 mmol, 100
mg) in 2 mL of dichloromethane was added at room temperature to a
solution of dicyclooctadiene rhodium(I) triflate (0.15 mmol, 71 mg) in
2 mL of dichloromethane. The mixture was stirred at room
temperature for 2 h, and then all volatiles were removed by vacuum
evaporation and the solid was washed with diethyl ether. This gave 130
mg (0.14 mmol, 93%) of a mixture of cis and trans isomers of complex
8b (the ratio according to 31P NMR is 1/0.1). The major isomer can
be obtained by slow crystallization from a dichloromethane solution of
the mixture, covered by pentane. This gives 75 mg (0.083 mmol, 55%)
of the cis isomer as deep red needles. 31P{1H} NMR of the crude
mixture of cis and trans isomers (signal ratio is 1/0.1; 101 MHz,
1
added. Mp: 181−183 °C (colorless crystals). H NMR (400 MHz,
CDCl3): 1.43 (q, 3H, 1JHB = 79.5 Hz, BH34), 1.55 (s, 6H, o-CH3(Mes)),
2.19 (s, 3H, p-CH3(Mes)), 6.63 (d, 2H, JHP = 2.8 Hz, m-Mes), 6.85
(s, 2H, H-1), 7.38 (d, 2H, 4JHP = 6.9 Hz, H-2), 7.48−7.57 (m, 4H, H-3,
H-4), 8.36−8.42 (m, 2H, H-5). 31P{1H} NMR (162 MHz, CDCl3):
1
10.4 (q, JPB = 74.1 Hz). 11B NMR (128 MHz, CDCl3): −39.8 (m).
13C{1H} NMR (101 MHz, CDCl3): 20.9 (s, p-CH3(Mes)), 22.8 (d,
1
3JCP = 5.5 Hz, o-CH3(Mes)), 121.5 (d, JCP = 57.2 Hz, i-Mes), 128.7
1
1
(d, 3JCP = 6.6 Hz, C-2), 129.2 (d, 3JCP = 13.0 Hz, C-4), 130.5 (d, 1JCP
=
CD2Cl2): 34.6 (d, JPRh = 149.0 Hz), 43.1 (d, JPRh = 127.3 Hz). Mp
243−246 °C (red solid). NMR spectra of the purified cis-8b[OTf]: 1H
NMR (400 MHz, CD2Cl2) 1.97 (s, 6H, o-CH3(Mes)), 2.32 (s, 3H, p-
CH3(Mes)), 6.20 (s, 2H, H-1), 6.70 (s, 2H, m-Mes), 7.09−7.14 (m,
2H, H-3/4/5), 7.21−7.26 (m, 4H, H-3/4/5), 7.51 (d, 2H, J = 4.6 Hz,
H-2); 31P{1H} NMR (162 MHz, CD2Cl2) 34.6 (d, 1JPRh = 149.0 Hz);
56.1 Hz, C-5a), 130.68 (C-3), 130.73 (d, 3JCP = 6.5 Hz, m-Mes), 131.7
3
2
(d, JCP = 1.7 Hz, C-1), 133.1 (d, JCP = 18.1 Hz, C-5), 138.4 (C-1a),
139.7 (d, 4JCP = 2.6 Hz, p-Mes), 142.0 (d, 2JCP = 8.5 Hz, o-Mes). IR: ν
̃
3058 (w), 3016 (w), 2970 (w), 2927 (w), 2382 (s), 2343 (m), 1451
(m), 1136 (w), 1058 (s), 804 (s), 775 (m), 743 (m), 460 (m). HR-
I
dx.doi.org/10.1021/om301012z | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
19F{1H} NMR (235 MHz, CD2Cl2) −78.7; 13C{1H} NMR (101 MHz,
CD2Cl2) 21.2 (p-CH3(Mes)), 27.3 (o-CH3(Mes)), 96.6 (C-1), 113.9
(d, 1JCP = 44.1 Hz, i-Mes), 121.5 (q, 1JCF = 318.8 Hz, CF3), 127.1 (C-
Chem. 1992, 428, 289−301. (e) Weding, N.; Hapke, M. Chem. Soc.
Rev. 2011, 40, 4525−4538.
(2) Reviews: (a) Herndon, J. W. Coord. Chem. Rev. 2010, 254, 103−
194. (b) de Bruin, B.; Hetterscheid, D. J. H. Eur. J. Inorg. Chem. 2007,
211−230. (c) Adams, R. D. Chem. Soc. Rev. 1994, 335−339. (d) Hahn,
C. Chem. Eur. J. 2004, 10, 5888−5899.
3
3
3/4/5), 127.5 (t, JCP ≈ JCRh = 7.0 Hz, C-2), 129.6 (C-3/4/5), 129.9
3
4
(C-3/4/5), 131.4 (t, JCP ≈ JCRh = 3.9 Hz, m-Mes), 134.9 (d, JCP
43.1 Hz, C-5a/1a), 142.0 (p-Mes), 142.5 (o-Mes), 144.2 (d, JCP = 22.6
Hz, C-1a/5a). IR: ν 3058 (w), 2966 (w), 2920 (w), 1455 (m), 1260
=
̃
(3) (a) Maire, P.; Deblon, S.; Breher, B.; Geier, J.; Bohler, C.;
̈
(s), 1154 (m), 1030 (s), 779 (m), 637 (s). HR-FAB MS: calcd for
C46H42P2Rh (M − OTf) 759.1817, found 759.1809; m/z (%) 759
(100) [M − OTf], 431 (50) [M − OTf − 6b], 311 (12), 154 (37),
136 (30).
Ruegger, H.; Schonberg, H.; Grutzmacher, H. Chem. Eur. J. 2004, 10,
̈
̈
̈
4198−4205. (b) Piras, E.; Lang, F.; Ruegger, H.; Stein, D.; Worle, M.;
Grutzmacher, H. Chem. Eur. J. 2006, 12, 5849−5858. (c) Ferrer, C.;
̈
̈
̈
̈
Benet-Buchholz, J.; Riera, A.; Verdaguer, X. Chem. Eur. J. 2010, 16,
8340−8346. (d) Hayashi, T.; Ueyama, K.; Tokunaga, N.; Yoshida, K. J.
Am. Chem. Soc. 2003, 125, 11508−11509. (e) Fischer, C.; Defieber, C.;
Suzuki, T.; Carreira, E. M. J. Am. Chem. Soc. 2004, 126, 1628−1629.
Bis(5-mesityl-5H-dibenzo[b,f ]phosphepine)rhodium(I) Tetrakis-
[3,5-(trifluoromethyl)phenyl]borate (8b[BArF]). OTf → BArF ex-
change was achieved according to the procedure of Grutzmacher et
̈
al.6a Complex 8b (X = OTf; 265 mg, 0.292 mmol) and NaBArF (269
mg, 0.304 mmol) were stirred in 10 mL of DCM (dry) overnight. A
white precipitate (NaOTf) was filtered off over Celite. The solvent was
removed under reduced pressure, and the red solid was dissolved in 3
mL of diethyl ether, layered with 10 mL of hexane, and crystallized
overnight. The product was filtered off and dried under reduced
pressure, which gave 376 mg (0.223 mmol, 80%) of 8b (X = BArF) as
a deep red crystalline product. The complex is air stable and soluble in
(f) Puschmann, F. F.; Grutzmacher, H.; de Bruin, B. J. Am. Chem. Soc.
̈
2010, 132, 73−75. (g) Bohler, C.; Avarvari, N.; Schonberg, H.; Worle,
̈
̈
̈
M.; Ruegger, H.; Grutzmacher, H. Helv. Chim. Acta 2001, 84, 3127−
3147. (h) Thoumazet, C.; Grutzmacher, H.; Deschamps, B.; Ricard, L.;
̈
̈
̈
le Floch, P. Eur. J. Inorg. Chem. 2006, 3911−3922. (i) Narui, R.;
Hayashi, S.; Otomo, H.; Shintani, R.; Hayashi, T. Tetrahedron:
Asymmetry 2012, 23, 284−293. (j) Bernard, M.; Guiral, V.; Delbecq,
F.; Fache, F.; Sautel, P.; Lemaire, M. J. Am. Chem. Soc. 1998, 120,
1441−1446.
1
DCM, THF, and diethyl ether. Mp: 205−210 °C dec (red solid). H
NMR (400 MHz, CD2Cl2): 2.00 (s, 12H, o-CH3(Mes)), 2.35 (s, 6H,
p-CH3(Mes)), 6.03 (s, 4H), 6.74 (s, 4H), 7.15−7.20 (m, 4H), 7.25−
7.32 (m, 8H), 7.45−7.49 (m, 4H), 7.60 (s, 4H), 7.77 (s, 8H). 31P{1H}
NMR (162 MHz, CD2Cl2): 33.3 (d, 1JPRh = 142.8 Hz). 11B NMR (128
MHz, CDCl3): −6.6 (s). 19F{1H} NMR (235 MHz, CD2Cl2): −62.8.
13C{1H} NMR (126 MHz, CD2Cl2): 20.7 (p-CH3(Mes)), 26.9 (o-
CH3(Mes)), 95.7, 113.0, 117.4, 124.6 (q, 1JCF = 278.4 Hz, CF3), 126.8,
(4) For a review on the use of chiral olefins and related chiral hetero
bidentate amine−olefin and phosphane−olefin ligands in asymmetric
catalysis, see: Defieber, C.; Grutzmacher, H.; Carreira, E. M. Angew.
̈
Chem., Int. Ed. 2008, 47, 4482−4502.
(5) Pregosin, P. S. Chem. Commun. 2008, 4875−4884.
(6) (a) Puschmann, F. F.; Harmer, J.; Stein, D.; Ruegger, H.; de
̈
Bruin, B.; Grutzmacher, H. Angew. Chem., Int. Ed. 2010, 49, 385−389.
̈
2
126.9, 128.9 (q, JCF = 30.8 Hz, C−CF3), 129.3, 129.8, 131.1, 134.4,
(b) Puschmann, F. F.; Grutzmacher, H.; de Bruin, B. J. Am. Chem. Soc.
̈
1
134.8, 141.9, 142.2, 143.4 (d, J = 20.3 Hz), 161.8 (q, JCB = 49.7 Hz,
2010, 132, 73−75.
̃
C−B). IR: ν 3072 (w), 2927 (w), 1610 (w), 1455 (w), 1352 (m), 1278
(7) Piras, E.; Lang, F.; Ruegger, H.; Stein, D.; Worle, M.;
̈
̈
̈
(s), 1126 (s), 881 (w). HR-FAB MS: calcd for C46H42P2Rh (M −
BArF) 759.1817, found 759.1828; m/z (%) 759 (100) [M − OTf],
431 (50) [M − BArF − 6b], 310 (12), 251 (6), 178 (4).
Grutzmacher, H. Chem. Eur. J. 2006, 12, 5649−5858.
̈
́
́
(8) Campos, J.; Lopez-Serrano, J.; Alvarez, E.; Carmona, E. J. Am.
Chem. Soc. 2012, 134, 7165−7175.
(9) (a) Lyaskovskyy, V.; Elders, N.; Ehlers, A. W.; Lutz, M.;
Slootweg, J. C.; Lammertsma, K. J. Am. Chem. Soc. 2011, 133, 9704−
9707. (b) Jansen, H.; Samuels, M. C.; Couzijn, E. P. A.; Slootweg, J.
C.; Ehlers, A. W.; Chen, P.; Lammertsma, K. Chem. Eur. J. 2010, 16,
1454−1458. (c) Borst, M. L. G.; Bulo, R. E.; Winkel, C. W.; Gibney,
D. J.; Ehlers, A. W.; Schakel, M.; Lutz, M.; Spek, A. L.; Lammertsma,
K. J. Am. Chem. Soc. 2005, 127, 5800−5801.
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving experimental synthetic
procedures, Cartesian coordinates and energies of all stationary
points (DFT), crystallographic data, NMR spectra of all new
compounds, EPR spectra of the compounds [6a•]− and [6b•]−,
and cyclic voltammograms of 6a, 8a, and 8b. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
(10) Markl, G.; Burger, W. Angew. Chem., Int. Ed. Engl. 1984, 23,
̈
894−895.
(11) (a) Jansen, H.; Slootweg, J. C.; Ehlers, A. W.; Lammertsma, K.
Organometallics 2010, 29, 6653−6659. (b) Yasuike, S.; Kiharada, T.;
Kurita, J.; Tsuchiya, T. Chem. Commun. 1996, 2183−2184. (c) Kassaee,
M. Z.; Cheshmehkani, A.; Musavi, S. M.; Majdi, M.; Motamedi, E. J.
Mol. Struct. (THEOCHEM) 2008, 865, 73−78.
AUTHOR INFORMATION
Corresponding Author
*E-mail: B.deBruin@uva.nl (B.d.B.); k.lammertsma@vu.nl
(K.L.).
■
(12) (a) Markl, G.; Burger, W. Tetrahedron Lett. 1983, 24, 2545−
̈
2548. (b) Yasuike, S.; Kiharada, T.; Tsuchiya, T.; Kurita. J. Chem.
Pharm. Bull. 2003, 51, 1283−1288. (c) Kurita, J.; Shiratori, S.; Yasuike,
S.; Tsuchiya, T. J. Chem. Soc., Chem. Commun. 1991, 1227−1228.
(13) Yasuike, S.; Ohta, H.; Shiratori, S.; Kurita, J.; Tsuchiya, T. J.
Chem. Soc., Chem. Commun. 1993, 1817−1819.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(14) Alternatively, dibenzophosphepines can be first converted to the
corresponding phosphane oxides, followed by purification and
subsequent reduction to the phosphanes with silanes: Segall, Y.;
Shirin, E.; Granoth, I. Phosphorus, Sulfur, Silicon Relat. Elem. 1980, 8,
243−254. A direct isolation (without the phosphorus protection step)
of the phosphepine 6a has also been reported,13 but the latter
procedure did not work in our hands.
(15) Dunne, B. J.; Orpen, A. J. Acta Crystallogr. 1991, C47, 345−347.
(16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.;
Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;
The work was financially supported by the Council for
Chemical Sciences of The Netherlands Organization for
Scientific Research (NWO/CW), the European Research
Council (Grant Agreement 202886), the University of
Amsterdam, and the VU University Amsterdam.
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dx.doi.org/10.1021/om301012z | Organometallics XXXX, XXX, XXX−XXX