Synthesis of rhodaboratranes bearing phosphine-tethered boranes: Evaluation of the metal-boron interaction

Article abstract of DOI:10.1021/om3000423

A series of rhodaboratranes [{o(Ph₂P)C₆H ₄}₃BRhH_n(CO)]^m (1, n = 1, m = 0; 4, n = 0, m = +1; 5, n = 0, m = -1) with different electron charges ranging from -1 to +1 have been synthesized. X-ray diffraction, IR, NMR, and DFT calculation studies have demonstrated that the σ-acceptor borane ligand produces a unique electron distribution in these systems and significantly weakens the Rh-L bond (L = CO, PR₃) trans to the boron. The reversible CO/PR ₃ (R = Me or Ph) substitution reactions of 1 and 5 are attributed to these properties.

Full text of DOI:10.1021/om3000423

Article
pubs.acs.org/Organometallics
Synthesis of Rhodaboratranes Bearing Phosphine-Tethered Boranes:
Evaluation of the Metal−Boron Interaction
Hajime Kameo, Yasuhiro Hashimoto, and Hiroshi Nakazawa*
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585,
Japan
S
* Supporting Information
ABSTRACT: A series of rhodaboratranes [{o(Ph2P)-
C6H4}3BRhHn(CO)]m (1, n = 1, m = 0; 4, n = 0, m = +1;
5, n = 0, m = −1) with different electron charges ranging from
−1 to +1 have been synthesized. X-ray diffraction, IR, NMR,
and DFT calculation studies have demonstrated that the σ-
acceptor borane ligand produces a unique electron distribution
in these systems and significantly weakens the Rh−L bond (L
= CO, PR3) trans to the boron. The reversible CO/PR3 (R =
Me or Ph) substitution reactions of 1 and 5 are attributed to these properties.
and theoretical analyses, indicating that the coordination of the
σ-acceptor ligand is not necessarily accompanied by formal two-
electron oxidation.5c They also quantified the electron-with-
drawing effect of the σ-accepting borane moiety through IR
spectroscopic analyses of the structurally similar rhodium
complexes (DPB)Rh(CO)Cl and (i-Pr2PhP)2Rh(CO)Cl.5d
Further studies of new frameworks are ongoing,6 and insight
into metal−boron interactions has been accumulated. Never-
theless, examples of metal-based reactivity in such compounds
are still scarce. To the best of our knowledge, there are only a
few examples of redox reactions that preserve the metal−boron
bond. One such example was reported by Hill and co-workers,
in which they demonstrated deprotonation of the [{B-
(mimMe)3}Pt(H)(PPh3)]+ (mimMe = 2-mercapto-1-methylimi-
dazolyl) and oxidative additions to the resulting complex
[{B(mimMe)3}Pt(PPh3)].3c,j Another interesting report has
been presented by Peters and Moret,7 in which they prepared
a (TPB)Fe(Br) complex (TPB = triphosphine-borane {(i-
Pr)2P(C6H4)}3B), which was converted into dinitrogen (TPB)-
Fe(N2) and Na[(TPB)Fe(N2)] complexes by reduction with
sodium naphthalenide under dinitrogen.7a The resulting
(TPB)Fe(N2) reacted with N3Ar (Ar = p-methoxyphenyl) to
give the imido complex (TPB)FeNAr. The series of isolated
iron complexes spanned four different electronic states ranging
from (Fe−B)6 to (Fe−B)9. The successful syntheses of the
(TPB)Fe complexes with the various electron configurations
are likely to have resulted from the ability of the Fe−B
interaction to respond to changes in the electronic properties of
the metal center. These results demonstrate one of the useful
properties of Z-type ligands, which is the ability to tune the
electron density of the metal center.
INTRODUCTION
■
Ligands of transition-metal complexes are generally classified as
L-type, X-type, or Z-type (Chart 1).1 L-type ligands serve as
Chart 1. Three Coordination Styles of Ligands
two-electron donors for transition metals and are represented
by amines, phosphines, CO, and alkenes. X-type ligands
typically are composed of halides, alkyls, and hydrides and
are considered to donate one electron to the central metal. In
comparison, Z-type ligands accept electrons from transition
metals through a σ-bonding interaction. Group 13 BR3 Lewis
acids are known to act as such σ electron acceptor ligands.
These types of ligands potentially have two useful functions:
(1) they can tune the electron density of the metal center, and
(2) they exhibit a strong trans-labilizing effect. However, Z-type
ligands are much less well understood than L- and X-type
ligands, and their functions are in the early stages of
investigation.
A structurally authenticated metallaboratrane was reported
by Hill et al. in 1999 for a cage complex of ruthenium.2 After
this historical and pioneering work, tri- and tetradentate 2-
mercapto-1-R-imidazolyl-tethered (R = Me, t-Bu) borane
ligands have become irreplaceable scaffolds in inducing
transition-metal−boron interactions.3,4 Another important
scaffold featuring ambiphilic ligands was developed by
Bourissou et al. Their phosphine-based system has widened
the field of the chemistry of Z-type ligands.5 For example, the
(DPB)AuCl complex (DPB = diphosphineborane {o-R2P-
(C6H4)}2BPh, R = i-Pr, Ph) provided informative evidence
Received: January 19, 2012
Published: March 22, 2012
for the d10 configuration of the Au atom through Mossbauer
̈
© 2012 American Chemical Society
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Parallel studies of complexes with σ-acceptor borane ligands
and their borane-free complexes would provide fresh insight
into the chemistry of Z-type ligands. Hence, we focused on the
carbonyl rhodium hydride [{o-(Ph2P)C6H4}3B]RhH(CO) (1),
bearing a new triphosphine-borane (TPB) ligand, which has a
framework similar to that of the well-known tris-
(triphenylphosphine) carbonyl rhodium hydride RhH(CO)-
(PPh3)3 (2), except for the borane moiety (Chart 2). The
Chart 2. TPB Complex 1 and Its Related Borane-Free
Complex 2
Figure 1. ORTEP drawings of 3 (left) and 1 (right) with 40%
probability ellipsoids. Hydrogen atoms and phenyl groups (except for
the ipso carbon) in 1 are omitted for clarity. Selected bond distances
(Å) and angles (deg) are as follows. 3: B1−C1 = 1.574(4), B1−C19 =
1.580(4), B1−C37 = 1.569(4); C1−B1−C19 = 120.2(2), C1−B1−
C37 = 122.0(2), C19−B1−C37 = 114.9(2). 1: Rh1−P1 = 2.3116(13),
Rh1−P2 = 2.2733(13), Rh1−P3 = 2.3355(13), Rh1−C1 = 1.960(6),
Rh1−B1 = 2.370(5), Rh1−H1 = 1.5367(4), C1−O1 = 1.113(7), B1−
C2 = 1.632(7), B1−C20 = 1.637(7), B1−C38 = 1.604(7); P1−Rh1−
P2 = 134.19(5), P1−Rh1−P3 = 105.30(5), P2−Rh1−P3 = 107.40(5),
P1−Rh1−C1 = 108.47(16), P2−Rh1−C1 = 100.10(18), P3−Rh1−C1
= 94.40(18), P1−Rh1−B1 = 78.90(13), P2−Rh1−B1 = 77.65(14),
P3−Rh1−B1 = 77.82(13), C1−Rh1−B1 = 170.6(2), Rh1−C1−O1 =
173.6(5), C2−B1−C20 = 114.4(4), C2−B1−C38 = 109.6(4), C20−
B1−C38 = 108.5(4).
comparison should clearly reveal the functions of a Z-type
ligand. We focused on the TPB ligand [{o-(Ph2P)C6H4}3B]
(3), similar to Bourissou’s TPB ligand,8 but with the isopropyl
groups replaced with phenyl groups. The preparation and
characterization of complex 1 allowed us to directly compare
the properties of the Rh→B interaction in 1 with that in 2.
is comparable in length to the sum of the covalent bonds (2.22
Å).11 These results indicate the presence of a strong Rh→B
interaction in 1. Furthermore, the Rh−C1 distance (1.960 Å) is
considerably longer than typical Rh−CO bond distances,12
which is attributed to the strong trans influence of boron.
To estimate the σ-electron acceptability of the Z-type borane
ligand through the Rh→B interaction, we attempted making
drastic changes to the electron density of the metal center of 1
through abstraction of H on rhodium as H− or H+. The H−
abstraction can be accomplished by treating 1 with a strong
Brønsted acid. Treatment of 1 with (HOEt2)(BF4) at ambient
temperature immediately afforded the cationic rhodium
complex [{o-(Ph2P)C6H4}3BRh(CO)](BF4) (4) and dihydro-
gen (Scheme 2). The formation of dihydrogen was confirmed
RESULTS AND DISCUSSION
■
Compound 3 was prepared in an excellent yield by treating o-
lithiated-phenyldiphenylphosphine with borane trichloride at
100 °C (Scheme 1). The 31P{1H} NMR resonance of 3 was
observed at −9.1 ppm as a sharp singlet, which is consistent
with an approximate C3v-symmetric structure observed in the
solid state. Allowing 3 to react with 2 at 50 °C gave the desired
rhodium complex 1 in a moderate yield via phosphine ligand
exchange. In the 31P{1H} NMR spectrum of 1 at −100 °C, two
broad resonances were observed at δ 55 and 65 with an
intensity ratio of 2:1, which coalesced near −60 °C (see the
Experimental Section).9 These results corresponded to a Cs-
symmetric geometry observed in the solid state of 1 (see
below) and also suggested the fluxionality of 1. The carbonyl
stretching frequency of 1 (1994 cm−1) was higher than that of 2
(1918 cm−1). The shift strongly implies that the borane ligand
withdraws a significant amount of electron density through its
interaction with rhodium.
1
by monitoring the reaction of 1 with (HOEt2)(BF4) using H
NMR spectroscopy (δ 4.59 in CD2Cl2). The 31P{1H} NMR
spectrum of 4 showed two mutually coupled resonances at δ
33.1 and 45.0 (JP−P = 33.0 Hz) in a 2:1 ratio, which is
consistent with the Cs-symmetric structure observed in the solid
state. The H+ abstraction of 1 was achieved by deprotonation
using a mixture of KH and 18-crown-6, and the anionic
complex [{o-(Ph2P)C6H4}3BRh(CO)][K(18-crown-6)] (5)
containing an electron-rich Rh center was obtained in a
moderate yield. The 31P{1H} NMR spectrum of 5 recorded at
23 °C showed a sharp doublet at 60.7 ppm (JRh−P = 183.6 Hz).
The molecular structures of 3 and 1 were confirmed by X-ray
analysis (Figure 1). Compound 3 has a planar boron
configuration (∑(C−B−C) = 357.2°).10 In contrast, the
geometry of the boron in 1 deviates noticeably from planarity
(∑(C−B−C) = 332.5°), and the Rh−B distance (2.370 Å) in 1
Scheme 1. Synthetic Pathway for the Carbonyl Rhodium Hydride [{o-(Ph2P)C6H4}3B]RhH(CO) (1)
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Scheme 2. Synthetic Pathways for Cationic and Anionic Complexes [{o-(Ph2P)C6H4}3BRh(CO)](BF4) (4) and [{o-
(Ph2P)C6H4}3BRh(CO)][K(18-crown-6)] (5)
In the 11B{1H} NMR spectra, the resonance shifted to a higher
magnetic field in the order 4 < 1 ≈ 5 (4, δ 0.0; 1, δ −7.5; 5, δ
−7.6), which implies the strengthening of the σ-electron
donation from the Rh center.
The crystallographically determined structure of compound 4
is shown in Figure 2. The nonequivalence of the phosphine
assumption of a structure featuring trigonal-bipyramidal
geometry is strongly supported by density functional theory
(DFT) calculations.13 In the 31P{1H} NMR spectrum of 5, only
one sharp doublet was observed, as mentioned above.
Furthermore, the full width at half-maximum of the signal
remained sharp even at −80 °C (w1/2 = 1.8 Hz), suggesting that
5 has an approximate C3v-symmetric structure. Hence, the
NMR data for 5 are entirely consistent with the structure
optimized by DFT calculations.
Hope et al. prepared the cationic complex [Rh(CO)-
(PPh3)3](BF4) (6).14 The structure was similar to that of 4,
except for the borane anchor (Chart 3). The related anionic
Chart 3. Structures of [Rh(CO)(PPh3)3][BF4] (6) and
[Rh(CO)(PPh3)3][K(18-crown-6)] (7)
Figure 2. ORTEP drawing of 4 with 40% probability ellipsoids. The
anion BF4, hydrogen atoms, and phenyl groups (except for ipso
carbon) are omitted for clarity. Selected bond distances (Å) and angles
(deg): Rh1−P1 = 2.3104(16), Rh1−P2 = 2.3276(15), Rh1−P3 =
2.3773(15), Rh1−C1 = 1.949(6), Rh1−B1 = 2.286(6), C1−O1 =
1.113(7), B1−C2 = 1.614(8), B1−C20 = 1.619(8), B1−C38 =
1.608(8); P1−Rh1−P2 = 93.61(5), P1−Rh1−P3 = 163.45(5), P2−
Rh1−P3 = 94.08(5), P1−Rh1−C1 = 86.99(18), P2−Rh1−C1 =
177.15(17), P3−Rh1−C1 = 84.64(18), P1−Rh1−B1 = 86.50(16),
P2−Rh1−B1 = 82.09(17), P3−Rh1−B1 = 80.07(16), C1−Rh1−B1 =
95.2(2), Rh1−C1−O1 = 176.3(5), C2−B1−C20 = 110.7(4), C2−
B1−C38 = 118.0(5), C20−B1−C38 = 113.0(5).
complex Na[Rh(CO)(PPh3)3] was characterized by IR spec-
troscopy studies,15 but its structure remained unconfirmed. In
this study, the novel [Rh(CO)(PPh3)3][K(18-crown-6)] (7)
(Chart 3) was synthesized by the deprotonation of 2 with a
mixture of KH and 18-crown-6, and the structure of the
rhodium d10 complex featuring tetrahedral geometry was
determined (detailed data from the structural analysis are
shown in the Supporting Information).
groups noted in the 31P{1H} NMR spectrum is consistent with
the observed square-pyramidal geometry. The sum of the C−
B−C angles is larger in 4 (341.7°) than in 1 (332.5°),
indicating that the amount of electron transfer from rhodium to
boron decreases, owing to the lower Lewis basicity of the Rh
center. This result corresponds to the observation made above
in the 11B{1H} NMR spectra. Curiously, the Rh−B bond is
shorter in the cationic complex 4 (2.286 Å) than in the neutral
complex 1 (2.370 Å), although other results indicated that the
borane ligand in 1 withdraws more electrons than that in 4
does. This appears to be due to the absence of a ligand in a
position trans to the borane moiety. The unexpected
shortening of the Rh−B distance from 1 to 4 can be also
explained by the fact that the Rh−B interaction in 1 has more
of a 5p orbital character than that in 4 (see the results of
NLMO analyses in the Experimental Section). The radius of
the 5p orbital is generally larger than that of the 4d orbital;
thus, the Rh−B bond increases in length with increasing 5p
orbital character of the hybridization.
The carbonyl stretching frequencies of 6 and 7 were found to
be at 1988 and 1800 cm−1, respectively. These values are
considerably lower than those of their ionic complexes bearing
borane anchors (4, ν(CO) 2086 cm−1; 5, ν(CO) 1887 cm−1).
These shifts are attributable to the presence of the strong Rh→
B interaction.
To investigate the rhodium−boron interaction in the series
of 1−5, we performed DFT calculations on the actual
mononuclear complexes 1−5 (Figure S9, Supporting Informa-
tion). The B3PW9116/SDD17(Rh),6-31G(d) (C, H, O, P, B)
level of theory was found to well reproduce the key features of
the complexes 1−4, and there were virtually no deviations in
the Rh−B distance and the boron pyramidalization relative to
those measured by X-ray crystallography (see the Supporting
Information). Natural population analysis data18 together with
ν(CO) data for 1−5 are summarized in Table 1.
A comparison of 1 (rhodaboratrane), 2 (borane-free Rh
complex), and 3 (metal-free borane compound) revealed
interesting features of the Rh→B interaction. The natural
charge of boron (qB) is lower for 1 than for 3 (0.43 for 1 and
0.88 for 3), indicating that the Rh→B electron donation occurs
Although the structure of 5 was not determined by X-ray
diffraction analysis because of poor crystal quality, the
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Table 1. NBO Atomic Charges (qRh and qB) and ν(CO)
1
2
3
4
5
qRh
−1.54
0.43
−1.56
−0.90
0.65
−1.56
0.38
qB
0.88
0
ΔqB
−0.45
1994
−0.23
2086
−0.50
1887
ν(CO) (cm−1
)
1918
in 1 to a considerable extent. As mentioned above, the Rh−B
distance (2.370 Å) in 1, corresponding to the sum of the
covalent bonds (2.22 Å), also suggests the occurrence of a
Rh→B interaction. Interestingly, in contrast, the qRh value of 1
(−1.54) is almost equal to that of 2 (−1.56), where the Rh→B
interaction is absent. This means that the Rh in 1 increases the
electron density of B, while the electron density of Rh does not
decrease. The apparent discrepancy may be explained by
considering π back-donation from Rh to CO. The value of
ν(CO) of 1 (1994 cm−1) is much greater than that of 2 (1918
cm−1), showing that π back-donation from Rh to CO is weaker
for 1 than for 2. Therefore, the Rh in 1 gives its electron density
to B at the expense of π back-donation to CO.
Next, we consider the NBO data of 4, 1, and 5 with negative,
neutral, and positive charges on the rhodaboratranes,
respectively. Comparison of 1 and 5 reveals that values of qRh
are very close (−1.54 and −1.56, respectively) irrespective of
the total charge variation (0 to −1). In addition, the values of qB
are not significantly different (0.43 for 1 and 0.38 for 5). This
means that the Rh→B electron donation is the same for 1 and
5, which is, as mentioned above, supported by the 11B{1H}
NMR chemical shift (−7.5 ppm for 1 and −7.6 ppm for 5). In
contrast, the value of ν(CO) for 5 (1887 cm−1) is more than
100 cm−1 lower than that for 1 (1994 cm−1). These data show
that anionization has only a slight contribution to the
enhancement of electron density of either Rh or B, but it
significantly strengthens the back-donation from Rh to CO.
Therefore, it is considered that the borane ligand in 1 has
accumulated almost the maximum possible electron density,
and the negative charge given in 5 is mainly distributed to the
CO ligand through π back-donation from the Rh center.
With 4 bearing a positive charge, the values of qRh (−0.90)
and qB (0.65) indicate lower electron densities of Rh and B
than for 1 (−1.54 and 0.43) (and also 5). The value of ν(CO)
in 4 (2086 cm−1) is about 90 cm−1 larger than that of 1 (1994
cm−1), indicating the decrease in π back-donation to CO.
Therefore, it is considered that the positive charge in 4 weakens
the Rh→B donation and the π back-donation of Rh→CO and
thus decreases the electron density of the Rh.
the exposure of 8 to CO immediately resulted in the almost
quantitative regeneration of 1. Neutral monocarbonyl com-
plexes of rhodium(I) rarely show the CO replacement reaction,
because of the strong back-donation from the metal center. For
example, Baird et al. reported that the rhodium(I) mono-
carbonyl complex RhMe(CO)(η3-MeC(CH2PPh2)3) on reac-
tion with PMe3 gave RhMe(CO)(PMe3)(η2-MeC(CH2PPh2)3)
via a process involving dissociation of one arm of the triphos
ligand, but the subsequent dissociation of CO has not been
observed.20 Therefore, the properties of the σ-acceptor borane
ligand certainly contribute to the CO elimination.
Surprisingly, the anionic complex 5 also exhibited a
substitution reaction of a carbonyl ligand with a phosphorus
ligand under relatively mild conditions (80 °C) to produce the
carbonyl-free anionic complex 9 (eq 2). This substitution
reaction is also reversible; 9 reacted with CO to regenerate 5. In
comparison, we examined the reaction of the boron-free
complex 7 with an excess amount of PPh3 at 100 °C and found
the recovery of 7. The strong trans-labilizing effect21 and σ-
electron acceptor properties of borane are probably responsible
for the weakening of the Rh−CO bonds in 1 and 5.
−
Rh(PPh3)4 containing an anionic charge has not been
reported. In fact, Rh(−I) complexes with no strong π-acceptor
ligand, such as a carbonyl ligand, are rare.22 The presence of a
σ-acceptor borane ligand is therefore responsible for the
stability of 9.
Single crystals of 8 suitable for X-ray diffraction analysis were
obtained from slow diffusion of hexane into a dichloromethane
solution of 8. The molecular framework of 8 is basically similar
to that of 1 (Figure 3). The Rh−B distance (2.354 Å) pointed
to the presence of a Rh→B interaction. The boron
pyramidalization in 8 (∑(C−B−C) = 330.8°) is slightly
more than that of 1, which is probably due to the enhancement
The unique electron flow induced by the Rh→B interaction
is expected to influence the reactivity of rhodaboratranes.
Hence, we examined the CO/PMe3 substitution reaction.
Complex 1 reacted with a slight excess of PMe3 even at ambient
temperature to afford the phosphine complex [{o-(Ph2P)-
C6H4}3B]RhH(PMe3) (8) through a CO/PMe3 exchange
1
reaction (eq 1). Compound 8 was fully characterized by H
and 31P{1H} NMR spectroscopy as well as elemental
analysis.9,19 The substitution reaction was perfectly reversible;
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EXPERIMENTAL SECTION
■
General Procedures. The compounds described below were
handled under a dinitrogen atmosphere, and air and water were
removed completely using Schlenk techniques. o-(PPh2)C6H4Br23 and
RhH(CO)(PPh3)3 (2)24 were prepared according to literature
procedures. Benzene-d6, toluene-d8, tetrahydrofuran-d8, diethyl ether,
and benzene were dried over sodium benzophenone ketyl and distilled
under the dinitrogen atmosphere. Tetrahydrofuran, hexane, and
toluene were purified using a two-column solid-state purification
system. Chloroform-d, dichloromethane-d2, and dichloromethane were
dried over P2O5 and stored over 4 Å molecular sieves. The other
reagents used in this study were purchased from commercial sources
and used without further purification. The 1H, 11B, 13C, and 31P NMR
spectra were recorded on a JNM-AL-400 spectrometer. H and 13C
1
Figure 3. ORTEP drawings of 8 (left) and 9 (right) with 40%
probability ellipsoids. Hydrogen atoms and phenyl groups (except for
ipso carbon) are omitted for clarity. Selected bond distances (Å) and
angles (deg) are as follows. 8: Rh1−P1 = 2.3463(10), Rh1−P2 =
2.3033(11), Rh1−P3 = 2.3002(11) Rh1−P4 = 2.4077(11), Rh1−B1 =
2.354(4), Rh1−H1 = 1.59(6), B1−C1 = 1.628(6), B1−C19 =
1.616(6), B1−C37 = 1.636(6), Rh1−H1 = 1.59(6); P1−Rh1−P2 =
102.79(4), P1−Rh1−P3 = 102.25(4), P1−Rh1−P4 = 105.42(4), P2−
Rh1−P3 = 142.16(4), P2−Rh1−P4 = 99.02(4), P3−Rh1−P4 =
101.13(4), P1−Rh1−B1 = 78.31(11), P2−Rh1−B1 = 81.05(12), P3−
Rh1−B1 = 76.78(11), P4−Rh1−B1 = 176.11(11), C1−B1−C19 =
112.8(3), C1−B1−C37 = 107.3(3), C19−B1−C37 = 110.7(3). 9:
Rh1−P1 = 2.332(4), Rh1−P2 = 2.270(4), Rh1−P3 = 2.284(3), Rh1−
P4 = 2.286(4), Rh1−B1 = 2.501(15), B1−C1 = 1.52(3), B1−C19 =
1.61(2), B1−C37 = 1.573(19); P1−Rh1−P2 = 109.17(14), P1−Rh1−
P3 = 108.36(13), P1−Rh1−P4 = 104.88(10), P2−Rh1−P3 =
121.61(10), P2−Rh1−P4 = 105.77(14), P3−Rh1−P4 = 105.77(13),
P1−Rh1−B1 = 72.5(4), P2−Rh1−B1 = 76.9(3), P3−Rh1−B1 =
73.8(3), P4−Rh1−B1 = 176.9(3), C1−B1−C19 = 108.9(13), C1−
B1−C37 = 112.0(13), C19−B1−C37 = 112.2(11).
NMR data were referenced to the residual peaks of the solvent, and
11B and 31P NMR chemical shifts were referenced to external BF3·Et2O
and 85% H3PO4 samples, respectively. IR spectra were recorded on a
Perkin-Elmer FTIR-Spectrum instrument. Elemental analyses were
recorded on a Perkin-Elmer 2400 II elemental analyzer.
Preparation of [o-PPh2(C6H4)]3B (3). A 200 mL Schlenk tube was
filled with 4.17 g of o-PPh2(C6H4)Br and 60 mL of dimethyl ether, and
the mixture was cooled to −80 °C. To the cold solution was slowly
added 7.7 mL of nBuLi (1.6 M in hexane, 12.3 mmol), and then the
mixture was gradually warmed to room temperature. Additionally, the
reaction was stirred at room temperature for 1 h, before the volatile
materials were removed under vacuum. The white residue was washed
with 2 mL of Et2O to afford 3.88 g of {o-PPh2(C6H4)}Li·Et2O (11.3
mmol) as a white solid in 93% yield. After {o-PPh2(C6H4)}Li·Et2O
(3.88 g, 11.3 mmol) was dissolved in toluene (50 mL), the solution
was cooled again to −80 °C. To this cooled toluene solution was
added a 1 M heptane solution of BCl3 (3.75 mL, 3.75 mmol), and the
reaction mixture was stirred at 110 °C for 12 h. The mixture was
cooled to room temperature, before the volatile materials were
removed under vacuum. The residue was washed with hexane (15 mL
× 1, 5 mL × 2) to afford 2.42 g of 3 (3.00 mmol) as a white solid in
82% yield. The white powder was used in the next step without further
purification. A sample suitable for elemental analysis was prepared by
crystallization by the slow diffusion of hexane into a dichloromethane
solution followed by washing with Et2O. 1H NMR (400 MHz,
CDCl3): δ 6.94−7.00 (m, 15H), 7.08−7.14 (m, 12H), 7.16−7.22 (m,
12H), 7.33−7.39 (m, 3H). 31P{1H} NMR (163 MHz, CDCl3): δ −9.1
(s). 13C{1H} NMR (100 MHz, CDCl3): δ 128.2, 129.4, 133.9, 134.0,
134.3, 134.7, 135.3, 138.9, 141.3, 157.4. 11B{1H} NMR (128 MHz,
CDCl3): δ −7.6. Anal. Found for C54H42P3B: C, 82.00; H, 5.71. Calcd:
C, 81.62; H, 5.33.
of the Lewis basicity of the rhodium center derived from a
better σ-donating ability of the aliphatic phosphine. In a fashion
similar to that for 1, the σ-acceptor borane elongates the Rh−P
bond (2.4077 Å) trans to the boron, which is likely to be at the
origin of the phosphine elimination.
The molecular structure of 9 was also determined by X-ray
crystallography using a single crystal obtained from a 8:1 mixed
solvent of 1,4-dioxane and tetrahydrofuran (Figure 3). The X-
ray study exhibited a distorted-trigonal-bipyramidal geometry
with P−Rh−P angles of 109.17(14), 108.36(13), and
121.61(10)°, and the phosphine and borane ligands occupied
the apical positions. In the 31P{1H} NMR spectrum of 9,
mutually coupled signals were observed at δ 34.6 and 45.5 ppm
(JP−P = 25.6 Hz) with an intensity ratio of 1:3, which is
consistent with a C3v-symmetric structure observed in the solid
state.
Preparation of [{o-PPh2(C6H4)}3B]RhH(CO) (1). A 100 mL
Schlenk tube was filled with 1.24 g of 3 (1.60 mmol), 1.15 g of 2 (1.30
mmol), and 20 mL of benzene, and the reaction mixture was stirred at
50 °C. After 10 h, the mixture was cooled to room temperature, and
the volatile materials were removed under vacuum. The orange residue
was extracted with 50 mL of Et2O, before the volatile materials were
removed under vacuum. The residue was washed with hexane (20 mL)
and cold Et2O to afford 0.760 g of 1 (0.80 mmol) as a pale brown solid
in 65% yield. Variable-temperature 31P{1H} NMR spectra of 1 are
shown in Figure 4. 1H NMR (400 MHz, CDCl3): δ −7.84 (dq, 1JH−Rh
SUMMARY
2
■
= 36.6 Hz, JH−P = 10.1 Hz, 1H, RhH), 6.66−6.76 (m, 12H, Ar),
6.76−6.84 (m, 12H, Ar), 6.93−7.06 (m, 12H, Ar), 7.27−7.34 (m, 3H,
Ar), 7.89−7.94 (m, 3H, Ar). 31P{1H} NMR (163 MHz, CDCl3): δ
54.8 (d, JP−Rh = 128.5 Hz). 13C{1H} NMR (100 MHz, CDCl3): δ
123.2, 124.4, 124.9, 126.5, 127.3, 127.7, 128.2, 128.5, 128.8, 129.7,
131.5, 132.0, 132.9, 133.4, 133.6, 136.4, 137.2, 140.0, 141.5, 169.8,
192.4. 11B{1H} NMR (128 MHz, THF-d8): δ −7.5. Anal. Found for
C55H43P3OBRh: C, 71.28; H, 5.04. Calcd: C, 71.29; H, 4.68. IR (KBr):
1994 cm−1 (νCO).
We synthesized a series of rhodaboratrane compounds with
different electron charges ranging from −1 to +1. These
compounds provide conclusive evidence for the adaptability of
σ-acceptor borane ligands to tune the electron density of the
metal center. Studies of the electron distribution using NBO
analysis show a unique electron flow induced by the σ-acceptor
borane ligand. Furthermore, we showed that the σ-acceptor
borane ligand potentially exhibits an extremely strong trans-
labilizing effect. The properties of these σ-acceptor borane
ligands make them promising candidates for the development
of molecular catalysts.
Preparation of [{o-PPh2(C6H4)}3B]Rh(CO)[BF4] (4). A 50 mL
Schlenk tube was filled with 117.2 mg of 1 (0.126 mmol) and 10 mL
of dichloromethane, and then 18 μL of tetrafluoroboric acid ether
complex (0.13 mmol) was added slowly to the solution. The reaction
mixture was stirred at room temperature for 1 h, before the volatile
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toluene, and then a 1 M toluene solution of PMe3 (300 μL, 0.30
mmol) was added slowly to the solution. The reaction mixture was
stirred at room temperature overnight, before the volatile materials
were removed under vacuum. The residue was washed with hexane (2
mL × 2) to afford 120.5 mg of 8 (0.124 mmol) as a white solid in 81%
yield. 1H NMR (400 MHz, CDCl3): δ −10.56 (ddq, 1JH,Rh = 31.7 Hz,
2JH,P = 14.6 Hz, 2JH,P = 6.1 Hz, 1H, RhH), 0.89 (d, 1JH,P = 4.9 Hz, 3H,
PMe3), 6.52−6.62 (m, 3H, Ar), 6.63−6.74 (m, 3H, Ar), 6.92−7.06 (m,
12H, Ar), 7.07−7.19 (m, 15H, Ar), 7.27−7.41 (m, 9H, Ar). 31P{1H}
NMR (163 MHz, CDCl3): δ −34.1 (d, JP−Rh = 91.8 Hz, PMe3), 52.1
(d, JP−Rh = 135.8 Hz, Ph2P). 13C{1H} NMR (100 MHz, CDCl3): δ
21.8, 123.0, 123.7, 125.3, 127.1, 127.4, 127.9, 128.4, 128.6, 129.0,
129.8, 131.5, 132.1, 133.0, 133.5, 133.6, 133.8, 137.4, 138.9, 144.8,
168.8. 11B{1H} NMR (128 MHz, CDCl3): δ −7.6. Anal. Found for
C57H52P4BRh: C, 70.14; H, 5.67. Calcd: C, 70.24; H, 5.38.
Preparation of [{o-PPh2(C6H4)}3B]Rh(PPh3)[K(18-crown-6)]
(9). Method A. A 50 mL Schlenk tube was filled with 39.7
mg of 5 (0.0323 mmol), 28.3 mg of PPh3 (0.108 mmol), and 3
mL of THF, and the reaction mixture was stirred at 80 °C.
After 5 h, the mixture was cooled to room temperature and
filtered with a cannula. Slow diffusion of hexane into the
mixture provided 12.1 g of 9 (0.00827 mmol) as red-brown
crystals in 26% yield.
Method B. A 50 mL Schlenk tube was filled with 104.3 mg of 1
(0.113 mmol), 57.5 mg of 18-crown-6 (0.218 mmol), 24.9 mg of KH
(0.621 mmol), 70.3 mg of PPh3 (0.268 mmol), and 4 mL of THF, and
the reaction mixture was stirred at 80 °C. After 3 h, the mixture was
cooled to room temperature and filtered with a cannula. Slow diffusion
of hexane into the filtrate provided 67.8 g of 9 (0.0463 mmol) as red-
brown crystals in 41% yield. 31P{1H} NMR (163 MHz, THF-d8): δ
Figure 4. Variable-temperature 31P{1H} NMR spectra (163 MHz,
THF-d8) of 1.
materials were removed under vacuum. The white residue was washed
with Et2O (1 mL × 2) to afford 115.4 mg of 4 (0.114 mmol) as a
white solid in 90% yield. A 13C NMR spectrum could not be obtained
1
because of its poor solubility. H NMR (400 MHz, CDCl3): δ 6.62−
6.71 (m, 3H), 6.76−6.88 (m, 12H), 6.96−7.08 (m, 9H), 7.16−7.23
(m, 3H), 7.28−7.44 (m, 6H), 7.48−7.64 (m, 6H), 7.92−8.00 (m, 3H).
31P{1H} NMR (163 MHz, CDCl3): δ 33.1 (d, JP−Rh = 110.1 Hz, JP−P
=
33.0 Hz), 45.0 (dt, JP−Rh = 124.8 Hz, JP−P = 33.0 Hz). 11B{1H} NMR
(128 MHz, THF-d8): δ −0.8 (BF4), 0.0 (BAr4). Anal. Found for
C55H43P3OBF4Rh: C, 65.19; H, 4.28. Calcd: C, 65.25; H, 4.18. IR
(KBr): 2086 cm−1 (νCO).
Preparation of [{o-PPh2(C6H4)}3B]Rh(CO)[K(18-crown-6)] (5).
A 50 mL Schlenk tube was filled with 41.2 mg of 1 (0.0445 mmol),
20.1 mg of 18-crown-6 (0.0760 mmol), 18.2 mg of KH (0.454 mmol),
and 4 mL of THF and the reaction mixture was stirred at 60 °C. After
3 h, the mixture was cooled to room temperature and filtered with a
cannula. Slow diffusion of hexane into the filtrate provided 23.3 mg of
34.6 (dq, JP−Rh = 100.4 Hz, JP−P = 25.6 Hz, PPh3), 45.5 (dd, JP−Rh
=
191.0 Hz, JP−P = 25.6 Hz, Ph2P). 13C{1H} NMR (100 MHz, THF-d8):
δ 70.3, 115.7, 121.1, 124.2, 124.6, 125.1, 126.1, 128.0, 128.5, 131.4,
133.1, 131.5, 134.6, 145.0, 145.7. 11B{1H} NMR (128 MHz, THF-d8):
δ −7.5. Anal. Found for C84H81P4O6BKRh: C, 69.01; H, 5.37. Calcd:
C, 68.95; H, 5.58.
1
5 (0.0190 mmol) as red-brown crystals in 43% yield. H NMR (400
MHz, THF-d8): δ 6.47−6.58 (m, 3H), 6.65−6.71 (m, 6H), 6.77−6.85
(m, 18H), 6.98−7.05 (m, 3H), 7.20−7.22 (m, 12H). 31P{1H} NMR
(163 MHz, THF-d8): δ 60.7 (d, JP−Rh = 183.6 Hz). 13C{1H} NMR
(100 MHz, THF-d8): δ 70.3, 121.1, 124.7, 125.1, 125.8, 128.0, 128.5,
131.2, 131.8, 133.1, 145.0, 172.2. 11B{1H} NMR (128 MHz, THF-d8):
δ −7.6. Anal. Found for C67H66P3O7BKRh: C, 65.56; H, 5.42. Calcd:
C, 65.48; H, 5.41. IR (KBr): 1887 cm−1 (νCO).
Structure Determination by X-ray Diffraction. Preparations of
single crystals are described in the Supporting Information. The
prepared single crystals were mounted on a CryoLoop with Palaton
oil, and the X-ray diffraction experiments were carried out using a
Rigaku/MSC Mercury CCD diffractometer with a graphite-mono-
chromated Mo Kα radiation source (λ = 0.710 69 Å). Cell refinement
and data reduction were carried out using the CrystalClear program.25
The intensity data were corrected for Lorentz−polarization effects and
empirical absorption. The structures were determined by direct
methods (SIR 97). All non-hydrogen atoms were found by a difference
Fourier synthesis and were refined anisotropically unless otherwise
stated. Refinement using the SHELXL-97 package26 was carried out by
least-squares methods based on F2 with all measured reflection data.
The crystal data and results of the analyses are given in Tables S1−S14
(Supporting Information). CCDC 859005, 859006, 859007, 859008,
859009, 859010, and 859011 contain 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.
Density Functional Theory (DFT) Calculations. Density
functional theory (DFT) calculations were carried out at the
B3PW91 level16 in conjunction with the Stuttgart/Dresden ECP17
and associated with triple-ξ basis sets for Rh. For H, B, C, O, and P,
631-G(d) was employed. All calculations were performed without
symmetry constraints utilizing the Gaussian09 program.27 Natural
localized molecular orbital (NLMO) analysis data for 1, 4, and 5 are
given in Table 2. NBO analysis data for 6 and 7 are given in the
Supporting Information (Table S15). Cartesian coordinates are given
in Tables S17−S23 (Supporting Information).
Preparation of [Rh(CO)(PPh3)3][BF4] (6). Compound 6 was
synthesized in a manner slightly different from that reported by Hope
et al.14 A 50 mL Schlenk tube was filled with 57.2 mg of 2 (0.0623
mmol) and 7 mL of dichloromethane, and then 9.0 μL of
tetrafluoroboric acid ether complex (0.066 mmol) was added slowly
to the solution. The reaction mixture was stirred at room temperature
for 2 h, before the volatile materials were removed under vacuum. The
residue was washed with Et2O (1 mL × 3) to afford 56.9 mg of 6
(0.0566 mmol) as a white solid in 91% yield. The NMR data were
identical with those reported by Hope et al.14
Preparation of [Rh(CO)(PPh3)3][K(18-crown-6)] (7). A 50 mL
Schlenk tube was filled with 53.1 mg of 2 (0.0578 mmol), 31.8 g of 18-
crown-6 (0.120 mmol), 23.7 mg of KH (0.591 mmol), and 2 mL of
THF, and the reaction mixture was stirred at 60 °C. After 3 h, the
mixture was cooled to room temperature, and filtered with a cannula.
Slow diffusion of hexane into the filtrate provided 35.0 g of 7 as red-
brown crystals in 50% yield. The IR spectral data were identical with
those reported by Zotti et al.15 1H NMR (400 MHz, THF-d8): δ 3.54
(s, 24H, C12H24O6), 6.71−6.75 (m, 15H, Ar), 6.83 (m, 6H, Ar), 7.21−
7.25 (m, 12H, Ar), 7.27−7.33 (m, 9H, Ar), 7.38−7.42 (m, 3H, Ar).
13C{1H} NMR (100 MHz, THF-d8): δ 70.3, 124.6, 125.7, 128.2, 133.7,
147.0. 31P{1H} NMR (163 MHz, THF-d8): δ 47.7 (d, JP−Rh = 171.2
Hz). Anal. Found for C67H69P3O7KRh: C, 65.90; H, 5.70. Calcd: C,
65.57; H, 5.33. IR (KBr): 1800 cm−1 (νCO).
Preparation of [{o-PPh2(C6H4)}3B]RhH(PMe3) (8). A 50 mL
Schlenk tube was filled with 141.5 mg of 1 (0.153 mmol) and 2 mL of
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Organometallics
Article
(h) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2006, 25,
289. (i) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2007,
26, 3891. (j) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics
2008, 27, 312. (k) Crossley, I. R.; Foreman, M. R. St.-J.; Hill, A. F.;
Owen, G. R.; White, A. J. P.; Williams, D. J.; Willis, A. C.
Organometallics 2008, 27, 381. (l) Crossley, I. R.; Hill, A. F. Dalton
Trans. 2008, 201. (m) Crossley, I. R.; Hill, A. F.; Willis, A. C.
Organometallics 2010, 29, 326.
Table 2. Hybridization of NLMO Involving the Rh and B
Atoms and NLMO/NPA Bond Orders28
1
4
5
NLMO d(Rh)→p(B)
% d(Rh)
% p(B)
76.21
19.14
86.19
11.84
56.69
30.36
Donor NBO d(Rh)
(4) (a) Mihalcik, D. J.; White, J. L.; Tanski, J. M.; Zakharov, L. N.;
Yap, G. P. A.; Incarvito, C. D.; Rheingold, A. L.; Rabinovitch, D.
Dalton Trans. 2004, 1626. (b) Landry, V. K.; Melnick, J. G.; Buccella,
D.; Pang, K.; Ulichny, J. C.; Parkin, G. Inorg. Chem. 2006, 45, 2588.
(c) Figueroa, J. S.; Melnick, J. G.; Parkin, G. Inorg. Chem. 2006, 45,
7056. (d) Blagg, R. J.; Charmant, J. P. H.; Connelly, N. G.; Haddow,
M. F.; Orpen, A. G. Chem. Commun. 2006, 2350. (e) Senda, S.; Ohki,
Y.; Hirayama, T.; Toda, D.; Chen, J.-L.; Matsumoto, T.; Kawaguchi,
H.; Tatsumi, K. Inorg. Chem. 2006, 45, 9914. (f) Pang, K.; Quan, S. M.;
Parkin, G. Chem. Commun. 2006, 5015. (g) Pang, K.; Tanski, J. M.;
Parkin, G. Chem. Commun. 2008, 1008.
(5) (a) Bontemps, S.; Gornitzka, H.; Bouhadir, G.; Miqueu, K.;
Bourissou, D. Angew. Chem., Int. Ed. 2006, 45, 1611. (b) Bontemps, S.;
Bouhadir, G.; Miqueu, K.; Bourissou, D. J. Am. Chem. Soc. 2006, 128,
12056. (c) Sircoglou, M.; Bontemps, S.; Mercy, M.; Saffon, N.;
Takahashi, M.; Bouhadir, G.; Maron, L.; Bourissou, D. Angew. Chem.,
Int. Ed. 2007, 46, 8583. (d) Bontemps, S.; Sircoglou, M.; Bouhadir, G.;
Puschmann, H.; Howard, J. A. K.; Dyer, P. W.; Miqueu, K.; Bourissou,
D. Chem. Eur. J. 2008, 14, 731. (e) Bontemps, S.; Bouhadir, G.; Gu,
W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Bourissou, D.
Angew. Chem., Int. Ed. 2008, 47, 1481. (f) Sircoglou, M.; Bontemps, S.;
Bouhadir, G.; Saffon, N.; Miqueu, K.; Gu, W.; Mercy, M.; Chen, C.-H.;
Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou, D. J. Am. Chem.
Soc. 2008, 130, 16729. (g) Sircoglou, M.; Bontemps, S.; Mercy, M.;
Miqueu, K.; Ladeira, S.; Saffon, N.; Maron, L.; Bouhadir, G.;
Bourissou, D. Inorg. Chem. 2010, 49, 3983.
% s(Rh)
% p(Rh)
% d(Rh)
0.23
8.14
91.62
1.83
2.34
0.43
21.31
76.35
97.74
Acceptor NBO p(B)
% s(B)
% p(B)
18.80
81.19
11.08
88.86
20.05
79.95
Rh−B Bond Order
0.571 0.443
0.582
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables, figures, and CIF files giving bond lengths and angles,
structure refinement details, and ORTEP drawings of 3, 1, 4, 6,
7, 8, and 9 and details of the DFT calculations. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: nakazawa@sci.osaka-cu.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(6) (a) Tsoureas, N.; Haddow, M. F.; Hamilton, A.; Owen, G. R.
Chem. Commun. 2009, 2538. (b) Tsoureas, N.; Bevis, T.; Butts, C. P.;
Hamilton, A.; Owen, G. R. Organometallics 2009, 28, 5222.
(7) (a) Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed. 2011, 50,
2063. (b) Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc. 2011, 133,
18118.
The present research was supported by a Challenging
Exploratory Research grant (No. 23655056) and a Grant-in-
Aid for Young Scientists(B) (No. 23750064) from Japan
Society of the Promotion of Science. H.K. acknowledges
financial support from the Sasakawa Scientific Research Grant
from the Japan Science Society. Finally, we thank Prof. A. F.
Hill, Prof. G. Parkin, and Prof. D. Bourissou for attracting our
attention to this field.
(8) Bontemps, S.; Bouadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou,
D. Inorg. Chem. 2007, 46, 5149.
(9) The low-temperature fluxionality of compounds 1 and 8 is
somewhat unexpected for octahedral complexes, and a possible
explanation would be transient migration of the hydride to the boron
atom to form a hydrogenoborate ligand (R3BH−). Related hydride
migrations are known; see for example: Tsoureas, N.; Kuo, Y.-Y.;
Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 484.
(10) Unlike Bourissou’s isopropyl analogue, in which the boron is
pyramidalized owing to the presence of an intramolecular P→B
interaction, compound 3 has planar boron (∑(C−B−C) = 357.2°).
This difference probably originates from the lower electron-donating
ability of the aromatic phosphine.
(11) Covalent radius: Rh, 134 pm; B, 88 pm: Atkins, P. W.; Overton,
T.; Rourke, J.; Weller, M.; Armstrong, F. Shriver and Atkins Inorganic
Chemistry, 4th ed.; Oxford University Press: New York, 2006; p 24.
(12) We checked 1943 rhodium complexes with a carbonyl ligand in
a terminal configuration via the Cambridge Structural database
(version 5.32) and found that only six complexes have a longer
Rh−CO bond (over 1.960 Å) than complex 1. Recently, Owen
reported an interesting explanation on the very large trans influence
observed in the octahedral system.1f
(13) Density functional theory (DFT) calculations were carried out
at the B3PW9116/(Rh, SDD;17 H, C, B, O, P, 6-31G*) level of theory.
To confirm that the aforementioned basis sets were able to reproduce
the geometry of the anionic complex 5, the geometry of anionic
complex 9, which is similar to that of 5 (whose structure was
accurately characterized by X-ray diffraction studies), was optimized.
This optimized geometry, shown in Figure S15 (Supporting
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