A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction with n-Butyllithium To Form a Stabilized Adduct by pπ-pπ Interaction

Article abstract of DOI:10.1002/anie.201607995

A boryl-substituted diphosphene was synthesized through the nucleophilic borylation of PCl₃with a borylzinc reagent, followed by a reduction with Mg. A combined analysis of the resulting diboryldiphosphene by single-crystal X-ray diffraction, DFT calculations, and UV/Vis spectroscopy revealed a σ-electron-donating effect for the boryl substituent that was slightly weaker than that of the 2,4,6-tri-tert-butylphenyl (Mes*) ligand. The reaction of this diboryldiphosphene withⁿBuLi afforded a boryl-substituted phosphinophosphide that was, in comparison with the thermally unstable Mes*-substituted diaryldiphosphene, stabilized by a π-electron-accepting effect of the boryl substituent.

Full text of DOI:10.1002/anie.201607995

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International Edition: DOI: 10.1002/anie.201607995
German Edition: DOI: 10.1002/ange.201607995
Diphosphene Hot Paper
A Boryl-Substituted Diphosphene: Synthesis, Structure, and Reaction
with n-Butyllithium To Form a Stabilized Adduct by pp-pp Interaction
Shun-suke Asami, Masafumi Okamoto, Katsunori Suzuki, and Makoto Yamashita*
Dedicated to Professor Takayuki Kawashima on the occasion of his 70th birthday
[
5m]
5o]
Abstract: A boryl-substituted diphosphene was synthesized
gen,
or the formation of divalent group 13 radical spe-
[
through the nucleophilic borylation of PCl with a borylzinc
cies. However, these studies on boryl anion-derived main-
group-element compounds did not reveal any information
regarding the substituent effect of boryl groups on the redox
chemistry.
3
reagent, followed by a reduction with Mg. A combined analysis
of the resulting diboryldiphosphene by single-crystal X-ray
diffraction, DFT calculations, and UV/Vis spectroscopy
revealed a s-electron-donating effect for the boryl substituent
that was slightly weaker than that of the 2,4,6-tri-tert-butyl-
phenyl (Mes*) ligand. The reaction of this diboryldiphosphene
Since the isolation of Mes*P=PMes* (1, Mes* = 2,4,6-tri-
tert-butylphenyl), diphosphenes, that is, compounds with a P=
P double bond, have attracted much attention on account of
their narrower HOMO–LUMO gap relative to that of diazo
(N=N) species. The photophysical properties of the P=P
n
with BuLi afforded a boryl-substituted phosphinophosphide
[
6]
that was, in comparison with the thermally unstable Mes*-
substituted diaryldiphosphene, stabilized by a p-electron-
accepting effect of the boryl substituent.
double bond is strongly influenced by the substituents on the
[6a,b]
[7]
[8]
[9]
phosphorus atoms. So far, aryl,
alkyl, silyl, thio, and
diphosphenes have been reported.
Although two computational studies on boryl-substituted
[
10]
amino-substituted
B
oryl substituents exhibit characteristic ambivalent sub-
[
11]
stituent effects as s-electron donors and p-electron acceptors.
The synthesis and photophysical properties of boron-contain-
ing organic molecules have been thoroughly investigated,
especially with respect to the substituent effects of the boryl
diphosphenes have been reported, such species have not
[
12]
yet been investigated experimentally. Herein, we report the
synthesis, structure, properties, and reactivity of a diboryldi-
phosphene. A characterization of this diphosphene by UV/
Vis spectroscopy, cyclic voltammetry, and DFT calculations
revealed that the boryl substituents act as s-donor substitu-
[1]
group. The p-acceptor properties of boryl substituents are
due to a vacant p-orbital, which may accept electrons from
adjacent atoms. The BÀN bond length of aminoboranes, for
n
ents. Moreover, this diboryldiphosphene reacts with BuLi to
example, is shorter than that of amine-boranes on account of
furnish an isolable phosphinophosphide, which is stabilized by
pp–pp interactions between the boron and the phosphorus
atoms. This behavior is thus markedly different to that of the
Mes*-substituted diarylphosphene 1.
Diboryldiphosphene 2 was synthesized from chlorobor-
ane 3 in four steps (Scheme 1). The transmetallation of the
boryllithium compound 4, which was prepared from 3, with
1.2 equivalents of ZnCl afforded THF-coordinated dimeric
borylzinc chloride (5·thf) using a procedure similar to that of
previously reported borylzinc species. This direct substitu-
tion at the main group element with a nucleophilic borylzinc
reagent stands in stark contrast to the reductive dehalogena-
tion of aminodibromobithmuthane(III), which is converted
[
2]
pp-pp interactions. But boryl substituents also exhibit
strong s-donor properties due to the relatively low electro-
negativity of the second-row element boron (2.04, Pauling),
which is reflected in for example, the strong trans influence of
[3]
boryl ligands in transition-metal-boryl complexes.
In contrast to the well-understood substituent effect of the
boryl group in organic and coordination chemistry, the effects
in main group chemistry remain relatively unexplored,
probably due to the limited availability of boryl-substituted
main-group-element compounds. After we had reported the
2
2
[
13]
[
4]
boryllithium compound as a boron nucleophile, others and
we used this compound to generate novel EÀB bonds using
I
the nucleophilic borylation of electrophilic main group
into a diborylbismuthene through a Bi intermediate, reported
by Jones et al. The reaction of isolated (5·thf) with PCl in
[
5]
[5q]
reagents. Some of the resulting compounds showed inter-
esting redox chemistry at the main group element, for
example, the direct reaction of a borylsilylene with dihydro-
2
3
[
*] S.-s. Asami, M. Okamoto, Dr. K. Suzuki, Prof. Dr. M. Yamashita
Department of Applied Chemistry, Faculty of Science and Engineer-
ing, Chuo University
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
E-mail: makoto@oec.chem.chuo-u.ac.jp
Homepage: http://www.chem.chuo-u.ac.jp/~element/
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201607995.
Scheme 1. Synthesis of diboryldiphosphene 2.
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acetonitrile resulted in the formation of boryldichlorophos-
details, see the Supporting Information): a) the HOMO
exhibits features of the nonbonding lone pair on the
phosphorus atoms, b) the HOMO-7 and the LUMO corre-
spond to the p and p* orbitals of the P=P double bond, and
phine 6. The observed BÀP bond length of 1.952(2) ꢀ in 6 is
close to the sum of the covalent radii of the B and P atoms
[
14]
(
1.98 ꢀ),
which suggests the absence of any pp–pp
interactions. A subsequent reduction of 6 with magnesium
in THF furnished diboryldiphosphene 2 in moderate yield. In
c) the HOMO-1 is predominantly characterized by contribu-
tions from the p-type orbital of the diazaborole moieties.
To better understand the electronic effects of the boryl
substituents in 2, the compound was subjected to UV/Vis
spectroscopy, cyclic voltammetry, and further DFT calcula-
tions. The UV/Vis spectrum of 2 showed two absorption
maxima at 310 nm (e = 5700) and 400 nm (e = 680; Fig-
ure S10), which are slightly blue-shifted relative to the
absorptions of Mes*-substituted diarylphosphene 1 (325,
3
1
the P NMR spectrum of 2 in C D , two magnetically
6
6
equivalent phosphorus nuclei were observed at d 605 ppm,
P
which are comparable, albeit slightly low-field-shifted, to the
resonances of carbon-substituted symmetrical trans-diphos-
[
6a,b,7,15]
phenes (d ꢀ 477–600 ppm).
The electropositive boryl
P
substituents may contribute to the low-field shift of these
resonances, considering that the chemical shifts of symmet-
rical diphosphenes with electropositive silyl substituents
[
6]
460–532 nm), indicating a larger HOMO–LUMO gap of
(
electronegativity of Si: 1.90, Pauling) are further down-
1 relative to that of 2. The cyclic voltammogram of 2 in THF
[
16]
11
n
field-shifted (d 735, 818 ppm). The B NMR spectrum of 2
showed a broad signal at d 30 ppm.
The centrosymmetric molecular structure of 2 was deter-
mined by single-crystal X-ray diffraction analysis (Figure 1).
The observed P=P bond length of 2.0655(17) ꢀ is slightly
(supporting electrolyte: 0.1m [ Bu N][BF ]) showed a rever-
P
4
4
+
sible reduction wave at À2.24 V (vs. Cp Fe/Cp Fe ; Figur-
B
2
2
[20]
es S16 and S17), which is lower than that of 1 (À2.36 V),
suggesting a lower LUMO level in 2 relative to that in 1. A
structural optimization of 1 and 2 at the B3LYP/6-31G(d)
level of theory was able to reproduce the crystallographically
obtained structures, albeit with slightly deviated dihedral
angles. For comparison, diphenyldiphosphene PhP=PPh (7)
longer than those of previously reported carbon-substituted
was calculated as a model compound (for details, see the
Supporting Information). These calculations revealed that the
presence of the boryl substituents in 2 raise the energy levels
of all P=P-related orbitals (n, p, p*) relative to those of 7. Due
to the introduction of three electron-donating tert-butyl
groups on each phenyl substituent in 1 compared to 7, the
change in energy levels of the corresponding orbitals is
consistent with the trends observed by UV/Vis spectroscopy
and cyclic voltammetry.
n
Addition of BuLi to diboryldiphosphene 2 at À358C in
THF afforded the stable adduct 8(thf) , which exhibited two
n
3
1
doublets in the P NMR spectrum at d À192 (P-Li) and d
P
P
n
À127 ppm (P- Bu; Scheme 2). The relatively large coupling
1
constant ( J = 208 Hz) is indicative of direct bonding
PP
Figure 1. Molecular structure of 2 (thermal ellipsoids set at 50%
probability, hydrogen atoms omitted for clarity; asterisks denote atoms
generated by symmetry operation), Selected bond lengths [ꢀ] and
angles [8]: P1–B1 1.936(3), N1–B1 1.418(4), N2–B1 1.424(4), P1–P1*
between two inequivalent phosphorus nuclei. Considering
1
1
7
that the B and Li NMR spectra of 8(thf) showed two
n
distinct broad signals and one sharp singlet signal, respec-
tively, the Li cation should be dissociated from the phosphide
center to form a solvent-separated ion pair. In THF, this
species was found to be stable at room temperature for 9.5 h.
2
.0655(17); P1*-P1-B1 95.83(9), N1-B1-N2 108.3(2), P1-P1-B1-N1 95.8-
(
2).
n
In contrast, treatment of 1 with BuLi at À358C in THF led to
[
17]
[18]
diphosphenes [1.985(3) –2.051(2) ꢀ ]. The two boron-
containing planes are oriented almost perpendicular to the
planar B-P=P-B moiety, exhibiting a dihedral N2-B1-P1-P1*
angle of À83.2(2)8. This structure stands in stark contrast to
the perpendicular and coplanar conformers that were
[5q]
observed for the recently reported diboryldibismuthene.
The B1–P1 bond length of 1.936(3) ꢀ in 2 is similar to the BÀ
P bond length in 6 [1.952(2) ꢀ], indicating a single-bond
character for this bond, due to little overlap between the 2p
and 3p orbitals of the boron and phosphorus atoms, respec-
tively. The observed B1-P1-P1* angle of 95.83(9)8 is relatively
small, but still within the range of the corresponding C-P-P
angles of symmetrical carbon-substituted diphosphenes
[
19]
[18]
[
3
93.22(5)8 -114.9(1)8 ]. DFT calculations at the B3LYP/6-
1G(d) level of theory revealed the following features (for
n
Scheme 2. Addition of BuLi to diboryldiphosphene 1 and 2.
2
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the appearance of two doublet signals at d À66 and d 8 ppm,
The presence of stabilizing pp–pp interactions between
the anionic phosphorus atom and the adjacent boron atom in
P
P
n
[21]
indicating the formation of BuLi adduct 9(thf) . However,
n
this species gradually decomposed at 08C over the course of
8(dme) was also corroborated by DFT calculations. The
structural optimization of the two independent structures
encountered in the unit cells afforded two energetically
similar structures, and an imaginary frequency was not found.
The HOMO and the HOMO-1 of 8(dme) correspond to the
P=B p-bond and a lone pair on the phosphorus atom,
7
h to generate unidentified species exhibiting two triplets in
3
1
the P NMR spectrum at dP À58 and d 3 ppm. The
P
extraordinary stability of 8(thf)n relative to 9(thf) should
n
most likely be attributed to the presence of pp–pp inter-
actions between the boryl substituent and the phosphide unit
in 8(thf) . Although the isolation of 8(thf) was not successful,
respectively (Figure 3). An NBO analysis of the resulting two
structures provided estimated second-order perturbation
n
n
recrystallization of 8(thf) from hexane in the presence of
n
À1
1
equiv of DME furnished 8(dme) as a crystalline solid. The
P NMR spectrum of 8(dme) in C D exhibited one doublet
energies of 46.78 and 54.67 kcalmol , which suggest the
3
1
existence of donor–acceptor interactions between the phos-
phorus and boron atoms. These interactions may very likely
be the key factor for the stabilization of boryl-substituted
phosphinophosphide 8(dme).
6
6
n
1
signal at d À116 ppm (P- Bu, J = 205 Hz) and one broad
P
PP
multiplet signal at d À226 ppm. For 8(dme), only one very
P
1
1
broad B NMR signal was observed at d 34 ppm, while the
B
7
Li NMR signal in C D exhibited a doublet of doublet at d
Li
6
6
1
À4.43 ppm. The observed coupling constants of J = 82 Hz
PLi
2
and J = 3 Hz should be attributed to interactions between
PLi
lithium and the two inequivalent phosphorus atoms. In their
entirety, the results of this multinuclear NMR analysis
indicates that 8(dme) exists in C D as a contact ion pair
6
6
with PÀLi bond in C D solution.
6
6
A single-crystal X-ray diffraction analysis on 8(dme)
suggested stabilizing pp-pp interactions between the anionic
phosphorus atom and the adjacent boron atom (Figure 2).
Single crystals of 8(dme) contained two independent mole-
cules of 8(dme) and one molecule of hexane per unit cell. The
two independent molecules differ with respect to each other
with regard to the orientation of the diazaborole ring
connected to the P2 atom. In 8(dme), the PÀB bond lengths
Figure 3. a) HOMO and b) HOMO-1 of 8(dme) (only one of the two
energetically similar structures is shown; hydrogen atoms omitted for
clarity; color code: gray=carbon, blue=nitrogen, orange=phospho-
rus, pale pink=boron, purple=lithium, red=oxygen).
within the phosphide moiety [1.896(3)/1.898(3) ꢀ] were
n
slightly shorter than those in the Bu-substituted phosphinyl
moiety in 8(dme) [1.933(3)/1.954(3) ꢀ]. The angle sum
around the phosphorus atoms (P1: 341.3/335.58, P2: 322.1/
In summary, boryl-substituted diphosphene 2 was synthe-
sized by a nucleophilic borylation of PCl using a borylzinc
3
3
12.78) demonstrate that the pyramidalization of P1 was less
reangent. A single-crystal X-ray diffraction analysis in
combination with DFT calculations and an analysis of UV/
Vis spectroscopic results revealed a s-donating effect of the
boryl substituents in 2 that is slightly weaker than that of the
pronounced than that of P2. These structural features indicate
the existence of pp-pp interactions between P1 and the boron
atom, probably due to the existence of two lone pairs on P1,
and reveal that 8(dme) exists as contact ion pair in the
crystalline state.
n
Mes* substituent. The reaction of 2 with BuLi resulted in the
formation of a phosphinophosphide that was stabilized by a p-
accepting effect of boryl substituent, which was confirmed by
single-crystal X-ray diffraction analysis and DFT calculations.
Acknowledgements
This research was supported by the Grants-in-Aid for
Scientific Research on Innovative Areas “Stimuli-responsive
Chemical Species for the Creation of Functional Molecules
(
No.2408)” (JSPS KAKENHI grant number JP24109012) and
for Scientific Research (B; MEXT KAKENHI grant number
6288019) as well as by research grants from The Science
2
Research Promotion Fund and The Asahi Glass Foundation.
We thank Prof. Tamejiro Hiyama (Chuo University) and Prof.
Yoshiaki Nishibayashi (University of Tokyo) for providing
access to an X-ray diffractometer and a mass spectrometer.
The theoretical calculations were carried out using resources
of the Research Center for Computational Science, Okazaki,
Japan.
Figure 2. Molecular structure of 8(dme) (thermal ellipsoids set at 50%
probability, hydrogen atoms omitted for clarity). Selected bond lengths
[
ꢀ] and angles [8]: N1–B1 1.448 (4), N2–B1 1.457 (4), P1–B1 1.896 (3);
N1-B1-N2 105.2(17) N3-B2 1.445 (4), N4-B2 1.434 (4), P2-B2 1.933
3), P1-P2 2.1775 (11).
(
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Keywords: boron · boryllithium · diphosphenes ·
main group chemistry · substituent effects
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Angewandte
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n
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adduct 9, which exhibits two doublets at d À9 and d 81 ppm
P
P
1
31
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pp
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[
[
[
[
3
4
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Received: August 16, 2016
1987, 249 – 251.
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Diphosphene
S.-s. Asami, M. Okamoto, K. Suzuki,
M. Yamashita*
&&&& — &&&&
A Boryl-Substituted Diphosphene:
Synthesis, Structure, and Reaction with n-
Butyllithium To Form a Stabilized Adduct
by pp-pp Interaction
A boryl-substituted diphosphene was
synthesized using a borylzinc reagent
(see picture). An analysis of the species
revealed an s-electron-donating effect of
the boryl substituents. The reaction of the
diboryldiphosphene with n-butyllithium
afforded a boryl-substituted phosphino-
phosphide that was stabilized by a p-
electron-accepting effect of the boryl
substituents.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!