A Phosphine-Coordinated Boron-Centered Gomberg-Type Radical

Article abstract of DOI:10.1002/anie.201504502

The P-coordinated boryl radical [Ph₂P(naphthyl)BMes]^. (Mes=mesityl) was prepared by (electro)chemical reduction of the corresponding borenium salt or bromoborane. Electron paramagnetic resonance (EPR) analysis in solution and DFT cal

Full text of DOI:10.1002/anie.201504502

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Angewandte
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International Edition: DOI: 10.1002/anie.201504502
German Edition: DOI: 10.1002/ange.201504502
Boryl Radicals
A Phosphine-Coordinated Boron-Centered Gomberg-Type Radical**
Amos J. Rosenthal, Marc Devillard, Karinne Miqueu,* Ghenwa Bouhadir,* and
Didier Bourissou*
Abstract: The P-coordinated boryl radical [Ph P-
2
(
naphthyl)BMes]C (Mes = mesityl) was prepared by (electro)-
chemical reduction of the corresponding borenium salt or
bromoborane. Electron paramagnetic resonance (EPR) anal-
ysis in solution and DFT calculations indicate large spin
density on boron (60–70%) and strong P–B interactions (P!
B s donation and B!P negative hyperconjugation). The
radical is persistent in solution and participates in a Gomberg-
type dimerization process. The associated quinoid-type dimer
has been characterized by single-crystal X-ray diffraction.
O
ver the last few years, significant progress has been
achieved in boron chemistry with the isolation of a variety of
compounds featuring original structures, exhibiting interest-
ing properties, and displaying rich reactivity. These develop-
ments concern highly reactive diamagnetic species (such as
Figure 1. Different types of EPR-characterized/isolated boryl radicals
A–E and the P-coordinated boryl radical F studied herein. Dipp=2,6-
diisopropylphenyl; Dur=2,3,5,6-tetramethylphenyl.
[
1]
[2]
boryl anions, diborynes and oxoboryls, 1,3-diboratacyclo-
butane-1,3-diyls, boriniums, and borylene adducts ) as
[3]
[4]
[5]
well as radicals for which unusual bonding situations (such as
[6]
one-electron BÀB s and p bonds) have been uncovered and
[
7]
important synthetic applications have emerged. In partic-
Some reported examples of boryl radicals are shown in
Figure 1. Basically, two different types can be distinguished:
ular, boryl radicals (L!BR )C, which can be considered as
2
[8a,b]
[8c]
[5c, 8d]
neutral tricoordinate boron radicals, have attracted consid-
erable attention.
1) acyclic systems, such as A,
B, and C,
in which
a five-electron BR C radical is stabilized by a strong Lewis
2
base, typically a N-heterocyclic carbene (NHC), and 2) cyclic
compounds, such as D and E, which are stabilized by
[
9]
[
*] Dr. A. J. Rosenthal, Dr. M. Devillard, Dr. G. Bouhadir,
Dr. D. Bourissou
p delocalization.
In this context, we report herein a persistent P-coordi-
nated boryl radical. A compound of type F (Figure 1) has
been prepared by (electro)chemical reduction. The structure
of the radical has been thoroughly analyzed by electron
paramagnetic resonance (EPR) spectroscopy and DFT cal-
culations. Single-crystal X-ray diffraction analysis revealed
that the molecule undergoes Gomberg-type dimerization by
B–C coupling. This B-centered radical is a new member in the
Laboratoire HØtØrochimie Fondamentale et AppliquØe
UniversitØ Paul Sabatier/CNRS UMR 5069
1
18 Route de Narbonne, 31062 Toulouse Cedex 09 (France)
E-mail: bouhadir@chimie.ups-tlse.fr
dbouriss@chimie.ups-tlse.fr
Homepage: http://lhfa.cnrs.fr/index.php/equipes/lbpb
Dr. K. Miqueu
Institut des Sciences Analytiques et de Physico-Chimie pour
l’Environnement et les MatØriaux/CNRS UMR 5254
Equipe Chimie Physique, UniversitØ de Pau et des Pays de l’Adour
HØlioparc, 2 Avenue du PrØsident Angot
[10]
family of naphthyl-bridged P/B compounds, and is a stable
version of the transient phosphine–boryl radicals
[11]
(
R PBH )C.
3
2
6
4053 Pau cedex 09 (France)
Taking advantage of the strong P!B interaction enforced
E-mail: karinne.miqueu@univ-pau.fr
[
10c–e]
by the naphthyl bridge,
we have recently isolated the P-
[
**] The Centre National de la Recherche Scientifique (CNRS) and the
UniversitØ Paul Sabatier (UPS) are acknowledged for financial
support of this work. For the theoretical work, access was granted to
the HPC resources of IDRIS by GENCI (Grand Equipement National
de Calcul Intensif) under allocation 2015 (i2015080045). The UPPA
and the MCIA (Mesocentre de Calcul Intensif Aquitain) are also
thanked for providing calculation facilities. Erik Schrader (ETH
Zürich), Lionel Rechignat (LCC Toulouse), and Yannick Coppel (LCC
Toulouse) are acknowledged for assistance with the EPR simula-
tions, EPR measurement, and solid-state NMR spectroscopic
analysis, respectively.
[10f]
coordinated borenium salt 1 (see Figure 2, inset).
This
prompted us to try to generate the corresponding boryl
radical. The reduction of 1 was first assessed electrochemi-
cally. The cyclic voltammogram showed a quasi-reversible
one-electron wave at À0.48 V (versus ferrocene/ferrocenium
+
[12]
Fc/Fc ) in fluorobenzene (Figure 2). This reduction poten-
tial is significantly more positive than that of other borenium/
boryl couples. For example, the NHC-stabilized radical B is
[
8c]
formed at much lower potential (E1/2 = À1.81 V) and the
lowest reported reduction waves occur at E = À0.86 V for 9-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504502.
1/2
[13]
acridinyl BMes2 derivatives.
As a result of its cationic
9
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the organic substituents (in particular the naphthyl moiety,
see DFT calculations described below) is indicated by the
1
necessity to include two H nuclei in the simulation with
11
couplings of 4 and 5 G. However, the large a( B) coupling
indicates that the spin density at boron is unusually high in
[16]
3.
The value measured for 3 falls in the upper range of those
reported for persistent/isolable boryl radicals (7.3–
[
5c,8]
1
3.2 G).
It is also higher than those of the triphenylboron
11
and trimesitylboron anion radicals (a( B) = 7.8 and 9.9 G,
respectively), which have approximately 60% spin density on
[
17]
boron.
To gain more insight into the structure of 3, DFT
calculations were performed. The geometry was optimized
Figure 2. Cyclic voltammogram of 1 in a fluorobenzene solution
containing [nBu N][NTf ] (0.06m) as the electrolyte. Scan rate:
[18]
4
2
À1
+
at the UB3PW91/6-31G** level of theory and the hyperfine
coupling constants were then computed at the B3PW91/
IGLO-II(P),EPR-II(H,B,C) level (see the Supporting Infor-
5
00 mVs ; potential reported against Fc/Fc as the internal standard.
[15]
character, 1 can also be reduced much more easily than
mation, Tables S1 and S2). The obtained values (35.3 and
31
11
boranes, including the highly electrophilic tris(pentafluoro-
10.3 G for P and B nuclei, respectively) match remarkably
well with those determined experimentally. DFT also nicely
[
14]
phenyl)borane (E1/2 = À1.17 V).
1
Encouraged by the electrochemical reduction of 1, we
then explored the chemical reduction of its precursor, the P-
coordinated bromoborane 2 (Scheme 1). Using zinc powder
reproduces the two couplings to H nuclei (5.7 and 5.5 G) and
enables us to assign the nuclei as the protons on the naphthyl
ring in the ortho and para positions with respect to boron
(suggesting some delocalization of spin density). The opti-
mized structure of 3 (Figure 4a) is also informative. The
Scheme 1. Synthesis of radical 3 by reduction of bromoborane 2.
or 1% Na(Hg) amalgam in THF, a dark-red solution was
formed from which compound 3 could be isolated by filtration
and precipitation over pentane (83% yield).
The X-band EPR spectrum of 3 in THF/toluene solution
at room temperature shows a complex but nicely resolved
signal centered at an isotropic g value of giso = 2.0026
(
Figure 3). No change was observed over days at room
temperature, indicating the highly persistent character of this
boryl radical. The hyperfine structure could be resolved by
[
15]
31
11
simulation, taking into account couplings to the P and B/
1
0
1
B nuclei, as well as to two H nuclei. The coupling constants
3
1
11
to P and B nuclei (31 and 11 G, respectively) fall in the
same range as those of transient phosphine–boryl radicals
[
11]
(
R PBH )C. Delocalization of the unpaired electron over
3
2
Figure 4. DFT analysis of compound 3: a) optimized geometry (dis-
tances in Š); b) SOMO (cutoff=0.04); c) spin density distribution
(isovalue=0.005 au); d) representation of the PB donation/back dona-
tion. H atoms are omitted for clarity.
boron center is in a trigonal planar environment (the sum of
the bond angles ÆB = 359.68) and the Mes group is twisted by
a
6
8.38 from the (naphthyl,P,B) plane, suggesting very little, if
any, delocalization of spin density over the mesityl ring.
Consistently, the BÀC bond (1.571 Š) is longer than the BÀ
Mes
C
bond (1.534 Š). Bond lengths in pm changed to Š to
naphthyl
Figure 3. Experimental (gray) and simulated (black) X-band EPR spec-
tra of 3 in toluene/THF solution at room temperature. G=gauss.
correspond to Figure 4a and throughout the manuscript for
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consistency. The presence of a strong P!B interaction is
apparent from the short PÀB distance (1.927 Š). Comparison
with the corresponding borenium salt 1 (2.006 Š) is interest-
ing and reveals some shortening of the bond in 3. This is
rather counterintuitive given that the boron center is less
electrophilic in the boryl radical 3 than in the borenium 1, and
thus reduction of 1 to form 3 would be expected to weaken the
P!B interaction.
The electronic structure of 3 was then carefully analyzed
[15]
by DFT calculations. The singly occupied molecular orbital
(
(
SOMO; Figure 4b) and the total spin density distribution
Figure 4c) are mainly localized on the B atom (60–70% spin
density) with small contributions of the P atom as well as the
Cipso, Cortho, and Cpara atoms of the naphthyl ring. Most
noticeable are the overlaps of the 2p(B) orbital with 1) the
adjacent 2p(Cipso) orbital (p bonding) and 2) the neighboring
s*(PC) orbitals (negative hyperconjugation). This picture is
corroborated by natural bond orbital analysis (NBO), which
gives at the first order a p(BCipso) orbital centered on boron
À
(
62%) with an occupancy of 0.86e . Stabilizing interactions
with p*(C=C)naphthyl and s*(PC) orbitals are found at the
Figure 5. OLEX2 plot of [3] with thermal ellipsoids set at 50%
2
second-order perturbation level (with NBO delocalization
probability. For clarity, the hydrogen atoms have been omitted.
Selected bond lengths [Š] and angles [8]: B1ÀC1 1.463(7), B1ÀP1
1.933(6), B2ÀC4 1.670(7), B2ÀP2 2.115(5), C1ÀC2 1.434(7), C3ÀC4
1.509(7), C2ÀC3 1.338(6); C1-B1-C11 131.4(5), C1-B1-P1 101.0(4),
À1
[15]
energies DEint = 15.3 and 9.5 kcalmol , respectively).
Interaction of the singly occupied 2p(B) with adjacent
vacant s*(PC) orbitals is reminiscent of the negative hyper-
conjugation occurring within P ylides (Figure 4d). The effect
of the negative hyperconjugation is probably to contribute to
the shortening of the PB distance in 3 (P!B donation/B!P
back donation) compared to borenium 1.
C11-B1-P1 127.5(3).
The stability and high spin density of 3 at the B atom
encouraged us to try to crystallize it. Our attempts were
finally rewarded and suitable crystals were obtained from
a THF solution layered with pentane at room temperature.
According to X-ray diffraction, the molecule adopts in fact
a dimeric quinoid-type structure [3] in the solid state
2
[
19]
(
Figure 5). The dimerization occurs at the boron atom of
one boryl unit and the Cpara(naphthyl) atom of the other boryl
unit. The length of the resulting BÀC bond (B2ÀC4 =
1
.670(7) Š) is typical of a single BÀC bond. The two boron
atoms are in different environments. The B atom directly
involved in the dimerization (B2) becomes tetrahedral and its
bond to phosphorus is elongated (2.115(5) Š) due to steric
shielding. The other boron atom (B1) remains trigonal planar.
Scheme 2. a) Monomer–dimer equilibrium of 3 and b) the Gomberg-
type dimerization of the triphenylmethyl radical (E=C) and the
À
triphenylboron anion radical (E=B ).
Atom B1 is engaged in
a
strong P!B interaction
[20]
(
1.933(6) Š)
and the bond towards Cipso(naphthyl) is
significantly shortened (1.463(7) Š), indicating that the
bond has double bond character.
rationalize the decrease of the EPR signal intensity observed
[24]
The formation of [3] is reminiscent of the dimerization of
upon cooling. In the case of boron compounds, Ph B was
2
3
[21]
the trityl radical Ph CC as reported by Gomberg (Scheme 2).
reported early on to dimerize under reductive conditions
3
[25]
Dimerization of triarylmethyl radicals is a particularly fasci-
nating transformation in organic chemistry. The reaction has
always attracted significant interest and undoubtedly repre-
(Na).
The ionic nature and instability of the transient
triphenylboron anion radical prevented detailed analysis of
the dimerization process, but a Gomberg-type dimeric
structure could be proposed based on NMR spectroscopic
[
22]
sents a founding principle of radical chemistry. Compara-
tively, very little is known about related transformations with
main-group radicals. Monomer/dimer interconversions have
been observed with p-block radicals (in particular with E = Si,
[
26]
data.
The dimerization process observed with the boryl
[27]
radical 3 is unique among persistent/isolable boryl radicals
and the B–Cpara coupling is probably favored by the same
factors as for the trityl radical: B–B coupling suffers from
strong steric congestion whereas C–C coupling is disfavored
by a) the low spin density at the Cpara position (20–25% versus
[
23]
Sn, P, Sb, Bi), but they usually involve EÀE bond formation/
cleavage. In a recent study on aryl-substituted silyl radicals,
Sekiguchi et al. proposed a Si–Cpara dimerization process to
9
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6
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structures).
9
Further characterization of [3] indicated complete dime-
2
rization of 3 in the solid state, as demonstrated by the fact that
the red powder obtained by crystallization is EPR silent.
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2
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CP-MAS) NMR spectrum, relatively sharp signals for
3
1
11
P and B nuclei were detected at expected chemical shifts
3
1
11
(
d( P) =+ 11.5 and À3.9 ppm; d( B) = 25.0 and 5.2 ppm).
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2
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In summary, a new P-coordinated boryl radical has been
prepared. According to EPR data and DFT calculations, the
spin density is mainly localized on the boron center, with
small contributions of the P atom and naphthyl unit. The
radical is persistent in solution and exists in equilibrium with
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Keywords: boron · cyclic voltammetry · EPR spectroscopy ·
phosphorus · radicals
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·
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3
2
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3
3
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