Reactivity of phosphaboradibenzofulvene toward hydrogen, acetonitrile, benzophenone, and 2,3-dimethylbutadiene

Article abstract of DOI:10.1021/om400553j

The reaction of 9-bromo-9-borafluorene (1) with Li[PtBu₂] in toluene gave quantitatively the corresponding di-tert-butyl- phosphaboradibenzofulvene (9-di-tert-butylphosphanyl-9-borafluorene, 2). Degradation of thf occurred by treatment of 2 with thf in toluene at room temperature. In this paper the reaction of 2 with gaseous H₂ in toluene solution at room temperature is described, by which the corresponding H₂ addition product 4 was formed. The hydrogen addition product 4 crystallizes from benzene in the monoclinic space group P2₁/n. Addition reactions of 2 with acetonitrile, benzophenone, and 2,3-dimethylbutadiene were also investigated. Treatment of 2 with a 20-fold excess of acetonitrile afforded the corresponding adduct, which itself dimerized to a mixture of cis and trans isomers of the corresponding cycloiminoborane 6. Cocrystals of cis-6 and trans-6 (ratio 2:1) were obtained from toluene in the presence of 20 equiv of acetonitrile at 6 C (monoclinic space group P2 ₁/c). The isolation of the pure trans-6 was achieved from toluene in the presence of 2 equiv of acetonitrile at -30 C (triclinic space group P1I..). Benzophenone reacted with the phosphaboradibenzofulvene 2, forming the corresponding addition product 7 (orthorhombic space group Pbca). The reaction of 2 with a 6-fold excess of 2,3-dimethylbutadiene gave the related Diels-Alder adduct 8 (monoclinic space group P2₁/c).

Full text of DOI:10.1021/om400553j

Article
pubs.acs.org/Organometallics
Reactivity of Phosphaboradibenzofulvene toward Hydrogen,
Acetonitrile, Benzophenone, and 2,3-Dimethylbutadiene
Jens Michael Breunig, Alexander Hubner, Michael Bolte, Matthias Wagner, and Hans-Wolfram Lerner*
̈
Institut fur Anorganische Chemie, Goethe-Universitat Frankfurt am Main, Max-von-Laue-Straße 7, 60438 Frankfurt am Main,
̈
̈
Germany
S
* Supporting Information
ABSTRACT: The reaction of 9-bromo-9-borafluorene (1) with Li[PtBu2] in toluene gave quantitatively the corresponding di-
tert-butyl-phosphaboradibenzofulvene (9-di-tert-butylphosphanyl-9-borafluorene, 2). Degradation of thf occurred by treatment of
2 with thf in toluene at room temperature. In this paper the reaction of 2 with gaseous H2 in toluene solution at room
temperature is described, by which the corresponding H2 addition product 4 was formed. The hydrogen addition product 4
crystallizes from benzene in the monoclinic space group P21/n. Addition reactions of 2 with acetonitrile, benzophenone, and 2,3-
dimethylbutadiene were also investigated. Treatment of 2 with a 20-fold excess of acetonitrile afforded the corresponding adduct,
which itself dimerized to a mixture of cis and trans isomers of the corresponding cycloiminoborane 6. Cocrystals of cis-6 and
trans-6 (ratio 2:1) were obtained from toluene in the presence of 20 equiv of acetonitrile at 6 °C (monoclinic space group P21/c).
The isolation of the pure trans-6 was achieved from toluene in the presence of 2 equiv of acetonitrile at −30 °C (triclinic space
group P1). Benzophenone reacted with the phosphaboradibenzofulvene 2, forming the corresponding addition product 7
̅
(orthorhombic space group Pbca). The reaction of 2 with a 6-fold excess of 2,3-dimethylbutadiene gave the related Diels−Alder
adduct 8 (monoclinic space group P21/c).
phosphinoboranes possess three-coordinate B and P centers.
This structural feature was also realized in the solid-state
structures of several triphosphatriborinane ring dervatives, BP
cumulenes with allene or butadiene structures, and borylated
lithium phosphides.3,5
A characteristic feature of boranylidene phosphanes as well as
of phosphinoboranes is that they have a strong tendency to
associate, in a head-to-tail manner, to give rings or oligomers
(BX2PR2)n, which have BP frameworks consisting of four-
coordinate B and P centers. The ring size of cyclo-
phosphinoboranes (BX2PR2)n depends on the method of
preparation and on the nature of the substituents R and X.12
Generally, phosphinoboranes exhibit spectroscopic and
structural evidence of significant BP π-bonding. It was reported
INTRODUCTION
■
Unsaturated BP compounds with two and four substituents are
rare, and only a few of these compounds were characterized by
X-ray crystallography.1−11 In most cases sterically bulky
substituents are necessary to suppress cyclization or oligome-
rization. In spite of the use of very bulky substituents, the
preparation of boranylidene phosphanes has still not yet been
possible. Noth and co-workers, however, could isolate the
̈
boranylidene phosphane complex (TMP)BP(CEt3)(Cr-
(CO)5) (TMP = 2,2,6,6-tetramethylpiperidino), by which the
Lewis-basic P center coordinates to the electrophilic Cr(CO)5
fragment.6 As a consequence, this complexed boranylidene
phosphane displays an allene structure in the solid state.6
Recently Power et al. have reported that the DMAP-supported
boranylidene phosphane (DMAP)(TMP)BPAr (Ar = 2,6-
(2,4,6-iPr3C6H2)2C6H3; DMAP = 4-dimethylaminopyridine)
can be synthesized by the reaction of ArPH-BBr(TMP) with 2
equiv of DMAP.7 In this compound the Lewis-acidic B center is
additionally coordinated by one DMAP molecule.7 Generally,
Special Issue: Applications of Electrophilic Main Group Organo-
metallic Molecules
Received: June 14, 2013
© XXXX American Chemical Society
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tBu, Cy, Mes) was made. Finally we report on addition
reactions of the phosphaboradibenzofulvene 2 with acetonitrile,
benzophenone, and 2,3-dimethylbutadiene (DMB), by which
the related trapping products could be isolated.
RESULTS AND DISCUSSION
■
When 9-bromo-9-borafluorene (1)19 in toluene was treated
with one molar equiv of Li[PtBu2],20 the reaction mixture
underwent a color change to red, while the NMR spectra of the
solution revealed new signals. These signals could be assigned
to phosphaboradibenzofulvene 2. After filtering and removing
the solvent, 2 was obtained as a red oil. In this context it should
be noted that the physical properties of the phosphabor-
adibenzofulvene 2 are comparable with those of the carbon
analogues.21 The phosphinoboranes (C6F5)2BPR2 (R = tBu,
Cy, Mes) are yellow compounds, whereas the phosphabor-
adibenzofulvene 2 has an impressive red color. The UV−vis
spectrum of 2 is characterized by a broad absorption band at
λmax = 478 nm (Figure 5S in the Supporting Information).
In addition we decided to prepare the thf-supported
phosphaboradibenzofulvene from 2 and thf; however, this
approach failed. Instead of thf-complexed 2 we isolated the
phosphonium salt 3 by this reaction, as shown in Scheme 1.
Apparently, the phosphaboradibenzofulvene 2 is able to initiate
Figure 1. BP compounds with two and four substituents.
that the BP bond lengths in phosphinoboranes, e.g.,
(C6F5)2BPtBu2,11 with electron-withdrawing ligands are
much shorter than those observed for the analogous
2
compounds (Mes)2BPR2 (R = tBu, Ph, Mes), in which
the B atom is coordinated by electron-rich mesityl groups.
Calculations reveal that the π-bonding HOMOs in the
phosphinoboranes were highly polarized (e.g., (C6F5)2B
PtBu2: 74% of the HOMO derived from the P atom but only
26% from the B atom).11 Additionally, Stephan et al. have
reported that the phosphinoboranes with pentafluorophenyl
substituents (C6F5)2BPR2 (R = tBu, Cy, Mes; Cy =
cyclohexyl) activate hydrogen due to the ambiphilic character
of their BP units.10,11
It is remarkable that the structural analogy of phosphinobor-
anes and silenes (1.786−1.859 Å for BP2,11 and 1.702−1.764
Å for SiC13−15) implies that the π-bonds in phosphinobor-
anes are similar to those in silenes. Therefore the isoelectronic
phosphinoborane (C6F5)2BPtBu2 is thus, in a sense, a mirror
of Wiberg’s silene.
Scheme 1. Reactions of the Bromoborafluorene 1 with
Li[PtBu2]: Synthesis of Phosphaboradibenzofulvene 2 and
the Formation of the Phosphonium Salt 3
Figure 2. Comparison Wiberg-type and Brook-type silenes.
In this context it should be noted that silenes of the Wiberg
type possess a more polar and shorter double bond than silenes
of the Brook type.15 Moreover it was reported that silenes of
the Wiberg type are more reactive than Brook-type silenes.15 In
contrast to the well-known reactivity of silenes toward enes and
dienes,16 electrocyclic reactions of phosphinoboranes have
rarely been studied up to now.
In the course of our investigations of unsaturated boron
compounds and boraaromatics we could demonstrate the
antiaromatic nature of borafluorene.17,18 This predicts very
Lewis-acidic B centers for this class of compounds. Therefore
we decided to synthesize phosphinoboranes with borafluorene
moieties (phosphaboradibenzofulvene) and to investigate their
reactivity.
Concretely we present the synthesis of the phosphabor-
adibenzofulvene 2 in this paper. Further, a comparison of the
hydrogenation of the BP bond of phosphaboradibenzofulvene 2
to that of Stephan’s phosphinoboranes (C6F5)2BPR2 (R =
B
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degradation of thf by which the O atom (formal as O2−) of the
thf molecule is transferred to the B center and the butylene unit
(formal as C4H82+) of thf to the phosphanyl group of the
phosphaboradibenzofulvene.
Scheme 3. Reaction of the Phosphaboradibenzofulvene 2
with Acetonitrile and Benzophenone
The phosphaboradibenzofulvene 2 is a highly reactive
compound, and we considered that it has the potential for
hydrogenation of the BP bond. When a solution of 2 in toluene
was stirred at room temperature in an atmosphere of H2 (1
atm), the red color of 2 disappeared after two hours. We
therefore concluded that H2 addition took place. In addition
the signals of 2 were no longer recognizable in the NMR
spectra of the reaction solution. Instead, resonances in the 11B
1
and 31P NMR spectra, showing J couplings to H atoms, were
attributable to the H2 addition product 4. Unfortunately we
could not isolate 4 by this approach. However, identification of
4 was confirmed by an independent synthesis, as treatment of
9H-9-borafluorene with HPtBu2 yielded quantitatively the
adduct 4. Interestingly, the phosphaboradibenzofulvene 2
apparently reacts faster with H2 than the related phosphinobor-
10
ane (C6F5)2BPtBu2 does (reaction time: 1 d vs 4 weeks at
room temperature). From this we can conclude that the
polarity of the BP bond of 2 and therefore the Lewis acidity
caused by the B center of the borafluorene moiety are higher
than those of the (C6F5)2B group in (C6F5)2BPtBu2.
Scheme 2. Reaction of Phosphaboradibenzofulvene 2 with
H2
ppm for a C-OB nucleus. Furthermore, the 11B shift (46.9
ppm) of 7 is in the range that was found for borinic esters.
By the reaction of 2 with a 6-fold excess of DMB the [4+2]
cycloadduct 8 was formed (Scheme 4). The Diels−Alder
Scheme 4. Reaction of the Phosphaboradibenzofulvene 2
with 2,3-Dimethylbutadiene
If indeed the BP bond in 2 possesses π-character, it should be
possible to trap phosphaboradibenzofulvene 2 by addition of
enes or dienes. This was attempted with acetonitrile,
benzophenone, and DMB. These compounds have long been
known to be excellent trapping agents for the silicon−carbon
double bond.16 Treatment of 2 with a 20-fold excess of
acetonitrile at ambient temperature afforded the related
trapping product 6 (Scheme 3). The formation of 6 suggested
to us the mechanism, as shown in Scheme 3: (i) The [2+2]
cycloadduct is formed. (ii) Then, the transient [2+2]
cycloadduct undergoes ring-opening to give the azaboraallene
5, which (iii) itself dimerizes to 6. The cyclodiboradiazane 6,
which results from a [2+2] cycloaddition of azaboraallene 5,
was identified from its NMR spectroscopic characteristics;
specifically the shift in the 11B NMR (δ = 7.4) spectrum is in
the range of already known four-membered B2N2 rings.26 The
most striking observation on the reaction of 2 with acetonitrile
is that cis and trans isomers of 6 are formed. Cocrystals of cis-6
and trans-6 in the ratio 2:1 were obtained from toluene in the
presence of 20 equiv of acetonitrile at 6 °C (monoclinic space
group P21/c). Pure trans-6 was isolated from toluene in the
presence of 2 equiv of acetonitrile at −30 °C.
adduct 8 was identified from its NMR spectra. The 13C NMR
spectrum of Diels−Alder adduct 8 displays resonances at 117.6
and 136.7 ppm, which could be assigned unambiguously to a
CC double bond. Additionally the structure of 8 was
confirmed by X-ray structure analysis. In 2007 Gilbert and
Bacharach have calculated that the phosphinoborane
(F3C)2BPtBu2 undergoes facile [2+2] cycloadditions with
acetylene and ethene as well as [4+2] cycloadditions with
butadiene derivatives.22,23 Indeed, we were successful in
preparing and isolating the trapping products of 2 with
CH3CN and Ph2CO as well as the [4+2] cycloaddition
product of 2 with 2,3-dimethylbutadiene.
The phosphaboradibenzofulvene 2 reacted rapidly with
benzophenone, forming the corresponding trapping product
7, which could be isolated from the reaction solution. The 13C
NMR spectrum of 7 reveals a characteristic resonance of 94.6
The connectivity of the thf degradation product 3 (Scheme
1) is supported by an X-ray diffraction study, the quality of
C
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which prevents its publication. A ball-and-stick presentation
(Figure 3S) can be found in the Supporting Information.
The hydrogen addition product 4 shown in Figure 3
crystallizes in the monoclinic space group P21/n (for selected
Figure 4. Solid-state structure of cis-6, which was obtained from the
reaction of 2 with acetonitrile (monoclinic, P21/c). Displacement
ellipsoids are drawn at the 50% probability level. H atoms are omitted
for clarity. Selected bond lengths (Å), bond angles (deg), and torsion
angles (deg): B(1)−N(1) = 1.581(7), B(1)−N(2) = 1.591(6), B(2)−
N(1) = 1.608(6), B(2)−N(2) = 1.596(6), N(1)−C(7) = 1.296(6),
N(2)−C(9) = 1.287(6), P(1)−C(7) = 1.844(5), P(2)−C(9) =
1.872(5), C(7)−C(8) = 1.518(6), C(9)−C(10) = 1.482(7); N(1)−
B(1)−N(2) = 87.2(3), N(1)−B(2)−N(2) = 86.2(3), B(1)−N(1)−
B(2) = 93.3(3), B(1)−N(2)−B(2) = 93.3(3), B(2)−N(1)−C(7) =
137.4(4), B(1)−N(1)−C(7) = 129.3(4), N(1)−C(7)−P(1) =
118.4(3), N(1)−C(7)−C(8) = 116.6(5), N(2)−C(9)−P(2) =
117.2(4), N(2)−C(9)−C(10) = 117.8(4); B(1)−N(1)−C(7)−P(1)
= −176.9(4), B(2)−N(1)−C(7)−P(1) = 4.6(9), B(1)−N(2)−C(9)−
P(2) = −178.5(4), B(2)−N(2)−C(9)−P(2) = −0.2(9).
Figure 3. Solid-state structure of the H2 addition product 4
(monoclinic, P21/n). Displacement ellipsoids are drawn at the 50%
probability level. H atoms, except for the two freely refined H atoms
on B1 and P1, are omitted for clarity. Selected bond lengths (Å) and
bond angles (deg): B(1)−C(1) = 1.617(2), B(1)−C(11) = 1.615(2),
B(1)−P(1) = 1.9733(16), P(1)−C(21) = 1.8593(14), P(1)−C(31) =
1.8624(14); C(1)−B(1)−C(11) = 100.35(11), C(1)−B(1)−P(1) =
117.36(10), C(11)−B(1)−P(1) = 108.36(9), C(21)−P(1)−C(31) =
115.15(7), C(21)−P(1)−B(1) = 116.25(6), C(31)−P(1)−B(1) =
112.11(7).
bond lengths and angles, see in the figure caption). As
anticipated, the structure of 4 has pseudotetrahedral P and B
centers. The B−P bond length of 1.9733(16) Å in 4 is
comparable with that found in (C6F5)2BH-PHtBu2 (1.966(9)
Å). It is interesting that the B−P bond lengths of 4 as well as
those of Stephan’s hydrogen addition products (C6F5)2BH-
PHR2 (R = Et, tBu, Cy, Ph, Mes) are significantly shorter (∼0.1
Å) than the B−P bonds of related dialkylphosphane borane
adducts.24 The molecular structures of (C6F5)2BH-PHR2 (R =
Et, tBu, Cy, Ph, Mes)11 reveal a trans orientation of the B−H
and P−H hydrogen atoms, whereas the solid-state structure of
4 exhibits a typically staggered conformation with syn
arrangement of the hydrogen atoms and a H−B−P−H torsion
angle of −81(1)°.
interesting to note that in contrast to the already structurally
characterized cycloiminoboranes,25,26 which feature a trans
orientation of their substituents, for the first time a cyclo-
iminoborane derivative had been structurally characterized that
displays a cis arrangement of its imino residues.
The benzophenone trapping product 7 (shown in Figure 7;
for selected bond lengths and angles, see the figure caption)
crystallizes in the orthorhombic space group Pbca. The trapping
product 7 consists of one borafluorene moiety and one tBu2P
group, which are connected by a central benzophenone unit. As
shown in Figure 7, the borinic acid ester 7 possesses a staggered
configuration with anti arrangement of borafluorene and tBu2P
substituents (B−O−C−P torsion angle of −171.6(3)°).
The [4+2] cycloadduct 8, shown in Figure 8, crystallizes in
the monoclinic P21/c space group (selected bond lengths in the
caption of Figure 8). Crystals of 8 were grown from a
concentrated toluene/pentane (1:1) solution at −30 °C. The
central structure motif of 8 is a heterocycle that is composed of
one P and one B atom and a 2-butenyl unit. In this structure
the BPC4 ring possesses a twist conformation. The B−P bond
length of 1.9882(17) Å in 8 varies only slightly from that found
in 4 (1.9733(16) Å). All other structural parameters of 8 are in
the expected range.24
Cocrystals of cis-6 and trans-6 in the ratio 2:1 were obtained
from toluene in the presence of 20 equiv of acetonitrile at 6 °C
(monoclinic space group P21/c). The crystal structure of cis-6 is
shown in Figure 4 and that of trans-6 in Figure 6; selected bond
lengths and angles can be found in the corresponding captions
(crystal packing of cocrystallized cis-6 and trans-6 is shown in
Figure 5). Crystallization of pure trans-6 was achieved from
toluene in the presence of 2 equiv of acetonitrile at −30 °C
(triclinic space group P1, Figure 6 and Figure 1S in the
̅
Supporting Information). The central core of both isomers of 6
is formed by a four-membered B2N2 ring (ring angles of cis-6:
B−N−B = 93.3(3)° and N−B−N = 86.2(3)°; ring angles of
trans-6: B−N−B = 93.5(3)° and N−B−N = 86.5(3)°). The N
atoms are thereby part of an imino group. The CN bond
lengths of the imino units of cis-6 as well as of trans-6 are
consistent with a full CN double bond.24 As expected, the
atoms directly linked to each CN unit are coplanar. It is
CONCLUSION
■
In summary, the reaction of the bromoborafluorene 1 with
Li[PtBu2] yielded quantitatively the phosphaboradibenzoful-
vene 2. We found that degradation of thf occurred by treatment
of 2 with thf in toluene at room temperature.
D
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Figure 5. Crystal packing of cocrystallized cis-6 and trans-6 (ratio 2:1; monoclinic, P21/c).
Our investigation of the chemical behavior of the
phosphaboradibenzofulvene 2 indeed confirms the ambiphilic
character of its BP bond. This could be impressively
demonstrated in the reaction of 2 with gaseous H2 in toluene
solution at room temperature, by which the H2 addition
product 4 was formed. Moreover we have shown addition
reactions of the phosphaboradibenzofulvene 2 with acetonitrile,
benzophenone, and 2,3-dimethylbutadiene. Therefore the
chemical behavior of 2 is comparable to that of Wiberg-type
silenes. Treatment of 2 with a 20-fold excess of acetonitrile
afforded at first the azaboraallene 5. However, this compound
undergoes dimerization to give the corresponding cyclo-
iminoborane 6 as a mixture of cis and trans isomers. Cocrystals
of cis-6 and trans-6 in the ratio 2:1 were obtained from toluene
in the presence of 20 equiv of acetonitrile at 6 °C, whereas pure
trans-6 could be isolated from toluene in the presence of 2
equiv of acetonitrile at −30 °C. Benzophenone reacted rapidly
with the phosphaboradibenzofulvene 2, forming the corre-
sponding trapping product 7. The reaction of 2 with a 6-fold
excess of 2,3-dimethylbutadiene yielded the Diels−Alder
adduct 8. We reported in this paper the isolation and X-ray
crystal structure of 8, which to our knowledge is the first
structurally characterized phosphinoborane Diels−Alder ad-
duct.
Figure 6. Solid-state structure of the cycloiminoborane trans-6
(triclinic, P1). Selected bond lengths (Å) and bond angles (deg):
̅
B(1)−N(1) = 1.577(6), N(1)−C(7) = 1.272(6), P(1)−C(7) =
1.866(5), C(7)−C(8) = 1.522(5); N(1)−B(1)−N(1B) = 86.5(3),
B(1)−N(1)−B(1B) = 93.5(3), B(1)−N(1)−C(7) = 131.4(3), N(1)−
C(7)−P(1) = 116.7(3), N(1)−C(7)−C(8) = 118.7(4); B(1)−N(1)−
C(7)−C(8) = −4.2(7), B(1)−N(1)−C(7)−P(1) = 175.6(4).
Symmetry transformation used to generate equivalent atoms: B −x
+1, −y+1, −z+1.
EXPERIMENTAL SECTION
■
General Procedures. All reactions and manipulations were carried
out under dry, oxygen-free nitrogen by using standard Schlenk
E
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Figure 8. Solid-state structure of the Diels−Alder adduct 8, which was
obtained from the reaction of 2 with 2,3-dimethylbutadiene
(monoclinic, P21/c). Displacement ellipsoids are drawn at the 50%
probability level. H atoms are omitted for clarity. Selected bond
lengths (Å), bond angles (deg), and torsion angles (deg): B(1)−P(1)
= 1.9882(17), B(1)−C(1) = 1.635(2), B(1)−C(11) = 1.620(2),
B(1)−C(21) = 1.625(2), P(1)−C(4) = 1.8141(16), P(1)−C(7) =
1.8831(16), P(1)−C(8) = 1.8903(16), C(1)−C(2) = 1.513(2),
C(2)−C(3) = 1.340(2), C(3)−C(4) = 1.519(2); C(11)−B(1)−
C(21) = 99.02(13), C(11)−B(1)−P(1) = 113.10(11), C(21)−B(1)−
P(1) = 111.01(11), C(1)−B(1)−C(21) = 110.12(13), C(7)−P(1)−
C(8) = 110.57(7), C(7)−P(1)−B(1) = 116.34(7), C(8)−P(1)−B(1)
= 115.73(7), C(8)−P(1)−C(4) = 105.53(8), C(1)−C(2)−C(3) =
126.48(14), C(2)−C(3)−C(4) = 124.97(14); C(1)−C(2)−C(3)−
C(4) = 4.7(3).
Figure 7. Solid-state structure of the trapping product 7, which was
obtained from the reaction of 2 with benzophenone (orthorhombic,
Pbca). Displacement ellipsoids are drawn at the 50% probability level.
H atoms are omitted for clarity. Selected bond lengths (Å), bond
angles (deg), and torsion angles (deg): B(1)−O(1) = 1.357(5), B(1)−
C(31) = 1.578(6), B(1)−C(41) = 1.580(6), O(1)−C(1) = 1.452(4),
P(1)−C(1) = 1.944(4), C(1)−C(11) = 1.545(5), C(1)−C(21) =
1.532(5); C(31)−B(1)−C(41) = 104.7(3), O(1)−B(1)−C(31) =
119.8(4), O(1)−B(1)−C(41) = 135.5(3), B(1)−O(1)−C(1) =
131.2(3), P(1)−C(1)−O(1) = 109.9(2), C(11)−C(1)−C(21) =
111.2(3); B(1)−O(1)−C(1)−P(1) = −171.6(3).
techniques or in an argon-filled M. Braun glovebox. The solvents thf,
pentane, toluene, and [D6]benzene were stirred over sodium/
benzophenone and distilled prior to use. 1,17,19 2,7-di-tert-butyl-9-
bromo-9-borafluorene,18 and Li[PtBu2]20 were prepared according to
the published procedures. The NMR spectra were recorded on a
Bruker AM 250, a Bruker DPX 250, a Bruker Avance 300, and a
Bruker Avance 400 spectrometer. NMR chemical shifts are reported in
ppm. Elemental analyses were performed by the Microanalytical
Laboratory of the Goethe University Frankfurt. Abbreviations: s =
singlet; d = doublet; m = multiplet; br = broad; n.o. = not observed.
Synthesis of 2. A solution of Li[PtBu2]20 (157 mg, 1.03 mmol) in
3.5 mL of toluene was cooled to −30 °C, and 9-bromo-9-borafluorene
(1)17,19 (250 mg, 1.03 mmol), which was dissolved in 7.5 mL toluene,
was added slowly. The reaction mixture was stirred at room
temperature overnight and filtered. After removing all volatiles in
(300.0 MHz, C6D6): δ 1.27 (s, 18 H, Ar−tBu), 1.49 (d, 3JHP = 11.9 Hz,
18 H, P−tBu), 7.17−7.18 (m, 4 H, Ar−H), 8.19−8.20 (m, 2 H, Ar−
H); see Figure 7S in the Supporting Information. 11B{1H} NMR (96.3
MHz): δ 78.4 (br). 31P{1H} NMR (121.5 MHz, C6D6): δ 21.0 (br).
13C{1H} NMR (75.4 MHz, C6D6): δ 31.4 (s, Ar−C(CH3)3), 33.0 (d,
2
1JPC = 16.3 Hz, P−C(CH3)3), 33.8 (d, JPC = 11.8 Hz, P−C(CH3)3),
34.9 (s, Ar−C(CH3)3), 119.1 (s, Ar−C), 130.9 (s, Ar−C), 135.2 (d,
3JPC = 3.2 Hz, Ar−C), 146.2 (br, B−C), 150.3 (d, 3JPC = 7.4 Hz, Ar−
C), 150.5 (s, Ar−C). For assignment see Figures 7S and 13S in the
Supporting Information.
Reaction of 2 with H2. A solution of the phosphaboradibenzo-
fulvene 2 (58 mg, 0.19 mmol) in 2 mL of toluene was stirred in an
atmosphere of H2 (1 atm) for 120 min at room temperature. The red
color of 2 immediately disappeared. The NMR spectra of the reaction
solution revealed that the hydrogen addition product 4 was formed in
50% yield.
1
vacuo the phosphaboradibenzofulvene 2 was obtained as a red oil. H
NMR (400.1 MHz, C6D6): δ 1.41 (d, 3JHP = 12.1 Hz, 18 H, tBu), 6.92
(m, 2 H, Ar−H), 7.00 (m, 2 H, Ar−H), 7.08 (m, 2 H, Ar−H), 8.01
(m, 2 H, Ar−H). 11B{1H} NMR (128.4 MHz): δ 79.1 (br). 31P{1H}
NMR (162.0 MHz, C6D6): δ 21.1 (br). 13C{1H} NMR (62.9 MHz,
Independent Synthesis of 4. A solution of HPtBu2 (0.3 mL, 0.41
M, 0.12 mmol) in C6D6 was added to a freshly prepared solution of
9H-9-borafluorene17 (0.12 mmol) in 0.3 mL of C6D6. The reaction
mixture was stirred at room temperature for 60 min. NMR
spectroscopic control showed a quantitative conversion to 4. All
volatiles were removed under vacuum. 4 was obtained by
crystallization from a 1:1 mixture of toluene/pentane at −30 °C.
Yield: 23 mg (0.07 mmol, 60%). X-ray quality crystals of 4 were
1
2
C6D6): δ 33.1 (d, JPC = 16.4 Hz, C(CH3)3), 33.8 (d, JPC = 11.8 Hz,
4
4
C(CH3)3), 119.6 (d, JPC = 0.4 Hz, Ar−C), 128.3 (d, JPC = 1.0 Hz,
Ar−C), 134.0 (d, 5JPC = 1.0 Hz, Ar−C), 137.4 (d, JPC = 3.4 Hz, Ar−
3
C), 145.9 (br, B−C), 152.7 (d, 3JPC = 7.4 Hz, Ar−C). For assignment
see Figures 6S and 12S in the Supporting Information. Anal. Calcd for
C20H26BP (308.21): C 77.94; H 8.50. Found: C 75.95; H 8.68. UV−
vis (toluene): λmax 478 nm. For spectrum see Figure 5S in the
Supporting Information.
1
obtained by slow evaporation of a benzene solution. H NMR (300.0
MHz, C6D6): δ 0.84 (d, 3JHP = 13.6 Hz, 18 H, tBu), 3.46 (br, 1 H, B−
Synthesis of the tBu Derivative of 2, 2,7-Di-tert-butyl-9-di-
tertbutylphosphanyl-9-borafluorene. A solution of 2,7-di-tert-butyl-
9-bromo-9-borafluorene18 (50 mg, 0.14 mmol) in 1 mL of C6D6 was
slowly added to one molar equiv of Li[PtBu2]20 (22 mg, 0.14 mmol)
dissolved in 0.5 mL of C6D6. The reaction mixture was stirred at room
temperature for 75 min under the exclusion of air and then filtered.
After removing all volatiles under vacuum 2,7-di-tert-butyl-9-di-tert-
1
H), 4.03 (d, JHP = 357 Hz, 1 H, P−H), 7.31−7.41 (m, 4 H, Ar−H),
7.74−7.78 (m, 2 H, Ar−H), 7.90−7.93 (m, 2 H, Ar−H). 11B{1H}
NMR (96.3 MHz): δ −20.5 (d, 1JBP = 45 Hz). 11B NMR (96.3 MHz):
δ −20.5 (dd, 1JBP = 45 Hz, 1JBH = 87 Hz). 31P{1H} NMR (121.5 MHz,
C6D6): δ 42.3 (d, 1JBP = 45 Hz). 31P NMR (121.5 MHz, C6D6): δ 42.3
1
(dm, JHP = 357 Hz). 13C{1H} NMR (75.4 MHz, C6D6): δ 29.2 (d,
1
butylphosphanyl-9-borafluorene was obtained as a red oil. H NMR
2JPC = 1.0 Hz, C(CH3)3), 32.6 (d, 1JPC = 22.0 Hz, C(CH3)3), 120.4 (s,
F
dx.doi.org/10.1021/om400553j | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
5
Ar−C), 125.9 (d, 4JPC = 2.6 Hz, Ar−C), 126.3 (d, JPC = 2.3 Hz, Ar−
6.2 Hz, Ar−C), 159.1 (br, Ar−C). For assignment see Figures 11S and
17S in the Supporting Information. Anal. Calcd for C26H36BP
(390.35): C 80.00; H 9.30. Found: C 80.46; H 9.49.
3
3
C), 132.0 (d, JPC = 2.4 Hz, Ar−C), 150.6 (d, JPC = 8.6 Hz; Ar−C),
154.7 (br, B−C). For assignment see Figures 8S and 14S in the
Supporting Information. Anal. Calcd for C20H28BP (310.22): C 77.43;
H 9.10. Found: C 76.19; H 9.03.
X-ray Crystallography of 3, 4, cis-6/trans-6, trans-6, 7, and 8.
The data for all structures were measured on a STOE IPDS-II
diffractometer using a Genix Microfocus X-ray source with mirror
optics and Mo Kα radiation. The data were corrected for absorption
with the frame-scaling procedure contained in the X-AREA package.
The structures were solved by direct methods using the program
SHELXS and refined against F2 with full-matrix least-squares
techniques using the program SHELXL.27 Hydrogen atoms were
placed on ideal positions and refined with fixed isotropic displacement
parameters using a riding model (for X-ray parameters see Table 1S
and Table 2S in the Supporting Information). In 4 the H atoms
bonded to B and P were freely refined. The toluene solvent in trans-6
is disordered about a center of inversion over two equally occupied
positions. The cif of 3 is available from the CSD, where it has been
Remark: No release of H2 was observable when 4 was heated to 120
°C.
Reaction of 2 with Acetonitrile. Acetonitrile (0.2 mL, 3.8 mmol)
was added via syringe to a solution of the phosphaboradibenzofulvene
2 (0.18 mmol) in 2 mL of toluene at room temperature with stirring,
whereupon the red solution turned colorless. The reaction mixture was
concentrated to 1 mL and stored at 6 °C to obtain X-ray quality
crystals of cis-6 and trans-6 (ratio 2:1) (monoclinic space group P21/
c).
Performing this reaction with slowly added 2 equiv (20 μL, 0.38
mmol) of acetonitrile and 2 (0.18 mmol) in 6 mL of toluene yielded
pure trans-6 by storing the concentrated solution (1 mL) at −30 °C
deposited as “Personal Communications”, J. M. Breunig, A. Hubner,
̈
(triclinic space group P1). In both reactions the yield was less than
̅
1
M. Bolte, M. Wagner, and H.-W. Lerner, 2013, CCDC 938237. CCDC
reference numbers: CCDC 938239 (4), CCDC 938238 (2:1 cocrystal
of cis-6 and trans-6), CCDC 943018 (trans-6), CCDC 941398 (7),
and CCDC 942579 (8).
10%. NMR spectra of trans-6: H NMR (250.1 MHz, C6D6): δ 0.75
(d, 3JHP = 11.6 Hz, 36 H, tBu), 1.69 (s, 6 H, CH3), 7.27−7.40 (m, 8 H,
Ar−H), 7.76−7.79 (m, 4 H, Ar−H), 8.14−8.16 (m, 4 H, Ar−H).
11B{1H} NMR (80.3 MHz, C6D6): δ 7.4 (br). 31P{1H} NMR (101.3
MHz, C6D6): δ 38.5 (br). 13C{1H} NMR (62.9 MHz, C6D6): δ 27.1
2
2
ASSOCIATED CONTENT
(d, JPC = 4.0 Hz, NC(PtBu2)CH3), 30.7 (d, JPC = 15.9 Hz,
C(CH3)3), 32.7 (d, 1JPC = 29.7 Hz, C(CH3)3), 120.4 (s, Ar−C), 126.6
(s, Ar−C), 128.3 (s, Ar−C), 131.5 (s, Ar−C), 151.5 (d, 5JPC = 3.6 Hz;
Ar−C), 188.5 (d, 1JPC = 48.2 Hz, NC(PtBu2)CH3), n.o. (C−B). For
assignment see Figures 9S and 15S in the Supporting Information.
Anal. Calcd for C44H58B2N2P2·C7H8 (790.65): C 77.47; H 8.41; N
3.54. Found: C 77.16; H 8.24; N 4.54.
■
S
* Supporting Information
Crystal packing of trans-6, solid-state structure of trans-6 in the
2:1 cocrystal of cis-6 and trans-6 (monoclinic, P21/c), ball-and-
stick presentation of 3, crystal packing of the phosphonium salt
1
3, UV−vis spectrum of 2, H NMR spectra of 2 and of tBu
Reaction of 2 with Benzophenone. A solution of the
phosphaboradibenzofulvene 2 (0.18 mmol) in 2 mL of toluene was
added to benzophenone (38 mg, 0.21 mmol). The red color of 2
disappeared within 30 s. The reaction mixture was stirred at room
temperature overnight and filtered. All volatiles of the filtrate were
removed under vacuum. X-ray quality crystals of 7 were grown from a
1:1 toluene/hexane solution at −30 °C. Yield: 18 mg (20%). 1H NMR
(300.0 MHz, C6D6): δ 1.20 (d, 3JHP = 10.4 Hz, 18 H, tBu), 6.76 (m, 2
H, Ar−H), 6.93 (m, 6H, m,p-Ph), 7.02 (m, 2 H, Ar−H), 7.29 (m, 2 H,
Ar−H), 7.67 (m, 2 H, Ar−H), 8.02 (m, 4 H, o-Ph). 11B{1H} NMR
(96.3 MHz, C6D6): δ 46.9 (h1/2 = 950 Hz). 31P{1H} NMR (121.5
MHz, C6D6): δ 91.1. 13C{1H} NMR (75.4 MHz, C6D6): δ 32.5 (d,
2JPC = 13.2 Hz, C(CH3)3), 36.4 (d, 1JPC = 36.2 Hz, C(CH3)2), 94.6 (d,
1JPC = 51.6 Hz; PC(O)Ph2), 119.7 (s, Ar−C), 127.4 (d, 4JPC = 1.7 Hz,
derivative of 2, 4, trans-6, 7, and 8, assignment of the signals of
the NMR spectra of 2, tBu derivative of 2, 4, trans-6, 7, and 8,
and the table of X-ray parameters. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +49-69798-29260. E-mail: lerner@chemie.uni-frankfurt.
de.
Notes
The authors declare no competing financial interest.
3
ACKNOWLEDGMENTS
Ar−C), 127.6 (s, Ar−C), 128.0 (s, Ar−C), 129.1 (d, JPC = 10.2 Hz,
■
Ar−C), 130.3 (s, Ar−C), 132.2 (s, Ar−C), 138.2 (s, Ar−C), 143.3 (d,
2JPC = 12.3 Hz, Ar−C), 153.1 (s, Ar−C). For assignment see Figures
10S and 16S in the Supporting Information. Anal. Calcd for
C33H36BOP (490.42): C 80.82; H 7.40. Found: C 79.02; H 7.44.
Reaction of 2 with DMB. A Schlenk tube was charged with the
phosphaboradibenzofulvene 2 (0.28 mmol). 2 was dissolved in 3 mL
of toluene, and DMB (145 mg, 1.77 mmol) was added. The red color
of 2 disappeared after 2 h at room temperature. After stirring the
reaction mixture overnight at room temperature all volatiles were
removed under vacuum. The NMR spectra showed quantitative
formation of 8. X-ray quality crystals were obtained from a
concentrated solution of toluene and pentane (1:1) at −30 °C.
A.H. wishes to thank the Fonds der Chemischen Industrie for a
Ph.D. grant. This work was supported by the Beilstein-Institut,
Frankfurt/Main (Germany), within the research collaboration
NanoBiC.
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dx.doi.org/10.1021/om400553j | Organometallics XXXX, XXX, XXX−XXX