Conclusive Evidence on the Mechanism of the Rhodium-Mediated Decyanative Borylation

Article abstract of DOI:10.1021/jacs.5b07357

The stoichiometric reactions proposed in the mechanism of the rhodium-mediated decyanative borylation have been performed and all relevant intermediates isolated and characterized including their X-ray structures. Complex RhCl{xant(PⁱPr₂)₂} (1, xant(PⁱPr₂)₂ = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) reacts with bis(pinacolato)diboron (B₂pin₂), in benzene, to give the rhodium(III) derivative RhHCl(Bpin){xant(PⁱPr₂)₂} (4) and PhBpin. The reaction involves the oxidative addition of B₂pin₂ to 1 to give RhCl(Bpin)₂{xant(PⁱPr₂)₂}, which eliminates ClBpin generating Rh(Bpin){xant(PⁱPr₂)₂} (2). The reaction of the latter with the solvent yields PhBpin and the monohydride RhH{xant(PⁱPr₂)₂} (6), which adds the eliminated ClBpin. Complex 4 and its catecholboryl counterpart RhHCl(Bcat){xant(PⁱPr₂)₂} (7) have also been obtained by oxidative addition of HBR₂ to 1. Complex 2 is the promoter of the decyanative borylation. Thus, benzonitrile and 4-(trifluoromethyl)benzonitrile insert into the Rh-B bond of 2 to form Rh{C(R-C₆H₄)-NBpin}{xant(PⁱPr₂)₂} (R = H (8), p-CF₃ (9)), which evolve into the aryl derivatives RhPh{xant(PⁱPr₂)₂} (3) and Rh(p-CF₃-C₆H₄){xant(PⁱPr₂)₂} (10), as a result of the extrusion of CNBpin. The reactions of 3 and 10 with B₂pin₂ yield the arylBpin products and regenerate 2.

Full text of DOI:10.1021/jacs.5b07357

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Article
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Conclusive Evidence on the Mechanism of the Rhodium-Mediated
Decyanative Borylation
Miguel A. Esteruelas,* Montserrat Olivan, and Andrea Velez
́
́
́ ́ ́ ́
Departamento de Química Inorganica, Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), Centro de Innovacion en
Química Avanzada (ORFEO−CINQA), Universidad de ZaragozaCSIC, 50009 Zaragoza, Spain
S
* Supporting Information
ABSTRACT: The stoichiometric reactions proposed in the mechanism of the rhodium-mediated decyanative borylation have
been performed and all relevant intermediates isolated and characterized including their X-ray structures. Complex
RhCl{xant(PiPr2)2} (1, xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) reacts with bis(pinacolato)diboron
(B2pin2), in benzene, to give the rhodium(III) derivative RhHCl(Bpin){xant(PiPr2)2} (4) and PhBpin. The reaction involves the
oxidative addition of B2pin2 to 1 to give RhCl(Bpin)2{xant(PiPr2)2}, which eliminates ClBpin generating Rh(Bpin){xant(PiPr2)2}
(2). The reaction of the latter with the solvent yields PhBpin and the monohydride RhH{xant(PiPr2)2} (6), which adds the
eliminated ClBpin. Complex 4 and its catecholboryl counterpart RhHCl(Bcat){xant(PiPr2)2} (7) have also been obtained by
oxidative addition of HBR2 to 1. Complex 2 is the promoter of the decyanative borylation. Thus, benzonitrile and 4-
(trifluoromethyl)benzonitrile insert into the Rh−B bond of 2 to form Rh{C(R-C6H4)NBpin}{xant(PiPr2)2} (R = H (8), p-
CF3 (9)), which evolve into the aryl derivatives RhPh{xant(PiPr2)2} (3) and Rh(p-CF3-C6H4){xant(PiPr2)2} (10), as a result of
the extrusion of CNBpin. The reactions of 3 and 10 with B2pin2 yield the arylBpin products and regenerate 2.
of 1,4-diazabicyclo[2.2.2]octane (DABCO)6 leads to arylbor-
INTRODUCTION
■
onic esters, eq 1.7 The catalysis is compatible with a variety of
Transition metal catalyzed borylation reactions are of great
interest because organoboronate esters and boronic acids are
useful intermediates in many processes that lead to molecular
frameworks with applications ranging from pharmaceutical and
agrochemicals to material science.1 In this context, a powerful
tool is the direct borylation of hydrocarbons. These reactions
provide products with site-selectivity for C−H bond cleavage
that depends upon steric factors.2
A major goal of the chemistry of this century is to develop
procedures for the selective functionalization of a specific
position of a particular organic fragment. A promising
alternative to the direct borylation is the use of a directing
group suitable to be replaced by a borate ester. The nitrile is a
reliable directing group, is common in many organic molecules,
and is abundant in nature. On the other hand, the C−CN bond
is relatively strong (120−135 kcal mol−1), and its selective
rupture remains difficult.3 As a consequence, the metal-
mediated decyanation of nitriles is a challenging target of
great importance.4
functional groups. Although this discovery is certainly notable,
the nature of the catalyst is not clear, and there is no evidence
about the reaction mechanism, mainly because experimental
work has not been performed in order to clarify the process.
DFT studies need experimental support to truly contribute
to the harmonious development of the field.8 In spite of this,
the groups of Chatani6 and Yao Fu9 have only performed
calculations, in the effort to gain insight into mechanistic
details, assuming the formation of the recently reported
complex RhCl(xantphos)10 as a precatalyst. In both cases, the
results suggest a catalytic cycle (Scheme 1) involving four steps:
(i) formation of a rhodium(I)-boryl complex resulting from the
Tobisu, Chatani, and co-workers5 discovered the decyanative
borylation in 2012. They observed that in toluene, at 100 °C,
the reaction of a wide range of aryl cyanides with diboranes in
the presence of 5 mol % of [Rh(μ-Cl)(η4-COD)]2 (COD =
1,5-cyclooctadiene), 10 mol % of xantphos (xantphos = 9,9-
dimethyl-4,5-bis(diphenylphosphino)xanthene), and 1.0 equiv
Received: July 15, 2015
© XXXX American Chemical Society
A
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POP compounds of groups 8 and 9.13 These 9,9-dimethyl-4,5-
bis(diisopropylphosphino)xanthene (xant(PiPr2)2) derivatives
are the Pr-counterparts of intermediates A, C, and F of the
Scheme 1
i
catalytic cycle shown in Scheme 1. In view of this, we decided
to investigate the stoichiometric reactions involved in steps i, ii,
and iii by using our complexes as models.
This paper shows the first experimental investigation of the
mechanism of the reaction of decyanative borylation,
discovered by Tobisu, Chatani, and co-workers in 2012, and
describes the formation of the catalytic intermediates, which are
isolated and characterized by X-ray diffraction analysis,
including N-boryl η1-iminoacyl complexes and the resulting
products of their C−C bond cleavage reactions via elimination
of boryl isocyanide.
RESULTS AND DISCUSSION
■
Formation of the Active Species: Rhodium(I)-Boryl
versus Hydride-Chloride-Rhodium(III)-Boryl. According to
the DFT calculation reported by the Chatani’s group,6 the
oxidative addition of diborane to A should afford a chloride-
rhodium(III)-bis(boryl) derivative (B in Scheme 1), which is
about 25 kcal mol−1 more stable than the reagents. The
addition has a small activation barrier of 2.6 kcal mol−1. This
rhodium(III) intermediate undergoes reductive elimination of
boryl chloride with an activation barrier of about 20 kcal mol−1
to give the square-planar rhodium(I)-boryl species C, which is
believed to be the true catalyst of the reaction. The reductive
elimination destabilizes the system by about 17 kcal mol−1.
There are marked differences between the expected one,
according to the theoretical calculations, and that experimen-
tally found. In contrast to the DFT results, the treatment of
benzene solutions of 1 with 1.0 equiv of bis(pinacolato)diboron
(B2pin2), at 90 °C, for 20 h leads to the hydride-chloride-
rhodium(III)-boryl derivative RhHCl(Bpin){xant(PiPr2)2} (4
in Scheme 2), which was isolated as a white solid in 78% yield,
and Ph-Bpin. Complex 4 was characterized by X-ray diffraction
analysis. Figure 1 gives a view of its structure. As expected for a
pincer coordination of the diphosphine, the Rh(POP) skeleton
is T-shaped with the metal center situated in the common
vertex and P(1)−Rh−P(2), P(1)−Rh−O(1), and P(2)−Rh−
O(1) angles of 157.30(3)°, 81.62(5)°, and 81.98(5)°,
respectively. Thus, the coordination geometry around the
rhodium atom can be rationalized as a distorted octahedron
with the boryl group trans-disposed to the oxygen atom of the
diphosphine (O(1)−Rh−B(1) = 176.06(12)°) and the hydride
trans-disposed to the chloride ligand (H(01)−Rh−Cl(1) =
176.5(12)°). The Rh−B bond length of 1.990(4) Å compares
well with the Rh(III)−B single bond distances previously
reported.14
reaction of RhCl(xantphos) with the diborane (from A to C),
(ii) insertion of the nitrile into the rhodium-boryl bond to form
an iminoacyl intermediate (from C to E), (iii) extrusion of the
boryl isocyanide to afford a rhodium(I)-aryl derivative (from E
to F), and (iv) reaction of the latter with the diborane to give
the aryl boronic ester and to regenerate the rhodium(I)-boryl
especies (from F to C).
The last step of the proposed cycle has been recently
corroborated by us.11 In addition, we have reported the
preparation and X-ray structure of complexes RhCl{xant-
(PiPr2)2} (1), Rh(Bpin){xant(PiPr2)2} (2), and RhPh{xant-
(PiPr2)2} (3)11,12 (Chart 1), as a part of a research program on
Chart 1
This structure is as that of the related silyl derivatives
RhHCl(SiR3){xant(PiPr2)2},13d with the Bpin ligand in the
position of the silyl group, in agreement with the marked
diagonal relationship between boron and silicon, which is also
Scheme 2
B
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Figure 1. ORTEP diagram of complex 4 (50% probability ellipsoids).
Hydrogen atoms (except the hydride) are omitted for clarity. Selected
bond lengths (Å) and angles (deg): Rh−P(1) = 2.2789(8), Rh−P(2)
= 2.2893(8), Rh−Cl(1) = 2.4595(8), Rh−O(1) = 2.3019(19), Rh−
B(1) = 1.990(4); P(1)−Rh−P(2) = 157.30(3), P(1)−Rh−O(1) =
81.62(5), P(2)−Rh−O(1) = 81.98(5), P(1)−Rh−Cl(1) = 97.57(3),
P(2)−Rh−Cl(1) = 96.51(3), P(1)−Rh−B(1) = 97.78(11), P(2)−
Rh−B(1) = 97.51(11), O(1)−Rh−Cl(1) = 84.98(5), Cl(1)−Rh−B(1)
= 98.97(11), B(1)−Rh−H(01) = 77.7(12), O(1)−Rh−B(1) =
176.06(12), O(1)−Rh−H(01) = 98.3(12), H(01)−Rh−Cl(1) =
176.5(12).
Figure 2. 31P{1H} NMR spectrum of the reaction of RhCl{xant-
(PiPr2)2} (1) with B2pin2, at 90 °C in benzene, after 2.5 h, showing the
formation of RhHCl(Bpin){xant(PiPr2)2} (4) via the bis(boryl)
intermediate RhCl(Bpin)2{xant(PiPr2)2} (5).
1
central heterocycle, at 1.16 and 1.06 ppm, in the H NMR
spectrum, strongly support the mutually cis disposition of the
boryl ligands. In the presence of 1 equiv of DABCO complex 4
is also formed. However, instead of 5, the square-planar
rhodium(I)-boryl derivative 2 is detected as an intermediate
(Figure 3). In order to corroborate the participation of the
evident in the chemistry of the platinum group metals.15 The
1H, 31P{1H}, and 11B{1H} NMR spectra of 4, in benzene-d6, at
room temperature are consistent with the structure shown in
Figure 1. The 1H NMR spectrum shows the hydride resonance
at −15.66 ppm as a double triplet with Rh−H and H−P
coupling constants of 26.6 and 15.4 Hz, respectively. As
expected for equivalent PiPr2 groups, the 31P{1H} NMR
spectrum contains at 52.8 ppm a doublet with a typical P−
Rh(III) coupling constant of 118.4 Hz. In the 11B{1H} NMR
spectrum, the Bpin group displays a broad signal at 36.0 ppm.
Scheme 2 collects the reactions involved in the formation of
4. In accordance with the theoretical results, complex 1 adds
B2pin2 to initially afford the bis(boryl)intermediate RhCl-
i
(Bpin)2{xant(PiPr2)2} (5), the Pr2 counterpart of B. This
intermediate undergoes reductive elimination of boryl chloride
to form the square-planar rhodium(I) boryl complex 2, which
promotes the direct borylation of arenes.11 In agreement with
this, it rapidly reacts with the solvent to give PhBpin (the
organic reaction product) and to generate the previously
described monohydride RhH{xant(PiPr2)2} (6),12,16 which also
is an active catalyst for the direct borylation of arenes with both
B2pin2 and HBpin.11 The oxidative addition of the previously
eliminated boryl chloride to 6 yields 4. In an effort to confirm
this sequence of reactions and to detect some reaction
intermediates, we followed the reaction by 31P{1H} NMR
spectroscopy. Figure 2 shows the spectrum after 2.5 h. It
contains a resonance at 41.0 ppm, in addition to the
characteristic signals of 1 and 4, which appeared to be due to
a rhodium(III) species according to the value of the P−Rh
coupling constant, 125.4 Hz. Thinking that this resonance
would correspond to the bis(boryl) intermediate 5 (δ11B, 32−
42), we added 1.0 equiv of boryl chloride to an NMR tube
containing 2 in benzene-d6. The spectra of the resulting
solution confirmed our suspicion and showed the formation of
5. The presence of four resonances for the methyl groups of the
isopropyl substituents of the phosphine, at 1.79, 1.73, 1.01, and
0.90 ppm, and two signals for the methyl substituents of the
Figure 3. 31P{1H} NMR spectra of the reaction of RhCl{xant(PiPr2)2}
(1) with B2pin2 in the presence of 1 equiv of DABCO after 5.5 h at 90
°C in benzene (a), and in benzene-d6 (b) showing the formation of
RhHCl(Bpin){xant(P Pr2)2} (4), via Rh(Bpin){xant(P Pr2)2} (2). ( )
Isotopomer RhDCl(Bpin){xant(PiPr2)2}. * indicates diphosphine
oxide.
i
i
●
monohydride 6, we also added boryl chloride to a NMR tube
containing a benzene-d6 solution of 6, and as expected, complex
4 was instantaneously formed. The participation of 6 in the
formation of 4 is also supported by the presence of about 0.5
deuterium atoms in the hydride position of the resulting
product from the addition of B2pin2 to 1 in benzene-d6. The
remaining 0.5 hydrogen atoms result from a D/H exchange
process between the intermediate RhD{xant(PiPr2)2} (6-d1)
and the methyl groups of the isopropyl substituents of the
phosphine.
C
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These results demonstrate that, in fact, the addition of
B2(OR)4 to A affords C via rhodium(III) intermediates
RhCl{B(OR)2}2(POP) (B) although, in the absence of nitrile,
rhodium(III) compounds RhHCl{B(OR)2}(POP) are formed
as a result of a side reaction of C with the solvent and
subsequent addition of Cl−B(OR)2 to the resulting square-
planar rhodium(I)-monohydride complexes. In order to
demonstrate that species C is certainly the catalyst of the
decyanative borylation we have performed the catalysis under
the same conditions as those employed by Tobisu, Chatani, and
co-workers using complex 2, 4-(trifluoromethyl)-benzonitrile,
and B2pin2. After 15 h, the decyanative borylation product 4-
(trifluoromethyl)phenyl boronic acid pinacol ester was formed
in quantitative yield.
from 2, which was prepared by reaction of 6 with HBpin
according to the previously described method.11 At room
temperature, the treatment of toluene solutions of this square-
planar rhodium(I) boryl complex with 1.0 equiv of benzonitrile
and 4-(trifluoromethyl)benzonitrile gave rise to the instanta-
neous formation of the unprecedented square-planar derivatives
Rh{C(R-C6H4)NBpin}{xant(PiPr2)2} (R = H (8), p-CF3
(9)), which were isolated as orange solids in 91% (8) and 74%
(9) yield, eq 3.
Complex 4 was also prepared in an isolated yield of 80% by
means of the HBpin oxidative addition to 1. Similarly, the
addition of 4.0 equiv of catecholborane (HBcat) to the latter
leads to the catecholboryl counterpart RhHCl(Bcat){xant-
(PiPr2)2} (7), which was isolated as a white solid in 78% yield,
Both compounds were characterized by X-ray diffraction
analysis. The structures prove their formation and reveal
marked differences with the optimized structures resulting from
the DFT calculations. The structure of 8 has two chemically
equivalent but crystallographically independent molecules in
the asymmetric unit. Figure 4 shows a drawing of one of them.
1
eq 2. In agreement with 4, the H NMR spectrum of 7, in
benzene-d6, at room temperature shows the hydride resonance
at −14.96 ppm as a double triplet with H−Rh and H−P
coupling constants of 26.5 and 14.4 Hz, respectively. In the
31P{1H} NMR spectrum, the equivalent PiPr2 groups display a
doublet (JP−Rh = 109.9 Hz) at 57.7 Hz, whereas the 11B{1H}
NMR spectrum contains a broad signal at 40.9 ppm. The high
stability of these rhodium(III) species toward the reduction of
their metal center must be pointed out. In contrast to 4 and 7,
the dihydride counterparts RhH2(BR2){xant(PiPr2)2} (BR2 =
Bpin, Bcat) eliminate molecular hydrogen to yield the
corresponding rhodium(I) boryl derivatives.11 This difference
in behavior is a new fact indicating that d6-Rh{xant(PiPr2)2}
species containing the π donor chloride ligand are more
reluctant to undergo reductive elimination than those without
any π donor ligand.12,13d
Insertion of the Nitrile: Nature of the Iminoacyl
Intermediate. In their original paper,5a Tobisu, Chatani, and
co-workers proposed that once the rhodium(I)-boryl species C
is formed, the insertion of the C−N triple bond of the substrate
into the Rh−B bond should lead to an “iminylrhodium” species
as a result of the migration of the boryl group to the
electrophilic carbon atom of the nitrile, in contrast with the
typical regiochemistry of the borylation of polar unsaturated
multiple bonds,17 and assuming a nucleophilic boron.18 Later,
on the basis of DFT calculations, the formation of a square-
planar η2-iminoacyl intermediate D, which evolved to the T-
shaped η1-iminoacyl species E, was proposed.6,9
Figure 4. ORTEP diagram of complex 8 (50% probability ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Rh(1)−P(1) = 2.2693(17), 2.2489(18); Rh(1)−P(2) =
2.2460(17), 2.2583(18); Rh(1)−C(1) = 1.943(6), 1.955(6); Rh(1)−
O(3) = 2.249(4), 2.287(4); C(1)−N(1) = 1.298(7), 1.279(7); N(1)−
B(1) = 1.388(8), 1.377(8); P(1)−Rh(1)−P(2) = 162.42(6),
162.34(6); P(1)−Rh(1)−O(3) = 81.84(11), 80.74(11); P(2)−
Rh(1)−O(3) = 81.49(11), 81.61(11); P(1)−Rh(1)−C(1) =
100.97(18), 99.99(18); P(2)−Rh(1)−C(1) = 96.01(18), 97.52(18);
O(3)−Rh(1)−C(1) = 175.6(2), 174.8(2); Rh(1)−C(1)−N(1) =
124.1(5), 125.2(5); C(1)−N(1)−B(1) = 143.3(6), 142.7(6); Rh(1)−
C(1)−C(2) = 121.8(4), 119.9(4); C(2)−C(1)−N(1) = 114.1(5),
114.9(5).
We were interested in knowing if N-boryl substituted η1-
iminoacyl intermediates could be really formed and their
nature. A few N-boryl substituted η2-iminoacyl transition metal
complexes had been previously isolated,19 and it has been
proven that N-silyl substituted η2-iminoacyl intermediates
played a main role in the C−C bond activation of alkyl and
aryl nitriles.20 Thus, we studied the insertion reaction starting
In contrast to the optimized structures of D and E, the
diphosphine acts as tridentate with P(1)−Rh(1)−P(2), P(1)−
Rh(1)−O(3), and P(2)−Rh(1)−O(3) angles of 162.42(6)°
and 162.34(6)°, 81.84(11) and 80.74(11)°, and 81.49(11)° and
81.61(11)°, respectively. Thus, the geometry around the
rhodium atom is almost square-planar with the η1-C donor
ligand trans disposed to the oxygen atom (C(1)−Rh(1)−O(3)
D
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= 175.6(2)° and 174.8(2)°). The greatest deviation from the
best plane through Rh(1), C(1), P(1), O(3), and P(2) atoms is
0.0868(16) Å in one molecule and −0.0488(18) Å in the other
one, involved with P(2) and Rh(1), respectively. The Rh(1)−
C(1) bond lengths of 1.943(6) and 1.955(6) Å and the C(1)−
N(1) distances of 1.298(7) and 1.279(7) Å are consistent with
single and double bonds between the respective atoms, whereas
the angles around C(1), between 125° and 114°, are in
accordance with a sp2-hybridization at C(1). The N(1)−B(1)
bond lengths of 1.388(8) and 1.377(8) Å, along with the
C(1)−N(1)−B(1) angles of 143.3(6)° and 142.7(6)°, suggest a
notable double bond character for the N(1)−B(1) bond and,
therefore, a significant allenic nature of the η1-C donor ligand.
This is a consequence of the empty p orbital at the boron,
which allows additional nitrogen to boron π donation.
Phenyl Migration. In spite of that DFT results indicate
that the formation of F from D and E is an endoergic process
by 21.0 and 14.0 kcal mol−1,6 respectively; the heating of
toluene solutions of 8 and 9, at 50 °C, for 72 h leads to the aryl
derivatives 3 and Rh(p-CF3-C6H4){xant(PiPr2)2} (10). Their
formation results from the extrusion of boryl isocyanide, which
polymerizes to afford an air-sensitive, insoluble, yellow solid
containing an IR band at 2116 cm−1, eq 4. In agreement with
The structure of 9 (Figure 5) resembles that of 8 with P(1)−
Rh−P(2), P(1)−Rh−O(1), P(2)−Rh−O(1), and C(1)−Rh−
the participation of these aryl derivatives in the catalysis, the
reactions of both 3 and 10 with B2pin2 yield the corresponding
aryl-Bpin and regenerate the rhodium(I)-boryl compound 2.
Complex 10 was isolated as a red solid in 72% yield and
characterized by X-ray diffraction analysis. The structure
(Figure 6) proves its formation. As expected, the coordination
Figure 5. ORTEP diagram of complex 9 (50% probability ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Rh−P(1) = 2.2606(18), Rh−P(2) = 2.2450(18), Rh−
C(1) = 1.946(6), Rh−O(1) = 2.246(4), C(1)−N(1) = 1.274(8),
N(1)−B(1) = 1.399(9); P(1)−Rh−P(2) = 160.30(6), P(1)−Rh−
O(1) = 82.44(12), P(2)−Rh−O(1) = 82.72(11), P(1)−Rh−C(1) =
96.39(18), P(2)−Rh−C(1) = 98.65(18), O(1)−Rh−C(1) = 178.5(2),
Rh−C(1)−N(1) = 129.2(5), Rh−C(1)−C(2) = 116.1(5), C(1)−
N(1)−B(1) = 155.5(6), C(2)−C(1)−N(1) = 114.6(6).
Figure 6. ORTEP diagram of complex 10 (50% probability ellipsoids).
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
angles (deg): Rh(1)−P(1) = 2.2434(10), Rh−C(1) = 1.983(6),
Rh(1)−O(1) = 2.221(4); P(1)−Rh(1)−P(1A) = 164.09(5), P(1)−
Rh(1)−O(1) = 82.24(2), P(1)−Rh(1)−C(1) = 97.71(2), O(1)−Rh−
C(1) = 178.6(2).
O(1) angles of 160.30(6)°, 82.44(12)°, 82.72(11)°, and
178.5(2)°, respectively. The Rh−C(1), C(1)−N(1), and
N(1)−B(1) distances of 1.946(6), 1.274(8), and 1.399(9) Å
are statistically identical to the respective bond lengths of 8.
The C(1)−N(1)−B(1) angle of 155.5(6)° also supports some
allenic nature for the C(p-CF3-Ph)NBpin ligand, and its
comparison with that of 8 reveals a marked influence of the
substituent at C(1) on the N−B bond.
geometry around the rhodium atom is almost square-planar
with the diphosphine coordinated in a mer-fashion (P(1)−Rh−
P(1A) = 164.09(5)° and P(1)−Rh−O(1) = 82.24(2)°) and the
aryl group trans disposed to the oxygen atom (C(1)−Rh−O(1)
= 178.6(2)°). The greatest deviation from the best plane
through Rh, C(1), O(1), P(1), and P(1A) atoms is 0.0174(24)
Å and involves Rh. The metalated aromatic ring lies
perpendicular to the coordination plane with a dihedral angle
of 90°. The Rh−C(1) distance of 1.983(6) Å compares well
with those found in the reported square-planar rhodium(I)-aryl
complexes (1.84−2.10 Å).11,21 In the 13C{1H} NMR spectrum
in benzene-d6, at room temperature the most noticeable
resonance is that corresponding to C(1), which appears at
173.8 ppm as a double triplet with C−Rh and C−P coupling
constants of 40.9 and 11.8 Hz, respectively. In agreement with
The 13C{1H}, 11B{1H}, and 31P{1H} NMR spectra of 8 and
9, in benzene-d6, at room temperature are consistent with the
structures shown in Figures 4 and 5. In the 13C{1H} NMR
spectra the resonance due to the metalated carbon atom C(1)
appears at about 209 ppm as a double triplet with C−Rh and
C−P coupling constants of about 43 and 10 Hz, respectively.
The Bpin group displays a broad resonance at about 27 ppm in
the 11B{1H} NMR spectra, whereas the equivalent iPr2P groups
give rise to a doublet (JP−Rh ≈ 187−191 Hz) at about 37 ppm
in the 31P{1H} NMR spectra.
i
equivalent Pr2P groups, the 31P{1H} NMR spectrum contains
at 37.5 ppm a doublet with a typical P−Rh(I) coupling
constant of 173.5 Hz.
E
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Journal of the American Chemical Society
Article
resulting solution was heated in an oil bath at 90 °C for 20 h. After this
time, it was concentrated to dryness to afford a pale brownish residue.
Addition of pentane (4 mL) afforded a white solid that was washed
with pentane (2 × 1 mL) and dried in vacuo. Yield: 219.0 mg (78%).
Anal. Calcd for C33H53BClO3P2Rh: C, 55.91; H, 7.54. Found: C,
56.32; H, 7.25. HRMS (electrospray, m/z) calcd for C33H53BO3P2Rh
[M − Cl]+: 673.2618; found: 673.2647. IR (cm−1): ν(Rh−H) 2094
(w), ν(C−O−C) 1109 (m). 1H NMR (400.16 MHz, C6D6, 298 K): δ
7.22 (m, 2H, CH-arom POP), 7.10 (dd, JH−H = 7.7, JH−H = 1.3, 2H,
CH-arom POP), 6.92 (dd, 2H, JH−H = 7.6, JH−H = 7.6, CH-arom
POP), 2.85 (m, 2H, PCH(CH3)2), 2.38 (m, 2H, PCH(CH3)2), 1.79
(dvt, JH−H = 7.3, N = 16.2, 6H, PCH(CH3)2), 1.63 (dvt, JH−H = 7.3, N
= 15.1, 6H, PCH(CH3)2), 1.40 (dvt, JH−H = 7.2, N = 15.8, 6H,
PCH(CH3)2), 1.35 (s, 3H, CH3), 1.26 (s, 12H, Bpin), 1.16 (s, 3H,
CH3), 0.95 (dvt, JH−H = 7.1, N = 14.9, 6H, PCH(CH3)2), −15.66 (dt,
JH−Rh = 26.6, JH−P = 15.4, 1H, Rh−H). 13C{1H} NMR (100.62 MHz,
C6D6, 298 K): δ 154.8 (vt, N = 12.3, C-arom POP), 132.2 (vt, N = 5.0,
C-arom POP), 130.8 (s, CH-arom POP), 127.4 (s, CH-arom POP),
126.1 (vt, N = 24.1, C-arom POP), 124.3 (vt, N = 4.8, CH-arom
POP), 82.0 (s, C Bpin), 34.9 (s, C(CH3)2), 34.4 (s, C(CH3)2), 29.4
(vt, N = 21.6, PCH(CH3)2), 28.2 (dvt, JRh−C = 3.0, N = 30.0,
PCH(CH3)2), 27.5 (s, C(CH3)2), 25.7 (s, CH3 Bpin), 21.5 (s,
PCH(CH3)2), 20.3 (vt, N = 5.6, PCH(CH3)2), 20.0 (s, PCH(CH3)2),
19.8 (vt, N = 5.6, PCH(CH3)2). 31P{1H} NMR (161.98 MHz, C6D6,
298 K): δ 52.8 (d, JP−Rh = 118.4). 11B{1H} NMR (128.38 MHz, C6D6,
298 K): δ 36.0 (br).
CONCLUDING REMARKS
■
This study shows the stoichiometric reactions forming the
catalytic cycle of the rhodium-mediated decyanative borylation
of nitriles and the X-ray structures of the main intermediates of
the catalysis.
The true catalyst of the process is certainly a square-planar
Rh{B(OR)2}(POP) derivative which is formed, via a rhodium-
(III)-bis(boryl) intermediate, through the oxidative addition of
B2(OR)4 to the catalytic precursor RhCl(POP) and subsequent
reductive elimination of boryl chloride. In the absence of
substrate the catalyst evolves into a rhodium(III) derivative
RhHCl{B(OR)2}(POP), as a consequence of the borylation of
the solvent (toluene) and the oxidative addition of the
eliminated boryl chloride to the resulting monohydride.
Once the catalyst is generated, the insertion of the C−N
triple bond of the substrates RCN into the Rh−B bond affords
square-planar Rh[C(R)N{B(OR)2}](POP) species, with the
diphosphine acting as mer-tridentated and the formed η1-C-
donor ligand showing a significant allenic nature. No evidence
of the participation of square-planar η2-iminoacyl and T-shaped
η1-iminoacyl intermediates has been found. Complexes Rh[C-
(R)N{B(OR)2}](POP) evolve into rhodium(I)-aryl deriva-
tives as a result of the extrusion of boryl isocyanide, which
polymerizes in the reaction medium. The reactions of the
rhodium(I)-aryl compounds with B2(OR)4 yield the products
of the catalysis, closing the cycle.
Too many catalytic cycles of relevant organic reactions are
reported on the basis of simple speculations, even some of
them without taking into account basic concepts of organo-
metallics. Other times, DFT calculations replace experimental
evidence, which can be easily obtained. In contrast to these
uses, here, we report the mechanism of a novel reaction
recently discovered on the basis of the experimental study of
each elemental stage of the process, the isolation of all relevant
intermediates, and their spectroscopic and structural character-
ization, including the X-ray structures.
Reaction of RhCl{xant(PiPr2)2} (1) with Bis(pinacolato)-
diboron in an NMR Tube: Detection of RhCl(Bpin)2{xant-
(PiPr2)2} (5). In an NMR tube a solution of 1 (40 mg, 0.069 mmol) in
benzene (0.5 mL) was treated with the stoichiometric amount of
B2pin2 (17.6 mg, 0.069 mmol), and the resulting solution was heated
at 90 °C in an oil bath. The reaction was periodically checked by
31P{1H} and 11B{1H} NMR spectroscopy. After 2.5 h the 31P{1H}
NMR spectrum shows signals corresponding to RhHCl(Bpin){xant-
(PiPr2)2} (4), RhCl(Bpin)2{xant(PiPr2)2} (5), and RhCl{xant(PiPr2)2}
(1) in a ratio 53:15:32. The 11B{1H} NMR spectrum show peaks
assigned to PhBpin (br, δ 30.9) and ClBpin (br, δ 27.5), in addition to
those assigned to 4 and 5. After 20 h the 31P{1H} NMR spectrum
shows the quantitative conversion to RhHCl(Bpin){xant(PiPr2)2} (4).
Reaction of Rh(Bpin){xant(PiPr2)2} (2) with ClBpin. Spectro-
scopic Detection of RhCl(Bpin)2{xant(PiPr2)2} (5). In an NMR
tube a solution of 2 (20 mg, 0.030 mmol) in benzene-d6 was treated
with ClBpin (4.8 μL, 0.030 mmol). The immediate and quantitative
EXPERIMENTAL SECTION
■
1
conversion to 5 was observed by H, 31P{1H}, and 11B{1H} NMR
General Information. All reactions were carried out with rigorous
exclusion of air using Schlenk-tube techniques or in a drybox. Pentane
was obtained oxygen- and water-free from an MBraun solvent
purification apparatus and was stored over P2O5 in the drybox, while
toluene, benzene, and benzonitrile were dried, distilled, and stored
under argon. Pinacolborane, B2pin2, and 4-(trifluoromethyl)-
benzonitrile were purchased from commercial sources and used
without further purification. Catecholborane was purchased from
1
spectroscopies. H NMR (300 MHz, C6D6, 298 K): δ 7.20 (m, 2H,
CH-arom POP), 7.12 (d, JH−H = 7.5, 2H, CH-arom POP), 6.96 (t, 2H,
JH−H = 7.5, CH-arom POP), 3.09 (m, 2H, PCH(CH3)2), 2.92 (m, 2H,
PCH(CH3)2), 1.79 (dvt, JH−H = 6.8, N = 14.9, 6H, PCH(CH3)2), 1.73
(dvt, JH−H = 7.5, N = 16.0, 6H, PCH(CH3)2), 1.37 (s, 12H, Bpin), 1.16
(s, 3H, CH3), 1.11 (s, 12H, Bpin), 1.06 (s, 3H, CH3), 1.01 (dvt, JH−H
= 8.1, N = 14.0, 6H, PCH(CH3)2), 0.90 (dvt, JH−H = 7.0, N = 13.4, 6H,
PCH(CH3)2). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 41.0 (d,
JP−Rh = 125.4). 11B{1H} NMR (96.29 MHz, C6D6, 298 K): δ 32−42
(very br).
1
commercial sources and distilled in a Kugelrohr distillation oven. H,
13C{1H}, 31P{1H}, 11B{1H}, and 19F NMR spectra were recorded on
Bruker 300 ARX, Bruker Avance 300 MHz, Bruker Avance 400 MHz,
or Bruker Avance 500 MHz instruments. Chemical shifts (expressed in
parts per million) are referenced to residual solvent peaks (1H,
13C{1H}), external 85% H3PO4 (31P{1H}), BF3·OEt2 (11B{1H}), or
Reaction of RhCl{xant(PiPr2)2} (1) with Bis(pinacolato)-
diboron in the Presence of DABCO. Two NMR tubes were
charged with 1 (40 mg, 0.047 mmol), B2pin2 (11.3 mg, 0.047 mmol),
and DABCO (5.0 mg, 0.069 mmol). To the first NMR tube was added
0.5 mL of benzene, and to the second was added 0.5 mL of benzene-
d6. Afer that, both tubes were introduced in an oil bath at 90 °C, and
the tubes were periodically checked by 31P{1H} NMR spectroscopy.
After 5.5 h the 31P{1H} NMR spectra of both tubes show the
formation of RhHCl(Bpin){xant(PiPr2)2} (4) and Rh(Bpin){xant-
(PiPr2)2} (2). Additionally, the 31P{1H} NMR spectrum of the
reaction performed in benzene-d6 showed the presence of the
isotopomer of 4 RhDCl(Bpin){xant(PiPr2)2}. Ratio 4:2 in benzene
is 94:6. Ratio 4 (plus its isotopomer):2 in benzene-d6 is 77:23.
Reaction of RhH{xant(PiPr2)2} (6) with ClBpin. A solution of 6
(37.0 mg, 0.067 mmol) in benzene-d6 (0.5 mL) in an NMR tube was
treated with ClBpin (70 μL, 0.070 mmol). The immediate and
CFCl3 (19F). Coupling constants J and N are given in hertz.
Attenuated total reflection infrared spectra (ATR-IR) of solid samples
were run on a PerkinElmer Spectrum 100 FT-IR spectrometer. C, H,
and N analyses were carried out in a PerkinElmer 2400 CHNS/O
analyzer. High-resolution electrospray mass spectra were acquired
using a MicroTOF-Q hybrid quadrupole time-of-flight spectrometer
(Bruker Daltonics, Bremen, Germany). RhCl{xant(PiPr2)2} (1),13d
Rh(Bpin){xant(PiPr2)2} (2),11 and ClBpin22 were prepared by
published methods.
Reaction of RhCl{xant(PiPr2)2} (1) with Bis(pinacolato)-
diboron: Preparation of RhHCl(Bpin){xant(PiPr2)2} (4). A solution
of 1 (230.1 mg, 0.40 mmol) in toluene was treated with the
stoichiometric amount of B2pin2 (100.7 mg, 0.40 mmol), and the
F
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quantitative conversion to RhHCl(Bpin){xant(PiPr2)2} (4) was
2H, CH Ph), 7.23−7.14 (m, 3H, 1H Ph + 2 CH-arom POP), 7.06 (d,
JH−H = 7.7, 2H, CH-arom POP), 6.86 (t, JH−H = 7.7, 2H, CH-arom
POP), 2.56 (m, 4H, PCH(CH3)2), 1.37 (s, 12H, CH3 Bpin), 1.30−
1.22 (m, 18H, PCH(CH3)2 + C(CH3)2), 1.16 (dvt, JH−H = 6.9, N =
14.0, 12H, PCH(CH3)2). 13C{1H} NMR (125.78 MHz, C6D6, 298 K):
δ 209.7 (dt, JC−Rh = 42.3, JC−P = 9.6, Rh−C), 155.2 (vt, N = 15.0,
Carom POP), 148.7 (dt, JC−Rh = 3.2, JC−P = 3.2, Cipso Ph), 131.2 (s,
CH Ph), 131.0 (s, CH-arom POP), 130.8 (vt, N = 4.9, Carom POP),
127.9, 127.6 (both s, CH Ph), 126.9 (s, CH-arom POP), 125.3 (vt, N
= 16.0, Carom POP), 123.9 (s, CH-arom POP), 80.8 (s, C Bpin), 34.2
(s, C(CH3)2), 32.8 (s, C(CH3)2), 25.6 (s, CH3 Bpin), 25.2 (dvt, JC−Rh
= 2.9, N = 17.8, PCH(CH3)2), 19.6 (br, PCH(CH3)2), 18.1 (s,
PCH(CH3)2). 31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 36.5 (d,
JP−Rh = 190.8). 11B NMR (92.29 MHz, C6D6, 298 K): δ 26.7 (br).
Reaction of Rh(Bpin){xant(Pi Pr2 )2 } (2) with 4-
(Trifluoromethyl)benzonitrile: Preparation of Rh{C(p-CF3−
C6H4)NBpin}{xant(PiPr2)2} (9). 4-(Trifluoromethyl)benzonitrile
(63.6 mg, 0.37 mmol) was added to a solution of 2 (250 mg, 0.37
mmol) in toluene (2 mL). The resulting mixture was stirred for 5 min
at room temperature. After this time, it was concentrated to dryness to
afford an orange residue. Addition of pentane (1 mL) afforded an
orange solid that was washed with pentane (1 × 0.5 mL) and dried in
vacuo. Yield: 231 mg (74%). Anal. Calcd for C41H56BF3NO3P2Rh: C,
58.38; H, 6.69. Found: C, 58.64; H, 6.31. HRMS (electrospray, m/z):
calcd for C35H44ONF3P2Rh [M − Bpin]+: 716.1900. Found: 716.1897.
1
observed by H and 31P{1H} NMR spectroscopies.
Decyanative Borylation of 4-(Trifluoromethyl)benzonitrile
with Bis(pinacolato)diboron Catalyzed by Rh(Bpin){xant-
(PiPr2)2} (2). In an argon filled glovebox an Ace pressure tube was
charged with 4-trifluoromethylbenzonitrile (85.6 mg, 0.5 mmol),
B2pin2 (254 mg, 1.0 mmol), 2 (67.2 mg, 0.1 mmol), DABCO (56.1
mg, 0.5 mmol), and toluene (0.5 mL). The resulting mixture was
stirred at 100 °C for 15 h. After this time the pressure tube was cooled
to room temperature, and the suspension was filtered. After the
1
volatiles were evaporated under reduced pressure, H, 11B{1H}, 19F,
and 13C{1H} NMR spectra showed the quantitative decyanative
borylation of 4-(trifluoromethyl)benzonitrile to give 4-
(trifluoromethyl)phenylboronic acid pinacol ester. 1H NMR (400
MHz, CDCl3, 298 K): δ 7.90 (d, JH−H = 7.7, 2H, CH), 7.58 (d, JH−H
=
7.7, 2H, CH), 1.33 (s, 12H, CH3). 13C{1H} NMR (100.5 MHz,
CDCl3, 298 K): δ 134.9 (s, CH), 132.5 (q, JC−F = 32.1, C-CF3), 125.3
(q, JC−F = 271.2, CF3), 124.0 (q, JC−F = 3.8, CH), 84.1 (s, C), 24.8 (s,
CH3). 11B{1H} NMR (128.30 MHz, CDCl3, 298 K): δ 30.7 (s). 19F
NMR (376.49 MHz, CDCl3, 298 K): δ −63.5. These data agree with
previously reported data.23
Reaction of RhCl{xant(PiPr2)2} (1) with Pinacolborane:
Preparation of RhHCl(Bpin){xant(PiPr2)2} (4). Pinacolborane (47
μL, 0.31 mmol) was added to a solution of 1 (121.4 mg, 0.21 mmol)
in toluene (0.5 mL). After the resulting solution was stirred for 10 min,
it was concentrated to dryness to afford a white residue. Addition of
pentane afforded a white solid that was washed with pentane (2 × 1
mL) and dried in vacuo. Yield: 116 mg (78%). 1H, 31P{1H}, and
11B{1H} NMR spectra are identical to those obtained for this
1
IR (cm−1): ν(CN) 1606 (w), ν(C−O−C) 1061 (m). H NMR
(300.13 MHz, C6D6, 298 K): δ 8.86 (d, JH−H = 7.9, 2H, CH, p-CF3-
C6H4), 7.59 (d, JH−H = 7.9, 2H, p-CF3-C6H4), 7.23 (m, 2H, CH-arom
POP), 7.04 (d, JH−H = 7.7, 2H, CH-arom POP), 6.84 (t, JH−H = 7.6,
2H, CH-arom POP), 2.50 (m, 4H, PCH(CH3)2), 1.36 (s, 12H, CH3),
1.23 (m, 18H, PCH(CH3)2 + C(CH3)2), 1.11 (dvt, JH−H = 6.8, N =
14.7, 12H, PCH(CH3)2). 13C{1H} NMR (125.78 MHz, C6D6, 298 K):
δ 209.3 (dt, JC−Rh = 43.6, JC−P = 9.9, Rh−C), 155.2 (vt, N = 16.0,
Carom POP), 151.1 (m, Cipso Ph), 139.7 (t, JC−F = 2.5, CH, Ph), 131.2
(s, CH-arom POP), 130.9 (s, CH-arom POP), 130.7 (s, Carom POP),
129.9 (q, JC−F = 31.4, C-CF3), 127.9 (q, JC−F = 271.9, −CF3), 124.8
(vt, N = 16.8, Carom POP), 124.1 (s, CH-arom POP), 123.8 (q, JC−F
= 3.7, CH, Ph), 81.1 (s, C Bpin), 34.2 (s, C(CH3)2), 32.8 (s,
C(CH3)2), 25.6 (s, CH3 Bpin), 25.2 (vt, N = 18.4, PCH(CH3)2), 19.3
(vt, N = 8.2, PCH(CH3)2), 18.6 (s, PCH(CH3)2). 31P{1H} NMR
(121.49 MHz, C6D6, 298 K): δ 37.0 (d, JP−Rh = 187.6). 19F{1H} NMR
(282.33 MHz, C6D6, 298 K): δ −61.4 (s). 11B NMR (92.29 MHz,
C6D6, 298 K): δ 26.5 (br).
compound starting from RhCl{xant(PiPr2)2} and B2pin2.
Reaction of RhCl{xant(PiPr2)2} (1) with Catecholborane:
Preparation of RhHCl(Bcat){xant(PiPr2)2} (7). Catecholborane
(109.4 μL, 1.02 mmol) was added to a solution of 1 (146.0 mg,
0.25 mmol) in toluene (0.5 mL). After the resulting solution was
stirred for 10 min, the solvent was removed in vacuo to afford a white
residue, which was washed with pentane (2 × 1 mL) and dried in
vacuo. Yield: 150 mg (78%). Anal. Calcd for C33H45BClRhO3P2: C,
56.56; H, 6.47. Found: C, 56.80; H, 6.52. HRMS (electrospray, m/z):
calcd for C33H45BO3P2Rh [M − Cl]+: 665.1992; found: 665.2122. IR
(cm−1): ν(Rh−H) 2108 (w); ν(C−O−C) 1092 (m). 1H NMR
(300.13 MHz, C6D6, 298 K): δ 7.23 (m, 2H, Bcat), 7.10 (dd, JH−H
=
7.5, JH−H = 1.4, 2H, CH-arom POP), 6.91 (dd, JH−H = 7.5, JH−H = 7.5,
2H, CH-arom POP), 6.89 (m, 2H, Bcat), 6.74 (m, 2H, CH-arom
POP), 2.72 (m, 2H, PCH(CH3)2), 2.18 (m, 2H, PCH(CH3)2), 1.61
(dvt, JH−H = 7.4, N = 15.9, 6H, PCH(CH3)2), 1.47 (dvt, JH−H = 7.4, N
= 16.4, 6H, PCH(CH3)2), 1.34 (s, 3H, CH3), 1.16 (s, 3H, CH3), 1.14
(dvt, JH−H = 7.3, N = 16.1, 6H, PCH(CH3)2), 0.93 (dvt, JH−H = 7.1, N
= 15.5, 6H, PCH(CH3)2), −14.96 (dt, JH−Rh = 26.5, JH−P = 14.4, 1H,
Rh−H). 13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 154.5 (vt, N =
12.7, Carom POP), 150.5 (s, C−O Bcat), 132.2 (vt, N = 5.4, Carom
POP), 130.9 (s, CH-arom POP), 128.1 (s, CH-arom POP), 125.0 (vt,
N = 25.8, Carom POP), 124.6 (vt, N = 5.3, CH-arom POP), 121.4 (s,
CH Bcat), 111.3 (s, CH Bcat), 34.8 (s, C(CH3)2), 34.4 (s, C(CH3)2),
Preparation of RhPh{xant(PiPr2)2} (3). Rh{C(Ph)NBpin}-
{xant(PiPr2)2} (8) (200 mg, 0.26 mmol) was dissolved in toluene (3
mL), and the resulting solution was heated at 50 °C for 72 h (the
progress of the reaction was periodically checked by 31P{1H} NMR
spectroscopy). During this time a yellow solid precipitated from the
red solution. The mixture was cooled to room temperature, and it was
filtered, obtaining a red solution that was evaporated to dryness.
Addition of pentane (5 mL) afforded a red solid, which was further
washed with pentane (6 × 1 mL) and was dried in vacuo. Yield: 53.0
mg (33%). 31P{1H} NMR spectroscopy shows that the reaction is
quantitative, but the isolated yield is low due to the high solubility of
the complex in pentane. 1H and 31P{1H} NMR spectra agree well with
those previously reported for this compound.11
22.8 (vt, N = 23.7, PCH(CH3)2), 28.1 (s, C(CH3)2), 27.9 (dvt, JC−Rh
=
2.3, N = 28.5, PCH(CH3)2), 21.7 (s, PCH(CH3)2), 19.8 (vt, N = 5.2,
PCH(CH3)2), 19.6 (s, PCH(CH3)2), 19.1 (vt, N = 6.7, PCH(CH3)2).
31P{1H} NMR (121.49 MHz, C6D6, 298 K): δ 57.7 (d, JP−Rh = 109.9).
11B{1H} NMR (96.29 MHz, C6D6, 298 K): δ 40.9 (br).
Preparation of Rh(p-CF3-C6H4){xant(PiPr2)2} (10). Rh{C(p-
CF3−C6H4)NBpin}{xant(PiPr2)2} (9) (200 mg, 0.24 mmol) was
dissolved in toluene (3 mL), and the resulting solution was heated at
50 °C for 72 h (the progress of the reaction was periodically checked
by 31P{1H} NMR spectroscopy). During this time a yellow solid
precipitated from the red solution. The mixture was cooled to room
temperature, and it was filtered, obtaining a red solution that was
evaporated to dryness. Addition of pentane (2 mL) afforded a red solid
that was washed with pentane (2 × 1 mL) and dried in vacuo. Yield:
68 mg (72%). Anal. Calcd for C34H44F3OP2Rh: C, 59.14; H, 6.42.
Found: C, 59.43; H, 6.18. HRMS (electrospray, m/z): calcd for
C34H44F3OP2Rh [M + H]+ 691.1962; found 691.1947. IR (cm−1):
ν(C−O−C) 1093 (m). 1H NMR (300.13 MHz, C6D6, 298 K): δ 8.07
(d, JH−H = 8.0, 2H, p-CF3-Ph), 7.39 (d, JH−H = 8.0, 2H, p-CF3-Ph),
Reaction of Rh(Bpin){xant(PiPr2)2} (2) with Benzonitrile:
Preparation of Rh{C(Ph)NBpin}{xant(PiPr2)2} (8). Benzonitrile
(46 μL, 0.45 mmol) was added to a solution of 2 (300 mg, 0.45 mmol)
in toluene (2 mL). The resulting mixture was stirred for 5 min at room
temperature. After this time, it was concentrated to dryness to afford
an orange residue. Addition of pentane (1 mL) afforded an orange
solid that was washed with pentane (1 × 0.5 mL) and dried in vacuo.
Yield: 315 mg (91%). Anal. Calcd for C40H57BNO3P2Rh: C, 61.95; H,
7.41; N, 1.81. Found: C, 61.63; H, 7.73; N, 1.98. HRMS (electrospray,
m/z): calcd for C34H45NOP2Rh [M − Bpin]+: 648.2026. Found:
1
648.1990. IR (cm−1): ν(CN) 1649 (m). H NMR (300.13 MHz,
C6D6, 298 K): δ 8.90 (d, JH−H = 7.8, 2H, CH Ph), 7.31 (t, JH−H = 7.8,
G
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7.22 (m, 2H, CH-arom POP), 7.03 (dd, JH−H = 7.6, JH−H = 1.6, 2H,
CH-arom POP), 6.84 (t, JH−H = 7.6, 2H, CH-arom POP), 2.28 (m,
4H, PCH(CH3)2), 1.21 (s, 6H, CH3), 1.11 (m, 24H, PCH(CH3)2).
13C{1H} NMR (75.47 MHz, C6D6, 298 K): δ 173.8 (dt, JC−Rh = 40.9,
15.142(3) Å, b = 7.5379(15) Å, c = 14.685(3) Å, V = 1676.2(6) Å3, Z
= 2, Dcalc = 1.368 g cm−3, F(000) = 716, T = 100(2) K, μ 0.646 mm−1.
There were 16 918 measured reflections (2θ: 4−60°, ω scans 0.3°),
3935 unique (Rint = 0.0418); minimum/maximum transmission factors
0.764/0.921. Flack parameter = 0.05(4). Final agreement factors were
R1 = 0.0412 (3436 observed reflections, I > 2σ(I)) and wR2 = 0.1047;
data/restraints/parameters 3935/1/208; GOF = 1.008. Largest peak
and hole 1.000 and −0.555 e/Å3.
J
C−P = 11.8, Rh−C), 155.7 (vt, N = 16.0, C-arom POP), 139.4 (t, JC−P
= 2.8, o-CH p-CF3-Ph), 131.0 (s, C-arom POP), 127.7 (s, CH-arom
POP), 127.5 (q, JC−F = 270.0, CF3), 124.9 (vt, N = 15.5, Carom),
123.9 (vt, N = 3.7, CH-arom POP), 120.6 (m, m-CH, p-CF3−Ph),
119.9 (q, JC−F = 31.1, C-CF3), 33.7 (s, C(CH3)2), 32.7 (s, C(CH3)2),
25.0 (dvt, JC−Rh = 2.9, N = 18.2, PCH(CH3)2), 19.0 (vt, N = 8.3,
PCH(CH3)2), 18.2 (s, PCH(CH3)2). 31P{1H} NMR (121.49 MHz,
C6D6, 298 K): δ 37.5 (d, JP−Rh = 173.5). 19F NMR (282.33 MHz,
C6D6, 298 K): δ −60.2 (t, JF−H = 2.0).
ASSOCIATED CONTENT
* Supporting Information
■
S
CIF files giving positional and displacement parameters,
crystallographic data, and bond lengths and angles of
compounds 4, 8, 9, and 10. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/jacs.5b07357.
Structural Analysis of Complexes 4, 8, 9, and 10. Crystals
suitable for the X-ray diffraction were obtained by slow evaporation of
pentane (8, 9, 10) or by diffusion of pentane into solutions of 4 in
toluene. X-ray data were collected on a Bruker Smart APEX or APEX
DUO difractometers equipped with a normal focus, 2.4 kW sealed
tube source (Mo radiation, λ = 0.710 73 Å) operating at 50 kV and 40
mA. Data were collected over the complete sphere by a combination of
four sets. Each frame exposure time was 10 s (8, 10), 20 s (4), 30 s (9)
covering 0.3° in ω. Data were corrected for absorption by using a
multiscan method applied with the SADABS program.24 The
structures were solved by the Patterson (Rh atoms) or direct methods
and conventional Fourier techniques and refined by full-matrix least-
squares on F2 with SHELXL97.25 Anisotropic parameters were used in
the last cycles of refinement for all non-hydrogen atoms. The
hydrogen atoms were observed or calculated and refined freely or
using a restricted riding model. The hydride ligand of complex 4 was
observed in the difference Fourier maps but refined too close to the
metal, so a restrained refinement fixing the RhH bond length to 1.59 Å
(CCDC) was used. The fluorine atoms of the CF3 group of complex 9
were observed disordered. This group was defined with three moieties,
complementary occupancy factors, isotropic atoms, and restrained
geometry. Complex 9 crystallizes with a half molecule of pentane that
was refined isotropically with restrained geometry. For all structures
the highest electronic residuals were observed in the close proximity of
the metal centers and make no chemical sense.
Crystal data for 4. C33H53BClO3P2Rh, MW 708.86, colorless,
irregular prism (0.16 × 0.14 × 0.06 mm), monoclinic, space group
P21/c, a = 19.0180(17) Å, b = 9.5701(9) Å, c = 19.2725(18) Å, β =
92.9000(10)°, V = 3503.2(6) Å3, Z = 4, Dcalc = 1.344 g cm−3, F(000) =
1488, T = 100(2) K, μ = 0.686 mm−1. There were 31 398 measured
reflections (2θ: 2−57°, ω scans 0.3°), 8330 unique (Rint = 0.0563);
minimum/maximum transmission factors 0.826/0.939. Final agree-
ment factors were R1 = 0.0403 (5830 observed reflections, I > 2σ(I))
and wR2 = 0.0851; data/restraints/parameters 8330/1/387; GOF =
0.928. Largest peak and hole 1.243 and −1.029 e/Å3.
Positional and displacement parameters, crystallographic
data, and bond lengths and angles for 10 (CIF)
Positional and displacement parameters, crystallographic
data, and bond lengths and angles for 9 (CIF)
Positional and displacement parameters, crystallographic
data, and bond lengths and angles for 8 (CIF)
Positional and displacement parameters, crystallographic
data, and bond lengths and angles for 4 (CIF)
NMR spectra of complexes 4 and 7−10 and IR spectrum
of polymeric boryl isocyanide (PDF)
AUTHOR INFORMATION
Corresponding Author
*maester@unizar.es
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support form the MINECO of Spain (Projects
CTQ2014-52799-P and CTQ2014-51912-REDC), the Diputa-
■
cion
́ ́
General de Aragon (E-35), FEDER, and the European
Social Fund is acknowledged. We are very grateful to Dr.
Enrique Onate for collecting the X-ray diffraction data.
̃
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