Bismuth Amides Mediate Facile and Highly Selective Pn–Pn Radical-Coupling Reactions (Pn=N, P, As)

Article abstract of DOI:10.1002/anie.202015514

The controlled release of well-defined radical species under mild conditions for subsequent use in selective reactions is an important and challenging task in synthetic chemistry. We show here that simple bismuth amide species [Bi(NAr₂)₃] readily release aminyl radicals [NAr₂]^. at ambient temperature in solution. These reactions yield the corresponding hydrazines, Ar₂N?NAr₂, as a result of highly selective N?N coupling. The exploitation of facile homolytic Bi?Pn bond cleavage for Pn?Pn bond formation was extended to higher homologues of the pnictogens (Pn=N–As): homoleptic bismuth amides mediate the highly selective dehydrocoupling of HPnR₂ to give R₂Pn?PnR₂. Analyses by NMR and EPR spectroscopy, single-crystal X-ray diffraction, and DFT calculations reveal low Bi?N homolytic bond-dissociation energies, suggest radical coupling in the coordination sphere of bismuth, and reveal electronic and steric parameters as effective tools to control these reactions.

Full text of DOI:10.1002/anie.202015514

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Bismuth Amides Mediate Facile and Highly Selective Pn–Pn
Radical Coupling Reactions (Pn = N, P, As)
Authors: Kai Oberdorf, Anna Hanft, Jacqueline Ramler, Ivo
Krummenacher, Matthias Bickelhaupt, Jordi Poater, and
Crispin Lichtenberg
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202015514
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10.1002/anie.202015514
Angewandte Chemie International Edition
COMMUNICATION
Bismuth Amides Mediate Facile and Highly Selective Pn–
Pn Radical Coupling Reactions (Pn = N, P, As)
Kai Oberdorf,[a] Anna Hanft,[a] Jacqueline Ramler,[a] Ivo Krummenacher,[a] F. Matthias
Bickelhaupt,[b, c] Jordi Poater,*[d] and Crispin Lichtenberg*[a]
[a]
M.Sc. K. Oberdorf, M.Sc. J. Ramler, M.Sc. A. Hanft, Dr. I. Krummenacher, Priv.-Doz. Dr. C. Lichtenberg
Department of Inorganic Chemistry
Julius-Maximilians-Universität, Würzburg
Am Hubland, 97074 Würzburg, Germany
E-mail: crispin.lichtenberg@uni-wuerzburg.de
Prof. Dr. F. M. Bickelhaupt
Department of Theoretical Chemistry, ACMM
Vrije Universiteit Amsterdam
Institute for Molecules and Materials
Radboud University
Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
Dr. J. Poater
[b]
[c]
[d]
Departament de Química Inorgànica i Orgànica & IQTCUB
Universitat de Barcelona & ICREA
Pg. Lluís Companys 23, 08010 Barcelona, Spain
E-mail: jordi.poater@ub.edu
Supporting information available
Abstract: The controlled release of well-defined radical species under
mild conditions for subsequent use in selective reactions is an
important and challenging task in synthetic chemistry. We show here
that simple bismuth amide species [Bi(NAr2)3] readily release aminyl
radicals [NAr2]● at ambient temperature in solution. These reactions
yield the corresponding hydrazines, Ar2N–NAr2, as a result of highly
selective N–N coupling. The exploitation of facile homolytic Bi–Pn
bond cleavage for Pn–Pn bond formation was extended to higher
homologs of the pnictogens (Pn = N-As): homoleptic bismuth amides
mediate the highly selective dehydrocoupling of HPnR2 to give R2Pn–
PnR2. Analyses by NMR and EPR spectroscopy, single crystal X-ray
diffraction and DFT calculations reveal low Bi–N homolytic bond
dissociation energies, suggest radical coupling in the coordination
sphere of bismuth, and reveal electronic and steric parameters as
effective tools to control these reactions.
ammoxidation of propene to acrylonitrile at a Bi2O3•MoO3 catalyst
(SOHIO process).[10]
Aminyl radicals, (NR2)●, play an important role in biological
processes,[11] and their emerging applications in synthetic
chemical protocols have attracted increasing interest.[12] However,
their use is still limited due to their high reactivity and the lack of
methods for their selective generation under mild conditions.[12a]
Thus, E–N bond homolysis would offer valuable prospects for the
generation of aminyl radical species. Indeed, E–N homolysis has
been suspected in applications such as the synthesis of highly
monodisperse nanoparticles from Bi(N(SiMe3)2)3 and CH
activation / C–C coupling sequences with BiCl2(N(SiMe3)(Mes*))
(Mes*
=
2,4,6-tBu3C6H2).[13] Pyrazolyl radicals have been
generated by oxidation of the pyrazolide anion with BiCl3.[14]
However, the liberation of aminyl radicals from well-defined
precursors through experimentally verified E–N homolysis under
mild reaction conditions and their subsequent utilization in
selective bond forming events has not been reported to date.
Here we show that bismuth amides [Bi(NAr2)3] readily release
aminyl radicals, allowing for highly selective N–N coupling under
mild conditions. The strategy of facile Bi–Pn homolysis could be
extended to higher homologs with Pn = N, P, As.
The controlled release of radical species and their subsequent
exploitation in selective bond forming events remains an
outstanding challenge in synthetic chemistry.[1] Recent progress
in the chemistry of heavy p-block elements has allowed a glimpse
of their potential in the development of controlled radical
reactions.[2],[3] Examples include CH activation reactions by tin
radicals,[4] the activation of P4 and S8 by bismuth radicals,[5]
bismuth-catalyzed radical dehydrocoupling reactions,[6] the
radical cyclo-isomerization of iodoolefins,[7] and controlled radical
olefin polymerization.[8] While recent developments in heavy p-
block chemistry exploit isolable radical species[9] and the
homolytic cleavage of E–E bonds,[3a, 3b] the homolysis of E–X
bonds as a source of radicals X● without the need for a radical
initiator is only little explored (E = heavy p-block element; X =
C,N,O). For instance, the homolytic cleavage of E–C bonds has
been exploited in the controlled radical polymerization of olefins,
which includes the addition of a carbon-centered radical to the
olefin monomer as the initiating step.[8] Bi–O homolysis has been
discussed as the rate-determining step in the industrially relevant
A limited number of homoleptic bismuth amides bearing
alkyl[15] or silyl[15b, 16] substituents is known. In addition, a few
bismuth compounds with amide ligands that contain one aryl
group, (NArR)–, have been reported and show unusual reactivity
patterns that have not been rationalized in many cases (R = H,
Si(alkyl)3; Supp. Inf.). In contrast, only a single homoleptic
bismuth amide of type [Bi(NAr2)3] has been described, which also
sticks out due to its simple substituents (Ar = Ph, 1-H).[17],[18]
We targeted the synthesis of a series of homoleptic bismuth
diaryl amides, [BiN(Ar2)3] (1-X), with simple aryl substituents that
do not impose extreme steric bulk on the overall complexes
(Scheme 1a; X = H, Me, OMe, Br, Ph, CF3). Different salt
elimination protocols (Routes A, B) and transamination
1
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10.1002/anie.202015514
Angewandte Chemie International Edition
COMMUNICATION
and Supp. Inf.). In agreement with the literature, isolated 3-Me
was found to be EPR-silent under identical conditions.[20b] This
indicates homolytic Bi–N bond cleavage as the source of the
aminyl radical and is, to the best of our knowledge, the first direct
experimental evidence of facile Bi–N homolysis under mild
conditions in the condensed phase. With different substituents X
in the para-position of the aryl backbone of compounds 1-X, the
electronic influence on the properties of this class of compounds
was examined. The half-life of each compound was determined
by 1H NMR spectroscopy and showed rapid reactions of electron-
rich 1-Me and 1-OMe in solution (t1/2 = 1.2-1.4 h). In contrast, 1-
H, 1-Br, and 1-Ph with less electron density in the phenylene
moieties reacted significantly slower (t1/2 = 63-84 h). Confirming
this trend, the electron-poor compound 1-CF3 is stable in solution
at 23 °C for days. In an intermolecular reaction, addition of 3
equiv. HN(4-Me-C6H4)2 to Bi(NMe2)3 (4) in benzene gave HNMe2
and 3-Me as the main benzene-soluble products (Supp. Inf.).
These findings on diaryl amides, [BiN(Ar2)3] (1-X), provide the
first thorough understanding of the properties and spectroscopic
data of this class of compounds. Closer investigation of the
previously pioneered 1-H showed exclusively the four expected
Scheme 1. a) Syntheses of aryl-substituted, homoleptic bismuth amides: Route
A: 1-H, 1-Me, 1-OMe; Route B: 1-OMe, 1-Br, 1-Ph; Route C: 1-CF3. b)
Molecular structure of 1-Me in the solid state. Displacement ellipsoids are shown
at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected
bond lengths [Å] and angles [°]: Bi1–N1, 2.1598(19); Bi1–N2, 2.158(2); Bi1–N3,
2.185(2); N1–Bi1–N2, 100.68(8); N1–Bi1–N3, 97.92(8); N2–Bi1–N3, 95.03(8).
signals in the 13C NMR spectrum in C6D6. Previously reported 13
C
NMR data in CD2Cl2 list seven signals,[17] which are in fact due to
a mixture of 1-H (4 signals) and the N–N coupling product 3-H (4
signals), with two resonances overlapping. Thus, a modified
synthetic protocol allowed for the multi-gram-scale isolation of
analytically pure material in 88% yield (improved from previously
reported 30%).
Mechanistic aspects of the reaction of 1-Me to give 3-Me were
investigated by DFT calculations (in this discussion, R = 4-Me-
C6H4). Homolytic Bi–N bond cleavage in 1-Me is clearly favored
over heterolytic Bi–N bond cleavage by ΔΔG = 36.8 kcal∙mol–1,
while the opposite is true for the protonated cationic derivative
[Bi(NR2)2(HNR2)]+ (ΔΔG = 24.1 kcal∙mol–1; for details see Supp.
approaches (Route C) were developed to account for differences
in solubility, reactivity and stability of starting materials, products
and by-products (Supp. Inf.). Compounds 1-X were isolated as
orange to violet solids in moderate (51%, X = OMe) to excellent
(93%, X = Ph) yields. The molecular structures of 1-H,[17] 1-Me, 1-
Br, and 1-Ph in the solid state were determined by single-crystal
X-ray diffraction, revealing isostructural relationships (triclinic
̅
space group 푃1 in all cases). The compounds form typical
molecular structures with trigonal pyramidal coordination
geometries around the central Bi atoms (N–Bi–N, 94.2°-107.7°;
see Scheme 1b for 1-Me and Supp. Inf. for details). 1-Me shows
intermolecular Bi∙∙∙Bi distances of 3.79 Å, suggesting weak Bi∙∙∙Bi
interactions in the solid state (Supp. Inf.).[19] The Bi–N bond
lengths in 1-H, 1-Me, and 1-Br are virtually independent of the
nature of the substituent X in para-position of the aryl groups
(2.15-2.19 Å). Slightly elongated Bi–N bonds in 1-Ph (2.16-2.23)
were ascribed to intramolecular π-stacking between two
[(N(C6H4Ph)2]– ligands. The
N
atoms are only weakly
pyramidalized (ƩC–N–C/Bi, 352°-360°). NMR spectroscopic
analyses of compounds 1-X are in agreement with six
magnetically equivalent aryl groups in each complex without
significant rotational barriers at ambient temperature in solution
(for 1H NMR spectrum of 1-Me see top of Scheme 2b). However,
it quickly became evident that compounds 1-X only have a limited
life-time in solution. At 23 °C in benzene, compounds 1-X undergo
a selective transformation with concomitant precipitation of a dark
1
solid (the H NMR spectroscopic monitoring of this process is
shown in Scheme 2b for 1-Me).[20] The products of these reactions
were identified as the hydrazines 3-X, demonstrating that
compounds 1-X are precursors for selective N–N coupling
reactions (Scheme 2a). The high (up to quantitative) yields
indicate that the dark precipitate is mainly or exclusively Bi0 based
on the atom balance of these reactions. Identical results were
obtained when the reaction 1-Me → 3-Me was performed under
exclusion of light, demonstrating that this transformation is
thermally-induced. EPR spectroscopic monitoring of the reaction
in the case of 1-Me revealed the aminyl radical [N(tol)2]● (2-Me)
as an intermediate of this reaction (tol = 4-Me-C6H4; Scheme 2c
Scheme 2. a) Selective formation of hydrazines 3-X from bismuth amides 1-X
via N–N radical coupling (yields determined by 1H NMR spectroscopy). b) NMR
spectroscopic monitoring of the reaction 1-Me → 3-Me (asterisk denotes
resonance of C6D6). c) EPR spectroscopic monitoring of the reaction 1-Me → 3-
Me with detection of [N(tol)2]● (2-Me) (for details see Supp. Inf.).
2
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10.1002/anie.202015514
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COMMUNICATION
Table 1. Reactions of bismuth amides with secondary phosphanes and arsane
proceed with highly selective P–P and As–As bond formation (R’: see table).
Inf.). An energy decomposition analysis of the Bi–N bond in 1-Me
(homolytic fragmentation) revealed similarly stabilizing
contributions by orbital interactions (45%) and electrostatic
interactions (46%) to the Bi–N bond, while dispersion interactions
contribute significantly (9%), but not decisively to the stability of
the Bi–N bond. As an initiating step of the reaction 2 1-Me → 2 Bi0
+ 3 3-Me, the elimination of 3-Me from 1-Me with formation of the
triplet-bismuthinidene 3[Bi(NR2)] is highly unlikely (ΔG = 46.3
kcal∙mol–1), with the corresponding singlet bismuthinidene and its
solvent adducts beingevenhigher in energy (Scheme 3 and Supp.
Inf.).[3c] In contrast, the formation of a dibismuthane according to
2 1-Me → [Bi2(NR2)4] + 3-Me is thermodynamically more
reasonable (ΔG = 12.7 kcal∙mol–1).[21] From [Bi2(NR2)4], the two-
step elimination of two equivalents of 3-Me to give dibismuthene
[Bi2(NR2)2] (ΔG = 36.4 kcal∙mol–1) and finally Bi2 (ΔG = 21.4
kcal∙mol–1) is not energetically viable. Instead, the ratio of Bi:NR2
is most likely increased by formation of bismacyclic compounds
according to reactions such as 2 [Bi2(NR2)4] → [Bi4(NR2)4] + 2 3-
Me (ΔG = 18.0 kcal∙mol–1), which may further proceed through
bismuth clusters[22] to finally give Bi0. Indeed, our calculations
suggest that the formation of bismuth metal is an important driving
force in these reactions (e.g.: [Bi4(NR2)4] → 4 Bi0(bulk) + 2 3-Me
(ΔG = –62.1 kcal∙mol–1)). The net reaction 2 1-Me → 2 Bi0(bulk) +
3 3-Me is exergonic by ΔG = –9.4 kcal∙mol–1. We suggest that the
single steps outlined above may well proceed through catalysis
by radicals such as (NR2)● (Supp. Inf.). Homolytic Bi–N bond
dissociation of 1-Me (ΔG = 25.4 kcal∙mol–1) or [Bi2(NR2)4] (ΔG =
28.9 kcal∙mol–1) would readily provide sufficient concentrations of
such radicals, which were detected by EPR spectroscopy (vide
supra). Investigations into the kinetics of the formation of 3-Me
from 1-Me confirmed the complexity and suggest a concentration-
dependency of the mechanism of this radical reaction (Supp. Inf.).
To expand the utilization of well-defined bismuth precursors
for selective homo-coupling reactions of lighter group 15 species,
we turned our attention to secondary phosphanes as substrates.
Dehydrocoupling of secondary phosphanes to give diphosphanes,
R2P–PR2 (important building blocks for the synthesis of bidentate
phosphane ligands)[23] have so far mostly relied on the use of
transition metal complexes.[24] Only recently, the first reports on
#
Reagent
1-Me
1-Me
4
Substrate
HPPh2
n
Conditions
RT, <7 min
RT, 3 h
Yield [%]
>99
90
1
1
[a]
2
HP(Xyl)2
2
3
HPMes2
2
RT, 16 h
>99
92
4
1-Me
1-Me
1-Me
1-Me
1-Me
4
HP(4-OMe-C6H4)2
HP(4-Cl-C6H4)2
HP(4-CF3-C6H4)2
HPCy2
1.5
1.2
2
RT, <2 h
RT, <2 h
RT, <3 h
RT, 1.5 h
RT, <6 h
RT, <3 h
60 °C, 1 d
60 °C, 2 d
–78 °C, 0.5 h
5
97
6
>99
>99
99
7
1
8
HPiPr2
1.2
2
9
HP(cyclo-pentyl)2
HPtBu2
94
10
11
12
4
2
>99
>99
80
[b]
4
HPAd2
6
1-Me
HAsPh2
1
Yields were determined by 1H and/or 31P NMR spectroscopy. [a] Xyl = 3,5-Me2-
C6H3. [b] Ad = adamantyl.
main group species in the context of this reaction have been
published. Carbenoids, NHCs, tri(aryl)boranes, the radical starter
“Vazo 88”, and KOtBu in the presence of different hydrogen
acceptors generate diphosphanes from secondary phosphanes,
but are all limited to substrates bearing at least one aryl
group.[25],[26],[27],[28] Only the organotin species Sn(C5Me5)2Cl2 has
been reported to also dehydrocouple HPCy2 as a single example
of a dialkylphosphane (Cy = cyclohexyl).[29] These reactions
proceed even in a catalytic manner (10 mol% Sn(C5Me5)2Cl2), but
require long reaction times of 3 d, elevated temperatures of 60 °C,
tedious protocols (periodical removal of H2), and result in low
yields of 40% Cy2P–PCy2. Reports on heavy main group element
compounds L1Bi–PPh2 (suggested) and L2Pb–PPh2 (isolated)
leading to the formation of the coupling product Ph2P–PPh2 (5-
Ph) spurred us to test the potential of bismuth amides as reagents
for dehydrocoupling reactions of secondary phosphanes (L1/L2 =
bulky mono-/dianionic ligand).[5a, 30] Indeed, reaction of 1-Me with
three equiv. HPPh2 at 23 °C resulted in the instant and
quantitative formation of the coupling product 5-Ph (Table 1, entry
1). A dark solid (presumably Bi0) precipitated, and HN(4-Me-
C6H4)2 was observed as the only benzene-soluble by-product.
Sterically more demanding phosphanes HP(Xyl)2 and HPMes2
also underwent coupling in high to quantitative yields at 23 °C,
albeit 2 equiv. Bi(NMe2)3 (4) as the bismuth amide were needed
and the latter case required extended reaction times (entries 2,3).
Diarylphosphanes with e–-donating or -withdrawing substituents
(OMe, Cl, CF3) in the para-position of their phenyl substituents
were well tolerated (yields: 92-99%, entries 4-6). The synthetically
challenging coupling of dialkyl phosphanes was also successful:
reaction of 1-Me with three equivalents of HPCy2 at 23 °C resulted
in quantitative formation of 5-Cy in 1.5 h (entry 7). Similarly,
Scheme 3. Potential steps in the formation of 3-Me from 1-Me (unfavourable
[more favourable] steps shown in blue [black]; energy levels scaled to ΔG);
Values for ΔH (regular font) and ΔG (italic) are given in kcal∙mol–1
.
3
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10.1002/anie.202015514
Angewandte Chemie International Edition
COMMUNICATION
2014, 53, 11172-11176; c) D. P. Mukhopadhyay, D.
Schleier, S. Wirsing, J. Ramler, D. Kaiser, E. Reusch, P.
Hemberger, T. Preitschopf, I. Krummenacher, B. Engels, I.
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Schwamm, M. Lein, M. P. Coles, C. M. Fitchett, J. Am.
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HPiPr2 and HP(cyclo-pentyl)2 underwent dehydrocoupling in
excellent yields (entries 8,9). Not only electron-donating
substituents at P (alkyl vs aryl), but also increased steric bulk
around the phosphorus atom tend to complicate P–P coupling
reactions.[25b, 25d, 28-29] Thus, it is remarkable that not only the aryl
species HPMes2 (entry 3), but even HPtBu2 and HPAd2 with their
extremely bulky alkyl groups could quantitatively be transformed
into the coupling products 5-tBu and 5-Ad with modified
experimental conditions (entries 10,11). Reactions of the
secondary arsane HAsPh2 with 1-Me (or 4) were performed as a
proof of principle and resulted in the selective formation of the
dehydrocoupling product 6-Ph in 80% yield (entry 12 and Supp.
Inf.). In the formation of Ph2P–PPh2, literature reports suggest
[4]
[5]
[6]
homolytic splitting of L1Bi–PPh2 and L2Pb–PPh2 bonds (vide
30]
supra).[5a,
Indeed, we detected a weak EPR spectroscopic
[7]
[8]
J. Ramler, I. Krummenacher, C. Lichtenberg, Angew.
Chem. Int. Ed. 2019, 58, 12924-12929.
signal in reactions of 4 with HPMes2, which was tentatively
assigned to a P-centered radical (giso = 2.007, a(31P) = 270 MHz
(96 G))[31] and thus suggests a radical nature of this reaction step
(for details and full discussion see Supp. Inf.). In contrast, polar
mechanisms have been discussed for most main group
compounds in phosphane dehydrocoupling.[25a-c, 26b, 28-29] Notably,
the only other main group species that has been reported for the
challenging dehydropouling of dialkylphosphanes has also been
suggested to operate via polar reaction pathways.[29] Transition-
metal-based catalytic protocols for phosphane dehydrocoupling
require elevated temperatures, longer reaction times, and/or
show limitations in their substrate scope (see Supp. Inf.).[24]
In summary, we have re-investigated the fundamental
bismuth diarylamide [Bi(NPh2)3] and provided data for the
thorough understanding of this class of compounds. As a striking
feature, they can easily release aminyl radicals under mild
reaction conditions and mediate highly selective N–N bond
formation. The first experimental proof of facile Bi–N bond
homolysis has been delivered, electronic parameters controlling
aminyl radical formation have been revealed, and fundamental
mechanistic aspects have been uncovered. Simple homoleptic
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[9]
[10]
[11]
[12]
[13]
bismuth
amides
efficiently
mediate
highly
selective
dehydrocoupling of HPnR2 to give R2Pn–PnR2 (Pn = N-As). These
findings reveal the potential of well-defined bismuth compounds
to be exploited for the controlled generation and synthetic
utilization of radicals such as [PnR2]●. The ability of bismuth to
readily release radical species and to easily accommodate very
bulky ligands due to its large atomic radius sets bismuth-based
methodologies apart from previously reported reagents in the field.
[14]
[15]
[16]
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Keywords: bismuth amide • aminyl radical • radical coupling •
heavier pnictogens • diphosphanes
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Entry for the Table of Contents
Bismuth radical chemistry: the first series of bismuth amides of type [Bi(NAr2)3] has been synthesized and characterized. Remarkably,
they readily release aminyl radicals (NAr2)● and facilitate highly selective radical coupling to give (NAr2)2. Bismuth amides mediate the
selective dehydrocoupling of HPnR2 (Pn = N, P, As), including very challenging substrates. Conditions, mechanisms, and substrate scopes
are complementary or superior to those of more established procedures.
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