Phosphine and carbene azido-cations: [(L)N3]+ and [(L)2N3]+

Article abstract of DOI:10.1039/c5sc02336j

The cationic N₃-species [(p-HC₆F₄)₃PN₃]⁺ (1) featuring a perfluoro-arene phosphonium group serves as a N₃⁺-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation of [(L)N₃]⁺ and [(L)₂N₃]⁺ cations (L = phosphine, carbene) with liberation of (p-HC₆F₄)₃P. X-Ray diffraction analysis and computational studies provide insight into the bonding in these remarkably stable azido-cations.

Full text of DOI:10.1039/c5sc02336j

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Phosphine and carbene azido-cations: [(L)N3]+ and
[(L)2N3]+†
Cite this: Chem. Sci., 2015, 6, 6367
Daniel Winkelhaus,‡a Michael H. Holthausen,‡a Roman Dobrovetskyb
a
*
and Douglas W. Stephan
The cationic N3-species [(p-HC6F4)3PN3]+ (1) featuring a perfluoro-arene phosphonium group serves as a
N3+-source in stoichiometric reactions with several Lewis bases (L) allowing for the stepwise formation
of [(L)N3]+ and [(L)2N3]+ cations (L ¼ phosphine, carbene) with liberation of (p-HC6F4)3P. X-Ray diffraction
analysis and computational studies provide insight into the bonding in these remarkably stable azido-
cations.
Received 28th June 2015
Accepted 20th July 2015
DOI: 10.1039/c5sc02336j
www.rsc.org/chemicalscience
azides. Herein we describe the synthesis of [(p-HC6F4)3PN3]+ (1)
and its use as a synthon to species of the form [(L)N3]+ and
[(L)2N3]+ (L ¼ phosphine, carbene).
Introduction
A major component of the recent renaissance in p-block
chemistry1 has been the use of neutral, two electron donors like
carbenes or phosphines for the stabilization of homoatomic,
low valent main group element fragments.2 Most recently
Cummins used an anthracene stabilized P2 for the synthesis of
an aromatic [P2N3].3 Species of the form (L)2E2 (E ¼ B,4 C,5 Si,6
Ge,7 Sn,8 N,9 P,10 As,11 Sb;12 L ¼ carbene, phosphine) have been
prepared affording ligand stabilization of unique diatomic
main group fragments. In addition, the groups of Robinson,
Bertrand and Roesky have exploited these donors for isolation
of dications and radical cations of type [(carbene)2E2]n+ (E ¼ C,5
P,13 As;14 n ¼ 1, 2) while Burford, Weigand, Jones and
Results and discussion
Compound [(p-HC6F4)3PN3][B(C6F5)4] (1) was readily prepared
in quantitative yield by the reaction of [(p-HC6F4)3PF][B(C6F5)4]
with Me3SiN3 in CH2Cl2 (Scheme 1).§ The 31P{1H} NMR spec-
trum of 1 in CD2Cl2 gives rise to a broad singlet resonance
(d(31P) ¼ 17.5 ppm, Dn1/2 ¼ 30 Hz).
The presence of the para hydrogen substituents in 1 is
crucial as the fully uorinated derivative [(C6F5)3PF]+ is prone to
nucleophilic attack at this position.21 The formulation of
compound 1 was subsequently conrmed by an X-ray crystal-
lographic study (Fig. 1).
+
Grutzmacher prepared [(carbene)2P3]
or bicyclic [(L)2P4]2+
15
¨
species (L ¼ carbene, AsPh3, PPh3).16 The latter compound is of
special interest since L can be exchanged for more basic donors.
A similar donor exchange was reported for [(Ph3P)PPh2]+
affording [(carbene)PPh2]+.17 The nature of these and related
systems, especially the L–E and E–E bonding, has sparked
vigorous debate.18
The molecular structure of 1 shows a distorted tetrahedral
environment about the phosphorus atom with angles at P
ranging from 102.3(1)–112.6(1)ꢀ. The P(1)–N(1) bond length of
˚
1.651(2) A is shorter than those observed in azido-substituted
Our recent studies of highly electrophilic phosphonium
cations (EPCs) have demonstrated that species such as the u-
orophosphonium cation [(C6F5)3PF]+19 exhibit remarkable Lewis
acidity and thus act as effective catalysts in a range of Lewis acid
mediated transformations.20 At the same time, we were moti-
vated to probe the utility of these EPCs as synthons for cationic
22
˚
phosphenium cations (1.67 A) and azido-phosphines (1.73
23
˚
A), consistent with the electron decient nature of the phos-
phonium center and the strongly polarized nature of the P–N
bond. The azido moiety shows an N(1)–N(2)–N(3) angle that
slightly deviates from the ideal 180ꢀ (172.4(2)ꢀ) while the N(1)–
˚
˚
N(2) (1.260(2) A) and N(2)–N(3) bond length (1.114(2) A) are in
the expected range for azide substituents. Compound 1 is a rare
example of a crystallographically characterized salt of a cationic
azido-phosphonium species24 while several neutral, azido-
substituted P(III) and P(V) derivatives have been reported to
date.25 Other examples of monocationic nitrogen species,
aDepartment of Chemistry, University of Toronto, 80 St. George St, Toronto, Ontario
M5S3H6, Canada. E-mail: dstephan@chem.utoronto.ca
bSchool of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv
University, Tel Aviv 69978, Israel
† Electronic supplementary information (ESI) available: Details on VT NMR
experiments, preparation of 5, quantum chemical calculations and X-ray
crystallography. CCDC 1403531–1403535. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5sc02336j
reported in the literature, include diazonium ions,26 the
homoleptic [N5]+ of Christe,27 [N2F] ,28 Hunig's stable tri-
+
¨
azenium ions [R2N3R2]+29 and aminodiazonium ions of the form
‡ These authors contributed equally.
[(R2N)N2]+30 (R ¼ H, silyl, alkyl, aryl).
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singlet resonance at low eld (d(31P) ¼ ꢁ85.9 ppm) which is in
the typical chemical shi range for an N-substituted trialkyl-
phosphonium derivative.31 Collectively, these observations
indicate the formal transfer of the N3+-moiety in 1 to t-Bu3P
yielding [t-Bu3PN3][B(C6F5)4] (2) which was isolated in 98%
yield. The nature of 2 was conrmed by X-ray single crystal-
lography and the metrics were found similar to 1 (see ESI†).
Interestingly, 2 and 1 are very stable and do not show any signs
of degradation on heating in C6D5Br solution to 100 ꢀC for 24 h.
Attempts to independently synthesize 2 from the reaction of
[t-Bu3PF][B(C6F5)4]19 with Me3SiN3 failed even aer prolonged
reaction times at 100 ꢀC in C6D5Br solution. This reects the
increased steric demand of the [t-Bu3PF]+ cation.
The corresponding reaction of 1 with Ph3P in CH2Cl2 was
monitored by 31P{1H} NMR spectroscopy revealing the complete
consumption of Ph3P and the liberation of only 0.5 equivalents
of (p-HC6F4)3P, leaving approximately 50% of 1 unreacted.
Addition of another equivalent of Ph3P resulted in its complete
consumption. Collectively, this indicates the formation of a bis-
adduct [(Ph3P)2(N3)][B(C6F5)4] (3, Scheme 1). While the
proposed intermediate [Ph3PN3]+ was independently synthe-
sized by the reaction of [Ph3PF][B(C6F5)4]20c and Me3SiN3 (see
ESI†), this species was not observed in the reaction of 1 with
Ph3P. This indicates that reaction of [Ph3PN3]+ with a second
equivalent of Ph3P is rapid, in agreement with Wiberg who
described the [(Ph3P)2(N3)]+ cation in 1967.24c Compound 3 was
isolated in high yields (97%). The t-Bu-substituted analog 4 was
obtained from 1 with two equivalents of t-Bu3P or by reaction of
2 with one equivalent of t-Bu3P (Scheme 1). Species 4 was also
obtained by reaction of 3 with two equivalents of t-Bu3P with
concurrent release of Ph3P. All methods furnish 4 in high yields
(76–99%). Compounds 3 and 4 show 31P{1H} NMR resonances
at 30.6 and 56.5 ppm that are between the chemical shi ranges
of phosphinimine and N-substituted phosphonium deriva-
tives.31 X-Ray structure determination conrmed the formula-
tions (Fig. 2). The P2N3-moiety adopts a W-shaped geometry
with all ve atoms almost located within one plane (largest
Scheme 1 Preparation of 1–6.
To gain further insights into the nature of 1, the geometry of
the cation was optimized at the wB97XD/def2-TZV level of theory
(see ESI†). The p-type HOMO of 1 is located at the P-bonded N
atom while the LUMO is part of two sets of degenerated p*-type
orbitals involving the p-HC6F4-groups (LUMO to LUMO+9). The
rst accessible acceptor orbital is a p*-type orbital located at the
terminal N2 moiety (LUMO+10). This stands in contrast to other
EPCs where the accessible acceptor orbital involves the P–F s*
orbital at the P center.19 The corresponding P–N s* orbital in 1 is
much higher in energy (LUMO+11). NBO analysis reveals that the
P–N single bond in 1 is occupied by 1.92 electrons, while the P-
bound N atom features two lone pairs of p and sp0.69 type.
Donation of electron density from these lone pairs into s* orbitals
of the adjacent P–C bonds occurs (LP(p)N1 / s*P–C7 11.2 kcal
˚
˚
deviation: 3: 0.013(2) A for N2, 4: 0.039(1) A for N2). The acute
N–N–N angles (3: 113.2(4)ꢀ, 4: 110.8(1)ꢀ) are in the typical range
for phosphazides.32 The two P–N–N angles in each compound
are 3: 114.5(3)ꢀ/114.7(3)ꢀ; 4: 118.1(1)ꢀ/118.9(1)ꢀ with the larger
angles in 4 reecting the increased steric congestion. Similarly,
˚
˚
˚
the P–N bond lengths of 1.648(3) A and 1.643(3) A and 1.675(2) A
˚
and 1.673(2) A seen in 3 and 4, respectively, range between those
of phosphinimine and N-substituted phosphonium deriva-
tives.31 The N–N bonds in 3 and 4 were found to be 1.298(4) A,
˚
28
˚
˚
˚
1.309(4) A and 1.309(2) A, 1.300(2) A, which are intermediate
between single and double bond distances. A similar confor-
mation is observed for several crystallographically characterized
organo-phosphazides R3PN3R,32 formed in the initial step of a
Staudinger reaction.33
molꢁ1, LP(p)N1 / s*P–C7 7.4 kcal molꢁ1, LP(sp0.69 N1 / sP*–C7 3.7
)
kcal molꢁ1). The secondary interactions of these lone pairs may
account for the observed shortening of the P–N bond in 1 in
comparison to other P-azido species.22,23
It is of interest to note that, similar to 1 and 2, 4 is thermally
stable and can be heated to 120 ꢀC in C6D5Br solution over
several days without decomposition. In contrast, compound 3 is
not temperature stable, decomposing quantitatively with
release of N2 within 3 h at 100 ꢀC.24c The 31P{1H} NMR spectrum
shows only one resonance at d(31P) ¼ 21.1 ppm assignable to
Compound 1 reacts with the Lewis base t-Bu3P in CH2Cl2
solution (Scheme 1).§ The 31P{1H} NMR spectrum of the reac-
tion mixture showed a septet resonance at d(31P) ¼ ꢁ72.3 ppm
(3JPF ¼ 36.4 Hz)19b which corresponds to (p-HC6F4)3P and a new
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Fig. 1 POV-ray depiction of the cation in 1 (top). Hydrogen atoms are
omitted for clarity. Selected bond distances and angles: P(1)–N(1)
1.651(2), P(1)–C(13) 1.783(2), P(1)–C(1) 1.789(2), P(1)–C(7) 1.789(2),
N(1)–N(2) 1.260(2), N(2)–N(3) 1.114(2), N(1)–P(1)–C(13) 111.6(2), N(1)–
P(1)–C(1) 102.3(2), C(13)–P(1)–C(1) 112.6(2), N(1)–P(1)–C(7) 109.8(2),
C(13)–P(1)–C(7) 110.5(2), C(1)–P(1)–C(7) 109.7(2), N(2)–N(1)–P(1)
120.9(2), N(3)–N(2)–N(1) 172.4(2). Graphical presentation of the
HOMO and LUMO+10 of 1 calculated at the wB97XD/def2-TZV level
of theory (surface iso-value ¼ 0.05).
Fig. 3 POV-ray depiction of the cation in 6. Hydrogen atoms are
omitted for clarity. Selected bond distances and angles: C(1)–N(4)
1.332(3), C(1)–N(5) 1.346(2), C(1)–N(1) 1.359(2), C(22)–N(7) 1.335(2),
C(22)–N(6) 1.337(2), C(22)–N(3) 1.369(2), N(1)–N(2) 1.309(2), N(2)–N(3)
1.311(2), N(4)–C(1)–N(5) 111.1(2), N(4)–C(1)–N(1) 119.3(2), N(5)–C(1)–
N(1) 129.4(2), N(7)–C(22)–N(6) 111.7(2), N(7)–C(22)–N(3) 119.9(2),
N(6)–C(22)–N(3) 128.2(2), N(2)–N(1)–C(1) 113.6(2), N(1)–N(2)–N(3)
110.4(2), N(2)–N(3)–C(22) 111.9(2).
Fig. 4 Graphical presentation of the LUMO (top) and HOMO (bottom)
of the cations in 3 and 6-Me calculated at the wB97XD/def2-TZVPP
level of theory (surface iso-value ¼ 0.05).
both isomers in a 4 : 1 ratio. The 31P{1H} NMR spectrum of the
minor isomer gave two resonances at d(31P) ¼ 28.4 and 11.0
ppm (4JPP ¼ 5.5 Hz). At elevated temperatures, rapid conversion
of the cation 3 to [(Ph3P)2N]+ 5 was observed. Nonetheless,
the ratio of the isomers of 3 is unchanged inferring the rate of
N2-loss is comparable to the rate of isomerization of 3.
Computations at the wB97XD/def2-TZV level of theory with
addition of the conductor-like polarizable continuum solvation
model (CPCM)35 showed only a small energy difference between
Fig. 2 POV-ray depiction of the cations in 3 (top) and 4 (bottom).
Hydrogen atoms are omitted for clarity. Selected bond distances and
angles for 3: N(1)–P(1) 1.648(3), N(3)–P(2) 1.643(3), N(1)–N(2) 1.298(4),
N(2)–N(3) 1.309(4), N(2)–N(1)–P(1) 114.5(3), N(1)–N(2)–N(3) 113.2(4),
N(2)–N(3)–P(2) 114.7(3), for 4: P(1)–N(3) 1.675(2), P(2)–N(1) 1.673(2),
N(1)–N(2) 1.309(2), N(3)–N(2) 1.230(2), N(2)–N(1)–P(2) 118.1(2), N(2)–
N(3)–P(1) 118.9(2), N(3)–N(2)–N(1) 110.8(2).
298
the W and U-shaped isomers of 3 (DGR ¼ ꢁ2.6 kcal molꢁ1).
Compound 1 also reacts with two equivalents of 1,3-dimesity-
limidazolidin-2-ylidene (SIMes) in C6D5Br solution with liberation
of (p-HC6F4)3P as evidenced by the 31P{1H} NMR spectrum. The
13C{1H} NMR spectrum shows a new resonance for the C-2 carbon
[(Ph3P)2N][B(C6F5)4] (5) (Scheme 1).34 N2 elimination is thought
to follow isomerization of the W-shaped 3 to a U-shaped isomer.
It is noteworthy that compound 3 was isolated as a mixture of
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atom at d(13C) ¼ 162.2 ppm.36 Collectively, the NMR data indicate
the formation of [(SIMes)2N3][B(C6F5)4] (6) which was isolated in
70% yield. Interestingly, 6 was the only product obtained from the
reaction using a 1 : 1 stoichiometry. This contrasts with the
reactivity observed for t-Bu3P and is likely a result of the low
solubility of 1 in bromobenzene. Interestingly, compound 6 was
also obtained in high yields by reaction of 3 or [Ph3PN3][B(C6F5)4]
with SIMes in 1 : 2 stoichiometry (see ESI†). Compound 6 is
thermally stable even under prolonged heating to 120 ꢀC for
several days in C6D5Br solution. The nature of compound 6 was
further conrmed by single crystal X-ray diffraction (Fig. 3). The
structure features a W-shaped N3-chain terminated by imidazoli-
diniumyl-groups with a N–N–N angle (110.4(2)ꢀ) comparable to
that seen for 3 and 4. The planes of the imidazole-rings are
skewed with respect to the N3-plane (38.3(7)ꢀ/41.2(5)ꢀ) and the
torsion angles involving the C–N bonds (N5–C1–N1–N2: 27.6ꢀ,
N6–C22–N3–N2: 35.0ꢀ) deviate from those expected for a C]N
double bond. This indicates that electron delocalization across
the imidazole and azide moiety may be hampered by the steric
1 (a) Y. Z. Wang and G. H. Robinson, Inorg. Chem., 2011, 50,
12326–12337; (b) C. D. Martin, M. Soleilhavoup and
G. Bertrand, Chem. Sci., 2013, 4, 3020–3030; (c) C. A. Dyker
and G. Bertrand, Science, 2008, 321, 1050–1051; (d)
Y. Wang and G. H. Robinson, Inorg. Chem., 2014, 53,
11815–11832.
2 D. J. D. Wilson, S. A. Couchman and J. L. Dutton, Inorg.
Chem., 2012, 51, 7657–7668.
3 A. Velian and C. C. Cummins, Science, 2015, 348, 1001–1004.
4 H. Braunschweig, R. D. Dewhurst, K. Hammond, J. Mies,
K. Radacki and A. Vargas, Science, 2012, 336, 1420–1422.
5 (a) Y. Li, K. C. Mondal, P. P. Samuel, H. Zhu, C. M. Orben,
S. Panneerselvam, B. Dittrich, B. Schwederski, W. Kaim,
T. Mondal, D. Koley and H. W. Roesky, Angew. Chem., Int.
Ed., 2014, 53, 4168–4172; Angew. Chem., 2014, 126, 4252–
4256; (b) L. Jin, M. Melaimi, L. Liu and G. Bertrand, Org.
Chem. Front., 2014, 1, 351–354.
6 Y. Z. Wang, Y. M. Xie, P. R. Wei, R. B. King, H. F. Schaefer,
P. V. Schleyer and G. H. Robinson, Science, 2008, 321,
1069–1071.
˚
˚
demands of the Mes substituents. The N–N (1.309(2) A/1.311(2) A)
˚
˚
and C–N (1.359(2) A/1.369(2) A) bond distances involving the W-
type fragment of 6 fall in the range between those expected for
single and double bonds.37
7 A. Sidiropoulos, C. Jones, A. Stasch, S. Klein and G. Frenking,
Angew. Chem., Int. Ed., 2009, 48, 9701–9704; Angew. Chem.,
2009, 121, 9881–9884.
DFT calculations (wB97XD/def2-TZV, see ESI†) carried out on
and the model compound 6-Me, (mesityl-substituents
replaced by methyl groups) showed similar molecular orbitals
for both compounds (Fig. 4). The HOMOs exhibit strong n
character comprised primarily of the lone pairs of the N-atoms.
8 C. Jones, A. Sidiropoulos, N. Holzmann, G. Frenking and
A. Stasch, Chem. Commun., 2012, 48, 9855–9857.
9 N. Holzmann, D. Dange, C. Jones and G. Frenking, Angew.
Chem., Int. Ed., 2013, 52, 3004–3008; Angew. Chem., 2013,
125, 3078–3082.
3
The LUMOs are p-type orbitals delocalized across the N3- 10 (a) Y. Z. Wang, Y. M. Xie, P. R. Wei, R. B. King, H. F. Schaefer,
linkage and the L donors. This is in contrast to the isolated P3-
allyl anion frontier orbitals reported for [(carbene)2P3]+.15
Interestingly these frontier orbitals are also reminiscent of the
HOMO and HOMOꢁ1 of neutral triazenes (NHC)N3R.38
P. V. Schleyer and G. H. Robinson, J. Am. Chem. Soc., 2008,
130, 14970–14971; (b) O. Back, G. Kuchenbeiser,
B. Donnadieu and G. Bertrand, Angew. Chem., Int. Ed.,
2009, 48, 5530–5533; Angew. Chem., 2009, 121, 5638–5641;
(c) J. D. Masuda, W. W. Schoeller, B. Donnadieu and
G. Bertrand, J. Am. Chem. Soc., 2007, 129, 14180–14181; (d)
J. D. Masuda, W. W. Schoeller, B. Donnadieu and
G. Bertrand, Angew. Chem., Int. Ed., 2007, 46, 7052–7055;
Angew. Chem., 2007, 119, 7182–7185.
Conclusions
In summary, the EPC [(p-C6F4H)3PF][B(C6F5)4] is used to prepare
the phosphonium ion salt 1 which serves as a precursor for the
formal transfer of [N3]+ to other donors affording stable and
isolable mono- and bis-adducts of the form [(L)N3]+ and [(L)2N3]+.
The reactivity of these species containing terminal and bridging
azide-fragments is the subject of continuing studies. In addition,
the exploration of the reactivity of EPCs as synthetic building
blocks for other unusual main group cations is ongoing.
11 M. Y. Abraham, Y. Z. Wang, Y. M. Xie, P. R. Wei,
H. F. Schaefer, P. V. Schleyer and G. H. Robinson, Chem.–
Eur. J., 2010, 16, 432–435.
12 R. Kretschmer, D. A. Ruiz, C. E. Moore, A. L. Rheingold and
G. Bertrand, Angew. Chem., Int. Ed., 2014, 53, 8176–8179;
Angew. Chem., 2014, 126, 8315–8318.
13 O. Back, B. Donnadieu, P. Parameswaran, G. Frenking and
G. Bertrand, Nat. Chem., 2010, 2, 369–373.
14 M. Y. Abraham, Y. Wang, Y. Xie, R. J. Gilliard Jr, P. Wei,
B. J. Vaccaro, M. K. Johnson, H. F. Schaefer III,
P. v. R. Schleyer and G. H. Robinson, J. Am. Chem. Soc.,
2013, 135, 2486–2488.
15 (a) M. H. Holthausen, S. K. Surmiak, P. Jerabek, G. Frenking
and J. J. Weigand, Angew. Chem., Int. Ed., 2013, 52, 11078–
11082; Angew. Chem., 2013, 125, 11284–11288; (b)
A. M. Tondreau, Z. Benko, J. R. Harmer and
Acknowledgements
D. W. S. gratefully acknowledges the nancial support of the
NSERC of Canada and the award of a Canada Research Chair.
M. H. H. thanks the Alexander von Humboldt Foundation for a
Feodor Lynen Research Fellowship.
Notes and references
¨
H. Grutzmacher, Chem. Sci., 2014, 5, 1545–1554.
§ Extra precaution must be taken when working with azides in CH2Cl2 solution as
they are potentially dangerous and can result in the possible formation of
dangerous and explosive diazidomethane.
16 (a) M. Donath, E. Conrad, P. Jerabek, G. Frenking,
¨
R. Frohlich, N. Burford and J. J. Weigand, Angew. Chem.,
6370 | Chem. Sci., 2015, 6, 6367–6372
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Edge Article
Int. Ed., 2012, 51, 2964–2967; Angew. Chem., 2012, 124, 3018–
Chemical Science
M. Farkens, A. Fischer, P. G. Jones and R. Schmutzler, Z.
Anorg. Allg. Chem., 1994, 620, 707–715; (g) A. H. Cowley,
F. P. Gabbaı, G. Bertrand, C. J. Carrano and M. R. Bond, J.
3021; (b) A. Sidiropoulos, B. Osborne, A. N. Simonov,
D. Dange, A. M. Bond, A. Stasch and C. Jones, Dalton
Trans., 2014, 43, 14858–14864.
¨
Organomet. Chem., 1995, 493, 95–99; (h) M. Larbig,
¨
17 N. Burford, T. S. Cameron and P. J. Ragogna, J. Am. Chem.
Soc., 2001, 123, 7947–7948.
M. Nieger, V. von der Gonna, A. V. Ruban and E. Niecke,
Angew. Chem., Int. Ed. Engl., 1995, 34, 460–462; Angew.
¨
18 (a) D. Himmel, I. Krossing and A. Schnepf, Angew. Chem., Int.
Ed., 2014, 53, 370–374; Angew. Chem., 2014, 126, 378–382; (b)
D. Himmel, I. Krossing and A. Schnepf, Angew. Chem., Int.
Ed., 2014, 53, 6047–6048; Angew. Chem., 2014, 126, 6159–
6160; (c) G. Frenking, Angew. Chem., Int. Ed., 2014, 53,
6040–6046; Angew. Chem., 2014, 126, 6152–6158.
19 (a) C. B. Caputo, L. J. Hounjet, R. Dobrovetsky and
D. W. Stephan, Science, 2013, 341, 1374–1377; (b)
C. B. Caputo, D. Winkelhaus, R. Dobrovetsky, L. J. Hounjet
and D. W. Stephan, Dalton Trans., 2015, 44, 12256–12264;
(c) M. Perez, Z. W. Qu, C. B. Caputo, V. Podgorny,
Chem., 1995, 107, 505; (i) C. Aubauer, T. M. Klapotke,
H. Noth, A. Schulz, M. Suter and J. Weigand, Chem.
Commun., 2000, 2491–2492; (j) N. Gotz, S. Herler, P. Mayer,
¨
A. Schulz, A. Villinger and J. J. Weigand, Eur. J. Inorg.
Chem., 2006, 2051–2057; (k) A. Schulz and A. Villinger, Eur.
J. Inorg. Chem., 2008, 4199–4203; (l) X. Zeng, M. Gerken,
H. Beckers and H. Willner, Inorg. Chem., 2010, 49, 3002–
3010; (m) X. Zeng, E. Bernhardt, H. Beckers and
H. Willner, Inorg. Chem., 2011, 50, 11235–11241; (n)
C. Hering, M. Hertrich, A. Schulz and A. Villinger, Inorg.
Chem., 2014, 53, 3880–3892.
˜
L. J. Hounjet, A. Hansen, R. Dobrovetsky, S. Grimme and 26 (a) A. Roglans, A. Pla-Quintana and M. Moreno-Manas,
D. W. Stephan, Chem.–Eur. J., 2015, 21, 6491–6500.
20 (a) M. Perez, C. B. Caputo, R. Dobrovetsky and
D. W. Stephan, Proc. Natl. Acad. Sci. U. S. A., 2014, 111,
Chem. Rev., 2006, 106, 4622–4643; (b) S. Mahouche-
Chergui, S. Gam-Derouich, C. Mangeney and
M. M. Chehimi, Chem. Soc. Rev., 2011, 40, 4143–4166.
10917–10921; (b) M. Perez, L. J. Hounjet, C. B. Caputo, 27 (a) K. O. Christe, W. W. Wilson, J. A. Sheehy and J. A. Boatz,
R. Dobrovetsky and D. W. Stephan, J. Am. Chem. Soc., 2013,
135, 18308–18310; (c) M. H. Holthausen, R. R. Hiranandani
and D. W. Stephan, Chem. Sci., 2015, 6, 2016–2021; (d)
Angew. Chem., Int. Ed., 1999, 38, 2004–2009; (b) K. O. Christe,
W. W Wilson, J. A. Sheehy and J. A. Boatz, Angew. Chem., Int.
Ed., 1999, 111, 2112–2118.
M. H. Holthausen, M. Mehta and D. W. Stephan, Angew. 28 D. Moy and A. R. Young, J. Am. Chem. Soc., 1965, 87, 1889–
Chem., Int. Ed., 2014, 53, 6538–6541; Angew. Chem., 2014,
126, 6656–6659.
1892.
¨
29 H. Hansen, S. Hunig and K.-I. Kishi, Chem. Ber., 1979, 112,
21 L. J. Hounjet, C. B. Caputo and D. W. Stephan, Dalton Trans.,
2013, 42, 2629–2635.
445–461.
30 (a) A. Mertens, K. Lammertsma, A. Massoud and G. A. Olah,
J. Am. Chem. Soc., 1983, 105, 5657–5660; (b) K. O. Christe,
W. W. Wilson, D. A. Dixon, S. I. Khan, R. Bau,
T. Metzenthin and R. Lu, J. Am. Chem. Soc., 1993, 115,
1836–1842; (c) A. Schulz and A. Villinger, Chem.–Eur. J.,
2010, 16, 7276–7281; (d) C. W. Huh and J. Aube, Chem.
Sci., 2014, 5, 699–706.
22 C. Hering, A. Schulz and A. Villinger, Angew. Chem., Int. Ed.,
2012, 51, 6241–6245; Angew. Chem., 2012, 124, 6345–6349.
23 (a) J. J. Weigand, K.-O. Feldmann and F. D. Henne, J. Am.
Chem. Soc., 2010, 132, 16321–16323; (b) D. Michalik,
A. Schulz, A. Villinger and N. Weding, Angew. Chem., Int.
Ed., 2008, 47, 6465–6468; Angew. Chem., 2008, 120, 6565–
6568.
31 M. H. Holthausen, I. Mallov and D. W. Stephan, Dalton
Trans., 2014, 43, 15201–15211.
24 Only two examples were crystallographically characterized:
(a) C. Hering, A. Schulz and A. Villinger, Inorg. Chem., 32 (a) L. LePichon and D. W. Stephan, Inorg. Chem., 2001, 40,
2013, 52, 5214–5225; for derivatives that were not
characterized by X-ray see: (b) K. Bott, Angew. Chem., Int.
Ed. Engl., 1965, 4, 695; Angew. Chem., 1965, 77, 683; (c)
N. Wiberg and K. H. Schmid, Angew. Chem., Int. Ed. Engl.,
1967, 6, 953–954; Angew. Chem., 1967, 79, 938; (d)
A. Schmidt, Chem. Ber., 1968, 101, 4015–4021; (e)
A. Schmidt, Chem. Ber., 1970, 103, 3923–3927; (f) W. Buder
and A. Schmidt, Chem. Ber., 1973, 106, 3812–3816; (g)
C. Hering, A. Schulz and A. Villinger, Angew. Chem., Int.
Ed., 2012, 51, 6241–6245.
3827–3829; (b) G. C. Fortman, B. Captain and C. D. Hoff,
´
Inorg. Chem., 2009, 48, 1808–1810; (c) P. Molina, C. Lopez-
´
Leonardo, J. Llamas-Botıa, C. Foces-Foces and
´
˜
C. Fernandez-Castano, Tetrahedron, 1996, 52, 9629–9642;
(d) R. D. Kennedy, Chem. Commun., 2010, 46, 4782–4784;
(e) C. G. Chidester, J. Szmuszkovicz, D. J. Duchamp,
L. G. Laurian and J. P. Freeman, Acta Crystallogr., Sect. C:
Cryst. Struct. Commun., 1988, 44, 1080–1083; (f) C. Reddy,
V. Reddy, B. M. Fetterly and J. G. Verkade, Energy Fuels,
2007, 21, 2466–2472; (g) M. W. P. Bebbington and
D. Bourissou, Coord. Chem. Rev., 2009, 253, 1248–1261.
¨
25 (a) U. Muller, Chem. Ber., 1977, 110, 788–791; (b)
H. W. Roesky, M. Noltemeyer and G. M. Sheldrick, Z. 33 H. Staudinger and J. Meyer, Helv. Chim. Acta, 1919, 2, 635.
Naturforsch., B: Chem. Sci., 1986, 41, 803–807; (c) 34 S. Cameron and M. Peach, J. Chem. Crystallogr., 1998, 28,
D. Schomburg, U. Wermuth and R. Schmutzler, Chem. Ber.,
919–924.
¨
1987, 120, 1713–1718; (d) A. H. Cowley, F. Gabbaı, 35 (a) V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–
R. Schluter and D. Atwood, J. Am. Chem. Soc., 1992, 114,
3142–3144; (e) U. Englert, P. Paetzold and E. Eversheim, Z.
Kristallogr., 1993, 208, 307–309; (f) J. R. Goerlich,
2001; (b) M. Cossi, N. Rega, G. Scalmani and V. Barone, J.
Comput. Chem., 2003, 24, 669–681; (c) J. Andzelm,
¨
C. Kolmel and A. Klamt, J. Chem. Phys., 1995, 103, 9312–
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¨¨
9320; (d) A. Klamt and G. Schuurmann, J. Chem. Soc., Perkin 38 (a) J. E. Leffer and R. D. Temple, J. Am. Chem. Soc., 1967, 89,
Trans. 2, 1993, 799.
5235–5246; (b) J. G. Gololobov, L. F. Kasukhin and
U. S. Petrenko, Phosphorus Sulfur Relat. Elem., 1987, 30,
36 M. Iglesias, D. J. Beetstra, J. C. Knight, L.-L. Ooi, A. Stasch,
S. Coles, L. Male, M. B. Hursthouse, K. J. Cavell, A. Dervisi
and I. A. Fallis, Organometallics, 2008, 27, 3279–3289.
37 A. F. Holleman and E. Wiberg, Lehrbuch der Anorganischen
Chemie, ed. von Nils Wiberg, de Gruyter, Berlin, 2007.
¨
393–396; (c) H. Bock and M. Schnoller, Angew. Chem., Int.
Ed. Engl., 1968, 7, 636–637; Angew. Chem., 1968, 80, 667.
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