Boron azides in Staudinger oxidations and cycloadditions

Article abstract of DOI:10.1039/c3dt50791b

Staudinger reactions of Cy₂BN₃ with tri-substituted phosphines (R₃P) yielded the boron-nitrogen-phosphorus linked systems Cy₂BNPR₃ (R = Et, ^tBu, Cy, Ph) (1a-1d respectively). Similarly, reaction of (C₆F₅) ₂BN₃ with the phosphines P^tBu₃, PPh₃, Ph₂PCCPh and Ph₂PCCPPh₂ yielded (C₆F₅)₂BNPR₃ (2a-d respectively). In contrast, the reaction of (C₆F₅) ₂BN₃ with Ph₂P-CCp-tol in the presence of excess Me₃SiN₃ afforded the bicyclic product 3 [1-(C ₆F₅)₂B-4-(p-tol)-1H-1,2,3-triazole-5-P(NH) Ph₂] in which both a Staudinger and a cycloaddition reaction has taken place. However, if the Staudinger reaction is prohibited via phosphine oxidation as in the case of (EtO)₂P(O)CCP(O)(OEt)₂ then the unusual dimeric product 4 [2-(C₆F₅) ₂B-4-(P(O)OEt₂)-2H-1,2,3-triazole-5-P(O)(OEt)(OB(C ₆F₅)₂)] is generated. The structures of 1b-d, 2b-d, 3 and 4 have been determined by X-ray diffraction.

Full text of DOI:10.1039/c3dt50791b

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Boron azides in Staudinger oxidations and
cycloadditions†
Cite this: Dalton Trans., 2013, 42, 8674
Rebecca L. Melen, Alan J. Lough and Douglas W. Stephan*
Staudinger reactions of Cy2BN3 with tri-substituted phosphines (R3P) yielded the boron–nitrogen–phos-
phorus linked systems Cy2BNvPR3 (R = Et, tBu, Cy, Ph) (1a–1d respectively). Similarly, reaction of
(C6F5)2BN3 with the phosphines PtBu3, PPh3, Ph2PCuCPh and Ph2PCuCPPh2 yielded (C6F5)2BNvPR3
(2a–d respectively). In contrast, the reaction of (C6F5)2BN3 with Ph2P–CuCp-tol in the presence of excess
Me3SiN3 afforded the bicyclic product 3 [1-(C6F5)2B-4-(p-tol)-1H-1,2,3-triazole-5-P(NH)Ph2] in which both
a Staudinger and a cycloaddition reaction has taken place. However, if the Staudinger reaction is prohibi-
ted via phosphine oxidation as in the case of (EtO)2P(O)CuCP(O)(OEt)2 then the unusual dimeric product
4 [2-(C6F5)2B-4-(P(O)OEt2)-2H-1,2,3-triazole-5-P(O)(OEt)(OB(C6F5)2)] is generated. The structures of 1b–d,
2b–d, 3 and 4 have been determined by X-ray diffraction.
Received 24th March 2013,
Accepted 23rd April 2013
DOI: 10.1039/c3dt50791b
www.rsc.org/dalton
investigating the ‘click’ chemistry9 of Cy2BN3 with alkynes,10
RCuCR′. These studies demonstrated that interesting boron
Introduction
Recently, there has been a surge in interest in the activation functionalised triazoles were accessible via such cycloaddition
and subsequent utilisation of small molecules and catalytic chemistry.
properties of main group compounds.1 The advent of such
The Staudinger reaction of a tertiary phosphine with an
metal-free processes has been stimulated to some extent by azide yielding a phosphinimine11 is a versatile tool in organic
the development of the concept of Frustrated Lewis Pairs synthesis affording amines from azides via a stable phosphini-
(FLPs). Most notably such species have been shown2 to activate mine intermediate (Scheme 1).12 The initial phosphinimine is
H2 reversibly leading to the development of metal-free hydro- formed by nucleophilic attack of the phosphine on the term-
genation catalysts.1c–g In addition to hydrogen activation and inal nitrogen of the azide, followed by the thermodynamically
hydrogenation reactions, FLPs have also been shown to acti- favourable elimination of N2 to afford the phosphinimines
vate a wide range of other small molecules including alkynes, (Scheme 1).12,13 Nevertheless, until recently use of boron
olefins, CO2,3 N2O,4 and NO5 among others.
azides in Staudinger reactions appears to be limited to the
In ongoing studies, we are continuing to explore the ‘click’ reaction of Ph2BN3 with Ph3P in pyridine at 200 °C, published
chemistry of main group azides with a view to making novel some 45 years ago.14 More recently, Erker et al.15 have reported
main group substituted heterocycles which potentially contain the Staudinger reaction at an intramolecular frustrated P–B
both Lewis acidic and Lewis basic sites. Of particular interest Lewis pair to generate a borane-stabilised phosphinimine.
is the cycloaddition reaction of a boron azide with alkynes. We
are particularly targeting systems bearing electron withdrawing
groups at B6 to enhance its Lewis acidity. Whilst several boron
azide compounds of the type R2BN3 (R = alkyl or aryl) have
been reported previously,7 their subsequent applications in
synthesis have barely been explored. In 2012 Curran et al.8
reported the cycloaddition chemistry of an N-heterocyclic
carbene-stabilised boryl azide. At the same time we were
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto,
Ontario, Canada, M5S 3H6. E-mail: dstephan@chem.utoronto.ca
†Electronic supplementary information (ESI) available: Selected NMR data, DFT
calculations and details of the crystal structure determination of 1b–d, 2a–d, 3
and 4. CCDC 929289–929295 and 929332–929333. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c3dt50791b
Scheme 1 Reaction pathway of Staudinger oxidation.
8674 | Dalton Trans., 2013, 42, 8674–8683
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 1 31P{1H} and 11B NMR chemical shifts for 1–4
In the present study, we report the facile synthesis of the
phosphinimines, R2BNvPR3 via Staudinger reactions and we
examine the competition between the possible Staudinger and
cycloaddition reaction pathways when using alkynyl phos-
phines leading to unique main group heterocyclic compounds.
Compound
31P{1H}/ppm
11B/ppm
1a (Et) −20
1b (tBu) 61
1c (Cy) 5
1d (Ph) −8
2a
14.7
33.3
17.6
0.8
43.6
50.3
49.0
48.5
52.0
31.4
37.0
36.1
38.2
2b
16.0
Results and discussion
2c
2d
−5.3
−7.1 [P(2)]
−33.8 [P(1)]
32.0
Initially we explored the reactions of the boron azides Cy2BN3
and (C6F5)2BN3 with a variety of tri-substituted phosphines.
The Staudinger reaction between the boron azide and the
phosphine readily afforded the boryl-phosphinimide species
3
4
−1.0
−0.5
2.6
7.9
−10.5
R1 R2PNBR2 1a–d and 2a–b (Fig. 1) in modest to excellent
2
recovered yields (44–99%) under mild conditions (25 °C, 18 h).
This is in stark contrast to Paetzold’s synthesis of Ph2BNPPh3
which required temperatures of 200 °C to afford the product.14
We found that the rate of the reaction is strongly dependent
upon both the electron-donating ability of the phosphine and
the electron-withdrawing ability of the groups attached to the
boron azide. Thus, the fastest reaction occurs for very electron-
rich phosphines and more electron-withdrawing azides which
is comparable to that seen for the typical Staudinger reac-
tion.16 Indeed, the rapidity of the reaction between (C6F5)2BN3
and PtBu3 can be observed by the effervescence of N2 upon
immediate addition of the phosphine to the boron azide.
Although the majority of these reactions appeared complete
within 30 min, based upon in situ 31P NMR spectroscopy all
reactions were left for 18 h to ensure complete conversion.
Notably no reaction was observed between Cy2BN3 and Mes3P,
presumably due to the steric hindrance at P which protects the
lone pair (Tolman cone angle for Mes3P is 212°).17
states and orbital overlap. The 11B NMR spectra of 1a–d exhibi-
ted a broad singlet between 48.5 and 52.0 ppm. Likewise, the
11B NMR chemical shift for 2a–b occurred at 31.4 and
37.0 ppm respectively.
High-resolution DART mass spectrometry of compounds
1a–d and 2a–b all showed peaks corresponding to either
[R3PvNH2]+ and/or [R3P–NH3]+ usually as the base peak, and
t
the Bu derivatives 1b and 2a showed a peak due to the mol-
ecular ion [M + H]+. The formation of the stable R3PvNH as
the base peak indicated that the B–N bond theoretically has
limited stability and addition of H2 to this could potentially
result in B–N bond breakage and the formation of R3PvNH
and R1 BH. However, attempts to activate H2 with 1a, 1c and
2
2a proved unsuccessful even after 4 weeks at 150 °C indicating
that the B–N bond remains intact.
In an analogous fashion phosphines bearing acetylene
functionalities were also reacted with boron azides to give the
species Ph2(RCuC)PNB(C6F5)2 (R = Ph 2c, PPh2 2d). It is note-
worthy that in the latter case, 2d does not react further with
azide. The isolated species exhibit 31P NMR signals at
−5.3 ppm and −7.1 ppm respectively while the 11B NMR reson-
ances appear at 36.1 ppm and 38.2 ppm (Table 1).
The structures of compounds (1b–d, 2a–d) were determined
by X-ray crystallography confirming the monomeric nature of
these PNB species (Fig. 2–4). A search of the CSD revealed no
structures of the from R2B–NvPR3 in the literature, although a
few adducts of the type R3B←NHvPR3 have been reported in
The 31P spectra were diagnostic of compounds 1a–d and
2a–b. The 31P chemical shifts are dependent upon the phos-
phine employed with the general trend that phosphines
bearing more electron donating groups were more deshielded.
In the case of 1 31P chemical shifts followed the trend tBu > Cy
> Et > Ph with the tBu derivative exhibiting the most downfield
shift at 33.3 ppm and the Ph derivative having the most
upfield shift at 0.8 ppm. Compounds 2a and 2b show analo-
gous trends in chemical shift in the 31P NMR spectra (Table 1).
These data are broadly comparable with trends seen for the
parent phosphines, R3P where the chemical shifts are domi-
nated by the paramagnetic shielding tensor.18 However, it is
notable that whilst the general trend is maintained some 31P
chemical shifts move downfield (1a, 1c and 1d) whereas others
move upfield (1b) relative to the parent phosphine reflecting
the complexity of the paramagnetic shielding tensor term
which involves (partial) charge, relative energies of excited
which there is
a boron–nitrogen dative bond (dB–N =
1.5645–1.7121 Å).15,19 The B–N bond lengths in the present
species (Table 2) span the range from 1.344(4) to 1.408(2) Å
which is shorter than conventional B–N single bonds
(1.51 Å)15,19 but longer than a normal BvN double bond
(1.31 Å), consistent with some degree of π-delocalisation in the
species 1 and 2. The B–N bond lengths are comparable to
those in conjugated allylic systems such as Ph2B–NvCtBu2
and Mes2B–NvCPh2 (1.366 Å and 1.376 Å respectively).20 The
sums of the angles at B are 360.0(2)°, consistent with the
expected trigonal planar geometry at B. The PvN bond
lengths reveal much smaller variations [average = 1.566 Å;
range 1.547(1)–1.581(2) Å, Table 2] and are consistent with
other R3PvNR ‘double’ bonds (1.584 Å),15,19 albeit one in
Fig. 1 Compounds 1 and 2.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8674–8683 | 8675

View Article Online
Paper
Dalton Transactions
Fig. 3 Structure of 2a (top) and 2b (bottom). Hydrogen atoms are omitted for
clarity. B: yellow-green, C: black, N: blue, F: green, P: orange.
Fig. 2 Structures of 1b (top), 1d (middle) and 1c (bottom). Hydrogen atoms
omitted for clarity. B: yellow-green, C: black, N: blue, F: green, P: orange.
which the zwitterionic contribution, R3P+–N−R, is the domi-
nant contribution to the resonance form.
The overall effect of the alkyl and aryl substituents on the
phosphorus and boron atoms in compounds 1 and 2 is to
produce a sterically-hindered central nitrogen atom. Indeed,
the solid state structures reveal B–NvP bond angles intermedi-
ate between the idealised 120° angle expected for an sp2 hybri-
dised nitrogen atom and 180° expected for an sp-hybrid
N with the angle becoming closer to 180° for the tBu derivative
(162.8° and 170.8° for 1b and 2a respectively) (Table 2). The
B–NvP angle increases with increasing size of the alkyl
groups on the phosphorus atom. Thus the tBu derivatives
show the largest B–N–P angles whereas the cyclohexyl and
phenyl compounds show a much smaller B–NvP angle in
general agreement with the Tolman cone angles (tBu3P 182°;
Cy3P 170°; Ph3P 145° and Et3P 132°).17 Notably the bond angle
at nitrogen varies widely for each of the individual molecules
in the asymmetric unit for 1c ranging between 134.9(2)° and
155.4(2)° which may indicate a shallow energy profile for dis-
tortion around the energy minimum for the BNP bond angle.
Fig. 4 Structures of 2c (top) and 2d (bottom). Hydrogen atoms are omitted for
clarity. B: yellow-green, C: black, N: blue, F: green, P: orange.
8676 | Dalton Trans., 2013, 42, 8674–8683
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Table 2 Selected bond lengths and angles for compounds 1–3
B–N bond
length/Å
PvN bond
length/Å
B–NvP bond
angle/°
Compound
1b
1c
1.388(2)
1.406(3)
1.391(3)
1.396(4)
1.408(2)
1.344(4)
1.362(3)
1.372(2)
1.373(8)
1.583(3)
1.547(1)
1.581(2)
1.565(2)
1.553(2)
1.5645(9)
1.560(3)
1.578(2)
1.575(2)
1.572(3)
1.609(2)
162.8(1)
134.9(2)
144.6(2)
155.4(2)
141.51(8)
170.8(3)
138.8(2)
135.5(1)
135.7(3)
118.3(2)
1d
2a
2b
2c
2d
3
Scheme 2 Synthesis of 3.
Fig. 5 Space filling model of 2a.
The increased steric hindrance at the nitrogen lone pair and at
boron in 2a can be seen in the space-filling representation
(Fig. 5). Indeed, in this case, the coordination of small mole-
cules such as MeCN to the boron centre was not possible.
While this may be a steric effect, the reduction of the Lewis
acidity at boron via π-donation from nitrogen may also have an
impact here.
Compounds 2c and 2d exhibit PvN and B–N bonds lengths
similar to those observed for 1b–d and 2a–b. However the B–
NvP bond angle is significantly smaller at 135°, closer to that
expected for the sp2 hybridised nitrogen atom which is pre-
sumably a result of the reduced steric hindrance about phos-
phorus with the alkyne substituent. In the case of 2c the solid
state structure shows that one of the C6F5 groups of the boron
atom lies over the alkyne moiety affording some π–π stabilis-
ation through interaction of the electron-rich alkyne and the
electron-poor C6F5 ring. The intramolecular distance from the
centroid of the C6F5 ring to the centre of the CuC bond in 2c
is 3.514 Å and is comparable with those reported for fluoro-
aryl⋯alkyne π–π stacking interactions which have been reported
elsewhere (average 3.49 Å).21 Likewise, 2b shows fluoroaryl⋯aryl
π–π stacking (Fig. 4) with a centroid to centroid distance of
3.581 Å, comparable to fluoroaryl⋯aryl π–π stacking (3.4–3.8 Å)
in compounds such as the co-crystal C6F6·C6H6.22
Fig. 6 Structure of 3. Hydrogen atoms omitted for clarity. B: yellow-green,
C: black, N: blue, F: green, P: Orange. Selected bond distances (Å) and angles (°).
3: B(1)–N(4), 1.541(4); P(2)–N(4), 1.609(2); B(1)–N(1), 1.583(3); P(1)–C(5),
1.781(3); N(1)–N(2), 1.319(4); N(2)–N(3), 1.327(2); N(3)–C(2), 1.361(4); C(1)–
C(2), 1.385(3); C(1)–N(1), 1.366(3); B(1)–N(4)–P(1), 118.3(2).
planar with a maximum deviation from planarity of 0.10 Å.
This permits π-delocalisation across the triazole ring, although
delocalisation is disrupted by the 4-coordinate B centre. Com-
pound 3 exhibits significant shortening of both the N(1)–N(2)
and N(2)–N(3) bonds at 1.319(4) Å and 1.327(2) Å respectively.
The C–N bonds in the triazole 3 also reveal shorter bond
lengths at 1.366(3) Å [N(3)–C(1)] and 1.361(4) Å [C(2)–N(1)]
compared to 1.383 Å and 1.367 Å in the average triazole. The
B(1)–N(4)–P(1) angle at 118.3(2)° is now more typical for an sp2
hybridised nitrogen atom presumably a reflection of the fact
that it is constrained to be a cyclic system. The B(1)–N(4) bond
at 1.541(4) Å and the P(2)–N(4) bond at 1.609(2) Å are much
longer than those observed for compounds 1–2 as a result of
(i) the reduced π-bonding to the sp3 hybridised boron atom
and (ii) the protonation of the nitrogen atom. The N–H group
acts as an intermolecular hydrogen-bond donor to the triazole
ring of an adjacent molecule with an intermolecular N(4)–
H⋯N(3) contact between molecules (dN(4)⋯N(3) = 2.852(4) Å).
The formation of 3 would appear to involve three discrete
The reaction of the phosphine Ph2P–CuCp-tol with the
boron azide (C6F5)2BN3 in the presence of excess TMSN3 in
toluene at 110 °C afforded the bicyclic compound 3 after
2 days in low yields (Scheme 2). The identity of 3 was con-
firmed crystallographically (Fig. 6) as Ph2P(vNH)(CvCp-tol)-
N3B(C6F5)2. The fused C2N4BP bicyclic framework is essentially
transformations: (i)
a
Staudinger reaction between the
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8674–8683 | 8677

View Article Online
Paper
Dalton Transactions
Scheme 3 Synthesis of 4.
phosphine and the boron azide; (ii) a subsequent cyclo-
addition reaction between the azide and an alkyne; (iii)
hydrolysis of the initial cycloaddition product to afford 3. The
observation that Ph2P–CuCPh reacts only very slowly with
Me3SiN3 at 85 °C in C6D6 after 3 days coupled with the obser-
vation that the similar products 2c and 2d were isolated after
just 18 h at room temperature supports our assertion that the
formation of 3 commences with the Staudinger oxidation of
the P centre. In addition, the presence of the electron-with-
drawing C6F5 groups activates the azide thus accelerating the
Staudinger reaction, i.e. the Staudinger oxidation using
(C6F5)2BN3 is expected to be substantially faster than with
Me3SiN3. Thus, the compounds 2c and 2d are plausible
analogs of the intermediate in the formation of 3.
To preclude the Staudinger oxidation, the O-protected phos-
phine-oxide (EtO)2P(O)CuCP(O)(OEt)2 was reacted with
(C6F5)2BN3. While the spectroscopic data clearly inferred the
formation of a new product 4, its nature was only fully deter-
mined via a crystallographic study which revealed it to be
[(EtO)2P(O)CvCN3B(C6F5)2P(O)(OEt)]2 (Scheme 3). The struc-
ture of 4 is composed of a bicyclic system comprising two
fused 5- and 7-membered rings which associate to form
dimers via a pair of dative PvO→B bonds to generate a central
C2N4O2B2P2 macrocycle (Fig. 7). Whilst the two crystallographi-
cally independent C2N3 triazole rings are essentially planar
(max. deviation 0.007 Å), the 7-membered C2P2O2B ring is
folded across the O⋯P transannular contact such that one O
and B atom lie considerably out of the plane by up to 0.88 Å.
The C–N bonds in the triazole rings are the same within error
(average = 1.346 Å), shorter than those in 3 and in convention-
al 1,2,3-triazoles and closer to that typical of a partially conju-
gated C–N bond such as that found in phenylamine (1.35 Å).23
Likewise the C–C bond lengths (average = 1.408 Å) are longer
than those in 3 and slightly longer than those in 1,2,3-triazoles
(Scheme 3).
Fig. 7 Structure of 4. Hydrogen atoms are omitted for clarity. B: yellow-green,
C: black, N: blue, F: green, P: orange, O: red.
(C6F5)2B group to the oxygen atoms on the phosphine. The
loss of the alkyl group from one of the ethyl groups of the
phosphate ester bears a strong similarity to the loss of alkyl
chains from ester groups when performing click reactions with
the boron azide Cy2BN3 with RCO2CuCCO2R (R = Me, Et,
tBu).10 Elimination of the ethyl group from the ester is thought
to proceed via coordination through the O-donor to the boron
of (C6F5)2BN3 inducing loss of EtN3. Nonetheless, the volatile
24
EtN3 was not observed by NMR spectroscopy. Notably EtN3
has been generated from diethyl sulfate and NaN3 supporting
the thesis that a similar mechanism operates here.25
Theoretical considerations
The near-linear geometry of the BNP unit permits the system
to perhaps be considered analogous to allene, H2CvCvCH2
with two σ-bonds and the central atom involved in two sets of
orthogonal π-type interactions. Indeed the relationship
between BvN and CvC is a textbook example of the isoelec-
tronic principle.26 In addition the R3P group can be considered
analogous to a singlet carbene CH2 in that both exhibit a lone
pair. However, whereas the singlet carbene offers a vacant
p-orbital, the R3P fragment implements R–P σ* acceptor orbi-
tals.27 The electronegativity differences between both B and N
and P and N are likely to lead to resonance forms of type A
(Fig. 8) in which there is a substantial build up of negative
charge at N, with two formal lone pairs. Stabilisation of those
two lone pairs can, to a greater or lesser extent be achieved
through (i) π-donation to the vacant p-orbital on B (B) and (ii)
donation to vacant P–C σ* orbitals on P (C). The efficiency of
these two processes is defined by the relative energies and
spatial overlaps of the N p-orbitals and unoccupied B p-orbital
and P–C σ* orbitals, with the former anticipated to be more
efficient, based on size and energy match.
In the absence of the phosphine centre, cycloaddition reac-
tion between the azide and alkyne occurs, followed by a 1,2-
migration of the (C6F5)2B functionality from the terminal nitro-
gen of the triazole to the central nitrogen atom. This pre-
sumably reduces steric hindrance, yielding
a
more
thermodynamically stable product than the terminal N-coordi-
nated isomer. The 7-membered C2P2O2B ring can be viewed as
arising from the coordination of a second Lewis acidic
Single point DFT calculations were undertaken at the
B3LYP/6-31G*+d level of theory on derivatives of both 1 and 2.
The three highest occupied molecular orbitals (Fig. 9) are
8678 | Dalton Trans., 2013, 42, 8674–8683
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
Fig. 8 Potential resonance forms for R2BNPR3 structures.
Fig. 10 HOMO for 2a; NBO partial charges (centre) and natural bond order
(right) analyses for 2a and 2b.
second order perturbation analysis associated with donation
of electron density from the N lone pair of p-character to the
vacant orbital on the P2+ centre (85% p-character) (259 kcal
mol−1) and from the BvN π-system to the P2+ centre (315 kcal
mol−1). Conversely the NBO analysis of 2b reflects substantial
PvN double bond character and a B–N single bond resulting
in a strongly Lewis acidic B-centre. In 2b there are again sub-
stantial second order perturbations arising from donation of
electron density from the P–N π-bond (implementing essen-
tially 50% p character and 50% d character on P and pure
p-character from N) to the vacant B p-orbital (86 kcal mol−1).
In both cases, the presence of significant second order pertur-
bations suggest a two centre two electron bonding model is an
inadequate description of the structure.
Fig. 9 (top) HOMO and HOMO − 1 for 1b; (bottom) NBO partial charges and
natural bond order analysis for the core of 1b.
similar for all derivatives of 1 in which the highest occupied
molecular orbital (HOMO) reflects the N p-orbital interacting
with P–C σ* with a significant additional antibonding com-
ponent through contributions from the B–C σ framework. On
the other hand, the HOMO − 1 shows a stronger BN π bond
with π-donation of the N lone pair to B. Notably the back
donation to P is maximised when the BNP angle is linear (sp-
hybrid N), thereby affording maximum spatial overlap as the
alternative sp2-hybridised N exhibits a non-bonding lone pair.
An NBO analysis of 1b (Fig. 9) reflects the strongly polar
nature of the bonding across the BNP fragment with both B
and P bearing substantial positive charges and the N signifi-
cantly negative. The presence of formal BvN double bond
character, coupled with the highly positively charged P centre
supports resonance form B (Fig. 8) as the major contributor to
the structure with a formal BvN double bond and ylidic PvN
bonding. Replacement of the mildly electron-releasing cyclo-
hexyl group by C6F5 leads to marked changes in the bonding
character and the net bonding appears extremely sensitive to
the substituents at P. The HOMO of 2a (Fig. 10) appears
similar in nature to that of 1b (Fig. 9). However the subsequent
highest occupied orbitals (HOMO − 1, HOMO − 2, and HOMO
− 3) are all based of predominantly C6F5 π-character.
Conclusions
The boron azides Cy2BN3 and (C6F5)2BN3 undergo Staudinger
reactions with primary phosphines to give the products
R2BNPR′ 1 and 2 in high to excellent yields. The structural par-
3
ameters determined from both X-ray diffraction and gas phase
DFT calculations reveal substantial B–N multiple bond charac-
ter and zwitterionic character to the P–N bond, with evidence
that the bonding is inadequately described through a simple
two centre, two electron resonance form. In the case of phos-
phines bearing an acetylene functionality an initial Staudinger
reaction takes place to generate the boranyl-phosphanimine in
preference to the cycloaddition of the azide with the alkyne.
However, in the presence of additional Me3SiN3, a subsequent
cycloaddition reaction occurs to afford a bicyclic product 3.
These preliminary studies demonstrate that “click chemistry”
of boron azides with ethynylphosphines occurs without the
need for a catalyst affording P/B substituted 1,2,3-triazoles and
is the target of current efforts.
Experimental section
General experimental
The NBO analysis shows that 2a (Fig. 10) retains the highly
polar nature of the BNP framework and the BvN multiple
bond character but leads to an apparent weakening of the P–N
bond such that the structure of 2a is better considered as
donor–acceptor adduct between discrete (C6F5)2BN and PtBu3
units with P–N bonding only apparent through a substantial
Unless otherwise stated, all reactions and manipulations were
carried out under an atmosphere of dry, O2-free, nitrogen
using standard double-manifold techniques with a rotary oil
pump. A nitrogen-filled glove box (MBRAUN) was used to
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8674–8683 | 8679

View Article Online
Paper
Dalton Transactions
manipulate solids. All solvents (including deuterated solvents) 27.4 (d, 3JPC = 12 Hz), 27.2 (d, 4JPC = 3 Hz), 26.6 (s). Elem. Anal.
were dried and stored over molecular sieves under a nitrogen calcd (%) for C30H55NPB: C 76.41; H 11.56; N 2.97; Found:
atmosphere before use. Cy2BCl (1 M in hexanes), TMSN3, C 76.48, H 12.00, N 1.67. DART MS, m/z: 297.3 (calcd for
PhCuCH, p-tol-CuCH, Ph2PCuCPPh2, Ph2PCl, (EtO)2P(O)- [Cy3P–NH3]+: 297.3), 296.3 (calcd for [Cy3PvNH2]+: 296.3), no
CuCP(O)(OEt)2 and all phosphines were commercially avail- peak assignable to the molecular ion was observed.
able and used as received. (C6F5)2BCl,28 (C6F5)2BN3,6
1d: off-white solid (207 mg, 91%, 0.46 mmol). 1H NMR
10
Ph2PCuCPh,29 Ph2PCuCp-tol30 and Cy2BN3 were prepared (400 MHz, d6-benzene, 298 K): 7.75 (m, 6H, o-Ph), 7.09 (m, br.,
according to literature methods. 1H, 13C, 11B and 31P NMR 9H, m-Ph and p-Ph), 1.75–1.89 (m, br., Cy), 1.56–1.65 (m, br.,
spectra were recorded on a Bruker Avance III or a Bruker Cy), 1.39–1.25 (m, br., Cy), 1.14 (t, br., Cy); 31P NMR (163 MHz,
Avance 500 spectrometer. A Perkin-Elmer analyser was used for d6-benzene, 298 K): 0.8 (septet, JPH = 11 Hz); 31P{1H} NMR
3
carbon, hydrogen and nitrogen elemental analyses. High (163 MHz, d6-benzene, 298 K): 0.8 (s); 11B NMR (128 MHz,
resolution mass spectrometry was performed in house employ- d6-benzene, 298 K): 52.0 (s, br.); 13C{1H} NMR (100 MHz,
1
ing DART or electrospray ionisation techniques in positive ion d6-benzene, 298 K): 134.4 (d, JPC = 100 Hz, i-Ph), 132.5 (d,
mode on an AB/Sciex QStarXL mass spectrometer (ESI) or a 2JPC = 10 Hz, o-Ph), 131.1 (d, br., JPC = 2 Hz, p-Ph), 128.3 (d,
4
JEOL AccuTOF model JMS-T1000LC mass spectrometer 3JPC = 12 Hz, m-Ph), 30.3 (s, Cy), 28.6 (s, Cy), 27.8 (s, Cy),
(DART). Caution: Covalent azides are potentially explosive and signals for boron-bound carbon atoms were not observed.
reactions were performed on small scale behind blast shields.
Elem. Anal. calcd (%) for C30H37NPB: C 79.47; H 8.23; N 3.09;
Synthesis of (Cy)2BNPR3 (R = Et 1a, tBu 1b, Cy 1c, Ph Found: C 79.59, H 8.58, N 2.80. DART MS, m/z: 279.1 (calcd for
1d). These compounds were prepared in a similar manner [Ph3P−NH3]+: 279.1), 278.1 (calcd for [Ph3PvNH2]+: 278.1), no
and thus only one preparation is detailed. Cy2BN3 (0.5 mmol) peak corresponding to the molecular ion was observed.
was prepared in situ in toluene (2 mL) and PEt3 (59 mg,
Synthesis of (C6F5)2BNPR3 (R = tBu3 2a, Ph 2b) and
0.5 mmol) was added. The reaction was left standing for 18 h (C6F5)2BNP(Ph)2CCPh (2c) and (C6F5)2BNP(Ph)2CCPPh2
and the solvent removed to give pure Cy2BNPEt3 as an off- (2d). These compounds were prepared in a similar manner
6
white oil which solidified in the freezer at −35 °C (150 mg, and thus only one preparation is detailed. 2a: (C6F5)2BN3 was
1
99%, 0.49 mmol). 1a: H NMR (400 MHz, C6D5Br, 298 K): 1.78 dissolved in toluene (2 mL) and PtBu3 (51 mg, 0.25 mmol) was
(m, br., Cy), 1.68 (s, br., Cy), 1.32 (s, br., Cy), 1.27–0.98 (m, br., added. The reaction was left standing for 18 h and the solvent
3
3
6H, –CH2CH3), 0.90 (dt, JPH = 17 Hz, JHH = 8 Hz, –CH2CH3); removed to give crude (C6F5)2BNPtBu3 which was washed with
11B NMR (128 MHz, C6D5Br, 298 K): 50.3 (s, br.); 31P NMR hexane (2 × 3 mL) to give pure (C6F5)2BNPtBu3 as an off-white
(163 MHz, C6D5Br, 298 K): 14.7 (s); 31P{1H} NMR (163 MHz, solid (110 mg, 92%, 0.23 mmol) which could be recrystallised
C6D5Br, 298 K): 14.7 (s, br.); 13C{1H} NMR (100 MHz, C6D5Br, from a saturated solution of toluene at −35 °C. 1H NMR
298 K) partial: 30.1 (s, Cy), 28.6 (s, Cy), 27.8 (s, Cy), 21.0 (d, (400 MHz, CD2Cl2, 298 K): 1.27 (s, 9H, 3JPH = 14 Hz, –C(CH3)3);
2
1JPC = 66.3 Hz, –CH2CH3), 6.3 (d, JPC = 4.5 Hz, –CH2CH3), 11B NMR (128 MHz, CD2Cl2, 298 K): 31.6 (s, br.); 31P NMR
signals for boron-bound carbon atoms were not observed. (163 MHz, CD2Cl2, 298 K): 43.9 (m); 31P{1H} NMR (163 MHz,
Elem. Anal. calcd (%) for C18H37NPB: C 69.90; H 12.06; N 4.53; C6D5Br, 298 K): 43.6 (s); 19F NMR (377 MHz, CD2Cl2, 298 K):
3
Found: C 69.76, H 11.91; N 4.05. DART MS, m/z: 134.1 −134.2 (m, 2F, o-F), −158.2 (t, 1F, JFF = 20.0 Hz p-F), −163.7
(calcd for [Et3PvNH2]+: 134.1), no molecular ion peak was (m, 2F, m-F); 13C NMR (100 MHz, CD2Cl2, 298 K): 146.0 (m,
observed.
C6F5), 140.6 (m, C6F5), 137.4 (m, C6F5), 40.4 (m, C(CH3)3), 29.4
1b: off-white solid (110 mg, 92%, 0.23 mmol). 1H (m, C(CH3)3), signals for boron-bound carbon atoms were not
(400 MHz, d8-toluene, 298 K): 1.84–1.35 (m, 22H, Cy), 1.17 (d, observed. Elem. Anal. calcd (%) for C24H27NPBF10: C 51.36; H
3JHP = 13 Hz, –C(CH3)3); 31P NMR (163 MHz, d8-toluene, 4.85; N 2.50; Found: C 50.56, H 4.84; N 2.67. DART MS, m/z:
298 K): 33.3 (m); 31P{1H} NMR (163 MHz, d8-toluene, 298 K): 562.2 (calcd for [M
+
H]+: 562.2), 218.2 (calcd for
33.4 (s); 11B NMR (128 MHz, d8-toluene, 298 K): 49.0 (s, br.); [tBu3PvNH2]+: 218.2), 202.2 (calcd for [tBu3PH]+: 202.2).
13C{1H} NMR (100 MHz, d8-toluene, 298 K) partial: 40.0 (d,
2b: colourless crystals of 2b (69 mg, 44%, 0.11 mmol). H
1
1JPC = 53 Hz), 30.1 (s), 29.8 (s), 28.8 (s), 28.1 (s), signals for NMR (400 MHz, CD2Cl2, 298 K): 7.58 (m, 2H, o-H PPh3), 7.46
boron-bound carbon atoms were not observed. Elem. Anal. (m, 1H, p-H PPh3), 7.36 (m, 2H, m-H PPh3); 11B NMR
calcd (%) for C24H27NPBF10: C 51.36; H 4.85; N 2.50; Found: (128 MHz, CD2Cl2, 298 K): 37.0 (s, br.); 31P NMR (163 MHz,
C 50.56, H 4.84; N 2.67. DART MS, m/z: 562.2 (calcd for CD2Cl2, 298 K): 16.0 (s, br.); 31P{1H} NMR (163 MHz, CD2Cl2,
[M + H]+: 562.2), 218.2 (calcd for [tBu3PvNH2]+: 218.2).
298 K): 16.0 (s); 19F NMR (377 MHz, CD2Cl2, 298 K): −134.0
1c: off-white solid (228 mg, 97%, 0.48 mmol). 1H NMR (dd, 2F, JFF = 24, 10 Hz, o-F), −156.9 (t, 1F, JFF = 20 Hz, p-F),
(400 MHz, d6-benzene, 298 K): 1.71 (m, br., Cy), 1.63 (m, br., −164.2 (m, 2F, m-F); 13C{1H} NMR (100 MHz, CD2Cl2, 298 K):
Cy), 1.53–1.46 (m, br., Cy), 1.39–1.25 (m, br., Cy), 1.15 (m, br., 146.2 (m, C6F5), 140.5 (m, C6F5), 136.7 (m, C6F5), 132.4 (d, JPC
Cy), 0.86–0.95 (m, br., Cy); 31P NMR (163 MHz, d6-benzene, = 10.6 Hz, Ph), 132.3 (d, JPC = 3 Hz, Ph), 128.5 (d, JPC = 13 Hz,
298 K): 17.6 (s, br.); 31P{1H} NMR (163 MHz, d6-benzene, Ph), signals for boron-bound carbon atoms were not observed.
298 K): 17.7 (s); 11B NMR (128 MHz, d6-benzene, 298 K): 48.5 Elem. Anal. calcd (%) for C30H15NPBF10: C 58.00; H 2.43;
(s, br.); 13C{1H} NMR (100 MHz, d6-benzene, 298 K): 61.5 (d, N 2.55; Found: C 57.27, H 2.66; N 2.23. DART MS, m/z: 279.1
2
1JPC = 62 Hz), 30.5 (s), 28.9 (s), 28.2 (d, JPC = 37 Hz), 28.1 (s), (calcd for [Ph3P–NH3]+: 279.1), 278.1 (calcd for [Ph3PvNH2]+:
8680 | Dalton Trans., 2013, 42, 8674–8683
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
278.1), 263.1 (calcd for [Ph3PH]+: 263.1), no peak correspond- (128 MHz, CD2Cl2, 298 K): −1.0 (s); 31P{1H} NMR (163 MHz,
ing to the molecular ion was observed.
CD2Cl2, 298 K): 32.0 (s); 19F NMR (377 MHz, CD2Cl2, 298 K):
2c: (127 mg, 0.2 mmol, 20%). 1H NMR (400 MHz, d6- −133.5 (dd, 2F, JFF = 24, 9 Hz, o-F), −156.6 (t, 1F, JFF = 21 Hz,
benzene, 298 K): 8.01–7.95 (m, 4H, o-C PPh2), 7.03–7.00 (m, p-F), −162.8 (m, 2F, m-F).
7H, m-C and p-C PPh2 and o-Ph –CuCPh), 6.88 (t, 1H, 3JHH = 8
Synthesis of [(EtO)2P(O)CvCN3B(C6F5)2P(O)(OEt)]2 4. (Et2O)-
3
Hz, p-C –CuCPh), 6.78 (t, 2H, JHH = 8 Hz, m-C –CuCPh); 31P P(O)CuCP(O)(OEt)2 (298 mg,
1
mmol) was added to
NMR (163 MHz, d6-benzene, 298 K): −5.3 (m); 11B NMR (C6F5)2BN3 (1 mmol) in toluene (5 mL). The resulting solution
(128 MHz, d6-benzene, 298 K): 36.1 (s, br.); 19F NMR was stirred at room temperature for 2 days. The solvent was
(377 MHz, d6-benzene, 298 K): −133.9 (dd, 2F, JFF = 24, 9 Hz, removed in vacuo to give crude solid 3 (132 mg, 29%,
o-F), −155.4 (t, 1F, JFF = 30 Hz, p-F), −164.0 (m, 2F, m-F); 0.14 mmol) which could be recrystallised from a saturated
13C{1H} NMR (100 MHz, d6-benzene, 298 K): 147.4 (m, C6F5), toluene solution (45 mg, 9%, 0.05 mmol). Crystalline 3 proved
137.4 (m, C6F5), 132.6 (d, JPC = 3 Hz, Ph), 132.1 (d, JPC = 2 Hz, to be stable at room temperature under an atmosphere of
Ph), 131.3 (d, JPC = 12 Hz, Ph), 131.1 (s, Ph), 130.9 (d, 1JPC = 131 nitrogen for several months. 1H NMR (400 MHz, d8-toluene,
Hz, i-C PPh2), 129 (s, Ph), 128.9 (s, Ph), 128.6 (s, Ph), 119.0 (d, 298 K): 3.86 (m, 4H, –P(O)(OCH2CH3)2), 3.76 (m, 2H, >P(O)-
3
2JPC = 4 Hz, –CuCPh), 108.2 (d, 1JPC = 23 Hz, –CuCPh), signals CH2CH3), 0.94 (t, 6H, JHH = 7 Hz, –P(O)(OCH2CH3)2), 0.72 (t,
3
for boron-bound carbon atoms were not observed. Elem. Anal. 3H, JHH
=
7
Hz, >P(O)CH2CH3); 31P NMR (163 MHz,
calcd (%) for C30H15NPBF10: C 59.57; H 2.34; N 2.17; Found: C d6-benzene, 298 K): 7.9 (t, JPP = 6 Hz), −10.5 (m); 11B NMR
59.13, H 2.59; N 2.37. DART MS, m/z: 646.1 (calcd for [M + H]+: (128 MHz, d6-benzene, 298 K): 2.6 (s, br.), −0.5 (s, br.);
646.1), 303.1 (calcd for [PhCC(Ph)2P–NH3]+: 303.1), 302.1 (calcd 19F NMR (377 MHz, d6-benzene, 298 K): −135.4 (dd, 2F, J = 24,
3
for [PhCC(Ph)2PvNH2]+: 302.1).
9 Hz, o-F), −135.7 (dd, br., 2F, JFF = 23 Hz, JFF = 8 Hz, o-F),
3
4
2d: (262 mg, 0.35 mmol, 35%). 1H NMR (400 MHz, d6- −154.1 (t, 1F, 3JFF = 21 Hz, p-F), −157.0 (t, 1F, 3JFF = 21 Hz, p-F),
benzene, 298 K): 7.87 (m, 4H, o-H P1Ph2), 7.33 (m, 4H, m-H −162.6 (m, 2F, m-F), −164.3 (m, 2F, m-F).
P1Ph2), 6.96–6.91 (m, 12H, p-H P1Ph2 and P2Ph2); 31P{1H} NMR
X-ray crystallography
3
(163 MHz, d6-benzene, 298 K): −7.1 (d, JPP = 6 Hz, –P2(Ph)2–);
−33.8 (d, br., –P1Ph2); 11B NMR (128 MHz, d6-benzene, 298 K):
Crystals were coated in paratone oil and mounted in a cryo-
loop. Data were collected on Nonius Kappa CCD or Bruker
APEX2 X-ray diffractometers using graphite monochromated
Mo-Kα radiation (0.71073 Å). The temperature was maintained
at 150(2) K using an Oxford cryostream cooler for both initial
indexing and full data collection. Data were collected using
Bruker APEX-2 software and processed using SAINT and an
38.2 (s, br.); 19F NMR (377 MHz, d6-benzene, 298 K): −133.6
3
(dd, 2F, JFF = 24, 10 Hz, o-C6F5), −154.2 (t, 1F, JFF = 21 Hz,
p-C6F5), −162.7 (m, br., 2F, m-C6F5); 13C{1H} NMR (100 MHz,
2
d6-benzene, 298 K): 147.1 (m, JCF = 244 Hz, C6F5), 141.1 (m,
2
2JCF = 256 Hz, C6F5), 137.1 (m, JCF = 251 Hz, C6F5), 132.8 (dd,
JPC = 6, 2 Hz, Ph), 132.6 (d, JPC = 22 Hz, Ph), 132.4 (d, JPC
3 Hz, Ph), 131.0 (d, JPC = 12 Hz, Ph), 130.3 (d, JPC = 130 Hz,
Ph), 129.7 (s, Ph), 128.9 (d, JPC = 8 Hz, Ph), 128.7 (d, JPC
=
absorption correction applied using multi-scan31 within the
APEX-2 program.32 The structures of 1b, 1c, 1d, 2a, 2b and 2c
were solved by direct methods within the SHELXTL package33
whereas the structure of 3 was solved using Superflip34 within
Crystals.35 All structures were refined against F2 using the
SHELXTL package. For 1c several of the cyclohexyl rings were
found to be disordered; the disordered cyclohexyl groups were
refined over two positions with common Uiso for chemically
equivalent C atoms and restrained cyclohexyl ring geometries
(SAME) whereas ordered cyclohexyl groups were refined freely
with anisotropic C atoms. H atoms were added at calculated posi-
tions and refined with a riding model for all structures. Unit cell
parameters and refinement statistics are presented in the ESI.†
=
1
2
14 Hz, Ph), 110.2 (dd, JPC = 32 Hz, JPC = 14 Hz, –CuCPPh2),
1
2
98.3 (dd, JPC = 98 Hz, JPC = 5 Hz, –CuCPPh2), signals for
boron-bound carbon atoms were not observed. Elem. Anal.
calcd (%) for C38H20NPBF10: C 60.59; H 2.68; N 1.86; Found:
C 60.24, H 2.89; N 1.97. DART MS, m/z: 754.1 (calcd for
[M + H]+: 754.1), 411.1 (calcd for [(Ph)2(CCPPh2)P–NH3]+:
411.1), 410.1 (calcd for [(Ph)2(CCPPh2)PvNH2]+: 412.1).
Synthesis of Ph2P(CvCp-tol)N3BN(C6F5)2 3. Excess tri-
methylsilylazide (300 mg, 2.6 mmol) in toluene (2 mL) was
added to (C6F5)2BCl (380 mg, 1 mmol) in toluene (25 mL) and
the resulting mixture stirred for 2 h. Ph2PCCp-tol (300 mg,
1 mmol) in toluene (10 mL) was added and the solution was
stirred at 90 °C for 3 days. The majority of the solvent
was removed until ca. 2 mL remained and CD2Cl2 (2 mL) was
added to redissolve any precipitate. The solvent was left to
evaporate over 2 days affording colourless crystals of 3 (30 mg,
4%, 0.04 mmol) which were stable at room temperature under
an atmosphere of nitrogen for several weeks. DART MS, m/z:
Computational studies
Single point energy calculations on derivatives of 1 and 2 were
undertaken using the crystallographically determined geome-
try using the B3LYP functional36 and 6-31G*+ Pople basis set
within Jaguar37 and the Natural Bond Orbital analysis under-
taken using the NBO 5.0 code.38
1
703.1 (calcd for [M + H]+: 703.1). H NMR (400 MHz, CD2Cl2,
3
298 K): 7.79 (tm, JHH = 7.5 Hz, 2H, p-H PPh2), 7.73–7.67 (m,
Acknowledgements
3
4H, o-H PPh2), 7.62–7.56 (m, 4H, m-H PPh2), 7.39 (d, JHH
=
3
8.1 Hz, 2H, o-H p-tol), 7.11 (d, JHH = 7.9 Hz, 2H, m-H p-tol), NSERC of Canada is thanked for financial support. DWS is
2.33 (s, 3H, CH3), the N–H proton was not observed; 11B NMR grateful for the award of a Canada Research Chair.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8674–8683 | 8681

View Article Online
Paper
Dalton Transactions
T. V. Bennigsen-Mackiewicz, Z. Anorg. Allg. Chem., 2004,
630, 2641; (e) P. I. Paetzold, Chem. Ber., 1983, 116, 1531;
(f) S. Biswas, H. F. Bettinger and I. M. Oppel, Inorg. Chem.,
2010, 49, 449.
8 E. Merling, V. Lamm, S. J. Geib, E. Lacôte and D. P. Curran,
Org. Lett., 2012, 14, 2690.
Notes and references
1 (a) P. P. Power, Nature, 2011, 463, 171; (b) R. J. Less,
R. L. Melen and D. S. Wright, RSC Adv., 2012, 2, 2191;
(c) D. W. Stephan, Org. Biomol. Chem., 2012, 10, 5740;
(d) D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase,
J. J. Hastie, S. J. Geier, J. M. Farrell, C. C. Brown,
Z. M. Heiden, G. C. Welch and M. Ullrich, Inorg. Chem.,
9 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem.,
Int. Ed., 2001, 40, 2004.
2011, 50, 12338; (e) D. W. Stephan and G. Erker, 10 R. L. Melen and D. W. Stephan, Dalton Trans., 2013, 42,
Angew. Chem., Int. Ed., 2010, 49, 46; (f) D. W. Stephan, 4795.
Dalton Trans., 2009, 3129; (g) D. W. Stephan, Org. 11 H. Staudinger and J. Meyer, Helv. Chim. Acta, 1919, 2,
Biomol. Chem., 2008, 6, 1535. 635.
2 (a) G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 12 F. L. Lin, H. M. Hoyt, H. V. Halbeek, R. G. Bergman and
129, 1880; (b) P. A. Chase, G. C. Welch, T. Jurca and C. R. Bertozzi, J. Am. Chem. Soc., 2005, 127, 2686.
D. W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 8050; 13 W. Q. Tian and Y. A. Wang, J. Org. Chem., 2004, 69, 4299.
(c) G. C. Welch, R. R. S. Juan, J. D. Masuda and 14 P. I. Paetzold, P. P. Habereder, G. Maier and M. Sandner,
D. W. Stephan, Science, 2006, 314, 1124.
3 (a) M. J. Sgro, J. Dömer and D. W. Stephan, Chem. 15 A. Stute, L. Heletta, R. Fröhlich, C. G. Daniliuc, G. Kehr
Commun., 2012, 48, 7253; (b) R. C. Neu, G. Ménard and and G. Erker, Chem. Commun., 2012, 48, 11739.
D. W. Stephan, Dalton Trans., 2012, 41, 9016; 16 J. E. Leffler and R. D. Temple, J. Am. Chem. Soc., 1967, 89,
(c) F. Lavigne, E. Maerten, G. Alcaraz, V. Branchadell, 5235.
Fortschr. Chem. Forsch., 1967, 8, 437.
N. Saffon-Merceron and A. Baceiredo, Angew. Chem., Int. 17 C. A. Tolman, Chem. Rev., 1977, 77, 313.
Ed., 2012, 51, 2489; (d) L. J. Hounjet, C. B. Caputo and 18 D. G. Gorenstein, Prog. Nucl. Magn. Reson. Spectrosc., 1983,
D. W. Stephan, Angew. Chem., Int. Ed., 2012, 51, 4714;
(e) G. Ménard and D. W. Stephan, Angew. Chem., Int. 19 (a) N. Kano, K. Yanaizumi, X. Meng and T. Kawashima, Het-
16, 98.
Ed., 2011, 50, 8396; (f) G. Ménard and D. W. Stephan,
J. Am. Chem. Soc., 2010, 132, 1796; (g) A. Berkefeld,
W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
10660; (h) A. Sibaouih, P. Ryan, M. Leskelä, B. Rieger
and T. Repo, Appl. Catal., A, 2009, 365, 194;
eroat. Chem., 2012, 23, 429; (b) M. W. P. Bebbington,
S. Bontemps, G. Bouhadir and D. Bourissou, Angew. Chem.,
Int. Ed., 2007, 46, 3333; (c) S. Courtenay, D. Walsh,
S. Hawkeswood, P. R. Wei, A. K. Das and D. W. Stephan,
Inorg. Chem., 2007, 46, 3623.
(i) C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich, 20 (a) G. J. Bullen, J. Chem. Soc., Dalton Trans., 1973, 858;
S. Grimme, D. W. Stephan and G. Erker, Angew. Chem.,
Int. Ed., 2009, 48, 6643; ( j) A. E. Ashley,
(b) W. Clegg, S. J. Bryan and K. Wade, Acta Crystallogr., Sect.
C: Cryst. Struct. Commun., 1984, 40, 2082.
A. L. Thompson and D. O’Hare, Angew. Chem., Int. Ed., 21 (a) G. Brizius, K. Billingsley, M. D. Smith and
2009, 48, 9839.
U. H. F. Bunz, Org. Lett., 2003, 5, 3951; (b) U. H. F. Bunz
and V. Enkelmann, Chem.–Eur. J., 1999, 5, 263; (c) A. Hori,
S. Takatani, T. K. Miyamoto and M. Hasegawa, CrystEng-
Comm, 2009, 11, 567.
4 (a) E. Otten, R. C. Neu and D. W. Stephan, J. Am. Chem.
Soc., 2009, 131, 9918; (b) R. C. Neu, E. Otten and
D. W. Stephan, Angew. Chem., Int. Ed., 2009, 48, 9709;
(c) R. C. Neu, E. Otten, A. Lough and D. W. Stephan, Chem. 22 (a) C. R. Patrick and G. S. Prosser, Nature, 1960, 187, 1021;
Sci., 2012, 2, 170.
(b) G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller,
E. B. Lobkovsky and R. H. Grubbs, J. Am. Chem. Soc., 1998,
120, 3641; (c) D. G. Naae, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1979, 35, 2765; (d) C. Dai,
P. Nguyen, T. B. Marder, A. J. Scott, W. Clegg and C. Viney,
Chem. Commun., 1999, 2493; (e) C. Allen, D. A. Haynes,
C. M. Pask and J. M. Rawson, CrystEngComm, 2009, 11,
2048; (f) J. H. Williams, Acc. Chem. Res., 1993, 26, 593;
(g) A. P. Weat, S. Mecozzi and D. A. Dougherty, J. Phys. Org.
Chem., 1997, 10, 347.
5 (a) M. Sajid, A. Stute, A. J. P. Cardenas, B. J. Culotta,
J. A. M. Hepperle, T. H. Warren, B. Schirmer, S. Grimme,
A. Studer, C. G. Daniliuc, R. Fröhlich, J. L. Petersen,
G. Kehr and G. Erker, J. Am. Chem. Soc., 2012, 134, 10156;
(b) A. J. P. Cardenas, B. J. Culotta, T. H. Warren, S. Grimme,
A. Stute, R. Fröhlich, G. Kehr and G. Erker, Angew. Chem.,
Int. Ed., 2011, 50, 7567.
6 (a) W. Fraenk, T. M. Klapötke, B. Krumm and P. Mayer,
Chem. Commun., 2000, 667; (b) W. Fraenk, T. M. Klapötke,
B. Krumm, P. Mayer, H. Nöth, H. Piotrowski and M. Suter, 23 J. D. Stark and H. G. Wallace, Chemistry Data Book, John
J. Fluorine Chem., 2001, 112, 73. Murray publ., London, 1980.
7 (a) P. I. Paetzold, Z. Anorg. Allg. Chem., 1966, 345, 79; 24 (a) H. Bock and R. Dammel, J. Am. Chem. Soc., 1988, 110,
(b) H. U. Meier, P. I. Paetzold and E. Schroeder, Chem. Ber.,
1984, 117, 1954; (c) P. I. Paetzold, P. P. Habereder and
5261; (b) H. O. Spauschus and J. M. Scott, J. Am. Chem.
Soc., 1951, 73, 208.
R. Muellbauer, J. Organomet. Chem., 1967, 7, 45; 25 H. Quast, M. Ach, J. Balthasar, T. Hergenröther, D. Regnat,
(d) J. Münster, P. I. Paetzold, E. Schroeder, H. Schwan and
J. Lehmann and K. Banert, Helv. Chim. Acta, 2005, 88, 1589.
8682 | Dalton Trans., 2013, 42, 8674–8683
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
26 N. N. Greenwood and A. Earnshaw, Chemistry of the 32 Bruker AXS Inc., Madison, WI.
Elements, Pergamon Press, Oxford, 1984.
27 (a) C. E. Housecroft and A. G. Sharpe, Inorganic Chemistry,
33 G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystal-
logr., 2008, 64, 112.
Pearson, Harlow, UK, 2008; (b) A. G. Orpen and 34 L. Palatinus and G. Chapuis, J. Appl. Crystallogr., 2007, 40,
N. G. Connelly, J. Chem. Soc., Chem. Commun., 1985, 1310. 786.
28 D. J. Parks, W. E. Piers and G. P. A. Yap, Organometallics, 35 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout
1998, 17, 5492. and D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
29 A. D. Miller, S. A. Johnson, K. A. Tupper, J. L. McBee and 36 (a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098; (b) C. Lee,
T. D. Tilley, Organometallics, 2009, 28, 1252.
30 E. Louattani, J. Suades and R. Mathieu, J. Organomet.
Chem., 1991, 421, 335.
W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter, 1988,
37, 785.
37 Jaguar version 7.7, Schrodinger, LLC, New York, NY, 2010.
31 R. H. Blessing, Acta Crystallogr., Sect. A: Fundam. Crystal- 38 NBO 5.0, Theoretical Chemistry Institute, University of Wis-
logr., 1995, 51, 33.
consin, Madison, WI, 2001.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 8674–8683 | 8683