Even the normal is abnormal: N-heterocyclic carbene C2 binding to a phosphaalkene without breaking the P=C π-bond

Article abstract of DOI:10.1039/c5cc08181e

The reaction of MesP=CPh₂ with the least sterically demanding N-heterocyclic carbene (NHC = IMe) results in formation of the 'abnormal' (C⁴-substituted) 4-phosphino-NHC (1). In contrast, reaction with Me₂IMe gives the unpr

Full text of DOI:10.1039/c5cc08181e

ChemComm
View Article Online
View Journal
COMMUNICATION
Even the normal is abnormal: N-heterocyclic
2
carbene C binding to a phosphaalkene without
Cite this:DOI: 10.1039/c5cc08181e
breaking the PQC p-bond†
Received 2nd October 2015,
Accepted 16th November 2015
ab
b
b
b
Paresh Kumar Majhi, Keith C. F. Chow, Tom H. H. Hsieh, Eric G. Bowes,
a
b
a
b
Gregor Schnakenburg, Pierre Kennepohl,* Rainer Streubel* and Derek P. Gates*
DOI: 10.1039/c5cc08181e
www.rsc.org/chemcomm
The reaction of MesPQCPh
with the least sterically demanding
2
N-heterocyclic carbene (NHC = IMe) results in formation of the
4
‘
abnormal’ (C -substituted) 4-phosphino-NHC (1). In contrast, reaction
2
with Me
2
IMe gives the unprecedented ‘normal’ C adduct, Me
2
IMe -
P(Mes )Q CPh
(2). Particularly striking is the asymmetric and weak
2
bonding of the NHC to the PQC moiety in 2. DFT calculations indicate
that the PQC natural bond order in 2 (1.54) still reflects significant
Fig. 1 Structures and numbering scheme for normal (NHC) and abnormal
imidazol-2-ylidenes (aNHC).
p-character to the bond (cf. MesPQCPh
computational analysis suggests that p-delocalization into the
remote C-phenyl substituents is key to stabilizing the NHC adduct. s-block metallation,
: NBO = 1.98). Further
2
17,18
19
and borate formation. An elegant
method to induce aNHC reactivity involves temporarily blocking
2
In just a few decades, N-heterocyclic carbenes (NHCs), the most C with an electrophile to permit base-assisted functionalization
1
4
4
important of which are the imidazol-2-ylidenes, have risen from selectively at C , thereby affording NHCs with C substituents such
20–22
relative obscurity to becoming competitive with phosphines as as: Br, Cl, C(O)R, OTf, PPh
important ligands in organometallic chemistry and catalysis. The
carbene carbon at the C position in such an NHC (Fig. 1) is by far stituents of IMes might impede the formation of the ‘‘normal’’
the most reactive site (so-called ‘‘classical’’ or ‘‘normal’’ binding).
A growing number of NHCs have been observed to react through to the ‘‘abnormal’’ (C ) 4-phosphino-NHC. To evaluate this
the C position (so-called ‘‘mesoionic’’ or ‘‘abnormal’’ binding). To hypothesis, we sought to probe the reactivity of less sterically-
2
obtain abnormal N-heterocyclic carbenes (aNHCs), the C -position is demanding NHCs with the phosphaalkene MesPQCPh .
usually blocked and the C -position bound to a d- or f-block element.
2
, SiMe
3
.
2
It was proposed that the sterically demanding N-Mes sub-
2
3
2
(C ) product in the reaction with phosphaalkenes thus leading
4
23
4
4–8
2
4
Significantly, we have now isolated the ‘‘abnormal’’ product 1
For example, the isolation of the first free aNHC by Bertrand and and the ‘‘normal’’ adduct 2, both of which have been crystallo-
2
5
co-workers required phenyl moieties at both the C and C
graphically characterized (1 as its BH
3
complex, 1ÁBH
3
). Adduct 2 is
5
positions. In 2009, we reported the ‘‘abnormal’’ reaction of particularly remarkable in that the metrical data along with DFT
the unblocked 1,3-dimesityl-imidazol-2-ylidene (IMes) with the calculations reveal that the NHC binds to the phosphaalkene p* with
phosphalkene MesPQCPh2 to afford the first example of an a low degree of disruption of the PQC p-bond.
The direct function-
alization of unprotected NHCs with p-block elements at the C
4
9,10
NHC with a phosphine substituent at C .
1,3-Dimethyl-imidazol-2-ylidene (IMe) was generated in situ in
4
24
THF and treated with MesPQCPh
2
(1 equiv.) at À78 1C followed
1
1,12
13
position has grown to include: halogenation,
deuteration,
by warming to room temperature (Scheme 1). Analysis of an aliquot
14
15
16
31
1
and silylation, addition of S iQ N and BQN compounds, removed from the reaction mixture by P{ H} NMR spectroscopy
31
2
revealed that the signal for MesPQCPh (d P = 234 ppm) was not
a
b
present and a new signal at À37.2 ppm had appeared, assigned to
Institut f u¨ r Anorganische Chemie, Rheinische Friedrich-Wilhelms-Universit ¨a t Bonn,
Gerhard-Domagk-Straße 1, D-53121 Bonn, Germany. E-mail: r.streubel@uni-bonn.de 1. Importantly, the chemical shift of this new signal is comparable
Department of Chemistry, University of British Columbia, 2036 Main Mall,
to the abnormal product derived from IMes and MesPQCPh2
Vancouver, British Columbia, V6T 1Z1, Canada. E-mail: dgates@chem.ubc.ca
9
(
d = À37.3 ppm). The isolated product (yield 78%) was analyzed
†
Electronic supplementary information (ESI) available: Experimental proce-
1
3
1
8
by C{ H} NMR spectroscopy in THF-d to reveal a broad
resonance at d = 222.5 ppm, assigned to the carbene carbon
(C ) atom of 1; the JP,C coupling constant could not be resolved.
dures, characterization data, X-ray data, DFT procedures and results, selected
) and 1428161 (2). For crystallographic data in CIF
spectra. CCDC 1428162 (1ÁBH
3
2
3
or other electronic format see DOI: 10.1039/c5cc08181e
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun.

View Article Online
Communication
ChemComm
27
31
1
(
2 2 2
Me IMe) was treated with MesPQCPh in Et O. P{ H} NMR
spectroscopic analysis of the reaction mixture revealed that the
signal for MesPQCPh (d = 234) had been replaced by a broad
2
signal at d = 206.1 (w1/2 = 124 Hz) along with about 7% of
unidentified products. Interestingly, the chemical shift is highly
dependent on the solvent in which the reaction is conducted
(THF: d = 185.1, w1/2 = 608 Hz; toluene: d = 213.4, w1/2 = 122 Hz).
Given the similarity in chemical shift to that of MesPQCPh , we
2
suspected that the product, regardless of solvent, retains significant
PQC bond character. In addition, the phosphaalkene is released
from 2 upon the addition of boranes [BR = BPh , B(C F ) ] to yield a
3
3
6 5 3
mixture of MesPQCPh and Me IMeÁBR .
2
2
3
X-ray crystallography permitted the product described above
to be identified as the unprecedented NHC-phosphaalkene
adduct 2 (Fig. 3). This ‘‘normal’’ NHC adduct 2 features a short
P–C bond [P(1)–C(17) = 1.7420(19) Å] that is closer to that of a
Scheme 1 Synthesis of compounds 1 and 2.
2
8
In order to obtain X-ray crystallographic confirmation of 1, several double bond (e.g. for MesPQCPh [1.692(3) Å] ) than that of a
2
9
2
complexes 1ÁML [ML = BH , Rh(cod)Cl, Cr(CO) , Mo(CO) , W(CO) ] typical P–C single bond (1.85 Å). In addition, the carbon atom
n
n
2
3
5
5
5
31
were prepared. The C bound complexes were characterized by P, C(17) retains a planar geometry S+C(17) = 359.71, which is
1
13
1
25
2
H, C{ H} NMR spectroscopy and mass spectrometry (see ESI†). consistent with retention of its sp hybridization. The distortion of
In the case of 1ÁBH , single crystals suitable for X-ray crystallography the PQC bond upon binding of NHC is small with a 36.41 twisting
were obtained from a saturated THF solution at À20 1C. The of the C(1)–P(1)–C(10) plane with respect to the C(18)–C(17)–C(24)
3
3
0
molecular structure (Fig. 2) confirms the formulation as the plane; for comparison, the same angle in MesPQCPh is 0.91 or
2
31
4
-phosphinyl-substituted NHC borane complex 1ÁBH
3
. The 7.51. Particularly striking is the almost perpendicular position of
metrical parameters in 1ÁBH
3
do not differ significantly from the NHC donor with respect to the best plane of the PQC bond
2
6
those of IMeÁBH and, hence, shall not be discussed further. (102.61 is max. angle between NHC–P bond and PQC best plane).
3
Despite monitoring the abnormal reaction of IMe with In addition, the C(10)–P(1) single bond length [P(1)–C(10) =
31
MesPQCPh2 by P NMR spectroscopy at variable temperatures 1.8512(18) Å] is long. These observations stand in stark contrast
3
2
(
À80 to 25 1C), no P-containing intermediates have been observed. to the Me IPr adduct of Mes*NQPCl which displays significant
2
In contrast, Braunschweig and co-workers have detected a ‘‘normal’’ torsion of the C
2
–NQP–Cl unit and in the Me IPr adduct of
C -adduct as a short-lived intermediate in the abnormal reaction of tBuNQBtBu where CtBu–NQB–CtBu and C are coplanar. The
Mes*
2
16
2
16
IPr with the iminoborane, tBuBQNtBu. In an effort to see whether molecular structures of related NHC-complexes to EQE bonds
2
33
the formation of the normal C adduct was possible with phos- also exhibit significant distortions at the EQE bond. The above
4
5
phaalkenes, the C /C -blocked 1,3,4,5-tetramethyl-imidazol-2-ylidene
Fig. 2 Molecular structure of complex 1ÁBH
3
. Disordered solvent molecule
Fig. 3 Molecular structure of complex 2. Hydrogen atoms have been
omitted for clarity (50% probability level); side view of the reduced
structure (top left). Selected bond distances (Å) and angles (1): P(1)–C(10)
1.8512(18), P(1)–C(17) 1.7420(19), P(1)–C(1) 1.8416(17), N(1)–C(10) 1.354(2),
N(2)–C(10) 1.345(2), C(17)–C(18) 1.479(2); N(1)–C(10)–N(2) 106.14(15),
N(1)–C(10)–P(1) 121.94(14), N(2)–C(10)–P(1) 128.95(13), C(10)–P(1)–C(1)
100.98(7), C(10)–P(1)–C(17) 112.73(9), C(18)–C(17)–C(24) 118.57(16).
(
THF) and hydrogen atoms (except B–H and C(6)–H) have been omitted for
clarity (50% probability level). Selected bond distances (Å) and angles (1):
C(2)–B 1.609(4), C(2)–N(1) 1.343(3), C(2)–N(2) 1.357(3), C(1)–C(3) 1.352(3),
C(1)–P 1.817(3), C(6)–P 1.881(2), C(1)–N(1) 1.407(3), C(3)–N(2) 1.374(3); N(1)–
C(2)–N(2) 104.7(2), N(1)–C(2)–B 128.9(2), N(2)–C(2)–B 126.4(3), N(1)–C(1)–P
1
21.21(18).
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015

View Article Online
ChemComm
Communication
3
1
metrical data for 2 combined with the P chemical shift and the Table 1 Calculated NPA atomic and fragment charges for MesPQCPh
2
,
2
Me IMe, and the resultant adduct 2. Dq is the change in charge between 2
ready displacement of the NHC with electrophiles are all consistent
with the interpretation of the NHC as a weak s-donor in 2.
The fascinating metrical parameters in 2 led us to probe the
and the sum of the charges in the reactants
NPA atomic/fragment charges
nature of bonding in this compound and in MesPQCPh by
2
Mes
Me
2
IMe
P
CPh
2
Phcis
Phtrans
À
computational methods (DFT-B3LYP/TZVP). The optimized gas
phase geometry was in good agreement with the solid state
molecular structure (see ESI†) and natural bond orbital (NBO)
analysis suggests that both P(1) and C(17) retain significant p-bond
MesPQCPh
2
À0.30
—
—
0.66
—
0.90
0.24
À0.38
0.02
—
0.01
—
IMe
2
4
0.00
0.35
0.35
—
À0.32
À0.65
À0.26
À0.16
À0.13
Dq
À0.01
À0.18
À0.14
character. On first glance, the data is consistent with adduct
2
formation between the C lone pair of Me IMe and the PQC p*
2
Analysis by natural resonance theory (NRT) suggests that the
major contributors to the canonical forms of 2 (R = Ph) are the
simple adduct (2A) and the asymmetric allylic form (2B) (Fig. 5).
It is interesting that the structure 2B showing p-delocalization
to Phcis is considerably more important than that involving
Phtrans (2D). Importantly, this result is consistent with the
crystal structure data described above. Moreover, this analysis
confirms that significant PQC character remains even though
charge is highly delocalized. Overall, the results are consistent
with an interpretation of the bonding within 2 where the pPQC
bond is perturbed by the NHC donor but is not broken entirely
resulting in the unique geometry of the isolated adduct.
Herein, we have shown that steric bulk at the N-substituents
of NHCs is not required for abnormal products to be formed in
their reactions with phosphaalkenes. By contrast, we find that
orbital of MesPQCPh . Surprisingly, this adduct formation only
leads to a small increase in the p* population in 2 (by 0.08 e
2
À
compared to MesPQCPh
the NBO PQC bond order (from 1.98 to 1.54). The PQC p bond
polarizes substantially upon binding of Me IMe (Fig. 4); the
2
) and displays a concomitant decrease in
2
phosphorus natural hybrid orbital (NHO) shows increased 3d
character in the pPC bond leading to pd hybridization, and this
pd hybrid functions as the acceptor orbital for the Me IMe - P s
2
bond. Nevertheless, the actual contributions from the putative pd
hybrid NHOs in both the pP(1)–C(17) and the sP(1)–C(10) orbitals are
quite small (11% and 23%, respectively).
The calculated atomic charges obtained by natural population
analysis (NPA) are shown in Table 1 and the changes (Dq) reveal the
overall effect of adduct formation. Predictably, there is an electron
redistribution towards the carbon of the phosphaalkene moiety that
is accompanied by a concomitant decrease in charge at phos-
phorus. The significant redistribution of charge density from the
NHC to the remote phenyl substituents was unanticipated, parti-
cularly when noting the asymmetry in negative charge accumula-
tion between the two substituents (see Dq for Phcis vs. Phtrans in
Table 1). However, these observations are in agreement with the
different angles between the PQC bond plane and each phenyl
substituent in 2 (cis, 17.51; trans, 51.91); the metrical data contrasts
4,5
blocking the C positions of the NHC leads to the formation of
2
an unprecedented weakly bound C -adduct that retains signifi-
cant PQC double bond character. The formation of such
adducts is predicated on the ability for charge delocalization
in the substituents of the phosphaalkene and suggests that
modulation of the stability of the NHC adduct is strongly
dependent on the nature of the phosphaalkene.
DG and PK are grateful to the Natural Sciences and Engineer-
ing Research Council (NSERC) of Canada for financial support of
this work. RS thanks the Deutsche Forschungsgemeinschaft
34
the analogous angles within the free MesPQCPh2 (Phcis
4
Phtrans). This finding is consistent with asymmetric allylic
(
(
DFG) SFB 813 ‘‘Chemistry at Spin Centers’’ and the COST action
CM1302). RS and PKM are grateful to the DAAD PPP Program
p-delocalization across the CPh moiety. In fact, a comparison to
2
2 2
the hypothetical Me IMe adduct of MesPQCMe shows that the
for Canada. Computation work was enabled by support by
WestGrid (www.westgrid.ca) and Compute Canada Calcul
Canada (www.computecanada.ca). We are grateful to Mr. S. Serin
phenyl substituents are critical to stabilizing 2. Namely, the
dimethyl derivative exhibits a much longer P–CNHC bond (2.00 Å
vs. 1.86 Å) and this bond has a significantly lower bond dissociation
À1
energy (B45 kJ mol lower than that for 2).
Fig. 5 Generalized canonical forms illustrating the bonding within
2 2
MesPQCR (A, R = Ph or Me) and a NHC - MesPQCR (2A–D, R = Ph
Fig. 4 NBO representation of the NBO-derived pPC orbital in MesPQCPh
2
or Me). The percent contributions are obtained from an NRT analysis of the
NBO computational results (see ESI† for details).
À
À3
(left) and 2 (right). Orbital surfaces are plotted at 0.05 e
Å
using WebMO.
This journal is ©The Royal Society of Chemistry 2015
Chem. Commun.

View Article Online
Communication
ChemComm
for assistance in the preparation of Fig. 1 and 2 and determina- 19 S. Kronig, E. Theuergarten, C. G. Daniliuc, P. G. Jones and M. Tamm,
Angew. Chem., Int. Ed., 2012, 51, 3240.
0 D. Mendoza-Espinosa, B. Donnadieu and G. Bertrand, J. Am. Chem.
tion of some metrical parameters.
2
Soc., 2010, 132, 7264.
Notes and references
21 J. Ruiz and A. F. Mesa, Chem. – Eur. J., 2012, 18, 4485.
22 K. Schwedtmann, M. H. Holthausen, K.-O. Feldmann and J. J. Weigand,
1
2
3
4
5
6
A. J. Arduengo III, R. L. Harlow and M. Kline, J. Am. Chem. Soc., 1991,
13, 361.
M. N. Hopkinson, C. Richter, M. Schedler and F. Glorius, Nature, 24 A. Wacker, H. Pritzkow and W. Siebert, Eur. J. Inorg. Chem., 1998,
014, 510, 485. 843.
D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev., 25 Consistent with that observed for the Rh(I) and Ir(I) complexes of 4-
000, 100, 39. phosphinyl NHC derived from IMes (see ref. 23), two signals were
Angew. Chem., Int. Ed., 2013, 52, 14204.
23 J. I. Bates and D. P. Gates, Organometallics, 2012, 31, 4529.
1
2
2
31
S. Grundemann, A. Kovacevic, M. Albrecht, J. W. Faller Robert and
H. Crabtree, Chem. Commun., 2001, 2274.
E. Aldeco-Perez, A. J. Rosenthal, B. Donnadieu, P. Parameswaran,
G. Frenking and G. Bertrand, Science, 2009, 326, 556.
observed in the P NMR spectra of complexes 1ÁML
n n 3
[ML = BH ,
Rh(cod)Cl, Cr(CO) , Mo(CO) , W(CO) ]. Although the origin of these
5
5
5
signals is not known, we tentatively attribute these signals to
isomers.
O. Schuster, L. Yang, H. G. Raubenheimer and M. Albrecht, Chem. 26 P. Bissinger, H. Braunschweig, T. Kupfer and K. Radacki, Organo-
Rev., 2009, 109, 3445.
metallics, 2010, 29, 3987.
7
8
9
P. L. Arnold and S. Pearson, Coord. Chem. Rev., 2007, 251, 596.
J. B. Waters and J. M. Goicoechea, Coord. Chem. Rev., 2015, 293, 80. 28 T. A. van der Knaap, T. C. Klebach, F. Visser, F. Bickelhaupt, P. Ros,
27 N. Kuhn and T. Kratz, Synthesis, 1993, 561.
J. I. Bates, P. Kennepohl and D. P. Gates, Angew. Chem., Int. Ed.,
009, 48, 9844.
0 NHCs bearing an anionic PR-substituent at C have subsequently
E. J. Baerends, C. H. Stam and M. Konijn, Tetrahedron, 1984, 40, 765.
29 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen and
R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
2
4
1
been reported: P. K. Majhi, G. Schnakenburg, Z. Kelemen, L. Nyulaszi, 30 T. A. Vanderknaap, T. C. Klebach, F. Visser, F. Bickelhaupt, P. Ros,
D. P. Gates and R. Streubel, Angew. Chem., Int. Ed., 2013, 52, 10080. E. J. Baerends, C. H. Stam and M. Konijn, Tetrahedron, 1984, 40, 765.
1 A. J. Arduengo III, F. Davidson, H. V. R. Dias, J. R. Goerlich, 31 O. Mundt, G. Becker, W. Uhl, W. Massa and M. Birkhahn, Z. Anorg.
1
D. Khasnis, W. J. Marshall and T. K. Prakasha, J. Am. Chem. Soc.,
997, 119, 12742.
2 M. L. Cole, C. Jones and P. C. Junk, New J. Chem., 2002, 26, 1296.
3 M. K. Denk and J. M. Rodezno, J. Organomet. Chem., 2001, 617–618, 737.
4 H. Cui, Y. Shao, X. Li, L. Kong and C. Cui, Organometallics, 2009, 33 For recent examples, see: (a) D. Geiss, M. I. Arz, M. Strassmann,
Allg. Chem., 1986, 541, 319.
1
32 N. Burford, T. S. Cameron, D. J. LeBlanc, A. D. Phillips, T. E.
Concolino, K. C. Lam and A. L. Rheingold, J. Am. Chem. Soc.,
2000, 122, 5413.
1
1
1
2
8, 5191.
G. Schnakenburg and A. C. Filippou, Angew. Chem., Int. Ed., 2015,
54, 2739; (b) S. Roy, P. Stollberg, R. Herbst-Irmer, D. Stalke,
D. M. Andrada, G. Frenking and H. W. Roesky, J. Am. Chem. Soc.,
2015, 137, 150; (c) F. Dahcheh, D. W. Stephan and G. Bertrand,
Chem. – Eur. J., 2015, 21, 199; (d) F. Dahcheh, D. Martin, D. W.
Stephan and G. Bertrand, Angew. Chem., Int. Ed., 2014, 53, 13159;
(e) A. Jana, V. Huch and D. Scheschkewitz, Angew. Chem., Int. Ed.,
2013, 52, 12179.
1
1
1
1
5 R. S. Ghadwal, H. W. Roesky, M. Granitzka and D. Stalke, J. Am.
Chem. Soc., 2010, 132, 10018.
6 H. Braunschweig, W. C. Ewing, K. Geetharani and M. Schafer,
Angew. Chem., Int. Ed., 2015, 54, 1662.
7 Y. Wang, Y. Xie, M. Y. Abraham, P. Wei, H. F. Schaefer, P. v. R. Schleyer
and G. H. Robinson, J. Am. Chem. Soc., 2010, 132, 14370.
8 D. R. Armstrong, S. E. Baillie, V. L. Blair, N. G. Chabloz, J. Diez,
J. Garcia-Alvarez, A. R. Kennedy, S. D. Robertson and E. Hevia, Chem. 34 M. Yam, J. H. Chong, C.-W. Tsang, B. O. Patrick, A. E. Lam and
Sci., 2013, 4, 4259. D. P. Gates, Inorg. Chem., 2006, 45, 5225.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2015