4-Phosphino-substituted N-heterocyclic carbenes (NHCs) from the abnormal reaction of NHCs with phosphaalkenes

Article abstract of DOI:10.1021/om3003174

The activation of the P=C bond of phosphaalkenes with N-heterocyclic carbenes (NHCs) offers a convenient means to introduce new functionality at the 4-position of an NHC. Treatment of MesP=CRR′ (2a: R = R′ = Ph; 2b: R = Ph, R′ = 2-C₅H₄N; 2c: R = R′ = 4-C ₆H₄F) with 1,3-dimesitylimidazol-2-ylidene [:C(NMesCH)₂, IMes, Mes = 2,4,6-Me₃C₆H ₂] affords 4-phosphino-2-carbenes:C(NMes) ₂CHC(PMesCHRR′)NMes [1a: R = R′ = Ph; 1b: R = Ph, R′ = 2-C₅H₄N; 1c: R = R′ = 4-C ₆H₄F]. Significantly, these functional NHCs retain the parent carbene functionality and, in addition, contain a phosphine moiety. The preparation and spectroscopic characterization of the complexes [(1a)M(cod)Cl] (4) and cis-[(1a)M(CO)₂Cl] (5) (M = Ir, Rh) are reported. The average CO stretching frequencies (νCOav) for 5_M=Ir, 5 _M=Rh, and authentic samples of cis-[(IMes)M(CO)₂Cl] (M = Ir, Rh) are presented as a means to evaluate the donor properties of 4-phosphino-2-carbene 1a. The average CO stretching frequency is lower energy for 5_M=Ir than for cis-[(IMes)Ir(CO)₂Cl] (ΔνCO_av = -2 cm^-1), whereas the opposite is observed between 5_M=Rh and cis-[(IMes)Ir(CO)₂Cl] (ΔνCO_av = 2 cm^-1). The molecular structures are reported for 1a, 1b, 1c, 4_M=Ir, and 5_M=Ir.

Full text of DOI:10.1021/om3003174

Article
pubs.acs.org/Organometallics
4-Phosphino-Substituted N-Heterocyclic Carbenes (NHCs) from the
Abnormal Reaction of NHCs with Phosphaalkenes
Joshua I. Bates and Derek P. Gates*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: The activation of the PC bond of phos-
phaalkenes with N-heterocyclic carbenes (NHCs) offers a
convenient means to introduce new functionality at the 4-
position of an NHC. Treatment of MesPCRR′ (2a: R = R′ =
Ph; 2b: R = Ph, R′ = 2-C5H4N; 2c: R = R′ = 4-C6H4F) with
1,3-dimesitylimidazol-2-ylidene [:C(NMesCH)2, IMes, Mes =
2,4,6-Me3C6H2] affords 4-phosphino-2-carbenes :C(NMes)2CHC(PMesCHRR′)NMes [1a: R = R′ = Ph; 1b: R = Ph, R′ = 2-
C5H4N; 1c: R = R′ = 4-C6H4F]. Significantly, these functional NHCs retain the parent carbene functionality and, in addition,
contain a phosphine moiety. The preparation and spectroscopic characterization of the complexes [(1a)M(cod)Cl] (4) and cis-
av
[(1a)M(CO)2Cl] (5) (M = Ir, Rh) are reported. The average CO stretching frequencies (ν ) for 5M=Ir, 5M=Rh, and authentic
CO
̅
samples of cis-[(IMes)M(CO)2Cl] (M = Ir, Rh) are presented as a means to evaluate the donor properties of 4-phosphino-2-
carbene 1a. The average CO stretching frequency is lower energy for 5M=Ir than for cis-[(IMes)Ir(CO)2Cl] (Δν = −2 cm−1),
av
CO
̅
av
whereas the opposite is observed between 5M=Rh and cis-[(IMes)Ir(CO)2Cl] (Δν = 2 cm−1). The molecular structures are
CO
̅
reported for 1a, 1b, 1c, 4M=Ir, and 5M=Ir
.
Chart 1. First aNHC Metal Complex (A) and the First
Isolable Metal-Free aNHC (B)
INTRODUCTION
■
The development of isolable neutral divalent carbon species
(carbenes) is an area of considerable current interest.1 Perhaps
the most widely studied are the N-heterocyclic carbenes
(NHCs) in which the divalent carbon moieties are flanked by
one or two π-donor nitrogen atoms within a five-membered
heterocycle. Since their first discovery,2 there has been rapid
growth in NHC chemistry due to their fundamental importance
and their attractive properties as ligands for metal-catalyzed
organic synthesis. Recently, NHCs have been having
considerable impact in the areas of organocatalysis3,4 and in
the field of p-block chemistry, particularly to stabilize unusual
low-coordinate species.5,6
facilitated by employing blocking phenyl moieties in the 2- and
5-positions.10
In 2009, we discovered an unusual and unexpected abnormal
reaction of an NHC. Specifically, the NHC 1,3-dimesitylimi-
dazol-2-ylidene (IMes) reacts with the phosphaalkene MesP
CPh2 to afford the bifunctional 4-phosphino-2-carbene (IMesP,
1a).11 To our knowledge, this compound was the first example
of a 4-phosphino-substituted NHC.
One general feature of NHCs is their tendency to bind
electrophiles at the most reactive site, namely, the 2-position of
the NHC ring.7 Therefore, it was particularly striking when the
structure of an Ir complex in which the NHC ligand was bound
the “wrong way” was reported (A in Chart 1).8 Since that
remarkable discovery, the coordination chemistry of these so-
called abnormal NHCs (aNHCs), where the carbene moiety
binds at the 4- or 5-position, has been reasonably well
established for d- and f-block elements.9 It should be noted
that, in many instances, the 2-position must be blocked or
bound to a metal to induce abnormal reactivity for NHCs. For
example, the isolation of the first metal-free aNHC (B) was
The direct functionalization of an NHC with retention of the
carbene moiety, although rarely observed, represents a powerful
and convenient methodology to functionalize the 4-position of
an NHC. Examples of 4-functionalized NHCs derived from the
Received: April 18, 2012
Published: May 31, 2012
© 2012 American Chemical Society
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Article
direct reaction of a free NHC are shown in Chart 2 and include
longer present and was replaced by a singlet resonance at −37.3
ppm. Remarkably, analysis of recrystallized product in C6D6
using 13C{1H} NMR spectroscopy suggested that the carbene
moiety was still present in the product [δ = 220.3 (d), 3JPC = 5
Hz, cf. IMes: δ = 220.3].2 The identity of the product was
confirmed by X-ray crystallography as the remarkable 4-
phosphino-substituted NHC 1a (Figure 1).
the halogenation of NHCs (C),12−15 the deuteration of ItBu
Chart 2. Examples of NHCs Bearing Functional Groups at
the C4 and/or C5 Positions That Are Derived from the
Abnormal Reaction of an Unprotected NHC
Figure 1. Molecular structure of 1a·1/2C6H6 (thermal ellipsoids set at
50% probability). The solvate and all hydrogen atoms except H(1) and
H(2) are omitted for clarity.
(D),16 the silylation of ItBu (E),17 the generation of F and G
from IDipp,18,19 the silylation of IDipp (H) (Dipp = 2,6-di-iso-
propylphenyl),20 the lithium-functionalized NHC (I),21 and the
synthesis of the anionic NHC (J).22 Notably, F and G represent
further examples of 4-phosphino-NHCs. These reactions are of
particular importance as they open the door to optimization
studies of catalytic activity by fine-tuning the 4- and 5-positions
of the NHC backbone.
Herein, we report the preparation, characterization, and
donor−acceptor properties of several novel 4-phosphino-
NHCs using the abnormal reaction of an NHC and a
phosphaalkene. The donor properties of these substituted
NHCs were investigated by preparing rhodium(I)- and
iridium(I)−carbonyl complexes.
The scope of this fascinating reaction of IMes was explored
by varying the phosphaalkene employed. In particular, IMes
was treated with the pyridyl-substituted phosphaalkene 2b in
THF at 70 °C. After several hours, the 31P NMR spectrum of
the reaction mixture revealed that the signal assigned to 2b [δ =
260 (E), 242 (Z)] had been replaced by two new signals
assigned to the diastereomers of 1b (δ = −33.7, −35.3 in 1:1
ratio). The analogous reaction of IMes with phosphaalkene 2c
similarly afforded 4-phosphino NHC 1c (δ = −37.4). In stark
contrast to the reaction of IMes with phosphaalkenes 2a or 2b,
which require elevated temperatures (70 °C), the reaction with
the electrophilic phosphaalkene 2c occurs at ambient temper-
ature. Both 1b and 1c were characterized by X-ray
crystallography, and the molecular structures are shown in
Figures 2 and 3. Attempts to detect the 13C{1H} NMR signal
assigned to the carbene moiety in 1b were unsuccessful due to
the low solubility of 1b combined with the long relaxation time
of the carbene moiety. The 13C{1H} NMR spectrum of 1c in
C6D6 revealed a broad resonance at 220.4 ppm from which the
3JPC coupling constant could not be ascertained.
RESULTS AND DISCUSSION
■
Abnormal Reactions of NHCs with Phosphaalkenes.
When we embarked on the present studies, phosphaalkenes had
been observed to react with fleeting electrophilic carbenes to
afford phosphiranes, neutral PC2 heterocycles.23 The reaction
of a phosphaalkene with an NHC had not been described.24 It
was known that NHCs can add to multiply bonded phosphorus
compounds, such as phosphaalkynes and iminophosphines, to
afford zwitterionic species.25 In the case of phosphaalkynes,
subsequent cyclizations are often observed. Our interest in the
reactions of PC bonds with initiators for polymerization26
provided the initial inspiration for the present investigations.
We imagined that the strongly donating NHCs might lead to
novel zwitterionic polymerizations or to novel heterocycles
analogous to those observed in our study of the reactions of
PC bonds with in situ-generated phosphenium (R2P+) ions,
which are isovalent to carbenes.27
With this hypothesis in mind, a THF solution of IMes was
treated with phosphaalkene 2a (1 equiv) in THF. Although no
reaction was observed at room temperature, when the reaction
mixture was heated to 70 °C for several hours, its 31P NMR
spectrum revealed that the signal for 2a (δ = 234) was no
The transformation of one NHC into another NHC is
surprising because the carbene moiety is by far the most
reactive site in the molecule. Because of this high reactivity, the
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Scheme 1. Steric Interaction between the Bulky IMes Donor
and the Bulky 2a Acceptor May Be Imagined as a Frustrated
Lewis Pair Leading to Donor−Acceptor Formation through
the Less Hindered Abnormal IMes
for 3 weeks and periodically monitoring the reaction progress
using 31P NMR spectroscopy.
We postulate that the frustrated NHC → 2a Lewis pair is
alleviated by invoking the abnormal NHC (aIMes or aItBu) as
a key intermediate in the formation of 1a−1c and 3a, as
depicted in Scheme 1. In addition to being a stronger
nucleophile, aIMes is significantly less hindered than IMes.
Thus, assuming that traces of aIMes may be present in
solutions of IMes (ΔEIMes→aIMes = 51.7 kJ/mol),11 it is plausible
that aIMes nucleophilically adds to the PC bond of 2a. If
indeed this hypothesis were correct, the subsequent proton
transfer from C2 to the −CPh2 moiety would afford 1a.
Electronic Properties of IMesP as a Ligand. It is well
established that the σ-donating properties of IMesX (X = Cl,
Br) bearing halides at the C4 and C5 positions are considerably
lower than those of IMes itself.12 Consequently, it would be
desirable to ascertain the impact of the 4-phosphino
functionality in 1a−1c on the donor−acceptor properties of
this species. The most widely used method to assess the donor
strength of carbene ligands involves comparing the stretching
Figure 2. Molecular structure of 1b·1/2C6H6 (thermal ellipsoids set at
50% probability). The solvate and all hydrogen atoms except H(1) and
H(2) are omitted for clarity.
frequencies of the carbonyl ligands (νCO) in their respective
̅
metal carbonyl complexes.34 Although numerous metal systems
have been utilized, and consequently different donor-strength
scales established, the most common are those based on
rhodium(I) and iridium(I) complexes of the type cis-[LM-
(CO)2Cl] (M = Rh, Ir).34−37 Therefore, 1a was treated with
[(cod)MCl]2 to afford complexes 4M=Rh and 4M=Ir, which, upon
treatment with CO, were converted to metal carbonyls 5M=Rh
and 5M=Ir. Complexes 4 and 5 were fully characterized
spectroscopically.
Figure 3. Molecular structure of 1c·1/2THF (thermal ellipsoids set at
50% probability). The solvate and all hydrogen atoms except H(1) and
H(2) are omitted for clarity.
first step in analogous reactions is believed to involve
temporarily blocking the C2 site of IMes through the formation
of a donor−acceptor adduct with one of the reagents. This
binding at C2 leads to higher acidity for the C4-H proton and
thus favors its subsequent deprotonation. This pathway has
been proposed for the selective chlorination of both C4 and C5
(C)12 and, more recently, for the selective substitution of either
C4 or C5 to afford F, G,10,18 or H.20 In contrast, our
preliminary calculations suggested that forming a zwitterionic
IMes → 2a donor−acceptor adduct is unfavorable due to the
steric influence of the two N substituents of IMes and the bulky
phosphaalkene.11 Essentially, the interaction of IMes and 2a
can be envisaged as a frustrated Lewis pair28 between the NHC
nucleophile and the phosphaalkene electrophile (Scheme 1).
The concept of frustrated Lewis pairs as a plausible rationale for
the observed abnormal reactions of NHCs with electron pair
acceptors has been considered recently for reactions involving
bulky NHCs and B(C6F5)3.22,29−32 Moreover, reaction of ItBu
with hindered phosphinidene complexes was proposed to
proceed through aItBu.33 Interestingly, treating ItBu with
phosphaalkene 2a in THF resulted in ca. 50% conversion to 3a
The 31P and 13C{1H} NMR spectroscopic data confirm that
1a binds though the carbene rather than the phosphine moiety.
Specifically, the 31P chemical shifts for the complexes are very
similar to those of the free NHC 1a. Furthermore, the 13C{1H}
NMR signals assigned to the carbene moiety are significantly
upfield for the complexes compared to those for free 1a. A
mixture of isomers (∼2:1 ratio) is observed in solution for each
of the complexes 4M=Rh and 4M=Ir, analogous to that observed
for [(IMes)Rh(cod)Cl].35 This is attributed to restricted
rotation about the C−M and C−P bonds. For complexes
31
(δ = −32.3) despite heating the reaction mixture to 100 °C
P
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Article
4M=Ir and 5M=Ir, the proposed carbene binding to the iridium(I)
center was confirmed by X-ray crystallographic analysis
(Figures 4 and 5) (vide infra).
Table 1. Summary of CO Stretching Frequencies for
Selected Group 9 Carbonyl Complexes of Selected NHCs
av
complex
5M=Rh
νCO (cm−1
)
νCO (cm−1
)
ref
̅
̅
ATR
2071, 1983
2066, 1984
2081, 1996
2056, 1967
2055, 1971
2066, 1980
2027
2025
2039
2011
2013
2023
this work
this work
35
IMesRh(CO)2Cl
IMesRh(CO)2Cl
5M=Ir
ATR
CH2Cl2
ATR
this work
this work
36
IMesIr(CO)2Cl
IMesIr(CO)2Cl
ATR
CH2Cl2
asymmetric CO stretching (νaCsOym), whereas the higher-energy
̅
frequencies (ca. 2070 cm−1) are assigned to the symmetric CO
stretching (νsCyOm).36 Typically, the average of both asymmetric
̅
av
and symmetric values (ν ) are used to evaluate the σ-donating
CO
̅
properties of a ligand (L) in cis-[LIr(CO)2Cl], although factors,
such as size of the ligand, also affect ν
34,37,38
av
CO
̅
av
The average CO stretching frequency for 5M=Ir (ν = 2011
CO
̅
cm−1) is weaker than that for [(IMes)Ir(CO)2Cl] (ν = 2013
av
CO
̅
cm−1) and suggests that IMesP 1a is a slightly stronger donor
than IMes. In contrast, comparison of the average values for
av
av
5M=Rh (ν = 2027 cm−1) and cis-[(IMes)Rh(CO)2Cl] (ν
=
CO
̅
CO
̅
2025 cm−1) suggests the opposite σ-donating properties.
Further illustrating the subtle differences in donor properties
asym
between IMesP and IMes, the ν
for cis-[(IMesP)M-
CO
̅
(CO)2Cl] (5M=Rh and 5M=Ir) is lower energy than that for cis-
[(IMes)M(CO)2Cl] [ΔνaCsOym: −1 cm−1 (M = Rh), −4 cm−1 (M
̅
Figure 4. Molecular structure of 4M=Ir·3/2CH2Cl2 (thermal ellipsoids
set at 50% probability). The solvate atoms and all hydrogen atoms are
omitted for clarity.
= Ir)], whereas ν shows the opposite trend [ΔνsCyOm: 5 cm−1
sym
CO
̅
̅
(M = Rh), 1 cm−1 (M = Ir)]. For comparison, the Δν
,
asym
CO
̅
ΔνsCyOm, and Δν values for complexes of backbone function-
av
CO
̅
̅
alized 4,5-dichlorocarbenes are ca. 4 cm−1 higher in energy
when compared with their respective nonchlorinated analogues,
reflecting the poorer donor properties of the former.12 To place
these differences in IR stretching frequencies into context, it
must be noted that NHC complexes tend to exhibit smaller
differences in their CO stretching frequencies compared with
their phosphine counterparts (e.g., cis-[(IMes)Ir(CO)2Cl] vs
cis-[(ItBu)Ir(CO)2Cl]: ΔνCO = 1 cm−1, whereas cis-[(PPh3)Ir-
̅
(CO)2Cl] vs cis-[(PCy3)Ir(CO)2Cl]: ΔνCO = 15 cm−1).36 In
̅
conclusion, the observed trends in CO stretching frequencies
for complexes IMesP suggest that the good σ-donating
properties of IMes are retained upon functionalization with a
phosphine moiety in the C4 position; however, the subtle
differences likely reflect the steric influence of IMesP upon the
metal carbonyl systems.
Molecular Structures and Metrical Parameters of
IMesP. Details of the crystal structure determinations under-
taken in the present study are given in Table 2, and important
metrical parameters for the new NHCs and their complexes are
given in Table 3. Only subtle perturbations in the metrical
parameters are observed between the 4-phosphino-substituted
NHCs (1a−1c) and IMes itself. For example, the endocyclic
angles at C(3) in 1a−1c [C(4)−C(3)−C(2): avg = 104.3(4)°]
are smaller than those in IMes (106.5(3)°).2 Furthermore, the
bond lengths to C(3) are slightly elongated with respect to
those in IMes. Presumably, these observations reflect the steric
pressure induced by the bulky P substituent at C(3).
For the most part, the bond lengths and angles within the
C3N2 ring do not change considerably upon coordination to
iridium(I). The C(2)−Ir bond length in 4M=Ir is similar to that
in [IMesIr(cod)Cl]36 [2.055(8) Å vs 2.052(7) Å, respectively].
Likewise, similar C−Ir bond lengths are observed in 5M=Ir and
[IMesIr(CO)2Cl]36 [2.071(2) Å vs 2.08(2) Å, respectively].
Figure 5. Molecular structure of 5M=Ir·C7H8 (thermal ellipsoids set at
50% probability). The solvate atoms and all hydrogen atoms are
omitted for clarity.
The ATR-IR (attenuated total reflectance) spectra of
carbonyl complexes 5M=Rh and 5M=Ir were collected and
compared to those of authentic samples of cis-[(IMes)Rh-
(CO)2Cl]35 and cis-[(IMes)Ir(CO)2Cl], respectively.36 The
results are summarized in Table 1. In such complexes, the
lower-energy frequencies (ca. 2000 cm−1) are assigned to the
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Table 2. Crystallographic Data for Compounds 1a, 1b, 1c, 4M=Ir, and 5M=Ir
1a·0.5C6H6
C46H48N2P
1b·0.5C6H6
1c·0.5C4H8O
4M=Ir·1.5CH2Cl2
5M=Ir·C7H8
formula
fw
C45H47N3P
660.83
monoclinic
C2/c
C45H47F2O0.5N2P
692.82
monoclinic
C2/c
C105H120Cl8Ir2N4P2
2167.99
C52H53ClIrN2O2P
996.58
orthorhombic
Pbcn
659.83
monoclinic
C2/c
cryst syst
space group
Z
triclinic
P1
̅
8
8
8
2
8
a (Å)
30.592(2)
15.388(1)
18.693(1)
90
30.427(4)
14.980(2)
18.825(2)
90
28.942(2)
15.805(1)
19.737(1)
90
15.941(1)
16.121(1)
20.855(2)
89.941(4)
68.341(5)
76.550(4)
4823.0(6)
100
29.159(1)
18.337(1)
17.068(1)
90
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
117.843(2)
90
117.373(5)
90
118.977(2)
90
90
90
7781.0(7)
173
7619.3(16)
173
7898.3(9)
173
9126.2(5)
100
T (K)
μ(Mo Kα) (cm−1
cryst size (mm3)
)
1.04
1.07
1.13
30.61
30.61
0.5 × 0.35 × 0.2
1.127
0.7 × 0.4 × 0.1
1.152
0.5 × 0.4 × 0.1
1.165
0.25 × 0.2 × 0.1
1.493
0.25 × 0.15 × 0.1
1.451
dcalcd (Mg m−3
)
2θ(max) (deg)
no. reflns
55.8
56
55.8
56.4
55.8
43 060
9259
34 727
9162
44 723
9340
116 130
36 890
67 757
10 883
0.029
no. unique reflns
R(int)
0.0492
20.12
0.1151
19.91
0.1000
19.75
0.0482
refln/param ratio
33.26
19.72
a
R1 [I > 2σ(I)]
0.0506
0.1401
1.016
0.0673
0.1673
0.972
0.0644
0.1561
0.972
0.0468
0.0234
0.0511
1.011
b
wR2 (all data)
0.1329
GOF
1.076
CCDC No.
748235
869364
869365
869368
869369
a
b
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 (F2 [all data]) = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2
.
a
Table 3. Selected Metrical Parameters for IMes and the New Compounds Prepared in This Study
a
IMes
1a
1b
1c
4M=Ir
5M=Ir
bond lengths (Å)
C(2)−N(1)
C(2)−N(2)
N(2)−C(3)
C(3)−C(4)
C(4)−N(1)
M(1)−C(2)
M(1)−Cl(1)
bond angles (deg)
C2−N1−C4
N1−C2−N2
C2−N2−C3
N2−C3−C4
N1−C4−C3
N1−C2−M1
N2−C2−M1
C2−M1−Cl1
1.365(4)
1.371(4)
1.378(4)
1.331(5)
1.381(4)
1.363(2)
1.366(3)
1.406(2)
1.349(3)
1.388(2)
1.367(3)
1.365(3)
1.403(3)
1.352(4)
1.390(3)
1.359(4)
1.368(3)
1.402(4)
1.352(4)
1.390(3)
1.36(1)
1.36(1)
1.41(1)
1.34(1)
1.39(1)
2.055(8)
2.371(3)
1.356(3)
1.361(3)
1.411(3)
1.354(3)
1.386(3)
2.071(2)
2.351(1)
112.8(3)
113.1(2)
113.1(2)
112.8(2)
111.2(7)
111.2(2)
104.3(2)
111.7(2)
104.9(2)
107.9(2)
126.6(2)
129.0(2)
90.9(2)
101.4(2)
112.8(3)
106.5(3)
106.5(3)
101.2(2)
114.1(2)
104.4(2)
107.2(2)
101.2(2)
114.2(2)
104.2(3)
107.4(3)
101.3(2)
114.2(2)
104.3(2)
107.4(3)
103.7(7)
112.0(7)
105.1(7)
108.1(7)
125.2(6)
131.1(6)
93.3(3)
a
See ref 2.
For the complexes, the N−C−N angles increase slightly and
the C−N−C angles contract slightly when compared with
those in 1a. Interestingly, a common feature of all the
complexes is the asymmetric binding of 1a to Ir(I). This is
evident upon examining the N−C−Ir bond angles, which are
larger on the side of the NHC closest to the 4-phosphino
substituent [ΔN−C−M: 5.9(9)° (4M=Ir), 2.4(3)° (5M=Ir)]. This
may not solely be a consequence of the presence of the bulky 4-
phosphino moiety since [(IMes)Ir(cod)Cl] displays a similar,
albeit smaller, asymmetrical distortion [ΔN−C−Ir: 2.9(8)°].36
Nevertheless, it is possible that these distortions are factors in
the unusual trends in CO stretching frequencies described
above.
SUMMARY
■
We have demonstrated that hindered phosphaalkenes will react
with bulky N-heterocyclic carbenes, such as IMes and ItBu, to
afford the corresponding NHC with a phosphine substituent in
the 4-position. We postulate that the reaction proceeds via the
coordination of an abnormal NHC intermediate to the
phosphaalkene, which is favored for these bulky substrates
since the donor−acceptor adduct of the normal NHC and
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3H), 1.50 (s, 3H). MS (EI, 70 eV): 622, 621 [3, 7; M+]; 454, 453 [36,
100; M+ − CHPh(C5H4N)]; 169, 168 [12, 60; CHPh(C5H4N)+].
Synthesis of 1c. To a solution of IMes (304 mg, 1 mmol) in THF
(2 mL) was added a solution of 2c (352 mg, 1 mmol) in THF (2 mL).
The mixture was stirred at room temperature overnight. An aliquot
was removed, and 31P NMR spectroscopy indicated quantitative
conversion of the phosphaalkene (δ = 234) to a new compound (δ =
−36.7). Slow evaporation of the solvent afforded crystals suitable for
X-ray crystallography. Yield (1c·1/2THF): 560 mg (81%).
phosphaalkene results in a frustrated Lewis pair. Significantly,
this unexpected reaction opens up a convenient route to highly
functionalized NHCs. The 4-phosphino-NHCs reported herein
have been utilized as ligands for rhodium(I) and iridium(I).
Noteworthy, the carbonyl stretching frequencies for the
rhodium(I)- and iridium(I)−carbonyl complexes of the 4-
phosphino-NHC suggests that its σ-donor ability is roughly
equal to that of unsubstituted IMes. A key difference between
IMes and the 4-phosphino-NHCs reported herein is the
potential application of the latter as bidentate bridging ligands,
which will form the basis for future investigations.
31P NMR (162 MHz, C6D6): δ = −37.4. 19F NMR (282 MHz,
1
C6D6): δ = −116.0, −116.1. H NMR (400 MHz, C6D6): δ = 7.28−
6.40 (m, 15H, 14 aromatic + 1 vinyl), 5.05 (d, JHP = 3.3 Hz, 1H,
CAr2H), 2.57 (s, 3H), 2.32 (s, 3H), 2.28 (s, 3H), 2.15 (s, 6H), 1.93 (s,
3H), 1.78 (d, JHP = 2.8 Hz, 3H), 1.77 (s, 3H), 1.49 (s, 3H). 13C{1H}
NMR (100 MHz, C6D6): δ = 220.4 (br s), 161.9 (dd, 5JCP = 3 Hz, 1JCF
= 246 Hz), 160.8 (dd, 5JCP = 3 Hz, 1JCF = 245 Hz), 147.7, 147.4, 144.3,
140.1, 139.0, 138.1 (d, JCP = 3 Hz), 137.9 (d, JCP = 3 Hz), 137.7, 137.6,
137.4, 137.3 (JCP = 3 Hz), 137.1, 136.5, 135.2, 130.6 (d, J = 8 Hz),
130.5, 130.4 (d, J = 7 Hz), 129.6 (d, J = 7 Hz), 129.5 (d, J = 8 Hz),
129.4 (d, J = 8 Hz), 129.3, 129.2, 129.1, 127.9, 125.2 (d, 1JCP = 20 Hz),
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise noted, all manipu-
lations were performed under an atmosphere of nitrogen. Hexanes,
dichloromethane, and toluene were deoxygenated with nitrogen and
dried by passing through a column containing activated alumina.
Tetrahydrofuran was distilled from Na/benzophenone. C6D6 was
stored over molecular sieves (4 Å) before use, and ampules of CD2Cl2
were used as received from CIL. IMes2 and ItBu39 and phosphaalkenes
2a−2c40 were prepared following literature procedures. The [M(cod)-
Cl]2 (M = Rh, Ir) were used as obtained from Strem. The CO was
2
2
1
115.7 (d, JCF = 21 Hz), 115.1 (d, JCF = 21 Hz), 46.3 (d, JCP = 10
Hz), 23.5 (d, JCP = 35 Hz), 21.5, 21.0 (2 × C coincident), 20.9, 18.8
(d, JCP = 7 Hz), 18.4, 17.3, 17.0. MS (EI, 70 eV): 657, 656 [3, 6; M+];
454, 453 [36, 100; M+ − CH(C6H4F)2]; 204, 203 [5, 13;
+
1
used as received from Praxair. 31P, H, and 13C{1H} NMR spectra
CH(C6H4F)2 ].
Synthesis of 3a. To a solution of ItBu (180 mg, 1 mmol) in THF
(2 mL) was added a solution of 2a (316 mg, 1 mmol) in THF (2 mL).
The mixture was heated at 100 °C in a closed vessel. Over the course
of 3 weeks, the mixture was cooled periodically to remove an aliquot
for 31P NMR spectroscopic analysis. The spectra revealed the growth
of a new signal (δ = −32.3), attributed to compound 3a, at the
expense of the signal attributed to the phosphaalkene (δ = 233). The
approximate ratio of signals was ∼1:1 after 1 week and remained
unchanged over the remainder of the experiment (ca. 3 weeks). The
separation of 3a from ItBu and 2a was not attempted.
were recorded at room temperature on Bruker Avance 300 or 400
MHz spectrometers with chemical shifts (δ) in parts per million
(ppm). Chemical shifts are referenced and reported relative to 85%
H3PO4 as an external standard (δ = 0.0 for 31P) or referenced to TMS
and measured relative to the residual solvent peak (C6HD5 or
1
CHDCl2: δ = 7.16 or 5.32 for H, respectively; C6D6 or CD2Cl2: δ =
128 or 53.8 for 13C, respectively). Mass spectra were recorded on a
Kratos MS 50 instrument in EI mode (70 eV). IR spectra were
recorded on a Thermo IR spectrometer with an attenuated total
reflection (ATR) attachment.
Synthesis of 4M=Rh. To a solution of [(cod)RhCl]2 (100 mg, 0.20
mmol) in THF (2 mL) was added a solution of 1a (266 mg, 0.41
mmol) in THF (2 mL). The mixture was stirred for 1 h, at which point
the volatiles were removed in vacuo. The crude product was
recrystallized from a mixture of THF and pentane. Yield: 310 mg
(88%).
Synthesis of 1a. To a solution of IMes (500 mg, 1.64 mmol) in
THF (2 mL) was added a solution of 2a (520 mg, 1.64 mmol) in THF
(2 mL). The mixture was heated at 70 °C overnight in a closed vessel.
Upon cooling, an aliquot was removed, and 31P NMR spectroscopy
indicated quantitative conversion of the phosphaalkene (δ = 233) to a
new compound (δ = −37.3). Volatiles were removed in vacuo, and the
product was recrystallized from benzene. Yield: 800 mg (78%).
31P NMR (162 MHz, CD2Cl2): δ = −38.4 (64%), −39.1 (36%). 1H
NMR (400 MHz, CD2Cl2) (major isomer): δ = 7.66−6.49 (m, 17H,
16 aromatic + 1 vinyl), 5.26 (d, JHP = 4 Hz, 1H, CPh2H), 4.33 (m, 2H,
CHcod), 3.24 (m, 2H, CHcod), 2.60 (s, 3H, CH3), 2.42 (s, 3H, CH3),
2.39 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.17 (s, 3H, CH3), 2.13 (s, 3H,
CH3), 1.74 (m, 4H, CH2cod), 1.58 (s, 3H, CH3), 1.51 (s, 3H, CH3),
1.50 (s, 4H, CH2cod), 1.41 (s, 3H, CH3). 1H NMR (400 MHz,
CD2Cl2) (minor isomer): δ = 7.64−6.52 (m, 17H, 16 aromatic + 1
vinyl), 5.24 (d, JHP = 4 Hz, 1H, CPh2H), 4.33 (m, 2H, CHcod), 3.33
(m, 2H, CHcod), 2.62 (s, 3H, CH3), 2.41 (s, 3H, CH3), 2.39 (s, 3H,
CH3), 2.37 (s, 3H, CH3), 2.17 (s, 3H, CH3), 2.13 (s, 3H, CH3), 1.86
(s, 3H, CH3), 1.74 (m, 4H, CH2cod), 1.59 (s, 3H, CH3), 1.50 (s, 4H,
CH2cod), 1.13 (s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ =
186.2 (d, JCRh = 51 Hz, major N−C−N), 185.5 (d, JCRh = 50 Hz,
minor N−C−N); other signals were not assigned. MS (EI, 70 eV):
869, 868, 867, 866 [10, 24, 28, 50; M+]; 831, 830 [4, 6; M+ − HCl];
723, 722 [5, 12; M+ − HCl − C8H12]; 623, 622, 621 [12, 50, 100;
1a·H+]; 455, 454 [22, 44; 1a·H+ − CHPh2]; 306, 305 [5, 22;
1
31P NMR (162 MHz, C6D6): δ = −37.9. H NMR (400 MHz,
C6D6): δ = 7.56−6.39 (m, 16H, aromatic), 6.64 (d, JHP = 2.0 Hz, 1H,
vinyl), 5.23 (d, JHP = 4.4 Hz, 1H, CPh2H), 2.64 (s, 3H), 2.35 (s, 3H),
2.34 (s, 3H), 2.14 (s, 6H), 1.92 (s, 3H), 1.86 (d, JHP = 2.4 Hz, 3H),
1.78 (s, 3H), 1.52 (s, 3H). 13C{1H} NMR (100 MHz, C6D6): δ =
220.3 (d, JCP = 5 Hz), 147.8, 147.4, 144.6 (d, JCP = 5 Hz), 142.6, 142.4,
141.9, 141.8, 139.7 (d, JCP = 2 Hz), 139.2, 137.9, 137.4, 137.16, 137.15,
136.7, 135.4, 130.4, 129.2, 129.1, 128.8, 128.7 (d, JCP = 2 Hz), 128.3,
128.2, 128.1, 128.0, 127.9, 126.8 (d, JCP = 2 Hz), 126.6 (d, JCP = 2 Hz),
125.8, 125.6, 48.2 (d, JCP = 10 Hz), 23.6 (d, JCP = 36 Hz), 21.6, 21.0 (2
× C coincident), 20.9, 18.8 (d, JCP = 8 Hz), 18.4, 17.6, 17.0. MS (EI,
70 eV): 621, 620 [2, 5; M+]; 455, 454, 453 [10, 50, 100; M+
CHPh2]; 168, 167 [18, 85; CHPh2 ]. Anal. Calcd for C43H45N2P: C,
83.19; H, 7.31; N, 4.51. Found: C, 83.10; H, 7.44; N, 4.61.
−
+
Synthesis of 1b. To a solution of IMes (104 mg, 0.34 mmol) in
C6D6 (2 mL) was added a solution of phosphaalkene E/Z-2b (106 mg,
0.33 mmol) in C6D6 (2 mL). The mixture was heated at 100 °C
overnight in a closed vessel. Upon cooling, an aliquot was removed,
and 31P NMR spectroscopy indicated quantitative conversion of the
E/Z-2b (δ = 261, 242) to new compounds (δ = −33.7, −35.3). Slow
evaporation of the solvent afforded crystals suitable for X-ray
crystallography. Yield (1b·1/2C6D6): 160 mg (73%).
+
IMesH+]; 304, 303 [14, 53; IMes+ − H]; 168, 167 [12, 52; CHPh2 ].
Anal. Calcd for C51H57ClN2PRh: C, 70.62; H, 6.62; N, 3.23. Found: C,
70.33; H, 6.57; N, 3.30.
Synthesis of 4M=Ir. To a solution of [(cod)IrCl]2 (50 mg, 0.07
mmol) in THF (2 mL) was added a solution of 1a (94 mg, 0.15
mmol) in THF (2 mL). The mixture was stirred for 1 h, at which point
the volatiles were removed in vacuo. The crude product was
recrystallized by slow diffusion of pentane into a dichloromethane
solution of the product. Yield: 113 mg (79%).
1
31P NMR (121 MHz, C6D6): δ = −33.7, −35.3. H NMR (300
MHz, C6D6): δ = 8.34−6.18 (m, 32H, 30 aromatic + 2 vinyl), 5.67 (d,
2JHP = 5 Hz, 1H, CAr2H), 5.52 (d, 2JHP = 5 Hz, 1H, CPh2H), 2.74 (s,
3H), 2.65 (s, 3H), 2.37 (s, 3H), 2.35 (br s, 3H), 2.29 (br s, 3H), 2.26
(s, 3H), 2.16 (s, 6H), 2.15 (s, 6H), 2.12 (d, JHP = 4 Hz, 3H), 1.93 (d,
JHP = 3 Hz, 3H), 1.91 (s, 3H), 1.89 (s, 3H), 1.81 (br s, 6H), 1.51 (s,
31P NMR (162 MHz, CD2Cl2): δ = −38.5 (62%), −39.3 (38%). 1H
NMR (400 MHz, CD2Cl2) (major isomer): δ = 7.67−6.49 (m, 17H,
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Organometallics
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16 aromatic + 1 vinyl), 5.27 (d, JHP = 4 Hz, 1H, CPh2H), 3.94 (m, 2H,
CHcod), 2.96 (br m, 2H, CHcod), 2.63 (s, 3H, CH3), 2.37 (s, 3H, CH3),
2.35 (s, 3H, CH3), 2.34 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.16 (s, 3H,
CH3), 1.57 (s, 3H, CH3), 1.56 (br m, 4H, CH2cod), 1.55 (s, 3H, CH3),
1.36 (s, 3H, CH3), 1.24 (s, 3H, CH2cod). 1H NMR (400 MHz,
CD2Cl2) (minor isomer): δ = 7.65−6.52 (m, 17H, 16 aromatic + 1
vinyl), 5.25 (d, JHP = 4 Hz, 1H, CPh2H), 3.91 (m, 2H, CHcod), 2.96
(br m, 2H, CHcod), 2.63 (s, 3H, CH3), 2.37 (s, 3H, CH3), 2.35 (s, 3H,
CH3), 2.34 (s, 3H, CH3), 2.17 (s, 6H, 2 × CH3), 1.83 (s, 3H, CH3),
1.57 (s, 3H, CH3), 1.56 (br m, 4H, CH2cod), 1.24 (s, 3H, CH2cod), 1.18
(s, 3H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ = 183.7 (s, major
N−C−N), 182.8 (s, minor N−C−N); other signals were not assigned.
MS (EI, 70 eV): 960, 959, 958, 957, 956, 955, 954 [5, 18, 42, 55, 100,
29, 50; M+]; 920, 919, 918 [2, 3, 6; M+ − HCl]; 792, 791, 790, 789,
788, 787 [2, 5, 6, 14, 4, 6; M+ − CHPh2]; 455, 454, 453 [12, 23, 30;
For 1c, it appears that the oxygen atom of the THF solvate is
disordered to every position of the ring; however, since the solvate
simply seems to be filling a void space in the crystala partial benzene
molecule in both 1a and 1b occupies the same sitethe partial THF
molecule was modeled as a single species. In 5M=Ir, the Cl and CO
ligands cis to the carbene ligand are disordered over both positions,
with the major isomer (83%) being shown in Figure 5. The crystal of
4M=Ir was a two-component twin, present in a 3:2 ratio, where the
minor component is related to the major component by a 180°
rotation about the (1 0 0) reciprocal axis.
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data as CIF files. This material is
■
S
+
1a+ − CHPh2]; 168, 167 [25, 99; CHPh2 ]. Anal. Calcd for
available free of charge via the Internet at http://pubs.acs.org.
C51H57ClN2PIr: C, 64.03; H, 6.01; N, 2.39. Found: C, 64.04; H,
6.00; N, 3.00.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dgates@chem.ubc.ca.
■
Synthesis of 5M=Rh. A stream of CO gas was bubbled through a
stirred solution of 4M=Rh (100 mg, 0.12 mmol) in dichloromethane (5
mL) for 15 min. The solvent was removed in vacuo, and the product
was washed with hexanes. Yield: 85 mg (90%).
Notes
1
31P NMR (162 MHz, CD2Cl2): δ = −38.6. H NMR (400 MHz,
The authors declare no competing financial interest.
CD2Cl2): δ = 7.67−6.51 (m, 17H, 16 aromatic + 1 vinyl), 5.29 (d, JHP
= 4 Hz, 1H, CPh2H), 2.63 (s, 3H, CH3), 2.36 (s, 3H, CH3), 2.34 (s,
3H, CH3), 2.26 (s, 3H, CH3), 2.20 (s, 3H, CH3), 2.17 (s, 3H, CH3),
1.68 (s, 3H, CH3), 1.56 (d, JHP = 3 Hz, 3H, CH3), 1.23 (s, 3H, CH3).
13C{1H} NMR (100 MHz, CD2Cl2): δ = 185.7 (d, JCRh = 54 Hz, CO),
183.4 (d, JCRh = 74 Hz, N−C−N), 178.6 (d, JCRh = 47 Hz, CO); other
ACKNOWLEDGMENTS
■
We thank the Natural Sciences and Engineering Research
Council (NSERC) of Canada for support of this work. J.I.B.
thanks NSERC for PGS M and D scholarships. We thank Dr.
Brian O. Patrick for helpful discussions regarding the X-ray
crystallography and Dr. Eamonn Conrad for useful discussions.
signals were not assigned. IR νCO (cm−1): 2070.5, 1983.2. Anal. Calcd
̅
for C45H45ClO2N2PRh: C, 66.30; H, 5.56; N, 3.44. Found: C, 66.34;
H, 5.82; N, 3.17.
Synthesis of 5M=Ir. A stream of CO gas was bubbled through a
stirred solution of 4M=Ir (50 mg, 0.05 mmol) in dichloromethane (5
mL) for 15 min. The solvent was removed in vacuo, and the crude
product was recrystallized from slow evaporation of a toluene solution.
Yield (5M=Ir·cod): 43 mg (81%).
REFERENCES
■
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31P NMR (162 MHz, CD2Cl2): δ = −38.8. H NMR (400 MHz,
́
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N), 169.1 (CO), 148.3, 148.0, 145.0 (d, JCP = 5 Hz), 141.8 (d, JCP = 13
Hz), 141.4 (d, JCP = 19 Hz), 141.3, 141.1, 140.2, 140.0, 137.3, 136.7,
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CHPh2]; 684, 683, 682, 681, 680, 679 [1, 3, 4, 9, 3, 8; 1a·IrCl+ −
+
CHPh2]; 168, 167 [28, 100; CHPh2 ]. IR νCO (cm−1): 2056.2, 1966.6.
̅
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́
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Anal. Calcd for C45H45ClN2O2PIr·C8H12: C, 62.67; H, 5.36; N, 2.81.
Found: C, 63.04; H, 5.42; N, 2.81.
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X-ray Crystallography. All single crystals were immersed in oil
and mounted on a glass fiber. Data were collected on a Bruker X8
APEX 2 diffractomer with graphite-monochromated Mo Kα radiation.
Data were collected and integrated using the Bruker SAINT41 software
package and corrected for absorption effect using TWINABS42 (for
4M=Ir) and SADABS43 (for all others). All data sets were corrected for
Lorentz and polarization effects. All structures were solved by direct
methods44 and subsequent Fourier difference techniques and refined
anisotropically for all non-hydrogen atoms using the SHELXTL45
crystallographic software package from Bruker-AXS. All data sets were
corrected for Lorentz and polarization effects. Additional crystal data
and details of the data collection and structure refinement are given in
Table 3.
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complications. The structures of 1c and 5M=Ir each exhibited disorder.
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