Indirect Access to Carbene Adducts of Bismuth- And Antimony-Substituted Phosphaketene and Their Unusual Thermal Transformation to Dipnictines and [(NHC)2OCP][OCP]

Article abstract of DOI:10.1021/acs.inorgchem.0c03683

The synthesis and thermal redox chemistry of the first antimony (Sb)- and bismuth (Bi)-phosphaketene adducts are described. When diphenylpnictogen chloride [Ph2PnCl (Pn = Sb or Bi)] is reacted with sodium 2-phosphaethynolate [Na[OCP]·(dioxane)x], tetraphe

Full text of DOI:10.1021/acs.inorgchem.0c03683

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Indirect Access to Carbene Adducts of Bismuth- and Antimony-
Substituted Phosphaketene and Their Unusual Thermal
Transformation to Dipnictines and [(NHC)2OCP][OCP]
̋
Jacob E. Walley, Levi S. Warring, Erik Kertész, Guocang Wang, Diane A. Dickie, Zoltán Benko,*
and Robert J. Gilliard, Jr.*
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ABSTRACT: The synthesis and thermal redox chemistry of the first antimony
(Sb)− and bismuth (Bi)−phosphaketene adducts are described. When
diphenylpnictogen chloride [Ph2PnCl (Pn = Sb or Bi)] is reacted with sodium
2-phosphaethynolate [Na[OCP]·(dioxane)x], tetraphenyldipnictogen (Ph2Pn−
PnPh2) compounds are produced, and an insoluble precipitate forms from
solution. In contrast, when the N-heterocyclic carbene adduct (NHC)−PnPh2Cl
is combined with [Na[OCP]·(dioxane)x], Sb− and Bi−phosphaketene
complexes are isolated. Thus, NHC serves as an essential mediator for the
reaction. Immediately after the formation of an intermediary pnictogen−
phosphaketene NHC adduct [NHC−PnPh2(PCO)], the NHC ligand transfers
from the Pn center to the phosphaketene carbon atom, forming NHC−C(O)P-PnPh2 [Pn = Sb (3) or Bi (4)]. In the solid state, 3
and 4 are dimeric with short intermolecular Pn−Pn interactions. When compounds 3 and 4 are heated in THF at 90 and 70 °C,
respectively, the pnictogen center PnIII is thermally reduced to PnII to form tetraphenyldipnictines (Ph2Pn−PnPh2) and an unusual
bis-carbene-supported OCP salt, [(NHC)2OCP][OCP] (5). The formation of compound 5 and Ph2Pn−PnPh2 from 3 or 4 is
unique in comparison to the known thermal reactivity for group 14 carbene−phosphaketene complexes, further highlighting the
diverse reactivity of [OCP]− with main-group elements. All new compounds have been fully characterized by single-crystal X-ray
diffraction, multinuclear NMR spectroscopy (1H, 13C, and 31P), infrared spectroscopy, and elemental analysis (1, 2, and 5). The
electronic structure of 5 and the mechanism of formation were investigated using density functional theory (DFT).
oxyphosphaalkyne isomer, while soft Lewis acidic elements, for
example, the heavy group 14 elements (Ge, Sn, Pb),4b,8 and
Ga4c,9 favor the phosphaketene isomer. Both O- and P-bound
isomers are known for B4a,10 and Si,4b illustrating the ambident
nature of the [OCP]− anion. (iii) Redox chemistry: the OCP
anion is prone to oxidation by many metals due to its reductive
nature and the electrophilic character of the metals.3a,11 While
point (i) can be circumvented using sterically demanding
substituents to stabilize the phosphaketene, points (ii) and (iii)
are more challenging to avoid because the inherent electro-
philic properties of the main-group elements differ widely
across the periodic table. Nevertheless, phosphaketene isomers
RPCO are usually more stable than their oxy-
phosphaalkyne ROCP analogues; thus (ii) is a less
common problem in synthetic routes. In this Article, we aim to
INTRODUCTION
■
Due to their unique electron distribution, heteroketenes show
versatile and fascinating chemistry. Phosphorus-containing
members of this family are phosphaketenes, RPCO.1
Although the first stable phosphaketene was reported nearly
four decades ago,2 the synthetic chemistry was experimentally
challenging, and various products were thermally unstable.
However, in the past decade, simple synthetic routes toward
such compounds have emerged, which has resulted in the rapid
development of the field. The utilization of the 2-
phosphaethynolate anion,3 [OCP]−, as a synthon has proved
to be an effective way to access phosphaketenes via
nucleophilic substitution.4 However, these synthetic processes
are not always straightforward, and phosphaketene stability and
reactivity may be hampered by a number of complications. The
most important of these are summarized as follows. (i) Dimer
formation: the PC bond of phosphaketenes is prone to
cycloaddition, which results in 4-membered rings; however,
this process can be minimized with the incorporation of bulky
substituents, or with heteroatoms.5 (ii) Formation of constitu-
tional isomers: due to its ambident reactivity, the [OCP]−
anion may bind through the P or the O center. Highly
oxophilic species, such as s-3c,6 or f-block elements,7 favor the
Received: December 17, 2020
Published: March 9, 2021
https://dx.doi.org/10.1021/acs.inorgchem.0c03683
© 2021 American Chemical Society
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offer a solution to the problem described in point (iii). Since
the heavier pnictogens are easily reduced, neutral donor
ligands such as N-heterocyclic carbenes (NHC) can be
employed to stabilize the phosphaketene motif, thereby
preventing reduction at the pnictogen center.
Phosphaketenes show rich and often unprecedented
chemistry. In recent years, the phosphanyl- and tetrel-
substituted phosphaketenes have attracted special interest.
ally.13 The reaction proceeds via an associative mechanism,
whereby the phosphine binds to the −PCO unit first, followed
by loss of CO.
Within the realm of main-group elements, the reactivity of
Na[OCP] has been established for group 2,6a,b group 13,4c,9,10
group 14,4b,e,8,14 and group 15.4e,15 For the lattermost, these
examples are limited to phosphorus and arscenic, with no
current reports describing reactions of Na[OCP] with the
heavier pnictogens (Sb and Bi). Nevertheless, the chemistry of
the heavier two Pn elements (Sb, Bi) has seen a substantial
increase in interest recently as novel bonding motifs, and new
applications in catalysis continue to be discovered.16
Bertrand, Su, and Grutzmacher discovered a unique reaction
̈
where OCP rearranges to OPC when an N-heterocyclic
phosphane (NHP)−phosphaketene adduct is reacted with
NHC (Figure 1A).4e Nucleophilic attack on the OCP carbon
Herein, we report the first reactions of Na[OCP] with
antimony and bismuth compounds, [NHC−Sb(Ph)2Cl]2 (1)
and [NHC−Bi(Ph)2Cl]2 (2). [NHC−PnPh2Cl]2 was com-
bined with [Na[OCP]·(dioxane)x] to afford Sb− and Bi−
phosphaketene complexes (3 and 4, respectively). Notably,
compounds 3 and 4 were susceptible to a thermal reduction
process where the PnIII center is reduced to PnII to form either
tetraphenyldistibine or tetraphenyldibismuthine and the
[(NHC)2OCP][OCP] salt (5). Compound 5 is a unique
example of a salt with an [OCP] moiety imbodying both the
cation and the anion. DFT calculations demonstrate that the
formation of the cationic unit in 5 occurs in a mechanistic step
where nucleophilic attack of a dissociated NHC on one unit of
4 leads to the loss of [Ph2Bi]− (Figure 1D).
RESULTS AND DISCUSSION
■
We initially performed the reaction of Na[OCP] with Ph2PnCl
(Pn = Sb or Bi) and observed the formation of
tetraphenyldipnictine and an insoluble unidentified precipitate.
Extending the scope of this reaction, NHC ligand 4,5-
dimethyl-1,3-diisopropylimidazolin-2-ylidene was reacted with
diphenylantimony chloride (Ph2SbCl) or diphenylbismuth
chloride (Ph2BiCl) in THF for 1 h at room temperature
(Scheme 1). Compounds 1 (Sb) and 2 (Bi) were obtained as
Scheme 1. Synthesis of Diphenylpnictogen Halide N-
Heterocyclic Carbene Complexes
Figure 1. (A) OCP to OPC rearrangement; (B) thermal loss of CO
from NHC−phosphaketene adducts of triphenyl-germanium or -tin to
form NHC−phosphinidenes; (C) phosphine-promoted CO dissoci-
ation; (D) This work: thermal reduction involving Sb and Bi
phosphaketene.
atom by the NHC results in a zwitterionic intermediate, which
is followed by migration of the NHP unit to oxygen.
Gru
̈
tzmacher et al. showed that the CO unit of the
white solids in 94% and 85% yield, respectively. The 1H NMR
spectrum of 1 in C6D6 shows a broad heptet at 4.69 ppm,
attributed to the NHC methine proton. This is shifted
downfield from the methine of the NHC ligand (3.96 ppm).
phosphaketene can be substituted by a carbene, demonstrating
similar phosphaketene reactivity with NHCs (Figure 1B).4d
Addition of NHC to a triphenylgermanium− or tin−
phosphaketene led to the formation of NHC−phosphaketene
adducts. When heated, the NHC transfers to phosphorus to
release CO, thereby forming NHC−phosphinidene germa-
nium and tin complexes. Similarly, the CO unit of a
phosphaketene can be exchanged by another donor. Bertrand
observed loss of CO from NHP−phosphaketenes when a
Lewis basic phosphine was introduced with moderate heating
(Figure 1C).12 It is noteworthy that the loss of CO from P−
CO-containing molecules has been explored computation-
1
Due to poor solubility in C6D6, the H NMR spectrum of
compound 2 was recorded in THF-d8, which showed a
broadened heptet at 4.51 ppm, attributed to the methine
protons of coordinated NHC.
Colorless crystals suitable for X-ray diffraction of both 1 and
2 were obtained from toluene/hexane (10:1) mixtures at −37
°C. The molecular structures of compounds 1 and 2 are
dimeric with distorted square pyramidal geometry around the
metal center (Figure 2). The C1−Sb1 bond distance in
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[OCP]− with other main group elements,3a we predicted the
formation of a pnictogen−phosphaketene adduct, Pn−PCO.
Therefore, we reacted compounds 1 and 2 with Na[OCP]·
(dioxane)x at −37 °C in THF (Scheme 2). The 31P NMR
spectra of the isolated complexes revealed shifts at 58.2 ppm
(Sb) and 82.2 ppm (Bi), which are downfield from known
main-group element Pn−PCO compounds (−441 to −225.8
ppm).3a Two doublets were observed in the 13C NMR spectra
for both the antimony [203.1 ppm (1JCP = 76.0 Hz) and 148.8
ppm (2JCP = 52.8 Hz)] and bismuth [203.6 ppm (2JCP = 81.7)
and 152.0 ppm (1JCP = 49.6 Hz)] complexes.
Single crystals of compounds 3 and 4 were obtained by
layering the original THF filtrate with hexanes in a 1:1 ratio at
−37 °C. Interestingly, the molecular structure revealed that the
NHC transferred from the pnictogen center to the
phosphaketene (Figure 3). Compounds 3 and 4 are unstable
at room temperature and −37 °C, respectively, and decompose
slowly in the solid state after a few days. The formation of
these products is consistent with the 13C NMR spectra. A
stretching frequency was not observed for the carbonyl group
in the IR spectrum; however, this is consistent with reported
NHC−phosphaketenyl species.4d The solid-state structures of
3 and 4 reveal pnictogen centers in a seesaw environment with
intermolecular Pn−Pn interactions at 3.9619(17) Å and
3.8204(6) Å, respectively. The Pn−P bond lengths for both
3 (2.5042(16) Å) and 4 (2.589(2)−2.594(2) Å) are close to
the sum of covalent radii for Sb and P (RSbP = 2.50 Å), as well
as for Bi and P (RBiP = 2.61 Å).19
Figure 2. Molecular structure of 1 (a): Thermal ellipsoids at 50%
probability; H atoms omitted for clarity. Selected bond distances (Å)
and angles (deg): Sb1−C1 2.356(3); Sb1−Cl1 2.8006(8); Sb1−Cl1′
3.9544(10); Sb1−C18 2.168(3); Sb1−C12 2.171(4). C18−Sb1−C12
102.19(14); C18−Sb1−C1 87.58(12); C12−Sb1−C1 86.33(12);
C18−Sb1−Cl1 87.00(8); C12−Sb1−Cl1 85.24(9); C1−Sb1−Cl1
168.80(9). Molecular structure of 2 (b): Thermal ellipsoids at 50%
probability; H atoms were omitted for clarity. Selected bond distances
(Å) and angles (deg): Bi1−C1 2.489(6); Bi1−Cl1 2.8696(16); Bi1−
Cl1′ 3.7211(17); Bi1−C18 2.257(6); Bi1−C12 2.267(6). C18−Bi1−
C12 99.0(2); C18−Bi1−C1 86.3(2); C12−Bi1−C1 88.1(2); C18−
Bi1−Cl1 86.26(16); C12−Bi1−Cl1 88.26(16); C1−Bi1−Cl1
171.05(15).
Recently, Grutzmacher and co-workers demonstrated that
̈
N-heterocyclic carbene (NHC)−phosphaketene adducts of
Ph3SnPCO and Ph3GePCO undergo a decar-
bonylation reaction when heated to form the phosphenidinyl
complexes NHC−P−SnPh3 and NHC−P−GePh3.4d We were
therefore interested in probing the thermal reactivity of
compounds 3 and 4 (Scheme 3), which can be considered
group 15 analogues of the aforementioned Sn and Ge
phosphaketene complexes. Compound 3 was heated to 90
compound 1 [2.356(3) Å] is outside the range of other NHCC−
Sb bonds (2.144−2.268 Å);16o,q,r,17 likewise, the C1−Bi1 bond
in compound 2 [2.489(6)] is slightly longer than the known
range for NHCC−Bi bonds (2.339−2.428 Å).17b,18 The Pn−Cl
bond lengths in 1 (2.8006(8) Å) and 2 (2.8696(16) Å) are
also significantly longer than those in reported complexes
containing Sb−Cl (2.332−2.402 Å)17a−c and Bi−Cl (2.437−
2.705 Å)18a,b bonds. The longer NHCC−Pn bonds results from
the weak Lewis acidity of Ph2PnCl compared to PhBiCl2 and
BiCl3. The intermolecular Pn−Cl distances in 1 [3.9544(10)
Å] are longer than those in 2 [3.7211(17) Å], which is due to
the pronounced Lewis acidity at the Bi center.
1
°C for 24 h in a J-Young NMR tube. The peaks in this H
NMR spectrum matched those reported in the literature for
tetraphenyldistibine (Figure S14).16s Compound 4, being less
stable than 3, was heated at 70 °C for 3 h in C6D6. Free NHC
emerged along with 100% conversion to tetraphenyldibismu-
thine (Figure S15). Single crystals suitable for X-ray diffraction
were grown from C6D6 inside the NMR tube. The solid-state
structure revealed a new polymorph of tetraphenyldibismu-
thine (6) (Figure S19). It is noteworthy that compounds 3 and
4 slowly covert to 5 and tetraphenyldipnictogen at room
temperature; therefore, heat was applied to escalate the
reactions as described.
For both compounds 1 and 2, we hypothesized that a
combination of electronic stabilization from the coordinated
NHC and steric protection from the two phenyl groups may
stabilize their OCP adducts. Based on the reactivity known for
Scheme 2. Synthesis of Antimony− and Bismuth−Phosphaketene Adducts
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revealed one broad and one well-defined heptet, suggesting
two distinct NHC ligand environments. The 31P NMR showed
a broad singlet at 22.1 ppm and a sharp singlet at −395.1 ppm.
The latter shift closely resembles the resonance of 2-
phosphaethynolate in D2O (−396.4 ppm).3b Four doublets
(δ = 200.7, 170.2, 150.2, 146.0) were observed in the 13C
NMR spectrum. Further supporting our assignment, the signal
at 170.2 ppm (1JCP = 63.4 Hz) agrees well with known 13C
NMR shifts for 2-phosphaethynolate, while the other signals
are attributed to three new 13C−31P coupling environments.
Similar to compounds 3 and 4, no signals were observed in the
IR spectrum for the CO stretch in the cationic unit of 5. Two
different stretches were observed for the phosphaalkyne at
1788 and 1768 cm−1, resulting from different orientations of
[OCP]− in the solid-state structure.
Orange single crystals of compound 5 suitable for X-ray
diffraction were obtained by heating a THF solution of 3 at 55
°C overnight. The crystal structure shows a cation containing
two NHCs coordinated to a [OCP] core with an [OCP]−
counteranion (Figure 4). A 2-fold rotation axis perpendicular
to the P1−C12 bond in the cation causes the two halves of the
molecule to be disordered by symmetry in the solid state. This
symmetry results in identical bond lengths and angles for both
NHC ligands. A similar disorder exists in the anion. There are
currently eight other molecular structures containing un-
coordinated [OCP]− counter-anions reported in the CSD
database.4c,6c,d,20 The C1−P1 bond (1.890(6) Å) is longer
than those in neutral NHC2P2 complexes (1.750−1.754 Å)21
and cationic [NHC2P2]+ complexes (1.795−1.841 Å).17c,21a,22
To gain insights into the formation mechanism leading to
the new compounds and the bonding situation thereof, we
carried out DFT calculations employing the ωB97XD range
separated functional with the def2-SVP and def2-TZVP basis
sets, which is similar to the level of theory used previously to
describe the bonding in carbene complexes of bismuth.17b
Relevant energies and structural parameters are shown in
Table 1.
The complex formation energy leading to adduct 2 is −26.2
kcal/mol (calculated with respect to the isolated carbene and
diphenyl bismuth chloride). This value is greater than the
values of −35.9 to −44.6 kcal/mol reported for NHC and
CAAC complexes of PhBiCl2,18b a stronger Lewis acid owing
to the presence of two chlorine atoms instead of one in
Ph2BiCl. This agrees nicely with the observations above on the
solid-state structures, which revealed rather long NHCC−Pn
bonds as a result of a weaker interaction. Compared to 2, the
antimony analogue 1 is slightly less stable (ΔE = −23.9 kcal/
mol), explainable by the weaker electron pair accepting
property of antimony than that of bismuth. The same
phenomenon is observed for the NHC−Ph2PnPCO com-
plexes, which are assumed as possible intermediates during the
replacement of the chlorides of 1 and 2 by phosphaethynolate
anion. However, the phosphaketene complexes are destabilized
compared to their chloro-analogues, due to the lower
electronegativity of P compared to Cl. Indeed, the partial
charge at the Bi center in the uncomplexed Ph2BiPCO and
Ph2BiCl is +1.010 and +1.226 e, respectively, in line with the
lower Lewis acidity of the former compared to the latter. The
reduced stability of the phosphaketene complexes compared to
analogous chloro-complexes is accompanied by the weakening
of the NHCC−Pn bonds; these bonds are longer and their
Wiberg bond indices (WBI), accounting for the covalent
character, are lower. Thus, the net charge transfer is smaller.
Figure 3. Molecular structure for 3 (a): Thermal ellipsoids at 50%
probability; H atoms omitted for clarity. Selected bond distances (Å)
and angles (deg): Sb1−C13 2.154(6); Sb1−C19 2.155(6); Sb1−P1
2.5042(16); Sb1−Sb1′ 3.9619(17); P1−C12 1.748(6); O1−C12
1.264(7); C1−C12 1.529(7). C13−Sb1−C19 97.7(2); C13−Sb1−P1
99.79(15); C19−Sb1−P1 91.77(15); Sb1′−Sb1−P1 94.602(40);
Sb1′−Sb1−C19 97.237(158); Sb1′−Sb1−C13 158.899(168). Mo-
lecular structure for 4 (b): Thermal ellipsoids at 50% probability; H
atoms were omitted for clarity. Selected bond distances (Å) and
angles (deg): Bi1−C19 2.229(6); Bi1−C13 2.272(7); Bi1−P1
2.589(2); Bi1−Bi2 3.8204(6); Bi2−C37 2.251(7); Bi2−C43
2.257(8); Bi2−P2 2.594(2); P1−C12 1.736(7); P2−C36 1.746(8);
O1−C12 1.255(8); O2−C36 1.258(8); C1−C12 1.525(11); C25−
C36 1.515(11). C19−Bi1−C13 94.2(2); C19−Bi1−P1 97.38(19);
C13−Bi1−P1 87.26(19); C37−Bi2−C43 94.8(3); C37−Bi2−P2
98.56(19); C43−Bi2−P2 90.99(19); Bi2−Bi1−P1 86.913(49);
Bi2−Bi1−C13 101.940(176); Bi2−Bi1−C19 163.528(178); Bi1−
Bi2−P2 89.365(43); Bi1−Bi2−C43 104.176(179); Bi1−Bi2−C37
159.346(177).
In addition to the formation of tetraphenyldipnictine, an
orange solid precipitated from the C6D6 solution. The orange
solid is insoluble in most common organic solvents except for
dichloromethane but decomposes within an hour after
dissolution. The 1H NMR spectrum of the orange solid
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Scheme 3. Thermal Reduction at Sb or Bi Center to Tetraphenyldipnictines and [(NHC)2OCP][OCP]
Figure 5. LUMO and HOMO of Ph2BiPCO.
Figure 4. Molecular structure of 5: Thermal ellipsoids at 50%
probability; H atoms omitted for clarity. Only one orientation of the
symmetry disordered [OCP]− anion is shown. Selected bond
distances (Å) and angles (deg): C1−P1 1.890(6); P1−C12
1.755(14); C12−O1 1.268(12); C12−C1′ 1.421(16). C1−P1−C12
98.3(5); O1−C12−C1′ 117.0(14).
The LUMO of Ph2BiPCO (Figure 5) shows main contribu-
tions both at the Bi center and the carbon atom of the PCO
moiety, explaining why this species may be complexed either at
Bi or on the phosphaketenyl carbon center. The rearranged
phosphaketene carbene adducts 3 and 4, in which the carbene
is coordinated to the PCO carbon atom, are significantly more
stable than the Pn-coordinated analogues. Thus, the driving
force for the carbene migration is the formation of a stronger
C−C bond instead of a dative C−Pn bond. These C−C bonds
show a high covalent character (WBI: 0.93) and remarkable
net charge transfer from the carbene to the PCO moiety of Δq
= 0.751 and 0.755, meaning that the carbenic unit possesses a
large partial positive charge. Furthermore, the WBIs of PC/CO
bonds (1.41/1.48 and 1.42/1.47 for 3 and 4, respectively)
indicate delocalization in the PCO fragment. Hence, the
structure of the C-coordinated Ph2PnPCO adducts 3 and 4 can
be best described as a superposition of two zwitterionic
resonance structures (Figure 6A). We also studied the
electronic structure and bonding of the cationic fragment of
Figure 6. Resonance structures for compounds 3/4 (A) and cation 5
(B).
compound 5. Even though [(NHC)2OCP]+ can be regarded
formally as an adduct of a cationic OCP+ unit and two
carbenes, the NPA charges and WBI values suggest the positive
charge is localized on the NHC ligands (Figure 6B). While the
sum of charges in the OCP core is −0.375e, both NHC
fragments possess high partial charges of 0.804e and 0.571e.
a
b
Table 1. Complex Formation Energies (ΔE) and Gibbs Free Energies (ΔG) , Geometrical Parameters , NPA Partial Charges
of Pn (q) in Electrons, and Net Charge Transfer in Electrons (Δq) at the ωB97XD/def2-TZVP Level
compound
1
2
NHC−SbPh2PCO
NHC−BiPh2PCO
3
4
ΔE
ΔG
−23.9
−9.4
2.572
0.37
−26.2
−11.7
2.715
0.31
−19.2
−3.0
2.638
0.32
−21.5
−7.1
2.801
0.26
−29.7
−12.4
1.513
0.93
−29.2
−12.0
1.513
0.93
d(Pn−Ccarbene/C−Ccarbene
WBI(Pn−Ccarbene/C−Ccarbene
)
)
q(Pn)
Δq
1.182
0.235
1.246
0.202
1.058
0.226
1.121
0.186
0.899
0.755
0.936
0.751
a
b
In kcal/mol. Bond length in Å/Wiberg bond indicies.
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The WBI of the P−C(carbene) and C−C(carbene) bonds of
0.93 indicate covalent character, and the PC/CO bonds show a
delocalization in the OCP moiety. The bis-zwitterionic charge
distribution of the [(NHC)2OCP]+ cation is also visible on the
molecular electrostatic potential (Figure 7).
homolytic dissociation at the P−Bi bond of adduct 4, or
alternatively, the free Ph2BiPCO. However, both reactions are
highly endothermic (ΔE = 53.0 and 51.7 kcal/mol,
respectively); thus, they are unlikely to happen even at higher
temperature. We considered further alternative pathways and a
plausible mechanism (Figure 8). The first step of the reaction
is the partial dissociation of adduct 4, resulting in the free
carbene and Ph2BiPCO. This reaction is rather endothermic
and proceeds via an activation barrier of 27.9 kcal/mol (Figure
9), resulting in a weakly bound complex of NHC and
Ph2BiPCO at the energy of 27.2 kcal/mol. Even though this
reaction is likely shifted toward the side of the starting adduct,
the formation of small amounts of free carbene is expected,
especially if the entropy factor is taken into account
(dissociation Gibbs free energy: 18.9 kcal/mol). This is further
supported by the experimental observation of uncoordinated
NHC during the reaction. The second step of the reaction is an
attack of the free carbene onto the P center of adduct 4,
delivering the contact ion pair of the [(NHC)2OCP]+ cation
with a diphenyl bismuthide ([BiPh2]−) counteranion. The
nucleophilic substitution at the phosphaketene P center is
known in the literature, and it has been shown that the attack
of Lewis bases (L) on the phosphorus center of phosphanyl
phosphaketenes RPCO results in the adduct RPL
and carbon monoxide. In our case, however, the C of the PCO
unit is occupied by the carbene fragments; thus, the
decarbonylation is hampered. Instead, the bismuthide anion
is released in a slightly exothermic reaction (ΔE = −10.1 kcal/
mol). Since all of our attempts to locate the transition state of
step 2 failed, we performed a relaxed optimization scan
connecting the structures at the two sides of the equation and
estimated a barrier of 1.4 kcal/mol via this approach. The
thermodynamic sink is obtained in reaction step 3, which is
strongly exothermic with a reaction energy of ΔE = −31.7
kcal/mol. Since we could not locate any transition states for
this step, we performed a relaxed scan computation which
Figure 7. Molecular electrostatic potential for the cationic moiety of
5.
We also aimed to understand the formation of the
Ph2PnPnPh2 dimers and compound 5; therefore, we
investigated possible reaction mechanisms by means of
computations. As the reactivity of 3 and 4 are rather similar,
we focused on the bismuth analogue. Because this reaction
proceeds in C6D6, the gas phase approximation seems to be
appropriate without solvent effects. In the following, we discuss
the energies obtained at the ωB97XD/def2-SVP level.
The formation of the tetraphenyldibismuthine may indicate
a radical mechanism, in which the first step would be the
Figure 8. Proposed mechanism for the formation of [(NHC)2OCP]+[OCP]−.
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Figure 9. Energy profile for the decomposition of 4 leading to 5. The three mechanistic steps illustrated in Figure 8 proceed on different potential
energy surfaces. For easier understanding, the energy levels of each initial mechanistic step are shifted to the energy of the previous step. (Atom
colors: C, black; O, red; N, blue; P, orange; Bi, purple). In step 3 the countercation is not shown for clarity but was included in the computations.
reported in parts per million (ppm) and are referenced using the
residual proton and carbon signals of the deuterated solvent (1H:
showed a continuous decrease in the energy; therefore, this
reaction step is assumed to proceed without barrier. In this
final step, the attack of the [BiPh2]− anion at the Bi center of
Ph2BiPCO formed in step 1 delivers the dibismuthine
Ph2BiBiPh2 as well as the [OCP]− anion for compound 5.
1
C6D6, δ 7.16; 13C: C6D6, δ 128.06; H: THF-d8, δ 3.58, 1.72; 13C:
THF-d8, δ 67.21, 25.31). 31P NMR chemical shifts are reported in
ppm and are referenced externally to an 85% H3PO4 solution.
Elemental analyses were performed at the University of Virginia and
Midwest Microlab, 7212 North Shadeland Avenue, Suite 110,
Indianapolis, IN 46250, USA. Single-crystal X-ray diffraction data
were collected on a Bruker Kappa APEXII Duo system. An Incoatec
Microfocus IμS (Cu Kα, λ = 1.54178 Å) and a multilayer mirror
monochromator were used for 1, 3, and 4, and a fine-focus sealed tube
(Mo Kα, λ = 0.71073 Å) and a graphite monochromator were used
for 2, 5, and 6. The frames were integrated with the Bruker SAINT
software package21 using a narrow-frame algorithm. Data were
corrected for absorption effects using the Multi-Scan method.21 The
structures were solved and refined using the Bruker SHELXTL
software package22 within APEX321 and OLEX2.23 Non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were placed in
geometrically calculated positions with Uiso = 1.2Uequiv of the parent
atom (Uiso = 1.5Uequiv for methyl). For 3, CELL_NOW24 was used to
identify a two-component twin. Starting with 1058 reflections, 889
reflections were fit to the first domain and 441 to the second domain
(165 exclusively), with 4 unindexed reflections remaining. The twin
domain was oriented at a 179.9° rotation about the reciprocal axis
0.003 0.500 1.000. The twin law was −0.994 0.009 0.004/0.622
−0.318 0.657/1.257 1.364 0.311. The structure was refined as a two-
component twin on HKLF5 data, with the BASF for the twin domain
refining to 0.12525. One isopropyl group was found to be disordered
over two positions. The relative occupancy was freely refined, and
constraints were used on the anisotropic displacement parameters of
one pair of disordered atoms. For 4, the relative occupancies of the
disordered isopropyl groups were freely refined. Constraints were
used on the anisotropic displacement parameters of the disordered
C6/C6a pair. For 5, one isopropyl group was disordered over two
positions. The relative occupancy was freely refined and restraints
were used on the anisotropic displacement parameters of the
disordered atoms. The OCP unit connecting the two carbenes was
disordered by symmetry and was therefore modeled at 50%
occupancy. The outer-sphere [OCP]− anion was disordered over
two positions, each of which was located on a symmetry element. The
relative occupancies of the different orientations were freely refined, at
CONCLUSION
■
The reaction of Ph2PnCl with Na[OCP] results in the
formation of tetraphenyldipnictine and an insoluble unidentifi-
able product. Therefore, we prepared the respective NHC-
supported Ph2PnCl compounds (1 and 2) and explored their
reactivity with Na[OCP]. In both cases, the NHC transfers
from the pnictogen center to the phosphaketene carbon atom.
The crystal structures of these two OCP complexes reveal
significant metal−metal interactions. Heating the NHC−
phosphaketene adducts 3 and 4 results in a formal reduction
at the pnictogen center, PnIII to PnII, resulting in the formation
of tetraphenyldipnictine and [(NHC)2OCP]+[OCP]− (5).
Notably, compound 5 represents the first example of an ionic
compound where the cation and anion each possess an OCP
unit. These results further demonstrate the utility of the 2-
phosphaethynolate ion as a reductant and contrast with the
chemistry observed for the group 14 (Sn and Ge) analogues,
which undergo decarbonylation to yield phosphinidenyl
species.
EXPERIMENTAL SECTION
■
General Considerations. All reactions were carried out under an
atmosphere of purified argon in a MBRAUN LABmaster glovebox
equipped with a −37 °C freezer. All solvents were distilled over
sodium/benzophenone. Glassware was oven-dried at 190 °C
overnight. Deuterated solvents were purchased from Acros Organics
and Cambridge Isotope Laboratories and were dried the same way as
their protic analogues. The NMR spectra were recorded at room
temperature on a Varian Inova 500 MHz (1H: 500.13 MHz and 31P:
202.46 MHz) and a Bruker Avance 800 MHz spectrometer (1H:
800.13 MHz, 13C: 201.19 MHz). 1H and 13C chemical shifts are
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50% occupancy to account for the symmetry and then with restraints
on the anisotropic displacement parameters and bond lengths of the
disordered atoms.
Hz, 2H, CH(CH3)2), 1.33 (s, 6H, C(backbone)−CH3), 1.06 (d, J =
7.1 Hz, 12H, CH(CH3)2). 13C{1H} NMR (201.19 MHz, C6D6): δ
203.58 (d, J = 81.7 Hz, CO), 152.03 (d, J = 49.6 Hz, CNHC), 151.04
(CPh‑i), 140.05 (CPh‑o), 130.15 (CPh‑m), 126.75 (CPh‑p), 123.45
(Cvinyl), 51.36 (N−CH−(CH3)2), 21.37 (N−CH−(CH3)2), 9.46
(Cvinyl−CH3). 31P{1H} NMR (202.46 MHz, C6D6): δ 82.17 (s, 1P).
IR: ν = 3033, 2975, 2932, 2869, 1625, 1569, 1418, 1371, 1312, 1217,
1110, 995, 928, 723, 697 cm−1. Suitable elemental analysis could not
be obtained due to solid-state instability. Thus, purity was assessed by
Synthesis of (NHC)SbPh2Cl (1). To a 20 mL scintillation vial,
Ph2SbCl (690 mg, 2.22 mmol) was added and stirred in toluene (5
mL). A toluene solution (5 mL) of NHC (400 mg, 2.22 mmol) was
added, and then, the reaction was allowed to stir for 1 h. After the
filtration, the crude solid was washed with hexanes and then dried in
vacuo. Compound 1 was obtained as a white solid (925 mg, 85%).
Crystals suitable for X-ray diffraction studies were obtained from a
toluene/hexane mixture at −37 °C. 1H NMR (C6D6, 500.13 MHz): δ
8.17 (t, 4H, CHortho), 7.22 (t, 4H, CHmeta), 7.12 (t, 2H, CHpara), 4.69
(Br, 2H, CH(CH3)2), 1.56 (s, 6H, C(backbone)−CH3), 0.81 (s, 12H,
CH(CH3)2). 13C{1H} NMR (THF-d8, 201.193 MHz): δ 146.53
(CPh‑i), 136.50 (CPh‑o), 128.51 (CPh‑m), 128.23 (CPh‑p), 125.33
(Cvinyl), 52.34 (N−CH−(CH3)2), 21.23 (N−CH−(CH3)2), 9.74
(Cvinyl−CH3). Anal. calcd for C23H30N2SbCl: C, 56.18; H, 6.15; N,
5.70%. Found: C, 55.95; H, 6.22; N, 5.68%.
Synthesis of (NHC)BiPh2Cl (2). To a 20 mL scintillation vial,
Ph2BiCl (1.111 g, 2.77 mmol) was added and stirred in toluene (5
mL). A toluene solution (5 mL) of NHC (500 mg, 2.77 mmol) was
added, and then, the reaction was allowed to stir for 1 h. After the
filtration, the crude solid was washed with hexanes and then dried in
vacuo. Compound 2 was obtained as a white solid (1.51 g, 94%).
Colorless crystals suitable for X-ray diffraction studies were obtained
from a toluene/hexane mixture at −37 °C. 1H NMR (THF-d8, 500.13
MHz): δ 8.35 (br, 4H, CHortho), 7.41 (t, J = 7.6 Hz, 4H, CHmeta), 7.21
(t, J = 7.3 Hz, 2H, CHpara), 4.51 (hept, J = 6.7 Hz, 2H, CH(CH3)2),
2.15 (s, 6H, C(backbone)−CH3), 1.17 (d, J = 7 Hz, 12H,
CH(CH3)2). 13C{1H} NMR (THF-d8, 201.19 MHz): δ 139.35
(CPh‑o), 131.45 (CPh‑m), 128.08 (CPh‑p), 126.48 (Cvinyl), 54.12 (N−
CH−(CH3)2), 22.75 (N−CH−(CH3)2), 10.28 (Cvinyl−CH3). Anal.
calcd for C23H30N2BiCl: C, 47.72; H, 5.22; N, 4.84%. Found: C,
47.37; H, 5.41; N, 4.77%.
1
immediately collecting the H, 13C, and 31P NMR data of a freshly
made sample of 4.
Synthesis of [NHC−PC(O)−(NHC)][OCP] (5). To a 20 mL
scintillation vial, (NHC)BiPh2Cl (505 mg, 869 μmol) was added and
suspended in 10 mL of dry THF. Na[OCP]·(dioxane)x (262 mg, 869
μmol) was added to the suspension, and the suspension was shaken
vigorously for 1 min. The reaction mixture was then extracted into a
100 mL Schlenk tube. Orange crystals of [(NHC)2OCP][OCP]
formed from the solution after sitting undisturbed at 55 °C overnight
1
(106 mg, 51%). H NMR (500.13 MHz, CD2Cl2): δ 5.35 (br, 4.99,
2H, CH(CH3)2) (hept, J = 7.0 Hz, 2H, CH(CH3)2), 2.38 (s, 6H,
C(backbone)−CH3), 2.35 (s, 6H, C(backbone)−CH3), 1.62 (d, J =
7.1 Hz, 12H, CH(CH3)2), 1.58 (d, J = 7.1 Hz, 12H, CH(CH3)2).
13C{1H} NMR (201.19 MHz, CD2Cl2): δ 200.71 (d, J = 64.2 Hz, C
O), 170.20 (d, J = 63.0 Hz, OCP), 150.24 (d, J = 86.9 Hz, CNHC−P),
146.04 (d, J = 67.5 Hz CNHC−CO), 128.8 (Cvinyl), 126.5 (Cvinyl),
53.9 (N−CH−(CH3)2), 52.5 (N−CH−(CH3)2), 21.9 (N−CH−
(CH3)2), 21.8 (N−CH−(CH3)2), 11.0 (Cvinyl−CH3), 10.7 (Cvinyl
−
CH3). 31P{1H} NMR (242.94 MHz, CD2Cl2): δ 22.72 (br, 1P,
[OCP]+), −395.09 (s, 1P, [OCP]−). IR: ν = 3044, 2973, 2934, 2873,
1788, 1768, 1634, 1574, 1429, 1371, 1312, 1217, 1051, 930, 729, 729,
699 cm−1. Anal. calcd for C24H40N4O2P2: C, 60.24; H, 8.43; N,
11.71%. Found: C, 60.09; H, 8.52; N, 11.70%.
Computational Details. The computations were carried out with
the Gaussian 09 suite of programs.23 The structures were optimized
using the ωB97XD functional in combination with the def2-SVP and
the def2-TZVP basis sets. At each of the optimized structures
vibrational analysis was accomplished to check whether the stationary
point located is a minimum or a saddle point of the potential energy
hypersurface. We neglected the solvent effect because toluene was
used as the solvent. For Wiberg Bond Indexes and NPA charges, the
NBO program version 5.0 was employed.24 The plotting of the
orbitals was carried out with the AVOGADRO program (www.
avogadro.cc).
Synthesis of NHC−C(O)P-SbPh2 (3). To a 20 mL vial,
(NHC)BiPh2Cl (97 mg, 0.197 mmol) was added and stirred in
THF. Na[OCP]•(dioxane)x (65 mg, 0.217 mmol) was added to the
stirring solution. upon addition, the solution immediately turned
yellow. After stirring for 5 min at room temperature, insoluble NaCl
was removed by filtration, and the yellow THF solution was layered
with hexanes in a 1:1 ratio and allowed to sit for 1 day at −37 °C.
After removal of the solvent and drying in vacuo, the product was
obtained as a yellow crystalline solid (50 mg, 49% yield). Note:
compound 3 decomposes to 5 and Ph4Bi2 at room temperature. Note:
compound 3 decomposes to 5 and Ph4Bi2 at −37 °C. 1H NMR
(C6D6, 500.13 MHz) δ 8.18 (d, J = 7.9 Hz, 4H, CHortho), 7.21−7.15
(m, 4H, CHmeta), 7.12 (t, J = 7.3 Hz, 2H CHpara), 5.10 (hept, J = 6.9
Hz, 2H, CH(CH3)2), 1.34 (s, 6H, C(backbone)−CH3), 1.06 (d, J =
7.1 Hz, 12H, CH(CH3)2). 13C{1H} NMR (201.19 MHz, C6D6) δ
203.13 (d, J = 76.0 Hz, CO), 148.84 (d, J = 52.8 Hz, CNHC), 141.01
(CPh‑i), 137.73 (CPh‑o), 128.28 (CPh‑m), 127.28 (CPh‑p), 123.30
(Cvinyl), 51.16 (N-CH-(CH3)2), 21.08 (N−CH-(CH3)2), 9.25
(Cvinyl-CH3). 31P{1H} NMR (202.46 MHz, C6D6) δ 58.18 (s, 1P).
IR: ν = 3040, 2971, 2934, 2854, 1625, 1574, 1427, 1371, 1310, 1217,
1105, 1051, 928, 729, 697 cm−1. Suitable elemental analysis could not
be obtained due to solid-state instability. Thus, purity was assessed by
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c03683.
NMR spectral data; IR spectral data; crystal structure of
compound 6; crystallographic refinement details; and
computational details (PDF)
Accession Codes
CCDC 1962839−1962843 and 1976639 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
immediately collecting the H, 13C, and 31P NMR data of a freshly
made sample of 3.
Synthesis of NHC−C(O)P-BiPh2 (4). To a 20 mL vial,
(NHC)BiPh2Cl (200 mg, 0.344 mmol) was added and stirred in
THF. Na[OCP]·(dioxane)x (302 mg, 0.344 mmol) was added to the
stirring solution. Immediately upon addition, the solution turned
yellow. After it stirred for 5 min at room temperature, insoluble NaCl
was removed by filtration and the yellow THF solution was layered
with hexanes in a 1:1 ratio and allowed to sit for 1 day at −37 °C.
After removal of the solvent and drying in vacuo, the product was
obtained as a yellow crystalline solid (92 mg, 48% yield). Note:
AUTHOR INFORMATION
■
Corresponding Authors
̋
Zoltán Benko − Department of Inorganic and Analytical
1
Chemistry, Budapest University of Technology and
Economics, H-1111 Budapest, Hungary; orcid.org/0000-
0001-6647-8320; Email: zbenko@mail.bme.hu
Compound 4 decomposes to 5 and Ph4Bi2 at −37 °C. H NMR
(C6D6, 500.13 MHz): δ 8.51 (d, J = 7.6 Hz, 4H, CHortho), 7.25 (t, J =
7.5 Hz, 4H, CHmeta), 7.20−7.13 (m, 2H, CHpara), 5.13 (hept, J = 6.9
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Article
Grutzmacher, H. N-Heterocyclic carbene phosphaketene adducts as
̈
Robert J. Gilliard, Jr. − Department of Chemistry, University
of Virginia, Charlottesville, Virginia 22903, United States;
orcid.org/0000-0002-8830-1064; Email: rjg8s@
virginia.edu
precursors to carbene−phosphinidene adducts and a rearranged π-
system. Chem. Commun. 2016, 52, 11343−11346. (e) Li, Z.; Chen,
̋
X.; Benko, Z.; Liu, L.; Ruiz, D. A.; Peltier, J. L.; Bertrand, G.; Su, C.-
Y.; Grutzmacher, H. N-Heterocyclic Carbenes as Promotors for the
̈
Rearrangement of Phosphaketenes to Phosphaheteroallenes: A Case
Study for OCP to OPC Constitutional Isomerism. Angew. Chem., Int.
Ed. 2016, 55, 6018−6022. (f) Weber, L. 2-Phospha- and 2-
Arsaethynolates − Versatile Building Blocks in Modern Synthetic
Chemistry. Eur. J. Inorg. Chem. 2018, 2018, 2175−2227.
Authors
Jacob E. Walley − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22903, United States;
orcid.org/0000-0003-1495-6823
Levi S. Warring − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22903, United States
Erik Kertész − Department of Inorganic and Analytical
Chemistry, Budapest University of Technology and
Economics, H-1111 Budapest, Hungary
(5) (a) Appel, R.; Laubach, B.; Siray, M. Einschiebungsreaktion von
kohlendioxid in disilylierte phosphane. Tetrahedron Lett. 1984, 25,
̋
4447−4448. (b) Heift, D.; Benko, Z.; Grutzmacher, H. Coulomb
̈
repulsion versus cycloaddition: formation of anionic four-membered
rings from sodium phosphaethynolate, Na(OCP). Dalton Trans.
2014, 43, 831−840.
Guocang Wang − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22903, United States;
orcid.org/0000-0002-4277-6056
̋
(6) (a) Gilliard, R. J.; Heift, D.; Benko, Z.; Keiser, J. M.; Rheingold,
A. L.; Grutzmacher, H.; Protasiewicz, J. D. An isolable magnesium
̈
diphosphaethynolate complex. Dalton Trans. 2018, 47, 666−669.
(b) Westerhausen, M.; Schneiderbauer, S.; Piotrowski, H.; Suter, M.;
Nöth, H. Synthesis of alkaline earth metal bis(2-phosphaethynolates).
J. Organomet. Chem. 2002, 643−644, 189−193. (c) Jupp, A. R.;
Goicoechea, J. M. The 2-Phosphaethynolate Anion: A Convenient
Synthesis and [2 + 2] Cycloaddition Chemistry. Angew. Chem., Int. Ed.
2013, 52, 10064−10067. (d) Hennersdorf, F.; Frötschel, J.; Weigand,
J. J. Selective Derivatization of a Hexaphosphane from Functionaliza-
tion of White Phosphorus. J. Am. Chem. Soc. 2017, 139, 14592−
14604.
Diane A. Dickie − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22903, United States;
orcid.org/0000-0003-0939-3309
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c03683
Notes
The authors declare no competing financial interest.
(7) (a) Hoerger, C. J.; Heinemann, F. W.; Louyriac, E.; Maron, L.;
ACKNOWLEDGMENTS
Grutzmacher, H.; Meyer, K. Formation of a Uranium-Bound η1-
̈
■
Cyaphide (CP−) Ligand via Activation and C−O Bond Cleavage of
Phosphaethynolate (OCP−). Organometallics 2017, 36, 4351−4354.
The authors are grateful to the University of Virginia for
support of this work. Further support came from NKFIH PD
116329, the Bolyai Research Fellowship of MTA, an UNKP
grant (UNKP-20-5-BME-317) and a BME Nanotechnology
and Materials Science TKP2020 IE grant from NKFIH
Hungary (BME IE-NAT TKP2020).
̀
(b) Camp, C.; Settineri, N.; Lefevre, J.; Jupp, A. R.; Goicoechea, J. M.;
Maron, L.; Arnold, J. Uranium and thorium complexes of the
phosphaethynolate ion. Chem. Sci. 2015, 6, 6379−6384. (c) Bestgen,
S.; Chen, Q.; Rees, N. H.; Goicoechea, J. M. Synthesis and reactivity
of rare-earth metal phosphaethynolates. Dalton Trans. 2018, 47,
13016−13024.
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