Reaction of carbon oxides with an ethylene-bridged PH/B Lewis pair

Article abstract of DOI:10.1039/d1dt00459j

The reaction of 2,4,6-tri(tert-butyl)phenyl vinyl phosphane with Piers’ borane [HB(C₆F₅)₂] gave the ethylene-bridged PH/B frustrated Lewis pair (FLP) system. It is a monomer at high temperature (>323 K), but exists as an associated 12-membered macrocyclic trimer below 273 K. The PH/B FLP splits dihydrogen and serves as a metal-free hydrogenation catalyst. It adds carbon dioxide. It serves as a PH/B template for the reduction of carbon monoxide by the HB(C₆F₅)₂borane to the formyl stage. The resulting six membered P/B/O containing heterocycle is opened upon treatment with pyridine and it reacts with benzaldehyde in a boron mediated Claisen-Tishchenko reaction.

Full text of DOI:10.1039/d1dt00459j

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Reaction of carbon oxides with an ethylene-
bridged PH/B Lewis pair†
Cite this: Dalton Trans., 2021, 50,
3523
Qiu Sun,
Constantin G. Daniliuc,
Gerald Kehr
and Gerhard Erker
*
The reaction of 2,4,6-tri(tert-butyl)phenyl vinyl phosphane with Piers’ borane [HB(C6F5)2] gave the ethyl-
ene-bridged PH/B frustrated Lewis pair (FLP) system. It is a monomer at high temperature (>323 K), but
exists as an associated 12-membered macrocyclic trimer below 273 K. The PH/B FLP splits dihydrogen
and serves as a metal-free hydrogenation catalyst. It adds carbon dioxide. It serves as a PH/B template for
the reduction of carbon monoxide by the HB(C6F5)2 borane to the formyl stage. The resulting six mem-
bered P/B/O containing heterocycle is opened upon treatment with pyridine and it reacts with benz-
aldehyde in a boron mediated Claisen-Tishchenko reaction.
Received 10th February 2021,
Accepted 11th February 2021
DOI: 10.1039/d1dt00459j
rsc.li/dalton
In a way the PH/B system 5 may be looked at as a missing
link between the FLPs 3 and 4. We have now prepared the
Introduction
Intramolecular phosphane/borane pairs have played a major ethylene-bridged PH/B compound 5 and investigated its behav-
role in the development of the chemistry of reactive frustrated iour toward a variety of small molecules, among them CO and
Lewis pairs (FLPs).1,2 The ethylene-bridged vicinal system 1 may CO2. These results will be described in this account.
be regarded as the parent compound3 of a series of internal
FLPs, designed for metal-free small molecular binding and acti-
vation. Early on in the development the system 1 served to find
Results and discussion
Preparation of the PH/B FLP 5 and first reactions
novel pathways in the chemistry of the carbon oxides.4 It was
one of a pair of first P/B frustrated Lewis pairs to bind carbon
dioxide,5 and it was used to develop metal-free pathways for the
conversion of carbon monoxide. Compound 1 served as one out
of a small family of intramolecular FLPs to serve as a template
for the reduction of the CO molecule to the formyl stage.6 This
was important as simple BH boranes cannot achieve that trans-
formation. They form borane carbonyls instead, unless cata-
lyzed.7 Compound 1 adds nitric oxide (NO) to form a persistent
FLPNO radical,8 it splits dihydrogen and serves as a metal-free
hydrogenation catalyst.
Compound 5 was prepared by reacting Mes*P(H)vinyl with
Piers’ borane [HB(C6F5)2]12 in a 1 : 1 molar ratio (pentane, r.t.,
15 h). This gave a suspension, and the product was isolated as
a white solid in 90% yield by filtration (Scheme 2). The NMR
and structural analysis revealed a strongly temperature depen-
dent equilibrium between the monomer 5 and an associated
cyclotrimeric structure (5)3.13 In the NMR spectra at r.t. both
components are present in solution (C6D6: 5 : (5)3 ∼8 : 1). At
323 K only the monomer 5 was observed. It showed the typical
heteronuclear magnetic resonance signals of an open, non-
interacting P/B pair [11B: δ 60.7, Δδ19Fm,p = 12.6 ppm, 31P: δ
−56.2 (d, 1JPH ∼ 250 Hz, 1H: δ 5.33), in d6-benzene] in addition
to the typical 1H/13C NMR resonances of the –CH2–CH2–
bridge and the bulky Mes* ligand at phosphorus.
Subsequently, the BH, respectively PH functionalized ethyl-
ene-bridged relatives 2–4 had been prepared and their chemistry
investigated.9–11 They showed similar FLP behaviour toward
small molecules as 1, but some of the observed reaction path-
ways were determined by the presence of the BH or PH func-
tional groups. Compound 3 stoichiometrically reduced CO to
the formyl stage and CO2 to the formaldehyde stage.10 The PH/
BH system 4 reacted with carbon monoxide by complete clea-
vage of the strong carbon oxygen bond.11
At 273 K the spectrum of the associated structure was
almost exclusively observed. We assume the analogous cyclotri-
meric composition as it was observed in the solid state by
X-ray diffraction (see below). In CD2Cl2 solution at 273 K com-
pound (5)3 showed a 31P NMR signal at δ −12.8 (br d, JPH
∼
1
375 Hz, 1H: δ 5.92) and a 11B NMR resonance at ca. δ −3. There
are the signals of the pairs of diastereotopic C6F5 substituents
at boron. The rotation around the B-C6F5 vectors are hindered,
so a total of each four 19F NMR o- and m-C6F5 as well as two
p-C6F5 signals were observed. The Δδ19Fm,p separations are
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149 Münster, Germany. E-mail: erker@uni-muenster.de
†Electronic supplementary information (ESI) available. CCDC 2054471–2054477.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1dt00459j
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Fig. 1 A view of the macrocyclic molecular structure of the cyclotri-
meric FLP associate (5)3 (thermal ellipsoids at 30% probability; only the
ipso-carbon atoms of the C6F5 groups at boron and the Mes* groups at
phosphorus are shown).
Scheme 1 Functionalized ethylene-bridged phosphorus/boron Lewis
pairs (Mes = mesityl, Mes* = 2,4,6-tri(tert-butyl)phenyl, Fmes = 2,4,6-
tris(trifluoromethyl)phenyl).
Scheme 2 Synthesis and aggregation of the PH/B FLP 5.
small as it is typical for a four-coordinate borate-type situation.
In the 1H NMR spectrum pairs of PCH2 and BCH2 proton reso-
nances were observed. The Mes*-P rotation is also hindered
and, consequently, separate signals of the pairs o-tBu and the
arene methine CH groups were monitored (the spectra are
depicted in the ESI†).
Scheme 3 Typical FLP reactions of compound 5.
Single crystals for the X-ray crystal structure analysis of (5)3
were obtained from pentane at r.t. It shows that the FLP 5 has
The FLP 5 splits dihydrogen reversibly. Under ambient con-
formed an associated cyclotrimeric structure featuring a ditions it reacts in dichloromethane with H2 to give a mixture of
twelve-membered heterocyclic core that contains three phos- compounds 5, (5)3 and the dihydrogen splitting product 6 (ca.
phorus and three boron atoms. It shows three newly formed P– 49 : 30 : 20; 31P) (Scheme 3). The product of heterolytic H2/FLP
B linkages with bond lengths of 2.091(5) Å (P1–B3), 2.131(4) Å splitting shows NMR signals of the [P]H2 phosphonium [1H: δ
1
(P2–B1) and 2.112(4) Å P3–B2. The P–C and B–C bond lengths 7.33 (dm, JPH = 488.3 Hz, 31P: δ −23.7 (t))] and the [B]H hydri-
are in a typical range (e.g., P1–C1 1.857(4) Å, B1–C2 1.650(5) Å). doborate [11B: δ −21.6 (1JBH ∼ 85 Hz)] in addition to the typical
The three connecting ethylene units show anti-periplanar con- NMR features of the Mes*PCH2CH2B(C6F5)2 framework. The 5/6
formations. The averaged bond angles at the heteroatoms equilibrium system serves as an active hydrogenation catalyst.
amount to ∑B1CCC = 338.2° and ∑P1BCC = 354.8°, with the The substrate N-tert-butyl-1-phenylmethanimine was rapidly
latter not deviating too much from co-planarity. The fourth reduced to benzyl-tert-butylamine by treatment with dihydrogen
substituent at P is hydrogen. In the solid state structure, the in the presence of 10 mol% of the FLP catalyst precursor 5
three P–H vectors are all-cis arranged. The molecule is close to (CD2Cl2, 1.5 bar H2, r.t., >99% conversion within 15 min).
(but not crystallographically) C3 symmetric (see Fig. 1). The
Compound
5
reacts rapidly with phenylacetylene
Mes* substituents at P are markedly bent (e.g. P1–C11⋯C14, (Scheme 3). Consecutively, two typical FLP reactions were
averaged 24°) as it is often observed in Mes* containing phos- observed. Within 30 min at r.t. the C–H activation15 product 7
1
1
phorus compounds.14
was formed [NMR; 13C: δ 97.1 (PhCu), H: δ 7.31 (ddd, JPH
=
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479.8 Hz, [P]H2 ), 31P: δ −21.8 (t), 11B: δ −18.1]. Compound 7
+
was isolated in 95% yield. Upon storage of a pentane solution
of compound 7 at r.t. for 3 days, crystals of the rearrangement
product 8 were obtained in almost quantitative yield. We
assume that the C–H activation reaction of 5 with phenyl-
acetylene to give 7 is actually reversible and that then the regio-
selective 1,2-P/B addition16 of the regenerated FLP 5 to the
alkyne gave 8. The heterocycle 8 was characterized by spec-
troscopy and by an X-ray crystal structure analysis (see the ESI†
for details and the depicted structure).
Reaction of the FLP 5 with carbon dioxide
Compound 5 was exposed to carbon dioxide5,17 (2.0 bar,
heptane solution, 80 °C, 15 min). Slow cooling of the reaction
mixture to r.t. gave colourless crystals of compound 9 that
were isolated in 90% yield (Scheme 3). The X-ray crystal struc-
ture analysis (see Fig. 2) showed that the PH/B FLP had added
to a CvO group of the CO2 molecule to form a six-membered
half-chair shaped heterocycle. It contains a tricoordinate phos-
phorus (P1) and a four-coordinate boron atom (B1). The
remaining carbonyl function was trapped by the B(C6F5)2
Lewis acid of a second molecule of 5 and a proton was trans-
ferred from P1 to P2 to form the observed adduct product 9.
The NMR spectra (CD2Cl2, 299 K) of compound 9 show two
sets of 1H NMR signals of the Mes* groups of each component
of the “dimer”. A single broad 11B NMR feature was observed
at ca. δ 1.7, whereas two separate 31P NMR signals were moni-
Fig. 3 Molecular structure of the CO reduction product 11. Only one of
the two independent molecules in the unit cell is depicted (thermal
ellipsoids at 15% probability). Selected bond lengths (Å) and angles (°):
P1A–C3A 1.838(3), C3A–B2A 1.681(5), C3A–O1A 1.453(4), O1A–B1A
1.349(4), B1A–C2A 1.573(5), C2A–C1A 1.546(5), C1A–P1A 1.822(3), P1A–
C11A 1.814(3), C1A–P1A–C3A 109.8(2), C3A–O1A–B1A 128.3(3), O1A–
B1A–C2A 123.8(3).
The X-ray crystal structure analysis of compound 11 (see
Fig. 3) revealed that a six-membered heterocyclic core had
been formed by formal addition of the CO carbon atom to
phosphorus and the oxygen atom to the boron atom of the
FLP reagent 5. Carbon atom C3 has the bulky B(C6F5)3 group
attached and a hydrogen atom. The adjacent phosphorus atom
P1 bears a hydrogen and represents a phosphonium unit. We
note that the boron atom B1 has only one C6F5 substituent
attached. The missing C6F5 group had apparently been trans-
ferred to B2 to build up the B(C6F5)3 building block.
We assume a pathway that involves the intermediate for-
mation of 10. This may have been formed after C6F5 for H
exchange between 5 and HB(C6F5)2.9,18 In a separate experi-
ment we found that a sizable amount of B(C6F5)3 was gener-
ated from 5 and HB(C6F5)2 in the absence of CO under other-
wise similar conditions. P/B FLP addition to the in situ gener-
ated borane carbonyl (C6F5)3B-CO7,19 might have provided a
pathway of the straightforward formation of 10. Internal
hydride reduction of the activated endocyclic CvO unit would
then directly open a pathway to 11 (see Scheme 4).
tored [δ −20.8 (tm, 1JPH ∼ 475 Hz, [P]H2 ); δ −21.5 [P]-CO]. The
+
[P]H2+ unit gives rise to a 1H NMR resonance at δ 7.02. The car-
bonyl 13C NMR signal of the former CO2 building block inside
the six-membered heterocyclic core was located at δ 196.0 (d,
1JPC = 37.4 Hz) (see the ESI† for further details).
Carbon monoxide reactions at the FLP template 5
Compound 5 itself did not react with carbon monoxide. However,
in the presence of Piers’ borane [HB(C6F5)2] the CO molecule was
reduced (2 bar CO, r.t., 12 h in dichloromethane) to give com-
pound 11 (isolated in 95% yield after workup; Scheme 4).
We note that compound 11 contains two stereogenic
centers, one at phosphorus and one at the adjacent carbon
Fig. 2 Molecular structure of compound 9, derived from the reaction
of the FLP 5 with carbon dioxide (thermal ellipsoids at 30% probability;
for clarity, only the ipso carbon atoms of the aryl substituents are
depicted) Selected bond lengths (Å) and angles (°): C2–B1 1.608(3), B1–
O1 1.593(3), O1–C5 1.282(2), C5–P1 1.835(2), C5–O2 1.277(2), O2–B2
1.549(3), C2–B1–O1 110.8(2), B1–O1–C5 127.6(2), C5–O2–B2 127.5(2),
∑P1CCC 320.1.
Scheme 4 Carbon monoxide reduction at the FLP template 5.
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atom C3. The isolated material contains only one of the poss- centers in the core chain. In the crystal the rel-3(R),(S)p dia-
ible diastereoisomers, namely the one that has the pair of the stereoisomer was observed.22,23
very bulky Mes* group at phosphorus and the B(C6F5)3 substi-
In solution (d2-dichloromethane, 213 K) the NMR signals of
tuent at the adjacent carbon atom trans-1,2-positioned.
two compounds were observed in a ca. 56 : 44 ratio. We assign to
In d2-dichloromethane solution compound 11 shows PH them the structures of the pair of possible diastereoisomers of
phosphonium NMR signals at δ 6.87 (1H) and δ −19.5 (31P), compound 12. The major isomer shows the signal of the [P]H
1
respectively, with a 1JPH ∼ 487 Hz coupling constant. There are moiety at δ 4.34 (1H, JPH = 221.5 Hz) and δ −69.6 (31P, both at
two 11B NMR resonances at δ 43.6 (OBC6F5) and δ −11.5 213 K). There is hindered rotation around the Mes*-P vector,
1
(B(C6F5)3). The H NMR signals of two pairs of diastereotopic giving rise to the observation of three 1H NMR tBu signals of 12-
methylene hydrogens were observed for the [P]CH2CH2[B] major at δ 1.18 (p), 1.30, 1.35 (o) and two arene methine CH reso-
unit, and the newly formed [P]CHO[B] unit gives rise to NMR nances. The C3–H NMR signal of 12-major occurs at δ 6.74. The
1
resonances at δ 6.31 (1H) and δ 69.0 (13C), respectively. Both 12-minor isomer shows the corresponding C3–H H NMR signal
1
the bulky Mes*-[P] and the [C]-B(C6F5)3 units show hindered at δ 6.79 (at 213 K) and the [P]H resonances at 4.62 (1H, JPH
=
rotation at 299 K.
221.5 Hz) and δ −69.0 (31P). There are also the NMR signals of
t
The six-membered heterocyclic core of compound 11 was three separate Bu substituents and a pair of arene CH signals.
opened when treated with pyridine (CD2Cl2, r.t., 2 h). Workup We observed a single set of pyridine NMR resonances and two
involving crystallization from pentane gave the ring opened 11B NMR signals [299 K, δ 47.9 (BO), δ −0.9 (B-pyr)] (the spectra
product 12, which was isolated as colourless crystals in 80% are depicted in the ESI†).
yield (see Scheme 4). Compound 11 is thermally stable for
The reaction of compound 11 with benzaldehyde also pro-
quite some time at room temperature. Therefore, we assume ceeded with migration of a C6F5 group from boron to the adja-
that the rather fast 11 to 12 transformation is probably pyri- cent carbonyl carbon atom. The reaction was carried out in a
dine initiated, but the detailed mechanism of this reaction has 1 : 2 molar ratio in d2-dichloromethane at r.t. (48 h reaction
remained unknown at this time.
time). The in situ NMR analysis indicated the formation of a
The X-ray crystal structure analysis of compound 12 showed 1 : 1 mixture of the products 14 and 15, that were apparently
the acyclic framework that contained the heteroatoms P1, B1, formed in a borane mediated Claisen-Tishchenko24 type dis-
O1 and B2 plus the pyridine donor attached at the latter boron proportionation reaction (see Scheme 5). Compound 14
atom. The internal B1–O1 bond is short indicating some π showed the signals of the Mes* substituent at phosphorus
interaction.20 The framework shows a sickle shaped confor- [PH: δ 4.77 (1JPH = 221.1 Hz, 1H), δ −65.9 (31P)]. It features a
mation (see Fig. 4). The formation of compound 12 involved 11B NMR signal at 48.1 and shows the 1H/13C NMR resonances
rupture of the phosphorus carbon bond and migration of one of the newly formed [B]-OCH2-Ph methylene group at δ 5.10 (s,
C6F5 substituent of the B(C6F5)3 group from boron to the adja- 2H, 1H) and δ 70.5 (13C). The ethylene-bridge shows the 1H
cent carbon atom. So, in the overall process the carbon monox- NMR signals of two pairs of diastereotopic hydrogen atoms
ide molecule was converted to a [B]-CH(C6F5)-O-[B] moiety in (13C NMR signals at δ 21.6, 19.5).
these consecutive steps.21 Compound 12 contains two chirality
The phosphorus free compound 15 was isolated from a sep-
arate experiment and isolated as a colourless crystalline solid.
It was characterized by an X-ray crystal structure analysis (crys-
tals from pentane at r.t.). It shows the presence of a planar
five-membered heterocyclic core with the phenyl substituent at
carbon atom C2 oriented coplanar with the core (the two inde-
pendent molecules found in the crystal are chemically equi-
valent). Both the O1–C2 (1.263(2) Å) and C2–O2 (1.297(2) Å) are
short, indicating a delocalized structure (see Fig. 5).
Fig. 4 Molecular structure of the ring-opened product 12 (thermal
ellipsoids at 30% probability). Selected bond lengths (Å) and angles (°):
B2–N71 1.617(3), B2–C3 1.663(3), C3–O1 1.459(2), O1–B1 1.351(3), B1–
C2 1.574(3), C2–C1 1.549(3), C1–P1 1.858(2), C1–P1–C11 102.3(1),
∑B1OCC 360.0, B1–O1–C3–B2–152.7(2), C2–B1–O1–C3–10.1(3), P1–
C1–C2–B1 48.0(2).
Scheme 5 Reaction of compound 11 with benzaldehyde.
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Acknowledgements
Q. S. thanks the Alexander von Humboldt-Stiftung for a post-
doctoral stipend.
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Conclusion
The PH/B compound 5 is a rather active frustrated Lewis pair.
The monomeric form has an open structure. Different from
the very parent compound of this series, the Mes2PCH2CH2B
(C6F5)2 system 13 (see Scheme 1) compound 5 has no internal
P⋯B interaction. However, that is offset at low temperature by
the entropically favoured intermolecular P–B bond formation
that led to the observation of the unique 12-membered macro-
cyclic associated structure (5)3. The chemistry of the system is,
however, dominated by the monomer. It undergoes a number of
typical frustrated Lewis pair reactions. It rapidly cleaves dihydro-
gen and serves as a rather active metal-free hydrogenation cata-
lyst. It C–H activates a terminal acetylene under kinetic control
and forms a heterocyclic product by P/B addition to the carbon–
carbon triple bond under thermodynamic control. The PH unit
is indirectly involved in the FLP reaction with CO2. The 2 : 1 FLP/
CO2 product is apparently stabilized by H+-transfer to the second
PH unit. In the CO reduction chemistry, the PH unit serves as
the essential FLP Lewis base component, but its P-bonded hydro-
gen is not directly involved in the overall reaction, aside from
influencing the rates of the reaction. Both the CO2 as well as the
CO chemistry at the PH/B FLP 5 show some new features, such
as the benzaldehyde disproportionation reaction under borane
control.
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Conflicts of interest
The authors declare no competing financial interests.
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