The [(NHC)B(H)C6F5]+ Cations and Their [B](H)?CO Borane Carbonyls

Article abstract of DOI:10.1002/anie.202009353

Hydride abstraction from the heterocyclic carbene borane adducts (NHC)BH₂C₆F₅ (NHC: IMes or IMe₄) gave the B?H containing [(NHC)B(H)C₆F₅]⁺ borenium cations. They added carbon monoxide to give the respective [(NHC)B(H)(C₆F₅)CO]⁺ boron carbonyl cations. Carbon nucleophiles add to the boron carbonyl to give [B](H) acyls. Hydride reduced the [B]CO cation to hydroxymethylborane derivatives.

Full text of DOI:10.1002/anie.202009353

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Accepted Article
Title: The [(NHC)B(H)C6F5]+ Cations and their [B](H)-CO Borane
Carbonyls
Authors: Gerhard Erker, Chaohuang Chen, Jun Li, Constantin G.
Daniliuc, Christian Mück-Lichtenfeld, and Gerald Kehr
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The [(NHC)B(H)C6F5]+ Cations and their [B](H)-CO Borane
Carbonyls
Chaohuang Chen,[a] Jun Li,[a] Constantin G. Daniliuc,[a] Christian Mück-Lichtenfeld,[a] Gerald Kehr,[a]
and Gerhard Erker*[a]
Dedication ((optional))
[a]
Dr. C. Chen, Dr. J. Li, Dr. C. G. Daniliuc, Dr. C. Mück-Lichtenfeld, Dr. G. Kehr, Prof. Dr. G. Erker
Organisch-Chemisches Institut
Westfälische Wilhelms-Universität Münster
Corrensstraße 40, 48149 Münster (Germany)
E-mail: erker@uni-muenster.de
Supporting information for this article is given via a link at the end of the document.
Abstract: Hydride abstraction from the heterocyclic carbene borane
adducts (NHC)BH2C6F5 (NHC: IMes or IMe4) gives the B–H
containing [(NHC)B(H)C6F5]+ borenium cations. They add carbon
monoxide to give the respective [(NHC)B(H)(C6F5)CO]+ boron
carbonyl cations. Carbon nucleophiles add to the boron carbonyl to
give [B](H) acyls. Hydride reduces the [B]CO cation to
hydroxymethylborane derivatives.
The borenium salt 3a was characterized by X-ray diffraction
(Figure 1). The cation features a planar-tricoordinate borenium
ion structure. The boron–C(NHC) and boron–C6F5 linkages are
about equal in length. The bulky mesityl substituents are both
rotated >70° from the central NHC plane, whereas the C6F5
plane is rotated a little less from the central B–NHC plane (ca.
40°). The [(IMe4)B(H)C6F5]+ borenium system 3b was generated
analogously. It was in situ characterized spectroscopically.
Borocations show an interesting chemistry.[1] They are
increasingly used as reactive boron containing reagents[2] and
as active catalysts in organic synthesis.[3] Two types of B–H
containing borocations were described, the tetracoordinated
boronium ions[4] and the less frequently encountered B–H
containing planar-tricoordinate borenium ions. The latter were
often only spectroscopically characterized[5,6] or postulated as
intermediates.[7] Alcarazo et al. described a rare example that
was characterized by X-ray diffraction, namely the
[(carbodiphosphorane)BH2]+ cation.[8] H. Wang had structurally
characterized the HSiEt3 adduct of an [(NHC)(σ-carboranyl)BH]+
cation recently.[9] We have now found a convenient entry to
strongly electrophilic [(NHC)B(H)C6F5]+ cations and have begun
to develop their typical chemistry.[10] These compounds are of
interest for studying CO reduction chemistry. The neutral B–H
boranes usually do not reduce carbon monoxide unless
catalyzed: they form borane carbonyls instead.[11,12] This posed
the question whether our new [(NHC)B(H)C6F5]+ B–H borenium
cations would behave similarly or if these strongly electrophilic
B–H cations would exhibit a different behavior toward the CO
molecule.
Scheme 1. Synthesis of B–H-containing borenium cations.
We reacted each of the N-heterocyclic carbenes (NHCs) 1,3-
dimesitylimidazol-2-ylidene (IMes, 1a) and 1,3,4,5-tetramethyl-
imidazol-2-ylidene (IMe4, 1b) with Lancaster’s reagent
[H2B(C6F5)SMe2][13] and isolated the NHC-borane adducts in
good yield [(IMes)BH2C6F5 2a, 80%; (IMe4)BH2C6F5, 2b, 72%].
Both were characterized spectroscopically [2a: 11B NMR: δ -32.0
(1JBH ~ 90 Hz); 13C NMR: δ 170.1 (“carbene-C”)] and by X-ray
diffraction (see the Supporting Information for details including
the depicted structures).
Figure 1. Molecular structure of compound 3a (only the cation is depicted;
thermal ellipsoids are set at 30% probability). Selected bond lengths (Å) and
angles (°): C1–B1 1.559(3), B1–C31 1.557(8), C1–N1 1.362(3), C1–N2
1.356(3); N1–C1–N2 105.6(2), C1–B1–C31 128.3(8), C1–N2–C21–C22
73.4(3), C2–N1–C11–C12 -71.8(3).
-
Both the compounds 3a and 3b are strong Lewis acids. The
simplified Gutmann Beckett method[14] gave a shifting of the 31P
NMR signal of the triethylphosphane donor (TPO) upon
coordination to 3a of Δδ = +35.6 ppm (in CD2Cl2 solution; 3b:
+37.6 ppm), which renders them more Lewis acidic than
Hydride abstraction from (IMes)BH2C6F5 (2a) with trityl
tetrakis(pentafluorophenyl)borate gave the [(IMes)B(H)C6F5]
1
[B(C6F5)4] salt 3a in good yield [11B: δ 27.1, H: δ 5.02 ([B]H) in
1
C6D5Br; 11B: δ 50.6, H: δ 5.90 ([B]H) in CD2Cl2] (Scheme 1).
1
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B(C6F5)3 (Δδ = +26.7 or Piers’ borane HB(C6F5)2 (Δδ = +31.4
ppm).
characterized by X-ray diffraction (see the Supporting
Information for details).
The
cation
of
3a
reacted
rapidly
at
r.t.
with
allyl(dimesityl)phosphane to give the cationic frustrated P/B
Lewis pair 8 [NMR in CD2Cl2: δ -22.6 (31P), δ 70.3 (11B)].
19
Together with the Δδ
F
m,p
= 12.4 ppm chemical shift difference
these data indicate Lewis acidic planar tricoordinate boron and,
consequently, an open FLP structure of compound 8 in solution.
The X-ray crystal structure analysis confirmed this (see Fig. 2).
The FLP 8 is an active metal-free hydrogen splitting reagent.
Reaction with dihydrogen (1.0 bar, r.t., 1h) gave the
[PH/BH(IMes)][B(C6F5)4] salt 9, isolated in 75% yield. It was
characterized by C,H,N elemental analysis, by NMR
spectroscopy [PH: δ 7.14 (1H), δ -12.3 (31P), 1JPH = 467 Hz, BH:
δ -22.8 (1JBH = 82 Hz)] and by an X-ray crystal structure analysis
(depicted in the Supporting Information). The reaction of the FLP
8 with dideuterium gave the respective product 9-D2. It showed
2
the corresponding PD/BD resonances in the H NMR spectrum.
The high dihydrogen splitting reactivity of the cationic FLP 8 is
remarkable since its –B(C6F5)2 containing neutral “parent”
trimethylene-linked P/B analogue shows no H2-splitting activity
due to a thermodynamic restriction.[15]
Scheme 2. Reactions of the B–H borenium ion 3a.
Scheme 3. Reaction of the borenium cations 3 with carbon monoxide.
Figure 2. A view of the trimethylene-bridged FLP salt 8 (only the cation is
depicted; thermal ellipsoids are set at 30% probability). Selected bond lengths
(Å) and angles (°): B1–C1 1.579(4), B1–C4 1.559(4), P1–C6 1.852(2), ΣB1CCC
= 359.8, ΣP1CCC = 319.3, P1–C6–C5–C4 178.3(2), C6–C5–C4–B1 61.2(3).
Both the [(NHC)B(H)C6F5]+ cations each add one additional
equivalent of the respective NHC to form the corresponding
[(NHC)2B(H)C6F5]+ boronium salts (4a,b; with [B(C6F5)4]- counter
anion). Compound 3a reacts slowly with CD2Cl2 (6h, r.t.) by
hydride for chloride exchange. A donor stabilized derivative
[(NHC)B(xylyl-isocyanide)(Cl)C6F5]+
(5a) was isolated and
characterized by X-ray diffraction. Compound 3a is thermally
labile. Keeping it for a prolonged period of time at 50 °C (48 h, in
C6H5Br) resulted in a C–H activation reaction at one of the
adjacent mesityl substituents with liberation of dihydrogen. The
resulting C–H activation compound (6a) was isolated and
characterized spectroscopically and by an X-ray crystal structure
analysis (see the Supporting Information for details of the
characterization of the compounds 4 to 6).
Compound 3a is an active hydroboration reagent. Its reaction
with 4-phenyl-1-butene (r.t., 5 min in C6H5Br) generated the
respective alkyl borenium compound. It was isolated as isonitrile
(CN–Xyl) stabilized adduct (7a, isolated in 91% yield). It was
Figure 3. A view of the molecular structure of the boron-carbonyl 10b (only the
cation is shown; thermal ellipsoids are set at 30% probability). Selected bond
lengths (Å) and angles (°): C1–N1 1.350(4), C1–N2 1.339(4), C1–B1 1.607(4),
B1–C8 1.608(5), C8–O1 1.111(4), B1–C11 1.611(4), N1–C1–N2 106.1(2),
C1–B1–C8 105.7(2), B1–C8–O1 173.3(3), C8–B1–C11 113.0(3), C8–B1–C1–
N1 -57.6(4), C11–B1–C1–N1 65.9(4), C8–B1–C11–C16 -5.3(4).
Both the (NHC)borenium compounds 3a and 3b react rapidly
with carbon monoxide (1 to 1.5 bar CO, 5 min at r.t.) to give the
respective B–H containing borenium carbonyls 10a and 10b,
respectively. Compound 10a shows a 13C NMR [B]–C≡O
resonance at δ 165.6. The 11B NMR resonance of the cationic
unit was located at δ -29.2 [1H NMR: [B]+(H) signal at δ 3.75].
2
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The B–H borenium carbonyl salt 10a was characterized by X-ray
diffraction. The structure is depicted in the Supporting
Information. The cation shows a pseudo tetrahedral structure.
formed by nucleophilic endiamine attack at the B–C≡O carbon
atom, in 94% yield. The X-ray crystal structure of compound 12
shows the presence of the newly formed IMes-substituted acetyl
group attached at boron. The boron coordination geometry is
distorted tetrahedral (Figure 4). In solution compound 12 shows
the 13C NMR acyl feature at δ 231.4 and the signals of the
adjacent –CH2–IMes unit at δ 41.0 (CH2) and δ 161.9 (central
IMes carbon), respectively. The –CH2– unit gives rise to an AB
1H NMR pattern at δ 3.40, 3.14 (2JHH = 17.6 Hz). The cation of
The sum of CCC angles at the boron atom amounts to ΣB1CCC
=
332.0°. The B–C–O angle is slightly bent (169.3(3)°) and the CO
bond is short (1.123(3) Å), actually slightly shorter than free CO.
Compound 10a shows a sharp IR C≡O band at ṽ = 2205 cm-1
(KBr).
The IMe4 containing B–H borenium carbonyl 10b was
characterized spectroscopically from the in situ experiment [1H
NMR: δ 4.18 (BH), δ 3.70 (NMe), δ 2.25 (=CMe); 13C NMR: δ
169.8 [B]–C≡O; 11B NMR: δ -30]. The X-ray crystal structure
analysis showed the structural framework of the parent
borenium ion (3b) to which the CO molecule had added. The
resulting new boron-carbon bond (B1–C8 1.608(5) Å) is in the
same order of magnitude as the adjacent B1–C1 and B1–C11
linkages to the IMe4 and the C6F5-substituents (Figure 3). The
boron carbonyl unit is close to linear. The C–O bond of the
boron-carbonyl unit is short. The coordination geometry at the
boron atom is distorted tetrahedral (ΣB1CCC = 330.7°). The
conformational orientation of the B–C≡O vector to the plane of
the IMe4 ligand is close to gauche, whereas it is oriented almost
in plane with the adjacent C6F5 substituent. The borenium
carbonyl 10b shows a sharp IR CO-stretching band at ṽ = 2223
cm-1 (KBr). We also reacted the borenium salts 3a,b with 13C
enriched carbon monoxide. As expected, the isotopologues 13C-
10a,b showed the IR-13CO stretching band at ṽ = 2155 cm-1 and
2172 cm-1, respectively.
compound 12 shows the 11B NMR BH signal at δ -22.1 (1JBH
~
85 Hz).
Figure 4. A view of the molecular structure of the substituted acetylboron
cation part of compound 12 (only the cation is shown; thermal ellipsoids at 15%
probability). Selected bond lengths (Å) and angles (°): C1–B1 1.627(10), B1–
C4 1.627(9), C4–O1 1.223(7), C4–C5 1.542(8), C5–C6 1.490(7); ΣB1CCC
335.8, ΣC4BCO 359.9.
The reaction of 10a with the (IMes)(C6F5)BH2 borane 2a (r.t. 24h
in C6D5Br) gave a single product to which we tentatively
assigned the composition of compound 14. The cation shows
11B NMR signals at δ 33.7 and -22.8, typical for the presence of
three- and four-coordinated boron. It features the 1H NMR
signals of diastereotopic hydrogens of the newly formed –CH2–O
unit (see Scheme 4). The reaction of 10a with the ethylene-
bridged FLP 13 made use of the B–H functionality. The reaction
(r.t., 24h) gave a major product to which we propose the
structure of 15 on the basis of the observed NMR spectra [11B: δ
71.9 (–B(C6F5)2), δ -13.8 (BC6F5); 1H/13C: δ 3.07 (2JPH = 25.8
Hz)/49.2 (1JPC ~ 58 Hz) (–CHO) in CD2Cl2].
Scheme 4. Reactions of the borenium carbonyl 10a.
The borenium carbonyl 10a was reduced with Schwartz’s
reagent [Cp2Zr(H)Cl] (17).[17] Treatment of 10a with the
zirconium hydride 17 (2 min, r.t., CD2Cl2) followed by trapping
with pyridine gave the B–zirconoxymethyl product 20 (Scheme
5). The X-ray crystal structure analysis revealed that the boron
bonded C≡O moiety of the starting material had become
reduced to a –CH2–O– unit at boron. The Zr(Cl)Cp2 moiety is
found attached at its oxygen atom. The boron atom B1 of the
cation has the newly formed –CH2–O[Zr], the C6F5 and –IMes
group and a pyridine donor bonded to it in a distorted tetrahedral
geometry.
The CO ligands in the BH borenium carbonyls 10a,b are
apparently inert to intramolecular reduction by the adjacent B–H
functionality. However, the carbonyl moiety at the positively
charged borenium core is activated for external nucleophilic
attack by the neutral carbon nucleophile IMes (1a) and its
endiamine-type derivative IMesCH2 (1c) (Scheme 4).
The N-heterocyclic carbene reacted rapidly with the borenium
carbonyl 10a (CD2Cl2, r.t., 2 min) to give the cationic acyl boron
1
compound 11 [11B NMR: δ -21.5 (br. d, JBH ~ 85 Hz), 13C: δ
We followed this reaction more closely in situ by NMR
spectroscopy and found that the reaction of 10a with the
zirconium hydride initially gave the product 19a (see Scheme 5).
217.2 (C=O)]. The X-ray crystal structure of compound 11 is
depicted in the Supporting Information.
The reaction of the borenium carbonyl salt 10a with the IMesCH2
reagent gave the substituted acetyl boron[16] cation system 12,
1
The H NMR spectrum (CD2Cl2 at 233 K) showed the presence
of a pair of diastereotopic Cp ligands at zirconium and a pair of
3
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diastereotopic methylene hydrogens adjacent to boron [11B
NMR: δ -9.5]. These data point to the presence of a three-
membered BOC substructure with a marked boron-oxygen
interaction.
generate 18 is strongly exergonic (-22 kcal mol-1) and then the
subsequent 1,2-hydride shift to give 19 provides an additional
thermodynamic driving force (by ca. -10 kcal mol-1). Compound
19a was then trapped with pyridine to give the isolated final
product 20.
The compounds 3a and 3b represent rare examples of [B]–H
containing trivalent borenium ions. They were readily prepared
from the (NHC)(C6F5)BH2 precursors 2a,b by hydride abstraction
with trityl cation. They show the typical behaviour of electrophilic
B–H species toward carbon monoxide. Similar to the chemistry
of neutral B–H boranes, which form neutral B–H borane
carbonyls,[11,12,19] both the compounds 3a and 3b reacted cleanly
with carbon monoxide to form the respective cationic borenium
carbonyls. The systems may superficially be reminiscent to
many metal carbonyls, but the IR spectra indicate a fundamental
difference: for both examples the C≡O stretching band is found
at wavenumbers that are higher than free C≡O. This is a typical
effect of CO coordination that is void of backbonding.[20-22]
Consequently, the C≡O bond lengths in the [B]+–carbonyls 10a
and 10b are found slightly below the carbon-oxygen bond length
in free CO (1.13 Å, although measured under different
conditions[23]). We showed that compound 10a contains a
reactive carbonyl group. It reacts readily with nucleophiles.
Addition of hydride from the Zr–H source even induces the
utilization of the internal boron-hydride for the CO reduction to
the –CH2–O– group at boron. Aside from the ability of forming
the interesting new cationic boron B(H)CO containing boron
carbonyls, the [B]+–H borenium systems may serve as reactive
reagents for the synthesis of various interesting new [B]+-
containing systems.
Scheme 5. Reaction of the borenium carbonyl ion 10a with Schwartz’ reagent
and DFT calculated Gibbs energies (in kcal mol-1) of the reaction of the virtual
model compound 10d with Cp2Zr(H)Cl [PW6B95-D3//TPSS-D3/def2-
TZVP+(COSMO-RS) (CH2Cl2)].
Acknowledgements
C. C. and J. L. thank the Alexander von Humboldt foundation for
postdoctoral fellowships.
Keywords: borocation • borenium • hydroboration • boron
carbonyl • CO reduction
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The B–H containing [(NHC)B(H)C6F5]+ borenium cations were obtained via hydride abstraction from the heterocyclic carbene borane
adducts (NHC)BH2C6F5. They cleanly add carbon monoxide to give the respective [(NHC)B(H)(C6F5)CO]+ boron carbonyl cations.
The activated C≡O can be attacked by carbon nucleophiles and reduced by external hydride.
6
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