Cationic Bismuth Amides: Accessibility, Structure, and Reactivity

Article abstract of DOI:10.1002/chem.201604117

The synthetic access to cationic bismuth compounds based on simple, monodentate, synthetically useful amido ligands, [Bi(NR₂)₂(L)_n]⁺, has been investigated (R=Me, iPr, Ph; L=neutral ligand). With [BPh₄]^?as a counteranion, the formation of contact ion pairs and subsequent phenyl transfer from B to Bi is observed. An intermediate of this reaction, [Bi(NMe₂)₂(HNMe₂)(BPh₄)] (1), could be isolated and fully characterized. The use of a fluorinated tetraarylborate as a counteranion leads to more stable cationic bismuth amides. The solvent-separated ion pairs [Bi₂(μ₂-NMe₂)₂(NMe₂)₂(thf)₆]²⁺(4) and [Bi(NiPr₂)₂(thf)₃]⁺(5) were fully characterized, with [B(3,5-C₆H₃(CF₃)₂)₄]^?anions balancing the positive charge. The coordination chemistry, aggregation in solution, and spectroscopic features of these compounds were investigated. Compounds 4 and 5 show an increased reactivity towards diisopropylcarbodiimide compared to their neutral parent compounds. These reactions result in formation of the first cationic bismuth guanidinates. Characterization techniques include¹H,¹¹B,¹³C,¹⁵N,¹⁹F, and³¹P (VT)NMR and IR spectroscopy, single crystal X-ray diffraction analysis, and DFT calculations.

Full text of DOI:10.1002/chem.201604117

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Title: Cationic Bismuth Amides: Accessibility, Structure, and Reactivity
Authors: Hannah Dengel and Crispin Lichtenberg
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Cationic Bismuth Amides: Accessibility, Structure, and Reactivity
Hannah Dengel[a] and Crispin Lichtenberg*[a]
Abstract: The synthetic access to cationic bismuth compounds
based on simple, monodentate, synthetically useful amido ligands,
i
[Bi(NR2)2(L)n]+, has been investigated (R = Me, Pr, Ph; L = neutral
ligand). With [BPh4]– as a counteranion, the formation of contact ion
pairs and subsequent phenyl transfer from B to Bi is observed. An
intermediate of this reaction, [Bi(NMe2)2(HNMe2)(BPh4)] (1), could be
isolated and fully characterized. The use of fluorinated
tetrarylborates as counteranions leads to more stable cationic
bismuth amides. The solvent-separated ion pairs [Bi2(μ2-
NMe2)2(NMe2)2(thf)6]2+ (4) and [Bi(NiPr2)2(thf)3]+ (5) were fully
characterized with [B(3,5-C6H3(CF3)2)4]– anions balancing the
positive charge. The coordination chemistry, aggregation in solution,
and spectroscopic features of these compounds were investigated.
Compounds
4 and 5 show an increased reactivity towards
Scheme 1. Examples of literature-known cationic bismuth amides.
diisopropylcarbodiimide compared to their neutral parent compounds.
These reactions result in formation of the first cationic bismuth
guanidinates. Characterization techniques include 1H, 11B, 13C, 15N,
19F and 31P (VT-)NMR and IR spectroscopy, single crystal X-ray
diffraction analysis, and DFT calculations.
supporting Bi–C bonds.[12,13] Early work in the field aimed at the
synthesis of low-coordinate bismuth cations stabilized by a
dianionic, bidentate silyldiamido ligand (Scheme 1, A).[14a] This
work has recently been extended by variation of the substitution
pattern of the silyldiamido ligand.[14b] Moreover, different
synthetic routes to access type A compounds were evaluated
and halide abstraction by a Lewis acid was determined to be the
only reliable method thus far.[14b] Using a different ligand scaffold,
a cationic bismuth compound bearing one chloro and one
silylamido ligand has been presented as an isolable complex en
route to the synthesis of a potential bismadiazonium ion
(Scheme 1, B).[15] The literature-known cationic bismuth amides
are based on bidentate or bulky ligands that confer stability to
these species by chelation effects, steric protection or secondary
metal-ligand interactions.[12-15] We became interested in cationic
bismuth species based on simple, monodentate amides. Such
compounds represent attractive synthetic targets, because these
“synthetically useful” amido groups can potentially be transferred
to electrophilic substrates or act as initiating groups in catalytic
transformations such as polymerization or hydroamination
reactions.
Introduction
Well-defined, cationic bismuth species have recently been
subjects of intense research efforts. They show an increased
Lewis acidity and electrophilicity compared to their neutral
parent compounds while maintaining a high functional group
tolerance. This makes them attractive targets for applications in
stoichiometric group transfer reactions and catalysis. Catalytic
transformations that have been realized to date include the
allylation
of
aldehydes,[1]
diastereoselective
aldol
condensations,[2] diastereoselective Mannich reactions,[3] and
the polymerization of activated olefins.[4]
Neutral bismuth amide compounds have been investigated
in some detail.[5] They are valuable and easily accessible
precursors for atomic layer deposition,[5e] for nanoparticle
synthesis,[6] and for the preparation of clusters[7] or bismuth
compounds featuring more complex ligand scaffolds.[8]
Furthermore, their reactivity towards substrates such as CH
acidic compounds,[9] silanes,[9] heterocumulenes,[5a,9,10] alkenes,
alkynes, aldehydes, and ketones[11] has been studied.
Herein, we report the synthesis, isolation, characterization,
and reactivity of the first examples of cationic bismuth
compounds featuring simple, synthetically useful amido ligands.
However, the considerable number of literature-known
bismuth cations featuring Bi–C bonds contrasts with the small
number of cationic bismuth amides that do not contain
Results and Discussion
Cationic Bi amides with [BPh4]– counterions
In order to generate a cationic bismuth amide, Bi(NMe2)3 was
treated with the Brønsted acid [HNEt3][BPh4] at ambient
temperature in THF (Scheme 2). A yellow crystalline product
could be isolated after immediately subjecting the reaction
mixture to solvent diffusion crystallization at –30 °C.
[a]
B. Sc. Hannah Dengel, Dr. Crispin Lichtenberg
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, D-97074 Würzburg, Germany
E-mail: crispin.lichtenberg@uni-wuerzburg.de
Supporting information for this article is available under
http://dx.doi.org/.
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supported by DFT calculations.[18] The Bi–Namido bond lengths
amount to 2.05-2.12 Å. The difference in Bi–Namido distances
goes along with a stronger pyramidalization of the nitrogen atom
N2 that is associated with the longer bond (sum of angles: 350°
(N2) vs. 360° (N1)). Despite the higher coordination number (CN
= 5), the Bi–N bonds in compound 1 are on average 0.10 Å
shorter than those in the neutral parent compound Bi(NMe2)3
(CN = 3; avg. Bi–N, 2.19 Å).[5b] This was ascribed to the cationic
charge of the [Bi(NMe2)2(HNMe2)]+ group and the relatively weak
Bi–π(Ph) bonding (vide supra). Compound 1 is the first example
of a monodentate amine interacting with a bismuth center. The
Bi1–N3 bond length of 2.42 Å is similar to those found in
[Bi(cyclen)(H2O)(ClO4)3] (2.38-2.41 Å).[19]
Scheme 2. Formation and degradation pathway of isolable contact ion pair
[Bi(NR2)2(HNMe2)(BPh4)] (1).
The 1H NMR spectrum of 1 in a mixture of C6D6/THF-d8
(1000:1)[20] shows three signals with relative intensities of 12:6:1
in the non-aromatic region. This confirms the presence of two
amido groups (δ = 3.42 (s, 12H) ppm) and one amine (δ = 1.91
(d, 6H), 5.06 (br s, 1H) ppm).[21] The [BPh4]– anion shows two
sets of resonances with relative intensities of 3:1, each of which
is characteristic for phenyl groups. This indicates interactions
between the bismuth atom and one phenyl substituent, i.e.
compound 1 also forms a contact ion pair in solution. This
coordination behavior was also observed at low temperatures of
–40 °C in a toluene-d8/THF-d8 solvent mixture.[49] 1H NMR
spectra of 1 in pure THF-d8 show only one set of resonances for
the [BPh4]– anion, suggesting that a solvent-separated ion pair
can also be formed under these conditions.[22,23]
When stored at –30 °C as a solid, compound 1 is stable for a
period of months. A reasonable solution NMR characterization
can be performed, when data acquisition is started immediately
after dissolving the compound. However, 1 shows first signs of
decomposition in solution at room temperature after ca. 10 min.
as determined by 1H NMR spectroscopy. The decomposition
pathway of 1 was investigated by 11B NMR spectroscopy in THF.
The intensity of the sharp resonance at –6.7 ppm that is
associated with the [BPh4]– anion gradually decreases while a
new, broad resonance at 0.6 ppm gradually increases in
intensity and becomes the dominating species after ca. 19 h.
This is in agreement with formation of an amine-borane,
[BPh3(NHMe2)] (2). Indeed, 2 could be isolated in 81% yield from
a solution of in situ generated compound 1 that had been kept at
ambient temperature for 2 d and was fully characterized (Supp.
Inf.).
Single crystal X-ray diffraction analysis (orthorhombic space
group Pna21, Z = 4) revealed the identity of this compound as
the contact ion pair [Bi(NMe2)2(HNMe2)(BPh4)] (1) (Figure 1).
The bismuth atom shows bonding interactions with two amido
groups, one amino group, and the π-electrons of two phenyl
groups. This results in
a distorted trigonal bipyramidal
coordination geometry around Bi with the amine and one phenyl
group in the axial positions (N3–Bi1–ct1, 166.0(3)°). The
distance between Bi1 and the centroid of the axial phenyl
substituent, ct1, amounts to 3.33 Å (Bi1–CPh1, 3.48-3.71 Å),
which is slightly larger than the corresponding distance reported
for an inverted sandwich complex featuring a Bi–toluene–Bi unit
(Bi–cttoluene, 3.26 Å).[16] The Bi1–ct2 distance (3.52 Å) is larger,
but the corresponding Bi–CPh distances are still below the sum
of the van der Waals radii of these elements (4.00 Å).[17] Thus,
weak Bi1–π(Ph2) bonding interactions are assigned, which is
Whereas a stable cationic aryl bismuth species with a [BPh4]–
counteranion has been described,[24] Ph/RCO2 exchange
between [BPh4]– and Bi(RCO2)3 has been reported as a method
for the preparation of aryl bismuth compounds.[25] In an attempt
to synthesize a cationic bismuth amide with a [BPh4]– counterion,
Ph/Cl exchange between Bi((NAryl)2SiMe2)Cl and Na[BPh4] has
been reported.[14b] Our results on the isolation of 1 and its
decomposition to 2 show that cationic bismuth amides with
[BPh4]– counterions are isolable, indeed, but represent
intermediates en route to B–C bond activation products.
Figure 1. Molecular structure of [Bi(NMe2)2(HNMe2)(BPh4)] (1) in the solid
state. Displacement ellipsoids are shown at the 50% probability level.
Hydrogen atoms (except for H3) are omitted for clarity. Centroids of phenyl
rings are indicated by “ct”. Selected bond lengths (Å) and angles (°): Bi1–N1,
2.050(14); Bi1–N2, 2.118(14); Bi1–N3, 2.419(10); Bi–ct1, 3.326(15); Bi–ct2,
3.520(12); N1–Bi1–N2, 93.0(6); N1–Bi1–N3, 88.6(6); N2–Bi1–N3, 89.0(6);
N1–Bi–ct1, 148.9(4); N2–Bi–ct1, 116.4(4); N3–Bi–ct1, 166.0(3); N1–Bi–ct2,
105.3(5); N2–Bi–ct2, 91.3(5); N3–Bi–ct2, 82.4(3); ct1–Bi–ct2, 84.9(3); ΣC–N1–
C/Bi, 360; ΣC–N2–C/Bi, 350; ΣC–N3–C/Bi, 325.
In order to evaluate the effect of the substituents at nitrogen on
the stability of [Bi(NR2)2(L)n(BPh4)], reactions between Bi(NR2)3
and [HNEt3][BPh4] were performed and monitored by NMR
spectroscopy (R = iPr, Ph; Scheme 3, Supp. Inf.).
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Figure 2. Molecular structure of [Bi(NiPr2)2(BPh4)] (3) in the solid state.
Displacement ellipsoids are shown at the 50% probability level. Hydrogen
atoms are omitted for clarity. Centroids of phenyl rings are indicated by “ct”.
Selected bond lengths (Å) and angles (°): Bi1–N1, 2.101(3); Bi1–ct, 3.228(5);
N1–Bi1–N1’, 95.71(18); N1–Bi1–ct, 115.94(12); N1–Bi1–ct’, 121.01(12); ct–
Bi1–ct’, 89.57(12); C7–B1–C7’, 101.88(19); C13–B1–C13’, 103.95(19); ΣC–
N1–C/Bi, 360.
[BPh4]– unit, each of which are crystallographically equivalent.
This results in an unusual (distorted) tetrahedral coordination
geometry around Bi1 (N/ct–Bi1–N/ct, 89.6°-121.0°). The Bi–
Namido bonds (2.10 Å) are intermediate between those found in 1
(2.05-2.12 Å). The nitrogen atoms show trigonal planar
coordination geometries (ΣC–N1–C/ct, 360°). The Bi1–ct
distances of 3.23 Å are intermediate between those found in
[BiCl2(C6Me6)(AlCl4)] (2.72 Å)[26,27] and an inverted sandwich
compound with a Bi-toluene-Bi unit (3.26 Å),[16a] and clearly
shorter than those of compound 1 (Bi-ct, 3.33-3.52 Å).[16b] This is
ascribed to the lower coordination number of the bismuth atom
in 3.
Scheme 3. Reaction of [HNEt3][BPh4] with Bi(NR2)3 (R = iPr, Ph).
It might have been anticipated that an increase of the steric
bulk around the nitrogen atoms leads to an increased stability of
the cationic species. However, rapid, quantitative consumption
of the starting materials with formation of [BPh3(thf)] was
1
observed in both cases (for details see Supp. Inf.). The H NMR
spectra recorded after 10 min reaction time revealed [BPh3(thf)]
and BiPh(NR2)2 as the main products. These compounds result
from transfer of a phenyl group from [BPh4]– to [Bi(NR2)2]+. With
increasing reaction times, Ph/NR2 exchange between boron and
bismuth species was also observed. The first exchange gives
compounds BPh2(NR2)/BiPh2(NR2) and was observed for R = iPr,
Ph. Detection of compounds BPh(NiPr2)2 and BiPh3 indicated
even a second Ph/NR2 exchange in the case of R = iPr.[50]
Work-up of the reaction between Bi(NiPr2)3 and
[HNEt3][BPh4] after short reaction times confirmed [BPh3(thf)] as
the main product with an isolated yield of 51%. Furthermore, a
few black crystals could be isolated from the bulk product by
manual separation. Using single crystal X-ray diffraction analysis,
this sample could be identified as the originally targeted cationic
bismuth species, which had formed as a solid in trace amounts.
Compound [Bi(NiPr2)2(BPh4)] (3) crystallizes in the orthorhombic
space group Pccn with Z = 4. As expected, 3 also forms a
contact ion pair in the solid state. In contrast to its methyl-
substituted derivative 1, however, there is no neutral amine
ligand present in compound 3. The metal center in 3 is
coordinated by two amido ligands and two phenyl groups of the
Cationic Bi amides with [B(3,5-C6H3(CF3)2)4]– counterions.
In order to generate more stable cationic bismuth species
based on simple, synthetically useful amido ligands, the effect of
a fluorinated borate counteranion was investigated. The bismuth
i
amides Bi(NR2)3 (R = Me, Pr) were reacted with the strong
Brønsted acid [H(OEt2)2][A] ([A]–
=
[B(3,5-C6H3(CF3)2)4]–)
(Scheme 4).[28] In the case of the methyl-substituted ligand, a
dinuclear, dicationic species, [Bi2(μ2-NMe2)2(NMe2)2(thf)6]2+ 2[A]–
(4), was obtained as a yellow solid, whereas use of sterically
more demanding isopropyl substituents gave a mononuclear
cationic species, [Bi(NiPr2)2(thf)3]+ [A]– (5), as an orange solid.
Both compounds are insoluble in hydrocarbons, but soluble in
polar solvents such as THF. In contrast to compounds 1 and 3,
no B–C bond activation was detected for compounds 4 and 5
due to their tetraarylborate counterions acting as true weakly
coordinating anions.
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Scheme 4. Reaction of Bi(NR2)3 with [H(OEt2)2]+[A]– (R = Me, iPr; [A]– = [B(3,5-
C6H3(CF3)2)4]–).
The molecular structures of 4 and 5 in the solid state were
determined by single crystal X-ray diffraction analyses. Both
compounds crystallized as solvent-separated ion pairs in the
monoclinic space group P21/c with Z = 4 (Figures 3, 4). The
bismuth-containing part of compound 4 is a dication consisting
of two crystallographically equivalent monomeric subunits. The
two metal centers are linked by two amido groups in bridging
coordination modes, resulting in an essentially planar Bi2N2 unit.
This aggregation behavior was ascribed to the low steric
demand of the methyl substituents at nitrogen. The coordination
sphere of each bismuth atom is saturated by interaction with one
terminal amido ligand and three THF ligands, which leads to
distorted octahedral coordination geometries around Bi.
Deviations from an ideal octahedral coordination geometry are
due to the bridging coordination mode of two amido groups and
Figure 4. Molecular structure of the cation of [Bi(NiPr2)2(thf)3] [B(3,5-
C6H3(CF3)2)4] (5) in the solid state. Hydrogen atoms are omitted for clarity.
Displacement ellipsoids are shown at the 50% probability level; carbon atoms
of THF ligands are shown as wireframe. Selected bond lengths (Å) and angles
(°): Bi1–N1, 2.091(7); Bi1–N2, 2.112(7); Bi1–O1, 2.654(6); Bi1–O2, 2.572(6);
Bi1–O3, 3.185(8); N1–Bi1–N2, 98.7(3); N1–Bi1–O1, 92.7(3); N2–Bi1–O3,
144.7(3); O1–Bi1–O2, 169.41(19); ΣC–N1–C/Bi, 360; ΣC–N2–C/Bi, 354.
to the weak bonding of one THF ligand (vide infra). In contrast to
other cationic bismuth compounds with monodentate ligands,
the bismuth-centered lone pair in 4 is not stereochemically
active.[4,29] The two crystallographically distinct Bi–N distances
that involve the bridging amido groups are identical within limits
of error (Bi1–N1/N1’, 2.34 Å) and elongated compared to
terminal Bi1–N2 bonds (2.12 Å). A shortening of the terminal
Bi1–N2 bond compared to the average Bi–N bond length in
neutral Bi(NMe2)3 (2.19 Å)[5b] was ascribed to the cationic
character of 4. The THF ligands in 4 occupy facial sites of the
coordination polyhedra. They differ with respect to the ligand
that is located in trans position relative to the OTHF atom, which is
either a bridging amido group in the case of O1/2 or a terminal
amido group in case of O3. Accordingly, the substantial
elongation of the Bi1–O3 distance (3.34 Å; sum of van der
Waals radii: 3.82 Å) compared to the Bi1–O1/2 distances (2.70-
2.71 Å) is due to a thermodynamic trans effect.
The higher steric bulk of the isopropyl substituents in 5 leads
to formation of a mononuclear compound in the solid state. The
bismuth center in 5 shows a coordination number of five and an
additional stereochemically active lone pair. A τ5 value of 0.41
indicates a distorted square pyramidal coordination geometry.[51]
The amido groups occupy the apical (N1) or an equatorial
position, respectively. However, despite their different
coordination environments, the Bi–N bond lengths are identical
within limits of error with an average value of 2.10 Å. The Bi–
O1/2 distances in 5 are 0.05-0.14 Å shorter than the Bi1–O1/2
distances in 4. This is due to the lower coordination number of
the bismuth center and the absence of strong donors trans to
these THF ligands in 5. The third THF ligand in compound 5 is
located trans to the equatorial amido ligand and experiences a
pronounced thermodynamic trans effect (Bi1–O3, 3.19 Å).
Figure 3. Molecular structure of the dicationic of [Bi2(μ2-NMe2)2(NMe2)2(thf)6]2+
2 [B(3,5-C6H3(CF3)2)4]– (4) in the solid state. Hydrogen atoms and two lattice-
bound THF molecules per formula unit are omitted for clarity. Displacement
ellipsoids are shown at the 50% probability level; carbon atoms of THF ligands
are shown as wireframe. Selected bond lengths (Å) and angles (°): Bi1–N1,
2.335(3); Bi1–N1’, 2.337(3); Bi1–N2, 2.120(3); Bi1–O1, 2.701(3); Bi1–O2,
2.711(3); Bi1–O3, 3.342(3); N1–Bi1–N1’, 77.59(12); N2–Bi1–O3, 162.12(13);
O1–Bi1–O2, 114.87(10); Bi1–N1–Bi1’, 102.41(12); ΣC–N2–C/Bi, 354.
NMR spectroscopic analysis of 4 in THF–d8 indicates
formation of solvent-separated ion pairs in solution. The 1H NMR
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spectrum at room temperature shows one resonance for the
(NMe2)– groups. Upon cooling the sample, this signal gradually
broadens and coalescence is observed at ca. –10 °C. Two well-
separated, sharp singlets of equal intensity and with similar
100 ppm is observed (δ = –172 ppm). Exchange of OPPh3
ligands for weaker THF ligands results in a downfield shift of an
additional 20 ppm ([Bi(NiPr2)2(thf)n]+: δ = –152 ppm). The 15N
NMR downfield shift upon going from neutral to cationic bismuth
chemical shifts are detected at a temperature of –60 °C (δ = 4.43, complexes of up to 116 ppm is significantly larger than the
4.53 ppm). These were ascribed to (NMe2)– groups in bridging
and terminal coordination modes of [Bi2(μ2-
NMe2)2(NMe2)2(thf)6]2+. The exchange rate at –10 °C is
approximated as k ≈ 70 s–1. Thus, the dimeric arrangement of 4
also represents a ground state in solution, with exchange
phenomena taking place at room temperature.
downfield shift of ca. 10-26 ppm observed upon protonation of
tertiary alkylamines.[34] Overall, this brief NMR spectroscopic
case study reveals that in a series of (diisopropyl)amido bismuth
compounds, the 15N NMR chemical shift shows a strong
response to the charge of the bismuth complex. More subtle
modifications of the electronic situation at nitrogen (induced by
different neutral donors bound to Bi) are also reflected.
NMR spectroscopic data of compound 5 is in agreement with
a mononuclear [Bi(NiPr2)2(thf)3]+ unit as the cationic part of a
solvent-separated ion pair. The 1H NMR spectrum shows a
characteristic septet for the CHMe2 groups at 6.03 ppm, which
corresponds to a downfield shift by 1.56 ppm compared to the
neutral starting material.[5e] The appearance of only one set of
DFT calculations
DFT calculations were performed on compounds 1, 3, 4, 5 and
related species in order to gain further insights into some of their
ground state properties and reactivity. Bonding parameters of
the experimentally observed compounds were reproduced
satisfactorily (Supp. Inf.).
1
signals for the isopropyl groups in the H and 13C NMR spectra
rules out the rigid square pyramidal coordination geometry that
was observed in the solid state. In agreement with this, the THF
ligands in compound 5 are substitutionally labile as shown by 1H
NMR spectroscopy in THF-d8. This is further supported by
substitution of the THF ligand in 5 by two equivalents of the
stronger donor OPPh3 to give [Bi(NiPr2)2(OPPh3)2][A] (6), which
was characterized spectroscopically.
(i) The experimental results on contact ion pairs
[Bi(NR2)2(L)n(BPh4)] showed that such compounds are isolable
intermediates in a B–C bond activation of the [BPh4]– counterion.
Somewhat counterintuitively, smaller substituents at nitrogen
increase the stability of these species. The computational results
suggest that B–C bond activation proceeds in all cases through
a transition state with a phenyl group in a bridging coordination
mode, [Bi(NR2)2(μ2-Ph)(BPh3)] (Supp. Inf.). For compound 3,
such a transition state is directly accessible. For compound 1,
however, the neutral amine HNMe2 must leave the coordination
sphere of Bi before an energetically accessible transition state
can be realized. This could take place by dissociation of HNMe2
either directly from the contact ion pair 1 or from the cation
15N NMR spectroscopy
In addition to standard NMR spectroscopic characterization, 15N
NMR spectroscopy can be a valuable tool for the investigation of
nitrogen containing compounds. However, such studies on
bismuth complexes are extremely rare.[30] We performed 15N
solution NMR spectroscopic experiments on a small series of
closely related amido bismuth complexes that differ with respect
to the overall complex charge or the donor strength of additional
neutral ligands bound to Bi (Scheme 5 and Supp. Inf.).[31]
Compound Bi(NiPr2)3 shows a 15N NMR chemical shift of –
268 ppm. This corresponds to a downfield shift of ca. 40 ppm or
ca. 16-23 ppm compared to HNiPr2 or amine adducts of LiNiPr2,
respectively.[32,33] Upon exchange of one anionic (NiPr2)– ligand
in Bi(NiPr2)3 for two equivalents of OPPh3 as a strong, neutral σ-
donor to give [Bi(NiPr2)2(OPPh3)2]+, a downfield shift of almost
[Bi(NMe2)2(HNMe2)(solv)n]+ after formation of
a
solvent-
separated ion pair. Thus, the ability to coordinate an additional
neutral donor ligand makes compound 1 more stable than
compound 3.
(ii) In order to gain further insights into the aggregation
behavior of cationic bismuth amides, thermodynamic parameters
of dissociation/association reactions were determined. The
formation of a solvent-separated ion pair according to the
reaction shown in Table 1 was calculated to be slightly
exothermic and endergonic with [BPh4]– as the counteranion [A]–
(entry 1). This is in agreement with the experimentally observed
Table 1. Calculated thermodynamic parameters for formation of contact vs.
solvent-separated ion pairs of species related to 1 and 4; L = HNMe2.
Entry
Borate “A”
ΔHrel [kcal·mol–1
]
ΔGrel [kcal·mol–1
]
1
2
BPh4
–3.9
+6.6
B(3,5-C6H3(CF3)2)4
–14.8
–11.9
Scheme 5. 15N NMR chemical shifts for neutral and cationic diisopropylamido
bismuth complexes in solution.
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behavior of 1 in THF. Exchanging the [BPh4]– anion for [B(3,5-
C6H3(CF3)2)4]– in this reaction makes the formation of a solvent-
separated ion pair exothermic and exergonic (entry 2). This was
also indicated by experimental observations. Similar results
were obtained for isopropyl substituted analogs (Supp. Inf.).
(iii) The thermodynamic parameters for the formation of
dinuclear dicationic amido-bismuth species were determined for
the amido ligands (NR2)– with R = Me, iPr. The reaction shown in
Table 2 is energetically favorable for R = Me (entry 1), but
disfavored for R = iPr (entry 2). This further supports the
interpretation of NMR spectroscopic results regarding the
aggregation behavior of compounds 4 and 5 in THF-d8 solution.
towards nitrogen (localization at N: 85-88% (4cat) and 83-84%
(5cat). Somewhat counterintuitive, the polarization of the Bi–N
bonds towards nitrogen is slightly less pronounced in the neutral
compounds (localization at N: 80-82% (Bi(NMe2)3) and 81-82%
(Bi(NiPr2)3).
Reactivity of cationic bismuth amides
As described above, the bismuth amide cations [Bi(NR2)2]+ react
with their [BPh4]– counteranions, leading to B–C bond activation
i
(R = Me, Pr, Ph). To compare the reactivity of these cationic
species with that of their neutral parent compounds, the bismuth
amides Bi(NR2)3 were reacted with stoichiometric amounts of
Na[BPh4] in THF at room temperature. In all cases, however, the
[BPh4]– anion remained intact within reaction times of ≥24 h as
detected by 11B NMR spectroscopy. This is a first example of the
increased reactivity of cationic bismuth amides compared to
their neutral counterparts.
Table 2. Calculated thermodynamic parameters for the dimerization of
amido-bismuth cations (species related to compounds 4 and 5).
Entry
R
ΔHrel [kcal·mol–1
]
ΔGrel [kcal·mol–1
]
The bismuth amides 4 and 5 with fluorinated tetraarylborate
anions show an increased stability, making them more suitable
for reactivity studies with external electrophiles. Carbodiimides
are attractive substrates for reactions with bismuth amides, as
they would yield the corresponding guanidinates. Lanthanoid
guanidinates have been investigated as homogeneous catalysts
in the polymerization of olefins and cyclic esters or as precursors
for atomic layer deposition (ALD) and chemical vapor deposition
(CVD).[35] Bismuth compounds based on bicyclic guanidinates
have also been reported as precursors for ALD and CVD.[36]
However, there is so far only a single example of the synthesis
of a bismuth guanidinate from a carbodiimide;[9] in this case, the
bismuth amide starting material contained one reactive
dimethylamido group. We chose diisopropylcarbodiimide,
(iPrN)2C, as a model substrate (Scheme 6).
1
2
Me
iPr
–21.7
–6.3
+14.3
+32.2
(iv) A frontier orbital analysis of the synthetically relevant
species (4cat) and
[Bi2(μ2-NMe2)2(NMe2)2(thf)6]2+
[Bi(NiPr2)2(thf)3]+ (5cat) revealed strong contributions of metal-
centered p-type orbitals to the LUMOs in both cases (Figure 5).
In both compounds, substitutionally labile THF ligands have to
be replaced by potential substrate molecules in order to realize a
sufficient orbital interaction with the LUMO and thus allow for
pre-coordination/activation of the substrate. As expected, the
HOMOs of the cationic bismuth species are mostly nitrogen
centered (Supp. Inf.).
Figure 5. LUMOs of cationic species [Bi2(μ2-NMe2)2(NMe2)2–(thf)6]2+ and
[Bi(NiPr2)(thf)3]+ at iso values of 0.04.
(v) An NBO analysis of Bi(NMe2)3, Bi(NiPr2)3, [Bi2(μ2-
NMe2)2(NMe2)2(thf)6]2+ and [Bi(NiPr2)2(thf)3]+ confirmed more
positive natural charges for the bismuth atoms in the cationic
complexes (4cat: +1.91e; 5cat: +1.83e) compared to the neutral
parent compounds (Bi(NMe2)3: +1.67e; Bi(NiPr2)3: +1.67e). The
Bi–N bonds in the cationic species are strongly polarized
Scheme 6. Reactions of neutral and cationic bismuth amides with
diisopropylcarbodiimide; [A]– = [B(3,5-C6H3(CF3)2)4]–.
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10.1002/chem.201604117
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Reaction of the neutral methyl-substituted bismuth amide,
Bi(NMe2)3, with three equivalents of (iPrN)2C (one equivalent per
amido group) gave the targeted insertion product 7 with ≥95%
selectivity as indicated by NMR spectroscopy. However,
extended reaction times of 11 h were necessary to obtain full
conversion. For the isopropyl-substituted neutral bismuth amide,
Bi(NiPr2)3, no reaction with (iPrN)2C was observed in 24 h at
room temperature. This was ascribed to the steric bulk of the
(NiPr2)– groups and an inability to activate the substrate by
precoordination. In contrast, the cationic bismuth amides 4 and 5
reacted with two equivalents of diisopropylcarbodiimide,
accompanied by an immediate color change, to give the
insertion products 8 and 9. This corresponds to a substantial
increase in reactivity for the cationic bismuth amide species
compared to their neutral parent compounds. Full conversion of
4 was observed within reaction times of ≤ 10 min. In the case of
5, insertion of the first equivalent of carbodiimide is fast (≤ 10
min), but the second insertion proceeds more slowly (full
conversion after 12 h). Compounds 7, 8, and 9 were isolated as
pale yellow (7) or colorless solids (8, 9) in yields of 75-96%. The
cationic complexes 8 and 9 are obtained free of THF, when
dried in vacuo for prolonged periods of time, but contain two
equivalents of THF per metal center, when dried in a stream of
argon. Compound 9 was analyzed by single crystal X-ray
diffraction (Figure 6; monoclinic space group P21 with Z = 2).
The bismuth atom is found in a distorted octahedral coordination
geometry. The distortion is due to the small bite angle of the
guanidinate ligand (58°) and leads to strong deviations of the
angles around Bi from those expected for an ideal octahedral
coordination geometry. Values of 58°-111° and 136°-168° were
observed for ligands in cis- and trans positions, respectively.
The THF ligands in 9 adopt a cis configuration. The Bi–O
distances of 2.99-3.00 Å are slightly shorter than the Bi1–O3
distance in compound 5. This suggests weak Bi-OTHF bonding
and is in agreement with the possibility to remove the THF
ligands under reduced pressure. Each of the guanidinate ligands
shows one short and one long bond to the Bi center (Bi1–N1/4,
2.21-2.22 Å; Bi1–N2/5, 2.35-2.39 Å). This was ascribed to a
thermodynamic trans effect, as the N atoms in trans position to
the weaker OTHF donor atom show shorter Bi–N bonds than
those in trans position to another N atom. The bonds of the
central carbon atom C1/14 to the nitrogen atoms N1-6 measure
1.31-1.39 Å, which is between values expected for C–N single
and double bonds suggesting electron delocalization within the
guanidinate ligands. This behavior was also observed in the only
other bismuth compound containing a guanidinate ligand.[9]
The isopropyl substituents of compounds 8 and 9 generate
broad 1H NMR resonances at ambient temperature in THF-d8
solution, indicating dynamic coordination behavior. At –40 °C,
low temperature limiting spectra were obtained with all
isopropyl groups of each guanidinate ligand being magnetically
inequivalent. For compound 9, this is in agreement with the solid
state structure being essentially maintained in solution. It is
suggested that 8 shows an analogous structure in THF-d8
solution.
Conclusions
Cationic bismuth compounds based on simple, synthetically
useful, monodentate amido ligands were investigated. The use
of [BPh4]– counteranions leads to contact ion pairs that readily
undergo phenyl transfer from B to Bi at ambient temperature in
i
solution (R = Me, Pr, Ph). Contact ion pairs [Bi(NR2)2(L)n(BPh4)]
are intermediates in this transformation (L = neutral ligand; n = 0,
1). One of these compounds, [Bi(NMe2)2(HNMe2)(BPh4)] (1),
could be isolated and fully characterized. The low steric bulk of
the methyl substituents in 1 allows coordination of the additional
neutral donor HNMe2, which decelerates its decomposition. A
more effective stabilization of bismuth amide cations is achieved
by use of the fluorinated counteranion [B(3,5-C6H3(CF3)2)4]–
(abbreviated as [A]–), which does not undergo aryl transfer from
B
to Bi. The solvent-separated ion pairs [Bi2(μ2-
NMe2)2(NMe2)2(thf)6]2+ 2[A]– (4) and [Bi(NiPr2)2(thf)3]+ [A]– (5)
were isolated and fully characterized. The analysis of 5 and
related compounds by 15N NMR spectroscopy revealed a strong
response of the 15N NMR chemical shift on the formal charge at
Bi. In a proof of principle study, compounds 4 and 5 were
reacted with diisopropylcarbodiimide as an electrophile. In both
cases, the cationic bismuth amides show a higher reactivity than
their neutral parent compounds. These selective insertion
reactions allowed for the synthesis and characterization of the
first cationic bismuth guanidinates. It is anticipated that these
results on the accessibility, characteristics, and reactivity of
cationic bismuth amides will aid in the future design of stable
complexes of this kind for possible applications in synthesis,
catalysis, and materials science.
Figure 6. Molecular structure of the cation of [Bi((iPrN)2CNiPr2)2(thf)2]+ [B(3,5-
C6H3(CF3)2)4]– ([9·2 thf]) in the solid state. Hydrogen atoms and one lattice-
bound THF molecule are omitted for clarity. Displacement ellipsoids are shown
at the 50% probability level; carbon atoms of THF ligands are shown as
wireframe. The asymmetric unit contains two formula units of [9·2 thf], one of
which is shown. Selected bond lengths (Å) and angles (°): Bi1–N1, 2.218(6);
Bi1–N2, 2.351(8); Bi1–N4, 2.213(7); Bi1–N5, 2.388(7); Bi1–O1, 2.981(7); Bi1–
O2, 2.998(9); C1–N1, 1.342(11); C1–N2, 1.360(12); C1–N3, 1.368(11); C14–
N4, 1.387(11); C14–N5, 1.311(12); C14–N6, 1.381(11); N1–Bi1–N2, 58.2(3);
N2–Bi1–N5, 136.0(3); N4–Bi1–O2, 168.2(3); N5–Bi1–O2, 110.6(3).
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7.16 (m, 3H, p-Ph1 (partially overlapping with solvent resonance), 7.27
(dd, 6H, 3JHH = 7.5 Hz, 3JHH = 7.5 Hz, m-Ph1), 7.34 (dd, 2H, 3JHH = 7.4 Hz,
Experimental Section
3
3JHH = 7.4 Hz, m-Ph2), 7.48 (t, 1H, JHH = 7.5 Hz, p-Ph2, partially
General considerations. All air- and moisture-sensitive manipulations
were carried out using standard vacuum line Schlenk techniques or in
inert atmosphere gloveboxes containing an atmosphere of purified argon.
Solvents were degassed and purified according to standard laboratory
procedures. NMR spectra were recorded on Bruker instruments
operating at 300, 400, or 500 MHz with respect to 1H. 1H and 13C NMR
chemical shifts are reported relative to SiMe4 using the residual 1H and
13C chemical shifts of the solvent as a secondary standard. 11B and 19F
NMR chemical shifts are reported relative to [BF3(OEt2)] and CFCl3,
respectively, as external standards. 15N NMR chemical shifts are
reported relative to CH3NO2 (90% in CDCl3) and were determined by
two-dimensional 1H-15N correlation NMR spectroscopic experiments (no
15N NMR resonances could be detected for compounds 1 and 4 using
this technique). If not otherwise noted, NMR spectra were recorded at
ambient temperature. Infrared spectra were collected on a Bruker FT-IR-
Alpha spectrometer. Elemental analyses were performed on a Leco or a
Carlo Erba instrument. Reproducibly satisfactory elemental analyses of 1
proved to be difficult to obtain, which was ascribed to the air and
temperature sensitivity of the compound. Instead, the Bi content of 1 was
3
overlapping with resonance due to o-Ph1), 7.52 (d, 6H, JHH = 7.5 Hz, o-
Ph1, partially overlapping with resonance due to p-Ph2), 7.86 (d, 2H, 3JHH
= 7.4 Hz, o-Ph2) ppm. 13C NMR (101 MHz, C6D6/THF-d8 (1000:1)): δ =
38.90 (s, HNMe2), 43.62[45] (br s, Bi(NMe2)2), 125.25 (s, p-Ph1), 127.45 (s,
m-Ph1), 130.89 (s, m-Ph2), 131.09[45] (s, p-Ph2), 134.55 (s, o-Ph1), 136.74
(s, o-Ph2) ppm. Resonances for ipso-Ph carbon atoms could not be
detected. 11B NMR (128 MHz, C6D6): δ = 1.1 (br s) ppm. 11B NMR (128
MHz, THF-d8): δ = –6.7 (s) ppm. ATI IR (neat): ῡ = 3244 (w, NH), 3053
(w), 2990 (w), 2866 (w), 2777 (w), 1577 (w), 1478 (m), 1457 (w), 1427
(m), 1308 (w), 1265 (s), 1257 (m), 1239 (s), 1189 (w), 1178 (w), 1137 (s),
1097 (w), 1067 (w), 1050 (w), 1035 (w), 1020 (w), 1011 (s), 995 (w), 981
(m), 968 (w), 952 (w), 913 (s), 903 (m) cm–1. Anal. calc. for C30H39BBiN3
(661.45 g/mol): Bi, 31.6; found: Bi, 31.9.
[Me2NHBPh3] (2). [HNEt3][BPh4] (29 mg, 69 µmol) was added to a
solution of Bi(NMe2)3 (24 mg, 70 µmol) in THF (1 mL). After 2 d, the
reaction mixture was filtered. The filtrate was layered with hexanes (3
mL) and cooled to –30 °C. After 3 d, colorless crystals of 2 were isolated
by filtration and dried in vacuo. Yield: 16 mg, 56 µmol, 81%.
determined in
procedures using
a
complexometric titration according to literature
solution of the disodium salt of
1H NMR (400 MHz, C6D6): δ = 1.63 (d, 6H, 3JHH = 5.9 Hz, Me), 3.53 (br s,
1H, NH), 7.15-7.19 (m, 3H, p-Ph), 7.26-7.30 (m, 6H, m-Ph), 7.39-7.41 (m,
6H, o-Ph) ppm. 13C NMR (101 MHz, C6D6): δ = 38.71 (s, Me), 125.46 (s,
p-Ph), 127.57 (s, m-Ph), 134.37 (s, o-Ph), 150.50 (br s, ipso-Ph) ppm.
11B NMR (128 MHz, C6D6): δ = 1.08 (s) ppm. 11B NMR (128 MHz, THF-
d8): δ = 0.56 (s) ppm. ATI IR (neat): ῡ = 3228 (w, NH), 3063 (w), 3018 (w),
3007 (w), 2994 (w), 2971 (w), 2949 (w), 1488 (w), 1470 (w), 1459 (w),
1429 (m), 1380 (m), 1338 (w), 1316 (w), 1290 (m), 1281 (w), 1271 (w),
1263 (w), 1246 (w), 1220 (w), 1188 (m), 1178 (m), 1151 (m), 1142 (w),
1131 (w), 1118 (w), 1102 (w), 1091 (m), 1074 (w), 1059 (w), 1044 (w),
1032 (m), 1012 (m), 1000 (m), 982 (w), 969 (w), 960 (w), 952(w), 924 (s),
914 (w), 905 (w) cm–1. Anal. calc. for C20H22BN (287.21 g/mol): C, 83.64;
H, 7.72; N, 4.88; found: C, 83.61; H, 7.78; N, 4.60.
a
ethylenediaminetetraacetic acid as a titrant and Xylenol Orange as an
indicator. Single crystals suitable for X-ray diffraction were coated with
polyisobutylene or perfluorinated polyether oil in a glovebox, transferred
to a nylon loop and then transferred to the goniometer of a diffractometer
equipped with a molybdenum X-ray tube (λ = 0.71073 Å). The structures
were solved using direct methods (SHELXS) completed by Fourier
synthesis and refined by full-matrix least-squares procedures. CCDC
1500494-1500499 contain the crystallographic information for this work.
DFT calculations. DFT calculations were performed with the Gaussian
program[37] using the 6-31G(d,p)[38] (H, B, C, N, O, F), 6-311G(d,p) (Al,
Cl), or the LanL2DZ[39] (Bi) basis set and the B3LYP functional.[40]
A
solvent model (smd,solvent=thf) was used for all calculations.[41] The D3
version of Grimme’s dispersion model with the original D3 damping
function was applied.[42] Frequency analyses of the reported structures
showed no imaginary frequencies for ground states and one imaginary
frequency for transition states. Thermodynamic parameters were
calculated at a temperature of 298.15 K and a pressure of 1.00 atm. NBO
analyses were performed using NBO 6.[43] NMR shielding tensors were
calculated using the gauge independent atomic orbitals method and 11B
NMR chemical shifts determined by comparison with BF3(OEt2).[44]
Reaction of Bi(NiPr2)3 with [HNEt3][BPh4].
A
(NMR scale):
[HNEt3][BPh4] (12.4 mg, 29 µmol) was added to a solution of Bi(NiPr2)3
(15.0 mg, 29 µmol) in THF-d8 (0.5 mL). The reaction mixture was
analyzed by NMR spectroscopy. The following species were identified
after 10 min: NEt3, [BPh3(thf)], HNiPr2, BiPh(NiPr2)2 (for further details see
Supp. Inf.).
BiPh(NiPr2)2: 1H NMR (400 MHz, THF-d8) δ = 1.22 (d, 24H, 3JHH = 6.6 Hz,
CHMe2), 4.45 (sept, 4H, 3JHH = 6.6 Hz, CHMe2), 7.23-7.27 (m, 1H, p-Ph),
7.54-7.57 (m, 2H, o-Ph), 7.30-7.33 (m, 2H, m-Ph) ppm.
B (lab scale): [HNEt3][BPh4] (34 mg, 81 µmol) was added to a solution of
Bi(NiPr2)3 (41 mg, 80 µmol) in THF (0.4 mL). The reaction mixture was
filtered. The filtrate was layered with hexanes (4 mL) and stored at –
30 °C for 2 days. A solid material was isolated by filtration and dried
under a stream of Argon. It consisted of a colorless solid (identified as
[BPh3(thf)]) and trace amounts of black crystals, which were separated
manually and identified as [Bi(NiPr2)2(BPh4)] (6) using single crystal X-ray
diffraction techniques.[46] Yield of [BPh3(thf)]: 13 mg, 41 µmol, 51%.
1H NMR (400 MHz, THF-d8): δ = 1.75-1.78 (m, 4H, β-THF), 3.59-3.63 (m,
4H, α-THF), 7.33-7.41 (m, 9H, p-Ph, m-Ph), 7.55 (d, 6H, 3JHH = 6.8 Hz, o-
Ph) ppm. 11B NMR (128 MHz, THF-d8): δ = 52.72 (br s) ppm. These and
further NMR spectroscopic analyses were in agreement with those
obtained for authentic sample. Unit cell (determined using single crystal
X-ray diffraction analysis at T = 170 K): monoclinic space group P21/c; a
= 9.0589(6) Å; b = 12.7256(9) Å; c = 15.4884(10) Å; α = γ = 90°; β =
104.808(2)°; these values are in agreement with previously reported
data.[47]
Bi(NMe2)3. Bi(NMe2)3 was synthesized according to the literature.[5b]
Analytical data were in agreement with the literature. Additional data: 15N
NMR (51 MHz, C6D6): δ = ‒318.5 (s, NMe2) ppm.
Bi(NiPr2)3. Bi(NiPr2)3 was synthesized according to the literature.[5e] Use
of sublimed BiCl3 as a starting material led to a slight increase in the
isolated yield for reactions performed on the 6 mmol scale with respect to
BiCl3 (74% (this work) vs. 69% (ref. 5e). Analytical data were in
agreement with the literature. Additional data: 15N NMR (51 MHz, C6D6):
δ = ‒267.6 (s, NiPr2) ppm.
[Bi(NMe2)2(HNMe2)(BPh4)] (1). [HNEt3][BPh4] (29 mg, 69 µmol) was
added to a solution of Bi(NMe2)3 (24 mg, 70 µmol) in THF (1 mL). The
reaction mixture was layered with pentane (5 mL) and cooled to –30 °C.
After 14 h, a yellow solid was isolated by filtration and dried in a stream
of argon. Yield: 42 mg, 63 µmol, 91%.
1H NMR (400 MHz, C6D6/THF-d8 (1000:1)): δ = 1.91 (br d, 6H, 3JHH = 4.7
Hz, HNMe2), 3.42 (br s, 12 H, Bi(NMe2)2), 5.06 (br s, 1H, HNMe2), 7.13-
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10.1002/chem.201604117
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Reaction of Bi(NPh2)3 with [HNEt3][BPh4]. [HNEt3][BPh4] (8.9 mg, 21
µmol) was added to a solution of Bi(NPh2)3 (15 mg, 21 µmol) in THF-d8
(0.5 mL). The reaction mixture was analyzed by NMR spectroscopy. NEt3
and [BPh3(thf)] were unambiguously identified after 10 min (for further
details see Supp. Inf.).
[Bi(NiPr2)2(OPPh3)2][B(3,5-C6H3(CF3)2)4)] (6). OPPh3 (6.4 mg, 23 µmol)
was added to a solution of 5 (17 mg, 11 µmol) in THF-d8 (0.5 mL). The
resulting orange solution was analyzed by NMR spectroscopy.
3
1H NMR (400 MHz, THF-d8): δ = 1.25 (d, 24H, JHH = 6.6 Hz, CHMe2),
3
5.75 (sept, 4H, JHH = 6.6 Hz, CHMe2), 7.52-7.56 (m, 12H, m-Ph), 7.57
(s, 4H, p-(3,5-C6H3(CF3)2), 7.64-7.70 (m, 18H, o,p-Ph), 7.79 (s, 8H, o-
(3,5-C6H3(CF3)2) ppm. 13C NMR (126 MHz, THF-d8): δ = 29.52 (s,
[Bi2(μ2-NMe2)2(NMe2)2(thf)6][B(3,5-C6H3(CF3)2)4)]2 (4).
A solution of
3
CHMe2), 53.12 (s, CHMe2), 118.36 (sept, JCF
= 3.9 Hz, p-(3,5-
[H(OEt2)2][B(3,5-C6H3(CF3)2)4)] (68 mg, 67 µmol) in THF (0.5 mL) was
added dropwise to a stirred solution of [Bi(NMe2)3] (24 mg, 70 µmol) in
THF (0.5 mL). The reaction mixture was layered with hexanes (4 mL) and
cooled to –30 °C. After 16 h, a yellow solid was isolated by filtration,
washed with hexanes (2 mL), and dried in vacuo. Yield: 82 mg, 30 µmol,
90%. The equivalents n of THF in the isolated compound has to be
checked individually for every batch. Values ranging from n = 4 to n = 6
(vacuum dried) or up to n = 8 (dried in a stream of Argon) have been
observed.
C6H3(CF3)2)), 125.69 (quart, 1JCF = 272.3 Hz, CF3), 130.02 (d, 3JCP = 12.3
2
4
Hz, m-Ph), 130.20 (quartquart, JCF = 31.8 Hz, JCF = 2.7 Hz, m-(3,5-
2
4
C6H3(CF3)2)), 133.35 (d, JCP = 10.4 Hz, o-Ph), 134.06 (d, JCP = 1.6 Hz,
1
p-Ph), 135.77 (br s, o-(3,5-C6H3(CF3)2)), 162.99 (quart, JBC = 49.9 Hz,
ipso-(3,5-C6H3(CF3)2)) ppm. Resonances for THF were also detected. A
13C resonance for the ipso-Ph atom could not be detected.[48] 11B NMR
(160 MHz, THF-d8): δ = –6.5 (s, B(C6H3(CF3)2)4) ppm. 31P NMR (162
MHz, THF-d8): δ = 32.4 (s) ppm. 15N NMR (41 MHz, THF-d8): δ = ‒171.8
(s, NiPr2) ppm.
1H NMR (300 MHz, THF-d8, 26 °C, c(monomer) = 0.01 M): δ = 1.75-1.80
(m, n×4H, β-THF), 3.59-3.64 (m, n×4H, α-THF), 4.49 (s, 12H, NMe2),
7.57 (s, 4H, p-(3,5-C6H3(CF3)2), 7.79 (s, 8H, o-(3,5-C6H3(CF3)2) ppm. 1H
NMR (300 MHz, THF-d8, –60 °C, c(monomer) = 0.01 M): δ = 1.76-1.81 (m,
n×4H, β-THF), 3.60-3.64 (m, n×4H, α-THF), 4.43 (s, 6H, μ2-NMe2), 4.53
(s, 6H, terminal NMe2), 7.69 (s, 4H, p-(3,5-C6H3(CF3)2), 7.87 (s, 8H, o-
(3,5-C6H3(CF3)2) ppm. 13C NMR (126 MHz, THF-d8, 26 °C, c(monomer) =
0.01 M): δ = 26.55 (s, β-THF), 43.92 (s, NMe2), 68.39 (s, α-THF), 118.35
i
Attempted reactions of Bi(NR2)3 with Na[BPh4] (R = Me, Pr, Ph). For
each reaction, one equivalent of Na[BPh4] was added to a solution of
Bi(NR2)3 in THF to give a yellow (R = Me, iPr) or orange (R = Ph) solution.
The reactions were monitored by 11B NMR spectroscopy. No conversion
of [BPh4]– could be detected after reaction times of >24 h. The
experiments were performed on a 30 µmol scale.
3
1
(sept, JCF = 3.9 Hz, p-(3,5-C6H3(CF3)2)), 125.68 (quart, JCF = 272.2 Hz,
2
4
i
[Bi((iPrN)2CNMe2)3] (7). PrN=C=NiPr (12 mg, 95 µmol) was added to a
CF3), 130.19 (quartquart, JCF = 31.5 Hz, JCF = 2.9 Hz, m-(3,5-
1
C6H3(CF3)2)), 135.76 (br s, o-(3,5-C6H3(CF3)2)), 162.97 (quart, JBC
=
solution of Bi(NMe2)3 (10 mg, 29 µmol) in C6D6 (0.5 mL). Full conversion
to Bi((NiPr)2CNMe2)3 (7) with a selectivity of ≥95% was detected after 11
h. The reaction was repeated using toluene as a solvent and doubling the
batch size (59 µmol of Bi(NMe2)3). After 14 h, all volatiles were removed
under reduced pressure to give a yellow solid, which was dried in vacuo.
Yield: 37 mg, 51 µmol, 86%.
59.8 Hz, ipso-(3,5-C6H3(CF3)2)) ppm. 13C NMR (75 MHz, THF-d8, –60 °C,
c(monomer) = 0.01 M): δ = 26.56 (s, β-THF), 43.30 (s, μ2-NMe2), 45.43 (s,
terminal NMe2), 68.40 (s, α-THF), 118.55 (br s, p-(3,5-C6H3(CF3)2)),
125.58 (quart, 1JCF = 272.3 Hz, CF3), 130.09 (br quart, 2JCF = 31.6 Hz, m-
(3,5-C6H3(CF3)2)), 135.55 (br s, o-(3,5-C6H3(CF3)2)), 163.07 (quart, 1JBC
=
48.2 Hz, ipso-(3,5-C6H3(CF3)2)) ppm. 11B NMR (156 MHz, THF-d8,
26 °C): δ = –6.7 (s) ppm. 19F NMR (376 MHz, THF-d8, 26 °C): δ = –63.4
(s) ppm. Anal. calc. for C72H48B2Bi2F48N4·(OC4H8)5 (2681.24 g/mol): C,
41.2; H, 3.3; N, 2.1; found: C, 41.3; H, 3.2; N, 1.9.
3
1H NMR (400 MHz, C6D6) δ = 1.37 (d, 36H, JHH = 6.4 Hz, CHMe2), 2.66
(s, 18H, NMe2), 4.46 (br sept, 6H, 3JHH = 6.4 Hz, CHMe2) ppm. 13C NMR
(126 MHz, C6D6): δ = 26.22 (br s, CHMe2), 40.95 (s, NMe2), 47.35 (s,
CHMe2), 165.56 (br s, (NiPr)2CNMe2) ppm. 15N NMR (41 MHz, THF-d8): δ
= ‒206.1 (s, (iPrN)2C(NMe2)) ppm. A resonance for (iPrN)2C(NMe2) was
[Bi(NiPr2)2(thf)3][B(3,5-C6H3(CF3)2)4)] (5). A solution of [H(OEt2)2][B(3,5-
C6H3(CF3)2)4] (54 mg, 53 µmol) in THF (0.5 mL) was added dropwise to a
stirred solution of Bi(NiPr2)3 (30 mg, 59 µmol) in THF (0.5 mL). The
solution was layered with hexanes (4 mL) and cooled to –30 °C. After 14
h, an orange crystalline material had formed, which was isolated by
filtration, washed with hexanes (2 mL) and dried in vacuo. Yield: 72 mg,
48 µmol, 91%.
not detected. Anal. calc. for C27H60BiN9 · (C6H14
45.3; H, 8.5; N, 17.4; found: C, 45.7; H, 8.8; N, 17.3.
)
(723.94 g/mol): C,
0.05
Reaction of Bi(NiPr2)3 with iPrN=C=NiPr. iPrN=C=NiPr (9.0 mg, 71
µmol) was added to a solution of Bi(NiPr2)3 (10 mg, 20 µmol) in C6D6 (0.5
mL). No conversion of the carbodiimide was detected by 1H NMR
spectroscopy after reaction times of up to 23 h.
The amount of THF in the isolated compound has to be checked
individually for every batch. Values ranging from n = 2 to n = 3 molecules
of THF per formula unit have been observed, depending on the time the
compound is exposed to reduced pressure.
i
[Bi((iPrN)2CNMe2)2][B(3,5-C6H3(CF3)2)4)] (8). PrN=C=NiPr (7.0 mg, 55
µmol) was added to a solution of 4 (35 mg, 25 µmol) in THF-d8 (0.5 mL).
The color of the reaction mixture changed immediately from intense
yellow to pale yellow. Full conversion to the insertion product 8 was
detected after 10 min by 1H NMR spectroscopy. The reaction was
repeated using non-deuterated THF as a solvent. After 4 h, the reaction
mixture was layered with hexanes (4 mL) and cooled to –30 °C to give a
colorless crystalline solid after 16 h. The solid was isolated by filtration,
washed with hexanes (2 × 1 mL) and dried in vacuo. Yield: 34 mg, 24
µmol, 96%.
3
1H NMR (400 MHz, THF-d8): δ = 1.41 (d, 24H, JHH = 6.6 Hz, CHMe2),
1.76-1.79 (m, 12H, β-THF), 3.60-3.63 (m, 12H, α-THF), 6.03 (sept, 4H,
3JHH = 6.6 Hz, CHMe2), 7.57 (s, 4H, p-(3,5-C6H3(CF3)2), 7.79 (s, 8H, o-
(3,5-C6H3(CF3)2) ppm. 13C NMR (126 MHz, THF-d8): δ = 26.54 (s, β-THF),
29.72 (s, CHMe2), 53.98 (s, CHMe2), 68.39 (s, α-THF), 118.35 (sept, 3JCF
1
= 3.9 Hz, p-(3,5-C6H3(CF3)2)), 125.69 (quart, JCF = 272.2 Hz, CF3),
2
4
130.20 (quartquart, JCF = 31.6 Hz, JCF = 2.9 Hz, m-(3,5-C6H3(CF3)2)),
1
135.77 (br s, o-(3,5-C6H3(CF3)2)), 162.98 (quart, JBC = 50.0 Hz, ipso-
1H NMR (400 MHz, THF-d8) δ = 1.23 (d, 24H, JHH = 6.4 Hz, CHMe2),
3
(3,5-C6H3(CF3)2)) ppm. 11B NMR (160 MHz, THF-d8): δ = –6.6 (s,
B(C6H3(CF3)2)4) ppm. 19F NMR (471 MHz, THF-d8): δ = –63.4 (s, CF3)
ppm. 15N NMR (51 MHz, THF-d8): δ = ‒151.6 (s, NiPr2) ppm. Anal. calc.
for C44H40B1Bi1F24N2 · (C4H8O)2.5 (1452.84 g/mol): C, 44.6; H, 4.2; N, 1.9;
found: C, 44.6; H, 4.0; N, 1.9.
2.95 (s, 12H, NMe2), 5.29 (br s, 4H, CHMe2), 7.57 (s, 4H, p-(3,5-
C6H3(CF3)2), 7.79 (s, 8H, o-(3,5-C6H3(CF3)2) ppm. 1H NMR (500 MHz,
THF-d8, –40 °C)
δ
=
1.05 (d, 6H, 3JHH
=
=
=
=
6.3 Hz,
6.3 Hz,
6.3 Hz,
6.3 Hz,
(MeMeCHN)(iPrN)C(NMe2)), 1.22 (d, 6H, 3JHH
(iPrN)(MeMeCHN)C(NMe2)), 1.26 (d, 6H, 3JHH
(MeMeCHN)(iPrN)C(NMe2)), 1.36 (d, 6H, 3JHH
(iPrN)(MeMeCHN)C(NMe2)), 2.95 (s, 12H, NMe2), 4.88 (br sept, 2H, 3JHH
6.3 Hz, 5.69 (br s, 2H,
(MeMeCHN)(iPrN)C(NMe2)),
=
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10.1002/chem.201604117
Chemistry - A European Journal
FULL PAPER
(iPrN)(MeMeCHN)C(NMe2)), 7.66 (s, 4H, p-(3,5-C6H3(CF3)2), 7.85 (s, 8H,
o-(3,5-C6H3(CF3)2) ppm. 13C NMR (101 MHz, THF-d8): δ = 25.58 (br s,
CHMe2, overlapping with solvent resonance), 40.64 (s, NMe2), 48.41 (br
(iPrN)2C(NiPr(CHMe2))), 4.77 (br sept, 2H, 3JHH
=
=
6.0 Hz,
6.3 Hz,
3
(Me2CHN)(iPrN)C(NiPr2)), 5.75 (br sept, 2H, JHH
(iPrN)(Me2CHN)C(NiPr2)), 7.66 (s, 4H, p-(3,5-C6H3(CF3)2), 7.85 (s, 8H, o-
(3,5-C6H3(CF3)2) ppm. 13C NMR (126 MHz, THF-d8, –40 °C): δ = 21.83 (s,
(iPrN)2C(N(CHMeMe)iPr)), 22.02 (s, (iPrN)2C(NiPr(CHMeMe))), 24.73 (s,
(iPrN)2C(N(CHMeMe)iPr)), 25.22 (s, (iPrN)(MeMeCHN)C(NiPr2)), 25.28
(s, (iPrN)(MeMeCHN)C(NiPr2)), 25.97 (s, (iPrN)2C(NiPr(CHMeMe))),
3
s, CHMe2), 118.36 (sept, JCF = 3.9 Hz, p-(3,5-C6H3(CF3)2)), 125.69
1
2
4
(quart, JCF = 272.3 Hz, CF3), 130.21 (quartquart, JCF = 31.6 Hz, JCF
=
2.9 Hz, m-(3,5-C6H3(CF3)2)), 135.77 (br s, o-(3,5-C6H3(CF3)2)), 162.99
1
(quart, JBC = 49.8 Hz, ipso-(3,5-C6H3(CF3)2)), 171.04 (s, (NiPr)2CNMe2)
ppm. 13C NMR (126 MHz, THF-d8, –40 °C):
δ
=
25.20 (s,
26.33
(s,
(iPrN)2C(N(CHMeMe)iPr)),
27.80
(s,
(MeMeCHN)(iPrN)C(NMe2)) 25.31 (s, (iPrN)(MeMeCHN)C(NMe2)), 25.34
(s, (MeMeCHN)(iPrN)C(NMe2)), 26.16 (s, (iPrN)(MeMeCHN)C(NMe2)),
40.51 (br s, NMe2), 47.49 (s, (Me2CHN)(iPrN)C(NMe2)), 49.05 (s,
(iPrN)(Me2CHN)C(NMe2)), 118.49 (br s, p-(3,5-C6H3(CF3)2)), 125.58
(MeMeCHN)(iPrN)C(NiPr2)), 48.55 (s, (Me2CHN)(iPrN)C(NiPr2)), 49.21 (s,
(iPrN)(Me2CHN)C(NiPr2)), 53.00 (s, (iPrN)2C(N(CHMe2)iPr), 53.72 (s,
(iPrN)2C(NiPr(CHMe2)), 118.48 (br s, p-(3,5-C6H3(CF3)2)), 125.58 (quart,
4
1JCF = 272.3 Hz, CF3), 130.09 (quartquart, 2JCF = 31.5 Hz, JCF = 2.7 Hz,
1
2
4
(quart, JCF = 272.3 Hz, CF3), 130.10 (quartquart, JCF = 31.6 Hz, JCF
=
m-(3,5-C6H3(CF3)2)), 135.58 (br s, o-(3,5-C6H3(CF3)2)), 163.03 (quart,
1JBC = 49.6 Hz, ipso-(3,5-C6H3(CF3)2)), 173.60 (s, (NiPr)2CNiPr2) ppm. 11
B
2.9 Hz, m-(3,5-C6H3(CF3)2)), 135.59 (br s, o-(3,5-C6H3(CF3)2)), 163.03
1
(quart, JBC = 49.8 Hz, ipso-(3,5-C6H3(CF3)2)), 170.67 (s, (NiPr)2CNMe2)
NMR (156 MHz, THF-d8, 26 °C): δ = –6.6 (s) ppm. 19F NMR (376 MHz,
THF-d8, 26 °C): δ = –63.4 (s) ppm. 15N NMR (51 MHz, THF-d8, –40 °C): δ
= ‒285.3 (s, (iPrN)2C(NiPr2)), ‒216.7 (s, (iPrN)(iPrN)C(NiPr2)), ‒205.1 (s,
(iPrN)(iPrN)C(NiPr2)) ppm. Anal. calc. for C58H68BBiF24N6 · (OC4H8)0.5
(1561.03 g/mol): C, 46.2; H, 4.7; N, 5.4; found: C, 46.1; H, 4.4; N, 5.1.
ppm. 11B NMR (160 MHz, THF-d8): δ = –6.6 (s, B(3,5-C6H3(CF3)2)4) ppm.
19F NMR (471 MHz, THF-d8): δ = –63.4 (s, CF3) ppm. 15N NMR (51 MHz,
THF-d8, –40 °C): δ = ‒312.8 (s, NMe2), ‒227.3 (s, (iPrN)(iPrN)C(NMe2)),
‒216.7 (s, (iPrN)(iPrN)C(NMe2)) ppm. Anal. calc. for C50H52BBiF24N6
(1412.76 g/mol): C, 42.5; H, 3.7; N, 6.0; found: C, 42.5; H, 3.3; N, 5.9.
[Bi((iPrN)2CNiPr2)2][B(3,5-C6H3(CF3)2)4)] (9). PrN=C=NiPr (6.0 mg, 48
i
Acknowledgements
µmol) was added to a solution of 5 (30 mg, 20 µmol) in THF-d8 (0.5 mL).
The color of the reaction mixture changed immediately from orange to
pale yellow. Full conversion to the mono-insertion product was detected
after 10 min by NMR spectroscopy. Full conversion to compound 9 was
detected after 12 h by NMR spectroscopy. The reaction was repeated
using non-deuterated THF as a solvent. After 14 h, the reaction mixture
was filtered and layered with hexanes (4 mL) and cooled to –30 °C. After
1 d, a colorless crystalline material had formed, which was isolated by
filtration, washed with hexanes (2 × 1 mL) and dried in vacuo. Yield: 23
mg, 15 µmol, 75%.
The authors thank Dr. Rüdiger Bertermann and Dr. Rian
Dewhurst for helpful discussions and Prof. Dr. Holger
Braunschweig for his support. Financial support by the Fonds
der Chemischen Industrie (Liebig fellowship to C.L.) is gratefully
acknowledged.
Keywords: Bismuth • Amides • Cationic Species • B–C Bond
Activation • Guanidinates
Data for intermediate compound [Bi((iPrN)2C(NiPr2))(NiPr2)] [B(3,5-
3
C6H3(CF3)2)4)]: 1H NMR (400 MHz, THF-d8) δ = 1.22 (d, 6H, JHH = 6.4
Hz, (MeMeCHN)2C(NiPr2)), 1.23 (d, 6H, 3JHH
=
6.4 Hz,
6.8 Hz,
[1]
[2]
[3]
X. Zhang, R. Qiu, N. Tan, S. Yin, J. Xia, S. Luo, C.-T. Au, Tetrahedron
Lett. 2010, 51, 153-156.
(MeMeCHN)2C(NiPr2)),
1.36
(d,
12H,
3JHH
=
3
(iPrN)2C(N(CHMe2)2)), 1.39 (d, 12H, JHH = 6.6 Hz, N(CHMe2)2), 3.74
(sept, 2H, 3JHH = 6.8 Hz, (iPrN)2C(N(CHMe2)2)), 5.36 (sept, 2H, 3JHH = 6.4
Hz, (Me2CHN)2C(NiPr2)), 5.98 (sept, 2H, 3JHH = 6.6 Hz, N(CHMe2)2), 7.57
(s, 4H, p-(3,5-C6H3(CF3)2), 7.79 (s, 8H, o-(3,5-C6H3(CF3)2) ppm. 13C NMR
(126 MHz, THF-d8): δ = 23.36 (s, (iPrN)2C(N(CHMe2)2)), 25.16 (s,
(MeMeCHN)2C(NiPr2)), 26.79 (s, (MeMeCHN)2C(NiPr2)), 29.35 (s,
R. Qiu, Y. Qiu, S. Yin, X. Song, Z. Meng, X. Xu, X. Zhang, S. Luo, C.-T.
Au, W.-Y. Wong, Green Chem. 2010, 12, 1767-1771.
a) R. Qiu, S. Yin, X. Zhang, J. Xia, X. Xu, S. Luo, Chem. Commun.
2009, 4759-4761; b) R. Qiu, S. Yin, X. Song, Z. Meng, Y. Qiu, N. Tan,
X. Xu, S. Luo, F.-R. Dai, C.-T. Au, W.-Y. Wong, Dalton Trans. 2011, 40,
9482-9489.
N(CHMe2)2),
48.00
(s,
(Me2CHN)2C(NiPr2)),
51.99
(s,
[4]
[5]
C. Lichtenberg, F. Pan, T. P. Spaniol, U. Englert, J. Okuda, Angew.
Chem. Int. Ed. 2012, 51, 13011-13015.
(iPrN)2C(N(CHMe2)2)), 53.43 (s, N(CHMe2)2), 118.35 (sept, 3JCF = 3.9 Hz,
1
p-(3,5-C6H3(CF3)2)), 125.69 (quart, JCF
(quartquart, JCF = 31.6 Hz, JCF = 2.9 Hz, m-(3,5-C6H3(CF3)2)), 135.77
(br s, o-(3,5-C6H3(CF3)2)), 162.99 (quart, JBC = 49.9 Hz, ipso-(3,5-
= 272.3 Hz, CF3), 130.21
a) F. Ando, T. Hayashi, K. Ohashi, J. Koketsu, J. Inorg. Nucl. Chem.
1975, 37, 2011-2013; b) W. Clegg, N. A. Compton, R. J. Errignton, G. A.
Fisher, M. E. Green, D. C. R. Hockless, N. C. Norman, Inorg. Chem.
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B. Rego, J. W. Ziller, Inorg. Chim. Acta 2007, 360, 1349-1353; e) M.
Vehkamäki, T. Hatanpää, M. Ritala, M. Leskelä, J. Mater. Chem. 2004,
14, 3191-3197.
2
4
1
C6H3(CF3)2)), 173.42 (s, (NiPr)2CNMe2) ppm. Resonances for THF and
excess iPrN=C=NiPr were also detected. 11B NMR (160 MHz, THF-d8): δ
= –6.6 (s, B(3,5-C6H3(CF3)2)4) ppm. 19F NMR (471 MHz, THF-d8): δ = –
63.4 (s, CF3) ppm. 15N NMR (41 MHz, THF-d8): δ = ‒282.6 (s,
(iPrN)2C(NiPr2)), ‒212.7 (s, (iPrN)2C(NiPr2)), ‒169.6 (s, Bi(NiPr2) ppm.
Data for compound 9: 1H NMR (400 MHz, THF-d8) δ = 1.30 (br s, 24H,
CHMe2), 1.35 (br s, 24H, CHMe2), 3.71 (br s, 4H, (iPrN)2C(N(CHMe2)2)),
4.95 (br s, 2H, (Me2CHN)(iPrN)C(NiPr2)), 5.71 (br s, 2H,
(iPrN)(Me2CHN)2C(NiPr2)), 7.57 (s, 4H, p-(3,5-C6H3(CF3)2), 7.78 (s, 8H,
o-(3,5-C6H3(CF3)2) ppm. 1H NMR (500 MHz, THF-d8, –40 °C) δ = 1.16 (d,
[6]
a) M. Yarema, M. V. Kovalenko, G. Hesser, D. V. Talapin, W. Heiss, J.
Am. Chem. Soc. 2010, 132, 15158-15159; b) G. Bendt, A. Weber, S.
Heimann, W. Assenmacher, O. Prymak, S. Schulz, Dalton Trans. 2015,
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[8]
M. García-Castro, A. Martín, M. Mena, C. Yélamos, Chem. Eur. J. 2009,
15, 7180-7191.
3
3
6H, JHH = 6.0 Hz, (MeMeCHN)(iPrN)C(NiPr2)), 1.23 (d, 6H, JHH = 7.0
Hz, (iPrN)2C(N(CHMeMe)iPr)), 1.27 (d, 6H, 3JHH
(iPrN)(MeMeCHN)C(NiPr2)),
1.31 (m,
=
6.3 Hz,
a) P. L. Shutov, S. S. Karlov, K. Harms, D. A. Tyurin, A. V. Churakov, J.
Lorberth, G. S. Zaitseva, Inorg. Chem. 2002, 41, 6147-6152; b) P. L.
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W. H. Watson, K. A. Brien, L. K. Jayarathna, T. A. Hanna, Inorg. Chem.
12H,
(MeMeCHN)(iPrN)C(NiPr(CHMeMe)) (overlapping)), 1.38 (d, 6H, JHH
=
3
6.3
Hz,
(iPrN)(MeMeCHN)C(NiPr2)),
1.45
(12H,
(iPrN)2C(N(CHMeMe)(CHMeMe)) (overlapping)), 3.66 (br sept, 2H, 3JHH
=
3
7.0 Hz, (iPrN)2C(N(CHMe2)iPr)), 3.73 (br sept, 2H, JHH = 6.6 Hz,
This article is protected by copyright. All rights reserved.

10.1002/chem.201604117
Chemistry - A European Journal
FULL PAPER
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[28] Attempts to generate cationic bismuth species starting from Bi(NPh2)3
and [H(OEt2)2][A] have so far led to isolation of [Bi(L)n] 3[A] along with
re-isolated Bi(NPh2)3.
[29] a) C. J. Carmalt, L. J. Farrugia, N. C. Norman, J. Chem. Soc., Dalton
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[30] Among other techniques, solid state 15N MAS NMR spectroscopy has
been used for the structure elucidation of [Bi(S2CN(C3H7)2)3]: A. V.
Ivanov, I. V. Egorova, M. A. Ivanov, O. N. Antzutkin, R. S. Tsvykh, Dokl.
Phys. Chem. 2014, 454, 16-20.
[31] Whereas a 15N NMR resonance for Bi(NMe2)3 could be detected (see
experimental), no 15N NMR resonance could be detected for compound
4, most likely due to a small 2JNH coupling constant.
[9]
B. Nekoueishahraki, P. P. Samuel, H. W.; Roesky, D. Stern, J.
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[32] Lithium compounds with terminal diisopropylamido ligands were chosen
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[14] a) M. Veith, B. Bertsch, V. Huch, Z. Anorg. Allg. Chem. 1988, 559, 73-
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Chem. Rev. 2013, 257, 2863-2879). For compounds 1 and 3, however,
arene→Bi interactions are assumed due to the electron rich nature of
the [BPh4]– anion and the high Lewis acidity of the [Bi(NR2)2]+ cation.
This is in agreement with the absence of M-arene interactions in
compounds 4 and 5 with their less electron rich [B(3,5-C6H3(CF3)2)4]–
counterions.
[36] T. Chen, W. Hunks, C. Xu, Bicyclic Guanidinates and Bridging
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0.12 and 0.02, respectively. The sum of Bi–CArene Wiberg bond indices
in [BiCl2(C6Me6)(AlCl4)] amounts to 0.55 (cf. note 26 and Supp. Inf.).
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[20] Small amounts of THF are necessary for solvation of 1 in benzene as a
primary solvent. In the 1H NMR spectrum of 1 in C6D6/THF-d8, the
resonances for THF-d8 are marginally downfield shifted compared to
those of free THF-d8 in C6D6 (Δ = 0.01-0.02 ppm), indicating that there
are no strong Bi–thf interactions.
[45] This resonance was only detected with a higher number of scans
resulting in resonances due to decomposition products being also
detected.
[21] The presence of an NH functionality was confirmed by IR spectroscopy
(ῡ = 3244 cm–1).
[46] Attempts to generate larger amounts of 6 by up-scaling this reaction
were so far unsuccessful.
[22] Near quantitative formation of [BPh3(NHMe2)] from 1 in THF solution
indicates that 1 can also form a contact ion pair under these conditions,
most likely in an equilibrium with a solvent-separated ion pair (vide
infra).
[23] This is also in agreement with 11B NMR shifts of compound 1 in THF (δ
= –6.65 (s) ppm) and benzene/THF (1000:1) (δ = 1.12 (br s) ppm).
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[48] This has previously been reported for other metal complexes with
OPPh3 ligands, e. g.: C. Lichtenberg, T. P. Spaniol, T. J. Okuda
Organometallics 2011, 30, 4409-4417.
[49] At elevated temperatures of up to +60 °C, sample decomposition
hampered an unequivocal interpretation of the Bi-arene interactions.
i
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[50] The higher degree of Ph/NR2 exchange for R = Pr compared to R = Ph
Organometallics 2007, 26, 6864-6866.
correlates with a higher basicity of NHiPr2 compared to NHPh2: M. B.
Smith, J. March, March’s Advanced Organic Chemistry, 6h ed., John
Wiley & Sons, Hoboken, New Jersey, 2007.
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both Bi–Ph interactions (cf. note 18 and Supp. Inf.).
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Chem. Soc. Dalton Trans. 1984, 1349-1356.
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10.1002/chem.201604117
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Hannah Dengel and Crispin
Lichtenberg*
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Cationic Bismuth Amides:
Accessibility, Structure, and
Reactivity
Text for Table of Contents
Highly reactive. Cationic bismuth amides undergo facile B–C bond activation with their [BPh4]– counteranions. The cationic
complexes can be stabilized by use of fluorinated tetra-arylborate counteranions. Compared to their neutral parent compounds, they
show an increased reactivity towards diisopropylcarbodiimide, allowing direct access to cationic bismuth guanidinates.
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