Bis(imino)carbazolate: A Master Key for Barium Chemistry

Article abstract of DOI:10.1002/anie.202001439

Reported here is a readily available bis(imino)carbazole-based proligand that constitutes a convenient entry point into the challenging synthetic molecular chemistry of barium. It enables the preparation of rare or even, up to now, unknown, solution-stabl

Full text of DOI:10.1002/anie.202001439

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Accepted Article
Title: Bis(imino)carbazolate: a master key for barium chemistry
Authors: Peter Chapple, Samia Kahlal, Julien Cartron, Thierry Roisnel,
Vincent Dorcet, Marie Cordier, Jean-Yves Saillard, Jean-
Francois Carpentier, and Yann Sarazin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202001439
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10.1002/anie.202001439
Angewandte Chemie International Edition
RESEARCH ARTICLE
Bis(imino)carbazolate: a master key for barium chemistry
Peter M. Chapple, Samia Kahlal, Julien Cartron, Thierry Roisnel, Vincent Dorcet, Marie Cordier, Jean-
Yves Saillard,* Jean-François Carpentier and Yann Sarazin*
Dedicated to Professor Cristian Silvestru on the occasion of his 65th birthday.
P. M. Chapple, Dr. S. Kahlal, J. Cartron, Prof. Dr. J.-Y. Saillard, Dr. T. Roisnel, Dr. V. Dorcet, M. Cordier, Prof. Dr. J.-F. Carpentier, Dr. Y. Sarazin
Univ Rennes, CNRS, ISCR-UMR 6226, 35000 Rennes (France)
E-mail: yann.sarazin@univ-rennes1.fr
jean-yves.saillard@unic-rennes1.fr
Supporting information for this article is given via a link at the end of the document.
Abstract: We report here on a readily available bis(imino)carbazole-
based proligand that constitutes a most convenient entry point into the
challenging synthetic molecular chemistry of barium. It enables the
preparation of rare or even, up to now, unknown, solution stable
heteroleptic barium complexes. The syntheses and structural features
for the first molecular Ba-fluoride and for the first Ba-stannylide with
an unsupported Ba-to-Sn bond are described, along with other
carbazolate barium species: an amide (both a remarkably stable
starting material and an excellent hydrophosphination precatalyst),
iodide, and silanylide. DFT analysis of bonding patterns in the Ba-
stannylide and Ba-silanylide highlights a prevailingly ionic Ba-to-
tetrelide bond with a small covalent contribution.
hydrotris(pyrazolyl)borate[17] or an iminoanilide,[18] and also
cyclopentadienyl derivatives.[19] However, due to either synthetic
difficulty or insufficient stabilising properties, none of these
ligands have yet been widely adopted.
Introduction
Although the organometallic chemistry of the heavier group 2
alkaline earths (Ae = Ca, Sr, Ba) has received considerable
attention in the past two decades,[1-6] it still lags behind the
chemistry of the transition metals and that of the lanthanides. This
is primarily due to the extreme oxophillic nature of the Ae metals,
their strong ionic character and hence their tendency to
redistribute ligands in solution through the Schlenk equilibrium.
These issues are generally overcome by using bulky chelating
ancillary ligands to inhibit Schlenk redistribution to homoleptic
species that are either unreactive or uncontrollably reactive, or to
insoluble polynuclear species. However, with the exception of a
few examples, the limited range of appropriate ligands have
hampered the use of these metals in organometallic processes,
especially for the largest and most electropositive barium. Despite
the problems associated with Ae metals, many examples exist in
the recent literature of catalytic processes using low Ae catalyst
loadings and mild conditions,[1-4] e.g. olefin hydroelementation,[7-9]
imine[10] or alkene[11] hydrogenation, and dehydrocoupling of
amines with hydrosilanes[12] or boranes.[13] For the chemistry of
the heavier Ae metals, no ligand is more ubiquitous than 2,6-bis-
diisopropylphenyl-β-diketiminate (BDIDiPP, Fig. 1). Although well
suited for calcium and to a lesser extent strontium complexes, for
barium this ligand quickly redistributes to the unreactive
homoleptic [Ba{BDIDiPP}2].[14] Several sterically congesting
monoanionic ligands have been reported that partially address
this problem, including N-based systems such as the β-
Figure 1. Examples of ligands used to stabilise heteroleptic Ba-complexes.
(Mes = mesityl, Ad = 1-adamantyl). NB: BDIDiPP is not suitable for barium.[14]
A variety of long sought heteroleptic calcium complexes have
recently been prepared by utilising the BDIDiPP ligand. Soluble Ca-
hydroxide,[20] cyanide,[21] fluoride,[22] and triphenylstannylide[23] are
hence now available. Related BDIDiPP-strontium hydroxides and
halides (including fluoride) have also been disclosed.[24-25] Yet, the
corresponding chemistry of barium is still to be initiated, as it has
been plagued by the absence of stable precursors. A handful of
homoleptic Ca-tetrelides related to Hill’s [{BDIDiPP}CaSnPh3]2 also
exist, e.g. [Ca(TetR3)2(donor)x] (Tet = Si, Ge, Sn), but congeneric
strontium and barium species are still rare and known only for Si
or Ge.[23,26-32] Ba-to-Sn bonds remain elusive, although ion pairs
such as [Ba(18-c-6).(HMPA)2][SnPh3]2 are known.[27]
Reported here is a tailor-made bis(imino)carbazolate capable
of kinetically stabilising a range of highly reactive complexes of
the heavier alkaline earths, including the first heteroleptic barium
silanylide and stannylide complexes, along with the first molecular
Ba-fluoride that completes the series of heavier Ae fluorides. The
nature of the Ba-to-Sn and Ba-to-Si bonding is discussed in the
light of DFT computations.
diketiminate derivative {BDIDiPeP}‒ [15] a dipyrromethene,[16]
a
,
1
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10.1002/anie.202001439
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RESEARCH ARTICLE
Results and Discussion
The unsolvated Ba-amide [{CarbDiPP}BaN(SiMe3)2] (1) is
conveniently obtained as a yellow solid upon stoichiometric
reaction of {CarbDiPP}H with [Ba{N(SiMe3)2}2]2. It is stable in
solution in [D6]benzene, without signs of decomposition or ligand
redistribution after 20 days at 60 °C (Fig. S24). Complex 1 forms
a four-coordinate monomer in the solid state, with a distorted
tetrahedral environment (Fig. 2). The various Ba-N interatomic
distances are commensurate with those in the iminoanilide
[{N^NDiPP}BaN(SiMe3)2.(thf)2],[18] arguably the closest relative to 1;
the Ba-Nimine and Ba-Ncarbazolate interatomic distances are very
similar (2.722(3)-2.741(3) Å).
The bulky proligand 1,8-bis-(2,6-diisopropylphenyl)imino-3,6-di-
tert-butyl-carbazole (aka {CarbDiPP}H) is synthesised in ca. 70%
yield and scales up to 25 g from the condensation between 2,6-
diisopropylaniline and the reported 1,8-diformyl-3,6-di-tert-butyl-
carbazole (Scheme 1).[33] The carbazole core and the two C=N
moieties are coplanar. The Nimine-N(H)-N’imine angle (97.31(8)°, SI
Fig. S23) is well below that in 2,5-bis-{(2,6-diisopropylphenyl)-
imino}pyrrole (133.16(5)°) used to obtain Ae-iodides.[34] We
anticipated that {CarbDiPP ‒
would provide a more sterically
}
sheltered coordination sphere around the large Ba2+ cation (rionic
= 1.35 Å) and afford stable complexes. The ligand was hence
used to synthesise yet unknown barium species through
protonolysis, transmetallation sequences (Scheme 1).
The resilience of 1 to ligand scrambling contrasts with the
known instability of [{BDIDiPP}BaN(SiMe3)2.(thf)2].[14] Gratifyingly,
the deliberate formation of the otherwise undesired homoleptic
[{CarbDiPP}2Ba] is only achieved under forcing conditions (80 °C,
16 h), with a three-fold excess of {CarbDiPP}H vs [Ba{N(SiMe3)2}2]2.
Yet, the reaction is not clean nor quantitative, as the product is
still contaminated by the presence of 1. It is likely that the
congested environment around the metal precludes the easy
coordination of a second bulky carbazolate.
Figure 2. Displacement ellipsoids (30% probability; H atoms omitted) for
[{CarbDiPP}BaN(SiMe3)2] (1). Only one of the two independent but similar
t
molecules in the asymmetric cell, and the main component of disordered Bu
and SiMe3 groups, are depicted. Representative interatomic distances (Å) and
angles (°): Ba1-N1 = 2.741(3), Ba1-N2 = 2.722(3), Ba1-N3 = 2.739(3), Ba1-N4
= 2.585(4); N1-Ba1-N3 = 133.13(10), N2-Ba1-N4 = 125.61(11).
Note also that THF-free 1 is prepared by a simple protonolysis
protocol carried out overnight at 20 °C, or within 1 h at 60 °C,
easier to implement than that for [{BDIDiPP}AeN(SiMe3)2.(thf)x].
These complexes, only stable in solution for Ae = Ca, are
prepared by salt metathesis between AeI2, {BDIDiPP}H and
KN(SiMe3)2 (2 eq.), and can only be isolated as thf solvates
following this route.[14] Harder’s bulkier {BDIDiPeP}H does generate
solvent-free [{BDIDiPeP}AeN(SiMe3)2] stable for Ca-Ba, but
requires heating at 140 °C for 5-9 days.[15]
The stability and availability of 1, combined to the absence of
coordinated solvent, make it an excellent precursor not only for
synthetic but also for catalysis purposes. For instance, in the
benchmark catalysis of the hydrophosphination of styrene with
HPPh2 (1:1 ratio, 3 mol-% 1) that yields regioselectively
Scheme 1. Synthesis of bis(imino)carbazolate-barium amide (1), halides (2-3)
and tetrelides (4-5), using the proligand {CarbDiPP}H, where DiPP = 2,6-iPr2-C6H3.
PhCH2CH2PPh2,
1 comes second to none. Its turnover
frequencies, as high as 23 h–1 at 25 °C, betters those of the best
2
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10.1002/anie.202001439
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RESEARCH ARTICLE
precatalysts known to date which, additionally, require higher
temperatures to be effective (60-75 °C, Table S25).[4,35-36]
K-N interatomic distances (2.6479(11)-2.7332(11) Å) within the
known ranges for potassium-diiminocarbazolates.[38] The
stoichiometric reaction of [{CarbDiPP}K.(thf)2] and BaI2 in thf affords
[{CarbDiPP}BaI.(thf)2] (3) as a yellow solid in near quantitative yield
(see Scheme 1). The complex crystallises as a di-solvated
monomer (Fig. 4). The six-coordinate metal centre rests in a
distorted anti-prismatic geometry. The three Ba-N interatomic
distances are fairly comparable, in the range 2.726(2)-2.801(2) Å,
and commensurate with those in 1. The Ba-I bond length,
3.4052(3) Å, matches that for reported barium iodides.[34,39-40]
Besides, 1 reacts with Me3SnF to return the dinuclear
[({CarbDiPP}Ba(μ-F).thf)2(μ-thf)] (2) as a yellow crystalline solid.
Complex 2 is the first molecular barium-fluoride, and completes
the series of available heavier Ae-F species, over 10 years after
the first soluble, molecular calcium-fluorides were unveiled.[22,25,37]
It can be seen as a well-defined derivative of BaF2 used for
applications as sensors or windows in spectrometry due to its
transparency from UV to far-IR. The C2-symmetric dinuclear 2 is
bridged by two F atoms and a thf molecule (Fig. 3). In addition to
a carbazolate ligand, coordinative saturation around each metal
is achieved by a thf molecule, making them seven-coordinated.
The Ba-Nimine interatomic distances in 2 (2.937(3)-2.955(3) Å) are
much longer than in 1, reflecting greater coordination number and
steric congestion about the metals in the fluoro complex. On the
other hand, the Ba-Ncarbazolate distances in the two complexes are
similar. For comparison, the coordination pattern in 2 differs from
those in Ca/Sr fluorides. [{BDIDiPP}Ca(μ-F).(thf)]2 is a symmetrical
dimer with one metal-bound thf molecule per Ca atom (Ca-F =
(2.180(2)av Å).[22] Its strontium analogue, [{BDIDiPP}Sr(thf)(μ-
F)2Sr{BDIDiPP}(thf)2], is asymmetrically ligated: one Sr atom bears
one coordinated thf molecule, and the other carries two of them
(Sr-F = 2.348(1)av Å).[25] The 19F NMR spectrum of 2 recorded in
[D6]benzene at 23 °C features a single resonance, a somewhat
broad singlet at δ19F = +10.1 ppm. It is considerably deshielded
compared to those in the abovementioned strontium (‒60.0 ppm
in [D8]thf) and calcium (‒78.0 ppm in [D6]benzene) fluorides. The
19F chemical shift in Ae-fluorides hence increases with metal
electropositivity and polarity of the Ae-to-F bond, but this trend
perhaps ought to be considered with caution as both coordination
environments and NMR solvents vary for the three complexes.
Figure 4. Displacement ellipsoids (30% probability; H atoms omitted) for
[{CarbDiPP}BaI.(thf)2] (3). Representative interatomic distances (Å) and angles
(°): Ba1-I1 = 3.4052(3), Ba1-N1 = 2.788(2), Ba1-N2 = 2.726(2), Ba1-N3 =
2.801(2), Ba1-O1 = 2.734(2), Ba1-O2 = 2.778(2); N2-Ba1-I1 = 141.39(4), N1-
Ba1-N3 = 127.69(6).
Compounds 2 and 3 were then tested as synthetic precursors
to obtain (yet unknown) heteroleptic barium-tetrelides. While the
strong Ba-F bond (ΔHf = 487(7) kJ mol‒1) in 2 proved reluctant to
react with NaNH2 or Si(SiMe3)4, 3 reacted cleanly in salt
metathesis reactions. Hence, [{CarbDiPP}BaSi(SiMe3)3.thf] (4), the
first heteroleptic Ba-silanylide and only the second example of a
Ba-silanylide,[32] was obtained by equimolar reaction of 3 with the
bulky [KSi(SiMe3)3.(thf)2].[41] Orange crystals suitable for XRD
analysis were isolated in a 36% (non-optimised) yield. The
monomeric 4 is soluble in common organic solvents, including
aliphatic hydrocarbons. The 29Si{1H}-INEPT NMR spectrum of 4
displays two sharp resonances, a major one at δ29Si = ‒5.8 ppm
for Si(SiMe3)3, and a minor one, at ‒158.3 ppm, assigned to
Si(SiMe3)3. The chemical shifts are consistent with those for the
homoleptic [Ae(Si(SiMe3)3)2.(thf)n],[26,29] although no value was
reported for the barium-bound Ba-Si(SiMe3)3 Si atom in this series.
The molecular structure of 4 established by XRD analysis is
depicted in Fig. 5. The crystallographically Cs-symmetrical
molecule is disordered over several sites including, very typically,
Si(SiMe3).[42] The geometry around the five-coordinate Ba atom
forms a distorted square pyramid (5 = 0.25), with the Si(SiMe3)3
in apical position. This square pyramidal arrangement driven by
the chelating nature of the carbazolate ligand is consistent with
the hypervalent 12-electron count of Ba. The Ba-Si interatomic
distance averaged over the two occupancy sites, 3.403(4)av Å, is
close to those in [Ba(Si(SiMe3)3)2.(thf)4] (3.419(1) and 3.461(1)
Å).[32] Significant pyramidalisation at the silicon atom Si1a, with
compressed Si(i)-Si1a-Si(j) angles (i, j = 2a, 3a, 4a) in the range
Figure 3. Displacement ellipsoids (30% probability; H atoms omitted) for
[({CarbDiPP}Ba(μ-F).thf)2(μ-thf)] (2). Only the main components of disordered thf
molecules and tBu and iPr groups depicted. tBu and iPr groups omitted for clarity.
Representative interatomic distances (Å) and angles (°): Ba1-F61 = 2.511(2),
Ba1-F61i = 2.518(2), Ba1-N1 = 2.955(3), Ba1-N25 = 2.735(3), Ba1-N37 =
2.937(3), Ba1-O51A = 2.795(3), Ba1-O71 = 3.047(3); N1-Ba1-N37 = 112.92(8).
Symmetry transformation: ‒x+1/2, y, ‒z+1/2.
The orange alkali salt [{CarbDiPP}K.(thf)2] was isolated in 96%
yield by metalation of {CarbDiPP}H with KH. Solvent coordination
appears to be loose, and thf can be removed under dynamic
vacuum. XRD analysis of the yellow [{CarbDiPP}K.thf] (Fig. S22)
revealed a distorted tetrahedral geometry around potassium, with
3
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10.1002/anie.202001439
Angewandte Chemie International Edition
RESEARCH ARTICLE
101.68(14)-101.93(14)° and, by opposition, large Ba1-Si1a-Si(i)
angles (107.01(12)-126.44(12)°) in 4, indicate substantial metal-
to-ligand charge transfer from Ba to Si1a.[43] As in 2, the Ba-
Ncarbazolate bond length in 4 (2.626(3) Å) is shorter than the Ba-
Nimine ones (2.776(4) and 2.795(3) Å). All corresponding Ba-N
bonds are shorter in the Ba-silanylide than in 2, reflecting lower
steric pressure and coordination number, and hence stronger
bonds, in the former.
the silicon atom Si(SiMe3)3 in 4. Hence, the Si(i)-Sn1a-Si(j) angles
all fall within 97.06(18)-99.17(18)°, i.e. relatively close to the value
of 90° expected for a three-coordinate tin(II) due to its lower ability
for hybridisation between s and p orbitals. Conversely, the Ba1-
Sn1a-Si(i) angles are large, between 107.63(14)-132.71(11)°.
Based on these structural data, it is therefore tempting to consider
an ionic model for 5, with barium-to-tin electron transfer resulting
in a barium cation and a negatively charged tin(II) anion.
Compression of the Si(i)-Sn1a-Si(j) angles and pyramidalisation
The Ba-iodide
3 also reacted with [KSn(SiMe3)3.(thf)n]
(prepared in situ[44]) to afford [{CarbDiPP}BaSn(SiMe3)3.thf] (5), the
in
5
are
greater
than
in
the
NHC-ligated
[{:C{N(iPr)C(Me)}2}Sn{Sn(SiMe3)3}2] (Si-Sn-Siav = 104.7(5)°) or in
[Mg{Sn(SiMe3)3}2.(thf)2] (103.9(5)°),[45-46] but similar to those in the
ionic salts [LiSn(SiMe3)3.(thf)3] (98.8(2)°)[48] and [{15-c-
5}NaSn(SiMe3)3] (99.1(3)°).[47] The Ncarbazolate-Ba-Sn angle in 5
(94.46(12)°) is narrower than the corresponding Ncarbazolate-Ba-Si
angle in 4 (98.12(9)°) due to the release of steric congestion
between SiMe3 and backbone tBu groups on switching from Si to
the larger Sn.
Figure 5. Displacement ellipsoids (30% probability; H atoms omitted) for
[{CarbDiPP}BaSi(SiMe3)3.thf] (4). Only the main components of the disordered thf,
i
tBu, Pr and Si(SiMe3)3 shown. Selected interatomic distances (Å) and angles
(°): Ba1-N1 = 2.776(4), Ba1-N2 = 2.626(3), Ba1-N3 = 2.795(3), Ba1-O1A =
2.729(4), Ba1-Si1A = 3.460(3), Ba1-Si1B = 3.346(4) (not shown), Si1A- Si2A =
2.362(3), Si1A-Si3A = 2.354(3), Si1A- Si4A = 2.358(3); Si2A-Si1A-Si3A =
101.93(14), Si2A-Si1A-Si4A = 101.68(14), Si3A-Si1A-Si4A = 101.79(14), Ba1-
Si1A-Si2A = 107.01(12), Ba1-Si1A- Si3A = 114.82(12), Ba1-Si1A-Si4A =
126.44(12), N2-Ba1-Si1A = 98.12(9), N1-Ba1-N3 = 122.86(12). Site occupancy
for Si(SiMe3)3 : a, 57% ; b, 43%.
Figure 6. Displacement ellipsoids (30% probability; H atoms omitted) for
[{CarbDiPP}BaSn(SiMe3)3.thf] (5). Only the main components of the disordered
thf, tBu, iPr and Sn(SiMe3)3 sites are shown. Representative interatomic
distances (Å) and angles (°): Ba1-N1 = 2.752(5), Ba1-N2 = 2.637(5), Ba1-N3 =
2.728(6), Ba1-O1A = 2.711(14), Ba1-Sn1A = 3.5813(18), Ba1-Sn1B = 3.461(3)
(not shown), Sn1A-Si1A = 2.572(4), Sn1A-Si2A = 2.575(5), Sn1A-Si3A =
2.580(5); Si1A-Sn1A-Si2A = 99.17(18), Si1A-Sn1A-Si3A = 97.06(18), Si2A-
Sn1A-Si3A = 97.57(17), Ba1-Sn1A-Si1A = 132.71(11), Ba1-Sn1A-Si2A =
107.63(14), Ba1-Sn1A-Si3A = 116.51(13), N2-Ba1-Sn1A = 94.46(12), N1-Ba1-
N3 = 126.43(19). Site occupancy for Sn(SiMe3)3 : a, 58% ; b, 42%.
first barium stannylide with an unsupported Ba-to-Sn bond, as an
orange solid upon removal of KI. Complex 5 is characterised by a
sharp resonance at δ119Sn = ‒758 ppm in its 119Sn{1H} NMR
spectrum. The 29Si{1H}-INEPT spectrum exhibits a sharp singlet
at δ29Si = ‒10.4 ppm, with clearly detectable 119Sn and 117Sn
1
satellites (1J29Si-119Sn = 83 Hz, J29Si-117Sn = 73 Hz). Both of these
resonances are substantially shifted towards lower fields
compared
to
the
crown-ether
adduct
[Li(12c4)]+[(Me3Si)3Sn]‒ (‒882 and ‒12.9 ppm) or
also to its heavier alkali congeners,[47] while the1J29Si-
1
Table 1. Representative structural features for complexes 1-5 (R = SiMe3).
and J29Si-117Sn coupling constants are much
119Sn
greater in this compound (204/195 Hz resp.) than in
5. The Ba-stannylide 5 is isomorphous with its
silanylide congener 4. The arrangement about Ba1
forms a Cs-symmetrical distorted square pyramid (5
= 0.28), with disorder over several positions (Fig. 6,
with 58% occupation for the main Sn(SiMe3)3 site).
The average Ba-Sn interatomic distances in 5 of
3.521(3)av Å is longer than the corresponding Ba-Si
one in 4 (3.403(4)av Å). This difference is lower than
expected on the basis of the covalent radii for Si and
Sn (1.20 and 1.39 Å), supposedly owing to the softer
and more polarisable nature of tin. The
pyramidalisation at Sn(SiMe3)3 in 5 is greater than for
Barium
complex
Nucl.[a]
C.N.[b]
THF/
Ba
Ni^Ncarb^N’i
Ni^Ba^N’i
Ba distance
to plane[e]
angle[c]
angle[d]
Ba-NR2
Ba-F
(1)
(2)
(3)
(4)
(5)
1
2
1
1
1
4
7
6
5
5
0
106.9(1)
103.9(1)
106.0(1)
103.7(1)
104.4(2)
133.1(1)
112.9(1)
127.7(1)
122.9(1)
126.4(2)
0.824
1.657
1.172
1.349
1.187
1.5
2
Ba-I
Ba-SiR3
Ba-SnR3
1
1
[a] Nuclearity. [b] Coordination number. [c] Nimine-Ncarbazolate-N’imine angle (°). [d] Nimine-Ba-N’imine
angle (°). [e] Distance (Å) to the best N(C)3N(C)3N backbone plane.
4
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RESEARCH ARTICLE
Examination of the coordination spheres in 1-5 reveals that
the carbazolate ligand {CarbDiPP}‒ gives access to a range of new
barium complexes with a diversity of structural patterns; the main
features are collated in Table 1. In particular, the data for
mononuclear complexes 1 and 3-5 show that the ligand readily
accommodates coordination numbers from 4 to 7, and is flexible
enough to widen the Nimine-Ncarbazolate-N’imine angle up to at least
106.9(1)°, that is, almost 10° more than in the proligand
{CarbDiPP}H. The increase of this angle is paired with that of the
Nimine-Ba-N’imine angle and, concomitantly, a decrease of the
distance from Ba to the best N(C)3N(C)3N plane.
the literature[51-54] correlate well with their experimental data (Fig.
S35) and reflect the prevailing ionic character of the Ba-to-Sn
bond in 5. A similar reasoning can be drawn for the Ba-Si bond in
4 (Fig. S36).
The bonding features in the Ba-tetrelide complexes 4 and 5
were probed by DFT calculations at the PBE0/TZP/Zora/D3 level
(see SI). Their DFT-optimised geometries reproduce nicely the
corresponding X-ray structures. The computed Ba-Si/Sn
distances (3.279 and 3.438 Å, respectively) are somewhat shorter
than their average X-ray counterparts, yet close to that found for
the sites of minor statistical occupation (Fig. 5 and 6). These
results suggest that the DFT Ba-Si/Sn distances in 4 and 5 are
likely to be more reliable than their averaged counterparts from
Figure 7. Plot of the HOMO of [{CarbDiPP}BaSn(SiMe3)3.thf] (5; isovalue = 0.025
a.u.). That of the Ba-silanide complex 4 looks very similar (see Fig. S37).
1
the disordered X-ray structures. Also, the computed H and 13C
NMR chemical shifts are in an excellent agreement with their
experimental counterparts (Fig. S31-S34). As expected, both 4
and 5 exhibit a substantial HOMO-LUMO gap (2.77 and 2.67 eV,
respectively), which is however smaller than in the non-tetrelide
complexes 1-3 (3.65, 3.75 and 3.74 eV, respectively). This is due
to the higher energy of the HOMOs of 4 and 5, which can be
identified as their (Ba-Si/Sn) bonding orbital (Fig. 7). HOMOs of
similar nature were found in related Ca-Sn species.23 This HOMO
is largely polarised on the tetrelide atom (84% and 73%, for Si/Sn
vs. 5% and 12% on Ba for 4 and 5, respectively). In any case, it
is sufficiently bonding to confer some covalent character to the
Ba-tetrel bond, as exemplified by the non-negligible Wiberg
indices of 0.155 and 0.192 in 4 and 5, respectively. These values
are larger than that of the Ba-N(SiMe3)2 bond in 1 (0.09), but
comparable to that of the Ba-I bond in 3 (0.150). It is also
supported by the Ba natural atomic orbital (NAO) charge (1.76
and 1.74 in 4 and 5, respectively), which is lower than in the more
ionic complexes 1 and 2 (1.81 and 1.83, respectively) but closer
to that in 3 (1.77). To further investigate the nature of the Ba-
tetrelide interaction, a Morokuma-Ziegler energy decomposition
analysis|49] was carried out, considering the interaction between
the [Si/Sn(SiMe3)3]- and [{CarbDiPP}Ba(thf)]+ fragments in 4 and 5.
Assuming this heterolytic cleavage, the contribution of the orbital
interaction to the total bonding energy amounts to 30% of that of
the electrostatic component in both 4 and 5 (Table S30). This
value, comparable to that for the Ba-I derivative in 3 (28%), is also
indicative of weak but non-negligible covalent bonding. This
(partly) covalent character of the Ba-to-tetrel bond can be
attributed to the softer (less electronegative) nature of Si, Sn and
I, as compared to the harder O- or N-ligands. The ionocovalent
character of the donor-acceptor tetrel-Ba bond is further
supported by a Quantum Theory of Atoms in Molecules
(QTAIM)[50] analysis of 4 and 5, where bond paths and bond
critical points (BCPs) are found for the Ba-tetrel contacts. These
BCPs are associated with weak electron densities (resp. 0.026
and 0.024 a.u.), small positive Laplacian values (resp. 0.030 and
0.027 a.u.), as well as small negative local energy density (resp.
‒0.004 and ‒0.003 a.u). Finally, the computed 119Sn NMR
chemical shifts for 5 (δ119Sn = ‒993 ppm) and a selection of more
covalent Sn-containing compounds (‒112/‒127 ppm) taken from
Conclusion
In conclusion, the carbazole-based proligand {CarbDiPP}H hence
constitutes a most convenient entry point in the synthetic
molecular chemistry of barium. Like the popular β-diketimine
{BDIDiPP}H that has proved so successful with calcium, it can be
readily synthesised on large scales (up to 25 g, not optimised) in
a few days. Yet, {CarbDiPP}H alone enables the preparation of a
variety barium species that are stable for weeks even at elevated
temperature; this cannot be achieved with the regular {BDIDiPP ‒
}
ligand. Along with the syntheses of starting materials like the
solvent-free Ba-amide 1 (also a very effective precatalyst for
styrene hydrophosphination that improves on state-of-the-art
systems) and the Ba-iodide 3, this has been illustrated upon
successful productions of the first molecular Ba-fluoride 2, and of
5, the first compound with an unsupported Ba-to-Sn bond. DFT
analysis of this Ba-stannylide 5 and its congeneric Ba-silanylide 4
indicates that the Ba-tetrelide bonds are chiefly ionic, although
they feature a non-negligible covalent component. The versatility
of {CarbDiPP}H can be exploited to generate other heteroleptic
species (e.g. alkyls, phosphides, hydrides) based on barium, but
also calcium and strontium, as will be detailed in a forthcoming
report. It thus opens up many opportunities for the further
implementation of heavy alkaline earths in molecular catalysis
and reactivity towards small molecules.
Acknowledgements
P.M.C, J.-F.C. and Y.S. thank the Agence Nationale de la
Recherche for the provision of a research grant (ANR-17-CE07-
0017-01). The GENCI (Grand Equipment National de Calcul
Intensif) is acknowledged for HPC resources (Project
A0050807367).
Keywords: barium • carbazolate • silanylide • stannylide •
fluoride
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Entry for the Table of Contents
A new carbazole-based ligand, readily available on large scales, enables the syntheses of yet unknown solution stable barium
complexes, e.g. Ba-fluoride and Ba-stannylide species, that display unusual bonding patterns.
7
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