Tetranuclear strontium and barium siloxide/amide clusters in metal-ligand cooperative catalysis

Article abstract of DOI:10.1002/ejic.201800231

One-pot reaction of 2,6-iPr₂-aniline (DIPP-NH₂) with (Me₂SiO)₃ and Sr[N(SiMe₃)₂]2 (SrN′′₂) gave a tetranuclear cluster consisting of four dianions [OSiMe₂N-DIPP]₂– and four Sr²⁺ ions solvated each by one THF ligand. The general applicability of this method was investigated by variation of amine and metal. Anilines with smaller substituents led to insoluble uncharacterized coordination polymers, whereas bulkier anilines gave soluble product mixtures that could not be purified. Primary alkylamines neither led to isolable products. Introduction of a tBu group in para-position of DIPP-NH₂, however, gave an isostructural cluster with increased solubility. Similar clusters could be obtained with barium. Both, the Sr and Ba clusters, were found to be active catalysts for a wide range of transformations: intramolecular alkene hydroamination, alkene hydrophoshination, pyridine hydroboration, pyridine hydrosilylation, and alkene hydrosilylation. The Ba catalysts were generally more active than the Sr catalysts. The tBu-substituent in para-position also had an accelerating effect, which is likely due to improved solubility.

Full text of DOI:10.1002/ejic.201800231

A Journal of
Accepted Article
Title: Tetranuclear Strontium and Barium Siloxide/Amide Clusters in
Metal-Ligand Cooperative Catalysis
Authors: Benjamin Freitag, Phillip Stegner, Katharina Thum, Christian
A Fischer, and Sjoerd Harder
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800231
Link to VoR: http://dx.doi.org/10.1002/ejic.201800231

European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Tetranuclear Strontium and Barium Siloxide/Amide Clusters
in Metal-Ligand Cooperative Catalysis
Benjamin Freitag, Phillip Stegner, Katharina Thum, Christian A. Fischer and Sjoerd Harder*
Inorganic and Organometallic Chemistry, University of Erlangen-Nürnberg. Egerlandstrasse 1, 91058 Erlangen, Germany.
Email: sjoerd.harder@fau.de, Homepage: http://www.harder-research.com
Abstract: One-pot reaction of 2,6-iPr
2
-aniline (DIPP-NH
2
) with (Me
2
SiO)
3
2+
and Sr[N(SiMe
3
)
2
]
2
(SrN’’
2
) gave a
2
tetranuclear cluster consisting of four dianions [OSiMe
2
N-DIPP] ˉ and four Sr ions solvated each by one THF ligand
(
1-Sr). The general applicability of this method was investigated by variation of amine and metal. Anilines with smaller
substituents led to insoluble uncharacterized coordination polymers whereas bulkier anilines gave soluble product
mixtures that could not be purified. Primary alkylamines neither led to isolable products. Introduction of a tBu group in
para-position of DIPP-NH , however, gave an isostructural cluster with increased solubility: 1-Sr(tBu). Similar clusters
2
could be obtained with barium: 1-Ba and 1-Ba(tBu). Both, the Sr and Ba clusters, were found to be active catalysts for a
wide range of transformations: intramolecular alkene hydroamination, alkene hydrophoshination, pyridine hydroboration,
pyridine hydrosilylation, and alkene hydrosilylation. The Ba catalysts were generally more active than the Sr catalysts.
The tBu-substituent in para-position also had an accelerating effect which is likely due to improved solubility.
Keywords: Alkaline Earth Metal – Strontium – Barium – Catalysis – Non-innocent ligand
Figure for Table of Contents
Text for Table of Contents: Serendipitously obtained tetranuclear Sr and Ba clusters can now be prepared
reproducibly. In various catalytic transformations, the siloxide/amide ligand combines the role of spectator and active
ligand.
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Introduction
Over the last decade, group 2 metal catalysis rapidly developed from obscurity to a mature field.[1] Due to the high
energy of the metal’s d-orbitals, substrate activation differs from that in transition metal catalysis.[2] The major principles,
however, are similar. In both fields the catalyst L-M-R generally consists of a spectator ligand L and a reactive group R.
We recently reported on the first examples of group 2 metal catalysts in which a dianionic bidentate ligand acts
cooperatively.[3] E.g. the bora-amidinate complex (NBN)Sr∙(THF)
is active in intramolecular alkene hydroamination (Scheme 1, top). We also found that reaction of NBN-H
dibenzylstrontium reagent Sr(DMAT) ∙(THF) (DMAT = ortho-Me N--Me Si-benzyl) in a Schlenk tube with well-greased
4
2
(NBN = HB[N(DIPP)] , DIPP = 2,6-diisopropylphenyl)
2
with the
2
2
2
3
joints led to the serendipitous formation of the tetranuclear strontium siloxide/amide cluster 1-Sr that could be isolated in
the form of colorless crystals in 24% yield (Scheme 1, bottom).[4] Nucleophilic depolymerization of silicone grease and
insertion of the Me
Me SiO) -unit in traces (2, 3% yield) supports this mechanism. There are several more unidentified compounds present.
2
SiO-unit plays a key role in the proposed mechanism.[5] Isolation of a second product with a
(
2
2
Cluster 1-Sr is a major product formed by self-organization which due to its high symmetry crystallized from the reaction
mixture.
4
Scheme 1. Top: Synthesis of (NBN)Sr∙(THF) and use as
catalyst for the intramolecular alkene hydroamination. Bottom:
Proposed mechanism for the serendipitous formation of the
tetranuclear strontium siloxide/amide cluster 1-Sr (only one
2
(
DIPP)NSiMe
2
O ˉ anion shown) and the minor side product 2.
Further research showed that the strontium complex 1-Sr can
also be obtained by replacing NBN-H for DIPP-NH , silicon
grease for the siloxane (Me SiO) and the dibenzylstrontium
reagent Sr(DMAT) ∙(THF) for the easier accessible complex
Sr[N(SiMe , abbreviated as SrN'’ . Although the yields are
2
2
2
3
2
2
3
)
2
]
2
2
low, the simplicity of the method tempts the question whether
this synthetic method can be generalized. This could give easy
access to a large variety of alkaline earth metal complexes
with siloxide/amide ligands. Another quest for our current
research is an evaluation of the potential of 1-Sr and related
complexes in catalysis. Similar to the NBN ligand in
(
4
NBN)Sr∙(THF) , the siloxide/amide ligand in 1-Sr could be
active as a cooperative ligand in catalysis by succesively
deprotonating the substrate and protonating the intermediate.
Herein we report investigations towards the generalization of
the synthetic method by variation of the amine or metal. In
addition, we present the use of these heavier group 2 metal
complexes in a wide range of catalytic conversions.
Results and Discussion
In order to broaden the scope of the straightforward synthesis
of tetranuclear strontium siloxide/amide clusters, a large
variety of different anilines and some primary and secondary
amines have been probed as building blocks for the one-pot
self-assembly reaction (see Scheme 2). In view of the
increasing interest in enantioselective group
2
metal
catalysts,[ also some chiral amines have been tested. A mixture of these anilines and amines with 0.6 equivalents of
6]
2 2
SrN'’ and excess siloxane in benzene was heated to 60 °C. This method gave for the aniline DIPP-NH (a) reproducible
formation of crystalline product 1-Sr that could be isolated by decantation. However, using the less sterically hindered
anilines b-d gave powdery precipitates that were also in THF insoluble and could not be recrystallized. Characterization
by X-ray diffraction or solution NMR methods was therefore not possible. Using anilines with smaller substituents
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
apparently gave rise to formation of insoluble polymeric salt structures. In contrast, using anilines with larger substituents
e-f) did not lead to crystallization of the product (also not at lower temperatures and upon concentration of the reaction
(
mixture or by changing the solvent). NMR investigations on raw product mixtures during synthesis indicate multiple
species to be present. The formation and crystallization of 1-Sr should therefore be regarded as a self-organization
process that is easily disturbed by small changes in the sterics of the aniline precursor. Use of a primary alkyl amine like
benzylamine (g) or the chiral secondary benzyl amines (S)-PhCH(Me)NH
2 2
(h) and (S)-CyCH(Me)NH (i) did not lead to
isolable products. Interestingly, using DIPP-NH aniline with an additional tBu-substituent in para-position (j) led to
2
isolation of the corresponding tetranuclear cluster, which we abbreviate in here as 1-Sr(tBu), in crystalline yield of 27%.
The product 1-Sr(tBu) is much better soluble in benzene than 1-Sr. In order to maximize the yield of 1-Sr(tBu), the
solvent benzene had to be replaced by n-hexane. Failure to isolate products from reactions with the amines b-i and the
successful formation of 1-Sr(tBu) demonstrates that this convenient one-pot procedure cannot be generalized. However,
2
changes in the periphery of aniline DIPP-NH are allowed and do not disturb the self-organizing process.
Scheme 2. Variation of anilines and amines for cluster syntheses.
The crystal structure of 1-Sr(tBu) is depicted in Figure 1 and selected bond distances and angles are listed in Table 1. In
4 1
contrast to the crystallographic S -symmetry observed for 1-Sr (space group P-42 c), complex 1-Sr(tBu) crystallizes in
the triclinic space group P-1 with one tetranuclear cluster in the asymmetric unit. Despite the lack of crystallographic
4
symmetry, the cluster displays approximate S -symmetry and expectedly distances and angles closely resemble those in
the higher symmetric 1-Sr (Table 1).
Fig. 1. Crystal structure of 1-Sr(tBu). (a) Hydrogen atoms and aryl
substituents (except Cipso) have been omitted for clarity. (b)
Coordination of the siloxide/amide and THF ligands to the Sr
cube.
4 4
O
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Table 1. Selected bond distances (Å) and angles (°) for crystal structures of clusters: 1-Sr,[4] 1-Sr(tBu), 1-Ba and
1-Ba(tBu); range and average values between [ ].
1-Sr
1-Sr(tBu)
1-Ba
1-Ba(tBu)
M-N
2.458(2)
2.674(1)
2.464(2)-2.475(2)
2.602(2)
2.600(2)-2.616(2)
[2.607(2)]
[
2.461(2)]
M-O within
2.620(2)-2.680(2)
[2.647(2)]
2.812(2)
2.805(2)-2.867(2)
[2.834(2)]
M-O-Si-N ring
M-O within
2.428(1)-2.444(1)
[2.436(1)]
2.409(2)-2.458(2)
[2.435(2)]
2.570(2)-2.594(2)
[2.582(2)]
2.557(2)-2.637(2)
[2.585(2)]
4 4
M O cube
M-O(THF)
2.544(2)
1.641(1)
1.674(2)
100.96(7)
2.518(2)-2.549(2)
2.751(2)
1.640(2)
1.671(2)
102.1(1)
2.735(2)-2.784(2)
[2.755(2)]
[
2.531(2)]
1.640(2)-1.649(2)
1.646(2)]
1.663(3)-1.690(2)
1.675(3)]
98.9(1)-100.9(1)
99.7(1)]
92.73(9)-94.73(9)
93.50(9)]
61.30(7) to 61.90(7)
94.72(7)]
127.6(2)-145.9(2)
135.9(2)]
104.0(1) to 104.9(1)
104.5(1)]
Si-O
1.636(2)-1.640(2)
[1.638(2)]
[
Si-N
1.658(2)-1.678(2)
[1.669(2)]
[
Si-N-M
Si-O-M
O-M-N
Si-N-Cipso
O-Si-N
101.3(1)-102.5(1)
[101.9(1)]
[
93.45(7)-94.50(7)
[94.02(8)]
93.68(6)
94.89(8)
57.91(6)
[
61.09(5)
134.8(1)
104.14(8)
57.38(6) to 58.09(5)
[57.80(6)]
[
141.7(2)
105.0(1)
131.5(2)-152.2(2)
[141.2(2)]
[
105.0(1)-106.6(1)
[105.8(1)]
[
Although the tBu substituent in para-position of the aniline moiety has no influence on the structure of the tetrameric
cluster, complex 1-Sr(tBu) shows a markedly better solubility in THF than 1-Sr and can even be dissolved to some
1
extent in benzene. H NMR investigations in THF reveal a remarkably simple spectrum with a single set of signals for the
iPr groups (one doublet, one septet), one singlet for the Me
symmetric cluster with free rotation of the DIPP-group around the N-Cipso axis. As previously discussed for 1-Sr,[4] the
tetranuclear cluster may form two different isomers (D and S ). Given the occurrence of only one signal set in the NMR
spectra, it is likely that complex 1-Sr(tBu) resides also in solution only in its S -symmetric solid-state form but fast
exchange between the D and S isomers cannot be excluded.
2
Si group and one aromatic singlet. This indicates a highly
2
4
4
2
4
The scope of the cluster synthesis was further extended by variation of the metal. Heating DIPP-NH
2
with with 0.6
equivalents of BaN'’ and excess siloxane in a mixture of benzene and THF for 2 days to 60 °C led to precipitation of
2
colorless crystals that were isolated by decantation giving 1-Ba in 13% yield. The crystal structure of 1-Ba is shown in
Figure 2 and selected bond distances and angles are listed in Table 1. Complex 1-Ba crystallized similar as 1-Sr as a
tetranuclear cluster in the highly symmetric tetragonal space group I4
1
2
/a with one [(OSiMe N(DIPP)]Ba∙THF fragment in
the asymmetric unit. Like 1-Sr, which crystallized in the space group P-42
1
c, cluster 1-Ba is crystallographically S
4
2
+
symmetric. Considering a difference of 0.18 Å in the ionic radii for Sr
2
+
[7]
(
1.18 Å) and Ba (1.36 Å), the Ba-O and Ba-N bond distances are
shorter and the Ba-O(THF) distances longer than expected. Changing
the metal from Sr to Ba has no major influence on the geometry of the
ligand. The only exception is the somewhat larger Si-N-Cipso angle in 1-
Ba. Complex 1-Ba is poorly soluble in benzene but dissolves top some
extent in THF-d
8
. 1H and 13C NMR spectra in this solvent are essentially
similar to those of 1-Sr. Due to the high symmetry of the complex,
remarkably few signals are observed (vide supra).
Figure 2. Crystal structure of 1-Ba viewed along the four-fold screw
axis. Hydrogen atoms and iPr groups have been omitted for clarity.
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Similar to 1-Sr(tBu), complex 1-Ba(tBu) could be isolated in 17 % yield. Although its crystal structure (Supplementary
Information) is similar to that of 1-Ba (see Table 1 for a comparison), the solubility of this tBu-substituted complex is
substantially higher. The H and 13C NMR spectra of 1-Ba(tBu) essentially match those of 1-Sr(tBu).
1
The ease of synthesis of the tetrameric clusters 1-Sr, 1-Sr(tBu), 1-Ba and 1-Ba(tBu) compensates for their generally low
yields (13-35%). The metal siloxide/amide clusters reported herein may, similar to (NBN)Sr∙(THF) , show catalytic
4
activity by a metal-ligand cooperative mechanism. Therefore, these clusters were tested in a variety of catalytic
transformations. The conversion times have been determined for essentially full conversion (> 99%) and products were
analyzed by NMR and GC-MS (Supporting Information).
The intramolecular alkene hydroamination mediated by group 2 metal complexes has been first reported by Hill et al.[8]
and is arguably one of the most investigated transformations.[9] It was found that the clusters 1-Sr, 1-Sr(tBu), 1-Ba and 1-
Ba(tBu) all catalyze the ring closure of the aminoalkene H
smoothly (Table 2, Entries 1-4). Although all catalysts tested reach essentially quantitative conversion, the reaction times
are rather long (12-32 h). Their catalytic activity, however, is similar to that of SrN’’ ∙(THF) (benzene, 2 mol% cat, 25 °C,
4 h, 98%).[9k] The Ba complexes are somewhat more active than the Sr complexes. The tBu-substituent in the para-
2 2 2 2 2
NCH CPh CH CH=CH , a substrate that generally reacts
2
2
2
position of the aryl group has an accelerating effect on catalysis (Entries 3-4). This is probably related to their increased
solubility (complexes 1-Sr and 1-Ba are hardly soluble in benzene). The poor catalytic activity of the Sr and Ba clusters
may be related to the weak Brønsted basicity of the siloxide/amide dianion: the negative charge on N is stabilized by
delocalization in the aryl substituent. In this respect, it is noteworthy that under the same reaction conditions the known
Sr complex[10] [(Me
Entry 5).
Si)(DIPP)N]
3
2
2 2 2 2 2 2
Sr∙(THF) (3) was found to be fully inactive for cyclization of H NCH CPh CH CH=CH
(
Group 2 metal mediated alkene hydrophosphination was first reported by Hill et al.[11] and was later extended to alkyne
substrates by Westerhausen et al..[12] It should be considered an attractive atom-efficient transformation for the
syntheses of phosphines. Carpentier et al. found increasing conversion rates for the heavier alkaline-earth metals: Ca <
Sr < Ba.[9i] Investigations are generally limited to conversion of an activated C=C bond like that in styrene or derivatives
thereof. Hydrophosphination of isolated (unactivated) alkenes or twofold substituted alkenes is significantly more
challenging. For this reason, hydrophosphination of myrcene gives only functionalization of the terminal (mono-
substituted and conjugated) C=C bond, leaving internal or unactivated double bonds unaffected.[9i] The siloxide/amide
clusters are also active in alkene hydrophosphination (Entries 6-12). Notably, the catalyst 1-Ba is able to convert
disubstituted alkenes like -Me-styrene (Entry 12) which is known to be slow.[9i] Disubstituted alkenes like 1,1-
diphenylethylene hitherto were not tackled by group 2 metal catalysis but can be fully converted by 1-Sr and 1-Ba, albeit
slow (Entries 10-11). Conversion rates increase along the row -Me-styrene < 1,1-diphenylethylene < styrene. The Ba
catalyst 1-Ba is generally faster than the Sr catalysts 1-Sr and 1-Sr(tBu). The activity of the latter Sr complexes is
substantially lower than that of heteroleptic catalysts of the type LSrR in which L is a spectator ligand and R is N’’ or
CH(SiMe
the homoleptic catalyst [(Me
quantitative. The poor activity of the latter homoleptic catalyst is due to the formation of insoluble Sr(PPh
explanation has been reported for the very poor catalytic activity of CaN’’ ∙(THF) for styrene/HPPh conversion (10
mol%, 75 °C, 36 h, 99%; precipation of Ca(PPh
was observed).[11] The spectator ligand therefore has a solubilizing
function. Along the lines of intramolecular alkene hydroamination, we propose for the catalysts 1-Sr and 1-Ba a
mechanism in which the siloxide/amide dianion acts as a cooperative ligand. Deprotonation of HPPh by the N-Ar arm is
followed by phosphide/alkene addition after which the intermediate is protonated again. This mechanism is corroborated
by the fact that aniline (pK (pK
30.7)[13] is less acidic than HPPh 21.7).[14]
3
)
2
. These generally show fast styrene/HPPh
Si)(DIPP)N] Sr∙(THF) (3, Entry 9) the conversions with the siloxide/amide catalysts are all
. A similar
addition (2 mol% cat, 60 °C, 3-15 min, 55-92%).[9i] In contrast to
2
3
2
2
2 2
)
2
2
2
2 2
)
2
a
2
a
Ca-mediated alkene hydroboration was first reported by Harder et al.[15] and extended to Mg-catalyzed hydroboration of
aldehydes, ketones and imines by the Hill group.[16] Mg-mediated hydroboration of pyridines is a special case of C=N
bond hydroboration in which the regioselectivity (1,2- or 1,4-reduction) is challenging.[17,18] Attempts to use Ca catalysts
in the latter reaction were frustrated by catalytic decomposition of the borane (pinacolborane, HBpin).[18] Herein we
report the catalytic hydroboration of pyridine and related substrates with tetranuclear siloxide/amide Sr and Ba clusters
(
1
Entries 13-21). Catalytic pyridine hydroboration is quantitative and somewhat faster for 1-Ba compared to 1-Sr (Entries
3-14). Rates are of the same order compared to a recently reported Mg catalyst (10 mol% cat, 70 °C, 17 h, 92%).[17]
The preference for 1,4-addition over 1,2-addition, which is for 1-Sr 95/5, is substantially better than that for the Mg
catalyst (1,4/1,2 = 63/37). As was previously reported for the Mg catalyst,[11] conversion of the activated substrates
quinoline and isoquinoline is much faster and fully selective. In isoquinoline hydroboration, the homoleptic catalyst
[
(Me
3
Si)(DIPP)N]
2
2
Sr∙(THF) (3) can compete with 1-Sr and 1-Ba (Entries 18-21). The catalyst 1-Ba enabled conversion
[
19]
of chlorinated pyridine (Entry 15), a substrate that so far has only been converted using a Fe catalyst.
[
20]
Ca-mediated alkene hydrosilylation was pioneered by Harder and coworkers. The Okuda group introduced calcium
[
21]
silanide catalysts.
Although mechanistically different, group 2 metal hydrosilylation catalysis has been extended to
ketones[ and pyridines.[23] Herein we report catalytic hydrosilylation of pyridines and alkenes with the siloxide/amide
22]
clusters of Sr and Ba (Entries 22-30). Hydrosilylation of styrene with PhSiH
3
is somewhat faster with the Ba catalyst 1-Ba
(
Entries 27-28) but still slower compared to that with a dibenzylstrontium catalyst (2.5 mol% cat, 20 °C, < 0.1 h,
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
>
99%).[20] Hydrosilylation of the more hindered 1,1-diphenylethylene is more challenging and slower (Entries 29-30).
Similar to conversion with a dibenzylstrontium catalyst,[20] the branched product Ph
CH(SiH Ph)CH is formed
selectively. It is at this stage not clear whether catalysis proceeds through a metal hydride intermediate (as proposed
earlier).[20] Attempts to isolate intermediates by reaction of 1-Sr with PhSiH
failed. Two possible routes could be
envisioned: (i) The dianionic siloxide/amide ligand ˉOSiMe (DIPP)Nˉ may react with PhSiH to give
ˉOSiMe (DIPP)NSiH Ph and Hˉ, or (ii) Forced proximity of the latter two anions gives a silicate containing a
pentacoordinate Si: ˉOSiMe (DIPP)NSiH Phˉ. Both, hydride or silicate, may reduce the C=C double bond by hydride
transfer. Hydrosilylation of quinoline and isoquinoline is selective and the latter substrate could also be converted by the
homoleptic catalyst [(Me Si)(DIPP)N] Sr∙(THF) (3); Entries 22-26.
2
2
3
3
2
3
2
2
2
3
3
2
2
Table 2. Various catalytic conversions with Sr and Ba catalysts (0.6 mol% based on tetramer or 2.4 mol% based on metal) in C
6
D
6
at
1
7
0 °C. Reactions were followed by H NMR and times have been given for >99% conversion (unless noted otherwise).
Entry
1
Cat.
Subtrates
Products
Time (h)
32a
1-Sr
a
2
3
4
5
6
1-Sr(tBu)
1-Ba
1-Ba(tBu)
3
,,
,,
,,
,,
,,
,,
,,
,,
16
28
a
a
12
a
-
1-Sr
7.25
7
8
9
1-Sr(tBu)
1-Ba
3
,,
,,
,,
,,
,,
,,
7
1.25
24
b
1
0
1-Sr
60
1
1
1
2
1-Ba
1-Ba
,,
,,
36
9 days
1
1
1
1
3
4
5
6
1-Sr
1-Ba
1-Ba
1-Sr
48c
23c
,,
5 daysc
4
1
1
7
8
1-Ba
1-Sr
,,
,,
4
1
1
2
2
2
9
0
1
2
1-Sr(tBu)
1-Ba
3
,,
,,
,,
,,
,,
,,
1
0.75
0.5
5
1-Sr
2
2
3
4
1-Ba
1-Sr
,,
,,
0.25
2
2
2
2
5
6
7
1-Ba
3
1-Sr
,,
,,
,,
,,
0.25
2
7.25
2
2
8
9
1-Ba
1-Sr
,,
,,
,,
,,
1.25
31
3
0
1-Ba
20
a
Reaction temperature 60 °C. b 20 % conversion due to precipitation of Sr(PPh . c Solvent is THF-d
)
2 2 8
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Conclusions
2
The serendipitous synthesis of 1-Sr is reproducible but slight changes of the aniline used in its synthesis (2,6-iPr -
aniline) gave complications. Using less sterically encumbered anilines gave products that were completely insoluble and
difficult to purify or characterize. Anilines with bulkier substituents led to highly soluble product mixtures. Addition of a
tBu group in the para-position of the aniline, however, led to the crystalline product 1-Sr(tBu) that could be fully
characterized. This shows that substitution in the periphery of the ligand does not disturb self assembly. The tBu
substituent in para-position increases the solubility of the cluster significantly. Exchanging the Sr metal for the larger Ba
led to isolation of the complexes 1-Ba and 1-Ba(tBu) which are isostructural to the analogue Sr complexes. Although
yields are low, the synthetic procedure using cheap starting materials is simple and gives generally crystalline products.
The tetranuclear siloxide/amide clusters were shown to be catalytically active in a wide range of substrate
functionalizations among which the intramolecular alkene hydroamination, alkene hydrophosphination, pyridine
hydroboration, pyridine hydrosilylation, and alkene hydrosilylation. Although in some cases the reaction times were long,
in the majority of experiments clean and full conversion was reached. The Ba catalysts react generally faster than the Sr
catalysts. Catalysts with tBu substituents in para-position of the aniline unit performed better than those without. The
latter is likely related to their better solubility which is often an issue. All catalytic conversions are highly selective, except
for the hydroboration of pyridine. The 1,4/1,2 product ratios (1-Sr: 95/5; 1-Ba: 82/18) are, however, better than that for a
Mg catalyst (63/37). It is noteworthy that the similar but mononuclear catalyst [(Me
3
2 2
Si)(DIPP)N] Sr∙(THF) (3) performed
much poorer in most conversions. This suggests that this class of cluster catalysts in which the ligand cooperates in the
catalytic cycle could be advantageous for a wide range of transformations. We continue to investigate group 2 metal
catalysis with ligands that combine the functions of spectator and reactive group.
Experimental Section
General Considerations
All experiments were performed under a nitrogen atmosphere by using standard Schlenk line and glove box
techniques. The solvents were dried on alumina columns and were degassed by bubbling nitrogen through the solvent
reservoir. Following compounds were prepared according to their reported literature procedures:
Sr[N(SiMe
commercially: hexamethylcyclotrisiloxane (Sigma Aldrich), aluminum chloride (ABCR), tert-butyl chloride (ABCR) and
,6-diisopropylaniline (ABCR). All chemicals were use as received, except tert-butyl chloride and 2,6-diisopropylaniline,
which were distilled prior to use. NMR spectra were recorded on BRUKER Avance III HD (400 MHz) and Avance III HD
3
)
2
]
2
∙(THF)
2
,[24] Ba[N(SiMe
3
)
2
]
2
,[25] and [(Me
3
Si)(DIPP)N]
2
Sr∙(THF)
2
.[10] Following compounds were obtained
2
(
(
600 MHz) spectrometer. Crystals were measured on an Agilent Supernova with an Atlas S2 CCD detector and Nova
Cu) and Mova (Mo) X-ray microsources. GC-MS analysis has been performed with a Thermo Scientic™ Trace™ 1310
gas chromatography system (carrier gas Helium) with detection by a Thermo Scientic™ ISQ™ LT Single Quadrupole
mass spectrometer (EI-MS, 70 eV). Elemental analyses on the complexes were performed with a Hekatech Eurovector
EA3000 analyzer. CH-Elemental analyses of complexes with Ca (or heavier alkaline earth metals like Sr and Ba) are
generally strongly affected by the very high air-sensitivity of the complexes, partial loss of coordinated or crystal lattice
solvent molecules, and particularly by formation of very stable metal carbide or metal carbonate products that also with
additional oxidizers like V
documented for the heavier alkaline-earth metal complexes of Ca, Sr and Ba.
2 5 2
O do not fully react to CO . This results in lower C-content values than expected and is well-
[
26]
CH-Elemental analyses are also
complicated by a difficult work-up procedure. During synthesis of the complexes from Sr and Ba amide bases, aniline
and excess siloxane, large amounts of oligo- and polysiloxanes are formed. Separation of the crystalline products from
these siloxanes is often tedious and in many cases contamination with silicones cannot be avoided. All cluster syntheses
can be up-scaled to circa 300 mg without a change in yield.
Synthetic procedures
Synthesis of 4-(tert-butyl)-2,6-diisopropylaniline. AlCl
25 mL) and cooled to 0°C. Subsequently, 2,6-diisopropylaniline (4.00 g, 22.6 mmol) was added and the reaction mixture
was cooled to -10°C. Freshly distilled tBuCl (dried over CaH ) (7.88 g, 85.2 mmol) was added dropwise over a period of
5 min, during which the solution turned dark brown. After having stirred the solution for one hour at -10 °C,
3
(4.00 g, 30.0 mmol) was dissolved in 1,2-dichloroethane
(
2
4
dichloromethane (50 mL) was added and the reaction mixture was poured on a mixture of ice (40 g) and aqueous
ammonia solution (28%, 12 mL). The white precipitate was filtered off. The organic phase was separated, washed with
water (2x40 mL) and brine (2x40 mL) and dried over MgSO . The solvent was removed under reduced pressure giving a
4
viscous yellow oil which was recrystallized from pentane (5 mL) at -20°C. The crystals were further purified by
sublimation (45°C, 3.0∙10- mbar) to give the pure product. Yield: 3.20 g, 13.7 mmol, 63%. H NMR (600 MHz,
5
1
3
), 2.72 (sept, 3J(H,H) = 6.8 Hz, 2H,
),
[
D
6
]benzene, 25°C): δ = 1.21 (d, J(H,H) = 6.9 Hz, 12H, C(CH
3
)
2
), 1.38 (s, 9H, C(CH
3 3
)
1
CH(CH
3
)
2
), 3.14 (s, 2H, NH
), 31.9 (C(CH
2
), 7.21 (s, 2H, CHarom) ppm; 13C{ H} NMR (151 MHz, [D
6
]benzene, 25°C): δ = 22.6 (C(CH )
3 2
2
8.4 (CH(CH
3
)
2
3
)
3
), 34.4 (C(CH
3
)
3
), 119.6 (Carom), 131.8 (CiPr), 138.2 (CN), 140.6 (Carom) ppm. GC/MS (EI-
+
+
+
+
MS 70 eV): m/z (%): 233.22 (100) [M] , 218.17 (448) [C15
H
24N] , 202.15 (16) [C14
H
20N] , 176.16 (36) [C12
H
18N] , 134.12
9
42) [C H
12N]+.
(
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Synthesis of 1-Sr. The complex was prepared according to the earlier procedure[4] but with slight variations giving an
improved yield. Schlenk tube was charged with Sr[N(SiMe ∙(THF) (52.0 mg, 94.1 µmol),
hexamethylcyclotrisiloxane (164 mg, 737 µmol) and 2,6-diisopropylaniline (25.0 mg, 141 µmol). Addition of benzene
1.12 mL) yielded a suspension that was subsequently heated to 60 °C for 3 days. The product was obtained in the form
A
3
)
2
]
2
2
(
of colorless crystals, which were isolated by decantation, washed with benzene (2 x 1.5 mL) and dried under reduced
pressure. Yield: 13.5 mg, 8.23 µmol, 35%. H and 13C NMR match the earlier reported product.[4]
1
Synthesis of 1-Sr(tBu).
A
Schlenk tube was charged with Sr[N(SiMe
3
)
2
]
2
∙(THF)
2
(104 mg, 188 µmol),
hexamethylcyclotrisiloxane (328 mg, 1.47 mmol) and 4-(tert-butyl)-2,6-diisopropylaniline (99.0 mg, 424 µmol). Addition of
hexane (1.12 mL) yielded a suspension that was subsequently heated to 60 °C for 3 days. The product was obtained in
the form of colorless crystals, which were isolated by decantation, washed with hexane (2 x 1.5 mL) and dried under
1
reduced pressure. Yield: 23.6 mg, 12.7 µmol, 27%. H NMR (400 MHz, [D
8
]THF, 25°C): δ = 0.11 (s, 6H, SiMe
2
), 1.22 (d,
3
3
J
3
H,H = 6.8 Hz, 12H, CH(CH
3
)
2
, 1.26 (s, 9H, C(CH
3
)
3
), 1.76–1.79 (m, 4H, THF), 2.98 (sept, JH,H = 6.8 Hz, 2H, CH(CH
3
)
2
),
), 23.2
), 68.4 (THF), 119.9 (Carom), 132.2 (Carom), 139.8
SiSr (1861.04): Calcd. C 56.79 H 8.45 N 3.01; found C 55.97 H 8.47 N 2.96.
.60–3.63 (m, 4H, THF), 6.97 (s, 2H, CHarom) ppm; 13C{ H} NMR (151 MHz, [D
1
]THF, 25°C): δ = 1.6 (SiMe
8
2
(
(
3 2 3 2 3 3 3 3
CH(CH ) ), 26.6 (THF), 28.9 (CH(CH ) ), 32.2 (C(CH ) ), 34.8 (C(CH )
C
arom), 140.1 (Carom) ppm. C88
H
156NO
2
Although the C value is outside the range viewed as establishing analytical purity, it is provided to illustrate the best
values obtained to date.
Synthesis of 1-Ba.
A
Schlenk tube was charged with Ba[N(SiMe
3
)
2
]
2
(78.1 mg, 176 µmol) and
hexamethylcyclotrisiloxane (39.2 mg, 176 µmol) and 2,6-diisopropylaniline (54.7 mg, 307 µmol). The educts were
dissolved in 1.12 mL of benzene and 30 µL of THF, yielding a clear solution. Subsequently, 2,6-diisopropylaniline (54.7
mg, 307 µmol) was added and the reaction was heated to 60 °C for 4 days. The product was obtained in the form of
colorless crystals, which were isolated by decantation, washed with benzene (2 x 1.5 mL) and dried under reduced
1
3
pressure. Yield: 10.5 mg, 5.72 µmol, 13%. H NMR (400 MHz, [D
8
]THF, 25°C): δ = 0.05 (s, 6H, SiMe
2
), 1.15 (d, JH,H
), 3.57 (m, 4H, THF), 6.52 (t,
]THF, 25°C): δ = 1.2
=
6.7 Hz, 12H, CH(CH
3
)
2
), 1.72 (m, 4H, THF), 2.91 (sept, 3JH,H = 6.1 Hz, 2H, CH(CH
H,H = 7.7 Hz, 1H, CHarom), 6.83 (d, 3JH,H = 7.3 Hz, 2H, CHarom) ppm; 13C{ H} NMR (100 MHz, [D
3
)
2
3
1
J
8
(
2 3 2 3 2
SiMe ), 22.8 (CH(CH ) ), 28.2 (CH(CH ) ), 117.9 (Carom), 122.8 (Carom), 132.2 (Carom), 142.0 (Carom) ppm. The crystalline
product 1-Ba cannot be easily separated from the polysiloxane side-product. Although the product could be clearly
identified by NMR analysis ( H and 13C NMR data for 1-Ba and 1-Sr are similar), we have not been able to obtain an
1
accurate CH-elemental analysis for 1-Ba. This is probably due to contamination with oligosiloxanes. A side-product could
1
be isolated from the washing liquid. The H NMR data are conclusive for formation of (DIPPNH)BaN(SiMe
Supporting Information Figure S8).
3 2 n
) ∙(THF) (see
Synthesis of 1-Ba(tBu).
A
Schlenk tube was charged with Ba[N(SiMe
3
)
2
]
2
(78.1 mg, 176 µmol),
hexamethylcyclotrisiloxane (39.2 mg, 176 µmol) and 4-(tert-butyl)-2,6-diisopropylaniline (73.0 mg, 308 µmol). Addition of
hexane (1.12 mL) and THF (50 µL) yielded a suspension that was subsequently heated to 60 °C for 2 days. The product
was obtained as white microcrystalline powder, which was isolated by centrifugation, washed with benzene (2 x 1.5 mL)
1
and dried under reduced pressure. Yield: 15.4 mg, 7.48 µmol, 17%. H NMR (600 MHz, [D
8
]THF, 25°C): δ = 0.11 (s, 6H,
SiMe
.85 (sept, 3JH,H = 6.7 Hz, 2H, CH(CH
SiMe ), 23.5 (CH(CH ), 26.3 (THF), 28.0 (CH(CH
36.8 (Carom), 139.9 (Carom), 147.8 (Carom) ppm. C88
2
), 1.17 (d, 3JH,H = 6.9 Hz, 12H, C(CH
3
)
2
), 1.26 (s, 9H, CH(CH)
3
), 1.76-1.79 (m, 4H, THF), 3.61-3.63 (m, 4H, THF),
1
3
3
)
2
), 6.94 (s, 2H, CHarom) ppm; 13C { H} NMR (151 MHz, [D
8
]THF, 25°C): δ = 1.6
), 68.8 (THF), 120.2 (Carom),
SiBa (2059.84): Calcd. C 51.31 H 7.63 N 2.72; found C 49.24
(
2
3
)
2
3 2 3 2 3 2
) ), 32.5 (CH(CH ) ), 34.8 (CH(CH )
1
H156NO
2
H 7.35 N 2.66. Although the C value is outside the range viewed as establishing analytical purity, it is provided to
illustrate the best values obtained to date.
Catalysis
All products were analyzed by H and 13C NMR as well as by GC-MS and compared to earlier reported analyses (see
1
Supporting Information).
Intramolecular alkene hydroamination. The catalyst (1.2 µmol, 0.6 mol% based on tetramer or 2.4 mol% based on
monomer) was added to a solution of the 2,2-diphenylpent-4-en-1-amine (47.5 mg, 0.200 mmol) in 500 µL benzene-d
6
.
The reaction mixture was heated to 60°C.
Alkene hydrophosphination. The catalyst (1.2 µmol, 0.6 mol% based on tetramer or 2.4 mol% based on monomer)
was added to a solution of the corresponding alkene (0.200 mmol) and HPPh (41.0 mg, 38.3 µL, 0.220 mmol) in 500
µL of benzene-d and the reaction mixture was heated to 70°C.
2
6
Hydroboration of pyridines. The catalyst (2.4 µmol, 0.6 mol% based on tetramer or 2.4 mol% based on monomer) was
added to a solution of the corresponding pyridine or quinoline (0.400 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(
56.3 mg, 63.8 µL, 0.440 mmol) in 500 µL of benzene-d
6
(in the case of 3-chloropyridine THF-d
8
was used as solvent).
The reaction mixture was heated to 70°C.
8
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201800231
Hydrosilylation of pyridines. The catalyst (2.4 µmol, 0.6 mol% based on tetramer or 2.4 mol% based on monomer)
was added to a solution of the corresponding pyridine or quinoline (0.400 mmol) and PhSiH (47.6 mg, 54.2 µL, 0.440
is reduced to 0.220 mmol) in 500 µL of benzene-d .The reaction
3
mmol) (in the case of isoquinoline the amount of PhSiH
3
6
mixture was heated to 70°C.
Hydrosilylation of alkenes. The catalyst (2.4 µmol, 0.6 mol% based on tetramer or 2.4 mol% based on monomer) was
added to a solution of the corresponding alkene (0.400 mmol) and PhSiH (47.6 mg, 54.2 µL, 0.440 mmol) in 500 µL of
benzene-d . The reaction mixture was heated to 70°C.
3
6
Crystal Structure Determination
Crystal data and details for refinement can be found in the Supporting Information. Crystallographic data (excluding
structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication
no. CCDC 1819032 1-Sr(tBu), 1819033 1-Ba, 1819034 1-Ba(tBu). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (+44)1223-336-033; E-mail:
deposit@ccdc.cam.ac.uk).
Supporting Information. Crystal data, details for structure determination, selected 1H NMR spectra and
analyses of products from catalytic reactions.
Acknowledgements
We kindly acknowledge Christina Wronna for numerous elemental analyses and the DFG for financial support (HA
3218/8-1).
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