Heteroleptic strontium complexes stabilized by donor-functionalized alkoxide and β-diketonate ligands

Article abstract of DOI:10.1039/c5dt01356a

Heteroleptic complexes of strontium (1-7) were prepared by employing β-diketonates and donor-functionalized alkoxides as coordinating ligands. The results illustrate the effect of alkoxide substituent groups on the overall structures of the complexes. The presence of a terminal methoxy group in the alkoxide ligands leads to the formation of trimeric complexes 1-4, whereas the substituents on the amine nitrogen prove to have less influence in determining the structure. The attempts to increase steric bulkiness of the aminoalkoxide ligands by introducing ethyl groups on the amine nitrogen and to the alkoxy carbon did not lead to a structural change from the dimeric form in 5-7 but resulted in structurally interesting strontium complexes. In trimeric complexes 2-4, the three strontium atoms were held together by two μ₃-O bonds using alkoxide oxygen atoms and two μ₂-O bonds using a combination of alkoxide and β-diketonate ligand oxygens. The strontium metal centers in these complexes exhibit seven-coordination states in 2 and 4, whereas 3 exhibits one six-coordinated and two seven-coordinated strontium metals in its structure. All of the complexes were characterized using FT-NMR, FT-IR, elemental analyses, and thermogravimetric (TG) analyses.

Full text of DOI:10.1039/c5dt01356a

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Heteroleptic strontium complexes stabilized by
donor-functionalized alkoxide and β-diketonate
ligands†
Cite this: Dalton Trans., 2015, 44,
14042
Sheby Mary George,a Hyo-Suk Kim,a Hyun Ji Oh,a Myoung Soo Lah,b
Dong Ju Jeon,a Bo Keun Park,a Jeong Hwan Han,a Chang Gyoun Kim*a and
Taek-Mo Chung*a
Heteroleptic complexes of strontium (1–7) were prepared by employing β-diketonates and donor-
functionalized alkoxides as coordinating ligands. The results illustrate the effect of alkoxide substituent
groups on the overall structures of the complexes. The presence of a terminal methoxy group in the alk-
oxide ligands leads to the formation of trimeric complexes 1–4, whereas the substituents on the amine
nitrogen prove to have less influence in determining the structure. The attempts to increase steric bulki-
ness of the aminoalkoxide ligands by introducing ethyl groups on the amine nitrogen and to the alkoxy
carbon did not lead to a structural change from the dimeric form in 5–7 but resulted in structurally inter-
esting strontium complexes. In trimeric complexes 2–4, the three strontium atoms were held together by
two μ3-O bonds using alkoxide oxygen atoms and two μ2-O bonds using a combination of alkoxide and
β-diketonate ligand oxygens. The strontium metal centers in these complexes exhibit seven-coordination
states in 2 and 4, whereas 3 exhibits one six-coordinated and two seven-coordinated strontium metals in
its structure. All of the complexes were characterized using FT-NMR, FT-IR, elemental analyses, and
thermogravimetric (TG) analyses.
Received 10th April 2015,
Accepted 25th June 2015
DOI: 10.1039/c5dt01356a
www.rsc.org/dalton
challenge. In the past, the formation of unfavorable oligomeric
compounds has been viewed as a main challenge, but that has
Introduction
The applications in the semiconductor industry and in cataly- been solved by using multi-dentate, sterically bulky ligands
sis are the driving forces for the large interest in group 2 metal that form strong bonds with strontium.2 However, the achieve-
chemistry. Among these, strontium compounds have a key role ments in the development of volatile strontium precursors did
in electronics due to the importance of strontium containing not ensure their successful application in the thin film indus-
metal oxides.1 These applications largely depend on the fabri- try because of several factors such as instability, unwanted
cation of strontium containing metal oxide thin films prefer- side reactions, and carbon contamination in films. These un-
ably through atomic layer deposition (ALD) or chemical vapor favorable factors associated with the existing precursors forced
deposition (CVD). For the deposition of thin films using these researchers to continue the development of new and better
techniques, strontium precursors with good stability and vola- strontium complexes to satisfy the industry requirements.
tility are essential. However, the large ionic radii and relatively
The most common method to develop a new precursor is
small charge (+2) associated with strontium ions make the using a newly designed ligand, or modifying the existing
development of volatile precursors with suitable properties a ligands by introducing functionalities to satisfy the needs. The
development of an entirely new ligand system is not only a
time consuming process but also does not ensure success. At
this point, recent developments in heteroleptic precursor
chemistry and its application are gaining importance.3 The
heteroleptic metal precursors, where the central metal atom is
bonded to different types of ligands, have a distinct advantage
of having a different dissociation pattern for each ligand,
which might be useful in the development of thin films. Even
though the development of heteroleptic metal precursors faces
similar challenges to those of homoleptic metal precursors
aThin Film Materials Research Center, Korea Research Institute of Chemical
Technology, P.O. Box 107, 141 Gajeong-Ro, Yuseong-Gu, Daejeon 305-600, Republic
of Korea. E-mail: cgkim@krict.re.kr, tmchung@krict.re.kr; Fax: (+82)42-861-4151
bInterdisciplinary School of Green Energy, Ulsan National University of Science and
Technology, UNIST-gil 50, Ulsan 689-798, Republic of Korea
†Electronic supplementary information (ESI) available: ORTEP diagrams of com-
plexes 5 and 6, selected bond angles of complexes 2–4, and the VT-NMR plots of
complexes 1, 5, and 7. CCDC 1034384–1034389 for 2–7. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c5dt01356a
14042 | Dalton Trans., 2015, 44, 14042–14053
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and depends on the selection of suitable combinations of
ligands, it can lead to much better results than those of homo-
leptic complexes.3,4 The availability of a large known ligand
pool from which to select ligands and the research results of
their homoleptic precursors make the development of hetero-
leptic metal precursors much easier.
Table 1 List of ligands used in complexes 1–7
Complex
Ligand
Ligand structure
1
memampH
Our recent results of a heteroleptic strontium complex
[Sr(demamp)(tmhd)]2 (8)4 with 1-{[2-(dimethylamino)ethyl]-
(methyl)amino}-2-methylpropan-2-olate (demamp) and 2,2,6,6-
tetramethyl-3,5-heptanedionate (tmhd) as coordinating
ligands and superior physical properties to homoleptic com-
plexes encouraged us to conduct further research into this
class of strontium complexes. The simple preparation of
[Sr(β-diketonate)(aminoalkoxide)] complexes by controlled sub-
stitution on strontium bis(trimethylsilyl)amides (Sr(btsa)2),
stability in various coordinating solvents as well as at high
temperatures, and commercial availability or simple prepara-
tive methods of the ligands are attractions of this class of com-
plexes. The good volatility and stability of the above complex
overcame the common drawbacks of dimeric/oligomeric struc-
tures, and the dimeric structure proved to be the stable form
for this class of complexes. Here in this paper we further
studied the structures and properties of this class of complexes
using potential tridentate donor-functionalized alkoxides in
combination with β-diketonate ligands. The results of the
chemistry and structural investigations of the new complexes
are described in this paper.
2
emeampH
3
4
imeampH
mempH
5
6
7
dmaeeampH
dmaemambH
dmaemampH
whereas the dmaemampH/dmhdH combination was used to
produce a complex that is molecularly similar to 8.
The complexes were prepared through a procedure similar
to that reported previously.5 The Sr[N(SiMe3)2]2·2DME solution
in toluene was first treated with the corresponding alcohol in
an equivalent ratio followed by the addition of tmhdH or
dmhdH (as shown in Scheme 1), and the resulting reaction
mixture was stirred overnight at room temperature to afford
the respective complexes 1–7. The X-ray quality crystals of com-
plexes 2–7 were grown from saturated hexane solutions of the
corresponding complexes at −30 °C. Efforts to make single
crystals of compound 1 resulted in needle type crystals that
were not suited for structural analysis.
Results and discussion
In this work, we introduced a variety of potential tridentate
ligands to study the effect of steric groups on coordinating
heteroatoms on the complex structure and its properties. The
tridentate ligands used (Table 1) in this work were 1-((2-meth-
oxyethyl)(methyl)amino)-2-methylpropan-2-ol
(memampH)
for 1, 1-(ethyl(2-methoxyethyl)amino)-2-methylpropan-2-ol
(emeampH) for 2, 1-(isopropyl(2-methoxyethyl)amino)-2-
methylpropan-2-ol (imeampH) for 3, 1-(2-methoxyethoxy)-2-
methylpropan-2-ol (mempH) for 4, 1-((2-(dimethylamino)
ethyl)(ethyl)amino)-2-methylpropan-2-ol (dmaeeampH) for 5,
Crystal structure
1-((2-(dimethylamino)ethyl)(methyl)amino)-2-methylbutan-2-ol The results obtained from the single-crystal structural analyses
(dmaemambH) for 6 and 3-(((2-(dimethylamino)ethyl)(methyl)- showed that complexes 2–4 crystallized as trimers, whereas
amino)methyl)pentan-3-ol (dmaemampH) for 7. In complexes complexes 5–7 displayed dimeric structures (Table 2). Complex
ˉ
1–6, 2,2,6,6-tetramethyl-3,5-heptanedione (tmhdH) was used 2, which crystallized in a triclinic P1 (Fig. 1) space group, exhi-
as a β-diketonate ligand whereas 2,6-dimethyl-3,5-heptane- bits heptacoordinated strontium metal centers. The strontium
dione (dmhdH) was used in 7. The tridentate memampH, atoms are directly bonded to one emeamp and one tmhd
emeampH, and imeampH ligands were introduced to investi- ligand each, and the three [Sr(emeamp)(tmhd)] fractions are
gate the effect of a terminal methoxy group in place of an held together by a combination of μ2- and μ3-oxygen bridging
amine group as well as the effect of progressively increased between the metal atoms. The emeamp ligands attached to Sr1
bulkiness on the tertiary amine groups in these ligands with and Sr2 undergo μ3-oxygen bridging with all three strontium
respect to the demampH ligand. The mempH was introduced atoms through the alkoxide oxygen atoms, whereas the
as a simple ligand with far less steric hindrance on the coordi- emeamp ligand attached to Sr3 engages in μ2-oxygen bridging
nating sites. The dmaeeampH and dmaemambH ligands were with the neighboring Sr1. The μ2-O bridging of the tmhd
intended to test the stable dimeric structures of this class of oxygen attached to Sr3 with Sr1 completes the metal–O–metal
ˉ
complexes by increasing bulkiness and introducing chirality, network. Complex 3 which crystallized in a similar triclinic P1
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Scheme 1 Synthesis of complexes 2 and 7.
Table 2 Crystallographic data and data collection parameters for 2–7
Compound
(2)
(3)
(4·0.5 Hex)
(5)
(6)
(7)
Formula weight
Temperature (K)
1335.42
100(1)
1377.50
100(1)
1297.30
100(1)
916.37
100(1)
916.37
100(1)
888.32
100(1)
Wavelength (Å)
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.71073
Monoclinic
P2(1)/c
10.6303(2)
22.0103(4)
11.6362(2)
90°
112.8670(10)
90°
2508.62(8)
2
1.213
2.169
976
0.16 × 0.12 ×
0.10
0.71073
Monoclinic
P2(1)/n
9.8488(3)
25.0965(7)
10.1883(3)
90°
101.478(2)
90°
2467.88(13)
2
1.233
2.205
976
0.10 × 0.08 ×
0.06
0.71073
Triclinic
P1
Crystal system
Space group
a (Å)
b (Å)
c (Å)
ˉ
ˉ
ˉ
ˉ
14.9083(2)
15.5225(2)
17.1464(2)
78.9340(10)
85.2550(10)
63.5860(10)
3487.59(8)
6
1.272
2.340
1416
0.20 × 0.16 ×
0.08
14.1918(2)
14.5190(2)
21.0895(3)
77.3070(10)
71.7630(10)
63.7730(10)
3685.39(9)
2
1.241
2.217
1464
0.10 × 0.08 ×
0.06
10.6143(5)
16.2868(7)
21.4751(10)
94.454(3)
103.164(2)
106.686(2)
3421.7(3)
2
1.259
2.385
1370
0.16 × 0.12 ×
0.06
10.7509(4)
15.1148(6)
16.3750(7)
93.525(2)
105.381(2)
108.420(2)
2403.25(17)
2
1.228
2.262
944
0.25 × 0.25 ×
0.22
α (°)
β (°)
γ (°)
V (Å3)
Z
ρcalcd (Mg m−3
)
μ (mm−1
F(000)
)
Crystal size (mm3)
Theta range for data
collection (°)
Index ranges
1.21 to 28.23
1.57 to 28.31
0.99 to 28.33
1.85 to 28.31
1.62 to 28.36
1.44 to 28.39
−19 ≤ h ≤ 19,
−20 ≤ k ≤ 20,
0 ≤ l ≤ 22
17 151
−17 ≤ h ≤ 18,
−18 ≤ k ≤ 19,
0 ≤ l ≤ 28
18 256
−14 ≤ h ≤ 13,
−21 ≤ k ≤ 21,
0 ≤ l ≤ 28
16 978
−14 ≤ h ≤ 13,
0 ≤ k ≤ 29,
0 ≤ l ≤ 15
6241
−13 ≤ h ≤ 12,
0 ≤ k ≤ 33,
0 ≤ l ≤ 13
6133
−14 ≤ h ≤ 14,
−20 ≤ k ≤ 20,
−21 ≤ l ≤ 21
62 066
Total reflns
Independent reflns (Rint
)
17 151 (0.00)
703
1.032
18 256 (0.00)
731
1.020
16 978(0.00)
685
1.031
6241 (0.00)
253
1.036
6133(0.00)
244
1.022
11 941 (0.0545)
469
1.225
Parameters
GOF on F2
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
0.0462
0.0770
0.0623
0.1179
0.0434
0.0838
0.0370
0.0758
0.0486
0.1071
0.0612
0.1602
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hexacoordinated state. To complete its coordination, Sr1
undergoes μ2-O bridging with Sr3 through the imeamp ligand
attached to Sr3 and μ2-O bridging with Sr2 through the tmhd
ligand attached to Sr2. The μ2-O bridging of a tmhd oxygen
atom from Sr3 with Sr2 completes the metal–O–metal bonding
in 3. Complex 4 (Fig. 3), which crystallized as a trimeric
ˉ
complex in a triclinic P1 space group, exhibits structural simi-
larity to 2. In a very similar fashion to 2, all three metal centers
are connected through μ3-oxygen bridging using the alkoxide
oxygens of two memp ligands. The μ2-O bridge between Sr1
and Sr2 is formed using an memp alkoxide O-atom and that
between Sr2 and Sr3 is formed using a tmhd O-atom. Com-
plexes 5–7, where the aminoalkoxide ligands exerted more
steric hindrance than in the above described complexes
because of the presence of terminal amine groups, crystallized
as dimers following a common trend in this class of com-
pounds. The metal centers in these complexes exhibit hexa-
coordinated, distorted octahedral structures and the two metal
centers are connected by two μ2-O bonds with the alkoxide
oxygen atoms from the alkoxide ligands.
Fig. 1 ORTEP diagram for complex 2.
The crystal structures of complexes 2–4 show that the pres-
ence of the terminal methoxy groups in the emeamp, imeamp,
and memp ligands decreases the steric hindrance exerted by
these ligands with respect to demamp in [Sr(demamp)(tmhd)]2
(8) and results in the formation of trimeric structures. The
ethyl group on the amine nitrogen in emeamp did not show
much influence on the structure of 2, whereas the isopropyl
group in the imeamp ligand had an influence on the structure
of 3. The imeamp ligand attached to Sr1 experienced strong
steric interactions from the tmhd tert-butyl group, which
forced imeamp to adopt a position that prevented coordi-
nation of the amine nitrogen and the terminal methoxy group
space group (Fig. 2), exhibits one hexacoordinated and two
heptacoordinated metal centers in its trimeric structure.
Unlike 2, the metal–O–metal network in 3 consists of two
μ3-oxygen bridges and three μ2-oxygen bridges. Of the two
imeamp ligands that undergo μ3-oxygen bridging in 3, one is
bonded to Sr2 utilizing all three available coordination sites
whereas the other displays an uncoordinated amine group and
an ether group. The steric interaction between the isopropyl
moieties attached to the amine nitrogen in imeamp and that
of the tert-butyl group of tmhd might have made the complete
coordination of imeamp with Sr1 impossible, leaving Sr1 in a
Fig. 2 ORTEP diagram for complex 3.
Fig. 3 ORTEP diagram for complex 4.
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with a strontium ion. This caused Sr1 to appear as the sole 164.98(6)°, 166.11(7)°, and 167.33(7)°, respectively which indi-
hexacoordinated metal center in all trimeric complexes. The cated that the respective oxygen atoms were in axial positions
memp ligand in complex 4 (Fig. 3) with only oxygen-based relative to the corresponding strontium atoms. The bonding
coordination sites poses fewer steric interactions with the constraints due to the μ2/μ3-bridging appear to be the main
neighboring groups and forms a trimeric complex isostruc- causes of the distortions from the idealized geometries and
tural with 2. The metal–metal nonbonding distances (Table 3) the bond angles involving μ2/μ3 oxygen atoms display sharp
of Sr2⋯Sr3 3.6273(5) and Sr1⋯Sr3 3.5256(5) Å in 2 and deviations from the ideal angles.
Sr2⋯Sr1 3.5566(4) Å and Sr1⋯Sr3 3.6067(4) Å in 4 were com-
The Sr–O bond lengths in the above complexes vary largely
parable to those in a similarly bonded complex [Sr3(tmhd)3(O- from 2.401(3) to 2.749(2) Å because of the μ2/μ3-bridging invol-
SiPh3)3]6 (average Sr⋯Sr is (3.588(2) Å), and these strontium ving oxygen atoms as well as steric interactions in the mole-
pairs were each connected through three oxygen bridges. On cule. The shortest bond lengths in 2 involve the emeamp
the other hand the Sr1⋯Sr2 in 2 and Sr2⋯Sr3 in 4 paired by alkoxy oxygen (O11), which bridges Sr1 and Sr3 (bond lengths
only two sets of O-bridges show longer metal–metal distances of 2.401(3) and 2.425(3) Å). The bonds involving μ3-oxygen
of 4.0468(5) and 3.9997(4) Å, respectively and these were still atoms in 2 display an average bond length of 2.557(2) Å. The
7
longer than the similarly paired Sr⋯Sr in [Sr(thd)2]3 (average bonds between strontium and non-bridging tmhd oxygen
Sr⋯Sr is (3.8264(1) Å) where metal atoms were paired by two atoms show an average length of 2.4888(3) Å, whereas the μ2-
sets of O-bridges. In complex 3 all three metal centers were bridging oxygen atom of the tmhd ligand shows bond lengths
connected by three sets of O-bridges, with metal–metal non- of 2.611(2) (Sr2–O10) and 2.744(3) Å (Sr3–O10). The methoxy
bonding distances of Sr2⋯Sr3 3.6604(6), Sr1⋯Sr3 3.5420(6), oxygens and strontium atoms form the Sr1–O4 (2.601(3) Å),
and Sr1⋯Sr2 3.6298(6) Å, which were comparable to those in Sr2–O8 (2.703(3) Å), and Sr3–O12 (2.650(3) Å) bonds, respect-
t
[Sr3(tmhd)3(OSiPh3)3].6 The strontium metal ions in 2 have dis- ively. The steric interactions between the Bu group of the
torted pentagonal–bipyramidal coordination environments tmhd ligand and the methoxy group are thought to be the
where O4 and O11 occupy the axial positions with respect to main factors behind the longer bond lengths for Sr2–O8 and
Sr1, O6 and O10 with respect to Sr2, and O7 and O12 with Sr3–O10.
respect to Sr3. In 3, Sr1 has a distorted octahedral geometry,
In 3, the bonds between μ3-O atoms and strontium metal
whereas Sr2 and Sr3 have pentagonal–bipyramidal structures. centers show an average length of 2.513(3) Å. The bonds invol-
The bond angles (Table S1†) of O3–Sr2–O8 and O7–Sr3–O12 ving tmhd μ2-O atoms show an average bond length of 2.643(5)
were 163.99(11)° and 168.43(10)°, respectively; these formed Å whereas, the bonds involving imeamp μ2-O atom show
the axial bond pairs with each strontium ion. In 4, the bond bond lengths of 2.415(3) (Sr1–O11) and 2.447(3) (Sr3–O11) Å.
angles of O13–Sr1–O5, O7–Sr2–O13, and O11–Sr3–O1 were The Sr–O bonds with non-bridging oxygen atoms of tmhd
Table 3 Selected bond lengths (Å) for 2, 3, and 4
[Sr(emeamp)(tmhd)]3 (2)
[Sr(imeamp)(tmhd)]3 (3)
[Sr(memp)(tmhd)]3 (4)
Bond lengths (Å)
Sr(1)–O(1)
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–O(4)
Sr(1)–O(7)
Sr(1)–O(10)
Sr(1)–N(1)
Sr(2)–O(3)
Sr(2)–O(5)
Sr(2)–O(6)
Sr(2)–O(7)
Sr(2)–O(8)
Sr(2)–O(11)
Sr(2)–N(2)
Sr(3)–O(3)
Sr(3)–O(7)
Sr(3)–O(9)
Sr(3)–O(10)
Sr(3)–O(11)
Sr(3)–O(12A)
Sr(3)–N(3A)
Sr(2)⋯Sr(3)
Sr(1)⋯Sr(3)
Sr(1)⋯Sr(2)
2.4577(19)
2.4895(19)
2.4977(18)
2.702(2)
2.5433(18)
2.6099(19)
2.803(2)
2.6252(18)
2.5178(19)
2.521(2)
2.6019(18)
2.5999(19)
2.3986(19)
2.913(2)
2.5417(18)
2.5223(18)
2.456(2)
2.742(2)
2.4240(19)
2.649(2)
Sr(1)–O(1)
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–O(5)
Sr(1)–O(7)
Sr(1)–O(11)
Sr(2)–O(3)
Sr(2)–O(5)
Sr(2)–O(6)
Sr(2)–O(7)
Sr(2)–O(8)
Sr(2)–O(9)
Sr(2)–N(2)
Sr(3)–O(3)
Sr(3)–O(7)
Sr(3)–O(9)
Sr(3)–O(10)
Sr(3)–O(11)
Sr(3)–O(12)
Sr(3)–N(3)
2.448(3)
2.448(3)
2.547(3)
2.705(3)
2.525(3)
2.414(3)
2.526(3)
2.539(3)
2.467(3)
2.456(3)
2.670(3)
2.597(3)
2.915(4)
2.540(3)
2.483(3)
2.731(3)
2.490(3)
2.447(3)
2.613(3)
2.834(4)
Sr(1)–O(1)
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–O(4)
Sr(1)–O(5)
Sr(1)–O(8)
Sr(1)–O(13)
Sr(2)–O(3)
Sr(2)–O(6)
Sr(2)–O(7)
Sr(2)–O(8)
Sr(2)–O(9)
Sr(2)–O(10)
Sr(2)–O(13)
Sr(3)–O(1)
Sr(3)–O(8)
Sr(3)–O(11)
Sr(3)–O(12)
Sr(3)–O(13)
Sr(3)–O(14)
Sr(3)–O(15)
Sr(1)⋯Sr(2)
Sr(1)⋯Sr(3)
Sr(2)⋯Sr(3)
2.6295(19)
2.484(2)
2.4244(19)
2.624(2)
2.704(2)
2.504(2)
2.5606(18)
2.399(2)
2.492(2)
2.479(2)
2.6202(18)
2.667(2)
2.749(2)
2.5498(19)
2.571(2)
2.5361(18)
2.440(2)
2.484(2)
2.4971(19)
2.661(2)
2.730(2)
3.5566(4)
3.6067(4)
3.9997(4)
2.870(3)
3.5256(5)
3.6273(5)
3.0468(5)
Sr(1)⋯Sr(3)
Sr(1)⋯Sr(2)
Sr(2)⋯Sr(3)
3.5420(6)
3.6298(6)
3.6604(6)
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ligands show a bond length of 2.464(3) Å. The bonds connect- lengths were 2.5217(17) and 2.7845(2) Å, respectively, which
ing strontium and methoxy oxygen atoms were 2.669(3) Å (Sr2– were marginally longer than those in 6, where the average
O8) and 2.615(3) Å (Sr3–O12), in length.
Sr–O and Sr–N bond lengths were 2.4264(2) and 2.734(3) Å,
The crystal structure of 4 is identical to that of 2, but has respectively, and 7, with average bond lengths of 2.4236(3) Å
fewer intramolecular steric interactions owing to the absence (Sr–O) and 2.735(5) Å (Sr–N). The bridging angles (Sr–O–Sr) in
of amine groups in the memp ligand. The Sr–O bonds with 5 and 7 were 100.87(6)° and 100.67(12)°, respectively, whereas
μ3-O atoms show an average length of 2.5446(19) Å. The Sr–O that in 6 was narrower at 97.17(8)°.
bonds with non-bridging oxygen atoms of the tmhd ligands
Complex 7, where the strontium is coordinated to amino-
show a bond length of 2.476(2) Å. The bonds involving tmhd/ alkoxide ligand dmaemamp and β-diketonate ligand dmhd,
memp μ2-O atoms are Sr1–O1 (2.6295(19) Å), Sr3–O1 (2.571(2) and complex 8 are structural isomers. Their structural features
Å), Sr1–O3 (2.4244(19) Å), and Sr2–O3 (2.399(2) Å). The bonds are nearly identical; they have dimeric structures, similar
between strontium and methoxy oxygen atoms display lengths average bond lengths (the average Sr–O and Sr–N bond lengths
of 2.704(2) (Sr1–O5), 2.749(2) (Sr2–O10), and 2.730(2)
(Sr3–O15).
Å
in 8 are (2.430(2) Å) and Sr–N (2.745(3) Å), respectively), and
similar Sr⋯Sr distances of 3.7100(9) Å for 7 and 3.7119(6) Å
The structural analyses of complexes 5–7 (Fig. S1 (5), S2 (6)† for 8.
and Fig. 4 (7)) demonstrated the formation of dimers quite
NMR
similar to those reported for complex 8. This shows that the
1
introduction of bulkier substituents on the amine or on the The recorded H NMR spectra of complexes 1–7 at room tem-
alkoxy carbon of these aminoalkoxide ligands does not perature generally exhibited broader peaks, especially for the
produce monomeric structures and the complexes stick to a aminoalkoxide/alkoxide ligands which made the identification
stable dimeric form. Previous attempts to synthesize mono- of individual peaks difficult. The tmhd/dmhd protons in the
meric [Sr(aminoalkoxide)(tmhd)] by introducing bulky substi- spectra displayed comparatively sharper and more identifiable
tuents on the ligands have been made; however here we tried peaks. The broad peaks in the spectra of these complexes may
to introduce an ethyl group to the amine nitrogen next to the be due to the slow site exchanges in solution at the NMR time
t
bridging alkoxy groups (in 5) and two different alkyl groups to scale. In complex 1, the Bu protons from the three tmhd
the alkoxy carbon (in 6), where the steric interactions may play groups exhibited two broad peaks at δ = 1.32 and 1.41 ppm in
an important role in the structure. The strontium ions in these a 4 : 2 ratio, whereas the β-CH protons appeared as a single
complexes exhibit a distorted octahedral geometry with one broad peak with two shoulders at 5.82 ppm. This observation
aminoalkoxide and one β-diketonate ligand bonded to each supports the assumption that 1 might have formed a trimer
t
strontium ion and the alkoxide oxygen undergoes μ2-O brid- similarly to complexes 2 and 3. In complex 2, the Bu protons
ging between strontium ions to complete the hexa-coordinate from the three tmhd groups appeared as three distinct peaks
environment for each metal center. The Sr⋯Sr nonbonding at δ = 1.32, 1.34, and 1.39 ppm in a 2 : 3 : 1 ratio and the three
distances in complexes 5–7 (Table 4) were 3.7183(4), 3.6350(6), β-CH protons appeared at δ = 5.83, 5.85, and 5.87 ppm in a
t
and 3.7100(9) Å, respectively. These inter-metal distances were 3 : 2 : 1 ratio. The spectrum of 3 showed two peaks for the Bu
longer than those of the tri-bridged metal pairs and shorter protons one at 1.32 ppm, which was broader and had a
than those of the di-bridged metal pairs in trimeric complexes shoulder on one side, and another at 1.37 ppm in a 3 : 3 ratio.
2–4, but in the ranges reported for other dimeric complexes of The β-CH protons followed the same pattern showing one
this class.4,5 In complex 5, the average Sr–O and Sr–N bond broad peak at 5.87 ppm and another at 5.88 ppm. The spec-
trum of complex 4, where the less bulky alkoxyalkoxides were
the co-ligands with tmhd, showed singlet peaks for the tmhd
tBu protons at 1.34 ppm and β-CH protons at 5.84 ppm. The
room-temperature NMR spectra of complexes 1–3 pointed
towards free site exchange interactions of ligands in solution,
resulting in the formation of the structural isomers. The dis-
tinct peaks of complexes 1–3 compared to those of 4 can be
related to the speed of these exchange reactions. In the first
three complexes, these changes must be faster than those in 4,
where they are slower with respect to the NMR time scale and
thus the peak appeared as a broad singlet.
For dimeric complexes 5 and 6, the tmhd tBu protons
appeared at δ = 1.35 and 1.34 ppm and the β-CH protons at
5.86 and 5.86 ppm, respectively. For complex 7, the iPr protons
appeared as a doublet at 1.23 ppm, and β-CH protons appeared
at 5.46 ppm. These broad peaks for β-diketonates in dimeric
complexes indicated slow-paced site exchanges in their solu-
Fig. 4 ORTEP diagram for complex 7.
tion phase compared to those of trimeric complexes.
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Table 4 Selected bond lengths (Å) and bond angles (°) for 5, 6, and 7
[Sr(dmaeeamp)[(tmhd)]2 (5)
[Sr(dmaemamb)(tmhd)]2 (6)
[Sr(dmaemamp)(tmpd)]2 (7)
Bond lengths (Å)
Sr(1)–O(3)i
Sr(1)–O(1)
Sr(1)–O(3)
Sr(1)–O(2)
Sr(1)–N(1)
Sr(1)–N(2)
Sr(1)i–O(3)
Sr(1)⋯Sr(1)i
2.3854(16)
2.4354(18)
2.4377(17)
2.4430(17)
2.748(2)
2.821(2)
2.3853(16)
3.7183(4)
Sr(1)–O(3)i
Sr(1)–O(1)
Sr(1)–O(2)
Sr(1)–O(3)
Sr(1)–N(2)
Sr(1)–N(1)
Sr(1)i–O(3)
Sr(1)⋯Sr(1)i
2.387(2)
2.439(2)
2.460(2)
2.460(2)
2.701(3)
2.767(3)
2.386(2)
3.6350(6)
Sr(1)–O(11)i
Sr(1)–O(11)
Sr(1)–O(2)
Sr(1)–O(1)
Sr(1)–N(2)
Sr(1)–N(11)
Sr(1)i–O(11)
Sr(1)⋯Sr(1)i
2.379(3)
2.438(3)
2.447(3)
2.471(3)
2.707(4)
2.744(4)
2.379(3)
3.7099(7)
Bond angles (°)
O(3)i–Sr(1)–O(1)
O(3)i–Sr(1)–O(3)
O(1)–Sr(1)–O(3)
O(3)i–Sr(1)–O(2)
O(1)–Sr(1)–O(2)
O(3)–Sr(1)–O(2)
O(3)i–Sr(1)–N(1)
O(1)–Sr(1)–N(1)
O(3)–Sr(1)–N(1)
O(2)–Sr(1)–N(1)
O(3)i–Sr(1)–N(2)
O(1)–Sr(1)–N(2)
O(3)–Sr(1)–N(2)
O(2)–Sr(1)–N(2)
N(1)–Sr(1)–N(2)
Sr(1)i–O(3)–Sr(1)
112.30(6)
79.13(6)
165.28(6)
113.06(6)
70.66(6)
96.68(6)
94.15(6)
86.65(6)
102.18(6)
149.41(6)
135.05(6)
106.92(6)
66.95(6)
99.81(6)
66.74(6)
100.87(6)
O(3)i–Sr(1)–O(1)
O(3)i–Sr(1)–O(2)
O(1)–Sr(1)–O(2)
O(3)i–Sr(1)–O(3)
O(1)–Sr(1)–O(3)
O(2)–Sr(1)–O(3)
O(3)i–Sr(1)–N(2)
O(1)–Sr(1)–N(2)
O(2)–Sr(1)–N(2)
O(3)–Sr(1)–N(2)
O(3)i–Sr(1)–N(1)
O(1)–Sr(1)–N(1)
O(2)–Sr(1)–N(1)
O(3)–Sr(1)–N(1)
N(2)–Sr(1)–N(1)
Sr(1)i–O(3)–Sr(1)
110.70(8)
126.14(8)
70.83(8)
82.83(8)
101.32(8)
151.03(8)
144.61(8)
94.87(9)
84.56(9)
68.03(8)
94.07(8)
153.49(9)
87.18(8)
90.73(8)
67.69(9)
97.17(8)
O(11)i–Sr(1)–O(11)
O(11)i–Sr(1)–O(2)
O(11)–Sr(1)–O(2)
O(11)i–Sr(1)–O(1)
O(11)–Sr(1)–O(1)
O(2)–Sr(1)–O(1)
O(11)i–Sr(1)–N(2)
O(11)–Sr(1)–N(2)
O(2)–Sr(1)–N(2)
O(1)–Sr(1)–N(2)
O(11)i–Sr(1)–N(11)
O(11)–Sr(1)–N(11)
O(2)–Sr(1)–N(11)
O(1)–Sr(1)–N(11)
N(2)–Sr(1)–N(11)
Sr(1)i–O(11)–Sr(1)
79.28(10)
106.89(10)
98.22(10)
119.59(10)
160.11(10)
71.39(10)
141.30(11)
67.00(11)
96.53(12)
96.72(12)
102.87(13)
100.40(14)
147.28(14)
82.34(14)
66.96(16)
100.72(10)
VT-NMR
1.32 (27H), and 1.47 (9H) ppm; the β-CH protons appeared at
δ = 5.67 (1H), 5.85 (1H), and 5.98 (1H) ppm.
To get a more detailed view of their structures in solution, the
In the spectrum of dimeric complex 5, the tmhd proton
NMR spectra of complexes 1–7 (Fig. S3†) were analyzed at low peaks did not show any changes in their characteristics and
temperatures with toluene-d8 as the solvent and reference. The remained as singlets for both the tBu protons and β-CH
low-temperature measurement was expected to give a clearer protons even at 213 K. For complex 6 at 213 K, the tBu protons
view of the structures as site exchange interactions will be from two tmhd ligands appeared as three peaks at δ = 1.33
limited at lower temperatures. The NMR spectrum of complex (9H), 1.34 (18H), and 1.34 (9H) ppm and the β-CH protons
1 at 213 K showed one singlet peak for each tmhd tBu group at appear at δ = 5.88 (0.5H), 5.89 (1H), and 5.91 (0.5H) ppm. The
δ = 1.18, 1.22, 1.29, 1.37, 1.47, and 1.50 ppm and three individ- results indicate retention of the dimeric structure in solution
ual peaks for β-CH protons at δ = 5.62, 5.84, and 5.99 ppm. with two possible structural isomeric forms coexisting in
t
This observation of six environmentally different Bu groups nearly equal proportions. We suggest that the isomers may be
and three β-CH protons supports its trimeric structure and a form of cis–trans isomers, where the trans form exists in the
confirms its structural stability in solution. The spectrum of crystal form and a cis form with both tmhd ligands on the
t
complex 2 exhibited twelve individual singlets for Bu groups same side of the dimer is the other. The spectrum of complex
and six singlets for β-CH protons at 213 K. This suggests the 7 at 253 K displayed two sets of two doublets each with a 2 : 1
existence of two closely related structural isomers at this tem- ratio, which was again an indication of the presence of two
perature, and the μ2-bridging of only one of the six tmhd isomers at that temperature. The first set for the major isomer,
oxygen atoms can produce a closely related structural isomer (which may be the stable trans form), appeared as two doublets
with site exchange interactions in solution for this complex. at δ = 1.18 and 1.19 ppm and a second set of doublets for the
t
The Bu protons in 3 appeared as six singlets at δ = 1.25, 1.27, minor isomer (cis form) appeared at δ = 1.21 and 1.22 ppm.
1.29, 1.37, 1.38 and 1.44 ppm, and three peaks for β-CH The β-CH protons appeared as a singlet peak at 5.40 ppm.
protons appeared at δ = 5.76, 5.90, and 5.98 ppm. These peaks However in the spectrum at 213 K, the β-CH protons of the two
for tmhd protons in 3 are broader than those observed for 1 isomers separated and existed as two singlets at δ = 5.50 and
and exhibited shoulders, which indicate the presence of 5.52 ppm with a 1 : 2 ratio. The spectral data obtained for the
closely related structural isomers in solution. The spectrum of above four trimeric complexes and three dimeric complexes
complex 4, which displayed a single peak at room temperature differed according to their structural features and ability to
t
t
for the Bu protons exhibited four broad peaks for six Bu undergo site exchanges in solution at a particular temperature
protons at 213 K. These appeared at δ = 1.13 (9H), 1.26 (9H), and pace at the NMR time scale.
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Mass spectra of complexes 1–6 display dominant peaks at observed mass losses were 40%, 34%, 41%, and 43% respect-
m/z = 725 and 271 which correspond to the [Sr2(tmhd)3]+ and ively. In the final step in the 370–550 °C range further mass
[Sr(tmhd)]+ species whereas, complex 7 displays peaks at m/z = losses of 4%, 8%, 7%, and 11% and residual masses of 25%,
641 ([Sr2(dmhd)3]+) and 243 ([Sr(dmhd)]+). The formation of 21%, 24%, and 31%, respectively, were observed. The first two
[Sr2(tmhd)3]+ and [Sr(tmhd)]+ type fragments during the mass steps with major mass losses at 75–300 °C and 300–370 °C
spectroscopic analysis of alkaline earth metal complexes that might be due to the partial evaporation of the complexes and
contain tmhd has been reported earlier. The reason for the possible fragmentation of the complexes. The final step, with
fragmentation has been described as a result of ion–molecule relatively small losses can be attributed to the evaporation of
reactions in the mass spectrometer source.9 In the elemental the portions of the decomposed products.
analyses of the complexes, complexes 4 and 6 show consider-
The TGA plots of compounds 5–7 displayed multiple steps
able deviation from the calculated values even with our best (Fig. 6). The complexes showed initial losses of 3% at 90 °C,
efforts and repeated experiments. All other complexes yielded and then 25% each for 5 and 7 and 30% for 6 up to 250 °C.
comparable values with those of the calculated ones. The sen- Further mass losses were observed for complexes 5 and 6 in
sitive nature of the complexes towards air and moisture has an multiple steps from 250 to 500 °C, with mass losses of 48% for
effect on the analyses where complete inertness could not be 5 and 45% for 6. In the case of complex 7, a 20% mass loss in
assured. FT-IR spectra of the complexes display characteristic the second step from 250 to 335 °C and a 19% mass reduction
peaks for this class of complexes. The absence of any –OH in the third step from 335 to 500 °C were observed. All com-
peaks confirmed complete reaction and any free ligand pres- plexes (1–7) displayed high nonvolatile residual masses of
ence. The sharp peaks displayed by the complexes around 25%, 21%, 24%, 31%, 24%, 21%, and 29%, respectively,
470 cm−1 confirm the diketonate-O–Sr bond formation.10 reflecting the decomposition of the compounds during the
Apart from the fragmentation in mass spectra, and elemental experiments and, thus the temperature instability. As all the
analyses where the high sensitivity of the complexes affected sample preparations and the experiments were performed
the results, the FT-NMR and FT-IR data proved the purity of under an air-free atmosphere, the final residue obtained ought
the bulk samples with no evidence of unreacted reagents, side to be a mixture of nonvolatile fractions from ligand decompo-
products or fragmentation.
sition and strontium oxide. The calculations based on mole-
cular weight and percentage of strontium in complex 1 show
the possible formation of Sr, SrO, and SrCO3 from the
decomposition as 20%, 24%, and 34%, respectively. The
experimental observation of 25% of the final residue in TGA
closely matches with the calculated SrO percentage. However,
in other complexes, the possible SrO and SrCO3 percentages
were calculated as 23% & 33% (2), 23% & 32% (3), 25% & 35%
(4), 23% & 32% (5), 23% & 32% (6), and 23% & 33% (7)
showing that the residues are possibly a mixture of strontium
oxide and ligand fractions. To prove the above calculations,
X-ray diffraction analysis on the residual powder of 1 was con-
Thermogravimetric analyses
Thermogravimetric analyses (TGA) of complexes 1–7 were con-
ducted from room temperature to 800 °C under a constant
flow of argon. TGA plots of the complexes display a 1–2% mass
loss up to 75 °C owing to the evaporation of any trapped sol-
vents in the samples. Complexes 1–4 showed similar charac-
ters in their TGA plots (Fig. 5), which demonstrated three-step
mass losses. In the first step between 75 and 300 °C, there
were mass losses of 30%, 34%, 26%, and 12%, respectively. In
the second step, in the 300–370 °C temperature range, the
Fig. 5 TGA plot of complexes 1 (black), 2 (red), 3 (blue), and 4 (violet).
Fig. 6 TGA plot of complexes 5 (black), 6 (red), and 7 (blue).
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ducted. The peaks obtained from the experiment (Fig. S4†) 2.58 (2H, t, CH3OCH2CH2), 3.06 (3H, s, CH3O), 3.15 (2H, t,
agree with the formation of SrO from the decomposition, and CH3OCH2CH2), 3.63 (1H, s, OH). Anal. Calcd for C9H21NO2: C,
show no evidence for the presence of SrCO3.
61.67; H, 12.08; N, 7.99. Found: C, 61.48; H, 11.98; N, 7.92.
1-(Isopropyl(2-methoxyethyl)amino)-2-methylpropan-2-ol
(imeampH). Isobutylene oxide (7.21 g, 0.1 mol) and N-(2-meth-
oxyethyl)propan-2-amine (11.7 g, 0.1 mol) were used. Bp
75 °C/0.5 Torr; yield 10.9 g (58%). FTIR (νmax/cm−1) 3457 (OH).
1H NMR (C6D6, 300 MHz): δH 0.84 (6H, d, N(CH(CH3)2)), 1.23
(6H, s, C(CH3)O), 2.23 (2H, s, CH2C(CH3)2), 2.53 (2H, t,
CH3OCH2CH2), 2.66 (1H, m, N(CH(CH3)2)), 3.07 (3H, s, CH3O),
3.16 (2H, t, CH3OCH2CH2), 3.89 (1H, s, OH). Anal. Calcd for
C10H23NO2: C, 63.45; H, 12.25; N, 7.40. Found: C, 63.21; H,
11.98; N, 7.28.
The study of the volatile character of the complexes was
conducted under reduced pressure (10−6 Torr) at room tem-
perature. The experiment resulted in the decomposition of
complexes 1 and 2 at 170 °C, whereas all other complexes (3–7)
displayed a stable character throughout the experiment.
Experimental
NMR spectra were recorded on a Bruker 300 MHz spectrometer
with C6D6 as the solvent and standard. IR spectra were
obtained with a Nicolet Nexus FT-IR spectrophotometer.
Elemental analyses were carried out on a ThermoScientific
OEA Flash 2000 analyzer. Thermo-gravimetric analyses were
conducted on a SETARAM 92-18 TG-DTA instrument with a
constant flow of nitrogen throughout the experiment.
(Sr(btsa)2·2DME),8 and the aminoalcohols, memampH,5
emeampH,5 imeampH,5 mempH,11 dmaeeampH,5 dmae-
mambH5 and dmaemampH5 were prepared by modified litera-
ture methods. All reactions except ligand preparations were
carried out under inert dry conditions using standard Schlenk
techniques or in an argon-filled glove box. Hexane and toluene
were purified with an Innovative Technology PS-MD-4 solvent
purification system. 2,2-Dimethyloxirane was purchased from
TCI, and all other chemicals were purchased from Aldrich and
used as received.
1-((2-(Dimethylamino)ethyl)(ethyl)amino)-2-methylpropan-2-ol
(dmaeeampH). Isobutylene oxide (7.21 g, 0.1 mol) and N1-
ethyl-N2,N2-dimethylethane-1,2-diamine (11.6 g, 0.1 mol) were
used. Bp 60 °C/0.5 Torr; yield 14.5 g (77%). FTIR (νmax/cm−1
)
3378 (OH). 1H NMR (C6D6, 300 MHz): δH 0.88 (3H, t,
N(CH2CH3)), 1.26 (6H, s, C(CH3)O), 2.02 (6H, s, N(CH3)2), 2.12
(2H, t, (CH3)2NCH2CH2), 2.26 (2H, s, CH2C(CH3)2), 2.33 (2H, t,
(CH3)2NCH2CH2), 2.44 (2H, q, N(CH2CH3)), 5.86 (1H, s, OH).
Anal. Calcd for C10H24N2O: C, 63.78; H, 12.85; N, 14.88.
Found: C, 63.47; H, 12.79; N, 14.79.
1-((2-(Dimethylamino)ethyl)(methyl)amino)-2-methylbutan-2-ol
(dmaemambH). 2-Ethyl-2-methyl-oxirane (0.860 g, 0.1 mol) and
N,N,N′-trimethylethylenediamine (10.2 g, 0.1 mol) were used.
Bp 80 °C/0.05 Torr; yield 12.3 g, 65%. FTIR (νmax/cm−1) 3209
(OH). 1H NMR (C6D6, 300 MHz): δH 0.90 (3H, t, C(CH3)
(CH2CH3)O), 1.07 (3H, s, C(CH3)(CH2CH3)O), 1.45 (2H, m,
C(CH3)(CH2CH3)O), 2.23 (6H, s, N(CH3)2), 2.35 (4H, m,
N(CH3)2CH2CH2), 2.40 (3H, s, CH2N(CH3)CH2), 2.55 (2H, m,
N(CH3)CH2), 5.78 (1H, s, OH). Anal. Calcd for C10H24N2O: C,
63.78; H, 12.85; N, 14.88. Found: C, 63.42; H, 12.80; N, 14.20.
1-(2-Methoxyethoxy)-2-methylpropan-2-ol (mempH). 2-Methoxy-
ethanol (7.61 g, 0.1 mol) was added dropwise at 0 °C with con-
stant stirring to a suspension of sodium hydride (2.40 g,
0.1 mol) in THF. The mixture was then allowed to reach room
temperature and stirring was continued for another 30 min.
Isobutylene oxide (10.8 g, 0.2 mol) was added dropwise to the
above mixture at 0 °C. The reaction mixture was slowly heated
to 90 °C and stirring was continued for another 12 h. After
cooling to room temperature the reaction was quenched with
ammonium chloride solution at 0 °C. The product was
extracted with ethyl acetate (3 × 50 mL), and the combined
organic layer was washed with brine. The solvent was removed,
and the crude product was distilled (60 °C/0.05 Torr) to
obtain the product as a colorless liquid (6.70 g, 45%). FTIR
(νmax/cm−1) 3450 (OH). 1H NMR (C6D6, 300 MHz): δH 1.18
(6H, s, C(CH3)2O), 2.48 (1H, s, OH), 3.07 (3H, s, CH3OCH2),
3.11 (2H, s, CH2CH2OCH2), 3.23 (2H, t, CH2CH2OCH2), 3.40
Syntheses
General procedure for the synthesis of aminoalcohols. Iso-
butylene oxide was added dropwise at 0 °C with constant stir-
ring to a 40% water solution of the corresponding amine in a
three-necked flask fitted with a reflux condenser. After the
addition, the reaction mixture was warmed to room tempera-
ture and stirring was continued for another 6 h. Then, the
product was extracted with diethyl ether three times (3 ×
50 mL), and the combined organic layer was washed with
brine. The solvent was removed, and the crude product was
distilled to obtain the pure product.
1-((2-Methoxyethyl)(methyl)amino)-2-methylpropan-2-ol
(memampH). Isobutylene oxide (7.21 g, 0.1 mol), and
2-methoxy-N-methylethan-1-amine (8.91 g, 0.1 mol) were used.
Bp 60 °C/0.5 Torr; yield 11.6 g (72%). FTIR (νmax/cm−1) 3463
(OH). 1H NMR (C6D6, 300 MHz): δH 1.18 (6H, s, C(CH3)O), 2.18
(2H, s, CH2C(CH3)2), 2.20 (3H, s, N(CH3)), 2.51 (2H, t,
CH3OCH2CH2), 3.10 (3H, s, CH3O), 3.14 (2H, t, CH3OCH2CH2),
3.44 (1H, s, OH). Anal. Calcd for C8H19NO2: C, 59.59; H, 11.88;
N, 8.69. Found: C, 59.54; H, 11.86; N, 8.65.
1-(Ethyl(2-methoxyethyl)amino)-2-methylpropan-2-ol (emeampH). (2H, t, CH3OCH2CH2). Anal. Calcd for C7H16O3; C, 56.73; H,
Isobutylene oxide (7.21 g, 0.1 mol), and N-ethyl-2-methoxy- 10.88. Found: C, 56.58; H, 10.80.
ethan-1-amine (10.3 g, 0.1 mol) were used. Bp 60 °C/0.5 Torr;
General procedure for [Sr(R-O)(β-diketonate)]x complexes.
yield 14.0 g (80%). FTIR (νmax/cm−1) 3458 (OH). 1H NMR The aminoalcohol/alkoxyalcohol in toluene (5 mL) was added
(C6D6, 300 MHz): δH 0.87 (3H, t, N(CH2CH3)), 1.20 (6H, s, dropwise at room temperature to a solution of Sr(btsa)2·2DME
C(CH3)O), 2.24 (2H, s, CH2C(CH3)2), 2.42 (2H, q, N(CH2CH3)), in toluene (15 mL) with stirring. After stirring for 5 h at room
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temperature, 2,2,6,6-tetramethyl-3,5-heptanedione (tmhdH) or CH2(CH3)2O), 3.30 (6H, s, br, CH3OCH2CH2), 3.37 (6H, s,
2,6-dimethyl-3,5-heptanedione (dmhdH) was added to the CH3OCH2CH2), 5.84 (3H, s, β-CH). Anal. Calcd for
reaction mixture, which was then stirred for another 15 h at C57H109O15Sr3: C, 51.71; H, 8.20. Found: C, 50.02; H, 8.13. MS:
room temperature. Then, the solvent was evaporated and the m/z calcd for [Sr(memp)(tmhd)]3: 1254.26 [M]+; found 726
residue was extracted with hexane, filtered, and dried to obtain [Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
[Sr(dmaeeamp)(tmhd)]2
1.0 mmol), 1-((2-(dimethylamino)ethyl)(ethyl)amino)-2-methyl-
(5). Sr(btsa)2·2DME
(0.590
g,
the product. X-ray quality crystals were grown from concen-
trated hexane solutions upon cooling.
[Sr(memamp)(tmhd)]3 (1). Sr(btsa)2·2DME (0.590 g, 1.0 propan-2-ol (dmaeeampH) (0.188 g, 1.0 mmol), and 2,2,6,6-
mmol), 1-((2-methoxyethyl)(methyl)amino)-2-methylpropan-2- tetramethyl-3,5-heptanedione (tmhdH) (0.184 g, 1.0 mmol)
ol (memampH) (0.161 g, 1.0 mmol), and 2,2,6,6-tetramethyl- were used. Yield 0.431 g (94%). M.p. 205 °C. FTIR (νmax/cm−1
)
3,5-heptanedione (tmhdH) (0.184 g, 1.0 mmol) were used. 2949m, 2862w, 2836w, 1589vs, 1577m, 1535w, 1504w, 1455m,
Yield 0.384 g (89%). M.p. 181 °C. FTIR (νmax/cm−1) 2950m, 1418vs, 1387w, 1355w, 1224w, 1184w, 982w, 863w, 470w. 1H
2867w, 1599s, 1577m, 1533w, 1503w, 1452m, 1423vs, 1357w, NMR (C6D6, 300 MHz): δH 0.85 (6H, t, NCH2CH3), 1.25 (6H, s,
1209w, 1126w, 1106w, 1036w, 863w, 470w. 1H NMR (C6D6, br, C(CH3)2O), 1.35 (36H, s, C(CH3)3), 1.48 (6H, s, br,
300 MHz): δH 1.32 (36H, br, C(CH3)3), 1.41 (18H, br, C(CH3)3), C(CH3)2O), 1.90–2.80 (28H, br), 5.86 (2H, s, β-CH). Anal. Calcd
1.42–1.90 (18H, broad peaks, C(CH3)2O), 1.92–3.40 (36H, br) for C42H84N4O6Sr2: C, 55.05; H, 9.24; N, 6.11. Found: C, 54.82;
5.82 (3H, br, β-CH). Anal. Calcd for C57H111N3O12Sr3: C, 52.93; H, 9.18; N, 6.01. MS: m/z calcd for [Sr(dmaeeamp)(tmhd)]2:
H, 8.65; N, 3.25. Found: C, 52.60; H, 8.62; N, 3.23. MS: m/z 916.37 [M]+; found 726 [Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
calcd for [Sr(memamp)(tmhd)]3: 1293.53 [M]+; found 726
[Sr(dmaemamb)(tmhd)]2 (6). Sr(btsa)2·2DME (0.590 g, 1.0
mmol), 1-((2-(dimethylamino)ethyl)(methyl)amino)-2-methyl-
[Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
[Sr(emeamp)(tmhd)]3
1.0 mmol), 1-(ethyl(2-methoxyethyl)amino)-2-methylpropan-2- tetramethyl-3,5-heptanedione (tmhdH) (0.184 g, 1.0 mmol)
ol (emeampH) (0.175 g, 1.0 mmol), and 2,2,6,6-tetramethyl- were used. Yield 0.407 g (89%). M.p. 154 °C. FTIR (νmax/cm−1
(2). Sr(btsa)2·2DME
(0.590
g, butan-2-ol (dmaemambH) (0.188 g, 1.0 mmol), and 2,2,6,6-
)
3,5-heptanedione (tmhdH) (0.184 g, 1.0 mmol) were used. 2958m, 2862w, 1590s, 1577w, 1535w, 1504w, 1448w, 1419vs,
Yield 0.382 g (86%). M.p. 178 °C. FTIR (νmax/cm−1) 2951m, 1356w, 1183w, 1128w, 1020w, 976w, 864w, 470w. 1H NMR
1599s, 1577m, 1533w, 1504w, 1452m, 1421vs, 1387w, 1358w, (C6D6, 300 MHz): δH 0.90 (6H, br, C(CH3)(CH2CH3)O),
1221w, 1181w, 1125w, 1056w, 973w, 863w, 471w. 1H NMR 1.15–1.28 (6H, br, C(CH3)(CH2CH3)O), 1.34 (36H, s, C(CH3)3),
(C6D6, 300 MHz): δH 0.92 (9H, t, br, NCH2CH3), 1.32 (18H, s, 1.41–1.66 (4H, m, br, C(CH3)(CH2CH3)O), 1.80–2.70 (30H, br),
br, C(CH3)3), 1.34 (27H, s, br, C(CH3)3), 1.39 (9H, s, br, 5.86 (2H, s, β-CH). Anal. Calcd for C42H84N4O6Sr2: C, 55.05; H,
C(CH3)3), 1.57–1.80 (18H, broad peaks, C(CH3)2O), 2.32–3.40 9.24; N, 6.11. Found: C, 52.55; H, 9.22; N, 6.05. MS: m/z
(33H, br), 5.83, 5.85 & 5.87 (3H, br, β-CH). Anal. Calcd for calcd for [Sr(dmaemamb)(tmhd)]2: 916.37 [M]+; found 726
C60H117N3O12Sr3: C, 53.96; H, 8.83; N, 3.15. Found: C, 53.24; [Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
[Sr(dmaemamp)(dmhd)]2 (7). Sr(btsa)2·2DME (0.590 g,
1.0 mmol), 3-(((2-(dimethylamino)ethyl)(methyl)amino)-
H, 8.73; N, 3.07. MS: m/z calcd for [Sr(emeamp)(tmhd)]3:
1335.58 [M]+; found 726 [Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
[Sr(imeamp)(tmhd)]3 (3). Sr(btsa)2·2DME (0.590 g, 1.0 mmol), methyl)-pentan-3-ol (dmaemampH) (0.202 g, 1.0 mmol), and
1-(isopropyl(2-methoxyethyl)amino)-2-methylpropan-2-ol 2,6-dimethyl-3,5-heptanedione (dmhdH) (0.156 g, 1.0 mmol)
were used. Yield 0.422 g (95%). M.p. 163 °C. FTIR (νmax/cm−1
)
(imeampH) (0.189 g, 1.0 mmol), and 2,2,6,6-tetramethyl-3,5-
heptanedione (tmhdH) (0.184 g, 1.0 mmol) were used. Yield
0.367 g (80%). M.p. 111 °C. FTIR (νmax/cm−1) 2964m, 2870w,
1593s, 1576m, 1534w, 1504w, 1452m, 1417vs, 1358w, 1127w,
2960m, 2926w, 2866w, 1604vs, 1525w, 1500w, 1450s, 1311w,
1
1167w, 1037w, 784w, 425w. H NMR (C6D6, 300 MHz): δH 0.89
(12H, br, C(CH2CH3)2O), 1.23 (24H, d, CH(CH3)2), 1.60 (4H, m,
br, CH(CH3)2), 1.80–2.4 (26H, br), 2.40–2.60 (12H, m, br,
C(CH2CH3)2O & (CH3)2NCH2), 5.46 (2H, s, β-CH). Anal. Calcd
for C40H80N4O6Sr2: C, 54.08; H, 9.08; N, 6.31. Found: C, 53.77;
H, 8.89; N, 6.27. MS: m/z calcd for [Sr(dmaemamp)(dmhd)]2:
888.32 [M]+; found 641 [Sr2(dmhd)3]+, 243 [Sr(dmhd)]+.
1
990w, 863w, 473w. H NMR (C6D6, 300 MHz): δH 0.70–1.10
(18H, br, CH(CH3)2) 1.32 (27H, s, br, C(CH3)3), 1.37 (27H, s,
C(CH3)3), 1.64 (18H, broad peaks, C(CH3)2O), 3.04 (3H, m,
CH(CH3)2), 2.10–3.50 (30H, br), 5.87 (1.5H, s, br, β-CH), 5.88
(1.5H, s, br, β-CH). Anal. Calcd for C63H123N3O12Sr3: C, 54.93;
H, 9.00; N, 3.05. Found: C, 54.27; H, 8.98; N, 3.03. MS: m/z
calcd for [Sr(imeamp)(tmhd)]3: 1377.5 [M]+; found 726
[Sr2(tmhd)3]+, 271 [Sr(tmhd)]+.
X-ray crystallography
Single crystals of all complexes were grown from saturated
[Sr(memp)(tmhd)]3 (4). Sr(btsa)2·2DME (0.590 g, 1.0 mmol), hexane solutions at −30 °C. A specimen of suitable size and
1-(2-methoxyethoxy)-2-methylpropan-2-ol (mempH) (0.148 g, quality was coated with Paratone oil and mounted onto a glass
1.0 mmol), and 2,2,6,6-tetramethyl-3,5-heptanedione (tmhdH) capillary. Reflection data were collected on a Bruker Apex
(0.184 g, 1.0 mmol) were used. Yield 0.368 g (88%). M.p. 93 °C. II-CCD area detector diffractometer, with graphite-monochro-
FTIR (νmax/cm−1) 2950m, 2866w, 1598s, 1576w, 1533w, 1503w, mated MoKα radiation (λ = 0.71073 Å). The hemisphere of
1455m, 1419vs, 1358w, 1224w, 1182w, 1127w, 1084w, 950w, reflection data was collected as ω scan frames with 0.3° per
863w, 471w. 1H NMR (C6D6, 300 MHz): δH 1.34 (54H, s, frame and an exposure time of 10 s per frame. The SAINT12
C(CH3)3), 1.61 (18H, s, br, C(CH3)2O), 3.13 (15H, s, br, CH3O & software was used for cell refinement and data deduction. The
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data were corrected for Lorentz and polarization effects. An
empirical absorption correction was applied using the SADABS
program.13 The structure was solved by direct methods and all
non-hydrogen atoms were subjected to anisotropic refinement
by full-matrix least-squares on F2 by using the SHELXTL/PC
package.14 Hydrogen atoms were placed at their geometrically
calculated positions and refined riding on the corresponding
carbon atoms with isotropic thermal parameters. CCDC
1034384–1034389 for complexes 2–7 contain the supplemen-
tary crystallographic data for this paper.
Notes and references
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Conclusion
The efforts to synthesize new heteroleptic strontium complexes
by introducing a variety of donor-functionalized alkoxide
ligands as well as different β-diketonate ligands were success-
ful. The introduction of terminal methoxy groups to the
ligands resulted in a decrease in steric effects of the ligands,
which led to the formation of trimeric complexes 1–4. The
structures of complexes 2–4 comprised three metal centers
connected by two sets of μ3-O bonding and two sets of μ2-O
bonding. All of the strontium atoms in complexes 2 and 4 were
seven-coordinated, whereas in 3 a mixture of six- and seven-co-
ordinated geometry was displayed. Complexes 5–7 displayed
dimeric geometry and showed similar structural features to the
known heteroleptic dimeric complexes. The attempts to
impose structural changes from the dimeric form by introdu-
cing ethyl groups on the amine nitrogen and the alkoxy carbon
of the aminoalkoxide ligands proved insufficient. VT-NMR
studies confirmed the stability of all the complexes in solu-
tions with no evidence of fragmentation. Thermo-gravimetric
analyses show major weight losses for all the trimeric and
dimeric complexes in two steps in the 75–370 °C region. Com-
plexes 3–7 exhibit good stability at higher temperatures in sub-
limation studies, and studies for the possible application of
these heteroleptic strontium complexes as potential precursors
for the growth of thin films of SrTiO3 (STO) and Ba0.5Sr0.5TiO3
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complexes in a variety of organic solvents makes them highly
suitable for the synthesis of STO nanoparticles in the solution
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Acknowledgements
We would like to thank the Center for Chemical Analysis at the
Korea Research Institute of Chemical Technology (KRICT) for
allowing the use of their facilities and Bruker SMART APEX II
for solving the molecular structures. This research was sup-
ported by the Converging Research Center Program through
the Ministry of Education, Science and Technology
(2013K000153).
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This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 14042–14053 | 14053