Synthesis and Crystal Structures of New Strontium Complexes with Aminoalkoxy and β-Diketonato Ligands.

New heteroleptic strontium complexes were synthesized using substitution reaction of bis(trimethylsilyl)amide of Sr(btsa)2·2DME with aminoalkoxide and β-diketonate ligands. The complexes [Sr(bdmp)(btsa)]2·2THF (1), [Sr(bdeamp)(btsa)]2 (2), [Sr(dadamb)(btsa)]2 (3), [Sr(bdmp)(hfac)]3 (4), [Sr(bdeamp)(hfac)]3 (5), [Sr(dadamb)(hfac)]3 (6), and [Sr3(dadamb)4(tmhd)2] (7) were prepared and characterized by means of various analysis techniques such as Fourier transform infrared, NMR, thermogravimetric analysis, and elemental analysis. Complexes 1-3 were further structurally confirmed by single-crystal X-ray crystallography, and they displayed dimeric structures in which strontium atoms were connected by alkoxide oxygen atoms of the μ2 type. Compound 1 has a trigonal prismatic structure, whereas 2 and 3 have a distorted square pyramidal structure. In complexes 5-7, trimeric structures were obtained with strontium atoms connected by μ3-O bonds of alkoxide oxygen atoms and μ2-O bonds of alkoxide and β-diketonate oxygen atoms. The crystal structures of 5, 6, and 7 showed distorted capped octahedral geometry, while 7 (middle Sr atom) displayed a distorted trigonal prism geometry. Complexes 5-7 displayed ∼70% mass loss in the temperature range from 25 to 315 °C.

Introduction

Among group 2 metals, strontium (Sr) is an attractive element because of its varied applications in electro-optic modulators, optical waveguides,[1] pyroelectric detectors, and electrically tunable microwave devices.[2] These applications require fabrication of strontium-containing metal oxide [BaSr1–TiO3 (BST)] thin films using metal–organic chemical vapor deposition.[3] To apply this technique, strontium precursors with thermal stability and good volatility are essential. However, the development of new strontium precursors is often challenged by the formation of oligomeric compounds with poor volatility because of their high coordination number, small charge, and large size. To prevent these problems, sterically bulky and multidentate ligands have been used to form fully saturated metal centers and strong bonds with Sr metal.[4]

Generally, β-diketonates have been used to synthesize homoleptic metal precursors such as Sr(mthd)2,[5] Sr(tfac)2,[6] and Sr(hfac)2,[6] with 1-methoxy-2,2,6,6-tetramethyl-3,5-heptanedionate (mthd), 1,1,1-trifluoropentanedionate (tfac), and 1,1,1,5,5,5-hexafluoropentanedionate (hfac). Unfortunately, homoleptic strontium complexes with β-diketonate ligands often polymerize or form bonds with neutral ligands or donor solvents because of the relatively large radius and extra coordination sites of Sr metal.[7] To inhibit polymerization, we have synthesized heteroleptic precursors with multidentate aminoalkoxide and β-diketonate ligands. Recently, our group reported the synthesis of heteroleptic Sr complexes, which showed improved volatility and thermal stability. In particular, [Sr(demamp)(tmhd)]2[8] is a suitable atomic layer deposition (ALD) precursor for SrTiO3 (STO) thin films because of its high vapor pressure.[9]

Although the careful design and selection of ligands for the synthesis of heteroleptic Sr precursors are difficult, the development of heteroleptic precursors is important because some of these compounds showed enhanced properties such as high volatility, good stability, and selective dissociation compared to homoleptic precursors.[10] Encouraged by our previous results, we conducted further research on heteroleptic Sr precursors using aminoalkoxide and β-diketonate ligands. Herein, we report the synthesis and characterization of new heteroleptic Sr complexes using 1,3-bis(dimethylamino)propan-2-ol (bdmpH), 1,3-bis(dimethylamino)-2-methylpropan-2-ol (bdeampH), and 1-(dimethylamino)-2-((dimethylamino)methyl)butan-2-ol (dadambH) ligands and β-diketonate ligands such as 2,2,6,6-tetramethylheptan-3,5-dionate (tmhd) and 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate (hfac). The complexes [Sr(bdmp)(btsa)]2·2THF (1), [Sr(bdeamp)(btsa)]2 (2), [Sr(dadamb)(btsa)]2 (3), [Sr(bdmp)(hfac)]3 (4), [Sr(bdeamp)(hfac)]3 (5), [Sr(dadamb)(hfac)]3 (6), and [Sr3(dadamp)4(tmhd)2] (7) were prepared by controlled ligand substitution on strontium bis(trimethylsilyl)amides (Sr(btsa)2)·2L (L = DME or THF). All strontium complexes were characterized by Fourier transform infrared (FT-IR), NMR, thermogravimetric analysis (TGA), elemental analysis, and single-crystal X-ray crystallography.

Results and Discussion

New heteroleptic strontium complexes were synthesized by controlled substitution reactions using strontium bis(trimethylsilyl)amide [Sr(btsa)2]·2L (L = DME or THF) and appropriate ligands to yield the desired product.[11] As shown in Scheme , complex 1 was synthesized using Sr(btsa)2·2THF, which is more reactive than the coordinatively saturated Sr(btsa)2·2DME. The reaction of Sr(btsa)2·2L (L = DME or THF) with aminoalkoxide ligands (bdmapH, bdeampH, and dadambH) in hexane resulted in the formation of [Sr(btsa)(bdmap)]2·2THF (1), [Sr(btsa)(bdeamp)]2 (2), and [Sr(btsa)(dadamb)]2 (3), respectively, (Schemes and 2), where complexes 1–3 showed moderate yield (70–83%) as a white solid. These complexes were purified by recrystallization from hexane at −30 °C. Complexes 1–3 were treated with hfacH in hexane, and the solution was subsequently concentrated in vacuo to dryness to obtain the products [Sr(bdmp)(hfac)]3 (4), [Sr(bdeamp)(hfac)]3 (5), and [Sr(dadamb)(hfac)]3 (6) (Schemes and 4). [Sr3(dadamb)4(tmhd)2] (7) was obtained by the reaction of complex 3 with the tmhdH ligand in hexane (Scheme ). Compared to compounds 1–3, complex 4 displayed similar yield (80%) as a white solid, whereas complexes 5–7 were observed in less yield (40–56%) than complexes 1–4. We attempted to produce [Sr3(bdmap)4(tmhd)2] and [Sr3(bdeamp)4(tmhd)2] by reacting 1 and 2 with the tmhdH ligand. However, several side products that could neither be purified nor identified were obtained. Complex 7 was purified by recrystallization from toluene at −30 °C.

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

NMR

1H NMR spectra of the synthesized complexes were recorded using C6D6 as both the solvent and standard at room temperature. In the spectra of all complexes, the −OH peak was absent, whereas the spectra of 4–6 displayed broad peaks for the protons of the aminoalkoxy ligand. In the 1H NMR spectrum of 7, the peaks of tmhd protons were relatively sharper and more easily identifiable than those of 4–6. The 1H NMR spectrum of 1 in C6D6 solution revealed an upfield shift of −Si(CH3)3 (from 0.36 to 0.26 ppm) compared to that of the btsa group in Sr(btsa)2·2THF, where eight protons of THF appeared at δH = 3.54 and 1.37 ppm. Protons of −N(CH3)2 and −N(CH2)CH groups resonated at δ = 2.17 and 1.68–1.72 ppm, respectively. These peaks showed an upfield shift compared to those of the free ligand. Complexes 2 and 3 showed peaks at δH = 0.35 and 0.34 ppm for −Si(CH3)3, respectively, which was an upfield shift from δH = 0.39 of the btsa group in Sr(btsa)2·2DME.

In the aminoalkoxide ligand part of the compounds, −N(CH3)2 and −N(CH2)– resonated at δH = 2.14 and 1.86–2.09 ppm in complex 2 and 2.13 and 1.91–2.08 ppm in complex 3, respectively. The 1H NMR spectra of complexes 4–7 showed singlet peaks at δH = 6.27, 6.29, 6.24, and 5.83 ppm for the β-CH proton of hfac and tmhd ligands, respectively. The −N(CH3)2 protons in complexes 4–6 appeared as single broad peaks at δH = 2.03, 2.08, 2.09, and 2.93 ppm. In the spectrum of 7, the −C(CH3)3 moieties of the tmhd ligand appeared at δ = 1.34 ppm.

FT-IR Spectra

The FT-IR spectra of the complexes 1–3 displayed strong peaks at ν = 2945 cm–1 (1), 2944 cm–1 (2), and 2943 cm–1 (3) (Si–CH3 rocking vibration) and middle peaks at ν = 1250 cm–1 (1), 1243 cm–1 (2), and 1242 cm–1 (3) (Si–CH3 rocking vibration), respectively, showing the presence of one btsa group in 1, 2, and 3. In complexes 4–7, the FT-IR spectra showed the absence of −OH or −NH stretching peaks but displayed peaks for C=O stretching in coordinated β-diketones at 1672, 1671, 1670, and 1598, respectively; this explained that the reaction proceeded successfully, and neither free aminoalcohol nor HDMS was coordinated.

Crystal Structures

X-ray-quality crystals of the complexes were obtained from saturated hexane (1–3, 5, and 6) and toluene (7) solutions at −30 °C, where all complexes were obtained as transparent crystals. Complexes 1, 2, and 6 crystallized in the triclinic space group, complexes 5 and 7 crystallized in the monoclinic space group, and complex 3 crystallized in the orthorhombic space group (Table ).

Table 1

Crystallographic Data and Data Collection Parameters for 1–7

compound	1	2	3	5	6	7
formula weight	930.68	814.56	842.58	1316.79	1403.87	1322.5
temperature (K)	100(1)	100(1)	100(1)	100(1)	100(1)	100(1)
wavelength (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
crystal system	triclinic	triclinic	orthorhombic	monoclinic	triclinic	monoclinic
space group	P1̅	P1̅	Pbca	P21/c	P1̅	C21/c
a (Å)	10.027(2)	9.0705(5)	12.2155(9)	17.8520(7)	12.058(3)	18.0400(14)
b (Å)	10.505(2)	10.4979(6)	18.9312(14)	14.2227(5)	12.438(3)	11.5817(9)
c (Å)	12.868(3)	12.2680(7)	20.4702(16)	23.1741(9)	21.422(6)	35.591(3)
α (deg)	86.037(12)	94.399(3)	90	90	93.087(16)	90
β (deg)	72.800(10)	106.772(3)	90	97.147(2)	101.598(14)	103.789(4)
γ (deg)	81.722(11)	96.640(3)	90	90	110.133(13)	90
V (Å3)	1280.8(5)	1103.51(11)	4733.8(6)	5750.5(4)	2928.4(12)	7221.9(10)
Z	2	2	8	4	2	8
ρcalcd. (mg/m3)	1.207	1.226	1.182	1.573	1.592	1.216
μ (mm–1)	2.212	2.555	2.384	2.882	2.832	2.258
F(000)	496	432	1792	2736	1416	2816
crystal size (mm3)	0.24 × 0.10 × 0.10	0.16 × 0.14 × 0.12	0.30 × 0.30 × 0.20	0.30 × 0.14 × 0.08	0.20 × 0.10 × 0.02	0.12 × 0.08 × 0.06
theta range (deg)	1.66 to 26.73	2.47 to 29.57	1.99 to 28.34	1.68 to 27.88	1.76 to 26.02	2.11 to 27.88
index ranges	–11 ≤ h≤12,	–12 ≤ h ≤ 12,	0 ≤ h ≤ 16,	–23 ≤ h ≤ 22,	–14 ≤ h ≤ 14,	–22 ≤ h ≤ 23,
	–13 ≤ k ≤ 13,	–14 ≤ k ≤ 14,	0 ≤ k ≤ 25,	0 ≤ k ≤ 18,	–15 ≤ k ≤ 15,	–15 ≤ k ≤ 0,
	–16 ≤ l ≤ 16	–17 ≤ l ≤ 17	0 ≤ l ≤ 27	0 ≤ l ≤ 30	0 ≤ l ≤ 26	–21 ≤ l ≤ 46
indep. refl. (Rint)	5366 (0.0000)	6067 (0.0408)	5873 (0.0000)	13,714 (0.0000)	11,446 (0.0000)	8578 (0.0761)
parameters	246	201	200	701	733	346
GOF on F2	1.105	0.966	1.045	1.035	1.013	1.026
R1 [I > 2σ(I)] (a)	0.0553	0.0553	0.0346	0.0566	0.0628	0.0527
wR2 [I > 2σ(I)] (b)	0.1473	0.1362	0.0783	0.1454	0.175	0.1219

Complexes 1–3 showed dimeric structures, whereas complexes 5–7 displayed trimeric structures. In complexes 1–3, each of the strontium metal ions was bonded to one btsa group and aminoalkoxide tridentate ligands, and THF was also coordinated to the metal center in complex 1 for stability. The crystal structures of 2 and 3 displayed penta-coordinated metal centers and distorted square pyramidal structures, whereas the crystal structure of 1 displayed a hexa-coordinated metal center and trigonal prismatic structure (Figure –3). These complexes were formed by μ2–O bridging, which is similar to the previously reported group 2 metal complexes [M(aminoalkoxide)(btsa)]2 (M = Mg, Sr, and Ca) and M(aminoalkoxide)(tmhd)]2 (M = Mg, Sr, Ca, and Ba).[8] The Sr···Sr distances in the dimeric structures of 1–3 were 3.9900 (11), 3.8739 (6), and 3.8610(4) Å, respectively. These distances are longer than those of previously reported μ2–O bridged strontium bis(trimethylsilyl)amides such as [Sr(demamp)(btsa)]2 (3.8193(3) Å)[9] and [Sr(L2)]2 (3.8360(4) Å), where L1H = 5-{[2-(dimethylamino)ethyl]amino}-2,2,6,6-tetramethylhept-4-en-3-one.[12] A gradual decrease in the bridging angle was observed in complexes 1–3 because of the gradual shortening of the non-bonding metal–metal distance [∠M–O–M = 110.83(11), 108.42(10), and 107.69(5) for 1–3, respectively] (Table S1). The Sr–N bond length of the btsa group was 2.478(3) Å in 2 and 2.491(2) Å in 3; these bond lengths are slightly shorter than those of 1 (Sr–N = 2.575(4) Å), which might be attributed to higher steric hindrance and higher coordination number of the strontium metal in 1 than in 2 and 3 owing to the THF coordination in 1 (Table S1).

Figure 1

Crystal structure of [Sr(bdmap)(btsa)]2·2THF (1). Hydrogens attached to carbon and atoms in the minor component of the disordered bdmap have been omitted for clarity.

Figure 3

Crystal structure of [Sr(dadamb)(btsa)]2 (3).

Crystal structure of [Sr(bdeamp)(btsa)]2 (2).

The crystal structures of 5 and 6 showed that these complexes were trimeric with more stable hepta-coordinated metal centers. In complexes 5 and 6, each of the strontium metal ions was bonded to one hfac ligand and one aminoalkoxide ligand (Figures and 5). In complex 5, the three [Sr(bdeamp)(hfac)] fractions were held together by a combination of μ2– and μ3–O bridges between the metal atoms.

Figure 4

Crystal structure of [Sr(bdeamp)(hfac)]3 (5). Hydrogens attached to carbon and atoms in the minor component of the disordered bdeamp have been omitted for clarity.

Figure 5

Crystal structure of [Sr(dadamb)(hfac)]3 (6). Fluorine atoms in the minor component of the disordered hfac ligand have been omitted for clarity.

The two oxygen atoms (O3 and O9) of bdeamp ligands underwent μ3–O bridging with all strontium atoms, whereas the other oxygen atom (O8) of the bdeamp ligand formed a μ2–O bridge with Sr2 and Sr3 where the bond length of Sr2–O8 (2.382(3) Å) is shorter than that of Sr3–O8 (2.402(3) Å) (Table S2). To satisfy the hepta-coordination of the three strontium metals, only one of the oxygen atoms among the six hfac oxygens underwent μ2 bridging between Sr1 and Sr2. Each of the nitrogen atoms from the bdeamp ligands was bonded to a different strontium atom, which means that the bdeamp ligands did not form chelating bonds using their nitrogen atoms. The non-bonding metal center distances were Sr1···Sr2 = 3.693(5) Å and Sr2···Sr3 = 3.556(5) Å in 5 and Sr1···Sr2 = 3.660(4) Å and Sr2···Sr3 = 3.547(11) Å in 6, which are similar to those of [Sr3(tmhd)3(OSiPh3)3] (Sr···Sr 3.588(2) Å).[12] These strontium pairs were connected by three oxygen bridges. In contrast, Sr1···Sr3 = 3.969(5) Å in 5 and Sr1···Sr2 = 3.977 Å in 6, with two sets of O-bridges, are longer than the other Sr···Sr non-bonding lengths and comparable to those in [Sr(emeamp)(tmhd)]3[13] and [Sr(memp)(tmhd)]3[13] (average Sr···Sr is 4.023 Å). In complexes 5 and 6, the metal–oxygen bond lengths range from 2.372(4) to 2.707(4) Å, with the shortest bond length in 6 (Sr3···O6). The average distances between strontium metal and μ3-bridging oxygen atoms were 2.546 Å in 5 and 2.543 Å in 6.

The Sr–O bonds with non-bridging oxygen atoms of the hfac ligands were 2.567(3) Å (Sr2···O4) in 5 and 2.532(4) Å (Sr3···O8) in 6, whereas the μ2-bridging oxygen atom of the hfac ligand showed a bond length of 2.662(3) Å (Sr2···O3) in 5 and 2.707(4) Å (Sr3···O7) in 6. Complex 7 showed a trimeric structure, where each Sr atom was either a hexa-coordinated (Sr1) or hepta-coordinated (Sr2 and Sr2i) metal center.

In complex 7, the side Sr ions (Sr2 and Sr2i) were bonded to one tmhd ligand and one dadamb ligand, whereas the middle Sr ion (Sr1) was bonded to two dadamb ligands (Figure ). The three strontium atoms were connected through four μ2–O bridging atoms using alkoxy oxygens from the dadamb ligands. In 7, the dadamb ligands were coordinated to metal centers in two different ways. The nitrogen atoms from the two dadamb ligands were coordinated to the same strontium metal centers (Sr2 and Sr2i), whereas the nitrogen atoms of other dadamb ligand were coordinated to two different strontium (Sr1 and Sr2 or Sr1 and Sr2i) metal centers, which is similar to the bonding in complexes 5 and 6. Metal centers Sr2 and Sr2i had a distorted octahedral geometry, whereas Sr1 had a distorted trigonal prismatic geometry. The non-bonding distance of Sr1···Sr2i (3.879(5) Å) in 7 was longer than that of strontium β-diketonate complexes [Sr3(thd)6] (3.826(1) Å)[14] and [Sr4(tmod)8] (3.785(4) Å).[7] In complex 7, the average distances between the μ2–O atoms and strontium metal centers were 2.457 Å. The Sr–O bonds of the tmhd ligands showed an average bond length of 2.477 Å.

Figure 6

Crystal structure of [Sr3(damdamb)4(tmhd)2] (7).

Thermogravimetric Analyses

TGA of complexes 4–7 was conducted from room temperature to 500 °C under a constant flow of nitrogen to avoid any air contact (Figure ). Complexes 4 and 6 showed similar mass loss patterns in their TGA curves, that is, a three-step mass loss. In the first step, the TGA plots of complexes 4 and 6 displayed 12 and 20% mass loss, respectively, from 25 to 150 °C. In the second step, in the 150–250 °C temperature range, the observed mass losses were 28 and 27%, respectively, for 4 and 6. In the final step (250–315 °C range), further mass losses of 20 and 21% and residual masses of 40 and 30% were observed, respectively. Thus, TGA showed that complexes 4 and 6 were thermally unstable, and the ligand fractions might have been broken at these temperatures.

Figure 7

TGA plots of complexes 4 (black), 5 (red), 6 (blue), and 7 (green).

The TGA plots of complexes 5 and 7 displayed two-step weight loss. In the first step, complexes 5 and 7 showed 22% mass loss from 25 to 78 °C and 18% mass loss from room 25 to 250 °C, respectively. Further mass losses of 42 and 50% were observed in these compounds up to 500 °C. Complexes 4–7 showed non-volatile residual masses of 40, 36, 30, and 29%, respectively, where the values for compounds 5–7 were similar to the calculated value of SrCO3 from deposition (SrO = 23, 22, 22, and 23% and SrCO3 = 33, 32, 32, and 33% in 4–7, respectively). To confirm the abovementioned calculations, X-ray diffraction analysis of the residual powder of 7 was conducted, which confirmed that SrCO3 was formed by thermal decomposition (Figure S1).

Experimental Section

General Remarks

NMR spectra were recorded with a Bruker 500 MHz spectrometer (1H) and a Bruker 500 MHz spectrometer (13C) with C6D6 as the solvent and reference. IR spectra were obtained using a Nicolet Nexus FTIR spectrophotometer. Elemental analyses were carried out using a Thermo Scientific OEA Flash 2000 analyzer. Thermogravimetric analyses were conducted using a SETARAM 92-18 TG-DTA instrument with a constant flow of nitrogen (500 mL/min) throughout the experiment.

Sr[N(SiMe3)]2·L2 (L = DME or THF) was prepared following previously reported methods.[11] All reactions were carried out under inert dry conditions in an argon-filled glove box. Hexane and toluene were purified using an Innovative Technology PS-MD-4 solvent purification system. All other chemicals were purchased from Aldrich and Alfa Aesar without further purification. Melting points were measured usingthe Stuart SMP40 automatic melting point apparatus.

General Procedure for the Synthesis of [Sr(aminoalkoxide ligand)(btsa)]2

A hexane solution (10 mL) of the aminoalcohol ligand was added dropwise to a solution of Sr[(btsa)]2·L2 (L= DME or THF) in hexane (20 mL) at room temperature with constant stirring. The reaction mixture was stirred for 15 h. Subsequently, it was filtered, and the volatiles were removed in vacuo to obtain the product as a white solid. X-ray-quality crystals were grown from a saturated solution in hexane at −30 °C.

[Sr(bdmap)(btsa)]2·2THF (1). Sr(btsa)2·2THF (0.55 g, 1.0 mmol) and bdmapH (0.14 g, 1.0 mmol) were used. Yield: 0.35 g (80%), mp 110 °C. 1H NMR (500 MHz, C6D6): δ 0.36 [s, 18H, 2Si(CH3)3], 1.37 [m, 4H, OCH2(CH2)2], 1.68–1.72 [m, 4H, N(CH2)CH], 2.17 [s, 12H, (CH3)2NCH], 3.54 [m, 4H, O(CH2)CH2], 4.12 [m, 1H, CH2(CH)CH2] ppm. 13C NMR (125 MHz, C6D6): δ 6.44 [Si(CH3)3], 25.55 [OCH2(CH2)], 44.05 [CH2N(CH3)2], 66.77 [N(CH2)CH], 68.43 [O(CH2)CH2], 69.13 [CH2(CH)CH2] ppm. FT-IR (KBr, cm–1): 2945 (s), 2854.55 (m), 2822 (s), 2783 (s), 1466 (m), 1250 (m), 1236 (w), 1134 (m), 990 (s), 821 (s), 737 (w). C34H86N6O4Si2Sr2 (874.49) Calcd: C, 43.88; H, 9.31; N, 9.03. Found: C, 43.50; H, 9.04; N, 9.73.

[Sr(bdeamp)(btsa)]2 (2). Sr(btsa)2·2DME (0.59 g, 1 mmol) and bdeampH (0.16 g, 1 mmol) were used. Yield: 0.29 g (73%), mp 130 °C. 1H NMR (500 MHz, C6D6): δ 0.34 [s, 18H, 2Si(CH3)3], 1.26 [s, 3H, OC(CH3)], 1.86–2.09 [m, 4H, N(CH2)CCH3], 2.14 [s, 12H, CH2N(CH3)2] ppm. 13C NMR (125 MHz, C6D6): δ 6.30 [Si(CH3)3], 30.97 [(CH3)CCH2N], 48.53 [CH2N(CH3)2], 73.06 [CH2(C)CH3], 74.41 [(CH3)2NCH2] ppm. FT-IR (KBr, cm–1): 2944 (s), 2859 (w), 2825 (w), 1457 (m), 1243 (m), 1079 (s), 881 (m), 822 (s). C28H74N6O2Si4Sr2 (814.51) Calcd: C, 41.29; H, 9.16; N, 10.32. Found: C, 41.53; H, 9.06; N, 9.47.

[Sr(dadamb)(btsa)]2 (3). Sr(btsa)2·2DME (0.59 g, 1 mmol) and dadambH (0.17 g, 1 mmol) were used. Yield: 0.35 g (83%), mp 130 °C. 1H NMR (500 MHz, C6D6): δ 0.35 [s, 18H, 2 Si(CH3)3], 0.80 [t, 3H, (CH3)CH2C], 1.62 [q, 2H, CH3(CH2)C], 1.91–2.08 [m, 4H, N(CH2)C], 2.13 [s, 12H, (CH3)2NCH2] ppm. 13C NMR (125 MHz, C6D6): δ 6.42 [Si(CH3)3], 10.24 [(CH3)CH2C], 35.59 [CH3(CH2)C], 48.59 [(CH3)2NC], 70.00 [CH2(C)CH2], 75.50 [N(CH2)C] ppm. FT-IR (KBr, cm–1): 2943 (s), 2861 (m), 2828 (m), 1463 (m), 1242 (m), 1146 (w), 1084 (m), 992 (w), 823 (m). C30H78N6O2Si4Sr2 (842.56) Calcd: C, 42.77; H, 9.33; N, 9.97. Found: C, 43.51; H, 9.23; N, 10.30.

General Procedure for the Synthesis of [Sr(aminoalkoxide)(hfac)]3

A hexane solution (10 mL) of hfacH was added dropwise to a solution of 1, 2, and 3 in hexane (30 mL) at room temperature with constant stirring. After stirring for 15 h, the reaction mixture was filtered, and the volatiles were removed in vacuo to obtain the product as a white solid. X-ray-quality crystals were grown from a saturated solution in hexane or toluene at −30 °C.

[Sr(bdmp)(hfac)]3 (4). 1 (0.80 g, 1 mmol) and hfacH (0.406 g, 2 mmol) were used. Yield: 0.59 g (80%), mp 118 °C. 1H NMR (500 MHz, C6D6): δ 1.82–2.14 [br m, 4H, C(CH)2N], 2.03 [s, 12H, (CH3)CCH2], 4.03 [s, 1H, CH2(CH)CH2], 6.23 [s, 1H, β-CH (Hfac)] ppm. 13C NMR (125 MHz, C6D6): δ 45.89 [CH2N(CH3)2], 67.11 [(CH3)2NCH2], 67.71 [CH2(C)CH3], 88.72 [CO(CH)CO], 120.11 [(CF3)COCH], 176.74 [CF3(CO)CH] ppm. 1F NMR (470.54 MHz, C6D6): δ −76.85 [C(CF3)] ppm. FT-IR (KBr, cm–1): 2958 (w), 2862 (w), 2832 (w), 2794 (w), 1672 (s), 1531 (s), 1469 (m), 1259 (s), 1191 (s), 1140 (s), 995 (m), 906 (w), 794 (w), 662 (w). C36H57N6O9Sr3F18 (1319.73) Calcd: C, 32.69; H, 4.34; N, 6.35. Found: C, 32.91; H, 4.51; N, 6.23.

[Sr(bdeamp)(hfac)]3 (5). 2 (0.81 g, 1 mmol) and hfacH (0.406 g, 2 mmol) were used. Yield: 0.51 g (56%), mp 120 °C. 1H NMR (500 MHz, C6D6): δ 1.29 [s, 3H, (CH3)CCH2], 2.08 [br s, 12H, (CH3)2NCH2], 2.08–2.28 [br m, 4H, C(CH)2N], 6.27 [s, 1H, β-CH (Hfac)] ppm. 13C NMR (125 MHz, C6D6): δ 30.39 [(CH3)CCH2], 48.45 [CH2N(CH3)2], 74.07 [CH2(C)CH3], 75.06 [(CH3)2NCH2], 89.11 [CO(CH)CO], 120.16 [(CF3)COCH], 176.08 [CF3(CO)CH] ppm. 1F NMR (470.54 MHz, C6D6): δ −76.61 [C(CF3)] ppm. FT-IR (KBr, cm–1): 2959 (w), 2863 (w), 2836 (w), 2793 (w), 1671 (s), 1530 (s), 1512 (s), 1459 (m), 1258 (s), 1204 (s), 1144 (s), 1088 (s), 1011 (w), 937 (w), 794 (w), 661 (m), 579 (w). C39H60N6O9Sr3F18 (1361.76) Calcd: C, 34.40; H, 4.44; N, 6.17. Found: C, 33.77; H, 4.45; N, 5.79.

[Sr(dadamb)(hfac)]3 (6). 3 (0.84 g, 1 mmol) and Hfac (0.406 g, 2 mmol) were used. Yield: 0.42 g (45%), mp 120 °C. 1H NMR (500 MHz, C6D6): δ 0.74 [br t, 3H, (CH3)CH2C], 1.73 [br q, 2H, CH3(CH2)C], 2.09 [br s, 12H, (CH)2NCH2], 2.21 [m, 4H, N(CH)C], 6.29 [s, 1H, β-CH (hfac)] ppm. 13C NMR (125 MHz, C6D6): δ 9.39 [(CH3)CH2C], 36.83 [CH3(CH2)C], 48.21 [(CH3)2NC], 69.57 [CH2(C)CH2], 77.16 [N(CH2)C], 89.11 [CF3(CO)CH], 120.13 [CF3(C)CH], 177.07 [CF3(CO)CH] ppm. 1F NMR (470.54 MHz, C6D6): δ −76.61 [C(CF3)] ppm. FT-IR (KBr, cm–1): 2972 (w), 2835 (w), 1670 (m), 1530 (m), 1515 (m), 1463 (w), 1257 (m), 1197 (m), 1144 (s), 793 (w), 660 (w), 579 (w). C42H66F18N6O9Sr3 (1403.84) Calcd: C, 35.93; H, 4.74; N, 5.99. Found: C, 35.84; H, 4.86; N, 5.78.

General Procedure for the Synthesis of [Sr3(aminoalkoxide)4(tmhd)2]

A hexane solution (10 mL) of tmhdH was added dropwise to a solution of 3 in hexane (30 mL) at room temperature with constant stirring. After stirring for 15 h, the reaction mixture was filtered, and the volatiles were removed in vacuo to obtain the product as a white solid. X-ray-quality crystals were grown from a saturated solution in toluene at −30 °C.

[Sr3(dadamb)4(tmhd)2] (7). 3 (0.84 g, 1 mmol) and tmhdH (0.368 g, 2 mmol) were used. Yield: 0.40 g (45%), mp 128 °C. 1H NMR (500 MHz, C6D6): δ 0.89 [t, 3H, (CH3)CH2C], 1.34 [s, 18H, (CH3)3CCH], 1.73 [q, 2H, CH3(CH2)C], 2.32–2.33 [m, 4H, N(CH2)C], 2.39 [s, 12H, (CH3)2NCH2], 5.83 [s, 1H, β-CH (Hfac)] ppm. 13C NMR (125 MHz, C6D6): δ 10.20 [(CH3)CH2C], 29.28 [(CH3)3CCO], 39.17 [(CH3)3CCO], 41.16 [CH3(CH2)C], 48.06 [(CH3)2NCH2], 49.14 [(CH3)2NCH2], 71.45 [CH3(CH2)C], 75.03 [N(CH2)C], 88.06 [CO(CH)CO], 197.38 [CH(CO)2C] ppm. FT-IR (KBr, cm–1): 2957 (s), 2862 (m), 2820 (m), 2784 (m), 1598 (m), 1536 (w), 1459 (s), 1414 (s), 1365 (m), 1204 (m), 1188 (m), 1008 (m), 976 (m), 862 (m), 469 (m). C56H148N8O8Sr3 (1322.49) Calcd: C, 52.67; H, 9.30; N, 8.47. Found: C, 52.55; H, 9.45; N, 7.63.

Crystallography

Single crystals of 1–3 and 5–7 complexes were grown from their saturated solutions in hexane and toluene at −30 °C. In a typical procedure, a specimen of suitable size and quality was obtained from the solution, coated with Paratone oil, and mounted on a glass capillary. Reflection data were collected using a Bruker SMART Apex II-CCD area detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The hemisphere of the reflection data was collected as ω-scan frames with 0.3° per frame and an exposure time of 10 s per frame. Cell parameters were determined and refined using the SMART program.[15] Data reduction was performed using SAINT software.[16] The data were corrected for Lorentz and polarization effects, and an empirical absorption correction was applied using the SADABS program.[17] The structures of the prepared compounds were solved by direct methods, and all non-hydrogen atoms were subjected to anisotropic refinement by the full-matrix least-squares method on F2 using the SHELXTL/PC package.[18] Hydrogen atoms were placed at their geometrically calculated positions and refined based on the corresponding carbon atoms with isotropic thermal parameters. CCDC 2052996–2053001 for complexes 1–3 and 5–7 contains the supplementary crystallographic data for this article.

Conclusions

New heteroleptic complexes of strontium with aminoalkoxide and β-diketonate ligands were successfully synthesized and characterized. All strontium complexes were prepared by a controlled substitution reaction with Sr(btsa)2·L2 (L = DME or THF). The partially substituted compounds 1–3 were dimers, whereas compounds 4–7 were obtained as trimers. In complexes 1–3, the penta- or hexa-coordinated metal centers appeared to have distorted square pyramidal (1 and 2) and trigonal prismatic geometry (3). In these complexes, the alkoxy oxygen of the aminoalkoxide acted as a μ2–O bridge between the two metal centers. In complexes 5–7, all strontium atoms were connected by μ2–O and μ3–O in hexa- and hepta-coordination states. Strontium metal centers appeared to be distorted capped octahedra [5–6 and 7 (side Sr)] and distorted trigonal prisms. The TGA curves for complexes 5–7 displayed two-step weight loss in the 25–350 °C region.