Alkaline-Earth Metal Bis[bis(trimethylsilyl)amide] Complexes with

Oct 24, 2018 - The reaction of [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 with 2,2,5,5-tetramethyltetrahydrofuran in pentane yields the mononuclear complexes ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Alkaline-Earth Metal Bis[bis(trimethylsilyl)amide] Complexes with Weakly Coordinating 2,2,5,5-Tetramethyltetrahydrofuran Ligands Sven Krieck,*,# Philipp Schüler,# Helmar Görls, and Matthias Westerhausen* Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstraße 8, D-07743 Jena, Germany

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S Supporting Information *

ABSTRACT: The reaction of [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 with 2,2,5,5-tetramethyltetrahydrofuran in pentane yields the mononuclear complexes [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Mg (1), Ca (2), Sr (3), and Ba (4)) with three-coordinate alkaline-earth metal centers. With increasing radius of the alkaline-earth metal atoms, the N−Ae−N bond angles decrease. These ether adducts significantly enhance the solubility of the bis(trimethylsilyl)amides of the alkaline-earth metals in hydrocarbon solvents. Contrary to the magnesium derivative 1, the heavier congeners dissociate into mononuclear [Ae{N(SiMe3)2}2] and free Me4THF without formation of sparingly soluble dinuclear [(Me3Si)2NAe{μ-N(SiMe3)2}]2.

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INTRODUCTION The bis(trimethylsilyl)amides of the alkaline-earth metals Ae are valuable and widely used reagents for diverse stoichiometric and catalytic applications like metalation and transamination reactions and as catalysts for polymerization and functionalization reactions.1−3 Due to the need for soluble alkaline-earth metal reagents to maintain homogeneous reaction conditions, several procedures for the synthesis of the alkaline-earth metal bis[bis(trimethylsilyl)amides] were developed independently in the beginning of the 1990s. Transmetalation of Hg[N(SiMe3)2]24 and Sn[N(SiMe3)2]25 as well as metathetical approaches using calcium alkoxides,6 sulfonates,7,8 and iodides9−11 represent suitable procedures. Direct metalation of HN(SiMe3)2 with the alkaline earth metals is challenging, but the use of activated calcium6 or of ammonia-containing solvents12,13 allow direct metalation protocols. This direct metalation can also be accelerated by addition of BiPh3 under ultrasonification.14 The metathetical approach using the alkaline-earth metal iodides requires a strict stoichiometric addition of KN(SiMe3)2 to avoid formation of calciates of the type K{Ca(NR2)3} (excess of KN(SiMe3)2) as well as isolation of halide- or pseudohalide-containing products (substoichiometric KN(SiMe3)2).15,16 The ether-free complexes of magnesium,17 calcium,18 strontium,19 and barium bis[bis(trimethylsilyl)amide]12 crystallize as dinuclear complexes of the type [(R2N)Ae(μ-NR2)]2 with a four-membered Ae2N2 ring and three-coordinate metal atoms. In tetrahydrofuran, the bis(thf) adducts of magnesium,4 calcium,20 strontium,20 and barium12 are formed. Removal of one thf ligand leads to the formation of dinuclear complexes of the type [(R2N)Ae(thf)(μ-NR2)]2 for the heavier alkalineearth metals.6,12 Despite the small coordination numbers, in all these derivatives the alkaline-earth metal atoms are effectively © XXXX American Chemical Society

shielded by the bulky amido and the thf ligands. This behavior presumably lowers the reactivity of these alkaline-earth metal bis[bis(trimethylsilyl)amides] as a result of the hindered coordination of substrate molecules. For solubility reasons, the metathetical approaches must be performed in ethereal solvents, and hence only ether adducts are isolated. Ether-free complexes maintain their dinuclear structures in aromatic hydrocarbons even though an exchange equilibrium between terminally bound and bridging amido ligands has been observed.5 In alkanes the solubility is significantly smaller. Maintenance of low coordination numbers succeeds with bulky and weakly binding ether molecules like 2,2,5,5tetramethyltetrahydrofuran (Me4thf). Thus, ether-free LiN(SiMe3)2 has been observed in this ether and its tetrameric congener with an eight-membered Li4N4 ring precipitates from this solvent (Scheme 1).21 The bis(trimethylsilyl)amides of the larger alkali metals A form dinuclear compounds of the type [(Me4thf)A{μ-N(SiMe3)2}]2 with one Me4thf ligand bound at each metal atom.22 In solution, these complexes dissociate, and free Me4thf molecules are found in hydrocarbons. This bulky cyclic ether attracted our interest also in the coordination chemistry of the alkaline-earth metal bis[bis(trimethylsilyl)amides] to stabilize low coordination numbers at the metal ions. The weak binding nature at electropositive metal ions could ease dissociation in solution making the Ae− N bonds more accessible to substrate molecules.

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RESULTS AND DISCUSSION Synthesis. The alkaline earth metal bis[bis(trimethylsilyl)amides] were prepared via the metathetical approach (Ca)

Received: August 31, 2018

A

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Outcome of the Crystallization of Alkali Metal Bis(trimethylsilyl)amides after Addition of 2,2,5,5Tetramethyltetrahydrofuran (see Text)

Molecular Structures. All derivatives show very similar molecular structures of the type [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Mg (1), Ca (2), Sr (3), and Ba (4)). The molecular structures of the calcium and barium compounds are depicted in Figures 1 and 2, respectively, whereas the structures of the

from AeI2 and 2 equiv of KN(SiMe3)2 in diethyl ether, via direct metalation of HN(SiMe3)2 with alkaline earth metals (Sr, Ba) or via magnesiation of bis(trimethylsilyl)amine with commercially available dibutylmagnesium in tetrahydrofuran (Scheme 2). Crystallization from a mixture of pentane and 2,2,5,5-tetramethyltetrahydrofuran yielded the mononuclear complexes of the type [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Mg (1), Ca (2), Sr (3), and Ba (4)), which were very soluble in aromatic and aliphatic hydrocarbons.

Figure 1. Molecular structure and atom labeling scheme of [(Me4thf)Ca{N(SiMe3)2}2] (2). The ellipsoids represent a probability of 30%, H atoms are neglected for the sake of clarity. Atoms generated by crystallographic C2 symmetry are marked with the letter “A”. Selected bond lengths (pm) of 2 (Ae = Ca) [and of isotypic 3 (Ae = Sr)]: Ae1−N1 226.66(12) [240.90(13)], Ae1−O1 234.27(14) [249.61(14)], N1−Si1 169.16(12) [168.43(14)], N1−Si2 169.67(13) [168.97(14)]; angles (deg.): N1−Ae1−N1A 128.34(6) [124.43(6)], N1−Ae1−O1 115.83(3) [117.79(3)], Ae1−N1−Si1 112.36(6) [113.35(7)], Ae1−N1−Si2 121.51(6) [117.98(7)], Si1− N1−Si2 125.81(7) [128.40(8)].

Scheme 2. Preparation of the Me4thf Adducts of the Alkaline-Earth Metal Bis[Bis(trimethylsilyl)amides] via Metalation (1) and Salt Metathesis Reactions (2−4)

magnesium and strontium congeners are shown in the Supporting Information. The three-coordinate alkaline-earth metal atoms are in distorted trigonal-planar environments, whereas the barium atom is in a slightly pyramidal coordination sphere (angle sum at Ba1 352.68°). Furthermore, the O1−Mg−N1/2 bond angles (115.54(10)° and 113.98(10)°)) are very similar; for the calcium and strontium complexes the O1−Ae1−N1/1A angles are identical due to crystallographic C2 symmetry. Contrary to this finding, the proximal and distal O1−Ba1−N1/2 bond angles differ significantly by 26.4°. Due to the small coordination number of three, free coordination sites are available at the metal atoms. Therefore, small Ae···Si distances are observed. With increasing radius of the alkaline-earth metal atom, available space for this kind of agostic interaction between the bis(trimethylsilyl)amido ligands and the metal centers increases. Therefore the differences between the proximal and distal Ae1-N-Si bond angles diminish with the size of the metal atoms (Mg 8.9°, Ca 9.2°, Sr 4.6°, Ba 1.3°) and the barium atom forms agostic bonds to all trimethylsilyl groups with rather similar Ba···Si distances between 358.05(4) and 364.46(5) pm, whereas calcium and strontium show rather different proximal and distal Ca···Si (330.40(5), 346.93(4) pm) and Sr···Si (344.32(7), 353.23(7) pm) values. The decreasing electronegativity of the alkaline-earth metals with increasing radius leads to enhanced ionic character of the B

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Table 1. Comparison of Selected Structural Parameters (Average Values) of [(Me4thf)Ae{N(SiMe3)2}2] (1−4) and [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 with Three-Coordinate Metal Centers as well as of [(thf)2Ae{N(SiMe3)2}2] with Tetra-Coordinate Ae Atoms Ae

Mg

[(Me4thf)Ae{N(SiMe3)2}2] Ae−N 199.1 Ae−O 204.8 N−Ae−N 130.5 N−Si 171.3 Si−N−Si 120.9 [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2a Ae−Nt 197.5 Ae−Nbr 215.1 Nt−Si 171.2 Nbr−Si 177.3 Si−Nt−Si 119.0 Si−Nbr−Si 110.8 ref 17 [(thf)2Ae{N(SiMe3)2}2] Ae−N 202.1 Ae−O 209.4 N−Ae−N 127.9 N−Si 170.6 Si−N−Si 121.0 ref 4

Figure 2. Molecular structure and atom labeling scheme of [(Me4thf)Ba{N(SiMe3)2}2] (4). The ellipsoids represent a probability of 30%, H atoms are omitted for clarity reasons. Selected bond lengths (pm): Ba1−N1 255.21(13), Ba1−N2 259.11(13), Ba1−O1 278.75(11), N1−Si1 168.20(13), N1−Si2 168.46(13), N2−Si3 167.92(14), N2−Si4 168.47(14); angles (deg.): N1−Ba1−N2 112.05(4), N1−Ba1−O1 107.14(4), N2−Ba1−O1 133.49(4), Ba1− N1−Si1 115.89(6), Ba1−N1−Si2 113.79(6), Si1−N1−Si2 130.31(8), Ba1−N2−Si3 115.53(6), Ba1−N2−Si4 114.97(7), Si3− N2−Si4 129.49(8).

Ca

Sr

Ba

226.7 234.3 128.3 169.4 125.8

240.9 249.6 124.4 168.7 128.4

257.2 278.8 112.1 168.3 129.9

227.5 247.5 169.7 173.3 122.3 117.3 18

243 263 169 171 123.2 128.2 19

257.6 282.2 168.8 170.3 127.5 131.1 12

230.2 237.7 121.3 168.6 126.3 20

245.8 253.4 120.6 167.4 132.3 20

259.2 273.1 116.8 168.0 131.7 12

a

Subscripts t or br mark terminally bound are bridging bis(trimethylsilyl)amido ligands.

Ae−N bonds from Mg to Ba. This enhanced heteropolar character enhances the negative charge on the nitrogen atoms and hence, the backdonation of charge from the p lone pair at the planar N atoms into the σ*(Si−C) orbitals of the trimethylsilyl groups. This interpretation is in agreement with the observation that the average N−Si bond lengths decrease from 1 to 4 (1: 171.3, 2: 169.4, 3: 168.7, and 4: 168.3 pm). This shortening also enhances the steric repulsion between the bulky trimethylsilyl substituents leading to larger Si−N−Si bond angles for more electropositive metals (1: 120.9°, 2: 125.8°, 3: 128.4°, and 4: 129.9°). In Table 1, selected structural parameters of these complexes are compared with those of [(thf)2Ae{N(SiMe3)2}2] with tetra-coordinate metal centers and those of coligand-free dinuclear complexes [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 with three-coordinate alkaline-earth metal atoms. Some peculiarities are remarkable. The Ae−N bonds in [(Me4thf)Ae{N(SiMe3)2}2] complexes with exception of the Mg derivative 1 are shorter than in the other complex types. In 1 intramolecular strain between the bulky bis(trimethylsilyl)amido ligands hinders a closer approach to the Mg center. Due to the small coordination number of three, smaller Ae−O distances are observed compared to [(thf)2Ae{N(SiMe3)2}2]. Here, the barium congener is an exception. The reason is lack of steric hindrance due to the large radius of the Ba2+ ions; therefore, the stronger Lewis base thf binds more tightly than the weaker base Me4thf. In all compounds decreasing electronegativity of the Ae atoms leads to a shortening of the N−Si bonds. This is also true for the bridging bis(trimethylsilyl)amido ligands in [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 with sp3 hybridized Nbr atoms due to an enhanced electrostatic attraction between the nitrogen bases and the silicon atoms.

The N−Ae−N bond angles decrease with increasing radius of the alkaline-earth metal ions. In general the smaller coordination number of three allows larger angles in comparison of the Me4thf adducts with the distorted tetrahedral complexes [(thf)2Ae{N(SiMe3)2}2]. The barium derivative 4 shows an exceptionally small N−Ba−N angle. Van-der-Waals attraction between the trimethylsilyl groups define an optimal distance between the bis(trimethylsilyl)amido ligands. Assuming a fixed distance between the trimethylsilyl groups, increasing Ae−N distances lead to more acute N−Ae−N angles. Furthermore, ab initio calculations at unsolvated monomeric [Ae(NH2)2] molecules predict linear N−Ca−N units but bent N−Ae−N moieties for Sr (131.7°) and Ba (118.4°) due to significant involvement of d-orbitals at the heavy alkaline-earth metals in the Ae−N bonding situations.23 Upon coordination of the four Lewis bases HF with restricted linear H−F−Ba alignments to exclude hydrogen bridges, the N−Ba−N bond angle (117.2°) in [(HF)4Ba(NH2)2] with the energetically favored structure remained nearly unchanged, which was interpreted in the sense that the more strongly bound anionic ligands determine the molecular structure, whereas neutral and more loosely bound coligands occupy vacant coordination sites.24 Coordination of electroneutral Lewis bases slightly reduces d-ordital participation at the alkaline-earth metal from 11% for [Ba(NH2)2] to approximately 8% for its tetra(HF) adduct. These ab initio calculations also verify a decreasing d-orbital influence for strontium and calcium congeners with a preferred linear N− Ca−N arrangement for [Ca(NH2)2]23 as well as [(HF)4Ca(NH2)2].24 In Table 2, the Ae−N bond lengths and N−Ae−N bond angles of the complexes 2, 3, and 4 are compared with C

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Comparison of Selected Structural Parameters of [(Me4thf)Ae{N(SiMe3)2}2] (2−4) with ab Initio Calculated Structures of the Corresponding Amides [Ae(NH2)2]23 and [(HF)4Ae(NH2)2]24 [(Me4thf)Ca{N(SiMe3)2}2] [Ca(NH2)2] [(HF)4Ca(NH2)2] [(Me4thf)Sr{N(SiMe3)2}2] [Sr(NH2)2] [(Me4thf)Ba{N(SiMe3)2}2] [Ba(NH2)2] [(HF)4Ba(NH2)2]

Ae−Na

N−Ae−N

226.7 222.0 227.3 240.9 234.7 257.2 248.7 252.7

128.3 180 177.8 124.4 131.7 112.1 118.4 117.2

Scheme 3. Dissociation Equilibria of the Complexes [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Ca, Sr, and Ba) in Hydrocarbon Solvents

ref 23 24 23 23 24

a

Bond lengths (pm) and bond angles (deg.).

In agreement with the DOSY spectra and the finding that the chemical shifts of Me4thf depend on the metal atom, an increasing dissociation with increasing radius of the alkalineearth metal is obvious. In the Mg complex 1 no dissociation takes place, whereas in the barium congener 4 a far-reaching dissociation is evident and the chemical 1H NMR shifts of the Me4thf ligands are nearly identical to those of the free ether. This trend is even more explicit for the resonances of the tertiary carbon atoms bound at the ether oxygen atom (Figure 3). Looking from molecular [(Me4thf)Mg{N(SiMe3)2}2] (1, δ

the values of the calculated structures of [Ae(NH2)2]23 and [(HF)4Ae(NH2)2] (Ae = Ca, Sr, and Ba).24 NMR Experiments. The NMR spectra were recorded at [D6]benzene solutions of [(Me4thf)Ae{N(SiMe3)2}2] (1−4); the values for the bis(trimethylsilyl)amido ligands are listed in Table 3. The chemical shifts are characteristic for terminally Table 3. Selected NMR Data of the Bis(trimethylsilyl)amido Groups of [(Me4thf)Ae{N(SiMe3)2}2] (1−4) Ae

Mg

Ca

Sr

Ba

δ( H)TMS δ(1H)Me,Me4thfb δ(1H)CH2,Me4thfb δ(13C{1H})TMSa δ(13C{1H})Me,Me4thfc δ(13C{1H})CH2,Me4thfc δ(13C{1H})OC,Me4thfc δ(29Si{1H}) 1 J(C,Si)

0.37 1.30 1.30 6.8 29.7 38.5 89.2 −7.0 52.6

0.34 1.13 1.29 6.0 29.8 38.2 85.7 −14.4 52.4

0.31 1.11 1.37 6.2 29.9 38.5 82.9 −16.7 n.o.d

0.27 1.14 1.52 5.5 30.0 38.8 81.4 −18.8 52.1

1

a

a Trimethylsilyl TMS. b1H NMR shifts of free Me4THF: δ 1.19 (Me), 1.59 (CH2). c13C{1H} NMR shifts of free Me4THF: δ 30.1 (Me), 39.1 (CH2), 80.6 (OC). dNot observed.

bound bis(trimethylsilyl)amido groups with significantly increasing high-field shift of the 29Si resonances. This behavior is less pronounced for the 1H and 13C{1H} NMR resonances with rather similar shifts for the Ca and Sr derivatives. The DOSY experiments of these complexes showed that the magnesium complex 1 remained intact in benzene solution based on the fact that ligated Me4thf and the trimethylsilyl groups exhibited the same diffusion constant. Contrary to this finding, the complexes of the heavier alkaline-earth metal complexes partly dissociated in solution and different diffusion coefficients were observed for Me4thf molecules and the SiMe3 groups (Scheme 3). The DOSY spectra of these complexes are depicted in the Supporting Information. However, dimerization of the alkaline-earth metal bis[bis(trimethylsilyl)amides] and formation of dinuclear compounds of the type [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 were not observed because only terminally bound bis(trimethylsilyl)amido ligands were detected in benzene solution. Furthermore, in aliphatic hydrocarbons the dinuclear complexes [(Me3Si)2N-Ae{μN(SiMe3)2}]2 are only sparingly soluble, but no precipitate is observed at concentrated solutions of [(Me4thf)Ae{N(SiMe3)2}2] in pentane.

Figure 3. Resonances of the tertiary carbon atoms of Me4thf in [(Me4thf)Ae{N(SiMe3)2}2] of magnesium (1, blue), calcium (2, red), strontium (3, violet), and barium (4, light green) and for comparison of free 2,2,5,5-tetramethyltetrahydrofuran (dark green).

= 89.2) to the heaviest homologous congener 4 (δ = 81.4), the increasing existence of the dissociated species is self-evident.

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CONCLUSION The complexes [(Me4thf)Ae{N(SiMe3)2}2] of magnesium (1), calcium (2), strontium (3), and barium (4) are available via addition of 2,2,5,5-tetramethyltetrahydrofuran to a suspension of [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 in pentane leading to a clear solution. This procedure verifies an enhanced solubility of these ether adducts compared to the unsolvated dinuclear complexes. In the crystalline state, the metal atoms are in distorted trigonal planar (Mg, Ca, and Sr) or slightly pyramidalized (Ba) environments. The N−Ae−N bond angles D

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

strontium, and barium. This behavior is in agreement with the rather low Lewis basicity of 2,2,5,5-tetramethyltetrahydrofuran. For organolithium complexes, it is well-known that smaller aggregates commonly exhibit enhanced reactivity.25 In future studies the reactivity of [(Me4thf)Ae{N(SiMe3)2}2] will be compared with unsolvated [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2 and with [(thf)2Ae{N(SiMe3)2}2] to elucidate the influence of aggregation degree and shielding of the alkaline-earth metals on chemical response.

decrease with increasing radius of the alkaline-earth metal atoms (Figure 4). In complexes of the type [(thf)2Ae{N-

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EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out in an inert nitrogen atmosphere using standard Schlenk techniques, if not otherwise noted. The solvents were dried over KOH and subsequently distilled over sodium/benzophenone in a nitrogen atmosphere prior to use. Deuterated solvents were dried over sodium, distilled, degassed, and stored under nitrogen over sodium. 1H, 29 Si{1H}, and 13C{1H} NMR spectra were recorded on a Bruker Avance III 400 spectrometer. Chemical shifts are reported in parts per million relative to SiMe4 as an external standard. All substrates were purchased from Alfa Aesar, abcr, Sigma-Aldrich or TCI and used without further purification. [Mg{N(SiMe3)2}2]25,17 and 2,2,5,5tetramethyltetrahydrofuran26 were prepared according to literature protocols. The yields given are crystalline yields and not optimized. Purity of the compounds was verified by NMR spectroscopy. General Procedure. [Ae{N(SiMe3)2}2]2 of Ca, Sr, and Ba were prepared according to variation of literature protocols.5,12,15,16 Two equivalents of KN(SiMe3)2 were reacted with 1 equiv of CaI2 in diethyl ether. After removal of precipitated KI the solvent was removed in vacuo. The residue was recrystallized twice from hexane and diethyl ether. Thorough drying in vacuo yielded [Ca{N(SiMe3)2}2]2. [Ae(hmds)2] with Ae = Sr and Ba were prepared via direct metalation of bis(trimethylsilyl)amine with Ae metal. [Ae{N(SiMe3)2}2]2 was suspended in 4 mL of n-pentane. Under vigorous stirring 2 equiv of Me4THF was added dropwise until a clear solution was obtained. This solution was stored at −20 °C for 24 h leading to crystallization of adducts as colorless crystals. Isolation and gentle drying in vacuo gave the adducts 1−4 as crystalline compounds in moderate yields. [(Me4thf)Mg{N(SiMe3)2}2] 1. Colorless crystals; yield: 636 mg, 2.0 mmol, 58%; 1H NMR (400 MHz, C6D6): δ 1.30 (s, 16H, CH2, CH3), 0.37 (s, 36H, H3CSi); 13C{1H} NMR (101 MHz, C6D6): δ 89.2, 38.5, 29.7, 6.8 (1JC,Si = 52.6 Hz); 29Si−1H-DEPT-NMR (79.41 MHz, C6D6): δ −7.0; IR (ATR, cm−1): 2954 (s), 2872 (m), 1244 (s), 979 (s), 930 (s), 825 (vs), 663 (m). [(Me4thf)Ca{N(SiMe3)2}2] 2. Colorless crystals; yield: 270 mg, 0.55 mmol, 82%; 1H NMR (400 MHz, C6D6): δ 1.29 (s, 4H, CH2), 1.13 (s, 12H, CH3), 0.34 (s, 36H, Si(CH3)3; 13C{1H} NMR (101 MHz, C6D6): δ 85.7, 38.2, 29.8, 6.0 (1JC,Si = 52.4 Hz); 29Si−1H-DEPT-NMR (79.14 MHz, C6D6, 297 K): δ −14.35. IR (ATR, cm−1): 2947 (m), 2895 (w), 1237 (m), 1034 (s), 818 (vs), 746 (m). [(Me4thf)Sr{N(SiMe3)2}2] 3. Colorless crystals; yield: 148 mg, 0.3 mmol, 75%; 1H NMR (400 MHz. C6D6): δ 1.37 (s, 4H, CH2), 1.11 (s, 12H, CH3),, 0.31 (s, 18H, Si(CH3)3); 13C{1H} NMR (101 MHz, C6D6): δ 82.9, 38.5, 29.9, 6.2; 29Si−1H-DEPT (79.41 MHz, C6D6): δ −20.9; IR (ATR, cm−1): 2945 (m), 2893 (w), 1234 (m), 1057 (s), 814 (vs), 745 (s). [(Me4thf)Ba{N(SiMe3)2}2] 4. Colorless crystals; yield: 217 mg, 0.37 mmol, 66%; 1H NMR (400 MHz, C6D6): δ 1.52 (s, 4H, CH2), 1.14 (s, 12H, CH3), 0.27 (s, 18H, Si(CH3)3); 13C{1H} NMR (101 MHz, C6D6): δ 81.4, 38.8, 30.0, 5.5 (JC,Si = 52.1 Hz); 29Si−1H-DEPT-NMR (79.14 MHz, C6D6, 297 K): δ −18.8; IR (ATR, cm−1): 2940 (m), 2892 (w), 1241 (m), 1064 (s), 809 (vs), 656 (s). Crystal Structure Determinations. The intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphite-monochromated Mo−Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semiempirical basis using multiple-scans.27−29 The

Figure 4. Comparison of Ae−N and Ae−O bond lengths (pm, bottom) and N−Ae−N bond angles (deg., top) of [(Me4thf)Ae{N(SiMe3)2}2] (blue), [(thf)2Ae{N(SiMe3)2}2] (red), and [Ae(NH2)2] (black).

(SiMe3)2}2] with tetra-coordinate metal atoms steric strain is present and reduction of the coordination number to three in [(Me4thf)Ae{N(SiMe3)2}2] reduces repulsion between the ligands. Due to the large radius of the barium atom, steric pressure is negligible in both complexes; this fact leads to a larger Ba−O distance in [(Me4thf)Ba{N(SiMe3)2}2] (4) compared to Ba−O values of [(thf)2Ba{N(SiMe3)2}2] (Figure 4), verifying that Me4thf is a significant weaker Lewis base than thf. In hydrocarbon solutions the complexes of the heavier alkaline-earth metals (Ca, Sr, and Ba) dissociate yielding the mononuclear unsolvated compounds [Ae{N(SiMe3)2}2], whereas the magnesium derivative with the smallest metal atom and the largest intramolecular strain remains unchanged in benzene. Even though that the complexes of the heavier alkaline-earth metals form the mononuclear unsolvated species, there is no evidence for dimerization and formation of dinuclear [(Me3Si)2N-Ae{μ-N(SiMe3)2}]2. Thus, the presence of 2,2,5,5-tetramethyltetrahydrofuran significantly enhances the solubility of [Ae{N(SiMe3)2}2] in hydrocarbon solvents and stabilizes the mononuclear unsolvated species for calcium, E

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry structures were solved by Direct Methods (SHELXS29) and refined by full-matrix least-squares techniques against Fo2 (SHELXL-9730). All hydrogen atoms were included at calculated positions with fixed thermal parameters. All non-hydrogen, nondisordered atoms were refined anisotropically.30 Crystallographic data as well as structure solution and refinement details are summarized in Table S1. XP (SIEMENS Analytical X-ray Instruments, Inc.)31 and POV-Ray32 were used for structure representations.

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(3) Selected reviews: (a) Zhong, Z.; Dijkstra, P. J.; Feijen, J. Controlled synthesis of biodegradable lactide polymers and copolymers using novel in situ generated or single-site stereoselective polymerization initiators. J. Biomater. Sci., Polym. Ed. 2004, 15, 929− 946. (b) Platel, R. H.; Hodgson, L. M.; Williams, C. K. Biocompatible initiators for lactide polymerization. Polym. Rev. 2008, 48, 11−63. (c) Labet, M.; Thielemans, W. Synthesis of polycaprolactone: a review. Chem. Soc. Rev. 2009, 38, 3484−3504. (d) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Complexes of Mg, Ca and Zn as homogeneous catalysts for lactide polymerization. Dalton Trans. 2009, 4832−4846. (e) Harder, S. From limestone to catalysis: Application of calcium compounds as homogeneous catalysts. Chem. Rev. 2010, 110, 3852−3876. (f) Barrett, A. G. M.; Crimmin, M. R.; Hill, M. S.; Procopiou, P. A. Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium. Proc. R. Soc. London, Ser. A 2010, 466, 927−963. (g) Kobayashi, S.; Yamashita, Y. Alkaline earth metal catalysts for asymmetric reactions. Acc. Chem. Res. 2011, 44, 58−71. (h) Reznichenko, A. L.; Hultzsch, K. C. Early transition metal (group 3−5, lanthanides and actinides) and main group metal (group 1, 2, and 13) catalyzed hydroamination. Top. Organomet. Chem. 2011, 43, 51−114. (i) Carpentier, J.-F.; Sarazin, Y. Alkalineearth metal complexes in homogeneous polymerization catalysis. Top. Organomet. Chem. 2013, 45, 141−190. (j) Crimmin, M. R.; Hill, M. S. Homogeneous catalysis with organometallic complexes of group 2. Top. Organomet. Chem. 2013, 45, 191−242. (k) Hill, M. S.; Liptrot, D. J.; Weetman, C. Alkaline earths as main group reagents in molecular catalysis. Chem. Soc. Rev. 2016, 45, 972−988. (4) Bradley, D. C.; Hursthouse, M. B.; Ibrahim, A. A.; Malik, K. M. A.; Motevalli, M.; Möseler, R.; Powell, H.; Runnacles, J. D.; Sullivan, A. C. Synthesis and chemistry of the bis(trimethylsilyl)amido bistetrahydrofuranates of the group 2 metals magnesium, calcium, strontium and barium. X-ray crystal structures of Mg[N(SiMe3)2]2· 2THF and related Mn[N(SiMe3)2]2·2THF. Polyhedron 1990, 9, 2959−2964. (5) Westerhausen, M. Synthesis and spectroscopic properties of bis(trimethylsilyl)amides of the alkaline-earth metals magnesium, calcium, strontium, and barium. Inorg. Chem. 1991, 30, 96−101. (6) Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A.; Royo, B. The synthesis and structure of the alkaline earth metal organic compounds [M(OAr)2(thf)n] [M = Ca, n = 3 (1) or 0; M = Ba, n = 4] and [{Ca(NR2)(μNR2)(thf)}2], and the X-ray structure of (1) (Ar = C6H2But2-2,6-Me-4; R = SiMe3; thf = OC4H8). J. Chem. Soc., Chem. Commun. 1990, 1141−1142. (7) Frankland, A. D.; Hitchcock, P. B.; Lappert, M. F.; Lawless, G. A. A novel high-yield route to organic derivatives of group 2 metals; Xray structure of [{Ba(OTf)2}4(pc)14]·py (OTf = OSO2CF3) and NMR spectral characterisation of [Ba(NR2)(μ-NR2)2Na(thf)2] (R = SiMe3). J. Chem. Soc., Chem. Commun. 1994, 2435−2436. (8) Frankland, A. D.; Lappert, M. F. Alkaline-earth-metal arenesulfonates as precursors to organic derivatives of group 2 metals. J. Chem. Soc., Dalton Trans. 1996, 4151−4152. (9) Tanner, P. S.; Burkey, D. J.; Hanusa, T. P. Polyhedron 1995, 14, 331−333. (10) Brady, E. D.; Hanusa, T. P.; Pink, M.; Young, V. G. Inorg. Chem. 2000, 39, 6028−6037. (11) He, X.; Noll, B. C.; Beatty, A.; Mulvey, R. E.; Henderson, K. W. J. Am. Chem. Soc. 2004, 126, 7444−7445. (12) Vaartstra, B. A.; Huffman, J. C.; Streib, W. E.; Caulton, K. G. Syntheses and structures of a series of very low coordinate barium compounds: Ba[N(SiMe3)2]2(THF)2, {Ba[N(SiMe3)2]2(THF)}2, and {Ba[N(SiMe3)2]2}2. Inorg. Chem. 1991, 30, 121−125. (13) Drake, S. R.; Otway, D. J. The synthesis of metal organic compounds of calcium, strontium and barium by ammonia gassaturated ethereal solvents. J. Chem. Soc., Chem. Commun. 1991, 517− 519. (14) Gillett-Kunnath, M. M.; MacLellan, J. G.; Forsyth, C. M.; Andrews, P. C.; Deacon, G. B.; Ruhlandt-Senge, K. BiPh3 - A convenient synthon for heavy alkaline-earth metal amides. Chem. Commun. 2008, 4490−4492.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02469. NMR and IR spectra of the complexes 1−4, figures of the Mg (1) and Sr (3) complexes, crystal and refinement data (PDF) Accession Codes

CCDC 1864053−1864056 (for 1−4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*(S.K.) E-mail: [email protected]. Fax: +49 3641-948110. Homepage: http://www.lsac1.uni-jena.de. *(M.W.) E-mail: [email protected]. Fax: +49 3641-9-48110. Homepage: http://www.lsac1.uni-jena.de ORCID

Matthias Westerhausen: 0000-0002-1520-2401 Author Contributions #

S.K. and P.S. are equal contributors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the valuable support of the NMR (www.nmr. uni-jena.de/) and mass spectrometry service platforms (www. ms.uni-jena.de/) of the Faculty of Chemistry and Earth Sciences of the Friedrich Schiller University, Jena, Germany.

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REFERENCES

(1) (a) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal and Metalloid Amides: Syntheses, Structures, and Physical and Chemical Properties; Ellis Horwood: Chichester, 1980. (b) Lappert, M.; Power, P.; Protchenko, A.; Seeber, A. Metal Amide Chemistry; Wiley: Chichester, 2009. (2) (a) Westerhausen, M. Dynamic behavior and structures of alkaline earth metal bis[bis(trimethylsilyl)amides] and -phosphanides]. Trends Organomet. Chem. 1997, 2, 89−105. (b) Westerhausen, M. Synthesis, properties and reactivity of alkaline earth metal bis[bis(trimethylsilyl)amides]. Coord. Chem. Rev. 1998, 176, 157− 210. (c) Westerhausen, M. Recent developments in the field of organic heterobimetallic compounds of the alkaline earth metals. Dalton Trans. 2006, 4755−4768. (d) Harrison-Marchand, A.; Mongin, F. Mixed AggregAte (MAA): A single concept for all dipolar organometallic aggregates. 1. Structural data. Chem. Rev. 2013, 113, 7470−7562. (e) Kays, D. L. Extremely bulky amide ligands in main group chemistry. Chem. Soc. Rev. 2016, 45, 1004−1018. F

DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry (15) Johns, A. M.; Chmely, S. C.; Hanusa, T. P. Solution Interaction of potassium and calcium bis(trimethylsilyl)amides; preparation of Ca[N(SiMe3)2]2 from dibenzylcalcium. Inorg. Chem. 2009, 48, 1380− 1384. (16) Müller, C.; Krieck, S.; Görls, H.; Westerhausen, M. Synthesis of Lewis base-adducts of barium bis[bis(trimethylsilyl)amide]. Eur. J. Inorg. Chem. 2016, 2016, 4637−4642. (17) Westerhausen, M.; Schwarz, W. Molekül- und Kristallstruktur des dimeren Magnesium-bis[bis(trimethylsilyl)amids]. Z. Anorg. Allg. Chem. 1992, 609, 39−44. (18) Westerhausen, M.; Schwarz, W. Molekül- und Kristallstrukturen des dimeren Calcium-bis[bis(trimethylsilyl)amids] und des Calcium-bis[bis(trimethylsilyl)amids]·DME. Z. Anorg. Allg. Chem. 1991, 604, 127−140. (19) Westerhausen, M.; Schwarz, W. Molekül- und Kristallstrukturen des dimeren Strontium-bis[bis(trimethylsilyl)amids] und des Strontium-bis[bis(trimethylsilyl)amids]·2DME. Z. Anorg. Allg. Chem. 1991, 606, 177−190. (20) Westerhausen, M.; Hartmann, M.; Makropoulos, N.; Wieneke, B.; Wieneke, M.; Schwarz, W.; Stalke, D. Synthese von Erdalkalimetallocenen aus Erdalkalimetall-bis[bis(trimethylsilyl)amid] und 6Methyl-6-phenylfulven. Z. Naturforsch., B: J. Chem. Sci. 1998, 53, 117−125. (21) Koch, A.; Görls, H.; Krieck, S.; Westerhausen, M. Coordination behavior of bidentate bis(carbenes) at alkali metal bis(trimethylsilyl)amides. Dalton Trans. 2017, 46, 9058−9067. (22) Krieck, S.; Schü ler, P.; Gö rls, H.; Westerhausen, M. Straightforward synthesis of rubidium bis(trimethylsilyl)amide and complexes of the alkali metal bis(trimethylsilyl)amides with weakly coordinating 2,2,5,5-tetramethyltetrahydrofuran. Dalton Trans. 2018, 47, 1256212569. (23) Kaupp, M.; Schleyer, P. v. R. The structural variations of monomeric alkaline earth MX2 compounds (M = Ca, Sr, Ba; X = Li, BeH, BH2, CH3, NH2, OH, F). An ab initio pseudopotential study. J. Am. Chem. Soc. 1992, 114, 491−497. (24) Mösges, G.; Hampel, F.; Kaupp, M.; Schleyer, P. v. R. Experimental and theoretical studies of alkaline-earth metal coordination: X-ray crystal structures of calcium, strontium, and barium carbazoles and ab initio model calculations. J. Am. Chem. Soc. 1992, 114, 10880−10889. (25) Elschenbroich, C.: Organometallchemie, 6th ed., Teubner, Wiesbaden, 2008. (26) Reppe, W. Ä thinylierung V. Reaktionen hydrierter Ä thinylierungsprodukte, Dehydratisierung von γ-Alkandiolen. Liebigs Ann. Chem. 1955, 596, 80−158. (27) Hooft, R. COLLECT, Data Collection Software; Nonius B.V., Netherlands, 1998. (28) Otwinowski, Z.; Minor, W. in Carter, C. W.; Sweet, R. M., Eds.; Methods in Enzymology; Academic Press: New York, 1997; Vol. 276, Macromolecular Crystallography, Part A, pp 307−326. (29) SADABS 2.10; Bruker-AXS Inc.: Madison, WI, USA, 2002. (30) Sheldrick, G. M. Crystal Structure Refinement with SHELXL. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (31) XP; Siemens Analytical X-Ray Instruments Inc., Karlsruhe, Germany, 1990; Madison, WI, USA, 1994. (32) POV-Ray, Persistence of Vision Raytracer: Victoria, Australia, 2007.

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DOI: 10.1021/acs.inorgchem.8b02469 Inorg. Chem. XXXX, XXX, XXX−XXX