Heteroleptic Heavier Alkaline Earth Metal Amide Complexes

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Heteroleptic Heavier Alkaline Earth Metal Amide Complexes Stabilized by a Superbulky β‑Diketiminate Ligand Thomas Xaver Gentner,† Bastian Rösch,† Katharina Thum,† Jens Langer,† Gerd Ballmann,† Jürgen Pahl,† Wolfgang A. Donaubauer,‡ Frank Hampel,‡ and Sjoerd Harder*,† †

Chair of Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany ‡ Chair of Organic Chemistry II, Department of Chemistry & Pharmacy, Friedrich-Alexander-Universität Erlangen Nürnberg, Nikolaus-Fiebiger-Straße 10, 91058 Erlangen, Germany Downloaded by UNIV AUTONOMA DE COAHUILA at 06:07:48:365 on May 29, 2019 from https://pubs.acs.org/doi/10.1021/acs.organomet.9b00211.

S Supporting Information *

ABSTRACT: Heteroleptic alkaline earth metal (Ae = Ca, Sr, Ba) amide complexes with the superbulky β-diketiminate ligand DIPePBDI (CH[C(Me)N-DIPeP]2, DIPeP = 2,6-di-isopentylphenyl) have been prepared by direct deprotonation of DIPeP BDI-H with either AeN′′2 or AeN′′2·(THF)2 (N′′ = N(SiMe3)2). Despite long reaction times of 5−14 days, this convenient one-step synthetic method has the major advantage that metal-pure products are obtained in generally quantitative yields. All (DIPePBDI)AeN′′ and (DIPePBDI)AeN′′· THF complexes are monomeric and stabilized by agostic metal···Me3Si and metal···iso-pentyl interactions. They are highly soluble in toluene and indefinitely stable toward ligand scrambling, even after 2 weeks at 140 °C. The same series with the smaller DIPPBDI ligand (CH[C(Me)N-DIPP]2, DIPP = 2,6-di-iso-propylphenyl) could, except for Ca, also be prepared by direct ligand deprotonation. The (DIPPBDI)CaN′′ and (DIPPBDI)CaN′′·THF complexes are stable toward ligand exchange up to 110 °C. Whereas THF-free (DIPPBDI)SrN′′ and (DIPPBDI)BaN′′ decompose at 50 and 20 °C, respectively, their THF adducts were found to be stable up to 60 °C. This is, however, strongly dependent on complex purity. Slight hydrolysis or contamination with KN′′ accelerates ligand scrambling. Therefore, partial hydrolysis and salt metathesis routes that involve KN′′ should be avoided when synthesizing heteroleptic complexes of the heavier Ae metals.

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INTRODUCTION The past decades have seen major developments in the organometallic chemistry of the heavier group 2 metals (Ae = Ca, Sr, Ba).1 The driving force for these investigations is partially due to the recognition that this compound class has enormous potential in homogeneous catalysis.2 Although there is a growing recognition that simple homoleptic complexes of type AeR2 can be versatile catalysts,3 most of the group 2 metal catalysts are of the heteroleptic type LAeR in which R is the reactive group (often the amide N(SiMe3)2 abbreviated in here as N′′) and L is a passive spectator ligand that controls the space and electronics at the metal. In some cases, the spectator ligand also functions as a solubilizing ligand, preventing the catalytically active species from precipitating, thus keeping the catalyst active.4 Since the bonds to Ae are generally ionic, long, and weak, ligand exchange reactions giving mixtures of homoand heteroleptic species is often an issue as demonstrated by the Schlenk equilibrium: 2 LAeR ⇄ L2Ae + AeR2. Very bulky multidentate spectator ligands can stabilize heteroleptic complexes by preventing such ligand exchange reactions (Scheme 1) and enable solubilization of otherwise typical salt-like compounds such as CaH2 or Ca(OH)2. Thus, hydrocarbon-soluble calcium hydride (LCaH)2, hydroxide © XXXX American Chemical Society

(LCaOH)2, or amide (LCaNH2)2 complexes have been isolated.5−7 Going down group 2, metal sizes increase (6coordinate: Ca2+ = 0.99 Å, Sr2+ = 1.18 Å, Ba2+ = 1.35 Å)8 and Ae−ligand bond strengths decrease rapidly. Especially for the heavier metals Sr and Ba, ligand exchange processes are very hard to control. Therefore, with some exceptions, the few examples of heteroleptic Ba amide complexes often lack stability at higher temperatures.9,10 One ligand that stabilizes heteroleptic amide complexes of most alkaline earth metals is the well-known β-diketiminate ligand DIPPBDI (CH[C(Me)N-DIPP]2, DIPP = 2,6-di-isopropylphenyl).9,11 This is not so much due to steric stress in the homoleptic complexes of this bulky ligand. In fact, homoleptic (DIPPBDI)2Ae complexes for the range Ae = Mg, Ca, Sr, and Ba are very stable on behalf of secondary C−H··· C(π) interactions between the DIPPBDI ligands.12 Therefore, the origin of stabilization is of kinetic nature and related to its ability to prevent ligand exchange through a binuclear intermediate (Scheme 1). Although the DIPPBDI ligand stabilizes the heteroleptic complex (DIPPBDI)CaN′′·THF,11 Received: March 27, 2019

A

DOI: 10.1021/acs.organomet.9b00211 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

We recently introduced the superbulky β-diketiminate ligand BDI (CH[C(Me)N-DIPeP]2, DIPeP = 2,6-di-iso-pentylphenyl) in low-valent Mg(I) chemistry.21 It was shown that this bulkier version of the DIPPBDI ligand, in which the isopropyl groups have been replaced for iso-pentyl moieties, gave a (DIPePBDI)Mg-Mg(DIPePBDI) complex with a considerably stretched Mg−Mg bond. Subsequently, we demonstrated that this ligand allows for the isolation of heteroleptic [(DIPePBDI)SrH]2 and [(DIPePBDI)SrEt]2 complexes, which are up to 70 °C stable toward ligand exchange reactions.22 Its precursor, (DIPePBDI)SrN″, was found to be indefinitely stable: a toluene-d8 solution at 140 °C did not decompose, even after 6 days. Herein, we describe synthesis and structures of the complete series of (DIPePBDI)AeN′′ and (DIPePBDI)AeN′′· THF with the heavier alkaline earth metals Ca, Sr, and Ba. For structural comparison, we extended the partially known series of complexes with the smaller DIPPBDI ligand and present convenient new synthetic routes to (DIPPBDI)AeN′′ and (DIPPBDI)AeN′′·THF (Ae = Ca, Sr, Ba). Not only their structures but also their stability toward ligand exchange is compared.

Scheme 1. Controlling the Schlenk Equilibrium for Heteroleptic Alkaline Earth Reagents with a Bulky Spectator Ligand

DIPeP

its heavier congeners have been reported as being extremely labile. Complex (DIPPBDI)BaN′′·THF (I, Scheme 2) has been

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RESULTS AND DISCUSSION Previously, heteroleptic complexes with the smaller DIPPBDI ligand have been synthesized by a salt metathesis route in THF: AeI2 + 2 KN′′ + DIPPBDI-H → (DIPPBDI)AeN′′· THF.9,11 This synthetic pathway needs a polar solvent, and therefore, only its ether adducts were accessible. The only THF-free complex in the series, (DIPPBDI)CaN′′, was obtained by performing the salt metathesis in diethyl ether and subsequently removing the ether ligand in (DIPPBDI)CaN′′· Et2O under high vacuum (10−2 mbar, 80 °C).23 The latter process, which has been described as being not always reproducible, is affected by formation of homoleptic (DIPPBDI)2Ca, resulting in rather low yields of (DIPPBDI)CaN′′ (41%). In contrast, the THF-free heteroleptic complexes with the bigger DIPePBDI ligand (1−3) can be conveniently synthesized in quantitative yields by direct ligand deprotonation (Scheme 3). Complexes (DIPePBDI)AeN′′ were obtained by heating a toluene solution of DIPePBDI-H and AeN′′2 in a pressure tube at 140 °C for 9 days for Ca (1), 6 days for Sr (2), and 5 days

Scheme 2. Selected Heteroleptic Ba Complexes

Scheme 3. Synthesis of Heteroleptic Ae Metal Amide Complexes with DIPePBDI and DIPPBDI Ligands isolated in the solid state, but it has been reported that, dissolved in C6D6, it rapidly forms the homoleptic complexes (DIPPBDI)2Ba and BaN′′2.9 Niemeyer and Deacon introduced superbulky triazenide ligands which stabilize the heteroleptic Ba complex II through metal encapsulation by π-interactions with the aryl substituents.13 Carpentier and co-workers reported a heteroleptic Ba complex (III) in which ligand exchange has been prevented by using the Anwander amide N(SiHMe2)2 which strongly binds Ba by secondary Ba···H−Si interactions.14 The same group also published a series of anilido-imine complexes (IV) with a bidentate ligand that closely resembles the β-diketiminate ligand but is considerably more stable to ligand exchange.14,15 Various other bulky ligands have been introduced to stabilize heteroleptic Ba complexes, among which include tris-pyrazolylborates,16,17 triscarbeneborates,18 bulky phosphides,19 bulky Cp ligands,20 or phenolates with pendant crown ether ligands.10 B


Article

Organometallics

Figure 1. Molecular structures of (a) (DIPePBDI)CaN′′ (1), (b) (DIPePBDI)SrN′′ (2),22 (c) (DIPePBDI)BaN′′ (3), (d) (DIPePBDI)CaN′′·THF (4), (e) (DIPePBDI)SrN′′·THF (5), and (f) (DIPePBDI)BaN′′·THF (6). iso-Pentyl groups and hydrogen atoms have been omitted for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) in the Crystal Structures of (DIPePBDI)AeN′′·THFn (n = 0, 1) Complexes 1−6 metal

1

222

3

4

5

6

Ca

Sr

Ba

Ca

Sr

Ba

THF

THF

THF

2.3907(10) 2.4253(11) 2.3388(9) 2.3283(12) 0.740(2) 3.2397(13) 3.5888(4) 3.3811(4) 80.53(4) 112.88(5) 124.45(5) 121.46(5) 358.79(5) 115.91(10) 118.51(10)

2.518(6) 2.529(5) 2.482(5) 2.590(4) 1.794(6) 3.304(7) 3.495(2) 3.7206(15) 79.99(17) 112.6(3) 124.0(3) 123.4(3) 360.0(3) 117.4(5) 118.0(6)

2.6744(13) 2.6690(19) 2.6007(15) 2.7620(17) 2.0815(16) 3.578(2) 3.7037(6) 3.6060(7) 73.26(5) 118.38(9) 112.81(7) 128.59(10) 359.78(9) 116.88(17) 117.88(15)

solvent BDI

M−N

M−N′′ M−OTHF M···NCCCNa M···C(Si) M···Si1 M···Si2 N1−M−N2 M−N3−Si1 M−N3−Si2 Si1−N3−Si2 Σ angles at Nb C−N−CAr

2.3024(12) 2.3217(14) 2.2811(12)

2.4916(12) 2.5334(12) 2.4733(12)

2.6823(14) 2.6564(14) 2.6073(15)

1.5005(16) 2.845(2) 3.4182(6) 3.1609(5) 88.13(5) 104.90(7) 118.05(7) 134.80(8) 357.75(7) 117.60(13) 118.25(14)

0.114(2) 3.0574(15) 3.8026(6) 3.3619(5) 73.17(4) 106.14(6) 130.52(6) 123.32(7) 359.98(6) 121.26(11) 120.03(11)

0.003(3) 3.390(2) 3.7016(6) 3.6163(6) 66.73(4) 113.17(7) 117.69(7) 129.09(9) 359.95(7) 121.77(14) 122.31(14)

a

Distance between the metal and NCCCN least-squared plane. bSum of the valence angles at N.

angles that strongly deviate from 120° (Figure 1). It is also demonstrated by large differences in Ae···Si distances: Ca···Si1 3.4182(6) Å and Ca···Si2 3.1609(5) Å; Sr···Si1 3.8026(6) Å and Sr···Si2 3.3619(5) Å; Ba···Si1 3.7016(6) Å and Ba···Si2 3.6163(6) Å. Similar agostic interactions have been found in (DIPPBDI)CaN′′ and in Ca amide complexes stabilized by a bulky amidinate ligand.23,24 The Sr complex 2 features an additional agostic interaction between an iso-pentyl group and the metal, as already discussed in a recent paper.22 The versatility of β-diketiminate ligands to bind to metals with large size differences12 is also apparent for the DIPePBDI ligand. The ligand bite angles become more acute with increasing metal size: N1−Ae−N2: Ca 88.13(5)°, Sr 73.17(4)°, and Ba 66.73(4)°. Also, the C−N−CAr angles

for Ba (3). Although reaction times are long, this synthetic route is remarkable. It clearly demonstrates that, even for (DIPePBDI)BaN′′, there is after 5 days at 140 °C no ligand scrambling. Compounds 1−3 were crystallized by storing saturated hexane or pentane solutions at −20 °C. Crystal structures are depicted in Figure 1, and selected geometrical data can be found in Table 1. All compounds crystallize as a monomer with tricoordinate metal centers and are stabilized by additional agostic SiMe···Ae contacts (shortest SiMe···Ae distance for: Ca 2.845(2) Å; Sr 3.0574(15) Å; Ba 3.390(2) Å); the latter agostic interaction to Ba should be considered weak. These agostic interactions cause asymmetric bonding of the amide ligand which is apparent from clearly different Ae−N3−Si C


Article

Organometallics

The high stability of these heteroleptic complexes of the heavier Ae metals at these conditions (140 °C, 2 weeks) is unique. In order to compare the structures and stabilities of the DIPeP BDI complexes 1−6 with those of complexes with the smaller DIPPBDI ligand (7−12), we completed the latter series with the syntheses of hitherto unknown THF-free complexes (DIPPBDI)SrN′′ (8) and (DIPPBDI)BaN′′ (9) (Scheme 3). We found that, also in this case, the convenient direct ligand deprotonation pathway with AeN′′2 is successful. However, the reaction temperature should be lowered to 70 °C and reaction times should be kept to a minimum in order to minimize ligand exchange reactions. For CaN′′2, complete conversion was observed after 4 days and, most important, no homoleptic byproducts were observed. Deprotonation with SrN′′2 and BaN′′2 is increasingly faster, and the reaction was finished after 16 and 6 h, respectively. Small amounts of homoleptic side products were observed (approximately 10%), but these could be removed by recrystallization. The THF adduct (DIPPBDI)CaN′′·THF (10) could not be obtained by direct deprotonation of the ligand with CaN′′2·(THF)2, again showing the retarding influence of THF on β-diketiminate deprotonation. At 70 °C, only a very slow conversion was observed. However, the more reactive SrN′′2·(THF)2 and BaN′′2·(THF)2 were able to deprotonate DIPPBDI-H in benzene at 70 °C, and complexes (DIPPBDI)SrN′′·THF (11) and (DIPPBDI)BaN′′· THF (12) could be isolated in moderate yields. Hill’s previous attempts to prepare 11 and 12 by direct ligand deprotonation at low temperature failed.9 The Roesky29 and Hill9 groups synthesized 11 and 12 according to Chisholm’s26 one-pot salt metathesis route using AeI2, KN′′ (2 equiv), and DIPPBDI-H. Although there are no reports on the stability of (DIPPBDI)SrN′′·THF (11), Hill and co-workers found (DIPPBDI)BaN′′· THF (12) to be very sensitive to ligand exchange already at room temperature.9 This strongly contrasts with our synthetic procedure for 11 and 12 at the higher temperature of 70 °C. Apparently, these heteroleptic Sr and Ba amides are more stable toward ligand exchange as observed previously (vide infra). Compound (DIPPBDI)SrN′′ (8) was crystallized by slow diffusion of pentane into a saturated benzene solution at room temperature, and (DIPPBDI)BaN′′ (9) was crystallized by storing a saturated benzene solution at 8 °C. Crystal structures of complexes 8 and 9 are depicted in Figure 2, and selected geometrical data can be found in Table 2. Whereas (DIPePBDI)AeN′′ complexes are monomeric, the smaller DIPP BDI ligand does not provide sufficient steric shielding. Similar to (DIPPBDI)CaN′′ (7),23 the THF-free (DIPPBDI)SrN′′ (8) crystallizes as a tetramer which is linked through a rather short SiMe···Sr agostic interaction of only 3.003(2) Å. The THF-free (DIPPBDI)BaN′′ (9) crystallizes as a polymeric chain connected through short SiMe···Ba agostic interactions of 3.337(3) Å. Apart from these intermolecular agostic interactions, also intramolecular agostic SiMe···Ae interactions with the N′′ ligand are evident: Ca 2.999(5) Å,23 Sr 3.171(2) Å, and Ba 3.457(2) Å. In all (DIPPBDI)AeN′′ structures, the metal is significantly moved out of the NCCCN plane. In contrast to the analogue DIPePBDI structures, the distance of the metal to the NCCCN plane is increasing from Ca to Ba: Ca 1.33(5) Å,23 Sr 1.6150(15) Å, and Ba 2.0645(18) Å. A comparison of the metal−ligand bond lengths in DIPePBDI and corresponding DIPPBDI complexes shows that there is hardly an influence of ligand bulk (Tables 1 and 2). Slight

become larger with increasing metal size: Ca 117.60(13)°/ 118.25(14)°, Sr 121.26(11)°/120.03(11)°, and Ba 121.77(14)°/122.31(14)°. In 1, the Ca center is significantly moved out of the NCCCN plane (Ca···NCCCN 1.5005(16) Å). This phenomenon was previously discussed for (DIPPBDI)CaN′′ and (DIPPBDI)CaN′′·solvent complexes.23,25 In contrast, the larger Sr and Ba metals are bound in the NCCCN plane with out-of-plane deviations of 0.114(2) Å (Sr) and 0.003(3) Å (Ba). To investigate the influence of THF on complex stability, we synthesized the corresponding (DIPePBDI)AeN′′·THF complexes 4 (Ae = Ca), 5 (Ae = Sr), and 6 (Ae = Ba). These can be obtained either by addition of THF to the Lewis base free adducts or directly by deprotonation of DIPePBDI-H with AeN′′2·(THF)2 reagents. Chisholm and co-workers showed that the latter route is not possible for the synthesis of (DIPPBDI)CaN′′·THF.26 For the extremely stable DIPePBDI complexes, however, this synthetic pathway is feasible: heating a toluene solution of DIPePBDI-H with AeN′′2·(THF)2 at 140 °C gave in quantitative yields the corresponding THF adducts (Scheme 3). The presence of THF nearly doubles the reaction times: 14 days for Ca (4), 12 days for Sr (5), and 9 days for Ba (6). This suggests that precoordination of the β-diketimine ligand to AeN′′2 is an essential first step. Despite the long reaction times, in all cases, no ligand exchange to homoleptic complexes was observed. Compounds 4−6 were crystallized from saturated hexane or pentane solutions at −20 °C. Their crystal structures are depicted in Figure 1, and selected geometrical data can be found in Table 1. All compounds crystallize as monomers with tetracoordinate metal centers and distorted tetrahedral geometries; the coordination sphere for Ca is most distorted and is close to a trigonal pyramid. In contrast to the THF-free complexes 1− 3, the metal cations in 4−6 are moved further out of the NCCCN plane with increasing metal size: Ca 0.740(2) Å, Sr 1.794(6) Å, and Ba 2.0815(16) Å. Coordination of THF leads in general to elongation of all metal−ligand bonds, but although this trend is clearly visible for the Ca complexes, there is hardly any metal−ligand bond elongation for the Sr and Ba complexes. The agostic SiMe···Ae interactions in the THF adducts are clearly less pronounced: Ca 3.2397(13) Å, Sr 3.304(7) Å, and Ba 3.578(2) Å. The Sr complex shows an additional agostic interaction of an iso-pentyl group to the metal (CH2···Sr: 3.261(7) Å). Complexes 1−6 are colorless compounds that are wellsoluble in aliphatic and aromatic solvents. Their 1H NMR and 13 C NMR spectra are simple and show, besides the signals of the DIPePBDI ligands, one resonance for the N′′ ligand group and, for 4−6, additional resonances for THF. The 1H, 13C, and 29 Si signals of the N′′ ligand moiety are listed in Table 3. These are high-field shifted with respect to those in homoleptic (AeN′′2)2 complexes and characteristic for terminally bound N′′ groups.14,25,27,28 The thermal stabilities of 1−6 were investigated by heating toluene solutions to 140 °C. Even after 2 weeks, no signs of decomposition were observed in the 1H NMR spectra. Attempted synthesis of the homoleptic complexes (DIPePBDI)2Ae by reaction of 2 equiv of DIPePBDIH with 1 equiv of AeN′′2 at 140 °C for 2 weeks resulted only in formation of the heteroleptic complexes (DIPPBDI)AeN′′. This indicates that formation of homoleptic complexes is hindered by ligand···ligand repulsion as depicted in Scheme 1. It also demonstrates the remarkable steric bulk and stabilizing properties of the DIPePBDI ligand to prevent ligand scrambling. D


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Organometallics

in a slight increase to 20% conversion. This is consistent with studies by Hill and co-workers on ligand exchange reactions in heteroleptic (DIPPBDI)BaN′′·THF to give BaN′′2·(THF)2 and (DIPPBDI)2Ba. Heating and cooling of this mixture had no effect on the product ratios, and they described this as an equilibrium.9 A reasonable explanation for this observation is that homoleptic and heteroleptic compounds form larger stable aggregates. During our investigations, we obtained on one occasion a small crop of single crystals of an aggregate consisting of two heteroleptic (DIPPBDI)SrN′′ and one homoleptic (DIPPBDI)2Sr unit (13, Figure 2c). These were obtained in an attempt to crystallize the rather labile complex (DIPPBDI)SrN′′ by slow diffusion of pentane in a saturated toluene solution of the crude product (DIPPBDI)SrN′′ at −20 °C. The homoleptic complex is connected to two heteroleptic compounds via CAr···Sr interactions of 3.0252(18) Å. It is currently not clear whether such interactions also exist in solution. The heavier heteroleptic Ba complex (DIPPBDI)BaN′′ (9) is less stable toward ligand exchange. A toluene solution already decomposed slowly at 20 °C. After 3 days, 33% of the heteroleptic complex is converted into (DIPPBDI)2Ba and BaN′′2. The THF adducts (DIPPBDI)AeN′′·THF (10−12) were found to be more stable than the THF-free complexes (7−9). Whereas a solution of the Ca complex (10) in toluene is also stable at higher temperatures, determination of an exact decomposition temperature for the Sr (11) and Ba (12) complexes was inconclusive. Depending on sample purity, decomposition at temperatures between 20 and 60 °C was found. Since especially samples that were slightly hydrolyzed showed ligand scrambling, this may be related to formation of metal hydroxide impurities that mediate ligand exchange processes. Very pure samples of 11 and 12 are at least stable up to 60 °C. The effect of impurities on complex stability may also explain the difference in observations made by us and Hill.9 For Ba complex (DIPPBDI)BaN′′·THF (12), Hill observed ligand scrambling already at room temperature, while we present a synthetic route at 70 °C. Assuming that the potassium amide precursor KN′′ used in Hill’s synthesis for 12 may influence complex decomposition, we performed stability tests of 11 and 12 in the presence of 5 mol % KN′′. The results of these studies unequivocally demonstrate that small quantities of a potassium impurity indeed influence the stability of these heteroleptic complexes: samples with KN′′ decomposed already at 20 °C (Figures S10 and S12), underscoring the importance of using potassium-free precursors. Johns and Hanusa showed that AeN′′2·(THF)2 containing up to 50 mol % KN′′ gives an averaged singlet signal for N′′ in the 1H NMR spectrum.30 Fast equilibria which involve the formation of the calcate complex (K+)(CaN′′3−) make the presence of KN′′ hard to detect. The influence of KN′′ on ligand exchange in (DIPPBDI)AeN′′ complexes may be explained by a similar formation of “ate” complexes. Reaction with KN′′ could give the intermediate (K+)[(DIPPBDI)AeN′′2−] which disproportionates in AeN′′2 and (DIPPBDI)K. The latter could react with (DIPPBDI)AeN′′ to give (DIPPBDI)2Ae and KN′′. Potassium therefore plays the role of a ligand transfer agent. Since the AeN′′2 and AeN′′2·(THF)2 reagents used in our studies have been prepared by reduction of SnN′′2 with the appropriate Ae metal, they are essentially free of potassium. This may explain why direct deprotonation of

Figure 2. Molecular structures of (a) (DIPPBDI)SrN′′ (8), (b) ( DIPP BDI)BaN′′ (9), and (c) ( DIPP BDI)SrN′′···( DIPP BDI)Sr(DIPPBDI)···(DIPPBDI)SrN′′] (13).

differences in bond lengths are likely caused by differences in the ligand−metal arrangement (in-plane vs out-of-plane) rather than by differences in ligand bulk. This indicates that the higher stability of the heteroleptic DIPePBDI complexes is mainly related to a kinetic stabilization of these complexes by the considerable bulk of this ligand (Scheme 1). All THF-free (DIPPBDI)AeN′′ complexes are colorless compounds and soluble in aromatic solvents. The chemical shifts of the amide moiety are listed in Table 3. Diffusionordered NMR spectroscopy (DOSY) showed that complexes 7 and 8 do not retain their tetrameric molecular structure in toluene-d8 solution but exist as a monomer: estimated molecular weight for Ca 498 g/mol (calcd 618.13 g/mol) and Sr 569 g/mol (calcd 665.67 g/mol). To investigate the stability toward the Schlenk equilibrium, solutions of 7−9 in toluene-d8 were heated until decomposition was observed. While the heteroleptic complex (DIPPBDI)CaN′′ (7) is in toluene quite stable toward ligand scrambling (2 days at 110 °C gave no changes in the 1H NMR spectrum), decomposition of the Sr complex 8 to homoleptic species started at 50 °C but stagnated after 2 days at 15% conversion. Increasing the temperature to 100 °C resulted only E


Article

Organometallics

Table 2. Selected Bond Distances (Å) and Angles (deg) in the Crystal Structures of (DIPPBDI)AeN′′·THFn (n = 0, 1) Complexes 7−12 metal

723

8

9

Ca

Sr

Ba

1011

solvent M−NBDI M−N3 M−OTHF M···NCCCNa M···C(Si) M···Si1 M···Si2 N1−M−N2 M−N3−Si1 M−N3−Si2 Si1−N3−Si2 Σ Angles at Nb C−N−CAr

2.331(4) 2.323(4) 2.299(3)

2.4691(11) 2.4549(12) 2.4286(12)

2.6324(16) 2.6367(18) 2.5830(16)

1.333(5) 2.999(5)c 2.964(4)d 3.1966(16) 3.5915(14) 82.10(12) 105.55(17) 128.05(19) 126.2(2) 359.8(2) 120.3(3) 119.0(3)

1.6150(15) 3.171(2)c 3.003(2)d 3.3585(10) 3.6051(7) 77.16(4) 108.08(6) 121.82(6) 130.05(8) 359.94 (6) 122.21(10) 120.24(11)

2.0645(18) 3.457(2)c 3.337(3)d 3.6210(6) 3.6727(6) 70.80(5) 114.74(8) 117.10(8) 128.12(10) 359.96(9) 121.51(15) 121.03(17)

a

Distance between the metal and NCCCN least-squared plane. Intermolecular agostic interaction.

d

b

δ(1H) SiMe3

δ(13C) SiMe3

δ(29Si) SiMe3

1 2 3 4a 5a 6a 7 8 9 1011 1129 129

0.14 0.00 −0.04 0.04 0.18 0.13 0.0423 0.07 0.16 0.18 0.14 0.16

5.5 5.7 5.6 6.0 6.2 5.8 5.123 5.8 6.2 n.d. 5.8 2.6

−15.26 −16.78 −19.16 −14.08 −15.86 −18.53 −16.11 −16.91 −18.08 n.d. −15.95 −18.2

129

Ca

Sr

Ba

THF

THF

THF

2.3697(18) 2.3521(14) 2.313(2) 2.3591(13) 1.218(3) 3.240(2)

2.514(2) 2.555(2) 2.446(2) 2.5361(19) 0.724(4) 3.127(3)

2.6477(13) 2.6829(13) 2.5930(14) 2.7659(11) 1.8801(16) 3.5409(18)

3.3639(7) 3.5609(11) 81.15(5) 113.16(9) 124.92(8) 121.52(12) 359.6(9) 120.73(18) 119.30(14)

3.6230(8) 3.3641(9) 74.13(7) 107.57(11) 121.57(11) 130.82(14) 359.96(12) 120.8(2) 118.2(2)

3.7067(5) 3.6187(4) 69.75(4) 113.79(6) 118.48(7) 127.18(9) 359.45(7) 120.50(13) 119.30(14)

Sum of the valence angles at N. cIntramolecular agostic interaction.

reports, direct deprotonation of the smaller DIPPBDI-H ligand is also possible, giving either THF-free (DIPPBDI)AeN′′ (7−9) or the THF adducts (DIPPBDI)AeN′′·THF (10−12). Crystal structures of the series of complexes with the superbulky DIPePBDI ligand (1−6) are compared to structures with the smaller DIPPBDI ligand (7−12), which were partially already known. Whereas the DIPePBDI ligand is considerably bulkier than the DIPPBDI ligand, there are no large differences in metal−ligand bond lengths. The most striking differences are found in the ligand−metal arrangements (in-plane vs outof-plane). The DIPePBDI complexes 1−6 show, in contrast to the DIPP BDI complexes 7−12, a remarkable stability toward ligand exchange by the Schlenk equilibrium. Solutions in toluene, even after 2 weeks at 140 °C, do not show any sign of ligand scrambling. Complexes with the smaller DIPPBDI ligand are clearly less stable. It was found that high sample purity is essential to stability. Slight sample hydrolysis or the presence of small amounts of the potassium reagent KN′′ accelerates ligand exchange, presumably by formation of intermediate “ate” complexes. For this reason, hydrolysis and salt metathesis should preferably be avoided when preparing heteroleptic complexes of the heavier Ae metals. The importance of complex purity may change conclusions on the reported stabilities of previously reported heteroleptic complexes. Having simple access to a large variety of heteroleptic amide complexes with DIPePBDI or DIPPBDI ligands, either THF-free or as a THF adduct, over the whole range of heavier Ae metals (Ca, Sr, Ba) will certainly assist further developments in the field of group 2 metal chemistry and is, especially for the study of ligand, metal, or solvent effects in catalysis, of enormous importance.

Table 3. Selected NMR Data for Complexes 1−12 (in C6D6 at 298 K)b compound

119

δ(1H) THF

3.80/1.45 3.51/1.27 3.39/1.25

3.37/1.11 3.31/1.11 3.26/1.17

a

In toluene-d8. bChemical shifts in ppm.

DIPP

BDI-H at higher temperatures gave the heavier Sr and Ba complexes in reasonable yields.

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CONCLUSIONS Stable heteroleptic complexes of the heavier alkaline earth metals Ca, Sr, and Ba are easily accessible by direct deprotonation of the very bulky β-diketiminate ligand DIPeP BDI-H with AeN′′2 in toluene at 140 °C. Although conditions are harsh and reaction times are long, the yields are quantitative and the (DIPePBDI)AeN′′ products (1−3) do not contain metal impurities from precursors. The salt metathesis pathway has the disadvantage that mixed-metal products (“ate” complexes) may be formed, a shortcoming which is not always immediately evident.30 Whereas the salt metathesis method generally needs to be performed in ethereal solvents, resulting in THF adducts, the direct deprotonation pathway has the additional advantage that THF-free complexes are obtained in a single step. The THF adducts (DIPePBDI)AeN′′·THF could be obtained either by THF addition or by direct ligand deprotonation with AeN′′2·(THF)2. In contrast to literature

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EXPERIMENTAL SECTION

All experiments were carried out under an atmosphere of N2 using standard Schlenk techniques, glovebox techniques (MBraun, Labmaster SP), and freshly dried and degassed solvent. All solvents were dried over a column (Solvent Purification System: Pure Solv 400−4− F


Article

Organometallics

Hz, 4H, CH), 3.79−3.81 (m, 4H, CH2-THF), 4.72 (s, 1H, CHbackbone), 6.98−7.07 (m, 6H, CH-arom) ppm. 13C NMR: (150.92 MHz, tol-d8, 298 K): δ = 6.0 (Si(CH3)3), 11.0 (CH3), 11.4 (CH3), 25.4 (CH2-THF), 26.1 (CH3-backbone), 26.4 (CH2), 27.2 (CH2), 40.6 (CH), 69.6 (CH2-THF), 92.4 (CH-backbone), 123.3 (C-arom), 126.1 (C-arom), 137.7 (C-arom), 150.2 (C-arom), 167.0 (CNbackbone) ppm. 29Si NMR: (119.22 MHz, tol-d8, 298 K): δ = −14.08 (Si(CH3)3) ppm. Elemental analysis: Calculated for C47H83CaN3OSi2 (M = 802.45 g/mol): C 70.35, H 10.43, N 5.24; Found: C 70.15, H 10.39, N 5.14. Synthesis of (DIPePBDI)SrN(SiMe3)2·THF (5). Method A: DIPeP BDI)SrN(SiMe3)2 (95.9 mg, 0.123 mmol) was dissolved in ( toluene-d8 (555 μL), THF (8.89 mg, 0.123 mmol, 10 μL) was added, and the solution was stirred for 1 h at room temperature. All volatiles were removed in vacuum at ambient temperature, yielding the title compound in quantitative yield as a pale yellow solid (101 mg, 0.119 mmol). Crystals suitable for X-ray diffraction analysis were grown by cooling a saturated hexane solution at −20 °C. Method B: DIPePBDI-H (122 mg, 0.23 mmol) and Sr[N(SiMe3)2]2·THF2 (127 mg, 0.23 mmol) were dissolved in toluene (400 μL) and stirred at 140 °C in a pressure tube. After 12 days, the conversion was quantitative. After removal of the solvent and drying at 60 °C in vacuum, the product was recrystallized from hexane (300 μL). Colorless crystals were obtained after 2 days at −20 °C. Yield (131 mg, 0.15 mmol, 67%). 1 H NMR: (400.13, tol-d8, 298 K): δ = 0.18 (s, 18H, Si(CH3)3), 0.85−0.93 (m, 24H, CH3), 1.26−1.29 (m, 4H, CH2-THF), 1.49−1.73 (m, 12H, CH2), 1.69 (s, 6H, CH3-backbone), 1.89−1.97 (m, 4H, CH2), 2.92 (quint, 3J = 6.5 Hz, 4H, CH), 3.49−3.52 (m, 4H, CH2THF), 4.75 (s, 1H, CH-backbone), 6.97−7.05 (m, 6H, CH-arom) ppm. 13C NMR: (100.62 MHz, tol-d8, 298 K): δ = 6.2 (Si(CH3)3), 10.5 (CH3), 11.7 (CH3), 25.2 (CH2-THF), 26.0 (CH3-backbone), 26.7 (CH2), 27.9 (CH2), 40.5 (CH), 69.1 (CH2-THF), 90.5 (CHbackbone), 122.7 (C-arom), 125.6 (C-arom), 136.7 (C-arom), 151.0 (C-arom), 165.2 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, told8, 298 K): δ = −15.86 (Si(CH3)3) ppm. Elemental analysis: Calculated for C47H83SrN3OSi2 (M = 849.99 g/mol): C 66.41, H 9.84, N 4.94; Found: C 66.10, H 9.69, N 4.61. Synthesis of (DIPePBDI)BaN(SiMe3)2·(THF) (6). (DIPePBDI)BaN(SiMe3)2 (102 mg, 0.123 mmol) was dissolved in toluene-d8 (555 μL), THF (8.89 mg, 0.123 mmol, 10 μL) was added, and the solution was stirred for 1 h at room temperature. All volatiles were removed in vacuum at ambient temperature, yielding the title compound in quantitative yield as a pale yellow solid (107 mg, 0.119 mmol). Crystals suitable for X-ray diffraction analysis were grown by cooling a saturated pentane solution at −20 °C. Method B: DIPePBDI-H (135 mg, 0.25 mmol) and Ba[N(SiMe3)2]2·(THF)2 (153 mg, 0.25 mmol) were dissolved in toluene (400 μL) and stirred at 140 °C in a pressure tube. After 9 days, the conversion was quantitative. After removal of the solvent and drying at 60 °C in vacuum, the product was recrystallized from hexane (400 μL). Colorless crystals were obtained after 2 days at −20 °C (164 mg, 0.18 mmol, 73%). 1 H NMR: (400.13, tol-d8, 298 K): δ = 0.13 (s, 18H, Si(CH3)3), 0.88−0.93 (m, 24H, CH3), 1.23−1.26 (m, 4H, CH2-THF), 1.52−1.68 (m, 12H, CH2), 1.72 (s, 6H, CH3-backbone),1.72−1.82 (m, 4H, CH2), 2.67 (quint, 3J = 6.5 Hz, 4H, CH), 3.38−3.41 (m, 4H, CH2THF), 4.78 (s, 1H, CH-backbone), 6.97−7.03 (m, 6H, CH-arom) ppm. 13C NMR: (100.62 MHz, tol-d8, 298 K): δ = 5.8 (Si(CH3)3), 11.4 (CH3), 12.0 (CH3), 25.2 (CH2-THF), 25.6 (CH3-backbone), 27.1 (CH2), 28.8 (CH2), 41.2 (CH), 68.5 (CH2-THF), 90.2 (CHbackbone), 122.8 (C-arom), 125.4 (C-arom), 136.8 (C-arom), 150.6 (C-arom), 162.8 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, told8, 298 K): δ = −18.53 (Si(CH3)3) ppm. Elemental analysis: Calculated for C47H83BaN3OSi2 (M = 899.70 g/mol): C 62.75, H 9.30, N 4.67; Found: C 62.35, H 9.24, N 4.35. Synthesis of (DIPPBDI)CaN(SiMe3)2 (7). DIPPBDI-H (768 mg, 1.83 mmol) and Ca[N(SiMe3)2]2 (695 mg, 1.93 mmol) were dissolved in C6H6 (5 mL) and heated at 70 °C. After 4 days, the conversion was completed and removal of the solvent at 60 °C in vacuum gave the title compound as an off-white powder in quantitative yields (1.10 g, 1.78 mmol). The 1H and 13C NMR data

MD, Innovative Technology), except for THF, which was dried over Na and redistilled. DIPePBDI-H,21 DIPPBDI-H,31 and Ae[N(SiMe3)2]232 were synthesized according to literature procedures. NMR spectra were recorded with a Bruker Avance III HD 400 MHz or a Bruker Avance III HD 600 MHz spectrometer. The spectra were referenced to the respective residual signals of the deuterated solvents. Elemental analysis were obtained with a Hekatech EuroVektor EA3000. Crystal structures have been measured on a SuperNova (Agilent) diffractometer with dual Cu and Mo microfocus sources and an Atlas S2 detector. Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC 1899349 (1), 1899350 (3), 1899351 (4), 1899352 (5), 1899353 (6), 1899354 (8), 1899355 (9), and 1899356 (13). Synthesis of (DIPePBDI)CaN(SiMe3)2 (1). DIPePBDI-H (655 mg, 1.23 mmol) and Ca[N(SiMe3)2]2 (467 mg, 1.29 mmol) were dissolved in toluene (7 mL) and stirred at 140 °C in a pressure tube. After 9 days, the conversion was quantitative and the solvent was removed in vacuum at 60 °C. The crude product was recrystallized from boiling hexane (2 mL). Colorless crystals were obtained after 2 days at −20 °C (753 mg, 1.03 mmol, 80%). 1 H NMR: (400.13, C6D6, 298 K): δ = 0.14 (s, 18H, Si(CH3)3), 0.88 (t, 3J = 7.4 Hz, 12H, CH3), 0.93 (t, 3J = 7.4 Hz, 12H, CH3), 1.62−1.75 (m, 16H, CH2), 1.66 (s, 6H, CH3-backbone), 2.94 (quint, 3 J = 6.6 Hz, 4H, CH), 4.70 (s, 1H, CH-backbone), 7.01−7.03 (m, 4H, CH-arom), 7.09 (dd, 2H, 3J = 8.6 Hz,, 2J = 6.5 Hz CH-arom) ppm. 13 C NMR: (100.62 MHz, C6D6, 298 K): δ = 5.5 (Si(CH3)3), 11.9 (CH3), 12.2 (CH3), 25.5 (CH3-backbone), 27.3 (CH2), 29.0 (CH2), 42.00 (CH), 90.2 (CH-backbone), 124.1 (C-arom), 125.5 (C-arom), 137.8 (C-arom), 149.0 (C-arom), 161.17 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, C6D6, 298 K): δ = −15.26 (Si(CH3)3) ppm. Elemental analysis: Calculated for C43H75CaN3Si2 (M = 730.34 g/ mol): C 70.72, H 10.35, N 5.75; Found: C 70.62, H 10.17, N 5.55. Synthesis of (DIPePBDI)BaN(SiMe3)2 (3). DIPePBDI-H (2.10 g, 3.96 mmol) and Ba[N(SiMe3)2]2 (1.81 g, 3.96 mmol) were dissolved in hexane (10 mL) and stirred at 140 °C. After 5 days, the conversion was quantitative and the solvent was removed in vacuum at 60 °C. The crude product was recrystallized from hexane (7 mL). Large colorless crystals were obtained after 2 days at room temperature. Yield: (2.69 g, 3.25 mmol, 82%). 1H NMR: (600.13, C6D6, 298 K): δ = −0.04 (s, 18H, Si(CH3)3), 0.85 (t, 3J = 7.4 Hz, 12H, CH3), 0.99 (t, 3 J = 7.4 Hz, 12H, CH3), 1.50−1.68 (m, 16H, CH2), 1.81 (s, 6H, CH3backbone), 2.67 (quint, 3J = 6.6 Hz, 4H, CH), 4.82 (s, 1H, CHbackbone), 7.05−7.08 (m, 6H, CH-arom) ppm. 13C NMR: (150.96 MHz, C6D6, 298 K): δ = 5.6 (Si(CH3)3), 12.0 (CH3), 13.4 (CH3), 24.7 (CH3-backbone), 27.3 (CH2), 29.2 (CH2), 42.2 (CH), 93.3 (CH-backbone), 123.8 (C-arom), 126.2 (C-arom), 137.9 (C-arom), 148.0 (C-arom), 162.0 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, C6D6, 298 K): δ = −19.16 (Si(CH3)3) ppm. Elemental analysis: Calculated for C43H75BaN3Si2 (M = 827.59 g/mol): C 62.41, H 9.13, N 5.08; Found: C 61.93, H 9.03, N 4.70. Although the C value is outside the range viewed as establishing analytical purity, it is provided to illustrate the best values obtained to date. Synthesis of (DIPePBDI)CaN(SiMe3)2·THF (4). Method A: (DIPePBDI)CaN(SiMe3)2 (90.0 mg, 0.123 mmol) was dissolved in toluene-d8 (555 μL), THF (8.89 mg, 0.123 mmol, 10 μL) was added, and the solution was stirred for 1 h at room temperature. All volatiles were removed in vacuum at ambient temperature, yielding the title compound in quantitative yields as a pale yellow solid (95.9 mg, 0.119 mmol). Crystals suitable for X-ray diffraction analysis were grown by cooling a saturated hexane solution at −20 °C. Method B: DIPePBDI-H (115 mg, 0.22 mmol) and Ca(N(SiMe3)2)2·THF2 (109 mg, 0.22 mmol) were dissolved in toluene (400 μL) and stirred at 140 °C in a pressure tube. After 14 days, the conversion was quantitative. After removal of the solvent and drying at 60 °C in vacuum, the product was recrystallized from hexane (300 μL). Colorless crystals were obtained after 2 days at −20 °C. Yield (127 mg, 0.15 mmol, 72%). 1 H NMR: (600.13, tol-d8, 298 K): δ = 0.04 (s, 18H, Si(CH3)3), 0.88 (t, 3J = 7.4 Hz, 12H, CH3), 0.96 (t, 3J = 7.4 Hz, 12H, CH3), 1.44−1.46 (m, 4H, CH2-THF), 1.62 (s, 6H, CH3-backbone), 1.64− 1.70 (m, 12H, CH2), 1.78−1.85 (m, 4H, CH2), 2.93 (quint, 3J = 6.1 G


Article

Organometallics fit to those reported earlier.23 29Si NMR: (119.22 MHz, C6D6, 298 K): δ = −16.11 (Si(CH3)3) ppm. Synthesis of (DIPPBDI)SrN(SiMe3)2 (8). DIPPBDI-H (834 mg, 1.99 mmol) and Sr[N(SiMe3)2]2 (839 mg, 2.05 mmol) were dissolved in C6H6 (5 mL) and heated at 70 °C. After 16 h, DIPPBDI-H was consumed and the solvent was removed at ambient temperatures in vacuum. After washing with pentane (5 mL), the compound was crystallized by slow diffusion of pentane into a saturated benzene solution (3 mL) at room temperature to yield the title compound as colorless crystals (821 mg, 1.23 mmol, 62%). 1 H NMR: (600.13, C6D6, 298 K): δ = 0.07 (s, 18H, Si(CH3)3), 1.26−1.30 (m, 24H, CH3), 1.75 (s, 6H, CH3-backbone), 3.17 (hept, 3 J = 6.7 Hz, 4H, CH), 4.85 (s, 1H, CH-backbone), 7.12−7.17(m, 6H, CH-arom) ppm. 13C NMR: (150.92 MHz, C6D6, 298 K): δ = 5.8 (Si(CH3)3), 24.5 (CH3), 25.0 (CH3-backbone), 25.7 (CH3), 28.7 (CH), 91.2 (CH-backbone), 124.1 (C-arom), 124.6 (C-arom), 140.7 (C-arom), 147.2 (C-arom), 162.1 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, C6D6, 298 K): δ = −16.91 (Si(CH3)3) ppm. Elemental analysis: Calculated for C35H59SrN3Si2 (M = 665.67g/mol): C 63.15, H 8.93, N 6.31; Found: C 63.02, H 9.08, N 5.98. Synthesis of (DIPPBDI)BaN(SiMe3)2 (9). DIPPBDI-H (130 mg, 0.31 mmol) and Ba[N(SiMe3)2]2 (142 mg, 0.31 mmol) were dissolved in C6D6 (600 μL) and heated at 70 °C. After 6 h, DIPP BDI-H was consumed and the solution was concentrated to half of its original volume. At 8 °C, colorless crystals were obtained (84 mg, 0.11 mmol, 38%). 1H NMR: (600.13, C6D6, 298 K): δ = 0.16 (s, 18H, Si(CH3)3), 1.27 (d, 3J = 6.7 Hz, 12H, CH3), 1.32 (d, 3J = 7.0 Hz, 12H, CH3), 1.73 (s, 6H, CH3-backbone), 3.10 (hept, 3J = 6.7 Hz, 4H, CH), 4.78 (s, 1H, CH-backbone), 7.11−7.17(m, 6H, CH-arom) ppm. 13 C NMR: (150.92 MHz, C6D6, 298 K): δ = 6.16 (Si(CH3)3), 24.5 (CH3), 24.7 (CH3-backbone), 25.6 (CH3), 28.6 (CH), 90.7 (CHbackbone), 123.9 (C-arom), 124.0 (C-arom), 140.3 (C-arom), 147.7 (C-arom), 162.5 (CN-backbone) ppm. 29Si NMR: (119.22 MHz, C6D6, 298 K): δ = −18.08 (Si(CH3)3) ppm. Elemental analysis: Calculated for C35H59BaN3Si2 (M = 715.39 g/mol): C 58.76, H 8.31, N 5.87; Found: C 59.36, H 7.90, N 5.54. Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. Synthesis of (DIPPBDI)SrN(SiMe3)2·THF (11). DIPPBDI-H (352 mg, 0.841 mmol) and Sr[N(SiMe3)2]2·THF2 (465 mg, 0.841 mmol) were dissolved in C6H6 (5 mL) and heated at 70 °C. After 5 days, DIPP BDI-H was consumed and the solvent was removed in vacuum. The crude product was dissolved in pentane (2 mL), and slow evaporation of the solvent at −20 °C led to precipitation of the desired product as a white powder in 52% yield (322 mg, 0.437 mmol). The 1H, 13C, and 29SiNMR data fit those reported earlier.29 Synthesis of (DIPPBDI)BaN(SiMe3)2·THF (12). DIPPBDI-H (244 mg, 0.58 mmol) and Ba[N(SiMe3)2]2·THF2 (351 mg, 0.58 mmol) were dissolved in C6H6 (5 mL) and heated at 70 °C. After 2 days, DIPP BDI-H was consumed and the solvent was removed in vacuum. The crude product was dissolved in pentane (1.5 mL) and stored at −20 °C. After 1 day, the desired product was obtained as colorless crystals in 52% yield (293 mg, 0.30 mmol). The 1H, 13C, and 29 SiNMR data fit those reported earlier.9

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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 Author

*E-mail: [email protected]. ORCID

Jens Langer: 0000-0002-2895-4979 Sjoerd Harder: 0000-0002-3997-1440 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Ms. C. Wronna and Ms. A. Roth (University of Erlangen-Nürnberg) for numerous CHN analyses and J. Schmidt and Dr. C. Färber (University of Erlangen-Nürnberg) for assistance with the NMR analyses.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00211. Details for the crystals structure determinations, selected NMR spectra, and an experimental description of investigations on complex stability (PDF) Accession Codes

CCDC 1899349−1899356 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 H


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Heteroleptic Heavier Alkaline Earth Metal Amide Complexes

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