Syntheses and Structures of Homo- and Heteroleptic Beryllium

Article pubs.acs.org/Organometallics

Syntheses and Structures of Homo- and Heteroleptic Beryllium Complexes Containing N,N′-Chelating Ligands Melike Bayram, Dominik Naglav, Christoph Wölper, and Stephan Schulz* Institute of Inorganic Chemistry and Center for Nanointegration Duisburg-Essen (CENIDE), University of Duisburg-Essen, 45117 Essen, Germany S Supporting Information *

ABSTRACT: Reactions of BeEt2 with S(NSiMe3)2 and carbodiimides RNCNR yielded homoleptic bisdiimidosulfinate [EtS(NSiMe3)2]2Be (1) and amidinate complexes [EtC(NAr)2]2Be (Ar = 2,6-i-Pr2-C6H3 2, SiMe3 3). In addition, the heteroleptic amidinate [t-BuC(NAr)2]BeEt (4) and β-diketiminate [HC(CMeNAr′)2]Be(i-Bu) (6) (Ar′ = 2,4,6-Me3C6H2) complexes were obtained by alkane elimination reactions of BeEt2 with t-BuC(NSiMe3)N(H)SiMe3 and Be(i-Bu)2 with β-diketimine [Ar′N(H)C(Me)CHC(Me)NAr′], respectively. In addition, 4 was found to react with dry oxygen with formation of the corresponding alkoxide [t-BuC(NAr)2]BeOEt (5). 1−6 were characterized by multinuclear NMR (1H, 9Be, 13C) and IR spectroscopy as well as by single-crystal X-ray diffraction (1, 2, 4, 5, 6).

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leading to a three-center two-electron π-bond stretching. The stability of the compound most likely originates from π backdonation from Be to the CAAC ligands. In addition, the search for isolable Be(I) compounds resulted in the synthesis of heteroleptic complexes of the general type LBeX, in which L represents N,N′,N″-chelating tris(pyrazolyl)borate A9 or N,N′chelating ligands such as β-diketiminate B,10 amidinate C,11 and bis(phosphinimino)methanide substituents D (Scheme 1).12

INTRODUCTION Metal organic as well as inorganic beryllium complexes, which have led a Cinderella-like existence over the last century, have received growing interest in recent years, and several research groups started to investigate their synthesis, structure, and reactivity in detail.1 Beryllium compounds are expected to show interesting chemical properties, which originate to a large extent from the small size of the Be atom, its high Lewis acidity, and its ability to form covalent bonds. This combination renders this metal very interesting for further applications in chemical synthesis. Quantum chemical calculations for instance showed that cationic 1-tris(pyrazolyl)borate beryllium complexes are even able to bind heavier noble gas atoms.2 Unfortunately, the high toxicity of beryllium and beryllium compounds3 hampered them from becoming widely applied in technical applications, in remarkable contrast to the widely used complexes of group 2 metals such as Grignard reagents as well as group 12 metals, in particular the well-established Zn chemistry. Metal organic beryllium chemistry started with the synthesis of Cp-substituted beryllium complexes by E. O. Fischer in 19594 and their structural characterization including quantum chemical calculations.5 Power et al. later on reported on Be complexes containing sterically demanding terphenyl ligands,6 while more recently the synthesis of base-stabilized beryllium complexes such as NHC-stabilized beryllium compounds was reported.7 Very recently, Braunschweig et al. synthesized the first subvalent beryllium compound BeL2 (L = cyclic (alkyl) (amino)carbenes “CAAC”) by reduction of the corresponding adducts of the type [Be(L)Cl2] in the presence of a second equivalent of π-acidic CAAC ligand.8 The Be−C bonds in this spectacular compound were described as a combination of donor−acceptor interactions between ground-state singlet MeL ligands and a Be(0) atom (1s22s02p2 electronic configuration), © XXXX American Chemical Society

Scheme 1. N,N′-Chelating Ligands A−D

These types of ligands have been successfully applied for the synthesis of the corresponding Mg(I) and Zn(I) complexes,13 but stable Be(I) compounds have not been obtained, to date, even though computational studies predicted monovalent Be(I) compounds containing Be−Be single bonds to be stable.14 We have a long-standing interest in zinc complexes containing N,N′-chelating ligands and started only recently to compare their structures and reactivity with those of comparable beryllium complexes. We structurally characterized heteroleptic and homoleptic beryllium complexes containing amide,15 bis(diphenylphosphinimino)methanide, and -methanediide12 as well as tris(pyrazolyl)borate ligands.9 We herein expand our studies on homoleptic and heteroleptic beryllium Received: November 17, 2016

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DOI: 10.1021/acs.organomet.6b00865 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

monoclinic space group C2/c with the molecule placed on a 2fold rotational axis (Figure 1). 2 (Figure 2) is a pentane solvate

complexes containing N,N′-chelating bisdiimidosulfinate, amidinate, and β-diketiminate ligands.

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RESULTS AND DISCUSSION Reactions of BeEt2 with two equivalents of eith S(NSiMe3)2 or C(NR)2 at −78 °C occurred with 2-fold insertion of the sulfur diimide and carbodiimide ligand into the Be−C bond and subsequent formation of the homoleptic bisdiimidosulfinate [EtS(NSiMe3)2]2Be (1) and bisamidinate complexes [EtC(NR)2]2Be (R = Ar = 2,6-i-Pr2-C6H3 2, SiMe3 3), respectively (Scheme 2). 1−3 were also obtained from equimolar reactions in less than 50% yield, and these reaction mixtures still contained the starting reagent BeEt2. Scheme 2. Synthesis of 1−3

Figure 1. Solid-state structure of 1. H atoms are omitted for clarity; thermal ellipsoids are shown at the 50% probability level. The part in pale colors is generated via 2-fold symmetry (−x, +y, 1/2−z).

and crystallizes in the orthorhombic space group Pna21. Since the Be atoms in 1 and 2 are each coordinated by two chelating ligands, the coordination number of Be in both complexes is 4. The planes of the ligand’s backbones are roughly orthogonal (1: 84.17(3)°, 2: 70.96(15)°), leading to a distorted tetragonal Compounds 1 to 3 are soluble in toluene and THF, respectively. The 1H and 13C NMR spectra show the expected resonances of the diimidosulfinate (1) and amidinate (2, 3) ligands, while no resonances due to the presence of a Be-Et group were observed. Interestingly the triplet for the methyl protons of the Cbb-Et group for 2 (0.27 ppm) is shifted to significantly higher field in comparison to that of 1 (1.02 ppm) and 3 (1.10 ppm), while the quartets of all three compounds are comparable (2.03 1, 2.07 2, 2.13 ppm 3). We are not aware of a comparable high-field shift for any metal complex containing this type of amidinate ligand ([EtC(NR)2]−). Only the aluminum and titanium complexes EtC(Ni-Pr)2AlMe2 (0.76 ppm)16 and [EtC(N(2-OCH3C6H5))2]2Ti[N(Me2)]2 (0.78 ppm)17 show high-field-shifted triplets, but their values are still far away from that observed for 2. The reason for the unusual resonance of 2 must originate from an electronic effect. Looking at the crystal structure of 2, we propose an ASIS-like effect (aromatic solvent-induced shif t), which is caused by the flanking Ar substituents.18 The Et group rotates between positions in which the Me group hovers over the π-system, causing an upfield shift in the 1H NMR spectrum. The 9Be NMR spectra of 1−3 each show one sharp singlet at 4.13 (1), 8.06 (2), and 5.22 ppm (3), respectively, which are significantly shifted to lower field compared to BeEt2 (15.75 ppm (C6D6)/ 17.09 ppm (tol-d8)) due to the deshielding effect of the N atoms of the diimidosulfinate (1) and amidinate (2, 3) ligands. Single crystals of 1 to 3 were obtained upon slow recrystallization from a solution in pentane (1, 3) and toluene (2). The crystals of 1 and 2 were of sufficient quality to determine their solid-state structure. 1 crystallizes in the

Figure 2. Solid-state structure of 2. H atoms and disordered solvent are omitted for clarity; thermal ellipsoids are shown at the 50% probability level; second component of the disorder is shown in pale color. B


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Organometallics Table 1. Selected Bond Lengths [Å] and Angles [deg] of 1, 2, 4, 5, and 6 1

2

4

Be−N

1.7544(10) 1.7240(9)

1.750(5) 1.731(5) 1.722(5) 1.761(5)

N−Cbb/Sa

1.6165(7) 1.6224(7)

1.333(4) 1.329(4) 1.319(4) 1.344(4)

C−Cbb/S

1.8092(9)

1.512(5) 1.522(11) 1.523(12)

1.661(3) 1.685(3) 1.663(3) 1.690(3) 1.664(3) 1.689(3) 1.347(2) 1.3369(19) 1.345(2) 1.3401(19) 1.347(2) 1.340(2) 1.525(2) 1.525(2) 1.527(2) 1.709(3) 1.710(3) 1.703(3) 104.71(14) 105.14(14) 105.20(15) 88.54(12) 87.86(13) 88.42(13) 87.47(13) 88.12(13) 87.31(13) 78.88(12) 78.97(13) 79.11(13)

Be−C

a

N−Cbb/S−N

93.59(3)

109.1(3) 109.0(3)

Cbb/S−N−Be

89.68(4) 90.56(4)

86.6(2) 87.3(2) 85.8(3) 88.2(3)

N−Be−N

129.60(10) 122.97(10) 85.48(3) 119.03(3)

C/Si−N−Be

136.65(5) 137.38(4)

137.0(3) 77.0(2) 77.0(2) 138.2(3) 117.3(3) 143.4(3) 144.4(3) 146.3(3) 145.5(3) 147.1(3)

139.41(14) 139.64(14) 139.46(15) 139.09(14) 139.95(14) 137.52(14)

5

6

1.758(2) 1.793(3)

1.6283(12) 1.6270(13)

1.338(2) 1.349(2)

1.3355(10) 1.3366(10)

1.547(2)

1.7350(13)

107.52(14)

88.77(12) 87.02(12)

122.04(7) 122.56(7)

75.23(10)

108.20(7)

140.60(13) 145.96(13)

118.56(6) 117.68(7)

Cbb = Cbackbone.

Since heteroleptic complexes were not accessible by insertion reaction, we investigated the alkane elimination reaction of BeEt2 with one equivalent of the amidine t-BuC(NAr)2H (Scheme 3). The reaction smoothly proceeded with gas elimination at −78 °C and formation of the heteroleptic amidinate complex [t-BuC(NAr)2]BeEt (4). Reactions with a 2-fold amount of the amidine also yielded only 4, most likely due to the sterically demanding amidinate ligand, which hampers the formation of the homoleptic complex [tBuC(NAr)2]2Be. In addition, 4 was found to react with oxygen with insertion into the Be−Et bond and subsequent formation of [{t-BuC(N-2,6-i-Pr2C6H3)2}BeOEt]2 (5). Moreover, an analogous reaction between Be(i-Bu)2 and the β-diketimine HC[C(Me)NAr′]2H (Ar′ = 2,4,6-Me3-C6H2) also occurred with elimination of isobutane and formation of the homoleptic complex HC[C(Me)NAr′]Be(i-Bu) (6). 1 H and 13C NMR spectra recorded in C6D6 show the expected BeEt resonances for the heteroleptic complex 4 (1H: 0.35, 1.34 ppm; 13C: 7.3, 16.1 ppm), while the 1H NMR

coordination sphere of the Be atoms. The endocyclic N−Be−N bond angles are smaller than the exocyclic ones (∼80° vs ∼130°, see Table 1), which is a result of the tetrahedron’s stretching along a (noncrystallographic) S4-axis. The Be−N bond lengths and exocyclic N−Be−N angles are comparable to those previously reported for four-membered BeN2C rings such as Be{(NSiMe3)2CPh}26b and (PPh4)[Be{(NSiMe3)2CPh}Cl2],11a respectively. The best planes of the four-membered BeN2C rings observed in the homoleptic complex Be{(NSiMe3)2CPh}2 are also approximately orthogonal (89.18°). The stronger distortion in 2 is likely caused by the higher steric demand of the 2,6-i-Pr2-C6H3 groups. Since no further Be bisdiimidosulfinate complexes are available for comparison in the CSD, a search for any metal complexes was conducted.19 The largest N−M−N intra-annular angles of ∼77° were found in cubane-type Li compounds. All N−M bonds are at least 1.9 Å long and thus more than 0.15 Å longer, which necessarily leads to a widening of the N−M−N angle in 1 compared to database results. C


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Organometallics Scheme 3. Synthesis of 4−6

resonances of 5 are significantly shifted to higher field (1.51 and 4.26 ppm). For compound 6 we observe a high-field shift of the i-Bu groups (1H: −0.23, 0.86, 1.26 ppm) in C6D6 and a sharp singlet for the Be resonance at 17.01 ppm. The beryllium resonances in the 9Be NMR spectra appear at 17.90 ppm (4), 3.61 ppm (5 in thf), and 17.06 ppm (6 in C6D6). These different values clearly prove the different coordination geometry of the central Be atom in 4 (c.n. 3) and 5 (c.n. 4). The low-field shift of 4 and 6 underlines the presence of a three-coordinated Be species in solution and excludes the formation of any bridged dimeric species. Crystals of 4 and 5 suitable for single-crystal X-ray diffraction studies were obtained upon slow recrystallization from solutions in pentane at ambient temperature. 4 crystallizes in the triclinic space group P1̅ with three independent molecules in the asymmetric unit, which are related by pseudoinversion and pseudotranslational symmetry. Thorough investigation of the reciprocal space did not yield any smaller unit cell to incorporate this symmetry without ignoring parts of the reflections. 5 crystallizes in the orthorhombic space group Pbca with the molecule placed on a center of inversion. Bond lengths and angles match well with those observed in 2 and the previously reported structures. The Be−N bond lengths in 4 are slightly shorter, but the difference is barely significant taking the 3σ criterion into account. The most striking difference is their conformation. 4 carries only one amidinate ligand and shows the least distorted conformation. Both Cbb−N−Cipso−Cortho torsions and the angles between the best planes of the central four-membered ring and the phenyl rings of the Ar residues are approximately 90° (Cbb−N−Cipso− C ortho 81.5(3)−104.0(3)°; angle between best planes 86.99(7)−89.73(7)°). The residual t-Bu group of the backbone is well aligned with the four-membered ring (deviation of central C from the rings best plane: 0.004(4), 0.021(4), 0.121(4) Å). The lower steric demand of the Et group in the backbone of 2 still allows a coordination of two amidinate ligands but leads to a distortion of the conformation. Cbb−N− Cipso−Cortho range from 63.0(5)° to 105.9(4)°, and the angles between the best planes range from 73.4(2)° to 88.2(2)°. The α-C of the Et group of the N1/N2 ligand lies well within the best plane of the four-membered ring (distance to best plane 0.09(8) Å); however, this value is not very precise. The two αC of the disordered Et group of the N3/N4 ligand significantly deviate from the best plane (0.107(14) and −0.273(14) Å). Steric hindrance seems to be a reasonable explanation for the

Figure 3. Solid-state structure of 4 showing one independent molecule. H atoms are omitted for clarity; thermal ellipsoids are shown at the 50% probability level.

Figure 4. Solid-state structure of 5. H atoms are omitted for clarity; thermal ellipsoids are shown at the 50% probability level. The part displayed in pale colors is generated via inversion symmetry (1−x, −y, 1−z).

disorder in this case. In 5 the bridging ethoxy ligands relieve the steric stress; hence the coordination of two amidinate ligands with backbone t-Bu groups is possible within this single molecule. The distortion in 5 isthough slightly stronger similar to that observed in 2 ((C bb −N−C ipso −C ortho 109.41(15)°, 116.31(18)°; angle between best planes 73.13(7)°, 65.78(7)°). However, the α-C atom of the t-Bu deviates further from the best plane of the four-membered ring (0.561(4) Å). Colorless crystals of 6, which were obtained from a solution in toluene at 6 °C after 1 day, belong to the triclinic crystal system with space group P1.̅ The asymmetric unit comprises one molecule. The Be atom is coordinated by one chelating βdiketiminate and one i-Bu group and adopts an almost ideal trigonal-planar coordination sphere (sum of the bond angles: D


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Organometallics 359.88°). The C−Be−N angles are ∼10° larger than the endocyclic N−Be−N angle (see Table 1). The six-membered C3N2Be ring in 6 is almost flat (rms deviation from the best plane 0.0264 Å), while a slightly distorted boat-type conformation was observed for the CP2N2Be metallacycle in the beryllium bis(diphenylphosphinimino)methanide complex [CH(PPh2NAr)2]BeEt.12 The Be−N, C−N, and C−C bond lengths and endocyclic N−Be−N, Be−N−C, and C−C−C angles within the β-diketiminate unit match well with those previously reported for β-diketiminate Be alkyl compounds.10 The Be−N bond lengths (av 1.628 Å) of 6 are about 5 pm shorter than those observed for 4 (av 1.673 Å) and roughly 13 pm shorter compared to the average value observed for 5 (av 1.775 Å), whereas the N−Be−N bond angle (108.20(7)°) of 6 is significantly larger compared to those of 4 (av 78.99°) and 5 (75.23(10)°), as is typical for six-membered rings compared to four-membered rings. In contrast, the Be−C bond length of 6 (1.7350(13) Å) is about 3 pm elongated compared to the average value of 4 (av 1.707 Å).

Colorless crystals of 1 were obtained after storage of the solution for 24 h at 18 °C. Yield: 0.21 g (91%). Mp: 84.7 °C (dec). 1H NMR (300 MHz, C6D6, 25 °C): δ = 0.29 (s, 36 H, SiMe3), 1.02 (t, 3JHH = 7.5 Hz, 6 H, SCH2CH3), 2.30 (sept, 3JHH = 7.5 Hz, 4 H, SCH2CH3). 9Be NMR (300 MHz, C6D6, 25 °C): δ = 4.13. 13C NMR (75 MHz, C6D6, 25 °C): 2.3 (SiMe3), 2.8 (SiMe3), 4.8 (SCH2CH3), 56.1 (SCH2CH3). ATR-IR: ν = 2947, 2894, 1570, 1458, 1420, 1381, 1238, 1016, 907, 825, 761, 744, 719, 669, 635, 595, 545, 516, 469, 399 cm−1. [EtC(NAr)2]2Be, 2. A 0.20 g (0.55 mmol) amount of C(NAr)2 was dissolved in 10 mL of pentane at −78 °C, and 0.018 g (0.28 mmol) of BeEt2 was added via syringe. The solution was warmed to ambient temperature within 12 h, and the solvent was reduced to 1 mL. Colorless crystals were obtained after storage for 24 h at 20 °C. Yield: 0.20 g (90%). Mp: >220 °C (dec). 1H NMR (300 MHz, C6D6, 25 °C): δ = 0.27 (t, 3JHH = 7.7 Hz, 6 H, CH2CH3), 0.37 (d, 3JHH = 6.8 Hz, 12 H, CH(CH3)2), 1.11 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.33 (d, 3 JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.50 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 2.07 (quart, 3JHH = 7.7 Hz, 4 H, CH2CH3), 3.11 (sept, 3 JHH = 6.7 Hz, 4 H, CH(CH3)2), 3.93 (sept, 3JHH = 6.7 Hz, 4 H, CH(CH3)2), 6.92 (d, 3JHH = 1.8 Hz, 2 H, m-Ar), 6.95 (d, 3JHH = 1.8 Hz, 2 H, m-Ar), 7.07 (t, 3JHH = 7.5 Hz, 4 H, p-Ar), 7.13 (d, 3JHH = 1.8 Hz, 2 H, m-Ar), 7.15 (d, 3JHH = 1.8 Hz, 2 H, m-Ar). 9Be NMR (300 MHz, C6D6, 25 °C): δ = 8.06. 13C NMR (75 MHz, C6D6, 25 °C): 9.0 (CCH2CH3), 23.7 (CCH2CH3), 24.0 (CCH2CH3), 24.6 (CH(CH3)2), 25.0 (CH(CH3)2), 25.1 (CCH2CH3), 28.1 (CH(CH3)2), 28.4 (CH(CH3)2), 123.9 (Ar), 124.3 (Ar), 125.5 (Ar), 141.6 (Ar), 144.5 (Ar), 144.8 (Ar), 183.0 (NCN). ATR-IR: ν = 3058, 2960, 2928, 2868, 1640, 1582, 1473, 1420, 1381, 1361, 1314, 1253, 1240, 1215, 1185, 1100, 1070, 1045, 1017, 981, 932, 922, 885, 842, 795, 772, 740, 701, 603, 543, 499, 438, 410 cm−1. [EtC(NSiMe 3)2] 2Be, 3. A 0.20 g (1.1 mmol) amount of C(NSiMe3)2 was dissolved in 15 mL of pentane at −78 °C, and 0.036 g (0.54 mmol) of BeEt2 was added via syringe. The solution was slowly warmed to ambient temperature within 12 h. 3 was obtained as a colorless liquid after evaporation of the solvent. Yield: 0.22 g (91%). 1 H NMR (300 MHz, C6D6, 25 °C): δ = 0.23 (s, 36 H, SiMe3), 1.10 (t, 3 JHH = 7.9 Hz, 6 H, CCH2CH3), 2.13 (quart, 3JHH = 7.7 Hz, 4 H, CCH2CH3). 9Be NMR (300 MHz, C6D6, 25 °C): δ = 5.22. 13C NMR (75 MHz, C6D6, 25 °C): 1.7 (SiMe3), 12.6 (CCH2CH3), 31.3 (CCH2CH3), 185.7 (NCN). ATR-IR: v = 2954, 2897, 1468, 1374, 1381, 1304, 1253, 1245, 1234, 1120, 1072, 969, 907, 826, 752, 681, 614, 489, 467 cm−1. [t-BuC(NAr)2]BeEt, 4. A 0.20 g (0.46 mmol) amount of (tBuC(NAr)2H was dissolved in 15 mL of toluene, the solution was cooled to −78 °C, and 0.032 g (0.46 mmol) of BeEt2 was added via syringe. The solution was slowly warmed to ambient temperature within 12 h. The solvent was evaporated, and the crude product was redissolved in a minimum amount of pentane. Colorless crystals were formed upon storage for 1 d at 20 °C. Yield: 0.19 g (93%). Mp: >220 °C (dec). 1H NMR (300 MHz, C6D6, 25 °C): δ = 0.35 (quart, 3JHH = 8.2 Hz, 2 H, BeCH2CH3), 0.87 (s, 9 H, CMe3), 1.29 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.31 (d, 3JHH = 6.9 Hz, 12 H, CH(CH3)2), 1.34 (t, 3 JHH = 6.6 Hz, 3 H, BeCH2CH3), 3.50 (sept, 3JHH = 6.9 Hz, 4 H, CH(CH3)2), 7.05−7.10 (m, 6 H, Ar). 9Be NMR (300 MHz, C6D6, 25 °C): δ = 17.94. 13C NMR (75 MHz, C6D6, 25 °C): δ = 7.3 (BeCH2CH3), 16.1 (BeCH2CH3), 27.5 (CH(CH3)2), 34.3 (CH(CH3)2), 43.9 (CMe3), 47.8 (CMe3), 129.1 (Ar), 131.8 (Ar), 134.7 (Ar), 146.2 (Ar), 195.2 (NCN). ATR-IR: ν = 3061, 2998, 2959, 2928, 2902, 2861, 2783, 1625, 1583, 1447, 1403, 1362, 1319, 1203, 1186, 1075, 1050, 1028, 986, 934, 838, 799, 754, 628, 423, 399 cm−1. [(t-BuC(NAr)2)BeOEt]2, 5. Very few (∼10) colorless crystals of 5 were isolated from a solution of 4 in pentane after storage for 7 d at 20 °C. Mp: > 220 °C (dec). The formation of 5 clearly resulted from a partial oxidation of 4 due to the presence of small amounts of oxygen. 1 H NMR (300 MHz, C6D6, 25 °C): δ = 0.95 (s, 18 H, CMe3), 1.04 (d, 3 JHH = 6.6 Hz, 24 H, CH(CH3)2), 1.31 (d, 3JHH = 6.8 Hz, 24 H, CH(CH3)2), 1.51 (t, 3JHH = 7.3 Hz, 4 H, OCH2CH3), 3.74 (sept, 3JHH = 6.8 Hz, 8 H, CH(CH3)2), 4.26 (quart, 3JHH = 6.9 Hz, 6 H, OCH2CH3), 7.00−7.10 (m, 12 H, Ar). 9Be NMR (300 MHz, THF, 25

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CONCLUSION Homoleptic Be complexes containing N,N′-chelating substituents were synthesized by an insertion reaction of BeEt2 with carbodiimides (C[NR]2) and sulfurdiimide S(NSiMe3)2, whereas analogous reactions of ZnEt2 yielded only the heteroleptic complexes LZnEt.20 These findings clearly prove the increased reactivity of BeEt2 compared to ZnEt2, resulting from the higher nucleophilic character of the Be-Et group compared to the Zn-Et group due to the higher bond polarity. In contrast, heteroleptic beryllium complexes LBeR containing 3-foldcoordinated Be atoms were obtained from alkane elimination reactions of beryllium dialkyls BeR2 with equimolar amounts of amidine and β-diketimine, as was previously reported for the corresponding ZnR2 reactions.21 Both reactions provide access to the desired complexes in almost quantitative yield, and since either no (insertion reaction) or only volatile side products (alkane elimination) are formed, the workup of the resulting complexes is very easy since filtration of any side products as is the case in salt elimination reactions is avoided.

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EXPERIMENTAL SECTION

General Procedures. Caution! Beryllium and organometallic beryllium compounds have to be handled with appropriate safety precautions since they are regarded as highly toxic and carcinogenic and have allergic potential when inhaled, with risk of causing chronic beryllium disease (CBD).1a,22 Beryllium dialkyl compounds are extremely reactive. They burn upon contact with air under the formation of extremely fine and toxic BeO dust. Therefore, all experiments were performed in a glovebox (MBraun) under an Ar atmosphere or with standard Schlenk techniques. Solvents were carefully dried over Na/K and degassed prior to use. S(NSiMe3)2,23 C(N-2,6-i-Pr2C6H3)2,24 tBuC(N-2,6-i-Pr2-C6H3)2H,21 BeEt2,25 and Be(i-Bu)225 were prepared according to literature procedures. NMR spectra were recorded on a Bruker Avance 300 spectrometer at 25 °C at 300 MHz (1H), 42 MHz (9Be), and 75 MHz (13C) and referenced to internal C6D5H (1H: δ = 7.154; 13C: δ = 128.0). IR spectra were recorded on a Bruker ALPHAT FT-IR spectrometer equipped with a single-reflection ATR sampling module. Melting points were measured in sealed capillaries and were not corrected. Elemental analyses were not determined due to the expected toxicity of 1−6. The purity of 1−6 was checked by NMR spectroscopy. [EtS(NSiMe3)2]2Be, 1. A 0.032 g (0.48 mmol) amount of BeEt2 was dissolved in 5 mL of pentane, the solution was cooled to −78 °C, and 0.20 g (0.96 mmol) of S(NSiMe3)2 was added via syringe. The solution was slowly warmed to ambient temperature within 12 h. E


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Organometallics °C): δ = 3.62. 13C NMR could not be recorded due to the poor solubility of 5 in thf-d8, C6D6, CD3CN, and CDCl3. ATR-IR: ν = 3340, 3062, 2960, 2928, 2868, 1656, 1613, 1585, 1460, 1433, 1393, 1383, 1362, 1320, 1211, 1182, 1172, 1111, 1101, 1046, 971, 933, 916, 858, 822, 797, 743, 692, 652, 613, 554, 541, 476, 435, 413, 388 cm−1. HC[C(Me)NAr′]Be(i-Bu), 6. A 335 mg (1 mmol) amount of HC[C(Me)NAr′]H was dissolved in 80 mL of toluene in a Schlenk vessel, which was then closed with a silicon rubber stopcock. A steel cannula was forced through the silicon stopcock to allow pressure release from the inside of the vessel. A 123 mg (1 mmol) sample of Be(i-Bu)2 was added dropwise to the stirred solution with a syringe. The orange solution quickly turned colorless, and the evolution of iBuH was observed. The solution was stirred for approximately 1 h until no gas evolution could be observed anymore. The solution was then concentrated to 5 mL and stored at 4 °C overnight, resulting in the formation of large colorless crystals of 6 in >99% yield. 1H NMR (300 MHz, C6D6, 25 °C): δ = −0.23 (d, 3JHH = 7.2 Hz, 1 H, Be(CH2C(H)Me2), 0.86 (d, 3JHH = 6.5 Hz, 6 H, Be(CH2C(H)Me2), 1.26 (m, 1 H, Be(CH2C(H)Me2), 1.60 (s, 6 H, HC[C(Me)N-2,4,6Me-C6H2]), 2.12 (s, 12 H, HC[C(Me)N-2,4,6-Me-C6H2]), 2.15 (s, 6 H, HC[C(Me)N-2,4,6-Me-C6H2]), 5.24 (s, 1 H, HC[C(Me)N-2,4,6Me-C6H2]), 6.85 (s, 4 H, HC[C(Me)N-2,4,6-Me-C6H2]). 9Be NMR (300 MHz, C6D6, 25 °C): δ = 17.06. 13C NMR (75 MHz, C6D6, 25 °C): δ = 18.93 (Be(CH2C(H)Me2), 21.32 (Be(CH2C(H)Me2), 21.90 (Be(CH2C(H)Me2), 27.51 (HC[C(Me)N-2,4,6-Me-C6H2]), 100.06 (HC[C(Me)N-2,4,6-Me-C6H2]), 132.31 (HC[C(Me)N-2,4,6-MeC6H2]), 135.03 (HC[C(Me)N-2,4,6-Me-C6H2]), 144.21 (HC[C(Me)N-2,4,6-Me-C6H2]), 167.30 (HC[C(Me)N-2,4,6-Me-C6H2]). Single-Crystal X-ray Analyses. Crystallographic data of 1, 2, and 4−6 were collected on a Bruker AXS APEX 2 diffractometer (Mo Kα radiation, λ = 0.710 73 Å) and are summarized in Table S1 (Supporting Information), while central bond lengths and angles are summarized in Table 1. Figures 1−5 show diagrams of the solid-state

The crystallographic data of 1, 2, 4, 5, and 6 (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-1516001 (1), CCDC-1516002 (2), CCDC-1516004 (4), CCDC-1516003 (5), and CCDC-1516000 (6). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge, CB21EZ (fax: (+44) 1223/336033; e-mail: [email protected]).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00865. 1 H, 9Be, 13C NMR and IR spectra as well as the crystallographic details of 1, 2, 4−6 (PDF) X-ray crystallographic data of 1−6 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 0201-1834635. Fax: +49 0201-1833830. E-mail: [email protected]. ORCID

Stephan Schulz: 0000-0003-2896-4488 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.S. thanks the University of Duisburg-Essen for financial support and Materion for providing beryllium metal. We would also like to thank Prof. U. Englert (RWTH Aachen) for a helpful discussion on the pseudosymmetric structures.

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REFERENCES

(1) For recent review articles see: (a) Naglav, D.; Buchner, M.; Bendt, G.; Kraus, F.; Schulz, S. Angew. Chem. 2016, 128, 10718− 10733; Angew. Chem., Int. Ed. 2016, 55, 10562−10576. (b) Iversen, K. J.; Couchman, S. A.; Wilson, D. J. D.; Dutton, J. L. Coord. Chem. Rev. 2015, 297−298, 40−48. (c) Bellemin-Laponnaz, S.; Dagorne, S. Chem. Rev. 2014, 114, 8747−8774. (d) Hill, M. S. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 2013, 109, 18−27. (2) Pan, S.; Saha, R.; Chattaraj, P. K. New J. Chem. 2015, 39, 6778− 6786. (3) Bruce, R. M.; Odin, M. Concise International Chemical Assessment Document 32: Beryllium and Beryllium Compounds; World Health Organization: Geneva, 2001. (4) Fischer, E. O.; Hofmann, H. P. Chem. Ber. 1959, 92, 482−486. (5) See the following and references therein: (a) Himmel, D.; Scherer, H.; Kratzert, D.; Krossing, I. Z. Anorg. Allg. Chem. 2015, 641, 655−659. (b) Naglav, D.; Tobey, B.; Neumann, A.; Bläser, D.; Wölper, C.; Schulz, S. Organometallics 2015, 34, 3072−3078. (c) Fernández, R.; Carmona, E. Eur. J. Inorg. Chem. 2005, 2005, 3197−3206. (d) Conejo, M. M.; Fernández, R.; Gutiérrez-Puebla, E.; Monge, Á .; Carmona, E. Angew. Chem. 2000, 112, 2025−2027; Angew. Chem., Int. Ed. 2000, 39, 1949−1951. (6) Ruhlandt-Senge, K.; Bartlett, R. A.; Olmstead, M. M.; Power, P. P. Inorg. Chem. 1993, 32, 1724−1728. (b) Niemeyer, M.; Power, P. P. Inorg. Chem. 1997, 36, 4688−4696. (7) (a) Nijesh, K.; De, S.; Parameswaran, P. Dalton Trans. 2016, 45, 7836−7846. (b) Arrowsmith, M.; Hill, M. S.; Kociok-Köhn, G. Organometallics 2015, 34, 653−662. (c) Iversen, K. J.; Wilson, D. J. D.;

Figure 5. Solid-state structure of 6. H atoms are omitted for clarity; thermal ellipsoids are shown at the 50% probability level. structures of 1, 2, 4, 5, and 6. Data were collected at 100(2) K. The structures were solved by direct methods (SHELXS-97) and refined anisotropically by full-matrix least-squares on F2 (SHELXL2013).26 Absorption corrections were performed semiempirically from equivalent reflections on the basis of multiscans (Bruker AXS APEX2). In 2 one of the ethyl groups is disordered over two positions. The included pentane molecule is disordered over several positions, of which the two largest could be modeled using isotropic refinement of the displacement parameters. RIGU, SADI, and DFIX restraints were employed in the refinement of the disordered parts. In 4 the structure contains pseudosymmetry (translation, inversion). Choosing the alternative origin leads to an R1 = 15% and disorder of the Ar groups. The ethyl groups are disordered over two positions. The smaller component could be refined only with isotropic displacement parameters. The ADP of C28_3 C29_3 suggests a disorder of the isopropyl group, which could not be resolved. F


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Syntheses and Structures of Homo- and Heteroleptic Beryllium

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