Jessica Wiederkehr, Christoph Wölper and Stephan Schulz

Synthesis, solid-state structures and reduction reactions of heteroleptic Ge(II) and Sn(II) β-ketoiminate complexes

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De Gruyter | Published online: October 10, 2017

Abstract

A series of new heteroleptic divalent germaniun and tin complexes of the general type L1,4GeN(SiMe3)2 (1, 2) and L1−4SnN(SiMe3)2 (36) were synthesized by reaction of β-ketimines L1−4H with Ge[N(SiMe3)2]2 and Sn[N(SiMe3)2]2, respectively. The reaction of 3 with the strong Mg(I) reductant L5Mg yielded the heteroleptic complex L1MgL57 after ligand transfer from tin to magnesium, whereas analogous reactions of L4GeN(SiMe3)22 and L4SnN(SiMe3)26 with L5Mg occurred with formation of insoluble precipitates, transfer of the amido substituent from the group 14 metal to magnesium and subsequent formation of the heteroleptic magnesium complex L5MgN(SiMe3)2 (8). 1–8 were characterized by heteronuclear NMR (1H, 13C, 119Sn) and IR spectroscopy, elemental analysis and single-crystal X-ray diffraction (L4SnN(SiMe3)26, L1MgL57).

1 Introduction

The interest in heavier homologues of carbenes (R2C:) has steadily increased over the last two decades and as a result, large numbers of silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:) have been synthesized and structurally characterized [1], [2], [3], [4], [5]. The stability of these divalent species, where the formal oxidation state of the tetrele atom is MII, was found to steadily increase with increasing atomic number as can already be seen when comparing the group 14 dihalides. While PbCl2, SnCl2 and GeCl2·(dioxane) are stable compounds in solution and in the solid state, SiCl2 was only recently isolated as a carbene-stabilized compound IPr-SiCl2 (IPr=1,3-bis(2,6-iPr2)imidazol-2-ylidene) [6], [7]. In addition, detailed reactivity studies of these heavier carbenes clearly revealed differences to carbenes, which mainly result from their different electronic structure. The ground state of H2C: is a triplet, whereas H2M: (M=Si, Ge, Sn, Pb) show a singlet ground state, resulting in an empty p orbital and a filled s orbital. According to quantum chemical calculations, the singlet−triplet energy differences ΔESTEST=E(triplet)−E(singlet)] steadily increase from −14.0 (M=C), 16.7 (M=Si), 21.8 (M=Ge), 24.8 (M=Sn) to finally 34.8 (M=Pb) kcal mol−1 [8]. In addition, the relative stabilities of the monomeric metal organic species R2M: (M=C–Pb; R=alkyl, aryl) compared to the corresponding dimeric species, R2M=MR2, was also found to increase with increasing atomic number of the tetrele atom.

The first stable metal organic germylenes and stannylenes [(Me3Si)2CH]2M (M=Ge, Sn) were reported by Lappert et al. [9], [10], [11] almost 40 years ago. Since then, a large variety of compounds of this type have been synthesized, and in particular N,N′-chelating organic substituents including sterically bulky amidinate, guanidinate and β-diketiminate ligands as well as N,O chelating substituents have been successfully used for the stabilization of the desired class of compounds [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. These compounds are of interest since they can both react as Lewis base due to the presence of an electron lone pair and as Lewis acid due to an empty (acceptor) orbital. They have been widely applied in different reactions with organic substrates and in catalysis [23], [24], in particular tin complexes containing N,N′- and N,O-chelating ligands have been shown to exhibit promising activities in ring-opening polymerization (ROP) of cyclic esters [25], [26], [27]. In addition, they can serve as suitable starting reagents in reduction reactions for the synthesis of low-valent, including cluster-type, compounds. For instance, the treatment of the heteroleptic β-diketiminate complex [HC(CMeNAr)2]SnCl with KC8 or LiAlH4 resulted in the formation of the homoleptic complex [HC(CMeNAr)2]2Sn [28], while reactions of the heteroleptic amidotin chlorides {Sn[N(dipp)SiMe2R](μ-Cl)}2 (dipp=2,6-iPr2C6H3; R=Me, Ph) with Li[BHs-Bu3] yielded two large metalloid Sb15 clusters [Sn9{SnN(dipp)SiMe2R)}6] containing unprecedented body-centered metal cores [29]. Bulky amido substituents generally show potential in the stabilization of low-valent main group metal and transition metal complexes including multiply-bonded and cluster-type species [30] as was previously shown for terphenyl [31], [32], [33], [34] and silyl substituents [35], [36], [37], [38].

We recently demonstrated that (trip2Sn)2 (trip=2,4,6-iPr3C6H2) reacts with the strong Mg(I) reductant (LMg)2 (L=HC[C(Me)N(dipp)]2) with Sn–C bond cleavage and formation of the novel metalloid tin cluster Sn10trip8 [39]. We therefore became interested to extend these reduction reaction studies to divalent heteroleptic germanium and tin compounds containing a sterically demanding, chelating β-ketiminate ligand, which was expected to be rather strongly bonded and to be able to stabilize low-valent Ge and Sn cluster species through its high steric demand, as well as a weakly bonded bis(trimethylsilyl)amide ligand as rather weakly bonded leaving group.

Herein we report on the synthesis of a series of heteroleptic divalent germanium and tin β-ketiminate amide complexes of the general type L1−4EN(SiMe3)2 (E=Ge, Sn). The β-ketiminate ligands either contain a sterically demanding substituent attached to the coordinating N atom or an alkyl group with a terminal NR2 moiety, which may serve as pendant side-arm donor and can further coordinate to the metal center. Selected complexes were then investigated in reduction reactions with a strong Mg(I) reductant.

2 Results and discussion

The germanium L1,4GeN(SiMe3)2 (1, 2) and tin β-ketiminate complexes L1−4SnN(SiMe3)2 (36) were synthesized (Scheme 1) by reaction of equimolar amounts of E[N(SiMe3)2]2 (E=Ge, Sn) with the corresponding β-ketimines L1−4H (L1=OC(Me)C(H)C(Me)NCH2CH2NMe2, L2=OC(Me)C(H)C(Me)NCH2CH2NEt2, L3=OC(Me)C(H)C(Me)NCH2CH2CH2NMe2, L4=OC(Me)C(H)C(Me)N(dipp)) at ambient temperature. The reactions proceeded smoothly with elimination of the amine HN(SiMe3)2 and subsequent formation of the expected heteroleptic β-ketiminate complexes (Scheme 2) in high yields (67–96%).

Scheme 1: Synthesis of the complexes 1–6.

Scheme 1:

Synthesis of the complexes 16.

Scheme 2: Schematic drawing of the complexes 1–6.

Scheme 2:

Schematic drawing of the complexes 16.

The 1H NMR spectra of 16 show the expected resonances of the chelating β-ketiminate unit (CH, NCMe, OCMe) and the expected resonances of the silyl groups between 0.30 ppm (L4SnN(SiMe3)26) and 0.45 ppm (L1SnN(SiMe3)23). Additional resonances due to the β-ketoiminate-imine-substituents (NR) were recorded. In the case of 2 and 6, two well separated septets and four distinguished doublets (Supporting information, Figs. S1, S19) with a relative intensity of 3:3:6:6:6:6 for the diastereotopic protons of the dipp substituent were observed, while the 1H NMR spectra of 1 and 35 show two multiplets of equal relative intensity for the methylene protons of the ethylamino and propylamino group (CH2CH2NR2, CH2CH2CH2NR2 R=Me, Et, Figs. S4, S7, S11, S15). These findings either result from the chirality of the molecules due to the pseudo-tetrahedral environment of the Ge and Sn atoms or point to a hindered rotation around the N–Cipso single bond (2, 6), according to which the iPr groups become magnetically inequivalent, or a coordination of the sidearm NR2 group in solution to the Lewis acidic germanium and tin atom (1, 35), respectively. The 119Sn NMR resonances of 36 were detected in the range between −89 ppm (L4SnN(SiMe3)26) and −221 ppm (L1SnN(SiMe3)23).

Orange crystals of 6 suitable for an X-ray structure determination were obtained after storage of a concentrated solution of 6 in n-hexane at −30°C. Compound 6 crystallizes in the monoclinic space group P21/n with one molecule in the asymmetric unit (Fig. 1).

Fig. 1: Solid-state structure of 6. Non-H atoms are shown as displacement ellipsoids at 50% probability levels, H atoms are omitted for clarity except for those engaged in intramolecular interactions. Selected bond lengths (Å) and angles (deg): Sn(1)–O(1) 2.1067(13), Sn(1)–N(2) 2.1264(14), Sn(1)–N(1) 2.2656(14); O(1)–Sn(1)–N(2) 96.93(5), O(1)–Sn(1)–N(1) 84.52(5), N(2)–Sn(1)–N(1) 99.77(5), C(1)–O(1)–Sn(1) 129.62(11), C(3)–N(1)–C(6) 120.89(13), C(3)–N(1)–Sn(1) 125.19(11), C(6)–N(1)–Sn(1) 113.58(10), Si(2)–N(2)–Si(3) 124.61(8), Si(2)–N(2)–Sn(1) 119.45(7), Si(3)–N(2)–Sn(1) 110.92(7), C(21)–H(21)c···O(1) 131.8, C(22)–H(22)a···O(1) 126.7.

Fig. 1:

Solid-state structure of 6. Non-H atoms are shown as displacement ellipsoids at 50% probability levels, H atoms are omitted for clarity except for those engaged in intramolecular interactions. Selected bond lengths (Å) and angles (deg): Sn(1)–O(1) 2.1067(13), Sn(1)–N(2) 2.1264(14), Sn(1)–N(1) 2.2656(14); O(1)–Sn(1)–N(2) 96.93(5), O(1)–Sn(1)–N(1) 84.52(5), N(2)–Sn(1)–N(1) 99.77(5), C(1)–O(1)–Sn(1) 129.62(11), C(3)–N(1)–C(6) 120.89(13), C(3)–N(1)–Sn(1) 125.19(11), C(6)–N(1)–Sn(1) 113.58(10), Si(2)–N(2)–Si(3) 124.61(8), Si(2)–N(2)–Sn(1) 119.45(7), Si(3)–N(2)–Sn(1) 110.92(7), C(21)–H(21)c···O(1) 131.8, C(22)–H(22)a···O(1) 126.7.

The central tin atom is coordinated by a chelating β-ketiminato ligand L4 and one amide ([N(SiMe3)2]) ligand. This leads to a trigonal pyramidal environment of the tin atom. Even though the Sn1–N1 bond (see caption Fig. 1) is longer than the average Sn–N bond in β-ketiminato complexes [40], [41], this value is still within the typical range. The Sn1–O1 bond length also is equal to the average value derived from the database, whereas the Sn1–N2 bond is longer than the average but well within the typical range of Sn–N bonds to [N(SiMe3)2] ligands [42]. This elongation may be attributed to the high steric demand of the SiMe3 groups. The three bonds are roughly perpendicular with N/O–Sn–N bond angles ranging between 84.52(5)° and 99.77(5)°. The central NOC3 unit of the β-ketiminato ligand is almost planar (r.m.s. deviation from best plane 0.0192 Å) and the tin atom is slightly shifted from this plane [0.330(3) Å] in the direction away from the [N(SiMe3)2] group. This may again be explained by the high steric demand of the SiMe3 groups. Two short intramolecular contacts (C21–H21c···O1 2.46 Å, C22–H22a···O1 2.67 Å) support the conformation of the molecule.

The potential capability of these heteroleptic complexes 16 to serve as starting reagents in reduction reactions for the synthesis of metalloid tin and germanium clusters was exemplarily investigated in the reaction of L1SnN(SiMe3)23 with the strong Mg(I) reductant [L5Mg]2 (L5=(dipp)NC(Me)C(H)C(Me)N(dipp)) [43], [44]. However, in addition to the formation of a black insoluble precipitate, most likely elemental tin, which clearly proves the reduction of 3, we observed the formation of the heteroleptic complex L1MgL57, which is formed by a ligand transfer reaction of L5 from the tin to the magnesium atom. Comparable results were obtained from the reactions of [L5Mg]2 with L4GeN(SiMe3)22 and L4SnN(SiMe3)26, which both yielded the known β-diketiminato magnesium amide L5MgN(SiMe3)28 as was demonstrated by in situ1H NMR spectroscopy [45]. In contrast to the reactions with 3, which occurred with transfer of the β-ketiminato ligand L1 to the magnesium atom, the reaction with 2 and 6 led to a transfer of the bis(trimethylsilyl)amido moiety from the group 14 metal center to magnesium. Again, both reactions proceeded with formation of insoluble precipitates, most likely elemental germanium and tin, as well as the formation of additional products according to in situ1H NMR spectroscopy. Unfortunately, these compounds could not be identified so far. However, these reactions clearly show that both Ge–N and Sn–N bonds can be cleaved by reaction with the strong Mg(I) reductant.

Crystals of 7 suitable for a single crystal X-ray diffraction study were obtained after storage of a solution of 7 in n-hexane for 2 d at −30°C (Fig. 2).

Fig. 2: Solid-state structure of 7. Non-H atoms are shown as displacement ellipsoids at 50% probability levels, H atoms and a second orientation of the disordered side-arm are omitted for clarity. Selected bond lengths (Å) and angles (deg): Mg(1)–O(1) 1.9903(7), Mg(1)–N(1) 2.1055(8), Mg(1)–N(2) 2.1144(8), Mg(1)–N(3′) 2.152(8), Mg(1)–N(3) 2.166(4), Mg(1)–N(4) 2.2669(9); O(1)–Mg(1)–N(1) 94.26(3), O(1)–Mg(1)–N(2) 93.25(3), N(1)–Mg(1)–N(2) 91.64(3), O(1)–Mg(1)–N(3′) 85.5(2), N(1)–Mg(1)–N(3′) 126.55(12), N(2)–Mg(1)–N(3′) 141.80(12), O(1)–Mg(1)–N(3) 84.58(10), N(1)–Mg(1)–N(3) 137.20(6), N(2)–Mg(1)–N(3) 131.17(6), O(1)–Mg(1)–N(4) 160.99(3), N(1)–Mg(1)–N(4) 98.89(3), N(2)–Mg(1)–N(4) 100.01(3), N(3′)–Mg(1)–N(4) 75.6(2), N(3)–Mg(1)–N(4) 76.44(10).

Fig. 2:

Solid-state structure of 7. Non-H atoms are shown as displacement ellipsoids at 50% probability levels, H atoms and a second orientation of the disordered side-arm are omitted for clarity. Selected bond lengths (Å) and angles (deg): Mg(1)–O(1) 1.9903(7), Mg(1)–N(1) 2.1055(8), Mg(1)–N(2) 2.1144(8), Mg(1)–N(3′) 2.152(8), Mg(1)–N(3) 2.166(4), Mg(1)–N(4) 2.2669(9); O(1)–Mg(1)–N(1) 94.26(3), O(1)–Mg(1)–N(2) 93.25(3), N(1)–Mg(1)–N(2) 91.64(3), O(1)–Mg(1)–N(3′) 85.5(2), N(1)–Mg(1)–N(3′) 126.55(12), N(2)–Mg(1)–N(3′) 141.80(12), O(1)–Mg(1)–N(3) 84.58(10), N(1)–Mg(1)–N(3) 137.20(6), N(2)–Mg(1)–N(3) 131.17(6), O(1)–Mg(1)–N(4) 160.99(3), N(1)–Mg(1)–N(4) 98.89(3), N(2)–Mg(1)–N(4) 100.01(3), N(3′)–Mg(1)–N(4) 75.6(2), N(3)–Mg(1)–N(4) 76.44(10).

Compound 7 crystallizes in the monoclinic space group C2/c with the molecule placed on a general position. The central Mg atom is coordinated by one L5 and one L1 ligands both in a chelating fashion. The resulting environment of the Mg atom is best described as distorted trigonal bipyramidal, in which O1 and N4 adopt the apical positions. The corresponding bond angle however deviates approximately 20° from the expected linearity. The distortion also manifests itself in the bond angles in the equatorial positions. The N1–Mg1–N2 bond angle is determined by the bite angle of the ligand and is thus smaller [91.64(3)°] than the ideal value of 120°. Since Mg is on the same plane with N1 and N3/N3′ (r.m.s. deviation 0.000 Å and 0.0049 Å, resp.) the other equatorial bond angles necessarily have to be larger than the ideal value. Mg1 is on the plane of the NOC3 backbone of L1 [deviation from best plane 0.016(5) Å and 0.350(3) Å, resp.], but significantly off the plane of the N2C3 backbone of L5 [deviation from best plane 0.9809(10) Å]. The bond lengths match well with results from the CSD [46]. As was expected the more weakly coordinated side-arm donor N4 shows an elongated bond to Mg1.

3 Conclusion

Heteroleptic divalent germanium L1,4GeN(SiMe3)2 (1, 2) and tin complexes L1−4SnN(SiMe3)2 (36) are easily accessible by amide elimination reactions of β-ketimines L1−4H with Ge[N(SiMe3)2]2 and Sn[N(SiMe3)2]2, respectively, under mild reaction conditions. 3 reacts with the strong Mg(I) reductant L5Mg with transfer of the β-ketiminate ligand (L1) from tin to magnesium and formation of the heteroleptic complex L1MgL57, whereas analogous reduction reactions of L4GeN(SiMe3)22 and L4SnN(SiMe3)26 occurred with transfer of the amido substituent and formation of the known heteroleptic magnesium complex L5MgN(SiMe3)2 (8).

4 Experimental section

All manipulations were performed under Ar. Solvents were carefully dried with Na/K and degassed prior to use. [L5Mg]2 was prepared according to literature methods [43], [44]. 1H NMR, 13C{1H} and 119Sn{1H} NMR spectra were recorded with a Bruker DMX 300 spectrometer and are referenced to internal C6D5H (1H: δ=7.16 ppm; 13C: δ=128.0 ppm) and Me4Sn (119Sn{1H}) [47]. IR spectra were recorded with an ALPHA-T FTIR spectrometer equipped with a single-reflection ATR sampling module. Melting points were measured in sealed capillaries. Elemental analyses were performed at the Elementaranalyse Labor of the University of Essen.

4.1 General synthesis of β-ketimines L1–4H

The β-ketimine ligands L1−4H with different additional side-arm donors where synthesized according to the following general procedure, which is slightly modified regarding the literature procedures using the corresponding (di)amine (L1H: N,N-dimethylethylenediamine, L2H: N,N-diethylethylenediamine, L3H: 3-(dimethylamino)-1-propylamine, L4H: 2,6-diisopropylaniline) [48], [49], [50]. L1–4H were obtained as orange liquids in almost quantitative yield.

4.1.1 L1H

N,N-Dimethylethylenediamine (9.92 mL, 0.091 mol) was added to acetyl acetone (10.26 mL, 0.100 mol). The mixture was stirred at ambient temperature for 3 h and L1H was isolated as an orange liquid after removal of the volatiles in vacuum. Yield: 15.3 g, (99%). – Elemental analysis: calcd. (found): H 62.5 (61.9), C 10.7 (10.8), N 14.5 (14.6). – 1H NMR (300 MHz, C6D6, 25°C): δ=1.86 (s, 6 H, OCCH3 and NCCH3), 2.18 (s, 6 H, N(CH3)2), 2.38 (t, 3JH,H=6.3 Hz, 2 H, CH2CH2NMe2), 3.24 (m, 3JH,H=6.3 Hz, 2 H, CH2CH2NMe2), 4.88 (s, 1 H, CH), 10.71 (s, 1 H, NH). – IR (ATR): ν=2943, 2861, 2819, 2767, 1609, 1564, 1513, 1440, 1285, 1193, 1110, 1019, 732, 652 cm−1.

4.1.2 L2H

Yield: 18.0 g, (100%). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.86 (t, 6 H, N(CH2CH3)2), 1.46 (s, 3 H, NCCH3), 1.82 (s, 3 H, OCCH3), 2.38 (m, 6 H, overlapping signals CH2CH2N(CH2CH3)2), 3.06 (m, 3JH,H=6.8 Hz, 1 H, CH2CH2NEt2), 3.32 (m, 3JH,H=6.8 Hz, 1 H, CH2CH2NEt2), 4.70 (s, 1 H, CH).

4.1.3 L3H

Yield: 15.9 g, (95%). – Elemental analysis: calcd. (found): H 65.2 (64.7), C 10.6 (10.2), N 15.2 (15.0). – 1H NMR (300 MHz, C6D6, 25°C): δ=1.63 (m, 3JH,H=7.3 Hz, 2 H, CH2CH2CH2NMe2), 1.87 (s, 6 H, OCCH3 and NCCH3), 2.12 (s, 6 H, N(CH3)2), 2.24 (t, 3JH,H=6.9 Hz, 2 H, CH2CH2CH2NMe2), 3.23 (q, 3JH,H=6.5 Hz, 2 H, CH2CH2CH2NMe2), 4.88 (s, 1 H, CH), 10.78 (s, 1 H, NH). – IR (ATR): ν=2944, 2860, 2817, 2766, 1608, 1574, 1511, 1439, 1295, 1260, 1097, 1016, 795, 734, 637 cm−1.

4.1.4 L4H

Yield: 21.2 g, (90%). – 1H NMR (300 MHz, C6D6, 25°C): δ=1.17 (d, 3JH,H=6.9 Hz, 6 H, CH(CH3)2), 1.25 (d, 3JH,H=7.0 Hz, 6 H, CH(CH3)2), 1.65 (s, 6 H, NCCH3), 2.10 (s, 6 H, OCCH3), 3.06 (sept, 3JH,H=6.9 Hz, 2 H, CH(CH3)2), 5.25 (s, 1 H, CH), 12.09 (s, 1 H, NH).

4.2 General synthesis of germanium complexes L1,4GeN(SiMe3)2

A solution of LH (L1H: 0.216 g, 1.3 mmol; L4H: 0.330 g, 1.3 mmol) in 10 mL of hexane was added slowly to a solution of Ge[N(SiMe3)2]2 (0.500 g, 1.3 mmol) in 10 mL of hexane at 0°C and stirred for 2 h. The solution was then allowed to warm to ambient temperature and stirred for another 2 h. All volatiles were removed in vacuum and 1 and 2 were obtained after extraction of the solid residue with toluene and evaporation of the solvent under vacuum as a red-brown waxy (1) and orange crystalline compound (2), respectively.

4.2.1 L1GeN(SiMe3)2 1

Yield: 0.501 g (96%). – Elemental analysis: calcd. (found): C 41.9 (41.9), H 7.7 (7.4), N 10.2 (10.0). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.44 (s, 18 H, Si(CH3)3), 1.38 (s, 3 H, NCCH3), 1.67 (s, 3 H, OCCH3), 2.01 (s, 6 H, N(CH3)2), 2.34 (m, 2 H, CH2CH2NMe2), 3.07 (m, 1 H, CH2CH2NMe2), 3.42 (m, 1 H, CH2CH2NMe2), 4.60 (s, 1 H, CH). – 13C NMR (75 MHz, C6D6, 25°C): δ=5.2 (Si(CH3)3), 20.6 (NCCH3), 25.2 (OCCH3), 45.1 (CH2CH2NMe2), 45.6 (CH2CH2N(CH3)2), 58.1 (CH2CH2NMe2), 99.3 (CH), 168.0 (NCCH3), 174.7 (OCCH3). – IR (ATR): ν=2962, 2819, 2769, 1738,1610,1575, 1513, 1455, 1259, 1088, 1015, 864, 795, 702, 540, 397 cm−1.

4.2.2 L4GeN(SiMe3)2 2

Yield: 0.581 g, (91%). – Elemental analysis: calcd. (found): C 54.1 (53.6), H 7.9 (8.5), N 5.1 (5.3). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.31 (s, 18 H, Si(CH3)3), 0.96 (d, 3JH,H=6.8 Hz, 3 H, CH(CH3)2), 1.01 (d, 3JH,H=6.8 Hz, 3 H, CH(CH3)2), 1.30 (s, 3 H, NCCH3), 1.33 (d, 3JH,H=6.9 Hz, 3 H, CH(CH3)2), 1.41 (d, 3JH,H=6.9 Hz, 3 H, CH(CH3)2), 1.74 (s, 3 H, OCCH3), 3.18 (sept, 3JH,H=6.8 Hz, 1 H, CH(CH3)2), 3.29 (sept, 3JH,H=6.9 Hz, 1 H, CH(CH3)2), 4.86 (s, 1 H, CH), 7.02–7.11 ppm (3 H, m/p-CH, overlapped with solvent signal). – 13C NMR (75 MHz, C6D6, 25°C): δ=5.5 (Si(CH3)3), 23.1, 24.3, 24.5, 24.7, 24.8, 25.4, 27.7, 28.8, 97.3 (backbone-CH), 124.3 (m-CH), 124.6 (m-CH), 139.1 (p-CH), 142.6 (p-CH), 142.8 (arom. dipp-CH), 169.8 (NCCH3), 176.1 (OCCH3). – IR (ATR): ν=3058, 2960, 2900, 2868, 1597, 1572, 1507, 1460, 1441, 1386, 1364, 1324, 1257, 1243, 1217, 1176, 1099, 1056, 1017, 913, 866, 833, 789, 770, 758, 703, 670, 630, 618, 555, 510, 445, 411 cm−1.

4.3 General synthesis of tin complexes L1–4SnN(SiMe3)2

A solution of LH (L1H: 0.750 g, 4.4 mmol; L2H: 0.451 g, 2.3 mmol; L3H: 0.800 g, 4.3 mmol; L4H: 1.000 g, 3.9 mmol) in 15 mL of hexane was added slowly to a solution of Sn[N(SiMe3)2]2 (1.936 g, 4.4 mmol ) in 15 mL of hexane at 0°C and stirred for 2 h. The solution was allowed to warm to ambient temperature, stirred for additional 2 h and then evaporated to dryness in vacuum. The heteroleptic complexes 36 were obtained as red waxy (3), red-brown waxy (4, 5) and yellow-orange crystalline solids (6), respectively, after extraction with toluene and removal of all volatiles in vacuum.

4.3.1 L1SnN(SiMe3)2 3

Yield: 1.569 g (80%). – Elemental analysis: calcd. (found): C 37.3 (37.5), H 6.8 (6.5), N 8.4 (8.2). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.46 (s, 18 H, Si(CH3)3), 1.36 (s, 3 H, NCCH3), 1.86 (s, 3 H, OCCH3), 1.94 (s, 6 H, N(CH3)2), 2.25 (m, 2 H, CH2CH2NMe2), 2.70 (m, 1 H, CH2CH2NMe2), 2.95 (m, 1 H, CH2CH2NMe2), 4.72 (s, 1 H, CH). – 13C NMR (75 MHz, C6D6, 25°C): δ=6.6 (Si(CH3)3), 23.2 (NCCH3), 27.5 (OCCH3), 44.9 (CH2CH2NMe2), 47.5 (N(CH3)2), 58.2 (CH2CH2NMe2), 99.2 (CH), 170.2 (NCCH3), 180.9 (OCCH3). – 119Sn NMR (111.9 MHz, C6D6, 25°C): δ=–221.4. – IR (ATR): ν=3055, 2952, 2860, 2819, 2770, 1646, 1603, 1567, 1508, 1455, 1422, 1393, 1340, 1246, 1175, 1124, 1097, 1016, 927, 867, 836, 780, 780, 758, 668, 618, 509, 411, 392 cm−1.

4.3.2 L2SnN(SiMe3)2 4

Yield: 0.834 g (76%). – Elemental analysis: calcd. (found): C 40.4 (41.0), H 6.5 (6.7), N 8.4 (8.7). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.43 (s, 18 H, Si(CH3)3), 0.86 (t, 3JH,H=7.1 Hz, 6 H, N(CH2CH3)2), 1.46 (s, 3 H, NCCH3), 1.82 (s, 3 H, OCCH3), 2.36 (m, 6 H, N(CH2CH3)2 overlapped with CH2CH2NEt2), 3.06 (m, 1 H, CH2CH2NEt2), 3.32 (m, 1 H, CH2CH2NEt2), 4.70 (s, 1 H, CH). – 13C NMR (75 MHz, C6D6, 25°C): δ=6.6 (Si(CH3)3), 12.0 (N(CH2CH3)2, 22.1 (NCCH3), 27.1 (OCCH3), 47.4 (N(CH2CH3)2), 48.4 (CH2CH2NEt2), 53.1 (CH2CH2NEt2), 100.2 (CH), 169.9 (NCCH3), 177.8 (OCCH3). – 119Sn NMR (111.9 MHz, C6D6, 25°C): δ=−150.7. – IR (ATR): ν=2964, 2933, 2872, 2809, 1647, 1608, 1565, 1510, 1467, 1438, 1397, 1372, 1341, 1286, 1248, 1200, 1176, 1124, 1065, 1015, 928, 883, 837, 821, 755, 733, 668, 617, 501, 395 cm−1

4.3.3 L3SnN(SiMe3)2 5

Yield: 1.597 g (81%). – Elemental analysis: calcd. (found): C 40.8 (41.1), H 6.4 (6.5), N 9.7 (9.5). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.41 (s, 18 H, Si(CH3)3), 1.46 (s, 3 H, NCCH3), 1.52 (m, 3JH,H=7.0 Hz, 2 H, CH2CH2CH2NMe2), 1.81 (s, 3 H, OCCH3), 1.98 (s, 6 H, N(CH3)2), 2.06 (m, 2 H, CH2CH2CH2NMe2), 3.08 (m, 1 H, CH2CH2CH2NMe2), 3.39 (m, 1 H, CH2CH2CH2NMe2), 4.69 (s, 1 H, CH). – 13C NMR (75 MHz, C6D6, 25°C): δ=4.35 (Si(CH3)3), 25.0 (NCCH3), 26.3 (OCCH3), 27.9 (CH2CH2CH2NMe2), 43.8 (CH2CH2CH2NMe2), 44.6 (N(CH3)2), 55.0 (CH2CH2CH2NMe2), 98.4 (CH), 167.4 (NCCH3), 175.0 (OCCH3). – 119Sn NMR (111.9 MHz, C6D6, 25°C): δ=−133.1. – IR (ATR): ν=2938, 2855, 2812, 2762, 1645, 1608, 1559, 1510, 1457, 1440, 1399, 1372, 1336, 1299, 1259, 1219, 1178, 1151, 1121, 1096, 1061, 1039, 965, 932, 820, 803, 743, 501, 460 cm−1.

4.3.4 L4SnN(SiMe3)2 6

Yield: 1.398 g (67%). – Elemental analysis: calcd. (found): H 51.3 (51.7), C 7.7 (8.0), N 5.3 (5.2). – 1H NMR (300 MHz, C6D6, 25°C): δ=0.30 (s, 18 H, Si(CH3)3), 0.96 (d, 3JH,H=6.7 Hz, 3 H, CH(CH3)2), 1.05 (d, 3JH,H=6.8 Hz, 3 H, CH(CH3)2), 1.22 (d, 3JH,H=6.7 Hz, 3 H, CH(CH3)2), 1.37 (s, 3 H, NCCH3), 1.43 (d, 3JH,H=6.8 Hz, 3 H, CH(CH3)2), 1.86 (s, 3 H, OCCH3), 2.99 (sept, 3JH,H=6.6 Hz, 1 H, CH(CH3)2), 3.32 (sept, 3JH,H=6.4 Hz, 1 H, CH(CH3)2), 4.86 (s, 1 H, CH), 6.95–7.25 (m, 3 H, m/p-CH, overlapped with solvent signal). – 13C NMR (75 MHz, C6D6, 25°C): δ=6.7 (Si(CH3)3), 24.2, 24.3, 24.9, 25.4, 25.6, 27.4, 28.3, 29.2, 98.9 (CH), 125.1 (dipp-CH), 125.3 (dipp-CH), 126.0, 141.1 (dipp-CH), 142.1 (dipp-CH), 143.7 (dipp-CH), 171.4 (NCCH3), 179.2 (OCCH3). – 119Sn NMR (111.9 MHz, C6D6, 25°C): δ=−88.8. – IR (ATR): ν=3055, 2962, 2896, 2869, 1723, 1575, 1502, 1460, 1440, 1386, 1364, 1322, 1240, 1176, 1101, 1054, 1042, 1014, 937, 864, 831, 795, 774, 758, 689, 668, 614, 595, 529, 506, 439, 405 cm−1.

4.4 Synthesis of L1MgL5 7

A mixture of 3 (94 mg, 0.2 mmol) and [L5Mg]2 (186 mg, 0.2 mmol) was dissolved at 0°C in 5 mL of toluene and stirred for 48 h at ambient temperature. The solvent was evaporated from the dark red-brown solution and the resulting crystalline solid was dissolved in pentane. Slightly orange crystals of 7 were formed within 2 d upon storage at –30°C. Yield: 30 mg (25%). – Elemental analysis: calcd. (found): C 74.7 (74.6), H 9.6 (9.7), N 9.2 (9.2). 1H NMR (300 MHz, C6D6): δ=1.21 (d, 3JHH=7.4 Hz, 6 H, CH(CH3)2), 1.26–1.35 (overlapping douplets, 18 H, CH(CH3)2), 1.59 (s, 3 H, NCCH3 (L1)), 1.72 (s, 6 H, CH3 (L5)), 1.79 (s, 6 H, N(CH3)2), 1.96 (s, 3 H, OCCH3 (L1)), 2.78 (m, 2 H, CH2CH2NMe2), 3.49 (m, 4 H, overlapping signals CH2CH2NMe2 and CH(CH3)2), 4.89 (s, 1 H, CH (L5)), 5.09 (s, 1 H, CH (L1)), 7.01–7.20 (m, 3 H, m/p-CH, overlapping with solvent signals). – 13C NMR (75 MHz, C6D6, 25°C): δ=21.9, 22.6, 23.5, 24.2, 24.5, 24.9, 25.2, 25.3, 27.4, 28.2, 28.5, 28.6, 45.4, 46.2, 59.5, 96.9 (CH), 97.7 (CH), 123.4, 123.6, 123.9, 124.0, 124.7, 125.9, 142.4 (dipp-CH), 142.8 (dipp-CH), 143.5 (dipp-CH), 148.1, 161.6 (NCCH3), 168.3 (NCCH3), 173.2 (NCCH3), 182.7 (OCCH3). – IR (ATR): ν=3053, 2959, 2922, 2865, 1621, 1581, 1537, 1516, 1458, 1430, 1400, 1337, 1312, 1261, 1225, 1172, 1098, 1052, 1014, 925, 841, 789, 759, 734, 699, 668, 646, 622, 596, 548, 516, 435, 417 cm−1.

4.5 Reaction of 2 with [L5Mg]2

Compound 2 (15 mg, 0.03 mmol) and [L5Mg]2 (13 mg, 0.015 mmol) were dissolved at –10°C in 0.5 mL toluene and stirred for 3 h at that temperature, slowly warmed up to ambient temperature and stirred for additional 12 h. Thereafter the reaction mixture was heated to 50°C for 6 h, yielding an dark solution with a black precipitate. The in-situ1H NMR spectrum of the solution showed the formation of L5MgN(SiMe3)28 [45]. Additional signals were either assigned to the starting reagent 2 or belong to so far unidentified products. The formation of a black solid indicates the reduction of 2 to elemental germanium 8: 1H NMR (300 MHz, C6D6, 25°C): δ=0.02 (s, 18 H, Si(CH3)3), 1.20 (d, 3JH,H=6.7 Hz, 12 H, CH(CH3)2), 1.40 (d, 3JH,H=6.8 Hz, 12 H, CH(CH3)2), 1.67 (s, 3 H, CH3), 3.23 (sept, 3JH,H=6.8 Hz, 4 H, CH(CH3)2), 4.83 (s, 1 H, CH), 7.02–7.05 ppm (m, 6 H, m/p-CH).

4.6 Reaction of 6 with [L5Mg]2

6 (64 mg, 0.1 mmol) and [L5Mg]2 (102 mg, 0.1 mmol) were dissolved at –10°C in 0.5 mL of toluene and stirred for 3 h at that temperature, resulting in the formation of a dark brown solution and a black precipitate. The mixture was warmed to ambient temperature and stirred for another 3 h, upon which the color did not change. The in-situ1H NMR spectrum of the reaction solution showed the formation of L5MgN(SiMe3)28. Additional signals could not been assigned to any known compound. The formation of a black solid indicates the formation of elemental tin.

5 Crystal structure determinations

The crystals of 6 and 7 were mounted on nylon loops in inert oil. Data were collected on a Bruker AXS D8 Kappa diffractometer with APEX2 detector (monochromated MoKα radiation, λ=0.71073 Å). The structures were solved by Direct Methods (Shelxs-97) [51], [52] and refined anisotropically by full-matrix least-squares on F2 (Shelxl-2014) [53], [54], [55], [56], [57]. Absorption corrections were performed semi-empirically from equivalent reflections on basis of multi-scans (Bruker AXS APEX2, Twinabs). Hydrogen atoms were refined using a riding model or rigid methyl groups. Compound 6 was non-merohedrally twinned and refined with two components against HKLF5 data. Compound 7 contained highly disordered solvent of unidentifiable nature. n-pentane, n-hexane and toluene were used but none of these or combinations thereof yielded a satisfying model for the residual density. Consequently, the refinement was done using “solvent-free” data from a Platon/SQEEZE run. Because the solvent could not be identified for certain it was ignored in the sum formula. The ligands side-arm is disordered over two positions.

CCDC-1555595 (6) and CCDC-1555596 (7) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

6 Supporting Information

1H and 13C NMR and IR spectra of 17, 119Sn NMR spectra of 36, in situ1H NMR spectra of the reduction reaction of 2 and 6 with [L5Mg]2, and crystallographic data of 6 and 7 are given as Supporting Information available online (https://doi.org/10.1515/znb-2017-0098).

Acknowledgments

S. Schulz thank the University of Duisburg-Essen for financial support. J. Wiederkehr is grateful to the Fonds der Chemischen Industrie for a doctoral fellowship. Ms. Haley Palm is acknowledge for contributing to the experimental work.

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Supplemental Material:

The online version of this article offers supplementary material (https://doi.org/10.1515/znb-2017-0098).

Received: 2017-6-16
Accepted: 2017-7-20
Published Online: 2017-10-10
Published in Print: 2017-11-27

©2017 Walter de Gruyter GmbH, Berlin/Boston