Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6-Diisopropyl-N-(trimethylsilyl)anilino Ligand

Abstract

Bulky amido ligands are precious in s-block chemistry, since they can implant complementary strong basic and weak nucleophilic properties within compounds. Recent work has shown the pivotal importance of the base structure with enhancement of basicity and extraordinary regioselectivities possible for cyclic alkali metal magnesiates containing mixed n-butyl/amido ligand sets. This work advances alkali metal and alkali metal magnesiate chemistry of the bulky arylsilyl amido ligand [N(SiMe₃ )(Dipp)]^- (Dipp=2,6-iPr₂ -C₆ H₃ ). Infinite chain structures of the parent sodium and potassium amides are disclosed, adding to the few known crystallographically characterised unsolvated s-block metal amides. Solvation by N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) or N,N,N',N'-tetramethylethylenediamine (TMEDA) gives molecular variants of the lithium and sodium amides; whereas for potassium, PMDETA gives a molecular structure, TMEDA affords a novel, hemi-solvated infinite chain. Crystal structures of the first magnesiate examples of this amide in [MMg{N(SiMe₃ )(Dipp)}₂ (μ-nBu)]_∞ (M=Na or K) are also revealed, though these breakdown to their homometallic components in donor solvents as revealed through NMR and DOSY studies.

Keywords: alkali metals; amides; bases; magnesiates; metalation; structure elucidation.

Figure 1

Low‐coordinate Ni and Fe complexes showing interaction of potassium with π‐system as reported by Tilley.14

Scheme 1

Alternative methods to prepare complexes 4 and 5.

Scheme 2

Method to prepare alkali metal magnesiates 9 and 10.

Figure 2

a) Infinite linear chain structure of [Na{N(SiMe₃)(Dipp)}]_∞ (1), showing atomic connectivity between the metal and the ligand. b) Packing diagram of complex 1 (viewed along a‐axis). Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. The dashed lines represent short Na⋅⋅⋅C contacts. Symmetry operation to generate equivalent atoms denoted ′: x, −y+1/2, z; ′′: x−1, y, z; ′′′: x−1, −y+1/2, z. Selected bond lengths [Å] and angles [°]: Na1−N1 2.2585(16); Na1−C4′′ 2.696(2); Na1−C3′′′ 2.7514(16); Na1−C1 3.1279(18); Si1−N1 1.6816(16); N1−C1 1.381(2); C1−C2 1.4260(17); N1‐Na1‐C4′′ 130.07(6); N1‐Na1‐C3′′′ 137.70(5); C1‐N1‐Si1 123.82(11); C1‐N1‐Na1 116.41(11); Si1‐N1‐Na1 119.77(8); N1‐C1‐C2 121.69(8).

Figure 3

a) Infinite zig‐zag chain structure of [K{N(SiMe₃)(Dipp)}]_∞ (2), showing atomic connectivity between the metal and the ligand. b) Packing diagram of complex 2 (viewed along c‐axis). Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. The dashed lines represent short K⋅⋅⋅C contacts. Symmetry operation to generate equivalent atoms denoted ′: x, −y+3/2, z−1/2. Selected bond lengths [Å] and angles [°]: K1−N1 2.6755(12); K1−C1 3.0735(14); K1−Centroid′ 2.81; Si1−N1 1.6615(14); N1−C1 1.355(2); C1−C2 1.445(2); C1‐N1‐Si1 137.47(10); C1‐N1‐K1 93.59(8); Si1‐N1‐K1 128.62(7); N1‐K1‐Centroid′ 154.53.

Figure 4

Molecular structure of [Na{N(SiMe₃)(Dipp)}(PMDETA)] (4). Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. The dashed lines illustrate the Na⋅⋅⋅C contact. Selected bond lengths [Å] and angles [°]: Na1−N1 2.3206(12); Na1−N2 2.4862(13); Na1−N3 2.4188(14); Na1−N4 2.5201(14); Na1−C1 3.0937(14); Si1−N1 1.6565(12); N1−C1 1.3729(17); C1−C2 1.4321(19); N1‐Na1‐N2 125.66(5); N1‐Na1‐N3 125.63(5); N3‐Na1‐N2 75.39(4); N1‐Na1‐N4 123.87(5); N3‐Na1‐N4 74.71(5); N2‐Na1‐N4 109.45(5); C1‐N1‐Si1 140.09(10); C1‐N1‐Na1 111.17(8); Si1‐N1‐Na1 107.98(6); N1‐C1‐C2 121.51(12).

Figure 5

Molecular structure of [Li{N(SiMe₃)(Dipp)}(PMDETA)] (3). Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°]: Li1−N1 2.020(3); Li1−N2 2.314(3); Li1−N3 2.148(3); Li1−N4 2.216(3); Si1−N1 1.6813(14); N1−C1 1.3898(19); C1−C2 1.429(2); N1‐Li1‐N2 122.13(13); N1‐Li1‐N3 125.62(14); N1‐Li1‐N4 122.30(14); N3‐Li1‐N4 84.00(11); N3‐Li1‐N2 83.16(10); N4‐Li1‐N2 108.19(13); C1‐N1‐Si1 128.77(11); C1‐N1‐Li1 115.86(13); Si1‐N1‐Li1 114.96(10); N1‐C1‐C2 122.22(14).

Figure 6

Molecular structure of [{K{N(SiMe₃)(Dipp)}(PMDETA)}₂] (5). Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. The dashed lines illustrate the K⋅⋅⋅C contacts. Symmetry operation to generate equivalent atoms denoted ′: −x+1, −y, −z. Selected bond lengths [Å] and angles [°]: K1−N1 2.7174(13); K1−N2 2.9203(15); K1−N3 2.9306(15); K1−N4 2.8230(15); K1−C1 2.9806(14); K1−C13′ 3.2272(17); K1−C20 3.2166(2); Si1−N1 1.6569(13); N1−C1 1.3716(18); C1−C2 1.432(2); N1‐K1‐N4 99.71(4); N1‐K1‐N2 127.56(4); N4‐K1‐N2 115.64(5); N1‐K1‐N3 157.12(4); N4‐K1‐N3 60.85(4); N2‐K1‐N3 60.58(4); N1‐K1‐C1 27.36(4); N1‐K1‐C13′ 90.96(4); C1‐K1‐C20 98.760(3); C1‐K1‐C13′ 109.373(3); C1‐N1‐Si1 133.05(10); C1‐N1‐K1 87.06(8); Si1‐N1‐K1 139.87(7); N1‐C1‐C2 122.09(13).

Figure 7

Molecular structure of [Li{N(SiMe₃)(Dipp)}(TMEDA)] (6). Thermal ellipsoids are displayed at 35 % probability, hydrogen atoms, and the disordered component of TMEDA ligand and two methyl groups of one iPr unit are omitted for clarity. Selected bond lengths [Å] and angles [°]: Li1−N1 1.905(4); N1−C1 1.388(2); Si1−N1 1.6756(17); Si1‐N1‐Li1 121.50(15); C1‐N1‐Li1 108.20(17); C1‐N1‐Si1 130.30(13).

Figure 8

Molecular structure of [{Na{N(SiMe₃)(Dipp)}(TMEDA)}₂] (7). Thermal ellipsoids are displayed at 30 % probability and hydrogen atoms have been omitted for clarity. Dashed lines illustrate the Na⋅⋅⋅C contacts. Symmetry operation to generate equivalent atoms denoted ′: −x+1, −y+2, −z+1. Selected bond lengths [Å] and angles [°]: Na1−N1 2.2847(14); Na1−N2 2.4726(17); Na1−N3 2.461(2); Na1−C14′ 2.869(2); Na1−C1 2.9379(16); Si1−N1 1.6631(13); N1−C1 1.3785(19); C1−C2 1.424(2); N1‐Na1‐N3 133.21(7); N1‐Na1‐N2 130.49(6); N3‐Na1‐N2 75.38(6); N1‐Na1‐C14′ 103.78(6); N3‐Na1‐C14′ 106.10(7); N2‐Na1‐C14′ 103.19(7); C1‐N1‐Si1 132.08(11); C1‐N1‐Na1 103.88(9); Si1‐N1‐Na1 124.02(7); N1‐C1‐C2 121.54(14).

Figure 9

a) Asymmetric unit of the structure of [{K{N(SiMe₃)(Dipp)}}₂(TMEDA)]_∞ (8). b) Section of extended framework structure showing atom connectivity between the metal and the [N(SiMe₃)(Dipp)] ligand. Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms have been omitted for clarity. Dashed lines denote the K⋅⋅⋅aryl contacts. Symmetry operation to generate equivalent atoms denoted ′: x+1/2, −y+2, z. Selected bond lengths [Å] and angles [°]: K1−Centroid1 2.7943(8); K1−Centroid2 2.7909(8); N1−C1 1.354(4); N1−Si1 1.663(3); Si2−N2 1.651(3); K2−N3 2.780(3); K2−N1′ 2.830(3); K2−N2 2.831(3); K2−N4 2.932(3); N2−C16 1.350(4); Centroid1‐K1−Centroid2 165.8; C1‐N1‐Si1 135.1(2); C1′‐N1′‐K2 112.00(19); Si1′‐N1′‐K2 110.56(12); N3‐K2‐N1′ 93.22(9); N3‐K2‐N2 103.45(9); N1′‐K2‐N2 148.95(9); N3‐K2‐N4 66.72(9); N1′‐K2‐N4 97.86(9); N2‐K2‐N4 112.70(9); C16‐N2‐Si2 143.5(2); C16‐N2‐K2 108.7(2); Si2‐N2‐K2 107.61(13).

Figure 10

a) Asymmetric unit of the structure of [NaMg{N(SiMe₃)(Dipp)}₂(μ‐nBu)]_∞ (9). Note that Na1 and Na2 are at half occupancy. b) Section of extended framework structure showing atomic connectivity between the metals, n‐butyl and connecting N atom of the [N(SiMe₃)(Dipp)] ligands. Thermal ellipsoids are displayed at 35 % probability and hydrogen atoms and iPr groups have been omitted for clarity. The dashed lines illustrate the Na⋅⋅⋅aryl contacts. Symmetry operation to generate equivalent atoms denoted ′: −x+1, y, −z+1/2. Selected bond lengths [Å] and angles [°]: Na1−C16 2.779(2); Na1−C4 2.793(2); Na1−C5 2.913(2); Na1−C3 3.2535(20); Na2−Centroid 2.5311(1); Mg1−N1 2.0263(16); Mg1−N2 2.0393(15); Mg1−C16 2.1533(19); Si1−N1 1.7030(16); Si2−N2 1.7042(16); N1−C1 1.414(2); N2−C20 1.405(2); centroid‐Na2‐centroid 180.0; Mg1‐C16‐Na1 126.94(8); C16′‐Na1‐C16 132.53(9); N1‐Mg1‐N2 129.36(7); N1‐Mg1‐C16 112.50(7); N2‐Mg1‐C16 117.83(7); C1‐N1‐Si1 120.85(12); C1‐N1‐Mg1 105.48(11); Si1‐N1‐Mg1 133.55(9); C20‐N2‐Si2 122.86(11); C20‐N2‐Mg1 109.11(11); Si2‐N2‐Mg1 127.99(8).

Figure 11

a) Structure of [KMg{N(SiMe₃)(Dipp)}₂(μ‐nBu)]_∞ (10), showing the contents of the asymmetric unit. b) Section of extended framework structure showing atomic connectivity between the metals, n‐butyl and connecting N atom of the [N(SiMe₃)(Dipp)] ligands. Thermal ellipsoids are displayed at 35 % probability, hydrogen atoms, iPr groups, one disordered component of a ‐SiMe₃ group and one disordered methylcyclohexane molecule of crystallisation have been omitted for clarity. The dashed lines illustrate the K⋅⋅⋅aryl contacts. Symmetry operation to generate equivalent atoms denoted ′: x, −y+1, z−1/2; ′′: x, −y+1, z+1/2. Selected bond lengths [Å] and angles [°]: K1−C31 3.046(3); K1−Centroid1 2.9162(1); K1−Centroid2 2.9239(1); Mg1−N1 2.0327(18); Mg1−N2′ 2.0341(17); Mg1−C31 2.150(2); Si1−N1 1.7092(17); N1−C1 1.415(2); N2−C16 1.411(2); N2−Mg1′′ 2.0340(17); Centroid1‐K1‐Centroid2 147.4; C31‐K1‐Centroid1 91.943(3); C31‐K1‐Centroid2 117.537(2); Mg1‐C31‐K1 94.26(9); N1‐Mg1‐N2′ 133.18(7); N1‐Mg1‐C31 110.36(9); N2′‐Mg1‐C31 116.34(9); C1‐N1‐Si1 121.87(13); C1‐N1‐Mg1 111.36(12); Si1‐N1‐Mg1 126.69(9); C16‐N2‐Mg1′′ 122.16(13).

Figure 12

Part of the polymeric helical chain of [NaMg{N(SiMe₃)(Dipp)}₂(μ‐nBu)]_∞ (9) that propagates parallel to the crystallographic c‐axis.

Figure 13

Part of the polymeric helical chain of [KMg{N(SiMe₃)(Dipp)}₂(μ‐nBu)]_∞ (10) propagating parallel to the c‐axis.

Scheme 3

Breakdown of the complexes 9 and 10 in [D₈]THF.