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

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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‐C6H3). 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.

No MeSH data available.


Part of the polymeric helical chain of [NaMg{N(SiMe3)(Dipp)}2(μ‐nBu)]∞ (9) that propagates parallel to the crystallographic c‐axis.
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chem201602683-fig-0012: Part of the polymeric helical chain of [NaMg{N(SiMe3)(Dipp)}2(μ‐nBu)]∞ (9) that propagates parallel to the crystallographic c‐axis.

Mentions: Sodium magnesiate 9 displays two distinct types of (half occupancy) Na atom separated by a magnesiate anion, neither of which engage with the amido N atom. Na1 lies upon a twofold rotation axis and bonds to two symmetrically equivalent butyl C atoms (C16/C16′, 2.779(2) Å) and two symmetrically equivalent aryl groups (C4/C4′ and C5/C5′) at the para/meta positions (2.793(2) Å and 2.913(2) Å, respectively), making it six‐coordinate overall. The next shortest distance between Na1 and the aryl group is to C3 (at 3.2535(20) Å), which seems too long, so the hapticity of the aryl substituent is best regarded as η2. Lying on a crystallographic inversion centre sandwiched between two aryls from neighbouring magnesiate ions, Na2 binds in a η6 arrangement to both of them (range of Na2−C bond lengths, 2.684(2)–3.1565(18) Å; Na−centroid distance 2.5311(1) Å; centroid‐Na‐centroid angle, 180.0°). Propagation of this contacted ion pair structure into an infinite helical chain (Figure 12) is through these distinct types of Na−C interaction. Mg1 occupies a N2C distorted trigonal planar coordination (sum of bond angles; 359.69°) comprising two amide ligands (mean Mg−N distance, 2.0328 Å) and one butyl ligand (Mg1−C16, 2.1533(19) Å). The helical chain structure of 10 (Figure 13) is less intricate than that of 9 as it contains only one alkali metal (K) site. Sandwiched between two aryl rings (K1−centroid1 2.9162(1) Å; K1−centroid2 2.9239(1) Å; centroid1‐K1‐centroid2, 147.4°), K1 also interacts with the α‐C of the Bu ligand (K1−C31 3.046(3) Å), while there is a contact with one of the CH3(Si) groups though the disorder in this group negates an accurate bond length. Consistent with the interaction between the alkali metal and the magnesiate ion being predominately electrostatic in nature the dimensions of the Mg1 centre in 10 (mean Mg−N, 2.0334 Å; Mg−C31, 2.150(2) Å; sum of bond angles subtending Mg1, 359.82°) match those in the sodium magnesiate 12.13


Structural Diversity in Alkali Metal and Alkali Metal Magnesiate Chemistry of the Bulky 2,6 ‐ Diisopropyl ‐ N ‐ (trimethylsilyl)anilino Ligand
Part of the polymeric helical chain of [NaMg{N(SiMe3)(Dipp)}2(μ‐nBu)]∞ (9) that propagates parallel to the crystallographic c‐axis.
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chem201602683-fig-0012: Part of the polymeric helical chain of [NaMg{N(SiMe3)(Dipp)}2(μ‐nBu)]∞ (9) that propagates parallel to the crystallographic c‐axis.
Mentions: Sodium magnesiate 9 displays two distinct types of (half occupancy) Na atom separated by a magnesiate anion, neither of which engage with the amido N atom. Na1 lies upon a twofold rotation axis and bonds to two symmetrically equivalent butyl C atoms (C16/C16′, 2.779(2) Å) and two symmetrically equivalent aryl groups (C4/C4′ and C5/C5′) at the para/meta positions (2.793(2) Å and 2.913(2) Å, respectively), making it six‐coordinate overall. The next shortest distance between Na1 and the aryl group is to C3 (at 3.2535(20) Å), which seems too long, so the hapticity of the aryl substituent is best regarded as η2. Lying on a crystallographic inversion centre sandwiched between two aryls from neighbouring magnesiate ions, Na2 binds in a η6 arrangement to both of them (range of Na2−C bond lengths, 2.684(2)–3.1565(18) Å; Na−centroid distance 2.5311(1) Å; centroid‐Na‐centroid angle, 180.0°). Propagation of this contacted ion pair structure into an infinite helical chain (Figure 12) is through these distinct types of Na−C interaction. Mg1 occupies a N2C distorted trigonal planar coordination (sum of bond angles; 359.69°) comprising two amide ligands (mean Mg−N distance, 2.0328 Å) and one butyl ligand (Mg1−C16, 2.1533(19) Å). The helical chain structure of 10 (Figure 13) is less intricate than that of 9 as it contains only one alkali metal (K) site. Sandwiched between two aryl rings (K1−centroid1 2.9162(1) Å; K1−centroid2 2.9239(1) Å; centroid1‐K1‐centroid2, 147.4°), K1 also interacts with the α‐C of the Bu ligand (K1−C31 3.046(3) Å), while there is a contact with one of the CH3(Si) groups though the disorder in this group negates an accurate bond length. Consistent with the interaction between the alkali metal and the magnesiate ion being predominately electrostatic in nature the dimensions of the Mg1 centre in 10 (mean Mg−N, 2.0334 Å; Mg−C31, 2.150(2) Å; sum of bond angles subtending Mg1, 359.82°) match those in the sodium magnesiate 12.13

View Article: PubMed Central - PubMed

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‐C6H3). 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.

No MeSH data available.