A Stable Calcium Alumanyl

Abstract

A seven-membered N,N'-heterocyclic potassium alumanyl nucleophile is introduced and utilised in the metathetical synthesis of Mg-Al and Ca-Al bonded derivatives. Both species have been characterised by experimental and theoretical means, allowing a rationalisation of the greater reactivity of the heavier group 2 species implied by an initial assay of their reactivity.

Keywords: alumanyl; calcium; density functional theory; magnesium; potassium.

Scheme 1

Reactivity of compound 2 with (^DippBDI)Ae reagents.

Figure 1

The potassium alumanyl derivatives, 6 and 7, and the Al−Mg bonded species, 8.

Scheme 2

Synthesis of compounds 10–12.

Figure 2

ORTEP representation of compound 12 (30 % probability ellipsoids). Hydrogen atoms and occluded molecule of diethyl ether solvent are omitted for clarity. Selected bond lengths [Å] and angles [°]: Al1−N1 1.887(2), Al1−N2 1.8889(19), Al2−N3 1.890(2), Al2−N4 1.892(2), Si1−N1 1.729(2), Si2−N2 1.7333(19), Si3−N3 1.731(2), Si4−N4 1.731(2); N1−Al1−N2 108.84(9), N3−Al2−N4 108.77(9).

Scheme 3

Synthesis of the magnesium and calcium alumanyl compounds, 15 and 16.

Figure 3

ORTEP representations of a) compound 15 and b) compound 16 (30 % probability ellipsoids). Hydrogen atoms, iso‐propyl carbon atoms and disordered molecules of solvent are omitted for clarity. Selected bond lengths [Å] and angles [°]: (15) Al1−Mg1 2.7980(6), Al1−N1 1.8482(13), Al1−N2 1.8402(13), Mg1−N3 2.0528(12), Mg1−N4 2.0721(12); N1−Al1−Mg1 125.94(5), N2−Al1−Mg1 121.91(4), N2−Al1−N1 111.87(6), N3−Mg1−Al1 132.59(4), N3−Mg1−N4 93.77(5), N4−Mg1−Al1 132.94(4); (16) Ca1−Al1 3.1229(5), Al1−N1 1.8973(13), Al1−N2 1.8544(14), Ca1−N3 2.3538(12), Ca1−N4 2.3682(13), Ca1−C1 3.1735(15), Ca1−C2 3.1333(16), Ca1−C3 3.0213(17), Ca1−C4 2.9790(17), Ca1−C5 3.0884(16), Ca1−C6 3.2215(15); N2−Al1−N1 111.08(6), N3−Ca1−N4 81.10(4), N1−Al1−Ca1 98.14(4), N2−Al1−Ca1 150.30(5), N3−Ca1−Al1 118.99(3), N4−Ca1−Al1 123.45(3).

Figure 4

Localized Pipek–Mezey orbitals of a) 15 and b) 16 showing Al−Ca and Al−Mg σ‐bonding orbitals, respectively. QTAIM molecular graphs of c) 15 and d) 16. The electron density contours are computed in the {Mg/Al/Si} planes with bond critical points (BCPs) shown as small red spheres. BCP electron densities (ρ(r) in e Å⁻³), values of the Laplacian of the electron density (∇² ρ(r) in e Å⁻⁵), ellipticities (ϵ) and total energy densities (H(r) in a.u.) are tabulated beneath the relevant figures.

Figure 5

ORTEP representations of (a) the aluminate component of compound 17 and (b) compound 18 (30 % probability ellipsoids). Hydrogen atoms and iso‐propyl carbon atoms, in both structures, and a disordered molecule of benzene solvent in 18 are omitted for clarity. Selected bond lengths [Å] and angles [°]: (17) Al1−O1 1.7637(16), Al1−N1 1.8997(18), Al1−N2 1.889(2), Al1−34 1.984(3); O1−Al1−N1 109.88(8), O1−Al1−N2 106.51(8), O1−Al1−C34 102.23(10), N1−Al1−C34 113.76(10), N2−Al1−N1 110.75(8), N2−Al1−C34 113.10(10); (18) Al1−N1 1.8085(17), Al1−N2 1.8200(16), Al1−C31 2.125(2), Al1−C32 2.123(2), Ca1−N3 2.3437(17), Ca1−N4 2.3831(18), Ca1−C31 2.7107(19), Ca1−C32 2.804(2), Ca1−C33 2.818(2), Ca1−C34 2.753(2), Ca1−35 2.693(2), Ca1−C36 2.695(2), Ca1−C37 2.706(2), Ca1−C38 2.698(2); N1−Al1−N2 119.27(8), N3−Ca1−N4 79.34(6).