CO2 Capture by 2-(Methylamino)pyridine Ligated Aluminum Alkyl Complexes

Abstract

A set of novel, easily synthesized aluminum complexes, Al[κ²-N,N-2-(methylamino)pyridine]₂R (R = Et, iBu) are reported. When subjected to 1 atm of CO₂ pressure, each hemilabile pyridine arm dissociates and facilitates cooperative activation of the CO₂ substrate reminiscent of a Frustrated Lewis Pair. This reaction has limited precedent for Al/N based Lewis Pair systems, and this is the first system readily shown to sequester multiple equivalents of CO₂ per aluminum center. The ethyl variant then reacts further, inserting a third equivalent of CO₂ into the aluminum alkyl to generate an aluminum carboxylate. Examples of this type of reactivity are rare under thermal conditions. A joint experimental/computational study supports the proposed reaction mechanism.

Keywords: Aluminum; Carbamate; Carbon dioxide; Frustrated Lewis pair; Insertion.

Figure 1.

Newly synthesized aluminum complexes. Stereoisomers drawn to match crystal structure.

Figure 2.

ORTEP representation of 1 (left) and 2 (right). Ellipsoids shown at 50 % probability. Hydrogen atoms omitted for clarity. Select bond lengths [Å] and angles [deg]: (1) N(1)pyridine–Al(1) 2.06(9), N(3)pyridine–Al(1) 2.11(0), N(2)–Al(1)–N(4) 119.33, C(13)–Al(1)–N(4) 117.26, C(13)–Al(1)–N(2) 123.23. (2) N(1)pyridine–Al(1) 2.085, N(3)pyridine–Al(1) 2.088, N(2)–Al(1)–N(4) 119.03, C(1)–Al(1)–N(2) 119.88, C(1)–Al(1)–N(4) 121.09. CCDC accession numbers 1987618, 1987619.

Figure 3.

Stacked ¹H-NMR spectrum representative of 1 (bottom) and 3 after addition of 1 atm CO₂ (top) in [D₆]benzene. Spectra referenced to residual solvent peak (7.16 ppm).

Figure 4.

Stacked ¹H-NMR spectrum representative of 2 (bottom) and 4 after addition of 1 atm CO₂ (top) in [D₆]benzene. Spectra referenced to residual solvent peak.

Figure 5.

Computed reaction pathways for sequential CO₂ insertion into complex 1.

Figure 6.

Computationally optimized structures of 1-trans (left) and its counterpart 1-cis (right), where the methyl substituents of the aminomethylpyridine are sterically unfavored. Hydrogen atoms are removed for clarity.

Figure 7.

Free energy diagram of proposed isomerization of reactant 1 from trans to cis form. The reference free energy (in kcal/mol) is that of the 1-trans structure.

Figure 8.

Free energy diagram of proposed activation of CO₂ to the aluminium-bound carboxylate where CO₂ insertion into alkyl group occurs first (Pathway 1 in Figure 5). The baseline free energy (in kcal/mol) is complex 1 + the free energy of CO₂ at infinite separation.

Figure 9.

Free energy diagram of proposed activation of CO₂ to the aluminum bound carboxylate (Pathway 3). The baseline free energy (in kcal/mol) is 1-trans + the free energy of CO₂ at infinite separation.

Scheme 1.

Insertion of CO₂ into complexes 1 and 2.