Steric Effects in Aluminum Compounds Containing Monoanionic Potentially Bidentate Ligands:  Toward a Quantitative Measure of Steric Bulk

Abstract

To develop an understanding of the factors that control the strength of the Lewis acidbase interactions in five-coordinate compounds of aluminum, dimeric dialkylaluminum compounds [R 2 Al{µ-O(CH 2 ) n ER′ x }] 2 (n ) 2, 3; ER′ x ) OR′, SR′, NR′ 2 ) have been prepared from AlR 3 and the appropriate substituted alcohol, thiol, or amine: [R The molecular structures of compounds 1, 2, 6, 7, 12, 13, 16, 18-22, and [Me 2 Al(µ-OCH 2 -CH 2 NMe 2 )] 2 have been determined by X-ray crystallography. The solution structures have been probed by 13 C NMR spectroscopy using the alkoxide derivatives, [R 2 Al(µ-O n Bu)] 2 , R ) t Bu (8), i Bu (9), and Et (10), as a comparison, which show that compounds 1-7 and 11-23 exist as an equilibrium mixture between the four-coordinate isomer and its five-coordinate isomer, where K eq ) [4-coord]/ [5-coord]. Factors that control the extent of this equilibrium and hence the coordination about aluminum include the steric bulk of the substituents at aluminum (R) and the Lewis base donor (R′), the basicity of the neutral donor group (ER′ x ), and the chelate ring size (as determined by n). The intramolecular bond dissociation energies (BDEs) of the Lewis base donor (ER′ x ), as determined by variable-temperature NMR spectroscopy, for compounds 1, 12, 15, 16, [Me 2 Al(µ-OCH 2 CH 2 OMe)] 2 , and [R 2 Al(µ-OCH 2 -CH 2 NMe 2 )] 2 (2.3-13.2 kJ‚mol -1 ) are significantly lower than observed for their fourcoordinate analogues, R 3 Al(ER′ x+1 ) (63.8-125.5 kJ‚mol -1 ). In addition, the relative donor ability (thioether > ether > amine) is the opposite of that expected. The similarity in BDE between compound 1 and 11 suggests that no cooperative effects are present. Ab initio calculations on the four-and five-coordinate isomers of the model compound [H 2 Al(µ-OCH 2 -CH 2 OH)] 2 indicate that the surprisingly weak fifth coordination ligation in [R 2 Al{µ-O(CH 2 ) n -ER′ x }] 2 is a consequence of significant steric hindrance between the aluminum alkyl groups (R) and the Lewis base donor group. A quantitative measure of the thermodynamic destabilization that occurs upon substitution of H with Me or t Bu is proposed on the basis of the derivation of the Lennard-Jones (12,6) potential from solution thermodynamic and X-ray structural data.

Ultra‐wideline 27Al NMR Investigation of Three‐ and Five‐Coordinate Aluminum Environments

“…The average AlÀN bond length is 1.813(3) with NÀAlÀN angles of 120 (1) Tables S5 and S6). The bond lengths and angles associated with the first coordination sphere of the aluminum atom in these compounds are analogous to similar compounds, such as [R 2 Al(m-OCH 2 CH 2 OMe)] 2, [69,70] [Me 2 Al(m-OCH 2 C 6 H 4 -2-OMe)] 2 , [71] and [Me 2 Al(m-OC 6 H 4 -2-OMe)] 2 . [71] The bridging oxygen atoms form a central (AlO) 2 ring, with average Theoretical Calculations of 27 …”

Is the Formation of Poly-CO2 Stabilized by Lewis Base Moieties in N- and S-Doped Porous Carbon?

“…The Al … O distance (1.881 Å) is within the range for typical Lewis acid-base interactions observed from X-ray crystallography (1.8-2.0 Å) [17][18][19]; however, there appears little effect on subsequent CO2 molecules. The strong Lewis acid nature of AlF3 and the Lewis basic nature of the fluorine ligands results in a more stable (27 kJ/mol) structure involving two CO2 molecules occupying the axial positions of a trigonal bipyramidal aluminum; again the Al … O distances (2.028, 2.049 Å) are within the range for such coordination geometries about aluminum (1.890(6)-2.283(2) Å) [20]. The additional CO2 molecules appear to show interactions with the fluorine ligands via Al-F … C interactions (2.44, 2.49 Å); however, again there is no poly-CO2 stabilization.…”

“…Optimization of all structures was carried out at the Hartree-Fock level with the 3-21G(*) basis set [29,30]. We have previously shown this basis set to give acceptable predictions of structural parameters for main group compounds [20,[31][32][33]. To determine the relative energy of each species with electron correlation included, second order Møller-Plesset (MP2) calculations were performed [29].…”

An Unusual N‐Bridged (Amido)(hydrido)(phenoxido)aluminium Dinuclear Compound – The Role of Nitrogen Substituents in Determining Nuclearity: A Combined Experimental and Theoretical Study

“…The stability of mononuclear or dinuclear aluminium derivatives with analogous N,O-donor ligands depending on electronic and steric factors have been previously studied. [25,60] In addition, a discussion of the factors affecting the coordination modes of amidate ligands to aluminium has been reported. [55] Thus, we started a computational study in order to clarify and rationalize the structural behaviour observed in the final products obtained in the treatment of AlH 3 ·NEtMe 2 with iminophenol and aminophenol compounds, depending on the nitrogen substituents.…”