Fluorination Effects on Rotational Correlation Times of Tris(β-diketonato)aluminum(III) in CO₂ by ²⁷Al NMR Relaxation Measurements

Abstract

(27)Al NMR longitudinal relaxation times, T(1,obs)((27)Al), of [Al(acac)(3)] and [Al(hfa)(3)] (Hacac = acetylacetone, Hhfa = hexafluoroacetylacetone) in CH(3)CN and CO(2) were measured over a wide range of temperature and pressure. The rotational correlation times, τ(r), of the tris(β-diketonato)aluminum(III) complexes were determined from T(1,obs)((27)Al) using (27)Al quadrupole coupling constants, eQq/h((27)Al), which were also obtained to be 3.11 and 3.22 MHz for [Al(acac)(3)] and [Al(hfa)(3)], respectively, in CD(3)CN by the dual spin probe technique in the present study. At each temperature, τ(r) increased almost linearly with increasing viscosity, η, in both CH(3)CN and CO(2); however, τ(r) in CO(2) at near critical densities deviated appreciably upward, as shown in a similar analogue of bis(acetylacetonato)beryllium(II), [Be(acac)(2)] (Umecky; et al. J. Phys. Chem. B 2002, 106, 11114). The η/T dependence of τ(r) was examined to discuss intermolecular interactions between the complexes and solvent molecules in terms of the fluorination and geometrical effects. The degree of solute-solvent interactions increases in the order [Be(acac)(2)] < [Al(hfa)(3)] < [Al(acac)(3)] in CH(3)CN and [Al(acac)(3)] < [Be(acac)(2)] < [Al(hfa)(3)] in CO(2). The results suggest that dipolar CH(3)CN molecules interact with negatively charged oxygen atoms in the complexes, whereas nonpolar CO(2) prefers fluorinated substituents as well as quasi-aromatic rings in the ligands. Moreover, the relationship between the rotational and translational motions of tris(acetylacetonato)metal(III), [M(III)(acac)(3)], in CO(2) was investigated.

Ligand-Modulated Chemical and Structural Implications in Four-, Five-, and Six-fold Coordinated Aluminum Heteroaryl Alkenolates

“…The octahedral environment is substantially distorted from ideal parameters in all three compounds, with cis and trans angles varying from 74.1(3)°to 98.64 (10) of the coordination geometry, the value τ, which measures how closely a distorted five-coordinate compound approximates either a trigonal bipyramidal or square pyramidal geometry, was calculated according to the reported method. 74 The value of τ = 0.82 suggested that the geometry around the aluminum centers is best described as distorted trigonal bipyramidal.…”

“…With continuing research interest and increasing technological importance of nanomaterials, particularly thin films of functional materials, the quest for new metal–organic compounds, enabling a controlled synthesis of new materials has gained significant impetus in recent years. − Aluminum compounds supported by monoanionic alkoxide and β-diketonate ligands are well-known and extensively investigated. − Among halide and alkyl aluminum complexes, they are the most widely used precursor classes for chemical vapor depositions of Al 2 O 3 despite the drawbacks of being prone to hydrolyses or showing low volatilities. − …”

“…The Al−N (2.10 and 2.08 Å) as well as the Al−O (1.84 Å) bond lengths involving the chelating ligands were similar to those found in [Al(PyTFP) 2 OH] 2 •2(C 3 H 6 O). The edgelinkage between the two polyhedra together with the steric demand of all ligands causes severe distortions of the octahedron as well as the tetrahedron evident in the acute angles of 76.35(10)°(O1−Al2−O2) deviating by approximately 14°from an ideal octahedron (90°) and even more by around 28°(81.05°; O1−Al1−O2) from values for a regular tetrahedron (109.47°).Chemical Vapor Deposition. First CVD experiments with [Al(PyTFP)(O t Bu) 2 ] 2 as precursor resulted in the formation of Al 2 O 3 films on Si substrates (Figure9).…”

Nuclear spin relaxation in liquids and gases

“…In a study already mentioned in section 3.2, Umecky and co-workers 199 presented measurements of 27 Al (I = 5/2) and 13 C relaxation in tris(acetylacetonato)aluminium(III) and its analogue with the methyls replaced by CF 3 groups, dissolved in supercritical carbon dioxide. The 27 Al T 1 -values were obtained over a range of temperatures and pressures and the dependences of the rotational correlation time on viscosity and density was investigated.…”