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

Abstract

Ultra-wideline 27Al NMR experiments are conducted on coordination compounds with 27Al nuclei possessing immense quadrupolar interactions that result from exceptionally nonspherical coordination environments. NMR spectra are acquired using a methodology involving frequency-stepped, piecewise acquisition of NMR spectra with Hahn-echo or quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequences, which is applicable to any half-integer quadrupolar nucleus with extremely broad NMR powder patterns. Despite the large breadth of these central transition powder patterns, ranging from 250 to 700 kHz, the total experimental times are an order of magnitude less than previously reported experiments on analogous complexes with smaller quadrupolar interactions. The complexes examined feature three- or five-coordinate aluminum sites: trismesitylaluminum (AlMes3), tris(bis(trimethylsilyl)amino)aluminum (Al(NTMS2)3), bis[dimethyl tetrahydrofurfuryloxide aluminum] ([Me2-Al(mu-OTHF)]2), and bis[diethyl tetrahydrofurfuryloxide aluminum] ([Et2-Al(mu-OTHF)]2). We report some of the largest 27Al quadrupolar coupling constants measured to date, with values of C(Q)(27Al) of 48.2(1), 36.3(1), 19.9(1), and 19.6(2) MHz for AlMes3, Al(NTMS2)3, [Me2-Al(mu-OTHF)]2, and [Et2-Al(mu-OTHF)]2, respectively. X-ray crystallographic data and theoretical (Hartree-Fock and DFT) calculations of 27Al electric field gradient (EFG) tensors are utilized to examine the relationships between the quadrupolar interactions and molecular structure; in particular, the origin of the immense quadrupolar interaction in the three-coordinate species is studied via analyses of molecular orbitals.

Analysis of electric field gradient tensors at quadrupolar nuclei in common structural motifs

“…In Fig. 6, a graphical representation of theV-tensor is shown for The sign pattern and orientation of the EFG tensor of the four-coordinate 27 Al compound shown in Fig. 4 matches exactly the predictions from this model; due to the electronegativity of the nitrogen ligands one expects less electron density in the aluminum sp 3 hybrids pointing towards nitrogen than in those pointing towards the carbons.…”

“…To visually compare EFG tensor magnitudes between different systems, such as the three-, four-, and five-coordinate aluminum centers that will be discussed in Section "Al EFG Tensors", it is best to use a common scaling factor. Figure 4 shows simulated SSNMR spectra for a set of hypothetical 27 Al compounds all having the same V 33 but varying η. Like these simulated spectra, the shapes of real spectra reveal the "shape" (asymmetry) of the EFG tensor.…”

“…This result is also easily obtained by considering (i) that the EFG contribution of an sp 3 orbital along its directional vector is − 3 /2, (ii) that with respect to this direction the other three orbitals are symmetry equivalent, and (iii) that for equal occupations of all four orbitals the diagonal tensor components must vanish. For the 4-coordinate 27 Al system shown in Fig. 4, the nearest neighbor atoms are two carbons on one side and two nitrogens on the other side.…”

“…More recent discussions have noted the connections between the quadrupolar interaction and the local symmetry, coordination numbers and bonding arrangements (27)(28)(29); however, only comparisons of series of structurally and/or chemically similar systems are valid in this context. Therefore, there is a need for the further development for systematic investigations of quadrupolar interactions for a variety of atom types in an assortment of coordination environments, as well as robust computational methods for exploring these systems.…”

Natural abundance 14N and 15N solid-state NMR of pharmaceuticals and their polymorphs

“…Due to the limited excitation bandwidths associated with the WURST pulses, all spectra were acquired using the previously mentioned VOCS method, 98,99 with frequency increments equal to an integer multiple of the spikelet spacings arising from the CPMG portion of the pulse sequence. 141 The experimental times ranged from as long as 40 hours to as short as 8.4 hours, with a mean of approximately 16 hours. Further experimental details are given in the Supporting Information (Tables S1 -S3).…”