Assignment of solid-state 13C and 1H NMR spectra of paramagnetic Ni(II) acetylacetonate complexes aided by first-principles computations

Abstract

Recent advances in computational methodology allowed for first-principles calculations of the nuclear shielding tensor for a series of paramagnetic nickel(II) acetylacetonate complexes, [Ni(acac)L] with L = HO, DO, NH, ND, and PMePh have provided detailed insight into the origin of the paramagnetic contributions to the total shift tensor. This was employed for the assignment of the solid-state H andC MAS NMR spectra of these compounds. The two major contributions to the isotropic shifts are by orbital (diamagnetic-like) and contact mechanism. The orbital shielding, contact, as well as dipolar terms all contribute to the anisotropic component. The calculations suggest reassignment of the C methyl and carbonyl resonances in the acac ligand [Inorg. Chem.53, 2014, 399] leading to isotropic paramagnetic shifts of δ(C) ≈ 800-1100 ppm and ≈180-300 ppm for C for the methyl and carbonyl carbons located three and two bonds away from the paramagnetic Ni(II) ion, respectively. Assignment using three different empirical correlations, i.e., paramagnetic shifts, shift anisotropy, and relaxation (T) were ambiguous, however the latter two support the computational results. Thus, solid-state NMR spectroscopy in combination with modern quantum-chemical calculations of paramagnetic shifts constitutes a promising tool for structural investigations of metal complexes and materials.

Remarkable reversal of 13C-NMR assignment in d1, d2 compared to d8, d9 acetylacetonate complexes: analysis and explanation based on solid-state MAS NMR and computations

“…First, the experimental 13 22 Then the experimental results will be compared with the computational results, followed by discussion of the combined observations.…”

“…The EPR parameters were computed following the previously reported approach, 16,17 which has been found to be efficient for these systems. 17,21,22 The D and g tensors were calculated using the ORCA program 65 at the NEVPT2 [66][67][68] level of theory, based on a state-averaged CAS(n,5)…”

“…Nevertheless, it has been successfully been applied to analyze a number of experimental situations. 15,21,22 Recently, the method was extended to periodic solids 23,24 and, combined with the long-range point-dipole approximation, it allowed ab initio pNMR calculations of an entire protein. 25 Important, so-far non-routine developments beyond the Kurland-McGarvey theory level include the pseudospin formulation 26,27 which is generally applicable, also in the case of strong spin-orbit coupling.…”

“…These acac complexes were chosen because the three types of carbons in the acac-ligand had distinct paramagnetic shift tensors in our previous studies of [Ni(II)(acac)2L2]. 22,45 In particular, the assignment of the 13 C-NMR shifts of CH3 and CO groups 45 had to be counter-intuitively reversed based on quantum-chemical calculations 22 employing the recipe of Ref. 16 .…”

Order in disorder: solution and solid-state studies of [MIII2MII5] wheels (MIII = Cr, Al; MII = Ni, Zn)

“…7(b)). The presence of paramagnetic Ni 2+ results in a contact shift in addition to the diamagnetic isotropic shift, 37 which move the resonances outside the conventional chemical shift range for diamagnetic materials. To a first approximation, the hyperfine shift in inorganic materials is proportional to the number of neighbouring Ni ions.…”