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

Abstract

The variation in 13C NMR paramagnetic shifts as a function of d-electron configuration was explained by NMR shielding calculations.

Crystal and Substituent Effects on Paramagnetic NMR Shifts in Transition-Metal Complexes

“…In contrast to previous reports, we succeeded in measuring all of the 13 C NMR resonances 22 of compound 1a and a very broad resonance 21 at around 900 ppm of 2a. The measured 1 H (solution) and 13 C (solid state) NMR shifts for compounds 1a−d and 2a−d are summarized in Table 1.…”

“…Note that the NMR shifts for 2a calculated in vacuo (Figure 3) agree nicely with those reported previously (+1153, +149, and −44 ppm). 21 The successful observation of all of the 13 C NMR resonances of parent compounds 1a and 2a (including those at around 1000 ppm) prompted us to investigate the set of compounds 1 and 2 shown in Figure 1 and to analyze the effects of substituents on the NMR shifts. However, for the chromium complexes 1, we obtained the NMR resonances of C1 (+1155 ppm) and C2 (+316 ppm) only for compound 1a.…”

“…Early transition-metal (TM) complexes with low d-electron occupancy (d 1 –d 3 ) exhibit negative g -shifts, whereas late transition-metal complexes (d 7 –d 9 ) typically have positive g -shifts. The sign of the isotropic g -shift for the Cr­(III) and Cu­(II) compounds being investigated (Figure ) can also be described by a linear spin–orbit approach to the g -tensor in the framework of crystal-field theory. , Recently, the relationship between the electron configuration and the sign of the hyperfine NMR shifts of the ligand has been demonstrated for a series of acetylacetonato vanadium, nickel, and copper complexes . Previous EPR and paramagnetic NMR studies of transition-metal complexes of acetylacetonate include those of Cr­(III), , Mn­(III), Fe­(III), , Ru­(III), − Ni­(II), , and Cu­(II) compounds.…”

A Quantum Chemistry View on Two Archetypical Paramagnetic Pentacoordinate Nickel(II) Complexes Offers a Fresh Look on Their NMR Spectra

“…Quantum chemical analysis of paramagnetic shifts (pNMR) in terms of the electronic structure of metal centers is gathering momentum, thanks to the effort of computational and experimental groups: the understanding of the electronic structure allows for a deeper understanding of the magnetic behavior of paramagnetic systems for the spectroscopic characterization of inorganic compounds, − for structure analysis in bioinorganic chemistry, − and also in view of the development of (e.g.) single-ion magnets or qubits. − With this work, we want to challenge state-of-the-art QC methods to accurately predict hyperfine shifts (both contact and pseudocontact) for an inorganic system the NMR properties of which have been studied over decades.…”

Hyperfine Coupling Constants in Cu-Based Crystalline Compounds: Solid-State NMR Spectroscopy and First-Principles Calculations with Isolated-Cluster and Extended Periodic-Lattice Models

“…Nevertheless, calculations of hyperfine couplings have so far mostly been carried out “in a gas phase”, i.e. , considering isolated motifs that were extracted from the materials’ extended periodic structure. − Some calculations were done with periodic boundary conditions, but they considered systems with relatively small crystallographic unit cells. ,− However, many interesting materials have structures described by large unit cells, and from some of them it can be difficult to extract characteristic isolated clusters that would adequately describe the extended periodic structures. Such structures can certainly be found in the family of metal–organic framework materials (MOFs), a structurally extremely diverse group of materials that is of great interest for various applications.…”