Abstract
Quantum chemical methods for calculating paramagnetic NMR observables are becoming increasingly accessible and are being included in the inorganic chemistry practice. Here, we test the performance of these methods in the prediction of proton hyperfine shifts of two archetypical high-spin pentacoordinate nickel(II) complexes (NiSAL-MeDPT and NiSAL-HDPT), which, for a variety of reasons, turned out to be perfectly suited to challenge the predictions to the finest level of detail. For NiSAL-MeDPT, new NMR experiments yield an assignment that perfectly matches the calculations. The slightly different hyperfine shifts from the two “halves” of the molecules related by a pseudo-C2 axis, which are experimentally divided into two well-defined spin systems, are also straightforwardly distinguished by the calculations. In the case of NiSAL-HDPT, for which no X-ray structure is available, the quality of the calculations allowed us to refine its structure using as a starting template the structure of NiSAL-MeDPT.
Highlights
IntroductionQuantum 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 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
Quantum chemical analysis of paramagnetic shifts 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,1−9 for structure analysis in bioinorganic chemistry,10−14 and in view of the development of (e.g.) single-ion magnets or qubits.15−23 With this work, we want to challenge state-of-the-art QC methods to accurately predict hyperfine shifts for an inorganic system the NMR properties of which have been studied over decades
We select complexes of nickel(II) coordinated by pentadentate salicylaldiminates with dipropylenetriamine bridges (SAL-DPT), such as NiSALMeDPT and NiSAL-HDPT (SAL = salicylaldiminate; DPT = dipropylenetriamine), which are archetypical of high-spin pentacoordinate complexes of this metal (Figure 1), being the first to be designed to enforce this at the time unusual coordination and spin state in nickel(II)
Summary
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 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. A recent QC treatment differs from the semiempirical approach based on the Spin Hamiltonian parameters for the inclusion of the nucleus-orbit coupling (called in the literature Paramagnetic Spin-Orbit contribution), and this difference breaks the link between the pseudocontact shifts and the magnetic susceptibility anisotropy tensor.. A recent QC treatment differs from the semiempirical approach based on the Spin Hamiltonian parameters for the inclusion of the nucleus-orbit coupling (called in the literature Paramagnetic Spin-Orbit contribution), and this difference breaks the link between the pseudocontact shifts and the magnetic susceptibility anisotropy tensor.29 This QC-based approach has been increasingly used to describe inorganic and bio-inorganic systems,− the Received: December 12, 2020 Published: January 21, 2021 All these features make these complexes an optimal benchmark for testing the prediction of QC methods: pNMR QC methods are not always in agreement with one another, as far as the calculation of the pseudocontact shift (PCS) contribution is concerned. A recent QC treatment differs from the semiempirical approach based on the Spin Hamiltonian parameters for the inclusion of the nucleus-orbit coupling (called in the literature Paramagnetic Spin-Orbit contribution), and this difference breaks the link between the pseudocontact shifts and the magnetic susceptibility anisotropy tensor. This QC-based approach has been increasingly used to describe inorganic and bio-inorganic systems,− the Received: December 12, 2020 Published: January 21, 2021
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