Electron-Spin Structure and Metal–Ligand Bonding in Open-Shell Systems from Relativistic EPR and NMR: A Case Study of Square-Planar Iridium Catalysts

Abstract

Electron and nuclear magnetic resonance spectroscopies are indispensable and powerful methods for investigating the molecular and electronic structures of open-shell systems. We demonstrate that the NMR and EPR parameters are extremely sensitive quantitative probes for the electronic spin density around heavy-metal atoms and the metal-ligand bonding. Using relativistic density-functional theory, we have analyzed the relation between the spin density and the EPR and NMR parameters in paramagnetic iridium(II/IV) complexes with a PNP pincer ligand. As the magnetic-response parameters for compounds containing 5d transition metal(s) are heavily affected by spin-orbit coupling, relativistic effects must be included in the calculations. We have used a recent implementation of the fully-relativistic Dirac-Kohn-Sham (DKS) method employing the hybrid PBE0 functional and an implicit solvent model to calculate EPR parameters and hyperfine NMR shifts. The modulation of the metal-ligand bond by the trans substituent (−Cl or ≡N) and the electronic spin structure around the central metal atom and ligands are shown to be reflected in the "long-range" through-bond Fermi-contact (FC) contributions to the ligand 13 C and 1 H hyperfine couplings. Interestingly, the hyperfine coupling constant of the ligand atom L () bonded directly to the iridium center changes its sign because of the dominating role of the paramagnetic spin-orbit (PSO) term. Furthermore, the electronic g-shift and the PSO contribution to the ligand are shown to invert their signs when nitrogen is substituted for chlorine, reflecting the different formal metal oxidation states and the change in metal-ligand bond character. A full understanding of the substituent effects is provided by using chemical bond concepts in combination with a molecular-orbital (MO) theory analysis of the second-order perturbation theory expression for the EPR parameters. Our findings are easily transferable to other systems containing d-block elements and beyond. Relativistic DFT calculations of magnetic-resonance parameters are expected to frequently assist in future experimental observations and the characterization of hitherto unknown unstable or exotic species.

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

“…Thus, calculated spin-density plots have been used to obtain a qualitative understanding of the sign and magnitude of the paramagnetic NMR shifts observed experimentally. 4,32,[37][38][39] Whether the two mechanisms are accessed in the computations, depends on the computational framework used.…”

“…Marek and coworkers have extensively studied 4d and 5d metal-organic complexes to explain the observed pNMR shifts based on electronic structure. 37,38,43,44 Recently, by variation of both ligand and metal oxidation states, the effects on the NMR shifts could be explained based on the contribution of the different SOMOs and total spin density plots. 38 The objective of the present work is to investigate the effect of the 3d-electron configuration in a series of structurally related acetyl acetonate (acac) complexes with different transition metal ions depicted in Fig.…”

“…37,38,43,44 Recently, by variation of both ligand and metal oxidation states, the effects on the NMR shifts could be explained based on the contribution of the different SOMOs and total spin density plots. 38 The objective of the present work is to investigate the effect of the 3d-electron configuration in a series of structurally related acetyl acetonate (acac) complexes with different transition metal ions depicted in Fig. 1: bis(acetylacetonato)vanadium(IV) oxide ([VO(acac)2]), tris(acetylacetonato)vanadium(III) ([V(acac)3]), tris(acetylacetonato)cobalt(III) ([Co(acac)3]), transbis(acetylacetonato)diaquanickel(II) ([Ni(acac)2(H2O)2]), and bis(acetylacetonato)copper(II) ([Cu(acac)2]) using a combined computational and experimental approach, where the total shift tensor is obtained.…”

Probing the Local Magnetic Structure of the [FeIII(Tp)(CN)3]− Building Block Via Solid‐State NMR Spectroscopy, Polarized Neutron Diffraction, and First‐Principle Calculations

“…in which β e , g N , β N , and k B represent the Bohr magneton, the nuclear g ‐factor, the nuclear magneton, and the Boltzmann constant, respectively, corresponds to the Zeeman coupling matrix and is calculated either by using wave‐function theory or by using KS‐DFT, and is the hyperfine coupling matrix and is obtained from KS‐DFT calculations (see the Computational Details). Such a computational strategy has already been applied successfully to a large panel of transition‐metal complexes . To assemble the Zeeman g ‐tensors, the hyperfine coupling matrices, and the orbital shielding, we have used the pNMRShift program developed by the Autschbach's group .…”

“…[89][90][91][92][93][94][95][96][97][98][99] To assemble the Zeeman g-tensors, the hyperfine coupling matrices, and the orbital shielding, we have used the pNMRShift program developed by the Autschbach's group. Such acomputational strategy has already been applied successfully to al arge panel of transition-metal complexes.…”