Vis sammendrag
Nuclear Magnetic Resonance (NMR) is a powerful spectroscopic technique to determine the structure of transition-metal complexes. In addition to structural information, NMR also offers insights into the electronic structure of these complexes. The chemical shift is, in fact, directly linked to the frontier molecular orbitals, and thus to reactivity [1,2]. Platinum complexes containing diimine ancillary ligands are promising candidates for C-H bonding activation under direct oxidative conditions [3]. Variations on the ancillary ligands modify the local environment of the Pt nucleus, producing a broad range of 195-Pt NMR chemical shifts. Hence, a molecular approach can help to rationalize the signature of the complexes and to establish correlations with descriptors such as the empirical Hammett’s parameters.
In this work, we investigate an extensive series of diimine and bipyridine complexes of neutral and cationic Pt(II) species. Calculations at the two-component relativistic level successfully reproduce the 195-Pt NMR chemical shifts experimental trends. We found a linear correlation with the empirical Hammett’s parameters for both neutral and cationic species. In addition, the Natural Chemical Shift (NCS) [4] analysis, reveals the most deshielded principal component of diimines lies parallel to the bonding plane in contrast to the perpendicular orientation reported for PtL4 systems. [5] Moreover, the (de)shielding across the series of Pt-species is clearly affected by the antibonding 𝜎*(Pt-L) orbital energy, which is determined by the electron-donating or –withdrawing character of the substituents in the diimine and bipyridine ligands, as well as the monodentate ligands bonded to Pt.
[1] M. Kaupp, M. Bühl, V. G. Malkin, Eds. In Calculation of NMR and EPR Parameters: Theory and applications. Wiley-VCH, Weinheim, 2004.
[2] C. P. Gordon, L. Lätsch, C. Copéret, J. Phys. Chem. Lett. 2021, 12, 8, 2072-2085.
[3] M. Lersch, M. Tilset, Chem. Rev. 2005, 105, 2471−2526.
[4] J. A. Bohmann, F. Weinhold, T. C. Farrar, J. Chem. Phys. 1997, 107, 1173–1184.
[5] A. Venkatesh, D. Gioffre, B.A. Atterberry, L. Rochlitz, S.L. Carnahan, Z. Wang, G. Menzildjian, A. Lesage, C. Coperet, A.J. Rossini, J. Am. Chem. Soc. 2022, 144, 13511-13525.