Giant Spin‐Orbit Effects on NMR Shifts in Diamagnetic Actinide Complexes: Guiding the Search of Uranium(VI) Hydride Complexes in the Correct Spectral Range

Abstract

Transition-and inner-transition metal hydride complexes are crucial reagents in a great variety of stoichiometric and catalytic transformations, including CÀH bond activation. [1] As hydrogen atoms near heavy-metal centers are difficult to locate by X-ray diffraction, often their prime characterization is by 1 H NMR spectroscopy, sometimes augmented by IR spectroscopy. A significant part of the utility of 1 H NMR spectroscopy in this field arises from the fact that the chemical shifts of metal-bound protons are characteristic and occupy extreme positions in the proton shift range, even for diamagnetic compounds. For instance, complexes with d 6 or d 8 metal configuration exhibit shifts below d = 0 ppm, in record cases down to below d = À50 ppm for iridium hydride complexes. [2,3] While this phenomenon was explained as early as the 1960s by Buckingham and Stephens as being due to offcenter paramagnetic ring currents [4] (see Ref.[5] for the earliest DFT results), we have recently shown that the largest low-frequency shifts of this kind are, to an appreciable part, caused by relativistic spin-orbit (SO) effects. [2] These heavyatom induced SO effects are mediated through the Fermi contact mechanism to which proton shifts are particularly susceptible, owing to the large hydrogen 1 s-orbital contributions to bonding (the transfer of SO-induced spin polarization to the NMR nucleus is decisive in this situation). [6] In contrast, d 10 metal hydride complexes of mercury or gold exhibit large high-frequency shifts up to d =+ 17 ppm, again predominantly because of SO coupling. [2] Some d 0 metal hydrides have been studied by 1 H NMR spectroscopy as well. Similarly to d 10 systems they often also exhibit shifts in the very highfrequency range (Scheme 1). [7] As shown in Scheme 1, SO effects again play a very important role in these NMR shift values, increasingly so moving down a group in the periodic table (cf. 1 H NMR shifts within the [H 2 MCp* 2 ] series, M = Ti, Zr, Hf, Cp* = h-C 5 Me 5 ). Probably the largest known shift value of such a d 0 complex is the d =+ 18.9 ppm of the tantalum complex in Scheme 1.We note in passing, that deshielding SO shifts are generally related to high-lying occupied orbitals with ssymmetry relative to the bond between the SO center and NMR atom (e.g. for the abovementioned d 10 and d 0 metal hydride complexes), whereas p-type occupied orbitals provide shielding SO contributions (e.g. in the d 6 and d 8 hydride complexes or for heavy halogen substituents). [8] In view of these observations, we wondered about the magnitude of hydride shifts when the d 0 transition-metal center is replaced by an actinide ion to form the corresponding f 0 species, as SO effects should be particularly large in this case. A literature survey provided only a few diamagnetic hydride complexes: the thorium systems 1-3 shown in Scheme 2 [9] and a few borohydride complexes of Th IV and UO 2 2+ . [10] The hydride shifts in 1-3 are in the high-frequency range, comparable to the abovementioned Ta complex (only some prot...

Paramagnetic NMR of Phenolic Oxime Copper Complexes: A Joint Experimental and Density Functional Study

“…Dimer ments, [26] but are small in this case as the molecules are rather far apart (Cu···Cu distance in the solid: 4.9 ). s orb valuesa ta ppropriate points in the space around 1 (at the "virtual" Ca nd Hp ositions of the nearest neighbour) are all less than d = 1ppm.…”

Insights into trans‐Ligand and Spin‐Orbit Effects on Electronic Structure and Ligand NMR Shifts in Transition‐Metal Complexes

“…In particular, the role of spin-orbit (SO) effectso nligand chemical shifts (a "heavya tom effect on the light-atom shielding", HALA) has been exposed in severals tudies. [1][2][3][4][5][6] Earlier studies suggested that SO shieldings on directly neighboring atoms induced by occupied MOs with s symmetry with respect to the bond between the heavy neighboring atom and that nucleus are generally negative, whereas those induced by p orbitals are positive. [7] Our recent analysiso ftrans ligand effects on 1 Hs hifts in as eries of square-planar Pt II complexes, [1] and ac omparison between 13 C and 14 Ns hifts in square-planar Au III and Pt II complexes by Vicha et al, [4] gave as omewhat more varied picture, whereby seemingly similarb onding arrangements mayg ive rise to either shieldingo rd eshielding SO effectsd epending on the trans ligand or the central metal.A dding the also frequently important paramagneticc ontributions (s p )t ot he shifts further com-plicatest he situation and makes general predictionsw ithout explicitq uantitative calculations more difficult.…”

The role of the exchange-correlation response kernel and scaling corrections in relativistic density functional nuclear magnetic shielding calculations with the zeroth-order regular approximation

“…In conjunction with density functional theory (DFT), ZORA is widely applied to calculations of NMR parameters. For a small selection of recent applications to a variety of problems in chemistry see, for instance [11][12][13][14][15][16]. A ZORA module for NMR shielding tensors has been part of the Amsterdam Density Functional (ADF) [17] package since 1999 [18,19], followed by an implementation of a module for calculations of indirect nuclear spin-spin coupling (J-coupling) tensors in the year 2000 [20,21].…”