State-of-the-art relativistic four-component DFT-GIAO-based calculations of (1)H NMR chemical shifts of a series of 3d, 4d, and 5d transition-metal hydrides have revealed significant spin-orbit-induced heavy atom effects on the hydride shifts, in particular for several 4d and 5d complexes. The spin-orbit (SO) effects provide substantial, in some cases even the dominant, contributions to the well-known characteristic high-field hydride shifts of complexes with a partially filled d-shell, and thereby augment the Buckingham-Stephens model of off-center paramagnetic ring currents. In contrast, complexes with a 4d(10) and 5d(10) configuration exhibit large deshielding SO effects on their hydride (1)H NMR shifts. The differences between the two classes of complexes are attributed to the dominance of π-type d-orbitals for the true transition-metal systems compared to σ-type orbitals for the d(10) systems.

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...

Empirical correlations between characteristic (1)H NMR shifts in Pt(II) hydrides with trans ligand influence series, Pt-H distances, and (195)Pt shifts are analyzed at various levels of including relativistic effects into density-functional calculations. A close examination of the trans ligand effects on hydride NMR shifts is shown to be dominated by spin-orbit shielding σ(SO). A rather complete understanding of the trends has been obtained by detailed molecular orbital (MO)-by-MO and localized MO analyses of the paramagnetic and spin-orbit (SO) contributions to the chemical shifts, noting that it is the perpendicular shift-tensor components that determine the trend of the (1)H hydride shifts. In contrast to previous assumptions, the change of the Pt-H distance in given complexes does not allow correlations between hydride shifts and metal-hydrogen bond length to be understood. Instead, variations in the polarization of metal 5d orbitals by the trans ligand affects the SO (and partly paramagnetic) shift contributions, as well as the Pt-H distances and the covalency of the metal-hydrogen bond (quantified, e.g., by natural atomic charges and delocalization indices from quantum theory atoms-in-molecules), resulting in a reasonable correlation of these structural/electronic quantities with hydride σ(SO) shieldings. Our analysis also shows that specific σ(p)- and σ(SO)-active MOs are not equally important across the entire series. This explains some outliers in the correlation for limited ranges of trans-influence ligands. Additionally, SO effects from heavy-halide ligands may further complicate trends, indicating some limitations of the simple one-parameter correlations. Strikingly, σ-donating/π-accepting ligands with a very strong trans influence are shown to invert the sign of the usually shielding σ(SO) contribution to the (1)H shifts, by a substantial reduction of the metal 5d orbital involvement in Pt-H bonding, and by involvement of metal 6p-type orbitals in the magnetic couplings, in violation of the Buckingham-Stephens "off-center ring-current" picture.

A series of dipolar and octupolar triphenylamine-derived dyes containing a benzothiazole positioned in the matched or mismatched fashion have been designed and synthesized via palladium-catalyzed Sonogashira cross-coupling reactions. Linear and nonlinear optical properties of the designed molecules were tuned by an additional electron-withdrawing group (EWG) and by changing the relative positions of the donor and acceptor substituents on the heterocyclic ring. This allowed us to examine the effect of positional isomerism and extend the structure-property relationships useful in the engineering of novel heteroaromatic-based systems with enhanced two-photon absorption (TPA). The TPA cross-sections (δ(TPA)) in the target compounds dramatically increased with the branching of the triphenylamine core and with the strength of the auxiliary acceptor. In addition, a change from the commonly used polarity in push-pull benzothiazoles to a reverse one has been revealed as a particularly useful strategy (regioisomeric control) for enhancing TPA cross-sections and shifting the absorption and emission maxima to longer wavelengths. The maximum TPA cross-sections of the star-shaped three-branched triphenylamines are ∼500-2300 GM in the near-infrared region (740-810 nm); thereby the molecular weight normalized δ(TPA)/MW values of the best performing dyes within the series (2.0-2.4 GM·g(-1)·mol) are comparable to those of the most efficient TPA chromophores reported to date. The large TPA cross-sections combined with high emission quantum yields and large Stokes shifts make these compounds excellent candidates for various TPA applications, including two-photon fluorescence microscopy.