Characteristic Spin−Orbit Induced ¹H(CH₂) Chemical Shifts upon Deprotonation of Group 9 Polyamine Aqua and Alcohol Complexes

Abstract

The recently observed nonintuitive pH dependence of methylene (1)H chemical shifts in cobalt(III) polyamine complexes upon deprotonation of coordinated aqua or (poly)alcohol coligands (J. Am. Chem. Soc. 2004, 126, 6728) was attributed to differential spin-orbit effects on the (1)H shifts transmitted over three bonds from the cobalt low-spin d(6) center. These remarkably large spin-orbit effects due to the comparably light Co center have now been examined closely by comparative computations for homologous Rh and Ir complexes, as well as by NMR titrations for a Rh complex. While larger spin-orbit effects (proportional to Z(2)) would have been expected for the heavier metal centers, the characteristic (1)H deshieldings upon deprotonation of [Rh(tren)(OH(2))(2)](3+) [tren = tris(2-aminoethyl)-amine] turn out to be smaller than for the Co homologous Co complex. Systematic computational studies ranging from smaller models to the full complexes confirm these results and extend them to the Ir homologues. Closer analysis indicates that the spin-orbit shift contributions do not follow the expected Z(2) behavior but are modulated dramatically by increasing energy denominators in the perturbation expressions. This is related to the increasing ligand-field splitting from 3d to 4d to 5d system, leading to almost identical differential spin-orbit shifts for the Co and Rh complexes and to only moderately larger effects for the Ir complex (by a factor of about two). Moreover, the differential nonspin-orbit deprotonation shifts cancel the spin-orbit induced contributions largely in the Rh complex, leading to the experimentally observed inverted behavior. The full multidentate polyamine complexes studied experimentally exhibit different three- and four-bond Fermi-contact pathways for transmission of the spin-orbit (1)H shifts. The novel four-bond pathways have different conformational dependencies than the Karplus-like three-bond pathways established previously. Both types of contributions are of similar magnitude. The (1)H NMR deprotonation shift patterns of [Ir(tren)(OH(2))(2)](3+) have been predicted computationally.

Atomic Contributions from Spin‐Orbit Coupling to 29Si NMR Chemical Shifts in Metallasilatrane Complexes

“…Similar conclusions have been drawn recently by Kaupp, Straka, et al. in a study of non‐monotonous SO trends induced in proton chemical shifts in structurally related Co, Rh, and Ir complexes 17. The significantly smaller ligand‐field splitting for Co caused large SO induced chemical shifts for nearby protons, with qualitatively similar trends for the Co, Rh, Ir triad as found here for the M=Ni, Pd, Pt metallasilatranes.…”

“…This conclusion is strongly supported by recent findings by Straka, Kaupp, et al. who reported similar “3d metal” anomalies in SO NMR effects on hydrogen chemical shifts in Group 9 complexes 17. In reference 17, the absence of heavier ligand atoms allowed to correlate SO effects directly with the metal and the associated ligand‐field splitting, without mandating an equivalent of the partial‐ZORA analysis adopted here.…”

“…Indeed, a recent study by Straka, Kaupp, et al. found similar non‐monotonous SO trends on proton shifts in Group 9 metal complexes and correlated the trends in the SO effects with calculated excitation energies and with the magnitude of orbital energy gaps of metal‐centered orbitals 17…”

Effects of stereoelectronic interactions on the relativistic spin–orbit and paramagnetic components of the 13C NMR shielding tensors of dihaloethenes

“…[22][23][24] The spin-orbit (SO) term for the nuclear shielding tensor is recognized to be primarily responsible for the relativistic effects on the chemical shift of a light nucleus in HALA systems. 1,25,26 The SO/FC is usually reported as the most important mechanism that explains the HA effect on the 13 C NMR chemical shift, whereas the SO/SD transmission mechanism is considered to be small. 25 The induced spin polarization can be transmitted through a covalent bond from a HA to a neighboring LA nucleus, resembling the mechanism for the transmission of the FC term in indirect spin-spin coupling constants (FC/FC mechanism).…”

“…The SO/FC transmission mechanism must depend significantly on the overlap and the energy gaps between the bonding and the antibonding orbitals, in which some studies [25][26][27][28][29] have found an inverse relationship among the energy gaps and s SO . Taking these considerations into account, any stereoelectronic interaction that increases a relevant energy gap between the ground and the excited states has the potential to decrease the corresponding SO contribution of the SO/FC and SO/SD cross terms.…”