Spin−Orbit-Induced Anomalous pH-Dependence in ¹H NMR Spectra of Co^III Amine Complexes:  A Diagnostic Tool for Structure Elucidation

Abstract

The pH-dependent (1)H NMR characteristics of a series of Co(III)-(polyamin)-aqua and Co(III)-(polyamin)-(polyalcohol) complexes, [Co(tach)(ino-kappa(3)-O(1,3,5))](3+) (1(3+)), [Co(tach)(ino-kappa(3)-Omicron(1,2,6))](3+) (2(3+)), [Co(tach)(taci-kappa-Nu(1)-kappa(2)-O(2,6))](3+) (3(3+)), [Co(ditame)(H(2)O)](3+) (4(3+)), and [Co(tren)(H(2)O)(2)](3+) (5(3+)), were studied in D(2)O by means of titration experiments (tach = all-cis-cyclohexane-1,3,5-triamine, ino = cis-inositol, taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, tren = tris(2-aminoethyl)amine, ditame = 2,2,6,6-tetrakis-(aminomethyl)-4-aza-heptane). A characteristic shift was observed for H(-C) hydrogen atoms in the alpha-position of a coordinated amino group upon deprotonation of a coordinated oxygen donor. For a cis-H-C-N-Co-O-H arrangement, deprotonation of the oxygen donor resulted in an additional shielding (shift to lower frequency) of the H(-C) proton, whereas for a trans-H-C-N-Co-O-H arrangement, deprotonation resulted in a deshielding (shift to higher frequency). The effect appears to be of rather general nature: it is observed for primary (1(3+)-5(3+)), secondary (4(3+)), and tertiary (5(3+)) amino groups, and for the deprotonation of an alcohol (1(3+)-3(3+)) or a water (4(3+), 5(3+)) ligand. Spin-orbit-corrected density functional calculations show that the high-frequency deprotonation shift for the trans-position is largely caused by a differential cobalt-centered spin-orbit effect on the hydrogen nuclear shielding. This effect is conformation dependent due to a Karplus-type behavior of the spin-orbit-induced Fermi-contact shift and thus only significant for an approximately antiperiplanar H-C-N-Co arrangement. The differential spin-orbit contribution to the deprotonation shift in the trans-position arises from the much larger spin-orbit shift for the protonated than for the deprotonated state. This is in turn due to a trans-effect of the deprotonated (hydroxo or alkoxo) ligand, which weakens the trans Co-N bond and thereby interrupts the Fermi-contact mechanism for transfer of the spin-orbit-induced spin polarization to the hydrogen nucleus in question. The unexpectedly large long-range spin-orbit effects found here for 3d metal complexes are traced back to small energy denominators in the perturbation theoretical expressions of the spin-orbit shifts.

Relativistic two-component calculations of electronic g-tensor for oxo-molybdenum(V) and oxo-tungsten(V) complexes: The important role of higher-order spin-orbit contributions

“…2). This suggests an underestimate of the SO effects by the SO-ECPs used, consistent with our recent experience with a similar comparison for SO effects on NMR chemical shifts [59]. This has to be kept in mind when using the computationally more expedient ECP/SO-ECP approach.…”

Simulation of 59Co NMR Chemical Shifts in Aqueous Solution

“…The reason for this seemingly small geometry change on deprotonation is that the contraction of the hydroxyl CO bond is compensated by elongation of the CoO bond to the water ligand in trans position (CP‐opt parameters are 1.868 and 2.051 Å, respectively). The computed δ ( 59 Co) resonance in gaseous 5 of 14 798 and 15 324 ppm at CP‐opt and CPMD levels, respectively, is significantly shielded with respect to that of free 2 (see values in Table 2), by up to about 2400 ppm for the CPMD‐derived average 61. It is interesting that in the simulations involving either 2 or 5 in water, this sizeable difference between the two forms essentially vanishes, and their simulated δ ( 59 Co) values are virtually indistinguishable.…”

“Through‐Space” Relativistic Effects on NMR Chemical Shifts of Pyridinium Halide Ionic Liquids

“…Accordingly, the authors concluded that ( i ) the involvement of valence n s orbitals of the LA in the bonding to the HA (α‐effect) enhances the SO shift akin to 1 J coupling, due to the nonzero density at the nucleus . In other words, the SO coupling induced shielding increases from sp 3 to sp 2 to sp hybridized carbon atoms as for iodoethane, iodoethylene, and iodoacetylene, respectively; ( ii ) the SO shift in γ‐position depends on the enclosed valence angle between the particular HA and LA and hence follows a Karplus relationship;, and that ( iii ) the energy gap between bonding and antibonding orbitals is inversely related to the SO shift ,…”