The Blue-Violet Color of Pentamethylbismuth: A Visible Spin-Orbit Effect

Relativity as a Synthesis Design Principle: A Comparative Study of [3 + 2] Cycloaddition of Technetium(VII) and Rhenium(VII) Trioxo Complexes with Olefins

The difference in [3 + 2] cycloaddition reactivity between fac-[MO₃(tacn)]⁺ (M = Re, ⁹⁹Tc; tacn = 1,4,7-triazacyclononane) complexes has been reexamined with a selection of unsaturated substrates including sodium 4-vinylbenzenesulfonate, norbornene, 2-butyne, and 2-methyl-3-butyn-2-ol (2MByOH). None of the substrates was found to react with the Re cation in water at room temperature, whereas the ⁹⁹Tc reagent cleanly yielded the [3 + 2] cycloadducts. Interestingly, a bis-adduct was obtained as the sole product for 2MByOH, reflecting the high reactivity of a ⁹⁹TcO-enediolato monoadduct. On the basis of scalar relativistic and nonrelativistic density functional theory calculations of the reaction pathways, the dramatic difference in reactivity between the two metals has now been substantially attributed to differences in relativistic effects, which are much larger for the 5d metal. Furthermore, scalar-relativistic ΔG values were found to decrease along the series propene > norbornene > 2-butyne > dimethylketene, indicating major variations in the thermodynamic driving force as a function of the unsaturated substrate. The suggestion is made that scalar-relativistic effects, consisting of greater destabilization of the valence electrons of the 5d elements compared with those of the 4d elements, be viewed as a new design principle for novel ^99mTc/Re radiopharmaceuticals, as well as more generally in heavy-element coordination chemistry.

Room temperature cycloaddition reactivity of fac-[⁹⁹TcO₃(tacn)]⁺ (tacn = 1,4,7-triazacyclononane) with a variety of unsaturated substrates and the lack of such reactivity for fac-[ReO₃(tacn)]⁺ appears largely attributable to much stronger relativistic effects for Re relative to Tc, based on relativistic density functional theory calculations.

Relativistic Effects on a Metal-Metal Bond: Osmium Corrole Dimers

A series of metal-metal bonded osmium corrole dimers, {Os[T pXPC]}₂, were synthesized in reasonably good yields (35-46%) via the interaction of the corresponding free-base meso-tris( p-X-phenyl)corroles (H₃[T pXPC], X = CF₃, H, CH₃, and OCH₃), Os₃(CO)₁₂, and potassium carbonate in 1,2,4-trichlorobenzene under an inert atmosphere at 180 °C over several hours. The complexes are only the second class of Os corroles reported to date (the first being Os^VIN corroles) and also the second class of metal-metal bonded metallocorrole dimers (the other being Ru corrole dimers). Comparison of the X-ray structures, redox potentials, and optical spectra of analogous Ru and Os corrole dimers, along with scalar-relativistic DFT calculations, has provided an experimentally calibrated account of relativistic effects in these complexes. Three of the Os corrole dimers (X = CF₃, H, and OCH₃) were analyzed with single-crystal X-ray diffraction analysis, revealing inversion-related corrole rings with eclipsed Os-N bonds and Os-Os distances of ∼2.24 Å that are ∼0.06 Å longer than the Ru-Ru distances in the analogous Ru corrole dimers. Interestingly, a comparison of scalar-relativistic and nonrelativistic DFT calculations indicates that this difference in metal-metal bond distance does not, in fact, reflect a differential relativistic effect. For a given corrole ligand, the Ru and Os corrole dimers exhibit nearly identical oxidation potentials but dramatically different reduction potentials, with the Os values ∼0.5 V lower relative to Ru, suggesting that whereas oxidation occurs in a ligand-centered manner, reduction is substantially metal-centered, which indeed was confirmed by scalar-relativistic calculations. The calculations further indicate that approximately a third of the ∼0.5 V difference in reduction potentials can be ascribed to relativity. The somewhat muted value of this relativistic effect appears to be related to the finding that reduction of an Os corrole dimer is not exclusively metal-based but that a significant amount of spin density is delocalized over to the corrole ligand; in contrast, reduction of an Ru corrole dimer occurs exclusively on the Ru-Ru linkage. For isoelectronic complexes, the Ru and Os corrole dimers exhibit substantially different UV-vis spectra. A key difference is a strong near-UV feature of the Os series, which in energy terms is blue-shifted by ∼0.55 V relative to the analogous feature of the Ru series. TDDFT calculations suggest that this difference may be related to higher-energy Os(5d)-based LUMOs in the Os case relative to analogous MOs for Ru.

The Blue-Violet Color of Pentamethylbismuth: A Visible Spin-Orbit Effect

Two-component relativistic time-dependent density functional theory calculations with spin-orbit coupling predict yellow and orange-red absorption for BiPh5 and BiMe5, respectively, providing an excellent explanation for their respective violet and blue-violet colors. According to the calculations, the visible absorption is clearly attributable to a single transition from a ligand-based HOMO to a low-energy LUMO with a significant contribution from a relativistically stabilized Bi 6s orbital. Surprisingly, scalar releativistic calculations completely fail to reproduce the observed visible absorption and place it at the violet/near-UV borderline instead.