Braband H, Benz M, Spingler B, Conradie J, Alberto R, Ghosh A. Relativity as a Synthesis Design Principle: A Comparative Study of [3 + 2] Cycloaddition of Technetium(VII) and Rhenium(VII) Trioxo Complexes with Olefins.
Inorg Chem 2021;
60:11090-11097. [PMID:
34255507 PMCID:
PMC8388117 DOI:
10.1021/acs.inorgchem.1c00995]
[Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022]
Abstract
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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.
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