Oxidorhenium(V) Complexes with Tetradentate Iminophenolate Ligands: Influence of Ligand Flexibility on the Coordination Motif and Oxygen-Atom-Transfer Activity

The synthesis of
oxidorhenium­(V) complexes 1–3 coordinated
by tetradentate iminophenolate ligands H2L1–H2L3 bearing backbones of different rigidity (alkyl, cycloalkyl,
and phenyl bridges) allows for the formation of distinct geometric
isomers, including a symmetric trans-oxidochlorido
coordination motif in complex 3. The complex employing
a cycloalkyl-bridged ligand (2) of intermediate rigidity
exhibits an interesting solvent- and temperature-dependent equilibrium
between a symmetric (trans) isomer and an asymmetric (cis) isomer
in solution. The occurrence of a symmetric isomer for 2 and 3 is confirmed by single-crystal X-ray diffraction
analysis. Chlorido abstraction from 2 with AgOTf yields
the corresponding cationic complex 2a, which does not
exhibit an isomeric equilibrium in solution but adopts the isomeric
form predominant for 2 in a given solvent. All complexes
were, furthermore, employed in three benchmark oxygen-atom-transfer
(OAT) reactions, namely, the reduction of perchlorate, the epoxidation
of cyclooctene, and OAT from dimethyl sulfoxide (DMSO) to triphenylphosphane
(PPh3), to assess the influence of the isomeric structure
on the reactivity in these reactions. In perchlorate reduction, a
clear structural influence was observed, where the trans arrangement
in 3 led to the complete absence of activity. In the
epoxidation reaction, all complexes led to comparable epoxide yields,
albeit higher catalytic activity but lower overall stability of the
catalysts with a trans arrangement was observed. In OAT from DMSO
to PPh3, also a clear structural dependence was observed,
where the trans complex 3 led to full phosphane conversion
with an excess of oxidant, while the cis compound 1 was
completely inactive.