Conspectus
Low-valent transition metalatesanionic,
electronic-rich
organometallic complexescomprise a class of highly reactive
chemical reagents that find integral applications in organic synthesis,
small-molecule activation, transient species stabilization, and M–E
bond formation, among others. The inherent reactivity of such electron-rich
metal centers has necessitated the widespread use of strong backbonding
ligands, particularly carbonyls, to aid in the isolation and handling
of metalate reagents, albeit sometimes at the expense of partially
masking their full reactivity. However, recent synthetic explorations
into transition-metalate complexes devoid of archetypic back-bonding
ligands have led to the discovery of highly reactive metalates capable
of performing a variety of novel chemical transformations.
Building
on our group’s long-standing interest in reactive
organometallic species, a series of rational progressions in early-to-middle
transition-metal chemistry ultimately led to our isolation of a rhenium(I)
β-diketiminate cyclopentadienide metalate that displays exceptional
reactivity. We have found this Re(I) metalate to be capable of small-molecule
activation; notably, the complex reversibly binds dinitrogen in solution
and can be utilized to trap N2 for the synthesis of functionalized
diazenido species. By employing isolobal analogues to N2 (CO and RNC), we were able to thoroughly monitor the mechanism of
activation and conclude that the metalate’s sodium counterion
plays an integral role in promoting dinitrogen activation through
a novel side-on interaction. The Re(I) metalate is also used in forming
a variety of M–E bonds, including a series of uncommon rhenium-tetrylene
(Si, Ge, and Sn) complexes that display varying degrees of multiple
bonding. These metal tetrylenes act to highlight deviations in chemical
properties within the group 14 elements. Our metalate’s utility
also applies to metal–metal bond formation, as demonstrated
through the synthesis of a heterotetrametallic rhenium–zinc
dimer. In this reaction, the Re(I) metalate performs a dual role as
a reductant and metalloligand to stabilize a transient Zn2
2+ core fragment. Finally, the metalate displays unique
reactivity with uranium(III) to yield the first transition metal–actinide
inverse-sandwich bonds, in this case with three rhenium fragments
bound through their Cp moieties surrounding the uranium center. Notably,
throughout these endeavors we demonstrate that the metalate displays
reactivity at multiple locations, including directly at the rhenium
metal center, at a Cp carbon, through a Cp-sandwich mode, or through
reversibly bound dinitrogen.
Overall, the rhenium(I) metalate
described herein demonstrates
utility in diverse applications: small-molecule activation, the stabilization
of reduced and/or unstable species, and the formation of unconventional
M–E/M–M bonds or heterometallic complexes. Moving forward,
we suggest that the continued discovery of noncarbonyl, electron-rich
transition-metal anions featuring new or unconve...
