Rhenium(I) Terpyridine π-Bases: Reversible η2-Coordination of Ketones, Aldehydes and Olefins in the Terpyridine Plane

The complex trans-[Re(terpy)(Br)(PPh₃)₂][OTf] (2) (terpy = 2,2‘:6‘,2‘ ‘-terpyridine) is a convenient precursor to the electron-rich π-basic fragment {(terpy)(L)₂Re}⁺ (L = ^tBuNC or PMe₃). Reduction of 2 with activated magnesium in the presence of unsaturated organic molecules and an excess of either ^tBuNC or PMe₃ yields complexes of the type trans-[(terpy)(L)₂Re(η²-π)][OTf] (L = ^tBuNC or PMe₃; π = olefin, aldehyde, or ketone). The dihapto-coordinated organic moieties show a preference for binding in the plane of the terpy ligand. Reaction of trans-[Re(terpy)(^tBuNC)₂(η²-acetone)][OTf] (9) with MeOTf yields an observable η²-ketonium complex. The electronic environment of these complexes has been probed by cyclic voltammetry, and the details of ligand exchange for the η²-ketone complexes are presented. Geometric features determined from X-ray crystal structure analyses of trans-[(terpy)(^tBuNC)₂Re(η²-cyclopentene)][OTf] (4) and trans-[(terpy)(^tBuNC)₂Re(η²-acetophenone)][OTf] (11) are reported.