π Ligands in Alkaline Earth Complexes

π ligands such
as olefins and alkynes bind intramolecularly
to the metal atom in d0 complexes of the
large alkaline earths (Ae) calcium and strontium supported by fluoroalkoxo
ligands with dangling unsaturated CC or CC bonds,
and having the amide N­(SiMe2H)2– as the coligand. These O-bridged dinuclear complexes
are further stabilized by secondary C–F→Ae and β-Si–H···Ae
interactions. In a set of structurally related Ca-olefin complexes,
the strength of these interactions gradually increases as the coordination
of the olefin onto Ca2+ becomes weaker (from η2-coordinated to η1 to fully dissociated)
upon increasing steric congestion, thus ensuring that overwhelming
electronic depletion does not occur at calcium. NMR data imply that
the olefins are metal-bound in [D8]­toluene solutions.
The Ae···Cπ, C–F→Ae,
and β-Si–H···Ae noncovalent interactions
are also strong in the parent
Ae-alkyne complexes, the first examples of non-acetylide Ae-alkynes
compounds. Calcium-arene complexes could not be made, as the aromatic
tether did not bind to the metal atom. Instead, a trinuclear complex
with noninteracting C6H5 groups was obtained.
It exhibits exceptionally strong C–F→Ca and β-Si–H···Ca
interactions. NMR data indicate that the congeneric calcium-allene
complex can be made, but it spontaneously isomerizes toward the more
stable Ca-alkyne via an unusual 1,3-hydride shift intramolecular process.