Ruthenium Carbene Complexes Analogous to Grubbs‑I Catalysts Featuring Germylenes as Ancillary Ligands

Reactions of the first-generation
Grubbs’ catalyst trans-[RuCl2(CHPh)­(PCy3)2] (1) with the amidinatogermylenes
Ge­(tBu2bzam)­R (R = tBu (L1), CH2SiMe3 (L2); tBu2bzam = N,N′-bis­(tertbutyl)­benzamidinate)
have allowed the isolation and full characterization of the first
specimens of Grubbs-type carbene complexes featuring heavier tetrylenes
as ancillary ligands, namely, the disubstituted derivatives trans-[RuCl2(CHPh)­(L1)2] (3) and cis-[RuCl2(CHPh)­(L2)2] (7), which curiously differ in the arrangement of
their germylene ligands. DFT calculations have revealed that the different
volumes of L1 and L2 (the former is larger than the latter)
are responsible for the different stereochemistry of 3 and 7. NMR-monitoring of the reaction solutions has
allowed the observation of the monosubstituted intermediates trans-[RuCl2(CHPh)­(L)­(PCy3)] (L = L1 (2), L2 (5)) and their evolution to
either the disubstituted final product (for L1) trans-[RuCl2(CHPh)­(L1)2] (3)
or the short-lived disubstituted intermediate (for L2) trans-[RuCl2(CHPh)­(L2)2] (6).
Complex 7 arises from a trans-to-cis isomerization of intermediate 6. As olefin
metathesis catalysts, both 3 and 7 promoted
the ring-closing metathesis of diethyl 2,2-diallylmalonate and the
ring-opening metathesis polymerization of norbornene, but their catalytic
activity decreased with the reaction time, indicating catalyst decomposition.