We synthesized two 4Me-PNP
ligands which block metal-ligand cooperation (MLC) with the Ru center and
compared their Ru complex chemistry to their two traditional analogues used in acceptorless
alcohol dehydrogenation catalysis. The corresponding 4Me-PNP complexes, which
do not undergo dearomatization upon addition of base, allowed us to obtain rare,
albeit unstable, 16 electron mono CO Ru(0) complexes. Reactivity with CO and H<sub>2</sub>
allows for stabilization and extensive characterization of bis CO Ru(0) 18
electron and Ru(II) cis and trans dihydride species that were also shown to be capable
of C(sp<sup>2</sup>)-H activation. Reactivity and catalysis are contrasted to
non-methylated Ru(II) species, showing that an MLC pathway is not necessary,
with dramatic differences in outcomes during catalysis between <sup>i</sup>Pr
and <sup>t</sup>Bu PNP complexes within each of the 4Me and non-methylated
backbone PNP series being observed. Unusual intermediates are characterized in one
of the new and one of the traditional complexes, and a common catalysis
deactivation pathway was identified.
Bulky PNP Ligands Blocking Metal-Ligand Cooperation Allow for Isolation of Ru(0), and Lead to Catalytically Active Ru Complexes in Acceptorless Alcohol Dehydrogenation
