[Cr(pincer 2- )] 2 as an electron shuttle for reductively promoted hydrazine disproportionation.

We describe here delivery of hydrazine to a reducing, low oxidation state chromium bound to a proton responsive ligand which has already been deprotonated. Reaction of PhHNNH2 at a 4 : 1 mole ratio with the bis-(pyrazolate)pyridyl pincer ligated reducing agent [(CrL)-L-II](2) gives prompt conversion to [CrIII2L(2)(PhNH2)(2)(mu-PhHNN)], with release of NH3 and C6H6, a new disproportionation of the hydrazine, with trapping of PhHNN as its dianion, bridging the two chromium centers. The redox balance of the reaction is discussed, and participation by Bronsted basic sites on the bis-(pyrazolate)pyridyl pincer ligand L2- is suggested, but no hydrazine protons remain on the pincer in the product.