Synthesis and Characterization of a New Thiophene-Bridged Diamidophosphine [NPN] Donor Set and Its Coordination Chemistry with Zirconium(IV): Unexpected Deprotonation-Lithiation Sequence with a Mesitylaminothiophene Precursor

The synthesis and characterization of the thiophene-bridged diamidophosphine proligand [NPN]H-S(2) (2) (where [NPN]H-S(2) = {[N-(2,4,6-Me3C6H2)(3-NH-SC4H2-2-)](2)PPh}) along with several [NPN](ZrX2)-Zr-S (X = NMe2, Cl, I) complexes are presented. The ligand precursor [NPN]H-S(2) was prepared from N-(2,4,6-Me3C6H2)(3-NH-4-Br-SC4H2) (1),(BuLi)-Bu-t, and PhPCl2 in Et2O in 50% yield; the stereochemistry of the ligand precursor was unexpected and likely resulted from a competitive bromine-lithium exchange and deprotonation sequence with the thiophene starting material I and (BuLi)-Bu-t. A mechanism to rationalize the observed stercochemistry of the product is proposed following deuteration experiments. [NPN]Zr-S(NMe2)(2) (8) can be prepared in 80% yield by the direct reaction of [NPN]H-S(2) and Zr(NMe2)(4) in toluene. Both [NPN](ZrCl2)-Zr-S (9) and [NPN](ZrI2)-Zr-S (10) are prepared in high yield by the reaction of 8 with excess Me3SiCl or Me3SiI, respectively.