Functional Group Chemistry at the Group 4 Bent Metallocene Frameworks: Formation and Metal-Free Catalytic Hydrogenation of Bis(imino-Cp)zirconium Complexes

Treatment of 6-dimethylaminofulvene 1 with lithio-2,6-diisopropylanilide 2 gave the lithiated 6-(2,6-diisopropylanilido)fulvene 3. Its treatment with Me3SiCl afforded the N-silylated derivative (C5H4)=CH-N(Ar)SiMe3 (4), which was characterized by X-ray diffraction. Protonation of 3 was effected by treatment with acetylacetone to yield a mixture of syn- and anti-5 [(C5H4)=CH-N(Ar)H] contaminated with some 2,6-diisopropylaniline. The minor isomer, syn-5, was also characterized by an X-ray crystal structure analysis. Treatment of this mixture with Zr(NMe2)(4) led to in situ deprotonation and formation of complex [(C5H4)-CH=NAr](2)Zr(NMe2)(2) (6) in a mixture with the exchange product [(C5H4)-CH=NAr](2)Zr(NMe2)(NH-Ar) (7) as a minor component (both characterized by X-ray diffraction). The corresponding reaction with the reagent Zr(benzyl)(4) or Cl2Zr(NMe2)(2)(THF)(2) resulted in the clean formation of the products [(C5H4)-CH=NAr](2)Zr(benzyl)(2) (8) (also characterized by X-ray diffraction) and [(C5H4)-CH=NAr](2)ZrCl2 (9), respectively. Complex 9 was subjected to a metal-free catalytic hydrogenation reaction by treatment with a substoichiometric amount of B(C6F5)(3) under mild conditions (2 bar H-2, rt) to yield the product [(C5H4)-CH2NH-Ar](2)ZrCl2 (10), which was also characterized by an X-ray crystal structure analysis. Treatment of 9 with 1 or 2 molar equiv of B(C6F5)(3) gave frustrated Lewis pairs that split dihydrogen heterolytically at near to ambient conditions to give the salts {[(C5H4)-CH2NH2+Ar][C5H4)-CH2NHAr]}ZrCl2/[HB(C6F5)(3)(-)] (11) and [(C5H4)CH2NH2+Ar](2)ZrCl2/2 [HB(C6F5)(3)(-)] (12), respectively. The system 12 itself was shown to be an active catalyst for the hydrogenation of bulky imines and of a bulky silyl enol ether.