MONOMERIC AND DIMERIC ZIRCONOCENE IMIDO COMPOUNDS - SYNTHESIS, STRUCTURE, AND REACTIVITY

This paper outlines-the generation of the transient monomeric imido complexes Cp2Zr=NR (1a-e), their successful trapping with unsaturated organic molecules and dative ligands, and their use in activating C-H bonds. The generation of Cp2Zr--NR has been accomplished by thermolysis of the alkyl amides Cp2Zr(NHR)(R') (2a-d) and bis(amides) Cp2Zr(NHR)2 (3a-c) via alpha-abstraction of alkane and amine, respectively. The imido intermediate 1a, generated from alkyl amide 2a (R = 4-tert-butylphenyl, R' = Me) in THF, resulted in the formation of the bridging imido dimer 4a in 70% yield. In contrast, the generation of intermediates 1b-d (1b R = tert-Bu; 1c R = 2,6-dimethylphenyl; 1d R = Si(CMe3)Me2) under similar conditions furnished the monomeric imido THF complexes 5b-d in 71, 69, and 72 % yields. Imido dimer 4a and monomer 5b were characterized crystallographically. The THF ligands of 5b-d are weakly bound, as demonstrated by the exchange of bound and free THF in 5c on the NMR time scale and by facile substitution of Ph3PO for THF in 5b, giving the triphenylphosphine oxide adduct Cp2Zr(NCMe3) (OPPh3) (6b). Thermolysis of the alkyl amides 2b and 2d at 85-95-degrees-C in benzene resulted in the addition of the C-H bond of benzene across the Zr-N linkage to provide the phenyl amides 7b and 7d. Similarly, the THF adduct 5b reacted with benzene under the same conditions to give the C-H activation product 7b. The imido intermediates Cp2Zr=NR undergo cycloadditon reactions with a variety of unsaturated organic compounds. In addition to formation of azametallacyclobutenes (e.g., 8b, 8c, 9c, 10b) from alkynes, certain olefins react with Cp2Zr=NR to give azametallacyclobutanes. Thus, upon reaction of norbornene or ethylene with imido complex 5b an equilibrium mixture of 5b, free olefin, free THF, and the metallacycles 12b and 13b, respectively, were observed. Addition of a large excess of norbornene shifted the equilibrium almost entirely to the azametallacyclobutane 12b and facilitated its isolation. The structure of 12b was determined after crystallization from hexamethyldisiloxane. Complex 5b also reacted with benzophenone and tert-butyl isocyanate to provide the tert-butylimine of benzophenone and di-tert-butylcarbodiimide in 89 and 65% yields, respectively. Monomer 5b reacted with tert-butyl isocyanide to provide the coupling/trapping product Cp2Zr(N(CMe3)C=NCMe3) (CNCMe3) (16), while reaction of 5b with isobutene oxide gave the epoxide-opened product Cp2Zr(NHCMe3)(OCH2C(Me)=CH2) (17). Finally, heating the alkyl amide Cp2Zr(NHCHPh2)(CH2CH2CMe3) (2e) (which contains hydrogens both alpha and beta to nitrogen) with diphenylacetylene yielded the azametallacyclobutene product formed via alpha-abstraction in 64 % yield.