Title: Reactions of Group 4 Diene Complexes with Isonitriles and Carbon Monoxide
Description: Organometallic chemistry is a powerful tool for the coupling of simple unsaturated molecules to form complex organic structures. Isonitriles (RNCs) are an attractive alternative to isoelectronic CO as C₁ sources in the pursuit of C,C coupling reactions. The electronic properties of the substituent on nitrogen in RNCs can bring about insertion chemistry that is different from the insertion reactions of CO. The insertion chemistry of RNCs with Group 4 (Ti, Zr, and Hf) butadiene complexes will be described. Cp*(Cl)Ti(2,3-dimethylbutadiene) (2.1) reacts with two equivalents of RNC to give η¹,η²-diimine complexes 2.2 (R = ^tBu) and 2.3 (R = 1-adamantyl). Cp*(Cl)Ti (N,N-di-^tBu-η¹,η²-diimine) (2.2), in the presence of pyridine, fragments to Cp*(Cl)Ti(NtBu)(NC₅H₅) (2.10) and an α-methylene cyclopent-3-enimine 2.11. The hafnium analog of 2.1, Cp*(Cl)Hf(2,3-dimethylbutadiene) (2.14), has been reported to give a cyclic amidine complex when treated with two equivalents of 2,6-dimethylphenyl isonitrile.

By X-ray crystallography, however, this work finds that 2.14 and 2,6-dimethylphenyl isonitrile give instead a 2,5-diazahafnacyclopentane that features a σ-interaction between the C¬–C bond of a cyclopropane ring and the Hf. Cp*₂Zr(2,3-dimethylbutadiene) (3.1) reacts with tert-alkyl isonitriles to give η²-iminoacyl complexes Cp*₂⏞(⏟(Zr[CH₂ C(CH₃)C(CH₃)CH₂C)(NR)] 3.2 (R = tBu) and 3.3 (R = 1-adamantyl). Treatment of 3.2 with excess isopropyl isonitrile gives the η¹,η²-bis(iminoacyl) complex Cp*₂⏞(⏟(Zr[C(N^i Pr)CH₂ C(CH₃)C(CH₃)CH₂C)(N)iPr)] (3.4) and free tert-butyl isonitrile. The reaction of 3.1 with 2 equivalents of isopropyl isonitrile also affords 3.4, through the intermediate Cp*₂⏞(⏟(Zr[CH₂ C(CH₃)C(CH₃)CH₂C)(N^iPr)] (3.5). Carbonylation of 3.2 affords the Zr formimidoyl cyclopentadienolate 3.6. Treatment of a Zr hydride cyclopentadienolate (3.7), obtained from the carbonylation of 3.1, with tert-butyl isonitrile also affords 3.6. Isotopic labeling shows that the insertion of tert-butyl isonitrile into 3.1 and 3.7 is reversible.