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The combination of VCl3(THF)3 and N,N-bis(trimethylsilyl)aniline (1a) is an efficient catalyst for the [2+2+1] coupling reaction of alkynes and azobenzenes, giving multisubstituted pyrroles. A plausible reaction mechanism involves the generation of a mono-(imido)vanadium(III) species as an initiation step, where 1a served as an imido source with concomitant release of 2 equiv of ClSiMe3, followed by a reaction with azobenzene to form a catalytically active bis(imido)-vanadium(V) species via N═N bond cleavage.

We report the syntheses of 2-pyridylimido complexes of tantalum and niobium by NN bond cleavage of 2,2′-azopyridine. Reaction of MCl5 (M = Ta and Nb) with 2,2′-azopyridine in the presence of 0.5 equiv of 1-methyl-3,6-bis­(trimethylsilyl)-1,4-cyclohexadiene (abbreviated Si-Me-CHD) afforded a dark red solution (for Ta) and a dark blue solution (for Nb) with some insoluble precipitates. After removing the solids, another 0.5 equiv of Si-Me-CHD was added to each solution, giving [M­(Npy)­Cl3] n (1a: M = Ta; 1b: M = Nb) through reductive cleavage of the NN bond of 2,2′-azopyridine. The initial products of the above reactions were determined to be 2,2′-azopyridine-bridged dinuclear complexes, [(MCl4)2(μ-pyNNpy)] (2a: M = Ta; 2b: M = Nb), which were isolated by treating MCl5 with 2,2′-azopyridine and Si-Me-CHD in a 2:1:1 molar ratio. In 2a and 2b, the NN bond was reduced to a single bond via two-electron reduction. Further reduction of complexes 2a and 2b with 1 equiv of Si-Me-CHD afforded complexes 1a and 1b. An anionic doubly μ-imido-bridged ditantalum complex, [ n Bu4N]­[Ta2(μ-Npy)2Cl7] (3a), was generated upon addition of n Bu4NCl to complex 1a, while addition of n Bu4NCl to niobium complex 1b gave a polymeric terminal imido complex, [ n Bu4N] n/2[{Nb­(Npy)­Cl3}2(μ-Cl)] n/2 (3b). Complexations of 1a and 1b with 1 equiv of 2,2′-bipyridine resulted in the formation of mononuclear 2-pyridylimido complexes, M­(Npy)­Cl3(bipy) (4a: M = Ta; 4b: M = Nb), whose main structural feature is intramolecular hydrogen bonding between the ortho hydrogen atom of 2,2′-bipyridine and the nitrogen atom of the pyridyl group on the imido ligand. Isolated 2-pyridylimido complexes 4a and 4b reacted with [RhCl­(cod)]2 to produce the corresponding early–late heterobimetallic complexes, (bipy)­MCl3(μ-Npy)­RhCl­(cod) (5a: M = Ta; 5b: M = Nb).

The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of its active sites. Based on a combination of IR, EPR spectroscopies, labelling experiments, and DFT modelling, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involves a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(III) alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analogue, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure as the UC catalyst, further supporting the proposed active site structure.

The Union Carbide(UC) catalyst, one of the first ethylene polymerization (EP) catalysts based on supported organometallic compounds, is based on chromocene dispersed on silica; it is distinct from the corresponding Phillips catalysts, based on supported chromium oxide, because it is responsive to H2 enabling tuning the resulting polymer. Yet, despite 50 years of research, the structure of its active sites has also been controversially discussed and remains elusive. Based on a combination of IR, EPR spectroscopies, labeling experiments, and modeling at the DFT level, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These active sites, with distinct EPR signatures, are exclusively formed at high Cr loadings because their formation requires a combination of congruent factors, namely the presence of grafted as well as adsorbed chromocene together with residual surface OH groups, and involves a C-H bond activation step of the Cp ring. These Cr hydrides readily initiate polymerization, yielding Cr(III) alkyl species that insert ethylene monomers through a Cossee Arlman-type mechanism, as evidenced by labeling and spectroscopic studies. This information inspired the design of a well-defined analog, that was prepared and characterized using SOMC by grafting CpCr(CH(SiMe3)2)2 on partially dehydroxylated silica. The resulting surface Cr sites show similar EPR signatures and produce similar polyethylene as the UC catalyst, further supporting the presence of similar active sites and opening the way to generating new classes of catalysts.