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We report the preparation, structure, and reactions of a stable metallapyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions.Monomeric Ta(dNC t BudCHC t BudCH)(OAr) 2 (THF) (5‚THF) is isolated upon thermolyzing the η 2 (N,C)-pyridine complex [η 2 (N,C)-2,4,6-NC 5 t Bu 3 H 2 ]Ta(OAr) 2 Me (2) in the presence of THF, while the metallapyridine dimer [Ta(µ-NC t BudCHC t BudCH)(OAr) 2 ] 2 (6) is isolated when this reaction is carried out in benzene. Complete NMR characterization of 5‚THF is described, along with its conversion into 6. The bis(pyridine) adduct Ta(dNC t BudCHCt BudCH)(OAr) 2 (py) 2 (5‚py) is also described. Ta(dNC t BudCHC t BudCH)(OAr) 2 (THF) (5‚THF) is shown to react with t BuNCO and i PrNCN i Pr to afford the σ (η 1 ) and π (η 3 ) insertion products respectively Ta[dNC t BudCHC t BudCHC(dN t Bu)O](OAr) 2 (7) and Ta[dNC t BudCHC t BudCH-(η 3 -C(dN i Pr) 2 )](OAr) 2 (8). The molecular structure of the metallapyridine Ta(dNC t BudCHCt BudCH)(OAr) 2 (THF) (5‚THF) was detemined by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon occupying equatorial positions. The TaNC 4 metallacycle is very nearly planar, and discrete single and double bonds are evident around the ring. This π localization clearly favors the imido form Ta(dNC t BudCHC t BudCH)(OAr) 2 (THF) rather than a carbene structure. The relevance of these compounds to hydrodenitrogenation catalysis is described.

The reaction of 4 equiv of LiNHAr (Ar = 2,6-06 3' 2) with a solution of ZrCl4(THF)2 in THF/pyridine yields Zr(=NAr)(NHAr)2(py)2 (1) in high yield. Zr(=NAr)(NHAr)2(py)2 reacts with 1 and 2 equiv of Me3SiCl in THF/pyridine to provide Zr(=NAr)(NHAr)Cl(py)2 (2) and Zr(=NAr)Cl2(py)3 (3), respectively. ZrCl4(THF)2 reacts with 4 equiv of LiNHAr, followed by 2 equiv of Me3SiCl (in THF), to afford Zr(=NAr)Cl2(THF)2 (4) in a high-yield, one-pot synthesis. Hf(=NAr)Cl2(THF)2 ( 5) is prepared similarly. Zr(=NAr)(NHAr)Cl(py)2 reacts with 1 equiv of K[N(SiMe3)2] in THF to provide Zr(=NAr)(NHAr) [N(SiMe3)2] (py)2 (6), which could not be converted into a bis(imido) species. Both Hf(=NAr)Cl2(THF)2 and Zr(=NAr)Cl2(THF)2 react with K2(C8H8) to provide [(?/8-C8H8)Zr(=NAr)]2 (7) and [(?;8-C8H8)Hf(=NAr)]2 (8) in high yield. Zr(=NAr)Cl2(THF)2 reacts with 1 equiv of Li(C3H4Me) in THF to provide [(r/5-C5H4Me)Zr(=NAr)Cl]2 (9) in moderate yield. An X-ray structural determination of this compound reveals a dimeric structure with bridging imido ligands. [(tj5-C5H4-