2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (^NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand ^NNLH reacts with [Zn{N(SiMe₃)₂}₂] and ZnEt₂ to give [(^NNL)ZnA] (A = N(SiMe₃)₂ (1), Et(2)). Both are inert to PhSiH₃ and H₂ but react with HBpin only through the internal Zn-N_anilide bond to give the borylated ligand ^NNLBpin (3). The reactions of 1 and 2 with Ph₃EOH (E = C, Si) afford a series of divergent compounds like [(^NNLH)Zn(OSiPh₃)₂] (4), [Zn₃(OSiPh₃)₄Et₂] (5), and [EtZn(OCPh₃)] (6). But in all cases, it is invariably the Zn-N_anilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(^NNL)Li] with ZnBr₂ gives [(^NNL)Zn(μ-Br)]₂ (7), which on successively treating with KOSiPh₃ and PhSiH₃ gives the desired [(^NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.