Heteroleptic Magnesium n-Butyl on a Chemically Non-innocent 2-Anilidomethylpyridine Ligand Leading to Diverse...

Molecular magnesium hydrides and hydride-rich clusters are of significant interest for applications ranging from catalysis and small molecule activation to hydrogen storage. Here, we investigate the 2-anilidomethylpyridine framework ^NNL as an ancillary support for magnesium organometallics with a special emphasis on hydrides. The proligand ^NNLH (N-[2,6-bis(1-methylethyl)phenyl]-α,6-diphenyl-2-pyridinemethanamine) gives [(^NNL)Mg(Bu)(thf)] (1) by butane elimination from Mg(Bu)₂(thf). A stronger donor such as DMAP replaces the THF from 1 to give [(^NNL)Mg(Bu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(^NNL)Mg(μ-OBu)]₂ (3). The homoleptic [(^NNL)₂Mg] (8) is made from 1 and a second equiv of ^NNLH. 1's terminal Bu group is selectively protonated by HN(SiMe₃)₂ to give [(^NNL)MgHMDS] (4; HMDS = N(SiMe₃)₂), whereas Ph₃SiOH partially protonates the backbone anilide as well to give a mixture of [(^NNL)Mg(OSiPh₃)(thf)] (5) and free ^NNLH. Like HN(SiMe₃)₂, aprotic MeOTf also reacts by selectively abstracting the Bu group from 1 to give [(^NNL)Mg(μ:κ²-O,O'-OTf)(thf)]₂ (6). Interestingly, screening the common synthetic routes for magnesium hydrides leads to diverse outcomes upon varying the Mg precursors and hydride sources. 1 and PhSiH₃ give the hydride cluster [{(^NNL)₂Mg₂(μ-H)}₂(μ-H)₄Mg] (7), whereas 2 and PhSiH₃ give the molecular complex [(^NNL^de)Mg(dmap)₂] (9) with a dearomatized pyridyl backbone. 1 and HBpin (pinacolborane) give a product mixture, from which a different hydride cluster [(^NNL)₂Mg₂(μ-H)}₂(μ:κ²-O,O'-O₂C₂Me₄)] (10) is identified, showing a rare instance of complete deborylation of a HBpin molecule. 1 and HBcat (catecholborane) also give a product mixture, one of which is the borylated ligand [(^NNL)Bcat] (11). HBpin with 4 as the Mg precursor takes the ligand borylation route more selectively to give [(^NNL)Bpin] (12). Last, 1 reacts with PrNH₂BH₃ to give [(^NNL)Mg{NH(Pr)BH₃}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(^NNL)₂Mg₂(μ-H){(μ-NH(Pr)BH₃}] (14) by partial β-hydride elimination. In comparison, [(^NNL)Mg(PrNHBH₃)(dmap)] (15) arising from the DMAP-bound 2 and PrNH₂BH₃ is stable toward such elimination.