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

Abstract

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( n Bu)(thf)] (1) by n butane elimination from Mg( n Bu)2(thf) n . A stronger donor such as DMAP replaces the THF from 1 to give [(NNL)Mg( n Bu)(dmap)] (2). Both are air-sensitive, and 1 is adventitiously oxidized into [(NNL)Mg(μ-O n Bu)]2 (3 2 ). The homoleptic [(NNL)2Mg] (8) is made from 1 and a second equiv of NNLH. 1’s terminal n 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 n Bu group from 1 to give [(NNL)Mg(μ:κ2-O,O’-OTf)(thf)]2 (6 2 ). 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 [(NNLde)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 i PrNH2BH3 to give [(NNL)Mg{NH( i Pr)BH3}] (13), which shows a slow and fractional conversion into the dinuclear mixed hydrido amidoborane [(NNL)2Mg2(μ-H){(μ-NH( i Pr)BH3}] (14) by partial β-hydride elimination. In comparison, [(NNL)Mg( i PrNHBH3)(dmap)] (15) arising from the DMAP-bound 2 and i PrNH2BH3 is stable toward such elimination.