Thermal Decomposition of Mono- and Bimetallic Magnesium Amidoborane Complexes

Complexes of the type [(DIPPnacnac)MgNH(R)BH(3)] have been prepared (DIPPnacnac=CH-{(CMe)(2,6-iPr(2)C(6)H(3)N)}(2)). The following substituents R have been used: H, Me, iPr, DIPP (DIPP =2,6-diisopropylphenyl). Complexes [(DIPPnacnac)MgNH(2)BH(3)]center dot THF, [{(DIPPnacnac)MgNH(iPr)BH(3))(2)] and [(DIPPnacnac)MgNH(DIPP)BH(3)] were structurally characterised. The Mg amidoborane complexes decompose at a significantly higher temperature (90-110 degrees C) than the corresponding Ca amidoborane complexes (20-110 degrees C). The complexes with the smaller R substituents (H, Me) gave a mixture of decomposition products of which one could be structurally characterised as [{(DIPPnacnac)Mg}(2)(H(3)B-NMe-BH-NMe)center dot THF. [{(DIPPnacnac)MgNH(iPr)BH(3)}(2)] cleanly decomposed to [(DIPPnacnac)MgH], which was characterised as a dimeric THF adduct. The amidoborane complex with the larger DlPP-substituent decomposed into a borylamide complex [(DIPPnacnac)MgN(DIPP)BH(2)], which was structurally characterised as its Ti-IF adduct. Bimetallic Mg amidoborane complexes decompose at lower temperatures (60-90 degrees C) and show a different decomposition pathway. The dinuclear Mg amidoborane complexes presented here are based on D1PPnacnac units that are either directly coupled through N N bonding (abbreviated NN) or through a 2,6-pyridylene bridge (abbreviated PYR). Crystal structures of [PYR-(Mg(nBu)}(2)], [PYR-{MgNH(iPr)BH(3))(2)], [NN-{MgNH(iPr)BH(3)}(2)]center dot THF and the decomposition products [PYR-Mg(2)(iPrN- BH-iPrN-BH(3))] and [NN-Mg(2)(iPrN-BH-iPrN-BH(3))]center dot THF are presented. The following conclusions can be drawn from these studies: i) The first step in the decomposition of a metal amidoborane complex is [3-hydride elimination, which results in formation of a metal hydride complex and R(H)N=BH(2), ii) depending on the nature of the metal, the metal hydride is either stable and can be isolated or it reacts further, iii) amidoborane anions with small R substituents decompose into the dianionic species (RN-BH-RN-BH(3))(2-), whereas large substituents result in formation of the borylamide RN-BH(2)(-), and iv) enforced proximity of two Mg amidoborane units results in decomposition at a significantly lower temperature and cleanly follows the BNBN pathway.