Synthesis and M−Cγ Hemilability of Group 2 Bis(phosphinimino)methanides

The sterically demanding bis(phosphinimino)methanide [CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 ] -, formed by deprotonation of the bis(phosphinimino)methane [CH 2 (Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 ], has been employed in the synthesis of a variety of homo- and heteroleptic complexes of the group 2 metals Mg, Ca, Sr, and Ba. The crystallographically characterized magnesium complexes, the dimer [{CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 }MgCl] 2 and the monomer [{CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 }MgCl(THF)], were synthesized in a similar manner by deprotonation of the ligand precursor with MeMgCl but crystallized either without or in the presence of THF. The synthetically useful [{CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 }K], formed from [CH 2 (Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 ] and [KN(SiMe 3 ) 2 ] in toluene, was shown in the solid state to consist of dimeric species formed by K-mesityl multihapto interactions. Homoleptic [{CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 } 2 Ae] (Ae = Ca and Ba) and heteroleptic [{CH(Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 }AeN(SiMe 3 ) 2 (THF)] (Ae = Ca, Sr) complexes of the heavier alkaline earth metals Ca, Sr, and Ba were most readily provided by a “one-pot” procedure utilizing the appropriate AeI 2 reagent and variation of [CH 2 (Ph 2 PNC 6 H 2 -2,4,6-Me 3 ) 2 ] and [KN(SiMe 3 ) 2 ] stoichiometry in THF. The complexes were shown by 31 P{ 1 H} NMR to adopt variable and hemilabile (Δ G ⧧ = ca. 50 kJ mol - 1 ) coordination geometries involving, most likely, M−C γ coordination and re-coordination. The ionic nature of the bonding was also implicated in our lack of success in the application of these complexes in reactivity studies with a variety of bulky protic reagents. This behavior was investigated by DFT (B3LYP/LANL2DZ) methods to highlight the differences in NBO charge distribution between model ionic bis(phosphinimino)methanide and β -diketiminato complexes.