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

Abstract

The sterically demanding bis(phosphinimino)methanide [CH(Ph2PNC6H2-2,4,6-Me3)2]-, formed by deprotonation of the bis(phosphinimino)methane [CH2(Ph2PNC6H2-2,4,6-Me3)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(Ph2PNC6H2-2,4,6-Me3)2}MgCl]2 and the monomer [{CH(Ph2PNC6H2-2,4,6-Me3)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(Ph2PNC6H2-2,4,6-Me3)2}K], formed from [CH2(Ph2PNC6H2-2,4,6-Me3)2] and [KN(SiMe3)2] in toluene, was shown in the solid state to consist of dimeric species formed by K-mesityl multihapto interactions. Homoleptic [{CH(Ph2PNC6H2-2,4,6-Me3)2}2Ae] (Ae = Ca and Ba) and heteroleptic [{CH(Ph2PNC6H2-2,4,6-Me3)2}AeN(SiMe3)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 AeI2 reagent and variation of [CH2(Ph2PNC6H2-2,4,6-Me3)2] and [KN(SiMe3)2] stoichiometry in THF. The complexes were shown by 31P{1H} NMR to adopt variable and hemilabile (ΔG⧧ = ca. 50 kJ mol-1) coordination geometries involving, most likely, M−Cγ 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.