Diamidophosphines with six-membered chelates and their coordination chemistry with group 4 metals: development of a trimethylene-methane-tethered [PN2]-type “molecular claw”

Abstract

The coordination chemistry of the phosphine-tethered diamidophosphine ligands PhP(CH₂CH₂CH₂NHPh)₂ (^pr[NPN]H₂) and PhP(1,2-CH₂-C₆H₄-NHSiMe₃)₂ (^bn[NPN]H₂) featuring six-membered N–C₃–P chelates was explored with group 4 metals, which allowed for the consecutive development of a new trimethylene-methane-tethered [PN₂] scaffold. In the case of the propylene-linked system ^pr[NPN]H₂, access to the sparingly soluble dibenzyl derivative ^pr[NPN]ZrBn₂ (3-Zr) was gained, while thermally sensitive zirconium and hafnium diiodo complexes ^bn[NPN]MI₂ (5-M, M = Zr, Hf) were isolated in the case of the benzylene-linked derivative ^bn[NPN]H₂. Despite the related phosphine-tethered backbone architectures of both of these ligands, their group 4 complexes were found to exhibit either C₁-symmetric (^bn[NPN]MX₂) or averaged C_S-symmetric (^pr[NPN]MX₂) structures in solution. To restrain the overall flexibility of these systems and thereby control the properties of the resulting complexes without disrupting the six-membered chelates, the new trimethylene-methane-tethered N,N′-di-(tert-butyl)-substituted [PN₂]H₂ protioligand was designed. This tripodal ligand system was prepared on a gram scale and its C_S-symmetric dichloro complexes [PN₂]MCl₂ (6-M, M = Ti, Zr, Hf) were isolated subsequently. The benzene-soluble dibenzyl derivative [PN₂]ZrBn₂ (7-Zr) was synthesised as well and characterised by X-ray diffraction. These results are discussed not only in conjunction with the known [NPN]-coordinated group 4 complexes incorporating five-membered chelates, but also in the context of “molecular claws” that are related to the new [PN₂] tripod.