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 CS-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 CS-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.

Closely Related Benzylene-Linked Diamidophosphine Scaffolds and Their Zirconium and Hafnium Complexes: How Small Changes of the Ligand Result in Different Complex Stabilities and Reactivities

“…Each 1 H and 13 C NMR spectrum of these dimethylamido complexes exhibited only one set of signals for the two ligand side arms of the respective amidophosphine and two geminally coupled doublets for the methylene protons indicative of C s -symmetric species in solution. However, NMR spectra measured at −80 °C showed a desymmetrization to C 1 , indicative of an antiparallel alignment of both ligand side arms upon cooling . Single crystals of [ Ph A]-1-Zr (see Supporting Information for an ORTEP diagram) and [B]-1-Hf (see Figure ) were subjected to X-ray diffraction, and the expected trigonal bipyramidal coordination geometries around the central metals were found in the refined C 1 -symmetric molecular structures.…”

“…For the synthesis of [ Ph A ]­H 2 , a retro-synthetic strategy that relied on the presence of the N -phenyl substituent prior to the introduction of the phosphine had to be developed, as numerous attempts to access the target molecule via Pd- or Cu-catalyzed coupling , of the unsubstituted ligand backbone (PhP­(CH 2 C 6 H 4 - o -NH 2 ) 2 ) were unsuccessful. Employing N -phenyl-substituted 2-aminobenzyl halides or anthranilic chlorides was found to be problematic as well, mainly due to the ease of 9,10-dihydroacridine and acridone formation via intramolecular electrophilic alkylation of the N -phenyl substituent .…”

“…In contrast to [ Si A ]­MI 2 ( [ Si A]-2-M , M = Zr, Hf), these complexes were found to be thermally stable at room temperature and even at +70 °C. That the titanium derivatives could be prepared seems noteworthy as well, as this was not the case for [ Si A ]­H 2 (see Supporting Information for experimental procedures and the ORTEP plots of the molecular structures of [ Ph A ]­Ti­(NMe) 2 , [ Ph A ]­TiI 2 , [ B ]­Ti­(NMe 2 ) 2 , and [ B ]­TiI 2 ) . In the 31 P­{ 1 H} NMR spectra of complexes [ Ph A]-2-M and [B]-2-M (M = Zr, Hf), the expected singlet resonances were found in each case.…”

“…In a previous report, we have noted that the simple zirconium diiodo complex [ Si A ]­ZrI 2 shown in Chart is fairly unstable at room temperature, while the hafnium homologue [ Si A ]­HfI 2 was found to exhibit a significantly enhanced thermal stability . On the basis of this finding, we decided to systematically probe for differences between [NPN]-coordinated hafnium and zirconium derivatives and the dependencies of these differences on subtle ligand alterations.…”

Hydroboration of Aldehydes, Ketones, and Carbodiimides Promoted by Mono(imidazolin‐2‐iminato) Hafnium Complexes

“…The Hf–N bond length in complex 1 (1.924(6) Å) is shorter than amido, diamido,, ketimido, and amidophosphine, hafnium complexes. In addition, an almost linear Hf–N–C angle of 176.2(3)° is observed.…”