Coordination and Reactivity Study of Group 4 and 10 Transition Metal Complexes ofN-Imidazol-2-ylidene-N′-p-tolylureate and Thioureate Ligands

Abstract

The synthesis and isolation of N-imidazol-2ylidene-N′-p-tolylureate and thioureate proligands is described. Titanium(IV), nickel(II), and palladium(II) complexes containing one or two equivalents of the ligand were prepared by transmetalation of the ureate or thioureate salts. The X-ray structure of titanium complexes corroborated our prediction based on spectroscopic data and showed the ureate binds in a bidentate fashion through the oxygen and nitrogen atoms of the isocyanate building block, leaving the imidazol-2-imine fragment uncoordinated. Carbon−nitrogen bond lengths indicate substantial electron delocalization from the imidazole ring to the ureate group. In contrast, the ligand adopts a different coordination mode and binds to nickel and palladium through the N imidazol-2-ylidene and N p-tolyl atoms. Surprisingly, despite the softer nature of sulfur, coordination of the thioureate was exclusively through the sulfur and nitrogen atoms of the isothiocyanate building block with all three metals studied. All complexes were tested for ethylene polymerization at ambient conditions using methylaluminoxane as cocatalyst. Titanium(IV) complexes were found to be the most active, with activities up to 60 kg PE mol −1 catalyst h −1 .

Coordination and Reactivity Study of Titanium and Zirconium Complexes of the First Imidazol-2-imine Ethenolate Ligand

“…As observed for compound 4a , the 1 H and 13 C NMR spectra of 4b show only one set of resonances for the enolate ligands, indicating the generation of a single coordination isomer. Although the structure of neither 4a nor 4b could be confirmed by X-ray diffraction studies, based on zirconium work using ureate and thioureate ligands and substituted salicylaldiminate ligands, − we would expect the chloride atoms to adopt a cis conformation, with both enolate ligands coordinated to the metal with the oxygen atoms trans to each other. However, in bis(salicylaldiminate) metal complexes with sterically demanding substituents on the imine nitrogen, a trans -N/ cis -O/ cis -Cl isomer is in fact preferred. ,, Considering that our ethenolate ligand bears most of the bulk on the nitrogen atom, we think that complexes 4a and 4b may also adopt a trans -N/ cis -O/ cis -Cl arrangement.…”

“…The lower activity observed in our complexes may therefore be a result of an unoptimized substitution pattern on the ethenolate ligand. Alternatively, it may also be a result of either the different ligand arrangement ( trans -N/ cis -O/ cis- Cl vs cis -N/ trans -O/ cis -Cl) or a less electrophilic metal center, a result of electron delocalization from the imidazole ring to the exocyclic nitrogen atom. ,− …”

“…While the ligand coordinates to Ti(IV) in the expected N ∧ N chelate, the use of Pd(II) led to the formation of a dimeric structure with monodentate coordination of the ligand to the metal through the terminal imine nitrogen. To enforce coordination of the ligand in a bidentate fashion to both early and late transition metals, we recently reported the related anionic ureate and thioureate ligands ( B , Figure ) …”

Reactivity study of low-coordinate phosphaalkene IMesPPh with Grubbs first-generation ruthenium benzylidene complexes

“…The electron-donating capability of the imidazole ring allows the formation of the zwitterionic structure C similar to related imidazol-2-imines. 8 As a result, these phosphaalkenes are expected to be good s-donors and poor p-acceptors, mimicking the electronics of NHCs used in Grubbs second-generation catalysts. Furthermore, they feature some unique combinations of sterics, with the phosphorus being low coordinate like NHCs, but with substituents protruding away from the metal centre as in tertiary phosphines.…”

The Use of Ureates as Activators for Samarium Diiodide

“…Although ureate complexes of Sm­(II) have not been reported, several transition metal and lanthanide metal cations do complex with ureates to form unique organometallic species. Ureates have been shown to coordinate with Ti­(IV), Fe­(III), Zr­(IV), Pd­(II), Hf­(IV), W­(VI), and U­(IV) . The Zr­(IV) ureates are known to catalyze hydroamination reactions, while the Pd­(II) ureates are efficient catalysts for the Heck reaction of aryl bromides …”