1,4-Diaza-1,3-diene (DAD) complexes of early transition elements. Syntheses, structures and molecular dynamics of mono- and bis(η 5-cyclopentadienyl)titanium-, zirconium- and hafnium(DAD) complexes. Crystal- and molecular structures of CpTi(DAD)CH 2Ph, [CpTi(DAD)] 2O, CpZr[(DAD)(N ∩O)] and Cp 2Hf(DAD)

Abstract

Treatment of CpTiCl 3 and CpZrCl 3(THF) 2 with one equivalent magnesium in the presence of 1,4-diaza-1,3-dienes (R 1NCR 2CR 2NR 1 (R 1,R 2-DAD; R 1=C 6H 4-2-Me, C 6H 4-4-Me, C 6H 4-4-OMe, R 2=H, Me, Ph) yields the monomeric titanium complexes CpTi(R 1,R 2-DAD)Cl ( 2, R 1=C 6H 4-4-OMe, R 2=H; 3, R 1=C 6H 4-2-Me, R 2=Me; 4, R 1=C 6H 4-4-OMe, R 2=Me; 5, R 1=C 6H 4-4-Me, R 2=Ph), and the chloro bridged dimeric zirconium complexes [CpZr(R 1,R 2-DAD)Cl] 2 ( 6, R 1=C 6H 4-4-OMe, R 2=H; 7, R 1=C 6H 4-4-OMe, R 2=Me; 8, R 1=C 6H 4-4-Me, R 2=Ph). Both the half-sandwich complexes of DAD ligands bearing alkyl ( 3, 4 and 7) and aryl ( 5, 8) substituents at the inner carbon atoms and the complexes without substituents at this DAD positions ( 2, 6) prefer the σ 2,π-coordination geometry with a supine conformation of the heterodiene. Alkylation of the new half-sandwich DAD complexes with one equivalent of PhCH 2MgCl or one equivalent of MeMgI affords the benzyl and methyl derivatives CpM(R 1,R 2-DAD)CH 2Ph ( 9, M=Ti, R 1=C 6H 4-4-OMe, R 2=Me; 10, M=Zr, R 1=C 6H 4-4-OMe, R 2=Me) and CpZr(R 1,R 2-DAD)Me ( 11, R 1=C 6H 4-4-Me, R 2=Ph). An X-ray study of the benzyl derivative 9 reveals that the alkylation does not change appreciably the DAD bonding parameters in comparison with the starting chloride complex 4. The monomeric half-sandwich zirconium complex CpZr(R 1,R 2-DAD)(N ∩O) ( 12, R 1=C 6H 4-4-OMe, R 2=H) which has been prepared by reaction of 6 with the chelating acetylacetoneiminate compound Na[(C 6H 4-4-Me)NC(Me)-CHC(Me)O] (Na[N ∩O]) as well as the oxygen bridged complex [CpTi(R 1,R 2-DAD)] 2O ( 13, R 1=C 6H 4-2-Me, R 2=Me) which has been formed by hydrolysis of 3 also keep the supine conformation of the heterodiene ligand with respect to the Cp group. Temperature dependent NMR spectra of a series of different titanocene DAD complexes Cp 2Ti(R 1,R 2-DAD) (R 1=Ph, C 6H 4-2-Me, C 6H 4-4-Me, C 6H 4-4-OMe, 1-C 10H 7, R 2=Me; 14– 18) have been used to estimate the energy barrier of the thermal induced inversion of the folded diazametallacyclopentene rings and to identify rotameric isomers derived from restricted rotation of the C 6H 4-2-Me and the 1-C 10H 7 group about the NC ipso bond of the DAD ligand. Accordingly, complexes 16 and 18 and also the half-sandwich complexes 3 and 13 adopt mixtures of meso and rac rotamers. Finally, the crystal structure of Cp 2Hf(R 1,R 2-DAD) ( 21, R 1=R 2=Ph) is reported.