1,4-Diaza-1,3-diene (DAD) complexes of early transition elements. Syntheses, structures and molecular dynamics of mono- and bis(η5-cyclopentadienyl)titanium-, zirconium- and hafnium(DAD) complexes. Crystal- and molecular structures of CpTi(DAD)CH2Ph, [CpT

Treatment of CpTiCl3 and CpZrCl3(THF)2 with one equivalent magnesium in the presence of 1,4-diaza-1,3-dienes (R1NCR2CR2NR1 (R1,R2-DAD; R1=C6H4-2-Me, C6H4-4-Me, C6H4-4-OMe, R2=H, Me, Ph) yields the monomeric titanium complexes CpTi(R1,R2-DAD)Cl (2, R1=C6H4-4-OMe, R2=H; 3, R1=C6H4-2-Me, R2=Me; 4, R1=C6H4-4-OMe, R2=Me; 5, R1=C6H4-4-Me, R2=Ph), and the chloro bridged dimeric zirconium complexes [CpZr(R1,R2-DAD)Cl]2 (6, R1=C6H4-4-OMe, R2=H; 7, R1=C6H4-4-OMe, R2=Me; 8, R1=C6H4-4-Me, R2=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 σ2,π-coordination geometry with a supine conformation of the heterodiene. Alkylation of the new half-sandwich DAD complexes with one equivalent of PhCH2MgCl or one equivalent of MeMgI affords the benzyl and methyl derivatives CpM(R1,R2-DAD)CH2Ph (9, M=Ti, R1=C6H4-4-OMe, R2=Me; 10, M=Zr, R1=C6H4-4-OMe, R2=Me) and CpZr(R1,R2-DAD)Me (11, R1=C6H4-4-Me, R2=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(R1,R2-DAD)(N∩O) (12, R1=C6H4-4-OMe, R2=H) which has been prepared by reaction of 6 with the chelating acetylacetoneiminate compound Na[(C6H4-4-Me)NC(Me)-CHC(Me)O] (Na[N∩O]) as well as the oxygen bridged complex [CpTi(R1,R2-DAD)]2O (13, R1=C6H4-2-Me, R2=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 Cp2Ti(R1,R2-DAD) (R1=Ph, C6H4-2-Me, C6H4-4-Me, C6H4-4-OMe, 1-C10H7, R2=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 C6H4-2-Me and the 1-C10H7 group about the NCipso 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 Cp2Hf(R1,R2-DAD) (21, R1=R2=Ph) is reported.