The secondary and tertiary 1,3,5-triazapenta-1,3-dienes 2 and 3 are easily obtained by the reaction of N-imidoyl chlorides with amidines or from ethyl N-imidoylimidoates 5 and primary amines. In a new synthetic pathway the compounds 5 were prepared by the reaction of imidoates in their protonated or free form with N-imidoyl chlorides. A quaternary 1,3,5-triazapenta-1,3-diene 4a is obtained by the reaction of the imidoate 5b with pyrrolidine. X-ray diffraction studies of the tertiary 1,3,5-triazapenta-1,3-diene 3c and its protonated form 3c·HCl allow the comparison of different patterns of intermolecular hydrogen bonding in the solid state. Two equivalents of a secondary 1,3,5-triazapenta-1,3-diene 2 were treated at room temperature after deprotonation with potassium tert-butoxide with CuCl 2 , Ni(ClO 4 ) 2 or with Pd(ac) 2 (without deprotonation) to give the corresponding neutral 2:1 chelate complexes (2a) 2 ·Cu, (2a) 2 ·Ni and (2b) 2 ·Pd, respectively. A fluorescent boron complex 2b·BF 2 was obtained by

Three novel N-acylamidines 1a−c with varying steric bulk and substitution patterns were synthesized and thoroughly characterized by X-ray diffraction. Compounds 1a and 1b, which contain two additional binding sites located at two pyridine substituents, were treated with equimolar amounts of cyclopentadienylzirconium(IV) trichloride at room temperature. The X-ray data of the resulting coordination compounds 7a and 7b indicate the formation of five-membered metallacycles with one of the pyridine nitrogen atoms and the carbonyl oxygen atom acting as binding sites. For the complexation of ligand 1c, a different route was chosen: 1c was first deprotonated to yield the polymeric potassium compound 8 with a very complex substitution pattern based on O , N, and aromatic interactions with the potassium ions. Transmetalation of 8 with cyclopentadienylzirconium(IV) trichloride gave amidinate complex 9, which is dimeric in the solid state but exists in solution in equilibrium with monomeric species. After addition of methylaluminoxane (MAO), the three novel cyclopentadienylzirconium complexes 7a, 7b, and 9 gave active homogeneous single-site catalysts for the polymerization of ethylene. Of these three systems, 9/MAO turned out to be the most efficient one, showing activities 3−5 times higher than 7a/MAO and 7b/MAO, respectively, and producing polymers with a well-defined "monomodal" molecular weight distribution. An important feature of these materials is their broader distribution in molecular weight (PDI > 3), which is best seen in the products of reactions at 53°C, with a "monomodal" main fraction at higher molecular weight and only small fractions of low molecular weight. ■ INTRODUCTIONAfter more than 30 years, the development of new single-site catalysts for olefin polymerization based on early transition metals continues to be one the fields of highest interest for industry and academia. In recent years, research has been centered on robust and efficient precatalysts that can be synthesized in few steps from common (not exotic) materials with the purpose of making them commercially more attractive.Precatalysts based on bi-, tri-, and tetradentate ligands in which carbon, nitrogen, or oxygen atoms efficiently stabilize an early transition metal are worth mentioning (Scheme 1). The advantage of such ligands is the easy access and great control of the steric and electronic environment of the atoms that coordinate the metal, characteristics that are essential for having access to thermally more stable and efficient catalysts in olefin homo-and copolymerization reactions. Among others, reviews 1−4 and articles from the groups of Edelmann, 5 Sita, 6 Hagadorn, 7 Eisen, 8 Rausch, 9 Green, 10 Lappert, 11 Kretschmer and Kempe, 12 and Jin 13 describe the work in this challenging field and deserve special attention.A second strategy to achieve greater competitiveness of the systems based on early transition metals has focused on reducing or eliminating the dependence of these systems on the use of methylaluminoxane (MAO) as an alk...