Reaction of the dichloro complexes [M(N(2) (TBS)N(py))Cl(2)] (M=Zr: 1, Hf: 2; TBS: tBuMe(2)Si; py: pyridine) with one molar equivalent of LiNHNPh(2) gave mixtures of the two diastereomeric chlorohydrazido(1-) complexes [M(N(2) (TBS)N(py))(NHNPh(2))Cl] (M=Zr: 3 a,b, Hf: 4 a,b) in which the diphenylhydrazido(1-) ligand adopts a bent kappa(1) coordination. This mixture of isomers could be cleanly converted into the deep green diphenylhydrazido(2-) complexes [Zr(N(2) (TBS)N(py))(NNPh(2))(py)] (5) and [Hf(N(2) (TBS)N(py))(NNPh(2))(py)] (6), respectively, by dehydrohalogenation with lithium hexamethyldisilazide (LiHMDS) in the presence of one molar equivalent of pyridine. Both complexes contain a linearly coordinated hydrazinediide for which a DFT-based frontier orbital analysis established bonding through one sigma and two pi orbitals. A high polarity of the M=N bond was found, in accordance with the description of hydrazinediide(2-) acting as a six-electron donor ligand. The pyridine ligand in [M(N(2) (TBS)N(py))(NNPh(2))(py)] (M=Zr: 5, Hf: 6) is substitutionally labile as established by line-shape analysis of the dynamic spectra (DeltaG(not equal)=19 kcal mol(-1)). A change in denticity of the hydrazido unit from kappa(1) to kappa(2) was studied by DFT methods. Both forms are calculated to be very close in energy and are only separated by shallow activation barriers, which supports the notion of a rapid kappa(1) to kappa(2) interconversion. This process is believed to happen early on in the N-N scission in the presence of coupling reagents. Frontier orbital and natural population analyses suggest that a primarily charge-controlled nucleophilic attack at N(alpha) is unlikely whereas interaction with an electrophile could play an important role. This hypothesis was tested by the reaction of 5 and 6 with one molar equivalent of B(C(6)F(5))(3) to give [Zr(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (7) and [Hf(N(2) (TBS)N(py))(NNPh(2)){B(C(6)F(5))(3)}] (8). In these products, B(C(6)F(5))(3) becomes attached to the N(alpha) atom of the side-on bound hydrazinediide and there is an additional interaction of an ortho-F atom of a C(6)F(5) ring with the metal centre.

Breaking the chain: Reaction of a zirconium hydrazinediide with organoazides gives 2-pentazene-1,4-diyl complexes, such as [Zr(N(2) (TBS)N(py)){N(Ad)N(3)NPh(2)}] (C gray, N blue, Si green, Zr turquoise), by formal [2+3] cycloaddition. Bonding within the N(5) chain is investigated using density functional calculations. These complexes thermally eject N(2) to give side-on bonded diazenides.

Dominos at zirconium: A cascade of N–N and C–H scissions and C–C and C–N coupling steps in the coordination sphere of zirconium directly converts alkynes and hydrazines into indoles. The reaction pathway differs fundamentally from that of the Fischer indole synthesis.

Single-site chromium catalysts for olefin polymerization with donor functionalized cyclopentadienyl (Cp) ligands have been modified in order to improve their incorporation ability for the comonomer 1-hexene into the polymer chain under maintenance of their very high catalytic activities. A trimethylsilyl substituent in combination with a fused thiophene ring at the Cp ligand has been identified as the best ligand so far, leading to a doubling in 1-hexene incorporation and polyethylene (PE) with up to 27% 1-hexene content (by weight) has been obtained. The complexes lead to PE with molecular weight in the range of 50 000 to 800 000 g mol −1 when used in homogeneous solution, however after supporting the complex on silica ultrahigh molecular weight polyethylene (UHMW-PE) is formed with 9.9% of 1-hexene incorporated into the chain. Although other known catalysts incorporate even more 1-hexene, the presented system is different as it combines considerable α-olefin incorporation with very high polymer molecular weights and very high catalytic activity. These improved single-site chromium catalysts maintain their advantageous properties on silica as solid support which makes them good candidates for their application in industrial processes for the synthesis of polyethylene materials with advanced properties.

Reaction of the zirconium dichloro complex [Zr(N 2 TBS N py )Cl 2 ] (1) with 1 molar equiv of ArNHLi (Ar = Mes, DIPP) yielded the zirconium imido complexes [Zr-(Ph )] (5) by addition of phenyl azide, whereas the reaction of 2 or 3 with mesityl azide gave the alternative product 7, in which the azide is coupled with the CH activated ancillary tripod ligand. Reaction of 1 molar equiv of trimethylsilyl azide or 1-adamantyl azide with the previously reported hydrazinediido complex [Zr(N 2 TBS N py )(NNPh 2 )(py)] (9) at ambient temperature resulted in the formation of the five-membered zirconaacacycles [Zr(N 2 TBS N py )(N TMS N 3 NPh 2 )] (10) and [Zr(N 2 TBS N py )(N Ad N 3 NPh 2 )] (11). Complex 11 was thermally converted into the diazenido complex 12 via loss of 1 molar equiv of molecular N 2 . The direct formation of the analogous side-on-bonded diazenido analogue 13 was observed upon reaction of 9 with 1 equiv of mesityl azide at ambient temperature. On the basis of 15 N labeling and DFT modeling (DFT(B3PW91/6-31 g(d))) a mechanism for the reaction pathway leading to 12 and 13 is proposed. ■ INTRODUCTIONTetraazadiene complexes have been studied for many d-block metals in different formal oxidation states. 1 The bonding within the N 4 fragment has been of special interest. It may be regarded as a neutral tetraazadiene donor (a), as part of a delocalized π system including the metal atom (b), or as a dianionic tetraazene-1,4-diido ligand with an isolated NN double bond (c).Evidence for all three cases has been reported in the literature on the basis of structural data as well as theoretical studies. 2 While the N 4 unit coordinated to late transition metals tends to be best represented by resonance structures a and b, the bonding situation in early-transition-metal complexes is adequately represented by resonance form c.Several preparative routes to tetraazadiene complexes have been described in the literature, including the coupling of diazonium cations at the metal center, 3 salt metathesis of dihalides with Li 2 N 4 R 2 , 4 andmost commonlythe cycloaddition of organic azides with imido complexes. 5 The latter may be either isolated starting materials or species generated in situ by thermal fragmentation of azides at low-valent metal centers and subsequent reaction with excess azide to give the cycloaddition product. Furthermore, ruthenium tetrazene intermediates have been proposed to play a key role in the catalyst deactivation in ruthenium-catalyzed "click"-azide− alkyne cycloadditions. 6 We also note the insertion of an azide into a nitrogen−silicon bond to form an N 4 species 7 as well as the formation of an iron hexazene complex by reductive dimerization of two molecules of azide reported by Holland and co-workers. 8 Bergman and co-workers first reported the synthesis of the zirconium tetraazadienido complex [Cp 2 Zr(N 4 t Bu 2 )] by reaction of [Cp 2 Zr(N t Bu)(thf)] with tert-butyl azide. 9 In the presence of phenyl azide the N-bonded phenyl and tertbutyl groups underwent exchange, a process which...