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Dinuclear dinitrogen and mononuclear paramagnetic complexes of zirconium Mylvaganam, Murugesapillai


The basic focus of this work is the use of the tridentate, mixed donor ligand, [N(SiMe₂CH₂PR₂)₂]⁻ abbreviated as PNP, to generate zirconium compounds that contain dinitrogen ligands in unusual bonding modes, or to allow the stabilization of the very rare zirconium(III) oxidation state. The approach that is used is to combine synthetic methods in organometallic chemistry with semi-empirical molecular orbital calculations as a means of designing new complexes and to rationalize bonding. Reduction of ZrCpCl₂[N(SiMe₂CH₂PPri₂)₂] 2.6, or ZrCl₂(OAr*)[N(SiMe₂CH₂ PPri₂)₂] (Ar* = (C₆H₃Me₂-2,6)) 2.11, under a dinitrogen atmosphere gave complexes { [(PPri₂CH₂SiMe₂)₂N] ZrCp } ₂(μ-N₂) 2.9, and ([(PPri₂CH₂SiMe₂)₂N] Zr(OAr*) }₂(μ-N₂) 2.12, respectively. The X-ray structure determination of these complexes confirmed the presence of an end-on bridging (μ-η¹:η¹-N₂) clinitrogen ligand in 2.9 whereas, 2.12 has a side-on bridging (μ- η²:η²-N₂) dinitrogen ligand. The observed nitrogen-nitrogen bond distances in 2.9 was 1.301 (3) Å and in 2.12, it was 1.528 (7) Å. The resonance Raman spectra of the solid and the solution state samples of 2.9 and 2.2 showed isotope sensitive peaks around 1200 and 730 cm⁻¹ respectively, assigned to the nitrogen-nitrogen stretching of these complexes. Also, the fact that these resonance Raman features are the same in the solid and in the solution states strongly suggest that the mode of bonding of the dinitrogen ligand are same for the respective complexes. The semi-empirical molecular orbital studies performed on the end-on derivative 2.9 and on the side-on derivatives 2.2 and 2.12 show that the π-acceptor interactions involving the π*orbitals of the dinitrogen ligand are significantly different. In the end-on case these interactions give rise to two π-MOs, whereas the side-on cases give rise to one δ -MO and one π-MO where the π-MO was found to be much lower in energy than the δ-MO. Mulliken population and Wiberg indices were calculated to show that in the side-on mode there is greater electron donation into the dinitrogen ligand than in the end-on cases and also the side-on bound N₂ ligand has a very weak nitrogen-nitrogen bond, which is corroborated by the bond length parameters and the resonance Raman data. An analysis of the frontier orbitals of the fragment [(H₃P)₂(H₂N)ZrX], where X = Cl, Cp or OH, shows that the metal-ligand dπ-pπ interactions influence the mode of dinitrogen coordination, i.e., end-on vs. side-on. The paramagnetic zirconium(ffl) complex, Zr(η⁵-C₅H₅)Cl[N(SiMe₂CH₂PPri₂)₂] 4.1, and the corresponding hafnium(III) derivative, Hf(η⁵-C₅H₅)Cl[N(SiMe₂CH₂PPri₂)₂] 4.1, were synthesized by the reduction of the respective dichioro precursors M(η⁵-C₅H₅)Cl₂- [N(SiMe₂-CH₂PPri₂)₂], where M = Zr or Hf. The results of this study show that complex 4.1 is a viable precursor to the synthesis of a variety of derivatives such as the first stable examples of alkyl, aryl and borohydride complexes of zirconium(III). X-ray structure elucidation has been carried out for an alkyl 4.8, phenyl 4.4 and borohydride complex, 4.17. Spectroscopic studies (IR and ESR) indicate that in the case of the alkyl derivatives a weak agostic type interaction may be present between one of the α-hydrogens and the metal. Hydrogenolysis of certain alkyl complexes shows a clean conversion to the mononuclear zirconium(Ill) hydride complex 4.14. This hydride complex has been shown to undergo an insertion reaction with ethylene. The hydride complex also undergoes hydrogen exchange reactions with H-C(sp²) and H-C(sp³) bonds, presumably by way of σ-bond metathesis. Complexes 4.1 and the zirconium(III) borohydride complex undergo reversible disproportionation reactions under a CO atmosphere to give zirconium(IV) and zirconium(ll) derivatives. It was also shown that CH₃CN reacts in a similar fashion with 4.1, however the reversibility of this reaction was not established. In the case of the borohydride complex the reaction with CO proceeds further to give a complex containing a “formyl-ylid” type ligand.

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