Zirconium complexes porphyrins

Zirconium porphyrin dicarboxy lates have been shown to catalyze the stereo- and regioselective ethylalumination of alkynes. Although little is known about likely reaction intermediates, NMR evidence forethy Izirconium complexes was observed for the products of the reaction of Zr(TPP)(02C-t-Bu)2 with AIEt. attributed to Zr(TPP)(02C-r-Bu)Et and [Zr(TPP)(/<-02C-/-Bu)(/t-Et) AIMe2." ... 

Other homoleptic Lu(III) sandwich complexes 23, 25 (78), and 27 (34), have been prepared accordingly, as in Scheme 6. Zirconium sandwich complexes have been prepared by the method employed by Kim et al. (80) for the synthesis of zirconium porphyrin double deckers reaction of Zr(lV-Et2)4 with the pz ligands 24 (35%) and 26 (61%) produced the corresponding sandwich complexes (28 and 29) (Scheme 6). 

All of the zirconium pz and porphyrin sandwich complexes have up to two reversible ring oxidations and three reversible ring reductions. The zirconium porphyrazines are harder to oxidize by -400 mV and easier to reduce by at least 400 mV than the analogous porphyrins, making them better oxidants and worse reductants. The heteroleptic pz-porphyrin sandwich complex has oxidation and reduction potentials between those measured for the porphyrin and the pz sandwiches, as expected (Table VII). 

This type of complex is derived from the mononuclear superoxo species via a further one-electron reduction of the dioxygen moiety. Cobalt is the only metal to form these complexes by reaction with dioxygen in the absence of a ligating porphyrin ring. Molybdenum and zirconium form peroxo-bridged complexes on reaction with hydrogen peroxide. In most cases the mononuclear dioxygen adducts of cobalt will react further to form the binuclear species unless specific steps are taken to prevent this. 

The alkylzirconium(m) octaethylporphyrin complex, (OEP)ZrCH2SiMe3 1, was prepared from the dialkylzirconium(rv) complex by reduction with H2 (1 atm) in toluene at 20 °C (Scheme 1). This reaction therefore appears to be a rather rare example of the chemical reduction of Zr(rv) to Zr(m) by H2. The structure of 1 was elucidated by single crystal X-ray diffraction and has a Zr-C bond length of 2.216(8) A. Although this complex formally contains zirconium in oxidation state hi, careful consideration of the structural and spectroscopic data led the authors to conclude that this was an overly simplistic view. At 77 K, an EPR signal typical of a metal-centered radical was observed, while no signal was detected at 293 K. The UV/Vis spectrum of 1 contains bands typical of a porphyrin anion. The electronic structure of 1 is therefore better described as a combination of two resonance forms a Zr(m) metal-based radical, and a zwitterionic form with a positively charged Zr(iv) center and a porphyrin radical anion. 

The reaction of (OEP)ZrCl2 with 3equiv. of LiC=CPh produces the alkynyl (OEP)Zr(lv) porphyrin complex (OEP)Zr(7]1-C=CPh)3Li(THF) 242194 (Equation (18)). The molecular structure shows that three alkynyl ligands are coordinated to the Zr center in a piano-stool fashion and that the lithium cation is bound to the pocket formed by three alkynyl ligands. Treatment of complex 242 with anhydrous HC1 produces a C-C bond-coupled product H2C=C(Ph)=CPh and HC=CPh quantitatively. This example shows different reactivity of the zirconium porphyrin from that of analogous metallocene complexes. 

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