Zirconium complexes with dicarbonyls

Cyclopentadienyl aryl complexes, with niobium, 5, 66 Cyclopentadienyl bis(phosphine)amide zirconium compounds, dicarbonyl complexes, 4, 702. -Cyclopentadienyl-carbanionic complexes, with Zr(IV),... 

Because of the low oxidation state of the metal [M(II)] in the group 4B metallocene dicarbonyl compounds, all of them, perhaps with the exception of (17—C5Me5)2Ti(CO)2 (27), are very air sensitive and decompose rapidly on exposure to air, forming a yellow solid for the titanium compounds and cream-colored solids for the zirconium and hafnium analogs. While the dicarbonyl 27 is indeed air sensitive, its decomposition appears qualitatively to be much slower relative to the other related complexes. 

In none of the cases discussed above is molecular hydrogen involved. The first report of the stoichiometric reduction of coordinated carbon monoxide by molecular hydrogen is that published by Bercaw et al. (35, 36). They reported that mononuclear carbonyl and hydride complexes of bis(pentamethylcyclopentadienyl)zirconium are capable of promoting stoichiometric H2 reduction of CO to methoxide under mild conditions. Thus, treatment of the dicarbonyl complex (rj5-C5Me5)2Zr(CO)2 with... 

Dicarbonyl coupling (8,483). This Ti-catalyzed coupling offers a useful route to cyclic sesquiterpenes such as humulene (4). The precursor is obtained by coupling a vinylic zirconium compound (1) with the u-allylpalladium complex (2) to give, after deprotection, the keto aldehyde 3 in 84% yield. This product couples to humulene as a single isomer in 60% yield. 

Addition of strong-field ligands such as carbon monoxide or alkynes to zirconium alkene complexes can also result in olefin displacement. Treatment of the monocyclopentadienyl complexes, [775-C5H3-(l,3-(SiMe2CH2PR2)2)]Zr( 72-G2H4)Br (R = Pr1, 87 R = Me, 88), with CO or alkynes results in ethylene loss and the formation of the corresponding dicarbonyl (R = Pf, 166 R = Me, 167) and alkyne (R = Pr 168 R = Me, 169) complexes (Scheme 26). For the alkyne addition, no metallacycle is observed, presumably due to the sterics of the ligand array. 

Reactivity studies reveal that the bis(indenyl)zirconium sandwich complexes serve as isolable, modular precursors to the rich chemistry of divalent zirconocene. Addition of 1 atm of carbon monoxide generates the bis(indenyl)-zirconocene dicarbonyl complex, 307, while treatment with N,N-dimethylaminopyridine results in C-H activation to form the zirconocene pyridyl hydride complex 308. Crystallographic characterization of both complexes as well as multinuclear NMR spectroscopy establishes that the more familiar 77s, 77s hapticity for the indenyl ligands has been restored. Alkyne coupling reactions are also observed as addition of 2-butyne to 302 allows observation of the alkyne complex, 208, and ultimately the zirconacyclopentadiene 309 (Scheme 50).104... 

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