Hafnocene complexes bonding

Two major factors are likely to be responsible for the relative stability of (5-rrani-i7 -diene)zirconocene and -hafnocene complexes. It seems that the characteristic bonding features of the bent metallocene unit thermodynamically favor coordination of the 5-trans over the i-cis conjugated-diene... 

Knobloch DJ, Semproni SP, Lobkovsky E, Chirik PJ. Studies into the mechanism of CO-induced N2 cleavage promoted by an cmsa-hafnocene complex and C-C bond formation from an observed intermediate. J Am Chem 5ot. 2012 134 3377-3386. 

The chemistry of group 4 silyl compounds is closely associated with the dehydrogenating coupling of hydrosilanes. This reaction was discovered by Harrod and coworkers who converted PhSiHa with a catalytic amount of Cp2TiMe2 to H(PhSiH) H [370]. Later Tilley and coworkers showed that if zirconocene and hafnocene complexes are used in the reaction a catalytic cycle is involved which consists of o-bond metathesis steps [371]. To enhance o-bond metathesis reactivity with Si-H and C-H bonds Cp2Hf(Me)Si(SiMe3)3 and related compounds were treated with B(C6F5)3 to obtain cationic Hf complexes [372]. 

Using the parent zirconocene-butadiene complex as a representative example, a typical bonding situation in these types of molecules is presented in Scheme 48. For 297, equilibration between the s-trans and the s-cis isomers occurs with a barrier of 23 kcal mol 1 at 283 K. The 72-olefin complex is believed to be a high-energy intermediate on the interconversion reaction surface. Significantly, structural data indicates that the s-cis complexes are best described as Zr(iv) compounds with a er2, ir ligand.158,175 The dynamic NMR measurements have also been extended to ansa-zirconocene and hafnocene butadiene complexes.176 Moreover, photoelectron spectroscopy has been used to determine the relative energetics of the two isomers for // -metallocenes.177... 

Alkene complexes of Ti, Zr and Hf have been intensively investigated with regard to the nature of bonding and the close relation to olefin oligomerization and polymerization. Alkene complexes of zirconocene and hafnocene are isolated as the trimethylphosphine adduct, Cp2M(T -alkene)(PMe3) (33) [92-94]. Cp 2Ti(CH2=CH2) (34) is a 16 electron ethylene complex with a rich reaction chemistry as summarized in Scheme 6.4 [95-99]. The reaction profile of 34 indicates that the metallacyclopropane canonical form makes an important contribution [100]. 

Mono-diene complexes of zirconocene and hafnocene have been prepared by two methods [129-131 ], viz the photochemical reaction of diphenylzirconocene in the presence of diene and the reaction of metallocene dichlorides with diene magnesium adduct. The structures and reactivity of s-cis-dicm complexes indicate that the metal-lacyclopentene (B) is the preferred canonical form. Complexes of the type Cp2Zr(j-/ra/25-1,3-diene), have been prepared they were the first examples of this mode of coordination (C). Insertion of unsaturated compounds into a diene coordinated to zirconocene results in regioselective C—C bond formation [132-136]. 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

Bernskoetter WH, Lohkovsky E, Chirik PJ. Nitrogen-carbon bond formation from N2 and CO2 promoted by a hafnocene dinitrogen complex yields a substituted hydrazine. Angew Chem Int Ed. 2007 46 2858-2861. 

Bernskoetter WH, Ohnos AV, PoolJA, Lobkovsky E, ChirikPJ. N-C bond formation promoted by a hafnocene dinitrogen complex comparison of zirconium and hafiiium congeners. J Am Chem Soc. 2006 128 10696-10697. 

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