Zirconium and Hafnium Complexes

FIGURE 8.37 Cyclophosphazene copper complex PgN5(NMe2)i2 CuCP. 

Another interesting complex can be made with hafnium. This complex can be employed to make various ring complexes as, for example, the (PC Bu) isomers in [24] 

Another isomer of the products in (8.325) can be made from the titanium complex. 

Numerous compounds containing Zr-P bonds have been synthesised, for example, (8.327). Some of these show promise for the synthesis of carbophosphorus compounds and other organic compounds [25]. 

When both N and P atoms are available for metal coordination, either, neither or both may be involved. This coordination generally involves V N or V P, both of which should have available lone pair electrons. 

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Bochmann M, Lancaster SJ (1995) Cationic group IV metal alkyl complexes and their role as olefin polymerization catalysts The formation of ethyl-bridged dinuclear and heterodinuclear zirconium and hafnium complex. J Org Chem 494 55-59... 

Fig. 27 Amino bis(phenolate)-supported zirconium and hafnium complexes and Al-fluorous complexes for isoselective lactide ROP...

Zirconium and hafnium complexes are highly fluxional. NMR studies indicate stereochemical nonrigidity, facile exchange of terminal and bridging ligands, and rapid intermolecular ligand exchange. 

These results explain previous observations of the greater stability of zirconium thiocyanate and selenocyanate complexes compared with their hafnium analogues, and the greater stability of zirconium and hafnium complexes in MeCN compared with DMF in terms of competition between the ligand and solvent molecules for co-ordination sites on the metal. Zirconium alkoxides have been prepared from ZrCl4 and aliphatic alcohols158 but with salicylaldehyde a Meerwein-Ponndorf... 

Ziegler Catalysts, G. Fink et al., Eds., Springer-Verlag, Berlin, 1995 (several chapters discuss various aspects of olefin polymerization catalysis by zirconium and hafnium complexes). 

Zirconium and hafnium complexes of an amine bis(phenolate) ligand with a THF side-arm donor have been shown to perform living polymerization of 1-hexene when activated with B(C6F5)3. The Zr catalysts (1) possess very high activities of 21 OOOgnunol cath. In neat olefin, the reaction is very exothermic and accompanied by various termination processes resulting in a higher polydispersity index than observed when the monomer is diluted in chlorobenzene. 

The zirconium and hafnium complexes of trifluoroacetyl-acetone are white crystalline solids, insoluble in water but soluble in benzene, cyclohexane, and carbon tetrachloride. The hafnium complex melts at 128 to 129° and the zirconium complex at 130 to 131°. The complexes have been subjected to gas-phase chromatography and may be sublimed at 115° at a pressure of 0.05 mm. The proton magnetic resonance spectra of the compounds dissolved in carbon tetrachloride show single peaks in the methyl and methylene regions. The peaks appear at 2.20 and 6.00 p.p.m. (5) relative to tetramethylsilane (internal reference) for the zirconium complex and at 2.20 and 5.97 p.p.m. for the hafnium complex. 

The effects of polymerization conditions on the molecular weight distribution of polyethylene synthesized with Cp2TiCl2/MAO catalysts have been studied in comparison with analogous zirconium and hafnium complexes.1206,1207... 

The molecular structures of Cp2TiCl(C6F5)1307 and Cp2TiMe21308 have been determined by X-ray diffraction. The dimethyl compound is isostructural with the analogous zirconium and hafnium complexes. The molecular structure of bis-Gp 2,2 -biphenyl titanium, determined by X-ray diffraction methods, has been reported.1309... 

Incorporation of sterically demanding aryl substituents allows isolation of bis(arene)zirconium and hafnium complexes. The bond enthalpies of (7]6-(l,3,5-tBu)3C6H3)2M (M = Ti, 1 Zr, 2 Hf, 3) have been measured by iodinolytic bath calorimetry and values of 49(1), 64(3), and 67(4) kcal mol-1 have been determined for the respective metal-arene bond enthalpies (Scheme l).4 Computational studies establish that the major metal-arene bonding interaction is a 5-backbond formed from the overlap of metal dxz-yz and orbitals with the appropriate linear combination of arene p-orbitals. The observed increase in metal-arene bond strength is consistent with increased backbonding down the... 

As expected, the strong-field 7r-acidic ligand, carbon monoxide, has also been used to isolate zero-valent zirconium and hafnium complexes. This area has been a subject of relatively long-standing interest and has been reviewed.7 Chemistry outside of this review will be the focus of this section. 

Stable early-late M-M metal-metal-bonded heterobimetallic complexes 1273 (M = Zr, Hf M = Fe, Ru) supported by tripodal amido ligands have been synthesized by the reaction of the zirconium and hafnium complexes MeSi(SiMe2NAr)3MCl 1272 with K[CpM (CO)2], as shown in Scheme 302.960,961 The molecular structure of the Zr-Fe complex (Ar =p-to y ) determined by X-ray diffraction establishes the presence of an unsupported metal-metal bond with a Zr-Fe bond distance of 2.605(2) A. The reaction of 1273 with isonitriles leads to insertion into the metal-metal bond and formation of metallaiminoacyl complexes 1274. 

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