Group 4 Titanium, Zirconium and Hafnium

The organometallic chemistry of titanium is dominated by complexes in the +IV oxidation state and in comparison there are relatively few examples of titanium complexes in the +III oxidation state. For information on organotitanium(iv) see Chapter 4.05. However, examples of titanium(lll) complexes are more common than examples of titanium complexes in lower oxidation states (for information on organotitanium in oxidation states 0 to II see Chapter 4.03) and titanium(m) chemistry is considerably more advanced than the chemistry of the heavier group 4 metals, zirconium and hafnium in the +m oxidation state. For information on organozirconium(m) and organohafnium(m) see Chapter 4.07. 

The effect of the metals used was then examined (Table 5.4). When the group 4 metals, titanium, zirconium, and hafnium, were screened it was found that a chiral hafnium catalyst gave high yields and enantioselectivity in the model reaction of aldimine lb with 7a, while lower yields and enantiomeric excesses were obtained using a chiral titanium catalyst [17]. 

We first studied group 4 metals (titanium, zirconium and hafnium) supported on a silica dehydroxylated especially at 700 °C (Table 3.8). Following the laboratory-developed strategy, surface-species have been well-characterized by classical techniques (IR, solid-state NMR gas evolvement, reactivity, etc.). Catalysis results show that titanium is the most active even if its activity is far less than that of homogeneous catalysts. In addition, an important amount of metal was lost by lixiviation even if this phenomenon seemed to stop after a certain time. 

Group 4 In ordet of increasing atomic number, ihese are titanium, zirconium, and hafnium. The elements of this group are characterized by the presence ol two electrons in an outer shell Although titanium and zirconium also have other valences, all of ihe dements in this group have u 4+ valence in eunimnn. 

In contrast to the monomeric nature of tertiary alkoxides of titanium, zirconium, and hafnium, the corresponding cerium and thorium lower tertiary alkoxides exhibit association, which decreases with increasing chain length of the groups attached to the tertiary carbon atom and finally Th(OCMeEtPr )4 and Ce(OCMeEtPr")4 show monomeric behaviour. On the basis of the above observations, the order of volatility of some quadrivalent metal alkoxides may be assigned Si(OR)4 > Ge(OR>4 > Ti(OR)4 > Hf(OR)4 > Zr(OR)4 > Ce(OR)4 > Th(OR)4. However, for monomeric tertiary alkoxides, the order of volatility is Hf(OR >4 > Zr(OR )4 > Ti(OR >4. 

IV. TRANSITION METAL COMPLEXES GROUP SURVEY A. Group 4 (IV B) Titanium, Zirconium, and Hafnium... 

Titanium, Zirconium and Hafnium. - Very little metal carbonyl related chemistry is ever reported for the metals in this group and the year 2000 was no exception. However, Ellis and co-workers (a team that have specialised in highly reduced carbonyls as Aubke has specialised in highly oxidised carbonyls) have produced the first hydroxo-carbonyl of titanium - [Ti2(p-OH)2(CO)8] . This interesting species was synthesised by the reaction of [Ti(CO)6] with azobenzene. 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

Klemn also reported that zirconium tetrabromide possessed a structure similar to the iodide but with tetragonal distortion. Berdonosov and co-workers (58), apparently unaware of Klemn s report, write that zirconium tetrabromide has a primitive cubic lattice with a= 10.95 0.01 A, and assigned the space group T, by analogy with zirconinm tetrachloride. Zirconium and hafnium tetrabromides seem to be structurally different than titanium tetrabromide. It is observed that titanium tetrabromide is soluble to the extent of 40% in chloroform and 30% in ethylene dichloride and carbon tetrachloride, whereas zirconium and hafnium tetrabromides are soluble to the extent of only 4% in chloroform and 0% in carbon tetrachloride (57). 

Monomeric species M OR-tert)x have been characterized for titanium, vanadium, chromium, zirconium, and hafnium (x = 4) and for niobium and tantalum (x == 5). With chromium it was found that limiting Cr(III) to coordination number 4 in the dimeric Cr2(OBu )e caused instability and a remarkable facility toward valency disproportionation or oxidation to the stable quadricovalent Cr(OBu )4 (8, 9). In contrast, molybdenum formed a stable dimeric tri-tert-butoxide (Bu O)3Mo=Mo-(OBu )3 which is diamagnetic and presumably bound by a metal-metal triple bond (10, II). Yet another interesting feature of chromium is the synthesis of a stable diamagnetic nitrosyl Cr(NO) (OBu )3 in which the nitric oxide is believed to act as a three-electron donor with formation of a four-coordinated low spin chromium (II) compound (12). The insta-bihty of Cr2(OBu )e and the stability of both Cr(NO) (OBu )3 and Cr(OBu )4 must result from the steric effects of the tertiary butoxo groups since the less bulky normal alkoxo groups form very stable polymeric [Cr(OR)3]a. compounds in which the Cr(III) has its usual coordination number of 6 (octahedral). 

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