Zirconium complexes oxidation states

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. 

Complex 2 proved to be an effective pre-catalyst for propene polymerization, when activated with methylalumi-noxane. However, very similar productivities were observed using the Zr(iv) complex as the pre-catalyst, suggesting that activation is accompanied by oxidation and that the active species contains zirconium in oxidation state rv. 

In the initial thiocyanate-complex Hquid—Hquid extraction process (42,43), the thiocyanate complexes of hafnium and zirconium were extracted with ether from a dilute sulfuric acid solution of zirconium and hafnium to obtain hafnium. This process was modified in 1949—1950 by an Oak Ridge team and is stiH used in the United States. A solution of thiocyanic acid in methyl isobutyl ketone (MIBK) is used to extract hafnium preferentially from a concentrated zirconium—hafnium oxide chloride solution which also contains thiocyanic acid. The separated metals are recovered by precipitation as basic zirconium sulfate and hydrous hafnium oxide, respectively, and calcined to the oxide (44,45). This process is used by Teledyne Wah Chang Albany Corporation and Western Zirconium Division of Westinghouse, and was used by Carbomndum Metals Company, Reactive Metals Inc., AMAX Specialty Metals, Toyo Zirconium in Japan, and Pechiney Ugine Kuhlmann in France. 

The zirconocene complex Zr(Si2Cp)2Cl2 (90) is a versatile starting material for a variety of zirconocene complexes with zirconium in the oxidation state IV, III,... 

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. 

Only a small number of zirconium(III) and hafnium(III) complexes are known. Nearly all of these are metal trihalide adducts with simple Lewis bases, and few are well characterized. Just one zirconium(III) complex has been characterized structurally by X-ray diffraction, the chlorine-bridged dimer [ ZrCl PBu,) ]- Although a number of reduced halides and organometallic compounds are known in which zirconium or hafnium exhibits an oxidation state less than III, coordination compounds of these metals in the II, I or 0 oxidation states are unknown, except for a few rather poorly characterized Zr° and Hf° compounds, viz. [M(bipy)3], [M(phen)3] and M Zr(CN)5 (M = Zr or Hf M = K or Rb). 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

A dimeric zirconium(IV) trihydride complex has been synthesized by reaction of LiBHEt3 with the tridentate triaryloxide zirconium chloride complex ZrCl(THF)2(t-Bu-L) (H3(t-Bu-L) = 2,6-bis(4-t-butyl-6-methylsalicyl)-4-f-butylphenol) (equation 14). The solid-state structure shows each zirconium center adopts a trigonal prismatic stracture with a Zr-Zr separation of 3.163(1) A. In solution, the three resulting hydrides appear equivalent on the NMR timescale. Analogous reactivity was observed for the titanium conger. However, in the resulting titanium trihydride, the metals are assigned formal Ti(III) oxidation states. ... 

Zirconium is the principal FP to arise in oxidation state (IV). Where Zircaloy clad fuel is involved, nonradioactive zirconium isotopes may also be present from fuel can residues. As with ruthenium, there may be a variety of nitrato complexes present in the solution including the aquated complexes Zr(N03)s where x = 1-6, and hydroxy nitrato complexes. However, species containing ZrO " " are not expected to be present since this ion is unstable in aqueous media and is rapidly hydrated to Zr(OH)2. The extraction chemistry is further complicated by the formation of inextractable polymeric species when the Zr" concentration exceeds ca. 10 M. An example of such oligomerization is afforded by the [Zr(0H)2(H20)4]4 ion which contains four Zr ions in a square arrangement linked by two /u-OH ligands on each square edge. Four water molecules complete the Zr coordination sphere in an approximately D2d dodecahedral geometry. 

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. 

Zirconium/Hafnium Complexes in Oxidation States +i and Below 697... 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

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