Zirconium hydride complexes

These examples all involve group IVa metal complexes, suggesting that the desired behavior might be possibly an attribute of early transition metal hydride complexes, an area that has been studied considerably less than complexes of metals that are further to the right of the early transition metals on the periodic table. Indeed, as discussed in this volume (see Chapter 10), a zirconium hydride complex recently has been found to reduce CO to methanol among other products. 

Heterobimetallic zirconocene hydride complexes of the type [M][(CsH4R)2ZrH2] were first characterized by ESR in 1991 (M = Li, Na, K R = Me, Bu, SiMe3).2,43,44 Aluminum-zirconium hydride complexes are involved in a number of catalytic and stoichiometric processes.4 The presence of Zr(m) species in reaction mixtures has been inferred from the color45 and, in the case of CpZr(//-I I)2AI(Mc)(2,4,6-tllu ((, I I2), detected by ESR.46... 

We have previously shown that the mononuclear zirconium hydride complexes 1 activate, under very mild conditions, the C-H bond of alkanes, including methane [7], The mechanism involves a four center intermediate, as proposed earlier for electrophilic activation of C-H bonds by group 3, 4 and lanthanides d° complexes [8], Given the similarities of the energies of dissociation of C-H and Si-H bonds, it is not surprising at ail that activation of Si-H bonds occurs with 1. Reactions of H/D exchange, followed by in situ IR spectroscopy, reveal that all types of silanes are activated, i.e. primary, secondary and even tertiary silanes [9],... 

Zirconium hydrides undergo 1,2-addition with 1,3-dienes to give y,5-unsaturated complexes in 80—90% yield. Treatment of these complexes with CO at 20°C and 345 kPa (50 psi) followed by hydrolysis gives y,5-unsaturated aldehydes (235). 

Note that the main difference between zirconium hydride and tantalum hydride is that tantalum hydride is formally a d 8-electron Ta complex. On the one hand, a direct oxidative addition of the carbon-carbon bond of ethane or other alkanes could explain the products such a type of elementary step is rare and is usually a high energy process. On the other hand, formation of tantalum alkyl intermediates via C - H bond activation, a process already ob-... 

Although a large variety of zirconium hydrides have been prepared, only very few reactions are described using these promising alternative complexes to 1. 

Bimetallic Transition Metal-Zirconocene Complexes from Zirconium Hydrides... 

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... 

In related studies, Cp2ZrCl2 has been found to catalyze at room temperature an aluminum hydride (i-Bu2AlH) reduction of CO to linear Ci-C5 alcohols (430). The system involves reaction of complex 55 with CO, which precipitates the starting zirconium(IV) complex and leaves a yellow solution, that on hydrolysis yields the alcohols. Toluene solutions of Cp2Ti(CO)2 complex under H2/CO effect Eq.(69), i.e., a homogeneous stoichiometric hydrogenation of carbon monoxide to methane (426). 

Zinc hydride, 4492 Zirconium hydride , 4493 See COMPLEX HYDRIDES, pyrophoric materials See entry LANTHANIDE-TRANSITION METAL ALLOY HYDRIDES... 

The Zr-H bonds for 7 are clearly hydridic, again in contrast to the group VIII transition metal hydrides that behave chemically more like protonated metal complex anions. Thus 7 readily reduces HCl, CH3I, and CH2O (Reactions 8-10). The Zr+-H- polarization of the zirconium hydride bonds for 7 is not altogether unexpected in light of the position of Zr in the periodic table. 

Zirconium hydride reactivity with carbon monoxide demonstrates the strong driving force toward products with a Zr-O bond. Indeed, the facility of the CO migratory insertion into Zr-C and especially Zr-H bonds may be from a carbonyl oxygen-zirconium interaction that stabilizes the transition state to the acyl and formyl complexes. 

Magnesium—nickel hydride, 4458 Plutonium(III) hydride, 4504 Poly(germanium dihydride), 4409 Poly(germanium monohydride), 4407 Potassium hydride, 4421 Rubidium hydride, 4444 Sodium hydride, 4438 f Stibine, 4505 Thorium dihydride, 4483 Thorium hydride, 4535 Titanium dihydride, 4484 Titanium—zirconium hydride, 4485 Trigermane, 4415 Uranium(III) hydride, 4506 Uranium(IV) hydride, 4536 Zinc hydride, 4486 Zirconium hydride , 4487 See COMPLEX HYDRIDES, PYROPHORIC MATERIALS See entry LANTHANIDE—TRANSITION METAL ALLOY HYDRIDES... 

The zirconium-benzyne complex 78 reacts with 2 equiv trimethylalumi-num57 or trimethylgallium58 to give 79 and 80, respectively [Eq. (17)]. As shown by X-ray analysis, these complexes contain a planar-tetracoordinate carbon center (C2) at the bridgehead position. Triethylaluminum and diiso-butylaluminum hydride react similarly.57... 

The bis(i7-pentamethylcyclopentadienyl)zirconium dinitrogen complex [tj-C5(CH3)5kZrj(N2)3 (22), in solution reversibly releases its dinitrogen in vacuo over several hours (Section III,E). The solutions remaining are said to consist mainly of [T7-C5(CH3)5][T7-C5(CH3)4CH2]ZrH (23) (33). Permethylzirconocene [tj-C5(CH3 )5l,Zr (24) may exist in these solutions in equilibrium with this hydride tautomer (as in the case of titanium). However, thus far permethylzirconocene has not been isolated as a discrete compound or otherwise characterized. It is very likely that permethylzirconocene is less stable and more prone to form complex metallocene hydrides than permethyltitanocene (16). 

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. ... 

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