Organometallic precursor molecules

Weakening of the carbon-metal bond in this manner is attributed to delocalization of the free-radical electronic charge. Even weaker carbon-metal bonds are formed for the allyl radical, where a double bond is formed, and the benzyl radical, where the central carbon atom is bonded to a benzene ring. A corollary to the rule described above is that the more stable the radical, the more rapidly it is formed. Thus, allyl groups forms radicals most rapidly and methyl groups form radicals least rapidly. This is reflected in the rates of pyrolysis of various organometallic precursor molecules. 

Since the 1970s, palladium and platinum complexes with dibenzylideneacetone Pd(dba)2 and M2(dba)3 (M = Pd, Pt) have been known to react under mild conditions with either hydrogen or carbon monoxide, with the formation of a metal [211]. Indeed, there exists a long series of examples where CO and H2 have been used to decompose organometallic precursor molecules [173-177,179-181,183,184,186-188, 212-216]. As an example, the decomposition Ru(COD)(COT) (COD = cyclo-octadiene COT =cydo-octatriene) in an atmosphere of hydrogen is worthy of mention [189,190]. (Scheme 3.20). In this case, the precursor molecule is dissolved in a methanol-THF mixture and is contacted with H2 (3 bar pressure) at room temperature for at least 45 min. Depending on the nature of the MeOH-THF mixture, the Ru particle size can be designed between 3 and 86 nm. 

A comparative study of a series of 28 single organometallic precursors to the binary ceramic phases TiC, TiN, VC, VN is reported. The design of the molecules, their thermal behavior, and their use in CVD applications are described. The influence on the film composition of (1) the nature of the precursor ligands (Cp, Cp, Cl, NR2 with R = Et, SiMe3) and (2) the CVD conditions (hot or cold wall, carrier gas, gas phase composition) is studied in the case of the best candidate precursors. 

Mechanistic evidence for outer sphere SET is not easy to obtain for reactions which lead to conventional products. Instrumental techniques such as EPR or CIDNP often fail to show direct evidence for radical production because of short radical life-times [55]. Spin trapping agents [56] may interfere with the reaction from the start, especially since the widely employed nitroso compounds are good ligands for coordinatively unsaturated organometallics [57]. Even if rather j rsistent radicals are established via instrumental detection it is not always certain that they arise directly from electron exchange between the precursor molecules or that they are even related to the major reaction [58]. 

Finally, a study of fairly monodisperse CdSe semiconducting nanorods functionalized with amphiphilic molecules in a suspension of cyclohexane has recently been published that shows preliminary results indicative of a typical nematic behavior [ 102]. Note that these nanorods have been synthesized by the pyrolysis of organometallic precursors of Cd and Se in hot surfactant mixture allowing a good control of the aspect ratio. This state of the art particle size and shape control method should allow one to customize accurately their semiconducting properties. 

Ferrocene, as a representative example for many organometallic compounds of interest here, can be first infiltrated into the MOF-5 cavities (see Fig. 3) and can then be removed without any change of the host material. Also, the dependence of the size of the precursor molecules on the loading was shown. MOF-5 cavities exhibit an opening diameter of 7.8 A. Ideally, only one of the three principal axes (x, y, z see Table 2) of the enveloping ellipsoid representing the van der Waals volume of the respective precursor molecule should exceed this diameter in order to allow diffusion into the MOF cavities. For that reason, [Cu(OCHMeCH3NMe2)2]... 

Dossi et al. [61] loaded platinum in KL zeolite by vapor deposition of Pt(hfa)2 (hfa hexafluoroacetylacetonate). The organometallic precursor was sublimated at 70 °C in a flow of argon and adsorbed on the dehydrated KL zeolite. The decomposition of Pt(hfa)2 was achieved at 350°C in a H2-atmosphere. In situ EXAFS measurements suggested the formation of small clusters (Pt-Pt coordination number of 5), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements using CO as the probe molecule indicated the formation of carbonyl clusters of the general formula [Pt3(CO)g]n (n= 1 -4). 

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