Infrared spectroscopy metal carbonyls

Chelation is not necessary to promote the abnormal metallation. When imidazo-lium salts with bulky substituents (e.g. iPr, tBu) are refluxed with pyridine and lrH5(PPh3)2 in tetrahydrofuran (THF), aNHC complexes are obtained in good yields, with the least sterically hindered of the three imidazolium carbons selectively bound to the metal (Scheme 3.13) [29]. Infrared spectroscopy on carbonyl... 

Coordinatively Unsaturated Metal Carbonyls in the Gas Phase via Time-Resolved Infrared Spectroscopy... 

The spectroscopy, reaction kinetics, and photophysics of coordinatively unsaturated metal carbonyls generated in the gas phase via UV photolysis are probed via transient infrared spectroscopy. The parent compounds that have been used to generate coordinatively unsaturated species are Fe(CO)5, Cr(C0)5 and Mn2(CO)io- In contrast to what is observed in solution phase, photolysis of these compounds produces a variety of coordinatively unsaturated photoproducts. 

Despite the considerable amount of information that has been garnered from more traditional methods of study it is clearly desirable to be able to generate, spectroscopically characterize and follow the reaction kinetics of coordinatively unsaturated species in real time. Since desired timescales for reaction will typically be in the microsecond to sub-microsecond range, a system with a rapid time response will be required. Transient absorption systems employing a visible or UV probe which meet this criterion have been developed and have provided valuable information for metal carbonyl systems [14,15,27]. However, since metal carbonyls are extremely photolabile and their UV-visible absorption spectra are not very structure sensitive, the preferred choice for a spectroscopic probe is time resolved infrared spectroscopy. Unfortunately, infrared detectors are enormously less sensitive and significantly slower... 

Varenne, A., Salmain, M., Brisson, C., and Jaouen, G. (1992) Transition metal carbonyl labeling of proteins. A novel approach to a solid-phase two-site immunoassay using Fourier transform infrared spectroscopy. Bioconjugate Chem. 3, 471-476. 

CO is an excellent probe molecule for probing the electronic environment of metals atoms either supported or exchanged in zeolites. Hadjiivanov and Vayssilov have published an extensive review of the characteristics and use of CO as a probe molecule for infrared spectroscopy [80]. The oxidation and coordination state of the metal atoms can be determined by the spectral features, stability and other characteristics of the metal-carbonyls that are formed. Depending on the electronic environment of the metal atoms, the vibrational frequency of the C-O bond can shift. When a CO molecule reacts with a metal atom, the metal can back-donate electron density into the anti-bonding pi-orbital. This weakens the C-O bond which results in a shift to lower vibrational frequencies (bathochromic) compared to the unperturbed gas phase CO value (2143 cm ) [62]. These carbonyls form and are stable at room temperature and low CO partial pressures, so low temperature capabilities are not necessary to make these measurements. 

High pressure infrared (HP IR) spectroscopy has now been used for over 30 years for the study of homogeneous transition metal catalysed processes. The technique is particularly useful for reactions involving carbon monoxide, for which transition metal carbonyl complexes are key intermediates in the catalytic mechanisms. Such complexes have one or more strong r(CO) absorptions, the frequencies and relative intensities of which provide information about the geometry and electronic character of the metal center. As well as probing the metal species, HP IR spectroscopy can also be used to monitor the depletion and formation of organic reactants and products if they have appropriate IR absorptions. 

Other important tests are for acid and alkalinity number and for water content (266), because water content and alkalinity of the polyether glycol can influence the reaction with isocyanates. The standard ASTM test for acid and alkalinity number, ASTM D4662 (267), is not sensitive enough for the low acidity and alkalinity numbers of PTMEG, and special methods have been developed. A useful alkalinity number (AN) has been defined as milliequivalents KOH per 30 kg of PTMEG, as titrated in methanol solution with 0.005 N HC1 (268). Other useful nonstandard tests are for heavy metals, sulfated ash, and peroxide. The peroxides formed initially in oxidations are quickly transformed into carbonyl groups, which are detectable by infrared spectroscopy. On oxidation, a small C—O peak develops at 1726 cm-1 and can be detected in thick (0.5-mm) films. A relative ratio of this peak against an internal standard peak at 2075 C—O is sometimes defined as the carbonyl ratio. 

Fundamental studies by reflection angle infrared spectroscopy of the bonding of EME coupling agents to metal oxides reveal a significant shift in the carbonyl absorbance band when the coupling agent is applied as a very thin layer on a metal oxide. The shift is reproducible and the extent varies with the type of oxide. These results were obtained both by use of copper mirrors and from CuzO powder coated with very thin layers of model compounds. The compounds were not removable by isopropanol, a solvent for the bulk compound. The thiol absorbances of thin layers of model compounds were also found to decrease in relative intensity with time. This illustrates that a specific chemical interaction has occurred. 

Dougherty TP, Heilweil EJ. Dual track picosecond infrared spectroscopy of metal-carbonyl photochemistry. In Lau A, Siebert F, Wemcke W, eds. Proceedings in Physics. Berlin Springer-Verlag, 1994 136-140. 

Infrared Spectroscopy of Organopalladium Compounds (See Transition Metal Carbonyls Infrared Spectra)... 

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