Metal carbonyl, kinetic studies

Kinetic studies on substitution reactions of carbonyl metal complexes. H. Werner, Angew. Chem., Int. Ed. Engl., 1968, 7,930-941 (106). 

No comparative kinetic study has been made on the same alkyl carbonyl system for two members of a given transition metal triad. Qualitative data show that the middle member is more reactive than the heaviest one e.g., CpMo(CO)jR > CpW(CO)jR (Section VI,B), Rh(III) > Ir(III) (Section VI,E), and Pd(II) > Pt(II) (Section VI,F). However, the extreme unreactivity of CpW(CO)jR and a considerable difference in lability between most alkyls of Rh(III) and Ir(III), as well as those of Pd(II) and Pt(II), have prevented detailed investigations. Surprisingly, no kinetic studies have been conducted on insertion reactions of RRe(CO)5, which would seem readily amenable to such investigations. 

There have been three primary motives behind the study of metal carbonyl photochemistry in the gas phase first, to discover the shapes of metal carbonyl fragments in the absence of perturbing solvents or matrices second, to probe the effect of uv photolysis wavelength on product distribution and third, to measure the reaction kinetics of carbonyl fragments. All three areas have already proved fruitful. The photochemistry of two molecules, Fe(CO)5 and Cr(CO)6, has been studied in detail. 

These IR kinetic experiments (75) were the first examples of vibrationally excited metal carbonyls to be observed. More detailed studies on the behavior of hot carbonyls should provide an intriguing insight into the photophysics of these molecules. We now look at metal carbonyl photochemistry in solution. 

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... 

These problems can be somewhat overcome by a study of reactions in solution where much greater densities are possible than in the gas phase and fast bimolecular reaction are diffusion limited [1,28,29]. However, since coordinatively unsaturated metal carbonyls have shown a great affinity for coordinating solvent we felt that the appropriate place to begin a study of the spectroscopy and kinetics of these species would be in a phase where there is no solvent the gas phase. In the gas phase, the observed spectrum is expected to be that of the "naked" coordinatively unsaturated species and reactions of these species with added ligands are addition reactions rather than displacement reactions. However, since many of the saturated metal carbonyls have limited vapor pressures, the gas phase places additional constraints on the sensitivity of the transient spectroscopy apparatus. 

Nevertheless, we were able to develop a transient absorption apparatus involving IR probe radiation that is suitable for gas phase studies, as have a number of other groups either coincident with or subsequent to our work [1]. In the remainder of this article we will discuss the apparatus and the results of our studies on three prototypical metal carbonyl species Fe(C0>5, Cr(C0>5 and Mn2(CO)] o The discussion in this article will center on the nature of the photolytically generated coordinatively unsaturated species, their kinetic behavior and photophysical information regarding these species. This latter information has enabled us to comment on the mechanism for photodissociation in these systems. Since most of the results that will be discussed have been presented elsewhere [3-10], we will concentrate on a presentation of data that illustrates the most important features that have come out of our research and directly related research regarding the kinetics, photophysics and photochemistry of coordinatively unsaturated metal carbonyls. 

The potential of the techniques described here is just beginning to be realized. Our approach is clearly well-suited to studying the photochemistry of virtually any (volatile) transition metal carbonyl and the spectroscopy of the fragments thus obtained. Kinetic data on these fragments is similarly accessible. Important areas for further research appear to be ... 

We have used such cells (J 4) to generate and measure the kinetic stability over a wide temperature range of a wide variety of unstable species. The easiest experiments - and the first performed - were simply the photolysis of a metal carbonyl in liquid xenon doped with dissolved N2 and hence replacement of CO by N2. The V(N-N) IR bands are little weaker than v(C-O) bands and hence detection and characterization are straightforward, particularly when use is made of previous matrix studies. Species examined include Cr (CO) (N )x x = Ni(CO)3(N ) ( 16). ... 

Metal carbonyls undergo reactions with a great many compounds to produce mixed carbonyl complexes. A large number of these reactions involve the replacement of one or more carbonyl groups by a substitution reaction. Such reactions have also been studied kinetically in some cases. 

Prior to our studies it was recognized that ion pairing with anionic metal carbonyls could promote CO insertion and related reactions (14-16). Both kinetic and non-kinetic evidence suggests the importance of ion pairs in these types of reactions (14,17). For example, a small cation was found to greatly accelerate the CO insertion reaction relative to the same reaction with a large cation, equation 6 (14). 

Systematic studies of the thermodynamic and kinetic acidity of metal hydrides in acetonitrile were carried out by Norton et al. [10, 11]. A review of the acidity of metal hydrides presents extensive tabulations of pKa data [12] only a few of the trends will be mentioned here. Metal hydrides span a wide range of pKa values considering only metal carbonyl hydrides shown in Table 7.1, the range exceeds 20 pfCa units. As expected, a substantial decrease in acidity is... 

As well as characterizing complexes involved in the main catalyst cycles, spectroscopy has contributed to the measurement of the kinetics of these cycles and to byproduct reactions. The major catalyst species present under working conditions of the catalyst systems have been identified for all the systems. Individual reaction steps involving interconversion of catalyst complexes have been isolated and studied in model reactions. IR has been very important in these studies with metal carbonyl species, including the identification of Ru promoter species in MeOH carbonylation. 

Kinetic studies on catalytic amine carbonylation reactions are scarce, although Brackman (13) has reported kinetics on a copper(I)-copper(II) catalyzed production of ureas from cyclic secondary amines using carbon monoxide-oxygen mixtures at ambient conditions. Saegusa and coworkers (14) used cuprous salts and other group IB and IIB metal compounds to car bony late piperidine to N-formylpiperidine under more severe conditions. We have published (15) a brief report involving some of the studies described in this paper. 

The hydrolysis of 8-acetoxyquinoline (159) is subject to catalysis by metal ions and detailed kinetic studies of the reaction have been reported.509 The metal ion could be bound to the carbonyl oxygen (160) or the ether oxygen (161) and the actual structure of the catalytically active complex was not unequivocally defined. 

The synthetic aspects of coordination catalysis still receive far more attention than mechanistic aspects, and precious little can be said with assurance concerning the mechanisms of the foregoing polymerization reactions. Almost the opposite is true of the free radical initiation of vinyl polymerization by metal carbonyls. Bamford et al. (10,11) have studied the kinetics of several such reactions and arrived at plausible interpretations of their results. The initiation of methyl methacrylate polymerization by tetracobalt dodecacarbonyl in the presence of carbon tetrachloride was studied in detail and the results were rationalized in terms of the following mechanism ... 

Within a few hours I accepted the offer and the next day saw Professor Hieber who gave me a cordial welcome and showed me the new lab. He also asked me about the research project for my Habilitation and for a moment I hesitated. It was known that he considered kinetic and mechanistic studies as a type of philosophy which, in his opinion, it was not worth doing.2 Nevertheless, he listened courteously and, after I told him that the class of compounds I was going to use as starting materials for my work were metal carbonyls, he wished me good luck. 

---