Elementary Rate Processes in the Dissociative CO for C2H4 Substitution Reactions of Organometallic Complexes in the Gas Phase

Dissociative substitution mechanisms abound in organometallic chemistry. For certain systems, such processes can be isolated in the gas phase, where, as sequences of elementary unimolecular and bimolecular reactions, their kinetics can convey informa- tion on fundamental energetics and dynamics of metal-centered chemical trans-formations. Methods of competitive kinetics, using time-resolved infrared absorption spectrometry, provide relative and absolute rate constants for comparatively fast reactions. Work yielding unimolecular decay and relative bimolecular production rate constants for selected bis- and tris-ethylene complexes of iron and chromium carbonyls is summarized together with a report of new work on the CO-for-C2H4 substitution kinetics of (C2H4)Cr(CO)5.


I. INTRODUCTION
The study of chemical change is a science of intermediates. Despite their importance, organometallic intermediates are among the most rarely observed. They are quite often highly reactive and exist only in infinitesimal concentration in steady state and catalytic sequences. In solution, coordinatively unsaturated species interact significantly with virtually all solvents including, for example, liquified rare gases. 2 Solution-phase characterization of uncomplexed intermediates is therefore beyond the reach of even the fastest spectroscopic methods, 3 leaving transient structures to be determined indirectly from such evidence as stereochemistry and isotope distributions.
Gas phase approaches to the problem of chemical transients do not suffer this limitation. Indeed, gas-phase detection and characterization of reactive intermediates has been a cornerstone of fundamental knowledge in main-group chemical kinetics for more than 50 years. 4 As exemplified by programs in place in our laboratory and elsewhere, a small but growing collection of comparable efforts are now underway which are concerned with the kinetic behavior of gas phase organometallic intermediates. 5 Some of the earliest of this work on gaseous organometallics extended simple flash photolysis/UV-visible absorption methods that had been extensively developed for solution phase systems. 6 By incorporating laser sources, such techniques achieve excellent sensitivity and time resolution. 7 However, the UV-visible absorption spectra of organometallics are generally too broad and featureless to characterize structures or definitively resolve contributions from spectrally similar compounds. 8 Infrared spectroscopy of organometallics is more informative. For carbonyl compounds in particular, which exhibit strong CO stretching transitions in the region from 2100 to 1900 cm-1, the number and relative intensities of the bands observed establish the group symmetry of the CO ligands. 9 Applicability to intermediates is well illustrated by the wealth of information provided by the infrared spectroscopy of organometallic transients captured in matrix isolation. 1 This latter work in particular has spurred the refinement of infrared methods for detection and identification of organometallic fragments. It has also provided a base of information critical to the assignment of infrared spectroscopic transients observed under other conditions. At present, recent technological advances in infrared sources and detectors have increased both sensitivity and time resolution of gas phase transient infrared experiments. As a result, fundamental data of great importance on the elementary properties of a number of key organometallic systems is now emerging. 5'11'12 Our laboratory has had a role in this progress. We have developed specialized kinetic and spectroscopic approaches, which we have extensively applied to studies of the chemical relaxation behavior of gas phase systems of pulsed laser prepared organometallic intermediates. We have focussed thus far on a class of reactions that occupies a place of central importance in homolytic organometallic chemistry, (1) This general scheme is representative of the broad class of organometallic reaction processes in which a stable substitution product forms by displacement of an abundant weakly bound ligand. For instance, a minimal mechanism in solution-phase photosubstitution necessarily includes the steps above with solvent coordinating and decoordinating as L'. To have efficient turnover, substrates in catalytic processes must not be too strongly bound, and thus the engineering of reversible coordination steps are key to sustaining catalytic activity. 3 Work in our laboratory on the kinetics of gas phase organometallic substitution 4-17 and homogeneous catalysis 13 extends the important place of weakly bound intermediates and dissociative substitution mechanisms to the elementary sequence of events in the conversion of isolated substrates and metal complexes. In the iron carbonyl catalyzed hydrogenation of ethylene, for example, the metastable complex (C2H4)EFe(CO)3 serves as a reservoir for the active catalyst, (CEH4)Fe(CO)3 13,17 The dissociative properties of this reservoir complex determine the overall rate of catalysis, while competing CO substitution regulates turnover numbers. A time-resolved FTIR examination of its dissociative substitution kinetics, isolated, 17 and in the presence of substrate HE, 13 has given us our most critical insights on the elementary mechanism and rate properties of the gas phase catalytic hydrogenation cycle.
In other work, we have shown that photogenerated chromium carbonyl olefin complexes are also effective hydrogenation catalysts in gas phase systems. TM Here again it is suspected that bis-olefin metastables serve as reservoirs that release coordinatively unsaturated catalytic intermediates for which substrates compete. Catalytic efficiency in chromium mediated hydrogenation is in general much lower than that found for iron complexes. This can, of course, be at least in part attributed to differences in electron density on d 6 chromium centers vs. d 8 iron, with accompanying differences in reactivity toward H2.
However, it is also the case, as shown by transient infrared absorption studies in our laboratory, 15'16 that bis-olefin complexes of chromium, typified by (c2a4)2Cr(CO)4, are less stable and intrinsically shorter lived with respect to dissociative substitution. Recognition of these differences underlines the means by which, through the understanding and accurate modeling of key elementary processes, we might design novel and kinetically optimized catalytic systems.
Studies of dissociative substitution chemistry also convey important fundamental information about the energetics of organometallic bond-forming and rearrangement reactions. For example, in the chromium bis-ethylene system, we observe infrared spectroscopic evidence for the presence only of the metastable cis isomer, cis-(C2H4)2Cr(CO)4. This spectral evidence persists despite the many gas-phase detachment and recombination cycles which occur over the course of relaxation of a typical complex, coupled with the fact that the rearranged isomer trans-(C2H2)2Cr(CO)4 is stable under our condio tions. 19 These results tell us: (1) that the recombination of C2H4 with unsaturated (C2H4)Cr(CO)4 is stereoselective, and (2) that the energy released in the isolated gas-phase molecule by the formation of the second (C2H4)-Cr bond is insufficient to overcome the barrier for isomerization: cis-(C2H4)2Cr(CO) 4 trans-(C2H4)2Cr(CO)4

II. EXPERIMENTAL APPROACH TO THE STUDY OF THE ELEMENTARY KINETICS OF DISSOCIATIVE SUBSTITUTION IN THE GAS PHASE
We extract rate information on the elementary reactions involved in dissociative substitution by transient infrared absorption spectrometry. The particular technique required depends upon the timescale of the process under investigation, which, given a specific system, depends on conditions: It is a convenient virtue of dissociative substitution that, within limits, one can tune the characteristic time for chemical relaxation from ML' to ML by varying partial pressures of L and L'. This in particular, permits highly accurate, robust spectroscopic techniques such as FTIR to be brought to bear on transient compounds associated with reactions which are intrinsically rather fast. The way in which this is realized is perhaps best seen by a brief examination of the dissociative substitution mechanism and its associated kinetics. The reactions again are" Steady-state analysis of the decay gives an overall rate law: dt in which kobsd is a phenomenological decay constant, which has a magnitude that depends inversely on [L']. Thus, overall kinetics can be slowed, and unstable ML' complexes can be preserved for spectroscopic study by high L' pressures. This puts reactions with even relatively large rate constants within reach of faster FTIR methods. We have had good success with this approach in the case, for example, of (C2H4)2Fe(CO)3.17 Time-resolved spectra of a system decaying by the reaction sequence: In these experiments metastable (C2Ha)2Fe(CO)3 is prepared b.y excimer (351 nm) laser irradiation of (C2Ha)Fe(CO)4 in the presence of ethylene. The subsequent acquisition of series of transient infrared spectra, such as those shown in Figure 1 cal decay constants can be obtained. From analysis of such data under conditions of systematically varying C2H4 and CO partial pressures, we obtain an intrinsic room temperature unimolecular decay rate for (C2H4):zFe(CO)3 of kl 2.9 x 10 -3 sec-1. We additionally find that competition between CO and C:zH4 for unsaturated (C:zHa)Fe(CO)3 favors CO by a factor k,/k:z 35. For reactions too fast to follow by FTIR, we have used a fast transient absorption apparatus which features a Nernst Glower source, 0.3M monochrometer, and a fast InSb photovoltaic detector. Though limited both in spectral resolution and sensitivity, this set-up has allowed us to obtain elementary rate information on systems with overall decay timescales in the range from tens of microseconds to tens of milliseconds. [4][5][6] Kinetic parameters obtained in our laboratory by time-resolved FTIR and dispersed absorption methods are summarized in Table I. The most recent addition to this table is (C2H4)Cr(CO)5. In the following section we present new experimental results on the dissociative substitution kinetics of this compound. Kinetic analysis gives the (C2H4)Cr(CO)5 is prepared for kinetic study in situ by low-power pulsed XeC1 excimer laser irradiation of gas phase samples, containing 120 rn torr Cr(CO)6 in combination with measured partial pressures of CO and C2H4 ranging from 3 to 120 torr and 500 to 1000 torr respectively. Progress of the photoconversion from Cr(CO)6 to (C2H4)(Cr)(CO)5 is followed by FTIR. Figure 2 shows a succession of spectra, taken under typical photopreparation conditions.
After sufficient (C2H4)Cr(CO)5 is produced, its decay by thermal reaction back to Cr(CO)6 is followed in time by successive FTIR scans. For concentrations of CO and C2H4 given above this process is slow, requiring intervals of observation ranging from 30 minutes to 2 hours. Loss of (C2H4)Cr(CO)5 proceeds at the same rate as recovery of Cr(CO)6, exhibiting simple exponential behavior in time.
Time constants for this relaxation by CO-for-C2H4 substitution are found to depend on both CO and C2H4 concentrations, the overall rate of reaction increasing to saturation With increasing [CO] and decreasing with increasing [C2H4]. Figure 3 shows a plot of kobsd VS. [CO] at a constant C2H4 pressure of 600 torr. Such rate behavior parallels entirely that found for other gas phase CO for olefin substitution reactions found in our laboratory. [3][4][5][6][7] In those cases, as with this, it is easily shown that rate saturation in [CO], coupled with an inverse dependence on [C2H4] signifies the mechanism of dissociative substitution: parably to the bis chromium complex, becoming limited by the rate of unimolecular decay at a higher CO pressure than for any of the iron systems. This simple, physical observation qualitatively places the relative rate constant for CO vs. C2H4 recombination, k3/k2, for Cr(CO)5 at a number close to one.
A more quantitative assessment can be made by considering the explicit steady-state rate law for the present system:  Figure 4 shows kobsd data plotted according to this linearized form as k-lobsd VS.
In this respect the recombination kinetics of the chromium pentacarbonyl fragment compares reasonably with the behavior of Cr(CO)4 as studied by Fletcher and Rosenfeld. 2 Using time resolved infrared laser absorption spectroscopy these workers directly measured Cr(CO)4 recombination with CO and with C2H4, finding rate constants in the ratio of 0.8 + 0.2, with both reactions near the gas kinetic limit. More recently Weitz  convert our ratio k3/k2 to an absolute extimate of the room temperature recombination rate of Cr(CO)5 with C2H4, thus finding k2 5 x 1012 cm 3 molec -1 sec-.