Evidence for the Formation of Free 16-Electron Species Rather than Solvate Complexes in the Ultraviolet Irradiation of CpCo(CO)2 in Liquefied Noble Gas Solvents

Abstract

It is now generally well established that photodissociation of a ligand from many transition metal complexes results in the formation of a solvent-stabilized metal center within picoseconds of ligand loss, even in saturated hydrocarbon solvents.1-6 For example, Rayner and co-workers have determined that, in the gas phase, interaction between the tungsten center and hydrocarbons (RH) in (CO)sW(RH) can be as high as 12 kcal/mol,1 234567 and Yang and co-workers have established an approximately 8 kcal/mol binding energy for the first-row metal complex Cp(CO)3Mn(n-heptane) in the liquid alkane.8 Even in unconventional solvents such as liquid Xe and Kr, there is compelling evidence for such metal-solvent interactions following the loss of a CO ligand from the metal complex.9•10The interaction energy between a metal center and a noble gas atom (Kr and Xe) has been estimated to be on the order of 5-10 kcal/mol, with the M-Xe bond a few kilocalories stronger than the M-Kr bond. Therefore, while noble gas atoms appear to bind to unsaturated metal centers somewhat less strongly than do hydrocarbons, the metal-noble gas bond energies are significant.Consistent with the above observations, recently we obtained strong evidence for the formation of Cp*Rh(CO)(Xe) and Cp*Rh(CO)(Kr) complexes after photolysis of Cp*Rh(CO)2 in liquid Xe and Kr, respectively.11•12 These molecules reacted with cyclohexane (or neopentane) to give an uninserted metalalkane complex, Cp*Rh(CO)(C6Hi2), which in a unimolecular step proceeded to yield the C-H activation product, Cp*Rh(CO)-(HXC5H11). In analogy to the rhodium (and analogous iridium) system, irradiation of the isoelectronic cobalt complex CpCo-(CO)2 results in CO loss. However, the intermediate generated in the cobalt reaction does not result in overall activation of the C-H bonds of hydrocarbons. Assuming that (in analogy to the Cr(CO)5, W(CO)s, and CpMn(CO)3 fragments studied earlier) cobalt-noble gas and cobalt-alkane complexes were probably formed in these reactions but were stable to further reaction, we decided to study the photochemistry of CpCo(CO)2 in liquid Kr and Xe in the presence of hydrocarbons in order to estimate directly the relative strengths of the CpCo(CO)-noble gas and CpCo(CO)-alkane interactions.8 However, we have found that in dramatic contrast to Cp*Rh(CO) and the coordinatively unsaturated organometallic complexes summarized above, CpCo(CO) and Cp*Co(CO) do not bind delectably to either a noble gas atom or cyclohexane.

Unusual behavior in the 308 nm flash photolysis of Vaska's complex

“…Another possibility is that this absorption is to a relatively long-lived excited triplet state of VC [18]. Excited triplet states of the coordinatively unsaturated complexes CpCo(CO) [19] and Fe(CO) 4 [20] do not appear to solvate, and the IR spectrum of solvated singlet Fe(CO) 4 has C Á/O stretches that are coincident or near-coincident with those of the unsolvated triplet [20].…”

Ultrafast Infrared Studies of Bond Activation in Organometallic Complexes

“…Further studies have shown that, in dilute solutions of alkanes in rare gas liquids, these rare gas molecules can exchange with alkane molecules to form a C-H-activated final product. Measurements of the exchange rates give a time for bond activation of ∼2 µs at a temperature of 73 K. 7 An interesting question in understanding and eventually making use of these reactions is what effect those ligands which are coordinated to the metal center have on the quantum yield of the reaction. The quantum yield of CpRh(CO) 2 is 1%, while the quantum yield of Cp*Rh-(CO) 2 (Cp ) C 5 H 5 ; Cp* ) C 5 (CH 3 ) 5 ) is only 0.2%.…”

“…Ultrafast Visible Spectroscopy. It was shown by Bengali et al and Weiller et al 6,7,18 that the reactive intermediate in the Cp*Rh(CO) 2 system is a monocarbonyl, Cp*Rh(CO). An important issue to be resolved then is the state of the 99% of the molecules which are unreactive.…”

Indenyl Ligands as Supports for Reactive, Low-Valent Cobalt(I) Fragments

“…16,17 For example, though 16-electron CpML (M = Rh, Ir) species readily break unactivated sp 3 CÀH bonds, the related cobalt version is unreactive, likely due to differences in spin state of the intermediate. 16,18 Caulton and co-workers have observed similar inertness of triplet, 16-electron PNPCo I pincer complexes toward alkane CÀH bonds. 19 However, recent development of the catalytic transfer dehydrogenation of silyl-protected amines by Brookhart and coworkers using Cp*Co(η 2 -H 2 CdCH(SiMe 3 )) 2 demonstrates sp 3 CÀH activation can be achieved using Co under certain conditions.…”