Photooxidation, halocarbon complexes

This binuclear photooxidative addition reaction is general for a number of halocarbons (Figure 3). While DCE and 1,2-dibromoethane react cleanly to give the dihalide metal dimers and ethylene, substrates such as bromobenzene or methylene chloride react through an alkyl or aryl intermediate. This intermediate reacts further to yield the dihalide d2-d2 metal complexes. 

Photochemical oxidation of square-planar bis(l,2-dithiolene) complexes of Ni, Pd, and Pt is by no means limited to IPCT excitation. Photooxidation also occurs in halocarbon solvents. In 1982, two separate reports addressed the photochemistry of metal bis(l,2-dithiolene) complexes. Vogler and Kunkely (80) investigated complexes of the type M(S2C2R-2)2, where M = Ni, Pd, Pt,... 

The long lifetimes of CT excited states of the Pt(diimine)(dithiolate) complexes allow for bimolecular photochemistry, often involving oxidation of the complex. The earliest report of photoreactivity of these complexes dealt with the photooxidation of Pt(bpy)(tdt) (20) following excitation at 577 nm in chloroform (118). The reaction proceeds with a quantum yield of < ) = 0.03 and was attributed to ET to the halocarbon solvent (Eq. 8) similar to the CTTS photooxidation chemistry of the platinum bis(dithiolate) dianions described above. 

ESR spectroscopy provided evidence for the radical ion of the oxidized tdt ligand, but the metal complex cation was not isolated nor were the products of halocarbon reduction identified. Interestingly, the related complex Ni(phen) (S2C2Ph2) was reported to undergo similar photooxidation when irradiated at higher energy, but not when irradiated in the low-energy CT band (118). 

It should be noted that similar "wavelength effects" have been observed for the photooxidation of ferrocene In halocarbon-contalning solvents (Section III-B-5). In these cases, however, the reaction is initiated by the excitation of ferrocene-solvent donor-acceptor complexes. As clear evidence against a similar mechanism was not presented for the ferrocene photooxidation involving N2O, this appears to remain a possibility. 

The photooxidation of the bis(mnt) complexes of Ni(II), Pd(II), and Pt(II) has been repoled by Vogler and Dooley [50,51]. Photolysis of dilute solutions (10" M) of [M(mnt)2] in halocarbon solvents leads to the fomation of the corresponding one-electron oxidized species, [M(S2C2(CN)2)2] (Scheme 7). These species are formed by excited state electron transfer to a solvent molecule, a rapid process that conq)etes with other modes of relaxation. The reaction rate... 

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