Rhenium complexes, photolysis

Photolysis of Cp Mn(CO)3 in THF leads to the solvent complex Cp Mn-(CO)2(THF). Removal of solvent at -20°C followed by warming to room temperature while maintaining reduced pressure results in dimerization of solvent complex, decarbonylation, and solvent loss to form the air-sensitive 8 (17,51,88). While not isolated, the related Cp complex 8 has been observed in the gas phase. It is seen, in the electron-impact mass spectrum of the THF complex CpMn(CO)2(THF), which shows a molecular ion and cracking pattern assignable to 8 rather than the THF complex itself (51). The rhenium complex 9 is formed on photolysis of Cp Re-(CO)3 in THF (18) and in the carbonylation (15-20 atm, THF or toluene) of Cp 2Re2(0)2(ju.-0)2 (89,90). 

Analogous alkoxy and aryloxyrhenium complexes do not show the same reactivity. However, Brown and Mayer discovered soon thereafter that a different rhenium complex could mediate C—O bond formation.68 Photolysis of the Re(V) complex 13 led to C—H activation and formation of a phenyl rhenium oxo, 14. Yields were improved from 30-40% to 90% upon addition of pyridine to the photolysis mixture. The role of pyridine was unclear, because other tertiary amines provided no such improvement in yield. Substituted benzenes showed a preference for para activation over meta fluorobenzene also gave a significant amount of ortho activation. 

Photolysis of the dimetallated l,2-bis(indene)ethane rhenium complex 20 (see Scheme 2) result in the formation of complex 21. The mechanism of this reaction is believed to proceed via loss of a carbonyl ligand from rhenium followed by C — H activation, hydrogen migration and finally re-coordination of carbon monoxide. 

Fig. 11 (a) Trinuclear rhenium carbonyl complexes 7 and 8 and (b) UV-Vis difference spectra (Aabs = At - At = 0) of compound 7 in CH3CN at 293 K after irradiation at 366 nm for 0, 1,2, 3, 4, 6 h, and inset shows the emission spectral changes before and after photolysis (reproduced with permission from [39])... 

This is, as expected, the rarest oxidation state as far as non-organometallic complexes of rhenium are concerned. The dirhenium phosphite complex Re2[P(OMe)3]10 has been prepared12,13 in low yield by several methods, viz. the potassium-potassium iodide reduction of ReOCl3(py)2 or an ReCLj-pyridine complex, followed by treatment with trimethyl phosphite. This yellow complex displays a 31P H) NMR spectrum that appears as a first order AB4 pattern, and is isoelectronic with Re2(CO)l0. It reacts with H2 upon photolysis in THF solution to produce hydridorhenium(III) and other species.13 The related triphenyl phosphite derivative Re2[P(OPh)3]l0 has been described14 as a product of the reaction between ReH3(PPh3)4 and P(OPh)3. 

Follow-up products may include metal oxides, be it in a mononuclear or in a di-and oligonuclear form (Eqs. 2-4). The tetranuclear rhenium(I) complex formed by photolysis (Eq. 4) has a cubane-type structure [7]. Related complexes 2a-d were made according to Scheme 2 [8],... 

Finally, photolysis of the Re(V) oxo-idodide compound (HBpz3)ReO(I)Cl in arene (ArH) solvents gives the aryl complexes (HBpz3)ReO(Ar)Cl [39], Substituted arenes react with electrophilic selectivity and the authors propose that the reactive species is a rhenium(V) 0x0 complex which can add aromatic C-H bonds aaoss the Re=0 linkage. 

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