Singlet-triplet equilibrium

The marked shift in the singlet-triplet equilibrium produced by sulfuric acid and aluminium chloride is important because it undermines one of the commonly applied criteria for polar mechanisms we can no longer be quite certain that an acid-catalyzed reaction is a polar one on the basis of the catalysis alone. Fortunately, the radical mechanism can often be ruled out on other grounds. 

Similar mixed-ligand complexes of the type (R, R2Dtc)2(MNT)Fe have been synthesized. The complexes were obtained initially as dianions, [(RiR2-Dtc)2(MNT)Fe]J", and were subsequently oxidized either by air or Cu(II) ions in acetonitrile (510). They also exhibit the singlet- triplet equilibrium however, they show a higher population of the triplet state than is found for the TFD analogues. The complexes are stereochemically nonrigid and display the same type of kinetic processes as their TFD counterparts. Thermodynamic activation parameters for inversion of the two complexes (TFD versus MNT) do not differ within experimental error. 

A second example is the recently reported singlet-triplet equilibrium in a Mo(II) complex, cis-[Mo(bipy)2(OPri)2] (31). The interpretation of the magnetism is complicated by the possibility that the equilibrium is not entirely metal centered, but involves charge transfer to the bi-pyridyl ligands. Because a strong field is required to spin pair the d4 configuration, however, it is not impossible that examples exist among Mo(II) chemistry. The possible dynamics are subject to considerations similar to those which apply to Mn(III). 

In (dtc)2Fe(mnt) and (dtc)2 Fe(tfd) (30) and their Ru analogs several novel observations were made 45 the complexes are stereochemically nonrigid, they show a temperature dependent, singlet-triplet equilibrium and they can be subjected to oxidation and reduction over a series of four steps (2— to 1 +), which, in the case of the Ru complexes, are fully reversible. 

Non-stereospecific cyclopropanation reactions of the diazafluorenylidene (10), generated by photolysis of the diazo compound, indicated a triplet carbene.18 Competition experiments suggested a singlet-triplet equilibrium at room temperature and a Hammett study of additions to substituted styrenes indicated that the carbene reacts as an electrophile (p = —0.65). 

The situation is not unlike the classic example of a spin singlet-triplet equilibrium in the dimeric carboxylates of copper, e.g., [Cu(OAc)2]2. See F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry A Comprehensive Test, 4th Ed., pp. 817-818. Wiley (Interscience), New York, 1980. 

In contrast with fluorenylidene, singlet-triplet equilibrium in the carbene derived by photoelimination of nitrogen from the diazodihydroboraanthracene (57) is slow. Direct irradiation affords singlet carbene which reacts with propan-2-ol to give the ether (58), whereas 2-acetonaphthone-sensitized irradiation yields triplet carbene which undergoes non-stereospecific addition to alkenes. Products obtained by irradiation of 3-diazo-2,5-diphenylpyrrole can arise via the intermediacy of singlet and triplet carbenes. Decomposition of the pyrrole (59) in the presence of benzene, for example, yields the 2H-cycloocta[c]pyiTole (60) as the major product. 

NKCjgHjoNJBrj-HjO room 1.5 singlet triplet equilibrium suggested 66B38... 

N(n-C4H9)4]3M03C1,2 291 2.5 NMR in CHjCU solution singlet<- triplet equilibrium suggested 74D8... 

Farad per complex singlet <-> triplet equilibrium suggested... 

Pyrene tethered by nine and ten carbon atoms Compounds 46 and 47, which have relatively long tethers, behave like 43 therefore, their two-electron reduction yielded anti-aromatic species 46b and 47b. The spectra of the dianions were broad and could be obtained only at low temperature. This is typical for systems that have a singlet-triplet equilibrium, with a higher contribution of the triplet state [124]. 

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