Diradical complexes

Many of the problems and misconceptions occurring for dithiolene compounds are related to the fact that the ligands are redox-active and can be oxidized to monoanionic radicals. Typical examples for this phenomenon are the mono and diradical complexes [Fe ( "bdt )( "bdt)(PMe3)] (9) and [Fe ( "bdf)2(PMe3)]-" (10) for which bdt and bdt are tcrt-butyl-dithiolene and its one-electron oxidized form. Originally, these and other bdt derivatives had been described as... 

The charge transfer complex resulting from the one-electron transfer from the electron donor monomer to the electron acceptor monomer has a stability which varies as a function of the internal resonance stabilization. The degree of stabilization apparently determines the ease with which the diradical complex opens, and consequently the stability of the complex determines whether the copolymerization occurs spontaneously or under the influence of heat, light, or free radical attack. 

The interactions of a-olefins or styrene with sulfur dioxide (16) or a-olefins (24, 58, 78), frans-stilbene (64), styrene (1,63), p-dioxene (52), 2,2-dimethyl-l,3-dioxole (17), or alkyl vinyl ethers (1, 63) with maleic anhydride yield charge transfer complexes which are stable and generally readily detectable either visually or by their ultraviolet absorption spectra. However, under the influence of a sufficiently energetic attack in the form of heat or free radicals, the diradical complexes open, and alternating copolymers are formed. 

In view of the failure to produce any polymer in the presence of peroxide and the absence of zinc chloride, probably caused by the slow rate of radical formation at 60°C., it is also unlikely that the excess polar monomer enters by the coupling of a growing chain with a radical from the diradical complex. It is significant, however, that a very small concentration of radicals is sufficient to open the complex. 

Reduction of the bis-pyridinium ion with various agents (Na(Hg), the 4 radical, or electrochemical reduction with ButtN ClO T as electrolyte) leads to the trans-cyclomer, trans-3. Bridged metal ion diradical complexes with Li or Mg (1 1) are diradical in spectra and chemistry. Extraction or distillation from the metal complexes yields a mixture of c -cyclomer A (cis-A-S) and tro 5-cyclomer (trans-3). 

Ugly facts, unfortunately, sometimes invalidate a beautiful mechanism. The Jacobsen epoxidation sometimes proceeds with loss of configurational purity of acyclic alkenes. This feature of the reaction can be explained by invoking radicals. Homolysis of the Mn C bond in the manganaoxetane intermediate would give a Mn(III) 1,4-diradical complex, and attack of the alkyl radical on O with displacement of Mn(II) would give the epoxide and regenerate the catalyst. 

The + + complex stability is dependent upon the nature of the anion and on temperature, with the activation energy for the disappearance of the complex (followed with the near infrared absorption) varying from 45 kcal/mole for the iodide to 26 kcal/mole for the tetraphenylborate. The epr signal disappears in parallel with the loss of the near infrared band. The mechanism for the loss fo the complex is explained as due to the dissociation of the initial con lex to a cation radical ( + ) which combines with another molecule of the initial complex to yield a diradical complex. (Eq. 15). Further changes in the spectrum ensue, which are presumed to result in the formation of higher conqplexes, with... 

The diradical is believed to be preceded on the reaction path by a complex of the alkene with excited-state ketone. This reaction, particularly its stereochemistry and regioselec-... 

In 1982 the present author discovered cyclic orbital interactions in acyclic conjugation, and showed that the orbital phase continuity controls acyclic systems as well as the cyclic systems [23]. The orbital phase theory has thus far expanded and is still expanding the scope of its applications. Among some typical examples are included relative stabilities of cross vs linear polyenes and conjugated diradicals in the singlet and triplet states, spin preference of diradicals, regioselectivities, conformational stabilities, acute coordination angle in metal complexes, and so on. 

The preceding orbital phase predictions of some topological units (like 1, 4-6) can be easily extended to more complex cychc diradicals [29], as shown in Fig. 12. On the basis of TMM sub-structure (1), diradicals 8-11 are predicted to be phase continuous in their triplet states. Such a triplet preference in their ground states is in agreement with calculation results and available experiments, as listed in Table... 

Fig. 12 The extension of some topological units into the more complex cychc jc-conjugated diradicals...

The anticancer activity of complex natural products having a cyclodecenediyne system [for a review see <96MI93>] has prompted the synthesis of 54 (X = CH2 and OCH2) <96CC749> and 55 (R = a-OH and p-OH) <95AG(E)2393> on the basis that such compounds are expected to develop anticancer activity as the P-lactam ring opens. This is because cycloaromatization can only occur in the monocyclic enediyne and the diradical intermediate in the cyclization is thought to be the cytotoxic species. 

In metal complexes of FTF5 and DPA, both single two-electron and double one-electron redox couples were observed. [(FTF5)Zn2] is diamagnetic, whereas the EPR spectmm of [(DPA)Zn2] was interpreted as that of a triplet that was complicated by aggregation. Why the ground electronic state of [(DPA)Zn2] is a triplet remains to be established a triplet porphyrin diradical was thought to be unique in porphyrin chemistry [LeMest et al., 1992]. 

While the structural and electronic properties of the dianions are consistent with the indicated Nin-enedithiolate description, two and four formulations are conceivable for the monoanion and the neutral complex, respectively. For the latter, one is a ligand diradical whose spins are antiferromagnetically coupled, consistent with diamagnetism. Numerous members of the series have been isolated and/or generated in solution by chemical or electrochemical reactions. Because redox potentials are markedly dependent on the nature of the R substituent, certain members of a given series have not been isolated in substance. 

Di-/-butyl-6-aminothiophenol gives complex (497) under strictly anaerobic conditions. In the presence of air, deep blue-violet solutions are obtained from which mixtures of solid deep blue-black cis/trans-(498) can be isolated.1345 For the square planar Ni complex, the cis-trans isomerization rate is fast. (498) is best described as a singlet diradical due to strong antiferromagnetic coupling of the benzosemiquinonate 7r-radicals.19, 134S... 

The platinum(0) complex [Pt(PhNO)(PPh3)2] reacts with C02 to afford the metallacyclic nitroso species [Pt 0N(Ph)C(0)0 (PPh3)2] (60), the first example of insertion of C02 into a Pt—N bond.186 Other unsaturated carbon compounds such as CS2 and electrophilic alkenes and alkynes react similarly. The diradical peril uoro-/V,/V -dimethylethane-l,2-bis(amino-oxyl) reacts readily by oxidative addition to the platinum(0) precursor Pt(PPh3)4 to afford the corresponding platinum(lI)-nitroso complex containing a seven-membered chelate ring (61). The resulting complex is stable in air for several days at room temperature.187... 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

Very recent calculations within the unrestricted ZORA-SORCI formalism on complexes indicated the neutral fe(l,2-dithiolene) complexes as consisting of two S,S-coordinated n radical monoanions strongly antiferro-magnetically coupled via the central diamagnetic Mn ion with a not negligible diradical character. In this context, the low-energy NIR transition should be considered as a LLCT band. See ref. 53. 

Exciplexesn6,nl) can be formed if the excitation energy B - B is higher than the one for A -> A in (1.8). Such an excited complex is associative in the excited state only, the corresponding ground state complex between A and B being dissociative (Fig. 9). Such exciplexes are important intermediates in e.g. cycloaddition reactions as precursors of diradicals 118) which are themselves precursors of the cyclized photoproducts. 

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