Metal-based radicals

A homolysis of metal-ligand or polar metal-metal bonds can be achieved by LMCT or MMCT excitation, respectively. The metal-based radicals which are thus obtained undergo a variety of secondary reactions. Many of these intermediates are kinetically labile and yield decomposition or substitution products. Moreover, frequently they are strong reductants or oxidants and engage in redox reactions. Owing to their radical nature they participate in radical coupling processes including recombination reactions. These typical properties of such metal-based radicals are illustrated by a few examples. 

LMCT excitation of Co (NH3)5X2 with X = halide or pseudohalide yields the complex fragment Co (NH3)52+ which is kinetically very labile [2,5,94]. Co(II)-ammine complexes decompose in aqueous solution to Co + and ammonia with a rate constant k lO s [95]. The LMCT photolysis of PtCl52- and other Pt(IV) complexes leads to the formation of Pt L5 intermediates [111-113] which are also kinetically labile and undergo facile ligand substitutions. Since these Pt(III) radicals can efficiently exchange electrons with Pt(IV) they catalyze the substitution of inert Pt(IV) complexes in a chain reaction [2,10]. The overall quantum yield for photosubstitution of PtClg - was shown to exceed unity. 

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In this study, the metal centred radicals were formed indirectly by the photolysis of (Me3CO)2 yielding Me3CO radicals which then abstracted a hydrogen atom from the (n-Bu)3MH (M = Ge or Sn). The resulting metal based radical reacts with the carbonyl compound by adding to the oxygen atom (reaction 3). 

The Pd—Pd bond can be cleaved using UV light. UV irradiation of a frozen solution of (5) in an EPR cavity indicated the presence of a metal-based radical with g — 2.12. Irradiation of a solution of (5) in CX4 (X = C1, Br) gave [PdnX(CNMe)3]PF6 and with [Pt2(CNMe6)](PF6)2 present the heterobimetallic complex [PdPt(CNMe)6](PF6)2 was observed."... 

The 17e metal-based radical Ta 3(CO)4(dppe) was formed via hydrogen atom extraction from TaIH(CO)4(dppe).659 In solution, this radical abstacted halogen atoms from many organic halides RX to give TaIX(CO)4(dppe). Ta°(CO)4(dppe) existed in solution as an equilibrium mixture of monomer and dimer [(dppe)(CO)3Ta](/i2-CO)2. The latter is the form stable in the solid state. While no crystal structures are available, DFT calculations indicated a stable pseudo-octahedral stereochemistry for the monomer and several possible (/i2-CO)2 structures for the dimer.656 Each of the latter featured linear, semi-bridging carbonyls supporting a weak, delocalized Ta—Ta interaction. 

The alkylzirconium(m) octaethylporphyrin complex, (OEP)ZrCH2SiMe3 1, was prepared from the dialkylzirconium(rv) complex by reduction with H2 (1 atm) in toluene at 20 °C (Scheme 1). This reaction therefore appears to be a rather rare example of the chemical reduction of Zr(rv) to Zr(m) by H2. The structure of 1 was elucidated by single crystal X-ray diffraction and has a Zr-C bond length of 2.216(8) A. Although this complex formally contains zirconium in oxidation state hi, careful consideration of the structural and spectroscopic data led the authors to conclude that this was an overly simplistic view. At 77 K, an EPR signal typical of a metal-centered radical was observed, while no signal was detected at 293 K. The UV/Vis spectrum of 1 contains bands typical of a porphyrin anion. The electronic structure of 1 is therefore better described as a combination of two resonance forms a Zr(m) metal-based radical, and a zwitterionic form with a positively charged Zr(iv) center and a porphyrin radical anion. 

The ESR spectra of the anions are particularly interesting. Both isomers of [Co(t/4-cod)Cp] are metal-based radicals but [Co(t/4-1,3-cot)Cp] - in frozen thf shows a more nearly isotropic spectrum with the singly occupied orbital possessing little cobalt d character. MO calculations are also consistent with localization of the free electron on the unbonded diene unit of [Co(r 4-l,3-cot)Cp] [cf. [Fe(CO)3(f/4-cot)], Section VI], but on the metal of [Cofa4-cod)Cp]" (255). 

Indeed, the neutral radicals can be prepared directly from nickelocene and [NiLJ (575). The ESR spectrum of [Ni(t/4-cod)Cp], generated by electrolytic reduction of the cation (E° = —0.46 V in CH2C12) at — 10°C, is consistent with a metal-based radical (575), and the 19-electron configuration was verified by the X-ray structure of the analog [Ni(bipy)Cp] (378). 

Chemically mduced dynamic nuclear polarization (cidnp) arises in some radical pair processes and is characterized by intensity inversion and enhancement of some of the resonances in the nmr spectra of radical combination products during a reaction.This provides definitive evidence for the involvement of radicals, but as with ESR, it is difficult to quantitatively estimate the fraction of the total reaction proceeding via the radical path. The conditions under which a cidnp effect will be observed are rather limited, and it has only been observed in a few cases with organometallics. In particular, situations in which a metal-based radical can be implicated are extremely rare. Nevertheless, with quantitative... 

Use of metal complexes with weaker Mt -H bonds (i.e., leading to a more stabilized metal-based radical) led to chain transfer catalytic activity in MMA polymerization. The penta-phenylcyclopentadienyl compound (C5Ph5)Cr (CO)3, XXXVIIIa, is a stable radical (no tendency to form dimer XLa) and the corresponding hydride complex (C5Ph5)Cr (CO)3H, XXXIXa, is also available (Figure 29). The reaction of XXXIXa with a large excess of MMA produces low-Mn PMMA, rather than hydrogenated MMA. The same process is initiated by thermal AIBN... 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

Most of the salts based on decamethylmetallocenium radical cations and on planar metal bisdichalcogenate radical anions reported so far present crystal structures with mixed linear chain basic motives. The only known exception is [Fe(Cp )2][Ni (mnt)2], which exhibits another type of crystal structure based on a D+ [A2]2 D+ repeat unit [28]. In the case of this compound the magnetic behavior is dominated by the intradimer antiferromagnetic interactions. 

Furthermore Catalytic effects produced by acids, bases, radical-producing compounds, metals and metal salts need to be considered. 

Scheme 6.27 considers other, formally confined, conformers of cycloocta-l,3,5,7-tetraene (COT) in complexes with metals. In the following text, M(l,5-COT) and M(l,3-COT) stand for the tube and chair structures, respectively. M(l,5-COT) is favored in neutral (18-electron) complexes with nickel, palladium, cobalt, or rhodium. One-electron reduction transforms these complexes into 19-electron forms, which we can identify as anion-radicals of metallocomplexes. Notably, the anion-radicals of the nickel and palladium complexes retain their M(l,5-COT) geometry in both the 18- and 19-electron forms. When the metal is cobalt or rhodium, transition in the 19-electron form causes quick conversion of M(l,5-COT) into M(l,3-COT) form (Shaw et al. 2004, reference therein). This difference should be connected with the manner of spin-charge distribution. The nickel and palladium complexes are essentially metal-based anion-radicals. In contrast, the SOMO is highly delocalized in the anion-radicals of cobalt and rhodium complexes, with at least half of the orbital residing in the COT ring. For this reason, cyclooctateraene flattens for a while and then acquires the conformation that is more favorable for the spatial structure of the whole complex, namely, M(l,3-COT) (see Schemes 6.1 and 6.27). 

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