Square-planar complexes radical formation

Studies have been carried out on the methylated complex [H3C-Niin(17)(H20)]2+, which is obtained from the reaction of methyl radicals (generated by pulse radiolysis) with [Ni(17)]2+. The volumes of activation are consistent with the coherent formation of Ni—C and Ni—OH2 bonds, as expected for the generation of a Ni111 complex from a square planar Ni11 precursor.152 The kinetics of reactions of [H3C-Niin(17)(H20)] + involving homolysis, 02 insertion and methyl transfer to Crn(aq) have been determined, and intermediates have been considered relevant as models for biological systems.153 Comparing different alkyl radicals, rate constants for the... 

Although only rarely luminescent in ambient fluid solutions, square-planar transition metal bis(dithiolene) complexes do display significant and varied photochemical reactivity. Much of the photoreactivity described above for dianionic bis(dithiolene) complexes involves excited-state oxidation and often leads to radical formation. In addition, the excited states of these complexes are receiving attention for their potential as materials for optical (15), nonlinear optical (10-13), and electrooptical (16) devices. The relevance of this work to those applications is addressed in other parts of chapter 8 in this volume (87b). 

The sterically crowded ligand (106) forms a very rare square-planar Co complex, as shown by an X-ray crystal structure, and a spin triplet (paramagnetic) ground state was also identified in the solid state. The Co complex of (107) catalyzes the epoxidation of norbomene with t-BuOOH or Phi as terminal oxidant, catalysis driven by formation of t-BuOO radicals employing a Co redox process. 

In the past, ESR studies on isoxazole(s) revealed the formation and structure of radical as well as the stereochemical information of metallic complexes. It has been reported that the metal appears to be 4-coordinate square planar in bis complexes and hexacoordinate in tetrakis complexes... 

A) Frozen solution EPR spectra of Fe (TPP)(4-MeIm) (top) and Fe (TPP)(4-MeIm)2 (bottom) prepared by addition of 4-methylimidazolate anion (4-MeIm ) to a solution of Fe(TPP)(SbF6). The top spectrum is characteristic of a high-spin Fe" -porphyrin complex, with a resonance atg = 6(g = 2.7, 2.3, and 1.8 are due to formation of a small amount of Fe" (TPP)(4-MeIm)2 ). The bottom spectrum is characteristic of a low-spin ferric-porphyrin bis(imidazole)-type complex.(B) Frozen solution EPR spectrum of Cu"(ImH)4-+ with gjl = 2.06, A = 183 G, and gj = 2.256 (courtesy of Dr. J. A. Roe). This type of spectrum is typical of square-planar Cu complexes, except that the ligand hyperfine splitting of the gi feature is frequently unresolved, especially in copper proteins (for example, see Figure 5.20). (C) Simulated EPR spectrum of a typical organic free radical with no hyperfine interaction. 

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