Metal Coordinated Phenoxyl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 290

t -Bu t -Bu t -Bu

t -Bu R

t -Bu N

H

t -Bu X t -Bu

t -Bu H R

OH N OH OH

OH

H 2 qui L 14 AP

X=S: H 2 L 15 S
X=NH: H 3 AP L 15 AP
X=Se: H 2 L 15 Se

R=H: H 2 L 14 AP
R=F: H 2 F L 14 AP
R=CF 3 : H 2 CF3 L 14 AP
R=tBu: H 2 t Bu L 14 AP t -Bu

t -Bu

t -Bu

O OH

N

t -Bu

t -Bu H

OH O

t -Bu X X t -Bu

t -Bu H H t -Bu

OH OH

t -Bu

H 4 Et L 16 oxm

OH HO

H 4 mPh L 16 N

t -Bu t -Bu

X=S: H 2 Ph L 16 S
X=NH: H 4 Ph L 16 N

t -Bu NH HN t -Bu

t -Bu NH HN t -Bu

OH HO

OH HO

t -Bu t -Bu

t -Bu t -Bu

H 4 Ph2 L 16 AP H 4 Pr L 16 AP

Figure8.9 Representativeaminophenolligands.

the radical species. Many of them can be isolated and crystallized in their radical form thus contributing
widely to their success. Some representative ligands are depicted in Figure 8.9.

8.4.2 Vanadium complexes

Among the metals presented in this section, the vanadium ion is the most subject to a metal-centered redox
activity in the usual potential range. As an example, the oxidative chemistry of vanadium - phenolate com-
plexes of the TACN ligands H 3 L 3 tBu or H 3 L 3 tBu,OMe involves exclusively the V III /V II ,V IV /V III ,V V /V IV
redox couples. 59,60 The electrochemical behavior of complexes of ligands involving aminophenolate moi-
eties (i.e. moieties that could be oxidized much more easily than other phenolates into radicals), such as
H 2 L 15 S and H 2 L 15 Se , also suggests metal-centered oxidations to V V or V IV O. 61 H 2 L 14 AP = is an excep-

tion, since the radical species [V V (L 14 AP ) 2 (L 14 ISQ ) • ] has been evidenced by X-ray diffraction. 62 Although

the classical phenoxyl radical band at ≈ 400 nm (see above) is absent in the visible spectrum of this species,

/ 2 system with a g-value of 2.0045 (the V V ion is diamagnetic).
The absence of hyperfine splitting with the 51 V ion further precludes the formulation as a paramagnetic d 1
V IV

the EPR spectrum is classical for an S =

1

) •• 2 ] + , whereas at lower potentials
reduction of the o -iminobenzosemiquinone radical followed by reduction of the metal ion is observed
according to Equation 8.1:

0.10 V affords the diradical [V V (L 14 AP (L 14 ISQ ion. Oxidation at − )

[V III L 14 AP ) 3 ] 3 − [V IV L 14 AP ) 3 ] 2 − [V V L 14 AP ) 3 ] − [V V L 14 AP L 14 ISQ ) • ] ( ( ( ( ) 2 (

[V V L 14 AP L ISQ

) •• 2 ] + ( )(

(8.1)