Formazanate Complexes of Hypervalent Group 14 Elements as Precursors to Electronically Stabilized Radicals

Abstract

The stability of molecular radicals containing main-group elements usually hinges on the presence of bulky substituents that shield the reactive radical center. We describe a family of Group 14 formazanate complexes whose chemical reduction allows access to radicals that are stabilized instead by geometric and electron-delocalization effects, specifically by the square-pyramidal coordination geometry adopted by the Group 14 atom (Si, Ge, Sn) within the framework of the heteroatom-rich formazanate ligands. The reduction potentials of the Si, Ge, and Sn complexes as determined by cyclic voltammetry become more negative in that order. Examination of the solid-state structures of these complexes suggested that their electron-accepting ability decreases with increasing size of the Group 14 atom because a larger central atom increases the nonplanarity of the ligand-based conjugated π-electron system of the complex. The experimental findings were supported by density-functional calculations on the parent complexes and the corresponding radical anions.

Formazanate Complexes of Bis-Cyclometalated Iridium

“…[27][28][29] Formazanate complexes of group 14 elements and ruthenium highlight the redox non-innocence of some classes of formazanate complexes. 16,30 Furthermore, boron chelates of formazanates not only exhibit tunable redox properties but are also in many cases photoluminescent, [31][32][33][34][35][36] finding applications as cell-imaging agents, 33,37 electrochemiluminescence emitters, 38,39 multifunctional polymers, 40,41 and precursors to a wide range of BN heterocycles. 42,43 In spite of these numerous examples, coordination complexes of formazans with third-row transition metals remain rare.…”

Structure and bonding in reduced boron and aluminium complexes with formazanate ligands

“…[38][39][40][41] The stability of this type of radical extends to inorganic systems. 4,9,10,14,17,26,30,31,42 In order to examine the effect on the N-C(Bn) bond dissociation energy (BDE) due to the replacement of B to the more electropositive Al in Clean exponential decay of the starting material and concomitant appearance of TEMPO-Bn were observed, which allowed the rate constants to be determined (Fig. S8 †).…”

Formazanate coordination compounds: synthesis, reactivity, and applications

“…The first examples of group 14 formazanate complexes were reported in 2018. 99 After deprotonating 1,5-bis(2-hydroxyphenyl)-3phenylformazan with NaH to generate its trianion, the formazanate was combined with the respective phenyl trichloride of Si, Ge, and Sn (EPhCl 3 ) in THF to form complexes 88a-c (Scheme 32), which contain hypervalent (5-coordinate) group 14 atoms. Unlike closely related hypervalent group 14 complexes of dipyrrin N 2 O 2 3À ligands that adopted a trigonal bipyramidal geometry, 100 88a-c adopt a distorted square pyramidal geometry.…”

“…Synthesis of group 14 complexes of formazanate ligands 88a-c and their corresponding radical anions 88a-c À 99. …”