Dioxygen activation by low valent cobalt complexes supported by ferrocenyl-substituted, redox active hydrotris(pyrazolyl)borate ligands

Sirianni, Eric R.
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University of Delaware
The successful synthesis and structural characterization of a new family of second and third generation ferrocenyl-substituted tris(pyrazolyl)borate ligands was achieved. These ligands employ ferrocenyl (Fc) moieties for increased C-H bond strengths as well as maintaining the steric demands necessary to remain "tetrahedral enforcers". The thallium(I) salts of the TP Fc* (1 ) and TpFc,Me* (2 ) ligands were isolated as N-confused asymmetric isomers. However, Tp Fc,iPr Tl (3 ) was isolated as the symmetric ligand as a dimer with an apparent Tl - Tl interaction. Cyclic voltammetry of 1, 2 , and 3 revealed single, reversible, three electron redox processes which suggests a lack of electronic communication between the ferrocenyl moieties when bound to a hydrotris(pyrazolyl)borate ligand. Conversely, TpCF3,Fc Tl (4 ) displays an irreversible redox process that is accompanied by decomposition during the oxidation wave of the CV. Thallium salts 1 and 2 undergo borotropic rearrangements to form the symmetric TpFc Tl (7 ) and Tp Fc,Me Tl (8 ), respectively. These ligands react with CoX 2 (X: Cl, I) to form their symmetric TpCoX complexes. Salt metathesis of 3 with CoI2 led to the isolation of TpFc,iPr CoI (13 ) as the only observable product. However, the reaction of 3 with CoCl2 yielded an N-confused Tp Fc,iPr* CoCl (14 ) complex. The electronic characterization of TpFc, (R: H, Me, i -Pr) has been sought through the synthesis of their corresponding copper(I) carbonyl complexes. Although the reaction of 7 and 8 with CuI under CO yields the symmetric TpFc Cu(CO) (16 ) and Tp Fc,Me Cu(CO) (17 ), respectively, the reaction of 3 with CuI under CO yielded the singly N-confused TpFc,iPr* Cu(CO) (19 ) and doubly N-confused TpFc,iPr** Cu(CO) (18 ) complexes. The TpFc,Me ligand has been employed in cobalt oxygen chemistry and has displayed its ability to resist C-H bond activation on the ligand. TpFc,Me Co(η3 -C3 H 5 ) (22 ) reacted with O2 to form an apparent TpFc,Me CoOOC3 H5 intermediate which underwent O - O bond homolysis to form TpFc,Me CoOH (21 ). 22 reacted with CO to form TpFc,Me Co(CO) (23 ). 23 reacts with O2 to form the side-on superoxo complex, TpFc,Me Co(O2 ), which has been structurally characterized by X-ray diffraction. Furthermore, the terminal imido complex, TpFc,Me CoNAd (25 ), displayed greater thermal stability than its analogous alkyl-substituted TptBu,Me CoNR complexes.