Cobalt(II) Complexes with N,N,N-Scorpionates and Bidentate Ligands: Comparison of Hydrotris(3,5-dimethylpyrazol-1-yl)borate Tp* vs. Phenyltris(4,4-dimethyloxazolin-2-yl)borate ToM to Control the Structural Properties and Reactivities of Cobalt Centers

Abstract

Scorpionate ligands Tp* (hydrotris(3,5-dimethylpyrazol-1-yl)borate) and ToM (tris(4,4-dimethyloxazolin-2-yl)phenylborate) complexes of cobalt(II) with bidentate ligands were synthesized. Both Tp* and ToM coordinate to cobalt(II) in a tridentate fashion when the bidentate ligand is the less hindered acetylacetonate. In crystal structures, the geometry of cobalt(II) supported by the N3O2 donor set in the Tp* complex is a square-pyramid, whereas that in the ToM complex is close to a trigonal-bipyramid. Both Tp*- and ToM-acac complexes exhibit solvatochromic behavior, although the changing structural equilibria of these complexes in MeCN are quite different. In the bis(1-methylimidazol-2-yl)methylphenylborate (LPh) complexes, Tp* retains the tridentate (к3) mode, whereas ToM functions as the bidentate (к2) ligand, giving the tetrahedral cobalt(II) complex. The bowl-shaped cavity derived from the six methyl groups on ToM lead to susceptibility to the bulkiness of the opposite bidentate ligand. The entitled scorpionate compounds mediate hydrocarbon oxidation with organic peroxides. Allylic oxidation of cyclohexene occurs mainly on the reaction with tert-butyl hydroperoxide (TBHP), although the catalytic efficiency of the scorpionate ligand complexes is lower than that of Co(OAc)2 and Co(acac)2. On cyclohexane oxidation with meta-chloroperbenzoic acid (mCPBA), both ToM and Tp* complexes function as catalysts for hydroxylation. The higher electron-donating ToM complexes show faster initial reaction rates compared to the corresponding Tp* complexes.

Organometallic 3d transition metal NHC complexes in oxidation catalysis

“…The catalytic performance of both complexes in the oxidation of cyclohexane was examined with mCPBA as the oxidant due to previous studies with comparable NHC free cobalt catalysts. [133][134][135] Compared to the NHC free cobalt catalysts Co II (acac) 2 , complex 19 exhibits enhanced selectivities towards alcohols at 25 °C (A/K = 8.7 and a TON of 540). 19 achieves an increased TOF of 1050 h −1 at 35 °C.…”

Tuning the O2 Binding Affinity of Cobalt(II) Centers by Changing the Structural and Electronic Properties of the Distal Substituents on Azole-Based Chelating Ligands

“…We have designed O 2 -binding mononuclear nonheme metallocomplexes involving “five azole-supported” transition-metal centers by combining facially capping tridentate N3 donors, hydrotris­(3,4,5-substituted-1-pyrazolyl)­borates (= [HB­(pz R ) 3 ] − ; Tp R ), , with bidentate N2 donors, bis­(1-methyl-2-imidazolyl)­borates (= [B­(Im N ‑Me ) 2 MeX] − ; L X ). − These chelating reagents act as monoanionic ligands composed of tetra-coordinated boron centers and azoles. Both ligands have great advantages for controlling the steric and electronic properties of the resulting metal complexes.…”