A cobalt–nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen

Abstract

The cobalt-nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3), and shows a linear Co-N-O unit. This complex was prepared by the reaction of the potassium salt of L3 with the cobalt-nitrosyl precursor [Co(NO)(tmeda)](BPh) (tmeda = N,N,N,'N'-tetramethylethylenediamine). The obtained cobalt-nitrosyl complex as well as the corresponding products from the reaction with dioxygen, [Co(η-ON)(L3)] and [Co(η-ONO)(L3)], were characterised by X-ray crystallography and a number of spectroscopic methods including IR/far-IR, UV-Vis, and NMR spectroscopy. We also performed MCD measurements and DFT calculations to further elucidate the electronic structure of [Co(NO)(L3)] and the optical properties of the complex. The MCD spectra reveal two NO-to-Co charge-transfer transitions with strong excited state displacements that give rise to vibrational progressions in the MCD spectra, indicative of a very covalent Co-NO bond. These results provide new insight into the properties of the Co-NO bond and the electron distribution in the complex, which is best described as [Co(NO)(L3)].

Structures and Properties of Dinitrosyl Iron and Cobalt Complexes Ligated by Bis(3,5-diisopropyl-1-pyrazolyl)methane

“…If the reaction with dioxygen is conducted in the solid state, [Fe(κ 2 -O2NO)(L3)] is now the main product and a small amount of [Fe(κ 2 -O2N)(L3)] is also formed. These reactivity patterns are in agreement with our [Co(NO)(L3)] results [33].…”

“…Therefore, [Co(NO) 2 (L1")](BF 4 ) is diamagnetic. This diamagnetic magnetic property of other {Co(NO) 2 } 10 dinitrosyl complexes is observed in (Et 4 N)[Co(NO) 2 (SPh) 2 ] [40], [Co(NO) 2 (Ar-nacnac)] [23], and [Co(NO) 2 (tmeda)](BPh 4 ) [33,41]. The origin of this diamagnetism has not been noted, to our best knowledge.…”

“…The origin of this diamagnetism has not been noted, to our best knowledge. However, from the detailed computational calculations of the mononitrosyl cobalt complexes {Co(NO)} 9 , we can see that this magnetism comes from the strongly antiferromagnetic interaction between d-electrons of the cobalt(II) center (S Co = 3/2) and the π* electrons of the triplet 3 NO − (S NO = 1), resulting in a total spin for the complex of S t = 1/2 in [Co(NO)(L3)] [33] and [Co(NO)(L0)] [42]. This suggests that the diamagnetic property in {Co(NO) 2 } 10 dinitrosyl complex [Co(NO) 2 (L1")](BF 4 ) can be described as a high-spin cobalt(II) center (S Co = 3/2) antiferromagnetically coupled to an overall (NO) 2 − (S NO = 3/2) or a high-spin cobalt(III) center (S Co = 2) antiferromagnetically coupled to two 3 NO − (each with S NO = 1).…”

“…In our previous works, a series of first row transition metal complexes with NO, [M(NO)(L3)] (M = Fe, Co, Ni, and Cu), were prepared, structurally characterized and ligated by the same hydrotris(pyrazolyl)borate coligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3 − , Scheme 1 left) [32][33][34][35]. Our synthetic strategy is to use exactly the same hindered supporting ligand to make four coordinate mono-nitrosyl transition metal complexes to directly compare their electronic properties and reactivities.…”

“…In the dinitrosyl cobalt complexes, the {Co(NO) 2 } 10 oxidation state is commonly encountered, as listed in Table 2 [23,33,[39][40][41][42][43]. Regarding the electronic structure of {Co(NO) 2 } core in the complexes, the reported {Co(NO) 2 } 10 core is of diamagnetic property (S t = 0) [23,33,40,41]. Moreover, there is the relatively wide range ν(N-O) frequency from 1900 cm −1 to 1700 cm −1 [23,39,40].…”