A multi-technique approach to unveil the redox behaviour and potentiality of homoleptic Cu^I complexes based on substituted bipyridine ligands in oxygenation reactions

Abstract

The effect of differently substituted 2,2’-bipyridine ligands (i.e. 6,6’-dimethyl-2,2’-bipyridine, 5,5’-dimethyl-2,2’-bipyridine, 6,6’-dimethoxy-2,2’-bipyridine and 2,2’-bipyridine) on the reversible oxidation of the resulting Cu(I) homoleptic complexes is investigated by means of a multitechnique...

Modulation of [CuOH/O]+ Properties in [2,2′-Bipyridine]2 Homoleptic Complexes through Substitution at the 6,6’ Position by Methyl Groups

“…Test calculations performed on CuBP cis BP trans -Me 2 H 2 , i.e., where one of the two chelating bpy molecules assumes the trans conformation (xyz coordinates of the optimized structure are listed in the Supporting Information, point B8), strongly suggest that this is the same structure that the complexes have in solution: in fact, the latter resulted to be less stable than CuBP-Me 2 H 2 of 31.0 kJ mol −1 . Also, the strong similarities observed between the Raman spectra recorded on dichloromethane solution and on the solid phase of the [Cu(2,2'-6,6′-Me 2 -bpy) 2 ] + [PF 6 ] − complex 5 S1, part b) but also the relative observed changes occurring between the two complexes.…”

“…All the Cu(I) complexes were synthesized by using the general procedure reported below and previously reported. 5 Synthesis and characterization of [Cu(2,2′-6,6′-Me 2 -bpy) 2 ] + [PF 6 ] − and [Cu-(2,2′-bpy) 2 ] + [PF 6 ] − were previously reported by our group. 5 The NMR spectra were recorded on a Jeol ECZ-R 600 MHz instrument, in deuterated acetone, using the residual solvent peak as an internal reference 1H: 2.05 ppm.…”

“…5 Synthesis and characterization of [Cu(2,2′-6,6′-Me 2 -bpy) 2 ] + [PF 6 ] − and [Cu-(2,2′-bpy) 2 ] + [PF 6 ] − were previously reported by our group. 5 The NMR spectra were recorded on a Jeol ECZ-R 600 MHz instrument, in deuterated acetone, using the residual solvent peak as an internal reference 1H: 2.05 ppm. 20 The chemical shifts are reported in delta (δ) units.…”

“…As a matter of fact, as it clearly emerged from the analysis of literature data concerning lightemitting electrochemical cells, 1−3 3 On passing to the well-known homoleptic complex [Cu(2,2′-6,6′-Me 2bpy) 2 ] + [PF 6 ] − , this showed very high performance as a redox mediator in dye-sensitized solar cells, the main reason being attributed to the low reorganization (flattening) occurring when the complex is reversibly oxidized to the [Cu(2,2′-6,6′-Me 2 -bpy) 2 ] 2+ [PF 6 ] − form compared to structural analogues (e.g., [Cu(2,2′-bpy) 2 ] 2+ [PF 6 ] − ), which is possibly due to steric hindrance from Me substituent groups at the 6,6' position. 4 It was recently shown by some of us 5 that such a feature can be exploited to employ the [Cu(2,2′-6,6′-Me 2 -bpy) 2 ] + [PF 6 ] − system (measured E 1/2 = 697 mV vs Ag + /Ag) as an active catalyst for allylic oxidation of cyclohexene by tert-butylhydroperoxide (hereafter tButOOH) in CH 2 Cl 2 (hereafter DCM) solutions; in fact, when contacted by tButOOH, [Cu(2,2'-6,6′-Me 2 -bpy) 2 ] + [PF 6 ] − transforms to oxidated species where Cu + passes to Cu 2+ , bearing OH/O groups that can be subsequently transferred to cyclohexene causing the backreduction of the oxidated complex to the original form containing Cu + . It is worth mentioning here that such a redox cycle was not observed for the [Cu(2,2′-bpy) 2 ] + [PF 6 ] − system (measured E 1/2 = 269 mV vs Ag + /Ag) and oxidation by tButOOH resulted in a quite fast but irreversible oxidative process, no back-reduction being observed after contacting with cyclohexene, so suggesting that [Cu(2,2′-bpy) 2 ] + [PF 6 ] − cannot be adopted as a useful catalytic system.…”

Copper and Zinc Complexes of 2,7-Bis(6-methyl-2-pyridyl)-1,8-naphthyridine─A Redox-Active, Dinucleating Bis(bipyridine) Ligand

“…As part of our work on dinucleating naphthyridine systems, we have revisited the chemistry of 2,7-bis­(2-pyridyl)-1,8-naphthyridine ,, ( H L ) and found that it binds only weakly and reversibly to Zn . Herein, we report the derivative 2,7-bis­(6-methyl-2-pyridyl)-1,8-naphthyridine ( Me L , Figure a), in which the inclusion of 6-Me substituents on the pyridyl groups is envisaged to (i) increase the donor strength, (ii) increase the steric presence proximal to the metal binding site, (iii) improve solubility, and (iv) enhance redox activity of the resulting complexes. − In polypyridyl ligand systems, such as 2,2′-bipyridine and 1,10-phenanthroline, substitution adjacent to the nitrogen donors is known to have a significant impact on the redox properties of the system. − For example, oxidation and reduction of [Cu­(phen) 2 ] + (phen = 1,10-phenanthroline) are reported to show poor electrochemical reversibility, whereas the 2,9-dimethyl analogue [Cu­(dmphen) 2 ] + (dmphen = 2,9-dimethyl-1,10-phenanthroline) exhibits reversible oxidative and reductive processes involving the Cu I /Cu II redox couple and the dmphen π-system, respectively . The inclusion of 2,9-substituents restricts geometric changes upon oxidation to Cu II , which is manifest in a positive shift in the oxidation potential …”