Karway

The synthesis and characterisation of the pentadentate ligand 1,4-di(picolyl)-7-(p-toluenesulfonyl)-1,4,7-triazacyclononane (Py₂^Tstacn) and their metal complexes of general formula [M(CF₃SO₃)(Py₂^Tstacn)][CF₃SO₃], (M=Fe (1_Fe), Co (1_Co) and Ni (1_Ni)) are reported. Complex 1_Co presents excellent H₂ photoproduction catalytic activity when using [Ir(ppy)₂(bpy)]PF₆ (PS_Ir) as photosensitiser (PS) and Et₃N as electron donor, but 1_Ni and 1_Fe result in a low activity and a complete lack of it, respectively. On the other hand, all three complexes have excellent electrocatalytic proton reduction activity in acetonitrile, when using trifluoroacetic acid (TFA) as a proton source with moderate overpotentials for 1_Co (0.59 V vs. SCE) and 1_Ni (0.56 V vs. SCE) and higher for 1_Fe (0.87 V vs. SCE). Under conditions of CH₃CN/H₂O/Et₃N (3:7:0.2), 1_Co (5 μM), with PS_Ir (100 μM) and irradiating at 447 nm gives a turnover number (TON) of 690 (n/n) and initial turnover frequency (TOF) (TON×t⁻¹) of 703 h⁻¹ for H₂ production. It should be noted that 1_Co retains 25 % of the catalytic activity for photoproduction of H₂ in the presence of O₂. The inexistence of a lag time for H₂ evolution and the absence of nanoparticles during the first 30 min of the reaction suggest that the main catalytic activity observed is derived from a molecular system. Kinetic studies show that the reaction is −0.7 order in catalyst, and time-dependent diffraction light scattering (DLS) experiments indicate formation of metal aggregates and then nanoparticles, leading to catalyst deactivation. By a combination of experimental and computational studies we found that the lack of activity in photochemical water reduction by 1_Fe can be attributed to the 1_Fe^II/I redox couple, which is significantly lower than the PS_Ir^III/II, while for 1_Ni the pK_a value (−0.4) is too small in comparison with the pH (11.9) imposed by the use of Et₃N as electron donor.

Cobalt is king! The Fe, Co, and Ni complexes based on the same ligand have shown excellent electrocatalytic H₂ activity (see figure). While only the Co complex has excellent H₂ photoproduction catalytic activity, when using [Ir(ppy)₂(bpy)]PF₆ (PS_Ir) as photosensitiser and Et₃N as electron donor. Analytic and kinetic studies show that the catalytic activity is molecular. The lack of photochemical activity of the Fe and Ni complexes is ascribed to their redox chemistry and the pK_a, respectively.

A new family of nickel(II) complexes of the type [Ni(L)(CH₃CN)](BPh₄)₂, where L=N-methyl-N,N′,N′-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N′,N′-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N′-bis(pyrid-2-ylmethyl)-N′-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N′-bis(pyrid-2-ylmethyl)-N′-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl-N,N′-bis(pyrid-2-ylmethyl)-N′-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L3)(CH₃CN)](BPh₄)₂ reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni^II/Ni^III redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1–10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH₃CN)](BPh₄)₂ by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH₃CN)](BPh₄)₂ or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH₃CN)](BPh₄)₂ and the quinolylmethyl arms ([Ni(L4)(CH₃CN)](BPh₄)₂, both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni^IIO^.] intermediate (ts1_hs, ts2_doublet), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni^II(L5)(OOCOC₆H₄Cl)]⁺ (5 a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6–11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni^II leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.

Interesting, interesting! Nickel(II) complexes with a strong π-backbonding ligand act as efficient catalysts for the oxidation of alkanes (see figure, m-CPBA=m-chloroperbenzoic acid) by stabilizing the NiO^. intermediate, whereas those with a better σ-donor ligand act as less efficient catalysts by destabilizing the reactive intermediate. The computed mechanism for cyclohexane hydroxylation reveals that a high-spin (S=3/2) [(L1/L5)Ni^IIO^.]⁺ species is the ground state.

A fundamental understanding of the mechanisms of both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in nonaqueous lithium–oxygen (Li–O₂) batteries is essential for the further development of these batteries. In this work, we systematically investigate the mechanisms of the ORR/OER reactions in nonaqueous Li–O₂ batteries by using electron paramagnetic resonance (EPR) spectroscopy, using 5,5-dimethyl-pyrroline N-oxide as a spin trap. The study provides direct verification of the formation of the superoxide radical anion (O₂^.−) as an intermediate in the ORR during the discharge process, while no O₂^.− was detected in the OER during the charge process. These findings provide insight into, and an understanding of, the fundamental reaction mechanisms involving oxygen and guide the further development of this field.

Chasing radicals: The fundamental understanding of the mechanisms for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in nonaqueous Li-O₂ batteries is essential for the further development of these batteries. Here, we systematically investigated the ORR/OER reaction mechanisms in nonaqueous Li-O₂ batteries using electron paramagnetic resonance spectroscopy.