A series of bioinspired polar atrane Cu−Al complexes were studied with a combined experimental and computational approach to assess the range and nature of Cu−Al interactions in these novel species. The aluminum metalloligand [Na{Me 2 Al(OPy-6-Me) 2 }] (2) was furnished in excellent yield (92%) from the nucleophilic attack of Na(OPy-6-Me) to AlMe 3 and the subsequent alkane elimination reaction with 6-methyl-2-hydroxypyridine. At the same time, the metalloligand [Al(OPy-6-Me) 3 ] (3) was isolated in an also excellent yield (95%) via alkane elimination of AlMe 3 with 6-methyl-2-hydroxypyridine. The zwitterionic Cu−Al atranes [Cu{MeAl(OPy-6-Me) 3 }] (5 Me ) and [Cu{MesAl(OPy-6-Me) 3 }] (5 Mes ) were isolated (73 and 97% yields) from metalloligands 2 and 3, respectively. [(Cu{Al(OPy-6-Me) 4 }) 2 (μ-Cu)] + ([6 + ][B(Ar CF3 ) 4 ]) was isolated via a reaction that involves alkane elimination and redistribution reacting from 5 Me with [H(OEt 2 ) 2 ][B(Ar CF3 ) 4 ] in benzene solution. Alkane elimination in benzene of either 5 Me or 5 Mes with [HNEt 3 ][B(Ar CF3 ) 4 ] renders [Cu{(Et 3 N)Al(OPy-6-Me) 3 }] + (Et 3 N-5 + ). The Lewis base-free cationic complex [Cu{Al(OPy-6-Me) 3 }] + (5 + ) was isolated in 68% yield upon reacting 3 with [Cu(COD) 2 ][B(Ar CF3 ) 4 ] in benzene. Metalloligands and complexes were fully characterized with an array of spectroscopic and analytical techniques that include multinuclear NMR, ATR-IR, ESI-spectrometry, combustion microanalysis, cyclic voltammetry (CV), and, whenever feasible, SCXRD. X-ray and DFT parameters indicate that the strength of the Cu→Al transannular interaction follows the trend 5 + > Et 3 N-5 + > [6 + ][B(Ar CF3) 4 ], 5 Me , and 5 Mes in a smooth transition from zwitterionic species where the Cu−Al interaction is nonexistent to moderate Cu−Al Z-type interactions. CV, in conjunction with DFT calculations of Et 3 N-5 + and 5 + , hint at the generation in the electrochemical cell of the radical species 5 rad at −1.82 V and the anionic complex 5 − at −2.32 V vs Fc/Fc + , respectively. The proposed species 5 rad exhibits 2-center/1-electron (2c/1e) σ bonding whereas 5 − a 2-center/2-electron (2c/2e) bond.

Five neutral nickel(II) bimetallic models of the active site of [NiFe]-hydrogenase supported by tridentate sulfur-rich RNS 2 ligands, were synthesized and tested as electrocatalysts for proton (H + ) reduction. Complexes were classified according to the À NR substituent (1: 1-methylpyrene; 2: 2-methylthiophene; 3: phenyl) and as type a for those without bulky substituents and type b for the analogues with voluminous groups. Solid state structures were determined for three dimers, revealing [Ni 2 (μ-SAr) 2 ] frameworks, in which the two coordination planes around the Ni centres define a dihedral angle (θ) that is influenced by the substituents on the ligands (2 a:Using CF 3 COOH as H + source, 1 b and 2 b exhibit catalytic activity at À 1.72 V (i cat /i p � 2.40) and À 1.80 V (i cat /i p � 2.89) vs the ferrocenium/ferrocene couple (Fc + / Fc), respectively. In contrast, type a complexes were not viable catalysts. This behaviour suggests a relationship between the dimer conformation and its activity, due to a Ni ••• Ni cooperative effect, which is favoured in angular molecules and appears to assist during electrocatalytic H + reduction.

Two nickel complexes supported by tridentate NS2 ligands, [Ni2(κ‐N,S,S,S′‐NPh{CH2(MeC6H2R′)S}2)2] (1; R′=3,5‐(CF3)2C6H3) and [Ni2(κ‐N,S,S,S′‐NiBu{CH2C6H4S}2)2] (2), were prepared as bioinspired models of the active site of [NiFe] hydrogenases. The solid‐state structure of 1 reveals that the [Ni2(μ‐ArS)2] core is bent, with the planes of the nickel centers at a hinge angle of 81.3(5)°, whereas 2 shows a coplanar arrangement between both nickel(II) ions in the dimeric structure. Complex 1 electrocatalyzes proton reduction from CF3COOH at −1.93 (overpotential of 1.04 V, with icat/ip≈21.8) and −1.47 V (overpotential of 580 mV, with icat/ip≈5.9) versus the ferrocene/ferrocenium redox couple. The electrochemical behavior of 1 relative to that of 2 may be related to the bent [Ni2(μ‐ArS)2] core, which allows proximity of the two Ni⋅⋅⋅Ni centers at 2.730(8) Å; thus possibly favoring H+ reduction. In contrast, the planar [Ni2(μ‐ArS)2] core of 2 results in a Ni⋅⋅⋅Ni distance of 3.364(4) Å and is unstable in the presence of acid.