Electrocatalytic Proton Reduction by Dimeric Nickel Complex of a Sterically Demanding Pincer-type NS₂ Aminobis(thiophenolate) Ligand

Abstract

Basic methanolysis of a sterically hindered aminobis(S-arylthiocarbamate) affords a novel aminobis(thiophenolate) pincer-type ligand NS22–; the in situ generated dianion reacts cleanly with Ni2+ and Zn2+ resulting in dimeric complexes with bridging thiophenolate ligands, as determined spectroscopically and by X-ray crystallography. The C2-symmetric [Ni(NS2)]2 dimer (1) has a square planar coordination geometry around the Ni2+ ions, while the [Zn(NS2)]2 analogue (2) is characterized by a distorted tetrahedral geometry around each independent Zn2+ ion. Addition of the neutral monodentate donor L = 2,6-xylylisocyanide to [Ni(NS2)]2 affords the monomeric complex [LNi(NS2)] (3), which is characterized in the solid state by a square planar geometry with the isocyanide donor trans to the tertiary amine of NS2. The pincer NS2 ligand provides redox plasticity to 1, manifested in the accessibility of the putative Ni+Ni+ and Ni3+Ni3+ dimeric complexes, based on comparative cyclic voltammetry studies with 2 and 3. The redox properties of 1 endow it with hydrogenase-type activity, as evidenced in the electrocatalytic reduction of protons in a mixed aqueous/organic phase, as well as the oxidation of hydrides from NaBH(OAc)3. Both 1 and 3 are resilient under protic and oxidative conditions, as evidenced in reactivity tests monitored by UV–vis spectroscopy.

Conformational Effects of [Ni2(μ‐ArS)2] Cores on Their Electrocatalytic Activity

“…Those planes around the nickel centers in 1 , defined by N1, S1, S2, and S4 (N1, S1, S2, and S1 bridge for 2 ), are joined along one edge by means of S1 and S2 (S1 and S1 bridge for 2 , see Table and Figure ); therefore, for both complexes, each monomeric unit has a terminal thiophenolate ligand that may be considered electron‐rich (S4 and S3 atoms for 1 and S2 for 2 in Figure ) . Additionally, the two coordination planes around the nickel centers in 1 define a dihedral angle ( θ ) of 81.3(5)° (hinge angle), leading to a Ni⋅⋅⋅Ni distance of 2.7295(8) Å, which is comparable to that previously reported for [Ni 2 ( κ ‐ N,S,S,S′ ‐N iBu {CH 2 (MeC 6 H 2 R′)S} 2 ) 2 ] ( 4 ; R′=3,5‐(CF 3 ) 2 C 6 H 3 , with a Ni⋅⋅⋅Ni distance of 2.702(8) Å and θ =85.5(4)°) . On the other hand, in 2 , the hinge angle is θ =180(2)° (Table and Figure ), resulting in a coplanar arrangement with a considerably longer Ni⋅⋅⋅Ni distance of 3.364(4) Å.…”

Redox and Acid–Base Properties of Binuclear 4‐Terphenyldithiophenolate Complexes of Nickel

“…Castillo reported on an N , N ‐bis(2‐thiophenol)methylisopropylamine that binds Ni II in the form of a dimeric thiolate‐bridged complex. Electrochemical reduction (oxidation) of the di‐Ni II precursor in THF has been suggested to afford di‐Ni I (di‐Ni III ) species that initiate electrocatalytic proton reduction (evolution) from HBF 4 (borohydride), as shown in Scheme . Although neither of the Ni species has been substantially characterized, these findings add to the general notion that low‐ and high‐valent binuclear complexes of Ni play a role in a variety of reactions…”

“… Electrochemical generation of putative di‐Ni I or di‐Ni III species from a (μ‐S) 2 Ni II 2 complex for electrocatalytic reduction and evolution of protons …”

Binuclear complexes of Ni(i) from 4-terphenyldithiophenol

“…One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,20 and biphenyldiyl 15,21,22 ligands, or in the form of unsupported Ni-Ni bonds. One can differentiate between structural motifs in which electronic coupling of the Ni(I) ions either occurs through bonding to (i) bridging conjugated π-systems in both syn-and antarafacial fashion, [1][2][3][4][5][6][7][8] (ii) bridging amido, 9 phosphido, 10 thiolato, 11,12 sulfido, 13 halogenido, [14][15][16][17] and hydrido 18 ligands in the form of Ni 2 (μ-X) 2 cores, (iii) bridging diphosphine, 19,…”

“…2,5,9 The reversible formation of reactive Ni(I) species appears to represent one of the two major modes of reactivity displayed by binuclear Ni(I) complexes. [10][11][12] In the context of a reactivity study of thiolate complexes of Ni(I), Tatsumi and co-workers reported the structure of a binuclear Ni(I) complex with bridging arene and thiolate groups, see II in Scheme 1. 26 Despite the exact mechanism of this process remaining unresolved, dinickel(I) complexes have been reported to catalyse reductive C-C bond formation in a cooperative fashion, 15,21,22 to act as precursors in bimetallic catalytic group transfer reactions to form carbodiimides and isocyanates, 17,27 and to activate secondary silanes for the catalytic hydrosilation of unsaturated substrates.…”