Hydride-Bridged NiRh Complexes with Tunable N3S2 Dithiolato Ligands and Their Utilization as Catalysts for Hydrogenation of Aldehydes and CO2 in Aqueous Media

New N3S2 dithiolato ligands, 1,4-bis­(2-mercaptoethyl)-7-R-1,4,7-triazacyclononane (RTACN-S2H2; R = Ts, iPr) were synthesized and reacted with NiII ion to give mononuclear complexes [Ni­(RTACN-S2)] (R = Ts (1a), iPr (1b)). Complexes 1a and 1b were further transformed by treatment with RhIII species into a series of NiIIRhIII heterodimetallic compounds, [Ni­(RTACN-S2)­RhCp*X]­X′ (R = Ts, X = Cl, X′ = Cl, OTf ([2a]­X′); R = Ts, X = X′ = NO3 ([3a]­NO3); R = iPr, X = Cl, X′ = Cl, NO3, PF6 ([2b]­X′)). Complexes [3a]­NO3 and [2b]­NO3 were readily reacted with H2 (0.1 MPa) in water at room temperature to afford hydride-bridged NiIIRhIII dinuclear complexes, [(RTACN-S2)­Ni­(μ-H)­RhCp*]­NO3 (R = Ts ([4a]­NO3), iPr ([4b]­NO3)), which were successfully characterized by X-ray crystallography. Upon the heterolytic activation of H2, the Ni–Rh interatomic distances were dramatically decreased from 3.2130(6)–3.262(2) Å ([2a]­OTf, [2b]­NO3, [3a]­NO3) to 2.6921(6)–2.7228(4) Å ([4a]­NO3, [4b]­NO3), resulting in semibridging Ni­(μ-H)­Rh cores. In addition, the stability of the hydride of [4a]+ and [4b]+ were interestingly tuned by varying the R group of the TACN ligand through trans influence at the NiII center (Ni–NR = 2.188(3) Å ([4a]+), 2.056(2) Å ([4b]+). In fact, [4a]+ was quite stable and capable of reducing benzaldehyde in water, although [4b]+ quickly decomposed under similar conditions. Catalytic hydrogenation of aldehydes and CO2 in water has been established by using [3a]­NO3 and [2a]Cl as precursors of an active species, [4a]+, and found to be interestingly contrasted to the inactive precursor [2b]­NO3 with R = iPr. These results indicated that the properties and reactivity of the Ni­(μ-H)Rh complexes can be controlled by changing the substituents of the N3S2 supporting ligands.