Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes

Abstract

Transition metal halide complexes are used as precursors for reductive N2 activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N2 splitting with the rhenium­(III) complexes [ReX2(PNP)] (PNP = N­(CH2CH2PtBu2)2 and X = Cl, Br, I) is presented. As previously reported for the chloride precursor (J. Am. Chem. Soc.20181407922), the heavier halides give rhenium­(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N2-bridged complexes, [{ReX­(PNP)}2(μ-N2)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and ReIII/ReI comproportionation, the iodide system also reacts via an alternative ReII/ReII-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N2 activation that was observed for the chloride precursor. Computational analysis of the relevant ReIII/II and ReII/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re–X bonding interactions over contributions from charge transfer.

Importance of Ligand Exchange in the Modulation of Molecular Catalysis: Mechanism of the Electrochemical Reduction of Nitrous Oxide with Rhenium Bipyridyl Carbonyl Complexes

“…This additional reduction regenerates the bi-reduced species [Re 0 (bpy •– )­(CO) 3 ] − via a “proton-coupled electron transfer”-type reaction (“PCET”, vide infra in the Discussion section), and catalysis can be sustained as summarized on Scheme . This process parallels the required PCETs to release NH 3 from metal nitride complexes in nitrogen reduction reaction catalyzed by molecular catalysts. − …”

“…The homogeneous electrochemical reduction of CO 2 to CO by the molecular catalyst [Ni­(cyclam)] 2+ provides a typical example in which the rate-limiting step during catalysis is CO decoordination from the deactivated intermediate [Ni­(cyclam)­(CO)] + as shown by Kubiak and co-workers . Even more drastic is the case of N 2 splitting by electrogenerated reduced rhenium or molybdenum complexes leading to stable nitride complexes, which then would require additional overpotential to release NH 3 via three proton-coupled electron transfers. − In other cases, catalysis is not inhibited but only self-modulated as shown for the reductive cleavage of chloroacetonitrile-catalyzed Co­(I) cobalamins and cobinamides where the production of free chlorides, as catalysis goes on, shifts the generation of the Co­(I) active species toward more negative potentials, thus slowing down catalysis . Due to their ubiquity, it is of prime importance to identify and analyze these phenomena in order to fully understand catalytic processes and, more importantly, provide strategies to avoid or delay catalyst deactivation or even design new reactions …”

(Electro)chemical N2 Splitting by a Molybdenum Complex with an Anionic PNP Pincer-Type Ligand

“…Overreduced Re­(I) and Re­(III) form the key Re­(II)–N 2 –Re­(II) intermediate, which splits N 2 to form the Re­(V) nitrido complex. Changing from chlorido to bromido and iodido ligands leads to less negative potentials for N 2 -splitting along the halide series . In contrast to the chlorido system, the iodido system II I reacts via a Re II /Re II -dimerization mechanism, due to the absence of the potential inversion after reduction and N 2 binding.…”

“…Changing from chlorido to bromido and iodido ligands leads to less negative potentials for N 2 -splitting along the halide series. 24 In contrast to the chlorido system, the iodido system II I reacts via a Re II /Re II -dimerization mechanism, due to the absence of the potential inversion after reduction and N 2 binding. Utilizing the analogous unsaturated PNP ligand (III) increases the potential for N 2 -splitting compared to II Cl with the saturated PNP ligand, but the nitrido complex was formed in a considerable lower yield (Scheme 1).…”

To Split or Not to Split: [AsCCAs]-Coordinated Mo, W, and Re Complexes and Their Reactivity toward Molecular Dinitrogen

“…For tungsten, however, only three N 2 splitting reactions have been discovered until now, suggesting that tungsten is not as privileged as molybdenum, at least with respect to this particular transformation. For the neighboring group 7 element, the groups of Schneider, Miller, and Holland clearly demonstrated that different [PNP] rhenium complexes may be employed to cleave N 2 , while related [PCP] or [PPP] rhenium derivatives have not yet been shown to exhibit this type of reactivity.…”