Broadly versus Barely Variable Complex Chromophores of Planar Nickel(II) from κ³-N,N′,C and κ³-N,N′,O Donor Platforms

Abstract

This work reports on complex chromophores of planar nickel(II) of the general formula [(κ 3 -N,N′,C/O)Ni−ER], which combines a κ 2 -N,N′-coordinate aromatic N-heterocycle with a pendant phenyl/phenol donor and the variable coligands ER (= carboxylato, phenolato, amido, and arylthiolato). The absorption properties of the planar complex chromophores vary broadly depending on the interplay of the three constituting components. Increasing the manifold of π*-orbitals at the N-heterocyclic moiety through π-extension along 1,10-phenanthroline, tetrahydrodipyridophenazine, and dipyridophenazine has only a small impact on the energies of charge transfer type transitions across the visible (vis) to near-infrared (NIR) spectral region and is due to an inversion among spatially distinct but energetically close unoccupied molecular orbitals. Substantial absorptivity across the visible spectral region rather associates with the increasing capability among carboxylato, phenolato, amido, and arylthiolato coligands to engage in Ni−O/N/S π-bonding. This adds a mechanism for electronic coupling of coligand RE − and π*-orbitals at the Nheterocycle fragment through occupied d orbitals at nickel, enabling light-induced charge transfer. Aryl-S − performs best in this regard, but {Ni−O/N/S}-to-(N-heterocycle-π*) charge transfer efficacy depends on donor−acceptor orbital alignment and appears to be kinetically hindered except for amido coligands owing to polar effects. Irrespective of the nature of coligand RE − , the choice of the pendant phenyl/phenol side arm rules overall absorptivity by setting the energy of occupied frontier molecular orbitals.

Role of the X Coligands in Cyclometalated [Ni(Phbpy)X] Complexes (HPhbpy = 6-Phenyl-2,2′-bipyridine)

“…The π-ligand-borne character of the virtual frontier MOs is conserved along the series [Ni­(Phbpy)­X], as is the energy of the essentially bpy-localized lowest unoccupied molecular orbital (LUMO). Very similar results have been recently also received with Ni complexes of related phenanthroline-derived C ∧ N ∧ N ligands . An excellent linear correlation of computed LUMO energies and experimental reduction potentials exists across a series of 17 planar Ni­(II) complexes (Figure S28 in the Supporting Information; additional data are from refs and ).…”

“…Very similar results have been recently also received with Ni complexes of related phenanthroline-derived C ∧ N ∧ N ligands . An excellent linear correlation of computed LUMO energies and experimental reduction potentials exists across a series of 17 planar Ni­(II) complexes (Figure S28 in the Supporting Information; additional data are from refs and ).…”

“…Computed metrics have been calibrated with experimental data, and an excellent match is generally observed (Table S6 in the Supporting Information). The elongated Ni–N bond located trans to the Ni–C bond is an artifact of the computation, as recently shown . Structures constrained to the experimental metrics are isoenergetic.…”

“…These Ni(0) intermediates were also observed for X = OMe, OTf, where the oxidative addition reaction leading to the target [Ni­(Phbpy)­X] complexes could not be initiated, even when the reaction mixture was heated under reflux (THF). This is remarkable in view of the recent reports of the successful nickelation of two closely related 4-( t Bu)-2-(1,10-phenanthrolin-2-yl)­phenol derivatives RO–( t Bu)­Phphen (R = triflate, pivalyl) using the same method at ambient temperature. , Presumably, the more rigid phenanthroline vs bipyridine precoordination and the t Bu group which probably stratifies the phenol group to the coordination plane in conjunction with its electron-releasing property are responsible for this success.…”

Sterically Encumbered Coordination Sites. Iron(II) Complexes of Jäger‐type ligands with a Terphenyl Backbone

“…Molecular orbitals and electronic properties were from single-point calculations in the optimised positions with the global hybrid functional TPSSh. [53] These settings provide reliable optical transition energies as shown recently for planar nickel(II) [21,54] and octahedral iron(II) complexes. [55] In order to assess the SCO energies, we used five derivatives of the well-established hybrid functional B3LYP [39,56] in single-point calculations.…”