Solution, Solid, and Gas Phase Studies on a Nickel Dithiolene System: Spectator Metal and Reactor Ligand

The syntheses of cationic nickel
complexes using N,N′-dimethyl
piperazine 2,3-dithione (Me2Dt0) and N,N′-diisopropyl piperazine 2,3-dithione
(iPr2Dt0) ligands
are reported. These ligands were used in synthesizing bis and tris­(dithione)­Ni­(II)
complexes as tetrafluoroborate or hexafluorophosphate salts, i.e.,
[Ni­(iPr2Dt0)2]­[BF4]2 ([1a]­[BF4]2), [Ni­(iPr2Dt0)2]­[PF6]2 ([1a]­[PF6]2), [Ni­(Me2Dt0)2]­[BF4]2 ([1b]­[BF4]2), [Ni­(iPr2Dt0)3]­[BF4]2 ([2a]­[BF4]2), and [Ni­(iPr2Dt0)3]­[PF6]2 ([2a]­[PF6]2), respectively. Complex
[2a]­[PF6]2 was isolated from a
methanolic solution of [1a]­[PF6]2. Compound [1a]­[BF4]2 crystallizes
in a trigonal crystal system (space group, P31/c) and exhibits unique packing features,
whereas [2a]­[BF4]2 crystallizes
in a monoclinic (P21/n) space group. Cyclic voltammograms of [1a]­[BF4]2 and [1b]­[BF4]2 are
indicative of four reduction processes associated with stepwise single-electron
reduction of the ligands. Spectroelectrochemical experiments on [1a]­[BF4]2 exhibit an intervalence charge
transfer (IVCT) transition as a spectroscopic signature of the mixed-valence
[Ni­(iPr2Dt0)­(iPr2Dt1–)]− species. Analysis of this IVCT band suggests that
this ligand based mixed valence complex, [Ni­(iPr2Dt0)­(iPr2Dt1–)]−, behaves
more like a traditional class II/III metal based mixed-valence complex.
The density functional theory (DFT) and time dependent DFT calculations
provide a theoretical framework for understanding the electronic structures
and the nature of excited states of the target compounds that are
consistent with their spectroscopic and redox properties. Vibrational
spectra of [1a]2+ and [2a]2+ were investigated as discrete species in the gas phase using
infrared multiple photon dissociation (IRMPD) spectroscopy.