Molecular and electronic structure of square-planar nickel II, nickel III and nickel III pi-cation radical complexes with a tetradentate o-phenylenedioxamidate redox-active ligand.
The molecular and electronic structures of the electron transfer series of four-coordinate square-planar nickel complexes with the ligand o-phenylenebis(N'-methyloxamidate), [NiL]z (z = 2-, 1-, 0), have been evaluated by DFT and TDDFT calculations, and most of their experimentally available structural and spectroscopic properties (X. Ottenwaelder et al., Dalton Trans., 2005, DOI: 10.1039/b502478a) have been reasonably reproduced at the B3LYP level of theory. The anionic species [NiL]2- and [NiL]- are genuine low-spin nickel II and nickel III complexes with diamagnetic singlet (S = 0) and paramagnetic doublet (S = 1/2) states, respectively. The nickel III complex presents shorter Ni-N(amidate) bond distances (1.85-1.90 A) than the parent nickel II complex (1.88-1.93 A) and characteristic LMCT bands in the NIR region (lambda max = 794 and 829 nm) while the analogous MLCT bands for the nickel(II) complex are in the UV region (lambda max = 346 and 349 nm). The neutral species [NiL] is a nickel III o-benzosemiquinonediimine pi-cation radical complex with a diamagnetic singlet (S = 0) and a paramagnetic triplet (S = 1) states fairly close in energy but fundamentally different in orbital configuration. The singlet metal-radical ground state results from the antiferromagnetic coupling between the 3d(yz) orbital of the Ni III ion (S(M) = 1/2) and the pi(b) orbital of the benzosemiquinone-type radical ligand (S(L) = 1/2), which have a large overlap and thus strong covalent bonding. The triplet metal-radical excited state involves the ferromagnetic coupling between the Ni III 3d(zx) orbital and the benzosemiquinone-type pi(b) orbital, which are orthogonal to each other. The singlet and triplet states of the nickel III pi-cation radical complex possess characteristic quinoid-type short-long-short alternating sequence of C-C bonds in the benzene ring, as well as intense MLCT transitions in the VIS (lambda max = 664 nm) and NIR (lambda max = 884 nm) regions, respectively.
Carrasco
Rosa
R
Departament de Química Inorgànica, Centre de Recerca en Química Teòrica (CERQT), Universitat de Barcelona, 08028, Barcelona, Spain.
Cano
Joan
J
Ottenwaelder
Xavier
X
Aukauloo
Ally
A
Journaux
Yves
Y
Ruiz-García
Rafael
R
D002412:Cations / Q000737:chemistry; D004583:Electrons; D005609:Free Radicals / Q000737:chemistry; D008024:Ligands; D008956:Models, Chemical; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010070:Oxalates / Q000737:chemistry*; D010084:Oxidation-Reduction; D010655:Phenylenediamines / Q000737:chemistry*; D011789:Quantum Theory
C516302:2-phenylenebis(N'-methyloxamidate); D002412:Cations; D005609:Free Radicals; D008024:Ligands; D009942:Organometallic Compounds; D010070:Oxalates; D010655:Phenylenediamines; D009532:Nickel