Computational studies of a paramagnetic planar dibenzotetraaza[14]annulene Ni(II) complex

Hassan Rabaâ; Hamid Khaledi; Marilyn M Olmstead; Dage Sundholm

doi:10.1021/jp509824z

Computational studies of a paramagnetic planar dibenzotetraaza[14]annulene Ni(II) complex

J Phys Chem A. 2015 May 28;119(21):5189-96. doi: 10.1021/jp509824z. Epub 2015 Jan 5.

Authors

Hassan Rabaâ¹, Hamid Khaledi², Marilyn M Olmstead³, Dage Sundholm⁴

Affiliations

¹ †Department of Chemistry, University Ibn Tofail, P.O. Box 133, 14000 Kenitra, Morocco.
² ‡Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia.
³ §Department of Chemistry, University of California, Davis, California 95616, United States.
⁴ ∥Department of Chemistry, P.O. Box 55 (A. I. Virtanens plats 1), FI-00014 University of Helsinki, Helsinki, Finland.

PMID: 25531241
DOI: 10.1021/jp509824z

Abstract

A square-planar Ni(II) dibenzotetraaza[14]annulene complex substituted with two 3,3-dimethylindolenine groups in the meso positions has recently been synthesized and characterized experimentally. In the solid-state, the Ni(II) complex forms linear π-interacting stacks with Ni···Ni separations of 3.448(2) Å. Measurements of the temperature dependence of the magnetic susceptibility revealed a drastic change in the magnetic properties at a temperature of 13 K, indicating a transition from low-to-high spin states. The molecular structures of the free-base ligand, the lowest singlet, and triplet states of the monomer and the dimer of the Ni complex have been studied computationally using density functional theory (DFT) and ab initio correlation levels of theory. In calculations at the second-order Møller-Plesset (MP2) perturbation theory level, a large energy of 260 kcal mol(-1) was obtained for the singlet-triplet splitting, suggesting that an alternative explanation of the observed magnetic properties is needed. The large energy splitting between the singlet and triplet states suggests that the observed change in the magnetism at very low temperatures is due to spin-orbit coupling effects originating from weak interactions between the fine-structure states of the Ni cations in the complex. The lowest electronic excitation energies of the dibenzotetraaza[14]annulene Ni(II) complex calculated at the time-dependent density functional theory (TDDFT) levels are in good agreement with values deduced from the experimental UV-vis spectrum. Calculations at the second-order algebraic-diagrammatic construction (ADC(2)) level on the dimer of the meso-substituted 3,3-dimethylindolenine dibenzotetraaza[14] annulene Ni(II) complex yielded Stokes shifts of 85-100 nm for the lowest excited singlet states. Calculations of the strength of the magnetically induced ring current for the free-base 3,3-dimethylindolenine-substituted dibenzotetraaza[14]annulene show that the annulene ring is very weakly antiaromatic, sustaining a paratropic ring-current strength of only -1.7 nA/T.