Flexibility is Key: Synthesis of a Tripyridylamine (TPA) Congener with a Phosphorus Apical Donor and Coordination to Cobalt(II)

Zachary Thammavongsy, Juliet F. Khosrowabadi Kotyk, Charlene Tsay, and Jenny Y. Yang*

Department of Chemistry, University of California, Irvine, California 92697, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.5b02133

Publication Date (Web): November 9, 2015

Copyright © 2015 American Chemical Society

*E-mail: j.yang@uci.edu (J.Y.).

http://pubs.acs.org/doi/10.1021/acs.inorgchem.5b02133

Synopsis

The synthesis and characterization of a tripyridylamine (TPA) congener bearing a phosphorus apical donor, tris(2-pyridylmethyl)proazaphosphatrane (TPAP), is described. Metalation of TPAP with a Co(II) ion results in the desired tetradentate coordination, whereas previous attempts of a TPA congener bearing a phosphorus apical donor resulted in tridentate coordination. The redox chemistry and electronic structure of [Co(TPAP)]2+ was compared to [Co(TPA)]2+ to investigate the effect of the phosphorus apical donor.

Abstract

Tripyridylamine (TPA), a tetradentate ligand that forms 5-membered chelate rings upon metal coordination, has demonstrated significant utility in synthetic inorganic chemistry. An analogue with a phosphorus apical donor is a desirable target for tuning electronic structure and enhancing reactivity. However, this congener has been synthetically elusive. Prior attempts have resulted in tridentate coordination to transition metal ions due to a lack of ligand flexibility. Herein, we report the successful synthesis of tris(2-pyridylmethyl)proazaphosphatrane (TPAP), a more accommodating tripyridyl ligand containing an apical phosphorus donor. The TPAP ligand forms 6-membered chelate rings upon coordination and binds in the desired tetradentate fashion to a Co(II) ion. Structural studies elucidate the importance of ligand flexibility in tripodal ligands featuring phosphorus donors. Cyclic voltammetry, UV–vis, and solution magnetic susceptibility experiments of [Co(TPAP)(CH3CN)]2+ are also reported and compared to [Co(TPA)(CH3CN)]2+. Notably, magnetic susceptibility measurements of [Co(TPAP)(CH3CN)]2+indicate a low spin electronic configuration, in contrast to [Co(TPA)(CH3CN)]2+, which is high spin.