Class III Delocalization and Exciton Coupling in a Bimetallic Bis‐ligand Radical Complex

Abstract

The geometric and electronic structure of an oxidized bimetallic Ni complex incorporating two redox-active Schiff-base ligands connected via a 1,2-phenylene linker has been investigated and compared to a monomeric analogue. Information from UV/Vis/NIR spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, electrochemistry, and density functional theory (DFT) calculations provides important information on the locus of oxidation for the bimetallic complex. The neutral bimetallic complex is conformationally dynamic at room temperature, which complicates characterization of the oxidized forms. Comparison to an oxidized monomer analogue 1 provides critical insight into the electronic structure of the oxidized bimetallic complex 2. Oxidation of 1 provides [1˙](+), which is characterized as a fully delocalized ligand radical complex; the spectroscopic signature of this derivative includes an intense NIR band at 4500 cm(-1). Oxidation of 2 to the bis-oxidized form affords a bis-ligand radical species [2˙˙](2+). Variable temperature EPR spectroscopy of [2˙˙](2+) shows no evidence of coupling, and the triplet and broken symmetry solutions afforded by theoretical calculations are essentially isoenergetic. [2˙˙](2+) is thus best described as incorporating two non-interacting ligand radicals. Interestingly, the intense NIR intervalence charge transfer band observed for the delocalized ligand-radical [1˙](+) exhibits exciton splitting in [2˙˙](2+), due to coupling of the monomer transition dipoles in the enforced oblique dimer geometry. Evaluating the splitting of the intense intervalence charge transfer band can thus provide significant geometric and electronic information in less rigid bis-ligand radical systems. Addition of excess pyridine to [2˙˙](2+) results in a shift in the oxidation locus from a bis-ligand radical species to the Ni(III) /Ni(III) derivative [2(py)4](2+), demonstrating that the ligand system can incorporate significant bulk in the axial positions.

Exciton Coupling in Redox‐Active Salen based Self‐Assembled Metallacycles

“…The extent of the blue-shift depends on the distance and angular orientation between the transition dipole moments in the metallacycles, [3 a] 2* + and [(3 b) 2 ] 8* + , in comparison to [4] * + . [59,63,70,71] For the metallacycle [3 a] 2* + , where the two salen units are cofacially aligned and 3.84 Å apart (NiÀ Ni distance for 3 a in the solid state), a 400 cm À 1 blue-shift vs. [4] * + is observed, consistent with the exciton model (Figures 1 and 7). This compares favourably to a previously reported Ni salen dimer complex, exhibiting a similar shift (330 cm À 1 ) with two salen units cofacially aligned and 3.98 Å apart.…”

Electronic Structure Evaluation of an Oxidized Tris(methoxy)‐Substituted Ni Salen Complex

“…Materials and Methods: All chemicals used were of the highest grade available and were further purified whenever necessary 24. The tris(2,4‐dibromophenyl)aminium hexafluoroantimonate radical chemical oxidant N(C 6 H 3 Br 2 ) 3 SbF 6 ( E 1/2 = 1.14 V, MeCN)25 was synthesized according to published protocols 7y,26. The tris‐methoxy salicylaldehyde was synthesized according to published methods 12.…”

“…Calculations: The geometry optimizations were performed with the Gaussian 09 program (Revision D.01),30 the B3LYP31 functional with a polarized continuum model (PCM) for CH 2 Cl 2 (dielectric ϵ = 8.94),32 and the 6‐31G(d) basis set on all atoms. This combination of functional and basis set have been previously used for structurally similar salen complexes, providing good matches to experimental metrical parameters 7h,7x,7y. A symmetric structure was used as a starting point for all geometry optimizations.…”

Quantum-chemical insights into mixed-valence systems: within and beyond the Robin–Day scheme

“…We note in passing the conceptual similarities of these metal-bridged TAA systems to other MV compounds where essentially organic radical ligands are coordinated to and thus bridged by a diamagnetic metal centre, such as for example certain salen-type complexes, where both class II and class III examples may be found. [41][42][43]182,183…”