A Strain-Deformation Nexus within Pincer Ligands: Application to the Spin States of Iron(II) Complexes

Abstract

The substitution of a pyrrolide ring for one (or more) pyridyl rings within the ubiquitous terpyridine (tpy, A) scaffold results in more open geometries of the pyridine-pyrrolide chelate ligands. DFT calculations (B3LYP-GD3BJ/6-31G**) demonstrate that the more open geometries of the unbound ligands are mismatched with the "pinched in" geometries required to chelate transition metal ions (e.g., Zn). The strain which builds within these ligands (Δ E) as they bind transition metal ions can be related to changes in a single geometric parameter: the separation between the two terminal N atoms (ρ). This relationship applies more generally to other three-ringed tridentate pincer ligands, including those with different donor groups. The approach was applied to homoleptic iron(II) complexes to investigate the contribution of the steric effects operating within the ligands to the different magnetic properties, including spin crossover (SCO) activities, of these systems.

Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

“…With no crystal packing or substrate effects operating in solutions, the same qualitative trend emerges from the NMR data, which agrees in part with a spin‐state behaviour identified by the magnetometry in the dried powders and by the UV/Vis spectroscopy in the dried films of [Fe( L R ) 2 ](ClO 4 ) 2 with differently‐sized substituents R (Cl<Me<Et) . In acetonitrile, instead of giving an LS iron(II) complex, as the above mesityl‐disubstituted 1‐bpp does (possibly, due to a higher strain associated with an N pz ⋅⋅⋅N pz separation upon an SCO), the ligand L Me favours its HS state more than the ligands L R with the smaller chlorine atom and the larger ethyl group (Table ). The latter two, however, show a well‐accepted steric influence on the SCO in the iron(II) complexes .…”

“…Comparing the spin‐state behaviour of the obtained iron(II) complexes with differently sized ortho ‐substituents (H<F<Cl<Me<Et) uncovered a structure‐function relationship between the ligand design and the spin state of the metal ion –. …”

“…As N,N’‐disubstituted 3‐bpp complexes have no pendant NH groups that make their spin state extremely sensitive to the environment, the proposed ligand design, which may be also applicable to isomeric 1‐bpp, or other families of popular bi‐, tri‐, and higher denticity ligands, opens the way for their molecular design as spin‐crossover compounds for future breakthrough applications …”

“…Tailoring it for the above applications relies on proper chemical modifications of the ligands identified mostly through systematic screening of solutions (by NMR spectroscopy that is behind the popular Evans method) of closely related iron(II) complexes with bi‐ and tridentate or higher denticity ligands. The resulting structure‐function relationships, which are behind a successful ‘truly molecular’ design of SCO compounds, established the stabilization of the HS state by bulky groups close to the donor nitrogen atoms, with a less clear‐cut role of subtle electronic or other effects of more remote substituents . The contradictory electronic effects have been recently reconciled in a solution‐state study of 2,6‐bis(pyrazol‐1‐yl)pyridines (1‐bpp; Scheme ), which are among the most popular ligands to induce an SCO behaviour in iron(II) complexes, and to control it with a judicious choice of substituents .…”

Zinc Complexes with Tridentate Pyridyl‐Pyrrole Ligands and their Use as Catalysts in CO2 Fixation into Cyclic Carbonates

“…Zn(PDP) 2 complexes are also totally different from homoleptic terpyridyl‐zinc analogues, which are most often reported as octahedral structures . Colbran and co‐workers theoretically predict that the largest deformations occur for the bispyridylpyrrole ligands with five‐membered pyrrolide core when modelling (B3LYP/6‐31G**) of hypothetical gas‐phase [Zn(ligand)] z + species , . According to the literature, the examples of zinc complexes with bispyridylpyrrole ligands are very rare, let alone their applications.…”