A Spectrochemical Walk:  Single-Site Perturbation within a Series of Six-Coordinate Ferrous Complexes

Abstract

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe(II)(PY5)(X)](n+), X = MeOH, H(2)O, MeCN, pyridine, Cl-, OBz-, N(3)-, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe(II)(PY5)](2+) unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK(a) of the exogenously ligated MeOH in [Fe(PY5)(MeOH)](2+) (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)](2+) to yield [Fe(PY5)(OMe)](1+) rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

Synthesis, characterization and DFT studies of the cobalt(iii) complex of a tetrapodal pentadentate N4S donor ligand

“…1- 6 The capacity of these ligands to invoke a single labile coordination site allows the possibility of their complexes to be investigated as, for example, analogues of metalloenzymes involved in substrate and O 2 activation. 5, 6 Other possible applications include catalyst formation and the synthesis of coordination polymers.…”

“…1,2,5, 6 The podal donor groups are all primary amines and pyridine nitrogens for pyN 4 and PY5, respectively. In the case of 2,2Ј-dimethyl-2,2Ј-iminodimethylenebis(1,3-propanediamine) (ditame), a secondary amine occupies the hinge site with four podal primary amine donors.…”

Synthesis, Characterization, and Catalytic Activity of Iron(II) and Nickel(II) Complexes Containing the Rigid Pentadentate Ligand PY5Me2

“…[20] [nBu 4 N]PF 6 (Aldrich) used for cyclic voltammetry was recrystallized three times from boiling ethanol and dried in a vacuum at 120°C for 24 h. Acetonitrile (Aldrich) was distilled over calcium hydride. All other chemicals were of reagent grade and used without further purification.…”

“…Cyclic voltammetry (CV) was performed with a PAR model 273 potentiostat. Cyclic voltammetry was performed in acetonitrile containing 0.1 m [nBu 4 N]PF 6 as the supporting electrode at room temperature. An Ag/AgNO 3 (0.1 m in CH 3 CN) electrode was used as reference electrode and the working electrode was a glassy carbon electrode (CH Instruments, Inc.) with a platinum-wire auxiliary electrode.…”

“…[2,3] Moreover, they are able to reinforce an octahedral environment and restrict ligand substitution to a single coordination site. [4] These properties are particularly important in controlling the reactivity of kinetically labile first row metals. A number of transition metal complexes (Mn, Fe, Co, Ni, Cu, and Zn) bearing these ligands have been reported and applied in a wide range of areas including single-molecule magnets, [5][6][7][8][9] and electrocatalytic water reduction.…”