Electronic Structure of Bis(imino)pyridine Iron Dichloride, Monochloride, and Neutral Ligand Complexes:  A Combined Structural, Spectroscopic, and Computational Study

Abstract

The electronic structure of a family of bis(imino)pyridine iron dihalide, monohalide, and neutral ligand compounds has been investigated by spectroscopic and computational methods. The metrical parameters combined with Mössbauer spectroscopic and magnetic data for ((i)PrPDI)FeCl(2) ((i)PrPDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N=CMe)(2)C(5)H(3)N) established a high-spin ferrous center ligated by a neutral bis(imino)pyridine ligand. Comparing these data to those for the single electron reduction product, ((i)PrPDI)FeCl, again demonstrated a high-spin ferrous ion, but in this case the S(Fe) = 2 metal center is antiferromagnetically coupled to a ligand-centered radical (S(L) = (1)/(2)), accounting for the experimentally observed S = (3)/(2) ground state. Continued reduction to ((i)PrPDI)FeL(n) (L = N(2), n = 1,2; CO, n = 2; 4-(N,N-dimethylamino)pyridine, n = 1) resulted in a doubly reduced bis(imino)pyridine diradical, preserving the ferrous ion. Both the computational and the experimental data for the N,N-(dimethylamino)pyridine compound demonstrate nearly isoenergetic singlet (S(L) = 0) and triplet (S(L) = 1) forms of the bis(imino)pyridine dianion. In both spin states, the iron is intermediate spin (S(Fe) = 1) ferrous. Experimentally, the compound has a spin singlet ground state (S = 0) due to antiferromagnetic coupling of iron and the ligand triplet state. Mixing of the singlet diradical excited state with the triplet ground state of the ligand via spin-orbit coupling results in temperature-independent paramagnetism and accounts for the large dispersion in (1)H NMR chemical shifts observed for the in-plane protons on the chelate. Overall, these studies establish that reduction of ((i)PrPDI)FeCl(2) with alkali metal or borohydride reagents results in sequential electron transfers to the conjugated pi-system of the ligand rather than to the metal center.

Self‐Coupling of a 4‐H‐Butatrienylidene Tungsten Complex

“…The deprotected compound [W(CO)(dppe) 2 {C=C=C=CH(p-C 6 H 4 tBu)}] (3) was fully characterized by NMR and IR spectroscopy and elemental analysis. The presence of the terminal CH(p-C 6 H 4 tBu) group in 3 was confirmed by ( 1 H, 13 C) correlation, 13 C-DEPT, and 1 H{ 31 P}-decoupling NMR experiments. The CH(p-C 6 H 4 tBu) protons appear at d = 3.74 ppm as a triplet with 6 J( 1 H, 31 P) = 4.0 Hz.…”

Reactivity and Properties of Metal Complexes Enabled by Flexible and Redox-Active Ligands with a Ferrocene Backbone

“…An example worth mentioning is a class of pincer type ligands with an aromatic backbone (most commonly, pyridine) and two neutral donor arms that can be imine, amine, phosphine, N-heterocyclic carbene, etc. [9][10][11][12] For example, Milstein and coworkers demonstrated the role of PCP and PNP pincer ligands in catalysis and small molecule activation and proposed mechanisms in which metal-ligand cooperation plays a major role (Chart 1c). [13][14][15][16][17][18][19][20][21] In addition, Chirik and coworkers exploited the potential of base metals in catalytic transformations by using flexible and redox non-innocent ligands to overcome the limitation of first row transition metals that usually undergo single electron processes (Chart 1d).…”

Unusual Electronic Features and Reactivity of the Dipyridylazaallyl Ligand: Characterizations of (smif)2M [M = Fe, Co, Co+, Ni; smif = {(2-py)CH}2N] and [(TMS)2NFe]2(smif)2

“…Its calculated spectrum, which appeared blue-shifted by ∼0.25 eV, revealed a minor MLCT component in these bands. The TDDFT study permitted a tentative identification of d-d bands at ∼18 000 and 25 000 cm Figure 3 indicates that smif may be redox-active [7][8][9][10][11][12] in the case of 2. For 1, nonbonding, carbon-based azaallyl orbitals are the HOMO and HOMO-1.…”