title: 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.
authors.affiliation: Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA.
authors.forename: Suzanne C
authors.initials: SC
authors.lastname: Bart
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Chłopek
authors.forename: Eckhard
authors.initials: E
authors.lastname: Bill
authors.forename: Marco W
authors.initials: MW
authors.lastname: Bouwkamp
authors.forename: Emil
authors.initials: E
authors.lastname: Lobkovsky
authors.forename: Frank
authors.initials: F
authors.lastname: Neese
authors.forename: Karl
authors.initials: K
authors.lastname: Wieghardt
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Chirik
mesh_terms: D000465:Algorithms; D000838:Anions / Q000737:chemistry; D001894:Borohydrides / Q000737:chemistry; D002712:Chlorides / Q000737:chemistry*; D004123:Dimethylamines / Q000737:chemistry; D004583:Electrons; D005296:Ferrous Compounds / Q000737:chemistry; D007097:Imines / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D011725:Pyridines / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D000838:Anions; D001894:Borohydrides; D002712:Chlorides; D004123:Dimethylamines; D005296:Ferrous Compounds; D007097:Imines; D008024:Ligands; D009942:Organometallic Compounds; D011725:Pyridines; D007501:Iron; C023666:pyridine