Mössbauer spectra of [LFe(II)X](0) (L = beta-diketiminate; X = Cl(-), CH(3)(-), NHTol(-), NHtBu(-)), 1.X, were recorded between 4.2 and 200 K in applied magnetic fields up to 8.0 T. A spin Hamiltonian analysis of these data revealed a spin S = 2 system with uniaxial magnetization properties, arising from a quasi-degenerate M(S) = +/-2 doublet that is separated from the next magnetic sublevels by very large zero-field splittings (3/D/ > 150 cm(-1)). The ground levels give rise to positive magnetic hyperfine fields of unprecedented magnitudes, B(int) = +82, +78, +72, and +62 T for 1.CH(3), 1.NHTol, 1.NHtBu, and 1.Cl, respectively. Parallel-mode EPR measurements at X-band gave effective g values that are considerably larger than the spin-only value 8, namely g(eff) = 10.9 (1.Cl) and 11.4 (1.CH(3)), suggesting the presence of unquenched orbital angular momenta. A qualitative crystal field analysis of g(eff) shows that these momenta originate from spin-orbit coupling between energetically closely spaced yz and z(2) 3d-orbital states at iron and that the spin of the M(S) = +/-2 doublet is quantized along x, where x is along the Fe-X vector and z is normal to the molecular plane. A quantitative analysis of g(eff) provides the magnitude of the crystal field splitting of the lowest two orbitals, /epsilon(yz) - epsilon(2)(z)/ = 452 (1.Cl) and 135 cm(-1) (1.CH(3)). A determination of the sign of the crystal field splitting was attempted by analyzing the electric field gradient (EFG) at the (57)Fe nuclei, taking into account explicitly the influence of spin-orbit coupling on the valence term and ligand contributions. This analysis, however, led to ambiguous results for the sign of epsilon(yz) - epsilon(2)(z). The ambiguity was resolved by analyzing the splitting Delta of the M(S) = +/-2 doublet; Delta = 0.3 cm(-1) for 1.Cl and Delta = 0.03 cm(-)(1) for 1.CH(3). This approach showed that z(2) is the ground state in both complexes and that epsilon(yz) - epsilon(2)(z) approximately 3500 cm(-1) for 1.Cl and 6000 cm(-1) for 1.CH(3). The crystal field states and energies were compared with the results obtained from time-dependent density functional theory (TD-DFT). The isomer shifts and electric field gradients in 1.X exhibit a remarkably strong dependence on ligand X. The ligand contributions to the EFG, denoted W, were expressed by assigning ligand-specific parameters: W(X) to ligands X and W(N) to the diketiminate nitrogens. The additivity and transferability hypotheses underlying this model were confirmed by DFT calculations. The analysis of the EFG data for 1.X yields the ordering W(N(diketiminate)) < W(Cl) < W(N'HR), W(CH(3)) and indicates that the diketiminate nitrogens perturb the iron wave function to a considerably lesser extent than the monodentate nitrogen donors do. Finally, our study of these synthetic model complexes suggests an explanation for the unusual values for the electric hyperfine parameters of the iron sites in the Fe-Mo cofactor of nitrogenase in the M(N) state.

Three-coordinate organometallic complexes are rare, especially with the prototypical methyl ligand. Using a hindered, rigid bidentate ligand (L), it is possible to create 12-electron methyliron(II) and 13-electron methylcobalt(II) complexes. These complexes are thermally stable, and 1 H NMR spectra suggest that the low coordination number is maintained in solution. Attempts to create the 14-electron LNiCH3 led instead to the three-coordinate nickel(I) complex LNi(THF). Single crystals of LMCH3 are isomorphous with the new three-coordinate chloride complexes LNiCl and LCoCl. Along with the recently reported LFeCl (Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001, 1542), these are the only examples of threecoordinate iron(II), cobalt(II), and nickel(II) complexes with terminal chloride ligands, enabling the systematic evaluation of the effect of coordination number and metal identity on M-Cl bond lengths. Electronic structure calculations predict the ground states of the trigonal complexes.

Reaction of the {LMeFe} fragment with adamantyl azide (AdN3) in the presence of 4‐tert‐butylpyridine (tBuPy) gives an S=3/2 species that has been examined by 1H NMR, IR, EPR, and Mössbauer spectroscopies. DFT calculations point toward an identification as [LMeFeNAd]. This species abstracts hydrogen atoms from the ligand or external reagents only through its pyridine adduct (see scheme; Ar=2,6‐iPr2C6H3).

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L(R)FeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and (1)H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L(R)FeNHAr may be oxidized using AgOTf, and the products L(R)Fe(NHAr)(OTf) are characterized with (19)F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L(R)Fe(NHAr)(OTf) and L(R)Fe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of L(R)FeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H(2)NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.

Hydrogen atom transfer (HAT, eq 1) is an elementary chemical transformation that results in the net transfer of both a proton and an electron. 1,2 MetalÀoxo complexes are widely used to abstract hydrogen atoms from organic compounds through HAT, which leads to the metal hydroxide (Scheme 1). 3 In oxidations by cytochrome P450, the mechanism is generally accepted to be HAT to an ironÀoxo species (FedO) followed by radical rebound. 4 Other enzymatic systems such as soluble methane monooxygenase, 5 ribonucleotide reductases and other B 12 -dependent enzymes, 6 lipoxygenases, 7 isopenicillin-N synthase, 8 and TauD 9 also utilize mechanisms with key HAT steps.Considerable effort has been devoted to synthesizing and studying biomimetic oxoiron complexes 10 in order to help elucidate the enzymatic mechanisms and to develop homogeneous iron-based oxidation catalysts. Studies on heme ironÀoxo complexes in the 1980s by Balch, La Mar, and Groves pioneered this field. 11 More recently, Que, Nam, and co-workers have reported isolable non-heme oxoiron(IV) complexes that react with hydrocarbons via HAT. 12 Reactions proceeding by HAT mechanisms have also been studied for terminal oxo complexes of Mn, Ru, Cr, and V. 13À19 In general, the selectivity of nonenzymatic HAT reactions is thermodynamically controlled, and reaction rates follow a linear correlation with the bond dissociation enthalpy (BDE) of the XÀH bond being broken (the BellÀEvansÀPolanyi relation), 1,20 i.e., homolytically weaker substrate bonds react more rapidly.Imido (NR 2À ) ligands are isoelectronic to oxo (O 2À ) ligands (Scheme 1), and imido complexes are often proposed as intermediates in hydrocarbon amination mechanisms in which the imido species performs the cleavage of the CÀH bond that precedes CÀN bond formation. Imido complexes are also more versatile than oxo complexes, because there is an opportunity to tune the steric and electronic properties of the complex by changing the nitrogen substituent. However, the HAT reactivity of imido complexes (MdNR) has not been investigated in as much detail as that of their oxo counterparts. There has been a recent renaissance of activity in the synthesis of imido complexes of the late transition metals (groups 8À11), 21 and this activity has resulted in the isolation of late transition metal complexes Received: January 18, 2011 ABSTRACT: In the literature, ironÀoxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. IronÀimido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L Me FeNAd (L Me = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ( t Bupy) to form a reactive fourcoordinate intermediate L Me Fe(NAd)( t Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very la...

Reaction of 1-adamantyl azide with iron(I) diketiminate precursors gives metastable but isolable imidoiron(III) complexes LFe=NAd (L = bulky β-diketiminate ligand; Ad = 1-adamantyl). This paper addresses: (1) the spectroscopic and structural characterization of the Fe=N multiple bond in these interesting three-coordinate iron imido complexes, and (2) the mechanism through which the imido complexes form. The iron(III) imido complexes have been examined by 1 H NMR and EPR spectroscopies and temperature-dependent magnetic susceptibility (SQUID), and structurally characterized by crystallography and/or X-ray absorption (EXAFS) measurements. These data show that the imido complexes have quartet ground states and short (1.68 ± 0.01 Å) iron-nitrogen bonds. The formation of the imido complexes proceeds through unobserved iron-RN 3 intermediates, which are indicated by QM/MM computations to be best described as iron(II) with an RN 3 radical anion. The radical character on the organoazide bends its NNN linkage to enable easy N 2 loss and imido complex formation. The product distribution between imidoiron(III) products and hexazene-bridged diiron(II) products is solvent-dependent, and the solvent dependence can be explained by coordination of certain solvents to the iron(I) precursor prior to interaction with the organoazide.

The metastable iron(III) imido species LtBuFeNAd catalyzes transfer of the nitrene fragment NAd from an organic azide to isocyanides or CO, forming unsymmetrical carbodiimides or isocyanates.

A three-coordinate diketiminate-nickel(I) complex with a carbonyl ligand has been characterized using EPR and IR spectroscopies and X-ray crystallography. The T geometry (bending from the sterically favored C 2v structure) contrasts with that of isosteric d 9 copper(II) complexes. DFT calculations on a truncated model reproduce experimental geometries, implying that the geometric differences are electronic in nature. Analysis of the charge distribution in the complexes shows that the geometry of the threecoordinate d 9 complexes is affected by differential charge donation of the ligands to the metal center.Three-coordinate complexes of transition metals with partially filled d shells have received attention because of their unusual reactivity and electronic structure. 1 The predominant geometry in crystallographically characterized three-coordinate complexes is trigonal-planar, with the ligands symmetrically distributed to minimize steric effects. The main exception to this generalization is with low-spin d 8 systems, which clearly favor a T-shaped geometry. 2 In recent papers, we described the synthesis and electronic structure of a series of three-coordinate complexes with d 6 , d 7 , d 8 , and d 9 electronic configurations at the metal center. 3,4 A bulky -diketiminate ligand ("L") was used, and a sterically favored Y geometry was evident at the metal in each case. In the Y geometry, the nickel coordination environment is idealized C 2V , with the non-diketiminate ligand on both mirror planes. The d 9 example, L tBu Ni(THF) (Figure 1, left), is notable because there are few examples of isolable three-coordinate nickel(I) complexes, 5 one series of three-coordinate copper(II) complexes, 6 and no threecoordinate d 9 complexes of heavier metals. Understanding of three-coordinate nickel(I) complexes is also biologically relevant because three-coordination is potentially accessible in the low-coordinate "proximal" nickel site of acetylcoenzyme A synthase (where methylcobalamin, CO, and coenzyme A are transformed into acetyl-coenzyme A). 7 Below, we use synthetic, crystallographic, and theoretical studies to show that the first three-coordinate nickel(I) carbonyl complex prefers a T geometry. We compare it to relevant nickel(I) and copper(II) complexes to arrive at new * To whom correspondence should be addressed. E-mail: tomc@unt.edu (T.R.C.), holland@chem.rochester.edu (P.L.H. . Ellipsoids are at 50% probability, and hydrogen atoms are omitted for clarity.