Heterolytic H_2 Activation Mediated by Low-Coordinate L_3Fe-(µ-N)-FeL_3 Complexes to Generate Fe(µ-NH)(µ-H)Fe Species

The diiron μ-nitride complexes, {L_3Fe^(II)(μ-N)Fe^(II)L_3}- and L_3Fe^(III)(μ-N)Fe^(II)L_3, heterolytically activate hydrogen (1 atm) at ambient temperature in solution (L_3 = [PhB(CH_2PPh_2)_3]-). These transformations lead to structurally unique {L_3Fe^(II)(μ-NH)(μ-H)Fe^(II)L^3}- and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3 products. X-ray data establish a marked reduction in the Fe−Fe distance upon H_2 uptake, and spectroscopic data establish both Fe^(II)Fe^(II) species to be diamagnetic, whereas the Fe^(III)Fe^(II) species, L_3Fe^(III)(μ-N)Fe^(II)L_3 and L_3Fe^(III)(μ-NH)(μ-H)Fe^(II)L_3, populate doublet ground states with thermally accessible higher spin states

Synthesis and Characterization of Three-Coordinate Ni(III)-Imide Complexes

A new family of low-coordinate nickel imides supported by 1,2-bis(di-tert-butylphosphino)ethane was synthesized. Oxidation of nickel(II) complexes led to the formation of both aryl- and alkyl-substituted nickel(III)-imides, and examples of both types have been isolated and fully characterized. The aryl substituent that proved most useful in stabilizing the Ni(III)-imide moiety was the bulky 2,6-dimesitylphenyl. The two Ni(III)-imide compounds showed different variable-temperature magnetic properties but analogous EPR spectra at low temperatures. To account for this discrepancy, a low-spin/high-spin equilibrium was proposed to take place for the alkyl-substituted Ni(III)-imide complex. This proposal was supported by DFT calculations. DFT calculations also indicated that the unpaired electron is mostly localized on the imide nitrogen for the Ni(III) complexes. The results of reactions carried out in the presence of hydrogen donors supported the findings from DFT calculations that the adamantyl substituent was a significantly more reactive hydrogen-atom abstractor. Interestingly, the steric properties of the 2,6-dimesitylphenyl substituent are important not only in protecting the Ni═N core but also in favoring one rotamer of the resulting Ni(III)-imide, by locking the phenyl ring in a perpendicular orientation with respect to the NiPP plane

High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands

Rare examples of monometallic high-spin and low-spin L_3Fe(H_3BH) complexes have been characterized, where the two L_3 ligands are [Tp^(Ph2)] and [PhBP3] ([Tp^(Ph2)] = [HB(3,5-Ph_2pz)_3]− and [PhBP_3] = [PhB(CH_2PPh_2)_3]−). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers. Despite the differences in spin states, short Fe–B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L_3)Fe(η^4-H_3BH) complexes

On the feasibility of N2 fixation via a single-site FeI/FeIV cycle: Spectroscopic studies of FeI(N2)FeI, FeIV=N, and related species

The electronic properties of an unusually redox-rich iron system, [PhBPR 3]FeNx (where [PhBPR 3] is [PhB(CH2PR2)3]−), are explored by Mössbauer, EPR, magnetization, and density-functional methods to gain a detailed picture regarding their oxidation states and electronic structures. The complexes of primary interest in this article are the two terminal iron(IV) nitride species, [PhBPiPr 3]FeN (3a) and [PhBPCH2Cy 3]FeN (3b), and the formally diiron(I) bridged-Fe(μ-N2)Fe species, {[PhBPiPr 3]Fe}2(μ-N2) (4). Complex 4 is chemically related to 3a via a spontaneous nitride coupling reaction. The diamagnetic iron(IV) nitrides 3a and 3b exhibit unique electronic environments that are reflected in their unusual Mössbauer parameters, including quadrupole-splitting values of 6.01(1) mm/s and isomer shift values of −0.34(1) mm/s. The data for 4 suggest that this complex can be described by a weak ferromagnetic interaction (J/D < 1) between two iron(I) centers. For comparison, four other relevant complexes also are characterized: a diamagnetic iron(IV) trihydride [PhBPiPr 3]Fe(H)3(PMe3) (5), an S = 3/2 iron(I) phosphine adduct [PhBPiPr 3]FePMe3 (6), and the S = 2 iron(II) precursors to 3a, [PhBPiPr 3]FeCl and [PhBPiPr 3]Fe-2,3:5,6-dibenzo-7-aza bicyclo[2.2.1]hepta-2,5-diene (dbabh). The electronic properties of these respective complexes also have been explored by density-functional methods to help corroborate our spectral assignments and to probe their electronic structures further

Vibrational Spectroscopy and Analysis of Pseudo-tetrahedral Complexes with Metal Imido Bonds

A number of assignments have been previously posited for the metal−nitrogen stretch (ν(M-NR)), the N−R stretch (ν(MN−R)), and possible ligand deformation modes associated with terminally bound imides. Here we examine mononuclear iron(III) and cobalt(III) imido complexes of the monoanionic tridentate ligand [PhBP_(3)] ([PhBP_(3)] = [PhB(CH_(2)PPh_(2))_(3)]^(-)) to clarify the vibrational features for these trivalent metal imides. We report the structures of [PhBP_(3)]Fe≡N^(t)Bu and [PhBP_(3)]Co≡N^(t)Bu. Pseudo-tetrahedral metal imides of these types exhibit short bond lengths (ca. 1.65 Å) and nearly linear angles about the M−N−C linkages, indicative of multiple bond character. Furthermore, these compounds give rise to intense, low-energy visible absorptions. Both the position and the intensity of the optical bands in the [PhBP_(3)]M≡NR complexes depend on whether the substituent is an alkyl or aryl group. Excitation into the low-energy bands of [PhBP_(3)]Fe≡N^(t)Bu gives rise to two Raman features at 1104 and 1233 cm^(-1), both of which are sensitive to ^(15)N and ^(2)H labeling. The isotope labeling suggests the 1104 cm^(-1) mode has the greatest Fe−N stretching character, while the 1233 cm^(-1) mode is affected to a lesser extent by ^(15)N substitution. The spectra of the deuterium-labeled imides further support this assertion. The data demonstrate that the observed peaks are not simple diatomic stretching modes but are extensively coupled to the vibrations of the ancillary organic group. Therefore, describing these complexes as simple diatomic or even triatomic oscillators is an oversimplification. Analogous studies of the corresponding cobalt(III) complex lead to a similar set of isotopically sensitive resonances at 1103 and 1238 cm^(-1), corroborating the assignments made in the iron imides. Very minimal changes in the vibrational frequencies are observed upon replacement of cobalt(III) for iron(III), suggesting similar force constants for the two compounds. This is consistent with the previously proposed electronic structure model in which the added electron resides in a relatively nonbonding orbital. Replacement of the tBu group with a phenyl ring leads to a significantly more complicated resonance Raman spectrum, presumably due to coupling with the vibrations of the phenyl ring. Polarization studies demonstrate that the observed modes have A1 symmetry. In this case, a clearer resonance enhancement of the signals is observed, supporting a charge transfer designation for the electronic transitions. A series of isotope-labeling experiments has been carried out, and the modes with the greatest metal−nitrogen stretching character have been assigned to peaks at 960 and 1300 cm^(-1) in both the iron and cobalt [PhBP_(3)]M≡NPh complexes. These results are consistent with a multiple M−N bond for these metal imides

Selectivity and Mechanism of Hydrogen Atom Transfer by an Isolable Imidoiron(III) Complex

This article discusses a mechanistic study of hydrogen atom transfer by an isolable iron (III) imido complex, LᴹᵉFeNAd (Lᴹᵉ = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl)

Bio-organometallic Approaches to Nitrogen Fixation Chemistry

Biological and industrial nitrogen fixation are the processes by which one of the most inert molecules, N_2, is transformed into a bioavailable nitrogen source (e.g. NH_3) that can be incorporated into all nitrogen-containing biomolecules [1]. As such, nitrogen fixation is essential to sustaining life on this planet, and has attracted intense scrutiny among biological and chemical communities for decades [2-4]. The mechanism by which nitrogenase enzymes promote the biological reduction of nitrogen under ambient conditions remains an unsolved and fascinating problem [4-6]. Nature's solution to fixing nitrogen stands in sharp contrast to that which humanity has adopted, i.e. the Haber-Bosch process for ammonia synthesis that is carried out at approximately 200 atm and 500°C. This Herculean feat of chemical engineering provides, in net, about 50% of the nitrogen atoms that wind up in all human beings on this planet [7]. From a chemists perspective, however, this remains a brute force and high-energy solution to the nitrogen fixation problem, especially given the knowledge that nature can fix N_2 under ambient conditions

Mid- to high-valent imido and nitrido complexes of iron

This short review summarizes recent advances in the synthesis and reactivity of iron imides and nitrides. Both monometallic and multimetallic assemblies featuring these moieties are discussed. Recent synthetic approaches have led to the availability of new mid- to high-valent iron imides and nitrides, allowing us to begin assembling models to describe the factors influencing their relative stabilities and reactivity patterns