Frank W. Heinemann

Copper–Nitrene Complexes in Catalytic CH Amination†

The direct catalytic transformation of typically inert carbon– hydrogen bonds into carbon–oxygen, carbon–nitrogen, and carbon–carbon bonds represents a significant challenge in organic synthesis and the chemical industry. Ideally, such reactions would not require the presence of a functional group that is discarded later in the course of the chemical transformation, but would proceed with atom and energy efficiency and be of minimal environmental impact. Chemical species capable of directly promoting this transformation often contain multiple bonds between late transition metals to oxygen, nitrogen, or carbon centers. Nature provides inspiration with enzymes such as cyctochrome P-450, which is efficient at inserting an oxygen atom into hydrocarbon C H bonds through an iron–oxo intermediate ([Fe]=O). The use of iminoiodanes (PhI=NR) has led to the generation of related metal–nitrene intermediates ([M]=NR), which in certain cases, exhibit stoichiometric reactivity with carbon–hydrogen bonds to give amines. Several catalytic protocols based on Rh, Ru, Ag, Au, Cu, and other metals for the amination of benzylic or allylic C H bonds with iminoiodanes bearing electrondeficient N substituents represent useful methodologies for the transformation of C H into C N bonds. 10,11] In these catalytic cases, metal–nitrene species are inferred, but their high reactivity makes their characterization as the active intermediates difficult. This lack of characterization is unfortunate, as a fundamental understanding of metal–nitrene species that mediate catalytic C N bond formation would aid the development of more general systems for C H bond functionalization. In this context, we recently isolated a dicopper complex [{(Me3NN)Cu}2(m-NAr)] (Ar = 3,5-dimethylphenyl) in which a nitrene is bound between two b-diketiminate copper fragments. Solution studies supported the intermediacy of terminal copper–nitrene [Cu] = [(Me3NN)Cu] through the dissociation of a b-diketiminate copper fragment from [{(Me3NN)Cu}2(m-NAr)]. The terminal nitrene could not be directly observed owing to an unfavorable dissociation constant [Eq. (1); [Cu] = [(Me3NN)Cu].

Synthesis, structure, and reactivity of an iron(V) nitride.

A reactive iron compound bound to nitrogen has been isolated in an unusually high oxidation state. Despite being implicated as important intermediates, iron(V) compounds have proven very challenging to isolate and characterize. Here, we report the preparation of the iron(V) nitrido complex, [PhB(tBuIm)3FeV≡N]BArF24 (PhB(tBuIm)3– = phenyltris(3-tert-butylimidazol-2-ylidene)borato, BArF24 = B(3,5-(CF3)2C6H3)4–), by one electron oxidation of the iron(IV) nitrido precursor. Single-crystal x-ray diffraction of the iron(V) complex reveals a four-coordinate metal ion with a terminal nitrido ligand. Mößbauer and electron paramagnetic resonance spectroscopic characterization, supported by electronic structure calculations, provide evidence for a d3 iron(V) metal center in a low spin (S = 1/2) electron configuration. Low-temperature reaction of the iron(V) nitrido complex with water under reducing conditions leads to high yields of ammonia with concomitant formation of an iron(II) species.

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)FeCl(2)] (1) (PNP = N[2-P(CHMe(2))(2)-4-methylphenyl](2)(-)), prepared from one-electron oxidation of [(PNP)FeCl] with ClCPh(3), displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of U(eff) = 32-36 cm(-1) (47-52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔE(Q) values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe-P bond lengths and increment in the Cl-Fe-Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d(z(2)) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet ((4)B) and the sextet ((6)A) states are close in energy.

An Iron Nitride Complex

Coordination compounds of iron in high oxidation states have been invoked as reactive intermediates in biocatalyses. Iron(IV) ferryl species are examples of such highly reactive species that have long been known to be at the catalytic centers of oxygenases. Supported by X-ray diffraction studies on nitrogenase, the iron nitride moiety has recently been suggested to be present at the site of biological nitrogen reduction. As a result, well-characterized high-valent iron complexes have been sought as biomimetic models for transformations mediated by iron-containing enzymes. To gain understanding of iron nitride reactivity and the possible role of such species in biocatalysis, insight into the molecular structure of complexes stabilizing the [FeN] synthon is highly desirable. Whereas significant progress has been made in the synthesis and spectroscopic elucidation of Fe=NR and Fe N species, X-ray crystallographic characterization of a complex with a terminal Fe N functionality has not been accomplished. The first mononuclear Fe=O entity crystallographically characterized was stabilized in an octahedral environment provided by a macrocyclic tetra-N-methylated cyclam ligand. Similar cyclam derivatives also allow the stabilization and detailed spectroscopic characterization of octahedral Fe and Fe nitride complexes in unusually high oxidation states. 10] Recently, Peters and Betley developed a stunningly redox-rich iron system employing the tripodal tris(phosphino)borate ligand system (PhBP3 ), which stabilizes tetrahedral L3Fe=Nx species in oxidation states ranging from + I to + IV. Remarkably, this ligand system enabled the first room-temperature spectroscopic characterization of a terminal Fe nitride species. Concentration-dependent coupling to the Fe-N2-Fe I dinuclear product, however, prevents crystallization of this nitride species. We herein present the synthesis, spectroscopy, and most significantly, the X-ray diffraction analysis of a discrete iron nitride complex stabilized by the sterically encumbering Nanchored tris(carbene) ligand, tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine (TIMEN, R= xylyl (xyl), mesityl (mes)). Structurally and electronically related to the tetrahedral phosphinoborate ligand system by Peters and Betley, this tripodal N-heterocyclic carbene (NHC) system coordinates a high-spin Fe center in a trigonal-planar fashion, thus forming four-coordinate complexes with the metal ion in trigonalpyramidal environments. Under inert atmosphere, treatment of TIMEN with one equivalent of anhydrous ferrous chloride in pyridine at room temperature yields the four-coordinate Fe complexes [(TIMEN)Fe(Cl)]Cl (1, 1) as analytically pure, white powders in 80% yield (Scheme 1).

Closed-shell and open-shell square-planar iridium nitrido complexes

Coupling reactions of nitrogen atoms represent elementary steps to many important heterogeneously catalysed reactions, such as the Haber-Bosch process or the selective catalytic reduction of NO(x) to give N(2). For molecular nitrido (and related oxo) complexes, it is well established that the intrinsic reactivity, for example nucleophilicity or electrophilicity of the nitrido (or oxo) ligand, can be attributed to M-N (M-O) ground-state bonding. In recent years, nitrogen (oxygen)-centred radical reactivity was ascribed to the possible redox non-innocence of nitrido (oxo) ligands. However, unequivocal spectroscopic characterization of such transient nitridyl {M=N(•)} (or oxyl {M-O(•)}) complexes remained elusive. Here we describe the synthesis and characterization of the novel, closed-shell and open-shell square-planar iridium nitrido complexes [IrN(L(t-Bu))](+) and [IrN(L(t-Bu))] (L(t-Bu)=N(CHCHP-t-Bu(2))(2)). Spectroscopic characterization and quantum chemical calculations for [IrN(L(t-Bu))] indicate a considerable nitridyl, {Ir=N(•)}, radical character. The clean formation of Ir(I)-N(2) complexes via binuclear coupling is rationalized in terms of nitrido redox non-innocence in [IrN(L(t-Bu))].

Gas-Phase C−H and N−H Bond Activation by a High Valent Nitrido-Iron Dication and 〈NH〉-Transfer to Activated Olefins

A tetrapodal pentadentate nitrogen ligand (2,6-bis(1,1-di(aminomethyl)ethyl)pyridine, 1) is used for the synthesis of the azido-iron(III) complex [(1)Fe(N3)]X2 where X is either Br or PF6. By means of electrospray ionization mass spectrometry, the dication [(1)Fe(N3)]2+ can be transferred into the gas phase as an intact entity. Upon collisional activation, [(1)Fe(N3)]2+ undergoes an expulsion of molecular nitrogen to afford the dicationic nitrido-iron species [(1)FeN]2+ as an intermediate, which upon further activation can intramolecularly activate C-H- and N-H bonds of the chelating ligand 1 or can transfer an NH unit in bimolecular reactions with activated olefins. The precursor dication [(1)Fe(N3)]2+, the resulting nitrido species [(1)FeN]2+, and its possible isomers are investigated by mass spectrometric experiments, isotopic labeling, and complementary computational studies using density functional theory.

The {FeIII[FeIII(L1)2]3} star-type single-molecule magnet

Star-shaped complex {FeIII[FeIII(L1)2]3} (3) was synthesized starting from N-methyldiethanolamine H2L1 (1) and ferric chloride in the presence of sodium hydride. For 3, two different high-spin iron(III) ion sites were confirmed by Mossbauer spectroscopy at 77 K. Single-crystal X-ray structure determination revealed that 3 crystallizes with four molecules of chloroform, but, with only three molecules of dichloromethane. The unit cell of 3·4CHCl3 contains the enantiomers (Δ)-[(S,S)(R,R)(R,R)] and (Λ)-[(R,R)(S,S)(S,S)], whereas in case of 3·3CH2Cl2 four independent molecules, forming pairs of the enantiomers [Λ-(R,R)(R,R)(R,R)]-3 and [Δ-(S,S)(S,S)(S,S)]-3, were observed in the unit cell. According to SQUID measurements, the antiferromagnetic intramolecular coupling of the iron(III) ions in 3 results in a S = 10/2 ground state multiplet. The anisotropy is of the easy-axis type. EPR measurements enabled an accurate determination of the ligand-field splitting parameters. The ferric star 3 is a single-molecule magnet (SMM) and shows hysteretic magnetization characteristics below a blocking temperature of about 1.2 K. However, weak intermolecular couplings, mediated in a chainlike fashion via solvent molecules, have a strong influence on the magnetic properties. Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) were used to determine the structural and electronic properties of star-type tetranuclear iron(III) complex 3. The molecules were deposited onto highly ordered pyrolytic graphite (HOPG). Small, regular molecule clusters, two-dimensional monolayers as well as separated single molecules were observed. In our STS measurements we found a rather large contrast at the expected locations of the metal centers of the molecules. This direct addressing of the metal centers was confirmed by DFT calculations.

Reversible Binding of Dioxygen by a Copper(I) Complex with Tris(2-dimethylaminoethyl)amine (Me6tren) as a Ligand

A reversible reaction with dioxygen is observed for the copper(I) complex [Cu(Me6tren)]+ (Me6tren=tris(2-dimethylaminoethyl)amine) at low temperatures. This complex displays an interesting crystal structure (depicted) and the interaction with dioxygen in solution was investigated by low-temperature UV/Vis spectroscopic methods.

Activation of elemental S, Se and Te with uranium(III): bridging U–E–U (E = S, Se) and diamond-core complexes U–(E)2–U (E = O, S, Se, Te)

Trivalent uranium complexes supported by tris(aryloxide) chelating ligands, [((t-BuArO)3tacn)U] and [((AdArO)3N)U], undergo activation of sulfur and selenium in their elemental forms, generating the mid-valent U(IV)/(U(IV) complexes of the type [{((t-BuArO)3tacn)U}2(μ-E)] and [{((AdArO)3N)U}2(μ-E)] (E = S, Se). Under reducing conditions, [((AdArO)3N)U] reacts with elemental sulfur, selenium and tellurium to yield the mid-valent dinuclear bis-μ-chalcogenide complexes [Na(DME)3]2[{((AdArO)3N)U}2(μ-E)2] (E = S, Se, Te) with the diamond-core structural motif and rare inorganic chalcogenide bridging ligands. For comparison, a unique high-valent U(V)/U(V) dinuclear complex [{((AdArO)3N)U}2(μ-O)2] was also synthesized. A short uranium–uranium distance in this complex with a U(μ-O)2U diamond-core may account for the unusual temperature-dependent magnetic behavior.

Synthesis and Characterization of a Uranium(II) Monoarene Complex Supported by δ Backbonding

The low-temperature (<-35 °C) reduction of the trivalent uranium monoarene complex [{((Ad,Me) ArO)3 mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][(((Ad,Me) ArO)3 mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature (1) H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f (4) electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc(+) is principally metal-centered.