[{(η5-C5Me5)Fe}2(μ-H)4]: A Novel Dinuclear Iron Tetrahydrido Complex

A Bis-Cyclopentadienyl Ligand-Supported Di-Iron Trihydride Motif as a Synthon for Access to Heterobimetallic Trinuclear Complexes

Herein, we report the synthesis of a flexible bis-cyclopentadienyl ligand L (the doubly deprotonated form of H2L (1,3-bis(2,4-di-tert-butylcyclopentadienyldimethylsilyl)benzene)), demonstrating its ability to stabilize a series of di-iron hydrido complexes. Notably, this ligand facilitates the isolation of an unprecedented anionic cyclopentadienyl ligand-supported di-iron trihydride complex, LFe2(μ-H)3Li(THF) (2), functioning as a synthon for the [Fe2(μ-H)3]- core and providing access to heterobimetallic complexes 4-6 with coinage metals.

The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{5CpFe}2(μ-H)4] Revisited by Hard X-Ray Spectroscopy

High energy resolution fluorescence detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (VtC-XES) are introduced as powerful tools to investigate hydride-iron interaction, the possible iron-iron bond, and iron spin state of the dinuclear tetra-hydrido complex [{⁵CpFe}₂(μ-H)₄] (1H, ⁵Cp = η⁵-C₅ iPr₅) by thoroughly accessing the geometric and electronic structure of this complex in comparison to the nonhydride reference [⁵CpCpFe] (1, Cp = C₅H₅). The so far observed most intense hydride induced signals in the pre-edge feature of the HERFD-XANES and in the VtC-XES spectra at the iron K-edge allow a precise analysis of the LUMO and HOMO states, respectively, by application of time-dependent density function theory (TD-DFT) and density functional theory (DFT) calculations. The results of these calculations are further employed to understand the oxidation state, spin states, and potential Fe-Fe bonds in this complex.

Interplay between Terminal and Bridging Diiron Hydrides in Neutral and Oxidized States

This study describes the structural, spectroscopic, and electrochemical properties of electronically unsymmetrical diiron hydrides. The terminal hydride Cp*Fe(pdt)Fe(dppe)(CO)H ([1(t-H)]⁰, Cp*^- = Me₅C₅^-, pdt^2- = CH₂(CH₂S^-)₂, dppe = Ph₂PC₂H₄PPh₂) was prepared by hydride reduction of [Cp*Fe(pdt)Fe(dppe)(CO)(NCMe)]⁺. As established by X-ray crystallography, [1(t-H)]⁰ features a terminal hydride ligand. Unlike previous examples of terminal diiron hydrides, [1(t-H)]⁰ does not isomerize to the bridging hydride [1(μ-H)]⁰. Oxidation of [1(t-H)]⁰ gives [1(t-H)]⁺, which was also characterized crystallographically as its BF₄^- salt. Density functional theory (DFT) calculations indicate that [1(t-H)]⁺ is best described as containing an Cp*Fe^III center. In solution, [1(t-H)]⁺ isomerizes to [1(μ-H)]⁺, as anticipated by DFT. Reduction of [1(μ-H)]⁺ by Cp₂Co afforded the diferrous bridging hydride [1(μ-H)]⁰. Electrochemical measurements and DFT calculations indicate that the couples [1(t-H)]^+/0 and [1(μ-H)]^+/0 differ by 210 mV. Qualitative measurements indicate that [1(t-H)]⁰ and [1(μ-H)]⁰ are close in free energy. Protonation of [1(t-H)]⁰ in MeCN solution affords H₂ even with weak acids via hydride transfer. In contrast, protonation of [1(μ-H)]⁰ yields 0.5 equiv of H₂ by a proposed protonation-induced electron transfer process. Isotopic labeling indicates that μ-H/D ligands are inert.

Binuclear cyclopentadienylrhenium hydride chemistry: terminal versus bridging hydride and cyclopentadienyl ligands

Theoretical studies predict the lowest energy structures of the binuclear cyclopentadienylrhenium hydrides Cp2Re2H n (Cp = η(5)-C5H5; n = 4, 6, 8) to have a central doubly bridged Re2(μ-H)2 unit with terminal η(5)-Cp rings and the remaining hydrides as terminal ligands. However, the lowest energy Cp2Re2H2 structure by more than 12 kcal mol(-1) has one terminal η(5)-Cp ring, a bridging η(3),η(2)-Cp ring, and two terminal hydride ligands bonded to the same Re atom. The lowest energy hydride-free Cp2Re2 structure is a perpendicular structure with two bridging η(3),η(2)-Cp rings. The previously predicted bent singlet Cp2Re2 structure with terminal η(5)-Cp rings and a formal Re-Re sextuple bond lies ∼37 kcal mol(-1) above this lowest energy (η(3),η(2)-Cp)2Re2 structure. The thermochemistry of the CpReH n and Cp2Re2H n systems is consistent with the reported synthesis of the permethylated derivatives Cp*ReH6 and Cp*2Re2H6 (Cp* = η(5)-Me5C5) as very stable compounds. Additionally, natural bond orbital analysis, atoms-in-molecules and overlap population density-of-state in AOMIX were applied to present the existence of rhenium-rhenium multiple bonds.

New reactions of terminal hydrides on a diiron dithiolate

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(term-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H](0)). Spectroscopic and computational studies indicate that [H1H](0) contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2](+) ([H1(CO)](+)), which undergoes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)(dppv)2](+) ([H1(NCMe)](+)). Upon treatment of [H1(NCMe)](+) with borohydride salts, the MeCN ligand is displaced to afford [H1H](0). DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s(-1) at -40 °C. The compound reacts with D2 to afford [D1D](0), but not mixed isotopomers such as [H1D](0). The dihydride undergoes oxidation with Fc(+) under CO to give [1(CO)](+) and H2. Protonation in MeCN solution gives [H1(NCMe)](+) and H2. Carbonylation converts [H1H](0) into [1(CO)](0).

Isomerization of the hydride complexes [HFe2(SR)2(PR3)(x)(CO)(6-x)]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

The reactivity patterns of low-coordinate iron-hydride complexes

We report a survey of the reactivity of the first isolable iron-hydride complexes with a coordination number less than 5. The high-spin iron(II) complexes [(beta-diketiminate)Fe(mu-H)] 2 react rapidly with representative cyanide, isocyanide, alkyne, N 2, alkene, diazene, azide, CO 2, carbodiimide, and Brønsted acid containing substrates. The reaction outcomes fall into three categories: (1) addition of Fe-H across a multiple bond of the substrate, (2) reductive elimination of H 2 to form iron(I) products, and (3) protonation of the hydride to form iron(II) products. The products include imide, isocyanide, vinyl, alkyl, azide, triazenido, benzo[ c]cinnoline, amidinate, formate, and hydroxo complexes. These results expand the range of known bond transformations at iron complexes. Additionally, they give insight into the elementary transformations that may be possible at the iron-molybdenum cofactor of nitrogenases, which may have hydride ligands on high-spin, low-coordinate metal atoms.

A Rare Dimer of Dimers Having Four Hydride Linkers Joining Two Quadruply Bonded Dimolybdenum Units

Hydride anions, H-, have been found to cause the assembly of dimolybdenum units [Mo2(cis-DAniF)2]2+, DAniF = N,N'-di(p-anisyl)formamidinate, forming a tetranuclear complex [Mo2(cis-DAniF)2]2(mu-H)4 (1) with an Mo4H4 core that may be described as an elongated tetrahedron in which the H atoms are along the four long edges of such tetrahedron and the Mo2 units are along the short edges. The two quadruply bonded dimolybdenum units, separated by only 2.718 A, are essentially orthogonal. This gives the shortest [Mo2]...[Mo2] distance known for complexes with multiple dimolybdenum units. DFT calculations indicate that the energy of a cuboidal isomer is only 3.8 kcal/mol above that of 1, but such an isomer has not been observed.

Electronic Coupling in Mixed-Valence Dinuclear Ferrocenes and Cobaltocenes with Saturated Bridging Groups

We have synthesised a series of new dinuclear metallocenes [{M(Cp*)(C5H4)}2X] (Cp* = eta5-pentamethylcyclopentadienyl; M = Fe, Co, X = CMe2, SiMe2, GeMe2; M = Fe, X = Si2Me4). For the neutral dicobalt complexes, magnetic susceptibility measurements reveal intramolecular antiferromagnetic interactions of -21 and -14 cm(-1) for SiMe2- and GeMe2-bridged species, respectively, but negligible interaction for the CMe2-bridged compound. In contrast, intervalence charge-transfer (IVCT) data for the mixed-valence monocations of both Fe and Co complexes show electronic coupling to decrease in the order CMe2 > SiMe2 > GeMe2. This suggests that electronic coupling is principally through-space in contrast to results found from previous studies. The IVCT data also show much stronger coupling in the dicobalt species versus their diiron analogues.

Gallium(III) Chloride-Catalyzed Double Insertion of Isocyanides into Epoxides

Gallium(III) chloride-catalyzed double insertion of aryl isocyanides into terminal and disubstituted epoxides leads to alpha,beta-unsaturated alpha-amino iminolactones (3-amino-2-iminio-2,5-dihydrofurans). [reaction: see text]

Sequential C-H activation and dinuclear insertion of ethylene promoted by a diiridium complex

Two ethylene molecules, bonded to different metal atoms of a diiridium(I) complex, can be converted into a bridging 3-butenyl-1-yl ligand and a terminal hydride. Such transformation is initiated by an ethylene C-H activation step and followed by an insertion of the second ethylene into a bridging vinyl ligand. This result illustrates the potentiality of diiridium(I) complexes to promote and support reaction sequences of alkene functionalization through C-H activation.