On the Single-Molecule Magnetic Behavior of Linear Iron(I) Arylsilylamides.

The rational design of 3d-metal-based single-molecule magnets (SMM) requires a fundamental understanding of their intrinsic electronic and structural properties and how they translate into experimentally observable features. Here, we determined the magnetic properties of the linear iron(I) silylamides K{crypt}[FeL2] and [KFeL2] (L = -N(Dipp)SiMe3; crypt = 4,7,13,16,21,24-Hexaoxa-1,10-diazabicyclo[8.8.8]hexacosan). For the former, slow-relaxation of the magnetization with a spin reversal barrier of Ueff = 152 cm-1 as well as a closed-waist magnetic hysteresis and magnetic blocking below 2.5 K are observed. For the more linear [KFeL2], in which the potassium cation is encapsulated by the aryl substituents of the amide ligands, the relaxation barrier and the blocking temperature increase to Ueff = 184 cm-1 and TB = 4.5 K, respectively. The increase is rationalized by a more pronounced axial anisotropy in [KFeL2] determined by dc-SQUID magnetometry. The effective relaxation barrier of [KFeL2] is in agreement with the energy spacing between the ground and first-excited magnetic states, as obtained by field-dependent IR-spectroscopy (178 cm-1), magnetic measurements (208 cm-1), as well as theoretical analysis (212 cm-1). In comparison with the literature, the results show that magnetic coercivity in linear iron(I) silylamides is driven by the degree of linearity in conjunction with steric encumbrance, whereas the ligand symmetry is a marginal factor.

Intricate Road to Linear Anionic Nickel(I) Hexamethyldisilazanide [Ni(N(SiMe3)2)2].

In this report, we present intricate pathways for the synthesis of linear nickel(I) silylamide K{m}[Ni(NR2)2] (NR2 = -N(SiMe3)2). This is achieved first via the reduction of nickel(II) trisamide Li(donor)4[Ni(NR2)3] (Li(thf)x[1]) with KC8 in the presence of 18-crown-6 or crypt.222. In due course, the behavior of Li(donor)4[Ni(NR2)3] as a source of masked two-coordinate nickel(II) hexamethyldisilazanide is explored, leading to the formation of nickel(I) and nickel(II) N-donor adducts, as well as metal-metal-bonded dinickel(I) trisamide K(toluene)[Ni2(NR2)3] (K(toluene)[5]). Finally, a convenient and reliable synthesis of K{m}[Ni(NR2)2] by ligand exchange of phosphines in [Ni(NR2)(PPh3)2] with K{m}(NR2) is presented. This allows for the comprehensive analysis of its electronic properties which reveals a fluxional behavior in solution with tight anion/cation interactions.

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3 )2- Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3 )2 }2 ].

Lowering the charge of Zintl anions by (element-)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge9 4- and Sn9 4- . We developed a new strategy for other anions, using low-coordinate 3d metal complexes as electrophiles. [K(crypt-222)]+ salts of (TrBi3 )2- anions dissolved in situ in Et2 O and/or THF when reacted with [Mn(hmds)2 ]. Work-up afforded soluble [K(crypt-222)]+ salts of [{(hmds)2 Mn}2 (TlBi3 )]2- (in 1), [{(hmds)2 Mn}2 (Bi2 )]2- (in 2), and [{(hmds)Mn}4 (Bi2 )2 ]2- (in 3) (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe3 )2 ), representing rare cases of Zintl clusters with open-shell metal atoms. 1 comprises the first coordination compound of the (TlBi3 )2- anion, 2 features a diamond-shaped {Pn2 M2 } unit, and 3 is a mixed-valent MnI /MnII compound. The uncommon electronic structures in 1-3 and magnetic coupling were studied by comprehensive DFT calculations.

A Molecular Low-Coordinate [Fe-S-Fe] Unit in Three Oxidation States.

A [Fe-S-Fe] subunit with a single sulfide bridging two low-coordinate iron ions is the supposed active site of the iron-molybdenum co-factor (FeMoco) of nitrogenase. Here we report a dinuclear monosulfido bridged diiron(II) complex with a similar complex geometry that can be oxidized stepwise to diiron(II/III) and diiron(III/III) complexes while retaining the [Fe-S-Fe] core. The series of complexes has been characterized crystallographically, and electronic structures have been studied using, inter alia, 57 Fe Mössbauer spectroscopy and SQUID magnetometry. Further, cleavage of the [Fe-S-Fe] unit by CS2 is presented.

Cobalt and Iron Stabilized Ketyl, Ketiminyl and Aldiminyl Radical Anions.

Carbonyl and iminyl based radical anions are reactive intermediates in a variety of transformations in organic synthesis. Herein, the isolation of ketyl, and more importantly unprecedented ketiminyl and aldiminyl radical anions coordinated to cobalt and iron complexes is presented. Insights into the electronic structure of these unusual metal bound radical anions is provided by X-Ray diffraction analysis, NMR, IR, UV/Vis and Mössbauer spectroscopy, solid and solution state magnetometry, as well as a by a detailed computational analysis. The metal bound radical anions are very reactive and facilitate the activation of intra- and intermolecular C-H bonds.

High-Spin Imido Cobalt Complexes with Imidyl Radical Character*.

We report on the synthesis of a variety of trigonal imido cobalt complexes [Co(NAryl)L2 ]- , (L=N(Dipp)SiMe3 ), Dipp=2,6-diisopropylphenyl) with very long Co-NAryl bonds of around 1.75 Å. Their electronic structure was interrogated using a variety of physical and spectroscopic methods such as EPR or X-Ray absorption spectroscopy which leads to their description as highly unusual imidyl cobalt complexes. Computational analyses corroborate these findings and further reveal that the high-spin state is responsible for the imidyl character. Exchange of the Dipp substituent on the imide by the smaller mesityl function (2,4,6-trimethylphenyl) effectuates the unexpected Me3 Si shift from the ancillary ligand set to the imidyl nitrogen, revealing a highly reactive, nucleophilic character of the imidyl unit.

Reactions of Alkynes with Quasi-Linear 3d Metal(I) Silylamides of Chromium to Cobalt: A Comparative Study.

This report describes a series of rare low-coordinate 3d transition metal alkyne complexes resulting from the reaction of quasi-linear metal(I) silylamides, K{18c6}[MX2] (18c6 = 18-crown-6; X = -N(SiMe3)2), -N(Dipp)SiMe3; Dipp = 2,6-diisopropylphenyl), of chromium, manganese, iron, and cobalt with aliphatic and aromatic alkynes. We evaluated the interaction of alkynes with quasi-linear metal complexes in dependence of the metal and the alkyne substituents. Whereas only a weak and reversible alkyne coordination is observed for cobalt, the formation of side-on alkyne complexes of the type [M(L2)(η2-RCCR)]- takes place readily for iron. In the case of manganese, we report the first example of a low-coordinate manganese alkyne complexes and, depending on the substrate, unique examples for the manganese mediated reduction of the alkyne to their dianions or even alkyne trimerization. For chromium, alkyne coordination or reduction to the respective alkyne dianions is also observed. Computational analysis of the series of [M(N(SiMe3)2)2(η2-PhCCPh)]- complexes (Cr-Co) using DFT and CASSCF methods reveals a partial reduction of the alkyne by the metal. This leads to the description of the electronic situation of all these complexes as formal metal(II) bound alkynyl radical anions. In the case of chromium, indications for further contributions of a metal(III) cyclopropene resonance structure were found. The computational analysis rationalizes the facile reduction to bis-metalated alkene dianions due to the radical anion character of the alkyne π-complexes.

C-H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex.

The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C-H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt-imido bond. It can cleanly cleave strong C-H bonds with a bond dissociation energy of up to 92 kcal mol-1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)2 (NHt Bu)]- . Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.

Homoleptic Two-Coordinate Silylamido Complexes of Chromium(I), Manganese(I), and Cobalt(I).

Anionic two-coordinate complexes of first-row transition-metal(I) centres are rare molecules that are expected to reveal new magnetic properties and reactivity. Recently, we demonstrated that a N(SiMe3)2(-) ligand set, which is unable to prevent dimerisation or extraneous ligand coordination at the +2 oxidation state of iron, was nonetheless able to stabilise anionic two-coordinate Fe(I) complexes even in the presence of a Lewis base. We now report analogous Cr(I) and Co(I) complexes with exclusively this amido ligand and the isolation of a [Mn(I){N(SiMe3)2}2]2(2-) dimer that features a Mn-Mn bond. Additionally, by increasing the steric hindrance of the ligand set, the two-coordinate complex [Mn(I){N(Dipp)(SiMe3)}2](-) was isolated (Dipp=2,6-iPr2-C6H3). Characterisation of these compounds by using X-ray crystallography, NMR spectroscopy, and magnetic susceptibility measurements is provided along with ligand-field analysis based on CASSCF/NEVPT2 ab initio calculations.

Ising-type Magnetic Anisotropy and Slow Relaxation of the Magnetization in Four-Coordinate Amido-Pyridine FeII Complexes.

A family of four-coordinate FeII complexes formed with N,N'-chelating amido-pyridine ligands was synthesized, and their magnetic properties were investigated. These distorted tetrahedral complexes exhibit significant magnetic anisotropy with zero-field splitting parameter D ranging between -17 and -12 cm-1. Ab initio calculations enabled identification of the structural factors that control the nature of the magnetic anisotropy and the rationalization of the variation of D in these complexes. It is shown that a reduced N-Fe-N angle involving the chelating nitrogen atoms of the ligands is at the origin of the negative D value and that the torsion between the two N-Fe-N planes imposed by steric hindrances further increases the |D| value. Field-induced slow relaxation of magnetization was observed for the three compounds, and a single-molecule magnet behavior with an energy barrier for magnetization flipping (Ueff) of 27 cm-1 could be evidenced for one of them.

Iron-Catalyzed Reduction of CO2 into Methylene: Formation of C-N, C-O, and C-C Bonds.

We report herein the use of the (dihydrido)iron catalyst, Fe(H)2(dmpe)2, for the selective reduction of CO2 into either bis(boryl)acetal or methoxyborane depending on the hydroborane used as a reductant. In a one-pot two-step procedure, the in situ generated bis(boryl)acetal was shown to be a reactive and versatile source of methylene to create new C-N but also C-O and C-C bonds.

Iron-catalyzed C-H borylation of arenes.

Well-defined iron bis(diphosphine) complexes are active catalysts for the dehydrogenative C-H borylation of aromatic and heteroaromatic derivatives with pinacolborane. The corresponding borylated compounds were isolated in moderate to good yields (25-73%) with a 5 mol% catalyst loading under UV irradiation (350 nm) at room temperature. Stoichiometric reactivity studies and isolation of an original trans-hydrido(boryl)iron complex, Fe(H)(Bpin)(dmpe)2, allowed us to propose a mechanism showing the role of some key catalytic species.

Two-coordinate iron(I) complex [Fe{N(SiMe3)2}2](-) : synthesis, properties, and redox activity.

First-row two-coordinate complexes are attracting much interest. Herein, we report the high-yield isolation of the linear two-coordinate iron(I) complex salt [K(L)][Fe{N(SiMe3 )2 }2 ] (L=18-crown-6 or crypt-222) through the reduction of either [Fe{N(SiMe3 )2 }2 ] or its three-coordinate phosphine adduct [Fe{N(SiMe3 )2 }2 (PCy3 )]. Detailed characterization is gained through X-ray diffraction, variable-temperature NMR spectroscopy, and magnetic susceptibility studies. One- and two-electron oxidation through reaction with I2 is further found to afford the corresponding iodo iron(II) and diiodo iron(III) complexes.

A low-coordinate iron organoazide complex.

A labile organoazide iron complex is reported. Under ambient conditions, the azide adduct is subject to a dissociation equilibrium in solution, yet also undergoes intramolecular C-H bond amination. Single-crystal irradiation of the azide at 80 K leads to partial N2-extrusion and formation of a putative imido iron intermediate, which was computationally identified as a highly covalent {FeNR}8 species.

Taming the stilbene radical anion.

Radical anions appear as intermediates in a variety of organic reductions and have recently garnered interest for their role as mediators for electron-driven catalysis as well as for organic electron conductor materials. Due to their unstable nature, the isolation of such organic radical anions is usually only possible by using extended aromatic systems, whereas non-aromatic unsaturated hydrocarbons have so far only been observed in situ. We herein report the first isolation, structure and spectroscopic characterization of a simple aryl substituted alkene radical anion, namely that of stilbene (1,2-diphenyl ethylene), achieved by encapsulation between two [K{18c6}] cations. The formation of the radical anion is accompanied by Z → E isomerization of the involved double bond, also on a catalytic scale. Employing the linear iron(i) complex [Fe(NR2)2]- as a reductant and coordination site also allows for this transformation, via formation of an iron(ii) bound radical anion. The use of the iron complex now also allows for Z → E isomerization of electron richer, simple alkenes bearing either mixed alkyl/aryl or even bis(alkyl) substitution.

A diarsene radical anion.

The isolation of the first diarsene radical anion by reduction of a neutral diarsene is presented. Comprehensive characterisation in conjunction with DFT calculations reveals unpaired spin density residing in the antibonding π*-orbital with involvement of the terphenyl ligands. First reactivity studies reveal no pronounced radical, but rather reducing properties.

Between imide, imidyl and nitrene - an imido iron complex in two oxidation states.

Imidyl and nitrene metal species play an important role in the N-functionalisation of unreactive C-H bonds as well as the aziridination of olefines. We report on the synthesis of the trigonal imido iron complexes [Fe(NMes)L2]0,- (L = -N{Dipp}SiMe3); Dipp = 2,6-diisopropyl-phenyl; Mes = (2,4,6-trimethylphenyl) via reaction of mesityl azide (MesN3) with the linear iron precursors [FeL2]0,-. UV-vis-, EPR-, 57Fe Mössbauer spectroscopy, magnetometry, and computational methods suggest for the reduced form an electronic structure as a ferromagnetically coupled iron(ii) imidyl radical, whereas oxidation leads to mixed iron(iii) imidyl and electrophilic iron(ii) nitrene character. Reactivity studies show that both complexes are capable of H atom abstraction from C-H bonds. Further, the reduced form [Fe(NMes)L2]- reacts nucleophilically with CS2 by inserting into the imido iron bond, as well as electrophilically with CO under nitrene transfer. The neutral [Fe(NMes)L2] complex shows enhanced electrophilic behavior as evidenced by nitrene transfer to a phosphine, yet in combination with an overall reduced reactivity.

Haloperoxidase activity of oxovanadium(V) thiobisphenolates.

By employing the 2,2'-thiobis(2,4-di-tert-butylphenolate) ligand ((S)L(2-)) a novel oxovanadium(V) complex, (PPh(4))(2)[(S)LV(O)(µ(2)-O)(2)V(O)(S)L] (1), was synthesised that exhibits haloperoxidase activity: on addition of H(2)O(2) a sequence of successive peroxide formation and intramolecular thioether oxidation events (sulfoxide and sulfone) led to a mixture of five products, which were all identified unambiguously, partly through an independent synthesis and characterisation. It was shown that internal thioether oxidation proceeds through peroxide formation, but the sulfoxidation of external thioether functions requires further activation of the peroxide function by protons or alkyl cations. Consistently, the employment of tBuOOH instead of H(2)O(2) led to a very active system for the catalytic sulfoxidation of thioethers.

Surface-inspired molecular vanadium oxide catalysts for the oxidative dehydrogenation of alcohols: evidence for metal cooperation and peroxide intermediates.

On the basis that thiacalix[4]arene (H(4)T4A) complex (PPh(4) )(2) [H(2)T4A(VO(2))](2) (Ia) was found to be an adequate functional model for surface species occurring on vanadium oxide based catalysts and itself catalyses the oxidative dehydrogenation (ODH) of alcohols, an analogue containing 2,2'-thiobis(2,4-di-tert-butylphenolate), (S)L(2-), as ligand, namely, (PPh(4))(2)[(S)LVO(2)](2) (II) was investigated in the same context. Despite the apparent similarity of Ia and II, studies on II revealed several novel insights, which are also valuable in connection with surfaces of vanadia catalysts: 1) For Ia and II similar turnover numbers (TONs) were found for the ODH of activated alcohols, which indicates that the additional OH units inherent to Ia do not contribute particularly to the activity of this complex, for instance, through prebinding of the alcohol. 2) On dissolution II enters into an equilibrium with a monomeric form, which is the predominant species in solution; nevertheless, ODH proceeds exclusively at the dimeric form, and this stresses the need for cooperation of two vanadium centres. 3) By omitting O(2) from the system during the oxidation of 9-fluorenol, the reduced form of the catalyst could be isolated and fully characterised (including single-crystal X-ray analysis). The corresponding intermediate had been elusive in case of thiacalixarene system Ia. 4) Reoxidation was found to proceed via a peroxide intermediate that also oxidises one alcohol equivalent. As the peroxide can also perform mono- and dioxygenation of the thioether group in II, after a number of turnovers the active catalyst contains a sulfone group. The reduced form of this ultimate catalyst was also isolated and structurally characterised. Possible implications of 1)-4) for the function of heterogeneous vanadia catalysts are discussed.

High-spin carbonyl complexes of iron(I) and cobalt(I).

Metal carbonyl complexes are almost exclusively found in a low-spin state due to the strong-field nature of the CO ligand. Here the characterisation of highly labile three-coordinate metal(I) monocarbonyl complexes of iron and cobalt is presented. Experimental and quantum chemical examinations reveal their high-spin configuration.