Μ-oxo Complex Research Articles

Tuning the visible colour of octahedral manganese(III) phthalocyanines via axial ligand exchange.

A series of [PcMnL2]SbF6 complexes (Pc = phthalocyanine) was synthesized and structurally characterized by stripping the chloride from PcMnCl with AgSbF6 in o-dichlorobenzene and adding a range of donor ligands (L = THF, pyridine, p-dimethylaminopyridine (DMAP), Ph3PO, N-methylimidazole (MeIm), MeCN) to the resulting solution. Addition of or exposure to water where L = heterocyclic amines yielded μ-oxo complexes of the form [PcMnL]2O, which were structurally characterized for L = DMAP and MeIm. The [PcMnL2]SbF6 complexes have an increased solubility in organic solvents, where the axial ligands inhibit the characteristic ring π-π aggregation of PcM complexes. A variety of colours were observed (blue/green to red/purple), with Q-band absorptions (excluding the μ-oxo species) spanning from 715-761 nm and LMCT-bands from 497-574 nm. The combination of the ligand-induced absorption shifts coupled with their relative intensities in the visible region is responsible for the observed colour range and illustrates that facile ligand exchange is a useful tool in producing materials with a variety of colours from PcMnCl.

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis.

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Structural and spectroscopical characterization of µ-oxo bridged Iron(III) bromide complexes of Pyclen ligands

Binuclear iron oxo-bridged complexes have found much interest since the early 80′s as synthetic models for relevant biological molecules, catalysts and magnetic probes. In this work, a series of iron(III)bromide-based oxo-bridged complexes of Pyclen ligands was prepared, with the aim of investigating the peculiar iron-oxygen bond features in different molecular environments. We observed the influence of the steric hindrance of the ligands employed in this study towards the formation of symmetrical and non-symmetrical µ-oxo complexes, and we confirmed this peculiar behaviour by single crystal X-ray diffraction and Raman spectroscopy. We report here structural and spectroscopical insight of three unprecedented oxo-iron complexes and their relative precursors, enlarging the existing database of these particular class of metal complexes.

Heterometallic iron(IV) μ-nitrido complexes supported by a tetradentate Schiff base ligand

A nitrido-bridged heterometallic iron(IV) chloride complex supported by a tetradentate Schiff base ligand has been synthesized and its chloride substitution reactions have been studied. Treatment of the Fe(II) Schiff base complex [Fe(Br4saloph)(py)] (H2Br4saloph = N,N’-bis(3,5-dibromosalicylidene)-1,2-phenylenediamine; py = pyridine) with the Ru(VI) nitride [Ru(N)Cl2(LOEt)] (1, LOEt− = [(η5-C5H5)Co{P(O)(OEt)2}3]−) afforded the μ-nitrido complex [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)Cl] (2-Cl). The short Fe-N(nitrido) [1.668(4) Å] and Ru-N(nitrido) [1.709(4) Å] distances and the roughly linear Fe-N-Ru linkage [172.9(3)o] in diamagnetic 2-Cl are indicative of the Fe(IV)=N=Ru(IV) bonding picture. Salt metathesis of 2-Cl with [Ag(CF3CO2)] and [Ag(ReO4)] afforded [{(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)(CF3CO2)}⋅Ag(CF3CO2)] (2-CF3CO2⋅Ag(CF3CO2)) and [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)(ReO4)] (2-ReO4), respectively. Chloride abstraction of 2-Cl with TlPF6, followed by recrystallization in air led to isolation of the μ-oxo complex [{2}2(μ3-O)Tl](PF6)2 (3), which is paramagnetic with a magnetic moment of 2.0 μB. Complex 3 features a [Ru-N-Fe-O(Tl)-Fe-N-Ru]2+ unit, the Tl atom of which is coordinated to the μ-oxo ligand and the oxygen atoms of the Schiff base ligands. The Ru-N(nitrido) (av. 1.709 Å), Fe-N(nitrido) (av. 1.659 Å) and Fe-O(oxo) (av. 1.991 Å) distances in 3 are consistent with M = N double bonds and Fe-O single bonds, respectively. Treatment of 2-Cl with [Tl(SC6F5)] afforded the Fe(III)/Ru(IV) nitrido complex [(LOEt)(py)ClRu(μ-N)Fe(Br4saloph)] (4) that has a longer Fe-N(nitrido) distance [1.753(4) Å] than the Fe(IV) congener 2-Cl does. The crystal structures of complexes 2-Cl, 2-CF3CO2⋅Ag(CF3CO2), 2-ReO4, 3, and 4 have been determined.

Dioxygen Reactivity of Copper(I)/Manganese(II)-Porphyrin Assemblies: Mechanistic Studies and Cooperative Activation of O2.

The oxidation of transition metals such as manganese and copper by dioxygen (O2) is of great interest to chemists and biochemists for fundamental and practical reasons. In this report, the O2 reactivities of 1:1 and 1:2 mixtures of [(TPP)MnII] (1; TPP: Tetraphenylporphyrin) and [(tmpa)CuI(MeCN)]+ (2; TMPA: Tris(2-pyridylmethyl)amine) in 2-methyltetrahydrofuran (MeTHF) are described. Variable-temperature (−110 °C to room temperature) absorption spectroscopic measurements support that, at low temperature, oxygenation of the (TPP)Mn/Cu mixtures leads to rapid formation of a cupric superoxo intermediate, [(tmpa)CuII(O2•–)]+ (3), independent of the presence of the manganese porphyrin complex (1). Complex 3 subsequently reacts with 1 to form a heterobinuclear μ-peroxo species, [(tmpa)CuII–(O22–)–MnIII(TPP)]+ (4; λmax = 443 nm), which thermally converts to a μ-oxo complex, [(tmpa)CuII–O–MnIII(TPP)]+ (5; λmax = 434 and 466 nm), confirmed by electrospray ionization mass spectrometry and nuclear magnetic resonance spectroscopy. In the 1:2 (TPP)Mn/Cu mixture, 4 is subsequently attacked by a second equivalent of 3, giving a bis-μ-peroxo species, i.e., [(tmpa)CuII−(O22−)−MnIV(TPP)−(O22−)−CuII(tmpa)]2+ (7; λmax = 420 nm and δpyrrolic = −44.90 ppm). The final decomposition product of the (TPP)Mn/Cu/O2 chemistry in MeTHF is [(TPP)MnIII(MeTHF)2]+ (6), whose X-ray structure is also presented and compared to literature analogs.

A novel μ-oxo-diruthenium(III,III)-ibuprofen-(4-aminopyridine) chloride derived from the diruthenium(II,III)-ibuprofen paddlewheel metallodrug shows anticancer properties

Diruthenium(II,III) metal-metal multiply bonded paddlewheel complexes bearing non-steroidal anti-inflammatory drugs are promising anticancer metallodrugs. The [Ru2(Ibp)4Cl] (Ibp, ibuprofenate anion from HIbp ibuprofen drug), free or encapsulated, shows anticancer activity against glioblastoma (in vitro, in vivo), and against human breast and prostate cancer cells. Herein we report the interaction of [Ru2(Ibp)4Cl] and of [Ru2(Ac)4(H2O)2]PF6 (Ac, acetate) with the 4-aminopyridine (4Apy) drug. The N-ligand was capable of cleaving the paddlewheel unit with oxidation of Ru2(II,III) to Ru2(III,III)O μ-oxo core in the ibuprofen complex while the acetate complex underwent axial substitution of water by 4Apy. Carefully designed synthetic and chromatographic methods succeeded in giving the novel [Ru2O(Ibp)2(4Apy)6]Cl2 metallodrug, the first diruthenium(III,III) μ-oxo having chloride as counterion. Characterization was performed by elemental analysis, mass spectrometry, thermogravimetric analysis, electronic absorption and vibrational spectroscopies, molar conductivity and cyclic voltammetry. Kinetic studies for the μ-oxo complex (in 50:50 v/v ethanol:water) suggested an aquation/complexation equilibrium in consecutive step reactions with the exchange of the two 4Apy trans to the μ-oxo bridge by water (aquation) and the back coordination of 4Apy in excess of the N-ligand (complexation). Trypan blue assays for the novel compound showed time- and dose- dependent antiproliferative effects (at 5–50 μmol L−1) and cytotoxicity (> 20 μmol L−1), and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide) assays gave IC50 value of 7.6 ± 1.5 μmol L−1 (at 48 h, 1–20 μmol L−1) against U87MG human glioblastoma cells (aggressive brain glioma cancer) pointing the metallodrug as potential candidate for novel therapies in gliomas.

Sunlight-induced dehydrogenation rearrangement of the dititanium complex [Ti(η5-C5HMe4)(μ-η1: η5-C5Me4)

Sunlight photolysis of the triplet state (S = 1) dititanium compound 1 induced its rearrangement to diamagnetic 2, which was a molecule containing two hydrogens less. The cage structure of 2 was determined by single crystal X-ray diffraction and was corroborated fully by 1H and 13C NMR spectra that identified three simple bridging moieties — a hydrogen atom, a methine carbon and a methylene carbon. Its extremely short Ti−Ti distance 2.7537(7) Å causes a strong distortion of the bridging fulvalene ligand, which becomes η5- bonded to one Ti(IV) and connected by two σ-bonds from nearest carbon atoms of its other ring to the other Ti(IV) of 2. Other photolytical byproducts in very minor amounts were also isolated, which differed from 2 by the absence of one bridging moiety: compound 4 lacked the methine group and compound 5 had no bridging hydrogen. In the latter cases, the Ti−Ti distance fell in the range 3.2077(8) – 3.5543(12) Å and the fulvalene ligand was coordinated in the common µ-η5: η5-mode to both titanium atoms. The octamethylfulvalene ligands in 4 and 5 had their ring planes rotated mutually by about 40° in order to relieve the steric hindrance between their methyl groups. The thermally robust µ-oxo complex 3 obtained from reacting 2 with water showed a very similar structure.

Diiron oxo reactivity in a weak-field environment.

Concomitant deprotonation and metalation of a dinucleating cofacial Pacman dipyrrin ligand platform tBudmxH2 with Fe2(Mes)4 results in formation of a diiron complex ( tBudmx)Fe2(Mes)2. Treatment of ( tBudmx)Fe2(Mes)2 with one equivalent of water yields the diiron μ-oxo complex ( tBudmx)Fe2(μ-O) and free mesitylene. A two-electron oxidation of ( tBudmx)Fe2(μ-O) gives rise to the diferric complex ( tBudmx)Fe2(μ-O)Cl2, and one-electron reduction from this FeIIIFeIII state allows for isolation of a mixed-valent species [Cp2Co][( tBudmx)Fe2(μ-O)Cl2]. Both ( tBudmx)Fe2(μ-O) and [Cp2Co][( tBudmx)Fe2(μ-O)Cl2] exhibit basic character at the bridging oxygen atom and can be protonated using weak acids to form bridging diferrous hydroxide species. The basicity of the diferrous oxo ( tBudmx)Fe2(μ-O) is quantified through studies of the pK a of its conjugate acid, [( tBudmx)Fe2(μ-OH)]+, which is determined to be 15.3(6); interestingly, upon coordination of neutral solvent ligands to yield ( tBudmx)Fe2(μ-O)(thf)2, the basicity is increased as observed through an increase in the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)(thf)2]+ to 26.8(6). In contrast, attempts to synthesize a diferric bridging hydroxide by two-electron oxidation of [( tBudmx)Fe2(μ-OH)(thf)2]+ resulted in isolation of ( tBudmx)Fe2(μ-O)Cl2 with concomitant loss of a proton, consistent with the pK a of the conjugate acid [( tBudmx)Fe2(μ-OH)Cl2]+ determined computationally to be -1.8(6). The foregoing results highlight the intricate interplay between oxidation state and reactivity in diiron μ-oxo units.

Influence of substituents in meso-aryl groups of iron μ-oxo porphyrins on their catalytic activity in the oxidation of cycloalkanes

The aim of this work was to study the effect of substituents on the catalytic activity of iron μ-oxo porphyrins in oxidation of cycloalkanes. Electron-donating or electron-withdrawing substituents were introduced in meso-aryl groups of iron μ-oxo porphyrins. An important part of the work was the characterization of the catalysts, in particular their redox properties. Catalytic performance and selectivity were evaluated using cyclopentane, cyclohexane, and cyclooctane as model compounds. It was shown that not only electron-withdrawing but also electron-donating substituents improved the catalytic performance of iron μ-oxo complexes. Moreover, catalytic activity of iron μ-oxo porphyrins with electron-withdrawing substituents correlated with the half-wave potential E1/2 while the catalytic activity of iron μ-oxo porphyrins with electron-donating substituents increased with the decrease of reduction potential ERED.

Effect of Inter-Porphyrin Distance on Spin-State in Diiron(III) μ-Hydroxo Bisporphyrins.

The synthesis, structure, and properties of bischloro, μ-oxo, and a family of μ-hydroxo complexes (with BF4 (-) , SbF6 (-) , and PF6 (-) counteranions) of diethylpyrrole-bridged diiron(III) bisporphyrins are reported. Spectroscopic characterization has revealed that the iron centers of the bischloro and μ-oxo complexes are in the high-spin state (S=(5) /2 ). However, the two iron centers in the diiron(III) μ-hydroxo complexes are equivalent with high spin (S=(5) /2 ) in the solid state and an intermediate-spin state (S=(3) /2 ) in solution. The molecules have been compared with previously known diiron(III) μ-hydroxo complexes of ethane-bridged bisporphyrin, in which two different spin states of iron were stabilized under the influence of counteranions. The dimanganese(III) analogues were also synthesized and spectroscopically characterized. A comparison of the X-ray structural parameters between diethylpyrrole and ethane-bridged μ-hydroxo bisporphyrins suggest an increased separation, and hence, less interactions between the two heme units of the former. As a result, unlike the ethane-bridged μ-hydroxo complex, both iron centers become equivalent in the diethylpyrrole-bridged complex and their spin state remains unresponsive to the change in counteranion. The iron(III) centers of the diethylpyrrole-bridged diiron(III) μ-oxo bisporphyrin undergo very strong antiferromagnetic interactions (J=-137.7 cm(-1) ), although the coupling constant is reduced to only a weak value in the μ-hydroxo complexes (J=-42.2, -44.1, and -42.4 cm(-1) for the BF4 , SbF6 , and PF6 complexes, respectively).

Revisiting oxo-centered carbonyl-triruthenium clusters: investigating CO photorelease and some spectroscopic and electrochemical correlations

We synthesized and characterized a series of oxo-centered carbonyl-triruthenium complexes with the general formula [Ru3O(CH3COO)6(L)2(CO)], where L = 2,6-dimethylpyrazine (dmpz) (1), isonicotinamide (adpy) (2), 4-acetylpyridine (acpy) (3), 3-methylpyridine (3-pic) (4), 4-methylpyridine (4-pic) (5), 4-tert-butylpyridine (4-tbpy) (6), 4-(dimethyl)aminopyridine (dmap) (7), or 4-aminopyridine (ampy) (8); we also investigated the photoreactivity of these complexes. Single-crystal X-ray diffraction helped to elucidate the structures of 1·H2O, 7·C2H4Cl2, and 8. The unit cell of 8 is composed of four cluster units; the hydrogen bonds between the amino groups of the terminal ligand of a neighboring molecule and the oxygen atoms of CO or acetate bridging ligands hold these cluster units together. The spectroscopic (NMR, UV-visible, and IR) and the electrochemical properties (cyclic voltammetry) of these complexes correlated with the ancillary ligands in terms of their σ-donating and π-accepting characteristics. The molecular orbital and the electronic localized description of the [Ru3O]-CO unit helped to rationalize the correlations. The photoreactivity of compounds 1-8 was investigated by laser excitation at 377 nm. Given the CO photorelease quantum yields, σ-donor ligands and aqueous medium (more polar) stabilized the charge-transfer excited state that culminated in CO photosubstitution, leading to higher Φ values.

Magnesium(I) Dimers Bearing Tripodal Diimine-Enolate Ligands: Proficient Reagents for the Controlled Reductive Activation of CO2 and SO2.

The first examples of magnesium(I) dimers bearing tripodal ligands, [(Mg{κ(3) -N,N',O-(ArNCMe)2 (OCCPh2 )CH})2 ] [Ar=2,6-iPr2 C6 H3 (Dip) 7, 2,6-Et2 C6 H3 (Dep) 8, or mesityl (Mes) 9] have been prepared by post-synthetic modification of the β-diketiminato ligands of previously reported magnesium(I) systems, using diphenylketene, OCCPh2 . In contrast, related reactions between β-diketiminato magnesium(I) dimers and the isoelectronic ketenimine, MesNCCPh2 , resulted in reductive insertion of the substrate into the MgMg bond of the magnesium(I) reactant, and formation of [{(Nacnac)Mg}2 {μ-κ(2) -N,C-(Mes)NCCPh2 }] (Nacnac=[(ArNCMe)2 CH](-) ; Ar=Dep 10 or Mes 11). Reactions of the four-coordinate magnesium(I) dimer 8 with excess CO2 are readily controlled, and cleanly give carbonate [(LMg)2 (μ-κ(2) :κ(2) -CO3 )] 12 (L=[κ(3) -N,N',O-(DepNCMe)2 (OCCPh2 )CH](-) ; thermodynamic product), or oxalate [(LMg)2 (μ-κ(2) :κ(2) -C2 O4 )] 13 (kinetic product), depending on the reaction temperature. Compound 12 and CO are formed by reductive disproportionation of CO2 , whereas 13 results from reductive coupling of two molecules of the gas. Treatment of 8 with an excess of N2 O cleanly gives the μ-oxo complex [(LMg)2 (μ-O)] 14, which reacts facilely with CO2 to give 12. This result presents the possibility that 14 is an intermediate in the formation of 12 from the reaction of 8 and CO2 . In contrast to its reactions with CO2 , 8 reacts with SO2 over a wide temperature range to give only one product; the first example of a magnesium dithionite complex, [(LMg)2 (μ-κ(2) :κ(2) -S2 O4 )] 16, which is formed by reductive coupling of two molecules of SO2 , and is closely related to f-block metal dithionite complexes derived from similar SO2 reductive coupling processes. On the whole, this study strengthens previously proposed analogies between the reactivities of magnesium(I) systems and low-valent f-block metal complexes, especially with respect to small molecule activations.

Formation and Subsequent Reactivity of a N2-Stabilized Cobalt–Hydride Complex

The reduced heterobimetallic Co/Zr complex N2Co(iPr2PNMes)3Zr(THF) (1) has been previously reported to react with the C═O bonds of CO2 and benzophenone to generate Zr/Co μ-oxo complexes OC-Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-CO2) and Ph2C═Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (1-Ph2CO), respectively. Herein, we report a similar reaction of 1 with pyridine-N-oxide to form an analogous complex (pyridine)Co(iPr2PNMes)2(μ-O)Zr(iPr2PNMes) (2) with a more labile ligand bound to cobalt. Much like 1-CO2 and 1-Ph2CO, compound 2 reacts with Ph3SiH via formation of a Si–O linkage to form (N2)(H)Co(iPr2PNMes)3ZrOSiPh3 (5). The dinitrogen ligand in 5 is weakly bound and can be readily removed in vacuo or displaced by other L-type ligands. This allows complex 5 to undergo insertion reactions with unsaturated substrates, including diphenyldiazomethane, CO2, benzonitrile, and phenylacetylene to give hydrazonato (Ph2C═NNH)Co(iPr2PNMes)3ZrOSiPh3 (7), formate (OC(H)O)Co(iPr2PNMes)3ZrOSiPh3 (8), ketimide (PhHC═N)Co(iPr2PNMes)3ZrO...

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing μ-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs.

While nitric oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO2(-)) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a3/CuB active site of cytochrome c oxidase. In this report, the oxidation of NO(g) to nitrite is shown to occur efficiently in new synthetic μ-oxo heme-Fe(III)-O-Cu(II)(L) constructs (L being a tridentate or tetradentate pyridyl/alkylamino ligand), and spectroscopic and kinetic investigations provide detailed mechanistic insights. Two new X-ray structures of μ-oxo complexes have been determined and compared to literature analogs. All μ-oxo complexes react with 2 mol equiv NO(g) to give 1:1 mixtures of discrete [(L)Cu(II)(NO2(-))](+) plus ferrous heme-nitrosyl compounds; when the first NO(g) equiv reduces the heme center and itself is oxidized to nitrite, the second equiv of NO(g) traps the ferrous heme thus formed. For one μ-oxo heme-Fe(III)-O-Cu(II)(L) compound, the reaction with NO(g) reveals an intermediate species ("intermediate"), formally a bis-NO adduct, [(NO)(porphyrinate)Fe(II)-(NO2(-))-Cu(II)(L)](+) (λmax = 433 nm), confirmed by cryo-spray ionization mass spectrometry and EPR spectroscopy, along with the observation that cooling a 1:1 mixture of [(L)Cu(II)(NO2(-))](+) and heme-Fe(II)(NO) to -125 °C leads to association and generation of the key 433 nm UV-vis feature. Kinetic-thermodynamic parameters obtained from low-temperature stopped-flow measurements are in excellent agreement with DFT calculations carried out which describe the sequential addition of NO(g) to the μ-oxo complex.

Reactions of a heme-superoxo complex toward a cuprous chelate and •NO(g): CcO and NOD chemistry.

Following up on the characterization of a new (heme)FeIII-superoxide species formed from the cryogenic oxygenation of a ferrous-heme (PPy)FeII (1) (PPy = a tetraarylporphyrinate with a covalently tethered pyridine group as a potential axial base), giving (PPy)FeIII-O2•- (2) (Li Y et al., Polyhedron 2013; 58: 60-64), we report here on (i) its use in forming a cytochrome c oxidase (CcO) model compound, or (ii) in a reaction with nitrogen monoxide (•NO; nitric oxide) to mimic nitric oxide dioxygenase (NOD) chemistry. Reaction of (2) with the cuprous chelate [CuI(AN)][B(C6F5)4] (AN = bis[3-(dimethylamino) propyl]amine) gives a meta-stable product [(PPy)FeIII-([Formula: see text])-CuII(AN)][B(C6F5)4] (3a), possessing a high-spin iron(III) and Cu(II) side-on bridged peroxo moiety with a μ-η2:η2-binding motif. This complex thermally decays to a corresponding μ-oxo complex [(PPy)FeIII-(O2-)-CuII(AN)][B(C6F5)4] (3). Both (3) and (3a) have been characterized by UV-vis, 2H NMR and EPR spectroscopies. When (2) is exposed to •NO(g), a ferric heme nitrato compound forms; if 2,4-di-tert-butylphenol is added prior to •NO(g) exposure, phenol ortho-nitration occurs with the iron product being the ferric hydroxide complex (PPy) FeIII(OH) (5). The latter reactions mimic the action of NOD's.

Synthetic and Mechanistic Study of the Catalytic Enantioselective Preparation of Primary β-Amino Ketones from Enones and a Fluorinated Gabriel Reagent

The salen μ-oxo complex of aluminum 1 catalyzes the asymmetric 1,4-addition of the novel ammonia equivalent 3,4,5,6-tetrafluorophthalimide to unsaturated ketones. All of the reagents are inexpensive and are readily available. The products are formed in up to 89% yield and up to 96% ee. The tetrafluorophthalimide group is removed under mild chemoselective conditions and in high yields to afford the free primary amines. Mechanistic studies suggest that the reaction occurs through a dual activation mechanism. A pre-equilibrium formation of a 1:1 complex between tetrafluorophthalimide and the catalyst is observed. The rate-determining step is the addition of tetrafluorophthalimide catalyst complex to the catalyst activated enone. These mechanistic studies provide important clues for the further development of catalytic asymmetric reactions.

The structural conversion of multinuclear titanium(IV) μ-oxo-complexes.

For the first time we report the structural conversion processes of hexanuclear μ-oxo-Ti(IV) complexes into tetranuclear ones. Single-crystal X-ray diffraction studies reveal that metastable hexanuclear μ-oxo complexes ([Ti6O6(O(t)Bu)(O2CR')6]) are formed in the first stage of reactions between [Ti(O(t)Bu)4] and branched carboxylic acids R'COOH (R' = C(Me)2Et, CH2(t)Bu, (t)Bu). In the next stage they convert into tetranuclear μ-oxo-Ti(IV) complexes of the formula [Ti4O4(O(t)Bu)4(O2CR')4]. Spectroscopic investigations ((13)C NMR, IR, and MS) proved that the conversion of hexanuclear clusters relies on the attachment of smaller units (e.g., [Ti3O(O(t)Bu)8(O2CR')2] or [Ti4O2(O(t)Bu)6(O2CR')6]) and intermediate species formation (e.g., [Ti9O8(O(t)Bu)14(O2CR')6]). The decomposition of intermediate systems in the next reaction stage leads to the formation of tetranuclear clusters. The type of solvent used in the synthesis of multinuclear oxo-Ti(IV) complexes is an important factor, which influences the kind of clusters formed.

Synthesis, Characterization, and Reactions of Ruthenium(II), -(III), and -(IV) Complexes with Sterically Demanding 1,2,4-Tri-tert-butylcyclopentadienyl Ligands

The reaction of RuCl3·3H2O with 1,3,5-C5(tBu)3H3 afforded a dimeric Ru(III) dichloride complex, [Cp‡RuCl(μ-Cl)]2 (1b; Cp‡ = η5-1,2,4-C5(tBu)3H2). The treatment of 1b with gaseous oxygen afforded the tetravalent ruthenium μ-oxo complex [Cp‡RuCl2]2(μ-O) (2b). The treatment of 2b with a primary or secondary alcohol regenerated 1b with concomitant dehydrogenative oxidation of the alcohol to afford the corresponding aldehyde or ketone, respectively. The treatment of 2b with refluxing 2-propanol or an excess of zinc powder in tetrahydrofuran efficiently afforded a novel dimeric Ru(II) monochloride with a butterfly structure, [Cp‡Ru(μ-Cl)]2 (4), with unsaturated ruthenium centers and high reactivity toward electron-donating reagents. The reaction of 1b with 4 afforded the mixed-valence Ru(III)/Ru(II) complex Cp‡Ru(μ-Cl)3RuCp‡ (7). The treatment of the appropriate precursor 1b or 2b with an excess of a hydride reagent such as LiEt3BH or LiAlH4 afforded the novel dinuclear ruthenium tetrahydride-bridged complex Cp‡Ru(μ-H)4RuCp‡ (9b). The treatment of a mixture of [Cp*Ru(μ3-Cl)]4 and 4 with LiEt3BH afforded the analogous dinuclear ruthenium tetrahydride-bridged complex bearing Cp* (Cp* = η5-C5Me5) and Cp‡ ligands, Cp*Ru(μ-H)4RuCp‡ (9c). The molecular structures of the newly synthesized complexes 1b, 2b, 4, 7, 8b, and 9b,c were determined by X-ray diffraction (XRD) studies. The reaction of 9b with ethylene exclusively afforded a μ-ethylidyne μ-ethylidene complex, Cp‡Ru(μ-CCH3)(μ-CHCH3)(μ-H)RuCp‡ (11), whereas the reaction of 9a with excess ethylene selectively afforded a dinuclear ruthenium diviny ethylene complex, Cp*Ru(μ-CHCH2)2(CH2CH2)RuCp* (10). The difference in the reactivities can be attributed to the difference in the spatial size of the reaction sites rather than their electronic properties such as electron density at the ruthenium centers. Irradiation of 11 with UV light (365 nm) effectively afforded a bis(μ-ethylidyne) complex, Cp‡Ru(μ-CCH3)2RuCp‡ (12). The structures of 11 and 12 were also determined by XRD studies.

Zirconium Hydrazides as Metallanitrene Synthons: Release of Molecular N2 from a Hydrazinediido Complex Induced by Oxidative N–N Bond Cleavage

The N–N bond in the zirconium hydrazinediido(2−) complex [Zr(N2TBSNpy)(NNPh2)(py)] (1) is readily cleaved by one-electron oxidation. Reacting [Zr(N2TBSNpy)(NNPh2)(py)] (1) with 0.5 molar equiv of iodine led to the release of molecular N2 and yielded the mixed diphenylamido/iodo complex [Zr(N2TBSNpyNPh2)(I)] (2). Exposure of hydrazinediide 1 to an excess of iodine resulted in further oxidation of the diphenylamido ligand, yielding the diiodo complex 3 and tetraphenylhydrazine. Similar reactivity was observed in the reaction of 1 with diphenyl diselenide and diaryl disulfides, which reacted to give the corresponding diphenylamido/arylchalcogenido complexes [Zr(N2TBSNpyNPh2)(SePh)] (4a) and [Zr(N2TBSNpy)(NPh2)(SAr)] (Ar = Ph (4b), C6F5 (4c)) along with N2. The reactions were also carried out on an NMR scale with a 15Nα-labeled hydrazido complex (1-15N). In all cases a single 15N NMR resonance at 310.16 ppm, assigned to 15N2, indicated the formation of dinitrogen from the Nα atom in the hydrazide. A crossover labeling experiment employing a 1:1 mixture of 1 and 15Nα-labeled 1-15N revealed that the isotope distribution is, as expected, statistical 1:2:1 (14N2: 14/15N2: 15N2), which is consistent with a reaction pathway involving a dinuclear intermediate in the dinitrogen-forming step. Complex 1 reacted with N2O to give a mixture of two compounds, the bis(diphenylamido) complex 6 and the doubly bridged μ-oxo complex 7. In contrast, reaction of 1 with 1 molar equiv of pyridinium N-oxide only gave the doubly bridged μ-oxo complex 7 along with 2,2′-bipyridine and diphenylamine.

A seven-coordinate iron platform and its oxo and nitrene reactivity

We present a new structurally determined seven-coordinate iron platform supported by the tris(2-picolyl)amine ligand 6,6′-(pyridin-2-ylmethylazanediyl)bis(methylene)bis( N- tert-butylpicolinamide) (TPA 2C(O)NH t Bu, 3) and its reactivity with oxo and nitrene transfer agents. Oxidation of the pentagonal bipyramidal, seven-coordinate iron(II)–triflate complex [TPA 2C(O)NH t BuFe II(OTf)][OTf] ( 4) with PhIO produces the corresponding diiron(III) μ-oxo complex [(TPA 2C(O)NH t BuFe III) 2(O)][OTf] 4 ( 5). Mössbauer and magnetic measurements on 5 in the solid-state establish antiferromagnetic coupling between its two Fe(III) centers. Reactions of 4 with the nitrene transfer agents PhINTs (Ts = p-MeC 6H 4SO 2) and PhINNs (Ns = p-NO 2C 6H 4SO 2) provide the corresponding iron(III)-amide congeners [TPA 2C(O)NH t BuFe III(NHTs)][OTf] 2 ( 6) and [TPA 2C(O)NH t BuFe III(NHNs)][OTf] 2 ( 7), respectively, affording a rare pair of isolable Fe(III)-amide compounds formed from nitrene transfer. By characterizing well-defined products in the crystalline form, derived from atom and group transfer to seven-coordinate iron, the collective data provide a starting point for the exploration of high-valent and metal–ligand multiply bonded species supported by approximate pentagonal-type ligand fields.