Heteropolymetallic [FeFe]-Hydrogenase Mimics: Synthesis and Electrochemical Properties.

The synthesis and electrochemical properties of tetranuclear [Fe2S2]-hydrogenase mimic species containing Pt(II), Ni(II), and Ru(II) complexes have been studied. To this end, a new tetranuclear [Fe2S2] complex containing a 5,5'-diisocyanide-2,2'-bipyridine bridging ligand has been designed and coordinated to the metal complexes through the bipyridine moiety. Thus, the tetranuclear [Fe2S2] complex (6) coordinates to Pt(II), Ni(II) and Ru(II) yielding the corresponding metal complexes. The new metal center in the bipyridine linker modulates the electronic communication between the redox-active [Fe2S2] units. Thus, electrochemical studies and DFT calculations have shown that the presence of metal complexes in the structure strongly affect the electronic communication between the [Fe2S2] centers. In the case of diphosphine platinum compounds 10, the structure of the phosphine ligand plays a crucial role to facilitate or to hinder the electronic communication between [Fe2S2] moieties. Compound 10a, bearing a dppe ligand, shows weak electronic communication (ΔE = 170 mV), whereas the interaction is much weaker in the Pt-dppp derivative 10b (ΔE = 80 mV) and virtually negligible in the Pt-dppf complex 10c. The electronic communication is facilitated by incorporation of a Ru-bis(bipyridine) complex, as observed in the BF4 salt 12 (ΔE = 210 mV) although the reduction of the [FeFe] centers occurs at more negative potentials. Overall, the experimental-computational procedure used in this work allows us to study the electronic interaction between the redox-active centers, which, in turn, can be modulated by a transition metal.

Synthesis, Structure, and Photophysical Properties of Platinum(II) (N,C,N') Pincer Complexes Derived from Purine Nucleobases†.

The synthesis of a series of Pt{κ3-N,C,N'-[L]}X (X = Cl, RC≡C) pincer complexes derived from purine and purine nucleosides is reported. In these complexes, the 6-phenylpurine skeleton provides the N,C-cyclometalated fragment, whereas an amine, imine, or pyridine substituent of the phenyl ring supplies the additional N'-coordination point to the pincer complex. The purine N,C-fragment has two coordination positions with the metal (N1 and N7), but the formation of the platinum complexes is totally regioselective. Coordination through the N7 position leads to the thermodynamically favored [6.5]-Pt{κ3-N7,C,N'-[L]}X complexes. However, the coordination through the N1 position is preferred by the amino derivatives, leading to the isomeric kinetic [5.5]-Pt{κ3-N1,C,N'-[L]}X complexes. Extension of the reported methodology to complexes having both pincer and acetylide ligands derived from nucleosides allows the preparation of novel heteroleptic bis-nucleoside compounds that could be regarded as organometallic models of Pt-induced interstrand cross-link. Complexes having amine or pyridine arms are green phosphorescence emitters upon photoexcitation at low concentrations in CH2Cl2 solution and in poly(methyl methacrylate) (PMMA) films. They undergo self-quenching at high concentrations due to molecular aggregation. The presence of intermolecular π-π stacking and weak Pt···Pt interactions was also observed in the solid state by X-ray diffraction analysis.

Two Types of σ-Allenyl Complexes from Reactions of Silylenes and Germylenes with Chromium Fischer Alkynyl(alkoxy)carbenes.

The reactivity of amidinatotetrylenes of the type E(tBu2 bzm)R1 (E=Si, Ge; tBu2 bzm=N,N'-bis(tertbutyl)benzamidinate; R1 =alkyl or aryl) with the chromium Fischer alkynylcarbene complexes [Cr{C(OEt)C2 R2 }(CO)5 ] (R2 =Ph; ferrocenyl, Fc) has been studied. At room temperature, two different reaction pathways have been identified: (a) attack of the amidinatotetrylene to the alkynyl C2 atom (γ-attack), which leads to σ-allenyl complexes in which the original Ccarbene atom maintains its attachment to the Cr(CO)5 and OEt groups (compounds 3 E - R 1 - R 2 ), and (b) attack of the amidinatotetrylene to the Ccarbene atom (α-attack), which ends in σ-allenyl complexes in which the original Ccarbene atom is not attached to the metal atom and has been inserted into an E-N bond of the amidinatotetrylene forming an E-C-N-C-N five-membered ring (compounds 4 E - R 1 - R 2 ). It has been found that compounds 3 E - R 1 - R 2 are thermodynamically less stable than their corresponding 4 E - R 1 - R 2 isomers and that some of the former (E=Ge; R1 =CH2 SiMe3 ) can be transformed into the latter upon heating. At high temperatures (>70 °C) the reactions involving bulky amidinatotetrylenes (R1 =Mes, tBu) end in the carbene-substitution products [Cr{E(tBu2 bzm)R1 }(CO)5 ].

Reversible Carbene Insertion into a Ge-N Bond and Insights into CO and Carbene Substitution Reactions Involving Amidinatogermylenes and Fischer Carbene Complexes.

The formation of the five-membered-ring germylene complexes [M(CO)5 {Ge(tBu2 bzamC(OEt)Me)tBu}] (3M ; M=Cr, W), which occurs readily at room temperature from the germylene Ge(tBu2 bzam)tBu (1tBu ) and Fischer carbenes [M(CO)5 {C(OEt)Me}] (2M ; M=Cr, W), has been found to be reversible. Upon heating at 60 °C, complexes 3M undergo epimerization to an equilibrium mixture of 3M and 3'M . At that temperature, the chromium epimers (but not the tungsten ones) release CO to end in the mixed germylene-Fischer carbene complexes [Cr(CO)4 {C(OEt)Me}{Ge(tBu2 bzam)tBu}] (cis-4Cr and trans-4Cr ). The latter decompose at 120 °C to [Cr(CO)5 {Ge(tBu2 bzam)tBu}] (6Cr ). Because the formation of cis-4Cr and trans-4Cr from 3Cr or 3'Cr requires the presence of free 1tBu and 2Cr in the reaction solutions, the reactions of 1tBu with 2M to give 3M (and 3'M at 60 °C) should be reversible. This proposal has been proven by germylene-exchange crossover reactions in which free 1tBu and [M(CO)5 {Ge(tBu2 bzamC(OEt)Me)CH2 SiMe3 }] (5'M ; M=Cr, W) were formed when complexes 3M were treated at room temperature with the germylene Ge(tBu2 bzam)CH2 SiMe3 (1tmsm ). A clear differential behavior between N-heterocyclic carbenes (NHCs) and amidinatogermylenes (1tBu and 1tmsm ) in their reactivity against group 6 metal Fischer carbene complexes is demonstrated. The higher electron-donor capacity of amidinatogermylenes with respect to NHCs and the bias of the former to get involved in ring expansion processes are responsible for this differential behavior.

Unexpected Zwitterionic Allenyls from Silylenes and a Fischer Alkynylcarbene: A Remarkable Silylene-Promoted Rearrangement.

The silylenes Si(tBu2 bzam)R (tBu2 bzam=N,N'-bis(tertbutyl)benzamidinate; R=mesityl, CH2 SiMe3 ) attack the Ccarbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)5 {C(OEt)C2 Ph}] to give, after a striking rearrangement, zwitterionic σ-allenyl complexes in which the original carbene C atom forms part of the allene C3 fragment and also of a Si-C-N-C-N five-membered ring after insertion into a Si-N bond of the original amidinatosilylene. These remarkable allenyl products, which contain two stereogenic groups, are selectively formed as single diastereomers.

Triazole-Containing [FeFe] Hydrogenase Mimics: Synthesis and Electrocatalytic Behavior.

Through a Cu-catalyzed Huisgen cycloaddition between terminal alkynes and azides (CuAAC) reaction, azide [(µ-SCH2)2N(4-N3C6H4)Fe2(CO)6] has demonstrated to be a robust and versatile reagent able to incorporate the [(µ-SR)2Fe2(CO)6] fragment on a wide range of substrates, ranging from aromatic compounds to nucleosides, metallocenes, or redox and luminescent markers. The [FeIFeI]/[Fe0FeI] and [Fe0FeI]/[Fe0Fe0] reduction potentials of the triazole derivatives prepared are comparable to those of other aminodithiolate (adt) Fe-Fe hydrogenase mimics. The presence of the triazole linker influences the electrochemical behavior of these complexes depending on the strength of the acid employed.

First Insertions of Carbene Ligands into Ge-N and Si-N Bonds.

The insertion of carbene ligands into Ge-N (three examples) and Si-N (one example) bonds has been achieved for the first time by treating Fischer carbene complexes (M=W, Cr) with bulky amidinatotetrylenes (E=Ge, Si). These reactions, which start with a nucleophilic attack of the amidinatotetrylene heavier group 14 atom to the carbene C atom, proceed through a stereoselective insertion of the carbene fragment into an E-N bond of the amidinatotetrylene ENCN four-membered ring, leading to [M(CO)5 L] derivatives in which L belongs to a novel family of tetrylene ligands comprising an ECNCN five-membered ring.

Sequential Reactions of Alkynes on an Iridium(III) Single Site.

Sequential insertion of terminal alkynes on IrIII cyclometalated complexes allow the formation of novel metallapolycycles in a controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, C(sp3 )-H activation, and reductive coupling) on a single IrIII center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure. The sequence is a multicomponent reaction in which the metal not only promotes the different steps but also determines their stereoselectivity. This is an elegant example of the synergy between a metal-promoting reaction and a symmetry-defined stereochemistry.

Effect of a κ1-Bonded-M-1,2,3-triazole (M = Co, Ru) on the Structure and Reactivity of Group 6 Alkoxy (Fischer) Carbenes.

The [3 + 2] cycloaddition of two different metal-bound azides, [(Me4cyclam)CoII(N3)]ClO4 and (η5-C5H5)(dppe)RuII(N3), (dppe = Ph2PCH2CH2PPh2) with Cr(0) and W(0) (ethoxy)(alkynyl) Fischer carbenes has been efficiently used for the preparation of polymetallic metal-carbene complexes. The presence of the κ1-bonded metal triazole causes a significant influence on the electronic properties, structure, and reactivity of this new class of Fischer alkoxycarbenes. For the Ru(II) derivatives, their chemical behavior is considerably influenced by the interaction of the (η5-C5H5)(dppe)RuII-triazole moiety with the empty p-carbene orbital that provokes a noticeable decrease in the electrophilicity of the M═C carbon (manifested by the shielding of the 13C NMR chemical shifts). In turn, in the Co(II) derivatives, the incorporation of the (Me4cyclam)CoII moiety diminishes the aromaticity of the triazole ring and has a marked effect on the energy and distribution of the LUSO orbital, mostly resident on the Co(II) fragment. The almost negligible participation of the carbene moiety in the LUSO makes this position unable to react with nucleophiles. The reactions reported in this work constitute the first examples of [3 + 2] cycloaddition of azides and alkynyl Fischer carbene complexes in solution.

An Entry to Mixed NHC-Fischer Carbene Complexes and Zwitterionic Group 6 Metal Alkenyls.

The addition of NHCs to α,ß-unsaturated Cr(0) and W(0) (Fischer) carbene complexes is strongly dependent on the electrophilicity of the carbene carbon. Electrophilic alkoxy-carbene complexes quantitatively react with NHCs to yield stable zwitterionic (racemic) Cr(0) - and W(0) -alkenyls with total regio- and E-stereoselectivity. Less electrophilic aminocarbenes react with NHCs to promote the displacement of a CO ligand and yield "mixed" NHC/Fischer biscarbenes in a process that is unprecedented in group 6 metal-carbene chemistry. In fact, the compounds prepared, are some of the scarce examples of Fischer bisylidenes reported in the literature. The electrochemistry of the zwitterionic Cr(0) - and W(0) -alkenylcomplexes made, show that these compounds have a strong reductor character, which is demonstrated in their reactions towards [Ph3 C][PF6 ]. The oxidation processes lead to new types of cationic Fischer mono- and biscarbene complexes having a charged NHC fragment in their structures, in a new example of the use of electron-transfer reactions as a method to prepare novel group 6 (Fischer) carbene complexes.

Mono- and bimetallic zwitterionic chromium(0) and tungsten(0) allenyls.

A series of stable chiral (racemic), formally neutral, zwitterionic mono- and bimetallic M(CO)5[C(OEt)═C═CR(NHC)] (M = Cr, W) σ-allenyls are ready available by the addition of N-heterocyclic carbenes (NHCs) to Cr(0) and W(0) alkynyl Fischer carbene complexes. Different classes of NHCs, (e.g., 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene, and their six- and seven-membered analogues and 1,3-bis(dimethyl)imidazol-2-ylidene) were employed as nucleophiles in these C-C bond-forming reactions yielding the novel complexes in essentially quantitative yields. A systematic experimental and computational study of the electronic properties of the Cr- and W-allenyls shows that their UV-vis spectra are directly influenced by the structure of the heterocyclic moiety derived from the NHC (ring size, substituents on the N atoms) and by the nature of the metal fragment (Cr/W). The electron-releasing nature of these complexes allows them to participate in electron-transfer reactions in the ground state, leading to a type of charged α,ß-unsaturated Fischer carbenes that incorporate an NHC fragment in their structure.

On the use of metal purine derivatives (M=Ir, Rh) for the selective labeling of nucleosides and nucleotides.

The reactions of neutral or cationic IrIII and RhIII derivatives of phenyl purine nucleobases with unsymmetrical alkynes produce new metallacycles in a predictable manner, which allows for the incorporation of either photoactive (anthracene or pyrene) or electroactive (ferrocene) labels in the nucleotide or nucleoside moiety. The reported methodology (metalation of the purine derivative and subsequent marker insertion) could be used for the postfunctionalization and unambiguous labeling of oligonucleotides.

The selective synthesis of metallanucleosides and metallanucleotides: a new tool for the functionalization of nucleic acids.

Nucleobases team up: the efficient and selective preparation of purine-derived metallanucleosides, metallanucleotides, and metalladinucleotides having M-C bonds (M=Ir(III), Rh(III)) is reported for the first time. The results presented may be applied to the synthesis of functionalized nucleic acids, or DNA/RNA-modified segments.

Biological activity of Fe(III) aquo-complexes towards ferric chelate reductase (FCR).

In this study we have obtained experimental evidence that confirms the high activity of aquo complexes III and IV towards the enzyme FCR, responsible for the reduction of Fe(III) to Fe(II) in the process of iron acquisition by plants. The in vivo FCR assays in roots of stressed cucumber plants have shown a higher efficiency of the family of complexes III and a striking structure-activity relationship with the nature of the substituent placed in a phenyl group far away from the metal center. The results obtained in this work demonstrate that all the aquo compounds tested interact efficiently with the enzyme FCR and hence constitute a new concept of iron chelates that could be of great use in agronomy.

Electrocatalytic Behavior of Tetrathiafulvalene (TTF) and Extended Tetrathiafulvalene (exTTF) [FeFe] Hydrogenase Mimics.

TTF- and exTTF-containing [(µ-S2)Fe2(CO)6] complexes have been prepared by the photochemical reaction of TTF or exTTF and [(µ-S2)Fe2(CO)6]. These complexes are able to interact with PAHs. In the absence of air and in acid media an electrocatalytic dihydrogen evolution reaction (HER) occurs, similarly to analogous [(µ-S2)Fe2(CO)6] complexes. However, in the presence of air, the TTF and exTTF organic moieties strongly influence the electrochemistry of these systems. The reported data may be valuable in the design of [FeFe] hydrogenase mimics able to combine the HER properties of the [FeFe] cores with the unique TTF properties.

Phosphorescent Ir(III) complexes derived from purine nucleobases.

We report the preparation and the study of new types of neutral and cationic phosphorescent heteroleptic Ir(III) complexes derived from 6-phenylpurine nucleosides and nucleotides. Neutral complexes of general formula Ir(C^N)2(acac) 7, and 8a-c (HC^N = 9-substituted-6-phenyl purine) are orange-red emissive upon photoexcitation, with short lifetimes and good quantum yields (0.42-0.65) in both PMMA films and 2-MeTHF at room temperature. In turn, cationic complexes [Ir(C^N)2(dtb-bpy)][PF6] 9, 12a and 12c (dtb-bpy = 4,4'-di-tert-butyl-2,2'-dipyridine) are yellow-green emitters with moderate quantum yields (0.24-0.32).

Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols.

Rhodium and iridium diolefin catalysts for the acceptorless and base-free dehydrogenation of secondary alcohols have been prepared, and their degradation has been investigated, during the study of the reactivity of the dimers [M(µ-Cl)(η4-C8H12)]2 (M = Rh (1), Ir (2)) and [M(µ-OH)(η4-C8H12)]2 (M = Rh (3), Ir (4)) with 1,3-bis(6'-methyl-2'-pyridylimino)isoindoline (HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane, to afford equilibrium mixtures of 1, the mononuclear derivative RhCl(η4-C8H12){κ1-N py-(HBMePHI)} (5), and the binuclear species [RhCl(η4-C8H12)]2{µ-N py,N py-(HBMePHI)} (6). Under the same conditions, complex 2 affords the iridium counterparts IrCl(η4-C8H12){κ1-N py-(HBMePHI)} (7) and [IrCl(η4-C8H12)]2{µ-N py,N py-(HBMePHI)} (8). In contrast to chloride, one of the hydroxide groups of 3 and 4 promotes the deprotonation of HBMePHI to give [M(η4-C8H12)]2(µ-OH){µ-N py,N iso-(BMePHI)} (M = Rh (9), Ir (10)), which are efficient precatalysts for the acceptorless and base-free dehydrogenation of secondary alcohols. In the presence of KO t Bu, the [BMePHI]- ligand undergoes three different degradations: alcoholysis of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N bond, and opening of the five-membered ring of the isoindoline core.

The electronic structure and photochemistry of group 6 bimetallic (Fischer) carbene complexes: beyond the photocarbonylation reaction.

The UV spectra of Group 6 metal carbene complexes bearing a CpM(CO)(3) (Cp = cyclopentadienyl) moiety bonded to the carbene carbon atom exhibit a redshift of the absorption maxima at higher wavelengths with respect to the parent monometallic complexes. This redshift is partly due to a higher occupation on the p(z) atomic orbital of the carbene carbon atom. Time-dependent DFT calculations accurately assign this band to a metal-to-ligand charge-transfer transition, thus showing that the presence of a second metal center does not affect the nature of the transition. However, the photochemical reactivity of Group 6 metal carbene complexes bearing a CpM(CO)(3) moiety strongly depends on the nature of this metal fragment. A new photoslippage reaction leading to fulvenes occurs when Mn-derived products 11 a, 11 b, and 12 a are irradiated (both Cr and W derivatives), whereas Re-derived product 11 c behaves like standard Fischer complexes and yields the usual photocarbonylation products. A new photoreduction process occurring in the metallacyclopropanone intermediate is also observed for these complexes. Both computational and deuteration experiments support this unprecedented photoslippage process. The key to this differential photoreactivity seems to be the M-Cp back-donation, which hampers the slippage process for Re derivatives and favors the carbonylation reaction.

The sequential building of chiral macrocyclic bis-beta-lactams by double staudinger-Cu-catalyzed azide-alkyne cycloadditions.

A novel approach for the synthesis of macrocyclic bis-beta-lactams based on the Cu-catalyzed alkyne-azide cycloaddition (CuAAC) is reported. The procedure is general and allows access to a full range of diastereomerically or enantiomerically pure macrocyclic cavities in good yields. The incorporation of chiral oxazolidinone fragments at C3 in the beta-lactam rings allows the total enantiocontrol of the process.

Synthesis and properties of mononuclear group 10 alkoxy-biscarbene complexes.

The synthesis, isolation, and complete characterization (NMR and X-Ray diffraction techniques) of stable mononuclear (palladium and platinum) alkoxy-biscarbene complexes is reported. These compounds are easily accessible through stoichiometric transmetalation reactions from Fischer alkoxy-chromium(0) carbene complexes. By a combination of experimental (UV/Vis spectroscopy, cyclic voltammetry) and computational methods (DFT), the structure, bonding situation, and the electronic and redox properties of these complexes are studied. Although they have structures quite similar, their electronic properties strongly depend on the metal and also on the substitution in the aromatic rings in the aniline moieties. Electron-withdrawing groups stabilize the HOMO causing the shifting of the oxidation waves to lower anodic potentials in the corresponding cyclic voltammograms.