Rhodium(I) carbonyl complexes with α-diimine ligands containing amino or hydroxy substituents

-Diimines, RN:C(R)C(R):NR(LL) derived from glyoxal, GLL (R=H) abbreviated as GAA (R= R=4-dimethylaminophenyl) or GHA (R= R=4-hydroxyphenyl), and derived from biacetyl, BLL (R=Me) abbreviated as BDH (R=R= NH₂), BOH (R=NH₂, R=OH) react with carbonylrhodium(I) compounds to give different products depending on the imino substituents in the ligand and/or the solvent employed. The reaction of -diimines bearing amino groups, such as GAA or BDH with [RhCl(CO)₂]₂ in acetone yields binuclear [RhCl(CO)₂]₂(-LL) while in CH₂Cl₂ ionic [Rh- (CO)₂(LL)]⁺[RhCl₂(CO)₂]^– species are obtained. In acetone [RhCl(CO)₂]₂(-GAA) exists as an equilibrium mixture between two different neutral binuclear species; [Rh(CO)₂(BDH)]⁺ exists as a mixture of two species containing chelate or monodentate bonded diimine respectively. GAA or BDH react in situ with [RhCl(CO)(C₂H₄)]₂ in benzene to yield tetracoordinated monocarbonylated [RhCl(CO)(LL)] compounds. -Diimines (LL) bearing hydroxy groups, such as GHA or BOH react with [RhCl(CO)₂]₂ or [RhCl(CO)(C₂H₄)]₂ to give pentacoordinated dicarbonylated [RhCl(CO)₂(LL)] compounds.     

Chelating or bridging? Halide-controlled binding mode of diamido donor ligands in iron(III) complexes

In a series of iron(III) halide complexes of the form {FeX[MesN(SiMe(2))]2O}2 (Mes = mesityl; X = Cl, Br, I), the ancillary diamidosilylether ligand can either chelate to one metal or instead bridge two metal centers, as a function of the halide coligand. The complexes are prepared from diamidosilyletheriron(II) precursors, which are oxidized with iodine, benzyl bromide, or PhICl2 to yield the appropriate iron(III) halide. The bromide analogue can also be synthesized by reacting the iron(II) precursor with a bromonium transfer agent (stabilized by adamantylideneadamantane). The latter reaction may proceed via an iron(IV) intermediate, which can oxidize the normally noncoordinating, inert [B(ArF)4]- counteranion [ArF = 3,5-(CF3)2Ph].     

Electron counting and bonding analysis in triruthenium clusters containing sulfoximido ligands: true or false electron-deficient systems?

Triruthenium clusters containing a methylphenylsulfoximido cap or bridge, Ru₃(CO)₉(μ₂-H)[μ₃-NS(O)MePh] (1), Ru₃(CO)₁₀(μ₂-H)[μ₃-NS(O)MePh] (2), Ru₃(CO)₈(μ₃-η²-CPhCHBu)[μ₃-NS(O)MePh] (3), Ru₃(CO)₉(μ₃-η²-PhCCCCHPh)[μ₂-NS(O)MePh] (4), and Ru₃(CO)₇(μ₂-CO)(μ₃-η²-PhCCCCHPh)[μ₃-NS(O)MePh] (5) have been examined by EHT and DFT calculations in order to analyze the bonding present in the clusters and to establish the electron counting. They clearly show that a μ₃-sulfoximido group is not a 3e⁻ ligand as one may be led to think at first sight, but rather acts as a three-orbital/5e⁻ system, i.e. should be considered as isolobal to an N---R⁻ ligand. Because of some delocalization of its π-type orbitals on the sulfur and oxygen atoms, it is expected to bind slightly less strongly to metal atoms than classical imido ligands. Once in a μ₂ coordination mode, the sulfoximido ligand retains a lone pair on its pyramidalized N atom and becomes a two-orbital/3e⁻ ligand. It follows that clusters 1, 2, 4 and 5 are electron-precise, whereas cluster 3 is electron deficient with respect to the 18e⁻ rule but obeys the polyhedral skeletal electron pair electron-counting rules. Consistently, all the calculated clusters exhibit large HOMO–LUMO gaps and no trace of electron deficiency can be found in their electronic structures.     

“Short-bite” ligands in cluster synthesis

The short-bite ligands CH₂(PR ₂)₂ or CH(PR ₂)₃ (R = Me, Ph),RN(PX ₂)₂ (R=H, Me, Et;X = F, OR (R= Me, Et, i-Pr, Ph), Ph),RE(CH₂ ER₂)₂ (E = P, As;R = Me, Ph ), Ph₂ P(2-C₅H₄N) and related species are particularly versatile for the synthesis of di- and polynuclear complexes which frequently possess metal-metal bonds. In addition to homometallic products, these ligands often permit the directed synthesis of heterometallic complexes. Selected aspects of the chemistry of these complexes are also reviewed.     

Filling gaps in the series of noninnocent hetero-1,3-diene chelate ligands: ruthenium complexes of redox-active α-azocarbonyl and α-azothiocarbonyl ligands RNNC(R')E, E = O or S

The series of 4-center unsaturated chelate ligands A═B-C═D with redox activity to yield (-)A-B═C-D(-) in two steps has been complemented by two new combinations RNNC(R')E, E = O or S, R = R' = Ph. The ligands N-benzoyl-N'-phenyldiazene = L(O), and N-thiobenzoyl-N'-phenyldiazene = L(S), (obtained in situ) form structurally characterized compounds [(acac)(2)Ru(L)], 1 with L = L(O), and 3 with L = L(S), and [(bpy)(2)Ru(L)](PF(6)), 2(PF(6)) with L = L(O), and 4(PF(6)) with L = L(S) (acac(-) = 2,4-pentanedionato; bpy = 2,2'-bipyridine). According to spectroscopy and the N-N distances around 1.35 ? and N-C bond lengths of about 1.33 ?, all complexes involve the monoanionic (radical) ligand form. For 1 and 3, the antiferromagnetic spin-spin coupling with electron transfer-generated Ru(III) leads to diamagnetic ground states of the neutral complexes, whereas the cations 2(+) and 4(+) are EPR-active radical ligand complexes of Ru(II). The complexes are reduced and oxidized in reversible one-electron steps. Electron paramagnetic resonance (EPR) and UV-vis-NIR spectroelectrochemistry in conjunction with time-dependent density functional theory (TD-DFT) calculations allowed us to assign the electronic transitions in the redox series, revealing mostly ligand-centered electron transfer: [(acac)(2)Ru(III)(L(0))](+) ? [(acac)(2)Ru(III)(L(?-))] ? [(acac)(2)Ru(III)(L(2-))](-)/[(acac)(2)Ru(II)(L(?-))](-), and [(bpy)(2)Ru(III)(L(?-))](2+)/[(bpy)(2)Ru(II)(L(0))](2+) ? [(bpy)(2)Ru(II)(L(?-))](+) ? [(bpy)(2)Ru(II)(L(2-))](0). The differences between the O and S containing compounds are rather small in comparison to the effects of the ancillary ligands, acac(-) versus bpy.     

Pincer ligands based on α-amino acids: I. Synthesis of polydentate ligands from pyrrole-2,5-dicarbaldehyde

New chiral pincer ligands having CH=N moieties were synthesized by condensation of 1H-pyrrole-2,5-dicarbaldehyde with l-methionine and l-histidine methyl esters. Their reduction under mild conditions (NaBH₄, ?30°C) gave the corresponding amine ligands in high yields. An improved procedure for the preparation of 1H-pyrrole-2,5-dicarbaldehyde was proposed.     

Application of chiral thiazolidine ligands to asymmetric hydrosilation

Seven chiral thiazolidines bound rhodium complexes were synthesized and their catalytic asymmetric hydrosilation properties were investigated It was found through investigation that the configuration of newly formed chiral centre C2 of substituted chiral thiazolidines prepared from L-cysteine or its esters has no effect on the final results of catalytic asymmetric hydrosilation.The direct reason for causing this phenomenon is reported by the present quantitative results for the first time:the rapid racemation of chiral center C2 of chiral thiazolidine ligands takes place under the catalysis of rhodium(Ⅰ) complex [Rh(COD)CI]2     

Amino-aryl-carbenes: alternative ligands for transition metals?

Despite the presence of a single amino substituent, an amino-anthryl-carbene was found to behave as a strong sigma-donor weak pi-acceptor ligand toward rhodium(I) fragments.     

Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands

~~Rhodium catalyzed asymmetric Pauson-Khand reaction using SDP ligands1. Khand, I. U., Knox, G R., Pauson, P. L. et al., Organocobalt complexes, Part Ⅱ. Reaction of acetylenehexacarbonyldicobalt complexes, (R1C2R2)Co2(CO)6, with norbomene and its deri…     

σ-Acceptor, Z-type ligands for transition metals

The ability of Lewis acids to coordinate to transition metals as σ-acceptor ligands was recognized as early as in the 1970's, but so-called Z-type ligands remained curiosities until the early 2000's. Over the last decade, significant progress has been made in this area, especially via the incorporation of Lewis acid moieties into multidentate, ambiphilic ligands. Our understanding of the nature and influence of TM → Z interactions has considerably improved and the scope of Lewis acids susceptible to behave as σ-acceptor ligands has been significantly extended. This feature article summarizes these recent achievements.     

Non-innocent ligands in bioinorganic chemistry—An overview

This review touches the most common instances where non-innocent (“suspect”) behaviour of redox-active ligands, either substrates or supporting components, is observed in a biochemical context. These ligands include the O₂/O₂^?/O₂^2?, NO⁺/NO/NO^?, o-quinone/o-semiquinone/catecholate and tyrosyl/tyrosinate redox systems, the tetrapyrrole (porphyrinic) ligands, the pterins and flavins, and the dithiolene/ene-dithiolate ligands in molybdo- and tungstopterin. These non-innocent ligands are discussed in their interaction with biological iron, copper, manganese, molybdenum or tungsten centers.     

Carbonyl substitution of the dicobalt-iron complex (μ3-S)FeCo2(CO)9 with monophosphane or diphosphane ligands

Eight new dicobalt-iron clusters have been synthesised and structurally characterized. Treatment of (μ₃-S)FeCo₂(CO)₉ (A) with monophosphane ligands tris(4-fluorophenyl)phosphane, tris(4-methoxyphenyl)phosphane, or tris(2-furyl)phosphane in the presence of Me₃NO?2H₂O afforded monosubstituted complexes (μ₃-S)FeCo₂(CO)₈L [L = P(4-C₆H₄F)₃, 1; P(4-C₆H₄OMe)₃, 3; P(2-C₄H₃O)₃, 5] and disubstituted complexes (μ₃-S)FeCo₂(CO)₇L₂ [L = P(4-C₆H₄F)₃, 2; P(4-C₆H₄OMe)₃, 4; P(2-C₄H₃O)₃, 6]. Reaction of complex A with Ph₂PN[CH(CH₃)₂]PPh₂ in refluxing toluene gave complex (μ₃-S)FeCo₂(CO)₇{Ph₂PN[CH(CH₃)₂]PPh₂} (7) with an intramolecular bridging diphosphane ligand. Reaction of complex A with trans-1,2-bis(diphenylphosphino)ethylene (trans-dppv) and Me₃NO?2H₂O yielded complex [(μ₃-S)FeCo₂(CO)₈]₂(trans-Ph₂PCH = CHPPh₂) (8) with an intermolecular bridging diphosphane ligand. The new complexes 1–8 were characterized by elemental analysis, IR, ¹H NMR, ³¹P{¹H} NMR, and ¹³C{¹H} NMR spectroscopy, particularly for 1, 3, and 6–8 by X-ray crystallography.     

Metrical oxidation states of 2-amidophenoxide and catecholate ligands: structural signatures of metal-ligand π bonding in potentially noninnocent ligands

Catecholates and 2-amidophenoxides are prototypical "noninnocent" ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)semiquinone or neutral (imino)quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical "metrical oxidation state" (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C-C, C-O, and C-N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d(0) metal complexes, particularly those of vanadium(V) and molybdenum(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.     

Small peptides as ligands for catalytic asymmetric alkylations of olefins. Rational design of catalysts or of searches that lead to them?

Amino acid-based chiral ligands have been developed for use in Cu-catalyzed enantioselective allylic alkylations and conjugate additions that allow access to optically enriched compounds that are otherwise difficult to prepare. These chiral ligands are easily modified and have been identified through mechanism-based library screening. The data presented point to the significance of the availability of a collection of catalysts, since subtle variations in substrate or nucleophile structure often call for a different optimal chiral ligand. Can a catalyst be truly "rationally designed" or do we design our search pathway that eventually leads us to such a catalyst? What is meant by a "general catalyst"? Do we need a class of effective catalysts instead? These and related questions are addressed in the context of the above studies.     

Electrolytic formation of technetium complexes with π-acceptor ligands

Electrolytic reduction of pertechnetate was performed in aqueous solution containing -acceptor ligands. Cyanide and 1,10-phenanthroline were the selected ligands. In both cases, electrolyses produced a cathodic TcO₂ deposit and soluble Tc complexes. When cyanide was the ligand, the complexes formed were [Tc (CN)₆]^5– and [TcO₂ (CN)₄]^3–. When working with the amine, [Tc (phen)₃]²⁺ and another positively charged species were found after reaction. Results are compared with previous studies with amines, and the usefulness of the electrolytic route to obtain Tc complexes is evaluated.     

Bidentate ligands by supramolecular chemistry--the future for catalysis?

In this Frontier Article we give our view on recent developments in transition-metal catalyst development that evolve from a combination of supramolecular strategies and traditional ligand design and development.     

Iridium metal–polymer complexes based on bipyridyl ligands

Novel polymer macroligands—copolyamides containing different quantity of bipyridyl groups—were obtained from 4,4′-diamino-2,2′-bipyridine, 4,4′-diaminodiphenyl ether and terephthaloyl-bis(3-methoxy-4-hydroxybenzoic) acid dichloroanhydride by low-temperature polycondensation. Metal–polymer complexes with different content of Ir(ppy)2 were obtained by the interaction between polymer ligand and [Ir(ppy)2Cl]₂ (ppy–2-phenylpyridine). The properties of films and coatings based on these materials were studied.