Metal "capture" by a heterotrimetalloligand, heterometallic d(10)-d(10) interactions, and unexpected iron-to-platinum silyl ligand migration: a combined experimental and theoretical study

The heterotrinuclear chain complex Hg[Fe{Si(OMe)(3)}(CO)(3)(dppm-P)](2) (dppm = Ph(2)PCH(2)PPh(2)) 1 which has a transoid arrangement of the phosphine donors was used as a versatile chelating metallodiphosphine ligand owing to the easy rotation of its metal core about the Fe-Hg sigma-bonds. Its reaction with the labile Pt(0) olefin complex [Pt(C(7)H(10))(3)] yielded [HgPt{Si(OMe)(3)}Fe(2)(CO)(6){Si(OMe)(3)}(mu-dppm)(2)] 5 which resulted, after coordination of the dangling phosphine donors to Pt, from an unprecedented intramolecular rearrangement involving a very rare example of silyl ligand migration between two different metal centers, and the first one in metal cluster chemistry. The major structural differences observed between the heterometallic complexes obtained from 1 and d(10) Cu(I), Pd(0), or Pt(0) precursors have been established by X-ray diffraction. The bonding situation in the silyl migrated Pt complex 5 was analyzed and compared to those in the isoelectronic, but structurally distinct complexes obtained from Cu(I) and Pd(0) precursors, [Hg{Fe[Si(OMe)(3)](CO)(3)(mu-dppm)}(2)Cu](+) (2) and [Hg{Fe[Si(OMe)(3)](CO)(3)(mu-dppm)}(2)Pd] (4), respectively, by means of extended Hückel interaction diagrams. DFT calculations then allowed the energy minima associated with the three structures to be compared for 2, 4, and 5. All three minima are in close competition for the Pd complex 4, but silyl migration is favored by approximately 10 kcal mol(-)(1) for 5, mainly due to the more electronegative character of Pt with respect to Pd.     

Synthesis of Long,Palladium End‐Capped Polyynes through the Use of Asymmetric 1‐Iodopolyynes

The synthesis of a unique series of long, asymmetric 1‐iodopolyynes ( 1 ‐C_nI and 2 ‐C_nI) with the sp‐hybridized carbon chain up to a decapentayne is reported. These compounds were then used as substrates in reactions with Pd(PPh₃)₄ leading to another series of palladium end‐capped polyynes, which were unstable in solution. Organometallic octatetraynes 1 ‐C₈[Pd]I, 2 ‐C₈[Pd]I, and decapentayne 1 ‐C₁₀[Pd]I are palladium end‐capped polyyne compounds with the longest carbon chains reported so far. All the complexes as well as their organic precursors were fully characterized by NMR, HRMS(ESI), IR, TGA‐DTA, and UV/Vis techniques, and the X‐ray crystal structures of two silyl‐protected precursors and one palladium complex are presented. The synthetic approach for palladium species is envisioned as a general route for the synthesis of labile organometallic polyynes.     

Reaction of bis(o-phosphinophenyl)silane with M(PPh3)4 (M = Ni, Pd, Pt): synthesis and structural analysis of η2-(Si-H) metal(0) and pentacoordinate silyl metal(II) hydride complexes of the Ni triad bearing a PSiP-pincer ligand

Reactions of bis(o-(diphenylphosphino)phenyl)methylsilane with M(PPh(3))(4) (M = Ni, Pd, Pt) were investigated. When M = Ni or Pd, synthesis and isolation of η(2)-(Si-H) complexes of mononuclear Ni(0) and Pd(0) were achieved for the first time as frozen intermediates for oxidative addition of the Si-H bond. Structural analysis by X-ray and NMR spectroscopy disclosed that their η(2)-(Si-H) structures were maintained in both solid and solution states and coordination of the Si-H bond to the metal center was relatively weak. On the other hand, reaction with a platinum(0) complex afforded two kinds of pentacoordinate silyl platinum(II) hydride complexes having a PSiP-pincer ligand, which underwent unique thermal isomerization from a square-pyramidal cis-H-Pt-Si to a trigonal-bipyramidal trans-H-Pt-Si isomer. Mechanistic investigations revealed that this isomerization proceeded via an intramolecular rearrangement process probably through a turnstile rotation.     

Pd, Pt, and Ru complexes of a pincer bis(amino)amide ligand

An improved synthesis of pincer ligand bis[(2-dimethylamino)phenyl]amine ((Me)N(2)NH) was reported. Reaction of the Li complex of (Me)N(2)N with suitable Pd, Pt, and Ru precursors gave the corresponding metal complexes. The structures of the Pd, Pt, and Ru complexes were determined. The Ru complex showed activity in catalytic transfer hydrogenation of aryl and alkyl ketones.     

Unsymmetrical, oxazolinyl-containing achiral and chiral NCN pincer ligand precursors and their complexes with palladium(II)

The unsymmetrical, achiral and chiral NCN pincer ligand precursors (3a-3d) with oxazoline and pyrazole as N donors as well as (3e) which has oxazolinyl and amino group have been synthesized in a facile manner in four steps starting from commercially available isophthalaldehyde. Direct C2 metallation of the precursors (3a-3e) with Pd(OAc)₂ in refluxing HOAc, followed by treatment with LiCl at room temperature provided convenient access to the corresponding pincer palladium(II) complexes (4a-4e). The molecular structure of complex 4e has been determined by X-ray single-crystal diffraction. The obtained Pd complexes exhibited good activities in the Suzuki reactions of aryl bromides and activated aryl chlorides with phenylboronic acid.     

Reductive elimination of ArCF3 from bidentate Pd(II) complexes: a computational study

The reductive eliminations of ArCF(3) from Pd(II) complexes bearing small- and large-bite-angle phosphane ligands have been investigated using computational methods. QM/QM' and QM/MM studies were applied and complemented with CP2K molecular dynamics investigations. The ligand substituents were varied and a decomposition analysis was performed to allow us to gain insights into the steric and electronic properties of the ligands. The greater reactivity of Xantphos-derived (Xantphos=4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) complexes in the reductive elimination of ArCF(3) is primarily due to the lower repulsive effect of the phoshine substituents in the transition state than in the reactant complex, combined with the increased electronic interaction in the transition state. For DPPE (1,2-bis(diphenylphosphino)ethane), the steric effect of the ligand substituents is greater in the transition state, leading to a higher reaction barrier overall for reductive elimination. There is no direct correlation of the reactivity with the bite angle of the reactant complexes. Only for complexes with large ligand substituents may the bite angle of the Pd complexes be used as a guide for reactivity.     

Palladium(II)-selenoether complexes as new single source precursors: first synthesis of Pd4Se and Pd7Se4 nanoparticles

Pd(4)Se and Pd(7)Se(4) nanoparticles (size 38-104 nm), protected by TOP, have been obtained for the first time using Pd(II) ligated with selenated primary and secondary amines (see 1 and 2) as single source precursors respectively. TEM, SEM, powder XRD and photoluminescence have been used to characterize them. 1 and 2 are also the first Pd(II)-selenoether complexes used for the synthesis of nanoparticles containing palladium and selenium.     

Novel phosphite palladium complexes and their application in C-P cross-coupling reactions

A mono- and a 1,3-bis-phosphite arene ligand based on 2,2′-biphenol have been synthesized in order to study the synthesis of the corresponding palladium(II) complexes starting from different Pd precursors. Novel bis-phosphite palladium complex 1 [PdCl₂(L)₂] (L = dibenzo[d,f][1,3,2]dioxaphosphepin, 6-phenoxy), C,P-chelate bonded monophosphite palladium complex 2 [Pd(κ₂-L)(μ-Cl)]₂, and PCP-pincer palladium complex 3 have been prepared from these ligands in promising to excellent yields (50-95%). Additionally, complexes 1 and 3 have been characterized by X-ray crystal structure determinations. The application of 2,6-bis-phosphite pincer palladium(II) complex 3 in C-P cross-coupling between diphenylphosphine-borane and a wide range of various aryl iodides under very mild conditions is reported. Kinetic investigations indicate that 3 merely acts as a pre-catalyst and that Pd nanoparticles are the actual catalytically active species.     

Synthesis, structural characterization, antimicrobial and cytotoxic effects of aziridine, 2-aminoethylaziridine and azirine complexes of copper(II) and palladium(II)

The synthesis, spectroscopic and X-ray structural characterization of copper(II) and palladium(II) complexes with aziridine ligands as 2-dimethylaziridine HNCH(2)CMe(2) (a), the bidentate N-(2-aminoethyl)aziridines C(2)H(4)NC(2)H(4)NH(2) (b) or CH(2)CMe(2)NCH(2)CMe(2)NH(2) (c) as well as the unsaturated azirine NCH(2)CPh (d) are reported. Cleavage of the cyclometallated Pd(II) dimer [μ-Cl(C(6)H(4)CHMeNMe(2)-C,N)Pd](2) with ligand a yielded compound [Cl(NHCH(2)CMe(2))(C(6)H(4)CHMe(2)NMe(2)-C,N)Pd] (1a). The reaction of the aziridine complex trans-[Cl(2)Pd(HNC(2)H(4))(2)] with an excess of aziridine in the presence of AgOTf gave the ionic chelate complex trans-[(C(2)H(4)NC(2)H(4)NH(2)-N,N')(2)Pd](OTf)(2) (2b) which contains the new ligand b formed by an unexpected insertion and ring opening reaction of two aziridines ("aziridine dimerization"). CuCl(2) reacted in pure HNC(2)H(4) or HNCH(2)CMe(2) (b) again by "dimerization" to give the tris-chelated ionic complex [Cu(C(2)H(4)NC(2)H(4)NH(2)-N,N')(3)]Cl(2) (3b) or the bis-chelated complex [CuCl(C(2)H(2)Me(2)NC(2)H(2)Me(2)NH(2)-N,N')(2)]Cl (4c). By addition of 2H-3-phenylazirine (d) to PdCl(2), trans-[Cl(2)Pd(NCH(2)CPh)(2)] (5d) was formed. All new compounds were characterized by NMR, IR and mass spectra and also by X-ray structure analyses (except 3b). Additionally the cytotoxic effects of these complexes were examined on HL-60 and NALM-6 human leukemia cells and melanoma WM-115 cells. The antimicrobial activity was also determined. The growth of Gram-positive bacterial strains (S. aureus, S. epidermidis, E. faecalis) was inhibited by almost all tested complexes at the concentrations of 37.5-300.0 μg mL(-1). However, MIC values of complexes obtained for Gram-negative E. coli and P. aeruginosa, as well as for C. albicans yeast, mostly exceeded 300 μg mL(-1). The highest antibacterial activity was achieved by complexes 1a and 2b. Complex 2b also inhibited the growth of Gram-negative bacteria.     

Di- and tetranuclear palladium complexes incorporating phospha- and diphosphaferrocenes as ligands

Monophosphaferrocenes and 4 react with [Pd(COD)Cl2] (COD = cyclooctadiene) to afford cis- [Pd(1 or 4)2Cl2] complexes that slowly decompose in solution to give dimeric complexes 3 and 6 of general formula [[Pd(1 or 4)Cl]2]. In these dimers, which incorporate a Pd-Pd bond, phosphaferrocenes act as four electron donors through the phosphorus-atom lone pair (mu2-bonded) and through one orbital of appropriate symmetry at iron. These dimers can also be more conventionally prepared from the reaction of cis- [Pd(1 or 4) Cl2] complexes with [Pd(dba)2] (dba = dibenzylidene acetone). The reaction of octaethyldiphosphaferrocene (7) with [Pd(COD)Cl2] yields a dinuclear complex [Pd2(7)2Cl4] (8) in which the two ligands 7 are coordinated in a trans fashion through the phosphorus-atom lone pairs. Decomposition of 8 in solution yields a dimeric dicationic complex of general formula [[Pd2(7)2Cl]2]2+[FeCl4]2- (9a) incorporating four palladium atoms. In each ligand. one phospholyl ring behaves as a two-electron donor through the phosphorus-atom lone pair whereas the second binds two palladium centers in a mu2-fashion. A plausible mechanism that explains the formation of dimers 3, 6, and 9a involves the preliminary oxidation of the mono- or diphosphaferrocene ligand. Parallel experiments aimed at confirming this hypothesis have shown that complex 9a can be synthesized from the reaction of FeCl2 with complex 8. Also presented is another synthetic approach to the synthesis of the tetranuclear complex 9b (counterion is GaCl4-) from the reaction of the palladium(0) complex [Pd(7)2] (10) with [Pd(COD)Cl2] the presence of GaCl3 as chloride abstractor.     

Preparation of benzyne complexes of group 10 metals by intramolecular suzuki coupling of ortho-metalated phenylboronic esters: molecular structure of the first benzyne-palladium(0) complex

A series of nickel(II) and palladium(II) aryl complexes substituted in the ortho position of the aromatic ring by a (pinacolato)boronic ester group, [MBr[o-C(6)H(4)B(pin)]L(2)] (M = Ni, L(2) = 2PPh(3) (2a), 2PCy(3) (2b), 2PEt(3) (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L(2) = 2PPh(3) (3a), 2PCy(3) (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with KO(t)Bu to form the corresponding benzyne complexes [M(eta(2)-C(6)H(4))L(2)] (M = Ni, L(2) = 2PPh(3) (4a), 2PCy(3) (4b), 2PEt(3) (4c), dcpe (4d); M = Pd, L(2) = 2PCy(3) (5b)). This reaction can be regarded as an intramolecular version of a Suzuki cross-coupling reaction, the driving force for which may be the steric interaction between the boronic ester group and the phosphine ligands present in the precursors 2 and 3. Complex 3d also reacts with KO(t)Bu, but in this case disproportionation of the initially formed eta(2)-C(6)H(4) complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(I) complex, [(dcpe)Pd(mu(2)-C(6)H(4))Pd(dcpe)] (6), and a 2,2'-biphenyldiyl complex, [Pd(2,2'-C(6)H(4)C(6)H(4))(dcpe)] (7d). Complexes 2a, 3b, 3d, 4b, 5b, 6, and 7d have been structurally characterized by X-ray diffraction; complex 5b is the first example of an isolated benzyne-palladium(0) species.     

A new synthesis for thermolabile low-valent palladium complexes by electron transfer reactions from nickel(0) to palladium(II) compounds

The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd₂X₂(cod)₂] (1: X = Cl, 2: X = Br), Pd(cod)₂ (3) and Pd(norbornene)₃ (4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)-Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC₄ tetrahedron with typical Pd-C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod)₂ (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to ¹H NMR measurements the complexes have the same structure in solution as found in the solid state.     

Synthesis of the β-lactam antibiotic (+)- thienamycin via an intermediate π-allyltricarbonyliron lactone complex

The π-allyltricarbonyliron lactone complex (7), formed by reaction of E-1,2-epoxy-2-methyl-6,6-dimethoxyhex-3-ene(5) with co-ordinatively unsaturated iron carbonyl species, was reacted with benzylamine to give a lactam complex (8) by an S_N'-like mechanism. This complex upon oxidation with Ce(IV) afforded cis-3-isopropenyl-4-[(2',2'-dim (9) which was chemically modified into trans-3-(1'-hydroxyethyl)-4-[(2',2-dimethoxy)ethyl] azetidin-2-one (13), a key intermediate previously used in the synthesis of the antibiotic thienamycin. Similar reaction with (S)-(-)--methylbenzylamine afforded a separable mixture of diastereoisomeric iron lactam complexes (16 and 17). These complexes could be individually converted to the corresponding optically active β-lactam derivatives (27 and 28) and, hence, are precursors for the synthesis of either natural (+)-thienamycin or unnatural (-)-thienamycin.     

Symmetrical and unsymmetrical pincer complexes with group 10 metals: synthesis via aryl C-H activation and some catalytic applications

Aryl-based pincer metal complexes with anionic terdentate ligands have been widely applied in organic synthesis, organometallic catalysis and other related areas. Synthetically, the most simple and convenient method for the construction of these complexes is the direct metal-induced C(aryl)-H bond activation, which can be fulfilled by choosing the appropriate functional donor groups in the two side arms of the aryl-based pincer preligands. In this perspective, we wish to summarize some results achieved by our group in this context. Successful examples include symmetrical chiral bis(imidazoline) NCN pincer complexes with Ni(II), Pd(II) and Pt(II), bis(phosphinite) and bis(phosphoramidite) PCP pincer Pd(II) complexes, unsymmetrical (pyrazolyl)phosphinite, (amino)phosphinite and (imino)phosphinite PCN pincer Pd(II) complexes, chiral (imidazolinyl)phosphinite and (imidazolinyl)phosphoramidite PCN pincer complexes with Ni(II) and Pd(II) as well as unsymmetrical (oxazolinyl)amine and (oxazolinyl)pyrazole NCN' pincer Pd(II) complexes. Among them, the P-donor containing complexes are efficiently synthesized by the "one-pot phosphorylation/metalation" method. The obtained symmetrical and unsymmetrical pincer complexes have been used as catalysts in Suzuki-Miyaura reaction (Pd), asymmetric Friedel-Crafts alkylation of indole with trans-β-nitrostyrene (Pt) as well as in asymmetric allylation of aldehyde and sulfonimine (Pd). In the Suzuki couplings conducted at 40-50 °C, some unsymmetrical Pd complexes exhibit much higher activity than the related symmetrical ones which can be attributed to their faster release of active Pd(0) species resulting from the hemilabile coordination of the ligands. Literature results on the synthesis of some related pincer complexes as well as their activities in the above catalytic reactions are also presented.     

Dynamic processes in silyl palladium complexes: evidence for intermediate Si-H and Si-Si sigma-complexes

The silyl palladium complexes (dcpe)PdH(SiHtBu2) and (dcpe)Pd(SiHMe2)2 display NMR spectra that vary with temperature. The dynamic NMR behavior is consistent with long-lived sigma-complexes as intermediates. In the case of (dcpe)PdH(SiHtBu2), the intermediate is believed to be a symmetric complex with doubly bridged hydrogen atoms between the silicon and palladium. Dynamic interchange of the two silicon atoms in (dcpe)Pd(SiHMe2)2 is consistent with an intermediate Si-Si sigma-complex.     

Structure of Complexes and Catalytic Oxidation of Triarylphosphine in the Reaction of 9-Phenyl-9-phosphafluorene with Bis(acetylacetonato)palladium

Complex formation of 9-phenyl-9-phosphafluorene with bis(acetylacetonato)palladium in benzene and acetonitrile was studied by means of NMR, IR, and UV spectroscopy. The structure of the resulting complexes, as well as specific features of catalytic oxidation of the arylphosphine and the reduction of Pd(II) to Pd(0) to form the complex Pd(PC_{1 8}H_{1 3})₄ were explored. 9-Phenyl-9-phosphafluorene gives two types of complexes with Pd(acac)₂). The first ones are similar to triphenylphosphine complexes, and the others ( complexes) are formed by coordination of the planar aromatic system of the phosphine to the plane of Pd(acac)₂.     

An efficient iridium catalyst for reduction of carbon dioxide to methane with trialkylsilanes

Cationic silane complexes of general structure (POCOP)Ir(H)(HSiR(3)) {POCOP = 2,6-[OP(tBu)(2)](2)C(6)H(3)} catalyze hydrosilylations of CO(2). Using bulky silanes results in formation of bis(silyl)acetals and methyl silyl ethers as well as siloxanes and CH(4). Using less bulky silanes such as Me(2)EtSiH or Me(2)PhSiH results in rapid formation of CH(4) and siloxane with no detection of bis(silyl)acetal and methyl silyl ether intermediates. The catalyst system is long-lived, and 8300 turnovers can be achieved using Me(2)PhSiH with a 0.0077 mol % loading of iridium. The proposed mechanism for the conversion of CO(2) to CH(4) involves initial formation of the unobserved HCOOSiR(3). This formate ester is then reduced sequentially to R(3)SiOCH(2)OSiR(3), then R(3)SiOCH(3), and finally to R(3)SiOSiR(3) and CH(4).     

The first stable mononuclear silyl palladium hydrides

The reaction of tertiary silanes with the low valent palladium complex [(mu-dcpe)Pd]2 affords equilibrium mixtures with mononuclear silyl palladium hydrides. These complexes have been characterized by NMR spectroscopy and, in one case, by X-ray crystallography. The silyl palladium hydride complexes rapidly interchange silicon and hydride coordination environments in solution which give rise to extraordinary temperature-dependent kinetic isotope effects for the fluxional process. An intermediate 2eta-Si-H complex is proposed for the interchange.     

A highly active and selective palladium pincer catalyst for the formation of α-aryl ketones via cross-coupling

Several air and moisture stable Pd(II) pincer complexes were synthesized via oxidative addition of Pd(0) to novel PheBox pincer ligand precursors. Low loadings (1 mol%) of the Pd complex [t-BuPhebox-Me(2)]PdBr are capable of efficiently promoting the selective α-monoarylation of a variety of ketones with numerous aryl bromides in only 1 h at 70 °C with 82-99% yields.     

Multifaceted palladium catalysts towards the tandem diboration-arylation reactions of alkenes

Novel Pd(2) (6+) compounds have been synthesized in high yield. These compounds and their Pd(2) (4+) counterparts as synthetic precursors mediate the diboration of vinylarenes and aliphatic 1-alkenes, and under mild and basic reaction conditions they produce a variety of 1,2-diboronate esters with excellent conversions and chemoselectivities. The presence of bis(catecholato)diboron (B(2)cat(2)) favours the reduction of Pd(III) to Pd(II), while the catalytic precursor of Pd(II) is transformed into Pd(0)-nanoparticles. An "in situ" catalytic tandem reaction has been designed to transform the diboronate intermediates into the monoarylated product, which after oxidative workup, provides the carbohydroxylated adduct. Eventually, the same catalyst performs both sequences with total conversion from the alkene.