Facile synthesis and bonding of 4-ferrocenyl-1,2,4-triazol-5-ylidene complexes.

Ferrocene-substituted carbenes have emerged as attractive, redox-active ligands. However, among the compounds studied to date, ferrocenylated 1,2,4-triazol-5-ylidenes, which are closely related to the archetypal imidazol-2-ylidenes, are still unknown. Here, we demonstrate that the triazolium salt [CHN(Me)NCHN(Fc)]I (2; Fc = ferrocenyl), obtained by alkylation of 4-ferrocenyl-4H-1,2,4-triazole (1) with MeI, reacts selectively with metal alkoxide/hydroxide precursors [(cod)Rh(OMe)]2 and [(IPr)Au(OH)] (cod = cycloocta-1,5-diene, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to produce the ferrocene-substituted 1,2,4-triazol-5-ylidene complexes [(cod)RhI{CN(Me)NCHN(Fc)}] and [(IPr)Au{CN(Me)NCHN(Fc)}]I in good yields. The complexes were characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry, and single-crystal X-ray diffraction analysis. Density function theory (DFT) calculations were used to rationalise the electrochemical behaviour of the carbene complexes and to elucidate the bonding situation in these compounds. An analysis using intrinsic bond orbitals (IBOs) revealed that the 1,2,4-triazol-5-ylidene ligand exerted a strong trans influence and showed a synergistic stabilisation by the negative inductive and positive π-donor effects of the nitrogen atoms adjacent to the carbene carbon atom; these effects were enhanced by conjugation with the CHN bond at the exterior, similar to that in imidazol-2-ylidenes.

Synthesis and reactivity of Pd(II) imidoyl complexes obtained by insertion of isocyanoferrocene into the Pd-C bonds of orthopalladated precursors.

While the multifaceted reactivity of organic isocyanides has been extensively demonstrated, that of their organometallic analogue, isocyanoferrocene (FcNC; Fc = ferrocenyl), has not yet been adequately explored. This contribution describes the syntheses of novel chelating Pd(II) imidoyl complexes, [(YCH2C6H4C(NFc)-κ2Y,C)PdCl(PR3)], by insertion of FcNC into the Pd-C bond of cyclopalladated precursors [(YCH2C6H4-κ2Y,C)PdCl(PR3)] (Y = Me2N, MeS, R = Ph, Me). The imidoyl complexes underwent facile alkylation with [Me3O][BF4] to produce the cationic aminocarbene complexes [{YCH2C6H4C(N(Me)Fc)-κ2Y,C}PdCl(PR3)][BF4]. All compounds were fully characterised using a combination of spectroscopic methods (NMR, FTIR and ESI MS), cyclic voltammetry and single-crystal X-ray crystallography. In addition, DFT calculations were used to describe the bonding in the two compound families. Analyses with intrinsic bond orbitals (IBOs) and the quantum theory of atoms in molecules (QTAIM) consistently pointed to the transformation of an X-type imidoyl C-ligand (σ-organyl) into an L-type carbene donor upon alkylation, which has only a minor structural consequence. Also reported is the unexpected conversion of the imidoyl complex [(Me2NCH2C6H4C(NFc)-κ2N,C)PdCl(PPh3)] into (Z)-2,2-dimethyl-1-(ferrocenylimino)isoindolin-2-ium tetrafluoroborate as a reductive elimination product, which was induced by Lewis and Brønsted acids.

Synthesis, Structure, Reactivity, and Intramolecular Donor-Acceptor Interactions in a Phosphinoferrocene Stibine and Its Corresponding Phosphine Chalcogenides and Stiboranes.

Ferrocene-based phosphines equipped with additional functional groups are versatile ligands for coordination chemistry and catalysis. This contribution describes a new compound of this type, combining phosphine and stibine groups at the ferrocene backbone, viz. 1-(diphenylphosphino)-1'-(diphenylstibino)ferrocene (1). Phosphinostibine 1 and the corresponding P-chalcogenide derivatives Ph2P(E)fcSbPh2 (1E, fc = ferrocene-1,1'-diyl, E = O, S, Se) were synthesized and further converted to the corresponding stiboranes Ph2P(E)fcSb(O2C6Cl4)Ph2 (6 and 6E) by oxidation with o-chloranil. All compounds were characterized by spectroscopic methods, X-ray diffraction analysis, cyclic voltammetry, and theoretical methods. Both NMR spectroscopy and DFT calculations confirmed the presence of P → Sb and P═O → Sb donor-acceptor interactions in 6 and 6O, triggered by the oxidation of the stibine moiety into Lewis acidic stiborane. The corresponding interactions in 6S and 6Se were of the same type but significantly weaker. A coordination study with AuCl as the model metal fragment revealed that the phosphine group acts as the "primary" coordination site, in line with its higher basicity. The obtained Au(I) complexes were applied as catalysts in the Au-catalyzed cyclization of N-propargylbenzamide and in the oxidative [2 + 2 + 1] cyclization of ethynylbenzene with acetonitrile and pyridine N-oxides. The catalytic results showed that the stibine complexes had worse catalytic performance than their phosphine counterparts, most likely due to the formation of weaker coordination bonds and hence poorer stabilization of the active metal species. Nevertheless, the stibine moiety could be used to fine-tune the properties of the ligated metal center by changing the oxidation state or substituents at the "remote" Sb atom.

Synthesis of Hybrid Ligands with Nitrile and Cage Phosphane Donor Groups and their Applications in Gold-Mediated Reactions.

Altering the donor properties of phosphane ligands through substituent variation is an established tool in coordination chemistry and catalysis. This contribution describes the synthesis of two new hybrid donors (L) combining 1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphaadamantane-8-yl (PCg) and nitrile donor groups at different molecular scaffolds, viz. ferrocene-1,1'-diyl (fc) and 1,2-phenylene. These ligands were used to prepare dimeric Au(I) complexes [Au2 (µ(P,N)-L)2 ][SbF6 ]2 , which were evaluated as silver-free, preformed catalysts in Au-mediated cycloisomerization of (Z)-3-methylpent-2-en-4-yn-1-ol to 2,3-dimethylfuran. The catalyst featuring the ferrocene-based ligand, viz., [Au2 (µ(P,N)-CgPfcCN)2 ][SbF6 ]2 , showed the best catalytic performance at low catalyst loading (0.5 or 0.15 mol.%), which exceeded that of its diphenylphosphanyl analog [Au2 (µ(P,N)-Ph2 PfcCN)2 ][SbF6 ]2 studied earlier and the prototypical Au(I) precatalyst [Au(PPh3 )(MeCN)][SbF6 ].

Beyond phosphorus: synthesis, reactivity, coordination behaviour and catalytic properties of 1,1'-bis(diphenylstibino)ferrocene.

Compared to their widely studied phosphine counterparts, ferrocene stibines have received only marginal attention thus far. This paper describes the synthesis of 1,1'-bis(diphenylstibino)ferrocene (1*), which is an antimony analogue of the ubiquitous dppf, and our investigations into the reactivity and coordination behaviour of this compound. Thus, distibine 1 was oxidised to stiboranes fc(SbPh2X2)2 (X = Cl, 2*; F, 6*; fc = ferrocene-1,1'-diyl) and to stibine-stiborane Ph2SbfcSbPh2F2 (5*). Compounds 2 and 6 were easily hydrolysed to produce ferrocenophanes fc[SbPh2XOSbPh2X] (X = Cl, 3*; F, 7*). Removing the halogen from 3 with silver(I) salts afforded the corresponding ferrocenophanes with O-bound oxyanions, fc[SbPh2ZOSbPh2Z] (Z = NO3, 4a*; ClO4, 4b*), which were alternatively prepared from 2 and also converted back to 2 by adding a chloride source. Through investigations into the coordination behaviour of distibine 1, the following compounds were isolated and characterised: [(µ-ClO4)2{Ag(1-κ2Sb,Sb')}2] (8*), [Ag(1-κ2Sb,Sb')2]X (X = ClO4, 9a; SbF6, 9b*), [(µ(Sb,Sb')-1){(arene)MCl2}2] (10*, 11*) and [(arene)MCl(1-κ2Sb,Sb')][PF6] (12*, 13*; 10, 12: M/arene = Ru/η6-p-cymene, 11, 13; Rh/η5-C5Me5), [(η5-C5Me5)RuCl(1-κ2Sb,Sb')] (14), [MCl2(1-κ2Sb,Sb')] (M = Pd, 15*; Pt, 16*), [Pd(1-κ2Sb,Sb')2]X2 (X = BF4, 17a; SbF6, 17b*), [Pd(η2-ma)(1-κ2Sb,Sb')] (18*; ma = maleic anhydride), [(µ(Sb,Sb')-1)(AuCl)2] (19*), and [Au(1-κ2Sb,Sb')2]X (X = AuCl2, 20a*; SbF6, 20b*). Inspection of the structural parameters suggested that complexes featuring 1 exhibit less sterically strained structures than their dppf analogues due to longer M-Sb and Sb-C bonds, which reduce crowding around the ligated metal centre. Cyclic voltammetry and DFT calculations revealed that the primary electrochemical oxidation of 1 is reversible and occurs at the ferrocene unit. Based on preliminary catalytic tests in Suzuki-Miyaura biaryl coupling, Pd-1 complexes exhibited a lower efficiency than their respective Pd-dppf analogues. (An asterisk indicates that the crystal structure has been determined.).

Metallation of a gold(I) metalloligand with P,C-bridging phosphinoferrocenyl groups enables the construction of defined multimetallic arrays.

The reactions of the gold(I) metalloligand [Au2{µ(P,C)-Ph2Pfc}2], where fc stands for ferrocene-1,1'-diyl, with bare or ligand-stabilised group 11 metal ions open access to diverse oligometallic clusters stabilised by Au-Au, Au-Ag and Au-Cu interactions. These capping reactions and the unique structures of the products stem from unparalleled properties of the bridging ferrocene groups, namely their structural flexibility and electron-rich nature, which enable accommodating the capping moieties and supporting ligands and facilitate electrophilic metalation, respectively. While the Au+ and Ag+ ions behave similarly, capping reactions with Cu+ proceed differently, with an accentuated role of the counter ions and other ligands in the system. Such behaviour reflects the relative strengths of the Au-M metallophilic interaction (M = Au, Ag and Cu), among which the Au-Cu interactions are the weakest, as confirmed by DFT calculations.

Preparation of High-Entropy (Ti, Zr, Hf, Ta, Nb) Carbide Powder via Solution Chemistry.

High-entropy ceramics is a new class of materials having a great potential and wide application. The carbide of Ti, Zr, Hf, Ta, Nb is a typical member of this group. It has been synthesized mostly through blending, milling, and high-temperature solid-state reaction of metal carbide precursors for each metal. This route needs extremely high temperature (2300 °C), which makes it energy and technology demanding. We have developed a chemical route for high-entropy carbide powder that needs a synthetic temperature that is several hundred degrees Celsius lower. A solution of desired metal citrates with an excess of citric acid was converted into a metal oxide/active carbon nanocomposite. Starting from a solution enabled ideal mixing of precursors on a molecular level, allowing us to skip any milling and blending steps. The nanocomposite was treated in vacuum at 1600 °C, giving a phase-pure high-entropy carbide. The intermediate compounds and products were characterized by means of solid-state analysis.

Cyclopalladation of a ferrocene acylphosphine and the reactivity of the C-H activated products.

Acylphosphines are an attractive subclass of phosphine ligands with specific reactivity and ligating properties. This study describes the synthesis of a ferrocene-based acylphosphine, FcC(O)PPh2 (1, Fc = ferrocenyl), the corresponding phosphine chalcogenides FcC(O)P(E)Ph2 (E = O, S and Se), and palladium(ii) complexes resulting from the orthometallation of 1, viz. [Pd(µ-X)(1- H)]2 (2-X, where X = Cl, Br and I), which in turn serve as a convenient entry point to a range of monopalladium complexes. Thus, the cleavage of 2-X with phosphines, 4-(dimethylamino)pyridine and in situ-generated 1,3-dimesitylimidazolin-2-ylidene (L) provided complexes [PdX(L)(1- H)] (3-5), which are typically obtained as single isomers by crystallisation but undergo spontaneous isomerisation in solution leading to equilibrium mixtures of cis and trans isomers. Upon treatment with sodium acetylacetonate (Na(acac)), 2-Cl was transformed into [Pd(acac)(1- H)] (6), which reacted with (diphenylphosphino)acetic acid under proton transfer to give [Pd(Ph2PCH2CO2-κ2O,P)(1- H)] (7), while the reaction with allylating agents produced the π-allyl complex [Pd(η3-C3H5)(1- H)] (9). Compound [PdCl(PMe3)(1- H)] was further used to prepare a series of diorganopalladium complexes [Pd(R)(PMe3)(1- H)] (8; R = Me, 4-C6H4Me, 4-C6H4CF3, CH[double bond, length as m-dash]CHC6H4Me-4 and C[triple bond, length as m-dash]CC6H4Me-4). All compounds were structurally characterised using spectroscopic methods and, in most cases, also by X-ray diffraction analysis. The structural data indicated the rigidity of the Pd(1- H)+ fragment and revealed variations in the Pd-donor distances dictated by an interplay between the trans influence and steric demands of the ligands surrounding Pd(ii). Moreover, some compounds were studied by DFT to rationalise their isomerisation equilibria and electrochemical properties, which were examined by cyclic voltammetry.

Synthesis and Reactivity of Multinuclear Gold Complexes with (Diphenylphosphanyl)ferrocene and Oxygen Ligands.

AuI complexes combining hard oxygen and soft (diphenylphosphanyl)ferrocene (L) ligands in their molecules were synthesized, viz. the gold hydroxides [Au(OH)(L-κP)] (5) and [{Au(L-κP)}2 (µ-OH)][BF4 ] (4), and the oxonium cluster [{Au(L-κP)}3 (µ3 -O)][BF4 ] (1). In-situ auration of 1 produced [{Au(L-κP)}4 (µ4 -O)][BF4 ]2 (2), which spontaneously converted into a dimeric tetragold complex featuring bridging phosphanylferrocenyl groups geminally diaurated in position 2 of the ferrocene scaffold. The same complex and its isomer incorporating ferrocene-1,1'-diyl bridges resulted similarly from 4. Upon crystallization, compound 5 underwent a redox reaction, producing a structurally unique, crown-like, mixed-valent Au0 /AuI cluster, [Au7 (L-κP)6 ]OH. Compounds 1 and 5 were used to prepare the analogous, N-bridged complexes, [{Au(L-κP)}3 (µ3 -NFc)][BF4 ] (Fc=ferrocenyl) and [{Au(L-κP)}4 (µ4 -N)][BF4 ]. The compounds were structurally characterized and further studied by DFT calculations.

Synthesis and Coordination Behavior of a Flexible Bis(phosphinoferrocene) Ligand.

A symmetrical flexible bis(phosphinoferrocene) derivative, viz. bis[1'-(diphenylphosphino)ferrocenyl]methane (1), was prepared and studied as a ligand in Pd(II) and Au(I) complexes. The reactions of 1 with [PdCl2(cod)] (cod = cycloocta-1,5-diene) and [Pd(µ-Cl)(LNC)]2 (LNC = [2-(dimethylamino-κN)methyl]phenyl-κC¹) produced bis(phosphine) complex trans-[PdCl2(1-κ²P,P')] (4), wherein the ligand spans trans positions in the square-planar coordination sphere of Pd(II) and the tetranuclear, P,P-bridged complex [(µ(P,P')-1){PdCl(LNC)}2] (5), respectively. In reactions with the Au(I) precursors [AuCl(tht)] and [Au(tht)2][SbF6] (tht = tetrahydrothiophene), ligand 1 gave rise to tetranuclear Au2Fe2 complex [(µ(P,P')-1)(AuCl)2] (6) and to symmetrical macrocyclic tetramer [Au4(µ(P,P')-1)4][SbF6]4 (7). All compounds were characterized by spectroscopic methods. In addition, the structures of compound 1, its synthetic precursor bis[1'-(diphenylphosphino)ferrocenyl]methanone (3), and all aforementioned Pd(II) and Au(I) complexes were determined by single-crystal X-ray diffraction analysis (some in solvated form).

Aurophilic Interactions in [(L)AuCl]...[(L')AuCl] Dimers: Calibration by Experiment and Theory.

Attractive metallophilic (aurophilic, argentophilic, cuprophilic, etc.) interactions play an important role in arrangement and stabilization of oligonuclear metal ion complexes. We report a combined experimental and theoretical assessment of aurophilic interactions in closed-shell gold(I) dimers. The experimental binding energies were obtained for charged [(LH)AuCl]+...[(L')AuCl] dimers (L is either a phosphine or an N-heterocyclic carbene ligand) in the gas phase. These energies served for benchmarking of correlated quantum chemical calculations (CCSD(T)-calibrated SCS-MP2/CBS method) that were then applied to neutral [(L)AuCl]...[(L')AuCl] dimers. The overall attractive interactions between monomeric units are in the order of 100-165 kJ mol-1 in the charged dimers and of 70-105 kJ mol-1 in the corresponding neutral dimers. In the neutral dimers, pure aurophilic interactions account for 25-30 kJ mol-1, the dipole-dipole interactions for 30-45 kJ mol-1, and the L···L' "inter-ligand" dispersion interactions for 5-25 kJ mol-1. Energy of the aurophilic interactions is thus comparable or even larger than that of strong hydrogen bonds.

Synthesis and characterization of 1'-(diphenylphosphino)-1-isocyanoferrocene, an organometallic ligand combining two different soft donor moieties, and its Group 11 metal complexes.

The development of a practical synthesis of 1'-(diphenylphosphino)-1-aminoferrocene (2) and its P-borane adduct (2B) allowed the facile preparation of 1'-(diphenylphosphino)-1-isocyanoferrocene (1). This compound combining two specific soft-donor moieties was studied as a ligand for univalent Group 11 metal ions. The reactions of 1 with AgCl at 1 : 1 and 2 : 1 molar ratios only led to the coordination polymer [Ag2(µ-Cl)2(µ(P,C)-1)]n (6), while those with Ag[SbF6] provided the dimer [Ag2(Me2CO-κO)2(µ(P,C)-1)2][SbF6]2 and the quadruply-bridged disilver complex [Ag2(µ(P,C)-1)4][SbF6]2 (8), respectively. Addition of 1 to [AuCl(tht)] (tht = tetrahydrothiophene) afforded the mono- and the digold complex, [AuCl(1-κP)] (9) and [(µ(P,C)-1)(AuCl)2] (10), depending on the reaction stoichiometry. Finally, the reaction of 1 with [Au(tht)2][SbF6] or halogenide removal from 9 with AgNTf2 led to cationic dimers [Au2(µ(P,C)-1)2]X2 (11, X = SbF6 (a) or NTf2 (b)). Catalytic tests in the Au-mediated isomerization of (Z)-3-methylpent-2-en-4-yn-1-ol to 2,3-dimethylfuran revealed that 11a and 11b are substantially less catalytically active than their analogues containing 1'-(diphenylphosphino)-1-cyanoferrocene as the ligand, most likely due to a stronger coordination of the isonitrile moiety, which prevents dissociation of the dimeric complexes into catalytically active monomeric species.

Formation of Oxazoles from Elusive Gold(I) α-Oxocarbenes: A Mechanistic Study.

The gold(I) catalyzed reaction between phenylacetylene, pyridine N-oxide and acetonitrile leading, via a putative gold-α-oxocarbene intermediate, towards an oxazole product has been investigated. A novel mass spectrometric method called "delayed reactant labeling" is used to track consecutive and parallel reactions. It clearly shows that the intramolecular formation of a pyridine adduct of gold-α-oxocarbene is in competition with the formation of the oxazole product. The reaction mechanism most probably corresponds to competition between acetonitrile and pyridine in an almost barrierless reaction with putative gold-α-oxocarbene within the solvent cage. The detected ionic species have been characterized by helium tagging infrared photodissociation spectroscopy.

Role of gold(I) α-oxo carbenes in the oxidation reactions of alkynes catalyzed by gold(I) complexes.

The gas phase structures of gold(I) complexes formed by intermolecular oxidation of selected terminal (phenylacetylene) and internal alkynes (2-butyne, 1-phenylpropyne, diphenylacetylene) were investigated using tandem mass spectrometry and ion spectroscopy in conjunction with quantum-chemical calculations. The experiments demonstrated that the primarily formed ß-gold(I) vinyloxypyridinium complexes readily undergo rearrangement, dependent on their substituents, to either gold(I) α-oxo carbenenoids (a synthetic surrogate of the α-oxo carbenes) or pyridine adducts of gold(I) enone complexes in the condensed phase and that the existence of naked α-oxo carbenes is highly improbable. Isotopic labeling experiments performed with the reaction mixtures clearly linked the species that exist in solution to the ions transferred to the gas phase. The ions were then fully characterized by CID experiments and IRMPD spectroscopy. The conclusions based on the experimental observations perfectly correspond with the results from quantum-chemical calculations.

Tri-µ-chlorido-bis-[(η-hexa-methyl-benzene)-ruthenium(II)] tetra-chlorido-ferrate(III).

The mol-ecular geometry of the complex cation in the title structure, [(µ-Cl)(3){Ru(II)(η(6)-C(6)Me(6))}(2)][Fe(III)Cl(4)], compares very well with that reported earlier for the corresponding PF(6) (-) salt [Pandey et al. (1999 ▶). J. Organomet. Chem.592, 278-282]. The [FeCl(4)](-) counter ion has a rather regular tetra-hedral geometry with Fe-Cl distances and Cl-Fe-Cl angles in the range 2.1891 (7)-2.2018 (8) Šand 107.10 (3)-110.56 (3)°, respectively. There are no significant inter-molecular inter-actions in the crystal except for some weak C-H⋯Cl contacts, which in turn indicates that the crystal packing is determined predominantly by electrostatic inter-actions between the ionic constituents.

The coordination behaviour of ferrocene-based pyridylphosphine ligands towards Zn(II), Cd(II) and Hg(II).

The reaction of Group 12 metal dihalides MX(2) with the P,N-ligands [Fe(C(5)H(4)-PPh(2))(C(5)H(4)-2-py)] (1) (2-py = pyrid-2-yl), [Fe(C(5)H(4)-PPh(2))(C(5)H(4)-CH(2)-2-py)] (2) and [Fe(C(5)H(4)-PPh(2))(C(5)H(4)-3-py)] (3) (3-py = pyrid-3-yl) was investigated. For a 1 : 1 molar ratio of MX(2) and the respective ligand, three structure types were found in the solid state, viz. chelate, cyclic dimer and chain-like coordination polymer. The M(II) coordination environment is distorted pseudo-tetrahedral in each case. The P-M-N angle is much larger in the chelates (≥119°) than in the ligand-bridged structures (≤109°). 1 prefers the formation of chelates [MX(2)(1-κ(2)N,P)]. 3 forms coordination polymers [MX(2)(µ-3)](n). With the more flexible 2 all three structure types can occur. Dynamic coordination equilibria were observed in solution for the molecular complexes obtained with 1 and 2. NMR data indicate that the N- and P-donor sites interact most strongly with Zn(II) and Hg(II), respectively. While the formation of bis(phosphine)mercury complexes (soft-soft) was easily achieved, no bis(pyridine)zinc complex (borderline-borderline) could be obtained, which is surprising in view of the HSAB principle.

1-Bromo-1'-(diphenyl-thio-phosphor-yl)-ferrocene.

The title compound, [Fe(C(5)H(4)Br)(C(17)H(14)PS)], crystallizes with two practically undistiguishable mol-ecules in the asymmetric unit, which are related by a non-space-group inversion. The ferrocene-1,1'-diyl units exhibit a regular geometry with negligible tilting and balanced Fe-ring centroid distances, and with the attached substituents assuming conformations close to ideal synclinal eclipsed.