Peter Kircher

Cooperative Transformations of Small Molecules at a Dinuclear Nickel(II) Site

The synergetic action of two nickel(II) ions embedded in a tunable dinucleating ligand matrix (illustrated) allows manifold cooperative reactions within the bimetallic pocket, such as the hydration of nitriles, the hydrolysis of esters, and the degradation of urea to cyanate. Related mononuclear complexes have been prepared in order to further elucidate mechanistic aspects of these transformations.

[{(CO)5Cr}6Ge6]2−, A Molecular Organometallic Derivative of the Unknown Zintl Ion [Ge6]2−

Octahedral clusters from p-block elements are rare; however, the only known molecular aggregate of this kind, [{(CO)5 Cr}6 Sn6 ]2- , has now been supplemented by the isoelectronic cluster [{(CO)5 Cr}6 Ge6 ]2- (1).

Cooperative Binding of Nitrile Moieties Within a Bimetallic Pocket: Enforcing Side-On π-Interaction With a High-Spin Nickel(II) Site

Different cooperative binding modes of nitriles within the bimetallic pocket of a pyrazolate-based compartmental dinickel(II) site have been studied. The H3O2-bridged dinuclear complex 1 reacts with cyanamide to yield 4, in which a secondary hydrogencyanamido(1–) bridge spans the two metal centers at an unusually short metal–metal distance imposed by the primary ligand matrix. In 5, a single 2-cyanoguanidine (cnge) molecule is N-bound to one nickel(II) ion through its nitrile part and is coordinated to the adjacent metal site through an amido nitrogen. The characteristics of the coordination spheres of the metal centers suggest an additional side-on π-bonding interaction of the nitrile moiety with the second high-spin nickel(II) ion. This unusual interaction is corroborated by comparing the IR bands for the ν(C≡N) stretching vibration of 5 with those of complex 6, which has two end-on bound cnge molecules, and those of the related mononuclear complex 7, which lacks a second nickel(II) ion. The nature of the π-bonding interaction in 5 is further analyzed by DFT calculations on relevant model systems. Even though the π-bonding is found to be very weak, it does include some backbonding from occupied 3d MOs at the second high-spin nickel(II) ion to the π* MOs of the nitrile. Such an unconventional π-interaction is suggested to be enforced by the constrained fixation of the nitrile unit within the highly organized coordination pocket of the bimetallic framework. In contrast, the bifunctional 2-hydroxybenzonitrile is accommodated by the distinct binding of the nitrile and phenolate functions to the different metal centers in 8, which confirms that the simultaneous binding of both an OR-function and an end-on bound nitrile is indeed feasible within the active site pocket. Such a situation is reminiscent of the bimetallic effect that has been assumed to enable the cooperative hydration of nitriles at the dinickel(II) site of 1. Complexes 4·(ClO4)2, 5·(ClO4)2, 6·(ClO4)3, 7·(ClO4)(BPh4), and 8·(ClO4)2 have been characterized structurally by X-ray crystallography.

[{(CO)5Cr}6Ge6]2−, ein molekulares metallorganisches Derivat des bislang unbekannten Zintl‐Ions [Ge6]2−

Oktaedrische Cluster aus Elementen des p-Blocks sind zwar selten, doch konnte nun dem einzigen bisher bekannten molekularen Aggregat dieser Art, [{(CO)5Cr}6Sn6]2−, der isoelektronische [{(CO)5Cr}6Ge6]2−-Cluster 1 zur Seite gestellt werden.

Syntheses and Ligating Properties of Molybdocene Alkoxides—The First Heterodimetallic Alkoxide Containing Molybdenum and Bismuth

Fluorinated alkoxide ligands RO(-) (R=CH(CF3 )2 ) are the key to the isolation of compounds of the type [Cp2 Mo(OR)2 ]. When electron-donating groups R are employed, the Mo(OR)2 moiety can, and necessarily has to, serve as a ligand for Lewis acidic fragments, allowing the isolation and structural characterization of the first heterodimetallic alkoxide containing a Bi and a Mo center (1).

Four-Coordinate Group-14 Elements in the Formal Oxidation State of Zero – Syntheses, Structures, and Dynamics of [{(CO)5Cr}2Sn(L2)] and Related Species

The sodium salts Na2[{(CO)5M}2EX2] (M = Cr, Mo, W; E = Ge, Sn, Pb; X = Cl, I, OOCCH3) react with 2,2′-bipyridine (bipy) to form neutral compounds [{(CO)5M}2E(bipy)] (E = Sn: 1a–1c; E = Ge: 3a; E = Pb: 4). 1,10-Phenanthroline (phen) analogues of compounds 1a–1c and 3a [{(CO)5M}2E(phen)] (E = Sn: 1d–1f, E = Ge: 3b) are as well accessible. The 2,2′-bipyridine ligand in 1 may be formally replaced by two pyridine (py) ligands resulting in [{(CO)5M}2Sn(py)2] (1g: M = Cr, 1h: M = W). The bis-bidentate ligand 2,2′-bipyrimidine (bpmd) is found to coordinate just one [{(CO)5M}2Sn] entity in [{(CO)5M}2Sn(bpmd)] (2b: M = Cr, 2c: M = W). The biimidazolato (biim) ligand binds two [{(CO)5Cr}2Sn] moieties in [{(CO)5Cr}2Sn(biim)Sn{Cr(CO)5}2]2–, 2a. It is shown by 1H-NMR that the pyrimidine entities in these compounds (2b, 2c) are able to rotate by a full 180° turn-around with respect to one another. This process must involve complete de-coordination of at least one of the two nitrogen donors in again at least one of the chelate cycles, the activation energy for this process being around 60 kJ/mol. By 119Sn-NMR spectroscopy of almost all of the tin compounds described it is shown that equilibria between [{(CO)5M}2Sn(L2)] and [{(CO)5M}2Sn(L)] + L exist in all cases. From the temperature dependence of the δ values it is concluded that the activation barriers for this association/dissociation process is below 10 kJ/mol. The structures of all new compounds are documented by X-ray analyses and all compounds are characterized by the usual analytical and spectroscopical techniques.

Self-Assembly of Tetranuclear Iron(II) Compounds by a Combination of Rod- and Clamp-Shaped Bridging Units

Dicyanomethanides, [RC(CN)2]− (1−), are bridging ligands with an angular clamp-type shape. In the tripod/iron(II) system, [tripod = CH3C(CH2PPh2)3], they form binuclear species with three of the clamp-type ligands bridging two iron centers. An appropriate stoichiometric mixture of the tripod ligand, iron(II) salts and the dicyanomethanide ligand leads to the exclusive formation of [tripodFe{μ-NC−C(R)−CN}3Fetripod]+ (3+). Altogether, seven constituent groups arrange themselves in this type of aggregate. The process of self-aggregation is distinctly modified by the presence of cyanide ions that may act as linear rod-type bridging ligands. Equimolar mixtures of tripod, iron(II) salts, dicyanomethanide ligands and cyanide form tetranuclear aggregates [{tripodFe}3{μ-NC−C(R)−CN}3{μ-CN}3FeX]+ (5+). In these species, the iron centers form a trigonal-pyramidal arrangement with three octahedrally coordinated low-spintripodiron(II) entities in the basal plane and a tetrahedrally coordinated high-spin iron(II) at the apex. The iron centers in the basal plane are connected by μ2-bridging dicyanomethanide ligands. The apical iron center is coordinated by the N-termini of three cyano ligands, which complete the octahedral coordination of the basal low-spin iron(II) centers. The fourth, external ligand X at the tetrahedrally coordinated high-spin iron(II) apex can be varied, but in most cases it is found to be a terminally coordinated dicyanomethanide entity. Without counting this variable ligand X, thirteen constituent groups of four different types selectively aggregate to form the cage compounds 5+. While the dinuclear compounds 3+ are diamagnetic, the tetranuclear species 5+ have magnetic moments close to the spin-only value of μs.o. = 4.9 μB, for tetrahedral high-spin iron(II). The Mossbauer spectra are in agreement with the description of 5+ above, which contain three low-spintripodiron(II) entities and one high-spin iron(II) center. The synthesis, spectroscopic data, cyclic voltammetric data, Mossbauer data and X-ray analytical data for a series of compounds of types 3+ and 5+ are presented in this paper.

Neopentane-Based Tripodal CpL2 Ligands: Synthesis and Reactions of CH3C(CH2-η5-C5H4)(CH2-η1-PPh2)2RuCl; Hindered Rotation of Vinylidene Ligands

The tripodal ligand [CH3C(CH2C5H4)(CH2PPh2)2]– reacts with RuCl2(PPh3)3 to produce CH3C(CH2-η5-C5H4)(CH2-η1-PPh2)2RuCl, [tripodCpL2RuCl], 1. Complex 1 undergoes substitution of the chlorine function with various nucleophiles L′ to produce [tripodCpL2RuL′]+. The carbonyl derivative (L′ = CO) 2, isonitrile (L′ = RNC) 3, nitrile compounds (L′ = RCN) 4, and a tolane adduct (L′ = η2-PhC≡CPh) 5 are obtained when 1 is treated with the appropriate ligands in polar solvents. Halide acceptors (e.g. TlPF6) are generally needed to promote these reactions. The cyanide derivative tripodCpL2RuCN (3a) is alkylated by F3CSO3CH3 to give the isonitrile derivative [tripodCpL2RuCNMe]+3b. Terminal alkynes HC≡CR produce vinylidene compounds [tripodCpL2RuL′]+, where L′ = CCHR (R = tBu, 7b; R = Ph, 7c), or allenylidene derivatives, L′ = CCCPh2 (6), depending on the nature of R (R = CPh2OH for synthesis of 6). Trimethylsilylacetylene gives the parent vinylidene species, L′ = CCH2 (7a), which is transformed to the Fischer-type carbene compound, L′ = C(OMe)Me (8), upon treatment with methanol. The vinylidene species 7 are deprotonated by NaOMe to produce the alkynyl compounds tripodCpL2RuC≡CR (9). Methylation of 9 with F3CSO3CH3 results in the vinylidene derivatives L′ = CC(Me)R (R = tBu, 7d; R = Ph, 7e), having two organic substituents at the terminal carbon centre. For all vinylidene compounds with two different substituents at their terminal carbon atom, hindered rotation of the single-faced vinylidene π-ligand about its Ru–C bond is observed. Analysis by 31P-NMR spectroscopic coalescence measurements as well as line-shape analyses reveals activation enthalpies of around 40 kJmol–1 for this rotation, with small activation entropies of around ±10 Jmol–1K–1. Solid-state structures of nine compounds of the type [tripodCpL2RuL′]+n (n = 0, 1) demonstrate the remarkable conformational rigidity of the tripodCpL2Ru template. They also show that the possible strain imposed by linking the Cp ligand and the two donor groups L in one and the same chelate scaffolding does not appear to impose a serious steric strain on these templates.

Reductive Activation of Tripod Cobalt Compounds: Oxidative Addition of H−H, P−H, and Sn−H Functions

Treatment of the tripod compounds tripodCoCl2 (1), and tripodCoCl (2), [tripod = CH3C(CH2PPh2)3] in THF solution under argon atmosphere with strong reducing agents such as KC8 leads to the generation of reactive species. While the dinitrogen compound tripodCo(N2)Cotripod (3), which is formed under N2 atmosphere, is rather unreactive, the species formed under argon atmosphere undergo selective reactions with compounds containing P−H or Sn−H functions. With PhPH2 as the reagent, the diphosphene compound tripodCo(η2-PhP=PPh) (8), is formed in a yield (44%) similar to that achieved in the preparation of 8 from 2 and PhPHNa (60%). With Ph3SnH as the reagent, tripodCoSnPh3 (9), is obtained (yield 61%), while reaction with Bu3SnH produces tripodCo(H)2SnBu3 (10a, 50%). The CoI species 9 undergoes oxidative addition of dihydrogen to produce the CoIII compound tripodCo(H)2SnPh3 (10b), which is an analogue of 10a. The properties of these new compounds are characterized by the usual analytical techniques, including NMR spectroscopy, cyclic voltammetry, and X-ray analysis.

Synthese und Koordinationseigenschaften von Molybdocenalkoxiden – das erste Heterodimetallalkoxid mit Molybdän und Bismut

Fluoriert mussen Alkoxidliganden RO− (R = CH(CF3)2) sein, dann lassen sich Verbindungen des Typs [Cp2Mo(OR)2] isolieren. Liefern die Reste R hingegen Elektronen, kann und mus die Mo(OR)2-Einheit als Ligand fur Lewis-saure Fragmente dienen, wie durch die Isolierung und strukturelle Charakterisierung des ersten Heterodimetallalkoxides mit einem Bi- und einem Mo-Zentrum (1) gezeigt wurde.