Alexander Hepp

Synthesis of Silver(I) and Gold(I) Complexes with Cyclic Tetra- and Hexacarbene Ligands

Cyclic polyimidazolium salts have been investigated with respect to their function as anion receptors and cyclophanelike tetraimidazolium tetracations have been shown to bind anions inside the ring cavity. Cyclic polyimidazolium cations can also serve as precursors for N-heterocyclic carbenes (NHCs) and their complexes. Complexes of cyclic di-NHC ligands have been generated from bisazolium cyclophanes like A. However, such dicarbene ligands coordinate in a cisfashion to the metal center. They act as classical bidentate ligands rather than encapsulating the metal in a manner typical for macrocyclic ligands. Only one cyclic tetradentate dicarbene ligand is known to form a saddle-shaped macrocyclic complex B with nickel(II). We have described the first complex with a macrocyclic tetracarbene ligand C. The rhenium complex of a tridentate, facially coordinated [11]ane-P2C NHC macrocycle has also been reported. Since the preparation of complex C in a metal-template controlled domino-reaction is cumbersome we became interested in the preparation of complexes with macrocyclic poly-NHC ligands from cyclic polyimidazolium cations. This approach has previously been applied successfully for the preparation of some mono (Pd) and dinuclear (Cu, Ag) carbene complexes from the cyclic tetraimidazolium salt D. We became particularly interested in the lutidine-bridged tetraimidazolium salt H4-1 ACHTUNGTRENNUNG(Br4)[8] which upon C deprotonation would yield a potentially hexadentate ligand possessing two C-N-C pincer-type binding sites. Here we describe the mono and dinuclear silver(I) complexes with the carbene ligands derived from H4-1(Br)4 and the transfer of the carbene ligand to yield the mono and dinuclear gold(I) complexes (Scheme 1). A slight modification in the synthesis protocol employed for the preparation of H4-1(Br)4 yields the cyclic hexaimidazolium salt H6-6Br ACHTUNGTRENNUNG(BPh4)5 (Scheme 2). The preparation and molecular structure of the hexanuclear silver(I)-NHC complex derived from this ligand is also ACHTUNGTRENNUNGdescribed. The tetraimidazolium salt H4-1(Br)4 is soluble in water. [8]

Metal template controlled formation of [11]ane-P2CNHC macrocycles

The synthesis of N-heterocyclic carbene-diphosphine macrocycles by metal template assisted cyclization reactions has been explored. Attempts to prepare the facial tungsten tricarbonyl precursor complex containing an NH,NH-functionalized carbene and a suitable diphosphine resulted in displacement of the coordinated carbene and the isolation of the corresponding diphosphine tungsten tetracarbonyl [3]. The Re(I) chloro tetracarbonyl complex bearing an NH,NH-functionalized carbene ligand [5] can be prepared and is a suitable precursor for the subsequent formation of the carbene-diphosphine tricarbonyl intermediate [H(2)-6]Cl bearing reactive 2-fluoro substituents at the phosphine-phenyl groups. Two of these fluoro substituents are displaced by a nucleophilic attack upon deprotonation of the coordinated NH,NH-functionalized carbene resulting in new C-N bonds resulting in the partially coupled intermediate, [10], followed by the desired complex with the macrocyclic ligand [8]Cl. Compounds [H-7]Cl and [8]Cl are also formed during the synthesis of [H(2)-6]Cl as a result of spontaneous HF elimination. Complex [8](+) may be converted to the neutral dicarbonyl chloro analog [11] by action of Me(3)NO. Related chemistry with analogous manganese complexes is observed. Thus, from the NH,NH-functionalized carbene manganese bromo tetracarbonyl [12], the diphosphine manganese carbene tricarbonyl cation [H(2)-13] may be readily prepared which provides the macrocyclic carbene-diphosphine tricarbonyl cation [14](+) following base promoted nucleophilic intramolecular displacement of fluoride. Again, [14](+) is converted to the neutral bromo dicarbonyl upon reaction with Me(3)NO. All complexes with the exception of the reaction intermediate [10] have been characterized by spectroscopic and analytical methods in addition to X-ray crystallographic structure determinations for complexes [3], [5], [H(2)-6]Cl, [H(2)-6][9], [8]Cl, [10], [11], [12], and [14]Br.

Dimeric aluminum–phosphorus compounds as masked frustrated Lewis pairs for small molecule activation

Hydroalumination of aryldialkynylphosphines RP(C≡C-(t)Bu)(2) (R = Ph, Mes) with equimolar quantities of diethylaluminum hydride afforded mixed alkenyl-alkynyl cyclic dimers in which the dative aluminum-phosphorus bonds are geminal to the exocyclic alkenyl groups. Addition of triethylaluminum to isolated 1 (R = Ph) or to the in situ generated species (R = Mes) caused diethylaluminum ethynide elimination to yield the arylethylphosphorus dimers 2 and 3. These possess a chair-like Al(2)C(2)P(2) heterocycle with intermolecular Al-P interactions. The boat conformation (4) was obtained by the reaction of (t)Bu-P(C≡C-(t)Bu)(2) with di(tert-butyl)aluminum hydride. Despite being dimeric, 2 behaves as a frustrated Lewis pair and activates small molecules. The reaction with carbon dioxide gave cis/trans isomeric AlPC(2)O heterocycles that differ only by the configuration of the exocyclic alkenyl unit. Four isomers resulted from the reaction with phenyl isocyanate. This is caused by cis/trans isomerization of the initial C=O adduct and subsequent rearrangement to the AlPC(2)N heterocycle, being the C=N adduct.

Trapping of Tin(II) and Lead(II) Homologues of Carbon Monoxide by a Benzannulated Lutidine-Bridged Bisstannylene

The reaction of the benzannulated bisstannylene ligand 2 with Sn O or Pb O generated in situ gave the pincer complexes 3 and 4. Both complexes have been characterized by X-ray diffraction and multinuclear NMR spectroscopy. A divalent state has been found by Mössbauer spectroscopy for the tin atoms in complexes 3 and 4.

Heterobimetallic carbene complexes by a single-step site-selective metalation of a tricarbene ligand.

The unsymmetrical tris(imidazolium) salt H3-1(Br)3, featuring a 1,2,4-substitution pattern of the central phenyl ring, after triple imidazolium C2 deprotonation reacts in a one-pot reaction with Pd(OAc)2 and [M(Cp*)(Cl)2]2 (M = Rh(III), Ir(III)) to yield heterobimetallic complexes [3] (M = Rh) and [4] (M = Ir), in which the Pd(II) ion is chelated by two ortho N-heterocyclic carbene (NHC) donors while the third NHC donor coordinates to the M(III) center, which orthometalates the central phenyl ring.

π‐Bonding in Complexes of Benzannulated Biscarbenes, ‐germylenes, and ‐stannylenes: An Experimental and Theoretical Study

Benzannulated bisstannylenes, exhibiting a CH(2)C(CH(3))(2)CH(2) linking unit and CH(2)tBu (1) or CH(2)CH(2)CH(2)NMe(2) (2) N-substituents, and their molybdenum tetacarbonyl complexes 3 and 4 have been prepared. The complexes 3 and 4 exhibit remarkably short Mo-E bond lengths compared to the related biscarbene and bisgermylene complexes. The experimentally determined bonding parameters of the molybdenum bisstannylene complexes are discussed based on DFT calculations.

Formation of cationic [RP5Cl](+)-cages via insertion of [RPCl](+)-cations into a P-P bond of the P4 tetrahedron.

Fluorobenzene solutions of RPCl(2) and a Lewis acid such as ECl(3) (E = Al, Ga) in a 1:1 ratio are used as reactive sources of chlorophosphenium cations [RPCl](+), which insert into P-P bonds of dissolved P(4). This general protocol represents a powerful strategy for the synthesis of new cationic chloro-substituted organophosphorus [RP(5)Cl](+)-cages as illustrated by the isolation of several monocations (21a-g(+)) in good to excellent yields. For singular reaction two possible reaction mechanisms are proposed on the basis of quantum chemical calculations. The intriguing NMR spectra and structures of the obtained cationic [RP(5)Cl](+)-cages are discussed. Furthermore, the reactions of dichlorophosphanes and the Lewis acid GaCl(3) in various stoichiometries are investigated to obtain a deeper understanding of the species involved in these reactions. The formation of intermediates such as RPCl(2)·GaCl(3) (14) adducts, dichlorophosphanylchlorophosphonium cations [RPCl(2)-RPCl](+) (16(+)) and [RPCl(2)-RPCl-GaCl(3)](+) (17(+)) in reaction mixtures of RPCl(2) and GaCl(3) in fluorobenzene strongly depends on the basicity of the dichlorophosphane RPCl(2) (R = tBu, Cy, iPr, Et, Me, Ph, C(6)F(5)) and the reaction stoichiometry.

An Al/P-Based Frustrated Lewis Pair as an Efficient Ambiphilic Ligand: Coordination of Boron Trihalides, Rearrangement, and Formation of HBX2 Complexes (X = Br, I)

The Al/P-based frustrated Lewis pair (FLP) Mes2P-C(═CH-Ph)-Al(CMe3)2 (1) reacted with boron halides BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF3 adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes2P-(μ-C═CH-Ph)(μ-HBX2)-AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2P-C(═CH-Ph)-AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.

Activation of Isocyanates and Carbon Dioxide by a Monomeric Aluminium Hydrazide as an Active Lewis Pair

The monomeric aluminium hydrazide H10 C5 NN(AltBu2 )Ad (4; Ad=adamantyl, NC5 H10 =piperidinyl) was obtained in high yield by hydroalumination of the corresponding hydrazone derivative 1. Compound 4 has a strained AlN2 heterocycle formed by a donor-acceptor bond between the β-nitrogen atom of the hydrazide group and the aluminium atom. Opening of this bond resulted in the formation of an active Lewis pair that was able to cooperatively activate carbon dioxide or isocyanates. Insertion of the heterocumulenes into the AlN bond selectively afforded a carbamate and two urea derivatives in high yield. In the first step, phenyl isocyanate gave the adduct 6, which has the oxygen atom coordinated to the aluminium atom and its central carbon atom bound to the nitrogen atom of the piperidine moiety. Adduct 6 represents a reasonable intermediate state for these activation processes. The applicability of hydroaluminated compounds, such as 4, in organic synthesis was demonstrated by the reaction with an imidoyl chloride, which gave the corresponding amidrazone derivative 9.

The influence of halogen substituents on the course of hydrogallation and hydroalumination reactions

Treatment of trimethylsilylethynylbenzene derivatives with HGaCl(2) afforded products, [C(6)H(6-x){C(H)=C(SiMe(3))GaCl(2)}(x)], in which by a very fast cis/trans-rearrangement the Ga and H atoms occupied opposite sides of the resulting C=C double bonds. The stability of the cis-forms considerably increased upon application of 1,3-dibromo- and pentafluorophenylalkyne derivatives. Two pairs of cis/trans-isomers could be characterized by crystal structure determinations and allow the direct comparison of structural parameters. For the first time an equilibrium was detected between cis- and trans-forms in solution. Treatment of 1,4-di(tert-butylalkynyl)benzene with HAlR(2) (R = CMe(3), CH(2)CMe(3)) afforded cyclophane-type molecules by the release of AlR(3). Only the neopentyl derivative could be isolated and characterized by crystal structure determination. In contrast, the dibromo compound, 1,4-Br(2)-2,5-(Me(3)CC[triple bond]C)(2)C(6)H(2), yielded the simple addition product, C(6)H(2)Br(2){C(AlR(2))=C(H)CMe(3)}(2) (R = CMe(3)). Condensation was hindered in this case by intramolecular Al-Br interactions. Surprisingly, the simple addition product was also isolated from the reaction of 1,4-(Me(3)CC[triple bond]C)(2)C(6)H(4) with the relatively small hydride HAlEt(2). Solid-state NMR spectra of the product revealed strong intermolecular Al-C interactions involving the negatively charged terminal vinylic carbon atoms, to give one-dimensional coordination polymers.