Ulrich Siemeling

Cyclopentadienyl compounds with nitrogen donors in the side-chain

Abstract The coordination chemistry of cyclopentadienyl ligands bearing either a dimethylaminoethyl side-chain or a pendant pyridyl group is described. The influence of these N donor functionalities on the structure and the physical as well as chemical properties of relevant metal complexes is discussed. In several cases, a relatively weak intramolecular coordination of the pendant amine is found; the hemilabile coordination profile of the N functionalised cyclopentadienyl ligands towards d-block elements may prove useful for the development of single-component catalysts.

N-heterocyclic carbenes which readily add ammonia, carbon monoxide and other small molecules,

N-Heterocyclic carbenes (NHCs) are extremely valuable as nucleophilic organocatalysts. They are widely applied as ligands in transition-metal catalysed reactions, where they are known as particularly potent σ-donors. They are commonly viewed as workhorses exhibiting reliable, but undramatic, chemical behaviour. The N → Ccarbene π-donation stabilises NHCs at the expense of low reactivity towards nucleophiles. In contrast to NHCs, stable (alkyl)(amino)carbenes exhibit spectacular reactivity, allowing, for example, the splitting of hydrogen and ammonia and the fixation of carbon monoxide. NHCs have been judged to be electronically not suitable for showing similar reactivity. Here, we demonstrate that a ferrocene-based NHC is able to add ammonia, methyl acrylate, tert-butyl isocyanide, and carbon monoxide—reactions typical of (alkyl)(amino)carbenes, but unprecedented for diaminocarbenes. We also show that even the simplest stable diaminocarbene, C(NiPr2)2, adds CO. This reaction affords a β-lactam by a subsequent intramolecular process involving a C–H activation. Our results shed new light on the chemistry of diaminocarbenes and offer great potential for synthetic chemistry and catalysis.

Spontaneous chiral resolution of a coordination polymer with distorted helical structure consisting of achiral building blocksDedicated to Professor Peter Jutzi on the occasion of his 65th birthday.

Reaction of achiral 2,5-diphenyl-3,4-di(3-pyridyl)cyclopenta-2,4-dien-1-one (2) with ZnCl2 and HgBr2, respectively, afforded the helically chiral coordination polymers [(2)ZnCl2]infinity and [(2)HgBr2]infinity, which show spontaneous chiral resolution, forming colonies of homochiral crystals.

1,1′-Di(heteroatom)-functionalised ferrocenes as [N,N], [O,O] and [S,S] chelate ligands in transition metal chemistry

Dppf is one of the most useful and popular ligands in coordination chemistry. Its overwhelming success has overshadowed and arguably even delayed the development and use of closely related ferrocene-based ligands with two ligating N, O or S atoms. Recently, however, dynamic progress concerning such homo-donor ligands can be noted. This tutorial review describes the main results obtained over the past decade in order to introduce the reader to an exciting field which currently shows particularly rapid development. The material is organised in sections according to ligand type, followed by a section which summarises the applications reported so far.

One-step synthesis of polysubstituted benzene derivatives by multi-component cyclization of α-bromoacetate, malononitrile and aromatic aldehydes

Polysubstituted benzene derivatives with an unprecedented substitution pattern are produced in a novel one-pot multi-component cyclization reaction from pyridine, ethyl alpha-bromoacetate, malononitrile and aromatic aldehyde in refluxing acetonitrile.

NHC-Based Self-Assembled Monolayers on Solid Gold Substrates

Thin films of 1,3-diethylbenzimidazol-2-ylidene (BIEt) were fabricated from THF solution on solid gold substrates and characterised by high-resolution X-ray photoelectron and near-edge X-ray absorption fine structure spectroscopy. The surface-analytical data are in accord with the formation of self-assembled monolayers of BIEt molecules exhibiting an approximately vertical orientation on the substrate. The crystal structure of (BIEt)(2) was also determined.

Correlation between the Molecular Structure and Photoresponse in Aliphatic Self-Assembled Monolayers with Azobenzene Tailgroups

We have compared the structural and photoisomerization properties of self-assembled monolayers (SAMs) comprising either the trans or cis isomers of azobenzene terminated dithiolane with in-chain amide unit, viz., 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide ( 1). These films were prepared on Au(111) from solutions of both isomers. Structure and composition of the SAMs were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The photoresponse of the films was monitored in real time by ellipsometry. SAMs fabricated from the trans isomer were found to be densely packed and highly ordered. These films did not show any discernible photoresponse upon irradiation with UV light, which, under favorable conditions, triggers the trans- cis isomerization. In contrast, films prepared from solutions containing predominantly the cis isomer were loosely packed and mostly disordered but exhibited reversible photoreactivity. The results confirm that steric effects, i.e., available free volume, play a dominant role for the photoresponse of aliphatic SAMs bearing the photoactive azobenzene group. The crystal structure of 1 ( trans isomer) exhibits a row-like aggregation of neighboring molecules by weak hydrogen bonds and can be taken as a model for the arrangement of 1 in the monolayer films. Further, in addition to the surface coordination behavior, we have also mimicked the chemisorption of the 1,2-dithiolane moiety onto the gold substrate in molecular coordination chemistry in oxidative addition reactions with the zero-valent platinum complex [Pt(PPh 3) 4].

Yes They Can: Small-molecule Activation with Stable Diaminocarbenes

Although small-molecule activation has long been the exclusive domain of transition metal chemistry, it has been increasingly realized that subvalent heavier main-group element compounds, too, are capable of activating H2, NH3, CO, etc. In a remarkable breakthrough, it was recently discovered that this is even possible with certain selected carbon(ii) compounds, viz. (alkyl)(amino)carbenes. However, N-heterocyclic carbenes and related diaminocarbenes, which represent the largest and most widely applied class of singlet carbenes, have been judged to be incapable of exhibiting similar reactivity. It now turns out that this is not quite true and that their reactivity needs to be reassessed.

Novel tripod ligands for prickly self-assembled monolayers

The new tridentate thioether ligands PhSi(CH2SMe)3 (1) and Ph-p-C6H4Si(CH2SMe)3 (2) have been synthesised and used for the preparation of the chelates fac-[W(kappa3-1)(CO)3] and fac-[W(kappa3-2)(CO)3], which were characterised by single-crystal X-ray diffraction. 1 and 2 were used as tripodal adsorbate molecules for the fabrication of self-assembled monolayers (SAMs) on gold. Film formation from solution was investigated in situ by second harmonic generation (SHG) and ellipsometry, which revealed a two-stepped process (fast adsorption, followed by slow film ordering). SAMs of 2 on gold were further investigated by ex situ methods, viz. high-resolution X-ray photoelectron spectroscopy (HRXPS), Fourier transform infrared reflection absorption spectroscopy (FTIRRAS), and scanning tunneling microscopy (STM). The latter two methods indicated dense packing of the tripodal anchor groups on the surface, with a substantially lower density of the biphenyl pricks. HRXPS showed three different binding states of sulfur, including a standard thiolate-type and a coordination-type state.

η2-Benzyne and η1-benzylidene complexes of niobium with ancillary imido ligands

Rare η2-benzyne and benzylidene complexes of niobium have been synthesised from [Nb(C5R5)(NR′)Cl2](R = H, Me; R′= 2,6-Pri2C6H3)via intermediate diphenyl and dibenzyl species and their molecular structures have been determined: these complexes are analogues of well-established zirconocene derivatives.