Organometallic Reactive Intermediates

Chiral Pentaorganylsilicates

Nucleophilic substitution of organosilicon compounds generally suffers from extensive racemization due to the intermediacy of elusive five-coordinate silicates. These species have a fluxional structure, interconverting rapidly between trigonal bipyramidal (TBP) and square pyramidal (SP) geometries. I have synthesized and characterized stable, isolable pentaorganylsilicates. Experimental and theoretical studies of the dynamic behavior of these compounds led me to design and synthesize pentaorganylsilicates that are no longer fluxional in solution. An NMR spectroscopic study on the ethyl analogue confirmed that the diastereotopic protons do not interchange, i.e. the anion assumes a rigid propeller-like shape. Efforts are under way to prepare these silicates as single enantiomers.

Configurationally Rigid Pentaorganosilicates

This research was published in the Journal of the American Chemical Society

Electrophilic Phosphinidenes

Electrophilic phosphinidenes, the phosphorus analogues of Fischer carbenes, are highly reactive intermediates which can undergo e.g. insertion and cycloaddition reactions. I used benzophosphepines as convenient phosphinidene precursors to synthesize the strained polycyclic phosphiranes shown below. The two isomers differ greatly in their 31P NMR chemical shieldings, which I explained using high-level calculations.

Fused Tricyclic Phosphoranes

This research was published in Chemistry − A European Journal

A Transient Electrophilic Phosphinidene Caught in the Act

This research was published in Chemistry − A European Journal

Seeing a phosphinidene fly − In a joint project of the groups of Prof. Chen and Prof. Lammertsma, I studied a cationic benzophosphepine by electrospray ionization tandem mass spectrometry (ESI-MS/MS). With this technique, I could directly observe the formation of the corresponding phosphinidene in the gas phase, its addition to alkenes, and the subsequent decomplexation of the resulting phosphirane (see right). This reactivity parallels that in solution as determined by Helen Jansen, my colleague in Amsterdam. Our results constitute the first direct experimental evidence for the existence of phosphinidenes.

Coinage Metal Nitrenes & Carbenes

The nitrogen analogues of carbenes are also elusive reactive intermediates. I collaborated with Alexey Fedorov, who was studying gold(I)carbene and -nitrene complexes by ESI-MS/MS. We assessed the importance of relativistic effects by a comparative experimental and computational study of the analogous copper(I)- and silver(I)nitrene complexes. To further confirm the structure of these species, the group of Prof. Rizzo (EPFL) measured highly resolved gas-phase IR spectra.

Structure and Bonding of Isoleptic Coinage Metal (Cu, Ag, Au) Dimethylaminonitrenes in the Gas Phase

This research was published in the Journal of the American Chemical Society

This research was published in ChemPhysChem