Conformational Isomerism in (p-RC₆H₄NC)₂W(dppe)₂:  Substantial Structural Changes Resulting from Subtle Differences in the π-Acidity ofp-RC₆H₄NC

Abstract

Bis(arylisocyanide) complexes of tungsten containing bulky bidentate aryl phosphine ligands adopt either cis or trans conformations, depending on the nature of the para-substituent group on the aryl isocyanide ligand. Ab initio calculations on a series of ligands, p-RC 6 H 4 NC, indicate that the energy of the LUMO is determined by the electron-withdrawing/donating capabilities of the substituent group, which determine the relative π-acidity of the ligand. Strong π acids, which contain electron-withdrawing groups, tend to polarize sufficient charge density away from the metal center to effect the formation of the sterically less favorable, but electronically stabilized, cis conformer. Weaker π acids, containing donor substituents, do not provide enough electronic stabilization to overcome steric repulsion, resulting in the adoption of trans geometry. (CNC 6 H 4 NC)W(dppe) 2 exists as the cis conformer, as does (CNC 6 H 2 (CH 3 ) 2 NC)W(dppe) 2 , while (CNC 6 (CH 3 ) 4 -NC)W(dppe) 2 , with increased steric bulk coupled with decreased π-acidity, exists as the trans conformer. Theoretical and experimental studies indicate that the energy match between isocyanide ligands and the metal center can be modified significantly by the selection of various substituents and that relatively subtle variations in the electronic structure of the ligands can have dramatic effects on the structures of the resulting complexes.

Formation of Germa-ketenimine on the Ge(100) Surface by Adsorption of tert-Butyl Isocyanide

“…Whereas IR absorption from organic functional groups between 1900 and 2000 cm –1 is uncommon, some isocyanide complexes that have undergone strong π back-donation show significantly red-shifted ν­(NC) absorbance in this range. − We assign the new features to a germa-ketenimine with a cumulative double bonded NCGe group, which occurs following cleavage of the Ge–Ge dimer bond, on the basis of the following reasons: With interruption of the 2 × 1 reconstruction by TBIC adsorbates, the resulting divalent surface atoms would become chemically analogous to germylene (R 2 Ge:), as observed for some atomic adsorbates on Ge(100). − Moreover, varying degrees of π back-donation have been observed for σ-donation complexes of isocyanides with molecular germylenes. , The proposed NCGe species would thus be characterized by π back-donation, consistent with a red-shifted absorbance near 1917–1963 cm –1 . In addition, DFT-calculated ν­(NC) frequencies at 1958 and 1863 cm –1 , assuming formation of the germa-ketenimine structure at a high coverage of TBIC (10 molecules on 9 Ge dimers), show good agreement with the position of these new features (Figure ).…”

Self-Assembled Diisocyanide Monolayer Films on Gold and Palladium

“…Likewise, the geometry of μ 2 -bridging isocyanides coordinated to two zerovalent metals or to Pd(111) at low surface coverage, which has ν(N⋮C) in the range of 1880−1580 cm -1 , , can also be eliminated. A bent μ 1 -η 2 geometry, such as is found in tungsten−isocyanide complexes that have dominant π back-donation from the metal, has been assigned to the ν(N⋮C) band position at 1960 cm -1 for PDI SAMs on palladium . However, another interpretation can be obtained by comparison with isocyanide complexes with open-faced trinuclear metal clusters, in which the metal triangles are locked in place in a planar arrangement.…”

“…The bulk (HATR) IR spectra of the diisocyanides show only one ν(N⋮C) band for each molecule, and Table shows the ν(N⋮C) peak position for each molecule. The addition of electron-donating methyl groups, which increases the π-acidity of the isocyanide, at both ortho positions, as in BXyDI, changes the frequency of this stretch by −6 cm -1 (ν(N⋮C) BXyDI − ν(N⋮C) BPDI ). When all positions are methyl substituted, as in TMPDI, the frequency is decreased by 19 cm -1 (ν(N⋮C) TMPDI − ν(N⋮C) PDI ).…”

Reactive Rigid-Rod Organometallic Polymers Involving Linear Triplatinum Units Connected by π-Conjugated Bisisocyanides

“…The contribution of structure B in Scheme may prevent further polymerization by reducing σ-donating ability of the terminal, uncoordinated CN groups. A similar phenomenon was observed in cis -[W(dppe) 2 (bisNC3) 2 ] (dppe = 1,2-bis(diphenylphosphino)ethane) . The poor solubility of compounds 8a , b prevented further detailed characterization.…”