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By What Mechanisms Are Metal Cyclobutadiene Complexes Formed from Alkynes?
Author(s) -
Veiros Luís F.,
Dazinger Georg,
Kirchner Karl,
Calhorda Maria José,
Schmid Roland
Publication year - 2004
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400523
Subject(s) - metallacycle , cyclobutadiene , chemistry , ligand (biochemistry) , ring (chemistry) , carbene , stereochemistry , acetylene , crystallography , computational chemistry , molecule , physics , catalysis , x ray crystallography , organic chemistry , receptor , biochemistry , diffraction , optics
The mechanism of formation of an η 4 ‐cyclobutadiene complex from a metallacycle, generated by oxidative coupling of two acetylenes with the fragments CpRuCl, [CpRu(PH 3 )] + , CpCo, and CpRh, was investigated by means of DFT/B3LYP calculations. Two distinct pathways can be envisaged. 1) A multistep reaction, which can be denoted the Vollhardt mechanism, proceeding via a cyclopropenyl carbene and a tetrahedrane‐type intermediate. 2) A one‐step transformation involving the formation of a third MC bond with rearrangement of the metallacyclic ring. Although path 2 is definitely favored over path 1, both pathways are energetically prohibitive unless substituents are present on the acetylene. For the CpRuCl system with HCCR the barrier varies with R in the series H≈Ph>Me≫SiMe 3 . On going from H to SiMe 3 , the barrier for path 2 drops from 41.1 to 26.8 kcal mol −1 . This latter value is already reachable, in agreement with experiment. Whereas the reaction mechanisms involving the fragments CpCo, CpRh, and CpRuCl are very similar (but not identical owing to the additional ligand in CpRuCl), those of [CpRu(PH 3 )] + reveal a modification with serious consequences. In both paths 1 and 2, the originally planar metallacycle experiences first a bending distortion induced by the σ‐donor strength of P (in contrast to Cl), which compensates the loss of electrons from the ring brought about by bending. The bent metallacycle is already electronically asymmetric and thus the further course of the reaction is facilitated.