Basen-induzierte Alkin-Spaltung in W(II)-Komplexen – ein neuer Weg zu niedervalenten Carbin-Komplexen aus W(CO)6 / Base-Induced Alkyne Cleavage in W(II) ComplexesaNew Way to Low Valent Carbyne Complexes from W(CO)6
Authors: Filippou Alexander Constantin , Völkl Christian , Grünleitner Walter , Kiprof Paul 
Published: March 2015

A new entry to low-valent, carbonyl-free tungsten phenylcarbyne complexes has been developed from W(CO)6 via tungsten(II) alkyne complexes. The first step includes thermal decarbonylation of W(CO)6 to fac-W(CO)3(MeCN)3 and subsequent substitution of the acetonitrile ligands by EtNC or t-BuNC to give fac-W(CO)3(EtNC)3 (1 a) or fac-W(CO)3(t-BuNC)3 (1 b) in almost quantitative yields. Advantages of this indirect synthesis of 1 a and 1 b from W(CO)6 are discussed in comparison to the previously described, CoCl2 catalyzed transformation of W(CO)6 with RNC to fac-W(CO)3(RNC)3. Compounds 1 a and 1 b are then quantitatively converted with Br2 to W(CO)2(EtNC)3(Br)2 (2 a) and W(CO)2(t-BuNC)3(Br)2 (2b). 2 a and 2b react subsequently with PhC=CBr to give upon elimination of both CO ligands the carbonylfree compounds W(EtNC)3(η2-PhC=CBr)(Br)2 (3a) and W(t-BuNC)3(η2-PhC=CBr)(Br)2 (3b) in good yield. 3 a and 3b represent rare examples of complexes containing a 1-haloalkyne ligand. They are susceptible to nucleophilic attack at this ligand. Thus reaction of 3 a and 3 b with

t-BuNH2 or Et2NH gives the 1-aminoalkyne complexes W(EtNC)3[η2-PhC=CNH(t-Bu)](Br)2 (4a), W(t-BuNC)3[η2-PhC≡CNH(t-Bu)](Br)2 (4 b) and W(t-BuNC)3(η2-PhC=CNEt2)(Br)2 (4c) in good yields. Finally a base-induced cleavage of the alkyne ligand is achieved in 4 a and 4b using LiPh and gives the carbyne complexes mer,trans-Br(t-BuNC)(EtNC)3W=CPh (5a) and trans-Br(t-BuNC)4W=CPh (5b). This decoupling reaction is the first example of an alkyne-bond cleavage to C1-fragments at a single transition metal center and the key step in the new synthetic procedure to 5 a and 5 b from W(CO)6. Complex 5 b has been previously prepared from W(CO)6 by the classical Fischer route via the carbonyl containing carbyne complex

trans-Br(CO)4W=CPh and has been shown to undergo the reverse of the decoupling reaction i.e. an H Br-induced coupling of a tert-butylisocyanide with the phenylcarbyne ligand to form 4b. The compositions and structures of the complexes 1 a-5b have been determined by total elemental analyses, IR, 1H NMR, 13C NMR and mass spectra. The spectroscopic results indicate a substantial bond delocalization in the 1-aminoalkyne complexes 4a-4c and a hindered rotation of the diethylamino group in the alkyne ligand of 4c. The barrier to this rotation is calculated to be ca. 14.7 kcal/mol and compared with previously reported diethylaminoalkyne complexes. An X-ray crystallographic study of 5 b has been carried out. The compound crystallizes in the monoclinic space group P 21. The unit cell contains two molecules of 5 b, which have a distorted octahedral coordination geometry with a trans-arrangement of the bromo and the phenylcarbyne ligand. The four isocyanide ligands are slightly bent out of the equatorial plane towards the bromo ligand. Striking features of the structure are the shortest W=Ccarbyne and the longest W —Br bond lengths known for low valent tungsten aryl- and alkylcarbyne complexes indicating a high electron density at tungsten. In addition two isocyanide ligands show a bent geometry at nitrogen, which has been so far reported only for electron-rich isocyanide complexes.