Coupling of alkynes with alkynes, isonitriles and carbon monoxide mediated by complexes of molybdenum, niobium and tantalum.

Abstract

Compound CpMo(PhCCPh)$\sb2$Cl 1a, at 100-110$\sp\circ$C gives CpMoCl(C$\sb4$Ph$\sb4$) 2, a folded metallacyclopentatriene. With two alkylidene Mo=C bonds (Av. 1.94 A) in the ring, complex 2 is folded 117$\sp\circ$. In addition, the reaction affords a dinuclear bicapped tetrahedrane compound, (CpMoCl)$\sb2$($\mu$-C$\sb4$Ph$\sb4$) 3a, that features an Mo$\sb2$C$\sb4$ core, along with a low yield of a paramagnetic compound, CpMo($\eta\sp4$-C$\sb4$Ph$\sb4$)Cl$\sb2$ 4a. The structure of 3a has a Mo-Mo single bond (distance 2.94 A) and two alkynes coupled to form a chain of four carbon atoms symmetrically bonded to both the Mo atoms. Similarly, the dinuclear clusters, (CpMoCl)$\sb2$($\mu$-C$\sb4$R$\sb2$R$\sp\prime\sb2$), $\{$3b, R = R$\sp\prime$ = p-tolyl; 3c, R = phenyl, R$\sp\prime$ = methyl$\}$ were also prepared from the corresponding bis(alkyne) complexes. Compound 3b has a structure similar to that of 3a. Reaction of the bis(alkyne) complex 1a with Co$\sb2$(CO)$\sb8$ at 110$\sp\circ$C gives a heterobinuclear complex CpMo(CO)$\sb2$Co(CO)$\sb2$($\mu$-C$\sb4$Ph$\sb4$) 5a. Similar reaction with CpMo(PhC$\sb2$Me)$\sb2$Cl gives a kinetic product CpMo(CO)$\sb2$Co(CO)$\sb2$($\mu$-CMeCPhCMeCPh) 5c. When heated, 5c gives the thermodynamic isomer CpMo(CO)$\sb2$Co(CO)$\sb2$($\mu$-CMeCPhCPhCMe) 5d. The Mo-Co bond distances are 2.68 A in 5a and 2.70 A in 5d. The Mo-Co bond distances are 2.68 A in 5a and 2.70 A in 5d. Reaction of Cp$\sp\*$Ta($\eta\sp2$-PhCCPh)(Me)$\sb2$ 7 with RNC gives Cp$\sp\*$Ta($\eta\sp2$-MeCNR) ($\eta\sp{2}$-PhCCPh)(Me) (8a R = tert-butyl, and 8b R = methyl). Compound 8a when heated in refluxing toluene forms the azatantallacyclopentatriene, Cp$\sp\*$Ta(CPhCPhCMeNR)Me. Reaction of 7 with CO gives the metallacycle Cp$\sp\*$Ta(CPhCPhCMeO)Me 10. The structure of 10 shows that the metallacycle is folded 124$\sp\circ$ along the C$\alpha$-O axis with an alkylidene Ta=C bond (distance = 1.97 A). Similarly, CpNb($\eta\sp2$-CPhCPh) ($\eta\sp2$-MeCNR) 15a, R = benzyl and 15b, R = tert-butyl were prepared by the reaction of the isonitriles with CpNb($\eta\sp2$-PhCCPh)Me$\sb2$. Complexes Cp$\sp\*$Ta($\eta\sp2$-PhCCPh)(X)(Y), $\{$11, X = Cl and Y = CH$\sb3$; 13, X = Cl and Y = N(CH$\sb3$)$\sb2$$\}$ were also prepared and spectroscopically characterized. Reduction of Cp$\sp\*$Ta($\eta\sp2$-PhCCPh)(Me)(Cl) 11 gives (Cp$\sp\*$Ta(PhCCPh)Cl) $\sb2$ as the only isolable product.