Bimetallic Complexes with Conjugated C₄, C₆, C₁₀, and C₁₄ Bridges:  Synthetic Routes to Alkynediyl-Bridged Bis(carbene) Complexes

Abstract

The reaction of [(CO)5WC(Ph)H] with Me2NC⋮CNMe2 affords two complexes: the binuclear complex [(CO)5WC(NMe2)(Me2N)CW(CO)5] (2) and the insertion product [(CO)5WC(NMe2)C(NMe2)C(Ph)H] (3). Sequential reaction of the (dimethylamino)ethynylcarbene complex [(CO)5WC(NMe2)C⋮CH] (4a) with nBuLi, W(CO)6, and F3CSO3Me yields the new ethynediyl-bridged binuclear alkoxycarbene−aminocarbene complex [(CO)5WC(NMe2)C⋮C(MeO)CW(CO)5] (5). Treatment of 4a and of [(CO)5CrC(NMe2)C⋮CH] (4b) with nBuLi and iodine gives the new iodine-substituted complexes [(CO)5MC(NMe2)C⋮CI] (M = W (6a), M = Cr (6b)). By Pd-catalyzed coupling of 6a,b with the C-stannylated ethynylcarbene complexes [(CO)5MC(NMe2)C⋮CSn(nBu)3] (7a,b) the butadiynediyl-bridged bis(carbene) complexes [(CO)5MC(NMe2)(C⋮C)2(Me2N)CM(CO)5] (8a,b) are obtained. Analogously, from [(CO)5WC(NMe2)(C⋮C) x H] (x = 2, 3) and nBuLi/I2 amino(iodobutadiynyl)carbene and amino(iodohexatriynyl)carbene complexes [(CO)5WC(NMe2)(C⋮C) x I] (x = 2 (10), 3 (14)) are obtained. Pd-catalyzed coupling with the corresponding C-stannylated alkynylcarbene complexes finally affords the binuclear C10(NMe2)2- and C14(NMe2)2-bridged complexes [(CO)5WC(NMe2)(C⋮C) x (Me2N)CW(CO)5] (x = 4 (12), 6 (16)). All alkynediyl-bridged bis(carbene) complexes are stable at room temperature. The spectroscopic data of 5 indicate that it is best described as a hybrid of a dipolar allenyl-type compound and an alkynylcarbene complex. In contrast, the C⋮C units in the binuclear complexes 8a,b, 12, and 16 are essentially localized.

Symmetric and Non-Symmetric Dinuclear Alkenyl-Bridged Carbene Complexes of Chromium and Tungsten

“…Thus, these data suggest that there is apparently no π delocalization occurring along the alkenyl bridges, in contrast to the recently described π-conjugated ethyne-bridged bis(carbene) complex (CO) 5 WϭC(NMe 2 )ϪCϵCϪ(OMe)CϭW(CO) 5 where the carbene resonances were found shifted upfield by more than 60 ppm [δ ϭ 235.9 and 181.7]. [14] Complexes 3a and 3b could be easily transformed into bis(aminocarbene) complexes by the classical aminolysis route. For example, addition of excess of piperazine, ethylenediamine or 2-(ethylamino)ethanol (3Ϫ5 equiv.)…”

“…Recently, H. Fischer et al reported an efficient synthesis of symmetrically substituted alkynediyl-bridged aminocarbene complexes by Pd coupling of two (CO) 5 MϭC(NR 2 )(CϵC) n units (n ϭ 1,2,3). [14] The same precursors were also used to prepare polynuclear complexes of the type [(CO) 5 MϭC(NR 2 )(CϵC)] x MЈL n . [15] By contrast, nonsymmetrical bis(carbene) complexes are scarce.…”

“…[13] An ethyne-bridged (alkoxycarbene)(aminocarbene)ditungsten complex was also recently obtained by the Fischer route starting from an ethynylcarbene complex. [14] We have previously described a convenient route to alkenyl(alkoxy)-and -(amino)carbene complexes of Groups 6 and 8 metals, by activation of propargyl alcohol derivatives. [16,17] With the use of dialkynols, this procedure was recently applied to the synthesis of alkenylcarbene complexes I containing a free propargyl alcohol substituent (Scheme 1).…”

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…Biscarbene complexes with heteroarene spacer units have been investigated [63][64][65][66][67][68]. Examples of biscarbene complexes with alternating unsaturated carbon-carbon bonds have been reported [69,70]. Metal carbene fragments can be electronically isolated as is the case with alkyl spacer units [71].…”

“…When applied in the area of Fischer carbene complexes of the type [M(CO) 5 {C(OR')R}], the carbene properties have either been modified by introducing metal-containing substituents to further activate the carbene carbon [65,96,98] or the carbene ligand is used as a connector to bridge the other transition metals [11,64,69,73,74]. In the ten years since Sierra's review article [11], very few applications of these multimetal complexes have been reported considering the interest in the cooperative mechanism of metals in catalytic transformations and the ability of the electropositive metals to effect modulation of reactive centres otherwise not possible by traditional organic fragments; and there is still little known about the reactivity of this type of Fischer carbenes.…”