Synthesis of heterobimetallic allene complexes

Abstract

The first dinuclear complex containing a four-electron p-q2,q2allene ligand was synthesized from the reaction of allene with [Cp(C0)2M0]2 (Cp = q5-C5H5) and was reported in 1977.1 Thereafter, a number of complexes of this type were prepared,2 notably by photolysis of dinuclear complexes with a bridging cyclopropylidene ligand.2b,c Several dinuclear and trinuclear metal complexes containing allenyl ligands in various bonding modes have been prepared by initially using a propargyl ligand as a t em~la t e .~ Despite these and other report^,^ methods that lead to formation of a heterodinuclar p-q2,q2-allene complex are still l a~k ing .~ In exploring the chemistry of a tungsten @-(chlorocarbony1)allylic complex Cp(C0)2W(q3-CH2C(COCl)CH2) (la)? we prepared several dinuclear complexes by replacing the C1 atom with another metal group and found photolysis of such complexes provided a systematic approach to the dinuclear p+,q2allene complexes. Herein, we report several complexes synthesized via this approach and their characterization. The reaction of Cp(CO)2W(q3-CH2C(COCl)CH2) (la)6 with CP(CO)~Win THF resulted in the formation of a homonuclear complex Cp( CO) 2W [ p-q3,q1-CH2C( CO)CH2] W(C0) 3Cp (2a). Heteronuclear analogues Cp(C0)2M(p-q3,q1-CH2C(CO)CH2)M’ (2b, M = Mo, M’ = W(CO)3Cp; 2c, M = W, M‘ = Fe(C0)zCp; 2d, M = W, M’ = Re(CO)5) were prepared similarly. However, attempts to prepare similar complexes using the Cp(C0)3Moanion were unsuccessful. We believe that metal anions with weak nucleophilicity7 may not be applicable for the preparation of 2 employing this synthetic strategy. On the basis of the EAN rule, the two metal centers in 2 bridged by a p-q3,q’-/3-carbonylallylic ligand should have no M-M bond. Complexes 2 were characterized through a combination of microanalytical data and IR, IH NMR, and I3C NMR spectroscopy.8 For example, in the ‘H NMR spectrum of 2a at room temperature, the two inequivalent Cp groups displayed a sharp resonance and a broad resonance and the allylic syn and anti protons appeared as two broad resonances at 6 2.76 and 1.38, respectively. These broad resonances are due to the fluxional behavior of the endo and exo configurations of the allylic group. At 240 K, the two broad resonances at 6 2.76 and 1.38 of 2a were resolved into two sets of allylic protons at 6 2.78, 1.16 and 2.68, 1.58,8 assignable to those of the endo and exo isomers, respectively. Photolysis of complexes 2a-c with UV irradiation in benzene resulted in the loss of two CO ligands and afforded the dinuclear