Transfer and coupling of zirconocene-bound alkenyl ligands. An alternative route to (s-trans-.eta.4-conjugated diene)ZrCp2 complexes

Abstract

Organometallics 1984,3, 945-947 945 We speculate that the two-electron character of eq 1 derives from creation of a single metal-metal bond and structural rearrangement within the binuclear unit upon forming the oxidized product. Complex 2 (R = t-Bu) is reportede to have an Mo-Mo distance of 2.98 A and an acute Mo-S-Mo bridge angle of ca. 74O. These facts are consistent with the presence of a single metal-metal bond in MO~(CO)~(SR)~ to produce an l&electron configuration about each Mo. [MO~(CO)~(SR)~]~-satisfies the 18-electron rule without a metal-metal bond and is expected to have a substantially different Mo2(SR), core structure.22 The related 18-electron phosphido-bridged complexes Fez-(CO),&PPh,), and [Fe2(Co),(PPh2),12-exhibit remarkably distinct core s t r u~t u r e s , 2~*~~ yet undergo reversible twoelectron transfer (E, , . 26 We believe the Mo2(SRI2 and I?e2(PPh2), complexes share a similar phenomenological explanation*,26 of their electrochemical behavior in which formation or cleavage of a metal-metal single bond coupled with structural rearrangement in a bridged bimetallic center leads to simultaneous transfer of two electrons. This behavior is a marked contrast to the stepwise transfer of single electrons that is commonly observed in strongly coupled bi-and polynuclear complexes.26Additional interesting chemistry associated with the Mo2(SR), centers is illustrated by the remaining voltammetric traces in Figure 1. In Figure lb, a small oxidation wave (Epa = -0.54 V) is observed prior to the major oxidation of 1 in MeCN. This wave is quenched when a voltammogram is run under CO, but the corresponding cathodic peak (E, = -0.59 V) is still apparent (Figure IC). This cathodic peak and a more negative one (Epc = -0.82 V) are observed when a voltammogram of 2 is run under CO (Figure Id). The -0.82-V peak is assigned to reduction of Mo2(CO),JMeCN),(SR), based on a voltammogram of 3 in the absence of CO (Figure le). In keeping with the chemistry of Scheme I, we attribute these observations to successive replacement of CO by MeCN in the Mo(1) oxidation state and assign the cathodic peaks at ca. -0.4,-0.6, and -0.8 V to reduction of Mo2(CO)&3R),, Mo2(C0),-(MeCN) (SR),, and MO,(CO)~(M~CN),(SR),, respectively. Purging with CO in MeCN does not completely reverse these reactions (Figure lc,d). In contrast to Mo(I), the Mo(0) species display a greater affinity for CO and are less susceptible to solvolytic reactions. The small anodic wave at -0.54 V (Figure lb), which we attribute to oxidation of [Mo~(CO),(M~CN)(SR),]~-, is the only evidence of Mo(0) solvolysis; this artifact is removed by purging briefly with CO (Figure IC). Furthermore, CV of 2 or 3 in the presence of CO (Figure Id) exhibits no anodic waves in conjunction with the cathodic peaks at -0.6 and -0.8 V, only the reoxidation of 1 at -0.35 v. Thus, the MeCN-substituted (22) Structural characterization of 1 is planned to determine the Mo-Mo distance and other appropriate dimensions in the M O~( S R )~ core. (23) Ginsberg, R. E.; Rothrock, R. K.; Fink...

Novel Dinuclear (μ-Hydrido)(μ-alkylideneamido)ruthenium Complexes, [(η5-C5Me5)Ru]2(μ-L)(μ-H)(μ-NCHR) (R = Me, Et; L = MeCN, Arene), as Precursors of Active Species for Bimetallic Activation. Dehydrogenative Coupling of Ethylene in a Dinuclear Ruthenium Complex

“…There is no precedent for such a coupling reaction of two vinyl groups as far as the dinuclear complex is concerned. However, examples of the reductive coupling of two vinyl ligands have already been reported for a mononuclear complex 3 …”

Carbon−Carbon Bond Formation between Adjacent Alkenyl Ligands: η3-Allyl Iridium(III) and η4-Butadiene Iridium(I) Complexes from a cis-Bis(alkenyl) Iridium(III) Complex

“…There are a few examples of proton-initiated C−C bond formation between adjacent alkenyl ligands to give η 3 -allyl complexes and of reductive C−C coupling between two cis -alkenyl ligands by thermal activation to give η 4 -butadiene complexes . Hardly does the conversion of η 3 -butenyl complex to η 4 -butadiene complex occur under normal conditions except one by photolysis …”

Reactivity of vinyl and alkynyl zirconium complexes with the di-ansa-[1,1′,2,2′-bis(dimethylsilanediyl) dicyclopentadienyl] ligand

“…Divinyl species may rearrange to form zirconacyclopent-1-enes through b-hydrogen elimination [10] and give butadiene derivatives [11,12] by C C coupling. The structure of such butadiene compounds has been extensively explored and their behaviour as h 4 -diene or s 2 -p-zirconacyclopent-2-ene species with a Scheme 1.…”