Dinuclear allyl and µ-vinyl complexes from the reactions of a µ-hydrido dimanganese and a µ-hydrido manganese–molybdenum complex with alkynes containing α-methyl or α-methylene substituents

Abstract

The photolytic reactions of [Mn,(p-H) (p-PPh,) (CO),] and of [(q-C,H,) (CO),Mo(p-H) -(p-PPh,) M n ( CO),] with alkynes containing a-methyl or a-methylene substituents have been studied. The dimanganese complex reacts with MeCECH to give, in comparable yields, the q3-allyl complex [Mn,(p-PPh,) (q3-C3H5) (CO),] and an inseparable mixture of the two p-vinyl complexes [Mn(p-PPh,),{p-o:q2-C( Me)=CH,}(CO),] and [Mn,{p-o:q2-C(H)=C(H)Me}(p-PPh,) (CO),]. With MeCzCMe, [Mn,(p-PPh,)(q3-syn-MeCHCHCH,) (CO),] and [Mn,{p-o:q2-C( Me)=C( H)Me}(p-PPh,) (CO),] are obtained, again in comparable yields. Reactions of the dimanganese complex with EtCrCH and PhC-CMe proceed analogously. The molybdenummanganese complex does not give p-vinyl complexes on reaction with MeCrCH, MeCrCMe, or E t C K H . Instead MeCrCH gives [(CO),] as the sole product whereas M e C r C M e and EtCGCH both give the same inseparable isomeric mixture of [ (q -C,H,) (CO),Mo(p-PPh,) Mn(q3-syn-MeCflCHCH,) (CO),] and [ (q-C5f15) (CO),Mo(p-PPh,) -Mn(q3-anti-MeCHCHCH,) (CO),]. Deuteriation studies show that the formation of the allyl complexes from the alkynes involves a 1,2 hydrogen shift within the ligand; a mechanistic scheme for the reactions of the dimanganese and molybdenum-manganese complexes with a-methyl-and x-methylene-substituted alkynes consistent w i t h such a shift is proposed.The reactions of alkynes with dinuclear and polynuclear complexes containing p-hydrido ligands generally give, in the first instance, insertion products containing p-vinyl ligands. In a previous paper we reported that the dimanganese coniplex [Mn,(p-H)(p-PPh,)(CO),] (1) reacts in this way with RCrCR' under photolysis to give the p-vinyl complexes [Mn,(p-o:q2- C(R)=C(H)R')(p-PPh,)(CO),1.6The range of alkynes studied was, however, restricted to P h C S P h , P h C g H , and H C g H and we have now extended our investigations to alkynes containing x-methyl or a-methylene substituents. In addition to giving p-vinyl complexes such alkynes also give q3-allyl complexes by a process which must involve hydrogen transfer within the ligands. In order to explore the generality of these hydrogen-transfer reactions we have also studied the reactions of alkynes containing a-methyl or a-methylene substituents with the p-hydrido heterodimetallic complex [(q-C,H,)(CO),Mo-(p-H)(p-PPh2)Mn(C0),1 (2).738 Complexes (1) and (2) react differently with acetylene itself, the former giving a simple pvinyl complex ' whereas the latter gives, as the major product,

A Versatile and Unprecedented Triply Bonded Dimolybdenum Carbonyl Anion

“…This requires a 2,1-H atom shift in the original allyl group at some stage of the reaction. Noticeably, this unexpected rearrangement is opposite the 1,2-shift (alkenyl to allyl) proposed to occur during the photolysis of alkynes at Mn 2 or MnMo centers . Compound 6 exists in solution as a mixture of cis- and trans -dicarbonyl isomers, as deduced from its IR spectrum.…”

“…Noticeably, this unexpected rearrangement is opposite the 1,2-shift (alkenyl to allyl) proposed to occur during the photolysis of alkynes at Mn 2 or MnMo centers. 20 Compound 6 exists in solution as a mixture of cis-and trans-dicarbonyl isomers, 7 as deduced from its IR spectrum. Interestingly, these isomers interconvert rapidly on the NMR time scale, a process not detected previously in the related alkenyl complexes [Mo 2 Cp 2 (µ-CMeCHMe)(µ-SR)(CO) 2 ].…”

Hydrocarbyl Ligand Transformations on Heterobimetallic Complexes

“…No alkenyl complexes were isolated, but mixtures of syn- and anti -allyl isomers 336a,b were obtained in a 1:1 molar ratio (Scheme ). Alkenyl complexes are proposed as intermediates in the formation of the allylic species, and indeed, the configuration of the allyl complex may depend on the geometry of the intermediate alkenyl species . Similar products 336 are obtained in the reactions of allene and 1,1-dimethylallene with complex 73 ; two isomers are obtained in each case, and a molybdenum bonded π-allyl bimetallic complex 337a is also obtained in low yield with allene.…”

Reactivity of the 30-electron dimolybdenum anion [Mo2(η5-C5H5)2(μ-PCy2)(μ-CO)2]− towards β,γ-unsaturated organic halides: Alkenyl, allenyl and alkoxycarbyne derivatives

“…Instead, the reverse process, that is, the isomerization of an alkenyl complex to render the corresponding allyl isomer, is well documented for mononuclear species [15], and there are also a few examples involving dinuclear compounds. Thus, the photochemical reactions of the hydrides [Mn 2 (l-H)(l-PPh 2 )(CO) 8 ] and [MnMoCp(l-H)(l-PPh 2 )(CO) 6 ] in the presence of 1-propyne have been shown to yield the allyl compounds [Mn 2 (g 3 -C 3 H 5 )-(l-PPh 2 )(CO) 7 ] and [MnMoCp(g 3 -C 3 H 5 )(l-PPh 2 )(CO) 5 ], respectively [16]. Although no vinyl precursor could be detected in the latter reactions, the authors propose the occurrence of a 1,2-H shift in a vinyl intermediate, ruling out the possibility of a 1,3-shift on the basis of additional experiments using deuterated alkynes.…”