Insertion, coupling and elimination processes in the reactions of the unsaturated alkyl-bridged complexes [Mo2(η5-C5H5)2(μ-CH2R)(μ-PCy2)(CO)2] (R = H, Ph) with isocyanides and secondary phosphines

Abstract

The methyl-bridged complex [Mo2Cp2(μ-Me)(μ-PCy2)(CO)2] (Cp = η(5)-C5H5) reacted with stoichiometric amounts of CN(t)Bu at 243 K to give the C,O:C,O-bridged acyl complex [Mo2Cp2{μ-C,O:C,O-C(O)Me}(μ-PCy2)(CN(t)Bu)(CO)], which at room temperature slowly rearranges into its iminoacyl-bridged isomer [Mo2Cp2(μ-C,N:C,N-MeCN(t)Bu)(μ-PCy2)(CO)2]. In contrast, the C:O-bridged acyl complex [Mo2Cp2{μ-C:O-C(O)Me}(μ-PCy2)(CN(t)Bu)(CO)] was the major product obtained when the above reaction was carried out at room temperature. Density Functional Theory (DFT) was used to find the most likely structures of all these isomers, of which the iminoacyl complex was the absolute minimum. In contrast to the above reactions, up to three molecules of the ligand added rapidly to the methyl complex when using the aryl isocyanides CNR (R = o-C6H4Me, p-C6H4OMe), triggering the coupling between the methyl ligand and one of the cyclopentadienyl groups to give the corresponding methylcyclopentadiene derivatives [Mo2Cp(η(4)-C5H5Me)(μ-PCy2)(CNR)3(CO)]. Carbonylation of the latter complex (R = o-C6H4Me) induced the displacement of the η(4)-bound ligand, but also gave small yields of the hydride derivative [Mo2Cp(η(5)-C5H4Me)(μ-H)(μ-PCy2){CN(o-C6H4Me)}(CO)3] (Mo-Mo = 3.2467(5) Å), the latter resulting from a C-H cleavage in the methylcyclopentadiene ligand. The reaction of the title complexes with phosphines HPR (R' = Et, Ph) gave two major products: the corresponding aldehyde complexes [Mo2Cp2(μ-PCy2)(μ-PR){η(2)-C(O)HR}(CO)] (Mo-Mo = 2.8288(5) Å when R = CH2Ph and R' = Et) and the dicarbonyl complexes [Mo2Cp2(μ-PCy2)(μ-PR)(CO)2], these following from alternative reductive elimination processes, from hydrogen and either acyl or alkyl ligands, respectively.

Chalcogenoacyl-bridged derivatives of the unsaturated carbyne complex [Mo 2 ( η 5 -C 5 H 5 ) 2 ( μ -CPh)( μ -PCy 2 )(CO) 2 ]

“…In a formal sense, for these 5-electron donor ligands we can view the chalcogen atom as providing the dimetal centre with 3 electrons, while the CR group accounts for 2 electrons (A in Chart 1). Interestingly, the same sort of spatial and electronic arrangement has been found previously in related cyclopentadienyl complexes of type [M 2 Cp 2 (-PR 2 )(-X)(CO) 2 ] when X is a related  2 : 2 -bound 5-electron donor such as a formimidoyl[8] or iminoacyl ligand (B in Chart 1)[9], or a diphosphenyl ligand (C in Chart 1)[10]. In all these complexes, the 3-electron donor part of the ligand (E, NR, PR) is placed specifically trans to the PCy 2 group, while the 2-electron donor part is placed cis to it.Such a geometric preference must have an electronic origin since, for instance, one might anticipate, on steric grounds alone, that the chalcogenoacyl ligand would rather prefer a conformation with the CR and E sites exchanged.…”

“…115 o ). This sort of distortion has been systematically found by us in cyclopentadienyl complexes of type [M 2 Cp 2 (-PR 2 )(-X)(CO) 2 ] with other  2 : 2bound X groups such as formimidoyl [8], iminoacyl [9] or diphosphenyl ligands [10]. is close to the reference single-bond figure of 1.78 Å for a (sp 2 )CS bond [12], and is significantly longer than the reference double-bond length of 1.62 Å [13].…”

Reactions of the Unsaturated Ditungsten Anion [W2Cp2(μ-PCy2)(μ-CO)2]− with C- and P-Based Electrophiles

“…However, the number of systems combining a metal–metal triple bond and a negative charge is quite limited so far; in fact, we can only quote the molybdenum pentachloride [Mo 2 Cp 2 (μ-Cl)­Cl 4 ] − reported by Rheingold and co-workers and the organophosphide-bridged anions [Mo 2 Cp 2 (μ-PR 2 )­(μ-CO) 2 ] − (R = Cy, Ph, OEt) prepared some time ago in our lab . Among all these anions, only the PCy 2 -bridged one has been the subject of a systematic study of reactivity, which revealed it as a highly active and versatile intermediate with two distinct nucleophilic sites located at the metal and oxygen atoms. , Moreover this anion turned out to be a key synthetic intermediate for the preparation of a variety of unsaturated derivatives such as the hydride [Mo 2 Cp 2 (μ-H)­(μ-PCy 2 )­(CO) 2 ], the methoxycarbyne [Mo 2 Cp 2 (μ-COMe)­(μ-PCy 2 )­(μ-CO)], or the agostic alkyls [Mo 2 Cp 2 (μ-CH 2 R)­(μ-PCy 2 )­(CO) 2 ] (R = H, Ph), all of them displaying a wide reactivity on their own, as shown by independent studies. − Encouraged by these remarkable results and to test the influence of the metal (W instead of Mo) on all this chemistry, more recently we developed an efficient synthetic route for the Na + salt of the related ditungsten anion [W 2 Cp 2 (μ-PCy 2 )­(μ-CO) 2 ] − ( 1 ) (Chart ) . Indeed our initial exploration on the reactivity of this anion revealed significant differences when compared with its dimolybdenum analogue.…”