Synthesis and reactivity of molybdenocene isocyanide complexes; crystal structure of (η⁵-C₅H₅)₂MoCN^tBu

Synthetic, structural and reactivity studies on Mo^IV and Mo^II alkyl isocyanide complexes of the types [Cp₂Mo(X)CNR]Y and Cp₂MoCNR (X = H, Me, Et, Cl, I; R = Me, Et, ^tBu; Y = I, BF₄, PF₆) are reported. Reaction of Cp₂Mo(H)I (2) with EtNC gives the hydrido- isocyanide complex [Cp₂Mo(H)CNEt]I (3) in high yield. Complex 3 is converted by CHI₃ into the corresponding iodo derivative [Cp₂Mo(I)CNEt]I (4). An alternative route to halo-isocyanide complexes of molybdenocene involves halide abstraction from Cp₂MoX₂ (5: X = Cl; 6: X = I) by TIPF₆ in the presence of RNC (R = Et, ^tBu); this affords the complexes [Cp₂Mo(X)CNR]PF₆ (7a-8b) (7: X = Cl, 8: X = I; a: R = Et, b: R =^tBu) in high yield. Reduction of 7a-8b by sodium amalgam in THF results in the formation of the Mo^II, isocyanide complexes Cp₂MoCNR (9a: R = Et, 9b: R =^tBu). An alternative high yield route to these compounds involves reaction of the acetonitrile complex Cp₂Mo(η²-MeCN) (10) with RNC. Alkylation of 9a with MeI or Et₃OBF₄ occurs exclusively at the metal centre to yield the Mo^IV alkyl complexes [Cp₂Mo(Me)CNEt]I (11) and [Cp₂ Mo(Et)CNEt]BF₄ (12), respectively. Similarly, complex 9a reacts with AuPPh₃Cl to give the heterobimetallic compound [Cp₂mo (AuPPh₃)CNEt]Cl (13). By contrast, a carbonyl/isocyanide exchange reaction occurs between 9a and Re(CO)₅Br, to give Cp₂MoCO (14) and cis-Re(CO)₄(CNEt)Br (15). The alkyl complexes 11 and 12, when heated in CH₂Cl₂ undergo a clean isocyanide insertion to give the Mo^IV iminoacyl complexes [Cp₂Mo[η₂-C(NEt)Me]I (16) and [Cp₂Mo[η₂-C(NEt)Et]BF₄ (17), respectively. Similarly the alkyl complexes [Cp₂Mo(R)CNMe]I (18: R = Me; 19: R = Et), which are obtained from Cp₂MoCNMe (9c) and RI, rearrange in refluxing CH₂Cl₂ to the η₂-iminoacyl complexes Cp₂Mo[η₂-C(NMe)R]I (20: R = Me; 21: R = Et), whereas the tert-butyl isocyanide derivative [Cp₂Mo(Me)CN^tBu]I (22), obtained from 9b and MeI, is stable even in refluxing acetonitrile. In the solid-state 9b consists of a bent molybdenocene fragment with an 'end on' bound tert-butyl isocyanide ligand. The isocyanide ligand lies within the mirror plane of the molecule, which is perpendicular to the Cp-Mo-Cp direction. A short bond from molybdenum to C_α of the isocyanide ligand, a long Cα-N bond, and extensive bending of the isocyanide ligand at the nitrogen atom are observed. On the basis of these structural features and the 'carbene like' character of the 16e Cp₂Mo fragment, complexes 9a-9c can be described as organometallic analogues of ketene imines. High-yield synthetic routes to Mo^IV and Mo^II isocyanide complexes of the type [Cp₂Mo(X)CNR]Y and Cp₂MoCNR (X = H, Me, Et, Cl, I; Y = I, BF₄, PF₆; R = Me, Et, ^tBu) have been developed, allowing detailed studies of the reactivity of these rare compounds. Reactions of the electron-rich molybdenocene isocyanide complexes Cp₂MoCNR with organic and inorganic electrophiles have been shown to be frontier-orbital controlled, i.e. the entering electrophiles accepts electron density from the high-lying metal-localized 1a₁ HOMO orbital, converting the organometallic substrate to a Mo^IV isocyanide complex. The latter displays interesting reactivity patterns, as shown for example by the clean isocyanide insertion rearrangement of the alkyl complexes [Cp₂Mo(R)CNR′]Y (R, R′ = Me, Et; Y = I, BF₄) to give η²-iminoacyl compounds.