Synthesis and reactivity of molybdenocene isocyanide complexes; crystal structure of (η 5-C 5H 5) 2MoCN tBu

Abstract

Synthetic, structural and reactivity studies on Mo IV and Mo II alkyl isocyanide complexes of the types [Cp 2Mo(X)CNR]Y and Cp 2MoCNR (X = H, Me, Et, Cl, I; R = Me, Et, tBu; Y = I, BF 4, PF 6) are reported. Reaction of Cp 2Mo(H)I ( 2) with EtNC gives the hydrido- isocyanide complex [Cp 2Mo(H)CNEt]I ( 3) in high yield. Complex 3 is converted by CHI 3 into the corresponding iodo derivative [Cp 2Mo(I)CNEt]I ( 4). An alternative route to halo-isocyanide complexes of molybdenocene involves halide abstraction from Cp 2MoX 2 ( 5: X = Cl; 6: X = I) by TIPF 6 in the presence of RNC (R = Et, tBu); this affords the complexes [Cp 2Mo(X)CNR]PF 6 ( 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 Cp 2MoCNR ( 9a: R = Et, 9b: R = tBu). An alternative high yield route to these compounds involves reaction of the acetonitrile complex Cp 2Mo(η 2-MeCN) ( 10) with RNC. Alkylation of 9a with MeI or Et 3OBF 4 occurs exclusively at the metal centre to yield the Mo IV alkyl complexes [Cp 2Mo(Me)CNEt]I ( 11) and [Cp 2 Mo(Et)CNEt]BF 4 ( 12), respectively. Similarly, complex 9a reacts with AuPPh 3Cl to give the heterobimetallic compound [Cp 2mo (AuPPh 3)CNEt]Cl ( 13). By contrast, a carbonyl/isocyanide exchange reaction occurs between 9a and Re(CO) 5Br, to give Cp 2MoCO ( 14) and cis-Re(CO) 4(CNEt)Br ( 15). The alkyl complexes 11 and 12, when heated in CH 2Cl 2 undergo a clean isocyanide insertion to give the Mo IV iminoacyl complexes [Cp 2Mo[η 2-C(NEt)Me]I ( 16) and [Cp 2Mo[η 2-C(NEt)Et]BF 4 ( 17), respectively. Similarly the alkyl complexes [Cp 2Mo(R)CNMe]I ( 18: R = Me; 19: R = Et), which are obtained from Cp 2MoCNMe ( 9c) and RI, rearrange in refluxing CH 2Cl 2 to the η 2-iminoacyl complexes Cp 2Mo[η 2-C(NMe)R]I ( 20: R = Me; 21: R = Et), whereas the tert-butyl isocyanide derivative [Cp 2Mo(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 Cp 2Mo 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 [Cp 2Mo(X)CNR]Y and Cp 2MoCNR (X = H, Me, Et, Cl, I; Y = I, BF 4, PF 6; 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 Cp 2MoCNR 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 1 a 1 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 [Cp 2Mo(R)CNR′]Y (R, R′ = Me, Et; Y = I, BF 4) to give η 2-iminoacyl compounds.