Synthetic, structural and reactivity studies on MoIV and MoII 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 MoII, 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 MoIV 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 MoIV 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)CNtBu]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 Cα 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 MoIV and MoII 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 MoIV 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.
Bibliographical noteFunding Information:
We thank Professor W.A. Herrmann for providing institute facilities, the Volkswagen and Leonhard-Lorenz Foundation for financial support, Dr. C. Whitacker for proof-readingt he manuscript,M . Barth and U. Graf for elemental