Reactions of Doubly SiMe2-Bridged Bis(cyclopentadienyl) Complexes of Molybdenum and Iron Carbonyls: Competitive Ring-to-Metal Migrations of Hydrogen and SiMe2

Reaction of the doubly bridged bis­(cyclopentadiene) (C5H4(SiMe2))2 (1) with Mo­(CO)6 in refluxing xylene gave the corresponding dinuclear molybdenum carbonyl complex [(η5-C5H3)2(SiMe2)2]­Mo2(CO)6 (3) and the desilylated product [(η5-C5H4)2(SiMe2)]­Mo2(CO)6 (6), together with the singly SiMe2SiMe2-bridged dinuclear molybdenum product [(η5-C5H4)2(SiMe2SiMe2)]­Mo2(CO)6 (5) and the novel complex [(SiMe2)­(η5-C5H4)­Mo­(CO)3]2 (4) containing two Mo–Si bonds. This diversity of products is caused by similar ring-to-metal migrating abilities of the hydrogen and SiMe2 groups and their competitive migration during the reactions. Reaction of the dihydride cis-[(η5-C5H3)2(SiMe2)2]­Mo2(CO)6(H)2 (7c) in refluxing xylene also afforded the above products except 4, which suggests that 7c might be intermediate precursor to 3, 5, and 6. Similar treatment of substituted doubly bridged bis­(cyclopentadiene) (C5H3tBu­(SiMe2))2 (2) with Mo­(CO)6 provided the corresponding derivatives (8–10) of the above products and the product [(η5-C5H4tBu)­Mo­(CO)3]2 (11), in which both SiMe2 groups are removed. Heating a mixture of 1 and Fe­(CO)5 in xylene yielded the normal dinuclear iron complex [(η5-C5H3)2(SiMe2)2]­Fe2(CO)2(μ-CO)2 (12) and the desilylated product [(η5-C5H4)2(SiMe2)]­Fe2(CO)2(μ-CO)2 (14), together with the novel complex (SiMe2)­(η5-C5H3)­(η5:η1-C5H3)­[(SiMe2)­Fe­(CO)2]­[Fe­(CO)2)] (13) containing one Fe–Si bond. However, similar treatment of 2 with Fe­(CO)5 provided only the corresponding dinuclear complex [(η5-C5H2tBu)2(SiMe2)2]­Fe2(CO)4 (15), in which the Fe–Fe bond (2.8205(9) Å) is the longest among all dimeric Cp′2Fe2(CO)4 analogues. A plausible mechanism for the formation of the different types of products is proposed. Molecular structures of 4, 9c, 13, and 15 determined by X-ray diffraction are also presented.