A SYNTHETIC AND X-RAY CRYSTALLOGRAPHIC STUDY OF COBALT AND MOLYBDENUMCLUSTER COMPLEXES OF THE 2-FENCHYL CATION - METAL-MEDIATED WAGNER-MEERWEIN REARRANGEMENTS

Treatment of 2-exo-propynylfenchol, 4a, with dicobalt octacarbonyl giv es the sterically crowded (2-exo-propynylfenchol)Co-2(CO)(6) cluster, 5a. Upon protonation, 5a yields the cobalt-stabilized 2-fenchyl cation 6, which is in equilibrium with its Wagner-Meervein rearranged 2-born yl isomer, 9, bearing the Co2C2 tetrahedral cluster moiety at the C(1) bridgehead position. Aqueous quenching of the mixture of cations 6 an d 9 yields. (2-endo-propynylfenchol)Co-2(CO)(6) 5b, together with two alkenes, 7a and 7b, derived by deprotonation of the 2-bornyl cation 9. Replacement of one Co(CO)(3) vertex in 7a by a (C5H5)Mo(CO)(2) moiety , and subsequent reprotonation, generates the Mo-Co cluster-substitute d 2-bornyl cation 19a, which undergoes Wagner-Meerwein rearrangement t o the more sterically demanding exo-fenchyl cation 17a. The isomerizat ion. of 19a to 17a is rationalized in terms of the enhanced ability of a (C5H5)Mo(C0)2 cluster vertex, relative to a Co(CO)(3) fragment, to alleviate the electronic deficiency at a cationic center. (2-endo-Prop ynylfenchol)Co-2(CO)(6), 5b, crystallizes in the orthorhombic space gr oup P2(1)2(1)2(1) with a = 7.408(2) Angstrom, b = 15.750(2) Angstrom, c = 17.598(2) Angstrom, and V = 2052.6(7) Angstrom(3) for Z = 4. [(2-e xo-propynylfenchyl)MoCo(CO)(5)(C5H5)] [BF4], 17a, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 12.569(2) Angstrom, b =13.491(6) Angstrom, and c = 141473(3) Angstrom, and V = 2454(1) Angst rom(3) for Z = 4.