METAL CLUSTER STABILIZED 2-BORNYL CATIONS - A SYNTHETIC, X-RAY CRYSTALLOGRAPHIC, AND EHMO STUDY

Metal cluster derivatives of the 2-bornyl cation, viz. (2-propynylbornyl)(M1-M2), where M1 and M2 can be (C5H5)MO(CO)2 or CO(CO)3, have been synthesized by protonation of suitable precursors. Unlike the uncomplexed 2-alkynylbornyl cations, these metal-stabilized systems do not undergo the Wagner-Meerwein rearrangement to the corresponding 4-alkynylcamphenyl cations. [(2-propynylbornyl)Mo2(CO)4(C5H5)2][BF4], 22, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 8.557(2) angstrom, b = 12.074(3) angstrom, c = 25.888(4) angstrom, and V = 2675(1) angstrom3 for Z = 4. [(2-propynylbornyl)Mo(CO)2(C5H5)-Co(CO)3][BF4], 23, crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 12.647(2) angstrom, b = 13.874(2) angstrom, c = 14.040(2) angstrom, and V = 2463.6(6) angstrom3 for Z = 4. In both cases, the 2-bornyl cation leans toward a molybdenum atom such that the Mo-C+ distance is 2.74 angstrom for 22 and 2.91 angstrom for 23. The preferential stabilization of the cation by the molybdenum vertex rather than by the tricarbonylcobalt fragment is rationalized by means of molecular orbital calculations at the extended Huckel level.