An NMR spectroscopic and EHMO investigation of cationic mixed metal clusters: X-ray crystal structure of (1,7,7-trimethyl-μ2-2-propynylnorbornene)-bis(cyclopentadienyl)tetracarbonyldimolybdenum

The reaction of (C5H5)2Mo2(CO)4 with 2-endo-propynylborneol yields the tetrahedral cluster (R ★CCMe)Mo2(CO)4(C5H5)2 (7), where R ★ is the 2-norbornenyl group. This chiral molecule crystallizes in the orthorhombic space group P212121 with a 8.938(4), b 14.074(4), c 19.510(6) Å, V 2268(1) Å3, Dc 1.65 g cm−3, Dm 1.64 g cm−3, for Z = 4, and R1 = 0.0290 (R2 = 0.0276) for 4350 unique reflections (R1 = 0.0236, R2 = 0.0245 for 3846 reflections with I > 2.5σ(I)). Protonation of 7 and of the related dicobalt cluster (2-endo-propynylborneol)Co2(CO)6 leads to metal stabilized cations which maintain their terpenoid skeletons and are stable towards carbocationic rearrangement. Protonation of the mixed metal clusters (C5H4Me)W(CO)2Co(CO)3(HCCCH2OH), (C5H5)Mo(CO)2Co(CO)3(HCCCH2OH), and (C5H5)Mo(CO)2Co(CO)3(HCCHEtOH) yields the corresponding cationic clusters [M-Co(CO)3(HCCCR2)]+. High field NMR spectroscopy reveals that the capping vinylidene moiety leands preferentially towards one vertex and the facile antarafacial migration which is observed in the homometallic complexes [M-M(HCCCH2)]+ (M = Co(CO)3, (C5H5)Mo(CO)2 or (C5H5)W(CO)2) has a very high barrier in these mixed metal systems. Variable-temperature NMR studies on the mixed trimetallic cluster cation [(C5H5)Mo(CO)2Co2(CO)6CC(CH3)2]+ confirm that the carbocationic centre is better stabilized on the molybdenum vertex rather than by the tricarbonylcobalt fragment. These results are supplemented by EHMO calculations which demonstrate not only that the positive charge is better tolerated at the (C5H5)Mo(CO)2 vertex but also that the transition state for antarafacial migration is strongly disfavored.