Michael F. D'Agostino

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

Abstract The reaction of (C 5 H 5 ) 2 Mo 2 (CO) 4 with 2- endo -propynylborneol yields the tetrahedral cluster (R ★ Cue5fcCMe)Mo 2 (CO) 4 (C 5 H 5 ) 2 ( 7 ), where R ★ is the 2-norbornenyl group. This chiral molecule crystallizes in the orthorhombic space group P 2 1 2 1 2 1 with a 8.938(4), b 14.074(4), c 19.510(6) A, V 2268(1) A 3 , D c 1.65 g cm −3 , D m 1.64 g cm −3 , for Z = 4, and R 1 = 0.0290 ( R 2 = 0.0276) for 4350 unique reflections ( R 1 = 0.0236, R 2 = 0.0245 for 3846 reflections with I > 2.5σ( I )). Protonation of 7 and of the related dicobalt cluster (2- endo -propynylborneol)Co 2 (CO) 6 leads to metal stabilized cations which maintain their terpenoid skeletons and are stable towards carbocationic rearrangement. Protonation of the mixed metal clusters (C 5 H 4 Me)W(CO) 2 Co(CO) 3 (HCue5fcCCH 2 OH), (C 5 H 5 )Mo(CO) 2 Co(CO) 3 (HCue5fcCCH 2 OH), and (C 5 H 5 )Mo(CO) 2 Co(CO) 3 (HCue5fcCHEtOH) yields the corresponding cationic clusters [M-Co(CO) 3 (HCue5fcCCR 2 )] + . 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(HCue5fcCCH 2 )] + (M = Co(CO) 3 , (C 5 H 5 )Mo(CO) 2 or (C 5 H 5 )W(CO) 2 ) has a very high barrier in these mixed metal systems. Variable-temperature NMR studies on the mixed trimetallic cluster cation [(C 5 H 5 )Mo(CO) 2 Co 2 (CO) 6 CC(CH 3 ) 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 (C 5 H 5 )Mo(CO) 2 vertex but also that the transition state for antarafacial migration is strongly disfavored.

Mixed metal acylium clusters: A chemical and spectroscopic study

Abstract The generation of mixed metal acylium cations [MCo2(CO)6CCO]+, M = (C5H5Mo(CO)2, (C5Me5)Mo(CO)2 and (C5H5, from the parent esters via treatment with HPF6 in propionic anhydride is described. These cations and also the related [(C5H5)2Co3(CO)4CCO]+ cluster react with alcohols to give esters and with indole or pyrrole to give Friedel-Crafts type products. 1H NMR, IR and FAB mass spectroscopic data are reported.

CpMoCo2(CO)8CCO+: Synthesis and characterization of a mixed metal acylium cation

Abstract CpMoCo2(CO)8CCO2CHMe2 in propionic anhydride reacts with HPF6 to yield the mixed metal acylium cation CpMoCo2(CO)8CCO+ which reacts with a variety of nucleophiles, e.g., ROH, R2NH, yielding cluster-bound esters and amides, respectively. The cation also reacts with PhNMe2 in a Friedel-Crafts process. In the presence of water, Co3(CO)9CCO+ can also decompose to yield the neutral dimer [(Co3(CO)9C]2.

Mixed metal clusters containing carbyne or ketenylidene bridges: High field NMR spectroscopy as a probe for structure and mechanism

Abstract High field NMR spectroscopy is shown to be exceedingly valuable as an aid to the determination of the structures of mixed metal clusters and also as a probe for molecular rearrangements. In particular, the incorporation of an isopropyl group into chiral organometallic clusters of the tetrahedral (M 2 C 2 ) and square-pyramidal (M 3 C 2 ) types allows the detection of “hidden processes” such as intramolecular racemization or ligand migrations. It is possible to slow, on the NMR time scale, local rotation of individual vertices (e.g., CpMo(CO) 2 or CO(CO) 3 ) in M 3 CR clusters, when the capping carbynyl group is very bulky. The cation [Co 3 (CO) 9 Cue5fbCue5fbO] + can be shown to adopt a C s rather than a C 3v geometry. Finally, the mechanism of the decarbonylation of Arue5f8COue5f8CCo 3 (CO) 9 clusters to the corresponding Arue5f8CCo 3 (CO) 9 molecules proceeds via initial loss of a cobalt carbonyl ligand rather than by elimination of the ketonic carbonyl.