Magnetic resonance studies of organometallic cations and clusters

Abstract

<p>This thesis describes the application of magnetic resonance methods to some problems in organometallic chemistry. The reactions of nitroso compounds with the (cyclohexadienyl)Fe(CO)₃ cation have been investigated by using ESR spectroscopy and yield nitroxides of the type (OC)₃Fe(C₆H₇)(Ar)N-O•, and also the corresponding C₆H₅(Ar)N-O• radical. With bulky nitrosoarenes, such as C₆Me₅NO, isomers are observed in which the aryl ring rotation is slow on the ESR time-scale. The analogous reactions with the cyclohexadienyl cation derived from the B ring of (ergosteryl acetate)Fe(CO)₃ lead to attack at the central carbon, i.e, at C-7. Subsequent hydrogen migration leads to the (5,7-diene)Fe(CO)₃ complex bearing the aryl nitroxide at the 7-position. In contrast, the reaction of ((η⁵-cyclohexadienyl)Fe(CO)₃)⁺BF₄⁻ with 2-methyl-2-nitrosopropane yields the bis(t-butyl)nitroxide radical and the most interesting observation is that the intensity of the ESR signal varies in an oscillatory manner with time. This is the first oscillating reaction that can be followed by ESR spectroscopy The experimental results and a mechanistic rationale are described. In the second part of the thesis, a 1- and 2-dimensional ¹³C NMR study on the carbonyl migration processes in trimetal clusters is discussed. The variable-temperature ¹³C NMR spectra of the clusters (C₅HｎMe₅₋n)MCo₂(CO)₈CCO₂-i-Pr, where n = 5, 4, and 0 and M = Mo and W, reveal the existence of two interconverting rotamers whereby the CpM(CO)₂ vertex is oriented either proximal or distal to the capping carbonyl group. Simulation of the carbonyl exchange process reveals that the migration of CO ligands between Co and Mo has the same activation energy barrier as the interconversion of proximal and distal cyclopentadienyl rings; it is proposed that these fluxional processes are correlated. Use of a capping fragment derived from a natural product such as menthol or podocarpic acid renders the cluster chiral and allows the detection of slowed carbonyl exchange between the diastereotopic cobalt centers. The 125.7 MHz ¹³C NMR spectrum of CH₃CCo₃(CO)₈P(cyclo-C₆H₁₁)₃ at -90°C exhibits a 1:2:1:2:2 carbonyl pattern consistent with the triply-bridged structure found in the solid state, showing that carbonyl exchange can be slowed on the NMR time scale. The coalescence behavior of the ¹³CO resonances can be simulated by invoking a mechanism in which a completely bridge-opened structure is required; local rotation of each Co(CO)₃ vertex is shown to be rapid. 2D-EXCHANGE spectroscopy is used as an independent probe for the fluxional processes and shows that, in accord with the bridge-opened model, all carbonyl sites do not exchange at the same rate. It is also demonstrated that rotation about the cobalt-phosphorus and P-C(ipso) bonds is slowed on the NMR time-scale.</p>