The Matrix
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6-aza-2-thiothymine, known by the acronym ATT, was used as a matrix for the experiments described in this poster. It absorbs the 337 nm light of the laser well, and it is known to be a good matrix for MALDI. The matrix was deposited by dissolving a small amount in methanol and transferring 0.5 mL of the solution to the target plate. After the methanol evaporated, leaving solid ATT, 0.5 mL of a dilute acetone solution of the analyte was added on top of the matrix. The rapid evaporation of acetone minimized the amount of time that ATT and the analyte could be in solution together. |
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The Results
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Cp*Re(CO)3 is a common starting material for organometallic chemistry that has found extensive use in the Casey group. (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl.) It is a pure, stable, and well-characterized compound, and it is therefore a suitable starting point to gauge the usefulness of MALDI-TOF for neutral organometallic chemicals. |
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| ...click to see spectrum full-size... |
Pentamethylcyclopentadienyl rhenium tricarbonyl |
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| In this experiment, it was possible to observe the parent peak (calcd 404.06, obsd 404.24) in cation detection mode. Since Cp*Re(CO)3 is a neutral compound, we are observing the ionized (loss of one electron) form of the complex in the mass spectrum. The isotope pattern (see sidebar) of the peak confirms the assignment of the sample structure to this signal. |
| Also observed was M+1 (assigned as Cp*Re(CO)3 plus a proton), M+115 ([Cp*Re(CO)3 - CO + ATT]+), and M+114 ([Cp*Re(CO)3 - CO + H]+). Loss of CO and replacement with another ligand (in this case, the matrix ATT) is understandable, since a photolysis reaction is commonly used to make Cp*Re(CO)2L complexes. The oxidation of the metal to ReIII required for the assignment of the M+114 peak is also reasonable in light of known reactions and the stability of rhenium in this oxidation state. |
