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MALDI-TOF Mass Spectrometry of Neutral Complexes

The Matrix

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.

The Results

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.

...click to see spectrum full-size... Pentamethylcyclopentadienyl rhenium tricarbonyl

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.

Cp*Re(CO)2I2 is another stable neutral complex of rhenium.

...click to see spectrum full-size... Pentamethylcyclopentadienyl rhenium dicarbonyl diiodide

In this experiment, the parent ion was not observed in cation mode. However, a peak with a mass of 503, consistent with M minus the iodide anion, was observed. We have not yet assigned the M - 110 peak or the resonances at higher masses, which appear to be dirhenium complexes based on the isotope pattern.
With the detector in anion mode, a monoisotopic rhenium resonance at 503 was also observed, consistent with M minus I+. In addition, resonances corresponding to I- and I3- were observed. This data complements the cation spectrum.

CpRe(CO)2Br2 is another organometallic rhenium halide, this time with a Cp (cyclopentadienyl) ligand in place of the Cp*.

...click to see spectrum full-size... Cyclopentadienyl rhenium dicarbonyl dibromide

This complex showed similar behavior to the iodide; no parent ion was visible, but the M - bromide anion signals are clearly there.
Again, the spectrum using anion detection showed the presence of the halide anion, in this case bromide. Note that since both Re and Br have two major isotopes two mass units apart, the pattern has three major lines: 385 (185Re & 79Br), 387 (185Re & 81Br, 187Re & 79Br), and 389 (187Re & 81Br).

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Send comments and questions to John R. Nash.