THE ANALYSIS OF ORGANOMETALLICCOMPOUNDS USING ELECTROSPRAY

IONIZATION MASS SPECTROMETRY

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Steve Preece1, Mike Ward2 and Mark Whiteley31Waters Corporation, 3 Tudor Road Altrincham, Cheshire, WA14 5RZ, UK.

2University of Bristol, School of Chemistry, Cantocks Close, Bristol, BS8 1TS, UK.3University of Manchester, Dept. of Chemistry, Brunswick Street, Manchester, M13 9PL, UK.

Introduction

The analysis of organometallic compounds by massspectrometry has traditionally been carried out usingFast Atom Bombardment (FAB) as the mode ofionization. The advent of electrospray ionizationoffers an alternative method of analysis with anumber of inherent advantages. This ApplicationNote demonstrates the analysis of syntheticcompounds using electrospray ionization on aMicromass® Platform LC (Waters Corporation,Altrincham, Cheshire, UK) benchtop massspectrometer.

Unlike FAB ionization, electrospray requires no matrixto aid the ionization process which means that thebackground ions relating to chemical noise are muchlower in intensity. All that is required for electrosprayanalysis is a suitable solvent for the compound, andthen a solution at the required concentration can beintroduced directly into the ionization source.As electrospray is an AtmosphericPressure Ionization (API)technique, it has a further

advantage over FAB in that there is no need to insertthe sample via a vacuum lock. Electrospray is wellknown for use with solvents used in reverse phasechromatography, such as water, methanol andacetonitrile, but it is also possible to use a widerrange of solvents, such as dichloromethane andtoluene, which may be more suitable fororganometallic compounds which are either insolublein other solvents, or require careful choice of solventto prevent ligand exchange reactions.

Once in solution, the sample can be introduced eitherby loop injection into a carrier solvent flowing to theelectrospray source or by direct introduction via aninfusion pump. Conventional electrospray can beoperated at flows as low as 5 µL/min, such that a100 µL aliquot of a sample solution can be infusedfor 20 minutes allowing easy optimization and dataacquisition. Several spectra, using different

conditions, can be acquired in ashort period of time

using less than amicrogram of

sample.

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As electrospray is a soft ionization technique whichtends to keep analytes intact and provide molecularweight information, it can be used to confirm themolecular weight of synthetic organometalliccompounds.

However, electrospray has an added advantage ofallowing fragmentation to be produced whenrequired. By increasing the source cone voltage itmay be possible to produce controlled andreproducible fragmentation, giving useful informationfor the elucidation or confirmation of the structure ofthe sample.

Experimental

For each sample, a suitable solvent was chosen anda solution was prepared in the concentration range25-50 ng/µL. The solution was then introduced intothe electrospray source at a flow rate of 5 µL/minusing a Harvard Apparatus™ 22 syringe pump. Theelectrospray source conditions were optimized for thegeneration of the molecular related ion and datawere acquired by accumulating several continuumspectra over a suitable mass range. Subsequentspectra were acquired at higher cone voltages togenerate in-source fragmentation.

Results

Sample 1Formula: C43H30N8RuReClO3.2PF6Average molecular weight = 1319.4Solvent: acetonitrileConcentration: 25 ng/µL(note : bipy = 2,2’-bipyridine, C10H8N2)

Molecular Weight Information: Sample 1 containsruthenium, rhenium and chlorine atoms so theexpected isotope pattern is complex. When usingelectrospray ionization in positive ion mode, with acone voltage of 30V, the compound loses a PF6-counter ion to give a singly charged cation with anaverage mass to charge ratio (m/z) of 1174.5.

Continuum spectra for the singly charged molecularions and the isotope model (Figure 1) show that theisotope profile for the acquired data closely matchesthat predicted by the isotope model.

1160 1162 1164 1166 1168 1170 1172 1174 1176 1178 1180 1182 1184 1186 1188m/z0

100

%

0

100

%

1175.0

1173.0

1172.0

1171.0

1170.01169.0

1177.0

1176.0

1178.0

1179.0

1180.0

1175.1

1173.0

1172.1

1171.01170.01169.0

1167.1

1177.0

1176.0

1178.0

1179.0

1180.0

Acquireddata

Calculatedprofile

Figure 1. The cost of drug development

N

N

N

N

Ru

Re(CO)3Cl

(bipy)2

Isotope Models: As organometallic compounds often containmetals with complex isotope patterns, the expected isotopepattern of the compound can be used to help confirm itsstructure. The MassLynx™ software, which controls the PlatformLC™ mass spectrometer, has an isotope pattern calculator whichcan model the expected isotope distribution. By entering theexpected chemical formula of the compound, a model of theisotopes can be produced in either continuum or centroid formatwhich can then be compared to the data acquired byelectrospray ionization to confirm the presence of the expectedatoms.

Resolution of Doubly Charged Ions: The sample also loses thesecond PF6- counter ion to give intense doubly charged ionswith an average mass of 1029.5 and thus an average mass tocharge (m/z) ratio of 515.7. Due to having two charges, theresulting spectrum shows isotopes which are separated by onlym/z 0.5. The Platform-LC is easily able to resolve doublycharged isotopes as shown in Figure 2.

508 509 510 511 512 513 514 515 516 517 518 519 520 521m/z0

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%

515.2

514.1

513.6

513.1

512.7

512.2

511.2

514.7516.2

515.7

516.6

517.1

517.6

Figure 2. Doubly charged ion produced by the loss of twocounter ions from Sample 1

140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560m/z0

100

%

284.1

283.5

283.1

282.6

157.1281.1

206.0

362.1

285.1

304.6

304.1

303.6

301.6

361.6

361.1

360.6305.6

359.1306.1

363.1

363.6

Figure 3. Induced fragmentation of Sample 1

Gaining Structural Information: As statedpreviously, it is possible to induce somefragmentation in the source in order togain some structural information about thesample. By increasing the sample conevoltage in the source to a value of 55V, thedoubly charged ion was completelydissociated to give a number of fragmentions as shown in Figure 3. The peakscentered around m/z 362 are due to adoubly charged fragment (charge statedetermined by the isotope separation)produced by the loss of the two PF6-counter ions and rhenium with itsassociated ligands, [M - 2(PF6

-) -Re(CO)3CI]2+. A further loss of a 2,2’-bipyridine ligand from the ruthenium centregives the intense group of doubly chargedions centered around m/z 284 and loss ofthe second (bipy) ligand gives a group ofions around m/z 206. The ions aroundm/z 304.6 are again due to a doublycharged species which has a mass of 609.The odd mass indicates that the fragmentcontains an odd number of nitrogen atomsand is likely to be due to the cleavage ofthe compound backbone to lose onenitrogen. There is also a singly chargedpeak at m/z 157 which can be assignedas protonated 2,2’-bipyridine.

Molecular Weight Information: Sample 2gives a molecular ion at m/z 738 (Figure4) which matches the expected isotopepattern generated by the software. Thebase peak of the spectrum at m/z 659 isdue to the facile loss of bromine from thecomplex which occurs even at a low conevoltage of 18V. Indeed, this fragmentationoccurs so easily that the intact molecularion for this complex has not previouslybeen observed using FAB.

500 520 540 560 580 600 620 640 660 680 700 720 740 760 780m/z0

100

%

659

657

656

653

661

738

737

662 735

732

740

741

Figure 4. Spectrum for Sample 2 at a cone voltage of 18V

Mo

P

R

P

BrCO

CO

R = 1-3-η : 5-6-C8H11P --- P = dppe = Ph2PCH2CH2PPh2

Sample 2Formula: C36H35O2BrMoP2Average molecular weight = 737.5Solvent: dichloromethaneConcentration: approx 50 ng/µL

Summary

The data presented show that electrospray is a suitable technique for the analysis of organometallic compounds. Like FAB, electrospraycan be used to confirm the molecular weight of a compound, but it has the added advantage of controlled and reproduciblefragmentation which can confirm an expected structure or can help with the elucidation of an unknown. The data were acquired on abenchtop mass spectrometer equipped with an electrospray ionization source which offers an advantage in cost while maintaining therequired performance. The high performance allows the facility to resolve multiply charged ions to help with interpretation of the data.

Acknowledgments

Thankyou to Dr. M. Ward of the School of Chemistry, University of Bristol and to Dr. M. Whiteley and Mr. D.M. Spencer of theDepartment of Chemistry, University of Manchester for providing samples.

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