gas-phase fragmentation of coordination compounds: loss of co2 from inorganic carbonato complexes to...

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 34, 1033–1039 (1999)

Gas-phase Fragmentation of CoordinationCompounds: Loss of CO2 from InorganicCarbonato Complexes to Give Metal Oxide Ions

Pia Dalgaard and Christine J. McKenzie*Department of Chemistry, University of Southern Denmark: Main Campus, Odense University, DK-5230 Odense M,Denmark

Using electrospray ionization mass spectrometry, novel transition metal oxide coordination complex ions areproposed as the products of the collision-induced dissociation (CID) of some carbonato complex ions throughthe loss of a mass equivalent to CO2. CID spectra of [(tpa)CoCO3]Y (tpa = tris(2-pyridylmethyl)methylamine),[(bispicMe2en)Fe(m-O)(m-CO3)Fe(bispicMe2en)]2Y (bispicMe2en = N ,N ′-dimethyl-N,N ′-bis(2-pyridylmethy)eth-ane-1,2-diamine) and [(bpbp)Cu2CO3]Y (bpbp− = bis[(bis-(2-pyridylmethyl)amino)methyl]-4-tertbutylpheno-lato(1−)), show peaks assigned to the mono- and dinuclear oxide cations, [(tpa)CoO]Y, [(bispicMe2en)2Fe2(O)2]2Y

and [(bpbp)Cu2O]Y, as the dominant species. These results can be likened to the reverse of typical syntheticreactions in which metal hydroxide compounds react with CO2 to give metal carbonato compounds. Because ofthe lack of available protons in the gas phase, novel oxide species rather than the more common hydroxide ionsare generated. These oxide ions are relevant to the highly oxidizing species proposed in oxygenation reactionscatalysed by metal oxides and metalloenzymes. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: carbonate; metal oxides; transition metal ions; electrospray ionization mass spectrometry; collision-induceddissociation.

INTRODUCTION

The ‘softness’ of electrospray ionization mass spectrom-etry (ESI-MS) makes possible the observation of coor-dination complex ions that typically decompose usingother methods of ionization.1–3 The last few years havewitnessed increasing application of this technique forthe characterization of coordination complexes. However,there is still relatively little known about the typicalfragmentation patterns of coordination and inorganic com-pounds. In our routine use of ESI-MS for partial charac-terization of coordination complexes synthesized in ourlaboratory, we observed that a significant peak assignedto a novel metal oxide complex cation appeared in theESI mass spectrum of a simple metal carbonato complex.This amounted to a mass loss equivalent to CO2. This dis-covery initiated the ESI-MS investigation described hereto ascertain whether or not this may be a general frag-mentation pathway for related carbonate compounds. TheESI-MS characterization of carbonato complexes has beenreported earlier but, to our knowledge, not the observationof oxide ion fragments and/or gas-phase CO2 loss.1,4

Using collision-induced dissociation (CID) in tan-dem MS/MS experiments, we studied the gas-phase

* Correspondence to: C. J. McKenzie, Department of Chemistry,University of Southern Denmark: Main Campus, Odense University,DK-5230 Odense M, Denmark.E-mail: [email protected]

Contract/grant sponsor: Danish Natural Science Research Council;Contract/grant number: 28808.

decomposition of three prototype inorganic carbonatocompounds, one mononuclear and two dinuclear. Lossof a mass equivalent to CO2 is indeed observed in theESI and tandem mass spectra of these compounds andprominent fragment ions are assigned to novel metal oxidecations. The relevance of these product ions to solution-and solid-state chemistry is discussed. Loss of neutral CO2fragments is well documented for carboxylic acids, anhy-drides, organic carbonates and esters,5 but to our knowl-edge this is the first report of similar losses from inorganiccarbonate ions.

The characterization of highly oxidizing metal oxospecies by mass spectrometry is difficult because of theease of reduction/decomposition of these species usingmost ionization techniques. However, ESI-MS has beenproved to be applicable for the detection of metal oxoanions in solutions containing permanganate, chromateand ruthenate.6 Metal oxides have also been produced byion–molecule reactions (IMRs) in the gas phase: singlycharged metal oxide ions have been generated by O atomtransfer from O2, N2O and CO2 to bare metal–metal clus-ters ions produced by laser ablation of metal surfaces orby CID of singly charged carbonyl complexes.7 Coordina-tively unsaturated metal ions, e.g. [M(bipy)2]2C (bipyD2,20-bipyridine), generated in the gas phase by CID werefound to undergo IMRs with molecular oxygen to produceions assigned to metal oxides, e.g. [M(bipy)2O2]2C (M DCr, Ru, Os).8 These latter species, and those produced bythe CID experiments described here, bear more relevanceto the solution and solid-state chemistry of high-valentmetal oxide ions compared with the (apparent low-valent)metal oxide ions obtained via laser ablation techniques.

CCC 1076–5174/99/101033–07 $17.50 Received 28 March 1999Copyright 1999 John Wiley & Sons, Ltd. Accepted 7 July 1999

1034 P. DALGAARD AND C. J. McKENZIE

EXPERIMENTAL

[Co(tpa)CO3]PF6,9 [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bi-spicMe2en)](CIO4)2,10 and bpbpH11 were prepared asdescribed elsewhere (abbreviations: tpaD tris(2-pyridyl-methyl)methylamine); bispicMe2enD N,N0-dimethyl-N,N0-bis(2-pyridylmethy)ethane-1,2-diamine; bpbp� D bis[(bis-(2-pyridylmethyl)amino)methyl]-4-tertbutylpheno-lato(1�); bpmp� D bis[(bis-(2-pyridylmethyl)amino)me-thyl]-4-methylphenolato(1�)). Solutions containing[(bpbp)Cu2CO3]C were obtained by bubbling CO2through a mixture of bpbpH and an excess of sus-pended Cu2O in acetonitrile for 20 min. Excess Cu2O wasremoved by filtration from the resultant green solution.

Mass spectral data were recorded on a Finnigan TSQ700 MAT triple-quadrupole instrument equipped with ananoelectrospray source.12 Spectra of [Co(tpa)CO3]PF6were obtained by spraying an acetonitrile solution froma coated needle held at 0.8 kV and introduced to the massspectrometer through the capillary tube heated to 150°C.Spectra of [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)](ClO4)2 in methanol were recorded with a capillarytube temperature of 140°C and a needle potential of0.8 kV. Spectra of [(bpbp)Cu2CO3]C in acetonitrile wererecorded with a capillary tube temperature of 100°Cand a needle potential of 1.4 kV. Sample concentrationswere typically 0.3 mM. CID experiments were performedtypically employing a 1 mTorr (1 TorrD 133.3 Pa) argonpressure in the collision cell. The region fromm/z 100 to

1000 was scanned for 2 s and the spectra were obtainedby averaging 60 scans.

RESULTS AND DISCUSSION

Inorganic carbonate (CO32�) is known to function as aligand for most metal ions using a variety of structuralbonding modes, since the carbonate anion can bond toone or more metal atoms using one, two or three of itsoxygen atoms. In combination with other ligands, thisgives rise to a large number of structural possibilities,three of which we studied by ESI-MS in the presentwork. These prototype complexes of aminopyridyl ligandscontaining ancillary bidentate and�2-bridging carbonatoligands are shown in Scheme 1. The complexes were cho-sen because of their availability in our laboratory andbecause, apart from the carbonate group (and, in the caseof the diiron complex, also the oxide group), only oneother ligand is bound to the metal ions. Hence potentialcomplications through ligand rearrangements, stepwiseligand loss, etc., are minimized. The dinuclear coordi-nation complexes are approximately symmetric, whichmakes the formation of symmetric fragment ions fromthe CID experiments probable, a fact that serves as anargument when suggesting structures for the oxide speciesproduced. Clearly it is not possible from the mass spec-trometric results to assign unambiguously the structuresfor the three oxide adducts depicted in the summary of

Scheme 1

Copyright 1999 John Wiley & Sons, Ltd. J. Mass Spectrom. 34, 1033–1039 (1999)

GAS-PHASE FRAGMENTATION OF COORDINATION COMPOUNDS 1035

Figure 1. Summary of the CID reactions observed. The ligandstpa and bispicMe2en are represented by N4 [Eqns (a) and (b)];bpbp� is represented by the N3,N3,O abbreviation in thedicopper structure [Eqn (c)]. The m/z values are given underthe relevant structures.

the gas-phasereactionsobservedfor thesecoordinationcomplexcationspresentedin Fig. 1.

The ESI mass spectrum of the simplest transitionmetal carbonatein Scheme1, [Co(tpa)CO3]PF6, showstheexpectedsignalfor thesingly chargedcomplexcationat m/z 408.9 as the major peak [Fig. 2(a)]. Althoughno x-ray structureof this compoundhas beenobtained,an �1 : �1 coordinationmode for carbonatein mononu-clear cobalt compoundsis well known and compari-son of spectroscopicdatawith structurallycharacterizedanalogues13 makes the assignmentof the coordinationmodeof the carbonateligand unambiguous.CID of thispeakinducesthe loss of 44 u (massequivalentto CO2),leadingto themajor fragmention atm/z 364.7[Fig. 2(b)]with the formulation[Co(tpa)O]C. Thepeaksatm/z 256.0and 271.9 in the CID spectrumare assignedto prod-ucts obtainedthrough loss of a pendantarm of the tpaligand andassociatedimine formation.[Co.C12H11N3/]Cand[CoO.C12H11N3/]C, respectively).

Theproposedcobaltoxo adduct,[Co(tpa)O]C, detectedin the CID experiment is extremely interesting if thecarbonato-derivedoxygen atom remains bound to thecobaltatom.However,the formulationof [Co(tpa)O]C asformally a cobalt(III)oxyl complex,or a Co(II) complex

Figure 2. (a) ESI mass spectrum of [Co(tpa)CO3]PF6; m/z 408.9 D [Co(tpa)CO3]C. (b) CID spectrum of the 408.9 m/z ion. Collisionenergy: 22 eV.

Copyright 1999JohnWiley & Sons,Ltd. J. MassSpectrom. 34, 1033–1039(1999)


with a bound oxygen atom radical, is clearly not possibleon the basis of our results. Such species are expected tobe very reactive. There is compelling evidence for thetransient generation of transition metal oxo species as cru-cial intermediates in oxygenation catalysts and enzymes.14

However, species containing this structural motif forcobalt have yet to be isolated. The alternative explana-tion of pyridine-N-oxide formation seems far less likelyowing to the amount of intramolecular rearrangement thatwould be required.

We have been interested in the characterization of diironand dicopper complexes containing one or more oxogroups linking the two metal atoms because of their rel-evance as structural mimics for the dimetallic enzymesinvolved in several biological oxidation reactions. Encour-aged by the results above with a simple mononuclearcobalt complex, we investigated possible CO2 loss fromthe carbonate-bridged diiron and dicopper complexes inScheme 1 to give novel and biologically relevant oxo-bridged complexes under CID conditions. ESI-MS hasbeen used previously in the characterization of oxo-bridged complexes, such as the diiron complex, for whichother mass spectrometric techniques proved too harsh forthe detection of molecular ions.1

The structure of the�-oxo-�-carbonato-bridged com-plex [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)](ClO4)2 has been solved by x-ray crystallogra-phy, thus establishing the carbonato binding mode.10

In Fig. 3(a) the major peak atm/z 364.1 inthe ESI mass spectrum of [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)](ClO4)2 is assigned to the doublycharged complex cation of this compound. Collisionsin the skimmer region resulted also in a minor peak44 u less. This ion corresponds to the formulation[(bispicMe2en)2Fe2(O)2]2C and its origin was verifiedby CID of the m/z 364.1 ion to produce them/z342.1 fragment ion as the major product. The iso-topic pattern of them/z 342.1 ion obtained in theESI mass spectrum (not the CID spectrum) in Fig. 4confirms that it is a doubly charged cation, support-ing the formulation of the di-�-oxo-diiron complexcation, [Fe2(bispicMe2en)2(O)2]2C, rather than the singlycharged alternative, [Fe(bispicMe2en)O]C. Not even asmall percentage of the signal intensity is due to[(bispicMe2en)FeO]C since this would result in a devi-ation from the observed isotopic distribution. Signifi-cantly, assignment of a di-�-oxo diiron complex cationrather than a simpler mononuclear iron oxide (analogous

Figure 3. (a) ESI mass spectrum of [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)](ClO4)2; m/z 364.1 D [(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)]2C. (b) CID spectrum of the m/z 364.1 ion. Collision energy: 45 eV.



Figure 4. Isotopic pattern of the ion at m/z 342.1 assignedto [.bispicMe2en/2Fe2.O/2]2C from the ESI mass spectrum.Theoretical isotopic patterns for (a) [.bispicMe2en/2Fe2.O/2]2C

and (b) [.bispicMe2en/FeO]C.

to the proposedcobalt oxo adduct above) corroborateswell with known solution- and solid-state iron chem-istry. Assignmentof the oxide groupsas bridging can-not be madeon the basisof massspectrometricresults.However, thisstructuralarrangementgivesthemoststablechargedistributionfor iron andsupportsthe fact thatonlydimericproductsareobserved.Althoughsuchspeciesareextremely rare, one exampleof a diiron complex withthe unique di-�-oxo-bridgedcore structureproposedinFig. 1(b) hasbeencharacterizedby x-ray crystallography(with a differentterminalcappingligand).15 Interestingly,thestartingcarbonatecomplex[(bispicMe2en)Fe(�-O)(�-CO3)Fe(bispicMe2en)](ClO4)2 is preparedfrom the reac-tion of [(bispicMe2en)Fe(�-O)(�-OH)Fe(bispicMe2en)](ClO4)2 with CO2.10 Thus loss of CO2 in the gasphasecanbelikenedto a reversalof thesyntheticreaction;how-ever,a lack of availableprotonsresultsin the formationof the proposeddi-�-oxo-bridgedproductratherthanthestarting�-oxo-�-hydroxide-bridgedcomplex.

We havenot isolatedthe dicoppercationin Scheme1,[(bpbp)Cu2CO3]C, as a pure solid compound;however,ESI-MS, UV –visible andESRspectroscopy16 wereusedto verify its presencein solution and in isolatedamor-phoussolids. However, the complex is chemically rea-sonableand a carbonato-bridgeddicoppercomplexof arelatedphenoxide-hingedligand has been isolated.17 Inaddition, the crystal structureof a structural analogue,[(bpmp)Cu2.CH3CO2/].ClO4/2,18 in which a syn O,O0-acetatogroupbridgesthe copperions, showsthat a geo-metrically similar syn O,O0-carbonatobridge is feasible.The ESI massspectrumof a reactionsolution contain-ing this speciesin Fig. 5(a) showstwo prominentpeaksdue to the desired[(bpbp)Cu2CO3]C (m/z 759.4)andthehydroxidocomplex[(bpbp)Cu2OH]2C (m/z 357.2).This is

consistentwith theexpectedsolutionequilibriumbetweenthe dicopperhydroxide ion and dicoppercarbonateionaccordingto the reaction

[(bpbp)Cu2OH]2C C CO2 ��⇀↽�� [(bpbp)Cu2CO3]C C HC

We have in fact isolated [(bpbp)Cu2OH]2C as a solidcompoundandobtainedthe crystalstructureof its homo-logue [(bpbp)Cu2OCH3]2C;16 therefore,the existenceofthe [(bpbp)Cu2OH]2C ion canbeverified in separateESI-MS experiments.In addition,a minor peakassignableto[(bpbp)Cu2O]C (m/z 713.2)is detectedand,in contrasttothecarbonatoandthehydroxidocomplexions,we believethis ion to bepresentonly in thegasphase.CID of them/z759.4ion showsthat them/z 713.2ion is a fragmention[Fig. 5(b)], correspondingto thelossof a massequivalentto CO2 from [(bpbp)Cu2CO3]C. We proposethe �-oxo-�-phenolatodicopperspeciesin Fig. 1(c) as a plausiblestructuralformulationfor this ion. However,coordinationcompoundswith copperoxidemoietiesareextremelyrare;copperhydroxide/aquamoietiesdominatein solutionandthe solid state.

CONCLUSIONS

Metal carbonatocomplexesare preparedoften by thereactionof metalhydroxideswith CO2.19 Using ESI-MSwe observedthat a massequivalentto CO2 can also bereleasedfrom inorganiccarbonatocompoundsunderCIDconditions.This is not a reversalof the synthetic reac-tion since a lack of availableprotons in the gas phasemakesthe regenerationof the starting hydroxide com-plexesimpossible;instead,peakswhich can be assignedto unusualmetaloxide speciesareproduced.Metal oxidespeciessuchas thesemay containstructuralmotifs pro-posedto be important in metal-catalysedoxidations oforganicmolecules.Also, althoughthey aremotifs soughtby syntheticchemists,precedencefrom solid-statechem-istry existsonly for the di-�-oxo diiron compound.Theproposeddiiron and dicoppermetal oxide adductshaveparticular relevanceto the field of active site structuralmimics for severalmetalloenzymesinvolved in oxidationandoxygeninsertionreactions.Forexample,similarstruc-tural motifs areproposedasimportantstructuralelementsin reactionscatalysedby thedinuclearnon-haemiron- andcopper-containingenzymesmethanemonooxygenaseandtyrosinase.20 Indeed,ananalogymight be drawnbetweentheenvironmentof complexedmetalionsin thegasphaseand the sites provided for metal ions by proteins,since(debilitating)interactionswith solventsareprevented,e.g.solvent oxidation and concomitantreduction/destructionof the metaloxide complexes.

Some initial experimentswith dinuclear carbonato-bridged complexesof cryptand ligands containing sec-ondary amine groupssuggest,in contrastto the resultsdescribedhere,that CO2 loss is not a relevantdecompo-sition pathway.In thesecasesa loss of massequivalentto carbonicacid is observed;the protonsarepresumablyderivedfrom oxidationof the cryptandligand, i.e. forma-tion of imine groupsfrom thesecondaryamines.21 Henceit cannotbe concludedthat all carbonatocomplexesloseCO2 to give oxidesunderCID conditions;theavailability



Figure 5. (a) ESI mass spectrum of [(bpbp)Cu2CO3]C; m/z 759.5 D [(bpbp)Cu2CO3]C; m/z 357.2 D [(bpbp)Cu2OH]2C. Expansions andcalculated isotopic patterns for the major ions are shown under the spectrum. (b) CID spectrum of the m/z 759.4 ion. Collisionenergy: 45 eV.

of hydrogenatomsfrom ancillary ligandsis apparentlyadeterminingfactor in the lossof eitherH2CO3 or CO2.

The results demonstratethat caution needs to beexercisedin assigningmetal-containingions to ‘solution’species in the ESI mass spectra of coordinationcompounds. Some (novel) speciesmay not be presentin solution but are generatedrelatively easily onlyafter entering the gas phase,even without using CIDconditions. The discovery of ‘standard’ fragmentationpathwaysfor coordinationandinorganiccompoundswillbe useful in the future characterizationof unknownmetal-containing ions, a practice well establishedin

mass spectrometricstudies of organic molecules.Theobservationswe describehereaddto the rapidly growingbody of knowledgeof massspectrometryand inorganiccompounds.

Acknowledgments

ProfessorPeterRoepstorff andDr RomanZubarevof the Universityof SouthernDenmarkaregratefully acknowledgedfor helpful adviceand discussions.This work was supportedby grantNo. 28808fromthe DanishNaturalScienceResearchCouncil.



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gas-phase fragmentation of coordination compounds: loss of co2 from inorganic carbonato complexes to...

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