research article electronic structure of ferrocene...

Research ArticleElectronic Structure of Ferrocene-Substituted CavitandsA QTAIM and NBO Study

Tiacutemea R Keacutegl Laacuteszloacute Kollaacuter and Tamaacutes Keacutegl

Department of Inorganic Chemistry and Janos Szentagothai Research Center University of PecsMTA-TKI Research Group for Selective Chemical Syntheses Ifjusag Utja 6 Pecs 7624 Hungary

Correspondence should be addressed to Tamas Kegl tkeglgammattkptehu

Received 1 November 2013 Accepted 24 December 2013 Published 10 February 2014

Academic Editor Anton Kokalj

Copyright copy 2014 Tımea R Kegl et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Ferrocene-substituted tetrakis(methyl)resorcin[4]arenes have been investigated by means of DFT calculations employing thegradient-corrected PBEPBE functional In comparison with ferrocene and simple ansa-ferrocenes containing 2ndash4 bridgingmethylene groups it was found that the tilt angle of the functionalized cyclopentadienyl (Cp) rings strongly influences the electrondensity distribution of the ferrocenyl moieties According to NBO analyses the iron atoms in the cavitands are more positive incomparison to those in ferrocene whereas they are less positive in ansa-ferrocenes The partial charges of carbon atoms belongingto Cp rings show some correlation with the tilt angle

1 Introduction

Metallocenes are among the first known organometallic com-pounds Since the discovery of ferrocene [1] and the elucida-tion of its bispentahapto sandwich structure biscyclopenta-dienyl-transition metal complexes have been in the focusof many important developments in modern organometallicchemistry

In biscyclopentadienyl-transition metal complexes thecyclopentadienyl (Cp) rings are relatively inert moieties inmost reactions The Cp rings can function as scaffolding ofthe upper and lower frames and define the reaction space ofthe attached metal Moreover the Cp ring can be varied inan almost unlimited number of ways in order to modify thesteric properties of the complex and the electronic propertiesof the metal A commonly used alteration to the metalloceneligand framework is the inclusion of a linking group betweenthe two Cp rings called an interannular bridge Complexes ofthis class were originally called metallocenophanes howeverthe term ldquoansa-metallocenerdquo is now more commonly used[2]The Latin prefix ansa (meaning ldquohandlerdquo) was introducedby Smith et al [3] referring to the bent handle functionalgroup connecting to the two Cp rings at both ends Ansa-metallocenes became very popular in the polymer industry

as for example an excellent catalyst in the synthesis of highlyisotactic polypropylene [4]

The amount of distortion from the normal metallocenegeometry that is caused by the bridging group X is depictedin Scheme 1 Several geometrical parameters can be defineddescribing the geometries of ansa-metallocenes [2] fromwhich we will focus on two fundamental values within thisstudyThedegree of ring tilt is represented by angle120572 whereasangle120573 denotes the deviation of the X-Cipso axis from the ringplane

The reactivity of metallocenes can substantially be influ-enced by the bridge Lentzner and Watts studied the effectof the bridge on the reactivity of ferrocenophanes [5] Thetetramethylene bridge enforces some bending of the Cprings and enhances the Lewis basicity of the iron centerin comparison to that in Cp

2Fe but makes the aromatic

rings resistant to Friedel-Crafts acylation and to lithiation byWilkinson et al [6]

The introduction of an ethylene bridge between Cp ringshas a strong effect on the chromocene system [7] Whereasthe carbonyl complex Cp

2CrCO cannot be isolated due

to the reversible binding of CO by chromocene [8] vari-ous ansa-chromocenes were prepared and crystallographi-cally characterized [9 10] DFT calculations indicated that

Hindawi Publishing CorporationJournal of Quantum ChemistryVolume 2014 Article ID 521037 5 pageshttpdxdoiorg1011552014521037

2 Journal of Quantum Chemistry

2022 1438

1438

14362036

20951448

1433

14462156

1 2

Figure 1 Computed structures of complexes 1 and 2 (from two viewpoints) Bond lengths are given in A

X M120572

120573

Scheme 1

the stabilization of the CO complex of ansa-chromocene isthe consequence of the stability of the triplet metalloceneproduct However in the ansa-bridged free chromocene therings are unable to relax to a near parallel structure thereforethe triplet state is of less advantage [11]

The resorcin[4]arenes and the related cavitands aremacrocyclic compounds consisting of multiple aromaticrings connected by different linkers [12] Cavitands areknown to offer excellent points of departure for the con-struction of large bowl-shaped molecular entities possessinga well-formed hydrophobic cavity Therefore cavitands andsimilar structures show promise with potential applicationsas gas sensors nanoreactors and drug delivery systems Cav-itands have also found awide application in the preparation ofcapsule-like self-assemblies Ferrocene-substituted resorcin-arenes were found to be redox-switchable dynamic systemsin which the ferrocene moieties serve as electroactive hydro-phobic fragments [13] Moreover decamethylruthenocenewas found to act as template for the self-assembly of a C-methylcalix[4]resorcinarenebipyridinedecamethylruthen-ocene supramolecular crystal [14]

The first goal of this study is to describe the geometryand electronic structure of tetrakis(methyl)resorcin[4]arene(1) and tetrakis(phenoxymethyl)resorcin[4]arene (2) con-taining ferrocene moieties The ferrocenyl groups providesome prediction for their stability and applicability by thefunctionalization of cavitands with cyclopentadiene followed

by deprotonation and metallocene formation by the addi-tion of Fe(II) salts The second purpose of this paper isto give some details for the electron density distributionof ferrocene-substituted cavitands in comparison to simpleansa-ferrocenes containing 2ndash4 bridging methylene groupsas model compounds

2 Computational Details

For all the calculations the PBEPBE gradient-corrected func-tional by Perdew et al [15] was selected using the Gaussian09 suite of programs [16] For iron the triple-zeta basisset by Schaefer and coworkers was applied and denotedas TZVP [17] whereas the 6-31G(dp) basis set [18] wasemployed for every other atom Local minimawere identifiedby the absence of the negative eigenvalues in the vibrationalfrequency analyses whereas the Hessian matrix of transitionstates has only one negative eigenvalue For the NBO calcu-lations the GENNBO 50 program was utilized [19] For theQTAIM studies the AIMAll software was employed [20]

3 Results and Discussion

The functionalization of resorcin[4]arenes with ferrocenes ispredicted to result in well-organized structures with symme-tries very close to C

2The computed geometries of ferrocene-

substituted tetrakis(methyl)resorcin[4]arene (1) and tet-rakis(phenoxymethyl)resorcin[4]arene (2) are depicted inFigure 1 For a better visibility complex 2 is shown from twoviewpoints emphasizing its peculiar structure arranged by theclosure of Cp rings around the iron centers forming formallya dimer of ansa-ferrocenes

For comparison the geometries of ferrocene and simpleansa-ferrocenes containing bridges formed from 2ndash4 CH

2

groups have been computed as well and illustrated in Figure2The unsubstituted ferrocene is denoted as 3 whereas ansa-ferrocenes with ethylene 13-propylene and 14-butylenebridges are denoted as 4 5 and 6 respectively For ferrocenethe eclipsed D

5h structure was found as global minimum at

Journal of Quantum Chemistry 3

2045

1455

19802075

1450

1438

1438

1438

2020

1446

1439

2051

1439

2038 2049

1444 1436

1 23

12 3

12 3

3 4 5 6

Figure 2 Computed structures of complexes 3 through 6 Bond lengths are given in A

the PBEPBE level in accord with the studies of Salzner [21]In all ansa-complexes the presence of the bridge breaks thesymmetry of the Cp ring leading to contracted CndashC bonds incomparison to those in ferrocene The C1ndashC2 bonds in theneighboring position with respect to the bridge are the leastelongated while the C2ndashC3 and C3ndashC31015840 bonds are somewhatshorter According to bond lengths the same tendency can beobserved for the cavitands 1 and 2 As expected the lengths ofFendashC bonds strongly depend on the bridge resulting in veryshort FendashC1 distance in complex 4 In complex 6 howeverwith the more flexible 14-butylene group the FendashC distancesare quite close to those in ferrocene It is remarkable that theFendashC distances in cavitands are somewhat longer comparedto ansa-ferrocenes For instance in complex 1 the FendashC3distance is 2156 A notably longer than that in complex 4with somewhat comparable tilt angle (see Table 1)

In Table 1 the results of natural population analysis havebeen collected as well The NPA charges of iron centersshow distinct values for cavitands and ansa-ferrocenes Incomplexes 1 and 2 the iron atoms are more positive thanthose in ferrocene In contrast in complexes 4ndash6 the metalcenter is more negative As expected the charge distributionin theCp rings strongly depends on the relative position of thebridge the partial charges increase gradually from the ipso-carbons to the C3 carbons The electron density distributionof the Cp rings in cavitands reveals similarity with that ofcomplexes 4ndash6 especially for 2 which presumably possessesa less rigid structure compared to complex 1

The electronic structure around the iron central atomhas been elucidated within the framework of the quantumtheory of atoms in molecules developed by Bader Oneof the three QTAIM descriptors taken into account is theelectron density at bond critical points (120588BCP) which issomewhat related bond strengths The delocalization index120575(AB) which is introduced by Bader and Stephens [22]describes the number of electron pairs delocalized betweentwo atomic basins The 120575(AB) is somewhat related to formalbond orders for an equally shared pair between two atoms ina polyatomic molecule however it is usually less than thatdue to delocalization over the other atoms in the moleculeThe ellipticity (120576) calculated from two negative eigenvalues(1205821and 120582

2) of the Hessian matrix of the electron density

function 120588(r) at the BCP is a measure of the deviation ofthe charge density from the axial symmetry of a chemicalbond and is defined as 120576 = 120582

11205822minus 1 Values close to zero

indicate cylindrical character whereas values greater than

Table 1 Structural parameters (in degrees see Scheme 1) andselected NPA charges of complexes 1ndash6

Complex 120572 120573 Q(Fe) Q(C1) Q(C2) Q(C3)1 minus141 minus97 0178 minus0072 minus0259 minus02632 minus66 minus46 0176 minus0080 minus0270 minus02713 0 na 0141 minus0281 na na4 210 148 0124 minus0043 minus0261 minus02975 84 44 0120 minus0061 minus0269 minus02836 18 minus24 0128 minus0065 minus0273 minus0277

zeromay indicate partial 120587-character of a bondThe 120588BCP and120576 values for complexes 1ndash6 are compiled in Table 2 whereasdelocalization indices 120575(AB) are presented in Table 3

The introduction of a bridge causes changes in FendashC bondstrengths due to the distortion of the metallocene structureFor the cavitand-based complexes the interaction betweenthe iron center and the Cp rings is somewhat weaker incomparison to ferrocene In ansa-metallocenes however thestrength of this interaction strongly depends on the bridgesize in the more strained complex 4 the FendashC1 and FendashC2 bonds become stronger while the FendashC3 bond is weakerin terms of delocalization indices as well as 120588BCP valuesAs expected the smallest change in the strength of FendashCinteractions can be observed for the least strained ansa-ferrocene 6

Inspecting however the changes in electron densitydistribution of the Cp rings it can be observed that theCndashC bond strengths in terms of delocalization indices areexpected to be increased upon the introduction of the ansa-bridge for complexes 1 and 5 in 120573-position whereas almostno change is predicted for complexes 2 4 and 6 For the C1ndashC2 bonds a decrease can be expected in all cases accordingto the 120588BCP and 120575(AB) values The bond ellipticities followthe distortion of the metallocene structure as well Upondecreasing the distance between the analogous carbon atomsin the Cp rings the double bond character is increasedwhereas it is decreased for the carbon atoms on the openingparts in the rings That is in complexes 1 and 2 the C2ndashC3bond is expected to behavewith a somewhat increased doublebond character which may result in a change of the reactivityof the rings on that site

It is concluded that the synthesis of ferrocene-substi-tuted tetrakis(methyl)resorcin[4]arene (1) and tetrakis(phe-noxymethyl)resorcin[4]arene (2) is likely to lead to success


Table 2 Electron densities and bond ellipticities in bond critical points 120588BCP for selected bonds in complexes 1ndash6

Complex 120588BCP(Fe C1) 120588BCP(Fe C2) 120588BCP(Fe C3) 120588BCP(C1 C2) 120576(C1 C2) 120588BCP(C2 C3) 120576(C2 C3)1 0080 0083 0091 0279 0220 0282 02352 0081 0089 0090 0277 0221 0282 02453 0089 na na 0281 0244 na na4 0103 0094 0086 0276 0264 0279 02435 0094 0091 0088 0278 0253 0282 02446 0090 0090 0088 0279 0239 0281 0244

Table 3 Delocalization indices 120575(AB) in complexes 1ndash6

Complex 120575(Fe C1) 120575(Fe C2) 120575(Fe C3) 120575(C1 C2) 120575(C2 C3)1 0467 0501 0503 1189 12232 0437 0501 0502 1171 12113 0503 na na 1211 na4 0517 0510 0491 1172 12105 0492 0508 0497 1178 12156 0491 0505 0501 1182 1211

These complexes with presumably somewhat activated fer-rocene moieties are possible candidates for further transfor-mations The increased electron density on the 120573-position ofcyclopentadienyl rings makes them possible candidates forfollowing functionalization

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

References

[1] T J Kealy and P L Pauson ldquoA new type of organo-ironcompoundrdquo Nature vol 168 no 4285 pp 1039ndash1040 1951

[2] P J Shapiro ldquoThe evolution of the ansa-bridge and its effecton the scope ofmetallocene chemistryrdquoCoordinationChemistryReviews vol 231 no 1-2 pp 67ndash81 2002

[3] J A Smith J von Seyerl G Huttner and H H Brintzingerldquoansa-Metallocene derivatives Molecular structure and pro-ton magnetic resonance spectra or methylene- and ethylene-bridged dicyclopentadienyltitanium compoundsrdquo Journal ofOrganometallic Chemistry vol 173 no 2 pp 175ndash185 1979

[4] W Kaminsky K Kulper H H Brintzinger and F R W PWild ldquoPolymerization of propene and butene with a chiralzirconocene and methylalumoxane as cocatalystrdquo AngewandteChemie International Edition in English vol 204 no 6 pp 507ndash508 1985

[5] H L Lentzner and W E Watts ldquoBridged ferrocenes-VII Thepreparation and properties of [2]ferrocenophanesrdquo Tetrahe-dron vol 27 no 18 pp 4343ndash4351 1971

[6] G Wilkinson M Rosenblum M C Whiting and R B Wood-ward ldquoThe structure of iron bis-cyclopentadienylrdquo Journal of theAmerican Chemical Society vol 74 no 8 pp 2125ndash2126 1952

[7] H Schwemlein L Zsolnai G Huttner and H H Brintzingerldquoansa-metallocene derivatives VI Synthesis and molecu-lar structure of a stable tetramethylethylene-bridged chro-mocene carbonyl complex (CH

3)4C2(C5H4)2Cr(CO)rdquo Journal

of Organometallic Chemistry vol 256 no 2 pp 285ndash289 1983

[8] K L T Wong and H H Brintzinger ldquoReactivity patterns ofchromocene molybdenocene and tungstenocene reaction sys-tems I Carbonyl complex formation as a probe of coordinativeunsaturationrdquo Journal of the American Chemical Society vol 97no 18 pp 5143ndash5146 1975

[9] F Schaper M Rentzsch M-H Prosenc U Rief K Schmidtand H-H Brintzinger ldquoansa-Metallocene derivatives XXXVIDimethylsilyl-bridged permethyl chromocene carbonyl com-plexesmdashsyntheses crystal structures and interconversion reac-tionsrdquo Journal of Organometallic Chemistry vol 534 no 1-2 pp67ndash79 1997

[10] G J Matare D M Foo K M Kane et al ldquoansa-Chromocenecomplexes 1 Synthesis and characterization of Cr(II) carbonyland tert-butyl isocyanide complexesrdquo Organometallics vol 19no 8 pp 1534ndash1539 2000

[11] J C Green and C N Jardine ldquoThermal stability of Group6 bis(cyclopentadienyl) and ethylene bridged bis(cyclopen-tadienyl)monocarbonyl complexes a theoretical studyrdquo Journalof the Chemical Society Dalton Transactions pp 3767ndash37701999

[12] Z Csok T Kegl Y Li et al ldquoSynthesis of elongated cavitandsvia click reactions and their use as chemosensorsrdquo Tetrahedronvol 69 no 38 pp 8186ndash8190 2013

[13] D E Korshin N V Nastapova S V Kharlamov et al ldquoElec-troswitchable self-assembly of tetraferrocene-resorcinarenerdquoMendeleev Communications vol 23 no 2 pp 71ndash73 2013

[14] Y Zhang C D Kim and P Coppens ldquoDoes C-methylca-lix[4]resorcinarene always adopt the crown shape confor-mation A resorcinarenebipyridinedecamethylruthenocenesupramolecular clathrate with a novel framework structurerdquoChemical Communications pp 2299ndash2300 2000

[15] J P Perdew K Burke andM Ernzerhof ldquoGeneralized gradientapproximation made simplerdquo Physical Review Letters vol 77no 18 pp 3865ndash3868 1996

[16] M J Frisch et al Gaussian 09 Revision C 01 GaussianWallingford Conn USA 2009

[17] A Schaefer C Huber and R Ahlrichs ldquoFully optimized con-tracted Gaussian basis sets of triple zeta valence quality foratoms Li to Krrdquo Journal of Chemical Physics vol 100 p 58291994


[18] R Ditchfield W J Hehre and J A Pople ldquoSelf-consistentmolecular-orbital methods IX An extended Gaussian-typebasis for molecular-orbital studies of organic moleculesrdquo Jour-nal of Chemical Physics vol 54 p 724 1971

[19] NBO 50 E D Glendening K Badenhoop et al TheoreticalChemistry Institute University of Wisconsin Madison WisUSA 2001

[20] AIMAll (Version 13 05 06) A Todd and T K Keith GristmillSoftware Overland Park Kan USA 2013

[21] U Salzner ldquoQuantitatively correct UV-vis spectrum of fer-rocene with TDB3LYPrdquo Journal of Chemical Theory and Com-putation vol 9 no 9 pp 4064ndash4073 2013

[22] R F W Bader and M E Stephens ldquoSpatial localization of theelectronic pair and number distributions in moleculesrdquo Journalof the American Chemical Society vol 97 no 26 pp 7391ndash73991975

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Bioinorganic Chemistry and ApplicationsHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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Applied ChemistryJournal of



Theoretical ChemistryJournal of


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Spectroscopy

Analytical ChemistryInternational Journal of


Journal of


Quantum Chemistry


Organic Chemistry International

ElectrochemistryInternational Journal of



CatalystsJournal of


2022 1438

1438

14362036

20951448

1433

14462156

1 2

Figure 1 Computed structures of complexes 1 and 2 (from two viewpoints) Bond lengths are given in A

X M120572

120573

Scheme 1

the stabilization of the CO complex of ansa-chromocene isthe consequence of the stability of the triplet metalloceneproduct However in the ansa-bridged free chromocene therings are unable to relax to a near parallel structure thereforethe triplet state is of less advantage [11]

The resorcin[4]arenes and the related cavitands aremacrocyclic compounds consisting of multiple aromaticrings connected by different linkers [12] Cavitands areknown to offer excellent points of departure for the con-struction of large bowl-shaped molecular entities possessinga well-formed hydrophobic cavity Therefore cavitands andsimilar structures show promise with potential applicationsas gas sensors nanoreactors and drug delivery systems Cav-itands have also found awide application in the preparation ofcapsule-like self-assemblies Ferrocene-substituted resorcin-arenes were found to be redox-switchable dynamic systemsin which the ferrocene moieties serve as electroactive hydro-phobic fragments [13] Moreover decamethylruthenocenewas found to act as template for the self-assembly of a C-methylcalix[4]resorcinarenebipyridinedecamethylruthen-ocene supramolecular crystal [14]

The first goal of this study is to describe the geometryand electronic structure of tetrakis(methyl)resorcin[4]arene(1) and tetrakis(phenoxymethyl)resorcin[4]arene (2) con-taining ferrocene moieties The ferrocenyl groups providesome prediction for their stability and applicability by thefunctionalization of cavitands with cyclopentadiene followed

by deprotonation and metallocene formation by the addi-tion of Fe(II) salts The second purpose of this paper isto give some details for the electron density distributionof ferrocene-substituted cavitands in comparison to simpleansa-ferrocenes containing 2ndash4 bridging methylene groupsas model compounds

2 Computational Details

For all the calculations the PBEPBE gradient-corrected func-tional by Perdew et al [15] was selected using the Gaussian09 suite of programs [16] For iron the triple-zeta basisset by Schaefer and coworkers was applied and denotedas TZVP [17] whereas the 6-31G(dp) basis set [18] wasemployed for every other atom Local minimawere identifiedby the absence of the negative eigenvalues in the vibrationalfrequency analyses whereas the Hessian matrix of transitionstates has only one negative eigenvalue For the NBO calcu-lations the GENNBO 50 program was utilized [19] For theQTAIM studies the AIMAll software was employed [20]

3 Results and Discussion

The functionalization of resorcin[4]arenes with ferrocenes ispredicted to result in well-organized structures with symme-tries very close to C

2The computed geometries of ferrocene-

substituted tetrakis(methyl)resorcin[4]arene (1) and tet-rakis(phenoxymethyl)resorcin[4]arene (2) are depicted inFigure 1 For a better visibility complex 2 is shown from twoviewpoints emphasizing its peculiar structure arranged by theclosure of Cp rings around the iron centers forming formallya dimer of ansa-ferrocenes

For comparison the geometries of ferrocene and simpleansa-ferrocenes containing bridges formed from 2ndash4 CH

2

groups have been computed as well and illustrated in Figure2The unsubstituted ferrocene is denoted as 3 whereas ansa-ferrocenes with ethylene 13-propylene and 14-butylenebridges are denoted as 4 5 and 6 respectively For ferrocenethe eclipsed D

5h structure was found as global minimum at


2045

1455

19802075

1450

1438

1438

1438

2020

1446

1439

2051

1439

2038 2049

1444 1436

1 23

12 3

12 3

3 4 5 6























References



































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


2045

1455

19802075

1450

1438

1438

1438

2020

1446

1439

2051

1439

2038 2049

1444 1436

1 23

12 3

12 3

3 4 5 6























References



































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of









References



































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of
















Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

research article electronic structure of ferrocene...

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