Intramolecular Hydrogen Bonds in Thiolato-, Sulfenato ... · Intramolecular Hydrogen Bonds in...

Intramolecular Hydrogen Bonds inThiolato-, Sulfenato-, and Sulfinato-Co(III) Complexes

MONIKA ARANYOSIOVA, OL’GA VOLLAROVA, JAN BENKO,IVAN CERNUSAKDepartment of Physical and Theoretical Chemistry, Faculty of Natural Sciences, ComeniusUniversity, Mlynska dolina CH1, SK-84215 Bratislava, Slovakia

Received 20 June 2004; accepted 23 November 2004Published online 13 October 2005 in Wiley InterScience (www.interscience.wiley.com).DOI 10.1002/qua.20839

ABSTRACT: The formation of intramolecular hydrogen bonds is investigated fortransition metal complexes in infrared (IR) spectroscopy experiments andB3LYP/LANL2DZ computational study. Three series of complexes are considered:[Co(en)2XCH2CH2NH2]2�, [Co(en)2XCH2CH(COO)NH2]�, and [Co(en)2XCH2COO]� (en �ONH2CH2CH2NH2O, XAS, SO, SOO). Both experimental and theoretical results confirmthe formation of the intramolecular hydrogen bond between the NH2 group of the ethylenediamine ligand and the oxygen of the COO group, as well as between the NH2 and oxygenadded to coordinated sulfur. In the second and third series, the interplay between SO (orSOO) and COO results in different patterns of red-shifted bands in model IR spectra. Therelationship between strength of the intramolecular hydrogen bond and some molecularparameters (H. . .O distance, IR red shift, size of the quasi-ring) is discussed. © 2005 WileyPeriodicals, Inc. Int J Quantum Chem 106: 747–763, 2006

Key words: transition metal complexes; intramolecular; hydrogen bond; IR spectra

Introduction

T ransition metal ion complexes exhibit a widevariety of structures and valences. The appro-

priate selection of ligands in the coordinationsphere allows different modifications of size,

charge, and geometry, as well as the hydrophilic/hydrophobic properties of metal complexes, andprovides different possibilities for interactions withthe surrounding solvent matrix. Hydrophilic/hy-drophobic properties have marked influence on theinteractions of these ions in mixed solvents. There-fore, transition metal complexes are popular targetsfor kinetic and thermodynamic studies in mixtures[1–3]. Solvation effects and reactivity are usuallydescribed by thermodynamic transfer functions inthese studies.

Correspondence to: I. Cernusak; e-mail: [email protected] grant sponsor: Slovak Grant Agency VEGA.Contract grant numbers: 1/0115/03; 1/1383/04.

International Journal of Quantum Chemistry, Vol 106, 747–763 (2006)© 2005 Wiley Periodicals, Inc.

Cobalt(III) complexes containing coordinatedsulfur are useful model systems for studying con-trolled oxidation [4–6]. In previous study [7], weinvestigated the kinetics of oxidation of thiolatoCo(III) complex ions in water–acetonitrile mixtures.These complex ions contain three chelated ligandsthat form puckered rings around a central cobaltatom. A specific kinetic solvent effect was found. Ahypochromatic shift in UV-VIS spectra (22 kJ �mol�1) on going from acetonitrile to water ofthe charge-transfer metal-to-ligand (CTML) bandCoOSOO of sulfenato Co(III) complex ion indi-cates the possibility of specific interaction in thevicinity of the sulfenato ligand and corresponds toan energy change comparable with the energy ofthe hydrogen bond. The changes in of thiolato,sulfenato, and sulfinato cobalt complexes dependstrongly on the nature of particular compound [8].In water–acetonitrile mixtures, the solvation ofCOO and SO groups by interaction of oxygen,which serves as the donor site for interaction withthe acceptor site of the protic solvent molecule, wasobserved. The transfer Gibbs energies indicate thatthere is preferential solvation of the COO group [7].But even in the absence of any solvent molecule,there is a possibility for the COO group as well asthe SO group to interact with a neighboring protonto form an intramolecular hydrogen bond.

Typically, an intramolecular hydrogen bondcloses a quasi-ring in the molecule [9, 10]; itsstrength depends on the size of the ring, participat-ing proton acceptor, and possibility to pucker thering (minimization of the strain). In the case of ourchelated complex ions, one must also consider the

competition between the neighboring protons fromthe NH2 groups. Larger, six- or seven-membered,quasi-rings [11, 12] can lead to intramolecular hy-drogen bonds whose participating bond lengths orinfrared (IR) wavenumbers and intensities are com-parable to intermolecular ones. For smaller quasi-rings [13], the bond length perturbations are lesspronounced, the IR red shifts are small, and perti-nent hydrogen bonds are weaker.

We have checked the possibility of the in-tramolecular hydrogen bond formation in thethiolato-, sulfenato-, and sulfinato-Co(III) com-plexes in preliminary density functional theory(DFT) calculations with minimal pseudo-poten-tial basis (DFT-B3LYP/LANL2MB for the series[Co(en)2XCH2CH(COO)NH2]�). These calcula-tions indicated that in Co(III) complexes, theCOO and SO groups (proton acceptors) and theneighboring NH2 groups (proton donors) are po-tential partners for intramolecular H-bonds.

Prompted by these findings, we have performedsystematic experimental and computational studyof IR spectra for a series of Co(III) complexes. Wehave combined DFT calculations of harmonic IRspectra with experimental spectral data to iden-tify the intramolecular H-bonds. The followingthiolato-, sulfenato-, and sulfinato-Co(III) com-plexes were considered: [Co(en)2XCH2CH2NH2]2�,[Co(en)2XCH2CH(COO)NH2]�, and [Co(en)2XCH2-COO]� (en � ONH2CH2CH2NH2� , XAS, SO,SOO). The detailed analysis of model IR spectracan provide deeper insight into various subtletiesand/or coupling of individual vibration modes andtheir role in H-bond formation.

TABLE I ______________________________________________________________________________________________Comparison of selected B3LYP wave numbers of stretching modes for NH2NH2 and CH3NH2 with experiment.

Mode irrep LANL2DZ cc-pvdz cc-pvtz Exper.a

NH2NH2

NH symm-stretch A 3525 3399 3455 3207NH asymm-stretch A 3671 3498 3551 3297NH symm-stretch B 3517 3381 3445 3356NH asymm-stretch B 3673 3507 3557 3390

CH3NH2

NH stretch A� 3694 3524 3566 3427NH stretch A� 3568 3453 3491 3361CH stretch A� 3130 3092 3087 2985CH stretch A� 3081 3051 3053 2961CH stretch A� 2961 2944 2961 2820

a Ref. [12].

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Experimental and ComputationalMethods

The thiolato-, sulfenato-, and sulfinato- complexsalts were prepared following published methods[4, 14]. The composition of the complexes waschecked by elemental analysis. C, H, and N con-tents were determined on a CHN analyzer (CarloErba; for details, see Ref. [7]). The IR spectra of thecompounds were measured on a Specord M-80

Carl Zeiss Jena spectrophotometer within a rangeof 400 – 4000 cm�1 with 1 cm�1 resolution. TheNujol technique was used. The sample cell wasequipped with KCl windows separated with a0.02-mm aluminum spacer. The cell with Nujolwas used as a reference, and the spectrophotom-eter was calibrated on the spectrum of polysty-rene versus the air. In all spectra, the Nujol bandis easily identifiable, lying at �3000 cm�1, and itscusp is cut. For [Co(en)2SOCH2CH(COO)NH2]�

FIGURE 1. Structure and IR spectra of [Co(en)2SCH2CH2NH2]2�.

INTRAMOLECULAR HYDROGEN BONDS

INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 749

and [Co(en)2SO2CH2COO]�, it was impossible torecord the spectra because the complexes wereunstable during recrystallization.

In our computations, we have used the Gaussian98 suite of programs [15] for the DFT calculationswith the hybrid B3LYP functional. Since our sys-tems are quite large, we have chosen the pseudo-potential basis set, LANL2DZ.

The geometries were determined with the GDIISprocedure, which is known to work well for opti-mizations with a large number of degrees offreedom and for systems with numerous low-frequency modes [16]. Optimized structures, calcu-lated harmonic vibrational frequencies, and modelIR spectra were analyzed with the graphical visu-alization program Molden 3.9 [17]. Identification of

FIGURE 2. Structure and IR spectra of [Co(en)2SOCH2CH2NH2]2�.

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intramolecular hydrogen bonds was based on thefollowing criteria: proton–oxygen distance, redshifts, and visual analysis of vibrational modes.

Results and Discussion

First, we checked the performance of pseudo-potentials with respect to more flexible cc-pvdz and

cc-pvtz basis sets for hydrazine and methylamine.In particular, we were interested in how B3LYP/LANL2DZ described NH and CH stretchings. Forboth molecules, we have optimized the geometrywith the B3LYP functional and series of basis setsand have performed vibrational analysis. Selectedharmonic frequencies in the series LANL2DZ, cc-pvdz, and cc-pvtz are compared in Table I. Althoughthe pseudo-potential basis tends to overestimate the

FIGURE 3. Structure and IR spectra of [Co(en)2SO2CH2CH2NH2]2�.



stretchings by 100–200 cm�1 (cf.o cc-pvdz and cc-pvtz), the ordering of the wavenumbers in a givensymmetry for both molecules is conserved. It appearsthat with the LANL2DZ basis, less polar CH bondsare better described than NH bonds. As expected,even the B3LYP/cc-pvtz harmonic wavenumbersovershoot the experimental fundamentals by approx-imately 50–150 cm�1. Thus, we are confident that thequalitative features of the harmonic IR spectra of our

complexes will follow the experimental ones withuniform shift toward larger values.

In Figures 1–9, we present optimal geometries(ball-and-stick models, top), model harmonic IRspectra (middle), and the observed experimental IRspectra (bottom, where available) of nine complexions. Thin dashed lines connect atoms bound byintramolecular H-bonds. The assignments of mostimportant vibrational modes are collected in Tables

FIGURE 4. Structure and IR spectra of [Co(en)2SCH2CH(COO)NH2]�.

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II–X. We did not use any scaling coefficients for thewavenumbers. Model IR spectra of complex ionsexhibit a slight shift to higher wavenumbers (50–100 cm�1) for typical NH and CH stretching com-pared with the experimental values. This is dueprimarily to harmonic approximation and limitedflexibility of the pseudo-potential basis set. Neverthe-less, we consider the computational level sufficient forthe qualitative comparison of theoretical and experi-mental spectra. The correspondence is quite good,facilitates interpretation of experimental spectra, andfulfills the primary goal of our analysis: identificationof intramolecular hydrogen bonds. In most H-bondcases, there is a well-developed (shifted) band, orband aggregates, in the high-frequency region, wellseparated from intact CH or NH bands. Note that CHgroups are not involved in H-bonds.

[CO(EN)2XCH2CH2NH2]2�, (XA S, SO, SO2)SERIES

Complex ion contains two ethylene diaminerings and one thiolate ring. The theoretical IR

spectra in this series can be split into three re-gions. The first one is an intensive band belong-ing to COC stretching in the ethansulfenaminatering at �1070 cm�1, followed by the CH2 andNH2 deformation band aggregate in the 1150 –1750 cm�1 region. In the third group are thesymmetric and antisymmetric CH2 and NH2stretches over 3000 cm�1 (considering local quasi-symmetry of AH2 groups). It is primarily thisthird group that we will discuss in detail in thefollowing paragraphs.

[CO(EN)2SCH2CH2NH2]2�

As expected, in this complex ion, neither exper-imental nor theoretical methods indicate the in-tramolecular hydrogen bonds (Table II). Populationanalysis showed that the sulfur atom in ethansulfe-naminate ring is positively charged (approximately�0.1), i.e., it is not a possible electron-donor site.Neighboring hydrogens on NH2 groups (electronacceptors) thus have no opportunity for intramolec-

FIGURE 5. Structure and IR spectrum of [Co(en)2SOCH2CH(COO)NH2]�.



ular hydrogen bond. Our previous results [7, 8]showed that there was no preferential solvation bywater in water–acetonitrile mixtures, but exclu-sively by acetonitrile (the Gibbs transfer energiesfrom water to acetonitrile was negative). It can beexpected that the preferential solvation in water–acetonitrile mixtures will proceed either via NH2groups or at the S atom or will be a compromisebetween them, the relaxation of chelate ring can be

involved too (minimization of repulsion). This topicis currently under study.

[CO(EN)2SOCH2CH2NH2]2�

Oxidation to the sulfenato- complex opens thepossibility for an intramolecular H-bond. The fre-quencies at 3144–3146 and 3303 cm�1 in the modelIR spectrum belong to two NH stretchings of hy-

FIGURE 6. Structure and IR spectra of [Co(en)2SO2CH2CH(COO)NH2]�.

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drogens from different neighboring ethylenedia-mine ligands coordinated toward the oxygen of SOgroup (Table III, Fig. 2). They form the six-mem-bered puckered ring CoONOH. . .O. . .HON, visi-ble in the foreground of Figure 2. Elongated SObond length (1.714 Å, cf. SO for similar compounds1.45�1.5 Å [18]) reflects its weakening due to two

intramolecular hydrogen bonds. The geometric per-turbations in NH bonds participating in H-bond(with respect to intact ones) range approximatelyfrom �0.01 Å to �0.02 Å. Now we can narrow theposition of the H-bonds in the experimental spec-trum based on overestimation of the harmonicwavenumbers from data in Table I. The band at

FIGURE 7. Structure and IR spectra of [Co(en)2SCH2COO]�.



�3100–3150 cm�1 has its experimental counterpartmost probably in the Nujol broad band and can notbe easily extracted from the experiment. In analogy,the latter harmonic band at �3300 cm�1 corre-sponds to the feature at �3200 cm�1 in the experi-mental spectrum. Population analysis for the struc-ture depicted in Figure 2 shows a strong chargeshift toward the oxygen atom on sulfur (�0.63e);

i.e., it is also a good candidate for hydration as theproton acceptor.

[CO(EN)2SO2CH2CH2NH2]2�

In the model IR spectrum, the intramolecularhydrogen bond stretchings are located at 3118 and

FIGURE 8. Structure and IR spectra of [Co(en)2SOCH2COO]�.

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3294 cm�1. They correspond to coordination of bothoxygen atoms by NH2 hydrogens from the sameethylenediamine ligand forming two large five-membered rings (NH. . .OSCo) visible in the fore-ground of the structure in Figure 3. The first (ac-commodating the 3118 cm�1 mode, i.e., slightlystronger H-bond) is almost planar, with a dihedralangle �(OSCoN) � �3°, the second one is signifi-cantly puckered (�(OSCoN) � 48.5°). This is differ-ent from the previous case, and the main reason isthe relaxation of the ethylenediamine ligand whosehydrogens participating in H-bond are well sepa-rated. Both the geometric perturbations in the NHbonds participating in the H-bond and the elonga-tions of the SO bonds (SO(37) � 1.661 Å, SO(36) �1.653 Å) exhibit trends similar to those in the pre-vious case. The corresponding bands in the exper-imental spectrum are assigned to 2900–2950 cm�1

(overlap with Nujol band) and �3150 cm�1 (Fig. 3,Table IV).

[CO(EN)2XCH2CH(COO)NH2]�, (XAS, SO,SO2) SERIES

This series was also studied experimentally inRef. [4], but the investigators did not report anyintramolecular H-bond. Complex ion contains twoethylene diamine rings and one cysteinate ring. Thecomposition of ligands introduces strong competi-tion in intramolecular H-bond formation betweenthe COO and SO (or SOO) groups, respectively. Thebands in the theoretical IR spectra can be dividedinto three groups. The first one is the intensive bandbetween 1065 and 1080 cm�1 belonging to the COCstretchings in ethylenediamine and cysteinaterings. The second is the group of CH2 and NH2

deformations in the 1150–1700 cm�1 range, wherewe could observe some bending of the intramolec-ular hydrogen bond. In the third group are the CH2

and NH2 symmetric and antisymmetric stretchingsat 2600 cm�1, including intramolecular hydrogenbond stretchings.

FIGURE 9. Structure and IR spectrum of [Co(en)2SO2CH2COO]�.



[CO(EN)2SCH2CH(COO)NH2]�

The fundamental frequency of the CAO groupin gas-phase carboxyl acids is �1780 cm�1 [19], andthe bond length lies at �1.20–1.21 Å. The model IRspectrum exhibits a series of deformation bandsranging from 1000 to 1700 cm�1 (twisting, rocking,and scissoring modes of the NH2 and CH2 groups,Table V). These bands correlate well with the ex-perimental bending bands. Vibration of the COgroup is shifted in the experimental spectrum tolower frequencies of �1550–1600 cm�1. This indi-cates weakening of the CO(12) bond and its elonga-

tion due to intramolecular hydrogen bond. Indeed,we obtained the bond length CO(12) � 1.328 Å inour calculation, while CO(11) � 1.247 Å. TheoreticalCAO stretching (�1650 cm�1) is slightly higher butstill apparently lower than expected at this compu-tational level. Both experiment and theory consis-

TABLE II ______________________________________Model harmonic IR spectrum of[Co(en)2SCH2CH2NH2]2�.

� (cm�1) Atoms Description

753–763 NH2 Rocking1062 COC Stretching (ethylenediamine

ring)1177–1277 CH2/NH2 Wagging/twisting1321–1346 CH2 Twisting1502–1535 CH2 Scissoring1665–1716 NH2 Scissoring

�3100–3170 CH2 Stretching3400–3530 NH2 Stretching

TABLE III _____________________________________Model harmonic IR spectrum of[Co(en)2SOCH2CH2NH2]2�.


�700 NH2 Rocking728 SO Stretching762–987 NH2/CH2 Rocking

�1070 COC Stretching(ethylenediaminering)

1096 NH2/CH2 Twisting1157–1345 NH2/CH2 Wagging/twisting1500–1540 CH2 Scissoring1680–1736 NH2 Scissoring3090 CH2 Stretching3144–3146 N(11)OH(29)

. . .O(5)a H-bond stretching

3303 N(7)OH(22). . .O(5)b H-bond stretching

3440–3530 NH2 Stretching

a R(NH) � 1.045 Å, R(H. . .O) � 1.949 Å, for the subscripts inparentheses refer to Figure 2.b R(NH) � 1.038 Å, R(H. . .O) � 2.012 Å.

TABLE IV _____________________________________Model harmonic IR spectrum of[Co(en)2SO2CH2CH2NH2]2�.


353 SOCoON Axial stretching764–877 NH2/CH2 Rocking

1001 CH2/NH2 Rocking/twisting(thiolate ring)

1070–1076 COC Stretching(EDA ring)

1150–1350 CH2/NH2 Twisting/wagging(four peaks)

1474–1540 CH2 Scissoring1672–1720 NH2 Scissoring3083–3092 CH2 Stretching3118 N(10)OH(35)

. . .O(37)a H-bond

stretching3294 N(13)OH(28)

. . .O(36)b H-bond

stretching3436–3532 NH2 Stretching

a R(NH) � 1.046 Å, R(H. . .O) � 1.868 Å, for subscripts inparentheses refer to Figure 3.b R(NH) � 1.0375 Å, R(H. . .O) � 2.167 Å.

TABLE V ______________________________________Model harmonic IR spectrum of[Co(en)2SCH2CH(COO)NH2]�.


381–839 NH2/CH2 Rocking1062 COC Stretching in

cysteine ring1069 COC Stretching in

ethylene diaminering

1150–1300 CH2/NH2 Twisting/wagging1512–1700 NH2 Twisting/scissoring2651 N(4)OH(24)

. . .O(12)a H-bond stretching

�3080 CH2 Stretching3247 N(6)OH(29)

. . .O(12)b H-bond stretching

a R(NH) � 1.075 Å, R(H. . .O) � 1.595 Å, for subscripts inparentheses refer to Figure 4.b R(NH) � 1.039 Å, R(H. . .O) � 1.926 Å.

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tently confirm that the exocyclic oxygen ofOOOCAO group is included in the intramolecularhydrogen bond. Indeed, the red-shifted strongband at 2561 cm�1 represents the largest shift dueto strong intramolecular hydrogen bond among allcases observed. This peak can be assigned to seven-membered ring with N(7)OH(23)

. . .O(38) sequence.Although the experimental spectrum does not con-tain clear features in the range 2000–2500 cm�1, theedge at �2400 cm�1 with its smaller peak is anindication of intramolecular hydrogen bond. Theslightly weaker hydrogen bond (NH originatingfrom cysteine ring) to the same oxygen ofOOOCAO group indicates a NOH stretchingband at 3247 cm�1, and its experimental counter-part is clearly visible just next to the Nujol band.

[CO(EN)2SOCH2CH(COO)NH2]�

The CC stretching at �1076 cm�1 in model IRspectrum is accompanied with an aggregate ofCH2/NH2 deformation bands at 1160–1700 cm�1.Three intramolecular hydrogen bonds stretchingswere found: (i) a weaker band in the five-mem-bered quasi-ring at 2886 cm�1 (stretching in theN(11)OH(29)

. . .O(5) sequence); (ii) a strong band at3007 cm�1 with N(7)OH(23)

. . .O(38) H-bond in sev-en-membered quasi-ring at carbonyl oxygen, withdominant participation of the hydrogen from eth-ylenediamine ring and weakly coupled hydrogenfrom cysteine ring; and (iii) a strong band in withN(12)OH(28)

. . .O(38) hydrogen bond in the five-membered quasi-ring at 3082 cm�1, where the sit-uation is reversed (Table VI). Thus, three alterna-tives for the hydrogen bond are possible: betweenthe oxygen of the COO group and the NH2 group(either from the neighboring ethylenediamine or itsown cysteinsulfenato ligand) and between the SOand ethylenediamine NH2 groups. One would ex-pect the seven-membered quasi-ring to have thelargest red shift in the IR spectrum, but it this is notthe case because of competition between protonsH(23) and H(28) that share the same oxygen atom.The lengths of the CO(38) (1.324 Å) and SO(5) (1.733Å) bonds reflect weakening due to H-bonds.

[CO(EN)2SO2CH2CH(COO)NH2]�

Addition of the second oxygen to sulfur resultsin four intramolecular hydrogen bonds, as revealedin the analysis of the model IR spectrum. In thiscase, the differences in experimental and theoreticalpatterns are surprisingly negligible, if we neglect

the differences in band intensities. The model spec-trum contains the N(12)OH(28)

. . .O(38) hydrogenbond deformation band at 1432 cm�1. Bands at 2995and 3057 cm�1 belong to “antisymmetric” and “sym-metric” stretching of N(7)OH(23)

. . .O(38) in the seven-membered quasi-ring and N(12)OH(28)

. . .O(38) in thefive-membered quasi-ring. Despite the seven-membered ring and the relatively small distanceH(23)

. . .O(38) � 1.690 Å, the band lies at a signif-icantly larger wavenumber than its analogue in[Co(en)2SOCH2CH(COO)NH2]�. The reason isagain the competition between H(23) and H(28),which must share the same proton acceptor O(38).The band at 3037 cm�1 (superimposed with theband at 3057 cm�1 to yield one intensive peak) andthe weak 3266 cm�1 feature belong to the hydrogenbond stretching in two five-membered quasi-ringsbetween both oxygen atoms of SO2 group and hy-drogens of different ethylenediamine NH2 groups(Fig. 6 and Table VII). Based on the known overes-timation of harmonic wavenumbers, the experi-mental counterparts of these bands probably lie inthe group at �2800 cm�1. We stress that the inten-sities of the peaks of intramolecular hydrogenbonds between oxygen of COO group and ethyl-enediamine NH2 group are higher (and the corre-sponding H. . .O distances smaller) that thoseformed by oxygens of SO2 group.

[CO(EN)2XCH2COO]�, (XAS, SO, SO2)SERIES

The theoretical IR spectra of complexes in this se-ries are also split into three parts. The first part con-

TABLE VI _____________________________________Model harmonic IR spectrum of[Co(en)2SOCH2CH(COO)NH2]�.


751–911 NH2/CH2 Rocking1076 COC Stretching1160–1180 CH2/NH2 Twisting/wagging1300–1400 CH2/NH2 Bending1550–1700 NH2 Scissoring2886 N(11)OH(29)OO(5)

a H-bond stretching3007 N(7)OH(23)OO(38)

b H-bond stretching3082 N(12)OH(28)OO(38)

c H-bond stretching

a R(NH) � 1.060 Å, R(H. . .O) � 1.760 Å, for numbers inparentheses refer to Figure 5.b R(NH) � 1.051 Å, R(H. . .O) � 1.740 Å.c R(NH) � 1.050 Å, R(H. . .O) � 1.793 Å, coupled with CH2

stretching.



tains the CH2 and NH2 deformation bands in the1000–1500 cm�1 region. The intensive band of COObond stretching in the mercaptoacetato ring is super-imposed with the scissoring mode of NH2 groups inthe range 1672–1699 cm�1. The third part includes asymmetric and antisymmetric stretchings of CH2 andNH2 groups at �3000 cm�1. For a series, there is atypical feature; i.e., the quasi-rings with intramolecu-lar hydrogen bonds are all five-membered.

[CO(EN)2SCH2COO]�

Neither experiment nor theory indicates any in-tramolecular hydrogen bonds (Table VIII). Owingto steric reasons, there is practically no chance forthe COO group to participate in the intramolecularhydrogen bond. Thus, one can expect the oxidationof sulfur to provide more flexible proton acceptors.

[CO(EN)2SOCH2COO]�

In the 1669–1724 cm�1 region, there is an inten-sive band of NH2 bending overlapping with CAOstretching (1678 cm�1). The SO bond is stretched to1.742 Å; its oxygen serves as a proton acceptor forboth NH2 groups of the same ethylenediamineforming two five-membered quasi-rings (Fig. 8).Analysis of model spectrum reveals that the respec-tive H-bonds correspond to the strong band at 2790cm�1 and the weak one at �3154–3167 cm�1 (seealso footnotes in Table IX). In this case, the experi-mental spectrum is more complicated, since the

complex was in the picrate salt form. We assignedthe perturbations in the Nujol band (3000–3150cm�1) to the weak NOH. . .O stretching and thefeature in experimental spectrum at �2500 cm�1

(Fig. 8) intensive band (2790 cm�1).

[CO(EN)2SO2CH2COO]�

In the third complex, the repulsion between ox-ygen lone pairs leads to the geometry of the sulfi-nato-ring that allows intramolecular hydrogenbonds only between oxygens of SO2 group andhydrogens of adjacent ethylenediamine NH2groups. The five-membered rings suffer from thering strain, therefore the H. . .O distances are sig-nificantly larger than in [Co(en)2SOCH2COO]�.The complex prefers to maintain the local quasi-Cs

symmetry of the CoSCCO ring leading to weakH-bond formation (Table X). The respective bandsat �3347–3404 cm�1 are therefore almost indistin-guishable from the unperturbed NH stretching ofthe other ethylenediamine ring (3450–3560 cm�1). Ex-perimental spectrum is absent in this case (Fig. 9).

Conclusions

Comparison of calculated model IR spectra withexperimental results shows that theoretical data arein fair agreement with the experimental findingsbased on UV-VIS spectroscopy and values of ther-modynamic transfer functions [7, 8]. The analysis of

TABLE VII ____________________________________________________________________________________________Model harmonic IR spectrum of [Co(en)2SO2CH2CH(COO)NH2]�.


800–1326 CH2/NH2 Twisting/wagging�1080 COC Stretching (ethylenediamine)

1432 N(12)OH(28)OO(38) H-bond bending1553–1723 NH2 Scissoring2995 N(7)OH(23)OO(38)

a/N(12)OH(28)OO(38) H-bond stretching (“antisymm”)3038 N(8)OH(36)OO(39)

b H-bond stretching3057 N(12)OH(28)OO(38)

c/N(7)OH(23)OO(38) H-bond stretching (“symm”)3087 CH2 Stretching3266 N(11)OH(29)OO(5)

d H-bond stretching3538 NH2 Stretching

a R(NH) � 1.051 Å, R(H. . .O) � 1.690 Å, for numbers in parentheses refer to Figure 6.b R(NH) � 1.051 Å, R(H. . .O) � 1.806 Å.c R(NH) � 1.053 Å, R(H. . .O) � 1.786 Å.d R(NH) � 1.039 Å, R(H. . .O) � 2.099 Å.

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gas-phase vibrational modes revealed the existenceof intramolecular hydrogen bonds that varystrongly with the competition between SO, SOO,and COO groups. In the first series of complexes,only the SO group (in [Co(en)2SOCH2CH2NH2]2�)or SOO group (in [Co(en)2SO2CH2CH2NH2]2�) actas proton acceptors and provide an intramolecularH-bond of comparable strengths (Tables III and IV).

We found the strongest H-bond in the secondseries, in complex [Co(en)2SCH2CH(COO)NH2]�. Itis formed between the amino group and the car-boxyl with R(NH) � 1.075 Å, R(H. . .O) � 1.595 Å,the largest geometry perturbations found among all

the complexes. It was also accompanied by thelargest red shift and intensity of the pertinent bandin the model spectrum (Fig. 4). Introducing oxygenatoms at sulfur results in a competition for protonsbetween the COO and SO (or SOO) groups,respectively. Intramolecular H-bonds weakenfrom [Co(en)2SOCH2CH(COO)NH2]� to [Co(en)2-SOOCH2CH(COO)NH2]�, this trend is docu-mented also in frequency shifts (Tables V–VII) andlower intensities (Figs. 5 and 6).

In the third series, the arrangement of COOgroup in the mercapto-ring of prevents any con-ceivable H-bond (the ring is quasi-planar, tendingto local Cs symmetry). Only the complex with theSO group permits slight distortion of the mercapto-ring from planarity (Fig. 8), which results in re-markable intramolecular H-bond (R(NH) � 1.064Å, R(H. . .O) � 1.695 Å). The oxidation to SOOrecovers the quasi-planar geometry of the ringand results only in weak intramolecular H-bonds(Fig. 9).

Comparison of the theoretical and experimentalresults leads to the following conclusions:

1. Amino groups (proton donors) and SO, SOO,or COO (proton acceptors) can form intramo-lecular hydrogen bonds in transition metalion complexes with flexible ligands (e.g., eth-ylenediamine). Intramolecular H-bonds canbe easily identified in model IR spectra (redshifts in NH stretchings).

2. Gas-phase calculations show that for theseries with relatively free carboxyl, i.e,[Co(en)2XCH2CH(COO)NH2]�, (XAS, SO,SOO), there is competition between the SO,SOO sites and the COO group in H-bondformation and COO group dominates. In the

TABLE VIII ____________________________________Model harmonic IR spectrum of[Co(en)2SCH2COO]�.


1031–1072 NH2/CH2 Wagging/rockinga

1142–1185 NH2/CH2 Wagging/twistingb

1198 OCO/NH2 Stretching/wagginga

1314, 1319 CH2 and NH2 Twisting1520–1533 CH2 Bendinga

1672 CAO/NH2 Stretching/bendinga

1675–1699 NH2 Scissoring3074–3171 CH2 Stretching3446–3564 NH2 Stretching

a Ethylenediamine ring.b Mercapto ring.

TABLE IX _____________________________________Model harmonic IR spectrum of[Co(en)2SOCH2COO]�.


1067–1075 CC/NC and NH2 Stretching andwagginga

1148–1194 CH2/NH2 Rocking/twisting1188 CO/CC Stretchingb

1258–1317 CH2/NH2 Twisting/rockinga

1519–1724 CH2/NH2 Scissoringa

2790 N(6)H(31). . .O(33)

c Stretching (H-bond)3080–3140 CH2 Stretching3154–3167 N(7)H(32)

. . .O(33)d Stretching (H-bond)


a Ethylenediamine ring.b Mercapto ring.c R(NH) � 1.064 Å, R(H. . .O) � 1.695 Å, for numbers inparentheses refer to Figure 8.d R(NH) � 1.044 Å, R(H. . .O) � 1.994 Å.

TABLE X ______________________________________Model harmonic IR spectrum of[Co(en)2SO2CH2COO]�.


1067–1084 CC Stretching1135–1330 CH2/NH2 Twisting/wagging1654–1728 NH2 Scissoring3078–3170 CH2 Stretching3347–3404 N(7)H(32)

. . .O(34) NH stretching (H-bond)N(6)H(31)

. . .O(33)

N(2)H(3). . .O(34)




series with carboxyl incorporated into the che-late ring (last complex listed), significantlyweaker intramolecular H-bonds occur.

3. The strength of the intramolecular hydrogenbond in this type of complexes can be charac-terized by geometric parameters (H. . .O dis-tance, SO or CO bond elongation), IR red shift(e.g., for pertinent NH, CO, or SO stretchings)with respect to isolated ones and by the size ofthe quasi-ring. In addition, one also has totake into account the competition between thehydrogens when bonding to common protonacceptor. Figure 10 illustrates the rather goodcorrelation of two of these parameters (theR2 coefficient of determination is acceptablekeeping in mind heterogeneity of the sample).

4. Our previous results from UV-VIS spectros-copy and values of thermodynamic transferfunctions [7, 8] indicate that most of this in-tramolecular H-bond can also persist in mixedsolvents (e.g., water–acetonitrile) and can playimportant role in preferential solvation. Forinstance, it appears that the interaction be-tween the water molecule (protic solvent) andthe sulfenate ligand occurs in solution. Inter-action is weaker in the case of complex ionswith SO and COO group together, carboxylicgroup is competitive with respect to SOgroup. UV-VIS spectra measurements showthe blue shift of absorption maximum by trans-fer from acetonitrile to water. The shift is more

significant for the complex ion without carbox-ylic group [Co(en)2SO2CH2CH2NH2]2�.

ACKNOWLEDGMENTS

The authors dedicate this study to our late col-league Associate Professor A. Perjessy for his helpwith IR spectra.

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FIGURE 10. Correlation between H. . .O hydrogen bond length and pertinent NH stretching wavenumber for thethree series of complex ions.

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