Unilateral Exclusion of Jahn–Teller-Inactive d5

Mn(H2O)4(C7H4NO3S)22+ Guests by Strongly Distorted Host d9

Cu(H2O)4(C7H4NO3S)22+ Lattice

Pance Naumov,*,†,‡,§ Ljupco Pejov,§ Gligor Jovanovski,§,⊥ Trajce Stafilov,§

Milena Taseska,§ and Emilija Stojanovska§

Frontier Research Base for Global Young Researchers, Graduate School of Engineering, OsakaUniVersity, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, National Institute for Materials Science,ICYS, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, Institute of Chemistry, Faculty of Science, SS.Cyril and Methodius UniVersity, P.O. Box 162, MK-1001 Skopje, Macedonia, and MacedonianAcademy of Sciences and Arts, P.O. Box 428, MK-1001, Skopje, Macedonia

ReceiVed NoVember 19, 2007; ReVised Manuscript ReceiVed December 16, 2007

ABSTRACT: The crystal lattice of the isomorphous tetraaquabis(saccharinate)metal(II) dihydrates was employed as a structurallyflexible coordination framework capable of sustaining large internal distortions to study the competitive inclusion of Jahn–Teller(JT) distorted d9 ions, [Cu(H2O)4(sac)2]2+, and JT-inactive d5 ions, [Mn(H2O)4(sac)2]2+, in binary solid solutions underthermodynamically controlled conditions of statistical mixing (sac ) saccharinate anion, C7H4NO3S-). Probing of the metal contentof the solid phase showed a two-regime inclusion profile: increasing the ratio of the distorted cation relative to the undistorted onein the solution phase of up to about 35% results in linear dependence and preferred inclusion of the former with maximum concentrationof 100% in the crystal and complete exclusion of the undistorted ion above that point. A mixed crystal with highest copper ratio of63% was obtained from solution with 25% copper, which under the P21/c crystal symmetry corresponds to sustainable integrity ofthe undistorted lattice by substitution of up to 2/3 of its sites. This stability limit shows that four out of the six sites around each[Mn(H2O)4(sac)2]2+ ion can be substituted by distorted [Cu(H2O)4(sac)2]2+ guests under conditions of thermodynamically controlled,statistically averaged exchange. The undistorted host is very tolerant toward inclusion of strongly distorted guests. When acting ashost, the distorted ion is more discriminatory toward the undistorted guest. Along with the expectation from the JT theory, structuralrefinement of seven crystals, including a mixed crystal with composition of [Cu0.126Mn0.874(H2O)4(C7H4NO3S)2](H2O)2, showed thatmetal–ligand distances are significantly affected by the metal substitution. Inclusion of the JT-active ion results in distortion of thecoordination polyhedron by increasing the bond length difference between the two metal-O(water) bonds and also causes shorteningof the M-N bond. Due to the rigidness caused by π-conjugation, the overall effect on the endocyclic geometry of the organic ligandis small. The anisotropic distortions around the metal ion are faithfully reflected in the stretching force constants of the coordinatedwater molecules and thus in the IR spectrum of the mixed crystals.

1. Introduction

Nonlinear molecules with a partially filled set of degenerateorbitals exhibit Jahn–Teller (JT) instability. The effect, whichis commonly referred to as “Jahn–Teller effect”, has beenemployed to explain some extraordinary properties of materialscontaining JT entities, such as high-temperature superconductiv-ity, colossal magnetoresistance, and creation of long-livedphotoinduced excited-state spin-trapped phases.1–5 Although aplethora of studies have been devoted to JT-related phenomena,their relation to the unusual physical properties of JT-activematerials continues to attract the attention of inorganic chemistsand solid-state physicists.

The isomorphous series of hexahydrated salts of saccharin[1,2-benzisothiazol-3(2H)-one 1,1-dioxide] with the first transi-tion row metals and cadmium [M(H2O)4(sac)2] ·2H2O [sac )saccharinate ligand, C7H4NO3S-; M ) V(II), Cr(II), Mn(II),Fe(II), Co(II), Ni(II), Cu(II), Zn(II), and Cd(II)] spans aremarkably large range of ionic radii, cell parameters, and intra-and intermolecular distances. The crystals have a monoclinicP21/c cell and are composed of octahedral [M(H2O)4(sac)2]2+

units and hydrogen bonded lattice–water molecules (Figure 1).

Being sufficiently flexible to sustain large lattice strains withoutcollapsing, this structure has frequently served as a probe intothe systematic structural trends concerning both intra- andintermolecular parameters, including bond distance-order re-lationships,6 structural correlations regarding the geometry ofthe isothiazole half of the saccharinate ligand,7,8 and vibrationalspectroscopic consequences of cation substitution.9,10 Outstand-ing members of the series with respect to their structuralproperties are the V(II),11 Cr(II),12 and Cu(II)13 compounds, forwhich the available structural and vibrational data indicate strongJT distortions that are reflected most evidently as anomalous

* To whom correspondence should be addressed. Tel: +81-(0)29-851-3354.E-mail: naumov.pance@nims.go.jp.

† Osaka University.‡ National Institute for Materials Science.§ SS. Cyril and Methodius University.⊥ Macedonian Academy of Sciences and Arts.

Figure 1. Molecular (A) and crystal (B) structure of the isomorphousseries of general formula [M(H2O)4(sac)2] ·2H2O with the atom labelingscheme.

CRYSTALGROWTH& DESIGN

2008VOL. 8, NO. 4

1319–1326

10.1021/cg701139y CCC: $40.75 2008 American Chemical SocietyPublished on Web 03/05/2008

metal–ligand distances. Based on the structural flexibility of thecrystal lattice, we presumed formation of stable solid solutionsin the binary system [CuxMn1-x(H2O)4(sac)2](H2O)2 includingthe strongly JT-distorted guest Cu(H2O)4(sac)2

2+ in the latticeof the undistorted host Mn(H2O)4(sac)2

2+ and vice versa. In thisstudy, the formation of solid solutions is demonstrated, and thestructural and spectroscopic consequences of metal substitutionare investigated on a series of novel mixed-metal saccharinatecrystals. The copper-manganese saccharinate system wasselected for this purpose based on a number of reasons: (a) theunit cell consists of a single formula unit per asymmetric unit,which simplifies greatly the interpretation of the correlationsbetween the spectroscopic and structural data; (b) the structureincludes both coordinated and noncoordinated water moleculesinvolved in hydrogen bonding ranging from weak to strong, sothat the influence on the JT instability on the primary as wellas on the second coordination shell can be conveniently assessed;(c) in addition to the water of hydration, the effect can be alsoinvestigated on the isothiazole and phenylene halves of theorganic ligand, that is, on both proximate and distant electroni-cally π-conjugated systems; (d) the saccharinate ligand can beused to probe the effects of the JT distortions on the electronicproperties of the carbonyl group and sulfonyl groups, twochemically very important functionalities.

2. Experimental Procedures

2.1. Preparation of the Mixed Crystals. Twelve aqueous solutionswere prepared with identical overall molar concentration of the saltand [Mn(H2O)4(sac)2] · 2H2O/[Cu(H2O)4(sac)2] · 2H2O molar ratios of100/0, 99/1, 97/3, 95/5, 75/25, 65/35, 50/50, 25/75, 5/95, 3/97, 1/99,and 0/100. Crystals from all solutions were obtained under identicalconditions by slow evaporation of the solvent at ambient temperature.The resulting crystals [Mn1-xCux(H2O)4(sac)2] ·2H2O (with x between0 and 1) differ by their size and habit, ranging from small well-definedcrystals in the case of large manganese content to large crystal aggregateblocks in the case of large copper content. The crystal color also dependson the cation ratio and varies between colorless and light blue.

2.2. FTIR Measurements. The 8000-400 cm-1 range of theFourier transform infrared spectra of the studied mixed crystals wasrecorded with a Perkin-Elmer System 2000 FT IR interferometer atroom temperature (RT) and at low temperature (∼100 K, LT). Avariable-temperature cell (Graseby-Specac) using liquid nitrogen wasused for the LT measurements. To obtain a good signal-to-noise ratio,128 spectra were collected and averaged at LT, while 64 scans appearedto be sufficient at RT.

2.3. Determination of the Metal Content. Copper and manganesecontents were determined by flame atomic absorption spectrometry withdeuterium correction using Thermo Elemental model Solaar 2 atomicabsorption spectrometer. Hollow cathode lamps were used as a source.In the case of copper, a wavelength of 324.8 nm, slit of 0.5 nm, andlamp current of 4 mA were used, while in the case of manganese, therespective parameters were 279.5 nm, 0.2 nm, and 5 mA. A gas mixtureof acetylene and air was used. The standard solutions were of p.a. grade,and doubly distilled water was used for preparation of the solutions.About 10 mg of samples was used to prepare the working solutions,and stock solutions for both Cu and Mn (γ ) 1000 mg L-1, SolutionPlus Inc.) were used as standards.

2.4. Uv–Vis Spectroscopy. The 300–800 nm range of the absorp-tion spectra was recorded in reflectance mode from mixtures ofpowdered samples of the crystals with KBr with a Jasco V-570spectrometer. The overall mass concentration of the sample in the matrixin all cases was about 1%.

2.5. X-ray Diffraction. Diffraction data were collected from sevencrystals obtained from solutions with the following metal ratios: 100/0, 99/1, 97/3, 95/5, 65/35, 50/50 and 5/95, corresponding to the realratios of 100/0, 94.7/5.3, 94.6/5.4, 87.4/12.6, 1.4/98.6, 1.7/98.3, and1.0:99.0, respectively. The crystals of the 75/25, 25/75, 3/97 and 1/99mixed salts were of insufficient quality for X-ray diffraction. The X-raydiffraction data on the pure host (Mn) crystal and the mixed crystals

were collected at room temperature on a Bruker AXS diffractometer,equipped with CCD detector. The frames were integrated with SAINTand further processed with SADABS and XPREP as part of theSHELXTL suite of programs.14 The structures were solved using directmethods15 and refined by least-squares method on F2.16 For the purecrystal, all non-H atoms were assigned anisotropic parameters. Due tothe small amount of the secondary component, all structures exceptfor the 87.4/12.6 crystal were treated as pure metal saccharinate crystalsin the refinement. Due to the close proximity of positions of the twocomponents, in the case of the mixed crystal the non-hydrogen atomswere refined as isotropic models, and the non-H atoms were includedas riding bodies. The inclusion of the second component of thedisordered saccharinate ligand resulted in a decrease of the primaryresidual value from about 18% to 5%, thus justifying the treatment ofthe structure as a two-component model. Restraints were applied totreat the disorder of the organic ligand: some of the bond distances ofthe minor component were restrained to their expected values in thepure crystal, and all atoms (except the sulfonyl oxygen atoms) wererestrained to be coplanar. The positions of the C1 atom of the twocomponents nearly coincide with each other and were treated as a singleatom. The displacement parameters of the nonprimed atoms wererestrained to similar values with the respective primed atoms. Oc-cupancies of the partially occupied atoms (87.4% and 12.6% for themajor and minor component, respectively) were fixed to the valuesdetermined by atomic absorption spectrometry. Hydrogen atoms of thewater molecules were placed at 0.85 Å from the respective parent non-Hatoms, and their coordinates were allowed to refine. The insufficientdata quality prevented separate treatment of the hydration water of thetwo components.

3. Results and Discussion

3.1. Formation of Solid Solutions. The metal contentdetermined by atomic absorption spectrometry showed nonuni-form inclusion of the two cations in the mixed crystals (Table1). At any cation ratio in solution phase, inclusion ofCu(H2O)4(sac)2

2+ in the solid phase is favored overMn(H2O)4(sac)2

2+. By increasing the molar ratio of Cu2+ cationsin the solution from 0% up to about one-third (increase of theratio Mn(H2O)4(sac)2

2+/Cu(H2O)4(sac)22+ from 100/0 to 65/35),

mixed crystals form with linear dependence (correlation coef-ficient 0.997) of their composition on the cation ratio in theliquid phase, and copper concentration in the solids rangingfrom about 0% to 100% (Figure 2). Higher concentrationsof copper result in “saturation” and crystallization of pure[Cu(H2O)4(sac)2](H2O)2. Under the thermodynamically con-trolled conditions of our experiment, the mixed crystal with thehighest copper ratio of 63% Cu was obtained from solution with25% Cu. The result clearly shows that the inclusion of the JTdistorted guest Cu(H2O)4(sac)2

2+ in the lattice of the undistortedhost Mn(H2O)4(sac)2

2+ is preferred over the inclusion of theundistorted guest in the lattice of the distorted host. The stabilityof the host lattice is sustainable if up to 2/3 of its sites are

Table 1. Metal Content of the Solution Phase and in the MixedCrystals

m(Mn)/m(Cu)in solution

w(Mn), % w(Cu), %m(Mn)/m(Cu)

in crystalscalcd found calcd found

100:0 10.42 9.545 0.0 0.023 99.8:0.0299:1 10.32 8.42 0.12 0.528 94.7:5.397:3 10.11 8.79 0.36 0.580 94.6:5.495:5 9.90 7.82 0.60 1.20 87.4:12.675:25 7.79 3.26 3.00 5.63 36.9:63.165:35 6.74 0.128 4.20 10.42 1.4:98.650:50 5.17 0.137 5.98 9.14 1.7:98.325:75 2.58 0.089 8.94 10.40 1.0:99.05:95 0.513 0.087 11.28 10.15 1.0:99.03:97 0.308 0.077 11.51 11.65 0.8:99.21:99 0.102 0.094 11.75 11.17 1.0:99.00:100 0.0 0.058 11.86 12.64 0.5:99.5

1320 Crystal Growth & Design, Vol. 8, No. 4, 2008 Naumov et al.

replaced by distorted guests. This limiting value correspondsto statistical replacement of four Cu(H2O)4(sac)2

2+ out of sixsites around each Mn(H2O)4(sac)2

2+ ion.Crystals of X-ray diffraction quality were obtained and diffrac-

tion data were collected from seven crystals with Mn(H2O)4(sac)22+/

Cu(H2O)4(sac)22+ ratios (from the atomic absorption spectro-

metric analysis) of 100/0, 94.7/5.3, 94.6/5.4, 87.4/12.6, 1.4/98.6,1.7/98.3, and 1.0/99.0. The lattice parameters exhibit significantchanges with cation substitution (Figure 3). By increasingsubstitution of the manganese sites with copper, the monoclinicunit cell undergoes compression of about 6% along the c axiscompensated by expansion of about 5% along the a axis and ofca. 1% along the unique axis b. The balanced compression alongthe a axis and stretching, mostly along the c axis, results innearly constant cell volume, which may be one of the reasonsfor the ability of the crystal to sustain its integrity upon cationsubstitution.

3.2. Effect of Metal Substitution on the Molecular andCrystal Structure. The crystallographic data of the sevenanalyzed crystals are listed in Table 2. Although all preparedsolutions afforded crystalline compounds, some were of insuf-ficient quality for X-ray diffraction analysis, while otherscontained too little of the secondary component to be includedin the refined models. According to the metal content resultsand the lattice parameters calculated from the X-ray diffractiondata (Table 2), the crystalline material obtained from solutionswith ratios 65/35, 50/50, and 5/95 corresponds to the pure coppersaccharinate. Four mixed crystals (94.7/5.3, 94.6/5.4, 87.4/12.6,and 36.9/63.1) contained notable amounts of both Mn and Cu,but only three (94.7/5.3, 94.6/5.4, and 87.4/12.6) crystallizedas well-diffracting samples and were analyzed with X-raydiffraction. The higher copper content of the 87.4/12.6 crystal(12.6%) relative to the other mixed crystals resulted in clearlydiscernible residual features in the difference Fourier electrondensity map assignable to the minor JT-active component.Therefore a crystal from that batch of composition

[Cu0.126Mn0.874(H2O)4(C7H4NO3S)2](H2O)2 was selected forstructure refinement in the two-component model. An ORTEP-style plot of the final refined structure is presented, togetherwith the structure of the pure Mn host, in Figure 4.

As expected from the comparison of the lattice parametersof the pure host (Mn only) and guest (Cu only) crystals (Figure3), inclusion of the JT-active copper atoms in the host latticecauses expansion of the cell along the a-axis and contractionalong the c-axis, resulting in nearly constant volume. Thecrystallographic b-axis is also stretched, but due to its doubledlength compared with the other two axes, the relative effect istwice as small. In the molecular coordinate system, expansionalong the crystallographic a-axis corresponds to expansion ofthe coordination octahedron in the direction between N1, O1W,and O2W, while compression along c is related to compressionalong the bisector of MO2WO1W′. In the crystal of the puremanganese saccharinate host, the Mn-OW and Mn-N bondsin the coordination octahedron are of similar length, Mn(1)-O(1W) ) 2.1593(13) Å, Mn(1)-O(2W) ) 2.2204(14) Å, andMn1-N1 ) 2.2844(13) Å. In the pure copper saccharinatecrystal, Cu-O2W is “normal” at 2.4918(18) Å, but due to thestrong action of the JT effect, the other two bonds aresignificantly shortened, Cu-N1 ) 2.0594(15) and Cu-O1W) 1.9491(14) Å. The bond shortening results in distortion ofthe coordination octahedron.

In the structure of the mixed crystal, space-averaged overthe crystal volume (Figure 4), the orientations of the two ligandcomponents are slightly twisted around the M-N1 bond,appearing as very close positions of the carbonyl C atom anddifferent positions of all other atoms. Due to the diffuse electrondensity of their disordered counterparts, the water molecules inthe mixed crystal were treated as single-molecule models.Accordingly, as expected from the presence of both components,their protons feature large isotropic displacement parameters,the overall effect being more pronounced in the case of the O2Wmolecule. Actually, the oxygen atom of O2W exhibits increasedthermal ellipsoids within the coordination plane even in the hostcrystal. The relevant intramolecular parameters are listed inTables 3 and 4. The metal–ligand distances are significantlyaffected by the metal substitution: M-O1W, M-O2W, andM-N1 change from 2.1593(13), 2.2204(14), and 2.2844(13)Å to 2.125(3), 2.246(3), and 2.252(13) Å, respectively. The JT-active Cu ion results in distortion of the coordination polyhedronby increase of the difference between the lengths of the twometal-O bonds and shortening of the M-N bond, which isconsistent with the expectations based on the crystals of thepure host and guest crystals.

Based on extensive structure and spectra-structure correla-tions, it has been concluded17–19 that the structure of theexocyclic groups (CO and SO2) of the saccharinate reflects wellthe changes in electron density caused by the covalency of theM-N1 bond. As expected from the mainly covalent characterin both components, comparison of the geometry of theC(O)-N-S(O2) fragment in pure manganese and coppersaccharinate crystals shows that the intraligand structure is notsignificantly affected: O1-C1 ) 1.239(2), S1-O3 ) 1.4364(13),S1-O2 ) 1.4437(13), S1-N1 ) 1.6340(13), and N1-C1 )1.359(2) Å in the Mn crystal, and O1-C1 ) 1.237(2), S1-O3) 1.4347(14), S1-O2 ) 1.4347(14), S1-N1 ) 1.6559(15),and C1-N1 ) 1.359(2) Å in the Cu crystal. In line with theseobservations, the geometry of the sulfonyl group and of thecarbonyl group in the mixed crystal are only slightly modified(S-O ) 1.427(4) and 1.497(5) Å; C-O ) 1.239(5) Å).Although the carbonyl groups of the guest molecule are slightly

Figure 2. Plot of the molar ratio of copper and manganese cations inthe crystallization solution and in the respective mixed crystals obtainedby slow evaporation.

Figure 3. Relative change of the cell parameters, a axis (blue), b axis(black), c axis (red), and cell volume (green) with the composition (mol% copper) of the crystals.

Jahn–Teller Distortions in Mixed Crystals Crystal Growth & Design, Vol. 8, No. 4, 2008 1321

Tab

le2.

Cry

stal

logr

aphi

cD

ata

for

the

Pur

eH

ost

Cry

stal

[Mn(

H2O

) 4(s

ac) 2

]·2H

2Oan

dSi

xM

ixed

Cry

stal

s[C

u xM

n 1-

x(H

2O) 4

(C7H

4NO

3S) 2

](H

2O) 2

mol

era

tioof

copp

er(I

I)in

the

met

alco

nten

t(x

)

00.

053

0.05

40.

126

0.98

60.

983

0.99

0

Mn/

Cu

ratio

(sol

utio

n)10

0/0

99/1

97/3

95/5

65/3

550

/50

5/95

rela

tive

FW52

7.38

527.

3852

7.38

(528

.47)

535.

9853

5.98

535.

98te

mp,

K29

5(2)

293(

2)29

3(2)

293(

2)29

3(2)

293(

2)29

3(2)

wav

elen

gth,

Å0.

7107

30.

7107

30.

7107

30.

7107

30.

7107

30.

7107

30.

7107

3cr

ysta

lsy

stem

mon

oclin

icm

onoc

linic

mon

oclin

icm

onoc

linic

mon

oclin

icm

onoc

linic

mon

oclin

icsp

ace

grou

pP

2 1/c

P2 1

/cP

2 1/c

P2 1

/cP

2 1/c

P2 1

/cP

2 1/c

a,Å

7.96

52(5

)7.

9693

(8)

7.96

4(7)

7.97

77(7

)8.

3837

(7)

8.38

37(5

)8.

3889

(10)

b,Å

16.1

469(

10)

16.1

474(

15)

16.1

35(1

4)16

.132

7(15

)16

.332

1(13

)16

.332

3(10

)16

.327

6(19

)c,

Å7.

7881

(5)

7.78

30(7

)7.

799(

7)7.

7742

(7)

7.33

69(6

)7.

3379

(4)

7.33

31(9

)�,

deg

99.6

990(

10)

99.7

69(2

)99

.836

(15)

99.9

64(2

)10

1.07

70(1

0)10

1.07

10(1

0)10

1.11

8(2)

V,

Å3

987.

34(1

1)98

7.02

(16)

987.

4(15

)98

5.46

(15)

985.

88(1

4)98

6.04

(10)

985.

6(2)

Z2

22

22

22

calc

dde

nsity

,g·c

m-

31.

774

1.77

51.

774

1.59

21.

806

1.80

51.

806

abs

coef

ficie

nt,

mm

-1

0.94

90.

950

0.94

90.

796

1.38

91.

389

1.38

9cr

ysta

lsi

ze,

mm

30.

30×

0.20

×0.

180.

30×

0.26

×0.

100.

20×

0.16

×0.

140.

30×

0.20

×0.

100.

40×

0.30

×0.

040.

38×

0.20

×0.

160.

30×

0.20

×0.

20θ

rang

e,de

g2.

52–2

7.49

2.52

–27.

492.

52–2

7.50

2.52

–27.

502.

48–2

7.50

2.48

–27.

502.

47–2

7.49

limiti

ngin

dice

s-

10e

he

10-

10e

he

10-

10e

he

9-

10e

he

10-

10e

he

10-

10e

he

10-

8e

he

10-

10e

ke

20-

10e

ke

20-

20e

ke

20-

20e

ke

20-

21e

ke

20-

21e

ke

21-

21e

ke

18-

10e

le

10-

10e

le

10-

4e

le

9-

10e

le

10-

9e

le

9-

9e

le

9-

9e

le

9re

flns

colle

cted

/uni

que

6009

/224

2(R

int)

0.01

78)

1124

3/22

58(R

int)

0.06

86)

6079

/224

1(R

int)

0.04

83)

1094

4/22

54(R

int)

0.06

42)

1117

6/22

43(R

int)

0.06

56)

1124

2/22

54(R

int)

0.06

77)

7602

/224

6(R

int)

0.06

48)

refin

emen

tm

etho

dFM

LS

onF

2FM

LS

onF

2FM

LS

onF

2FM

LS

onF

2FM

LS

onF

2FM

LS

onF

2FM

LS

onF

2

data

/res

trai

nts/

para

ms

2242

/10/

182

2258

/6/1

6622

41/6

/166

2254

/25/

118

2243

/6/1

6622

54/6

/166

2246

/6/1

66G

OF

(F2)

1.03

81.

058

1.06

10.

975

1.08

51.

081

1.05

7fin

alR

indi

ces

[I>

2σ(I

)]R

1)

0.02

86;

wR

2)

0.07

42R

1)

0.02

66;

wR

2)

0.07

18R

1)

0.03

45;

wR

2)

0.08

21R

1)

0.05

63;

wR

2)

0.14

69R

1)

0.02

96;

wR

2)

0.07

94R

1)

0.03

16;

wR

2)

0.08

39R

1)

0.03

44;

wR

2)

0.08

50R

indi

ces

(all

data

)R

1)

0.03

10;

wR

2)

0.07

58R

1)

0.02

66;

wR

2)

0.07

18R

1)

0.04

29;

wR

2)

0.08

57R

1)

0.06

14;

wR

2)

0.15

05R

1)

0.03

10;

wR

2)

0.08

04R

1)

0.03

42;

wR

2)

0.08

52R

1)

0.03

92;

wR

2)

0.08

74la

rges

tdi

ff.

peak

and

hole

,e·Å

-3

0.32

4/-

0.36

50.

325/-

0.48

40.

525/-

0.41

21.

474/-

1.01

60.

340/-

0.59

30.

351/-

0.38

60.

343/-

0.79

8

1322 Crystal Growth & Design, Vol. 8, No. 4, 2008 Naumov et al.

stretched (1.268(18) Å) as a result of the change of the covalentnature of the metal–ligand coordination bond, compared withthe effect on the metal–ligand bonds, the overall effect on theendocyclic geometry of the saccharinate ligand is much smaller.

The UV–vis spectra of the mixed crystals with Mn/Cu ratiosof 0.8/99.2, 1.0/99.0, 1.4/98.6, and 1.7/98.3 are deposited asSupporting Information (Figure S1). The pure manganesecomplex does not have an absorption band in the visible region.The blue color of the copper-containing samples is due to thebroad d-d transition band with a maximum in the 730–770nm region. Increase of the concentration of manganese of 1%in the copper(II) saccharinate crystals results in shift of theabsorption maximum of several tens of nanometers, decreasedband intensity, and shift of the UV absorption edge. Theseobservations represent a joint result of the decreased strain ofthe ligand field around the Jahn–Teller-distorted copper(II) ion,which affects the energy difference of the electronic d-levels,and decreased population of the light-absorbing species.

3.3. The Inherent Jahn–Teller Instability and VibronicInteractions. To provide a more profound insight into thestructural and especially spectroscopic properties of the titlemixed crystals, certain specific aspects related to vibronicinteractions in the constituent species must be taken into account.Actually, complexes of high-spin ionic species such as Cr2+

(d4) and Cu2+ (d9) are typical examples of the validity of theJahn–Teller theorem in practice. This is due to the odd numberof eg electrons, which in an idealized octahedral environmentwould give rise to E electronic state. According to Jahn andTeller,20 as a result of the breakdown of the Born–Oppenheimer(BO) approximation, there will be a nuclear motion leading toremoval of the degeneracy of the electronic states, the overalleffect of which will be the lowering of the energy of the system.Geometrically, a distortion of the ideal octahedron occurs.Accounting for the symmetry of the electronic state (E), the

coupled nuclear mode through which the removal of degeneracywill occur must be of e symmetry. Such systems with coupleddegenerate states are usually named as E X e or Jahn–Teller(JT) or vibronic systems (due to the vibrational-electroniccoupling, which is in the essence of the effect). In second-quantization formalism, the E X e coupling Hamiltonian isusually written in the form

hEXe ) ∑R)(

ω0(aR+aR+

12)+ gω0∑

R)((a-R+ aR

+)c-R+ cR (1)

where the generation/annihilation operators âR and cR correspondto the phonon and electron degenerate states, correspondingly.When the environment of the JT central ion is not ideallyoctahedral (e.g., MY6), that is, when one deals with, for example,complexes of the type MY4Y2′, the point symmetry is lowerand degenerate representations do not occur. However, thesesystems are structurally very similar to the totally substituted“parent” compounds, where degenerate electronic states arepossible at the undistorted geometry, and one intuitively expects

Figure 4. ORTEP-style plots (50% probability level) of the crystalstructures of the pure host crystal [Mn(H2O)4(sac)2] ·2H2O (top) andof the mixed crystal [Cu0.126Mn0.874(H2O)4(C7H4NO3S)2](H2O)2 (bottom).

Table 3. Selected Bond Distances (Å) and Angles (deg) in theStructure of the Pure Host Crystal [Mn(H2O)4(sac)2] ·2H2Oa

Mn(1)-O(1W)#1 2.1593(13)Mn(1)-O(1W) 2.1593(13)Mn(1)-O(2W) 2.2204(14)Mn(1)-O(2W)#1 2.2204(14)Mn(1)-N(1) 2.2844(13)Mn(1)-N(1)#1 2.2844(13)S(1)-O(3) 1.4364(13)S(1)-O(2) 1.4437(13)S(1)-N(1) 1.6340(13)S(1)-C(7) 1.7498(16)N(1)-C(1) 1.359(2)O(1)-C(1) 1.239(2)O(1W)-H(1W1) 0.848(10)O(1W)-H(1W2) 0.845(10)O(2W)-H(2W1) 0.844(10)O(2W)-H(2W2) 0.842(10)O(3W)-H(3W1) 0.841(10)O(3W)-H(3W2) 0.844(10)O(1W)#1-Mn(1)-O(1W) 180.0O(1W)#1-Mn(1)-O(2W) 90.63(6)O(1W)-Mn(1)-O(2W) 89.37(6)O(1W)#1-Mn(1)-O(2W)#1 89.37(6)O(1W)-Mn(1)-O(2W)#1 90.63(6)O(2W)-Mn(1)-O(2W)#1 180.0O(1W)#1-Mn(1)-N(1) 86.75(5)O(1W)-Mn(1)-N(1) 93.25(5)O(2W)-Mn(1)-N(1) 93.26(5)O(2W)#1-Mn(1)-N(1) 86.74(5)O(1W)#1-Mn(1)-N(1)#1 93.25(5)O(1W)-Mn(1)-N(1)#1 86.75(5)O(2W)-Mn(1)-N(1)#1 86.74(5)O(2W)#1-Mn(1)-N(1)#1 93.26(5)N(1)-Mn(1)-N(1)#1 180.0O(3)-S(1)-O(2) 116.11(8)O(3)-S(1)-N(1) 110.85(8)O(2)-S(1)-N(1) 109.63(7)O(3)-S(1)-C(7) 111.13(8)O(2)-S(1)-C(7) 110.54(8)N(1)-S(1)-C(7) 96.92(7)Mn(1)-O(1W)-H(1W1) 117.7(17)Mn(1)-O(1W)-H(1W2) 101.6(18)H(1W1)-O(1W)-H(1W2) 107(2)Mn(1)-O(2W)-H(2W1) 131(2)Mn(1)-O(2W)-H(2W2) 117.5(17)H(2W1)-O(2W)-H(2W2) 112(3)H(3W1)-O(3W)-H(3W2) 108(3)C(1)-N(1)-S(1) 111.10(11)C(1)-N(1)-Mn(1) 129.22(11)S(1)-N(1)-Mn(1) 119.56(7)

a Symmetry code: (#1) -x + 1, -y + 1, -z +1.

Jahn–Teller Distortions in Mixed Crystals Crystal Growth & Design, Vol. 8, No. 4, 2008 1323

that effects reminiscent of the JT effect should appear in thesecases, too. In cases when the first-order JT theorem does notapply (due to the lack of actual degeneracy) but the states arenearly degenerate, higher-order variants of the JT theorem arein fact applicable. These often allow for rationalization of thestructural results. Just as an illustrative example, a perturbationtheoretic expression for the energy of the ground electronic statesreads

E0 ) ⟨0|Hq|0⟩q+ 12[⟨0|Hqq|0⟩ - 2∑

n

|⟨0|Hq|n⟩ |2

∆E0n ]q2 (2)

In eq 2, q is the distortion coordinate, whereas Hq and Hqq arethe first and second derivatives of the electronic Hamiltonianwith respect to q. While the first term on the right-hand side ofeq 2, containing q as a multiplier is the true or first-order JTcontribution (nonzero only in the case of degenerate electronicstates), the second term, containing q2, is responsible for the

second-order or pseudo-JT contribution. It is exactly this termthat leads to a stabilization of the system upon distortionprovided that there exists a low-lying state n (i.e., the value of∆E0n is very small) having the “correct” symmetry so that⟨0|q|n⟩ has a nonzero value.

It is exactly this second-order (or pseudo-) JT contri-bution that governs the structural characteristics of the[Cu(H2O)4(sac)2] ·2H2O compound (a d9 system), as discussedin our previous studies.9,10,21,22 We have demonstrated that thestructural changes of the coordination sphere around the centralCu2+ ion mostly influence the hydrogen-bonding network inthis member of a series of isomorphous [M(H2O)4(sac)2] ·2H2Ocompounds. Due to the structural differences between thiscompound and the other members of the isomorphous series,certain spectroscopic manifestations of the second-order JTeffect were observed and discussed in our previous papers.9,10,21,22

Similarly to the case of [Cu(H2O)4(sac)2] ·2H2O, also the

Table 4. Selected Bond Distances (Å) and Angles (deg) in the Structure of the Mixed Crystal [Cu0.126Mn0.874(H2O)4(C7H4NO3S)2](H2O)2 (M )Mn or Cu)a

M(1)-O(1W) 2.125(3) O(2W)#1-M(1)-N(1′) 84.8(5)M(1)-O(1W)#1 2.125(3) N(1′)#1-M(1)-N(1′) 180(4)M(1)-N(1)#1 2.252(13) O(1W)-M(1)-H(2W2) 91(2)M(1)-N(1) 2.252(13) O(1W)#1-M(1)-H(2W2) 89(2)M(1)-O(2W) 2.246(3) N(1)#1-M(1)-H(2W2) 98.6(19)M(1)-O(2W)#1 2.246(3) N(1)-M(1)-H(2W2) 81.4(19)M(1)-N(1′)#1 2.28(9) O(2W)-M(1)-H(2W2) 11.7(19)M(1)-N(1′) 2.28(9) O(2W)#1-M(1)-H(2W2) 168.3(19)M(1)-H(2W2) 2.92(6) N(1′)#1-M(1)-H(2W2) 96.5(19)O(1W)-H(1W2) 0.852(10) N(1′)-M(1)-H(2W2) 83.5(19)O(1W)-H(1W1) 0.853(10) M(1)-O(1W)-H(1W2) 106(4)O(2W)-H(2W1) 0.849(10) M(1)-O(1W)-H(1W1) 123(4)O(2W)-H(2W2) 0.851(10) H(1W2)-O(1W)-H(1W1) 105(5)C(1)-O(1) 1.239(5) M(1)-O(2W)-H(2W1) 119(7)C(1)-O(1′) 1.268(18) M(1)-O(2W)-H(2W2) 136(8)C(1)-N(1′) 1.36(9) H(2W1)-O(2W)-H(2W2) 103(9)C(1)-N(1) 1.365(13) O(1)-C(1)-O(1′) 15.6(9)S(1)-O(2) 1.427(4) O(1)-C(1)-N(1′) 124(2)S(1)-O(3) 1.497(5) O(1′)-C(1)-N(1′) 126(2)S(1)-N(1) 1.632(7) O(1)-C(1)-N(1) 122.9(5)S(1)-C(7) 1.763(5) O(1′)-C(1)-N(1) 123.6(9)N(1′)-S(1′) 1.66(4) N(1′)-C(1)-N(1) 3.9(11)O(2′)-S(1′) 1.485(18) O(1)-C(1)-C(2) 123.9(4)S(1′)-O(3′) 1.12(2) O(1′)-C(1)-C(2) 121.0(9)S(1′)-C(7′) 1.714(17) N(1′)-C(1)-C(2) 112(2)O(3W)-H(3W2) 0.848(10) N(1)-C(1)-C(2) 113.2(5)O(3W)-H(3W1) 0.851(10) O(1)-C(1)-C(2′) 125.0(10)O(1W)-M(1)-O(1W)#1 180.0 O(1′)-C(1)-C(2′) 120.6(13)O(1W)-M(1)-N(1)#1 87.1(2) N(1′)-C(1)-C(2′) 111(3)O(1W)#1-M(1)-N(1)#1 92.9(2) N(1)-C(1)-C(2′) 112.0(11)O(1W)-M(1)-N(1) 92.9(2) C(2)-C(1)-C(2′) 5.1(6)O(1W)#1-M(1)-N(1) 87.1(2) O(2)-S(1)-O(3) 119.1(2)N(1)#1-M(1)-N(1) 180.0(5) O(2)-S(1)-N(1) 110.8(3)O(1W)-M(1)-O(2W) 89.15(11) O(3)-S(1)-N(1) 107.7(3)O(1W)#1-M(1)-O(2W) 86.90(17) O(2)-S(1)-C(7) 111.3(3)N(1)-M(1)-O(2W) 93.10(17) O(3)-S(1)-C(7) 108.5(3)O(1W)-M(1)-O(2W)#1 90.85(11) N(1)-S(1)-C(7) 97.3(5)O(1W)#1-M(1)-O(2W)#1 89.15(11) C(6)-C(7)-C(2) 124.4(4)N(1)#1-M(1)-O(2W)#1 93.10(17) C(6)-C(7)-S(1) 129.0(4)N(1)-M(1)-O(2W)#1 86.90(17) C(2)-C(7)-S(1) 106.6(4)O(2W)-M(1)-O(2W)#1 180.0 C(1)-N(1)-S(1) 110.8(8)O(1W)-M(1)-N(1′)#1 87.9(16) C(1)-N(1)-M(1) 129.5(5)O(1W)#1-M(1)-N(1′)#1 92.1(16) S(1)-N(1)-M(1) 119.6(7)N(1)#1-M(1)-N(1′)#1 2.2(7) C(1)-N(1′)-S(1′) 113(5)N(1)-M(1)-N(1′)#1 177.8(7) C(1)-N(1′)-M(1) 128(3)O(2W)-M(1)-N(1′)#1 84.8(5) S(1′)-N(1′)-M(1) 119(5)O(2W)#1-M(1)-N(1′)#1 95.2(5) O(3′)-S(1′)-O(2′) 96.5(16)O(1W)-M(1)-N(1′) 92.1(16) O(3′)-S(1′)-N(1′) 121(3)O(1W)#1-M(1)-N(1′) 87.9(16) O(2′)-S(1′)-N(1′) 111.4(16)N(1)#1-M(1)-N(1′) 177.8(7) O(3′)-S(1′)-C(7′) 123.6(16)N(1)-M(1)-N(1′) 2.2(7) O(2′)-S(1′)-C(7′) 109.2(11)O(2W)-M(1)-N(1′) 95.2(5) N(1′)-S(1′)-C(7′) 95(4)

a Symmetry code: (#1) -x + 1, -y + 1, -z + 1.

1324 Crystal Growth & Design, Vol. 8, No. 4, 2008 Naumov et al.

corresponding chromium compound [Cr(H2O)4(sac)2] ·2H2Ocontains a JT d4 ion in a nonideal octahedral environment. X-raycrystallographic studies of [Cr(H2O)4(sac)2] ·2H2O12 inevitablyshowed the manifestation of the second-order JT effect in thissolid-state system. The coordination sphere around the centralCr2+ ion is characterized by a centrosymmetric arrangement oftwo nitrogen atoms of saccharinate ligands and four oxygenatoms from coordinated water molecules. As a direct conse-quence of the breakdown of the BO approximation to thesecond-order in a perturbation-theoretic sense, this arrangementmanifests deviations from the regular pattern found in the othermembers of the series, similar to the case of the copper analogue:one M-O and the M-N bond are significantly shorter, whilethe other M-O bond is much longer. Such distortions in the caseof the latter compound are two times smaller than the corre-sponding geometrical changes in the copper analogue. Due to

the relative flexibility of the hydrogen-bonding network in thecrystals, the distortions within the coordination sphere of thecentral ion cause significant changes in the proton donor-protonacceptor distances.

As mentioned above, the compounds of the type[M(H2O)4(sac)2] ·2H2O form an isomorphous series. Sincemixed crystals containing various metal ions may be relativelyeasily obtained, this series of isomorphous compounds is actuallyvery suitable for studying the cooperative vibronic effects, whichappear when a Jahn–Teller active ion is present as a substitutent(forming a substituent-type solid solution) in a non-Jahn–Tellermatrix. According to the previous discussion, in the present casewe actually deal with cooperative pseudo-Jahn–Teller effect.The coupling between JT centers involves elastic flexibility ofthe host lattice, so that in a sense, matrix isolation of JT ions ina non-JT matrix may be used as a probe to test the matrix elasticproperties. Accounting for the relatively complex hydrogen-bonding network in the studied compounds and the mentioneddifferences in such network between JT-active and JT-inactivecompounds, we expect that the cooperative pseudo-JT effectwill mostly affect the hydrogen-bonding interaction in the hostmatrix. Aside from the approaches involving crystal structuredetermination, perhaps the easiest and the most straightforwardmethod for probing the hydrogen-bonding network is vibrationalspectroscopy, which could certainly shed some light at leaston the range of hydrogen bond strength in the studied com-pounds. As a test system, the Cu-doped [Mn(H2O)4(sac)2] ·2H2O,was chosen, and a series of mixed crystals of the form[Mn1-xCux(H2O)4(sac)2] ·2H2O where x ranges from 0 to 1 werestudied.

3.4. Vibrational Spectroscopy of the O-H StretchingModes. In Figure 5, the O-H stretching region in the LT FTIRspectra of the protiated analogues of [Mn(H2O)4(sac)2] ·2H2Oand [Cu(H2O)4(sac)2] ·2H2O are presented. The highest andlowest frequency bands in the case of the copper compoundappear at about 3560 and 2980 cm-1 respectively. The corre-sponding numbers in the case of the manganese compound are3480 and 3230 cm-1, respectively. These spectroscopic dataclearly indicate that the hydrogen-bonding network in the caseof the copper compound involves a much wider range ofhydrogen bond strengths. In Figure 6, the O-H stretching regionin the LT FTIR spectra of several [Mn1-xCux(H2O)4(sac)2] ·2H2Omixed crystals with varying x are shown (for protiated com-pounds). Comparison with the corresponding spectral region inthe LT FTIR spectra of the pure copper compound invariablyshows that the hydrogen bonding network with wide extensionof hydrogen bond strengths characteristic for the case of the JTcopper compound is achieved in the manganese host matrix evenat values of x of 0.35. This observation represents clearmanifestation of the cooperative JT effect in the O-H stretchingvibrational spectra of solid hydrates. To the best of ourknowledge, this is the first reported evidence of the phenomenon.The spectroscopic manifestation of the effect is so remarkablethat all other possible reasons for rearrangement of the hydrogen-bonding network (e.g., variations of the lattice constants due tothe inclusion of the guest cation with different ionic radius, etc.)may be ruled out. This is a very good example showing thestructural flexibility of a non-JT host crystal lattice and its abilityto accommodate to the pseudo-JT deformations of guest JT ioniccenters through rearrangement of the hydrogen-bonding network.The result can be relevant for application in the design of newcrystals with exotic properties within the frame of the crystalengineering.

Figure 5. The O-H stretching region in the LT FTIR spectra of theprotiated analogues of [Cu(H2O)4(sac)2] ·2H2O (upper curve) and[Mn(H2O)4(sac)2] ·2H2O (lower curve).

Figure 6. The O-H stretching region in the LT FTIR spectra of theprotiated analogues of several [Mn1-xCux(H2O)4(sac)2] · 2H2O mixedcrystals, where x is 0 (a), 0.35 (b), 0.50 (c), and 1 (d).

Jahn–Teller Distortions in Mixed Crystals Crystal Growth & Design, Vol. 8, No. 4, 2008 1325

Acknowledgment. We thank Prof. S. W. Ng (University ofMalaya) for the useful discussions on the crystallography, Dr.T. Fujita (National Institute for Materials Science) for recordingthe UV–vis spectra, and Dr. Xiang Gao Meng from the CentralChina Normal University for the collection of the X-ray data.

Supporting Information Available: Plot of the UV-visible spectra.This material is available free of charge via the Internet at http://pubs.acs.org.

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