A re-examination of the infrared spectra of first transition series metal(II) and metal(III) tropolonates

Spectrochimica Am,

Vol. 45A, No. II, pp.

Printedin Great Britain.

0584.8539189 %3.00+0.00

1179-1186, 1989. 0

1989 Pergamon Press plc

A re-examination of the infrared spectra of first transition series metal (II) and metal (III) tropolonates KATHRYN Department

J. BURDEN,

of Inorganic

DAVID

Chemistry,

A. THORNTON

University

(Received 22 February 1989; infinalform

and

GARETH

M. WATKINS

of Cape Town, Rondebosch

7700, South Africa

13 May 1989; accepted 13 June 1989)

Abstract-The infrared spectra of first transition series metal(U) and metal tropolonates, [MT,], (T = tropolonate anion, M = Mn, Co, Ni, Cu, Zn) and [MT,] (M = SC, Ti, V, Cr, Mn, Fe, Co, Ga) have been re-examined with extension of the range to 1600-50 cm-‘. The metal sensitivities of the i.r. bands have been determined in relation to the change in CFSE which accompanies metal ion substitution. Those bands which exhibit maximum sensitivity towards metal ion substitution are also those which have previously been found most sensitive towards metal isotope substitution. They are assigned to vM-0. The metal-ligand bending modes are some 75% less sensitive to metal ion substitution. Bands with frequencies > 400 cm-’ (which are less metal sensitive and have previously been assigned to pure vM-0 modes) are probably vM-0 coupled with skeletal ring deformations. Band splitting in the spectrum of [MnT,] is discussed in relation to Jahn-Teller distortion.

INTRODUCTION

The i.r. spectra of metal complexes of tropolone have received less attention than those of acetylacetone and related /?-ketoenols. Nevertheless, they have been studied by three different techniques: ‘so-labelling Cl], metal ion substitution [2] and metal isotope labelling [3] with a view particularly to obtaining satisfactory assignments for the metal-oxygen stretching modes. Unfortunately, the instrumental facilities available to the various workers precluded measurements beyond 400, 300 and 150cm-’ for the three respective techniques used. In the metal isotope labelling study, HUTCHINSON et al. [3] have assigned two bands within the range 25@400 cm- ’ to vM-0 on the grounds that these bands are far more sensitive to metal isotope labelling than any other spectral bands. In the 180-labelling study, JUNGE [l] found that two bands between 500 and 600 cm- ’ were shifted most by 180-labelling of copper(H) tropolonate and were therefore assigned to the principal vCu-0 bands. However, since their measurements did not extend below 400 cm- I, the “O-shifts of low frequency bands are not available. Furthermore, although HULETT and THORNTON [2] found that the bands corresponding with those assigned by JUNGE to vCu-0 in a series of 14 metal tropolonates exhibited the maximum sensitivity to the metal ion, their study also did not extend to the vM-0 bands assigned by HUTCHINSON et al. Availability of superior instrumental facilities has now enabled us to re-investigate these spectra down to 50 cm-‘. EXPERIMENTAL

The complexes were synthesized by the reported methods [Z]. Purity was established by microanalysis (C,H). Infrared spectra were determined on Nujol mulls between caesium iodide discs from 2000 to 1500 and 1300 to 200 cm - ’ and as hexachlorobutadiene mulls from 1500 to 1300 cm-’ on a Perkin-Elmer 983 spectrophotometer and on Nujol mulls

between polyethylene discs from 500 to 50 cmFTS 16B/D interferometer.

’ on a Digilab

RESULTS AND DISCUSSION

The spectra are depicted in Fig. 1 and the frequencies (together with those previously reported) and assignments are given in Tables 1 and 2. Tracings of representative spectra (MnT, and ScT,) are shown in Figs 2 and 3. Assignments of bands in the spectra of the free ligand are based on the report by REDINGTON and REDINGTON [4]. In the spectra of the complexes we recognize the following features. There are several bands below 8OOcm-’ which are shifted by substitution of the metal ion. Comparison with the spectrum of the free ligand (also shown in Fig. 1) suggests that some of these bands are ligand modes which have acquired metal sensitivity by coordination while others occur in regions free from ligand absorption. Bands between 660 and 180 cm- ’ which are significantly affected by metal ion substitution (in order of decreasing shifts) are those numbered Vet, v3,, vZ4 5 v16. All but vZ6 occur in a region free from ligand absorption and therefore qualify as relatively uncoupled vM-0 bands. From the reported metal isotope shift data [3], it is clear that vS1 and v32 are also far more sensitive than towards isotopic substitution, the obv24 and v26 served shifts being within the range 2.0-6.1 cm-‘. Hence, the shifts which occur on metal isotope labelling and metal ion substitution are in excellent agreement with respect to assigning the bands vJ, and v32 to the principal vM-0 modes, with vZ6 probably being the third i.r.-active metal-ligand stretching mode required for these complexes in terms of their D, point group symmetry. However, the vz6 band is almost certainly coupled with the skeletal C-C-C/C-C-O deformation at 535 cm-’ in the ligand spectrum [4]. The ‘*O-labelling study did not 1179

KATHRYNJ. BURDENet al.

1180

I MT2I,,

Ti V

Mn

Ga

Fig. 1. Infrared

spectra

of metal(H)

extend beyond 400 cm- ’ so that none of the presentlyassigned vM-0 bands occur within the range studied. However, the observed shift of 26 cm- ’ on **Olabelhng of the band at 587 cm-’ in copper tropolonate (corresponding with v& is close to the theoretical ‘*O-shift of an isolated diatomic molecule Cu-0 of a (Av,,,r: = 27 cm- ‘), so that the ‘80-sensitivity band of lower frequency (i.e. vjl or VJ is hardly likely to be much closer to the theoretical shift than that of vz6. Nevertheless, an extension of the ‘*O-labelhng study to frequencies below 4OOcm-’ would be of interest, especially in view of the fact that the observed isotopic shifts often exceed the theoretical values calculated from the diatomic molecule relationship.

and metal(II1)

The

tropolonate

observed

complexes

metal

isotope shifts reported by are a case in point. For instance, the theoretical shift in a vCu-0 band at 332 cm-’ resulting from substitution of 65Cu for 63Cu is 1.1 cm-’ whereas the observed shift is 2.7 cm-‘. This appears to be a general feature of metal isotope shifts of vibrationally pure metal-ligand modes. HUTCHINSON et al. [3]

Evaluation of metal sensitivities The metal sensitive bands in the spectra of the metal tropolonates are in the Irving-Williams stability sequence Mn < Fe Zn [S]. Except for the position of copper, this is also the sequence of crystal field stabilization energies (CFSEs)

1181

First transition series metal(H) and metal(II1) tropolonates Table 1. Infrared frequencies for [MT,], complexes (cm-‘) Mn

co

Nil

1594 1571 1513 1471 1434 1409 1383 1334 1248 1222 1074 1005 987 974 924 918 876 867 763 734 696 583

1590tt 1569 1510 1470 1430 1414 1379 1335 1247 1235 1219 1074 1015 996 912 934 915 874 860 760 726 698 595

535 436 376

535 516 413

545 530 416

347

379 265 252 213

383 268 255 217 185 159 138 128 104 12 62

1594 1569 1513 1469 1430 1410tt 1382 1335 1247 1232 1217 1074 1011 984 971 926 914 877 856 758 728 699 609 (0.1) 594(n.o.)$f 56O(n.o.)$f 542(-H 422(0.3) 408 (0.0) 389(-)§§ 291(6.1) 267(3.6) 228 185 163 134 (134)

HTt 1613(2) 1547(11) 1470(3) 1437(4) 1315(7) 1298(2) 1266(7) 1249(6) 1209 (3) 955 (2)

92 l(0) 874(l) 859(2) 768(l) 713(O) 676(-)

199 122 12

147 135 120 88

1590{1}

1594

1515 (2) 1469 (2) 1425 1412 1369(3} 1344(16) 1247tt (2) 1231{4} 1220{3} 1076 {3) 999 {O)

1517 1472 1434 1411 1350 1336 1251

z

Assignment 11 II

Znll

Cu§

[:I

918 (O} 875 (1) 851(O) 753{1) 732{+2} 712{ 14) 636(0.4) {21} 587 (0.3) 126) 423(0.1)(6) 4OtWO.3) {5) 371 (n.o.)$S 332 (2.7) 308(1.9) 223(1.1) 185(n.o.)$j (l85)(n.o.W 136 161(1.0) 110 85 64

“CC “C-O +

vc-c

“C-c

K-H X-0

broad

6C-H 1222 1075 1005 988 974 934 918 877

“CC +X-H

762 733 697 (0.3) 593(-)&j

k-H

553 (0.4) 532 (0.4) 4 17(0.2) 379 (0.0) 254(n.o.)$f 235(3.5) 189(2.9) 213(-)

sccc/acco coupled vM-0

159(1.4) 129 148(1.1) 114 82 65

oop

“M-0 coupled “M-0

I 6 ring 1 “M-0 I y ring i +60-M-O 60-M-0 y ring 60-M-0 i 1 lattice

t Figure in parentheses is the low-frequency shift on ‘*O-labelling [l]. $Figure in parentheses is the frequency shift on 58~62Ni-labelling [3]. §Figure in parentheses is the frequency shift on 63*65Cu-labelling [3] and figure in braces is the low-frequency shift on ‘80-labelling [l]. jlFigure in parentheses is the frequency shift on 64~68Zn-labelling [3]. ttMean of doublet. ISBand not reported in metal isotope labelling study [3]. @Metal isotope shift could not be measured accurately due to low intensity of the band [3]. (1IIInternal ligand mode assignments are those of REDINGTON and REDINGTON [4].

[6]. The fact that copper has the highest vM-0 value is a reflection of its lower coordination number in [CUT,]. All of the other metal (II) tropolonates exhibit polymeric octahedral coordination [2]. The metal sensitivities are determined by plotting the band frequencies against d-orbital population. The points for Mn(II) and Zn(II) (which have zero CFSE) are joined to yield an interpolation line which represents the frequencies (vO) “free from crystal field stabilization” [6]. The difference between the observed and interpolated frequencies (v - ve) is a measure of the crystal field effect. Values of (v-ve) for v14, vz6, vJ, and vj2 are shown in Table 3 from which it is clear that vjl and

vj2 exhibit the highest sensitivities to the changes in CFSE as one metal ion replaces another. Figure4 shows the plot of frequency against d-orbital population for the most metal-sensitive band, vS2. Metal-ligand

bending modes

At frequencies below those of vM-0, the 6 ring ligand band at 199 cm - ’ in the spectrum of tropolone exhibits some metal-sensitivity and recurs in the spectra of the complexes within the range 210-260 cm-’ (v&. This band is therefore assigned to the 6 ring mode coupled with SO-M-O. Below this frequency range it is possible to identify two bands in the spectra

1182

KATHRYNJ. BURDENet al. Table 2. Infrared frequencies of [MT,] complexes (cm-‘) SC

Ti

V

1590 1518

1586 1517

1427 1413 1358 1342 1260 1233 1210 1076 1019

1430 1417 1348 1329 1261 1230 1210 1079 1018

1346 1320 1261 1233 1209 1079 1017

973 956

970 955

970 953

875

876

875

754 720 709 594 582

753 730 718

753 730 721

591

619

crt

Mn

FeS

co

Ga

Assignmentjj

1589 1517

1586 1512

1586 1512

1585 1513

1588 1517

“C-C vca + “C-C

1425

1419 1410 1350 1342 1255 1234 1220 1076 1010 993 970 943 919Q: 877

1586 1513 1466 1424

1408

1330 1312 1255 (1255) 1212 1074 1012

1424 1413 1353 - (sh)

VC-C N-H vC-0 (broad)

968 941 927 875

1354 1343 1259 1235 1217 1075 1019 1005 968 954 927 875

1424 1410 1347

764 735 727

754 727 705

755 739 709

625 (0.4)

627 608

592(-)/l

8605

587 537 446 428 410

552 446 421 410

561 (n.o.)tt 429 (0.5) 4OO(n.o.)tt

421 (421)

343 277

346 306

365 319

360(2.9) 332(2.0)

221 186 149

229 195 161

226 183 160

231 210 173

132

126 113 64

129 117 62

152

415 340 334 265 205 177 (177) 167 135

60

57

418

“31 “32 “32‘2 “33 “34 “35 “350 ‘36 “38

67 53

v39 “40

759 737 724 674 656

1256 1224 1263 1078 1019 1004 970 935 927 875 866 776 734§ 711

X-H i “C-C +X-H

X-H oop i { ~ccc/scco coupled “M-0 vMn-0 xtra bd sccc/acco

605

6295 579 524

541

1269 1256 1215 1078 1012 999 970 937 918 878

550 (0.2)

584

578

415 (0.0)

452 420

421 409 385

313 (5.0) 258(4.2) .

402 371

287 230

221(1.6) 189 160

259 176 (176)

258 188 151

142 109 64

145 115 70

133 108 65

vMn-0 xtra bd coupled vM_O c

I

6 ring vMn-0 vM-0

xtra bd

vMn-0 xtra bd y ring + 60-M-0 \ 60-M-0 60-Mn-0 xtra bd? y ring 60-M-0 lattice I

tFigure in parentheses is the frequency shift on 50.53Cr-labelling [3]. $ Figure in parentheses is the frequency shift on 54~57Fe-labelling [3]. $Mean of doublet. I/Metal isotope shift could not be measured accurately due to low intensity of the band [3]. ttBand not reported in metal isotope labelling study [3]. $$.Internal ligand mode assignments are those of REDINGTON and REDINGTON [4].

of the metal

complexes (vS5 and vS7) and one band in the spectra of the metal(II1) complexes (v& which are significantly metal sensitive in the CFSE sequence and therefore qualify for assignment to the 60-M-0 bending modes. Table 2 also includes the (v-~6) values of the more sensitive of the 60-M-0 bands (v&. It is observed that the mean value of (v - vO) for both the metal(I1) and metal(II1) series is approximately 25% of the mean value of (v - ~6) for the most metal sensitive vM-0 band (Vet). In the single example of a 60-M-0 band for which its sensitivity to a heavier metal isotope was determined (Vet in ZnT,),

the observed shift (Av = 1.4 cm-‘) was also considerably smaller than that observed for the most sensitive

vZn-0 band (Av = 3.5 cm- ‘). Reduced sensitivity of metal-ligand bending modes (relative to the stretching modes) on metal ion substitution or metal isotope labelling appears to be a general feature of the i.r. spectra of metal complexes. The spectrum of manganese(II1) tropolonate

The band splitting in the Mn(II1) complex warrants some comment. Since FORMAN and ORGEL [7] first pointed out that Jahn-Teller distortion of Mn(II1) complexes should lead to splitting of vMn-L i.r. bands and found evidence of this in the spectrum of Mn(II1) acetylacetonate, there have been a few further reports of such band splitting [6] including that of

First transition series metal(II) and metal(II1) tropolonates

1183

1184

KATHRYN J. BURDENet al.

-----

-

First transition Table 3. Frequencies 3d0

3d’

3d2

series metal(H)

and (v-vo) 3d’

and metal(W)

tropolonates

1185

values for vZ,,, vzB. Yap, vS2 and va5 (cm-‘) 3d4

3dS

3d6

3d7

3ds

3d9

3d’O

MUI)

Mn

co

Ni

cu

Zn

“24

583 0 516 0 265 0 252 0 147 0

595 10 530 8 268 14 255 29 159 8

609 21 542 16 291 45 261 56 163 9

636 46 587 59 332 91 308 107 185 29

593 0 532 0 235 0 189 0 159 0

(v-vo) “26

(v-vo) “31

(v-yo) “32

(v-vo) “35

(v-yo) M(II1) “24 (v - vo) v26 (v-vo) “31 (v-vo) “32 (v - vo) “35 (v -

vd

SC

Ti

V

Cr

Mn

Fe

co

Ga

582 0 541 0 343 0 217 0 149 0

591 I 537 -6 346 8 306 32 161 11

619 34 552 8 365 33 319 49 160 8

625 38 561 15 360 35 332 70 173 19

617t 21 5511 3 340 20 3005 38 17211 15

592 0 550 0 313 0 258 0 160 0

656 61 584 29 402 94 371 120 176 19

605 0 578 0 287 0 230 0 151 0

tMean of doublet v2.,, vzda. $Mean of doublet vz6, v16,,. !jMean of doublet va2, vjz,,. 11 Mean of doublet Y,~, v3so.

1

380

360

360

X0-

340

320-

323

7

E

-----1 A-----\.

3% i

m-

P260-

260

%

*I

---___

e y so-

240

220 -

220

ZCQ-

2ca

---___

--__

I, - _:

1

l&I

180-

/

d-G-bltol

populotlon-

d-OrbItal populotlon-

Fig. 4. Plot of frequency of va2 against d-orbital population for metal(I1) tropolonate (left diagram) and metal(II1) tropolonates (right diagram). The vertical lines represent (v- vo). The frequency for [MnT,] is the mean of Y,~ and vjZa.

1186

KATHRYNJ. BURDENet al.

[MnT,] where vMn-0, then assigned to the bands now observed at 627 and 524 cm-‘, was split into two doublets with separations of 19 and 55 cm-‘, respectively [Z]. This is consistent with the X-ray structural determination of [MnT,] which yields Mn-0 bond lengths of 1.94 and 2.13 A [S]. We might imagine that the newly-assigned vM-0 bands would show even greater. splitting

in the Mn(II1)

complex

in view of

their greater vibrational purity. Reference to Fig. 1 shows that one of the two vM-0 bands acquires a pronounced shoulder in the spectrum of the Mn(III) complex yielding, effectively, three bands at 340, 334 and 265 cm-‘. Since the vM-0 band of lower frequency is the more sensitive towards both metal substitution and metal isotope labelling, we consider that the bands at 334 and 265 cm- ’ are the two components of vMn-0 yielded by the lowering of symmetry from D, to D,, which occurs on tetragonal distortion. In this event, the separation between the two components is 68 cm-’ which is indeed greater than that observed for the bands previously assigned to vMn-0. However it is not so easy to find a high (or low) frequency partner for the band at 340 cm-‘. The

shoulder at 415 cm-’ ponent in which case Certainly, the splitting bands is not as clear-cut range 52&630 cm- l.

appears to be a possible comthe splitting here is 75 cm-‘. of the newly assigned vMn-0 as that of the bands within the

Acknowledgements-We thank the Foundation for Research Development of the CSIR and the University of Cape Town

Research Committee for financial assistance.

REFERENCES

Cl1 H. JUNGE,Spectrochim.

Acta 24A, 1957 (1968). L. G. HULETTand D. A. THORNTON,Spectrochim. Acta 27A, 2089 (1971). D. EVERDYKand S. OLBRICHT.SDectro131B. HUTCHINSON. chim. Acta 30A, 1605 (1974). c41R. L. REDINGTONand T. E. REDINGTON,J. molec. Spectrosc. 78, 229 (1979). PI H. IRVING and R. J. P. WILLIAMS,J. them. Sot. 3192 (1953). D. A. THORNTON,Coord. Chem. Rev. 55, 113 (1984). ;;; A. FORMANand L. E. ORGEL,Molec. Phys. 2,362 (1959). PI A. AVDEEF,J. A. COSTAMAGNAand J. P. FACKLER,Inorg. Chem. 13, 1854 (1974).

PI