8pectrochimic.a Acta, Vol. SOA,~~.1606 to 1614. Peqamon Press 1974. Printed In h'orthern Ireland

A comparison of three alignment techniques for metal-ligand stretching bands-a lowhequency inkwed tidy of some transition metal his,-

tris- and tetr~tropolonato complexes

BBINNETT HUTCHINSON,* DAVID EVERSDYK and SUZANKB OLBRIUECT Department of Chemistry, Abilene Christian College, Abilene, Texas 79601, U.S.A.

(Received 21 Septmh 1973)

Abrtrsot-_The low-frequency infrared spectra (600-160 cm-l) of several his- and trie- transition metal tropolonates were measured and the metal-oxygen stretching bands aesigned baeed on the metal-isotope substitution method. Since other assignment methods for these complexes have plaoed the metal-oxygen stretching bands more than 3OO~rn-~ higher, the differenoes were discussed. Right-coordinate tropolonato oomplexea were also investigated in the fer-infra- red region and M-G stret&ing assignmenta for these complexes presented.

INTRODUCTION

METAL-ligand stretching bands in metal complexes are currently aligned using several different methods. Normal coordinate analysis, high pressure techniques, and lsO-l*O isotopio substitution methods have been employed in the assignment of the metal-oxygen stretching bands [l-4]. The metal-oxygen stretching modes in 14 metal tropolonates of st.oi&iometry MT2 and MT,, where M is the coordinate metal and T is the tropolone anion ligand (Fig. l), were assigned based on data from l*O- labelled tropolone and the relationship between metal-oxygen stretching modes and crystal field stabilization energy (CFSE) [5].

This work extends the measured infrared spectrum to 150 cm-l for the transition metal tropolonates studied by HULETT and THORNTON and adds the far-infrared spectra of some MT4 complexes (600-160 cm-l). The metal-ligand vibrations are assigned using the metal-isotope technique in which one compares the spectra of a pair of metal complexes with the metal mass altered. The bands which show isotopic shifts are assigned as modes in whioh there is movement of the metal.

Tropolone displays the characteristic properties of a @liketone and forms a wide range of transition metal complexes with a variety of structures. Both FAUKLER and THOMPSON have reviewed the recent work in @liketone ohelate chemistry [6,7].

EXPERIMENT&

The bis- and tris- tropolonato complexes were prepared by methods described in the literature and referenced by HULETT and THORNTON [a]. ZrT, was prepared by mixing a solution of 0.067 g Zr(S04)1 in 5 ml of 50% water-methanol solution with 0.11 g tropolone in 2 ml 60% water-methanol. The solution was refluxed for one

[l] M. MIKAMI, I. NAKAUAWA and T. SI~IMANOUCHI, &wctrochim. Acta =A, 1037 (1967). [2] E. R. LIPPINCOTT, F. E. WELBH and C. E. WEIR, Anal. Chem. m, 137 (1961). [3] C. POSTMUS, K. NAKAMOTO and J. R. F~RBARO, Inorg. Ckm. 6, 2194 (1967). [4] H. JUNGE and H. Musso, Spectrmhim. Acta a4A, 1219 (1968). [S] L. G. Hm and D. A. THOBNTON, Spectmhim. Acta !#A, 2089 (1971). [B] D. W. THOMPSON, Structure mad Bonding 9,27 (1971). [7] J. P. FACKLER, Prop. Inorg. C?mn. 7, 361 (1966).

1606

1606 BENNETT HUTCHINSON, DAVID EVERSD~ snd SUZANNE OLBRICHT

a /\ O’

'0

Fig. 1. The tropolonato &on.

hour, then allowed to cool to room temperature and suction filtered. A similar method was used for the corresponding Hf complex except the starting material was HfOCl,. PbT, was prepsred following the method of MUETTERTIEB and WRIGHT [S].

The isotopic metal complexes were prepared as described above. The metal isotopes were purchased from Oak Ridge National Laboratories, Oak Ridge, Ten- nessee.

The infrared spectra of the metal complexes were recorded as Nujol mulls on a Hitachi Perk&Elmer FIS3 far-in&red spectrophotometer using polyethylene plates and on a Beckman IR-12 spectrophotometer using CsI plates a;t Texas A & M University. All spectra were measured at room temperature and calibrations were performed using polystyrene and water vapor. The frequencies of the bands meas- ured for isotopic species are reproducible to ho.5 cm-l.

Analytical results for C, H and N are performed on each complex by M-H-W Laboratories, Garden City, Michigan. The infrared spectra of the isotopic metal complexes correspond to the naturally abundant complexes.

RESULTS

MT,

H. Junge investigated the 2009-390 cm-1 region of the CUT, infrared spectrum using 180 labeling of the keto-enol oxygen atoms of tropolone [lo]. Based on the 180 isotopic shifts he assigned the coupled Cu-0 stretching and bending bands in the 400-700 cm-l region. Table 1 shows the ISO- 180 shift for each bsnd in this region.

The @CU-~SCUT, spectra in Table 1 show that bands in the 369-650 cm-l region move to slightly lower wavenumber when 6%~ is substituted for 6%~. These small shifts are close to the limit of experimental error, but are all in the same direction, suggesting some metal-isotope effect. Considering both x60-l*O and S3Cu-~SCu data for CUT, and the free ligand band assignments [l l] in this region we assign the bands at 399.3, 422.8, 688.0 and 637.6 cm-l for BSCuT, as ligand bands with some bands vibrationally coupled with the Cu-0 stretching mode. The amount of coupling for each band is probably small, but a more quantitative determination would require a normal coordinate analysis beyond the scope of this investigation. The shifts for these bands are not as large as those which occur in Cu-0 and Cu-N stretching bands. Previously assigned Cu-0 and Cu-N bands shift from 1.3 and 3.5 cm-l upon 68Cu-eSCu substitution [12, 131.

[8] E. L. MTJETTERTIES and C. M. WEIUET, J. AmeT. Chem. 5%~. 86,6132 (1964). [9] E. L. MUE!~JXXTIES, H. ROESKY and C. M. WRIQEIT, J. Amer. Chem. Sot. 88,4866 (1966).

[lo] H. JUNQE, f+ptx~oc~im. Actu 84A, 1967 (1968). [ll] Y. IXEGAMI, Bull. Chem. Sot. Jqvan 80,lllS (1963). [12] Y. SAITO, J. TAKEMOTO, B. HTJTCEINSON, K. NAIUMOTO, Ivwrg. Chem. 11,2003 (1972). [13] K. NAKAMOTO, C. UDO~ICH md J. TAKEMOTO, J. Amer. Chem. Sot. Sa, 3973 (1970).

Tab

le

1.

Far

-infr

ared

ba

nds

of

tropolo

ne

and M

(tro

polo

ne)

, co

mple

xes

(c

m-‘)

T’

fiT

* C

OT

. N

iT,

[NiT

,.H

,O],

CU

T,

Zn

T,

896 5

676 w

664 V

W

679 m

561 V

W

611s

434 m

410 m

404 m

376s

380 m

346 m

636 w

417 m

382 w

270s

262 8

611.0

m

610.9

644.2

m

643

638 s

h

0.1

II

420.6

m

420.2

407.7

w

407.7

389 w

, ah

0.3

Oil

0

292.6

m

286.4

6.1

268.6

m

266.0

3.6

240 w

II

606.1

m

604.8

696.0

m

694.7

647.2

m

647.0

414.9

In

414.6

393 w

, sh

342.0

m

340.8

300.6

* 296.1

266.0

8

262.1

239.6

w

238.3

0.3

0.3

0.2

0.2

II 1.2

4.1

3.9

1.2

667 V

W(O

) 636 m

(-21)

637.6

s

686 ~

~(-2

6)

688.0

a

687.7

423 m

(-6)

422.8

s

422.7

399 w

(-5)

399.8

w

331.6

87

309.1

m

224.8

B

162.6

m

637.2

399.6

328.9

307.2

1.9

223.7

161.6

G97.3

697.6

0.4

0.3

690 w

664.3

m

633.9

632.8

m

632.4

0.1

416.2

s

416.0

0.3

‘3

77.4

m

377.4

2.7

11 244.1

8

240.6

--

1*1

1 g

* 189.6

1.0

11 1

61.2

~ 169.8

161.0

70

149.9

0.3

II 0.4

0.4

0.2

0.0

3.6

II 2.9

1.4

1.1

+ T

aken

fro

m r

efer

moe

11.

t v(

s*N

i)-v

(“*N

i).

II Is

oto

pe

shif

ts c

ann

ot

be

mee

sum

d

due

to l

ow

in

ten

sity

or

poor

shap

e of

the

ban

d.

Th

e fm

quen

oie

s obse

rved

fo

r th

e n

atura

l ab

un

dan

ce

com

ple

x

am

report

ed.

7 U

nder

lin

ed

ban

ds

are

aeai

gned

as

M-O

at

ret&

ing

ban

ds.

l *

Tak

en

from

re

fere

nce

10.

Num

bers

in

p-t

hes

is

den

ote

th

e on

rl

shif

t of

‘@O

-lab

elin

g of

CU

T 1

’

1608 BENNE~C H UTOEINEON, DAVID EVERSDYK end

KC ____-----\ /\ /--

I I :; iG------x \ , \

su!‘ANNE OLBBIOET

/c--w

/ /’ ‘\ I I--_’

/

Frequency. cm-l

Fig. 2. The infme spectre of %(tropolone), and %(tropolone),.

We have extended the CUT, i.r. spectrum to 150 cm-l to see if we could locate the primary Cu-0 stretching modes. Figure 2 shows four bands appear below 399 cm-l and two bands show metal-isotope shifts of 1.9 and 2.7 om-l at 331.6 and 309.1 cm-r respectively. Crystal cell data show that cell contains two molecules per cell. Since single-crystal X-ray analysis indicates CuTa to be a planar molecule with D,, symmetry [l4], these two bands are the B,, and B,, Cu-0 stretching modes respec- tively. Bands due to bending modes of the type O-M-O, where M is the central metal, are found at lower wavenumber than the stretching modes and show smaller metal-isotope shifts [12]. Two medium-intensity bands at 224.8 and 162.6 cm-1 meet the above criteria and are assigned as O-Cu-0 bending vibrations. Lattice modes do appear in the far-infrared spectral region, but in coordination compounds have usually been found below 140 cm-l [El.

Table 1 shows two rather broad bands for ZnT, have 04Zn-68Zn shifts of 3.6 and 2.9 cm-l respectively. Two other bands at 161.2 and 161.0 show metal-isotope shifts of 1.4 and 1.1 cm-l respectively. The bands at 244.1 and 192.6 cm-l are assigned as the Zn-0 stretching bands while the 161.2 and 161.0 cm-l bands are due to bending modes. Above 360 cm-l we observe no bands with metal-isotope shifts larger than 0.4 cm-l. The bands in this region which do show small shifts may arise from vibrational coupling between ligand and metal-ligand modes as discussed in the CUT, case.

NiTa and [NiT,.HPO], [ 161 each show two bands which are assigned as Ni-0 (tropolone) stretching modes based on metal-isotope substitution data. An X-ray crystal structure of the di-+opolonato-bis[aquo(tropolonato)-nickel(II)] shows a dimerio molecule with oxygen atoms of the tropolonato ligands bridging the nickel atoms and no nickel-nickel interaction [la]. In Fig. 3 [NiT,*H,O], exhibits a band at 340 cm-1 which is not present in the NiT, spectrum. 68Ni-‘J*Ni substitution shows a 1.2 cm-1 shift for this band and it is assigned as a Ni-OH, stretohing vibration. A weak band appears at about 240 cm-l in both complexes and shifts 1.2 cm-l in the [NiT,*H,O], complex spectrum. We assign this band as a Ni-O(tropolone) bending

[14] w. rd. MAOINTYR E,J.M.ROBEBTSON andR.F. ZAEROBSKY, J.C?wn.Soc., 1966, 161. [la] I. NAMQAWA and T. SEIUNOUOHI, i$~ectro&m Acta, 28A, 2099 (1969). [IS] R. J. &vINU, M. L. POST and D. C. POVEY, J. Chem. Sot., D&on, 1973, 697.

A comparison of three assignment techniques for metal-ligand stretching bands 1609

Frequency, cm-’

Fig. 3. The infrared speatra of Ni(tropolone)* and [Ni(tropolone)n*J&O],.

mode. Bands between 350 cm-l and 650 cm-1 show no significant isotopic shifts and are assigned as primarily ligand bands. I

Cobalt and manganese isotopes are not available with suiEcient mass separation for metal-isotopic studies, but we have extended the i.r. spectra ti 160 cm-l. These bands are also reported in Table 1. R. J. IRVING et al. have recently suggested that the cobalt complex with tropolone is a tetramer [ 101. Below 380 cm-l only two bands appear at 270 and 252 cm-l for COT,. Since these bands do not occur in the spectrum of the pure ligand and are compatible with the M-O stretching frequencies assigned, we suggest these bands are due to Co-O stretching modes.

There is disagreement concerning the structure of MnT,; HULETT and THORNTON consider it as four-coordinate [5], while MTJETTERTIES et al., state that MnT, has a coordination number larger than four [9]. The low-frequency i.r. spectra exhibit 3 strong bands in the 200-300 cm-l region for this complex. These bands also occur in t4e general region established for metal-tropolone stretching, but could include a bending band. Definite assignments of each band are not made without isotopic data.

1610 BENNETT HTJTCXIINSON, DAVID EVERSDYK and SUZANNE OLBRICHT

MT, Conz$exees

Isotopic studies of six-coordinate complexes with D, symmetry have shown two bands with large metal-isotopic shifts [12]. The FeT, complex has an overall D, symmetry [17] and the other MT, complexes are thought to have the same structure. Figure 4 shows the low-frequency infrared spectra of FeT, and CrT,. The 317.3 and 259.9 cm-l bands in 6*FeT, are assigned as Fe-O stretching bands since they shift 5.0 and 4.2 cm-l respectively upon iron isotope substitution. The 222.7 cm-l band shifts 1.1 cm-l upon metal-isotope substitution and is probably due to O-Fe-O bending. Only two bands appear below 400 cm-l at 361.3 and 333.5 cm-l for SOCrT,; both show isotope shifts greater than 2.0 cm-l and are assigned as due to G-0 stretching modes.

MnT,, COT, and VT, spectral bands are listed in Table 2. Both MnT, and COT, have far-infrared spectra with two bands between 377 and 268 cm-l. These bands are assigned as M-O stretching bands. The COT, complex spectra shows one band and a shoulder in this region at 371 and 360 cm-l respectively. Although the separa- tion between these two bands is not as large as found in other MT, complexes we assign these bands as Co-O stretching bands.

MT, Com&exes

Table 3 show the low-frequency infrared bands for several MT, complexes. Al- though 4 infrared active metal-ligand stretching frequencies are predicted, metal- isotope substitution for ZrT, indicates that only one band in the low-frequency

440 400 370 330 290 250 210

Frequency. cm-’

Fig. 4. The infrared spectra, of60Cr(tropolone),,Wr(tropolone)3, “Fe(tropolon~), and 67Fe(tropolone)3.

[ 171 T. A. HAMOR and D. J. WATKIN, Chem. Cornmu~., 1969, 440.

A comparison of three assignment techniques for metal-ligand stretching bands 1611

Table 2. Far-infrared bands of M(tropolone)c complexes (cm-l)

VT, CrT, &TO FeT, COT,

6% Wr Av* %-e 6’Fe Avt

627.8 m 627.4 0.4 684 m 581 m 590 ah $ 586 s 554 8 552.7 B 652.5 0.2 556 VW

5298 443 m 462 8 412 m 429.4 m 428.9 0.5 418 m 416.8 m 416.8 0.0 41sw

407 sh t 402 w 377 m# 361.3 m 358.4 2.9 371 s

333.5 m 331.6 2.0 338 m 360 sh

319s 317.3 312.3 5.0

268 m 259.9 xv, 266.7 4.2 ---

223 m 222.7 w 221.6 w 1.6 191 w 172 w

l v(“ocr~(6Q). t v(~Fe)--v(~‘Fe). $ Isotope shift cannot be measured accurately due to low intensity of the band. The frequenoy reported is

thet of the natural abundanoe oomplex. 8 Underlined bands are assigned es M-O stretohing bands.

Table 3. Far-infrared banda of M(trcpolone)* complexes (cm-‘)

g”ZrT4

690 w 533.7 I3

420.8m

288.0 vat

.

04ZrT4

633.4

420.2

283.9

Av*

2 0.3

0.6

4.1

HfTa

686 w 640 s

410 m

291 w 246 s

PbT,

655 8 470 m

380 w

222 s

* v(wZr)+(BBZr). t Underlined bands are assigned 88 M-O stretohing bands. $ Isotope shifts cannot be measured accurately due to low intensity of the band.

The frequencies reported arc for the natural abundance complex.

spectral region is shifted by metal-isotopio substitution. This strong band at 288.0 cm-1 is assigned as a metal-oxygen stretching band. As in the spectra of other tropolone complexes the region above 400 cm-l shows no bands which are signifi- cantly affected by isotopic substitution of the metal and thus are identiCed as ligand bands.

Single-crystal X-ray data for eight-coordinate Zr complexes have shown dodeca- hedral or square-antipriamic structures. DAY and PINN.A,V~ have assigned two strong Zr-0 bands in the square antiprism K,[Zr(acac)J5H,O [18], while infrared

[ 181 R. C. FAY and T. J. PINNAVAIA, Inorg. CIwrn. 7, 608 (1968).

1612 BENNE~ HUTCHINSON, DAVID EVERSDYK and Suum Ommcm

study of several dodecahedral complexes, indicates only one broad metal-ligand stretching band [ 191. On the basis of the similarity of low-frequency infrared spectra of ZrT, and dodecahedral complexes, the dodecahedral structure is suggested for ZrT,.

HfT, and PbT, show a strong band at 245 and 222 cm-l respectively which we assign as M-O stretching bands. These M-O stretching bands occur at progres- sively lower energy as would be expected since the mass of the metals are 148.49 and 207.19 amu for Hf and Pb respectively, compared to 90.0 and 94.0 amu for Zr isotopes.

k9CUSSION

Table 1 compares the infrared data for lS-l80 and 6a-ebCu substitution of CUT,. The bands which show the largest shifts with l+leO substitution do not show shifts larger than 0.4 cm-1 with s*-s%u substitution. The Cu-0 stretching bands for CUT, at 331.6 and 309.1 cm-l were not measured with 160. It has been shown using acetylacetone (acac) complexes that the metal-isotope assigned M-O stretching do not show the large ls-160 band shifts [13]. We conclude that the metal-ligand stretching vibrations cannot be uniquely assigned by data from the substitution of the oxygen-atoms of the ligand.

Hulett and Thornton based their assignments of the M-O stretching bands for MT, and MT, on the ls--160 assignments of CUT, and the relationship between the metal-ligand bands and CFSE. They proposed for a series of isostructural complexes of isovalent first row transition metal ions the metal-ligand stretching bands vary with d-orbital population in a way which parallels the variation of CFSE [6]. The least-coupled or purest M-O stretching band is assigned to the band with the largest metal-sensitivity in the CFSE order [20]. The metal-sensitivity is obtained by sub- tracting the wavenumber for the M-O stretching bands predicted if there is no CFSE from the actual value of the M-O stretching band.

For the MT, complexes, six bands are assigned in each complex as coupled M-O stretching bands [5]. The least-coupled or purest M-O stretching band is assigned at 621, 627, 611, 594 and 656 cm-l for V, Ck, Mn, Fe and Co respectively since this series of bands showed the largest differences between the value predicted for no CFSE and the actual value [20]. The two metal-isotope assigned M-O stretching bands parallel the CFSE order except for the 377 cm-l VT, band. FeT, has no CFSE and its metal-isotope assigned Fe-O stretching bands position can be sub- tracted from the values measured from the VBT, CrT,, MnT, and COT, shown in Fig. 5. In each case the metal-sensitivity is greater than that of the least-coupled stretching band assigned by Hulett and Thornton. For example, the 50Cr-0 stretch- ing bands at 361.3 and 333.5 cm-l are 44.0 and 73.6 cm-l above the corresponding bands for “*FeT, which has no CFSE. These differences can be compared to a differ- ence of 37 cm-l for the 627 cm-l band in CrT, [5].

The MT, complexes, MnT, through ZnT,, exhibit both four and six coordination numbers, as well as monomeric and polymeric forms. These differences within the

[I91 B. HUTCHINSON, A. SUTHERLAND, M. NEAL and S. OLBRICHT, Spectrochim. ‘Acta 2BA, 2001 (1973).

[20] L. G. HULE~ and D. A. THORNTON, &w&wchim. Acta 2@A, 767 (1973).

A comparison of three SBsignment teohniques for metal-ligand etret&ing bands 1613

390

t

370-

350-

; 330 -

5

E z 310-

s ?I Lb

290-

270 -

2501

Fig. 6. Relation&ip between the M-0 stretohing bands and d orbital population in the spectra of ikf(tropolone)a complexes.

MT, series can effect the position of the M-O stretching bands msking straight- forward applicstion of the CFSE relationship difficult. Even so, it should be men- tioned that the metal-isotope assigned bands parallel the CFSE order with the exception of CUT,.

The assignments made by the metal-isotope technique for the M-O stretching bands in the tropolone complexes are compared to M-O stretching frequencies in acac complexes in Table 4. For the tris-complexes the M-O stretching frequencies are 35 to 100 cm-1 higher for &c&c complexes. These infrared results for the Fe complexes

Table 4. Metal-oxygen stretching bands for MT,, MT,, M(aac), and ikf(aoac), (om-l)

463.4 361.3 436.0* 317.3 368.4 333.5 300.6 269.9

mNi (LUXC)~ 68NiT, “Cu(==), “CUT,

421.1* 292.5 466.0 331.6 228.6 268.6 290.6 309.1

* This band is a coupled vibration of M-O and the C-CH, stretching modea

1814 Bmrr HUTCHINSON, DAVID E~E%LSDYK and SUZANNE OLBRICJCI

are compatible with the X-ray analysis data that the Fe-O bond distance in Fe(acac), is 0.05A shorter than in Fe,T [l, 171. For bis-complexes measured in this study the two M-O stretching bands occur between the corresponding M-O stretching bands in the acac complexes. X-ray analyses for the Cu(acac), and CUT, complexes give an average bond distance of 1.91 A for Cu-0 for both complexes [16,21].

Acknowledgements--We wish to this& Texas A & M University for the use of their infrared spectrophotometers. We gratefully aoknowledge the support of this work by the Robert A. Welch Foundation, Houston, Texa9, Grant Number R-483.

[21] Z. A. STARIKOVA and E. A. Swuus, Zh. Struct. Khim. 10, 294 (1969).