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Metal isotope effect on metal-ligand vibrations—VIII
Spa&o&micaActs,Vol.%A, pp.1469 to1466.Pemon Pren# 1072.Printed InNorthern Ireland
Metal isotope effect on metal-ligand vibrations-VIII Far-infrared and Raman spectra of zinc halide complexes of pyridine* YUTAKA SAITO, MARC= CORDES~ and KAZUO NAUMOTOS Department of Chemistry, Marquette University, Milwaukee, Wisoonsin 53233 (Received21 Jdy
1971; Revised 14 Februuy 1972)
Ab&ad-The far-infrared ad Ramas spectra of the Zn(pyridine),X, (X = Cl, Br or I) type complexes hsve been measured. Band assignmentshave been made besed on isotope shifts due to the @Zn-6sZn and pyridin+pyridine(d5) substitutions. INTRODUCTION
spectra of Zn(py),X, (py, pyridine; X, a halogen) have been studied by several investiga;tors. CLARK and WILLUS [ 11carried out an extensive i.r. study (down to 200 cm-l) on metal halide complexes of pyridine and related ligands including the Zn(py)*X, series. POSTMUS et al. [2] measured the far-i.r. spectra of the Zn(py),X,, Zn(bipy)X, and Zn(terpy)X, (bipy, 2,2’-bipyridine; terpy,2,2’2”terpyridine) type compounds, and assigned the Zn-N and Zn-X stretching as well as skeletal bending modes. BRADBURY et al. [3] also assigned these modes for the Zn(py),X, and Zn(py)X,- type complexes including their triphenylphosphine analogs. However, the band assignments proposed by these investigators are markedly different from each other for most of the bands. The main purpose of this work is to measure the i.r. and Reman spectra of the Zn(py),X, type complexes, and to assign all the skeletal modes based on observed isotopic shifts due to the 64Zn-@Zn and pyridine-pyridine-d5 substitutions. Such isotopic data were not, available in previous investigations. THE VIBRATIONAL
EXPERIMENTAL
Preparation of compounds All the complexes were prepared by using the literature method [a]. Metal complexes containing pure metal isotopes were prepared on a milligram scale by using metal isotopes purchased from Oak Ridge National Laboratory. The purity of metal isotopes was: 64Zn(99.66 ‘A) and ‘J*Zn(99.60°A). The metal isotopes were received as the oxides and converted to fhe halides by dissolving fhem in desired hydrogen halides. The py-dE,complexes were prepared by using py-d, which was purchased from Aldrich Chemical Co., Milwaukee, Wisconsin. * This work was supported by an ACS-PRF unrestrictedresearchgrant (331%C3, 6). t Department of Chemistry, Creighton University, Om&a, Nebraska 68131. $ To whom correspondenceshould be sddressed. [I] R. J. H. CLARKmd C. S. WILLIAMS, Inorg. Chena. 4, 350 (1966). [2] C. Posms, J. R. FERRAROand W. WOZNIAK, Inorg. Chem. 6.2030 (1967). [3] J. BRADBURY, K. P. FOREST, R. H. NTJTTAIJ, and D. W. A. SW, &-xctrochim. Acta asA, 2701 (1907). [4] W. LANG, Chem. Ber. 21, 1678 (1888).
1
1469
1460
YUTAKA SAITO, MARCIACOEDEEIand KAZUO NAKAMOTO
Spectral measzlrements Infrared spectra were measured on a Beckman IR 12(4000-260 cm-l) and a Hitachi-Perk&Elmer FIS-3 (410-33 cm-l) i.r. spectrophotometer. A Nujol mull with polyethylene plates was used to obtain the spectra of solid samples. The spectra were run on an expanded scale with a scanning speed of l-2 cm-l/mm Calibration of frequency readings was made by using polystyrene film, water vapor and 1,2,4trichlorobenzene . Reman spectra were recorded on a Spex model 1401 Raman spectrophotometer equipped with an Argon ion laser. The green excitation (6146A) was used. Calibration of frequency readings was made by using indene and carbon tetrachloride. Reproducibility of i.r. and Raman spectra was checked by multiple scans over the desired frequency range. The average error in frequency reading was &O.S cm-l. Figures l-4 show actual tracings of far-i.r. and Raman spectra of the Zn(py),X, type compounds obtained in this work.
360
320
260
240
200
Frecquency.
160
120
cm“
Fig. 1. Far-ix. spectra of 6*Zn(py)&2 (X = Cl, Br and I) and their b*Zn analogs.
RESVLTSAND DISCUSSION The tetrahedral Zn(py),X, type complex (C,, symmetry) gives nine normal vibrations if a pyridine ring is regarded as a single atom having the mass of the C,H,N group. These vibrations are : a, species : r,(Zn-X) ; r,(Zn-N) a2 species: B(XZnN) b, species: r,(Zn-N) b, species: r,(Zn-X);
; d(XZnN)
6(XZnN)
; 8(NZnN);
S(XZnX)
Metal isotope &‘ect on m&al-ligad
320
360
280
240
Fre~~uency,
viir&iona--YiII
200
160
120
cm”
Fig. 2. Far4.r. Bpectraof Zn(py)&8 and Zn(py-d,)&n, (X = Cl, Br and If.
340
320
300
260
260
240
Frequency.
220
200
I60
160
140
cm-f
Fig. 3. Raman spectra of *4Zn(py)&Yh(X = Cl and Br) and their 6*Zn malo@.
YUTA.KASAXTO,Maac~ll CORDES and Kuuo
1462
NA.KAMOTO
-Zn(py),C, ___...._ (j 5
.-_-.-._.._- ~-~~......_......1_.______.___.~________.____~_.._~.
I,
I,,
340
320
,
300
,
260
,
I, 260
I 240
Freauency.
Fig. 4. Baman spectra of Zn(py)&
, , ( , , , , , 220 200 160 160
,
14<
cnrl
and Znfpy-ds)&
(X = Cl, Br and I).
where v*, v, and 8 denote ant~~met~c, symmet~c stretohing and bending modes, respectively. All these modes are ir. as well as Raman active except the c~smode which is only Raman active. Previous investigators have assigned some of these modes based on far-i.r. data alone. In this work, we have measured the i.r. and Raman spectra of a series of Zn(py}~X~ type oomplex containing pure metal isotopes (%3n and 6aZn) and the deuterated ligand (py-~~), and made assignments based on the following criteria: (I) Betal isotope ejfect. All four stretching modes should be sensitive to the 64Zn-68Zn substitution since they involve the motion of the Zn atom. The metal isotope shift of the antisymmetric mode is expected to be larger than that of the symmetric mode. In general, bending modes show much smaller isotopio shifts (2-O cm-l) than the stretch~ modes (8-2 cm-l) [5$ (2) Ligand ~e~er~~~~ ejj’ect. All the modes involving the motion of the pyridine ring should be shifted by the py-py(d,) substitution. Thus the Zn-N stretching, NZnN bending and XZnN bending modes should be shifted while the Zn-X stretching and XZnX bending modes should not be shifted by such substitution. [a] K.
NAKAXOTO, K. SHOBATAKE fsnd B. ~%TTCEINSON, Chem Coewrmr~ 1451 (1909); K. SHOBATAIW and K. NAKAMOTO, J. Am. Glsem. Sot. i&3332,3339 (1970);B. HUTCHINSON, J. TAEEMOTO tendK. NAKAXOTO, J. Am. Chem. Sot. 92, 3336 (1970); K. Nwont, C. UDOVICEI rendJ. TAKEMOTO, J. Am. C&m. Soo. 92, 3973 (1970); J. TAYCEMOTO rend K. NAKAMOTO, C&m.Commzcn. X017 (1970);N.OEKAKU andK.N~xoro, Inorg.Chem. 10,
798 (1971).
1403
Metal isotope effect on metal-ligand vibrations-VIII
(3) Halogen substitution eflect. All the modes involving the motion of the halogen atom should be shifted markedly when the halogen atom is changed from Cl to Br to I. Thus, the Zn-X stretohing, XZnN and XZnX bending modes should be markedly sensitive to the halogen substitution. (4) Vibra&maZ coupling. The above criteria do not hold rigorously if vibrational coupling occurs between some of these modes. For example, r,(Zn-X) and v,(Zn-N) may couple in the a, species. Then, even the former will be shifted slightly by the py-py(d,) substitution. Zn-X
stretching mode.~
Both antisymmetric (b,) and symmetric (al) stretching modes can be assigned relatively easily. In general, they are strong in the i.r. and weak in the Raman spectrum. They show relatively large shifts by the BqZn-66Znsubstitution and relatively small shifts by the py-py (d6) substitution. In agreement with previous work [l-3] these modes are assigned at ca. 330-290, 260-220 and 230-210 cm-l for the chloro, bromo and iodo complexes, respectively. The antisymmetric mode is expected to be stronger in the infrared and weaker in the Raman spectrum than the symmetric mode. However, the 217.0 cm-l (i.r.) of @Zn(py),I, cannot be assigned to the Zn-I stretching mode since this band gives a relatively large shift (3.1 cm-l) by the py-py(d,) substitution. It is probably due to the Zn-N stretching mode as indicated in Table 1. Then the 227.4 cm-l band may be an overlap of two Zn-I stretching modes. Alternatively, the 217.0 cm-1 band may be considered as an overlap of the symmetric Zn-I and antisymmetric Zn-N stretching modes. Zn-N
&etching mode.s
The Zn-N stretching modes are more difilcult to assign than the Zn-X stretching modes since the former are relatively weak both in i.r. and Reman spectra. Furthermore, they are closely located to the Zn-X stretching modes in the bromo and iodo complexes. In general, the Zn-N stretching bands are expected to show Table 1. Infrared frequencies, isotopic shifts and and assignments of tetrahedral Zn(py)& complexes (cm-l) Zn(PY)Pl, 4Zn LZ(VS) 1.6(s) 1.4(m) :.9(w)
I.Q(rn)
Z~(PY)&%
Av’+ Apt 4.8 2.4 3.6 2.4
0.0
3.8 1.7 4.4 2.2
0.3
AC$ 0.8 0.2 3.8 4.1
0.7
o*ZIl
AC’
267.O(vs) 223.O(vs) --p 184.6(w)
6.3 3.7 0.7
77.0(m)
0.0
Zn(py),I,
Ay’t AF$ 6.4 4.4 4.7 0.2
0.2
1.7 1.3 3.2
0.6
64ZI4
Assignment
A?* Apt M$
227.4(vs) -_O 217.O(vs) 1613.8(m) 16l.O(sh) 126.0(w)
4.4 2.6 0.7 0.2
B&b(m)
0.0
6.9 4.6 4.6 0.4 0.1 0.2 0.4 0.1
Q(e’Zn)~(“*Zn)--obsrved. 4(EPZn)~(e*Zn)~alauleted. No shift is predicted for the aa mode. : 4(py)-Y’(py-.&)-observed. i Hidden or overlapped by other bends. 1Isotopic shifts oould not be determined beaauee of poor shape of band maxima. / This coupling does not occur in the chloro complex. ibbreviations: Y, stretching; 6, bending; VB. very strong; B. strong; m, medium;
type
1.Q 3.1 7.2 6.0 8.1
B(NZnN), a, &XZnN). b,, b,
0.3
G(XZnx). 4
a’(Zn-X), Y
(Zn-X)
v(Zn-N),
VW-N)
b, f
b1
v(Zn-N).
+ v(Zn-X).
o 4
’
w, weak; VW, very weak; eh, should
1464
YVTAKA SAITO, MARCIA COBDES and KAZUO NAKAMOTO
relatively large shifts by the MZn-s*Zn and py-py(&) substitutions. However, they should not be sensitive to the change in halogens unless they are coupled with halogen-sensitive modes such as Zn-X stretching modes. In sgreement with previous assignments [2, 31, two bands at 222.4 and 203.9 cm-l (ir.) of Y&i(py)&h can be assigned to the ctntisymmetrio(b,) and symmetrio(a,) Zn-N stretching modes, respectively, since these frequencies are sensitive to both substitutions and the 222.4 om-l band is stronger in the i.r. and weaker in the &man spectrum than the 203.9 cm-1 band. In Zn(py),Br,, the antisymmetric Zn-N stretching band is at cu. 224 cm-l in the Raman although it is overlapped by the 223 cm-l band (Zn-Br stretch) in the i.r. spectrum. Both i.r. and Raman spectra show a band near 185 cm-l. As is seen in Tables 1 and 2, this band gives a relatively large shift (3.2 cm-l) by the py-py(d,) Table 2. Ramen frequencies, ieotopic shifts and band a&gnments Zn(py)&, type complexes (cm-l) Zn(PY)*Cl, “ZZn 330.3(VW)
AG*
Apt
3.9
294.8(w) 224.l(vw)
1.3 4.4
200(sh) 161.2(vs)
N 0 0.0
207.13(w)
Z~(FY),%
1.0 0.4 6.0
3.2
4.2 -
10 6.6
“Zn 267.4(vw)
217.8(w) 224(sh) lES.B(ve) -$ 160.0(w)
A+’ 4.9
2.2
N 3 0.6 ---(I 0.2
of t&r&&al
Zn(pJT)J; H*zn
Av’t
1.3 --p
- I -
- II -
-_O
-
Act
3.4 -7 8.B
168(sh) 160(sh) 149.6(vs)
N6 - 8 3
Assignment
v(Zn-X), r(Zn-X)
v(Zn-N),
b, + v(Zn-N),
4**
b1
* B(“Zn-y’(‘*Zn). t Y’(PY)-)Y’Py_d,). $ Hidden by the neighboring band. 0 Isotopio shifts on deuteration could not be determined because of poor shape of band maxima. 11Too weak to be observed. The intensity of the exciting line had to be deoreased drastically to avoid the decomposition of the compound. 7 Zn(py-d,),Br, exhibits two Raman bands at 182 and 177(sh) cm-l. The latter may oorrwpond to the 186.6 cm-1 band of the non-deuterated complex. ** This ooupling ooours in the bromo and iodo complexes.
substitution. However, it cannot be assigned to a pure Zn-N stretching mode since it is sensitive to the halogen substitution. It is most probably due to a coupled vibration between Zn-N and Zn-X stretching modes (both a, species). This coupling occurs only in the bromo and iodo complexes because the Zn-N(a,) and Zn-Br(a,) or Zn -I@,) stretching frequencies are relatively close to eech other. As is seen in Fig. 3, the 186 cm-l band of Zn(py),Br, is extremely strong in the Raman spectrum. This result probably indicates that it is almost a totally symmetric breathing vibration because the masses of Br and py are similar in this case. This interpretation can also eccount for a relatively small metal isotope shift of this mode both in i.r. and Raman spectra. The above assignments are different from either of previous workers [2, 31. The 217 cm-l (i.r.) band of Zn(py),I, is sensitive to both types of isotopic substitutions, and can reasonably be assigned to the antisymmetric Zn-N stretching mode in agreement with the previous assignment [2]. However, the 169 cm-1 (i.r.) band must be assigned to the Zn-N stretching coupled with the Zn-I stretching mode because of the same reason given for the 186 cm-l band of the bromo complex.
M&alisotopeeffect on metal-ligandVibrations-VIII
1466
NZnN bending mode This mode should be sensitive to the py-py&) substitution. However, it should be relatively insensitive to the metal isotope [a] or halogen substitution. Thus, the bands between 161 and 150 cm-i of wZn(py),X, (X = Cl, Br and I) are all assigned to this mode in the i.r. and Raman spectra. It is relatively strong in the Reman and weak in the i.r. spectrum as expected from its ai symmetry. This assignment is in good agreement with that of previous workers [Z]. XZnN bending mode There are three XZnN type bending modes which are distributed in the a,, b, and b, species. These modes are expected to be sensitive to the change in halogens and to the py-py(d,) substitution. However, they would show very little shifts by the metal isotope substitution since they are not the stretching modes. Although both b, and b, modes are i.r. active, there is an indication that these two frequencies are relatively close to each other [S]. Thus the bands at ca. 143,132 and 126 cm-i (i.r.) of the chloro, bromo and iodo complexes, respectively, have been assigned to an overlap of these two bending modes. These assignments are entirely different from either of previous investigators [2, 31. Although both modes are also Raman active, they were not observed in the Reman spectra because they are relatively close to the strong exciting line. The a,XZnN bending mode is observed only in the Raman spectrum ; it appears at ca. 200 cm-l as a shoulder band in Zn(py),Cl, (64Zn, e*Zn and NAZn) and as a distinct band in Zn(py-d&&l,. In Zn(py-&),Br,, this mode appears as a shoulder band at 177 cm-l although it is overlapped by the 185 cm-l band in other bromo complexes. In Zn(py),I,, this az mode is observed at 160 cm-l as a shoulder and is shifted to ca. 153 cm-l upon deuteration of the py ligand. As seen in Tables 1 and 2, the bands corresponding to this a8 mode do not appear in the i.r. spectra. XZnX bending mode This mode is not expected to show any appreciable shift by the 84Zn-6sZn or py-py(&) substitution. However, it should be sensitive to the change in halogens. The bands at 110, 77 and 66 cm-i of the chloro, bromo and iodo complexes, respectively, have been assigned to this mode. These assignments are in good agreement with those of BRADBURY et al. [3]. Approximate normal coordin.ateanalysis In the previous sections, we have assigned the skeletal vibrations of the Zn(py)sX, type compounds based on the observed isotopic shifts. In order to confirm these results, we have carried out approximate normal coordinate analyses by assuming that the pyridine ligand is a single atom having the mass of CSH,N. The molecular parameters used were: Zn-N, 2.2 A and Zn-X, 2.4 A [7]. The six angles around the Zn atom were assumed to be tetrahedral. The potential energy was expressed using the Urey-Bradley field [S]. The initial set of force constants was taken from [6] E. MASLOWSKY. JR. and K. NAXAMOTO,AppZ. Spectry. 26,187 (1971). [7] M. A. PORAI-KOSHITS, L. 0. ATOVMY~ and G. N. TIS~~EIENJCO, 2%. Strukt. Khim. (1960). [S] T. SHIMANOUCEI,J. Chem. Phya. 17, 246 (1949).
1, 337
1466
YUTAKA SAITO, MABOIA
CORDESand -0
NAKAHOTO
those of related compounds (ammine and halogenoa complexes) and refined by the least-square method. The final set of force constants was: K(Zn-N), 0.55; K(Zn-X), 0.91; H(X-Zn-X), 0.03; H(N-Zn-N), 0.12; H(X-Zn-N), 0.11; F(X . . . X), 0.10; F(N . . . N), 0.50; F(N . . . X), 0.30; K, -0.10. (All in units of mdyn/A except K (mdyn A)). Theoretically, all the force constants involving the X atom should be slightly different for each halogen. No attempts have been made, however, to adjust these values rigorously since our main purpose was to confirm band assignments by normal coordinate analysis. Table 1 compares the theoretical and observed shifts due to the aZn-68Zn substitution. The agreement is quite satisfactory in spite of the approximations used. In order to co&m band assignments, we have calculated the potential energy distribution for each normal vibration. The results are in good agreement with those predicted from the observed isotopic shifts. As is shown in the last column of Table 1, both Zn-X and Zn-N stretching modes are coupled strongly in the bromo and iodo complexes. The bands at co. 186 cm- l of the bromo complex and at co. 169 cm-l of the iodo complex show rather small metal isotope shifts since this mode is similar to the totally symmetric stretching mode (Raman active) of a tetrahedral X Y,-type molecule. Ackncwledg~menta-The authors are grateful to Prof. K. MAOIDA of Kyoto University and Dr. C. W. SC~~PIPIER of Marquette University for their helpful discussions. Thanks are also due to the staff of the Kyoto University Computbion Centerfor the use of the FACOM 230-60 computer for this work.
[Q]I. NAKAQAWA,JASCO Rep.9,2 (1965).
Metal isotope effect on metal-ligand vibrations-VIII Far-infrared and Raman spectra of zinc halide complexes of pyridine* YUTAKA SAITO, MARC= CORDES~ and KAZUO NAUMOTOS Department of Chemistry, Marquette University, Milwaukee, Wisoonsin 53233 (Received21 Jdy
1971; Revised 14 Februuy 1972)
Ab&ad-The far-infrared ad Ramas spectra of the Zn(pyridine),X, (X = Cl, Br or I) type complexes hsve been measured. Band assignmentshave been made besed on isotope shifts due to the @Zn-6sZn and pyridin+pyridine(d5) substitutions. INTRODUCTION
spectra of Zn(py),X, (py, pyridine; X, a halogen) have been studied by several investiga;tors. CLARK and WILLUS [ 11carried out an extensive i.r. study (down to 200 cm-l) on metal halide complexes of pyridine and related ligands including the Zn(py)*X, series. POSTMUS et al. [2] measured the far-i.r. spectra of the Zn(py),X,, Zn(bipy)X, and Zn(terpy)X, (bipy, 2,2’-bipyridine; terpy,2,2’2”terpyridine) type compounds, and assigned the Zn-N and Zn-X stretching as well as skeletal bending modes. BRADBURY et al. [3] also assigned these modes for the Zn(py),X, and Zn(py)X,- type complexes including their triphenylphosphine analogs. However, the band assignments proposed by these investigators are markedly different from each other for most of the bands. The main purpose of this work is to measure the i.r. and Reman spectra of the Zn(py),X, type complexes, and to assign all the skeletal modes based on observed isotopic shifts due to the 64Zn-@Zn and pyridine-pyridine-d5 substitutions. Such isotopic data were not, available in previous investigations. THE VIBRATIONAL
EXPERIMENTAL
Preparation of compounds All the complexes were prepared by using the literature method [a]. Metal complexes containing pure metal isotopes were prepared on a milligram scale by using metal isotopes purchased from Oak Ridge National Laboratory. The purity of metal isotopes was: 64Zn(99.66 ‘A) and ‘J*Zn(99.60°A). The metal isotopes were received as the oxides and converted to fhe halides by dissolving fhem in desired hydrogen halides. The py-dE,complexes were prepared by using py-d, which was purchased from Aldrich Chemical Co., Milwaukee, Wisconsin. * This work was supported by an ACS-PRF unrestrictedresearchgrant (331%C3, 6). t Department of Chemistry, Creighton University, Om&a, Nebraska 68131. $ To whom correspondenceshould be sddressed. [I] R. J. H. CLARKmd C. S. WILLIAMS, Inorg. Chena. 4, 350 (1966). [2] C. Posms, J. R. FERRAROand W. WOZNIAK, Inorg. Chem. 6.2030 (1967). [3] J. BRADBURY, K. P. FOREST, R. H. NTJTTAIJ, and D. W. A. SW, &-xctrochim. Acta asA, 2701 (1907). [4] W. LANG, Chem. Ber. 21, 1678 (1888).
1
1469
1460
YUTAKA SAITO, MARCIACOEDEEIand KAZUO NAKAMOTO
Spectral measzlrements Infrared spectra were measured on a Beckman IR 12(4000-260 cm-l) and a Hitachi-Perk&Elmer FIS-3 (410-33 cm-l) i.r. spectrophotometer. A Nujol mull with polyethylene plates was used to obtain the spectra of solid samples. The spectra were run on an expanded scale with a scanning speed of l-2 cm-l/mm Calibration of frequency readings was made by using polystyrene film, water vapor and 1,2,4trichlorobenzene . Reman spectra were recorded on a Spex model 1401 Raman spectrophotometer equipped with an Argon ion laser. The green excitation (6146A) was used. Calibration of frequency readings was made by using indene and carbon tetrachloride. Reproducibility of i.r. and Raman spectra was checked by multiple scans over the desired frequency range. The average error in frequency reading was &O.S cm-l. Figures l-4 show actual tracings of far-i.r. and Raman spectra of the Zn(py),X, type compounds obtained in this work.
360
320
260
240
200
Frecquency.
160
120
cm“
Fig. 1. Far-ix. spectra of 6*Zn(py)&2 (X = Cl, Br and I) and their b*Zn analogs.
RESVLTSAND DISCUSSION The tetrahedral Zn(py),X, type complex (C,, symmetry) gives nine normal vibrations if a pyridine ring is regarded as a single atom having the mass of the C,H,N group. These vibrations are : a, species : r,(Zn-X) ; r,(Zn-N) a2 species: B(XZnN) b, species: r,(Zn-N) b, species: r,(Zn-X);
; d(XZnN)
6(XZnN)
; 8(NZnN);
S(XZnX)
Metal isotope &‘ect on m&al-ligad
320
360
280
240
Fre~~uency,
viir&iona--YiII
200
160
120
cm”
Fig. 2. Far4.r. Bpectraof Zn(py)&8 and Zn(py-d,)&n, (X = Cl, Br and If.
340
320
300
260
260
240
Frequency.
220
200
I60
160
140
cm-f
Fig. 3. Raman spectra of *4Zn(py)&Yh(X = Cl and Br) and their 6*Zn malo@.
YUTA.KASAXTO,Maac~ll CORDES and Kuuo
1462
NA.KAMOTO
-Zn(py),C, ___...._ (j 5
.-_-.-._.._- ~-~~......_......1_.______.___.~________.____~_.._~.
I,
I,,
340
320
,
300
,
260
,
I, 260
I 240
Freauency.
Fig. 4. Baman spectra of Zn(py)&
, , ( , , , , , 220 200 160 160
,
14<
cnrl
and Znfpy-ds)&
(X = Cl, Br and I).
where v*, v, and 8 denote ant~~met~c, symmet~c stretohing and bending modes, respectively. All these modes are ir. as well as Raman active except the c~smode which is only Raman active. Previous investigators have assigned some of these modes based on far-i.r. data alone. In this work, we have measured the i.r. and Raman spectra of a series of Zn(py}~X~ type oomplex containing pure metal isotopes (%3n and 6aZn) and the deuterated ligand (py-~~), and made assignments based on the following criteria: (I) Betal isotope ejfect. All four stretching modes should be sensitive to the 64Zn-68Zn substitution since they involve the motion of the Zn atom. The metal isotope shift of the antisymmetric mode is expected to be larger than that of the symmetric mode. In general, bending modes show much smaller isotopio shifts (2-O cm-l) than the stretch~ modes (8-2 cm-l) [5$ (2) Ligand ~e~er~~~~ ejj’ect. All the modes involving the motion of the pyridine ring should be shifted by the py-py(d,) substitution. Thus the Zn-N stretching, NZnN bending and XZnN bending modes should be shifted while the Zn-X stretching and XZnX bending modes should not be shifted by such substitution. [a] K.
NAKAXOTO, K. SHOBATAKE fsnd B. ~%TTCEINSON, Chem Coewrmr~ 1451 (1909); K. SHOBATAIW and K. NAKAMOTO, J. Am. Glsem. Sot. i&3332,3339 (1970);B. HUTCHINSON, J. TAEEMOTO tendK. NAKAXOTO, J. Am. Chem. Sot. 92, 3336 (1970); K. Nwont, C. UDOVICEI rendJ. TAKEMOTO, J. Am. C&m. Soo. 92, 3973 (1970); J. TAYCEMOTO rend K. NAKAMOTO, C&m.Commzcn. X017 (1970);N.OEKAKU andK.N~xoro, Inorg.Chem. 10,
798 (1971).
1403
Metal isotope effect on metal-ligand vibrations-VIII
(3) Halogen substitution eflect. All the modes involving the motion of the halogen atom should be shifted markedly when the halogen atom is changed from Cl to Br to I. Thus, the Zn-X stretohing, XZnN and XZnX bending modes should be markedly sensitive to the halogen substitution. (4) Vibra&maZ coupling. The above criteria do not hold rigorously if vibrational coupling occurs between some of these modes. For example, r,(Zn-X) and v,(Zn-N) may couple in the a, species. Then, even the former will be shifted slightly by the py-py(d,) substitution. Zn-X
stretching mode.~
Both antisymmetric (b,) and symmetric (al) stretching modes can be assigned relatively easily. In general, they are strong in the i.r. and weak in the Raman spectrum. They show relatively large shifts by the BqZn-66Znsubstitution and relatively small shifts by the py-py (d6) substitution. In agreement with previous work [l-3] these modes are assigned at ca. 330-290, 260-220 and 230-210 cm-l for the chloro, bromo and iodo complexes, respectively. The antisymmetric mode is expected to be stronger in the infrared and weaker in the Raman spectrum than the symmetric mode. However, the 217.0 cm-l (i.r.) of @Zn(py),I, cannot be assigned to the Zn-I stretching mode since this band gives a relatively large shift (3.1 cm-l) by the py-py(d,) substitution. It is probably due to the Zn-N stretching mode as indicated in Table 1. Then the 227.4 cm-l band may be an overlap of two Zn-I stretching modes. Alternatively, the 217.0 cm-1 band may be considered as an overlap of the symmetric Zn-I and antisymmetric Zn-N stretching modes. Zn-N
&etching mode.s
The Zn-N stretching modes are more difilcult to assign than the Zn-X stretching modes since the former are relatively weak both in i.r. and Reman spectra. Furthermore, they are closely located to the Zn-X stretching modes in the bromo and iodo complexes. In general, the Zn-N stretching bands are expected to show Table 1. Infrared frequencies, isotopic shifts and and assignments of tetrahedral Zn(py)& complexes (cm-l) Zn(PY)Pl, 4Zn LZ(VS) 1.6(s) 1.4(m) :.9(w)
I.Q(rn)
Z~(PY)&%
Av’+ Apt 4.8 2.4 3.6 2.4
0.0
3.8 1.7 4.4 2.2
0.3
AC$ 0.8 0.2 3.8 4.1
0.7
o*ZIl
AC’
267.O(vs) 223.O(vs) --p 184.6(w)
6.3 3.7 0.7
77.0(m)
0.0
Zn(py),I,
Ay’t AF$ 6.4 4.4 4.7 0.2
0.2
1.7 1.3 3.2
0.6
64ZI4
Assignment
A?* Apt M$
227.4(vs) -_O 217.O(vs) 1613.8(m) 16l.O(sh) 126.0(w)
4.4 2.6 0.7 0.2
B&b(m)
0.0
6.9 4.6 4.6 0.4 0.1 0.2 0.4 0.1
Q(e’Zn)~(“*Zn)--obsrved. 4(EPZn)~(e*Zn)~alauleted. No shift is predicted for the aa mode. : 4(py)-Y’(py-.&)-observed. i Hidden or overlapped by other bends. 1Isotopic shifts oould not be determined beaauee of poor shape of band maxima. / This coupling does not occur in the chloro complex. ibbreviations: Y, stretching; 6, bending; VB. very strong; B. strong; m, medium;
type
1.Q 3.1 7.2 6.0 8.1
B(NZnN), a, &XZnN). b,, b,
0.3
G(XZnx). 4
a’(Zn-X), Y
(Zn-X)
v(Zn-N),
VW-N)
b, f
b1
v(Zn-N).
+ v(Zn-X).
o 4
’
w, weak; VW, very weak; eh, should
1464
YVTAKA SAITO, MARCIA COBDES and KAZUO NAKAMOTO
relatively large shifts by the MZn-s*Zn and py-py(&) substitutions. However, they should not be sensitive to the change in halogens unless they are coupled with halogen-sensitive modes such as Zn-X stretching modes. In sgreement with previous assignments [2, 31, two bands at 222.4 and 203.9 cm-l (ir.) of Y&i(py)&h can be assigned to the ctntisymmetrio(b,) and symmetrio(a,) Zn-N stretching modes, respectively, since these frequencies are sensitive to both substitutions and the 222.4 om-l band is stronger in the i.r. and weaker in the &man spectrum than the 203.9 cm-1 band. In Zn(py),Br,, the antisymmetric Zn-N stretching band is at cu. 224 cm-l in the Raman although it is overlapped by the 223 cm-l band (Zn-Br stretch) in the i.r. spectrum. Both i.r. and Raman spectra show a band near 185 cm-l. As is seen in Tables 1 and 2, this band gives a relatively large shift (3.2 cm-l) by the py-py(d,) Table 2. Ramen frequencies, ieotopic shifts and band a&gnments Zn(py)&, type complexes (cm-l) Zn(PY)*Cl, “ZZn 330.3(VW)
AG*
Apt
3.9
294.8(w) 224.l(vw)
1.3 4.4
200(sh) 161.2(vs)
N 0 0.0
207.13(w)
Z~(FY),%
1.0 0.4 6.0
3.2
4.2 -
10 6.6
“Zn 267.4(vw)
217.8(w) 224(sh) lES.B(ve) -$ 160.0(w)
A+’ 4.9
2.2
N 3 0.6 ---(I 0.2
of t&r&&al
Zn(pJT)J; H*zn
Av’t
1.3 --p
- I -
- II -
-_O
-
Act
3.4 -7 8.B
168(sh) 160(sh) 149.6(vs)
N6 - 8 3
Assignment
v(Zn-X), r(Zn-X)
v(Zn-N),
b, + v(Zn-N),
4**
b1
* B(“Zn-y’(‘*Zn). t Y’(PY)-)Y’Py_d,). $ Hidden by the neighboring band. 0 Isotopio shifts on deuteration could not be determined because of poor shape of band maxima. 11Too weak to be observed. The intensity of the exciting line had to be deoreased drastically to avoid the decomposition of the compound. 7 Zn(py-d,),Br, exhibits two Raman bands at 182 and 177(sh) cm-l. The latter may oorrwpond to the 186.6 cm-1 band of the non-deuterated complex. ** This ooupling ooours in the bromo and iodo complexes.
substitution. However, it cannot be assigned to a pure Zn-N stretching mode since it is sensitive to the halogen substitution. It is most probably due to a coupled vibration between Zn-N and Zn-X stretching modes (both a, species). This coupling occurs only in the bromo and iodo complexes because the Zn-N(a,) and Zn-Br(a,) or Zn -I@,) stretching frequencies are relatively close to eech other. As is seen in Fig. 3, the 186 cm-l band of Zn(py),Br, is extremely strong in the Raman spectrum. This result probably indicates that it is almost a totally symmetric breathing vibration because the masses of Br and py are similar in this case. This interpretation can also eccount for a relatively small metal isotope shift of this mode both in i.r. and Raman spectra. The above assignments are different from either of previous workers [2, 31. The 217 cm-l (i.r.) band of Zn(py),I, is sensitive to both types of isotopic substitutions, and can reasonably be assigned to the antisymmetric Zn-N stretching mode in agreement with the previous assignment [2]. However, the 169 cm-1 (i.r.) band must be assigned to the Zn-N stretching coupled with the Zn-I stretching mode because of the same reason given for the 186 cm-l band of the bromo complex.
M&alisotopeeffect on metal-ligandVibrations-VIII
1466
NZnN bending mode This mode should be sensitive to the py-py&) substitution. However, it should be relatively insensitive to the metal isotope [a] or halogen substitution. Thus, the bands between 161 and 150 cm-i of wZn(py),X, (X = Cl, Br and I) are all assigned to this mode in the i.r. and Raman spectra. It is relatively strong in the Reman and weak in the i.r. spectrum as expected from its ai symmetry. This assignment is in good agreement with that of previous workers [Z]. XZnN bending mode There are three XZnN type bending modes which are distributed in the a,, b, and b, species. These modes are expected to be sensitive to the change in halogens and to the py-py(d,) substitution. However, they would show very little shifts by the metal isotope substitution since they are not the stretching modes. Although both b, and b, modes are i.r. active, there is an indication that these two frequencies are relatively close to each other [S]. Thus the bands at ca. 143,132 and 126 cm-i (i.r.) of the chloro, bromo and iodo complexes, respectively, have been assigned to an overlap of these two bending modes. These assignments are entirely different from either of previous investigators [2, 31. Although both modes are also Raman active, they were not observed in the Reman spectra because they are relatively close to the strong exciting line. The a,XZnN bending mode is observed only in the Raman spectrum ; it appears at ca. 200 cm-l as a shoulder band in Zn(py),Cl, (64Zn, e*Zn and NAZn) and as a distinct band in Zn(py-d&&l,. In Zn(py-&),Br,, this mode appears as a shoulder band at 177 cm-l although it is overlapped by the 185 cm-l band in other bromo complexes. In Zn(py),I,, this az mode is observed at 160 cm-l as a shoulder and is shifted to ca. 153 cm-l upon deuteration of the py ligand. As seen in Tables 1 and 2, the bands corresponding to this a8 mode do not appear in the i.r. spectra. XZnX bending mode This mode is not expected to show any appreciable shift by the 84Zn-6sZn or py-py(&) substitution. However, it should be sensitive to the change in halogens. The bands at 110, 77 and 66 cm-i of the chloro, bromo and iodo complexes, respectively, have been assigned to this mode. These assignments are in good agreement with those of BRADBURY et al. [3]. Approximate normal coordin.ateanalysis In the previous sections, we have assigned the skeletal vibrations of the Zn(py)sX, type compounds based on the observed isotopic shifts. In order to confirm these results, we have carried out approximate normal coordinate analyses by assuming that the pyridine ligand is a single atom having the mass of CSH,N. The molecular parameters used were: Zn-N, 2.2 A and Zn-X, 2.4 A [7]. The six angles around the Zn atom were assumed to be tetrahedral. The potential energy was expressed using the Urey-Bradley field [S]. The initial set of force constants was taken from [6] E. MASLOWSKY. JR. and K. NAXAMOTO,AppZ. Spectry. 26,187 (1971). [7] M. A. PORAI-KOSHITS, L. 0. ATOVMY~ and G. N. TIS~~EIENJCO, 2%. Strukt. Khim. (1960). [S] T. SHIMANOUCEI,J. Chem. Phya. 17, 246 (1949).
1, 337
1466
YUTAKA SAITO, MABOIA
CORDESand -0
NAKAHOTO
those of related compounds (ammine and halogenoa complexes) and refined by the least-square method. The final set of force constants was: K(Zn-N), 0.55; K(Zn-X), 0.91; H(X-Zn-X), 0.03; H(N-Zn-N), 0.12; H(X-Zn-N), 0.11; F(X . . . X), 0.10; F(N . . . N), 0.50; F(N . . . X), 0.30; K, -0.10. (All in units of mdyn/A except K (mdyn A)). Theoretically, all the force constants involving the X atom should be slightly different for each halogen. No attempts have been made, however, to adjust these values rigorously since our main purpose was to confirm band assignments by normal coordinate analysis. Table 1 compares the theoretical and observed shifts due to the aZn-68Zn substitution. The agreement is quite satisfactory in spite of the approximations used. In order to co&m band assignments, we have calculated the potential energy distribution for each normal vibration. The results are in good agreement with those predicted from the observed isotopic shifts. As is shown in the last column of Table 1, both Zn-X and Zn-N stretching modes are coupled strongly in the bromo and iodo complexes. The bands at co. 186 cm- l of the bromo complex and at co. 169 cm-l of the iodo complex show rather small metal isotope shifts since this mode is similar to the totally symmetric stretching mode (Raman active) of a tetrahedral X Y,-type molecule. Ackncwledg~menta-The authors are grateful to Prof. K. MAOIDA of Kyoto University and Dr. C. W. SC~~PIPIER of Marquette University for their helpful discussions. Thanks are also due to the staff of the Kyoto University Computbion Centerfor the use of the FACOM 230-60 computer for this work.
[Q]I. NAKAQAWA,JASCO Rep.9,2 (1965).