346
KAZUO
NAICAMOTO, Y'UxlIYOSfI MORIMOTO
AYJ)
ARTHUR'E. MARTELL
V O ~66 ,
INFRARED SPECTRA OF METAL CHELATE COMPOUNDS. V, EFFECT OF SUBSTITUEKTS ON THE INFRARED SPECTRA OF METAL ACETYLACETONATESl BY KAZUONAKAMOTO,~ YUKIYOSWI MORIMOTO,~ AND ARTHURE. MART ELL^ Department of Chemistry, Clark University, Vorcester, Massachusetts Received October 6, 1961
The infrared spectra of the metal chelate compounds of hexafluoro- and trifluoro-acetylacetone, benzoylacetone, and dibenzoylmethane with Cu(I1) and Ni(I1) have been measured in the range between 4000 and 300 cm.-'. Vibrational frequencies of these compounds were calculated by the use of the perturbation method on the results of a previous normal coijrdinate treatment of bis-acetylacetono-Cu(I1). The calculated force constants provide a quantitative measure of the electronic effect of the trifluoromethyl and phenyl groups on the chelate rings.
Introduction In the previous papers,s the effect of changing the metal on the infrared spectra of metal acetylacetonates has been calculated by the use of the perturbation method. As a next step, this paper deals with the effect of replacement of the methyl groups of the metal acetylacetonates on the infrared spectra. The compounds studied are represented by the general formula H
I
R,
Lti\c,R'
C
:I h
R, R' = CF3, CH3 and C ~ H S M = Cu and Ni
I Changes of the electronic structures of the chelate rings resulting from substitution in the ligand can be detected from the infrared spectra, and more quantitative information can be deduced from calculations of the force conshants by the use of the perturbation method. It should be noted that these substituted compounds exhibit the absorptions characteristic of the chelatering as well as of the substituents themselves. Therefore, caution must be taken in differentiating between these two kinds of absorption. The sorting out of the many infrared bands of these compounds ia very difficult in the sodium chloride region, where overlapping of the bands is serious. On the other hand the spectra of the low frequency region are helpful in following substitution effects, since the organic substituents have very few strong absorptions below 600 crn.-l. Although the infrared spectra of some of the compounds described by formula I have been rep ~ r t e d , ~the - ~previous papers have been concerned (1) This work was supported by the U. S. Army Research Office (Durham) under Contract No. DA-19-020-ORD-5119 and grant No. DA-ORD-31-124-61-G61. ( 2 ) Department of Chemistry, Illinois Institute of Technology, Technology Center, Chicago 16, Illinois. (3) K. Nakamoto, P. J. MoCarthy, A. Ruby and A. E. Martell, J . Am. Chem. Soc., 83, 1066, 1272 (1961). (4) L. 3. Bellamy and R. B. Branch, J . Chem. Soc., 4487 (1954). (5) R. L. Belford, A. E. Martell and M. Calvin, J . Inorg. & Nuclear Chem., 2 , 11 (1956). (6) H. F. Holtsclaw, Jr., and J. P. Collman, J . Am. Chem. SOC.,79, 3318 (1957). ( 7 ) R. P. Dryden and A. Winston, J . Phys. Chem., 6 2 , 635 (1958). (8) G. A. Guter and G. S. Hammond, J . Am. Chem. Soc., 81,4686 (1959).
primarily with the carbonyl stretching frequencies and did not give spectra in the KBr and CsBr regions.
Experimental Spectral Measurements.-A Perkin-Elmer Model 21 infrared spectrophotometer equipped with NaCl, KBr, and CsBr optics was used to obtain spectra in the range between 4000 and 300 crn.-'. The KBr disk method was employed for the NaCl and KBr regions whereas the nujol mull technique was used for the CsBr region. Calibration of the frequency reading wm made with polystyrene film (NaC1 region), with 1,2,4-trichlorobenzene (KBr region), and with water vapor for all regions. Preparation of Compounds.-All the compounds studied were prepared according to the methods described in the literature.4-8JO Purity of each compound was checked by measurement of the melting point and of the ultraviolet spectrum.
Results and Discussion I. Trifluoromethyl Group.-Figures
1 and 2 show the infrared spectra of Cu(1I) and Ni(I1) complexes of substituted acetylacetonates indicated by formula I. The band assignments given for the acetylacetonates originate in the previous normal coordinate treatment." I n order to identify the GF8 group absorptions, the infrared spectrum of gaseous C F P is shown by dotted curves on those of hexafluoroacetylacetanates. From the comparison of the spectrum of C R with that of a hexafluoroacetylacetonato complex, the bands near 1200,900, 700, and 500 crn.-I in the latter were empirically assigned as the bands characteristic of the CFa group. The bands characteristic of the chelate ring in the fluoro complex then were connected by dotted lines to those of the correspon+ng acetylacetonato complex. It is seen from Flg. 1 and 2 that CF8 substitution causes marked shifts of the C=C(vV) and C-O(vl) stretching bands to higher frequencies, and of the M-O(vg) stretching band to lower frequency. Table I summarizes the frequencies of these bands together with the stability constants of the substituted compounds. In order to confirm these empirical band assignments, perturbation calculations2 were made for hexafluoroacetylacetonato complexes of Cu(I1) and Ni(I1) in which the CF, group was considered to be a single atom having the same mass as that of the group. The parameters used for the calcula(9) J. Charette and P. TeyssiB, Spectrochim. Acta, 16, 689 (1960). (10) H. Schlesinger, H. Brown, J. Kats, S. Aroher and R. Lad, J . A m . Chem. Soc., 76, 2448 (1953). (11) K. Nakamoto and A. E. Martell, J . Chem. Phys., 38, 588 (1960). (12) P.J . H. Woltz and A. H. Nielsen, ibid., 20, 307 (1952).
(i
T,og K I K zin 75'% diosane-1120 mixture at 30". (L. G. Van IJitert,, W. C. I ~ e r n n l i u x ,and B. 12. I)ou~las,J , Am. Chem. 75, 457, 27% (19.53).)
SOC.,
TABLEI1 MASSO F SURSTITLJENT, METAL-OXYffEN Boxn Metal
R
C1'3
Cu(I1)
Ni(I1)
Suhstitrient
R'
CF3 CH3 CaHs CF3 CHs (:GI16
Mass of aiihubituent R (atomic weight)
69 15 77 69
15 77
1>ISTANCF: A N D FORCZ
Met& oxygen
A.
K(C-0)
CONSTANT
Force ronstant (105 dyn~/orn.)
K(C=C)
K(C-R)
K(M-0)
1.05 1.95 1.93
7 90 6.90 0.82
5.81 5.35 5.49
1.80 3.60 3 .60
1.75 2.20
2.05 2.05 2.05
7.72 7 65 7 ,55
5.88 5.35 5 BO
3.60
bstuncc,
1.80 3.60
2.31
1.35 2.05 2.25
tions are listed in Tnblc 11. Since no structural dat)a arc available for these substituted compounds, it was assumed that the bond distances in lhe chelate ring do not change appreciably by substitution. Although all the force constants are expected to be changed by substitution, only the relatively more important C=C, C 4 , C-R, and M-0 stretching force constants were adjusted to fit the observed frcquencics. Evidently such an assumption does not give completcly satisfactory results, especially 5 for the vibrations which are neglcctcd. For simof Cu(I1) complexes of vmious plicity in the calculations and because the centcr of Fig. 1.-Infrared spectra &dike tones. interest is in the C=C, C=O, and M-0 stretching bands, only the force constants of thcse three bands plus that of the adjacent C-11 group were employed for the perturbat,ion calculation. Table I11 compares the observed frequcncies with the calculated valucs obtained by using the parameters given in 'Tablo 11. The agreements are quite satjisfactory for VA, vl, vs, vI1, arid v6 bands and less s:it,isfactory for other bands, as expected, since their frequencies arc functions of other force conI 00 I500 ICC 90C 700 5CC stants such as those involving angle dcformation. ,- I C N It is to hc noted that coupling between various Fig. S.--Infrared spectra of Ni(I1) complexes of various vibrational rnodcs also C:LIISCS difficulty in Rdj usting 8-dilictoncs. some of these frequencies. Nevarthclcss, it, is clear l h t tho substitiithn of trifluorometliyl ill placc of n tonates is intermediatc between thosr, of the acemethyl group iiic:ro:iscs t,hc C=C and C==O and tylacetonato and hexafluoroacet,ylac:ctonnto comdccrcsses the C-lt and M-0 stretching form con- plexes, since tho spectra of the trifluorotlcetylncc1,~St~:LIll,S. nates are intermcdi&tc bctwccn those of the latter Onr. woiild predict on the basis of clectroiiic two compounds. theory of bonding tha,t the strong positive inductive 11. Phenyl Group.-Figures 1 arid 2 show tfic cffcct of the C1c3 group strengthens t,he C=C and infrared spectra of Cu(I1) and Ni(I1) complexes of C=O bonds and weakens the J I - 0 bonds. There- benzoylacetone and dibenzoylmethane. The specfore, the resulk of the present infrared study pro- trum of benzene also is shown by dotted curves vide experimental evidence confirming this predic- superimposed on the spectra of dibenzoylmethane tion. The wcakcning of the M-0 bond by CF3 complexes. It is seen that overlapping of the bensubstil ution is also in agreement mit,h the lowcr zene bands with those of the chelate rings is serious stabilit,y constants, shown in Table I, of the tri- in the whole range of the benzene spectrum, and fluoromethyl derivatives. Finally, it is seen that empirical assignments therefore are virtually imevidence is given in Table I11 indicating that, tho possible. As is seen in Fig. 1 and 2, however, the strength of the M-0 bond in the trifluoroacetylace- C=C(v8), C-@(VI),and M-Ojv5)stretching bands (CY
l3CO
I
3C3
K ~ z u oNAKAMOTO, YVKIYQSHI MORIMOTO AXD ARTHURE. MARTELL
348
Vol. 66
TABLE111 COMPARISON OF CALCULATED AUD OBSERVEDFREQUENCIES IS SUBBTITUTED ACETYLACETONATES ( CRI.-1) CB,,CBa, Cu Obsd. Calcd.
CFn, CFa, Ni Obsd. Calcd.
1644 1614 1563 1537 1357 1145
1643 1613 1563 1535 1349 1145
..
1647 1614 1505 1191 1208 *.
..
e .
1644 1612 1498
Ce%, CeI-fr,C U C&lod.
Obsd.
1693 1544 1526
1592 1543 1486
1188 1204
..
..
..
869 874 805 746 730 746 809 .. 807 618 608 588 597 602 588 415 416 397 371 320 355 All the vibrations characteristic of
1295 1233 944 932
1270 1226 937 902
..
..
873 730
..
..
A
..
744 707 663 694 634 462 462 337 373 the CFa and CeH5 groups are not a .
593 580 397 371
are out of this range. Therefore the shifts of these bands resulting from phenyl substitution may be discussed without ambiguity. Table I compares the frequencies of 2’8, vl, and VI, bands and the stability constant for the complexes of acetylacetone, benzoylacetone, and dibenzoylmethane. It is seen that phenyl substitution shifts the M-0 stretching (v5) of the Cu(I1) and Ni(I1) complexes and the C=C stretching (”8) of the Cu(I1) complex to higher frequencies. The shift of the C=O stretching ( Y J band is somewhat irregular. In order t o confirm the foregoing empirical band assignments, perturbation calculations were carried out for the dibenzoylmethane complexes of Cu(I1) and Ni(II), with the same assumptions as those made previously for CFBsubstitution. The results shown in Table 111indicate that phenyl substitution slightly increases the C=C and M-0 stretching force constants and slightly decreases the C=O stretching force constant. Neglecting the weak negative inductive effect of the aromatic ring, one may look for the principal electronic effects in the mesomeric interactions of the phenyl group with the semi-aromatic metal chelate ring, The result of remnance shift of n-electrons to the chelate ring can be visualized by asauming small contributions from resonance forms such as A, B, and C shown below O z M/
1595 1593 1644 1522 1293 1230 940 929
Assignments
1268 1234 945 905
..
..
745 719 686 458 337 listed.
659 632 460 3 73
C=C Btr. (vg) C-0 str. ( V I ) CO str. CH bend. ( va)
1595 1695 1515
+ CC str. + CR str.
~
..
/ +
