109
ANISOTROPIC ESRPARAMETERS IN COPPER ACETYLACETONATES
Effect of Substituents on the Anisotropic Electron Spin Resonance Parameters in Copper Acetylacetonates'a
by Henry A. Kuska, Department of Chemistry, The University of Akron, Akron, Ohio
4304
Max T. Rogers, and R. E. Drullinger Department of Chemistry, Michigan State University, East Lansing, Michigan 448B8 (Received September BY, 1966)
In a previous paper it was found that the isotropic electron spin resonance (esr) metal hyperfine parameters for a series of substituted copper actylacetonates gave the opposite ordering of covalency from that predicted by other physical methods. The discrepancy was attributed to a contribution to the Fermi contact term of 4s character of opposite sign to the 3d contribution. Since the anisotropic esr metal hyperfine splittings can be used to calculate the covalency independently of the value of the isotropic splittings, we have investigated the anisotropic esr spectra for the same systems as studied in the previous investigation. The anisotropic data do give the correct ordering for the relative covalencies for this series. However, the model of considering only the 3d and 4s contribution to the hyperfine splittings could not be extended to some of the systems examined from the literature.
Introduction I n a previous paper' it was found that the isotropic electron spin resonance (esr) metal hyperfine parameters for a series of substituted copper actylacetonates gave the opposite ordering of covalency from that predicted by other physical methods. The discrepancy was attributed to a contribution to the Fermi contact term of 4s character of opposite sign to the 3d contribution. Since the anisotropic esr metal hyperfine splittings can be used to calculate the covalency independently of the value of the isotropic splittings, we have investigated the anisotropic esr spectra for the same systems as studied in the previous investigation.
mm. The anisotropic data were taken from frozen solution spectra. gL and A Iwere calculated from 1 9 = $71
+ 2591
The molecular orbitals involved are
Experimental Section The esr apparatus and the compounds studied were the same as described previouslylb except that a recent publication2 has shown that the compound that we considered as bis(hexafluoroacety1acetonatojcopper(11) is actually the dihydrate of this complex. We were able to prepare the blue anhydrous compound from the green dihydrate by subliming a t 90' and 1
(1) (a) Supported by the U. S. Army Research Office (Durham): (b) H. A. Kuska and M. T. Rogers, J . Chem. Phys., 43, 1744 (1965). (2) J. A. Bertrand and R. I. Kaplan, Inorg. Chem., 5, 489 (1966).
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H. A. KUSKA,M. T. ROGERS, AND R. E. DRULLINGER
110
where the y axis bisects the chelate rings. An effective DZhsymmetry is assumed as was suggested by Gersmann and Swalen3 and found for bis(dipivaloy1methanido)copper(II) from a recent investigation of the polarized optical spectra, esr, and extended Hiickel molecular orbital calculation^.^ The magnetic parameters as given by Gersmann and Swalen are 8X A,z-yz
- -[dp2
gz = 2.0023
f(p)
+ aff’p(1 - p”*”T(n)/2
= aff’P2S
g, = 2.0023
- f(@)]
-
2x
-[[~r~ti”~
-
2x
(10)
- c~a’S8‘~]
(11)
-[[(U~S’~
A,,
(9)
- [~ra’S6’‘~]
Azz
gy = 2.0023
(8)
vary over a range of approximately ~k0.005 cm-l; however, for a relative comparison of the covalency of a closely related series, the assumption of a constant value for P appears warranted. K is the Fermi contact term and is usually given a value of 0.43. However, in ref l b it was concluded that K cannot be considered a constant since the use of a single constant value of K leads to a relative ordering of covalency opposite that obtained by other physical methods. E’ortunately eq 12, 13, and 14 can be rewritten in a form which permits the use of the isotropic il value in place of K . This equation is
The corresponding equation for obtaining isotropic A value is
a2 from
the
If the assumption that the error in using eq 16 is due to a 4s contribution is correct, then eq 15 and 16 can be rewritten in the forms
a2
=
-_.
+-/Ai + g - 2 0023 PK K
(1 - f2)0.0975
+ -
PK
(16’)
is the fraction of 3d character in the copper 3d-4s ground state. I n eq 16’ an electron in a 4s orbital has been assigned a value of 0.0975 cm-l. This is the value determined theoretically from unrestricted Hartree-Fock theory by Freeman and Watson.s In obtaining eq 15’ and 16’ we have assumed that the ground state is composed only of d,, and 4s metal character. This approximation is satisfactory for very small admixtures of 4s character since a small admixA, = .[a2($ - K ) - ture has a large effect in eq 16’ but only a small effect 14 AUZ in eq 15’. However, for larger 4s admixture one must S is the group overlap integral between the copper consider that the 4s admixture may be due to some d,, orbital and the assumed sp2hybrid ligand u orbitals. d,, character in the ground state, since it is the d,, Its value has been determined by Kivelson and i‘l~eiman.~ and not the d,, orbital which is of the correct symmetry They obtained S = 0.076 for a copper-oxygen distance of 1.9 A. This distance is approximately in the middle (3) H. R. Gersmann and J. D. Swalen, J . Chem. Phys., 36, 3221 (1962). of the range of copper-oxygen distances reported6J (4) J. J. Wise, Ph.D. Thesis, Chemistry Department, Massachusetts for substituted copper acetylacetonate complexes. Institute of Technology, 1965. See also F. A. Cotton and J. J. Wise, T(n) is in integral over the ligand functions and is J . Am. Chem. SOC.,88, 3451 (1966). (5) D. Kivelson and R. Neiman, J . Chem. Phys., 35, 149 (1961). equal to 0.ZZ0.5 (6) P. K. Hon, C. E. Pfluger, and R. L. Belford, Inorg. Chem., 5 , x is the spin-orbit coupling constant for the free 516 (1966), and references cited therein. ion and is equal to -828 crn-l. P is the free-ion di(7) G. A. Barclay and A. Cooper, J. Chem. Soc., 3746 (1965), and pole term proportional to l / r 3 and is given a value of references cited therein. 0.036 cm-l. Estimates for this term in the literature (8) A. J. Freeman and R. E. WLitson, Phys. Rez., 41, 2027 (1961). f2
~
The Journal of Physical Chemistry
ANISOTROPIC ESRPARAMETERS IN COPPERACETYLACETONATES
111
Table I : Esr Data for Substituted Copper(I1) Acetylacetonates
1 2 3 4 5 6 7 8 9
H H H H
4
DYl
H H H H
H
10
H
11
CH3
142 173 168 167 176 164 172 175 160
186
11.2 24.5 23.4 25.2 26.6 3.5 13.1 28.2 19.5
25
2.351 2.306 2.310 2.308 2.281 2.302 2.293 2,285 2.266
2.050 2.051 2.042 2.040 2.046 2.071 2.061 2.042 2.053
2,254
2.075
2.269
2.047
Dihydrate in CHC13 Anhydrous in CHCl3 CHCI, CHCI, CHCli Pyridine Dimethylformamide CHC13' Diluted single crystal" Undiluted single crystald CHCl,
Reference 15. B. R. McGarvey, J. Phys. Chem., 60.71 ' Structure is R-CO-CHR'-COR". Calculated from eq 1 and 2. (1956). e Wilson and Kivelson, ref lob, find A , , = 173.1, A L = 22.7, g,, = 2.2875, and 9 1 = 2.031.
to mix with the 4s orbital. As discussed by O'Brieng the anisotropic esr parameters are dependent on the admixture of z 2 orbital with the zy orbital. If the wave function is
then eq 15 is
and eq 8, 10, and 11 are of the forms
gz = 2.0023
- -(cos2 8X A,,
;)[a2BP- f@)]
e>2
cos 2
[ ( U W
-
(8')
a(UrPs] (11l)
Results The esr data are given in Table I. For the diphenylsubstituted acetylacetonate complex we were unable to obtain values of Ail and 911 owing to the appearance of a superhyperfine splitting of approximately 4 gauss. I t was not possible to determine the exact number of lines and relative intensities owing to overlapping of
the sets from each Ai I copper line. As shown in Figure 1, between copper lines two and three, thirty lines were observed. The gL and AL values were calculated from eq 1 and 2 since it is difficult t o measure these parameters accurately from powder spectra. The use of these equations involves the assumption that the freezing of the solvent does not change the local crystal field in a manner that would affect the isotropic A and g values. Falls and Luckhurst suggest that this approximation may lead to an error in the value of A , as high as 20%.loa Fortunately Wilson and Kivelsonlob have measured the temperature dependence of copper acetylacetonate in chloroform and have also measured g1 and AL directly from the low-temperature spectrum. As can be seen from a comparison of the molecular orbital coefficients obtained from our data in Table I1 (the values for compound 8) with the values obtained from Wilson and Kivelson's data (8') the change in the A values appears to be nearly compensated for by a corresponding change in the g values. The u molecular orbital coefficients are given in Table 11. The coefficients obtained from the anisotropic data do give the expected trend in covalency. Electron-withdrawing substituents in the acetylacetonate skeleton decrease the covalency as do electrondonating solvents which presumably interact along the x axis. Compound 11, which is the only compound substituted in the R" position, appears to be less covalent M.O'Brien, Proc. Roy. Soc. (London), A281, 323 (1964). (10) (a) H. R. Falls and G. R. Luckhurst, Mol. Phys., 10,597 (1966); (b) R. Wilson and D. Kivelson, J. Chem. Phys., 44,4445 (1966). (9) M. C.
Volume 71,Number 1
January 1967
H. A. KUSKA,M. T. ROQERS,AND R. E. DRULLINGER
112
even though a methyl group is expected to be electron donating. The smaller covalency is consistent with the literature stability constant data" but is inconsistent with the polarographic reduction data cited in our earlier paper.' Another change from the earlier paper is that from the isotropic esr data in the earlier paper the phenyl group appeared to be more electron withdrawing than a methyl group while from the present anisotropic data they appear to be about equal as shown by the data for compounds 5 and 8 in Table 11.
Table 11: u Molecular Orbital Coefficient for Substituted Acetylacetonate Complexes Compd
no."
a ' b
1 2 3 4 5 6 7 8 8' 9 11
0.698 0.757 0.763 0.763 0.777 0.706 0.743 0.779 (0.771)' 0.712 0.784
a9
0.812 0.840 0.813 0.802 0.793 0.843 0.831 0.791 (0.802)' 0.744 0.816
a ' d
P'
0.828 0.853 0,820 0.808 0.795 0.865
0.981 0.985 0.991 0.993 0.997 0.975 0.985 0.997
0.844
0.793 ... 0.748 0.821
...
0.994 0.994
The numbers correspond to the numbering in Table I. Calculated from the isotropic data, eq 16. Calculated from the anisotropic data, eq 15. Calculated taking into account a 4s contribution, eq 15' and 16'. * fB is the fraction of 3d character in the 3 d 4 s ground state assumed in writing eq 15' and 16'. Calculated with data from ref lob.
'
The in-plane ?r-bonding molecular orbital coefficient,
b2, can be obtained from eq 8 if the value of Azt--yl is known. Wise4 estimates that this transition should occur at =16,800 cm-' in pure copper acetylacetonate. While AllenI2 assigns the 18,000-cm-' band as the x2 - y2 transition. In copper acetylacetonate solution spectra at most only two peaks are observed. In CHCIP the peaks are at 14,900 and 17,900 cm-l; however, a.s the solvent becomes more polar, the peaks move together until in pyridine only a single peak is observed which has been resolved by Gaussian analysis into three peaks at 12,100, 14,800, and 15,900 Cm-'. Using the assignments given in Table I11 for Az*+, the in-plane bonding appears to be small' This is in agreement with the Of p2 calculated Wise4 using an extended Huckel molecular orbital theory. From the frozen solution spectra it w&s not possible to Obtain separate for and For the single crystal, Maki and McGarvey16 observed a small The Journal of Physical Chemktry
Table I11 : In-Plane r-Bonding Molecular Orbital Coefficients A&'
assignLigand
Acetylacetone in CHClr Acetylacetone in pyridine Hexafluoroacetylacetone in CHCls (hydrated) Dipivaloylmethane (single crystal) Tetraphenylporphine Pyridine (single crystal) N-Propylsalicylaldimime (single crystal)
ment, cm -1
8'
Ref
16,800 17,900 16,000
0.96 1.02 0.89
a
14,500
0.98
a
18,200
0.94
b
22,000 15,600 16,800
31 0.92 0.72
d e
a
C
a T his work. Reference 4. J. M. Assour, J. Chem. Phys., W. Schneider and A. V. Zelewsky, Helv. 43, 2477 (1965). Chim. Acta, 48,1529 (1965). C. W. Riemann, G . F. Kokozska, and H. C. Allen, Jr., J. Res. Natl. Bur. Std., in press.
amount of rhombic character, but they were unable to assign gz and gy accurately. From his theoretical calculations Wise' predicts g, = 2.069 and gu = 2.062 for Cu(AcAc)*. It will be interesting to repeat the single-crystal work with a higher frequency spectrometer to see what the gz and gu values are since the g2 gu separation in magnetic field is proportional to the klystron frequency used.
Discussion Proton Splittings. The large number of lines observed for the diphenylacetylacetonate complex suggests that the splittings are due to the three types of inequivalent protons on the phenyl groups. There were three possible mechanisms considered for this splitting. The first was that even in dilute solution the chelates existed as dimers with the phenyl group of one complex situated above the z axis of the second complex. However, this model is considered improbable since the splittings persisted in polar solvents such as dimethylformamide. Also, the unpaired electron is in the plane of the complex, and copper esr (11) M. Calvin and K. W. Wilson, J . Am. Chem. SOC., 67, 2003 17.4; for Cu(y-CHaAcAc)p, (1945). For Cu(AcAc)a, KlKz KIKI 6 16.4. (12) H. C. Allen, J . Chem. Phgs., 45,553 (1966). (13) J. P. Fackler, Jr., F. A. Cotton, and D. W. Barnum, Inorg. Chem., 2, 97 (1963). (14) R. L. Belford, A. E. Martell, and M. Calvin, J . Inorg. Nucl. Chem.. 2 , 11 (1956). (15) A, H. Maki and B . R. McGarvey, J . Chem, p h y s . , 2 9 , 31 (1958).
ANISOTROPIC ESRPARAMETERS IN COPPER ACETYLACETONATES
1'
113
'
0
0
Figure 1. Hyperfine structure between g,, l i e s 2 and 3 in a frozen CHCls esr spectrum of copper(I1) dibenzoylmethane.
spectra apparently do not exhibit further splittings from ligands coordinated perpendicular to this plane. For example, nitrogen splittings are not observed for solutions of copper acetylacetonate in pyridine, and datalb indicate a greater z-axis interaction for the acetylacetonate in pyridine than for the diphenylacetyl acetonate in chloroform. Another possible mechanism is u delocalization. Wise' found from his extended Huckel molecular orbital calculation that the unpaired electron spent 2.2% of its time at each position, which in our case would correspond to a phenyl group. For the phenyl radical, where the unpaired electron is also in a u orbitd, the A values were found to bel6 18.1, 6.4, and -0 gauss for the cy, 0, and y protons, respectively. Using this radical as a model we have approximated the total spin density required in each phenyl group cfi) to give an cy-proton splitting of 4 gauss by a simple proportion, fs/4 = 1/18.1. This givesf, = 0.2, which is too large since it predicts a total spin density in the four phenyl groups of 0.8. The actual spin density on each cy proton can be estimated from the calculated molecular orbital coefficients of Dixon17for the phenyl radical and the observed splittings by the proportion fsH/4 = 0.026/18.1. This gives a spin density of 0.0057 at each a proton. There are several other reported cases of proton splittings in this position for similar chelates. Maki and McGarveyl* found a 5.5gauss proton splitting from the starred proton in copper(11)bissalicylaldehydimine
usH)
Larin, Panova, and Rukhadzel9 observed a proton splitting from positions R1 and Ra of A I I = 7 gauss and
s
4
I
6
0
0 0
7
6
9
Cf Figure 2. Plot of u molecular orbital coefficient us. nitrogen hyperfine splitting value for a group of copper complexes (data from literature).
A l = 6 gauss for a series of compounds of the general formula
where RI, Rz, Ra = H or CHa. These experimental A H values give a spin density of approximately 1% at the proton in relative agreement with the 2% value calculated by Wise. The fact that the proton splittings were not observed when only one methyl group was substituted by a phenyl group (compound 5, Table I) is difficult to explain with a model of u delocalization but could be explained by s delocalization. Further experiments with deuterated phenyl groups and higher frequency esr equipment are contemplated to see if gv does indicate a higher A covalency for the diphenylacetylacetonate complex than for the other substituted complexes. At present we are not able to distinguish between the two alternatives: (1) that the electron spin density is u delocalized and that the net spin density in the phenyl group is small due to a cancellation of positive and negative spin density; (2) that (16) J. E. Bennett, B. Mile, and A. Thomas, Chem. Cormnun., 286 (1986). (17) W. T. Dixon, Mol. Phys., 9 , 201 (1965). (18) A. H. Maki and B. R. McGarvey, J . Chem. Phys., 29, 35 (1968). (19) G. M. Larin, G . V. Panova, and E. G. Rukhadze, Zh. Strukt. Khim., 6, 699 (1966).
Volume 71, Number 1 January 1967
114
H.A. KUSKA,M. T. ROGERS, AND R. E. DRULLINGER
the spin density is ?r delocalized through a configuration interaction mechanism similar to that proposed by Kivelson and Lee20 and by Fortman and Hayes.21 Esr Theory. Although the revised equations (15’) and (16’), which take into account the copper 4s contribution do give the apparent correct ordering of covalency for this series, the agreement cannot be extended to other copper complexes. For example, Figure 2 is a plot of the value of the nitrogen hyperfine splitting values, A N us. the a2 values calculated from eq 15’ and 16‘. If the sp2 hybridization of the nitrogens in different copper chelates remains constant, then the A N values are expected to be good indications of the covalency; however, no apparent correlation is found in Figure 2. Also, in Table IV there are listed a number of compounds for which eq 15 predicts a more covalent bond than predicted by eq 16. This discrepancy cannot be explained by a 4s contribution. It could be attributed to admixture of z2 and/or 4p orbital into the xy ground state. The possibility of z2 admixture could be checked by solvent effect esr data on a square-planar copper chelate which has only copper-nitrogen bonds to see if the change in covalency as predicted from eq 15”, 8’, lo’, and 11’ correlated with the change in covalency as predicted from the nitrogen splittings with
the use of O’Brien’s equationss for the dependence of the ligand splittings on 6.
a**
a2 f
Ref
Bisdithiocarbamate Bis-%hydroxyquinoline Tris-l,l0-phenanthroline Bismaleonitriledithiolate
0.582 0.818 0.804 0.617
0.474 0.763 0.697 0.489
Q
b C
d
’ R. Pettermon and T. Vannghrd, Arkiv Kmi, 17,249 (1960). G. F. Kokoszka, H. C. Allen, Jr., and G. Gordon, J . Chem. Phys., 42, 3730 (1965). e H. C. Allen, Jr., G. F. Kokoszka, and R. G. Inskeep, J . Am. Chem. SOC.,86, 1023 (1964). A. H. Maki, N. Edelstein, A. Davison, and R. H. Holm, ibid., 86, 4580 (1964). Calculated from the isotropic data with eq 16. Calculated from the anisotropic data with eq 15.
of
Physical Chemktry
B. R. MCGARVEY (Polytechnic Institute of Brooklyn, Brooklyn). I do not feel that a vibronic interaction could introduce enough 4s character to account for your results, but, if we were to accept your explanation, your results would indicate that the 4s character increases as the bond becomes more ionic. Do you have any comments? H. A. KUSKA. The admixture is directly proportional to the metal-ligand vibrational frequency and is inversely proportional to the xy to 211 energy separation. From compound 8 in Table I to compound 1 the metal-ligand vibrational frequency d e creases from 455 to 415 cm-1 and the zy to z* energy separation decreases from -14,500 to -12,000 cm-*. It would appear that the decrease in the energy separation is the dominant effect since the 4s contribution increases. In Table I1 the f* values indicate that for the highest symmetry complex (compound 8) only 0.003 4s character is required, and this amount would be reduced if either our approximations and choice of values for the constants were such that O L ~ is too low or as is too high. If the 4s character is due to a vibronic mechanism, we would expect an inverse dependence of A on temperature. An inverse dependence has been reported for copper acetylacetonate; see ref 10.
N. Y.). Could the 4s character, required in the ground state of the copper complexas to account for the observed hyperfine
Ligand
The J o u r d
Discussion
K. KRIST(Fairchild Camera and Instrument Corp., Syosset,
Table IV: Esr Data for Copper Complexes Which Cannot Be Explained by Only a 4s Correction
’
Acknowledgment. We wish to thank Professor J. P. Fackler, Case Institute of Technology, and Professor R. H. Holm, University of Wisconsin, for helpful discussions.
splittings, arise because of zero-order orbital mixing between the Cu 4s and 3d,, orbitals? Since it has been proposed that d,, and d,, orbitals are split in the complex, it would seem that the symmetry is low enough for this to be the case.
H. A. KUSKA. In D2h zy contains the unpaired electron and transforms as Bt,; the z* orbital transforms as At,. In CsV (which would be the symmetry if one solvent molecule coordinates along z ) xy is A2 and 2%is AI. In Clh (0-CR‘-CHCR”-O with R’ # R”) both xy and z* transform as A, and would mix. (20) D. Kivelson and 9. K. Lee,J . C h m . Phys., 41, 1896 (1964). (21) J. J. Fortman and R. G . Hayes, &id., 43, 15 (1965).
