1728
C. H. HE,R. J. KURLAND, C. I. LIN,AND N.
c. LI
Nuclear Magnetic Resonance Isotropic Shifts in 4-Methylpyridine and 4-Methylpyridine N-Oxide Complexed with Copper(I1) fi-Diketonatesl
by C. H. Ke,2aR. J. Kurland,*" C. I. Lin,2band N. C. LiZb Departments of Chemistry, Duquesne University, Pittsburgh, Pennsylvania 16219 and State University of New York at Bu$alo, Bufalo, New Y O T I4U.G ~ (Received October ,@Y, 1969)
Isotropic nuclear magnetic resonance shifts have been measured for 4-methylpyridine and 4-methylpyridine N-oxide complexed with Cu(p-dik)z in CDCls (p-dik = P-diketonate). The P-diketonates chosen are acetylacetonate (AA), trifluoroacetylacetonate (TFA), and hexafluoroacetylacetonate (HFA). In both 4-methylpyridine and 4-methylpyridineN-oxide complexes, the magnitude of the contact shifts lie in the order CU(HFA)~ > Cu(TFA)z > Cu(AA)z. This order is the same as that for the stability constants for the adduct formation found from spectrophotometric studies. For a given Cu(p-dik)2, 4-methylpyridine (4-MePy) forms a stronger adduct than 4-methylpyridine N-oxide (4-MePyO). The isotropic shifts vary linearly with the reciprocal of the absolute temperature for Cu(HFA)z.2(4-MePy) and Cu(HFA)z-2(4-MePyO) throughout the temperature range of -50 to +50°, However, anomalous temperature variation was found for Cu(AA)z, 4-MePy, CU(TFA)~* 4-MePyO, and Cu(TFA)g-$-MePy.
Nuclear magnetic resonance isotropic shift studies of paramagnetic transition metal complexes can provide useful information on the electronic structure. 3 Relatively small isotropic shifts are found in studies reported for copper(I1) c~rnplexes,~-~ compared to those for analogous nickel(I1) and cobalt(I1) complexes. Noreover, in copper(I1) compounds the isotropic shifts in some cases do not follow a Curie law temperature d e p e n d e n ~ e . ~ , ~ Copper(I1) P-diketonates are known to form 1 : l and 1:2 adducts with pyridine and pyridine P\'-oxide type ligand^.^^^-" Such adduct formation through axial coordination has a dramatic influence on the electronic spectra,l2-l4 esr parameters,15J6 and solvent extraction properties." Stoichiometric and equiIibrium studies have shown that fluorine substitution in the acetylacetone moiety markedly enhances adduct formation.11118 Thus, in a benzene solution containing 4-methylpyridine, copper(I1) bisacetylacetonate, Cu(AA)*, forms only a 1: 1 adduct, whereas copper(I1) his(hexafluoroacetylacetonate), CU(HFA)~,and copper (11) bis (trifluoroacetylace tonate), Cu (TFA)2 , form both 1: 1 and 1 :2 adducts. The present study mas undertaken to investigate the effect of fluorine substitution in acetylacetone on the nuclear magnetic resonance isotropic shifts of 4-methylpyridine (4-&IePy), and 4-methylpyridine N-oxide (4-MePyO) coordinated t o copper(I1) p-diketonates. The temperature variation of the isotropic shifts was examined for all systems and the results were compared with the stoichiometry of the adduct species in the solution. The pyridine nitrogen group in the ligand The Journal of Physical Chemistry
also serves as a useful model for the corresponding group in the biologically important compounds. Experimental Section
Materials. 4-Methylpyridine was obtained from Eastman Organic Go. and was purified by distillation. (1) This investigation was supported by National Science Foundation Grant No. GB 8237 and by Atomic Energy Commission Contract No. AT(30-1)-1922 at DU, and by National Science Foundation Grant No. GP 10463 at SUNYAB. (2) (a) State University of New York at Buffalo: (b) Duquesne University. (3) D. R. Eaton and W. D. Phillips, Advan. Magn. Resonance, I, 102 (1965). (4) Z. Luz and R. G. Shulman, J . Chem. Phys., 43, 3750 (1965). (5) R. W. KIuiber and W. D. Horrocks, Jr., Inorg. Chem., 6 , 1427 (1967). (6) C. C. Hinckley, ibid., 7, 396 (1968). (7) A. F. Garito and B. B. Wayland, J. Amer. Chem. tSoe., 91, 866 (1969) (8) D. P. Graddon and E. C. Watton, J . Inorg. Nucl. Chem., 21, 49 (1961). (9) R. D. Gillard and G. Wilkinson, J . Chem. Soc., 5885 (1963). (10) W. R. May and M. M. Jones, J . Inorg. Nucl. Chem., 2 5 , 507 (1963). (11) W. R. Walker and N. C. Li, ibid., 27, 2255 (1965). (12) R. L. Belford, A. E. Martell, and M. Calvin, J . Chem. Phys., 2 6 , 1165 (1957). (13) D. P. Graddon, J . Inorg. Nucl. Chem., 14, 161 (1960). (14) L. L. Funk and T. R. Ortolando, Inorg. Chem., 7, 567 (1968). (15) B. R. McGarvey, J . Phys. Chem., 6 0 , 71 (1956). (16) (a) H. A. Kuska, M. T. Rogers, and R . F. Drullinger, ibid., 71, 109 (1967); (b) H. A. Kuska and M. T. Rogers, J . Chem. Phys., 43, 1744 (1965). (17) H. Irving and D. E. Edginton, J. Inorg. Nucl. Chem., 27, 1359 (1965). (18) C. H. Ke and N. C. Li, ibid., 2 8 , 2265 (1966). a
ISOTROPIC SHIFTS I N
4-l/IETHYLPYRIDINE AND
4-METHYLPYRIDINE
4-Methylpyridine N-oxide was purchased from K arid K Laboratories and was recrptallized twice from benzene and dried in a vacuum desiccator over PzOa. Cu(AA)z, Cu(TFA)2, and CU(HFA)~ were prepared and analyzed as previously described.18 All the copper chelates were dried in a vacuum desiccator over PzOs. Chloroform was purified as previously described.I6 Nmr Measurements. Proton nrnr spectra were obtained at 60 MHz via Varian A-60 and HA-60 nrnr spectrometers. Deuteriochloroform was used as a solvent and TMS as an internal reference. The chemical shifts were obtained by side band interpolation techniques. For line width and variable temperature experiments, the Varian HA-60 nmr spectrometer was used in the field-frequency controlled mode. Temperature measurements were calibrated by a thermistor19 and should be accurate to & l o . The line widths, measured as the full width at the half maximum peak intensity, and the isotropic shifts, defined as the difference between the observed chemical shifts in the presence of copper chelate and the chemical shifts of the free ligand, are accurate to 0.2 HZ or 275, whichever is greater. Upfield shifts are defined as positive and downfield shifts as negative. Optical Spectra and Stoichiometry. Optical spectra were recorded with a Cary 14 recording spectrophotometer at room temperature. Purified chloroform was used as solvent. The stoichiometry of the adduct species in solution was deduced from the spectral change as a function of the molar ratios of ligand to copper chelate. Electron Spin Resonance. Esr spectra of chloroform solutions of CU(TFA)~,CU(HFA)~,and 4-methglpyridine or 4-methylpyridine N-oxide were obtained using a Varian V4500-10A spectrometer operating at about 9.2 kMHz.
1729
N-OXIDE
20
0
-
20
AJ
..
\
40
\
\ \
\ \
-
60
-
80
\ \
----
\
CU(AA):!
\ \
CU(AFI)~
\\
CU(TFA),
\\
Q' -100
\ \
,
I
0.02
0.04
0.06
I
,o.o8
I
0.10
C U ( B A ) ~or C U ( T F A ) ~ , I
Figure 1. Variation of isotropic shifts of 4-methylpyridine in CDCL with Cu(che1ate)t concentration; (a) upper scale for Cu(HFA)S in 3 M 4-methylpyridine; (b) lower scale for CU(AA)~ and Cu(TFA)z in 2 M 4-methylpyridine at 36".
glassy "solutions" a t about - 180". Isotropic and anisotropic q values were calculated directly from the observed spectra. The g values obtained are essenResults and Discussion tially the same for all the complexes. Thus gll and g l Adduct formation of CU(TFA)~ and Cu(HFA)Z with are: 2.32 and 2.07 for C U ( T F A ) ~ * ~ - M2.34 ~ P and ~, 4-MePy and 4-MePyO in chloroform solution was 2.06 for C U ( T F A ) ~ . ~ - M ~ 2.36 P ~ Oand , 2.07 for Cuobserved spectrophotometrically. CU(AA)~had been (HFA)2.2(4-MePy), 2.39 and 2.05 for CU(HFA)~. reported to form only a 1 : 1 adduct with 4-MePy in 2(CMePyO). These results compare well with the chloroform.8 No adduct formation of CU(AA)~with values 2.30 and 2.07 for Cu(AA)2 in pyridine,lea and 4-MePyO in chloroform 'was detected. The stoichiom2.28 and 2.08 for C U ( T F A ) ~ . ~ - M ~ P ~ O . ~ etry of the adduct species in solution was deduced Isotropic proton magnetic resonance shifts of (a) from the spectral change as a function of the molar 4-MePy coordinated with CU(AA)~,Cu(TFA)t, and ratio of ligand to copper chelate. Thus, CU(TFA)~ Cu(HFA)z and (b) 4-MePyO coordinated with Cuforms mainly 1 : 1 adducts with 4-MePy and 4-MePy0 (TFA)z and CU(HFA)~ were obtained in CDCL solution ~ of 100:1, and Cuup to a ligand to C U ( T F A ) ratio samples. The lines of 4-MePyO did not shift upon (HFAh forms mainly 1:2 adducts with 4-MePy and addition of Cu(AA)z; this result is consistent with the 4-MePyO when the ligand to CU(HFA)~ ratio exceeds optical results which showed that CU(AA)~does not 5 : 1 and 20: 1, respectively. These results are conform an adduct with 4-MePyO in chloroform solution. sistent with other spectrophotometric results.6z8t11 Typical nrnr data are shown in Figures 1 and 2. Electron spin resonance spectra of the 1: 1 adducts of I n all cases studied the ligand concentration was kept Cu(TFA)z with 4-MePy and CMePyO and the 1 : 2 in excess. The fast-exchange limit prevailed in the adducts of Cu(HFA), with 4-MePy and 4-MePyO were obtained for chloroform solutions at 20" and for frozen, (19) A. L. Van Geet, Anal, Chem., 40, 2227 (1968). Votume 74, Number 8 April 16, 1970
C.H. KE,R. J. KURLAND, C. I. LIN, AND N. C. LI
1730 temperature range of -40" to +50", as shown by the linear plot of the log of the line width (the y C H , peak of the ligand) us. the reciprocal of the absolute temperature, as shown in Figure 3, and by the fact that the observed shifts depended only on the ratio of metal ionto-adduct concentrations. The temperature variation of the nmr shifts is illustrated in Figures 4 and 5. The isotropic shifts are found to vary linearly, under the experimental conditions chosen, with increasing copper chelate concentration as shown in Figures 1 and
50
3.L
3.2
3.6
ld/T'
4.2
L.0
3.8
K
0
Figure 3. Plots of log w1/2 (wl,2 is the full line width of the y-CH3 peak measured at half-maximum peak intensity) vs. 108/T"K in CDC1, for: (1)0.03814 M Cu(TFA)2 in 1 M 4-methylpyridine N-oxide, (2) 0.01131 M Cu(HFA)a in 1 M 4-methylpyridine N-oxide, (3) 0.0871 M Cu(AA)z in 2 M 4-methylpyridine, (4) 0.0782 M Cu(TFA)2 in 2 M 4-methylpyridine, (5) 0.0331 M Cu(HFA)$ in 2 M 4-methylpyridine. '
AJ (HZ)
- 50
;/\;;
-100
\
-150
0.02
0.04
0.06
0.38
0.10
C U ( H F A ) ~or CU(TFA)~, M
Figure 2. Variation of isotropic shifts of 1 M 4-methylpyridine N-oxide in CDCL with Cu(che1ate)z concentration at 36'.
2. Furthermore, from room temperature measurements, the slope of such a plot is reduced to one-half if the total ligand concentration is doubled; the slope was calculated from the portions where the ligand to copper chelate ratio is high. These results suggest that under the experimental conditions used, one adduct species predominates in the solution. However, the presence of a small amount of other species cannot be ruled out. The spectrophotometric results indicate that the following species are responsible for the observed isotropic shifts in CDCla solution: c~(AA)~q.l-MePy,CuThe Journal of Physical Chemistry
1,
I
3.2
3.4
I
3.6
I
I
?.E
L.0
Id/T
Figure 4. Temperature effect on isotropic shifts of 2 M 4-methylpyridine in CDCls: (a) 0.0870 M Cu(AA)z; (b) 0.0782 M Cu(TFA)z, (0) 0.0436 M Cu(HFA)a.
ISOTROPIC SHIFTS I N
4-l/IETHYLPYRIDINEAND 4-METHYLPYRIDINE
30
I
II "
contact contribution to the isotropic paramagnetic shift; xxx,xYU, xZZare the principal axis components of the molecular susceptibility tensor; R i is the length of a vector from the metal ion center to nucleus i; Q is the polar angle and # is the azimuthal angle between +his vector and the principal axes of the susceptibility tensor; and d is the rotationally averaged susceptibility [3 = 1/3(xzx xYu xzz)]- The lowtemperature esr results indicate that the g tensor and, thus, the susceptibility tensor are axially symmetric, so that the pseudocontact shift can be put in terms of gill the g tensor component along the symmetry axis and g l , the component perpendicular to the symmetry axis, = 1/3 (gl1 2g,), as in eq 3.
+
0
6."
1731
N-OXIDE
+
+
(Avi)D
=
+ 1) m - Sl) (1 - 3
YP2S(S 9lc T
3.2
3.L
3.6
3.6
L.0
L.2
ldh
Figure 5. Temperature effect on isotropic shifts of 1 M Cmethylpyridine N-oxide in CDCL: (a) 0.09413 M Cu(TFA)2; (b) 0.0165 M CU(HFA)~.
(TFA), 4-R/IePy1 Cu (HFA), 2 (4-MePy) , Cu(TFA), 4-MePyO, and CU(HFA)~. 2(4-MePy0). The isotropic nmr shift may be decomposed into the Fermi contact and pseudocontact contributions.a I n the fast-exchange limit, if (1) the rotational correlation time is short compared to the g-tensor anisotropy, (2) no more than one Kramers multiplet is populated, and (3) there is no orbital contribution to the paramagnetism from unpaired electron spin in orbitals centered a t the nucleus in question, then the Fermi contact I n eq 1, (hvi)F is the contribution is given by eq 1.20~21 +
Fermi contact contribution to the paramagnetic nmr shift for nucleus i; Ai is the Fermi contact constant for this proton; ye and yn are the gyromagnetic ratios for electron and nuclear spins, respectively; S is the spin quantum number for the complex; g is the rotationally averaged value of the g tensor; p is the value of the Bohr magneton (in ergs/G); v is the nuclear Larmor frequency; and P, is the fraction of ligand coordinated to the metal ion. Under the conditions listed above, the pseudocontact shift is given by eq 2.'l I n eq 2 , (AVi)D is the pseudo-
COS2
Ri3
D
) E (3)
The observed isotropic shifts for the copper complexes are given in Tables I and 11. They are relatively small compared to the analogous nickel(I1) and cobalt(I1) bi~acetylacetonates.~2~~3 In the 4-metbylpyridine systems the a-H and p-H are shifted downfield whereas the y C H 3 is shifted upfield. The isotropic shifts attenuate regularly as the number of bonds between the proton and the metal increases, as can be seen in Table I. The y-H in pyridine is shifted downfield in contrast to the upfield shift of the y C H 3 in 4-MePy. All the proton lines are broadened to at least some extent by the paramagnetic copper chelate. The broadening increases with the isotropic shift. Thus, the or-H of the 4-MePy in the Cu(TFA), and CU(HFA)~ systems can be observed only at low concentrations of copper chelate and high concentrations of 4-MePy. In the 4-methylpyridine N-oxide systems the a-H line is shifted upfield whereas the p-H and y-CH3 lines are shifted downfield. Again, the y-H line in pyridine IT-oxide is shifted in a direction opposite to that of the y-CH3 line in 4-MePyO. Although the broadening of the proton lines in the 4-MePyO systems is less than in the 4-RIePy systems, the or-H line in the CU(HFA)~ system is broadened so much that it can be observed only a t low concentrations of CU(HFA)~. The magnitude of the isotropic shifts of the Cu(HFA),. 4-MePyO adduct decreases in the order y-CH3 > a-H > 0-H, while with Cu(TFA),.4-MePyO the order is yCH8 > p-H > a-H. The fact that the isotropic shift for the a-H line is smaller than for p-H in this system cannot be rationalized in terms of a pseudocontact contribution. (20) H.M.MoConnell and D. B. Chesnut, J. Chem. Phys., 2 8 , 107 (1958). (21) R. J. Kurland and B. R. MoGarvey, Advan. Magn. Resonance, in press. (22) J. A. Happe and R. L. Ward, J. Chem. Phys., 39, 1211 (1963). (23) R. W. Kluiber and W. D. Horrocks, Jr., J . Amer. Chem. SOC., 87, 5350 (1965). Volume 74, Number 8 April 16, 1970
1732
C. H. KE, R. J. KURLAND, C. I. LIN, AND N. C. LI
Table I : Isotropic Shifts of 4-Methylpyridine Systems in CDCla at 36'
M-------, M(che1ate)p
Ligand proton
-Conon, Ligand
1.000
0,0241
2.000
0,0244
1.000
0.0130
Cu(TFA)z
LY-H P-H r-CHs LY-H @-H r-CHs LY-Hb 6-H r-CHa LY-H
2.000
0.0094
Cu(HFA)z
@-E r-CHs LY-Hb
3.000
0.0134
M(ohe1ate)i
Cu(AA)z CLI(AA)Z Cu(TFA)z
Cu(HFA)a Ni(AA)!p Co(AA)z"
P-H r-CHa LY-Hb P-H Y-CHa LY-H P-H r-CHa LY-H
Avobsdi
Hz
-9.0 -3.8 -0.3 -5.7 -2.0 -0.4 -16.8 4.6 -18.2 -5.7 1.8 -13.6 3.7
3.000
0.0316 -34.3 9.5 -10.00 -2.78 0.90 -10.00 -0.88 4.61
P-H
Paeudocontact shift, HzC
-2.3 -1.3 -0.8 -1.4 -0.8 -0.6 -1.5 -0.8 -0.5 -0.6 -0.3 -0.1 -1.1 -0.6 -0.3 -2.6 -1.5 -0.8
j3 ratioe
Av,ord
-6.7 -2.5 0.5 -4.3 -1.2 0.2
0.37 1.oo -5.0 0.28 1.00 -6.0
-16.0 5.1 -17.6 -5.4 1.9
1.00 -3.14 0.31 1.00 -2.84
-13.0 4.0
1.00 -3.25
-32.8 10.3
1.00 -3.18 0.28 1.00 -3.09 0.30 1.00 -2.64
-10.00 -2.97 1.17
y-CHa 0 Taken from ref 22. The isotropic shifts are expressed in relative magnitude giving the isotropic shift of the LY proton equal to 10.00. b The a-proton signals in these concentrations are not observed. They can be observed at much lower M(chelate)z concentrations. 0 Pseudocontact shifts are estimated from eq 3, using the geometric factor found for Ni(AA)2 (ref 22) and the experimentally . g anisotropy for Cu(AA)z.4-MePy is found g anisotropy for Cu(AA)a*4MePy,Cu(TFA)a'd-MePy, and Cu(HFA)z. ( 4 - M e p ~ ) ~The taken from ref 14a. Furthermore, a formation constant of 2.0 (estimated from ref 6 and 8) for the reaction Cu(AA), 4-MePy = Cu(AA),.k-MePy in chloroform and 36' is used to calculate the Po value of this system. Isotropic shift corrected for pseudocontact shift to give contact shift. a The ratio of @-protoncontact shift to that of the given proton contact shift.
+
Table I1 : Isotropic Shifts of 4-Methylpyridine N-Oxide Systems in CDCls at 36" Pseudocontact M(che1ate)a
Cu(TFA)z Cu(TFA)z Cu(HFA)z CU(HFA)z Ni(AA)Z" Co(AA)e"
Ligand proton
a-H P-H r-CHs a-H 0-H r-CHs LY-H 6-H y-CHa CY-H P-H r-CHa LY-H 6-H r-CHa LY-H P-H a-CHa
-Conon, Ligand
M7 M (chelate)1
1.00
0.0390
2.00
0.0920
1.00
0.0118
2.00
0.0103
2.33
0.097
2.33
0.097
AVobnd,
HE
HE
Avoor'
@ ratiod
-1.71 1.00 0.46 -2.54 1.00 0.45 -0.43 1.00 0.40 -0.43 1.00
1.9 -3.8 -7.9 1.0 -3.6 -7.4 20.9 -9.2 -23.9 10.8 -4.8
-0.2 -0.2 -0.1 -0.3 -0.3 -0.1 -0.1 -0.1 -0.1 -0.1 -0.1
2.1 -3.6 -7.8 1.3 -3.3 -7.3 21 .o -9.1 -23.0 10.9 -4.7
-11.3
-0.1
-11.2
95 -50 - 119 113 28 - 100
-
20 20 11
95 -50 -119 93 -48 -111
0.42
-0.53 1.00 0.42 -0.52 1.00 0.43
Taken from ref 23. b Pseudocontact shifts are estimated from eq 3 using the geometric factor found in ref 23 and the experimentally 0 Isotropic shift corrected for pseudocontact shift to obtained g anisotropy for C U ( T F A ) ~ . ~ - M ~and P ~ Cu(HFA)p.2(4-MePyO). O give contact shift. d The ratio of @-protoncontact shift to that of the given proton contact shift. a
T h e Journal of Physical Chemistry
ISOTROPIC SHIFTS IN
4-METHYLPYRIDINE
AND
~~
~~~
Table 111: Estimated Pseudocontact Shifts in Ha (for Po = 1) Complex
Cu(AA)?:.4-MePy Cu(TFA)t 4-MePy Cu(HFA)r.2(4-MePy) Cu(TFA)z.4-MePyO Cu (HFA)z * 2 (4-MePyO)
0-H
8-H
7-CHs
- 145 -113 - 126
-81
- 47 - 36 - 41
-5 -6
-63 -71 -6
-7
1733
4-METHYLPYRIDINE N-OXIDE
-3 -4
into the ligand via the n-molecular orbitals of the 4-methylpyridine N-oxide molecule. Recently, Byers, et u1.,26have reported that the lowest energy M --+ L charge transfer band of the essentially octahedral copper-pyridine N-oxide complex arises from an eg -+ n* transition. I n tetragonally distorted octahedral adducts of CU(TFA)~ -4-NIePyO and CU(HFA)~. (4MePyO)2, the unpaired electron resides in the eg orbital; thus, the eg n* charge-transfer process may well account for the observed isotropic shifts. A qualitative comparison can be drawn between the magnitude of the isotropic shifts and the tendency for adduct formation in these copper chelates. Thus, from the equation Avobad = Av,PCz6 (where Avobed is the observed contact shift, after correction of pseudocontact shift, and Avo is a "unit" contact shift), the contact shifts of the 4-methglpyridine complexes at 36" are found to be (for the p-H of 4-JIePy) : - 101 Hz, - 1190 Hz, and - 1506 Hz for C U ( A A ) ~ - ~ - R W CU ~ (, T E ' A ) ~ - ~ JiIePy, and C U ( H F A ) ~ . ~ ( ~ - M respectively. ~P~) Similarly, unit contact shifts obtained for the 7-CH3 proton of the 4-methylpyridine N-oxide complexes at 36" are -1036 Hz for -180 Hz for C U ( T F A ) ~ . ~ - M ~and P~O CU(HFA)~. (4-MePy0)2. I n both 4-methylpyridine and 4-methylpyridine N-oxide complexes the magnitudes of the contact shifts lie in the order C U ( H F A )> ~ CU(TFA)~ > CU(AA)~. This order is the same as that for the stability constants for adduct formation found from spectrophotometric studies.11B18 It is interesting to note that Garito and Wayland' have recently reported that in copper t-butylacetoacetate-pyridine type ligand complexes, the isotropic shifts increase as the basicity of the ligand is increased. For a system in which the Fermi contact interaction is dominant, the isotropic shifts should vary linearly with the reciprocal of the absolute temperature, 1/T, i.e., shows a Curie law dependence. However, the temperature effects in these copper complexes are not all linear, as can be seen from Figures 4 and 5 . We have found a correlation between the temperature effect and the stoichiometry of these copper complexes,' although such a correlation is not obvious from a priori considerations. We have found (a) a normal temperature effect with six-coordinated bis adducts of Cu(HFA)z*2(4-MePy) and CU(HFA)~.~(~-LM~P~O) throughout the temperature range of -50 to +50", (b) an abnormal temperature effect, i.e., the isotropic shifts increase as the temperature is increased, with five-coordinated mono adducts of CU(AA)~ -4-MePy and C U ( T F A ) ~ . ~ - M ~ and P ~ O(c) , a nonlinear temperature effect, the isotropic shifts decreasing with in-+
The fact that the isotropic shift changes sign when a methyl group is substituted at the y position in pyridine or pyridine N-oxide suggests that the major contribution to the observed isotropic shifts in these copper complexes is theFcrmicontact interaction. According to eq 3, if the pseudocontact interaction is dominant, then lines of protons with approximately the same geometric factor, (1 - 3 cos2 a/Ri3),would be shifted in the same direction. Furthermore, esr measurements of these copper chelateslBand their 4-MePy and 4-R'lePyO adducts in chloroform have shown that the g anisotropy (91 I - g l ) is about 0.3, so that one would not expect a large pseudocontact interaction in these copper complexes. However, as shown in Tables I and 11, the relative contribution of the pseudocontact interaction is more important in those systems where the total isotropic shift is small. Without knowing the exact geometry of these copper complexes, one cannot calculate exactly the correction for the pseudocontact shift. If one uses the geometric factors of the corresponding nickel(I1) bisacetylacetonate c o r n p l e x e ~ one , ~ ~can ~~~ estimate the pseudocontact shifts of the copper complexes with the results listed in Table 111. In Tables I and 11, corrections for the pseudocontact interaction are included. It is clear that for the 4-MePyO adducts of Cu(TFA)2 and Cu(HFA)2, the pseudocontact contribution to the observed isotropic shifts may be neglected. However, in the 4-MePy adducts the pseudocontact contribution is appreciable, especially in the CU(AA)~ -4-MePycomplex. The downfield shifts of a-H and p-H and the regular attenuation of the p-H and y-CH3 shifts in the 4-methylpyridine systems clearly indicate that unpaired electron spin density has been delocalized to the ligand via the a-molecular orbitals of the 4-methylpyridine molecules. The upfield shift of the y-CH3 in this ligand indicates that there is negative spin density at this position, and the Fermi contact interaction reflects probably a U-T correlation mechani~m.~Delocalization of unpaired spin into the 4-methylpyridine ligand occurs via a u mechanism, consistent with the fact that pyridine type ligands are poor recipients of rr-electron density from transition metal ions, as found by infrared The pattern of the isotropic shifts, both in magnitude and sign, found in the 4-methylpyridine N-oxide systems is that of a typical conjugated aromatic system. This suggests that unpaired spin is delocalized
(24) W. D. Horrocks, Jr., and R. C. Taylor, Inorg. Chem., 2, 723 (1963). (25) W. Byers, B. Fa-chun Chou, A. B. P. Lever, and R. V. Parish, J. Amer. Chem. Soc., 91, 1329 (1969). (26) B. B. Wayland and R. 9. Drago, ibid., 87, 2372 (1965).
Volume 74, Number 8 April 16, 1970
1734
N.E.VANDERBORGH, N. R.ARMSTRONG, AND W. D. SPALL
creasing temperature, but not varying linearly with l / T , in the system of C U ( T F A ) ~ . ~ - > I ~AsP ~eq. 1 implies, the hyperfine coupling constant, A , can be obtained from the Iinear plot of contact shift us. I / T . For the C U ( H F A ) ~ . ~ ( ~ - Mand ~ P ~CU(HFA)~. ) 2(4-RIePyO) complexes, the calculated hyperfine coupling constants for y-CHa are, respectively, +3.0 X 105 Hz and +7.7 X 105 Hz. Kluiber and Horrocks5 have reported a similar abnormal temperature effect for Cu(TFA)t .4-MePyO complex. They have proposed that a five-coordinated complex of low symmetry or a tetrahedral four-coordinated complex (with the opening of one TFA ring) accounts for the observed temperature variation a t elevated temperature (above 30") for this complex. However, this model of Kluiber and Horrocks is not consistent with our results on the Cu(HFA)z complexes. C U ( H F A )forms ~ tetragonally distorted octahedral bis
adducts with 4-methylpyridine and 4-methylpyridine K-oxide in chloroform solution. Since CU(HFA)~is less stable than C U ( T F A ) ~ ,one ~ ' would expect ring opening to be easier for CU(HFA)~ than CU(TFA)~ at elevated temperatures. The fact that CU(HFA)~. 2(4-MePy) and C U ( H F A ) ~ . ~ ( ~ - X I ~show P ~ O ) appreciable spin delocalization and a normal temperature effect is not consistent, then, with a tetrahedral species being present for Cu(HFA)* complexes. Thus, since this tetrahedrally coordinated complex does not occur for the Cu(HFA)z complexes, we think it even less likely to occur for the CU(TFA)~ complexes. Acknowledgment. The authors thank Dr. A. Allendoerfer of the State University of New York at Buffalo for his assistance in obtaining the esr g values. (27) L.G.Van Uitert, W. C. Fernelius, and B. E. Douglas, J.Amer. Chem. Soc., 75,457 (1953).
A Cryoscopic Study of the Association of Phenolic Compounds in Benzene by Nicholas E. Vanderborgh, NeaI R. Armstrong, and W. DaIe Spa11 Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87106
(Received June I d , 1969)
The cryoscopic behavior of phenol, positional isomers of chlorophenol and cresol, and 2,5-, 2,6-, 3,4-, and 3,5dimethyl phenol were studied in benzene, and equilibrium constants describing this behavior in terms of association were determined for the concentration range 0-0.8 rn. The results indicate that substitution of a ring hydrogen of phenol by either chlorine or methyl decreases the amount of association of the parent phenol, chloro isomers having less association than methyl isomers. The degree of association is qualitatively related to the effects of the substituents on the T electron cloud of the phenyl ring. The measurement of colligative properties of liquid solutions, those properties which depend upon the number and not the type of dissolved species, has long been recognized as an important method for the study of molecular association. Of the several colligative property techniques, one of the most accurate and experimentally simple is the depression of the freezing point, cryoscopy. Early workers studying this technique discovered that many organic solutes, when used to depress the freezing points of aprotic solvents, showed smaller freezing point depressions than would be predicted on the basis of their formula weights. The most commonly used explanations for this observed nonideal behavior are the formation of solid solutions, changes in activity coefficientswith concentration, and molecular association. The Journal of Physical Chemistry
Solid solution formation greatly complicates the interpretation of cryoscopic data. The existence of solid solutions may be detected either by chemical analysis of the frozen solid or by the method of Van Bijlert. Compounds suspected of forming solid solutions or mixed crystals in benzene solutions include acetic a ~ i d z -and ~ pheno1.2J However, in these previous studies, the existence of solid solutions was not verified by chemical analysis. The usual method for correcting cryoscopic data for the effects of solid solu(1) A. Van Bijlert, 2.Phgs. Chem. (Leipzig), 8, 343 (1891). (2) C.R. Bury and H. 0. Jenkins, J . Chem. SOC.,688 (1934). (3) A. G.Milligan, J . Phys. Chem., 33, 1363 (1929). (4) R.Marc and W.'Wenk, Z. Phys. Chem. (Leipzig), 68, 104 (1910). (5) F. Garelli, ibid., 21, 122 (1890). (6) J. A, Davison, J. Amer. Chem. Soc., 67, 222 (1945).
