molecules in the aquo complex of aluminum(II1) cannot be displaced by 2-methyl-8-quinolino1, whereas they can be displaced by quinolinol. Also, the water molecules in the aquo complex of gallium(II1) can be displaced by both 8-quinolinol and 2-methyl-8-quinolinol. In nonaqueous solutions, however, the solvating molecules of chloroform or dimethyl sulfoxide, can be displaced by either ligand. Moreover, the formation of an aluminum(III)-2-methyl-8-quinolinol complex from hydrated aluminum sulfate in DMSO suggests that the aluminum ion is preferentially solvated by the DMSO in the presence of water and that the solvated DMSO molecules are displaced by the 2-methyl-8-quinolinol to form the aluminum(II1) complex. The inability of the 2-methyl-8-quinolinol to replace the
water molecules that solvate the aluminum(II1) ion is the obvious reason for the apparent nonreactivity of 2-methyl-8quinolinol with aluminum(II1) in aqueous solutions. Whether this is the result of steric effects alone, or other effects arising from the 2-methyl group is the important question, This question can only be resolved when the bond distances in the gallium(II1) and aluminum(II1) chelates of 2-methyl-8quinolinol are known and when equilibrium and kinetic studies on the 2-methyl-8-quinolinol-aluminum(III)system in nonaqueous and partially aqueous media have been carried out. RECEIVED for review June 24, 1968. Accepted August 21, 1968. Work supported by the U. S. Atomic Energy Commission.
Proton Nuclear Magnetic Resonance Studies of 8-Quinolinol and Several of Its Metal Complexes Blair C. Baker and Donald T. Sawyer Department of Chemistry, University of California, Riverside, Calq. 92502
The proton NMR spectrum for 8-quinolinol has been analyzed and assigned with the aid of computer techniques and substituted derivatives of oxine. NMR spectra also have been recorded for the oxine chelates of Pt(ll), Zn(ll), Mg(ll), Sn(lI), Pb(ll), Hg(ll), AI(III), Co(lll), and Rh(lll) in dimethylsulfoxide. Consideration of the chemical shift changes of the phenolicring and hetero-ring protons of the ligand in the metal chelates relative to their positions for the free ligand provides a measure of the strength (or lability) of the coordinate bonds to the oxygen and nitrogen atoms of the ligand. Structures for the metal complexes are proposed which are consistent with the experimental data.
~ - Q U I N O L I N(oxine, OL Cs,HiNO, mol wt = 145.16) has been used extensively as a complexing agent in a wide variety of analytical techniques. As a result, both oxine and its metal complexes have been the subject of numerous studies ( I , 2). However, only limited attention has been given to the chelate structures and their metal-ligand interactions. The crystal structures of the “solvated” uranium(V1) oxinate (3), as well as the dihydrates of the copper ( 4 ) and zinc (5, 6) oxinates, have been determined using X-ray methods. A number of oxine chelates have been studied by infrared spectrometry, both in the sodium chloride region (7, 8) (2-16 p ) and the potassium bromide region (9, 10) (12-40 p ) . These studies have raised a number of questions which make (1) J. P. Phillips, Chem. Rec., 56, 271 (1955). (2) R. G. W. Hollingshead, “Oxine and Its Derivatives,” Butterworths, London, 1954, part I. (3) D. Hall, A. D. Rae, and T. N. Waters, Acta Crystallogr., 22, 258 (1967). (4) R. Kruh and C. W. Dwiggins, J . Amer. Chem. SOC.,77, 806 ( 1955). (5) L. Merritt, ANAL.CHEM., 25, 718 (1953). 16) L. Merritt. R. T. Cadv, .. and B. W. Mundy, . . Acta Crystallorg., - . 7, 473 (1954). 17) K. G . Stone. J . Amer. Chem. SOC..76. 4997 (1954). R. G. Charles, H. Freiser, R. Freidel, L. E. Hillard, and W. D. Johnston, Spectrochim. Acta, 8, 1 (1956). (9) R. J. Magee and L. Gordon, Tulanru, 10, 851, 961, 967 (1963). (10) J. E. Tackett and D. T. Sawyer, Znorg. Chem., 3,692 (1964). \
~
,
further study of the bonding and structural characteristics of oxine and its metal complexes desirable. Proton NMR spectrometry provides an effective means for studying the bonding and structures of organic molecules. Although the protons of the oxine molecule are not involved in metal ion coordination, inductive effects at their positions on the aromatic rings allow the electronic environment of the coordination sites to be monitored. Also, interactions of the ring protons with neighboring ligands affect the N M R spectra to a degree which is dependent on the geometry of the chelate molecule. Thus, changes in the chemical shifts for the ring protons upon chelation provide insight to the nature of the metal-ligand bonds and the gross structure of the chelate species. Metal-oxine complexes have not been studied by NMR, possibly because of their limited solubility in many solvents. The proton NMR spectrum of oxine has been analyzed only partially (11). The present paper summarizes the results of a detailed proton NMR study of 8-quinolinol and of several metaloxine complexes in terms of their bonding and structural characteristics. Spectra for the oxinate complexes of Pt(II), Zn(II), Mg(II), Hg(II), Pb(II), Sn(II), Al(III), Rh(III), and Co(II1) have been recorded and assigned. EXPERIMENTAL
Spectra were recorded with a Varian Model HA-I00 N M R spectrometer equipped with a 100 MHz oscillator and a variable temperature probe. Reagent grade dimethylsulfoxide (DMSO), obtained from J. T. Baker and Co., was used as the solvent for all spectra; it also served as a convenient internal lock signal for the spectrometer. Chemical shifts were measured downfield from the DMSO lock signal with a frequency counter. The DMSO resonance line is 2.840 i 0.002 ppm downfield from an external TMS reference. 8-Hydroxyquinoline (mp 74-76 “C) was obtained from (11) L. W. Reeves and K. 0. Strprnrne, Can. J . Chem., 39, 2318 (1961). VOL. 40, NO. 13, NOVEMBER 1968
1945
to the oxine and quinoline solutions, and 0.76 ml of the acid to the a-naphthol and naphthalene solutions. This gave 0.5F solutions of the protonated (or neutral) species which were 1.OF in HCl. The basic solutions were prepared in a similar manner by adding 0.85 ml of a 50% aqueous sodium hydroxide solution t o the oxine and a-naphthol solutions, and 0.57 ml of base to the quinoline and naphthalene solutions. This gave 0.5F solutions of the anionic (or neutral) species which were 1.OFin NaOH. The line spectrum for oxine (Figure 10) and the spectral parameters in Table I were calculated using the LAOCOON I1 computer program (I3), modified by Maddox (14) and Sudmeier (15), on an TBM 7040 computer. RESULTS
I O W
I
’ 6.4
6.0
5.6
6 , pprn
8.2 VI
4.8
44
DMSO
Figure 1. Proton NMR spectra for 8-quinolinol (Curve C) and its 5-chloro- (Curve A ) and 2-methyl(Curve B ) derivatives in dimethylsulfoxide. Curve D represents the computed line spectrum for oxine Matheson, Coleman, and Bell and the 2-methyl- and 5chloro- derivatives from Columbia Organic Chemical Co., Inc. All other materials were reagent grade. Mercury(I1) oxinate was prepared by adding 1.5 grams of oxine dissolved in 25 ml of acetone to 50 ml of aqueous 0.1F mercuric nitrate. Cobalt(II1) oxinate was prepared by hydrogen peroxide oxidation (2). Platinum(I1) oxinate WRS obtained by dissolving 0.05 gram of potassium chloroplatinate (KIPtCla) in 25 mi of water, then adding 0.07 gram of oxine in 25% acetic acid and neutralizing the solution to pH 6.5 with ammonium hydroxide. Orange crystals formed after heating o n a steam bath. All of the other metal oxinates were prepared by adding increments of dilute ammonium hydroxide to an acetic acid solution containing oxine and the metal nitrate salt (12). The precipitates were filtered after heating in solution for several hours at 90 “C. The isolated complexes were washed several times with methanol to remove any excess oxine and then air dried at room temperature for at least 24 hours before their spectra were recorded. The neutral solutions discussed in Table I were prepared by dissolving 1.8 grams of oxine, 1.8 grams of a-naphthol, 1.6 grams of quinoline, and 1.6 grams of naphthalene, respectively, in 25 ml of DMSO. The acidic solutions were prepared by taking the same weight of each compound in 25 ml of DMSO and adding 1.15 ml of 12F hydrochloric acid (12) W. F. Hillebrand, G. E. F. Lundell, H. A. Bright, and J. I. Hoffman, “Applied Inorganic Analysis,” 2nd ed., Wiley, New York, N. Y.,1953, pp 122-5. 1946
ANALYTICAL CHEMISTRY
Oxine. The proton N M R spectra for oxine and its 2methyl- and 5-chloro- derivatives are illustrated in Figure 1. From the known structure of the oxine molecule (Figure 1) and a consideration of the inductive effects of the hydroxyl group and the ring nitrogen, the protons of the hetero ring are expected to be deshielded relative to those of the phenolic ring. This is verified by considering the spectra of the 2methyl- (methyl resonance not shown) and 5-chloro- derivatives which aid in identifying the protons of each ring. The specific assignments for each proton have been made by assuming that values for the coupling constants are similar to those in quinoline (16)and that the relative strengths of the inductive effects are ortho > para > meta. After plausible values for the chemical shifts and coupling constants have been selected by first order analysis, the LAOCOON I1 computer program has been used to obtain refinements on these parameters (13-15). Part I of the program computes a preliminary spectrum using the approximate parameters, and then the calculdted line positions are compared with those of the actual spectrum. Part I1 carries out a n iterative computation by which the parameters are varied until the best least-squares fit is obtained. The program output consists of the best values for the chemical shifts and coupling constants, a comparison of the theoretical and actual line positions, and a statistical analysis of the errors for the values of the parameters. The calculated chemical shifts and coupling constants for oxine and its 2-methyl- and 5-chloro- derivatives are summarized in Section A of Table I; fhe probable errors are less than ~ t 0 . 2Hz. This table also summdrizes the effects o n the spectral parameters when strong acid and strong base are added t o oxine in DMSO. For these conditions the probable errors are *0.5 Hz because of the near equivalence of protons 5, 6, and 7. A line model of the computed theoretical spectrum for oxine is given in Section D of Figure 1. A comparison between the chemical shifts of oxine, quinoline, a-naphthol, and naphthalene in basic, neutrdl, and acidic media also is summarized in Table I. Because of hydrogen bonding, the chemical shift of DMSO in the acidic solutions lies 0.061 ppm downfield from its position in basic and neutral solutions. This correction is included in the reported chemical shifts so that they are all referenced t o the neutral DMSO resonance signal. Cross ring coupling is not observed in the oxine spectrum because of substitution at the 1 and 8 positions. As a result, (13) S. Castellano and A. A. Bothner-By, J . Clzern. Phys., 41, 3863 (1964). (14) M. Maddox, Ph.D. Thesis, U.C.L.A., 1966. (15) J. L. Sudmeier, University of California, Los Angeles, private communication, 1967. (16) P. J. Black and M. L. Heffernan, Aust. J. Cliern., 17, 558 (1964).
Table I. Proton NMR Parameters for Neutral, Anionic, and Cationic Species of Oxine, 2-Methyl Oxine, and 5-Chloro Oxine, and for Several Related Compounds Compound Proton
Chemical shifts (6), ppm cs. DMSO 1
A. Oxine and its derivatives Oxine (HOx) ,..
ox-
H20xt 2-Me-HOx 5-CI-HOx
...
...
... ...
2
3
6.268 4.945 5.903 4.674 6.527 5.531 . . . 4.802 6.365 5.134
Coupling constants, Hz
4
5
6
7
8
J?,4
J2,3
J3,4
Jj.0
J6,7
Js.7
5.714 5.434 6.594 5.567 5.893
4.816 3.993 5.154 4.754
4.872 4.605 5.149 4.804 5.022
4.578 3.985 5.017 4.537 4.536
... ,.. ... ...
1.63 1.69 1.28
4.14 4.09 5.38
7.95 7.71 8.77 7.75
4.00
...
7.82 8.04 7.77 7.23 8.30
1.10 1.24 1.02 1.45
1'.'39
8.29 8.18 8.41 8.40 8.53
5.78 5.78 6.63 5.25 4.95 5.25
5.41 5.41 5.82 4.79 3.97 4.79
5.02 5.02 5.39 4.75 4.50 4.75
5.19 5.19 5.60 4.36 3.69 4.44
5.48 5.48 5.93
1.77
4.25
8.34
8.07
6.82
1.49
1.37
6.03
8.58
1.81
4.31
8.39
8.48
7.46
1.35
...
, , ,
..,
B. Related compounds
Quinoline,a neutral Quinoline, basic Quinoline, acidic a-Naphthol,b neutral a-Naphthol, basic a-Naphthol, acidic Naphthalenqc neutral Naphthalene, basic Naphthalene, acidic 8-N02-Quinolinea
...
6.34 6.34 6.76 5.62 4.90 5.72 4.48 5.62 4.90 5.35 4.96 5.35 4.96 5.38 4.99
... ...
4.95 4.95 5.55 4.90 4.64 4.90
... ... ...
C . Oxine, corrected for hydrogen bonding
... a
c
6.32
4.95
5.71
4.82
4.87
...
4.73
Reference (16) for coupling constants. Numbering changed to correspond to oxine. Reference (17) for coupling constants.
the spectrum is divided into an ABC (phenolic ring) and an AMX (hetero ring) pattern. Reeves and Stromme (ZI), using a 40-MHz instrument, were able to resolve the low-field (hetero ring) half of the spectrum and reported coupling constant values of 52,3,4.4 Hz; J2,4, 1.7 H z ; and 53,4, 8.3 H Z which are in agreement with the present data (Table I). The use of a 100-MHz instrument affords increased resolution of the spectrum, and has simplified the measurement and assignment of the remaining parameters. The proton chemical shifts for the oxine anion (Ox-), the neutral species (HOx), and the protonated oxine cation (HzOx+) provide a reference base for studying the effect of metal ion coordination upon the electronic environment of the donor groups. Thus, deductions concerning the nature of the metal-oxine bonds can be made from the proton NMR spectra of the metal complexes. Although the coupling constants are not affected greatly by chelation, the chemical shift values for the ligand protons are shifted substantially. The extent of the shift changes depends o n the nature of the metalligand bonds and also on the geometry of the chelate in question. Such effects are apparent in the chemical shifts, obtained by first order analysis, for the metal oxinates summarized in Table 11. Divalent Metal Oxinates. Metal-oxine chelates have been prepared for the divalent ions of Ba, Ca, Hg, Mg, Pb, Pd, Pt, Sn, and Zn. However, the chelates of Pd and the alkaline earth ions are not sufficiently soluble in DMSO to obtain useful spectra. The magnesium oxinate spectrum has been recorded using a Computer of Average Transients to overcome the problem of its low solubility. After 50 scans, the signal-to-noise ratio is such that the chemical shifts can be measured. The spectra for the soluble divalent metal oxinates all consist of six proton resonances with well resolved splittings, corresponding to the six ring protons of the ligand (Table 11). This establishes that the two ligands about the central metal atom are magnetically equivalent in the NMR time scale.
Table 11. Chemical Shifts for Metal Oxinatesa Compound 6, ppm GS. DMSO (+0.01 ppm) Proton 2 3 4 5 6 7 HOx 6.27 4.94 5.71 4.82 4.87 4.58 Pt(0x)z 6.38 5.20 6.10 4.64 4.94 4.47 Zn(OxI2 5.95 4.84 5.73 4.42 4.72 4.33 ~
Mg(W2
Sn(Ox)t Pb(0x)z Hg(Ox)n Al(OX)3 A B C AI(Ox)o(115 "C) CO(OX),A
B C
Rh(0X)a A B
5.64 6.40 6.32 6.30 6.19 6.06 4.58 5.68 5.90 5.83 4.65 5.85 5.81 4.76
4.64 5.13 5.13 5.04 5.13 5.02 4.91 4.94 5.07 5.03 4.86 5.02 4.98 4.85
5.50 5.90 5.83 5.80 6.00 5.94 5.94
5.88 5.85 5.81 5.77 5.85 5.85 5.83
4.22 4.61 4.45 4.84 4.61 4.61 4.61 4.58 4.52 4.52 4.50 4.52 4.52 4.50
C Spectra run at normal probe temperature (-35 otherwise indicated.
4.68 4.86 4.90 4.90 4.96 4.96 4.96 4.92 4.86 4.86 4.86 4.89 4.89 4.89
a
4.19 4.27 4.23 4.56 4.38 4.36 4.23 4.32 4.45 4.44 4.38 4.42 4.42 4.34
"C) unless
Trivalent Metal Oxinates. Metal oxinates have been prepared for the trivalent ions of AI, Co, and Rh. The spectrum for each chelate has three separate sets of ligand resonances (Table 11), which indicates that the three ligands are situated around the metal atom such that each is magnetically distinct. When the Al(Ox), and Co(Ox>, spectra are recorded at a probe temperature of 115 "C,the aluminum spectrum collapses to a single set of ligand resonances. However, the cobalt spectrum remains unchanged, confirming the inert character of the cobalt(II1)-ligand bonding.
DISCUSSION AND CONCLUSIONS Oxine. Chemical Shifts and Coupling Constants. The N M R parameters for quinoline have been measured previously (16) in acetone and are in excellent agreement with the VOL. 40, NO. 13, NOVEMBER 1968
1947
present work in DMSO. A comparison of the chemical shifts for oxine with those for quinoline indicate a close parallel for the effects in the hetero ring. The slight differences presumably are due t o inductive and hydrogen bonding effects from the hydroxyl group of oxine. I n the phenolic ring of oxine, the protons ortho, para, and meta to the hydroxyl group are shifted upfield 0.61, 0.59, and 0.15 ppm, respectively, compared t o their positions in quinoline. This reflects the predominately ortho-para directing nature of the hydroxyl ring substituent. Comparison of both oxine and quinoline with their hydrocarbon analog, naphthalene (Table I), indicates the magnitude of the effect when an aromatic C-H is replaced by nitrogen. Again it is predominately an ortho-para effect, but now one of deshielding. The large shift for proton 2 is due to its close proximity to the magnetically anisotropic nitrogen atom. Coupling constants in the hetero ring are essentially the same for oxine and quinoline (16) but definite differences exist in the other ring (Table I). J5,6is decreased by 0.1 Hz, J5,7is decreased by 0.4 Hz, and J s , is ~ increased by 1.0 Hz because of the addition of the hydroxyl group. Measurements on 8-nitroquinoline (16) also show a decrease in J 5 , i and an increase in JF,,from those in quinoline, although the magnitudes of the changes are about half those for oxine. The inductive effects of the hydroxyl and nitro groups show opposite influences on the chemical shifts of the ring protons; this effect is transferred predominately through the pi-electron system (17). However, the effect of these substituents on coupling constants, an effect influenced by the sigma-electron structure (19,is parallel; this indicates that polarization of the sigma bonds by each group is similar. The effect on coupling constants by the nitrogen atom of the hetero ring has been shown (16) to be opposite to that for a nitro substituent (and thus for a hydroxyl group), implying that sigma bond polarization due to a ring nitrogen is opposite to that for these substituents. Acidic and Basic Properties. Oxine in solution exhibits both acidic and basic properties which contribute to its chelating ability. Both protonation of the nitrogen and removal of the hydroxyl proton are resonance stabilized by the presence of the aromatic rings. In some solvents oxine appears to exist as keto-enol tautomers (18). Infrared and dipole moment studies (19) have shown that the hydroxyl proton is actually involved in strong intramolecular hydrogen bonding, even in the presence of strongly hydrogen bonding solvents, I n DMSO, the bonding probably is predominately intramolecular also. By examining the effects on chemical shifts in basic, neutral, and acidic solutions of oxine, quinoline, and a-naphthol, the magnitude of the effects of the intramolecular hydrogen bond in oxine can be evaluated. In basic solution the N M R spectrum of quinoline is identical to that in neutral solution, which indicates that there is no interaction between hydroxide ion and the quinoline molecule. This implies that any effects on chemical shifts in going from the neutral species of oxine to the anionic form will be due to changes at the hydroxyl group of the phenolic ring. Removal of the proton results in the bonding electrons being drawn into the ring system and a consequent shielding of all the ring protons. In both oxine and a-naphthol, the ortho-para directing nature of the oxygen causes protons 5 and 7 to be (17) J. A. Pople, W. G. Schneider, and H. J. Bernstein, “High-
resolution Nuclear Magnetic Resonance,” McGraw-Hill, New York, N. Y.,1959. (18) M. Seguin, Bull. SOC.Cllirn., 1946,566. (19) J. H. Richards and S. Walker, Trans. Faraday Soc., 57, 399 ( 1961). 1948
ANALYTICAL CHEMISTRY
shielded most strongly. Protons 3, 4, and 6 are shifted upfield nearly equally (about 0.28 ppm) by the increase in electron density throughout the pi system; proton 2 is shifted by a larger amount. This is observed in both spectra so the effect is not due t o the neighboring nitrogen atom in oxine. The cause of the large shifts of protons 5 and 7 appears to affect proton 2 also, possibly through the sigma electron system. The total shift for proton 2 in going from the anion of a-naphthol to the neutral species (hydroxyl proton completely bonded) is 0.42 ppm; the shift for oxine is 0.37 ppm. The difference, 0.05 ppm, can be attributed to hydrogen bonding with the oxine nitrogen. Similarly, the total shift for proton 7 of a-naphthol is 0.75 ppm and that for oxine is 0.60 ppm. Thus, partial removal of the hydroxyl proton and the subsequent electron migration results in a shielding of 0.15 ppm. By appling these differences t o the observed chemical shifts of oxine, a set of chemical shifts is obtained which is corrected for hydrogen bonding effects (Section C of Table I). The shifts due t o hydrogen intramolecular bonding (0.05 ppm for proton 2 and 0.15 ppm for proton 7), when compared with the total expected shifts (0.42 and 0.75 ppm, respectively), indicate a 15-20x conversion to the keto tautomer. The close similarity between oxine and a-naphthol for the chemical shift changes in going from basic to neutral solution indicates that, in an aromatic ring, nitrogen and carbon are equal in their ability to transmit inductive effects through the sigma- and pi-electron systems. Thus, the changes which are observed are merely the inductive effects from the hydroxyl group transmitted through the system. In strong acid the nitrogens of oxine and quinoline are protonated, which results in deshielding of all of the ring protons. The effects which contribute to the observed chemical shift changes when the cation is formed are more complex than those in anion formation. Upon protonation, bond formation by the nitrogen electrons with the proton results in changes in the diamagnetic effects of the dipole and magnetic anisotropy associated with the unprotonated nitrogen. Gil and Murrell (20) have calculated the magnitude of these changes and have shown that the dipole and anisotropy effects due to the N-H+ group are nearly the same as those for a C-H group. Thus, in acid solutions the deshielding effects due to the aza nitrogen are completely removed. For pyridine the upfield shifts due to protonation are calculated (20) to be +0.72 pprn (proton 21, t 0 . 2 1 pprn (proton 3), and +0.12 ppm (proton 4). At the same time, formation of the N-H’ bond causes a n inductive deshielding of all of the ring protons. The chemical shift changes that result from the formation of the quinolinium cation show the combined influences of these two effects. The result is a net deshielding in the order 4 > 3 > 2, whereas the expected order due to purely inductive effects by an orthopara directing substituent is 2 > 4 > 3. By comparing the chemical shift changes of oxine and quinoline when both are protonated, differences are apparent at positions 2, 5, 6, and 7. Because the two effects from protonation occur in both oxine and quinoline, the discrepancies must be attributed to the presence of the hydroxyl substituent in oxine. Additivity of Shielding Effects. The previous observations, that the chemical shift changes for oxine in going from neutral to acid solution parallel those for quinoline and the changes in going to basic solution parallel those for a-naph(20) V. M. S. Gil and J. N. Murrell, ibid.,60, 248 (1964).
thol, imply that the overall effects in oxine are merely the sum of the separate effects jn quinoline and a-naphthol. This is illustrated when the chemical shifts of the three compounds are compared with the analogous aromatic hydrocarbon naphthalene (Table 111). The chemical shift of each ring position in naphthalene has been subtracted from its counterpart in each of the other three compounds (+ indicating that the resonance lies downfield of its corresponding position in naphthalene). I n this way, the sum of the shift differences for quinoline and a-naphthol give the net shift for an “ideal” oxine molecule. Considering that the error for each measurement is about +0.01 ppm, the agreement between the oxime spectrum and the ideal spectrum for bdsic and neutral solutions is excellent. In acid solution, the differences are larger, which implies that inductive effects are not transmitted equally through nitrogen and carbon when the nitrogens (in oxine and quinoline) are protonated. Metal Complexes. The effects which contribute t o the observed N M R spectrum of a metal chelate are numerous and, unfortunately, not completely understood. However, some of the effects are due to ligand-metal interactions analogous t o the ligand-proton interactions discussed above. Because bonding to the oxygen and nitrogen of oxine are independent effects, five types of metal-ligand interactions are possible as limiting cases. The chemical shifts for each case are listed in Table IVA. CASE1. When the metaloxygen bond is covalent and the nitrogen is coordinated strongly, the resulting effect, compared with the neutral oxine species, is equivalent t o total protonation of both oxygen and nitrogen. The resulting spectrum will be that of oxine in acid solution. CASE2. When the metal-oxygen bond is ionic and the nitrogen is coordinated strongly, the situation will be intermediate to oxine in acid and oxine in base. The net effect will be the sum of the effects when oxine is taken from neutral to acid solution and when it is taken from neutral to basic solution. CASE3. When the metal-oxygen bond is ionic and there is no bonding through the nitrogen, the complex will be essentially a n ionic salt. The spectrum will be that of oxine in basic solution. CASE4. When the metaloxygen bond is covalent and there is no coordination t o nitrogen, the result will be similar t o oxine itself, but without the effects of hydrogen bonding. The spectrum will be that for the neutral nonhydrogen bonded oxine molecule (Table IC). CASE5. This is an intermediate bonding case, obtained by averaging Cases 1 and 3 (or 2 and 4), in which both ligand bonds contain partially delocalized electrons. Other effects which influence the proton N M R spectra are due to the close proximity of the ligands to each other in a rigid chelate structure. The interaction of local magnetic fields of one ligand with the protons of a neighboring ligand are sufficient to cause shielding o r deshielding of those protons, depending o n the geometrical configuration of the chelate. The causes of such local magnetic fields have been enumerated in various works attempting to quantitatively evaluate observed chemical shifts (20-30). The only application t o N M R studies of metal chelates has been made for the iron(II)-2,2’-bipyridyl complex (28). Shielding contributions from induced electron currents in the aromatic rings have been evaluated by using the tables of Johnson and Bovey (25) and assuming that the electron currents in the homocyclic and heterocyclic rings are equivalent (22, 28). For calculations of the effects from magnetic anisotropies associated with the lone electron pair o n nitrogen, a value (28) of -7.2 X cm3/moleculehas been used for the anisotropy, AX. The anisotropy for oxygen previously
Table 111. Chemical Shifts (ppm) Relative to Naphthalene for Oxine, Quinoline, and a-Naphthol in Basic, Neutral, and Acidic-Media Quinoline a-Naphthol Proton (Q) (N) Oxine(0x) Z Q S N A. Basic solution 2 3 4 5 6 7
+1.38 -0.01 +0.43 f0.06 $0.06 +0.23
2 3 4 5 6 7
f1.38 -0.01 +0.43 f0.06 $0.06 +0.23
2 3 4 5 6 7
+1.77 +O. 56 +1.25 $0.44 +0.40 +0.61
+0.94 -0.29 +0.08 -1.36 -0.36 -0.98
+o. 90
+1.31 -0.01 $0.36 -0.53 -0.09 -0.38
+1.32 -0.07 $0.33 -0.50 -0.15 -0.37
$1.54 $ 0 . 54 +1.21 -0.23 $0.16 +0.03
+1.68 +0.47 + I . 12 -0.15 +O. 16 $0.06
-0.48 -0.32 -0.40 -1.38 -0.46 -1.27
-0.33 +0.03 -1.32 -0.40 -1.04
B. Neutral solution -0.06 -0.06 -0.10 -0.56 -0.21 -0.60
C . Acidic solution -0.09 -0.09 -0.13 -0.59 -0.24 -0.55
Table IV. Predicted Chemical Shifts for Oxine Ligand in Limiting Bonding Cases and Two Geometrical Configurations Proton A. Ligand Case 1 2 3 4 5
2
6.52 6.16 5.90 6.32 6.23
3
5.53 5.25 4.67 4.95 5.10
6 cs. DMSO, ppm 4 5
6.59 6.31 5.43 5.71 6.01
5.15 4.32 3.99 4.82 4.57
6
5.15 4.88 4.60 4.87 4.88
B. Tetrahedral(T) and square planar(SP) configurations T- 1 6.48 5.53 6.61 5.16 5.16 T-2 6.13 5.25 6.33 4.33 4.89 T-3 5.86 4.67 5.45 4.00 4.61 T-4 6.28 4.95 5.73 4.83 4.88 T-5 6.19 5.10 6.03 4.58 4.89 SP-1 SP-2 SP-3 SP-4 SP-5
6.97 6.61 6.35 6.72 6.68
5.67 5.39 4.81 5.09 5.24
6.64 6.36 5.48 5.76 6.06
5.17 4..34 4.01 4.84 4.59
5.19 4.92 4.64 4.91 4.91
7
5.02 4.42 3.98 4.73 4.54
5.06 4.46 4.02 4.77 4.58 5.18 4.58 4.14 4.89 4.70
(21) A. H. Gawer and B. P. Dailey, J . Chem. Phys., 42, 2658 (1965). (22) P. J. Black, R. D. Brown, and M. L. Hefferman, Aust. J . Chem., 20, 1305 (1967). (23) A. D. Buckingham, Can. J . Chem., 38, 300 (1960). (24) J. I. Musher, J . Chem. Phys., 37, 34 (1962). (25) C. E. Johnson, Jr., and F. A. Bovey, J. Chem. Phys., 29, 1012 (1958). (26) W. T. Raynes, A. D. Buckingham, and H. J. Bernstein, ibid., 36, 3481 (1962). (27) J. A. Pople, ibid., 37, 60(1962). (28) S. Castellano, H. Gunther, and S. Ebersole, J. Phys. Chem., 69, 4166 (1965). (29) P. J. Black, R. D. Brown, and M. L. Heffernan, Aust. J . Chem., 20, 1325 (1967). (30) D. J. Blears and S. S . Danyluk, Tetrahedron, 23, 2927 (1967). VOL. 40, NO. 13, NOVEMBER 1968
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has been assumed to be approximately that for nitrogen (29)) but in the present work a value of - 8.4 X cm3/molecule has been used (26, 27) in a direction perpendicular to the C-0-M bond plane. This assumes tetrahedral hybrid orbitals on oxygen with two of them bonding and two nonbonding (containing the lone pairs). In general, shielding due to the magnetic anisotropy and dipole of the lone electron pair o n nitrogen will not occur because coordination through these electrons will remove these effects, as in the case when the nitrogen is protonated. Effects due to changes outside of the chelate molecule cannot be evaluated and include differences in concentrations when the chelate and the free ligand are compared, variations in the “reaction field” (23) at the solvent-solute interface, and specific interactions of one chelate molecule with another in solution. Divalent Metal Oxinates. To facilitate a semiquantitative evaluation of the effects on chemical shifts due to magnetic anisotropies and ring currents, idealized models have been constructed for square planar and tetrahedral configurations. The structure of the ligand itself has been well established (31); iverage metal-oxygen and metal-nitrogen bond lengths of 2.0 A have been used in each case. The calculated shielding shifts (in ppm) for the two configurations are: Tetrahedral; -0.04, 0.00, f0.02, +0.01, +0.01, +0.04; Square planar; +0.45, +0.14, f0.05, f0.02, +0.04, +0.16 for protons 2 through 7, respectively (+ indicates a downfield shift). These corrections have been applied to the theoretical spectra for the five bonding cases summarized in Section A of Table IV to give predicted spectra that are tabulated in Section B of the table. Thus, for a given bonding case and a given configuration, the observed metal oxinate spectrum should parallel one of those listed in Table IV. Large deviations from the data in Table IV may occur for spectra of square planar systems due to specific interactions between separate chelate molecules. In the crystalline state the platinum (32) and palladium (33) oxinates have been shown to be square planar with the metal atom of one molecule interacting with the aromatic pi-electron systems of molecules in the planes immediately above and below its own, somewhat analogous t o pi bonding in the metallocenes. For copper oxinate (34) the interaction is between the metal atom in one plane and a ligand oxygen in the adjacent plane. If the same sorts of interactions exist to some extent in solution, the ligand protons may experience additional shielding which is dependent on their distance and relative position t o the neighboring molecules. Such interactions are not expected to be as great in a tetrahedral molecule because the metal atom is shielded more effectively by the ligands. PtOxs. This chelate is square planar in the solid state and is assumed to have the same configuration in solution. The fact that Pt-N and Pt-0 bond strengths are high implies that the spectrum should be similar to Case SP-1 or perhaps SP-5. The correlation with the latter is good except for deviations at protons 2 and 7. This may be due t o specific interactions between the metal atom and solvent molecules which would affect protons 2 and 7 most because of their close proximity. However, if interplanar interactions be-
Figure 2. Trans (1,2,6) octahedral configuration of M(III)(Ox)3 tween chelate molecules exist (similar to those in the solid with an interplanar separation of about 4 the chemical shifts of all the ring protons should lie upfield of their calculated positions by about 0.2 to 0.5 ppm. This is the situation when the PtOxs spectrum is compared with Case 1. For either condition, the ligand should be coordinated strongly through nitrogen with a slightly more labile metal-oxygen bond. ZnOx2. The spectrum for the zinc oxinate most closely resembles a condition intermediate to Cases T-3 and T-5; the deviations for each proton are only about 0.05 ppm or less. This implies that both metal-ligand bonds are quite labile in solution. Independent evidence for labile bonding between zinc and oxine exists from isotopic exchange studies (35) which indicate that the chelate exchanges its metal content rapidly in the presence of excess zinc ion. Zinc oxinate can be either tetrahedral (36) or octahedral (ligands square planar with water in the axial positions) (35, 37)) but the low tendency of zinc ion to coordinate with oxygen decreases the chances to form the octahedral configuration in solution. Hence, zinc oxinate is concluded to be in a tetrahedral configuration in solution. MgOx2. The magnesium chelate also has been shown to exchange readily with excess magnesium ion (38). The NMR spectrum parallels Case T-3, but is shifted downfield (about 0.1 ppm) which indicates weak bonding to oxygen and nitrogen. The only deviation is at proton 2 which is shifted about 0.3 ppm upfield from its calculated position. This proton is much closer t o the metal atom than any of the others so that if any specific solvent interactions d o occur they will be felt most at proton 2. The data indicate that the magnesium oxinate is tetrahedral in solution with highly labile bonding. SnOx2 and PbOxz. The tin and lead oxinates are similar to each other and both are intermediate between Case T-4
A.),
-
Kamenar. C . K. Prout, and J. D. Wright, - J . Chem. Sac., Sect. A , 661 (1966). (32) J. E. Lydon and M. R. Truter, J. Chem. Sac., 6899 (1965). (33) C. K. Prout and A. G. Wheeler, J . Chern. SOC.,Sect. A , 1286
(31) -, B. \-
(i966). (34) G. J. Palenik, Acra Crystallogr., 17, 687 (1964). ~
1950
e
ANALYTICAL CHEMISTRY
(35) D. C. Atkins, Jr., and C. S . Garner, J. Amer. Chem. SOC.,74, 3527 (1952). (36) J. C. I. Liu and J. C. Bailar, Jr., ibid.,73, 5432 (1951). (37) G. J. Palenik, Acra CrystaNogr., 17, 696 (1964). (38) S . Ruben, M. D. Kamen, M. B. Allen, and P. Nahinsky, J . Amer. Chem. SOC.,64,2297 (1942).
and Case T-5. The hetero-ring resonances for each are shifted slightly downfield, the effect being largest a t proton 2. This may again be attributed t o specific solvent effects. Thus the chelates are both assumed to be tetrahedral with the metalnitrogen bond being quite labile and the metal-oxygen bond much more inert. Trivalent Metal Oxinates. The normal tris metal oxinates are expected t o exist in octahedral configurations. Because oxine is an unsymmetrical bidentate ligand, the chelates can exist in cis (1,2,3 isomer) and trans (1,2,6 isomer) forms. In the cis isomer, all three chelate rings are geometrically and magnetically equivalent. In contrast, the trans isomer has three distinct axes of symmetry such that each proton position on the ligand theoretically should exhibit three separate resonances (see Figure 2). AIOxs, CoOxs, and RhOxo. The aluminum(III), cobalt (111), and rhodium(II1) oxinates, on the basis of their NMR spectra, exist in the trans configuration in solution (Figure 2). To evaluate the observed chemical shifts a model of the trans structure has been constructed, assuming metal-oxygen and metal-nitrogen bond lengths of 1.9 A. Then, by methods similar to those outlined for the divalent ions, the contributions to the chemical shifts of each proton caused by ring currents and magnetic anisotropies from the two neighboring ligands have been determined. The calculated shielding shifts (in ppm) for the three different ligands of Figure 2 are: Ligand A ; -0.36, -0.26, -0.16, -0.05, -0.04, -0.08; Ligand B ; -0.41, -0.28, -0.19, -0.05, -0.04, -0.07; Ligand C ; -1.60, -0.43, -0.17, -0.00, +0.01, -0.16 for protons 2 through 7, respectively (- indicates an upfield shift). These shielding shifts have been applied as corrections to the five limiting cases of ligand bonding to give the predicted spectra summarized in Table V. The spectra of all three chelates are remarkably similar t o each other and closely related t o Case 2 and Case 5, which implies coordination through the ligand nitrogen and a highly labile (or ionic) metal-oxygen bond. When the aluminum chelate is heated in solution, the spectrum collapses to a single set of ligand resonances. This could be attributed to conversion t o the cis isomer which should give only a single ligand spectrum. However, in the cis isomer, proton 2 on each ligand is quite close to the heterocyclic ring of the neighboring ligand. This is identical to the relationship of proton 2 on ligand C to ligand A in the trans isomer. Therefore, the chemical shift for proton 2 should be in the region of 4.6 ppm from DMSO. If the collapse of the spectrum is due to rapid exchange of the ligands in the trans isomer about the aluminum ion, the spectrum will merely be the average of the three separate ligand spectra observed at room temperature. Comparison of the observed high temperature spectrum with the averaged room temperature spectrum confirms that the collapse is due t o rapid exchange in the trans isomer.
Table V. Predicted Chemical Shifts for Oxine Ligands in the Limiting Cases for Trans Octahedral M(II1) (Ox), 6 , ppm cs. DMSO
Proton
~
m
L
5
4
5
6
7
Ligand A Case I Case 2 Case 3 Case 4 Case 5
6.16 5.80 5.54 4.96 5.87
5.27 4.99 4.41 4.69 4.84
6.43 6.15 5.27 5.55 5.85
5.10 4.27 3.94 4.77 4.52
5.11 4.84 4.56 4.83 4.84
4.94 4.34 3.90 4.65 4.46
Ligand B Case 1 Case 2 Case 3 Case 4 Case 5
6.11 5.75 5.49 5.91 5.82
5.25 4.97 4.39 4.67 4.82
6.40 6.12 5.24 5.52 5.82
5.10 4.27 3.94 4.77 4.52
5.11 4.84 4.56 4.83 4.84
4.95 4.35 3.91 4.66 4.47
Ligand C Case 1 Case 2 Case 3 Case 4 Case 5
4.92 4.56 4.30 4.72 4.63
5.10 4.82 4.24 4.52 4.67
6.42 6.14 5.26 5.54 5.84
5.15 4.32 3.99 4.82 4.57
5.16 4.89 4.61 4.88 4.89
4.86 4.26 3.82 4.57 4.38
Although the bonding in the aluminum(II1) and cobalt(II1) oxinates are similar, the fact that the cobalt spectrum does not collapse at high temperature establishes that the bonds t o cobalt are much more stable than those to aluminum. The large upfield shift of proton 2 in ligand C is analogous to observations made for the tris chelates of iron(I1) with 1,lOphenanthroline (39) and 2,2’-bipyridyl (28). The proton alpha t o nitrogen in each of these chelates lies close t o the hetero-ring of a neighboring ligand, similar to the position of proton 2 in ligand C relative t o ligand A (Figure 2). For the 1,lo-phenanthroline chelate, the large shift has been attributed t o a direct metal-proton interaction, but the present work indicates that the entire shift is due t o ring currents in the ddjacent ligand. This conclusion is supported by a recent discussion concerning bipyridyl complexes (40, 41). A similar ring current effect has been assumed to be the cause of the shift in the iron(II)-2,2’-bipyridyl complex (28). RECEIVED for review May 22, 1968. Accepted August 26, 1968. Work supported by the U S . Atomic Energy Commission under Contract No. AT-(11-1)-34, Project No. 45.
(39) J. D. Miller and R. H. Prince, J. Clzem. SOC.,4706 (1965). (40) S. M. Castellano and H. Giinther, J . Pliys. Chem., 71, 2368 (1967). (41) T. Saito, M. Araki, Y.Uchida, and A. Misono, ibid., p 2371.
VOL. 40, NO. 13, NOVEMBER 1968
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