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Electronic Structure of Pseudotetrahedral Copper(II) Schiff Base Complexes J o h n R . W a ss o n , H . W a y n e R ic h a r d s o n , a n d W illia m E. H a tfie ld Department of Chemistry, University of North Carolina, Chapel Hill, N. C. 27514, USA (Z. Naturforsch. 32b, 551-561 [1977); received December 2, 1976/February 12, 1977) Copper(II) Complexes, Electron Paramagnetic Resonance, Schiff Base Complexes, Electronic Spectroscopy, Hyperfine Coupling Constants Copper(II) complexes with the Schiff bases formed by condensing i-butyl-, isopropyl-, cyclohexyl- and diphenylmethylamines with salicylaldehyde and 3-methoxy- and 5-nitrosalicyl -aldehyde have been prepared and characterized. The isotropic nuclear hyperfine coup ling constants increase in the order t -butyl < isopropyl < cyclohexyl < diphenylmethyl for the N-substituents in the three series of complexes while isotropic g-values increased in the opposite direction. Spectra structure correlations were examined and discussed. Introduction Pseudotetrahedral copper(II) compounds have been known for several years and have been investigated with varying degrees of thorough ness1-31. In principle, operation of the Jahn-Teller effect32 precludes the existence of purely tetrahedral (Ta) copper(II) complexes and lower symmetries are found for CuLa (L = monodentate ligand) species, e.g., CuCl2- and CuBr|- 11. Molecular orbital studies of distortions of the CuCU2- ion have shown that total energy versus distortion angle curves are rather flat bottomed8. This has been interpreted to mean that a variety of geometries (“distortion isomers” 30-33) are possible for a given tetrahedral CuLi complex depending on ionic and van der Waals forces as well as steric factors present in a given compound. The tetrachlorocuprate(II) anion is well-known to exhibit square planar and a variety of “tetrahedral” geometries depending on the nature of the cation10-28’34. The rather general occurence of distortion isomerism in copper(II) compounds has been reviewed33. For pseudotetrahedral copper(II) compounds of the type CuL2L2' (L ^ L') the possibility of distortion isomerism is an additional complication33 to an already lowr-symmetry prob lem. Copper(II) Schiff base complexes have been employed as enzyme models35’36. Inspection of Requests for reprints should be sent to Professor E. H a t f i e l d , Department of Chemistry, University of North Carolina, Chapel H ill, North Carolina 27514, USA. W illia m available ESR data suggests that pseudotetrahedral Schiff5’ 19- 25’ 29’ 37 base compounds might serve as models for the “blue” copper(II) proteins 38-39. This study was initiated to further explore that pos sibility as well as more generally probe the electronic structures of pseudotetrahedral Schiff base com plexes. It is well-established that the “d -p ” mixing is the principal mechanism giving rise to the elec tronic spectral intensities of d-d bands of tetra hedral transition metal compounds. The d-d band intensities can arise from metal 4p orbital partici pation in the ground and/or excited states40. ESR studies of pseudotetrahedral copper(II) compounds have shown that they exhibit much reduced electron spin-nuclear spin hyperfine coupling constants compared to complexes with other geometries. The small values of the hyperfine coupling constants have been associated with metal 4p orbital partici pation in the ground state41-44. These considerations suggested to us that there should be a relation between the oscillator strengths of the d-d absorp tion bands and the nuclear hyperfine coupling constants of pseudotetrahedral copper(II) com plexes. To investigate this matter we prepared a variety of copper(II) Schiff base complexes (1) and characterized them spectroscopically. H 'C = N= I R = (CH3 )2 C-, (CH3 )3 C-, C6 H n H I and (C6 H 5 )2 CX — H, 3-methoxy, 5-nitro Unauthenticated Download Date | 6/19/17 2:01 AM 552 J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes Isopropyl (P), £-butyl (T), cyclohexyl (C) and diphenylmethyl (D) groups substituted on the azomethine nitrogen atom were chosen since crystallographic studies 3 ’ 17 have shown that bulky groups lead to pseudotetrahedral geometries for copper(II) salicylaldimine complexes. For convenience, the complexes are designated Cu(TsalOCH3 )2, etc., where the capital letter indicates the substituent group and the latter part of the abbreviation indicates the appropriate salicylaldimine. Experimental The complexes employed in this study were prepared and purified by standard methods and gave satisfactory elemental analyses (C, H and N from Chemalytics, Inc., Tempe, Arizona and copper by EDTA titration). Crystals of most of the complexes could be growrn from acetone or chloro form solution. Electronic spectra were obtained with a Cary Mod el 17 recording spectrophotometer using matched 1.0 cm cells. Mull (transmittance) spectra were obtained by a technique described previously45. Electron spin resonance spectra were recorded with a Varian E-3 spectrometer at room temperature or at 77 °K. Quartz sample tubes were employed for powders and chloroform solutions. Spectra were calibrated using diphenylpicrylhydrazyl (DPPH, g = 2.0036) as a field marker. A sample of polycrystalline DPPH taped to a tube containing oxobis(2,4-pentane-dionato)vanadium(IV) in ben zene served as a double standard for checking field strength, frequency and sweep rate settings37. Table I. Electronic spectral data (chloroform solutions). 0 4 3 1 6 8 7 0 10 12 11 9 Compound 3 v(kK)b E mc f X 10+3d Cu(Psal) 2 Cu(Csal ) 2 Cu(Tsal) 2 Cu(Dsal)2 Cu(PsalOCH 3 ) 2 Cu(CsalOCH3 ) 2 Cu(TsalOCH3 ) 2 Cu(DsalOCH 3 ) 2 15.63 16.21 13.16 16.75 15.15 15.63 13.64 16.67 15.77 16.18 13.30 16.55 165 157 4.17 4.56 3.64 3.96 3.90 4.63 5.18 4.90 4.92 5.10 4.74 4.70 Cu(PsalN 0 2 ) 2 Cu(CsalN 0 2 ) 2 Cu(TsalN 0 2 ) 2 Cu(DsalN 0 2 ) 2 (15.27) (16.53) 2 0 2 (16.58) (16.67) 157 166 188 234 189 2 0 2 (16.45) (16.53) 189 261 192 a P, C, T and D refer to isopropyl, cyclohexyl, <-butvl and diphenylmethyl groups attached to the azometliine nitrogen atom (1 ). sal, salOCH 3 and salN 0 2 refer to the salicylaldimine, 3-methoxysalicylaldimine and 5-nitrosalicylaldimine groups, respec tively. b 1 kK — 1000 cm - 1 values in parentheses are the band maxima in the mulled solids. c Molar absorptivity. d Oscillator strengths, f, were calculated using the expression: f = 4.60 X 10- 9 E max ^1 / 2 where E max is the molar absorptivity of the band maximum and i>i/ 2 is the band width at half-height expressed in wave numbers: C. J. B a llh a u s e n , Proer. Inorg. Chem. 2, 251 [I960]. Results and Discussion Electronic spectra Table I summarizes the electronic spectra of the copper(II) Schiff base complexes in chloroform solution and representative spectra are shown in Fig. 1. Apparent oscillator strengths for the mani fold of d-d transitions, which provide a better measure of band intensity than the molar ab sorptivity of the band maximum, were determined in order to obtain a comparison, discussed below, of band intensities and isotropic hyperfine coupling constants. The molar absorptivities of the Schiff base complexes are all about 200 M-1 cm-1 and the oscillator strengths are in the range 3-5 X 10-3. For pseudotetrahedral complexes the major contribution to the intensities of the d-d bands is expected to be ,ld -p ” mixing whereas the vibronic mechanism is operative in planar complexes. Although the 4p orbital participation in the ground and excited states might be anticipated to increase on going Fig. ( . . . . V (kK) , Electronic spectra ofCu(Dsal ) 2 (— —), Cu(Csal)2 , Cu(Psal) 2 ( - • ---- ) and Cu(Tsal)2 (- - - ) in chloroform. from effective planar (~D 2h or D411) to pseudotetra hedral (C2, C2V or D2d) geometries, the degree of mixing is not necessarily a smooth function of the degree of distortion. Iterative extended Huckel molecular orbital calculations8 for the CuCU2- anion show that small distortions from D4h toward Td symmetry are more likely to introduce appreciable metal 4p orbital character into the ground state than are larger distortions. Crystal structure data for pseudotetrahedral Schiff base complexes, sum Unauthenticated Download Date | 6/19/17 2:01 AM J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase Com plexes marized by W e i 17, show that Cu(Psal)2 is more tetrahedral than Cu(Tsal)2. Just the opposite conclusion is implied by the band maxima and oscillator strength data presented in Table I. The oscillator strengths are not always in accord with the expectation that isopropyl-substituted salicylaldimines are more tetrahedral than t-butyl substituted compounds. Apparently, the solution species are not always identical to those existing in the solid state as evidenced by the differences in the electronic spectra of solutions and mulled solids (Table I). Unfortunately, crystal structures are not available for compounds other than Cu(Psal)2 and Cu(Tsal)2. Since the apparent oscillator strengths involve all of the bands (d-d, intraligand and chargetransfer tail contributions) it is not surprising that they do not yield a simple spectra-structure correla tion although, generally, the £-butyl derivatives have larger molar absorptivities at the band maxima and larger oscillator strengths. The spread in the d-d band centers is probably the dominant contribu tion to the failure of oscillator strength-structure correlations. The band maxima for each series of compounds indicate that the degree of tetrahedral distortion of the complexes in solution decreases in the order: £-butyl > isopropyl > cyclohexyl > diphenylmethyl. There is a general increase in inten sities on going from sal to salOCH3 to salN 0 2 groups suggesting covalency contributions to the oscillator strengths are also of significance. For a complex with Td, D2d. C2 or lower symmetry the wave function for the ground state can be expressed by yg.s. == a3d ~f- b4s -f- c4p — j- dL (1) where a, b, c, and d are ground state orbital coefficients for metal 3d, 4s, and 4p and ligand L orbitals, respectively. A given excited state might have an eigenfunction given b y : Vex = a'3 d -f b '4s + c'4p + d'L. (2) The transition moments will then consist of a sum of terms of the types ac'(3d|f|4p), a'c (4p|r |3d>, be'(4s|r|4p), b'c(4p[f|4s> and (metal orbital|r|L> and dd' (L|r jL). Simple calculations 40 show that d-p terms can readily lead to oscillator strengths of 1 0 -5 or larger, Fig. 2 shows results of some calculations of transition moment integrals for copper com pounds. The integrals were evaluated using the general expression of M c G l y n n et a l .46. The effective nuclear charges for copper 3d, 4s and 4p orbitals were estimated from the overlap matched orbital 553 % Effective Nuclear Charge Fig. 2. Transition moment integrals and their squares as a function of effective nuclear charge on copper. exponents of Z e r n e r and G o u t e r m a n 47 by multi plying the exponents by the appropriate principal quantum number. These were then reduced in increments of 1 0 % in order to simulate the effect of charge reduction by complexation of the metal ion. Using the expression48 f — 1.096 x 10u v(cm_1) <yex|r|y>g.r) 2 (3) it is easily shown that for a d-d transition appearing at 1 0 ,0 0 0 cm-1 the d-p contribution to the oscillator strength can range from 0.016 to 0.062 while the s-p contribution only varies from 6.3 X 10- 5 —4.4 x 10-4 for a corresponding 50% reduction in the effective nuclear charge on the copper ion. It is noted that the oscillator strengths of the electronic absorption bands of the “blue” copper proteins range 38 from 1 0 _3 to 1 0 -1 which has led to the suggestion of the presence of a pseudotetrahedral chromophore. Data for planar copper(II) complexes with sulfur donors49 implies that the presence of sulfur atoms leads to a predominance of (metal orbital|r|S) and (LjrlL'} contributions, particularly the former, to the transition moments. By considering “d -p ” mixing and covalency contributions to the transition moments, a semiquantitative account of the “d-d” band intensities of pseudotetrahedral copper(II) complexes is attainable. Crystal field calculations were performed using the explicit approach of C o m p a n io n and K o m a - Unauthenticated Download Date | 6/19/17 2:01 AM 554 J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase Complexes and a computer program described else where51. Calculations were performed for a CuXj complex having the geometry (2 ) with the angles X 1CUX2 and X 3CUX4 being 90°. 1 z r y n s k y 50 j /*1 Cu— —x ■ * / 1\ ( 3 a> X2 — Cu ^ X, — x ^ 2 3 The ligands Xi and X 2 were held in the x z plane while the X 3CUX4 plane was rotated about the 2 -axis: the angle between the XiCuX 2 and X 3CUX4 planes was designated as to (3). The calculations were performed for various values of the crystal field splitting parameter a 4 (014 = 6 Dq for a cubic complex) for each of the ligands while holding the a 2/a4 ratio equal to 0.9. Fig. 3 shows that small distortions from planar to tetrahedral type geo metries result in a slight increase in the d-d transi tion energies but these generally decrease as co approaches 90° (D2d geometry). Increasing the tetrahedral geometries to complexes in the order of /-butyl > isopropyl > cyclohexyl > diphenylmethyl substituents is in accord with the phenome nological crystal field results. It is noted that changes in geometry change the populations of the various d-orbitals in the ground and excited states. Thus, to discuss the electronic energy levels of a particular compound in great detail requires extensive calculational and experimental effort. Fig. 4. Calculated electronic transition energies as a function of 0 4 parameters for two C2 geometries. Electron Spin Resonance Spectra Fig. 5 compares the ESR spectra of polycrystalline Cu(Tsal)2 and Cu(Psal)2. Although qualitatively similar, the g-values are better resolved for Cu(Psal)2 which is more tetrahedral in the solid state. Fig. 6 compares the ESR spectra of polycrystalline samples of salOCH3 derivatives; again, the isopropyl derivative shows the best resolved g-values. The ESR spectra were interpreted by standard proce- Fig. 3. Calculated electronic transition energies as a function of dihedral distortion. crystal field strength of the ligands increases the band separations. In optimal cases where the band widths are rather small ( < ~2kK ) and the bands are 2-3 kK apart it may be possible to resolve three of the four d-d bands. Fig. 4 which shows calculated transition energies as a function of cu for two distorted (C2) geometries shows that resolution of more than three of the four absorption bands cannot be readily anticipated. Although resolution of d-d bands were not observed for the copper(II) Schiff' base complexes, the assignment of decreasingly Fig. 5. Electron spin resonance spectra of p oly crystalline Cu(Psal) 2 (— ) and Cu(Tsal)2 (----- ) at room temperature. Unauthenticated Download Date | 6/19/17 2:01 AM J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase Com plexes dures. In several instances the A|| or A z nuclear hyperfine coupling constants were estimated by taking one third of the peak width at half-height. Experiments with copper-doped samples of the corresponding zinc complexes did not appear to result in improved resolution of g and A values and TsalOCI HlOOoe^ Fig. 6 . Electron spin resonance spectra of poly crystalline o-vanillin Schiff base complexes with copper(II). Room temperature. 555 were thus not performed for all of the complexes. The spin-Hamiltonian parameters for the Schiff base complexes are summarized in Table II. The average g-values calculated from the anisotropic powder data generally tend to be lower than the isotropic values obtained from the chloroform solutions. This strongly supports the conclusion that the species present in solution are not those found in the solid state and is in accord with the electronic spectral data discussed in the preceding section (see also, Table I). The isotropic nuclear hj^perfine coupling constants increase in the order £-butyl < isopropyl < cyclohexyl < diphenylmethyl for the N-substituents in the three series of complexes while isotropic g-values increased in the opposite direction. Thus, as a complex becomes more tetra hedral, the isotropic g-values increases and the Ao value decreases. Table II. Electron spin resonance data. j X 104 cm- \ Compound Solvent go g|| gjAi gX) gyJ Ao Cu(Psal )o Chloroform powder Chloroform 2.118 2.094 2.116 2.092 2.138 _ _ 2.180 2.051 61.4 64.8 2.169 2.053 Cu(Csal)-2 Cu(Tsal) 2 Cu(Dsal) 2 Cu(PsalOCH 3 ) 2 Cu(CsalOCH3 ) 2 Cu(TsalOCH3 ) 2 Cu(DsalOCH 3 ) 2 Cu(PsalN 0 2 ) 2 Cu(CsalN 0 2 ) 2 Cu (TsalN O2 ) 2 Cu(DsalN 0 2 ) 2 Chloroform Zn(II) complex powder Chloroform Zn(II) complex powder Chloroform powder 2 .1 2 0 2.090 2.094 - 2.276 2.167 - 2.132 2.217 2.151 2.262 Chloroform powder 2.116 2.104 2.205 Chloroform powder Chloroform powder Chloroform powder Chloroform powder Chloroform powder 2.143 2.097 2.115 2.075 2.119 2.116 2.118 2.182 2.116 2.224 Chloroform powder 2.114 2.086 2 .1 2 1 2 .1 0 2 2.141 2.132 2.114 2.083 41.6 2.028 2.056 2.051 2.062 2.054 2.047 2.086 2.044 2.062 2.055 2.055 - 6 8 .0 2.178 2.064 - - 2.256 2.054 2.086 - 2.051 _ 9.3 - 17.6 - 2.147 - 61.9 2.062 - _ - 69.2 67.9 - 50.0 A l( t ) _ 118.2 59.0 - - 50.0 68.9 64.9 - - A|| - ’ 42.9 36.1 40.4 - 49.4 - 36.6 -3 2 -3 2 25.5 13.4 17.6 1 2 .2 - 6 .0 9.4 24.0 12.7 - 1 0 .0 - 1A - 8 .0 17.6 69.9 - - - - 18.6 * See Table I for abbreviations. Unauthenticated Download Date | 6/19/17 2:01 AM 556 J. R. Wasson et al. • Electronic Structure of Copper(II) Schiff Base Complexes Table III. ESR data for pseudotetrahedral copper(II) complexes. |— X 104 cm - 1 ZnW 0 4 Reference g||(g*) 2.072 53 2.072 57, 58 2 .0 0 1 CdW 0 4 58 2 .0 1 2 M gW 0 4 58 2.013 BeO ZnO 54 55 CdS 52 ZuS N H 4F 59 2 2 1.709 0.7392 0.7392 0.74 2.240 1.930 2.470 Cs2 CuCl4 2 0 2.384 Cs2 ZnCl4 2 0 2.445 Compound or host lattice CaW 0 4 g i \gvJ 2.286 2.286 2.341 2.382 2.302 2.496 2.334 2.385 2.379 1.5182 1.5182 1.531 1.75 2.14 2.077 2.104 2.083 2.105 2.083 Ax ( Ay) All 55.6 38.5 59.5 41.1 76.5 - - 9.2 76.5 - -1 9 .3 82 15 2 0 2.462 2.078 Comment for 63Cu for 65Cu 0 76 0 18 50 108 199.6 224.1 240.1 213.8 231 195 99 ± 5 28.1 for 63Cu for 65Cu 2 1 .1 < 4 <4 - - 25 + 5 2 .1 0 0 [(CH3 )4 N] 2 ZnCl4 | 17 + 5 several centers 51 + 5 46 + 5 51+5 2 .1 0 1 [(C4H 9 )4N ]2CuB r4 Cu(dmp) (CH3 C0 2 )2 4 0 0 2 .2 2 0 2.324 Cubich (CH2 C 0 2 ) 2 60 2.333 Cu2+ penicillamine 62 2.27 2.14 Cu(II)-doped L-histidine hydrochloride monohydrate 64 2.013 Cu(MPG) (violet) Cu(MPG) (green) Cu(dapy)2 (C104 ) 2 (red) Cu(dapv)2 (C104 ) 2 (orange) - 39 34 30 30 2.185 . 2.259 2.467 2.451 Cu(dapy)2 (BF 4 ) 2 30 2.455 Cu(dapyp)2 (C104 ) 2 30 2.465 Cu(dapyp)2 (BF 4 ) 2 30 2.399 6 6 2.291 Cu(OAs{C6 H 5 }3 )2 Br2 Cu (OP{CßHö }3 )2 Br2 6 6 2.224 ZnHg(SCN ) 4 ZnHg(SCN ) 4 CdHg(SCN ) 4 ZnCd(SCN ) 4 CdCd(SCN) 4 13 56 56 56 56 2.40 2.440 2.447 2.270 2.245 2.053 < 145 2.068 168 2.068 2.078 2 .1 1 2.03 2.262 2.153 2.037 2.040 2 .1 0 1 2.061 2.093 2.078 2.107 2.053 2.145 2.046 2.076 2 .0 2 1 2.070 2.096 2.167 2.07 2.095 2.095 2.036 2.036 129 149 140 131.7 171 8 6 128.3 135.4 135.0 Ao — 54.1 x 10'4 cm _ 1 25.9 _ dmp = 2,9-dimethyl1 , 1 0 -phenanthroline A0 = 30 + 0.5 X 10~4cm bich = 2 ,2 '-biquinoline Solution species. Solution species. 62.4 ~31 _ 11.7 12.3 17.4 MPG = u-merceptopropionylglycine dapy = diantipyrylmethane 8 .1 18.0 108.0 2 0 .6 30.5 165.2 1 0 .2 dapyp = diantipvrvlpropylmethane 11.3 Zn(II) lattice 170.5 «7.3 74 77 1 0 C 14 7 Z n(ll) lattice 11.5 29 29 13 60 Unauthenticated Download Date | 6/19/17 2:01 AM J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes 557 Table III (continued) I— X 104 cm - 1 —| Compound or host lattice Reference g||(gz) Catena-// -bis (1,2 dipheny 1-phosphinyl)ethan dichloiocopper(II) 6 2.413 Cu({C6 H 5 }sPO)2 Cl2 65 2.408 Cu({C6 H 5 }3 PO)2 Cl2 6 6 2.292 Zn(l,10-phenanthroline)Cl2 23 2.297 Cu( 1 -sparteine)Cl2 24 2.299 Cu( 1-sparteine)Br 2 24 2.27 Cu( 1-benzene-azo-N-phenyl2 -naphtylamine )2 Cu(2 ,2 /-bipyridvlamine) 2 (C1 0 4 ) 2 7 14 2.158 2.244 Cu(N-<-butylpyrole2 -carboxaldimine ) 2 „tetragonal form 37 29 2.156 2.209 ,,triclinic form 29 2.266 CuL2 Cl2 L = [(C6 H 5 )2 PO] 2 CH2 = [(CjHg^PO^CH^ = [(C4 H 9 0)2PO]2 CH2 = [(C4 H 9 )2 PO](CH 2 ) 6 63 63 63 63 2.538 2.495 2.445 2.422 Dipyrromethanes 3,3',4,4'-tetramethyl 3,3\5-trim ethyl 3,4,5-trimethyl 3,3',5,5'-tetramethyl 3,3',4,4',5,5'-hexamethyl 5,5'-diphenyl 5,5'-dibromo Cu(N -R -sal ) 2 R = isopropyl = <-butyl = s-butyl = cyclohexyl 21 21 21 21 21 21 41 25, 5 19 25, 5 19 25 25 Cu(PBO ) 2 a Cu(PBS ) 2 b 67 67 Cu(Im ) 2 c Cu(Benzim ) 2 d Cu(II) in: Zn(a-picoline)2 (NCS ) 2 Zu(pyridine) 2 (NCS ) 2 Zn(ß-picoline)2 (NCS ) 2 Zn(y-picoline)2 (NCS ) 2 6 6 Cu{Me2 Ga(dmpz) 2 } 2 e 69 6 6 68 6 8 6 8 6 8 , 61 g± 2.067 2.073 2.064 2.071 2.026 2.079 2.058 2.062 2.075 2.050 2.071 2.042 2.041 2.059 2.069 2.089 2.086 2.045 2.090 2.065 2 .1 1 0 2 .1 0 0 2.091 2.103 A | Pure crystal studied Zn(II) lattice 170.6 123 ± 4 131.4 123 ± 0 - Zn(II) host 20.1 Adopted inNi (II)complex Pure crystal studied A 0 - 2 0 .9 x 1 0 - 4 cm Pure crystal studied 1 Pure crystal studied 67.4 74.9 91.2 118 2.053 2.067 9.068 156 164.9 145 119.3 156 156 2.117 2.162 2.135 2.108 2.041 Zn(II) host 29.8 - - 2.253 2.229 2.270 2.273 2.253 2.253 2.155 2.181 2.426 2.406 2.385 2.370 2.316 9.4 14.5 8.4 14.9 - -1 7 .1 154 136 136 108 108 103 2.198 2.178 9±4 97.0 2.039 2.050 2.057 2.062 2.062 2.070 2.084 2.069 2.055 2.062 2.075 2.079 2.050 Comment Pure crystal studied 2.224 2.248 2.245 2.272 2.278 2.279 2.283 - Aj^ \AjJ -1 4 - - - - - 158 183 197 57 1 1 .8 8 - -1 7 -1 6 30 29 44 - 81 97 95 —50 63 63 63 6 8 6 8 - -1 6 - a> 73 - <50 < 16 < 18 <30 6 8 Ao X 104 cm - 1 55.6 56.1 56.2 82.3 83.1 - Toluene 1 , 1 , 2 ,2 -tetrachlorothene Toluene 1 , 1 , 2 ,2 -tetrachloroethene Toluene Toluene Zn(II) lattice Zn(II) lattice Zn(II) lattice Zn(II) lattice a PBO = anion from 2-(0-hydroxyphenyl)benzoxazole, b PBS = anion from 2-(0-hydroxyphenyl)benzoxazole, CIM = imidazolate anion, d benzim = benzimidazolate, e Me2 Ga(dmpz ) 2 = anion of 3,5-dimethylpyrazoyldimethylgallium. Unauthenticated Download Date | 6/19/17 2:01 AM 558 J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase C om plexes Table III summarizes ESR data for pseudotetrahedral copper(II) complexes. The ESR spectra of Cu(II) doped into binary compounds such as BeO pose special problems associated with the JahnTeller effect. In tungstates and other compounds the g-values vary widely with the g, or g|| values becoming as large as 2.5. The copper nuclear hyperfine constants of pseudotetrahedral com pounds also vary widely but they take values less than 150 X 10-4 cm-1 which are significantly smaller than A2 or A 11 values found for planar and pseudotetrahedral Cu(II) compounds. Table III provides a brief review of ESR studies of pseudotetrahedral Cu(II) compounds and is included for the reader’s convenience as well as to demonstrate the observed variation of spin-Hamiltonian parameters. The large value of g2 or g|| of many pseudotetra hedral Cu(II) compounds is readily accounted for in terms of the theoretical expressions for the g-values70. The mixing of d levels, as noted in the crystal field results, can be such that explicit expressions for the g-values must be derived for a particular compound. Table VIII of reference 70 lists g-value expressions for a number of geometries having various ground states. Taking selected expressions from that tabulation many ground states and geometries as well as intermediate situations can be considered. Fig. 7 shows how g2 or g|| varies with the lowest electronic transition energy and the effective spinorbit coupling constant. The particular label for the electronic transition applies to a C2 complex with a d rz ground state24. However, the results in Fig. 7 are applicable to a great number of complexes possessing a non-degenerate and “unmixed” elec- Fig. 7. gz or g11 as a function of electronic transition energies and spin-orbit coupling constants. tronic ground state70. An alternate expression for gzis‘2 k ,2A g2 = 2 .0 0 2 (4) AE where k 22 is the orbital reduction factor, kz2/ replacing ?. the effective spin-orbit coupling constant. k -2 can be expressed in terms of molecular orbital coefficients70-71 and it expresses the well-known reduction in the spin-orbit coupling constant when a metal ion is complexed. Keeping in mind that X is negative for Cu(Il) and rearranging the preceding expression, it is found th a t: ZlgZlE k 2A (5) Table IV. Orbital reduction factors for pseudotetrahedral copper(II) Schiff base complexes 3 Part A k z2 k? Ligand 1.63 1.31 1.53 1.30 1.28 1.14 1.24 PsalOCHg CsalOCHg TsalOCHg DsalOCHg Ligand k,2 k2 Ligand Psal Csal Tsai Dsal 0.42 0.41 0.33 0.38 0.65 0.63 0.57 0.62 PsalOCHg CsalOCHg TsalOCHg DsalOCHg Ligand Psal Csal Tsai Dsal k?2 1 .6 8 2.38 1.92 1.48 1.15 Part B k, 2 0.60 0.48 0.37 0.29 kz Ligand kz 2 1.54 1.39 2 .1 1 1.07 P sa lN 0 2 CsalN 0 2 T salN 0 2 D sa lN 0 2 1.72 2.04 1.45 kz Ligand kz2 k2 0.77 0.69 0.61 0.54 PsalOCHg CsalOCHg TsalOCHg DsalOCHg 0.53 0.43 0.51 0.36 0.73 1 .2 2 a Values found using data from Tables I and II and Equation (5) Unauthenticated Download Date | 6/19/17 2:01 AM k2 1.45 1.31 1.43 1 .2 0 0 .6 6 0.71 0.60 J. R . W asson et al. • Electronic Structure o f Copper(II) Schiff B ase C om plexes Table IV, Part A shows that the optical and ESR data in Tables I and II lead to orbital reduction factors which are significantly greater than a value of 1 .0 which is considered to be the orbital reduction factor when the unpaired electron is completely localized on the metal ion. As covalency increases, kz2 normally decreases. Since the 10-20% reduction (more if sulfur and selenium ligands are involved) of spin-orbit coupling constants in complexed ions is a well-established phenomenon, the anomalous results listed in Table IV need to be accounted for. An alternate expression for g-values also covering many pseudotetrahedral geometries having non degenerate and “unmixed” ground states is: * ■ "2002 ~ ^ (fi) where the symbols have their usual meanings. Similarly to equation (5), the orbital reduction factor can be evaluated using k 2 A= zlgzlE (7 ) Fig. 8 demonstrates how equation (6 ) accounts for many of the rat her large gz or g| | values encountered. Table IV, Part B, shows how equation (7) gives a more reasonable account of the data for the com plexes encountered herein. 559 d xy orbitals. However, it is common practice for a complex to be treated as though it had a higher symmetry and a pure “d” ground state. This practice is convenient and will undoubtedly con tinue. Two other common sources of error in the interpretation of the ESR spectra are: a) electronic band assignments and b) higher order contributions to the g-values. Fig. 4 show's that the assignment of the band maximum in the electronic absorption spectrum to ZlE employed in equations (4) to (7) is a very tenuous practice. For a complex having electronic transitions below 10 kK the errors in k 22 can be enormous. Hence, estimation of meaningful orbital reduction (or covalency factors when a molecular orbital approach is involved) necessitates accurate band assignments. When very small electronic transition energies are encountered, higher order contributions of the type (A/zIE) 2 to the g-values must be considered72. Fig. 9 shows that these contributions became highly significant for only very small transition energies and can probably be neglected in all but the most exacting wrork and wrhen very weak field ligands are of concern. For pseudotetrahedral Cu(II) complexes writh Schiff base Fig. 9. Second-order corrections to g-values as a function of electronic transition energies and spinorbit coupling constants. ?. — -— 828 cm-1. (a) 0.5A, (b) 0.62, (c) 0.7Ä, (d) 0.8A, (e) 0.9A, (f) 1.0A. Fig. 8 . gz or g|| as a function of electronic transition energies and spin-orbit coupling constants. For lowr-symmetry complexes expressions for the g-values frequently have to be derived for a particular situation, e.g., a complex with C2 sym metry and a ground state made up of dr2, d r2_)/2 and ligands the above considerations show that accurate electronic band assignments are vital to meaningful discussions of electronic structures. Several cor relations of spectral data for the compounds listed in Table I wrere attempted. A plot of emax vs Ais 0 showed a general increase of £max with decreasing Also but no quantitative relationship. A plot of giso vs Also showed that giS0 increases with decreasing Aiso but, again, no quantitative correlations w^ere Unauthenticated Download Date | 6/19/17 2:01 AM 560 J. R . W asson et al. • E lectronic Structure o f Copper(II) Schiff B ase C om plexes found and. similarly, for a plot of f vs AjS0. Fig. 10 shows that a plot of the band maximum vs the isotropic hyperfine coupling constants is reasonably linear for each family of Schiff base complexes where the azomethine nitrogen R groups are being substituted. In the absence of extensive crvstallo- Fig. 10. Electronic band maxima in chloroform vs isotropic nuclear hyperfine coupling constants for three groups of copper(II) Schiff base compleses. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 E. H a t f i e l d and R. W h y m a n , Transition Metal Chem. 5, 47 [1969]. R. H. H o l m , A. C h a k r a v e r t y . and L . J. T h e r i o t , Inorg. C h e m . o, 625 [1966]. L . S a c c o n i , Coord. Chem. Revs. 1, 126 [1966] and references therein; H. S . M a s l e n and T. N. W a t e r s , Coord. Chem. Revs. 17, 137 [1975]. R . A. V a u g h a n , Phys. State. Sol. b49, 247 [1972]. V. K . V o r o n k o v a , M. M. Z a r i p o v , V. A. K o g a n , and Y u . V. Y a b l o k o v , Phys. Stat. Sol. boo, 747 [1973]. B. J. H a t h a w a y and P. 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W e i , Inorg. Chem. 11, 2315 [1972]. W . graphic data additional structure-spectra correla tions were not attempted. Estimation of isotropic nuclear hyperfine coupling constants for the “blue” copper proteins 73 from available data 38 shows that they should lie in the range 20-40 X 10~4 cm-1. The ESR data in Table II shows that the butyl salicylaldimine chelates have isotropic coupling constants at the upper end of this range. Hence, the complexes discussed herein are not good models for the “blue” copper proteins - the hyperfine coupling constants are too large and the “d -d ” band intensities are too weak. By going to the corresponding sulfur substituted derivatives we expect to remedy both of these deficiencies and the next step for validation of the compounds as reasonable models for the “blue” copper proteins will be an investigation of the redox reactions. Such work is in progress and will be described subse quently. 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