Some Dicopper Complexes of Benzimidazole-Containing Ligands Ji R. TZOU,Yuan M. Lou, Sung M. Wang, and Norman C. Li JRT, NCL. Department of Chemistry, Duquesne University, Pittsburgh, Pennsylvania. -YML, SMW. Department of Chemistry, National Tsing Hua University, Hsinchu,
Taiwan
ABSTRACT Dicopper complexes of the following benzimidazole-containing ligands have been studied as possible models for the active site of hemocyanin: EDTB (N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-l,2ethanediamine), EGTB (1, 1 ,lO, lo-tetrakis-(2-benzimidazolylmethyl)-l , lo-diaza-4,7-dioxadecane), and The iniMEGTB (1, 1 ,lO, lo-tetrakis-( 1-methylbenzimidazol-2-ylmethyl)l , lO-diaza-4,7-dioxadecane). tial oxygenation product of Cu,(EDTB)(ClO,), in Me,SO gives optical absorption maxima at 315 nm (E = 3750 M-’ cm-‘) and 690 nm (E = 100 M- ’ cm-‘). The fluorescence emission intensities of Cu,(EDTB)(ClO,), at 400 and 700 nm (excitation at 350 nm) decreases rapidly on exposure to air. This suggests oxidation of &,(I) to Cu,(II). The x-ray absorption edge spectra suggest that both coppers in the oxygenation product, analyzed as Cu,(EDTB)(ClO,),(O) * 3H,O, are Cu(I1). From spectrophotometric titration of Cu,(MEGTB)Cl, with azide, formation constant of the Cu,(MEGTB)N,Cl, complex has been obtained. Data from cyclic voltamrnetry experiments suggest that in the presence of azide, Cu(II)(N,)Cu(II) species is present.
INTRODUCTION Several authors [1,2] have reported the study of Cu,(EDTB)(ClO,), and where EDTB = N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-1,2Cu,(EDTB)Cl,, ethanediamine, (C,H,N,CH,-),N-CH,-CH,-N(-CH,-C,H,N,),, as possible models for the active site of hemocyanin and oxidized hemocyanin. The purpose of the present research is to further study the dicopper(1) and dicopper(I1) complexes of EDTB and extend the studies to dicopper complexes of other benzimidazole-containing ligands: EGTB, (EGTB = 1,l ,lO, lo-tetrakis-(2-benzimidazolylmethyl)-l , lodiaza-4,7,-dioxadecane, (C,H,N,CH,),N-(CH,),O-(CH),O-CH,-CH,N(-CH,C,H,N*), 3 and MEGTB (1 , 1,lO, lo-tetrakis-( l-methylbenzimidazol-2-ylmethyl)-
Address reprint requests to: Dr. Sung M. Wang, University, Hsinchu, Taiwan, ROC 300.
Department
Journal of Inorganic Biochemistry, 43, 723-729 (1991) 0 1991 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas,
of Chemistry,
National
Tsing Hua 723
NY, NY 10010
0162-0134/91/$3.50
724
J. R. Tzou et al.
l,lO-diaza-4,7-dioxadecane) [3]. The dicopper(1) complexes of EDTB. EGTB. and MEGTB in Me,SO show dioxygen uptake with 0, /Cu$) = 0.9. Useful reports on copper-dioxygen chemistry 141 and a peroxy complex [S] have been published. Scheme 1 gives the structural formula of the ligands. Methemocyanin readily binds azide to form met azido hemocyanin. We have therefore studied the binding of azide to the copper complexes, and obtained the formation constant of an azide complex.
MATERIALS
AND METHODS
Preparations EDTB, CU,(EDTB)(CIO,)~ 20, and
and its oxygenated
product
CU,(EDTB)(CIO,)~(O)
. nH :O. an erhanol solution containing
46.88 (5.31);
N, 16.05 (16.84);
t i
t-1 EDTB
SCHEME 1.
0,
Ligand structures.
9.36 (9.62).
For
.
SOME DICOPPER
COMPLEXES
[Cu,(PDTB)(C10,),(OH,),I(C104)~(H,0),. By analogy, waters of crystallization or chloride ions are coordinated. Physical
OF LIGANDS
we believe
725
some of the
Measurements UV-Vis spectra were obtained on a Perkin-Elmer Lambda 5 spectrophotometer or a Shimadzu UV-Visible recording spectrophotometer, UV-265. Fluorescence spectra were obtained with a Perkin Model LS-5 luminescence spectrometer. The x-ray absorption edge spectra of dicopper complexes were obtained by G. 0. Tan, K. 0. Hodgson, and E. I. Solomon at Beam Line I-5 at the Stanford Synchrotron Radiation Laboratory, and interpreted in accordance with their previous paper 1181.
RESULTS
AND DISCUSSION
The initial oxygenation product of Cu,(EDTB)(ClO,), in Me,SO gives optical absorption maxima at 3 15 nm (E = 3750 M - ’ cm- ’ ) and 690 nm (E =: 100). Because
.,CJ .z
FIGURE 1. Absorbance of 2.0 x 1o-4 M Cu,(EDTB)(ClO,), in Me,SO vs time of exposure to air, 5 min/cycle, 250-400 nm.
726
J. R. Tzou et ai.
of the 38-fold higher extinction coefficient at 315 nm over that at 690 nrn. the adsorption at 3 IS nm has not been reported until now. Figure I gives absorption spectra of 2.0 x 10 ’ M Cu2(EDTB)(CI0,),, 250-400 run, showing an increase in absorbance at 315 nm with time of exposure to air. Figure 2 shows the cRects of adding ascorbic acid and subsequent bubbling with oxygen on absorbance of green solutions of 1) Cu2(MEGTB)CI,, 2) Cuz(EGTBjCI,. and 3) Cu,iEDTB)Cl, in Me,SO. Bubbling O2 through the solution results in a rise in absorbance at 690 nm. whereas addition of ascorbic acid lowers its absorbance. For Cu z IX1 ,$I 1 := MEGTB, EGTB, EDTB). addition of’ ascorbic acid also lowers the absorbance at 700 nm. However, there is only a slight increase in absorbance on exposure IO dioxbgen. These findings suggest that Cuz LCl, reacts with dioxygcn. whereas (‘a, L.Cl; doe5 riot.In deoxy hemocyanin, the coppers are Cu(I) and react with tlioxbgen. whereas in oxidized hemocyanin. the coppers are Cu(II) and do not react with 0,. We have obtained fluorescence emission spectra (excitation at 350 nmi’of 0.005 M CU~(EDTB)(CIO,)~ in Mc,SO, exposed to air at t = 0, 15. am! 45 min. The fluorescence emission intensities at both 400 nm and 700 nm decrease rapidly \vith time of exposure to air. This is definite evidence that Cu(lj has been oxidized to Cu(lI). which is a strong quencher of fluorescence. Cu,(EDTBjCI, gives no fluorescence emission spectra because both coppers are Cu(llj. The emission spectrurn of Cu2(EDTB)(C10,j?, at t = 0 resembles the spectrum of EDTB. The ligand itself. however, shows no decrease in fluorescence intensity at 400 and 700 nm on exposure to air. In our previous paper 161. we presented cyclic voltammograms of‘ 0.005 M Cu,(EDTB)Cl, + 0. I M [(C,H,),N]BF, in Me,SO, which showed two cathodic reduction peaks and two anodic oxidation peaks. The more positive of the cathodic peaks corresponds to reduction of Cu(II)Cu(II) to Cu(I)Cu(Il). In the present research, we added NaN, to Cu2(EDTB)Cl, and now fird that the more positive cathodic peak disappears. In the presence of azide, we have Cu(IIj(N,)C’u(IIj instead of Cu(II)Cu(lIj, and there is, therefore, no simple reduction of CutIljCu(IXj to cu(I)Cu(II). For solution of 0.00036 M Cu,(MEGTB)Cl, in MelSO in the presence of azide, a visible absorption maximum at 720 nm and a IJV maximum at 380 nm occur: for both wavelengths the peak height increases with increasing azide ctrncentration. Similar absorption spectra arc obtained for CuT(EDTB)CIJ and Cu,(EGTB)Cl, with addition of azide. The absorption peak at 380 nm is probably the azide-to-Cu(II) charge-transfer transition 191. while that at 720 nm is d-d. \:nn Kijn i7j has shown from crystal structure studies of Cu,(MEGTB)CI,(H,O), that only two of the chloride ions coordinate to Cuz( I Cl to 1 Cu) and that the other two chloride ions are within the crystal lattice together with the water molecules. l’hc compound rnq therefore be written [Cu2(MEGTB)Clz]Clz. The reaction with azidc may be \vritten [Cu2(MEGTB)Cl,]Cl, + “I, = [Cu,(MEGTB)N?CI !]Cl~ + Cl , and [Cu,(MEGTB)N,Cl,]. 1 _._-_
the following 1 -__-
_y -
equation
is written:
I
A-- A! A linear plot of l/(A - A,) vs l/IN, 1 was obtained. The ratio of intercept to slope was used to calculate a value of K’ = 2300 with an uncertainty in K’ of + 10%. By reacting Cu,LCI, (I., = MEGTB, EGTB. EDTB) with sodium aride in alcohol. we have isolated and analyzed Cu,(MEGTB)N,Cl, .4.5 HZO, Cu,(EGTB)N,Cl aI 3.5 H,O, and Cu2(EDTB)N,C13 .2H,O. The asymmetric azide stretching frequency in the IR spectra of all three dicopper(II) compounds appear at 3040 cm ’ _ similar to the asymmetric azide stretching frequency in the resonance Raman spectrum ol metazido hemocyanin [9]. Instead of azide. imidazolate may also bridge the two Cu(I1) ions, and we have isolated and analyzed Cu,(MEGTB)(Im ICI,. 5H ,O Cu,(EGTB)(Im )Cl, .4H,O, and Cu,(EDTB)(Im )C”l: * ?H,O. Figure 3 (curve a) shows a sharp x-ray absorption edge peak at 8983.5 eV (normalized absorption coefficient of 1.00) for Cu~(EDTB)(CiOJjI. Kau ct al. [8] have assigned this as the Is -+4px,y electric dipole-allowed transition m linear 2-coordinate Cu(1) complex. The linear 2-coordinate Cut?) m CU~(EDTB)(CIO~)~ has previously been demonstrated by crystal structure [ 7 j, Hith the N-01-N angle of 170.9”. Figure 3 (curve b) shows the edge spectrum of the oxygenated complex. Cuz(EDTB)(C1O,),(0) I 3H,O. while curve c is the edge spectrum ot” Cu(Il) complex, Cu,(EDTB)CI,. The edge structures (Fig. 3b) and (Fig. 3c j suggest that in the oxygenated complex, both coppers are Cu(Il). Figures !b and ?c show that the results are consistent with Cl being coordinated to Cu in C”u,(EDTB)CI,. because the edge in (Figure 3c) rises at - 1 eV lower in energy than that of the oxygenated complex in (Figure 3b). Covalency in Cu coordination such as occurs in bonding tc
EV
FIGURE 3. Normalized edge spectra: (a) 2-coordinate &(I) complex CuZ(tU~~B)(C104j2; (b) oxygenated complex CU~(EDTB)(CIO,)~(O). 3H,O: (CJ Cu(II) complex Cu,(EDTBjCI,. (Results of G. 0. Tan. K. 0. Hodgson. and E. I. Solomon)
SOME DICOPPER
COMPLEXES
OF LIGANDS
729
S gives rise to a shakedown transition that lowers the edge energy. Crystal structure data have shown that in Cu,(MEGTB)Cl,, each of the two coppers is bound to one chloride. Hence the x-ray edge data is consistent with an analogous crystal structure.
CONCLUSIONS Benzimidazole-containing ligands, such as EDTB, EGTB, and MEGTB give stable dicopper(1) and dicopper(I1) coordination compounds which resemble, in some respects, the active site in hemocyanin and oxidized hemocyanin. In this paper we report for the first time the 315 nm absorption maximum for Cu,(EDTB)(ClO,), when exposed to air, compared to the 345 nm band of oxyhemocyanin, which corresponds to a charge-transfer from oxygen to copper. We also report for the first time the fluorescence properties of a benzimidazole-containing ligand and its dicopper complexes. The fluorescence emission intensities of Cu,(EDTB)(CIO,), at 400 nm and 700 nm (excitation at 350 nm) decrease rapidly on exposure to air. This suggests that Cu,(I) has been oxidized to Cu *(II), similar to the decrease in fluorescence emission intensity when deoxyhemocyanin (Cu,(I)) is changed to oxyand MetHc (Cu,(II)). The conversion of Cu,(I) to Cu,(II) in the model compound on oxygenation is also supported from x-ray absorption edge experiments. The linear 2-coordinate Cu(1) in Cu,(EDTB)(ClO,), , established from x-ray absorption edge spectra, is in agreement with the x-ray crystal structure. We have carried out spectrophotometric titration of Cu,(MEGTB)Cl, with azide and obtained formation constants of the [Cu,(MEGTB)N,ClJ complex. Azide complexes, together with the imidazolate complexes, have also been isolated and analyzed. In cyclic voltammetry experiments, the more positive cathodic peak is ascribed to reduction of Cu(II)Cu(II) to Cu(I)Cu(II). With addition of azide, this peak disappears, suggesting that there is no longer a simple Cu(II), species. In the presence of azide, we have Cu(II)(N,)Cu(II) instead of Cu(II)Cu(II), similar to the Cu(II)(N,)Cu(II) species in azido methemocyanin. The authors are very much indebted to Grace 0. Tan, Professors K. 0. Hodgson and E. I. Solomon for the x-ray absorption edge data which were obtained at the Stanford Synchrotron Radiation Laboratory, supported by the Department of Energy, Division of Chemical Sciences, and the National Institutes of Health, Biomedical Resource Technology Program, Division of Research Resources. The authors also thank Professors J. Reedijk and Paul Stein for useful discussions and the National Science Council for partial support.
REFERENCES 1. H. M. J. Hendriks,
P. J. M. W. L. Birker, J. van Rijn, G. C. Verschoor,
and J. Reedijk,
J. Am. Chem. Sot. 104, 3607 (1982); P. J. M. W. L. Birker, H. M. J. Hendriks, J. Reedijk, and G. C. Verschoor, Znorg. Chem. 20, 2408 (1981). 2. S. M. Wang, J. R. Tzou, M. N. Chen, and N. C. Li, Znorg. Chim. Acta 125, 229 (1986). 3. J. R. Tzou, N. C. Li, Y. M. Lou, and S. M. Wang, J. Znorg. Biochem. 36, 334, Abstract LO78, (1989).
730
J. R. Tzou et ni.
4. ‘Z. Tyeklar and K. D. Karlin, Act. Chem. Res. 22, 241 (1989). 5. N. Kitajima, T. Koda. S. Hashimnto. T. Kitagawa. Y. More-oka. J. Chem. SOL..Chem. Commun., 151 (1988). 6. N. C. Li, J. R. Tzou, S. W. Chen, S. M. Wang, Y. C. Chou, Ii, 7‘. Ltang. and H. J. Lin, Inorg. Chim. Actu 138. 121 (1987); Elemental analyses of the oxygenated compound of Cu,(EDTB)(CIO,~), give the formula CU~(EDTB)(CIO,),~~~). ZH,O or Cu,(EDTB)(CIO,),(O) 3H,O. Since the oxygenated compound lacks a dio~ygen stretch. we chose the latter formulation. with an 0x0 group possibly bridging the iW0 coppers. We pian to look for a Cu-O-Cu stretching mode using JR and resonance Ranrnn techniyues. 7. J. van Ri.jn, PhD thesis. University of Lciden. 1985; P. 1. M. W. L. B~rher I-l. 41. J Hendriks, and J. Reedijk, Inorg. Chim. Actu 55. L17 (1981). 8. L.-S. Kau, D. J. Spira-Solomon, J. E. Penner-Hahn, K 0. Hodgson. and E. 1, Solomon. J. Am. Chem. SOL.. 109. 6433 (1987). 9. J. E. Pate. P. K. Ross. ‘I‘. J. Thamann. C. A. Reed, K. D. Karlin. T. N. Sorrell, and E. I. Solomon. .I, Am. C’izcm. Sot. 111, 5198 (1989). J. E. Pate. ‘T. J Thamann, and E. I. Solomon. Specwmchim. 4ctu 42A. 3 13 (19%).
Received July 25, 1990; uccepred
October 2.5, 1990
Taiwan
ABSTRACT Dicopper complexes of the following benzimidazole-containing ligands have been studied as possible models for the active site of hemocyanin: EDTB (N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-l,2ethanediamine), EGTB (1, 1 ,lO, lo-tetrakis-(2-benzimidazolylmethyl)-l , lo-diaza-4,7-dioxadecane), and The iniMEGTB (1, 1 ,lO, lo-tetrakis-( 1-methylbenzimidazol-2-ylmethyl)l , lO-diaza-4,7-dioxadecane). tial oxygenation product of Cu,(EDTB)(ClO,), in Me,SO gives optical absorption maxima at 315 nm (E = 3750 M-’ cm-‘) and 690 nm (E = 100 M- ’ cm-‘). The fluorescence emission intensities of Cu,(EDTB)(ClO,), at 400 and 700 nm (excitation at 350 nm) decreases rapidly on exposure to air. This suggests oxidation of &,(I) to Cu,(II). The x-ray absorption edge spectra suggest that both coppers in the oxygenation product, analyzed as Cu,(EDTB)(ClO,),(O) * 3H,O, are Cu(I1). From spectrophotometric titration of Cu,(MEGTB)Cl, with azide, formation constant of the Cu,(MEGTB)N,Cl, complex has been obtained. Data from cyclic voltamrnetry experiments suggest that in the presence of azide, Cu(II)(N,)Cu(II) species is present.
INTRODUCTION Several authors [1,2] have reported the study of Cu,(EDTB)(ClO,), and where EDTB = N,N,N’,N’-tetrakis-(2-benzimidazolylmethyl)-1,2Cu,(EDTB)Cl,, ethanediamine, (C,H,N,CH,-),N-CH,-CH,-N(-CH,-C,H,N,),, as possible models for the active site of hemocyanin and oxidized hemocyanin. The purpose of the present research is to further study the dicopper(1) and dicopper(I1) complexes of EDTB and extend the studies to dicopper complexes of other benzimidazole-containing ligands: EGTB, (EGTB = 1,l ,lO, lo-tetrakis-(2-benzimidazolylmethyl)-l , lodiaza-4,7,-dioxadecane, (C,H,N,CH,),N-(CH,),O-(CH),O-CH,-CH,N(-CH,C,H,N*), 3 and MEGTB (1 , 1,lO, lo-tetrakis-( l-methylbenzimidazol-2-ylmethyl)-
Address reprint requests to: Dr. Sung M. Wang, University, Hsinchu, Taiwan, ROC 300.
Department
Journal of Inorganic Biochemistry, 43, 723-729 (1991) 0 1991 Elsevier Science Publishing Co., Inc., 655 Avenue of the Americas,
of Chemistry,
National
Tsing Hua 723
NY, NY 10010
0162-0134/91/$3.50
724
J. R. Tzou et al.
l,lO-diaza-4,7-dioxadecane) [3]. The dicopper(1) complexes of EDTB. EGTB. and MEGTB in Me,SO show dioxygen uptake with 0, /Cu$) = 0.9. Useful reports on copper-dioxygen chemistry 141 and a peroxy complex [S] have been published. Scheme 1 gives the structural formula of the ligands. Methemocyanin readily binds azide to form met azido hemocyanin. We have therefore studied the binding of azide to the copper complexes, and obtained the formation constant of an azide complex.
MATERIALS
AND METHODS
Preparations EDTB, CU,(EDTB)(CIO,)~ 20, and
and its oxygenated
product
CU,(EDTB)(CIO,)~(O)
. nH :O. an erhanol solution containing
46.88 (5.31);
N, 16.05 (16.84);
t i
t-1 EDTB
SCHEME 1.
0,
Ligand structures.
9.36 (9.62).
For
.
SOME DICOPPER
COMPLEXES
[Cu,(PDTB)(C10,),(OH,),I(C104)~(H,0),. By analogy, waters of crystallization or chloride ions are coordinated. Physical
OF LIGANDS
we believe
725
some of the
Measurements UV-Vis spectra were obtained on a Perkin-Elmer Lambda 5 spectrophotometer or a Shimadzu UV-Visible recording spectrophotometer, UV-265. Fluorescence spectra were obtained with a Perkin Model LS-5 luminescence spectrometer. The x-ray absorption edge spectra of dicopper complexes were obtained by G. 0. Tan, K. 0. Hodgson, and E. I. Solomon at Beam Line I-5 at the Stanford Synchrotron Radiation Laboratory, and interpreted in accordance with their previous paper 1181.
RESULTS
AND DISCUSSION
The initial oxygenation product of Cu,(EDTB)(ClO,), in Me,SO gives optical absorption maxima at 3 15 nm (E = 3750 M - ’ cm- ’ ) and 690 nm (E =: 100). Because
.,CJ .z
FIGURE 1. Absorbance of 2.0 x 1o-4 M Cu,(EDTB)(ClO,), in Me,SO vs time of exposure to air, 5 min/cycle, 250-400 nm.
726
J. R. Tzou et ai.
of the 38-fold higher extinction coefficient at 315 nm over that at 690 nrn. the adsorption at 3 IS nm has not been reported until now. Figure I gives absorption spectra of 2.0 x 10 ’ M Cu2(EDTB)(CI0,),, 250-400 run, showing an increase in absorbance at 315 nm with time of exposure to air. Figure 2 shows the cRects of adding ascorbic acid and subsequent bubbling with oxygen on absorbance of green solutions of 1) Cu2(MEGTB)CI,, 2) Cuz(EGTBjCI,. and 3) Cu,iEDTB)Cl, in Me,SO. Bubbling O2 through the solution results in a rise in absorbance at 690 nm. whereas addition of ascorbic acid lowers its absorbance. For Cu z IX1 ,$I 1 := MEGTB, EGTB, EDTB). addition of’ ascorbic acid also lowers the absorbance at 700 nm. However, there is only a slight increase in absorbance on exposure IO dioxbgen. These findings suggest that Cuz LCl, reacts with dioxygcn. whereas (‘a, L.Cl; doe5 riot.In deoxy hemocyanin, the coppers are Cu(I) and react with tlioxbgen. whereas in oxidized hemocyanin. the coppers are Cu(II) and do not react with 0,. We have obtained fluorescence emission spectra (excitation at 350 nmi’of 0.005 M CU~(EDTB)(CIO,)~ in Mc,SO, exposed to air at t = 0, 15. am! 45 min. The fluorescence emission intensities at both 400 nm and 700 nm decrease rapidly \vith time of exposure to air. This is definite evidence that Cu(lj has been oxidized to Cu(lI). which is a strong quencher of fluorescence. Cu,(EDTBjCI, gives no fluorescence emission spectra because both coppers are Cu(llj. The emission spectrurn of Cu2(EDTB)(C10,j?, at t = 0 resembles the spectrum of EDTB. The ligand itself. however, shows no decrease in fluorescence intensity at 400 and 700 nm on exposure to air. In our previous paper 161. we presented cyclic voltammograms of‘ 0.005 M Cu,(EDTB)Cl, + 0. I M [(C,H,),N]BF, in Me,SO, which showed two cathodic reduction peaks and two anodic oxidation peaks. The more positive of the cathodic peaks corresponds to reduction of Cu(II)Cu(II) to Cu(I)Cu(Il). In the present research, we added NaN, to Cu2(EDTB)Cl, and now fird that the more positive cathodic peak disappears. In the presence of azide, we have Cu(IIj(N,)C’u(IIj instead of Cu(II)Cu(lIj, and there is, therefore, no simple reduction of CutIljCu(IXj to cu(I)Cu(II). For solution of 0.00036 M Cu,(MEGTB)Cl, in MelSO in the presence of azide, a visible absorption maximum at 720 nm and a IJV maximum at 380 nm occur: for both wavelengths the peak height increases with increasing azide ctrncentration. Similar absorption spectra arc obtained for CuT(EDTB)CIJ and Cu,(EGTB)Cl, with addition of azide. The absorption peak at 380 nm is probably the azide-to-Cu(II) charge-transfer transition 191. while that at 720 nm is d-d. \:nn Kijn i7j has shown from crystal structure studies of Cu,(MEGTB)CI,(H,O), that only two of the chloride ions coordinate to Cuz( I Cl to 1 Cu) and that the other two chloride ions are within the crystal lattice together with the water molecules. l’hc compound rnq therefore be written [Cu2(MEGTB)Clz]Clz. The reaction with azidc may be \vritten [Cu2(MEGTB)Cl,]Cl, + “I, = [Cu,(MEGTB)N?CI !]Cl~ + Cl , and [Cu,(MEGTB)N,Cl,]. 1 _._-_
the following 1 -__-
_y -
equation
is written:
I
A-- A! A linear plot of l/(A - A,) vs l/IN, 1 was obtained. The ratio of intercept to slope was used to calculate a value of K’ = 2300 with an uncertainty in K’ of + 10%. By reacting Cu,LCI, (I., = MEGTB, EGTB. EDTB) with sodium aride in alcohol. we have isolated and analyzed Cu,(MEGTB)N,Cl, .4.5 HZO, Cu,(EGTB)N,Cl aI 3.5 H,O, and Cu2(EDTB)N,C13 .2H,O. The asymmetric azide stretching frequency in the IR spectra of all three dicopper(II) compounds appear at 3040 cm ’ _ similar to the asymmetric azide stretching frequency in the resonance Raman spectrum ol metazido hemocyanin [9]. Instead of azide. imidazolate may also bridge the two Cu(I1) ions, and we have isolated and analyzed Cu,(MEGTB)(Im ICI,. 5H ,O Cu,(EGTB)(Im )Cl, .4H,O, and Cu,(EDTB)(Im )C”l: * ?H,O. Figure 3 (curve a) shows a sharp x-ray absorption edge peak at 8983.5 eV (normalized absorption coefficient of 1.00) for Cu~(EDTB)(CiOJjI. Kau ct al. [8] have assigned this as the Is -+4px,y electric dipole-allowed transition m linear 2-coordinate Cu(1) complex. The linear 2-coordinate Cut?) m CU~(EDTB)(CIO~)~ has previously been demonstrated by crystal structure [ 7 j, Hith the N-01-N angle of 170.9”. Figure 3 (curve b) shows the edge spectrum of the oxygenated complex. Cuz(EDTB)(C1O,),(0) I 3H,O. while curve c is the edge spectrum ot” Cu(Il) complex, Cu,(EDTB)CI,. The edge structures (Fig. 3b) and (Fig. 3c j suggest that in the oxygenated complex, both coppers are Cu(Il). Figures !b and ?c show that the results are consistent with Cl being coordinated to Cu in C”u,(EDTB)CI,. because the edge in (Figure 3c) rises at - 1 eV lower in energy than that of the oxygenated complex in (Figure 3b). Covalency in Cu coordination such as occurs in bonding tc
EV
FIGURE 3. Normalized edge spectra: (a) 2-coordinate &(I) complex CuZ(tU~~B)(C104j2; (b) oxygenated complex CU~(EDTB)(CIO,)~(O). 3H,O: (CJ Cu(II) complex Cu,(EDTBjCI,. (Results of G. 0. Tan. K. 0. Hodgson. and E. I. Solomon)
SOME DICOPPER
COMPLEXES
OF LIGANDS
729
S gives rise to a shakedown transition that lowers the edge energy. Crystal structure data have shown that in Cu,(MEGTB)Cl,, each of the two coppers is bound to one chloride. Hence the x-ray edge data is consistent with an analogous crystal structure.
CONCLUSIONS Benzimidazole-containing ligands, such as EDTB, EGTB, and MEGTB give stable dicopper(1) and dicopper(I1) coordination compounds which resemble, in some respects, the active site in hemocyanin and oxidized hemocyanin. In this paper we report for the first time the 315 nm absorption maximum for Cu,(EDTB)(ClO,), when exposed to air, compared to the 345 nm band of oxyhemocyanin, which corresponds to a charge-transfer from oxygen to copper. We also report for the first time the fluorescence properties of a benzimidazole-containing ligand and its dicopper complexes. The fluorescence emission intensities of Cu,(EDTB)(CIO,), at 400 nm and 700 nm (excitation at 350 nm) decrease rapidly on exposure to air. This suggests that Cu,(I) has been oxidized to Cu *(II), similar to the decrease in fluorescence emission intensity when deoxyhemocyanin (Cu,(I)) is changed to oxyand MetHc (Cu,(II)). The conversion of Cu,(I) to Cu,(II) in the model compound on oxygenation is also supported from x-ray absorption edge experiments. The linear 2-coordinate Cu(1) in Cu,(EDTB)(ClO,), , established from x-ray absorption edge spectra, is in agreement with the x-ray crystal structure. We have carried out spectrophotometric titration of Cu,(MEGTB)Cl, with azide and obtained formation constants of the [Cu,(MEGTB)N,ClJ complex. Azide complexes, together with the imidazolate complexes, have also been isolated and analyzed. In cyclic voltammetry experiments, the more positive cathodic peak is ascribed to reduction of Cu(II)Cu(II) to Cu(I)Cu(II). With addition of azide, this peak disappears, suggesting that there is no longer a simple Cu(II), species. In the presence of azide, we have Cu(II)(N,)Cu(II) instead of Cu(II)Cu(II), similar to the Cu(II)(N,)Cu(II) species in azido methemocyanin. The authors are very much indebted to Grace 0. Tan, Professors K. 0. Hodgson and E. I. Solomon for the x-ray absorption edge data which were obtained at the Stanford Synchrotron Radiation Laboratory, supported by the Department of Energy, Division of Chemical Sciences, and the National Institutes of Health, Biomedical Resource Technology Program, Division of Research Resources. The authors also thank Professors J. Reedijk and Paul Stein for useful discussions and the National Science Council for partial support.
REFERENCES 1. H. M. J. Hendriks,
P. J. M. W. L. Birker, J. van Rijn, G. C. Verschoor,
and J. Reedijk,
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Received July 25, 1990; uccepred
October 2.5, 1990
