Structural and electronic effects on the polarographic half-wave potentials of copper (II) chelate complexes

BIOINORGANIC

CHEMISTRY

4.257-275

(1975)

257

Structural and Electronic Effects on the Polarographic Half-Wave Potentials of Copper (II) Chelate Complexes G. S. PATTERSON

Department Cambridge,

and R. H. HOLM

of Chemistry, Massachusetts Massachusetts 02139

Institute

of Technology.

ABSTRACT Polarosaphic

measurements

in

dimethylformamide

solution

have

been

performed on 37 bis-chelate Cu(II) complexes containing primarily salicylaidimine, p-ketoamine, p-iminoamine, and pyrrole-Zaldimine ligand systems. The complexes were selected in order to reveal the effect of stereochemicxl and donor atom variations on half-wave potentials. hiono- and binuclear

complexes undergo one- and two-electron reductions, respectively, which were established for representative cases by controlled potential coulometry. The response of Cu(II)/Cu(I) potentials to these variations reveals that nonplanar bis-chelate complexes are easier to reduce than their planar analogs, rigid phnar tetradentate or related planar bis-chelate complexes, and comp:eses differing only in donor atoms are more readily reduced in the order N, < NzOt < N2S2 _ Binuclear complexes are reduced in one twoelectron or two resolvable one-electron steps depending on ligand structure_ Potentials of these and other complexes previously examined are considered in relation to the markedly positive potentials of “blue” copper proteins and &and structural features which might afford Cu(II)/Cu(I) potentials in the protein range are noted. INTRODUCTION

Among the distinctive features of “blue” copper sites in proteins and enzymes [l-3] are markedly positive Cu(II)/Cu(I) redox potentials (ca. 1-02 to to.8 V [1,2,4-S] at pH-7 vs the standard hydrogen electrode), indicating a more effective stabilization of Cu(1) vs Cu(II) than is usually the case for complexes of synthetic origin (vide infra). in the absence of high resolution X-ray diffraction information, the composition and geometry of these sites are unknown. However, the presence of a cysteinyl residue appears necessary for the occurrence of “blue” copper [P-l 21 with some or all of the remaining coordination positions possibly occupied by nitrogen ligands [ 131 in coordination units of tetrogonal or lower [14,15] ligand field symmetry. Possible geometries such as five-coordinate or pseudotetrahedral are difficult to dis0 American Elsevier Publishing Company,

Inc.. 197.5

258

G. S. PATTERSON

AND

R. H. HOLM

tinguish because of their low symmetry and the viability of the latter has been Evidence for a nonplanar, possibly considered in some detail [l-3,161. four-coordinate site with one sulfur ligand for “blue” copper is indirect at best. This or any other description of the coordination unit and its protein environment must ultimately be reconcilable with the relatively positive redox potentials, which reach a maximum with Polyporus lactase (tie = t-O.77 V’ ; compare Ee = +0.17 V for aqueous Cu*+’ 3. The origin of the high protein potentials is a well recognized problem [ 1,2,17] _ Previous qualitative considerations [ 17,181 together with potentials for complexes in aqueous solution [ 17,19,3_0] reveal that reduction potentials can be shifted to more positive values by the use of ligands which sterically or electronically destabilize tetragonal Cu(II) and/or enhance the stabilization of (presumably tetrahedral) Cu(I). In devising models of the “blue” copper sites or, possibly, in interpreting protein potentials, it could be advantageous to have knolwedge of the sensitivity of Cu(II)/Cu(i) potentials to controlled structural and ligand electronic variations, even if the complexes are derived from nonphysiological ligands and their potentials do not occur in the protein range. Existing redox data on copper complexes, while extensive, are not well-suited to disclosure of such effects due to differences in methods of potential measurements, reference electrodes, solvent, pH, &and systems, and, in nearly all cases, the lack of definite structural information for the oxidized and reduced forms. In this investigation, a systematic study of polarographically determined Cu(II)/Cu(I) potentials has been undertaken utilizing series of complexes seiected to reveal the effects of stereochemistry of one or both oxidation states and, to a lesser extent, ligand electronic properties on the values of these potentials. The complexes examined are represented in Fig. 1 and are listed in Table 1 together with their abbreviated names_ They are not intended as models for ligation of “blue” copper inasmuch as the Iigands employed are obviously of a nonphysiological nature. Except for 35-37 the ligands are of the essential Schiff base type, the principal advantage of which is the ability to vary stereochemistry by alteration of N-substituents. As will become evident, the potentials for these complexes do not fall in the protein range. Rather, it is changes in potentials relative to specified reference compIexes which provide the more useful information_

EXPERIMENTAL Preparation

SECTION

of compounds

The following complexes (tf- Table 1) were synthesized by methods: I, 2 1221; 3. 4 1231; 5. 6 [24]; 7. 8 [25];14 [26];15. 17-22 [28];23 [29];24. 25 [30];28,29 [31];30 [32]f31 [26];32 [34];34 [26];35 [35];36.37[36].

published 16 [27]; [33];33

COPPER

CHELATE

COMPLEXES

259

Ph

k

13

(L-s)

”

(L:VH)

R

,o \ N
2

-0.

CH3

B

--24 -26

E”s

l7.3

CR

=CH31

19.20

CR =

2”s

CR

23

Ph)

= CF3)

G \N

\

cu/2

-N’

CH,

A

R -30 Me

Me

0

Me 0

8

NE

O”IH

-35

8:

(CH,),

(en)

(CH,),

(tn)

G-G 00

Y

Y

Y=

H

Y = CH,

(bo)

8-B o o

cH;c’z+

\C”/2

(bnl

00

(bmp)

FIG- 1. Numbers and structural formulas of Cu(II) complexes.

3

;; 29 30 31

;: 24 2s 26

12 13 14 15 16 17 18 19 20 21

11

10

43 i H’ 9

:

NO.

Cu(W, IIMc), tn Cu(S~McllMc), CI~ Cu(PhHH), cn Cu(PhHII), tn Cu(l’l~IllI), bp Cu(S.l%l~II), Cl1 Cu(Mc,-McHMc), Cu(l%,-MeHMc), Cu(McllMc), (cn), Cu(llqxl),

Cu(McJ’hIlMc), Cu(ilWWlMe), Cu(McHMc),cn Cu(McHMc), tn Cu(PbHMe), en Cu(l’hHMc), tn Cu(CI:, NMc), en

Cu(odJ),co

cll(ssill), Cl1

Cu,(S~lllu-sal),(rtr.pllcn),

Cu,(s~ll),(bp)

Cu(5-/l3u-sal),bll

Cu(5431l-sal), bmp

Cll(snl),bp Cll(Sill),imp

CU(Sill),Cl1 CU(S;ll),-0~pllCll

Cu(Mcal), Cu(Et*sal), Cu(iPml), Cu(IBu-WI),

Nmnc

colllpoullci

TABLE 1 Elcctrochernic~l, v”

-0.90 -0,RG -0,74 -0,6G -I,21 -I,1 I -0,75 -0.78 -o,u3 -0,78 -0,7 lr -0,83’ -0,7tP -0.83 -I,54 -0096 -0,78 -1.50 -1.22 -I,41 -1,14 -1.08 -0.74 -1.07 -1.26 -1.08 -0.82 -0.87 -1.52 -0.88 -2.26 -0.69

El/2

Structural,

:; 58 53 69 65 57

-49 66 58 57 54 56 55 51 55 54 58 58

55; 5G

69 62 65 56 48 5R 56

IlW

Slob'c

1.02

I .05 0.92

1.05

0

(I$ D up -01 1’ I’ P P I’ 1’ P P uP I’ np w I’ 0”

41k

flP nP nP

371

nnglc(dcg)

Uhcdrnl

14

-17,500 (50) -17,200 (109) -15,900 (140) 13,200 (243)t 17,900 (213) -17,900 (485) -I 1,800 (41) -I 1,800 (59) -I 1,400 (60) -I 1,400 (51)

DMl:

Complexes

-11,400 (104) 12,800 (120) 131boo (107) 14,600 (170) 14,800 (203) 16,200 (81) 16,000 (92) -13,900 (108) 13,400 (163) 18,400 (177)’ 18,300 (2O7)x 16,800 (I 15) 16,600 (119) 18,200 (I 86)” 18,300 (223)“’ 16,500 (131) 16,300 (95) 18,000 (ISI) 17,600 (I 32) 16,500 (94) 16,300 (76) 13,800 (36) 13,900 (41) 18,300 (321) 18,100 (326) -14,700 (52) -14,700 (59) 12,000 (47) I I,800 (46) -14,300 (107) -14,000 (88) 7,500 (6G) 7,500 (63) 5,900 (4 1y 6,100 (41) 16,200 (118) 16,300 (163) -19,200 (119) -19,200 (103)

-17,200 (154) -16,900 (127) -15,900 (138) 13,200 (236) l7,AOO(231) -17,500 (350) -12,300 (38) -12,000 (57) -12,100 (59) -I 1,900 (49) -12,000 (96)

crd,

I’ cm-(CM/J)

Dutu for ITour-Coordinete Copp~$Jl)

2.16 1.25’1 1.00 1.09

0.94 1.08 1.03 1.07

na

und Spectnl

Cu(Mc-pa), Cu(tBI1.pi]), Cu(pa),en Cu(Mc,-dipym), [Cu(2,9-Mc,phcn),1(GIG,I1 [Cu(2,9-Me,phcn),] (CIO,)

-0.G1 -0.46 -0.74 -0.70 to,64 +0,64

61 12 57 1.02 0.93 0.98 1 1

ho

-16,700 (130) -16,900 (129) 15,800(279) 15,400(234) 18,600(305y 18,100(256) 10,900(278) 11,000(268)

(6% 2: :: rrp 60 37 I1P %MF solution. bApparentv;ducs, uncorrcctcd for underlyingihsurption. -form: E. C. Linpfcllcr, G. L, Simmons,B, Morosin,C. Shcringcr,i1ndC. Prciburg,Aclu Cr~~s~ullo~r. 14, 1222(1961). fpI ncrage value of a-fornl: G. R. Clark,D. Hall, and ‘I’.N. Waters,J, Clre~e.Sot A, 2808 (1969). ‘Fform: C. P;m:ittotli,G. Bombicri,snd R. Grnzinrli,Acta C’rys~ullogr. 23, 537 (1967); E. N. Ihkcr, G. R, Clark,D. Hull,and T. N. Waters,./. ht. Sot. A, 251 (1967), f 1’.L. Orioli and L, Sacconi,J, Arncr.Chn. Sue. 88, 271 (1966). ‘T, P. Chccscm;1rl, D. Hall,and T. N. Wutcrs,J. C/ICI~I. S’OC. A, 685 (19GG). “1:l con~plcxwith trinitrobcnzcnc:E. E. Castclho, 0. J. R. Iloddcr, C. K. Prout, and P. J. Qdler,J. Clroa Sot, A, 2620 (1971). !p.nitrophcnol,CJICI,adducls: E. N. Daker,D. Hall,and T. N. Walers,J, Chon, Sot. A, 400,406 (1970). IT. I’,Clic~~~rnan,D. Ilnll,nnd T. N. W;itcrs,J.C/w?. Sot. A, 1396(19h6). ‘C. A. Bear,J. M. Waters, ond T. N. W;1tcrs,J.Chr~. SOC. A, 2494(1970). ‘G. R. Chk, D. Ilall, and T, N. Wafers,J,C’hetn.Sot. A, 223 (19GR);E. N. Baker,D. Ilull, and T. N. Watcrs,J.Clrerrt.SOC.A, 396 (1970). “‘Cu(oab),pn: D. Hull,‘I’.N. Waters,imdP. E. Wright,J. C. S, fh/l~ot~, 1508 (1913), “R. Tcwnri:md R. C. Srivashvn,AC&ICr~sfalloh~,U27, 1644 (I 911), oC.VH. Wci,Jrrorg Cltcn~.11, 2315 (1972). his (3 3’ 5 5’-tctran~ctf~yI_rl,4’-dic;~rboctlroxydipyrrornctllcnido) Cu(Il): M. Elder and Il. R. Pcnfold,J. Cllcn~. Sot. A, 2556 (1969). “Iliglihb’i1ttributed to rcduciblcimpurity at ci1.-1.1 v. ‘One-electronreductionsobscrvcdby PCpoJ;~rography. ‘ScPariitcreduction SICPS not clearly rcsolvcdby i1cpol;\rogri\plly. ‘Weilk shoulder iIt -8,700 CIII-’ (C 27). “Low cncrgy hid not dctcctcd. “Shoulderat _ 16,400(e 109). “Shoulder nt -16,700 (c 215). “Sbouldcrat -16,000 (c 63) (CHCI,), 16,400 (c 90) (IIMF). YSllouldcriIt 14,500(C 93). ‘CCI, solution.

;; 34

262 Cu(S-rBu-sa&

G. S. PATTERSON

AND R. H. HOLM

bmp(9)

The Schiff base was prepared in the same manner as for related compounds [37] and the complex was obtained by the method reported for 8 (25 1. It was recrystallized from chloroform-heptane and dried in VQCUO at 140° for 2 days; mp 283-28?. Anal. talc for C36H3aN202C~: C, 72.76; H, 6.45; N, 4.71. Found: C, 72.00; H, 6.43; N, 4.45.

Cu(S-tBu-sa&

bn (IO)

The complex was prepared from stoichiometric quantities of the Schiff base [37] in chloroform and CU(OAC)~ in hot ethanol and was isolated after partial remova of solvent and addition of n-heptane. The product was recrystallized from chIoroformethano1; rnp”277O (dec). Anal- Calc for C42H38N202C~: C, 75.71; H, 5.75; N, 4.20_ Found: C, 75.73; H, 5.96; N, 4.18.

2,2’,6,6’-Tetranitrobiphenyl was prepared as described [381 and converted to the tetraamine 1381 by the procedure used for 2,2’-diaminobiphenyl [39] _ The I&and was obtained by adding the tetraaminobiphenyl (0.86 g, 0.004 mol) in hot ethanol to saiicylaidehyde (I .95 g, 0.016 mol) in ethanol and refluxing the resukng solution untit a yellow crystalline precipitate appeared. The mixture was cooled to room temperature and the product was collected. The &and was recrystallized from chloroiorm-ethanol and dried at 140” in vacua for one day; mp 23I-234O. Anal. talc for C40H30N404: C, 76-18; H, 4.79; N, 8.88_ Found: C, 76.67; H, 4.58; N, 8.96. The complex was prepared by the same technique which WZKused for 7 [25J, recrystallized from chloroform-ethanol, and dried at 140° in sucuo for two days; mp > 360°_ Anal. talc for CqOHz6NO04Cu~: C, 63.74; H, 3.48; N, 7.43; Cu, 16.86. Found: C, 63.75; H, 3.42; N, 7.52; Cu, 16.81.

Cu2(5-rBu-sa&(m-phen)z

(12)

The Iigand was formed in siru by combining 5-t-butyl-sticylaidkhyde (1.78 g, 0.01 mol) and m-phenyIenediamine (0.54 g, 0.005 mol) in chloroform and then adding an equal volume of ethanol. After the mixture was refluxed for several IZ+, a hot ethanolic solution of C~(OAC)~ (0.005 mol) was added. The red-brown complex formed immediately and crystallized after partial removal of solvent and cooling to room temperature_ Recrystallization from chloroform-ethanol afforded the product as the chloroform monosolvate. Aml. talc

COPPER(H) CHELATE COMPLEXES for Cs7H61ClaN404C~~: N, 5.01.

C, 62_26;H,

5_59;N,

5.10. Found: C, 62.19;H,

263 6.05;

Cu(Ssal)s en (13)

This compound was prepared by the method briefly reported for Cu(p-MeOCs.Hq-S.sal)a [40]. Operations up to and including complex formation were conducted under a nitrogen atmosphere. A solution of o-thiocyanatobenzaldehyde (1.63 g, 0.0 1 mol) and sodium sulfide nonahydrate (2.50 g, 0.01 mol) in 25 ml of n-butanol was warmed to -50” for 10 min. Anhydrous sodium sulfate was added to remove the water and the solution was filtered_ Ethylenediamine (0.3 g, 0.005 mol) in 10 ml ethanol was added to the filtrate, and the resulting rrr.&ure was warmed for 5 min. Complex formation occurred when copper acetate (I .O g, 0.005 mol) was introduced to the resulting Schiff base. After a short time the complex and copper sulfide precipitated from solution, the precipitate was collected and the complex was separated by extraction with acetone followed by filtration. As the acetone was removed from the filtrate, the complex appeared as purple crystals; mp 231-233O. Anal. talc for C16Hr4N2S2Cu: C, 53-09; H, 3.90; N, 7.74. Found: C, 52.96; H, 3.87; N, 7.63.

Cu(PhHH)* bp (26) The sodium salt of I-phenyl-propane-l,3-dione (6.8 g, 0.04 mol) and 2,2’diaminobiphenyl (3.7 g, 0.02 mol) were dissolved in 150 ml of methanol and ca. 2 g of 6N HzS04 in a small volume of methanol was added_ The reaction mixture was stirred for 2 h, filtered, and the collected solid extracted with chloroform. The extract was combined with the filtrate, the solvent removed, and the residue extracted with hot benzene. The Schiff base crystallized upon cooling the benzene solution and was recrystallized from carbon tetrachloride; mp 2ll-213O. The complex was prepared by the reaction of stoichiometric amounts of the base in chloroform and CU(OAC)~ in ethanol. It was recrystallized from acetonitrile and dried in vacua at SO” for 24 h; mp 233-235 _ Anal. talc for CsoHaa N202Cu: C, 71.20; H, 4.38; N, 5.54. Found: C, 70.68; H, 4.38; N, 5.10.

Cu(S-PhHH)z en (27) The &and [41] (0.7 g, 0.002 mol) was partially dissolved in 50 ml of hot chloroform, to which.was added a solution of Cu(OAc)z Hz0 (0.4 g, 0.002 mol) in ethanol. The complex precipitated as golden-brown crystals, which were recrystallized from DMF-ethanol and dried in vacua at 80° for 24 h; mp

264

G. S. PATTERSON

AND

R. H. HOLM

232-235” (dec). Anal. talc for CaeHraNaSaCu: Found: C, 57.81; H, 4.39; N, 6.52.

C, 58.02;

H, 4.38;

N, 6.77.

Physical measurements A Princeton Model 170 Electrochemistry System was employed for polarographic, cyclic voltammetric, and controlled potential coulometric experiments_ Poiarographic measurements were performed at 25.0 t O-IO; a dropping mercury electrode was empioyed except for complexes 36 and 37, which were measured at a rotating platinum electrode. Solutions were -1 mM in complex and 0.1 M in tetra-n-propylammonium perchlorate as supporting electrolyte. Slopes and half-wave potentials were evaluated from plots of log[i/(id_i)] vs E; E,lz values are referenced to a saturated calomel electrode. Coulometric determinations were checked against the stable reversible one-electron couple Ru(acac), W [421 in DMF solution_ DMF was purified by distillation from sodium anthracenide, a procedure which increases its cathodic range compared to the usual distillation from anhydrous CuSO4 _ Electronic spectra were recorded on a Cary Model 14 spectrophotometer. RESULTS

AND

DISCUSSION

Types and structures of complexes The complexes selected for redox examination are neutral four-coordinate cheIate species which contain Cu(II) in their isolated forms. The only exceptions are the cationic species 36 and 37 [361, with the latter being a stable CuU) complex_ The complexes may be organized into categories defined by their &and structures (Fig 1 and TabIe 1): bis-chelate and tetradentate salicylaldiminates (I-22) [including several binuclear species (II. 12)], P-ketoaminates (15-Z. B-26), &iminoaminates (Z&30), and pyrrole-2-aldiminates (31-34); tetradentate thiosalicylaldiminat (IS), o-aminobenzaldiminate (Z4), and fl-thioaminates (23, 27); bis(dipyrrometbenide) (35); and bis(2,9-dimethylo-phenanthrolines) (36, 37). NaOa donor atom sets occur in ali complexes except 13,23. and 27 (N,S,) and 14. 28-37(Nq). Nearly all complexes in each category whose structures are known from X-ray diffraction results have been included in the investigation. The structural feature of principal importance is the departure from planar stereochemistry inherently preferred by Cu(I1). A measure of this feature is the dihedral angle between coordination planes or chelate rings [21]_ Reported values of either angle are set out in Table 1. Differences between these angles are generally small and are of no concern here. In cases where no direct structural information is available, the probabie structures of the Cu(II) complexes are entered in Table 1 as “p” (planar or nearly so) and “np” (nonplanar). These designations follow from known structures of related complexes, ligand stereochemical constraints, cr electronic spectral and magnetic information_

COPPER(U)

CKELATE

COMPLEXES

265

Much of this information on which the structural designations in Table 1 are based has been reviewed elsewhere [ 21,431 and will not be included here. However, several points are noted. From the available evidence it is hot clear to what extent the dihedral angles of the b&chelates 2-4 are retained in solution 1211. However, ligand field spectra have been interpreted to indicate that 3 and 4 1231, as well as 28, 29 [31] , and 33 [ 341, are nonplanar in solutions of noncoordinating solvents. Likewise, spectral data strongly suggest that 16 is nonplanar whereas 1.5, with the smaller N-substituent, is planar in such solvents 1271. The steric features of the hindered bridging groups 2,?‘-biphenylyl (bp), (bn), when incor6,6’-dimethyl-2,2*-biphenylyl (bmp), and 2,2’-binaphthylyl porated in tetradentate complexes with planar 6-membered chelate rings, are such as to impose a degree of nonplanar coordination [21,25,37], especially in solution [37J _ Pseudotetrahedral structures have been found for Cu(s&bp (Table 1) and Co(sal)zbmp (441 (dihedral angle 68O). Ligand field spectral similarities amongst 7-12 and 26 support a nonplanar structure for each in solution_ Remaining tetradentate Cu(11) complexes with dimethylene (en) or trimethylene (tn) bridges, which lack steric features promoting nonplanar coordination, are known or expected to be essentially planar.’

Electrochemical

behavior

Conventional dc polarography was applied to some 35 neutral chelate complexes in DMF solution. Electronic spectral comparisons in cNoroform and DMF solutions gave no indication of significant structural changes in the noncoordinating and potentially weakly coordinating solvents &though some intensity differences, generaliy small, were found. Spectral data for the lowest energy resolved ligand field bands in the two solvents are included in Table 1 together with slopes and half-wave potentials determined from least squares FlOtS of log[ij(i&i)] vs E_ Precision of El/z values is at least 0.010 v (determined at a d.m.e_)_ Measurements were confined to cathodic processes. Slopes obtained from current-voltage curves recorded under the diffusion-controlled conditions prevalent in the dc polarographic measurements are tolerably close to the theoretical value of 59 mV for a one-electron process at 2s”. Controlled potential coulometric determinations were performed on representative complexes; potentials applied to a Pt working electrode were ca 0.2 V cathodic of El/2 values. The n-values listed in Table i demonstrate oneand two-eIectron reductions for mono- and binuclear species, respectively. I An exception is bis(7,-hydroxyacetophenone)trimethylenediiminatoCu(II). whose dihedral angle between coordination planes is 34O 1451. In this case, it would appear that steric interactions between methyl and bridge methylene groups are important in stabilizing nonplanar stereochemistry. Similar interactions may obtain in 18. 20. and 22. but are absent in 25. For this reason the lower energy spectral shifts between pairs of en-m complexes flable 1) are not considered to signal a major change in stereochemistry. However, some extent of distortion from planarity for tn-bridged compbxes in their Cu(II) form cannot be excluded. This nonplanar complex did not yield well-defined electrochemical behavior_

G. S. PATTERSON

266 Exceptions

are I and 4, which appeared

AND R. H. HOLM to be reduced

to h(O)

under conditions

of the coulometric experiment_ This behavior may result from the use of a Pt electrode and/or the electrolysis time (ca 3045 min) required for complete reduction_ For these complexes and for others not subjected to coulometry, polarographic one-electron reductions at the d.m.e_ were established by comparison of diffusion currents_ The cyclic voltammetry of selected complexes was also examined_ In a number of cases (e.g., 6. 7, 23, 33, 35) the observed behavior was consistent with reversible charge transfer [46] at sweep rates Y of lO-1,000 mV/sec, as illustrated by the following data (Up, mV; ipc/ipa; 6, 61(4), 1.01(0_02), 13.5(0.2); 7, 63(7), O-99(0.02), ip c Iv %, r_tA VH se?): 16_8(0.9); 35, 63(7), 0_95(0.01), 10.4(0.3). Standard deviations are given m parentheses. Voltammograms of 7 and 23 are shown in Fig_ 2_ In other instances current-voltage characteristics indicated charge-transfer processes which were either quasi-reversible or were succeeded by irreversible chemical reactions [46] _ In view of these results and the decidedly negative potentials of all neutral complexes, no attempts were made to isolate the reduced forms. Reversibility of the couple [Cu(2,9-Mea phen)z]z*‘+ (36.37) was demonstrated by voltammetry of both forms. The electrochemical results are considered adequate to demonstrate that the various Cu(II) complexes undergo one-electrcn reductions which, at least under diffusion controlled conditions, are reversible or sufficiently close thereto to allow a valid internal comparison of half-wave potentials. The great majority of the data set out in Table 1 is new. Previously reported potentials for I 7 and derivatives [ 47 ] ,23 [ 48 ] , and 36. 37 [ 49 ] in nonaqueous media are in adequate agreement with those determined in this work and were shown to correspond to Cu(II)/Cu(I) redox. These results are consistent with others, accumulated in studies too numerous to document in full here, which demonstrate that Cu(I1) coordinated in diverse bis-chelate, tetradentate, and macrocyclic complexes can be reduced to Cu(i) in processes that are often chemically and electrochemically reversible.

Structural

effects

on half-wave

potentials

With reference to Table 1 and assuming that Cu(I) is four-coordinate ligand structure permitting, will tend to assume a degree of tetrahedrality to or exceeding that of Cu(II), the following stereochemical changes occur redox processes: p-cu(II) + e-=+p-cu(I) p-Cu( II) + e-+np-Cu( I) np-Cu(II) + e-+np-Cu(I)

and, equal in the

(1) (2) (3)

If it applies in any case Reaction (1) occurs in the reduction of 30, whose macrocyclic structure should prevent significantly nonplanar Cu(I) coordination_ Reaction (2) is considered likely for other complexes which are planar in their

COPPER(U)

CHELATE -0.793

Cu(Sol12

COMPLEXES

267

v

bp CiJ

100 mV/sec

redt 0x.-

c

redt

FIG- 2. Cyclic voltammograms of complexes 7 and 23 in DMF solution recorded at Y = 100 mV/sec. Cu(I1) forms, including tetradentate complexes (e.g., 5. 17). The structures of Co(sal),en(acac) I501 and Co2 [(3MeO_sal)senls 15 1 I indicate that tetradentate ligands .can be deformed from coordination planarity in response to the stereochemical tendency of the metal- This reaction shouId aIso apply to his-chelate complexes which are planar in their oxidized forms, inasmuch as there are no constraining effects toward planarity in the Cu(I) forms. Reaction (3) obviously applies to complexes sterically constrained from planarity in their oxidized forms. Inspection of the data in Table 1 reveals some gross effects of structure. The most negative potential (-2-26 V) is observed for 30, Reaction (1). With the exception of 28 potentials corresponding to Reaction (3) occur in

268

G. S. PATTERSON

AND R. H. HOLM

the intervalof -0.46 to -0.88 V for neutral

Cu(II) species. The majority of potentials assigned to Reaction (3) fall in the range of ca. -0-6 to - IS V, partially overlapping the range for Reaction (3), but much less cathodic than for the assumed single example of Reaction (1). are derived from data on complexes with The preceeding observations different ligand structures_ Hence, the potentials are presumably influenced by both ligand structural and electronic properties. To provide a more meaningful assessment of the sensitivity of Cu(II)/Cu(I) potentials to systematic ligand changes which appear principally to alter the stereochemistry of the oxidized and/or reduced members of the various couples, the complexes have been organized into six series A-F specified in Table 2. In order to allow reasonable comparisons of potentials, the compIexes in each series belong to a common ligand category and a reference Cu(II) compound, known or assumed to be essentially planar, is defined for each. These compounds have been utilized as planar standards in previous deductions of solution stereochemistry of related complexes from ligand field spectra [3_1]_ Nonplanar structures have been associated with red shifts of ligand field bands relative to these standards (Table 1). It is apparent that in each series the potentials assigned to Reactions (2) and (3) become less cathodic in the order (3) < (2).If the potentials are accorded thermodynamic significance in that El/22= E. under the same physical conditions [52] and AGO =-nFEo (Nemstian reversibility), then in an approximate sense the reference compounds could be considered to represent the free energy content of the real or hypothetical (sterically unstrained) planar Cu(I1) members of each couple in a given series. Ascending positive values of Ml/2 in Table 2 reflect the increased relative stabilization of Cu(I) afforded by ligand structural features such as large N-substituents (series A, C, E, and F) and bp, bmp, and bn bridging units (series B and D), al1 of which promote the stability of a nonplanar structure_ Another interesting example of this effect is found in comparisons of potentials for complexes which contain en or tn bridges, but are othenvise identical_ Two such cases are included in series C and I) and two others are furnished by 17-18 and 21-22. AEl/3 values (El/z (tn) - EI;z(en) ) vary from 0.18 V (21-25) to 0.34 V (21-22) and are interpreted in terms of enhanced stability of the nonplanar Cu(1) forms afforded by the longer and more flexible (CHa)s bridge.’ The data given in Table 1 and the comparisons offered in Table 2 provide the first adequately documented evidence that destabilization of planar Cu(I1) and/or stabilization of nonplanar Cu(1). effected predominantly by ligand steric constraints, produces substantial anodic shifts in Cu(II)/Cu(l) potentials. The dipyrromethenide complex 3.5 is closely related to bis(3,3’,5,5’-tetramethyl~,s’_icarboethoxydipyrro ido)Cu(II), which has the largest dihedral angle known for any Cu(I1) complex (Table 1). However, other than noting that its potential falls in the range for Reaction (3), the actual potential value is difficult to interpret in the absence of a planar reference. TWO binuclear complexes, 11 and 12, were included in this investigation in order to see whether any structural changes accompanying reduction at one

COPPER(H)

Comparative Series Structural A B C D E F Electronic

G H

Structural

CHELATE

TABLE and Electronic

Reference

269

COMPLEXES 2 Effects

on Half-Wave

Series members

(&

Potentials 112, V)

effects Cu(.sa& en (5) Cu(sal)z -o-phen (6) Cu(PhHMe)a en (19) Cu(PhHH)z en (2 4) Cu(MeHMeIz (en)2 (30) Cu(pa)z en (34)

1(0_31), 2(0.35), 3(0_47), 4(0_55) 9(0.28), 8,10(0.33); 7(0.36),12(0.41) 20(0_27), 15(0_45), 16(0.63) 25(0.18), 26CO.44) 28(0.74), 29( 1.38) 3r(O_OS), 32(0.13), 33tO.28)

effects - ring substituents Cu(MeHMejz en (I 7) Cu( MeHMe), tn(i8)

19(0_09), 20(0.08),

2 z (0.42) Z(O.48)

Donor atomsb I J K

Cu(sal);? en W* Cu(MeHMe)., en (I 7)* Cu(PhHH)zen (24)*

I4*(-O-53), 30? (-O-76), 27f (0.39)

13% (0.38) 23% (0.43)

El,2 = El 12 (complex)-~1/2 @ef)‘Donor atom sets: *N,O? ; *N, ;%,S,.

‘A

metal site would affect the redox potential of the other site. Both sites are nonplanar and the structure of Cuz (sal)s (m-phen)2 has been determined (Table 1). (t-Bu groups were included in 12 in order to render the complex adequately soluble_) DC polarography of the two complexes and ac polarography of 12 showed a single two-electron reduction step. However, the latter method when applied to 11 resolved the peak potentials for two oneelectron processes separated by 0.12 V, as shown in Fig_ 3_ In both cases, the effects of reduction at one site are not transmitted very effectively to the other. The Cu...Cu distance in II, estimated to be ca. 6 A from molecular models, is somewhat shorter than the 7.4 8, separation calculated from structural data for C~~(sal)~(m-phen):!. The longer distance, together with the two separate m-phen bridges compared to a common bridge unit in I I, appears to attenuate markedly structural changes in 12 such that both centers effectively have the same potential. *The ability of the tn bridge to sustain a distorted tetrahedral the spectra and high-spin nature of Co(x& tn (53]_ Co(sal),en

coordination is supported is low-spin and planar.

by

G. S. PATTERSON

270 I

I

AND R. H. HOLM I

1

I

- 0.71

I - 0.4

I -0.5

I -0.6

-d.7

I

I

I -0.9

I -1.0

I

-0.83

I -0.8

I - 1.1

E (volts) FIG_ 3_ AC pohro~gam of Cu,(saI), bp (II) in DMF solution recorded under the following experimental conditions: dc scan rate , ZmV/sec; ac frequency, 500 Hz; applied ac voltage, 10 mV peak-to-peak: current-sampled mode.

Electronic effects on half-wave potentials (a) Ring Substituents

Although the effect of substituent variation on half-wave potentials for compIexes of apparently constant Cu(Ii) stereochemistry was not extensively studied; several features are evident in the data of Table 1. Two recent studies (42,541, among others, have shown that potentials of metal-centered redox processes are affected by chelate ring substituents in a manner which affords iinear correlations of El/2 with the appropriate substituent constants. Examples of similar behavior are indicated in series G and H of Table 2. Here the potentials shift to more negative values in the order CFs < Ph Me for 28-29 (series E) may in part derive from electronic effects.

COPPER(I1) (b)

Donor

CHELATE

COMPLEXES

271

Atoms

The influence of donor atom changes in planar Cu(II) complexes is expressed in series I, J, and K of Table 2. where species with NaOz donor sets are taken as references. The order of increasingly anodic potential shift is N:, < NaOa < Nz S2 , where only the donor atom portion of the ligand has been varied. The most interesting feature is the substantial and nearly constant anodic shift of NO.4 V upon replacing oxygen by sulfur. Attempts to prepare stable Cu(Ii) complexes containing both an NzSz donor set and nonplanar stereochemistry were unsuccessful_ However, the available data do show that, relative to the same or similar reference compounds, sulfur donors promote anodic shifts to an extent comparable with those afforded by nonplanar structures_ Lastly, attention is directed to Fig. 4, which compares potentials for synthetic complexes obtained in this and other studies with those of the “blue” copper sites in proteins. To facilitate comparisons potentials not reported vs the standard hydrogen electrode have been referenced to it, requiring addition of +0.25 V to potentials obtained reIative to the s.c.e. The data included have been obtained by direct potentiometry or voltammetry, often in different solvents and, in aqueous solution, at near-neutral pH, and no allowance has been made Hence, the comparisons are imprecise, but for junction potential effects_ nonetheiess illustrative. Included are potentials for the most cathodic and anodic members of the series which reveal structural and donor atom effects. These effects are summarized as follows: (i) nonplanar bis-cheiate or tetradentate complexes are easier to reduce than their planar analogs; (ii) rigid planar tetradentate complexes (en-bridged) are more difficult to reduce than less rigid tetradentate (tn-bridged) or electronically similar planar bis-chelate complexes; (iii) complexes which differ only in donor atom sets are more readily reduced in the order N4 < N202 < NaSz- On the basis of limited cyclic voltammetric data, it is tentatively concluded that (iv) current-voltage characteristics for nonplanar (especially tetradentate) complexes tend to approach more closely those for reversible charge transfer than is usually the case for planar complexes of the same series_ With regard to the values of the potentials relative to a common reference, it is observed, as pointed out above. that the values for the neutral complexes examined here fall well below the protein range. Indeed, a survey of potentials reported for diverse types of Cu(II) complexes reveal that the great majority occur on the negative side of 0 V vs s.h.e. Some of the more positive potentials are included in Fig. 4. While no entirely adequate generalization concerning the relation between ligand type and potential values is apparent from the data, we note that with the exception of the ammine system, the most positive potentials are afforded by unsaturated nitrogendonor ligands such as imidazole, phenanthrolines, and bipyridyls [ 17,19,20,64] _ This point has been raised earlier [ 171 and in the interim much evidence has been collected bearing on the ability

of such ligands to stabilize low oxidation potential

for 33 is the least negative

states, including Cu(I) [65,661.

of ail neutral

species examined

The

here, and the

G. S. PATTERSON

272

AND R. H. HOLM

1.0 -II

e

-

_+“blue” Cu proteins -m f,i,j -h cu2+/+

0.5

0

-

=,g

-d -b -e

E

-k

- 0.5

-C

-335 -e

1%

-

-

16

-

33

-

34 26 29 -2

-6 -5

- I.0

-

1: 2

-

I 2

24

IS

Ii I’

-

-1.5

-2.0 Other

Complexes

A-C D-F C-------Series--------J

I-K

FIG. 4. Schematic tepresentation of Cu(ll)/Cu(l) potentials (vs s.h.e.) of “blue” Cu (3) dodecatungstate, jS6J ; (b) proteins (ref. 1) and synthetic complexes. Lignds: u-dikeronebis(thio~mic=xrbazones). 1.571: (c) hemiporphyr-azine, l.581; (d) dithiotropolone, (591; (e) N, macrocycles, [60-62] ; (0 TAAB. (~1 TAXB (OIIe), . [63j ; (h) py, (i) NH,, 6) [64] ; (m) imidazole. bentimidszole. (k) en. [?Ol; (I). 2,9-Me,phen (potentiometric), 2CIphen. [64] ; (n) 2,2’-biquinolyl, [ 19 j _

cases appear potentials for the bis(3,9-hiezphen) (36-37) and bis( ?,I?‘-biquinolyl) to be the most positive known for synthetic complexes_ In these instances the Iigand structures should destabilize planar Cu(l1) and stabilize tetrahedral Cu(f) in a structure possibly quite similar to that of (Cam I’ [6?) _

COPPER(H)

CHELATE

COMPLEXES

273

If the study of “blue” copper sites is to be pursued by the model system approach and the attainment of high reduction potentials is considered obligatory to a realistic model, the comparisons of absolute and relative potentials given here might serve as a useful guide. Assuming four-coordinate Cu(I) in the model and the reduced proteins, a potentially suitable ligand structure would include one thiolate sulfur with some or al! of the remaining ligands being histidyl groups, and the entire ligand structure constrained toward nonplanar coordination in both redox forms. However, a cautionary note is in order. Based on experience with ferredoxin synthetic analogs [68,69], structural, spectroscopic, and magnetic features of fixed protein oxidation states can be closely approached in models while failing to match redox potentials by as much as ca. 0.5 to 1 V. Not surprisingly, it is much more difficult to reproduce energetics dependent upon free energy changes of the entire protein molecule than to simulate features of a resting oxidation state. This research

was supported

by NIH Grant

Gill-1 54 71.

REFERENCES 1. R. Makin and B. G. Malmstrijm,Advan. Enzymol. 33, 177 (1970). 2. R. Malkin, in Inorganic Biochemistry (G. Eichhorn, Ed.), Elsevier 3. 4. 5.

6_ 7. 8_ 9. 10.

Publish@ Co., Amsterdam, 1973, Vol. 2, Chap. 21. T. Vtingard, in Biological Aspects of Electron Spin Resonance. Wiley-Interscience, New York, 1972, Chap. 9. L. E. Bennett,Frogr_ Inorg. Chem. 18, 1 (1973). J. Deinum and T_ Winngard, Biochem. Biophyr Acta 310, 321 (1973). 8. R. M. Reinhimmar and T. Xinngard, Eur. J. Biochem. 18,463 (197 1). H. lwasaki and T. Matsubara, J. Biochem. (Tokyo) 73,659 (1973). T. S tigbrbrand,FEBS Letters 23.4 I (1977). J_ Kelly and R_ P_ Ambler, Biochem. Sot. Trans. 1, 164 (1973). M. D. Scawen, 3. A. M Ramshaw, R. H. Brown, and D. Boulter. Eur. J. Biochem. 44,

299 (1974). A_ F. Xgr&, G. Rotilio, D. Barrd, and B. blondovi, Biochemistry 13, 804 (1974). 12. D. R. Mclillin. R. A. Holwerda, and H. B. Gray. Proc. Nat. Acad. Sci_ USA 71, 1339 (1974). 13. 0. Siiman, N. M. Young, and P_ R. Carey,1 Amer. Chem. Sot. 96.5583 (1974). 14. B. G_ MaImstr6m, B. Reinhtimar, and T. Viinngard, Biochem. Biophys. Acta 205,48 11.

M. T. Graziani,

(1970). IS_ B_ G. MaImstriim, 16. 17. 18_ 19. 20. 21.

22.

B. ReinhZmmar, and T- Vtingard, Biochem. Biophys. Acta 156, 67 (1968); R. hfalkin, B. G. MalstrBm, and T. Winngard, FEBS Letfers I, 50 (1968). A. S. Brill and G. F. Bryce.J. Chem. Whys. 48.4398 (1968). A. Brill, R. B. Martin, and R_ J. P. WiIliams, in Electronic Aspects of Biochemistry (B. Pullman, Ed.). Academic Press, New York, 1964, p_ 5 19. K. Wiithrich. Helv. Chim. Acta 49. 1400 (1966). B. R. James and R. J. P. Williams, J. Chem. Ser. 2007 (196 1). C. J. Hawkins and D. D. Perrin.1 CIzent Sot. 1351 (1962). R. H. Helm and &ii. J. O’Connor, Progr. Inorg. Chem. 14, 241 (1971). R. G_ CharIes,J_ Org. Chem. 22,677 (19.57).

G. S. PATTERSON

274 23. 24. 25_ 26.

AND R. H. HOLM

L. Sacconi and M. Ciampohni,J. Chem. Sot_. 276 (1964) P. Pfeiffer, E. Breith. E. Liibbe. and T. Tsumaki, Ann. 503,84 (1933). F. Lions and K. V_ Martin, J. Amer. Chem. Sot- 79,1273 (1957). P_ Pfeiffer, T_ Hesse, H. Pfitzinger, W. Scholl, and H. Thielert,J. h&f_

Chem. 149,217

(L937)_ 27_ D. H. Gerlach and R. H. Hahn, Inorg. Chem 9,588 (1970). 28. A_ E. Martell, R. L. Belford, and M. Calvin,J. Inorg Nud. Chem. 5, 170 (1958). 29. P_ R. Blum, R_ M. C. Wei, and S. C. Cummings, frrorg. Chem. 13,450 (1974). 30. Y. Nakao. N. Nonagase, and A_ Nakahara, Bull. Chem Sac_ Japan 42.452 (1969). 31. S. G. McGeachin, Can-J. Chem. 46,1903 (1968). 32. T. J_Truex and R. H. Holm,i. Amer_ Chem Sot. 94,4529 (1972). 33_ A_ Chakravorty and T. S. Kannan, J. Inorg. Nucl. Chem. 29,1691(1967). 34. R. H. Holm, A.Chakravorty, and L. J. Tberior.Inorg_ Chem 5,625 (1966). 3~. Y_ hfunkami and K. Sakata.fnorg_ Cfzim. Ada. 2,273 (1968). 36. J. R. Hall, N. K. Marchant, and R. A. Plowman, Austr. J. Cizem. 15,480 (1962); 16,34 (1963). 37. M_ J. O’Connor, R. E. Ernst, and R. H. Helm, J. Amer. Chem. Sot. 90,456l (1968). 3% H. MM. L. Dieteren and C. Konigsbeger, Rec. Trav. Chim Puys-Eas 82, S (1963). 39. D. Lloyd and R. H. McDougall,J. Chem. Sot-. 4136 (1960). 40_ L. P. Olekhnovich, V. P. Kurbatov, 0. A. Osipov. L. S. hlinkina, and V. 1. Minkin, J. Gen. Chem. U_S_S.R. 38,2512 (1968). 41. S. C. Tang, S. Koch, G. N. Weinstein, R. W_ Lane, and R. H. Hohn, Itrorg. Chem_ 12, 2589 (1973). 42. G. S. Patterson and R. H. Hahn, Irzorg. Chem. II,2285 (1972); G. S. Patterson, Ph.D. Thesis, M.I.T., 1974. 43. MI.D_ Hobday and T. D_ Smith, Coot-d_ Chem. Rev. 9.3 11 (1972/73). 44. L. H. PignoIet, R. P. TayIor, and W. D. Horr0cks.J. Amer_ Chem Sot. 91,5457 (1969). 45 R. Iida, I. Oonishi, A. Nakahara, and Y. Komiyama, Bull. Chem. Sot. Japan 43, 2347 (1970). 46. E. R_ Brown and R. F. Large, in Techniques of Chemistry, (A. Weissberger, Ed.), Vol. I, Part HA. Electrochemical Methods, Wiley-Interscience, New York, 197 1, Chap. Vi. 47. M_ Kodama, Y. Fujii. and hf. Sakurai, Bufl_ Chem Sot. Ju~Q~ 45, 1729 (1972). 48. W. R_ Pangratz. F. L. Urbach, P. R. Blum, and S. C. Cummings, Znorg Nucl. Chem Lett. 9. 1141 (1973). 49. P. Thomas, H. Spindler, D_ Rehorek, and 0. Giirtler, Z_ Chem. 13,225 (1973). 50. M_ Calligaris, G. Manzini, G. Nardin, and L. Randaccio, JCS Dalton, 543 (1972). Sl- M. Caliigaris, G. Nardin, and L. Randaccio, Chem_ Commun., IO79 (1970). 52_ J. B. Headridge, EIectrochemical Techniques for Inorganic Chemists. Academic Press, New York, lS69, Chap. 3. 53. 3. Manassert, Irzorg. Chem. 9,966 (I 970). 54. R. F. Handy and R_ L. Lintvedt,Inorg_ CJzem 13,893 (1974). 55. A. J. Gordon and R. A. Ford. The Chemist’5 Companion, Wiley and Sons, Inc., New York, 1972, pp- 14415.5. 56. D. R. WexeU and 31. T. Pope, C/rem. Commun.. 886 (197 I). 57_ D. H. Petering, Sioinorg. Chem. 1 . 255 (1972); D. G. Davis and P. Armatis. AnaL 58.

59. 60.

Letters 6, 115 (1973). C. G_ Birch and R. T. Iwamoto, Zrzorg.Chent 12.66 (1973). C. E. Forbes and R. H. Hoim, J. Amer. Chem. Sot. 92,2297

(1970). L. F. Lindoy, N. E. Tokei. L. B. Anderson, and D. H_ Busch, J. Cooni_ Chem. 1, 7 (1971).

COPPER(I1)

CHELATE

COMPLEXES

275

61. J. M. Palmer, E. Papaconstantinou, and J. F. Endicott, frzorg. Chem 8, 1516 (1969). 62. D. P. Riliema, J. F. Endicott, and E. Papaconstantinou,Zrzorg Chem. lo,1739 (1971). 63. N. Takvoryan, K. Farmery, V. Katovic, F. V. Lovecchio, E. S. Gore, L. B. Anderson, and D. H. Busch,J. Amer. Chem. Sot. 96,731 (1974). 64. C. J. Hawkins and D. D. Perrin, .I_ Chem. Sot_. 2996 (1963). 65. W. R. McWhinnie and J. D. Miller,Advan. Inorg. Radio&em. 12, 135 (1969). 66. R. J. Sundberg and R. B. Martin. Chem. Rev. 74,47 1 (1974). 67. A. H. Lewin. R. J. Michl, P. Ganis, V. Lepore, and G. AvitabiIe, Chem. Commurz., 1400 (197 1). 68. J. J. Mayerle, R. B. Frankel. R. H. Hahn, J. A. Ibers, W. D. Phillips, and J. F. Weiher, Proc- Nat. Acad. Sci. U_ S. 70,2429 (1973). 69. B. V. DePamphilis, B. A. Averill, T. Herskovitz, L. Que, Jr., and R. H. Holm,J. Amer. Chem. Sot. 96,4159 (1974). Received

Sept. I 9, I9 74; revised Oct. 30, I974