6562
The Monomer-Dimer Equilibria of N-Hydroxyet h ylet hylenediaminetriacet atovanadium(I1I) [V( HEDTA)] Frank J. Kristine and Rex E. Shepherd* Contribution from the Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260. Received March 17, 1977
Abstract: Electronic spectra and the equilibria for formation of the V(III.111) dimers of HEDTA3- and EDTA4- are reported. The pK,'s and dimerization constants ( ~ K Ddimerization , from two aquo monomers; PKd, dimerization from two hydroxy monomers) have been determined at p = 0.20 and 1.00 NaC104, 25.0 O C , for species at equilibrium with VL(H2O)I-" ( L = HEDTA3-, n = I ; L = EDTA4-, n = 2): pK, = 6.59 f 0.05; 9.57 f 0.05, ~ K =D9.16 f 0.02; 15.99 f 0.02, pKd = -4.01 f 0.07; -3.14 f 0.07 at p = 1 .OO. [enHz] [(HEDTA)VOV(HEDTA)I.H20 was isolated. The salt exhibits no antiferromagnetic coupling (S = 2) in the solid state (Faraday method) or in solution (NMR method) where between pH 3 and 10 the effective moment was 2.75-2.82 p~/V(111).Equilibria between the oxo-bridged structure, LV0VL"- (I), a dihydroxy structure LV(OH)2VLn- (II), and hydroxy-aquo bridged moiety LV(OH)(OH2)VL('-") (VI) are required for the kinetic parameters determined for formation of the V(II1, III)(HEDTA) dimer and for its monomerization by a preequilibrium hydrogen ion pathway (type I). A kinetically indistinguishable hydrogen ion assisted rupture of strained structures of equivalent composition (type 2) is possible. Monomerization of V(II1, 111) (HEDTA) proceeds at p = 0.20, 25.0 O C , with k l K h y d = 4.0 s - I and kZKKhyd = 2.51 X IO3 M-I s - I (type 1) with a preassociation constant for H30+, K = 7.8 M-1. The acid-catalyzed path exhibits activation parameters of AH* = 14.5 f 0.1 kcal/mol, AS* = -1 1.3 f 3 eu, p = 0.20. Formation of the V(II1, 111)(HEDTA) dimer occurs with k-1 = 360 f 10 M-l s-l via hydroxy monomers and k-2 = 428 & 30 M-' s - I via a hydroxy and an aquo monomer and 25 IM-'s-I through a separate diaquo path. The kinetics and solution structure of V(II1, 111) ions are discussed with respect to the [ Fe(HEDTA)]202- complex.
A considerable interest exists in inorganic, geochemical, and biochemical disciplines for the p-oxo-bridged polymeric clusters and hydroxy polymers of tripositive ions. Examples of the simple aquo ion species include the A12(OH)24+, CrOCr4+, VOV4+, and Fe2(OH)z4+ cations. W e recently communicated the observation of a V(II1) binuclear complex formed either by the inner-sphere electron transfer reaction between V(HEDTA)- and VO(HEDTA)- or by a slower association of 2 mol of V(HEDTA) formed by the outer-sphere component of the same cross-reaction.Is2The inner-sphere path has allowed detection of a precursor complex of transitory existence having the oxidation state assignment (11, IV). Intramolecular electron transfer within the precursor complex produces a (111, 111) dimer, (HEDTA)VOV(HEDTA)2-. Analogous Fe(II1) binuclear ions of the EDTA family (EDTA4-, HEDTA3-, CyDTA4-) have been characterized by the studies of Walling and Gray,3 Schugar, Anson, and Gray,4 and the early explorations of Gustafson and MarThe (HEDTA)FeOFe(HEDTA)2- complex is antiferromagnetically coupled with S = 1.3 The formation and dissociation of these binuclear complexes are important as models for inorganic polymerization processes. The association and monomerization steps for the [Fe(EDTA)]2O4- and [Fe( HEDTA)]202- dimers have been studied by Wilkins and Yellin6 and by Martell's group.' The dominant kinetic pathway for the Fe(II1) dimers appears to involve dehydration of an intermediate formed by the combination of aquo and monohydroxy monomer complexes. N o oxo or dihydroxy binuclear complexes of the EDTA family of ligands have been detected for Cr(II1) in spite of the well-known tendency of the simple ammine complexes of Co( 111) and Cr(II1) to form bridged binuclear structures.11,12 Dihydroxy structures are common for the Co(II1) ammines. A binuclear N T A complex K ~ [ C O ~ ( N T A ) ~ ( OisHknown )~] and the acid-dependent monomerization has been studied.3Y.4D The Fe(lI1) EDTA series of complexes were considered as early models for ferritin and as excellent spectral models for hemerythrin.* Hemerythrin is a binuclear iron 0 2 carrier that Journal of the American Chemical Society
uses only protein side-chain residues for binding two Fe(1I) centers in close proximity. The biological function of the related hemovanadin is less ~ e r t a i nA. ~complex of the formula V(C16H17N301 I ) ( S O ~ )can ~ ~ be - isolated from cytolysis of vanadocytes. The low molecular weight organic ligand binds four of the coordination sites on the V(III) center.I0 Swinehart et al. have characterized the oxidation state of vanadium found in tunicates from California coastal waters. The oxidation state in the cells of Ascidia ceratodes is V(III).43 Members of the order of Aplousobranchia have predominantly V(IV) in the intact cells.43Kustin et al. have found that the cellular blood of Ascidia nigra collected in Bermuda waters has vanadium in the I11 oxidation state.44 Attempts to isolate the V(II1) protein are frequently accompanied with production of EPRactive V02+. Ehrenberg and Boeri obtained a yellow-brown lysate solution from tunicate cell; the absorbing material is most likely to be a hydrolytic dimer of V ( I H ) . ~Kustin's ~ blood spectrum from Ascidia exhibits maxima a t about 280 and 335 11111.~~ The spectrum is not similar to that of the known V(II1) complexes and it would be useful to have spectra of V(II1) complexes in various ligand fields, particularly the N2O4 and N 3 0 3 donor atom set, for comparison. Swinehart has observed a nonprotein fraction from chromatographed Ascidia certodes plasma which reduces V(V) to V(II1). Because tunicates must convert vanadates from sea water to its usable V(II1) form, the interconversion of various oxidation states of vanadium attached t o chelating ligands is of interest. The (111,111) dimer of this report may be generated by a redox path and the vanadium(II1) monomers may be obtained by acid-catalyzed cleavage of the dimers. The issue is germane since the cellular organelles which have the highest concentration of V(II1) also contain high levels of H 3 0 + (-0.10 M). These findings from the vanadocyte chemistry stimulate an interest in the binding of V(II1) to polydentate ligands, particularly those which leave one or two readily exchangeable coordination positions for solvent or other small molecule donors. The similarities of the Fe(II1) and V(II1) systems and the novelty that the V(II1) dimer may be formed by a redox path has prompted us to characterize the monomeric and dimeric species of the general formulas VL(H20)'-", VL(OH)"-,
/ 99:20 / September 28, 1977
6563
I
veilow
/I-
magenta
W
\
f
I 20 1
5 00
GOO
700
900
800
Volume NaOH
Figure 1. Titration of V(HEDTA)(H20) with NaOH, [V(III)ltot= 2.00 X M, p = 0.20, T = 25.0 OC, [NaOH] = 0.0486 M.
I 01
300
L
Species Monomer Monomer Monomer Dimer Dimer
A,, 454 (21.8) 454 (27.8) 460 (27.4) 469 (28.1) 406 (30.5)
nm
(c,
50'~ :nm)
6 00
700
Figure 2. Ultraviolet-visible absorption spectra: (A) [(HEDTA)V]202-, pH 5.97; (B) [(HEDTA)V(H20)], pH 2.34; (C) [(DTPA)VI2-, pH 6.62 ( p = 0.1 1 NaC104, T = 25.0 "C).
Table I. Wavelength Maxima and Molar Absorptivity Data for V L ( H Z O ) I - ~and LV0VL2"- Species
HEDTA3EDTA4DTPA5HEDTA3EDTA4-
. . I
400
M-I cm-I)O
530 (12.6) 535 (10.0) 533 (17.7) 559 (27.4) 492 (41.1)
760 (20.3) 708 (16.5)
C/V(IIl).
LV0VL2"-, and VL(OH)2("+I)- (L = EDTA4-, n = 2; L = HEDTA3-, n = 1). These species have been prepared in solution from VC13 as an authentic source of V(II1) rather than by the indirect redox r0ute.l The hydrogen ion dependencies for dimer association and dissociation are presented in this report. The (111,111) dimer has been isolated as a solid of the formulation [enH2] [(HEDTA)VOV(HEDTA)].H20. The solid exhibits no antiferromagnetic coupling as determined by the Faraday method. The Evan's N M R method shows that the dimer is also paramagnetic with S = 2 in solution. The magnetic properties of the weakly coupled V(II1) centers of the (111, 111) dimer are likely to be of theoretical interest.46a The low-temperature moments and the structural arrangement of the (111, 111) complex will be the subject of a separate report .46b
Results Dimer Equilibria and Spectra. The equilibria shown in eq 1-6 are observed when stoichiometric amounts of VCl3 and H3HEDTA or NaZHzEDTA are titrated under an inert atmosphere. Similar equilibria occur for the corresponding Fe( 111) c ~ m p l e x e s . ~The J oxo-bridged formulation has been selected for the reasons cited in the Discussion section and because of the existence of the analogous (H2O)5VOV( H20)s4+ ion. V(H20)63+
Kr + L(*+")- + VL(H20)'-"
(1)
+ H20 &VL(0H)"- + H@+ (2) 2VL(H20)I-" + 2 H 2 0 +LV0VL2"- + 2 H 3 0 + (3) 2VL(OH)"- +LVOVL2"- + H20 (4) VL(H20)'-" + VL(0H)"- +LVOVL2"- + H30+ ( 5 ) VL(0H)"- + 2 H 2 0 +VL(OH)2(fl+1)-+ H 3 0 + (6) VL(H*O)'-"
KD
Kd
KdKa
K,'
Kristine, Shepherd
X (nm)
Figure 3. Ultraviolet-visible absorption spectra: ( A ) [(EDTA)V]204-, pH 10.96; (B) [(EDTA)V(H@-], pH 3.40 ( p = 0.10, T = 25.0 "C).
A typical titration curve obtained as described in the Experimental Section is shown in Figure 1. Initially only the dark yellow monomeric complexes, VL(H20)'-" and VL(OH)"-, species are detected. With increasing pH the magenta LV0VL2"- species is formed. The dimer equilibria shift in favor of VL(OH)*("+')- only a t high pH; the solution returns to a yellow brown hue as equilibrium 6 predominates a t high pH. The spectra of the monomeric VL(H20)I-" and the corresponding LVOVL2"- species (Table I) are shown in Figures 2 and 3 for L = HEDTA3- and L = EDTA4-, respectively. Spectra are recorded in the pH domain where either the monomer or dimer is the only species of appreciable concentration in solution. The monomer complexes have very approximate octahedral ligand fields. The 454-nm band of the monomer appears to be split by the lowered symmetry in the LVOVL*"dimer. The e's per V(II1) of the dimer species do not exceed a factor of 2 for those of the monomer species. Transitions in [V(HEDTA)]*O*- occur shifted by about 65 nm to lower energy from those of the same bands in [V(EDTA)]*O4-. It may be inferred that the higher electrostatic field of EDTA4relative to HEDTA3- causes a larger ligand field for EDTA4-. The Fe(II1) dimers have electronic spectra which have been interpreted as two high-spin Fe(II1) centers in approximately octahedral coordination,
/ N-Hydroxyethylethylenediaminetriacetatovanadium(II1)
6564 Table Hydrolytic Equilibrium Constants for V(II1) and Fe(II1) Complexes of HEDTA3- and EDTA4-
7ljlil I
22
V(HEDTA) pK,
V(EDTA)-
6.59f0.05 6.39 f O . l O b ~ K D 9.16 f 0.02 9.05 f 0.02b pKd -4.01 f 0.07 -3.74 f O . l O b a00
900
1000
9.57f0.05 4.11f0.07C 7 S d 10.16 f 0.01 15.99 f 0.02 5.84 f O . O 1 12.21 16.71 f 0.01 -3.14 f 0.07 -2.38 f 0.08 -2.53 -3.62 f 0.02b
" p = 1.00, T = 25.0 "C.
1100
5.
~ [ V L ( H ~ O ~o*l/ C ~= 3 d
Figure 4. Titration data from V(HEDTA) system: P = 1.OO, T = 25.0 "C, and 0 represent separate experiments.
Fe(HEDTA) Fe(EDTA)-
p
= 0.20, T = 25.0 "C. References 3,
Reference 4.
Table 111. Acid Dissociation Constants for Monomeric Complexes of M( I II)L(H2O)'-" Ionic radii ______ M(II1)
ShannonPauling Prewitt HEDTA
Ru V
0.14
0.70 0.64
Rh Cr
0.68 0.63
0.67 0.62
Mn Fe
0.66 0.64
0.65 0.65
co AI
0.63 0.51
0.53 0.53
6.59
*
PK, EDTA
Ref
7.63 10.2
42 This work
0.10
6.13 f 0.10 3.7" 4.11 f 0.07
9.2 7.39f 0.03 5.5" 7.58
16 4.5
5.92
17 18
-8
14 15
" May be coordination number 7. Table IV. Data from the Hydrogen Ion Induced Monomerization of (HEDTA)VOV(HEDTA)2-u 1/kobsd
Figure 5. Acid monomerization rate data treated by eq 11
Combination of the equilibria 1-6 together with mass balance and charge balance expressions provides [VL(H20)'-"] = TM- TOH- [H30+]
log Kd = 2pK,
-~
K D
+ [OH-]
(7)
(9)
where T M represents the total V(II1) concentration in all forms, and TOHis the concentration of N a O H added beyond the formation of the VL(H20)'-" monomeric c o m p l e ~A. ~plot of
+
2[VL(H20)'-"] [ H ~ O + ] ( T O H [H30+] - [OH-]) vs. [VL(HzO) I-"] [H@+] will yield a straight line of slope K D and intercept K , . Potentiometric titration data a t 25.0 O C , p = 0.20 and 1.00, were obtained for HEDTA3- and EDTA4- systems of V(II1) under N2. Representative data for the V(HEDTA) system are shown in Figure 4 a t p = 1.00. The accumulated equilibrium constants that have been determined by this technique are listed in Table I1 with the existing data for Fe(II1) dimers. Values for the pK, of reaction 2 where the V(II1) is replaced by other tripositive cations are given in Table 111. Kinetics of the Monomerization Process. Dissociation of [V(HEDTA)]202- was induced by mixing solutions of the fully formed dimer with solutions of varying hydrogen ion a t constant ionic strength ( p = 0.20, T = 25.0 "C). The disapJournal of the American Chemical Society
[H30+1
kobsd
x
0.1 I5 0.102 0.095 1 0.0820 0.0690 0.0620 0.0520 0.0436 0.032i 0.0229
158 144 126 135 121 107 98.8 84.7 69.0 50.8
6.33 6.94 7.94 7.41 8.26 7.94 10.0 11.8 14.5 19.7
lo3
kobsd
1/[H30+1 8.70 9.80 10.5 12.2 14.5 16.1 19.2 22.9 31.1 43.7
( 1 -t
Kz[H3O+I) 300. 259. 219. 221. 186. 159. 139. 114. 86.3 59.8
"p K2
= 0.20 NaC104, T = 25.0 "C; average of three or more runs, = 7.79 M-'.
pearance of the dimer absorbance a t 557 nm was monitored in a stopped-flow spectrophotometer. The decrease in absorbance obeys first-order kinetics in [dimer],,, and an apparent saturation effect in [H30+]. The data for the monomerization reaction are given in Table IV. These data conform to a 1/[H30+] vs. l/kobsd linear dependence as shown in Figure 5. The general mathematical form for kobsd that is implied by this relationship is given by a b[H3O+] kobsd = 1 c[H~O+] The solution structure of the (111,111) dimer is still uncertain as to whether the V(II1) centers are joined by an oxo ligand or by dihydroxy bridging ligands (see the Discussion section). Since (111, 111) is produced by the intramolecular electron transfer bleaching of (11, IV), the oxo structure must exist a t least as a kinetic transient. The reverse reaction of (111, 111) monomerization exhibits a rate term dependent on two molecules of the hydroxy monomer and it is certain that the dihy-
/ 99:20 1 September 28, 1977
+ +
6565 Scheme I
H
1
/o\ LM ‘0’
+ H+
ML2-
H
BLMOH-
LMOH-
/ 2 H + . fast
+ LM(H,O)
1
H+, fast
2LM(H20) 2LM(H,O)
Scheme I1
u
‘0
@2 04
C6
08
10
12
[H30+3
H
/o\ LM
H
ML’-
/o\ ML2F--L LM, k‘‘o / k0
0
‘0’ H
H
Figure 6. Acid monomerization rate data treated by eq 12.
droxy structure also exists at least as a kinetic transient. The two species differ only by the components of a solvent molecule and the hydration equilibrium 11 accounts for the interconH
LMOML*-
+ H,O
Khrd
F.-L
Am*LM, ’0’
I
,
H
I1 version. Two kinetic pathways are compatible with the general law of eq 10. These include the following: (1) preequilibrium protonation catalysis (eq 12-14) and (2) proton-assisted rupture of a strained system (eq 15-17).47 In these general schemes D represents the (111,111) either as structure I or I1 and D* represents a strained bond structure of the (111,111) dimer. Structures 111, I V , and V readily come to mind as H
1H’.
+ LM(H,O) fast
( I ) addition of H2O may occur concerted with kl or k2 steps which break the dimer structure. Similarly, the proton-assisted rupture of a strained system is shown in Scheme I1 with structure V with the potentially concerted hydration of 111 as an equivalent path. Application of the usual mass balance and equilibrium equations to Scheme I (type 1) yields expression 18. Application of mass balance equations, the steady-state approximation on the concentration of D*, and the assumption that the pseudoequilibrium constant (ko/k-0) favors structure D yields eq 19 for Scheme I1 (type 2). The steady-state and (ko/k-0) 0.02 M.
kl3
D *monomers
(13)
k14
DH+ -monomers
(14)
kis
D S D * k-15
k16
-
D* -monomers H3O+
+ D*
k17
monomers
(15)
(16) (17)
The protonation preequilibrium path is shown in Scheme I for the dihydroxy structure (11); for an oxo bridged species Kristine, Shepherd
The intercept, c, is determined as 7.8 M-l from Figure 5. The slope, 2.64 X lo3 M-I s - I , serves as an initial estimate of b although a refinement is obtained from eq 21 with c taken to be 7.8 M-I. (1
+ ~ [ H 3 0 + ]=) a + b[H3O+]
(21) The monomerization data in the form of eq 21 are shown in Figure 6. The values of a and b are established as 4.0 s-I and 2.5 1 X lo3 M-I s-l, respectively. The validity of the statement a/[H30+] 0.02 M is established; a / [H3O+Imax< 200 M-I s-’ or
