Inorganic Chemistry, Vol. 16, No. I, 1977 93
Bis(ethylenediamine)cobalt(III) Complexes
Contribution from the Departments of Chemistry, University of Florida, Gainesville, Florida 32601, and San Diego State University, San Diego, California 92182
Synthesis, Characterization, and Redox Studies of Thiolate, Thioether, Alkoxy, and Carboxylate Chelate Complexes with Bis(ethylenediamine)cobalt(III)la ROBERT H. LANE, FRANK A. SEDOR, MICHAEL J. GILROY, PETER F. EISENHARDT, JAMES P. BENNETT, Jr., RALPH X. EWALL, and LARRY E. BENNETT*lb
Received October 21, 1975
AIC50768M
The preparation and properties (ion exchange, electronic, and IH N M R absorption) of Co111(en)2(X-Y)"+complexes with X-Y = (H)ECH2CH2NH2, (H)ECH2CH20H, (H)ECHzCOO, (H)ECH(CH3)COO (E = S, 0),CH3SCH2CH2NH2, CH3SCH2CO0, and NH2CH2COO as chelate ligands are described in conjunction with kinetic studies of their reactions with Cr(H20)62+, V(H20)62+, and Ru(NH3)b2+ as reductants. Inner-sphere rate enhancement factors of 35-3400 are observed for thiolate bridging over analogous alkoxide bridging to Cr(1I) and attributed to a diminished steric accessibility of the oxygen atom and, perhaps, a ground-state elongation of the C e N bond trans to an efficient sulfur bridge. Substitution control is evident in parallel inner-sphere reductions by V(H20)62+,whereas a comparative kinetic advantage for the thiolate class is again evident in outer-sphere reduction by Ru(NH3)a2+ although at a low reactivity level. Reduction pathways utilizing carboxylate bridging functions are found subject to both chelate and substituent steric effects and nonbridging ligand electronic effects, particularly from a cis thioether donor. In the absence of facile bridging ligands, the latter function appears to engender a relatively high degree of outer-sphere reactivity toward Cr(I1). Rate laws, constants, and, where accessible, activation parameters are reported and discussed in the context of the limited data presently available for this class of chelating ligand.
Introduction Our interest in how coordinated thiolate and thioether functions affect the redox behavior of transition metal centers was aroused by the widespread occurrence of such coordination in iron-sulfur2 a n d cytochrome c3 electron-transfer proteins4 as well as heme m o n o o ~ y g e n a s e s . Our ~ early studies5 revealed that, relative to its alcoholate analogue, a thiolate coordinated to cobalt(II1) functions extremely efficiently as a bridge in inner-sphere reactions, perhaps related to t h e efficient transfer mediation subsequently reported for sulfur-bridging thiocyanate.6 We have since discovered high reactivities for thiolate functions in outer-sphere reactions also, at least for the tetrahedrally distorted Fe(I1)-Fe(II1) couple of rubred o ~ i n . Additionally, ~~?~ thioether coordination to cobalt(II1) appears to bestow unique kinetic redox features.8 It is the purpose of this p a p e r to amplify and extend observations in this area of coordinated sulfur redox chemistry, including alcoholate and carboxylate comparison reactions. Certain features of t h e complexes are notable aside from possible biological relevance. T h e use of a divalent chalcogen donor atom, unlike one f r o m groups 5 and 7, permits t h e introduction of substituents on t h a t atom without precluding its function as an electron-transfer bridge. T h i s allows exploration of the effects of chelation and steric modification on static and dynamic properties. For example, we have found chelated alcohol complexes to be substantially more acidic than closely similar monodentate c o m p l e ~ e s .Kinetic ~ anomalies also a p p e a r t o arise from chelation in certain of our redox studies to b e discussed. In t h e accompanying paper,1° which deals with the substitution behavior of the chromium(II1) product complexes,l l J 2 evidence is presented for redox catalysis t h a t is enhanced by pendent chelate functions. Experimental Section Equipment. Visible and ultraviolet spectra (Table I) were measured on a Cary Model 14 spectrophotometer. Infrared spectra were measured in KBr pellets with a Perkin-Elmer Model 337 recording spectrophotometer. IH N M R spectra (Table 11) were obtained at 37 OC using a Varian A-60A spectrometer. Fast reactions were analyzed using Polaroid photographs from a thermostated Durrum-Gibson Model D-1 10 stopped-flow spectrophotometer. Aside from flushing the system with several rinses of deaerated reactant solutions, no special precautions were necessary where air-sensitive M. Slower reactions reductant concentrations exceeded - 5 X were followed using an all-glass mixing device13 in the thermostated cell compartment of the Cary 14.
Table I. Spectral Parameters of Various Cobalt(II1) Complexesa Entrv
Species
h. (e)
1 Co(en),(SCH,COO)+ 2 Co(en),(OCH,-
282 (11 700)
518 (132)
360 (140)
499 (113)
348 (122)
4 Co(en),(SCH517 (153) (CH,)COOY 5 Co(en),(OCH517 (139) (CH,)COO)' 6 Co(en),(HOCH499 (113) (CH, )COO)'+
360 sh (350) 360 (150)
COO)+
3 Co(en),(HOCH,COO),+
-
h, (e)
h, (e)
518 (152)
349 (123)
370 sh (283) 490 (98.7) 347 (104)
.. . ... 282 (12 350)
... ...
7 Co(en),(SCH CH,NH,)~+~ 8 Co(en),(OCH,CH,NH,)ZC 9 Co(en),(HOCH,CH, NH,)3+
482 (138)
282 (13 200)
486 (79.4) 344 (87.8)
.. . .. .
10 Co(en),(SCH,CH, OY 11 Co(en),(SCH,CH,OH)'+ 12 Co(en),(OCH,CH, O)+ 13 Co(en),(HOCH,CH,0H)3'
524 (139)
283 (10 840)
520 (156)
283 (14 560)
523 (137)
382 (195?)
500 (112)
355 (82)
. .. .. .
14 Co(en),(CH,487 (172) 350 sh 282 (7800) .~ SCH,-CH,NH, 13+ (276) 15 Co(en),(CH,499 (168) 360 sh 280 (7300) SCH,COO)2+ (250) Wavelengths, X, of maxima or shoulders (sh) are in nm; molar extinction coefficients, e, given in parentheses, are in M-' cm-I , signifies no band observed; no entry signifies shoulder not observed. Intense absorption was noted in certain complexes at higher energy (-240 nm) but not studied. At 600 nm a shoulder (e 41.4) was detected as similar to that at 593 nm (e 38) in the analogous N-S cysteine complex.28
". . ."
Analyses. Analyses for C, H, S, and N were performed by Galbraith Laboratories, Inc., Knoxville, Tenn. Cobalt was determined using 2,2',2"-terpyridine (terpy)14following complex decomposition by liquid fire (7 parts 70% HC104-3 parts 70% "03). Development and measurement of absorption at 505 nm due to C0(terpy)2~+were accomplished anaerobically, and a molar absorbitivity of 1386 M-1 cm-l determined with known solutions15was used rather than the
94 Inorganic Chemistry, Vol. 16, No. 1, 1977
Bennett et al.
Table 11. ‘H NMR of Various Protons Adjacent to Chalcogensa Methylene protons Entry
Species
1 2 3
NaOOCCH, SD Co(en), (OOCCH,S)’ Co(en), (OOCCH,S)t
4 5
DOOCCH,OD Co(en), (OOCCH,OD)’i
6
I
Medium
A. Thiolato and Alkoxy Species 3.5 1 D,O 3.10 DZO D,O-CF,COOD 3.10
4.26
DSCH,CH,NH,.DCl
D,O
2.85
D2O
2.20 mult
D,O
3.54
...
...
b
D,O-NaOD
3.19
...
...
C
D ,O-CF ,COOD
3.60
...
...
C
3.68 3.00 3.15
...
*..
...
Co(en),(SCH,CH,NH,)2’ DOCH,CH,NH, Co(en), (OCH,CH,NH)” Co(en), (DOCHzCHzNHz)3t X
12 13 14
DOCH,CH,OD Co(en), (OCH,CH,O)+ Co(en), (DOCH,CH, OD) ’+
15
DOCH,CH, SD xo xs Co(en), (OCH,CH,S)’ xo xs Co(en) (DOCH, CH ,S) ” xo xs
11
18
19 20 21 22 23 24 25
21 28 29 30
...
b
...
C
...
...
C
...
*..
b C
...
3.69 t (xo) 2.68 t (xs) 3.00 mult (xo) 1.92 mult (xs) -3.6 mult (xo) -2.1 mult (xs)
... ... ... ...
b C C
b C C
DOOCCH(CH,)SD Y Co(en), (OOCCH(CH,)S)+ Co(en), (OOCCH(CH,)S)’
... ...
1.48
3.65
C
...
1.32 1.32
3.51 3.51
C C
NaOOCCH(CH,)OD Y Co(en),(OOCCH(CH,)O)’ Co(en), (OOCCH(CH,)OD)’+
...
1.30
4.09
b
D,O D,O-CF,COOD
..* ...
1.25 1.32
3.92 4.36
C C
B. Thioether Species DZO
2.08
CH, SCH,CH,COOCH, Y CH, (SCH,COOH), X
26
b C C
D,O-CF,COOD
X
16
...
... ...
Co(en), (OOCCH,0D)2+
X
11
...
...
X
10
*.. ... ...
Ref
4.29 3.90
X
9
Methinyl protons
DZO D,O
X
8
X
Methyl protons Y
CF ,COOD
3.62
b
...
b
COOCCH(CH,SCH,)ND, x v Co(en), (OOCCHCCH, SCH,)NH,)’+
D, 0
3.21
2.22
4.5 1
b
D, 0
3.05
2.22
3.62
d
Co(en),(OOCCH,SCH,)2t Co (en) ,(OOCCH, SCH ,) ” Co(en) ,(CH, SCH,CH,NH,)’+
DZO D,O-CF,COOD D,O
3.15 3.15 e
2.38 2.38 2.32
X
Y
C C C
a Absorption downfield from external TMS in ppm. Where insufficiently soluble, perchlorate salts were solubilized using a slight deficiency of tetraphenylarsonium chloride. Acidification was usually with trifluoroacetic anhydride. Sadtler NMR tables. This work. Reference 28, using 6 ~ 4.60 0 ppm ~ downfield from TMS. e Signal obscured by en absorption at 2.6-3.2 ppm.
reported value. Total chromium was determined spectrally by alkaline peroxide oxidation to chromate(V1) ion which was monitored at 373 nm where t was 4815 f 15.1s Chromium(I1) was determined from the excess Co(NH3)sCl2+ I7-l9 absorption at 534 nm remaining after anaerobic reaction and allowance for absorbance due to Co2+ 2o and CrC12+.*1 Kinetic Determinations. Except where noted kinetic runs were made under pseudo-first-order conditions for the consumption of the cobalt(II1) complex (usually at the low-energy maximum) in aqueous acidic solutions maintained with LiC104-HC104 at Z = 1.00 M. Standard plots of log (Al - A m ) vs. t were linear through at least 900h reaction. The cobalt complexes were not as sensitive to room light as their chromium(II1) analogues.12cEyring treatments of log (k/ T) vs. 1 / T yielded activation parameters. Error limits are standard deviations from least-squares analyses using a Hewlett-Packard HP-65 calculator and program. The units for rate constants are molar and seconds. For two unmeasurably fast reactions a deficiency of reductant, R, was added to a solution containing equimolar amounts of the oxidant under investigation, 0 1 , and a second oxidant, 0 2 , whose rate of
reaction, k2, with the reductant is kinetically parallel and known for the experimental conditions employed (e.g., [R]0:[01]0:[02]0= 0.9: 1.O: 1.O). The appropriate second-order differential rate laws can be rearranged and integrated to yield
[ O l l t - kl log---log-
P2lt
[OIlO
[0210
k2
Thus, if [Oilmand [O2lmare determined by ion-exchange product isolation, then kl may be found by solving at t = m. Ion-Exchange Procedures. The charges of ions were inferred from their elution characteristics from cation-exchange columns of Bio-Rad AG (or Dowex) 5OW-X2,20&400 mesh (Na+ or Li+ form) calibrated with cis-Co(en)2C12+,C0(H20)6~+,and C r ( H ~ 0 ) 6 ~ions. + Nitrogen pressure (1 atm) was used to increase the flow rate through the columns. Stoichiometry Studies. After the reactions were allowed to occur with cobalt(II1) in excess, with chromium(I1) in excess and, in the case of the carboxylate chelates, with equal amounts of each, the
Bis(ethylenediamine)cobalt(III) Complexes
Inorganic Chemistry, VoZ. 16, No. 1, 1977 95
N , 14.64; S, 8.36; Co, 15.40. Found: C, 21.75; H, 5.34; N , 14.61; mixture was aerated and charged onto a calibrated ion-exchange S, 8.32; Co, 15.15. column. With the carboxylate chelates the monomeric chromium(II1) product was selectively eluted and analyzed as chromate(V1). With (2-Mercaptoethanolato-O,S)bis(ethylenediamine)cobalt(III) perchlorate was prepared by adding 61 g of trans-[Co(en)~C12]Clto a the amino chelates the remaining excess reagent was analyzed spectrally after elution of either the cobalt complex or the dimeric solution of 13 g of N a O H and 17 g of 2-mercaptoethanol in 250 ml of water. After 10 rnin the solution was filtered and excess sodium chromium(II1) product of chromium(I1) air oxidation.22 The last perchlorate added. The resulting solid was filtered and recrystallized approach gave results which were usually -20% below those expected by dissolution in minimal hot water, addition of sodium perchlorate, whereas the others were routinely within 10%. Gas Analysis. After reaction of C O ( ~ ~ ) ~ ( N H ~ C H ~ C H ~ S Cand H ~cooling. ) ~ + Anal. Calcd for [Co(en)2(SCHzCH20)]C104: C, 20.32; H , 5.68; N, 15.80; S, 9.03; Co, 16.61. Found: C, 20.32; H, 5.70; (6.0 pmol) with excess chromium(I1) in 0.5 M acidic solution, the N , 15.91; S, 9.22; Co, 16.30. At least as indicated by elemental gastight reaction vessel was attached to a high-vacuum line and any analysis, (2-mercaptoethylamine-N,S)(triethylenetetramine)cobalt(III) volatile gaseous products were collected. The contents of the collection perchlorate can apparently be prepared analogously. Sodium hydroxide vessel were examined by standard vapor-phase chromatography. The (4.0 g), cysteamine hydrochloride (5.7 g), and [Co(trien)Clz]Cl (15.4 authors are grateful to H. E. O N e a l for assistance with this analysis. g) were mixed as solids, 200 ml of water was added, followed by excess Materials. Common chemicals were of reagent grade unless sodium perchlorate after 15 min, and the resulting solid was reotherwise specified. Deionized water was redistilled from alkaline crystallized as before. Anal. Calcd for [Co(trien)permanganate and stored in glass bottles. Nitrogen for deaerating (SCH2CH2NH2)][C104]2: C, 19.93; H, 5.00; N , 14.52; S, 6.65. air-sensitive solutions was passed through two Cr(I1) scrubbing towers. Found: C, 19.83; H, 5.17; N , 14.53; S. 6.83. Lithium perchlorate was occasionally checked for purity by passage Elemental analyses suggest that silver and mercury adduct of solutions through an acidic cation-exchange column followed by complexes can be prepared. Addition of mercuric chloride to 1 g of titration of the eluent with sodium hydroxide. Substitution of solutions [Co(en)2(0CH2CHzS)]C104 in 40 ml of water led to a red-orange prepared from lithium carbonate and excess perchloric acid revealed color after about 2 rnin and a precipitate after about 5 rnin which no kinetic anomalies. was filtered after another 5 min, washed, and dried. One-fourth of Stock solutions of Cr(II1) (from chromium(II1) perchlorate) at the filtrate from the previous synthesis was treated with -0.25 g of acidities between and 1 M were anaerobically reduced to Cr(I1) K3 [Co(CN)6]. The immediate precipitate was isolated as usual. using mossy or 20-mesh zinc which had been amalgamated with Reaction of 0.1 g of [ C O ( ~ ~ ) ~ ( S C H ~ C H ~ in O )minimal ] C ~ O water ~ tetrachloromercurate ion in 1 M HC1 for 10 s following activation with 3 ml of 1 M silver perchlorate was followed after 15 rnin with with 3 M HCl. Hexaammineruthenium(II1) chloride (Mattheyaddition of excess lithium perchlorate and, after stirring of the mixture Bishop) was similarly reduced anaerobically to R u ( N H ~ ) following ~~+ overnight, a dark pink solid was isolated. In D2O the ' H N M R repeated recrystallization from hydrochloric acid to spectral integrity.23 spectrum observed retained methylene (0)absorption almost unshifted Vanadium(I1) solutions were obtained anaerobically by the zinc (6 -3.1 ppm) while methylene (S) absorption had shifted downfield amalgam reduction of vanadyl trifluoroacetate (from vanadyl sulfate (6 -2.55 ppm). The nature of the reactants, reaction times required, and barium trifluoroacetate) in acidic solution. ' H N M R spectra and analyses lead us to tentatively formulate these 2-Aminoethyl disulfide dihydrochloride was prepared24 by the three salts as follows. Anal. Calcd for [Co(en)z(HzO)oxidation of 2-aminoethanethiol (0.1 mol of the hydrochloride (OCH2CH2SHg)l [C104]3: C, 9.33; H, 2.87; N, 7.27. Found: C, neutralized with 0.1 mol sodium hydroxide in 30 ml of water) by 9.08; H , 2.84; N , 6.92. Calcd for [Co(en)z(HzO)hydrogen peroxide (lo%, 10 ml). After 30 min, ten 100-ml ether (OCH2CH2SHg)l [CO(CN)6]: C, 20.92; H, 3.22; N, 20.33. Found: extracts were evaporated at 40 "C. The resulting oil was treated with C, 20.97; H, 3.45; N , 20.09. Calcd for [Co(en)z(HzO)25 ml of 12 M HCI and 200 ml of ethanol to yield a white solid which (OCH2CH2SAg)l [C104]2: C, 12.42; H, 3.82; N, 9.66; S, 5.53. Found was filtered, washed with ethanol and ether, and suction-dried. C, 12.38; H, 3.77; N , 9.45; S, 5.29. 2,2'-Dithiopropionic acid was prepared25 by the addition of 1 mmol 2-Methylthioethylaminebis(ethylenediimine)cobalt(ILI) perchlorate of 2-mercaptopropionic acid to 1 mmol of iron(II1) perchlorate in was prepared by adding a tenfold excess of methyl iodide to 0.82 g 200 ml of water. After 1 h several drops of concentrated H2S04 were of [CO(~~)~(SCH~CH~NH~)][C~O~]~ in 60 ml of 66% (v/v) added and the reaction mixture was extracted with three 50-ml portions methanol-water. The flask was sealed and stirred vigorously for 36 of diethyl ether which was evaporated to dryness under a stream of h, with a change from dark brown to orange, followed by evaporation nitrogen to yield a white solid (mp 113 "C uncor). to dryness overnight from a beaker at 35 "C. The crude product was Preparation of Cobalt Complexes. Syntheses for the alkoxy recrystallized twice from water (80 "Cj by the addition of excess complexes have been d e ~ c r i b e d .(Mercaptoacetato-O,S)bis(ethyl~ lithium perchlorate followed by cooling. Anal. Calcd for [Coenediamine)cobalt(III) perchlorate, (2-mercaptopropionato-0,s)( ~ ~ ) ~ ( C H ~ S C H ~ C H[C104]3: ~ N H Z )C, ] 14.78; H, 4.43; N , 12.31; bis(ethylenediamine)cobalt(III) perchlorate, and (2-mercaptoethylS, 5.64. Found: C, 14.69; H, 4.36; N , 12.11; S, 5.49. amine-N,S)bis(ethylenediamine)cobalt(III) perchlorate were prepared Methylthioacetatobis(ethylenediamine)cobalt(III) perchlorate was by similar procedures of which the following is representative. prepared by adding a 25-fold excess of methyl iodide to a suspension Deoxygenated 98-1 00% ethylenediamine (0.057 mol) was added via of 1.8 g of [Co(en)z(OOCCH2S)]Cl04 in 300 ml of 90% methasyringe to a stirring, deoxygenated solution of CO(H20)6(C104)2 (0.028 nol-water. After stirring of the mixture for 24 h in a closed flask mol) in 30 ml of water. After 15-30 rnin the solid disulfide (0.014 and for 3 h in an open flask a light pink solid was isolated and mol) or, in the case of ( N H ~ C H ~ C H ~ S - ) Y ~aHneutralized C~, recrystallized twice from hot (90 "C) water by addition of sodium deoxygenated 15-ml solution was added. Isolation procedures after perchlorate until precipitation began followed by cooling. Anal. Calcd 30 rnin under nitrogen differed slightly. The mercaptoethylamine C, 17.40; H, 4.38; N, 11.60; for [Co(en)2(00CCHzSCH3)][C104]2: complex solution was filtered and, after addition of solid sodium S, 6.64. Found: C, 17.42; H, 4.26; N , 11.64; S, 6.75. perchlorate, yielded a purple-brown solid which was recrystallized Glycinatobis(ethylenediamine)cobalt(III) perchlorate was prepared from 0.1 M perchloric acid with slow cooling to yield dark brown by adding slowly with stirring a solution of 6.8 g of [Co(en)2needles. Anal. Calcd for [ C O ( ~ ~ ) ~ ( N H ~ C H ~[C104]2: C H ~ S )C, ] (00CCH2NH2)]C1226 in 40 ml of water to a solution of 2.4 g of 15.87; H, 4.88; N, 15.42; S, 7.06. Found: C, 15.91; H, 5.07; N, 15.49; AgC104 in 2 M HC104. The filtrate from silver chloride was reduced S, 7.02. From the mercaptoacetate reaction mixture a brown, solid to a thick syrup on a rotary evaporator at 45 OC. After the mixture mass was filtered and redissolved in 140 ml of hot (90 "C) water. stood for several days, the resultant solid mass was filtered and dried After filtration, 30 g of NaClOvH20 was added with stirring and for 2 days under aspirator suction and for 24 h in vacuo. Anal. Calcd burgundy crystals separated on cooling. Recrystallization was acC, 15.94; H, 4.43; N , 15.49; for [CO(~~)~(O~CCH~NH~)][C~O~]~: complished by addition of 20 g of N a C 1 0 e H 2 0 to 120 ml of hot Co, 13.04. Found C, 16.13; H, 4.52; N, 15.52; Co, 13.25. Agreement solution. Anal. Calcd for [Co(en)2(00CCH2S)]C104: C, 19.5; H, was found with reported spectra.26 4.88; N , 15.2; S, 8.67. Found: C, 19.5; H, 4.84; N , 15.13; S, 8.53. The mercaptopropionate product mixture was concentrated with Results stirring to 20 ml in a 150-ml beaker at 100 "C under a stream of Characterization of the Cobalt(II1) Complexes. Infrared nitrogen and cooled. After filtration and washing with 5 ml of hot spectra confirmed the coordination of carboxylate (strong water, the collected solid was dissolved in 100 ml of hot (90 "C) water, absorption near 1635 and 1350 cm-l) and the absence of the and the solution was filtered and cooled yielding light purple needles. Anal. Calcd for [Co(en)2(OOCCH(CH3)S)]ClO4:C, 21.97; H, 5.27; -S-H function (no 2500-cm-l absorption). The charges of
-
96 Inorganic Chemistry, Vol. 16, No. 1, 1977
Bennett et al.
all species in Table I were confirmed by ion-exchange the ethylene glycol chelate also occurred slowly. In acidic chromatography with the exception of entries 10-13 for which solution, C O ( ~ ~ ) ~ ( S C H ~ C very H ~ ~slowly H ) ~changed + color the formula weights of their isolated salts were confirmed to what may be a mixture of hydrolysis and redox products.2e (within 3.5%) by calibrated vapor-phase osmometry. Thus, all complexes retain their integrity in solution over the times of the redox reactions described below. Chromatographic, electronic, and 'H NMR absorption data Reduction by Chromium(I1) Ion. The stoichiometric ratio for the alkoxy chelates were collected under conditions apA[Co(III)]/A[Cr(II)] typically ranged from 0.87 to 0.99 with propriate to their acid-base characteristic^.^ Titrations 0.93 f 0.04 as a mean for the reactions studied; with Coconfirmed that Co(en)z(SCH2CH20)+ is monobasic with a (en)2(HOCH2CH20H)3+ and C O ( ~ ~ ) ~ ( H O C H ~ C aH ~ S ) ~ + pKa = 6.35 ( I = 0.30 M) for its conjugate oxy acid in line with 1:l ratio was assumed. Only for the reaction of Co(en)zprevious a r g ~ m e n t s .No ~ evidence for the protonation of our (NH2CH2CH2SCH3)3+could no evidence be found for ligand coordinated thiolates was encountered according to previous transfer to ~ h r o m i u m ( I I I ) .In ~ ~all other cases the primary criteria.5 reactions are inner sphere as indicated by mechanistic features This conclusion is supported by the 'H NMR results in and ion-exchange and/or spectral characteristics of the priTable 11. As with C O ( N H ~ ) ~ O C Hproton ~ ~ + ,resonances ~~ mary or secondary chromium(II1) products which, except with on the carbon adjacent to deprotonated alcoholate oxygen or the C O ( ~ ~ ) ~ ( H O C H ~ C H ~ O and H ) ~Co(en)z+ thiolate sulfur bound to cobalt(II1) are characteristically ( S C H Z C H ~ O H ) reactions, ~ + were isolated by ion exchange. shifted upfield from the corresponding resonance in the These products are treated subsequently1° and will be discussed protonated free ligand. Methylene and methinyl resonances here only inasmuch as mechanistic issues demand. are shifted 0.35-0.70 and -0.15 ppm, respectively. ProAll of the reactions were found to be pseudo first order in tonation of alcoholate oxygen shifts the adjacent resonance the cobalt(II1) complex and first order in chromium(II), except back toward or beyond the free ligand resonance. No such where noted, usually over a tenfold variation in the latter in change is detectable on acidification of thiolate complex the vicinity of M. Thus, the rate law solutions. Thus, spectral, potentiometric, and 'H NMR measurements all reflect the very low basicity of coordinated -d [Co(III)]/dt = k [Co(III)] [Cr(II)] thiolate sulfur in these cationic complexes. @-Methylresoobtains at any particular acidity. nances undergo less dramatic upfield shifts (see entries After reaction with Co(en)2(SCH2COO)+, Co(en)z(11-19,22) (I1 refers to Table 11). In contrast, the thioether (SCH(CH3)COO)+, and C O ( ~ ~ ) ~ ( S C H ~ C H ~(and NH~)~+ methyl resonance shifts downfield on coordination. presumably C O ( ~ ~ ) ~ ( S C H ~ C H ~although O H ) ~ this + was not Electronic absorption studies complement other results in checked) the isolated chromium(II1) product exhibited the confirming the chelate formulations of Table I which, for ultraviolet absorption characteristic of chromium(111)-thiolate Co(en)z(SCH2COO)+ and C O ( ~ ~ ) ~ ( S C H ~ C H ~are N H ~ ) ~coordination.1°J2 +, This and the absence of alternative bonding also supported by x-ray s t r ~ c t u r e s . As ~ ~ with chromiumsites for chromium(I1) on C O ( ~ ~ ) ~ ( S C H ~ C H ~es-N H ~ ) ~ + (II1),loJ2thiolate coordination gives rise characteristically to tablished a thiolate-bridging mechanism for this reaction. The intense ultraviolet sulfur-to-metal charge-transfer transitions. rate, k = 3.3 X lo4 M-' s-l at 25 OC, was independent of With Co111(en)2the energies are insensitive to variation in the acidity over the range studied, 0.040-0.50 M. Linear reauxiliary chelate function and, surprisingly, to methylation at gression of 23 individual runs at four temperatures between sulfur. 8 and 25 OC led to values of A P = 7.26 f 0.28 kcal/mol and In their XI ('AIg lTzg) and X2 (lAlg lTlg) d-d AS*= -13.5 f 1.0 eu which reproduced experimental averages transitions the alkoxy chelates exhibit higher molar absorpwithin 1.6% except at 15 OC where the experimental average tivities than their M"'(en)2(monodentate)2 analogues. For was ~ 4 lower. % example, X (e) for cis-Co(en)z(OH)(OOCCH3)+(512 (102), Rates with Co(en)2(SCH2COO)+ and Co(en)2365 (85)) and c~~-CO(~~)~(H~O)(OOCCH~)~+ (498 (loo), (SCH2CH20H)2+ called for competitive experiments against 360 (77.5))31can be compared with data for entries I-2,3,5,6 truns-Co(en)2C12+ which is reported to react with Cr(I1) with and their monodentate hydrolysis products; vide infra. Data k2 = (6.6 f 0.4) X lo6 M-l s-l at [H+] = 0,010 M, I = 0.10 are a ~ a i l a b l e for ~ ~ ,similar ~ ~ comparisons with entries IM (HC104-NaC104), and 18.6 0C.36 Under these conditions 8,9,12,13. The thiolato and thioether complexes have still and assuming second-order rate laws, the respective rate higher extinction coefficients, presumably through intensity constants were found to be kl = (6.4 f 0.5) X lo6 and kl = borrowing from the ultraviolet absorption. The data suggest (6.0 f 0.8) X lo6 M-' s-1.36 No variation with acidity was very similar ligand field strengths for thiolate and alcoholate detectable down to 0.005 M. Thiolate bridging is almost functions and a shift to higher energies on methylation of sulfur certainly operative for both fast reactions. An alcohol bridge reminiscent of that occurring on protonation of oxygen. The is expected to be even less effective than a water bridge.37 The acidity characteristicsg and the nature of the reactions derate of 2.7 X lo2 M-l s-l recorded below for Co(en)zscribed below are also consistent with the formulations (OOCCH2SCH3)2+reduction represents, at the least, an upper presented. limit for carboxylate bridging. Carboxylate bridging with Co(en)2(00CCH2S)+ would require an unprecedented Decomposition Reactions of the Cobalt(II1) Complexes. nonbridging ligand acceleration by 2 X lo4 above this rate. None of the complexes incorporated water on recrystallization. With C O ( ~ ~ ) ~ ( H O C H ~ C H ~ and N H ~Co(en)z)~+ Moreover, in the accompanying paperlo we shall describe a ( C H ~ S C H ~ C H Z N Hno ~ )anated ~ + products could be isolated fleeting chromium(II1) product of this reaction, believed to from hot (90 "C) 3 M HBr34 nor was there appreciable be Cr(Hz0)sSCHzCOOH2+, which gives rise to chelated spectral change for the former complex over a period of 2 Cr(H20)4(SCH2C00)+ and then, under the same conditions, weeks in 1.0 M HC104. Co(en)2(SCH2COO)+ and Coto C ~ ( H ~ O ) S ( O O C C H ~ S H ) ~ + . The rate with a 2-methyl substituent in Co(en)z(SCH(en)2(SCH2CH2NH2)2+ undergo negligible decomposition (CH3)COO)+, k = 1.54 X lo5 M-' s-l at 25 OC and I = 0.10 over extended periods in acid,12csuggesting a comparable M, could be followed at 282 nm and is independent of [H+] inertness for Co(en)2(SCH(CH3)COO)+. In 0.1 M HC104, 0.10 and 0.010 M. Studies at four temperatures CO(~~)~(HOCH~CO and O )C~O+( ~ ~ ) ~ ( H O C H ( C H ~ ) C O O ) ~between + s-l) to monoapparently dechelate slowly ( k i= 1.8 X between 25 and 45 OC yielded AlP = 1.1 f 0.2 kcal/mol and AS*= -31 f 1 eu. Evidence analogous to that adduced for dentate aquocarboxylate products as judged by their spectral parameters X (e) (498 (-loo), 360 (-80)). Hydrolysis of the preceding case supports a thiolate-bridged mechanism.
-
-
Inorganic Chemistry, Vol. 16, No. 1 , 1977 97
Bis(ethylenediamine)cobalt(III) Complexes Analogous alcoholate-bridged pathways occur for the reduction of each of the alcohol complexes for which reaction of a protonated and a deprotonated complex can be envisaged, i.e. k
Co(en),ROH"+
+ Cr2+a
with Co(en),RO(n'L)+
+ Cr2+k d
+
Co(en),ROHn+ = Co(en),RO("-')+
+ H+
K,
The appropriate differential rate expression is -d[ColI1]/dt = kobsd[COIII]total where kobsd = kz[Cr(II)]. In general kz =
+ k d K a / [Htl 1 + K a / [H+I
kp
Cr(I1) is independent of acid concentration over the range 0.010-0.10 M with k = 274 f 9 M-I at 25 OC. Studies at four temperatures between 20 and 37 OC yielded AHS = 8.44 f 0.05 kcal/moi and AS* = -19.1 f 0.2 eu. Cr(H20)5(00CCH2SCH3)2+ was the only reaction product isolable by ion exchange (-90%). Under conditions where t1/2 c 0.03 s for the redox reaction ([Cr(II)] = 0.093 M) no absorbancy changes were detectable on the Cary 14 after 30 s at 570 nm, 60 s at 440 nm ([Co(III)] = 4.1 X or 120 s at 285 nm ([Co(III)] = 1.6 X These observations appear to exclude the intermediates, Cr(H20)5(CH3SCH2COOH)3+ and Cr(H20)4(CH3SCH2C00)2+, required by a thioether-bridged mechanism yielding the observed product and, supported by earlier arguments,* to substantiate a carboxylate bridging mechanism. The rate of reduction of C O ( ~ ~ ) ~ ( N H ~ C H ~ C H ~ S C H ~ ) ~ by chromous ion was found to be independent of acid between 0.050 and 0.50 M with k = 0.381 f 0.012 M-l at 25 OC. Three or four runs at each of three other temperatures between 12 and 32 OC yielded 0 = 5.48 f 0.19 kcal/mol and AS* = -42.1 f 0.6 eu. Only Cr(H20)~j~+ could be isolated by ion exchange. This and the absence of any evidence for subsequent reaction when the redox reaction was monitored at 487 nm for 3-6 half-lives under first-order conditions where t1/2 70 s are consistent with the reaction being outer sphere. Nevertheless, because of the slowness of the reaction and the rapidity of chromium(III)-carbon bond hydrolysis38we felt obliged to test the possibility that the redox reaction might be initiated by chromium(I1) cleavage of a carbon-sulfur bond, i.e. Crl+ t CO(~~),(CH,SCH,CH,NH,)~+
which reduces to k2 = kp + kdKa[H+]-' when Ka/[H+] > 1. With C O ( ~ ~ ) ~ ( H O C H ~ C H reaction ~ N H ~ is) ~inner + sphere (4+ chromium product by ion exchange) with no alternative to alkoxy bridging. The inefficiency of the protonated path37 is underscored by 19 runs at 0.50 M > [H+] > 0.050 M which conformed to the equation k2 = (-0.041 f 0.022) (0.166 f 0.002) [H+]-l, the first term being indistinguishable from k, = 0. At the lowest feasible initial acidity, 5.5 X M, M (to avoid complications from and [Co(III)] = 6.0 X metal hydroxides) k2 = 935 f 50 M-I s-l N kd. Eleven runs at three temperatures between 12 and 25 OC yielded A H d * = 5.14 f 0.54 kcal/mol and A&* = -27.7 f 1.9 eu which reproduced experimental averages within 1.4%. From the kinetic data an apparent Ka = 1.78 X at I = 1.0 M can CrCH,'+ + CO(~~),(.SCH,CH,NH,)~+ be derived in reasonable agreement with values of Ka = 3.47 X lo4 ( I = 0.10 M) and 2.58 X ( I = ?) obtained directly Three hours after initiation of the reaction described in the at lower ionic strength^.^^^^ Experimental Section no appreciable methane (or evacuable In less extensive experiments, data on Co(en)2gas other than argon) could be detected by standard high(HOCH2CH20H)2+ over the range 0.10 M > [H+] > 0,010 vacuum and gas chromatographic procedures. Accepting this M conformed to the equation k2 = (-16.2 f 11.2) + (18.0 reaction as being outer sphere in the absence of contrary f 0.2) [H+]-l, the first term again being indistinguishable from evidence, the kinetic parameters of the chromous reductions determined at I = 0.10 M9 was used zero. A Ka = 2.5 X are tabulated by reaction category and considered further to estimate kd N 7.2 X lo3 M-l s-l at 25 OC. under Discussion. Over about the same acid range for Co(en)2Reduction by Vanadium(II) and Hexaammineruthenium(I1) (HOCH2C00)2+, k2 = (38.4 f 0.7) (0.988 f 0.015)Ions. The results with these reductants are summarized in [H+]-I, while for CO(~~)~(HOCH(CH~)COO)~+, k2 = (7.44 Table 111. The stoichiometricratio A[Co(III)]/A[V(II)] was f 0.12) (0.0221 f 0.0014)[H+]-1. The first term for each found to range from 1.03 to 1.06 by ion-exchange separation must be associated with carboxylate bridging in view of the and spectral analysis of the excess cobalt(II1) complex. The bridging inefficiency documented earlier for the protonated rate laws were first order in [V(H20)62t] over 4-5-fold alkoxy function and arguments advanced previ~usly.~A variations near M and independent of [Ht] over the directly measured Ka = 4.36 X ( I = 1.0 M) (which did ranges indicated. Reasonable bridging groups are present in not vary significantly with ionic strength down to I = 0.10 M the first three complexes, and, according to well-established as has been found elsewhere34)was used to estimate kd = 51 riter ria,^^?^^ all rate parameters are consistent with innerM-l s-l for the latter complex. At an initial [H+] = 8.0 X sphere reactions governed by rate-limiting substitution on lov5 M direct rates of kd = 50 M-' were observed but with vanadium(I1). With Co(en)2(HOCHzCHzOH)3+, however, 20% fluctuations in value. With C O ( ~ ~ ) ~ ( H O C H ~ C O Othe ) ~activation ~ parameters are similar to those observed with a directly measured Ka = 1.0 X lo3 ( I = 1.0 M) yielded kd U022+ 41 on which basis an outer-sphere mechanism has been = 0.99 X lo3 M-' s-I whereas direct kinetic measurements assigned.40 Our results presumably reflect again the bridging at [H+] = 1.0 X lo4 M gave kd = 1.9 X lo3 M-ls-l. Limited inefficiency of the ROH function and an outer-sphere retemperature studies between 15 and 35 OC under the latter duction of C O ( ~ ~ ) ~ ( H O C H ~ C H ~ O H ) ~ + . conditions indicated that AHd* = 2.2 kcal/mol and ASd* c Slow reductions by R u ( N H ~ ) ~were ~ + complicated by its -36 eu for the deprotonated path. The kinetic comparisons slow decomposition at higher ~oncentrations.13~~2 Stoinow available with noncarboxylate alkoxy chelate ligands chiometries were assumed to be 1:l and only data obtained ensure that this path utilizes alkoxide bridging. within the first hour after preparation were considered reliable The rate of reduction of C O ( ~ ~ ) ~ ( O O C C H ~ Nk H=~ ) ~ +for , determination of the rate constant under second-order 2.22 M-' s-l at 25 OC, was independent of acidit between conditions. 0.010 and 0.10 M and Cr(H20)5(00CCH2NH3)Y+was the Discussion initial product.1° Studies at three temperatures between 13 and 34 "C yielded A H ' = 8.93 f 0.13 kcal/mol and AS* = Synthesis of the Cobalt(II1) Complexes. Our thiolate -27.0 f 0.4 eu. syntheses were designed5on the basis of principles underlying The rate of reduction of C O ( ~ ~ ) ~ ( O O C C H ~ S C byH ~ ) ~the + preparation of CrSH2+,27i.e., the l-equiv oxidative ad-
+
-
+
+
+
N
98 Inorganic Chemistry, Vol. 16, No. I, 1977 Table 111. Kinetic Results with V(H,O),
Bennett et al.
'+ and Ru(NH,), '+
A. V(H,O), '+,Z= 1.00 M (LiClO,-HClO,)
k(25 "c), M-' Complex Co(en), (OOCCH, S)'
[H+], M 0.05-0.5
Co(en)z(OOCCH,OH)'t
0.01-0.5
15.3
Co(en),(SCH,CH,0H)2+
0.01-0.5
6.5
s"
9.6
C O ( ~ ~ ) ~ ( H O C H ~ C H0.1-0.5 ~ O H ) ~ 40.6 ~
Table IV. Kinetic Parameters for Outer-Sphere Chromium(II)-Cobalt(III) Pathways
AH+,
kcall mol 11.6 i 0.5 10.2 i 0.4 12.0 i 0.7 7.4 i 0.3
1
AS*,eu -15.1 * 1.6 -19.0 * 1.5 -14.5 t 2.4 -26.4 * 1.0
2
HC1-NaC1
I, M
[H+], M
10, [CO- i o 3[RU- k(2s "c), (III)], M (II)], M M-' S-'
(a) Co(en), (OOCCH,0H)2+ 0.15 0.100 5.00 2.33
3.2
(b) Co(en) (OOCCH, O)+
NH,Cl-NH, NH,Cl-NH,
0.50 0.50
-5 X lo-'' -5 X lo-''
47.5 45.9
HC1-NaC1 HCl-NaC1 HCl-NaCl
(c) Co(en),(OOCH,S)+ 0.15 0.10 5.00 0.10 0.0010 0.198 0.10 0.0010 0.195
30.0 12.5 1.83 0.136 0.140
