Observations on Atom-Transfer Reactions - ACS Symposium Series

7 Observations on Atom-Transfer Reactions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 13, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch007

HENRY TAUBE Stanford University, Department of Chemistry, Stanford, CA 94305

Some of the issues involved i n trying to understand atom transfer processes are i l l u s t r a t e d by a discussion of the observations on the reduc t i o n of perchlorate i o n . No examples are known of oxygen removal from ClO - under mild conditions by 4

orthodox nucleophilic agents, and all the reac tions which have been studied i n any d e t a i l in volve metal ions. The stability of the bond to oxygen, formed i n the oxidized product, appears to be very important i n affecting rates. In all of the cases i n which reaction takes place at room temperature at a reasonable rate, the for mation of an "yl" product--vanadyl, for example, 2+

when V (aq) i s the reducing agent--is possible. A r a t i o n a l i z a t i o n i s offered of the unsymmetrical coordination which i s characteristic of "yl" ions. The t i t l e i n d i c a t e s a r e t u r n on my p a r t to a c t i v e research i n a c l a s s o f redox r e a c t i o n s to which I devoted c o n s i d e r a b l e a t t e n t i o n q u i t e e a r l y i n my c a r e e r . Tracer experiments by H a l p e r i n (1) i n 1950 showed t h a t when C 1 0 " and C 1 0 " i n a c i d i c 3

2

aqueous s o l u t i o n r e a c t w i t h S ( I V ) , r e s p e c t i v e l y 2.4 and 1.4 of the oxygen atoms c a r r i e d by the oxidants appear i n the p r o duct s u l f a t e . We concluded from these and a d d i t i o n a l r e s u l t s t h a t the redox changes i n v o l v e i n t i m a t e contact o f the reagents, and

that

oxygen

CK>2

CIO

plete

transfer

transfer

is essentially occurring i n

in

the

steps

ClO^

•+ ClO^

and

q u a n t i t a t i v e , the defect from com the

H0C1 stage.

The f i r s t

step

can be understood then on the b a s i s t h a t SO^ adds to an oxygen of CIO,.

(S(IV) i n a c i d i c s o l u t i o n i s present l a r g e l y as S 0 ) ; 9

0097-6156/82/0198-0151$08.25/0 © 1982 American Chemical Society In Mechanistic Aspects of Inorganic Reactions; Rorabacher, David B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

MECHANISTIC ASPECTS OF

152 s u b s t i t u t i o n on produce

ClO^

INORGANIC REACTIONS

, even i n a c i d , i s slow (vide

i n f r a ) to

:θ: :Ô: :ë:Cl:«:S:0: which then can decompose to C i e 2-

+ 8 S 0 , or by involvement of 9

+

H 0, d i r e c t l y to C l & + QSO^ + 2H . These conclusions about mechanism though s a t i s f y i n g and i n accord w i t h the observations, f a i l to engage the b a s i c i s s u e of how the r e a c t i o n r a t e can be p r e d i c t e d from p r o p e r t i e s of the r e a c t a n t s . Before proceeding f u r t h e r , the sense i n which "atom t r a n s f e r " w i l l be used i n t h i s paper should be defined. The term i m p l i e s t h a t an atom o r i g i n a t i n g i n the o x i d i z i n g agent i s t r a n s f e r r e d to the reducing agent so t h a t i n the a c t i v a t e d com p l e x both centers are bound to the atom being t r a n s f e r r e d . In u s i n g the term i n t h i s way, no p o s i t i o n i s taken on the q u e s t i o n of whether the e n t i t y t r a n s f e r r e d i s i n an atomic s t a t e , or whether the r e a c t i o n can be b e t t e r understood as i n v o l v i n g e l e c t r o n flow from reducing agent to the o x i d i z i n g agent concomitant w i t h the motion of the l i g a n d i o n from the oxidant to the r e ductant. This d i s t i n c t i o n i s meaningful only i f one p o i n t of view, as a f i r s t approximation, more r e a d i l y c o r r e l a t e s a set of observations. The term "atom" i s used o n l y t o d i s t i n g u i s h the r e a c t i o n s being considered from the c l a s s i n which l a r g e b r i d g i n g l i g a n d s are t r a n s f e r r e d by remote a t t a c k ( 2 ) . Many of the l a t t e r i n v o l v e only weak e l e c t r o n i c i n t e r a c t i o n s i n the a c t i vated complexes and, f o r t h i s reason, are s e t apart from the cases where o x i d i z i n g and reducing centers are separated by s i n g l e atoms, so t h a t e l e c t r o n i c c o u p l i n g i n the a c t i v a t e d com p l e x e s can be v e r y l a r g e . In some of the examples which w i l l be considered, i t appears to be very l a r g e indeed. Atom t r a n s f e r r e a c t i o n s , as d e f i n e d , are w i d e l y recognized as b e i n g extremely important, both i n the sense t h a t they pose b a s i c s i g n i f i c a n t questions, and, p a r t i c u l a r l y where carbon compounds are i n v o l v e d , as being extremely u s e f u l i n a p r a c t i c a l sense. N e v e r t h e l e s s , much more a t t e n t i o n has been devoted to the c l a s s of redox r e a c t i o n s l o o s e l y described as i n v o l v i n g "electron transfer." The reasons f o r t h i s are q u i t e under standable. I t i s d i f f i c u l t to estimate e l e c t r o n d e r e a l i z a t i o n energies from f i r s t p r i n c i p l e s , at l e a s t when the i n t e r a c t i o n s are l a r g e . For many outer-sphere e l e c t r o n t r a n s f e r r e a c t i o n s , the e l e c t r o n i c coupling i s weak enough so t h a t the energy of the a c t i v a t e d complex i s l i t t l e a f f e c t e d by e l e c t r o n d e r e a l i z a t i o n , but strong enough to ensure t h a t the r e a c t i o n s are a d i a b a t i c . This s i m p l i f i c a t i o n does not seem t o apply to most of the reac t i o n s which we w i l l consider. The focus of t h i s paper i n terms of d e s c r i p t i v e m a t e r i a l i s 2

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 13, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch007

o

2

In Mechanistic Aspects of Inorganic Reactions; Rorabacher, David B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

7.

TAUBE

Observations on Atom-Transfer

Reactions

153


on oxygen atom t r a n s f e r , s p e c i f i c a l l y i n the r e d u c t i o n o f ClO^ . This choice was made f o r s e v e r a l reasons. P e r c h l o r a t e i o n i s a strong reducing agent and there are many r e a c t i o n s which a r e f a v o r a b l e i n terms o f AG°, but which, n e v e r t h e l e s s , can be very slow. The r a t e s o f r e a c t i o n s d i f f e r enormously, ranging from those t h a t a r e "too slow t o measure" t o some which proceed r e a d i l y under m i l d c o n d i t i o n s . Those t h a t have been measured show no obvious c o r r e l a t i o n w i t h d r i v i n g f o r c e . I t seems l i k e l y t h a t ideas o f general import w i l l emerge from a study o f the r e a c t i o n s . Such study i s i n progress i n my l a b o r a t o r i e s , though I must hasten t o add t h a t t h i s has not advanced f a r enough t o be featured s i g n i f i c a n t l y here. But before d e a l i n g w i t h the s p e c i f i c chemistry, i t seems appropriate t o make a few general observations by way o f p r e p a r a t i o n . Atom Transfer i n 2e

Changes

An important d i s t i n c t i o n i n redox r e a c t i o n s devolves on the question o f whether r e a c t i o n occurs by a l e o r 2e change. One-electron changes can take p l a c e by inner-sphere o r outersphere mechanisms, but most 2e changes seem t o i n v o l v e atom transfer. When the l i g a n d s themselves a r e not e a s i l y o x i d i z e d or reduced, as i s the case f o r most o r d i n a r y s a t u r a t e d ones, 2e changes c a l l f o r d r a s t i c r e o r g a n i z a t i o n o f the f i r s t co o r d i n a t i o n spheres o f the r e a c t i n g p a r t n e r s — e . g . , C l ( V I I ) C1(V) - C l ( I I I ) - C1(I) - C l ( - I ) ; Sn(IV) - S n ( I I ) ; P t ( I V ) P t ( I I ) ; V(IV) - V ( I I ) ; C 0 - H C 0 - H CO - ChÔH - CE^. When 2

2

2

2

the changes i n the c o o r d i n a t i o n spheres o f the r e a c t i o n p a r t n e r s are complementary, atom t r a n s f e r i s an economical way f o r r e a c t i o n t o be consummated, economical i n the sense t h a t the c r i t i c a l events i n v o l v i n g the changes i n the c o o r d i n a t i o n spheres a t the two centers a r e c o r r e l a t e d . The r e s u l t s described by Basolo and Pearson, e t a l . ( 3 , 5 ) , and by M a r t i n , e t a l . ( 4 ) , on the o x i d a t i o n o f P t ( I I ) by P t ( I V ) provides an elegant i l l u s 2+ tration of this point. The r e a c t i o n between Pt(NBL), and 2+ trans-Pt(NH^)^Cl i n v o l v e s a l s o C l and can be represented as f o l l o w s : 2

Cl

M i Pt

/\

\/iv 1 1

1 V

ClPt Cl

/\

\4v l v

ClPt Cl

/\

\/n Pt

1 1

Cl

/\

The simple motion o f a b r i d g i n g c h l o r i d e from P t ( I V ) t o P t ( I I ) concomitant w i t h motions o f the e x t e r i o r c h l o r i d e ions e f f e c t s the r e v e r s a l o f the o x i d a t i o n s t a t e s o f the metal. S a t i s f a c t o r y though the i n s i g h t i m p l i e d by the above may be, i t does not cover the important question o f what the a c t i v a t i o n b a r r i e r t o


MECHANISTIC ASPECTS OF INORGANIC REACTIONS

154

the r e a c t i o n i s , and how t h i s w i l l change from one system t o a r e l a t e d one. R e c i p r o c i t y of the changes a t the r e a c t i n g center by no means guarantees a f a c i l e r e a c t i o n . The process

Θ αβ~ + cio ~


3

3

-> ci$ ~ + ecio " 3

3

i s extremely slow (though j u s t how slow i s not known). The b a r r i e r i s undoubtedly o n l y p a r t l y e l e c t r o s t a t i c ; even i n the absence of such a b a r r i e r , as f o r example i n

ci« " + cio = cie + ecio ~ 3

2

2

2

r e a c t i o n can be extremely slow ( 6 ) . N u c l e o p h i l i c S u b s t i t u t i o n i n R e l a t i o n t o Redox Changes C e r t a i n redox changes i n v o l v i n g atom t r a n f e r can u s e f u l l y be d e a l t w i t h a p p l y i n g the ideas which have been developed f o r n u c l e o p h i l i c s u b s t i t u t i o n . There i s , i n f a c t , no sharp d i s t i n c t i o n between a 2e redox change i n v o l v i n g atom t r a n s f e r and an orthodox n u c l e o p h i l i c s u b s t i t u t i o n . This p o i n t i s i l l u s t r a t e d by the two r e a c t i o n s IBr + C l " =

ICI + Br"

BrCl + I"

IBr + C l "

=

where the second o n l y i s a redox process as o r d i n a r i l y defined. Orthodox n u c l e o p h i l i c s u b s t i t u t i o n i m p l i e s t h a t the n u c l e o p h i l e acts on a center which i s more e l e c t r o p o s i t i v e than the nucleo p h i l e ; when the center i s more e l e c t r o n e g a t i v e (Br more e l e c t r o negative than I ) the r e a c t i o n i s c l a s s i f i e d , f o l l o w i n g accepted conventions, as a redox change. This p o i n t was recognized q u i t e e a r l y by Edwards (7) i n c o r r e l a t i n g the rates a t which H 0 reacts w i t h a s e r i e s of nucleophiles. The Edwards equation 2

log ^

= OfE + βΗ ο where E^ and Η are p r o p e r t i e s

2

n

of the n u c l e o p h i l e

and or and β o f

the s u b s t r a t e , s u c c e s s f u l l y c o r r e l a t e s orthodox s u b s t i t u t i o n by nucleophiles a t carbon and a t H 0 (most o f the l a t t e r are 2

2

c l a s s i f i e d as i n v o l v i n g redox changes). According t o the an a l y s i s , the d i f f e r e n c e between C and 0 as substrates r e s i d e s i n α and β being l a r g e r i n magnitude f o r the l a t t e r . Typical values f o r the former are 2.0 and 0.07; f o r H 0 , α and β are 2

6.31 and -0.394.

2

The s p e c i f i c r a t e s f o r the r e a c t i o n s of I


and

7.

TAUBE


Br" w i t h H 0 9

Reactions

155

a r e , r e s p e c t i v e l y , 0.7 and 2.3 x 1θ" M " S " , the 5

9

1

1

r a t e r a t i o being 3 χ 10 . F o r s u b s t i t u t i o n a t C, t y p i c a l r a t i o s are 10 t o 100. I n Edwards' a n a l y s i s , no allowance f o r the con t r i b u t i o n t o the r a t e by d i f f e r e n c e s i n d r i v i n g f o r c e i s made. The value o f -AG° f o r the r e a c t i o n o f I w i t h H 0 i s g r e a t e r 2

2


than t h a t o f Br , the hypohalous a c i d and hydroxide i o n being taken t o be the products, by 15.8 k c a l mol f o r substitution i n CHÔH, the d i f f e r e n c e i s about zero ( 8 ) . I f i t were p o s s i b l e to f a c t o r out e f f e c t s a r i s i n g from d i f f e r e n c e s i n d r i v i n g f o r c e , the r a t e p a t t e r n f o r n u c l e o p h i l i c s u b s t i t u t i o n a t oxygen i n peroxides might prove t o be l i t t l e d i f f e r e n t from t h a t a t c a r bon. A particularly

important

r e a c t i o n o f HÔ^

i s oxygen ex

change between HÔ^ and HÔ, a r e a c t i o n which has f o r many years been known t o be slow enough t o make f e a s i b l e d e f i n i t i v e oxygen t r a c e r s t u d i e s on r e a c t i o n s o f HÔ^ wards'

i n aqueous s o l u t i o n . Ed

c o r r e l a t i o n y i e l d s 10 *^s * as the s p e c i f i c

rate f o r

t h i s exchange.

I t i s i n t e r e s t i n g t o note t h a t Anbar and Gutt-9 -1 mann (9) measured t h i s r a t e as 0.5 χ 10 s a t 60°. I f the a c t i v a t i o n energy i s assumed t o be 30 k c a l , a reasonable e s t i mate i n view o f the slowness o f the exchange, the s p e c i f i c r a t e a t 25C becomes 10 **s i n remarkably good agreement w i t h Edwards' estimate. Reduction o f CIO^ - General

not

U n f o r t u n a t e l y , the ideas on n u c l e o p h i l i c s u b s t i t u t i o n a r e p a r t i c u l a r l y u s e f u l i n t r y i n g t o understand the e x i s t i n g

data on the r e d u c t i o n o f CIO, . The reductions which have been s t u d i e d i n v o l v e metal i o n s . The mode o f a c t i o n o f s e v e r a l such p o t e n t i a l r e d u c t a n t s — S n ( I I ) a n d low s p i n C o ( I ) , f o r e x a m p l e — c a n p r o f i t a b l y be viewed i n the l i g h t o f ideas developed f o r nucleo p h i l i c s u b s t i t u t i o n , b u t t h i s i s not o b v i o u s l y the case f o r many o t h e r s — T i ( I I I ) o r C r ( I I ) , f o r example. Before i n t r o d u c i n g a d i s c u s s i o n o f the observations on t h e r e d u c t i o n o f CIO^ , some p r o p e r t i e s o f t h i s and r e l a t e d species which a r e germane t o the c e n t r a l i s s u e w i l l be considered. Of paramount importance a r e data on the s t a b i l i t y o f the s p e c i e s : how does t h e s t r e n g t h o f t h e bond between halogen and oxygen vary w i t h o x i d a t i o n s t a t e ? The e x i s t i n g data a r e summarized i n Table I . The e n t r i e s p e r t a i n t o the r e a c t i o n :



156

xo


n

+

*>

2

*

xo

INORGANIC REACTIONS

n+1

values of AG° r a t h e r than of ΔΗ° being chosen because the former are more complete. One trend i n the data seems reasonable enough: the i n c r e a s ing s t r e n g t h of the XO bond i n the s e r i e s CIO , BrO , 10 , the d i f f e r e n c e i n e l e c t r o n e g a t i v i t y i n c r e a s i n g i n the same order. Less easy t o r a t i o n a l i z e are the data on successive a d d i t i o n s of to the same halogen. The increase i n bond s t r e n g t h i n the s e r i e s CIO

, C10

2

and ClO^" might a t f i r s t s i g h t seem anomalous

because i t can reasonably be argued t h a t an i n c r e a s i n g content of the e l e c t r o n e g a t i v e oxygen w i l l c o n t r a c t the lone p a i r o r b i t a l s on halogen and, thus, s u c c e s s i v e l y weaken the Cl-0 bond. By way of q u a l i t a t i v e r a t i o n a l i z a t i o n , compensating counter v a i l i n g e f f e c t s can be found. Two such which have gained some currency i n d i s c u s s i o n s of t h i s k i n d are these: ( i ) lone p a i r r e p u l s i o n s w i l l decrease as the number of lone p a i r s on c h l o r i n e are decreased; ( i i ) the 3d o r b i t a l s become i n c r e a s i n g s t a b l e as the o x i d a t i o n number of the c e n t r a l atom i n c r e a s e s . As to the l a t t e r p o i n t , bond d i s t a n c e s are i n l i n e w i t h the idea t h a t the Cl-0

bond

character.

in

ClO^

has

especially significant

ClO^

great

deal

of

i s weaker than i n ClO^ i n view of the

i s reduced r e l a t i v e t o CIO^

.

Cl 22.5

X0~ •> X 0 "

11.6

X0 "

-3.7

2

2

-> X 0 " 3

with

which

F i n a l l y , we draw a t t e n t i o n e x p l a n a t i o n of

Table I Values of AG°(25C) f o r Reaction w i t h

-» X0~

bond

, a fact that

facility

to the f a c t t h a t there i s no ready simple data shown i n the l a s t row of Table I .

X

double

These arguments do not s u f f i c e to e x p l a i n the f a c t

t h a t the 0 bond i n CIO^" is

a

0~ 2

Br 16.6

3.8

(ΧΟ' -> X0 ~

7.9

12.5

-23.8)

XO ~ + XO," 3 4

0.1

24.6

19.1

3

-

the

Ref. 8, except f o r e n t r i e s i n bottom row, which are taken from r e f . 10.



7.

TAUBE

Observations

on Atom-Transfer

Reactions

157

This d i s c u s s i o n o f the e n e r g e t i c s o f the halogenate ions i s i n no way a d i g r e s s i o n . I n the f i r s t p l a c e , when the reac t i o n coordinate w i t h i n a s e r i e s i s the same, a connection be tween d r i v i n g f o r c e and r a t e can be expected. I n the second p l a c e , i t i s important t o r e a l i z e t h a t , i f we are unable t o understand the e q u i l i b r i u m s t a b i l i t y i n a b a s i c way, we a r e h a r d l y i n p o s i t i o n t o d e a l e f f e c t i v e l y w i t h the s t a b i l i t i e s of the a c t i v a t e d complexes. Increased concern w i t h the s t a b i l i t i e s o f t h i s important f a m i l y of molecules, both a t the ex perimental and t h e o r e t i c a l l e v e l s , i s j u s t i f i e d . A l s o important f o r understanding the redox chemistry o f the oxyanions i s t h e i r s u b s t i t u t i o n l a b i l i t y . This can be i n f e r r e d from the r a t e o f exchange o f oxygen between the oxy anions and water, and f o r the more h i g h l y o x i d i z e d members o f the f a m i l y t h i s i s v i r t u a l l y the o n l y s u b s t i t u t i o n r e a c t i o n which lends i t s e l f t o d i r e c t study. As t o p e r c h l o r a t e i o n : s u b s t i t u t i o n takes place extremely s l o w l y , so s l o w l y t h a t the r a t e o f t h i s r e a c t i o n has never been determined. Hoering, e t a l . ( 1 1 ) , r e p o r t t h a t there i s no exchange i n 63 days a t 100C i n 9.5 M HC10,, and the s p e c i f i c r a t e can s a f e l y be s e t as -8-1 < 10 s . As i s the case w i t h other oxyanions, the r a t e i s expected t o decrease s h a r p l y w i t h a c i d i t y , and thus the r a t e of exchange i n 1 M HCIO^ would be very slow indeed. I ti s remarkable reducing extremely

that

agents

oxygen exchange much

slow.

f o r BrO^ , which r e a c t s

more r a p i d l y than does CIO^ , i s a l s o

Appelman, e t a l . ( 1 2 ) , r e p o r t

exchange between Br0^~ and hÔ i n 0.06 M HNO^ a t 94C.

The s u b s t i t u t i o n l a b i l i t y o f C10^

than t h a t o f CIO^ , suggesting the

with

two systems i s d i f f e r e n t .

l e s s than 1% a f t e r 19 days

(11) i s much g r e a t e r

t h a t the a c t i v a t i o n process f o r A p o s s i b i l i t y f o r ClO^ , which

i s much l e s s l i k e l y f o r CIO^ , i s a t t a c k by the incoming nucleo p h i l e , here HÔ, on c h l o r i n e , i n an S 2 process. Hoering, e t a l . (11), have shown t h a t , on t h i s b a s i s , a s u c c e s s f u l c o r r e l a t i o n o f the r a t e o f oxygen exchange w i t h the r a t e s o f r e d u c t i o n by the h a l i d e ions i s achieved. I n Table I I , t h e i r r e s u l t s are summarized. Since oxygen exchange must i n v o l v e s u b s t i t u t i o n a t c h l o r i n e , the success o f the c o r r e l a t i o n i m p l i e s t h a t the h a l i d e s , i n being o x i d i z e d , a l s o a t t a c k a t c h l o r i n e . I n the ab sence o f evidence t o the c o n t r a r y , the redox r e a c t i o n s could a l s o reasonably be taken as proceeding by a t t a c k a t oxygen. Before c o n s i d e r i n g the few systems i n which the r a t e o f N

reduction its

of CIO^

reducibility

has been measured, some general are i n order.

Perchlorate

comments on

ion i s resistant



158

t o a t t a c k by e l e c t r o n s i n l i q u i d ammonia and i n water. This i s not a l t o g e t h e r s u r p r i s i n g . On the one hand, the i n t a c t mol ecule has no l o w - l y i n g o r b i t a l s t o accommodate the e l e c t r o n ; on the o t h e r , the process CIO," + e" = C10 + 4 3


o

2

0"

cannot be expected unless some group i s a v a i l a b l e t o s t a b i l i z e 220 . Protons would, o f course, s t a b i l i z e 0 but are incompat ible with e or e P e r c h l o r a t e r e s i s t s a t t a c k by orthodox am aq n u c l e o p h i l i c reagents, a t l e a s t under m i l d c o n d i t i o n s . Espe c i a l l y important among the n u c l e o p h i l i c r e a c t i o n s i s the process J

C1*0," + CIO " = C1*0 " + 4 3 3

CIO," 4

Table I I N u c l e o p h i l i c A t t a c k on ClO^

Rate Law:

+

k[Nucl][H ][C10 "] 3

1

Nucl.

-

k , , M" s" obs*

1

k

1

, -, M" s" calc

9

H0

1.6 X ΙΟ"

ci" £

4.5 X ΙΟ"

5

4.7 X 10"

5

Br" £

9.3 X ΙΟ"

5

7.1 X 10"

5

Γ

2.0 X 10"

1.8 X 1θ"

3

2

£

1

3

- At 25C - Using l o g k/k

Q

= crE^ + βΗ and v a l u e s of Ε

β

and Η

as t a b u l a t e d by Edwards. £

Ref. 13.

which i s the analog i n atom t r a n s f e r t o the " s e l f exchange" reactions f o r electron transfer. Conceivably, an e l a b o r a t i o n of the Marcus type of c o r r e l a t i o n a l s o u n d e r l i e s atom t r a n s f e r processes, though i t may be d i f f i c u l t t o formulate i f , as seems l i k e l y , many o f these r e a c t i o n s do not conform t o the weak over l a p l i m i t . For t h i s reason alone, i t i s important t o have meas-


7.

TAUBE


Reactions

159

urements of r a t e s such as these, slow though the r e a c t i o n s are. ( I t i s i n t e r e s t i n g t o note i n p a s s i n g t h a t the o x o c h l o r i n e system of species a l s o provides atom t r a n s f e r process

access t o an example of the l e

ci*o + cio " = ci*o ~ + cio 2

3


General experience,

3

2

u s i n g p e r c h l o r a t e media f o r redox r e a c t i o n s 2-

of other reagents, teaches t h a t the r e d u c t i o n of ClO^

by S 0

3

,

P ( O R ) , S b ( I I I ) , or S n ( I I ) i s a l s o extremely slow, so slow t h a t 3

complications

a t t r i b u t a b l e t o the r e a c t i o n of CIO^

w i t h these

reagents has not been reported. Slow though these r e a c t i o n s a r e , they must occur a t a f i n i t e r a t e , and i t would be both i n t e r e s t i n g and u s e f u l i f the r a t e s were known. Much can un doubtedly be surmised about the r a t e s o f r e a c t i o n of CIO^ with " n u c l e o p h i l i c " reducing agents from the r e s u l t s which have been obtained w i t h BrO^ as o x i d i z i n g agent (12), s e v e r a l of these r e a c t i o n s being r a p i d enough f o r convenient study. I n the two cases among those s t u d i e d i n which d e f i n i t i v e oxygen t r a c e r experiments are p o s s i b l e , namely w i t h S(IV) and A s ( I I I ) as r e ductants, i t was found t h a t one atom of oxygen i s t r a n s f e r r e d to the

reducing

agent when i t i s o x i d i z e d by BrO^

.

The

spe

c i f i c r a t e , k, i n the r a t e law diBrO "] = k[S0

Z 3

] [Br0

]

4

-3 -1 -1 i s 5.8 χ 10 M s a t 25C, which seems remarkably r a p i d con s i d e r i n g the charge types of the reagents. The r e a c t i o n of S(IV) w i t h BrO^" i s 24.5 k c a l m o l " more exothermic than t h a t 1

w i t h ClO^

.

I t i s reasonable t o suppose t h a t the r e a c t i o n co

ordinates

f o r the two

r e a c t i o n s are s i m i l a r .

I f i t i s assumed

then t h a t h a l f of the d i f f e r e n c e i n d r i v i n g f o r c e i s r e f l e c t e d 2i n the r a t e , the s p e c i f i c r a t e f o r the r e a c t i o n of SO~ with CIO, would be 10 Ms which corresponds to a h a l f - l i f e f o r 23 ClO^ i n 1 M S0 (aq) of 2 χ 10 y r . The r e a c t i o n i s probably 3

more r a p i d than t h i s and the r a t e may w e l l be measureable i f a j u d i c i o u s choice of c o n d i t i o n s i s made. The

r e a c t i o n of C10 " 3

w i t h a number of the n u c l e o p h i l i c


160


INORGANIC REACTIONS

reagents mentioned and w i t h C l , Br , and I a l l proceed at r a t e s which are r a t h e r conveniently measureable. In a l l the cases which have been s t u d i e d , o n l y the paths i n v o l v i n g a l s o protons i n the a c t i v a t e d complex have been uncovered. This comment takes on s i g n i f i c a n c e i n the l i g h t of the observations


(12)

made on

the

o x i d a t i o n by

BrO^~

or

B r 0 ~ of S ( I V ) . 3

In

a c i d i c s o l u t i o n , the l a t t e r r e a c t s more r a p i d l y than the former, but i n a l k a l i n e s o l u t i o n the reverse i s t r u e . These observa t i o n s are c o n s i s t e n t w i t h the idea t h a t r e d u c t i o n of BrO^ i n v o l v e s s u b s t i t u t i o n on oxygen, w h i l e r e d u c t i o n of BrO^ in volves a t t a c k on Br. There i s no p a r t i c u l a r reason f o r protons to a s s i s t i n the former case; i n the l a t t e r , protons would be 2u s e f u l i n weakening the Br(V)-0 bond \ S:

-»

/

:6: H :Br | :6:H :0: ·'

|

Since the r e l a t i v e r e a c t i v i t i e s of ClO^ and C10^ do not r e f l e c t the r e l a t i v e strengths of the Cl-0 bonds i n the two mole c u l e s , i t i s reasonable to suppose t h a t a d i f f e r e n c e i n mecha nism s i m i l a r t o t h a t proposed f o r BrO, vs. Br0« holds f o r the C10 '-C10 " case. 4

4

0

3

Reduction of ClO^ We

t u r n now

by Metal Ions to the r e d u c t i o n of C10^

by t r a n s i t i o n metal

i o n s , s e v e r a l of which are known to r e a c t under q u i t e m i l d con ditions. A l i s t i n g of a number of t r a n s i t i o n metal i o n redox couples appears i n Table I I I . These are c l a s s i f i e d i n t o the c a t e g o r i e s : lanthanides and a c t i n i d e s , f i r s t row transition s e r i e s , second row t r a n s i t i o n s e r i e s . A survey of the r a t e data f o r the reducing agents which are featured i n Table I I I f o l l o w s , but the r a t e data f o r the ruthe nium couples are reserved f o r l a t e r d i s c u s s i o n because they f e a t u r e some p o i n t s of s p e c i a l i n t e r e s t . I n d i s c u s s i n g the r e maining couples, we w i l l d e a l f i r s t w i t h those systems i n which data on the r a t e of r e d u c t i o n of CIO, have been recorded. As 4 an exception to the behavior of s e v e r a l much stronger reducing 3+ agents l i s t e d i n Table I I I , we have T i , which reduces ClO^, r a p i d l y enough so t h a t an mination 25C,

of CIO,

μ = 1.0,

a n a l y t i c a l procedure f o r the

i s based on

t h i s process.

For

deter

r e a c t i o n at

the r a t e law:



4 +

+

3 +

= Eu

3 +

= U

= Yb

3 +

2

+

2 +

2 +

3 +

+

= V

2 +

+ Cr

3 +

= Cr

2 +

2 +

2 +

3 +

3 +

3 +

= V

= V

= V

3 +

= U

2e" + Cr(IV) = C r

e

2

+

2 +

2 +

2e" + 4 H + V 0

+

e" + 2 H + V 0

+

2e" + 2 H + V 0

e" + V

e" + T i ( I V ) = T i

2e" + 4 H + U 0

e" + U

e" + E u

e" + Y b

2

+ 2H 0

2

+ H0

2

+ H0

2

+ 2H 0

Ref,

4

1 M HC10

-0.13

>0.9

b

-0.41

0.68

Continued on next page.

8

8

8

8

0.05 0.36

8

8

16

15

-0.25

-0.1

4

1 M HC10

-0.63

8

Conditions

-0.43

a

14

f

-1.15

E° o r E

Table I I I A Roster o f Redox Couples



2

5

2

6

2 +

2

2 +

2 + 2

2

2+

= Ru(bpy) pyOH

= Ru(bpy) pyOH

2

+ Ru(bpy) py0

2

3+

3

Ru(NH ) H 0

6

2 +

2

2+

0.89

0.78

0.23

0.066

0.051

f

a

μ = 0.10

μ = 0.10

μ = 0.2

0.1 M NaBF, 4

Conditions

(c) The e l e c t r o c h e m i c a l r e s u l t s c i t e d i n Ref. 14 are i n good agreement w i t h those reported by Matsubara and Ford (18).

3+ (b) Estimated on the b a s i s o f the o b s e r v a t i o n (17) t h a t Cr i n a c i d i c s o l u t i o n i s much l e s s e f f i c i e n t i n scavenging atomic c h l o r i n e than i s Co 2+ , Ce 3+ or even P r 3+

(a) E° understood unless c o n d i t i o n s given.

2e" * + 2H

+

2

+ Ru(bpy) pyOH

=

= Ru(H 0)

e

6

2

3 +

2

+ Ru(H 0)

5

3 +

3

= Ru(NH )

e

3

+ Ru(NH ) H 0

6

e

3

+ Ru(NH )

e

3 +

E° or E

Table I I I (cont.) A Roster o f Redox Couples


21

21

20

18

18

Ref,


TAUBE

7.

163

Reactions

3+ ]

-

3 +

= [ T i ] [ C 1 0 " ] ( 1 . 8 9 χ 10~

4

4

+

+ 1.25 χ l ( f [ H ] )

4

has been reported ( 2 2 ) , and these d a t a , a t l e a s t i n s o f a r as the f i r s t term i s concerned, are c o n s i s t e n t w i t h e a r l i e r ones ob2+ t a i n e d a t 40C (23). The chemistry o f V i s often studied i n ClO^ medium, but the r e d u c t i o n of CK> i s r a p i d enough so Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 13, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch007

4

t h a t , i n experiments over an extended p e r i o d , allowance f o r i t must be made. A t 25C and μ = 2.0, i n the r a t e law ( V ( I I ) •> V(III))

- . ^ k

= [v ][cio -] 2+

k

4

( h e r e i n a f t e r used to represent the c o e f f i c i e n t f o r the r a t e

term

[Reductant][CK> ~],

the

4

d[reductant].

e x c e

pt f

o

rate

being

defined

c e r t a i n o f the ruthenium systems, C l -7 -1 -1

r

i s the r e d u c t i o n product) i s r e p o r t e d (24) as 5.3 χ 10 +

and i s independent of [H ] over a l i m i t e d range. has M and King the

by

M

Deutsch

s

,

(25) 7

recorded f o r k a t 25C and μ = 1.0 a value of 7.6 χ 10 \ which i s i n s a t i s f a c t o r y agreement w i t h t h a t of A d i n Sykes. These data are c o n s i s t e n t w i t h those obtained by and Garner (26) a t 50C. The l a t t e r authors r e p o r t f o r k v a l u e 2.8 χ 1θ" M" s" , and f o r the r e a c t i o n o f V ( I I I ) 6

1

1

1

1

w i t h CIO,", the v a l u e 3.0 χ 1θ"^ M" s" . I n both cases a weak + dependency of k on [H ] i s mentioned, but i n view o f the f a c t t h a t the i o n i c s t r e n g t h was not kept c o n s t a n t , i t i s by no means c e r t a i n t h a t a p r o t o n dependent term i s p r e s e n t . I n f a c t , the 3+ p r o t o n dependent term i n the r e a c t i o n o f C10 with T i also may not be r e a l . I o n i c s t r e n g t h was maintained (22) u s i n g NaC10 t o replace HC10 and spurious r a t e terms can a r i s e s i m p l y because the a c t i v i t y c o e f f i c i e n t s o f the r e a c t a n t s do not remain 4

4

4

4

+

+

constant i n such mixtures as the r a t i o o f Na /h* i s changed. 3+ I t i s l i k e l y that U a l s o reduces C10 a t room temperature. Uranium(III) is known to disappear more rapidly in a s o l u t i o n o f HC10 than i n one o f HC1. The o b s e r v a t i o n s by 4

4

Sato (27) on t h i s p o i n t are c o n s i s t e n t w i t h the r a t e measure ments o f P e r e t r u k h i n , e t a l . ( 2 8 ) , f o r the r e a c t i o n a t 22C which 3+ y i e l d e d , f o r the disappearance of U i n 0.5 M HC10,, the v a l u e -5 -1 o f 1.8 χ 10 s . However, s i n c e i n n e i t h e r case are products


164


3+ s p e c i f i e d , i t i s , of course, p o s s i b l e t h a t the o x i d a t i o n o f U by H was being f o l l o w e d i n p e r c h l o r i c a c i d media. 2+ 2+ Despite Eu being a stronger reducing agent than V , 3+ 3+ V or T i , i t r e a c t s w i t h ClO^ much l e s s r a p i d l y than does any member of the l a t t e r group. From Adin and Sykes (24) 2+ o b s e r v a t i o n s , k f o r the r e a c t i o n of CIO, w i t h Eu a t 25C i s —8 -1 ™1 o+ l e s s than 3 χ 10 M s . Even Yb (aq), which i s a v e r y s t r o n g reducing agent indeed, r e a c t s w i t h CIO, s l o w l y enough 2+ so t h a t Yb can be prepared and s t u d i e d i n ClO^ media without +


1

i n t e r f e r e n c e from the r e a c t i o n of the reducing agent w i t h CIO (29) .

General experience w i t h s o l u t i o n s of C r

Z +

4 i n aqueous per

c h l o r a t e suggests t h a t k f o r the r e d u c t i o n of ClO^ 10~

8

i s l e s s than

1

M'V a t 25C. A s i g n i f i c a n t f e a t u r e of the r e a c t i o n k i n e t i c s o u t l i n e d i s t h a t , f o r each reducing agent f o r which the r e a c t i o n r a t e was measured, the a c t i v a t e d complex f o r the dominant path contains o n l y the reducing agent, p e r c h l o r a t e i o n , and no protons. I n the absence of protons t o s t a b i l i z e the oxide i o n which i s r e l e a s e d , whether the r e d u c t i o n i s by a l e or 2e change: C10 "

+ e" = C10

C10 "

+ 2e" = C10 "

4

4

+

3

3

2

0" +

2

0"

an important requirement must be t h a t the o x i d i z e d metal product makes a s t r o n g bond t o the oxide being r e l e a s e d . Unusually strong metal t o oxygen bonds are found i n the y l i o n s . A r e markable f e a t u r e of the chemistry of metal ions i n the aquo or ammono system i s the abrupt change i n a c i d i t y which can occur a t a c r i t i c a l stage i n the stepwise i n c r e a s e i n o x i d a t i o n s t a t e . 2+ Two examples w i l l s u f f i c e f o r i l l u s t r a t i o n . When V ( H 0 ) , is 3+ o x i d i z e d t o VOÔ)^ , nothing p a r t i c u l a r l y remarkable happens, and the i n c r e a s e i n a c i d i t y of the aquo i o n i s perhaps a f a c t o r of 10 5 , the d i s s o c i a t i o n constant of V ( H 0 ) , 3+ being about 10 -3 f ,

f f

o

o

Ζ

ο

(30) . But on i n c r e a s i n g the o x i d a t i o n number by one more u n i t , the enhancement i n a c i d i t y i s v e r y great indeed, there being 2+ complete d i s s o c i a t i o n of two protons t o y i e l d VO ( i . e . , V0 2+ (H 0) 2

4

).

striking.

The behavior of the aquo uranium system i s even more Here the

change

i n o x i d a t i o n s t a t e from 4+


t o 5"»*

7.

TAUBE


165

Reactions

changes the a c i d i t y even more d r a s t i c a l l y . The d i s s o c i a t i o n 4+ -2 constant f o r U (aq) i s about 10 ( 3 0 ) . I n c r e a s i n g the o x i d a t i o n s t a t e by one more u n i t , i n the absence o f s p e c i a l e f f e c t s , would l e a d t o the e x p e c t a t i o n t h a t perhaps one t o two protons would be r e l e a s e d . Instead, the o x i d a t i o n leads t o t h e complete


+

r e l e a s e o f f o u r protons, U0^ being the product. The sharp i n crease i n a c i d i t y i s a consequence o f the f a c t t h a t the metal ion e x e r t s a s e l e c t i v e p o l a r i z a t i o n o f t h e surrounding waters. Thus V ( I V ) s e l e c t s one water molecule from the environment and i n t e r a c t s w i t h i t p a r t i c u l a r l y s t r o n g l y , l e a d i n g t o complete proton d i s s o c i a t i o n f o r t h a t water, r a t h e r than gathering e l e c t r o n d e n s i t y evenly from the surrounding molecules. The b a s i c d r i v i n g f o r c e f o r the formation o f " y l " ions can be understood by t a k i n g p o l a r i z a t i o n e f f e c t s i n t o account. Thus z+ consider the two s t a t e s f o r the species M Z

H0M *0H

^H^

1

Z+

,

?0 M 0

A Β To produce s t a t e Β from s t a t e A w i l l K

cost

a

RT l n - ^ K

0H + where the Κ * s a r e the a s s o c i a t i o n constants f o r H on c o o r d i 2nated 0 and on coordinated OH . However, the p o l a r i z a b i l i t y 2of the combination 0 + H 0 i s g r e a t e r than t h a t o f 20H ( t h e 2p o l a r i z a b i l i t y of 0 decreases enormously w i t h the a d d i t i o n o f the f i r s t proton, much l e s s w i t h the a d d i t i o n o f the second), and s t a t e Β i s s t a b i l i z e d r e l a t i v e t o A by a term 2 2

a

?

r where ΔΡ i s the p o l a r i z a b i l i t y o f Β l e s s t h a t o f A. The i n crease i n the second term r e l a t i v e t o the f i r s t as ζ increases can be^very sharp. Not only does the p o l a r i z a t i o n term increase w i t h ζ , b u t , as ζ i n c r e a s e s , the radius r decreases and, f u r thermore, the f a c t o r K

0H

K

0H

2



166


f f

becomes s m a l l e r . Thus, the abrupt appearance of the " y l form i n general terms can be understood i n terms of p o l a r i z a t i o n e f f e c t s , the p o l a r i z a t i o n l e a d i n g to non-equivalence or d i s p r o p o r t i o n a t i o n of the l i g a n d s i n the f i r s t c o o r d i n a t i o n sphere. I t i s not unreasonable to suppose t h a t the p o l a r i z a t i o n term on changing the o x i d a t i o n number by one u n i t i n c r e a s e s enough not o n l y t o account f o r the s t a b i l i z a t i o n of Β w i t h respect t o A, z+ but a l s o w i t h respect t o M (OH), which might be the dominant form i n the absence of s e l e c t i v e i n t e r a c t i o n s . I n a p p l y i n g these i d e a s , p o l a r i z i n g power and p o l a r i z a b i l i t y must be i n t e r p r e t e d i n terms of the e l e c t r o n i c s t r u c t u r e s of the c e n t r a l metal and the l i g a n d s . P o l a r i z i n g power i s not simply a f u n c t i o n of z/r [thus note t h a t the i o n i z i n g po t e n t i a l of Ag i s 7.58 V ( r a d i u s of A g 1 . 1 3 8 ) w h i l e t h a t of Na i s 5.38V ( r a d i u s 0.988)], nor i s the p o l a r i z a b i l i t y of the l i g and, i n the sense i n which i t i s important here, simply a func t i o n of charge and s i z e (the y l ions are s t a b i l i z e d by m u l t i p l e bond formation and 71 bonds f o r oxygen tend to be stronger than for s u l f u r ) . To understand the chemistry of the y l i o n s , the e l e c t r o n i c s t r u c t u r e of the c e n t r a l i o n must be taken i n t o acz+ count. In producing the m u l t i p l e bonded u n i t M 0 or l i n e a r z+ +

OM

0,

the

dy

z

and

d

o r b i t a l s are promoted i n energy,

z x

r e s u l t i n g h i e r a r c h y of d - o r b i t a l energies

the

being

2 ζ 2 2 x -y xy

C10^

_The observations which have been made on the r e d u c t i o n of by metal i o n reducing agents conform to the idea t h a t

those which r e a c t r e l a t i v e l y r a p i d l y can produce an " y l " o x i d i z e d product. This i s t r u e of T i ( I V ) , V ( I I ) ( i f a t t a c k i s by a 2e

change), V ( I I I ) ( i f a t t a c k i s by

or 2e change), 2+ 2+ and U ( I I I ) ( i f a t t a c k i s by a 2e change). For Eu and Yb , the o n l y a c c e s s i b l e higher o x i d a t i o n s t a t e i s the 3+, and the 3+ i o n does not lend i t s e l f t o the formation of the " y l " bond. Chromium(II) as reducing agent c a l l s f o r s p e c i a l mention. On 2+ 3+ the b a s i s of the to

ideas lead

introduced,

not

expected

to

z/r

f o r C r ( I I I ) i s not

the

a le

change Cr

r a p i d r e d u c t i o n of C10^

-> Cr : the

h i g h enough t o produce the

is ratio

" y l " bond



7.

Observations

TAUBE

on A torn-Transfer

167

Reactions

and, furthermore, t h e r e i s a b a r r i e r a r i s i n g from e l e c t r o n i c destabilization. The formation o f an y l bond would c a l l f o r the promotion o f two e l e c t r o n s , o r i f t h e d , d against d yζ zx s p l i t t i n g i s great enough, f o r d e s t a b i l i z a t i o n by e l e c t r o n p a i r i n g . Chromium(IV) can presumably form an " y l " i o n , b u t , i n forming i t , a p e n a l t y i n e l e c t r o n d e s t a b i l i z a t i o n i s exacted, and t h i s may be a f a c t o r i n accounting f o r the f a c t t h a t the C r ( I I ) - C r ( I V ) couple i s so weakly reducing. No such p e n a l t y i s exacted i n the case o f T i ( I V ) o r V ( I V ) . The

observations

on the r e a c t i o n s o f ruthenium(II) com

plexes w i t h ClO^ are i n accord w i t h the i d e a t h a t the formation o f an " y l " i o n i n t h e product i s an important f a c t o r . The p r o nounced a c i d i t y o f an ammineruthenium(IV) s p e c i e s was demon s t r a t e d by Rudd, e t a l . (31), and the c h a r a c t e r i z a t i o n o f Ru(IV) b e a r i n g an oxygen as an " y l " s p e c i e s has been c o n v i n c i n g l y completed by Meyer and coworkers (21, 32, 33). Quite e a r l y i n the r a t h e r recent resurgence o f i n t e r e s t i n t h e chemistry o f ruthenium i n the lower o x i d a t i o n s t a t e s , i t was noted t h a t both Ru(NH ) and R u ( N H ) H 0 a r e incompatible w i t h C10 ". The 2 +

3

2 +

6

3

5

2

4

s p e c i f i c r a t e , k, f o r the r e a c t i o n o f t h e l a t t e r i o n w i t h CIO, -2 -1 -1 was reported (34, 35) as 2.6 χ 10 M s . A t the time t h i s work was done, i t was assumed t h a t , once the f i r s t stage o f r e d u c t i o n had been achieved, the subsequent r e d u c t i o n o f C 1 0 would be r a p i d . B u t , i n view o f the f a c t t h a t s u b s t i t u t i o n i n 2+ Ru(NH^)_H 0 i s by no means instantaneous, CIO., may a c t u a l l y -2-1-1 accumulate. A t any r a t e , 2.6 χ 10 M s i s an upper l i m i t f o r the r a t e a t which the f i r s t r e a c t i o n stage occurs. I f the net product i s C10 , the s p e c i f i c r a t e f o r t h e r a t e determining -2 -1 -1 step i s 1.6 χ 10 M s , and, i f i t i s C l , t h e s p e c i f i c r a t e -3 -1 -1 becomes 3.3 χ 10 M s . I n any event, t h i s r a t e i s slower 2+ than expected f o r s u b s t i t u t i o n on RuiNH^ÎÔ where a s p e c i f i c r a t e i n the range 1 t o 10 i s expected f o r a mononegative e n t e r ing group (18). T h i s p o i n t takes on i n t e r e s t i n the context o f 2+ 3

9

q

the r e s u l t s which have been reported f o r RuOÔ)^ ducing agent.

as the r e

K a l l e n and E a r l e y (36) have compared the s p e c i f i c 2+

r a t e o f the f i r s t step i n the r e a c t i o n o f RuOÔ)^

w i t h ClO^

(37) w i t h the r a t e o f s u b s t i t u t i o n i n t o the aquo i o n by C l , Br , I . 1θ"

3

1

M" s"

These 1

rates

are a l l s i m i l a r , 3

1

f o r C10 " t o 9.7 χ 1θ" ϋ" V" 4

ranging

from 3.2 χ

f o r B r " , and t h e ac-



168

INORGANIC REACTIONS

t i v a t i o n parameters AS* and AH* are very much a l i k e f o r the series. In t h i s case i t appears t h a t s u b s t i t u t i o n i s r a t e de t e r m i n i n g f o r the r e d u c t i o n of CK> . 4

Reduction of CK> strongly

reducing

Ru(bpy) pyH 0 Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 13, 2016 | http://pubs.acs.org Publication Date: September 27, 1982 | doi: 10.1021/bk-1982-0198.ch007

2

2

In f a c t , the

complexes

(38).

Nor

but

is

reported

i s the a c t i v i t y

rather

also

for

l i m i t e d to Ru(II).

i n an e a r l y study of c a t a l y s i s by ruthenium s a l t s

reduction

reached

2+

4

by R u ( I I ) i s not l i m i t e d t o the

(39)

of

ClO^

by

HBr^

(98C),

t h a t a R u ( I I I ) species

the

conclusion

of was

i s responsible f o r attack 2+

on ClO^ . Broomhead, e t a l . (40), r e p o r t t h a t R u ( N H ) C l is converted by 0.10 M HC10 t o [ R u ( N H ) ( 0 H ) N 0 ] i n a matter of 3

5

2+

4

3

4

hours

on a steam bath. The corresponding reaction with 3+ Ru(NH )^ i s much slower, suggesting t h a t C10 occupies a normal c o o r d i n a t i o n s i t e on being reduced by ruthenium. Osmium s a l t s are a l s o known (41) to be c a t a l y s t s f o r the r e d u c t i o n of p e r c h l o r a t e , and experience w i t h the chemistry of the osmium i n 3

4

lower o x i d a t i o n s t a t e s shows t h a t CIO, can be i n v o l v e d as an 4 oxidant. Osmium a l s o makes very strong bonds to oxygen i n the higher o x i d a t i o n s t a t e s , and an " y l " oxygen species may w e l l r e a c t f o r Os(IV) as an i n t e r m e d i a t e . I t has been reported (35) t h a t Ru(NH )^ a l s o reduces C10 , the i m p l i c a t i o n being t h a t the open normal c o o r d i n a t i o n 2 +

3

4

s i t e i s not necessary f o r f a c i l e r e a c t i o n . Though an a t t a c k by a t r a n s f e r of an atom of oxygen to a face of the octahedron, 0 2being converted t o 0 , would confer some s t a b i l i t y on the immediate product of the r e a c t i o n , i t i s d i f f i c u l t t o see how a 2strong Ru(IV)-0 bond could be formed i n t h i s s i t u t a t i o n . The system i s being r e i n v e s t i g a t e d , a c e t o n i t r i l e being added t o the 2+ r e a c t i o n s o l u t i o n so as to scavenge Ru(NH )^H 0 which may be present i n i t i a l l y , or which w i l l be formed by slow l o s s of NH 2+ from Ru(NH )£ . We (42) now f i n d t h a t the d i r e c t r e a c t i o n of 3 6 — Ru(NH ). i s a t l e a s t a f a c t o r of 20 slower than our e a r l i e r 2+ r e s u l t s i n d i c a t e d . A contamination of 1% of Ru(NH ),.H 0 in the hexaammine used i n our e a r l i e r work would account f o r the published rates. The observations c i t e d do seem t o support the p o i n t of view t h a t , f o r f a c i l e r e d u c t i o n , i t i s important t h a t an e s p e c i a l l y 3

2

q

0

2 +

0

3


2


7.

Observations on A torn-Transfer

TAUBE

Reactions

169

2s t a b l e bond t o 0 be formed by the o x i d i z e d product. This con c l u s i o n , even i f accepted, i s by no means tantamount to under standing the r e d u c t i o n of p e r c h l o r a t e and, a t b e s t , represents the beginning of an understanding. The e l e c t r o n i c s t r u c t u r e s and how they a f f e c t the energy of a c t i v a t i o n must be understood much more completely i f we are to p r o v i d e s a t i s f a c t o r y answers 3+ to questions such as these. Why does V , which i s a much 2+ 2+ weaker reducing agent than V , r e a c t as r a p i d l y as does V ? 3+ Why

is Ti

so much more r e a c t i v e than e i t h e r ?

i s r e s t r i c t e d to a l e

Titanium(III)

change and i n reducing ClO^

c h l o r i n e i n the v e r y unstable s t a t e , ClO^.

Why

must leave

are the R u ( I I )

s p e c i e s , though by f a r not the s t r o n g e s t r e d u c t a n t s , so remark a b l y r e a c t i v e i n reducing ClO^ ? I n c o n v e r t i n g R u ( I I ) to a r u t h e n y l ( I V ) s p e c i e s , there i s a s p i n change. The s t a b i l i t y gained by i m p a i r i n g e l e c t r o n s may compensate f o r the promotion of the d y , d o r b i t a l s accompanying m u l t i p l e bond f o r m a t i o n , but there i s the p o s s i b i l i t y t h a t the s p i n change i t s e l f can a f f e c t the r e a c t i o n r a t e a d v e r s e l y . z

The roster

z x

f o r e g o i n g d i s c u s s i o n by of metal

readily.

no

means covers

the

i o n species which are known to reduce

entire ClO^

Considerable a t t e n t i o n has been devoted t o the i n t e r

a c t i o n of ClO^ w i t h a v e r y r e a c t i v e molybdenum s p e c i e s , but there i s disagreement about i t s i d e n t i t y (43-47). A d i f f i c u l t y i s t h a t the Mo(IV) and Mo(V) species o r d i n a r i l y s t u d i e d are b i or p o l y n u c l e a r , w h i l e the r e a c t i v e species may a c t u a l l y be mono nuclear. A p r i o r i , mononuclear Mo(IV) would appear t o be a b e t t e r prospect than Mo(V) f o r f a c i l e r e a c t i o n w i t h ClO^ . I n the

former

case,

water would be 2OH

or 0

replacement

r e q u i r e d , but,

by

ClO^

o n l y of a coordinated

i n the l a t t e r , of a coordinated

i f a normal c o o r d i n a t i o n s i t e i s t o be used f o r r e

d u c t i o n . There are numerous examples of the r e d u c t i o n of ClO^ i n heterogeneous systems. The l i k e l i h o o d of understanding these on the s h o r t term i s v e r y s m a l l , c o n s i d e r i n g the d i f f i c u l t i e s we face even i n cases where the compositions of the a c t i v a t e d complexes are known. Though the r e d u c t i o n of ClO^ i s o n l y one o f many atom t r a n s f e r processes t h a t might be considered and i s f a r from the most important one, the f o r e g o i n g d i s c u s s i o n serves to show the need f o r a systematic experimental approach and does, perhaps,


170


i l l u s t r a t e the p o i n t t h a t such a study can lead to ideas which can be u s e f u l i n a much wider c o n t e x t .


Literature Cited 1. Halperin, J.; Taube, H. J. Am. Chem. Soc. 1950, 72, 3319; ibid., 1952, 74, 375. 2. Taube, H.; Gould, E. S. Acc. Chem. Res. 1969, 2, 321. 3. Basolo, F.; Willis, P. H.; Pearson, R. G.; Wilkins, R. G. J. Inorg. Nucl. Chem. 1958, 6, 161. 4. Cox, L. T.; Collins, S. B.; Martin, D. S. J. Inorg. Nucl. Chem. 1961, 17, 383. 5. Basolo, F.; Morris, M. L.; Pearson, R. G. Discuss. Faraday Soc. 1960, 29, 80. 6. Taube, H.; Dodgen, H. J. Am. Chem. Soc. 1949, 71, 3330. 7. Edwards, J. O. J. Am. Chem. Soc. 1954, 76, 1540. 8. Latimer, W. M. "Oxidation Potentials"; 2nd ed., PrenticeHall: Englewood Cliffs, N.J., 1952. 9. Anbar, M.; Guttmann, S. J. Am. Chem. Soc. 1961, 83, 2035. 10. Johnson, G. K.; Smith, P. N.; Appelman, Ε. H.; Hubbard, W. N. Inorg. Chem. 1970, 9, 119. 11. Hoering, T. C.; Ishimori, F. T.; McDonald, H. O. J. Am. Chem. Soc. 1958, 80, 3876. 12. Appelman, E. H. Kläning, U. K.; Thompson, R. C. J. Am. Chem. Soc. 1979, 101, 929. 13. Skrabal, Α.; Schreiner, H. Monatsh. Chem. 1935, 65, 213. 14. Laitinen, H. A. J. Am. Chem. Soc. 1942, 64, 1133. 15. Kritchevsky, E. S.; Hindman, J. D. J. Am. Chem. Soc. 1949, 71, 2096. 16. Newton, T. W. "The Kinetics of Actinide Oxidation Reduc tion Reactions" of Erda Critical Review Series TID - 26506, 1975. 17. Taube, H. J. Am. Chem. Soc. 1943, 65, 1880. 18. Lim, H. S.; Barclay, D. J.; Anson, F. C. Inorg. Chem. 1972, 11, 1460. 19. Matsubara, T.; Ford, P. C. Inorg. Chem. 1976, 15, 1107. 20. Buckley, R. R.; Mercer, E. E. J. Phys. Chem. 1966, 70, 3103. 21. Moyers, Β. Α.; Meyer, T. J. J. Am. Chem. Soc. 1978, 100, 3601. 22. Cope, V. W.; Miller, R. G.; Fraser, R. T. M. J. Chem. Soc. A 1967, 301. 23. Duke, F. R.; Quinney, P. D. J. Am. Chem. Soc. 1954, 76, 3800. 24. Adin, Α.; Sykes, A. G. J. Chem. Soc. A 1966, 1230. 25. Deutsch, Ε. Α., Ph.D. Dissertation, Stanford University, 1967. 26. King, W. R., Jr.; Garner, C. S. J. Phys. Chem. 1954, 58, 29.



7.

TAUBE


Reactions

171

27. Sato, A. Bull. Chem. Soc. Japan, 1967, 40, 2107. 28. Peretrukhin, V. F.; Krot, N. N.; Gelman, A. D. Sov. Radiochem. (Engl. Transl.) 1970, 12, 85. 29. Christensen, R. J . ; Espenson, J. H.; Butcher, A. B. Inorg. Chem. 1973, 12, 574. 30. "Stability Constants of Metal Ion Complexes." Special Pub. 17 & 25, The Chemical Society: London (1964, 1971). 31. Rudd, DeF. P.; Taube, H. Inorg. Chem. 1971, 10, 1543. 32. Moyer, Β. Α.; Meyer, T. J. J. Am. Chem. Soc. 1979, 101, 1326. 33. Moyer, Β. Α.; Thompson, M. S.; Meyer, T. J. J. Am. Chem. Soc. 1980, 102, 2310. 34. Endicott, J. F.; Taube, H. J. Am. Chem. Soc. 1962, 84, 4984. 35. Endicott, J. F.; Taube, H. Inorg. Chem. 1965, 4, 437. 36. Kallen, T. W.; Earley, J. E. Inorg. Chem. 1971, 10, 1149. 37. Seewald, D.; Sutin, N.; Watkins, K.O.J. Am. Chem. Soc. 1969, 91, 7307. 38. Referred to in Moyer, Β. Α.; Sipe, Β. K.; Meyer, T. J . , submitted for publication. 39. Crowell, W. R.; Yost, D. M.; Carter, J. M. J. Am. Chem. Soc. 1929, 51, 986. 40. Broomhead, J. Α.; Taube, H. J. Am. Chem. Soc. 1969, 91, 1261. 41. Crowell, W. R.; Yost, D. M.; Roberts, J. D. J. Am. Chem. Soc. 1940, 62, 2176. 42. Louis, P., work in progress, Stanford University. 43. Bredig, G.; Michel, J. Z. Phys. Chem. 1922, 100, 124. 44. Haight, G. P., Jr.; Sager, W. F. J. Am. Chem. Soc. 1952, 74, 6056. 45. Haight, G. P., Jr. Acta Chem. Scand. 1961, 15, 2012. 46. Rechnitz, G. Α.; Laitinen, H. A. Anal. Chem. 1961, 33, 1473. 47. Kolthoff, M.; Hodara, I. J. Electroanal. Chem. 1963, 5, 2. RECEIVED April 21, 1982.


General Discussion—Observations on Atom-Transfer Reactions Leader: John M a l i n

DR. JOHN MALIN ( N a t i o n a l Science Foundation): I wanted t o ask Dr. Taube whether he thought t h a t , i n the case o f the reac t i o n o f vanadium(II) w i t h p e r c h l o r a t e , the r e a c t i o n might a l s o be f a c i l i t a t e d by an a t t a c k a t the t orbital. Of 2g course, the d r i v i n g f o r c e i s p r o v i d e d by the f o r m a t i o n o f vanadium(IV) oxo i o n . 0


y

DR. TAUBE: The number o f d e l e c t r o n s i n the reducing agents does seem t o be an important f a c t o r , and s i n c e above a c e r t a i n number both σ and π o r b i t a l s a r e p o p u l a t e d , i n t h a t sense a t l e a s t o r b i t a l symmetry p l a y s a r o l e . Perhaps the 3+ most i n s t r u c t i v e jexample i s T i which has a s i n g l e 7td e l e c t r o n and reduces CIO, w i t h c o n s i d e r a b l e ease. T h i s i s t o be con4 t r a s t e d w i t h orthodox e l e c t r o p h i l i c donors, none o f which appear to be v e r y e f f e c t i v e . As p o i n t e d out i n the paper, T i ( I V ) can make a s t a b l e " y l ' i o n because i t has l o w - l y i n g nd o r b i t a l s t o 2accommodate the π bonds between the metal i o n and 0 . When the number o f d e l e c t r o n s i n c r e a s e s , a n t i - b o n d i n g e f f e c t s l i m i t the s t a b i l i t y o f the " y l " i o n . An i n t e r e s t i n g example t o study would be Co ( I ) which i s known t o be a good e l e c t r o p h i l e , but because o f t h e number o f d e l e c t r o n s which would be l e f t even on 2e o x i d a t i o n i s not expected t o form a s t a b l e " y l " i o n . 1

DR. JOSEPH EARLEY (Georgetown U n i v e r s i t y ) : Would you care to comment on the use o f t i n ( I I ) as a reducing agent f o r perchlorate? DR. TAUBE: I don't t h i n k t h a t S n ( I I ) w i l l reduce ClO^" a t a l l rapidly. I n saying t h i s , I m r e l y i n g on the experience w i t h other reducing agents which a r e orthodox n u c l e o p h i l e s . Because o n l y outer d o r b i t a l s a r e a v a i l a b l e f o r π bonding, Sn(IV) i s n ' t expected t o have the tendency t h a t T i ( I V ) has t o make an " y l " i o n — t h a t i s a product i n which one oxide i s v e r y t i g h t l y bound, even a t the expense o f loosening the b i n d i n g t o the metal o f o t h e r l i g a n d s . f

DR. GILBERT HAIGHT ( U n i v e r s i t y o f I l l i n o i s ) : Some o f you may r e c a l l t h a t I reduced p e r c h l o r a t e w i t h t i n ( I I ) , but i n the presence o f molybdates. And we came t o the c o n c l u s i o n t h a t mo lybdenum (IV) was produced but i t was not the reducing agent f o r p e r c h l o r a t e ; i t was c a t a l y t i c . R e c e n t l y we've been a b l e t o make a tin(II)-molybdenum(IV) complex and add i t t o p e r c h l o r i c a c i d . The t i n i s o x i d i z e d extremely r a p i d l y and the molybdenum i s not. So the molybdenum i s s e r v i n g as a pathway f o r e l e c t r o n s to get through an oxygen b r i d g e , and then i t holds the oxygen.



7.

TAUBE

Observations on A torn-Transfer

Reactions

173

DR. TAUBE: A problem w i t h the molybdenum c a t a l y s i s i s t h a t the species which r e a c t s r e a d i l y w i t h ClO^ may be an un s t a b l e , p o s s i b l y mononuclear, species r a t h e r than one of the condensed forms which we o r d i n a r i l y encounter. P a f f e t t and Anson [ P a f f e t t , M. T.; Anson, F. C. Inorg. Chem. 1981 20, 3967] have shown t h a t mononuclear Mo(V) r e a c t s moderately r a p i d l y w i t h C10^ . My guess i s t h a t mononuclear Mo(IV) would be even more r e a c t i v e . Mononuclear Mo(IV) i s probably a t y p i c a l " y l " 2i o n , t h a t i s , i t has both HLO and 0 coordinated t o i t . Mo(V) 2on the other hand _ w i l l have OH and 0 as l i g a n d s Replace ment of HÔ by C10^ i s e a s i e r than replacement of OH . DR. HAIGHT: We are now about t o t a c k l e the problem of d i s t i n g u i s h i n g between the r e a c t i v i t i e s of mononuclear and p o l y n u c l e a r molybdenum s p e c i e s . We added mononuclear molybdenum-tin(II) complexes to water and to p e r c h l o r a t e and our evidence r i g h t now i s t h a t i t becomes a c l u s t e r very f a s t when you do t h i s . L

DR. EAKLEY: We had expected t o f i n d r u t h e n i u m ( I I I ) dimers as products of the r e a c t i o n of R u ( I I ) w i t h p e r c h l o r a t e i o n but we found none. We had expected ruthenium(II) to go t o ruthenium(IV) and the Ru(IV) t o r e a c t w i t h a second R u ( I I ) t o form a dimer, but t h i s d i d not occur. DR. THOMAS MEYER ( U n i v e r s i t y of North C a r o l i n a ) : Wagner de Giovanni, i n my l a b , i s s t u d y i n g the r e a c t i o n between an oxo ruthenium compound and NO^ : I V

(bpy) (py)Ru =0

2 +

2

+ N0 " 2

H

(bpy) (py)Ru -ON0 2

+ + 2

•>

J2° ( b p y ) ( p y ) R u O H 2

2+ 2

+

N0 " 3

When you c a r r y out the r e a c t i o n i n t h i s d i r e c t i o n you can ac t u a l l y see the n i t r a t e complex b u i l d up i n the s o l u t i o n . The 2 -1 -1 r a t e constant f o r i t s formation i s about 10 M s The next step i n v o l v e s an aquation and the r a t e constant i s about 0.1 s * I t i s a stoichiometrically clean reaction y i e l d i n g n i trate. The r e a c t i o n provides a reinforcement of Dr. E a r l e y ' s p o i n t . However, one other appealing aspect o f t h i s r e a c t i o n i s t h a t the f r e e energy change i s c l o s e t o zero. The c r i t i c a l step i n the forward d i r e c t i o n i s a t t a c k of the coordinated oxo l i g a n d on n i t r i t e . The s p e c i f i c r a t e constant f o r t h a t step i s a c t u a l l y p r e t t y l a r g e . A p p l y i n g m i c r o s c o p i c r e v e r s i b i l i t y takes us back to Dr. E a r l e y ' s p o i n t t h a t , f o r the reverse r e a c t i o n , the slow step i s the s u b s t i t u t i o n and not the redox step.


174



DR. JAMES ESPENSON (Iowa S t a t e U n i v e r s i t y ) : Murmann's data [Murmann, R. K. Inorg. China. Acta 1977, 25 L43] on the exchange of the oxo oxygen o f vanadium(IV) would c e r t a i n l y suggest t h a t i t would be p o s s i b l e t o prove whether o r n o t t h e oxygen was r e a l l y t r a n s f e r r e d from p e r c h l o r a t e during i t s r e a c t i o n w i t h vanadium(II). Has t h a t experiment a c t u a l l y been done? DR. RICHARD THOMPSON ( U n i v e r s i t y o f M i s s o u r i ) : I do not b e l i e v e t h a t experiment w i l l work. I n a d d i t i o n t o the presum a b l y slow r e d u c t i o n o f the p e r c h l o r a t e i o n by vanadium(II), the V ( I I ) - V ( I V ) r e a c t i o n would be r a p i d enough t o preclude the 2+ generation o f much VO [Newton, T. W. ; Baker, F. B. Inorg. Chem. 1964, 3, 5 6 9 ] . Another p o i n t t o remember i s t h a t some o f the p r e c i p i 18 t a t i o n s f o r 0 a n a l y s i s can be v e r y troublesome, and vanadium(IV) i s no e x c e p t i o n . However, the work o f Murmann, which you have c i t e d , has opened the p o s s i b i l i t y o f t e s t i n g f o r oxygen atom t r a n s f e r t o vanadium(II), provided the redox r e a c t i o n i s rapid. DR. TAUBE: Murmann s p u b l i s h e d exchange s t u d i e s on V(V) are l i m i t e d t o condensed species and the r e s u l t s are not a p p l i cable t o an immediate product o f o x i d a t i o n , which presumably i s a mononuclear s p e c i e s . He has some s t i ^ l l unpublished r e s u l t s (K. Rahmoeller, as coworker) on V 0 and f i n d s oxygen -1 exchange t o be q u i t e r a p i d w i t h k ~0.03 s a t 0°. The spe2+ c i f i c r a t e f o r exchange between VO and water a t 0° i s 2.9 χ 10 s * [Murmann, R. K. ; o p . c i t . ]. Both r a t e s are so r a p i d 3+ 2+ compared t o the r e a c t i o n s o f V or V w i t h ClO^ as t o i n v a l i date the oxygen t r a c e r approach t o determining the mechanism. DR. ESPENSON: Carboxylate r a d i c a l anion from r a d i a t i o n chemistry ought t o be w e l l s e t up i n the formation o f carbonate i o n t o meet j u s t t h i s requirement. Y e t I'm under the impression t h a t the c a r b o x y l a t e r a d i c a l anion doesn't r e a c t w i t h p e r c h l o rate. 1

9

DR. TAUBE: With unstable i n t e r m e d i a t e s , there i s the problem o f competing r a t e s . My guess i s t h a t , i f you could keep the c a r b o x y l a t e anion long enough, i t would reduce CIO. . The 9-1-1 s p e c i f i c r a t e i s c e r t a i n l y not 10 M s , but i t might be as l a r g e as f o r some o f the other reagents f o r which measurements have been made. DR. ROBERT BALAHURA ( U n i v e r s i t y o f Guelph) : When you make chromium(II) s o l u t i o n s by the Taube method, you u s u a l l y f i n d a


7.

TAUBE


175

Reactions

l o t of c h l o r i d e i o n i n the s o l u t i o n . What do you envisage as the r e l e a s i n g agent: chromium(I) o r some combination o f chrom ium ( I ) and hydrogen?


ClO^

JDR. TAUBE: There are numerous examples of r e d u c t i o n o f a t the surfaces of s o l i d s , i n c l u d i n g one from work i n my

own l a b o r a t o r i e s done by Zabin [Zabin, Β. Α.; Taube, H. Inorg. 2+ Chem. 1964, 3, 963]. When Cr reduces MnO^ i n the presence o f ClO^ , t h e r e i s copious p r o d u c t i o n of C l . The heterogeneous r e d u c t i o n s are even l e s s w e l l understood than the homogeneous, and f o r t h i s reason I have omitted them from my account. I n the systems you r e f e r t o , t h e r e i s always a s o l i d phase p r e s e n t . What i s your experience on the d u r a b i l i t y of a chromium(II) s o l u t i o n a f t e r the r e a c t i o n becomes homogeneous? DR. BALAHURA: We have analyzed chromium(II) s o l u t i o n s two or t h r e e times a week and they seem t o be s t a b l e f o r about two weeks w i t h the c o n c e n t r a t i o n s we're u s i n g (0.2-0.4 M). But we a l s o have l i t h i u m p e r c h l o r a t e p r e s e n t t o suppress some of those reactions. DR. KENNETH KUSTIN (Brandeis U n i v e r s i t y ) : You mentioned t h a t p e r c h l o r a t e i s r e s i s t a n t t o a t t a c k by the hydrated e l e c tron. I t h i n k t h a t i s even more remarkable when you r e a l i z e that a s i m i l a r l y usually i n e r t anion, n i t r a t e , i s rapidly a t t a c k e d by the hydrated e l e c t r o n . Even though the hydrated 2e l e c t r o n has no p l a c e i n which t o go, i t adds on, forming N0^ which hangs around long enough f o r the r a d i a t i o n chemists t o monitor i t s decay mode. And t h a t doesn't happen a t a l l w i t h p e r c h l o r a t e , which I do not understand. DR. REX SHEPHERD ( U n i v e r s i t y of P i t t s b u r g h ) : About a year ago, we s t a r t e d t o look a t the r e a c t i o n of hydrogen p e r o x i d e with

(NHg),.Ru**I.

complexes

where

L

could

be

ammonia, water

and o t h e r n i t r o g e n bases. A c t u a l l y , Dr. Taube had

t r i e d p r e v i o u s l y t o measure the 2+ r a t e of o x i d a t i o n o f Ru(NH^)^ by HÔ^. As a secondary r e a c t i o n a f t e r the i n i t i a l o x i d a t i o n o f hexaammine by m o l e c u l a r oxygen and under h i s c o n d i t i o n s i n a c i d s o l u t i o n , he obtained zero-order k i n e t i c s . We employed a phosphate b u f f e r , and were able t o see the i n t r i n s i c r e a c t i o n f o r peroxide and these ruthenium(II) complexes. There are two i n t e r e s t i n g r e s u l t s . The f i r s t i s t h a t the r a t e s a t u r a t e s i n [ H 0 ] . The second i s Φ Φ t h a t we found AH t o be n e a r l y independent of L and AS t o be v e r y n e g a t i v e (work i n c o l l a b o r a t i o n w i t h Mr. C R . Johnson). The v a l u e s obtained are shown i n Table I . 9

9




176

Table I K i n e t i c A c t i v a t i o n Parameters f o r the O x i d a t i o n o f Ruthenium(II) Complexes by Hydrogen Peroxide

2 Ru(NH ) L 3

2 +

+ H 0

5

2

L

3

2Ru(NH ) L 3

5

3+

+ 2H 0 2

AH* (kcal/mole)

AS* (eu)

3

5.87 ± 1.27

-41.9 ± 4.3

H0

5.32 ± 0.36

-36.8 ± 1.2

5.46 ± 0.48

-50.6 ± 1.6

NH 2

Ν

W Ref:

2 2

NCH

0

3

K r i s t i n e , F. J . ; Johnson, C. R. ; O'Donnell, S.; Shepherd, R. E. Inorg. Chem. 1980, 19, 2280.


7.

TAUBE

Observations


We hypothesized

on A torn-Transfer

Reactions

177

•u 2+ t h a t AS « 0 could r e s u l t i f Ru(NH > 3

6

were undergoing some type o f d i s t r i b u t i o n process which allowed f o r seven c o o r d i n a t i o n w i t h peroxide a t t a c k i n g on a face t o produce a chemical intermediate (Scheme I ) . This species could s u f f e r a number o f p o s s i b l e f a t e s : (A) the formation o f t h e ruthenium(IV)-oxyl l i g a n d p l u s water; (B) the formation o f coor dinated hydroxide w i t h e x p u l s i o n o f h y d r o x y l ; o r (C) t h e f o r mation o f coordinated h y d r o x y l w i t h f r e e hydroxide. The l a t t e r two pathways u l t i m a t e l y y i e l d the h y d r o x y l r a d i c a l . The completion o f the mechanism i s the r e d u c t i o n o f e i t h e r 2+ h y d r o x y l o r ruthenium(IV) by a second mole o f Ru(NH )^ going on t o product. We wanted t o see i f we could f i n d anything t h a t would t e l l us whether we had t h e case f o r ruthenium(IV) as op posed t o t h e h y d r o x y l r a d i c a l pathways. To accomplish t h i s we employed the organic r a d i c a l - t r a p p i n g reagents, PBN and DMPO. The r a d i c a l a d d i t i o n products a r e n i t r o x y l r a d i c a l s which are r e a d i l y c h a r a c t e r i z e d by means o f t h e i r EPR s p e c t r a . We used both r a d i c a l t r a p p i n g agents t o c a r r y out some o f the s t u d ies. The PBN system i s l e s s d i a g n o s t i c because i t i s not as s e n s i t i v e t o t r a p p i n g a l l the r a d i c a l species as DMPO. The data i n Table I I a r e f o r t h e DMPO t r a p . We a l s o generated h y d r o x y l 3

2

by means o f two other reagents, Fe(EDTA) " and Ti(EDTA)(hÔ)", which are a l l e g e d t o make h y d r o x y l i n t h e i r r e a c t i o n s w i t h HÔ^. I f there's nothing i n t h e s o l u t i o n except t h e r e a c t a n t s , the t r a p p i n g agent and t h e s o l v e n t , we f i n d t h a t we t r a p a species which i s t h e same f o r a l l three reagents (the two con2+ t r o i s and Ru(NH ), w i t h H 0„) which i s s i m i l a r t o t h a t o f t h e 0

DO

o

Ζ Ζ

h y d r o x y l r a d i c a l adduct o f DMPO. I n the presence o f a l c o h o l s o r i n an acetone s o l v e n t we found evidence o n l y f o r trapped hy d r o x y l o r f o r a k i n e t i c a l l y determined d i s t r i b u t i o n o f h y d r o x y l and t h e r a d i c a l formed by hydrogen atom a b s t r a c t i o n from t h e solvent. 2+ F i n a l l y , we nave a l s o i n v e s t i g a t e d t h e RuiNH^)^ /ClO^ r e a c t i o n reported by E n d i c o t t and Taube, and thought t o i n v o l v e Ό-atom t r a n s f e r [ E n d i c o t t , J . F. ; Taube, H. Inorg. Chem. 1965, 4, 437; Taube, Η., " E l e c t r o n T r a n s f e r Reactions o f Complex Ions i n S o l u t i o n , " Academic P r e s s : New York, 1970]. I n t h i s e x p e r i ment we were unable t o t r a p any r a d i c a l . So, presumably, t h e r a d i c a l pathway i s blocked and o n l y t h e Ru(IV) pathway i s i n v o l v e d (Scheme I I ) .


178

MECHANISTIC ASPECTS OF INORGANIC

Scheme I

Ru(NH ) 3

2 +

^==i

6

*Ru(NH ) 3

2 + 6

-1 2+ *Ru(NH ) " T


3

+

6

H 0 2

2 +

[Ru(NH ) (H 0 ) ]

2

3

I V

(NH ) Ru 0 3

2 +

[Ru(NH ) (H 0 ) ] — 2 3

6

2

^

2

6

2 +

II3[

o

2

0H

L

2+

+ HO-

»

(NH ) Ru 3

I]CI

6

OH

L-(NH ) Ru 3

2

+ H0

6

(NH )^Ru

2

OH

3+

+ OH"

3+

+ Η0·

6

Scheme I I

Ru(NH ) 3

2 +

+

6

I V Ii v V

2

( N H ) R u 0« 3

6

2 +

C10 "

->-C10 "

4

3

Ru(NH ) 3 6 2H

+

„

(M ) RuOH

2

3 +

R u ( I f f l

+

6

Ru(NH ) 3

3

2 +

6

2+

+

3

I V

(NH ) Ru 0

3 + 6

+

3 3 6

+

+

2

OH"

HO-


REACTIONS

Observations

TAUBE

7.

on Atom-Transfer

179

Reactions

Table I I R e s u l t s o f R a d i c a l Trapping Studies

T i = Τi(EDTA)(HÔ)"

2


Fe = Fe(EDTA) '

H 0

Ru = R u ( N H ) 3

2 + 6

DMPO R a d i c a l

Substrate

Reagent

(Gauss)

*H (Gauss)

H 0

Fe

15.07

15.07

Ti

15.1

15.1

Ru

15.05

15.05

Fe

16.0

22.7

HOChy

Ti

16.4

22.6

(14.7, 20.7)*

Ru

16.2

23.2

Fe

16.0

22.8

Ti

16.2

23.1

Ru

16.5

23.9

Fe

16.5 [15.0]

24.1 [15.0Γ

Fe

15.0

15.0

Ti

15.0 [16.1]

15.0 [23.8]

HO•CH CO(CH )

15.0 [16.5]

15.0 [23.8]

HO•CH CO(CH )

15.0

15.0

2

2

CH OH 3

CH CH OH 3

2

2 +

Trapped

HO(15.3, 15.3)*

CH CHOH 3

(CH ) COH 3

3

(CH ) CO 3

2

Ru

0 +Ru DMPO =

/

α Η

(15.0, 22.5)*

•CH C(CH.) 0H HO9

9

J

HO-

2

2

3

3

HO-

V

37^ Λι Ν

CH 0

? 0

Φ [ ] - Denotes minor s p e c i e s , * Janzen, E,; L i u , F. J . Mag. Res, 1973, 9, 510 (Values i n benzene)


Observations on Atom-Transfer Reactions - ACS Symposium Series

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