The Chemistry of Organometallic Cations in Aqueous Media - ACS

9

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on February 18, 2015 | http://pubs.acs.org Publication Date: January 12, 1979 | doi: 10.1021/bk-1978-0082.ch009

The Chemistry of Organometallic Cations in Aqueous Media R. STUART TOBIAS Department of Chemistry, Purdue University, West Lafayette, IN 47907

A number of organometallic cations that are at least moderately stable in aqueous solution are known, and among these are the following species: Group IVB, R Ge , R Sn , R Sn , R Pb , R Pb ; Group IIIB, R Ga , R In , R Tl ; Group IIB, RHg ; Group IB, R2Au ; Group VIIIA, R Pt . Several of the methyl derivatives, R = CH , can be produced by the action of methanogenic bacteria or by reaction of methylcobalamin with inorganic compounds of the appropriate metal, but data on methylation under environmental conditions are mainly limited to mercury(II). Most of the information available on the aqueous chemistry pertains to the methyl species since these have the highest s o l u b i l i t y . Characteri s t i c a l l y these ions are sigma-bonded carbanion complexes of the metal in i t s maximum oxidation state. In 1966, the aqueous chemistry of organometallic cations was reviewed (1), but interest in environmental effects of several of these species in recent years has stimulated a good deal of new work. Although the reactions of metal ions in biological systems are exceedingly complex, a knowledge of the hydrolysis constants and s t a b i l i t y constants with a limited number of ligand types will permit a number of predictions about the ionic binding and transport. [In this discussion, unless otherwise indicated, hydrolysis will refer to proton transfer from a coordinated water molecule rather than M-C or M-X bond cleavage in the presence of water.] Thermodynamics will play a large part in governing the reactions of these organometallic species. Without exception, the methyl derivatives are highly labile to substitution at the metal center. In certain cases with bulky alkyl groups, reactions may proceed more slowly because of steric effects. Heterogeneous reactions, e.g. dissolution, are slower with large alkyl groups. Because of their hydrophobic nature, they r e s t r i c t attack at the metal center. +

+

3

2+

+

2

3

+

2+

+

2

3

+

2

2

+

+

2

+

3

3

0-8412-0461-6/78/47-082-130$05.00/0 © 1978 American Chemical Society In Organometals and Organometalloids; Brinckman, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


9.

Organometallic Cations

TOBIAS

131

This review w i l l concentrate on the i n t e r a c t i o n of these organometallic c a t i o n s with the solvent water, and a t t e n t i o n w i l l be focused on those ions o f environmental i n t e r e s t , namely the d e r i v a t i v e s o f mercury(II), t i n ( I V ) , and l e a d ( I V ) . The i n t e r a c t i o n with water i s a strong one, i t i s not normally encountered i n the organometallic chemistry o f these elements, and i t has a major influence on r e a c t i o n s a t the metal c e n t e r . Some o f the more a c i d i c species e x h i b i t amphoteric behavior, i . e . the organometallic oxides d i s s o l v e i n a c i d to give c a t i o n i c and i n base to give a n i o n i c s p e c i e s . Strong solvent i n t e r a c t i o n s are responsible f o r the absence from the l i s t above o f organometallic c a t i o n s o f c e r t a i n p e r f e c t l y s t a b l e organometallic m o i e t i e s , e . g . R2Ge(IV) and RSn(IV). These i n t e r a c t so strongly with water that they normally are encountered only as the hydrated o x i d e s . An even more extreme example would be c a c o d y l i c a c i d , (CH3)2As0(0H), which e x i s t s i n s o l u t i o n only i n the neutral or a n i o n i c form, (CH ) As0 ". U n t i l about a decade ago, i t g e n e r a l l y was f e l t that these metal-carbon bonds were r a t h e r weak and that t h e i r s t a b i l i t y was l a r g e l y k i n e t i c and not thermodynamic (2). Recent s y n t h e t i c work on metal a l k y l s i n d i c a t e s that the metal-carbon sigma bonds o f even h i g h l y r e a c t i v e metal a l k y l s can be quite s t r o n g . For example, the mean bond d i s s o c i a t i o n energy o f the Ta-C bond i n TaiCHjîs has been determined to be 62 ± 2 kcal m o l " ( 3 ) . The observation o f b i o l o g i c a l methylation o f Hg(II) (4,5) showed t h a t the synthesis o f a l e a s t some metal-carbon bonds was p o s s i b l e even in aqueous media. Nevertheless, in s p i t e o f the strength o f the metal-carbon bonds, cleavage r e a c t i o n s i n v o l v i n g water g e n e r a l l y w i l l be thermodynamically f a v o r a b l e , because both metal-oxygen and carbon-hydrogen bonds are formed. 3

2

2

1

Reactions Involving Cleavage o f the Metal-Carbon Bonds Very l i t t l e information i s a v a i l a b l e on the pathways by which the metal-carbon bonds are cleaved i n aqueous systems. For example, most o f these c a t i o n s are s t r o n g l y r e s i s t a n t to concentrated aqueous a c i d s . For a n a l y s i s , CH3Hg(II) compounds have been decomposed by heating with concentrated n i t r i c a c i d i n a bomb a t 150° for an hour (6). Preparative data and mechanistic s t u d i e s with nonaqueous media, some o f which are described elsewhere in t h i s volume ( 7 J , can give some general g u i d e l i n e s to f a c t o r s a f f e c t i n g s t a b i l i t y . From studies on p r o t o n o l y s i s of d i a l k y l m e r c u r y ( I I ) compounds, i t has been found that the rate o f the r e a c t i o n depends upon the e l e c t r o n d e n s i t y i n the Hg-C bond being cleaved and the p o l a r i z a b i l i t y o f the metal center ( 8 ) . With, for example, the very s t a b l e l i n e a r d i a l k y l t i n ( I V ) c a t i o n s , p r o t o n o l y s i s should be slow because the R'Sn(IV) residue i s o f low p o l a r i z a b i l i t y .

In Organometals and Organometalloids; Brinckman, F., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

ORGANOMETALS

132

R — S n — R '

—**R-H

+

ORGANOMETALLOIDS

— S n — R'

s/ t\


AND

[1]

l\

Η Χ X In a d d i t i o n , there are no strong e l e c t r o n i c e f f e c t s r e l e a s i n g e l e c t r o n density i n the t r a n s i t i o n s t a t e as occurs with c e r t a i n t r a n s i t i o n metal compounds that e x h i b i t strong trans e f f e c t s . For example, strong a c i d s , HX, cleave one a l k y l group r a p i d l y from R3AUL (9), but the product R AuXL i s s t a b l e so long as X i s a reasonable good l i g a n d . The high r e a c t i v i t y o f the t r i a l k y l g o l d ( I I I ) compounds i s due t o high trans e f f e c t o f a l k y l groups which f a c i l i t a t e s e l e c t r o n release to the proton i n the t r a n s i t i o n state i n attack on R3AUL. R Strong trans I e f f e c t ligand J 2

R

R ——Air—R

•

R' — Η

+

— A u — R

[2]

H

Χ Χ Most o f the decomposition r e a c t i o n s that have been studied in organic solvents are d i s s o c i a t i v e and involve as the f i r s t step the loss o f one o r more o f the ligands besides the carbanion from the f i r s t c o o r d i n a t i o n sphere. R.ML RML, + L n--m n (m-l) n (m-l)—^Decomposition products R

R

ML

1

X

Γ

M L

3 L

ΐ J

An example i s the p r o t o n o l y s i s o f the R3AUL compounds discussed above which involves d i s s o c i a t i o n o f L i n the f i r s t s t e p . An a l t e r n a t i v e mechanism that has been suggested f o r p r o t o n o l y s i s involves an o x i d a t i v e a d d i t i o n o f HX, i . e . a p r i o r protonation o f an e l e c t r o n - r i c h metal c e n t e r . This i s not a reasonable path f o r o r g a n o - m e r c u r y ( I I ) , - t i n ( I V ) , o r - l e a d ( I V ) compounds, because there i s no stable o x i d a t i o n state two u n i t s higher. Such a mechanism may operate i n the r e a c t i o n o f a l k y l p l a t i n u m ( I I ) compounds with HC1 ( 1 0 ) . CH Pt ciL n

3

+

2

HC1—*>CH (H)Pt Cl L I V

3

Ptnci L 2

2

2

[4]

2

+ CH

4

Another type o f r e a c t i o n t h a t i s observed i n the decomp o s i t i o n o f a l k y l metal complexes i s reductive e l i m i n a t i o n , [ 5 ] . R ML n

*

m

R

n

M L

(m-l)

+

L

L5J n

(m-1 )

^

2

(n-2)

(m-1)

This Intramolecular mechanism requires a r e l a t i v e l y

stable



9.


TOBIAS

133

o x i d a t i o n state two units lower than that c h a r a c t e r i s t i c o f the a l k y l complex. For example R2AUL2"*" c a t i o n s decompose r a p i d l y to R2 + AuL-2 when L i s a r e l a t i v e l y weak donor (11 ). In aqueous s o l u t i o n , [ ( C H ^ A u f O ^ ) ? ] * decomposes a t 25° i n hours with the production o f c o l l o i d a l gold and ethane. The low thermodynamic s t a b i l i t y o f t i n ( I I ) r e l a t i v e to t i n ( I V ) makes t h i s decomposition path e n e r g e t i c a l l y unfavorable f o r a l k y l t i n ( I V ) compounds and accounts i n part f o r t h e i r high stability. I t i s p o s s i b l e that coupling occurs with d i a l k y l lead(IV) s o l u t i o n s which are much l e s s stable than t h e i r tin(IV) counterparts. The l e a d ( I I ) i o n i s one o f the observed decomposition products. Most o f the organometallic ions are q u i t e r e s i s t a n t to nucleophilic attack. In s t r o n g l y a l k a l i n e s o l u t i o n , d i m e t h y l t i n ( I V ) has been observed to decompose according to [6] (12). 3(CH ) Sn 3

2+

120H

3(CH ) Sn(0H)

ΊψΓ"

2

3

2

24

[6]

J Slow 2(CH ) SnOH 3

3

+

Sn(0H)g

40H"

2

Reactions i n Which the Metal-Carbon Bonds Remain Intact · Proton Transfer from Coordinated Solvent: Hydrolysis. The aqueous chemistry o f many o f these species i s dominated by the h y d r o l y s i s r e a c t i o n s , i . e . by proton t r a n s f e r from coordinated water molecules, and by subsequent condensation r e a c t i o n s o f the hydroxo complexes to form c l u s t e r s with several organometallic c a t i o n s . + The h y d r o l y s i s r e a c t i o n s o f the RHg , R3Sn , and R2Sn ions were studied i n some d e t a i l over ten years ago, and t h i s and other e a r l y work was reviewed i n 1966. (1) The hydrolyses o f the a l k y l m e r c u r y ( I I ) , - t i n ( I V ) , and-lead^IV) species are the only r e a c t i o n s o f organometallic c a t i o n s included i n the recent monograph on the h y d r o l y s i s o f c a t i o n s by Baes and Mesmer (13). These r e a c t i o n s are somewhat simpler than i s c h a r a c t e r i s t i c o f s t r o n g l y hydrolyzing monatomic metal s p e c i e s , because the a l k y l groups e f f e c t i v e l y block c o o r d i n a t i o n s i t e s a t the metal c e n t e r . Consequently, the c h a r a c t e r i s t i c p o l y condensation r e a c t i o n s tend to give smaller c l u s t e r s with fewer metal atoms. The a l k y l m e r c u r y ( I I ) ions are unusually strong acids f o r u n i v a l e n t c a t i o n s , comparable to H g , r e a c t i o n [ 7 ] . Α

+

+

2

2 +

R

"

H 9 +

(aq)

+

¥ ( 1 )

^=i=

R H

9° (aq) H

+

H

3 (aq) 0 +

^

E q u i l i b r i u m constants f o r R = CH3, C2H5, n-C^Hj, and n-C/jHg are c o l l e c t e d i n Table I. These ions have pK = 5 and are a c i d s comparable i n strength to the c a r b o x y l i c a c i d s . As a a


134

ORGANOMETALS

AND

ORGANOMETALLOIDS

consequence, a t pH 7 the r a t i o [C^HgOH]: [Ch^Hg ] very much favors the hydroxide, and the c a t i o n i s a r e l a t i v e l y unimportant species. At pH values i n the v i c i n i t y o f the p K , condensed species a l s o are important, r e a c t i o n [8]. The f r a c t i o n o f the a


C H

3 9 (aq) H

+

+

CF^HgOH ^ H C H g ( 0 H ) H g C H 3

3 ( a q )

log Κ = 2.37 [ 8 ] ( l £ )

+

organometal i n the polycondensed species depends upon both pH and t o t a l organometal c o n c e n t r a t i o n . Very concentrated s o l u t i o n s appear to contain small amounts o f (CH3Hg) 0 , but t h i s i s not s i g n i f i c a n t below 0.2 M (15). In general p o l y nuclear complexes are unimportant below t o t a l metal c o n c e n t r a t i o n s o f ca_. 1 0 * M, so they probably play l i t t l e p a r t i n the r e a c t i o n s of mercurials i n the environment. The d i s t r i b u t i o n o f CH Hg(II) species as a function of pH i s i l l u s t r a t e d i n Figure 1. 3

+

4

3

Table I.

H y d r o l y s i s Constants f o r Univalent Cations at 2 5 °

Organometallic

a

Species CH HgOH (CH Hg) 0H (CH Hg) 0

log * K

-4.59 . CH Hg + CH Hg0H^(CH Hg) 0H log Κ = 2.37 CH HgOH + ( C H H g ) 0 H ^ (CH Hg) 0* + H 0 log Κ = 0 . 3 - 0 . 7 -4.98 -5.12 -5.17 -6.60 -6.81 -9.1

2

3

3

+

7

9

3

2

3

3

3

3

3

5

3

+

3

C H HgOH n-C H Hg0H n-C4H HgOH (CH3) SnOH (C H5) SnOH (CH ) PbOH 2

(14) Q4)

b

3

3

Reference

p q

3

3

+

2

+

2

D

3

3

2

(15) (44) (44) (44.) (16) (16) (17)

a-The symbols used throughout f o r the e q u i l i b r i u m constants are those of the Chemical S o c i e t y Tables o f S t a b i l i t y Constants (37.); b - 20°. Most o f the other u n i v a l e n t organometallic ions are r a t h e r weak aquo a c i d s . T y p i c a l are the t r i a l k y l t i n ( I V ) species which behave as simple monoprotic a c i d s , r e a c t i o n [9]. These have R

3Sn ( +

a q )

+

H 0 2

( 1

j ^

R SnOH 3

( a q )

+

H 0 3

+

( a q )

[9]

Q_6j

pK values between 6 and 7, i . e . a c i d strengths comparable to the weak oxyacids, e . g . HC10. The pK values increase s l i g h t l y with higher a l k y l groups as a l s o i s observed with the RHg ions and as would be expected from i n d u c t i v e e f f e c t s . At neutral pH, the r a t i o [R3SnOH]:[R3Sn ] s t i l l favors the hydroxo complex as i s i l l u s t r a t e d i n the d i s t r i b u t i o n diagram for (CH3)3Sn(IV) i n Figure 1. Since the R3SnOH i s uncharged i t should f a c i l i t a t e a

a

+

+



9.

TOBIAS


135

Figure 1. Species distributions as a junction of pH for 10~ M solutions of (CH ) Sn\ CH Hg\ and (CH ) Pb\ The fraction of organometal in the binuclear complexes will increase with increasing organometal concentration. Distribution diagrams for more concentrated solutions of CH$Hg* and (CH ) Pb* can be found in Refs. 40 and 117, respectively. 3

s

3

s

s

s

3

3


136

ORGANOMETALS

A N D ORGANOMETALLOIDS


the d i s t r i b u t i o n o f the organotin species into hydrophobic phases. T h i s may be o f importance f o r membrane t r a n s p o r t o f these s p e c i e s . There i s l i t t l e tendency to form polynuclear species. Data on the h y d r o l y s i s constants are c o l l e c t e d i n Table I. As i s to be expected, the t r i a l k y l l e a d ( I V ) c a t i o n s are weaker a c i d s than the t i n analogs and w i l l e x i s t as the aquated c a t i o n s a t neutral pH. For r e a c t i o n [10], the e q u i l i b r i u m (

C H

3)3Pb

+

( a q )

+

constant l o g * K

1 1

H 0 2

( 1

p-(CH ) PbOH 3

i s 9.1 (17).

3

( a q )

+

H 0 3

[10]

+

Some condensation has been

observed with concentrated s o l u t i o n s , r e a c t i o n

[11].

Figure 1 a l s o i l l u s t r a t e s the species d i s t r i b u t i o n f o r a m i l l i molar s o l u t i o n o f (CH3) Pb . The d i p o s i t i v e d i a l k y l t i n ( I V ) ions are much stronger aqup a c i d s than the t r i a l k y l s , and a t concentrations above ç a . 1 0 " M they form s i g n i f i c a n t f r a c t i o n s o f polynuclear h y d r o l y s i s products (1J5,18,19). In g e n e r a l , t h e i r strength as a c i d s i s comparable to that o f S n , and many s o l u t i o n p r o p e r t i e s o f the R2Sn ions are s i m i l a r to those o f Sn^ . The l o g value o f ( C H ) ? S n , £ 3 . 5 , (16), comparable to that f o r n i t r o u s a c i d . This does not t e l l the whole s t o r y , however, because much o f the hydrolyzed organotin i s i n the form o f species such as [ ( ( C H ) S n ) 2 ( 0 H ) 2 ] » [ ( ( C H ) S n ) ( 0 H ) ] , e t c . i n more concentrated s o l u t i o n s . Figure 2 i l l u s t r a t e s the e f f e c t o f c o n c e n t r a t i o n on the species d i s t r i b u t i o n , and Table II l i s t s the h y d r o l y s i s c o n s t a n t s . While these polynuclear complexes should be o f l i t t l e importance i n most environmental or b i o l o g i c a l systems, they w i l l make up a large f r a c t i o n o f the d i a l k y l t i n ( I V ) species i n r e a c t i o n s designed to model these systems a t 3 ^ pH ^ 8 i f the organotin concentration i s as high as m i l l i m o l a r . I t should a l s o be noted t h a t the sets o f e q u i l i b r i u m constants such as those i n Tables I and II normally do not include s o l u b i l i t y product d a t a . While a species d i s t r i b u t i o n can be c a l c u l a t e d with them f o r the e n t i r e pH range, i t w i l l be meaningless a t higher concentrations i f p r e c i p i t a t i o n o c c u r s . (See Figure 15.7 (a), r e f . 13.) This e f f e c t i s i l l u s t r a t e d i n Figure 2 f o r the 10 mM s o l u t i o n . This f i g u r e a l s o c l e a r l y i l l u s t r a t e s the amphoteric behavior o f (CH ) Sn *. Again, the analogous dimethyllead(IV) ions are much weaker a c i d s than the t i n analogs (20), and the e q u i l i b r i u m constants are c o l l e c t e d in Table I I . The p r a c t i c a l consequence o f the magnitude o f these constants i s t h a t very l i t t l e ( C H ) 2 P b e x i s t s i n s o l u t i o n a t pH 7. Approximately one equivalent o f a c i d must be t i t r a t e d to reach pH 7. 3

5

2

2+

2 +

3

3

3

2

1

2 +

2

S

3

2

4

6

2 +

2

3


2+


9.

TOBIAS

137


Figure 2.

Species distribution as a function of pH for 10 M and 10' M solutions of (CH ) Sn * 5

2

s

t

2


138

ORGANOMETALS

Table I I .

Summary o f ( C H ) S n 3

Species


3

+

2

2

2

2

3

4

[(CH )|M] (0H) (CH ) M(0H) (CH ) M(0H) 23

3

2

3

2

4

XoÔ -8.29 -4.83

3

2+

0

4

H y d r o l y s i s a t 25°

5-7.4 -15.54 -10.83

-24.31

2

6

2 +

pq

[(CH ) M] (0H) 2 3

2

ORGANOMETALLOIDS

log *K ,M=Pb(20)

pq

2

3

and ( C H ) P b

2 +

log *K ,M=Sn(18.)

(CH ) M(0H) (CH3)2M(0H)o [(CH ) M] (0H) ^ 3

2

AND

-16.17 -20.0 -32.2

-28.52

B. Complexation o f Organometallic Cations i n Aqueous Solution. Information on s t a b i l i t y constants of organometallic cations i n aqueous s o l u t i o n makes i t p o s s i b l e to p r e d i c t a number o f t h i n g s . If p a r t i c u l a r l y s t a b l e complexes are formed with a c e r t a i n f u n c t i o n a l group that i s present i n environmental o r b i o l o g i c a l systems, these groups are l i k e l y to be involved i n the t r a n s p o r t and p h y s i o l o g i c a l a c t i v i t y o f the organometal. Often organometals are introduced, as i n the case o f t i n ( I V ) compounds, as the h a l i d e s ; and a knowledge o f the s t a b i l i t y constants f o r complexes with h a l i d e ions w i l l permit a p r e d i c t i o n o f whether the h a l i d e w i l l p e r s i s t o r aquate i n natural waters. With organometallic compounds, syntheses often are c a r r i e d out with non-aqueous s o l v e n t s , a p r a c t i c e t h a t i s l e s s common with simple inorganic s p e c i e s . S t a b i l i t y data can allow p r e d i c t i o n s o f whether model compounds have s u f f i c i e n t s t a b i l i t y to be important species i n d i l u t e aqueous s o l u t i o n . Trends i n S t a b i l i t y : Hard and Soft A c i d Character. The organom e t a l l i c c a t i o n f o r which the most systematic studies on complex s t a b i l i t y have been made i s CH3Hg . It i s a very s o f t a c i d and tends to bind s t r o n g l y to s o f t bases, i . e . to heavier donor atoms ( 2 1 , 2 2 ) . This i s demonstrated by the e q u i l i b r i u m constants for r e a c t i o n [ 1 2 ] with X = halide i o n . These increase markedly +

C H

3 9 H

+

+

X'(aq)

C H

3

H g X

(aq)

[

1

2

]

with heavier h a l i d e s : log K i = 1.50 ( F " ) , 5 . 2 5 ( C I " ) , 6 . 6 2 ( B r " ) , 8 . 6 0 (I") (14). Because o f the high s t a b i l i t y o f the c h l o r i d e , bromide, and iodide complexes, these compounds have quite low aqueous s o l u b i l i t y . The p r i n c i p a l c o o r d i n a t i o n number o f mercury(II) i s two, and the CH3HgX molecules do not i n t e r a c t s t r o n g l y with water. A d d i t i o n o f C I " or B r " can be used to reverse the binding of CH3Hg to n u c l e i c a c i d s , e . g . i n agarose e l e c t r o p h o r e s i s with CH3HgOH as a denaturing agent ( 2 3 ) . +


9.


TOBIAS

139


In c o n t r a s t to the aqueous s o l u b i l i t y , the CI^HgX molecules should have reasonable l i p i d s o l u b i l i t y , and chloro complexing may be important i n f a c i l i t a t i n g transport o f methylmercury(II) in b i o l o g i c a l systems. The d i s t r i b u t i o n c o e f f i c i e n t o f CH3HgCl between toluene and an aqueous phase was found to be 1 1 . 1 , r e a c t i o n [13] (24). C H

3 9 H

C 1

(aq)

C H

3 9 H

C 1

(toluene)

™

The a l k y l t i n ( i v ) s p e c i e s behave as hard a c i d s , and i n t h i s respect are very d i f f e r e n t from the alkylmercury(II) ions which are prototype soft acids (21). Experimentally t h i s i s demonstrated by the trend i n the s t a b i l i t y constants o f the h a l i d e ion complexes, r e a c t i o n s [14]: log K] = 2.3 ( F " ) , -0.17 (CI") (25). The constants with bromide and iodide are Me

!

+ Me

Me

+

X'

(

a

q

)

^=5:

{[(CH ) Sn(0H ) ] -x} 3

!

3

2

2

+

j|

(aq)

llr

-H 0

[14]

2

X i ,Sn

/ 1^ Me

I

%

(aq) Me

Me so small t h a t values have not been determined. I t i s l i k e l y that the complexation i n s o l u t i o n i s mainly o f the outer sphere type, because the formation o f inner sphere (CH3)3SnX requires dehydration and a s t r u c t u r a l change from a planar to a pyramidal skeleton f o r ( O ^ ^ S n * . These data together with the knowledge that s u b s t i t u t i o n a t the t i n center occurs r a p i d l y t e l l us that d i s s o l u t i o n o f ( C ^ ^ S n X , X = C I , B r , I, i n water w i l l y i e l d the same aquated species w i t h i n the time o f mixing. Even i n sea water, mean [Cl~] - 0.5 M o r blood 0.103 M, (CH3)3Sn w i l l be present p r i n c i p a l l y as the aquo c a t i o n a t pH < 5 and (CHj^SnOH at pH 7. The low e q u i l i b r i u m concentration o f (CH3)oSnCi may be important i n processes t h a t involve e x t r a c t i o n into l i p i d phases, e . g . i n membrane t r a n s p o r t , although the hydroxide w i l l a l s o be quite l i p i d s o l u b l e . Studies on the permeability o f mitochondrial membranes show that f^Sn ions mediate the transport o f C I " and OH" across the membrane and provide a means f o r r a p i d e q u i l i b r a t i o n o f pH (26). The d i s c u s s i o n o f the chemistry i n t h i s reference i s somewhat obscured by neglect o f the h y d r o l y s i s o f these ions which was discussed i n 4

+


140

ORGANOMETALS AND ORGANOMETALLOIDS

the previous s e c t i o n . The R3Pb ions form only very unstable complexes with h a l i d e i o n s : Ki (not log) (CFh^Pb" ^ 6.5 ( F " ) , 2.1 ( C I " ) ; ( C H ) P b = 3.5 ( F " J , 3.7 (CI") (27,28). Also they are not so hard, since K P / K Q I i s only 3.1 for (CH3) Pb , while i t i s 3 χ 1 0 with ( C H ) S n . The R Sn^ ions form somewhat more stable complexes with a n i o n i c 1igands because o f t h e i r d i p o s i t i v e charge, but the h a l i d e complex s t a b i l i t y constants i n d i c a t e that they, too, are very hard a c i d s . For r e a c t i o n [15], the values l o g Ki are 3.70 ( F " l (25), 0.38 (CI") (29), < - 0 . 5 (Br") (29). In the ( C H ) S n - C I " system, Raman spectra demonstrate t h a t the complexing i s mainly o f the outer sphere type, and inner sphere complexing only occurs with very concentrated s o l u t i o n s where the water a c t i v i t y i s reduced to a low value. +

1

2

5

+

3

3

2

3

+

3


2

3

z

2

Me I

2+

'Sn:

+ X"

Ί

^[(CH ) Sn(0H ) ] -X"} 3

(aq)

11

2

Me

2

4

2 +

" ¥

[15]

[(CH ) SnX(0H ) f 3

2

2

n

These data c l e a r l y show the much l e s s favorable displacement o f water by the heavier h a l i d e s in the case o f R S n and R S n compared to the RHg i o n s . A p r a c t i c a l consequence i s t h a t the o r g a n o t i n ( I V ) c a t i o n s w i l l tend to i n t e r a c t r e l a t i v e l y more s t r o n g l y w i t h nitrogen and oxygen donors, p a r t i c u l a r l y n e g a t i v e l y charged ones, compared to s u l f u r donors, while the converse w i l l be true f o r the a l k y l m e r c u r y ( I I ) s p e c i e s . The R Pb* ions form much l e s s s t a b l e complexes than the t i n analogs: (CH ) Pb , 54.0 ( F " ) , 5.8 ( C I " ) ; ( C H ) ? P b 35.0 ( F " ) , 9.2 (CI") (£7,28). Thev a l s o are much l e s s hard than the t i n analogs. For~TCH ) Sn' K P / K Q ] i s C J K 2.1 χ 1 0 , while the r a t i o f o r ( C H ) P b is 9.3. S t a b i l i t y with Biomolecules and Related Ligands. The hard o r s o f t a c i d c h a r a c t e r i s o f use i n p r e d i c t i n g the types o f donors t h a t w i l l give the most s t a b l e complexes. Of a t l e a s t comparable importance i s the i n t r i n s i c b a s i c i t y o f the donor; f o r example, a mereaptide w i l l form much more stable complexes with C H H g than a t h i o e t h e r even though both ligands are s u l f u r donors. With a given type o f donor, reasonably good l i n e a r f r e e energy r e l a t i o n s e x i s t f o r protonation and m e t h y l m e r c u r i a t i o n as observed o r i g i n a l l y by Simpson (30) f o r nitrogen donors and examined more g e n e r a l l y by Erni and Geier (J31_,_32). T h i s i s i l l u s t r a t e d in Figure 3 f o r some oxygen and s u l f u r donors. As expected from the s o f t a c i d c h a r a c t e r o f CH3Hg , the i n t e r c e p t in the l o g K C H v s . log K H L P l o t i s much l a r g e r f o r the s u l f u r than the oxygen donors. S t a b i l i t y con stants f o r a number o f CH3Hg complexes are c o l l e c t e d in Table I I I . +

3

2 +

2

+

2

3

2

2 +

2

3

3

3

2

2

5

2 +

2 +

3

2 +

+

+

H

g

L

+


TOBIAS



9.


141

142

ORGANOMETALS


Table

III.

CHoHg

Selected S t a b i l i t y Constants o f Organometallic Cations with Organic Ligands.

H0CH CH S2

2

1 P H 1 6 ^ 6 3 " ' glutathionate C3H6Ô2NS-, c y s t e i n e t e C6H5S-, thiophenolate CCHÎOANOS-, 2,4-dinitrothiof (&2N)2C=S

C

N

S

S(CH?CH C0 S(CH CH 0H 2

2

)2

2

^ 7 ^ 5 ^ 2 ~ tropolonate C5H5Û"phenolate C6H4O3N" 4 - n i t r o p h e n o l a t e CH C0 " HC0 C1 HC0 " 3

2

2

2

2

3 3 2 ~ imidazolate C H 4 N imidazole H2NCH?CH2NH2 CH3NH2 (Cfl3)3N N H 2 C H C 0 " glycinate 1 2 8 2 1,10-phenanthroline C5H5N p y r i d i n e

C

H

N

3

2

2

C

(CH ) Pb 3

3

+

H

2

N

2+

2

2+

11.8 7.1 8.2 7.6 5.0 7.9 7.2 4.7

(32) (32) (14)

(51) (51) (51) (52) (53)

(35) (35) (35)

CH C0 "

2.6

(54)

2

3

(49) (32) (32) (50) (50) (50)

3.9 5.3 6.0

2

2

(CH ) Pb

5.4 5.5 3.8 3.2 2.7 1.1

(45) (46) (47) (32) (47) (48)

C-ioHoNo 1,10-phenanthroline C6fl40 N- picolinate C5H70 "acetylacetonate

2

2

(14) (45)

1.2 (U) 0.97,0.54(1_7,54) 0.86 (II) 0.52 (U)

2

2

3

16.1 15.9 15.7 14.7 10.5 6.9 4.2 3.1

C c H 9 0 2 " pivalate CH C0 " HC0 ~ C1CH C0 " 3

(CH ) Sn

A N D ORG A N O M E T A L L O IDS

3

2

(log K

2


1.0)

9.

TOBIAS

Organometallie Cations

143

Few data are a v a i l a b l e f o r s t a b i l i t y constants o f other organometallic ions with such l i g a n d s . Recently, some data f o r complexes o f (CH3)3Pb with carboxylates have been reported by Sayer e t a]_. (17). A rough l i n e a r f r e e energy r e l a t i o n i s observed between complexàtion by ( C H j ^ P b * and p r o t o n a t i o n , and t h i s i s i l l u s t r a t e d i n Figure 4. S i m i l a r behavior i s to be expected f o r the R3$n i o n s , although the s t a b i l i t y constants with hard bases should be s l i g h t l y l a r g e r . Consistent with t h i s i s the observation t h a t phosphate b u f f e r s , pH 7 . 4 , decrease the binding o f ^ H c ^ S n * to r a t hemoglobin (33). Phosphate buffers a l s o were observed to decrease the e x t r a c t i o n o f (C2H5h Sn(IV) i n t o a chloroform phase. In a q u a l i t a t i v e study o f complexing o f R3Sn species with a number o f b i o l o g i c a l molecules, no evidence could be found f o r complexes with ATP, g l u t a t h i o n e , a r g i n i n e , o r l y s i n e i n a borate-EDTA buffer a t pH 8.4 (34). The R£Sn and R 2 P b species probably form a much l a r g e r v a r i e t y o f complexes than the t r i a l k y l s p e c i e s , but the extensive h y d r o l y s i s o f the c a t i o n s has made the accurate d e t e r mination o f s t a b i l i t y constants by p o t e n s o m e t r i c t i t r a t i o n s very d i f f i c u l t . While t h i s i s the c l a s s i c a l technique f o r such determinations, i t r e q u i r e s an accurate knowledge o f a l l the h y d r o l y s i s constants as well as the l i g a n d protonation constants. To complicate matters, mixed hydroxide-1igand complexes, e . g . [R2$n(0H) L ], often w i l l be formed g i v i n g a l a r g e number o f p o s s i b l e species and rendering the a n a l y s i s o f t i t r a t i o n data even more d i f f i c u l t . While CF^Hg"*" i s e x t e n s i v e l y hydrolyzed i n s o l u t i o n s with pH > 1, i t s e s s e n t i a l l y monofunctional c h a r a c t e r precludes the formation o f mixed hydroxo-1igand complexes and permits the r e l a t i v e l y s t r a i g h t forward determination o f s t a b i l i t y c o n s t a n t s . A few data have been obtained f o r complexes o f (CH3)2$n with bidentate l i g a n d s , using a computer to analyze the data (35,36). As i s expected from the hard a c i d c h a r a c t e r o f ( C H 3 l ^ S n , the most s t a b l e complex i s formed with the negative bidentate oxygen donor, a c e t y l acetonate, l o g Ki 6 . 0 ; and the l e a s t s t a b l e complex was formed with the neutral bidentate nitrogen donor 1,10 phenanthroline, l o g K] 3 . 9 . The s t a b i l i t y o f the a c e t y l acetonate complex i s s i m i l a r to t h a t o f the N i , complex, l o g Κ ι ^ 6.0 ( 3 7 ) » while the phenanthroline complex i s much l e s s s t a b l e than f o r N i , l o g Κ] ^ 8.6 (37). +


+

4

+

2+

2+

n

m

2+

2+

2 +

2 +

Future Prospects For the organometallic c a t i o n s t h a t are e x t e n s i v e l y hydrolyzed i n s o l u t i o n , s p e c t r o s c o p i c techniques can be used to determine both e q u i l i b r i u m constants and i n many cases the binding s i t e s . While n e i t h e r the organometallic c a t i o n s nor most o f the l i g a n d s o f i n t e r e s t have s u i t a b l e e l e c t r o n i c t r a n s i t i o n s , both Raman and nmr spectroscopy have been used s u c c e s s f u l l y . Resonances o f both the c a t i o n and



ORGANOMETALS

AND

ORGANOMETALLOIDS

pivalate

propionate

Ο

/ 0

1.0

/ HC0 "

ί

2

Ο

I y

CH3C02"

)

-

acetylgiyc i n a t e - ^

/ /

Ο

0.5 '2.5

°

CICH C0 " 2

3.0

2

3.5

4.0 logK

4.5

50

H L

Figure 4. Stability constants for (CH ) Pb* with carboxylic acids; log K ) PbL vs. log K . Data from Kef. 17. s

(CHs

S

s

HL


TOBIAS


9.

Organometallie Cations

145

CH^OH

1201

572'

CH HgOH 3

2

• Ç

'CH^g-N^.H

α 1616 1 ^ 1 5 7 9 ^

Figure 5. Raman perturbation differ ence spectra for the CH Hg(II)-pyridine system at pH 8.4, 6.5, and 4.4. In each set, A is the CH Hg(H) + pyridine vs. CH Hg(II) difference spectrum, while Β is the CH Hg(II) + pyridine vs. pyridine difference. Rehtive ordinate expansions are indicated at the right. s

s

3

1800

1600

1400

1007 1200 1000 800' 600 ' FREQUENCY (CM-1)

s

400


ORGANOMETALS AND

146

ORGANOMETALLOIDS

1igand protons have been used to study many r e a c t i o n s o f CH3Hg with amino acids (38) and with the nucleoside i n o s i n e (39). Raman p e r t u r b a t i o n d i f f e r e n c e spectroscopy has been used to study the r e a c t i o n s o f CH^Hg" " with pyrimidine (40,42,43) and purine (39,41,42) n u c l e o t i d e s as well as with c a l f thymus DNA (43). Figure 5 i l l u s t r a t e s the a p p l i c a t i o n o f the Raman technique to the simple C H o H g i l l J - p y r i d i n e system. A t pH 8 . 4 , the CH Hg(II) + p y r i d i n e v s . CH Hg(II) d i f f e r e n c e gives j u s t the spectrum o f p y r i d i n e showing that no r e a c t i o n o f the methylmercury c a t i o n has taken p l a c e . S i m i l a r l y the CH3Hg(II)+ p y r i d i n e ys_. p y r i d i n e d i f f e r e n c e gives j u s t the spectrum o f CH3HgOH. At pH 6.5 and 4.4 both spectra show extensive perturbations. The 1021 c n H band o f the complex i s s u f f i c i e n t l y well resolved from the 1003 cm** band o f p y r i d i n e o r the 1008 cm-1 band o f the pyridinium ion to permit measurements o f the concentrations o f unreacted 1igand using the Raman i n t e n s i t i e s . While 100% i s unreacted a t pH 8 . 4 , the values are 20% and ç a . 0%, r e s p e c t i v e l y a t pH 6.5 and 4 . 4 . This i s i n good agreement with values c a l c u l a t e d from the known s t a b i l i t y constant. The Raman d i f f e r e n c e technique i s p a r t i c u l a r l y s e n s i t i v e in d e t e c t i n g small amounts o f r e a c t i o r ^ a n d both Raman and nmr can be used to determine the product d i s t r i b u t i o n . These s p e c t r o s c o p i c techniques should be e q u a l l y s u i t a b l e f o r the study o f R Sn and R 2 S n i n t e r a c t i o n s , and a s t a r t a l r e a d y has been made with R3Pb chemistry (17).


1

3

3

1

2+

3

+

Acknow!edgements. This work has been supported, i n p a r t , by the National Science Foundation Grant CHE 76-18591 and by the P u b l i c Health S e r v i c e , Grant AM-16101 from the National I n s t i t u t e f o r A r t h r i t i s , Metabolism, and D i g e s t i v e Diseases. The author a l s o would l i k e to express h i s a p p r e c i a t i o n to the O f f i c e o f Naval Research, as p a r t o f t h i s material was presented a t a workshop on organotin chemistry i n February, 1978. Thanks are due Mary R. M o l l e r f o r the C H H g ( I I ) - p y r i d i n e spectra. 3

Literature Cited 1. 2. 3.

4. 5.

T o b i a s , R. S., Organometal. Chem. Revs. (1966), 1, 93. Coates, G. E. "Organometallic Compounds," p 72, Methuen, London, 1960. A d e d e j i , F . Α . , Connor, J. Α . , Skinner, Η. Α . , G a l y e r , L . and W i l k i n s o n , G . , J. Chem. Soc. Chem. Commun. (1976), 159. Wood, J. M . , Kennedy, F . S . , and Rosén, C . G . , Nature (London) (1968), 220, 173. Jensen, S. and Jernelöv, Α . , Nature (London) (1969), 223, 753.


9.

TOBIAS



6.

Β.

147

Schellenberg, M., Ph.D. Thesis, Eidgenössische Technische Hochschule Zürich, 1963. 7. Kochi, J. K., Factors Involved in the Stability of Alkyl— Metal Bonds, this volume. 8. Nugent, W. A. and Kochi, J. K., J. Amer. Chem. Soc. (1976), 98, 273. 9. Komiya S. and Kochi, J. K., J. Amer. Chem. Soc. (1976), 98, 7599. 10. Belluco, U., Giustiniani, M., and Graziani, M., J. Amer. Chem. Soc. (1967), 89, 6494. 11. Kuch, P. L. and Tobias, R. S., J. Organometal. Chem. (1976), 122, 429. 12. Tobias, R. S. and Freidline, C. E., Inorg. Chem. (1965), 4, 215. 13. Baes, C. F., J r . and Mesmer, R. E. "The Hydrolysis of Cations," Wiley, New York, 1976. 14. Schwarzenbach, G. and Schellenberg, M., Helv. Chim. Acta. (1965), 48, 28. 15. Rabenstein, D. L., Evans, C. Α., Tourangeau, M. C., and Fairhurst, M. T., Anal. Chem. (1975), 47, 338. 16. Tobias, R. S., Farrer, H. N., Hughes, M. B., and Nevett, Α., Inorg. Chem. (1966), 5, 2052. 17. Sayer, T. L., Backs, S., Evans, C. Α., M i l l a r , Ε. K., and Rabenstein, D. L., Can. J. Chem. (1977), 55, 3255. 18. Tobias, R. S. and Yasuda, M., J. Phys. Chem. (1964), 68, 1820. 19. Tobias, R. S., Ogrins, I., and Nevett, Β. Α., Inorg. Chem. (1962), 1, 638. 20. Freidline, C. E. and Tobias, R. S., Inorg. Chem. (1966), 5, 354. 21. "Hard and Soft Acids and Bases," R. G. Pearson, E d . , Dowden, Hutchinson, and Ross, Strondsburg, Pa., 1973. 22. Pearson, R. G., J. Chem. Educ. (1968), 45, 581. 23. Bailey, J. M. and Davidson, N., Anal. Biochem. (1976), 70, 75. 24. Gruenwedel, D. W. and Davidson, N., J. Mol. B i o l . (1966), 21, 129. 25. Cassol, A., Magon, L., and Barbieri, R., Inorg. Nuclear. Chem. Lett. (1967), 3, 25. 26. Selwyn, M. J., Dawson, A. P., Stockdale, M., and Gains, Ν., Eur. J. Biochem. (1970), 14, 120. 27. P i l l o n i , G. and Magno, F., Inorg. Chim. Acta. (1970), 4, 105. 28. P i l l o n i , G. and Magno, F., Inorg. Chim. Acta. (1971), 5, 30. 29. Farrer, H. N., McGrady, M. M., and Tobias, R. S., J. Amer. Chem. Soc. (1965), 87, 5019. 30. Simpson, R. B., J. Amer. Chem. Soc. (1964), 86, 2059. 31. Geier, G., Erni, J., and Steiner, R., Helv. Chim. Acta. (1977), 60, 9. American Chemical S o c i e t y Library 16th St. N . Brinckman, W. In Organometals1155 and Organometalloids; F., et al.; ACS Symposium Series; American Chemical Washington, DC, 1979. Washington, D. C. Society: 20036

ORGANOMETALS AND

148

32. 33. 34.


35. 36. 37.

38. 39. 40. 41. 42. 43.

44. 45. 46. 47. 48. 49. 50. 51.

52. 53. 54.

ORGANOMETALLOIDS

E r n i , I. W., Ph.D. T h e s i s , Eidgenössische Technische Hochschule Z u r i c h , 1977. Rose, M. S., Biochem. J. (1969), 111, 129. A l d r i d g e , W. N. and S t r e e t , B. W., Biochem. J. (1964), 91, 287. Tobias, R. S. and Yasuda, M., Inorg. Chem. (1963), 2, 1307. Yasuda, M. and Tobias, R. S., Inorg. Chem. (1963), 2 , 207. " S t a b i l i t y Constants o f Metal-Ion Complexes," L. G. S i l l é n and A. E. M a r t e l l , e d s . , Special P u b l i c a t i o n No. 17, 1964, and No. 25, 1971, The Chemical S o c i e t y , London. Rabenstein, D. L., A c c t s . Chem. Res. (1978), 11, 100. Mansy, S. and Tobias, R. S., Biochemistry (1975), 14, 2952. Mansy, S., Wood, T. E., Sprowles, J. C., and Tobias, R. S., J. Amer. Chem. Soc. (1974), 96, 1762. Mansy, S. and Tobias, R. S., J. Amer. Chem. Soc. (1974), 96, 6874. Mansy, S., F r i c k , J. P., and T o b i a s , R. S., Biochim. Biophys. A c t a . (1975), 378, 319. Chrisman, R. W., Mansy, S., P e r e s i e , H. J., Ranade, Α., Berg, Τ. Α., and Tobias, R. S., B i o i n o r g . Chem. (1977) 7, 245. Z a n e l l a , P., Plazzogna, G., and Tagliavini, G., Inorg. Chim. A c t a . (1968), 2, 340. Simpson, R. B., J. Amer. Chem. Soc. (1961), 8 3 , 4711. Schwarzenbach, G. and K a r l e n , U., quoted i n ref. 32. Gross, H., Diplomarbeit, ΕΤΗ Zürich, quoted i n ref. 32. Fehr, R., Diplomarbeit, ΕΤΗ Zürich, quoted i n ref. 32. S t e i n e r , R., quoted i n ref. 32. L i b i c h , S. and Rabenstein, D. L., A n a l . Chem. (1973), 45, 118. Rabenstein, D. L., Ozubko, R., L i b i c h , S., Evans, C. Α., F a i r h u r s t , M. T., and Suvanprakorn, C., J. Coordn. Chem. (1974), 3, 263. Anderegg, G., Helv. Chim. A c t a . (1974), 57, 1340. Hohl, H., Diplomarbeit, ΕΤΗ Zürich, (1968), quoted i n ref. 32. Pilloni, G., M i l a n i , F., Inorg. Chim. A c t a . (1969), 3, 689.

R E C E I V E D August 28,

1978.


The Chemistry of Organometallic Cations in Aqueous Media - ACS

Recommend Documents