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11 Metal Chelate Compounds as Acid Catalysts in Solvolysis Reactions ARTHUR E. MARTELL Illinois Institute of Technology, Chicago 16, Ill.

Examples of metal chelate catalysis are given in

Downloaded by UNIV OF PITTSBURGH on June 12, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1963-0037.ch011

which the metal ion acts as a Lewis acid in the activation of the reaction between a substrate and a nucleophilic reagent.

Requirements for maxi

mum activity of Cu(II) chelates in the catalysis of the

solvation of diisopropylphosphorofluoridate

(DFP),

and

isopropylmethylphosphonofluoridate

(Sarin) are maximum positive charge on the com plex, minimum number of coordination sites of the metal ion occupied by the ligand, and minimum formation of μ-dihydroxo binuclear species. Ca talysis of salicyl phosphate hydrolysis is described for the aquo Cu(II), bipyridine-Cu(II) ion, N-hy droxyethylethylenediamine-Cu(II),

and

VO(IV)

ions, and the 1:1 and 2:1 vanadyl chelates of 3,5-disulfopyrocatechol.

A general

mechanism

proposed for metal ion and metal chelate-cat alyzed hydrolysis is a combination of the substrate with the metal ion in such a manner that intramo lecular nucleophilic attack of the phosphate group by the carboxylate group is note prevented. The inactivity of the mixed salicyl phosphate bipyri dine-CU(II) complex and the activity of the corre sponding 1,3-dicarboxyphenyl-2-phosphate com plex are in accord with this mechanism.

T h i s investigation of the catalysis of solvolysis reactions b y metal chelate com pounds is an outgrowth of our previous studies of solution equilibria and stabili ties of metal chelates. O f particular interest as catalysts are the chelates i n w h i c h the ligand does not completely satisfy the coordination requirements of the metal ion. Compounds of this type undergo interesting reactions such as hydrolysis (hydroxo complex formation), and olation (bridging of metal ions b y hydroxyl ions), to give polynuclear complexes. T h e residual coordinating tendencies of the metal ion i n these metal chelate compounds, w h i c h are responsible for their hydrolysis and olation reactions, also impart catalytic activity to these complexes.

loi Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.


162

ADVANCES IN CHEMISTRY SERIES

The catalytic effect is achieved through the weak L e w i s acid properties of the metal ion as the "active site" i n the metal chelate compound. T h e residual L e w i s acid activity of aquo metal ions a n d "incompletely coordinated" metal ions i n complexes a n d chelates i n aqueous solution is actually very weak compared to that of the hydrogen i o n ; on the other hand, metal ions a n d complexes are avail able i n solution at h i g h p H values, where the concentration of hydrogen ions is so low that their catalytic effect cannot be significant. O f particular interest as catalysts are the incompletely coordinated metal chelate compounds, w h i c h are sufficiently stabilized b y the ligand to be stable i n solution at p H values m u c h higher than that at w h i c h the aquo metal i o n w o u l d precipitate as the hydroxide a n d thus to become unavailable for homogeneous catalysis. Such a metal chelate w o u l d be particularly effective as a catalyst for the activation of a substrate w h i c h can coordinate to the metal ion i n the chelate compound. T h e interaction of the substrate w i t h the metal ion w o u l d increase its reactivity toward nucleophilic reagents such as solvent molecules or hydroxyl ions. in accordance w i t h the following scheme:

It is apparent that coordination of the substrate w i t h the metal i o n w o u l d increase its reactivity toward hydroxyl ions and other nucleophilic reagents. Because the electronic interaction of the metal i o n w i t h the substrate is considerably lower than that of the hydrogen ion, it is proposed that the metal ion be considered a "subproton" i n these catalytic réactions. Thus, the catalytic effect expected of the hydrogen ions, i f available at the same concentration (and conditions) as the metal i o n or metal chelate, w o u l d be expected to be much greater. Although the metal chelate is a n extremely weak L e w i s acid, it has some special properties not possessed b y the proton. B y virtue of its coordina tion number, size, and the steric requirements of its coordinate bonds, it w o u l d be expected to have a high degree of specificity, w i t h respect to both the nature of the substrate w i t h w h i c h it combines a n d the selectivity of interaction w i t h specific groups within the ligand. T h e considerable specificity that is theoretically possible i n metal chelate catalysis makes this type of study of considerable interest for the study of reaction mechanisms. Metal Chelate Catalysis in Solvolysis of Fluorophosphates T h e solvolysis of fluorophosphates such as methylisopropylphosphonofluoridate (Sarin) a n d diisopropylphosphorofluoridate ( D F P ) has been found to be catalyzed b y a number of metal ions and metal chelates such as I to V I ( 5 ) . T h e catalytic solvolysis reactions are generally first order i n metal chelate concentra tion, i n substrate concentration, a n d i n hydroxyl ion. A s is suggested b y these examples, the catalytic activity decreases as the negative charge of the ligand increases, a n d as the number of coordination positions on the metal i o n which are satisfied b y the l i g a n d increase. F o r a completely coordinated a n d stable

Busch; Reactions of Coordinated Ligands Advances in Chemistry; American Chemical Society: Washington, DC, 1962.

MARTELL

163

Chelates as Catalysts in Solvolysis

metal chelate, such as C u ( I I ) - t r i e n ( V I ) , there is very little activity. T h e aquo metal ion is generally the most effective catalyst, but its p H range of solubility is so low that very little free C u + ion is i n solution i n the p H range ( 7 or higher) 2

Half-Times of First-Order Sarin Hydrolysis (t =

2 5 ° ; -log [ H ]

=

+

7.0;

[Sarin] =

[metal chelate] =

-12+

10"

^ C H — C H 2

Ai)

8

2

NHCH; CH

2

CH

2

NHCH

3


II N-HydroxyethylethylenediamineC u ( I I ) i o n ; tm= 15 m i n .

Ν,Ν'-Dimethylethylenediamine-

C u ( I I ) i o n ; ii/2 = 3.5 m i n .

-12 + ^CH —CH 2

HO-CH2-CH:

2

,CH

2

C H

2

N, N - D i h y d r o x y e t h y l g l y c i n o - C u ( II ) i o n ; fi/2 = 25 m i n .

D i e t h y l e n e t r i a m i n e - C u ( II ) i o n ; = 25 m i n .

ti/2

- 2 +

HOCH2CH2

C H

2

- C O

CH —CH

v

2

2

NH CH I „CH NH

N-HydroxyethyliminodiacetatoC u ( I I ) i o n ; tm = 57 m i n .

Triethylenetetramine-Cu — 65 m i n .

ti/2

2

2

2

( II )

ion;

where the hydroxyl ion is available i n sufficient concentrations to give a reason ably h i g h rate of hydrolysis. F o r metal chelates and complexes of C u ( I I ) , it is necessary to have at least a bidentate donor coordinated to the metal ion to achieve reasonable stability i n dilute solution, so that the most effective C u ( I I ) complexes are those having a 1 to 1 molar ratio of a bidentate neutral ligand to the metal ion. Experimental measurement (6, 8) of the solvolysis of Sarin and D F P i n the presence of 1 to 1 diamine-metal chelates, such as those listed i n Table I, under varying solution conditions showed that the catalytic effect was not proportional to, or a simple function of, the total metal chelate species i n solution. A detailed analysis of the variation of rate w i t h composition of the solutions indicated (4,7) the presence of hydroxo and dihydroxo mononuclear forms of the chelate c o m pound, as w e l l as a binuclear μ,-dihydroxo species. If the possible reactivities of all catalytic species are taken into consideration, the rate expression w o u l d have the form: *obad

= WCuL^HOH-]

+ *CU[OHIL[GU(OH)L«]

+ ^oH^tCuiiOH)^^

WOH) L[GU(OH) L] + * c [ C u ^ ] [ O H - ] + *HO 2

2

u

2


+

ADVANCES IN CHEMISTRY SERIES

164

Near the neutral region ( p H 6 to 8) the concentration of the base, C u ( O H ) L , is negligible. The constants k and fc can be determined independently, while 2

H20

Cu

the influence of concentration on fc can be employed to show & C U 2 ( O H ) 2 L > catalytic effect of the binuclear form, to be negligibly small. Thus it is possible to determine k + & C U ( O H ) L > which are seen to be indistinguishable, since they are related to each other by the hydrolysis constant of the normal chelate compound— i.e., W ^ [ C u L + 2 ] [ O H - ] = * U < O H ) J C u ( O H ) L + ] , where K = [ C u ( O H ) L+]/[CuL+ ][OH-]). T

obsd

N

E

2

GuIj

B

B

C


2

H

2

U

JN

^

CuL

Figure 1.

Catalysis of Sarin and OF? hydrolysis by Cu(II) complexes plus OH~ and/or hydroxo Cu(H) complexes

The results of this type of analysis are expressed in Table I for the catalytic effects of Cu(II) chelates on the hydrolysis of Sarin (8) and D F P (6, 8 ) . Although there has been considerable disagreement in the literature as to whether the catalytically active species is C u L + or C u ( O H ) L + , it is seen from the mechanism outlined in Figure 1 that such distinctions are meaningless. Since the rate-determining step is probably the breaking of the phosphorus-fluorine bond (XI), the substrate (VII), metal chelate ( C u L ) , and hydroxyl ion are involved in a number of interdependent pre-equilibria, all leading to the same monoprotonated reactive intermediate, which exists in two (or more) tautomeric forms, Xa and Xb. More basic catalysts indicated in the general rate law lead to a less protonated intermediate, or react by direct attack on the phosphorus atom. The constants listed in Table I reflect the significance of the hydrolysis and 2


MARTELL Table Κ

Chelates as Catalysts in Solvolysis

165

Third-Order Rate Constants Assigned to 1:1 Cu(ll) Chelates as Catalysts in the Hydrolysis of Sarin and DFP at 25° Ligand

DFP

h

TMEN, H 0 DMEN, H 0 DIPY, H 0 PHEN, H 0 HEN, H 0 DHEN, H 0

7.0 2.3 7.4 4.9 6.4 4.0

2

2

2

2

2

2

X 10« X 106 X 10» X 106 X 10* X 10*

Sarin* 1 . 0 X 10» 3 . 2 Χ 10 3.1 X 10? 1 χ i 7 9.3 X 10« 5 . 2 X 106 7

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