Electrochemical Surface Science - ACS Publications - American

Chapter 31

Electrosynthesis and Electrochemistry of Metalloporphyrins Containing a Metal—Carbon σ-Bond Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 27, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch031

Reactions of Rhodium, Cobalt, Germanium, and Silicon Complexes 1

K. M . Kadish, Q. Y. Xu, and J . E. Anderson

Department of Chemistry, University of Houston, Houston, TX 77004 The electrosynthesis of metalloporphyrins which con t a i n a metal-carbon σ-bond is reviewed i n t h i s paper. The electron transfer mechanisms of σ-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and c o n t r o l l e d - p o t e n t i a l electrooxidation/reduction. The four described electrochemical systems demonstrate the v e r s a t i l i t y and s e l e c t i v i t y of electrochemical methods f o r the synthesis and c h a r a c t e r i z a t i o n of metal-carbon σbonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the com monly used CH2Cl2 is also discussed. Metalloporphyrins containing a metal-carbon σ-bond are currently l i m i t e d to complexes with eight d i f f e r e n t t r a n s i t i o n metals ( T i , N i , Fe, Ru, Co, Rh, I r and Zn) and seven d i f f e r e n t non-transition metals ( A l , Ga, I n , T l , S i , Ge, and Sn). These compounds have been the subject of several recent reviews(l-3) which have discussed t h e i r synthesis and physicochemical proper ties. The synthesis of metalloporphyrins which contain a metalcarbon σ-bond can be accomplished by a number of d i f f e r e n t methods(_1^2). One common synthetic method involves reaction of a Grignard reagent or a l k y l ( a r y l ) l i t h i u m with (P)MX or (P)M(X) where Ρ i s the dianion of a porphyrin macrocycle and X i s a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an a l k y l or a r y l h a l i d e . This l a t t e r technique i s s i m i l a r to methods described i n t h i s paper for electrosynthesis of cobalt and rhodium σ-bonded complexes. However, the p r e v a i l i n g mechanisms and the chemical reactions 2

1

Current address: Department of Chemistry, Boston College, Chestnut Hill, M A 02167

β

0097-6156/88/0378-0451$06.00/0 1988 American Chemical Society

In Electrochemical Surface Science; Soriaga, Manuel P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

E L E C T R O C H E M I C A L SURFACE SCIENCE

Downloaded by UNIV OF MASSACHUSETTS AMHERST on February 27, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch031

452

following electrogeneration may d i f f e r due to the difference i n reaction conditions for generation of the reactive species. Two aspects of porphyrin electrosynthesis w i l l be discussed i n t h i s paper. The f i r s t i s the use of c o n t r o l l e d p o t e n t i a l electroreduction to produce metal-carbon σ-bonded porphyrins of rhodium and cobalt. This e l e c t r o s y n t h e t i c method i s more selec t i v e than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with c y c l i c voltammetry, can be used to determine the various reaction path ways involved i n the synthesis. The e l e c t r o s y n t h e t i c method can also lead to a simultaneous or stepwise formation of d i f f e r e n t products and several examples o f t h i s w i l l be presented. The second type of porphyrin electrosynthesis discussed i n t h i s paper i s c o n t r o l l e d p o t e n t i a l electrooxidation of σ-bonded b i s - a l k y l or b i s - a r y l porphyrins of Ge(IV) and S i ( I V ) . This electrooxidation r e s u l t s i n formation of σ-bonded mono-alkyl or mono-aryl complexes which can be i s o l a t e d and characterized i n s i t u . Again, c y c l i c voltammetry can be coupled with t h i s method and w i l l lead to an understanding of the various reaction path ways involved i n the electrosynthesis. Dozens of electrochemical and spectroelectrochemical papers on t r a n s i t i o n metal and main group metal-carbon σ-bonded metallo porphyrins were published between 1984 and 1987 and a summary of these r e s u l t s are well covered i n three recent reviews(l-3). Therefore, a characterization of chemically synthesized metalcarbon porphyrins w i l l not be discussed i n t h i s paper. Rhodium Porphyrins. Chemical syntheses of [(P)Rh] and (P)Rh(R) complexes are w e l l known(A-ll). Electrochemical techniques have also been used to synthesize dimeric metal-metal bonded L(TPP)Rh] as well as monomeric metal-carbon σ-bonded (TPP)Rh(R) and (OEP)Rh(RX12-16). The e l e c t r o s y n t h e t i c and chemical synthe t i c methods are both based on formation of a highly reactive monomeric rhodium(II) species, (P)Rh. This chemically or electrochemically generated monomer r a p i d l y dimerizes i n the absence of another reagent as shown i n Equation 1. 2

2

(P)Rh

*

1/2 [(P)Rh]

2

(1)

However i n the presence of hydrogen gas, (P)Rh(H) can be formed as shown i n Equation 2. (P)Rh + 1/2 H

2

*

(P)Rh(H)

(2)

Reactions 1 and 2 involve an equilibrium which i s s h i f t e d to the r i g h t . The methods generally u t i l i z e d for chemical genera t i o n of (P)Rh i n i t i a l l y involve a dimeric Rh(II) species or the Rh(III) hydride, and r e l a t i v e l y high temperatures and/or long reaction times are required to s y n t h e t i c a l l y generate (P)Rh(4-9). For t h i s reason, much of the mechanistic information i n v o l v i n g (P)Rh reactions i s l o s t and a detection of reaction intermediates i n the synthesis of the σ-bonded (P)Rh(R) complexes i s generally not possible.


31.

Metalloporphyrins Containing Metal-Carbon σ-Bonds

KADISH ET AL.

The method for electrosynthetic generation of (P)Rh(R) involves an i n i t i a l formation of (P)Rh and i s accomplished by the one electron reduction of an i o n i c Rh(III) porphyrin s p e c i e s ( l A ) . The generated Rh(II) porphyrin can dimerize as shown i n Equation 1 but, i n the presence of an a l k y l halide RX, w i l l react to form (P)Rh(R). The o v e r a l l reaction between RX and ( P ) R h i s given by Equation 3, where S i s an electron source other than the electrode. 11


(P)Rh

11

+ RX + S + (P)Rh(R) + X"

+ S

+

(3)

The reactions associated with Equation 3 can be monitored by c y c l i c voltammetry which give current voltage curves of the type shown i n Figures l a and lb for the case of RX = CH I. The f i r s t reduction i n Figure l b i s the Rh(III) Rh(II) t r a n s i t i o n and the second i s the r e v e r s i b l e reduction of (TPP)Rh(CH ). An analysis of these c y c l i c voltammograms and other s i m i l a r voltammetric/ spectroelectrochemical data gave the following reaction sequence where L i s dimethylamine(14). 3

3

Scheme I [(TPP)Rh(L)J

+

+

(TPP)Rh + RX

+

(TPP)Rh + RX X*

+

[(TPP)Rh(R)]

+

-

e

(TPP)Rh(R) + X* [(TPP)Rh(R)]

S +

+ S

(TPP)Rh + 2L

X" -

+

S

+

(5a)

+ X"

(5b)

+

(TPP)Rh(R) + S

(4)

(6a) +

(6b)

After electrogeneration of (TPP)Rh (Equation 4) and the attack at the R-X bond, either a loss of X* and formation of ÇTPP)Rh(R) (Equation 5a) or a loss of X- with formation of L(TPP)Rh(R)J (Equation 5b) w i l l occur. Reaction pathways 5a and 6a occur for RX complexes where X = I or Br while reactions 5b and 6b occur for RX complexes where X = CI or F. The mechanism shown i n Scheme I was supported by coulometric determinations, the formation of X- as one of the reaction products, and the absence of any (TPP)RhX as a f i n a l product(14). Equations 6a and 6b i n Scheme I indicate that a second electron source other than the electrode i s involved i n the o v e r a l l react i o n and was postulated on the basis of coulometric data which indicated that only one electron was electrochemically transferred. This was true despite the fact that the o v e r a l l reaction requires a t o t a l of two electrons. A f u l l discussion of these r e s u l t s are presented i n the o r i g i n a l report(14). An electrosynthetic method was used to generate the 25 d i f ferent (P)Rh(R) complexes l i s t e d i n Table 1(14,16). Many of the complexes i n t h i s Table had not been previously reported, espec i a l l y (P)Rh(RX). In a l l cases, bulk e l e c t r o l y s i s of +


453

454



I 0.0

ι

ι -0.4

ι

ι -0.6

ι

ι -1.2

ι

ι -1.6

I

I -2.0

POTENTIAL, (V vs SCO 4

Figure 1. Cyclic voltammograms obtained at Pt electrode: a, 9.9 χ Ι Ο " M [(TPP)Rh(L)2] Cr; b, 9.9 χ Ι Ο " M [ ( T P P J R h ^ J + C r and 1.0 equivalent of C H I ; c, (TPP)Co; and d, (TPP)Co and 1.5 equivalent of C H I in T H F containing 0.1 M TBAP. Adapted from refs. 14 and 26. +

3

4

3


31.

Metalloporphyrins Containing Metal—Carbon σ-Bonds

KADISH E T AL.

Table I. The Redox Potentials and UV-vis Spectral Data for the Electrogenerated (TPP)Rh(R) Complexes in THF Containing 0.2 M TBAP a

R Group


Complex (R)Rh(R)

CH

l/2

—

U 3

0.97 0.97

-1.90 -1.90

1.33

-1.42

-1.91

418(22.9) 525(2.3) 411(21.6) 524(1.9) 411(17.7) 524(1.5)

1.3A

-1.42

-1.92

411(21.5) 524(1.7)

0.98

1.34

-1.42

-1.91

411(25.2) 524(2.3)

0.97

1.34

-1.42

-1.91

411(27.9) 524(2.8)

—

—

—

—

— — —

— — —

-1.41

-1.86

411(27.2) 524(2.5)

-1.41

-1.88

411(27.9) 524(2.7)

-1.42

-1.90

1.12

CH

7

H

49 H

5 11

C

H

6 13 CH(CH,)CH'2"'3 CH. C(CH ) 0

e

3

C

H

C 1

8

C H C1 5

10

CjHgBr C H Br A

C

8

H

Br

5 10

W C H, 12 6

A

9

1.14

411(29.2) 524(3.2)

e

411(23.7) 524(1.9)

-1.40

-1.85 -2.01 -1.89

f

-1.40

0.98

1.35

-1.38

0.97

1.34

-1.39

-1.78^ -1.99

0.98

1.35

-1.40

-1.8lf -1.88

0.99

1.35

-1.41

-1.82^ -1.88

—

—

-1.42

f

411(26.9) 524(2.2) f

411(30.9) 524(2.6) 411(27.7) 524(2.2) 411(27.2) 524(2.2) 415(20.0) 527(2.0)

-

413(21.9) 526(2.2) 415(18.5) 527(1.8)

-1.41

418(20.8) 529(2.2)

-

-1.43

418(15.5) 528(2.0)

CC1

-1.41

421(21.2) 533(2.1)

CI,

-1.43

416(11.8) 526(1.7)

-1.43

--

-

CHC1

2

CHIo 3

411(24.7) 524(2.4)

f

-1.81 -1.83

-1.43

2

(P)Rh(CX )

411(24.9) 524(2.0) f

-1.39

—

2

CH I 2

f

530

415

1.149

CH Br

(P)Rh(CHX )

417

3

3 6 C H C1 A

UV-Vis

( R e d )

-1.41

5

C

l/2

-1.45

2

C

E

( 0 x ) b

— —

CH 3

(P)Rh(RX)

c

E

3

From reference 12, 13, 14. Measured in PhCN. nm (e χ 10-4, - l cm-1). M

Measured in PhCN. Complete formation of (P)Rh(R) is not observed. E

p measured at 0.1 V/s. Measured in THF.


455

456



+

[(TPP)Rh(L)J C1~ i n the presence of an a l k y l halide leads to a given (P)Rh(R) or (P)Rh(RX) complex. The y i e l d was nearly quant i t a t i v e (>80%) i n most cases based on the rhodium porphyrin s t a r t i n g species. However, i t should be noted that excess a l k y l halide was used i n Equation 3 i n order to suppress the competing dimerization reaction shown i n Equation 1. The ultimate (P)Rh(R) products generated by electrosynthesis were also characterized by H NMR, which demonstrated the formation of only one porphyrin product(14). No reaction i s observed between (P)Rh and a r y l halides but this i s expected from chemical reactivity studies(10,15). Table I also presents e l e c t r o n i c absorption spectra and the reduction and oxidation potentials of the electrogenerated (P)Rh(R) complexes. C y c l i c voltammetry was used to monitor the mechanism for (TPP)Rh(R) formation according to Equation 3 where RX i s a t e r minal a l k y l halide. The current for reduction of electrogenerated (P)Rh(R) or (P)Rh(RX) species i s a measure of the concentration of t h i s product at the electrode surface, and hence a measure of i t s rate of formation. The current must be standardized for experimental factors such as scan rate, i n i t i a l concentration of [(TPP)Rh(L)J cr (or other i n i t i a l Rh(III) species), electrode area, and concentration of the a l k y l halide reactant. The r e s u l t s of k i n e t i c experiments with terminal a l k y l h a l i des demonstrate that the o v e r a l l reaction rate between (P)Rh and RX i s dependent upon the s i z e of the a l k y l h a l i d e , the number of halide groups (RX or RX ) and on the type of halide. The react i o n rate of RX follows the trend I > Br > F = C1(1A). Also, the larger the a l k y l group, the slower i s the chemical reaction(14). Mechanistic studies of homogenous chemical reacTTons involving formation of (P)Rh(R) from (P)Rh and RX demonstrate a r a d i c a l pathway(9). These studies were c a r r i e d out under d i f ferent experimental conditions from those i n the electrosynthes i s . Thus, the difference between the proposed mechanism using chemical and electrochemical synthetic methods may be due to d i f ferences related to the p a r t i c u l a r investigated a l k y l halides i n the two d i f f e r e n t studies or a l t e r n a t i v e l y to the d i f f e r e n t react i o n conditions between the two sets of experiments. However, i t should be noted that the electrochemical method for generating the reactive species i s under conditions which allow for a greater s e l e c t i v i t y and control of the reaction products. The highly reactive nature of (P)Rh i s perhaps best demonstrated by the reaction of electrochemically generated (TPP)Rh with terminal alkenes and alkynes. The o v e r a l l reaction with alkynes i s given by Equation 7 and the suggested mechanism +

2

(P)Rh + HC"CR ·* (P)Rh(R) + products

(7)

i s presented i n Scheme 11(15). A s i m i l a r reaction mechanism was also found to occur with aïl (aq) 4

•2 -β C(0EP)Si(C H5)0H] 6

===== [(OEPJSKCeHsJOH]

=====

^y•2 [(OEPJSKCeHgJCIOj

-

—

- [(OEP)Si(C H )CIOJ 6

5

• -β ======

> [(0EP)SKC H5)0H]"

(0EP)SKC H5)0H

e

e

OH* HCK>4(aq)

'

\ \

β

\

(OEP)SKC H5)^4=======C(OeP)SJ(C H5)Cl04J 6


6

463


464

A comparison of Schemes V and VI show that the electroche mistry of (0EP)Ge(C H )0H and (0EP)Si(C H )0H are s i m i l a r . The f i r s t oxidation of (0EP)Si(C H )0H occurs at the hydroxyl ligand and generates (0EP)Si(C H )ClCU as a f i n a l product. This com pound can be r e v e r s i b l y oxidized by up to two electrons at more p o s i t i v e p o t e n t i a l s and gives a porphyrin π cation r a d i c a l and dication(36). In summary, the four chemical systems described i n t h i s paper demonstrate the v e r s a t i l i t y and s e l e c t i v i t y of electrochemical methods f o r synthesis and characterization of metal-carbon σbonded metalloporphyrins. The described rhodium and cobalt systems demonstrate s i g n i f i c a n t differences with respect to t h e i r formation, s t a b i l i t y and to some extend, r e a c t i v i t y of the low valent species. On the other hand, properties of the electrochemically generated mono-alkyl or mono-aryl germanium and s i l i c o n systems are s i m i l a r to each other. 6

5

6

6


6

5

5

5

Acknowledgment. The support of the National Science Foundation [Grant CHE-8515411) i s g r a t e f u l l y acknowledged. We also acknowledge numerous discussions with our c o l l a b o r a t o r , Dr. Roger Guilard, from the University of Dijon i n France. L i t e r a t u r e Cited ( 1) Guilard, R.; Lecomte, C.; Kadish, Κ. M. Struct. Bond. 1987, 64, 205-268. ( 2) Brothers, P. J . ; Collman, J . P. Acc. Chem. Res. 1986, 19, 209. ( 3) Kadish, Κ. M. Prog Inorg. Chem. 1986, 34, 435-605. ( 4) Highes, R. P. Comprehensive Organometallic Chemistry; Wilkinson, G. Ed.; Pergamon Press: Ν. Y., 1982, V o l . 5, p. 277. ( 5) Wayland, Β. B.; Woods, Β. Α.; C o f f i n , V. L. Organometallics 1986, 5, 1059. ( 6) Del Rossi, K.; Wayland, Β. B. J. Am. Chem. Soc. 1985, 107, 7941. ( 7) Wayland, Β. B.; Woods, Β. Α.; Pierce, R J. Am. Chem. Soc. 1982, 104, 302. ( 8) Setsune, J . ; Yoshida, Z.; Ogoshi, H. J . Chem. Soc. Perkin Trans. 1982, 983. ( 9) Paonessa, R. S.; Thomas, N. C.; Halpern, J . J. Am. Chem. Soc. 1985, 107, 4333. (10) Aoyama, Y.; Yoshida, T.; Sakurui, K.-I.; Ogoshi, H. J. Chem. Soc. Chem. Comm. 1983, 478. (11) Aoyama, Y.; Yoshida, T.; Sakurui, K.-I.; Ogoshi, H. Organometallics 1986, 5, 168. (12) Kadish, K. M.; Yao, C.-L.; Anderson, J . E.; Cocolios, P. Inorg. Chem. 1985, 24, 4515. (13) Anderson, J . E. Yao, C.-L.; Kadish, Κ. M. Inorg. Chem. 1986, 25, 718. (14) Anderson, J . E.; Yao, C.-L.; Kadish, Κ. M. J. Am. Chem. Soc. 1987, 109, 1106. ;


31.

KADISH ET AL.

(15) (16) (17) (18)


(19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) RECEIVED

Metalloporphyrins Containing Metal-Carbon σ-Bonds

Anderson, J . Ε.; Yao, C.-L.; Kadish, Κ. M. Organometallics 1987, 6, 706. Anderson, J . E.; L i u , Y. H.; Kadish, Κ. M. Inorg. Chem. 1987, 26, 4174. Collman, J . P.; Barnes, C. E.; Swepston, P. N.; Ibers, J . J . Am. Chem. Soc. 1984, 106, 3500. Farnor, M. D.; Woods, Β. Α.; Wayland, Β. B. J . Am. Chem. Soc. 1986, 108, 3659. Dolphin, D.; Halko, D. J . ; Johnson, E. Inorg. Chem. 1981, 30, 4348. Lexa, D.; Saveant, J.-M.; S o u f f l e t , J.P. J. Electroanal. Chem. I n t e r f a c i a l Electrochem. 1979, 100, 159. Perree-Fauvet, M.; Gaudemer, Α.; Boucly, P.; Devynck, J. J . Organomet. Chem. 1976, 120, 439. Clarke, D. Α.; Grigg, R.; Johnson, A. W.; Pinnock, H. A. J. Chem. Soc. Chem. Comm. 1967, 305. Clarke, D. Α.; Dolphin, D.; Grigg, R.; Johnson, A. W.; Pinnock, H. A. J. Chem. Soc. 1968, 881. Ogoshi, H.; Wantanabe, E.; Koketsu, N.; Yoshida, Z. B u l l . Chem. Soc. Jpn. 1976, 49, 2529. Momenteau, M.; Fournier, M.; Rougee, M. J. Chim Phys. 1970, 67, 926. Kadish, K. M.; L i n , X. Q.; Han, B. C. Inorg. Chem. 1987, 26, 4161. Kobayashi, H.; Hara, T.; Kaizu, Y. B u l l . Chem. Soc. Jpn. 1972, 45, 2148. Maskasky J . E.; Kenney, M. E. J . Am. Chem. Soc. 1971, 93, 2060. Maskasky, J. E.; Kenney, M. E. J . Am. Chem. Soc. 1973, 95, 1443. Cloutour, C.; Lafargue, D.; Richards, J . Α.; Pommier, J . C. J. Organomet. Chem. 1977, 137, 157. Cloutour, C.; Debaig-Valade, C.; Pommier, J . c.; Dabosi, G.; Marthineau, J . J . Organomet. Chem. 1981, 220, 21. Cloutour, C.; Lafargue, D.; Pommier, J . C. J. Organomet. Chem. 1983, 190, 35. Cloutour, C.; Debaig-Valade, C.; Gacherieu, C.; Pommier, J . C. J. Organomet. Chem. 1984, 269, 239. Cloutour, C.; Lafargue, D.; Pommier, J . C. J . Organomet. Chem. 1978, 161, 327. Kadish, K. M.; Xu, Q. Y.; Barbe, J.-M.; Anderson, J . E.; Wang, E.; Guilard, R. J . Am. Chem. Soc. 1987, 109, 7705. Kadish, Κ. M.; Xu, Q. Y.; Barbe, J.-M.; Guilard, R. Inorg. Chem. 1988, 27, 1191. July 5, 1988


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