Basicity and Nucleophilicity of Transition Metal Complexes - American

16 Basicity and Nucleophilicity of Transition Metal Complexes Ralph G. Pearson

Downloaded by UNIV QUEENSLAND on April 16, 2013 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch016

Department of Chemistry, University of California, Santa Barbara, CA 93106

Rates of reaction of transition metal nucleophiles correlate both with oxidation potentials for MLn- and with the pKa values of the corre sponding acids, HMLn. Therefore, the two parameters, E° and H, in the Edwards equation are not independent parameters. The same result is found for other nucleophiles, if the donor atom is C, N, O, or F. However, for bases with heavier donor atoms, E° and H are not as correlated with each other. For transition metal complexes, soft ligands, L, increase acidity and decrease nucleophilic reactivity. Hard ligands have the opposite effect.

T R A N S I T I O N M E T A L C O M P L E X E S , either as anions or as neutral molecules, are often very good nucleophiles for alkyl halides and sulfonates. Some of them, such as vitamin B , a cobalt (I) species, and the solvent-separated ion pair, Na :S:Fe(CO) ~, where S is N-methylpyrrolidinone, are among the most reactive known ( i , 2). Reactions are of several types, for example 1 2 s

+

2

4

RX + M L — R M L n

+ n

+ X-

R X + M L -> R M X L n

(1) (2)

n

where M is the metal atom and L stands for η ligands bound to M . A variety of mechanisms have been found (3). Simple S 2 substitution mechanisms are most common, followed by free-radical pathways. The latter occur by two mechanisms: (a) removal of X as an atom, followed by reactions of R-; and (b) single-electron transfer from M L to RX, to give R \ Hydride ion transfer reactions are also known (4). Reactions believed to be simple S 2 processes in which R X is methyl iodide are discussed here. n

N

n

N

RX + H M L

n

— RH + X" + M L

+ n

0065-2393/87/0215-0233$06.0G70 © 1987 American Chemical Society

In Nucleophilicity; Harris, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

(3)

234

NUCLEOPHILICITY

Characteristically, transition metal nucleophiles react much faster with methyl iodide than with methyl tosylate. Rate constant ratios ranging from 30 to 3 Χ 10 have been found (3). Such behavior qualifies transition metal complexes to be called supersoft nucleophiles (5). Even larger ratios are found for reagents such as Co(CN) ~, up to 10 . Such large ratios are found only for free-radical pathways (6) and may be used as a mechanistic probe. 5

3

9

5

Nucleophilic Reactivity and Redox Potential A pioneering study of transition metal nucleophiles was made by Dessy et al. (7). These workers measured not only rates of reaction of various M L " with C H I but also the oxidation potentials at a platinum electrode. A good linear relation was found when log k was plotted against E for various nu cleophiles. n


3

2

m

ML„ — M L + e

E

n

(4)

w

Such a finding was not unexpected because Edwards (8) had already put forward his equation log (k/k ) = α £ ° + β #

(5)

0

relating nucleophilicity to two properties of the nucleophile: the ease of electron loss, as given by E°, the redox potential, and the proton basicity, as given by Η (Η = ρΚ + 1.74). Soft nucleophiles would be expected to have large values of α and small values of β. A n even better reason exists to expect that the nucleophilicity of a transition metal complex would depend strongly on its ease of oxidation. Reaction with an alkyl halide, according to either reaction 1 or 2, is an example of oxidative addition. The oxidation state of the metal increases by two units. Taking a definite example, we find that the change is more than a formal one. The reactant is a typical d complex, whereas the product is a definite d complex. The product has the right coordination number, squareplanar structure, and v i s i b l e - U V spectra found for similar Pt(II) complexes. α

1 0

8

0

P t [ P ( C H C H ) ] 3 + C H B r — C ^ P r ^ C r ^ C H ^ ^ + B r " (6) 3

2

3

3

The more readily the metal atom in the complex M L can be oxidized, the more rapidly it can react with alkyl halides. Consideration of a large amount of experimental data on oxidative additions, in general, leads to a metal ordering: Os° > Ru° > Fe° » Ir > R h > C o » Pt > P d » N i (9). For any one metal, M > M° > M and so on. The associated ligands can play an equally important role to that of the metal. Ligands that stabilize the reduced form of a metal will deactivate the n

1

1

1

1

11

11

1


1 1

16.

Basicity and Nucleophilicity of Transition Metal Complexes 235

PEARSON

metal complex toward nucleophilic behavior. Such ligands are C O , P R , C H , and other soft bases. Hard bases such as amines and alcohols will activate the complex by stabilizing the oxidized state. A complex such as R h C l ( C O ) [ P ( C H ) ] is a very poor nucleophile, but R h ( C D O B F ) is extremely reactive (10). C D O B F is a complex ligand that has four nitrogen donor atoms. Tjiese conclusions are somewhat unexpected. Soft ligands are commonly thought to put more negative charge density on the metal atom than hard ligands and thus make the metal atom a better electron donor. However, Kubota (11) made a detailed study of the reaction 3

2

4

I

I

6

5

3

2

2


2

IrX(CO)[P(C H ) ] + C H I 6

5

2

2

3

2

3

CH IrIX(CO)[P(C H ) ] 3

6

5

3

(7)

2

-

The order of rates found for different Xs was F " > N ~ > C l > Br~ > N C O - > Γ > N C S ~ with hard F " reacting 100 times faster than soft (Sbonded) N C S " . 3

Nucleophilicity and Br0nsted Basicity At the time of the study by Dessy et al. (7), little was known about the Br0nsted basicity of M L ~ in a quantitative sense. Therefore, what role, if any, was played by the parameter H in the Edwards equation was unclear. Recently, a number of p K values were measured for transition metal hydrides, both neutral, H M L , and cationic, H M L . The solvents used were chiefly methanol (12) and acetonitrile (13; J. R. Norton and J. Sullivan, personal communication). n

a

+

n

n

M L " + H+ — HML„ n

pK

(8)

a

Table I gives a listing of relative rate constants for reaction with C H I for a number of transition metal bases. The values are normalized to that for C o ( C O ) " set equal to unity and are valid for solvents such as tetrahydrofuran (THF) or glyme. Also given are the pK values for the conjugate acids in acetonitrile. In some cases, pK values in methanol were adjusted to acetonitrile by using references of the same charge type. A very strong correlation between nucleophilic reactivity and Br0nsted basicity occurs. The correlation is not perfect, but it cannot be expected to be, because a variety of types of nucleophiles are represented, and the solvents also vary. Clearly, the dependence on H in equation 5 is as great as the dependence on E°. Some data seem to contradict this conclusion. The basicity of P t [ P ( C H C H ) ] is much greater than that of P d [ P ( C H C H ) ] or N i [ P ( C H C H ) ] (14). The rate constants for reaction with C H I are exactly opposite (3). However, these reactions are quite complicated. The actual 3

4

fl

fl

3

3

2

2

3

3

3

3

3

3


2

3

3

236

NUCLEOPHILICITY

Table I.

Nucleophilic Reactivity and Basicity of a Series of Transitional Metal Bases

Base

3

C H Fe(CO) -

70,000,000

19.4

C H Ru(CO) -

7,500,000

20.2

5

5

5

2

5

2

Co(dmgH)(P[CH (CH )3]3)-

57,000

17.5
H C r ( C O ) [ P ( O C H ) ] - > HW(CO) - > CpV(CO) H- > HCr(CO) - > HRu(CO) " > H F e ( C O ) [ P ( O C H ] - » H F e ( C O ) " . This order is also that of decreasing basicity for these anions. Whether the mechanism for reaction 3 is a direct H ~ transfer to R X or oxidative addition of R X occurs first, followed by reductive elimination of R H , is unclear. Although the available data leave some questions, another good reason to believe that a strong correlation exists between nucleophilic reactivity and basicity follows. The protonation reaction 8 for M L " , or M L , is also an oxidative addition. For example 3

2

3

4

5

3

3

3

33

2

3

3

2

3

2

4

3

5

3

4

4

n

Pt° [ P ( C H C H ) ] 3

2

3

3

+ H+ -

n

H P t " [P(CH CH ) ] + 3

2

3

3


(9)

16.


PEARSON

Therefore, any predictions about the effect of changing the metal, or the ligands, L , are exactly the same for reaction 1 or 2 and reaction 9. Accordingly, we predict that soft bases, such as C O , P ( O C H ) , and C H will increase acidity, and hard bases, such as N R and R 0 , will decrease acidity. These predictions are borne out very well (12). Soft bases will stabilize the lower oxidation state, for example 3

2

4

3

n

H Fe (CO) 2

HFe°(CO) - + H

4

3

2

(10)

+

4

Alternatively, ττ-bonding ligands, such as C O , stabilize the anion by delocalizing the lone electron pair. A striking example of the role of the ligands is shown by Rh (bipy) , studied by Sutin and co-workers (17). The protonated form, R h ( b i p y ) H has a pK of 7.3 in water. In contrast, R h ( N H ) H has a pK > 14.0 and R h ( C N R ) H has a p K < 0 (18). The isocyanide ligand is similar to carbon monoxide. The high basicity of Rh(bipy) correlates with the high nu cleophilic reactivity of R h ( C D O B F ) . I

+


2

m

2+

2

m

2 +

fl

3

5

a

2 +

4

a

+

2

2

Relationship between E ° and Η The arguments given state that easily oxidized bases will be good nu cleophiles toward alkyl halides and also strong bases toward the proton. This statement means that E° and Η in the Edwards equation are no longer independent parameters but essentially one and the same property for transition metal bases. For bases where the donor atom is a nonmetal, this correlation is normally not the case. The fluoride ion is a stronger base than the iodide ion but is more difficult to oxidize. Still, examination of bases of the representative elements more closely to see if E° and Η are truly independent is worthwhile. Actually E°, the redox potential in water, is not the best parameter to use. E° is measurable only for a few nucleophiles and is complicated by the nature of the products formed. For example, in 21- = I + 2e

(Π)

2

the iodine-iodine bond strength as a factor is not desirable. A much better parameter to use is the ionization potential (IP) of the nucleophile (EA is electron affinity). 1(g)" = 1(g) + e NH (g) = NH +(g) + e 3

3

(12a)

EA IP

(12b)

Ritchie (20) recently emphasized this point and showed how the correspond ing values can be obtained in aqueous solution


238

NUCLEOPHILICITY

I-(aq) = I(aq) + e NH (aq) = N H 3

+ 3

E°'

(13a)

(aq) + e

E°'

(13b)

For the present purposes, a method equivalent to Ritchie's may be used. For a series of anionic nucleophiles, the energies of the two gas-phase reactions are compared (PA is proton affinity): X-(g) = X(g) + e +

H (g)

+ X-(g) = HX(g)

EA

(14)

PA

(15)


For a series of neutral nucleophiles, the corresponding reactions are +

B(g) = B (g) + e +

H (g)

IP

+

+ B(g) = BH (g)

(16)

PA

(17)

Any linear relationship between equations 14 and 15, or between equations 16 and 17 will also be found in solution. Solvation energies and entropies will either be constant, as for the H in equation 15, or will cancel as for X in equations 14 and 15, or nearly cancel, as for B and B H in equations 16 and 17. The reasoning involved in making these statements, and supporting literature citations, were given by Ritchie (19). Figure 1 shows a plot of the gas-phase electron affinities of a number of anions plotted against the gas-phase proton affinities. The circles refer to anions where the donor atom is a second-row element, C , N , O, or F. The straight line is a least-squares fit to these values only. The slope is - 0.87 and the correlation coefficient is —0.923. In spite of the scatter, clearly a strong negative correlation between the electron affinity of a radical and the proton affinity of its anion occurs. A strong base, such as C H ~ , loses its electron readily, whereas P 0 ~ (metaphosphate ion) is a weak base and is difficult to oxidize. The crosses refer to anions where the donor atom is from the third, fourth, or fifth row. These data points fall regularly below the line: I > Br~ C l " ; S e H " > S H " ; A s H " > P H " ; and G e H ~ > S i H " . These nu cleophiles are all weaker bases than their ease of oxidation would suggest. This behavior is easily traced to low values of the homolytic bond energy of HX. If the homolytic bond energy (D ) was constant for all H X +

-

+

+

3

3

-

2

2

3

3

0

HX(g) = H-(g) + X-(g)

D

0

(18)

then a perfect linear relationship would occur in Figure 1, and the negative slope would be unity. However, D does not have to be constant to obtain a straight line. For simple H X molecules, D and E A values are as follows: 0

0


PEARSON

Basicity and Nucleophilicity of Transition Metal Complexes

239


16.

0

1

2

3

4

5

E A , volts

Figure 1. Plot of the electron affinities of anions against their proton affinities. Circles are for C, IV, O, and F donor atoms, and crosses are for heavier donor atoms. Data are from reference 32 for proton affinities and references 20, 31, and 32 for electron affinities.


240

NUCLEOPHILICITY

Molecule

D of H X (kcal)

Ε A of X· (eV)

HF H 0 NH CH

136 119 107 105

3.80 1.83 0.74 0.08

0

2

3

4

The resulting opposite trends of D and PA are responsible for the negative slope of Figure 1 being less than unity. Also, the scatter of the anions of the second-row elements is due to variations in D not related to the electron affinity. Figure 2 is a plot of the ionization potential against proton affinity for a number of neutral molecules, where again the donor atom is C, N , O, or F. The molecules include H F , H 0 , N H , N , C O , singlet C H , and a sampling of ethers, alcohols, amines, esters, amides, nitriles, and other organic mole cules. The latter compounds are considered to represent several hundred such molecules for which both IP and PA are known (20-21). The straight line drawn has a slope of —0.55, and the correlation coefficient is 0.939. For these neutral bases where the donor atom is a second-row element, again a strong correlation between the basicity and the ease of losing an electron exists. The larger scatter in Figure 1 is due to random variations in the homolytic bond energy. 0


0

2

+

3

2

B H ( g ) = B-(g) + H-(g)

2

D

0

(19)

Such scatter is expected in view of the wide variety of molecules. If a series of closely related molecules, such as substituted anilines, had been chosen, then a good straight line would be obtained with a negative slope close to 1 (22) . If molecules in which the donor atom is in the third, fourth or fifth row of the periodic table are added to Figure 2, then similar results to those of Figure 1 are found. The heavier donor atoms lie below the line, correspond ing to weak homolytic bond energies. Exceptions occur for alkyl phosphines and phosphites, which lie above the line. These exceptions are related to the fact that the ionization potentials of these molecules are anomalously large (23) . As explained earlier, Figures 1 and 2 will not be greatly changed if data in solution are used. Individual bases will move up or down, parallel to the lines shown. Therefore, under the normal conditions for nucleophilic sub stitution, ease of oxidation (Ε°') and Br0nsted basicity are not independent parameters for bases where the donor atom is a second-row element. Either parameter may be used as a measure of nucleophilic reactivity. For heavier donor atoms, the basicity is much lower than the E°' values would suggest. Nucleophilic reactivity would probably correlate with the E°' values, but not with pK . This statement is based on the assumption that a


Figure 2. Plot of ionization potentials of neutral molecules against their proton affinities. Molecules are C, N, O, and F donors. Data are from references 20 and 21.


16. PEARSON

Basicity and Nucleophilicity of Transition Metal Complexes


241

242

NUCLEOPHILICITY

partial electron transfer to the substrate from the nucleophile is substantial in the transition state. This situation would be the case for reactions with C H I and most other alkyl halides. Thus, for substrates of this nature, the ease of electron loss, or ease of oxidation, is the important property deter mining nucleophilic reactivity. This discussion helps to explain some interesting results recently re ported by Bordwell and Hughes (24). Large différences in rate constants are found for reactions of carbanions, nitranions, and oxanions with benzyl chlorides. But these differences are largely eliminated if comparisons are made at the same proton basicities. The remaining differences are rather small and are substrate-dependent. The order is C " > Ο " > Ν " for a chloride leaving group but changes to Ο " > N " > C~ for a tosylate leaving group (25). That basicity is the dominant factor in determining nucleophilic reac tivity in these cases cannot be concluded from these results. Because the nucleophiles are structurally similar, very probably E°' would vary in the same way as pK does for all of them. If the oxygen atom is replaced by sulfur, the resultant ion is K P - I O times more reactive, even though the sulfanion is much less basic. This result strongly suggests that electron transfer plays the dominant role. The correlation between proton affinity and electron affinity, as in reactions 14 and 15, has its origin in two factors. Within each ion, a certain distribution of charges exists. The potential of these charges will affect the removal of an electron or the addition of a proton almost equally. This relationship is a classical electrostatic effect. The partial transfer of a pair of bonding electrons from the nucleophile to the proton leads to covalent bonding between the two. Easy electron loss means strong proton binding, if other quantum mechanical requirements are met, such as good orbital overlap. The failure of the heavier donor atoms to follow the same pattern as the lighter ones must be largely an atom size effect. The diffuse orbital of an iodide ion will not overlap well with that of a proton, at least in comparison to that of fluoride ion. As for nucleophilic reactivity, the relative reactivity of F~ and I" will be substrate-dependent. If covalent bonding in the transition state is large, and if the accepting orbital of the electrophile is diffuse, then I" will do very well. Alkyl halides are usually in this category. For other electrophiles, such as esters, much less covalent bonding in the transition state may occur and ionic bonding may be large. The accepting orbitals may be concentrated in space, similar to that of the proton. Clearly, I~ will not be a good nucleophile, compared to F~ (for an interesting analysis of nucleophile-electrophile interactions that does not use orbital theory, see reference 26). Returning to transition metal nucleophiles, the experimental evidence already cited indicates that very likely pK and E°' are strongly correlated.


3

a

5

a


16.

PEARSON


Some additional evidence is available, at least for anionic M L ~ . The homo lytic bond energy, D , is known for a number of H M L molecules (12). The D values all fall within a narrow range, between 54 and 73 kcal/mol. Furthermore, the variations that exist are related to pK . Molecules that are strong acids have weak M - H bonds (12). These two observations mean that a good negative correlation between E A and PA will certainly exist. For neutral M L bases, the situation is not so clear. A number of gasphase proton affinities have been measured for neutral complexes where ionization potentials are also known (27). A larger range of homolytic bond energies is found, between 53 and 87 kcal/mol. A plot of PA versus IP resembles Figure 2, but the correlation coefficient is only "0.694. However, in some cases gas-phase protonation occurs on the ligand and not on the metal. In such cases, the homolytic bond energy is not that of M - H bond. Compared to the representative elements, the transition metals are remarkable in that little variation in atomic sizes occurs in going from the first to the second and third series. Orbital sizes do not change greatly, and the strength of covalent bonds to ligands remains much more constant (28). This statement means that the atom size factor, which separates out the heavier donor atoms from C , N , O, and F, will not be present for the transition metals. n

0

n

0

fl


n

Acknowledgment Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this work.

Literature

Cited

1. Schrauzer, G. L.; Deutsch, E . ; Windgassen, R. J. J. Am. Chem. Soc. 1968, 90, 2441. 2. Collman, J. P.; Finke, R. G.; Cawse, J. N . ; Brauman, J. I. J. Am. Chem. Soc. 1977, 99, 2515. 3. Pearson, R. G.; Figdore, P. E. J. Am. Chem. Soc. 1980, 102, 1541. 4. Kao, S. C.; Spillett, C.; Ash, C.; Lusk, R.; Park, Y. K.; Darensbourg, M . Y. Organometallics 1985, 4, 83. 5. Pearson, R. G.; Songstad, J. J. Org. Chem. 1967, 32, 2899. 6. Pearson, R. G.; Gregory, C. D. J. Am. Chem. Soc. 1976, 98, 4098. 7. Dessy, R. E.; Pohl, R. L.; King, R. B. J. Am. Chem. Soc. 1966, 88, 5121. 8. Edwards, J. O. J. Am. Chem. Soc. 1954, 76, 1540. 9. Lukehart, C. M . Fundamental Transition Metal Organometallic Chemistry; Brooks-Cole: Monterey, CA; 1985;p228. 10. Collman, J. P.; Mac Laury, M . R. J. Am. Chem. Soc. 1974, 96, 3019. 11. Kubota, M . Inorg. Chim. Acta 1973, 7, 195. 12. Pearson, R. G. Chem. Rev. 1985, 85, 41. 13. Norton, J. R.; Jordan, R. F. J. Am. Chem. Soc. 1982, 104, 1255. 14. Yoshida, T.; Matsuda, T.; Okano, T.; Tetsumi, K; Otsuka, S. J. Am. Chem. Soc. 1979, 101, 2027.


NUCLEOPHILICITY


244

15. Pearson, R. G.; Jayaraman, R. Inorg. Chem. 1974, 13, 246. 16. Kochi, J. K. Organometallic Mechanisms and Catalysis; Academic: New York, 1978; Chapter 7. 17. Chou, M . ; Creutz, C.; Mahajan, D.; Sutin, N . ; Zipp, A. P. Inorg. Chem. 1982, 21, 3989. 18. Sigal, I. S.; Gray, H . B. J. Am. Chem. Soc. 1981, 103, 2220. 19. Ritchie, C. D. J. Am. Chem. Soc. 1983, 105, 7313. 20. Rosenstock, H . M . ; Draxl, K.; Steiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data 1977, 6, Suppl. No. 1. 21. Lias, S. G.; Liebman, J. F.; Levin, R. D. J Phys. Chem. Ref. Data 1984, 13, 695. 22. Lias, S. G.; Ausloos, P. Ion-Molecule Reactions: Their Role in Radiation Chem istry; American Chemical Society: Washington, DC, 1975; p 91. 23. Elbel, S.; Bergmann, H . ; Ensslin, W. J. Chem. Soc., Faraday Trans. 2 1974, 555. 24. Bordwell, F. G.; Hughes, D. L. J. Am. Chem. Soc. 1984, 106, 3234. 25. Bordwell, F. G.; Hughes, D. L. J. Org. Chem. 1982, 47, 3224. 26. Bader, R. F. W.; MacDougall, P. J.; Lau, C. D. H. J. Am. Chem. Soc. 1984, 106, 1594. 27. Stevens, A. E.; Beauchamp, J. L. J. Am. Chem. Soc. 1981, 103, 190. 28. Pearson, R. G. Inorg. Chem. 1984, 23, 4675. 29. Calderazzo, F. In Catalytic Transition Metal Hydrides; Slocum, D. W.; Moser, W. R., Eds.; New York Academy of Sciences: New York, 1983; p 37. 30. Bartmess, John E . ; Scott, J. Α.; McIver, R. T., Jr. J. Am. Chem. Soc. 1979, 101, 6046. 31. DePuy, C. H . ; Bierbaum, V. M.; Damrauer, R. J. Am. Chem. Soc. 1984, 106, 4051. 32. Henchman, M . ; Viggiano, A. A; Paulson, J. F.; Freedman, Α.; Wormhoudt, J. J. Am. Chem. Soc. 1985, 107, 1453. RECEIVED

for

review October 21, 1985.

ACCEPTED

January 27, 1986.


Basicity and Nucleophilicity of Transition Metal Complexes - American

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