Electron Transfer Reactions - American Chemical Society

Downloaded by UNIV QUEENSLAND on September 27, 2013 | http://pubs.acs.org Publication Date: May 5, 1997 | doi: 10.1021/ba-1997-0253.ch018

18 Hydrolysis of Coordinated Nitriles and Linkage Isomerization Reactions in Ruthenium Ammine Complexes with Nitriles and Amides Zênis Novais da Rocha , Glaico Chiericato,Jr.,and Elia Tfouni* 1

Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. dos Bandeirantes, 3900, 14.040-901, Ribeirão Preto, São Paulo, Brazil

This chapter surveys the hydrolysis of coordinated nitriles to amides in ruthenium ammine complexes, and the subsequent afterreduction of the amido-Ru(III) complex. Nitriles coordinated to Ru(II) can undergo hydrolysis to amides after oxidation of Ru(II) to Ru(III).The hydrolysis of the Ru(II/III)-1-R-cyanopyridinium complex leads to pure amide, whereas hydrolysis of the free ligand results in a mixture of amide and pyridone. The rate constants for the hydrolysis of different coordinated nitriles to amides span several orders of magnitude. Catalytic systems can be designed. Upon reduction of the resulting Ru(III)-amide, the isonicotinamide, nicotinamide, or acrylamide complexes undergo aqua-tion and isomerization to form the pyridyl or the olefin-bonded complex. On reduction of the Ru(III)-2-picolinamido complex, aquation and chelation occur to form [Ru(NH ) (H O)] and picolinamide, and cis[Ru(NH ) (2-picolinamide)] , respectively. The spectral properties of the nitrile and amide complexes of Ru(II) and Ru(III) ammines and the preferred coordination site in ambidentate ligands are also presented. 3 5

3 4

2

2+

2+

THE HYDROLYSIS OF NITRILES TO AMIDES

is well-known (I, 2), and the poly mers, such as acrylamide and polyacrylamide, have widespread use. However, the conditions of hydrolysis of nitriles to amides are usually extreme, resulting in low yields and mixtures of products. Current address: Departamento de Quimica, Universidade Federal da Bahia, Salvador, Bahia, Brazil. 1

*Corresponding author

© 1997 American Chemical Society

In Electron Transfer Reactions; Isied, S.; Advances in Chemistry; American Chemical Society: Washington, DC, 1997.

297

298

E L E C T R O N TRANSFER REACTIONS

Coordinated nitriles can undergo hydrolysis to amides with widely varying rate constants that are often higher than those in the corresponding uncoordi nated nitriles. For some coordinated nitriles, selective hydrolysis of nitriles to amides is possible, and catalytic hydrolysis systems can even be designed. In addition to this important potential application, ruthenium-coordinated nitriles and amides show interesting properties, such as the isomerization reaction that occurs after the reduction of [Ru (NH ) (amide)] (where the amide is bound at the nitrogen) (3-25). This chapter is not intended as a thorough review of the literature, but rather presents some important aspects of the chemistry of Ru(II) and Ru(III) ammines with coordinated nitriles and amides (3-25). This chapter focuses, among other possible reactions, mostly on the hydrolysis of coordinated nitriles in ruthenium ammine complexes, and the reactions occurring upon the reduction of the resulting Ru(III)-amide complexes.


ln

3

5

Ruthenium Amines with Coordinated Nitriles A number of Ru(II)/(III) nitrile complexes with pentaammine and polypyridyl auxiliary ligands have been reported (3, 7-9, 12-38). In these complexes, Ru(II) acts as a σ-acceptor and π-donor, while Ru(III) is a σ- and π-acceptor. Nitrile ligands, such as acetonitrile (acn), benzonitrile (bzn), or cyanopyridines, which have low-lying empty π* orbitals of appropriate symmetry to interact with d -metal orbitals can coordinate to either Ru(III) or Ru(II), resulting in substitutionally inert complexes. In cationic cyanopyridinium ligands, the charge on the ligands imparts additional properties to the ruthenium com plexes {12,13-21, 30, 36). Polynitriles as ligands have also been reviewed with respect to their electric, magnetic, and spectroscopic properties (39). 7C

Spectral and Redox Properties In Ru(II)-nitrile complexes, as in the analogous pyridines (py-X) and pyrazine (pz) complexes, π-backbonding from the metal to the ligand plays a key role in the properties of the nitrile complexes and their derivatives. The metal-to-ligand charge-transfer ( M L C T ) band energies (Table I) and the Ru(III/II) redox potentials (Table II) in these [ R u ( N H ) L ] complexes (L = substituted aromatic nitrogen heterocycles such as pyridines, pyrazines, and cyanopyridines) depend on the properties of the aromatic ring substituent of the ligand and on the solvent (26, 40, 48). The more electron-withdrawing the substituent is, the lower the M L C T absorption energy and the higher the redox potential will be. This trend in the electronegativity of the ligand sub stituent is related to the higher π-backbonding ability of the Ru(II) complex. However, the redox potentials (Table II) are indicative of the Ru(III)/Ru(II) affinity ratios for the nitrile ligands. Thus, they cannot be taken as an estimate of the π-backbonding ability of Ru(II), and, furthermore, the relationship of 3

5

2+


R O C H A ET AL.

18.

Hydrolysis and Isomerization in Ru Ammine Complexes

299

Table I. Electronic Absorption Data of [Ru (NH ) L] Complexes n

3

5

^innmfbgs) Ref.

407 397 479 427

(3.89) (3.89) (4.02) (3.78)

40 2M (3.66) 40,41a 244 (3.66) 40, 41b,41c 260 (3.66) 254 (3.72), 212 (3.84) 40, 41b, 41c

(4.05)

253 (4.22), 212 (4.09)

(3.91) (4.14)

260 (3.93)

1 -me thyl-4 - cyanopyridinium (4-mcp)° 4-mcp

422 500 532 534 542 545

(4.26)

3-NCpy (nitrile bonded) 3-NCpyH (nitrile bonded)**

398 (3.98) 460 (3.67)

267 sh,257 sh, 242 (4.23) 255 (4.29), 218 (4.05) 327 sh, 261 (4.10), 218(3.83) 258 (4.16), 210 (4.04) 254 (4.43), 214 (4.71) 256 (4.23) 253 (3.78)

0

0

+


IL

MLCT

L pyridine (py)° 4-picoline (4-pic)° isonicotinamide (isn)° nicotinamide (nic)° 4-cyanopyridine (4-NCpy) (nitrile bonded) 4-NCpy (pyridyl bonded) 4-NCpyH (nitrile bonded)**

e

0

+

3-mcp [(NH3) Ru (3-NCpy)] (nitrile bonded) [(NH3) Ru (3-NCpy)] (pyridyl bonded/ pyrazine (pz)° pzH 1-methylpyrazinium (pzCH )s l-decyl-4-cyanopyridinium (4-decp) l-dcdecyl-4-cyanopyridiimim (4-docp) l-benzyl-4-cyanopyridinium (4-bcp) [(NH ) Ru (4-NCpy)] (nitrile bonded) [(NH ) Ru (4-NCpy)] (pyridyl bonded/ e

n

5

0

n

5

+ d

3

+

e

e

e

3

5

3

5

n

n

n i (2-cyanopyridine) 2-NCpy (nitrile [(NH3) (pyridyl bonded) 5Rh(4-NCpy)] bonded) 2 - N C p y H (nitrile bonded/ 1 -methy1-2-cyanopyridinium (2-mcp) [(NH3) Ru"(pz)]° +

462 (3.89) 422 (4.27) 438 (3.92) 472(4.03) 529 (4.08) 877 (2.30) 545 (4.35) 545 (4.31) 556 (4.31) 518 (4.35), 416 sh 492 (4.01) 488 (20.5)

1

e

5

[(NH3) Ru"i(pz)] [(NH3) Rhin(pz)]° benzonitrile (bzn) û

5

5

4-toluenenitrile (4-tln) 1,4-dicyanobenzene (1,4-dcb) acetonitrile (acn) propionitrile (prn) 2-cyanoethyldiphenylphosphine (2-cedp)

270(3.81) 244 (4.22) 245 (4.11) 240 (4.22), 256 (4.26) 254 (4.22),206 (4.08)

28 28 12 30 30

—b 28 30

_J>

34 40,41b 40 42,43 30 30 30

—b 34 35

406 (3.97) 505 (3.91) 517 (4.18) 547 (4.48) 565 (4.32) 528 (4.25) 376(3.93), 347 (3.84) sh 367 (3.86), 347 (3.8) sh 462(21.6) 229 (4.19) 262 (4.18)

256 (4.18), 222 (3.94) 217 (4.14) 275 (4.36), 219 (4.08) 254 (3.43) 270 sh, 252 (3.75) 263 (3.81)

—b

249 (4.21), 226 (4.17)

27

350 (2.40) (LF) 350(2.38) (LF)

27 27 27 23

334 sh(2.58) (LF)

307 (3.46) (IL)

31

28 30 44 44 44

NOTE: is the wavelength of maximum absorption in nanometers; ε is the molar absorptivity; sh means shoulder; M L C T is metal-to-ligand charge transfer band; IL is internal-ligand band; L F is ligand-field band. °Dilute aqueous solution. *This work. Spectrum from [(NH^RuHOH^] (pH = 6) with excess 4-NCpy. l M HC1 solution. ^Acetonitrile solution, fl M H S 0 solution. «The spectrum of this complex displays an absorption band at 538 nm (log ε = 3.20) (ref. 43), to which an associated covalent character was assigned ( λ ^ = 540 nm; log ε = 4.20) (ref. 42) 2M HC1. c

d

2

4

h


300


Table II. Electrode Potentials of Some Ruthenium Nitrile Complexes, [Ru(NH ) L] ^ 3

L

3+

5

+

E° ' (mV vs. NHE)

C H C N (acn) C H C N (bzn)

426* 485* 510° 475* 609* 592* 573 692* 637 476>" 390, 657* 409, 684 682 327 57& 4U 160,385 [(NH ) Ru (NHC(0)R)] + H+ 3

ni

5

2

3

The hydrolysis rates of coordinated nitriles are greater than the rates of the corresponding uncoordinated nitrile and are dependent on the charge and nature of the metal center (3, 5, 6, 22, 62), nature of the nitrile, and the p H . The hydrolysis rates of Ru(III) nitrile complexes and those for the free nitriles and related species are listed in Table III. The hydroxide-catalyzed hydrolyses of [ R h ( N H ) L ] , [ R u ( N H ) L ] , and [ R u ( N H ) L ] (L = acn or bzn) to an amide complex have been studied by Zanella and Ford (3). The [ R u ( N H ) L ] complexes (L = acn or bzn) have rate constants that are 10 higher than those of the free nitriles and 10 higher than for the analogous Co(III) and Rh(III) complexes (3). For the Ru(II)-acetonitrile complex, where the rate constant is 10 smaller (Table III), a less dra matic change is seen. The decrease in the rate constants for the Ru(II) com plexes relative to Ru(III) is attributed to both the lower charge of the cation and the Ru(II) π-backbonding to the nitrile, which increases the electronic density on the nitrile carbon and makes it less prone to nucleophilic attack (3). A mild selective conversion of nitriles to amides was proposed by Taube for [ R u ( N H ) ( N C R ) P + (7-9). The [ R u ( N H ) N C R ] complex is oxidized to Ru(III), which undergoes hydrolysis to Ru(III)-amide complex. After reduction of Ru(III)-amide to the Ru(II) complex, amide aquation occurs, resulting in free amide i n very high yields and [ R u ( N H ) ( H 0 ) ] , which can be recycled by the addition of more nitrile to make the reaction catalytic in ruthenium. The hydrolysis of the coordinated nitriles in the binuclear [(CN) Fe(pyCN)Ru(NH ) ] (4- and 3-NCpy isomers) and [(NH ) Ru(4-NCpy)Ru( N H ) ] complexes have been investigated (12, 64). The chemically oxidized binuclear complexes hydrolyze at a faster rate than the corresponding mono nuclear Ru(III) complexes. Recently, the hydrolysis of [Ru (NH ) (rcp)] complexes with 1-R-cyanopyridinium cations (rep) (R = methyl, decyl, dodecyl, or benzyl) has been stud ied (30). These [Ru (NH ) (rcp)] complexes differ from the binuclear com plexes ( N H ) R u L M by the presence of R rather than M bonded to the pyridine nitrogen, which hinders intramolecular electron-transfer to another metal center. These complexes are rather stable in acidic medium as expected, but i n basic medium the hydrolysis of the coordinated nitrile results exclu sively in [Ru (NH ) (amide)] , with no pyridone being formed (13). In basic solution the rate of reaction is pseudo-first-order in hydroxide (13). This may be a good example of inhibition of hydrolysis upon coordination to a metal cen ter i n comparison to the free nitrile and illustrates the importance of backbonding. In alkaline medium, it is difficult to know if the hydrolysis occurred before, after, or simultaneously with the oxidation of the Ru center (13). The 3


(2)

III

5

3

5

3+

3

2+

5

3+

5

3

3+

5

8

2

6

3

5

3

5

3

5

2+

2

2+

5

3

3

5

5

3

5

n+

n

n

3

5

3+

3+

5

n +

5

m

3

3

3

5

2+


304


Table III. Rate Constants of the Hydrolysis of [Ru (NH ) (L)} and Some Related Species in

Species

ία (s- ) 1

[Ru(NH ) (4-NCpy)] (NC bonded) 3

3+

5

(2.85 ±0.02) χ 103.9 χ ίο(4.23 ±0.04) χ 1(Η (0.667 ± 0.007) χ ΙΟ 1.7χ1(Η (2.7 ±0.1) χ ΙΟ" 2.0 χ ΙΟ- " (1.38 ±0.01) χ ισ- * (160) χ ΙΟ(210±1)χ1(Η (235 ± 1) χ 1(Η (283) χ ΙΟ" (388 ±8) χ ΙΟ710 χ ΙΟ530χ ΙΟ" * 330 χ ΙΟ" * 163 χ ΙΟ" * (19±3)xl0" 17.5 χ 1025 χ ΙΟ- " (5.49 ±0.03) χ ΙΟ(2.77 ±0.01) χ ΙΟ- * (10.4 ±0.1) χ ΙΟ(3.41 ±0.03) χ ΙΟ" 220

Electron Transfer Reactions - American Chemical Society

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