Activation of Carbon Monoxide by Metalloradicals - American

17 Activation of Carbon Monoxide by Metalloradicals

Downloaded by FUDAN UNIV on March 3, 2017 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch017

Bradford B. Wayland, Alan E . Sherry, and Virginia L . Coffin Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323

Rhodium(II) porphyrin derivatives are observed to activate carbon monoxide toward one-electron reactions at the carbonyl carbon. Re ductive coupling of carbon monoxide to form dimetal diketone com plexes (M-C(O)-C(O)-M) is one prominent example of this type of reactivity. Thermodynamic and kinetic-mechanistic studies for re actions of CO with a series of cobalt, rhodium, and iridium por phyrins provide insights into criteria for obtaining CO coupling. Seventeen-electron monocarbonyl complexes, (porphyrin)Rh-CO, are implicated as reactive intermediates. Electron paramagnetic res onance studies of the tetramesitylporphyrin derivative (TMP)Rh-CO illustrate how this type of complex directs radicallike reactivity to the carbon center.

xiiCTiVATiON O F co

BY O N E - E L E C T R O N S T E P S is illustrated by the reaction

of methyl radicals with C O to form a transient intermediate acyl ( C H C O ) radical. This intermediate subsequently takes part in a second one-electron reaction at the carbonyl carbon to produce acetone, ( C H ) C O , and biacetyl, C H - C ( 0 ) - C ( 0 ) - C H (eqs 1-3) (1-3). Related reactions of alkyl radicals and atomic species like Η· and CI* are also well known (3-5). 3

3

3

2

3

C H . + C O -> C H C O 3

3

C H C O + C H * -> ( C H ) C O 3

3

3

2

2 C H C O - » C H -C(0)-C(0)-CH 0065-2393/92/0230-0249$06.00/0 © 1992 American Chemical Society

Moser and Slocum; Homogeneous Transition Metal Catalyzed Reactions Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

(1) (2) (3)

250

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

In transition metal chemistry one-electron activation of C O has been accomplished by reduction of 18-electron metal carbonyl complexes to form transient " 1 9 - e " species like [(CO) FeCO] ~ (6-11). One important reaction e

4

of 19-e" carbonyl complexes like [ ( C O ) F e C O ] " is hydrogen atom abstraction #

4

from

metal hydrides to form

transient

18-e" metalloformyl species,

[ F e ( C O ) C H O ] ~ (eq 4) (6). Reactions 2-4 serve as models for several types 4

of reactions that are characteristic of one-electron activated carbon monoxide. [(CO) FeCO]-

+ H-M


4

( C O ) F e - C ( 0 ) H + [Μ·]

(4)

4

As part of a program to evaluate the reactivity patterns of metalloradicals with small molecules, we have been studying the reactions of Co(II), Rh(II), and Ir(II) porphyrins with C O . O n e of our objectives was to determine whether metalloradicals (Μ·) can react with C O to form intermediate com plexes, [MCO]% that exhibit one-electron carbonyl carbon reactions in anal ogy with the acyl radical ( C H C O ) (eqs 1-3). A feature that distinguishes this study from previous work is that the one-electron activating species (Μ·) is present at thermal equilibrium. This characteristic provides an opportunity to have an equilibrium source of one-electron activated C O , (MCO). A series of Group 9 metalloporphyrin complexes [(POR)M(II); M is Co(II), Rh(II), Ir(II)] were selected as sources of metalloradicals. Monomeric d metal ion complexes of porphyrins are expected to be low-spin (s = V2) complexes with the unpaired electron effectively localized in the d orbital normal to the porphyrin plane. Species of this type are often referred to as metalloradicals because there is a single metal-based unpaired electron (s = V2) in an orbital (d ) capable of giving one-electron reactions. 3

7

z 2

z2

Cobalt(II) porphyrins are invariably low-spin monomeric complexes with the expected d , d xy2

d i electron configuration (12). Rhodium(II) and

,

z2

xzyz4

iridium(II) porphyrin complexes are usually diamagnetic M - M bonded di mers, but function as sources of the paramagnetic monomers (13, 14). The large steric requirements of tetramesitylporphyrin (TMP) preclude dimeri zation through M - M bonding and provide a stable monomeric Rh(II) com plex, (TMP)Rh(II) (15). Electron paramagnetic resonance (EPR) spectra for (TMP)Rh(II) in toluene glass (90 K) clearly indicate that this low-spin d

7

(s = V2) complex has a d i electron configuration (Figure la). z2

Figure 1. EPR spectra for (TMP)Rh(ll) and (TMP)Rh-CO in toluene. Key: a, anisotropic EPR spectrum for (TMP)Rh(II) (90 K) [g = 2.65, g, = 1.915; K Rh(gi.2) = 197 MHz, A Rh(&) = 158 MHz]; b, anisotropic EPR spectrum that results from exposing a frozen solution of (TMP)Rh(II) to CO (P o = 200 torr, Τ = 100 Κ), warming to 195 Κ and refreezing to 100 K; c, anisotropic EPR spectrum for (TMP)Rh- CO (90 K) /g, ~ 307 MHz; & = 2.14, Α 0(&) = 330 MHz; g3 = 1.995, A Rh(&) = 299 MHz; A Rh(g3) = 67 MHz]; d, isotropic spectrum for (TMP)Rh-™CO (90 K) l = 2.101; = 312 MHz]. 12

m

m

12

C

13

m

13

103

iso


13

WAYLAND ET AL.

Activation of Carbon Monoxide by Metalloradicals


17.


251

252


Table I. Thermochemical Estimates for Generalized Reactions of X - X and X· with C O T o F o r m Ketone and 1,2-Ethanedionyl Compounds

Reactions X-X 2Χ· X-X 2X.

AG (298 K) (kcal mol )

Δ Η (kcal mol- )

+ C O τ± X-C(0)-X + C O τ± X-C(0)-X + 2CO i=± X-C(0)-C(0)-X + 2CO τ± X-C(0)-C(0)-X

0

1

0

1

(X-X) - 2[X-C(0)] + 78 -2[X-C(0)] + 86 (X-X) - 2[X-C(0)] + 86 -2[X-C(0)] + 94

(X-X) - 2[X-C(0)] + 70 -2[X-C(0)] + 70 (X-X) - 2[X-C(0)] + 70 -2[X-C(0)] + 70

N O T E : Bond energies from reference 22 were used in deriving the table: (C=C)-(C = O), 70 kcal mol" , and C(0)-C(0), 70 kcal mol" . Bond energies for X - X species were discussed in the chapter: H - H , 104; H C - C H , 85; (OEP)Rh-Rh(OEP), 15; (TXP)Rh-Rh(TXP), 12; (OEP)Ir-Ir(OEP), 23; and (TXP)Ir-Ir(TXP), 20 kcal m o l . 1

1

3

3


1

Interaction of a half-occupied metal σ orbital (d i) with the C O σ orbitals z2

places spin density at the carbonyl carbon and should also result in a bending of the M - C O unit (16). As the M - C covalent bond strength increases, the carbonyl carbon will rehybridize toward s p , the C - O bond order will de 2

crease, the M - C - O angle will decrease from 180° toward 120°, and the carbon spin density will increase. The limiting case is approximated by CH -CO, 3

which is properly depicted as C H - C = 0 . Evidence for one3

electron metalloradical activation can come from observation of these struc tural and spin density changes, but further requires the characteristic overall reactivity depicted by eqs 1-4. Guideline thermodynamic criteria for the metal analogs of reactions 2 and 3 are given in Table I.

Reactions of (Porphyrin)M(H) Complexes with CO Co(II) Porphyrin.

Monomeric cobalt(II) complexes of porphyrins

have the low-spin d electron configuration, which is frequently associated 7

with metalloradical species, and accomplish one-electron reactions with N O and 0

2

(eqs 5 and 6) (16). (POR)Co + N O ? ± ( P O R ) C o - N O (POR)Co + 0

2

«± (POR)Co-0

Tetraphenylporphyrincobalt(II),

2

(s = 0)

(5)

(s = V2)

(6)

(TPP)Co, binds C O in toluene glass

(90 K) to form a monocarbonyl complex [(TPP)CoCO] (eq 7) (16). (TPP)Co + C O τ± (TPP)CoCO EPR

(7)

spectra of ( T P P ) C o C O were used in probing the properties of 13

this species. A n isotropic

1 3

C hyperfine coupling constant of 170 M H z yields

a Csjj spin density of 0.054. A nonlinear C o - C - O fragment should be a fundamental feature of a half-filled d However, the E P R g values and the

z 2 1 3

orbital forming a σ bond with C O . C hyperfine coupling constants are


17.


WAYLAND ET AL.

253

compatible with axial symmetry (A) for (TPP)CoCO (g,, = 2.017; g = 2.217; A„ C = 179 M H z ; A C = 166 M H z ) . T h e anisotropic C coupling constants yield a C spin density of 0.049 for an axially symmetric complex. They are used to deduce that the carbonyl carbon exhibits near-sp hybrid ization (pea/Pa^ = 1.1) and that the total C O spin density is —0.13. T h e x

1 3

±

1 3

1 3

2 p

odd electron in (TPP)CoCO is thus largely based on cobalt. The C o - C O covalent bonding is too weak to produce sufficient carbonyl carbon rehybridization for the associated bending of the C o - C O unit to be detected in the E P R spectrum. The C o - C bond energies determined for cobalt por phyrin alkyls (20-30 kcal mol" ) (17) are far too small to produce reactions analogous to reactions 2-4, which result in reduction of the C O bond order to 2.


1

Rh(II) Porphyrin.

The search for metal complexes capable of one-

electron activation of C O was subsequently extended to Rh(II) and Ir(II) porphyrins, which are expected to form stronger M - C bonds than Co(II) complexes. Reactivity studies of C O with octaethylporphyrinrhodium(II) d i mer, [(OEP)Rh] , revealed the formation of dimetal ketone [(OEP)RhC(0)-Rh(OEP)] and dimetal α-diketone [(OEP)Rh-C(0)-C(0)-Rh(OEP)] complexes (eqs 8 and 9) (18, 29). 2

[(OEP)Rh] + C O « ± ( O E P ) R h - C ( 0 ) - R h ( O E P )

(8)

2

[(OEP)Rh] + 2 C O * ± ( O E P ) R h - C ( 0 ) - C ( 0 ) - R h ( O E P ) 2

(9)

These products have a formal relationship to acetone ( C H - C ( 0 ) - C H 3 ) and biacetyl ( C H - C ( 0 ) - C ( 0 ) - C H 3 ) that results from the reaction of C H with C O (eqs 2 and 3). The desired type of C O complex, ( O E P ) R h - C O , could be an intermediate in reactions 8 and 9. The insertion of C O into the R h - H bond in ( O E P ) R h - H to produce a metalloformyl complex ( O E P ) R h - C ( 0 ) H (eq 10) (20) involves a radicallike pathway in benzene. 3

3

3

( O E P ) R h - H + C O +± ( O E P ) R h - C ( 0 ) H

(10)

A detailed kinetic study of the [(OEP)Rh] catalysis of reaction 10 (21) demonstrated a rate law consistent with a radical chain process in which ( O E P ) R h - C O functions as an intermediate (eqs 11-13). 2

[(OEP)Rh] +± 2(OEP)Rh2

(OEP)Rh- + C O * ± ( O E P ) R h - C O ( O E P ) R h - C O + ( O E P ) R h - H * ± ( O E P ) R h - C ( 0 ) H + (OEP)Rh-

(11) (12) (13)

Our efforts to observe the proposed intermediate complex, ( O E P ) R h - C O , directly by E P R spectroscopy were not successful. However,


254


C N M R spectra of bulk C O in solutions of [(OEP)Rh] with C O manifest large line broadening of the C O peak as the temperature increases. This broadening provides qualitative evidence for C O exchange with a paramag netic complex, ( O E P ) R h - C O (22). Subsequent investigations of the reaction of C O with the monomeric tetramesitylrhodium(II) complex, (TMP)Rh% have resulted in the direct ob servation of ( T M P ) R h - C O . This observation led to evaluation of some general structural and electronic features of this 17-electron monocarbonyl complex (15). 1 3

l 3

1 3

2

1 3

(TMP)Rh- (1) reacts with C O to form a mono-CO complex, ( T M P ) R h - C O


(2), and an α-diketone ( T M P ) R h - C ( 0 ) - C ( 0 ) - R h ( T M P ) (3) (eqs 14 and 15) (15). (14)

(TMP)Rh- + C O ? ± ( T M P ) R h - C O

(15)

2(TMP)Rh. + 2 C O / 3 1 1 0 M H z ) compared to p = 0.12 in H C O . Anisotropy of the C coupling constants on the g value transitions cannot be used to extract the C spin density because the principal values of the A C tensor are not observable in the glass spectrum. Presence of a bent R h - C O unit implies substantial rehybridization at the carbonyl carbon from sp toward sp . The carbonyl carbon spin density is thus —0.2-0.3, which implies that the total C O spin density is -0.25-0.35. The C O spin density in ( T M P ) R h - C O is 2 to 3 times larger than in ( T P P ) C o - C O but probably substantially less than that in H C O 1 3

1 3

z

2

13

1 3

1 3

l

3

2

1 3

2

e

4

1 3

13

4

#

1 3

2 s

13

2s

1 3

2 s

2 p

I 3

2


17.

WAYLAND E T AL.

Activation of Carbon Monoxide by Metalloradicak

255

(Pco = 1 - P H - 0.75) (23, 24). ( T M P ) R h - C O thus contains a bent, partially rehybridized R h - C O unit in which the unpaired electron occupies a σ mo lecular orbital (d2 + C O J that has - 7 0 % d and - 3 0 % Ο Ο character. The electronic structure can be depicted as a resonance hybrid of limiting electron structures that localize the odd electron in the rhodium d 2 (•Rh-C^O) or a carbon s p hybrid ( R h - C = 0 ) . T h e 17-electron complex, ( T M P ) R h - C O , is thus poised for a second one-electron reaction at either the rhodium or carbonyl carbon sites. Reaction with a one-electron species X· at the metal would produce an 18-electron Rh(III) carbon monoxide com plex, (TMP)Rh(X)(CO). Correspondingly, reaction at the carbonyl carbon forms a 16-electron Rh(III) complex, (TMP)Rh-C(0)X. Reversible dimerization of ( T M P ) R h - C O (1) through C - C bond for mation to produce a 1,2-ethanedionyl complex, ( T M P ) R h - C ( 0 ) - C ( 0 ) Rh(TMP) (2), illustrates a carbonyl carbon-centered one-electron reaction that can be viewed as an organometallic analog of formyl radical coupling [2H C O H - C ( 0 ) - C ( 0 > - H ] (eq 16). 1s

z

z 2

σ

z


2

2 ( T M P ) R h - C O τ± (TMP)Rh-C(0>-C(0)-Rh(TMP)

(16)

The type of C O reductive coupling depicted by eq 16 is presently known only for reactions of (POR)Rh(II) complexes with C O . Temperature depen dence of the isotropic E P R intensity for ( T M P ) R h C O (1) has been analyzed to provide the enthalpy change for reaction 16 ( Δ Η ° = -18.3 ± 0 . 8 kcal mol" ) (15). 1 6

1

Temperature dependence of the pyrrole H line broadening of (TMP)Rh-C(0)-C(0>-Rh(TMP) (2) is ascribed to exchange of the diamagnetic dimer (2) with the paramagnetic monomer (1) and used in deriving the activation energy for the dissociation of 2 into 1 (ΔΗ* = 21.3 ± 0.8 kcal mol" ) (Figure 2). A n energy profile for the reversible dimerization of ( T M P ) R h - C O is illustrated in Figure 3. The activation energy for dimeri zation ( Δ Η * - 3 ± 1 kcal mol" ) is derived from Δ Η ° (-18.3 kcal mol* ) and Δ Η _ * ( + 21.3 ± 0.8 kcal m o l ) , which is close to the value calculated for a diffusion-controlled process in benzene (3.1 kcal). l

1

1

16

1

1 6

1

16

Assuming that the dionyl unit [-C(0)-C(0)-] in 3 is similar to the organic analogs like biformyl, the dissociation of 3 into 2 results in cleavage of a 65-70-kcal C - C bond. The experimental dissociation energy ( Δ Η _ ° = 18.3 ± 0 . 8 kcal m o l ) is considerably smaller than the expected C - C bond energy in 3 (—65-70 kcal m o l ) (26). The energy difference indicates that —50 kcal of the energy change associated with reduction of the C O bond order in forming the α-diketone complex [2(C=0)-2(C=0) — 140 kcal mol" ] is regained in forming ( T M P ) R h - C O . This observation supports the E P R evi dence that ( T M P ) R h - C O contains only a partially rehybridized carbonyl unit with a bond order between 2 and 3 ( R h - C = 0 ) . Thermodynamic studies for reactions of [(OEP)Rh] and ( O E P ) R h - H with C O have been used in deriving values for the R h - C ( 0 ) X bond energies 16

1

1

1

2


256


-24 (TMR)Rh-C(0)-C(0)-Rh(TMP) ^

Δ Η » r 21.3 ± 0.8 kcal mol-1

-25 H


* e

(TMP)Rh-CO

2

-26

-27 H

-28 0.0033

1

0.0034

0.0036

0.0035

Figure 2. Temperature dependence of the pyrrole Ή line broadening for (TMP)Rh-C(0)-C(0)-Rh(TMP) in C D . 6

(TMP)Rh-C(0)-C(0)-Rh(TMP)

6

2 (TMP)Rh-CO

o

[

4

(TMP)Rh-C* 1 ^ ^C-RhfTMPJJ

k

t 3.0 kcal

2(TMP)Rh-CO

ΔΗ

21.3 ±0.8 kcal

18.3 ±0.8 kcal

[(TMP)Rh-CO]

2

Reaction Coordinate Figure 3. Enthalpy profile for the dissociation of (TMP)Rh-C(0)-C(Oy Rh(TMP) into two (TMP)Rh-CO in C D . 6

6


17.

W A YL A N D E T A L .


257

in ( O E P ) R h - C ( 0 ) H (58 kcal m o l ) (13), (OEP)Rh-C(0)-C(0}-Rh(OEP) (54 kcal m o l ) , and (OEP)Rh-C(0)Rh(OEP) (49 kcal mol" ) (16). The current maximum R h - C ( 0 ) X bond energy of —58 kcal mol" is insufficient to justify full rehybridization of C ^ O to a C = 0 unit [ ( C = 0 ) - ( C = 0 ) « 70 kcal mol" ). 1

1

1

1

1

More complete C O rehybridization occurs in the formyl radical because of the stronger H - C ( O ) bond. The bond strength and directional character of the C H - C ( 0 ) bond produces virtually complete C O rehybridization in the acyl radical (Table II). In ( P O R ) R h - C O complexes, reduction of the C - O bond order to 2 (C=0) requires the formation of a second bond at the carbonyl carbon. This bond formation is observed to be effectively accom plished in forming (POR)Rh-C(0)H, (POR)Rh-C(0)-Rh(POR), and (POR)Rh-C(0)-C(0)-Rh(POR) complexes, where the observed C O stretch ing frequencies for these species fall in the range for C - O double bonds (1700-1770 c m ) (18, 21, 29).


3

1

( P O R ) R h - C O is the probable central intermediate in reactions of rho dium porphyrins that produce formyl, ketone, and α-diketone complexes. The bent and partially rehybridized R h - C O fragment provides a relatively low-energy pathway for reversible one-electron reactions at the carbonyl carbon. Thermodynamic and kinetic studies of the dissociation of (TMP)Rh-C(0)-C(0)-Rh(TMP) provide a detailed view of how the nature of ( T M P ) R h - C O provides a facile pathway for (O)C-C(O) bond homolysis. The stretching and ultimate cleavage of the C - C bond is synchronized with an increase in the C O bond order such that the activation energy for dis sociation (ΔΗ* = 21 kcal mol" ) is - 4 3 - 4 8 kcal mol" less than the C - C bond energy (—65-70 kcal m o l ) in the α-diketone complex. The R h - C O units in the transition state are thus similar to that present in the monomer, (TMP)Rh-CO. 1

1

1

Most of the reorganization energy expended for rehybridization of the carbonyl unit in forming the C - C bond is regained in the transition state, in which an increase in the C - O bond order occurs at the expense of breaking Table II. Reorganization Energies for the X C O Units in the Homolysis of the (O)C-C(O) Bonded Dimers XC(0)C(0)X (kcal mot )

Reorganization Energy"/XCO (kcal mol )

69.20 69.9 18.3

0 0 -25

ΔΗ

X CH H

0

1

3

(TMP)Rh

2XCO 1

T h e reorganization energy per X C O unit is defined here as onehalf of the difference between the enthalpy of dissociation (24, 27, 28) and the enthalpy for the bond homolysis without electronic or structural reorganization in the resulting fragments. The (O)C-C(O) bond energies in biformyl and biacetyl are 69-70 kcal mol" and assumed to be similar in dimetal α-diketone species (25). 1


258


the C - C bond. Similarly, homolytic dissociation energies and potentially associated activation enthalpies for dissociation of any X· from (POR)Rh-C(0)X should be - 2 5 kcal m o l less than the C ( 0 ) - X bond energy. This feature of (POR)Rh-C(0)X species can be contrasted with organic an alogs C H - C ( 0 ) - X , in which the C ( 0 ) - X bond homolysis enthalpies ap 1

3

proach the values for the C ( 0 ) - X bond energies (Table II) (30). Ir(H) P o r p h y r i n . [(OEP)Ir] and ( O E P ) I r - H are in many ways closely related to the corresponding (OEP)Rh complexes. However, their reactions with C O are surprisingly different (eqs 17 and 18). 2


[(OEP)Ir]

2

+ C O * ± [(OEP)Ir] (CO) 2

n

(n = 1, 2)

( O E P ) I r - H + C O * ± (OEP)Ir-(H)(CO)

(17) (18)

Carbon monoxide reacts with [(OEP)Ir] and ( O E P ) I r - H (31) to form only terminal C O complexes. Reactions that give overall insertion of C O into Ir-Ir and I r - H bonds, which are prominent types of reactions for rho dium porphyrins, are not observed for (OEP)Ir derivatives. These observa tions indicate that increases in the M - M and M - H bond energies in going from Rh to Ir are not compensated by corresponding increases in the I r - C ( O ) - bond energies. Weakening of the Ir-Ir bond by increasing the ligand steric requirements should permit formation of dimetal ketone and α-diketone complexes. However, testing of this supposition must await re sults from studies utilizing tetramesitylporphyriniridium(II). 2

Summary Reactions of carbon monoxide with Group 9 M(II) porphyrin complexes (M is C o , Rh, Ir) have been evaluated. At present only rhodium(II) porphyrin complexes have manifested the characteristics of one-electron C O activation. Cobalt-carbon bond energies (—20-30 kcal m o l ) are too small and the Ir-Ir and I r - H bond energies are too large for the compounds studied to produce observable equilibrium quantities of acyl derivatives. Rhodium(II) porphyrin complexes react with C O to form equilibrium distributions of dimetal ketone and dimetal α-diketone complexes. The porphyrin rhodium hydrides are also known to form metalloformyl complexes. 1

Studies of the reaction of (TMP)Rh(II) with C O have provided direct observation of the monocarbonyl species ( T M P ) R h C O , which contains a nonlinear R h C O unit and substantial carbon spin density. (POR)RhCO spe cies contain one-electron activated carbon monoxide that provides low-en ergy pathways for a second one-electron reaction to occur at the carbonyl carbon. (POR)RhCO species can thus function as productive intermediates in the sequential two-electron reduction of C O manifested in reactions of C O that produce metalloformyl, dimetal ketone, and dimetal diketone spe cies.


17.

WAYLAND ET AL.


259

Acknowledgment We gratefully acknowledge support of this work by the Department of E n ergy, Division of Chemical Sciences, Offices of Basic Energy Sciences, through Grant DE-FG02-86ER13615.


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RECEIVED

for review October 19, 1990.

ACCEPTED

revised manuscript July 17, 1991.


Activation of Carbon Monoxide by Metalloradicals - American

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