Distal metal effects in cobalt porphyrins related to CcO

Inorg. Chem. 2002, 41, 6583−6596

Distal Metal Effects in Cobalt Porphyrins Related to CcO

James P. Collman,* Katja E. Berg, Christopher J. Sunderland, Ally Aukauloo, Michael A. Vance, and

Edward I. Solomon

Department of Chemistry, Stanford UniVersity, Stanford, California 94305-5080

Received June 12, 2002

Cobalt(II) porphyrins were studied to determine the influence of distal site metalation and superstructure upon

II

dioxygen reactivity in active site models of cytochrome c oxidase (CcO). Monometallic, Co (P) complexes when

ligated by an axial imidazole react with dioxygen to form reversible Co-superoxide adducts, which were characterized

I

by EPR and resonance Raman (RR). Unexpectedly, certain Co porphyrins with Cu metalated imidazole pickets do

III

II

not form µ-peroxo Co /Cu products even though the calculated intermetallic distance suggests this is possible.

I

Instead, cobalt-porphyrin-superoxide complexes are obtained with the distal copper remaining as Cu. Moreover,

I

II

distal metals (Cu or Zn ) greatly enhance the stability of the dioxygen adduct, such that Co superoxides of bimetallic

complexes demonstrate minimal reversibility. The “trapping” of dioxygen by a second metal is attributed to structural

and electrostatic changes within the distal pocket upon metalation. EPR evidence suggests that the terminal oxygen

in these bimetallic Co-superoxide systems is H-bonded to the NH of an imidazole picket amide linker, which may

contribute to enthalpic stabilization of the dioxygen adduct. Stabilization of the dioxygen adduct in these bimetallic

systems suggests one possible role for the distal copper in the Fe/Cu bimetallic active site of terminal oxidases,

which form a heme-superoxide/copper(I) adduct upon oxygenation.

Introduction

have helped unravel the contributions of metal, porphyrin,

proximal base, distal amino acid residue, and solvation to

the binding and stabilization of heme-bound dioxygen.^1,6,7

In cellular respiration, the reduction of dioxygen is

mediated by terminal oxidases, such as cytochrome c oxidase

(CcO).^8-10At the active site of these enzymes is a highly

conserved Fe/Cu (heme/trishistidyl-Cu) center at which

dioxygen binding and reduction occurs. Extensive spectro-

scopic and crystallographic studies of CcO have illuminated

many characteristics that lead to its efficient 4e^-reduction

of dioxygen, although many controversial issues remain,

especially concerning the role of Cu. Consequently, a number

of synthetic Fe/Cu models have been prepared,^11-16although

Hemes (iron porphyrins) are widely used prosthetic groups

for dioxygen transport and activation in biological systems.^1,2

Despite the absence of naturally occurring cobalt porphyrins,

they have significantly added to the interpretation of heme

protein-dioxygen chemistry. This contribution derives in part

from the attenuated reactivity of the (P)CoO₂(cobalt super-

oxide) unit compared to its heme counterpart, as manifested

by a greater stability toward decomposition reactions. In addi-

tion, the single odd-electron configuration of d⁷(P)Co(II)

•-

species allows EPR studies of the (P)Co(III)O₂products,

whereas (P)FeO₂is EPR silent. This point is illustrated by

the cobalt analogues of hemoglobin and myoglobin (CoHb,

CoMb) and corresponding synthetic models.^3-5Such studies

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2907.

* To whom correspondence should be addressed. E-mail: jpc@

stanford.edu. Fax 650-7250259.

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188-195.

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M.; Schwenninger, R.; Boitrel, B. Angew. Chem., Int. Ed. Engl. 1998,

37, 3397-3400.

(13) Collman, J. P.; Rapta, M.; Broring, M.; Raptova, L.; Schwenninger,

R.; Boitrel, B.; Fu, L.; L’Her, M. J. Am. Chem. Soc. 1999, 121, 1387-

1388.

Published on Web 11/13/2002

Inorganic Chemistry, Vol. 41, No. 25, 2002 6583

Collman et al.

Chart 1

little kinetic and mechanistic information has been obtained

because of the high reactivity and limited stability of these

complexes. Despite the importance of simple picket and

capped Co porphyrins to understanding hemoproteins, little

progress has been made in extending such investigations to

cobalt porphyrins which have distal ligand systems that

provide covalently linked bimetallic Co/Cu complexes. The

only published Co(porphyrin)/Cu CcO model was described

by Collman in 1997 using a tetraphenylporphyrin system

covalently linked to triazacyclononane (TACN).^17,18A few

examples of cobalt porphyrins having Ru, Zn, or Co in a

distal amine structure have been reported with some spec-

troscopic characterization.^19-22The synthesis of imidazole-

picket porphyrins^23-25(see Chart 1) provided an opportunity

to study Co-porphyrin complexes that are chemically and

structurally similar to the active site in CcO.

Herein we report the synthesis of a series of mono- and

bimetallic (P)Co models having varying superstructural

complexity and products deriving from their reaction with

dioxygen. Benefiting from greater chemical stability and

using resonance Raman (RR) and EPR spectroscopy, we

have been able to define the starting (P)Co(II) species as

well as their oxygenated adducts. The dioxygen affinities

and characterization of the oxygen adducts reveal the effects

of a distal metal (Cu, Zn, or Ag) on dioxygen binding. We

propose that certain characteristics demonstrated by these

compounds may be relevant to dioxygen chemistry in CcO.

Results and Discussion

(14) Collman, J. P.; Schwenninger, R.; Rapta, M.; Broring, M.; Fu, L. Chem.

Commun. 1999, 137-138.

Structure and Geometry. X-ray diffraction structures of

the binuclear Fe_a3-Cu_Bactive site in CcO have been

determined for both bovine heart^26-28and Paracoccus

denitrificans,^29,30revealing a trishistidyl ligated distal Cu

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N. J. Inorg. Biochem. 2001, 83, 239-246.

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Transition Metal Complexes; Meunier, B., Ed.; Imperial College

Press: London, 2000; pp 309-362.

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275, 949-951.

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Org. Chem. 1998, 63, 8084-8085.

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107, 4904-4911.

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41, 2282-2291.

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Science 1996, 272, 1136-1144.

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(22) Andrioletti, B.; Ricard, D.; Boitrel, B. New J. Chem. 1999, 23, 1143-

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Straumanis, A. J. Org. Chem. 1998, 63, 8082-8083.

(27) Yoshikawa, S.; ShinzawaItoh, K.; Nakashima, R.; Yaono, R.; Ya-

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2000, 82, 1-7.

6584 Inorganic Chemistry, Vol. 41, No. 25, 2002

Cobalt Porphyrins Related to CcO

present case, cobalt acetate was used to deprotonate and

metalate the macrocycle without need for additional base,

which was found to give product mixtures. Initial metalation

of the pickets instead of the macrocycle was found in

porphyrins with a secondary structure (1, 3-5). Heating for

an hour led to formation of the Co macrocycle, as indicated

by a color change from purple-red to red-orange. Subsequent

extraction utilizing aqueous EDTA removes the distal cobalt,

leaving only the cobalt porphyrin in the organic phase. In

earlier work, scrambling of the pickets was observed during

metalation with cobalt under reflux.³⁶To ensure that atro-

pisomerization was not taking place, Zn metalation of the

porphyrin was carried out under the same conditions. This

reaction proceeded cleanly, yielding only the R₃-metallopor-

Figure 1. Comparison between the geometries of the active site of CcO

(crystal structure from ref 28) and the designed (P)Co mimics.

placed about 4.5 Å above the Fe center in the heme (Figure

1). A proximal histidine is bound to Fe, and a redox-active

tyrosine (Tyr244) extends from one of the distal histidine

residues toward the catalytic pocket. To minimize synthetic

efforts, earlier models of the active site made use of N-donor

capping ligands (such as TACN, tpa, or tren) and exogenous

proximal bases. Since the improved synthesis of R₃â-picket

phenylporphyrins in 1998,²³a number of Fe/Cu porphyrins

have been reported with this distinctive geometry.^31,32Refin-

ing this concept to include covalently linked proximal

imidazole and distal imidazole pickets, the ensuing model

porphyrins closely mimic the structure of the CcO active

site. These models were designed to evaluate the influence

of the distal environment on ligand (O₂) binding. Previous

results show differences in O₂reactivity depending on the

distal superstructure;^33,34thus, we have chosen to examine a

series of distal ligands, shown in Chart 1.

Synthesis and Metalation. The free base, Fe, Zn, Fe/Cu,

and Zn/Cu complexes of R₃[imidazole]â[CF₃T] porphyrins

3 and 4 were prepared and characterized by Collman et al.²⁵

The R₃geometry denotes three distal imidazole pickets,

whereas â indicates a proximal axial imidazole ligand. For

bimetallic porphyrins, the second metal of the pair (M₁/M₂)

identifies the distal metal. In this work, Co and Co/Cu

complexes of H₂3 and H₂4 have been investigated (see

Scheme 1). Also, porphyrins with a â-acetylamide group H₂1

as well as the R₃-acetamide picket porphyrin H₂2 have been

prepared. The imidazole-substituted free base porphyrins

were found to be light sensitive. TACN-capped porphyrins

such as 5 have been previously reported.^17,18

1

phyrin, as determined by TLC and H NMR spectroscopy.

Several bimetallic complexes are reported herein. The

combinations Co/Zn and Co/Ag have not previously been

prepared. Whereas tetrahedral coordination of N-donor atoms

to Zn(II) and Cu(I) is well-documented, only recently has

the same preference been shown with silver in Ag(I)

pyrazolylborate compounds.^37,38The choice of solvent and

counterion is important for introduction of the second metal.

Usually, a 1-to-1 addition of a second metal to the metal-

loporphyrin was used with a solvent system (often mixtures

of THF and MeOH) that is capable of dissolving the

monometalated porphyrin, the metal salt, and the bismeta-

lated product. To ensure better solubility and favor coordina-

tion to the distal pickets, metal triflates and hexafluorophos-

phates were used exclusively. Complete removal of the

second metal was achieved by running the crude product

through a column of aluminum oxide and/or by aq EDTA

extraction. For [TACN]-capped systems, extraction of the

second (distal) cobalt was achieved by addition of an excess

of chelating amine (tetramethylethylenediamine) in THF and

purification via chromatography. All mono- and bismetalated

compounds are highly air-sensitive, and so, manipulation of

these was carried out in a glovebox. Furthermore, certain

combinations of metal oxidation states were observed to

undergo redox reactions, even under an inert atmosphere (see

later).

Spectroscopic Characterization. Several spectroscopic

techniques were employed to determine the purity and

homogeneity of these compounds prior to oxygen binding

studies. The compositions of all Co metalated complexes

were confirmed by ESI-MS and compared to calculated

simulations of their isotope patterns. EPR spectroscopy was

used to characterize these (d⁷, S ) ¹/₂) Co(II) porphyrins. In

every Co-only complex with an axial base, spectra typical

of 5-coordinate Co(II) with axial symmetry were observed

at 77 K, showing the expected hyperfine and superhyperfine

coupling. In contrast to the commonly encountered 6-coor-

dinate Co(III) porphyrin, (P)Co(II) usually exists as a 4- or

5-coordinate species.³⁵

Several methods have been reported for cobalt metalation

of porphyrins.³⁵Often, a cobalt(II) salt (e.g., Cl^-, AcO^-,

-

NO₃) and a hindered base have been employed. In the

(29) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature 1995, 376,

660-669.

(30) Ostermeier, C.; Harrenga, A.; Ermler, U.; Michel, H. Proc. Natl. Acad.

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144-148.

(32) Collman, J. P.; Boulatov, R.; Sunderland, C. J. In The Porphyrin

Handbook; Kadish, K. M., Smith, K., Guilard, R., Eds.; Academic

Press: San Diego, CA; volume in press.

(33) Collman, J. P.; Fu, L. Acc. Chem. Res. 1999, 32, 455-463.

(34) Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659-698.

(35) Sanders, J. K. M.; Bampos, N.; Clyde-Watson, Z.; Darling, S. L.;

Hawley, J. C.; Kim, H.-J.; Ching Mak, C.; Webb, S. J. In The

Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;

Academic Press: San Diego, CA, 2000; Vol. 3, p 23.

(36) Collman, J. P.; Zhang, X.; Wong, K.; Brauman, J. I. J. Am. Chem.

Soc. 1994, 116, 6245-6251.

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6215.

Inorganic Chemistry, Vol. 41, No. 25, 2002 6585

Collman et al.

Scheme 1. Synthetic Scheme of the Tris-imidazole Picketed Co and Co/Cu Porphyrins.

NMR spectroscopy was used to ensure purity and to gain

insight into solution conformation. The paramagnetic Co(II)

complex, [Co3], was oxidized to a diamagnetic Co(III)

derivative, which was studied by NMR. Because of the low-

coordination sphere on the NMR time scale. This NMR

spectrum also shows that the distal imidazole H4’s point

toward the porphyrin plane (with the imidazole N-methyl

groups out). Therefore, in these Fe and Co cases, the

relatively nonshifted Im_R-H2’s together with the upfield shifts

of the Im_R-H4’s imply that the imidazole-(N3) lone pairs

point into the cavity. For the Co porphyrin, the chemical

shift change for Im_RH4’s (as compared to the free base) was

1.2 ppm upfield. The proximal Im_â-H2 and Im_â-H4 protons

were observed to move 5-5.5 and 5.5-6.3 ppm upfield for

the Fe(II)-CO and Co(III) adducts, respectively. We conclude

that the gross structures of the picket imidazole porphyrins

appear to be comparable regardless of the identity of the

porphyrin metal.

7

2

spin d configuration (unpaired electron in the d_zorbital),

¹H NMR spectra of Co(II) species yield broad signals.³⁹In

contrast to the planar and symmetric R₄-type Co(II) porphy-

rins,⁴⁰each Co porphyrin reported here has complex distal

and proximal superstructures resulting in broad signals and

poorly resolved spectra. A CF₃-marker in the tail (axial base)

portion of the porphyrin was included to monitor purity as

well as to quantify Cu(I) binding. The latter was determined

by integration of the porphyrin-bound CF₃-phenylimidazole

against the F-containing copper counterion (hexafluorophos-

phate or triflate), using ¹⁹F NMR spectroscopy. Upon

oxidation of [Co3] to [Co3]⁺, the ¹⁹F signal of the paramag-

netic species shifted from -66.84 to -67.60 ppm with

narrowing of the line width at half-height to half of its

previous value, as expected upon change to a diamagnetic

species. Other ¹⁹F NMR signals were not observed (Figure

2). The stereochemical homogeneity of [Co3]⁺was assayed

using 1D and 2D ¹H NMR spectra. Interestingly, [Co3]⁺and

[Fe3]CO (Co(III) is isoelectronic with low-spin Fe(II)) show

similar chemical shift changes upon metalation of H₂3.²⁵As

shown in Figure 2, upfield shifted signals were found for

the proximal imidazole and for the distal imidazole H4’s,

because of proximity to the porphyrin ring current. From

this, it is concluded that the proximal imidazole is coordi-

nated to cobalt and not exchanging in and out of the

It was important to establish insertion of Cu(I) into the

distal site before evaluating oxygen binding. The presence

of Cu(I) was evidenced by a combination of several tech-

niques. EPR spectra of Co(II)/Cu(I) porphyrins have not been

previously reported. Although Cu(I) (d¹⁰, S ) 0) is diamag-

netic, small changes in the Co(II) g-values and hyperfine

coupling were observed, probably because of electronic ef-

fects caused by copper (see Tables 2 and 3). When the

Co(II)/Cu(I) species were subjected to 1 atm CO, typical

CuCtO IR stretching frequencies were observed. Conclusive

ESI-MS ions were observed for the Co/Cu species and com-

pared to the simulated copper-containing isotope patterns.

CO Binding. In dichloromethane solution, under an

atmosphere of CO, [Co1Cu] and [Co3Cu] demonstrate CO

stretching frequencies of 2085 and 2082 cm^-1, respectively

(Table 1). These frequencies are consistent with carbon

monoxide bound to a Cu(I) center, and with Cu having a

tris-N donor (rather than a bis-N donor) environment. The

coordination environment can be deduced from the frequency

(39) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K.

M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Vol.

5, pp 108, 160.

(40) Clayden, N. J.; Moore, G. R.; Williams, R. J. P.; Baldwin, J. E.;

Crossley, M. J. J. Chem. Soc., Perkin Trans. 2 1983, 1863-1868.

6586 Inorganic Chemistry, Vol. 41, No. 25, 2002

Cobalt Porphyrins Related to CcO

Figure 2. ¹H NMR spectra of 3, recorded in CDCl₃at RT. Free base (upper), Fe(II)CO (middle), and Co(III)X (lower). ¹⁹F NMR spectra of Co(II) and

Co(III) metalated 3 as an inset. Bz: protons of benzoyl tail linking the proximal imidazole to porphyrin. ImH4 or NMe R2 designation for mirror symmetric

distal imidazole protons; R1 for unique central distal imidazole. Detailed assignments are in the Supporting Information.

Table 1. Infrared Stretching Frequencies for CO Bound in Cu(I)(N)₃

Table 2. EPR Parameters for 4- and 5-Coordinated Co and Co/Cu

Chelates

Species

complex

[Co3Cu]^a

CuC-O stretch (cm^-1

)

A_|(⁵⁹Co) A_|(¹⁴N)^d

porphyrin^a

[Co1] (4-coord)

[Co1]{NMeIm}_ax(5-coord) 2.306 2.306 2.036

g(x)

g(y)

g(z)

(mT)

2082^h

[Co3Cu]^a

[Co4Cu]^a

[Fe3Cu]^b

[Fe4Cu]^b

[Zn3Cu]^b

[Zn4Cu]^b

Cu(2-MeIm)₃

2

2085^h

2.321 2.320 2.222

h

-

7.89

8.00

8.15

8.09

8.00

7.95

7.70

8.12

8.01

7.6

1.76

1.7

1.78

1.75

1.70

1.7

2085^h

[Co2]

[Co3]

[Co4]

[Co5]

[Co3Cu]

[Co4Cu]

[Co4Ag]

[Co4Zn]

2.286 2.295 2.025

2.302 2.302 2.030

2.303 2.303 2.031

2.287 2.295 2.021

2.291 2.303 2.031

2.274 2.303 2.027

2.302 2.302 2.032

2.299 2.301 2.027

2.293 2.287 2.022

2.310 2.310 2.027

h

-

2087^h

2079(w)^h

2067^h

c

Cu[P(Im^iPr)₃]

2083^h

1.7

d

[Co5Cu]^a

2096^h

1.70

1.79

1.73

1.7

[Fe5Cu]^e

2092(w)^h

2020ⁱ

Cu[Me₃TACN]^f

Cu[ⁱPr₃TACN]^g

Cu[Bn₃TACN]^g

[Co5Cu]

2067ⁱ

CoMb^b(5-coord conf

by X-ray)

Co[T_pivPP]{NMeIm}_ax

2084ⁱ

c

2.31

2.04

7.5

^aThis work. ^bReference 25. ^cReference 82. ^dP(Im^iPr2

) ) tris[2-(1,4-

3

diisopropylimidazoyl)]phosphine.⁸³^eReference 84. ^fReference 47. ^gRef-

erence 48. ^hIn CH₂Cl₂. ⁱKBr pellet.

From this work, unless otherwise stated. Recorded in CH₂Cl₂at 77 K.

[n] ) 2 mmol. X-band EPR. Simulations of EPR spectra performed with

XSophe. ^bReference 66. ^cReference 64. ^dNumber of decimals depends on

the resolution of the hyperfine coupling.

a

of the vibration, because (N)₂Cu-CO carbonyl stretches are

observed to be >2110 cm^-1, whereas a range 1950-2100

cm^-1has been observed for a number of (N)₃CuCO

complexes.⁴¹Additionally, these ν(CO) values are consistent

with those of [Fe3Cu]CO and [Zn3Cu]CO, which display

stretches at 2085 and 2087 cm^-1, respectively.²⁵In all cases

(Fe/Cu, Zn/Cu or Co/Cu), CO binding to copper is reversible.

Formation of a CO adduct was not observed for the Co-

only complexes. This observation, along with the appearance

of only a single carbonyl stretch and the similarity to ν(CO)

of [Fe3Cu]CO, supports the conclusion that CO is not

binding to the Co(II) center, although CO binding has been

observed in some Co(II) porphyrins at low temperatures.^42-44

By contrast, cobalt(III) porphyrins bind CO readily.^45,46

[Co4Cu] does not bind carbon monoxide at 1 atm CO

pressure, as was noted for the corresponding Fe porphyrin.

It was suggested that a steric effect from the propyl groups

at the 2-position of NMePrIm leads to decreased affinity for

CO at the Cu-imidazole site.²⁵Thus, it appears that the copper

environment in Co(II)/Cu(I) metalated 3 and 4 is similar to

that claimed for the corresponding Fe(II)/Cu(I) and Zn(II)/

Cu(I) complexes. We conclude, as depicted in Figure 1, that

(41) Rondelez, Y.; Seneque, O.; Rager, M. N.; Duprat, A. F.; Reinaud, O.

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Inorganic Chemistry, Vol. 41, No. 25, 2002 6587

Collman et al.

O₂‚‚‚Cu(II)]⁺(P-state) has been suggested, but little evidence

supports this theory.^56,57If the peroxide state is formed, it

exists as a short-lived intermediate or in small amounts,

difficult to detect. Nonetheless, formation of peroxides has

been observed for Fe/Cu models.^12,17,58-63Usually, this is

found for Fe/Cu models where the donor ability and

geometry of the capping ligand are able to stabilize Cu(II),

favoring formation of a peroxide intermediate. Similar

reactivity was expected for the R₃[imidazole]â[imidazole]

porphyrins. As a first spectroscopic assessment of oxygen

reactivity with these complexes, a choice of cobalt over iron

was made to gain kinetic and product stability. Little

information on cobalt porphyrin models of CcO exists as

only two types of superstructured (and bimetallic) Co-por-

phyrins have been reported.^18,22Upon oxygenation, our Co-

only porphyrins reversibly form Co superoxides. Because

our efforts were targeted toward understanding the bimetallic

structure-function relationships in CcO, and especially the

role of copper, a redox-active copper was introduced into

the pickets of the cobalt complexes of 1, 3, and 4. Hitherto,

in the sole Co/Cu porphyrin system studied, [Co5Cu], a stable

Co-O₂-Cu peroxide bridge was formed (ν(O-O) ) 802

cm^-1; ∆ν(¹⁶O₂-¹⁸O₂) ) 48 cm^-1).¹⁸Peroxide formation has

also been reported for a cyclam-strapped Co/Co porphyrin,

yielding a stable Co-O₂-Co adduct.²²

Co and Co/Cu Porphyrins before Oxygenation. The

EPR spectrum of the 4-coordinated Co(II) porphyrin [Co1]

(g_|) 2.22, g ) 2.32) shows neither hyperfine nor

superhyperfine structure (examples of EPR spectra for

various coordinations are shown in Figure 3). Introduction

of a base (NMeIm) alters the electron distribution (especially

in the parallel direction to the field), and coupling is ob-

served (Table 2). A decrease in g_|(∼2.03) and the appear-

ance of hyperfine (A_|(⁵⁹Co) ∼8 mT) as well as superhyperfine

(A_|(¹⁴N) ∼1.7-1.8 mT) couplings were monitored. The

resulting EPR spectrum of {NMeIm}_ax[Co1] displays axial

symmetry, characteristic of a 5-coordinate Co(II) complex

with a nitrogenous base axial ligand parallel to the magnetic

field (g_|). Because of interaction between ⁵⁹Co (I ) ⁷/₂) and

¹⁴N (I ) 1), an octet hyperfine structure with a triplet super-

hyperfine coupling pattern is observed, ruling out a second

coordinated base even at large excess of NMeIm. Previous

studies of cobalt(II) porphyrins and NMeIm as base have

never demonstrated double occupancy of the axial positions.³⁵

Table 3. EPR Parameters for Oxygenated Co and Co/Cu Species.

a₁

a₂

a₃

porphyrin^a

g(x)

g(y)

g(z) (mT) (mT) (mT)

b

[CoTPP]{NMeIm}_axO₂

[Co2]{NMeIm}_axO₂

[Co2]O₂

[Co3]O₂

[Co4]O₂

[Co5]O₂

[Co3Cu]O₂

[Co4Cu]O₂

[Co4Ag]O₂

2.083

2.081

2.084

2.077

2.072

2.082

2.084

2.082

2.084

2.08

2.004

2.002

2.004

2.005

2.003

1.998

2.007

2.006

2.002

2.007

2.03

1.985 2.1

1.995 1.7

0.5

1.1

0.4

0.6

1.981 1.7^c1.1^c1.0^c

1.991 1.7^c1.1^c0.9^c

1.986 1.7^c1.1^c0.9^c

1.985 1.7^c1.1^c0.9^c

1.986 1.71 1.0

1.993 1.71 1.0

0.9

1.0

1.987 1.68 1.03 0.86

1.990 1.68 0.98 0.83

1.98 2.32 0.62 0.72

1.983 1.73 0.82 0.77

2.0

[Co4Zn]O₂

CoMbO₂(Type 1)^d

CoMbO₂(Type 2)^d

2.085

2.008

e

[CoT_pivPP]{NMeIm}_axO₂2.09 (|) 2.01 ( )

a

From this work, unless otherwise stated. Recorded in CH₂Cl₂at 77

K. [n] ) 2 mmol. X-band EPR. Simulations of EPR spectra performed

b

with XSophe. Reference 67. ^cNonresolved hyperfine. ^dReference 85.

^eReference 64.

the distal copper appears to be in a tris-imidazole coordina-

tion sphere above the porphyrin plane.

For [Co5Cu]CO, ν(CO) was observed at 2096 cm^-1(1

atm CO/CH₂Cl₂). As for the imidazole-picket porphyrins

discussed earlier in the text, CO binding to Cu is reversible.

While the CO stretch is within the range associated with

(N)₃CuCO species, it is somewhat higher than the highest

frequency reported to date for (R₃TACN)CuCO complexes

(2020-2092 cm^-1).^47-50Higher ν(CO) values indicate

weakened metal-to-CO back-bonding. The CO frequency of

(R₃TACN)CuCO species demonstrates dependence on elec-

tron donation from the R-groups as well as differences due

to steric-induced structural changes. The high CO stretching

frequency of [Co5Cu]CO may partly be from decreased

N-donor basicity due to the electron-withdrawing amido

(tethering) substituents. Given the location of the TACN

above the porphyrin ring, distortion of the Cu-CO bonding

due to conformational constraints at the TACN, and steric

interaction with the porphyrin ring, may also be contributing

factors. Transfer of electron density onto the CO may also

be diminished because of electrostatic interaction of the

M-CO oxygen with the porphyrin π system or other lone

pairs, a phenomenon observed with heme-CO complexes,^51-55

but not systematically studied in copper-CO complexes.

Oxygen Binding. Throughout the catalytic cycle in CcO,

Cu_Bis subjected to changes in oxidation state and coordina-

tion geometry. Along this cycle, a peroxide state [Fe(III)-

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M. Biophys. J. 2001, 81, 435-445.

6588 Inorganic Chemistry, Vol. 41, No. 25, 2002

Cobalt Porphyrins Related to CcO

bases) show 5-coordinated cobalt prior to oxygenation, the

O₂in the Co-superoxo species is most likely located inside

the distal cavity (and likewise for {NMeIm}_ax[Co1]), as

has previously been shown for R₄picket fence Co porphy-

rins.^36,64All Co-superoxo porphyrins herein are con-

sistent with literature values of a B-Co(P)-O₂core,⁶as

demonstrated in Table 3. All EPR samples were examined

by ESI-MS to confirm the formation of Co-oxy species.

The molecular peaks for the Co-O₂parent ions were weak,

but Co-O₂peaks were observed for the following com-

pounds: {NMeIm}_ax[Co1]O₂) 1235 (30%), [Co2]O₂)

1217 (18%), [Co3]O₂) 1417 (3%).

Oxygenation of [Co3Cu] and [Co4Cu] (1 atm O₂) gave

an unexpected result. Instead of the anticipated Cu(II) signal

of a Co-O₂-Cu species (with the unpaired electron on

copper), an EPR spectrum characteristic of a Co-superoxo

species was observed (Table 3). At lower concentrations of

oxygen (to slow oxygenation), overlapping signals of Co-

superoxo and Co-only species were detected. Cu(II) was

never observed, even when the oxygenation was quenched

immediately after addition of oxygen by freezing the sample

to 77 K. No EPR evidence for a peroxo-bridged species was

seen.

Figure 3. Examples of EPR spectra for 4-, 5-, and 6-coordinated species.

However, it is possible that a coordinating solvent molecule,

such as water, might be retained inside the distal pocket and

held within this cavity by weak coordination to cobalt and/

or hydrogen bonding. In earlier studies of picket fence cobalt

porphyrins with excess NMeIm, an exclusively proximal

substitution of the exogenous base was suggested, explained

by the steric hindrance of the tert-butyl pickets.^36,64It is also

likely that, in [Co1], distal axial ligation is suppressed by

the steric bulk of the imidazoles, although binding in the

cavity cannot be excluded.

All estimated g-values and coupling constants obtained

from simulations of the recorded EPR spectra are consistent

with literature values for Co-porphyrin models as well as

for cobalt-substituted enzymes, for example, cobalt heme

oxygenase,^65,66cobalt hemoglobin,⁶⁷and cobalt myoglobin⁶⁸

(Table 2). EPR spectra of [Co2], [Co3], and [Co4] are each

consistent with 5-coordinate cobalt having one Co-bound

imidazole (the endogenous CF₃T tail). The presence of

Cu(I), Ag(I), or Zn(II) in the distal pocket causes some

changes in g values, especially in g , and/or in hyperfine

coupling. The presence of Cu(I), for example, appears to

induce a higher degree of anisotropy (g ) together with a

decrease in A_|(⁵⁹Co). Broadening of this signal was observed,

possibly because of a bridging solvent or simply reshuffling

of the electronic distribution due to the presence of a positive

metal center in the cap.

The capacity of the three imidazole pickets to coordinate

1

3

Cu(II) was studied. Generally, copper(II) (S ) /₂, I ) /₂)

gives characteristic spectra where the positions of g_|versus

g indicate the geometry of Cu(II) and the (super)hyperfine

coupling suggests the number and nature of the coordinated

atoms.⁷⁰CuTf₂was titrated into the diamagnetic [Co3]⁺

porphyrin, and a signal typical for square planar (or

pyramidal) Cu(II) was obtained, although it was poorly

resolved. However, addition of CuTf₂to [Zn3] resulted in a

well-resolved spectrum characteristic of Cu(II) in a square

planar arrangement where g_|) 2.281, g ) 2.053, and

A_|(Cu) ) 16.5 mT. The superhyperfine coupling pattern of

the g signal (A_|(¹⁴N))1.5 mT) is consistent with at least 3

nitrogen donor atoms coupled to copper. A similar spectrum

was obtained for the previously reported [Zn3Cu^IICl] (g_|)

2.28, g ) 2.07, A_|(Cu) ) 17.5 mT, A_|(¹⁴N) ) 1.5 mT, A

) 1.3 mT).²⁵Free Cu(II) triflate (g_|) 2.344 and g ) 2.073,

A_|(Cu) ) 14.6 mT) was not observed in the M₁/Cu(II)

samples.

Co-O₂Complexes. When 5-coordinate Co(II) porphyrins

are exposed to oxygen, large changes in g-values and

coupling constants are observed. Simulations of the EPR

spectra indicated free-radical-like signals (g ∼ 2) and smaller

A_|(⁵⁹Co) (∼1.7 mT) (Table 3). This feature is charac-

teristic of monomeric cobalt porphyrins having one O₂

Reversibility. Generally, cobalt-O₂porphyrins show com-

plete or partial reversibility upon either purging, pumping,

heating, or irradiating the complex.^36,71According to earlier

reports,⁷²at low temperatures Co-O₂equilibria are shifted

in favor of the Co-O₂complex. The Co-only systems

reported herein bind dioxygen reversibly at RT as demon-

strated in the EPR spectra shown in Figure 5. [Co4] placed

under oxygen for 2 h gave a spectrum of coexisting Co-

only and Co-O₂. The same sample held for 1.5 h under

vacuum at RT yielded >90% Co-only. After 15 h under

molecule bound in a bent end-on fashion, formally denoted

•- 69

as Co(III)-O₂

.

Because [Co3] and [Co4] (endogenous

(64) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.; Hayes,

S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761-2766.

(65) Chu, G. C.; Tomita, T.; Sonnichsen, F. D.; Yoshida, T.; Ikeda-Saito,

M. J. Biol. Chem. 1999, 274, 24490-24496.

(66) Fujii, H.; Dou, Y.; Zhou, H.; Yoshida, T.; Ikeda-Saito, M. J. Am. Chem.

Soc. 1998, 120, 8251-8252.

(67) Bowen, J. H.; Shokhirev, N. V.; Raitsimring, A. M.; Buttlaire, D. H.;

Walker, F. A. J. Phys. Chem. B 1997, 101, 8683-8691.

(68) Brucker, E. A.; Olson, J. S.; Phillips, G. N.; Dou, Y.; Ikeda-Saito, M.

J. Biol. Chem. 1996, 271, 25419-25422.

(69) VanDoorslaer, S.; Bachmann, R.; Schweiger, A. J. Phys. Chem. A

1999, 103, 5446-5455.

(70) Hathaway, B. J. ComprehensiVe Coordination Chemistry; Pergamon

Press: New York, 1987; Vol. 5.

(71) Walker, F. A. J. Am. Chem. Soc. 1970, 92, 4235-4244.

(72) Salmon, L.; Biedcharreton, C.; Gaudemer, A.; Moisy, P.; Bedioui,

F.; Devynck, J. Inorg. Chem. 1990, 29, 2734-2740.

Inorganic Chemistry, Vol. 41, No. 25, 2002 6589

Collman et al.

oxygen, this equilibrium went to 100% completion; subse-

quent removal of the oxygen head-gas regenerated the

5-coordinate Co(II) porphyrin. Because of some background

oxidation that forms a Co(III)-X adduct over time (indis-

tinguishable from Co superoxide by UV-vis spectroscopy),

quantitative equilibrium constants could not be obtained. In

the higher concentrations used with EPR experiments (mM),

this oxidation process is insignificant, as compared to the

low-concentration UV-vis measurements (µM).

•-

[Co(II)Por][B] + O₂h [Co(III)Por][B]O₂

(1)

The final products obtained from reaction of O₂with

Co/Cu, Co/Ag, and Co/Zn complexes each showed spectra

characteristic of a Co superoxide, with well-resolved hyper-

fine features (Figure 4). All these oxygenations were

seemingly irreversible, which would have been expected for

peroxide-bridged but not superoxide-type complexes.

•-

[Co(II)/Cu(I)Por][B] + O₂u [Co(III)/Cu(I)Por][B]O₂

(2)

Interestingly, the [Co2] complex shows reversible O₂

binding, but with a higher dioxygen affinity than the

imidazole-picket Co-only porphyrins of 1, 3, and 4. Oxygen

affinity studies have shown that, in porphyrin models where

amides or other groups capable of H-bonding are bound close

to the coordinated oxygen, a large increase in oxygen affinity

is observed.^36,73This enhancement was accounted for by

H-bonding and/or dipole-dipole interaction in the cavity,

which has also been demonstrated by other groups.^65,66,74As

the O₂molecule binds to the Co(II) center, the complex

formally written as Co(III)-O₂^•-has a strong dipole,⁶⁹where

Co(III) is in a low-spin d⁶state and the electronegative

terminal oxygen of the superoxide may additionally be

stabilized by neighboring δ⁺charges. We propose that the

high oxygen affinity observed for the Co/Cu complexes may

be due to conformational changes in the distal superstructure

upon metalation of the picket imidazoles and/or electrostatic

effects caused by the presence of the second metal. Cor-

respondingly, high oxygen affinity was found for the Co/Zn

species. In contrast, the imidazole lone pairs (δ^-charge) in

the Co-only complexes are directed into the cavity and may

destabilize the superoxide adduct. It should be noted that

simple Co porphyrins function as O₂carriers similar to Mb

and Hb, whereas the superstructured Co/Cu porphyrins retain

the O₂molecule in the active site, paralleling characteristics

of bimetallic CcO where O₂is retained for reduction.

Electrochemical potentials and catalytic reductions of these

CcO analogues will be discussed in a following paper.

H-Bonding. Oxygenation of the Co-only complexes [Co3]

and [Co4] leads to EPR spectra with poorly resolved hyper-

fine coupling. This phenomenon may be explained either as

superimposed Co-superoxo signals originating from a mix-

ture of frozen Co-superoxide conformations, or by a freely

rotating O₂-unit in the distal cavity. When distal Cu(I) is

Figure 4. Examples of experimental EPR spectra (upper trace) and

simulations (lower trace) of [Co4] (A) and [Co4Cu]O₂(B) porphyrins. H

vs D experiment for [Co4Cu]O₂(C).

(73) Tang, H.; Dolphin, D. Inorg. Chem. 1996, 35, 6539-6545.

(74) Chu, G. C.; Tomita, T.; Sonnichsen, F.; Yoshida, T.; Ikeda-Saito, M.

Biophys. J. 1999, 76, A426-A426.

6590 Inorganic Chemistry, Vol. 41, No. 25, 2002

Cobalt Porphyrins Related to CcO

Figure 6. Illustration of plausible H-bonding in the cavity.

ing, which may be explained as Co-superoxide signals

manifesting hindered oxygen rotations due to H-bonding to

the amide and/or the imposed tighter distal pocket structure

when Cu or Zn is present to assemble the capping imidazoles

(Figure 6).

Figure 5. Demonstrated reversibility of O₂binding for the Co-only

porphyrin [Co4].

Parameters for the imidazole pickets of a zinc R₄[NMeIm]

structure⁷⁶combined with parameters for the Co-superoxo

core from the solved CoMb structure⁶⁸were utilized in

molecular mechanics calculations (MM3; MM3 minimization

performed with software program CAChe Worksystem 4.5,

Cambridge, MA) of a CoR₃[NMeIm]â[Im]-O₂species. In

the resulting minimized structures, a hydrogen bond distance

of about 2.6-2.8 Å was found between the amide hydrogen

and the terminal oxygen of the superoxide. A strong

H-bonding interaction between the terminal electronegative

oxygen and the amide is probably the driving force stabilizing

the Co-superoxide core in our Co/Cu complexes. Similarly,

in a recent structure of Co(T_pivPP)(NO₂)(1-MeIm), a H-bond

was indicated between the amide of the picket fence and

the oxygen of the nitrite,⁷⁷which was found to point toward

the amide hydrogen at a distance of 2.96 Å. Comparable

H-bond interactions have been observed for other complexes

such as Fe(T_pivPP) porphyrins, but to somewhat lesser extent

because of electronic differences between the oxy states of

the two metals. In heme-enzymes, H-bonding has been

shown to occur between O₂and distal residues, such as

histidine, glutamine, or tyrosine. For example, a remarkable

affinity for O₂, a factor of 10⁴higher than for mammalian

Hb, was reported for Hb from the bloodworm Ascaris.⁷⁸This

effect is believed to originate from strong H-bonding between

Tyr 30 to the terminal oxygen as well as between Gln 64

and the heme-bound oxygen. In the oxy forms of native and

mutated (Co) myoglobin, the oxygen-distal histidine (His 64)

distances range between 2.72 and 3.01 Å.^68,73

present and the bimetallic complexes are oxygenated, an EPR

spectrum with resolved hyperfine coupling was observed.

Walker and Bowen investigated hydrogen bond donation

from neighboring acetamides to the terminal oxygen in

Co-O₂porphyrins.^67,75Enthalpic stabilization due to the

amide N-H bonding to bound O₂was shown to depend on

the substitution position of the acetamide (ortho vs para).

This effect was observed as a loss or appearance of hyperfine

coupling in the spectra of various Co-O₂adducts as a

function of temperature. At low temperatures, an ortho-

acetamide H-bonds to O₂, locking that conformation and

giving rise to hyperfine coupling, which is lost at elevated

temperatures. With a para-acetamide substitution where such

H-bonding to O₂is not possible, an almost completely

motionally averaged EPR spectrum is observed, consistent

with the oxygen molecule freely rotating about the Co-O

bond.

A paramagnetic center surrounded by ions with nuclear

magnetic moments, such as protons, experiences magnetic

dipole-dipole interactions leading to spectral line broaden-

ing. In comparison to the proton, the smaller nuclear

magnetic moment of deuterium may reduce the line width

by a factor of 33%. If exchangeable protons are near the

paramagnetic Co-O₂center, a deuterated environment

should result in reduction of the line width yielding a

sharpened hyperfine structure. For the oxy-Co analogues of

heme oxygenase and a related heme degradation enzyme

Hmu O,^65,66H-bonding to a neighboring amide was examined

by H/D exchange experiments. Comparison of the EPR

spectra of the H and D systems revealed different coupling

patterns for the Co-superoxide signals. Specifically, hyperfine

coupling was found to be more pronounced in the D

experiments. Similarly, in the present studies, differences in

the hyperfine coupling of H- and D-substituted amides were

observed for both [Co4Cu]O₂(Figure 4C) and [Co4Zn]O₂

complexes, whereas no change was seen in an analogous

experiment using [Co4]O₂.

Raman Spectroscopy. Raman spectra of the oxygenated

[Co4] complex show a band at 1148 cm^-1that is not present

in the non-oxygenated samples (Figure 7). This peak is

isotopically sensitive and shifts to 1077 cm^-1upon ¹⁸O₂

substitution (∆ ) 71 cm^-1), which is indicative of ν(O-O)

for cobalt-porphyrin-superoxide species.^4,5

As depicted in Figure 8, Raman spectra of oxygenated

[Co4Cu] show an isotope-sensitive band in the superoxide

region at 1143 cm^-1that shifts to 1070 cm^-1upon ¹⁸O₂

substitution (∆ ) 73 cm^-1). Superoxide stretching frequen-

In conclusion, O₂appears to be freely rotating in the [Co3]-

O₂and [Co4]O₂complexes. The Co/Cu and Co/Zn com-

plexes of both porphyrins demonstrate evidence for H-bond-

(76) See Supporting Information.

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Inorganic Chemistry, Vol. 41, No. 25, 2002 6591

Collman et al.

Figure 7. RR spectra of [Co4]O₂with ¹⁶O₂and ¹⁸O₂.

Figure 8. RR spectra of [Co4Cu]O₂with ¹⁶O₂and ¹⁸O₂.

cies for oxygenated [Co4Co] and [Co4Zn] species were

found at 1147 cm^-1(∆ ) 73 cm^-1) and 1146 cm^-1(∆ ) 72

cm^-1), respectively, demonstrating the resemblance of the

two systems. The notably lower ν(O-O) (1143 cm^-1) of

the oxygenated [Co4Cu] complex indicates a slightly dif-

ferent environment for the superoxide species in Co/Cu,

possibly because of differences in distal metal coordination

geometry and/or charge.

ligand as observed by Proniewicz et al.⁷⁹Asymmetry of the

¹⁸O₂peak indicates the potential for mode mixing with the

broad peak at 1086 cm^-1, which would cause the ¹⁸O₂

superoxide peak to shift down in energy and lead to the

unusually large isotope shift.

[TACN][PhIm] System. TACN superstructured Co por-

phyrins have previously been prepared,^17,18and it was found

that upon oxygenation the Co-only derivative binds O₂

reversibly, while the bimetallic TACN adduct forms a

bridged Co-O₂-Cu peroxide complex, which undergoes

The magnitudes of the measured isotope shifts were found

to be ∼10% higher than the predicted shift of ∆ ) 65 cm^-1

for dioxygen alone. The reason for this difference is likely

due to coupling with vibrational modes of the axial imidazole

(79) Proniewicz, L. M.; Bruha, A.; Nakamoto, K.; Kyuno, E.; Kincaid, J.

R. J. Am. Chem. Soc. 1989, 111, 7050-7056.

6592 Inorganic Chemistry, Vol. 41, No. 25, 2002

Cobalt Porphyrins Related to CcO

reduction of O₂when 4 equiv of Co(Cp)₂is added. For

comparison to the present work, the [Co5] and [Co5Cu]

porphyrins were synthesized and studied. In reaction of

[Co5Cu] with O₂, Co superoxide was neither detected by

EPR nor RR spectroscopy at any point during the course of

the reaction. Consequently, we suggest that a Co-O₂-Cu

adduct is formed. However, the observed bridged Cu(II) EPR

signal could not be resolved from another Cu(II) signal

which, after a day of reaction, was the only remaining EPR-

active species. As evidenced by EPR, the final product

consists of a Co(III)/Cu(II) couple (EPR-inactive cobalt).

When the Co-only derivative was treated with oxygen, a

reversible [Co5]O₂(superoxide) adduct was confirmed by

both spectroscopic techniques, yielding a RR ν(O-O) stretch

at 1147 cm^-1(∆ ) 73 cm^-1) and the characteristic Co-

superoxide EPR signal at g ∼ 1.99 and g_|∼ 2.07 (Table

3). Contrary to the Co/Cu adduct, a cobalt superoxide is

formed when treating [Co5Zn] with O₂, which was evidenced

by EPR and RR spectroscopy (ν(O-O) ) 1150 cm^-1; ∆ )

73 cm^-1). Little change is observed between cobalt-porphy-

rin-superoxide stretching frequencies with different distal

superstructures (TACN vs tris-imidazole). However, redox-

versus non-redox-active distal metals may have a different

influence on Co/O₂chemistry as observed for the TACN-

capped porphyrins.

Further Reactivity. After oxygenation, regardless of

system, the Co-superoxide species in concentrated solutions

were found to be stable. At RT under dry conditions, the

Co-superoxide derivative stayed intact for as long as 10 days.

Exposure to protons, from MeOH or acid, caused an imme-

diate (<seconds) decomposition of the Co superoxide (which

could explain the difficulty in observing Co-superoxide

species by ESI-MS). The reduction of O₂to OOH^•/H₂O₂is

clearly promoted by protons.^80,81The final stable product is

EPR inactive suggesting a Co(III)-X species, where X

denotes the corresponding base (MeO^-, Cl^-, Ac^-, OH^-, etc.).

UV-vis spectra showed distinct changes in the Soret bands

when the Co-superoxide complexes were treated with a

variety of acids (HCl, HOAc, organic acid, etc.). Addition

of base, such as aqueous sodium hydroxide, had no apparent

effect on the decomposition of Co superoxide. With water

present, the Co superoxide was less long-lived. Moreover,

certain RR laser excitation wavelengths caused some disin-

tegration of the Co-superoxide complex. This decomposition

was most prominent for the Co-only porphyrins. Detailed

analyses of these reactions were not undertaken.

Redox-active, oxyphilic Cu(I) or non-oxyphilic redox-stable

metals (Ag(I) and Zn(II)) have been coordinated in the distal

superstructure (tris-imidazole pickets), and the oxygenated

products of both mono- and bimetallic porphyrins have been

characterized by EPR, RR, and ESI-MS spectroscopy.

Unexpectedly, Co superoxide was found to be the end

product for all these porphyrin complexes. This is the first

example of model compounds for CcO where two metals

that readily react with dioxygen, placed at a favorable

intermetallic distance, result in a metal-superoxide product

rather than a metal-metal µ-peroxo or µ-oxo/hydroxo

species. However, the distal metal in these models is not

merely an innocent spectator. For the Co/Cu and Co/Zn

porphyrins, H-bonding of the dioxygen unit to the picket-

amide-NH is indicated by H/D EPR experiments. Negligible

reversibility of O₂binding is observed for all bimetallic

porphyrins, whereas the Co-only porphyrins show reversible

dioxygen binding. We propose that several factors, a tight

distal cavity, H-bonding, and electrostatic effects, strongly

influence the dioxygen affinity and the stabilization of bound

superoxide. In comparison to native enzymes, the Co-only

porphyrins resemble the Mb/Hb type of hemes (or CoMb/

CoHb), where reversible superoxide formation is observed.

However, the Co/Cu models show limited reversibility after

oxygenation, thus approaching the nature of the active site

of CcO. These results indicate that one possible role of Cu_B

in CcO is to aid retention of O₂and to stabilize a superoxide

intermediate. Dioxygen binding kinetics, mechanistic studies,

and electrochemistry of these Co porphyrins will be presented

in a later publication.

Experimental Section

Materials and Methods. Reagents and solvents, purchased from

Aldrich, Fluka, Alfa, Strem, or Lancaster, were of commercial

reagent quality unless otherwise stated. Huenig base was stored

over molecular sieves, and the SiO₂and Al₂O₃chromatography

gels were used as received from EM Science. Under an inert

atmosphere, tetrahydrofuran, benzene, and diethyl ether were

distilled from sodium benzophenone ketyl while acetonitrile and

dichloromethane were distilled from CaH₂and methanol was

distilled from magnesium prior to use. For NMR studies, CDCl₃

was vacuum transferred from 4 Å molecular sieves into flame-

dried NMR tubes immediately prior to use. Synthesis of air-sensitive

porphyrin compounds and all metalations were performed in a

Vacuum Atmospheres glovebox (VAC) under a N₂atmosphere with

<1 ppm O₂content. All porphyrin and metalloporphyrin NMR

samples were prepared under an inert (N₂) atmosphere in 5 mm

NMR tubes fitted with PTFE valves. RR, EPR, IR, and UV-vis

samples were prepared in a glovebox. Oxygenations were performed

via vacuum transfer of 100% ¹⁶O₂(gas cylinder) or 100% ¹⁸O₂

(bulb) to samples (1 atm).

Conclusions

A series of (P)Co(II) models with structural and chemical

resemblance to the active site in CcO has been synthesized.

Instrumentation. All NMR spectra were recorded on a Varian

Unity 500 MHz spectrometer. ¹H NMR spectra were referenced to

residual protio-solvent signal, and ¹⁹F NMR spectra were referenced

to CFCl₃(0 ppm). IR spectra were recorded on a Mattheson Infinity

Series FTIR in a NaCl solution cell (path length 1.0 mm) of ∼2

mM concentrations. EPR spectra were recorded on a Bruker EMX

spectrometer (X-band) at 77 K in a 3 mm (o.d.) quartz tube fitted

with a PTFE valve of analyte at millimolar concentration in

glassified CH₂Cl₂. Data treatment and simulations of EPR spectra

(80) Steiger, B.; Anson, F. C. Inorg. Chem. 2000, 39, 4579-4585.

(81) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New York,

1991.

(82) Sorrell, T. N.; Jameson, D. L. J. Am. Chem. Soc. 1983, 105, 6013-

6018.

(83) Sorrell, T. N.; Allen, W. E.; White, P. S. Inorg. Chem. 1995, 34, 952-

960.

(84) Collman, J. P.; Berg, K. E.; Sunderland, C. J. Unpublished results.

(85) Hori, H.; Ikeda-Saito, M.; Yonetani, T. J. Biol. Chem. 1982, 257,

3636-3642.

Inorganic Chemistry, Vol. 41, No. 25, 2002 6593

Collman et al.

were performed with the Bruker software packages XSophe,

eprView, and WinEPR. UV-vis spectra were recorded on a

Hewlett-Packard 8452A spectrometer at micromolar concentration.

ESI-MS spectra were recorded on a Finnigan LCQ mass spectrom-

eter at the PAN facility at Stanford University. FAB-MS spectra

were measured by the Mass Spectrometry Facility at the University

of California, San Francisco. Resonance Raman spectra were

obtained using 458 nm Ar⁺(Coherent Sabre 25/7) ion lasers with

incident power in the 5-25 mW range using a 135 backscattering

arrangement. The scattered light was dispersed by a triple mono-

chromator (Spex 1877 CP, equipped with 1200, 1800, and 2400

groove/mm gratings) and detected with a back-illuminated CCD

camera (Princeton Instruments ST-135). Frozen solution samples

in PTFE valve quartz NMR tubes were immersed in liquid nitrogen

using an EPR finger dewar.

Ph_R1H6, NH_â), 8.01 (1H, d, Ph_âH6), 7.88 (3H, td, Ph_R2H4,

Ph_R1H4), 7.84-7.81 (3H, m, Ph_âH4, NH_R2), 7.64-7.60 (3H, m,

Ph_R2H5, Ph_R1H5), 7.51(1H, t, Ph_âH5), 6.96 (1H, s, Im_R1H2), 6.94

(2H, s, Im_R2H2), 6.66 (1H, s, NH_R1), 5.43 (3H, s, Im_R1H4,

Im_R2H4), 3.62 (3H, s, Im_R1CH3), 3.58 (6H, s, Im_R2CH3), -2.56

(2H, s, porNH). ESI-MS: m/z 1041.5 [(C₆₁H₄₉N₁₄O₄)]⁺or

H₂[NMeIm][Ac]⁺(100%). UV (toluene, 298 K) (λ, ꢀ): 334 (44 ×

10³), 428 (257 × 10³), 520 (21 × 10³), 554 (7 × 10³), 594 (7 ×

10³), 650 (4 × 10³).

Synthesis of H₂2. As a combination of the descriptions in this

paper and earlier work, this acetylamide-capped porphyrin was

synthesized. The free base porphyrin together with its cobaltated

complex are being reported and characterized here for the first time.

ESI-MS: m/z [C₆₈H₅₂F₃N₁₀O₄]⁺1129.4 (100%). ¹H NMR (DMSO,

δ): 9.36 (s, 1H, NH_R1), 9.11 (s, 2H, NH_R2), 8.77-8.76 (m, 4H,

pyrrH), 8.67-8.61 (m, 5H, Im_âH4, pyrrH), 8.10 (d, 1H, Ph_âH3),

8.05 (m, 3H, Ph_âH6, Ph_R2H3), 7.88 (t, 1H, Ph_âH4), 7.85 (d, 2H,

Ph_R2H6), 7.81-7.77 (m, 3H, Ph_R1H3, Ph_R2H4), 7.72 (t, 1H,

Ph_âH5), 7.67 (d, 1H, Ph_R1H6), 7.54-7.50 (m, 4 H, Ph_R2H5,

Ph_R1H4, Ph_R1H5), 7.44 (s, 1H, NH_â), 7.13 (d, 2H, CF₃Ph-3,5-H),

6.95 (d, 2H, CF₃Ph-2,6-H), 6.84 (s, 1H, Im_âH2), 6.51 (s, 1H,

benzoylH2), 6.44-6.41 (m, 2H, benzoylH5, benzoylH4), 6.21

(m, 1H, benzoylH6), 4.71 (s, 2H, benzoylCH₂), 1.28 (s, 3H,

R-amideCH₃), 1.17 (s, 6H, R2-amideCH₃), -2.74 (s, 2H, porNH).

Synthesis of H₂3. Synthesis and characterization of H₂3 has

previously been reported in a paper by Collman et al.²⁵Here, we

only report the ESI-MS data as well as the 1H NMR assignment

for comparison with other related porphyrins described in this paper.

ESI-MS: m/z 1327.5 [C₇₇H₅₈F₃N₁₆O₄]⁺(100%). ¹H NMR (CDCl₃,

δ): 8.92 (d, 2H, pyrrH), 8.90 (d, 2H, pyrrH), 8.87 (d, 2H, pyrrH),

8.84 (d, 2H, pyrrH), 8.80 (d, 1H, Ph_âH3), 8.53 (d, 2H, Ph_R2H3),

8.42 (d, 1H, Ph_R1H3), 8.10 (d, 1H, Ph_âH6), 8.10 (s, 1H, NH_R1),

8.06 (d, 1H, Ph_R1H6), 8.00 (d, 2H, Ph_R2H6), 7.87-7.83 (m, 4H,

Ph_âH4, Ph_R1H4, Ph_R2H4), 7.80 (s, 2H, NH_R2) 7.61 (t, 1H,

Ph_R1H5), 7.57 (t, 1H, Ph_âH5), 7.54 (t, 1H, Ph_R2H5), 7.45 (s, 1H,

NH_â), 7.06 (d, 2H, CF₃Ph-3,5-H), 6.94 (s, 3H, Im_R1H2, Im_R2H2),

6.88 (s, 1H, Im_âH2), 6.80 (s, 1H, Im_âH4), 6.66 (d, 2H, CF₃Ph-

2,6-H), 6.37 (s, 1H, benzoylH₂), 6.29 (t, 1H, benzoylH5), 6.28 (d,

1H, benzoylH4), 6.11 (d, 1H, benzoylH6), 5.41 (s, 2H, Im_R2H4),

5.36 (s, 1H, Im_R1H4), 4.11 (s, 2H, benzoylCH₂), 3.60 (s, 3H, Me_R1),

3.58 (s, 6H, Me_R2), -2.51 (s, 2H, porNH).

Synthesis. Synthesis of H₂1. Synthesis of H₂1 was carried out

according to an earlier description,²⁵except for the two separate

coupling reactions of imidazole and acetyl onto the distal and the

proximal sides of the porphyrin, respectively. These two reactions

are described in following paragraphs.

Coupling Reaction of Acetyl Chloride to H₂[TFA][NH₂] and

Deprotection of the TFA. H₂[TFA][NH₂] (250 mg, 0.23 mmol)

was dissolved in acetic acid (40 mL), and acetyl chloride (80 µL,

1.13 mmol) was added into the dark-purple solution, immediately

rendering the solution dark-green. The reaction mixture was left

stirring under nitrogen at RT in darkness for 3 h, whereupon a

solution of NaOAc/HOAc (3.85 mL, 5 g anhyd NaOAc in 100 mL

HOAc) was added slowly via syringe. CH₂Cl₂(100 mL) was added

into the reaction mixture, and the organic phase was extracted twice

with 100 mL of distilled water and once with 100 mL dil NaHCO₃

(aq). After drying the organic phase with K₂CO₃, filtering, and

evaporating the solvent, we obtained one major purple product

(TLC: SiO₂; CH₂Cl₂bubbled with NH₃). The crude H₂[TFA][Ac]

was dissolved in 10 mL of CH₂Cl₂followed by addition of 5 mL

of a solution of MeOH prebubbled with NH₃(g) and cooled. The

solution was left stirring for 1-2 days at RT in darkness. After

evaporation of the solvent, the purple microcrystalline powder of

H₂[NH₂][Ac] was purified by column chromatography using SiO₂

gel (230-400 mesh) and NH₃-bubbled CH₂Cl₂as eluent. Yield:

1

144 mg (76%). H NMR (CDCl₃, δ): 8.94-8.92 (6H, m, pyrrH),

8.78 (2H, d, pyrrH), 8.71 (1H, d, Ph_âH3), 7.98 (1H, d, Ph_âH6),

7.88 (2H, dd, Ph_R2H3), 7.85-7.80 (2H, m, Ph_âH4, Ph_R1H3), 7.64-

7.61 (3H, td, Ph_R2H6, Ph_R1H6), 7.49 (1H, t, Ph_âH5), 7.21-7.16

(5H, m, Ph_R2H4, Ph_R1H4, RNH₂), 7.15-7.12 (5H, m, Ph_R2H5,

Ph_R1H5, RNH₂), 6.75 (3H, s, RNH₂, NH_â), 3.60 (3H, s, âCOCH₃),

-2.66 (2H, s, porNH).

Synthesis of H₂4. Synthesis was carried out as described in a

Article Doi

Distal metal effects in cobalt porphyrins related to CcO

DOI: 10.1021/ic020395i

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic020395i

Full text of DOI:10.1021/ic020395i

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