Ind. Eng. Chem. Res. 2000, 39, 3429-3435
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Studies on 24-Membered Macrocyclic Mononuclear and Dinuclear Iron Complexes: Stability and Catalytic Hydroxylation of Adamantane by Divalent Iron Complexes Deyuan Kong, Arthur E. Martell,* and Ramunas J. Motekaitis Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012
The dinucleating 24-membered hexaazadiphenol macrocyclic ligand 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyltricyclo[23.3.1.111,15]triaconta-1(28),11,13,15(30),25,26-hexaene (L or [24]BDBPH) is prepared by the NaBH4 reduction of the Schiff base obtained from [2 + 2] template condensation of 2,6-diformyl-p-cresol with diethyltriamine. The ligand maintains dinuclear integrity for both iron(II,II) and iron(III,III) complexes, while facilitating the formation of bridging phenolate diiron cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear iron(II) and iron(III) complexes form from p[H] 2-11 in an aqueous solution. The protonation constants and stability constants of the 1:1, 1:2 [ligand/ iron(II) or ligand/iron(III)], and 1:1:1 [ligand/iron(III)/iron(II)] complexes were determined in a KCl supporting electrolyte (µ ) 0.100 M) at 25 °C. The mechanisms for the formation of dinuclear iron(II), iron(III), and mixed-valence iron(II,III) complexes are described. Preliminary results showed that the dinuclear iron(II) complexes catalyze hydroxylation of adamantane in the presence of H2S as a two-electron reductant. Introduction Oxo- and hydroxo-bridged diiron units are of wide occurrence in biology and perform a range of activities. Dinuclear iron centers have been found in hemerythrin, methane monoxygnenase, and the B2 subunits of ribonucleotide reductase.1 These proteins have elicited interest because of their widespread occurrence and the diverse nature of their functions, including reversible O2 binding, alkane hydroxylation, and DNA biosynthesis.2 Synthetic structurally different models for these proteins using several types of ligands have been reported in recent years.3 Tetradentate ligands,4 alkoxobridging polypodal ligands,5 and a few dinucleating macrocyclic ligands have been described.6 Also, two 20membered Schiff base macrocyclic ligands (H2LI and H2LII) were used to study diiron complexes as well as other dinuclear transition-metal complexes. This work contributed much to our understanding of the behavior of coupled metal systems, in which LIM2+M2+, LIIM2+Fe3+, and LIIFe3+(OH)2Fe3+LII species were reported.7 To widen the studies on the various oxidation states of the dinuclear iron site, ligands that are larger and have a special affinity for ferrous and ferric centers need to be synthesized. In this paper, a new 24-membered ligand (L) has been synthesized and compared with an analogous ligand, [24]RBPyBC(L′). This new ligand has more flexible donor groups than L′, which can provide a larger cavity for dinuclear centers. Also an amino group is a more basic group than pyridine nitrogen and may allow metal-metal spatial distances and coordination geometry to vary from one intermediate to the next. The direct functionalization of saturated hydrocarbons usually requires drastic conditions (i.e., high temperature, high pressure, strong electrophilicity, or radical reagents) and gives mixtures of products including polyfunctionalized compounds.8 The oxidation of saturated hydrocarbons under mild conditions is an
intellectually stimulating and industrially important objective of current relevance. Many biological systems are able to hydroxylate nonactivated hydrocarbon bonds. Of particular interest are the monooxygenases, of which cytochrome P450 (cP450) constitutes the most studied group. These consist of a family of isozymes, which are active in the oxidation of numerous drugs, xenobiotics, and endogenous compounds.9,10 Among these transformations, the attractive model of hydroxylation of saturated hydrocarbons, such as GIF systems11 proposed by D. H. Barton’s group, strongly supports an FeVdO species,12 which probably acts as a hydrogen abstractor to give a discrete carbon radical. In our system, we use molecular oxygen, pyridine, the designed and synthesized macrocyclic ligand, and ferrous iron, in the presence of a two-electron reductant (H2S) for facile oxygenation of adamantane. Results and Discussion Potentiometric Studies. (a) Stability of Mononuclear and Dinuclear Iron(II) Complexes. The fully protonated species is designated as H8L6+ and the fully deprotonated species as L2-. Log KHn values of eight successive stepwise protonation constants of the ligand obtained in this paper are 11.46, 10.13, 10.05, 9.60, 7.06, 4.42, 3.67, and 3.28 (KHn ) [HnL(n-2)+]/ [Hn-1L(n-3)+][H], n ) 1-8) and are in good agreement with those of previous work, especially in last six constants.13 When p[H] < 2, the ligand exists in the fully protonated form, H8L6+. As the p[H] is increased, the ligand loses its protons from amino nitrogens to become H7L5+, H6L4+ H5L3+, H4L2+, H3L+, and H2L species, respectively. The neutral ligand H2L reaches its maximum concentration at p[H] ) 10.6 (61%). The potentiometric data obtained from solutions containing H2L‚6HBr and ferrous ions are illustrated in Figure 1. The inflections at a ) 7 and 9 indicate the formation of the mononuclear and dinuclear complexes,
10.1021/ie000058m CCC: $19.00 © 2000 American Chemical Society Published on Web 08/18/2000
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Figure 1. Potentiometric equilibrium curves for the L/FeII system in argon at 25.0 ( 0.1 °C and µ ) 0.100 M KCl (a ) moles of KOH added per mole of ligand). Table 1. Overall and Stepwise Stability Constants for the L/FeII System [µ ) 0.10 M KCl, 25 °C, under Argon] stoichiometry L
Fe
H
log β
stepwise quotient, K
log Ka
1 1 1 1 1 1 1 1
1 1 1 1 1 2 2 2
0 1 2 3 4 0 -1 -2
18.88 29.31 32.59 37.28 48.84 26.38 18.34 -8.38
[FeL]/[Fe][L] [FeHL]/[FeL][H] [FeH2L]/[FeHL][H] [FeH3L]/[FeH2L][H] [FeH4L]/[FeH3L][H] [Fe2L]/[FeL][Fe] [Fe2(OH)L][H]/[Fe2L] [Fe2(OH)2L][H]/[Fe2(OH)L]
18.88 10.43 3.29 4.69 11.55 7.50 -16.27 -11.70
a
Estimated error ) (0.02-0.04.
respectively. The p[H] titration curves were employed to calculate 1:1 and 1:2 ligand/metal binding constants. The stability constants involving protonated, deprotonated, and hydroxo-bridged Fe(II) species are shown in Table 1. For mononuclear systems, three major species, [FeIIH4L]4+, [FeIIHL]+, and [FeIIL], were identified with high concentrations of species and fairly high stability constants. The formation constant of log K[FeL] is 18.88, which is larger than that of [24]RBPyBC (L′) with log K[FeL′] ) 15.32. [FeL] begins to dominate when the p[H] is higher than 8, and it reaches its highest concentration at p[H] ) 11.7 (77%). In the dinuclear system, [Fe2L]2+ and two hydroxo-bridged species, [Fe2(OH)L]3+ and [Fe2(OH)2L]2+, dominate in the range p[H] ) 6-12. Dinuclear complexes easily form hydroxo-bridged hydrolytic species. (b) Stability of Mononuclear and Dinuclear Iron(III) Complexes. Potentiometric equilibrium curves having 1:1 and 1:2 molar ratios of ligand to ferric ion are shown in Figure 2. For the 1:1 system, the strong inflection at a ) 6 indicates the formation of the mononuclear ferric complex [FeIIIH2L]3+, which reaches
maximum concentration, 99.9%, at p[H] ) 4. The logarithm of the stability constant of [FeIIIL]+ is 31.77, which is also somewhat less than log K[FeIIIL′]+ ) 32.02 by about 0.25 log units. For the 1:2 system, a strong inflection occurs at a ) 10, indicating that 2 mol of hydrogen ions was neutralized in addition to the 8 mol of hydrogen released from the ligand. This observation is evidence for the formation of a µ-oxo bridge between two ferric ions in the dinuclear species, even under fairly acidic conditions. The overall and stepwise stability constants for the ligand/FeIII system are listed in Table 2. The data in Table 2 show that the addition of a second metal ion to [FeIIIL]+ to form [FeIII2L]4+ is characterized by a lower stability constant than that of [FeIIIL]+, because of the destabilization resulting from the coubombic repulsion between the two metal ions in the limited macrocyclic cavity. Martell et al.15 also reported this trend in similar work on Fe(III) and [24]RByBC, (L′); the stability constants changed from 32.02 (log K[FeIIIL′]+) to 12.89 (log K[FeIII2L′]4+). Two mononuclear and four dinuclear complexes were identified with high stability constants. The formation constant of the dinuclear complex of [24]RByBC(L′) is log K[FeIII2L′]3+ ) 12.89, compared with that of L of log K[FeIII2L]3+ ) 15.42, is lower by 2.54 log units. This means the flexible pendant groups in L are more suitable for the formation and high stability of dinuclear iron complexes. The species distribution diagram of the system H8L/2FeIII is shown in Figure 4. It is seen that the mononuclear [FeIIIH2L]3+ complex predominates from p[H] 2 to 3. Then the other ferric ion enters into the macrocyclic cavity to form the dinuclear ferric complex [FeIII2L]4+ and reaches a maximum concentration (44%) at p[H] ) 2.8. When the p[H] is raised, a µ-hydroxo ferric complex [Fe2(OH)L]3+ forms and reaches a maximum concentration of 95.6% at p[H]
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3431
Figure 2. Potentiometric equilibrium curves for L/FeIII,II systems in argon at 25.0 ( 0.1 °C and µ ) 0.100 M KCl (a ) moles of KOH added per mole of ligand). Table 2. Overall and Stepwise Stability Constants for the L/FeIII and the L/FeIII/FeII Systems [µ ) 0.10 M KCl, 25 °C, under Argon] stoichiometry
a
L
Fe2+
Fe3+
H
log β
stepwise quotient, K
log Ka
1 1 1 1 1 1 1 1 1 1 1 1
0 0 0 0 0 0 0 0 1 1 1 1
1 1 1 2 2 2 2 2 1 1 1 1
0 1 2 0 -1 -2 -3 -4 0 -1 -2 -3
31.77 42.02 48.65 47.19 44.01 38.96 29.21 19.18 37.33 35.99 28.52 17.31
[FeL]/[Fe][L] [FeHL]/[FeL][H] [FeH2L]/[FeHL][H] [Fe2L]/[FeL][Fe] [Fe2(µ-OH)L][H]/[Fe2L] [Fe2(µ-O)L][H]/[Fe2((µ-OH)L] [Fe2(µ-O)(OH)L][H]/[Fe2(µ-O)L] [Fe2(µ-O)(OH)2L][H]/[Fe2(µ-O)(OH)L] [Fe3+Fe2+L]/[Fe3+L][Fe2+] [Fe3+Fe2+(µ-OH)L][H]/[Fe3+Fe2+L] [Fe3+Fe2+(OH)2L][H]/[Fe3+Fe2+(µ-OH)L] [Fe3+Fe2+(µ-O)(OH)2L][H]/[Fe3+Fe2+(OH)2L]
31.77 10.25 6.63 15.42 -3.18 -5.05 -9.75 -10.03 5.56 -1.34 -7.47 -8.22
Estimated error ) (0.02-0.04.
4.9. Between p[H] 4 and 7, the stable µ-hydroxo-bridged diferric complex [FeIII2(µ-OH)L]3+ dominates. Above p[H] > 7-10, the hydrolytic species [FeIII2(µ-O)L]2+, [FeIII2(µ-O,µ-OH)L]+, and [FeIII2(µ-O)2L] are formed successively. The µ-oxo-bridged diferric complexes become the main components in an aqueous solution at high p[H]. (c) Stability of Mixed-Valence Dinuclear Iron(II,III) Complexes. The pH profile of the 1:1:1 solutions of the ligand with Fe3+ and Fe2+ shows an inflection at a ) 6, indicating the initial formation of the mononuclear [FeIIIH2L]3+ complex, and an inflection at a ) 9, indicating the formation of a mixed-valence [FeIIIFeII(OH)L]2+ complex. In addition, [FeIIIFeII(OH)2L]2+ and the corresponding µ-oxo species [FeIIIFeII(µ-O)(OH)2L] are also identified, and stability constants for these species are included in Table 2. From Chart 1, it is seen that this ligand contains six nitrogens able to act as donor atoms in complexes. However, they are arranged
as two subunits separated by two phenolic bridging donor groups. It is expected that a mononulear complex [FeIIIH2L]3+ will be formed by the coordination of the ferric ion to one of the subunits. When the p[H] is raised, the remaining amino groups deprotonate and the ferrous ion coordinates to the donor groups on the other side of the macrocycle to form the mixed-valence complex. Finally, the µ-hydroxo-bridged species and further hydrolytic mixed-valence species are subsequently formed in an alkaline solution. The stability of the mixed-valence complex formed from dinuclear iron(III) and iron(II) is indicated by its comproportionation constant for the following equilibrium.
[FeIII2L]4+ + [FeII2L]2+ a 2[FeIIIFeIIL]3+
(1)
Kcom ) ([FeIIIFeIIL]3+)2/[FeIII2L]4+[FeII2L]2+
(2)
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Figure 3. Species distribution diagram for the L/2FeII system as a function of p[H] (FedFe2+) (% ) percentage of species distribution in solution).
Figure 4. Species distribution diagram for the L/2FeIII system as a function of p[H] (FedFe3+) (% ) percentage of species distribution in solution).
From the stability constants of dinuclear ferrous, ferric, and mixed-valence complexes listed in Tables 1 and 2, the comproportionation constant Kcom ) 12.3 is calculated for equilibrium (1) in an aqueous solution. The magnitude of Kcom is an example of the fact that single-valence dinuclear complexes are far less stable than the mixed-valence diiron complexes of the polypodal ligands containing phenolate bridging groups. For example, Kcom ) 4 × 106 for the [FeIIFeIII(BBPPNOL)(µ-OAc)2]+ complex [BBPPNOL ) N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-pyridylmethyl-2-hydroxy)-1,3-propanediamine] and Kcom ) 8 × 109 for the [FeIIFeIII(BBPMP)(µ-OAc)2]+ complex [BBPMP ) 2,6-bis{[(2hydroxybenzyl)(2-pyridylmethyl)amino]methyl}-4-methylphenol].14 (d) Catalytic Hydroxylation to Adamantane. Compared with L′, L has a more flexible cavity and can
accommodate the two iron ions at lower Coulombic repulsions. Now it has been discovered that the new synthesized dinuclear ferrous complex can catalyze the hydroxylation of adamantane in the presence of a twoelectron reductant (H2S). The overall oxidation reactions are shown in Scheme 1, which shows the organic substrate and reaction products. The results obtained for the hydroxylation of adamantane are summarized in Table 3. The turnover numbers have been calculated as the following: turnovers ) the sum of millimoles of products/millimoles of catalyst. The turnover numbers show that this macrocyclic iron complex is less active than L′ and approximately at the same level of catalyst: [30]RBBPyBC{20,41-dimethyl-3,16,24,27,43,44,46,47-octaazaheptacyclo[37.3.1.1. 5,91.10,141.18,221.26,301.31,35]-octatetraconta-5,7,9(43),12,12,14(44),18,20,22-
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3433 Scheme 1. Oxidation of Adamantane by Molecular Dioxygen with Dinuclear Iron(II) Complexes as Catalysts
same as that observed for the GIF system. The regioselectivity of 1-OH and 2-OH + 2-O varied with time. From 2 to 6 h, the selectivity of C2/C3 is nearly 3.6. When the reaction time was prolonged, the C2 products increase slowly compared with the C3 product (1-OH) and the ratio of C2/C3 was reduced to 1.5. Further detailed studies to optimize the catalytic conditions and the detailed mechanism will be investigated.
Chart 1. Poly(azadiphenol) Dinucleating Macrocyclic Ligands
Experimental Section
Table 3. Hydroxylation of Adamantane in the Presence of H2S-Oxygen solvent CH3CN
CH3COCH3
time/ h
1-OH/ mmol
2-OH/ mmol
2dO/ mmol
turnover
1 2 4 6 8 1 4 6 8
0.0223 0.0234 0.0302 0.120 0.141 0.0315 0.0466 0.0644 0.112
0.0647 0.0714 0.0925 0.114 0.136 0.0709 0.108 0.154 0.237
0.0133 0.0153 0.0195 0.0685 0.0775 0 0 0 0
4.01 4.40 5.69 12.11 14.18 4.10 6.18 8.74 14.00
(45),26,28,30(46),31,33,35(47),39,41,1(48)-octadecene45,48-diol}.15 It is interesting that the oxidation of adamantane produces not only hydroxylation but also ketonization products. Martell15 and Kitajima16 reported that no or only a trace of ketonization products was formed with different macrocyclic ligands as catalysts. However, for cyclohexane, Martell et al. also identified cyclohexanone as an oxidation product that suggested a different mechanism of these two substrates. When the solvent was changed to acetone, in similar conditions, no ketonization products were detected, and the turnover also was somewhat lower. In that case, pyridine was added as extra ligand, which seems to be necessary in this kind of catalytic reaction. This observation is the
Materials. 2,6-Diformyl-p-cresol and diethylenetriamine were supplied by Aldrich Chemical Co. and were redistilled before use. All other chemicals used in the synthetic work were obtained in high purity from Aldrich Chemical Co. and were used without further purification. Synthesis of Ligand. A mixture of 2,6-diformyl-pcresol (0.66 g, 4.0 mmol) and Pb(SCN)2 (1.35 g, 8.2 mmol) in 200 mL of methanol was heated at 50-60 °C until most of Pb(SCN)2 was dissolved. Diethylenetriamine (0.42 g, 4.0 mmol) in 40 mL of methanol was added dropwise over 1 h. The reaction mixture was stirred overnight and then cooled to room temperature. The powdery yellow precipitate was filtered and washed with methanol. The precipitate was suspended in 100 mL of methanol, 2.0 g of NaBH4 was added during a period of 1 h, and the reaction mixture obtained was filtered. The clear filtrate was diluted with 100 mL of water and then acidified with cold 8 M H2SO4. The white precipitate of PbSO4 was filtered off, and the filtrate was treated with ammonia until the solution became basic. The solution was extracted with chloroform, and the organic phase was dried with anhydrous Na2SO4 overnight. The solution was filtered and evaporated under reduced pressure to remove all of the solvent, the purple product was dissolved in a minimum amount of methanol, and 10 mL of 48% HBr was added slowly. Cold ether was then added slowly to complete the formation of the precipitate. In the final procedure, a methanol/ether mixed solution was used to recrystallize the ligand to give a white crystalline powder. Spectra data for the ligand: 1H NMR (300 MHz, D2O) δ 7.3 (s, aryl, 4H), 4.35 (s, PhCH2-, 8H), 3.5-3.4 (m, CH2-CH2-, 16H), 2.3 (s, CH3, 6H); 13C NMR (300 MHz, D2O) δ 152.8, 135.9, 134.5, 48.6, 45.4, 43.7, 20.9; FABMS m/z 471 [M + 1]+ (in free base, without any bromide anions). Potentiometric Determinations. A Corning model 350 pH digital meter fitted with a blue-glass electrode and a calomel reference electrode was calibrated with standard dilute strong acid at 0.10 M ionic strength to read the hydrogen concentration directly so that the measured quantity was -log [H]+, designated as p[H]. Hydrogen ion activities (pH) were not used in this research. Potentiometric p[H] measurements and com-
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putation of the protonation constants and the stabilities of the iron complexes were carried out by procedures described in detail elsewhere.17 The p[H] measurements were made at 25.0 ( 0.1 °C, and the ionic strength was adjusted with 0.10 M KCl. Analytical-grade FeCl3 in the presence of 0.01 M HCl was used, and the concentration of the stock solution was quantified by cation-exchange hydrogen techniques. FeCl2‚4H2O was used in solid state. All systems were investigated under anaerobic conditions; oxygen and carbon dioxide were excluded from the reaction mixture by maintaining a slight positive pressure of purified argon gas in the reaction vessel. The equilibrium constants were determined with the program BEST.17 The species distributions were calculated from equilibrium constants with the help of program SPE and plotted with SPEPLOT developed by Martell et al.17 log Kw, defined as log([H+][OH-]), was found to be -13.78. Catalytic Oxidation of Adamantane. A total of 15 mL of KOH (0.01 M) was added to 0.025 mmol of the macrocyclic ligand to neutralize the hydrogen bromide. The solution obtained was evaporated to dryness under vacuum. The light yellow oil was dissolved in 40 mL of CH3CN, and then 0.05 mmol of FeCl2‚4H2O was added to initiate the iron complex formation. After the mixture was stirred for 10 min at room temperature, the solution turned dark violet, signifying the formation of the dinuclear iron complex [Fe2L]2+. While stirring was continued, 15 mmol of adamantane was added, and then 1.0 mL of pyridine was added. Hydrogen sulfide (2 mL/ min) and dioxygen (20 mL/min) were simultaneously passed through the solution. After successive 1 or 2 h periods, 1 mL of the reaction mixture was filtered from the deposited sulfur and the solution was analyzed. The sample was quantitatively analyzed with a HP-5890 series II gas chromatograph with solid naphthalene as the internal standard. Acknowledgment This research was supported by the Robert A. Welch Foundation. Literature Cited (1) (a) Vincent, J. B.; Olivier-Lilley, G. L.; Averill, B. A. Proteins Containing Oxo-Bridged Dinuclear Iron Centers: A Bioinorganic Perspective. Chem. Rev. 1990, 90, 1447. (b) Lipscomb, J. D. Biochemistry of the Soluble Methane Monooxygenase. Annu. Rev. Microbiol. 1994, 48, 371. (2) (a) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Structural and Functional Aspects of Metal Sites in Biology. Chem. Rev. 1996, 96, 2239. (b) Feig, A. L.; Lippard, S. J. Reactions of Non-Heme Iron(II) Centers with Dioxygen in Biology and Chemistry. Chem. Rev. 1994, 94, 759. (c) Karlin, K. D. Metalloenzymes, Structural Motifs, and Inorganic Models. Science 1993, 261, 701. (3) Stassinopoulos, A.; Mukejee, S.; Caradonna, J. P. Mechanistic Bioinorganic Chemistry; Plenum Press: New York, 1995; p 84. (b) Mckee, V. Macrocyclic Complexes as Models for Nonporphine Metalloproteins. Adv. Inorg. Chem. 1993, 40, 323. (4) (a) Armstrong, W. H.; Spool, A.; Papaefthymiou, G. C.; Frankel, R. B.; Lippard, S. J. Assembly and Characterization of an Accurate Model for the Diiron Center in Hemerythrin. J. Am. Chem. Soc. 1984, 106, 3653. (b) Mengae, S.; Zhang, Y.; Hendrich, M. P.; Que, L., Jr. Structure and Reactivity of a Bis(µ-acetatoO,O′)diiron(II) Complex, [Fe2(O2(CH3)2(TPA)2)](BPh4)2. A Model for the Diferrous Core of Ribonucleotide Reductase. J. Am. Chem. Soc. 1992, 114, 7786. (5) (a) Borovik, A. S.; Que, L., Jr. Models for the FeIIFeIII and FeIIFeII Forms of Iron-Oxo Proteins, J. Am. Chem. Soc. 1988, 110, 2345. (b) Campbell, V. D.; Parsons, E. J.; Pennington, W. T. Diiron
and Dicobalt Complexes of a Phenolate-Bridged Binucleating Ligand with Mixed Phenolate and Pyridine Podands. Inorg. Chem. 1993, 32, 1773. (c) Suzuki, M.; Uehara, A.; Oshio, H.; Endo, K.; Yanaga, M.; Kida, S.; Saito, K. Synthese and Characterization of Dinuclear Iron(II,II) and Iron(II,III) Complexes with a Dinucleating Ligand, 2,6-Bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate(1-). Bull. Chem. Soc. Jpn. 1987, 60, 3547. (6) (a) Spiro, C. L.; Lambert, S. L.; Smith, T. J.; Duesler, E. N.; Gange, R. R.; Henderickson, D. N. Binuclear Complexes of Macrocyclic Ligands: Variation of Magnetic Exchange Interaction in a Series of Six-Coordinate Iron(II), Cobalt(II) and Nickel(II) Complexes and the X-ray Structure of a Binuclear Iron(II) Macrocyclic Ligand Complex. Inorg. Chem. 1981, 20, 1229. (b) Mountford, H. S.; MacQueen, D. B.; Li, A.; Otvos, J. W.; Clavin, M.; Frankel, R. B.; Spreer, L. O. Spectroscopic and Electrochemical Characterization of a Bis-Macrocyclic Diiron Compound. Inorg. Chem. 1994, 3, 1748. (c) Motekaitis, R. J.; Utley, W. B.; Martell, A. E. Iron(II) and Sulfate Binding by the Binuclear Ligands O-BISDIEN, O-BISTREN, and O-BISBAMP. Inorg. Chim. Acta 1993, 212, 15. (7) Mandal, S. K.; Thompson, L. K.; Nag, K.; Charland, J. P.; Gabe, E. J. Synthesis, Structure, and Electrochemistry of a Novel Macrocyclic Dicopper(II) Complex. Four One-Electron-Transfer Steps Producing Binuclear Copper(III) and Copper(I) Species and Mixed-Valence-State Species. Inorg. Chem. 1987, 26, 1391. (b) Das, R.; Nanda, K. K.; Venkatsubramanian, K.; Paul, P.; Nag, K. Carboxylate Bridging of Amino Acids in Dinuclear Macrocyclic Nickel(II) Complexes. J. Chem. Soc., Dalton Trans. 1992, 1253. (c) Nanda, K. K.; Dutta, S. K.; Baitalik, S.; Venkatsubramanian, K.; Nag, K. Hydroxide-Bridged Diiron(III) Complexes of Tetraaminodiphenol Macrocyclic Ligands: Structure and Properties. J. Chem. Soc., Dalton Trans. 1995, 1239. (d) Dutta, S. K.; Werner, R.; Mohanta Florke, S.; Nanda, K. K.; Haase, W.; Nag, K. Model Compounds for Iron Proteins. Structures and Magnetic, Spectroscopic and Redox Properties of FeIIIMII and [CoIIIFeIII]2O Complexes with (µ-Carboxylato)bis(µ-phenoxo)dimetalate and (µ-oxo)diiron(III) Cores. Inorg. Chem. 1996, 35, 2292. (e) Nanda, K. K.; Thompson, L. K.; Bridson, J. N.; Nag, K. Linear Dependence of Spin Exchange Coupling Constant on Bridge Angle on PhenoxyBridged Dinickel(II) Complexes. J. Chem. Soc., Chem. Commun. 1994, 1337. (8) (a) Barton, D. H. R.; Ollis, W. D.; Stoddart, J. F. Comprehensive Organic Chemistry; Pergamon Press: Oxford, U.K., 1979; Vol. 1, p 108. (b) Reich, L.; Stivala, S. S. Auto-oxidation of Hydrocarbons and Polyolefins; Dekker: New York, 1969. (9) (a) Alexander, L. S.; Goff, M. M. Chemicals, Cancer, and Cytochrome P-450. J. Chem. Educ. 1982, 59, 179. (b) Guengerich, F. P.; Mcdonald, T. L. Chemical Mechanism of Catalysis by Cytochromes P-450: A Unified View. Acc. Chem. Res. 1984, 17, 9. (10) . Molecular Mechanism of Oxygen Activiation; Hayaishi, O., Ed.; Academic Press: New York, 1974. (11) (a) Barton, D. H. R.; Doller, D. The Selective Functionalization of Saturated Hydrocarbons: GIF and All That. Pure Appl. Chem. 1991, 63, 1567. (b) Barton, D. H. R.; Csuhai, E.; Doller, D.; Geletii, Y. V. The Functionalization of Saturated Hydrocarbons. Part XIX. Oxidation of Alkanes by H2O2 in Pyridine Catalyzed by Copper(II) Complexes. A GIF-Type Reaction. Tetrahedron 1991, 47, 6561. (c) Barton, D. H. R.; Li, T. S. The Functionalization of Saturated Hydrocarbons. Part 43. Modified GIF Oxidation in Acetonitrile. Tetrahedron 1998, 54, 1735. (12) Ullrich, V. Enzymatic Hydroxylation with Molecular Oxygen. Angew. Chem., Int. Ed. Engl. 1972, 11, 701. (13) Shangguan, G. Q.; Martell, A. E.; Zhang, Z. R.; Reibenspies, J. The Synthesis, Crystal Structure and Metal Complexes of a New Macrocyclic Dinucleating Ligand, 3,6,9,17,20,23-hexaaza-29,30dihydroxy-13,27-dimethyltricyclo[23.3.1.111,15]triaconta-1(28),11,13,15(30),25,26-hexane. Inorg. Chim. Acta 2000, 299, 47. (14) Suzuki, M.; Oshio, H.; Uehara, A.; Endo, K.; Yanaga, M.; Kida, S.; Saito, K. Syntheses and Characterization of Dinuclear High-Spin Iron(II,III) and -(III,III) Complexes with 2,6-Bis[bis[(2-benzimidazolylmethyl)amino]methyl]-4-methylphenolate(1-). Bull. Chem. Soc. Jpn. 1988, 61, 3907. (15) (a) Wang, Z.; Martell, A. E.; Motekaitis, R. J. Hydroxylation of Alkanes by Molecular Oxygen with Dinuclear FeII Macrocyclic Complexes as Catalysts. J. Chem. Soc., Chem. Commun. 1998, 1523. (b) Wang, Z.; Martell, A. E.; Motekaitis, R. J.; Reibenspies, J. The First Systematic Stability Study of Mononuclear and Dinuclear Iron(II) and Iron(III) Complexes Incorporating a Di-
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Received for review January 18, 2000 Revised manuscript received April 18, 2000 Accepted April 18, 2000 IE000058M
