Factors That Control the Reactivity of Cobalt(III)\xe2\x80\x93Nitrosyl Complexes in Nitric Oxide Transfer and Dioxygenation Reactions: A Combined Experimental and Theoretical Investigation
Metal\xe2\x80\x93nitrosyl complexes are key intermediates involved in many biological and physiological processes of nitric oxide (NO) activation by metalloproteins. In this study, we report the reactivities of mononuclear cobalt(III)\xe2\x80\x93nitrosyl complexes bearing N-tetramethylated cyclam (TMC) ligands, [(14-TMC)CoIII(NO)]2+ and [(12-TMC)CoIII(NO)]2+, in NO-transfer and dioxygenation reactions. The Co(III)\xe2\x80\x93nitrosyl complex bearing 14-TMC ligand, [(14-TMC)CoIII(NO)]2+, transfers the bound nitrosyl ligand to [(12-TMC)CoII]2+ via a dissociative pathway, {[(14-TMC)CoIII(NO)]2+ \xe2\x86\x92 {(14-TMC)-Co\xe2\x8b\xafNO}2+}, thus affording [(12-TMC)CoIII(NO)]2+ and [(14-TMC)CoII]2+ as products. The dissociation of NO from the [(14-TMC)CoIII(NO)]2+ complex prior to NO-transfer is supported experimentally and theoretically. In contrast, the reverse reaction, which is the NO-transfer from [(12-TMC)CoIII(NO)]2+ to [(14-TMC)CoII]2+, does not occur. In addition to the NO-transfer reaction, dioxygenation of [(14-TMC)CoIII(NO)]2+ by O2 produces [(14-TMC)CoII(NO3)]+, which possesses an O,O-chelated nitrato ligand and where, based on an experiment using 18O-labeled O2, two of the three O-atoms in the [(14-TMC)CoII(NO3)]+ product derive from O2. The dioxygenation reaction is proposed to occur via a dissociative pathway, as proposed in the NO-transfer reaction, and via the formation of a Co(II)\xe2\x80\x93peroxynitrite intermediate, based on the observation of phenol ring nitration. In contrast, [(12-TMC)CoIII(NO)]2+ does not react with O2. Thus, the present results demonstrate unambiguously that the NO-transfer/dioxygenation reactivity of the cobalt(III)\xe2\x80\x93nitrosyl complexes bearing TMC ligands is significantly influenced by the ring size of the TMC ligands and/or the spin state of the cobalt ion.
INTRODUCTION<!>Generation and Characterization of [(14-TMC)-CoIII(NO)]2+ (1)<!>NO-Transfer Reactions<!>Mechanistic Insight into the NO-Transfer Reaction<!>DFT Calculations for Mechanistic Insight into the NO-Transfer Reaction of 1 with [(12-TMC)CoII]2+<!>Dioxygenation Reactivity of Co(III)\xe2\x80\x93Nitrosyl Complex, 1<!>CONCLUSION<!>Materials<!>Instrumentation<!>Purification of Nitric Oxide<!>X-ray Crystallography<!>Reactivity Studies<!>EPR Spectroscopy<!>Solution IR Spectroscopy<!>Synthesis of [(14-TMC)CoII(NO3)](BPh4)
<p>As a free radical nitrogen–oxide molecule, NO (nitric oxide, nitrogen monoxide) is known to play diverse roles in biological processes.1 In natural systems, the biosynthesis of NO and its various biological and physiological reactions come about through the interaction with metalloproteins of iron and/or copper.2 Thus, reactions of NO through its coordination with transition metal centers are of great interest for chemists and biochemists.3 In addition, there is a long history of the study of transition metal–NO interactions, to understand fundamental aspects of the structure, bonding, and reactivity of metal–NO species.3</p><p>There are a few reports on the intermolecular transfer of metal-coordinated ligands such as O2, O2−, and NO,4,5 and it has been shown that NO-transfer between two metal complexes can occur, depending on the NO-to-metal binding constant, ligand geometry, and metal oxidation state. The prior literature on metal complex NO-transfer chemistry suggests at least two proposed pathways, but no unambiguous mechanistic conclusion has ever been reached. In this regard, one of the early examples involves the acid-promoted intermolecular transfer of NO from CoII to CrII, involving the formation of a labile aqua complex at lower pH.5a In another case, protonation of an axial ligand transforms the geometry of coordinated NO toward a more bent conformation, allowing NO-transfer.5b Caulton and co-workers undertook a detailed investigation of nitrosyl transfer from Co(NO)(dmgH)2 (dmgH = monoanion of dimethylglyoxime (dmgH2)) to the various metal complexes of Fe, Co, Ni, and Ru.5c For the first time, they suggested a μ-bridged nitrosyl formation prior to the transfer of NO. A dissociative pathway of NO-transfer was described for a Co(NO)(dmgH)2 complex in the presence of hemoglobin (Hb), with first dissociation of NO followed by formation of HbNO.6 Lippard and co-workers also suggested a dissociative pathway of NO-transfer from manganese to iron bearing a tropocoronand ligand framework.7 Recently, intermolecular transfer of a ruthenium-bound nitrosyl to a coordinatively unsaturated iron complex was reported to occur through a heterobimetallic μ-bridged nitrosyl intermediate.8 Thus, on the basis of insights coming from all of these reactions, two mechanisms have been proposed (Scheme 1, M and M′ are different metal ions): one is a dissociative pathway of NO-transfer (Scheme 1a),6 and the other involves an associative pathway (Scheme 1b).8</p><p>In addition to the mechanism described above, the study of the reactions of both NO and O2 or their derivatives with transition metal complexes has attracted much attention. Since the coordination chemistry of metal–nitrosyl complexes has a long history, various reactions with metal–nitrosyl complexes have been reported.3 These species can be exceedingly stable, but many of them do in fact react with O2, and metal nitrite or nitrate complexes are frequently obtained as the products, depending on the system at hand.9–11 Clarkson and Basolo described for the first time the chemistry of certain cobalt–nitrosyl complexes, proposing that a peroxynitrite (PN, −OON=O) intermediate may form and thermally decay to give a cobalt nitrite complex product.9 Many other examples supported this work.10 In the case of dinitrosyl-iron species (DNICs), the metal–nitrosyl complex reacts with O2 to form a metal nitrate complex as the final product, putatively via the isomerization of a PN intermediate.11 It should be noted that, in aqueous chemistry, (i) peroxynitrite is the diffusion-controlled reaction product of NO plus superoxide anion (O2•−),12 and (ii) the primary decomposition product of peroxynitrous acid is nitric acid derived from a very fast isomerization reaction.13</p><p>In what is turning out to be chemistry that is complementary, the study of the reactivity of metal–O2 species with NO has been of great interest,14,15 in part due to its relevance in understanding the chemistry involved in the enzyme NO dioxygenase (NOD). NOD activity aids the maintenance of appropriate cellular NO balance by converting excesses to give the biologically benign nitrate ion.13 Thus, oxyhemoglobin, oxymyoglobin, and bacterial heme–O2 adducts ("oxy-ferrous" or formally FeIII–superoxide) are NODs. These reactions are thought to proceed through FeIII–PN intermediates, prior to nitrate formation;14,15 nitrogen dioxide (NO2) may otherwise be released.16 Thus, metal–PN generation may occur via metal–O2 plus NO or metal–NO plus O2 reactions, as shown recently in coordination complexes of chromium,17a,b iron,17c copper,18 hemes,19 and cobalt–porphyrins.20</p><p>Nam and co-workers have described an extensive series of first-row transition metal–superoxo, –peroxo, and –oxo species bearing N-tetramethylated cyclam (TMC) ligands,21 in which structure, bonding, and reactivity of the metal–oxygen complexes are greatly influenced by the ring size of the macrocycles. In relation to the NOD reactivity, TMC chromium–superoxo, chromium–peroxo, and iron–peroxo complexes were treated with NO or nitrosonium (NO+) ion, and in all cases metal–peroxynitrite intermediates were implicated as transient intermediates, leading to the formation of nitrite or nitrate metal complexes as final products.17</p><p>We report herein for the first time that the NO-transfer and dioxygen reactivity of CoIII–nitrosyl complexes bearing TMC ligands are tuned by the ring size of the supporting ligands and/or the spin state of CoII ion. More specifically, a new CoIII–nitrosyl complex bearing a 14-TMC ligand, [(14-TMC)-CoIII(NO)]2+ (1) (14-TMC = 1,4,8,11-tetramethy-1,4,8,11-tetraazacyclotetradecane), is generated and characterized spectroscopically and structurally. Its reactivities are investigated in the NO-transfer reaction from 1 to [(12-TMC)-CoII]2+ (12-TMC = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and in the reaction with O2, and are compared with those of the previously reported [(12-TMC)CoIII(NO)]2+ (2) complex.22 We propose the mechanism of the NO-transfer and dioxygen reactions as occurring via a dissociative pathway. Density functional theory (DFT) calculations further clarify the detailed mechanism of NO-transfer from 1 to [(12-TMC)-CoII]2+, supporting the experimental implication of a NO dissociation pathway.</p><!><p>The CoIII–nitrosyl complex bearing a 14-TMC ligand, [(14-TMC)CoIII(NO)]2+ (1), was synthesized by adding NO(g) to a solution of [(14-TMC)CoII]2+. As shown in Figure 1a, addition of NO (1 equiv) to the solution of [(14-TMC)CoII]2+ under an Ar atmosphere in CH3CN at −40 °C resulted in the formation of a new species with absorption bands at 395 nm (ε = 690 M−1 cm−1) and 555 nm (ε = 220 M−1 cm−1), which are similar to those of [(12-TMC)-CoIII(NO)]2+ (2; 368 and 538 nm) and [(13-TMC)-CoIII(NO)]2+ (375 and 550 nm).22 A solution IR spectrum of 1 exhibited a characteristic peak at 1715 cm−1 (Figure 1a, inset), which is almost identical to NO stretching bands of 2 (1712 cm−1) and [(13-TMC)CoIII(NO)]2+ (1716 cm−1).22 An electron paramagnetic resonance (EPR) spectrum of 1 is silent, suggesting the trivalency of the cobalt center in 1 (Figure 1b). In addition to EPR, a 1H NMR spectrum of 1 supports that the oxidation state of cobalt center in 1 is 3+ with a spin state of S = 0 (Figure S1 in Supporting Information (SI)). The results described above demonstrate that the formally CoIII(NO−) or {Co(NO)}8 was formed in the reaction of [(14-TMC)CoII]2+ with NO.</p><p>In addition to the above spectroscopic characterization, the electronic structure of the Co(III)–nitrosyl complexes was investigated by carrying out DFT calculations to understand NO binding to the cobalt center, revealing that both 1 and 2 have an open-shell singlet ground state (Figure 2). The DFT-optimized geometry of 2 is in a good agreement with the previously reported X-ray crystal structure of 2-(ClO4)2 (Figure 2c).22 The triplet states of 1 and 2 are 3.5 and 0.3 kcal/mol higher than the corresponding singlet states, respectively. Moreover, the nearly linear Co(1)–N(5)–O(1) geometry in the DFT-optimized structure of the triplet state of 2 suggests that the triplet state is not likely to be the ground state. We also calculated the quintet states of 1 and 2, which are higher in energy than the corresponding ground singlet states of 1 and 2 by 6.9 and 14.0 kcal/mol, respectively. The calculated Co(1)–N(5)–O(1) angle of about 150° in the quintet state of 2 is also inconsistent with the X-ray crystal structure of 2-(ClO4)2. The open-shell character of the singlet states of 1 and 2 is depicted by the two singly occupied orbitals as shown in Figure 2, in which the two unpaired electrons couple antiferromagnetically. Thus, all spectroscopic data (Figure 1) with the DFT-optimized structure (Figure 2) support that 1 is a cobalt(III)–nitrosyl complex binding the NO ligand in an end-on fashion.</p><!><p>The reactivity of the CoIII–nitrosyl complexes was investigated in NO-transfer reactions under an Ar atmosphere in CH3CN at −40 °C. Upon addition of one equiv of [(12-TMC)CoII]2+ to a CH3CN solution of [(14-TMC)CoIII(NO)]2+ (1, black line in Figure 3a), a new UV–vis spectrum was obtained immediately (blue line in Figure 3a), and following this, the blue line spectrum slowly changed to the red line spectrum with an absorption band at 368 nm (Figure 3a, inset, red circles), which corresponds to the characteristic absorption band of [(12-TMC)CoIII(NO)]2+ (2) (Scheme 2, reaction a).22 The spectrum with blue line was confirmed to be the sum of UV–vis spectra of 1 and [(12-TMC)CoII]2+ (Figure S2a in SI). It should be noted that, in the absence of [(12-TMC)CoII]2+, no spectral change was observed (Figure 3a, inset, black circles), indicating that 1 is highly stable under an Ar atmosphere at −40 °C.23 The final spectrum (red line in Figure 3a), which was confirmed to be the sum of those spectra of 2 and [(14-TMC)CoII]2+ (Figure S2b in SI), indicating that 2 was generated quantitatively (>95%) based on the ε (M−1 cm−1) value at 368 nm due to 2, since the [(14-TMC)CoII]2+ product has almost no absorption at 368 nm as shown in Figure 1a (pink line). This NO-transfer from 1 to [(12-TMC)CoII]2+ for the formation of 2 was confirmed by addition of [(12-TMC)CoII]2+ to a solution of 1 in increments of 0.2 equiv in CH3CN at −40 °C (Figure 4). The spectroscopic titration at 368 nm (Figure 4, inset) for the formation of 2 indicates that the reaction stoichiometry of 1 and [(12-TMC)CoII]2+ is 1:1, and 1 equiv of [(12-TMC)-CoII]2+ is required for the full conversion of 1 to 2. In contrast to the NO-transfer from 1 to [(12-TMC)CoII]2+, no NO-transfer from 2 to [(14-TMC)CoII]2+ was observed (Figure S3 in SI), indicating that the binding affinity of NO to [(12-TMC)CoII]2+ is higher than that to [(14-TMC)CoII]2+. In addition, the formation rates of 1 and 2 in the reactions of [(14-TMC)CoII]2+ and [(12-TMC)CoII]2+ with NO, respectively, were determined to be the second-order rate constants of 3.3(3) and 12(1) M−1 s−1, respectively (Figure S4 in SI), indicating that the formation rate of 2 is ~4 times faster than that of 1.</p><p>The intermolecular NO-transfer from 1 to [(12-TMC)-CoII]2+ was further confirmed by examination of the associated solution IR, ESI-MS, and EPR spectroscopies. According to solution IR measurements, a peak at 1715 cm−1 for the NO stretching frequency of 1 (Figure 1a, inset) was changed to 1712 cm−1 due to formation of 2 (Figure 5a), which is identical to the NO stretching frequency of 2,22 upon addition of [(12-TMC)CoII]2+ to a solution of 1. An ESI-MS of the reaction solution exhibited prominent mass peaks at m/z = 416.0, 414.2, and 178.1 (Figure 3b), whose mass and isotopic distribution patterns correspond to [(12-TMC)CoIII(NO)(ClO4)]+ (calcd m/z = 416.1), [(14-TMC)CoII(ClO4)]+ (calcd m/z = 414.1), and [(14-TMC)CoII(CH3CN)]2+ (calcd m/z = 178.1), respectively, indicating that 2 and [(14-TMC)CoII]2+ were formed in this reaction. An EPR spectrum of the final reaction solution revealed that a high-spin cobalt(II) complex formed, which is identical to that of [(14-TMC)CoII]2+ (see Figure 1b). Based on the above spectroscopic characterization of the reaction solution, we further conclude that 2 and [(14-TMC)CoII]2+ were produced by reacting 1 with [(12-TMC)CoII]2+ (Scheme 2, reaction a).</p><!><p>We then investigated the concentration effect of [(12-TMC)CoII]2+ on the NO-transfer reaction of 1 to [(12-TMC)CoII]2+ and found that the NO-transfer reaction was independent of the [(12-TMC)CoII]2+ concentration (Figure S5 in SI). As shown in Figure 3a, the reaction of 1 with [(12-TMC)CoII]2+ did not afford the formation of a new associated intermediate, indicating that this reaction does not occur via an associative pathway (vide supra). In addition, there is an equilibrium between 1 and a cage molecule, {(14-TMC)Co⋯NO}2+, prior to the reaction with [(12-TMC)CoII]2+, as shown in Scheme 3. Since the first step of the reaction (Scheme 3, pathway a) is the rate-determining step, the overall rate constant (kobs) of 8.2(7) × 10−4 s−1 (Figure 3a, inset) is equal to the rate constant of the dissociation to the cage molecule (k1). The second-order rate constant of 3.3(3) M−1 s−1 determined in the reaction of [(14-TMC)CoII]2+ with NO (Figure S4 in SI) indicates that the rate of re-formation to 1 (k−1) might be at least more than 103-fold faster than that of dissociation to the cage molecule (k1). In addition, the rate of the formation of 2 (k2; Scheme 3, pathway b) might be faster than that of the re-formation of 1 (k−1), because the NO binding affinity of [(12-TMC)CoII]2+ is greater than that of [(14-TMC)CoII]2+. Therefore, as shown in Scheme 3, we propose that the NO-transfer reaction of 1 with [(12-TMC)CoII]2+ occurs via a dissociative pathway6,7 through a putative cage molecule, {(12-TMC)CoII⋯NO}2+, and the order of rates for each step is k2 > k−1 >> k1.</p><!><p>DFT calculations were conducted to investigate why a dissociative cage molecule mechanism is preferred to an associative mechanism in the NO-transfer from 1 to [(12-TMC)CoII]2+. First, for the dissociative pathway, the calculated NO dissociation profile starting from the ground singlet state of 1 is depicted in Figure 6. We can see that the barrier of NO dissociation is only 3.2 kcal/mol. This small barrier is associated with a large stretching of Co-NO bond in a NO-dissociative transition state (TS) by about 1.4 Å from 1, as shown in Figure 7. After NO dissociation from 1, the NO coordination to [(12-TMC)CoII]2+ is found to be a barrierless process.</p><p>In comparison, the NO-transfer profile from 1 to [(12-TMC)CoII]2+ for the associative pathway is shown in Figure 8. Here, we employed a model of {[(14-TMC)CoIII-NO-CoII(12-TMC)](ClO4)3}+. In the absence of three perchlorate counterions, the [(14-TMC)CoIII-NO-CoII(12-TMC)]4+ is not stable, and the [(12-TMC)CoII]2+ fragment dissociates from the [(14-TMC)CoIII(NO)]2+ fragment, possibly due to the Coulombic repulsion between the two +2 charged Co fragments. As shown in Figure 8, the associative mechanism is initiated from the μ-NO-bridged intermediate 3 by NO-transfer from the [(14-TMC)CoII]2+ fragment to the [(12-TMC)-CoII]2+ fragment, resulting in intermediate 4 with an NO coordinated to the [(12-TMC)CoII]2+ fragment by its O-atom. Then, O- to N-atom coordination isomerization occurs, leading to the final NO-transfer product 5. The highest-lying TS (TS4–5) along the reaction profile is 9.3 kcal/mol higher than 3, which is significantly higher than the 3.2 kcal/mol barrier in the dissociative pathway, indicating that the latter is actually the more preferred NO-transfer pathway from 1 to [(12-TMC)-CoII]2+.</p><p>Considering the experimental results observed in solution IR and EPR experiments and taking account of the UV–vis kinetic measurements as well as the computational results, the reaction for NO-transfer is likely to occur via a dissociative pathway through a cage molecule prior to NO dissociation from cobalt center. Therefore, it is highly logical to assume that NO-transfer between 1 and [(12-TMC)CoII]2+ takes place through NO dissociation pathway as shown in Scheme 3. The affinity of cobalt ion toward NO is the key factor and controls the mode of NO-transfer mechanism, which can be explained on the basis of the TMC ring size and the spin state of CoII ion. To understand the TMC ring size effect, we compare the rate of the formation of 1 and 2 (vide supra), suggesting the formation rate of 2 is ~4 time faster than that of 1 (Figure S4 in SI). This explains the high affinity of 2 toward NO and suggests a higher stability of 2 compared to 1. Our computational results for 1 and 2 also directly showed their relative stabilities, showing that 2 is more stable than 1 by 3.6 kcal/mol in Gibbs free energy. Therefore, this difference of the stability of 1 and 2 is the thermodynamic driving force for the NO-transfer reaction from 1 bearing a larger TMC ligand to [(12-TMC)CoII]2+ bearing a smaller TMC ligand.</p><p>The spin state of the cobalt ions in [(12-TMC)CoII]2+ and [(14-TMC)CoII]2+ was also determined. An X-band EPR spectrum of [(12-TMC)CoII]2+ shows signals at g1 = 2.34, g2 = 2.31, and g3 = 2.05, which are very typical for low spin (S = 1/2) CoII species (Figure 5b, black line), while that of [(14-TMC)CoII]2+ exhibits signals at g1 = 7.4, g2 = 2.17, and g3 = 1.50, which are the characteristic values of a high-spin (S = 3/2) CoII species (Figure 1b, pink line). This ground spin state difference between [(12-TMC)CoII]2+ and [(14-TMC)CoII]2+ was also sensitively monitored in our DFT calculations (Figure S6 in SI). The spin state of the cobalt centers influences the reactivity toward the NO coordination, indicating that [(12-TMC)CoII]2+ (low-spin S = 1/2) has very high affinity for NO, compared to the [(14-TMC)CoII]2+ species (high-spin S = 3/2). Consequently, as the ring size of the TMC ligand increases, the affinity of the cobalt center toward NO coordination decreases, and the reactivity of CoIII–nitrosyl increases, which hence facilitates the NO-transfer from 1 to [(12-TMC)CoII]2+.</p><!><p>In addition to the NO-transfer reactions, the O2 reactivity of [14-(TMC)CoIII(NO)]2+ (1) was investigated. Upon addition of an excess of O2 to a CH3CN solution of 1, the absorption band at 395 nm due to 1 decayed with first-order kinetics (Figure 9), indicating that 1 reacted with O2, whereas 2 did not show any reactivity toward O2 as reported previously.22 The first-order rate constant was determined to be 7.6(7) × 10−4 s−1 (Figure S7 in SI), which is quite similar to that obtained in the NO-transfer reaction from 1 to [(12-TMC)CoII]2+ (vide supra). We also found that the reaction rate of 1 with O2 was independent of the concentration of O2 (Figure S7 in SI), indicating that the mechanism of the reaction of 1 with O2 is the same as that of the NO-transfer reaction from 1 to [(12-TMC)CoII]2+ (vide supra). There is an equilibrium between 1 and a cage molecule, {(14-TMC)Co⋯NO}2+, prior to the reaction with O2, as shown in Scheme 4. Since the first step of the reaction (Scheme 4, step a) is the rate-determining step, the overall rate constant (kobs) of 7.6(7) × 10−4 s−1 is equal to the rate constant of the dissociation to the cage molecule because the rates of re-formation of 1 and reaction with O2 are much faster than that of the dissociation to the cage molecule.24</p><p>The products obtained in the reaction of 1 with O2 were analyzed by solution IR, ESI-MS, and EPR spectroscopies. Upon addition of O2 to a solution of 1, an IR peak at 1715 cm−1 for the NO stretching vibration of 1 disappeared, and concomitantly a new IR peak at 1302 cm−1 due to a nitrate NO3− stretching vibration25 was formed (Figure 9b). ESI-MS spectrum of the reaction solution exhibited prominent mass peaks at m/z = 377.1 and 178.1 (Figure 9a, inset), whose mass and isotopic distribution patterns correspond to [(14-TMC)-CoII(NO3)]+ (6, calcd m/z = 377.1) and [(14-TMC)-CoII(CH3CN)]2+ (calcd m/z = 178.1). When 16O2 was replaced by isotopically labeled 18O2, the mass peak at m/z = 377.1 was shifted to m/z = 381.1. (blue and red lines in Figure 9a, inset). The observed shift of four mass units indicates that two of the three O atms in the NO3− ligand of 6 are derived from O2. An EPR spectrum of the complete reaction solution exhibited signals at g1 = 7.0, g2 = 2.09, and g3 = 1.52, indicating that a high-spin cobalt(II) complex was the final product formed (Figure S8 in SI).26</p><p>Finally, 6 was structurally characterized using X-ray crystallography. Single crystals of 6 were grown from the solution obtained in the reaction of 1 with O2. The X-ray crystal structure of 6 revealed that the nitrate anion coordinates to Co center in a bidentate fashion to produce a product with pseudo-octahedral geometry (Figure 10). Co–O distances of 2.234 and 2.225 Å indicate that two O atoms of the nitrate anion coordinate quite symmetrically to the Co center. The crystallographic data and selected bond distances and angles for 6 are listed in Tables S1–S3 in the SI.</p><p>According to the above results, we propose a reaction mechanism as shown in Scheme 4. The first step should be the dissociation of NO ligand from 1 to form a cage molecule, {(14-TMC)Co⋯NO}2+, indicating that the CoIII center was reduced to CoII. Then, this cage molecule, {(14-TMC)Co⋯ NO}2+, reacts with O2 to form a putative CoII peroxynitrite (PN) intermediate, CoII(OON=O−) (Scheme 4, step b). Subsequently, isomerization of CoII–PN would generate the CoII–NO3 complex (6) (Scheme 4, step c),27 as reported previously in metal-bound PN intermediates.14,15,28 In the dioxygenation reaction of 1, we observed the formation of nitrate anion, which is the product formed by the isomerization of PN.14,15 Since metal-PNs are known to be extremely unstable,19 there are only few reports about the metal-bound PN complexes.29,30 However, PN is often proven to be present by its ability to effect phenol ring nitration.31 Thus, 2,4-di-tert-butylphenol (DTBP) was added to a solution of 1 prior to addition of dioxygen to support our proposed mechanism (Scheme 5). We found that 2,4-di-tert-butyl-6-nitrophenol (nitro-DTBP) and dimerized DTBP (2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylphenol) were produced with the yields of 60(5)% and 10(4)%, respectively (see Experimental Section). Thus, the experimental results, such as the formation of nitro-DTBP with a good yield, strongly support the proposed reaction mechanism, which is the formation of a Co-PN intermediate in the reaction of 1 with O2.</p><!><p>We have demonstrated for the first time that the NO-transfer and dioxygen reactivity of CoIII–nitrosyl complexes bearing N-tetramethylated cyclam ligands can be tuned by varying the ring size of the supporting ligands and/or the spin state of CoII ion. In the NO-transfer reaction and the reaction with O2, the reactivity of [(14-TMC)CoIII(NO)]2+ (1) was compared with that of a smaller ring-size congener, [(12-TMC)CoIII(NO)]2+ (2). We found that the NO moiety in 1 was transferred to [(12-TMC)CoII]2+ to form 2, whereas no NO-transfer reaction occurred between 2 and [(14-TMC)CoII]2+; these results were interpreted as being due to the higher NO binding affinity of [(12-TMC)CoII]2+ compared to that of [(14-TMC)CoII]2+. The NO-transfer reaction was independent of the [(12-TMC)CoII]2+ concentration, leading us to suggest that the mechanism of the NO-transfer from 1 to [(12-TMC)CoII]2+ occurs via dissociation of the NO ligand from 1 to form a cage molecule, {(14-TMC)Co⋯NO}2+, prior to its reaction with [(12-TMC)CoII]2+ to produce 2. DFT calculations supported the experimental observation for the NO dissociation pathway. In addition to the NO-transfer reaction, the reactivity of 1 toward O2 was investigated. A cobalt–nitrato complex, [(14-TMC)CoII(NO3)]+, which was structurally characterized by X-ray crystallography, was produced as a sole product in the reaction of 1 and O2, while 2 was inert toward O2.22 The reaction rate of 1 with O2 was independent of the O2 concentration, indicating that the reaction mechanism of 1 and O2 is similar to that of the NO-transfer reaction from 1 to [(12-TMC)CoII]2+, which occurs via a dissociation of NO ligand from 1 to form a cage molecule, {(14-TMC)Co⋯NO}2+, prior to the reaction with O2. The reaction of the cage molecule, {(14-TMC)Co⋯NO}2+, with O2 produces a putative peroxynitrite (PN) intermediate, CoII(OON=O−), which is converted to the final CoII–NO3 product. As a conclusion, we have demonstrated unambiguously that the reactivities of the cobalt(III)–nitrosyl complexes bearing TMC ligands in the NO-transfer and dioxygenation reactions are significantly influenced by the spin state of the cobalt(II) center, caused by the ring size of the TMC ligands. Such explanations have also been provided and published for other first-row transition metal ions, where the chemistries are compared using metal complexes bearing different ring-size TMC ligands.32</p><!><p>All chemicals obtained from Aldrich Chemical Co. were of the best available purity and used without further purification unless otherwise indicated. Solvents were dried according to published procedures and distilled under Ar prior to use.33 The 14-TMC ligand and 1,4,7,10-tetraazacyclododecane compound were purchased from Aldrich Chemical Co. 12-TMC was prepared by reacting excess amounts of formaldehyde and formic acid with 1,4,7,10-tetraazacyclododecane.34 [(14-TMC)CoII]2+ and [(12-TMC)CoII]2+ complexes were synthesized and isolated according to the literature methods.21b Co(III)–nitrosyl complexes (1 and 2) were synthesized by reported procedures.22</p><!><p>UV–vis spectra were recorded on a Hewlett-Packard 8453 diode array spectrometer equipped with a UNISOKU Scientific Instruments for low-temperature experiments. Coldspray ionization time-of-flight mass (CSI-TOF MS) spectral data were collected on a JMS-T100CS (JEOL) mass spectrometer equipped with the CSI source. Typical measurement conditions are as follows: needle voltage, 2.2 kV; orifice 1 current, 50–500 nA; orifice 1 voltage, 0–20 V; ring lens voltage, 10 V; ion source temperature, 5 °C; spray temperature, −40 °C. Electrospray ionization mass spectra (ESI-MS) were collected on a Thermo Finnigan (San Jose, CA, USA) LCQTM Advantage MAX quadrupole ion trap instrument, by infusing samples directly into the source using a manual method. The spray voltage was set at 4.2 kV and the capillary temperature at 80 °C. EPR spectra were recorded at 5 K using an X-band Bruker EMX-plus spectrometer equipped with a dual-mode cavity (ER 4116DM). Low temperatures were achieved and controlled using an Oxford Instruments ESR900 liquid He quartz cryostat with an Oxford Instruments ITC503 temperature and gas flow controller. Low-temperature solution IR spectra were recorded on Remspec 626 FT-IR spectrometer. 1H NMR spectra were measured with a Bruker model digital AVANCE III 400 FT-NMR spectrometer. Crystallographic data collections were carried out on a Bruker SMART AXS diffractometer equipped with a monochromator in the Mo Kα (λ = 0.71073 Å) incident beam.</p><!><p>Nitric oxide gas (NO(g)) was purchased from Dong-A Specialty Gases in Korea and purified as follows:22 NO(g) was first passed through two columns filled with NaOH beads and molecular sieves to remove higher nitrogen oxide and moisture impurities and collected in a frozen form in a first trap cooled at 77 K with liquid N2. Further purification was done by distillation of frozen NO(g) (as crystalline N2O2) by warming at −80 °C (acetone/dry ice mixture, −80 °C), collecting it in a second trap cooled at 77 K with liquid N2. This second trap was again warmed to −80 °C, and the highly purified NO(g) was collected in another vacuumed Schlenk flask fitted with a rubber septum (free from oxygen, after several cycles of vacuum and Ar purging). The NO(g) should be at high pressure in the Schlenk flash (>1 atm; the septum bulges outward due to high pressures inside the Schlenk flask). Then, NO(g) was purged to the distilled and degassed CH3CN (15 mL) in a Schlenk flask, which was already connected with an oil bubbler. NO(g) purging with vigorous stirring under 1 atm for 20 min is required to make NO-saturated CH3CN solution. The concentration of NO-saturated in CH3CN solution at 20 °C was approximated to be 14 mM based on published values for NO(g) solubility.35 According to the literature, NO(g) solubility in diethyl ether is 27 mM, while for most other non-ethereal organic solvents the values are in the range of 11–18 mM.35</p><!><p>The single crystals of 6-(BPh4), which were formed by anion exchange with BPh4− from 6-(ClO4) complex, were obtained by slow diffusion of Et2O into the CH3CN solution at −30 °C. We tried to grow the suitable crystals of 6-(ClO4), but we failed to grow them because of high degree of disorder in perchlorate anion. Thus, to grow suitable crystals, ClO4 anion was exchanged to BPh4 anion. A single crystal of 6-(BPh4) was picked from solution by a nylon loop (Hampton Research Co.) and mounted on a goniometer head in a N2(g) cryostream. Data collections were carried out on a Bruker SMART AXS diffractometer equipped with a monochromator in the Mo Kα (λ = 0.71073 Å) incident beam. The CCD data for 6-(BPh4) were integrated and scaled using the Bruker-SAINT software package, and the structure was solved and refined using SHELXTL V 6.12.25.36 The crystallographic data and selected bond distances and angles for 6-(BPh4) are listed in Tables S1–S3 in the SI. CCDC-1449724 for 6-(BPh4) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (or from the Cambridge Crystallographic Data Centre, 12 Union Rd., Cambridge CB2 1EZ, UK; fax (+44) 1223-336-033, or deposit@ccdc.cam.ac.uk).</p><!><p>All the reactions were run in a 1 cm UV cuvette, by monitoring the UV–vis spectral changes of the reaction solutions, and the rate constants were determined by fitting the changes in absorbance at 393 and 368 nm for 1 and 2, respectively. Reactions were run at least in triplicate, and the data reported represent the average of these reactions. Complex 1 was prepared in situ by reaction of [(14-TMC)CoII]2+ with an equal amount of NO(g) under an Ar atmosphere in CH3CN at −40 °C. 1 (0.50 mM) was then used in different reactivity studies, such as NO-transfer and NO dioxygenation reactions under above reaction conditions. The NO-transfer reactions were carried out by adding different amounts of [(12-TMC)CoII]2+ (0.50, 2.5, and 5.0 mM) to a solution of 1 (0.50 mM), which was in situ prepared under an Ar atmosphere in CH3CN at −40 °C. UV–vis spectral changes were monitored at 368 nm due to 2, and rate constants were determined by fitting the changes in absorbance at 368 nm. In NO-transfer reactions, the yields were measured by comparing molar extinction coefficient (ε) of 2 at 368 nm with authentic sample.22 The reactions of 1 with O2 were carried out by purging excess O2 (0.10 M) to a CH3CN solution of 1 (0.50 mM) at −40 °C. The reactions were followed by monitoring the change in absorbance at 395 nm.</p><p>Reaction of 1 with 2,4-di-tert-butylphenol (DTBP) were carried out in CH3CN at −40 °C. 1 (2.0 mM, 5.0 mL) was prepared under an Ar atmosphere in CH3CN at −40 °C in a 30 mL Schlenk flask fitted with a septum. Upon addition of DTBP to a solution of 1, no reaction occurred. However, upon addition of excess oxygen to a solution containing 1 and DTBP in CH3CN at −40 °C, the absorption band at 395 nm due to 1 disappeared. The reaction mixture was stirred for 3 h at −40 °C before warming to room temperature. The products were analyzed by injecting reaction mixture directly into a GC and were identified by comparison with authentic samples. The product yields were determined to be 60(5)% for 2,4-di-tert-butyl-6-nitrophenol and 10(4)% for 2,2′-dihydroxy-3,3′,5,5′-tetra-tert-butylbiphenol by comparison against standard curves prepared with authentic samples and using decane as an internal standard.</p><!><p>EPR spectra were recorded for different reactions, such as generation of 1, NO-transfer reaction from 1 to [(12-TMC)CoII]2+, and dioxygenation reaction of 1. All the reactions were performed under an Ar atmosphere in CH3CN at −40 °C, and then EPR spectra were recorded at 5 K. In the case of NO-transfer reaction, EPR spectral changes as a function of time (0–170 min) have been recorded at 5K for the reaction of 1 (2.0 mM) with equivalent amount of [(12-TMC)CoII]2+ in CH3CN at −40 °C. Time-dependent EPR measurements revealed the clear conversion of 1 to 2.</p><!><p>Solution IR spectra for samples were recorded with a sophisticated setup, attached to an IR instrument (Remspec model 626) equipped with a very sensitive probe under an Ar atmosphere in CH3CN at −40 °C. The formation of Co(III)–nitrosyl complexes (1 and 2), NO-transfer reactions, and dioxygenation reactivity of 1 were followed by monitoring the characteristic nitrosyl stretching vibration bands. 1 (3.0 mM, 2.0 mL) was in situ prepared under the above experimental conditions/setup and showed the formation of a peak at 1715 cm−1, which corresponds to nitrosyl stretching in solution medium. Solution IR spectral changes for the reaction of 1 (3.0 mM, 2.0 mL) with equal amount of [(12-TMC)CoII]2+ show the shifting of characteristic nitrosyl stretching of 1 (1715 cm−1) to 1712 cm−1 (corresponding to 2). In addition, solution IR spectral changes for reaction of 1 (3.0 mM, 2.0 mL) with dioxygen were also recorded in CH3CN at −40 °C, showing that the decay of the peak at 1715 cm−1 due to 1, and concomitantly the formation of a new peak at 1302 cm−1, which corresponds to a NO3− stretching vibration,25 was observed.</p><!><p>Upon addition of an aqueous solution of sodium tetraphenylborate (52 mg, 0.15 mmol) to a CH3OH solution of [(14-TMC)CoII]2+ (56 mg, 0.1 mmol) at room temperature, a very light-colored precipitate of [(14-TMC)CoII]-(BPh4)2 was produced within 10 min. The precipitate was filtered, washed with cold methanol, and then dried over CaCl2 under vacuum. [(14-TMC)CoIII(NO)]+ was also generated by reacting [(14-TMC)-CoII](BPh4)2 (3.0 mM, 10 mL) with an equal amount of NO, and then [(14-TMC)CoII(NO3)](BPh4) was obtained by reacting with excess dioxygen in CH3CN at −40 °C over 3 h. The reaction mixture containing [(14-TMC)CoII(NO3)](BPh4) was layered with Ar-saturated Et2O for several days at −30 °C to obtain the light pink-colored crystals of 6-(BPh4).</p>