New Types of CO-Releasing Molecules (CO-RMs), Based on Iron

Article pubs.acs.org/Organometallics

New Types of CO-Releasing Molecules (CO-RMs), Based on Iron Dithiocarbamate Complexes and [Fe(CO)3I(S2COEt)] Lindsay Hewison,† Sian H. Crook,† Brian E. Mann,*,† Anthony J. H. M. Meijer,† Harry Adams,† Philip Sawle,‡ and Roberto A. Motterlini§ †

Department of Chemistry, University of Sheffield, Sheffield, United Kingdom S3 7HF Vascular Biology Unit, Department of Surgical Research, Northwick Park, Institute for Medical Research, Harrow, Middlesex, United Kingdom. § INSERM U955, Equipe 3, Faculty of Medicine, University Paris Est, 94010, Creteil, France ‡

S Supporting Information *

ABSTRACT: New CO-releasing molecules, [Fe(CO)3X(S2CNR2)] and [Fe(CO)3I(S2COEt)], are reported. [Fe(CO)3X(S2CNR2)] releases the first two carbonyls rapidly to myoglobin (t1/2 < 1 min) and the third carbonyl more slowly. In the case of [Fe(CO)3I(S2COEt)], only 0.4 mol of CO are lost. [Fe(CO)3Br(S2CNEt2)] has low toxicity. CO loss is much slower from [Fe(CO)2(S2CNR2)2] (R2 = Me2, Et2, (CH2CH2)2O, (CH2CH2)2, (CH2CH2OH)2; t1/2 >24 h) and is not observed from [Fe(CO)2(S2COEt)2]. The mechanism of CO loss was investigated using Gaussian 09 calculations for [Fe(CO)3Br(S2CNMe2)] and [Fe(CO)2(S2CNMe2)2]. The X-ray structures of [Fe(CO)3Br(S2CNMe2)] and [Fe(CO)3I(S2CNEt2)]2I2 were determined.

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INTRODUCTION Although carbon monoxide (CO) is highly toxic, it has been shown to be an important signaling molecule in many living organisms ranging from bacteria to mammals.1 Considerable interest has been shown in using CO in animal experimental models of disease to ameliorate several pathological disorders affecting the cardiovascular system but also as a pharmacological tool to control inflammation as well as a bactericidal agent.2 There are clinical trials using CO gas administered to patients by inhalation through the lungs,3−8 but due to its renowned toxicity and the inability to precisely control its delivery in the gaseous form, CO presents dangers to both the patients and the medical staff. Therefore, administration of CO in the form of CO-releasing molecules (CO-RMs) offers a safer method, since CO can be carried and delivered where required in easily controllable quantities. In addition, although the concentration of CO delivered locally to cells and tissues can be relatively high, the whole amount of CO at which a human subject would be exposed is still within safe levels.9 As a result, there has been considerable effort on the development of CORMs. The first reports appeared in 2002 for [Mn2(CO)10], [Fe(CO)5], and [Ru2(CO)6Cl4].10−12 These compounds presented problems, with [Mn2(CO)10] and [Fe(CO)5] requiring photolysis to release CO while [Ru2(CO)6Cl4] is dissolved in DMSO. In 2003, a water-soluble derivative of [Ru2(CO)6Cl4], [Ru(CO)3Cl(glycinate)], was reported.13 © 2012 American Chemical Society

Subsequently, many more CO-RMs have been identified and recently reviewed.14,15 Virtually all the CO-RMs involve transition metals, but Na[H3BCO2H] and related compounds also release CO as a function of pH.16−18 Over 200 papers describing the biological applications of CO-RMs have been published, but most have been restricted to using [Ru2(CO)6Cl4], [Ru(CO)3Cl(glycinate)], and Na[H3BCO2H], which are known in the biological field respectively as CORM2, CORM-3, and CORM-A1.2,14,15 It is important to identify ligand systems which labilize coordinated CO for the development of CO-RMs. Recently we have shown that CO is rapidly lost from [Mn(CO)4(S2CNMeCH2CO2H] despite very slow loss from [Mn(CO) 4(bipy)]+,19,20 showing that in this case the dithiocarbamate ligand labilizes CO. In this paper, a number of iron carbonyls with dithiocarbamate or xanthate ligands of the types [Fe(CO)2(S2CY)2] and [Fe(CO)3(S2CY2)X] (X = halide, Y = NR2, OR) were examined as CO-RMs. Iron-based CO-RMs have already attracted attention, as iron is naturally present in the body and is a product of heme catabolism by heme oxygenase to produce CO.21 As a result, a wide range of Special Issue: Organometallics in Biology and Medicine Received: May 1, 2012 Published: June 18, 2012 5823

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Table 1. t1/2, ν(CO), and δ(13CO) Values for [Fe(CO)2(S2CNR2)2] compd

t1/2/min

[Fe(CO)2(S2CNMe2)2] [Fe(CO)2(S2CNEt2)2] [Fe(CO)2(S2CN{CH2CH2}2O)2] [Fe(CO)2(S2CN{CH2CH2}2)2] [Fe(CO)2(S2CN{CH2CH2OH}2)2] [Fe(CO)2(S2COCH2CH3)2]

4000 12000 6000 4000 1600 N/A (oil)

ν(CO)/cm−1 2030, 2028, 2027, 2028, 2024, 2041,

iron CO-RMs have been reported, including [Fe(CO)5],10,12 [Fe(CO)4I2],22 [CpFe(CO)3]+ and its derivatives,12,23 [(η5indenyl)Fe(CO)3]+ and its derivatives,24 complexes of the type [(η 4-2-pyrone)Fe(CO) 3],25 [(η4-cyclohexadiene ester)Fe(CO)3] complexes,26 [FeI2(CO)3(PR3)],22 [FeI2(CO)2{P(OMe)3}2],22 [Fe(SPh)2(CO)2(NH2CH2CH2NH2)],12 [Fe(SPh)2(CO)2(2,2′-bipyridyl)],12 [Fe(CO)2(cysteinate)2],12,27 [Fe(CO) 2 (SCH 2 CH 2 NH 2 ) 2 ], 28 carbonmonoxy-hemoglobin,29,30 and some iron−nitrogen compounds, [Fe(CO)L]2+, where L is a pentadentate nitrogen ligand.31,32 A number of complexes [Fe(CO)2(S2CNR2)2] (R = alkyl, aryl, pyridinyl, pyrrolidinyl, morpholinyl, piperidinyl) and [Fe(CO)3(S2CNR2)X] (X = halide) have been reported previously but not examined as CO-RMs.33,34 Transmetalation of [CpFe(CO)2(S2CNR2)] with [Fe2(CO)9] gives cis-[Fe(CO) 2 (S 2 CNR 2 ) 2 ], 35 while transmetalation of [CpW(CO)3(S2CNMe2)] with either [Fe2(CO)9] or [Fe3(CO)12] gives cis-[Fe(CO)2(S2CNMe)2] and [Fe(S2CNMe2)2].36 Previous syntheses of [Fe(CO)3(S2CNR2)X] include complexes where X = I and [S2CNR2]− is limited to dimethyldithiocarbamate and diethyldithiocarbamate.37−39 These complexes were made by starting from [Fe(CO)4I2] and the relevant [S2CNR2]− and from [Fe3(CO)12] and R2NC(S)S2C(S)NR2.33

1977 1976 1971 1975 1969 1993

av ν(CO)/cm−1

δ(13C)/ppm

2003.5 2002.0 1999.5 2001.5 1996.5 2017.0

214.02 214.12 212.74 214.31 214.00 226.21

No correlation is observed between t1/2 and the average ν(CO) or δ(13CO) value. [Fe(CO)2(S2CNMe2)2], [Fe(CO)2(S2CNEt2)2], and [Fe(CO)2(S2CN{CH2CH2}2O)2] were tested for cell viability and cytotoxicity using murine RAW264.7 macrophages as previously described (see Table 2).25,46 All three compounds Table 2. Biological Data on [Fe(CO)2(S2CNR2)2] (R2 = Me2, Et2, (CH2CH2)2O) measured in RAW264.7 Macrophages Incubated for 24 h with 10, 50, and 100 μM of Each Compounda compd [Fe(CO)2(S2CNMe2)2] (CORM413) [Fe(CO)2(S2CNEt2)2] (CORM414) [Fe(CO)2(S2CN{CH2CH2}2O)2] (CORM-415)

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RESULTS AND DISCUSSION [Fe(CO)2(S2CNR2)2]. Two synthetic routes were used to prepare [Fe(CO)2(S2CNR2)2]. In the first, [Fe(CO)4I2] was treated with [R2NCS2]−,37 while in the second R2NC(S)S2C(S)NR2 was oxidatively added to [Fe3(CO)12].33 Purification is easier for the disulfide route. In principle, there are two possible isomers for [Fe(CO)2(S2CNR2)2], 1 and 2. The spectroscopic data show that the isolated compound is the cis isomer, 2. The IR spectrum (ν(CO)) shows two signals of approximately equal strength at ca. 2030 and 1970 cm (see Table 1), consistent with a cis-dicarbonyl. The 1H and 13C NMR spectra show two sets of signals for the two R groups of each NR2 group (see Figures S1 and S2 in the Supporting Information for R = Et). This observation requires rotation about the S2C−NR2 bond to be slow on the NMR time scale at room temperature, as has been reported previously.34,40−45 The inequivalence of the R groups in NR2 is consistent with a cis-dicarbonyl, 2, while the R groups would be equivalent in the trans isomer, 1. In principle, the CH2 protons in [Fe(CO)2(S2CNEt2)2] are prochiral and the ethyl group should give a ABX3 pattern. This is not observed, presumably due to the group being too far away from the chiral iron center. [Fe(CO) 2 (S 2 CNR 2 ) 2 ] complexes (R 2 = Me 2 , Et 2 , {CH2CH2}2O, (CH2CH2)2, (CH2CH2OH)2) were examined for CO release to myoglobin. Release was just at the detectable limit with half-lives of around 5000 min (see Table 1). [Fe(CO)2(S2CN(CH2CH2OH)2)2] is the fastest of these compounds but is still very slow with a half-life of 1600 min.

cell viability at 100 μM/%

toxic effect on cells at 100 μM/%

100

25

95

10

100

0

a

The results are expressed as a percentage of the control as previously described.25,46

showed excellent cell viability profile with no toxicity, even at 100 μM. [Fe(CO)2(S2CNMe2)2] showed some cytotoxicity (ca. 25%) at 100 μM. The cytotoxicity was reduced to less than 10% with [Fe(CO)2(S2CNEt2)2] at 100 μM, while there was 0% cytotoxicity with [Fe(CO)2(S2CN{CH2CH2}2O)2] at 100 μM. [Fe(CO)3X(S2CNR2)]. Prior to this work, the only [Fe(CO)3X(S2CNR2)] complexes reported in the literature were [Fe(CO)3I(S2CNMe2)] and [Fe(CO)3I(S2CNEt2)]. There are also xanthates: [Fe(CO) 3 I(S 2 COEt)] and [Fe(CO) 3 I(S2COCy)]. The complexes were synthesized by treating [Fe(CO)4X2] (X = Br, I) with [S2CY]− (Y = NMe2, NEt2, N(CH2CH2)2O, OEt, OCy), respectively.37,38 There are two structures possible for [Fe(CO)3X(S2CNR2)] complexes, 3 and 4. The IR (ν(CO)) is consistent with the fac isomer, 4. It shows three ν(CO) bands of approximately equal intensity. In contrast, the mer isomer, 3, would be expected to show two strong ν(CO) bands with a weak ν(CO) signal for the symmetric stretch of the mutually trans carbonyls. The NMR spectra only show one set of signals for the R groups (see Figures S3 and S4 in the Supporting Information for [Fe(CO)3Br(S2CNEt2)]). This is consistent with the fac isomer, 4. In principle, the two R groups in the mer isomer, 5824

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3, should be inequivalent with one R cis to a CO group and the other cis to a halide. However, it is possible that rotation about the R2N−CS2 bond is fast on the NMR time scale, although rotation is slow in 2.

Table 4. Bond Distances (Å) and Angles (deg) in the Metal Coordination Spheres of [Fe(CO)3I(S2CNEt2)]2I2 Bond Distances Fe1−C1 Fe1−C2 Fe1−C3 Fe1−S1 C(1)−Fe(1)−C(3) C(1)−Fe(1)−C(2) C(3)−Fe(1)−C(2) C(1)−Fe(1)−S(2) C(3)−Fe(1)−S(2) C(2)−Fe(1)−S(2) C(1)−Fe(1)−S(1) C(3)−Fe(1)−S(1) C(2)−Fe(1)−S(1) S(2)−Fe(1)−S(1)

Confirmation of the fac structure, 4, comes from the crystal structures of [Fe(CO) 3 I(S 2 CNEt 2 )] and [Fe(CO) 3 Br(S2CNMe2)] (see Figures 1 and 2), and bond lengths and

Figure 2. Structure and labeling scheme of [Fe(CO)3I(S2CNEt2)]2I2 with ellipsoids at the 50% probability level.

angles involving iron are given in Tables 3 and 4. The Fe−C bond length is greater for the carbonyls trans to S than for those Table 3. Bond Distances (Å) and Angles (deg) in the Metal Coordination Spheres of [Fe(CO)3Br(S2CNEt2)] Bond Distances

C(1)−Fe(1)−C(3) C(1)−Fe(1)−C(2) C(3)−Fe(1)−C(2) C(1)−Fe(1)−S(2) C(3)−Fe(1)−S(2) C(2)−Fe(1)−S(2) C(1)−Fe(1)−S(1) C(3)−Fe(1)−S(1)

1.7865(16) Fe1−S1 1.8209(18) Fe1−S2 1.8123(17) Fe1−Br1 Bond Angles 94.32(8) 95.65(8) 96.41(8) 87.36(5) 94.36(6) 168.56(6) 88.36(5) 169.46(6)

C(2)−Fe(1)−S(1) S(2)−Fe(1)−S(1) C(1)−Fe(1)−Br(1) C(3)−Fe(1)−Br(1) C(2)−Fe(1)−Br(1) S(2)−Fe(1)−Br(1) S(1)−Fe(1)−Br(1)

94.3(2) 95.84(19) 94.06(19) 85.61(14) 97.29(14) 168.42(14) 88.63(14) 171.79(14) 93.27(14) 75.25(4)

C(1)−Fe(1)−I(1) C(3)−Fe(1)−I(1) C(2)−Fe(1)−I(1) S(2)−Fe(1)−I(1) S(1)−Fe(1)−I(1) C(4)−S(1)−Fe(1) C(4)−S(2)−Fe(1) Fe(1)−I(1)−I(2) I(1)−I(2)−I(2)

2.3161(13) 2.6618(7) 3.488 2.7526(7) 176.01(15) 84.64(14) 88.07(14) 90.71(4) 91.93(4) 87.00(15) 87.01(15) 93.28 174.4

trans to halide. This is consistent with the relative trans influence of sulfur compared with that of halide. The closest literature structure is of [Fe(CO)3(CN)(S2CS)]−, which has mer carbonyls.47 There is I2 within the crystal of [Fe(CO)3I(S2CNEt2)]. It forms an approximately linear unit, Fe−I−I−I−I−Fe, linking pairs of molecules (see Figure 2). This linkage has been reported previously for [(η 5-C 5 Me5 )Fe(CO) 2I(μ-I 2)IFe(CO)2(η5-C5Me5)],48 [···I(η5-C5Me5)Ir(μ-I)2Ir(η5-C5Me5)I− I−I···]n,49 [···IPd(Ph2PCHCHPPh2)I−I−I···]n,50 and [C5H3N2,6-(NMe=NNH2)2CuI(I−I−I−I)CuI{2,6(NMe=NNH2)2C5H3N}].51 The bond lengths and angles of the Fe−I−I−I−I−Fe unit are within the previously observed range (see Table 5). Unlike [Fe(CO) 2 (S 2 CNR 2 ) 2 ], where CO release to myoglobin is very slow, for [Fe(CO)3X(S2CNR2)] it is very fast for the first two carbonyls and up to three carbonyls are lost. In the case of [Fe(CO)3I(S2COEt)] only 0.4 CO is lost. A typical plot of carbon monoxy myoglobin versus time for CO capture is shown in Figure 3. The data have been analyzed on the basis of three first-order rate constants with irreversible reactions using an extension of a literature treatment (see the Supporting Information).52 The computer fit of the data gives values for the half-life of loss for the first, second, and third carbonyls from [Fe(CO)3X(S2CNR2)], t1/2(1) ≤ t1/2(2) < t1/2(3), where t1/2(1), t1/2(2), and t1/2(3) are half-lives for the loss of the first, second, and third carbonyls (see Table 6). As the CO release measurements were made every 30 s, the t1/2 values are probably subject to an error of at least 10 s. The complex [Fe(CO)3(S2CNEt2)Br] releases 3 mol of CO (see Figure 3). The first 2 mol of CO is released rapidly with effectively the same rate. The third mole of CO is released more slowly with a t1/2 value of 2 min. Similar release is observed for the other bromide compounds in Table 6. The iodo complexes lose the third carbonyl very slowly. [Fe(CO)3I(S2COCH2CH3)] is an exception, where only 0.4 mol of CO is released. It is probable that the electron-withdrawing ability of O results in stronger CO bonding and less electron density to stabilize the five-coordinate intermediate. Reversible CO release has been demonstrated for [Mn(CO)4(S2CNMeCH2CO2H)].20 It is probable that the complexes reported here behave similarly. This would explain why [Fe(CO)3I(S2COEt)] only appears to release 0.4 mol of CO.

Figure 1. Structure and labeling scheme of [Fe(CO)3Br(S2CNMe2)] with ellipsoids at the 50% probability level.

Fe1−C1 Fe1−C2 Fe1−C3

1.799(5) Fe1−S1 1.821(5) Fe1−I1 1.816(5) I1−I2 2.3183(12) I2−I2 Bond Angles

2.3119(4) 2.3127(5) 2.4738(3) 93.47(5) 75.557(15) 178.04(5) 84.60(6) 86.10(6) 91.081(14) 92.417(14)

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Table 5. Bond Lengths and Angles of the M−I−I−I−I−M Unit from the Literature and This Work d(I(1)−I(2))/Å

compd [(η5-C5Me5)Fe(CO)2I(μ-I2)IFe(CO)2(η5-C5Me5)]48 [···I(η5-C5Me5)Ir(μ-I)2Ir(η5-C5Me5)I−I−I···]n49

d(I(2)−I(2))/Å

3.391 3.243 3.568 3.527(1), 3.483(1)

2.776 2.794

[C5H3N-2,6-(NMeNNH2)2CuI(I−I−I−I)CuI{2,6-(NMe NNH2)2C5H3N}]51

3.353

2.806

[Fe(CO)3I(S2CNEt2)]2I2 (this work)

3.488

2.7526(7)

[···IPd(Ph2PCHCHPPh2)I−I−I···]n50

∠(M−I(1)−I(2))/ deg

∠(I(1)−I(2)−I(2))/ deg

102.55 111.80 126.36 84.6(1) 72.6(1) 98.30

176.08 173.72 167.80 176.1(1)

2.745(1)

98.65 93.28

173.21

174.4

pseudorotation on the reaction pathway will have a similarly small barrier. After completing these calculations, McLean discovered that dithionite is involved in the release of CO from [Ru( C O ) 3 C l 2 ] 2 , [ R u ( CO ) 3 C l ( g l y c i n a t e ) ] , a n d [ M n (CO)4(S2CNMeCH2CO2H)].53 It was shown that CO is released in the presence of the dithionite. Previously we had shown that [Mn(CO)4(S2CNMeCH2CO2H)] loses CO reversibly and that the same occurs with other CO-RMs.20 The dithionite is in equilibrium with sulfite, and it is probable that recoordination of the CO is being prevented by SO2 coordination. As a consequence the mechanism for CO release to myoglobin in the presence of dithionite may be complicated by SO2 coordination rather than water in Schemes 1−6. However, it was shown that when hemoglobin is used to determine CO release, dithionite is not used and Schemes 1−6 remain applicable.53 It is probable that myoglobin will now be replaced by hemoglobin. There are two different carbonyls in [Fe(CO) 3 Br(S2CNMe2)], trans to S or to Br, and both possible losses are modeled. We will focus on Gibbs energy changes. However, if we had just considered energy changes, the discussion would have been identical. Replacement of a CO in [Fe(CO)3Br(S2CNMe2)] (4) can give four possible products, 5−8, with 6 being the most stable. CO loss from [Fe(CO)3Br(S2CNMe2)] trans to Br generates 4a. There are two possible subsequent pathways; direct substitution gives 5, while a Berry pseudorotation via 4b gives a choice of 6−8 (see Scheme 1). The isomer 6 has the lowest Gibbs energy, and the pathway from 4b via 4c to 6 has a lower Gibbs energy than that via 4d to 7 or that from 4b via 4c and then 4e to 8. It should be noted here that 4d itself is also a transition state on our potential energy surface. The isomerization of 4a to 4b involves an increase in Gibbs energy, giving

Figure 3. Plot of time (minutes) against the number of moles (μM) of heme-CO formed from the addition of [Fe(CO)3(S2CNEt2)Br] to a solution containing myoglobin, 0.1% sodium dithionite, and phosphate-buffered saline (pH 7.4) at 37 °C to achieve a final [Fe(CO)3(S2CNEt2)Br] concentration of 10 μM.

If the equilibrium constant for CO release is similar to that of carboxymyoglobin, an equilibrium would develop rather than complete transfer of CO. Theoretical Modeling of CO Loss. Information on the mechanism of CO release comes from calculations on the stability of the species involved in sequential CO loss from [Fe(CO)3Br(S2CNMe2)] and [Fe(CO)2(S2CNMe2)2] (see Schemes 1−6). The calculations focused on which sixcoordinate species formed after replacement of CO by water. The reactions are reversible and will result in the formation of the most stable six-coordinate species. The transition state was determined in two cases: (Scheme 1) 4a to 4b (transition state) to 4c and (Scheme 4) 11a to 11b (transition state) to 11c/11e. As 4b is only 8 kJ mol−1 higher in energy than 4a and 11b is only 17 kJ mol−1 higher in energy than 11a, both transition states are accessible. It is assumed that any other Berry

Table 6. t1/2, ν(CO), and δ(13CO) Values for [Fe(CO)3X(S2CR)]a t1/2/s compd

t1/2(1)

t1/2(2)

t1/2(3)

[Fe(CO)3Br(S2CNMe2)] (CORM-809) [Fe(CO)3Br(S2CNEt2)] (CORM-804) [Fe(CO)3Br(S2CN{CH2CH2}2O)] (CORM-807) [Fe(CO)3I(S2CNMe2)] (CORM-806) [Fe(CO)3I(S2CNEt2)] (CORM-386, -801) [Fe(CO)3I(S2CN(CH2CH2)2O)] (CORM-808) [Fe(CO)3I(S2COCH2CH3)] (CORM-816)

7 8 2 7