Characterization of Metalloporphines: Iron(II) Carbonyls and

5 days ago - Synopsis. The synthesis of two iron(II) porphine derivatives is reported. The new six-coordinate species have CO and 1-methylimidazole or...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Characterization of Metalloporphines: Iron(II) Carbonyls and Environmental Effects on νCO Ming Li, Allen G. Oliver, and W. Robert Scheidt* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The synthesis and characterization of two new iron(II) porphine complexes is described. Porphine, the simplest porphyrin derivative, has been studied less than other synthetic porphyrins owing to synthetic difficulties and solubility issues. The subjects of this study are two sixcoordinate iron(II) species further coordinated by CO and an imidazole ligand (either 1-methylimidazole or 2-methylimidazole). The two species have very different CO stretching frequencies, with the 2-methylimidazole complex having a very low stretching frequency of 1923 cm−1 compared to the more usual 1957 cm−1 for the 1-methylimidazole derivative. The very low frequency is the result of environmental effects; the oxygen atom of the carbonyl forms a hydrogen bond with an adjacent coordinated imidazole with a hydrogen atom from the N−H group. The two species, with their differing C−O stretches, also display substantial differences in the values of the Fe−C and C−O bond distances, as determined by their X-ray structures. The two bond distances are strongly correlated (R = 0.98) in the direction expected for the classical π-backbonding model. The two bond distances are also strongly correlated with the C−O stretching frequencies. We can conclude that the Fe−C and C−O stretches are quite representative of the observed bond distances; their stretching frequencies are not affected by substantial mode mixing.

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INTRODUCTION The carbonyl (CO) ligand has had profound and wide-ranging influences on classical inorganic and organometallic chemistry, as well as bioinorganic chemistry, with the latter including both small molecule and protein studies. CO can be used as a probe in the characterization of metal−ligand bonding and for understanding the nature of the environment of prosthetic groups in heme proteins. Such studies are possible because of the well-established π-backbonding relationship between the strength of the metal−carbon and carbon−oxygen bonds.1 The relationship is inverse with an effect such that strengthening the M−C bond leads to a weakening of the C−O bond and vice versa. This model of π-backbonding has been widely used for complexes with π-acceptor (π-acid) ligands. Such inverse πbackbonding effects have been noted for heme derivatives as well.2,3 Moreover, in heme proteins, CO is used as a probe for an empty coordination site in iron(II) derivatives where the resulting C−O stretch is readily observable in the infrared. CO is also used to probe the nature of the environment of heme protein prosthetic groups. Such studies are possible because of the high affinity of CO for iron(II) and the well-established inverse π-backbonding relationship between the Fe−C and C− O bonds. The heme proteins myoglobin (Mb) and hemoglobin (Hb) have been among the most studied CO-ligated proteins. In the CO derivatives of native Mb and Hb, up to four distinct CO stretching frequencies can be observed.4,5 These bands have © XXXX American Chemical Society

been designated A0, A1, A2, and A3 in Mb, and the range of values is between 1938 and 1965 cm−1. This range of frequencies has been attributed to varying conformational states and, subsequently, electrostatic environments in the heme pocket.6−8 A complete understanding of these bands is of interest since they provide information about the relationship of conformational substates with functional states of the protein.9−15 A large number of mutation studies have been dedicated to understanding the multiple peak phenomena, differing environments, and substrate selectivity (CO vs O2) in myoglobin.16,17 These investigations have demonstrated that CO frequency changes can be the result of very specific interactions with ligand pocket residues, the general electrostatic environment of the pocket, or both. The π-backbonding correlation was both established and enhanced by vibrational studies and has generally been thought to be sufficiently subtle so as to be observable only via vibrational spectra showing the M−C and C−O stretching vibrations. Indeed, trends such as that found for an isoelectronic series of carbonyl species are accordingly wellknown phenomena due to vibrational studies.19 However, we recently showed, with four high precision X-ray structure determinations of four six-coordinate heme carbonyl derivatives, that the experimental values of the Fe−C and C−O bond distances displayed an inverse relation with an extremely high Received: March 7, 2018

A

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Brief Crystallographic Data and Data Collection Parameters formula FW, amu a, Å b, Å c, Å β, deg V, Å3 space group Z, Z ′ Dc, g/cm3 F(000) μ, mm−1 absorption correction radiation, Mo Kα, λ̅, Å temperature, K unique data unique observed data [I > 2σ(I)] absolute structure factor refinement method final R indices [I > 2σ(I)] final R indices (all data)

[Fe(porphine)(CO)(2-MeHIm)]

[Fe(porphine)(CO)(1-MeIm)]

C25H18FeN4O 474.30 9.972(2) 14.941(3) 14.561(3) 108.99(3) 2051.4(8) P21/n 4, 1 1.536 976 0.768 multiscan 0.71073 120(2) 5544 4707 − full-matrix least-squares on F2 R1 = 0.0299, wR2 = 0.0788 R1 = 0.0324, wR2 = 0.0807

C25H18FeN4O 474.30 8.2949(8) 17.1310(16) 29.184(3) 90 4147.1(7) P212121 8, 2 1.519 1952 0.760 multiscan 0.71073 120(2) 10290 9762 0.007(9) full-matrix least-squares on F2 R1 = 0.0328, wR2 = 0.0861 R1 = 0.0415, wR2 = 0.0920

correlation.20 We have expanded these bond distance studies, making use of the now more readily available porphyrin macrocycle porphine. We have synthesized two new derivatives of the form [Fe(porphine)(CO)(R-Im)], where R-Im is either 1-methylimidazole or 2-methylimidazole. The two derivatives have a large difference in C−O stretching frequency, 1923 cm−1 for the 2-methylimidazole species and 1957 cm−1 for 1methylimidazole. Moreover, the correlation of Fe−C and C−O distances further substantiates the earlier inverse bond distance correlation.20

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Synthesis of [Fe(porphine)]. [Fe(porphine)] was prepared by stirring a suspension of [Fe(porphine)]2O and excess ethanethiol in benzene for 3 days.24 The suspension color changed from black to brown. Synthesis of [Fe(porphine)(2-MeHIm)(CO)]. A solution of 2methylimidazole (25 mg, 0.29 mmol) in dichloromethane (3 mL) was transferred into a suspension of [Fe(porphine)] (15 mg, 0.035 mmol) in toluene (3 mL) via cannula and was stirred for 1 h. The reaction solution was purged with CO for 30 min and stirred overnight under a CO atmosphere. The solution was then cannula-transferred into 8 mm glass tubes and carefully layered with hexanes, and the tubes were flame-sealed. X-ray quality crystals of [Fe(porphine)(2-MeHIm)(CO)] were isolated after several days. IR: νCO 1923 cm−1. A brief selection of crystallographic data and data collection parameters are provided in Table 1. Synthesis of [Fe(porphine)(1-MeIm)(CO)]. A solution of 1methylimidazole (0.02 mL) in CH2Cl2 (0.5 mL) was introduced into a suspension of [Fe(porphine)] (12 mg, 0.03 mmol) in toluene (4 mL) via cannula and was stirred for 1 h. The reaction solution was purged with CO for 30 min and stirred overnight under a CO atmosphere. The solution was then cannula-transferred into 8 mm glass tubes and carefully layered with hexanes, and the tubes were flame-sealed. X-ray quality crystals of [Fe(porphine)(1-MeIm)(CO)] were isolated after several days. IR: νCO 1957 cm−1. A brief selection of crystallographic data and data collection parameters are provided in Table 1. X-ray Structure Determinations. Crystal data were collected and integrated using a Bruker Apex system, with graphite monochromated Mo Kα (λ = 0.71071 Å) radiation at 120 K (700 Series Oxford Cryostream) for all complexes. Data were corrected for absorption using multiscan methods.25 The structures were solved by direct methods in SHELXS-97;26 both structures were refined using SHELXL-14. All nonsolvent atoms were found after successive fullmatrix least-squares refinement cycles on F2 and were refined with anisotropic thermal parameters. Hydrogen atom positions were idealized with a riding model and fixed thermal parameters [Uij = 1.2Uij(eq) or 1.5Uij(eq)] for the atom to which they are bonded. There is one disordered unit in [Fe(porphine)(2-MeHIm)(CO)]. The disordered 2-methylimidazole was refined as two rigid imidazole rings with 50% occupancies in each position.

EXPERIMENTAL SECTION

General Information. All reactions and manipulations were carried out under an argon atmosphere using a double manifold vacuum line, Schlenkware, and cannula techniques. Porphine was synthesized by literature methods.21 Dichloromethane, toluene, and hexanes were distilled under nitrogen from CaH2 and sodium/ benzophenone. Ethanethiol (ACROS) was used as received, and 2methylimidazole was purchased from Aldrich, recrystallized from toluene, and dried under vacuum. Infrared spectra were recorded on a Nicolet Nexus 870 FTIR spectrometer with both KBr pellets and Nujol mulls. UV−vis spectra were recorded on a PerkinElmer Lambda 19 UV−vis/near-IR spectrometer. Synthesis of [Fe(porphine)Cl]. [Fe(porphine)Cl] was prepared using the modified metalation method described by Adler22 due to solubility. A mixture of porphine (0.75 g, 2.4 mmol) and FeCl2 (0.7 g, 5.5 mmol) in DMF (75 mL) was refluxed, and the reaction was monitored using UV−visible spectroscopy. After the reaction was finished, the solution was cooled to room temperature, and water was added to the residue upon removal of the solvent under vacuum and then filtered. The obtained solid was washed with hot water (∼50 °C) until the filtrate was colorless and was then dried under vacuum. Yield: 0.72 g, 75%. UV−vis/near-IR (CH2Cl2 solution) λmax: 363, 394, 492, 518, 623 nm. Synthesis of [Fe(porphine)]2O. [Fe(porphine)]2O was prepared by reaction of a suspension of [Fe(porphine)Cl] in dichloromethane with a 2 M aqueous sodium hydroxide solution for 2 days.23 The organic suspension was collected upon removal of the solvent, and the obtained solid was washed with water and dried under vacuum. UV− vis/near-IR (DMF solution) λmax: 307, 386, 557 nm. IR(KBr): ν(Fe− O−Fe) 861 (sh), 851 cm−1 (s). B

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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RESULTS

The crystal and molecular structures of two iron porphine complexes, the simplest porphyrin, have been determined at relatively high precision. The axial ligands are carbon monoxide and a trans imidazole. A correlation between the values of the Fe−C and C−O bond lengths was noted and compared with the same values from earlier high precision structure determinations of four distinct carbonyl heme derivatives.20

Figure 3. Formal diagram of the porphinato core in [Fe(porphine)(CO)(2-MeHIm)] displaying the displacement of each unique atom from the 24-atom mean plane. All displacements are given in units of 0.01 Å. Negative values of atom displacements are toward the CO ligand. The orientation of the imidazole ligand with respect to the core atoms is shown by the line; the position of the methyl group is indicated by the circle. The value of the dihedral angle between the imidazole and the closest Fe−Np bond is shown. Also entered on the diagram are the averaged values of all bond distances and angles of the core.

apparent response to the steric demands of the 2methylimidazole ligand. The two [Fe(porphine)(CO)(1MeIm)] molecules have more nearly planar core conformations. Also given are averaged values for all distinct groups of bond distances and angles. The standard uncertainty following each averaged value is calculated on the assumption that all values were drawn from the same population. It can be seen that all bond distance and angle values found for the two independent molecules in the crystals of [Fe(porphine)(CO)(1-MeIm)] are identical to within less than one standard uncertainty. Accordingly, we will report a single value using the averaged values from the two molecules as appropriate. The observed bond distances and angles in the three unique cores are unremarkable.27

Figure 1. ORTEP diagram of [Fe(porphine)(CO)(2-MeHIm)]. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. Only one of the two equally populated imidazole ligands is shown.

Figures 1 and 2 display ORTEP drawings of the molecules. For the [Fe(porphine)(CO)(1-MeIm)] derivative, there are two independent molecules that will always be shown side by side, with molecule 1 on the left and molecule 2 on the right. Figures 3 and 4 show mean plane diagrams illustrating the displacement of the core atoms from the mean plane of the 24atom cores. The core conformation of the [Fe(porphine)(CO)(2-MeHIm)] derivative shows a modest ruffling that is an

Figure 2. ORTEP diagrams of [Fe(porphine)(CO)(1-MeIm)]. Molecule 1 is on the left, and molecule 2 is on the right. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity. C

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Formal diagrams of the porphinato cores in [Fe(porphine)(CO)(1-MeIm)] displaying the displacement of each unique atom from the 24atom mean plane. All displacements are given in units of 0.01 Å. Negative values of atom displacements are toward the CO ligand. The orientation of the imidazole ligand is shown by the line; the position of the methyl group is indicated by the circle. The value of the dihedral angle between the imidazole and the closest Fe−Np bond is shown. Also entered on the diagram are the averaged values of all bond distances and angles of the core. Molecule 1 is on the left, and molecule 2 is on the right.

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DISCUSSION

However, even the distances found for the 1-methylimidazole complexes (average 2.053(13) Å) are significantly longer than those found for a series of bis-ligated six-coordinate iron(II) porphyrinate complexes bonded to imidazole ligands.47 The average value for the Fe−NIm bond distance is 2.012(7) Å, and thus, a lengthening of ≥0.04 Å is inferred. This bond lengthening is somewhat smaller than that observed for the thiocarbonyliron(II) porphyrinates where a lengthening of about 0.10 Å was inferred48 or that in iron(II) nitrosyl porphyrinates where the Fe−N(L) distance of the ligand trans to nitrosyl is lengthened by >0.2 Å.49−53 The carbonyl ligand thus has a significant structural trans effect but one that is smaller than those seen in a number of other diatomic ligands coordinated to iron(II) porphyrinates. The core conformations of the two porphine derivatives are shown in Figures 3 and 4. The orientation of the imidazole is also shown in these diagrams. The porphine core of [Fe(porphine)(CO)(2-MeHIm)] is seen to be distinctly nonplanar with an S4 ruffling. As seen in Figure 3, the ruffling results from the steric effects of the hindering 2-methyl group of the imidazole that occur even with the Fe−Nax bond distance elongation. The steric requirements of the 2-methyl group also explain the near bisecting of the nearest Np−Fe−Np angle by the imidazole planes with ϕ angles of 40° and 42°. The slightly shorter average Fe−Np distance in [Fe(porphine)(CO)(2MeHIm)] is consistent with the ruffling of the core. The porphine cores in [Fe(porphine)(CO)(1-MeIm)] are much closer to planarity, where one has modest saddling whereas the other has very modest ruffling (Figure 4). The two porphine complexes have very different C−O stretches with the 1923 cm−1 value observed for [Fe(porphine)(CO)(2-MeHIm)] being the lowest stretching frequency observed for a structurally characterized heme CO complex. The 1957 cm−1 value observed for [Fe(porphine)(CO)(1-MeIm)] is a more typically observed value of the C−O stretch. These IR measurements were made on Nujol mulls of carefully selected single crystals. It has been recognized since the pioneering work of Caughey54 that protein environmental effects have substantial influences on the C−O stretching frequency. These studies were carried out on MbCO. The relative intensities of the modes are also found to be affected by pH, temperature, and

The synthesis of the [Fe(porphine)(CO)(R-Im)] derivatives was straightforward after solving solubility issues, although it is known that the CO binding constant is substantially smaller when the trans imidazole ligand is sterically hindered.28 We were able to prepare derivatives with 1-methylimidazole, 2methylimidazole, and 1,2-dimethylimidazole, but we were unable to prepare single crystals of the latter derivative. We report the crystal structures of two new porphine derivatives, [Fe(porphine)(CO)(1-MeIm)] and [Fe(porphine)(CO)(2-MeHIm)], at sufficiently high precision and accuracy, that permit us to make structural and spectroscopic correlations along with results from an earlier study.20 Bond lengths and angles with uncertainties for the LIm −Fe−C−O coordination group for these two new structures and, for comparison, all known heme carbonyl structures are given in Table 2. The structural parameters of the two new derivatives follow within the framework of earlier results. The Fe−C−O group is close to linearity with the largest deviations found for the derivatives with a trans hindered imidazole, i.e., [Fe(porphine)(CO)(2-MeHIm)] (175.03(14)°), [Fe(TPP)(CO)(2MeHIm)]·C7H8 (175.96(13)°), and [Fe(TPP)(CO)(1,2Me2Im)]·C7H8 (175.95(14)°). Although the deviations are small, the differences from exact linearity are significant at the 3σ level. The nonlinearity is a combination of a 3.3° off-axis tilt and a bending of the FeCO group; the combination of tilt and bend is found from DFT calculations to be a relatively low energy distortion.45 The nonlinearity can be seen in Figure 1, which also shows that the axial Fe−N2MeIm bond is also tilted off the heme normal. The average off-axis tilt is 7.1°, which is the average from two differing orientations of the 2-MeHIm ligand. The off-axis tilt is the result of the bulky 2-methyl substituent which also leads to a ∼12° difference in the two Fe−N−C(Im) angles. The differences in the Fe−N−C(Im) angles are even larger than the off-axis tilt. The bulky substituents in 2-methyl- and 1,2-dimethylimidazoles also lead to a substantial increase (approximately 0.07 Å) in the Fe−NIm bond length over the values found for the unhindered 1methylimidazole derivatives.46 D

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Carbonyl Stretching Frequencies and Notable Structural Features for the Lax−Fe−C−O Group in [Fe(porphine)(CO)(2-MeHIm)], [Fe(porphine)(CO)(1-MeIm)], and Related Compounds Fe−Ca [Fe(porphine)(CO)(1-MeIm)] [Fe(porphine)(CO)(2-MeHIm)] [Fe(porphine)(CO)(1,2-Me2Im)] [Fe(TPP)(CO)(2-MeHIm)]·C7H8 [Fe(TPP)(CO)(2-MeHIm)] [Fe(TPP)(CO)(1,2-Me2Im)]·C7H8 [Fe(TPP)(CO)(1,2-Me2Im)]·C7H8f [Fe(TPP)(CO)(1,2-Me2Im)] [Fe(TPP)(CO)(1,2-Me2Im)] [Fe(TPP)(CO)(1-MeIm)]·C6H6 [Fe(TPP)(CO)(1-MeIm)]·C7H8 [Fe(TPP)(CO)(1-MeIm)]·2.5C7H8 [Fe(TPP)(CO)(1-MeIm)] [Fe(C2Cap)(CO)(1-MeIm)] [Fe(C2Cap)(CO)(1-MeIm)] [Fe(OC3OPor)(CO)(1-MeIm)]·1.5C7H8 [Fe(OC3OPor)(CO)(1,2-Me2Im)]· 2CHCl3 [Fe(Tpiv2C12P)(CO)(1-MeIm)] [Fe(TpivPP)(CO)(1-MeIm)] [Fe(OEP)(CO)(1-MeIm)] [Fe(C3Cap)(CO)(1,2-Me2Im)] [Fe(C3Cap)(CO)(1,2-Me2Im)] [Fe(C2Cap)(CO)(1,2-Me2Im)] [Fe(β-PocPivP)(CO)(1,2-Me2Im)] [Fe(TPP)(CO)(Py)] [Fe(OEP)(CO)(Py)] [Fe(Deut)(CO)(THF)] [Fe(TPP)(CO)(THF)] [Fe(TpivPP)(CO)(THF)] [Fe(TPP)(CO)(SEt)] [Fe(TpivPP)(CO)(THT)] HbCO MbCO

Fe−C−Ob

Fe−Laxa

C−Fe−Laxb

C−Oa

νC−Oc d

νC−Oc

ref

1.765(3) 1.7449(16)

177.1(2) 175.03(14)

2.049(2) 2.124(10) 2.069(11)

178.95(11) 170.61(17) 178.07(19)

1.139(3) 1.1463(19)

1957 1923d

this work this work

1.7410(14)

175.96(13)

2.1018(12)

175.42(5)

1.1488(17)

1961d 1926d

this work 20 20 20 20 20 20, 29 20 30 31 20 32 32 33 33

1972e 1.7537(15) 1.764(2)

175.95(14) 180g

2.0779(11) 2.1402(18) 2.133(2)

176.77(6) 168.73(8) 173.85(9)

1.1408(19) 1.138(3)

d

1948, 1953 1948, 1953d 1963d 1972e

1.7600(17) 1.793(3) 1.7636(13)

177.03(15) 179.3(3) 178.76(13)

2.0503(14) 2.071(2) 2.0400(11)

176.78(6) 178.3(3) 178.38(5)

1.139(2) 1.061(3) 1.1437(17)

d

1968

1969h 1967d 1969i

1.742(7) 1.748(7) 1.748(7) 1.713(8)

172.9(6) 175.9(6) 173.9(7) 180g

2.043(6) 2.041(5) 2.027(5) 2.102(6)

1.728(6)

180g

2.062(5)

1.744(5) 1.800(13)

1.768(7) 1.77(2) 1.706(5)

175.1(4) 178.0(13)

172.5(6) 179(2) 178.3(14)

1.78(1)

174.7(3) 177.8(3) 174.5(3) 173.0(2) 180g

1.161(8) 1.158(8) 1.171(8) 1.161(10) 1.149(6)

2.077(3) 2.046(10)

176.8(2) 178.9(5)

1.158(5) 1.107(13)

2.079(5) 2.10(1)

176.3(3) 177.5(8)

1.148(7) 1.12(2)

2.127(4)

177.4(9)

1.144(5)

2.352(2)

1.17(1)

d

2000 2000d 1978d 1974d 1958i 1965i 1965h 1984e 1984j 1999j 1980k 1967 1955l 1955k 1961k 1920m 1970i 1951n 1933n 1945n 1967n

34 35, 36 37 38 29 29 18 39 40 41 41 42 43 42 44 6 6 6

Angstroms (Å). bDegrees (°). ccm−1. dNujol mull. eToluene solution saturated with base. fThe second conformation of the 1,2-Me2Im is approximately 38% occupied. gSymmetry imposed linearity. hCDCl2 solution. iBenzene solution. jToluene solution. kPyridine solution. l Tetrahydrofuran solution. mChlorobenzene solution. nAqueous solution. a

electrostatic field from the acidic hydrogen atom of the imidazole.58 Distances more typical of hydrogen bonding might lead to a lower value of the C−O stretching frequency, perhaps even as low as the 1904 cm−1 value observed for horseradish peroxidase.59 It is to be noted that the contact distances in [Fe(porphine)(CO)(2-MeHIm)] are similar to the close contact distances found for [Fe(TPP)(CO)(2-MeHIm)]·C7H8 (2.38 and 3.16 Å) where the observed C−O stretch found was 1926 cm−1. Since the N−H group that forms the weak hydrogen bond to the carbonyl oxygen atom is a coordinated imidazole with limited mobility, the interaction distance is likely to be constrained by the packing of molecules in the crystal structure. It might be expected that packing of [Fe(porphine)(CO)(2-MeHIm)] and [Fe(TPP)(CO)(2-MeHIm)]·C7H8 could be substantially different. Is the similarity in the N−H···O interactions in the systems a coincidence or a reflection of the energetics of the interaction? We regard this as interesting speculation that we are unable to defend or discuss further.

pressure. 55 The resulting changes in C−O stretching frequencies are judged to be the result of both interactions with specific residues in the ligand binding pocket and generalized electrostatic effects. Indeed, Phillips and co-workers have demonstrated that the stretching frequency of carbonyl can act as a gauge of the electrostatic fields near the ligand binding site.16 A complete understanding of such effects could assist in determining how discrimination in binding of the diatomic ligands NO, CO, and O2 in heme proteins arises.9,16 Franzen, based on a theoretical study,56 suggested that specific orientations of the ligand binding pocket groups play an important role in CO/O2 discrimination. The carbonyl oxygen intermolecular contacts found for [Fe(porphine)(CO)(2-MeHIm)] are illustrated in Figure 5; the closest contact is the H−N atom of a coordinated imidazole. The O···H distance of 2.42 Å and the O···N distance of 3.21 Å are both longer than the standard hydrogen bonding distances of ∼2.0 and 2.9 Å, respectively.57 Nonetheless, the carbonyl oxygen atom must still experience a distinct E

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. Plot of Fe−C vs C−O distances. Data illustrated in the figure are for (1) [Fe(porphine)(CO)(1-MeIm)], (2) [Fe(TPP)(CO)(1,2-Me2Im)], (3) [Fe(TPP)(CO)(1-MeIm)]·C6H6, (4) [Fe(TPP)(CO)(1,2-Me 2 Im)]·C 7 H 8 , (5) [Fe(porphine)(CO)(2MeHIm)], and (6) [Fe(TPP)(CO)(2-MeHIm)]·C7H8. Error bars on distances are shown. The linear fit illustrated for six unique structures has a correlation coefficient of 0.98.60 Values were obtained from this work and ref 20.

Figure 5. Diagram illustrating the environment of the carbonyl oxygen atom in [Fe(porphine)(CO)(2-MeHIm)] with important contacts illustrated. The alternate orientation of the disordered imidazole leads to a N−H···O distance of 2.54 Å. The O···O separation is 2.94 Å.

with a very high correlation coefficient of the fit given by a Pearson’s R of 0.98.60 It would plainly be expected that the two types of bond distances would also be correlated with the C−O stretching frequency. These two correlations are shown in the two panels of Figure 8 that show satisfactory agreement. One interesting conclusion to be drawn from these bond distance and vibrational correlations is that both the C−O stretch and the Fe−C stretch are quite representative of their respective bond distances with relatively small amounts of mixing with other modes. This is particularly significant for the Fe−C vibrations found at sufficiently low frequencies such that significant mode mixing might have been expected.

The close carbonyl oxygen contacts in the two [Fe(porphine)(CO)(1-MeIm)] molecules are shown in Figure 6. The most important contact of molecule 1 is with a hydrogen from a β-pyrrole carbon. Molecule 2 contacts involve the hydrogen atoms from C−H groups of a coordinated imidazole. The effects on the C−O stretching frequency of the two molecules are very small as the solid-state CO band is very sharp. Based on earlier work with TPP derivatives and C−H type interactions, some small differences might have been expected20 but were not observed. Figure 7 presents the experimental correlation of the Fe−C vs the C−O distance. An observed decrease in the Fe−C bond length is paralleled by an increase in the C−O bond length, exactly as expected from the vibrational frequency correlation. Clear evidence of the inverse π-bonding relationship is shown

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SUMMARY The syntheses of iron(II) porphine derivatives of the form [Fe(porphine)(CO)(R-Im)], where R-Im is either 2-methylimidazole or 1-methylimidazole, are reported. The two

Figure 6. Diagram illustrating the environment of the carbonyl oxygen atom in [Fe(porphine)(CO)(1-MeIm)] with important close contacts illustrated. Molecule 1 is on the left (a), and molecule 2 is on the right (b). F

DOI: 10.1021/acs.inorgchem.8b00599 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 8. Plot (left panel) showing the relationship between C−O distances (Å) and νC−O values (cm−1). The linear fit illustrated has a correlation coefficient of 0.96.60 Plot (right panel) showing the relationship between Fe−C distances (Å) and νC−O values (cm−1). The linear fit illustrated has a correlation coefficient of 0.91.60 Data illustrated in the two panels are for (1) [Fe(porphine)(CO)(1-MeIm)], (2) [Fe(TPP)(CO)(2-MeHIm)], (3) [Fe(TPP)(CO)(1,2-Me2Im)]·C7H8, (4) [Fe(TPP)(CO)(1,2-Me2Im)]·C7H8 (second νCO), (5) [Fe(porphine)(CO)(1-MeIm)], (6) [Fe(TPP)(CO)(1,2-Me2Im)], and (7) [Fe(TPP)(CO)(1-MeIm)]·C6H6. Error bars on distances are shown. Values were obtained from this work and ref 20.

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ACKNOWLEDGMENTS We thank the National Institutes of Health for support of this research (GM-38401).

derivatives were found to have distinctly different C−O stretching frequencies. X-ray crystallographic characterization of the two species shows that the differing C−O stretching frequencies reflect the solid-state environment of the carbonyl oxygen atom. The crystal structures also show that the Fe−C and C−O bond distances are substantially different. The correlation between the Fe−C and C−O distances, along with values previously reported by us, are consistent and demonstrate the classical π-backbonding relationship with very high confidence.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00599. Complete crystallographic details, coordinates, bond distances and angles, and thermal parameters (Tables S1−S12); diagram illustrating the disordered imidazole hydrogen bonded to the carbonyl oxygen atom (Figure 1) (PDF) Accession Codes

CCDC 1827169−1827170 (([Fe(porphine(CO)(2-MeHIm)] and ([Fe(porphine)(CO)(1-MeHIm)], respectively) contain 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 by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

W. Robert Scheidt: 0000-0002-6643-2995 Notes

The authors declare no competing financial interest. G

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Inorganic Chemistry

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