Six-Coordinate Ferrous Nitrosyl Complex FeII(TTP)(PMe3)(NO) (TTP

Sep 19, 2016 - Synopsis. The preparation and spectroscopic characterization (Fourier transform infrared and UV−vis) of the six-coordinate complex Fe...
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Six-Coordinate Ferrous Nitrosyl Complex FeII(TTP)(PMe3)(NO) (TTP = meso-Tetra‑p‑tolylporphyrinato Dianion) Tigran S. Kurtikyan,*,† Astghik A. Hovhannisyan,† and Peter C. Ford*,‡ †

Molecule Structure Research Centre of the Scientific and Technological Centre of Organic and Pharmaceutical Chemistry NAS, 0014 Yerevan, Armenia ‡ Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United States S Supporting Information *

phosphine−imidazole complexes of FeII(TPP).11 Regarding diatomic ligands, the carbonyl complexes with trans-phosphine ligands FeII(TPP)(CO)(PR3) and FeII(Me2PPIX)(CO)(PR3) were obtained in situ by stirring carbon monoxide (CO)saturated methylene chloride solutions with the corresponding bis(phosphine) complexes.12 This reaction is reversible, and the mixed phosphine−carbonyl complexes were not isolated. Phosphine−carbonyl complexes have also been characterized by UV−vis and magnetic circular dichroism spectroscopy for mutant H93G myoglobin.8b For this protein, the dioxygen complex with a proximal phosphine was characterized by Dawson et al. in a low-temperature buffer solution.8b However, to our knowledge, no mixed phosphine−nitrosyl complexes of the type FeII(Por)(PR3)(NO) have been characterized with simple iron porphyrins or with mutant hemoproteins with open axial sites. Our previous studies have shown that metal tetraarylporphyrinato complexes sublime onto a low-temperature substrate to form amorphous microporous layers. The reaction of these with NO plus other ligands allows one to prepare a variety of five- and six-coordinate nitrosyl complexes7,13 and to use in situ IR and UV−vis spectroscopy to characterize their reactivities without solvent interference. In the present study, the binding of trimethylphosphine to the FeII(TTP)(NO) center is elaborated by vibrational and electronic absorption spectroscopies. Furthermore, it will be shown that the Fe−N(NO) bond of the FeII(TTP)(PMe3)(NO) complex formed at low temperatures is strongly destabilized and that NO readily dissociates upon warming. Exposure of microporous FeII(TTP) layers to excess NO gas gives an IR spectrum with a very strong band at 1676 cm−1, which shifts to 1645 cm−1 when 15NO is used. This band was assigned as the ν(NO) stretching frequency of the {FeNO}7 complex FeII(TTP)(NO) with a bent Fe−NO unit.14 Figure 1 shows Fourier transform infrared (FTIR) spectral changes observed upon stepwise addition of PMe3 vapors into a cryostat containing FeII(TTP)(NO) in low-temperature layers. The ν(NO) band shifts from 1676 to 1634 cm−1 (from 1645 to 1600 cm−1 for 15NO; Figure S1), indicating formation of the six-coordinate adduct FeII(TTP)(PMe3)(NO) (Scheme 1). This is in agreement with previous spectroscopic results and quantum-chemical calculations, showing that the ν(NO)

ABSTRACT: Low-temperature in situ Fourier transform infrared and UV−vis measurements show that trimethylphosphine (PMe3) reacts with microporous layers of FeII(TTP)(NO) (TTP = meso-tetra-p-tolylporphyrinato dianion; NO = nitric oxide) to form the previously unknown six-coordinate complex FeII(TTP)(PMe3)(NO). Upon warming this compound to room temperature in the presence of excess phosphine, the NO ligand is completely replaced by phosphine, resulting in formation of the bis(trimethylphosphine) complex FeII(TTP)(PMe3)2. Simultaneously, the NO released oxidizes free PMe3 to the corresponding phosphine oxide (OPMe3) with concomitant formation of nitrous oxide (N2O).

N

itric oxide (NO) performs important physiological functions, many closely tied to its interaction with the iron porphyrin center of heme proteins.1 Because most heme proteins contain an axial electron-donor ligand in the proximal site, 6-coordinate nitrosyl complexes are of particular interest. Six-coordinate iron(II) porphyrin nitrosyl complexes with axial N-donor ligands have been extensively studied to model the proximal histidine ligation of soluble guanylyl cyclase, hemoglobin, and myoglobin.2−4 Although S- and O-donor ligands have received less attention, there have been limited studies of thioether and ether ligation to ferrous porphyrin nitrosyls. For example, Yoshimura used EPR to probe the interaction of FeII(Me2PPIX)(NO) (Me2PPIX = protoporphyrin IX dimethyl ester) with various S- and O-donor ligands at low temperatures.5 Analogous studies by Lehnert et al.6 examined thiophenolate and thioether coordination to a FeII(TPP)(NO) (TPP = meso-tetraphenylporphyrinato dianion). Quantitative vibrational spectroscopy has been used by Martirosyan et al.7 to interrogate interactions of ethers and thioethers with FeII(TTP)(NO) both in low-temperature layered solids and in a toluene solution. These studies show that the interactions of the FeII(Por)(NO) moiety (Por = porphyrinato dianion) with neutral S- and O-donor ligands are relatively weak. Although P-donor ligands, such as phosphines, are not native, they have been used as structural and functional probes of heme protein active sites.8,9 A number of mixed sixcoordinate ferrous porphyrin complexes containing phosphine and σ-donor ligands have been described, 10 including © XXXX American Chemical Society

Received: July 24, 2016

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DOI: 10.1021/acs.inorgchem.6b01744 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Table 1. ν(NO) and Soret Band Values for the SixCoordinate Nitrosyl Complexes FeII(TTP)(L)(NO) complexa FeII(TTP)(NO) FeII(TTP)(THF) (NO) FeII(TTP)(THT) (NO) FeII(TTP)(Pyrr) (NO) FeII(TTP)(PMe3) (NO)

ν(NO),b cm−1

Δν, cm−1

Soret band, nm

1676 (1646) 1651 (1626)

25 (20)

411 425

7 7

1648 (1622)

28 (24)

427

7

1636 (1605)

40 (41)

427

7

1634 (1601)

42 (45)

437

this work

ref

a

Figure 1. FTIR spectral changes in the range of ν(NO) upon the stepwise addition of PMe3 vapors to the cryostat containing layered FeII(TTP)(NO) at 120 K and annealed to 180 K.

THF = tetrahydrofuran; THT = tetrahydrothiophene; Pyrr = pyrrolidine; PMe3 = trimethylphosphine. bThe data for the 15NO isotopomer are given in parentheses.

Scheme 1

frequency in the six-coordinate FeII(Por)(L)(NO) systems occurs at lower frequencies than that in the five-coordinate analogues.3a,4a,5,15 Detailed structural,13 spectroscopic, and density functional theory (DFT) investigations4 have shown that this frequency lowering is due to a strong σ-trans-interaction between NO and its trans ligand. DFT calculations show that binding of a Ndonor ligand in the trans position to NO reduces mixing of the singly occupied π* orbital of NO with the dz2 orbital of iron due to a σ-trans-interaction with the proximal N-donor ligand. The resulting higher electron population in the π*(NO) orbital leads to corresponding decreases in ν(NO). The change in σ bonding is also responsible for weakening of the Fe−NO bond and explains the experimentally observed correlation of the N− O and Fe−NO bond strengths in the six-coordinate porphyrin nitrosyls.4c This consideration also operates for the trans-S- and -O-donor ligands7 and, according to data described in the present work, for the P-donor ligand as well. The spectra shown in Figure 1 also show that some fivecoordinate FeII(TTP)(NO) remains in the layer [ν(NO) at 1676 cm−1]. Attempts to completely transform this to sixcoordinate FeII(TTP)(PMe3)(NO) were unsuccessful. Introducing more PMe3 into the cryostat leads not only to consumption of the former but also to a decrease of the intensity of the ν(NO) frequency of the latter species because of further transformations in the layer (see below). As seen in Table 1, coordination of O-, S-, N-, and P-donor ligands results in the shift of the ν(NO) frequency for FeII(TTP)(NO) to lower frequency by 25, 28 40 and 42 cm−1, respectively, a trend that qualitatively parallels the basicities of these ligands. Yoshimura showed this trend FeII(Me2PPIX)(NO) with a series of N-donor ligands,15a and Choi and Ryan also noted the strong correlation of the ligand pKa and binding constants for iron porphyrin nitrosyl interactions with N bases.16a When a layer containing predominantly FeII(TTP)(PMe3)(NO) was warmed from 180 K to room temperature (RT), the result was the complete disappearance of the ν(NO) band characteristic of this six-coordinate complex (Figure 2). However, this process was not accompanied by regeneration

Figure 2. FTIR spectral changes upon warming of the layer, containing FeII(TTP)(PMe3)(NO) from 180 to 293 K. Three spectra are shown; the last of these indicates that the ν(NO) band at 1634 cm−1 has completely disappeared. Asterisks indicate bands of coordinated PMe3.

of the five-coordinate nitrosyl species, as has been observed in the case of weaker electron-donor trans ligands for which formation of the six-coordinate complexes was completely reversible. 7 For example, warming the six-coordinate FeII(TTP)(L)(NO) (L = THF or THT) complexes formed in similar low-temperature porous layers completely restored the FTIR and UV−vis spectra of FeII(TTP)(NO). With strong N-donor ligands,15a and the pattern is somewhat different.16 Choi and Ryan reported that the excess N-donor used to form six-coordinate nitrosyl complexes facilitated NO dissociation and formation of bis-N-donor-ligated iron porphyrin complexes.16a Lehnert and co-workers have also noted4a that the tendency toward denitrosylation corresponds to the strength of the N-donor binding constants, and computational studies by Heinecke et al.4d point to the weakening of the Fe−NO bond in model FeII(Por)(PMe3)(NO) complexes. Bohle and Hung have also found that dissociation of NO promoted by N bases depends on the iron porphyrin substituents and, for some derivatives, is very rapid at RT.16b In the present case, the P-donor PMe3 is a relatively strong base (pKa 8.65) and the irreversible NO dissociation seen here is additionally maintained by the consumption of released NO in a reaction with PMe3 to form trimethylphosphine oxide OPMe3 and N2O (Scheme 2). These products are evident in the RT IR spectrum recorded after warming the layers containing FeII(TTP)(NO) and PMe3 (Figure 2) in which new bands at ∼1170 cm−1 (overlapping with the B

DOI: 10.1021/acs.inorgchem.6b01744 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

disappearance of the nitrosyl ν(NO) band, and this is accompanied by dramatic changes in the UV−vis spectrum. The final electronic spectrum (Figure 3, dotted line) exhibits the properties of a hyperporphyrin complex with a red-shifted Soret band at 459 nm and a near-UV band at 350 nm along with relative intensity changes in the visible bands. This spectrum is very similar to that measured by Ohia et al.21 for the bis(phosphine) complex FeII(TPP)(PBu3)2 in CH2Cl2− PBu3 solution. Thus, we conclude that the product is FeII(TTP)(PMe3)2. Furthermore, the interaction of PMe3 with microporous of FeII(TTP) gives the same electronic spectrum as that seen upon warming of the layers containing FeII(TTP)(PMe3)(NO), FeII(TTP)(NO) (some), and PMe3 (excess) to RT (Figure 3, dotted line). In summary, the low-temperature interaction of PMe3 with layered FeII(TTP)(NO) generates the mixed six-coordinate FeII(TTP)(PMe3)(NO) complex, which was characterized by in situ FTIR and UV−vis spectroscopies. Coordination of the phosphine ligand in the trans position to NO leads to a strong weakening of the Fe−NO bond and its cleavage even at rather low temperatures. Presumably, this is a major reason why mixed phosphine−nitrosyl complexes of iron porphyrins have not been characterized previously. Experimental details for the preparation and reactions of FeII(TTP) sublimed layers and characterization of the reaction products by FTIR and electronic spectroscopy are described in the Supporting Information.

Scheme 2

porphyrin band in this range) and in the vicinity of 2220 cm−1 appear. The former can be assigned as the ν(PO) stretching of OPMe3 and the latter as νa(NNO) of gas-phase N2O.17,18 When 15NO was used, the latter band shifted to 2165 cm−1 in accordance with this assignment. Oxidation of PPh3 by NO in solution to form OPPh3 and N2O was shown by Drago and coworkers many years ago,19 and subsequently mechanistic studies of this reaction were carried out by Lim et al.20 The present study indicates that the analogous reaction can take place in the solid state. The direct reaction of PMe3 vapors with the porous FeII(TTP) layers was studied by FTIR spectroscopy in order to confirm the identification of FeII(TTP)(PMe3)2 as the product formed upon warming FeII(TTP)(PMe3)(NO) layers containing excess PMe3 from 180 K to RT. While formation of the bis-coordinated PMe3 complex is difficult to confirm by FTIR spectroscopy solely on the behavior of coordinated PMe3 bands, there are prominent changes in the porphyrin bands that, together with UV−vis spectra (see below), unambiguously support formation of the bis(phosphine) species (Figure S2). The intense porphyrin band at 1003 cm−1 shifts to 992 cm−1, and the porphyrin band at ∼1540 cm−1 sharply grows in intensity in a manner that is inherent to the six-coordinate complexes of iron tetraarylporphyrins with basic ligands. The UV−vis spectrum of the layered FeII(TTP)(NO) also undergoes changes in the course of reaction with PMe3. These spectra were recorded on the samples for which the FTIR spectra indicated formation of FeII(TTP)(NO), followed by reaction with PMe3 to give a lower-frequency ν(NO). As seen in Figure 3, the Soret band of FeII(TTP)(NO) at 411 nm (solid

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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.6b01744. Figure S1 representing FTIR spectra of FeII(TTP)(PMe3)(NO) and FeII(TTP)(PMe3)(15NO) in the range of ν(NO), Figure S2 demonstrating the formation of FeII(TTP)(PMe3)2 upon interaction of PMe3 with layered FeII(TTP) and experimental procedures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by SCS RA (Grant 15T-1D172). Figure 3. UV−vis spectra of layered FeII(TTP)(NO) (solid line) after the stepwise addition of PMe3 portions into the cryostat at 120 K and annealing to 180 K to give FeII(TTP)(PMe3)(NO) (dashed line) and further warming to RT to give FeII(TTP)(PMe3)2 (dotted line).

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DOI: 10.1021/acs.inorgchem.6b01744 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01744 Inorg. Chem. XXXX, XXX, XXX−XXX