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Aza-Diphosphido-Bridged Di-Iron Complexes Related to the [FeFe]-Hydrogenases

Pankaj Das
Catherine Elleouet
François Y. Pétillon
2 and
Philippe Schollhammer
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
Laboratoire de Chimie, Electrochimie Moléculaires et Chimie Analytique, UMR 6521 CNRS, Université de Bretagne Occidentale, CS 93837—6 Avenue Le Gorgeu, 29238 Brest Cedex 3, France
Author to whom correspondence should be addressed.
Molbank 2024, 2024(2), M1797;
Submission received: 19 February 2024 / Revised: 15 March 2024 / Accepted: 18 March 2024 / Published: 25 March 2024


The reaction of the dianionic species [Fe2(CO)6(μ-PPh)2]2− with tBuN(CH2Cl)2 gives the di-iron carbonyl aza-diphosphido-bridged complex [Fe2(CO)6(µ-{P(Ph)CH2}2NtBu)] (1). Attempts to prepare 1 by click-chemistry by reacting [Fe2(CO)6(μ-PHPh)2] with CH2O and tBuNH2 afforded a bis-phosphido compound [Fe2(CO)6(µ-P(Ph)CH2NHtBu)2] (2) which exists as two, syn and anti, isolable isomers depending on the relative orientation of the groups carried by the phosphorus atoms. In the presence of HBF4.Et2O, in dichloromethane, 1 leads to the stabilized ammonium species [Fe2(CO)6(µ-{P(Ph)CH2}2NHtBu)](BF4) (3). The derivatives 13 were characterized by IR and 1H, 31P-{1H} NMR spectroscopies. Their structures in a solid state were determined by X-ray diffraction analyses, which accord with their spectroscopic characteristics.

Graphical Abstract

1. Introduction

In recent decades, the chemistry of carbonyl iron complexes has undergone intensive development within the field of biomimicry. Alongside numerous reports on carbonyl dithiolato-bridged di-iron complexes structurally close to the active site of [FeFe]-hydrogenases (H-cluster) (Scheme 1a) [1,2,3,4,5], phosphido-bridged analogues (Scheme 1b,c) were studied in order to compare the electronic and stereochemical effects of the replacement of the µ-S2R bridge with the µ-PR2 group on the reactivity and redox properties of such carbonyl di-iron systems [6,7,8,9,10,11,12,13,14,15,16,17,18,19]. The chemistry of bis-phosphido-bridged di-iron complexes of general formula [Fe2(CO)6(µ-PR2)2] and linked-diphosphido [Fe2(CO)6(µ-(PR)2R’)] dates back much earlier [20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45]. Electrochemical and theoretical calculations on robust complexes [Fe2(CO)6(µ-PRR’)(µ-PRR″)] were carried out because most of these compounds present a reversible two-electron reduction involving a potential inversion [15,16,44] depending on R, R’ and R″ groups which affect Fe-P distances and the geometry of the {Fe2P2} core. Among these complexes with a {Fe2P2} core, diphosphido compounds featuring a µ-η2:η2-(RP-CH2-R’-CH2-PR″) bridge were more recently the subject of interest (Scheme 1c) [11,12,13,14,15,16,17]. To the best of our knowledge, the compound [Fe2(CO)6(µ-{P(Ph)CH2}2N(CH2)2OCH3] (Scheme 1d) is the sole example of an azadiphosphido-bridged complex [11]. We report herein an extension of our work for preparing such scarce µ-aza-diphosphido complexes. The syntheses, spectroscopic and structural characterizations of the complex [Fe2(CO)6(µ-{P(Ph)CH2}2NtBu)] (1), its protonated form (3) and a bis-phosphido side-product [Fe2(CO)6(µ-P(Ph)CH2NHtBu)2] (2) are presented.

2. Results and Discussion

The complex [Fe2(CO)6(µ-{P(Ph)CH2}2NtBu)] (1) was obtained from the reaction of tBuN(CH2Cl)2 with the dianion [Fe2(CO)6(µ-PPh)2]2− (Scheme 2, path (a)). This latter dianion was prepared by the reaction of [Fe2(CO)6(µ-PHPh)2] with two equivalents of methyllithium [21,31,41]. In the presence of 1 equivalent of HBF4, Et2O, the protonation of 1 occurred at the nitrogen atom of the azadiphosphido bridge, which produced the cationic complex [Fe2(CO)6(µ-{P(Ph)CH2}2NHtBu)](BF4) (3). Attempts to prepare 1 through a one-pot reaction following a click process were also performed (Scheme 2, path (b)). The two isomers 2-anti and 2-syn were isolated by reacting a solution of the complex [Fe2(CO)6(µ-PHPh)2] in ethanol with an aqueous solution of CH2O, followed by the addition of tBuNH2. Compounds 13 were characterized by spectroscopic analyses (IR, NMR), and their structures were confirmed by X-ray analyses of single crystals (see Appendix A and Tables S1–S3). The strong bands observed at 2053, 2013, 1984 and 1970 cm−1 in the carbonyl region of the infrared spectrum of a CH2Cl2 solution of 1 are typical of hexacarbonyl diiron (FeIFeI) complexes with a diphosphido bridge [11]. The protonation at the amine function in 1, giving the ammonium species 3, is revealed by a shift of ca 12 cm−1 (average) of the ν(CO) bands at higher wavenumbers compared to those of 1, which is typical of a protonation in the second sphere of coordination of the di-iron centre. It is worth noting that few examples of isolated N-protonated forms of thiolato analogues have been reported [2,5]. The IR spectra of 2-syn/anti are similar to that of 1. The 1H NMR spectrum of 1 in CDCl3 exhibits the expected set of resonances for the aza-diphosphido bridge {P(Ph)CH2}2NtBu} according to a symmetrical molecule. Similar signals were observed in the 1H NMR spectrum of 3 in CD3CN, but shifted at higher chemical shifts due to its cationic nature. The proton of the ammonium group could not be observed. The 1H NMR spectra of 2-syn/anti in CDCl3 differ from those of 1 and 3 in the tBu/CH2/C6H5 ratio, which shows that there are two tBu groups instead of one. In the case of the syn isomer, a singlet is observed for the tBu groups, while the 1H NMR spectrum of the anti isomer displays two inequivalent singlets. Symmetrical species 1, 2-syn, 3 present in their 31P-{1H} NMR spectra a singlet above 110 ppm. Interestingly, in the case of the 2-anti species, an AB pattern with a coupling constant of 130 Hz was detected at 147.7 ppm. These chemical shifts are in the range of those observed when phosphido groups bridge two metal centres connected by a metal–metal interaction [46].
Single crystals suitable for X-ray diffraction studies were obtained, at −15 °C, from CH2Cl2: Et2O or EtOH: Et2O solutions for compounds 1, 3 and 2, respectively (Figure 1 and Table 1). The overall geometry of the three complexes 13 is similar to those of analogous compounds with a butterfly {Fe2P2} core, two eclipsed square-pyramidal {Fe(CO)3} moieties bridged by two phosphido groups and an Fe—Fe interaction [11]. The distances of the Fe-Fe bond in 1 and 3, 2.6361(5) Å and 2.6242(8) Å, respectively, are close to that observed in [Fe2(CO)6(µ-{P(Ph)CH2}2N(CH2)2OMe)] (2.631(1) Å), while such a distance is much longer in 2 (2.6693(5) Å) (Table 1). The intramolecular P1···P2 contact is significantly shorter in complexes 1 and 3 (2.6880(9) Å and 2.6816(14) Å, respectively)] relative to that determined in 2-anti (2.8899 Å) due to the steric constraint imposed by the aza-diphosphido bridge. The Fe2—P2—C8—N1—C7—P1 metalloheterocycle in 1 and 3 adopts a chair conformation with the tBu substituent in an equatorial position. The geometry around N1 in 1 is a nearly trigonal pyramid (see C8—N1—C7, C8—N1—C9, C7—N1—C9 angles in Table 1). Indirect hints of the N-protonation in 3 are revealed by the opening of the Fe(1)—Fe(2)—C(1) angle of 7° to minimize H/CO steric clash (Fe1—Fe2—C1 = 152.48(16)° in 3 and 144.79(9)° in 1) and by a slight lengthening of N-C bonds in 3 compared with those in 1 (Table 1). In addition, the N—H proton is involved in a stabilizing hydrogen bond interaction to a hydrous H2O…F-BF3 anion [d(N1-H1…O1W) = 2.7056(1) Å, (N1—H1—O1W) = 172.418(4)°, d(F4…O1W) = 2.7081(1) Å, (F4—H1W—O1W) = 160.799(4)°].

3. Materials and Methods

All the experiments were carried out under an inert atmosphere, using Schlenk techniques for the syntheses. Solvents were deoxygenated and dried according to standard procedures. A literature method was used for the preparation of the starting compound [Fe2(CO)6(μ-PHPh)2] [47] and tBuN(CH2Cl)2 [48]. All other reagents were commercially available and used as purchased. NMR spectra (1H, 31P-{1H}) were recorded at room temperature with Bruker AMX 400 or AC 300 spectrometers (Bruker, Billerica, MA, USA) of the “Service général des plateformes, Université de Bretagne Occidentale, Brest” and were referenced to SiMe4 (1H) and H3PO4 (31P). The infrared spectra were recorded with Bruker Vertex 70 and FT IR Perkin Elmer spectrum 2 spectrometer (PerkinElmer Inc., Waltham, MA, USA). Crystal data of compounds 13 were collected with an Oxford Diffraction X—Calibur −2 CCD diffractometer (Agilent Technologies Inc., Santa Clara, CA, USA), equipped with a jet cooler device and graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved and refined using standard procedures [49]. Deposition numbers CCDC 2330851, 2330852 and 2330853 contain the supplementary crystallographic data for 1, 2 and 3. These data can also be obtained free of charge from the Cambridge Crystallographic Data Centre via (accessed on 4 February 2024).

4. Conclusions

These results show the feasibility of extending the preparation of aza-diphosphido complexes [Fe2(CO)6(µ-{P(Ph)CH2}2NR)] with a steric crowding amine function (R = tBu) through a synthetic pathway involving the handy precursor [Fe2(CO)6(μ-PHPh)2]. The protonation of 1 is directed to the amine site, cleanly affording its stable N-protonated form. X-ray study of the ammonium complex 3 reveals an interesting structural feature with stabilizing hydrogen interactions {F3BFH2OHN} involving a water molecule between the counter-anion BF4 and the ammonium function. The one-pot reaction of [Fe2(CO)6(μ-PHPh)2] in the presence of CH2O and tBuNH2 turned out differently and afforded the separable anti and syn isomers of the bis-phosphido compound [Fe2(CO)6(µ-P(Ph)CH2NHtBu)2] (2) with ‘PCH2NHtBu’ linkages. The intermediate formation of the hydroxo species [Fe2(CO)6{μ-PPh(CH2OH)}2] is involved in this reaction. Its preparation with the aim of investigating its reactivity towards amine should help to understand and improve this click process.

Supplementary Materials

Table S1: Crystal data and structure refinement for complex 1; Table S2: Crystal data and structure refinement for complex 2; Table S3: Crystal data and structure refinement for complex 3; Figure S1: IR spectrum in CH2Cl2 of 1; Figure S2: 1H NMR spectrum (CDCl3) of 1; Figure S3: 31P-{1H} NMR spectrum (CDCl3) of 1; Figure S4: IR spectrum in CH2Cl2 of 2-anti; Figure S5: 1H NMR spectrum (CDCl3) of 2-anti; Figure S6: 31P-{1H} NMR spectrum (CDCl3) of 2-anti; Figure S7: IR spectrum in CH2Cl2 of 2-syn; Figure S8: 1H NMR spectrum (CDCl3) of 2-syn; Figure S9: 31P-{1H} NMR spectrum (CDCl3) of 2-syn; Figure S10: IR spectrum in CH2Cl2 of 3; Figure S11: 31P-{1H} NMR spectrum (CDCl3) of 3; Figure S12: 1H NMR spectrum (CDCl3) of 3.

Author Contributions

P.S. and P.D. supervised the syntheses and X-ray, spectroscopic characterizations of the compounds 13. C.E., F.Y.P., P.D. and P.S. contributed to the writing of the manuscript. All authors have read and agreed to the published version of the manuscript.


CNRS (Centre National de la Recherche Scientifique) and the Université de Bretagne Occidentale are acknowledged for financial support.

Data Availability Statement

Data are contained within the article and Supplementary Materials.


We thank K.W. Muir and the University of Glasgow for providing crystallographic measurements of 13. We are grateful to F. Michaud for his help with the resolution of X-ray structures.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Preparation of [Fe2(CO)6(µ-{P(Ph)CH2}2NtBu)] (1). A THF solution (50 mL) of Li2[Fe2(CO)6(μ-PPh)2], generated in situ (30 min stirring), at −78 °C, from [Fe2(CO)6(μ-PHPh)2] (0.4 g, 0.80 mmol) with 2 equiv (1.6 mmol) of MeLi (1.1 mL, 1.5 M in Et2O) was treated with a THF solution of tBuN(CH2Cl)2 (0.15 g, 0.88 mmol). After 30 mn, the orange-brown solution was allowed to warm to room temperature and stirred for 1h. The solvent was then removed, and the crude product was extracted with dichloromethane (2 × 25 mL). The extracts were evaporated to dryness, and the residue was chromatographed on a silica gel column. Elution with hexane-dichloromethane (1:1) afforded a bright yellow solution of 1, which was evaporated under vacuum. Yellow crystals grew from CH2Cl2-Et2O (3:1) mixtures, yield 0.196 g (41%). Elemental analysis (%) calculated for C24H23Fe2NO6P2: C 48.44, H 3.89, N 2.35; found; C 46.46, H 4.18, N 1.42. Elemental analysis (%) calculated for 1. ½CH2Cl2 C24.5H24ClFe2NO6P2: C 46.15, H 3.79, N 2.19. 1H NMR (CDCl3, 25 °C): δ = 7.72–7.47 (m, 10H, C6H5), 2.92 (s, 4H, P-CH2-N), 0.97 (s, 9H, N-C(CH3)3); 31P-{1H} NMR (CDCl3, 25 °C): δ = 141.79 (s); IR (CH2Cl2): ν(CO) = 2053(s), 2013(vs), 1984(s), 1970(s) cm−1.
Reaction of [Fe2(CO)6(μ-PHPh)2] with CH2O and tBuNH2. A solution of [Fe2(CO)6(μ-PHPh)2] (0.2 g, 0.40 mmol) in 30 mL of ethanol was treated with 0.1 mL (1.23 mmol) of an aqueous solution of CH2O (37%). After stirring for 30 min at room temperature, 0.5 mL of tBuNH2 (4.8 mmol) was added to the resulting orange-red solution. After the reaction mixture turned brown, the solvent was evaporated to dryness and the crude product was extracted with 3 × 15 mL of Et2O. After filtration and evaporation of Et2O, the resulting residue was chromatographed on a silica gel column with mixture of hexane-CH2Cl2. Elution with dichloromethane afforded a yellow fraction of anti-[Fe2(CO)6(µ-P(Ph)CH2NtBu)2] (2-anti) as the major product of the reaction. A second yellow fraction of the minor isomer syn-[Fe2(CO)6(µ-P(Ph)CH2NHtBu)2] (2-syn) was collected with a mixture of solvents CH2Cl2-THF (9:1).
Yields: 2-anti: 136 mg (51%); 2-syn: 34 mg (13%). Single crystals of 2-anti grew from EtOH-Et2O (1:1) mixtures.
2-anti: Elemental analysis (%) calculated for C28H34Fe2N2O6P2: C 50.33, H 5.13, N 4.19; found; C 50.22, H 5.13, N 4.03. 1H NMR (CDCl3, 25 °C): δ = 7.71–7.35 (m, 10H, C6H5), 3.38 (s, 4H, P-CH2-N), 0.82 (s, 9H, C(CH3)3), 0.36 (s, 9H, C(CH3)3), NH not assigned; 31P-{1H} NMR (CDCl3, 25 °C): δ = 147.7 (AB, JPP = 130.0 Hz); IR (CH2Cl2): ν(CO) = 2051(s), 2012(vs), 1981(s), 1964(s) cm−1.
2-syn: Elemental analysis (%) calculated for C28H34Fe2N2O6P2: C 50.33, H 5.13, N 4.19; found; C 49.61, H 5.11, N 3.89. 1H NMR (CDCl3, 25 °C): δ = 7.10–6.77 (m, 10H, C6H5), 3.33 (s, 4H, P-CH2-N), 0.78 (s, 18H, C(CH3)3), NH not assigned; 31P-{1H} NMR (CDCl3, 25 °C): δ = 136.8 (s); IR (CH2Cl2): ν(CO) = 2050(s), 2011(vs), 1979(s), 1964(s) cm−1.
Protonation of 1. A solution of 1 (0.1 g, 0.17 mmol) in dichloromethane (10 mL) was treated with 1 equiv (23 µL) of HBF4. Et2O (1.19 g/mL). The mixture was stirred for 1 h. The volume was then reduced under vacuum, and diethyl ether was added to precipitate a yellow powder of 3. Yield 0.11 g (95%). Single crystals grew from CH2Cl2-Et2O (3:1) mixtures. 1H NMR (CDCl3, 25 °C): δ = 7.79–7.57 (m, 10H, C6H5), 3.50 (br, 4H, P-CH2-N), 1.41 (s, 3H, C(CH3)3), NH not assigned; 31P-{1H} NMR (CDCl3, 25 °C): δ = 113.8 (s); IR (CH2Cl2): ν(CO) = 2070(s), 2032(vs), 2004(s), 1963(s) cm−1.


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Scheme 1. Representations of (a) the H-cluster, (b) carbonyl di-iron bis-phosphido complexes, (c) carbonyl diphosphido complexes and (d) aza-diphosphido complexes.
Scheme 1. Representations of (a) the H-cluster, (b) carbonyl di-iron bis-phosphido complexes, (c) carbonyl diphosphido complexes and (d) aza-diphosphido complexes.
Molbank 2024 m1797 sch001
Scheme 2. Synthetic pathways for complexes 13: (a) Sequential steps pathway and (b) one-pot pathway.
Scheme 2. Synthetic pathways for complexes 13: (a) Sequential steps pathway and (b) one-pot pathway.
Molbank 2024 m1797 sch002
Figure 1. Molecular structure of compounds 13 with thermal ellipsoids at 20% probability.
Figure 1. Molecular structure of compounds 13 with thermal ellipsoids at 20% probability.
Molbank 2024 m1797 g001
Table 1. Selected bond lengths (Å) and angles (°) of 13.
Table 1. Selected bond lengths (Å) and angles (°) of 13.
N(1)—C(8)1.455(3) 1.486(5)
N(2)—C(8) 1.442(3)
P(1)—Fe(1)—P(2)74.84(3)81.33 (3)75.18(4)
C(7)—N(1)—C(8)111.1(2) 119.1(3)
C(8)—N(1)—C(9)116.69(19) 113.3(3)
C(8)—N(2)—C(25) 117.2 (2)
N(1)—C(8)—P(2)111.44(16) 120.1(3)
N(2)—C(8)—P(2) 111.84(17)
C(19)—P(2)—P(1)170.23(8) 174.05(12)
C(13)—P(1)—P(2)174.1(9) 171.98(13)
H(1W)—O(1W)—H(2W) 110(4)
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Das, P.; Elleouet, C.; Pétillon, F.Y.; Schollhammer, P. Aza-Diphosphido-Bridged Di-Iron Complexes Related to the [FeFe]-Hydrogenases. Molbank 2024, 2024, M1797.

AMA Style

Das P, Elleouet C, Pétillon FY, Schollhammer P. Aza-Diphosphido-Bridged Di-Iron Complexes Related to the [FeFe]-Hydrogenases. Molbank. 2024; 2024(2):M1797.

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Das, Pankaj, Catherine Elleouet, François Y. Pétillon, and Philippe Schollhammer. 2024. "Aza-Diphosphido-Bridged Di-Iron Complexes Related to the [FeFe]-Hydrogenases" Molbank 2024, no. 2: M1797.

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