Redox behaviour of ([fc(NPiPr2)2]Fe)2, formation of an iron-iron bond and cleavage of azobenzene

Article abstract of DOI:10.1039/c8dt00828k

The redox behaviour of the dimeric tetrairon complex, ([fc(NPⁱPr₂)₂]Fe)₂ (where fc(NPⁱPr₂)₂ = 1,1′-(C₅H₄NPⁱPr₂)₂Fe) has been investigated. Upon reduction with KC₈ an Fe-Fe bond is formed with the complex maintaining a high spin configuration and having the formula [K(THF)₆]([fc(NPⁱPr₂)₂]Fe)₂. In contrast, oxidation of the complex is ligand based; for example, addition of the 1,2-diiodoethane (I₂ equivalent) results in the formation of the monomeric iron(ii) diiodide [fc(NⁱPr₂I)₂]FeI₂ wherein the phosphine is oxidized. The dimeric tetrairon complex reacts photolytically with azobenzene, cleaving the NN double bond and forming the new monomeric bis(phosphoramidate) iron complex. [fc(NP(NPh)ⁱPr₂)₂]Fe. Characterization of these paramagnetic complexes was accomplished by magnetic susceptibility studies and X-ray analyses.

Full text of DOI:10.1039/c8dt00828k

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Redox behaviour of ([fc(NPiPr2)2]Fe)2, formation of
an iron–iron bond and cleavage of azobenzene†‡
Cite this: DOI: 10.1039/c8dt00828k
a
Fraser S. Pick,a Daniel B. Leznoff *b and Michael D. Fryzuk
*
The redox behaviour of the dimeric tetrairon complex, ([fc(NPiPr2)2]Fe)2 (where fc(NPiPr2)2 = 1,1’-
(C5H4NPiPr2)2Fe) has been investigated. Upon reduction with KC8 an Fe–Fe bond is formed with the
complex maintaining a high spin configuration and having the formula [K(THF)6]([fc(NPiPr2)2]Fe)2. In con-
trast, oxidation of the complex is ligand based; for example, addition of the 1,2-diiodoethane (I2 equi-
valent) results in the formation of the monomeric iron(II) diiodide [fc(NiPr2I)2]FeI2 wherein the phosphine
is oxidized. The dimeric tetrairon complex reacts photolytically with azobenzene, cleaving the NvN
double bond and forming the new monomeric bis(phosphoramidate) iron complex. [fc(NP(NPh)iPr2)2]Fe.
Characterization of these paramagnetic complexes was accomplished by magnetic susceptibility studies
and X-ray analyses.
Received 3rd March 2018,
Accepted 26th June 2018
DOI: 10.1039/c8dt00828k
rsc.li/dalton
Betley and coworkers have contributed to this area with the
isolation of the trinuclear, high-spin cluster of Fe(II) centers (A
Introduction
The impressive transformations facilitated by heterogeneous in Scheme 1), which is capable of cleaving the N–N bond of
catalysts1–6 and multimetallic co-factors in metalloenzymes, azobenzene (PhNvNPh) with no external reductant.28 We have
such as nitrogenase,7–10 have led to interest in the study of previously reported the tetrairon dimer 1 that contains two
polynuclear molecular systems.11–14 The ability of metallo- ferrocenyl diphosphinoamides supporting two high spin Fe(II)
enzymes to perform multielectron reductions15,16 using iron- ions; this ancillary ligand system also can be used to generate
based cofactors is particularly noteworthy considering the con- a mixed tetranuclear Fe2–Co2 system that displays weak Fe–Co
ditions of the intracellular environment. Despite significant interactions.34 Based on reports from our group and others
efforts, the binding and activation of substrates by these poly- that show how dinitrogen can be activated and functionalized
nuclear sites remain poorly understood.16–20 Attempts to by dinuclear or trinuclear complexes,35–41 we investigated the
create synthetic models of these polyiron sites has been of redox reactivity of 1 to examine structural changes, as well as
increasing interest17,21–26 as such studies may reveal insights interaction with small molecules related to dinitrogen fixation.
into complex redox processes. Of particular note, recent In this paper, we report the oxidation and reduction of 1, and
reports of polyiron complexes supported by a biological cleavage of azobenzene by 1.
ligands detail the activation of substrates relevant to nitrogen
fixation.27–31 Such studies are important because these syn-
thetic systems can be more easily studied than the naturally
occurring enzymes, which allows for a more detailed descrip-
Results and discussion
tion of their electronic structures, factors affecting metal–
Initially we sought to use cyclic voltammetry to study the redox
behaviour of 1 but the results were poor with multiple irrevers-
ible waveforms observed in both the reductive and oxidative
regimes (see the ESI‡). We then examined chemical oxidants
and reductants. Although 1 reacts rapidly with numerous oxi-
dants of the type Ag(Y) and X2 (Y = OTf−, Cl−, BPh4−, X = Br, I)
the cleanest reaction was obtained using the molecular iodine
equivalent 1,2-diiodoethane. Treatment of 1 with four equiva-
lents of 1,2-diiodoethane resulted in conversion to a new para-
magnetic product. The stoichiometry of the reaction suggested
that the oxidation was more than a simple Fe(II)/Fe(III) event.
Recrystallization from toluene/hexanes mixture results in the
formation of dark red crystals of a new paramagnetic product,
metal bonding, and substrate binding.32,33
aDepartment of Chemistry, The University of British Columbia, 2036 Main Mall,
Vancouver, BC, Canada, V6T 1Z1. E-mail: fryzuk@chem.ubc.ca;
Fax: +1 604-822-2847; Tel: +1 604 -822-2897
bDeprtment of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby,
BC, Canada V5A 1S6. E-mail: dleznoff@sfu.ca; Fax: +1 778-782-3765;
Tel: +1 778 -782-4887
†Dedicated to Professor Richard A. Andersen on the occasion of his 75th
Birthday.
‡Electronic supplementary information (ESI) available. CCDC 1822229–1822231.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c8dt00828k
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Scheme 1 The triiron complex A from the Betley group (ref. 28) and the dimeric tetrairon complex 1 discussed in this work.
2, with an empirical formula of [fc(NPiPr2)2]FeI4 (56%). X-ray the ligand must twist to become perpendicular to each other
analysis of 2 revealed the solid-state molecular structure, (cp plane to cp plane angle = 84.75°–86.05°). In addition, the
shown in Fig. 1. The P–N bond shortens from 1 (1.6993(12) Å) iron–nitrogen bonds elongate slightly in the reduced derivative
to 2 (1.603(7) Å) demonstrating that the electron-rich phosphino- 3 to 1.975(3) and 1.983(3) Å from 1.9217(12) and 1.9459(12) Å
amide arms of the ligand have been oxidized to generate as found in starting 1. Interestingly, the iron–phosphine bond
iodophosphinime units while both iron atoms remain in the distances decrease when 1 is reduced; in 1, the Fe–P bonds are
formal Fe(II) oxidation state. We suggest that irreversible oxi- 2.4443(4) Å whereas in the reduced species 3, they are shorter,
dation of the P–N bond is contributing to the poor quality of i.e., Fe2–P4 is 2.3612(10) and Fe3–P2 is 2.3606(11) Å. This is
the aforementioned CV data in the oxidative regime.
likely due to increased back bonding to the phosphines in the
Attempts to generate a dinitrogen complex by reduction of reduced form 3 whereas the iron–amido (Fe–N) bonds increase
1 under mild conditions (4 atm N2) were unsuccessful. Upon slightly due to the increased negative charge shared between
reduction with KC8 1 does not coordinate N2 but rather under- the two iron centers.
goes a rearrangement to form an iron–iron bond between the
The Mössbauer spectrum of 3 and the fits are shown in
two internal iron centers (Fig. 1). The Fe1–Fe2 distance Fig. 2. Formally, complex 3 is a mixed-valent species with the
decreases from 3.9241(5) Å in 1 to 2.4760(6) Å in 3, similar to core iron centers with the Fe–Fe bond being Fe(I)/Fe(II) sur-
the diiron (FeI/FeII) tris(phosphinoamide) reported by the rounded by two Fe(II) ferrocence units in the ligand framework.
Thomas group (2.4645 Å).29 In order for the iron centers in 3 We observe two doublets, one with an isomer shift of
to get close enough to form a bond the ferrocene backbones of +0.45 mm s−1 and ΔEq of 2.23 mm s−1 (doublet 1, blue fit in
i
Fig. 1 ORTEP diagram of 2 (left) and anionic portion of 3 (right) with ellipsoids drawn at 50% probability. All H atoms and Pr methyl groups have
been omitted for clarity. Selected bond lengths (Å) and angles (°) of 2: Fe1–Fe2 3.4836(17); N1–P1 1.607(7); N2–P2 1.603(7); Fe1–I1 2.6889(13); Fe1–
I2 2.6428(13); P1–I3 2.408(3); P2–I4 2.409(3); I1–Fe–I2 104.97(4); N1–Fe–N2 115.7(3); cp tilt42,43 2.78; and 3: Fe1–Fe2 3.7016(10); Fe2–Fe3 2.4755(7);
Fe3–Fe4 3.6954(10); N1–P1 1.690(3); N2–P2 1.651(3); Fe2–N2 1.975(3); Fe2–N1 1.983(3); Fe2–P4 2.3612(10); Fe3–P2 2.3607(10); N1–Fe2–N2 11.49(11);
N2–Fe2–P4 110.22(8); N1–Fe2–Fe3 136.97(8); cp tilt42,43 for Fe1 1.18 and Fe4 0.88.
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reduction. Compound 1 and A in represent rare examples of
iron–iron interactions that increase upon reduction while
maintaining a high-spin state, and therefore populating the
Fe–Fe anti-bonding orbitals. The reactivity of 3 with nitrogen
rich substrates such as PhNvNPh has so far led to compli-
cated mixtures, no doubt due to the high reactivity of this
complex.
While 1 was unable to coordinate dinitrogen even under
reducing conditions, we wondered how 1 would react with the
NvN double bond in azobenzene. Cleavage of PhNvNPh is
known for iron49 and ruthenium50 carbonyl clusters, both of
which are proposed to involve transfer of the putative metal
imidos to coordinated CO to form isocyanates. More recently a
Fig. 2 Zero-field Mössbauer spectrum of 4 fit as a pair of quadrupolar
doublets, with δ = +0.45 mm s−1, ΔEQ = 2.23 mm s−1 (blue line, doublet trinuclear ruthenium hydride51 and
a
trinuclear iron
1 fit) and δ = +0.21 mm s−1, ΔEQ = 0.77 mm s−1 (red line, doublet 2 fit).
The total fit is shown as a green line.
complex28 have been shown to cleave PhNvNPh into metal
imidos. We envisioned that complex 1, containing four iron(II)
centers, might also have the available reducing equivalents to
cleave the PhNvNPh double bond.
Fig. 2); this doublet corresponds to the two ferrocene Fe(II)
Exposing compound 1 to azobenzene does not result in a
centers and matches our previous assignment34 of the ferro- reaction as evidenced by NMR spectroscopy. Even heating the
cene centers in starting 1. The remaining doublet (doublet 2, mixture to 70 °C for 12 hours does not result in any new
red fit) with an isomer shift of +0.23 mm s−1 and ΔEq of signals in either the paramagnetic or diamagnetic 1H NMR
0.77 mm s−1 is assigned to the core Fe centers, and because spectra. However, when a solution of 1 and PhNvNPh in C6D6
there is only one doublet, this is consistent with a delocalized was irradiated with UV light (350 nm) a new paramagnetic
mixed-valent system. The broadness of the inner doublet com- product was detected by NMR spectroscopy (compound 4 in
pared to the outer doublet is also consistent with a mixed- Scheme 2). Single crystals were obtained by cooling a solution
valent electronic structure for these inner core Fe centers.44 of 4 in pentane, and the solid-state structure is shown in
The isomer shift and ΔEq values do not match other somewhat Fig. 3.
related complexes in the literature. For example, a diiron tris
Unexpectedly, two new P–N bonds were formed with similar
(diphenylformamidate) species45 is formally a mixed-valent P1–N1 and P1–N3 bond lengths of 1.6119(14) Å and 1.6260(14)
Fe(I)/Fe(II) system with an isomer shift of +0.65 mm s−1 and ΔEQ Å respectively, indicate a delocalized phosphoramidate anion.
of + 0.32 mm s−1; however, the very different ligand environ- Interestingly the ferrocene linker forces the iron center to
ment and high symmetry of this latter tris(formamidate) adopt
52
a
distorted square-planar geometry (τ4
= 0.19).
complex make comparisons difficult. Analogy to other systems Compound 4 displays a room temperature magnetic moment
has not been helpful either as there is considerable variation of 2.9μB, consistent with the spin only value of two unpaired
of Mössbauer parameters for other Fe derivatives.46–48
electrons (S = 1) and other square planar Fe(II) complexes.53
The iron–iron bond, formed upon reduction of 1, illustrates Phosphoramidates have been used to support metal–metal
the potential for the storage of reducing equivalents in this bonds in alkali earth metals,54–56 rare earth element polymer-
system; however, we were unable to capitalize on this as com- ization catalysts57–61 and group 10 cross-coupling catalysts.62–64
pound 3 is unstable in solution: a C6D6 solution 3 reverts back To the best of our knowledge this is the first report of a bis
to over 50% compound 1 in less than 24 hours. In the solid- (phosphoramidate)iron complex, however, a diiron system
state, however, 3 is stable in the glovebox freezer (−35 °C) for with
up to a week. The magnetic moment of 3 (7.8μB) indicates that reported.65
the complex maintains a maximally high spin S = 7/2 ground
Since there was no reaction with the readily available trans
a bridging phosphoramidate backbone has been
state. We also tried without success to reduce 3 further by form of azobenzene, we initially hypothesized that 1 could
addition of more KC8, but this did not result in any change as only react with cis-PhNvNPh due to steric congestion. To test
evidenced by NMR spectroscopy. In fact, a K mirror in the this we produced a mixture of cis/trans-PhNvNPh by photo-
NMR tube extended the lifetime of 3 in C6D6 solution as evi- lysis and then added compound 1 in the absence of UV light.
dence by the persistence of the paramagnetically shifted peaks No reaction was observed, although 1 does seems to catalyze
due to 3.
the cis to trans isomerization of azobenzene (see ESI‡).
It has been observed that diiron systems displaying a M–M Compound 4 could only be produced when both reactants are
bond contract upon oxidation47 with the rationale being a exposed to UV light together, indicating that the role of UV
depopulation of the M–M anti-bonding orbitals. However, in a radiation is more than just isomerization of azobenzene.
recent contribution, Betley and coworkers report33 that the
While phosphoramidates are most commonly synthesized
same triiron system, which activates PhNvNPh (A in from phosphines and organic azides, synthesis from azo-
Scheme 1) contracts the Fe–Fe distance (0.13 Å) upon benzene is precedented. Recently, a reaction between the Ti/Co
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Scheme 2
phosphinoamide complex, (THF)Ti(μ-ArNPR2)3Co(N2), and
azobenzene has been reported, which resulted in cleavage of
azobenzene and formation of one phosphoramidate and one
bridging metal imido.66 Given the observations of an imido
unit in the Ti–Co system above, it is likely that an iron imido is
produced in the reaction between PhNvNPh and compound
1, but this imido quickly reacts with a phosphinoamide arm of
the ligand. The complex most similar to 4 is Fe(ArNPR2)3Fe
(PMe3),29 which reacts with organic azides to produce an iron
imido. However, this imido is not reported to react with the
bound amidophosphine ligands and we suggest that the geo-
metric differences between the two complexes, namely the
presence of η1-N-phosphinoamides, are responsible for the
divergent reaction profiles of these similar complexes.
Conclusions
In this report we present a polyiron complex that, under photo-
lytic conditions, cleaves azobenzene through a proposed but
undetected iron imido species. We also note that 1 undergoes
a ligand rearrangement forming an iron–iron bond upon
reduction while maintaining a high-spin S = 7/2 state. From a
ligand design perspective it appears that amidophosphine
Fig. 3 ORTEP diagram of 4 with ellipsoids drawn at 50% probability. All
H atoms and Pr methyl groups have been omitted for clarity. Selected
bond lengths (Å) and angles (°): Fe1–Fe2 4.0103(4); N1–P1 1.6142(13);
P1–N3 1.6260(14); N1–Fe1 2.0558(13); P1–Fe1 2.7176(5); N3–Fe1 2.0634
(14); N1–P1–N3 96.90(7); N1–Fe1–N2 98.04(5); N1–Fe1–N3 72.13(5);
N3–Fe1–N4 118.01(5).
i
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1
donors are too electron-rich to study oxidation of high-spin 55.7%. H NMR (δ in ppm, C6D6, 293 K, 400 MHz) 18.36 (s),
iron clusters. Further work will involve determining whether 4.74 (s), 3.19 (s), 0.88 (s), −9.33 (s). Anal. calcd for
the proposed imido, generated during the reaction of 1 with C29H44Fe2I4N2P2: C, 31.61; H, 4.02; N, 2.54. Found: C, 31.94; H,
PhNvNPh, can be trapped before transfer to the phosphine. 4.12; N, 2.50. μeff (solution 293 K) 5.5μB.
New ligand designs will focus on scaffolds that can support
(K(THF)6)(Fe[fc(NPiPr2)2])2 (3). To an oven-dried Schlenk
higher nuclearity iron systems that are more redox-innocent, flask was added ([fc(NPiPr2)2]Fe)2 (0.100 g, 0.0996 mmol) and
and stay dimeric throughout redox processes and their reac- toluene (5 mL) and chilled to −30 °C. In a separate Schlenk
tions with nitrogen-rich small molecules.
flask KC8 (0.054 g, 0.3984 mmol) was suspended in toluene
(5 mL) and chilled to −30 °C. The([fc(NPiPr2)2]Fe)2 solution
was cannula transferred on to the KC8 slurry and allowed to
warm to room temperature with stirring for 6 h. The resulting
slurry is filtered through Celite and the volatiles are removed
in vacuo leaving a black residue. The residue is dissolved in a
Experimental
General experimental procedures
All manipulations were performed under an atmosphere of minimal amount of THF, layered with hexanes and cooled to
dry and oxygen-free dinitrogen by means of standard −35 °C resulting in the formation of black crystals, which were
Schlenk or Glovebox techniques. Anhydrous diethylether, filtered and dried in vacuo. Yield: 0.094 g, 0.064 mmol, 64%.
toluene, hexanes and THF were purchased from Sigma- 1H NMR (δ in ppm, C6D6, 293 K, 400 MHz) 188.42 (s), 36.80
Aldrich, sparged with dinitrogen and dried further by (s), 34.10 (s), 25.32 (s), 15.14 (s), 12.08 (s), 11.51 (s), −2.75 (s),
passage through towers containing activated alumina and −17.86 (s), −29.72 (s). Anal. calcd for C68H120Fe4KN4O6P4: C,
molecular sieves. Benzene-d6, THF-d8 and pentane were 55.33; H, 8.19; N, 3.80. Found: C, 55.11; H, 7.82; N, 3.98. μeff
refluxed over sodium benzophenone ketyl, vacuum trans- (solution 293 K) 7.8μB.
ferred and freeze–pump–thaw degassed. 1,2-Diiodoethane
[fc(NPiPr2NPh)2]Fe (4). An oven dried bomb was loaded with
and PhNvNPh were purchased from Sigma-Aldrich and ([fc(NPiPr2)2]Fe)2 (0.100 g, 0.100 mmol), PhNvNPh (0.020 g,
used as received. KC8 and compound 134 were prepared by 0.110 mmol) and toluene (10 mL). The bomb was sealed,
67
literature methods.
removed from the glovebox and irradiated with UV-light
1H, 31P and 13C NMR spectra were recorded on a Bruker (350 nm) for 20 hours. The solution was then filtered, remov-
1
AV-300 or AV-400 MHz spectrometer at room temperature. H ing minimal solids, and reduced in vacuo to 1 mL. Pentane
NMR spectra were referenced to residual proton signals in (1 mL) was added and the solution was cooled to −40 °C
C6D6 (7.16 ppm) and C4D8O (3.58 ppm); 31P NMR spectra were resulting in formation of an orange powder. Yield: 0.057 g,
1
referenced to external 85% H3PO4 (0.0 ppm); microanalyses (C, 0.083 mmol, 83%. H NMR (δ in ppm, C6D6, 293 K, 400 MHz)
H, N) and mass spectroscopy (low resolution EI) were per- 109.02, 103.18, 98.59, 33.36, 29.81, 28.06, 23.23, 22.36, 19.92,
formed at the Department of Chemistry at the University of 19.08, 17.06, 14.66, 14.13, 13.24, 12.35, 11.19, 10.91, −2.08,
British Columbia. Magnetic moments in solution were −4.67, −5.73, −14.75, −18.43, −22.89, −23.74. Anal. calcd for
obtained using Evans NMR method.68,69 The 57Fe Mössbauer C34H46FeN4P2: C, 64.97; H, 7.38; N, 8.91. Found: C, 64.55; H,
spectrum of 3 was recorded at Simon Fraser University using a 7.24; N, 8.65. μeff (solution 293 K) 2.9μB.
W.E.B. Research Mössbauer spectroscopy system at room
temperature under a He atmosphere. A 57Co (in rhodium
matrix) source with a strength of approx. 10 mCi was used. Author contributions
The detector was
a Reuters–Stokes Kr/CO2 proportional
This manuscript was written through contributions of all
authors.
counter. The sample powder was loaded in a glovebox into a
high-density polyethylene flat washer, wrapped in parafilm,
and secured with Kapton tape. The velocity was scanned at a
rate between 4 mm s−1 and −4 mm s−1, using a constant accel-
eration triangle waveform, and calibrated against iron foil,
measured at 295 K in zero magnetic field. All isomer shifts (δ)
are relative to the iron foil. Fitting of the data was performed
using WMOSS software, which is available free of charge at
http://wmoss.org/.
Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
[fc(NPiPr2I)2]FeI2 (2). To an oven-dried Schlenk flask was
added ([fc(NPiPr2)2]Fe)2 (0.100 g, 0.0996 mmol) and toluene We thank the Natural Science and Engineering Research
(5 mL). A solution of ICH2CH2I (0.118 g, 0.419 mmol) in Council of Canada (NSERC) for a Discovery Grant (M. D. F. &
toluene (5 mL) was added dropwise and the resulting solution D. B. L.) and a postgraduate scholarship (F. S. P.). The authors
was allowed to stir for 16 h. The volatiles were removed would like to thank Professor Glenn Sammis and his group for
in vacuo and the resulting solids were recrystallized from a the use of their light enclosure, and John R. Thompson for col-
50/50 mixture of toluene and hexanes resulting in dark red lecting the Mössbauer data. M. D. F. especially wants to
crystalline solids which were dried in vacuo. Yield 0.112 g, acknowledge Richard (Dick) Andersen who inspired a lot of
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