Investigation of formally zerovalent Triphos iron complexes

Article abstract of DOI:10.1039/c2cc33926a

The reduction of Triphos [PhP(CH₂CH₂PPh ₂)₂] iron halide complexes has been explored, yielding formally zerovalent (κ³-Triphos)Fe(κ²-Triphos) and (κ³-Triphos)Fe(κ²-Bpy). Electrochemical analysis, coupled with the metrical parameters of (κ³-Triphos) Fe(κ²-Bpy), reveal an electronic structure consistent with a π-radical monoanion bipyridine chelate that is antiferromagnetically coupled to a low spin, Fe(i) metal center.

Full text of DOI:10.1039/c2cc33926a

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COMMUNICATION
Investigation of formally zerovalent Triphos iron complexesw
Tufan K. Mukhopadhyay,a Russell K. Feller,b Francisca N. Rein,c Neil J. Henson,d
Nathan C. Smythe,c Ryan J. Trovitch*ac and John C. Gordon*c
Received 31st May 2012, Accepted 6th July 2012
DOI: 10.1039/c2cc33926a
The reduction of Triphos [PhP(CH2CH2PPh2)2] iron halide
complexes has been explored, yielding formally zerovalent
(j3-Triphos)Fe(j2-Triphos) and (j3-Triphos)Fe(j2-Bpy). Electro-
chemical analysis, coupled with the metrical parameters of
(j3-Triphos)Fe(j2-Bpy), reveal an electronic structure consistent
with a p-radical monoanion bipyridine chelate that is antiferro-
magnetically coupled to a low spin, Fe(I) metal center.
leads to facile decomposition), we turned our attention towards
iron halide complexes bearing the highly flexible Triphos ligand,
PhP(CH2CH2PPh2)2, and the study of their reduction under N2.
As Triphos is known to coordinate in either a fac- or mer-
fashion within an octahedral metal environment,8 we hypothesized
that its coordinative flexibility may allow for the preparation
of an iron(0) dinitrogen complex featuring a near tetrahedral
geometry. Although such a compound could not be prepared,
this contribution offers a detailed investigation of the formally
zerovalent (Triphos)Fe complexes that have been synthesized
under reducing conditions.
Due to the perceived importance of iron in biological nitrogen
fixation1 and its utilization in the Haber–Bosch process,2 the
study of dinitrogen coordination to well-defined iron complexes
remains critical for guiding the development of inexpensive N2
reduction catalysts.3 In theory, the multi-electron reduction of
N2 between two iron centers, or its direct cleavage to form two
equivalents of LnFeRN,4 could facilitate the formation of
N–H bonds from H2 and lead to a simplified, low-energy input
NH3 production cycle.
This study commenced with the preparation of a series of
iron halide complexes of the general formula (Triphos)FeXn
(X = Cl, Br; n = 2, 3) (1-Cl2, 1-Br2, 1-Cl3, 1-Br3) via the
stoichiometric addition of Triphos to a THF solution of the
respective iron halide starting material. Although 1-Cl2 has been
previously reported,9 the structure of this complex in both
solution and as a solid remains unknown. Attempts to crystallize
1-Cl2 resulted in the isolation of slightly opaque colorless
crystals, which have been identified as (k2-Triphos)2FeCl2 by
single crystal X-ray diffraction (Fig. S1of the ESIw).10
One approach to this challenge has involved the study of
reducing Fe(0) complexes that are supported solely by electron-
donating phosphine ligands. Iron dinitrogen complexes featuring
two bidentate phosphine ligands have been studied,5 although
they have yet to afford significant reduction of the NRN
bond. This has led researchers towards phosphine-based ligand
scaffolds that could occupy four sites (3 basal and 1 apical) of
a trigonal bipyramidal coordination environment, leaving
one apical site for N2 coordination. This methodology has
allowed the characterization of [k4-N(CH2CH2PPh2)3]Fe(N2),6
[k4-P(CH2CH2PMe2)3]Fe(N2) and its related bridging complex
([k4-P(CH2CH2PMe2)3]Fe)2(m2,Z1,Z1-N2).7 Because the trans-
influence imparted by the apical chelate substituent in these
complexes acts to deter strong nitrogen coordination (the greater
trans-effect of the central phosphine in [P(CH2CH2PMe2)3]Fe(N2)
The crystallization of 1-Br2, 1-Cl3, and 1-Br3 was also
unsuccessful; however, each of these complexes could be
effectively characterized by NMR spectroscopy. The ambient
temperature 1H NMR spectra of 1-Br2 and 1-Cl3 feature
paramagnetically broadened resonances over a 110 or 70 ppm
range, respectively, with the Triphos methylene resonances
appearing the most shifted from their diamagnetic reference
values. In contrast, evidence for Triphos lability was found when
investigating 1-Br3 by 1H and 31P NMR spectroscopy. This
1
complex displays a relatively featureless H NMR spectrum in
THF-d8 at 23 1C with only one significant paramagnetically
shifted resonance at 27.51 ppm. Surprisingly, 31P NMR spectro-
scopy revealed the presence of a paramagnetically broadened
free Triphos phosphine arm at ꢀ22.10 ppm (Fig. S2 of the
ESIw). This resonance is comparable in shift to those observed
for uncoordinated Triphos (ꢀ16.87 and ꢀ20.43 ppm, THF-d8);
however, it has a peak width at half-height of 449.5 Hz.
Although 31P resonances are typically not observed for para-
magnetic complexes, the weaker ligand field strength of 1-Br3
when compared to 1-Cl3,11 permits an arm of the Triphos
chelate to dissociate upon dissolution into THF.
a Department of Chemistry & Biochemistry, Arizona State University,
Tempe, AZ 85287, USA. E-mail: ryan.trovitch@asu.edu;
Fax: +1 480 965 2747; Tel: +1 480 727 8930
b Materials Physics and Applications Division, Los Alamos National
Laboratory, Los Alamos, NM 87545, USA
c Chemistry Division, Los Alamos National Laboratory, Los Alamos,
NM 87545, USA. E-mail: jgordon@lanl.gov;
Fax: +1 505 667 9905; Tel: +1 505 665 6962
d Theoretical Division, Los Alamos National Laboratory, Los Alamos,
NM 87545, USA
w Electronic supplementary information (ESI) available: Full experi-
mental details, supporting figures, and cystallographic data. CCDC
884463, 884464. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2cc33926a
The reduction chemistry of each halide starting material was
explored with the hope of isolating a Triphos iron complex
featuring N2 coordination. Adding an excess (5 eq.) of freshly
c
8670 Chem. Commun., 2012, 48, 8670–8672
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cut Na metal to 1-Br2 in THF solution under 1 atm. of N2
afforded the bis(chelate) complex, (k3-Triphos)Fe(k2-Triphos)
[1-(k2-Triphos)], as judged by multinuclear NMR spectroscopy
(eqn (1)). The 1H NMR spectrum of this complex features CH2-
and aryl-resonances that are near their diamagnetic reference
values. Additionally, the 31P NMR spectrum of 1-(k2-Triphos)
revealed five Fe-coordinated phosphorous environments
(resonances at 119.23, 99.11, 82.74, 76.95, and 68.24 ppm)
and a free phosphine arm at ꢀ12.08 ppm (Fig. S3 of the ESIw).
This NMR data, coupled with the lack of a magnetic moment
for 1-(k2-Triphos) at ambient temperature, is consistent with
the formulation of this complex as a low spin Fe(0) complex.
The connectivity of 1-(k2-Triphos) was confirmed by single
crystal X-ray diffraction; however, data worthy of publication
could not be obtained.
Conducting the same reaction with 5 eq. of bpy allowed the
isolation of pure 1-Bpy following recrystallization from a
concentrated ether solution at ꢀ35 1C (eqn (2)). Analysis of
this complex by 1H, 13C, and 31P NMR spectroscopy revealed
that 1-Bpy displays peaks characteristic of a diamagnetic
1
complex. As expected, the H and 13C NMR spectra of 1-Bpy
suggest inequivalence between the two bpy ring environments.
The 31P NMR spectrum of 1-Bpy displayed only 2 resonances
at 112.93 and 91.59 ppm, revealing that the two arms of the
Triphos chelate remain equivalent.
2,20-Bipyridine has thus far enjoyed a rich history in the field
of coordination chemistry13 and its ability to act as a weak
p-acceptor has been well documented in the literature.14
Although 1-Bpy can formally be described as a diamagnetic,
zerovalent iron complex, it was decided that a more detailed
electronic structure investigation of 1-Bpy was warranted. A single
crystal obtained from the recrystallization of 1-Bpy from Et2O at
ꢀ35 1C was analyzed by X-ray diffraction and the molecular
structure determined for this complex is displayed in Fig. 1.
ð1Þ
Alternatively, 1-(k2-Triphos) was prepared from 1-Br3 and
1-Cl3. Although the reduction of 1-Br3 with Na metal in THF
solution proceeded in a similar fashion, the reduction of 1-Cl3
in THF did not proceed readily when Na was used as the
reductant. Replacing Na with 5 eq. of K allowed the formation of
1-(k2-Triphos) from 1-Cl3 within 6 hours at 23 1C. The reduction
of 1-Br2 to 1-(k2-Triphos) was also successful when as little as
2.5 eq. of K was added to a diethyl ether slurry of 1-Br2. Adding
an extra equivalent of Triphos ligand to the Na reduction of 1-Br2
still afforded 1-(k2-Triphos); however, excess free ligand remained
in the product mixture, complicating product purification.
ð2Þ
The metrical parameters of 1-Bpy are consistent with an
approximate square pyramidal coordination geometry about
the iron with P(1)–Fe(1)–P/N angles of 116.38(3), 104.83(7),
95.43(7), and 85.02(3) for P(3), N(2), N(1), and P(2), respec-
tively. More importantly, bpy chelate reduction is also evident
with bond distances of 1.383(3) and 1.399(3) A for N(1)–C(5)
Based on literature precedent, the failure of these reactions to
yield an isolable Fe–N2 complex was disappointing, but perhaps
not overly surprising. Even though Field and co-workers7 were
able to spectroscopically identify [k4-P(CH2CH2PMe2)3]Fe(N2)
and ([k4-P(CH2CH2PMe2)3]Fe)2(m2-N2), attempts to isolate these
complexes were unsuccessful. Upon slow solvent evaporation,
loss of the dinitrogen ligand occurred, resulting in the formation
of an iron(0) tetramer that is held together by the free arms of a
central bis(chelate) moiety closely related to 1-(k2-Triphos).7
Because N2 coordination was not achieved in the experi-
ments described above, the reduction of 1-Br2 in the presence
of more efficient trapping ligands was investigated. Although
we were unable to obtain a single, well-defined iron complex
when 1-Br2 was reduced with excess Na metal in the presence
of 1 atm. of carbon monoxide or 10 eq. of diphenylacetylene,12
adding only one eq. of 2,20-bipyridine (bpy) to the reaction
mixture afforded a mixture of 1-(k2-Triphos) and the newly
identified complex (k3-Triphos)Fe(k2-2,20-bipyridine) (1-Bpy).
Fig. 1 The solid state structure of 1-Bpy at 30% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Relevant bond distances and
angles are mentioned where appropriate within the text.
c
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Chem. Commun., 2012, 48, 8670–8672 8671

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oxidation of the metal center. In addition to challenging
traditional coordination complex redox chemistry, this obser-
vation highlights the importance of electronic structure deter-
mination when studying low-valent metal complexes.
X-ray crystallographic data for complexes 1-Bpy (CCDC
884463) and (k2-Triphos)2FeCl2 (CCDC 884464) in CIF format
have been deposited at the Cambridge Crystallographic Data
Centre. We would like to thank the LANL LDRD Program for
financial support and Dr Brian L. Scott for help with X-ray
crystallography.
Fig. 2 Cyclic voltammogram of 1-Bpy in THF (electrolyte = 0.1 M
NBu4PF6; scan rate = 100 mV sꢀ1). The mid-potentials for bpy
(E1/2(L)) and Fe (E1/2(M)) redox processes are ꢀ1.61 V and ꢀ0.82 V,
respectively.
Notes and references
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and N(2)–C(6), respectively, along with a significantly shortened
C(5)–C(6) bond distance of 1.420(4) A. A striking, comprehensive
electronic structure investigation of eighteen bpy-supported tran-
sition metal complexes was recently reported by Scarborough and
Wieghardt15 providing extensive insight for linking the bond
distances found for 1-Bpy with an accurate electronic structure
description. As described in their report, the bond distances
determined for uncoordinated neutral bpy (bpy0),16 the potassium
salt of bpy monoanion (bpyꢁꢀ),17 and the disodium salt of bpy
dianion (bpy2ꢀ),18 serve as representative examples for each
degree of bpy reduction. Upon comparing each set of reference
distances to the metrical parameters determined for 1-Bpy, it is
clear that this complex features a p-radical monoanionic bpy
ligand. As Scarborough and Wieghardt point out,15 previously
reported bpy complexes that truly possess this chelate as a
p-radical monoanion feature C(5)–C(6) bonds in the narrow
range between 1.41–1.43 A.19 Although most of the 1-Bpy
bipyridine bond lengths are statistically indistinguishable from
the same distances found for K(2,20-bpy)(en),17 the C(5)–C(6)
distance of 1.420(4) A determined for 1-Bpy remains the best
structural indication that the bpy ligand should be described as
a p-radical monoanion.
7 L. D. Field, R. W. Guest and P. Turner, Inorg. Chem., 2010, 49,
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10 The ambient temperature structure of this complex with two
co-crystallized acetone molecules has been reported. M. Di Vaira,
S. Midollini and L. Sacconi, Inorg. Chem., 1981, 20, 3430–3435.
11 R. H. Crabtree, The Organometallic Chemistry of the Transition
Metals, Wiley & Sons, Inc., Hoboken, NJ, 4th edn, 2005, p. 12.
12 Reduction of 1-Br2 with Na in the presence of CO resulted in a
mixture of complexes believed to be of the type previously described
in R. B. King, P. N. Kapoor and R. N. Kapoor, Inorg. Chem.,
1971, 10, 1841–1850. Reduction in the presence of 10 eq. of
diphenylacetylene afforded 1-(k2-Triphos).
13 E. C. Constable, Adv. Inorg. Chem., 1989, 34, 1–37.
14 H. Tom Dieck, K.-D. Franz and F. Hohmann, Chem. Ber., 1975,
108, 163–173.
15 C. C. Scarborough and K. Wieghardt, Inorg. Chem., 2011, 50,
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16 M. H. Chisholm, J. C. Huffman, I. P. Rothwell, P. G. Bradley, N. Kress
and W. H. Woodruff, J. Am. Chem. Soc., 1981, 103, 4945–4947.
17 E. Gore-Randall, M. Irwin, M. S. Denning and J. M. Goicoechea,
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Remembering that 1-Bpy is diamagnetic, the electronic
structure of this complex should be assigned as having a singly
reduced p-radical bpy ligand that is antiferromagnetically coupled
to a low spin Fe(I) center.20 To further bolster this claim, 1-Bpy
was also investigated by electrochemistry. Starting from isolated
1-Bpy, cyclic voltammetry shows two reversible oxidations as the
potentials are scanned from ꢀ2.4 to 0.0 V (Fig. 2). The first, with
mid-potential at ꢀ1.61 V, is typical of the bpyꢁꢀ/bpy0 couple
(E1/2(L)),21 while the second process at ꢀ0.82 V corresponds to
the metal-based FeI/FeII couple (E1/2(M)). Compared to 1-Br2
(M)
(E1/2
= ꢀ1.20 V), the positive shift of >0.4 V for the
FeI/FeII redox potential of 1-Bpy is consistent with the repla-
cement of the charge-donating anionic ligands in 1-Br2 by the
charge-accepting bidentate ligand in 1-Bpy. Additionally, since
no evidence for reduction of Fe(I) to Fe(0) was found in a
range of potentials as low as ꢀ2.5 V, the formulation of 1-Bpy
as Fe(I)-(bpyꢁꢀ) instead of Fe(0)-(bpy0) is fully supported.
Importantly, heating a solution of 1-(k2-Triphos) in the presence
of 5 eq. of bpy to 80 1C for 23.5 hours resulted in clean conversion
to 1-Bpy and Triphos as judged by multinuclear NMR spectro-
scopy. Based on the electronic structure description of 1-Bpy,
this chelate substitution therefore leads to a one electron
c
8672 Chem. Commun., 2012, 48, 8670–8672
This journal is The Royal Society of Chemistry 2012