Reactivity of three-coordinate iron-NHC complexes towards phenylselenol and lithium phenylselenide

Article abstract of DOI:10.1039/c3dt53203h

The three-coordinate iron(ii) NHC complexes [(IPr)Fe(N′′) ₂] (1) and [(I^tBu)Fe(N′′)₂] (3) (N′′ = N(SiMe₃)₂) react with PhSeH or LiSePh to give the iron(ii) selenolates [(IPr)Fe(N′′)(SePh)] (6) and [I<

Full text of DOI:10.1039/c3dt53203h

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Reactivity of three-coordinate iron–NHC
complexes towards phenylselenol and lithium
phenylselenide†
Cite this: Dalton Trans., 2014, 43,
4251
Received 13th November 2013,
Accepted 27th November 2013
Thomas Pugh and Richard A. Layfield*
DOI: 10.1039/c3dt53203h
www.rsc.org/dalton
The three-coordinate iron(II) NHC complexes [(IPr)Fe(N’’)2] (1) and
[(ItBu)Fe(N’’)2] (3) (N’’ = N(SiMe3)2) react with PhSeH or LiSePh to
give the iron(II) selenolates [(IPr)Fe(N’’)(SePh)] (6) and [ItBu(H)]-
[(aItBu)Fe(SePh)3], [ItBu(H)][7], with complex 7 containing an abnor-
mal NHC ligand.
N-heterocyclic carbene (NHC) complexes of iron have been
known for over 40 years,1 yet, despite these early studies, iron–
NHC chemistry has evolved at slow pace relative to the NHC
chemistry of late transition metals.2 However, in the last five
years, iron–NHC chemistry has grown at a more rapid rate,3
which is due largely to the increasing number of applications
of iron–NHC complexes in homogeneous catalysis.4 In
addition to catalytic applications of iron–NHC complexes,
their chemistry has also been used to develop structural and/
or functional models of iron coordination environments of
relevance to biological systems, such as nitrogenase enzymes5
and iron nitrosyl complexes.6 Tripodal NHC ligands have also
enabled the detailed structural characterization of iron coordi-
nation environments that were otherwise difficult or imposs-
ible to stabilize, such as a terminal nitride complex of iron(V).7
Reflecting on this emerging field as a whole, it is apparent that
many significant advances in iron chemistry have been
achieved through the use of NHC ligands.
Three-coordinate iron complexes have provided a source of
intrigue for many years.8 Although several examples of three-
coordinate iron NHC complexes have been structurally charac-
terized,9 their reactivity has not yet been studied to any great
extent. Our own work has focused on the bulky three-coordi-
nate complexes [(NHC)Fe(N″)2] (N″ = N(SiMe3)2), in which the
NHC ligands are 1,3-bis(2,6-diisopropylphenyl)imidazole-2-
ylidene (IPr, complex 1), 1,3-bis(mesityl)imidazole-2-ylidene
(IMes, complex 2) and 1,3-bis(tert-butyl)imidazole-2-ylidene
Chart 1 N’’ = N(SiMe3)2.
(ItBu, complex 3) (Chart 1).10 In a recent study, we have shown
that the NHC ligands in 1 and 3 are thermally unstable.11 In
the case of the sterically hindered complex [(IPr)Fe(N″)2] (1),
refluxing in toluene for 2–3 hours results in a rearrangement
of the normal IPr ligand to its abnormal/meso-ionic isomer
[(aIPr)Fe(N″)2] (4) in order to relieve steric pressure. In con-
trast, refluxing [(ItBu)Fe(N″)2] (3) in toluene for prolonged
periods results in one tert-butyl substituent per NHC ligand
being eliminated as isobutene, in what appears to be a con-
secutive C–H/C–N bond activation process, and which results
in the formation of the tetrahedral iron(II) bis(imidazole)
complex [(tBuIm)2Fe(N″)2] (5).
Having established that 1 and 3 are sensitive to heat, we
have now begun to explore their chemical reactivity, particu-
larly towards Brønsted acids and organolithium reagents. The
reactions of iron–NHC complexes towards thiols are well estab-
lished, especially in the context of bio-mimetic chemistry,5,6,12
however analogous chemistry with selenium is not well develo-
ped. Our initial reactivity studies have therefore focused on the
reactions of 1 and 3 with phenylselenol, PhSeH, and lithium
phenylselenide, PhSeLi (Scheme 1).
The reaction of 1 with two stoichiometric equivalents of
PhSeH or PhSeLi in toluene solvent was undertaken in order
to synthesize the three-coordinate iron bis(selenolate) complex
[(IPr)Fe(SePh)2] (5). However, the product of these reactions
was the mono-substituted complex [(IPr)Fe(N″)(SePh)] (6),
which was isolated as colourless crystals. Compound 6 can
also be obtained from the reaction of 1 with one equivalent of
PhSeE (E = H, Li). The molecular structure of 6 (Fig. 1) was
School of Chemistry, The University of Manchester, Oxford Road, Manchester,
M13 9PL, UK. E-mail: Richard.Layfield@manchester.ac.uk
†Electronic supplementary information (ESI) available: Synthetic details and
characterization. Crystallographic details in .cif format. CCDC 971588–971590.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53203h
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contain a selenium donor ligand. The structures of two
dimetallic iron–NHC complexes with additional selenium ligation
have been reported previously, both of which contain five-
coordinate iron and an iron–iron bond.15
The observation that double substitution of the amido
ligands in 1 to give [(IPr)Fe(SePh)2] (5) does not occur is intri-
guing. To investigate the possibility that the extreme steric
bulk imparted by the IPr ligand influences the reactivity of 1
towards phenylselenol, the analogous reaction with another
bulky NHC, ItBu, was carried out.16 Surprisingly, the reaction
of 3 with 0.75 equivalents of phenylselenol in toluene solvent,
followed by work-up in thf, resulted in the formation of [ItBu(H)]-
[(aItBu)Fe(SePh)3]·thf, [ItBu(H)][7]·thf, as light-yellow crys-
tals (Scheme 1, Fig. 2 and Table S1†). In the monoanion 7, the
iron centre is complexed by an abnormal, or meso-ionic, aItBu
ligand in addition to three phenylselenolate ligands. The iron
centre occupies a distorted tetrahedral environment, with an
Fe(1)–C(1) distance of 2.099(5) Å and Fe–Se distances in the
range 2.4812(8)–2.4995(8) Å. The angles formed by the ligand
donor atoms and iron are 88.96(3) and 107.39(11)–115.89(13)°
(average 109.1°).
Scheme 1 E = H or Li.
1
The H NMR spectrum of [ItBu(H)][7] in acetonitrile-d3 fea-
tures a series of resonances in the range δ(1H) = +33.99 to
−17.25 ppm, including several very broad, low-intensity reso-
nances. The ortho- and meta- 1H environments on the PhSe
ligands in 7 occur at δ(1H) = 17.86, 16.83, −16.29 and
1
−17.25 ppm, with the resonances due to the para H environ-
ments probably occurring at δ(1H) = −9.17 and −11.23 ppm
(the rationale for these assignments is provided by obser-
vations on complex 8 – see below). The resonances due to
[ItBu(H)]+ cation occur at 1.58, 6.70 and 7.46 ppm, suggesting that
the other resonances in the spectrum should be due to the
aItBu ligand. We tentatively assign the resonance at δ(1H) =
33.99 ppm to the imidazolylidene 1H bonded to the carbon
Fig. 1 Molecular structure of 6 (50% thermal ellipsoids). Hydrogen
atoms not shown. Unlabelled atoms are silicon (grey) and carbon
(black).
determined by X-ray crystallography (Table S1†), and features that is adjacent to the carbene donor atom, which is in agree-
an iron centre in a distorted trigonal planar environment, with ment with our previous studies on iron(II) NHC complexes.10,11
Fe(1)–C(1), Fe(1)–N(3) and Fe(1)–Se(1) bond distances of 2.124(3), The two inequivalent tert-butyl groups in 7 occur at δ(1H) =
1.926(2), and 2.4351(3) Å, respectively, and C–Fe–N, N–Fe– 11.60 and 4.26 ppm, on the basis that they have relatively high
Se and Se–Fe–C bond angles of 125.53(11), 119.99(8) and intensity and are broad, which indicates that the methyl
114.52(8)°, respectively (sum of 360°). Although the Fe–C bond groups are magnetically inequivalent on the NMR timescale at
1
in 6 is approximately 0.06 Å shorter than that in 1, it is still room temperature; these features were also observed in the H
relatively long for an iron–NHC complex, according to the NMR spectrum of 3. The only remaining resonance is the back-
1
average distance of 1.994 Å in the Cambridge Structural Data- bone imidazolylidene H at δ(1H) = −2.39 ppm. The solution-
base (CSD).13
1
The H NMR spectrum of 6 in toluene-d8 at room tempera-
ture reflects the low-symmetry of the molecule (Fig. S1†). A
series of broad, overlapping resonances occur in the range
δ(1H) = 0.29–9.69 ppm, in addition to a very broad, weak down-
field resonance at δ(1H) = 51.97 ppm and three sharper up-
field resonances at δ(1H) = −16.25, −19.12 and −42.12 ppm.
The solution-phase effective magnetic moment of 6 was deter-
mined by the Evans method to be μeff = 5.2(6)μB,14 which indi-
cates that the iron(II) centre has
a high-spin S = 2
configuration, with a contribution from spin–orbit coupling.
Complex 6 extends the relatively small family of three-co-
Fig. 2 Molecular structure of 7 (50% thermal ellipsoids). Hydrogen
atoms are not shown. Unlabelled atoms are silicon (grey) and carbon
ordinate iron–NHC complexes, and it is the first example to (black).
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phase effective magnetic moment for [ItBu(H)][7] is μeff
=
a rapid rate relative to the rate of the rearrangement of ItBu to
5.9(6)μB, which is consistent with high-spin tetrahedral iron(II).17 aItBu. In attempting to relate complexes 6–8, complex 8 can, in
In contrast to the deprotonation of the tert-butyl substitu- a thermodynamic sense, be regarded as the end product of a
ents by the [(Me3Si)2N]− ligands in 3 upon heating, the substi- sequence of reactions in which the amido and NHC ligands
tuents on the ItBu ligands in 3 are retained in complex 7. This are sequentially replaced by selenolate ligands. The mono-sele-
suggests that deprotonation of PhSeH by the [(Me3Si)2N]− nolate complex 6 can therefore be regarded as the first step in
ligands is relatively rapid, and also that the [PhSe]− ligands in the formation of 8, and the tris(selenolate) complex 7 can be
7 are too weakly basic to deprotonate the tert-butyl substitu- regarded as an intermediate species. The missing link for this
ents and, hence, initiate elimination of isobutene. The chemistry is therefore a bis(selenolate) complex of iron(II),
normal-to-abnormal rearrangement of the ItBu ligand probably which has yet to reveal itself to us, although our on-going
occurs in order to reduce steric congestion around the iron studies will attempt to identify such a species.
coordination environment, which is consistent with obser-
vations made on the 2,6-dipp-substituted normal NHC
complex 1 and its abnormal isomer 4.11 Although a reduction
in steric bulk is likely to initiate the normal-to-abnormal
Conclusions
rearrangement of 3 to 7, the Fe–C bond in 7 is approximately
Our initial studies on the chemistry of the three-coordinate
0.05 Å shorter than that of 2.151(2) Å in 3, which, coupled with
iron(II) complexes [(NHC)Fe(N″)2] have provided evidence of
the stronger Lewis basicity of the abnormal ItBu ligand relative
complicated reactivity towards phenylselenol and lithium phenyl-
to its normal isomer, should also produce a stronger Fe–C
selenide. In addition to the tendency of the NHC ligands to
bond.
undergo normal-to-abnormal rearrangements, it is also appar-
Although transition metal complexes of abnormal, or meso-
ent that the relatively weak Fe–C bonds can be cleaved by the
ionic, NHC ligands are well known in late transition metal
Brønsted acid PhSeH. Future work will study the reactivity of
chemistry,18 abnormal iron–NHC complexes are uncom-
[(NHC)Fe(N″)2] with a range of other substrates.
mon.9h,19,20 The abnormal ItBu ligand has not previously been
observed for iron, however it is known in late transition metal
chemistry, with examples including osmium and ruthenium
carbonyl clusters21 and the square-planar platinum(II) complex
Notes and references
[(ItBu)(aItBu)PtMe2].22 Examples of the aItBu ligand in main
group chemistry include the tetrahedral aluminium complex
[(aItBu)AlMe3],23 and a sulphide-bridged dimetallic tin(IV)
cation ligated by two aItBu ligands.24
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of this compound could be achieved by combining complex 3
with ItBu and PhSeH in the ‘correct’ stoichiometric ratio of
1 : 1 : 3. However, yet another surprising outcome was
observed, in which the only isolable product of the reaction
was [ItBu(H)]2[Fe(SePh)4]·(2MeCN), the structure of which was
determined by X-ray crystallography (Fig. S3 and Table S1†).
Upon reaction with phenylselenol, all the amido and NHC
ligands in 3 are replaced by phenylselenolate ligands to give
the tetrahedral complex anion [Fe(SePh)4]2− (8) (Scheme 1).
Complex 8 has no remarkable structural features (Fig. S3†),
with the Fe(1)–Se(1/1A) and Fe(1)–Se(2/2A) bond lengths of
2.4559(5) Å and 2.4522(5) Å, respectively, being very similar to
those in the only other two examples of [Q][Fe(SePh)4] com-
pounds in the CSD, where Q = [(NEt4)+]2 or [FeII(1,10-
phen)3]2+.25 The 1H NMR spectrum of [ItBu(H)]2[8] in aceto-
nitrile-d3 is relatively straightforward to interpret (Fig. S4†),
with the resonances due to the [ItBu(H)]+ cation being
observed at δ(1H) = 8.31, 7.44 and 1.56 ppm, and the reso-
nances due to 8 occurring at δ(1H) = 17.85 and −16.29 ppm
(ortho and meta Ph), and δ(1H) = −9.16 ppm (para Ph).
The solution-phase effective magnetic moment of 8 is μeff
=
5.4(2)μB, which is similar to that of 7.
The formation of [ItBu(H)]2[8] suggests that phenylselenol
is deprotonated by the amido ligands in 3, and also by ItBu, at
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