Hydroboration of terminal olefins with pinacolborane catalyzed by new 2-iminopyrrolyl iron(ii) complexes

Article abstract of DOI:10.1039/c8cy02319k

Four paramagnetic 14-electron tetracoordinated Fe(ii) complexes of 5-substituted-2-iminopyrrolyl ligands of the type [Fe{κ²N,N′-5-R-NC₄H₂-2-C(H)N(2,6-ⁱPr₂-C₆H₃)}(Py)Cl], with R = 2,6-Me₂-C₆H₃ (1a), 2,4,6-ⁱPr₃-C₆H₂ (1b), 2,4,6-Ph₃-C₆H₃ (1c) and CPh₃ (1d), were synthesized in moderate yields by reacting the respective 5-substituted-2-iminopyrrolyl potassium salts KLa-d with FeCl₂(Py)₄ in toluene. Complexes 1a-d were characterized by ¹H NMR, FTIR spectroscopies, elemental analysis and by the Evans method, the corresponding effective magnetic moments showing a high-spin electronic nature. X-ray diffraction studies on complexes 1a and 1c showed distorted tetrahedral coordination geometries. Complexes 1a-c, activated with K(HBEt₃), were efficient catalyst systems for the hydroboration of several terminal alkenes with pinacolborane in good to high yields (50-90%). This system mainly yielded the respective anti-Markovnikov addition products, except when styrenes were used. A screening of the hydroboration of styrene catalyzed by complexes 1a-c activated with K(HBEt₃) showed that the selectivity in the Markovnikov product increased with increasing steric bulkiness of the R group, exhibiting selectivities up to 91%. Additionally, the stoichiometric reaction of complex 1b with K(HBEt₃) over 30 minutes yielded the mixture of hydride species 2 and 2₂ (mixture I). On the other hand, when reacting the same components over 16 h, the Fe(i) complex 3 was also identified in the mixture, in addition to 2 + 2₂ (mixture II). These mixtures were characterized in solution by the Evans method and in the solid state by elemental analysis, ⁵⁷Fe M?ssbauer and FTIR spectroscopies, compounds 2₂ and 3 being also analyzed by X-ray diffraction. These results suggest that the corresponding catalytic cycle follows the borane oxidative addition route to a Fe(i) species.

Full text of DOI:10.1039/c8cy02319k

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Page 1 of 31
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DOI: 10.1039/C8CY02319K
Hydroboration of terminal olefins with pinacolborane catalyzed
by new 2-iminopyrrolyl iron(II) complexes
Tiago F. C. Cruz,a Laura C. J. Pereira,b João C. Waerenborgh,b Luís F. Veirosa and
Pedro T. Gomes*a
a Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior
Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal.
b C2TN-Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico,
Universidade de Lisboa, 2695-066 Bobadela LRS, Portugal.
ABSTRACT: Four paramagnetic 14-electron tetracoordinated Fe(II) complexes of 5-
substituted-2-iminopyrrolyl ligands of the type [Fe{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-iPr2-
C6H3)}(Py)Cl], with R = 2,6-Me2-C6H3 (1a), 2,4,6-iPr3-C6H2 (1b), 2,4,6-Ph3-C6H3 (1c) and
CPh3 (1d), were synthesized in moderate yields by reacting the respective 5-substituted-2-
iminopyrrolyl potassium salts KLa-d with FeCl2(Py)4 in toluene. Complexes 1a-d were
1
characterized by H NMR and FTIR spectroscopies, elemental analysis, and by the Evans
method, the corresponding effective magnetic moments showing a high-spin electronic
nature. X-ray diffraction studies on complexes 1a and 1c showed distorted tetrahedral
coordination geometries. Complexes 1a-c, activated with K(HBEt3), were efficient catalyst
systems for the hydroboration of several terminal alkenes with pinacolborane in good to high
yields (50-90 %). This system mainly yielded the respective anti-Markovnikov addition
products, except when styrenes were used. A screening of the hydroboration of styrene
catalyzed by complexes 1a-c activated with K(HBEt3) showed that the selectivity in the
Markovnikov product increased with increasing steric bulkiness of the R group, exhibiting
selectivities up to 91%. Additionally, the stoichiometric reaction of complex 1b with
K(HBEt3) over 30 minutes yielded the mixture of hydride species 2 and 22 (mixture I). On the
other hand, when reacting the same components over 16 h, the Fe(I) complex 3 was also
identified in the mixture, in addition to 2 + 22 (mixture II). These mixtures were characterized
in solution by the Evans method and in the solid state by elemental analysis, 57Fe Mössbauer
and FTIR spectroscopies, compounds 22 and 3 being also analyzed by X-ray diffraction.
These results suggest that the corresponding catalytic cycle follows the borane oxidative
addition route to a Fe(I) species.
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DOI: 10.1039/C8CY02319K
Keywords: 2-iminopyrrolyl ligands; tetracoordinated iron(II) complexes; hydroboration of
terminal alkenes; iron(II) hydride complexes; iron(I) arene complexes.
2

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Introduction
DOI: 10.1039/C8CY02319K
The use of organoboron compounds in organic synthesis paves the way to carbon-heteroatom
bond formation, for example via stereospecific cross-coupling or oxidation reactions, making
them an important class of reagents.1 An important source of organoboranes is the
hydroboration of alkenes, which has often been catalyzed by platinum group elements, such
as rhodium or iridium.2 The need for metal-catalyzed hydroboration is justified as it allows
the use of rather unreactive boranes, such as pinacolborane (HBPin), which are often more
selective in subsequent organic synthetic steps. The expensive and rather toxic characteristics
of the Rh or Ir elements have led researchers to develop alternative cheap and abundant
mediators.3 For these reasons, iron and cobalt have been increasingly used as metals in
catalytic alkene hydroboration reactions.4
As far as utilizing iron as the metal in complexes capable of catalyzing hydroboration of
alkenes with HBPin is concerned, some works are to be noted. Chirik and co-workers
developed and used [bis(imino)pyridine]Fe(N2)2 or [bis(imino)pyridine]FeCl2 complexes (A,
Chart 1), the latter activated by Na(HBEt3), in the hydroboration of alkenes with HBPin.4a
Iron complexes using monoanionic ligands have also been explored in hydroboration
reactions. Szymczak et al., used
pyridylimino)isoindolate]FeBr for hydroboration regulated by an outer-sphere effect,
activated by Also, P,N bidentate complex (N-
K(HBEt3).4k
a
N,N,N tridentate [1,3-bis(6’-methyl-2’-
a
phosphinoamidinate)Fe[N(SiMe3)2] (B, Chart 1), reported by Turculet et al., catalyzed
hydroborations in neat and mild conditions.4e All the examples mentioned above led to an
anti-Markovnikov selectivity of the addition products.
In contrast, Thomas et al. used an alkoxy-tethered N-heterocyclic carbene Fe(II) complex
(C, Chart 1) that proved to be Markovnikov-selective towards the hydroboration of alkenes.4g
In a similar approach, Webster and co-workers reiterated the importance of ligand design in
the selectivity of hydroboration products, using LFe(CH2SiMe3) complexes (D, Chart 1), with
L being a β-diketiminate ligand of varying stereochemical bulkiness.4j
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DOI: 10.1039/C8CY02319K
Chart 1 Iron complexes used in the hydroboration of terminal alkenes.
The coordination chemistry of Fe with a single monoanionic bidentate N,N ligand bearing a
chloride co-ligand is not extensively explored. The N,N β-diketiminate ligand system has
allowed for the successful preparation of complexes of the type L1FeCl and L2Fe(µ-
Cl)2Li(THF)2, or [L3FeX]2, with X=Cl or Br, with L being bulky β-diketiminate ligands (A,
Chart 2), reported by Holland et al.5 With a different system, the complexes of the type
[LFeCl]2, with L being a bulky amidinate ligand (B, Chart 2), were prepared by Jones et. al.6
Betley and co-workers also synthesized complexes of the type LFeClPy bearing
dipyrromethane ligand derivatives (C, Chart 2).7
The chemistry of iron bearing the 2-iminopyrrolyl system is very limited and only
involves the inclusion of two ligands of that type. The first reported case was the bis[2-N-
(arylimino)pyrrolyl] iron complex, FeL2, with L being 2,6-bis(imino)pyrrolyl ligands with
only one of the imine arms coordinated to Fe (D ,Chart 2), by Bochmann and co-workers.8
Sun et al. also reported the FeL2(PMe3)2 complexes, with L being a 2-benzyliminopyrrolyl
ligand, via a NH bond activation reaction with Fe(PMe3)4.9 Lately, we have been interested in
the coordination chemistry of complexes bearing 2-iminopyrrolyl ligands, having prepared
compounds of the late-transition metals Co,10 Ni and Cu11 and Zn,12 all having at least two 2-
iminopyrrolyl or 2-iminophenanthropyrrolyl ligands. We have also prepared Na13 and B14
compounds of these ligands and have been able to crystalographically characterize the Fe(III)
oxo complexes bearing two 2-iminopyrrolyl ligands, highlighting the high sensitivity to air
and moisture of the putative Fe[2-iminopyrrolyl]2. However, we managed to stabilize that
fragment with the addition of a pyridine (Py) ligand (E, Chart 2).11
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DOI: 10.1039/C8CY02319K
Chart 2 Low-coordinate iron(II) halide complexes (A–C) and 2-iminopyrrolyl iron(II) (D–F) and cobalt(II) (F, G)
complexes.
Our group has recently reported the preparation of bulky 5-substituted-2-iminopyrrole ligand
precursors, from their corresponding 5-substituted-2-formylpyrroles,15 and subsequently used
them to prepare homoleptic [M(5-substituted-2-iminopyrrolyl)2] complexes of Fe(II) and
Co(II) (F, Chart 2).16 In a subsequent work, we were able to avoid the synthesis of bis
chelates and prepared a family of mono(5-substituted-2-iminopyrrolyl) Co(II) complexes (G,
Chart 2), which proved to be active in the hydroboration of terminal alkenes upon activation
by K(HBEt3),17 as well as aryl Ni(II) complexes containing a single 5-substituted-2-
iminopyrrolyl ligand that are active in the production of hyperbranched polyethylene.18
Taking these results into account, we now present the extension of the coordination chemistry
of the bulky 5-substituted-2-iminopyrrolyl ligands to iron, and the application of these new
mono(2-iminopyrrolyl) complexes as efficient precatalysts for the hydroboration of terminal
alkenes with HBPin.
Results and discussion
Synthesis and characterization of the pyridine chloride iron(II) precatalysts: The Fe(II)
complexes of the type [Fe{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl] (1a-d) were
prepared in moderate yields by the metathetic salt reaction of FeCl2(Py)4 with the respective
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potassium salts KLa-d17 in toluene, at 80 ºC (Scheme 1). Complexes 1a-d were isolated as
DOI: 10.1039/C8CY02319K
orange-red powders or crystals from concentrated toluene/n-hexane solutions after standard
work-up procedures.
Scheme 1 Synthesis of the pyridine chloride iron(II) complexes 1a-d.
All complexes are paramagnetic and are very sensitive to air and moisture both in solution
and in solid state, since they are highly unsaturated 14-electron species, with two potentially
labile cis coordination sites. These complexes are partially soluble in n-hexane and Et2O and
soluble in toluene. Preliminary synthesis attempts of chloride Fe(II) complexes of this ligand
system were made by reacting the referred potassium salts with FeCl2 in THF. This pathway,
however, was marred by very troublesome purification procedures and unexpected
decomposition of the reaction intermediates. The respective combustion analyses of these
reactions proved to be inconclusive, pointing to adducts of the mono chelated chloride
complexes with KCl and THF.
Complexes 1a-d were characterized in solution by 1H NMR spectroscopy, their respective
spectra being shown in Figures S1-S4 of the ESI. Complexes 1a-d display paramagnetically
1
shifted H NMR spectra, which resonances can tentatively be assigned to their respective 5-
substituted-2-iminopyrrolyl chelates and to the pyridine ligand.
In toluene-d8 solutions, complexes 1a-d showed effective magnetic moments (measured
by the Evans method19) in the range of 4.6-5.3 µB (see Table 1), which is characteristic of a d6
metal ion in the high-spin state (S = 2; µeff (spin-only) = 4.9 µB) with some degree of spin-
orbit coupling effects.20 Further studies in the solid state were performed on complex 1a by
SQUID magnetometry (see Figure S14 of the ESI). As expected for a tetracoordinated Fe(II)
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complex, this compound is paramagnetic, with S = 2. At room temperature, μeff = 4.86 μB, in
DOI: 10.1039/C8CY02319K
good agreement with the value obtained in solution.
Table 1 Effective magnetic moments μeff (μB) for the Fe(II) complexes 1a-d, measured in toluene-d8 solution (Evans
method), at room temperature.
Complex μeff (μB)
1a
1b
1c
1d
5.3
4.8
4.6
4.7
Complexes 1a-d were characterized by FTIR spectroscopy, the respective spectra being
shown in Figures S7-S10 of the ESI. As expected, the spectra of the four complexes are very
similar, being possible to observe the typically sharp C=N bond stretching vibration bands, at
1561-1573 cm-1.
Complexes 1a and 1c were also characterized in the solid state by X-ray diffraction.
Crystals of 1a and 1c were obtained from a toluene:n-hexane solution (1:3 in volume) cooled
to -20 ºC, and crystalized in the monoclinic system in the P21 and P21/c space groups,
respectively. The structure of complex 1a is shown in Figure 1, with selected bond lengths
and angles being listed in Table S1 of the ESI. The structure of complex 1c, owing to the
poor quality of its diffraction data, is only shown in the ESI (Figure S13 and Table S1) as
additional structural evidence.
In complex 1a, it is possible to observe that a single 5-aryl-2-iminopyrrolyl ligand is
coordinated to the metal center in a bidentate fashion through the pyrrolyl (N1) and iminic
(N2) nitrogen atoms in a near planar five-membered chelate (Fe1−N1−C2−C6−N2). The
tetracoordinated metal centers are further bonded to a chlorine atom and a pyridine ligand.
The Fe−N bond lengths are in the range of 2.0308(19)−2.112(2) Å, in the order
Fe1−N1<Fe1−N3<Fe1−N2, very likely associated with the decreasing degree of the σ
character of the respective bonds. The Fe1−N bond lengths are longer than in the analogous
Co(II) complexes [Co{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl] (R=2,6-Me2-
C6H3 or 2,4,6-Ph3-C6H2) reported by our group,17 attributed to the lower electronegativity
(higher ionic radius) of the Fe(II) center. The τ4 parameter for complex 1a is 0.74 (τ4 = 0 for
an ideal square planar geometry and τ4 = 1 for an ideal tetrahedral geometry21), thus
corresponding to a distorted tetrahedral geometry.
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DOI: 10.1039/C8CY02319K
Figure 1 ORTEP-3 diagram for complex 1a showing 30% probability ellipsoids. All hydrogen atoms were omitted for
clarity.
The molecular structure of the complexes presented above was probed by DFT
calculations.22 Geometry optimizations were performed for the family of complexes 1a-d in
the high spin electronic state, S = 2. The atomic coordinates of all optimized complexes are
presented in the ESI. All complexes exhibit a distorted tetrahedral geometry and their τ4
parameters are in the range 0.73−0.77, comparing well with the experimentally determined
structures of complexes 1a and 1c. The calculated Fe-N bond lengths lie in the range
2.025−2.184 Å and in the order Fe−N1<Fe−N3<Fe−N2. On the other hand, the Fe−Cl bond
lengths are in the range 2.241−2.260 Å, being almost invariant with the 5-substituent. When
comparing the calculated bond lengths with the ones determined by X-ray diffraction (in
complexes 1a and 1c) one can see a maximum absolute deviation of 0.08 Å. These results are
in good agreement and reinforce that the experimentally determined high-spin electronic
configurations are corroborated by DFT calculations.
Complex 1a was also characterized by Mössbauer spectroscopy. The Mössbauer spectra
taken at 4 and 295 K (Figure S15 of the ESI) consist of symmetric two-peak patterns, which
are fitted with single quadrupole doublets in agreement with a single crystallographic site for
Fe in 1a. The estimated isomer shifts, IS = 0.88 mm s-1 at 4 K and 0.77 mm s-1 at 295 K
(Table S5 of the ESI), are consistent with high-spin Fe(II) (S = 2).23,24 The increase of IS with
increasing temperature is explained by the second order Doppler shift. The quadrupole
splitting values (QS = 2.41 mm/s at 4 K and 2.16 mm/s at 295 K) as well as their temperature
dependence are also typical of high-spin Fe(II).
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Catalytic hydroboration studies: The coordinative and electronic unsaturation of the Fe(II)
DOI: 10.1039/C8CY02319K
complexes 1a-d showed that they are good candidates to precatalysts for
hydrofunctionalization reactions. We were also driven by the encouraging results obtained in
the hydroboration of terminal alkenes with pinacolborane (HBPin) precatalyzed by our
analogous Co(II) complexes, activated by K(HBEt3).17 Firstly, we analyzed the α-olefin
scope with the 1a/K(HBEt3) catalytic system. The results for the substrate scope study with
1
the 1a/K(HBEt3) system are presented in Table 2 and the H NMR spectra of the
alkylboronate products are shown in Figures S16-S26 of the ESI. The system is
hydroboration inactive either in the absence of activation by K(HBEt3) or in the absence of
complexes 1a-d.
Table 2 Substrate scope of the hydroboration of terminal alkenes catalyzed by the system 1a/K(HBEt3).
Substratea
Yield (%)b
92
Selectivity (a-Mk:Mk)c
0.39:1
54
54
0.36:1
0.38:1
58
59
2.23:1
1.32:1
51
36
2.13:1
-
a Conditions: 1 mol% of 1a, 3 mol% of K(HBEt3), 2 mmol of substrate, 2.5 mmol
b
of HBPin, reaction time: 16 h, temperature: 25 ºC. Yields determined by
weighing the isolated reaction products. c Calculated by 1H NMR.
It is possible to see that the 1a/K(HBEt3) catalyst system gives rise to a mixture of anti-
Markovnikov/Markovnikov addition products of the respective terminal alkenes (a-Mk/Mk)
in good yields (51-92%), under neat and mild conditions. No isomerization was observed in
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allylic olefins such as 1-hexene or allyltrimethylsilane. This catalyst system also hDyOdI:r1o0b.1o03Vr9iae/wtCeA8srCticYl0e2O3n1l9inKe
cyclohexene in moderate yield.
It is also notably observed that by screening styrenes para-substituted with electronically
differentiated groups (1-fluoro-4-vinylbenzene, 1-methyl-4-vinylbenzene) in the same
hydroboration reaction, the regioselectivity remains unaltered. Additionally, the present
catalyst system was tested with the sterically hindered 1,3,5-trimethyl-2-vinylbenzene,
revealing very low conversion (9%) and selectivity towards hydroboration, other
uncharacterized products being present after work-up of the catalytic reaction (see Figures
S19 of the ESI), being though possible to detect a a-Mk/Mk ratio of 0.35:1. Therefore, the
styrene substrate scope shows that this catalyst system is sensitive to stereochemical
constraints but seems to be independent of electronic variations in the substrate, the
hydroboration regioselectivity depending solely on the bulkiness of the chelating ligand of
the precatalyst used.
In general, from the substrate scope analysis, it is clear that this system has the tendency to
systematically generate a notable amount of the Mk addition product, a fact that was not
observed in our Co(II) system, which always showed exclusive a-Mk selectivity.17 However,
in this mixture of a-Mk and Mk addition products, the 1a/K(HBEt3) system mainly produces
the a-Mk one, except in the case of styrene, in which the selectivity is reversed, the major
isomer produced being the Mk addition product.
As far as styrene is concerned, a major selectivity in the Mk addition product was
observed. Therefore, similarly to our Co(II) system, we evaluated the effect of the different
precatalysts 1a-d in the hydroboration of styrene. The results for the hydroboration of styrene
with HBPin using the different Fe(II) precatalysts 1a-d activated by K(HBEt3) are shown in
Table 3. Also, a superimposition of the 1H NMR spectra of the α-protons relative to the boron
atom of the products of the hydroboration of styrene catalyzed by 1a-d/K(HBEt3) is shown in
Figure S27 of the ESI.
The Fe(II) pre-catalysts 1a-d activated by K(HBEt3) yielded mixtures of products in
moderate to good yields, where the Mk product is systematically the major isomer. It can be
concluded that by increasing the steric bulkiness of the ortho groups of the 5-aryl substituent,
going from methyl (in complex 1a), to isopropyl (in complex 1b) to phenyl (in complex 1c),
the system increases the selectivity in the Markovnikov product. In fact, precatalyst 1c gave
rise to a 91% selectivity in the Mk product. Complex 1d did give rise to a 0.15:1 a-MK:Mk
ratio of addition products of styrene. However, catalysis by this latter complex was not
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completely selective in the hydroboration of styrene, yielding a complex mixtureDoOfI: p10r.1o0d39u/cC8tsCY02319K
(the 1H NMR spectrum of the isolated products is presented in Figure S22 of the ESI).
The yields in the organoboranes produced in this work are in the range of those obtained
by other authors that reported iron-catalyzed hydroboration of the same substrates,4e,g,j,k,l but
can only be considered as moderate when compared with the best results reported to date. In
fact, some authors reported yields as high as 98 % for some of the substrates presented in this
work, in less than an hour, in similar reaction conditions.4a,m
Table 3 Structure/selectivity relationship for the hydroboration of styrene catalyzed by the system 1a-d/K(HBEt3).
Complexa
Yield (%)b
Selectivity (a-Mk:Mk)c
1a
1b
1c
1d
92
92
0.39:1
0.32:1
0.09:1
0.15:1d
51
31d
a Conditions: 1 mol% of 1a-d 3 mol% of K(HBEt3), 2 mmol of styrene, 2.5 mmol
b
of HBPin, reaction time: 16 h, temperature: 25 ºC. Yields determined by
c
1
d
weighing the isolated reaction products. Calculated by H NMR. The yield
obtained with complex 1d corresponds to the mixture of addition products
presented above, but was not completely selective towards the hydroboration of
styrene, as other unidentified products were observed.
The most common selectivity found in the literature for iron-catalyzed hydroboration of
alkenes is anti-Markovnikov, with several examples showing total selectivity in this type of
products.4a,e,k,m By contrast, we systematically observed a mixture of addition products,
yielding nearly Markovnikov-exclusive products by using precatalyst 1c. Markovnikov
selectivity in iron-catalyzed hydroboration of alkenes is not common and, to date, only few
cases have been reported, with a-MK:Mk ratios as low as 0.02:1.4g,j,l
Stoichiometric reactions of precatalysts 1a-d with K(HBEt3) and mechanistic insights:
To understand the mode of activation of the Fe(II) pyridine chloride complexes, we explored
their stoichiometric reactivity with the super hydride K(HBEt3). After several attempts, the
only reaction that produced isolable materials was the one involving complex 1b. In fact, the
reaction of complex 1b with one equivalent of K(HBEt3) for 30 minutes, in toluene, led, after
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a standard work-up procedure, to the formation of a dark red-brown crystDaOllI:in10e.103s9o/Cli8dCY02319K
identified as a mixture of Fe hydride complexes, the monomer 2 and the dimer 22, with
concomitant formation of KCl, BEt3 and pyridine (Scheme 2, mixture I). This mixture of
hydride complexes was characterized by elemental analysis, measurement of magnetic
susceptibility in solution (by the Evans method19), and by FTIR and Mössbauer
spectroscopies. This observation proved slightly puzzling at first, since it contradicted the
results previously reported by our group with cobalt, in which the same reaction conditions
cleanly led to the formation of the 6-toluene Co(I) complex [Co{κ2N,N’-5-(2,4,6-iPr3-C6H2)-
NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(η6-C6H5CH3)].25
In light of these results, we attempted to obtain the analogous Fe(I) arene complex by
further forcing the reaction conditions, using a reaction time of 16 hours. This time around,
we were indeed able to identify the reduced complex [Fe{κ2N,N’-5-(2,4,6-iPr3-C6H2)-NC4H2-
2-C(H)=N(2,6-iPr2-C6H3)}(η6-C6H5CH3)] (3), albeit only corresponding to 10% of the
isolated mixture, the remainder 90% still corresponding to the mixture 2 + 22 (Scheme 2,
mixture II). Mixture II of complexes was characterized by elemental analysis, by the Evans
method,19 and by FTIR and Mössbauer spectroscopies. Attempts to isolate analytically pure
samples of complex 3 were made through more rational synthetic procedures. Unfortunately,
chemical reduction reactions of complex 1b with Na(Hg) or with KC8 in toluene only led to
decomposition of the reaction mixtures, contrasting with the clean generation of the
analogous η6-toluene Co(I) complex reported previously by our group.25
Scheme 2 Reaction of complex 1b with K(HBEt3) at different reaction times, leading,to mixtures I (complexes 2 + 22) and II
(complexes 2 + 22 + 3).
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DOI: 10.1039/C8CY02319K
Mixtures I and II are very soluble in n-hexane and very sensitive to air and moisture, both in
solution and in the solid state, readily decomposing with effervescence in protic solvents.
These mixtures are of impossible separation, owing to similar solubilities of their respective
components. A similar reactivity seems to be observed when the Fe complexes 1a or 1c were
used as precursors, but the lack of crystallinity of the samples frustrated any X-ray diffraction
studies or suitable combustion analyses, preventing an analogous report.
Mössbauer spectroscopy proved instrumental in the qualitative and quantitative
characterization of mixtures I and II. The Mössbauer spectra of mixtures I (complexes 2 + 22)
and II (complexes 2 + 22 + 3) taken between 4 and 150 K are presented in Figure 3 and may
only be adequately fitted if three (for mixture I, Figure 3a) or four (for mixture II, Figure 3b)
doublets are refined.
Figure 2 Mössbauer spectra of mixture I (a) and of mixture II (b) taken at different temperatures. Lines above the
experimental points are the sum of three (for mixture I) or four (for mixture II) doublets (see also Table S3 of the ESI),
shown slightly shifted for clarity.
In mixture I, considering that two molecular entities, a monomeric compound 2, bearing a
terminal hydride ligand, and a dimeric compound 22, containing µ-bridging hydrides, are
present in the sample, one of the doublets may be attributed to the complex that has only one
crystallographic site for Fe (2, terminal hydride), and the remaining two doublets, with the
same relative areas within experimental error (Table S3, shown in the ESI), are assigned to
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both crystallographic positions in the dimer (22, bridging hydrides). The relativeDOarI:e1a0.1o03Vf9ie/wtCh8AerCticYl0e2O3n1l9inKe
doublet attributed to compound 2 corresponds to 35%, and the sum of the relative areas
corresponding to 22 is 65% (Table S3 of the ESI). This molar composition of the mixture
2+22, retrieved from the Mössbauer spectrum at 4 K, perfectly matches the values obtained
for the elemental analysis of the sample (within the experimental error), taking into account
that 2 really has the formulation suggested in Scheme 2. In fact, in the mixture
(C37H49FeN3)0.35(C69H93Fe2N5)0.65, the combustion analysis values obtained and calculated
(the latter between parentheses) are: C 75.12 (75.07), H 8.50 (8.46), N 6.45 (6.51). The
estimated IS values are too large or too low for low-spin or high-spin monovalent Fe,
respectively.23 They are close to the low end of the isomer shifts range of high-spin Fe(II).23
These low values are usually found for Fe(II) complexes with at least one hydride as a ligand
due to the better σ-donating ability of the hydride as compared to the other organic ligands.26
The temperature dependence of the quadrupole splittings is also consistent with high-spin
Fe(II).23 The lowest measured IS values are due to tetracoordinated Fe(II) in the terminal
hydride Fe complex 2 and in the µ-bridging hydrides di-iron complex 22 and the highest IS to
pentacoordinated Fe(II) in dimer 22, in agreement with the estimated relative areas.
On the other hand, the Mössbauer spectra of mixture II are somewhat different from those
of mixture I taken at the same temperatures. These differences are accounted for, in addition
to the doublets assigned to the monomeric compound 2, bearing a terminal hydride ligand,
and the dimeric compound 22, containing μ-bridging hydrides, the appearance of a fourth
doublet, corresponding to a 10 % molar contribution to the mixture, at 4 K. The estimated IS
values for this doublet at 4 K and 150 K (Table S3, shown in the ESI) are within the range of
IS values that have been reported in the literature for high-spin Fe(I) complexes (S = 3/2).27
In addition, as observed for mixture I, the combustion analysis values of mixture B are in
accordance with the molar ratios determined by Mössbauer spectroscopy, the obtained and
calculated
(the
latter
between
parentheses)
values
for
the
entity
(C37H49FeN3)0.34(C69H93Fe2N5)0.56(C39H51FeN2)0.1(-OSi(CH3)2-)0.5, being C 72.93 (73.48), H
8.47 (8.45), N 6.05 (6.14).
The FTIR spectra of mixtures I and II are presented in Figures S11 and S12, respectively,
being nearly superimposable. The C=N stretching bands of the mixtures are present at 1565
cm-1, irrespective of the observed mixtures of complexes. Despite the apparent low sensitivity
of this technique towards C=N bond stretching vibrations in these mixtures, the FTIR
spectrum of mixture II clearly displays three bands at 799, 788 and 776 cm-1, which were not
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Catalysis Science & Technology
observed in the FT-IR spectrum of mixture I, being diagnostic of the additional tolVuieewnAerticle Online
DOI: 10.1039/C8CY02319K
moiety of complex 3 present in mixture II.
Complexes 22 and 3 were further characterized by X-ray diffraction, having crystallized in
the triclinic system, in the P-1 space group. The molecular structures of complexes 22 and 3
are shown in Figure 2, with selected bond lengths and angles being presented in Tables S2
and S3 of the ESI. Complex 22 is an asymmetric Fe hydride dimer with two µ-bridging
hydride ligands, where the Fe1 center is pentacoordinated and the Fe2 center is
tetracoordinated. The hydrogen atoms corresponding to the hydride ligands in complex 22
(H1A and H1B) were located in the electron density map. The modes of coordination of the
5-aryl-2-iminopyrrolyl ligands in complex 22 are the same as in the previous complexes. The
tetracoordinated Fe2 center has the Fe2−N4 and Fe2−N5 bond lengths equal to 1.990(2) Å
and 2.025(2) Å, respectively. The Fe2−H1 bond lengths are around 1.55 Å and the τ4
parameter in complex 22 is 0.07, revealing a square planar geometry around Fe2. The
pentacoordinated Fe1 center has a coordinated pyridine ligand, aside from the 5-aryl-2-
iminopyrrolyl and the bridging hydride moieties. In this case, the Fe1−N bond lengths are in
the range of 2.059−2.180 Å, slightly longer than in the tetracoordinated Fe2 center. The
Fe1−H1 bond lengths are around 1.68−1.69 Å, also longer than that observed for the Fe2
center. The τ5 parameter in the Fe1 center in complex 22 is 0.60 (τ5= 1 for an ideal trigonal
bipyramid and τ5= 0 for an ideal square pyramid28), thus revealing to be an intermediate case
between trigonal bipyramidal and square pyramidal geometries. The different bond lengths
involving the two Fe centers, which are larger in Fe1 than in Fe2, point to different electronic
environments.
On the other hand, complex 3 is a Fe(I) complex with a η6-coordinated toluene molecule
in which the Fe1−C33 to C38 bond lengths are quite similar, in the range of 2.060(11)−
2.149(10) Å (Δ ≈ 0.089 Å). The Fe1−centroid distance (the centroid being defined as the
center of the six-membered aromatic ring formed by atoms C33 to C38) is equal to 1.548(6)
Å. The Fe1−N bond lengths are in the range 1.971(7)−1.977(8) Å, where the 5-aryl-2-
formiminopyrrolyl ligand exhibits the typical bidentate coordination mode. The Fe1−N bond
lengths are shorter than in the analogous Co(I) complex [Co{κ2N,N’-5-(2,4,6-iPr3-C6H2)-
NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(η6-C6H5CH3)] reported by our group.25 Both the Fe1−N
and the Fe1-centroid bond lengths are similar to the crystalographically characterized Fe(I)
complexes of the type [LFe(toluene)], where L is a bidentate chelating ligand.6,29 Complex 3
displays a pseudo-tetrahedral coordination geometry (with τ4 = 0.74), if one considers the
tetrahedron formed by the N1, N2, C33 and C36 atoms bonded to Fe1. The η6-arene moiety is
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virtually perpendicular to the chelation plane (87.46º). The torsions of the 5-aryl DaOnId: 10t.h10e3V9iNe/wC2A8rC-ticYl0e2O3n1l9inKe
aryl rings relative to the 2-iminopyrrolyl plane are 83.37º and 81.75º, respectively.
Additionally, a DFT geometry optimization has been performed for complex 3 in the high
spin electronic state, S = 3/2, its optimized atomic coordinates being presented in the ESI.
The theoretical structure of complex 3 compares well with its experimentally determined
structure.
22
3
Figure 3 ORTEP-3 diagrams for complexes 22 and 3 showing 30% probability ellipsoids. All hydrogen atoms except for
H1A and H1B were omitted for clarity.
In order to understand the electronic structure of complex 22, geometry optimizations were
performed, using DFT calculations. The deviations between calculated and experimental
coordination bond lengths of complex 22 are presented in Table 4. The geometry optimized
for complex 22 is similar to the one determined by X-ray diffraction. The calculated Fe–N
distances are slightly overestimated (except for Fe2–N5), with a maximum deviation of 4.7%,
but the Fe1−N bond lengths are longer than the ones calculate for the Fe2 center, as observed
by X-ray diffraction. The larger overestimation (8.2 %) is found for the Fe1−H bond length
(average), but still the trend observed in the experimental structure is reproduced, with longer
Fe–H distances for the Fe1 center. Those values are reasonable for the level of theory
employed that is constrained by the size of the molecule. The τ geometry parameters are also
well reproduced in the calculated structure of 22.
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Table 4 Comparison of selected bond lengths (in Å) and the appropriate τ geometry parameters in complex 2D2 OdeI:t1e0rm.10in39e/dCb8yCY02319K
single crystal X-ray diffraction and by DFT calculations.
Parameter
Fe1−N1
Fe1−N2
Fe1−N3
Fe2−N4
Fe2−N5
Fe1−H1 a
Fe2−H1 a
Fe1−Fe2
τ4
Experimental
2.059(2)
2.155(2)
2.180(3)
1.990(2)
2.025(2)
1.69(4)
Calculated
2.14
Δ
Δ (
3.7
0.08
0.11
0.05
0.02
2.26
4.7
2.23
2.2
2.01
1.0
2.01
-0.02 -0.74
1.84
0.15
0.03
0.08
8.2
1.9
3.2
1.55(4)
1.58
2.4874(5)
0.07
2.57
0.10
τ5
0.60
0.69
a The average distance is considered.
The electronic structure of complex 22 was further studied by a Natural Population Analysis30
(NPA, see Computational Details) and the results are summarized in Figure 4. The
coordination asymmetry of the molecule is reflected in both its electronic structure and spin
density. The Wiberg indices31 involving the Fe1 pentacoordinated center are smaller than the
ones observed in the Fe2 tetracoordinated center, indicating weaker Fe1–X bonds, when
compared with Fe2–X, in accordance with the bond lengths discussed above. The spin
density of the complex is essentially located on the two metal atoms (Figure 4b) but most of
it is centered on Fe1 (3.7) with a much lower participation of Fe2 (0.4) where the
coordination geometry approaches a square planar arrangement.
(a)
(b)
Figure 4 Selected Wiberg indices (a) and calculated spin density (b) in complex 22.
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The isolated mixtures of complexes I and II were further tested in the hydroboration of
DOI: 10.1039/C8CY02319K
styrene with HBPin, by reproducing the experimental conditions used in the 1a-d/K(HBEt3)
system. The corresponding set of catalytic experiments performed is summarized in Table 5.
In neat conditions, we have observed that the catalytic run using styrene, HBPin and 1 mol%
of mixture I (2 + 22) has almost exclusively yielded PinB-BPin (B2Pin2) and a very complex
mixture of very minor products, which included the expected mixture of boranes in a a-
Mk:Mk ratio of 0.37:1 (Figure S28 of the ESI). In light of this somewhat puzzling result, we
also performed other control experiments, using only styrene or only HBPin in the catalytic
runs, also in neat conditions. We realized that the reaction of mixture I with HBPin also led to
the formation of B2Pin2. Catalysis with 1 mol% of mixture II in neat conditions has led to
virtually the same a-Mk:Mk ratio (0.40:1), with a much higher selectivity towards
hydroboration than that obtained with mixture I in neat conditions (Figure S28 of the ESI).
Finally, we also performed a catalytic run with mixture I in the presence of a large excess of
toluene (4:1 in volume with respect to styrene) and, gratifyingly, observed a much more
hydroboration-selective system, with the same a-Mk:Mk ratio (0.36:1) (Figure S28 of the
ESI).
Table 5 Summary of the catalytic experiments performed with mixtures I and II (see Scheme 2).
Selectivity
[Fe] catalyst
Amountsa
Solvent
Yieldb
(a-Mk:Mk:B2Pin2)c
Mixture I
Mixture I
Mixture I
Mixture II
Mixture I
n=1; m=0
n=0; m=1
n=1; m=1
n=1; m=1
Neat
Neat
Neat
Neat
44
0
0:0:1
-
52
41
47
60
0.37:1:8.7
0.40:1:1.6
0.36:1:1.2
0.37:1:0
n=1; m=1 Toluene
1b/K(HBEt3) n=1; m=1 Toluene
a Conditions: 1 mol% of mixture I (2 + 22) or 1 mol% of mixture II (2 + 22 + 3) or 1 mol% of 1b +
3 mol% of K(HBEt3), 2 mmol of styrene, 2.5 mmol of HBPin, reaction time: 16 h, temperature:
25 ºC. b Yields determined by weighing the isolated reaction products. c Calculated by 1H NMR.
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As far as mechanistic considerations for the present system are concerned, we propose this
DOI: 10.1039/C8CY02319K
catalyst system follows the oxidative addition pathway represented in Scheme 3.
Scheme 3 Proposed catalytic cycle.
Even though the reaction of the pyridine chloride complexes 1a-d with K(HBEt3) in toluene
seems to favor the formation of Fe(II) hydride complexes (cf. mixture I of Scheme 2),
prolonged stirring in the same solvent leads to an increased concentration of low valent Fe(I)
species (cf. mixture II of Scheme 2). Furthermore, when testing the catalytic performance of
the 2 + 22 mixture (mixture I, see Scheme 2), we observed that it was virtually non-selective
towards hydroboration and such selectivity increased dramatically when the catalytic runs
were performed in toluene.
Considering these observations, we propose that in the activation by K(HBEt3), while
giving rise to a hydride complex (NN’)Fe(II)H, the coordinative pressure of olefins/arenes
during catalysis very likely generates a low oxidation state species “(NN’)Fe(I)” that, by
coordination of the respective substrate generates the Fe(I) intermediate (NN’)Fe(I)(η2-
CH2=CHR) (Scheme 3). This Fe(I) entity is prone to oxidative addition of HBPin, possibly
giving rise to (NN’)Fe(III)(H)(BPin)(η2-CH2=CHR). The resulting hydride-boryl species
generates
the
intermediates
(NN’)Fe(III)(CH2CHBPinR)(H)
and/or
(NN’)Fe(III)[CH(R)CH2BPin](H), possibly via a migratory insertion of the BPin boryl
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moiety. Alternatively, a migratory insertion of the hydride can occur, generatingVietwhAerticle Online
DOI: 10.1039/C8CY02319K
intermediates (NN’)Fe(III)(CH2CH2R)(BPin) and/or (NN’)Fe(III)[CH(R)CH3](BPin).
Finally, a reductive elimination reaction yields the reaction products and regenerates the
active “(NN’)Fe(I)” species. The high Markovnikov selectivity in the case of styrene
substrates is very likely justified by a more favorable 2,1-insertion of the boryl or hydride
ligands (as opposed to the near quantitative 1,2-insertion in the remaining substrates), thus
increasing
the
amount
of
the
(NN’)Co(III)[CH(R)CH2BPin](H)
or
(NN’)Co(III)[CH(R)CH3](BPin) intermediate species.
Conclusions
Four new paramagnetic Fe(II) complexes of the type [Fe{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-
iPr2-C6H3)}(Py)Cl] (1a-d) were prepared via salt metathesis with the respective potassium
salts KLa-d in moderate yields. The magnetic susceptibilities of complexes 1a-d were
characterized in solution by the Evans method, displaying high-spin electronic
configurations. Complexes 1a and 1c were characterized by single crystal X-ray diffraction,
highlighting distorted tetrahedral geometries. Complex 1a was additionally characterized in
57
the solid state by SQUID magnetometry and Fe Mössbauer spectroscopy, and complexes
1a-d were studied by DFT calculations, all these techniques reinforcing their high-spin
nature.
Complexes 1a-d are precatalysts for the hydroboration of terminal α-olefins with
pinacolborane, when activated by K(HBEt3). The system composed by 1a/K(HBEt3) was able
to hydroborate several terminal alkenes to their respective boronate esters in good yields,
mainly yielding the respective anti-Markovnikov addition products. However, the screening
of the catalytic system 1a-d /K(HBEt3) with styrene showed that the selectivity in the
Markovnikov product increased with the increasing steric bulkiness of the R group, with 1c
(R = 2,4,6-Ph3-C6H3) being 91% selective in the latter product.
The reactivity of complexes 1a-d with K(HBEt3) was studied, the only isolable product
coming from complex 1b, for which a mixture of Fe(II) hydrides, monomeric species 2 and
dimeric 22, was obtained (mixture I). Prolonged stirring of 1d and K(HBEt3) in toluene led to
the identification of a third species, identified as the Fe(I) complex 3, along with 2 + 22
(mixture II). 57Fe Mössbauer spectroscopy performed on mixtures I and II indicated the
presence of three and four different doublets, respectively, a set of two doublets
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corresponding to the two-different iron centers in 22, another doublet corresponding Vtieow Aarticle Online
DOI: 10.1039/C8CY02319K
Fe(II) high-spin in 2 and, only in mixture II, a further doublet, assigned as the high-spin Fe(I)
complex 3. The DFT calculated structure of complex 22 shed light on its asymmetry, as both
iron centers showed different spin density distributions.
Taking these results into account and considering that the presence of olefinic media is the
driving force for the observation of Fe(I) complexes, it is very likely that the present catalytic
process follows a route involving the borane oxidative addition to a Fe(I) species.
Experimental
General considerations: All operations were performed under dry dinitrogen atmosphere
using standard glovebox and Schlenk techniques unless otherwise stated. Solvents were pre-
dried with activated 4 Å molecular sieves and distilled by refluxing under dinitrogen for
several hours over suitable drying agents (sodium/benzophenone for diethyl ether, THF and
toluene; CaH2 for n-hexane). The substrates used in the catalytic runs were dried over CaH2
and purified by trap-to-trap distillation. Solvents and solutions were transferred using a
positive pressure of dinitrogen through stainless steel cannulae and mixtures were filtered in a
similar way using modified cannulae that could be fitted with glass fiber filter disks.
K(HBEt3) was purchased in THF solutions and was used as a solid by recrystallization from
the same solvent, being stored at 4 ºC. Pinacolborane was purified by trap-to-trap distillation
prior to use. The potassium salts KLa-d17 and FeCl2(Py)432 were prepared as described in the
literature. All other reagents were acquired commercially and used without further
purification. FTIR measurements were conducted on a Bruker Alpha II ATR IR spectrometer
located inside a glovebox. Elemental analyses were obtained from the IST elemental analysis
services.
General method for the synthesis of [Fe{κ2N,N’-5-R-NC4H2-2-C(H)=N(2,6-iPr2-
C6H3)}(Py)Cl] (complexes 1a-d): Toluene was added to a solid mixture of FeCl2(Py)4 and
the respective potassium salt KLa-d, at room temperature. The mixture was stirred overnight
at 80 ºC, leaving an orange-red suspension. All volatiles were evaporated to dryness, leaving
an orange-red residue. The residue was washed with n-hexane and extracted with toluene.
The combined orange-red extracts were concentrated, carefully layered with n-hexane (1:3)
and stored at -20 ºC, precipitating the title complexes as orange-red powders or crystals.
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DOI: 10.1039/C8CY02319K
[Fe{κ2N,N’-5-(2,6-Me2-C6H3)-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl] (1a): The general
procedure was followed, using 0.32 g (0.8 mmol) of the potassium salt KLa and 0.39 g (0.8
mmol) of FeCl2(Py)4, yielding an orange-red crystalline solid. Yield: 0.21 g (49 %).
Anal. Calc. for C30H34ClFeN3, obtained (calculated): C 68.65 (68.25), H 6.69 (6.49), N 7.65
(7.96). µeff (toluene-d8) = 5.3 µB. 1H NMR (300 MHz, C6D6): δ 50.14, 5.32, 2.11, 1.19, 0.24, -
2.30, -12.11, -22.68. FTIR (ATR, cm−1) = 1561 (s, C=N).
[Fe{κ2N,N’-5-(2,4,6-iPr3-C6H2)-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl]
(1b):
The
general procedure was followed, using 0.37 g (0.75 mmol) of the potassium salt KLb and
0.37 g (0.75 mmol) of FeCl2(Py)4, yielding an orange-red powder. Yield: 0.117 g (25 %).
Anal. Calc. for C37H48ClFeN3, obtained (calculated): C 70.89 (70.98), H 8.04 (7.73), N 6.32
(6.71). µeff (toluene-d8) = 4.8 µB. 1H NMR (300 MHz, C6D6) δ 70.18, 38.79, 10.27, 3.07, 1.24,
-0.72, -1.40, -1.85, -2.29, -4.76, -7.87, -9.36, -19.71. FTIR (ATR, cm−1) = 1565 (s, C=N).
[Fe{κ2N,N’-5-(2,4,6-Ph3-C6H2)-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl]
(1c):
The
general procedure was followed, using 0.43 g (0.72 mmol) of the potassium salt KLc and
0.35 g (0.72 mmol) of FeCl2(Py)4, yielding an orange-red crystalline solid. Yield: 0.13 g (25
%).
Anal. Calc. for C46H42ClFeN3, obtained (calculated): C 75.51 (75.88), H 5.93 (5.81), N 5.60
1
(5.77). µeff (toluene-d8) = 4.6 µB. H NMR (300 MHz, C6D6) δ 71.28, 36.86, 16.00, 11.33,
8.56, 6.06, 3.74, 2.11, 1.11, -0.07, -3.63, -11.24, -19.78. FTIR (ATR, cm−1) = 1563 (s, C=N).
[Fe{κ2N,N’-5-(triphenylmethyl)-NC4H2-2-C(H)=N(2,6-iPr2-C6H3)}(Py)Cl] (1d): The
general procedure was followed, using 0.53 g (1 mmol) of the potassium KLd and 0.49 g (1
mmol) of FeCl2(Py)4, yielding an orange-red powder. Yield: 0.16 g (24 %).
Anal. Calc. for C41H40ClFeN3, obtained (calculated): C 73.56 (73.93), H 6.29 (6.05), N 5.92
1
(6.31). µeff (toluene-d8) = 4.7 µB. H NMR (300 MHz, CDCl3) δ 69.23, 38.91, 21.93, 18.59,
10.77, 9.15, 7.27, 4.33, 3.03, -0.53, -3.02, -13.95. FTIR (ATR, cm−1) = 1573 (s, C=N).
General procedure for catalytic hydroboration reactions: A Schlenk flask was charged
with the desired amount of complex 1a-d (1% mol) and K(HBEt3) (3% mol), after which a
solution of the appropriate substrate (2 mmol) and pinacolborane (2.5 mmol) was added. The
mixture was stirred at 25 ºC for 16 h and quenched by adding ca. 15 mL of n-hexane and
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exposing the mixture to air. The solution was filtered through a plug of silica and the solvent
DOI: 10.1039/C8CY02319K
evaporated to dryness. The resulting pale-yellow oil was eluted with n-hexane through a
Pasteur pipette mounted with silica. The hydroboration products were isolated as near
1
colorless oils. The selectivity of the products was determined by H NMR and their yields
were determined by weighing the isolated reaction products.
Isolation of mixture I (2 + 22): Toluene was added to a solid mixture of complex 1b (0.30 g,
0.49 mmol) and K(HBEt3) (0.072 g, 0.54 mmol) at room temperature, to give a dark red-
brown solution. The mixture was stirred at room temperature for 30 min, to yield a dark red-
brown suspension. All volatile materials were evaporated under reduced pressure, to give an
oily dark red-brown residue. The residue was redissolved in n-hexane with separation of a
pale precipitate and a negligible dark residue. The dark red-brown solution was concentrated
and stored at -20 ºC, giving rise to a dark red-brown crystalline solid suitable for X-ray
diffraction. Yield: 44 % (0.12 g).
Anal. Calc. for (C37H49FeN3)0.35(C69H93Fe2N5)0.65 obtained (calculated): C 75.12 (75.07), H
8.50 (8.46), N 6.45 (6.51). µeff (toluene-d8) = 4.9 µB, considering the formula
(C37H49FeN3)0.35(C69H93Fe2N5)0.65
.
Isolation of mixture II (2 + 22 + 3): Toluene was added to a solid mixture of complex 1b
(0.30 g, 0.49 mmol) and K(HBEt3) (0.072 g, 0.54 mmol) at room temperature, to give a dark
red-brown solution. The mixture was stirred at room temperature for 16 h, to yield a dark red-
brown suspension. All volatile materials were evaporated under reduced pressure, to give an
oily dark red-brown residue. The residue was redissolved in n-hexane with separation of a
pale precipitate and a negligible dark residue. The dark red-brown solution was concentrated
and stored at -20 ºC, giving rise to a dark red-brown crystalline solid suitable for X-ray
diffraction. Yield: 55 % (0.15 g).
Anal. Calc. for (C37H49FeN3)0.34(C69H93Fe2N5)0.56(C39H51FeN2)0.1(-Osi(CH3)2-)0.5 obtained
(calculated): C 72.93 (73.48), H 8.47 (8.45), N 6.05 (6.14). µeff (toluene-d8) = 5.0 µB,
considering the formula (C37H49FeN3)0.34(C69H93Fe2N5)0.56(C39H51FeN2)0.1(OSiMe2)0.5.
NMR measurements: NMR spectra were recorded on a Bruker “AVANCE III” 300 MHz, at
299.995 MHz (1H), and referenced internally using the residual protio-resonances of the
corresponding solvents to tetramethysilane (δ = 0). Deuterated solvents were dried over
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activated 4 Å molecular sieves and degassed by the freeze-pump-thaw techniqueD,OaI:n1d0.1s0t3o9/rCe8dCY02319K
under dinitrogen in J. Young ampoules.
Magnetic susceptibility measurements in solution were carried out using the Evans method,19
using a 3% solution of hexamethyldisiloxane in toluene-d8. These solutions were prepared in
a glovebox in J. Young NMR tubes containing capillary tubes filled with the same solvent
mixture, in which the hexamethyldisiloxane is the external reference.
X-ray diffraction: Crystallographic and experimental details of crystal structure
determinations are listed in Table S1 of the ESI. The crystals were selected under dinitrogen,
covered with polyfluoroether oil and mounted on a nylon loop. Crystallographic data were
collected using graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) on a Bruker AXS-
KAPPA APEX II diffractometer equipped with an Oxford Cryosystem open-flow nitrogen
cryostat, at 150 K. Cell parameters were retrieved using Bruker SMART33 software and
refined using Bruker SAINT34 on all observed reflections. Absorption corrections were
applied using SADABS.35 Structure solution and refinement were performed using direct
methods with the programs SIR201436 and SHELXL37 included in the package of programs
WINGX-Version 2014.1.38 Crystals of complex 1c were of poor quality, presenting a
relatively low ratio of observed/unique reflections. Though the structure of complex 1c was
refined to a perfect convergence, it is only presented in Figure S6 of the ESI as a proof of its
connectivity. All hydrogen atoms were inserted in idealized positions and allowed to refine
riding on the parent carbon atom, except for the hydride hydrogen atoms H1A and H1B of
complex 22, which were located on the electron density map. All the structures refined to a
perfect convergence. Graphic presentations were prepared with ORTEP-3.38b,39 Data was
deposited in CCDC under the deposit numbers 1875120 for 1a, 1875121 for 1c, 1875122 for
22 and 1908799 for 3.
Solid state magnetic measurements: The magnetic properties of complex 1a were studied
using a 6.5 T S700X SQUID (Cryogenic Ltd.) magnetometer. The magnetic susceptibility
was measured as a function of temperature in increasing temperature range 5-300 K using a
DC magnetic field of 500 Oe. The paramagnetic data was obtained after the correction for the
core diamagnetism estimated using Pascal’s constants, giving χD= -370.210-6 emu/mol.
24

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Catalysis Science & Technology
Mössbauer spectroscopy: Mössbauer spectra for complexes 1a and the mixtures I and VIieIwoArfticle Online
DOI: 10.1039/C8CY02319K
Fe complexes were collected between 295 and 4 K in transmission mode using a
57
conventional constant-acceleration spectrometer and a 25 mCi Co source in a Rh matrix.
The velocity scale was calibrated using α-Fe foil. Isomer shifts, IS, are given relative to this
standard at room temperature. The absorbers were obtained by gently packing the powdered
samples (5 mg of natural Fe cm-2) into Perspex holders. Low-temperature measurements were
performed with the sample immersed in liquid He in a bath cryostat for measurements at 4.1
K and in He exchange gas in the same cryostat for temperatures higher than 4.1 K. The
spectra were fitted to Lorentzian lines using a non-linear least-squares method.40
Computational details: All calculations were performed using the Gaussian 09 software
package41 and the OPBE functional. OPBE combines the Handy’s OPTX modification of
Becke’s exchange functional42 and the gradient-corrected correlation functional of Perdew,
Burke and Ernzerhof,43 and was shown to be accurate in the calculation of spin state energy
splitting for first transition row species.44 The geometry optimizations were accomplished
without symmetry constraints using a standard 6-31G** basis set45 for all atoms except for
iron, that used the LanL2DZ46 (for complexes 1a-d and 3) or Effective Core Potential SDD47
(for complexes 2 and 22) basis set with a f-polarization function for Fe.48 A Natural
Population Analysis (NPA)30 and the resulting Wiberg indices31 were used to study the
electronic structure and bonding of the optimized species. The spin density plot of 22 was
represented using ChemCraft.49
Acknowledgements
We thank Fundação para a Ciência e a Tecnologia for the financial support (Projects
UID/QUI/00100/2019 and UID/Multi/04349/2019) and the fellowships to T.F.C.C. and
P.T.G. (PD/BD/52372/2013 – CATSUS PhD Program and SFRH/BSAB/140115/2018,
respectively).
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