Hydroboration of Terminal Olefins with Pinacolborane Catalyzed by New Mono(2-Iminopyrrolyl) Cobalt(II) Complexes

Article abstract of DOI:10.1021/acs.inorgchem.8b00568

The 5-substituted 2-aryliminopyrrolyl ligand precursors of the type 5-R-2-[N-(2,6-diisopropylphenyl)formimino]-1H-pyrrole (R = 2,6-Me₂-C₆H₃ (1a), 2,4,6-ⁱPr₃-C₆H₂ (1b), 2,4,6-Ph

Full text of DOI:10.1021/acs.inorgchem.8b00568

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Hydroboration of Terminal Olefins with Pinacolborane Catalyzed by
New Mono(2-Iminopyrrolyl) Cobalt(II) Complexes
Tiago F. C. Cruz,† Patrícia S. Lopes,† Laura C. J. Pereira,‡ Luís F. Veiros,† and Pedro T. Gomes*,†
†
́
Centro de Química Estrutural, Departamento de Engenharia Química, Instituto Superior Tecnico, Universidade de Lisboa, Av.
Rovisco Pais 1, 1049-001 Lisboa, Portugal
‡
2
̂
́
C TN-Centro de Ciencias e Tecnologias Nucleares, Instituto Superior Tecnico, Universidade de Lisboa, 2695-066 Bobadela LRS,
Portugal
S
* Supporting Information
ABSTRACT: The 5-substituted 2-aryliminopyrrolyl ligand pre-
cursors of the type 5-R-2-[N-(2,6-diisopropylphenyl)formimino]-
1H-pyrrole (R = 2,6-Me2-C6H3 (1a), 2,4,6-iPr3-C6H2 (1b), 2,4,6-
Ph3-C6H3 (1c; reported in this work), anthracen-9-yl (1d), CPh3
(1e; reported in this work)) were treated with K[N(SiMe3)2] in
toluene to yield the respective 5-R-2-[N-(2,6-diisopropylphenyl)-
formimino]pyrrolyl potassium salts 2a−e in high yields. The
paramagnetic 15-electron Co(II) complexes of the type [Co-
{κ2N,N′-5-R-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}(Py)Cl] (3a−
e; Py = pyridine) were prepared by salt metathesis of CoCl2(Py)4
with the respective potassium salts 2a−e in moderate to good
yields. When the CoCl2(THF)1.5 precursor was combined with
the in situ prepared sodium salt of ligand precursor 1b, the trinuclear complex [Co{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-
C(H)N(2,6-iPr2-C6H3)}(μ-Cl)]2[(μ-Cl)2Co(THF)2] (4) was obtained in high yields. Complexes 3a−e have high-spin
electronic configurations both in solution and in the solid state. X-ray diffraction studies of complexes 3a,e confirmed distorted
tetrahedral coordination geometries. Complex 4, on the other hand, is a linear trinuclear Co(II)−Co(II)−Co(II) complex with
two terminal distorted tetrahedral four-coordinate sites and a central octahedral six-coordinate site, all in the high-spin state, S =
3/2, as confirmed by the magnetization measurements and DFT calculations. Solid-state magnetic measurements in both
complexes 3a and 4 point to paramagnetic behavior with a significant contribution of spin−orbit coupling. Additionally,
intramolecular antiferromagnetic coupling of the adjacent cobalt atoms is observed in 4. The Co(II) family 3a−d, on activation
with K(HBEt3), catalyzed the hydroboration of several α-olefins with pinacolborane, in good to high yields (50−80%). This
system almost exclusively yielded the anti-Markovnikov (a-Mk) addition product, except when styrene was used, where the
selectivity in the Markovnikov (Mk) product increased with increasing steric bulkiness of the 5-R-2-iminopyrrolyl substituent,
with the a-Mk:Mk molar ratio varying from 2.33:1 (3a, R = 2,6-Me2-C6H3) to 0.75:1 (3c, R = 2,4,6-Ph3-C6H3). Preliminary
mechanistic studies indicate that the activation by K(HBEt3) gave rise to a Co(I) species, the catalyst system likely following an
oxidative addition pathway.
Specifically considering cobalt catalyzed hydroboration of
alkenes, Chirik et al. have developed groundbreaking work in
the hydroboration of alkenes by using tridentate (bis(imino)-
pyridine)CoMe precatalysts (Chart 1, A), achieving the
respective organoboranes under neat, mild conditions.4b In
an example with neutral tridentate 6-phosphino-2,2′-bipyridine
ligands, Huang et al. reported that PNN pincer complexes (6-
phosphino-2,2′-bipyridine)CoCl2 (Chart 1, B) catalyzed the
hydroboration of alkenes with very high activities, when
activated by K(HBEt3).4k The cases of catalyst precursors
containing anionic ligands are scarce, and one can consider the
work of Turculet et al. using a P,N-bidentate (N-
phosphinoamidinate)Co(N(SiMe3)2) complex (Chart 1, C)
INTRODUCTION
■
The hydroboration of olefins consists of the formal addition of
a H−B fragment to the unsaturated bond and is a viable source
of organoboron reagents that are often used in cross-coupling,
oxidation, or other important organic chemistry reactions.1
Hydroboration has been typically catalyzed by the expensive
platinum-group elements, more specifically by rhodium and
iridium.2 The development of metal-catalyzed hydroboration
has allowed more selective reactions by enabling the use of
previously unreactive boranes, such as pinacolborane (HBPin).
When this is taken into account, it is important to develop
alternative cheap and abundant mediators that can perform
selective and versatile transformations.3 Recently, iron and
cobalt have been increasingly used in catalytic systems for the
hydroboration of alkenes4 and alkynes.5
Received: March 5, 2018
© XXXX American Chemical Society
A
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Chart 1
Scheme 1. Preparation of the 5-Substituted 2-Iminopyrrolyl Ligand Precursors 1a−e Used in This Work
to catalyze hydroboration reactions in neat and mild
conditions.4f,g In all of the reports mentioned above, the
reactions were directed toward the anti-Markovnikov addition
products. However, Thomas and co-workers reported the
uncommon Markovnikov-selective hydroboration of alkenes
with a bipyridyl−oxazoline cobalt(II) complex (Chart 1, D),
activated with NaOtBu.4n
role ligand precursors,15d from their corresponding 5-
substituted-2-formylpyrroles,17 we report herein the synthesis
and complete characterization of new mono(5-substituted 2-
iminopyrrolyl) Co(II) complexes encompassing varying
degrees of steric bulkiness. These Co(II) complexes were
tested as catalysts for the hydroboration of terminal α-olefins
upon activation by K(HBEt3), enabling the establishment of a
structure/reactivity relationship.
The chemistry of Co bearing one anionic bidentate N,N
ligand is limited. A chloride complex containing the N,N β-
diketiminate framework, LCoCl (L = [{2,6-diisopropylphenyl-
NC(tBu)}2CH]−) (Chart 1, E), has been reported by Holland
et al.6 In fact, arene Co complexes of β-diketiminate ligands
have been reported as active mediators of hydrosilylation
reactions of terminal alkenes, but not yet in hydroboration.7 In
a different work, the amidinato/guanidinato scaffold has been
used to stabilize Co complexes of the type [LCoCl]2 (with L =
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Unsolvated Ligand
Potassium Salts. The 5-substituted 2-iminopyrrole ligand
precursors 1a−e used in this work were prepared by a
multistep strategy (Scheme 1) reported by our group for the
synthesis of 1a.15d The syntheses of the ligand precursors 1b−e
are reported in the present work. These ligand precursors were
t
[(2,6-diisopropylphenyl-N)2CR]−, with R being Bu, NiPr2, or
1
characterized by H and 13C{1H} NMR spectroscopy and
NCy2) (Chart 1, F).8 On a similar note, Betley and co-workers
reported a tetracoordinated Co(II) complex containing a
dipyrromethene bidentate framework (Chart 1, G).9
elemental analysis and for two of them (1c,e) by single-crystal
X-ray diffraction.
1
The H and 13C{1H} NMR spectra of 1b−e (Figures S4−
We have been interested in the coordination/organometallic
chemistry of complexes bearing 2-iminopyrrolyl ligands, having
prepared complexes of the late transition metals Fe, Ni, and
Cu10,11 and Zn,12 as well as of the main-group elements Na13
and B.14 Our group also reported some homoleptic bis(2-
iminopyrrolyl) complexes of Co with different steric environ-
ments.11,15 Thus far, the only case of a mono-chelated 2-
iminopyrrolyl cobalt(II) complex, [LCoCl2][Li(THF)4]
S11 in the Supporting Information) show the expected
resonances for the respective moieties, with the NH protons
being present as broad resonances at 8.59−9.21 ppm and the
iminic protons appearing at 7.66−8.04 ppm. The pyrrolyl
protons at positions 3 and 4 appear at 6.31 and 5.67 ppm,
respectively, for 1c, whereas 1b,d,e display those resonances at
6.54−6.97 and 6.12−6.66 ppm. The shift of the 4-pyrrolyl
proton of 1c to higher field is an indication of ring current
anisotropy effects induced by the phenyl groups in the ortho
positions of the 5-(2,4,6-triphenylphenyl) substituent, in
contrast with previously prepared 5-aryl-2-iminopyrrolyl ligand
precursors.15d The molecular structures of ligand precursors
(Chart 1, H), with the bis(arylimino)pyrrolyl ligand L−
=
2,5-([CH = N(2,6-iPr2-C6H3)2]C4H2N−, was that reported by
Bochmann et al.16 Taking into account that we have previously
described the preparation of bulky 5-substituted 2-iminopyr-
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1c,e obtained from single-crystal X-ray diffraction are
presented in Figure S27 of the Supporting Information.
In order to obtain unsolvated 5-substituted 2-iminopyrrolyl
alkali-metal salts as precursors of this ligand system, the 5-
substituted 2-iminopyrroles 1a−e were deprotonated with
K[N(SiMe3)2] in toluene to form the respective 5-substituted
2-iminopyrrolyl potassium salts 2a−e (Scheme 2). The
of the solvent was necessary for its isolation. Compounds 2a−e
1
were characterized by H and 13C{1H} NMR spectroscopy.
The NMR spectra of the potassium salts 2a−e are presented in
Figures S12−S21 of the Supporting Information.
In THF-d8, the 1H and 13C{1H} NMR spectra of
compounds 2a−e show the expected resonances for their
respective moieties; in general, the isopropyl protons are
equivalent and the iminic protons appear in the range 7.52−
7.71 ppm. The protons of the pyrrolyl moiety in positions 3
and 4 appear in the ranges 6.45−6.55 and 5.85−5.92 ppm,
respectively, for 2a,b,e. In a different observation, the same
pyrrole 3- and 4-proton resonances are shifted to higher fields
at 6.25 and 5.48 ppm for 2c and to lower fields, at 6.79 and
6.38 ppm, in 2d. As in the case of the parent 2-iminopyrrole
1c, the shift of the pyrrolyl 4-proton of 2c to higher field is an
indication of ring current anisotropy induced by the 5-(2,4,6-
triphenylphenyl) substituent.
Scheme 2. Synthesis of the 5-Substituted 2-Iminopyrrolyl
Potassium Salts 2a−e
Compound 2d was also characterized by X-ray diffraction,
crystallizing in the monoclinic system, in the I2/a space group.
The asymmetric unit is composed of two independent 5-
anthracen-9-yl-2-iminopyrrolyl ligands and two potassium
atoms, cocrystallized with one toluene molecule. The
ORTEP-3 diagrams relevant to the molecular structure of 2d
are shown in Figure 1, a selection of bond lengths and angles
being shown in Table S2 of the Supporting Information. In this
structure, the potassium atom is coordinated to one 5-
(anthracen-9-yl)-2-iminopyrrolyl ligand through the iminic and
pyrrolyl nitrogen atoms and to the pyrrolyl ring of the next
fragment in an η5 coordination mode. The structure self-
potassium salts 2a−c,e gradually precipitated from toluene
and were isolated by filtering off the respective supernatants. In
the case of 2d, which was very soluble in toluene, evaporation
Figure 1. ORTEP-3 diagrams of compound 2d: (a) representation of a repeating unit of 2d; (b) representation of the asymmetric unit of 2d; (c)
representation of an octamer of 2d, with the anthracen-9-yl and the 2,6-iPr2 groups omitted for clarity (bottom). All diagrams were drawn showing
20% probability ellipsoids, the hydrogen atoms and the cocrystallized toluene molecule being omitted for clarity.
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assembles in a 1D-coordination polymer, with the 5-
(anthracen-9-yl)-2-iminopyrrolyl moieties and the potassium
atoms being sequentially arranged in a zigzag conformation.
The K−Npyrrolyl and K−Nimine bond distances of both
fragments in the asymmetric unit are in the ranges of 2.641−
2.656 and 2.822−2.874 Å, respectively, with a bite angle in the
range of 61.89−63.42°. This observation is related to the
higher σ-donor character of the K−Npyrrolyl bond. The
potassium distance to the centroid of the η5-coordinated
pyrrolyl rings is in the range of 2.836−3.048 Å. This
observation is comparable to those of the 2-iminopyrrolyl
sodium salt features reported by our group,13 as well as other
compounds containing a potassium η5-coordinated to pyrrolyl
anions.18 The torsion of the 2,6-diisopropylphenyl ring relative
to the 2-iminopyrrolyl moiety is in the range of 74.58−89.12°.
In the K1 center, the five-membered chelate is relatively close
to planarity (the angle between the planes N1−K1−N2 and
N1−C2−C6−N2 is 10.90°), giving rise to a relatively
orthogonal anthracen-9-yl group, with a torsion of 64.57°. In
the K1A center, the five-membered chelate is well away from
planarity, with a severe pyramidalization of the Npyrrolyl atom,
the angle between the planes N1A−K1A−N2A and N1A−
C2A−C6A−N2A being 21.77°.
Synthesis and Characterization of the Pyridine
Chloride Co(II) Complexes. The metathetical exchange
reaction of the CoCl2(Py)4 (Py = pyridine) starting material
with the respective potassium salts 2a−e, in toluene at 80 °C,
afforded after standard workup procedures the corresponding
pyridine chloride Co(II) complexes [Co{κ2N,N′-5-R-NC4H2-
2-C(H)N(2,6-iPr2-C6H3)}(Py)Cl] (3a−e) (Scheme 3) in
moderate to good yields as dark blue-violet microcrystalline
solids, which precipitated from concentrated toluene/n-hexane
solutions.
Table 1. Effective Magnetic Moments μeff (μB) for the
Co(II) Complexes 3a−e, Measured in Toluene-d8 Solution
(Evans Method), at Room Temperature
complex
μeff (μB)
3a
3b
3c
3d
3e
4.1
4.5
4.5
3.7
4.8
μeff(spin only) = 3.88 μB), with spin−orbit coupling effects.19
Complex 3a was additionally characterized in the solid state by
SQUID variable-temperature magnetometry (see Figure S36 of
the Supporting Information) At room temperature, χT is 2.58
emu K mol−1 and corresponds to an effective moment, μeff, of
4.55 μB/Co and g = 2.35. This value is higher than the spin-
only value of Co(II) in the high-spin configuration (S = 3/2;
μeff = 3.88 μB; g = 2), suggesting a significant contribution of
the unquenched orbital angular momentum due to the spin−
orbit coupling, typical of Co(II),19 and agrees well with the
magnetic moments determined by the Evans method (Table
1).
The Co(II) complexes 3a,c were characterized by X-ray
diffraction, having crystallized in the monoclinic system in the
P21 and P21/c space groups, respectively. The ORTEP-3
drawings of complexes 3a,c are presented in Figure 2, a
selection of bond distances and angles being given in Table S3
of the Supporting Information. In both structures, it is possible
to observe that a single 5-R-2-iminopyrrolyl ligand is
coordinated to the Co atom in a bidentate fashion through
the pyrrolyl and iminic nitrogen atoms.
The tetracoordinated Co centers are further bonded to a
chlorine atom and a pyridine ligand. The Co−N bond
distances are in the range of 1.982−2.064 Å, in the order
Co−Npyrrolyl < Co−Npyridine < Co−Nimine in both complexes,
very likely associated with the decreasing degree of the σ-donor
character of the respective bonds. The Co−Cl bond distances
are in the range of 2.211−2.227 Å. Considering the τ4
parameters of 3a (0.78) and 3c (0.75),20 the coordination
geometries are best described as distorted tetrahedral and
trigonal pyramidal, respectively. In both complexes, the torsion
angles of the aryl rings bonded to the iminic nitrogen are 86.83
and 87.60°, respectively, making them nearly perpendicular to
the iminopyrrolyl moiety. On the other hand, the dihedral
angle between the 5-aryl group relative to the iminopyrrolyl
moiety presents a significant deviation in both complexes. In
complex 3a, the torsion of the 5-(2,6-Me2-C6H3) ring is
64.51°. In the case of 3c, two different dihedral angles can be
defined since a slight dearomatization of the C5 atom of the
pyrrolyl ring is observed. In fact, this carbon deviates 9.27−
11.17° from planarity (considering the C3−C4−C5−C1 and
C2−N1−C5−C1 dihedrals). This gives rise to a torsion of the
5-(2,4,6-Ph3-C6H2) substituent of 55.00−60.01° relative to the
2-iminopyrrolyl moiety. The less pronounced torsion of the 5-
(2,4,6-Ph3-C6H2) group in comparison to that of 5-(2,6-Me2-
C6H3) leads to a slight pyramidalization of the pyrrolyl
nitrogen. The latter observation is quantified by an angle
between the plane defined by N1−Co1−N2 and the 2-
iminopyrrolyl plane (defined by N1−C2−C6−N2) of 11.23°
(in comparison to the near-planar chelate in 3a).
Scheme 3. Synthesis of the Co(II) Complexes 3a−e
Complexes 3a−e are paramagnetic, and their solutions are
sensitive to air and moisture, being formally unsaturated 15-
electron compounds. These complexes are partially soluble in
n-hexane and Et2O and soluble in toluene. Complexes 3a−e
1
show paramagnetically shifted and broad H NMR spectra
(presented in Figures S22−S26 of the Supporting Informa-
tion). Although the resonances corresponding to 5-substituted
2-iminopyrrolyl and pyridine ligand moieties are present, no
accurate attributions can be made. In toluene-d8 solutions,
complexes 3a−e show effective magnetic moments in the
range of 3.7−4.8 μB (Table 1), a typical observation for d7
electronic distributions in the high-spin state (S = 3/2;
The structures of complexes 3a−e were also studied by DFT
calculations, the bond parameters determined by X-ray
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Figure 2. ORTEP-3 diagrams of the X-ray diffraction molecular structures of 3a (left) and 3c (right) showing 30% probability ellipsoids. Hydrogen
atoms are omitted for clarity.
Scheme 4. Isolation Attempt of the Complex N,N′CoCl(THF), Leading to the Isolation of Complex 4
diffraction for 3a,c being consistently reproduced (see pages
S29−S33 of the Supporting Information).
stead, after a workup procedure that did not involve the
evaporation of volatile materials, the trinuclear complex
[Co{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-C(H)N(2,6-iPr2-
C6H3)}(μ-Cl)]2[(μ-Cl)2Co(THF)2] (4) was the only isolated
product (Scheme 4). Reactions of the same components in
THF, with evaporation of volatile materials to dryness, point to
the formation of impure Co(II) entities containing the
respective 2-iminopyrrolyl and chloride ligands, likely con-
taminated with NaCl(THF)x, considering the respective
combustion analysis. The formation of these multinuclear
species is a consequence of irreproducible workup conditions,
the nonexistence of a strongly coordinating ligand in the
reaction medium, and the sensitivity of some reaction
intermediates to vacuum conditions.
Isolation of a Trinuclear Chloride Co(II) Complex in
the Absence of the Pyridine Donor. The reaction between
the respective sodium salts of the 5-substituted 2-iminopyrrole
ligand precursors 1a−e prepared in situ (by reacting 1a−e with
NaH in refluxing THF) with CoCl2(Py)4 in THF solutions
also afforded, after standard workup conditions, complexes
3a−e in comparable yields, albeit with more troublesome and
somewhat irreproducible purification procedures. In prelimi-
nary attempts of the preparation of Co(II) complexes bearing a
single 5-substituted 2-iminopyrrolyl ligand, the reaction of the
in situ prepared sodium salt of 1b with an excess of
CoCl2(THF)1.5 was performed, in toluene at 80 °C. With
this reaction, we sought the isolation of the THF chloride
Co(II) complex [Co{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-
C(H)N(2,6-iPr2-C6H3)}(THF)Cl] (N,N′CoCl(THF)). In-
The trinuclear complex 4 exhibits a total effective magnetic
moment in solution of 8.3 μB, which is higher than the value
expected for a spin-only system of three uncoupled Co(II)
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with S = 3/2 (μeff = g × [3 × (3/2 × (3/2 + 1))]1/2 ≅ 6.71 μB/
fu, g = 2) but well within the expected experimental values
corresponding to a ground state (S = 3 × 3/2 = 9/2), with
some degree of spin−orbit coupling. Complex 4 was
additionally characterized by variable-temperature SQUID
magnetometry (see Figures S36 and S37 of the Supporting
Information), χT exhibiting a continuous decrease as the
temperature decreases, from 8.85 emu K mol−1 at 300 K down
to 6.2 emu K mol−1 at 16.5 K. At room temperature, the value
sites are in the range of 1.968−2.070 Å, with the Co−Npyrrolyl
bonds being, on average, 0.09 Å shorter than the Co−Nimine
bonds.
The Co−Cl bond distances are in the range of 2.274−2.285
Å for the tetracoordinated Co(II) centers and around 2.474−
2.503 Å for the hexacoordinated Co(II) center. The
significantly longer Co2−Cl bond distances in the latter case
(0.21 Å longer, on average, in relation to those of Co1 and
Co3) is attributed to the coordinative supersaturation of the
central Co(II) atom. The observed long Co2−Cl bond
distances fall in the 5% longest Co−Cl bonds crystalo-
graphically characterized in Co chloride complexes.22 In fact,
most of the compounds with a Co−Cl bond distance in this
range correspond to bis chloride octahedral Co(II) com-
plexes.23
per Co(II) trimer corresponds to an effective moment μeff
=
8.42 μB/fu with g = 2.51, in agreement with the value
determined by the Evans method. According to the theoretical
calculations for the electronic structure (see below), this
reveals a significant orbital contribution, as frequently observed
in other linear trinuclear cobalt(II) complexes.21 Below 16 K
and upon cooling, χT sharply increases to 8.94 emu K mol−1
with the appearance of a narrow peak at 5.6 K, followed by a
rapid decrease down to 7.92 emu K mol−1 at 2 K. This
maximum suggests that the appearance of magnetic exchange
interactions between the Co(II) centers is expected to be
antiferromagnetic (AFM). These results are still preliminary,
and a complete interpretation of these data will be the subject
of a new publication.
In order to understand the spin-state distribution of the Co
atoms of the trinuclear complex 4, geometry optimizations on
two spin isomers were performed by DFT calculations. The
spin isomers chosen were that corresponding to the three Co
atoms in the high-spin state (S = 3 × 3/2 = 9/2), 49/2, and that
corresponding to two terminal high-spin Co atoms and one
central low-spin Co atom (S = 2 × 3/2 + 1/2 = 7/2), 47/2. The
coordinates of the optimized structures are shown in pages
S33−S36 of the Supporting Information. The energy balance
between both structures indicates that isomer 49/2 is 24 kcal
The molecular structure of complex 4 was determined by X-
ray diffraction (Figure 3), a selection of bond distances and
mol−1 more stable than isomer 47/2
.
The spin density calculated for 49/2 (Figure 4) is essentially
centered on the Co atoms, with small contributions from the
Figure 3. ORTEP-3 diagram of the X-ray diffraction structure of 4
showing 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
Figure 4. Total spin density plot of the optimized 49/2 spin isomer (S
= 9/2) of the trinuclear complex 4.
angles being shown in Table S4 of the Supporting Information.
Complex 4 crystallized in the monoclinic system, in the P21/c
space group. This compound is an example of a linear
trinuclear Co complex, the Co−Co−Co angle being
175.3144(6)°. Complex 4 is composed of two tetracoordinated
15-electron Co(II) centers (Co1 and Co3) and one
hexacoordinated 19-electron Co(II) center (Co2). The
tetracoordinated Co(II) centers have distorted tetrahedral
coordination geometries and are bonded to a 5-(2,4,6-
triisopropyl)-2-iminopyrrolyl moiety in the usual bidentate
chelation mode and two bridging chlorides to the hexacoordi-
nated Co2 center. The central Co2 atom is further coordinated
to two THF molecules, displaying a nearly octahedral
geometry. The Co−Co distances are in the range of 3.319−
3.328 Å. The Co−N bond distances in the tetracoordinated
coordinating nitrogen and oxygen atoms, being equally
distributed by the three Co atoms, in accordance with the
same high-spin nature for the three metal centers.
In summary, the DFT results corroborate the experimental
data obtained by both the Evans method and SQUID
magnetochemistry studies, in which the trinuclear complex 4
exhibits a global high-spin S = 9/2 electronic configuration.
Hydroboration of Terminal Alkenes Catalyzed by
Complexes 3a−e Activated by K(HBEt3). Bearing in mind
that the synthesized tetracoordinated Co(II) complexes 3a−e
are coordinatively unsaturated (formally 15-electron species)
and contain potentially labile or reactive ligands, we envisioned
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their use as precatalysts for the hydroboration of α-olefins with
pinacolborane (HBPin). A preliminary hydroboration of
styrene was attempted, using 1 mol % of 3a with 3 mol % of
K(HBEt3) as the catalytic system, under neat conditions, and it
was possible to observe that, in 16 h, the mixture contained
exclusively the respective addition products in a 2.33:1 anti-
Markovnikov:Markovnikov molar ratio (a-Mk:Mk) in 63%
yield (Table 3). Encouraged by this result, we decided to
analyze the α-olefin substrate scope with the catalyst system
Table 3. Structure/Selectivity Relationship for the
Hydroboration of Styrene Catalyzed by the System 3a−e/
K(HBEt3)
a
b
c
complex
yield (%)
selectivity (a-Mk:Mk)
1
3a/K(HBEt3). These results are presented in Table 2, the H
3a
3b
3c
3d
3e
63
35
44
62
d
2.33:1
2.14:1
0.75:1
1:1
NMR spectra of the alkylboronates being shown in Figures
S28−S32 of the Supporting Information. The system is
inactive in the absence of activation by K(HBEt3).
d
0.78:1
Table 2. Substrate Scope of the Hydroboration of Terminal
Alkenes Catalyzed by the System 3a/K(HBEt3)
a
Conditions: 1 mol % of 3a−e, 3 mol % of K(HBEt3), 2 mmol of
styrene, 2.5 mmol of HBPin, reaction time 16 h, temperature 25 °C.
b
Isolated yields determined by weighing the isolated reaction
c
d
products. Calculated by 1H NMR. Complex 3e was not completely
selective in the hydroboration of styrene.
but it can be seen that precatalyst 3c, containing the highly
encumbering 5-(2,4,6-triphenylphenyl) substituent, reverses
the a-Mk:Mk molar ratio, favoring the Markovnikov addition
1
product. A superimposition of the H NMR spectra of the
products obtained by 3a−d/K(HBEt3) in the region of the α
protons (relative to the boron atom) is shown in Figure S33 of
the Supporting Information, revealing the increasing selectivity
in the Markovnikov product in the order 3a < 3b < 3d < 3c. In
fact, it can be observed that increasing the steric bulkiness of
the ortho groups of the precatalyst 5-aryl substituent, going
from methyl (3a) to isopropyl (3b) and phenyl (3c), the
catalyst system becomes more selective in the Markovnikov
product.
The use of the also bulky, although flat, 5-(anthracen-9-yl)
substituent (precatalyst 3d) leads to an equimolar mixture of
addition products. Complex 3e, the only 5-alkyl derivative in
the series, exhibited a lower selectivity toward hydroboration,
yielding a complex mixture of reaction products, the two
desired addition products of hydroboration being obtained in a
0.78:1 a-MK:Mk molar ratio, an evidence of the also high steric
bulkiness of the CPh3 group.
a
Conditions: 1 mol % of 3a, 3 mol % of K(HBEt3), 2 mmol of
substrate, 2.5 mmol of HBPin, reaction time 16 h, temperature 25 °C.
b
Yields determined by weighing the isolated reaction products.
c
d
1
Calculated by H NMR. Owing to the symmetry of cyclohexene, a
single reaction product is obtained.
Under neat and mild conditions, this catalyst system leads to
good yields (62−75%) in addition products of HBPin to the
respective α-olefins, being almost exclusively selective in the
anti-Markovnikov (a-Mk) products, except for the case of
styrene. The system 3a/K(HBEt3) is also suitable for the
hydroboration of cyclohexane, albeit in a more modest yield.
In this work, the organoboranes were produced in yields that
are in the range of those obtained by some authors reporting
cobalt-catalyzed hydroboration of a similar substrate scope.4c,d
Although respectable, the yields obtained in the present work
are lower than the best results reported to date. In fact, some
authors have obtained yields higher than 90% for most of the
substrates presently considered in less than 1 h, under similar
reaction conditions.4b,f,k In our case (except for styrene) and in
those reported by other authors, the reactions were exclusively
selective in the anti-Markovnikov products.
Markovnikov-rich cobalt-catalyzed hydroboration of alkenes
is not common and the present system showed moderate
selectivity, by stereochemical tuning of the complexes used.
Only very recently did some authors report nearly
Markovnikov exclusive cobalt-based systems for these reac-
tions, with ratios as high as 2:98 (a-Mk:Mk).4n−p
Preliminary stoichiometric reactions of some of the
precatalysts with K(HBEt3) were attempted. In particular, a
NMR-scale reaction of complex 3b with 3 equiv of K(HBEt3)
in C6D6 was performed, giving rise to a sudden color change
from dark blue-violet to dark red with concomitant
1
effervescence. The H NMR spectrum reveals an excess of
K(HBEt3) and a single paramagnetic species, whereas the
11B{1H} spectrum also exhibits the excess of K(HBEt3) and
another species at 0.98 ppm that can be assigned to a Py−BEt3
adduct24 (the NMR spectra of this reaction are presented in
Figures S34 and S35 of the Supporting Information). These
observations point to the formation of a putative Co(II)
hydride complex that readily undergoes reductive elimination
of molecular dihydrogen to form a potential Co(I) species,
Aside from the substrate scope, we evaluated the effect of
the different precatalysts in the hydroboration of styrene,
which was the substrate that revealed regioselectivity problems.
The results for the hydroboration of styrene with HBPin using
the different Co(II) precatalysts 3a−e activated by K(HBEt3)
are shown in Table 3.
The Co(II) precatalysts activated by K(HBEt3) systemati-
cally yielded a mixture of products in moderate to good yields,
G
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Inorganic Chemistry
Article
with simultaneous elimination of pyridine via the formation of
the Py−BEt3 adduct.
(NN′)CoIIH hydride complex. In such a case, the coordination
of the olefin to that hydride species and subsequent migratory
insertion generates the intermediate complexes (NN′)-
CoII(CH2CH2R) and (NN′)CoII[CH(R)(CH3)]. These in-
termediates then react with HBPin, possibly via a four-
membered concerted step, to yield the corresponding mixture
of products, regenerating the hydride active species.
Taking the latter results into account, we propose that the
activation by K(HBEt3) very likely generates a low-oxidation-
state “(NN′)CoI” entity (Scheme 5, cycle A). Considering
Scheme 5. Proposed Catalytic Cycles of the Hydroboration
of Alkenes Catalyzed by 3a−d/K(HBEt3)
CONCLUSIONS
■
A family of new tetracoordinated Co(II) complexes bearing a
single 5-substituted 2-iminopyrrolyl ligand and its catalytic
application in the hydroboration of terminal alkenes was
reported. First, the sterically demanding ligand precursors 1a−
e were treated with K[N(SiMe3)2] to afford the respective
potassium salts 2a−e in high yields. The integrity of
compounds 2a−e was probed in solution by NMR spectros-
copy and, in the case of 2d, in the solid state by X-ray
diffraction, revealing a 1D-coordination polymeric structure.
Subsequently, by salt metathesis of 2a−e and CoCl2(Py)4, the
pyridine chloride 5-substituted 2-iminopyrrolyl Co(II) com-
plexes 3a−e were prepared in moderate to good yields. In
solution, these compounds exhibit 1H NMR spectra with
paramagnetically shifted and broad resonances, displaying by
the Evans method a high-spin configuration. Complexes 3a,e
were also characterized by X-ray diffraction, demonstrating
distorted tetrahedral geometries. It was also observed that,
when traces of THF are present in the reaction media, adducts
such as the linear trinuclear complex 4 can be formed and
isolated, which exhibited high-spin behavior for all its Co(II)
centers, as shown by the magnetic properties and confirmed by
DFT calculations, and antiferromagnetic coupling between
adjacent Co(II) atoms.
Complexes 3a−d served as efficient precatalysts for the
hydroboration of α-olefins with HBPin. On activation with
K(HBEt3), complex 3a exclusively yielded the anti-Markovni-
kov and Markovnikov alkylboronate addition products, in 50−
80% yields, except for the case of styrene. Specifically for this
latter substrate, by using the different precatalysts 3a−d it was
possible to observe that the selectivity in the Markovnikov
addition product increased with the increase in steric bulkiness
of the substituent at the position 5 of the 5-aryl-2-
iminopyrrolyl ligand, anti-Markovnikov:Markovnikov ratios of
2.33:1 (3a) to 0.75:1 (3c) being obtained. Considering these
encouraging results, it is imperative to modify position 5 of the
pyrrole, to favor the scarcer Markovnikov product or even turn
the attention to less reactive substrates. Preliminary mecha-
nistic studies indicate that activation by K(HBEt3) gave rise to
a putative Co(I) species, meaning that the catalytic system
likely followed an oxidative addition pathway. These studies
are essential to the understanding of the system and will be
reported in a timely fashion.
mechanistic discussions proposed by other authors for cobalt-
catalyzed hydroboration of alkenes,4d,h the “(NN′)CoI” entity
is prone to coordination of the olefin and subsequent oxidative
addition of HBPin to that center, giving rise to (NN′)-
CoIII(H)(BPin)(η2-CH2CHR). The resulting hydride−
boryl species generates the intermediates (NN′)-
CoIII(CH2CHBPinR)(H) and/or (NN′)CoIII[CH(R)-
CH2BPin](H), possibly via a migratory insertion of the BPin
boryl moiety. Alternatively, a migratory insertion of the
hydride can occur, generating the intermediates (NN′)-
CoIII(CH2CH2R)(BPin) and/or (NN′)CoIII[CH(R)CH3]-
(BPin). Finally, a reductive elimination reaction yields the
reaction products and regenerates the active “(NN′)CoI”
species. The enhanced Markovnikov selectivity in the case of
styrene 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), which
might result from the preferential η3 stabilization of the
resulting styryl group through η3-benzylic coordination, thus
increasing the amount of the (NN′)CoIII[CH(R)CH2BPin]-
(H) or (NN′)CoIII[CH(R)CH3](BPin) intermediate species.
One cannot discard the possibility of a hydride route
mechanism (Scheme 5, cycle B),4b in which the activation of
the Co(II) complexes by K(HBEt3) generates a putative
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under a
dry dinitrogen atmosphere using standard glovebox and Schlenk
techniques unless otherwise noted. Dinitrogen gas for all operations
(purity <1 ppm of O2 and H2O) was supplied by Air Liquide and
purified by passage through 4 Å molecular sieves. Solvents were
predried with activated 4 Å molecular sieves and distilled by heating
under dinitrogen over suitable drying agents (sodium/benzophenone
for toluene, diethyl ether, and THF; CaH2 for n-hexane). Solvents and
solutions were transferred using a positive pressure of nitrogen
through stainless steel cannulae, and mixtures were filtered in a similar
H
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
way using modified cannulae that could be fitted with glass fiber filter
disks.
extracted with diethyl ether, and evaporated to dryness. A second
washing with n-hexane gave the title compound as a golden yellow
solid. Yield: 2.03 g (68%).
The ligand precursor 1a15d and starting materials CoCl2(Py)425 and
26
CoCl2(THF)1.5 were prepared as described in the literature.
General Procedure for the Synthesis of the Ligand
Precursors 1b−e. A mixture of the respective 5-R-2-formyl-1H-
pyrrole and 2,6-diisopropylaniline, with a catalytic amount of p-
toluenesulfonic acid, was dissolved in ca. 50 mL of toluene and
refluxed in a flask equipped with a Soxhlet apparatus charged with
activated molecular sieves and a CaCl2 guard tube, until full
consumption of the reagents (as indicated by 1H NMR analysis).
The mixture was filtered, and the resulting solution was evaporated to
dryness. The crude product was purified by conventional solvent
recrystallization or column chromatography.
5-(2,4,6-Triisopropylphenyl)-2-[N-(2,6-diisopropylphenyl)-
formimino]-1H-pyrrole (1b). The general procedure was followed
using 5-(2,4,6-triisopropylphenyl)-2-formyl-1H-pyrrole17b (1.4 g, 4.8
mmol) and 2,6-diisopropylaniline (0.85 g, 4.5 mmol). The crude
mixture was extracted with n-hexane, concentrated, and stored at −20
°C, precipitating as a brown powder. Yield: 1.60 g (77%).
Anal. Calcd for C31H30N2·0.25C4H10O, obtained (calculated): C,
1
85.31 (85.58); H, 7.84 (7.29); N, 6.01 (6.24). H NMR (300 MHz,
CDCl3): δ 8.56 (s, 1H, H13), 8.14−8.04 (m, 5H, CHN + H8 +
H11), 7.56−7.48 (m, 4H, H9 + H10), 7.23−7.17 (m, 3H, N-Ph-Hmeta
3
+ N-Ph-Hpara), 6.97 (br, 1H, H3), 6.66 (d, JHH = 3.3 Hz, 1H, H4),
3
3
3.15 (sept, JHH = 6.9 Hz, 2H, CH(CH3)2), 1.25 (d, JHH = 6.9 Hz,
12H, CH(CH3)2), NH resonance absent. 13C{1H} NMR (75 MHz,
CDCl3): δ 151.7 (CHN), 131.5 (C12), 131.4 (C7), 128.7 (C8),
128.4 (C13), 126.4 (C9), 126.3 (C10), 125.5 (C11), 123.3 (N-Ph-
Cmeta + N-Ph-Cpara), 116.2 (C3), 113.7 (C4) 28.1 (CH(CH3)2), 23.8
(CH(CH3)2), N-Ph-Cipso, N-Ph-Cortho, C2, C5, and C6 resonances
absent.
Anal. Calcd for C32H44N2, obtained (calculated): C, 84.14 (84.16);
H, 9.30 (9.71); N, 6.00 (6.13). 1H NMR (400 MHz, CDCl3): δ 9.21
(br, 1H, NH), 7.96 (s, 1H, NCH), 7.17−7.15 (m, 2H, 5-Ph-Hmeta),
7.11−7.07 (m, 3H, N-Ph-Hmeta and N-Ph-Hpara), 6.69 (br, 1H, H3),
5-Triphenylmethyl-2-[N-(2,6-diisopropylphenyl)formimino]-1H-
pyrrole (1e). The general procedure was followed using 5-
triphenylmethyl-2-formyl-1H-pyrrole (1.4 g, 4.2 mmol) (synthesis
described in the Supporting Information) and 2,6-diisopropylaniline
(0.61 g, 4.0 mmol). The crude product was purified by column
chromatography using as eluent n-hexane/ethyl acetate (4/1) to yield
the product as a pale yellow powder. Yield: 1.02 g (51%).
3
6.23 (br, 1H, H4), 3.06 (h, JHH = 6.2 Hz, 2H, N-Phortho
-
3
(CH(CH3)2)), 2.97 (h, JHH = 6.9 Hz, 1H, 5-Phpara-(CH(CH3)2),
2.83 (h, 3JHH = 6.2 Hz, 2H, 5-Phortho(CH(CH3)2)), 1.33 (d, 3JHH = 6.9
Hz, 6H, 5-Phpara-(CH(CH3)2)), 1.22−1.18 (m, 24H, 5-Phortho
-
(CH(CH3)2 and N-Phortho-(CH(CH3)2). 13C{1H} NMR (101 MHz,
CDCl3): δ 151.3 (NCH), 150.0 (5-Ph-Cpara), 149.3 (N-Ph-Cipso
and 5-Ph-Cortho), 138.3 (N-Ph-Cortho), 134.6 (C5), 130.0 (C2), 127.8
(5-Ph-Cipso), 123.9 (N-Ph-Cpara), 123.1 (5-Ph-Cmeta), 121.0 (N-Ph-
Cmeta), 115.6 (C3), 111.4 (C4), 34.6 (5-Phpara-(CH(CH3)2), 30.94 (5-
Anal. Calcd for C36H36N2 obtained (calculated): C, 86.78 (87.05);
H, 7.24 (7.31); N, 5.41 (5.64). 1H NMR (CDCl3, 400 MHz): δ 8.95
(br, 1H, NH), 7.85 (s, 1H, NCH), 7.33−7.24 (m, 9H, 5-CPh3-
Hortho + CPh3-Hpara), 7.20−7.18 (m, 6H, C(Ph)3-Hmeta)), 7.11−7.04
(m, 3H, N-Ph-Hmeta + N-Ph-Hpara), 6.54 (d, 1H, 3JHH = 3.7 Hz, H3),
Phortho-(CH(CH3)2), 28.1 (N-Phortho-(CH(CH3)2), 24.7 (5-Phortho
-
(CH(CH3)2), 24.2 (5-Phpara-(CH(CH3)2), 23.5 (N-Phortho-(CH-
(CH3)2).
3
3
6.12 (d, 1H, JHH = 3.7 Hz, H4), 2.92 (sept, 2H, JHH = 6.6 Hz,
CH(CH3)2), 1.12 (d, 12H, JHH = 6.8 Hz, CH(CH3)2). 13C{1H}
3
5-(2,4,6-Triphenylphenyl)-2-[N-(2,6-diisopropylphenyl)-
formimino]-1H-pyrrole (1c). The general procedure was followed
using 5-(2,4,6-triphenylphenyl)-2-formyl-1H-pyrrole (2.0 g, 5.0
mmol) (synthesis described in the Supporting Information) and
2,6-diisopropylaniline (0.87 g, 4.9 mmol). The crude mixture was
washed with n-hexane and recrystallized from a concentrated toluene/
n-hexane solution (1/4 in volume), to yield the product as a pale
yellow powder. Yield: 1.19 g (43%).
NMR (CDCl3, 75 MHz): δ 151.4 (NCH), 148.8 (N-Ph-Cipso),
145.5 (CPh3-Cipso), 142.2 (C5), 138.4 (N-Ph-Cortho), 130.5 (CPh3-
Cortho), 130.2 (C2), 127.9 (CPh3-Cmeta), 126.9 (CPh3-Cpara), 124.0
(N-Ph-Cpara), 123.2 (N-Ph-Cmeta), 115.3 (C3), 112.8 (C4), 60.96
(CPh3), 28.0 (CH(CH3)2), 23.6 (CH(CH3)2).
General Procedure for the Synthesis of the 5-R-2-[N-(2,6-
diisopropylphenyl)formimino]pyrrolyl Potassium Salts 2a−e.
Toluene solutions of K[N(SiMe3)2] (1.1 equiv) and the respective 5-
substituted 2-iminopyrrolyl ligand precursor (1a−e, one equivalent)
were combined, giving rise to a thick suspension. The suspension was
stirred for 2 h, the supernatant was filtered off, and the solid was
washed three times with n-hexane. The pale yellow to off-white
powders were dried under vacuum to yield the title salts.
Potassium 5-(2,6-Dimethylphenyl)-2-[N-(2,6-diisopropylphenyl)-
formimino]pyrrolyl (2a). The general procedure was followed, using
3.0 g (8.37 mmol) of ligand precursor 1a and 1.83 g (9.20 mmol) of
K[N(SiMe3)2], yielding an off-white powder. Yield: 2.69 g (81%).
1H NMR (300 MHz, THF-d8): δ δ 7.72 (s, 1H, NCH), 7.06−
6.99 (m, 2H, 5-Ph-Hpara + N-Ph-Hpara), 6.97−6.84 (m, 4H, 5-Ph-Hmeta
+ N-Ph-Hmeta), 6.55 (br, 1H, H3), 5.92 (br, 1H, H4), 3.28 (br, 2H,
CH(CH3)2), 2.23 (s, 6H, CH3), 1.15 (d, 12H, CH(CH3)2, 3JHH = 4.2
Hz). 13C{1H} NMR (75 MHz, THF-d8): δ 159.9 (NCH), 154.1
(N-Ph-Cipso), 148.1 (C5), 143.9 (C2), 140.6 (5-Ph-Cipso), 139.6 (5-
Ph-Cortho), 138.2 (N-Ph-Cortho), 127.5 (5-Ph-Cmeta), 125.7 (5-Ph-
Cpara), 123.4 (N-Ph-Cpara), 122.9 (N-Ph-Cmeta), 121.9 (C3), 110.5
(C4), 28.6 (CH(CH3)2), 24.6 (CH(CH3)2).
Anal. Calcd for C41H38N2, obtained (calculated): C, 88.04 (88.13);
H, 6.65 (6.86); N, 4.93 (5.01). 1H NMR (300 MHz, CDCl3): δ 8.59
(br, 1H, NH), 7.72 (m, 5H, H8 + H15 + NCH). 7.48 (t, 2H, H13,
3JHH = 7.8 Hz), 7.39 (t, 1H, H17, 3JHH = 7.2 Hz), 7.38−7.30 (m, 10H,
H11 + H12 + H16), 7.15−7.05 (m, 3H, N-Ph-Hmeta + N-Ph-Hpara),
3
3
6.32 (d, 1H, H3, JHH = 3.3 Hz), 5.67 (d, 1H, H4, JHH = 3.3 Hz),
3
2.78 (h, 2H, CH(CH3)2, JHH = 6.6 Hz), 1.14 (d, 12H, CH(CH3)2,
3JHH = 6.6 Hz), NH resonance absent. 13C{1H} NMR (75 MHz,
CDCl3): δ 151.3 (NCH), 148.9 (N-Ph-Cipso), 142.9 (C6), 141.8
(C10), 140.9 (C9), 140.2 (C14), 138.4 (N-Ph-Cortho), 133.5 (C2),
129.8 (C5), 129.4 (C11), 129.0 (C16), 128.7 (C8), 128.2 (C12),
127.9 (C17), 127.3 (C15), 127.1 (C13), 123.9 (N-Ph-Cpara), 123.0
(N-Ph-Cmeta), 115.8 (C3), 113.9 (C4), 27.6 (CH(CH3)2), 23.8
(CH(CH3)2), C7 resonance absent.
5-(Anthracen-9-yl)-2-[N-2,6-(diisopropylphenyl)formimino]-1H-
pyrrole (1d). The general procedure was followed using 5-(anthracen-
9-yl)-2-formyl-1H-pyrrole (1.9 g, 7.0 mmol) and 2,6-diisopropylani-
line (1.2 g, 6.9 mmol). The crude mixture was washed with n-hexane,
Potassium 5-(2,4,6-Triisopropylphenyl)-2-[N-(2,6-
diisopropylphenyl)formimino]pyrrolyl (2b). The general procedure
was followed, using 1.18 g (2.59 mmol) of ligand precursor 1b and
0.57 g (2.85 mmol) of K[N(SiMe3)2], yielding an off-white powder.
Yield: 0.989 g (78%).
1H NMR (300 MHz, THF-d8): δ 7.70 (s, 1H, NCH), 7.06−6.98
(d, 2H, N-Ph-Hmeta, 3JHH = 7.5 Hz), 6.95 (s, 2H, 5-Ph-Hmeta), 6.88 (t,
I
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3
1H, N-Ph-Hpara, 3JHH = 7.5 Hz), 6.53 (d, 1H, H3, 3JHH = 3.0 Hz), 5.91
(d, 1H, H4, 3JHH = 3.3 Hz), 3.27 (h, 2H, N-Ph-CH(CH3)2, 3JHH = 6.9
H9 + H10 + N-Ph-Hmeta), 6.94 (t, 1H, N-Ph-Hpara, JHH = 6.0 Hz),
6.79 (br, 1H, H3), 6.38 (br, 1H, H4), 3.36 (br, 2H, CH(CH3)2), 1.22
(br, 12H, CH(CH3)2). 13C{1H} NMR (75 MHz, THF-d8): δ 160.3
(NCH), 153.9 (N-Ph-Cipso), 145.3 (C2), 140.9 (C6), 140.5 (C5),
133.4 (C12), 133.2 (C7), 130.6 (C11), 128.6 (C8), 125.5 (C9),
124.6 (C13), 124.4 (C10), 123.4 (N-Ph-Cortho + N-Ph-Cmeta), 123.1
(N-Ph-Cpara), 121.6 (C3), 114.4 (C4), 28.6 (CH(CH3)2), 24.8
(CH(CH3)2).
3
Hz), 3.19 (h, 2H, 5-Ph-CH(CH3)2 ortho, JHH = 6.9 Hz), 2.87 (h, 1H,
5-Ph-CH(CH3)2 para,
3JHH = 6.9 Hz), 1.27 (d, 6H, 5-Ph-CH-
3
(CH3)2 para, JHH = 6.9 Hz) 1.17−1.05 (m, 24H, N-Ph-CH(CH3)2 +
5-Ph-CH(CH3)2 ortho). 13C{1H} NMR (75 MHz, THF-d8): δ 159.7
(NCH), 154.1 (N-Ph-Cipso), 149.2 (5-Ph-Cortho), 147.9 (C5), 146.7
(5-Ph-Cpara), 140.5 (N-Ph-Cortho), 139.9 (5-Ph-Cipso), 139.3 (C2),
123.4 (N-Ph-Cmeta), 122.8 (N-Ph-Cpara), 122.0 (C3), 120.3 (5-Ph-
Cmeta), 111.5 (C4), 35.6 (5-Ph-CH(CH3)2 para), 30.9 (5-Ph-CH-
(CH3)2 ortho), 28.7 (N-Ph-CH(CH3)2), 25.3 (N-Ph-CH(CH3)2 + 5-
Ph-CH(CH3)2 ortho), 24.9 (5-Ph-CH(CH3)2 para).
Potassium 5-(Triphenylmethyl)-2-[N-(2,6-diisopropylphenyl)-
formimino]pyrrolyl (2e). The general procedure was followed,
using 2.05 g (4.13 mmol) of ligand precursor 1e and 0.91 g (4.54
mmol) of K[N(SiMe3)2], yielding an off-white powder. Yield: 1.70 g
(77%).
Potassium 5-(2, 4, 6-Triphenylphen yl)-2-[N-(2, 6-
diisopropylphenyl)formimino]pyrrolyl (2c). The general procedure
was followed, using 1.19 g (2.13 mmol) of ligand precursor 1c and
0.47 g (2.34 mmol) of K[N(SiMe3)2], yielding a pale yellow
fluorescent powder. Yield: 0.813 g (64%).
1H NMR (300 MHz, THF-d8): δ 7.65 (s, 1H, NCH), 7.35−7.24
(br, 6H, CPh3-Hortho), 7.22−7.11 (br, 9H, CPh3-Hmeta + CPh3-Hpara),
7.00 (d, 2H, N-Ph-Hmeta, 3JHH = 7.5 Hz), 6.86 (t, 1H, N-Ph-Hpara, 3JHH
3
3
= 7.2 Hz), 6.45 (d, 1H, H3, JHH = 2.7 Hz), 5.85 (d, 1H, H4, JHH
=
3
2.7 Hz), 3.18 (h, 2H, CH(CH3)2, JHH = 6.6 Hz), 1.11 (d, 12H,
CH(CH3)2, JHH = 6.9 Hz). 13C{1H} NMR (75 MHz, THF-d8): δ
3
156.4 (NCH), 153.4 (CPh3) 140.4 (N-Ph-Cortho), 138.2 (CPh3-
Cipso), 132.4 (CPh3-Cortho), 127.3 (CPh3-Cmeta), 125.9 (CPh3-Cpara),
123.3 (N-Ph-Cipso + N-Ph-Cmeta), 122.9 (N-Ph-Cpara), 121.0 (C3),
112.7 (C4), 28.4 (CH(CH3)2), 24.5 (CH(CH3)2).
General Procedure for the Synthesis of [Co{κ2N,N′-5-R-
NC4H2-2-C(H)N(2,6-iPr2-C6H3)}(Py)Cl] (3a−e). Toluene was
added to a solid mixture of CoCl2(Py)4 (1 equiv) and the appropriate
potassium salt (2a−e, 1 equiv). The mixture was stirred overnight at
80 °C, leaving a dark violet suspension. The dark violet solution was
filtered, and all volatiles were evaporated to dryness, leaving a dark
blue-violet residue. The residue was washed with n-hexane and
extracted with toluene, the extracts being combined, concentrated,
carefully layered with n-hexane (1/3), and stored at −20 °C. After a
few days, the title complexes precipitated as dark blue-violet powders
or crystals.
1H NMR (300 MHz, THF-d8): δ 7.72 (d, 2H, H15, 3JHH = 7.5 Hz),
7.63 (s, 2H, H8), 7.52 (s, 1H, NCH), 7.42 (t, 2H, H16, 3JHH = 7.2
Hz), 7.31−7.12 (m, 11H, H11 + H12 + H15 + H17), 6.99 (d, 2H, N-
[Co{κ2N,N′-5-(2,6-Me2-C6H3)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}-
(Py)Cl] (3a). The general procedure was followed, using 0.32 g (0.8
mmol) of the potassium salt 2a and 0.40 g (0.8 mmol) of CoCl2(Py)4,
yielding a dark blue-violet crystalline solid. Yield: 0.322 g (76%).
Anal. Calcd for C30H34ClCoN3, obtained (calculated): C, 67.77
3
3
Ph-Hmeta, JHH = 7.5 Hz), 6.86 (t, 1H, N-Ph-Hpara, JHH = 7.5 Hz),
6.25 (br, 1H, H3), 5.48 (br, 1H, H4), 3.05 (h, 2H, CH(CH3)2, 3JHH
=
3
6.6 Hz), 1.11 (d, 12H, CH(CH3)2, JHH = 6.6 Hz). 13C{1H} NMR
(75 MHz, THF-d8): δ 159.6 (NCH), 153.3 (N-Ph-Cipso), 145.5
(C10), 145.4 (C2), 142.8 (C6), 142.3 (C14), 140.4 (C7), 139.4
(C5), 138.9 (N-Ph-Cortho), 130.6 (C11 + C15), 129.6 (C16), 129.0
(C8), 127.9 (N-Ph-Cmeta), 127.7 (C11 + C16), 127.6 (C13 + C17),
126.3 (N-Ph-Cpara), 123.2 (C3), 114.2 (C4), 28.2 (CH(CH3)2), 24.7
(CH(CH3)2), C9 resonance absent.
1
(67.86); H, 6.53 (6.45); N, 7.69 (7.91). μeff (toluene-d8): 4.1 μB. H
NMR (300 MHz, C6D6): δ 50.14, 5.31, 1.18, 0.83, 0.23, −2.30,
−12.31, −22.68.
[Co{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}-
(Py)Cl] (3b). The general procedure was followed, using 0.39 g (0.8
mmol) of the potassium salt 2b and 0.40 g (0.8 mmol) of CoCl2(Py)4,
yielding a dark blue-violet powder. Yield: 0.259 g (52%).
Anal. Calcd for C37H48ClCoN3, obtained (calculated): C, 69.87
Potassium 5-(Anthracen-9-yl)-2-[N-(2,6-diisopropylphenyl)-
formimino]pyrrolyl (2d). The general procedure was followed,
using 1.51 g (3.50 mmol) of ligand precursor 1d and 0.77 g (3.85
mmol) of K[N(SiMe3)2]. Since the reaction mixture was very soluble
in toluene, all volatiles were removed under reduced pressure until an
oil was formed. n-Hexane (ca. 30 mL) was added to the oil,
immediately precipitating a dark brown powder that was further
washed three times with n-hexane. Crystals suitable for X-ray
diffraction were obtained from a concentrated toluene solution
layered with 3-fold n-hexane at room temperature. Yield: 1.18 g
(72%).
1
(70.63); H, 7.79 (7.69); N, 6.66 (6.68). μeff (toluene-d8): 4.5 μB. H
NMR (300 MHz, C6D6): δ 49.56, 6.56, 6.04, 2.66, 2.37, 1.59, 1.09,
0.88, 0.16, −11.51, −23.62.
[Co{κ2N,N′-5-(2,4,6-Ph3-C6H2)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}-
(Py)Cl] (3c). The general procedure was followed, using 0.48 g (0.8
mmol) of the potassium salt 2c and 0.40 g (0.8 mmol) of CoCl2(Py)4,
yielding a dark green-violet microcrystalline solid. Yield: 0.164 g
(52%).
Anal. Calcd for C46H42ClCoN3·0.5C7H8, obtained (calculated): C,
76.45 (76.49); H, 6.65 (5.97); N, 5.02 (5.41). μeff (toluene-d8): 4.5
1
μB. H NMR (300 MHz, C6D6): 51.66, 11.84, 4.63, 4.34, 2.84, 1.22,
0.87, 0.74, −4.46, −8.98, −13.51, −23.14, −46.81.
[Co{κ2N,N′-5-(anthracen-9-yl)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}-
(Py)Cl] (3d). The general procedure was followed, using 0.47 g (1
mmol) of the potassium salt 2d and 0.49 g (1 mmol) of CoCl2(Py)4,
yielding a dark green-brown crystalline solid. Yield: 0.448 g (75%).
Anal. Calcd for C36H34ClCoN3, obtained (calculated): C, 71.95
1
(71.70); H, 5.73 (5.68); N, 6.72 (6.97). μeff (toluene-d8): 3.7 μB. H
NMR (300 MHz, C6D6): 102.57, 50.80, 5.16, 1.94, 0.81, 0.23, −0.73,
−0.86, −12.22, −22.36.
1H NMR (300 MHz, THF-d8): δ 8.52 (d, 2H, H11, 3JHH = 7.2 Hz),
8.29 (s, 1H, NCH), 8.52 (m, 3H, H8 + H13), 7.48−7.02 (m, 6H,
[Co{κ2N,N′-5-(triphenylmethyl)-NC4H2-2-C(H)N(2,6-iPr2-C6H3)}-
(Py)Cl] (3e). The general procedure was followed, using 0.43 g (0.8
J
DOI: 10.1021/acs.inorgchem.8b00568
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mmol) of the potassium salt 2e and 0.40 g (0.8 mmol) of CoCl2(Py)4,
yielding a dark blue-violet powder. Yield: 0.325 g (61%).
temperatures (1.7, 5, 10, and 30 K) were also performed. The
diamagnetism correction for the experimental susceptibility data was
estimated using the Pascal constants, corresponding to χD = −325.5 ×
10−6 emu mol−1 and χD −868.0 × 10−6 emu mol−1 for 3a and 4,
respectively.
Anal. Calcd for C41H40ClCoN3, obtained (calculated): C, 73.79
1
(73.59); H, 6.33 (6.03); N, 5.92 (6.28). μeff (toluene-d8): 4.8 μB. H
NMR (300 MHz, C6D6): 105.13, 52.45, 10.33, 5.11, 2.31, 1.13, 0.01,
−2.04, −4.18, −9.99, −23.88, −92.32.
X-ray Diffraction. Crystallographic and experimental details of
crystal structure determinations are given in Tables S5 and S6 of the
Supporting Information. The crystals were selected under an inert
atmosphere, covered with dry and degassed 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 SMART28 software and refined using
Bruker SAINT29 on all observed reflections. Absorption corrections
were applied using SADABS.30 Structure solution and refinement
were performed using direct methods with the programs SIR201431
and SHELXL32 included in the package of programs WINGX-Version
2014.1.33 All non-hydrogen atoms were refined anisotropically, and
the hydrogen atoms were inserted in idealized positions and allowed
to refine riding on the parent carbon atom. Graphic presentations
were prepared with ORTEP-3.34 Data were deposited with the CCDC
under the deposit numbers 1827273 for 1c (shown in the Supporting
Information), 1827274 for 1e (shown in the Supporting Information),
1827275 for 2d, 1827276 for 3a, 1827277 for 3c and 1827278 for 4.
Computational Details. Calculations were performed using the
Gaussian 09 software package35 and the OPBE functional without
symmetry constraints. This functional combines Handy’s OPTX
modification of Becke’s exchange functional36 with the gradient-
corrected correlation functional of Perdew, Burke, and Ernzerhof37
and was shown to be accurate in the calculation of spin state energy
splitting for first-transition-row species.38 The geometry optimizations
were accomplished without symmetry constraints using a standard 6-
31G** basis set39 for all atoms except for cobalt. For this element a
SDD basis set40 was used for the calculation of the spin isomers of 4
and a LanL2DZ41 basis set for the calculations of the mononuclear
complexes 3a−e, with an added f polarization function,42 in both
cases. The spin density plots were represented using Chemcraft.
[Co{κ2N,N′-5-(2,4,6-iPr3-C6H2)-NC4H2-2-C(H)N(2,6-iPr2-
C6H3)}(μ-Cl)]2[(μ-Cl)2Co(THF)2] (4). Ligand precursor 1b (0.46 g, 1
mmol) was added as a solid to a THF suspension of NaH (1.1 mmol,
0.026 g), and the mixture was refluxed for 3 h, after which the solution
was filtered. All volatile materials were removed under reduced
pressure to give a pale red waxy solid. The resulting solid was
redissolved in toluene and added to a blue toluene suspension of
CoCl2(THF)1.5 (1.5 mmol, 0.36 g). The mixture was stirred overnight
at 80 °C, forming a dark blue-violet suspension. The dark blue-violet
toluene solution was filtered at room temperature, concentrated under
reduced pressure, and stored at −20 °C, from which dark blue-violet
crystals suitable for X-ray diffraction were obtained. Yield: 0.43 g
(67%).
Anal. Calcd for C72H102Cl4Co3N4O2, obtained (calculated): C,
62.97 (62.93); H, 7.31 (7.48); N, 3.88 (4.08). μeff (toluene-d8): 8.3
μB.
General Procedure for Catalytic Hydroboration Experi-
ments. A Schlenk flask was charged with the desired amount of
complex 3a−e (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 °C for 16 h and
quenched by adding ca. 15 mL of n-hexane and exposing the mixture
to air. The solution was filtered through a plug of silica and the
solvent evaporated to dryness. The resulting pale yellow oil was eluted
with n-hexane through a Pasteur pipet with a plug of silica. The
hydroboration products (already described in the literature4a) were
isolated as nearly colorless oils. The selectivity of the products was
determined by 1H NMR spectroscopy (highlighted in the Supporting
Information), and their yields were determined by weighing the
isolated reaction products.
NMR-Scale Reaction of Complex 3b with K(HBEt3). Inside a
glovebox, C6D6 was carefully added to a solid mixture of complex 3b
(0.026 g, 0.0413 mmol) and K(HBEt3) (0.0169 g, 0.123 mmol) inside
a vial. An immediate color change to dark red was observed,
accompanied by effervescence. The mixture was transferred to a J.
Young tube and the NMR spectra acquired.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR (300 MHz, C6D6): δ 63.03, 18.67, 17.33, 13.72, 10.46,
8.15 (Py−BEt3), 7.89 (Py−BEt3), 6.02, 5.26, 4.69, 3.14, 2.38, 1.94,
1.26 (K(HBEt3)), 1.06 (K(HBEt3)), 0.50 to −0.20 (Py−BEt3), −4.82,
−37.08. 11B{1H} NMR (30 MHz, C6D6): δ 0.98 (Py−BEt3), −13.26
(K(HBEt3)). The unassigned peaks correspond exclusively to the
putative Co(I) species.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00568.
Synthetic details, characterization data, NMR spectra,
crystallographic data, preliminary mechanistic studies,
magnetic data, and details of the calculations (PDF)
NMR Measurements. NMR spectra of complexes and polymers
were recorded on a Bruker AVANCE III 300 MHz spectrometer at
299.995 MHz (1H) and 75.4296 MHz (13C), referenced to the
solvent residual protio resonances (1H) and to the solvent carbon
(13C) resonances. Solution samples were prepared in dried and
degassed deuterated solvents at room temperature, using standard
NMR tubes. For air-/moisture-sensitive compounds, the samples were
prepared in a glovebox in J. Young NMR tubes. Magnetic
susceptibility measurements in solution were performed on a Bruker
AVANCE III 300 MHz spectrometer at 298 K using the Evans
method,27 the solution samples being prepared in toluene-d8 with 3%
of hexadimethylsiloxane. These solutions were prepared in a glovebox
in J. Young NMR tubes containing capillary tubes filled with the same
solvent mixture, in which hexamethyldisiloxane is the external
reference.
Magnetic Measurements in the Solid State. Magnetic
measurements by the dc extraction method were performed on
polycrystalline samples (10−20 mg) of complexes 3a and 4 using a
SQUID magnetometer, S700X (Cryogenic Ltd.). The temperature
dependence of the magnetic susceptibility was measured under
magnetic fields of 5 T in the temperature range 5−300 K. The
isothermal magnetization measurements up to 5 T, taken at different
Accession Codes
CCDC 1827273−1827278 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for P.T.G.: pedro.t.gomes@tecnico.ulisboa.pt.
ORCID
Luís F. Veiros: 0000-0001-5841-3519
Pedro T. Gomes: 0000-0001-8406-8763
Notes
The authors declare no competing financial interest.
K
DOI: 10.1021/acs.inorgchem.8b00568
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
11605−11608. (m) Tseng, K.-N. T.; Kampf, J. W.; Szymczak, N. K.
Regulation of Iron-Catalyzed Olefin Hydroboration by Ligand
Modifications at a Remote Site. ACS Catal. 2015, 5, 411−415.
(n) Peng, J.; Docherty, J. H.; Dominey, A. P.; Thomas, S. P. Cobalt-
catalysed Markovnikov selective hydroboration of vinylarenes. Chem.
Commun. 2017, 53, 4726−4729. (o) Zuo, Z.; Yang, J.; Huang, Z.
Cobalt-Catalyzed Alkyne Hydrosilylation and Sequential Vinylsilane
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ACKNOWLEDGMENTS
■
̂
We thank the Fundaca
̧
o
̃
para a Ciencia e a Tecnologia for
financial support (Projects UID/QUI/00100/2013 and UID/
Multi/04349/2013) and fellowships to T.F.C.C., P.S.L., and
P.T.G. (PD/BD/52372/201-CATSUS Ph.D. Program, SFRH/
BD/88639/2012, and SFRH/BSAB/140115/2018, respec-
tively).
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