Cobalt(I) Complexes of 5-Aryl-2-iminopyrrolyl Ligands: Synthesis, Spin Isomerism, and Application in Catalytic Hydroboration

Article abstract of DOI:10.1021/acs.inorgchem.8b02392

This work reports the first successful isolation and full characterization of cobalt(I) complexes of 5-aryl-2-iminopyrrolyl ligands. In one approach, when [Co{κ²N,N′-5-(2,4,6-R₃-C₆H₂)-NC₄H₂

Full text of DOI:10.1021/acs.inorgchem.8b02392

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Cobalt(I) Complexes of 5‑Aryl-2-iminopyrrolyl Ligands: Synthesis,
Spin Isomerism, and Application in Catalytic Hydroboration
Tiago F. C. Cruz, 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
*
S Supporting Information
ABSTRACT: This work reports the first successful isolation and full char-
acterization of cobalt(I) complexes of 5-aryl-2-iminopyrrolyl ligands.
2
In one approach, when [Co{κ N,N′-5-(2,4,6-R -C H )-NC H -2-C(H)
3
6
2
4
2
i
i
N(2,6- Pr -C H )}(Py)Cl] (R = Pr, 1a; R = Ph, 1b) were reacted with
2
6
3
2
K(HBEt ) or Na(Hg) in toluene, the Co(I) arene complexes [Co{κ N,N′-
3
i
i
6
5
-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -C H )}(η -C H CH )]
3
6
2
4
2
2
6
3
6
5
3
2 6
(
2a) and [Co{κ N,N′-5-[2′-(κ:η -C H )-C H -4′,6′-Ph ]-NC H -2-C(H)
6
5
6
2
2
4
2
i
N(2,6-Pr -C H )}] (2b) were formed. The reaction of complex 1a with KC
2
6
3
8
2
i
in Et O yielded the [Co{κ N,N′-5-(2,4,6-Pr -C H )-NC H -2-C(H)
2
3
6
2
4
2
i
N(2,6- Pr -C H )}] (3). On another approach, the metathesis of potas-
2
6
3
2
sium 5-(2,4,6-triisopropylphenyl)-2-(N-2,6-diisopropylphenylformimino)-
pyrrolyl (KLa) with CoCl(PMe ) yielded the bis(trimethylphosphine)
3
3
2
i
i
complex [Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
C H )}(PMe ) ] (4) in a good yield. Complexes 2a, 3, and 4 are paramagnetic, high-spin species, while 2b is a diamagnetic
6
3
3 2
1
complex. Compound 2b exhibited a spin isomerism behavior (S = 0 ↔ S = 1) as determined by variable-temperature H NMR
−
1
−1
experiments (ΔH° = 7.7 kcal mol ), being also supported by computational studies (ΔE = 4.2 kcal mol ). All complexes were
tested in the hydroboration of styrene with pinacolborane (HBPin), with complex 4 exclusively yielding the respective anti-
Markovnikov addition product. Additionally, all complexes catalyzed the fast and quantitative hydroboration of benzaldehyde
with HBPin.
INTRODUCTION
observed. Interestingly, an LCo(THF) complex, with L being
a bulky β-diketiminate chelate and THF being tetrahydrofuran,
undergoes an isomerization process ending up in an unusual
■
The isolation of transition-metal complexes in low oxidation
state and low coordination number has been pivotal in diverse
fields, from small molecule activation and biomimetics to cata-
lysis involving the activation of hydrocarbons or ketones. Many
advances in the isolation of transition-metal complexes in low
oxidation states with bidentate chelating ligands have been made,
6
1
slipped κN,η -arene binding mode in the resulting new LCo
8
2
complex (Chart 1, E). A redox noninnocence was also verified
when isolating the Co(0) complex LCo(COD), with L being an
α-diiminate ligand (derived from 1,2-bis(2,6-diisopropyl-
3
phenylimino)acenaphthene (BIAN)) and COD = 1,5-cyclo-
most notably with the β-diketiminate framework.
9
octadiene (Chart 1, F), with applications in P activations.
4
Specifically considering Co complexes in low oxidation states
with bidentate chelating ligands, some remarkable results are to
be noted on the way to their isolation. One case, reported by
Holland et al., is the binding of dinitrogen yielding the com-
plexes [LCoN CoL] or [M {LCoN CoL}], with L being a bulky
The synthesis of low-valent Co complexes has recently paved
the way to well-defined catalytic systems. For instance, owing to
the importance of boronate esters in organic synthesis, late
transition-metal complexes have been increasingly used in cata-
2
2
2
10
4
lytic systems for the hydroboration of alkenes and, sparsely, of
β-diketiminate bidentate ligand and M = Na, K (Chart 1, A).
Another interesting result is the formation of a Co−Co inter-
action in the complex [Co L ], (with L being amidinate or
11
12
aldehydes and ketones. In an example of a well-defined
Co(I) catalyst in alkene hydroboration, Chirik and co-workers
reported good yields of the respective organoborane reaction
products with anti-Markovnikov selectivity, under neat and mild
conditions, using a tridentate (bis(arylimino)pyridine)CoMe
2
2
t
i
guanidinate bidentate ligands (R = Bu, N( Pr) , or N(Cy) ;
2
2
5
Chart 1, B), reported by Jones and co-workers. Other notable
results obtained when isolating low-valent Co complexes have
resulted from noninnocent behavior of the supporting chelating
ligand. For example, a coordination of an aryl ring of a chelating
10b
complex.
The great majority of first-row transition-metal complexes
bearing 2-iminopyrrolyl ligands involve the coordination of
imine in a [LCo] arrangement, with L being an α-diiminate
2
ligand (derived from N,N′-bis(2,6-diisopropylphenyl)-2,
6
3
-dimethyl-1,4-diazabuta-1,3-diene) (Chart 1, C) or a bulky
Received: August 23, 2018
7
β-diketiminate bidentate ligand (Chart 1, D) have been
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02392
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Inorganic Chemistry
Article
Chart 1. Examples of Low-Valent Co Complexes (A−F) Containing Monoanionic Bidentate Chelates
two of these ligands. Many complexes of the type [M(2-imino-
pyrrolyl)2], including some made in our group, have been
activation of the pyridine chloride Co(II) complexes with super
hydrides, we attempted some stochiometric reactions with
1
3
14
15
reported (Chart 2, A). To avoid the bis-homoleptic coordination
K(HBEt ) in toluene (Scheme 1, route a). On the one hand,
3
Chart 2. Homoleptic (A) and Heteroleptic Co(II) (B)
Scheme 1. Synthesis of Complexes 2a and 2b from 1a and 1b,
Respectively
a
2
-Iminopyrrolyl Complexes
chemistry thermodynamic sink, we recently reported the pre-
paration of bulky 5-aryl-2-iminopyrrole ligand precursors. These
15
compounds allowed the preparation of Co(II) complexes
Chart 2, B) of a single chelating 5-substituted-2-iminopyrrolyl
(
ligand, which were also prepared in our group, having proved to
be active in the hydroboration of terminal α-olefins, upon
activation by K(HBEt3).
The latter Co(II) complexes served as the starting point for
the present study. Initially, the goal of this work was the under-
standing of the activation mode of these complexes (Chart 2, B)
a
Route a: Reactions of 1a and 1b with K(HBEt ) in toluene. Route b:
3
Reactions of 1a and 1b with Na(Hg), in toluene.
with K(HBEt ) in the hydroboration of terminal α-olefins, which
3
culminated in the synthesis and characterization of the first examples
of Co(I) complexes bearing a 5-aryl-2-iminopyrrolyl ligand. The
present study also addresses the high dependence of their electronic
structures on the nature of the 5-aryl substituents employed and
their behavior as efficient well-defined catalysts for the hydro-
boration of terminal alkenes and arylaldehydes with pinacolborane.
the reaction of a dark blue-violet toluene solution of complex 1a
i
(
[
R = Pr) with K(HBEt ) afforded the dark red complex
3
2
i
i
Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
6
C H )}(η -C H CH )] (2a) from a cooled dark red n-hexane
6
3
6
5
3
solution in moderate yields, after workup of the reaction mix-
ture. On the other hand, treating a dark olive-green toluene
solution of 1b (R = Ph) with K(HBEt ), the very dark green
RESULTS AND DISCUSSION
Synthesis and Characterization of Co(I) Complexes
Stabilized by Arene Ligands. To understand the mode of
3
■
2
6
complex [Co{κ N,N′-5-[2′-(κ:η -C H )-C H -4′,6′-Ph ]-
6
5
6
2
2
i
NC H -2-C(H)N(2,6- Pr -C H )}] (2b) was isolated from
4
2
2
6
3
B
DOI: 10.1021/acs.inorgchem.8b02392
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a cooled very dark green toluene solution in moderate yields. All
respective 5-(2,4,6-triphenylphenyl)-2-iminopyrrolyl moiety
and the absence of free toluene, in contrast with the observations
made for complex 2a. The pyrrolyl protons 3 and 4 appear at
7.64−7.12 and 5.97 ppm, respectively, being slightly shifted to a
lower field with respect to the ligand precursor. This same
observation is valid for the iminic and the meta protons of the central
phenyl ring of the 5-(2,4,6-triphenylphenyl) group (7.64−7.12 and
reactions with K(HBEt ) were systematically accompanied by
3
effervescence.
Alternatively, complexes 2a and 2b were obtained when the
respective precursor complexes 1a and 1b, respectively, were
reacted with a stoichiometric amount of freshly prepared sodium
amalgam in toluene solution, also in moderate yields (Scheme 1,
route b).
6
7.97 ppm, respectively). The resonance for the η -coordinated
The pathways for the formation of complexes 2a and 2b may
be discussed considering their independent synthetic proce-
dures. The synthesis of complexes 2a and 2b via reaction of 1a and
phenyl ring para proton appears very shifted to a higher field,
1
at 2.00 ppm, in the H NMR spectrum, and 104.7 ppm in the
1
3
1
C{ H} NMR spectrum (Figure S4 of the Supporting Infor-
1
b with K(HBEt ) likely involved the formation of a putative
mation), the cross-peak of which being clearly observed in the
3
2
1
13
1
Co(II) hydride bridging dimer complex, [{κ N,N′-5-(2,4,6-R -
H− C{ H} HSQC 2D correlation. This observation is very
likelyattributed tocontact chemical shifts induced by the observed
Co(I) complex spin isomerism (see below).
3
i
C H )-NC H -2-C(H)N(2,6-Pr -C H )} Co (μH,H)] (with
6
2
4
2
2
6
3
2
2
i
15
R = Pr orPh), with formation of KCland the Py → BEt adduct.
3
This presumed intermediate readily undergoes hydride reductive
elimination, with the liberation of molecular dihydrogen (as
hinted by the systematic effervescence in these reactions) and
subsequent stabilization by coordination of an η -arene moiety,
either from the reaction solvent, toluene, to form complex 2a, or
Complexes 2a and 2b were characterized by single-crystal
X-ray diffraction, having crystallized in the monoclinic system, in
the P2 /c and P2 space groups, respectively. The structure of
1
1
6
complex 2b has two independent molecules in the asymmetric
unit and two cocrystallized toluene molecules. The molecular
structures of complexes 2a and 2b are presented in Figure 1, and a
selection of their bond lengths and angles is presented in Table 1.
Complex 2a is a Co(I) arene complex with a coordinated
6
via the intramolecular η -coordination of one ortho phenyl ring
present in the 5-(2,4,6-triphenylphenyl) substituent, to form
complex 2b. The synthesis of complexes 2a and 2b by the
reaction of 1a and 1b with sodium amalgam is a one-electron
reduction reaction, via the abstraction of the chloride ligand by
metallic sodium, with the concomitant inter- or intramolecular
substitution of the pyridine ligand by the η -arene moiety,
respectively.
Complexes 2a and 2b are very sensitive to air and moisture,
bothinthe solidstateand in solution, despite they may be formally
regarded as 18-electron compounds. Complex 2a is paramagnetic,
having a high-spin electronic configuration, with a solution mag-
netic moment of 3.4 μ , in accordance with an S = 1 species.
Complex 2a is soluble in n-hexane, Et O, and toluene, and the
6
toluene molecule through its aromatic system in an η -coordi-
nation mode, considering that the Co1−Cn (with Cn corre-
sponding to C33 to C38) bond lengths are quite close and fall in
the range of 2.150(4)−2.194(3) Å (Δ ≈ 0.038 Å). The Co1−
centroid distance (with the centroid being defined as the center
of the six-membered ring formed by atoms C33 to C38) is equal
to 1.6613(16) Å. The Co1−N bond lengths are in the range of
2.025(3)−2.093(3) Å, where the 5-aryl-2-formimino-pyrrolyl
ligand exhibits the typical bidentate coordination mode. Both
the Co1−N and the Co1−centroid bond lengths are similar to the
6
16
B
previously crystalographically characterized Co(I) complexes of
2
6
5,6,17
lability of the η -toluene ligand in 2a is demonstrated by its easy
the type [LCo(toluene)].
Complex 2a has a trigonal planar
displacement by C D in solutions of this deuterated solvent,
geometry or, considering the tetrahedron formed by the N1, N2,
6
6
with the observation of the complex 2a-C D and a free toluene
C33, and C36 atoms bonded to Co1, a pseudotetrahedral coor-
6
6
6
molecule (Scheme 2). The species 2a-C D , that is, the species
dination geometry (with τ = 0.74). The η -toluene moiety is
6
6
4
perpendicular to the chelation plane (89.11°). The torsions of
the 5-aryl and the N2-aryl rings relative to the 2-iminopyrrolyl
plane are 80.2(3)° and 83.7(3)°, respectively.
Complex 2b is also a Co(I) arene complex, resulting from the
intramolecular coordination of the ortho phenyl ring of the
Scheme 2. Quantitative Displacement of the Toluene Ligand
in 2a by C D in Benzene-d Solution
6
6
6
5
-(2,4,6-triphenylphenyl) substituent of the 2-iminopyrrolyl
moiety. Complex 2b contains, therefore, the first case of a 5-aryl-
2
6
2
-iminopyrrolyl with a κ N,N′+κC:η -coordination mode. The
6
η -coordination mode is slightly distorted, as the Co1−Cn (with
Cn corresponding to C12 to C17) bond lengths fall in the range
of 2.002(9)−2.140(10) Å, with a mean absolute deviation of
0
.120 Å for both molecules of the asymmetric unit. The distorted
6
η -coordination mode happens at the expense of the rotation of
the central ring of 5-(2,4,6-triphenylphenyl) substituent, giving
rise to a near-planar π system spanning over the 2-iminopyrrolyl
core and the six-membered Co1−N1−C5−C1−C11−C12
chelate. The Co1−centroid distances (with the centroid being
defined as the center of the six-membered ring formed by atoms
C12 to C17) are in the range of 1.536−1.551 Å. The coordina-
tion geometry of complex 2b can be rationalized as pseudo-
square planar, if we consider the plane formed by the N1, N2,
monitored in C D solutions of complex 2a, displays a para-
6
6
1
magnetically shifted H NMR spectrum with broad resonances
accounting for the iminopyrrolyl moiety and a free toluene
molecule (Figure S1 of the Supporting Information). In cyclo-
1
hexane-d , the H NMR spectrum of 2a shows a similar pattern
1
2
to that of 2a-C D (Figure S2 of the Supporting Information).
6
6
However, complex 2b is partially soluble in n-hexane and
C12, and C15 atoms, bonded to Co1 (average τ = 0.10). The
4
Et O and very soluble in toluene, being diamagnetic at room
Co1−N1 and Co1−N2 bond lengths are in the ranges of
1.847(8)−1.859(7) and 1.883(8)−1.894(7) Å, respectively. The
intramolecular arene stabilization observed in 2b is rare and has
2
1
temperature. Its H NMR spectrum in C D (Figure S3 of the
6
6
Supporting Information) shows the expected resonances of the
C
DOI: 10.1021/acs.inorgchem.8b02392
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Figure 1. (top) ORTEP-3 diagrams of the X-ray diffraction structures of complexes 2a (left) and 2b (right) showing 30% probability ellipsoids.
Hydrogen atoms, cocrystallized toluene molecules, and the second molecule in asymmetric unit in the structure of 2b are omitted for clarity. (bottom)
Frontier orbitals of complexes 2a (left, HOMO) and 2b (right, LUMO).
been documented in Co complexes bearing the isocyanide
molecular orbital (HOMO) of a tetrahedral complex with the
same electron count.
On the one hand, considering the superimposition of both
18
CNArMes (with ArMes = 2,6-(2,4,6-Me C H ) -C H ).
2
2
3
6
2 2
6
3
The frontier orbitals of the two species 2a and 2b (their
frontier orbitals (d-splitting) are presented in Figures S9−S11 of
the Supporting Information) confirm the structural discussion
above. The LUMO of the closed shell complex (2b) can be
viewed as the 2b orbital of a square planar molecule, based on
2-iminopyrrolyl Co fragments of 2a and 2b, it is very clear that
6
the corresponding η -coordinated arenes are staggered, being
rotated in relation to each other by ca. 82°. On the other hand, it is
critically observed that the bond lengths involving the Co atom of
complex 2b are clearly shorter than those observed for complex 2a
(in average: Δ(Co1−N1) = 0.172 Å, Δ(Co1−N2) = 0.202 Å,
1
g
2
2
8
metal x −y , and empty in a d species. Conversely, in 2a the
equivalent orbital becomes the single occupied highest occupied
D
DOI: 10.1021/acs.inorgchem.8b02392
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Inorganic Chemistry
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7
8
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 2a and 2b
highlighting a lower oxidation state (going from a d to a d
configuration).
To obtain a toluene-free analogue of complex 2a, bearing the
2b
(I)
2
i
very reactive “[Co {κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-
3
6
2
4
2
i
2
a
molecule 1
molecule 2
C(H)N(2,6- Pr -C H )}]” scaffold, some reactions were
2
6
3
distances (Å)
Co1−N1
Co1−N2
Co1−C33/C12
Co1−C38/C17
Co1−C37/C13
Co1−C34/C16
Co1−C36/C15
Co1−C35/C14
N1−C5
N1−C2
N2−C6
N2−Cipso
C2−C3
C2−C6
C3−C4
C4−C5
attempted in the absence of that solvent. Initially, complex 1a
2.025(3)
2.093(3)
2.150(4)
2.154(4)
2.171(4)
2.184(4)
2.188(3)
2.194(3)
1.357(4)
1.382(4)
1.293(4)
1.433(4)
1.386(4)
1.403(4)
1.383(4)
1.404(4)
1.859(7)
1.894(7)
2.022(9)
2.085(9)
2.088(9)
2.114(8)
2.106(9)
2.140(10)
1.360(11)
1.362(11)
1.312(11)
1.449(11)
1.400(12)
1.411(12)
1.371(13)
1.406(12)
1.847(8)
1.883(8)
2.002(9)
was reacted with K(HBEt ) in n-hexane, from which a color
change from dark blue-violet to dark red was observed. After
workup, an uncharacterizable mixture of reaction products was
observed by H NMR spectroscopy. The chemical reduction
reactions of complex 1ain n-hexane or Et O with sodiumamalgam
only led to the decomposition of the respective reaction mixtures.
Ultimately, the reaction of complex 1a with KC in Et O afforded
3
1
2.068(10)
2.092(10)
2.123(10)
2.095(11)
2.110(8)
1.372(11)
1.388(12)
1.313(12)
1.423(11)
1.400(13)
1.405(12)
1.368(13)
1.428(12)
2
8
2
2
the isolation of the dark red binuclear complex [Co{κ N,N′-5-
i
i
(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -C H )}] (3)
3
6
2
4
2
2
6
3
2
(Scheme 3, route a). Complex 3 is alternatively prepared by heating
complex 2a in refluxing n-hexane (Scheme 3, route b).
The synthesis of complex 3 via reaction of 1a with KC is a
one-electron chemical reduction and shows the preferred stabi-
lization of this system by an arene moiety, as opposed to the
coordination of Et O. The latter route demonstrated the lability
of the toluene ligand of complex 2a at higher temperatures. The
8
2
absence of an arene moiety causes the self-assembling of the
2
i
i
angles (deg)
[
Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
N1−Co1−N2
N1−Co1−C33/C12
N2−Co1−C36/C15
C33/C12−Co1−C36/C15
N2−Co1−C33
N1−Co1−C36
C5−N1−C2
N1−C2−C3
N1−C2−C6
C3−C2−C6
C4−C3−C2
80.56(10)
127.52(12)
123.63(11)
81.39(14)
122.27(14)
127.70(12)
106.4(3)
110.0(3)
116.7(3)
133.2(3)
106.9(3)
83.8(3)
89.1(3)
101.8(4)
85.4(4)
84.1(3)
89.6(4)
100.9(4)
85.8(4)
C H )}] synthon into the dimeric complex 3. As observed for
6 3
complexes 2a,b, complex 3 is very sensitive to air and moisture
and is paramagnetic. In contrast with complex 2a, which is very
soluble in n-hexane, complex 3 is only partially soluble in that
solvent. The partial solubility of complex 3 in cyclohexane-d
1
2
1
allowed the detection of its H NMR spectrum (Figure S5 of the
Supporting Information), showing paramagnetically shifted
resonances. These facts frustrated the determination of the solu-
tion magnetic moment of complex 3 in cyclohexane-d . The
108.0(8)
109.1(8)
112.7(9)
138.2(10)
106.9(8)
107.5(8)
108.5(8)
115.8(9)
110.1(8)
107.4(9)
112.7(10)
139.8(10)
107.6(9)
108.9(11)
105.6(8)
115.0(10)
1
2
systematic isolation of Co(I) arene complexes strongly suggests
that this system is not suitable for the bonding or activation of
C3−C4−C5
N1−C5−C4
N2−C6−C2
106.9(3)
109.9(3)
119.5(3)
dinitrogen or weakly coordinating molecules, such as Et O. This
2
observation contrasts with the propensity of certain Co com-
plexes bearing N,N bidentate anionic ligands for bonding or
4
activating dinitrogen under similar reaction conditions.
Δ(Co1−N1) = 0.085 Å), attributed to a low-spin electronic
state of the former. These two observations reinforce the differ-
ent electronic structures of 2a and 2b.
Complex 3 crystallized in the triclinic system, in the P1
̅
space
group, and is composed by an asymmetric unit containing the
2
i
i
[Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
The Co1−N bond lengths in complexes 2a and 2b are shorter
C H )}] moiety. Complex 3 is generated by an −x, −y, −z
symmetry operation of the previous fragment, with an inversion
6
3
15
than in the family of their Co(II) precursor complexes,
a
Scheme 3. Synthesis of the Dimeric Complex 3
a
Either by chemical reduction of complex 1a in Et O (route a) or by stirring complex 2a in refluxing n-hexane (route b).
2
E
DOI: 10.1021/acs.inorgchem.8b02392
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Inorganic Chemistry
Article
Figure 2. ORTEP-3 diagram of the X-ray diffraction structure of complex 3 showing 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): Co1−N1: 2.020(2), Co1−N2: 2.1691(19), Co1−centroid: 1.6818(9), Co1−C21: 2.183(2), Co1−C22: 2.170(2),
Co1−C23: 2.194(2), Co1−C24: 2.260(2), Co1−C25: 2.190(2), Co1−C26: 2.180(2), N1−C5: 1.359(3), N1−C2: 1.373(3),
N2−C6: 1.302(3), N2−C21_1: 1.429(3), C1−C5: 1.486(3), C2−C3: 1.403(3), C2−C6: 1.408(3), C3−C4: 1.394(4), C4−C5: 1.405(3). Selected
bond angles (deg): N1−Co1−N2: 79.97(8), N1−Co1−C21: 177.25(8), N2−Co1−C21: 97.56(8), N1−Co1−C24: 103.47(9), N2−Co1−C24:
1
75.57(8), C21−Co1−C24: 79.05(9), C5−N1−C2: 106.13(19), C6−N2−C : 117.0(19), C7−C1−C5: 119.5(2), N1−C2−C3: 110.7(2),
ipso
N1−C2−C6: 116.7(2), C3−C2−C6: 132.5(2), C4−C3−C2: 105.8(2), C3−C4−C5: 106.8(2), N1−C5−C4: 110.5(2), N1−C5−C1: 120.0(2),
C4−C5−C1: 129.5(2), N2−C6−C2: 120.8(2), C20−C18−C19: 109.5(2), C22−C21−C26: 120.8(2), C23−C22−C21: 118.4(2), C24−C23−C22:
1
22.0(2), C25−C24−C23: 118.6(2), C24−C25−C26: 122.0(2), C25−C26−C21: 118.0(2).
center at [0, 0, 0]. The molecular structure of complex 3 is pre-
sented in Figure 2, including selected bond lengths and angles.
Complex 3 is a dimer, where the Co atoms are bonded to one
Synthesis and Characterization of a Co(I) Complex
Stabilized by Trimethylphosphine. The stabilization of the
Co(I) complexes with 5-aryl-2-iminopyrrolyl ligands was also
explored with other neutral ligands, such as trimethylphosphine.
The salt metathesis reaction of the Co(I) starting material
CoCl(PMe ) with the potassium 5-(2,4,6-triisopropylphenyl)-
2
5
-aryl-2-iminopyrrolyl ligand in the κ N,N′ mode and further
6
stabilized by the η -coordination of the N-2,6-diisopropylphenyl
ring of the other unit. The Co1−Cn (with Cn corresponding to
C21 to C26) bond lengths are relatively close and fall in the
range of 2.170−2.256 Å (Δ ≈ 0.086 Å). The Co1−centroid dis-
tance (with the centroid being defined as the center of the six-
membered ring formed by carbon atoms C21 to C26) is equal to
3
3
2-(N-2,6-diisopropylphenylformimino)pyrrolyl (KLa) smoothly
2
afforded the dark red-brown Co(I) complex [Co{κ N,N′-5-
i
i
(2,4,6-Pr -C H )-NC H -2-C(H)N(2,6- Pr -C H )}(PMe ) ]
3
6
2
4
2
2
6
3
3 2
(4) in good yields, from a cooled n-hexane solution (Scheme 4,
route a). Complex 4 can be alternatively prepared by the reaction
of Co(PMe ) with the ligand precursor 5-(2,4,6-triisopropyl-
1
.6818(9) Å. Complex 3 is structurally analogous to complex 2a,
the important variation being the nature of the arene ligand.
In the case of complex 3, the Co1−N1 and Co1−N2 bond
lengths are 2.020(2) and 2.1691(19), respectively. This observa-
tion contrasts with the very similar Co1−N1 and Co1−N2 bond
lengths in complex 2a. This difference is attributed to the rigidity
of the 2,6-diisopropylphenyl ring and the stereochemical stress
imparted by the isopropyl groups closer to N2. The torsions of
the 5-aryl and the N2-aryl rings relative to the 2-iminopyrrolyl
plane are 84.7(3)° and 89.0(2)°, respectively. These results
are in accordance with the very few Co(I) complexes of
3
4
phenyl)-2-(N-2,6-diisopropylphenylformimino)-1H-pyrrole
(HLa) (Scheme 4, route b). In this latter case, it is proposed that
a NH oxidative addition to the Co(0) center in Co(PMe3)4
occurs, yielding a putative unobserved Co(II) hydride complex,
that readily undergoes reductive elimination, with liberation of
molecular dihydrogen, to form complex 4. This latter observa-
tion was similarly reported in the synthesis of bis(trimethyl-
13h
phosphine) bis(2-iminopyrrolyl) Fe complexes by Sun et al.
This latter result reiterates the instability of potential Co(II)
hydride complexes to the reaction conditions.
6
,7
this type.
a
Scheme 4. Synthesis of Complex 4
a
Route a: starting from KLa and CoCl(PMe ) . Route b: starting from HLa and Co(PMe ) .
3
3
3 4
F
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Inorganic Chemistry
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Complex 4 is very sensitive to air in solution, but very crys-
talline samples are relatively stable in the solid state. Complex 4
lengths are in the range of 2.2198(11)−2.2463(12) Å, which is
typical of other crystalographically characterized Co(I) complexes
is formally a 16-electron species, with a solution magnetic
bearing a monoanionic bi- or tridentate chelate and two trimethyl-
8
19
moment equal to 3.4 μ , which is expected for a d ion in the
phosphine ligands in cis positions. Complex 4 has a τ parameter
B
4
1
6
high-spin state and is in accordance with its paramagnetically
equal to 0.66, which is nearly an intermediate case between a
square planar and tetrahedral coordination geometry, probably
best described as trigonal pyramidal.
1
shifted and broad H NMR spectrum (Figure S6 of the
Supporting Information). Despite this paramagnetism, sparse
coupling patterns are observed, allowing for partial unambig-
In C D solutions, the conversions of complex 2a into 4 and
6
6
uous assignments. The PMe proton resonance appears at a very
complex 4 into 2a-C D can be clearly monitored by NMR
3
6 6
low field, at 69.6 ppm, and displays a broad characteristic, with
spectroscopy. In one experiment, treatment of a solid mixture of
Δν equal to 93 Hz. This compound does not display a
complex 4 with 1 equiv of 1-azidoadamantane (AdN ) in C D
1
/2
3
6
6
3
1
1
P{ H} NMR spectrum.
resulted in immediate effervescence of N and formation of complex
2
Complex 4 was characterized by single-crystal X-ray diffrac-
2a-C D (Scheme 5), as pointed out by the corresponding
6
6
tion, having crystallized in the triclinic system, in the P1
group. The molecular structure of 4 is presented in Figure 3,
̅
space
Scheme 5. Chemical Interexchange between Complexes
2
a/2a-C D and 4
6 6
1
H NMR spectrum, accompanied by formation of AdNPMe ,
3
31
1
which is assigned by its P{ H} NMR spectrum (a singlet at
20
1
4.3 ppm−Figure S7 of the Supporting Information). Con-
versely, complex 2a is rationally converted to complex 4 by
treating the former with 2 equiv of PMe in C D , as shown by
3
6
6
1
H NMR spectroscopy (Scheme 5).
Spin Isomerism in Complex 2b−Experimental Deter-
mination of the Energy between Spin Isomers by
Variable Temperature NMR Spectroscopy. Spin isomerism
4
7
in d to d metal complexes has been reported over the years,
Figure 3. ORTEP-3 diagram of the X-ray diffraction structure of complex
showing 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å): Co1−N1: 2.026(3), Co1−N2:
21
22
mainly for Co(III) and for Fe(II) and Fe(III) compounds.
More recently, Co(II) complexes have also been reported to
4
23
exhibit spin isomerism behavior. However, spin isomerism in
2
.113(3), Co1−P1: 2.2198(11), Co1−P2: 2.2463(12), N1−C5:
Co(I) is virtually unknown, except in an example reported by
Holland and co-workers of an LCo(CO) complex, with L being
a bulky β-diketiminate bidentate ligand identical to that of com-
pound A (see Chart 1). In that case, the mechanism for the spin
1
.365(5), N1−C2: 1.378(5), N2−C6: 1.296(5), N2−C : 1.431(5),
ipso
C2−C3: 1.387(5), C2−C6: 1.414(5), C3−C4: 1.384(6), C4−C5:
1
8
.399(5), C5−C1: 1.494(5). Selected bond angles (deg): N1−Co1−N2:
2.80(11), N1−Co1−P1: 151.89(9), N2−Co1−P1: 102.18(9), N1−
24
Co1−P2: 99.84(9), N2−Co1−P2: 114.59(9), P1−Co1−P2: 102.94(4),
isomerism was a reversible arene slippage stabilization.
C5−N1−C2: 106.1(3), C6−N2−C : 116.9(3), C3−C2−N1:
ipso
The unexpected diamagnetism of complex 2b prompted us to
110.7(3), C3−C2−C6: 131.4(3), N1−C2−C6: 118.0(3), C2−C3−
undergo further studies. First, variable-temperature (VT)
C4: 106.1(3), C3−C4−C5: 107.6(3), N1−C5−C4: 109.6(3), N1−
C5−C1: 123.6(3), C4−C5−C1: 126.7(3), N2−C6−C2: 121.4(3).
1
H NMR experiments were performed and are presented in
Figure S8 of the Supporting Information. According to the VT
1
H NMR spectra of 2b it is possible to observe the presence of
including selected bond lengths and angles. Complex 4 is a
tetracoordinated complex with a κ N,N′-coordinated 5-aryl-2-
contact shifts in several resonances as the temperature rises.
At 30 °C, the spectrum starts to shift away from the diamagnetic
region. As the temperature rises between 30 and 90 °C, some
paramagnetic resonances start to appear and become more
intense. At 90 °C, complex 2b displays a paramagnetically
2
iminopyrrolyl ligand and two adjacent trimethylphosphine
ligands. The Co1−N1 and Co1−N2 bond lengths are 2.026(3)
and 2.113(3) Å, respectively, being slightly longer than the ones
observed for complex 2a. This observation is a testament to
the higher electronic density in complex 4 due to the higher
σ-donating properties of the two trimethylphosphine ligands,
giving rise to the largest N1−Co−N2 bite angle of these
two monomeric complexes (82.80(11)°). The Co1−P bond
1
shifted H NMR spectrum and a loss of resolution, suggesting
that an equilibrium between a low-spin (S = 0; singlet, 2bS=0
)
and high-spin (S = 1; triplet, 2bS=1) isomer is occurring. This
effect is reversible, since the spectrum of 2b at room temperature
after cooling is perfectly reproducible.
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According to this assumption, the chemical shifts of the
H NMR spectra can be modeled considering a Boltzmann
1
distribution of spin states (details of the model are depicted in
21,25,26
the Supporting Information).
The experimental chemical
shifts fitted to equation S1 of the Supporting Information as a
function of temperature, for three different resonances of com-
plex 2b, are shown in Figure 4. The thermodynamic data derived
from the model for the 2bS=0 ↔ 2bS=1 equilibrium is presented in
Table 2.
Figure 4. Fittings of the experimental chemical shifts (points), δ (ppm),
to the calculated chemical shifts determined by equation S1 of the
Supporting Information (solid lines) vs temperature T (K), for different
1
H NMR (300 MHz, toluene-d ) resonances. Blue, green, and red
8
correspond to H8, H4, and H13a protons, respectively, according to the
numbering scheme of the Experimental Section.
Figure 5. Optimized geometries of the low-spin (bottom, 2bS=0) and
high-spin (top, 2bS=1) isomers, with selected bond lengths (Å) and
angles (deg) in italic.
Table 2. Calculated Values of ΔH°, ΔS°, and ΔG° (at 298 K)
from the Fittings Using Different Resonances and Their
Average Values
stretch in the bond lengths in 2bS=1 is an expected observation
for a Co(I) complex in the high-spin state and is achieved at the
expense of a slight tilting of the arene ring from orthogonality
with the N1−Co1−N2 plane (teapot’s or Aladdin lamp’s hinged
lid type of motion), from 89.98° in 2b to 83.84° in 2b . The
Resonances
Parameter
H8 (blue) H4 (green) H13a (red)
Average
S=0
S=1
ΔH° (kcal mol−1)
8.4
9.5
5.5
7.3
5.9
5.6
7.5
5.9
5.8
7.7
7.1
5.6
average increase in the Co1−C
bond lengths (from 2bS=0 to
arene
ΔS° (cal mol−
1
K
−1)
2bS=1) is 0.076 Å, which is lower than the other differences,
possibly associated with a slight change of the arene coordi-
nation mode in the spin isomers. The frontier orbitals of com-
ΔG° (kcal mol−1)
It can be observed from Figure 4 that the experimental data fit
plex 2b are similar to the ones calculated for 2a, being typical
of a d tetrahedral complex (see Supporting Information). The
S=1
8
well with the proposed distribution and allowed the calculation
−
1
of an average positive value of ΔG° = 5.6 kcal mol , at 298 K.
τ4 parameters of 2bS=0 and 2bS=1 increase from 0.13 to 0.19,
respectively. This subtle difference indicates a higher tetrahedral
distortion of the latter isomer. The experimental bond lengths
of complex 2b, determined by X-ray diffraction, are better
reproduced in 2bS=0 (maximum absolute variation equal to
0.02 Å). A comparison of experimental and calculated selected
structural parameters, for both spin isomers, is presented in
Table 3.
This result indicates that this process favors 2b , being
S=0
−
1
endothermic (ΔH° = 7.7 kcal mol ). The positive value for
−
1
−1
ΔS° (7.1 cal mol K ) may be explained by a general increase
in the metal-to-ligand bond lengths in the conversion process of
2
bS=0 into 2bS=1 (see below in next subsection).
Spin Isomerism in Complex 2b−DFT Calculations and
Determination of the Minimum-Energy Crossing Point.
Complex 2bS=0 is 4.2 kcal mol− more stable than its high-spin
counterpart 2bS=1 (ΔE) corroborating the low-spin state observed
for the complex and in fair accordance with the experimental
1
It was experimentally established that complex 2b exhibits tem-
1
perature-dependent H NMR spectra, being translated in the
2
bS=0 ↔ 2bS=1 spin equilibrium. We started by examining the
structures of both spin isomers by performing the respective
geometry optimizations utilizing density functional theory
−
1
stability difference (ΔH° = 7.7 kcal mol ).
Analogous calculations were performed for complex 2a, where
the geometries of the spin isomers 2aS=0 and 2aS=1 were opti-
mized, their respective atomic coordinates being presented in
the Supporting Information. In contrast with the spin isomers of
2b, it was established for 2a that the high-spin isomer, 2aS=1, is
(
DFT) calculations, using the OPBE functional. The optimized
geometries for both the low-spin (2bS=0) and high-spin (2bS=1
)
isomers are presented in Figure 5, and the respective atomic
coordinates are listed in the Supporting Information.
Looking at the two structures, it can be observed that the
−
1
5.6 kcal mol below the low-spin one (ΔE), 2aS=0. In addition,
the experimental bond lengths determined for complex 2a lie
closer to those calculated for the high-spin isomer, 2aS=1, with a
maximum absolute variation of 0.097 Å (as opposed to a
Co1−N1 and Co1−N2 bond lengths in 2b are 0.124 and
S=1
0
.229 Å longer than in 2bS=0. Similarly, the Co1−C13a distance
in 2bS=1 is 0.128 Å longer than that of 2bS=0. The observed
H
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Table 3. Comparison of Selected Structural Parameters of the
Single-Crystal X-ray Diffraction Structure of 2b and the
Optimized Structures of 2bS=0 and 2bS=1
2
b (av exp)
2bS=0
2bS=1
a
a
Co1−N1
Co1−N2
1.853(8)
1.888(8)
2.101(10)
84.0(3)
101.4(4)
0.10
1.847
1.921
2.114
84.01
101.73
0.13
2.150
1.971
2.242
80.96
106.35
0.19
a
Co1−C13a
N1−Co1−N2
N2−Co1−C13a
b
b
c
τ4
a
b
c
In angstroms. In degrees. Calculated considering the square
defined by N1−N2−C13a−Cipso (see Figure 5).
maximum Δ(2a−2aS=0) of 0.166 Å). These observations
support the experimental isolation of complexes 2a and 2b in
two different spin states.
The change in spin state in 2b from 2bS=1 to 2bS=0 corre-
sponds to a spin-forbidden reaction, whereby such a profile goes
through a minimum-energy crossing point (MECP) of the two
potential energy surfaces (PES) involved, the ones of 2b and
S=0
2
7
of 2bS=1
.
The structure corresponding to the MECP is 2bCP.
In this MECP, both the energy and the geometry of the molecule
are the same in the two surfaces. Once that point is reached along
the reaction coordinate, there isa certainprobabilityfor the system
to change spin state and hop from one PES to the other, giving
28
rise to the “spin-forbidden” reaction. A schematic representation
of the spin-state transition alongside the structure of 2b is pre-
CP
−1
sented in Figure 6. The energy of 2bCP is 1.6 kcal mol above
bS=1 indicating a low barrier for the process of spin exchange,
2
in accordance with its experimental observation. The observed
low barrier is attributed to the subtle structural changes between
the three different structures considered for 2b (see Table 3).
Hydroboration of Terminal Alkenes and Arylalde-
hydes Catalyzed by Complexes 2a,b, 3, and 4. With the
prepared Co complexes 2a,b, 3, and 4 being coordinatively unsatu-
rated, in a low oxidation state, and containing two potentially labile
ligands in adjacent positions, we decided to test them as catalysts
for hydroboration of terminal alkenes and arylaldehydes with
pinacolborane (HBPin).
Preliminary hydroboration reactions of styrene with 1% molar
of these complexes were attempted, in neat conditions. It was
observed that only complex 4 was hydroboration-selective, exclu-
sively yielding the anti-Markovnikov (a-Mk) addition product in
Figure 6. Representation of the MECP of the two PES, with the
respective energy balance in italics (in kcal mol ) (top) and the
optimized structure of 2bCP (bottom).
−
1
selectivity observed in this work follows a common tendency
found in the literature surrounding this topic.
All complexes were also tested in the hydroboration of
benzaldehyde, in C D solutions. The complexes proved to be
6
6
6
4% yield of isolated product. It should be emphasized that the
extremely effective in this reaction, displaying full conversion in
less than an hour, with high yields of isolated products. The
results of these reactions are presented in Table 5, and the
same reaction performed in the presence of the related catalyst
system 1a/K(HBEt ), in similar reaction conditions, led to a
much lower yield (35%) and selectivity (a-Mk/Mk = 2.14:1).
3
15
1
H NMR spectra of the obtained boronate esters are shown in
However, complexes 2a,b and 3 yielded unidentified products.
Encouraged by the nearly quantitative selectivity of complex 4,
we tested different terminal alkenes in this reaction. The results
for the substrate scope study catalyzed by complex 4 are pre-
the Supporting Information.
Given the high reactivity of these complexes toward the
hydroboration of benzaldehyde, the substrate scope of arylalde-
hydes was evaluated with complex 4. These results are presented
1
1
sented in Table 4, and the H NMR spectra of the alkylboronates
in Table 6, the H NMR spectra of the obtained boronate esters
are shown in Figures S12−S15 of the Supporting Information.
As observed with styrene, complex 4 catalyzed the hydro-
boration of other terminal olefins in high yields and with total
selectivity in the anti-Markovnikov addition product.
being shown in Figures S16−S20 of the Supporting Information.
It can be observed that complex 4 is equally effective with other
arylaldehydes, also giving rise to fast and complete conversions
and high yields in isolated products. Incontrast, the hydroboration
of acetophenone always gave conversions below 10% after 24 h of
reaction. The nearly quantitative yields obtained for this aldehyde
scope are in the same range of those reported in the literature for
The yields obtained in this work are in the range of those
found by authors reporting cobalt-catalyzed hydroboration of
1
0c,d
the same alkenes.
Even though the presently obtained yields
11
are respectable, some authors have reported yields higher than
systems containing late transition-metal-based catalytic systems.
9
0%, for most of the substrates and in less than an hour, in similar
Although we have not yet found experimental evidence for the
catalytic pathway of these hydroboration reactions, we can propose
1
0b,f,k
reaction conditions.
The exclusive anti-Markovnikov
I
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Table 4. Scope of the Hydroboration of Terminal Alkenes
Catalyzed by Complex 4
Table 6. Scope of the Hydroboration of Arylaldehydes
Catalyzed by Complex 4 in C D6
6
a
Conditions: 1 mol % of 4, 2 mmol of substrate, 2.5 mmol of HBPin.
b
Reaction time: 16 h, temperature: 25 °C. Yields determined by
weighing the isolated reaction products. Calculated by H NMR.
c
1
Table 5. Hydroboration of Benzaldehyde Catalyzed by
Complexes 2a,b, 3, and 4 in C D6
6
a
b
complex
conversion (yield) (%)
2
2
3
4
a
b
>99 (85)
>99 (90)
>99 (90)
>99 (91)
a
Conditions: 1 mol % of 4, 1 mmol of benzaldehyde, 1.1 mmol of
b
HBPin. Reaction time: 1 h, temperature: 25 °C. Conversion
calculated by H NMR and yields determined by weighing the
isolated reaction products. Conversion after 24 h.
1
a
Conditions: 1 mol % of 2a-b, 3, or 4, 1 mmol of benzaldehyde,
.1 mmol of HBPin. Reaction time: 1 h, temperature: 25 °C. Con-
version calculated by H NMR and yields determined by gravimetry
weighing the isolated reaction products.
c
b
1
1
Scheme 6. Proposed Catalytic Cycle of the Hydroboration
of Terminal Alkenes and Arylaldehydes Catalyzed by 2a,b,
3
a
, or 4
a mechanism whereby there is a partial or full substitution of the
Co(I) precatalysts neutral ligands by the organic substrates,
which become activated, followed by reaction with HBPin to
generate a hypothetic Co(III) intermediate species, possibly via
a concerted migratory insertion. A final reductive elimination
step, yielding the corresponding organic product and concomi-
tant regeneration of the Co(I) initial species, closes the catalytic
cycle (Scheme 6). The improved selectivity in the hydrobor-
ation reaction of styrene performed in the presence of complex
4
1
, when compared to that observed with the catalyst system
1
5
a/K(HBEt ) from an a-Mk/Mk ratio of 2.14:1 to greater
3
than 99:1is justified by the presence of strongly donating
PMe ligands, which are not present in the former case. The
3
PMe ligands in complex 4 very likely stabilize the coordination
3
sphere of the intermediate complexes of the respective catalytic
cycle, their presence strongly promoting (nearly quantitatively)
the formation of terminal addition products.
a
1
29
For aldehydes η -coordination of the substrate is more favored.
CONCLUSIONS
■
or with Na(Hg) in toluene. Complex 3 was prepared from the
reaction of complex 1a with KC in Et O. Finally, complex 4 was
prepared in moderate to good yields by salt metathesis of
Four new Co(I) complexes bearing a 5-aryl-2-iminopyrrolyl
ligand and arene or trimethylphosphine ligands have been
synthesized, characterized, and tested in the hydroboration of
several substrates such as terminal alkenes and arylaldehydes.
Complexes 2a and 2b were prepared in moderate yields from
thereactionsofcomplexes 1aand 1b, respectively, with K(HBEt3)
8
2
CoCl(PMe ) with the potassium salt KLa or by reacting
3
3
Co(PMe ) with the ligand precursor HLa. Complexes 2a, 3, and
3
4
4 are paramagnetic high-spin species and were characterized by
1
elemental analysis, H NMR spectroscopy, and X-ray diffraction.
J
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However, complex 2b revealed to be unexpectedly diamag-
Anal. Calcd for C39
77.20), H 9.59 (8.47), N 4.62 (4.62)%. μeff (toluene-d ): 3.4 μ .
H
51CoN
2
, obtained (calculated): C 77.26
1
13
1
(
netic, and it was fully characterized by H and C{ H} NMR
spectroscopy, elemental analysis, and X-ray diffraction. The dia-
magnetism of 2b proved to display spin isomerism, as indicated
8 B
1
H NMR (300 MHz, C D ): δ 63.03 (s, 1H), 18.67 (br, 1H, CH(CH ) ),
6
6
3 2
1
7.33 (br, 2H), 13.72 (br, 2H), 10.46 (br, 2H, CH(CH ) ), 6.02 (br, 6H,
3 2
1
CH(CH ) ), 5.26 (br, 6H, CH(CH ) ), 4.69 (br, 1H), 3.14 (br, 6H,
3 2 3 2
by VT H NMR spectroscopy and DFT studies. The VT
CH(CH ) ), 2.38 (br, 6H, CH(CH ) ), 1.94 (br, 6H, CH(CH ) ),
1
3
2
3
2
3 2
H NMR experiments showed the presence of contact shifts
1
−
4.82 (br, 1H), −37.08 (br, 1H). H NMR (300 MHz, cyclohexane-d ):
δ 61.87 (s, 1H), 18.61 (br, 1H, CH(CH ) ), 17.32 (br, 2H), 13.44 (br,
2H), 9.73 (br, 2H, CH(CH ) ), 6.11 (br, 6H, CH(CH ) ), 5.28 (br,
6H, CH(CH ) ), 4.68 (br, 1H), 3.03 (br, 6H, CH(CH ) ), 2.35 (br, 6H,
3 2 3 2
1
2
−
1
above 30 °C, with ΔG° (298 K) = 5.6 kcal mol , in an endo-
3
2
−
1
thermic (ΔH° = 7.7 kcal mol ) process. Computational studies
3 2 3 2
on complex 2b showed that the low-spin S = 0 isomer (2b ) is
S=0
−1
the most stable (ΔE = 4.2 kcal mol ). The minimum-energy
crossing point (2b ) was determined, standing 1.6 kcal mol
CH(CH
3
)
2
2
), 2.02 (br, 6H, CH(CH
3
) ), −4.97 (s, 1H), −36.28 (s, 1H).
2
−
1
6
CP
[Co{κ N,N′-5-[2′-(κ:η -C
6
H
5
-C
6
H
2
-4′,6′-Ph )]-NC
2
H
4 2
-2-C(H)
i
above the high-spin isomer (2bS=1).
N(2,6- Pr
2
-C H
6
3
)}]
2
(2b). Route a. Toluene was added to a solid
mixture of complex 1b (0.09 g, 0.12 mmol) and K(HBEt ) (0.002 g,
.14 mmol) at room temperature, to give a very dark green solution.
All complexes were tested in the hydroboration of styrene, with
complex 4 exclusively yielding the respective anti-Markovnikov
addition products in high yields. Furthermore, all complexes effi-
ciently catalyzed the hydroboration of benzaldehyde in nearly
quantitative yields, in 1 h. Complex 4 was also capable of cata-
lyzing the hydroboration of other terminal alkenes and arylalde-
hydes in high yields, but the corresponding reaction with aceto-
phenone led to low yields even after 24 h of reaction.
3
0
The mixture was stirred at room temperature for 2.5 h, to yield a very
dark green suspension. All volatile materials were evaporated under
reduced pressure, to give a very dark green residue. The residue was
washed with n-hexane and extracted with toluene, with separation of a
pale precipitate and a negligible dark residue. The very dark green
solution was concentrated and stored at −20 °C, giving rise to a very dark
green crystalline solid suitable for X-ray diffraction. Yield: 0.08 g (40%).
Route b. A toluene solution of complex 1b (0.35 g, 0.48 mmol) was
added to freshly prepared 5% Na(Hg) (0.025 g, 1 mmol of Na; 0.3 mL,
In a complementary perspective, this work provided a good
insight of the previously reported reactivity of our pyridine
15
2
0 mmol of Hg). The dark olive-green solution gradually turned to a
chloride Co(II) complexes 1a,b to form Co(I) species.
very dark green suspension. The mixture was stirred for 6 h at room
temperature. The solution was filtered and the volatile materials were
evaporated to dryness to give a very dark green residue. The residue was
washed with n-hexane, extracted with toluene and the extracts concen-
trated and stored at −20 °C, yielding a very dark green powder. Yield:
0.10 g (35%).
EXPERIMENTAL SECTION
■
General Considerations. All operations were performed under
purified dinitrogen atmosphere using standard glovebox and Schlenk
techniques unless otherwise stated. Solvents were predried with activ-
ated 4 Å molecular sieves and distilled by refluxing under dinitrogen for
several hours over suitable drying agents (sodium/benzophenone for
Anal. Calcd for C41H37CoN obtained (calculated): C 79.55 (79.85),
2
1
H 6.00 (6.05), N 4.42 (4.54)%. H NMR (300 MHz, toluene-d
,
8
diethyl ether and toluene; CaH for n-hexane), being stored under
298 K): δ 7.97 (br, 2H, H8a + H12), 7.76 (br, 2H, H8 + H12a),
7.64−7.12 (m, 11H, N = CH + H3 + H11 + H13 + H15−17 + H11a),
2
dinitrogen. Solvents and solutions were transferred using a positive pres-
sure 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. Trimethylphosphine was purchased in 1 M toluene
3
6.82 (t, 1H, N-Ph-Hpara, JHH = 6.9 Hz), 5.97 (br, 1H, H4), 3.96 (sept,
3
2H, CH(CH
)
2
, JHH = 5.4 Hz), 2.00 (br, 1H, H13a), 1.40 (d, 6H,
3
3
3
CH(CH
)
2
, JHH = 3.0 Hz), 1.03 (d, 6H, CH(CH
H NMR (300 MHz, toluene-d , 353 K): δ 12.01, 11.08, 10.32, 9.68,
) , JHH = 3.0 Hz).
3 2
3
1
solutions and used as received. K(HBEt ) was purchased in THF
8
3
solutions and was used as a solid by recrystallization from the same solvent,
being stored at 4 °C. Sodium amalgam was prepared inside the glovebox
by carefully adding freshly cut sodium metal to mercury with stirring. KC8
was prepared by mixing freshly cut potassium metal and graphite in the
correct stoichiometry inside the glovebox and heating the mixture in a
Schlenk tube to 150 °C for 2 h, until a golden solid was observed. The
8.63, 8.22, 6.24, 4.60, 4.25, 1.73, 1.27, 1.10, 0.87, 0.24, −2.18, −2.87,
1
−7.61, −10.11, −12.33, −19.17, −23.72, −32.39, −57.46. H NMR
(300 MHz, C D , 298 K): δ 7.99 (s, 1H, H8a), 7.90 (br, 2H, H12a),
6 6
7.81 (s, 1H, H8), 7.79 (s, 1H, NCH), 7.71 (br, 1H, H3), 7.62−7.38
(m, 6H, H11 + H15 + H13 H11a), 7.35−7.19 (m, 5H, H12 + H16 +
3
H17), 7.14−7.06 (m, 2H, N-Ph-Hmeta), 6.88 (t, 1H, N-Ph-Hpara, JHH
=
1
5
15
2
3
ligand precursor HLa, the potassium salt KLa, [Co{κ N,N′-5-
7.2 Hz), 6.07 (br, 1H, H4), 3.95 (sept, 2H, CH(CH ) , JHH = 6.6 Hz)
3 2
i
i
15
3
(
[
(
2,4,6-Pr -C H )-NC H -2-C(H)N(2,6-Pr -C H )}(Py)Cl] (1a),
2.00 (br, 1H, H12a), 1.38 (d, 6H, CH(CH
3
)
2
,
J
HH = 4.8 Hz),
, JHH = 4.5 Hz). C{ H} NMR (75 MHz,
C D , 298 K): δ 151.6 (C10), 145.37 (C14), 144.2 (N-Ph−Cortho),
6 6
3
6
2
4
2
2
6
3
2
i
3
13
1
Co{κ N,N′-5-(2,4,6-Ph -C H )-NC H -2-C(H)N(2,6-Pr -C H )}-
1.01 (d, 6H, CH(CH )
3 2
3
6
2
4
2
2
6
3
15
30
30
Py)Cl] (1b), CoCl(PMe ) , Co(PMe ) , and 1-azidoadaman-
3
3
3 4
31
tane were prepared as described in the literature. All other reagents were
142.4 (C9), 142.6 (C8), 139.9 (C10a), 137.2 (C2 or C5), 136.08 (C2
or C5), 135.1 (C8a), 130.37 (C17), 129.92 (C13), 129.7 (C4),
129.3 (C3), 129.1 (NCH), 128.99 (C11 + C11a), 127.5 (N-Ph−
acquired commercially and used without further purification.
2
i
i
[
Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
6
C H )}(η -C H CH )] (2a). Route a. Toluene was added to a solid
C
), 127.4 (N-Ph−C ), 126.48 (C16), 122.1 (C12 + C12a),
6
3 6 5 3
para
meta
mixture of complex 1a (0.20 g, 0.33 mmol) and K(HBEt ) (0.051 g,
119.2 (C15) 104.7 (C13a), 27.30 (CH(CH ) ), 26.99 (CH(CH ) ),
3
3 2 3 2
0.36 mmol) at room temperature, to give a dark red solution. The mix-
24.04 (CH(CH ) ), C6, C7 resonances absent.
3 2
2
i
i
ture was stirred at room temperature for 2.5 h, to yield a dark red sus-
pension. All volatile materials were evaporated under reduced pressure,
to give a dark red residue. The residue was extracted in n-hexane with
separation of a pale precipitate and a negligible dark residue. The
resulting dark red solution was concentrated and stored at −20 °C,
giving rise to a dark red crystalline solid suitable for X-ray diffraction.
Yield: 0.08 g (42%).
[Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
3
6
2
4
2
2
C H )}] (3). Route a. A solid mixture of complex 1a (0.40 g,
6
3 2
0.64 mmol) and KC (0.095 g, 0.70 mmol) was suspended in Et O. The
8
2
mixture was stirred for 3 h at room temperature, going from a gray-blue
to gray-red suspension. The dark red supernatant was filtered, and
the volatile materials were evaporated to dryness to give a red solid. The
residue was extracted with Et O, and the extracts were concentrated
2
Route b. A toluene solution of complex 1a (0.25 g, 0.4 mmol) was
added to freshly prepared 5% Na(Hg) (0.025 g, 1 mmol of Na; 0.3 mL,
and stored at −20 °C, to give a red powder. Yield: 0.13 g (40%).
Route b. Complex 2a (0.35 g, 0.58 mmol) was dissolved in ca. 10 mL
of n-hexane, and the mixture was stirred for 2 d, at 80 °C. The mixture
was stirred while it cooled to room temperature. The dark red super-
natant was filtered off at −20 °C, and the red solid was washed with
n-hexane and dried under vacuum. Yield: 0.20 g (68%).
20 mmol of Hg). The dark blue-violet solution gradually turned to a
dark red suspension. The mixture was stirred for 4 h at room tem-
perature. The solution was filtered, and the volatile materials were
evaporatedtodryness to give a darkredresidue. The residue was extracted
with n-hexane, and the extracts were concentrated and stored at −20 °C,
to yield a dark red solid. Yield: 0.09 g (37%).
Anal. Calcd for C H Co N , obtained (calculated): C 74.08
6
4
86
2
4
1
(74.68), H 8.56 (8.42), N 5.35 (5.44)%. H NMR (300 MHz,
K
DOI: 10.1021/acs.inorgchem.8b02392
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cyclohexane-d ): δ 81.06 (br, 2H), 53.07 (br, 2H), 33.67 (br, 2H),
(δ = 0) and referenced externally using H PO 85% (δ = 0) for
P spectra. Deuterated solvents were dried over activated 4 Å mole-
cular sieves and degassed by the freeze−pump−thaw technique. All
samples were prepared inside a glovebox and transferred to J. Young
NMR tubes.
12
3
4
3
1
1
1
4.66 (br, 12H, CH(CH ) ), 14.10 (s, 4H, 5-Ph-Hmeta), 12.25 (br, 4H),
3
2
0.56 (br, 4H), 7.62 (br, 12H, CH(CH ) ), 7.02 (br, 12H, CH-
3
2
(CH ) ), 5.27 (br, 2H), 3.74 (br, 12H, CH(CH ) ), 2.81 (br, 12H,
3
2
3
2
CH(CH ) ), 2.27 (br, 2H), −37.28 (br, 2H), −56.19 (br, 2H).
3
2
2
i
i
[
Co{κ N,N′-5-(2,4,6- Pr -C H )-NC H -2-C(H)N(2,6- Pr -
Magnetic susceptibility measurements in solution were performed
3
6
2
4
2
2
32
C H )}(PMe ) ] (4). Route a. An Et O solution of the potassium ligand
according to the Evans method, using a 3% solution of hexamethyl-
6
3
3 2
2
salt KLa (0.90 mmol, 0.45 g) was added dropwise to a dark blue sus-
pension of CoCl(PMe ) (0.90 mmol, 0.29 g) in Et O, at −80 °C. The
mixture was stirred overnight while slowly warming to room tempera-
ture. The volatile materials were removed under vacuum to give a dark
brown powder. The residue was extracted with n-hexane, and the com-
bined extracts were concentrated and stored at −20 °C, yielding a dark red-
brown crystalline solid suitable for X-ray diffraction. Yield: 0.37 g (63%).
disiloxane in toluene-d . These solutions were prepared in a glovebox in J.
8
3
3
2
Young NMR tubes containing capillary tubes filled with the same solvent
mixture, in which the hexamethyldisiloxane is the external reference.
X-ray Crystallography. The crystals were selected under an
inert atmosphere, covered with dry and degassed poly(fluoroether) oil
(FOMBLIN) and mounted on a nylon loop. Crystallographic data were
collected using graphite monochromated Mo Kαradiation (λ=0.710 73Å)
on a Bruker AXS-KAPPA APEX II diffractometer equipped with an
Oxford Cryosystem open-flow nitrogen cryostat, at 150 K. Cell
Route b. An Et O solution of the ligand precursor HLa (0.9 mmol,
2
0
.41 g) was added dropwise to an Et O solution of Co(PMe ) (0.9 mmol,
2 3 4
33
0
.33 g) at −80 °C. The mixture was stirred for 1 h, slowly warming to room
parameters were retrieved using Bruker SMART software and refined
34
temperature, and was further stirred at room temperature for 3 h, to give
a dark red-brown solution. The volatile materials were removed under
vacuum to give a dark brown residue. The residue was redissolved in
n-hexane, and the dark red-brown solution was filtered, concentrated,
and stored at −20 °C, from which a dark red-brown crystalline solid
precipitated. Yield: 0.25 g (42%).
using Bruker SAINT on all observed reflections. Absorption corre-
35
ctions were applied using SADABS. Structure solution and refinement
36
were performed using direct methods with the programs SIR2014 and
37
SHELXL included in the package of programs WINGX-Version
38
2014.1. 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
Anal. Calcd for C H CoN P , obtained (calculated): C 68.18
3
8
61
2 2
39
(
68.45), H 8.71 (9.22), N 4.08 (4.20)%. μeff (toluene-d ): 3.4 μ .
prepared with ORTEP-3. The crystallographic data for all complexes is
presented in Table S1 of the Supporting Information. Data were
deposited in CCDC under the deposit numbers 1851719 for 2a, 1851720
for 2b, 1851721 for 3, and 1851722 for 4. These data are provided free of
charge by the Cambridge Crystallographic Data Centre.
8
B
1
H NMR (300 MHz, C D ): δ 73.1 (br, 1H), 69.6 (br, 18H, P(CH ) ),
6
6
3 3
3
6
3
(
8.6 (s, 1H), 36.3 (br, 1H), 5.78 (d, 2H, N-Ph-Hmeta, J = 6.3 Hz),
HH
.98 (s, 2H, 5-Ph-Hmeta), 3.80 (br, 6H, CH(CH ) ), 0.91 (br, 6H, CH-
3
2
3
CH ) ), 0.58 (sept, 1H, CH(CH ) , J = 5.7 Hz), −0.49 (br, 6H,
3
2
3
2
HH
3
CH(CH ) ), −1.15 (d, 6H, CH(CH ) , J = 6.3 Hz), −1.40 (br, 6H,
Computational Details. All calculations were performed using the
3
2
3
2
HH
40
CH(CH ) ), −4.18 (br, 1H), −11.6 (br, 1H), −14.3 (br, 1H), −42.70
Gaussian 09 software package and the OPBE functional. OPBE com-
41
3
2
(
br, 1H).
bines the Handy’s OPTX modification of Becke’s exchange functional
and the gradient-corrected correlation functional of Perdew, Burke, and
Generation of 4 from 2a. A dark red C D solution of complex 2a
6
6
42
(
0.018 g, 0.029 mmol) was treated with PMe (0.064 mmol, 0.064 mL
Ernzerhof. The geometry optimizations were accomplished without
3
43
of a 1 M solution in toluene), to yield a dark red-brown solution. The
symmetry constraints using a standard 6-31G** basis set for all atoms
44
1
except for cobalt, which used a LanL2DZ basis set with a f-polarization
H NMR spectra was recorded, corresponding to complex 4.
45
function. The MECP(2b ) betweenthe spinsinglet (S = 0, 2bS=0) and
Generation of 2a-C D from 4. C D was carefully added to a solid
CP
6
6
6
6
the spin triplet (S = 1, 2bS=1) PES was determined using a code developed
mixture of complex 4 (0.023 g, 0.035 mmol) and 1-azidoadamantane
0.006 g, 0.035 mmol), giving rise to a dark red solution, with an observable
46
by Harvey et al. This code consists of a set of shell scripts and Fortran
programs that uses the Gaussian 09 results of energies and gradients of
both spin states to produce an effective gradient pointing toward the
MECP. Since the MECP is not a stationary point, a standard frequency
analysis is not applicable.
(
1
evolution of gas. The H NMR spectra was recorded, corresponding to
complex 2a-C D , jointly with AdNPMe . Data for AdNPMe :
6
6
3
3
1
H NMR (300 MHz, C D ): δ 2.24−153 (m, 15H, AdNP(CH ) ),
6
6
3 3
3
1
1
1
.00 (br, 9H, AdNP(CH ) ). P{ H} NMR (121 MHz, C D ):
3
3
6
6
δ 14.3 (AdNP(CH ) ).
3 3
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b02392.
General Procedure for the Hydroboration of Terminal
■
Alkenes Catalyzed by Complex 4. The desired amount of complex
*
S
(
1% mol) was placed in a small Schlenk tube, a solution of the appro-
priate substrate (2.5 mmol) and pinacolborane (2.75 mmol) was added,
and the mixture was stirred for 16 h, at 25 °C. The reaction mixture was
quenched by exposing it to air and treating it with n-hexane. The
solution was filtered through a plug of silica mounted on a Pasteur pipet,
Characterization data, NMR spectra, crystallographic
data, spin isomerism NMR studies, and details of the
DFT calculations (PDF)
and the solvent was evaporated to dryness to yield nearly colorless oils
1
(
H NMR spectra in the Supporting Information). The corresponding
Accession Codes
yields were determined by weighing the isolated reaction products.
General Procedure for the Hydroboration of Arylaldehydes
Catalyzed by Complexes 2a, 2b, 3, and 4. A C D solution of the
CCDC 1851719−1851722 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
6
6
desired amount of complex (1% mol) was added to a solution of the
appropriate substrate (1 mmol) and pinacolborane (1.1 mmol), and the
mixture was transferred to a J. Young NMR tube. The conversion was
1
1
measured by H NMR after 1 h of reaction ( H NMR spectra in the
Supporting Information). The mixture was quenched by exposing it to
air and treating it with n-hexane. The solution was filtered through a
plug of silica mounted on a Pasteur pipet, and the solvent was
evaporated to dryness to yield nearly colorless oils. The corresponding
yields were determined by weighing the isolated reaction products.
NMR Spectroscopy Measurements. NMR spectra were recorded
on a Bruker “AVANCE III” 300 MHz spectrometer at 299.995 MHz
AUTHOR INFORMATION
Corresponding Author
*E-mail: pedro.t.gomes@tecnico.ulisboa.pt.
■
ORCID
1
13
31
Tiago F. C. Cruz: 0000-0001-9461-9493
Luís F. Veiros: 0000-0001-5841-3519
Pedro T. Gomes: 0000-0001-8406-8763
(
H), 75.4296 MHz ( C), and 121.439 MHz ( P), referenced inter-
1
nally using the residual protio-resonances ( H) and the solvent carbon
13
(
C) resonances of the corresponding solvents to tetramethysilane
L
DOI: 10.1021/acs.inorgchem.8b02392
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
J.; Sydora, O. L.; Small, B. L.; Stradiotto, M.; Turculet, L. (N-
Phosphinoamidinate)cobalt-Catalyzed Hydroboration: Alkene Iso-
merization Affords Terminal Selectivity. Chem. - Eur. J. 2014, 20,
The authors declare no competing financial interest.
1
3918−13922. (g) Zhang, H.; Lu, Z. Dual-Stereocontrol Asymmetric
ACKNOWLEDGMENTS
■
Cobalt-Catalyzed Hydroboration of Sterically Hindered Styrenes. ACS
Catal. 2016, 6, 6596−6600. (h) Zhang, L.; Zuo, Z.; Leng, X.; Huang, Z.
A Cobalt-Catalyzed Alkene Hydroboration with Pinacolborane. Angew.
Chem., Int. Ed. 2014, 53, 2696−2700. (i) Peng, J.; Docherty, J. H.;
Dominey, A. P.; Thomas, S. P. Cobalt-catalysed Markovnikov selective
hydroboration of vinylarenes. Chem. Commun. 2017, 53, 4726−4729.
We thank Fundaca
̧
o
̃
para a Cien
̂
cia e a Tecnologia for the
financial support (Project No. UID/QUI/00100/2013) and a
fellowship to T.F.C.C. and P.T.G. (PD/BD/52372/2013−
CATSUS Ph.D. Program and SFRH/BSAB/140115/2018,
respectively).
(j) Zuo, Z.; Yang, J.; Huang, Z. Cobalt-Catalyzed Alkyne Hydro-
silylation and Sequential Vinylsilane Hydroboration with Markovnikov
Selectivity. Angew. Chem., Int. Ed. 2017, 55, 10839−10843. (k) Zhang,
G.; Wu, J.; Wang, M.; Zeng, H.; Cheng, J.; Neary, M. C.; Zheng, S.
Cobalt-Catalyzed Regioselective Hydroboration of Terminal Alkenes.
Eur. J. Org. Chem. 2017, 2017 (38), 5814−5818. (l) Chen, J.; Xi, T.;
Ren, X.; Cheng, B.; Guo, J.; Lu, Z. Asymmetric cobalt catalysts for
hydroboration of 1,1-disubstituted alkenes. Org. Chem. Front. 2014, 1,
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