Reactivity of a T-shaped cobalt(i) pincer-complex

Article abstract of DOI:10.1039/d1dt00277e

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

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Reactivity of a T-shaped cobalt(I) pincer-complex†
Regina Matveeva, Clemens K. Blasius, Hubert Wadepohl and Lutz H. Gade
*
Cite this: DOI: 10.1039/d1dt00277e
The reactivity of a paramagnetic T-shaped cobalt(I) complex, [(iPrboxmi)Co], stabilised by a monoanionic
bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon
monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)],
accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment
with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was
observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and
diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)],
[(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-
supported cobalt(I) complex to homolytically cleave bonds and thus its distinct one-electron reactivity.
Furthermore, a square-planar cobalt(II) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main
product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene.
Putting such species in the context of the previously investigated hydroboration catalysis, its stoichio-
metric reaction with pinacolborane revealed its potential conversion into a cobalt(II) hydride complex,
thus confirming its original attribution as off-cycle species.
Received 27th January 2021,
Accepted 24th March 2021
DOI: 10.1039/d1dt00277e
rsc.li/dalton
complexes, as they have a readily accessible vacant coordi-
nation site, which is essential for the activation of inert
Introduction
The reactive potential of low-coordinate low-valent transition chemical bonds. Only few T-shaped cobalt(I) complexes have
metal complexes is well established.1–7 Their unsaturated char- been reported to date which can be traced back to their reac-
acter, both coordinatively and electronically, renders them tivity, in particular, their profound tendency to fill the vacant
“activators” of small molecules such as O2,8–11 N2O,12 coordination site. The first T-shaped amidophosphine-stabil-
CO2,12–14 and N2.15,16 Moreover, they have also found broad ized cobalt(I) complex was reported by the Caulton group.37
application in a variety of stoichiometric and catalytic group In addition, Fryzuk and co-workers published a phosphine-
transfer reactions relevant to biological processes.17,18
enamidoiminophosphorane stabilised cobalt(I) complex,
As part of the research efforts involving earth-abundant 3d which in solution is in equilibrium with the corresponding
metals,19–23 the synthesis and exploration of new reaction nitrogen complex and catalyses the conversion of dinitrogen
pathways of monovalent, coordinatively unsaturated cobalt to N(SiMe3)2.38 Other examples have been reported by our
compounds has recently attracted increasing interest.24–31 group while attempting to isolate the corresponding PNP
Their kinetic stabilisation is usually provided by bulky multi- cobalt(II) hydride complex39 as well as Ozerov and
dentate ligands. Besides nitrogen donor based spectator coworkers.40
ligands such as nacnac32–35 and bis(imino)pyridine deriva-
We recently synthesized a coordinatively unsaturated high-
tives,36 amidophosphine ligands combining hard and soft spin cobalt(I) complex (1) employing the boxmi45,46 ligand
ligating units have been found to stabilize electron-rich cobalt family, which is dimeric in the solid-state, (1)2, representing a
(I) complexes.37–40
novel stabilization mode (Scheme 1).47 In solution, the dimer
Pincer ligands have been shown to be very suitable for the was found to be in equilibrium with the T-shaped monomeric
stabilization of coordinatively unsaturated cobalt centres.41–44 species as observed by paramagnetic NMR spectroscopy.
Particular attention has been attributed to T-shaped cobalt(I)
Given the reactive potential of such coordinatively and elec-
tronically unsaturated complexes as the boxmi cobalt(I) com-
pound 1, we recently embarked on a systematic investigation
of its reaction patterns. In this first study, the addition of
neutral ligands and the resulting coordination geometries and
spin states were of interest. In view of its reactivity as a
“metallo-radical” single-electron bond cleavage reactions were
Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 270,
69120 Heidelberg, Germany. E-mail: lutz.gade@uni-heidelberg.de
†Electronic supplementary information (ESI) available. CCDC 2056218–2056224.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/d1dt00277e
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carbonyl complexes based on PCP or PNP ligands (νCO
=
1885–1899 cm−1),39,43,48 the stretching frequency of complex 2
with a monoanionic NNN pincer ligand (νCO = 1918 cm−1) and
hence the metal π-basicity of the parent cobalt(I) species 1 is
slightly lower. The solid-state structure of 2 was determined by
X-ray diffraction, with the cobalt centre adopting a square-
planar coordination geometry (rms deviation ≤0.01 Å, Fig. 1).
All structural parameters are within the range expected for
such a compound.
Scheme 1 The T-shaped cobalt(I) complex [(iPrboxmi)Co] (1) in equili-
brium with its dimer (1)2 in solution.
The addition of an excess of PMe3 to a solution of 1 in C6D6
was accompanied by a rapid change of colour from dark
brown to dark red and yielded complex [(iPrboxmi)Co(PMe3)]
(3) in almost quantitative yield (Scheme 2, bottom).
of particular interest. Following the observed formation of
complex 1 under the conditions of the catalytic hydroboration
of terminal alkynes, we also looked into the reactions of an
alkynyl species which represents an off-cycle species in the
catalytic transformation.
In contrast to the carbonyl analogue, complex
[(iPrboxmi)Co(PMe3)] was found to be paramagnetic. The
measurement of the magnetic susceptibility of a C6D6 solution
of 3 revealed an effective magnetic moment of 2.99µB, indicat-
ing a high-spin S = 1 ground state. This value is consistent with
other previously reported magnetic moments of similar cobalt
(I) high-spin complexes,37,49 the slight deviation from the spin-
only value (d8: µSO = 2.82 µB) being due to the contribution of
the orbital angular momentum to the magnetic moment.49
The molecular structure of complex 3 in the solid-state was
established by X-ray diffraction (Fig. 2). A slightly distorted
tetrahedral coordination geometry was found, as reflected in
the (N–Co–P) angles of 99.03(4)–103.07(4)°. As indicated above,
the difference in the coordination geometry between com-
plexes 1 and 2 can be attributed to a greater steric demand
of trimethyl phosphine compared to the carbonyl ligand.
The mean P–C bond length (1.8242(2) Å) found for complex
3 is shorter than that in free PMe3 (1.846(3) Å) while the mean
C–P–C angle is larger (102.29(12)° as compared to 98.6(3)°).
This indicates a higher s-character of the P–C bonds in
[(iPrboxmi)Co(PMe3)] compared to free PMe3.50
Results and discussion
Addition of neutral donors to the coordinatively unsaturated
T-shaped cobalt(I) complex 1
As starting point for our investigation on the reactivity of
boxmi cobalt(I) complex 1, we examined the propensity of 1 to
bind neutral two electron donors. Whereas hard σ-donors
such as dialkylethers and trialkylamines were found to be
unreactive towards the T-shaped cobalt compound, soft
ligands with variable degrees of π-acidity were found to readily
coordinate to 1. The resulting coordination geometry and spin
state were primarily determined by the steric demand of the
added ligand.
Charging a solution of 1 in C6D6 with 5 bar of CO resulted
in a rapid change of colour from dark brown to green
(Scheme 2, top). The 1H NMR spectrum revealed the formation
of a diamagnetic compound indicating a d8 low-spin complex,
[(iPrboxmi)Co(CO)] (2). The infrared spectrum of 2 exhibits one
sharp intense vibrational band, which can be assigned to the
CO stretching vibration. Compared to similar cobalt(I) mono-
Fig. 1 Molecular structure of monocarbonyl complex [(iPrboxmi)Co
(CO)] (2) in the solid-state. Hydrogen atoms have been omitted for
clarity; displacement ellipsoids are set at 50% probability level; only one
of the three independent molecules is shown for 2. Selected bond
lengths [Å] and angles [°] given as range for all independent molecules:
Co–N1 1.889(6)⋯1.903(6), Co–N2 1.935(5)⋯1.940(6), Co–N3 1.893(6)
⋯1.908(6), Co–C23 1.710(8)⋯1.724(7), O23–C23 1.155(8)⋯1.170(9),
N1–Co–N2 90.6(2)⋯91.1(2), N1–Co–N3 177.7(3)⋯178.7(2), N3–Co–N2
90.4(2)⋯90.8(2), C23–Co–N2 177.5(4)⋯178.9(3).
Scheme 2 Synthesis of cobalt(I) monocarbonyl and triphenylpho-
sphane complexes 2 (top) and 3 (bottom).
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Fig. 3 Molecular structure of iodide complex 4 in the solid state.
Hydrogen atoms have been omitted for clarity; displacement ellipsoids
are set at 50% probability level; only one of the two independent mole-
cules is shown for 4. Selected bond lengths [Å] and angles [°], values in
square brackets refer to the second independent molecule: Co–I 2.6197
(3) [2.5966(3)], Co–N1 1.999(2) [2.000(2)], Co–N2 1.953(2) [1.954(2)],
Co–N3 1.997(2) [1.999(2)], N1–Co–I 101.05(6) [104.87(6)], N2–Co–I
120.45(6) [118.25(6)], N3–Co–I 107.98(7) [110.07(6)], N1–Co–N2 92.94
(10) [92.74(10)], N3–Co–N1 142.68(10) [137.41(9)], N3–Co–N2 92.13(10)
[91.60(9)].
Fig. 2 Molecular structure of trimethylphosphane complex [(iPrboxmi)
Co(PMe3)] (3) in the solid-state. Hydrogen atoms have been omitted for
clarity; displacement ellipsoids are set at 50% probability level. Selected
bond lengths [Å] and angles [°]: Co–P 2.3309(5), Co–N1 1.9349(13),
Co–N2 1.9793(14), Co–N3 1.9299(14), P–C23 1.8216(19), P–C24 1.823(2),
P–C25 1.828(2), N1–Co–P 100.79(4), N2–Co–P 99.03(4), N3–Co–P 103.07
(4), N1–Co–N2 92.26(6), N3–Co–N1 154.25(6), N3–Co–N2 93.43(6),
C23–P–C24 101.71(12), C23–P–C25 101.94(10), C24–P–C25 103.22(15).
distorted
tetrahedral
geometry around
the
cobalt
Single-electron bond cleavage reactions
atom. Remarkably, there is a notable difference between the
N2–Co–X (X = I, Cl) angles for the respective iodide and chlor-
ide cobalt complexes. In case of [(iPrboxmi)CoI], the halide
ligand is more strongly tilted towards the isoindolate back-
bone, as evinced by the N2–Co–X angle of 120.45(6)° (X = I)
compared to 127.79° of [(iPrboxmi)CoCl].52 This difference can
be attributed to the stronger steric demand of iodide com-
pared to the chloride ligand in its repulsive interaction with
the isopropyl substituents of the wingtip oxazoline units.
O–O-bond cleavage of organic peroxides. Organic peroxides
are known to readily undergo (redox-induced) bond cleavage
reactions.54,55 Therefore, [(iPrboxmi)Co] was next reacted
First-row transition metals are generally known for their ten-
dency to favour one-electron over two-electron processes (as
e.g. associated with oxidative additions).39,48 We were therefore
interested to probe the potential one-electron redox chemistry
associated with homolytic bond cleavage of various substrates
(I–I, O–O and S–S).
I–I-bond cleavage of iodine. Compound 1 was first reacted
with molecular iodine, characterized by its very low I–I bond
dissociation energy of 36 kcal mol−1 51
resulting in an immedi-
,
ate change of colour from dark brown to deep red (Scheme 3).
In the 1H NMR spectrum, a single set of paramagnetic
signals was observed which were very broad compared to
those previously obtained for the chlorido complex
[(iPrboxmi)Co(Cl)].52 The high temperature limiting spectrum
recorded above 80 °C displayed a signal pattern consistent
with C2 symmetry in solution (see ESI†). By means of the
Evans method, the magnetic moment of complex 4 in solu-
tion was determined to be µeff = 4.27 µB, consistent with three
unpaired electrons and indicating a high-spin cobalt(II) d7
complex. The value is in the range of other known high-spin
cobalt(II) pincer complexes.39,52,53
with bis(tert-butyl)peroxide (BDE O–O 45.2 kcal mol−1 56
and
)
bis(trimethylsilyl)peroxide (BDE O–O 54.8 kcal mol−1),56 result-
ing in the corresponding [(iPrboxmi)Co(OtBu)] (5a) and
[(iPrboxmi)Co(OSiMe3)] (5b) complexes, respectively (Scheme 4).
Their paramagnetic susceptibilities in solution were deter-
mined by the Evans method (5a: µeff = 4.05µB; 5b: µeff = 4.11µB)
and found to be in agreement with the expected values for d7
high-spin complexes.39,52,53
The
solid-state
structures
of
both
complexes,
[(iPrboxmi)Co(OtBu)] and [(iPrboxmi)Co(OSiMe3)], were deter-
mined by X-ray diffraction (Fig. 4). As for the tetracoordinated
iodide complex 4, distorted tetrahedral coordination geome-
For complex [(iPrboxmi)CoI], the solid state structure was
determined by X-ray diffraction (Fig. 3). The molecular struc-
ture confirmed the anticipated four-coordinated cobalt centre
coordinated by the boxmi and iodide ligands and showed a
Scheme 4 Synthesis of complexes [(iPrboxmi)Co(OtBu)] (5a) and
[(iPrboxmi)Co(OSiMe3)] (5b).
Scheme 3 Synthesis of cobalt(II) iodido complex 4.
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Fig. 4 Molecular structure of complexes 5a (left) and 5b (right) in the
solid state. Hydrogen atoms have been omitted for clarity; displacement
ellipsoids are set at 50% probability level; only one of the two indepen-
dent molecules is shown for 5a and 5b. Selected bond lengths [Å] and
angles [°], values in square brackets refer to the second independent
molecule, for 5a: Co–O3 1.855(3) [1.865(3)], Co–N1 2.000(3),‡ Co–N2
1.989(3) [1.986(4)], Co–N3 2.012(3) [2.016(4)], O3–Co–N1 110.15(12),‡
O3–Co–N2 128.35(12) [125.25(13)], O3–Co–N3 109.66(12) [107.92(13)],
N1–Co–N3 129.18(13) [129.2(4), 140.8(4)], N2–Co–N1 89.4(6) [86.4(4),
93.8(4)], N2–Co–N3 89.81(12) [90.33(16)]; for 5b: Co–O3 1.867(3) [1.878
(4)], Co–N1 1.996(4) [1.993(5)], Co–N2 1.986(4) [1.980(5)], Co–N3 2.001
(4) [2.000(5)], O3–Co–N1 107.74(17) [105.33(18)], O3–Co–N2 128.71(17)
[124.71(18)], O3–Co–N3 108.40(17) [107.54(17)], N1–Co–N3 132.88(18)
[138.60(18)], N2–Co–N1 89.78(19) [90.32(19)], N2–Co–N3 90.52(18)
[91.17(19)]. ‡ Co–N1/2 distances not meaningful due to equal distance
restraints because of apparent disorder of the boxmi ligand.
Fig. 5 Molecular structure of thiolate complex 6 in the solid state.
Hydrogen atoms have been omitted for clarity; displacement ellipsoids
are set at 50% probability level. Selected bond lengths [Å] and angles [°]:
Co–S 2.2894(10), Co–N1 2.002(3), Co–N2 1.956(3), Co–N3 2.011(3),
N1–Co–S 100.71(8), N2–Co–S 132.40(9), N3–Co–S 105.77(9), N1–Co–N2
91.02(11), N3–Co–N1 140.27(13), N3–Co–N2 92.45(12), C23–S–Co
104.06(11), C12–C13–C14–N3 5.5(5), N1–C3–C4–C5 −1.0(4).
thiolato pincer complex,39 while the N2–Co–S angle of
[(iPrboxmi)Co(SPh)] appears to be significantly larger. This
difference between the two related complexes can be attributed
to the rigidity of the boxmi system compared to the carbazole-
based pincer ligand.39
tries around the cobalt atom were observed for 5a and 5b. The
deformation of the ligand framework is the result of the out-
of-plane coordination of the cobalt atom with respect to the
plane spanned by the three nitrogen atoms (5a: d = 0.794(2) Å;‡
5b: d = 0.739(3) Å [0.657(3) Å]). Based on all metric parameters
this deformation was found to be more pronounced for the
[(iPrboxmi)Co(OtBu)] derivative.
Precatalyst activation pathways in cobalt-catalysed
hydroborations of terminal alkynes
In previous studies, we examined the cobalt-catalysed hydro-
boration of terminal alkynes, selectively furnishing branched
alkenyl boronate esters.47 As part of the mechanistic investi-
gations, cobalt(I) complex 1 was found as an off-cycle inter-
mediate, which derives from the catalytically active cobalt(II)
hydride in the absence of alkynes. Since complex 1 was found
to act as a precatalyst for the hydroboration reaction in its own
right,47 we were interested in further activation pathways.
Treating cobalt(I) complex 1 with stoichiometric amounts of
4-fluoro-1-ethinylbenzene, i.e. a terminal alkyne, led to an
unselective reaction as apparent from 19F NMR spectroscopy
(Scheme 6, top and ESI† for details). However, cobalt(II)
alkynyl complex 7 was isolated as the major component of the
reaction mixture (approx. 37% of the resulting material based
on NMR) and characterized by X-ray diffraction analysis. Its
S–S-bond cleavage of diphenyl disulphide. Finally, the thio-
lato complex [(iPrboxmi)Co(SPh)] (6) was synthesized by the
reaction of [(iPrboxmi)Co] with a stoichiometric amount of
Ph2S2 (BDE 55 kcal mol−1 57
in benzene, accompanied by a
)
rapid change of colour from dark brown to ocre (Scheme 5).
The magnetic moment of 4.30µB of complex 6, as determined
by the Evans method, is consistent with a S = 3/2 ground state,
indicating a high-spin cobalt(II) d7 system.
The molecular structure of 6 was established by X-ray diffr-
action (Fig. 5) and was found to be characterized by a distorted
tetrahedral coordination geometry. Notably, the coordination
of the SPh fragment leads only to a slight deformation of the
boxmi ligand as reflected by torsion angles C12–C13–C14–N3
5.5(5) and N1–C3–C4–C5 −1.0(4)° between the central isoindo-
lato unit and the two flanking oxazoline cycles. The Co–S bond
length in 6 is similar to that of a previously reported cobalt(II)
Scheme 5 Synthesis of cobalt(II) thiolato complex 6.
Scheme 6 Synthesis of cobalt(II) alkynyl complex 7.
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independent and selective synthesis was subsequently accom-
plished by alkane elimination of the cobalt(II) alkyl complex 8
in the reaction with 4-fluoro-1-ethinylbenzene (Scheme 6,
bottom).
In contrast to other tetracoordinated CoII complexes in this
study, complex [(iPrboxmi)Co(CCArF)] features a d7 low-spin
ground state (µeff = 2.39µB, Evans method). Accordingly,
alkynyl complex 7 exhibits an almost perfect (rmsd = 0.017(1)
Å) square-planar coordination geometry in the solid-state
(Fig. 6), with the boxmi ligand coordinating in the
expected meridional fashion and a Co–C23 bond length of
d = 1.899(2) Å.
Resulting from the π-accepting properties of the alkynyl
ligand, this electronic state is in accordance with a structurally
related and previously described alkenyl complex.47 As appar-
ent from DFT calculations, the unpaired spin density is
efficiently delocalized on the alkynyl ligand, the nitrogen
donor atoms and also on the π-system of the boxmi ligand
(Fig. 7). In accordance with the comparatively small Fermi
contact shifts observed for most of the signals in low-spin
complex 7, the spin density on the ligand framework is mainly
located in π-type orbitals. With respect to the metal centre,
Scheme 7 Role of cobalt(II) alkynyl complex 7 in the hydroboration of
terminal alkynes. Relative fractions determined by 19F NMR spec-
troscopy. ArF denotes
iPrboxmi)Co fragment.
a 4-fluorophenyl group and [Co] represents
(
unpaired spin density is predominantly found in the
d
yz orbital [x = principal molecular axis].
In order to probe the role of alkynyl complex 7 in the
cobalt-catalysed hydroboration of terminal alkynes, its stoi-
chiometric reaction with pinacolborane was studied
(Scheme 7). Monitoring the reaction via 19F NMR spectroscopy,
initially the emergence of one new species was detected, puta-
tively assigned as a cobalt alkenyl complex with a pro-(Z) con-
figuration (see schematic illustration and ESI† for details).
In the course of the reaction, the gradual conversion of
such species into the corresponding pro-(E)-configurated
product was observed. The occurrence of such (E)-(Z)-isomeri-
zation of an alkenyl complex has been also described in pre-
vious studies.47,58 Additionally, this assignment could be con-
firmed after hydrolytic work-up of the mixture, furnishing the
two isomeric linear alkenyl boronate esters as sole products in
the expected ratio of approximately 1 : 1 (Scheme 7). Since the
formation of product (Z)-9 was not observed under catalytic
conditions with precatalyst 8, complex [(iPrboxmi)Co(CCArF)] is
thought to play an off-cycle role in the hydroboration reaction.
However, its conversion into a catalytically active cobalt(II)
hydride intermediate via a σ-bond metathesis reaction with
pinacolborane and thus its re-entry into the main catalytic
cycle was found to be feasible.
Fig. 6 Molecular structure of complex 7 in the solid state. Hydrogen
atoms have been omitted for clarity; displacement ellipsoids are set at
50% probability level. Selected bond lengths [Å] and angles [°]: Co–C23
1.899(2), Co–N1 1.9218(19), Co–N2 1.9487(18), Co–N3 1.9265(19),
N2–Co–C23 178.36(8), N1–Co–N3 178.59(8).
Conclusion
In this work, we have investigated the coordination behaviour
and reducing capability of a coordinatively unsaturated cobalt
(I) complex 1 generated in solution from its aggregated dimer.
A pronounced dependency of the coordination geometry on
the respective neutral two-electron donor ligand (CO versus
PMe3) was found for d8 configurated, tetracoordinated cobalt(I)
complexes. Furthermore, a strong tendency of the cobalt(I)
complex 1 to engage in one-electron redox processes was
demonstrated, resulting in various cobalt(II) species. Such
behaviour was both observed in homolytic E–E bond cleavage
reactions (E = heteroelement) and in the reaction with a term-
inal alkyne, giving rise to an alkynyl species.
Fig. 7 Visualisation of the spin density distribution in complex
7 (isovalue = 0.0004, positive green, negative orange).
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CDCl3, 295 K): δ 165.1, 156.9, 139.3, 128.3, 120.3, 81.2, 76.6,
Experimental section
General remarks
66.8, 35.6, 19.0, 14.2. IR (ATR): νCO = 1918 cm−1
.
[(iPrboxmi)Co(PMe3)] (3): Compound [(iPrboxmi)Co] (1)
All experiments were carried out under an argon atmosphere (20 mg, 47.2 μmol, 1.0 eq.) was dissolved in 600 μL benzene
(Argon 5.0 purchased from Messer Group GmbH and dried and an excess of trimethylphosphine (50 μL, 35.9 mg,
over Granusic phosphorus pentoxide granulate prior to use) by 472 μmol, 10 eq.) was added. The dark red solution was freed
means of standard glove box or Schlenk techniques. Solvents from any volatiles in vacuo and washed with n-pentane, yield-
were dried by passing through activated alumina columns ing the product 3 as a red solid (19.6 mg, 39.2 μmol, 83%).
(M. Braun SPS 800) and stored in glass ampules under argon Single crystals of 3 suitable for X-ray crystallography were
atmosphere. Deuterated solvents were purchased from Deutero obtained by slow diffusion of n-pentane into a saturated solu-
GmbH, dried with potassium (C6D6) or natrium (Tol-d8), tion of 3 in toluene at −40 °C. C25H35CoN3O2P (449.48) calcd:
vacuum distilled, degassed and were stored in glass ampules C 60.12, H 7.06, N 8.41; found: C 60.38, H 6.98, N 8.40. 1H
in a glove box. Cobalt(I) complex 1 and cobalt(II) alkyl complex NMR (600.13 MHz, C6D6, 295 K): δ = 103.91 (s, 2H), 67.74 (s,
8 were synthesized according to literature procedures.47 All 9H), 33.77 (s, 2H), 19.79 (s, 2H), 3.33 (s, 2H), 1.79 (s, 6H),
other reagents were purchased from commercial suppliers and −3.35 (s, 2H), −7.66 (s, 2H), −17.93 (s, 6H), −19.42 (s, 2H) ppm.
employed without further purification unless explicitly stated 13C NMR (150.90 MHz, C6D6, 295 K): δ = 349.5 (s), 235.8
1
1
otherwise. All liquids were degassed by three freeze–pump– (d, JCH = 131.3 Hz), 166.6 (s), 124.1 (d, JCH = 156.8 Hz) ppm.
thaw cycles before use. Iodine was sublimated prior to use. µeff = 2.99 µB (C6D6, Evans, 295 K).
NMR spectra were recorded on a Bruker Avance (200 MHz), a
[(iPrboxmi)CoI] (4):
A solution of iodine (5.99 mg,
Bruker Avance II (400 MHz) or a Bruker Avance III (600 MHz, 23.6 μmol, 0.5 eq.) in benzene (2 mL) was added to a solution
equipped with a QNP Cryo Probe) spectrometer. Chemical of compound 1 (20.0 mg, 47.2 μmol, 1.0 eq.) in 1.5 mL
shifts (δ) are given in parts per million (ppm) relative to tetra- benzene at ambient temperature. The dark red solution was
methylsilane and are referenced to residual protio solvent stirred for 1 h and was freed from any volatiles in vacuo. The
signals (1H) or carbon resonances (13C) or to CCl3F (19F) as residue was washed with n-pentane to yield 4 as a dark red
external standard.59 In order to indicate the signal multiplici- solid (18.0 mg, 26.6 μmol, 56%). Single crystals of 4 suitable
ties, the following abbreviations were used: singlet (s), doublet for X-ray crystallography were obtained by slow diffusion of
(d), triplet (t), multiplet (m). Mass spectra were acquired on n-pentane into a saturated solution of 4 in toluene at −40 °C.
JEOL AccuTOF GC time-of-flight (EI) spectrometer at the mass C22H26CoIN3O2 (550.31) calcd: C 48.02, H 4.76, N 7.64; found:
1
spectrometry facility of the Organic Department at the C 47.90, H 4.73, N 7.60. H NMR (600.13 MHz, C6D6, 295 K):
University of Heidelberg. Elemental analysis measurements δ = 174.4, 90.5, 23.0, 14.0, 11.8, −6.5, −21.4, −34.0, −23.9,
were performed on a Vario MICRO Cube elemental analyser at −63.7, −77.9 ppm. 13C NMR (150.90 MHz, C6D6, 295 K):
the Microanalysis Laboratory of the Faculty of Chemistry and δ = 813.3 (s), 211.0 (d, 1JCH = 153.5 Hz), 193.5 (d, 1JCH = 160.5 Hz),
Earth Sciences at the University of Heidelberg. Magnetic sus- 28.4 (s) ppm. µeff = 4.27 µB (Tol-d8, Evans, 295 K).
ceptibilities in solution were determined employing the Evans
[(iPrboxmi)Co(OtBu)] (5a): Complex 1 (20.0 mg, 47.2 μmol,
Method according to literature procedures.60,61 UV/vis spectra 1.0 eq.) was dissolved in toluene (1.0 mL) and cooled to
were acquired on a Cary 5000 UV/vis/NIR spectrometer and −40 °C. Di-tert-butyl peroxide (4.37 μL, 3.45 mg, 23.6 μmol,
were baseline and solvent corrected. Single crystal X-ray diffrac- 0.5 eq.) was added dropwise. The reaction mixture was
tion experiments were performed on an Agilent SuperNova-E allowed to warm up to 0 °C and was stirred for 1 h at
CCD at low temperature with Mo-Kα or Cu-Kα radiation at lab- ambient temperature. Then, the solvent was removed in
oratory for structural analysis of the Institute of Inorganic vacuo and the residue was washed with n-pentane to yield 5a
Chemistry at the University of Heidelberg.
as a dark brown solid (16.1 mg, 32.5 μmol, 69%). Single crys-
[
iPrboxmiCo(CO)] (2): Compound [(iPrboxmi)Co] (1) tals of 5a suitable for X-ray crystallography were obtained as
(15.0 mg, 35.4 µmol, 1.0 eq.) was dissolved in 600 µL C6D6 and brown needles by slow diffusion of n-pentane into a satu-
exposed to a carbon monoxide atmosphere (5 bar) in a high- rated solution of 5a in toluene at −40 °C. C26H35CoN3O3
pressure tube. Instantaneously, a colour change to dark green (496.51) calcd: C 62.90, H 7.11, N 8.46; found: C: 62.18, H:
was observed. The reaction mixture was filtered and freed from 6.97, N: 8.08 (due to the thermal instability of the compound
any volatiles in vacuo, yielding the product as green solid we were unable to obtain more accurate analytical data in
(14.4 mg, 31.9 μmol, 90%). Single crystals of 2 suitable for this case). 1H NMR (399.89 MHz, Tol-d8, 295 K): δ = 113.7,
X-ray crystallography were obtained by slow evaporation of a 64.3, 19.6, 19.3, 16.2, 14.0, 9.6, 8.6, 6.7, 5.3, 5.2, 4.7, 4.4,
saturated solution in n-pentane/diethylether. C23H26CoN3O3 −4.1, −12.5, −14.5, −21.0, −51.6, −56.4 ppm. 13C NMR
(451.41) calcd: C 61.20, H: 5.81, N: 9.31; found: C: 60.62, H: (150.90 MHz, Tol-d8, 295 K): δ = 830.0 (s), 583.5 (s), 488.8 (br
5.36, N: 9.39. 1H NMR (600.13 MHz, CDCl3, 295 K): δ = s), 480.5 (br s), 475.6 (br s), 379.1 (s), 342.8 (s), 233.8 (s),
1
1
7.57–7.55 (m, 2H), 7.13–7.11 (m, 2H), 6.52 (s, 2H), 4.47 (dt, J = 201.9 (d, JCH = 157.0 Hz), 192.5 (d, JCH = 154.5 Hz), 185.2
1
1
8.5 Hz, J = 2.7 Hz, 2H), 3.91 (dd, J = 8.5 Hz, J = 2.6 Hz, 2H), (d, JCH = 162.8 Hz), 178.9 (d, JCH = 162.7 Hz), 174.8 (br s),
3.74 (t, J = 8.5 Hz, 2H), 2.77–2.70 (m, 2H), 0.76 (d, J = 7.0 Hz, 150.8 (br s), 82.4 (br s), 8.9 (br s), 2.9 (br s), −23.0 (br s) ppm.
6H), 0.51 (d, J = 7.0 Hz, 6H) ppm. 13C NMR (150.90 MHz, µeff = 4.05µB (Tol-d8, Evans, 295 K).
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[(iPrboxmi)Co(OSiMe3)] (5b): Bis(trimethylsilyl)peroxide any symmetry restrictions.62 The B3LYP exchange–correlation
(5.08 μL, 4.21 mg, 23.6 μmol, 0.5 eq.) was added dropwise to a functional63–65 combined with Ahlrich’s def2-TZVP (for cobalt
solution of compound 1 (20.0 mg, 47.2 μmol, 1.0 eq.) in and nitrogen) and def2-SVP basis set was employed.66,67
benzene (1.5 mL) and stirred for 1 h at ambient temperature. Frequency calculations at the same level of theory were per-
Subsequently, the solvent was removed under reduced pressure formed to identify the optimized structures as stationary
and the residue was washed with n-pentane to yield 5b as a points.
dark brown solid (17.2 mg, 33.6 μmol, 71%). Single crystals of
X-ray diffraction studies
5b suitable for X-ray crystallography were obtained as brown
needles by slow diffusion of n-pentane into a saturated solu-
tion of 5b in toluene at −40 °C. C25H35CoN3O3Si (512.59)
calcd: C: 58.58, H: 6.88, N: 8.20; found: C: 58.64, H: 6.66, N:
Details of the structure determinations are provided in the
ESI.† CCDC 2056218 (for 2), 2056219 (for 3), 2056220 (for 4),
2056221 (for 5a), 2056222 (for 5b), 2056223 (for 6), 2056224
(for 7)† contain the supplementary crystallographic data for
this paper.
1
7.93. H NMR (600.13 MHz, C6D6, 295 K): δ = 113.4, 69.1, 20.5,
16.6, 15.4, 9.8, 4.8, −3.8, −16.2, −18.9, −28.7, −52.1,
−57.9 ppm. 13C NMR (150.90 MHz, C6D6, 295 K): δ = 310.0 (s),
1
1
1
201.4 (d, JCH = 155.0 Hz, JCH = 152.8 Hz), 185.1 (d, JCH
153.2 Hz), 157.4 (s) ppm. µeff = 4.11 µB (Tol-d8, Evans, 295 K).
=
Conflicts of interest
[(iPrboxmi)Co(SPh)] (6): To a stirred solution of compound 1
(20.1 mg, 47.5 μmol, 1.0 eq.) in benzene (1.0 mL) diphenyl di-
sulfide (5.18 mg, 23.7 μmol, 0.5 eq.) was added. The reaction
mixture was stirred for 3 h and then freed from any volatiles in
vacuo. The crude product was washed with n-pentane to afford
6 as a brown-red solid (24.9 mg, 46.7 μmol, 98%). Single crys-
tals of 6 suitable for X-ray crystallography were obtained by
slow diffusion of n-pentane into a saturated solution of 6 in
toluene at −40 °C. C28H31CoN3O2S (532.57) calcd: C: 63.15, H
5.87, N 7.89; found: C: 63.27, H: 6.24, N: 7.55. 1H NMR
(600.13 MHz, C6D6, 295 K): δ = 124.0, 74.6, 21.4, 21.0, 14.0,
11.5, 4.9, −1.7, −5.9, −21.7, −24.1, −28.5, −29.3, −50.5, −64.3,
−74.2 ppm. 13C NMR (150.90 MHz, C6D6, 295 K): δ = 934.5 (s),
There are no conflicts to declare.
Acknowledgements
We acknowledge funding by the University of Heidelberg and
the Deutsche Forschungsgemeinschaft (DFG-Ga488/9-2) as
well as support by the bwHPC initiative and the bwHPC-C5
project.
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C28H31CoN3O2S+. µeff = 4.30 µB (C6D6, Evans, 295 K).
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