Concerted aryl-sulfur reductive elimination from PNP pincer-supported Co(iii) and subsequent Co(i)/Co(iii) comproportionation

Article abstract of DOI:10.1039/d0sc01813a

This report discloses a combined experimental and computational study aimed at understanding C-S reductive elimination from Co(iii) supported by a diarylamido/bis(phosphine) PNP pincer ligand. Divalent (PNP)Co-aryl complexes could be easily oxidized to five-coordinate Co(iii) derivatives, and anion metathesis provided five-coordinate (PNP)Co(Ar)(SAr′) complexes of Co(iii). In contrast to their previously described (POCOP)Co(Ar)(SAr′) analogs, but similarly to the (PNP)Rh(Ar)(SAr′) and (POCOP)Rh(Ar)(SAr′) analogs, (PNP)Co(Ar)(SAr′) undergo C-S reductive elimination with the formation of the desired diarylsulfide product ArSAr′. DFT studies and experimental observations are consistent with a concerted process. However, in contrast to the Rh analogs, the immediate product of such reductive elimination, the unobserved Co(i) complex (PNP)Co, un-dergoes rapid comproportionation with the (PNP)Co(Ar)(SAr′) starting material to give Co(ii) compounds (PNP)Co-Ar and (PNP)Co-SAr′.

Full text of DOI:10.1039/d0sc01813a

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DOI: 10.1039/D0SC01813A
ARTICLE
Concerted Aryl-Sulfur Reductive Elimination from PNP Pincer-
Supported Co(III) and Subsequent Co(I)/Co(III)
Comproportionation
Received 00th January 20xx,
Accepted 00th January 20xx
Bryan J. Foleya, Chandra Mouli Palita, Nattamai Bhuvanesha, Jia Zhoub*, and Oleg V. Ozerova*
DOI: 10.1039/x0xx00000x
This report discloses a combined experimental and computational study aimed at understanding C-S reductive elimination
from Co(III) supported by a diarylamido/bis(phosphine) PNP pincer ligand. Divalent (PNP)Co-Aryl complexes could be easily
oxidized to five-coordinate Co(III) derivatives, and anion metathesis provided five-coordinate (PNP)Co(Ar)(SAr’) complexes
of Co(III). In contrast to their previously described (POCOP)Co(Ar)(SAr’) analogs, but similarly to the (PNP)Rh(Ar)(SAr’) and
(POCOP)Rh(Ar)(SAr’) analogs, (PNP)Co(Ar)(SAr’) undergo C-S reductive elimination with the formation of the desired dia-
rylsulfide product ArSAr’. DFT studies and experimental observations are consistent with a concerted process. However, in
contrast to the Rh analogs, the immediate product of such reductive elimination, the unobserved Co(I) complex (PNP)Co,
un-dergoes rapid comproportionation with the (PNP)Co(Ar)(SAr’) starting material to give Co(II) compounds (PNP)Co-Ar and
(PNP)Co-SAr’.
Cross-coupling reactions of aryl (pseudo)halides with
nucleophiles typically rely on the oxidative addition (OA) –
transmetallation (TM) – reductive elimination (RE) cycles such
Introduction
Carbon-heteroatom cross coupling has become an immensely
powerful synthetic tool in recent years.14 The existing art on
cross-coupling reactions is historically dominated by
palladium,5 with additional prominence by another group 10
metal Ni,6,7 as well as a group 11 metal Cu.8,9 There has recently
been a renewed push to find alternatives to homogeneous
precious metal catalysts from among cheaper, more Earth-
abundant metals.10-12
as depicted in Figure 1. The OA and RE steps are two-electron
processes that are well established for Pd. Our group has been
interested in the potential of the analogous OA-TM-RE cycle to
enable cross-coupling catalysis by group 9 metals. Pd (and Ni)
go through the Pd0/PdII oxidation states corresponding to the
d10/d8 configurations. For group 9 metals, we have targeted the
MI/MIII oxidation states (d8/d6). In particular, we were able to
establish that a T-shaped RhI center supported by an anionic
pincer ligand possesses a rather striking similarity in its
reactivity to the LPd0 fragment.1318 This approach with Rh
proved especially fruitful in catalytic C-S coupling.19 Catalytic C-
S coupling with Pd has received a considerable amount of
attention.20,21
PiPr2
Ar-Nu
LnPd0
Ar-Hal
X
M
Ar-Nu
Ar-Hal
PiPr2
Reductive
Elimination
Reductive
Elimination
Oxidative
Addition
Oxidative
Addition
PiPr2
MI/MIII Cycle
M = Rh, Co
Pd0/PdII Cycle
Ar
Ar
It is easy to envisage the steps of the analogous pincer-
supported CoI/III cycle (Figure 1). However, in the chemistry of
3d metals, competition from one-electron pathways to the
desired two-electron steps is something that must be closely
considered.22 In principle, there is a substantial body of
literature describing Co-based cross-coupling catalysis,23 and
PiPr2
Ar
LnPd
LnPd
Ar
M
X
Nu
Hal
Nu
X
M
PiPr2
Hal
PiPr2
Transmetallation
Hal- "Nu-"
Nu = Ar, NHR, SR...
Hal = Cl, Br, I, OSO2R...
X = Anionic Ligand
Transmetallation
Hal- "Nu-"
Figure 1. Cross-coupling mechanistic cycles for Pd0/PdII (left) and for RhI/RhIII or CoI/CoIII
(right).
CoI/CoIII cycles are often proposed.2428
However, firm
mechanistic information remains rather limited. Fout and
coworkers analyzed a Co-catalyzed aryl halide amination system
in 201429 where the CoI/CoIII cycle was strongly implicated but
the individual steps of OA and RE were not observed. Chirik et
al. reported on the C-C coupling of aryl triflates in 2016, where
it appears that the CoI/CoIII cycle should operate but the details
were not uncovered.30 Bernskoetter’s group reported a well-
defined example of C-C RE from CoIII in 2011,31 but this involved
coupling of two CH3 groups, only indirectly related to aryl halide
a. Department of Chemistry, Texas A&M University, 3255 TAMU, College Station,
Texas 77842, USA.
b. School of Science, Harbin Institute of Technology, Shenzhen 518055, China.
Electronic Supplementary Information (ESI) available: experimental details and
pictorial NMR spectra, details of the computational studies and the coordinate files.
Crystallographic
information:
CCDC
1868266-1868268.
See
DOI: 10.1039/x0xx00000x
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reactions. However, outside of aryl halide coupling reactions,
there have in recent years appeared examples of homogeneous
catalysis by pincer-supported Co complexes where two-
electron OA/RE steps are either well understood or strongly
suggested.3242
R1
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NaS
xs.
M
DOI: 10.1039/D0SC01813A
PiPr2
PiPr2
PiPr2
Co Cl
PiPr2
R1
R2
R
N
Co
S
N
Co
R
N
THF, RT
-NaCl
THF or Toluene
-35o C to RT
-MCl
PiPr2
PiPr2
R2
M
R
Equiv. Yield
R1
R2 Yield
1
3a
H
H
iPr
H
F
74%
85%
2a
2b
2c
Li
Li
H
CH3
F
1.0
1.0
2.0
55%
85%
78%
PiPr2
Ph
O
PiPr2
3b
3c
O
Ph
Ph
H
63%
MgBr
Keq = 65
Rh
Rh
S
Scheme 2. Transmetallation reactions among (PNP)CoIIX.
SPh
PiPr2
O
PiPr2
O
PR2
PiPr2
PiPr2
PR2
O
O
PR2
Ph
Ph
Keq = 1
Ph
N
Co X
N Rh
N Rh
S
N
Co X
Co X
PR2
SPh
PiPr2
PiPr2
PR2
PR2
S = 1/2
S = 1/2
S = 1/2
(PNP1)CoX
(POCOP)CoX
(PNP2)CoX
Heinekey:
R = tBu, X = Cl
Ozerov:
Tonzetich:
PiPr2
Ph
Arnold, Hazari:
R = iPr; X = Cl, Me, Ph
O
PiPr2
Ph
SPh
PiPr2
O
O
R = Cy; X = Br, Ph

Co
Nishibayashi:
R = tBu, Cy; X = Cl, Me
- "CoSPh"
R = iPr, X = Cl, Ph
PiPr2
O
tBu
PR2
PiPr2
PiPr2
N Co Ar
PiPr2
PiPr2
PR2
Co
PR2
Me2Si
Me2Si

Ar
N
Co X
N
X
+
N Co
N Co SAr
PiPr2
SAr
- Ar2S
PiPr2
PR2
tBu
(PNP4)CoX
(PNP3)CoX
Scheme 1. C-S reductive coupling observed for the (POCOP)Rh(Ph)(SPh) and
(PNP)Rh(Ph)(SPh) complexes (refs. 18 and 19, top), C-C coupling with the pincer carbon
in (POCOP)Co(Ph)(SPh) (ref. 43, middle), and the subject of this work (bottom).
Fryzuk: S = 3/2 if R = Ph; X = Cl, I
S = 1/2 if R = Ph; X = CH2Ph
Caulton: S = 3/2 if R = tBu; X = I
Gade: S = 3/2 if R = iPr, tBu; X = Cl
S = 1/2 if R = iPr; X = H
In 2018, we reported on the reactivity of (POCOP)Co(Ph)(SPh).43
In contrast to (PNP)Rh(Ph)(SPh)18 or (POCOP)Rh(Ph)(SPh),19 it
did not undergo C-S RE but instead a RE of the phenyl with the
pincer aryl (Scheme 1). Because of this, the POCOP system did
not allow for the investigation of the C-S RE. We surmised that
the analogous RE with the amido of a PNP pincer should be less
likely and set off to examine the reactivity of (PNP)Co(Ar)(SAr)
complexes. The present report details our efforts in the
synthesis of five-coordinate CoIII-aryl/thiolate complexes
supported by the PNP ligand, their propensity to undergo
concerted C-S RE, and the subsequent comproportionation
reactivity that again diverges from the Rh system.
PiPr2
N Co CO
PiPr2
PiPr2
N Co X
PiPr2
(PN*P)CoX
(PNP)CoCO
Chirik:
Mindiola:
S = 0
S = 1 if X = Cl, OPh
S = 0 if X = Me, Ph
Figure 2. High and low spin dichotomy in POCOP and PNP pincer-supported four-
coordinate complexes of CoII (top) and CoI (bottom).
Examination of the literature shows that four-coordinate Co(II)
complexes of anionic pincer ligands are known with both a low-
spin S= ½ configuration (square-planar geometry) and a high-
Results and Discussion
Synthesis and Characterization of (PNP)CoII complexes
3
spin S = /2 configuration (pseudotetrahedral geometry).43-52
Treatment of the previously reported square planar, low spin, S
= ½ (PNP)CoCl44 (1) with selected aryl nucleophiles resulted in
the formation of the corresponding CoII aryl complexes 2a-c
(Scheme 2). Clean transmetallation of 1 was also accomplished
using sodium thiophenolate reagents to give Co(II) thiolate
complexes 3a-c (Scheme 2). Complexes 2a-c and 3a-c were
green to dark teal in color. They exhibited paramagnetically
Low-spin, square-planar Co(II) complexes give rise to
1
paramagnetically shifted H NMR resonances that are broad
compared to diamagnetic compounds, but are typically
interpretable in terms of their relative integration and chemical
(in)equivalence. High-spin Co(II) compounds tend to produce
1H NMR spectra that are broadened beyond useful
interpretation. Complexes 2a-c and 3a-c in the present work
and the (POCOP)CoX complexes (Figure 2) recently reported by
us and Heinekey et al.45 are all low-spin compounds. The same
is true for the (PNP1)CoX complexes of the pyrrolyl-based PNP
ligand by Tonzetich et al.46 and Nishibayashi et al.,47 as well as
the (PNP2)CoX complexes reported by Arnold et al. and Hazari
1
shifted H NMR resonances contained in the +40 to -30 ppm
range, except for the resonances at around -90 ppm in
complexes 2a-2c which we tentatively assign as ortho-
hydrogens of the Co-bound aryl rings. No 31P NMR resonances
were detected for these complexes.
2 | J. Name., 2012, 00, 1-3
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et al.49 In contrast, Co(II) halide complexes of the Fryzuk-type and aryl/-acetato complexes 4a-4b, sharp resonances with well-
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PNP3 and Gade’s carbazole-based PNP4 ligands are high-spin.50- resolved fine structure were observed.
On the other hand, (PNP3)Co(CH2Ph) and (PNP4)CoH are low
spin.50,52 It appears that the presence of even one very strong-
DOI: 10.1039/D0SC01813A
52
Synthesis and Characterization of (PNP)CoI complexes
field ligand such as hydride or aryl/alkyl is sufficient for the low- The dimeric compound [(PNP)Co]2 (7) (Scheme 4) was
spin preference. It is interesting that in their absence, the previously reported by Mindiola et al.44 We were also able to
various CoII complexes in Figure 2 contain low and high-spin observe a CoI complex (PNP)Co(PPh3) (8) by treatment of
complexes with essentially the same set of donors (e.g., high- (Ph3P)2CoN(SiMe3)2 (9)29 with (PNP)H (10). This reaction
spin (PNP3)CoCl vs low-spin (PNP)CoCl (1) or (PNP1)CoBr or liberated triphenyl phosphine and HN(SiMe3)2 (Scheme 4). A
(PNP2)CoCl). All the complexes in Figure 2 possess trans- wide 1H NMR spectral window revealed a new set of
1
disposed phosphines or phosphinites in the pincer, and it does paramagnetically shifted H NMR resonances which we have
not appear that their presence alone is sufficient to ensure low- assigned to 8. Compound 8 was not isolated as it appears to be
spin configurations. We surmise that the geometric constraint in equilibrium with diamagnetic 7 on the timescale of
of the ligand plays a role in enforcing the corresponding experimental handling. For example, freshly made 8 was
geometry and thus spin state, with the less flexible PNP and observed to produce 7 when left overnight in a -35 °C freezer,
PNP1 favoring square planar, low-spin CoII.53 The PNP4 ligand while addition of 12 equiv. of PPh3 to 7led to the observation of
contains the most π-donating (dialkylamido) central N donor of
8
(Scheme 4). Addition of 12 eq. of tris(4-
the selection in Figure 2, which may help stabilize a low-spin methoxyphenyl)phosphine to this mixture gave rise to a second
configuration. Amido/bis(phosphine) PNP pincer ligands can be set of distinct but very similar paramagnetically shifted 1H NMR
oxidized at the ligand and thus are potentially redox non- resonances we interpret as belonging to 11 (Scheme 4, Figure
innocent,54 but we see no evidence of it in this work.
S12). This observation supports the notion that 8 is a PPh3-
bound Co complex.
Synthesis and Characterization of (PNP)CoIII(Ar)(X) Complexes
As is the case in our (POCOP)Co system,43 reacting (PNP)Co(Aryl)
complexes 2a-b with 0.5 eq. of PhI(OAc)2 led to the clean
formation of (PNP)Co(Aryl)(OAc) (4a-b), isolated in good yield
as tan solids after workup (Scheme 3). Treatment of 4a with
Me3SiI furnished dark blue-green (PNP)Co(Ph)(I) (5a). 5a can
also be prepared via reaction of 2a with 0.5 equiv. of I2.
Transmetallation from 5a using sodium thiophenolate
proceeded smoothly providing (PNP)Co(Ph)(SPh) complex 6a as
a dark blue solid in good yields after recrystallization (Scheme
3).55 The analogous synthesis of (PNP)Co(Ar)(SAr’) complexes
6b-c from 4b-c can be achieved without the isolation of the
intermediate 5b-c.
PiPr2
P
9
( )
+(Ph3P)2CoN(SiMe3)2
PPh3
NH
PiPr2
10
N
Co
-HN(SiMe3)2
-PPh3
P
- PPh3
+ PPh3
8
iPr2P
PiPr2
PiPr2
N
Co Co
N
Ar' =
iPr2P
O
+ PAr'3
P
- PAr'3
7
N
Co
PAr'3
P
11
Scheme 4. Formation of proposed (PNP)Co(PPh3) and equilibrium between (PNP)2Co2
and (PNP)Co(P(Ar)3)
Ar
Ar
C6H5
4-CH3C6H4 65%
Yield
44%
P
4a
4b
N
Co OAc
P
(AcO)2IPh
0.5 eq.
1 eq. Me3SiI
Arene Solvent, RT
-Me3SiOAc
1) 1 eq. Me3SiI
2) xs. NaSC6H4R
Toluene, RT
Based on the paramagnetically shifted 1H NMR spectra, we
assume that both 8 and 11 possess an S = 1 ground state. As
with four-coordinate Co(II), there are examples of both high-
spin and low-spin pincer complexes of Co(I) (S = 1 or 0) (Figure
2, bottom). With the pyridine-centered PN*P ligand, complexes
substituted with stronger field Me or Ph are low-spin, while Cl
or OAr as the fourth donor are high-spin.30,57 With the PNP
ligand, CO in place of PPh3 in 8 was reported to give a low-spin
carbonyl complex (PNP)Co(CO). Thus, it appears that the
presence of at least one strong-field ligand (CO, or
hydride/alkyl/aryl) is needed to ensure an S = 0 ground state.
-PhI
NaS
Ar
Ar
P
P
xs.
P
0.5 eq. I2
THF, RT, 4 h.
R
N
Co Ar
N
Co S-p-C6H4R
N
Co
I
THF
-NaI
P
P
P
Ar
R
H
F
F
Yield
64%
88%
Ar
Ar
Yield
6a
C6H5
2a C6H5
5a
C6H5
60-90%
6b
6c
2b 4-CH3C6H4
2c 4-FC6H4
4-CH3C6H4
C6H5
Scheme 3. Oxidation of (PNP)CoII and synthesis of target (PNP)Co(Ar)(S-Ar)
The (PNP)Co(Ar)(X) compounds (4a-b, 5a, and 6a-c) gave rise to
1H, 31P, and 13C NMR spectra expected for these diamagnetic
complexes. The resonances arising from the Co-bound aryl
group exhibited inequivalence between the two ortho- and
between the two meta-hydrogens, characteristic of restricted
rotation of the metal-bound aryl oriented cis to the central
donor of a pincer ligand with two side –PiPr2 arms.14,43,56 In the
cases of the aryl/-thiolate complexes 6a-6c, these aromatic
resonances were broad humps, whereas in the aryl/-halide 5a
X-ray Structural Studies
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DOI: 10.1039/D0SC01813A
Figure 3. POV-Ray renditions of ORTEP drawings (50% probability ellipsoids) of (PNP)Co(Tol), (PNP)Co(Ph)(OAc), and (PNP)Co(Ph)(SPh). All hydrogen atoms
omitted. Selected bond distances (Å) and angles (degrees) for (PNP)Co(Tol) (2b, left): N1–Co1, 1.9262(17); P1–Co1, 2.1756(7); P2–Co1, 2.1849(7); C15–Co1,
1.939(2); N1–Co1–C15, 178.99(9); P1–Co1–P2, 172.63(3). Selected bond distances (Å) and angles (degrees) for (PNP)Co(Ph)(OAc) (4a, middle): N1–Co1,
1.9333(12); O1–Co1, 1.9896(12); O2–Co1, 2.1166(11); P1–Co1, 2.2619(7); P2–Co1, 2.2353(6); C29–Co1, 1.9403(15); C29–Co1–N1, 97.59(6); N1–Co1–O1,
165.17(5). Selected bond distances (Å) and angles (degrees) for (PNP)Co(Ph)(SPh) (6a, right): N1–Co1, 1.9497(18); P1–Co1, 2.2597(7); P2–Co1, 2.2313(7); C33–
Co1, 1.933(2); S1–Co1, 2.2069(6); N1–Co1–C33, 98.54(8); N1–Co1–S1, 149.60(6).
Single crystals of 2b, 4a, and 6a suitable for X-ray diffraction generation of free aryl radicals. Similarly, thermolysis of 6b in
were grown from hydrocarbon solvents at -35 °C (Figure 3). The benzene resulted in the formation of 2b, 3b, and C in a 1:1:1
geometry about the cobalt center in the solid-state structure of ratio (Scheme 5, bottom; Figure S3 & S4).
2b is slightly distorted square planar. The Co-bound tolyl ring in Attempting to determine whether the C-S bond formation step
2b is approximately perpendicular to the Co/P/N/P plane.
happened at a single Co center, thermolysis of 6a in a 1:1 ratio
The structure of 4a is pseudo-octahedral about the metal with 6b was carried out. In principle, strict unimolecular C-S
center, with a κ2 acetate coordination. The two oxygens of the reductive elimination should lead to only two diarylasulfide
acetate are bound trans to two donors of markedly different products as a result of this thermolysis. In the event, formation
trans-influence (amido N vs phenyl C), which is reflected in the of four diarylsulfides was instead observed, along with the four
large difference between the two Co-O bond distances (ca. 0.13 expected CoII products: 2a, 2b, 3a, and 3b (Scheme 6). 19F NMR
Å). The geometry about Co in 6a is intermediate between analysis during the course of the reaction at 80 °C revealed the
square-pyramidal with the phenyl trans to the empty site and Y- formation of the Co(III) crossover product 6c (Figure S5). This
shaped (with the thiolate at the base of the Y). The preference suggested that during the thermolysis, thiolate ligands can
of low-spin five-coordinate d6 complexes for square-pyramidal exchange between the Co(III) centers prior to RE. This exchange
and Y-shaped geometries have been discussed elsewhere.58 would then lead to the formation of crossover diarylsulfides,
The angles, bond lengths, and orientation of the thioaryl ligand even if RE happens unimolecularly, and thus prevent us from
about the cobalt center for 6a are similar to those that we firmly excluding crossover via other pathways. Performing this
reported for (POCOP)Co(Ph)(SPh).43
reaction at double the initial concentration of CoIII complexes
still showed 6c during thermolysis and a very similar distribution
of CoII products and diaryl sulfides after the reaction had
completed.
Ph
80 o
C
P
P
P
S
A
+
+
N
Co SPh
N
Co Ph
N
Co SPh
C6D6
P
P
P
6a
2a
3a
:
:
Quantitative conversion 1 1 1 ratio
S
A
Ph
P
P
P
Tol
N
Co SPh
N
Co Ph
+
N
Co SPh
80 o
C6D6
C
P
P
P
S
D
+
P
P
P
S
S
+
N
Co Tol
N
Co S-p-C6H4F
N
Co S-p-C6H4F
6a
+
2a
3a
B
C
F
P
P
P
6b
2b
3b
+
:
:
Quantitative conversion 1 1 1 ratio
P
P
Tol
P
Scheme 5. Top: Thermolysis of 6a and observed products. Bottom: Thermolysis of 6b and
observed products.
N
Co Tol
+
N
Co S-p-C6H4F
F
F
N
Co S-p-C6H4F
P
P
P
6b
2b
3b
S
D
Thermolysis of (PNP)CoIII(Ar)(SAr) Complexes
Ratio by GC-FID
A:B:C:D
Thermolysis of 6a in benzene led to the formation of 2a, 3a, and
A in a 1:1:1 ratio (Scheme 5, top). Further investigation showed
that this process is first order in 6a (Figure S1). Thermolysis of
this complex in the presence of 1 eq. of 2,6-di-tert-butyl-4-
methylphenol (BHT) resulted in the same distribution of
products in the same time period, providing evidence against
Tol
Ph
P
= 3:1:1:3
P
+
N
Co SPh
N
Co S-p-C6H4F
Ratio by 1H NMR
2a:2b:3a:3b
P
P
6d
6c
:
:
:
= 1.4 1.0 1.1 1.1
Scheme 6. Thermolysis of a mixture of 6a and 6b.
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The thiolate exchange between Co(III) complexes could occur in case, free triphenylphosphine was also observeVdiewbAyrti1cHle Oanlninde
at least59 two non-exclusive ways: 1) direct thiolate exchange 31P{1H} NMR spectroscopy.
DOI: 10.1039/D0SC01813A
between two Co(III) complexes, 2) via exchange between Co(II)
and Co(III) thiolate complexes. To probe the ability of Co(III) to
exchange thiolate ligands with Co(II), 6a was thermolyzed in the
Reversibility of C-S RE?
Treatment of 7 with diphenyl sulfide (A) and heating overnight63
in a 55 °C oil bath resulted in the formation of 2a and 3a in a 1:1
ratio (Scheme 8). This experiment shows that Co(I) here can
presence of 1 eq. 3b. In situ 19F NMR observation at 80 °C
revealed the formation 6c during the reaction (Scheme 7, top).
After the thermolysis was complete, 6% of the total starting
fluorinated thiolate was found as C demonstrating that CoII and
cleave a C-S bond in a diarylsulfide, suggesting that C-S RE might
CoIII can swap thiolates. The conditions required for the
be reversible. A related observation is that thermolysis of 6b in
the presence of A resulted in the formation of a small amount
of C in addition to D (Scheme 7, bottom); which can be
interpreted as occasional trapping of a Co(I) species formed in
observation of thiolate swapping between two Co(III)
complexes inevitably led to at least some Co(II) thiolate, which
prevented us from establishing whether Co(III) thiolate
complexes can exchange thiolates without the involvement of
the RE of D by Ph2S (A) as opposed to by the Co(III) starting
Co(II).
material 6b. By way of a control experiment, thermolysis of 2b
and 3b with A at 80 °C for 7 d resulted in no detectable change,
establishing that CoII compounds do not react with
diarylsulfide.
P
P
+
N
Co Ph
N
Co SPh
a
Ph
P
P
P
80 o
C
P
2a
3a
+
N
Co S-p-C6H4F
N
Co SPh
6a
2a:3a
:
= 0.9 1
C6D6, 12 h
P
3b
P
S
S
+
Ph
P
F
80 oC, 3 hr.
N
Co SPh
A
C
P
6% of total fluorine
- 0.5 eq. Ph2S
6a
P
Ph
P
P
N
Co S-p-C6H4F
S
55 oC, 24 hr.
+
N
Co SPh
[(PNP)Co]2
+
+
N
Co S-p-C6H4F
6c
P
3b
(94% Remains)
P
3a
7
P
A
(observed by 19F NMR)
P
P
+
N
Co Ph
N
Co SPh
Ph
P
P
Immediate
- PPh3
P
P
N
Co PPh3
2a
3a
N
Co SPh
P
P
P
P
8
6a
N
Co Tol
N
Co S-p-C6H4F
+
P
P
Tol
2b
3b
P
80 o
C6D6, 12 h
C
S
Ph
P
2b:3b
:
= 1 1
+ 2 eq.
N
Co S-p-C6H4F
6b
Immediate
(observed by 1H NMR)
[(PNP)Co]2
N
Co SPh
+
P
P
7
A
6a
S
S
Scheme 8. Reaction pathways shown to generate 1:1 mixture of 2a and 3a.
+
F
F
C
D
C:D
:
= 2 98
(observed by 19F NMR)
In order to gain some insight into whether this C-S cleavage by
Co(I) occurs via concerted OA, we subjected 7 to thermolysis
with 2-isopropylphenyl-4’-fluorophenyl sulfide (E). Based on
what we learned of the preferences of the (pincer)Rh systems
in OA with aryl halides,13,14,19 it seemed reasonable to assume
that the concerted OA mechanism with (PNP)Co should favor
the C-S bond unencumbered by the ortho-isopropyl substituent
(CF-S, Scheme 9). DFT calculations (see SI) predicted that
homolytic cleavage of the CF-S bond is 1.6 kcal/mol less
thermodynamically favorable than the cleavage of the C-S bond
connecting to the 2-isopropylphenyl substituent (CR-S, Scheme
9). Thus, a radical abstraction mechanism for the C-S cleavage
might be expected to favor the cleavage of CR-S.
S
A
(>96% Remains)
(observed by 1H NMR)
Scheme 7. Top: Thermolysis of (PNP)Co(Ph)(SPh) (6a) in the presence of (PNP)Co(S-p-
C6H4F) (3b) and observed products. Bottom: Thermolysis of 6b in the presence of A and
observed products.
Comproportionation hypothesis and reactions with CoI
compounds
By analogy with our work on pincer rhodium complexes,13,15,19
we envisioned that after the concerted C-S reductive
elimination from 6a, an unsaturated (PNP)Co fragment (12)
would be generated. We further hypothesized that this
unsaturated (PNP)Co species 12 undergoes rapid
comproprotionation with the remaining 6a to generate the
observed Co(II) products.6062 To test this hypothesis, 7 was
combined with 6a in benzene at ambient temperature. An
immediate color change to green was observed upon mixing,
indicating the formation of (PNP)CoII complexes (Scheme 8). 1H
NMR spectroscopic observation confirmed the formation of 2a
and 3a in a 1:1 ratio. Similarly, mixing freshly made 8 with 6a
resulted in an immediate comproportionation producing 2a and
1
3a in a 1:1 ratio by H NMR spectroscopy (Scheme 8). In this
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P
P
Ph
-9.9
-8.3
0.4
3.8
P
P
P
N
Co p-C6H4F
N
Co p-C6H4F
DOI: 10.1039/D0SC01813A
[(PNP)Co]2
N
Co SPh
6a
N
Co SPh2
N
Co SPh2
P
P
2c
2c
P
7
13s
singlet
13t
triplet
P
P
+
+
80 oC, 3 days
HCl (non aq.)
+
-22.1
-12.7
P
Co Cl
1
P
-4.1
8.3
E
1.5
14.6
S
N
N
Co
S
P
P
F
3c
-15.5
-14.7
P
P
CR-S
CF-S
P
2c:1
:
P
= 1 0.6
1
1
2c:3c
:
= 1 0.6
+
N
Co Ph
N
/
2
P
Co SPh
/
N
Co
+ SPh2
2
N
Co
+ SPh2
+
P
2a
3a
P
P
12t
triplet
A
P
S
12s
singlet
A
HS
1
+ /2 SPh2
N
Co
P
-2.8
3b
-11.9
F
Detected by
GC-MS and
APCI-MS
Not Observed
1
/
2 [(PNP)Co]2 + SPh2
Ph
P
1
7
A
1
+
/
4 [(PNP)Co]2
Scheme 9. Thermolysis of [(PNP)Co]2 (7) with 2-isoropylphenyl-4’-fluorophenyl sulfide
(E) and the following treatment with non-aqueous HCl.
N
Co SPh
6a
/
2
-13.9
-8.7
7
P
-15.3
-14.6
Ph
P
Heating the mixture of 7 with E in at 80 °C for three days
resulted in the complete consumption of 7 with the formation
of a 1:1 mixture of 2c and 3c (Scheme 9). Compound 3b was not
P
P
P
1
2 N Co
1
1
1
/
+
+
N
Co Ph
N
/
2
P
Co SPh
/
N
Co SPh
6a
/
2
2
P
12t
P
2a
3a
P
-5.7
-4.7
1
observed by either H or 19F NMR spectroscopy. Treatment of
1
1
/
2 [(PNP)Co]2
+
/
2 SPh2
this solution with anhydrous HCl resulted in the formation of 1
along with 2c with presumed liberation of 2-
isopropylthiophenol. GC-MS and APCI-MS analysis of this
solution revealed the formation of 2-isopropylthiophenol and E
as the only two volatile components; p-FC6H4SH was not
detected. We did not detect any biaryl or bisulfide products by
GC-MS. These observations show that only the CF–S bond was
cleaved, consistent with a concerted OA C–S activation
pathway.64
7
A
Scheme 10. DFT calculated energies for the various transformations. Reaction free
energies (at 298 K) are given over the arrows on top; reaction enthalpies in italic below.
All the energies are in kcal/mol, normalized per one mole of Co.
We then considered the monomeric Co(I) intermediates in the
reaction. Both the “naked” (PNP)Co fragment 12 and its SPh2
adduct 13 were calculated to favor a triplet ground state (by
15.5 kcal/mol and 9.9 kcal/mol in free energy, respectively).
The geometry of the triplet (PNP)Co(SPh2) (13t) about Co is
decidedly not planar, and can be described as attempting to
approach tetrahedral within the constraint of the pincer. The
array of donor atoms in 13t is the same as in the low-spin
doublet 3a, but all the calculated bond distances to Co are
considerably longer, especially that for the C-S bond (2.538 Å in
13t vs 2.264 Å in 3a).
DFT calculations
In order to gain better understanding of the system, DFT
calculations were carried out with the the Gaussian 0965
program to address a few salient points (Scheme 10). The
geometries were optimized using [B3LYP/LANL2DZ/6-31G(d)]
level of theory66 in the gas phase and the energies for these
geometries were then determined with the [M06/SDD/6-
311+G(d,p)] method67 incorporating the benzene solvent effect
via the SMD model.68 Further details are given in the Supporting
Information. We first evaluated the thermodynamics of the
overall observed reaction. Conversion of 1 equiv of 6a into a
0.5:0.5:0.5 mixture of 2a:3a:A was calculated to be favorable by
-22.1 kcal/mol in free energy. The geometries of 2a and 3a in
the doublet ground state were approximately square-planar
geometry and consistent with the X-ray structure of 2b. The
calculated structure of 6a reproduced the overall geometry
determined in the XRD study, as well. Figure 4 shows the
calculated SOMO’s and spin density profiles fo 2a and 3a.
Interestingly, the nature of the SOMO’s in these two
compounds differs. In 2a, it is essentially a pure dz2, whereas in
3a it is primarily dxz (x axis along Co-S) with small contributions
from the amido and thiolate ligands. This disparity reflects the
fact that a stronger σ-donor Ph elevates the energy of dz2 in 2a
relative to dxz. In either case, the SOMO if firmly metal-based.
6 | J. Name., 2012, 00, 1-3
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Figure 4. Depictions of the calculated singly occupied molecular orbital (SOMO) for 2a
and 3a (left, isovalue 0.04), and of the calculated spin-density plots for 2a and 3a (right,
isovalue 0.004). Hydrogen atoms are omitted for clarity.
out that even its adduct with PPh3 (8) does not present as a low-
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spin complex based on the appearance ofDiOtsI:N10M.10R3s9p/De0cStCra0.18I1t3iAs
possible that the (PNP5)Co system needs to be reexamined
more closely.
The reductive C-S coupling in 6a to give singlet (PNP)Co(SPh2)
(13s) is nearly ergoneutral, but the conversion of 6a to 13t is
favorable on the enthalpy (by 4.5 kcal/mol) and on the free
energy (by 9.5 kcal/mol) surfaces. The dissociation of SPh2 from
13t to give triplet 12t and free SPh2 is endothermic, but is of
course favored entropically, resulting in a favorable free energy
of dissociation. Thus the complete RE from 6a to give 12t and
A is exoergic by 13.6 kcal/mol, but that is less favorable than the
formation of a mixture of 2a:3a:A.
The dimerization of 12t to form 7 was calculated to be
enthalpically favorable, but disfavored entropically and overall
slightly exoergic (by 2.8 kcal/mol per Co). This is consistent with
the experimental observation of the dimer 7 as the ground
state.
P
Ph
N
Co
P
SPh
G
H
TS
P
Co
P
24.8
23.5
P
Ph
N
N
Co SPh2
P
SPh
P
Co
P
13s
N
6a
0.4
3.8
SPh2
0.0
0.0
13t
-9.5
-4.6
Figure 6. Representation of the reaction coordinate for the RE of SPh2 (A) from
(PNP)Co(Ph)(SPh) (6a). Energy values are given below the bars: free energy on top,
enthalpy on the bottom in italic.
The thermodynamics of the standalone Co(I)/Co(III)
comproprotionation reactions were calculated to be consistent
with our hypothesis outlined above. The reaction of 12t with 6a
to give 2a and 3a was found to be exothermic and exoergic (by
For the C-S reductive coupling en route to 13 from 6a, a
transition state was found, lying 24.8 kcal/mol above 6a in free
energy (TS, Figure 5). This transformation requires spin
crossover in the process, which we propose happens after the
singlet TS on the reaction coordinate. A singlet state of the
reductive coupling product (PNP)Co(SPh2) (13s) is only 0.4
kcal/mol endergonic relative to 6a. However, 13t is lower in
energy still. The activation barrier magnitude calculated by DFT
agrees reasonably well with the experimental observations.
The observed half-life of 0.6 h at 80 °C for 6a corresponds to ca.
26 kcal/mol in free energy barrier (ΔG‡298).
Examination of the geometry of TS (Figure 6) shows that it can
be thought of as reflecting the migration of the Co-bound Ph
group onto the S atom, which in turn is brought more closely
into the plane defined by P/N/P/Co. The Co-S distance in TS is
actually slightly shorter than in 6a, and much shorter than
claculated in 13t. The Co-CPh distance elongates by ca. 0.13 Å in
TS (2.055 Å) vs 6a (1.929 Å), while the newly forming C-S
distance (2.085 Å) is about 0.29 Å longer than the expected C–S
distances of ca. 1.80 Å in Ph2S or its complexes. The other
geometric feature of TS that needs to be emphasized is the
necessary rotation of the Co-bound phenyl ring from edge-on
relative to S in 6a to side-on in TS. The hindrance of this rotation
by the iPr groups is a major contributor to the magnitude of the
activation barrier. This is a rather general observation for the
reductive elimination of R-X from five-coordinate d6 complexes
(pincer)M(R)(X) where R = aryl or alkenyl, first articulated by
Goldman and Krogh-Jespersen for the (PCP)Ir system.71 We
previously discussed this issue for the closely related RE
reactions from (pincer)Rh(Ar)(X) complexes.15,16,19
-14.6 and -15.3 kcal/mol, respectively).
comproportionation starting from 7 instead of 12t was also
found to be favorable (by -13.9 kcal/mol per mole of Co).
A
similar
tBu
PBut2
PiPr2
N Co
PiPr2
PBut2
Co
PBu2
PiPr2
N Co
PiPr2
Me2Si
Me2Si
N Co
PBut2
N
tBu
(PNP)Co
(PNP5)Co
(PNP3)Co
(PNP4)Co
This work
Caulton 2006
Gade 2019
S = 1
experimentally
established
Lee 2019
S = 1
experimentally
established
S = 1
suggested
by DFT
S = 0
proposed
Figure 5. Examples of three-coordinate Co complexes supported by various anionic PNP
ligands.
The substantial (ca. 15 kcal/mol) calculated preference for the
triplet state of (PNP)Co (12t) is at odds with the recent report
by Lee and coworkers, which presented three-coordinate
(PNP5)Co as a singlet species (Figure 4).69 This intepretation by
Lee et al. is also at odds with the unambiguously established
triplet ground states for the (PNP3)Co and (PNP4)Co by the
Caulton70 and Gade groups,52 respectively (Figure 4). Both
(PNP3)Co70 and (PNP4)Co52 were isolated and fully
characterized, including by X-ray crystallography, magnetic
moment measurement, as well as by elemental analysis for
(PNP4)Co. (PNP5)Co was purported to be isolated, but no
structural determination or magnetic moment was reported,
and satisfactory elemental analysis was not obtained. (PNP5)Co
was analyzed by DFT calculations as a singlet, but the
calculations examining the viability of the triplet state were not
carried out. Given these facts and the very close similarity of
the PNP and PNP4 ligands, it would be very surprising indeed if
they led to different spin state preferences in 12 vs (PNP5)Co.
Although we do not observe free 12, it is also worth pointing
Lastly, we considered the experimental observations of the
apparent reversibility of C-S RE in reactions of A with Co(I)
complexes. Thermodynamically, the experimentally observed
reaction of [(PNP)Co]2 with Ph2S to give 2a and 3a was indeed
calculated to be favorable (Scheme 10). At first glance, the
microscopic reverse C-S OA might appear kinetically feasible as
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the energy of 13s is similar to that of 6a. However, given that metals in the d10/d8 manifold, as well as the contrast between
View Article Online
DOI: 10.1039/D0SC01813A
1) 13t, 2) 12t + free SPh2, and 3) 7 + free SPh2 are all considerably the 3d metals (Co or Ni comproportionate) and the 4d metals
lower in energy than 13s, the barriers for the microscopic (Rh or Pd do not comproprotionate) within the same group.
reverse C-S OA starting from these states are prohibitively high.
Conflicts of interest
There are no conflicts to declare.
C
N
C
N
Co
P
Acknowledgements
Co
P
P
P
We are grateful to the Welch Foundation (grant A-1717 to
O.V.O.) for support of this research. This work was supported by
the startup of Harbin Institute of Technology (Shenzhen)
through the Talent Development Starting Fund from Shenzhen
Government. Computer time made available by the National
Supercomputing Center of China in Shenzhen (Shenzhen Cloud
Computing Center) is gratefully acknowledged. We thank
Samuel R. Lee for assistance with the composition of the
manuscript.
S
S
Figure 7. Top: DFT calculated structures of 6a (left) and TS (right). Bottom: key distances
(in Å) in 6a and TS; bottom structures not to scale.
In rationalization, two possibilities might be considered. First, it
is possible that our DFT calculations do not accurately describe
the relative energies of compounds in different spin states. The
second option is that the reaction of A with 7 proceeds as an C-S
OA within the dimer, without the formation of monomeric
intermediates. The putative single C-S OA dicobalt product may
then comproportionate intramolecularly to give 2a and 3a
without the intermediacy of free 6a. This reaction pathway
would thus not be a microscopic reverse of the monomolecular
C-S RE. The complexity of the many potential pathways that
would need to be considered to properly analyze the reaction
of [(PNP)Co]2 with A has deterred us from pursuing this problem
computationally within the scope of this report.
Conclusion
In summary, (PNP)Co complexes in the +1, +2, and +3 oxidations
states relevant to potential cross-coupling reactions were
prepared and fully characterized. A switch to a PNP ligand
prevented intramolecular reductive elimination of the Co–Ar
unit with the central donor of the pincer and permitted
observation of concerted C-S reductive elimination. However,
it appears that the PNP supporting ligand does not have a strong
enough ligand field strength to prevent promotion of an
electron from the (PNP)Co fragment to a triplet ground state.
This fundamental realization is probably related to the swift
Co(I)/Co(III) comproportionation investigated in this work,
which removes potentially catalytically competent odd-
oxidation state cobalt complexes from the reaction.
Interestingly, the Co(I)/Co(III) comproportionation observed
here directly mimics the Ni(0)/Ni(II) comproportionation
observed and studied by the Hazari group.72 The similarity
further underscores the close parallels in reactivity that exist
between group 9 metals in the d8/d6 manifold and the group 10
8 | J. Name., 2012, 00, 1-3
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Notes and references
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DOI: 10.1039/D0SC01813A
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