Synthesis, characterization and reactivity of iron- and cobalt-pincer complexes

Article abstract of DOI:10.1016/j.poly.2015.12.037

The ^tBuPONOP (2,6-bis(di-tert-butyl-phosphinito)pyridine) complexes of iron and cobalt, (^tBuPONOP)FeCl₂ (1) and (^tBuPONOP)CoCl₂ (2)) have been prepared. Both complexes are paramagnetic and the solid-state structures of 1 and 2 were determined by single crystal X-ray diffraction studies. Analogous Fe and Co complexes of the ^tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) ligand (3 and 4, respectively) were prepared to allow comparison between the closely related pincer ligands in the hydrosilylation of carbonyl moieties. All four complexes were found to be catalytically active when treated with NaBEt₃H, which was assumed to generate a metal-hydride species in-situ.

Full text of DOI:10.1016/j.poly.2015.12.037

Polyhedron xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization and reactivity of iron- and cobalt-pincer
complexes
⇑
Ashleigh D. Smith, Anu Saini, Laci M. Singer, Neha Phadke, Michael Findlater
Department of Chemistry & Biochemistry, Texas Tech University, Lubbock, TX 79409, USA
a r t i c l e i n f o
a b s t r a c t
The tBuPONOP (2,6-bis(di-tert-butyl-phosphinito)pyridine) complexes of iron and cobalt, (tBuPONOP)FeCl2
(1) and (tBuPONOP)CoCl2 (2)) have been prepared. Both complexes are paramagnetic and the solid-state
structures of 1 and 2 were determined by single crystal X-ray diffraction studies. Analogous Fe and Co
complexes of the tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) ligand (3 and 4, respectively)
were prepared to allow comparison between the closely related pincer ligands in the hydrosilylation of
carbonyl moieties. All four complexes were found to be catalytically active when treated with NaBEt3H,
which was assumed to generate a metal-hydride species in-situ.
Article history:
Received 22 October 2015
Accepted 18 December 2015
Available online xxxx
Keywords:
Co complexes
Fe complexes
Pincer ligands
Solid state structures
Hydrosilylation
Ó 2016 Published by Elsevier Ltd.
1. Introduction
systems, is depicted in Scheme 1 [18–26]. It should be noted that
several related iron and cobalt complexes containing anionic pin-
Pincer ligands have emerged as a privileged class of ligand in
organometallic chemistry and have seen widespread use in cataly-
sis [1–8]. In particular, Ir-based pincers have been shown to be
effective catalysts in a range of valuable catalytic transformations
[9–12]. Among these pincer ligands, neutral tridentate systems
such as PNP and PONOP have emerged as leading platforms for
the study of catalytic reactions [13,14]. Given the high cost of pre-
cious metals such as Ir and Rh, the application of such ligands in
base metal catalysis is no surprise and a number of reports have
appeared in which such base–metal complexes were deployed in
fields ranging from energy science to asymmetric catalysis [15,16].
Given our interest in the hydrosilylation of carbonyl moieties
using iron [17], we turned our attention to the synthesis and reac-
tivity of first-row metal PONOP and PNP pincer complexes. These
types of PONOP and PNP ligands are readily available via salt
metathesis reactions between 2,6-dihydroxypyridines or 2,6-bis
(bromomethyl)pyridines and dichlorophosphines to afford either
PONOP or PNP pincer ligands, respectively. As befitting the status
of a privileged class of ligands, the modular fashion in which they
can be synthesized allows versatility in ligand platform design that
is virtually unmatched. Given such versatility, it is surprising that
only a few Fe- and Co-PONOP and PNP pincer complexes have been
reported in the literature, although related systems have also been
disclosed. An overview of cobalt and iron PNP and PONOP pincer
cer-type PCP, POCOP, and PNpyrrole
described [27–33].
P frameworks have been
In this report, the synthesis, characterization, and reactivity of
new (PONOP)M (M = Fe and Co) complexes are examined. Reac-
tions of these metal PONOP complexes with NaBEt3H result in
catalytically competent systems for the hydrosilylation of carbonyl
groups. The synthesis and characterization of the, presumably,
metal hydride complexes proved to be difficult because of the
instability of these complexes, even at room temperature under
inert atmosphere.
2. Results and discussion
Treatment of anhydrous FeCl2 or CoCl2 with the ligand tBuPO-
NOP in THF affords the 16e and 17e complexes (tBuPONOP)FeCl2
(1) and (tBuPONOP)CoCl2 (2), respectively, in low yields after crys-
tallization (Scheme 2). Both complexes exhibit broad NMR reso-
nances consistent with paramagnetic compounds. Single crystals
of both 1 and 2 were grown from concentrated toluene solutions
at low temperature and analyzed by X-ray crystallography. The
solid-state structures of 1 and 2 are shown in Fig. 1.
As we were preparing our manuscript, Schrodi and coworkers
disclosed the solid-state structure of complex 1.[26] However, sig-
nificant differences exist between the two structures. In our hands,
1 crystallized in the monoclinic space group rather than the previ-
ously reported triclinic system. Overall, the geometry is consistent
with the previously reported crystal structure and with the
⇑
Corresponding author.
E-mail address: michael.findlater@ttu.edu (M. Findlater).
http://dx.doi.org/10.1016/j.poly.2015.12.037
0277-5387/Ó 2016 Published by Elsevier Ltd.
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2
A.D. Smith et al. / Polyhedron xxx (2016) xxx–xxx
analogous structure, (tBuPNP)FeCl2 (3) [34]. However, the geometry
HN
HN
PPh2
Cl
Cl
P(iPr)2
CO
CO
P(tBu)2
Cl
P(iPr)2
Br
around the iron center is best described as a distorted square pyra-
mid with one chloride in the apical position and the nitrogen of the
pyridine ring, the two phosphorus atoms and the remaining chlo-
rine forming the basal plane. The structural distortion in complex 1
arises from an iron center, which is raised above the basal plane by
a distance of 0.433 Å. The Fe–Cl(1) and Fe–Cl(2) distances are very
different, 2.365(2) and 2.268(2) Å, respectively. Similarly, N–Fe–
Cl(1) and N–Fe–Cl(2) bond angles are also dissimilar, 136.4(3)°
and 117.1(3)°, respectively. Close examination of the packing dia-
gram reveals the cause to be the presence of secondary bonding
interactions between the chloride ligands and adjacent ligand-
based H-atoms (Fig. 1b). Curiously, a bifurcated non-bonding inter-
action is present involving both chlorides and a second interaction
is observed only with Cl(1). This additional secondary bonding
interaction accounts for the lengthening of the Fe–Cl(1) bond
distance.
N Fe
N Fe
N Fe
N Fe CO
H
Cl
PPh2
P(iPr)2
P(tBu)2
P(iPr)2
N
H
N
H
Kirchner
2007
Chirik 2006
Goldman 2008
Milstein
2011
Kirchner
2013
O
P(iPr)2
Cl
P(iPr)2
P(tBu)2
H
N Fe CO
H
Cl
Cl
P(tBu)2
N Co
N Fe
Cl
P(iPr)2
P(iPr)2
O
Chirik
2014
Milstein
2014
Schrodi
2014
Scheme 1. Overview of Fe/Co PNP and PONOP complexes reported in the literature.
The solid-state structure of 2 (Fig. 1c) is essentially isostructural
with that of 1, only less distorted. Like 1, analysis of the bonding in
2 unearths inequivalent cobalt-chloride bond lengths for the apical
and the basal sites (2.389(1) and 2.253(1) Å, respectively). Once
more, N–Co–Cl(1) and N–Co–Cl(2) bond angles also vary widely,
97.94(7)° and 160.72(8)°, respectively. We attributed the distor-
tions from ideal square pyramidal geometry in 1 to intermolecular
non-bonding interactions. As anticipated, upon examination of the
extended packing of 2 we observed a similar network of inter-
molecular interactions (Fig. 1d). Thus, the presence of non-bonding
interactions between the chloride ligands and adjacent ligand-
based H-atoms is revealed. However, unlike 1 there are no ‘chelating’
interactions between both chloride ligands and an adjacent
O
N
O
O
N
O
P(tBu)2
P(tBu)2
Cl
MCl2
M
Cl
P(tBu)2
THF, 12 h,
RT
P(tBu)2
P(tBu)2
M = Fe (1); Co (2)
P(tBu)2
Cl
MCl2
N
N M
Cl
P(tBu)2
THF, 12 h,
RT
P(tBu)2
M = Fe (3); Co (4)
Scheme 2. Synthesis of (PONOP)M and (PNP)M complexes.
Fig. 1. (a) Solid-state structure of 1. Hydrogen atoms omitted for clarity. Thermal ellipsoids at 30% probability. Key bond lengths (Å) and bond angles (°): N1–Fe1 2.268(8),
P1–Fe1 2.526(2), P2–Fe1 2.503(2), Fe1–Cl1 2.365(2), Fe–Cl2 2.268(2), P1–Fe1–P2 144.68(8), N1–Fe1–Cl1 136.4(3), N1–Fe1–Cl2 117.1(3), Cl1–Fe1–Cl2 106.53(7).
(b) H-bonding contacts to chloride ligands account for distortion of square–pyramidal metal center geometry. (c) Solid-state structure of 2. Hydrogen atoms omitted for
clarity. Thermal ellipsoids at 30% probability. Key bond lengths (Å) and bond angles (°): N1–Co1 1.971(2), P1–Co1 2.281(1), P2–Co1 2.277(1), Co1–Cl1 2.389(1), Co1–Cl2
2.253(1), P1–Co1–P2 160.38(3), N1–Co1–Cl1 97.94(7), N1–Co1–Cl2 160.72(8), Cl1–Co1–Cl2 101.34(3). (d) H-bonding contacts to chloride ligands account for distortion of
square–pyramidal metal center geometry.
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A.D. Smith et al. / Polyhedron xxx (2016) xxx–xxx
3
Table 1
H-atom. We believe this is the reason the overall structure in 2 is
less distorted that that of 1.
Solvent effects in the hydrosilylation of acetophenone.a
Solvent
Temperature (°C)
Conversion (%)b
2.1. Hydrosilylation studies
THF
66
35
110
40
100
60
58
54
80
82
Ether
Toluene
DCM
Neat
The reduction of the carbonyl, imine, or alkene functionality
using a hydrosilylation strategy has emerged as a fundamental
transformation in organometallic chemistry [1]. Prevalent among
the transition metal elements capable of such transformations
are rare and expensive metals such as ruthenium, rhodium, and
iridium [35]. In recent years, much effort has been devoted to
the development of cheaper, first-row, transition metal based cat-
alysts with cobalt and iron complexes proving a particular focus
[36]. Guan et al. synthesized phosphinite based pincer complexes
of Fe [30,31,37], which were shown to be effective catalysts for
reducing various aldehydes and ketones. Recently Li and coworkers
disclosed imine based Fe hydrido complexes capable of catalyzing
the reduction of the carbonyl moiety [38]. In contrast, hydrosilyla-
tion catalysts based upon Co have received only limited attention
[39].
a
Reaction conditions: FeCl2 (1 mol%), tBuPNP (1 mol%), acetophenone (0.154 g,
1.28 mmol), (EtO)3SiH (0.213 g, 1.29 mmol), 1 M NaBEt3H solution in THF (2 mol%).
b
Calculated using GC–MS analysis.
conditions and found all complexes were effective hydrosilylation
pre-catalysts. Subsequently, we discovered that the formation of
undesired by-products could be reduced by conducting the
hydrosilylation reactions under slightly modified conditions; room
temperature for 18–24 h. After optimizing the reaction conditions,
several aldehydes and ketones were reduced to demonstrate the
generality of the catalytic hydrosilylation method under solvent-
free conditions (Table 2).
Very recently Chirik and co-workers have reported on the
catalytic hydrosilylation [40] and hydroboration [41] of carbon–
oxygen and carbon–carbon multiple bonds employing PNP pincer
cobalt complexes. However, to our knowledge no such studies
have been conducted using the analogous PONOP systems. Thus,
we decided to study the ability of complexes 1 and 2 to function
as pre-catalysts in the hydrosilylation of aldehydes and ketones.
In an effort to compare the relative effects of PONOP- versus
PNP-ligation to the metal center on catalytic performance we pre-
pared the known (PNP)MCl2 (M = Fe and Co, 3 and 4 respectively)
complexes using literature procedures [20,34].
2.3. Hydrosilylation of ketones and aldehydes
A broad range of ketone substitution patterns is tolerated under
hydrosilylation conditions: long-chain aliphatics (2-dodecanone),
cyclic aliphatics (cyclohexanone), and aromatic substituents (ace-
tophenone and dibenzoketone). Typically yields are satisfactory
and range from ꢀ40% to 80%, generally the sterically encumbered
aliphatic ketones afforded the lowest yield. Similarly, aromatic
aldehydes were cleanly reduced to siloxyalcohols by all catalyst
variations (ꢀ50–96%). Heterocyclic ketones and aldehydes
(2-acetylthiophene and 2-thiophenecarboxaldehyde, respectively)
are also reduced, albeit in moderate yield. Some substrates,
SiR3
O
O
ð1Þ
1
(1 mol%)
HSiR3
H
NaBEt3H (2 mol%)
Table 2
Hydrosilylation of aldehydes and ketones.a
2.2. Optimization of reaction conditions
Si(OEt)3
R'
[Cat]
NaBEt3H (2 mol%)
O
O
OH
R'
To probe for optimal reaction conditions we chose acetophe-
none as a model substrate. tBuPNP complex 1 was synthesized
using literature protocols and used to rapidly screen for optimized
hydrosilylation conditions. At first, no reaction was observed, how-
ever in situ activation of the dihalide with 2 equivalents of NaBEt3H
(1 M THF solution) afforded a catalytically competent system
giving the reduced product as shown in Eq. (1). The reaction is
conveniently monitored by gas chromatography coupled with
mass spectrometry (GC–MS). Both triethylsilane and triethoxysi-
lane proved to be viable reducing agents for acetophenone (13%
and 60%, respectively, conversion to silylated product). Given the
superior performance and lower cost of triethoxysilane we chose
to pursue further studies with it in preference to triethylsilane.
As shown in Table 1, we screened a variety of organic solvents
but found that the highest conversions to silylated product could
be obtained by running the reaction in neat triethoxysilane.
Initially, our catalytic experiments were conducted with iso-
lated pre-catalysts 1–4. However, identical conversions are
obtained using the operationally more convenient approach of
generating the active catalyst in situ. Thus, reduced products can
be accessed through simply mixing FeCl2 and ligand in a 1:1 ratio
under the optimized reaction conditions. Importantly, no reaction
products are observed if any component is withheld, i.e. the
absence of ligand, metal dihalide or NaBEt3H results in isolation
of unreacted starting materials. Based upon these results we
screened the remaining pre-catalysts (2–4) for activity in the
hydrosilylation of acetophenone employing our optimized reaction
2M NaOH
(EtO)3SiH
(2)
R
R'
R
R
rt, MeOH, stir 1d
neat, rt, stir 24 h
O
O
7
O
O
O
69d,25d,30,21
60,59,45,78
89,80,77,60b
52d,32d,43,62
O
39,41,24,22
(22,45,74,42)c
O
O
O
F3C
82d,80d,42,38
82,92,34,51
71,43,36,41
89,63,37,37
(57,47,87,91)
O
H
O
O
H
H
MeO
70,96,65,52
F3C
78,76,77,52
50,20,56,39
(53,54,56,26)
H
S
S
O
O
61,66,59,33
51,50,54,41
(28,30,34,21)
[Cat] = Fe-tBuPNP, Co-tBuPNP, Fe-tBuPONOP, Co-tBuPONOP; bConversion
a
numbers calculated using GC–MS analysis; cIsolated yields in the parentheses of the
corresponding alcohols; dAfter heating for 12 h.
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4
A.D. Smith et al. / Polyhedron xxx (2016) xxx–xxx
O
O
N
O
O
N
(tBu)2P
P(tBu)2
H
H
H
M
M
(EtO)3Si
H
Si(OEt)3
OSi(OEt)3
(EtO)3Si
H
(tBu)2P
O
P(tBu)2
O
H
SILANE
SILANE
H
(EtO)3Si
O
O
N
O
O
N
O
P(tBu)2
M
P(tBu)2
OSi(OEt)3
H
- H2
H
M
H
P(tBu)2
P(tBu)2
H
(EtO)3Si
CARBONYL
CARBONYL
OSi(OEt)3
O
O
N
O
N
O
P(tBu)2
H
(tBu)2P
H
M
O
O
M
H
P(tBu)2
H
(tBu)2P
O
Si(OEt)3
Scheme 3. Proposed hydrosilylation mechanism.
denoted below, were heated to 100 °C for an additional 18–24 h to
further maximize yields. Furthermore, to demonstrate the poten-
tial of our reaction protocol, we chose to hydrolyze a representa-
tive sample of our substrates to isolate the desired alcohol
products. Thus, hydrolysis of the siloxy-intermediates using 2 M
NaOH (2 mL) in MeOH (2 mL) followed by evaporation of solvent
afforded the corresponding alcohols. Purification via flash column
chromatography was only required when starting material was
remaining in the reaction mixture (isolated yields shown as paren-
theses in Table 2).
2.5. Proposed mechanism
The crucial role played by NaBEt3H in activating the metal diha-
lide precatalysts (1–4) led us to propose the mechanism shown
below (Scheme 3). Crucially, similar observations were reported
by Huang’s group in their related investigations of Fe- and Co-cat-
alyzed alkene hydroboration [42]. Thus, salt metathesis of the
halogens with the hydride reagent affords a metal(II) dihydride
which may either retain (Scheme 3, red) or reductively eliminate
(Scheme 3, blue) dihydrogen to afford a catalyst based upon a M
(II) or M(0) complex, respectively. Either metal is capable of pre-
coordinating (activating) either the silane or carbonyl prior to
hydrosilylation [37]. At this stage, the mechanism beyond
activation of the dichloride remains a mystery to us and further
examination of the mechanism is ongoing in our laboratory. In
either mechanistic scenario the binding of a carbonyl substrate in
2.4. Competition experiment
To delineate the effects of electron withdrawing/donating sub-
stituents on the reactivity of aldehydes, competition reactions
were performed using complex 1 and appropriately substituted
benzaldehydes as shown in Eq. (3). In the competition experiment
an equimolar mixture of all three substrates involved in hydrosily-
lation were mixed with a sub-stoichiometric amount (2.0 equiv.) of
(EtO)3SiH. The progress of the reaction was monitored by GC–MS
analysis, which revealed the silyl-ether products to be formed in
a 1:2.5:4.5 ratio for the –OMe:–H:–CF3 substituted aldehydes.
Thus, the relative rate of aldehyde hydrosilylation increases
with electron-withdrawing groups in the para position, where
the electron-deficient substrate is reduced most rapidly.
either an
g
-1 or
g
-2 manner is possible, although based upon
-1 binding mode.
recent theoretical calculations,[43] we favor the
g
Complicating this picture is the report of Kundu and Jones [44] in
which they disclosed the reaction of [(PONOP)MCl][Cl] (M = Ni,
Pd, Pt) with LiBEt3H affords a dearomatized ligand via hydride
attack at the para-position of the pincer, not the anticipated
metal-hydride complex. Notably, these systems proved to remain
active in the hydrosilylation of aldehydes and ketones. As of yet,
we have no evidence of Jones-type ligand non-innocence in our
complexes.
OSi(OEt)3
H
O
1
1
1
1
H
O
MeO
3. Conclusion
1 (1 mol %)
OSi(OEt)3
(EtO)3SiH (2 eq)
2.5
4.5
H
H
H
ð3Þ
In summary, we have developed an efficient catalyst system
based on cheap, earth abundant and environment friendly transi-
tion metals (Fe and Co) for hydrosilylation of carbonyl compounds
which is more appealing as compared to the currently used expen-
sive methods. The active catalyst generated in situ conveniently
reduces aldehydes and ketones. The details of the hydrosilylation
mechanism are at present unknown and this remains an area of
ongoing research in our group.
NaBEt3H (2 mol %)
MeO
F3C
OSi(OEt)3
H
O
R = CF3 > H > OMe
F3C
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A.D. Smith et al. / Polyhedron xxx (2016) xxx–xxx
5
4.5. General method for hydrosilylation
4. Experimental
To a known amount of mesitylene (GC standard) in a vial
tBuPNP/tBuPONOP (1 mol%), FeCl2/CoCl2 (1 mol%), substrate
(1.27 mmol), (EtO)3SiH (1 eq) and NaBEt3H (2 mol%) as a 1 M solu-
tion in THF were added. After stirring this reaction mixture for
24 h, GC–MS was taken and the mixture was hydrolytically
quenched with 2 M NaOH (2 mL) and MeOH (2 mL). After stirring
for 1 d, the mixture was neutralized with 2 M HCl. The product
mixture was extracted with diethyl ether (3 ꢂ 5 mL) and the
organic layers were combined, washed with brine (2 ꢂ 5 mL), dried
over MgSO4 and concentrated under reduced pressure to afford the
corresponding alcohol as a crude product. Flash column chro-
matography with 5–20% (ethyl acetate: hexane) yielded analyti-
cally pure alcohol. 1H NMR spectra of the isolated alcohols are
provided as Supplementary information.
4.1. General synthetic procedures
All reagents were purchased from commercial vendors, and
were used without further purification unless otherwise noted.
All syntheses were performed under inert atmosphere using
standard glove-box or Schlenk techniques. Pincer ligand PNP is
commercially available, PONOP was synthesized following the lit-
erature procedure [14a]. Complex 1, PONOP-FeCl2, was prepared
according to the procedure reported by Schrodi and co-workers
[26]. Complexes 3 and 4 were synthesized according to procedures
published by Goldman [20] and Chirik [19,23]. Aldehyde and
ketone substrates were obtained commercially, dried and degassed
using the freeze–pump–thaw method before being stored in an
argon-filled glovebox. Proton and carbon nuclear magnetic reso-
nance spectra (1H NMR and 13C NMR) were recorded on a Jeol
400 MHz spectrometer with Me4Si or solvent resonance as the
internal standard (1H NMR, Me4Si at 0 ppm, CHCl3 at 7.24 ppm;
13C NMR, Me4Si at 0 ppm, CDCl3 at 77.0 ppm). 1H NMR data are
reported as follows: chemical shift, multiplicity (s = singlet,
d = doublet, t = triplet, q = quartet, quint = quintet, sext = sextet,
sept = septet, br = broad, m = multiplet), coupling constants (Hz),
and integration. Gas chromatography–mass spectrometry
(GC–MS) was performed on an electron ionization time-of-flight
(EI-TOF) mass spectrometer.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgments
The authors acknowledge the Robert A. Welch Foundation
(under Award No. D-1807), and Texas Tech University (Undergrad-
uate Research Fellowship for A.D.S.) for financial support. The
authors thank the National Science Foundation for funding the pur-
chase of NMR instrumentation (Grant No. CHE-1048553) used in
this project.
4.2. X-ray crystallography
Data for PONOP-FeCl2 and PONOP-CoCl2 complexes were
obtained on a Bruker Smart Apex II CCD diffractometer. All data
were collected at room temperature using graphite-monochro-
Appendix A. Supplementary data
mated Mo
collected using
K
a
radiation (k = 0.71073 Å). Intensity data were
-steps accumulating area detector images span-
CCDC 1431434 and 1431435 contains the supplementary crystal-
lographic data for 1 and 2. These data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: deposit@ccdc.cam.
ac.uk. Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.poly.2015.12.037.
x
ning at least a hemisphere of reciprocal space. All the data were
corrected for Lorentz polarization effects. A multi-scan absorption
correction was applied using SADABS [45] or CrystalClear [46].
Structures were solved by direct methods and refined by full-
matrix least-squares against F2
(SHELXTL [47]). All hydrogen atoms
were assigned riding isotropic displacement parameters and
constrained to idealized geometries.
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