Sterically encumbered β-diketonates and base metal catalysis

Article abstract of DOI:10.1039/c9dt02293g

Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone (esac), are reported for Co, Ni, Cu, and Zn. All four form ML₂-type complexes with typical coordination behavior for late-metal β-diketonates, however the effects on established electrochemistry and reactivity vary somewhat per metal. For example, the striking chemical and electrochemical inertness of Co^II(esac)₂ to oxidation and disproportionation is atypical. Conversely, the behavior of Cu^II(esac)₂ is rather typical relative to other Cu^II(β-diketonate)₂ complexes. These data suggest a relative disfavoring of certain reaction pathways, and represent an important step in modulating the catalysis of the base metals via sterically hindered β-diketonates.

Full text of DOI:10.1039/c9dt02293g

Dalton
Transactions
View Article Online
View Journal
PAPER
Sterically encumbered β-diketonates and base
metal catalysis†
Cite this: DOI: 10.1039/c9dt02293g
Sebastian M. Krajewski,a Aaron S. Crossman, a Eser S. Akturk,a Tim Suhrbier,b
a,c
Steven J. Scappaticci,a Maxwell W. Staaba and Michael P. Marshak
*
Metal coordination complexes of the sterically hindered β-diketonate, 2,6-dimesitylbenzoyl pinacolone
(esac), are reported for Co, Ni, Cu, and Zn. All four form ML2-type complexes with typical coordination
behavior for late-metal β-diketonates, however the effects on established electrochemistry and reactivity
vary somewhat per metal. For example, the striking chemical and electrochemical inertness of CoII(esac)2
to oxidation and disproportionation is atypical. Conversely, the behavior of CuII(esac)2 is rather typical
relative to other CuII(β-diketonate)2 complexes. These data suggest a relative disfavoring of certain reac-
tion pathways, and represent an important step in modulating the catalysis of the base metals via sterically
hindered β-diketonates.
Received 30th May 2019,
Accepted 20th June 2019
DOI: 10.1039/c9dt02293g
rsc.li/dalton
described as pseudo-aromatic, speaking to the efficacy of its
Introduction
interaction with ligated metals.18–20 Reactions continue to be
Precious metal catalysts are indispensable in modern organic discovered using base metal β-diketonates, and it is often
synthesis, but suffer from inherent shortcomings such as scar- noted that even modestly sterically hindered ligands offer
city and cytotoxicity which are less prevalent in the base improvements over the parent acetylacetone (Fig. 2).12–14,21–23
metals (e.g., Fe, Co, Ni, Cu, etc.).1–3 The development of steri- The hindered ligands identified often feature tert-butyl or iso-
cally hindered ligands, such as phosphines and N-heterocyclic propyl substituents, which remain the most common “hin-
carbenes (NHCs), was a marked improvement for the precious dered” ligands due to synthetic limitations of the Claisen
metal systems, imparting robust and often enantioselective condensation.24,25 Consequently, mechanisms involving base
reactivity to the active catalyst.4–7 A common motif has become metal β-diketonates remain poorly understood, inhibiting
apparent for affording steric protection to reactive metal further improvement (e.g., reducing the high catalyst loadings
centers and isolating low-coordinate, highly-reactive species —10 mol% vs. ppm/ppb levels for precious metals).26–28
(Fig. 1). However, the ligand frameworks which have been opti- Notably, an iron(III) acetylacetonate pre-catalyst was employed
mized for second- and third-row metals are often inadequate in 30–100 mol% to effect an intramolecular reaction.29
to the task of taming their first row congeners.6,8
The addition of steric bulk to ancillary ligands is a common
Conversely, β-diketones are a class of ligands capable of approach to improving catalyst activity and selectivity, which
supporting a number of unique and important catalytic reac- has yet to be applied to β-diketones (Fig. 1). Previous efforts
tions using the base metals (Fig. 2).9–16 The β-diketonates
provide an exceptionally weak field, ionic interaction with the
metal ion, offering a very different electronic environment in
comparison with strong-field ligand platforms.17 While at the
same time, the ligand framework is π-acidic and has been
aDepartment of Chemistry, University of Colorado Boulder, 80309, USA.
E-mail: michael.marshak@colorado.edu
bInstitut für Chemie, Universität Rostock, Albert-Einstein-Str. 3a, 18059 Rostock,
Germany
cRenewable & Sustainable Energy Institute, Boulder, Colorado, 80309, USA
†Electronic supplementary information (ESI) available: Full experimental
details, characterisation, NMR and selected crystallographic X-ray data. CCDC
1868585–1868591. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c9dt02293g
Fig. 1 The hindered β-diketone ligand, esac (far right) next to other
popular ligand frameworks, including NacNacs, NHCs, and m-terphe-
nyls, illustrating the comparable steric profiles of each ligand.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
lized in high purity. Analysis of these materials by X-Ray crys-
tallography shows planar structures for Ni and Cu, and
pseudo-tetrahedral structures for Co and Zn (Fig. 3). These iso-
morphous pairs are essentially identical to the known
MII(DPM)2 complexes and highlight the ligand’s deference for
the metal’s preferred coordination geometry. Two m-terphenyl
moieties are easily accommodated about the metal center, as
was observed for the early members of the first row using the
same esac ligand.35
The cobalt complex 2 was crystallized from tetrahydrofuran
(THF) yielding two different pseudo-octahedral structures of
Co(esac)2(THF)2. Both isomers feature planar bis-β-diketonate
coordination with THF molecules occupying axial positions;
however, one features the bulky aryl groups in a cis configur-
ation while they adopt a trans configuration in the other (see
ESI†). Together with 2, these three structural observations
suggest the two esac ligands can rotate about the cobalt
center, with σ-donor ligands like THF trapping the pseudo-
octahedral geometry. Related solvent-dependent fluxionality
has been observed in other cobalt(II) bis-β-diketonate com-
pounds.36 The nickel(II) complex 3 was also found to adopt a
trans configuration in absence of coordinating solvent and a
cis configuration as the THF adduct, which suggests a similar
freedom of rotation in the solution phase.
These crystal-phase observations offer further evidence that
the steric influence of the esac ligand can accommodate
additional solvent/substrate binding and does not significantly
influence the coordination environment of cobalt beyond its
ability to prevent aggregation of Co(esac)2 or formation of
Co(esac)3. However, catalytic reactions occur in the solution-
phase, and so other methods are needed to probe this
domain.
Fig. 2 Many important, base-metal catalyzed reactions are supported
by the parent acetylacetone (acac), but improvements and novel reactiv-
ity are being discovered with moderately hindered ligands such as dipi-
valoylmethane (DPM).
using dipivaloylmethane (DPM) and similarly-sized ligands
were effective in preventing the oligomerization observed in
the crystal and solution phases, but left the coordination
chemistry essentially unchanged.30–33 DPM obviates the first
step in the pre-catalyst activation sequence and offers slightly
more steric shielding of the reactive species, while leaving the
manifold reactivity problem unsolved. Truly hindered
β-diketones have only recently been made synthetically accessi-
ble,34 and their coordination and catalytic support is of high
interest.
Previously, we reported that the use of a bulky m-terphenyl-
substituted β-diketonate, esac (Fig. 1), sterically blocks the for-
mation of more stable M(acac)3 complexes and enables iso-
lation of monomeric MIII(β-diketonate)2Cl (M = Ti, V, Cr) com-
plexes for the first time.35 Here we report the crystallography,
electrochemistry, and reactivity of related late-metal
β-diketonate complexes, MII(esac)2 (M = Co, Ni, Cu, Zn), and
the catalytic behavior of the cobalt and copper examples in
Mukaiyama (Co) and Ullmann (Cu) reactions. Additionally,
their resistance to both oxidation and reduction, the conse-
quences for outer-sphere redox events and disproportionation
mechanisms, and the impact on mechanistic pathways is also
discussed.
Spectroscopy and electrochemistry
UV-Vis-NIR spectroscopy was used to confirm the pseudo-tetra-
hedral environment about cobalt(II), and thus the applicable
Tanabe–Sugano analysis of the absorption spectrum results in
a ligand field splitting parameter Δt of 3930 cm−1 and a Racah
parameter B of 873 cm−1 (Fig. 4). These results are consistent
with Cotton’s analysis of Co(dpm)2 where a B value of
880 cm−1 was found.33 This B value is rather high in compari-
son with other Co(II) complexes and is close to the free ion
value (967 cm−1), suggesting little mixing between the metal
and ligand orbitals. Furthermore, the weak orbital mixing
rationalizes the weak intensities of the d–d transitions com-
pared with other tetrahedral cobalt(II) complexes.33 The high-
spin nature of 2 contrasts with a low-spin and planar bis-
NacNac cobalt complex, highlighting the considerable differ-
ence in ligand field strength between related NacNac and acac
Results and discussion
Synthesis and crystallography
The ligand H(esac) can be deprotonated with potassium tert- ligand motifs.37 Magnetic moments, as determined by Evans’
butoxide to yield K(esac) (1) in 94% yield, and subsequent method, indicate 2–4 display typical, high-spin behavior under
metalation with MCl2 (M = Co, Ni, Cu, Zn) gives crystalline most circumstances, characteristic of known β-diketonate com-
M(esac)2 (M = Co (2), Ni (3), Cu (4), Zn (5)) in good yields plexes. A notable exception is complex 3 in the absence of
(78%, 40%, 71%, and 78% respectively). The compounds are coordinating solvents which is diamagnetic, as was similarly
conveniently filtered from the alkali halide salts and recrystal- observed for Ni(DPM)2.31
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
Fig. 4 Tanabe–Sugano analysis of the absorption spectrum of
Co(esac)2 and assignment of electronic transitions.
The electrochemical behavior of 2 was investigated by cyclic
voltammetry (Fig. 5A), which lacked a well-defined oxidation
event. This stands in contrast with the parent complex
[CoII(acac)2], which shows such an oxidation at ca. 0.5 V vs.
Fc/Fc+ (see ESI†). Neither complex shows a reduction event in
dichloromethane (DCM) out to −2 V vs. Fc/Fc+. The absorption
spectrum of 2 was unchanged upon exposure to pure oxygen
gas in toluene at both ambient temperature and −20 °C. Cyclic
voltammetry of 3 revealed this complex to be redox-inert
between −2 and +1 V vs. Fc/Fc+ in DCM. On the other hand,
the copper complex 4 was reduced irreversibly at −2 V vs. Fc/
Fc+ (Fig. 5B)—low even for copper(II) β-diketonates.38 An irre-
versible oxidation event was thereafter detected at about 0 V.
This behavior is rationalized as a CuII/I reduction followed by
disproportionation of the CuI species to Cu0 and CuII.39 The
irreversible oxidation at about 0 V is consistent with the oxi-
dative stripping of metallic copper from the electrode.
That these one-electron electrochemical events occur at
such relatively negative potentials, or not at all, is surprising.
Further, the electrochemical consequences of the same hin-
dered ligand appear to differ among members of the late, first-
row series. Thus while cobalt is effectively precluded from oxi-
dation by sterics, copper will be much more difficult to afflict
with steric hindrance, as precedent suggests.40 Given that
electrochemical techniques are often used to probe a com-
plex’s facility for redox events as they might occur during a
reaction, this outer-sphere electronic stability was further
probed by direct chemical means.
Fig. 3 X-Ray structures of (A) Co (2), (B) Ni (3), (C) Cu (4), and (D) Zn (5).
The mesityl substituents have been omitted from 3, 4, and 5 for clarity,
as have all hydrogen atoms. Ellipsoids have been drawn at the 50%
confidence level.
Reactivity and catalysis
Incredibly, 2 could not be chemically oxidized by the strong
oxidant [(4-BrC6H5)3N]SbCl6, even in the presence of excess
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
porting ligands.44,45 Nevertheless, complexes 2 and 4 were
employed as pre-catalysts in well-known conditions to deter-
mine catalytic viability.
The Mukaiyama reduction–oxidation system is a versatile
method for the preparation of a variety of products from
common olefin starting materials, the most notable example
of which is the hydration reaction. 4-Phenylbutene and styrene
are two of the original model systems employed by Mukaiyama
during reaction development,9 and thus represent ideal
models for comparative purposes (Fig. 6A and B). As expected,
conducting the reaction in isopropanol under an atmosphere
of dioxygen resulted in the consumption of starting materials,
with the silane, solvent, and complexes thereof acting as
sources of dihydrogen.46,47 That only trace secondary alcohol
or ketone was observed is consistent with the empirical obser-
vation that Co(β-diketonate)2 complexes are not active
hydration catalysts if the CoII/III oxidation potential occurred
above +0.5 V vs. Ag/AgCl.48 However, volatile reduction pro-
ducts were observed to have formed, which Co(acac)n (n = 2, 3)
is known to accomplish under similar, anaerobic reaction
conditions.49–51 The importance of metal disproportionation
and/or ligand dissociation is unclear in this case, and it is
possible that the rates of dioxygen uptake versus reductive sol-
volysis are also sensitive to steric accessibility, coordinative
environment, oxidation states, and donating ability of the
solvent.
Conversely, an Ullmann-type coupling reaction was con-
ducted smoothly under known reaction conditions. This reac-
tion has received much renewed interest for its ability to form
a variety of carbon bonds, such as C–X (X = C, N, O, S)
bonds which are currently prepared using precious metal
catalysts.12,16,52 The optimization of such a general carbon
bond-forming reaction using an earth abundant metal would
represent a considerable step forward for homogeneous cataly-
Fig. 5 Electrochemical behavior of (A) 2 in DCM and (B) 4 in THF.
Background scans are shown in grey. All voltammograms are referenced
versus Fc/Fc+.
K(esac). The stability of 2 towards oxidation is striking, given sis, and is a current research focus of the academic and indus-
that CoII(DPM)2 readily oxidizes to form diamagnetic try communities.1,8,53 Using
CoIII(DPM)3 in air.41 Although, similar steric influences have
been noted for hindered phosphine ligands.42 Despite that
CoIII(esac)3 resisted all efforts at formation, it is still possible
that the redox events are very sensitive to the presence of oxi-
dizing or reducing adducts which might form during a reac-
tion. This lack of stabilization afforded the cobalt(II) center by
the acac-type ligand, as established by UV-Vis spectroscopy,
also makes the involvement of electron-rich adducts very
probable.
4 as pre-catalyst, 2-phenoxy
The stabilization of a copper(I) β-diketonate is of practical
interest, given its well-documented instability which implies
its possibility as a catalytically relevant species.39,43 The syn-
thesis of CuI(esac) was attempted by stoichiometric reaction of
the protonated ligand, H(esac), with mesityl copper(I);
however, the reaction yielded 4 along with colloidal red-brown
metallic copper. The desired copper(I) compound, although
likely formed in situ, remains unstable in solution with respect
to disproportionation to Cu0 and CuII(esac)2. This longstand-
Fig. 6 Catalytic activity of MII(esac)2 complexes, 2 and 4, using reported
ing challenge of unsupported copper(I) β-diketonates has pre-
vented their isolation, except in the cases of strong-field sup- conditions.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
naphthalene, a diaryl ether, was obtained in 35% isolated ping parameters controlling the operability of each manifold,
yield, with residual starting materials recovered during the such as solvent choice or redox facility afforded by the specific
purification (Fig. 6C). The rather low yield is consistent with β-diketonate. The situation is somewhat simplified using the
either the system’s increased steric bulk slowing down the sterically hindered esac ligand. It is noteworthy that not only
turnover rate or activation of the pre-catalyst to give an did the cobalt(II) complex 2 resist chemical and electro-
unstable CuI species.39,43
chemical oxidation, but there was no evidence of the for-
While these pre-catalysts seem inferior at face value, the mation of K[CoII(esac)3] either. This suggests that the steric
counter-intuitive benefits of steric hindrance have long since protection afforded by the esac platform effectively suppresses
been remarked.21,54 On the contrary, the shuttling of active the formation of tris-chelates among the first-row transition
species along well-defined reaction pathways represents a metals generally, which results in commensurate modification
major step forward for these base metal systems.
of well-established reaction pathways.
The electrochemical behavior of 2 and 3 suggests that
outer-sphere single electron transfer (SET) processes are inac-
cessible for these bis-β-diketonate complexes, while 4 exhibits
a more negative potential barrier than other Cu(β-diketonate)2
compounds for an electrochemical reduction.38 Nonetheless,
these compounds are capable of effecting organic transform-
ations, as examined above. These results suggest that while
electrochemical methods are adequate for probing outer-
sphere electronic processes, it is a poor indicator for inner-
sphere, tandem coordination-redox events. Furthermore, the
steric shielding afforded by the esac ligand effectively inhibits
outer-sphere redox events, and thus de-activates one of the
many observed reaction pathways for these compounds.
Thus, restricted to inner-sphere reaction paths, this rep-
resents a fundamental improvement in the catalytic ability of
the β-diketonates. Further, the geometry of the complex is sen-
sitive to the availability of donating ligands (e.g., solvent, sub-
strate, nucleophile, etc.). It is reasonable to assume that in
both reactions studied this large β-diketonate ligand functions
as a true ancillary ligand, as other large β-diketonates have
also been described.12,14,21 The size of the ligand also prevents
disproportionation to tris-complexes. Specifically, two
CoII(esac)2 could react to form CoI(esac) and CoIII(esac)3 but do
not, unlike their DPM counterpart.41 This may be important in
pre-catalyst activation in Mukaiyama hydration reactions,
among various reaction pathways. On the other hand, esac is
not sterically encumbering enough to prevent the disproportio-
nation of CuI(esac) to CuII(esac)2 and Cu0. Further research
towards understanding the steric influence of the β-diketones
to control diverse reactivities of the pre-catalysts, improve the
stability and efficiency of the active species, and gain insight
into catalytic mechanism is warranted.
Mechanistic implications
Unlike the well-behaved precious metal catalysts, the base
metals have been reported to operate via several possible
mechanisms, two of which are described in Fig. 7.27,43 The
situation is made incredibly complex by the multiple, overlap-
Conclusions
The approach described has enabled well-defined, albeit pre-
liminary, investigations into the accessibility and stability of
acac-supported reactive species. The steric encumbrance of the
esac ligand leaves the complexes’ dynamic solvent interactions
and preferred coordination geometries intact. Conversely,
MII(esac)2 or MIII(esac)2Cl compounds have been previously
reported for the majority of the first row. Taken with these
results, a reluctance to form the tris-esac complexes among
the first-row transition series is confirmed—a novel develop-
Fig. 7 Comparison of the one- and two-electron mechanisms often
invoked to explain the catalytic cycles, where R represents a generalized
nucleophile (R = C, N, O, S; M’ = H, Li, Mg, Na, Zn, Sn, B(OR)3, etc.).
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
ment for acac-type ligands. This interference in preferred reac- Cu(esac)2 experiments were conducted in tetrahydrofuran for
tivity paradigms of ligated metals is unprecedented using the solubility reasons. All electrochemical data was corrected to
β-diketonate framework. Off-cycle catalyst deactivation path- the external reference of the Fc/Fc+ couple. X-ray crystal struc-
ways are thus particularly disfavoured in the cobalt(II) tures were collected using a Bruker D8 Quest Eco three circle
complex, while the copper(II) analogue is not as immune.
goniometer platform equipped with a Bruker APEX-II CCD
Interestingly, the esac ligand system revealed a disfavoring detector. A graphite monochromator was employed for wave-
of SET redox behavior in M(esac)2 complexes (M = Co, Ni, Cu) length selection of the Mo Kα radiation (λ = 0.71073 Å). The
per cyclic voltammetry, suggesting the lack of ligand electronic data were processed using APEX III software provided by
donation and crowding out of other Lewis bases steepens the Bruker. Except as noted below, structures were solved by intrin-
barrier to forming a charged species. Furthermore, the ligands sic phasing in SHELXT57 and refined by standard difference
enforce a high-spin electronic configuration of the metal Fourier techniques with SHELXL58 within the OLEX259 soft-
center, which may be crucial for their unique catalytic behav- ware package. Hydrogen atoms were placed in calculated posi-
ior relative to strong-field ligands. These compounds were tions using the standard riding model and refined isotropi-
shown to be competent pre-catalysts in typical β-diketonate cally except as noted below.
reactions, albeit relatively sluggish. These preliminary results
suggest definite perturbations of the well-studied β-diketonate
Synthesis and characterization of complexes
ligand framework, although much remains to be achieved.
K(esac) (1). To a 250 mL round bottom flask, potassium tert-
Therefore, ligands which prevent metals from forming bis-acac butoxide (2.77 g, 24.7 mmol), esacH (10.9 g, 24.7 mmol), and
type complexes [e.g., CuI(acac), CoII(acac)X] are important syn- 100 mL toluene were added. 25 drops of dimethylformamide
thetic targets for further development.34,55
were introduced to the solution, which was fitted with a
rubber septum and subsequently heated to 80 °C for 48 hours.
The solution was concentrated to an orange oil under vacuum
and 50 mL of hexanes were added. The flask was cooled in a
freezer at −35 °C for 48 hours, during which a white solid pre-
cipitated out of the solution. The solution was then filtered,
Experimental
General experimental methods
Syntheses were performed in a nitrogen-filled glovebox or in a and the precipitate was washed with hexanes and dried under
fume hood using standard air-free Schlenk techniques. Metal vacuum yielding a white solid (11.1 g, 23.2 mmol, 94%). 1H
halides were purchased from Strem Chemical and used as NMR (400 MHz, Benzene-d6) δ 7.22–7.14 (m, 1H), 6.94 (d, J =
received. Potassium tert-butoxide was purchased from Alfa 7.6 Hz, 1H), 6.82 (s, 2H), 5.16 (s, 0H), 2.25 (s, 6H), 2.14 (s, 3H),
Aesar and used as received. CoII(acac)2 was prepared according 1.03 (s, 4H). 13C{1H} NMR (101 MHz, C6D6) δ 197.58 (C–O),
to a literature procedure.56 Deuterated solvents were purchased 187.42 (C–O), 144.51, 139.80, 137.91, 136.68, 135.91, 94.08
from Cambridge Isotope Labs and stored over 4 Å molecular (OCCHCO), 40.65, 28.40 (C(CH3)3), 21.13 (p-CH3), 20.75
sieves. All other solvents were purchased in anhydrous form (o-CH3). IR (neat solid): 2951 (m), 2918 (m), 1583 (s), 1431 (vs),
from Sigma Aldrich and further dried with and stored over 4 Å 1375 (s), 846 (s), 764 (s). UV-Vis (THF): λmax (ε) 311 nm (16 500
molecular sieves. esacH was synthesized according to literature M−1 cm−1). HRMS (ESI) (m/z): [M − H]+ Calcd for C31H35O2K,
procedures.35 NMR spectra were recorded on a Bruker Avance- 479.2352; Found, 479.2349.
III 300 NMR spectrometer (300 MHz) or Bruker Avance-III 400
Co(esac)2 (2). To a 300 mL thick walled glass bomb reactor,
NMR spectrometer (400 MHz), EPR spectra were recorded on a anhydrous cobalt(II) chloride (0.240 g, 1.85 mmol), K(esac)
Bruker Elexsys E 500 EPR spectrometer (X-band, 9.869 GHz). (1.720 g, 3.60 mmol), and 60 mL toluene were added. The
IR spectra were recorded on an Agilent Cary 630 FTIR with mixture was stirred at 130 °C for 24 hours, during which the
ATR attachment, UV–vis were recorded on an Agilent Cary solution changed color from light blue to pink. The solution
5000 UV-Vis-NIR spectrophotometer, High-Resolution Mass was filtered, and the solvent removed under vacuum, resulting
Spectrometry (HRMS) data were recorded at the University of in a pink-purple crystalline solid (1.321 g, 1.41 mmol, 78%).
Colorado Central Analytical Laboratory on a Waters Synapt G2 IR (neat solid, cm−1): 2959 (m), 2918 (m), 1540 (s), 1506 (s),
(ESI) by Dr Danijel Djucovic. Magnetic data were collected in 1361 (vs), 854 (s), 772 (s). UV-Vis (CH2Cl2): λmax (ε) 599 nm
benzene or THF solutions at room temperature using Evans’ (36 M−1 cm−1). µeff = 4.43(3) (Evans’ Method, C6D6, 25 °C).
method.2 The reported value is the mean of three separately Anal. Calcd for C62H70O4Co: C, 79.38; H, 7.52. Found: C, 79.13;
prepared and measured solutions with the standard deviation H, 7.62.
reported in parenthesis. Magnetic susceptibility data were cor-
Co(esac)2(THF)2. To a 100 mL Schlenk flask, anhydrous
rected for diamagnetic contributions for the core diamagnet- cobalt(II) chloride (0.108 g, 0.83 mmol), K(esac) (0.750 g,
ism of each sample (estimated using Pascal’s constants). 1.56 mmol), and anhydrous tetrahydrofuran (40 mL) were
Elemental Analyses were performed by Midwest Microlab LLC. added. The mixture was stirred at 60 °C for 24 hours, during
Electrochemical data were gathered using a Gamry Interface which the reaction mixture changed color from a light blue to
1000B, with the analyte in 5 mM concentration and 0.1 M dark purple. The solution was filtered, and the solvent
TBAPF6 as the electrolyte. Electrochemical experiments of removed under vacuum. The resulting purple residue was
Co(esac)2 and Ni(esac)2 were conducted in CH2Cl2, while extracted with diethyl ether, and upon removal of solvent a
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
purple solid was obtained (0.607 g, 0.56 mmol, 72%). IR (neat then washed with tetrahydrofuran and filtered, leaving behind
solid): 2970 (m), 2918 (m), 1569 (s), 1514 (s), 1417 (vs), 1380 elemental copper as a fine brown powder. The resulting blue
(vs), 851 (s), 780 (s). UV-Vis (THF): λmax (ε) 609 nm (24 M−1 filtrate was concentrated under vacuum to form light blue
cm−1). µeff = 4.48(2) (Evans’ Method, THF/C6D6, 25 °C).
flakes of Cu(esac)2 (0.210 g, 0.228 mmol, 33%).
Ni(esac)2 (3). Nickel(II) chloride dimethoxyethane adduct
Reaction of Co(esac)2 with K(esac) and [(4-BrC6H4)3N]SbCl6.
(0.228 g, 1.038 mmol) and K(esac) (1.000 g, 2.084 mmol) in To a reddish-purple solution of Co(esac)2 in CH2Cl2 (0.150 g,
anhydrous tetrahydrofuran (40 mL) were heated and stirred at 0.160 mmol), K(esac) (0.077 g, 0.161 mmol) and tris(4-bromo-
60 °C for 24 hours, during which the solution turned a light phenyl)ammoniumyl
hexachloroantimonate
(0.137
g,
green color. The solution was filtered, then solvent was 0.168 mmol) were added. The reaction mixture was stirred at
removed under vacuum to yield a light green solid. Upon the room temperature for 2 hours, then the solvent was removed
addition of anhydrous benzene, a dark purple suspension was under vacuum. The resulting dark blue solids were triturated
formed. The benzene was removed under vacuum to yield a with hexanes and filtered, resulting in the recovery of Co(esac)2
purple diamagnetic solid (0.388 g, 0.413 mmol, 40%). 1H NMR as a pink solid.
(400 MHz, CDCl3) δ 7.33 (t, J = 7.6 Hz, 1H), 7.04 (s, 3H), 6.98
Reaction of Co(esac)2 and O2. A pink solution of Co(esac)2 in
(d, J = 7.6 Hz, 2H), 4.67 (s, 1H), 2.47 (s, 6H), 1.98 (s, 12H), 0.61 toluene was sparged with dry oxygen for 15 minutes, to no
(s, 9H), 13C NMR (101 MHz, CDCl3) δ 193.58 (C–O), 182.86 visible effect. A sample was withdrawn, and a UV-Vis spectrum
(C–O), 138.53, 137.59, 136.54, 128.64, 127.95, 97.89 recorded at both room temperature and −20 °C. Both spectra
(OCCHCO), 27.41 (C(CH3)3), 21.46 (p-CH3), 20.91 (o-CH3). IR were identical to a spectrum of Co(esac)2 in toluene taken
(neat solid): 2963 (m), 2918 (m), 1524 (vs), 1397 (vs), 842 (s), under nitrogen.
768 (s). UV-Vis (CH2Cl2): λmax (ε) 545 nm (63 M−1 cm−1). µeff
3.5(2) (Evans’ Method, THF-d8, 25 °C). Anal. Calcd for solution of Ni(esac)2 (0.100 g, 0.106 mmol) in CH2Cl2, tris
C62H70O4Ni: C, 79.40; H, 7.52. Found: C, 78.66; H, 7.75. (4-bromophenyl)ammoniumyl hexachloroantimonate (0.087 g,
=
Reaction of Ni(esac)2 and [(4-BrC6H4)3N]SbCl6. To a purple
Cu(esac)2 (4). A suspension of copper(II) chloride (0.110 g, 0.106 mmol) was added. The dark blue reaction mixture was
0.82 mmol) and K(esac) (0.750 g, 1.56 mmol) in anhydrous stirred at room temperature for 24 hours, then the solvent was
tetrahydrofuran (40 mL) was prepared. Shortly after addition of removed under vacuum. The resulting dark blue solids were
the solvent, the solution was observed to turn a dark blue triturated with toluene and filtered, resulting in the recovery of
color. The solution was heated and stirred at 60 °C for Ni(esac)2 as a purple solid.
24 hours, then filtered. Upon removal of volatiles under
Synthesis of 2-phenoxynaphthalene. To a 100 mL round
vacuum, a blue solid remained. This was further washed with bottom flask, Cu(esac)2 (0.47 g, 0.50 mmol), 2-hydroxy-
water and hexanes, then dried, resulting in a grey powder. naphthalene (1.08 g, 7.5 mmol), potassium carbonate (1.38 g,
(0.545 g, 0.58 mmol, 71%). IR (neat solid): 2966 (m), 2918 (m), 10.0 mmol), bromobenzene (0.5 mL, 5 mmol) and 15 mL
1551 (s), 1532 (s), 1406 (vs), 1376 (s), 851 (s), 784 (s). UV-Vis anhydrous toluene were added. The mixture was heated to
(toluene): λmax (ε) 620 nm (66 M−1 cm−1). µeff = 2.17(5) (Evans’ 90 °C for 16 hours, then the mixture was washed successively
Method, C6D6, 25 °C). Anal. Calcd for C62H70O4Co: C, 78.99; H, with DI H2O (10 mL), brine (10 mL), and 1 N HCl (10 mL). The
7.48. Found: C, 79.31; H, 8.01.
solvent was removed in vacuo, and then the residue was tritu-
Zn(esac)2 (5). To a 250 mL round bottom flask, anhydrous rated in hot hexane and purified by column chromatography
Zinc(II) Bromide (0.360 g, 1.07 mmol), K(esac) (1.530 g, (isocratic 20% DCM/hexane). A waxy white solid was obtained
2.15 mmol), and 80 mL anhydrous tetrahydrofuran were (0.385 g, 1.75 mmol, 35%). The spectroscopic data matches
added. The mixture was heated to 60 °C for 24 hours, then the that previously reported.60
solvent was removed under vacuum. The solids were extracted
into toluene, then filtered. Upon removal of the solvent under
1
vacuum, a white solid was obtained (1.186 g, mmol, 78%). H
Conflicts of interest
NMR (400 MHz, Chloroform-d) δ 7.42 (t, J = 7.6 Hz, 1H), 7.09
(d, J = 7.6 Hz, 2H), 6.78 (s, 4H), 5.02 (s, 1H), 2.25 (s, 6H), 1.99 M.P.M. is an inventor on U.S. Patent 9676693, which comprises
(s, 12H), 0.78 (s, 9H). 13C NMR (101 MHz, CDCl3) δ 203.45 various bulky β-diketones, other related ligands, and their
(C–O), 190.67 (C–O), 140.86, 138.99, 137.86, 136.03, 128.62, metal complexes. There are no conflicts to declare.
127.68, 97.30 (OCCHCO), 41.14 (C(CH3)3), 27.58 (C(CH3)3),
26.79, 20.98 (p-CH3), 20.65 (o-CH3). IR (neat solid): 2970 (m),
2921 (m), 1577 (s), 1428 (vs), 851 (s), 780 (s).
Reaction of Cu(Mes) and esacH. To a 100 mL round bottom
Acknowledgements
flask, copper(I) mesityl (0.250 g, 1.37 mmol), esacH (0.603 g, This work was supported by laboratory start-up funds
1.37 mmol), and hexanes (35 mL) were added. The reaction provided by the University of Colorado, Boulder,
mixture was stirred at room temperature for 12 hours, during CO. S. M. K. acknowledges a summer UROP award from
which brown solids precipitated from the reaction mixture. CU-Boulder. T. S. acknowledges German Academic Exchange
The solids were separated from the reaction mixture via fil- Service (Deutscher Akademischer Austauschdienst) for finan-
tration and washed with pentane. The solids in the frit were cial support.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Experimental work was conducted by S.M.K., E.S.A., and 15 R. L. Clough, P. Mison and J. D. Roberts, Coupling of
T.S. Catalytic demonstrations conducted by A.S.C. and
M.W.S. EPR data collected by S.J.S. The manuscript was
written by A.S.C. with assistance from M.P.M. The work was
supervised by M.P.M.
Nonequivalent Aromatic Rings by Soluble Nickel Catalysts.
A General Route to the 1,8-DiaryInaphthalene, J. Org.
Chem., 1976, 41, 2252–2255.
16 S.-K. Kang, S.-H. Lee and D. Lee, Copper-Catalyzed N
-Arylation of Amines with Hypervalent Iodonium Salts,
Synlett, 2000, 1022–1024.
17 A. M. Fatta and R. L. Lintvedt, Nephelauxetic and spectro-
chemical series for 1,3-diketonates. Ligand field spectra of
some tris(1,3-diketonato)chromium(III) chelates, Inorg.
Chem., 1971, 10, 478–481.
18 J. Aggett and A. L. Odell, The reactivity of co-ordinated acet-
ylacetone. Part I. Oxygen-18 exchange and aquation of trisa-
cetylacetonatochromium(III), J. Chem. Soc. A, 1966, 1820–
1822.
19 M. Palusiak, S. Simon and M. Solà, Interplay between
Intramolecular Resonance-Assisted Hydrogen Bonding and
Local Aromaticity. II. 1,3-Dihydroxyaryl-2-aldehydes, J. Org.
Chem., 2009, 74, 2059–2066.
Notes and references
1 P. Chirik and R. Morris, Getting Down to Earth: The
Renaissance of Catalysis with Abundant Metals, Acc. Chem.
Res., 2015, 48, 2495–2495.
2 C. C. C. Johansson Seechurn, M. O. Kitching, T. J. Colacot
and V. Snieckus, Palladium-Catalyzed Cross-Coupling: A
Historical Contextual Perspective to the 2010 Nobel Prize,
Angew. Chem., Int. Ed., 2012, 51, 5062–5085.
3 Metal Sustainability: Global Challenges, Consequences, and
Prospects, ed. R. M. Izatt, John Wiley
Chichester, UK, 2016.
& Sons, Ltd,
4 J. M. Dennis, N. A. White, R. Y. Liu and S. L. Buchwald, 20 H. Masui, Metalloaromaticity, Coord. Chem. Rev., 2001,
Breaking the Base Barrier: An Electron-Deficient Palladium 219–221, 957–992.
Catalyst Enables the Use of a Common Soluble Base in C–N 21 P. M. O’Neill, S. Hindley, M. D. Pugh, J. Davies, P. G. Bray,
Coupling, J. Am. Chem. Soc., 2018, 140, 4721–4725.
5 D. Zhang and Q. Wang, Palladium catalyzed asymmetric
Suzuki–Miyaura coupling reactions to axially chiral biaryl
compounds: Chiral ligands and recent advances, Coord.
Chem. Rev., 2015, 286, 1–16.
B. K. Park, D. S. Kapu, S. A. Ward and P. A. Stocks, Co(thd)
2: a superior catalyst for aerobic epoxidation and hydroper-
oxysilylation of unactivated alkenes: application to the syn-
thesis of spiro-1,2,4-trioxanes, Tetrahedron Lett., 2003, 44,
8135–8138.
6 N-Heterocyclic Carbenes: From Laboratory Curiosities to 22 E. Buck, Z. J. Song, D. Tschaen, P. G. Dormer, R. P. Volante
Efficient Synthetic Tools, ed. S. Diez-Gonzalez, Royal Society
of Chemistry, Cambridge, 2nd edn, 2016.
7 R. H. Crabtree, NHC ligands versus cyclopentadienyls and
and P. J. Reider, Ullmann Diaryl Ether Synthesis: Rate
Acceleration by 2,2,6,6-Tetramethylheptane-3,5-dione, Org.
Lett., 2002, 4, 1623–1626.
phosphines as spectator ligands in organometallic cataly- 23 M. Stalpaert and D. De Vos, Stabilizing Effect of Bulky
sis, J. Organomet. Chem., 2005, 690, 5451–5457.
8 J. D. Hayler, D. K. Leahy and E. M. Simmons,
Pharmaceutical Industry Perspective on Sustainable Metal
Catalysis, Organometallics, 2018, 38, 36–46.
9 S. Isayama and T. Mukaiyama, Hydration of Olefins with
Molecular Oxygen and Triethylsilane Catalyzed by Bis(tri-
β-Diketones on Homogeneous Mo Catalysts for
Deoxydehydration, ACS Sustainable Chem. Eng., 2018, 6,
12197–12204.
A
24 K. R. Kopecky, D. Nonhebel, G. Morris and
G. S. Hammond, Preparation of Dipivaloylmethane, J. Org.
Chem., 1962, 27, 1036–1037.
fluoroacetylacetonato)cobalt(II), Chem. Lett., 1989, 18, 569– 25 N. S. Nandurkar, M. J. Bhanushali, D. S. Patil and
572.
B. M. Bhanage, Synthesis of Sterically Hindered 1,3-
10 T.-G. Baik, A. L. Luis, L.-C. Wang and M. J. Krische,
Diketones, Synth. Commun., 2007, 37, 4111–4115.
Diastereoselective Cobalt-Catalyzed Aldol and Michael 26 F. Monnier and M. Taillefer, Catalytic C-C, C-N, and C-O
Cycloreductions, J. Am. Chem. Soc., 2001, 123, 5112–5113.
11 B. D. Sherry and A. Fürstner, The Promise and Challenge of
Ullmann-Type Coupling Reactions, Angew. Chem., Int. Ed.,
2009, 48, 6954–6971.
Iron-Catalyzed Cross Coupling, Acc. Chem. Res., 2008, 41, 27 S. W. M. Crossley, C. Obradors, R. M. Martinez and
1500–1511.
R. A. Shenvi, Mn-, Fe-, and Co-Catalyzed Radical
Hydrofunctionalizations of Olefins, Chem. Rev., 2016, 116,
8912–9000.
12 A. Shafir and S. L. Buchwald, Highly Selective Room-
Temperature Copper-Catalyzed C−N Coupling Reactions,
J. Am. Chem. Soc., 2006, 128, 8742–8743.
13 J. C. Lo, J. Gui, Y. Yabe, C.-M. Pan and P. S. Baran,
Functionalized olefin cross-coupling to construct carbon–
carbon bonds, Nature, 2014, 516, 343–348.
14 D. N. Primer and G. A. Molander, Enabling the Cross-
Coupling of Tertiary Organoboron Nucleophiles through
Radical-Mediated Alkyl Transfer, J. Am. Chem. Soc., 2017,
139, 9847–9850.
28 D. Roy and Y. Uozumi, Recent Advances in Palladium-
Catalyzed Cross-Coupling Reactions at ppm to ppb Molar
Catalyst Loadings, Adv. Synth. Catal., 2018, 360, 602–625.
29 J. C. Lo, D. Kim, C.-M. Pan, J. T. Edwards, Y. Yabe, J. Gui,
T. Qin, S. Gutiérrez, J. Giacoboni, M. W. Smith,
P. L. Holland and P. S. Baran, Fe-Catalyzed C–C Bond
Construction from Olefins via Radicals, J. Am. Chem. Soc.,
2017, 139, 2484–2503.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Dalton Transactions
Paper
30 E. Uhlemann and E. Frank, Stabilitätsuntersuchungen an 46 T. Yamada and T. Mukaiyama, Recent Advances in Aerobic
Chelaten sterisch behinderter β-Diketone, Z. Anorg. Allg.
Chem., 1965, 340, 319–324.
Oxygenation, Bull. Chem. Soc. Jpn., 1995, 17–35.
47 C. Obradors, R. M. Martinez and R. A. Shenvi, Ph(i -PrO)
31 F. A. Cotton and J. J. Wise, The Crystal and Molecular
Structure of Bis(2,2,6,6-tetramethylheptane-3,5-dionato)
nickel(II), Inorg. Chem., 1966, 5, 1200–1207.
SiH
: An Exceptional Reductant for Metal-Catalyzed
2
Hydrogen Atom Transfers, J. Am. Chem. Soc., 2016, 138,
4962–4971.
32 F. A. Cotton and J. S. Wood, The Crystal and Molecular 48 K. Kato, T. Yamada, T. Takai, S. Inoki and S. Isayama,
Structure of Bis(dipivaloylmethanido)zinc(II), Inorg. Chem.,
1964, 3, 245–251.
33 F. A. Cotton and R. H. Soderberg, Absorption Intensities
and Electronic Structures of Tetrahedral Cobalt(II)
Complexes, J. Am. Chem. Soc., 1962, 84, 872–873.
Catalytic Oxidation–Reduction Hydration of Olefin with
Molecular Oxygen in the Presence of Bis(1,3-diketonato)
cobalt(II) Complexes, Bull. Chem. Soc. Jpn., 1990, 63, 179–
186.
49 X. Ma and S. B. Herzon, Non-classical selectivities in the
reduction of alkenes by cobalt-mediated hydrogen atom
transfer, Chem. Sci., 2015, 6, 6250–6255.
34 A. S. Crossman, A. T. Larson, J. X. Shi, S. M. Krajewski,
E. S. Akturk and M. P. Marshak, Synthesis of Sterically
Hindered β-Diketones via Condensation of Acid Chlorides 50 X. Ma and S. B. Herzon, Cobalt bis(acetylacetonate)– tert
with Enolates, J. Org. Chem., 2019, 84, 7434–7442.
35 E. S. Akturk, S. J. Scappaticci, R. N. Seals and
M. P. Marshak, Bulky β-Diketones Enabling New Lewis
Acidic Ligand Platforms, Inorg. Chem., 2017, 56, 11466–
11469.
-butyl hydroperoxide–triethylsilane: a general reagent com-
bination for the Markovnikov-selective hydrofunctionaliza-
tion of alkenes by hydrogen atom transfer, Beilstein J. Org.
Chem., 2018, 14, 2259–2265.
51 M. F. Sloan, A. S. Matlack and D. S. Breslow, Soluble
Catalysts for the Hydrogenation of Olefins, J. Am. Chem.
Soc., 1963, 85, 4014–4018.
36 K. Yang and O. Chang-Eon, Conformational Change of
Optically Active [Co(acac) 2 (diamine)]+Complexes in Some
Organic Solvents, Bull. Korean Chem. Soc., 1987, 8, 441–444. 52 I. P. Beletskaya and A. V. Cheprakov, Copper in cross-coup-
37 M. P. Marshak, M. B. Chambers and D. G. Nocera, Cobalt
in a Bis-β-diketiminate Environment, Inorg. Chem., 2012,
51, 11190–11197.
38 E. Chiyindiko and J. Conradie, Redox behaviour of bis
(β-diketonato)copper(II) complexes, J. Electroanal. Chem.,
2019, 837, 76–85.
39 B. Cheng, H. Yi, C. He, C. Liu and A. Lei, Revealing the
Ligand Effect on Copper(I) Disproportionation via
Operando IR Spectra, Organometallics, 2015, 34, 206–211.
ling reactions The post-Ullmann chemistry, Coord. Chem.
Rev., 2004, 248, 2337–2346.
53 W. M. Czaplik, M. Mayer, J. Cvengroš and A. J. von
Wangelin, Coming of Age: Sustainable Iron-Catalyzed Cross-
Coupling Reactions, ChemSusChem, 2009, 2, 396–417.
54 I. Kazuaki, I. Etsuro and Y. Yuroku, Effects of N-Methylated
Diamine on Ion Pair Formation of Bis(2,4-pentanedionato)
(diamine) Cobalt(III) Bromides in Nitrobenzene by
Conductometry, Bull. Chem. Soc. Jpn., 1982, 55, 3190–3192.
40 S. A. Gromilov and I. A. Baidina, Regularities of Crystal 55 L.-C. Wang, H.-Y. Jang, Y. Roh, V. Lynch, A. J. Schultz,
Structures of Cu(II) beta-Diketonates, J. Struct. Chem., 2004,
45, 1031–1081.
41 F. A. Cotton and R. H. Soderberg, A Spectroscopic Study of
the Polymeric Nature of Bis(acetylacetonato)cobalt(II),
Inorg. Chem., 1964, 3, 1–5.
X. Wang and M. J. Krische, Diastereoselective
Cycloreductions and Cycloadditions Catalyzed by Co(dpm)
2-Silane (dpm = 2,2,6,6-tetramethylheptane-3,5-dionate):
Mechanism and Partitioning of Hydrometallative versus
Anion Radical Pathways, J. Am. Chem. Soc., 2002, 124,
9448–9453.
42 S. M. Socol and J. G. Verkade, Steric and electronic effects
in cobalt(II) disproportionation with phosphorus ligands, 56 J. B. Ellern, R. O. Ragsdale, R. J. Allen, A. B. Chatterjee and
Inorg. Chem., 1986, 25, 2658–2663.
43 G. O. Jones, P. Liu, K. N. Houk and S. L. Buchwald,
D. F. Martin, in Inorganic Syntheses, John Wiley & Sons, Ltd,
1968, vol. 11, pp. 82–89.
Computational Explorations of Mechanisms and Ligand- 57 G. M. Sheldrick, SHELXT – Integrated space-group and
Directed Selectivities of Copper-Catalyzed Ullmann-Type
Reactions, J. Am. Chem. Soc., 2010, 132, 6205–6213.
crystal-structure determination, Acta Crystallogr., Sect. A:
Found. Adv., 2015, 71, 3–8.
44 H.-K. Shin, M. J. Hampden-Smith, E. N. Duesler and 58 G. M. Sheldrick, Crystal structure refinement with SHELXL,
T. T. Kodas, The chemistry of copper(I) β-diketonate com- Acta Crystallogr., Sect. C: Struct. Chem., 2015, 71, 3–8.
pounds. Part V. Syntheses and characterization of 59 O. V. Dolomanov, L. J. Bourhis and R. J. Gildea, J. a. K.
(β-diketonate)CuL
species where L = PBu3, PPh3, and
Howard and H. Puschmann, OLEX2: a complete structure
solution, refinement and analysis program, J. Appl.
Crystallogr., 2009, 42, 339–341.
n
PCy3; n = 1 and 2; crystal and molecular structures of (acac)
Cu(PCy3), (tfac)Cu(PCy3), (hfac)Cu(PCy3), and (hfac)Cu
(PCy3)2, Can. J. Chem., 1992, 70, 2954–2966.
60 K. Swapna, S. N. Murthy, M. T. Jyothi and
Y. V. D. Nageswar, Recyclable heterogeneous copper oxide
on alumina catalyzed coupling of phenols and alcohols
with aryl halides under ligand-free conditions, Org. Biomol.
Chem., 2011, 9, 5978.
45 B. Emmert, H. Gsottschneider and H. Stanger, Über einige
innere Komplexsalze des zweiwertigen Mangans und
einwertigen Kupfers, Ber. Dtsch. Chem. Ges., 1936, 69,
1319–1321.
This journal is © The Royal Society of Chemistry 2019
Dalton Trans.