Selective metalation of phenol-type proligands for preparative organometallic chemistry

Article abstract of DOI:10.1039/d0cc01254h

The selective C-metalation of phenol ester derived proligands is a readily applicable addition to state-of-the-art protocols toward cyclometalated structures, in particular of the base metals. The approach is demonstrated by the ortho-nickelation of a 1,10-phenanthroline based strong-field ligand platform that is a sought-after motif in the field of base-metal photochemistry.

Full text of DOI:10.1039/d0cc01254h

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Selective Metalation of Phenol-Type Proligands for Preparative
Organometallic Chemistry†
Andreas Berkefeld,*a Markus Fröhlich,†a Mike Kordan,†a Gerald Hörner,b and Hartmut Schuberta
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The selective C-metalation of phenol ester derived proligands is a
readily applicable addition to state-of-the-art protocols toward
cyclometalated structures, in particular of the base metals. The
approach is demonstrated by the ortho-nickelation of a 1,10-
phenanthroline based strong-field ligand platform that is a sought-
after motif in the field of base-metal photochemistry.
Cyclometalated ligands that comprise an aryl σ-donor moiety
linked to a κn-heteroaromatic donor platform are a prominent
motif in transition-metal organometallic chemistry. The broad
occurrence of cyclometalated structures relates to their pivotal
roles in a variety of contexts such as the functional group
directed transition-metal catalysed functionalisation of C-H
bonds1-7 and photochemistry that evolves primarily from
complex dyes of the platinum group metals.8-14 Ground- and
excited-state state redox chemistry and photoluminescence are
important characteristics that relate closely to the strong
ligand-field, the σ-donor/π-acceptor properties and the
coplanar coordination affected by this particular ligand type.
Notwithstanding the broad interest in cyclometalated
structures, substantial limitations prevail in the search for
modular synthetic protocols that are amenable to both base
and noble metals. Herein, we report that the use of C-O type
proligands opens the door to a general strategy toward
cyclometalated organometallic structures as indicated in
Scheme 1. This approach complements established protocols
such as directed ortho-C-H bond metalation that proceeds
straightforwardly for platinum group metals3, 12 but inefficiently
for base metals such as Ni.15 Specific exceptions are the reactive
central C-H bonds in pincer type proligands16-18 or those that
feature at least one additional activating donor group.3, 7, 19-27
Scheme. 1 Strategies toward preparative cyclometalation.
The use of C-halide type proligands28-31 certainly addresses this
limitation but hinges on the implementation of protocols for the
regioselective introduction of a C-X bond to any proligand. In
addition to their good availability owing to a broad structural
pool, one of the key benefits to phenols is the ready use of their
methyl ethers as building blocks in proligand synthesis. The
introduction of an activating group R* renders the C-O bond
susceptible to oxidative addition.32 This strategy is reminiscent
to the well-established use of phenol-derived building blocks in
catalytic C-C bond forming homo- and cross-coupling.32-34 The
ortho-C-O functionalised 2-aryl-1,10-phenanthroline shown in
Scheme 1 suits as a representative proligand framework in this
context for two reasons. First, its κ3-N,N’,C-type coordination to
any 3d-metal is yet unreported, most likely due to the lack of a
suitable proligand. Second, calculations predict beneficial use of
this framework in base-metal complex photochemistry.35, 36 The
fourth ligand R*O in product Ni complexes can be varied
a. Institut für Anorganische Chemie, Eberhard Karls Universität Tübingen, Auf der
Morgenstelle 18, 72076 Tübingen, Germany.
E-mail: andreas.berkefeld@uni-tuebingen.de
b. Anorganische Chemie IV, Universität Bayreuth, Universitätsstraße 30, 95440
Bayreuth, Germany.
† M.F. and M.K. contributed equally. Electronic Supplementary Information (ESI)
available: Experimental, analytical and computational details. See
DOI: 10.1039/x0xx00000x
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straightforwardly, which has a substantial impact on the coproduct 2-(µ-Tf) verified ligand coordination in a κ3-N,N’,O
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DOI: 10.1039/D0CC01254H
composition and energetic order of frontier orbitals and thus fashion. 2-(µ-Tf) is dimeric in solution and the solid state, with
the electronic properties of this family of novel compounds.
Tf and the phenolate acting as bridging ligands whereas a non-
A scalable two-step protocol37 starting from lithiated 2-bromo- coordinate TfO, which results from C-O bond cleavage,
4-tert-butylanisol and 1,10-phenanthroline afforded the phenol balances the net positive charge. The presence of sulfinate and
precursor after demethylation with NaSC2H5 in 73 % yield. At sulfonate ions in bulk samples of 2-(µ-Tf) is corroborated by IR
this point, a variety of activating functional groups OR* can be spectra in KBr, which show distinct and strong bands at 1158
introduced, in this work these are the triflate 0-OTf, Tf = CF3SO2, and 1147 cm-1 for µ2-O,O’-Tf and 1299, 1278, 1264 and 636
and the pivaloate 0-OPiv, Piv = C(O)tBu. Combining 0-OTf and cm-1 for non-coordinating TfO.38 Axial binding of thf completes
Ni(cod)2 in THF affords a yellow-green solution from which octahedral S = 1 Ni(II) sites that couple ferromagnetically, in line
orange needles separate within 2 hours at room temperature with an earlier report on a complex prepared from 2-(1,10-
(r.t.). Slow diffusion of pentane vapour into the filtrate phenanthroline-2-yl)phenol and Ni(II) acetate.39 For THF as the
deposited a light green crystalline solid. Both products have reaction medium O-S bond scission by Ni(0) dominates over C-
been characterised by single-crystal X-ray diffraction, XRD, and O bond addition, affording Tf that scrambles between the two
molecular structures are included in Fig. 1. Orange 1-Tf is the products owing to the lower coordination ability of the TfO
product from C-O bond oxidative addition to Ni(0). The ligand congener formed by C-O bond addition to Ni(0).
coordinates in a κ3-N,N’,C fashion and the bond distances Contrary to THF, a light green solid separated from reaction
between Ni and the peripheral and central N-donors differ by mixtures in diethyl ether or toluene, which gradually converted
0.181 Å, pointing to the trans-influence of the σ-aryl donor. into a dark yellow material that is sparingly soluble even in THF.
Bond angles at Ni add up to 360° as expected for square-planar To identify product composition, ligand metathesis was done by
coordination. Unexpectedly, the fourth ligand at Ni(II) is κ-O- reacting NaN(SiMe3)2 and the yellow material in diethyl ether
CF3SO2 , Tf, rather than κ-O-CF3SO3 , TfO. IR spectra of which afforded dark red solutions instantaneously. 1H NMR
crystalline 1-Tf in KBr show a group of strong bands centred at spectroscopic analysis of isolated red powders showed two
1158 cm-1 that are reminiscent of those observed for CF3SO2Na, distinct products 1-N(SiMe3)2 and 2-N(SiMe3)2 whose molecular
confirming the presence of Tf throughout the bulk material. structures and characteristic metrics are included in Fig. 2. Both
The solubility of 1-Tf is poor in common low-polarity organic products were substantiated from independent preparations


1
solvents but H, 13C{1H} and 19F NMR data recorded in THF-d8 starting from isolated 1-Tf and 2-(µ-Tf). The yield of the κ3-
solution corroborate the structure found in the solid state. N,N’,C-metalated product, quantified in form of 1-N(SiMe3)2,
Analysis of NMR data points to a dynamic behaviour of 1-Tf, nearly doubles to 57 % in toluene as indicated in Fig. 1.
indicating partial solvolysis of the Ni-O bond but studies at Accordingly, the yellow solid material formed in either toluene
lower temperatures are precluded owing to poor solubility.
or diethyl ether appears to consist of a mixture of products 1-Tf,
1-OTf and 2-Tf that convert into the respective amido
complexes 1-N(SiMe3)2 and 2-N(SiMe3)2.
Fig. 1 Preparation and metalation selectivity of C-O type proligands, and ORTEP
representations of molecular structures of κ3-N,N’,C/O-metalated products (thermal
displacement ellipsoids set at 50 % probability level, H-atoms omitted for clarity).
Fig. 2 Diversification of the fourth donor of C- and O-metalated precursors (thermal
displacement ellipsoids in ORTEPs set at 50 % probability, H-atoms omitted for clarity).
The marked solvent effect on the chemoselectivity of the
metalation provides an insight into the reaction mechanism:
Ni(cod)2 and proligand react to form a precursor complex or
complexes that undergo metalation through two independent
pathways. The first is oxidative addition of Ni(0) to the ortho-C-
Binding of Tf rather than TfO to Ni(II) in product 1-Tf indicates
that C-O bond cleavage occurs in competition with O-S bond
scission and that resulting Tf- and TfO-ions scramble between
the products. Indeed, the molecular structure of the green
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O bond and appears to be kinetically challenged, which agrees ligand) charge transfer (CT) transitions. RelevantVbieawnAdrtsicleinOntlhinee
DOI: 10.1039/D0CC01254H
with the general notion that ortho-substituted aryl triflates visible spectrum integrate significant contributions of several
show a lower reactivity in catalytic C-C bond formation.40 The mainly MLCT-like (d(Ni) π*(Phen)) and intra-ligand CT-like
second pathway involves single-electron transfer induced O-S transitions (π(Ph/O-Ph)π*(Phen)) for all complexes, with

bond cleavage of the triflate group to F3CSO2 and a phenoxyl relative energies and intensities in good qualitative agreement
radical that eventually forms the phenolate donor.40 Even with experiment. {Ni-X}-to-π*(Phen) CT transitions for X =
though S-O bond cleavage is irreversible, the lower preference tBuC(O)O and (Me3Si)2N reflect the availability of non-bonded
for this pathway in solvents that provide limited solubility to the electron pairs at these moieties. This qualitative difference from
initially formed precursor complex(es) indicates that Ni-to- X = Tf gives rise to diagnostic shifts of the HOMO character and
triflate SET occurs more preferentially in a bimolecular rather energy with X, whereas the phenanthroline borne π*-character
than in an intramolecular reaction.
of relevant acceptor orbitals (LUMO to LUMO+2) is conserved
Circumstantial evidence for this mechanistic picture derives throughout the series. As shown in the bottom part of Fig. 3, the
from the metalation of the pivaloate 0-OPiv by Ni(cod)2. A dark HOMO energies increase by >1.0 V along the series that features
green reaction mixture in THF formed within minutes whose the antibonding combination of Ni 3d and π-orbitals of the σ-
colour changed gradually to dark red and fine dark red needles aryl moiety in 1-Tf (close to degenerate with a nonbonding
separated. Ligand metathesis afforded 1-N(SiMe3)2 exclusively nickel d-orbital), a non-bonding combination Ni 3d-O 2p in 1-
whereas slow diffusion of pentane vapour into the dark red THF OPiv, and largely non-bonding Ni 3d-N 2p-orbital in 1-N(SiMe3)2
solution gave 1-OPiv in form of a red crystalline solid in 59 % and 2-N(SiMe3)2. More broadly, DFT analysis corroborates that
yield. Products 1-N(SiMe3)2, 2-N(SiMe3)2 and 1-OPiv have been a change of the characteristics of variable coligand X from σ-
characterised by NMR and UV/vis spectroscopies, and Fig. 2 donor to σ-/π-donor changes the frontier orbital characteristics
provides pertinent structure metrics that have been derived in these systems as inferred qualitatively from experimental
from single-crystal XRD analyses; respective data of 1-OPiv are UV/vis absorption data.
provided in Fig. S1, ESI.† NMR characteristics are unexceptional
other than the 13C NMR chemical shifts of the Ni-Caryl nuclei that
fall into the 147-149 ppm range. The products of C-metalation
1-N(SiMe3)2 and 1-OPiv are isostructural, and the trans-
influence of the σ-aryl donor likely results in the Δd(Ni-N/N’) =
0.203 Å found for 1-N(SiMe3)2 as indicated in Fig. 2; Δd(Ni-N/N’)
= 0.186 Å for 1-OPiv. In case of 2-N(SiMe3)2, both SiMe3 groups
are bent toward the Ni-O moiety, which results in a tetrahedral
distortion at N(SiMe3)2 as indicated by the sum of bond angles
of 353° whereas trigonal-planar bonding at nitrogen prevails for
1-N(SiMe3)2. The perpendicular orientation of the Si-N-Si moiety
with regard to the 1,10-phenanthroline backbone aligns the
second electron pair at nitrogen parallel to the coordination
plane.
The selective and efficient two-step synthesis of 1-X according
to Fig. 1 allows the ready diversification with respect to X.
Varying the electronic properties of the anionic donor X from
sulfinato to carboxylato to amido results in the continuous
redshift of a set of three bands that locate in the 480-600 nm
spectral region as shown in Fig. 3. Except for the red colour of
2-N(SiMe3)2 that results from multiple overlapping bands in the
visible spectrum, the overall appearance of the series of spectra
Fig.
3 Top: UV-vis absorption spectra. Bottom: Energy of highest occupied MOs
is very similar, which points to qualitatively similar electronic
structures that are fine-tuned through the donor characteristics
of X. This view is corroborated by TD-DFT computations of 1-X
and 2-N(SiMe3)2 at the TPSSh-D3/TZVP/COSMO(THF) level of
theory. In all cases, DFT-optimized structures adequately mirror
the measured structural metrics; computational details are
provided in the ESI.† In general, HOMO-LUMO type transitions
play a negligible role with calculated fosc<103 that are
reminiscent to complexes of a 6-phenyl-2,2’-bipyridine ligand
platform;29 e.g. a predicted transition 600 nm for 1-Tf is hardly
noticeable even in concentrated THF solutions as shown in Fig.
S2, ESI.† The experimentally observed electronic transitions at
lowest energies are best described as {Ni-X}-to-(π*-κ3-N,N’,C/O-
computed at TPSSh-D3/TZVP/COSMO(THF) level of theory; pertinent MO types are
depicted for 1-N(SiMe3)2.
In summary, the use of C-O type proligands complements state-
of-the-art approaches that use C-H and C-halide type ligand
precursors for the preparation of cyclometalated transition-
metal coordination compounds. Structure diversification of C-O
type proligands benefits from the substantial structural pool of
phenols and the broad variety of established protocols for
proligand assembly. Metalation selectivity by a low-valent base
metal such as Ni is controlled by the choice of an activating
functional group R*, and ligand metathesis of the resulting R*O
donor in
a one-pot procedure makes a variety of
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scope of preparative strategies toward cyclometalated
structures for both base and noble metals,‡ and aids research
on organometallic structures that are relevant for catalytic bond
manipulations or new transition-metal based chromophore
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DOI: 10.1039/D0CC01254H
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‡ The reaction of 0-OTf and (nbn)3Pt(0), nbn = norbornene, in THF
solution afforded the κ3-N,N',C-metalated product exclusively in
which κ-N-binding of unreacted 0-OTf compensates for the poor
coordination ability of triflate.
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