Thermodynamics of a μ-oxo Dicopper(II) Complex for Hydrogen Atom Abstraction

Article abstract of DOI:10.1021/jacs.7b10833

The mono-μ-hydroxo complex {[Cu(tmpa)]₂-(μ-OH)}³⁺ (1) can undergo reversible deprotonation at -30 °C to yield {[Cu(tmpa)]₂-(μ-O)}²⁺ (2). This species is basic with a pK_a of 24.3. 2 is competent for concerted proton-electron transfer from TEMPOH, but is an intrinsically poor hydrogen atom abstractor (BDFE(OH) of 77.2 kcal/mol) based on kinetic and thermodynamic analyses. Nonetheless, DFT calculations experimentally calibrated against 2 reveal that [Cu₂O]²⁺ is likely thermodynamically viable in copper-dependent methane monoxygenase enzymes.

Full text of DOI:10.1021/jacs.7b10833

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Communication
Thermodynamics of a µ-oxo Dicopper(II)
Complex for Hydrogen Atom Abstraction
Ghazanfar Ali, Peter E. VanNatta, David A. Ramirez, Kenneth M. Light, and Matthew T. Kieber-Emmons
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10833 • Publication Date (Web): 06 Dec 2017
Downloaded from http://pubs.acs.org on December 6, 2017
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Thermodynamics of a µ-oxo Dicopper(II) Complex for Hydro-
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Ghazanfar Ali, Peter E. VanNatta, David A. Ramirez, Kenneth M. Light, Matthew T. Kieberꢀ
Emmons*
Department of Chemistry, University of Utah, Salt Lake City, UT 84112ꢀ0850
Supporting Information Placeholder
experimental thermodynamic information has been
available to validate the feasibility for initial hydroꢀ
gen atom abstraction from strong CꢀH bonds. Since
this putative intermediate has not been trapped in
pMMO, model studies are required to evaluate facꢀ
tors that would enable the initial hydrogen atom
abstraction. Herein we report {[Cu(tmpa)]2ꢀ(μꢀ
OH)}3+ (1, tmpa = tris(2ꢀpyridylmethyl)amine),
which could be reversibly deprotonated to generate
a [Cu2O]2+ core which is functional for Hꢀatom abꢀ
straction from exogenous substrates (Scheme 1).
This finding enables the thermodynamic and kinetic
evaluation of the feasibility of a [Cu2O]2+ core as
the functional oxidant in pMMO.
ABSTRACT:
The
monoꢀμꢀhydroxo
complex
{[Cu(tmpa)]2ꢀ(μꢀOH)}3+ (1) can undergo reversible
deprotonation at –30°C to yield {[Cu(tmpa)]2ꢀ(μꢀ
O)}2+ (2). This species is basic with a pKa of 24.3. 2
is competent for concerted protonꢀelectron transfer
from TEMPOH, but is an intrinsically poor hydrogen
atom abstractor (BDFE(OH) of 77.2 kcal/mol)
based on kinetic and thermodynamic analyses.
Nonetheless, DFT calculations experimentally caliꢀ
brated against 2 reveal that [Cu2O]2+ is likely therꢀ
modynamically viable in copperꢀdependent meꢀ
thane monoxygenase enzymes.
Scheme 1. Interconversion of dicopper-O(H)
complexes reported herein.
A major contemporary challenge is the selective
catalytic conversion of fossil feedstocks. Methane,
the principle component of natural gas, poses a
particular challenge because of its strong CꢀH
bonds and tendency to be over oxidized to carbon
dioxide. Advances in extraction from underground
sources has dramatically increased access to meꢀ
thane, and thus efficient conversion of methane
into easily transportable products, such as methaꢀ
nol, is a significant need. Methanotrophic organꢀ
isms are well known to consume methane through
the action of methane monoxygenase enzymes
that catalyze the direct oxidation of methane to
methanol;1 however, the nature of the active site in
copperꢀdependent
methane
monoxygenase
(pMMO) has eluded unambiguous assignment.2
Mechanistic insight for pMMO has come from comꢀ
putational3,4 and model studies, of which copper
exchanged zeolites have been highly instructive
due to good selectivity for oxidation of methane to
methanol and ability to function under catalytic
conditions.5 Under stoichiometric conditions,
[Cu2O]2+ species6 and [Cu3O2]2+ species7 have
been implicated as the active oxidants, which parꢀ
allels proposals in copper dependent MMOs.8 The
dicopper(II) μꢀoxo ([Cu2O]2+) species has been of
particular recent interest (Figure 1);9,10 however, no
Figure 1. Comparison of the activeꢀsite architecture in
pMMO with the defined [Cu2O]2+ cores in CuꢀZSMꢀ5
and in the present study.
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Complex 1 was prepared as the OTf– salt (OTf =
Mꢀ1cmꢀ1) Cu(II) d→d transitions respectively on the
basis of energy and intensity.
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trifluoromethansulfonate) by addition of CuOTf2 to
tmpa in methanol under nitrogen, followed by addiꢀ
tion of excess H2O and NEt3 to generate a blue
product (62% yield, see Supporting Information for
details). Crystals suitable for Xꢀray diffraction analꢀ
ysis were generated by vapor diffusion of diethyl
ether into the methanol solution that yielded the
dicopper(II) monoꢀμꢀOH complex 1 (Figure 2). Asꢀ
signment of the bridge as an OH group was based
on the overall charge of the cation which was +3
and on the CuꢀO(H) bond lengths of 1.918(2) Å and
1.915(3) Å, consistent with those of monoꢀμꢀOH
complexes which range from 1.87 Å to 1.93 Å (Taꢀ
ble S2). The coordination environment about the
copper is trigonal bipyramidal with the tmpa amine
apical, the pyridine arms in the equatorial positions
with an average CuꢀN(py) distance of 2.084 Å, and
the OH– occupying the basal axial position. The
CuꢀO(H)ꢀCu core is bent, with the CuꢀO(H)ꢀCu anꢀ
gle at 144.50(14)°.
The reduction of complex 1 was studied by cyclic
voltammetry (CV). Complex 1 was prepared at 0.52
mM in MeCN with 0.1 M TBAPF6 (tetrabutylammoꢀ
nium hexafluorophosphate) as supporting electroꢀ
lyte. At –30°C, a reversible 1 e– wave was obꢀ
served at –0.48 V vs Fc/Fc+ (Figure 3) and a seꢀ
cond, irreversible wave was observed at –0.70 V vs
Fc/Fc+ (Figure S6). These waves are assigned as
the Cu2II,II/II,I couple and tentatively the Cu2II,I/I,I couꢀ
ple respectively based on comparison to
[Cu2(BPMAN)(μꢀOH)](OTf)3 (BPMAN = 2,7ꢀ[bis(2ꢀ
pyridylmethyl)aminomethyl]ꢀ1,8ꢀnaphthyridine),
where an irreversible anodic half wave was obꢀ
served at –0.44 V vs Fc/Fc+ and an irreversible caꢀ
thodic half wave was observed at –0.78 V vs
Fc/Fc+.11
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Figure 3. Cyclic voltammograms of 1 in MeCN at –
30°C with scan direction denoted by the arrow.
Figure 2. ORTEP of the trication of 1 from Xꢀray crysꢀ
tallographic data at 35% ellipsoid probability.
Deprotonation of complex 1 yielded a new, therꢀ
mally sensitive species 2. Specifically, addition of
one equivalent of triisobutylꢀsubstituted Verkade’s
Further evidence for assignment of 1 as a monoꢀ
μꢀOH comes from FTIR where a ν(OꢀH) peak was
observed at 3450 cm–1 that downshifts to 2560 cm–
superbase
(2,8,9ꢀTriisobutylꢀ2,5,8,9ꢀtetraazaꢀ1ꢀ
1
phosphabicyclo[3.3.3]undecane) to 1 in MeCN at –
30°C (Figure S9) results in appearance of a new
optical feature at 290 nm (2750 Mꢀ1cmꢀ1). This reacꢀ
tion was reversible; addition of [LutH](OTf) (LutH =
2,6ꢀdimethylpyridinꢀ1ꢀium) resulted in reappearꢀ
ance of the optical bands associated with 1. (Figure
in the OD isotopologue, consistent with that exꢀ
pected (2510 cm–1) from reduced mass calculaꢀ
tions on an isolated harmonic oscillator. Complex 1
is EPR silent, yet displays a paramagnetic 1HꢀNMR
spectrum (Figure S8), which indicates an
integer
spin electronic structure derived from two magnetiꢀ
cally coupled d9 Cu(II) ions, and thus a dinuclear
geometric structure in solution. Variable temperaꢀ
1
S9). 2 displays a diamagnetic HꢀNMR spectrum
(Figure S8) and is EPR silent, which together sugꢀ
gest an integer spin electronic structure and thus a
dinuclear geometric structure. These data suggest
formulation of 2 as the dicopper(II)ꢀμꢀO adduct
{[Cu(tmpa)]2ꢀ(μꢀO)}2+.12
Since the [Cu2O]2+ core of 2 has been implicated
as analogous to the active oxidant in pMMO, we
sought experimental insight into the thermodynamꢀ
ics for hydrogen atom abstraction by 2. Thus, we
measured the pKa of 1 at –30°C in MeCN. Titration
of 1 with triisobutylꢀsubstituted Verkade’s superꢀ
base (pKa = 33.53) resulted in stoichiometric conꢀ
version of 1 into 2, indicating a weaker base would
be required to access an equilibrium condition
1
ture HꢀNMR (Figure S4) revealed antiꢀCurie magꢀ
netic
behavior
resultant
from
an
antiꢀ
ferromagnetically coupled ground state with a lowꢀ
lying ferromagnetically coupled excited state. This
behavior was modeled using the effective spin
Hamiltonian H = –2JS1⋅S2 to yield a magnetic couꢀ
pling parameter –2J = –223 cmꢀ1 in agreement with
that expected from the 144° CuꢀOꢀCu angle (Figure
S3). The electronic absorption spectrum of complex
1 in acetonitrile (MeCN, Figure 4) displays features
at 290 nm (2750 Mꢀ1cmꢀ1), and 350 nm (5460 Mꢀ
1cmꢀ1) that are assigned as OH→Cu(II) ligandꢀtoꢀ
metal charge transfer transitions and 822 nm (320
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Figure 5. Thermodynamic cycle which relates the
(Figure S9). No reaction was observed with NEt3
(pKa = 18.46), but sigmoidal shaped conversion of
1 to 2 was observed for titration of 1 with DBU (1,8ꢀ
diazabicyclo[5.4.0]undecꢀ7ꢀene, pKa = 24.33, Figꢀ
ure 4). This equilibrium behavior was modeled to
yield a room temperature corrected pKa of 24.3 ±
1.9 for complex 1. This is much more basic than a
recently reported dicopper(II)ꢀμꢀO adduct in which
the pKa was determined to be between 4.5 and
14,13 but is similar to a dicopper(II)ꢀsuperoxo adꢀ
duct, which displayed a pKa of 22.2,14 and sugꢀ
gests an electron rich oxo moiety.
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oxidation of complex 1 to the pKa of complex 1 to
yield the BDFE(OH) of complex 3. The hypothetical
species 4 is illustrated in gray.
Despite the weaker BDFE of 2 compared to Cuꢀ
ZSMꢀ5, we were encouraged because 2 decomꢀ
poses slowly to unidentified products at –30°C in
MeCN with a t1/2 of ~3.5 hrs. We tested the reactiviꢀ
ty of 2 towards weak CꢀH bonds at –30°C, and
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found
2
reacted very slowly with 1,4ꢀ
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cyclohexadiene (BDFE 72.9 kcal mol–1) to generate
benzene (75% yield by NMR). Nonetheless, 2 was
moderately reactive towards the OꢀH bond of
TEMPOH to generate TEMPO• in quantitative yield
by EPR spin quantitation (Figure S16). Kinetic
analysis of the reaction of 2 generated in situ with 2
eq DBU and TEMPOH in MeCN (Figure 6)16 yieldꢀ
ed an ΔH‡ of 5.3 ± 2.8 kcal mol–1 and a ΔS‡ of ꢀ44 ±
11 cal mol–1 which translates to a ΔG‡ (298 K) of 18
± 2.8 kcal mol–1. While these kinetics are compliꢀ
cated by subsequent sideꢀreactions (see Suppleꢀ
mentary Information for details), we verified that
these kinetic parameters are probing the Hꢀatom
abstraction by reaction with TEMPOD, which yieldꢀ
ed a small, primary kinetic isotope effect (KIE =
kH/kD) of 1.6 ± 0.3. The barrier and KIE of this reacꢀ
tion reflect a concerted hydrogen atom transfer
(HAT) process rather than an initial proton or elecꢀ
tron transfer, which are disfavored due to the pKa
and E0 of TEMPOH. A concerted HAT would reꢀ
quire formation of an encounter complex, which the
large entropy of activation and small enthalpy of
activation supports. The small KIE further supports
this notion, indicating a transition state late with
respect to Hꢀatom transfer. Thus, the rationale for
the lack of reactivity towards weak CꢀH bonds is
the higher steric demands of the CꢀH substrates. To
corroborate the role of sterics, DFT calculations
were performed on 2. DFT predicts an antiꢀ
ferromagnetically coupled and linear [Cu2O]2+ core
to be the lowest energy structure after spin purificaꢀ
tion of the brokenꢀsymmetry singlet (Figure S20).
Evaluation of the spaceꢀfilling model of 2 suggests
that the pseudoꢀC3v symmetric copper coordination
environment with interdigitated ligand arms preꢀ
cludes access to the [Cu2O]2+ core (Figure S18).
Figure 4. Titration of 1 (—) with DBU at –30°C in
MeCN resulting in formation of 2 (—); Inset: titration
monitored at 350 nm.
Using the thermodynamic parameters E1/2 (–0.48
V vs Fc/Fc+) and pKa (24.3) determined above, a
thermodynamic cycle was constructed (Figure 5)
that allows determination of the BDFE of the O–H
bond of 3. This value is derived according to the
equation:
ꢃ
ꢀ
ꢁ
BDFE OH = 1.37pKꢂ + 23.06E + ꢄꢅ
where CG represents the H+/H• reduction potential
in MeCN (54.9 kcal/mol).15 E1/2 as an approximaꢀ
tion of E0 yields a BDFE(OH) of 77.2 ± 2.3 kcal molꢀ
1. Density functional theory calculations were perꢀ
formed to benchmark this value. Specifically, an
isodesmic Hꢀatom abstraction from 2,4,6ꢀ(triꢀtertꢀ
butyl)phenol was calculated. This phenol has a
known –OH BDFE (80.3 kcal molꢀ1), which yields a
calculated BDFE(OH) in 3 of 81.6 kcal molꢀ1, in
reasonable agreement with the experimentally deꢀ
termined value. This value is substantially lower
than that in CuꢀZSMꢀ5 where the analogous BDFE
is predicted on the basis of DFT to be 90 kcal molꢀ
1 6
.
H
O
The weak BDFE of 3 suggests that even in abꢀ
sence of sterics, the [Cu2O]2+ core of 2 is not intrinꢀ
sically very reactive; for example, the minimum barꢀ
rier for reaction of 2 with methane (BDFE(CH) 105
kcal mol–1) would be ~28 kcal mol–1. This is in conꢀ
trast to the reactivity of the [Cu2O]2+ core demonꢀ
strated by CuꢀZSMꢀ5, albeit with its abiological ligꢀ
and set, and thus we questioned whether this type
of core could function in the biological context of
pMMO. To address this question, we applied our
LCuII CuIL
O
CuIL
LCuII
3
4
E0 = –0.48 V
(vs Fc/Fc+)
-
1
H
O
pKa(OH) = 24.3
LCuII CuIIL
LCuII
CuIIL
O
1
2
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calibrated DFT methodology to the BDFE(OH) of
Page 4 of 5
dynamics of a [Cu2O]2+ species. While it has been
generally held that these species would be intrinsiꢀ
cally reactive,8 the oxo group is highly basic and
thus bears considerable electron density, which
leads to weak HAT ability. Thus, the pMMO active
site must generate a less electron rich [Cu2O]2+
moiety to drive HAT from the strong CꢀH bonds of
methane. Ongoing studies are aimed to elucidate
factors that enable this process.
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3
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7
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an active site model of pMMO (Figure S19). In
pMMO, the CuꢀCu distances are relatively short
(2.58 – 2.71 Å, Table S4) compared to binuclear
Type 3 Cu proteins in the deoxy state (2.2 – 4.5
Å),17 which implies the protein constrains the Cuꢀ
Cu distance. In our computational model containing
the [Cu2O]2+ core, the constraints lead to a CuꢀCu
distance of 3.475 Å, comparable to previous oxoꢀ
pMMO models (Table S5), which further leads to a
bent Cu(II)ꢀOꢀCu(II) core (165.1°). In pMMO, this
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ASSOCIATED CONTENT
core is much more accessible than in
bidentate coordination environment about each
copper. Using our calibrated DFT methodology and
2 due to the
Supporting Information. The Supporting Information
is available free of charge via the internet at
http://pubs.acs.org.
benchmarked by
3, we computed an isodesmic
Crystallographic data (cif).
reaction with 2,4,6ꢀ(triꢀtertꢀbutyl)phenol on the
pMMO model. On the singlet surface, the
BDFE(OH) of the pMMO model is predicted to be
89.9 kcal mol–1 (Figure S21). While our methodoloꢀ
gy slightly overestimates this value compared to
experiment, it nonetheless indicates that a putative
[Cu2O]2+ core is likely strong enough to abstract an
Hꢀatom from methane in pMMO. An unresolved
question is what specific geometric and electronic
structure factors would lead to the enhanced HAT
thermodynamics of a [Cu2O]2+ core in pMMO? Toꢀ
wards that end, the CuꢀOꢀCu bend plays a minor
Detailed materials and methods, crystalloꢀ
graphic information, determination of the pKa
of 1, and computational methods. (pdf).
AUTHOR INFORMATION
Corresponding Author
matthew.kieberꢀemmons@utah.edu
ORCID
Matthew T. KieberꢀEmmons: 0000ꢀ0002ꢀ6357ꢀ5579
Notes
The authors declare no competing financial interests.
role (~2 kcal/mol) based on DFT of
2 (Figure S21 &
S22).18 Other factors will likely include a combinaꢀ
tion of charge and nature of the donors, coordinaꢀ
tion number, and geometry, all of which are yet to
be explored.
ACKNOWLEDGMENTS
The authors acknowledge Nathan Buehler for the synꢀ
thesis and crystallization of 1, Asmita Shrestha for
early observations on the deprotonation of 1, and Arif
Atta for solving the structure of 1. Computer time was
provided by the Extreme Science and Engineering
Discovery Environment (XSEDE), which is supported
by National Science Foundation grant number ACIꢀ
1053575 (TGꢀCHE130047), and the Center for High
Performance Computing at the University of Utah.
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Figure 6. A: Reaction of 10 eq TEMPOH with 2 (—)
prepared in situ by addition of 2 eq DBU to 1 in MeCN
at –30°C, time between lines is 10 sec; B: change in
A350 vs time for reaction of 2 with TEMPOH (—) and
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In summary, we have synthesized a dicopper(II)ꢀ
μꢀOH adduct that enabled for the first time the deꢀ
termination of hydrogen atom abstraction thermoꢀ
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18. Mahyuddin, M. H., Staykov, A., Shiota, Y., Miyanishi, M.,
and Yoshizawa, K. ACS Catal. 2017, 7, 3741–3751.
Authors are required to submit a graphic entry for the Table of Contents (TOC) that, in conjunction with the manꢀ
uscript title, should give the reader a representative idea of one of the following: A key structure, reaction, equaꢀ
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for TOC graphic specifications.
H
O
LCuII CuIL
O
CuIL
LCuII
3
4
B
D
F
E
=
7
E0 = –0.48 V
(vs Fc/Fc+)
7
.
2
k
c
a
l
m
o
-
1
l
H
O
pKa(OH) = 24.3
LCuII CuIIL
LCuII
CuIIL
O
{[(tmpa)Cu]2ꢀ(ꢁꢀOH)}3+
1
2
ACS Paragon Plus Environment