T.M. Khazanov et al.
Inorganica Chimica Acta 516 (2021) 120168
Recently we reported experimental and theoretical results that
carried out with the density functional BP86 [31,32] using the crystal
structures of 1 and 2 as a starting point. Calculations employed the def2-
TZVP basis set [33,34] for copper and directly coordinated atoms;
remaining atoms were calculated using the def2-SV(P) basis set [35].
Dispersion effects were included using the atom-pairwise correction
with Becke-Johnson damping scheme (D3BJ) as it has been adopted in
the ORCA program [36,37]. The density fitting and “chain of spheres”
(RIJCOSX) [38] approximations were employed in conjunction with the
auxiliary basis set def2/J [39]. Stationary points were verified by ana-
lytic computation of vibrational frequencies. Damping parameters were
adjusted as needed in order to reach convergence using the keyword
SlowConv. Electron paramagnetic resonance (EPR) parameters were
obtained from single-point calculations on the previously optimized
geometries using the hybrid density functional B3LYP [40,41]. Scalar
relativistic effects were included with the zeroth order regular approx-
imation (ZORA) [42–44] employing the core property basis set CP(PPP)
on copper, the ZORA-def2-TZVP(-f) basis set [33,34] on nitrogen and
oxygen atoms, and the ZORA-def2-SVP basis set [35] on all remaining
atoms. Auxiliary basis sets were generated automatically (AutoAux)
[45]. Picture change effects were included. Electronic absorption
spectra were calculated for 1–3 within the time-dependent density
functional theory (TDDFT) formalism employing the Tamm-Dancoff
approximation (TDA) [46] with the same functional and combination
of basis sets. Consistent with experiment, solvation effects associated
with acetonitrile were accounted for by using the conductor-like
polarizable continuum model (CPCM) as implemented in ORCA [30].
+
implicated a [CuO] species as the reactive intermediate for C
–
H bond
activation [25]. Reactive intermediates were generated from mono-
II
nuclear Cu complexes supported by tetradentate polypyridyl ligands
2 2
with H O in an analogous manner to the biological peroxide shunt
pathway, bypassing the need for external electrons [26]. In these ex-
periments, substrate oxidation was observed when each copper complex
was treated with H
CHD) or dihydroanthracene (DHA). On the basis of isotope labeling
studies, spectroscopic analysis of degradation products, and density
2 2
O and base in the presence of either cyclohexadiene
(
functional theory (DFT) calculations, a mechanism for C
–
H bond
+
oxidation by a [CuO] species largely analogous to the radical rebound
mechanism of other high-valent metal-oxo species was proposed [27].
Herein we report the reactivity of three mononuclear copper com-
plexes supported by bispicen ligand variants (Fig. 1) toward C H bond
–
activation. This study further refines our understanding of the electronic
+
structure of the [CuO] species generated from 1 to 3 with H
2
O
2
. In
addition to C H bond activation, we use an ab initio quantum me-
–
chanical investigation into the electronic structure of the proposed
+
[
CuO] intermediates and the knowledge of the electronic structure of
well-understood epoxidation catalysts to predict complexes 1–3 as being
able to perform O-atom transfer to alkenes to form epoxides. As a proof
of concept, we further support this claim with reactivity studies of 1–3
toward exogenous alkene substrates.
2
. Experimental
+
+
+
The geometries of [1-O] , [2-O] , and [3-O] were optimized on the
broken symmetry (BS) potential energy surface. Single-point calcula-
tions were carried out to obtain singlet–triplet gaps and quasi-restricted
orbitals (QROs) using the same combination of basis sets described
2
.1. Materials and methods
All reagents and solvents were purchased from MilliporeSigma and
Fisher Scientific and directly used for synthesis without further purifi-
cation unless mentioned otherwise. Copper complexes and ligands were
synthesized following reported procedures [28,29]. 9,10-dihydroan-
thracene (DHA) was purchased from MilliporeSigma and recrystallized
from ethanol prior to use. All electronic absorption spectra were
measured using an Agilent Cary 8454 UV–Vis Spectrophotometer.
Samples for optical spectroscopy were prepared in quartz cells with an
optical path length of 1 cm. Graphical representation and data analysis
were performed in Origin. Quantification of the oxidized products was
carried out using an Agilent 6890 Gas Chromatograph (GC) equipped
with an Agilent 5973 Network Mass Selective Detector and Flame
Ionization Detector (FID) using naphthalene as an internal standard.
above at the B3LYP, M06 [47], and
ω
B97X [48] levels of theory. To
correct for biradical character in the singlet structures, wavefunctions
were calculated employing the complete active space self-consistent
field (CASSCF) [49] method. In the CASSCF calculations, an active
space consisting of twelve electrons in seven orbitals was chosen (CAS
+
+
(12,7)). QROs from the single-point calculations of [1-O] , [2-O] , and
+
[3-O] were used as starting orbitals for the CASSCF calculations.
Dynamical correlation effects were explicitly introduced as described by
second-order N-electron valence perturbation theory (NEVPT2) to
obtain singlet–triplet splittings [50,51]. To better account for dynamical
correlation effects, the Cu-4d orbitals were also included, giving rise to a
final active space of 12 electrons in 12 orbitals (CAS(12,12)) for the
CASSCF/NEVPT2 calculations.
Alternative active spaces and multireference configuration interac-
tion procedures were explored. An active space consisting of fourteen
electrons in the five Cu-3d based molecular orbitals (MOs) and three O-
2
.2. General procedure for substrate oxidation
Forty equivalents of substrate with respect to the complex were
2
p orbitals was chosen (CAS(14,8)). Orbitals with an occupation less
added to solutions of complexes 1–3, followed by the addition of H
and NEt solutions in acetonitrile (10 eq. with respect to the complex)
2 2
O
than 1.98 and greater than 0.02 from the CAS(14,8) calculations were
used to construct reference spaces of 10 electrons in 6 orbitals (10,6) for
subsequent spectroscopy oriented configuration interaction calculations
3
under aerobic conditions at 298 K. Reaction progress was monitored via
electronic absorption spectroscopy and products were identified after
removal of the copper complex using a microscale alumina column.
Product yields were calculated with respect to the copper complexes.
(
SORCI) [52]. While (8,5) reference spaces may have been sufficient for
+
+
[
1-O] and [3-O] , a (10,6) reference space was required to describe the
+
electronic structure of [2-O] based on orbital occupation, so (10,6)
reference spaces were constructed for all three compounds for consis-
tency. All CASSCF and SORCI calculations were carried out with the
ZORA-def2-TZVP(-f) basis set on copper, nitrogen, and oxygen atoms
and the ZORA-def2-SVP basis set on all other atoms.
2
.3. Computational methods
All computations were carried out using the computational chemis-
try software package ORCA 4.2 [30]. Geometry optimizations were
2
.4. X-ray crystallography
Single-crystal X-ray diffraction was performed on a Rigaku-Oxford
Diffraction Synergy-S diffractometer equipped with a HyPix detector
and a Mo-K
α
radiation source (λ = 0.71073 Å). In a typical experiment, a
single crystal was suspended in Parabar® oil (Hampton Research) and
mounted on a cryoloop that was cooled to the desired temperature in an
◦
Fig. 1. Copper complexes studied in the present work.
N
2
cold stream. The data sets were recorded as
ω
-scans at 0.5 step width
2