Effects of denticity and ligand rigidity on reactivity of copper complexes with cumyl hydroperoxide

Article abstract of DOI:10.1016/j.ica.2018.07.035

Cu(II) complexes bearing N2/Py2 tetradentate ligands consisting of two pyridyl arms and a flexible ethyldiamine backbone, [(BPMEN)Cu(ClO₄)₂] (1), with rigid cyclohexyl backbone [(BPMCN)Cu(ClO₄)₂] (2), and substituted with bispyrrolidyl [(PDP)Cu(ClO₄)₂] (3), and Cu(II) complex bearing N2/Py3 pentadentate ligand, [(TPMEN)Cu(ClO₄)₂] (4), were synthesized and structurally characterized. Reactivity of 1–4 with cumyl hydroperoxide was investigated to study the effects of ligand rigidity and denticity on the mechanism of O–O bond cleavage. Results presented herein illustrate that 1–3 favors homolysis, however 4 showed little to almost no impact on O–O bond cleavage.

Full text of DOI:10.1016/j.ica.2018.07.035

Accepted Manuscript
Research paper
Effects of Denticity and Ligand Rigidity on Reactivity of Copper Complexes
with Cumyl Hydroperoxide
Bruce J. Pella, Jens Niklas, Oleg G. Poluektov, Anusree Mukherjee
PII:
S0020-1693(18)30916-2
https://doi.org/10.1016/j.ica.2018.07.035
ICA 18381
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
14 June 2018
22 July 2018
23 July 2018
Please cite this article as: B.J. Pella, J. Niklas, O.G. Poluektov, A. Mukherjee, Effects of Denticity and Ligand
Rigidity on Reactivity of Copper Complexes with Cumyl Hydroperoxide, Inorganica Chimica Acta (2018), doi:
https://doi.org/10.1016/j.ica.2018.07.035
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Effects of Denticity and Ligand Rigidity on
Reactivity of Copper Complexes with Cumyl
Hydroperoxide
Bruce J. Pella1, Jens Niklas2, Oleg G. Poluektov2, Anusree Mukherjee1*,
1. Department of Chemistry, University of Alabama in Huntsville, 301 Sparkman Dr NW,
Huntsville, AL 35899
2. Chemical Science and Engineering Division, Argonne National Laboratory, 9700 S. Cass
Avenue, Lemont, IL 60439
KEYWORDS: Copper coordination chemistry, oxidation reactions, complex characterization,
O-O bond cleavage mechanism, cumyl hydroperoxide
ABSTRACT
Cu(II) complexes bearing N2/Py2 tetradentate ligands consisting of two pyridyl arms and a
flexible ethyldiamine backbone, [(BPMEN)Cu(ClO4)2] (1), with rigid cyclohexyl backbone
[(BPMCN)Cu(ClO4)2] (2), and substituted with bispyrrolidyl [ (PDP)Cu(ClO4)2] (3), and Cu(II)
complex bearing N2/Py3 pentadentate ligand, [(TPMEN)Cu(ClO4)2] (4), were synthesized and
structurally characterized. Reactivity of 1-4 with cumyl hydroperoxide was investigated to study
the effects of ligand rigidity and denticity on the mechanism of O-O bond cleavage. Results
presented herein illustrate that 1-3 favors homolysis, however 4 showed little to almost no
impact on O-O bond cleavage.

Introduction
Synthetic copper-coupled oxidations have attracted considerable attention in multiple
scientific disciplines due in part to their prevalence in biological systems and capability to
perform diverse functional group transformations.1-3 The diversity of biological and synthetic
Cu-coupled oxidation processes demonstrates Cu center reactivity can be tuned with coordinated
ligands through steric and electronic effects.4 Among many important roles exhibited by Cu in
biology of particular interest are two noncoupled binuclear copper enzymes, dopamine-β-
monooxygenase (DβM) and peptidyl-α-hydroxylating monooxygenase (PHM), due to their
ability to activate O2 and accomplish hydroxylation of carbon-hydrogen bonds of their respective
substrates.5,6 Although the mechanism of enzymatic action for PHM and DβM has been
thoroughly investigated, the identity and formation of the reactive intermediate responsible for
C-H bond hydroxylation is still under debate.7 Particularly the mechanism of the cleavage of the
O-O bond by mono-nuclear Cu-center remains to be established.
In literature synthetic metal complexes and their reactivity toward oxygen has shed
important insights about enzyme mechanism. Moreover, metal alkylperoxo complexes have been
synthesized to understand mechanism of O-O bond scission (Figure 1). Investigations into
Cu(II)-alkylperoxo species may elucidate reaction pathways involved in dioxygen activation by
enzymes and synthetic small molecule copper complexes. In 1993, Kitajima et al. characterized
the first crystal structure for monomeric Cu-alkylperoxo intermediate (LCu(II)-OOR, where R =
Cm) by reacting cumyl hydroperoxide (CmOOH) with a bis(µ-hydroxo)copper(II) complex,
[CuII(HB(3,5-iPr2pz)3)]2(OH)2 in pentane/octane mixture.8 Since then, considerable research with
LCu(II)-OOR complexes has been conducted in order to determine the relationship between
structure and reactivity.9-14 These LCu(II)-OOR complexes have been generated at sub-zero

temperatures and display electrophilic reactivity with external substrates such as 1,4-
cyclohexadiene upon thermolytic cleavage of the peroxide (O-O) bond.
Figure 1: LCuII(ClO4)2 homolytic and heterolytic O-O bond cleavage mechanisms.
In this study, we investigated the reactivity of cumyl hydroperoxide with three Cu(II)
complexes bearing bispicen(N2/Py2) ligands and one Cu(II) complex bearing an N2/Py3 ligand.
In order to examine the effect of ligand strain on O-O bond cleavage, the three Cu(II) complexes
with N2/Py2 ligands were strategically designed with varied backbone rigidity. The Cu(II) center
of [Cu(BPMEN)(ClO4)2] (1, Figure 2) is ligated to two pendant arms bearing heterocyclic
pyridines and to two tertiary amines of a flexible ethyldiamine backbone. In
[Cu(BPMCN)(ClO4)2] (2), the ethyldiamine backbone was substituted with a cyclohexyl variant
and [Cu(PDP)(ClO4)2] (3) was designed with rigid cyclic substituents to replace the flexible

ethyldiamine backbone present in 1. Additionally, an N5-pentadentate Cu complex bearing an
additional pendant arm, [Cu(TPMEN)(ClO4)2] (4), was designed to evaluate the effect of
increased denticity on O-O bond cleavage pathway.
Figure 2. N-polydentate copper complexes, [Cu(BPMEN)(ClO4)2] (1), [Cu(BPMCN)(ClO4)2]
(2), [Cu(PDP)(ClO4)2] (3) and [Cu(TPMEN)(ClO4)2] (4) presented in this work.
Experimental Methods
1.1 Materials
All chemicals and solvents used for syntheses and reactivity studies were acquired from
standard commercial suppliers (Fisher Scientific and Millipore Sigma) and used without

1
further purification. H-NMR spectra were recorded with a 500 MHz Varian INOVA
spectrometer equipped with a 5 mm Penta (H, C, N, P, D) PFG VT probe using listed
deuterated solvents at 25 °C. Mass spectra were recorded with a LTQ Orbitrap XL Hybrid FT
Mass Spectrometer in positive ion mode using MeCN or MeOH as solvent. UV-vis
absorption spectra were recorded using an Agilent Cary 8454 UV-vis spectrophotometer
with dual deuterium and tungsten lamps equipped with Unisoku USP-203 cryostat. Reaction
products were characterized and identified using an Agilent 6890 Gas Chromatograph
equipped with Agilent 5973 Network Mass Selective Detector and Flame Ionization
Detector. Continuous wave (CW) X-band (9 - 10 GHz) EPR experiments were carried out
with a Bruker ELEXSYS II E500 EPR spectrometer (Bruker Biospin, Rheinstetten,
Germany), equipped with a TE102 rectangular EPR resonator (Bruker ER 4102ST). A helium
gas-flow cryostat (ICE Oxford, UK) and an intelligent temperature controller (ITC503;
Oxford Instruments, UK) were used for measurements at cryogenic temperatures (T = 50 K).
Modulation amplitude was 0.6 mT and microwave power was 0.5 mW. Data processing was
done using Xepr (Bruker BioSpin, Rheinstetten) and Matlab 7.11.2 (The MathWorks, Inc.,
Natick) environment. Simulations were performed using the EasySpin software package.15
Elemental analyses were conducted by Atlantic Microlab, Inc. located in Norcross, GA. X-
Ray crystallographic data were collected using a XtaLAB Synergy, Dualflex, HyPix
diffractometer equipped with Oxford Cryosystems low-temperature device, operating at T =
100.01(10) K, located at the Emory University – X-Ray crystallographic facility.
Caution! Although no problems occurred in this work, perchlorate salts mixed with
organic compounds can be explosive. All necessary safety precautions should be
followed.

1.2 Synthesis BPMEN
Synthesis of [N,N’-Dimethyl-N,N’-bis-(pyridine-2-ylmethyl)-1,2-diaminoethane] was taken
from a previously reported procedure.16 2-(chloromethyl)pyridine hydrochloride (1.501 g,
9.15 mmol) dissolved in 5 mL deionized (DI) water was added dropwise to an aqueous
solution containing K2CO3 (2.556 g, 18.49 mmol) dissolved in 7.5 mL DI water. The
resulting mixture was stirred for 30 minutes. The mixture was extracted with CH2Cl2 (3 × 10
mL). The organic phase was collected and dried with anhydrous Na2SO4. The dried solution
was concentrated in vacuo to afford orange oil. A solution containing N,N’-
dimethylethylenediamine (0.471 mL, 4.38 mmol) in 15 mL CH2Cl2 was added dropwise to
the aforementioned orange oil dissolved in 5 mL CH2Cl2. An aqueous solution containing
NaOH (0.311 g, 7.78 mmol) dissolved in 7.6 mL DI water was slowly added to organic
mixture and stirred at room temperature. After 60 hours, a second portion of NaOH solution
(0.318 g, 7.95 mmol) was quickly added to the mixture. The combined mixture was extracted
with CH2Cl2 (3 × 20 mL) and dried with anhydrous Na2SO4. The organic solution was
1
concentrated in vacuo to afford a brown oil, BPMEN (Yield: 0.631 g, 2.33 mmol, 70%) H-
NMR (500 MHz, CD2Cl2) δ 8.46 (dt, 2H, pyridine ring), 7.80 (m, 2H, pyridine ring), 7.51
(m, 2H, pyridine ring), 7.30 (m, 2H, pyridine ring), 3.70 (m, 4H, -CH2), 2.66 (m, 4H, -CH2),
2.27 (s, 6H, -CH3). ESI-MS (MeOH). Observed m/z 271.25 [BPMEN+H+] (z = 1); simulated
m/z 271.19.
1.3 Synthesis of BPMCN
Synthesis of [N1,N2-Dimethyl-N1,N2-bis(pyridin-2-ylmethyl)cyclohexane-1,2-diamine] was
achieved following a previously reported procedure.17 1,2-dimethylcyclohexane-1,2-diamine
(164 mg, 1.15 mmol) was dissolved in MeCN (15 mL) in a round-bottom flask.

Triethylamine (780 μL, 5.60 mmol) was added to the solution followed by the addition of 2-
(chloromethyl)-pyridine (375 mg, 2.28 mmol). The combined solution was refluxed for 18
hours then cooled to room temperature and concentrated in vacuo to yield a crude solid. The
crude solid was dissolved in 30 mL CH2Cl2 and washed with saturated NaHCO3 (aq, 30 mL).
The aqueous wash was extracted with CH2Cl2 (2 × 30 mL). The combined organic phase was
dried with anhydrous Na2SO4. The dried solution was concentrated in vacuo to yield a brown
oil. The crude product was purified using silica gel chromatography with 86% EtOAc/ 10%
MeOH/ 4% NH4OH to yield a light brown oil, BPMCN. (Yield: 0.143 g, 0.400 mmol, 38%)
1H-NMR (500 MHz, CDCl3) δ 8.48 (t, 2H, pyridine ring), 7.57 (m, 2H, pyridine ring), 7.11
(m, 4H, pyridine ring), 3.81 (m, 4H, -CH2), 2.65 (m, 2H, -CH), 2.28 (s, 6H, -CH3), 2.01 (m,
2H, -CH2), 1.75 (m, 2H, -CH2). ESI-MS (MeCN). Observed m/z 325.24 [BPMCN+H+] (z =
1); simulated m/z 325.24.
1.4 Synthesis of PDP
Synthesis of [(-)-2-(((S)-2-((S)-1-(pyridine-2-yl)pyrrolidin-1-yl)methyl)pyridine] was taken
from a previously reported procedure.18 A 50 mL round-bottom flask was charged with (S,S)-
2,2’-bispyrrolidine tartrate (1.007 g, 2.92 mmol), 30 mL DI water, and 30 mL CH2Cl2. Solid
NaOH pellets (0.887 g, 22.18 mmol) were added and dissolved. 2-picolyl chloride•HCl
(1.245 g, 7.59 mmol) was dissolved in the solution. After stirring for 18 hours at room
temperature, the reaction mixture was diluted with 1 M NaOH. The aqueous layer was
extracted with CH2Cl2 (3 × 50 mL). The combined organic phase was dried with anhydrous
MgSO4 and concentrated in vacuo to afford a brown oil. The crude ligand was purified using
silica gel chromatography with 93% CH2Cl2/ 5% MeOH/ 2% NH4OH, washed with 1 M
NaOH, dried over anhydrous MgSO4, and concentrated in vacuo to yield a yellow oil, PDP.

(Yield: 0.454 g, 1.41 mmol, 48%) 1H-NMR (500 MHz, CDCl3) δ 8.50 (m, 2H, pyridine ring),
7.60 (td, 2H, pyridine ring), 7.40 (d, 2H, pyridine ring), 7.11 (dd, 2H, pyridine ring), 4.19 (d,
2H, -CH2), 3.51 (d, 2H, -CH2 ), 3.00 (p, 2H, pyrrolidine –CH2), 2.80 (p, 2H, pyrrolidine –
CH2), 2.24 (q, 2H, pyrrolidine –CH2), 1.72 (m, 8H, pyrrolidine –CH2). ESI-MS (MeCN).
Observed m/z 323.22 [PDP+H+] (z = 1); simulated m/z 323.22.
1.5 Synthesis of TPMEN
Synthesis of [N,N,N’-Tris(2-pyridylmethyl)-N’-methylethylenediamine] was taken from a
previously reported procedure.19 2-Picolyl chloride hydrochloride (2.99 g, 30.4 mmol) was
dissolved in 23 mL 10 N NaOH (9.19 g, 230 mmol) aqueous solution. N-
Methylethylenediamine (0.473 mL, 38.2 mmol) was added dropwise to the mixture which
was then stirred at room temperature for three days. The upper oil layer was separated and
dissolved in 20 mL CH2Cl2, washed with DI water (3 × 20 mL). The organic solution was
dried with anhydrous MgSO4 and concentrated in vacuo to yield a brown oil. The crude
ligand was purified using silica gel chromatography with 90% CHCl3/ 10% MeOH and
concentrated in vacuo to yield a yellow oil, TPMEN. (Yield: 0.174 g, 0.500 mmol, 13%). 1H-
NMR (500 MHz, CD2Cl2) δ 8.49 (m, 3H, pyridine ring), 7.64 (m, 6H, pyridine ring), 3.83
(m, 4H, -CH2), 3.64 (s, 2H, -CH2), 2.72 (m, 4H, -CH2), 2.21 (s, 3H, -CH3). ESI-MS (MeOH).
Observed m/z 348.33 [TPMEN+H+] (z = 1); simulated m/z 348.22.
1.6 Synthesis of [Cu(BPMEN)(ClO4)2] (1)
The ligand BPMEN (0.271 g, 1.00 mmol) was dissolved in 10 mL MeOH and combined with
a 5 mL MeOH solution containing Cu(II) (ClO4)2 •6H2O (0.370 g, 1.00 mmol) and stirred for

30 minutes. The combined solution was allowed to slowly evaporate to produce
[(BPMEN)Cu(ClO4)2]. (Yield: 0.307 g, 0.576 mmol, 58%) UV-vis (MeCN) λmax 624 nm (ε =
260 M-1 cm-1). ESI-MS (MeCN). Observed m/z 333.11 [(BPMEN)Cu] (z = 1), 432.06
[(BPMEN)Cu(ClO4)] (z = 1) ; simulated m/z 333.11, 432.06.
1.7 Synthesis of [Cu(BPMCN)(ClO4)2] (2)
The ligand, (BPMCN) (0.143 g, 0.441 mmol) was dissolved in 3 mL MeOH and combined
with a 3 mL MeOH solution containing Cu(II) (ClO4)2 •6H2O (0.163 g, 0.440 mmol) and
stirred for two hours. A light blue precipitate was filtered and washed with Et2O and dried in
vacuo to produce [(BPMCN)Cu(ClO4)2]. (Yield: 0.125 g, 0.211 mmol, 48%) UV-vis
(MeCN) λmax 606 nm (ε = 255 M-1 cm-1). ESI-MS (MeCN): Observed m/z 486.11
[(BPMCN)Cu(ClO4)] (z = 1); simulated m/z 486.11. Elemental Analysis: Observed - C,
40.96; H, 4.74; Cl, 11.93; N, 9.58. Calculated for C20H28Cl2CuN4O8 - C, 40.93; H, 4.81; Cl,
12.08; N, 9.55.
1.8 Synthesis of [Cu(PDP)(ClO4)2] (3)
The ligand, (PDP) (0.454 g, 1.41 mmol) was dissolved in 5 mL MeOH and combined with a
5 mL MeOH solution containing Cu(II) (ClO4)2 •6H2O (0.532 g, 1.41 mmol) and stirred for
two hours. A purple precipitate was filtered and washed with Et2O and dried in vacuo. The
purple precipitate was dissolved in MeOH and recrystallized by slow diffusion with THF to
produce [(PDP)Cu(ClO4)2]. (Yield: 0.487 g, 0.833 mmol, 59%) UV-vis (MeCN) λmax 587 nm
(ε = 211 M-1 cm-1). ESI-MS (MeCN): Observed m/z 484.09 [(PDP)Cu(ClO4)+] (z = 1);

simulated m/z 484.09. Elemental Analysis: Observed – C, 39.96; H, 4.52; Cl, 11.70; N, 9.37.
Calculated for [CuPDP](ClO4)2•(H2O) – C, 39.84; H, 4.68; Cl, 11.76; N, 9.29.
1.9 Synthesis of [Cu(TPMEN)(ClO4)2 (4)
The ligand, (TPMEN) (0.161 g, 0.463 mmol) was dissolved in 10 mL MeOH and combined
with a 10 mL MeOH solution containing Cu(II) (ClO4)2 •6H2O (0.177 g, 0.477 mmol) and
stirred for 30 minutes. A light blue precipitate was filtered and washed with Et2O and dried
in vacuo to produce [(TPMEN)Cu(ClO4)2]. (Yield: 0.161 g, 0.264 mmol, 57%) UV-vis
(MeCN) λmax 687 nm (ε = 268 M-1 cm-1). ESI-MS (MeCN): Observed m/z 509.09
[(TPMEN)Cu(ClO4)] (z = 1); simulated m/z 509.09.
1.10
Cumyl hydroperoxide reactivity at low temperature
Anhydrous MeCN was purged with N2 gas with stirring for 30 minutes to establish anaerobic
conditions. To a solution containing the 1 (1 mM) in anhydrous MeCN in a quartz 1 cm path
length UV-vis cell sealed with a rubber septum and purged with N2 gas placed in cryostat
holder at -20 °C for several minutes to reach thermal equilibrium (± 0.5 °C), a solution
containing triethylamine (1 mM) in anhydrous MeCN was injected with gastight syringe
followed by an injection of a solution containing CmOOH (1 mM) in anhydrous MeCN with
gastight syringe. The reaction was monitored for UV-vis spectroscopic changes. The final
product solution, where the peak at 385 nm has reached maximum intensity, was taken for an
ESI-MS analysis. An aliquot of reaction solution (100 μL) was diluted with anhydrous
MeCN (1 mL) and stored at -20 °C. Mass spectra were taken using ESI-MS in positive ion
mode, from this cold solution.

In preparation for GC-FID analysis, the reaction solution was warmed to 50 °C until the λmax
= ~380 nm peak completely decayed. To this final solution anisole was added as an internal
standard (4.5 mM). This solution was passed through a mini-silica column to remove the
metal complex and eluted with MeCN (3 mL). The resulting solution was further diluted with
MeCN in 1:5 ratio and measured using GC-FID to determine concentration of acetophenone
(AP) and cumyl alcohol (CmOH). For 2-4 similar procedure was adopted.
1.11
X-ray crystallography
Single blue plate-shaped crystals of 3, [(PDP)Cu(ClO4)2] were recrystallized from methanol
by slow evaporation. A suitable crystal (0.18×0.11×0.07 mm3) was cut from a larger
specimen and mounted on a loop with paratone oil on a XtaLAB Synergy, Dualflex, HyPix
diffractometer. The crystal was cooled to T = 100.01(10) K during data collection. The
structure was solved with the ShelXT20 structure solution program using the Intrinsic Phasing
solution method and by using Olex221 as the graphical interface. The model was refined with
version 2014/7 of ShelXL-2014 using Least Squares minimisation.20,22

2. Results and Discussion
Synthesis and Characterization
The syntheses of ligands were performed by following published procedures.16-19 Copper
complexes supported by polydentate amine ligands are known in literature and 1-4 were obtained
using similar techniques.16,23 The mixing of equimolar amounts of ligand and copper perchlorate
salt in methanol at room temperature resulted solid precipitation that was isolated by filtration.
All complexes 1-4 were analyzed using different spectroscopic and analytical techniques. Crystal
structures of complexes 1 and 2 are reported in literature and they were both isolated in square
pyramidal (SP) geometry (Table 1). A previously reported crystal structure of 1 displays the
flexible N2/Py2 tetradentate ligand coordinates with the Cu(II) metal center in a distorted square
pyramidal geometry (τ = 0.36; where τ is distortion index, and typically defined as 0 for square
pyramidal geometry and 1 for trigonal bipyramidal geometry24).16 The crystal structure of 2
displays a square pyramidal geometry (τ = 0.02) with four nitrogen atoms occupying equatorial
positions and an oxygen atom from H2O occupying an axial position.25 Single crystals of
complex 3 suitable for X-ray diffraction resulted from a solution of methanol – tetrahydrofuran
at room temperature. Single blue plate-shaped crystals of 3 were isolated and X-ray
crystallographic analysis revealed 3 adopts a distorted square pyramidal geometry (τ = 0.17) with
four nitrogen atoms occupying equatorial positions and an oxygen atom from H2O occupying an
axial position (Figure 3). To our knowledge this is the first example of Cu-complex supported by
PDP ligand and perchlorate counter ion. The Cu-N bond lengths of complex 3 are consistent with
reported N4-tetradentate Cu(II) complexes with distorted square pyramidal geometry (Table 1
and Table S2).26-28 Despite exhaustive efforts, crystals suitable for X-ray crystallographic
analysis have not been obtained for 4.

Table 1: Selected structural parameters for complexes, 1-3.
Complex
1
2
3
Ligand
BPMEN
BPMCN
PDP
Bond Length (Å)
Cu-Namine
2.026(2)
2.025(2)
1.990(2)
1.998(1)
2.011(4)
2.010(4)
1.994(4)
1.987(4)
2.002(6)
2.030(5)
2.023(6)
1.996(6)
Cu-Npyridine
Bond Angle(Degrees)
a
Cu-O1
2.245(2)
2.354 (3)
2.284(6)
Npyr-Cu-Namine
167.8
146.4
166.9
165.6
166.8
156.4
τ
0.36
16
0.02
25
0.17
Reference
This work
a. Cu-O1 bond distances for 1-2 derived from ClO4 and 3 derived from H2O.

Figure 3: ORTEP view of complex 3 [Cu(PDP)(ClO4)2]. Hydrogen bonding interactions
between axially coordinated water molecule and one of the perchlorate ions are shown by dotted
lines.
Optical Spectroscopy
The optical spectra of the complexes 1 and 4 in MeCN are known .16,23 Complexes 1-3 in MeCN
display broad absorption bands in the visible region at 625nm, 606 nm and 587 nm (Figure 4,
Table 2), respectively, that correspond to d-d transitions. Complexes 1-3 may coordinate with
MeCN in solution to yield a five coordinate complex. The UV-vis spectrum of 4 in MeCN
displays a maximum absorption band at 687 nm. While the assignment of coordination geometry

in solution is difficult to determine via optical spectroscopy, the electronic spectra of complexes
1–3 are consistent with reported mononuclear Cu(II) complexes in similar ligand
environment.29,30 Electronic spectra of 4 have been published elsewhere and are consistent with
previously reported Cu(II) complexes with N5-pentadentate ligand.23,31-34
Figure 4: UV-vis absorption spectra of 1-4 (1 mM) in MeCN at room temperature with
optical pathlength of 1 cm..

Table 2: Optical and Magnetic parameters for complexes 1 - 4
Complex
Absorption
Absorption
Maxima nm, ( 
g valuesa
Cu hyperfine coupling
constants (10-4 cm-1)
Maxima nm, ( 
M-1 cm-1) /MeOH M-1 cm-1) /MeCN
g║
g┴
A║
175
180
186
A┴
31
20
20
624 (233)
609 (268)
593 (191)
625 (260)
606 (255)
587 (211)
2.213
2.216
2.212
2.053
2.051
2.047
1
2
3
4
687 (242),
910 (sh)
687 (268),
907 (sh)
2.209, 2.092,
2.023
158, 50, 35
a. All EPR measurements were performed in DCM:MeCN solvent mixture (3:1). Cu hyperfine coupling
constants are given in 10-4 cm-1 for the 63Cu isotope. The perpendicular hyperfine coupling constants have a
larger error than the parallel hyperfine coupling constants, since the hyperfine structure is not resolved. It was
assumed for the simulation that the principal g-tensor axes are collinear with the Cu hyperfine coupling tensor
principal axes.
EPR Spectroscopy
EPR spectra of complex 1 was reported from our group.16 EPR spectra for complex 2-4
were obtained from frozen solution of the complexes in the same solvent system as in our
previous study (DCM:MeCN, 3:1, by volume). All three spectra are characteristic of
mononuclear Cu(II) complexes in d9 configuration with S = ½ (Fig. 4). The frozen solution
EPR spectra of all Cu(II)-complexes showed a g|| value (>2.2) significantly higher than the

other two g-values (<2.1) and resolved Cu hyperfine structure for A|| (Cu has two magnetic
63
65
isotopes, Cu and Cu, both with nuclear spin I = 3/2, which have very similar magnetic
moments and are not resolved in our spectra). Both complexes 2 and 3 showed axial g-
tensors and Cu hyperfine tensors. The magnetic resonance parameters are consistent with an
electronic ground state, where the unpaired electron resides mainly in the dx2-y2 orbital.35
Complex 3 shows for the perpendicular orientation a partially resolved superhyperfine
structure, which we attribute to the four nitrogen atoms of the ligand (I = 1 for 14N). The EPR
spectrum of complex 4 exhibits some differences from those of complexes 2 & 3. The
spectrum is not axial anymore, but shows some degree of rhombic distortion. This can be
attributed to the presence of five coordinating nitrogen atoms in the ligand vs. four in the
other two complexes.

Figure 5: CW X-band EPR spectra obtained from frozen solutions (T = 50 K) of complexes
2, 3, and 4 in dichloromethane-acetonitrile solvent mixture (3:1). Spectra shown in black are
the experimental spectra. Spectra in red are the corresponding simulations. Simulation
parameters are summarized in Table 2.

Investigation of self-decomposition of cumyl hydroperoxide (CmOOH) with 1-4 at room
temperature
It is well documented in literature that metal-alkylperoxo adduct can shed light about O-O
bond cleavage mechanism.9,11,12 Quantification of CmOOH decomposition products can help
determine the nature of O-O bond cleavage. The homolytic and heterolytic cleavage of the
O-O bond yield different decomposition products; homolytic O-O bond scission yields
radical cumyl-oxyl species that undergoes β-scission to form primarily acetophenone (AP),
while heterolytic O-O bond scission yield exclusively cumyl alcohol (CmOH) (Figure 1). To
elucidate the effects of coordination and varied ligand rigidity on self-decomposition of
LCu(II)-OOCm adduct, reaction of complexes 1-4 in MeCN with excess CmOOH (5 mM) in
the presence of base, TEA (2 mM), at room temperature were monitored with UV-visible
spectroscopy. Addition of CmOOH to solutions of 1-4 charged with TEA, resulted in slow
decay of the peaks associated with the starting material. New absorption features at the 410
nm region developed over the period of 10 hours (Figure 6, S13-15). When the reaction
solutions were heated to 50 °C after peak growth at 412 - 415 nm reached maximum
intensity, no further spectral changes were observed. Furthermore, it was established that the
peak in the 414 nm region does not show significant loss of intensity if the solution was
stored at room temperature for a long period of time (24 hrs). Although the peak at 410 nm
region is similar to other known LCu(II)-OOCm adduct, no evidence of adduct formation
were noted in mass spectrometric analysis of the final solution. The extraordinary stability of
the final UV-vis spectra suggests that the species responsible for the maximum absorption
bands (λmax = 412 - 415 nm) is a decomposition product produced by the thermal
decomposition of the cumylperoxo adduct (LCu(II)-OOCm) derived from complexes 1-4.

Product analysis of this final solution showed evidence of Cm-OH as a major product in all
the cases (Table S1). When the reaction was carried out with 1 equivalent of CmOOH in
order to minimize CmOOH sole influence in the decomposition, better results were obtained
(Table 3). Comparing the results of decomposition product yields by complexes 1-4 with
CmOOH decay in absence of any Cu salt, suggests that complexes 1-3 all enhance yield of
acetophenone (AP) supporting more homolytic cleavage, with the exception of complex 4.
This is probably not very surprising as pentadentate ligand in complex 4 may prevent access
of the CmOOH toward the metal center, thus exerting no effect. In order to corroborate our
analysis about 4, CmOOH (1 mM) decomposition was probed in the presence of Cu(ClO4)2
(1 mM), which primarily produced the heterolytic product, cumyl alcohol (Table S1),
comparable to CmOOH decomposition in the presence of 4.
Figure 6: Electronic spectral changes for the reaction of 1 (1 mM) with CmOOH (5 mM) in the
presence of TEA (2 mM) in MeCN at 25 °C: initial (black), final (blue) measured in a quartz
cuvette (l = 1 cm).

Table 3: Yield of decomposition products from the reactivity of 1-4 with 1 equivalents of
CmOOH in the presence of TEA.
Low Temperature
Room Temperature
( -20 °C)
CD3CN (-20 °C)
AP CmOH
68(2) 31(6)
Complex
AP
CmOH
AP
49(2)
CmOH
51(6)
38(3)
30(2)
33(3)
26(2)
18(2)
62(3)
70(2)
67(4)
74(3)
82(3)
1
2
3
39(3)
28(1)
22(2)
61(1)
72(4)
78(2)
72(4) 28(2)
68(8) 32(2)
4
CmOOH
Reactivity of CmOOH with complexes 1-4 at low temperature
When complexes 1–4 were treated with stoichiometric CmOOH in the presence of base, TEA, in
MeCN at -20 °C under anaerobic conditions, different spectral signatures were obtained. Upon
addition of CmOOH (1 mM), the electronic spectra of 1-4 displayed new maximum absorption
bands at 397 nm, 398 nm, 398 nm, and 395 nm, respectively, with a concomitant loss of intensity
of d-d transition absorption bands, 625 nm, 606 nm, 587 nm, and 687 nm, respectively (Figure
7a, S17a-19a). The newly formed absorption bands are consistent with maximum absorption
bands for previously reported Cu(II)-cumylperoxo complexes and are attributed to a ligand to
metal charge transfer (LMCT) transitions. The mass spectrum of the reaction solution of 1 with
stoichiometric CmOOH displayed a prominent ion cluster at 484.16 m/z assigned to
[(BPMEN)Cu(OOCm)]+ (simulated: 484.19 m/z) (Figure S20). However, despite multiple
attempts mass spectrometric analysis of the product solution resulting from complexes 2-4 at low
temperature showed no evidence of an LCu(II)-OOCm adduct. When solutions of 1-4-

cumylperoxo complexes were heated to 50 °C, decay of the LMCT absorption bands occurred
(Figure 7b, S17b-19b).
Interestingly, when the product solutions of these samples were analyzed after warming to 50 °C,
different yield of the decomposition products were obtained (Table 3). Complexes 1 and 2
showed more acetophenone than observed at room temperature; however, complexes 3 and 4
showed no distinct trends in the yield. Furthermore, when these reactions of 1- 3 were performed
using 1 equivalent of CmOOH, in deuterated acetonitrile, yield of the acetophenone enhanced
further (Table 3). These observation are in accord with LCu(II)-OOCm self-decomposition
mechanism that suggest that acetophenone formation is in competition with H-atom abstraction
(Figure 1) and solvent can be a source for H-atom. Thus, deuteration of the solvent, significantly
enhances acetophenone yield. Complex 4 was not investigated in CD3CN as the reactivity trend
suggests that 4 has less to no impact with decomposition yield.
a)

b)
Figure 7: a) Electronic spectra for the reaction of 1 (1 mM) with CmOOH (1 mM) in the
presence of TEA (1 mM) in MeCN at -20 °C under anaerobic conditions: initial (black), final
(blue). b) Electronic spectra for the thermal decomposition of 1-cumylperoxo in MeCN at 50 °C
under anaerobic conditions over the course of 1 hour: initial (black), final (blue).
Table 4: Reactivity of 1-4 and previously researched Cu(II) complex with CmOOH in the
presence of TEA at low temperatures and nature of O-O bond cleavage.
Reference
Complex
λmax (nm)
383, 397
Decomposition Products
AP
CmOH
51(6)
61(1)
72(4)
78(2)
82(3)
This work
This work
This work
This work
49(2)
39(3)
28(1)
22(2)
18(2)
92
1
2
3
4
398
398
395
This work
CmOOH (1 equiv.)
12
[Cu(bpma)(MeCN)(OOCm)]
[Cu(bpa)(MeCN)(OOCm)]
[Cu(tpa)(OOCm)]
465
388
430
475
9
9
76
83
40
15
15
49
11
[Cu(CHDAP)(MeCN)(OOCm)]

Conclusion
In summary we have synthesized four Cu(II)-complexes supported by polypyridyl ligand with
different denticity and backbone rigidity. This paper reports first crystal structure of 3. Reactivity
of these complexes with regard to O-O bond cleavage were pursued using cumyl hydroperoxide
as oxidant. The results obtained in this data set suggest complex 4 has no pronounced impact on
O-O bond cleavage mechanism of CmOOH, although formation of the near UV band was
observed. Among, Cu(II)-complexes with tetradentate ligand (1–3), various observation were
noted. Complexes 1-3 all showed preference toward homolysis, the acetophenone yields are
significantly improved when solvent is deuterated. However, homolysis yield with these
complexes are significantly lower than other known literature evidence (Table 4) but comparable
to a recent publication where ligand geometries are restrained.14 This report illustrates another set
of complexes where ligand electronics and steric play important role in O-O bond scission
mechanism.
Acknowledgements
B.J.P. and A.M. acknowledge support from University of Alabama in Huntsville. This material is
based upon work supported by the National Science Foundation under CHE – 1626172 ( X-ray
crystallographic analysis). The EPR study was supported by the U.S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences,
Geosciences, and Biosciences, under contract number DE-AC02-06CH11357 at Argonne
National Laboratory (J.N. and O.G.P.).
Supporting Information

ESI-MS, NMR spectra, figures for reactivity and crystallographic data and the cif file for 3 are
available.
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



Syntheses of N-polydentate copper(II) complexes with varied rigidity and denticity
Reaction with cumyl hydroperoxide generated absorption bands from 380-420 nm
Heterolytic cleavage predominates in the presence of tetradentate copper complexes
Ligand rigidity has a minor effect on cleavage pathway