Interplay between the Conformational Flexibility and Photoluminescent Properties of Mononuclear Pyridinophanecopper(I) Complexes

Article

pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

Interplay between the Conformational Flexibility and

Photoluminescent Properties of Mononuclear

Pyridinophanecopper(I) Complexes

†

‡

́

Pradnya H. Patil, Georgy A. Filonenko, Sebastien Lapointe, Robert R. Fayzullin,

and Julia R. Khusnutdinova*^,†

^†Coordination Chemistry and Catalysis Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha,

Onna-son, Kunigami-gun, Okinawa 904-0495, Japan

^‡Arbuzov Institute of Organic and Physical Chemistry, Federal Research Center, Kazan Scientiﬁc Center, Russian Academy of

Sciences, 8 Arbuzov Street, Kazan 420088, Russian Federation

S

* Supporting Information

ABSTRACT: The macrocyclic ligand conformational behavior in solution,

solid-state structures and the photophysical properties of copper(I)

cationic and neutral mononuclear complexes supported by tetradentate

N,N′-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane ligands ^RN4 (R = H, Me,

i

ⁱBu, ^secBu, ^neoPent, Pr, Ts) were investigated in detail. Steric properties of

the alkyl group at the axial amine in the ^RN4 ligand were found to strongly

aﬀect the conformational preferences and dynamic behavior in solution.

Several types of conformational exchange processes were revealed by

variable-temperature NMR and 2D exchange spectroscopy, including

degenerative exchange in a pseudotetrahedral species as well as exchange

between two isomers with diﬀerent conformers of tri- and tetracoordinate

^RN4 ligands. These exchange processes are slower for the complexes containing bulky alkyl groups at the amine compared to

less sterically demanding analogues. A clear correlation is also observed between the steric bulk of the alkyl substituents and the

photoluminescent properties of the derived complexes, with less dynamic complexes bearing bulkier alkyl substituents exhibiting

higher absolute photoluminescence quantum yield (PLQY) in solution and the solid state: PLQY in solution increases in the

order Me < ^neoPent < ⁱBu < ^secBu ≈ ⁱPr < ^tBu. The electrochemical properties of the cationic complexes [(^RN4)Cu^I(MeCN)]X

(X = BF₄, PF₆) were also dependent on the steric properties of the amine substituent.

ligand.^45,55,56However, one of the common problems that can

limit the practical application of copper(I) complexes is their

INTRODUCTION

■

During the past decades, photoluminescent (PL) materials have

lability in solution, leading to dissociation of polynuclear species

been widely utilized as sensors, electroluminescent displays, and

probes of biological systems.¹⁻¹⁷Among these materials, d⁶and

or, in the case of heteroleptic mononuclear complexes, ligand

dissociation and exchange.^53,57−59

d⁸transition-metal complexes such as iridium(III), ruthenium-

(II), osmium(II), rhenium(I), and platinum(II) are extensively

used because of their stability, tunability, and high eﬃcien-

cies.¹⁸⁻²⁹Another important class of PL compounds contains

d¹⁰coinage metals such as silver(I), gold(I), and copper(I).³⁰⁻³⁶

Among these compounds, copper(I) complexes are of

considerable interest because of their low price and availability

as an alternative to more expensive precious metal-based PL

materials.^32,37−39Over the last several decades, a great variety of

copper(I) PL complexes have been developed that include

copper clusters, halide-bridged complexes, mono- and poly-

nuclear phosphine complexes, homo- or heteroleptic species

with diimine-type ligands, and recently developed N-hetero-

cyclic carbene and amide complexes.⁴⁰⁻⁵²A variety of strategies

were developed to control the emissive properties of copper(I)

complexes, mostly based on variation of the supporting ligand

electronic properties^42,53,54or, in some cases, control of the

conﬁgurational changes using the steric properties of the

We have recently reported a series of solution-stable, PL

mononuclear copper(I) complexes supported by the tetraden-

tate ligand N,N′-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane

(^RN4).⁶⁰These N-donor ligands are synthetically easily

accessible and allow for various structural modiﬁcations by

varying the nature of the amine substituent, while macrocycle

coordination to a Cu center leads to the formation of well-

deﬁned mononuclear complexes.⁶¹In particular, complexes A

and B (Scheme 1a) were found to be emissive in the solid state,

with the absolute photoluminescence quantum yield (PLQY)

reaching 0.78 for complex A but only 0.08 for complex B.

Moreover, complex A also showed emission in a dichloro-

methane solution at 20 °C, while complex B was not emissive.

Received: April 30, 2018

A

DOI: 10.1021/acs.inorgchem.8b01181

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Inorganic Chemistry

Article

Scheme 1. Previously Reported (^RN4)Cu^II Complexes⁶⁰

at the axial amine donor rather than their electronic properties.

Moreover, the greater ﬂexibility of complex B could contribute

t

to the absence of emission in solution in contrast to the Bu-

substituted complex A. However, the eﬀect of the ester

functional group in complex B could not be completely excluded

because the length of the ester-containing substituent is

suﬃciently long to allow “wrapping around” the metal center,

resulting in weak interaction of the metal with the ester

functional group and “exciplex” quenching.^38,51,64,65Therefore,

a comparison over a more diverse range of ligands is required to

elucidate the role of structural factors in determining the

photophysical properties of the derived complexes.

We recently reported that the ^RN4 ligand dynamic behavior in

copper(I) complexes has an important implications in the

development of mechanoresponsive polymer materials. We

showed that the analogous ^RN4-based copper(I) complexes

containing a bulky alkyl group at the amine can be covalently

attached to the polyurethane linear chain, acting as a stress-

responsive PL probe showing fast and reversible emission

intensity changes in response to tensile stress in polyurethane

ﬁlms.⁶⁶Further studies showed that this is likely due to

suppression of the nonradiative decay pathway in samples

subjected to stress, likely reﬂecting the dynamics within the

macrocyclic ligand.⁶⁶

In order to better design mechanoresponsive materials, we

decided to carry out a systematic study of the main factors that

aﬀect the photophysical properties of complexes similar to the

ones used in the copper(I)-incorporated polyurethane study.

In the current work, we investigated in detail the solid-state

structures, redox and photophysical properties, solution

dynamic behavior, and conformational equilibria in a series of

cationic and neutral copper(I) complexes supported by the

tetradentate pyridinophane-type ligands. We assessed the steric

eﬀect of the amine substituents on the conformational

preference and dynamics in the series of cationic [(^RN4)-

Cu^I(MeCN)]⁺and neutral (^RN4)Cu^II complexes where R =

hydrogen (H), methyl (Me), isobutyl (ⁱBu), sec-butyl (^secBu),

neo-pentyl (^neoPent), and isopropyl (ⁱPr) (Chart 1). In addition,

the strength of the copper(I) interaction with axial amine donors

was varied using electron-poor tosyl (Ts)-substituted complexes

[(^TsN4)Cu^I(MeCN)]⁺and (^TsN4)Cu^II (Chart 1). The

comparison of these complexes demonstrates that the steric

properties of the amine substituent are the main factors

controlling the conformational preference and dynamics in

solution as well as the photophysical properties of the derived

complexes. 2D exchange spectroscopy (EXSY) and variable-

temperature (VT) NMR spectroscopy studies also revealed

several types of exchange processes involved in the conforma-

tional equilibria in solution. For both cationic and neutral

complex series, increasing steric hindrance is associated with

Another notable feature of these complexes was the

conformational ﬂexibility of the macrocyclic ligand in

solution,^62,63leading to the formation of two isomeric

complexes (Scheme 1b). In particular, we have reported that

the cationic complexes [(^RN4)Cu^I(MeCN)]⁺(MeCN =

acetonitrile) and the neutral complexes A and B exist as two

R

isomers in solution, in which the N4 ligand binds to the Cu

center with three N donors or with all four N donors

coordinating in a κ³or κ⁴fashion, respectively.⁶⁰A comparison

of the solution behavior of A and B studied by NMR

spectroscopy showed that the tert-butyl (^tBu)-substituted

complex A exists only as a tetracoordinate species in solution

t

with a κ³-bound ^BuN4 ligand, while complex B exists in an

equilibrium between two isomers with κ³- and κ⁴-bound ligands

in a 61:39 ratio, respectively, at −35 °C. We hypothesized that

this is due to the diﬀerence in steric bulk of the alkyl substituents

Chart 1. Ligands Studied in This Work

B

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Inorganic Chemistry

Article

module: numerical absorption correction based on Gaussian

integration over a multifaceted crystal model and empirical absorption

correction based on spherical harmonics according to the point group

symmetry using equivalent reﬂections. The GRAL module and ASSIGN

SPACEGROUP routine of the WinGX suite⁷²were used for analysis of

systematic absences and space group determination. The structures

were solved by direct methods using SHELXT⁷³and reﬁned by full-

matrix least squares on F²using SHELXL.⁷⁴The non-H atoms were

reﬁned anisotropically. The H atoms were inserted at the calculated

positions and reﬁned as riding atoms. The positions of the H atoms of

the methyl groups were found using rotating group reﬁnement with

idealized tetrahedral angles. Complex 6 crystallizes with two molecules

A and B in the asymmetric cell (Z′ = 2); complex 9 crystallizes with the

molecule bisected by a mirror plane (Z′ = 0.5). N4B-pivot ^neoPent

(symmetrically independent molecule B) and N3-pivot ⁱBu substituents

of 6 and 8, respectively, are disordered into two positions. In the case of

7, the ^secBu fragment at atom N3 and the N4-^secBu moiety are involved in

substitutional disordering with diﬀerent conﬁgurations of chiral C

atoms; the racemic composition of the whole unit cell is provided by

crystallographic inversion symmetry. Interestingly, as a result of the

mentioned disorder, the noncoordinated N4 atom shows either a

distorted pyramidal (N41) or an almost planar (N42) conﬁguration.

The disorder was resolved using free variables and reasonable restraints

on geometry and anisotropic displacement parameters. Achiral

complexes 1 and 3 crystallize in the Sohncke space group P2₁2₁2₁,

and the absolute structures of the crystals were determined by means of

the Flack parameter.⁷⁵All of the compounds studied have no unusual

bond lengths and angles. Section VI of the Supporting Information

contains full experimental details regarding data collection and

structure determination.

greater preference for the tetracoordinate complexes in solution.

Moreover, steric hindrance was found to aﬀect the redox

properties of the cationic complexes. The current study

demonstrates that a synthetically simple modiﬁcation of the

axial amine steric properties leads to signiﬁcant variation of the

emissive properties. This can be used as a strategy to design

solution-stable mononuclear copper(I) complexes with macro-

cyclic N-donor ligands, by contrast to many other transition-

metal-based systems where synthetically demanding variation of

the electronic properties of the surrounding ligands is needed.

EXPERIMENTAL DETAILS

■

General Speciﬁcations. All manipulations were carried out under

an argon atmosphere using standard Schlenk and MBRAUN glovebox

techniques if not indicated otherwise. All reagents for which the

synthesis is not given were commercially available from Sigma-Aldrich,

TCI, and Nacalai Tesque and were used as received without further

puriﬁcation. Anhydrous solvents were dispensed from an MBRAUN

solvent puriﬁcation system and degassed prior to use. Anhydrous

deuterated solvents were purchased from Euriso-top and stored over 4

Å molecular sieves.

Ligands N,N′-diisopropyl-2,11-diaza[3,3](2,6)pyridinophane

i

(

^PrN4),⁶⁷N,N′-dimethyl-2,11-diaza[3,3](2,6)pyridinophane

^MeN4),⁶⁸2,11-diaza[3,3](2,6)pyridinophane (^HN4),⁶⁸and N,N′-

ditosyl-2,11-diaza[3,3](2,6)pyridinophane (^TsN4)⁶⁸were prepared

according to the literature procedures. [Cu(MeCN)₄]⁺precursors

were prepared by dissolving Cu₂O in an MeCN solvent in the presence

of aqueous HBF₄or HPF₆, followed by two consecutive recrystalliza-

tions from cold MeCN.⁶⁹NMR spectra were recorded on JEOL

ECZ400S 400 MHz and ECZ600R 600 MHz spectrometers. Chemical

shifts are referenced internally to the residual solvent signals. 2D EXSY

experiments were performed using nuclear Overhauser spectroscopy

(NOESY) pulse sequence; spin-saturation-transfer (SST) experiments

were performed according to a literature procedure.^70,71Full spectra

and complete characterization data for all complexes are available in the

Supporting Information. The signal abbreviation is as follows: s, singlet;

d, doublet; t, triplet; q, quartet; quin, quintet; sept, septet; m, multiplet;

br, broad; Ar−H, aromatic proton; quat, quaternary. Elemental analyses

were performed using an Exeter Analytical CE440 instrument. UV−vis

spectra were recorded on an Agilent Cary 60 spectrophotometer and

Fourier transform infrared (FT-IR) spectra were recorded on a Cary

630 with an attenuated-total-reﬂectance (ATR) module. The PL

measurements at varying concentrations were performed using a

Hamamatsu Quantaurus-QY plus apparatus; the measurements at 298

K were performed in degassed dichloromethane solutions and at liquid

nitrogen temperature in 2-methyltetrahydrofuran (MeTHF). The PL

lifetime was measured using the second harmonics of a Spectra-Physics

Mai Tai pulsed laser and a Hamamatsu Photonics Streak Scope camera.

The decay data were ﬁtted with a single-exponential decay function

unless speciﬁed otherwise. The quantum yield was measured using a

Hamamatsu Photonics Quantaurus-QY system, which established

variations in the absolute QY to be within 5% for solid and solution

samples (CH₂Cl₂, c = 1−5 μM) at 298 and 77 K. Cyclic voltammetry

(CV) experiments were performed using ALS/CHI electrochemical

analyzers 660E and 760E Electrochemical-grade ⁿBu₄NPF₆and

ⁿBu₄NBF₄(Fluka) were used the supporting electrolytes. Electro-

chemical measurements were performed in an argon-ﬁlled glovebox. A

platinum disk electrode (d = 1.6 mm) was used as the working electrode

and a platinum wire as the auxiliary electrode. The nonaqueous silver-

wire reference electrode assembly was ﬁlled with a 0.01 M AgNO₃/0.1

M ⁿBu₄NClO₄/MeCN solution and calibrated against Cp₂Fe (Fc).

X-ray Structure Determination. The X-ray diﬀraction (XRD)

data for the single crystals 1−10 were collected on a Rigaku XtaLab

PRO instrument (ω-scan mode) with a PILATUS3 R 200 K hybrid

pixel array detector and a MicroMax-003 microfocus X-ray tube using

Mo Kα (0.71073 Å) radiation at low temperature. Images were indexed

and integrated using the CrysAlis^Prodata reduction package. Data were

corrected for systematic errors and absorption using the ABSPACK

Synthesis of N,N′-Diisobutyl-2,11-diaza[3,3](2,6)-

pyridinophane (ⁱ^BuN4). Synthesis of 2,6-Bis[(isobutylamino)-

methyl]pyridine. To a solution of 2,6-bis(bromomethyl)pyridine

(2.1 g, 7.8 mmol, 1 equiv) in MeCN (10 mL) was added dropwise

the solution of isobutylamine (5.74 g, 78.5 mmol, 10 equiv) in MeCN

(10 mL) over 25 min at room temperature. The mixture was stirred at

room temperature for 17 h. MeCN was then removed on a rotary

evaporator, and residual waxy oil was dissolved in dichloromethane and

washed with concentrated aqueous potassium carbonate. The organic

phase was collected, dried with sodium sulfate, and concentrated to

dryness. The resulting oil was distilled under vacuum at 65 °C (120

mTorr) to give the diamine product that was used in the next step

1

without additional puriﬁcation. Yield: 1.23 g (63%). H NMR (600

MHz, 25 °C, CDCl₃): δ 7.58 (t, ³J_HH= 7.6 Hz, p-H_Py, 1H), 7.16 (d, ³J_HH

= 7.7 Hz, m-H_Py, 2H), 3.88 (s, −PyCH₂NH−, 4H), 2.46 (d, ³J_HH= 6.9

Hz, −NHCH₂CH−, 4H), 1.87 (s, NH−, 2H), 1.84−1.74 (m,

3

−CH₂CH-(CH₃)₂, 2H), 0.93 (d, J_HH= 6.6 Hz, −CH(CH₃)₂−,

12H). ¹³C NMR (151 MHz, 25 °C, CDCl₃): δ 159.7 (quat, C_Py), 136.9

(p-C_Py), 120.5 (m-C_Py), 57.9 (−NHCH₂CH), 55.6 (−PyCH₂NH−),

28.7 (−CH₂CH(CH₃)₂−), 20.9 (−CH(CH₃)₂−).

i

Synthesis of ^BuN4. 2,6-Bis[(isobutylamino)methyl]pyridine (1.23

g, 4.93 mmol, 1.05 equiv) was placed in a round-bottomed ﬂask

containing 20 mL of benzene and 30 mL of a 10% Na₂CO₃aqueous

solution. The mixture was heated to 80 °C, and the solution of 2,6-

bis(bromomethyl)pyridine (1.24 g, 4.69 mmol, 1 equiv) was added

dropwise to the hot reaction mixture over the period of 30 min. The

reaction mixture was heated at 80 °C for 16 h. After cooling to room

temperature, the organic phase was collected using a separatory funnel.

Evaporation of the solvent furnished a white waxy solid containing the

target compound. The crude was then extracted three times with 10 mL

of a 1/1 (v/v) dichloromethane/hexane mixture. The extracts were

then evaporated to dryness to furnish the target macrocycle. Combined

yield: 1.03 g (58%). ¹H NMR (600 MHz, 25 °C, CDCl₃): δ 7.10 (t, ³J_HH

3

= 7.5 Hz, p-H_Py, 2H), 6.76 (d, J_HH= 7.6 Hz, m-H_Py, 4H), 3.86 (s,

−PyCH₂N−, 8H), 2.56 (d, ³J_HH= 7.2 Hz, −NCH₂CH−, 4H), 1.98 (m,

−CH₂CH(CH₃)₂, 2H), 1.08 (d, ³J_HH= 6.6 Hz, −CH(CH₃)₂−, 12H).

¹³C NMR (151 MHz, 25 °C, CDCl₃): δ 158.4 (quat, C_Py), 135.5 (p-

C_Py), 122.8 (m-C_Py), 69.0 (−NCH₂CH), 64.9 (−PyCH₂N−), 27.4

(−CH₂CH(CH₃)₂−), 21.1 (−CH(CH₃)₂−). ESI-HRMS. Calcd for

C

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[C₂₂H₃₂N₄·H⁺]: m/z 353.2705. Found for C₂₂H₃₂N₄·H⁺: m/z

353.2702.

temperature. Crystalline solids were collected, washed with ether and

hexane, and dried under vacuum. Crystals suitable for X-ray analysis

were obtained by diethyl ether vapor diﬀusion into a MeCN solution of

the complex. The details are given below for individual complexes, and

the full characterization is given in the Supporting Information.

Synthesis of N,N′-Di-sec-butyl-2,11-diaza[3,3](2,6)-

^secBu

pyridinophane ( N4). ^HN4 (0.200 g, 0.83 mmol, 1.0 equiv), sec-

butyl bromide (5.7 g, 41.6 mmol, 50.0 equiv; used as racemate),

anhydrous K₂CO₃(1.38 g, 9.96 mmol, 12.0 equiv), and dry MeCN (50

mL) were charged into a 100 mL round-bottomed ﬂask with a magnetic

stirring bar. The reaction mixture was reﬂuxed under dinitrogen for 3

days. The solution was cooled to room temperature (RT) and the

solvent removed under reduced pressure. The residue was suspended in

50 mL of CH₂Cl₂and then washed with 1 M NaOH and water. The

CH₂Cl₂layer was isolated, dried over anhydrous K₂CO₃, evaporated,

and further dried under vacuum to give a pale-yellow powder. Yield:

[(ⁱ^PrN4)Cu^I(MeCN)]PF₆(1). Orange crystalline solid. Isolated yield:

111 mg (63%). At −30 °C, [(κ⁴-ⁱ^PrN4)Cu^I(MeCN)]PF₆and

[(κ³-ⁱ^PrN4)Cu^I(MeCN)]PF₆isomers were present in a CD₃CN

solution in a 93.5:6.5 ratio by NMR integration. κ⁴-1, major isomer.

¹H NMR (600 MHz, −30 °C, CD₃CN): δ 7.32 (t, ³J_HH= 7.7 Hz, p-H_Py,

3

2

2H), 6.77 (d, J_HH= 7.6 Hz, m-H_Py, 4H), 4.14 (d, J_HH= 15.3 Hz,

−PyCH₂N−, 4H), 3.60−3.53 (m, −NCH(CH₃)₂−, 2H), 3.54 (d, ²J_HH

= 15.3 Hz, −PyCH₂N−, 4H), 1.96 (s, CH₃CN, 3H), 1.31 (d, ³J_HH= 6.6

Hz, −CH(CH₃)₂−, 12H). ¹³C NMR (151 MHz, −30 °C, CD₃CN): δ

157.2 (quat, C_Py), 137.3 (p-C_Py), 122.8 (m-C_Py), 58.0 (−CH(CH₃)₂−),

208 mg (71%). ¹H NMR (400 MHz, 25 °C, CDCl₃): δ 7.08 (t, ³J_HH

=

7.7 Hz, p-H_Py, 2H), 6.74 (d, ³J_HH= 7.7 Hz, m-H_Py, 4H), 3.96 (d, ²J_HH

2

12.2 Hz, PyCH₂N−, 4H), 3.81 (d, J_HH= 12.2 Hz, PyCH₂N−, 4H),

2.95−2.87 (m, −NCH(CH₃)(CH₂CH₃), 2H), 1.83−1.72 (m,

−CHCHHCH₃, 2H), 1.52−1.41 (m, −CHCHHCH₃, 2H), 1.19 (d,

³J_HH= 6.4 Hz, NCHCH₃, 6H), 1.10 (t, ³J_HH= 7.3 Hz, −CH₂CH₃, 6H).

¹³C NMR (100 MHz, 25 °C, CDCl₃): δ 159.3 (quat, C_Py), 135.8 (p-

C_Py), 122.9 (m-C_Py), 65.70 (−NCH(CH₃)(CH₂CH₃)), 65.67 (−NCH-

(CH₃)(CH₂CH₃)−), 60.9 (br, PyCH₂N−), 28.1 (−CHCH₂CH₃−),

28.0 (−CHCH₂CH₃), 15.81 (−CHCH₃), 15.76 (−CHCH₃), 12.5

(−CH₂CH₃); two sets of signals were observed in equal ratio for the

N−CH C atom of the sec-butyl group and adjacent C atoms due to the

presence of two diastereomers. ESI-HRMS. Calcd for [C₂₂H₃₂N₄·H⁺]:

1

57.7 (−PyCH₂N−), 18.9 (−CH(CH₃)₂−). κ³-1, minor isomer. H

NMR (600 MHz, −30 °C, CD₃CN): δ 7.40 (t, ³J_HH= 7.7 Hz, p-H_Py,

2H), 6.99 (d, ³J_HH= 7.7 Hz, m-H_Py, 2H), 6.82 (d, ³J_HH= 7.7 Hz, m-H_Py,

2H), 4.29 (d, ²J_HH= 15.2 Hz, −PyCH₂N−, 2H), 4.22 (d, ²J_HH= 12.4

Hz, −PyCH₂N−, 2H), 4.02 (d, ²J_HH= 12.4 Hz, −PyCH₂N−, 2H), 3.74

2

3

(d, J_HH= 15.2 Hz, −PyCH₂N−, 2H), 1.39 (d, J_HH= 6.5 Hz,

−CH(CH₃)₂−, 6H), 1.22 (d, ³J_HH= 6.6 Hz, −CH(CH₃)₂−, 6H). The

−CH(CH₃)₂− peak could not be observed because of its low intensity.

¹³C NMR (151 MHz, −30 °C, CD₃CN): δ 159.2 (quat, C_Py), 155.3

(quat, C_Py), 138.3 (p-C_Py), 124.9 (m-C_Py), 122.4 (m-C_Py), 62.5

(−PyCH₂N− or −CH(CH₃)₂−), 60.2 (−CH(CH₃)₂− or

−PyCH₂N−), 59.6 (−PyCH₂N− or −CH(CH₃)₂−), 19.0 (−CH-

(CH₃)₂−), 18.7 (−CH(CH₃)₂−). Anal. Found (calcd for

C₂₂H₃₁CuF₆N₅P): C, 45.68 (46.03); H, 5.30 (5.44); N, 11.99 (12.20).

[(^MeN4)Cu^I(MeCN)]BF₄(2). Orange crystalline solid. Isolated yield:

55 mg (65%). At −26 °C, [(κ⁴-^MeN4)Cu^I(MeCN)]BF₄and

[(κ³-^MeN4)Cu^I(MeCN)]BF₄isomers were present in a CD₃CN

solution in a 96.6:3.38 ratio by NMR integration. κ⁴-2, major isomer.

¹H NMR (600 MHz, −26 °C, CD₃CN): δ 7.31 (t, ³J_HH= 7.5 Hz, p-H_Py,

m/z 353.2700. Found for C₂₂H₃₂N₄·H⁺: m/z 353.2700.

neo

Synthesis of ^PentN4. Synthesis of 2,6-Bis[(neo-pentylamino)-

methyl]pyridine. 2,6-Bis(bromomethyl)pyridine (1.20 g, 4.53 mmol,

1.0 equiv) was added slowly to the neo-pentylamine (7.90 g, 90.6 mmol,

20.0 equiv) at RT with stirring. The mixture was stirred at room

temperature for 17 h. Extra neo-pentylamine was then removed on a

rotary evaporator, and the residual waxy oil was dissolved in

dichloromethane and washed with concentrated aqueous potassium

carbonate. The organic phase was collected, dried with sodium sulfate,

and concentrated to dryness. The resulting waxy solid was used in the

next step without additional puriﬁcation. Yield: 1.24 g (98%). ¹H NMR

(600 MHz, 25 °C, CDCl₃): δ 7.61 (t, ³J_HH= 7.6 Hz, p-H_Py, 1H), 7.21 (d,

³J_HH= 7.7 Hz, m-H_Py, 2H), 3.96 (s, PyCH₂NH−, 4H), 2.84 (br s, 2H),

2.43 (s, −NHCH₂C−, 4H), 0.96 (s, −CH₂C(CH₃)₃−, 18H). ¹³C NMR

(151 MHz, 25 °C, CDCl₃): δ 158.9 (quat, C_Py), 137.4 (p-C_Py), 120.9

(m-C_Py), 62.1 (PyCH₂NH−), 55.7 (−NHCH₂C−), 31.9

3

2

2H), 6.73 (d, J_HH= 7.7 Hz, m-H_Py, 4H), 4.04 (d, J_HH= 15.1 Hz,

−PyCH₂N−, 4H), 3.54 (d, ²J_HH= 15.1 Hz, −PyCH₂N−, 4H), 2.91 (s,

−NCH₃, 6H), 1.97 (s, CH₃CN, 3H). ¹³C NMR (151 MHz, −26 °C,

CD₃CN): δ 156.7 (quat, C_Py), 137.7 (p-C_Py), 122.8 (m-C_Py), 64.6

(−PyCH₂N−), 49.0 (−NCH₃). κ³-2, minor conformer. ¹H NMR (600

MHz, −30 °C, CD₃CN): δ 7.45 (t, ³J_HH= 7.5 Hz, p-H_Py, 2H), 7.00 (d,

³J_HH= 7.9 Hz, m-H_Py, 2H), 6.89 (d, ³J_HH= 7.3 Hz, m-H_Py, 2H), 4.34 (d,

(−CH₂C(CH₃)₃), 28.2 (−CH₂C(CH₃)₂).

2

²J_HH= 13.5 Hz, −PyCH₂N−, 2H), 4.29 (d, J_HH= 15.6 Hz,

neo

Synthesis of ^PentN4. 2,6-Bis[(neo-pentylamino)methyl]pyridine

2

−PyCH₂N−, 2H), 4.19 (d, J_HH= 13.5 Hz, −PyCH₂N−, 2H), 3.76

(d, ²J_HH= 15.6 Hz, −PyCH₂N−, 2H), 3.03 (s, −NCH₃, 6H). The ¹³

C

(1.24 g, 4.47 mmol, 1.0 equiv) was placed in a round-bottomed ﬂask

containing benzene (20 mL) and a 10% Na₂CO₃(30 mL) aqueous

solution. The mixture was heated to 80 °C, and the solution of 2,6-

bis(bromomethyl)pyridine (1.13 g, 4.25 mmol, 0.95 equiv) was added

dropwise to the hot reaction mixture over a period of 30 min. The

reaction mixture was heated at 80 °C for 16 h. After cooling to room

temperature, the organic phase was collected using a separatory funnel.

Evaporation of the solvent furnished a white, waxy solid containing the

target compound. The crude was then treated with MeCN (15 mL) and

ﬁltered to remove an insoluble white solid. The MeCN ﬁltrate was

extracted with hexane. The hexane layer was concentrated to give the

peaks of [(κ³-^MeN4)Cu^I(CH₃CN)]BF₄could not be detected because

of their low intensity. Anal. Found (calcd for C₁₈H₂₃CuF₄N₅B): C,

46.60 (47.02); H, 4.93 (5.04); N, 14.76 (15.23).

[(^HN4)Cu^I(MeCN)]PF₆(3). Orange crystalline solid. Isolated yield: 35

mg (17%). At −30 °C, [(κ⁴-^HN4)Cu^I(MeCN)]PF₆and [(κ³-^HN4)-

Cu^I(MeCN)]PF₆isomers were present in a CD₃CN solution in a

87.3:12.7 ratio by NMR integration. κ⁴-3, major isomer. ¹H NMR (600

MHz, −30 °C, CD₃CN): δ 7.33 (t, ³J_HH= 7.7 Hz, p-H_Py, 2H), 6.81 (d,

2

³J_HH= 7.7 Hz, m-H_Py, 4H), 4.27 (2 doublets, J_HH= 15.9 Hz,

−PyCH₂N−, 4H), 3.64 (d, ²J_HH= 15.9 Hz, −PyCH₂N−, 4H), 3.60 (br

m, −CH₂NH, 2H), 1.96 (s, CH₃CN, 3H). ¹³C NMR (151 MHz, −30

°C, CD₃CN): δ 157.4 (quat, C_Py), 137.1 (p-C_Py), 122.6 (m-C_Py), 55.8

(−PyCH₂N−). κ³-3, minor conformer. ¹H NMR (600 MHz, −30 °C,

CD₃CN): δ 7.47 (t, ³J_HH= 7.6 Hz, p-H_Py, 2H), 7.06 (d, ³J_HH= 7.8 Hz,

m-H_Py, 2H), 6.91 (d, ³J_HH= 7.7 Hz, m-H_Py, 2H), 4.42 (d, ²J_HH= 14.3 Hz,

−PyCH₂N−, 2H), 4.41 (d, ²J_HH= 16.1 Hz, −PyCH₂N−, 2H), 4.18 (d,

1

target product as a white powder. Combined yield: 0.129 g (7%). H

NMR (400 MHz, 25 °C, CDCl₃): δ 7.11 (t, ³J_HH= 7.6 Hz, p-H_Py, 2H),

6.90 (d, ³J_HH= 7.7 Hz, m-H_Py, 4H), 3.97 (s, −PyCH₂N−, 8H), 2.58 (s,

−NCH₂C−, 4H), 1.09 (s, −C(CH₃)₃, 18H). ¹³C NMR (100 MHz, 25

°C, CDCl₃): δ 158.4 (quat, C_Py), 135.5 (p-C_Py), 122.8 (m-C_Py), 72.5

(−NCH₂C−), 67.7 (PyCH₂N−), 34.2 (−C(CH₃)₃), 28.1 (−C-

(CH₃)₃−). ESI-HRMS. Calcd for [C₂₄H₃₇N₄·H⁺]: m/z 381.3013.

Found for C₂₄H₃₇N₄·H⁺: m/z 381.3010.

2

²J_HH= 14.3 Hz, −PyCH₂N−, 2H), 3.80 (d, J_HH= 16.1 Hz,

−PyCH₂N−, 2H). ¹³C NMR (151 MHz, −30 °C, CD₃CN): δ 155.7

(quat, C_Py), 138.8 (p-C_Py), 124.1 (m-C_Py), 122.4 (m-C_Py), 57.6

(−PyCH₂N−). Second inequivalent resonance of quat C_Pyand

−PyCH₂−N groups could not be detected in ¹³C NMR because of

overlap with other peaks and small intensity. Anal. Found (calcd for

C₁₆H₁₉CuF₆N₅P): C, 39.02 (39.23); H, 3.71 (3.91); N, 13.79 (14.30).

General Procedure for the Synthesis of [(^RN4)Cu^I(MeCN)]X (X

= PF₆, BF₄). To a stirred solution of the ligand (1.0 equiv) in MeCN

was added [Cu(MeCN)₄]PF₆or [Cu(MeCN)₄]BF₄(1.0 equiv), which

immediately produced a red-orange solution. The reaction was stirred

for 30 min, ﬁltered through a Celite plug, and then recrystallized by the

slow diﬀusion of diethyl ether vapor over 1−2 days at room

D

DOI: 10.1021/acs.inorgchem.8b01181

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Inorganic Chemistry

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sec

[(^TsN4)Cu^I(MeCN)]PF₆(4). Yellow crystalline solid. Isolated yield: 29

mg (20%, low yield because the product was sparingly soluble in

MeCN). Complex 4 remains ﬂuxional in a CD₃CN solution even when

cooled to −30 °C. The eﬀective symmetry of the ligand is C_2v, which

could be due to the presence of only the κ⁴isomer or due to a fast,

unresolved exchange process. ¹H NMR (600 MHz, −30 °C, CD₃CN):

δ 7.89 (d, ³J_HH= 7.8 Hz, −(SO₂)CCHCH−_Ar, 4H), 7.66−7.42 (br m, p-

(

^BuN4)Cu^II (7). Bright-yellow crystalline solid. Isolated yield: 64 mg

(42%). Complex 7 exists as a single isomer (κ³-^sec^BuN4)Cu^II in a CD₂Cl₂

solution at −30 °C. κ³-7. ¹H NMR (600 MHz, −30 °C, CD₂Cl₂): δ 7.26

(t, ³J_HH= 7.6 Hz, p-H_Py, 2H), 6.91 (d, ³J_HH= 7.6 Hz, m-H_Py, 1H), 6.85

3

(d, J_HH= 7.9 Hz, m-H_Py, 1H), 6.7−6.6 (m, two overlapping m-H_Py,

2H), 5.02 (dd, ²J_HH= 2.4 and 13.0 Hz, −PyCH₂N−, 1H), 4.76 (d, ²J_HH

= 13.0 Hz, −PyCH₂N−, 1H), 4.32−4.24 (m, two overlapping CH₂N,

2H), 3.96−3.87 (m, two overlapping CH₂N, 2H), 3.60 (vd, ²J_HH= 15.0

Hz, two overlapping CH₂N, 2H), 3.28−3.23 (m, −NCH(CH₃)-

(CH₂CH₃), 1H), 2.91−2.85 (m, −NCH(CH₃)(CH₂CH₃)−, 1H),

2.54−2.48 (m, −CHCH₂CH₃, 1H), 1.74−1.67 (m, −CHCH₂CH₃,

H

_Pyand −CHCCH₃−_Ar, 6H), 7.08−6.83 (br m, m-H_Py, 4H), 5.29 (br s,

−PyCH₂N−, 4H), 3.59 (br s, −PyCH₂N−, 4H), 2.50 (s, CH₃−_Ar, 6H),

1.96 (s, CH₃CN, 3H). ¹³C NMR (151 MHz, −30 °C, CD₃CN): δ 153.9

(quat, C_Py), 146.2 (−CHCCH₃−_Ar), 140.2 (−(SO₂)CCH−_Ar), 138.9

(p-C_Py), 131.1 (CHCCH₃−_Ar), 129.1 (−(SO₂)CCHCH−_Ar), 124.6

(m-C_Py), 56.8 (−PyCH₂N−), 21.5 (CH₃−_Ar). Anal. Found (calcd for

C₃₀H₃₁CuF₆N₅O₄PS₂): C, 44.67 (45.14); H, 3.88 (3.91); N, 8.60

(8.77).

3

1H), 1.47−1.35 (m, −CHCH₂CH₃, 2H), 1.42 (d, J_HH= 6.7 Hz,

−CHCH₃, 3H), 1.20 (d, ³J_HH= 6.5 Hz, −CHCH₃−, 3H), 1.01 (t, ³J_HH

= 7.4 Hz, −CH₂CH₃, 3H), 1.00 (t, ³J_HH= 7.3 Hz, −CH₂CH₃, 3H); four

partially overlapping signals of m-H of Py and eight signals for PyCH₂−

arms are observed because of the asymmetric environment caused by

the presence of sec-butyl and κ³coordination of the ligand. ¹³C NMR

(151 MHz, −30 °C, CD₂Cl₂): δ 159.97, 159.92, 159.88 (quat, C_Py; two

signals are not resolved due to overlap), 154.79, 154.70, 154.65, 154.56

(quat, C_Py), 136.69 (p-C_Py), 124.60, 124.53, 124.46, 124.39 (m-C_Py),

121.57, 121.49, 121.41, 121.33 (m-C_Py), 66.39, 65.52 (−NCHCH₃),

62.94, 62.82, 60.89 (br), 59.53 (br), 58.25, 58.12 (−PyCH₂N), 27.63,

27.62 (−CH₂CH₃), 15.83, 15.61 (−CHCH₃), 11.87 (−CH₂CH₃);

because of the asymmetric environment caused by sec-butyl groups and

the presence of two diastereomers, four sets of signals were observed for

meta and para protons of Py and several overlapping sets of signals for

aliphatic protons. Anal. Found (calcd for C₂₂H₃₂N₄CuI): C, 48.30

General Procedure for the Synthesis of (^RN4)Cu^II (5−10). To a

stirred solution of ^RN4 (1.0 equiv) in dry tetrahydrofuran (THF) was

added CuI (0.95 equiv) to immediately produce a bright-yellow

suspension. Within 10 min, the suspended solids dissolved, and after 2−

5 h, a bright-yellow solid appeared. THF was removed by vacuum

R

i

evaporation, and dichloromethane (for N4 complexes; R = Me, Bu,

^secBu, ^neoPent, Pr) or a dichloromethane/methanol solution (for the

i

^TsN4 complex) was added to dissolve all of the reaction mixture. The

solution was passed through a Celite plug and allowed to crystallize by

diethyl ether vapor diﬀusion over 1−2 days. Crystalline solids were

collected, washed with ether and hexane, and dried under vacuum.

Crystals suitable for X-ray analysis were obtained by the slow diﬀusion

of diethyl ether vapors into dichloromethane (5−9) or a dichloro-

methane/methanol solution (10).

(48.67); H, 5.80 (5.94); N, 10.02 (10.32).

i

^BuN4)Cu^II (8). Bright-yellow crystalline solid. Isolated yield: 78 mg

(ⁱ^PrN4)Cu^II (5). Bright-yellow crystalline solid. Isolated yield: 120 mg

(

(51%). Complex 6 exists as a single isomer (κ³-ⁱ^BuN4)Cu^II in a CD₂Cl₂

solution at −30 °C. κ³-8. ¹H NMR (600 MHz, −30 °C, CD₂Cl₂): δ 7.30

(t, ³J_HH= 7.6 Hz, p-H_Py, 2H), 6.84 (d, ³J_HH= 7.6 Hz, m-H_Py, 2H), 6.73

(d, ³J_HH= 7.5 Hz, m-H_Py, 2H), 4.94 (d, ²J_HH= 13.5 Hz, −PyCH₂N−,

2H), 4.35 (d, ²J_HH= 15.1 Hz, −PyCH₂N−, 2H), 4.05 (d, ²J_HH= 13.5

Hz, −PyCH₂N−, 2H), 3.85 (d, ²J_HH= 15.1 Hz, −PyCH₂N−, 2H), 3.30

(76%). At −30 °C, [(κ³-ⁱ^PrN4)Cu^II] and [(κ⁴-ⁱ^PrN4)Cu^II] isomers were

present in a CD₂Cl₂solution in a 91.2:8.8 ratio by NMR integration. κ³-

5, major isomer. ¹H NMR (600 MHz, −30 °C, CD₂Cl₂): δ 7.27 (t, ³J_HH

= 7.7 Hz, p-H_Py, 2H), 6.89 (d, ³J_HH= 7.8 Hz, m-H_Py, 2H), 6.68 (d, ³J_HH

=

7.8 Hz, m-H_Py, 2H), 4.85 (d, ²J_HH= 12.9 Hz, −PyCH₂N−, 2H), 4.32 (d,

2

²J_HH= 14.8 Hz, −PyCH₂N−, 2H), 3.94 (d, J_HH= 12.9 Hz,

3

(d, J_HH= 6.3 Hz, −NCH₂CH−, 2H), 2.41 (d, J_HH= 7.3 Hz,

−NCH₂CH−, 2H), 2.32−2.26 (m, −CH₂CH(CH₃)₂, 1H), 2.01−1.95

(m, −CH₂CH(CH₃)₂, 1H), 1.09 (d, ³J _HH= 6.8 Hz, −CH(CH₃)₂, 6H),

0.99 (d, ³J _HH= 6.6 Hz, −CH(CH₃)₂, 6H). ¹³C NMR (151 MHz, −30

°C, CD₂Cl₂): δ 156.9 (quat, C_Py), 155.0 (quat, C_Py), 136.4 (p-C_Py),

124.0 (m-C_Py), 121.3 (m-C_Py), 71.3 (−NCH₂CH−), 63.7

(−NCH₂CH− and −PyCH₂C), 63.3 (−PyCH₂C), 26.9 (−CH₂CH-

(CH₃)₂), 26.1 (−CH₂CH(CH₃)₂), 22.8 (−CH(CH₃)₂), 20.4 (−CH-

(CH₃)₂). Anal. Found (calcd for C₂₃H₃₄N₄CuI): C, 48.72 (48.67); H,

5.89 (5.94); N, 10.35 (10.32).

2

−PyCH₂N−, 2H), 3.66−3.62 (m, −CH(CH₃)₂−, 1H), 3.56 (d, J_HH

= 14.8 Hz, −PyCH₂N−, 2H), 3.26−3.21 (m, −CH(CH₃)₂−, 1H), 1.46

3

(d, J

= 6.7 Hz, −CH(CH₃)₂, 6H), 1.23 (d, J

= 6.7 Hz,

HH

−CH(CH₃)₂, 6H). ¹³C NMR (151 MHz, −30 °C, CD₂Cl₂): δ 159.8

(quat, C_Py), 154.6 (quat, C_Py), 136.7 (p-C_Py), 124.4 (m-C_Py), 121.4 (m-

C_Py), 60.1 (−PyCH₂N−), 59.9 (−PyCH₂N−), 59.4 (−CH(CH₃)₂−),

59.1 (−CH(CH₃)₂−), 19.4 (−CH(CH₃)₂), 19.2 (−CH(CH₃)₂). κ⁴-5,

minor isomer. ¹H NMR (600 MHz, −30 °C, CD₂Cl₂): δ 7.22 (t, ³J_HH

=

7.7 Hz, p-H_Py, 2H), 4.14 (d, ²J_HH= 14.8 Hz, −PyCH₂N−, 4H), 3.74−

2

3.70 (m, −CH(CH₃)₂−, 2H), 3.42 (d, J_HH= 14.8 Hz, −PyCH₂N−,

(

^MeN4)Cu^II (9). Bright-yellow crystalline solid. Isolated yield: 101 mg

4H), 1.32 (br d, −CH(CH₃)₂, 12H). The peaks of the meta protons of

pyridine cannot be detected because of their low intensity. ¹³C NMR

(151 MHz, −30 °C, CD₂Cl₂): δ 156.9 (quat, C_Py), 135.9 (p-C_Py), 121.9

(m-C_Py), 56.1 (−CH(CH₃)₂−). The PyCH₂N− and methyl peaks

might be merging with another isomer peak. Anal. Found (calcd for

CH₂Cl₂·3C₂₀H₂₈CuIN₄): C, 44.82 (44.96); H, 5.17 (5.32); N, 10.12

(59%). At −30 °C, [(κ⁴-^MeN4)Cu^II] and [(κ³-^MeN4)Cu^II] isomers

were present in a CD₂Cl₂solution in a 63.5:36.5 ratio by NMR

integration. κ⁴-9, major isomer. ¹H NMR (600 MHz, −30 °C, CD₂Cl₂):

δ 7.20 (t, ³J_HH= 7.9 Hz, p-H_Py, 2H), 6.61 (d, ³J_HH= 7.4 Hz, m-H_Py, 4H),

2

4.03 (d, J_HH= 14.8 Hz, −PyCH₂N−, 4H), 3.40 (d, J_HH= 14.8 Hz,

−PyCH₂N−, 4H), 2.90 (s, −NCH₃, 6H). ¹³C NMR (151 MHz, −30

°C, CD₂Cl₂): δ 156.2 (quat, C_Py), 136.0 (p-C_Py), 121.8 (m-C_Py), 64.5

(−PyCH₂N−), 50.1 (−NCH₃). κ³-7, minor conformer. ¹H NMR (600

MHz, −30 °C, CD₂Cl₂): δ 7.32 (t, ³J_HH= 7.7 Hz, p-H_Py, 2H), 6.88 (d,

³J_HH= 7.4 Hz, m-H_Py, 2H), 6.75 (d, ³J_HH= 7.4 Hz, m-H_Py, 2H), 4.92 (d,

(10.31).

neo

(

^PentN4)Cu^II (6). Bright-orange crystalline solid. Isolated yield: 99

mg (66%). Complex 6 exists as a single isomer (κ³-^neo^PentN4)Cu^II in a

CD₂Cl₂solution at −30 °C. κ³-6. H NMR (600 MHz, −30 °C,

1

²J_HH= 13.6 Hz, −PyCH₂N−, 2H), 4.29 (d, J_HH= 15.3 Hz,

2

CD₂Cl₂): δ 7.27 (t, ³J_HH= 7.5 Hz, p-H_Py, 2H), 7.01 (d, ³J_HH= 7.5 Hz, m-

H_Py, 2H), 6.66 (d, ³J_HH= 7.5 Hz, m-H_Py, 2H), 4.94 (d, ²J_HH= 12.8 Hz,

−PyCH₂N−, 2H), 4.45 (d, ²J_HH= 14.9 Hz, −PyCH₂N−, 2H), 4.08 (d,

²J_HH= 14.9 Hz, −PyCH₂N−, 2H), 4.01 (d, ²J_HH= 12.8 Hz, −PyCH₂N,

2H), 3.48 (s, −NCH₂C(CH₃)−, 2H), 2.55 (s, −NCH₂C(CH₃)₃−,

2H), 1.12 (s, −C(CH₃)₃, 9H), 0.99 (s, −C(CH₃)₃, 9H). ¹³C NMR

(151 MHz, −30 °C, CD₂Cl₂): δ 158.4 (quat, C_Py), 155.5 (quat. C_Py),

136.7 (p-C_Py), 124.6 (m-C_Py), 121.7 (m-C_Py), 75.7 (−NCH₂C−), 71.2

(−NCH₂C−), 67.7 (−PyCH₂N), 63.7 (−PyCH₂N), 36.4

(−CH₂C(CH₃)₃), 34.2 (−CH₂C(CH₃)₃), 30.3 (−C(CH₃)₃), 27.7

(−C(CH₃)₃). Anal. Found (calcd for C₂₂H₃₂N₄CuI): C, 50.49 (50.48);

H, 6.22 (6.35); N, 9.56 (9.81).

2

−PyCH₂N−, 2H), 4.02 (d, J_HH= 13.6 Hz, −PyCH₂N−, 2H), 3.62

(d, ²J_HH= 15.3 Hz, −PyCH₂N−, 2H), 3.02 (s, −NCH₃, 3H), 2.51 (s,

−NCH₃, 3H). ¹³C NMR (151 MHz, −30 °C, CD₂Cl₂): δ 155.6 (quat,

C_Py), 154.8 (quat, C_Py), 136.4 (p-C_Py), 124.1 (m-C_Py), 121.3 (m-C_Py),

66.2 (−PyCH₂N−), 64.9 (−PyCH₂N−), 50.7 (−NCH₃), 43.9

(−NCH₃). Anal. Found (calcd for C₁₆H₂₀N₄CuI): C, 41.77 (41.89);

H, 4.40 (4.39); N, 11.88 (12.21).

(^TsN4)Cu^II (10). Bright-yellow crystalline solid. Isolated yield: 41 mg

(30%, low yield due to low solubility in dichloromethane). In CD₂Cl₂,

complex 10 remains ﬂuxional even at −80 °C. The eﬀective symmetry

of the ligand is C_2v, which could be due to either the presence of only

E

DOI: 10.1021/acs.inorgchem.8b01181

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Inorganic Chemistry

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one isomer, (κ⁴-^TsN4)Cu^II or (κ²-^TsN4)Cu^II, or due to unresolved fast

exchange. Single isomer. ¹H NMR (600 MHz, −80 °C, CD₂Cl₂): δ 7.71

were characterized by elemental analysis, NMR, UV−vis, and

FT-IR spectroscopy, and their solid-state structures were

determined by single-crystal XRD (Figure 1). Although complex

2 with a PF₆⁻counterion did not yield crystals suitable for X-ray

analysis, the analogous complex 2 with BF₄⁻as a counterion was

3

(d, ³J_HH= 7.7 Hz, −(SO₂)CCHCH−_Ar, 4H), 7.38 (d, J_HH= 7.7 Hz,

−CHCCH₃−_Ar, 4H), 7.32 (t, ³J_HH= 7.3 Hz, p-H_Py, 2H), 7.06 (br s, m-

H_Py, 4H), 5.01 (br d, ²J_HH= 11.2 Hz, −PyCH₂N−, 4H), 3.60 (br d, ²J_HH

= 11.2 Hz, −PyCH₂N−, 4H), 2.41 (s, CH₃−_Ar, 6H). ¹³C NMR (151

MHz, −80 °C, CD₂Cl₂): δ 154.2 (quat, C_Py), 143.8 (CHCCH₃−_Ar),

137.0 (p-C_Py), 134.2 (−(SO₂)CCH−_Ar), 129.7 (−CHCCH₃−_Ar), 126.5

(−(SO₂)CCHCH₃−_Ar), 122.8 (m-C_Py), 56.0 (−PyCH₂N−), 21.2

(CH₃−_Ar). Anal. Found (calcd for CH₂Cl₂·C₂₈H₂₈CuIN₄O₄S₂): C,

42.17 (42.27); H, 3.65 (3.67); N, 6.53 (6.80).

analyzed by XRD. The attempted preparation of the cationic

i

complex with ^BuN4 failed to give the desired product in an

analytically pure form under the same conditions even after

multiple recrystallizations.

X-ray analysis reveals that the pyridinophane ligand ^RN4

adopts a syn-boat−boat conformation^62,63in cationic copper(I)

complexes, featuring stronger coordination of the two pyridine

rings and one MeCN ligand and weaker interactions with the

two axial amines. The coordination geometry of the Cu centers

can be formally described as distorted square-pyramidal,⁶¹with

τ₅parameter values closer to 0 expected from the ideal square-

pyramidal geometry; however, the Cu−N_aminedistances are

signiﬁcantly elongated compared to the Cu−N_pydistances. A

comparison of the bond distances in complexes [(^RN4)-

Cu^I(MeCN)]⁺1−4 given in Table 1 shows that the Cu−N_py

and Cu−N_MeCNbond distances are in a range of 1.88−2.10 Å

typical for Cu^I−N bond lengths,^{60,61,76−79}while the Cu−N_amine

distances are signiﬁcantly longer (Cu−N_amine2.35−2.49 Å).

These structures resemble those previously reported for other

^RN4-type pyridinophanecopper(I) complexes.^60,61The compar-

ison of two analogous cationic complexes with diﬀerent

RESULTS AND DISCUSSION

■

Synthesis and Solid-State Structures. Cationic Com-

plexes. Cationic copper(I) complexes with a series of

pyridinophane ligands were synthesized by reacting [Cu-

(MeCN)₄]X (X = PF₆, BF₄) with 1 equiv of the pyridinophane

ligand (Scheme 2); they were isolated in an analytically pure

Scheme 2. Synthesis of Cationic Copper(I) Complexes

counteranions, [(^t^BuN4)Cu^I(MeCN)](PF₆) and [(^t^BuN4)-

Cu^I(MeCN)](BF₄), shows that bond distances and coordina-

tion geometries are quite similar for both complexes.

The comparison of Cu−N_aminebond lengths (Cu1−N3 and

Cu1−N4, Table 1) in complexes [(^RN4)Cu^I(MeCN)]X (1−4)

as well as a comparison with the published structures of

form as orange (1−3) or yellow (4) crystalline solids in 17−66%

yield. All of these complexes were completely soluble in polar

solvents such as CH₃CN and acetone and poorly soluble in

nonpolar solvents. All [(^RN4)Cu^I(MeCN)]PF₆/BF₄(1−4)

complexes reacted slowly with chlorinated solvents, CH₂Cl₂and

CHCl₃, to form the respective (^RN4)Cu^IIchloride complexes.

All cationic complexes were highly unstable in the presence of air

both in solution and in the solid state. All obtained complexes

t

[(^BuN4)Cu^I(MeCN)]X with bulky Bu groups at the amines

reveals that greater steric hindrance at the axial amine

substituents leads to noticeable elongation of the Cu−N_amine

bond lengths in the following order: R = ^tBu > ⁱPr > Me > H. At

the same time, complex 4 with a Ts-substituted amine features

the longest Cu−axial amine distances, likely due to weak

interactions with poorly donating N−Ts groups.

Figure 1. ORTEP projections of MeCN complexes 1−4 showing anisotropic displacement ellipsoids at the 50% probability level. H atoms and

counterions are omitted for clarity.

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a

Table 1. Selected Cu−N Bond Distances and τ₅Geometrical Indices in Complexes [(^RN4)Cu^I(MeCN)]X (1−4) and

[(^t^BuN4)Cu^I(MeCN)]X (X = PF₆, BF₄)

a

complex

Cu1−N1, Å

Cu1−N2, Å

Cu1−N3, Å

Cu1−N4, Å

Cu1−N5, Å

τ₅

t

b

[(^BuN4)Cu^I(MeCN)](PF₆)

2.089(2)

2.0920(18)

2.098(3)

2.0782(10)

2.0892(15)

2.0641(11)

2.099(2)

2.0819(16)

2.082(3)

2.0784(10)

2.0937(16)

2.0749(11)

2.444(2)

2.4249(16)

2.418(3)

2.3722(10)

2.3510(17)

2.4219(11)

2.470(2)

2.4570(17)

2.400(3)

2.3577(10)

2.3653(17)

2.4902(11)

1.901(2)

1.8957(19)

1.890(3)

1.8804(10)

1.8824(16)

1.8828(12)

0.04

0.08

0.14

0.03

0.13

0.15

t

[(^BuN4)Cu^I(MeCN)](BF₄)

[(ⁱ^PrN4)Cu^I(MeCN)](PF₆) (1)

[(^MeN4)Cu^I(MeCN)](BF₄) (2)

[(^HN4)Cu^I(MeCN)](PF₆) (3)

[(^TsN4)Cu^I(MeCN)](PF₆) (4)

a

b

The geometrical indices τ₅for pentacoordinate structures are calculated according to refs 80−82. From ref 60, the atom numbering corresponds

to that from Figure 1.

The ATR FT-IR spectra of polycrystalline samples of

complexes 1, 3, and 4 are consistent with the presence of a

noncoordinated PF₆⁻counterion characterized by the 820−831

cm⁻¹band, while complex 2 shows bands between 1047 and

1003 cm⁻¹that are typical for the BF₄⁻counterion. The N−H

stretching bands of the ^HN4 ligand appear at 3400 cm⁻¹for

complex 3. For complex 4, the characteristic SO and N−S

stretching bands appear at 1088 and 992 cm⁻¹, respectively.

Neutral Copper(I) Iodide Complexes. The neutral copper(I)

iodide complexes were synthesized by mixing pyridinophane

ligands with anhydrous copper iodide in dry THF to provide

yellow (5 and 7−10) and orange (6) complexes (^RN4)Cu^II

(Scheme 3), which were isolated in an analytically pure form in

Scheme 3. Synthesis of Neutral Copper(I) Iodide Complexes

Figure 2. ORTEP projections of iodide complexes (^RN4)Cu^II (5−10)

showing anisotropic displacement ellipsoids at the 50% probability level

if not indicated otherwise. For complex 6, only one independent

molecule A is shown. For complex 7, displacement ellipsoids are shown

at the 30% probability level for clarity. H atoms, minor components of

disorder, and a solvent molecule in the case of 10 are omitted for clarity.

15−76% yields and characterized by elemental analysis, NMR,

and FT-IR spectroscopy. All of these complexes were completely

soluble in CH₂Cl₂and acetone and poorly soluble in nonpolar

solvents such as hexane, toluene, benzene, and diethyl ether.

These complexes show some limited stability in the crystalline

state under air but decompose in solution in the presence of

oxygen. The ligand ^HN4 failed to give the desired product in a

pure form under the same reaction conditions: a mixture of

several complexes was obtained. Although the presence of the

desired product (^HN4)Cu^II was conﬁrmed by XRD, it could not

be separated from undesired byproducts and could not be fully

characterized.⁸³

tion. These structures also resemble the previously reported

tetracoordinate (^t^BuN4)Cu^II (A; Scheme 1a).⁶⁰The selected

bond lengths for complexes 5−8 and a previously reported

tetracoordinate complex A with a ^tBu-substituted pyridinophane

ligand are summarized in Table 2. The Cu−axial amine

distances in the solid state vary from 2.065 to 2.155 Å; however,

no clear correlation with the steric properties of the alkylamine is

observed, which could reﬂect the eﬀect of crystal packing and

intermolecular interactions, especially in the complexes where a

The X-ray crystal structures of complexes (^RN4)Cu^II (5−10)

display distorted tetrahedral coordination geometry around the

Cu^Icenter, with the pyridinophane ligand binding in a κ³fashion

(Figure 2). The τ₄and τ_4′parameters for tetracoordinate Cu

centers in these complexes are in the range 0.53−0.68, indicative

of a noticeable deviation from the ideal tetrahedral geometry

where τ₄and τ_4′values equal to 1 are expected. In all complexes,

the pyridinophane ligand adopts a syn-boat−chair conforma-

remote steric hindrance is present, (^ι^ΒuN4)Cu^II and 6. It should

i

be noted that the ^secBu and Bu fragments at atom N3 for

complexes 7 and 8 are disordered into two positions. Similar to

complexes with alkyl-substituted ligands, 10 features a much

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Table 2. Selected Cu−N and Cu−I Bond Distances and Geometrical Indices (τ₄and τ_4′) in Complexes 5−10 and A

a

τ₄and τ_4′values

complex

Cu1−N1, Å

Cu1−N2, Å

Cu1−N3, Å

Cu1−I1, Å

τ₄

τ_4′

b

(

^tBuN4)Cu^II (A)

2.1292(16)

2.1554(16)

2.1163(12)

2.1136(12)

2.120(5)

2.0658(10)

2.0876(13)

2.0967(16)

2.1518(15)

2.1118(15)

2.1025(12)

2.0688(12)

2.117(5)

2.2158(16)

2.2068(15)

2.2404(12)

2.2560(12)

2.234(5)

2.4907(3)

2.4881(2)

0.67

0.62

0.63

0.62

0.66

0.68

0.65

0.66

0.60

0.62

0.53

0.62

0.64

0.68

0.61

i

^PrN4)Cu^II (5)

neo

c

^PentN4)Cu^II (6A)

2.46511(19)

2.46202(19)

2.4737(8)

2.46123(18)

2.4755(3)

neo

sec

^PentN4)Cu^II (6B)

^BuN4)Cu^II (7)

^BuN4)Cu^II (8)

i

2.0555(10)

2.1978(17)

2.2487(11)

d

^MeN4)Cu^II (9)

(^TsN4)Cu^II (10)

2.0849(16)

2.3467(17)

2.4600(3)

a

b

The geometrical indices τ₄and τ_4′for tetracoordinate structures are calculated according to refs 80−82. From ref 60, the atom numbering

c

d

corresponds to Figure 2. There are two molecules present in the asymmetric part of the unit cell. The distance is related to the Cu−N_aminebond

(see the numbering in Figure 2). Complex 9 crystallizes with the molecule bisected by a mirror plane; hence, the asymmetric cell contains half a

molecule.

longer Cu−axial amine bond owing to weak coordination of the

tosylamide N atom.

Scheme 4. Isomers of [(^RN4)Cu^I(MeCN)]⁺(R = ⁱPr, Me, H)

Present in a MeCN Solution at −30 °C

Macrocyclic Ligand Conformational Flexibility in

Solution and NMR Studies. Cationic Complexes. The

dynamic behavior of the complexes was investigated by a

solution VT NMR study. Complexes [(^RN4)Cu^I(MeCN)]X (X

= PF₆, BF₄), where R = ⁱPr (1), Me (2), and H (3), all featured

slightly broadened ¹H resonances at RT indicative of the

eﬀective C_2vsymmetry in solution. However, when the

temperature was lowered to −30 °C, two isomers could be

distinguished in solution under slow exchange conditions with

clearly resolved, sharp proton resonances, except for complex 4,

which remains ﬂuxional (vide infra). The major isomer was

assigned as a C_2v-symmetric [(κ⁴-^RN4)Cu^I(MeCN)]⁺, in which

the ^RN4 ligand adopts a syn-boat−boat conformation binding to

the Cu center with both pyridine and both axial amine N atoms.

This is evident from the presence of only one pair of doublets of

the CH₂groups showing geminal coupling (J ≈ 15 Hz) and only

be resolved upon cooling, with the coalescence temperature-

dependent on the substituent. The predominant isomer for

one aromatic doublet of the pyridine meta proton (H_meta

)

t

i

complexes (^RN4)Cu^II (R = Bu, Pr, ^neoPent, ^secBu, Bu) in a

CD₂Cl₂solution was a pseudotetrahedral complex (κ³-^RN4)-

Cu^II, in which the pyridinophane ligand binds only by three N

atoms, while one of the amine arms remains free. In the case of

integrating as 2:1 relative to the triplet of the para proton of the

pyridine, H_para. The minor isomer was assigned as the complex

[(κ³-^RN4)Cu^I(MeCN)]⁺, where the ligand coordinates only

with three N atoms of pyridinophane, while one of the axial

amines remains free. Accordingly, the CH₂groups feature two

pairs of geminally coupled doublets and two inequivalent

pyridine H_metaresonances are observed, consistent with an

eﬀective C_ssymmetry of the ligand in solution. The ratio of

[(κ⁴-^RN4)Cu^I(MeCN)]⁺to [(κ³-^RN4)Cu^I(MeCN)]⁺at −30

°C is dependent on the axial amine substituent; however, in all

cases, the [(κ⁴-^RN4)Cu^I(MeCN)]⁺isomer remains predom-

inant (Scheme 4).

Complex 4 remains highly ﬂuxional in solution even at −30

°C featuring broad, unresolved signals of methylene protons and

pyridyl aromatic protons. This is consistent with fast conﬁgura-

tional exchange due to the weak interactions of the Cu center

with poorly donating NTs axial donor groups that were observed

in the solid state.

ⁱPr-substituted complex 5, the minor isomer, (κ⁴-ⁱ^PrN4)Cu^II, was

also present in solution, with a (κ⁴-ⁱ^PrN4)Cu^II/(κ³-ⁱ^PrN4)Cu^II

ratio of 8.8:91.2. By contrast, complex 9 with the less sterically

demanding Me-substituted ligand ^MeN4 shows the opposite

conformational preference, with the (κ⁴-^MeN4)Cu^II complex

being the predominant isomer with a (κ⁴-^MeN4)Cu^II/

(κ³-^MeN4)Cu^II ratio of 63.5:36.5 (Scheme 5).

To further study the nature of processes involved in

conformational exchange in complexes (^RN4)Cu^II, 2D EXSY/

NOESY NMR measurements were performed at low temper-

1

ature under slow exchange conditions. Although the H NMR

spectrum of 8, measured at −20 °C, features only one

conformer, (κ³-ⁱ^BuN4)Cu^II, the EXSY spectrum clearly reveals

the exchange cross peaks between two inequivalent resonances

of pyridine meta protons H_mand H_m′that appear in the same

phase as the diagonal peaks (Figure 3). In addition, exchange

cross peaks are also present between two pairs of methylene

doublets, H_a/H_band H_a′/H_b′, that belong to CH₂groups of the

coordinated and noncoordinated axial amines (Figure 3).

Accordingly, exchange cross peaks are also seen between two

sets of corresponding multiplets of the isobutyl groups. This is

indicative of the degenerative mutual-site exchange process

Neutral Copper(I) Iodide Complexes. The solution behavior

and exchange processes were studied in more detail for the series

t

i

of copper iodide complexes (^RN4)Cu^II (5−10; R = Bu, Pr,

^neoPent, ^secBu, ⁱBu, Me). In contrast to the cationic complexes, the

copper(I) iodide series displays PL not only in the solid state but

also in solution. The PL properties are likely strongly aﬀected by

the complexes’ ﬂuxional behavior (vide infra).

All above-mentioned complexes 5−10 are ﬂuxional at RT

featuring signiﬁcantly broadened ligand resonances, which can

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Scheme 5. Isomers of (κ³-^RN4)Cu^II Present in a CD₂Cl₂

Solution (R = ^tBu, ⁱPr, ^neoPent, ^secBu, ⁱBu, Me) at −30 °C

Scheme 6. Exchange Process in a Solution of 5−10 Involving

Coordinated and Noncoordinated Amines (R = ^tBu, ⁱPr,

^neoPent, ^secBu, ⁱBu, Me)

i

to that of the less bulky complex 6. Accordingly, in Pr-

substituted complex 5, the coalescence temperature is ca. 35 °C,

t

6 showed a coalescence temperature at ca. 30 °C, and in Bu-

substituted complex A and ^secBu-substituted complex 7,

coalescence was not observed even at 35 °C. Because the

chemical shift diﬀerence between H_mand H_m′remains similar

(126 Hz) both for complexes A and 5,⁸⁴this indicates that

complex A with a more bulky ^tBu substituent is less ﬂuxional in

solution compared to 5.

This was further conﬁrmed by VT SST NMR experiments in

the case of complexes A, 8, and 6 featuring a simple mutual-site

exchange process.⁸⁵The SST experiment was performed by

irradiating one of the doublets of the meta protons, which leads

to a decrease in the intensity of the signal of the second meta

proton, with the degree of suppression determined by the rate

constant of exchange (Tables 3−5). The activation parameters

involving coordinated and free amine arms in complex

(κ³-ⁱ^BuN4)Cu^II, as shown in Scheme 6. Upon warming,

coalescence is observed between the resonances of H_mand

H_m′, which merge into a single broad signal at ca. 23 °C (Figure

4).

Similar exchange peaks were also observed by EXSY at −20

°C in complexes A, 6, and 7 and for a major isomer of 5

i

containing a κ³-bound ^PrN4 ligand. However, the coalescence

temperature for H_mand H_m′in these systems is higher compared

Figure 3. EXSY spectrum of complex 8 at −20 °C (mixing time 0.2 s). Exchange cross peaks are shown in red, in the same phase as the diagonal peaks

(NOE cross peaks are shown in blue, in the opposite phase to the diagonal peaks).

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Figure 4. ¹H VT NMR spectra of complex 8 in CD₂Cl₂.

Table 3. Rate Constant at Various Temperatures for Complex

A in CD₂Cl₂

Table 6. Activation Parameters for Complexes A, 8, and 6

Determined from VT SST Experiments

a

temperature

(K)

rate constant (k,

s⁻¹

temperature

(K)

rate constant (k,

s⁻¹

A

8

6

)

ΔH^⧧(kcal mol⁻¹

)

8.5 0.8

6.7 0.6

11.9 0.5

−7.4 2.1

14.2 0.5

12.4 0.5

ΔS^⧧(cal mol⁻¹K⁻¹

)

−23

3

−27

2

263

258

253

248

5.37

3.66

2.57

1.61

243

238

233

1.14

0.839

0.591

b

ΔG^⧧(kcal mol⁻¹

)

15.3 0.8

9.0 0.8

14.7 0.6

7.2 0.6

c

E_A(kcal mol⁻¹

)

d

ln A

19

2

17

1

26

1

a

All experiments were repeated three times at each temperature; see

b

Table 4. Rate Constants at Various Temperatures for

Complex 8 in CD₂Cl₂

details in the Supporting Information. Because measurements could

not be done at RT directly, ΔG^⧧was extrapolated to 298 K by using

c

the ΔH^⧧and ΔS^⧧values obtained above. Arrhenius activation

temperature

(K)

rate constant (k,

s⁻¹

temperature

(K)

rate constant (k,

s⁻¹

d

energy. Natural logarithm of the preexponential factor.

)

253

248

243

240.5

12.4

8.52

6.86

5.47

238

235.5

233

4.62

3.98

3.56

3.07

the isobutyl and neo-pentyl groups, 8 and 6, showed generally

faster isomer exchange rates.

As discussed above, complex 9 features two isomers in

solution, (κ⁴-^MeN4)Cu^II and (κ³-^MeN4)Cu^II, in comparable

amounts, which allows for the observation of two types of

exchange by EXSY NMR at −20 °C. First, similar to complexes

5, 8, and A, the degenerative exchange between coordinated and

noncoordinated amines is evident in the (κ³-^MeN4)Cu^II isomer,

with the coalescence between H_mand H_m′observed at ca. 15 °C,

showing that the Me-substituted complex undergoes faster

230.5

Table 5. Rate Constants at Various Temperatures for

Complex 6 in CD₂Cl₂

temperature

(K)

rate constant (k,

s⁻¹

temperature

(K)

rate constant (k,

s⁻¹

)

255

253

251

249

7.15

5.88

4.73

4.06

247

243

241

3.26

2.07

1.74

t

i

conformational exchange compared to the bulkier Bu-, Pr-,

i

^neoPent-, ^secBu-, and Bu-substituted analogues A and 5−8. The

EXSY experiment also exhibits interconversion that occurs

between the complexes with two diﬀerent conformers of the

ligand, (κ⁴-^MeN4)Cu^II and (κ³-^MeN4)Cu^II (Figure 5). For

example, exchange cross peaks are observed between two para

protons of pyridines, H_pand H_p′, that belong to isomers

are given in Table 6. Complex A was the least ﬂuxional among

the series, while complexes featuring remote steric hindrance in

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Figure 5. EXSY spectrum of complex 9 at −20 °C (mixing time of 0.2 s). Exchange cross peaks are shown in red in the same phase as the diagonal peaks

(NOE cross peaks are shown in blue in the opposite phase to the diagonal peaks).

(κ³-^MeN4)Cu^II and (κ⁴-^MeN4)Cu^II, respectively. Accordingly,

both meta protons of pyridines in (κ³-^MeN4)Cu^II, H_mand H_m′,

exchange, with the signal of H_m′′corresponding to pyridine meta

protons of the C_2v-symmetrical isomer (κ⁴-^MeN4)Cu^II (Scheme

7). Similarly, exchange cross peaks are clearly seen between N−

Compared to the alkylamine analogues, the Ts-substituted

complex 10 undergoes much faster conformational exchange, so

that slow exchange conditions could not be reached even at −80

°C. This is likely due to weak coordination of the electron-

deﬁcient N−Ts group to the Cu^Icenter, also consistent with the

signiﬁcantly longer Cu^I−N bond distances observed in complex

10 compared to the alkylamine derivatives (even the ones with

the bulky ^tBu and ⁱPr groups). The ¹H NMR spectrum of 10 at

−80 °C features two broadened doublets of CH₂groups and

only one signal corresponding to the meta protons of pyridines,

indicative of the eﬀective C_2vsymmetry in solution. While this

could be consistent with κ⁴(or κ²) coordination of the ^TsN4

ligand in solution, the high ﬂuxionality of this complex does not

allow unambiguous assignment of the conformational prefer-

ence because the observed signals could also result from

exchange between several conﬁgurational isomers, leading to

eﬀective averaging of the signals.

Scheme 7. Isomer Interconversion Involving (κ³-^MeN4)Cu^II

and (κ⁴-^MeN4)Cu^II

Electrochemical Properties. The electrochemical proper-

ties of cationic and neutral pyridinophane complexes were

studied by CV (Figure 7). The cationic complexes were

examined by CV in a MeCN solution using ⁿBu₄NPF₆(for 1, 3,

Me groups that belong to the κ³and κ⁴isomers, Me_a, Me_b, and

Me_c. Upon warming, the resonances of N−Me of both isomers

coalesce at ca. 21 °C (Figure 6).

Overall, a qualitative comparison of the solution behavior of

complexes with alkylamine axial donors, (^RN4)Cu^II, indicates

that the steric bulk of the alkyl group of the axial amines has a

dual eﬀect on the conformational behavior of the derived

complexes: (i) increasing the steric bulk of the R substituent

leads to higher preference toward the isomer with a κ³-bound

^RN4 ligand; (ii) increasing the steric bulk of the alkyl R groups

slows down conformational exchange in these systems.

n

and 4) and Bu₄NBF₄(for 2) as supporting electrolytes: their

electrochemical properties are summarized in Table 7. For all

cationic complexes except Ts-substituted compound 4, the

chemically reversible oxidation wave assigned to a Cu^I/Cu^II

oxidation event was observed. The separation between the

forward and reverse peaks was in a range of 55−89 mV, larger

than expected for an electrochemically reversible process for the

majority of the complexes except the least sterically hindered

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Figure 6. ¹H VT NMR spectra of complex 9 in CD₂Cl₂.

Table 7. Electrochemical Properties of Complexes

a

[(^RN4)Cu^I(MeCN)]X (1−4; X = PF₆, BF₄)

E_pf

(mV)

E_pr

(mV)

ΔE

E

^1/2e

b

c

d

complex

(mV)

[(ⁱ^PrN4)Cu^I(MeCN)](PF₆)

(1)

−163

−241

−289

674

−249

−316

−344

−205

89

75

55

−206

[(^MeN4)Cu^I(MeCN)](BF₄)

−279

−317

f

(2)

[(^HN4)Cu^I(MeCN)](PF₆)

(3)

[(^TsN4)Cu^I(MeCN)](PF₆)

g

(4)

a

Cyclic voltammograms for complexes 1−4 (1 mM) in a 0.1 M

solution of ⁿBu₄NPF₆(for 1, 3, and 4) or ⁿBu₄NBF₄(for 2) as

supporting electrolytes in MeCN at 23 °C [100 mV s⁻¹scan rate; Pt

disk electrode (d = 1.6 mm); all peaks were referenced versus

b

c

Figure 7. Cyclic voltammograms of complexes 1, 3, and 4 (1 mM) in

0.1 M ⁿBu₄NPF₆/MeCN and 2 in a 0.1 M ⁿBu₄NBF₄/MeCN solution

at 23 °C (scan rate 100 mV s⁻¹; 1.6 mm Pt disk working electrode; the

arrow indicates the initial scan direction).

ferrocene]. Potential of the forward peak. Potential of the return

d

peak. The peak-to-peak separation ΔE was calculated as E_pf− E_pr.

e

f

E_1/2was estimated as ¹/₂(E_pf+ E_pr). ⁿBu₄NBF₄was used as an

g

electrolyte. Irreversible oxidation.

3.⁸⁶This is likely due to signiﬁcant structural changes that occur

during oxidation of the pentacoordinate Cu^Icenter to form

hexacoordinate Cu^IIspecies. According to a literature example, a

similar large separation between the anodic and cathodic peaks

Cu−axial amine bond distances compared to its copper(I)

analogue [(^t^BuN4)Cu^I(MeCN)](OTf).⁶¹

Interestingly, the estimated E_1/2of the Cu^I/Cu^IIoxidation

wave varies depending on the substituents at the amine axial

donors, with the most positive E_1/2being observed for the

complex with the bulkiest ⁱPr substituent, 1, and most negative

E_1/2observed for the least sterically hindered complex 3. This

trend is diﬀerent from what could have been expected based

solely on the donor properties of the axial amines, and it likely

reﬂects the degree of stabilization of the copper(II) product due

was observed in complex [(^t^BuN4)Cu^I(MeCN)]X (X = PF₆,

OTf).⁶¹In this earlier work, the oxidation product, copper(II)

complex [(^t^BuN4)Cu^II(MeCN)₂](OTf)₂, was previously ob-

tained and structurally characterized. Complex [(^t^BuN4)-

Cu^II(MeCN)₂](OTf)₂featured a distorted octahedral Cu^II

center with two MeCN ligands and with signiﬁcantly shortened

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i

to diﬀerences in the steric bulk of the axial donor. Because

signiﬁcantly shorter Cu−N_aminebond distances are expected for

the oxidized copper(II) complex, more eﬀective stabilization is

expected from the least bulky ligands.

Compared to complexes 1−3, the Ts-substituted complex 4

featured an irreversible oxidation wave at a much higher

potential, which could be a result of the poor stability of the

copper(II) product in the presence of weakly coordinating axial

NTs groups.

Compared to cationic complexes, the electrochemical

behavior of neutral copper(I) iodide complexes (^RN4)Cu^II

(5−10) is generally more complex and is likely aﬀected by the

redox activity of a free iodide ion that can be released into

solution in the presence of an electrolyte, which prevented us

from carrying out detailed electrochemical studies on the copper

iodide complex series.⁷¹

Photophysical Properties. The UV−vis absorption

spectra of complexes 1−4 in a MeCN solution and complexes

5−10 in dichloromethane were recorded (Figure 8). All

complexes feature an intense absorption band at 230−300 nm

assigned as the ligand-centered transition. Similarly, free

pyridinophane ligands also show an absorption band in this

region.⁶⁰^RN4-supported complexes 1−3 and 5−9, where R = H,

Me, Bu, ^secBu, ^neoPent, and Pr, are also characterized by a less

intense metal-to-ligand charge-transfer (MLCT) band (ε ≈ 10⁴

M⁻¹cm⁻¹) at 390−430 nm (Table 9). By contrast to complexes

with alkyl-substituted pyridinophane ligands, the Ts-substituted

analogues 4 and 10 did not exhibit any absorption bands above

300 nm.

The emission properties of the cationic and neutral complexes

were then studied in the solid state and in solution to elucidate

interplay between the ligand steric and photophysical properties

of copper(I) complexes in a series containing Bu, Pr, ^neoPent,

^secBu, ⁱBu, and Me substituents at the amine. While the cationic

MeCN complexes 1−4 showed negligible emission in the solid

state upon excitation at 400 nm (typically <5%), the neutral

copper(I) iodide complexes 5−9 bearing alkyl substituents at

the amine donors exhibited broad emission bands with the

maximum in the range of 540−589 nm at 298 K, indicative of the

charge-transfer character of the emissive excited states^49,54,57,87

and consistent with previously reported time-dependent density

functional theory studies (Figure 9).⁶⁰The photophysical

t

i

Figure 9. Normalized emission spectra of complexes A and 5−9 in the

solid state at 298 K. Excitation at 400 nm.

properties of complexes 5−9 as well as a comparison with the

previously reported complex A at room temperature are

summarized in Table 8. In all cases, the average emission

lifetimes were in the microsecond range, varying from 6.3 to

20.02 μs, indicating phosphorescense from the triplet excited

state (Figure 10), although thermally activated delayed

ﬂuorescence (TADF) cannot be completely excluded.^60,88

Complex 9 showed only negligible emission (<3%) under the

same conditions. The emission decay proﬁle could be ﬁt with a

monoexponential curve, except for complex 9, in which a

biexponential ﬁt gave two components with lifetimes of 1.70 and

8.39 μs, with an intensity-weighted average value of 6.31 μs.

A comparison of the PLQYs in Table 8 shows that the highest

PLQY was observed for the most bulky ^tBu-substituted complex

A, while the least sterically hindered complex 9 showed the

lowest PLQY. Accordingly, the nonradiative decay rate constant

k_nrwas lowest for complex A (1.39 × 10⁴s⁻¹) and highest for

complex 9 (15.68 × 10⁴s⁻¹) in this series. Complexes with

intermediate steric hindrance at the axial amines, 5−8, exhibit

PLQYs in the range of 0.39−0.55 with k_nrfalling in the

intermediate range [(2.25−4.65) × 10⁴s⁻¹].

Figure 8. UV−vis absorbance spectra for complexes 1−4 in MeCN (a)

and 5−10 in dichloromethane (b).

M

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Inorganic Chemistry

Article

Table 8. Photophysical Properties of Complexes 5−9 and A

a

in the Solid State at 298 K

k_r×

k_nr

×

CIE color

λ

10⁻⁴

coordinates

^emib

c

d

e

f

g

complex

(nm)

Φ

τ (μs)

(s⁻¹

)

(s⁻¹

)

(x, y)

t

i

(

^BuN4)

585

0.78

0.58

0.55

0.40

0.39

0.02

15.79

4.94

4.97

2.75

3.10

2.34

0.32

1.39

3.59

2.25

4.65

3.66

15.68

0.514, 0.476

0.487, 0.506

0.549, 0.439

0.323, 0.268

0.414, 0.562

0.470, 0.512

h

Cu^II (A)

^PrN4)Cu^II

576

589

570

546

570

11.66

20.02

12.96

16.69

6.31

(5)

neo

^PentN4)

Cu^II (6)

sec

^BuN4)

Cu^II (7)

i

^BuN4)

Cu^II (8)

i

^MeN4)

Cu^II (9)

a

b

d

All measurements were performed with excitation at 400 nm.

c

Emission maximum. PLQYs at 298 K (excitation at 400 nm).

e

Figure 11. Normalized emission spectra of complexes A and 5−8 in a

CH₂Cl₂solution at 298 K. Excitation at 400 nm.

Emission lifetime at 298 K. Radiative decay rate constants were

f

estimated as Φ/τ. Nonradiative decay rate constants were calculated

g

h

as k_r(1 − Φ)/Φ. CIE 1931 chromaticity diagram coordinates. From

i

ref 60. Intensity-weighted average value based on a biexponential ﬁt

(calculated from α₁τ₁+ α₂τ₂/(α₁+ α₂), where τ₁= 1.70 and τ₂= 8.39

are the decay components and α₁= (A₁/A₁+ A₂) and α₂= (A₂/A₁+

A₂) are the respective amplitudes, where A₁= 0.301 and A₂= 0.668

are the respective contributions.

While the radiative rate constants remain in a range similar to

that in the solid state, the solution-state k_nrincreases by about an

order of magnitude, indicative of faster nonradiative decay

pathways in the solutions of conformationally ﬂexible complexes

(^RN4)Cu^II. Complex 10 showed only negligible emission in the

solid state (PLQY < 0.01) and was not emissive in solution,

which could be attributed to its high ﬂuxionality in solution

(vide infra) as well as a diﬀerent electronic structure consistent

with the lack of MLCT bands at ca. 400 nm; variation of the

excitation source wavelength to higher energies also did not

result in any observable emission. In particular, conformation of

the ^TsN4 ligand in complex 10 in solution can diﬀer from other

alkyl-substituted complexes, although solution NMR studies did

not allow us to unambiguously conﬁrm the conformational

assignment because of the high ﬂuxionality of this complex (vide

supra).

The frozen MeTHF solutions of complexes A and 5−10 at 77

K show a dramatic increase of the PLQY compared to RT

measurements, reaching 0.79−0.91 for complexes (^RN4)Cu^II

(R = Bu, Pr, ^neoPent, ^secBu, Bu) and 0.25 for 9, which was

nonemissive at RT (Figure 13). This resembles the behavior of

2,9-disubstituted bis(phenanthroline)copper(I) complexes with

long-alkyl-chain substituents in frozen solutions, which was

attributed to the ability of long alkyl chains to prevent signiﬁcant

distortions of the ground and excited states in a rigid matrix.⁹³

The observed correlation between the steric bulk of the amine

donor and the PLQY could be aﬀected by two main factors.

First, as shown by the NMR studies, the steric bulk of the alkyl

group aﬀected the rate of the conformational exchange processes

in solutions of (^RN4)Cu^II (including degenerative exchange),

with slower conformational exchange observed for the

complexes bearing the more bulky alkyl groups at the amines

(vide supra). Second, the steric bulk of the alkyl group also

aﬀected the relative stabilities of two diﬀerent isomers in

solution, with tetracoordinate (κ³-^RN4)Cu^II species being more

favorable for more sterically demanding R groups. However, in

the case of complexes A, 6, and 8, only one conformer,

(κ³-^RN4)Cu^II, was present in solution in both cases, while the

solution quantum yield decreased drastically for less bulky and

more ﬂuxional complexes 6 and 8 compared to ^tBu-substituted

complex A, showing that the ﬂuxionality may be the

predominant factor aﬀecting the solution PLQY.

t

i

Figure 10. Normalized PL decay proﬁles for complexes 5−10 (500−

800 nm range) in the solid state at 298 K. Excitation at 400 nm.

To exclude the eﬀects of crystal packing and aggregation on

the photophysical properties, we examined the emission

properties in solution at 298 K and in a frozen solution at 77

K. To our satisfaction, solutions of complexes bearing ⁱ^PrN4 (5),

neo

sec

i

^PentN4 (6), ^BuN4 (7), and ^BuN4 (8) in CH₂Cl₂at 298 K

showed emission at 600−615 nm (Figure 11), and only the

complex with the least bulky Me group (9) showed negligible

emission with PLQY < 0.01. In polar-coordinating solvents such

as alcohols or nitriles, the emission was negligible, while low

solubility prevented measurements in benzene and toluene

solutions. A comparison of the CH₂Cl₂solution PLQY data for

5−9 and A shows that the PLQY gradually decreases in the order

^tBu > ⁱPr ≈ ^secBu > ⁱBu > ^neoPent > Me (Table 9), thus showing a

clear correlation with the steric bulk.⁸⁹⁻⁹²

Compared to the solid state, the emission lifetimes at 298 K

are signiﬁcantly shorter, falling to a 0.7−6 μs range (Figure 12).

N

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Inorg. Chem. XXXX, XXX, XXX−XXX

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Article

a

Table 9. Photophysical Properties of Complexes 5−10 and A in Solution

d

g

298 K

77 K

b

λ_abs(nm) [ε (M⁻¹

λ

τ

k_r× 10⁻⁴

k_nr× 10⁻⁵

CIE color coordinates (x,

c

^emie

f

^emie

f

h

i

j

k

complex

cm⁻¹)]

(nm)

Φ

(nm)

Φ

(μs)

(s⁻¹

)

(s⁻¹

)

y)

t

i

l

(

^BuN4)Cu^II (A)

^PrN4)Cu^II (5)

^PentN4)Cu^II (6)

^BuN4)Cu^II (7)

^BuN4)Cu^II (8)

^MeN4)Cu^II (9)

(^TsN4)Cu^II (10)

419 [2031]

415 [1190]

434 [345]

407 [2380]

417 [811]

432 [1090]

n.d.

600

612

613

609

610

n.d.

0.28

568

572

567

575

559

571

499

0.81

0.79

0.91

0.72

0.79

0.25

0.53

4.3

6.51

1.54

3.57

3.29

2.73

n.d.

1.67

1.47

13.9

2.66

4.35

n.d.

0.372, 0.245

0.388, 0.252

0.377, 0.242

0.389, 0.258

0.261, 0.126

n.d.

0.095

0.025

0.11

0.059

<0.01

6.17

0.70

3.34

2.16

n.d.

neo

sec

i

n.d.

a

b

c

d

All measurements were performed with excitation at 400 nm; n.d. = not determined. Absorption maximum. Extinction coeﬃcient. In degassed

e

f

g

h

i

dichloromethane at 298 K. Emission maximum. PLQY. In MeTHF at 77 K. Emission lifetime at 298 K in CH₂Cl₂. Radiative decay rate

constants were estimated as Φ/τ; measured at 298 K in CH₂Cl₂. Nonradiative decay rate constants were calculated as k_r(1 − Φ)/Φ; measured at

298 K in CH₂Cl₂. CIE 1931 chromaticity diagram coordinates determined for the emission spectrum at 298 K in CH₂Cl₂. From ref 60.

j

k

l

diphosphine). Introducing bulky substituents at the phenan-

throline backbone leads to a signiﬁcant increase in the emission

lifetime and PLQY. The detailed investigations of these systems

showed that such a strong eﬀect of the ligand steric properties

arises because of suppression of the ﬂattening distortion in the

charge-transfer state, thus preventing nonradiative structural

relaxation.^{30,37,51,56,94−96}Although the geometrical changes

involved in nonradiative relaxation may diﬀer, suppression of

the ligand dynamics achieved by introducing bulky substituents

at the weakly interacting amine donors in (^RN4)Cu^II complexes

is also the main factor that determines the PLQY in this series.

Although the ligand dynamic process observed by NMR

involving degenerative exchange between two axial amines is

much slower than the nonradiative decay, the introduction of

the steric bulk at amine substituents might aﬀect the ﬂuxionality

of the ligand in the excited state in a similar manner. In addition,

dynamic processes in the excited state might feature faster rate

constants because of charge buildup at the Cu center and at the

Figure 12. Normalized PL decay proﬁles for copper iodide complexes

5−8 (500−800 nm range) in a CH₂Cl₂solution at 298 K. Excitation at

400 nm.

pyridinophane ligand.⁶⁰

In addition, it has been reported that, in the case of

tetracoordinate complex [Cu^I(dmp)₂]⁺(dmp = 2,9-dimethyl-

1,10-phenanthroline), coordination of a Lewis basic solvent

such as MeCN to the MLCT excited state leads to “exciplex”

quenching.^38,51,64,65In the present (^RN4)Cu^II system, the free

axial amine moiety can coordinate to the Cu center in the

absence of coordinating solvents. Thus, suppression of the

nucleophilic attack by the pendant amine donor to the Cu center

in the excited state due to the presence of bulky alkyl

substituents could be another mechanism responsible for the

higher PLQY observed for complexes with bulky substituents at

the amine donors. A higher propensity of complexes with less

bulky substituents to undergo pendant amine donor coordina-

tion is observed in the solution behavior of (^RN4)Cu^II studied

by NMR, which shows that only 9 exhibits a substantial fraction

of a pentacoordinate [(κ⁴-^RN4)Cu^II], and isomer interconver-

t

i

sion is faster compared to that of the Bu- and Pr-substituted

analogues. Moreover, the trend in the oxidation potentials of

pentacoordinate complexes 1−4 also conﬁrms that stabilization

of a Cu^IIcenter is favored by less bulky amine donors because of

the more eﬃcient coordination of the axial amines (vide supra).

Figure 13. Normalized emission spectra of complexes A and 5−10 in

frozen MeTHF at 77 K. Excitation at 400 nm.

t

i

Interestingly, the PLQYs in frozen solutions for Bu-, Pr-,

i

^neoPent-, ^secBu-, and Bu-substituted complexes are very similar,

0.72−0.91, while the Me-substituted complex 9 shows a lower

PLQY of 0.25. Similarly, Armaroli and co-workers reported high

emission intensity in frozen solution 2,9-disubstituted bis-

(phenanthroline)copper(I) complexes with long-alkyl-chain

Similarly, the eﬀect of the ligand steric bulk on the

photophysical properties was described for a series of copper(I)

+

complexes with phenanthroline-based ligands Cu(NN)₂and

Cu(NN)(PP) (NN = substituted phenanthroline; PP =

O

DOI: 10.1021/acs.inorgchem.8b01181

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

substituents.⁹³At the same time, the Me-substituted bis-

(phenanthroline)copper(I) complex remained a weak emitter

in a frozen solution, and it was proposed that smaller Me

substituents do not prevent excited-state distortions even in a

rigid matrix. By analogy, one could propose that at 77 K

conformational exchange and nucleophilic attack by pendant

amine are essentially “frozen”, and therefore no signiﬁcant

diﬀerentiation of the PLQY is observed for complexes

of organometallic comonomer required for visual observation of

the mechanical stress.

ASSOCIATED CONTENT

* Supporting Information

■

S

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.8b01181.

(^RN4)Cu^II (R = Bu, Pr, ^neoPent, ^secBu, Bu), all containing

bulky alkyl groups that can eﬀectively prevent distortions in the

excited state, leading to leveling oﬀ the PLQY to similarly high

values of 0.72−0.91. Interestingly, even Ts-substituted complex

10, which was highly ﬂuxional in solution at RT, becomes

emissive in a frozen solution with a PLQY of 0.53, which further

conﬁrms the importance of “freezing” the conformational

lability for emissive properties.

Overall, although the Cu−N_aminedistances are signiﬁcantly

longer than the Cu−N_pyridinedistances, the emissive properties of

complexes 5−9 are very sensitive to steric requirements of the

axial amine arms, which determine the dynamic properties of the

macrocyclic pyridinophane ligand. Such sensitivity to subtle

changes at the axial amine environment and its eﬀect on the

ligand ﬂuxionality are likely the key features that enabled the use

of analogous, covalently copolymerized complexes as emissive

probes for detecting stress in polymer ﬁlms.

t

i

Synthesis, characterization, and full NMR, UV−vis, and

FT-IR spectra of complexes 1−10, spin-saturation

transfer kinetic experiments, cyclic voltammograms of

complexes 5−10, and X-ray structure determination

details (PDF)

Accession Codes

CCDC 1818217−1818224 and 1851237−1851238 contain the

supplementary crystallographic data for this paper. These data

can be obtained free of charge via www.ccdc.cam.ac.uk/

data_request/cif, or by emailing data_request@ccdc.cam.ac.

uk, or by contacting The Cambridge Crystallographic Data

Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44

1223 336033.

AUTHOR INFORMATION

Corresponding Author

*E-mail: juliak@oist.jp (J.R.K.).

■

CONCLUSION

■

ORCID

In summary, we have explored in detail the structure, ligand

conformational behavior, and redox and photophysical proper-

ties of a series of copper(I) complexes with a conformationally

ﬂuxional ^RN4 pyridinophane ligand. Steric requirements of the

axial amine substituents in these complexes were shown to play a

key role in determining the solution behavior and emissive

properties of these complexes. The solution NMR studies

revealed two types of exchange processes that exist in solutions

of (^RN4)Cu^II, one involving degenerative exchange between

two axial amino groups in a κ³-coordinated ligand and another

being the exchange between the κ³- and κ⁴-bound ligand

isomers. The dynamics of exchange were directly correlated to

the steric bulk of the alkyl group at the amines, with complexes

containing more bulky alkyl groups at the amines showing

slower conformational exchange.

Robert R. Fayzullin: 0000-0002-3740-9833

Julia R. Khusnutdinova: 0000-0002-5911-4382

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

We thank Dr. Hirohiko Watanabe (Hamamatsu Photonics) for

kind assistance with measurement of the PLQYs and Dr. Kieran

Deasy (Mechanical Engineering & Microfabrication support

section, OIST) for help with the emission lifetime measure-

ments, and special thanks go to Dr. Michael Roy (technical

support for NMR, mass spectrometry, and elemental analysis).

We are also sincerely grateful to Dr. Pavlos Stampoulis (JEOL

RESONANCE Inc.) and Prof. Ilya Gridnev (Tohoku Uni-

versity) for discussions and suggestions regarding NMR

measurements. We also thank Dr. Ayumu Karimata and Dr.

Nancy Singhal for helpful discussions. The authors acknowledge

the Okinawa Institute of Science and Technology Graduate

University for start-up funding. This work was supported by

JSPS KAKENHI Grant JP18K05247.

Moreover, we have shown that the mononuclear complexes

supported by an N-donor macrocyclic ligand, (^RN4)Cu^II, show

emissive properties in solution and in the solid state, with the

PLQY highly sensitive to the steric bulk of the amine

substituents, likely through suppression of the nonradiative

decay pathways in less ﬂuxional complexes bearing bulkier alkyl

groups. Because the steric bulk of the alkyl group at the amine

can be easily controlled by simple synthetic modiﬁcations, this

oﬀers a convenient strategy for control of the photophysical

properties of simple, solution-stable mononuclear copper(I)

complexes. Importantly, the current study showed that, although

increasing the steric bulk eﬀects the axial amine−Cu^Icenter

distance, it does not lead to faster exchange equilibria, and the

quantum yield and emission lifetime only improve in the series

of simple alkyl-substituted complexes studied here. These

photophysical properties are adversely aﬀected when an alkyl

group is substituted for an electron-withdrawing Ts moiety.

These ﬁndings have a direct implication in the design of future

dynamic polymer probes,⁶⁶where we will seek to introduce even

bulkier, alkyl-based substituents in order to decrease the amount

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