Fluxional motion in a dinuclear copper(i) complex with a propeller-type ligand: metal hopping on both sides

Article abstract of DOI:10.1039/C8DT03884H

The first study of fluxional motion in [Cu₂(2-HPB)(MeCN)₂Cl₂] (1,2-HPB = hexa(2-pyridyl)benzene) is presented. For detailed examination of the fluxional motion mechanism, a monofluorinated derivative of the ligand (MFHPB)

Full text of DOI:10.1039/C8DT03884H

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Fluxional motion in a dinuclear copper(I) complex
with a propeller-type ligand: metal hopping on
both sides†
Cite this: Dalton Trans., 2018, 47,
17206
Received 26th September 2018,
Accepted 13th November 2018
a,b
Hyunchul Kwona and Eunsung Lee
*
DOI: 10.1039/c8dt03884h
rsc.li/dalton
The first study of fluxional motion in [Cu2(2-HPB)(MeCN)2Cl2] (1,2- the central benzene ring, while two remaining pyridines are
HPB = hexa(2-pyridyl)benzene) is presented. For detailed examin- uncoordinated (Fig. 1).
ation of the fluxional motion mechanism, a monofluorinated
In this paper, we present a more detailed mechanistic study
derivative of the ligand (MFHPB) and its copper(I) complexes were of the fluxional behaviour of [Cu2(2-HPB)(MeCN)2Cl2] complex,
synthesized and characterized. The solution NMR studies of 1, which was cursorily presented in the previously published
monofluoro copper(I) complex 1a suggest the existence of five article.22 We successfully synthesized monofluorohexa(2-
species in equilibrium. NMR spectra and DFT calculations suggest pyridyl)benzene (MFHPB) for detailed mechanism elucidation
the fluxional motion of 1 resulting in the “metal hopping process” of the fluxional behaviour. Among six pyridine rings in this
of two copper(I) ions.
ligand, one contains a fluorine atom in the meta-position to
nitrogen, while the other five pyridine rings remain unsubsti-
tuted. The presence of the fluorine atom was expected to give
rise to isomers of the corresponding dicopper(I) complex by
breaking the symmetry of the ligand (from D3d for 2-HPB to Cs
for MFHPB), thus providing additional information about the
fluxional processes of 1 (Scheme 1). We also propose the struc-
tures of the intermediate species formed in the course of the
fluxional motion of 1 based on 19F NMR data and other experi-
mental evidence. Finally, the mechanism of the fluxional
motion is also supported by density functional theory (DFT)
calculations.
The study of fluxional processes in coordination chemistry pro-
vides a comprehensive explanation of the bonding between the
metals and ligands involved1–8 and subsequently assists in
rationalizing chemical reactivity patterns.9–16 Several fluxional
coordination systems were previously reported. (See Table S3†)
Among them, metal complexes with highly symmetrical multi-
dentate ligands often exhibit rich fluxional behaviour, while
metal atoms interchange between symmetry-equivalent
positions.17–23 In this context, hexapyridylbenzene (HPB)
derivatives with six identical pyridyl units are highly attractive
symmetric ligands that can coordinate to metal centers
forming complexes exhibiting fluxional behaviour.24–29 The
Shionoya group reported a trinuclear Ag(I) sandwich complex
that exhibits ‘ball bearing’ molecular behaviour.30–34 In our
group, coordination properties of the 2-HPB ligand in copper
(I) complexes of variable stoichiometry have been studied as
well as the dynamic behaviour of the dinuclear 2-HPB copper
(I) complexes.22 In the previous article, we briefly described the
fluxional behaviour of complex 1, [Cu2(2-HPB)(MeCN)2Cl2]. In
complex 1, the 2-HPB ligand acts as a bis-bidentate ligand for
two Cu(I) centers which are located on the opposite sides of
The 1H NMR spectra of complex 1 in d3-acetonitrile solu-
tion at 323 K showed only four signals for 24 protons instead
of twelve signals for coordinated and non-coordinated pyridine
aDepartment of Chemistry, Pohang University of Science and Technology,
77 Cheongam-Ro, 37673 Pohang, Republic of Korea. E-mail: eslee@postech.ac.kr
bDivision of Advanced Materials Science, Pohang University of Science and
Technology, 77 Cheongam-Ro, 37673 Pohang, Republic of Korea
†Electronic supplementary information (ESI) available. CCDC 1552119 and
1874373. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c8dt03884h
Fig. 1 Top view (a) and side view (b) of [Cu2(2-HPB)(MeCN)2Cl2], 1.
Ellipsoids are shown at the 50% probability level and hydrogen atoms
are omitted for clarity.
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1
The H NMR spectrum of the MFHPB ligand is similar to
that of the unsubstituted 2-HPB ligand with the integration
ratio of 5 : 1 for the ortho-proton region, due to one 5-fluoropyr-
idine unit. The MFHPB ligand with 2 equivalents of copper(I)
chloride gives the corresponding complex [Cu2(MFHPB)
(MeCN)2Cl2], 1a which has an identical coordination mode to
1 in d3-acetonitrile solution (Fig. 2). At 323 K, the 1H NMR
spectrum of 1a is similar to that of the free ligand MFHPB,
suggesting rapid dynamic motion in the complex. As the tem-
perature decreases, the signals become broad and the coalesc-
ence temperature was determined to be 273 K. At 223 K, all
proton signals split, showing a pattern analogous to that of 1
at the same temperature (Fig. 2(c) and (f)). In the complex,
signals arising from six protons in the ortho-position to nitro-
gen were considered. At 323 K, the six protons integrate as 5 : 1
in the 1H NMR spectrum (Fig. 2(e)). At 223 K, the signals from
the ortho-protons split into five signals in the (3 : 3) : (1 : 1 : 1)
ratio (i.e., 2 : 1 ratio for coordinated pyridine and non-co-
ordinated pyridine) (Fig. S10†). At 323 K, the 19F NMR spec-
trum of 1a shows only one signal at −131.03 ppm which is
downfield shifted compared to the signal from the free ligand
(Fig. S7†). The coalescence of the signal was observed at 273 K.
At 223 K, the signal split into five peaks at −127.7, −129.6,
−129.8, −130.4, and −130.7 ppm with the integration ratio of
1.00 : 0.21 : 0.12 : 0.24 : 1.82 (Fig. 3(a)). Due to high resolution
of the fluorine nuclei, the signals from the intermediate
species were now distinguishable. In an NMR titration experi-
ment, the 19F NMR signals in the solution of MFHPB with one
equivalent of copper(I) chloride differ from the signals of 1a,
while signals in solution containing three equivalents of
copper(I) chloride remain the same as those in 1a. This
implies that all five different isomeric species contain two
copper(I) atoms per one ligand molecule (Fig. S11†). In 19F 2D
EXSY, cross signals between the five fluorine signals were
observed (Fig. 3(b)).
Scheme 1 Synthesis of hexa(2-pyridyl)benzene (2-HPB), penta(2-
pyridyl)benzene (2-PPB), and monofluorohexa(2-pyridyl)benzene
(MFHPB).
rings, which indicates dynamic motion in the complex
(Fig. 2(b)). At 223 K, the signals were separated in accordance
with the proton ratio in the solid state structure (Fig. 2(c)). The
two-dimensional exchange spectroscopy (2D-EXSY) spectrum
of 1 shows the exchange process between coordinated and
uncoordinated pyridines with the following activation para-
meters: ΔH‡ = 16.47 0.02 kcal mol−1, ΔS‡ = 13.40 0.16 e.u.,
ΔG‡ = 13.36
0.11 kcal mol−1, and Ea = 16.9 kcal mol−1
(Fig. S8 and S18†). The positive ΔS‡ indicates that the tran-
sition state is more disordered than the starting material.
Therefore, it is proposed that in the course of the fluxional
motion, one of the copper(I) atoms dissociates to facilitate the
exchange, i.e., the mechanism is dissociative. In addition to
the signals of 1, cross peaks from the unassigned broad
signals in the range of 8.3 to 8.4 ppm were observed indicating
the presence of an intermediate in the course of fluxional
behaviour. The other 1H signals of this species probably
overlap with signals of 1 (Fig. S9†).22
The proposed structures of the five different isomeric
species of 1a are shown in Fig. 4. The structures of 1a_1 and
1a_2 are analogous to the structure of 1 obtained by single
crystal X-ray diffraction (SCXRD) and the fluorine atom is
attached to the non-coordinated ring and coordinated ring,
respectively. The structure of 1a obtained by SCXRD clearly
indicates that the fluorine atom is disordered in six pyridine
Fig. 2 1H NMR spectra of 2-HPB (a), complex 1 at 323 K (b), complex 1
at 223 K (c), MFHPB (d), 1a at 323 K (e), and 1b at 223 K (f) in
d3-acetonitrile.
Fig. 3 VT 19F NMR spectra of 1a (a) and 19F NMR 2D-EXSY of 1a at
223 K (τ = 200 ms) (b).
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Fig. 4 The calculated rate constants for fluxional motion (s−1) in 1a.
rings (Fig. S15†). Our earlier study of the coordination modes
of 2-HPB allows us to suggest the structures 1b_1, 1b_2, and
1b_3 for the three intermediate species observed in the low
temperature 19F NMR spectrum.23 In those complexes, two
copper(I) atoms located on the opposite sides of the benzene
ring coordinate with four pyridines in a row leaving two pyri-
dines in the ortho-position to each other non-coordinated
(Fig. 5(b)). This coordination mode can explain the presence of
three non-equivalent fluorine nuclei in the same ligand con-
formation (Fig. S12†). By correlating the integration values of
Fig. 5 The optimized structures of the proposed intermediates 1B_1 (a)
and 1B_2 (b). Energy diagram for the proposed metal hopping process
(c) (pathway (ii) – dotted line, pathway (iii) – solid line). Atom colors in
(a) and (b): C, gray; N, blue; Cl, green; Cu, brown. Hydrogen atoms are
omitted for clarity. In (c), red labeled atoms are pointing up from the
plane. Blue labeled atoms are pointing downside from the plane. The
ΔG° values are given in kcal mol−1
.
1
19F NMR and H NMR data, we could surely perceive that two
major signals at −127.7 ppm and −130.7 ppm correspond to
1a_1 and 1a_2, respectively. Based on the chemical shifts in
19F NMR spectra, we propose that the three signals at −129.6,
−129.8, and −130.4 ppm correspond to 1b_1, 1b_2 and 1b_3,
respectively. The rate constants for the exchange between the
two major isomers 1a_1 and 1a_2 are 0.153 0.044 s−1 and
0.338
0.048 s−1, while the exchange rate of 1a (1a_1 and
Fig. 6 The proposed mechanism for the fluxional motion of 1. Red
labeled atoms are pointing up from the plane. Blue labeled atoms are
pointing down from the plane.
1a_2) to 1b (1b_1, 1b_2 and 1b_3) ranged from 1.02 0.19 to
5.03 0.10 s−1 (Fig. 4 and Fig. S17†).
Complex 1 is a dinuclear copper(I) complex with six pyri-
dine binding sites. Equal population of all pyridine binding
sites can be achieved via a rapid 180 degree rotation of the
non-coordinated pyridine rings, followed by a dissociation- of both copper centers, this pathway proceeds via the relatively
recombination process. In such a “hopping” process, one of stable intermediate 1B_2 (Fig. 5(a) and (b)).
the copper(I) centers (red in Fig. 6) disconnects itself from the
19F 2D EXSY of 1a at 223 K shows that five different dicop-
pyridine ring 1 and then coordinates to the ring 3. The two per(I) species are involved in the fluxional motion of the
copper(I) centers in 1 can move between the pyridine nitrogen complex. By considering the presence of the intermediate
atoms following three pathways: (i) concerted hopping of the species, we can rule out the concerted pathway (i). The path-
two copper(I) atoms by simultaneous dissociation of the two ways (ii) and (iii) can be considered since the conformations of
copper-nitrogen bonds, followed by bond formation with pre- both the proposed intermediates 1B_1 and 1B_2 would give
viously non-coordinated pyridine rings, or (ii) dissociation of three different signals in 19F NMR spectra when MFHPB is
copper–nitrogen bonds on both copper(I) centers (settling into used as a ligand (Fig. S13†). To explain the experimental obser-
the relatively unstable intermediate 1B_1), followed by recom- vations and to provide information about the thermodynamic
bination with new pyridine rings, or (iii) subsequent hopping stability of possible intermediate species, DFT calculations at
17208 | Dalton Trans., 2018, 47, 17206–17210
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the B3LYP/6-31G(d,p) level of theory were performed (see the In addition, the rotational barriers for the non-coordinated
ESI†). In the calculation of ΔG°, solvation free energies (aceto- pyridine ring in 1 and 1B_2 were calculated. We conclude that
nitrile) using SMD were included. The calculated structures the hopping process of two copper atoms between six pyri-
and energy profiles for the exchange between 1 and two pro- dines occurs along with a rotation of the pyridine ring in the
posed intermediate species 1B_1 and 1B_2 are shown in Fig. 5. complex 1.
Fig. 5(c) shows the calculated energy diagram for two poss-
ible pathways for the fluxional motion in complex 1. In
pathway (ii) (dotted line), one copper(I)–nitrogen bond dis-
sociates (1 to 1A), and then dissociation of another copper(I)–
Conflicts of interest
nitrogen bond occurs (1A to 1B_1). After that, one copper atom There are no conflicts to declare.
forms a copper(I)–nitrogen bond with a new pyridine (1B_1 to
1C). This is followed by second copper(I)–nitrogen bond for-
mation, thus restoring the original conformation (1C to 1). On
the other hand, in pathway (iii) (solid line), one copper(I)–
Acknowledgements
nitrogen bond dissociates (1 to 1A) and the copper atom This work was supported by the Institute for Basic Science
associates with a new pyridine (1A to 1B_2), and then 1B_2 (IBS) [IBSR007-D1], MSIP and PAL, Korea. The X-ray crystallo-
converts to 1 via the intermediate 1C.
graphy analysis with synchrotron radiation was performed at
The calculated energy difference between 1 and 1B_2 is the Pohang Accelerator Laboratory (PLS-II BL2D SMC). We
0.34 kcal mol−1 indicating comparable stability of both com- thank MSc ETH Ewa Pietrasiak, Dr Young Ho Ko, and
plexes and implies that the two isomers should exist in notice- Prof. Donghwan Lee for helpful discussions.
able amounts in equilibrium. This is supported by the fact
that the signal from isomer 1B_2 was observed in the 1H 2D
EXSY. Likewise, the isomeric species 1b_1, 1b_2 and 1b_3 were
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