Ethylene-Bridged Tetradentate Bis(amidines): Supramolecular Assemblies through Hydrogen Bonding and Photoluminescence upon Deprotonation

Article abstract of DOI:10.1002/ejoc.202000207

Sterically crowded tetradentate bis(amidines) encapsulate their N–H functionalities or unveil them to undergo inter- and intramolecular hydrogen bonding both in solid state and solution, depending on a subtle interplay between the amidine backbone substituents. X-ray crystallography reveals for four distinct ZZ(syn/syn) and EE(syn/syn) bis(amidines) that bulky terminal N-Mes groups in combination with N₂C-^tBu or N₂C-Ph substituents result in steric protection of the N–H moieties, whereas less crowded terminal p-^tBu(C₆H₄) groups either show encapsulation (N₂C-^tBu) or hydrogen bonding (N₂C-Ph), the latter resulting in a bis(amidine) dimer formed by inter- and intramolecular hydrogen bonds. Moreover, a supramolecular solvent adduct consisting of one bis(amidine) and four ethanol molecules is presented. DFT calculations show that both the dimerization and formation of the solvent adduct is associated with a significant energy gain (dimerization: ΔE = –27.7 kcal/mol; formation of ethanol adduct: ΔE = –64.3 kcal/mol). The corresponding four Li bis(amidinates) are weakly blue to green-emissive in THF solution. Overall, a new series of highly flexible bis(amidines) has been examined.

Full text of DOI:10.1002/ejoc.202000207

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Ethylene-Bridged Bis(amidines)
Ethylene-Bridged Tetradentate Bis(amidines): Supramolecular
Assemblies through Hydrogen Bonding and Photoluminescence
upon Deprotonation
Alvaro Calderón-Díaz,[a] Janet Arras,[a] Ethan T. Miller,[a] Nattamai Bhuvanesh,[b]
Colin D. McMillen,[c] and Michael Stollenz*[a]
Dedicated to Professor Uwe Rosenthal on the occasion of his 70th birthday
Abstract: Sterically crowded tetradentate bis(amidines) encap- ter resulting in a bis(amidine) dimer formed by inter- and intra-
sulate their N–H functionalities or unveil them to undergo inter- molecular hydrogen bonds. Moreover, a supramolecular solvent
and intramolecular hydrogen bonding both in solid state and adduct consisting of one bis(amidine) and four ethanol mole-
solution, depending on a subtle interplay between the amidine cules is presented. DFT calculations show that both the dimeri-
backbone substituents. X-ray crystallography reveals for four zation and formation of the solvent adduct is associated with
distinct ZZ(syn/syn) and EE(syn/syn) bis(amidines) that bulky ter- a significant energy gain (dimerization: ΔE = –27.7 kcal/mol;
minal N-Mes groups in combination with N2C-tBu or N2C-Ph formation of ethanol adduct: ΔE = –64.3 kcal/mol). The corre-
substituents result in steric protection of the N–H moieties, sponding four Li bis(amidinates) are weakly blue to green-emis-
whereas less crowded terminal p-tBu(C6H4) groups either show sive in THF solution. Overall, a new series of highly flexible
encapsulation (N2C-tBu) or hydrogen bonding (N2C-Ph), the lat- bis(amidines) has been examined.
Introduction
mediate catalytic ring-opening polymerizations/copolymeriza-
tions of cyclic esters[9,10] and amidations of aldehydes with
amines.[11] Tetradentate bis(amidines) are also capable of em-
bedding multiple late transition metal ions.[12–14]
We have recently presented a new class of flexible ethylene-
linked bis(amidines) 1–5 that bear additional terminal N-donor
sites (Figure 1).[15] These aromatic variants of the closely related
and extremely versatile bridged 1,3,5-triazapentadienes[16] fea-
ture variable inter- and intramolecular NH···N′ hydrogen bonds
in the solid state that are also preserved in solution (Figure 1).
Upon deprotonation using nBuLi or NaN(SiMe3)2, the corre-
sponding blue-light emitting bis(amidinates) [1–5Li2] are
formed in THF solution.
Amidines represent a versatile class of isoelectronic N-analogs
of carboxylic acids and esters. They have attracted considerable
attention as medical and biochemical reagents, as building
blocks for redox-switchable chromophores/fluorophores, and
also as important ligands in coordination chemistry.[1–3] Associ-
ated applications include homogeneous catalysis[3] as well as
utilizing amidinate complexes of transition[4] and rare earth
metal[3e,3h] amidinates as precursors for producing thin metal/
metal oxide films by atomic layer deposition (ALD).
Although known since the early 1950s,[5] tethered bis-
(amidines) remained underrepresented in the literature until
their striking analogy to bis(cyclopentadienyl) ligand scaffolds
in “constrained” ansa metallocenes became apparent.[6] In par-
ticular, bis(amidines) linked to alkylene and arylene backbones
have been identified as excellent ligands for alkali metal,
lanthanide, and Group 4 metal complexes,[7,8] some of which
[a] A. Calderón-Díaz, Dr. J. Arras, E. T. Miller, Prof. Dr. M. Stollenz
Department of Chemistry and Biochemistry, Kennesaw State University,
370 Paulding Avenue NW, MD1203, Kennesaw, GA 30144, USA
E-mail: Michael.Stollenz@kennesaw.edu
http://facultyweb.kennesaw.edu/mstollen/
[b] Department of Chemistry, Texas A&M University,
580 Ross Street, P.O. Box 30012, College Station, TX 77842-3012, USA
[c] Department of Chemistry, Clemson University,
Figure 1. Ethylene-bridged N,N′-disubstituted bis(amidines) 1–9.
Our ongoing interest in tetradentate bis(amidines) has arisen
from designing a suitable ligand framework for defined linear
CuI cluster assemblies[17] that were obtained from 8 and mesit-
ylcopper.[18]
379 Hunter Laboratories, Clemson, SC 29634-0973, USA
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.202000207.
Part of the French Chemical Society DCC Prizewinner Collection.
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Although ethylene-bridged bis(amidines) were the first re-
Compared to the more electron-deficient bis(amidines) 8
ported examples of tetradentate bis(amidines) interconnected and 9, the CH2 and NH signals of 6 and of 7 are substantially
by a flexible alkylene linker,[5] still little is known about their upfield-shifted by up to 1.44 ppm (CH2) and 3.37 ppm (NH),
molecular structures both in solid state and solution. Only the respectively. Only the N–H peak of 7 is very broad in the CDCl3
related trans-1,2-diaminocyclohexane-linked tetradentate (t-1,2- (δ = 4.53 ppm) and in the C6D6 spectra (δ = 4.61 ppm) whereas
DACH-)bis(amidines) have been in the focus of two earlier re- 6 indicates only slight line broadening in C6D6 (at δ =
ports.[7] No accompanying computational studies have been 3.91 ppm), accompanied by an upfield-shift of about 0.1 ppm
published yet.
for that signal, in comparison with the CDCl3 spectrum. This is
in clear contrast to bis(amidine) 8 that produces broad reso-
nance signals in its C6D6-1H NMR spectrum at room tempera-
ture.[17] Additional broadened signals at δ = 3.02, 5.72, and
7.89 ppm were found and suggest that C=N double-bond
stereoisomers (EE, EZ/ZE, and ZZ) and their syn/anti rotational
isomers (c) exist in solution. These isomers can interconvert
through tautomerism (a) and C–N single-bond rotation (b) at
the amidine moieties (Scheme 2).
This report focuses on the synthesis and molecular structures
of the new tetradentate bis(amidines) 6, 7, and 9 that exhibit
sterically encapsulated amidine moieties or NH groups amena-
ble to form remarkable networks of inter- and intramolecular
NH···N′ hydrogen bonds in the solid state, depending on the
nature of the amidine substituents (Figure 1). An example of a
supramolecular solvent adduct (9·EtOH) is also presented.
Moreover, we found strong evidence for hydrogen bonding re-
tained in solution for bis(amidine) 9 that shows hydrogen bond-
ing in the solid state. A comprehensive computational study
(gas phase) supports these observations. Similar to the hexad-
entate bis(amidines) 1–5, also their tetradentate congeners 6–9
become emissive in THF solutions upon deprotonation through
nBuLi – a phenomenon that has never been reported for simple
bis(amidinates) of this kind yet.
If the predominant tautomer retains the –NH(CH2)2NH– di-
amine core motif, then eight distinct and four additional pairs
of identical stereoisomers are possible for ethylene-bridged
N,N′-disubstituted bis(amidines). This was confirmed for the
solid state by the XRD structures of 1–5,[15] 6, 7, 8,[17]
9·0.8CH2Cl2, and 9·EtOH that all show the ethylenediamine tau-
tomeric form (vide infra).
The underlying prototropic exchange is considerably sup-
pressed in 8 if [D6]DMSO is used, as only one set of resonance
signals becomes apparent.[17] For 6, 7, and 9, line width of the
broad N–H resonance signals decreases significantly in
[D6]DMSO and they are also downfield-shifted by 1.71 ppm (6),
1.04 ppm (7), 1.76 ppm (8),[17,19] and 1.41–1.46 ppm (9),[20] if
Results and Discussion
The synthesis of 6–9 followed a proven three-step protocol
(Scheme 1).[15,17] Two benzoylation methods of ethylene-
diamine (A and B) were identified to provide the diamides I
and II in excellent yields (up to 91 %). Chlorination using PCl5
afforded the bis(imidoyl chlorides) III and IV (yields: 70–86 %)
that were subjected to aminolysis with the corresponding aryl-
amines to afford the target bis(amidines) 6–9 as microcrystal-
line solids in good yields (62–72 %).
1
compared to their respective H NMR spectra in the least polar
solvent (C6D6) within this series (Table S7). Most evidently, the
N–H signal of 9 in C6D6 shows a notable downfield shift (by up
to 0.60 ppm) in comparison with 8.[19] This difference in chemi-
cal shift is more pronounced than for the CH2 resonances
(|Δδ| = 0.13 ppm)[21] and the phenyl group proton signals (|Δδ|
≈ up to 0.1 ppm) of 8 and 9. Therefore, sole electronic effects
of the terminal aryl substituents are not exclusively responsible
for the large difference between the N–H shifts. Similar to the
hexadentate bis(amidines) 1–5,[15] the large |Δδ| value strongly
suggests the presence of hydrogen bonds in solution.[22] The
deshielding effect of hydrogen bonds is more distinct in less
polar C6D6 than in CDCl3 or [D6]DMSO, as the latter two serve
as potential hydrogen-bond donor/acceptors. Temperature-de-
pendent 1H NMR spectra of 9 in CDCl3 show increased deshield-
ing of the N–H protons with decreasing temperature (by
+0.67 ppm from 27 °C to –60 °C), which is also consistent with
the formation of hydrogen bonds in solution (Figure S31).
t
t
Scheme 1. Three-step synthesis of 6–9 (6: R = Bu, Ar = Mes; 7: R = Bu, Ar =
p-tBu(C6H4); 8: R = Ph, Ar = Mes; 9: R = Ph; Ar = p-tBu(C6H4)).
X-ray crystallography confirmed hydrogen bonding for the
solid-state structure of 9 and the existence of hydrogen bonds
in solution was supported by density functional theory (DFT)
calculations (vide infra).
1
The H NMR spectra of the tetradentate bis(amidines) 6, 7,
and 9 (recorded in CDCl3, C6D6, and [D6]DMSO, respectively)
each show one set of sharp resonances for all C–H protons that
The 13C{1H} NMR spectra of 6 and 7 in all three deuterated
suggest the presence of one tautomeric form in solution (see solvents are very similar and show the diagnostic signals of
Table S7 and Figures S13–S31 in the Supporting Information, the common –N=C(tBu)NH(CH2)2NH(tBu)C=N– core motif at δ =
t
SI). The bulky Bu groups attached to the amidine moieties are 28.8–29.4, 38.3–39.0 (tBu), 42.0–44.3 (CH2), and 158.0–
indicated by sharp singlets between δ = 1.09 and 1.20 ppm, 160.9 ppm (amidine signal, Table S8). Notably, the CH2 reso-
thus suggesting fast rotation.
nance of 7 is clearly broader than of 6, which is consistent with
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the corresponding broader proton signals of 7 in CDCl3, C6D6,
Single crystals of 9 suitable for XRD analysis were isolated
and [D6]DMSO, respectively. Bis(amidines) 8 and 9 show similar from a concentrated CH2Cl2 solution (the result is shown in Fig-
CH2 shifts (δ = 40.6–43.7 ppm) and similar quaternary carbon ure 2, Figures S7–S9, and Table S2. This bis(amidine) crystallizes
signals of the amidine moieties (δ = 155.5–158.4 ppm).
as a solvate 9·0.8CH2Cl2, without any significant hydrogen
bonding interactions between CH2Cl2 and the bis(amidine) moi-
eties (Figure S9; closest donor/acceptor distance for N–H···Cl is
3.780(3) Å). However, as opposed to 6–8, bis(amidine) 9 does
form intramolecular hydrogen bonds between the two tethered
amidine compartments to constitute a seven-membered ring.
In addition, intermolecular hydrogen bonds are observed be-
tween two molecular entities forming a 14-membered ring
within the resulting Ci-symmetrical dimer of 9. This phenome-
non was earlier observed for a tetradentate (t-1,2-DACH-
)bis(amidine)[7a] and the hexadentate congener 5.[15] Recently,
a related bis(guanidine) featuring an n-propylene bridge has
been reported that shows inter- and intramolecular hydrogen
bonding as well.[23]
In order to enlighten the structural features of 6, 7, and 9 in
the solid state, single crystals were grown and subjected to
X-ray crystallographic structure determinations. Suitable single
crystals of 6 and 7 were obtained from hexanes solutions (see
Experimental Section). The results for 6 and 7 are shown in
Figure 2, Figures S1–S6, and Table S1; key crystallographic dis-
tances and angles are outlined in Table S3. In contrast to the
previously reported solid-state EE (syn,syn) form of bis(amidine)
8, the XRD analyses reveal that 6 and 7 exclusively exist as
ZZ(syn, syn) stereoisomers in the crystalline state. This is consist-
ent with the increased steric repulsion between the terminal
aryl substituents and the bulky tert-butyl group. Similar to 8,
the bulky substituents of the amidine components of 6 and 7
also hamper the formation of intramolecular hydrogen bonds
According to Jeffery's classification,[26] all hydrogen bonds of
that were earlier observed in the less crowded hexadentate 9 can be considered as being moderately strong (NNH···N=C:
bis(amidines) 1–5.[15]
2.86–3.06 Å; NH···N: 2.12–2.21 Å; N–H···N angles of 141–161°
Scheme 2. a: Tautomerism in 6–9; b: syn/anti rotational isomers through C–N single-bond rotation; c: ZZ (syn/syn), ZE/EZ (syn/syn), and EE (syn/syn) isomers
of 6–9 including two rotational isomers, III and V, that show intramolecular hydrogen bonds and their relative stability (kcal/mol) in the gas phase. Structure
representations denoted with an asterisk are geometry-optimized structures based on X-ray data. The remaining 13 syn/anti and anti/anti rotational isomers
that correspond to I, II, and IV[24] are not shown.[25] The computed structures of I–V depicted are the rotational isomers of 6 as representative examples for
6–9. See SI for more information.
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angles of 6–9 and 9·4EtOH is substantial – they are larger by
6–10° in the ZZ isomers than in the EE isomers. This is consistent
with the increased steric constraints on the amidine moieties in
the ZZ isomers, as observed for 6 and 7.
The relative stability (kcal/mol) for the molecular structures
of 6–9 (single-crystal X-ray structures I, IV and V and additional
ZE (syn/syn) isomers II and III) was modeled in the gas phase
using DFT at the RI-B3LYP-D3/def2-TZVP level of theory
(Scheme 2; see the SI for more information).
This study helps us to understand which rotational isomers
could be accessible in solution and if the internal hydrogen
bonding in III and V would give stability preferences. For 6 and
t
7, the compounds with the bulky Bu substituents (ZZ (syn/syn)
isomer I) is energetically favored (in the gas phase by 6.6 kcal/
mol (6) and 3.7 kcal/mol (7) over IV, and by 3.8 kcal/mol (6)
and 5.1 kcal/mol (7) over II) as evidenced by XRD analysis and
predicted by DFT calculations. The less bulky Ph substituent in
8 and 9 results in lower energies of the EE (syn/syn) isomers IV
(8) and V (9) (in the gas phase by 1.1 kcal/mol (8) and 6.7 kcal/
mol (9) in relation to I, and by 0.2 kcal/mol (8) and 5.1 kcal/mol
(9) in relation to II). In the gas phase for 9, form IV is the lowest
energy structure (by about 0.8 kcal/mol energetically favored
over V). For all isomers II–V, 8 is higher in energy than 9. In
order to understand the effect of the intermolecular hydrogen
bonding in 9, we calculated the dimerization energy and the
formation of 9·4EtOH from 9 + 4 EtOH. Both reactions are signif-
icantly exothermic (dimerization: ΔE = –27.7 kcal/mol; forma-
tion of 9·4EtOH: ΔE = –64.3 kcal/mol).
Figure 2. XRD molecular structures of 6, 7, 9, and 9·EtOH.
The unexpected photoluminescence behavior of the hexa-
dentate bis(amidinates) [1–5Li2] encouraged us to examine the
corresponding Li complexes of 6–9.[15] There are only a few
examples of isolated Li complexes with tetradentate bis(amidin-
ates) described in the literature and no photophysical proper-
ties have been reported yet.[7,8c–8e,8j] To our surprise, the
tetradentate bis(amidines) 6–9 become weakly emissive in THF
solution upon deprotonation using nBuLi to form [6–9Li2]
(Scheme 3).
(Table S5). Consequently, the IR ν(N–H) stretching frequency of
9 (3272 cm–1) is redshifted by 132–175 cm–1 relative to the
bis(amidines) 6–8 that do not show evidence of hydrogen
bonding in the solid state (Table S6). This is in turn consistent
with the ν(N–H) values of 1–5 as these have hydrogen bonds
which lie in a similar range (3252–3309 cm–1).[15]
The fragile network of hydrogen bonds is susceptible to al-
ternative hydrogen-bond donor/acceptor molecules, as demon-
strated by the formation of solvent adduct 9·4EtOH (see Fig-
ure 2 Figures S10–S12, and Table S2).
This supramolecular assembly was obtained by recrystalliza-
tion of 9 from a mixture of ethanol and acetonitrile (see Experi-
mental Section). Both amidine units accept overall four ethanol
molecules in an approximate D2h-symmetrical arrangement;
two of which are undergoing intermolecular O–H···O hydrogen
bonding interconnect the amine NH donor of one amidine
compartment with the imine acceptor of the second one
through N–H···O and O–H···N hydrogen bonds. This gives rise
to two 11-membered rings. While all hydrogen bonds in
9·4EtOH are moderately strong, the O–H···O and O–H···N bonds
are significantly shorter (by up to 0.09 Å) than the N–H···O
bonds (Table S5). The pronounced N–C single bond character
of the central –NH(CH2)2NH– diamine bridge gives rise to large
ΔCN parameters[1, 27] for 6–8 (0.073–0.956 Å, Table S4). Bis-
(amidine) 9 and its solvent adduct 9·4EtOH have clearly smaller
values (0.059 and 0.0602 Å), thus indicating an influence of
hydrogen bonding on increased delocalization within the amid-
ine moieties. The impact of E/Z isomerization on the N=C–NH
Scheme 3. Deprotonation of 6–9 and steady-state emission spectra of
[6–9Li2] in THF.
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98 %), and n-butyllithium (Acros, 1.6
M in hexanes) used as received
The steady-state photoluminescence spectra of [6–9Li2]
without further purification. The syntheses of N,N′-1,2-ethanediyl-
bis(2,2-dimethylpropanimidoyl chloride)[15] and 8[17] were previ-
ously described.
show emission maxima between 456–502 nm that are clearly
redshifted (by up to 81 nm), in relation to [1–5Li2] (Figure S40–
S43, Table S35, and Ref. 15). The Stokes shifts of [6Li2] (0.50 eV)
and [7Li2] (0.55 eV) are comparable to [1–4Li2], whereas the
aryl-substituted bis(amidinates) [8Li2] and [9Li2] have smaller
values (0.40 eV and 0.25 eV, respectively), tending to be similar
to the enhanced-conjugated Li bis(amidinate) [5Li2] (0.16 eV).
This further supports our initial hypothesis that the bis(amid-
inate) moiety is the origin of the emission in THF solution,
which is predominantly affected by the individual amidinate
substituents.
Elemental analyses were performed by Atlantic Microlab, Inc. Melt-
ing points were determined with an SRS (Stanford Research Sys-
tems) Digi Melt instrument using open capillaries; values are uncor-
rected (the heating rate was 2 K/min). NMR measurements were
recorded on a Bruker Avance III 400 spectrometer at ambient probe
temperatures unless otherwise noted. 13C{1H} NMR resonances were
obtained with proton broadband decoupling and referenced to the
solvent signals of [D6]DMSO at 39.5, CDCl3 at 77.0, and C6D6 at
128.0 (1H NMR: 2.50 (DMSO), 7.24 (CHCl3), and 7.15 (benzene), re-
spectively). 13C{1H} NMR assignments are based on DEPT 135, and
the following 2D experiments: COSY, NOESY, HSQC, and HMBC.
Mass-spectrometric analyses were performed on a Waters Q-Tof API
US quadrupole time of flight MS system (low resolution ESI) and on
a Thermo Obitrap Velos Pro MS system (high resolution ESI). IR spec-
tra were measured on a PerkinElmer Spectrum One FT-IR Spectrom-
eter equipped with a Universal ATR Sampling Accessory. UV/Vis
spectra of solutions of 6–9 and [6–9Li2] in THF were measured with
a Cary 60 spectrometer. Steady-state emission spectra of solutions
of [6–9Li2] in THF were recorded on a PTI Picomaster 1 fluorescence
spectrometer system.
Conclusion
Three new ethylene-bridged tetradentate bis(amidines), 6, 7,
and 9, were reported. X-ray crystallography reveals that these
bis(amidines) form either distinct ZZ(syn/syn) isomers in the
t
solid state if bulky Bu groups serve as central substituents or
EE(syn/syn) isomers, if Ph substituents are incorporated into the
bis(amidine) scaffold. DFT calculations support this observation
for the gas phase. While the N–H functional groups of 6, 7,
and the closely related bis(amidine) 8 are encapsulated in the
sterically protected amidine compartments, 9 shows a versatile
network of intra- and intermolecular hydrogen bonds. Hydro-
gen bonding of 9 is retained in solution, which was confirmed
by significant NH-proton downfield shifts in the 1H NMR spectra
and supported by DFT gas phase calculations. Moreover, 9 is
capable of forming supramolecular assemblies with protic sol-
vents such as ethanol, as demonstrated by the solvent adduct
9·4EtOH. All bis(amidines) 6–9 form weakly luminescent Li
bis(amidinato) complexes [6–9Li2] through deprotonation in
THF solution. Ongoing studies and future projects will be de-
voted to explore the versatile coordination chemistry of 6–9
with respect to new photoluminescent materials.
Synthesis of N,N′-1,2-ethanediylbis(benzenecarboximidoyl
chloride): This compound was prepared in analogy to a method
described earlier, with modifications.[17] N,N′-1,2-ethanediylbis-
(benzamide) (9.887 g, 36.85 mmol) and PCl5 (15.347 g, 73.70 mmol)
were suspended in CH2Cl2 (100 mL) with stirring to give a clear
yellow solution. After 3 d, triethylamine (10.3 mL, 7.48 g, 73.9 mmol)
was added dropwise via syringe over a period of 5 min to form a
colorless precipitate. The suspension was reduced to 40 mL in oil
pump vacuum and filtered. The filter cake was washed with diethyl
ether (3 × 10 mL). Additional diethyl ether (30 mL) was added to
the filtrate to form more colorless precipitate. This was removed by
filtration and the filter cake was washed with cold (0 °C) diethyl
ether (3 × 10 mL). The filtrate was reduced to dryness using oil
pump vacuum, then redissolved in CH2Cl2 (20 mL), and finally
stored at –35 °C. After 2 d, pale yellow crystals formed that were
isolated by filtration, rinsed with cold (–78 °C) CH2Cl2 (3 × 3 mL),
and dried in oil pump vacuum for 18 h to yield a light yellow pow-
der. Yield: 7.918 g (25.94 mmol, 70 %).
Experimental Section
General methods: All synthetic procedures involving air- and mois-
ture-sensitive compounds were carried out by using Schlenk tech-
niques under an atmosphere of dry argon. Glassware was heat-
sealed with a heat gun under vacuum.
General Procedure for the Preparation of 6, 7, and 9: A solution
of arylamine (19.42 mmol; 9: 5.88 mmol) in toluene (10 mL) was
added dropwise to a solution of N,N′-1,2-ethanediylbis(2,2-dimeth-
ylpropanimidoyl chloride) (9.80 mmol; 9: 2.94 mmol) in toluene (40–
50 mL) with stirring. The reaction mixture was heated to reflux for
24 h, and then cooled to room temperature, and filtered. The filter
cake was first washed with cold (0 °C) toluene (3–5 × 10 mL), then
with diethyl ether (3 × 5 mL), and finally suspended in a mixture of
diethyl ether (50 mL) and a saturated aqueous Na2CO3 solution
(50 mL) which was stirred for 30 min. The organic phase was sepa-
rated, washed with water (3–5 × 100 mL; 9: 3 × 25 mL), and dried
with anhydrous Na2SO4. Subsequent filtration, removal of all vola-
tiles from the filtrate by rotary evaporation resulted in a colorless
solid that was recrystallized from hexanes (6–9) or from a mixture
of CH2Cl2 and diethyl ether (1:1 v/v) and dried in oil pump vacuum
for 14–24 h.
Solvents: Prior to use, tetrahydrofuran (THF), diethyl ether, and tolu-
ene were freshly distilled from sodium/benzophenone. CH2Cl2 was
distilled from CaH2. Alternatively, the aforementioned solvents were
purified using a PPT Solvent Purification System. For non-inert ma-
nipulations, CH2Cl2, diethyl ether, and hexanes (mixture of isomers)
were used as received without further purification.
Deuterated solvents: CDCl3 (Cambridge Isotope Laboratories, Inc., D,
99.8 % + 0.03 % v/v tetramethylsilane, TMS), C6D6 (Cambridge Iso-
tope Laboratories, Inc., D, 99.5 %), and [D6]DMSO (Cambridge Iso-
tope Laboratories, Inc., D, 99.9 %) were used as received without
further purification.
Reactants: Triethylamine (Alfa Aesar, 99 %) was distilled from so-
dium before use. Ethylene diamine (Alfa Aesar, >99 %), pivaloyl
chloride (Acros, 99 %), benzoic acid (Alfa Aesar, 99 %), N,N′-carbon-
yldiimidazole (Oakwood Chemical, 98+%), PCl5 (Alfa Aesar, 98 %),
mesitylamine (TCI, >99 %), p-tert-butylaniline (Oakwood Chemicals,
Synthesis of 6: This bis(amidine) was obtained by recrystallization
from hexanes at 5 °C within 2 d as a colorless crystalline solid. Yield:
72 %; Mp: 95.3–96.4 °C. Anal. Calcd for C30H46N4: C, 77.87; H, 10.02;
N, 12.11; found C, 77.95; H, 10.29; N, 12.08. 1H NMR (400.1 MHz,
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t
CDCl3): δ = 1.20 (s, 18 H, CH3, Bu), 1.98 (s, 12 H, CH3, o-Mes), 2.18 169.5–171.4 °C. Anal. Calcd for C36H42N4: C, 81.47; H, 7.98; N, 10.56;
(s, 6 H, CH3, p-Mes), 2.45 (s, 4 H, CH2), 4.05 (broad s, 2 H, NH), 6.68
(s, 4 H, CH). H NMR (400.1 MHz, C6D6): δ = 1.12 (s, 18 H, CH3, Bu), 1.19 (s, 18 H; CH3, Bu), 3.79 (s, 4 H; CH2), 6.18 (broad s, 2 H; NH),
found C, 81.29; H, 7.86; N, 10.73. 1H NMR (400.1 MHz, CDCl3): δ =
1
t
t
3
3
2.11 (s, 12 H, CH3, o-Mes), 2.21 (s, 6 H, CH3, p-Mes), 2.42 (broad s, 4
6.34 (d, JH,H = 6.3 Hz, 4 H; CH, o-C6H4), 6.96 (d, JH,H = 7.3 Hz, 4 H;
H, CH2), 3.91 (broad s, 2 H, NH), 6.77 (s, 4 H, CH). 1H NMR (400.1 MHz,
CH, m-C6H4), 7.19–7.26 (m, 10 H; CH, Ph). 1H NMR 400.1 MHz, C6D6):
δ = 1.17 (s, 18 H; CH3, Bu), 3.62 (s, 4 H; CH2), 5.82 (broad s, 2 H;
t
t
[D6]DMSO): δ = 1.15 (s, 18 H; CH3, Bu), 1.86 (s, 12 H; CH3, o-Mes),
3
2.12 (s, 6 H; CH3, p-Mes), 2.35 (broad s, 4H; CH2), 5.62 (broad s, 2H; NH), 6.74 (d, JH,H = 8.1 Hz, 4 H; CH, o-C6H4), 6.84–6.90 (m, 6 H; CH,
NH), 6.59 (s, 4H; CH). 13C{1H} NMR (100.6 MHz, CDCl3): δ = 18.3 (CH3,
m-, p-Ph), 7.08 (d, JH,H = 8.3 Hz, 4 H; CH, m-C6H4), 7.20 (d, JH,H =
3
3
t
t
1
o-Mes), 20.6 (CH3, p-Mes), 29.1 (CH3, Bu), 38.5 (C, Bu), 43.0 (CH2),
126.9 (C, o-Mes), 127.8 (CH), 129.8 (C, p-Mes), 145.3 (C, i-Mes), 158.4
(C, CN2). 13C{1H} NMR (100.6 Hz, C6D6): δ = 18.7 (CH3, o-Mes), 20.9
(CH3, p-Mes), 29.1 (CH3, tBu), 38.7 (C, tBu), 43.4 (CH2), 126.8 (C,
o-Mes), 128.4 (CH), 129.5 (C, p-Mes), 146.3 (C, i-Mes), 158.0 (C, CN2).
6.5 Hz, 4 H; CH, o-Ph). H NMR (400.1 MHz, [D6]DMSO): δ = 1.16 (s,
18 H; CH3, tBu), 3.61 (s, 4 H; CH2), 6.34 (d, 3JH,H = 7.1 Hz, 4 H; CH, o-
3
C6H4), 6.97 (d, JH,H = 7.8 Hz, 4 H; CH, m-C6H4), 7.23–7.28 (m, 12 H;
CH, Ph, NH). 13C{1H} NMR (100.6 MHz, CDCl3): δ = 31.4 (CH3, Bu),
t
t
34.0 (C, Bu), 43.4 (CH2), 122.5 (CH, o-C6H4), 125.0 (CH, m-C6H4),
13C{1H} NMR (100.6 MHz, [D6]DMSO): δ = 18.1 (CH3, o-Mes), 20.3 128.2 (CH, m-Ph), 128.7 (CH, o-Ph), 129.0 (CH, p-Ph), 135.2 (C,
t
t
(CH3, p-Mes), 28.9 (CH3, Bu), 38.5 (C, Bu), 42.0 (CH2), 125.7 (C, o-
Mes), 127.3 (CH), 127.6 (C, p-Mes), 145.9 (C, i-Mes), 158.1 (C, CN2).
MS (ESI(+)): m/z (relative intensity): 463 (13) [M + H]+, 232 (100) [M
+ 2 H]2+. HRMS (ESI(+)): m/z calcd. for C30H47N4 [M + H]+ 463.3801,
i-Ph), 143.8 (C, p-C6H4), 147.6 (C, i-C6H4), 158.4 (C, CN2). 13C{1H} NMR
(100.6 MHz, C6D6): δ = 31.6 (CH3), 34.0 (C), 43.7 (CH2), 123.2 (CH, o-
C6H4), 125.6 (CH, m-C6H4), 128.3 (CH, o-, m-Ph), 2 × 129.1 (CH, p-Ph,
CH, o-, m-Ph), 135.8 (C, i-Ph), 143.7 (C, p-C6H4), 148.8 (C, i-C6H4),
found 463.3792. IR (neat, cm–1): ν = 3447 (w, ν(N–H)), 2961, 2941, 158.3 (C, CN2). 13C{1H} NMR (100.6 MHz, [D6]DMSO): δ = 31.2 (CH3,
˜
2919, 2909 (m, ν(C–H)), 2864 (w, ν(C–H)), 2730 (w), 1644 (vs), 1604
(m), 1513, 1474 (s), 1451, 1435, 1393, 1374, 1362 (m), 1300 (w),
1279, 1266 (m), 1226 (s), 1179 (m), 1139 (w), 1035, 1007 (m), 936
tBu), 33.5 (C, tBu), 41.3 (CH2), 122.1 (CH, o-C6H4), 124.6 (CH, m-C6H4),
127.9, 128.5 (CH, o-, m-Ph), 128.7 (CH, p-Ph), 135.2 (C, i-Ph), 142.2
(C, p-C6H4), 148.4 (C, i-C6H4), 157.3 (C, CN2). MS (ESI(+) in MeOH):
(w), 898, 882 (m), 856 (s), 825 (w), 767, 760 (m), 710 (s). UV/Vis (THF): m/z (relative intensity): 531 (30) [M + H]+, 266 (100) [M + 2H]2+
.
λ [nm] (ε [L · mol–1 · cm–1]) 243, 1.92 × 104; ≈294, 4.83 × 103 (br
HRMS (ESI(+)): m/z calcd. for C36H43N4 [M + H]+ 531.3488, found
shoulder).
531.3481. IR (neat, cm–1): ν = 3272 (m, ν(N–H)), 3080, 3056, 3028
˜
(w) 2956, 2916, 2866 (m, ν(CH)), 1616 (s), 1592 (vs), 1576 (s), 1522,
1502 (vs), 1460 (s), 1446, 1422, 1394, 1362, 1314, 1290 (m), 1252 (s),
1202, 1186, 1158, 1142, 1110, 1072 (m), 1050 (w), 1026 (m), 1014,
1002, 968, 924, 880, 862 (w), 836 (s), 772 (m), 750 (m), 696 (vs). UV/
Vis (THF): λ [nm] (ε [L · mol–1 · cm–1]) 247, 3.26 × 104; ≈305,
9.07 × 103 (br shoulder).
Synthesis of 7: This bis(amidine) was obtained by recrystallization
from hexanes at 5 °C within 3 d as a colorless crystalline solid. Yield:
72 %; Mp: 159.5–161.0 °C. Anal. Calcd for C32H50N4: C, 78.31; H,
10.27; N, 11.42; found C, 78.40; H, 10.36; N, 11.46. 1H NMR
t
(400.1 MHz, CDCl3): δ = 1.14 (s, 18 H; CH3, Bu–CN2), 1.26 (s, 18 H;
CH3, tBu–C6H4), 2.80 (broad s, 4 H; CH2), 4.53 (broad s, 2 H; NH), 6.63
3
3
Deprotonation Studies of 6–9: A 1.6
M solution of n-butyllithium
(d, JH,H = 8.0 Hz, 4 H; CH, m-C6H4), 7.16 (d, JH,H = 7.4 Hz, 4 H; CH,
o-C6H4). 1H NMR (400.1 MHz,C6D6): δ = 1.11 (s, 18 H; CH3, tBu–CN2),
1.28 (s, 18 H; CH3, Bu–C6H4), 2.84 (s, 4 H; CH2), 4.61 (broad s, 2 H;
in hexanes (0.06 mL, 0.10 mmol) was added dropwise to a solution
of 6–9 (0.048 mmol) in THF (12 mL) at –78 °C with stirring. After
15 min, the reaction mixture was warmed to room temperature and
t
3
3
NH), 6.91 (d, JH,H = 7.7 Hz, 4 H; CH, m-C6H4), 7.24 (d, JH,H = 7.9 Hz,
stirred for additional 6 h (UV/Vis: 6.7 × 10–6–3.4 × 10–5
M, excitation
4 H; CH, o-C6H4). 1H NMR (400.1 MHz, [D6]DMSO): δ = 1.09 (s, 18 H;
and emission spectra: 6.0–7.5 × 10–3
M; see Table S35).
t
t
CH3, Bu–CN2), 1.23 (s, 18 H; CH3, Bu–C6H4), 2.54 (s, 4 H; CH2), 5.65
3
UV/Vis (THF): λ [nm] (ε [L · mol–1 · cm–1]) [6Li2]: 240, 2.72 × 104;
≈290, 9.96 × 103 (br shoulder); [7Li2]: 243, 3.15 × 104; ≈293,
1.19 × 104 (br shoulder); [8Li2]: ≈244, 1.41 × 105 (br shoulder); ≈273,
6.16 × 104 (br shoulder); ≈286, 5.17 × 104 (br shoulder); 311,
5.29 × 104; [9Li2]: 246, 1.26 × 105; ≈309, 3.66 × 104 (br shoulder);
412, 7.74 × 103.
(broad s, 2 H; NH), 6.45 (d, JH,H = 7.8 Hz, 4 H; CH, m-C6H4), 7.11 (d,
3JH,H = 7.8 Hz, 4 H; CH, o-C6H4). 13C{1H} NMR (100.6 MHz, CDCl3):
t
t
t
δ = 29.2 (CH3, Bu–CN2), 31.5 (CH3, Bu–C6H4), 34.0 (C, Bu–C6H4),
38.7 (C, tBu–CN2), 43.3 (CH2), 120.3 (CH, m-C6H4), 125.1 (CH, o-C6H4),
143.4 (C, p-C6H4), 148.5 (C, i-C6H4), 160.9 (C, CN2). 13C{1H} NMR
t
t
(100.6 MHz, C6D6): δ = 29.4 (CH3, Bu–CN2), 31.8 (CH3, Bu–C6H4),
34.1 (C, tBu–C6H4), 39.0 (C, tBu–CN2), 44.3 (CH2), 121.1 (CH, m-C6H4),
125.4 (CH, o-C6H4), 143.1 (C, p-C6H4), 149.5 (C, i-C6H4), 160.7 (C, CN2).
X-ray Crystallography
13C{1H} NMR (100.6 MHz, [D6]DMSO): δ = 28.8 (CH3, Bu–CN2), 31.4
t
Single crystals of 6, 7, and 9·0.8CH2Cl2 were obtained as colorless
plates (6), blocks (7), or needles (9·0.8CH2Cl2) from slowly concen-
trating hexanes (6 and 7) or CH2Cl2 (9·0.8CH2Cl2) solutions at room
temperature. Colorless blocks of 9·EtOH were grown from a concen-
trated ethanol/acetonitrile solution at –35 °C. X-ray data for 6,
9·0.8CH2Cl2, and 9·EtOH were collected on a Bruker D8 Venture
diffractometer and for 6 on a Bruker D8 Quest X-ray diffractometer
(CuKa radiation, λ = 1.54178 Å or MoKa radiation, λ = 0.71073 Å) by
using ω and ꢀ scans at 100 K (6 and 9·EtOH), 110 K (6) or 140 K
(9·0.8CH2Cl2, Table S1 and S2). The integrated intensities for each
reflection were obtained by reduction of the data frames with the
program APEX3.[28] Cell parameters were obtained and refined with
44601 (5289 unique, 6), 34113 (3844 unique, 7), 39871 (6376
unique, 9·0.8CH2Cl2), and 17988 (3862 unique, 9·EtOH) reflections,
respectively. The integrated intensity information for each reflection
was obtained by reduction of the data frames by using the SAINT
algorithm of APEX3. The integrated data were corrected for absorp-
tion by using SADABS.[29] The structures of 6, 7, and 9·0.8CH2Cl2
t
t
t
(CH3, Bu–C6H4), 33.6 (C, Bu–C6H4), 38.3 (C, Bu–CN2), 42.6 (CH2),
119.7 (CH, m-C6H4), 124.6 (CH, o-C6H4), 141.6 (C, p-C6H4), 149.0 (C,
i-C6H4), 160.3 (C, CN2). MS (ESI(+)): m/z (relative intensity): 491 (20)
[M + H]+, 246 (100) [M + 2 H]2+. HRMS (ESI(+)): m/z calcd. for
C32H51N4 [M + H]+ 491.4114, found 491.4104. IR (neat, cm–1): ν =
˜
3446 (w, ν(N–H)), 2954 (s, ν(C–H)), 2902, 2866 (m, ν(C–H)), 1624 (vs),
1602, 1504 (s), 1458 (m), 1392 (w), 1364 (m), 1314 (w), 1282, 1270
(s), 1226 (s), 1204, 1186, 1164, 1110 (m), 1050, 1032 (w), 1012 (m),
940, 928, 903 (w), 854, 840, 812 (s), 802, 754 (m), 726 (s), 674 (m).
UV/Vis (THF): λ [nm] (ε [L · mol–1 · cm–1]) 244, 2.26 × 104; ≈294,
7.34 × 103 (br shoulder).
UV/Vis spectrum of 8:[17] UV/Vis (THF): λ [nm] (ε [L · mol–1 · cm–1])
242, 2.89 × 104 (br shoulder).
Synthesis of 9: This bis(amidine) was obtained by recrystallization
from a mixture of CH2Cl2 (4 mL) and diethyl ether (4 mL, 1:1 v/v) at
5 °C within 3 d as a colorless crystalline solid. Yield: 66 %. Mp:
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1997, 339, 729–734; d) D. Müller, R. Beckert, H. Görls, Synthesis 2001, 4,
601–606; e) M. Matschke, R. Beckert, L. Kubicova, C. Biskup, Synthesis
2008, 18, 2957–2962; f) R. Strathausen, R. Beckert, J. Fleischhauer, D.
Müller, H. Görls, Z. Naturforsch. B 2014, 69, 641–649.
were solved by direct methods and refined (weighted least-squares
refinement on F2) by using SHELXL-97.[30] The structure of 9·4EtOH
was solved by intrinsic phasing (SHELXT) and refined on F2 using
SHELXL-2016. The hydrogen atoms were placed in idealized posi-
tions, and refined by using a riding model. In the case of 9·4EtOH,
hydrogen atoms attached to the nitrogen atoms of the bis(amidine)
molecule and the solvent ethanol molecules were readily identified
from the difference electron density map, and were fully refined.
Non-hydrogen atoms were refined with anisotropic thermal param-
eters. For 9·0.8CH2Cl2, elongated thermal ellipsoids on the CH2Cl2
atoms indicated disorder, which was modeled successfully between
two positions each with an occupancy ratio of 0.67:0.13. Appropri-
ate restraints were added to keep the bond lengths, angles,
and thermal ellipsoids meaningful. For all structures, absence of
additional symmetry and voids was confirmed using PLATON
(ADDSYM).[31]
[3]
a) F. T. Edelmann, Coord. Chem. Rev. 1994, 137, 403–481; b) J. Barker, M.
Kilner, Coord. Chem. Rev. 1994, 133, 219–300; c) M. P. Coles, Dalton Trans.
2006, 985–1001; d) F. T. Edelmann in Advances in Organometallic Chemis-
try (Eds.: A. F. Hill, M. J. Fink), Elsevier; Amsterdam, 2008, Vol 57, pp. 183–
352; e) F. T. Edelmann, Chem. Soc. Rev. 2009, 38, 2253–2268; f) S. Collins,
Coord. Chem. Rev. 2011, 255, 118–138; g) F. T. Edelmann in Struct. Bond-
ing (Eds.: D. M. P. Mingos, P. W. Roesky), Springer; Berlin, Heidelberg,
2010, Vol 137, pp. 109–163; h) F. T. Edelmann, Chem. Soc. Rev. 2012, 41,
7657–7672; i) R. Kretschmer, Chem. Eur. J. 2020, 26, 2099–2119.
a) B. S. Lim, A. Rahtu, J.-S. Park, R. G. Gordon, Inorg. Chem. 2003, 42,
7951–7958; b) Z. Li, S. T. Barry, R. G. Gordon, Inorg. Chem. 2005, 44,
1728–1735.
A. J. Hill, J. V. Johnston, J. Am. Chem. Soc. 1954, 76, 920–922.
a) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R. M. Waymouth,
Angew. Chem. Int. Ed. Engl. 1995, 34, 1143–1170; Angew. Chem. 1995,
107, 1255–1283; b) R. L. Halterman, Chem. Rev. 1992, 92, 965–994; c) P. J.
Shapiro, Coord. Chem. Rev. 2002, 231, 67–81; d) A. H. Hoveyda, J. P.
Morken, Angew. Chem. Int. Ed. Engl. 1996, 35, 1262–1284; Angew. Chem.
1996, 108, 1378–1401; e) Y. Qian, J. Huang, M. D. Bala, B. Lian, H. Zhang,
H. Zhang, Chem. Rev. 2003, 103, 2633–2690; f) S. Prashar, A. Antiñolo, A.
Otero, Coord. Chem. Rev. 2006, 250, 133–154; g) C. Cobzaru, S. Hild, A.
Boger, C. Troll, B. Rieger, Coord. Chem. Rev. 2006, 250, 189–211; h) B.
Wang, Coord. Chem. Rev. 2006, 250, 242–258; i) G. Erker, Coord. Chem.
Rev. 2006, 250, 1056–1070; j) U. Rosenthal, Angew. Chem. Int. Ed. 2018,
57, 14718–14735; Angew. Chem. 2018, 130, 14932–14950.
[4]
[5]
[6]
Deposition Number(s) CCDC 1976262 (for 6), 1976264 (for 7),
1976263 (for 9·0.8CH2Cl2), and 1935519 (for 9·EtOH) contain(s) the
supplementary crystallographic data for this paper. These data are
provided free of charge by the joint Cambridge Crystallographic
Data Centre and Fachinformationszentrum Karlsruhe Access Struc-
tures service www.ccdc.cam.ac.uk/structures.
Computational details: All structures (6–9) were fully geometry-
optimized using TURBOMOLE v7.3.1.[32] Density functional theory
was used with the B3LYP functional.[33] together with the RIDFT
module,[34] dispersion correction[35] and a triple-ꢀ quality basis set
def2-TZVP including polarization functions.[36] No symmetry con-
straints were imposed on any structures during optimization. The
energetic minimum of the calculated structures was confirmed by
vibration analyses performed analytically (aoforce).[37] For NMR
computations, the gauge-independent atomic orbital (GIAO)
method was used as implemented in TURBOMOLE v7.3.1. The
B3LYP[33] functional was used together with the RIDFT module[34]
and the basis set def2-TZVP.[36] See Table S9 and Table S10 for se-
[7]
[8]
a) G. D. Whitener, J. R. Hagadorn, J. Arnold, J. Chem. Soc., Dalton Trans.
1999, 1249–1255; b) M. S. Hill, P. B. Hitchcock, S. M. Mansell, Dalton Trans.
2006, 1544–1553.
a) J. R. Hagadorn, J. Arnold, Angew. Chem. Int. Ed. 1998, 37, 1729–1731;
Angew. Chem. 1998, 110, 1813–1815; b) J. R. Babcock, C. Incarvito, A. L.
Rheingold, J. C. Fettinger, L. R. Sita, Organometallics 1999, 18, 5729–5732;
c) S. Bambirra, A. Meetsma, B. Hessen, J. H. Teuben, Organometallics
2001, 20, 782–785; d) J.-F. Li, L.-H. Weng, X.-H. Wei, D.-S. Liu, J. Chem.
Soc., Dalton Trans. 2002, 1401–1405; e) S.-D. Bai, J.-P. Guo, D.-S. Liu, Dal-
ton Trans. 2006, 2244–2250; f) J.-F. Li, S.-P. Huang, L.-H. Weng, D.-S. Liu,
J. Organomet. Chem. 2006, 691, 3003–3010; g) J. Wang, T. Cai, Y. Yao, Y.
Zhang, Q. Shen, Dalton Trans. 2007, 5275–5281; h) L. Yan, H. Liu, J. Wang,
Y. Zhang, Q. Shen, Inorg. Chem. 2012, 51, 4151–4160; i) A. O. Tolpygin,
A. S. Shavyrin, A. V. Cherkasov, G. K. Fukin, A. A. Trifonov, Organometallics
2012, 31, 5405–5413; j) G. G. Skvortsov, G. K. Fukin, S. Y. Ketkov, A. V.
Cherkasov, K. A. Lyssenko, A. A. Trifonov, Eur. J. Inorg. Chem. 2013, 4173–
4183; k) A. O. Tolpygin, A. V. Cherkasov, G. K. Fukin, A. A. Trifonov, Inorg.
Chem. 2014, 53, 1537–1543; l) G. G. Skvortsov, A. O. Tolpygin, G. K. Fukin,
J. Long, J. Larionova, A. V. Cherkasov, A. A. Trifonov, Eur. J. Inorg. Chem.
2017, 2017, 4275–4284.
a) J. Wang, H. Sun, Y. Yao, Y. Zhang, Q. Shen, Polyhedron 2008, 27, 1977–
1982; b) W. Li, M. Xue, J. Tu, Y. Zhang, Q. Shen, Dalton Trans. 2012, 41,
7258–7265; c) A. O. Tolpygin, G. G. Skvortsov, A. V. Cherkasov, G. K. Fukin,
T. A. Glukhova, A. A. Trifonov, Eur. J. Inorg. Chem. 2013, 2013, 6009–6018.
D. Osorio Meléndez, J. A. Castro-Osma, A. Lara-Sánchez, R. S. Rojas, A.
Otero, J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2397–2407.
J. Wang, J. Li, F. Xu, Q. Shen, Adv. Synth. Catal. 2009, 351, 1363–1370.
a) M. Ohashi, A. Yagyu, T. Yamagata, K. Mashima, Chem. Commun. 2007,
3103–3105; b) S. Tanaka, K. Mashima, Inorg. Chem. 2011, 50, 11384–
11393.
1
lected computed H NMR chemical shifts referenced to TMS.
Acknowledgments
We thank Connor O'Dea, Omar Ugarte Trejo, Nimia Zoé Maya,
Ofumere Omokhodion, and Nattasith Siwabut for experimental
assistance. Dr. Michael D. Walla and Dr. William E. Cotham (Mass
Spectrometry Center at the University of South Carolina) are
acknowledged for recording mass spectra. We are grateful to
Andreas Ehnbom for his help on computational problems. The
Department of Chemistry and Biochemistry and the College of
Sciences and Mathematics (CSM) at Kennesaw State University
are acknowledged for financial support (New Faculty Startup
Fund (No. 08020) and the CSM Research Stimulus Program). This
material is based upon work supported by the National Science
Foundation under Grant No. CHE-1800332.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
Keywords: Amidines · Isomers · Hydrogen bonds · Density
functional calculations · Luminescence
H. Kawaguchi, T. Matsuo, Chem. Commun. 2002, 958–959.
M. Stollenz, Chem. Eur. J. 2019, 25, 4274–4298.
C. O'Dea, O. Ugarte Trejo, J. Arras, A. Ehnbom, N. Bhuvanesh, M. Stollenz,
J. Org. Chem. 2019, 84, 14217–14226.
[16]
[1] G. Häfelinger, F. K. H. Kuske in The Chemistry of Amidines and Imidates
(Eds.: S. Patai, Z. Rappoport), John Wiley & Sons, Ltd.; Chichester, UK,
1991, pp. 1–100.
[2] a) D. Lindauer, R. Beckert, M. Döring, P. Fehling, H. Görls, J. Prakt. Chem.
1995, 337, 143–152; b) D. Lindauer, R. Beckert, T. Billert, M. Döring, H.
Görls, J. Prakt. Chem. 1995, 337, 508–515; c) J. Atzrodt, J. Brandenburg,
C. Käpplinger, R. Beckert, W. Günther, H. Görls, J. Fabian, J. Prakt. Chem.
a) E. A. Marihart, J.-B. Greving, R. Fröhlich, E.-U. Würthwein, Eur. J. Org.
Chem. 2007, 2007, 5071–5081; b) J.-B. Greving, H. Behrens, R. Fröhlich,
E.-U. Würthwein, Eur. J. Org. Chem. 2008, 2008, 5740–5754; c) J. I. Clodt,
C. Wigbers, R. Reiermann, R. Fröhlich, R. E.-U. Würthwein, Eur. J. Org.
Chem. 2011, 2011, 3197–3209; d) C. Glotzbach, N. Gödeke, R. Fröhlich,
C.-G. Daniliuc, S. Saito, S. Yamaguchi, E.-U. Würthwein, Dalton Trans.
2015, 44, 9659–9671.
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[17] M. Stollenz, J. E. Raymond, L. M. Pérez, J. Wiederkehr, N. Bhuvanesh,
Chem. Eur. J. 2016, 22, 2396–2405.
[18] M. Stollenz, F. Meyer, Organometallics 2012, 31, 7708–7727.
[19] Two NH signals at δ = 5.22 ppm (major isomer) and 5.72 ppm (minor
isomer) are observed for 8; the downfield shift refers to the dominant
species.
[20] The NH resonance signal is overlaid by the Ph group signals.
[21] Two CH2 signals at δ = 3.02 ppm (minor isomer) and 3.75 ppm (major
isomer) are observed for 8; the downfield shift refers to the dominant
species.
[28] a) APEX2: Program for Data Collection on Area Detectors; BRUKER AXS
Inc., 5465 East Cheryl Parkway, Madison, WI 53711–5373 USA; b) APEX3:
Program for Data Collection on Area Detectors; BRUKER AXS Inc., 5465
East Cheryl Parkway, Madison, WI 53711–5373 USA.
[29] G. M. Sheldrick, SADABS: Program for Absorption Correction of Area De-
tector Frames; University of Göttingen, Göttingen, Germany, 2008.
[30] a) G. M. Sheldrick, Acta Crystallogr., Sect. A 2008, 64, 112–122; b) G. M.
Sheldrick, Acta Crystallogr., Sect. C 2015, 71, 3–8.
[31] a) A. L. Spek, PLATON: A Multipurpose Crystallographic Tool, Utrecht Uni-
versity, Utrecht, The Netherlands 2008; b) A. L. Spek, J. Appl. Crystallogr.
2003, 36, 7–13.
[32] a) R. Ahlrichs, M. Bär, M. Häser, H. Horn, C. Kölmel, Chem. Phys. Lett. 1989,
162, 165–169; b) F. Furche, R. Ahlrichs, C. Hättig, W. Klopper, M. Sierka, F.
Weigend, Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2014, 4, 91–100; c)
TURBOMOLE V7.3 2018, A development of University of Karlsruhe and
Forschungszentrum Karlsruhe GmbH, 1989–2007; TURBOMOLE GmbH,
2007. http://www.turbomole.com.
[33] a) A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100; b) A. D. Becke, J. Chem.
Phys. 1993, 98, 5648–5652; c) C. Lee, W. Yang, R. G. Parr, Phys. Rev. B
1988, 37, 785–789.
[34] a) K. Eichkorn, O. Treutler, H. Öhm, M. Häser, R. Ahlrichs, Chem. Phys. Lett.
1995, 242, 652–660; b) K. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs,
Theor. Chem. Acc. 1997, 97, 119–124; c) M. Sierka, A. Hogekamp, R. Ahl-
richs, J. Chem. Phys. 2003, 118, 9136–9148.
[35] a) S. Grimme, J. Comput. Chem. 2004, 25, 1463–1473; b) S. Grimme, J.
Comput. Chem. 2006, 27, 1787–1799; c) S. Grimme, J. Antony, S. Ehrlich,
H. Krieg, J. Chem. Phys. 2010, 132, 154104.
[22] For earlier reports about hydrogen bonds on correlations between 1H
NMR chemical shifts and crystallographic data see: a) V. Bertolasi, P. Gilli,
V. Ferretti, G. Gilli, J. Chem. Soc., Perkin Trans. 2 1997, 945–952; b) T. K.
Harris, Q. Zhao, A. S. Mildvan, J. Mol. Struct. 2000, 552, 97–109.
[23] V. Vass, M. Dehmel, F. Lehni, R. Kretschmer, Eur. J. Org. Chem. 2017, 2017,
5066–5073. The authors only reported an intramolecular hydrogen bond
of a bis(guanidine). However, a closer inspection of the molecular struc-
ture revealed the existence of intermolecular hydrogen bonding as well
(NNH···N′: 2.9500(17) Å; N–H···N′: 2.100(19) Å; N–H···N′: 154(2)°) that re-
sults in a polymeric arrangement of the individual bis(guanidine) mole-
cules in a helical fashion.
[24] E/Z(syn/syn) and Z/E(syn/syn) represent one identical pair. In total, six
pairs of identical stereoisomers and four unique stereoisomers are possi-
ble for 6–9, if each predominant tautomer retains the −NH(CH2)2NH−
diamine core motif.
[25] There is no evidence for the corresponding anti rotational isomers in the
solid-state structures of 1–8, 9·0.8CH2Cl2, and 9·EtOH.
[36] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297–3305.
[37] P. Deglmann, F. Furche, R. Ahlrichs, Chem. Phys. Lett. 2002, 362, 511–518.
[26] G. A. Jeffrey, in An Introduction to Hydrogen Bonding, Oxford University
Press, New York, 1997.
[27] F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen, R. Taylor,
J. Chem. Soc., Perkin Trans. 2 1987, S1–S19.
Received: February 19, 2020
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Ethylene-Bridged Bis(amidines)
A. Calderón-Díaz, J. Arras, E. T. Miller,
N. Bhuvanesh, C. D. McMillen,
M. Stollenz* .......................................... 1–9
Ethylene-Bridged
Tetradentate
Supramolecular
through Hydrogen
Bis(amidines):
Assemblies
Bonding and Photoluminescence
upon Deprotonation
Flexible ethylene-bridged tetradentate hydrogen bonds both in the solid state
bis(amidines) either encapsulate their and in solution. They also produce
N–H moieties through steric protec- blue to green emissions upon depro-
tion or form versatile networks of tonation.
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