Magnesium bis(amidinate) and bis(guanidinate) complexes: Impact of the ligand backbone and bridging groups on the coordination behaviour

Article abstract of DOI:10.1039/d0dt01923b

A library of ten dinucleating bis(amidine) and bis(guanidine) ligands, in which the bridging groups, terminal rests, and backbone substituents were systematically altered, has been synthesized and subsequently applied in metallation reactions using three

Full text of DOI:10.1039/d0dt01923b

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Magnesium bis(amidinate) and bis(guanidinate)
complexes: impact of the ligand backbone and
bridging groups on the coordination behaviour†
Cite this: DOI: 10.1039/d0dt01923b
Andreas Rösch,a Simon H. F. Schreiner,a Philipp Schüler,b Helmar Görlsb and
b,c
Robert Kretschmer
*
A library of ten dinucleating bis(amidine) and bis(guanidine) ligands, in which the bridging groups, terminal
rests, and backbone substituents were systematically altered, has been synthesized and subsequently
applied in metallation reactions using three different magnesium sources. Eight Mg complexes could be
isolated and fully characterized, and in seven cases their solid-state structure could be determined by
means of single crystal X-ray diffraction analysis. The results evidence a versatile coordination behaviour,
which ranges from the first dinuclear heteroleptic magnesium iodide complex to dinuclear homoleptic
complexes. These findings indicate the crucial impact of both the ligand and the magnesium source not
only on the accessibility of well-defined dinuclear complexes but also on the aggregation in solution and
in the solid state.
Received 28th May 2020,
Accepted 21st August 2020
DOI: 10.1039/d0dt01923b
rsc.li/dalton
metal separation and the relative orientation of both active
sites, and electronic aspects such as the capability of the back-
Introduction
Over the past decades, researchers from various fields of chem- bone to donate or withdraw electron density, play a crucial role
istry have become more and more interested in bimetallic and and may be tuned by adapting the ligand framework. While
dinuclear compounds, in which two active sites are framed dinucleating ligands have been regularly used for the com-
within a single molecule.1 These molecules often display plexation of transition metals and rare earth elements,
characteristics that cannot be accessed using their mono- examples of complexes containing early main-group elements
nuclear congeners, due to cooperative effects.2 However, the remain rare.3,9 This is surprising, considering the catalytic
efficient synthesis of homo- and heterodinuclear compounds potential of mononuclear complexes of s- and p-block
remains challenging and is often affected by an appropriate metals.10 Due to their high abundance and low toxicity, the
selection of the dinucleating ligand. Among the very relevant group 2 elements magnesium and calcium are particularly
kinds of N,N-dinucleating ligands,3 most research studies have attractive,10b,c,f,g,11 but only a small number of related dinuc-
been devoted to bis(amidine)s,4 while bis(β-diketimine)s5 and lear complexes has been reported so far.5l,12
bis(guanidine)s6 only recently received particular attention. Bis
Among these, the Harder group has investigated mag-
(amidinate)-based compounds have found numerous appli- nesium and calcium complexes based on bis(salicylaldimine)s
cations in materials science and catalysis,7 and the related bi- and bis(β-diketimine)s, respectively, possessing rigid bridging
metallic complexes are associated with reactivities and selectiv- groups such as hydrazine, 1,3- and 1,4-phenylene, 2,3,5,6-tetra-
ities that could not be achieved using their mononuclear rela- methyl-1,4-phenylene, and 2,6-pyridylene.5l,12b,d–f,h,i When
tives.8 In this regard, the geometric features, i.e., the metal– using bis(salicylaldimine)s, the authors observed in the case of
magnesium, dinuclear heteroleptic complexes of type I, while
for calcium a mixture of heteroleptic and intermolecular
aInstitute of Inorganic Chemistry, University of Regensburg, Universitätsstraße 31,
homoleptic (type II) dinuclear complexes was obtained besides
93053 Regensburg, Germany
polymeric aggregates, Fig. 1.5g,12b In the case of bis
bInstitute of Inorganic and Analytical Chemistry (IAAC), Friedrich Schiller University
(β-diketiminate)s, several heteroleptic (I)5e,g and homoleptic
Jena, Humboldtstraße 8, 07743 Jena, Germany
cJena Center for Soft Matter (JCSM), Friedrich Schiller University Jena,
(II)5e,g,12e complexes were observed for both, calcium and mag-
nesium, while for the latter di- (III and IV)12d and tetranuclear
Philosophenweg 7, 07743 Jena, Germany. E-mail: robert.kretschmer@uni-jena.de
†Electronic supplementary information (ESI) available: For crystallographic
(V and VI)12h,i complexes were also reported, in which
data, experimental details concerning the ligand syntheses and NMR as well as
additional ligands such as n-butyl, amidoborane or hydrides
IR spectra. CCDC 2000854–2000860. For ESI and crystallographic data in CIF or
act as intra- or intermolecular bridges. Notably, in the case of
other electronic format see DOI: 10.1039/D0DT01923B
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(carbodiimide)s, the instability of the latter limits the scope of
the reaction. Hence, we targeted the synthesis of N-bridged bis
(amidine)s 1a–d through aminolysis of the related bis(imi-
doylchloride)s, Scheme 1, and also investigated the reaction of
imidoylchlorides with the primary diamines. However, only
the aminolysis of bis(imidoylchloride)s allowed for the iso-
lation of the related bis(amidine)s, 1a–d, which are obtained
in yields ranging from 45 to 87%. In contrast, treating the sec-
ondary diamines with two equivalents of imidoylchloride gave
rise to complex reaction mixtures. Bis(guanidines) 2b–e were
synthesized in good to excellent yields using the one-pot pro-
cedure recently reported by us,6h which however gives 2a only
as a side-product in 18% yield along with the related 3,4-ethyl-
ene-bridged 1,1,2,5-tetrasubstituted biguanide.6i 2f was alter-
natively synthesized from the related bis(carbodiimide) upon
reaction with pyrrolidine, as the reaction directly originating
from the respective bis(thiourea) gave a complex reaction
mixture most likely due to the absence of steric protection,
Scheme 2.
Fig. 1 Known coordination modes of calcium and magnesium bis
(β-diketiminate) and bis(salicylaldiminate) complexes. DHP
=
dihydropyridine.
Synthesis of mono- and dinuclear magnesium compounds
the 2,6-pyridylene-bridged magnesium bis(β-diketiminate)
complex additional but unique binding modes were obser-
ved.12h We wondered how the coordination behaviour changes
on going from the well-established bis(β-diketiminate)s to (bis
(amidinate)s and bis(guanidinate)s. Hence, we synthesized the
related nitrogen-bridged protio-ligands and investigated their
reactivity towards magnesium precursors; our findings are
reported herein. Please note that dinuclear magnesium com-
plexes incorporating backbone-bridged bis(amidinate)s have
been reported before.4k
Aiming to access dinuclear magnesium bis(amidinate) and bis
(guanidinate) complexes, the respective protio-ligands were
treated with various magnesium reagents including mag-
nesium bis(hexamethyldisilazide) Mg(HMDS)2, di-n-butylmag-
nesium Mg(n-Bu)2 as well as methylmagnesium iodide
CH3MgI. Please note that the first two have been used pre-
viously to derive magnesium bis(β-diketiminate) and bis(salicy-
laldiminate) complexes, while reports about the applications
of CH3MgI to derive dinuclear heteroleptic magnesium iodide
complexes have no precedence in the literature.
Reacting the bis(amidine)s 1 with Mg(HMDS)2 (1a–d), Mg
(n-Bu)2 (1a and 1d) or CH3MgI (1a–d) affords rather complex
1
reaction mixtures as evidenced by the H NMR spectra of the
crude products. Despite the repeated attempts to obtain crys-
tals under various conditions, we were only able to isolate
Results and discussion
Protio-ligand synthesis
Nitrogen-bridged bis(amidine)s are generally available by clean products in two cases, Scheme 3. Adding a few drops of
various routes,3 from which those originating from either bis tetrahydrofuran to the crude mixture obtained from the reac-
(imidoylchloride)s or imidoylchlorides are most versatile. tion of 1a with two equivalents of methyl magnesium iodide
Although bis(amidine)s are also accessible starting from bis causes instantaneous formation of the crystalline material,
Scheme 1 Synthesis of bis(amidine)s 1a–d employed in this work. Dipp = 2,6-diisopropylphenyl, Dipp* = 2,6-dibenzhydryl-4-methylphenyl.
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Scheme 2 Synthesis of bis(guanidines)s 2a–f employed in this work. Dipp = 2,6-diisopropylphenyl, DMP = 2,6-dimethylphenyl.
Scheme 3 Synthesis of the heteroleptic (3) and homoleptic (4) bis(amidinate) complexes from the respective protio-ligands upon treatment with
either methylmagnesium iodide or magnesium bis(hexamethyldisilazide). Dipp = 2,6-diisopropylphenyl.
from which we were able to separate single crystals suitable for
X-ray diffraction analysis. The related solid-state structure of
3a, Fig. 2, consists of a heteroleptic dinuclear magnesium(II)
iodide complex and its structural parameters are discussed
further below.
Although the solubility of 3a in benzene is rather low, a
simple room temperature 1H NMR spectrum, i.e., one septet
and two doublets corresponding to the methine and methyl
resonances of the Dipp groups and an averaged singlet
accounting for the four methylene protons of the bridge, is
observed. The fast exchange and averaging on the NMR time-
scale indicate that the heteroleptic complex persists in solu-
tion. Notably, two molecules of THF are loosely bound and
Fig. 2 Solid-state structure of 3a (hydrogen atoms are omitted for the
readily removed in a vacuum as evidenced by the integration
1
sake of clarity) with selected bond lengths [Å] and angles [°]: Mg1–Mg1’
7.1542(13), Mg1–I1 2.7659(7), Mg1–N1 2.0733(17), Mg1–N2 2.1262(19),
Mg1–O1 2.1465(16), Mg1–O2 2.0423(19), N1–Mg1–N2 63.26(7), O1–
Mg1–O2 88.55(7).
of the H NMR spectrum. A simple pattern of resonances was
also observed if the 1H NMR spectra of the freshly prepared
samples in THF-d8 were recorded. However, over time the pre-
cipitation of the crystalline material and the appearance of
new signals in the 1H NMR spectrum were observed, Fig. S42.†
This is in good agreement with the Schlenk equilibrium that is
shifted over time towards the intermolecular homoleptic THF-d8, Fig. S43.† The structure of 4a in the solid state was
complex, 4a, Scheme 4. Time-dependent 1H NMR spectra unambiguously confirmed by X-ray diffraction analysis,
reveal a half-life of about 18 hours (0.023 mol L−1) for 3a in Fig. 3a.
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gave rise to 4a. Using more polar solvents such as THF does
not appear reasonable due to the shift of the Schlenk equili-
brium, as discussed before. Notably, the cyclic voltammograms
recorded from the freshly prepared solutions of 3a in THF do
not reveal any reduction event above −3.30 V (against Fc/Fc+),
Fig. S84.†
4b is the only other yet isolated magnesium complex incor-
porating a bis(amidinate), which was obtained in very good
yield by reacting bis(amidine) 1b with two equivalents of mag-
nesium bis(hexamethyldisilazide). An X-ray diffraction analysis
of the single crystals of 4b established its molecular structure
in the solid state, Fig. 3b, which will be discussed further
below.
Reacting bis(guanidine)s 2 with di-n-butyl magnesium gave
only in three cases (2a,b,d) well-defined species, Scheme 5,
while no product could be isolated with the other protio-
ligands. However, upon using Mg(HMDS)2 (with 2b–e) or
Scheme 4 Schlenk equilibrium between 3a and 4a. Dipp
diisopropylphenyl.
= 2,6-
As magnesium iodide complexes are suitable precursors to
access the related dinuclear Mg(I) derivatives possessing inter-
molecular Mg–Mg bonds,13 we were interested to see whether
3a can be reduced to a low-valent derivative featuring an intra-
molecular magnesium–magnesium bond. Repeated attempts
to reduce 3a using KC8, potassium mirror, or elemental
caesium in hydrocarbon solvents remained unsuccessful and
Fig. 3 Solid-state structures (hydrogen atoms and in the case of 4b also co-crystallized toluene are omitted for the sake of clarity) with selected
bond lengths [Å] and angles [°]: (a) 4a: Mg1–Mg2 4.3350(9), Mg1–N1 2.0514(11), Mg1–N2 2.0555(12), Mg1–N5 2.0629(12), Mg1–N6 2.0361(13), Mg2–
N3 2.0507(11), Mg2–N4 2.0436(10), Mg2–N7 2.0322(9), Mg2–N8 2.0569(11), N1–C13 1.3443(14), N2–C13 1.3401(14), N3–C20 1.3413(15), N4–C20
1.3385(16), N5–C56 1.3424(18), N6–C56 1.3412(16), N7–C49 1.3414(14), N8–C49 1.3442(12), N1–Mg1–N2 65.49(4), N5–Mg1–N6 65.46(4), N3–
Mg2–N4 65.56(4), N7–Mg2–N8 65.57(4); (b) 4b: Mg1–Mg2 5.1714(14), Mg1–N1 2.053(3), Mg1–N2 2.084(3), Mg1–N5 2.0403(19), Mg1–N6 2.055(3),
Mg2–N3 2.045(3), Mg2–N4 2.060(2), Mg2–N7 2.066(3), Mg2–N8 2.0380(18), N1–C13 1.328(3), N2–C13 1.325(5), N3–C21 1.331(4), N4–C21 1.339(3),
N5–C50 1.334(4), N6–C50 1.331(2), N7–C58 1.349(4), N8–C58 1.349(4), N1–Mg1–N2 63.58(13), N5–Mg1–N6 64.70(9), N3–Mg2–N4 64.83(10), N7–
Mg2–N8 66.82(9).
Scheme 5 Synthesis of the dinuclear homoleptic bis(guanidinate) magnesium complexes 5 and 6 from the respective protio-ligand 2 using various
magnesium precursors. Dipp = 2,6-diisopropylphenyl and DMP = 2,6-dimethylphenyl.
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Solid-state structures
The heteroleptic bis(amidinate) complex, 3a, Fig. 2, possesses
two equivalent magnesium(II) centres that are each penta-co-
ordinated to one N,N-chelating amidinate unit, an iodide
ligand, and two molecules of THF. In the case of the two
homoleptic bis(amidinate) complexes, 4a and 4b, Fig. 3, each
of the two magnesium ions is four-fold coordinated by two
N,N-chelating amidinate units, which belong to two different
bis(amidinate) ligands. The Mg–N bond lengths (2.0733(17)–
2.1262(19) Å) and the N–Mg–N bite angle (63.26(7)°) within 3a
are comparable to those of the only yet reported mononuclear
amidinate magnesium iodide complex14 (Mg–N 2.085(6) and
2.140(6) Å; N–Mg–N 64.3(2)°). The corresponding Mg–N bond
lengths within 4a and 4b are slightly shorter and range from
2.0322(9) to 2.084(3) Å, consistent with values previously
Fig. 4 Solid-state structure (hydrogen atoms are omitted for the sake
of clarity) of 6b with selected bond lengths [Å] and angles [°]: Mg1–Mg2 reported for mononuclear homoleptic magnesium amidinate
complexes (2.039(3) to 2.079(18) Å).15 The same holds true for
the N–Mg–N angles that fall in between 63.58(13)° and 66.82
5.1725(8), Mg1–N1 2.0451(12), Mg1–N2 2.0363(11), Mg2–N4 2.0367(12),
Mg2–N5 2.0365(13), Mg1–N7 2.0527(12), Mg1–N8 2.0301(11), Mg2–N10
2.0326(14), Mg2–N11 2.0378(12), N1–Mg1–N2 66.48(5), N4–Mg2–N5
66.77(5), N7–Mg1–N8 66.46(5), N10–Mg2–N11 66.56(5).
(9)°, Table 1. The nitrogen–carbon bond lengths of the amidi-
nate backbone (1.323(3)–1.352(2) Å) are almost equal in all
three cases indicating the delocalized nitrogen–carbon
bonding. Notably, the Mg–I bond length of 2.7659(7) Å within
CH3MgI (with 2b–d,f), two more products, i.e., 5e and 5f,
could be isolated. Among these five species, single crystals of
5a, 5e, 5f, and 6b suitable for X-ray diffraction analyses could
be obtained after a short work-up and their molecular struc-
tures are given in Fig. 4 and 5. Although the product of the
reaction of the 1,3-xylylene-bridged bis(guanidine) 2d with di-
n-butyl-magnesium was obtained as a colourless solid (7),
recrystallization attempts from various solvents as well as
solvent combinations did not allow us to obtain single crystals
suitable for X-ray diffraction analysis.
3a resembles the value of the related mononuclear complex
(2.783(2) Å).
The dinuclear complex 6b is reminiscent of 4b, but the two
magnesium ions are each framed by two N,N-chelating guani-
dinate units of two bis(guanidinate) ligands. The magnesium–
nitrogen bond lengths within 6b are comparable to those in
4b, but the N–Mg–N guanidinate bite angles are more obtuse
than those of their amidinate relatives.
While the values discussed before remain largely unaffected
by varying the linker group, the metal–metal separation
Fig. 5 Solid-state structures (hydrogen atoms and in the cases of 5a and 5e co-crystallized toluene are omitted for the sake of clarity) with the
selected bond lengths [Å] and angles [°]: (a) 5a: Mg1–Mg1’ 3.0348(8), Mg1–N1 2.1541(11), Mg1–N2 2.2703(11), Mg1–N4 2.0646(13), Mg1–N5 2.1521
(12), Mg1–N2’ 2.0736(12), N1–Mg1–N2 62.05(4), N4–Mg1–N5 64.29(4), N2’–Mg1–N2 91.48(4); (b) 5e: Mg1–Mg1’ 2.9929(8), Mg1–N1 2.1005(14),
Mg1–N2 2.2336(14), Mg1–N4 2.0604(11), Mg1–N5 2.1298(13), Mg1–N2’ 2.0949(11), N1–Mg1–N2 62.95(5), N4–Mg1–N5 64.47(5), N2’–Mg1–N2 92.57
(5); (c) 5f: Mg1–Mg1’ 3.0823(10), Mg1–N1 2.0789(12), Mg1–N2 2.2913(12), Mg1–N4 2.0897(13), Mg1–N5 2.1366(13), Mg1–N2’ 2.1276(13), N1–Mg1–
N2 61.94(5), N4–Mg1–N5 64.34(5), N2’–Mg1–N2 91.62(5).
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Table 1 Selected bond lengths [Å] and angles [°] of the mono- and
dinuclear magnesium bis(amidinate) and bis(guanidinate) complexes
values of 2.040(3) to 2.052(2) Å reported for homoleptic mono-
nuclear guanidinate complexes.16,19 However, the non-brid-
ging Mg–N4 interactions (2.0604(11)–2.0897(13) Å) resemble
these values, while the bonding between the magnesium and
the lateral bridging nitrogen atoms, i.e., Mg–N2, is signifi-
cantly affected by the bridging interaction as expressed by sig-
nificantly longer bond distances of 2.2336(14) to 2.2913(12) Å.
The intermolecular Mg–N2′ and Mg′–N2 bond lengths increase
on going from 5a (2.0736(12) Å) to 5e (2.0949(11) Å) to 5f
(2.1276(13) Å), which might be either due to the increasing
length of the linker group or due to a decrease in the disper-
sive interaction upon reducing the bulkiness of the terminal
aryl substituent. The magnesium–magnesium separation,
finally, amounts to 3.0348(8) (5a), 2.9929(8) (5e), and 3.0823
(10) Å (5f). The bite angles of the non-bridging guanidinate
units are more acute (61.94(5)–62.95(5)°) as compared to those
of the bridging ones (64.29(4)–64.47(5)°) and closer to those of
the mononuclear guanidinate complexes with values between
65.90(10) and 66.67(10)°.16,19
Compound Mg–Mg
Mg–N
N–Mg–N
3a
4a
4b
5a
5e
5f
7.1542(13) 2.0733(17)–2.1262(19) 63.26(7)
4.3350(9)
5.1714(14) 2.0380(18)–2.084(3)
2.0322(9)–2.0629(13)
65.46(4)–65.57(4)
63.58(13)–66.82(9)
3.0348(8)
2.9929(8)
2.0646(13)–2.2703(11) 62.05(4)–91.48(4)
2.0604(11)–2.2336(14) 62.95(5)–92.57(5)
3.0823(10) 2.0789(12)–2.2913(12) 61.94(5)–91.62(5)
5.1725(8) 2.0301(11)–2.0527(12) 66.46(5)–66.77(5)
6b
increases on going from 4a (4.3350(9) Å) to 4b (5.1714(14) Å)
and 6b (5.1725(8) Å), respectively, as a result of the additional
CH2 group of the bridge. The separation is even further
increased in the case of 3a (7.1542(13) Å) due to the opposing
orientation of the two magnesium centres. The shorter ethyl-
ene bridge confines the overall flexibility of 4a, and the angles
between the two MgNCN metallacycle planes of each mag-
nesium ion, which are inclined by 77.4° and 79.4°, are more
obtuse than those of 4b, with values of 67.6° and 77.7°.
However, in the case of 6b, the angles between the two
MgNCN metallacycles are closer to orthogonality (78.3° and
81.4°) as compared to those of 4b. Notably, in both cases,
these angles are moved towards orthogonality compared to
those reported for related mononuclear amidinate and guani-
dinate complexes (54°–80°).15b,c,16
The homoleptic complexes 5a, 5e, and 5f crystallize as
centrosymmetric dimers consisting of two mononuclear homo-
leptic magnesium bis(guanidinate) fragments, Fig. 4. Hence,
each magnesium ion possesses a coordination number of five
and comprises four magnesium–nitrogen bonds to the chelat-
ing bis(guanidinate) and an additional intermolecular Mg–N
bond to a lateral nitrogen atom of one of the guanidine units.
Thus, a primary transoid ladder17 of i or S2 symmetry with a
central four-membered Mg2N2-ring is formed and the two gua-
nidinate units of each ligand are unequally bonded in a µ-
1′κ1N2:1κ2N1N2 and 1κ2N4,N5 fashion, respectively. While the
overall coordination mode has no precedence in the literature
for any metallated nitrogen-based ditopic ligand,3 it is reminis-
cent of the dimeric calcium and magnesium complexes based
on guanidinate and amidinate ligands reported before by the
groups of Hill and Winter.18 The bond distances between the
magnesium and the terminal nitrogen atoms, i.e., Mg–N1 and
Mg–N5, are longer (2.0789(12)–2.1541(11) Å) compared to the
Behaviour in solution
1
As discussed before, 3a resembles an averaged H NMR spec-
trum at room temperature, but on going from 3a to 4a, the
rotation about the C–C bond of the ethylene bridge is effec-
tively cancelled out and the respective methylene protons
become inequivalent causing the appearance of two multiplets
instead of one singlet in the 1H NMR spectrum (C6D6),
Table 2. Furthermore, two multiplets and three doublets (ratio
1 : 2 : 1) for the respective methine and methyl proton reso-
nances of the 2,6-diisopropylphenyl substituents are observed.
In contrast to the well-resolved room-temperature 1H NMR
spectrum of 4a, the respective spectrum of 4b contains a mul-
tiplet and a number of broadened resonances, which likely
arise from the slow conformational exchange of the complex
on the NMR timescale due to the more flexible propylene
bridge. Upon cooling to 193 K, these resonances shifted
slightly but did not resolve into separate peaks, Fig. S50.†
However, in the case of 6b, the bis(guanidinate) relative of 4b,
1
well-resolved H and 13C NMR spectra were obtained and the
pattern of the resonances for the Dipp groups, i.e., four methyl
doublets and two methine septets, is in good agreement with
the solid-state structure.
The 1H NMR spectra of 5a, 5e, and 5f are consequently
affected by the NMR solvent. The low solubility of 5a in apolar
Table 2 1H NMR shift [ppm] of the mono- and dinuclear magnesium bis(amidinate) and bis(guanidinate) complexes
Compound (solvent)
CH(CH3)2
CH(CH3)2
Linker
3a (C6D6)
3a (THF-d8)
4a (C6D6)
4b (THF-d8)
5a (C6D6/THF-d8)
6b (C6D6)
d (1.28), m (1.35–1.37)
d (1.16), d (1.25)
d (0.43), m (1.24–1.26), d (1.35)
m (1.07–1.21)
dd (1.20)
d (0.54), d (1.16)
m (3.58–3.65)
sept (3.45)
sept (3.16), m (3.50–3.54)
br s (3.13–3.31)
sept (3.21)
m (3.06–3.13), m (3.89–3.96)
m (3.50–3.64)
s (4.11)
s (3.79)
m (3.50–3.54), m (4.45–4.50)
m (1.92–2.09), m (3.62–3.74)
m (3.14–3.16)
m (2.34–2.41), m (3.45–3.52), m (3.61–3.67)
s (4.36), m (7.30–7.56)
s (4.30), m (6.98–7.45)
7 (C6D6)
7 (toluene-d8)
m (1.07–1.13), d (1.28)
br (1.00), d (1.25)
br (3.51)
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solvents necessitates the use of THF-d8 at least as a co-solvent.
1
One septet and two doublets in the respective H NMR spec-
trum account for the Dipp isopropyl groups indicating the con-
formational averaging on the NMR timescale. The increased
1
solubility of 5e allows obtaining the H NMR spectra in both
toluene-d8 and THF-d8. In the first case, the methyl groups
within 5e appear as four singlets evidencing that the complex
is not symmetrical in solution. In THF-d8, two singlet reso-
nances are observed, which indicates a higher symmetry likely
due to the coordination of THF-d8 to the magnesium centre. 5f
1
does not contain a valuable H NMR probe as in the cases of
5a and 5e, but shows only broadened resonances for the
protons of the linker group and the backbone-based pyrroli-
dine unit.
Finally, the room temperature 1H NMR spectrum of 7 in
deuterated benzene or toluene-d8 includes one doublet and
one broadened resonance for the Dipp methyl groups and one
broad resonance for the respective methine proton, which
indicates a symmetric or averaged structure in solution.
Adding about 20 equivalents of THF to this solution does not
significantly affect the spectrum but the dissolution of 7 in
THF-d8 gives a significantly more complex pattern of reso-
nances with partially overlapping signal groups, which
impedes a distinct assignment, Fig. S67–S71.†
Fig. 6 Conceivable structures of the solution of complex 7.
toluene-d8, only one major component was observed and the
diffusion coefficient (D = 4.25 × 10−10 m2 s−1) corresponds to a
molecular weight of 1255 g mol−1. Notably, a comparable value
is observed when about 20 equivalents of THF are added.
These findings indicate that 7 either resembles a structure
comparable to that of complex 4 or exists as a dimeric aggre-
gate as observed for the complexes 5 in the solid state. In both
cases, the calculated molecular weight amounts to 1342.6 g
mol−1. In THF-d8, two similar species with a diffusion coeffi-
cient of about 5.2 × 10−10 m2 s−1 and a molecular weight of
1045 g mol−1 were observed. This number does not match that
of either a mono- or a dinuclear complex but falls well in
between the values of the two species (average molecular mass
1046 g mol−1), Fig. 6. Please note that depending on the
exchange rate, converging and even averaging diffusion coeffi-
cients have been reported,20 which further indicates a THF-
dependent equilibrium between the mono- and dinuclear
complexes. Indeed, the 1H exchange spectroscopy (EXSY)
experiments using a mixing time of 1 s revealed distinct cross-
peaks indicating chemical exchange at room temperature,
Fig. S79–S82.†
To gain further insight into the behaviour in solution,
diffusion-ordered NMR spectroscopy (DOSY) experiments have
been conducted for 4b, 5e, 5f, and 7 using tetrakis(trimethyl-
silyl)silane as an internal reference, Table 3. In the case of 4b,
the experimentally obtained diffusion coefficient (D = 5.52 ×
10−10 m2 −1) reveals an experimental molecular weight of
s
1225 g mol−1 when assuming highly compact spheres, which
is close to the calculated value of 1166.41 g mol−1. In the case
of 5e, the impact of the NMR solvent on the aggregation has
been probed with both, C6D6 and THF-d8, but the same
diffusion coefficient (D = 7.75 × 10−10 m2 s−1) and hence mole-
cular weight (536 g mol−1) were observed. This indicates the
presence of a monomeric species (a molecular weight of
496.99 and 577.14 g mol−1 without and with considering a co-
ordinated THF-d8 molecule, respectively) in contrast to the
solid-state dimer. The same holds true for 5f (D = 1.2 × 10−9
m2 s−1) for which the experimental molecular weight of 546 g
mol−1 is in good agreement with the calculated value of
535.06 g mol−1 assuming one coordinated molecule of THF-d8.
As mentioned before, the behaviour of compound 7 in solu-
tion is strongly solvent dependent and so are the experi-
mentally determined diffusion coefficients: in the case of
Conclusions
The application of four bis(amidine) and six bis(guanidine)
ligands allowed for the synthesis of overall eight magnesium
complexes possessing different coordination modes. The
results illustrate how crucially the electronic and steric features
of the dinucleating ligand on the one hand and the metal
source on the other affect the complex formation and the
related coordination behaviour. Only in one case, i.e., upon
using ethylene-bridged bis(amidine) along with CH3MgI, a het-
eroleptic magnesium iodide complex could be obtained.
However, while complex 3a is stable in hydrocarbon solutions,
Table 3 Experimental values of the DOSY experiments
Compound (solvent)
Diffusion coefficient
Calc. mol weight
4b (THF-d8)
5e (toluene-d8)
5e (THF-d8)
5f (THF-d8)
7 (THF-d8)
5.52 × 10−10
7.75 × 10−10
7.75 × 10−10
1.12 × 10−9
5.20 × 10−10
4.25 × 10−10
1225
536
536
546
1045
1255
7 (toluene-d8)
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the dissolution in THF shifts the Schlenk equilibrium and (715 mg, 18.3 mmol) and toluene (30 mL) was added to reach
gives rise to the respective dinuclear homoleptic complex 4a. all spots of the potassium mirror. After stirring for two days at
Such a kind of complex was also observed in the cases of pro- room temperature, the black suspension was filtered and the
pylene-bridged bis(amidinate) 4b and the bis(guanidinate) 6b, filtrate was concentrated in a vacuum to obtain a yellowish
which share comparable structural features indicating a negli- solid. Recrystallization from Et2O gave 4a (577 mg, 0.51 mmol,
gible impact of the backbone substituent. Furthermore, the bis 37%) as clear, colourless crystals.
(guanidine)s, 2a, 2e, and 2f, possessing various linker groups
1H NMR (400 MHz, C6D6): δ (ppm) = 0.43 (d, J = 4.5 Hz, 6H,
and terminal substituents afford intramolecular homoleptic CH(CH3)2), 1.24–1.26 (m, 24H, CH(CH3)2 & C(CH3)3), 1.35 (d, J
complexes that form dimers in the solid state. Overall, and due = 4.6 Hz, 12H, CH(CH3)2), 3.16 (sept, J = 4.6 Hz, 2H,
to the Schlenk equilibrium, bis(amidinate)s and bis(guanidi- CH(CH3)2), 3.50–3.54 (m, 4H, CH(CH3)2 & CH2CH2), 4.45–4.50
nate)s containing flexible linker groups appear to be not very (m, 2H, CH2CH2), 6.94–6.95 (m, 2H, C6H3), 7.01–7.07 (m, 4H,
suitable for deriving heteroleptic dinuclear magnesium com- C6H3); 13C{1H} NMR (101 MHz, C6D6): δ (ppm) = 22.5 (CH
plexes, which could be used as ligands for other metal(loid)s (CH3)2), 22.8 (CH(CH3)2), 23.6 (CH(CH3)2), 25.2 (CH(CH3)2),
or adopted for further applications in catalysis.
28.2 (CH(CH3)2), 28.8 (CH(CH3)2), 29.7 (C(CH3)3), 41.4
(C(CH3)3), 54.9 (CH2CH2), 122.9 (p-C6H3), 123.2 (m-C6H3),
123.3 (m-C6H3), 140.9 (o-C6H3), 142.1 (o-C6H3), 145.5 (i-C6H3),
179.8 (NCN); IR (ATR): ˜ν [cm−1] = 2960 (m), 2868 (w), 1623 (m),
1585 (m), 1512 (m), 1429 (m), 1216 (m), 765 (m); anal. calc.
(found) for [C72H112Mg2N8 0.95 toluene]: C 77.06 (76.73), H
Experimental section
General considerations
All preparations were performed under an inert atmosphere of 9.83 (9.48), N 9.14 (9.28).
dinitrogen by means of standard Schlenk-line techniques,
4b: CH3MgI (2.10 mL, 6.18 mmol, 3.0 M in Et2O) was added
while the samples for analytical tests were handled in a glove- to a stirred solution of 1b (1.65 g, 2.94 mmol) in toluene
box (GS-Systemtechnik and MBraun). All solvents (toluene, (10 mL) followed by stirring at room temperature overnight,
n-pentane, n-hexane, and tetrahydrofuran) were distilled from yielding a yellow to brown solution and a white precipitate.
Na/benzophenone prior to use while C6D6, THF-d8 and The solids were filtered off and the filtrate was concentrated to
toluene-d8 were dried using molecular sieves.
one third, causing the formation of crystals at room tempera-
3a: CH3MgI (0.39 mL, 1.10 mmol, 3.0 M in Et2O) was added ture overnight. The crystals were collected and washed with
to a stirred solution of 1a (300 mg, 0.55 mmol) in toluene n-pentane (2 × 5 mL) to obtain 4b (2.85 g, 2.44 mmol, 83%) as
(3 mL) followed by stirring at room temperature overnight clear, colourless blocks.
yielding a white slurry. The solids were filtered off and a few
1H NMR (400 MHz, THF-d8): δ (ppm) = 1.07–1.21 (m, 84H,
drops of THF were added to the filtrate causing the instant for- CH(CH3)2 & C(CH3)3), 1.92–2.09 (br s, 4H, CH2CH2CH2),
mation of clear crystals. The filtration and washing of the crys- 3.13–3.31 (br s, 8H, CH(CH3)2), 3.62–3.74 (br s, 8H,
tals with n-pentane gave 3a (450 mg, 0.39 mmol, 72%) as clear, CH2CH2CH2), 6.72–6.80 (m, 12H, C6H3); 13C{1H} NMR
colourless crystals.
(101 MHz, THF-d8): δ (ppm) = 23.8 (CH(CH3)2), 24.6 (C(CH3)3),
1H NMR (400 MHz, C6D6): δ (ppm) = 1.28 (d, J = 6.8 Hz, 28.5 (CH(CH3)2), 31.3 (C(CH3)3), 41.2 (CH2(CH2)CH2), 48.0
12H, CH(CH3)2), 1.37 (m, 12H, CH(CH3)2) 1.43 (s, 18H, (CH2(CH2)CH2), 122.0 (p-C6H3), 123.2 (m-C6H3), 142.3
C(CH3)3), 3.58–3.65 (m, 4H, CH(CH3)2), 4.11 (s, 4H, CH2CH2) (o-C6H3), 149.1 (i-C6H3), 178.0 (NCN); IR (ATR): ˜ν [cm−1] = 2955
1
6.94–7.08 (m, 6H, C6H3). H NMR (400 MHz, THF-d8): δ (ppm) (m), 2923 (w), 2866 (w), 1651 (w), 1411 (vs), 1340 (s), 1322 (s),
= 1.16 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.23 (s, 18H, C(CH3)3) 1171 (m), 765 (s), 728 (m); anal. calc. (found) for
1.25 (d, J = 7.0 Hz, 12H, CH(CH3)2), 3.45 (sept, J = 6.9 Hz, 4H, [C74H116Mg2N8 1.40 toluene]: C 77.70 (77.31), H 9.90 (9.47), N
CH(CH3)2), 3.79 (s, 4H, CH2CH2) 6.79–6.83 (m, 2H, C6H3), 8.69 (8.65).
6.90–6.93 (m, 4H, C6H3); 13C{1H} NMR (101 MHz, THF-d8): δ
5a: Mg(n-Bu)2 (1.50 mL, 1.50 mmol, 1.0 M in heptane) was
(ppm) = 24.1 (CH(CH3)2), 25.5 (CH(CH3)2), 28.6 (CH(CH3)2), added to a stirred solution of 2a (400 mg, 0.70 mmol) in
31.0 (C(CH3)3), 41.5 (C(CH3)3, 52.6Dipp (CH2CH2), 122.5 toluene (4 mL) followed by stirring at 105 °C overnight. After
(p-C6H3), 123.6 (m-C6H3), 143.0 (o-C6H3), 148.8 (i-C6H3), 178.0 the solvent was concentrated to one-third and standing at
(NCN). Notably, the resonance at 25.5 ppm overlaps with the room temperature for two weeks, 5a (262 mg, 0.44 mmol, 63%)
solvent peak, but its presence was confirmed by 13C-DEPT was obtained as colourless crystals.
NMR spectroscopy; IR (ATR): ˜ν [cm−1] = 2959 (m), 2868 (w),
1H NMR (400 MHz, C6D6/THF-d8): δ (ppm) = 1.20 (dd, J =
1652 (w), 1625 (w), 1458 (s), 1402 (s), 1286 (m), 1170 (m), 1023 9.0 Hz, 24H, CH(CH3)2), 1.37–1.40 (m, 8H, NCH2(CH2)2CH2),
(s), 876 (s), 762 (s); anal. calc. (found) for [C52H88I2Mg2N4O4 2.97–3.00 (m, 8H, NCH2(CH2)2CH2), 3.14–3.16 (m, 4H,
0.15 toluene]: C 55.43 (55.56), H 7.82 (7.56), N 4.87 (4.74).
CH2CH2), 3.21 (sept, J = 7.0 Hz, 4H, CH(CH3)2), 6.89–6.93 (m,
4a: CH3MgI (0.96 mL, 2.88 mmol, 3.0 M in Et2O) was added 2H, p-C6H3), 7.03–7.05 (m, 4H, m-C6H3); 13C{1H} NMR
to a stirred solution of 1a (750 mg, 1.37 mmol) in toluene (101 MHz, C6D6/THF-d8): δ (ppm) = 23.3 (CH(CH3)2), 23.7 (CH
(10 mL) followed by stirring at room temperature overnight, (CH3)2),
25.6
(CH2CH2),
28.6
(CH(CH3)2),
44.5
yielding a white slurry. The solution was filtered directly into a (NCH2(CH2)2CH2), 48.1 (NCH2(CH2)2CH2), 121.5 (p-C6H3),
flask loaded with
a
freshly prepared potassium mirror 122.8 (m-C6H3), 139.8 (o-C6H3), 146.7 (i-C6H3), 149.8 (NCN); IR
Dalton Trans.
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(ATR): ˜ν [cm−1] = 2955 (m), 2863 (w), 1615 (m), 1582 (m), 1354 (CH3)2), 23.3 (CH(CH3)2), 24.4 (CH(CH3)2), 25.1 (CH(CH3)2),
(m), 749 (m); despite repeated attempts no suitable elemental 25.8 (NCH2(CH2)2CH2), 27.5 (CH(CH3)2), 28.8 (CH(CH3)2), 40.8
analysis could be achieved.
(CH2(CH2)CH2), 46.5 (CH2(CH2)CH2), 48.5 (NCH2(CH2)2CH2),
5e: Mg(HMDS)2 (79.0 mg, 0.46 mmol) was added to a 122.2 (p-C6H3), 122.6 & 122.9 (m-C6H3), 142.0 & 143.4 (o-C6H3),
stirred solution of 2e (100 mg, 0.21 mmol) in toluene (5 mL) 145.3 (i-C6H3), 166.0 (NCN); IR (ATR): ˜ν [cm−1] = 2960 (w), 2866
and stirred at room temperature overnight. Solvent removal (vw), 1417 (vs), 1357 (m), 1336 (s), 772 (m), 731 (s); anal. calc.
and recrystallization from n-hexane/Et2O (7 : 1) at room temp- (found) for [C74H112N12Mg2]: C 72.95 (72.86), H 9.27 (9.13), N
erature gave 5e (64 mg, 0.13 mmol, 61%) as clear colourless 13.80 (13.39).
crystals.
7: Mg(n-Bu)2 (0.31 mL, 0.31 mmol, 1.0 M in heptane) was
1H NMR (400 MHz, toluene-d8): δ (ppm) = 1.13–1.21 (m, added to a stirred solution of 2d (200 mg, 0.31 mmol) in
4H, CH2(CH2)CH2), 1.34–1.47 (m, 8H, N(CH2(CH2)2CH2), 2.04 toluene (5 mL) followed by stirring at room temperature over-
(s, 3H, C6H3(CH3)2), 2.20 (s, 3H, C6H3(CH3)2), 2.56 (s, 3H, night. Solvent removal gave a yellow solid, which was redis-
C6H3(CH3)2), 2.64 (s, 3H, C6H3(CH3)2), 3.09–3.25 (m, 8H, solved in pentane. After some minutes, 7 (103 mg, 0.15 mmol,
N(CH2(CH2)2CH2), 3.35–3.55 (m, 2H, CH2(CH2)CH2), 6.65–7.08 50%) precipitated as a colourless solid. Despite repeated
(m, 6H, C6H3); due to the low solubility of 5e no proper 13C attempts using various solvents and solvent combinations we
NMR spectrum could be obtained; IR (ATR): ˜ν [cm−1] = 2967 could not obtain single crystals.
(w), 2847 (w), 2912 (w), 2878 (w), 1505 (vs), 1487 (s), 1392 (vs),
1H NMR (400 MHz, C6D6): δ (ppm) = 1.07–1.13 (m, 20H, CH
1363 (m), 1331 (m), 857 (w), 755 (s), 729 (s); anal. calc. (found) (CH3)2 & N(CH2(CH2)2CH2)), 1.28 (d, J = 6.9 Hz, 12H, CH
for [C29H40MgN6 0.4 toluene]: C 71.55 (71.15), H 8.16 (8.10), N (CH3)2), 2.97–3.00 (m, 8H, N(CH2(CH2)2CH2), 3.50–3.64 (m,
15.74 (15.42).
4H, CH(CH3)2), 4.36 (s, 4H, CH2(C6H4)), 7.03–7.10 (m, 6H,
5f: CH3MgI (0.54 mL, 1.62 mmol, 3.0 M in Et2O) was added C6H3), 7.30–7.56 (m, 4H, C6H4); 1H NMR (400 MHz, toluene-
to a stirred solution of 2f (350 mg, 0.81 mmol) in THF (10 mL) d8): δ (ppm) = 1.00 (br, 12H, CH(CH3)2), 1.16–1.20 (m, 8H,
followed by stirring at room temperature overnight. The white N(CH2(CH2)2CH2), 1.25 (d, J = 6.8 Hz, 12H, CH(CH3)2),
precipitate formed was filtered off and the solvent of the fil- 2.96–3.00 (m, 8H, N(CH2(CH2)2CH2), 3.51 (br, 4H, CH(CH3)2,
1
trate was removed. The crude product was recrystallized from a 4.30 (s, 4H, CH2(C6H4)), 6.98–7.45 (m, 10H, C6H3 + C6H4); H
mixture of toluene/THF (7 : 1) layered with n-pentane at NMR (400 MHz, THF-d8): signals have not been assigned due
−20 °C. 5f (282 mg, 0.62 mmol, 76%) was obtained as clear col- to complexity, for details see fig. S70;† 13C{1H} NMR
ourless blocks.
(101 MHz, C6D6): δ (ppm) = 23.1 (CH(CH3)2), 24.7 (CH(CH3)2),
(NCH2(CH2)2CH2), 28.1 (CH(CH3)2), 48.5
1H NMR (400 MHz, THF-d8): δ (ppm) = 1.59 (br, 4H, 25.6
CH2(CH2)2CH2), 1.72–1.75 (m, 8H, N(CH2(CH2)2CH2), 3.08 (br, (NCH2(CH2)2CH2), 52.1 (CH2C6H4), 123.0 (p-C6H3), 123.1
4H, CH2(CH2)2CH2), 3.23–3.26 (m, 8H, N(CH2(CH2)2CH2), (m-C6H3), 124.9 (o-C6H4), 125.8 (o-C6H4), 128.7 (m-C6H4), 142.9
6.45–6.49 (m, 2H, p-C6H5), 6.64–6.66 (m, 4H, m-C6H5), (o-C6H3), 144.8 (i-C6H4), 144.9 (i-C6H3), 166.8 (NCN); IR (ATR):
6.93–6.97 (m, 4H, o-C6H5); 13C{1H} NMR (101 MHz, THF-d8): δ ˜ν [cm−1] = 2958 (w), 2866 (vw), 1452 (vs), 1325 (m), 1282 (m),
(ppm) = 26.2 (NCH2(CH2)2CH2), 30.8 (CH2(CH2)2CH2), 48.9 776 (m), 728 (m); anal. calc. (found) for [C42H58N6Mg]: C 75.15
(CH2(CH2)2CH2), 49.5 (NCH2(CH2)2CH2), 117.4 (p-C6H5), 121.5 (75.03), H 8.71 (8.87), N 12.52 (12.27).
(m-C6H5), 129.1 (o-C6H5), 154.6 (i-C6H5), 166.4 (NHCN); IR
X-ray crystallography
(ATR): ˜ν [cm−1] = 3055 (vw), 2970 (vw), 2920 (vw), 1594 (w),
1506 (m), 1473 (m), 1400 (s), 1334 (m), 1275 (m), 993 (m), 736 The intensity data were collected using a GV-50 diffractometer
(vs), 697 (vs); anal. calc. (found) for [C26H34MgN6 0.10 with a TitanS2 detector from Rigaku Oxford Diffraction (for-
toluene]: C 69.10 (69.09), H 7.56 (7.46), N 18.11 (18.02).
merly Agilent Technologies) applying Cu-Kα radiation (λ =
6b: Mg(n-Bu)2 (0.35 mL, 0.35 mmol, 1.0 M in heptane) was 1.54184 Å). The data were corrected for Lorentz and polariz-
added to a stirred solution of 2b (200 mg, 0.35 mmol) in ation effects; analytical absorption corrections were applied to
toluene (5 mL) followed by stirring at room temperature over- the data.21 The structures were solved by direct methods
night. Solvent removal gave a yellow solid, which was further (SHELXT)22 and refined by full-matrix least squares techniques
2
washed with n-pentane to obtain 6b (142 mg, 0.23 mmol, 67%) against Fo (SHELXL-2018).23 All hydrogen atoms were
as a white crystalline solid. Crystals suitable for X-ray diffrac- included at calculated positions with fixed thermal para-
tion analysis grew from the n-pentane washing solution.
meters. The crystals of 4a and 5a contain large voids, filled
1H NMR (400 MHz, C6D6): δ (ppm) = 0.54 (d, J = 6.7 Hz, with disordered solvent molecules. The size of the voids is 310
12H, CH(CH3)2), 1.16 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.24–1.29 and 307 Å3 per unit cell, respectively. Their contribution to the
(m, 16H, N(CH2(CH2)2CH2)), 1.38 (d, J = 7.0 Hz, 12H, CH structural factors was secured by back-Fourier transformation
(CH3)2), 1.53 (d, J = 6.7 Hz, 12H, CH(CH3)2), 2.34–2.41 (m, 4H, using the SQUEEZE routine of the program PLATON24 result-
CH2(CH2)CH2), 2.96–3.00 (m, 8H, N(CH2(CH2)2CH2)), 3.06–3.13 ing in 54 and 52 electrons per unit cell, respectively. All non-
(m, 12H, CH(CH3)2 & N(CH2(CH2)CH2)), 3.45–3.52 (m, 4H, hydrogen atoms were refined anisotropically.23 The crystallo-
CH2(CH2)CH2), 3.61–3.67 (m, 4H, CH2(CH2)CH2), 3.89–3.96 (m, graphic data as well as structure solution and refinement
4H, CH(CH3)2), 7.00–7.07 (m, 8H, m-C6H3), 7.14–7.21 (m, 4H, details are summarized in Table S1 of the ESI.† Olex2 was
p-C6H3); 13C{1H} NMR (101 MHz, C6D6): δ (ppm) = 23.0 (CH used for structure representations.25
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The crystallographic data (excluding structural factors) have
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication CCDC 2000854 for 3a,
2000855 for 4a, 2000856 for 4b, 2000857 for 5a, 2000858 for
5e, 2000859 for 5f, and 2000860 for 6b.†
Dalton Transactions
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The work is dedicated to Christian Robl on the occasion of his
65th birthday. The project was financially supported by the
Deutsche Forschungsgemeinschaft (DFG, KR4782/3-1), the
Friedrich Schiller University Jena and the Elite Network of
Bavaria. We thank Friederike Pielenz and Philipp Rinke for
assistance with the NMR experiments and Dr Peter Bellstedt
for helpful discussions.
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