Alkaline Earth Metal-Carbene Complexes with the Versatile Tridentate 2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine Ligand

Article abstract of DOI:10.1021/acs.organomet.6b00914

Diffusion of 2,6-bis(3-mesitylimidazol-2-ylidene)pyridine (Car^MesPyCar^Mes, 2) into a solution of CaI₂ in THF leads to microcrystalline [(Car^MesPyCar^Mes)(thf)CaI₂] (3), in one case containin

Full text of DOI:10.1021/acs.organomet.6b00914

Article
pubs.acs.org/Organometallics
Alkaline Earth Metal−Carbene Complexes with the Versatile
Tridentate 2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine Ligand
Alexander Koch, Sven Krieck, Helmar Gorls, and Matthias Westerhausen*
̈
Institute of Inorganic and Analytical Chemistry, Friedrich-Schiller-University Jena, Humboldtstraße 8, D-07743 Jena, Germany
S
* Supporting Information
ABSTRACT: Diffusion of 2,6-bis(3-mesitylimidazol-2-ylidene)-
pyridine (CarMesPyCarMes, 2) into a solution of CaI2 in THF leads
to microcrystalline [(CarMesPyCarMes)(thf)CaI2] (3), in one case
containing a few single crystals of the unique separated ion pair
[(CarMesPyCarMes)2(thf)Ca]I2 (4) with four Ca−C bonds. However,
isolation of single-crystalline [(CarMesPyCarMes)(thf)CaI2] (3) suc-
ceeds via the addition of 2 to a solution of [(thf)5CaI]+[BPh4]− due to
a subsequent dismutation. Two modifications with the shapes of
needles and cubes crystallize simultaneously. In contrast to this
finding, the reaction of [(thf)5CaI]+[AlPh4]− with 2 yields solvent
separated [(CarMesPyCarMes)(thf)2CaI]+[AlPh4]− (5). The Ca−CNHC
bond lengths lie in a typical range of Ca−CAryl σ bonds and represent
the shortest Ca−CNHC bonds known to date. Soluble
[(CarMesPyCarMes)(thf)Ca(NPh2)2] (6) can be prepared via a
metathetical approach from 4 and KNPh2 as well as via the addition of 2 to Ca(NPh2)2 in tetrahydrofuran. The bulkier
amido ligands lead to elongated bonds between the calcium center and the ligand 2. Furthermore, the reactions of MI2 (M = Sr,
Ba) with 2 yield [(CarMesPyCarMes)(thf)2MI2] (M = Sr (7), Ba (8)).
Scheme 1. Representation of Commonly Used Neutral NHC
Ligands in the Field of s-Block Metal Chemistry
INTRODUCTION
■
a
Since the pioneering work of Wanzlick,1 subsequently leading
to the first isolation of a thermally stable, free N-heterocyclic
carbene (NHC) by Arduengo and co-workers,2 this compound
class has become one of the most important Lewis bases in
organometallic chemistry. These NHCs offer unique features
resulting in applications in various fields of chemistry. In
contrast to the fruitful and vast coordination chemistry of
carbenes with transition metals, s-block metal−carbene
complexes have been studied mainly with cyclopentadienyl,3
bis(trimethylsilyl)amido (HMDS),4 halide, and alkyl counter-
ions.5
Typical carbene ligands are shown in Scheme 1. Addition of
an anionic anchor group to the carbene moiety establishes
another possibility to investigate s-block metal ion−carbene
interactions.6 Very recently, the metalation of NHCs with
superbases has been studied.7 However, all of these types of
complexes are often indispensable intermediates in the
synthesis of transition-metal complexes applying free carbenes
or salt-metathetical approaches.
The strong σ-donor binding capabilities of carbenes
encouraged us to reinvestigate these Lewis bases as ligands
for the coordination chemistry of the alkaline earth metals. Hill
and co-workers suggested a similar coordination strength of thf
and monodentate NHC ligands.4b Schumann et al. demon-
strated that ligated ethers can easily be replaced by carbenes at
alkaline earth metallocenes.3a Consequently, enhancement of
the denticity of the carbene-based Lewis base should represent
a
Abbreviations: Ad, adamantyl; Dipp, 2,6-diisopropylphenyl; Mes,
mesityl, 2,4,6-trimethylphenyl.
a very capable class of neutral coligands in the field of s-block
metal chemistry. However, to date only a few examples of such
complexes are known, such as bis- and tris(imidazol-2-yliden-1-
yl)borate complexes,8,9 which degrade in the vicinity of heavy
group II metal ions, and also some lithium halide adducts of
methylene-bridged bis(NHC) ligands.10 Due to the small bite
angle (CNHC···CNHC distance) of the latter Lewis base a ligation
of heavier s-block metals seems to be difficult. Design of a more
Received: December 9, 2016
© XXXX American Chemical Society
A
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Scheme 2. Reaction Diversity of CarMesPyCarMes (2) with the
Iodides of the Heavy Alkaline Earth Metals Calcium,
Strontium, and Barium, Yielding the Complexes 3−7
suitable binding pocket and enhancement of the denticity are
advantageous to stabilize such complexes. Therefore, we
investigated 2,6-bis(3-mesitylimidazol-2-ylidene)pyridine (2,
CarMesPyCarMes) as a tridentate bis(carbene)-based ligand.
Here we present the results of the coordination studies with
MI2 of the heavy alkaline earth metals calcium, strontium, and
barium.
RESULTS AND DISCUSSION
■
Structure of CarMesPyCarMes. The microcrystalline waxy
solid of CarMesPyCarMes (2)11,12 was recrystallized from
concentrated THF/toluene mixtures, allowing structural
characterization by X-ray diffraction experiments. The asym-
metric unit of compound 2 contains two molecules A and B of
2 and one toluene molecule; molecule A is depicted in Figure 1.
Intramolecular π stacking of the pyridyl moiety of 2a3 and
one mesityl group of 2b3 leads to different Ca−CNHC bond
lengths (2.565(4)/2.577(4) Å vs 2.701(4)/2.631(4) Å) as well
as different Ca−N distances (2.524(3) Å vs 2.607(3) Å) of the
two CarMesPyCarMes ligands (Figure 2). However, the Ca−
Figure 1. Molecular structure and numbering scheme of molecule A of
CarMesPyCarMes (2). Thermal ellipsoids are shown at a probability level
of 30%. H atoms are omitted for clarity. Selected bond lengths (Å),
bond angles (deg), and dihedral angles (deg): C6A−N2A 1.380(3),
C6A−N3A 1.363(3), C8A−C7A 1.341 (3), C18A−N4A 1.376(3),
C18A−N5A 1.362(3), C19A−C20A 1.340(3), C1A−N1A 1.341(3),
N1A−C5A 1.332(3); N3A−C6A−N2A 101.28(19), N5A−C18A−
N4A 101.53(18), C1A−N1A−C5A 116.88(19); C18A−N4A−C1A−
N1A 164.78, C6A−N2A−C5A−N1A +164.43.
The carbene moieties are twisted by 13° with respect to the
central pyridine unit and show anti arrangements due to
electrostatic repulsion between the CNHC and NPy lone pairs.
The bond lengths and angles exhibit values comparable to
those observed earlier for other unstrained carbenes.13 This
tridentate molecule attracted our attention as a potent ligand
for heavy alkaline earth ions.
Coordination of CarMesPyCarMes to Group II Metals. We
started our investigations with the addition of a dilute solution
of anhydrous CaI2 in THF to a solution of 2 in THF at room
temperature. Immediately a white solid, which was collected
and dried, precipitated with characteristic yields greater than
95% with respect to CaI2. The white precipitate was insoluble
in THF and, hence, NMR spectroscopic characterization failed.
However, elementary analysis supports a composition of
[(CarMesPyCarMes)(thf)CaI2] ([(2)(thf)CaI2], 3) with traces
of coprecipitated [(thf)4CaI2]. Due to the insolubility of the
material, diffusion experiments were performed and a solution
of 2 in THF was layered with a dilute solution of CaI2 in THF.
Most commonly, microcrystalline solids of the composition
[(CarMesPyCarMes)(thf)CaI2] were obtained, but in one case we
were able to also grow a few single crystals of
[(CarMesPyCarMes)2(thf)Ca]I2 ([(2)2(thf)Ca]I2, 4) (Scheme
2). Complex 4 represents one of the rare examples of a
separated ion pair based on CaI2;14 the calcium ion has a
coordination number of 7 via the coordination of two tridentate
ligands 2, in the following distinguished as 2a3 and 2b3, and one
THF base (according to the formulation [(2a3)(2b3)(thf)Ca]-
I2). Both iodide ions are intercalated between the bulky
[(CarMesPyCarMes)2(thf)Ca]2+ cations.
Figure 2. Molecular structure and numbering scheme of the cation of
4. The ellipsoids represent a probability of 30%. H atoms are omitted
for the sake of clarity. Selected bond lengths (Å), bond angles (deg),
and dihedral angles (deg): Ca1−C18 2.565(4), Ca1−C1 2.577(4),
Ca1−C30 2.701(4), Ca1−C47 2.631(4), Ca1−N1 2.524(3), Ca1−N6
2.607(3), Ca1−O1 2.506(3); N3−C1−N2 102.8(3), N5−C18−N4
102.6(3), N8−C30−N7 102.1(3), N10−C47−N9 102.2(3), C13−
N1−C17 117.5(3), C42−N6−C46 116.4(3), N1−Ca1−N6
138.28(10), O1−Ca1−N1 73.14(10), O1−Ca1−N6 90.38(11);
C18−N4−C17−N1−11.87, C1−N2−C13−N1 + 11.41, C30−N7−
C42−N6 +17.52, C47−N9−C46−N6 +3.00.
CNHC bonds of 2a3 are the shortest reported for the interaction
of a calcium ion with carbene ligands15 and lie in a typical range
of Ca−CAryl/CAlkyl σ bonds, verifying the lack of severe
intramolecular strain.16 In contrast, the Ca−CNHC contacts of
2b3 are among the longest known.15 Furthermore, the
aforementioned π stacking opens the coordination sphere at
Ca for an additional thf molecule, however, with a large Ca−O
distance of 2.506(3) Å, suggesting a rather weak interaction
between Ca and this ether base. The dihedral angles between
the pyridyl and carbene moieties are comparable to those of
free CarMesPyCarMes (2) (however, here a syn conformation is
B
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realized), as are the other structural parameters. The finding
that binding of calcium in the donor pocket of 2 leaves the
geometry nearly unchanged excludes steric pressure and
supports a strong bond between CarMesPyCarMes (2) and the
calcium ion. This interpretation is further supported by a
beneficial entropic effect, finally allowing the coordination of
two molecules of 2 and leading to the formation of the
separated ion pair 4. Unfortunately, no further analytical data
can be provided because the major compound consisted of
[(CarMesPyCarMes)(thf)CaI2].
contact remains unchanged. The Ca−O distance is in the
expected order of magnitude.16 The insolubility of the complex
may be explained by an intermolecular π stacking in the crystal
involving one carbene and one pyridyl moiety of different
molecules.
In contrast to the reaction of 2 with [(thf)5CaI]+[BPh4]−, the
addition of 2 to [(thf)5CaI]+[AlPh4]− in THF (prepared via the
addition of [(thf)4CaI(Ph)] to AlPh317) yielded
[(CarMesPyCarMes)(thf)2CaI]+[AlPh4]− (5), which was soluble
1
enough to allow H NMR experiments. The reaction mixture
Due to the insolubility of 4, we attempted the substitution of
shows only a single set of resonances for 5. However, the
concentration was too low to record reliable 13C{1H} NMR
spectra. Addition of Et2O to the reaction mixture led to slow
crystallization of 5 at room temperature. The hexacoordinate
calcium ion of 5 (Figure 4) is in an octahedral environment
−
one iodide anion by one noncoordinating anion such as MPh4
(M = B, Al). The heteroleptic complex [(thf)5CaI]+[BPh4]−
was synthesized from CaBr2, NaI, and NaBPh4 in THF and
then reacted in high dilution with 1 equiv of 2, also dissolved in
THF. Due to the high dilution, NMR studies of the reaction
solution failed; however, addition of a few drops of Et2O
initiated crystallization. Instead of the desired [(CarMes
-
PyCarMes)(thf)2CaI]+[BPh4]−, [(CarMesPyCarMes)(thf)CaI2]
(3) was isolated in a yield of 37% with respect to calcium.
This observation suggests a ligand scrambling of
[(CarMesPyCarMes)(thf)2CaI]+[BPh4]−, yielding 3 and
[(CarMesPyCarMes)x(thf)yCa]2+[BPh4]− , the latter forming an
2
insoluble amorphous solid. Two modifications of 3, appearing
as long needles as well as cubes, were obtained in the same
crystalline batch. The molecular structure and numbering
scheme of one modification of 3 is depicted in Figure 3; the
Figure 4. Molecular structure and numbering scheme of the cation of
5. Thermal ellipsoids represent a probability of 30%. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å), bond angles (deg),
and dihedral angles (deg): Ca1−C1 2.573(2), Ca1−C18 2.546(2),
Ca1−N3 2.530(2), Ca1−O1 2.363(2), Ca1−O2 2.3356(19), Ca1−I1
3.0701(5); O1−Ca1−O2 83.48(7), N3−Ca1−O2 174.84(7), O1−
Ca1−I1 173.13(6); C1−N1−C13−N3 −17.89, C18−N4−C17−N3
+5.07.
which is distorted due to the constrained binding pocket of 2.
One iodide anion and a thf molecule occupy the axial positions
and enclose an angle of 173.13(6)° with Ca1. Furthermore, the
coordination sphere is saturated by a second thf ligand. The
asymmetric unit contains two additional THF molecules and
for electroneutrality reasons also the [AlPh4]− ion. Substitution
of one iodide ion by a THF molecule leads to slightly elongated
Ca−CNHC contacts (2.525(3)/2.537(3) Å in 3 vs 2.546(2)/
2.573(2) Å in 5]. Again, the Ca1−O1/2 distances are in the
characteristic range.
Figure 3. Molecular structure and numbering scheme of 3. The
ellipsoids represent a probability of 30%. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å), bond angles (deg), and dihedral
angles (deg): Ca1−C1 2.525(3), Ca1−C11 2.537(3), Ca1−N3
2.527(3), Ca1−O1 2.349(2), Ca1−I1 3.0480(7), Ca1−I2 3.0580(7);
I1−Ca1−I2 174.60(2), C1−Ca1−O1 108.07(9), O1−Ca1−N3
171.67(9); C1−N2−C4−N3 2.07, C11−N4−C8−N3 10.58.
Extension of this method and the use of
−
[(thf)6Ca]2+[MPh4]2 (M = B, synthesis by interaction of a
other (3′) is depicted in the Supporting Information. The
calcium ion shows a coordination number of 6 with one
tridentate molecule of 2 and two iodide anions as well as one
thf base. The environment of the calcium atom is best
described as a distorted octahedron with I1 and I2 in a trans
arrangement with an I1−Ca1−I2 bond angle of 174.60(2)°.
Due to the smaller coordination number in comparison to
complex 4, shorter Ca−CNHC bonds of 2.525(3)/2.537(3) Å
are found in 3 than in 4 (2.565(4)/2.577(4) and 2.701(4)/
2.631(4) Å). In contrast to these smaller values, the Ca−N
2:1 mixture of NaBPh4 and CaBr2 in THF; M = Al, synthesis by
addition of [(thf)4CaPh2] to 2 equiv of AlPh3 in THF) in a very
similar reaction with 2 yielded insoluble amorphous materials
of unknown compositions.
These experiments demonstrate that purification and
characterization of these calcium complexes is quite challeng-
ing, due to the fact that they are only sparingly soluble or even
insoluble in common organic solvents and that reliable C, H, N
analytical data are difficult to obtain because the content of
solvent molecules as well as of coprecipitated CaI2 cannot safely
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be determined. Therefore, we decided to prepare a soluble
derivative, namely [(CarMesPyCarMes)2(thf)Ca(NPh2)2]
([(2)2(thf)Ca(NPh2)2], 6) (Scheme 3), to also further verify
The binding pocket of CarMesPyCarMes (2) seems to be
ideally suited for calcium ions and even allows binding of rather
bulky diphenylamido ligands, maintaining the preferred
hexacoordination of this alkaline earth ion. We were also
interested in how this tridentate base adapts to larger alkaline
earth ions. Therefore, we slowly combined a THF solution of
MI2 (M = Sr, Ba) with a solution of 2, yielding single crystals
with the composition [(CarMesPyCarMes)(thf)2MI2]·THF (M =
Sr (7, Figure 6), Ba (8, see the Supporting Information)). In
Scheme 3. Synthesis of Soluble
[(CarMesPyCarMes)2(thf)Ca(NPh2)2] (6) via a Metathetical
Approach of KNPh2 with 3 and via Addition of 2 to
Ca(NPh2)2 in THF
the constitution of microcrystalline 3. Complex 6 was accessible
18
from the reaction of 2 with Ca(NPh2)2 in tetrahydrofuran.
Crystallization and X-ray diffraction studies verified the
composition. The reaction of KNPh2 with the amorphous
and insoluble solids, described above and assigned to
[(CarMesPyCarMes)(thf)CaI2] ([(2)(thf)CaI2], 3), also yielded
complex 6, as has been verified by NMR spectroscopy. The
carbene resonance was observed at a characteristic shift of δ
202.9 ppm.
Compound [(2)(thf)Ca(NPh2)2] (6) crystallized with two
molecules A and B in the asymmetric unit. The molecular
structure and numbering scheme of molecule A is depicted in
Figure 5. The hexacoordinate calcium atom is in a distorted-
octahedral environment. The larger size of the amido ligands
leads to an elongation of the bond lengths between Ca1 and the
donor atoms of ligand 2 as well as of the ligated thf ligand. The
nitrogen atoms of the diphenylamido anions are in a trigonal-
planar coordination sphere due to charge back-donation from
the lone pair at N into the phenyl groups.
Figure 6. Molecular structure of 7 (M = Sr) [8 (M = Ba)]. Thermal
ellipsoids represent a probability of 30%. Hydrogen atoms are omitted
for clarity. Selected bond lengths (Å), bond angles (deg), and dihedral
angles (deg): Sr1−C1 2.779(3) [2.888(5)], Sr1−C11 2.808(3)
[2.925(4)], Sr1−N3 2.803(3) [2.962(3)], Sr1−O1 2.625(2)
[2.745(3)], Sr1−O2 2.578(2) [2.713(3)], Sr1−I1 3.2302(3)
[3.3696(4)], Sr1−I2 3.3493(3) [3.4855(4)]; I1−Sr1−I2
177.130(11) [175.147(11)], O1−Sr1−O2 73.14(7) [73.23(12)],
C1−Sr1−N3 60.41(8) [57.63(11)], C11−Sr1−N3 60.28(8)
[57.78(10)], N4−C11−N5 101.6(2) [102.5(4)], N1−C1−N2
101.6(3) [102.1(4)], C4−N3−C8 116.2 [116.8(4)]; C1−N2−C4−
N3 8.84 [8.84], C11−N4−C8−N3 −15.32 [−15.45].
the crystalline state both compounds are isotypic. In both
complexes the metal ions have distorted pentagonal-bipyr-
amidal coordination spheres consisting of 2 and two thf
molecules as well as two iodide counterions. The axial positions
are occupied by the two halide ions, which span I1−M1−I2
angles of 177.13(1) and 175.15(1)°, respectively, for 7 and 8.
The M−CNHC bonds are in a range similar to that observed for
monodentate carbene ligands at Sr (2.731−2.768 Å) and Ba
(2.914−3.1209 Å).3a,4b,19 The expected symmetry of the
molecule is broken by a slight shift of the metal ion to one
side of the κ2CNHC,κ1N pocket of 2, as indicated by different
M−CNHC bond lengths (7 (M = Sr), 2.779(3) and 2.808(3) Å;
8 (M = Ba), 2.888(5) and 2.925(4) Å) and dihedral angles,
with one carbene ring turning out of the ligand plane.
It is noteworthy that the M−I2 bond is significantly longer
than the M−I1 bond by 11.9 pm (M = Sr1, 3.7% elongation)
and 11.6 pm (M = Ba1, 3.4% elongation). In both complexes,
the atom I2 shows intermolecular contacts of 3.064 Å to the
hydrogen atom at C7 in the meta position of the pyridyl
fragment, whereas I1 is intramolecularly more effectively
shielded by the bulky aryl groups. An I···H contact of 3.064
Å is smaller than the sum of the van der Waals radii (3.5 Å)20
and could polarize the very soft iodide, leading to a lengthening
of the M−I bonds. It has been reported earlier that the Sr−I21
and Ba−I distances22 in complexes of strontium and barium
Figure 5. Molecular structure and numbering scheme of molecule A of
6. Thermal ellipsoids represent a probability of 30%. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and bond angles
(deg) for molecule A [molecule B]: Ca1−C1 2.614(5) [2.664(5)],
Ca1−C18 2.667(5) [2.618(5)], Ca1−N3 2.595(4) [2.616(4)], Ca1−
O1 2.477(3) [2.474(3)], Ca1−N6 2.447(4) [2.428(4)], Ca1−N7
2.443(4) [2.434(4)]; N6−Ca1−N7 160.68(14) [157.43(14)], N3−
Ca1−O1 147.43(12) [149.41(12)], C1−Ca1−C18 127.73(15)
[127.25(15)].
D
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iodides vary within the large ranges of 322−349 and 337−354
pm, respectively, depending on the coordination numbers of
the alkaline earth ions and iodide as well as on the donor
strength and bulkiness of the coligands.
EXPERIMENTAL SECTION
■
General Remarks. All manipulations were carried out under an
inert nitrogen atmosphere using standard Schlenk techniques, if not
otherwise stated. The solvents were dried over KOH and subsequently
distilled over sodium/benzophenone under a nitrogen atmosphere
prior to use. Deuterated solvents were dried over sodium, distilled,
degassed, and stored under nitrogen over sodium. If the solubility of
CONCLUDING REMARKS
■
2,6-Bis(3-mesitylimidazol-2-ylidene)pyridine (2) exhibits an
ideal binding pocket for heavy alkaline earth ions that has
been studied by the complexation of the iodides of calcium,
strontium, and barium with 2. However, due to intermolecular
π stacking of the aromatic groups most of these adducts are
only sparingly soluble or even insoluble in common organic
solvents. Therefore, diffusion experiments seem to be the
preferred procedure to isolate crystalline complexes. Thus, CaI2
reacts with 2 in a diffusion experiment, yielding a few crystals of
the unique separated ion pair [(CarMesPyCarMes)2(thf)Ca]I2
(4), containing two molecules of 2 which show significantly
different Ca−C/N bond lengths due to intramolecular π
stacking. Reaction of 2 with [(thf)5CaI]+[BPh4]− in THF
induced dismutation, leading to two modifications of
[(CarMesPyCarMes)(thf)CaI2] (3 and 3′) as well as insoluble
and poorly characterized [(CarMesPyCarMes)x(thf)yCa]2+-
1
the compounds was sufficient, H and 13C{1H} NMR spectra were
recorded on Bruker Avance 400 and Avance III 600 spectrometers.
Chemical shifts are reported in parts per million relative to SiMe4 as an
external standard referenced to the solvent’s residual proton signal. All
substrates were purchased from TCI or Alfa Aesar and used without
further purification. KHMDS was obtained from KH and H-HMDS in
toluene. The complex [(thf)5CaI][AlPh4] was prepared following a
protocol published by Krieck et al.17 The compound [(thf)5CaI]-
[BPh4] was prepared from CaBr2, NaBPh4, and NaI in THF. The
compound [(thf)4Ca(NPh2)2] was prepared according to the
procedure of Glock et al.18 The yields given are not optimized.
Ligand 2 was prepared according to literature procedures,11,12,23 as
also specified in the Supporting Information. The complexes of the
heavy alkaline earth metal complexes with 2 were extremely sensitive
toward air and turned dark immediately upon exposure to air. This fact
together with the insolubility in common organic solvents hampered
1
the elemental analyses. Purity and yields were verified by H NMR
[BPh4]− . In contrast to this finding, the reaction of 2 with
2
spectroscopy for soluble complexes and by titration of the iodine
content of an HNO3 pulping of the insoluble derivatives 7 and 8.
Synthesis of [(CarMesPyCarMes)(thf)CaI2] (3) and
[(CarMesPyCarMes)2(thf)Ca]I2 (4). A 254 mg portion of CaI2 (0.86
mmol, 1 equiv) was suspended in 20 mL of THF, and 10.7 mL of a
0.08 M solution of CarMesPyCarMes (0.86 mmol, 1 equiv) in THF was
added. The suspension was stirred for 1 day at room temperature,
forming a light brown solution and a white precipitate, which was
collected and dried. Yield: 664 mg (95%) of [(CarMesPyCarMes)(thf)-
CaI2]. Single crystals suitable for X-ray diffraction were obtained by the
following procedure. A dilute solution of CaI2 in THF was layered
with a dilute solution of CarMesPyCarMes in THF. Various attempts
yielded only microcrystalline [(CarMesPyCarMes)(thf)CaI2], and finally
a few crystals of [(CarMesPyCarMes)2(thf)Ca]I2 (4) were isolated,
which were suitable for X-ray diffraction studies. However, no further
analytical data were accessible. Single crystals of [(CarMesPyCarMes)-
(thf)CaI2] were obtained as follows. A 130 mg portion of
[(thf)5CaI][BPh4] (0.15 mmol, 1 equiv) was dissolved in 30 mL of
THF. To this solution was added 1.93 mL of a 0.08 M solution of
CarMesPyCarMes (0.15 mmol, 1 equiv), and the mixture was filtered.
Addition of Et2O to the filtrate led to slow crystallization of
[(CarMesPyCarMes)(thf)CaI2] (37%, 46 mg of 3), suitable for X-ray
diffraction experiments, as well as of an amorphous solid of the same
constitution. Anal. Calcd for C33H37CaI2N5O (813.56 g mol−1): C,
48.72; H, 4.58; N, 8.36; Ca, 4.93; I, 31.20. Found: C, 47.89; H, 4.50;
N, 8.36; Ca, 4.87; I, 31.83. Mp: 146 °C dec. IR (ATR, cm−1): ν 3053
w, 2973 w, 2945 w, 2872 w, 1610 m, 1587 m, 1461 s, 1392 m, 1270 m,
877 m, 799 m, 737 m.
[(thf)5CaI]+[AlPh4]− in THF yields the desired complex
[(CarMesPyCarMes)(thf)2CaI]+[AlPh4]− (5). For solubility
reasons, we substituted the halide ions by diphenylamido
ligands, leading to soluble [(CarMesPyCarMes)2(thf)Ca(NPh2)2]
(6), allowing NMR spectroscopic characterization. We also
investigated the coordination behavior of 2 toward the heavier
alkaline earth ions. Therefore, compound 2 was added to MI2,
yielding isostructural [(CarMesPyCarMes)(thf)2MI2]·THF for M
= Sr (7), Ba (8).
All crystalline compounds have been characterized by X-ray
diffraction studies (Table 1). These derivatives contain short
Table 1. Comparison of Average Values of Selected Bond
Lengths (Å) of the Alkaline Earth Metal Complexes,
Containing the Ligand CarMesPyCarMes (2)
complex
M−NPy
M−CNHC
M−Othf
M−I
3
2.527
2.545
2.522
2.566
2.530
2.606
2.803
2.962
2.531
2.550
2.531
2.619
2.560
2.641
2.794
2.907
2.349
2.357
2.332
2.506
2.349
2.476
2.602
2.729
3.053
3.056
3.075
a
3′A
3′B
4
5
6
7
8
a
3.070
3.290
3.428
Synthesis of [(CarMesPyCarMes)(thf)2CaI][AlPh4] (5). A 162 mg
portion of [(thf)5CaI][AlPh4] (0.18 mmol, 1 equiv) was dissolved in
15 mL of THF, and 2.35 mL of a 0.08 M solution of CarMesPyCarMes in
THF was added. The mixture was stirred for 2 h and filtered. Addition
of 5 mL of Et2O at room temperature resulted in slow crystallization of
105 mg of [(CarMesPyCarMes)(thf)2CaI][AlPh4] (47%). The obtained
crystals were suitable for X-ray diffraction experiments. Anal. Calcd for
C69H81CaIO4AlN5 (1238.35 g mol−1): C, 66.92; H, 6.59; N, 5.66.
Found: C, 64.60; H, 6.25; N, 5.89. The carbon content is too low most
likely due to formation of Al4C3. Mp: 135 °C dec. IR (ATR, cm−1): ν
3047 w, 2967 w, 2861 w, 1608 m, 1586 m, 1464 s, 1392 m, 1271 m,
1072 s, 705 s, 662 s, 479 s. NMR spectra were recorded of the reaction
mixture due to the insolubility of the crystals. However, the
concentration was too low for 13C{1H} NMR measurements. 1H
NMR (600 MHz, 25 °C, [D8]THF): δ 1.82 (m, THF), 2.11 (6H,s),
a
The modification 3′ contains two crystallographically independent
molecules A and B (see the Supporting Information).
Ca−CNHC bond lengths which are in a typical range of Ca−C σ
bonds of organocalcium reagents. There are only very few
complexes of strontium and barium with M−C σ bonds known,
but the values for complexes 7 and 8 are also rather small.
Consequently, the Lewis base 2 has verified its nature as a
strong tridentate ligand for heavy alkaline earth ions being able
to substitute thf ligands in the coordination spheres of heavy
alkaline earth ions, whereas the halides remain bonded to the
metal centers. Only for the calcium derivative 4 is substitution
of the halide by a second tridentate ligand 2 observed. With the
exception of this complex, rather short M−O bond lengths
support the lack of severe intramolecular steric strain.
2.27 (12H, s), 3.66 (m, THF), 6.91 (4H, tt, 3JH−H = 7.26 Hz, 3JH−H
=
3
1.50 Hz), 6.97 (8H, t, JH−H = 7.26 Hz), 7.00 (4H, s), 7.31 (2H, d,
E
DOI: 10.1021/acs.organomet.6b00914
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
3
3JH−H = 1.92 Hz), 7.72 (8H, bs), 7.84 (2H, d, JH−H = 8.20 Hz), 8.19
Crystallographic data as well as structure solution and refinement
details are summarized in the Supporting Information. XP30 and POV-
Ray31 were used for structure representations.
(2H, d, JH−H = 1.92 Hz), 8.24 (1H, t, JH−H = 8.20 Hz) ppm. 27Al
NMR (104.2 MHz, 25 °C, [D8]THF): δ 133.03 ppm.
3
3
Synthesis of [(CarMesPyCarMes)(thf)Ca(NPh2)2] (6). A 2.2 mL
portion of a 0.147 M solution of [(thf)4Ca(NPh2)2] (0.32 mmol, 1
equiv) in THF was diluted with 4 mL of THF. To this solution was
added 4.1 mL of a 0.08 M solution of CarMesPyCarMes (0.32 mmol, 1
equiv) in THF. The volume was reduced by half. Storage of the
solution at −40 °C led to the formation of crystals, which were suitable
for X-ray diffraction ([(CarMesPyCarMes)(thf)Ca(NPh2)2]·4THF). The
product was collected, which resulted in a color change from red to
orange. NMR spectra were recorded of this solid, verifying the purity
of the compound and also the loss of 1.75 THF molecules during
collection and drying of the crystals. Yield of [(CarMesPyCarMes)(thf)-
Ca(NPh2)2]·2.25THF: 67% (211 mg).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00914. Crystallographic data (excluding structure
factors) have also been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
CCDC-1519060 for 2, CCDC-1519061 for 4, CCDC-1519062
for 3, CCDC-1519063 for modification 3′, CCDC-1519064 for
5, CCDC-1530221 for 6, CCDC-1519065 for 7, and CCDC-
1519066 for 8. Copies of the data can be obtained free of
charge on application to the CCDC, 12 Union Road,
Cambridge CB2 1EZ, U.K. (e-mail deposit@ccdc.cam.ac.uk).
Due to the easy loss of cocrystallized solvent, no reliable elemental
analysis data were obtained. IR (ATR, cm−1): ν 3042 w, 2972 w, 2914
w, 2855 w, 1588 m, 1573 m, 1461 m, 1440m, 1342 m, 1308 m, 1269
m, 1167 m, 983 m, 743 m, 688 m, 519 m, 500 m. 1H NMR (400 MHz,
25 °C, [D8]THF): δ 1.81 (13H, m, THF), 1.83 (12H, s), 2.31 (6H, s),
3.65 (13H, m, THF), 6.08 (4H, bt), 6.35 (8H, bd), 6.54 (8H, bt), 6.79
(4H, s), 7.17 (2H, d, 3JH−H = 1.90 Hz), 7.66 (2H, d, 3JH−H = 8.22 Hz),
Crystallographic data (CIF)
Synthesis of compound 2, structure representations of
modification 4′ and 7, and crystallographic and refine-
ment details (PDF)
3
3
8.07 (2H, d, JH−H = 1.90 Hz), 8.11 (1H, t, JH−H = 8.22 Hz) ppm.
13C{1H} NMR (100.6 MHz, 25 °C, [D8]THF): δ 17.0, 20.1, 25.4,
67.2, 109.7, 112.7, 116.7, 118.6, 124.0, 127.9, 128.7, 135.4, 136.7,
137.6, 142.9, 151.3, 156.9, 202.8 ppm.
AUTHOR INFORMATION
Corresponding Author
*M.W.: e-mail, m.we@uni-jena.de; web, http://www.lsac1.uni-
jena.de; fax, +49 3641 9-48132.
■
Synthesis of [(CarMesPyCarMes)(thf)2SrI2].THF (7). A 152 mg
portion of SrI2 (0.44 mmol) was suspended in 20 mL of THF, and
5.61 mL of a 0.08 M solution of CarMesPyCarMes (0.44 mmol, 1 equiv)
in THF was added. The suspension was stirred for 1 day at room
temperature, forming a light brown solution and a white precipitate,
which was collected and dried. Yield: 410 mg (93%) of
[(CarMesPyCarMes)(thf)2SrI2]. Single crystals suitable for X-ray
diffraction were obtained by the following procedure. A dilute solution
of SrI2 in THF was layered with a dilute solution of CarMesPyCarMes
(2) in THF. After a few days, crystals of [(CarMesPyCarMes)-
(thf)2SrI2].THF were obtained that were suitable for X-ray diffraction
analysis. No reliable C, H, N analytical data were obtained. HNO3
pulping: I, calcd 25.25, found 25.71. Mp: 135 °C dec. IR (ATR, cm−1):
ν 3023 w, 2944 w, 2859 w, 1605 m, 1592 m, 1459 s, 1390 m, 1273 s,
1258 m, 1038 m, 801 m, 735 m, 720 m.
ORCID
Matthias Westerhausen: 0000-0002-1520-2401
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We appreciate the financial support of the Fonds der
Chemischen Industrie im Verband der Chemischen Industrie
e.V. (FCI/VCI, Frankfurt/Main, Germany; fund no. 510259).
A.K. thanks the Fonds der Chemischen Industrie im Verband
der Chemischen Industrie e.V. for a generous Ph.D. stipend.
Synthesis of [(CarMesPyCarMes)(thf)2BaI2] (8). A 202 mg portion
of BaI2 (0.52 mmol) werewas suspended in 20 mL of THF, and 6.5
mL of a 0.08 M solution of CarMesPyCarMes (0.52 mmol, 1 equiv) in
THF was added. The suspension was stirred for 1 day at room
temperature, giving a light brown solution and a white precipitate,
which was collected and dried, yielding 350 mg (70%) of
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