π ligands in alkaline earth complexes

Article abstract of DOI:10.1021/acs.organomet.7b00006

π ligands such as olefins and alkynes bind intramolecularly to the metal atom in d0 complexes of the large alkaline earths (Ae) calcium and strontium supported by fluoroalkoxo ligands with dangling unsaturated C=C or C=C bonds, and having the amide N(SiMe

Full text of DOI:10.1021/acs.organomet.7b00006

Article
pubs.acs.org/Organometallics
π Ligands in Alkaline Earth Complexes
Sorin-Claudiu Rosc
and Yann Sarazin*
̧
a, Elsa Caytan, Vincent Dorcet, Thierry Roisnel, Jean-Franco
̧
is Carpentier,*
Universite
France
́
de Rennes 1, CNRS, UMR 6226, Institut des Sciences Chimiques de Rennes, Campus de Beaulieu, 35042 Rennes Cedex,
S
* Supporting Information
ABSTRACT: π ligands such as olefins and alkynes bind
intramolecularly to the metal atom in d0 complexes of the large
alkaline earths (Ae) calcium and strontium supported by
fluoroalkoxo ligands with dangling unsaturated CC or CC
bonds, and having the amide N(SiMe2H)2− as the coligand. These
O-bridged dinuclear complexes are further stabilized by secondary
C−F→Ae and β-Si−H···Ae interactions. In a set of structurally
related Ca-olefin complexes, the strength of these interactions
gradually increases as the coordination of the olefin onto Ca2+
becomes weaker (from η2-coordinated to η1 to fully dissociated)
upon increasing steric congestion, thus ensuring that over-
whelming electronic depletion does not occur at calcium. NMR data imply that the olefins are metal-bound in [D8]toluene
solutions. The Ae···Cπ, C−F→Ae, and β-Si−H···Ae noncovalent interactions are also strong in the parent Ae-alkyne complexes,
the first examples of non-acetylide Ae-alkynes compounds. Calcium-arene complexes could not be made, as the aromatic tether
did not bind to the metal atom. Instead, a trinuclear complex with noninteracting C6H5 groups was obtained. It exhibits
exceptionally strong C−F→Ca and β-Si−H···Ca interactions. NMR data indicate that the congeneric calcium-allene complex can
be made, but it spontaneously isomerizes toward the more stable Ca-alkyne via an unusual 1,3-hydride shift intramolecular
process.
[{DippNacNac}CaN(η2-C6H5)(C6H5)·THF] (DippNacNac =
INTRODUCTION
■
CH[CMeN(2,6-iPr2-C6H3)]2),15 and in the bis(guanidinate)
barium complex [{MesN{C(NCy2)}NMes}2Ba], where Mes =
2,4,6-Me3C6H2 and Cy = cyclohexyl.16 Other types of Ae2+···
arene interactions have been reported in Ae2Odpp4 (Ae = Ca-
Ba, η1 to η6 interactions),17 and in the iodocalcium-cuprate(I)
[ICa-η1,η6-(Mes2Cu)]4.18 Ae-alkyne complexes are somewhat
more common. [{C5Me5}2Ca{Me3Si−CC−CC−SiMe3}]
was the first monomeric diyne complex of a main-group
metal.19 Since then, “side-on” interactions between Ae metal
centers and CC triple bonds have been observed in dimeric
acetylides20 such as [{DippNacNac}Ca(μ-CC−R)]2 (R = Ph,
p-tol) and [{DippNacNac}Ca(μ-CC−CH2−E)]2 (E = OMe,
OPh, NMe2),21 or in the THF solvate [{DippNacNac}Ca(μ-C
C−Ph)·(THF)]2.22 A single side-on barium-alkyne complex,
obtained by treatment of [Ba{P(SiMe3)2}2·(THF)4] with
diphenylbutadiyne, is known.23 Finally, no example of π-
interaction in Ae-allene complexes has appeared to date.
Following a landmark review article by Ruhlandt-Senge,24 we
have used secondary interactions to stabilize otherwise
kinetically labile Ae heteroleptic complexes by β-Si−H···Ae
agostic distortions25 or by cation-dipole C−F→Ae interac-
tions.26,27 We have then become keen on exploiting olefins and
other π ligands to synthesize model Ae complexes which could
Divalent cations Ae2+ of the alkaline earths (Ae) calcium,
strontium, and barium are large (effective ionic radii: 1.00, 1.18,
and 1.35 Å for C. N. = 6), polarizable, and electrophilic. They
form d0 complexes that feature ionic, nondirectional bonding
with no propensity for π bonding due to the inability to convey
d−π* back-donation. Following Westerhausen’s syntheses of
mesitylcalcium iodide,1 diarylcalcium,2 arylcalcium cations,3 and
1-alkenyl and alkyl calcium iodides,4 the organometallic
chemistry of calcium has achieved a significant status.5
However, noncovalent interactions between alkaline earths
and π ligands remain little explored. Hence, very few Ae-olefin
complexes of Ca-Ba are known.6 Until recently,7 the only
structurally authenticated Ae-olefin complexes were Schu-
mann’s [Ae(C5Me4CH2CH2CHCH2)2] (Ae = Ca-Ba)8 and
Roesky’s [Ba(C5Me5)(C5Me4CH2C5Me5)].9 Even if Ae-cyclo-
pentadienyl and -allyl complexes are known,10 the paucity of
pure Ae-olefin complexes is surprising. Similarly, although π
interactions between Ae2+ divalent cations and charge-neutral
arenes have been probed computationally,11 Ae-arene adducts
are rare. In the known examples, Ae2+···arene η6-interactions are
most often encountered, as in [{N3ArAr′}AeC6F5] (Ae = Ca-
Ba) bearing aryl-substituted triazenides,12 in bis[N-(2,6-diiso-
propylphenyl)-N′-(2-pyridylmethyl)pival-amidinate] and bis-
[N-diisopropylphenyl-N′-(8-quinolyl)pivalamidinate] calcium
complexes,13 in the heterobimetallic [Ca2Odpp3]+·[LnOdpp4]−
(Ln = Nd, Ho, Yb; Odpp = 2,6-diphenyl-phenolate),14 in
Received: January 4, 2017
© XXXX American Chemical Society
A
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a
Scheme 1. Structures of Complexes [{ROx}AeN(SiMe2H)2]2, with Secondary Interactions as Dashed Lines
a
x = 1, Ae = Ca, [1-Ca]2; x = 1, Ae = Sr, [1-Sr]2; x = 2, Ae = Ca, [2-Ca]2; x = 3, Ae = Ca, [3-Ca]2; x = 4, Ae = Ca, [4-Ca]2. Positions of the C atoms
labeled in the frame. For the syntheses and characterization of [1-Ca]2, [1-Sr]2, [2-Ca]2, and [3-Ca]2, see ref 7.
help unravel the mechanisms of Ae-promoted olefin and alkyne
hydroelementations.
RESULTS
■
General Synthetic Features. A range of calcium and
We have earlier shown that the calcium- and strontium-olefin
complexes [{RO1}CaN(SiMe2H)2]2 ([1-Ca]2) and [{RO1}-
SrN(SiMe2H)2]2 ([1-Sr]2) stabilized by intramolecular β-Si−
H···Ae and C−F→Ae interactions could be synthesized upon
treatment of [Ca[N(SiMe2H)2]2·(THF)] or [Sr[N-
(SiMe2H)2]2·(THF)2/3] with the α,α-(CF3)2-alcohol {RO1}H
having a single dangling terminal olefinic tether (Scheme 1).7
The related [{RO2}CaN(SiMe2H)2]2 ([2-Ca]2) and [{RO3}-
CaN(SiMe2H)2]2 ([3-Ca]2), where the olefin in the ligands
{RO2}− and {RO3}− is substituted, respectively, by one (Z
configuration) and two methyl groups on the carbon atom in
the δ position from the Namine atom, were also obtained.
Coordination of the internal olefin onto the alkaline earths in
[1-Ca]2, [1-Sr]2, and [2-Ca]2 was evidenced by crystallographic
methods. It was shown to be thermodynamically favored over
the coordination of external Lewis bases (THF). NMR data
and DFT computations further attested to the existence of Ae···
olefin interactions. Upon increasing steric hindrance about the
olefin, olefin coordination switched from η2 in [1-Ca]2 and [1-
Sr]2 to η1 in [2-Ca]2 and to complete dissociation in [3-Ca]2.
This was, however, compensated by the other β-Si−H···Ae and
C−F→Ae interactions, which conversely became increasingly
strong.
strontium complexes supported by fluorinated aminoalkoxides
{ROx}− bearing pendant alkene, alkyne, arene, or allene tethers,
−
and incorporating the amide N(SiMe2H)2 as a coligand, were
synthesized (Scheme 2); specifics are given further. Typical
syntheses involved the 1:1 reaction of [Ae[N(SiMe2H)2]2·
(THF)n] (Ae = Ca, n = 1; Sr, n = 2/3) with the proteo-ligand
{ROx}H in diethyl ether (Ae precursor used in slight excess to
avoid forming [{ROx}2Ae]), with temperature rising from −78
to 25 °C. The heteroleptic, dinuclear complexes [{ROx}AeN-
(SiMe2H)2]2 were generally obtained in moderate to high
Scheme 2. General Representation of the Reactions between
[Ae[N(SiMe2H)2]2·(THF)n] and Fluorinated Aminoalcohols
{ROx}H with Tethered π ligands, Yielding the Heteroleptic
[{ROx}AeN(SiMe2H)2]2
We report here on related alkene, alkyne, arene, and allene
fluoroalkoxo calcium and strontium complexes showing
interactions between the π ligands and the alkaline earths.
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yields upon release of HN(SiMe2H)2 and displacement of
THF. All complexes were isolated as very air- and moisture-
sensitive, colorless, analytically pure solids. Synthetic and
characterization details for all compounds are given in the
Supporting Information.
coordination of one olefinic moiety per Ca2+ ion (Ca1−
C25#1 = 2.972(2) Å and Ca1−C26#1 = 3.121(2) Å). Out of the
two olefinic tethers in each alkoxo ligand, only one binds to the
metallic ion, in the same slightly dissymmetric η2-coordination
mode (with a shorter distance to the internal Cπ atom, aka
C25) as that seen in [1-Ca]2.
Olefin Complexes. Elaborating on our syntheses of [1-
Ca]2−[3-Ca]2,7 complex [{RO4}CaN(SiMe2H)2]2 ([4-Ca]2;
see Scheme 1), where each ligand possesses two side-arms with
terminal olefins, was obtained in 52% yield and free of
coordinated THF from [Ca[N(SiMe2H)2]2·(THF)]. It crystal-
lized from toluene at −26 °C. Its formulation was established
on the basis of crystallographic data, and its purity was
confirmed by NMR spectroscopy and combustion analyses.28
The proteo-ligands akin to {RO2}H but where the methyl
group is either in the γ position to the Namine atom (=
vinylidene end-group) or in the δ position with an E
configuration, were synthesized. However, the resulting
complexes were obtained as oily materials, and although their
fluxional 1H and 19F NMR data were encouraging, we could not
obtain firm evidence for the formations of the desired
heteroleptic complexes. Like for [1-Ca]2−[3-Ca]2,7 X-ray
diffraction crystallography shows [4-Ca]2 to exist as an O-
bridged dinuclear complex (Figure 1). It features η2-
The second olefin, which is disordered over two sites, is
remote from calcium (Ca−Cπ distances > 5.7 Å). The binding
of this second olefin is prohibited presumably on account of
excessive steric congestion about the eight-coordinate metal,
resulting in unfavorable entropic considerations. In addition to
Ca···olefin interactions, the complex is stabilized by one
intramolecular C−F→Ca interaction (Ca(1)−F(15) =
2.9309(15) Å) and one β-Si−H···Ae agostic distortion
(Ca(1)−H(2) = 3.083(25) Å), by which we mean the largely
electrostatic 2-electron-3-center interaction between the
electro-deficient Ae2+ ion and polarized (δ+)Si−H(δ−) bond.
The narrow Ca1−N1−Si2 (111.96(10)°), the short distances
from Ca1 to H2 (3.083(25) Å; the positions of the SiH
hydrogen atoms were located in the electronic density map),
and the narrow Ca1−N20−Si2−H2 torsion angles (8.3°) are
reliable indications of β-Si−H···Ca agostic distortions.25,29 Key
metric parameters for [1-Ca]2−[4-Ca]2 and [1-Sr]2 are given
in Table 1.
The geometric features in [4-Ca]2 combine with reasonable
accuracy those seen in [1-Ca]2 and [3-Ca]2. The metric
parameters collated in Table 1 show that the strength of C−
F→Ca and β-Si−H···Ae interactions increase when the Ca···
olefin interactions become weaker, i.e.. on moving from [1-Ca]2
(olefin η2-coordinated) to [2-Ca]2 (olefin η1-coordinated) and
[3-Ca]2 (olefin fully dissociated): stronger C−F→Ca and β-
Si−H···Ca contacts make up for the loss of electronic density
provoked by the gradual dissociation of the olefin. Such a
unique pattern of multiple secondary interactions ensures the
stability of these heteroleptic calcium complexes by preventing
aggregation, decomposition, or ligand redistribution via Schlenk
equilibria.
Figure 1. ORTEP diagram of the molecular solid-state structure of [4-
Ca]2. Ellipsoids at 50% probability. Only the main components of the
disordered noncoordinated olefins (C29AC30A) are represented. H
atoms other than SiH omitted for clarity. Ca interactions with F, H,
and Cπ atoms in dashed lines. Selected bond lengths (Å) and angles
Solution NMR Studies. We turned to NMR spectroscopy to
inform ourselves on the solution behavior of [1-Ca]2−[4-Ca]2
and [1-Sr]2. Their NMR data were recorded in [D6]benzene, or
1
in [D8]toluene for low-temperature experiments. H DOSY
(deg): Ca1−N1 = 2.288(2), Ca1−O11 = 2.302(1), Ca1−O11#1
=
diffusion coefficient-formula weight analyses revealed that all
complexes remain dinuclear in [D6]benzene.30 Further
evidence is provided by comparison of the hydrodynamic
2.3127(15), Ca1−N22#1 = 2.597(2), Ca1−F15 = 2.9309(15), Ca1−
C25#1 = 2.972(2), Ca1−C26#1 = 3.121(2), Ca1−C29A#1 = 5.780(10),
Ca1−C30A#1 = 6.707(11), Ca1−H2 = 3.083(25), Ca1−H1 =
3.335(33); Si2−N1−Ca1 = 111.96(10), Si1−N1−Ca1 = 119.60(10).
1
radii determined crystallographically and in solution by H
DOSY NMR.31 The 1H and 19F VT NMR data for [1-Ca]2−[4-
Ca]2 and [1-Sr]2 show that, in solution, the compounds are
involved in complex dynamic phenomena, even at low
Table 1. Summary of Key Metric Parameters in Ae-Olefin Complexes (Distances in Å, Angles in deg)
a
[1-Ca]2
[2-Ca]2
[3-Ca]2
[4-Ca]2
[1-Sr]2
Ae−Cπ(int)
Ae−Cπ(ext)
Ae−F
2.958(3)
3.102(3)
3.050(2)
2.933(2)−2.978(2)
3.334(2)−3.390(2)
2.834(1)−2.949(1)
5.053(3)
6.226(3)
2.605(1)
2.972(2)
3.121(2)
2.9309(15)
3.061(3)
3.165(3)
3.050(2)
3.136(2)
b
Ae−HSi(1)
Ae−HSi(2)
3.12(4)
2.983(27)−3.037(27)
2.77(2)
3.083(25)
3.092(34)
3.169(37)
111.70(13)
113.99(14)
b
3.19(3)
∠Ca−N−Si(1)
∠Ca−N−Si(2)
115.22(14)
116.69(14)
110.54(7)−112.52(7)
119.18(8)−122.06(8)
104.93(9)
125.80(9)
111.96(10)
119.60(10)
a
b
The two metallic ions are inequivalent in this complex; only one Ca···H−Si 2e3c interaction per Ca2+. Positions of H atoms located in the
electronic density map, not idealized.
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a
1
Table 2. Summary of Key H NMR Data in Ae-Olefin Complexes
b
c
d
e
f
[1-Ca]2
[2-Ca]2
[3-Ca]2
[4-Ca]2
[1-Sr]2
A
B
A
B
A
B
Hint
δ1H
5.96
0.49
0.981
−67%
H
6.13
0.66
0.883
−70%
H
5.97
5.87
0.37
1.162
−48%
H
5.1
5.1
5.87
0.49
1.151
−37%
H
6.19
0.71
1.252
−66%
H
g
Δδ1H
0.47
0.16
1.104
0%
0.16
1.104
0%
T1 1H
1.003
−55%
H
h
h
h
ΔT1 1H
i
cis group
Me
Me
δ1H
5.47
0.52
0.591
−73%
H
5.37
0.42
0.655
−70%
H
5.72
5.55
0.37
1.56
0.06
1.215
+7%
Me
1.54
0.04
1.197
+5%
Me
5.28
0.37
0.809
−42%
H
5.38
0.43
0.616
−81%
H
g
Δδ1H
0.54
j
T1 1H
0.811
−57%
Me
n.d.
j
ΔT1 1H
n.d.
k
trans group
Me
δ1H
5.31
0.34
0.586
−73%
5.29
0.32
0.586
−73%
1.53
1.58
0.09
0.93
−50%
1.69
0.02
0.839
+14%
1.69
0.02
0.839
+14%
5.21
0.33
0.755
−44%
5.22
0.28
0.592
−82%
g
Δδ1H
0.04
T1 1H
0.774
−58%
ΔT1 1H
a
All data recorded in [D8]toluene at the temperature of best resolution. δ1H and Δδ1H given in ppm. T1 given in s. For complexes [1-Ca]2−[3-Ca]2,
b
c
d
e
the data for the two distinct forms A and B are given. Data recorded at 263 K. Data recorded at 253 K. Data recorded at 233 K. Data recorded at
f
g
243 K. Data recorded at 298 K. Variation of chemical shift compared to the resonance for the same group (H/Me) in the parent proteo-ligand.
h
i
Percentage of variation of T1 compared to the same group (H/Me) in the parent proteo-ligand. For the group (H or Me) in position cis with
j
k
respect to Hint. Could not be determined due to overlap. For the group (H or Me) in position trans with respect to Hint.
temperatures;7 this was confirmed by 2D NMR (see below). As
1H−1H NOESY experiments evidenced dipolar coupling
1
a result, H and 19F spectra alone cannot demonstrate the
(spatial proximity) between olefinic hydrogens (or terminal
methyl groups) and SiH for [1-Ca]2, [2-Ca]2, [4-Ca]2, and [1-
Sr]2 (Supporting Information, Figures S20−S26). This
observation is again consistent with the retention of the Ae···
olefin interactions in solution with these compounds. The
dynamic behavior of the complexes due to conformational
exchange already observed by VT experiments was confirmed
persistence of the Ae···olefin interactions in aromatic solvents.
1
Nonetheless, the deshieldings of H resonances for olefinic
Holefin atoms (Table 2) on going from the proteo-ligand to the
Ae complex are greater for the complexes having a coordinated
olefin in the solid state (Δδ1H = 0.30−0.70 ppm) than for [3-
Ca]2 where olefin binding is not observed in the XRD structure
(Δδ1H < 0.10 ppm).32 This is seen as an indication that, in
solution, the olefins remain coordinated in [1-Ca]2, [2-Ca]2,
[4-Ca]2, and, by extrapolation, [1-Sr]2.
1
by correlation cross-peaks in the H−1H NOESY spectra: (i)
i
the rotation of Pr groups is detected in each species but [4-
Ca]2; (ii) inversion of chiral nitrogen configuration is suggested
by the following observations: at low temperature, two species
are detected for [1-Ca]2, [2-Ca]2, and [3-Ca]2; conformational
exchange between these two species is shown by positive
1
The measurement of H T1 spin−lattice relaxation times
helped our assessment of olefin coordination in solution. The
participation of a small molecule or organic fragment to the
coordination sphere of a large metal complex increases its
rotational correlation time, which in turn results in a decrease
of the relaxation time T1.33 In our case, the variations of T1
from proteo-ligands to Ae complexes are in line with the
observed presence or not of a coordinated olefin in the solid-
state structures on the complexes. In [1-Ca]2, [1-Sr]2, and [2-
Ca]2, all T1 for olefinic hydrogens are decreased by ca. 60−80%
with respect to the parent proteo-ligands, whereas no decrease
or even slight increases were observed for [3-Ca]2 (Table 2). In
each case, two conformational isomers A and B exhibited
comparable values of T1 for [1-Ca]2, [2-Ca]2, and [3-Ca]2. T1
also diminished by 50−60% for the methyl group in [2-Ca]2.
The decrease of T1 can be linked to anisotropic motion of the
olefin upon coordination to Ca2+, and it is an indication for the
retention of olefin binding in solution in these complexes.
For [4-Ca]2, which features one coordinated and one
dissociated olefins in the solid state and is involved in a rapid
equilibrium in solution (vide infra), presumably as a result of
exchange between coordinated and dissociated olefin moieties,
the observed T1 relaxation times for olefinic hydrogens decrease
by ca. 37−43% with respect to {RO4}H; it is roughly the
average of the values measured for [1-Ca]2 (one η2-interacting
olefin) and [3-Ca]2 (one noninteracting olefin). See Table 2 for
details.
i
correlation peaks for Pr and CH2 groups in the α′ position to
the Namine atom (see Scheme 1 for labeling), and (iii) at
ambient temperature, for [1-Sr]2, even if only one mean
resonance is observed for each site, positive correlations are
seen for nonequivalent CH2 groups. In addition, one more
conformational change seems to occur for [1-Ca]2 and [2-Ca]2:
at low temperature, two exchanging signals are seen for olefinic
protons, suggesting a change of enantioface for the coordinated
olefin.34 All pertinent NMR spectra are given in the Supporting
Information.
EXSY NMR experiments on [1-Ca]2 and [2-Ca]2 confirmed
that two distinct conformational exchanges occur concom-
itantly. For [1-Ca]2 studied at 263 K, the two exchange rates
kN,1 and kN,−1 estimated for CH2 (α and α′ positions vs Namine
)
and iPr groups were in the range 0.48−0.55 0.01 s−1, whereas
the exchange rates kolefin,1 and kolefin,−1 for the olefinic protons
were in the range 1.41−1.55
0.01 s−1. These different
exchange rates suggest that two independent, competitive
events, thought to be chiral nitrogen inversion and change of
coordinated enantioface of the olefin,35 take place simulta-
neously. The same scenario was established for [2-Ca]2, albeit
at different rate constants (kN,1 and kN,−1 = 0.21−0.38 0.01
s−1, kolefin,1 and kolefin,−1 = 0.07−0.14 0.01 s−1) (Supporting
Information, Tables S3 and S4).
D
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Scheme 3. Synthesis and Representations of the Dimeric [5-Ca·(THF)]2 and Trinuclear [5-Ca]3 Complexes Showing Selected
Secondary Interactions as Dashed Lines
1
Attempts to use H−15N HMBC NMR spectroscopy to
establish through-metal scalar coupling between the chiral
Namine atom with CHolefin hydrogens in [1-Ca]2, or even to
simply detect the Namine atom, were unsuccessful; only the
coupling between the terminal Namido and SiH atoms was
visualized. Since coupling between Namine and various CH
atoms was visible in {RO1}H, this result also pleads for rapid
(on the NMR time scale) inversion of the configuration at the
chiral nitrogen in [1-Ca]2.
[[{RO5}CaN(SiMe2H)2]2·Ca[N(SiMe2H)2]2] ([5-Ca]3), in-
corporating two alkoxo and four amido ligands and devoid of
coordinated solvent, was obtained in 62% yield with respect to
Ca (Scheme 3). The molecular solid-state structure of [5-Ca]3
exhibits a complicated pattern of strong to extremely strong C−
F→Ca and β-Si−H···Ca interactions, but no Ca···Cπ(arene)
contacts (Figure 2). The three calcium atoms are crystallo-
graphically inequivalent, even though the arrangements about
the two terminal atoms Ca2 and Ca3 are very similar; hence,
only that around Ca2 is discussed below.
Arene Complexes. The syntheses of calcium complexes
showing Ca···arene interactions were attempted by equimolar
reaction of [Ca[N(SiMe2H)2]2·(THF)] with the proteo-ligand
{RO5}H bearing a pendant phenethyl substituent and a methyl
substituent on the Namine atom; however, this attempt only
resulted in the formation of the THF-solvated complex
[{RO5}CaN(SiMe2H)2·(THF)]2 ([5-Ca·(THF)]2) (Scheme
3).
The coordination sphere around Ca2 includes one terminal
Namide atom (Ca2−N1 = 2.295(1) Å), one μ2-bridging Namide
(Ca2−N2 = 2.519(1) Å), and one μ2-bridging Oalkoxide (Ca2−
The molecular solid state of this O-bridged dimeric complex
was established by X-ray diffraction crystallography (Supporting
Information). It showed that there is no interaction between
the calcium center and the tethered aromatic moieties; the
other expected C−F→Ca contacts and β-Si−H···Ca agostic
distortions are seen, despite each Ca2+ being coordinated by a
molecule of THF. Complex [5-Ca·(THF)]2 was characterized
by NMR spectroscopy. In [D8]toluene solution, it remains
dimeric (1H DOSY NMR, Dt = 6.90 × 10−10 m2 s−1) and
undergoes fluxional processes (1H, 19F NMR). The DOSY
NMR data also indicated that the THF molecules (Dt = 1.40 ×
10−9 m2 s−1) dissociate to some extent in solution, or that they
engage in a fast (for the NMR time scale) coordination−
dissociation process.36 It is assumed that the coordination of
THF molecules instead of that of the aromatic groups in [5-Ca·
(THF)]2 stems from an unfavorable entropic factor and steric
constraints associated with the folding of the CH2−CH2−C6H5
side-arm in comparison with the smaller CH2−CH2−CH
CH2 in, for instance, [1-Ca]2.
Figure 2. ORTEP diagram of the molecular solid-state structure of [5-
Ca]3. Ellipsoids at 50% probability. H atoms other than SiH omitted
for clarity. Ca interactions with F and H atoms in dashed lines.
Selected bond lengths (Å) and angles (deg): Ca1−O1 = 2.307(5),
Ca1−O2 = 2.286(1), Ca1−N2 = 2.480(1), Ca1−N3 = 2.462(4),
Ca1−F16 = 2.740(1), Ca1−F36 = 3.080(1), Ca1−F40 = 2.949(1),
Ca1−H4 = 2.66(2), Ca1−H5 = 2.63(2), Ca2−O1 = 2.268(1), Ca2−
N1 = 2.295(1), Ca2−N2 = 2.519(1), Ca2−N9 = 2.585(1), Ca2−H2 =
2.64(2), Ca2−H3 = 2.39(2), Ca3−O2 = 2.310(1), Ca3−N3 =
2.504(1), Ca3−N4 = 2.306(1), Ca3−N29 = 2.573(1), Ca3−H6 =
2.44(2), Ca3−H7 = 2.97(2); Ca1−N2−Si3 = 130.00(7), Ca1−N2−
Si4 = 98.77(7), Ca1−N3−Si5 = 98.50(7), Ca1−N3−Si6 = 128.59(7),
Ca2−N1−Si1 = 128.38(9), Ca2−N1−Si2 = 102.16(8), Ca2−N2−Si3
= 91.87(6), Ca2−N2−Si4 = 123.33(7), Ca3−N3−Si5 = 118.11(7),
Ca3−N3−Si6 = 92.28(7), Ca3−N4−Si7 = 111.12(9), Ca3−N4−Si8 =
124.37(9).
To alleviate the problem stemming from the presence of
THF in [Ca[N(SiMe2H)2]2·(THF)] and, as a result, in [5-Ca·
(THF)]2, {RO5}H was reacted instead with an equimolar
amount of the THF-free [Ca[N(SiMe2H)2]2].25c However,
instead of the expected dimeric complex [5-Ca]2 showing
coordination of the aryl rings onto calcium, the trinuclear
E
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O1 = 2.268(1) Å) atom, and one Namine atom (Ca2−N9 =
2.585(1) Å), all with bond lengths as expected for such bonds.
It is completed by two strong β-Si−H···Ca agostic contacts (H2
and H3), taking the coordination number to 6. Prominent
features are the extremely short Ca2−H3 distance (2.39(2) Å)
together with the corresponding remarkably acute Ca2−N2−
Si3 angle (91.87(6)°), which both testify to a distortion of
unique amplitude for a calcium complex, stronger than any of
those reported to date. By comparison, although still unusually
pronounced (compare for instance to the characteristic data in
Table 1), the Ca2···H2 agostic interaction (Ca2−H2 = 2.64(2)
Å, Ca2−N1−Si2 = 102.16(8)°) looks significantly weaker. The
strength of these two β-Si−H···Ca2 agostic contacts must stem
from the absence of other secondary interactions about Ca2
with either Cπ or F atoms. The geometry around the central
calcium atom Ca1 differs much. It comprises two μ2-bridging
with [Ca[N(SiMe2H)2]2·(THF)] or [Sr[N(SiMe2H)2]2·
(THF)2/3] (Scheme 4). Although they show slightly broad
Scheme 4. Synthesis and Representations of the Dimeric [6-
Ca]2 and [6-Sr]2 Ae-Alkyne Complexes Showing Selected
Secondary Interactions as Dashed Lines
N
amide (Ca1−N2 = 2.480(1) Å and Ca1−N3 = 2.462(4) Å) and
1
resonances at room temperature, their H and 19F{1H} NMR
two μ2-bridging Oalkoxide (Ca1−O1 = 2.307(5) Å, Ca1−O2 =
2.286(1) Å) atoms, and three typical C−F→Ca1 interactions
(Ca1−F16 = 2.740(1) Å, Ca1−F36 = 3.080(1) Å, and Ca1−
F40 = 2.949(1) Å); two very strong β-Si−H···Ca1 agostic
distortions (Ca1−H4 = 2.66(2) Å and Ca1−H5 = 2.63(2) Å)
are also detected.
data are well resolved compared to those of their congeneric
olefin complexes [1-Ca]2 and [1-Sr]2. The 19F NMR spectrum
for [6-Sr]2 in [D6]benzene (298 K) exhibits two quartets of
equal intensities at −77.98 and −77.15 ppm. The broad 1H and
19F resonances in the NMR spectra for [6-Ca]2 recorded at 298
K in [D8]toluene are much improved upon heating to 333 K.
The dinuclear [6-Ca]2 crystallized as a centro-symmetric O-
bridged dimer (Figure 3). Its key structural feature is the
1H DOSY NMR data (diffusion-molecular weight analysis:
expected mol wt 1278 g mol−1, found 1201 g mol−1;
hydrodynamic radii: XRD 6.13 Å, DOSY NMR 7.42 Å)
suggest that the trinuclear structure of [5-Ca]3 is maintained in
solution (Figure S19). The 1D NMR data for [5-Ca]3 are
complicated as expected in view of its structure, and in
particular, the multiplicity of resonances for SiH hydrogens
1
precludes the determination of JSiH coupling constants. They
were established instead by a 2D 1H−29Si HMQC NMR
experiment without broadband decoupling, which reveals the
presence of four different SiH environments in solution,
1
characterized by JSiH values of 132, 145, 161, and 161 Hz
(Figure S29).37 The JSiH value of 132 Hz indicates an
1
extremely strong Ca···H−Si β-agostic interaction. That of 145
Hz is unusually strong for a calcium complex, whereas those of
161 Hz for mild Ca···H−Si contacts are more typical. These
spectroscopic data are consistent with the crystallographic data.
The yield in the synthesis of [5-Ca]3 is improved by
deliberate reaction of [Ca[N(SiMe2H)2]2] and {RO5}H in the
appropriate 3:2 stoichiometry. The complex can also be
prepared by ligand redistribution in the 2:1 reaction of the
two homoleptic precursors [Ca[N(SiMe2H)2]2] and {RO5}2Ca
(a complex prepared for this purpose upon treatment of
[Ca[N(SiMe3)2]2·(THF)2] with 2 equiv of {RO5}H). How-
ever, we have so far failed in obtaining complexes featuring
intramolecular Ca···Cπ(arene) interactions with our method-
ology. The substitution of the nitrogen-bound Me group in
{RO5}H by an iPr one made no difference, as a trinuclear
complex is returned in the same manner as that seen for [5-
Ca]3. The addition of 1 equiv of {RO5}H to [5-Ca]3 failed to
deliver the targeted complex of expected formula [{RO5}CaN-
(SiMe2H)2]2. Finally, the 1:1 reaction of [Ca[N(SiMe2H)2]2]
and {RO5}2Ca returned [5-Ca]3 with another unidentified
calcium complex.
Figure 3. ORTEP diagram of the molecular solid-state structure of [6-
Ca]2. Ellipsoids at 50% probability. H atoms other than SiH omitted
for clarity. Ca interactions with Cπ, F, and H atoms in dashed lines.
Selected bond lengths (Å) and angles (deg): Ca1−O20 = 2.305(1),
Ca1−O20#1 = 2.364(1), Ca1−N1 = 2.341(1), Ca1−N6 = 2.618(1),
Ca1−H1 = 2.65(3), Ca1−F15 = 2.753(1), Ca1−C2 = 3.020(2), Ca1−
C3 = 2.875(2), C2−C3 = 1.199(3); Ca1−N1−Si1 = 101.37(9), Ca1−
N1−Si2 = 131.6(1); Torsion angle Ca(1)−N(1)−Si(1)−H(1) =
0.6(11).
asymmetric η2-coordination of the alkyne groups onto the
metallic ions, as indicated by short Ca−Cπ distances (Ca1−C3
= 2.875(2) Å, Ca1−C2 = 3.020(2) Å) both diagnostic of Ca···
Cπ interactions.39 As in [1-Ca]2, the interaction is, therefore,
substantially stronger with the internal Cπ than with the
external one. The Ca−Cπ distances are unsurprisingly
significantly shorter in the Ca-alkyne complex than in the Ca-
olefin one, but they are longer than those reported for calcium
complexes containing Ca···Cπ(alkyne) interactions with bridg-
ing acetylides (ca. 2.53−2.82 Å).20−22 The binding mode of the
alkyne in [6-Ca]2 is unique, and this compound constitutes the
only “non-acetylide” example of a calcium-alkyne complex
Alkyne Complexes. The proteo-ligand {RO6}H having a
dangling methyl-substituted alkyne group was obtained in ca.
90% yield by reaction of N-isopropylpent-3-yne-1-amine38 with
2,2-bis(trifluoromethyl)oxirane. The dinuclear complexes
[{RO6}AeN(SiMe2H)2]2 (Ae = Ca, [6-Ca]2, 62%; Sr, [6-Sr]2,
48%) were isolated after amine release by reacting {RO6}H
F
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Figure 4. Representation of the coordination sphere around the calcium centers in the calcium-alkyne complex [6-Ca]2 and calcium-olefin
complexes [1-Ca]2 and [2-Ca]2. Bond distances are given in Å.
Scheme 5. Synthesis of the Calcium-Allene Complex [7-Ca]n with a Putative Structure for n = 2, and Its Isomerization via 1,3-
Hydride Shift to [6-Ca]2
known to date. The C2−C3 bond length of 1.199(3) Å is
unremarkable.
tether (Scheme 5); {RO7}H was first prepared by the Crabbe
synthesis of penta-3,4-dien-1-ol,40 and its conversion to N-
isopropylpenta-3,4-dien-1-amine38 to ring-open 2,2-bis-
(trifluoromethyl)-oxirane. The protonolysis reaction afforded
a colorless compound, the 1H and 19F{1H} NMR data
([D6]benzene, 298 K) of which were consistent with the
formulation [{RO7}CaN(SiMe2H)2]n ([7-Ca]n). However, on
the basis of these NMR data alone, the nuclearity n could not
be clearly ascertained, and the putative structure given for n = 2
in Scheme 5 solely relies on a probable analogy with the other
calcium complexes. The NMR data for crude [7-Ca]n also
unambiguously showed contamination by a substantial amount
of the calcium-alkyne complex [6-Ca]2. After 12 h, full
conversion of the allene-containing complex [7-Ca]n into [6-
Ca]2 had taken place in [D6]benzene at room temperature. We
assume that isomerization of the calcium allene compound [7-
Ca]n into the more stable calcium alkyne complex occurs via
1,3-hydride shift as depicted in Scheme 5.
While the isomerization of alkynes to allenes is well
documented,41 reports of the conversion of allenes into alkynes
are seldom. 1-Propyne can be converted to 1,2-propadiene in
the presence of catalytic amounts of CaH2.42 Very recently,
Mashima and co-workers employed [{N^N}MgCH2Ph]2
({N^N}H = N1-(1,2-diphenylethyl)-N2,N2-dimethylethane-
1,2-diamine) to catalyze the isomerization of 3-phenyl-1-
propyne to phenylallene, which was subsequently isomerized
to 1-phenyl-1-propyne.43 Because a pure sample of [7-Ca]n
could not be obtained and owing to its instability in solution,
we could not obtain X-ray quality crystals to support the
proposed formulation.
As with the other complexes, stabilization of [6-Ca]2 is
further warranted by a pronounced C−F→Ca interaction
(Ca1−F15 = 2.753(1) Å) and a strong β-Si−H···Ca distortion
(Ca1−H1 = 2.65(3) Å, Ca1−N1−Si1 = 101.37(9)° vs Ca1−
N1−Si2 = 131.6(1)°, torsion angle Ca(1)−N(1)−Si(1)−H(1)
= 0.6(11)°).
The molecular structure of the centro-symmetric [6-Sr]2, the
first non-acetylide strontium-alkyne complex, was also estab-
lished (Figure S28). With its asymmetric Sr···Cπ η2-
coordination of the alkyne (Sr−Cπ = 3.001(5) and 3.094(5)
Å), strong Sr···H agostic (Sr−H = 2.882(49) Å to the H atom
in idealized position, with the corresponding Sr−N−Si angle of
103.1(3)° and Sr−Si distance of 3.282(1) Å) and Sr···F−C
(Sr−F = 2.806(3) Å) interactions, it resembles very closely that
of its calcium parent.
Because the alkyne in [6-Ca]2 is more electron-donating and
less hindered than the olefins in [1-Ca]2 and [2-Ca]2, that the
Ca−Cπ distances in [6-Ca]2 are shorter than in the olefins
complexes is unsurprising. However, the crystallographic data
summarized in Figure 4 highlight that C−F→Ca contacts and
β-Si−H···Ca agostic distortions are also noticeably weaker in
the calcium-olefin complexes. This is consistent with the
observation that, in solution, [6-Ca]2 (and even more so [6-
Sr]2) undergoes fewer dynamic phenomena than [1-Ca]2 and
[2-Ca]2. The results detailed below with the calcium-allene
complex emphasize the particular stability of the alkyne
complex [6-Ca]2.
Toward Allene Complexes. The synthesis of a calcium-
allene complex allowing Ca···Cπ(allene) interactions was
carried out by reacting [Ca[N(SiMe2H)2]2·(THF)] with 1
equiv of the proteo-ligand {RO7}H having a dangling allene
G
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Article
of the CC double bonds in the Ca-olefin complexes [1-
Ca]2−[4-Ca]2 are perhaps the best illustrations of the
paramount importance of secondary interactions in alkaline
earth chemistry, as highlighted by Ruhlandt-Senge.24
CONCLUSION
■
The results presented here, in conjunction with our earlier
communication,7 show that π ligands, or at least alkenes and
alkynes, can be made to work efficiently in alkaline earth
chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
The structures of a range of calcium- and strontium-olefin
S
complexes supported by amino-fluoroalkoxides and bearing the
−
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00006.
N(SiMe2H)2 amido coligand were established. The presence
of an Ae2+···olefin interaction is clear in these complexes,
although it gradually diminishes from η2 to complete
dissociation when the steric bulk around the CC double
bond increases. Since the stability of these complexes is further
warranted by pattern of C−F→Ae and β-Si−H···Ae agostic
interactions, compensation takes place and these two types of
interactions become stronger as the Ae···olefin interaction
loosens.
Synthetic procedures and product characterizations for
all compounds; 2D NMR data; tables of NMR data;
comparison of hydrodynamic radii; VT NMR data, and
tables of crystallographic data for CCDC nos. 1508047−
1508550, 1508553, and 1508556 (PDF)
Crystallographic data (CIF)
Considering the oxophilic nature of calcium and strontium,
the absence of coordinated THF in [1-Ca]2, [2-Ca]2, [4-Ca]2,
and [1-Sr]2 is remarkable, since they were made from the THF-
containing precursor Ae[N(SiMe2H)2]2·(THF)n or by crystal-
lization in the presence of THF.44 The results presented here
show that, against accepted wisdom, the intramolecular binding
of an olefin onto calcium can be favored over the coordination
of external THF molecule(s). Related to this, we have reported
before that combining C−F→Ae and β-Si−H···Ae interactions
in aminoether−fluoroalkoxo calcium complexes was more
efficient at stabilizing the metal than the binding of an
additional chelating methoxy arm.27a This is a convincing
evidence for the key role of the secondary interactions in
alkalino-earth chemistry.24
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jean-francois.carpentier@univ-rennes1.fr (J.-F.C.).
*E-mail: yann.sarazin@univ-rennes1.fr (Y. S.).
ORCID
Yann Sarazin: 0000-0003-1121-0292
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
S.-C. R. thanks the French Agence Nationale de la Recherche
for a Ph.D. grant (GreenLAkE, ANR-11-BS07-009-01).
■
DFT computations have shown that the calcium···olefin
interaction in [1-Ca]2 is almost purely electrostatic (like the
C−F→Ae and β-Si−H···Ae interactions),7 and the solution
NMR data are consistent with this interpretation. We have
implemented several NMR techniques to assess whether the
olefin remains metal-bound in solution in aromatic solvents.
Taken collectively, the NMR spectroscopic data militate in
favor of the retention of Ae···olefin interactions in solution.
Besides, a complex pattern of fluxional processes was also
demonstrated. Inversion of configuration at the chiral nitrogen
atoms, reversible dissociation of the olefin, which may involve
an inversion of the coordinated enantioface, or a monomer−
dimer equilibrium for which we could not, however, find any
concrete indication, could constitute plausible reasons for these
phenomena.
Non-acetylide alkaline earth-alkyne complexes were synthe-
sized for the first time; the evidence for the Ae···Cπ(alkyne) η2-
interaction is plain from crystallographic studies. The calcium-
alkyne complex is particularly stable, so much so that the
isomerization of the allene derivative [7-Ca]2 into [6-Ca]2,
presumably by intramolecular 1,3-hydride shift, is very facile;
NMR monitoring seemed to indicate that there is no significant
kinetic barrier to the isomerization. Such an isomerization
process is unusual, and we are endeavoring to take advantage
from the stoichiometric reaction and turn it into a viable
catalytic process.
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(37) Due to its structural complexity, we are unable to explain further
the pattern of resonances and proceed to assignment for this complex.
All indications point to the existence of a multinuclear compound also
in solution. Several different silicon environments exist, some of which
are involved in very strong SiH···Ca agostic interactions as attested to
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1
by the 2D H−29Si HMQC NMR experiment.
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5692. Considering the many similarities between Ae2+ and Ln3+ ions,
we think it most appropriate to adopt this notation for Ae complexes.
(40) Kuang, J.; Ma, S. J. Org. Chem. 2009, 74, 1763−1765.
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(44) Attempts to grow crystals of a THF-solvated species by
crystallization of [1-Ca]2 from a pentane solution containing
approximately 1−2 equiv of THF per metal returned crystalline [1-
Ca]2, whereas no crystal could be isolated when 10 equiv of THF was
used; see ref 7.
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(28) The solution NMR data for [1-Ca]2−[4-Ca]2 and [1-Sr]2 are
complicated. For instance, except for the regions of olefinic hydrogens
1
and −N(SiMe2H)2 moieties, the H NMR spectrum of [1-Ca]2 could
hardly be exploited due to the fluxional behavior between two
distinguishable forms A and B in conformational exchange; the
scenario was identical for [2-Ca]2 and [3-Ca]2. Also, we have found no
way of knowing exactly the pattern of C−F→Ca interactions in
solution. We cannot be certain either of the dynamic processes that are
occurring in solution or about the exact structure of the compound in
solution.
(29) Michel, O.; Tornroos, K. W.; Maichle-Mossmer, C.; Anwander,
̈
̈
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I
DOI: 10.1021/acs.organomet.7b00006
Organometallics XXXX, XXX, XXX−XXX