Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium

Article abstract of DOI:10.1039/c8dt04905j

Metalation of the formamidine Dipp-N═C(H)-N(H)-C₂H₄-Py (1a) and benzamidine Dipp-N═C(Ph)-N(H)-C₂H₄-Py (1b) with [(thf)₂M{N(SiMe₃)₂}₂] (M = Ca, Sr) yields the corresponding homoleptic complexes [M{Dipp-N═C(R)-N-C₂H₄-Py}₂] (M/R = Ca/H (2a), Ca/Ph (2b), and Sr/Ph (3b)) regardless of the applied stoichiometry. Only during calciation of Dipp-N═C(H)-N(H)-C₂H₄-Py (1a), the heteroleptic intermediate [{(Me₃Si)₂N}Ca{Dipp-N═C(R)-N-C₂H₄-Py}]₂ (2a′) has been observed. The formamidinate complex of strontium crystallizes as a tmeda adduct of the type [(tmeda)Sr{Dipp-N═C(H)-N-C₂H₄-Py}₂] (3a). Metalation of the pivalamidine Dipp-N═C(tBu)-N(H)-C₂H₄-Py (1c) leads to the formation of heteroleptic mononuclear [{(Me₃Si)₂N}M{Dipp-N═C(tBu)-N(H)-C₂H₄-Py}] (M = Ca (2c) and Sr (3c)) with a side-on bonding of the Dipp group to the alkaline earth metals. Calciation of chiral Dipp-N═C(tBu)-N(H)-CH₂CH(Me)-Py (R)-1d and (S)-1d with [(thf)₂Ca{N(SiMe₃)₂}₂] yields the homoleptic complexes [Ca{Dipp-N═C(tBu)-N-CH₂CH(Me)-Py}₂] with the enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. The complexes 2c and 2d catalyze the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-methyl-4,4-diphenylpyrrolidine but the stereochemistry cannot be influenced by the chiral compounds (R,R)-2d and (S,S)-2d.

Full text of DOI:10.1039/c8dt04905j

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DOI: 10.1039/C8DT04905J
Synthesis and Catalytic Activity of Tridentate N-(2-Pyridylethyl)-
Substituted Bulky Amidinates of Calcium and Strontium
Sven Krieck,*a,§ Diana Kalden,a,§ Ansgar Oberheide,b Lydia Seyfarth,b Hans-Dieter Arndt,b Helmar Görls,a and
Matthias Westerhausen*a
a Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University
Humboldtstraße 8, 07743 Jena (Germany)
http://www.lsac1.uni-jena.de; e-mail: m.we@uni-jena.de
b Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller University
Humboldtstraße 10, 07743 Jena (Germany)
Abstract.
Metalation of the formamidine Dipp−N=C(H)−N(H)−C2H4−Py (1a) and benzamidine
Dipp−N=C(Ph)−N(H)−C2H4−Py (1b) with [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) yields the corresponding homoleptic
complexes [M{Dipp−N=C(R)−N−C2H4−Py}2] (M/R = Ca/H (2a), Ca/Ph (2b), and Sr/Ph (3b)) regardless of the
applied stoichiometry. Only during calciation of Dipp−N=C(H)−N(H)−C2H4−Py (1a), the heteroleptic intermediate
[{(Me3Si)2N}Ca{Dipp−N=C(R)−N−C2H4−Py}]2 (2a’) has been observed. The formamidinate complex of strontium
crystallizes as a tmeda adduct of the type [(tmeda)Sr{Dipp−N=C(H)−N−C2H4−Py}2] (3a). Metalation of the
pivalamidine Dipp−N=C(tBu)−N(H)−C2H4−Py (1c) leads to the formation of heteroleptic mononuclear
[{(Me3Si)2N}M{Dipp−N=C(tBu)−N(H)−C2H4−Py}] (M = Ca (2c) and Sr (3c)) with a side-on bonding of the Dipp
group to the alkaline earth metals. Calciation of chiral Dipp−N=C(tBu)−N(H)−CH2CH(Me)−Py (R)-1d and (S)-1d
with [(thf)2Ca{N(SiMe3)2}2] yields the homoleptic complexes [Ca{Dipp−N=C(tBu)−N−CH2CH(Me)−Py}2] with the
enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. The complexes 2c and 2d
catalyze the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-
methyl-4,4-diphenylpyrrolidine but the stereochemistry cannot be influenced by the chiral compounds (R,R)-2d
and (S,S)-2d.
Introduction
Bidentate monoanionic ligands with nitrogen donors1 like β-diketiminates,2 amidinates and
guanidinates3 as well as triazenides4 represent very beneficial ligands because the nature of the N-
bound groups (bulkiness, electronic properties) significantly influence the coordination chemistry of
these anions. Amidinate complexes of divalent metal ions can be classified into four categories. On the
one hand homoleptic complexes (the expressions homoleptic and heteroleptic refer only to the anionic
bases neglecting neutral coligands) of the type [{RC(NR')2}2M(L)n] (A, Scheme 1) contain two symmetric
amidinate ligands with neutral bases occupying available coordination sites. Heteroleptic complexes of
the type [{RC(NR')2}MX(L)n] (B) contain one amidinate ligand and any other anionic Lewis base X (such
as e.g. halide, hydride, bis(trimethylsilyl)amide). On the other hand, homo- and heteroleptic complexes
can have asymmetric amidinate ligands with two different substituents at the nitrogen bases, leading
to the complex types [{RC(NR')(NR'')}2M(L)n] (C) and [{RC(NR')(NR'')}MX(L)n] (D) as clarified in Scheme 1.

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DOI: 10.1039/C8DT04905J
R'
R'
X
N
N
N
N
M(L)n
M(L)n
R
R
R'
R'
R'
R'
A
B
2
X
N
N
N
N
M(L)n
M(L)n
R
R
R''
R''
C
D
2
Scheme 1. Different types of amidinate complexes of divalent metal ions M2+ according to homoleptic (A and C,
left column) and heteroleptic nature of the complexes (B and D, right column) with symmetric (A and B, top row)
and asymmetric (C and D, bottom row) amidinate ligands (L = electroneutral Lewis base, X = anionic ligand; see
text).
Due to the fact that the steric requirements and electronic properties of amidinate ligands can easily
be altered by the substitution pattern (R, R' and R''), calcium complexes (and to a lesser extent the
strontium congeners) with these anions gained tremendous attention in stoichiometric and catalytic
applications in recent years. Advantageously, diverse synthetic protocols allow the isolation of such
6
complexes,5-16 including calciation and strontiation of amidines with intermediately prepared Ca(C6F5)2
or easily available [{(Me3Si)2N}2M(thf)2] (M = Ca, Sr)7-11 and dibenzylcalcium,12 addition of MR2 across
carbodiimides,11,13,14 and also metathetical approaches.15,16
Here we present the synthesis of calcium and strontium complexes with asymmetric tridentate N-(2-
pyridylethyl)-substituted bulky amidinate ligands. Contrary to numerous complexes of these alkaline
earth metals with symmetric amidinate ligands (complex types A and B, see Scheme 1), the congeners
with asymmetrically substituted amidinates (complex types C and D) are quite scarce up to now.
Examples are limited to asymmetric N-(2,6-diisopropylphenyl)amidinates with rigid 2-methoxyphenyl,9
2-pyridyl,17 8-quinolyl,17 and 2-pyridylmethyl groups18 at the other N'-Lewis base (Table 1). These stiff
tridentate Lewis bases are unable to bind all coordination sites at once to one metal ion and hence,
these asymmetric amidinates act as bi- or monodentate ligands toward calcium and strontium ions.
Increasing flexibility of the Lewis basic side-arm can be achieved by using the 2-pyridylethyl substituent
(Table 1).19 Despite the fact that alternative reaction pathways like metalation at the ethylene linker
followed by a deamidation sequence have been observed during metalation with alkali metal
metalation reagents (Scheme 2), [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) exclusively showed N-metalation ,
quantitatively yielding the tridentate amidinate complexes.19
Dipp
Dipp
Dipp
N
N
N
N
M
N
N
R
R
M
N
R
H
+ MR'
- RH
+ MR'
- RH
H
H2C
+
CH2
N
N
Scheme 2. Reaction pattern of N-(2,6-diisopropylphenyl)-N'-(2-pyridylethyl)amidine with s-block metalation
reagents yielding either the amidinate via N-metalation (attack at the position marked with a red arrow) or to an
amidinate and 2-vinylpyridine via C-metalation at the ethylene linker (marked with a green arrow) followed by a
deamidation reaction sequence.

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Table 1. Structurally authenticated complexes of calcium and strontium with asymmetric amidinates of the
types C (homoleptic) and D (heteroleptic), see Scheme 1.a
M
Ca
Type
C
C
C
C
C
C
C
D
D
D
D
D
R
R'
R''
H
X
-
-
-
-
-
-
-
L
n
4
1
2
-
1
-
1
-
-
Ref.
19
9
tBu
tBu
tBu
tBu
tBu
tBu
tBu
tBu
Ad
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
Dipp
thf
dme
thf
-
thf
-
C6H4-2-OMe
2-Py
8-Quinolyl
CH2-Fu
CH2-Py
CH2-Py
(CH2)2Py
C6H2-4-iPr-2,6-(CHPh2)2
(CH2)2Py
C6H2-4-iPr-2,6-(CHPh2)2
C6H2-4-iPr-2,6-(CHPh2)2
17
17
18
18
18
19
16
19
16
16
dme
b
HMDS
HMDS
HMDS
H
-
-
-
-
b
Sr
tBu
Ad
Ad
-
-
-
c
HMDS
-
a Ad = 1-adamantyl; Dipp = 2,6-diisopropylphenyl; Fu = 2-furanyl; dme = 1,2-dimethoxyethane; HMDS =
hexamethyldisilazanide, bis(trimethylsilyl)amide, N(SiMe3)2; Py = 2-pyridyl. b The Dipp group shows a side-on
coordination of the π-system. c Dinuclear complexes.
We investigated the influence of a larger denticity of the amidinato ion on the catalytic intramolecular
hydroamination. For our study we have chosen the intramolecular cyclohydroamination of the aminoalkene
2,2-diphenylpent-4-enylamine, yielding 2-methyl-4,4-diphenylpyrrolidine, as a widely used benchmark
reaction. In former catalytic studies the calcium and strontium complexes commonly contain one innocent
spectator ligand such as bulky β-diketiminate or amidinate ligands and another catalytically active
monodentate Lewis base like the bis(trimethylsilyl)amido ligand. The hitherto only exceptions from this
principle are homoleptic [(thf)2M{N(SiMe3)2}2],24b,24c [M{N(SiMe3)2}2]2,24c and [(thf)2M{CH(SiMe3)2}2]24c as well as
boraamidinate complexes, the latter showing moderate activity.20 In general it is well accepted that only the
N(SiMe3)2 ligand acts as the active site whereas the bulky β-diketiminate or amidinate ligands remain inactive
metal-bound spectator ligands. In addition we intended to clarify if homoleptic calcium bis(amidinates) also
exhibit catalytic hydroamination activity to introduce stereochemical contral during the catalytic process.
Results and discussion
Synthesis
For our study we chose amidines with hydrogen (formamidine),21 phenyl (benzamidine),17 and tert-butyl
substituents (pivalamidines)21 bound at the carbon atom of the 1,3-diazaallyl moiety (R in Scheme 1).
Formamidine Dipp−N=C(H)−N(H)−C2H4−Py (1a), benzamidine Dipp−N=C(Ph)−N(H)−C2H4−Py (1b), and
pivalamidine Dipp−N=C(tBu)−N(H)−C2H4−Py (1c) were prepared according to published protocols.19,21 In a very
similar manner, the reaction of racemic H2NCH2CH(CH3)−Py with Dipp−N=C(tBu)Cl in toluene at 85 °C yielded
racemic Dipp−N=C(tBu)−N(H)−CH2CH(CH3)−Py (1d). Separation of the enantiomers succeeded with chiral HPLC
procedures (see Supporting Information).
Metalation of these tridentate amidines with an equimolar amount of [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr; see
Scheme 2) afforded the homoleptic metal complexes with the exception of already known
[{(Me3Si)2N}M{Dipp−N=C(tBu)-N−C2H4−Py}]2 (M = Ca (2c), Sr (3c)) (Scheme 3).19 The reaction of the
formamidine Dipp−N=C(H)-N(H)−C2H4−Py (1a) with [(thf)2Ca{N(SiMe3)2}2] enabled the crystallization of the
intermediately formed heteroleptic complex [{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2 (2a') presumably
because the formation of a dinuclear compound decelerated the formation of the homoleptic mononuclear
congener. However, in solution only the homoleptic complex [Ca{Dipp−N=C(H)-N−C2H4−Py}2] was observed
(Scheme 4). In order to obtain single-crystals of the strontium congener 3a, 1,2-bis(dimethylamino)ethane

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(tmeda) was added to the mother liquor. Thus the following crystalline compounds were obtained and their
DOI: 10.1039/C8DT04905J
molecular structures elucidated: [{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2 (2a'), [Ca{Dipp−N=C(Ph)-
N−C2H4−Py}2] (2b), [{(Me3Si)2N}Ca{Dipp−N=C(tBu)-N−C2H4−Py}]2 and [Ca{Dipp−N=C(tBu)-
(2c),19
N−CH2CH(CH3)−Py}2] (2d), as well as [(tmeda)Sr{Dipp−N=C(H)-N−C2H4−Py}2] (3a), [Sr{Dipp−N=C(Ph)-
N−C2H4−Py}2] (3b), and [{(Me3Si)2N}Sr{Dipp−N=C(tBu)-N−C2H4−Py}]2 (3c).19 It is noteworthy that an equimolar
reaction of amidine 1c with [(thf)2Ca{N(SiMe3)2}2] yielded complex 2c with a side-on bound Dipp group,
sterically protecting the Ca-N bonds and preventing aggregation (as observed for complex 2a') whereas the
slightly more bulky amidine 1d leads to the formation of homoleptic 2d regardless of the stoichiometric ratio.
N
N
N
N
N
N
N
N
N
N
Sr
Ca
H
H
H
H
N
N
iPr
iPr
N
N
iPr
iPr
iPr
iPr
iPr
iPr
3a
2a
N
N
N
N
N
N
N
N
N
Ca
Sr
Ph
iPr
Ph
Ph
Ph
iPr
N
N
N
iPr
iPr
iPr
iPr
iPr iPr
2b
3b
N
N
N
N(SiMe3)2
N
tBu
tBu
iPr Ca
N(SiMe3)2
iPr Sr
N
iPr
N
2c
iPr
3c
Me
Me
N
N
N
N
Ca
tBu
iPr
tBu
iPr
N
N
iPr
iPr
2d
Scheme 3. Metalation of the amidines with equimolar amounts of [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) in toluene
yields the depicted amidinates of calcium (2a-2d, left column) and strontium (3a-3c, right column); see text.

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N
N
N
N
iPr
iPr
N
H
N
N
Ca
(Me3Si)2N
H
H
H
NH
Ca
N
2
N
N
N
Ca
N
- 2 HN(SiMe3)2
N(SiMe3)2
iPr
iPr
iPr
iPr
iPr
N
N
H
iPr
iPr
iPr
2a
2a'
+
+
2 Ca{N(SiMe3)2}2
Ca{N(SiMe3)2}2
Scheme 4. Metalation of formamidine Dipp−N=C(H)-N(H)−C2H4−Py (1a) with [(thf)2Ca{N(SiMe3)2}2] in toluene
yielding intermediately heteroleptic [{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2 (2a') and finally homoleptic
[Ca{Dipp−N=C(H)-N−C2H4−Py}2]2 (2a).
Molecular Structures
Molecular structure and atom labeling scheme of dinuclear [{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2 (2a') are
depicted in Figure 1. The parameter τ (calculated with the equation (α-β)/60° with α and β being the largest
and second largest N-Ca1-N bond angles) characterizes the coordination sphere of penta-coordinate metal
atoms and varies between 0 for a square pyramid and 1 for a trigonal bipyramid.22 Here a value of τ = 0.6 for
the calcium atom hints toward a severely distorted trigonal bipyramidal environment. The negative charge of
the amidinate is delocalized within the N1-C8-N3 fragment with the middle nitrogen base N1 occupying a
bridging position between the calcium atoms. Whereas all Ca1-N bond lengths of the tridentate amidinate
anion vary in a very small range between 247.7(2) and 251.2(2) pm, the Ca1-N4 bond to the
bis(trimethylsilyl)amide is significantly shorter due to the enhanced electrostatic attraction. Negative
hyperconjugation leads to rather short N4-Si bonds with an average value of 168.5 pm.

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Figure 1. Molecular structure and atom labeling scheme of dinuclear [{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2
DOI: 10.1039/C8DT04905J
(2a'). The ellipsoids represent a probability of 30 %, H atoms are omitted for the sake of clarity. Symmetry-related
atoms (-x+1, -y+1, -z+1) are marked with the letter “A”. Selected bond lengths (pm): Ca1-N1 247.70(17), Ca1-N1A
251.16(17), Ca1-N2A 251.20(19), Ca1-N3A 248.69(17), Ca1-N4 232.16(18), N4-Si1 168.4(2), N4-Si2 168.6(2), N1-C7
147.2(3), N1-C8 133.6(3), N2-C1 135.1(3), N2-C5 134.5(3), N3-C8 131.1(3), N3-C9 143.8(3), C5-C6 150.2(4), C6-C7
152.6(4), C8-H8 97(2); bond angles (deg.): N1-Ca1-N1A 84.16(6), Ca1-N4-Si1 122.49(10), Ca1-N4-Si2 112.31(10),
Si1-N4-Si2 125.01(11), C7-N1-C8 114.76(17), C8-N3-C9 117.26(18), N1-C8-N3 121.76(18), N1-C8-H8 117.9(14), N3-
C8-H8 120.3(14).
The complexes [Ca{Dipp−N=C(Ph)-N−C2H4−Py}2] (2b) and [Sr{Dipp−N=C(Ph)-N−C2H4−Py}2] (3b) crystallized
isotypic with very similar bonding parameters. The molecular structure and the atom labeling scheme of 2b are
shown in Figure 2. Due to the small bites of the amidinate functionalities the octahedral coordination spheres
are distorted. The middle nitrogen bases N2 and N5 show the smallest M-N bond lengths with average values
of 238.9 (M = Ca) and 254.5 pm (M = Sr).
Figure 2. Molecular structure and atom labeling scheme of [Ca{Dipp−N=C(Ph)-N−C2H4−Py}2] (2b). The ellipsoids
represent a probability of 30 %, H atoms are neglected for clarity reasons. Selected bond lengths of 2b (M = Ca) [of
isotypic 3b (M = Sr)] (pm): M1-N1 246.7(3) [256.94(19)], M1-N2 238.7(3) [254.8(2)], M1-N3 255.1(3) [267.2(2)],
M1-N4 243.5(3) [256.6(2)], M1-N5 239.1(3) [254.1(2)], M1-N6 256.5(3) [270.7(2)], N1-C1 133.1(4) [133.7(3)], N1-
C8 141.2(4) [140.6(3)], N2-C1 134.1(4) [132.3(3)], N2-C20 143.5(4) [144.6(3)], C1-C2 149.5(4) [150.1(3)]; bond
angles (deg.): N1-M1-N2 55.58(8) [52.60(6)], N1-M1-N3 131.43(9) [125.66(6)], N2-M1-N3 76.14(9) [73.16(6)], N4-
M1-N5 55.59(9) [52.38(6)], N4-M1-N6 130.95(9) [125.22(6)], N5-M1-N6 75.65(9) [73.01(6)], C8-N1-C1 122.2(3)
[122.30(19)], N1-C1-N2 115.9(3) [116.9(2)], N1-C1-C2 122.7(3) [121.9(2)], N2-C1-C2 121.4(3) [121.1(2)], C1-N2-C20
123.6(3) [122.6(2)], C34-N4-C27 122.8(3) [124.1(2)], N4-C27-N5 115.2(3) [116.3(2)], N4-C27-C28 121.8(3)
[121.3(2)], N5-C27-C28 122.9(3) [122.3(2)], C27-N5-C46 123.2(3) [123.2(2)].
The chiral complexes [Ca{Dipp−N=C(tBu)-N−CH2CH(CH3)−Py}2] (R,R)-2d and (S,S)-2d crystallized in the
asymmetric monoclinic space group P21 allowing the determination of the absolute structures of the amidinate
ligands. The molecular structure and atom numbering scheme of (R,R)-2d are depicted in Figure 3. Surprisingly,
the structure is rather similar to 2b with a distorted octahedral coordination sphere for the calcium atoms. The
additional methyl group at the ethylene fragment hinders isolation of a heteroleptic complex and no π-
interaction of calcium to the Dipp group was observed in the solid state.

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Figure 3. Molecular structure and atom labeling scheme of [Ca{Dipp−N=C(tBu)-N−CH2CH(CH3)−Py}2] (S,S)-2d. The
ellipsoids represent a probability of 30 %, H atoms are not shown. Selected bond lengths of (S,S)-2d [of (R,R)-2d]
(pm): Ca1-N1 244.08(15) [243.33(14)], Ca1-N2 238.48(15) [238.28(14)], Ca1-N3 253.86(16) [254.40(15)], Ca1-N4
243.39(15) [244.07(14)], Ca1-N5 238.50(15) [238.38(14)], Ca1-N6 254.60(16) [253.39(15)], N1-C1 134.6(2)
[134.5(2)], N1-C6 140.5(2) [140.8(2)], N2-C1 133.3(2) [132.9(2)], N2-C18 145.5(2) [145.2(2)], C1-C2 155.6(3)
[156.3(2)]; bond angles (deg.): N1-Ca1-N2 55.17(5) [55.07(5)], N1-Ca1-N3 90.57(5) [92.94(5)], N2-Ca1-N3 73.76(5)
[75.63(5)], N4-Ca1-N5 55.04(5) [55.18(5)], N4-Ca1-N6 92.88(5) [90.59(5)], N5-Ca1-N6 75.58(5) [73.75(5)], C6-N1-C1
130.34(15) [131.91(14)], N1-C1-N2 113.07(15) [112.79(14)], N1-C1-C2 123.67(16) [123.80(15)], N2-C1-C2
123.01(16) [123.21(15)], C1-N2-C18 128.21(16) [127.86(14)], C31-N4-C26 131.93(16) [130.27(14)], N4-C26-N5
112.82(15) [113.13(15)], N4-C26-C27 123.58(16) [123.46(15)], N5-C26-C27 123.38(16) [123.14(15)], C26-N5-C43
127.98(15) [128.24(15)].
The larger strontium atom allows the coordination of additional Lewis bases and a larger coordination number
of eight for [(tmeda)Sr{Dipp−N=C(H)-N−C2H4−Py}2] (3a) is realized. The molecular structure and atom labeling
scheme are depicted in Figure 4. Again, the middle nitrogen bases N1 and N5 of the tridentate amidinate
ligands show the smallest Sr-N distances with an average value of 256.7 pm. The larger coordination number
leads to a significant elongation of the other Sr-N bond lengths which vary between 276.5(2) and 297.5(2) pm.

Dalton Transactions
Page 8 of 18
View Article Online
Figure 4. Molecular structure and atom labeling scheme of [(tmeda)Sr{Dipp−N=C(H)-N−C2H4−Py}2] (3a). The
DOI: 10.1039/C8DT04905J
ellipsoids represent a probability of 30 %, H atoms are omitted for clarity reasons. Selected bond lengths (pm):
Sr1-N1 256.90(18), Sr1-N2 297.51(18), Sr1-N3 285.27(18), Sr1-N4 292.17(17), Sr1-N5 256.43(17), Sr1-N6
280.90(18), Sr1-N7 276.49(19), Sr1-N8 279.61(19).
Selected structural parameters are compared in Table 1. The negative charge is largely delocalized within the
NDipp-CNCN-NCH2 diazaallylic system regardless of the coordination geometry of this ligand. For steric reasons the
large Dipp substituents are nearly perpendicular to the allylic system and hence, this group does not participate
in the delocalization of the negative charge. The shortest M-NCH2 distances are observed to the middle nitrogen
base of the tridentate ligand with the exception being complex 2a' due to the bridging nature of this Lewis
base. In most cases, the bulkiness of the group R at the carbon atom CNCN significantly influences the NDipp-CNCN
-
NCH2 bond angle with the smallest values for R = tBu. The exception is complex 3b because the bidentate tmeda
base pushes the Dipp and (CH2)2Py groups off which slightly distorts the amidinate moiety. Furthermore, this
steric inter-ligand repulsion and the large coordination number of eight for the strontium center leads to an
enormous elongation of the Sr-NDipp and Sr-NPy bonds by 38 pm and 14 pm, respectively.
Table 2. Selected structural parameters (average values without estimated standard deviations) of
[{(Me3Si)2N}Ca{Dipp−N=C(H)-N−C2H4−Py}]2 (2a') and [M{Dipp−N=C(R)-N−CH2CH(CH3)−Py}2].
2a'
Ca
2b
Ca
(R,R)-2d
Ca
(S,S)-2d
Ca
3a
Sr
3b
Sr
M
R
H
Ph
tBu
tBu
Ph
H
C.N.(M)
M-NDipp
M-NCH2
M-NPy
M-NHMDS
M-Ntmeda
NDipp-CDipp
NDipp-CNCN
NCH2-CNCN
NCH2-CCH2
CDipp-NDipp-CNCN
NDipp-CNCN-NCH2
CNCN-NCH2-CCH2
5
6
6
6
8
6
248.7(2)
249.5
251.2(2)
232.2(2)
-
143.8(3)
131.1(3)
133.6(3)
147.2(3)
117.3(2)
121.8(2)
114.8(2)
245.1
238.9
255.8
-
243.7
238.3
253.9
-
243.7
238.5
253.2
-
294.8
256.7
283.1
-
278.1
141.6
132.6
131.4
145.7
116.8
121.8
116.9
256.8
254.5
269.0
-
-
-
-
-
141.1
133.8
133.1
144.0
122.5
115.6
123.4
140.6
134.6
133.1
145.3
131.1
113.0
128.1
140.7
134.7
133.0
145.4
131.1
113.0
128.1
140.7
133.7
131.9
142.7
123.2
116.6
122.9
Catalytic Activity
We tested the mononuclear complexes 2c and 3c as well as (R)-2d und (S)-2d as precatalysts for the
intramolecular hydroamination of aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-methyl-4,4-
diphenylpyrrolidine (Scheme 5), a commonly used benchmark test for catalytic hydroamination activity.23-25
Furthermore, first efforts to stereochemically influence the intramolecular hydroamination of aminoalkenes
showed only very limited success26 and therefore, we also tested the chiral complexes (R)-2d und (S)-2d.
Me
precat.
Ph Ph
*
NH
NH2
C6D6
Ph
Ph
Scheme 5. Catalyzed intramolecular hydroamination of 2,2-diphenylpent-4-enylamine, yielding 2-methyl-4,4-
diphenylpyrrolidine, in [D6]benzene to elucidate the catalytic activity of the calcium complexes 2c, (R,R)-2d, and (S,S)-2d.

Page 9 of 18
Dalton Transactions
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In a typical procedure, 2 to 4 mol-% of precatalyst were added to a solution of the aminoalkene in [D6]benzene.
As a standard for quantitative elucidation of the conversion of the aminoalkene to 2,2-dDipOhI:e1n0y.1l0p3e9n/Ct-84D-T04905J
enylamine, a definite amount of tetrakis(trimethylsilyl)silane (TMSS) was added. The conversion was monitored
by NMR spectroscopy (see Supporting Information, Figure S4). We studied the influence of metal, temperature,
and precatalyst concentration on the conversion.
1
From the H NMR spectra the decreasing concentration of the substrate and increasing concentration of the
product was determined. The dependency of the conversion from the concentration of the calcium-based
precatalyst 2c is shown in the Supporting Information (Figure S5). After 16 hours at 24 °C, conversion rates of
38, 77 and 73 % were observed in the presence of 2, 3 and 4 mol-% of the calcium-based precatalyst, whereas
conversion rates of approx. 80 % were already achieved with 2 mol-% of the strontium congener 3c. As long as
the pyrrolidine concentration was much smaller than the concentration of the aminoalkene substrate, a
quasilinear relation and zero-order reaction was found (Figure 5). The pyrrolidine product exhibits a
significantly higher basicity than the aminoalkene substrate and hence, the pyrrolidine base interacts with the
catalyst, blocking coordination sites in competition to the substrate binding. From the gradient of the linear
relation the rate constants k of 0.008, 0.020 and 0.026 mol L-1 h-1 can be deduced for precatalyst loadings of 2,
3 and 4 mol-%, respectively. The turn-over frequencies with values around 3 h-1 for precatalyst 2c were quite
small compared to a heteroleptic β-diketiminatocalcium bis(trimethylsilyl)amide (54.5 h-1)24b but in the same
region as observed for some lanthanoide tris[bis(trimethylsilyl)amide] precatalyst systems.27
Figure 5. Formation of 2-methyl-4,4-diphenylpyrrolidine via intramolecular hydroamination of the aminoalkene 2,2-
diphenylpent-4-enylamine in a 0.24 M solution in [D6]benzene. At the top 2, 3 and 4 mol-% of the precatalyst
[{(Me3Si)2N}Ca{Dipp−N=C(tBu)-N−C2H4−Py}]2 (2c) were applied at room temperature. At the bottom the conversion was
recorded at different temperatures with 3 mol-% of precatalyst 2c.
To further elucidate thermodynamic parameters the influence of the temperature on the rate of conversion
was investigated (Figure 5). Again a quasilinear relation was derived from these graphical representations as
long as the pyrrolidine concentration was much smaller than the concentration of the aminoalkene substrate.
The slopes of the graphs give the rate constants k·105 with values of 5.0, 5.3, 11.3, 36.3, and 110 min-1. These
data allow to elucidate the activation energy with the Arrhenius equation, shown in Figure 6. The slope of the
graph gives -Ea/R (with R = ideal gas constant) which allows to determine an activation energy of 59±6 kJ mol-1.

Dalton Transactions
Page 10 of 18
This energy is in agreement with values of intramolecular (65±2 and 81±1 kJ mol-1)24c and intermolecular
View Article Online
hydroamination of an activated alkene like a styrene derivative (54±5 kJ mol-1)25 with calcium cDoOmI:p1l0e.1x0e3s9./CT8hDeT04905J
strontium precatalyst 3c exhibits enlarged catalytic reactivity and a lower energy barrier of 38±5 kJ mol-1 (the
Supporting Information also includes the catalytic cyclohydroamination studies performed with the strontium
complex 3c).
Figure 6. Graphical determination of the parameters of substrate activation according to the Arrhenius equation using the
equation of linear regression y = m x + b (3 mol-% of 2c, [D6]benzene, T = 10, 15, 24, 40, and 50 °C).
The proposed reaction mechanism is shown in Scheme 6. Depending on the pKa values of the aminoalkene
substrate (pKa approx.. 30),28 hexamethyldisilazane (pKa = 25.8),29 the pyrrolidine product (pKa approx. 44),28a
and the amidines (pKa in the range of 23-27),30 the first reaction step is the transamination of 2c yielding
intermediate A as shown at the top of Scheme 6. Thereafter, intramolecular addition of the Ca-N bond onto the
alkene functionality leads to cyclization and formation of intermediate B with a highly reactive Ca-C bond.
Deprotonation of the aminoalkene substrate by intermediate B liberates 2-methyl-4,4-diphenylpyrrolidine and
regains complex A. With increasing concentration of the pyrrolidine product, this Lewis basic cyclic secondary
amine competes with the vacant coordination sites at the calcium ion and hence, decelerates the calcium-
mediated intramolecular hydroamination of the 2,2-diphenylpent-4-enylamine substrate.

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Dalton Transactions
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DOI: 10.1039/C8DT04905J
Ph Ph
NH2
+
Ph Ph
H
N
N
N
N
N
N(SiMe3)2
- HN(SiMe3)2
tBu
tBu
iPr Ca
iPr Ca
N
N
A
iPr
iPr
2c
*
NH
Ph
Ph
Ph Ph
NH2
N
N
H
tBu
N
Ph
Ph
iPr Ca
N
B
iPr
Scheme 6. Proposed mechanism for the calcium-mediated intramolecular hydroamination of 2,2-diphenylpent-4-enylamine
via an initial transamination reaction followed by cyclization and protonation finally yielding 2-methyl-4,4-
diphenylpyrrolidine.
Deviation of the first order rate laws are not only caused by the interfering coordination chemistry of the
evolving pyrrolidine base. During the proceeding hydroamination, the formation of free amidine was observed
in the 1H NMR spectra and hence, we conclude that also homoleptic calcium bis(2,2-diphenylpent-4-
enylamides) can be formed during this catalytic cycle. Therefore, we were interested in the possibility to also
catalyze the intramolecular hydroamination with homoleptic calcium amidinates and to influence the
stereochemistry of this conversion. Therefore we prepared and isolated the calcium complexes (R,R)-2d and
(S,S)-2d and used these compounds as precatalysts in the benchmark hydroamination of 2,2-diphenylpent-4-
enylamine. Typically 4-5 mol-% of precatalyst were applied at room temperature and after approximately one
week a conversion of 89 % was achieved. The product 2-methyl-4,4-diphenylpyrrolidine was isolated and the
ratio of the (R)/(S)-enantiomers determined with chiral HPLC methods. The results are compared with the
findings using 2c as precatalyst. The chromatograms are depicted in the Supporting Information and it is
evident that the chiral backbone of 2d has a negligible influence on the stereochemistry of the hydroamination
reaction. These results verify that the amidinato ligands not only act as spectator ligands but they are also able
to take over an active role.
Conclusions
The metalation of amidines of the type Dipp−N=C(R)−N(H)−C2H4−Py (R = H (1a), Ph (1b)) with
[(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) yields the corresponding homoleptic amidinate complexes of the type
[M{Dipp−N=C(R)−N−C2H4−Py}2] (M/R = Ca/H (2a), Ca/Ph (2b), Sr/H, and Sr/Ph (3b)) regardless of the
stoichiometry. In order to grow single-crystals of the strontium complex with formamidinate ligands, tmeda
can be added leading to the formation of [(tmeda)Sr{Dipp−N=C(H)−N−C2H4−Py}2] (3a). During the synthesis of
derivative 2a it is feasible to grow crystals of heteroleptic dinuclear [{(Me3Si)2N}Ca{Dipp−N=C(H)−N−C2H4−Py}2]2
(2a’) with a central Ca2N2 ring and with the bulky bis(trimethylsilyl)amido ligands in terminal positions.
Aggregation decelerates ligand exchange reactions enabling the isolation of this heteroleptic congener.

Dalton Transactions
Page 12 of 18
View Article Online
The bulky tert-butyl group at the carbon atom of the 1,3-diazaallylic system (forming the pivalamidine
DOI: 10.1039/C8DT04905J
Dipp−N=C(tBu)−N(H)−C2H4−Py) stabilizes the heteroleptic complexes of calcium and strontium and metalation
of Dipp−N=C(tBu)−N(H)−C2H4−Py (1c) with [(thf)2M{N(SiMe3)2}2] (M Ca, Sr) yields mononuclear
=
[{(Me3Si)2N}M{Dipp−N=C(tBu)−N−C2H4−Py}2]2 (M = Ca (2c) and Sr (3c)).21 Ligand exchange reactions are
prevented by the side-on π-coordination of the Dipp group which leads to an effective steric shielding of the
reactive M-N bonds to the bis(trimethylsilyl)amido ligand. Despite this steric shielding the calcium complex 2c
catalyzes the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-
methyl-4,4-diphenylpyrrolidine. The calcium-mediated conversion is rather slow allowing kinetic and
thermodynamic studies. The energy barrier of 59±6 kJ mol-1 for this intramolecular hydroamination lies in a
typical range as also observed for other calcium-mediated hydroamination reactions.
Addition of another methyl group at the ethylene unit of the pyridylethyl side-arm leads to racemic
Dipp−N=C(tBu)−N(H)−CH2CH(Me)−Py of (R)-1d and (S)-1d which can be separated by chiral HPLC procedures.
Calciation with [(thf)2Ca{N(SiMe3)2}2] yields the homoleptic complexes [Ca{Dipp−N=C(tBu)−N−CH2CH(Me)−Py}2]
with the enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. Heteroleptic
complexes as isolated for [{(Me3Si)2N}Ca{Dipp−N=C(tBu)−N−C2H4−Py}2]2 (2c) are not observed. Complexes (R,R)-
2d and (S,S)-2d again catalyze the intramolecular hydroamination of 2,2-diphenylpent-4-enylamine, however,
the influence of the stereochemistry on the intramolecular addition of N-H bonds across the alkene is
negligible.
In summary, we could show that a third Lewis basic donor site attached to the amidinate ligand decelerates the
hydroamination activity due to reduction of the accessibility of the Lewis acidic metal ion. Furthermore, the
amidinato bases act not as pure spectator ligands but also influence the catalytic activity. Stereocontrol of
calcium-mediated intramolecular cyclohydroamination remains a challenging task for future projects.
Experimental
General Considerations. All manipulations were carried out in a nitrogen atmosphere using standard Schlenk
techniques. The solvents THF and toluene were dried over KOH and subsequently distilled over sodium/
benzophenone under a nitrogen atmosphere prior to use. Deuterated solvents were dried over sodium,
degassed, and saturated with nitrogen. The indicated yields are not optimized and refer to crystalline yields. 1H
and 13C{1H} NMR spectra were recorded on Bruker Avance 600, 400 and Avance 250 spectrometers. Chemical
1
1
shifts are reported in parts per million relative to SiMe4 as external standard. H,13C{1H}-HSQC, H,13C{1H}-
HMBC, and H,H-COSY NMR experiments were performed for the assignment of the resonances. The residual
signals of [D8]THF, CDCl3 and C6D6 were used as internal standards. For mass spectrometric investigations the
spectrometers ThermoFinnigan MAT95XL and Finnigan SSQ710 were at our disposal. IR spectra were recorded
with a Bruker ALPHA FT-IR spectrometer. All amines were dried over CaH2 and distilled before use and
purchased from TCI GmbH, Fluorochem or abcr GmbH. Solvents with HPLC gradient grade were purchased
from Carl Roth Company. The synthesis and full characterization of the used compounds
[({Me3Si)2N}Ae{Dipp−N=C(tBu)N−C2H4−Py}] with Ae = Ca (2c) and Ae = Sr (3c) as well as the formamidine
Dipp−N=C(H)−N(H)−C2H4−Py (1a) were already described in our previous work.21 The used benzamidine
Dipp−N=C(Ph)−N(H)−C2H4−Py (1b) was also published very recently.17
N-(2,6-diisopropylphenyl)-N’-(2-pyridylpropan-2-yl)pival amidine, Dipp−N=C(tBu)−N(H)−CH2CH(CH3)−Py (1d).
Dipp−N=C(tBu)Cl was prepared according to our previous work.21 To a toluene (4 ml) solution of
NH2CH2CH(CH3)−Py (0.4 g, 2.9 mmol) was added dropwise Dipp−N=C(tBu)Cl (0.82 g, 2.9 mmol) at 0 °C. The
reaction mixture was stirred at 85 °C for 4 h to give a clear orange solution. Subsequently, all volatiles were
removed in in vacuo. The residuum was dissolved in 50 ml of dichloromethane and washed two times with
aqueous Na2CO3 solution (20 ml, 2%). Removal of the solvent led to formation of 614 mg of 1d (55%) as a

Page 13 of 18
Dalton Transactions
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collorless oil. The chiral resolution was carried out by chiral HPLC (Chiralpak® IA, n-Hexan/EtOH 99.98:0.02,
0.1% add. DEA, 1 ml/min, 254 nm) and the absolute configuration was determined after metalDlaOtiIo: 1n0.1r0e3a9c/tCio8DnT04905J
21
and backtracking. (R)-1d: t = 6.62 min (HPLC); 21 = -49.2°, 1.26 g/l (DCM). (S)-1d: t = 8.20 min (HPLC);
=
α
[ ]D
[α]D
r
r
1
3
4
43.4°, 1.48 g/l (DCM). H NMR (CDCl3, 297.0 K, 600.15 MHz): δ 8.49 (dd, JH,H = 4.8 Hz, JH,H = 1.8 Hz, 1H, Py N–
CH), 7.58 (td, 3JH,H = 7.2 Hz, 4JH,H = 1.8 Hz, 1H, Py m-CH), 7.13–7.11 (m, 1H, Py p-CH), 6.99–6.96 (m, 3H, Dipp m-
3
3
CH, Py o-CH), 6.88 (t, JH,H = 7.8 Hz, 1H, Dipp p-CH), 5.40 (s, broad, 1H, NH), 2.97 (sept, JH,H = 6.6 Hz, 2H,
CH(CH3)2), 2.89–2.82 (m, 2H, N−CH2−), 2.79–2.75 (m, 1H, Py−CHCH3), 1.22 (s, broad, 9H, C(CH3)3), 1.17–1.13 (m,
12H, CH(CH3)2), 1.05 (d, 3JH,H = 6.6 Hz, 3H, Py−CHCH3). 13C{1H} NMR (CDCl3, 297.0 K, 150.93 MHz): δ 163.9 (Py i-
C), 157.3 (NC(tBu)N), 149.0 (Py N–CH), 146.9 (Dipp i-C), 137.7 (Dipp o-C), 137.4 (Dipp o-C), 136.8 (Py m-C), 122.1
(Dipp m-C), 121.9 (Py o-C), 121.8 (Py p-C), 120.8 (Dipp p-C), 48.4 (N−CH2−), 41.3 (Py−CHCH3), 38.8 (C(CH3)3), 29.1
(C(CH3)3), 28.5 (CH(CH3)2), 28.4 (CH(CH3)2), 23.3 (CH(CH3)2), 23.3 (CH(CH3)2), 22.6 (CH(CH3)2), 22.4 (CH(CH3)2),
1
17.9 (Py−CHCH3). H NMR (C6D6, 297.0 K, 400.13 MHz): δ 8.27–8.25 (m, 1H, Py N–CH), 7.18–7.17 (m, 2H, Dipp
m-CH), 7.10–7.06 (m, 1H, Dipp p-CH), 6.96 (td, 3JH,H = 8.0 Hz, JH,H = 1.8 Hz, 1H, Py m-CH), 6.54–6.51 (m, 2H, Py
4
p-CH, Py o-CH), 5.88 (s, broad, 1H, NH), 3.32–3.23 (m, 2H, CH(CH3)2), 3.06–3.02 (m, 2H, N−CH2−), 2.52–2.48 (m,
3
1H, Py−CHCH3), 1.35 (s, 9H, C(CH3)3), 1.34–1.29 (m, 12H, CH(CH3)2), 0.98 (d, JH,H = 7.2 Hz, 3H, Py−CHCH3).
13C{1H} NMR (C6D6, 297.0 K, 100.62 MHz): δ 164.4 (Py i-C), 157.2 (NC(tBu)N), 149.1 (Py N–CH), 147.6 (Dipp i-C),
137.8 (Dipp o-C), 137.4 (Dipp o-C), 136.5 (Py m-C), 122.4 (Py o-C), 122.1 (Dipp m-C), 121.6 (Dipp p-C), 121.5 (Py
p-C), 48.7 (N−CH2−), 41.0 (Py−CHCH3), 39.2 (C(CH3)3), 29.3 (C(CH3)3), 29.0 (CH(CH3)2), 28.9 (CH(CH3)2), 23.7
(CH(CH3)2), 23.7 (CH(CH3)2), 22.6 (CH(CH3)2), 22.4 (CH(CH3)2), 18.0 (Py−CHCH3). MS (DEI): m/z (%) 379 (100,
[M]+), 336 (30, [M-C3H7]+), 322 (10, [M-C3H7-CH3]+; [M-C4H9]+), 289 (20, [M-2xC3H7]+), 261 (5, [C17H29N2]+), 244
(20, [C16H24N2]+), 229 (30, [C15H21N2]+), 217 (20, [C14H21N2]+), 204 (15, [C13H20N2]+), 186 (10, [C12H14N2]+), 146 (5,
[C H N ]+; [C H N]+), 120 (5, [C H N]+), 106 (5, [C H N]+), 57 (5, [C H ]+). IR (pure, /cm-1): 3350 (vw), 3058
υ
9
10
2
10 12
8
10
7
8
4 9
(vw), 2958 (m) 2867 (m), 1685 (m), 1651 (m), 1618 (s), 1590 (m), 1570 (m), 1519 (m), 1474 (m), 1463 (m), 1433
(m), 1394 (m), 1377 (m), 1363 (m), 1329 (m), 1299 (m), 1258 (m), 1221 (m), 1199 (m), 1169 (m), 1125 (m), 1055
(m), 1044 (m), 993 (m), 957 (w), 935 (w), 901 (w), 883 (w), 858 (w), 827 (m), 786 (m), 747 (s), 717 (m), 681 (w),
626 (w), 598 (w), 564 (m), 544 (w), 516 (w), 484 (w), 465 (w). Anal. Calcd. for C25H37N3 (379.59 g/mol): C, 79.10;
H, 9.83; N, 11.07. Found: C, 78.82; H, 9.92; N, 10.82.
[{(Me3Si)2N}Ca{Dipp−N=C(H)N−C2H4−Py}]2 (2a). [{(Me3Si)2N}Ca{Dipp−N=C(H)N−C2H4−Py}]2 was prepared in
analogy to 2b using 238 mg (0.77 mmol) of 1b and toluene (10 mL) as solvent. Recrystallization from a mixture
of n-pentane, toluene and THF (3:1:1) at -20 °C led to formation of 56 mg of 2a' as pale yellow crystals (16%). A
complete rearrangement to homoleptic [Ca{Dipp−N=C(H)N−C2H4−Py}2] is indicated by analytical data in solution
1
3
and hindered the full characterization. H NMR (C6D6, 297.0 K, 400.22 MHz): δ 9.05 (d, JH,H = 5.2 Hz, 2H, Py
3
N−CH), 7.90 (s, 1H, NC(H)N), 7.15–7.14 (m, 4H, Dipp m-CH), 7.13–7.07 (m, 2H, Dipp p-CH), 6.94 (td, JH,H = 7.6
Hz, JH,H = 2.0 Hz, 2H, Py m-CH), 6.68–6.65 (m, 2H, Py p-CH), 6.55 (d, JH,H = 7.6 Hz, 2H, Py o-CH), 3.53 (t, JH,H
4
3
3
=
5.6 Hz, 4H, N−CH2−), 3.39 (sept, 3JH,H = 6.8 Hz, 4H, CH(CH3)2), 2.79–2.76 (m, 4H, Py−CH2−), 1.07 (d, 3JH,H = 7.2 Hz,
24H, CH(CH3)2). 13C{1H} NMR (C6D6, 297.0 K, 100.63 MHz): δ 167.7 (NC(H)N), 162.2 (Py i-C), 150.0 (Dipp i-C),
149.7 (Py N−CH), 142.9 (Dipp o-C), 137.8 (Py m-C), 124.7 (Py o-C), 123.1 (Dipp m-C), 122.4 (Dipp p-C), 121.5 (Py
p-C), 53.2 (N−CH2−), 41.9 (Py−CH2−), 28.4 (CH(CH3)2), 24.4 (CH(CH3)2). MS (DEI): m/z (%) 364 (< 5, [C20H28CaN4]+),
309 (90, [L]+), 266 (30, [L-C3H7]+), 230 (40, [L-C5H4N]+), 203 (90, [C13H19N2]+), 188 (40, [C12H16N2]+), 172 (70,
[C11H12N2]+), 146 (55, [C10H12N]+), 106 (100, [C7H8N]+), 93 (60, [C6H7N]+), 78 (15, [C5H4N]+), 43 (25, [C3H7]+).
[Ca{Dipp−N=C(Ph)N−C2H4−Py}2] (2b). A mixture of amidine 1b (294 mg, 0.76 mmol) and [(thf)2Ca{N(SiMe3)2}2]
(385 mg, 0.76 mmol) was stirred in toluene (18 ml) at room temperature for 24 h. Thereafter all volatiles were
removed in vacuo and 145 mg of 2b (47%) as yellow crystals were isolated after recrystallization at r.t from a
mixture of toluene (3 ml) and THF (1 ml). M. p. 112−114 °C (dec.). 1H NMR (C6D6, 297.0 K, 400.13 MHz): δ 8.86–
8.85 (m, 4H, Py N−CH), 7.22 (d, 3JH,H = 6.8 Hz, 4H, Ph o-CH), 7.06–7.02 (m, 6H, Py m-CH, Ph m-CH), 6.96 (d, 3JH,H
=
3
7.2 Hz, 4H, Dipp m-CH), 6.91–6.86 (m, 4H, Dipp p-CH, Ph p-CH), 6.73–6.69 (m, 2H, Py p-CH), 6.60 (d, JH,H = 7.6
Hz, 2H, Py o-CH), 3.60–3.55 (m, 8H, N−CH2−, CH(CH3)2), 2.66–2.63 (m, 4H, Py−CH2−), 1.14 (d, 3JH,H = 7.2 Hz, 12H,
3
CH(CH3)2), 0.88 (d, JH,H = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (C6D6, 297.0 K, 100.61 MHz): δ 172.6 (NC(Ph)N),
163.9 (Py i-C), 148.8 (Py N−CH), 147.8 (Dipp i-C), 141.4 (Dipp o-C), 138.0 (Py m-C), 136.3 (Ph i-C), 129.5 (Ph o-C),
127.5 (Ph m-C), 125.1 (Py o-C), 122.8 (Dipp m-C), 121.5 (Dipp p-C), 121.3 (Py p-C), 49.8 (N−CH2−), 42.3

Dalton Transactions
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DOI: 10.1039/C8DT04905J
(Py−CH2−), 28.4 (CH(CH3)2), 25.0 (CH(CH3)2), 22.9 (CH(CH3)2). MS (DEI): m/z (%) 808 (< 1, [M] ), 765 (< 5, [M-
C3H7]+), 424 (5, [CaL]+), 385 (45, [L]+), 342 (30, [L-C3H7]+), 292 (15, [C20H24N2]+), 279 (20, [C19H23N2]+), 249 (40,
[C17H17N2]+), 235 (50, [C16H15N2]+), 177 (40, [C12H19N]+), 162 (100, [C12H18]+), 106 (50, [C7H8N]+), 103 (40, [C8H7]+),
79 (30, [C H N]+), 44 (25, [C H ]+). IR (pure, /cm-1): 2860 (m), 2810 (m), 1625 (m), 1597 (m), 1568 (w), 1545 (w),
υ
5
5
3 8
1535 (w), 1511 (w), 1479 (m), 1460 (s), 1429 (m), 1413 (m), 1380 (m), 1344 (m), 1320 (m), 1308 (m), 1252 (m),
1224 (m), 1214 (w), 1178 (m), 1141 (m), 1099 (m), 1080 (m), 1055 (m), 1038 (m), 1023 (m), 1007 (m), 994 (w),
975 (m), 933 (m), 914 (vw), 886 (vw), 837 (vw), 815 (w), 801 (w), 779 (m), 758 (m), 750 (m), 699 (m), 663 (w),
647 (w), 634 (m), 616 (vw), 601 (w), 588 (m), 552 (w), 536 (w), 510 (m), 460 (m). Due to the sensitivity of this
compound during handling and weighing no reliable CHN analytical data have been obtained.
[Ca{Dipp−N=C(tBu)N−CH2CH(CH3)−Py}2] (2d). [Ca{Dipp−N=C(tBu)N−CH2CH(CH3)−Py}2] was prepared in analogy
to 2b using 186 mg (0.49 mmol) of (R)-1d, respectively 183 mg (0.48 mmol) of (S)-1d and 287 mg (0.57 mmol)
of [(thf)2Ca{N(SiMe3)2}2] in toluene (20 mL). Recrystallization of the obtained solid from toluene (4 ml) at 5 °C
led to formation of 50 mg (26%) of (R,R)-2d, respectively 31 mg (16%) of (S,S)-2d as pale yellow crystals. M. p.
1
3
4
203−208 °C (dec.). H NMR (C6D6, 297.0 K, 400.22 MHz): δ 8.56 (dd, JH,H = 5.2 Hz, JH,H = 2.0 Hz, 2H, Py N–CH),
3
4
3
4
7.23 (dd, JH,H = 7.6 Hz, JH,H = 2.0 Hz, 2H, Dipp m-CH), 7.11 (dd, JH,H = 7.6 Hz, JH,H = 2.0 Hz, 2H, Dipp m-CH),
7.07–7.02 (m, 4H, Py m-CH, Dipp p-CH), 6.61 (d, JH,H = 8.0 Hz, 2H, Py o-CH), 6.67–6.64 (m, 2H, Py p-CH), 3.69–
3
3.61 (m, 4H, CH(CH3)2, N−CH2−), 3.55 (sept, 3JH,H = 6.8 Hz, 2H, CH(CH3)2), 3.34–3.28 (m, 2H, N−CH2−), 2.67–2.62
(m, 2H, Py−CHCH3), 1.65 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2), 1.51 (d, 3JH,H = 6.8 Hz, 6H, CH(CH3)2), 1.22 (d, 3JH,H = 6.8
3
3
Hz, 6H, CH(CH3)2), 0.98 (d, JH,H = 6.8 Hz, 6H, CH(CH3)2), 0.92 (d, JH,H = 6.8 Hz, 6H, Py−CHCH3), 0.84 (s, 18H,
C(CH3)3). 13C{1H} NMR (C6D6, 297.0 K, 100.65 MHz): δ 172.9 (NC(tBu)N), 167.5 (Py i-C), 150.2 (Dipp i-C), 146.1
(Py N–CH), 141.1 (Dipp o-C), 138.1 (Py m-C), 123.0 (Dipp m-C), 122.8 (Dipp m-C), 121.3 (Py o-C), 120.9 (Py p-C),
119.6 (Dipp p-C), 57.6 (N−CH2−), 45.2 (Py−CHCH3), 41.7 (C(CH3)3), 29.6 (C(CH3)3), 28.3 (CH(CH3)2), 27.8
(CH(CH3)2), 24.5 (CH(CH3)2), 24.3 (CH(CH3)2), 23.9 (CH(CH3)2), 23.3 (CH(CH3)2), 15.2 (Py−CHCH3). MS (DEI): m/z
(%) 796 (5, [M]+), 753 (40, [M-C3H7]+), 418 (5, [CaL]+), 379 (55, [L]+), 336 (35, [L-C3H7]+), 322 (15, [L-C3H7-CH3]+),
259 (5, [L-C8H11N]+), 244 (45, [C16H24N2]+; [C17H26N]+), 229 (85, [C16H23N]+), 217 (60, [C14H20N2]+), 203 (30,
[C13H19N2]+), 146 (10, [C10H12N]+), 120 (100, [C7H8N2]+; [C8H10N]+), 106 (10, [C7H8N]+), 57 (10, [C4H9]+), 41 (5,
[C H N]+). IR (pure, /cm-1): 2955 (m), 2867 (w), 1651 (m), 1590 (w), 1568 (w), 1514 (w), 1470 (m), 1452 (m),
υ
2
3
1426 (m), 1379 (m), 1357 (m), 1331 (w), 1312 (m), 1258 (w), 1235 (w), 1178 (w), 1158 (w), 1097 (w), 1056 (w),
1039 (w), 1020 (w), 1004 (w), 995 (w), 932 (w), 897 (w), 861 (w), 829 (w), 782 (m), 750 (m), 713 (m), 681 (w),
647 (w), 636 (w), 585 (w), 550 (w), 522 (w). Anal. Calcd. for C50H72CaN6 (797.22 g/mol): C, 75.33; H, 9.10; N,
10.54. Found: C, 75.05; H, 9.13; N, 10.34.
[(tmeda)Sr{Dipp−N=C(H)N−C2H4−Py}2] (3a). [(tmeda)Sr{Dipp−N=C(H)N−C2H4−Py}2] was prepared in analogy to
2b using 249 mg (0.80 mmol) of 1a and 445 mg (0.81 mmol) of [(thf)2Sr{N(SiMe3)2}2] in toluene (16 mL).
Recrystallization of the obtained solid from a mixture of toluene (1 ml) and TMEDA (0.2 ml) at r.t. led to
formation of 126 mg (38%) of 3a as pale yellow crystals. Physical and analytical data show cocrystalline toluene
(1 eq), which is partly lost by drying in vacuo M. p. 75−77 °C (dec.). 1H NMR (C6D6, 296.9 K, 250.13 MHz): δ 8.78
(s, broad, 2H, Py N−CH), 7.99 (s, 2H, NC(H)N), 7.19 (d, 3JH,H = 6.8 Hz, 4H, Dipp m-CH), 7.13–7.09 (m, 4H, Dipp p-
CH, Tol m-CH), 7.06–6.99 (m, 4H, Py m-CH, Tol o-CH, Tol p-CH), 6.76–6.74 (m, 2H, Py o-CH), 6.67–6.65 (m, 2H,
Py p-CH), 3.67 (s, broad, 4H, N−CH2−), 3.43 (s, broad, 4H, CH(CH3)2), 2.96 (t, 3JH,H = 6.8 Hz, 4H, Py−CH2−), 2.17 (s,
broad, 4H, tmeda N−CH2−), 2.14 (s, broad, 12H, tmeda N(CH3)2), 2.11 (s, broad, 3H, Tol CH3), 1.16–1.14 (m, 24H,
CH(CH3)2). 13C{1H} NMR (C6D6, 296.9 K, 62.90 MHz): δ 168.4 (NC(H)N), 162.1 (Py i-C), 151.2 (Dipp i-C), 149.9 (Py
N−CH), 142.8 (Dipp o-C), 137.9 (Tol i-C), 136.7 (Py m-C), 129.3 (Tol o-C), 128.6 (Tol m-C), 125.7 (Tol p-C), 123.7
(Py o-C), 123.2 (Dipp m-C), 122.0 (Dipp p-C), 121.1 (Py p-C), 57.7 (tmeda N−CH2−), 55.1 (N−CH2−), 45.9 (tmeda
N(CH3)2), 43.2 (Py−CH2−), 28.2 (CH(CH3)2), 24.8 (CH(CH3)2), 21.4 (Tol CH3). MS (DEI): m/z (%) 704 (< 1, [M-
tmeda]+), 661 (< 1, [M-tmeda-C3H7]+), 309 (60, [L]+), 266 (20, [L-C3H7]+), 230 (40, [L-C5H4N]+), 203 (95,
[C13H19N2]+), 172 (60, [C11H12N2]+), 146 (55, [C10H12N]+), 106 (100, [C7H8N]+), 93 (80, [C6H7N]+), 78 (15, [C5H4N]+),
43 (25, [C H ]+). IR (pure, /cm-1): 3050 (vw), 2961 (w), 2929 (w), 1660 (m), 1628 (w), 1584 (w), 1567 (w), 1459
υ
3
7
(w), 1430 (w), 1395 (w), 1380 (w), 1362 (w), 1317 (w), 1257 (w), 1235 (w), 1220 (w), 1186 (w), 1157 (w), 1107
(w), 1096 (w), 1076 (w), 1051 (w), 1025 (w), 986 (w), 937 (w), 888 (w), 845 (w), 822 (w), 795 (w), 782 (w), 770

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(m), 748 (w), 701 (w), 669 (w), 629 (w), 603 (w), 592 (w), 566 (w), 534 (w), 503 (w). Anal. Calcd. for C46H68SrN8
DOI: 10.1039/C8DT04905J
(866.53 g/mol): C, 68.59; H, 8.37; N, 12.93. Found: C, 68.20; H, 8.35; N, 12.38.
[Sr{Dipp−N=C(Ph)N−C2H4−Py}2] (3b). [Sr{Dipp−N=C(Ph)N−C2H4−Py}2] was prepared in analogy to 2b using 294
mg (0.65 mmol) of 1b and 357 mg (0.65 mmol) of [(thf)2Sr{N(SiMe3)2}2] in toluene (20 ml). Recrystallization of
the obtained solid from toluene (4 ml) at r.t. led to formation of 143 mg (51%) of 3b as pale yellow crystals. M.
p. 110−113 °C (dec.). 1H NMR (C6D6, 297.0 K, 400.13 MHz): δ 8.68–8.67 (m, 2H, Py N−CH), 7.04 (d, 3JH,H = 6.8 Hz,
4H, Ph o-CH), 7.01–6.96 (m, 10H, Py m-CH, Ph m-CH, Dipp m-CH), 6.90–6.83 (m, 4H, Dipp p-CH, Ph p-CH), 6.66–
6.63 (m, 2H, Py p-CH), 6.55 (d, 3JH,H = 7.6 Hz, 2H, Py o-CH), 3.62–3.53 (m, 8H, N−CH2−, CH(CH3)2), 2.75–2.72 (m,
3
3
4H, Py−CH2−), 1.19 (d, JH,H = 6.8 Hz, 12H, CH(CH3)2), 1.09 (d, JH,H = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (C6D6,
297.0 K, 100.61 MHz): δ 172.2 (NC(H)N), 163.8 (Py i-C), 148.3 (Py N−CH), 147.8 (Dipp i-C), 141.4 (Dipp o-C),
137.9 (Py m-C), 137.3 (Ph i-C), 128.7 (Ph o-C), 127.4 (Ph m-C), 127.3 (Ph p-C), 124.9 (Py o-C), 122.5 (Dipp m-C),
121.5 (Dipp p-C), 121.4 (Py p-C), 50.7 (N−CH2−), 42.8 (Py−CH2−), 28.5 (CH(CH3)2), 25.5 (CH(CH3)2), 22.6
(CH(CH3)2). MS (DEI): m/z (%) 857 (< 5, [M]+), 813 (10, [M-C3H7]+), 751 (< 5, [M-C7H8N]+), 708 (5, [C24H45N5Sr]+),
472 (10, [SrL]+), 385 (55, [L]+), 342 (35, [L-C3H7]+), 292 (10, [C20H24N2]+), 280 (20, [C19H24N2]+), 249 (40,
[C17H17N2]+), 235 (75, [C16H15N2]+), 162 (40, [C12H18]+), 106 (100, [C7H8N]+), 103 (25, [C8H7]+), 79 (20, [C5H5N]+), 43
(10, [C H ]+). IR (pure, /cm-1): 3239 (w), 3047 (w), 2954 (m), 2862 (w), 1624 (m), 1596 (m), 1567 (m), 1536 (m),
υ
3
7
1500 (w), 1475 (m), 1459 (m), 1435 (m), 1380 (w), 1356 (w), 1340 (w), 1314 (m), 1301 (m), 1281 (m), 1256 (m),
1145 (m), 1098 (w), 1055 (w), 1039 (w), 1008 (w), 992 (w), 955 (w), 934 (w), 825 (w), 805 (w), 784 (w), 773 (m),
761 (m), 696 (m), 662 (w), 619 (w), 583 (w), 535 (w), 498 (w). Due to the sensitivity of this compound during
handling and weighing no reliable CHN analytical data have been obtained.
Chromatographic separation of (R)-1d and (S)-1d. Analyses were performed on a SHIMADZU system consisting of
a system controller (SLC-10AVP), a column oven (CTO-10ACVP), an auto-injector (SIL-10ADVP), a degasser
(DGU-14A), two pumps (LC-10ATVP), a diode array detector (SPD-M20A) and a UV-vis-detector (SPD-10AVP).
For the analysis of the enantiomeric ratio of 2-methyl-4,4-diphenyl-pyrrolidine an isocratic flow (3 mL/min) of
n-hexane/EtOH (90:10 v/v or 99.98:0.02 v/v) containing 0.1% diethylamine and a CHIRALCEL® OJ column (250 ×
10 mm ID, 10 µm, DAICEL) was used. For more details refer to the Supporting Information.
Crystal Structure Determinations. The intensity data for the compounds were collected on a Nonius KappaCCD
diffractometer using graphite-monochromated Mo-K radiation. Data were corrected for Lorentz and
polarization effects; absorption was taken into account on a semi-empirical basis using multiple-scans.31-33 The
2
structures were solved by direct methods (SHELXS)34 and refined by full-matrix least squares techniques against Fo
(SHELXL-9734 and SHELXL-2014).35 The hydrogen atoms bonded to the amidinate carbon atom C8 of 2a' and carbon
atoms C1, C21 of 3a were located by difference Fourier synthesis and refined isotropically. All other hydrogen
atoms were included at calculated positions with fixed thermal parameters. All non-hydrogen, non-disordered
atoms were refined anisotropically.34,35 The crystal of 2a' contains large voids, filled with disordered solvent
molecules. The size of the voids are 414 Å3. Their contribution to the structure factors was secured by back-
Fourier transformation using the SQUEEZE routine of the program PLATON36 resulting in 108 electrons/unit cell.
Crystallographic data as well as structure solution and refinement details are summarized in Table S2
(Supporting Information). XP37 and POV-Ray38 were used for structure representations.
Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgements
We acknowledge the support by the NMR service platform (www.nmr.uni-jena.de) and by the mass
spectrometry platform (www.ms.uni-jena.de) of the Faculty of Chemistry and Earth Sciences of the
Friedrich Schiller University Jena.

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DOI: 10.1039/C8DT04905J
Notes and references
§ Equal contributors.
‡ Crystallographic data (excluding structure factors) has been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication CCDC-1884727 for 2a', CCDC-1884728 for 2b,
CCDC-1884729 for (R,R)-2d, CCDC-1884730 for (S,S)-2d, CCDC-1884731 for 3a, and CCDC-1884732 for 3b.
Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [E- mail: deposit@ccdc.cam.ac.uk].
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DOI: 10.1039/C8DT04905J
TOC Picture
Sterically protected tridentate amidinates of calcium have been prepared to study the
intramolecular hydroamination of an aminoalkene.