Lanthanum dibromide complexes of sterically demanding aminopyridinato and amidinate ligands

Article abstract of DOI:10.1002/zaac.200600142

It is report on synthesis and structure of Ap*LaBr ₂(THF)₃ and Am*LaBr₂(THF)₃ (Ap*-H = {(2,6-diisopropyl-phenyl)-[6-(2,4,6-triisopropyl-phenyl)-pyridin- 2-yl]-amine}, Am*-H = N,N′-bis-(2,6-diisopropylphenyl)benzamidine). X-ray crystal structure analyses of the two seven coordinated complexes were carried out to compare the steric demand of the two amido ligands. A similar overall primary coordination site bulkiness for both ligands and distinct differences regarding this bulkiness for different directions were observed. A better shielding of the second coordination sphere was observed for the aminopyridinate.

Full text of DOI:10.1002/zaac.200600142

DOI: 10.1002/zaac.200600142
Lanthanum Dibromide Complexes of Sterically Demanding Aminopyridinato
and Amidinate Ligands1)
Winfried P. Kretschmera,*, Auke Meetsmaa, Bart Hessena, Natalie M. Scottb, Sadaf Qayyumb, and Rhett
Kempeb,*
a Groningen/The Netherlands, Stratingh Institut for Chemistry and Chemical Engineering, University of Groningen
b Bayreuth, Lehrstuhl Anorganische Chemie II, der Universität Bayreuth und Rostock, Leibniz-Institut für Organische Katalyse
Received May 5th, 2006.
Dedicated to Professor Glen Deacon on the Occasion of his 70th Birthday
Abstract. It is report on synthesis and structure of Ap*LaBr2(THF)3
and Am*LaBr2(THF)3 (Ap*-H {(2,6-diisopropyl-phenyl)-
[6-(2,4,6-triisopropyl-phenyl)-pyridin-2-yl]-amine}, Am*-H
and distinct differences regarding this bulkiness for different direc-
tions were observed. A better shielding of the second coordination
sphere was observed for the aminopyridinate.
ϭ
ϭ
N,NЈ-bis-(2,6-diisopropylphenyl)benzamidine). X-ray crystal struc-
ture analyses of the two seven coordinated complexes were carried
out to compare the steric demand of the two amido ligands. A
similar overall primary coordination site bulkiness for both ligands
Keywords: Amidinate ligands; Aminopyridinato ligands; Lanthani-
des; Lanthanum
Recently, sterically demanding N,N-bidentate anionic ligands be-
came very popular in amido [1, 2] lanthanide chemistry. Among
such ligands selected amidinate [3] and aminopyridinato [4, 5] li-
gands are of special interest since they bind a large variety of rare
earth metals forming species of the type (L)LnX2(THF)n (L ϭ
N,N-bidentate mono anionic amido ligands, Ln lanthanide metal,
X ϭ halide, n ϭ 1,2,3,...) [6, 7]. In this communication we report on
synthesis and structure of Ap*LaBr2(THF)3 and Am*LaBr2(THF)3
(Ap*-H ϭ {(2,6-diisopropyl-phenyl)-[6-(2,4,6-triisopropyl-phenyl)-
pyridin-2-yl]-amine}, Am*-H ϭ N,NЈ-bis-(2,6-diisopropylphenyl)-
benzamidine).
Both compounds are accessible via salt elimination reaction in
moderate to good yields. The reaction of Ap*-H or Am*-H with
KH leads to the potassiated aminopyridinate or amidinate, respec-
tively, which then can undergo transmetalation (Schema 1). For 1
no reaction is observed with lithiated Ap*-H. The potassium salt
of Ap*-H is a polymer and the lithium salt a three coordinated
monomer. Additional coordination of one solvent molecule was
found for the lithium salt [8]. This observation is in contrast to the
reaction of lithiated Am*-H with LaBr3(THF)4. We succeeded in
the formation of the corresponding lanthanum amidinate, however
no complete LiBr separation could be achieved. Furthermore, no
Scheme 1 Synthesis of 1 and 2.
formation of a bisaminopyridinato complex could be observed by
reacting two equiv. of Ap*K with LaBr3. NMR spectroscopy of
the two lanthanum complexes revealed the coordination of three
additional THF ligands. Due to the additional coordination of
three THF ligands we expected mononuclear seven coordinated
compounds in solution. Such complexes should be ideal to com-
pare the steric bulk of the aminopyridinate and the amidinate li-
gand since the angles between the ancillary ligands are quite sensi-
tive to the steric “pressure” of the amido ligands in at least two
directions. Crystals of 1 and 2 suitable for X-ray crystal structure
analysis could be grown from hexane (1) or THF (2) solutions. The
molecular structures of 1 and 2 (ORTEP plots) are shown in Figure 1
* Dr. Winfried Kretschmer
Stratingh Institut for Chemistry and Chemical Engineering
University of Groningen
Nijenborgh 4
NL-9747 AG Groningen (The Netherlands)
Fax: ϩ31 50 3634315
e-mail: W.P.Kretschmer@rug.nl
* Prof. Dr. Rhett Kempe
Lehrstuhl Anorganische Chemie II
Universität Bayreuth
D-95440 Bayreuth
e-mail: kempe@uni-bayreuth.de
1) Paper presented at the XVIIIth Tage der Seltenen Erden (Terrae
Rarae 2005) at Bonn-Röttgen/Germany, November 30th Ϫ Decem-
ber 2nd, 2005 (www.Terrae-rarae.de).
1936
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 632, 1936Ϫ1938

Lanthanum Dibromide Complexes
Figure 1 Molecular structure of 1 (ellipsoids [non carbon atoms]
correspond to the 50 % probability level).
˚
Selected bond lengths/A and angles/°: La-N1 2.447(3), La-O1 2.560(2), La-
O3 2.579(2), La-O2 2.634(3), La-N2 2.645(3), La-Br1 2.9028(6), La-Br2
2.9116(6); N1-La-O1 76.60(9), N1-La-O3 142.25(9), O1-La-O3 140.96(8),
N1-La-O2 146.27(10), O1-La-O2 69.68(10), O3-La-O2 71.44(10), N1-La-N2
52.98(8), O1-La-N2 129.56(8), O3-La-N2 89.31(9), O2-La-N2 160.75(9), N1-
La-Br1 96.58(7), O1-La-Br1 91.08(6), O3-La-Br1 87.98(7), O2-La-Br1
83.62(10), N2-La-Br1 95.61(6), N1-La-Br2 96.55(7), O1-La-Br2 88.29(6),
O3-La-Br2 83.95(7), O2-La-Br2 83.38(10), N2-La-Br2 95.29(6), Br1-La-Br2
166.342(17), O1-La-O3 140.96(8).
Figure 2 Molecular structure of 2 (ellipsoids [non carbon atoms]
correspond to the 50 % probability level).
and 2, respectively. Both compounds are monomeric in the solid
state and the coordination can be described best as pentagonal bi-
pyramides in which the two bromo ligands occupy the axial posi-
tions. The equatorial sites are populated by the three oxygen atoms
of the THF ligands as well as the two N-atoms. The two N-atoms
˚
Selected bond lengths/A and angles/°: La-N 2.5254(18), La-O1 2.5571(17),
La-O2 2.620(3), La-Br 2.9053(2); N-La-Br 99.18(4), NЈ-La-Br 99.44(4), N-
La-O1 80.46(6), N-La-O2 153.84(4), N-La-O1Ј 132.77(6), NЈ-La-O1
132.77(6), NЈ-La-O2 153.84(4), O1-La-O2 73.38(4), O1-La-O1Ј 146.77(6), N-
La-NЈ 52.33(6), O1-La-Br 85.62(4), O2-La-Br 79.62(1), Br-La-BrЈ 159.24(1).
˚
in 2 are equally bonded to the metal center [La-N 2.5254(18) A],
˚
sterically more demanding in d-direction which could be under-
stood by the fact that the 2,6-isopropyl-phenyl substituent linked
to the pyridine ring is pointing downwards (Figure 1). In e-direc-
tion the amidinate ligand is bulkier. The differences in the Br-La-
Br und O-La-O angles are similar 7.1° and 5.8°, respectively. Thus
we assume a similar overall primary coordination site bulkiness for
both ligands but distinct differences in the d- and e-directions. The
consequences of these differences in terms of the reactivity of the
corresponding early transition metal and lanthanide complexes are
going to be investigated.
while the La-N distances of
1
[La-N1 2.447(3) A; La-N2
˚
2.645(3) A] indicate a localization of the anionic function at the
amido N-atom. The binding of Ap* to the lanthanum atom is de-
scribed best as a donorfuntionalized amido metal bond. The maxi-
mum atom to atom distances in accordance to Scheme 2 express
the overall steric bulk of the two amido ligands or the second coor-
˚
dination sphere bulkiness. These distances are: d ϭ 15.1 A, e ϭ
8.8 A for 1 and d ϭ 11.9 A, e ϭ 8.6 A for 2. Which means 1 is the
˚
˚
˚
more demanding in this regard.
Experimental Section
All reactions and manipulations with air-sensitive compounds were
performed under dry argon, using standard Schlenk and drybox
techniques. Solvents were distilled from sodium benzophenone ketyl.
Deuterated solvents were obtained from Cambridge Isotope
Laboratories and were degassed, dried (CaH2) and distilled prior
to use. NMR spectra were obtained using either a Bruker ARX
250, Bruker DRX 500, Varian Unity Inova 400 or VXR 300 spec-
trometer. Chemical shifts are reported in ppm relative to the deute-
urated solvent. Elemental analyses were carried out using an Ele-
mentar Vario EL III. Ap*-H [8] and Am*-H [7b] were synthesised
following literature procedures. LaBr3(THF)4 was prepared by con-
tinuous extraction of anhydrous LaBr3 [9]. All other starting mate-
rials were purchased from commercial suppliers. X-ray crystal
structure analyses were performed using a STOE-IPDS II (1) or a
Brucker SMART APEX CCD (2) equipped with a low temperature
Scheme 2 Description of the steric demand of the amido ligands
by using the parameter d and e (maximum H-atom-H-atom distan-
ces perpendicular to each other).
Furthermore, the Br-La-Br bond angles and the nearly linear O-
La-O angle can be used to describe the steric “pressure” which is
put on the ancillary ligands (bromo and THF ligands). We call this
the primary coordination site bulkiness. For 1 these angles are: Br1-
La-Br2 166.342(17)°, O1-La-O3 140.96(8)° and for 2: Br-La-BrЈ
159.24(1)°, O1-La-O1Ј 146.77(6)°. The aminopyridinato ligand is
Z. Anorg. Allg. Chem. 2006, 1936Ϫ1938
 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
1937

W. P. Kretschmer, A. Meetsma, B. Hessen, N. M. Scott, S. Qayyum, R. Kempe
After a few hours yellow crystals of the title complex were formed
which were filtered of and dried under reduced pressure (1.68 g,
88 %). Analysis: C, 51.94; H, 6.39; N, 2.82 %. C43H63Br2LaN2O3
requires C, 54.10; H, 6.65; N, 2.93 %).
1H NMR (400 MHz, THF-d8, 298 K): δ ϭ 0.70 (d, 12H, 3J(H,H) ϭ 6.6 Hz,
CH3), 1.18 (d, 12H, 3J(H,H) ϭ 6.6 Hz, CH3), 1.72 (br, 12H; β-CH2, THF),
3.56 (br, 12H; α-CH2, THF), 3.71 (sept, 4 H, 3J(H,H) ϭ 6.6 Hz, CHMe2),
6.69 Ϫ 6.97 (m, 11H, C6H5, C6H3) ppm. 13C NMR (100 MHz, THF-d8,
298 K): 25.19 (CH3), 26.56 (CH3), 27.34 (THF), 29.68 (CHMe2), 69.22
(THF), 124.98 (Ar C), 125.14 (Ar C), 127.93 (Ar C), 130.23 (Ar C), 133.23
(Ar C), 143.56 (Ar C), 147.40 (Ar C), 174.25 (NCN) ppm.
Table 1 Details of the X-ray crystal structure analyses of 1 and 2.
compound
1
2
crystal system
space group
monoclinic
P21/n
orthorhombic
Pbcn
17.0544(9)
17.393(1)
14.6421(8)
˚
a, A
14.455(3)
17.772(4)
18.361(4)
95.13(3)
4697.9(16)
4
˚
b, A
˚
c, A
β, deg
3
˚
V, A
4343.2(4)
4
Z
crystal size, mm
ρcalcd, g cmϪ3
µ, mmϪ1 (Mo Kα)
T, K
0.48 x 0.36 x 0.34
1.372
2.647
193(2)
1.60 to 26.31
9341
8148
469
0.0849
0.0333
0.49 x 0.46 x 0.39
1.460
2.862
100(1)
2.34 to 29.68
5384
4661
358
0.0618
0.0286
Acknowledgement. Financial support of the Deutsche Forschungs-
gemeinschaft (Schwerpunktprogramm 1166 “Lanthanoidspezifische
Funktionalitäten in Molekül und Material“), the Fonds der Chem-
ischen Industrie and the NWO is gratefully acknowledged.
θ range, deg
no. of reflecions unique
no. of reflections obs. [I > 2σ (I)]
no. of parameters
wR2 (all data)
R value [I>2σ (I)]
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unit. Structure solution and refinement was accomplished using
SIR97 [10], SHELXL97 [11] and WinGX [12]. Crystallographic de-
tails are summarized in Table 1. CCDC-602184 (compound 1)
and -602602 (compound 2) contain the supplementary crystallo-
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Preparation of the lanthanide complexes
Ap*LaBr2(THF)3 (1)
LaBr3 (0.80 g, 2.10 mmol), [K(Ap*)]
(1.04 g, 2.10 mmol) and THF (40 cm3)
were added to a flask, and the mixture
was stirred for 15 h. The solvent was
removed under vacuum and hexane
was added (30 cm3). The yellow reac-
tion mixture was filtered and on stand-
ing at room temperature for 24 h, yel-
low crystals (partially suitable for X-
ray analysis) of 1 were formed (0.80 g,
40 %). Analysis: C, 54.03; H, 6.70; N,
2.70 %; C44H67Br2LaN2O3 requires C,
54.44; H, 6.96; N, 2.89 %.
1H NMR (250 MHz, C6D6, 298 K): δ ϭ 1.17 (d, 6H, H28,29,32,33), 1.22 (d,
6H, H30,31), 1.29 (d, 6H, H24,25,26,27), 1.44 (br, 4H, β-CH2, THF), 1.49 (d,
6H, H24,25,26,27), 1.57 (d, 6H, H28,29,32,33), 2.78 (sept, 1H, H15), 3.44 (sept,
2H, H13,14), 3.58 (br, 12H, α-CH2, THF), 4.22 (sept, 2H, H22,23), 5.78 (d,
1H, H3), 6.03 (d, 1H, H5), 6.86 (t, 1H, H4), 7.18 (m, 2H, H18,20), 7.24 (m,
1H, H19), 7.29 ppm (m, 2H, H9,11). 13C NMR (C6D6, 298 K): δ ϭ 21.36
(C28,29,32,33), 24.39 (C24,25,26,27), 24.64 (C28,29,32,33), 25.23 (β-CH2, THF),
25.98 (C24,25,26,27), 26.17 (C30,31), 28.64 (C22,23), 30.81 (C13,14), 34.75 (C15),
70.65 (α-CH2, THF), 107.70 (C3), 111.18 (C5), 121.04 (C9,11), 124.10 (C18,20),
125.60 (C19), 137.83 (C7), 138.75 (C4), 144.45 (C17,21), 147.13 (C16), 148.66
(C8,12), 149.07 (C10), 155.92 (C6), 170.77 (C2) ppm.
[8] N. M. Scott, T. Schareina, O. Tok, R. Kempe, Eur. J. Inorg.
Chem. 2004, 3297Ϫ3304.
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Am*LaBr2(THF)3] (2)
Am*-H (0.88 g, 2.00 mmol) was added to a slurry of KH (0.08 g,
2.00 mmol) in THF (25 mL) and stirred to become a clear solution.
After adding LaBr3(THF)4 (1.33 g, 2.00 mmol) the mixture was
heated under reflux for several minutes to become slurry again. The
hot mixture was filtered and slowly cooled to room temperature.
1938
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 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2006, 1936Ϫ1938