Syntheses and characterization of samarium and erbium borohydrido complexes supported by N-aryliminopyrrole ligand

Article abstract of DOI:10.1002/zaac.201200426

Reaction of potassium salt of N-aryliminopyrrole ligand [2-(2, 6-iPr ₂C₆H₃N=CH)-C₄H₃NK] (1) with samarium tris-boro-hydride [Sm(BH₄)₃(THF) ₃] gave a samarium ate complex [η²-{2-(2, 6-iPr ₂C₆H₃N=CH)-C₄H₃N} ₃Sm(η¹-BH₄){K(THF)₆] (2); whereas similar treatment with erbium borohydride [Er(BH₄) ₃(THF)₃] afforded the mono(iminopyrrolyl) complex [η²-{2-(2, 6-iPr₂C₆H₃N=CH)- C₄H₃N}Er(η³-BH₄) ₂(THF)₂] (3). In the solid-state structures, the samarium complex 2 shows a rarely observed η¹ and the erbium complex 3 shows a usual η³ coordination mode of the borohydrido ligand. Copyright

Full text of DOI:10.1002/zaac.201200426

ARTICLE
DOI: 10.1002/zaac.201200426
Syntheses and Characterization of Samarium and Erbium Borohydrido
Complexes Supported by N-Aryliminopyrrole Ligand
Kishor Naktode,[a] Ravi K. Kottalanka,[a] and Tarun K. Panda*[a]
Dedicated to Dr. Arogya Varam Saha on the Occasion of His 60th Birthday
Keywords: Iminopyrrole; Samarium; Erbium; Borohydrides; Steric crowding; Metallacyle
Abstract. Reaction of potassium salt of N-aryliminopyrrole ligand the mono(iminopyrrolyl) complex [η2-{2-(2,6-iPr2C6H3N=CH)–
[2-(2,6-iPr2C6H3N=CH)–C4H3NK] (1) with samarium tris-boro-hy-
dride [Sm(BH4)3(THF)3] gave a samarium ate complex [η2-{2-(2,6-
iPr2C6H3N=CH)–C4H3N}3Sm(η1-BH4){K(THF)6] (2); whereas sim-
ilar treatment with erbium borohydride [Er(BH4)3(THF)3] afforded
C4H3N}Er(η3-BH4)2(THF)2] (3). In the solid-state structures, the sa-
marium complex 2 shows a rarely observed η1 and the erbium complex
3 shows a usual η3 coordination mode of the borohydrido ligand.
ene carbonate (TMC).[23] Secondly, a large number of lantha-
nide borohydride derivatives, which also have a high catalytic
potential are prepared from [Ln(BH4)3(THF)n] making lantha-
nide trisborohydrides as valuable starting materials. Even
though various diamagnetic borohydride complexes with other
lanthanide elements are studied extensively, the studies of sa-
marium and erbium borohydride complexes are limited due to
the paramagnetic nature of the complexes.[24] To have con-
trolled reactivity of the lanthanide borohydrido complexes a
supporting ligand plays an important role. In this context the
tunable ancillary ligands such as pyrrolyl-based ligands have
been proved wide range of versatility in organolanthanide
chemistry.[25] Recent reports from Hou et al. demonstrated that
Introduction
In the last 20 years, organolanthanide complexes are used as
catalysts extensively, especially in favor of polymerization.[1,2]
Even though the lanthanide hydrides have already been proved
to be powerful catalysts for many interesting chemical trans-
formations, the challenging problem remains for their access,
since they are mostly reactive.[3] In recent time, the search for
alternatives to the ubiquitous [Ln]–H or [Ln]–R functions, i.e.
the in situ alkylation of a pre-catalyst, or the use of borane
supported hydrides [Ln]–HBR3 or borohydrides [Ln]–BH4
have appeared as new trends.[4,5] In these alternative methods,
complexes bearing a [Ln]–BH4 function are easily accessible
and more robust in contrast to [Ln]–H compounds.[6] In gene- the lanthanide aminobenzyl complexes having pyrrolide moi-
ety count for a high catalytic activity and stereocontrol ability
for the polymerization of styrene, and in contrast, metal ami-
nobenzyl complexes showed no catalytic activity for the styr-
ene polymerization.[26] Mashima and co-workers also reported
that the rare-earth metal amides with pyrrolyl moieties showed
high catalytic activities on the ROP of ε-caprolactone, but
homoleptic tris(pyrrolyl) yttrium complexes had no activity,
indicating initiation step did not proceed.[27] Wang et al. also
demonstrated various rare-earth metal complexes with the tris
iminopyrrolyl ligands and their catalytic activities on the ROP
of ε-caprolactone.[28] Roesky et al. also showed clearly that
pyrrolyl ligands can serve as unique monoanionic nitrogen li-
gands in coordination chemistry of transition metal and rare
earth metals.[29] Thus to have lanthanide borohydrido com-
plexes having pyrrolyl imine ligands into the coordination
sphere can serve as unique example of hybrid pyrrole imino-
borohydrido lanthanide complex.
ral borohydride compounds of various metals have been re-
cently proved their merits as potential hydrogen storage mate-
rial.[7] The lanthanide trisborohydrides were first prepared in
the early 1950s by reaction of rare earth metal alkoxides with
diborane B2H6.[8] In the 1980s Mirsaidov et al. reported a more
straight forward approach to access, [Ln(BH4)3(THF)3] from
the starting materials LnCl3 (Ln = La, Ce, Pr, Nd, Sm) and
NaBH4.[9–11] Today in preparative chemistry, the use of lantha-
nide trisborohydrides [Ln(BH4)3(THF)n] are restricted mainly
in two fields.[6] Firstly, they are widely applied as efficient
catalysts for the polymerization of isoprene,[12–15] styr-
ene,[14,16] and some polar monomers such as ε-caprolactone
(CL),[17–19] methyl methacrylate (MMA),[20–22] and trimethyl-
* Dr. T. K. Panda
Fax: + 91-40-23016032
E-Mail: tpanda@iith.ac.in
[a] Department of Chemistry
Indian Institute of Technology Hyderabad
Ordnance Factory Estate
Herewith we describe the preparation and structural charac-
terization of samarium and erbium borohydride complexes
supported by N-aryliminopyrrole ligand.
Yeddumailaram 502205, Andhra Pradesh, India
Z. Anorg. Allg. Chem. 2013, 639, (1), 73–76
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
73

K. Naktode, R. K. Kottalanka, T. K. Panda
ARTICLE
Table 1. Crystallographic details of [η2-{2-(2,6-iPr2C6H3N=CH)–
C4H3N}3Sm(η1-BH4){K(THF)6] (2) and [η2-{2-(2,6-iPr2C6H3-N=CH)–
Results and Discussion
Reaction of [2-(2,6-iPr2C6H3N=CH)–C4H3NK] (1) with sa- C4H3N}Er(η3-BH4)2(THF)2] (3)a).
marium tris-borohydride [Sm(BH4)3(THF)3][30] in THF at am-
2
3
bient temperature followed by filtration and evaporation of the
Formula
Formula weight
Crystal system
Space group
a /Å
b /Å
c /Å
α /°
C79H124BKN6O7Sm C25H45B2Er N2O2
solvent resulted in a samarium ate complex [η2-{2-(2,6-
iPr2C6H3N=CH)–C4H3N}3Sm(η1-BH4){K(THF)6}] (2) as col-
orless crystals in good yield after re-crystallization from THF/
pentane (Scheme 1). The title compound was characterized by
standard analytical/spectroscopic techniques and the solid-state
structure was established by single-crystal X-ray diffraction
analysis.
1470.11
orthorhombic
Pna21
594.51
trigonal
¯
R3
18.4419(5)
34.7908(9)
12.3954(4)
90
23.9043(11)
23.9043(11)
27.4845(12)
90
β /°
90
90
γ /°
90
2252.4(2)
4
1.257
150(2)
120
13601.0(11)
18
1.307
150(2)
V /Å3
Z
Density /mg·m3
T /K
Radiation
CuKα
CuKα
(λ = 1.54184Å)
6.427
(λ = 1.54184Å)
5.296
μ /mm–1
F(000)
3128
5454
Absorption correction
Reflections collected
Unique reflections
empirical
21908
12239 [Rint
0.0523]
98.2%
empirical
10520
5656 [Rint = 0.0343]
=
Completeness to
GOF
96.9%
1.10
1.052
Scheme 1. Synthesis of samarium and erbium complexes.
Refinement method
Full-matrix least-
Full-matrix least-
squares on F2
0.0479; 0.1549
squares on F2
1
Even though compound 2 is slightly paramagnetic, H and
R1; wR2
0.0721; 0.2058
13C{1H} NMR can be measured for characterization of 2 along
a) Programs used: SHELXS-97,[37] SHELXL-97.[38]
1
with elemental analysis. In the H NMR spectrum, a doublet
at δ = 1.14 ppm corresponds to 36 methyl protons. A septet
signal at δ = 3.18 is observed due to resonance of two iso- of 2.600(10) Å. To the best of our knowledge this the first
propyl methine protons. The iminopyrolyl moiety shows sig- example where borohydride anion shows η1 coordination
nals at expected regions for the protons attached to it. The mode towards central samarium(III) atoms.[24] Such kind of
–
most characteristic signal is the BH4 anion which shows a coordination modes are rarely observed in transition metal
broad peak at δ = –2.41 ppm which is significantly low fre- chemistry as well and one example is [Ru(H)(BH4)(CO)2-
quency shifted compared to that of some borohydride com- (P(C6H11)3)2].[33] A steric congestion around the central metal
plexes like [(C5HiPr4)Sm(BH4)2(THF)] (δ = –10.5 ppm),[31] atom presumably prevents the higher coordination mode for
[Cp*ЈSm{(p-tol)NN}(BH4)] (δ
=
–9.3 ppm) (Cp*Ј
=
the borohydride moieties. The central metal samarium is hepta-
C5Me4(nPr), (p-tol)NN
=
(p-tol)–NC(Me)CHC(Me)N(p- coordinated and adopts distorted capped octahedral arrange-
–
tol).[32] The solid-state structure was established by single ment, where BH4 group acts as a cap to the octahedron. The
crystal X-ray diffraction analysis. The structural parameters are average Sm–Npyr [Sm1–N1 2.533(7), Sm1–N3 2.433(7), Sm1–
given in Table 1. All hydrogen atoms are located in the differ- N5 2.460(7) Å] and Sm–Nimine distances [Sm1–N2 2.6227,
ence Fourier map and subsequently refined.
Sm1–N4 2.610(7), Sm1–N6 2.655(7) Å] are in the range of the
The samarium complex 2 crystallizes in the orthorhombic reported values [2.373(5) –2.561(5) Å] observed in analogous
space group Pna21 with four molecules of 2 and one molecule samarium complexes [(L-L)Sm2(THF)4] (L-L = C–C-bonded
of THF in the unit cell (Figure 1). The molecular structure of 1,2-benzenediamine N,NЈ-bis(pyrrolyl)methylene dimer).[34] In
2 consists of an ion pair composed of a [K(THF)6]+ cation the cationic part of the molecule, the potassium atom is hexa-
and an [η2-{2-(2,6-iPr2C6H3N=CH)–C4H3N}3Sm(η1-BH4)]– coordinate by six THF molecules to adopt an octahedron ar-
anion. In the anionic part of the molecule, the central metal rangement.
samarium is coordinated by three bidentate N-aryliminopyrro-
When compound 1 was treated with erbium tris-boro-
lyl ligand moieties through the chelation of two nitrogen atoms hydride [Er(BH4)3(THF)3][9] in THF at ambient temperature,
of the each ligand. Thus three five member rings (Sm1–N1– an erbium borohydride complex of composition [η2-{2-(2,6-
C4–C5–N2; Sm1–N3–C21–C22–N4; Sm1–N5–C38–C39–N6) iPr2C6H3N=CH)C4H3N}Er(η3-BH4)2(THF)2] (3) was obtained
are formed due to the coordination of the three iminopyrrolyl in good yield as pink crystals (Scheme 1).
ligands onto the samarium atom. Most interesting is the coordi-
As the title compound 3 is highly paramagnetic, 1H or
–
nation mode of the BH4 group onto the central samarium 13C{1H}NMR spectra were not obtained and therefore 3 was
–
atom. It is observed that BH4 group is coordinated to the sa- fully characterized by FT-IR spectroscopy, elemental analysis,
marium atom through η1 fashion with a Sm1–B1 bond length and the solid-state structure was established by single crystal
74
www.zaac.wiley-vch.de
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2013, 73–76

Sm and Er Borohydrido Complexes Supported by N-Aryliminopyrrole
Figure 1. Molecular structure of compound 2 showing atom labeling
scheme, ellipsoids are drawn to encompass 30% probability, hydrogen
atoms are omitted except H1, H2na, H2nb, H2Nc. Shown is only the
anionic part of the molecule. Selected bond lengths /Å or angles /°:
Sm1–N1 2.453(7), Sm1–N2 2.622(7), Sm1–N3 2.433(7), Sm1–N4
Figure 2. Molecular structure of compound 3 showing atom labeling
scheme, ellipsoids are drawn to encompass 30% probability, hydrogen
2.610(7), Sm1–N5 2.460(7), Sm1–N6 2.655(7), Sm1–H1 1.23(5), atoms are omitted except H2, H1a, H1d, H1e. Selected bond lengths
Sm1–B1 2.600(10), N1–Sm1–N2 66.9(2), N3–Sm1–N4 66.7(2), N5– /Å or angles /°: Er1–N1 2.336(5), Er1–N2 2.482(4), Er1–B1 2.535(7),
Sm1–N6 66.7(2), N1–Sm1–N3 77.3(2), N1–Sm1–N4 142.2(2), N1– Er1–B2 2.535(7), Er1–O1 2.345(4), Er1–O2 2.353(4), Er1–H1a
Sm1–N5 76.4(3), N1–Sm–N6 83.8(2).
2.53(8), Er1–H1d 2.38(8), Er1–H1e 2.39(8), Er1–H1c 2.30(7), Er1–
H1f 2.37(7), Er1–H1b 2.34(6), N1–Er1–N2 69.65(15), N1–Er1–B1
158.0(2), N1–Er1–B2 96.4(2), N2–Er1–B1 88.4(2), N2–Er1–B2
166.0(2), N1–Er1–O1 83.15(14), N1–Er1–O2 83.41(15), N2–Er1–O1
87.96(13), N2–Er1–O2 87.10(14), O1–Er1–O2 166.56(14).
X-ray diffraction analysis. In the FT-IR spectrum a strong ab-
sorption at 2460–2200 cm–1 was observed and assigned as the
characteristic peak for ν(B–H) and indicates η3-coordination
–
mode of the BH4 anions.[24] The erbium complex 3 crys-
Experimental Section
¯
tallizes in the trigonal space group R3 having eighteen mole-
cules in the unit cell (Figure 2). All hydrogen atoms are located All manipulations of air-sensitive materials were performed with the
rigorous exclusion of oxygen and moisture in flame-dried Schlenk-
type glassware either on a dual manifold Schlenk line, interfaced to a
high vacuum (10–4 Torr) line, or in an argon-filled M. Braun glove
box. THF was pre-dried with sodium wire and distilled in a nitrogen
atmosphere from sodium and benzophenone ketyl prior to use. Hydro-
carbon solvents (toluene and n-pentane) were distilled in a nitrogen
atmosphere from LiAlH4 and stored in the glove box. 1H NMR
(400 MHz) and 31P{1H} NMR (161.9 MHz) spectra were recorded
with a BRUKER AVANCE III-400 spectrometer. BRUKER ALPHA
FT-IR was used for FT-IR measurement. Elemental analyses were per-
in the difference Fourier map and subsequently refined. Unlike
samarium complex 2, complex 3 is a neutral molecule and only
one bidentate iminopyrrolyl ligand is coordinated to the central
erbium atom to form the coordination polyhedron. Thus one
five-membered ring Er1–N1–C4–C5–N2 is formed due to co-
ordination of pyrrolyl nitrogen and imine nitrogen atoms. The
distances of Er1–N1 [2.336(5) Å] and Er1–N2 [2.482(4) Å] are
in the range of reported values of 2.32(8) and 2.559(4) Å for
[{(Me3Si)2NC(NCy)2}Er(BH4)2(THF)2].[35] Additionally two
borohydride ligands are attached to the erbium atom in a η3- formed with a BRUKER EURO EA at the Indian Institute of Technol-
ogy Hyderabad. [(ImPDipp)K] (1),[34] [Sm(BH4)3(THF)3],[30] and
fashion showing a bond length of Er1–B1 2.535(7) Å. This
kind of coordination mode is typical for Ln–BH4 com-
[Er(BH4)3(THF)3][9] were prepared according to the literature pro-
pounds.[6,24,36] In the coordination polyhedron two THF mole-
cedure.
cules are also binding to the erbium metal [Er1–O1 2.345(4) Å,
Preparation of [η2-{2-(2,6-iPr2C6H3N=CH)–C4H3N}3Sm(η1-BH4)-
Er1–O2 2.353(4) Å] to give an overall pseudo-octahedral ar-
{K(THF)6] (2): (ImPDipp)K (1) (300 mg, 1.03 mmol) and (141 mg,
0.34 mmol) of [Sm(BH4)3(THF)3] were dissolved in THF and the reac-
tion mixture was stirred at ambient temperature for 18 h. White pre-
cipitate separated by filtration, and the solvent was evaporated in vacuo
giving a pale yellow powder. Yield 225 mg, 47%. Single crystals were
obtained by recrystallization from THF/pentane at –35 °C. 1H NMR
(400 MHz, C6D6, 25 °C) : δ = 1.14 (d, JH,H = 6.68 Hz, 36 H, CHMe2),
3.18 (sept, JH,H = 6.5 Hz, 6 H, CHMe2), 6.08 (s, 3 H, 4-pyr), 6.15 (s,
3 H, 3-pyr), 6.41 (s, 3 H, 5-pyr), 6.93 (m, 9 H, m,p-C6H3), 7.77 (s, 3
H, N=C–H), 0.29 (s, 3 H, H–BH3), –2.41 (s, 1 H, Sm-H-BH3), 3.55
(br., 24 H, thf), 1.39(br., 24 H, thf) ppm. 13C NMR (101.6 MHz, C6D6,
25 °C): δ = 24.36 (12C, CHMe2), 26.94 (6C, CHMe2), 108.94 (3C, 5-
pyr), 115.23 (3C, 4-pyr), 122.35 (3C, 3-pyr), 122.09 (6C, m-C6H3),
123.36 (3C, p-C6H3), 129.12 (3C, ipso-pyr), 137.44 (6C, o-C6H3),
–
rangement around the central erbium atom considering BH4
as pseudo monodentate ligands.
Conclusions
We have prepared samarium and erbium borohydride com-
plexes supported by N-aryliminopyrrole ligand. In the samar-
ium borohydride complex we observe an unusual η1-coordina-
tion mode of BH4– ligand, whereas for the erbium complex we
–
observe usual η3-cordination mode of the same BH4 group
presumably due to the difference in ion radii of the central
metal atoms.
Z. Anorg. Allg. Chem. 2013, 73–76
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
75

K. Naktode, R. K. Kottalanka, T. K. Panda
ARTICLE
[9] U. Mirsaidov, G. N. Boiko, A. Kurbonbekov, A. Rakhimova,
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148.15 (3C, ipso-C6H3), 151.21(3C, N=C–H), 24.26(12C, thf),
66.45(12C, thf) ppm. IR (selected peaks): ν˜ = 860 (m), 1034 (s), 1134
(m), 1311 (w), 1625 (vs), 2279 (w), 2867 (m), 2960 (s).
C79H124BKN6O7Sm (1470.85): calcd. C 64.54, H 8.50, N 5.72%;
found C 63.97, H 8.14, N 5.32%.
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(THF)2] (3): (ImPDipp)K (1) (100 mg, 0.34 mmol) and [Er(BH4)3-
(THF)3] (146.37 mg, 0.34 mmol) were dissolved in THF and the reac-
tion mixture was stirred at ambient temperature for another 18 h. Fil-
tration and evaporation of thf in vacuo gives a pink color crystal. Sin-
gle crystals were obtained by recrystallization from THF/pentane at
–35 °C. Yield 142 mg, 70%. IR (selected peaks): ν˜ = 801 (s), 1086
(m), 1166 (s), 1458 (m), 1573 (vs), 2227 (m), 2297 (w), 2446 (m),
2866 (s), 2961 (vs). C25H45B2ErN2O2 (594.5): calcd. C 50.51, H 7.63,
N 4.71%; found C 49.89, H 7.45, N 4.22%.
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X-ray Crystallographic Analyses: Single crystals of compounds 2
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of suitable dimensions was mounted on a CryoLoop (Hampton Re-
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stream at 150(2) K. All measurements were made with a Oxford Su-
pernova X-Calibur Eos CCD detector with graphite-monochromatic
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2
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2
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2
2 2
4
tion R1 and wR2 were (Σ||Fo|–|Fc||)/Σ|Fo| and [Σw(Fo –Fc ) /Σ(wFo )]1/2
,
respectively. The ORTEP-3 program was used to draw the molecule.
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Crystallographic data (excluding structure factors) for the structures in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
numbers CCDC-898517 (2) and CCDC-898518 (3) (Fax: +44-1223-
336-033; E-Mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Acknowledgements
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