The synthesis of a sterically hindered samarium(II) bis(amidinate) and conversion to its homoleptic trivalent congener

Article abstract of DOI:10.1039/b501447f

The first divalent samarium bis(amidinate) has been prepared and aspects of its novel chemistry, including the preparation of a sterically hindered homoleptic Sm(III) tris(amidinate), explored. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b501447f

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The synthesis of a sterically hindered samarium(II) bis(amidinate) and
conversion to its homoleptic trivalent congener{
Marcus L. Cole{ and Peter C. Junk*
Received (in Cambridge, UK) 28th January 2005, Accepted 17th March 2005
First published as an Advance Article on the web 7th April 2005
DOI: 10.1039/b501447f
2.395(4)–2.426(4) s,9 six-coordinate ionic radii; Sm2+ 1.18 s,
The first divalent samarium bis(amidinate) has been prepared
Sm3+ 0.96 s).10
and aspects of its novel chemistry, including the preparation of
a
sterically hindered homoleptic Sm(III) tris(amidinate),
explored.
Samarium compounds have dominated research in the area of
divalent lanthanoid organometallic species ever since Kagan’s
seminal report of SmI2 as a coupling/reducing agent in organic
synthesis.1 To this end, the samarocene family of compounds
(SmCp92, Cp9 5 a cyclopentadienide), in particular the decaalkyl-
samarocenes, have attracted considerable attention as one-electron
reductants in the organometallic arena.2 By contrast, developments
using other ligand supports have been sparse. Amidinates
[{R1NC(R2)LNR1}2] represent a sterically and electronically
tuneable family of ligands that, owing to commensurate size–
charge characteristics to the (C5R5)2 donor set, can be considered
Cp9 analogues.3,4 Surprisingly, no divalent samarium bis(amidi-
nate) complexes have been reported.5 Given the ease by which
amidinates can be modified (e.g. inclusion of chiral, electron
withdrawing or sterically demanding moieties)3 and the proposed
participation of organosamarium species in several Sm(II)
mediated C–C coupling reactions (e.g. Barbier and
Reformatzky),6 paths to this compound class are attractive to a
broad synthetic audience. Herein we describe the three-way
synthesis of a sterically hindered Sm(II) bis(amidinate) and some
preliminary studies of its novel chemistry.
Scheme 1 Reagents and conditions: (i) X 5 Na, 1.0 eq. [Sm(I)2(THF)2],
22.0 eq. NaI, THF, RT, 2 h; (ii) X 5 H, .1.0 eq. Sm0, 1.0 eq.
[Hg(C6F5)2], 21.0 eq. Hg0, 22.0 eq. C6F5H, THF, RT, 12 h; (iii) X 5 H,
1.0 eq. [Sm{N(SiMe3)2}2(THF)2], 22.0 eq. HN(SiMe3)2, THF, RT, 2 h;
(iv) 0.5 eq. [Sm(I)2(THF)2], 1.0 eq. NaI, 20.5 eq. ‘‘Sm0’’, THF, RT, 1 day;
(v) hexane, 21/3 eq. [SmI3(THF)3.5], 21.0 eq. NaI, 35 uC–RT; (vi) 0.5 eq.
[Hg(C6F5)2], 0.5 eq. DippFormH, 21.0 eq. ‘‘C6F4’’, THF, RT, 24 h.
As illustrated in Scheme 1, [Sm(DippForm)2(THF)2] (1)
[DippForm 5 {(2,6-iPr2C6H3)NC(H)LN(2,6-iPr2C6H3)}2] can be
prepared in high yield by reaction of sodium metalated DippForm
with [Sm(I)2(THF)2],§ the one-pot reaction of excess samarium
metal with bis(pentafluorophenyl)mercury7 and DippFormH,
or
transamination
of
[Sm{N(SiMe3)2}2(THF)2]
and
DippFormH in tetrahydrofuran.{ Structural data indicate dark
green 1 consists of cisoid-[Sm(DippForm)2(THF)2] units (see
Fig. 1){" that are isomorphous to the related alkaline earth
compounds [M(DippForm)(THF)2], where M 5 Sr or Ba.8
The DippForm ligands coordinate in an g2-fashion with samarium
to nitrogen bond lengths [Sm(1)–N(1) 2.529(3) s, Sm(1)–N(4)
2.617(3) s] that are necessarily longer than those observed in
related trivalent samarium guanidinate species (e.g. five-
coordinate
[Sm(C{N(SiMe3)2}{N(c-C6H11)}2)2{CH(SiMe3)2}];
Fig. 1 Molecular structure of 1, POV-RAY illustration, 40% thermal
ellipsoids, all hydrogen atoms omitted for clarity. Selected bond lengths
(s) and angles (u): Sm(1)–N(1) 2.529(3), Sm(1)–N(4) 2.617(3), Sm(1)–O(1)
2.560(3), Sm(1)–O(2) 2.599(3), N(1)–C(25) 1.323(4), N(2)–C(25) 1.317(4),
N(1)–Sm(1)–N(2) 52.9(1), N(1)–C(25)–N(2) 120.6(3), O(1)–Sm(1)–O(2)
79.1(1) O(1)–Sm(1)–C(25) 103.3(1), O(1)–Sm(1)–C(50) 111.1(1), C(25)–
Sm(1)–C(50) 134.9(1).
{ Electronic supplementary information (ESI) available: full experimental
and X-ray structure determination data for compounds 1–4. See http://
www.rsc.org/suppdata/cc/b5/b501447f/
{ Present address: Department of Chemistry, University of Adelaide,
South Australia 5005, Australia.
*Peter.Junk@sci.monash.edu.au
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During syntheses of 1 by salt elimination [Scheme 1 (i)] small
quantities (ca. 5%) of colourless crystalline co-product (2) were
repeatedly isolated after further work-up of reaction media.
Further to 1H NMR spectra, which indicate an approximate
THF:DippForm ratio of ca. 3:1,{ and a coloration indicative of
trivalent samarium (i.e. loss of dark divalent colour), 2 was
spatial bulk of the Schiff bases contribute to the redox process
observed. Further, the steric congestion of 3, as illustrated in
Fig. 2(b), and our failure to generate homoleptic lanthanoid
complexes of DippForm by salt elimination16 make the redis-
tribution step noteworthy.
Due to the unexpected generation of 3 from 2, direct synthesis of
3 using divalent 1 [see Scheme 1 (vi)]{ in redox transmetallation/
ligand exchange was attempted.7 Previous studies of this type using
HDippForm, the Ln (5 lanthanoid) elements La, Nd or Tm
and [Hg(C6F5)2] in a 3:1:1.5 ratio provide the monomeric
fluoride bis(amidinate) complexes [Ln(F)(DippForm)2(THF)], by
identified
by
XRD
methods
as
the
samarate
[Na(THF)5][Sm(I)2(DippForm)2(THF)].{" Compound 2 presum-
ably arises from coordination of sodium iodide to a trivalent
[Sm(I)(DippForm)2(THF)n] intermediate. It is possible that this is
generated by disproportionation of excess SmI2 with 1 to yield
elemental samarium as a co-product,11 however at this early stage
other redox paths cannot be discounted. To further investigate, 0.5
molar equivalents of [Sm(I)2(THF)2] were added to a pre-prepared
solution of 1 [generated in situ by (i), Scheme 1 under meticulously
anaerobic conditions] as an intentional synthesis of 2.{ This
resulted in gradual loss of the dark green colour of 1 over a period
of 24 hours to give 2 in moderate yield.12{ Dissolution in hexane,
to effect loss of NaI from 2, resulted in redistribution§ to give
homoleptic [Sm(DippForm)3] (3) with concomitant precipitation
of NaI and [Sm(I)3(THF)3.5].13 Recrystallisation of the mother
liquor from toluene yielded samples of 3 suitable for X-ray
structure determination (see Fig. 2).{"
The considerable buttressing about the samarium of 3 is
evidenced by extended Sm–N bonds relative to other six-
coordinate trivalent samarium compounds [Sm(1)–N(3) 2.462(6)
s, Sm(1)–N(6) 2.467(6) s], and uncharacteristic twisting14 of the
2,6-iPr2C6H3 groups, such that they lie non-perpendicular to the
SmNCN metalacyclic planes [1; C(1)–C(6) ring 59.5(2)u, C(13)–
C(18) ring 56.8(2)u]. Indeed, the geometry about the samarium
centre is near trigonal planar if one considers the DippForm
ligands single point donors located at the carbon of the 1,3-
diazaallyl unit [S C–Sm–C angles 5 359.9(6)u].
heterolytic cleavage of
a
2-position C–F bond of
a
[Ln(C6F5)(DippForm)2(THF)n] intermediate,17 however these
metals do not possess a typically stable divalent oxidation
state. Unfortunately, as per the aforementioned metals, a
[Ln(F)(DippForm)2(THF)] species, where Ln 5 Sm (4),{ was
isolated in high yield indicating an analogous C–F activation
mechanism.17 As depicted in Fig. 3, complex 4 is a discrete
monomer of composition [Sm(F)(DippForm)2(THF)] with similar
geometry to 1.{" Akin to 1 and 3, the coordination environment
about the metal centre can also be described using the 1,3-
diazaallyl carbons as point donors. This provides a near
tetrahedral geometry [1 C(25)–Sm(1)–O(1) 103.3(1)u, C(50)–
Sm(1)–O(1) 111.1(1)u; 4 C(25)–Sm(1)–F(1) 105.0(1)u, C(25)–
Sm(1)–O(1) 111.9(1)u], in which the Sm–N bond lengths differ
from those of 1 in a manner consistent with a transition from di-
to trivalent samarium{ [Sm(1)–N(1) 2.443(3) s, Sm(1)–N(2)
2.454(3) s].9 The Sm–F bond compares well to the only literature
example of a terminal samarium fluoride, a seven coordinate bis-
Tp [Tp 5 hydrido tris(pyrazolyl)borate] supported complex from
Takats and Sella [2.090(7) s; six coordinate 4 2.093(2) s].18
Location of a single broad 19F{1H} NMR resonance at 224.8 ppm
The redistribution of bis(amide) supported lanthanoid halides
when extracted into low polarity solvents, as per the formation of 3
from 2, is not unusual;15 however disproportionation of divalent to
tri- and zero valent samarium, tentatively the source of 2,12 has
limited precedent, there being one literature example using
tetradentate Schiff bases as support ligands for Sm(II).11
Interestingly, the authors of this report suggest that the significant
Fig. 3 Molecular structure of 4, POV-RAY illustration, 40% thermal
ellipsoids, all hydrogen atoms omitted for clarity. Selected bond lengths
(s) and angles (u): Sm(1)–F(1) 2.093(2), Sm(1)–O(1) 2.457(2), Sm(1)–N(1)
2.443(3), Sm(1)–N(2) 2.454(3), N(1)–C(25) 1.320(4), N(2)–C(25) 1.340(4),
O(1)–Sm(1)–F(1) 82.2(1), N(1)–Sm(1)–N(2) 55.6(1), N(1)–C(25)–N(2)
118.1(3), O(1)–Sm(1)–C(25) 111.9(1), O(1)–Sm(1)–C(50) 105.1(1), F(1)–
Sm(1)–C(25) 105.0(1), F(1)–Sm(1)–C(50) 117.9(1), C(25)–Sm(1)–C(50)
126.2(1).
Fig. 2 (a) Molecular structure of 3, POV-RAY illustration, 40% thermal
ellipsoids, all hydrogen atoms and lattice solvent omitted for clarity.
Selected bond lengths (s) and angles (u): Sm(1)–N(1) 2.448(6), Sm(1)–N(3)
2.462(6), Sm(1)–N(5) 2.467(6), N(1)–C(25) 1.331(8), N(2)–C(25) 1.351(8),
N(1)–Sm(1)–N(2) 56.5(2), N(1)–C(25)–N(2) 120.1(8), C(25)–Sm(1)–C(50)
120.0(2), C(25)–Sm(1)–C(75) 118.9(2), C(50)–Sm(1)–C(75) 121.0(2). (b)
Space filling depiction of 3 [same perspective as (a)].
2696 | Chem. Commun., 2005, 2695–2697
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8189.3(3) s3, Z = 4, Dc = 1.193 g cm23, F000 = 3136, m = 0.765 mm21
,
confirms the inclusion of a fluoride ligand.{ This differs
considerably to the reported 19F NMR resonance of the
aforementioned Tp compound by some margin (2172.26 ppm).18
However, this is not unexpected due to the direct metal contact of
the fluoride to paramagnetic samarium(III) in both instances.
We would like to thank the Australian Research Council for
continued financial support and Prof. Glen B. Deacon for
invaluable discussions.
2hmax = 56.74u, 44716 reflections collected, 18160 unique (Rint = 0.1993).
Final GooF = 0.956, R1 = 0.0938, wR2 = 0.1435, R indices based on 6734
reflections with I > 2s(I) (refinement on F2), 939 parameters, 0 restraints.
Crystal data for 4: C54H78FN4OSm, M = 968.55, monoclinic, P21/n (No.
14), a = 20.4714(2), b = 12.1996(2), c = 21.6593(3) s, b = 110.0650(10)u, V =
5080.93(12) s3, Z = 4, Dc = 1.266 g cm23, F000 = 2036, m = 1.199 mm21
,
2hmax = 56.48u, 32459 reflections collected, 12173 unique (Rint = 0.0518).
Final GooF = 1.044, R1 = 0.0605, wR2 = 0.1572, R indices based on 10645
reflections with I > 2s(I) (refinement on F2), 566 parameters, 0 restraints.
CCDC 262796–262799. See http://www.rsc.org/suppdata/cc/b5/b501447f/
for crystallographic data in CIF or other electronic format.
Marcus L. Cole{ and Peter C. Junk*
School of Chemistry, Monash University, Victoria 3800, Australia.
E-mail: Peter.Junk@sci.monash.edu.au; Fax: +61 (0)3 9905 4597
1 J.-L. Namy, P. Girard and H. B. Kagan, Nouv. J. Chim., 1977, 1, 5.
2 W. J. Evans, I. Bloom, W. E. Hunter and J. L. Atwood, J. Am. Chem.
Soc., 1981, 103, 6507; W. J. Evans, R. A. Keyer and J. W. Ziller,
J. Organomet. Chem., 1990, 394, 87.
Notes and references
§ Method (i) for preparation of 1: a tetrahydrofuran (40 cm3) solution of
[Na(DippForm)(THF)3] (0.72 g, 1.19 mmol) was added dropwise to a
cooled (ca. 0 uC) deep blue solution of [Sm(I)2(THF)2] (0.33 g, 0.60 mmol),
also in tetrahydrofuran (50 cm3). The resulting deep green solution was
gradually warmed to ambient temperature and stirred for two hours.
Filtration, followed by removal of all volatiles in vacuo, gave a green
powder that was extracted into toluene (10 cm3) and placed at 210 uC
overnight to yield deep green rhombohedral plates of 1 [0.43 g, 70% by
{Na(DippForm)(THF)3}], m.p. 201 uC (dec.). Samarium analysis (%) calcd
for C58H86N4O2Sm: Sm 14.72; found: Sm 14.58; IR (Nujol): 1932 w sh,
1866 w sh, 1798 w sh, 1667 m sh, 1602 m, 1468 s br, 1389 s, 1365 s, 1272 m,
1231 s, 1108 m, 1009 m, 946 m, 917 m, 873 w, 829 w, 798 m sh, 756 s sh,
728 s sh, 687 s sh cm21; 1H NMR (C6D6, 300 K): d 5 8.90 (br s, 8H; CH,
iPr), 7.45–6.9 (br m, 12 H; Ar–H), 6.3 [br s, 2H; NC(H)N], 3.45 (s br, 8H;
OCH2, THF), 3.22 (br s, 48H; CH3, iPr), 1.65 (br s, 8H; CH2, THF).
Method for preparation of 3: dissolution of 2 (0.43 g, 0.27 mmol) into
warm (35 uC) hexane (40 cm3) resulted in immediate precipitation of NaI
and [SmI3(THF)3.5] to leave 3 in solution. Filtration, followed by removal
of volatiles in vacuo, yielded colourless 3 as a fine powder. Extraction into
toluene (10 cm3), followed by placement at 210 uC overnight, gave 3 as
small, light yellow, irregular prisms (0.19 g, 72%), m.p. 221 uC. Samarium
analysis (%) calcd for C75H105N6Sm (3 without lattice toluene): Sm 12.12;
found: Sm 11.89; IR (Nujol): 1932 w sh, 1865 w sh, 1798 w sh, 1665 m br,
1567 m, 1478 m, 1380 m sh, 1362 m, 1331 m sh, 1286 m, 1257 m, 1234 m
sh, 1000 w sh, 956 w sh, 820 m, 797 s sh, 766 m sh, 753 s sh cm21; 1H NMR
(C6D6, 300 K): d 5 10.01 [br s, 3H; NC(H)N], 7.40–6.87 (br m, 18H; Ar–
H), 4.01 (br s, 12H; CH, iPr), 1.45 (br s, 72H; CH3, iPr).
3 F. T. Edelmann, Coord. Chem. Rev., 1994, 137, 403.
4 R. Kempe, Angew. Chem. Int. Ed., 2000, 39, 468.
5 Divalent ytterbium complexes supported by 1,3-bis(trimethylsilyl)ben-
zamidinate ligands have been reported: M. Wedler, M. Noltemeyer,
U. Pieper, H.-G. Schmidt, D. Stalke and F. T. Edelmann, Angew.
Chem., Int. Ed. Engl., 1990, 29, 894. The synthetic path described utilises
in situ prepared sodium 1,3-bis(trimethylsilyl)benzamidinates.
6 G. A. Molander and C. Kenny, J. Org. Chem., 1991, 56, 1439;
D. P. Curran and M. J. Totleben, J. Am. Chem. Soc., 1992, 114, 6050;
J.-L. Namy, J. Collin, C. Bied and H. B. Kagan, Synlett, 1992,
733.
7 G. B. Deacon, C. M. Forsyth and S. Nickel, J. Organomet. Chem., 2002,
647, 50.
8 M. L. Cole and P. C. Junk, New J. Chem., 2005, 135.
9 Y. Zhou, G. P. A. Yap and D. S. Richeson, Organometallics, 1998, 17,
4397.
10 E. Fluck and K. G. Heumann, Periodic Table of the Elements, VCH,
Weinheim, Germany, 1991.
11 C. D. Be´rube´, S. Gambarotta, G. P. A. Yap and P. G. Cozzi,
Organometallics, 2003, 22, 434.
12 Aqueous quenching of the reaction medium for 2, followed by organic
work-up gave solely DippFormH. No ‘‘DippFormH’’ dimer com-
pounds, as would be consistent with reductive imine coupling by Sm(II),
were observed.
13 As confirmed by unit cell determination of single crystals from the
recrystallised precipitate from Scheme 1, (v): Z. Xie, K. Chiu, B. Wu and
T. C. W. Mak, Inorg. Chem., 1996, 35, 5957.
¯
" Crystal data for 1: C58H86N4O2Sm, M = 1021.66, triclinic, P1 (No. 2),
a = 12.1023(2), b = 12.7993(3), c = 19.6691(5) s, a = 84.4520(10), b =
14 M. L. Cole, A. J. Davies, C. Jones and P. C. Junk, J. Organomet. Chem.,
2004, 689, 3093.
86.800(2), c = 63.9530(10)u, V = 2724.19(10) s3, Z = 2, Dc = 1.246 g cm23
,
15 M. L. Cole and P. C. Junk, New J. Chem., 2003, 1032.
16 Preliminary data indicates the reaction of LnCl3 with three equivalents
of [Na(DippForm)(THF)3] in THF at reflux (Ln 5 La, Nd, Sm and Tb)
fails to provide lanthanoid tris(amidinates), instead yielding
[Ln(Cl)(DippForm)2(THF)n] species where n 5 1 (Ln 5 Nd, Sm and
Tb) or n 5 2 (Ln 5 La): M. L. Cole and P. C. Junk, unpublished
material.
17 M. L. Cole, G. B. Deacon, P. C. Junk and K. Konstas, Chem.
Commun., 2005, 1581–1583.
18 A. C. Hillier, X.-W. Zhang, G. H. Maunder, S. Y. Liu,
F000 = 1080, m = 1.121 mm21, 2hmax = 56.56u, 24871 reflections collected,
13029 unique (Rint = 0.0571). Final GooF = 1.044, R1 = 0.0542, wR2
=
0.904, R indices based on 10328 reflections with I > 2s(I) (refinement on
F2), 602 parameters, 0 restraints.
Crystal data for 2: C148H236I4N8O12Sm2, M = 3173.73, monoclinic, P21/c
(No. 15), a = 26.7632(3), b = 14.3697(2), c = 41.5794(5) s, b =
91.7860(10)u, V = 15982.8(3) s3, Z = 4, Dc = 1.319 g cm23, F000 = 6536, m =
1.560 mm21, 2hmax = 56.60u, 61917 reflections collected, 30037 unique
(Rint = 0.0572). Final GooF = 1.031, R1 = 0.0603, wR2 = 0.1742, R indices
based on 16943 reflections with I > 2s(I) (refinement on F2), 1618
parameters, 0 restraints.
ˆ
T. A. Eberspacher, M. V. Metz, R. McDonald, A. Domingos,
M. Marques, V. W. Day, A. Sella and J. Takats, Inorg. Chem., 2001,
40, 5106.
Crystal data for 3: C92.5H125N6Sm, M = 1471.34, monoclinic, P21/n (No.
14), a = 13.1662(2), b = 37.9133(8), c = 16.4994(3) s, b = 96.1070(10)u, V =
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