Synthesis and structural characterization of lithium and trimethyltin complexes of 2,6-bis(oxazolinyl)phenyl

Article abstract of DOI:10.1021/om049267n

The treatment of 2,6-bis(oxazolinyl)phenyl bromide (Phebox-Br) with n-BuLi affords a Phebox-Li complex. Subsequent transmetalation with [SnClMe ₃] affords a Phebox-Sn complex. The Phebox ligand can coordinate to a transition metal in various terdentate fashions; both the oxazoline oxygen and the imine nitrogen are perfectly positioned for chelation; NCN , OCO , or mixed terdentate coordination modes are theoretically possible using this ligand. The structural properties and NMR spectra of [Sn(Me,Me-Phebox)Me₃] (2) and [Li(R,R′-Phebox)] complexes 3a (R = R′ = Me), 3b (R = iPr, R′ = H), and 3c (R = tBu, R′ = H) were investigated. It was found that 2 exhibits no chelation of the Phebox ligand to the Sn center in this case. The [Li(R,R′-Phebox)] complex 3a has been crystallographically characterized and is in the form of a molecular dimer (i.e. [Li(Phebox)]₂), containing two formally three-center - two-electron bonds in a four-membered Li₂C₂ ring. The formal Phebox anion is bonded to the lithium cation via the two ortho imine N centers and the intraannular aromatic C atom. The ¹³C{¹H} NMR signal of C_ipso, being a seven-line pattern with coupling constant ¹J(¹³C-⁷Li) = 18 Hz, confirms that the dimeric structure is maintained in solution at room temperature. Variable-temperature (VT) NMR studies of 3a indicate that a fluxional process is occurring at room temperature, which can be frozen out below -16°C (ΔG^? = 56 kJ/mol). This fluxional process is not observed in VT-NMR studies on 3b,c. This is likely due to the presence of bulky (iPr or tBu) substituants that effectively shut down the pathways to rapid inversion of the puckering of the five-membered chelate ring.

Full text of DOI:10.1021/om049267n

Organometallics 2005, 24, 743-749
743
Synthesis and Structural Characterization of Lithium
and Trimethyltin Complexes of 2,6-Bis(oxazolinyl)phenyl
Marianne Stol,†,‡ Dennis J. M. Snelders,† Jeroen J. M. de Pater,†
Gerard P. M. van Klink,† Huub Kooijman,§ Anthony L. Spek,§,| and
Gerard van Koten*,†
Dutch Polymer Institute and Debye Institute, Department of Metal-Mediated Synthesis, and
Bijvoet Center for Biomolecular Research, Department of Crystal and Structural Chemistry,
Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
Received September 21, 2004
The treatment of 2,6-bis(oxazolinyl)phenyl bromide (Phebox-Br) with n-BuLi affords a
Phebox-Li complex. Subsequent transmetalation with [SnClMe3] affords a Phebox-Sn
complex. The Phebox ligand can coordinate to a transition metal in various terdentate
fashions; both the oxazoline oxygen and the imine nitrogen are perfectly positioned for
chelation; “NCN”, “OCO”, or mixed terdentate coordination modes are theoretically possible
using this ligand. The structural properties and NMR spectra of [Sn(Me,Me-Phebox)Me3]
(2) and [Li(R,R′-Phebox)] complexes 3a (R ) R′ ) Me), 3b (R ) iPr, R′ ) H), and 3c (R )
tBu, R′ ) H) were investigated. It was found that 2 exhibits no chelation of the Phebox
ligand to the Sn center in this case. The [Li(R,R′-Phebox)] complex 3a has been crystallo-
graphically characterized and is in the form of a molecular dimer (i.e. [Li(Phebox)]2),
containing two formally three-center-two-electron bonds in a four-membered Li2C2 ring.
The formal Phebox anion is bonded to the lithium cation via the two ortho imine N centers
and the intraannular aromatic C atom. The 13C{1H} NMR signal of Cipso, being a seven-line
1
pattern with coupling constant J(13C-7Li) ) 18 Hz, confirms that the dimeric structure is
maintained in solution at room temperature. Variable-temperature (VT) NMR studies of 3a
indicate that a fluxional process is occurring at room temperature, which can be frozen out
below -16 °C (∆Gq ) 56 kJ/mol). This fluxional process is not observed in VT-NMR studies
on 3b,c. This is likely due to the presence of bulky (iPr or tBu) substituents that effectively
shut down the pathways to rapid inversion of the puckering of the five-membered chelate
ring.
Introduction
class of metal binding agents. In the terdentate bonding
mode, Phebox ligands have three C, and O- or N-donor
sites in fixed positions. This is due to the fact that the
ortho-chelating ligands are connected to the aryl ring
by an sp2 C and one sp2 N or sp3 O center. For this
reason, the binding is predicted to be less flexible in
comparison to the wide variety of bonding modes
encountered in the aryldiamine ligands, the so-called
“NCN-pincer” ligands.1b,4 We were interested in the
structural properties, both in the solid state and in
solution, and the mode of coordination (C,N- vs C,O-
chelation) of the Phebox ligand in simple model com-
plexes incorporating Li or Sn metal.3e In a triorganotin
Chelating aryl ligands are an important class of
ligands in homogeneous catalysis1 and also in other
fields of interest.1b,2 One of the classes of chelating
ligands which is applied in homogeneous catalysis is the
monoanionic bis(oxazolinyl)phenyl or Phebox ligand. An
interesting feature of Phebox ligands is the ease of
synthesis of a number of chiral analogues. Phebox
ligands have been previously shown to coordinate in a
terdentate fashion;3 both the oxazoline oxygen and
nitrogen can potentially be positioned for chelation, and
hence, “NCN”, “OCO”, or mixed “NCO” terdentate
coordination modes are theoretically possible using this
(3) (a) Motoyama, Y.; Makihara, N.; Mikami, Y.; Aoki, K.; Nish-
iyama, H. Chem. Lett. 1997, 951-952. (b) Denmark, S. E.; Stavenger,
R. A.; Faucher, A. -M.; Edwards, J. P. J. Org. Chem. 1997, 62, 3375-
3389. (c) Motoyama, Y.; Mikami, Y.; Kawakami, H.; Aoki, K.; Nish-
iyama, H. Organometallics 1999, 18, 3584-3588. (d) Stark, M. A.;
Jones, G.; Richards, C. J. Organometallics 2000, 19, 1282-1291. (e)
Motoyama, Y.; Okano, M.; Narusawa, H.; Makihara, N.; Aoki, K.;
Nishiyama, H. Organometallics 2001, 20, 1580-1591. (f) Motoyama,
Y.; Shimozono, K.; Aoki, K.; Nishiyama, H. Organometallics 2002, 21,
1684-1696. (g) Motoyama, Y.; Kawakami, H.; Shimozono, K.; Aoki,
K.; Nishiyama, H. Organometallics 2002, 21, 3408-3416. (h) Mo-
toyama, Y.; Koga, Y.; Kobayashi, K.; Aoki, K.; Nishiyama, H. Chem.
Eur. J. 2002, 8, 2968-2975. (i) Fossey, J. S.; Richards, C. J. Organo-
metallics 2004, 23, 367-373.
* To whom correspondence should be addressed. Tel: +3130
2533120. Fax: +3130 2523615. E-mail: g.vankoten@chem.uu.nl.
† Debye Institute.
‡ Dutch Polymer Institute.
§ Bijvoet Center for Biomolecular Research.
| Address correspondence pertaining to crystallographic studies to
this author. E-mail: A.L.Spek@chem.uu.nl.
(1) For leading references see: (a) Singleton, J. T. Tetrahedron 2003,
59, 1837-1857. (b) Albrecht, M.; van Koten, G. Angew. Chem., Int.
Ed. 2001, 40, 3750-3781. (c) van der Boom, M. E.; Milstein, D. Chem.
Rev. 2003, 103, 1759-1792.
(2) (a) Albrecht, M.; Rodr´ıguez, G.; Schoenmaker, J.; van Koten, G.
Org. Lett. 2000, 2, 3461-3464. (b) Guillena, G.; Kruithof, C. A.; Casado,
M. A.; Egmond, M. R.; van Koten, G. J. Organomet. Chem. 2003, 668,
3-7.
(4) Rietveld, M. H. P.; Grove, D. M.; van Koten, G. New J. Chem.
1997, 21, 751-771.
10.1021/om049267n CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/15/2005

744 Organometallics, Vol. 24, No. 4, 2005
Stol et al.
Scheme 1. Synthesis of [Sn(Me,Me-Phebox)Me3]
in Apolar and Polar Solvents
Figure 1. Achiral and chiral Phebox-Br ligands.
Phebox complex, the Phebox fragment will be attached
to the Sn nucleus via a direct aryl C-Sn bond. The
question then arises as to how the ortho substituents
orient themselves relative to the electrophilic Sn center.
In an isolated Phebox-Li compound, a dimeric structure
is anticipated,5 although steric constraints could give
rise to novel or unusual structural features.
Results and Discussion
Three different Phebox ligands were synthesized in
this work. One achiral Phebox ligand (1a) was derived
from 2-amino-2-methyl-1-propanol, and two chiral Phe-
box ligands (1b,c, respectively; Figure 1) were obtained
from L-valinol and L-tert-leucinol.
2
pounds.8 The observed JH-Sn values indicate that the
tin center in 2 is tetracoordinated9 (i.e. with no signifi-
cant Sn-O or Sn-N coordination). This conclusion is
further corroborated by the molecular structure of 2 in
the solid state. Single crystals of 2 were obtained by slow
evaporation of a pentane solution of the complex at room
temperature. A view of the molecular structure is shown
in Figure 2.
2,6-Bis(4′,4′-dimethyl-2′-oxazolinyl)phenyl bromide
([Me,Me-PheboxBr], 1a), was synthesized using modi-
fied literature procedures.3e,6 Compound 1a can be
lithiated, via selective Li-Br exchange, with n-BuLi in
THF solution.3e After the product was treated with
[SnClMe3], [Sn(Me,Me-Phebox)Me3] (2) was obtained.3e
To study a possible solvent dependence of this reaction,
[Me,Me-PheboxBr] was reacted with n-BuLi in both
polar coordinating solvents (THF and diethyl ether) and
apolar solvents (pentane and toluene) (Scheme 1). The
reaction mixture was quenched with [SnClMe3]3e and
The molecule has an approximate local plane of
symmetry containing C(4), C(1), Sn, and C(18) (the
torsion angle C(18)-Sn-C(1)-C(6) is 99.63(17)°) (RMS
deviation of ideal positions (0.08 Å). The configuration
at tin can be described as a distorted tetrahedron, as is
shown by the bond angles around tin, where the angle
C(1)-Sn-C(18) is opened to 113.6° and the angle
C(17)-Sn-C(19) is compressed to 103.70(8)°. The Sn-
Me and Sn-Ar bond lengths correspond well with bond
lengths reported in the literature (2.08-2.19 and 2.15-
2.18 Å) for similar tin compounds.10 The oxazoline
substituents are rotated with respect to the plane
defined by the aryl ring by 27.19(10)° (substituent
containing O(1)) and 22.81(10)° (substituent containing
O(2)). The absence of additional coordination of the Sn
center by the oxazoline moieties is further indicated by
the position of the Sn center out of the plane of the aryl
ring. The angle C(4)-C(1)-Sn is 164.68(10)° (i.e. the
Sn center is “lifted” by ∼0.612(1) Å out of the least-
squares aryl plane). These structural features point to
steric interaction between the SnMe3 group and the
oxazoline ring substituents. There is no evidence of the
oxazoline N or O atoms interacting with the Sn center,
although this is a feature in other tetravalent Sn
compounds.10a,d The distances between the oxazoline
oxygens and the Sn center are 2.9842(13) and 3.0271-
(14) Å, respectively. These lengths are distinctively
1
the conversion studied by H NMR spectroscopy.
In all cases, selective Li-Br exchange occurred and
quantitative formation of 2 was observed. Thus, sup-
pression of the directed ortho metalation of 1a (i.e.
metalation at the aryl C-3 position) is accomplished by
these synthetic protocols.7
The 1H NMR spectrum of a solution of 2 (C6D6, room
temperature) reveals singlet resonances for both the
oxazoline methyl groups and ring methylene protons (δ
1.17 and 3.70 ppm, respectively). The Sn-Me groups
are centered at δ 0.46 ppm, with the diagnostic 117Sn/
119Sn satellites (2JH-Sn ) 26.7, 27.8 Hz) confirming the
presence of the SnMe3 group. It is known that the
magnitude of 1H-Sn spin-spin coupling constants are
a function of the coordination number in tin(IV) com-
(5) (a) Smeets, W. J. J.; Spek, A. L. Acta Crystallogr. 1987, C43,
1429-1430. (b) van der Zeijden, A. A. H.; van Koten, G. Recl. Trav.
Chim. Pays-Bas 1988, 107, 431-433. (c) Donkervoort, J. G.; Vicario,
J. L.; Rijnberg, E.; Jastrzebski, J. T. B. H.; Kooijman, H.; Spek, A. L.;
van Koten, G. J. Organomet. Chem. 1998, 550, 463-467. (d) Steen-
winkel, P.; James, S. L.; Grove, D. M.; Veldman, N.; Spek, A. L.; van
Koten, G. Chem. Eur. J. 1996, 2, 1440-1445. (e) Schlengerman, R.;
Sieler, J.; Jelonek, S.; Hey-Hawkins, E. Chem. Commun. 1997, 197-
198. (f) Stey, T.; Stalke, D. In The Chemistry of Organolithium
Compounds; Rappoport, Z., Marek, I., Eds.; Wiley: London, 2004. (g)
Gossage, R. A.; Jastrzebski, J. T. B. H.; van Koten, G. Angew. Chem.,
Int. Ed., in press.
(6) (a) Rodr´ıguez, G.; Albrecht, M.; Schoenmaker, J.; Ford, A.; Lutz,
M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2002, 124, 5127-
5138. (b) Vorbru¨ggen, H.; Krolikiewicz, K. Tetrahedron 1993, 49, 9353-
9372.
(7) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J.
1998, 4, 759-762.
(8) Jastrzebski, J. T. B. H.; van Koten, G. Adv. Organomet. Chem.
1993, 35, 257.
(9) 2JH-Sn is 26.2 and 27.5 Hz for [SnPhMe3].
(10) (a) Davenport, A. J.; Davies, D. L.; Fawcett, J.; Russel, D. R.
Dalton 2002, 3260-3264. (b) Schulte, M.; Gabbai, F. P. Chem. Eur. J.
2002, 8, 3802-3806. (c) Schilling, B.; Kaiser, V.; Kaufmann, D. E.
Chem. Ber. 1997, 130, 923-932. (d) Steenwinkel, P.; Jastrzebski, J.
T. B. H.; Deelman, B.-J.; Grove, D. M.; Kooijman, H.; Veldman, N.;
Smeets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics 1997,
16, 5486-5498.

2,6-Bis(oxazolinyl)phenyl Complexes of Li and Sn
Organometallics, Vol. 24, No. 4, 2005 745
Figure 2. Top and side-on views of the crystal structure of [Sn(Me,Me-Phebox)Me3]. Hydrogen atoms have been omitted
for clarity. Relevant bond distances (Å) and angles (deg): Sn-C(1) ) 2.1810(18), Sn-C(17) ) 2.150(2), Sn-C(18) ) 2.129-
(3), Sn-C(19) ) 2.153(2), Sn-O(1) ) 2.9842(13), Sn-O(2) ) 3.0271(14); C(1)-Sn-C(17) ) 106.03(8), C(1)-Sn-C(18) )
113.64(8), C(1)-Sn-C(19) ) 108.45(7), C(17)-Sn-C(18) ) 112.18(9), C(17)-Sn-C(19) ) 103.70(8), C(18)-Sn-C(19) )
112.18(9).
Scheme 2. Synthesis of [Li(R,R′-Phebox)]
From the crystal structure data, it is clear that [Me,-
Me-PheboxLi]2 (3a) is present as aggregate type A in
the solid state. In the related compound [(Et2O)LiC6H4C-
(O)N(iPr)2]2,12 CdO donation is preferred over iPr2N
donation. In our case, imine N coordination is preferred
over O donation, which is, to our knowledge, the first
crystallographically characterized organolithium com-
pound with intramolecular CdN chelates present in the
structure.13 Synthetically, it also appears that Li-Br
exchange is favored over n-BuLi addition to the CdN
double bond.14 In addition, the product ArLi species are
obviously also unreactive toward the imine functional-
ities. The Cipso-Li bond lengths (2.25-2.30 Å) are
slightly longer than in comparable structures with two
donating amine arms (2.19-2.24 Å).5 The opposite trend
is observed for the Li-N bond lengths. These are
somewhat shorter (1.99-2.01 Å) than distances found
in the literature for amine donor systems (2.07-2.13
Å).5 The Li‚‚‚Li distance (2.333(4) Å) is also shorter than
in the bis(amino)aryl systems (2.40-2.48 Å).5 The angles
between the nonplanar C2Li2 rhombus and the aryl
planes (42.39(11) and 42.74(11)°) are smaller than found
in other NCN chelating systems (56-59°).5 Elongation
of the Cipso-Li bond distances and shortening of the
Li-N and Li‚‚‚Li distances may be the result of a
stronger interaction of the Li centers with the imine
groups as opposed to the interaction with tertiary
amines. The acute angle Li-Cipso-Li (62°) is smaller
than comparable literature values (66-68°),5 as a result
of reduced repulsion between the Li atoms.
Complexes
longer than corresponding Sn-O distances reported
previously for tetraorganotin compounds, where weak
Sn-O interactions (2.770, 2.781 Å)11 are inferred. The
oxazoline N is most likely too sterically hindered to
approach the tin center, by virtue of the adjacent Me
groups in the ring, while the oxazoline O donor site is
probably too weak a donor for coordination.
For the structural characterization of [Li(R,R′-Phe-
box)]2 complexes, 3a (R ) R′ ) Me), 3b (R ) iPr, R′ )
H), and 3c (R ) tBu, R′ ) H) were synthesized and
isolated (Scheme 2).
To address the structural motif(s) of these organo-
lithium species, single crystals were obtained from a
saturated solution of 3a (Et2O, -30 °C). The molecular
structure, depicted in Figure 3, confirms the anticipated
dimeric form and shows distinct NCN chelation (cf. 2)
of the Phebox ligand.
Both aryl groups bridge the two lithium atoms via a
three-center-two-electron CipsoLi2 bonding interaction.
Both lithium atoms have distorted-tetrahedral geom-
etries, and the two Phebox ligands are almost perpen-
dicular to each other (angle beween the least-squares
planes through the ring atoms is 84.67(7)°) with the two
Li atoms above and below the plane of the aryl rings.
This type of structure can be in the form of three
different modes of chelation (Figure 4) with two different
donor atoms (O, N).
The 1H and 13C{1H} NMR spectra (room temperature)
of 3a-c suggest that all three [Li(R,R′-Phebox)]n com-
plexes have distinct dimeric structures in solution. The
13C{1H} NMR signal for Cipso is observed as a seven-
line pattern (intensities 1:2:3:4:3:2:1) and a coupling
1
constant J(13C-7Li) of 18 Hz. This pattern, as well as
1
the magnitude of JC-Li, is direct proof that Cipso is
bonded to two Li atoms. The above coupling constant is
similar to the value found for dimeric [Li(C6H3(CH2-
(12) Clayden, J.; Davies, R. P.; Hendy, M. A.; Snaith, R.; Wheatley,
A. E. H. Angew. Chem., Int. Ed. 2001, 40, 1238-1240.
(13) We are aware of the compound [(Et2O){LiC6H4P(Ph)2NSiMe3}2],
which has a PdN bond present: Steiner, A.; Stalke, D. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1752.
(11) (a) Willemsen, L. C.; van der Kerk, G. J. M. Investigations in
the Field of Organolead Chemistry; International Lead Zinc Research
Organisation: Utrecht, The Netherlands, 1965. (b) Nefedov, O. M.;
Kolesnikov, S. P.; Ioffe, A. I. J. Organomet. Chem. Libr. 1981, 5, 181.
(14) Nu¨ckel, S.; Burger, P. Organometallics 2000, 19, 3305-3311.

746 Organometallics, Vol. 24, No. 4, 2005
Stol et al.
Figure 3. Top and side-on views of the crystal structure of [Li(Me,Me-Phebox)]2. Hydrogen atoms have been omitted for
clarity. Relevant bond distances (Å) and angles (deg): C(1)-Li(1) ) 2.268(3), C(1)-Li(2) ) 2.289(3), C(21)-Li(1) ) 2.247-
(3), C(21)-Li(2) ) 2.299(3), Li(1)-N(1) ) 2.004(3), Li(1)-N(21) ) 1.987(3), Li(2)-N(2) ) 1.989(3), Li(2)-N(22) ) 2.006(3);
Li(1)-C(1)-Li(2) ) 61.61(10), Li(1)-C(21)-Li(2) ) 61.74(10), C(1)-Li(1)-C(21) ) 117.99(13), C(1)-Li(1)-N(1) ) 84.46-
(10), C(1)-Li(1)-N(21) ) 114.11(14), N(1)-Li(1)-N(21) ) 121.39(14), N(21)-Li(1)-C(21) ) 85.76(11), N(1)-Li(1)-C(21)
) 135.61(15), C(1)-Li(2)-C(21) ) 115.03(12), C(1)-Li(2)-N(2) ) 84.78(11), C(1)-Li(2)-N(22) ) 139.79(14), N(2)-Li(2)-
N(22) ) 118.46(14), N(22)-Li(2)-C(21) ) 84.24(11), N(2)-Li(2)-C(21) ) 117.84(13).
puckering by rotation (wagging) about the C(4)-Cipso
-
C′ipso-C(24) axis provides 3a with an apparent molec-
ular symmetry plane containing the methylene and the
C(Me2) carbon centers. Consequently, this inversion
process renders the CH2 protons and CMe2 methyl
groupings enantiotopic. This view is corroborated by the
observation that the CH2 resonances coalesce (1H NMR)
at -16 °C, which corresponds to ∆Gq ) 57 kJ/mol,16
while the related CMe2 resonances coalesce at -14 °C
and hence a ∆Gq value of 55 kJ/mol is calculated.16 The
1H and 13C{1H} NMR spectra of 3b,c (toluene-d8) do not
exhibit the above-mentioned temperature dependence
for 3a at the temperatures studied (-60 to 25 °C). On
the basis of the above discussion of 3a compounds 3b,c
are expected to exist in solution as Λ SS or the ∆ SS
Figure 4. Possible modes of chelation.
NMe2)2-2,6)]2 (20.5 Hz).15 Moreover, these features are
temperature independent, indicating that the dimeric
structure is apparently preserved over the temperature
range -60 to 25 °C. Hence, the three-center-two-
electron C-Li bonding motif (cf. X-ray data of 3a) is also
retained in solution.
The remaining resonances in the 1H and 13C{1H}
NMR spectra of 3a (toluene-d8), however, exhibit tem-
perature dependence. At 25 °C, the methylene protons
1
diastereoisomers (see Figure 5), i.e. exhibiting two H
and 13C NMR resonance patterns. Most interestingly,
only one resonance pattern is observed, which suggests
that only one diastereoisomer is present in solution over
a wide temperature range: i.e., the inversion process,
operative in 3a, is blocked in 3b,c, resulting in the
presence of either the Λ SS or the ∆ SS diastereoisomer.
It is conceivable that steric interference between the
bulky ring substituents iPr (3b) and tBu (3c) lock the
wagging process about the C(4)-Cipso-C′ipso-C(24) axis,
leading to the stabilization of one diastereoisomer over
the other.
1
and methyl protons appear as singlets in the H NMR
spectrum; the 13C{1H} NMR spectrum also has two
singlet signals for the methylene carbon and methyl
substituents, respectively. At -50 °C, the CH2 grouping
now appears (1H NMR) as an AB pattern (δA 3.55 ppm,
δB 3.58 ppm, JAB ) 8.6 Hz) and the CMe2 groups are
separated into two singlets (at 0.73 and 0.83 ppm,
respectively). This aspect is also apparent in the 13C-
{1H} NMR spectrum (-60 °C), where two signals at 28.3
and 28.2 ppm are found for the CMe2 groups.
The described observations can be explained by
considering the ring puckering present in the four five-
membered chelate rings, as demonstrated by the struc-
ture in the solid state. The ring puckering of the four
chelate rings is coupled, and all have either the Λ or
the ∆ configuration. As a result, 3a exists in the slow-
exchange limit as a 1:1 mixture of two enantiomers. In
the fast-exchange limit rapid inversion of the ring
Both 1H and 7Li NMR signals of 3a are insensitive to
the addition of 1 equiv of diethyl ether, THF, or
triethylamine. This suggests that the dimeric structure
is not disrupted by these simple bases, and hence,
stronger coordination of the bis(oxazoline) substituents
is implied.
(15) Jastrzebski, J. T. B. H.; van Koten, G.; Konijn, M.; Stam, C. H.
J. Am. Chem. Soc. 1982, 104, 5490-5492.
(16) Friebolin, H. Basic One- and Two-Dimensional NMR Spectros-
copy; Wiley-VCH: Weinheim, Germany, 1998.

2,6-Bis(oxazolinyl)phenyl Complexes of Li and Sn
Organometallics, Vol. 24, No. 4, 2005 747
LiCl (1.0 M) in D2O as an external standard. 2-Bromoiso-
phthalic acid,6a L-valinol, and L-tert-leucinol18 were synthesized
according to literature procedures. (R,R′-Phebox)Br was syn-
thesized according to a combination of literature procedures.3e,6
All other chemicals were purchased from either Acros or
Aldrich and used as received. Elemental analysis were per-
formed by Kolbe, Mikroanalytisches Laboratorium, Mu¨lheim/
Ruhr, Germany.
Synthesis of [R,R′-PheboxBr] Compounds. (a) [Me,Me-
PheboxBr] (1a). Thionyl chloride (12 mL, 164 mmol) was
added to 2-bromoisophthalic acid (9.8 g, 40 mmol). The mixture
was refluxed overnight, and the residual SOCl2 was removed
in vacuo. A portion of the resulting 2-bromoisophthalic acid
chloride (6.9 g, 25 mmol) was weighed into a 500 mL one-
necked round-bottom flask, equipped with a nitrogen inlet. The
acid was dissolved in CH2Cl2 (25 mL), and the temperature of
the flask was reduced to 0 °C (ice/water bath). In a separate
Schlenk vessel, 2-amino-2-methyl-1-propanol (5.07 g, 56.3
mmol) and NEt3 (13.9 mL, 100 mmol) were dissolved in CH2-
Cl2 (35 mL) and slowly added by syringe to the cooled
2-bromoisophthalic acid chloride solution. The reaction mixture
was stirred overnight, after which time the solvent was
removed in vacuo. Acetonitrile (200 mL), PPh3 (23.7 g, 90.6
mmol), and Et3N (15.0 mL, 108 mmol) were added to the
residue. The temperature was reduced to 0 °C, after which
time CCl4 (20.7 mL, 215 mmol) was slowly added via a syringe.
After it was stirred overnight, the mixture was quenched with
H2O (10 mL) and the volatiles were removed in vacuo. The
residue was dissolved in H2O (200 mL) and ethyl acetate (200
mL). After separation of the layers, the organic layer was
washed with H2O (100 mL). The combined aqueous layers were
extracted with ethyl acetate (3 × 100 mL), dried (MgSO4), and
filtered. After the solvent was removed in vacuo, the residue
was stirred for 3 h in Et2O (250 mL). The suspension was
filtered, the residue was washed with additional Et2O (200
mL), and the filtrate was dried in vacuo. The crude product
was filtered over alumina (neutral Merck 90: Et2O; Rf 0.25),
resulting in the recovery of 6.95 g (20 mmol, 81%) of the white
Figure 5. Schematic representation of the structure of one
of the proposed diastereoisomers (Λ SS) of 3b,c.
Conclusions
The synthesis and structural features of [Sn(Me,Me-
Phebox)Me3] (2) and [Li(R,R′-Phebox)] complexes 3a (R
) R′ ) Me), 3b (R ) iPr, R′ ) H), and 3c (R ) tBu, R′
) H) were investigated. It was found that the Phebox
ligand in 2 is η1(C)-bonded to the Sn center: i.e., no
chelation of the bis(oxazoline) substituents occurs. The
[Li(R,R′-Phebox)] complex 3a (R ) R′ ) Me) exists both
in solution (13C NMR) and in the solid state (X-ray) as
a dimer containing formal three-center-two-electron
bonds in a Li2C2 central ring. The Phebox ligand is η3-
(N,C,N)-bonded to the lithium centers, presenting the
first structurally characterized aryllithium complex
containing intramolecular imine N-lithium coordina-
tion. The [Li(R,R′-Phebox)] complexes can be prepared
in high yield and are excellent starting materials for
the preparation of R,R′-Phebox-metal complexes which
can be used as transmetalating reagents ([Au(Me,Me-
Phebox)(PPh3)],17a [(Au(Me,Me-Phebox))2(dppbp)])17b and
as homogeneous catalysts (R,R′-Phebox-Pd,3b,d,g,i -Pt,3c,g,i
-Rh3e,f,h).
1
solid product. H NMR (300 MHz, acetone-d6): δ 1.35 (s, 12
3
H, CMe2), 4.14 (s, 4H, CH2), 7.52 (t, 1H, JH-H ) 7.7 Hz, Ar
H), 7.67 (d, 2H, 3JH-H ) 7.5 Hz, Ar H). 13C{1H} NMR (75 MHz,
acetone-d6): δ 28.4 (CMe2), 69.1 (CMe2), 79.9 (OCH2), 121.7
(Ar C), 128.1 (Ar C), 133.2 (Ar C), 133.7 (Ar C), 161.7 (OCN).
Anal. Calcd for C16H19BrN2O2: C, 54.71; H, 5.45; N, 7.98.
Found: C, 54.63; H, 5.38; N, 7.96.
Synthesis of [(S,S)-iPr,H-PheboxBr] (1b). A procedure
analogous to the synthesis of 1a was employed using 2-bromo-
isophthalic acid (5.05 g, 21 mmol) and L-valinol (4.91 g, 48
mmol), giving a light yellow oil. Yield: 5.16 g (67%). Rf value
(Et2O): 0.28. [R]20D ) -63.2° (c ) 0.10 g/mL, CH2Cl2). 1H NMR
Experimental Section
3
(300 MHz, CDCl3): δ 0.98 (d, 6H, JH-H ) 6.6 Hz, CHMe2),
3
3
All experiments were carried out under a dry, oxygen-free
nitrogen atmosphere, using standard Schlenk techniques.
Solvents were dried and distilled from sodium (pentane,
toluene), Na/benzophenone (diethyl ether, tetrahydrofuran),
CaH2 (CH2Cl2, CH3CN), or KOH (Et3N) prior to use. n-BuLi
(1.6 M in hexanes) was supplied by Acros and used as received.
1H and 13C{1H} NMR spectra were recorded on a Varian Inova
300 spectrometer. 7Li NMR (5 mm tube) and NMR experi-
ments with diethyl ether, tetrahydrofuran, and triethylamine
(10 mm tube) were recorded on a Varian Mercury 200
spectrometer. Benzene-d6 and toluene-d8 were purchased from
Cambridge Isotope Laboratories, distilled from Na, and stored
in a nitrogen-filled M. Braun glovebox. Other NMR solvents
(acetone-d6, CDCl3) were used as received. Chemical shifts (δ)
are reported in ppm relative to the residual solvent signal as
an internal standard. The 7Li NMR spectra are referenced to
1.04 (d, 6H, JH-H ) 6.6 Hz, CHMe2), 1.91 (septet, 2H, JH-H
) 6.6 Hz, CHMe2), 4.18 (m, 4H, OCH2), 4.45 (m, 2H, CHN),
3
3
7.37 (t, 1H, JH-H ) 7.7 Hz, Ar H), 7.65 (d, 2H, JH-H ) 7.8
Hz, Ar H). 13C{1H} NMR (CDCl3, 75 MHz): δ 32.3 (CMe2), 60.0
(CMe2), 70.2 (CHN), 72.6 (OCH2), 121.0 (Ar C), 126.6 (Ar C),
132.0 (Ar C), 132.2 (Ar C), 162.6 (OCN). Anal. Calcd for C18H23-
BrN2O2: C, 57.00; H, 6.11; N, 7.39. Found: C, 56.88; H, 6.07;
N, 7.26.
Synthesis of [(S,S) tBu,H-PheboxBr] (1c). A procedure
analogous to the synthesis of 1a was employed using 2-bromo-
isophthalic acid (4.1 g, 15 mmol) and L-tert-leucinol (3.5 g, 30
mmol), giving a colorless oil. Yield: 4.31 g (73%). Rf value (1:1
diethyl ether/ethyl acetate): 0.71. [R]20 ) -60.2° (c ) 0.11
D
g/mL, EtOH). 1H NMR (300 MHz, CDCl3): δ 0.96 (s, 18H,
CMe3), 4.06 (dd, 2H, 3JH-H ) 10.1 Hz, 3JH-H ) 8.3 Hz, OCH2),
4.22 (t, 2H, 3JH-H ) 8.7 Hz, CHN), 4.37 (dd, 1H, 3JH-H ) 10.1
3
3
Hz, JH-H ) 8.9 Hz, OCH2), 7.33 (t, 1H, JH-H ) 7.7 Hz, Ar
(17) (a) Stol, M.; Lutz, M.; Spek, A. L.; van Klink, G. P. M.; van
Koten, G. To be submitted for publication. (b) Stol, M.; Snelders, D.
M.; Kooijman, H.; Spek, A. L.; van Klink, G. P. M.; van Koten, G. To
be submitted for publication.
(18) McKennon, M. J.; Meyers, A. I. J. Org. Chem. 1993, 58, 3568-
3571.

748 Organometallics, Vol. 24, No. 4, 2005
Stol et al.
Table 1. Crystallographic Data for Crystal
H), 7.60 (d, 2H, 3JH-H ) 7.2 Hz, Ar H). 13C{1H} NMR (75 MHz,
CDCl3): δ 26.1 (CMe3), 34.0 (CMe3), 69.2 (CHN), 76.7 (OCH2),
121.4 (Ar C), 127.0 (Ar C), 132.4 (Ar C), 132.6 (Ar C), 163.0
(OCN). Anal. Calcd for C20H27BrN2O2: C, 58.97; H, 6.68; N,
6.88. Found: C, 59.10; H, 6.74; N, 6.76.
Structure Determinations of 2 and 3a
2
3a
formula
mol wt
cryst syst
space group
a (Å)
C
19H28N2O2Sn
C32H38Li2N4O4
556.54
435.15
Synthesis of [Sn(Me,Me-Phebox)Me3] (2). A portion of
n-BuLi (0.70 mL, 1.1 mmol, 1.6 M solution in hexanes) was
added to a solution of 1a (0.35 g, 1.0 mmol) in an appropriate
solvent (20 mL, THF, diethyl ether, pentane, or toluene;
Scheme 1) at -78 °C. After the solution was stirred and slowly
warmed to room temperature, SnClMe3 (2.8 mL, 1.0 mmol,
0.36 M solution in pentane) was added, and the resulting
mixture was stirred for 1 h. The mixture was then quenched
with water (10 mL) and the layers separated. The organic layer
was washed with H2O (10 mL), dried (Na2SO4), and filtered,
and the solvent was removed in vacuo. A yellow oil was
obtained, which crystallized rapidly. The yields in the following
solvents were as follows: THF (97%), diethyl ether (97%),
monoclinic
P21/c (No. 14)
11.1077(10)
11.3324(10)
18.4916(19)
121.130(6)
1992.5(3)
1.4506(2)
4
888
1.295
colorless
0.15 × 0.30 × 0.30
1.0, 27.6
50
monoclinic
P21/c (No. 14)
11.4446(10)
17.530(2)
17.622(3)
115.945(11)
3179.1(7)
1.1628(3)
4
1184
0.076
pale yellow
0.2 × 0.3 × 0.3
1.0, 27.5
40
b (Å)
c (Å)
â (deg)
V (Å3)
Dcalcd (g cm-3
Z
)
F(000)
µ(Mo KR) (mm-1
cryst color
)
cryst size (mm)
θmin, θmax (deg)
dist from cryst to
detector (mm)
data set (hkl)
1
pentane (98%), toluene (99%). H NMR (300 MHz, C6D6): δ
0.46 (s, 9H, 117Sn (7.7%) and 119Sn (8.4%) satellites, 2J ) 26.7
Hz, 27.8 Hz, SnMe3), 1.17 (s, 12H, CMe2), 3.67 (s, 4H, OCH2),
-14 to +14,
-14 to +14,
-24 to +23
0.596-0.875
39 706, 4589
-14 to +14,
-22 to +22,
-22 to +22
3
3
8.11 (t, 1H, JH-H ) 7.8 Hz, Ar H), 7.96 (d, 2H, JH-H
)
abs cor range
total no. of data,
no. of unique data
Rint
7.5 Hz, Ar H). 13C{1H} NMR (75 MHz, CDCl3): δ -2.0
(1J(117,119Sn-13C) ) 190 Hz, SnMe3), 28.6 (CMe2), 68.1 (CMe2),
79.2 (OCH2), 128.0 (Ar C), 131.3 (1J(117,119Sn-13C) ) 16 Hz,
Ar C), 137.0 (Ar C), 148.2 (Ar C), 163.6 (OCN). Anal. Calcd
for C19H28N2O2Sn: C, 52.44; H, 6.49; N, 6.44. Found: C, 52.53;
H, 6.57; N, 6.37.
82 804, 7294
0.1012
224
0.1016
387
no. of refined
params
final R1a
0.0242 (4199,
I > 2σ(I))
0.0627
0.0493 (5095,
I > 2σ(I))
0.1275
final wR2b
goodness of fit
w-1 c
Synthesis of [Li(Me,Me-Phebox)]2 (3a). To a solution of
1a (1.01 g, 2.89 mmol) in pentane (50 mL) was added n-BuLi
(1.8 mL, 2.9 mmol, 1.6 M in hexane) at -78 °C. The reaction
mixture was warmed to room temperature and stirred for 3
h, after which the volatile components were removed in vacuo.
The residue was washed twice with pentane (4 mL), which
1.029
1.034
σ2(Fo2) + (0.0313P)2 + σ2(Fo2) + (0.0506P)2 +
0.89P
1.00P
-0.24, 0.22
min, max residual
-1.14, 0.93
(near Sn)
density (e Å-3
)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
.
1
2
c P ) (Max(Fo2,0) + 2Fc )/3.
afforded an off-white solid product (yield: 531 mg, 66%). H
NMR (300 MHz, C6D6): δ 0.92 (s, 12H, CMe2), 3.76 (s, 4H,
3
3
3
OCH2), 7.20 (t, 1H, JH-H ) 7.7 Hz, Ar H), 8.09 (d, 2H, JH-H
) 7.8 Hz, Ar H). 13C{1H} NMR (75 MHz, C6D6): δ 28.4 (CMe2),
65.6 (CMe2), 79.9 (OCH2), 124.0 (Ar C), 127.7 (Ar C), 139.7
) 8.4 Hz, JH-H(AX + BX) ) 19.8 Hz, OCH2, NCH), 7.23 (t,
3
3
1H, JH-H ) 7.7 Hz, Ar H), 8.14 (2H, JH-H ) 7.8 Hz, Ar H).
13C{1H} NMR (75 MHz, C6D6): δ 26.0 (CMe3), 33.4 (CMe3),
69.4 (NCH), 75.4 (OCH2), 124.1 (Ar C), 128.4 (Ar C), 139.7 (Ar
1
(Ar C), 172.2 (OCN), 199.4 (septet, J(13C,7Li) ) 17.7 Hz, Ar
C). 7Li NMR (78 MHz, toluene-d8): δ 1.50. Anal. Calcd for
C16H19LiN2O2: C, 69.06; H, 6.88; N, 10.07. Found: C, 68.83;
H, 6.95; N, 9.83.
C), 173.8 (OCN), 198.1 (sp, J(13C,7Li) ) 17.8 Hz, Ar C). Li
NMR (78 MHz, toluene-d8): δ 1.74. Anal. Calcd for C20H27-
LiN2O2: C, 71.84; H, 8.14; N, 8.38. Found: C, 71.64; H, 8.02;
N, 8.22.
1
7
Synthesis of [Li(iPr,H-Phebox)]2 (3b). To a solution of
1b (0.47 g, 1.24 mmol) in pentane (25 mL) was added n-BuLi
(0.9 mL, 1.44 mmol, 1.6 M in hexane) at a temperature of -78
°C. The reaction mixture was warmed to room temperature
and stirred for 4 h, after which the suspension was decanted
after centrifugation. The residue was washed with pentane
(10 mL), which afforded a white solid product (yield: 370 mg,
NMR Experiments of 3a with Subsequent Addition of
Diethyl Ether, Tetrahydrofuran, and Triethylamine. In
a nitrogen-filled glovebox, 3a (128 mg, 0.46 mmol) was weighed
in a 10 mm NMR tube and dissolved in toluene-d8. After the
NMR spectra were recorded without additional coordinating
solvents present, the NMR tube was placed under nitrogen in
a modified Schlenk tube and Et2O (45 µL, 0.46 mmol) was
98%). 1H NMR (300 MHz, toluene-d8): δ 0.63 (d, 6H, 3JH-H
)
3
6.6 Hz, CHMe2), 0.70 (d, JH-H ) 6.6 Hz, CHMe2), 1.41 (sp,
3
1
7
added. H, Li{1H}, and 13C{1H} NMR spectra were recorded
at room temperature and 1H and 7Li{1H} NMR spectra at -60
°C. The NMR tube was again placed under nitrogen before
adding THF (40 µL, 0.49 mmol) to the mixture. 1H and 7Li
NMR spectra were again recorded at -60 °C and at room
temperature. Following the same procedure, Et3N (65 µL, 0.47
2H, JH-H ) 6.6 Hz, CHMe2), 3.76, 4.02, 4.06 (ABX pattern,
2
3
6H, JH-H(AB) ) 8.4 Hz, JH-H(AX + BX) ) 15.8 Hz, OCH2,
NCH) 7.21 (t, 1H, 3JH-H ) 7.8 Hz, Ar H), 8.04 (d, 3JH-H ) 7.8
Hz, Ar H). 13C{1H} NMR (75 MHz, C6D6): δ 18.5 (CHMe2),
18.9 (CHMe2), 33.4 (CHMe2), 71.4 (NCH), 71.6 (OCH2), 124.2
(Ar C), 128.6 (Ar C), 139.7 (Ar C), 173.9 (OCN), 198.8 (sp,
1J(13C,7Li) ) 17.6 Hz, Ar C).). 7Li NMR (78 MHz, toluene-d8):
δ 1.64. Anal. Calcd for C19H23LiN2O2: C, 70.58; H, 7.57; N,
9.14. Found: C, 70.15; H, 7.49; N, 8.86.
mmol) was added to the mixture and H and Li{1H} spectra
were recorded of the mixture at room temperature. No change
was observed in the chemical shift of the 7Li signal for 3a.
7Li{1H} NMR (78 MHz, toluene-d8, Et2O): 298 K, δ 1.48; 213
K, δ 1.50. 7Li{1H} NMR (78 MHz, toluene-d8, Et2O, THF): 298
1
7
Synthesis of [Li(tBu,H-Phebox)]2 (3c). To a solution of
1c (0.24 g, 0.59 mmol) in diethyl ether (20 mL) was added
n-BuLi (0.5 mL, 0.8 mmol, 1.6 M in hexane) at -78 °C. The
reaction mixture was stirred at this temperature for 45 min,
after which time it was warmed to room temperature and
stirred for another 3 h. The volatile components of the mixture
were then removed in vacuo. The residue was washed with
pentane (2 × 5 mL), which afforded a white solid product
K, δ 1.48; 213 K, δ 1.48. Li{1H} NMR (78 MHz, toluene-d8,
Et2O, THF, Et3N, 298 K): δ 1.46.
7
X-ray Crystal Structure Analyses. Pertinent data for the
structure determinations are given in Table 1. Data were
collected at 150 K on a Nonius KappaCCD diffractometer on
a rotating anode (graphite-monochromated Mo KR radiation,
λ ) 0.710 73 Å). The unit-cell parameters were checked for
1
(yield: 83 mg, 42%). H NMR (300 MHz, toluene-d8): δ 0.67
(s, 18H, CMe3), 3.79, 4.067, 4.116 (ABX pattern, 6H, 2JH-H(AB)

2,6-Bis(oxazolinyl)phenyl Complexes of Li and Sn
Organometallics, Vol. 24, No. 4, 2005 749
the presence of higher lattice symmetry.19 The intensity data
of 2 were corrected for absorption using an algorithm based
on multiple measurements of symmetry-related reflections
incorporated in Platon.20 The structures were solved with
direct methods using SHELXS8621 and refined on F2 using
SHELXL-9722 (no observance criterion was applied during
refinement). Hydrogen atoms were included on calculated
positions riding on their carrier atoms. The methyl groups
were allowed to rotate along the X-CH3 bond. Non-hydrogen
atoms were refined with anisotropic displacement parameters.
The isotropic displacement parameters of the hydrogen atoms
were linked to the value of the equivalent isotropic displace-
ment parameters of their carrier atoms. Neutral atom scat-
tering factors and anomalous dispersion corrections were taken
from ref 23. Validation, geometrical calculations, and illustra-
tions were performed with PLATON.20
Acknowledgment. Prof. Dr. B. Hessen, Dr. R. A.
Gossage, and Dr. P. Chase are acknowledged for their
suggestions and stimulating discussion. This work was
supported in part (A.L.S.) by the Council for the Chemi-
cal Sciences of The Netherlands Organization for Sci-
entific Research (CW-NWO).
(19) Spek, A. L. J. Appl. Crystallogr. 1988, 21, 578.
Supporting Information Available: Further details on
the crystal structures of complexes 2 and 3a, including tables
of atomic coordinates, displacement parameters, bond lengths,
and bond angles. This material is available free of charge via
the Internet at http://pubs.acs.org.
(20) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
(21) Sheldrick, G. M. SHELXS86: Program for Crystal Structure
Determination; University of Go¨ttingen, Go¨ttingen, Germany, 1986.
(22) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure
Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997.
(23) Wilson, A. J. C., Ed. International Tables for Crystallography;
Kluwer Academic: Dordrecht, The Netherlands, 1992; Vol. C.
OM049267N