Intramolecular nucleophilic addition to the 2 position of coordinated 2,2′-bipyridine by a deprotonated dimethyl sulfide ligand

Article abstract of DOI:10.1021/ic401065h

Deprotonation of the dimethyl sulfide ligand in [Re(bipy)(CO) ₃(SMe₂)][OTf] (1) by KN(SiMe₃)₂ afforded a mixture of two diastereomers (2M and 2m) in which a C-C bond has been formed between the S-bonded CH₂ group and the 2 position of 2,2′-bipyridine. The solid-state structure of the more stable 2,6- ⁱPr-BIAN analogue could be determined by X-ray diffraction.

Full text of DOI:10.1021/ic401065h

Communication
pubs.acs.org/IC
Intramolecular Nucleophilic Addition to the 2 Position of
Coordinated 2,2′-Bipyridine by a Deprotonated Dimethyl Sulfide
Ligand†
Rebeca Arevalo,‡ Julio Perez,*,‡ and Lucía Riera§
́
́
‡Departamento de Química Organ
́
ica e Inorgan
́
ica-IUQOEM, Universidad de Oviedo-CSIC, c/Julian
́
Clavería 8, 33006 Oviedo, Spain
§Instituto de Síntesis Química y Catal
c/Pedro Cerbuna 12, 50009 Zaragoza, Spain
́
isis Homogen
́
ea, Departamento de Química Inorganica, Universidad de Zaragoza-CSIC,
́
S
* Supporting Information
with IR ν(CO) bands at lower wavenumbers (2012, 1910, and
1893 cm−1 in THF). Attempts to crystallize the product under a
variety of conditions failed because of limited thermal stability.9
However, THF-d8 solutions of the residue, obtained by
evaporation of volatiles in vacuo, afforded informative NMR
spectra.7 To aid in the signal assignment, the labeled analogue
[Re(bipy)(CO)3(S(13CH3)2)][OTf] (1*) was similarly pre-
pared, characterized, and deprotonated.7 The NMR spectra of
the deprotonation crudes are consistent with deprotonation of
one dimethyl sulfide CH3 group and attack of the resulting
methylene onto one of the C atoms of the bipy ligand (see
Scheme 1).
ABSTRACT: Deprotonation of the dimethyl sulfide
ligand in [Re(bipy)(CO)3(SMe2)][OTf] (1) by KN-
(SiMe3)2 afforded a mixture of two diastereomers (2M
and 2m) in which a C−C bond has been formed between
the S-bonded CH2 group and the 2 position of 2,2′-
bipyridine. The solid-state structure of the more stable
2,6-iPr-BIAN analogue could be determined by X-ray
diffraction.
oordination of organic molecules to a Lewis acid through a
Cheteroatom can have two effects on the α groups: an
increase in the acidity of the CH groups1 and an increase in the
electrophilicity of C atoms forming multiple bonds to the
heteroatom.2 If the same Lewis acid center could bind in
proximal positions a molecule with α-CH groups and a molecule
with an electrophilic α-C, deprotonation of the former by an
external base could trigger an intramolecular nucleophilic
addition.3 The marked geometrical preferences of some
transition-metal centers, along with the relative stability of
their coordinative bonds, make them ideal Lewis acid centers for
this purpose. Herein we illustrate this approach using the very
weak acid dimethyl sulfide4 and 2,2′-bipyridine (bipy) as the
electrophilic counterpart. bipy is a diimine and, therefore,
potentially electrophilic; however, throughout its extensive
coordination chemistry,5 it has been found to be a remarkably
inert ligand.
Scheme 1. Deprotonation of 1 and Reaction of the
Diastereomeric Products (Only One Enantiomer of Each Is
Shown) with PMe3
Compound [Re(bipy)(CO)3(SMe2)][OTf] (1) was prepared
by the reaction of [Re(bipy)(CO)3(OTf)]6 with excess dimethyl
sulfide in a CH2Cl2 solution and characterized by IR and NMR.7
The IR ν(CO) bands [2036, 1939, and 1928 cm−1 in
tetrahydrofuran (THF)] showed that 1 is a fac-tricarbonyl, as
is typical of rhenium tricarbonyl complexes; therefore, SMe2
must be cis to each 2-pyridyl group. Because attempts to obtain
X-ray-quality single crystals of 1 failed, [Re(bipy)-
(CO)3(SMe2)][BAr′4] [1′; Ar′ = 3,5-bis(trifluoromethyl)-
phenyl] was prepared from 1 and Na[BAr′4]8 and characterized
spectroscopically and by X-ray diffraction.7 Both the solution
spectra and the results of the solid-state structure character-
ization confirmed the composition given above for 1′ and hence
support that proposed for 1.
These spectra show the presence of two species with the same
connectivity, which are proposed to be two diastereomers (2M
and 2m or 2M* and 2m* for the 13C-labeled mixture), resulting
from the presence of four stereocenters in the product (Re, S, the
attacked C, and its adjacent N), two of which (Re and S) can
adopt either configuration (at C and N, the configuration results
from the attack being on a given face of bipy). As a result, the
methylene H atoms (which do not show coupling with bipy H
atoms) are diasterotopic and appear as two doublets (2JHH = 12.1
Hz). A 2M/2m ratio of 2.2 was consistently found in several
A freshly prepared THF suspension of 1 cooled to −78 °C
reacted with a slight excess of KN(SiMe3)2 to afford a species
Received: April 29, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic401065h | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
deprotonation reactions. For both 2M and 2m, the presence of
1
eight H NMR signals indicates desymmetrization of the bipy
ligand. For each compound, half of these signals occur at
chemical shifts within the 4−7 ppm range, diagnostic of
dearomatization of one of the 2-pyridyl rings.3a,7 The same
features are apparent in the 13C NMR spectrum of the 2M/2m
mixture, consisting of 10 signals for each species, 5 of which occur
in the 70−130 ppm range. The NMR spectra demonstrate that
the methylene C atom is bonded to the bipy C2 atom. Thus, in
Figure 1. 2,6-iPr-BIAN.
1
the H NMR spectra of the 2M*/2m* mixture, the signals at
2.96, 2.41, 2.15, and 1.50 ppm were assigned to the methylene
groups and those at 2.67 and 1.48 ppm to the methyl groups on
the grounds of their couplings with 13C (1JCH = 144.1 Hz and 3JCH
= 5.2 Hz). On the basis of their higher intensity and opposite
phase in the DEPT-135 spectrum, the 13C NMR signals at 33.8
and 32.4 ppm were assigned to the CH2 groups and those at 25.0
and 18.4 ppm to the CH3 groups. A 2D HMBC NMR spectrum
of the 2M/2m mixture showed a two-bond correlation between
the methylene H atom at 2.96 ppm and the 13C NMR signal at
78.0 ppm, assigned (DEPT-135) to a bipy quaternary C atom
and a three-bond correlation to the other quaternary C NMR
signal at 165.5 ppm. The 13C NMR spectrum of the 2M*/2m*
mixture showed the quaternary C atom bipy signals at 78.0 and
76.1 ppm as doublets because of coupling with the 13CH2 groups
(1JCC = 31.5 Hz).
The 2M/2m mixture reacts with 1 equiv of trimethylphos-
phine to afford a single fac-tricarbonyl compound 3 with a PMe3
ligand (31P NMR singlet at −25.7 ppm) and with NMR signals
corresponding to a single diastereomer (see Scheme 1). This
indicates that the Re-bonded S atom in 2M and 2m was displaced
by PMe3, leading to the loss of the S stereocenter and hence to
the formation of a single diastereomer. 13C NMR of the
analogous 13C-labeled compound, 3*, confirmed that the
methylene group remains bonded to the quaternary C atom
because its signal is a doublet at 72.6 ppm (1JCC = 35.0 Hz).
Analogously, the mixture 2M/2m reacted with the diphosphine
bis(dimethylphosphino)methane (dmpm) to afford a single
product (4) similar to 3, i.e., containing a monodentate dmpm
Figure 2. Molecular structures of compounds 5 (a) and 6 (b). 2,6-
Bis(isopropyl)phenyl (Ar) groups have been omitted for clarity in 6.
solvent diffusion furnished crystals of 6, which was characterized
spectroscopically and by single-crystal X-ray diffraction (see
Figure 2b).
The 1H and 13C NMR spectra reflected the loss of a molecular
mirror plane in the formation of 6. The 13C NMR featured a
signal at 192.8 ppm attributable to the imine C atom, whereas the
signal at 91.2 ppm was assigned to the sp3 C atom formed as a
result of the nucleophilic addition. Two- and three-bond
correlations like those encountered for 2M and 2m (see
above) were observed in the 2D HMBC spectrum of 6.7
These results demonstrated that a reaction like that proposed
above for the bipy analogue 1 had occurred. Thus, the methylene
C atom of a S(CH3)CH2 moiety (resulting from deprotonation
of coordinated dimethyl sulfide) is bonded to the C atom of one
of the N-coordinated C−N groups. As a consequence, the C−N
distance in that group [1.446(4) Å,14 consistent with a single
bond] is longer than the C−N distance at the intact imine group
[1.288(4) Å], and the Re−N distance [2.149(3) Å, consistent
with a rhenium amido moiety15] is shorter than that at the intact
imine donor [2.247(3) Å]. The sum of the angles about the
amido N atom is 359.9°, indicating a planar geometry and,
therefore, delocalization of the N lone electron pair, a feature
previously encountered in most amido complexes.16 In an
organometallic compound like 6, a dative component in the Re−
N bond would lead to a high-energy 20 electron metal center. In
some such species, N lone-pair delocalization is made possible
through aryl substituents at the N atom.15 Amido complexes with
a localized N lone pair and pyramidal geometry about the N atom
are highly reactive species.17 The instability of the 2M and 2m
compounds could be due, at least in part, to the presence of this
sort of ligand. The full characterization of 6 demonstrates
ligand. The 31P NMR spectrum of 4 displays two doublets (2JPP
=
32.3 Hz) for the coordinated (−18.7 ppm) and uncoordinated
(−59.0 ppm) P atoms. Complexes 3 and 4 are, like their
precursors, thermally unstable. No product with a chelating
dmpm ligand could be detected by NMR despite the high
nucleophilicity of this diphosphine, showing that the instability of
2M, 2m, and 3 was not due to the lability of the ligands (e.g.,
decarbonylation).
These results are remarkable in that (a) a bipy ligand is
dearomatized, (b) the site of nucleophilic attack is the bipy 2
position, for which there is no precedent, and (c) the employed
fragment, fac-{Re(bipy)(CO)3}, which has been widely
employed in the areas of bioinorganic chemistry,10 supra-
molecular chemistry,11 and catalysis of CO2 reduction,12 is
notoriously inert.
Analogous results were obtained with complexes of other
diimines, but the products suffered also from low stability.
Therefore, we turned our attention to the bulky nonaromatic
diimine bis{2,8-(2,6-diisopropylphenylimino)acenaphthene
(2,6-iPr-BIAN; Figure 1).13 Compound [Re(2,6-iPr-BIAN)-
(CO)3(SMe2)][BAr′4] (5) was prepared similarly to 1 and
characterized spectroscopically and by X-ray diffraction (see
Figure 2a).7 The reaction of 5 with KN(SiMe3)2 afforded the
neutral complex 6, which was found to consist of a single
diastereomer and to be quite thermally stable in solution. Slow
B
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Inorganic Chemistry
Communication
(2) Constable, E. C. Metals and Ligand Reactivity; VCH: Weinheim,
Germany, 1990; p 193.
deprotonation of the SMe2 ligand, and the similarity in the
spectral changes supports the structure assignment of the
unstable bipy analogue.
(3) (a) Huertos, M. A.; Per
5662. (b) Espinal Viguri, M.; Huertos, M. A.; Per
Am. Chem. Soc. 2012, 134, 20326.
́
ez, J.; Riera, L. J. Am. Chem. Soc. 2008, 130,
́
ez, J.; Riera, L.; Ara, I. J.
Squires et al. found that gas-phase deprotonation of the
Me2S−BH3 adduct afforded a stable carbanion that does not
rearrange to the B−C species.17 Gladysz et al. found that
deprotonation of cationic rhenium dialkyl sulfide complexes
yielded the corresponding alkylthioalkyl products via a [2,3]-
sigmatropic rearrangement.18 However, in the case of dimethyl
sulfide, a multitude of noncharacterized products were
formed.18b To our knowledge, no product of the deprotonation
of transition-metal-coordinated dimethyl sulfide has been
characterized. Nucleophilic attack on coordinated bipy has
been a matter of controversy for a long time,2,19 and in the very
few cases where products have been unambiguously charac-
terized, the attack took place at the 6 position.3a,20
(4) For the standard method to deprotonate dimethyl sulfide, see:
Peterson, D. J. J. Am. Chem. Soc. 1967, 32, 1717.
(5) (a) Kaes, C.; Katz, A.; Hosseini, M. W. Chem. Rev. 2000, 100, 3553.
(b) Newkome, G. R.; Patri, A. K.; Holder, E.; Schubert, U. S. Eur. J. Org.
Chem. 2004, 235.
́
(6) Hevia, E.; Perez, J.; Riera, L.; Riera, V.; Del Río, I.; García-Granda,
S.; Miguel, D. Chem.Eur. J. 2002, 8, 4510.
(7) See the Supporting Information for experimental details.
(8) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11,
3920.
(9) Crystallization attempts by the slow diffusion of solvents afforded
noncrystalline, sparingly soluble solids that could not be characterized.
(10) (a) Gabrielsson, A.; Hartl, F.; Zhang, H.; Lindsay Smith, J. R.;
Towrie, M.; Vlcek, A., Jr.; Perutz, R. N. J. Am. Chem. Soc. 2006, 128,
4253. (b) Belliston-Bittner, W.; Dunn, A. R.; Nguyen, Y. H. L.; Stuehr,
D. J.; Winkler, J. R.; Gray, H. B. J. Am. Chem. Soc. 2005, 127, 15907.
(11) Deye, J. R.; Shiveley, A. N.; Goins, S. M.; Rizzo, L.; Oehrle, S. A.;
Walters, K. A. Inorg. Chem. 2008, 47, 23.
(12) (a) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. J. Am. Chem.
Soc. 2003, 125, 11976. (b) Koike, K.; Okoshi, N.; Hori, H.; Takeuchi, K.;
Ishitani, O.; Tsubaki, H.; Clark, I. P.; George, M. W.; Johnson, F. P. A.;
Turner, J. J. J. Am. Chem. Soc. 2002, 124, 11448.
(13) Coventry, D. N.; Batsanov, A. S.; Goeta, A. E.; Howard, J. A. K.;
Marder, T. B. Polyhedron 2004, 23, 2789.
(14) The results of the structural determination showed the presence
of two independent molecules of 6 in the asymmetric unit. Average
values of the bond distances are used in the discussion.
In summary, we have demonstrated that the coordination of
+
dimethyl sulfide to the Lewis acid Re(bipy)(CO)3 makes
deprotonation with the commercially available solution of
potassium bis(trimethylsilyl)amide in toluene possible. More-
over, the deprotonated SMe2 ligand is nucleophilic enough to
add to a proximal bipy ligand, which undergoes dearomatization
of one of the 2-pyridyl rings. The product is a mixture of two
diastereomers due to the creation of four stereocenters in the
reaction. The addition of a phosphine, either PMe3 or dmpm, to
this mixture yields a single diastereomer due to substitution of
the S-donor atom by the P atom. Deprotonation of a similar
2,6-iPr-BIAN complex affords a stable product, which could be
crystallized and fully characterized, including an X-ray diffraction
structural determination.
(15) Hevia, E.; Per
1966.
́
ez, J.; Riera, V.; Miguel, D. Organometallics 2002, 21,
(16) (a) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc.
Chem. Res. 2002, 35, 44. (b) Blue, E. D.; Davis, A.; Conner, D.; Gunnoe,
T. B.; Boyle, P. D.; White, P. S. J. Am. Chem. Soc. 2003, 125, 9435.
(c) Webb, J. R.; Munro-Leighton, C.; Pierpont, A. W.; Gurkin, J. T.;
Gunnoe, T. B.; Cundari, T. R.; Sabat, M.; Petersen, J. L.; Boyle, P. D.
Inorg. Chem. 2011, 50, 4195.
ASSOCIATED CONTENT
* Supporting Information
Crystallographic information of compounds 1′, 5, and 6 and full
experimental details for 1−6. This material is available free of
charge via the Internet at http://pubs.acs.org.
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(17) Squires, R. R.; Workman, D. B.; Ren, J. Angew. Chem., Int. Ed. Engl.
1997, 36, 2230.
(18) (a) Cagle, P. C.; Meyer, O.; Weickhardt, K.; Arif, A. M.; Gladysz, J.
A. J. Am. Chem. Soc. 1995, 117, 11730. (b) Cagle, P. C.; Meyer, O.;
Vichard, D.; Weickhardt, K.; Arif, A. M.; Gladysz, J. A. Organometallics
1996, 15, 194.
(19) (a) Gillard, R. D.; Lyons, J. R. J. Chem. Soc., Chem. Commun. 1973,
585. (b) Gillard, R. D. Coord. Chem. Rev. 1975, 16, 67. (c) Henry, M. S.;
Hoffman, M. Z. J. Am. Chem. Soc. 1977, 99, 5201. (d) Farver, O.;
Monsted, O.; Nord, G. J. Am. Chem. Soc. 1979, 101, 6118. (e) Nord, G.;
Hazell, A. C.; Hazell, R. G.; Farver, O. Inorg. Chem. 1983, 22, 3429.
(f) Spellane, P. J.; Watts, R. J.; Curtis, C. J. Inorg. Chem. 1983, 22, 4060.
(g) Lay, P. A. Inorg. Chem. 1984, 23, 4775. (h) Blackman, A. G. Adv.
Heterocycl. Chem. 1993, 58, 123. (i) Cotton, F. A.; Wilkinson, G.;
Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry; John Wiley
& Sons: New York, 1999; p 351. (j) McInnes, C. S.; Clare, B. R.;
Redmond, W. R.; Clark, C. R.; Blackman, A. G. Dalton Trans. 2003,
2215.
AUTHOR INFORMATION
Corresponding Author
*E-mail: japm@uniovi.es. Phone: +34 985 103465. Fax: +34 985
103446.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. Eva Hevia (University of Strathclyde) for the
preparation of compound 1 and preliminary studies. Financial
support from Ministerio de Ciencia e Innovacion (Grant
CTQ2009-12366) and Ministerio de Economia y Competitivi-
dad (Grants CTQ2012-37379-C02-01 and CTQ2012-37379-
C02-02) is gratefully acknowledged. R.A. thanks the Universidad
de Oviedo (Beca de Excelencia del Campus de Excelencia
Internacional), Principado de Asturias (Beca Severo Ochoa), and
́
Ministerio de Educacion, Cultura y Deporte (Beca FPU), for
graduate studentships.
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́
́
(20) (a) Zhang, X.; Tong, M.; Chen, X. Angew. Chem., Int. Ed. 2002, 41,
1029. (b) For an example of an intramolecular attack on a coordinated
1,10-phenanthroline, see: Cuesta, L.; Hevia, E.; Morales, D.; Per
́
ez, J.;
Riera, V.; Seitz, M.; Miguel, D. Organometallics 2005, 24, 1772.
DEDICATION
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†Dedicated to Professor Antonio Laguna on the occasion of his
65th birthday.
REFERENCES
(1) Kessar, S. V.; Singh, P. Chem. Rev. 1997, 97, 721.
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