Expedient method for the transmetalation of Zn(II)-centered salphen complexes

Article abstract of DOI:10.1021/ic701245r

Various mono- and bis-Zn^II complexes derived from salphen ligands have been conveniently transmetalated with a number of transition metal (TM) acetates (M = Ni, Pd, Mn) in THF to afford their respective TM-salphen counterparts in excellent isolated yields (80-100%). This new transmetalation procedure allows in situ switching between supramolecular and catalytic functions of the metallosalphen complex.

Full text of DOI:10.1021/ic701245r

Inorg. Chem. 2007, 46, 7265−7267
Expedient Method for the Transmetalation of Zn(II)-Centered Salphen
Complexes
Eduardo C. Escudero-Ada´n,§ Jordi Benet-Buchholz,§ and Arjan W. Kleij*,§,†
Institute of Chemical Research of Catalonia (ICIQ), AV. Pa¨ısos Catalans 16,
43007 Tarragona, Spain, and Institucio´ Catalana de Recerca i Estudis AVanc¸ats (ICREA),
Pg. Llu´ıs Companys 23, 08010 Barcelona, Spain
Received June 25, 2007
Various mono- and bis-ZnII complexes derived from salphen ligands
have been conveniently transmetalated with a number of transition
ated by a change in or accommodation of a higher oxidation
state of the metal ion complexed by the salen ligand.3
Recently, we and others have been active in the develop-
ment of new applications for salen frameworks with an
emphasis on its supramolecular potential.4 In particular, ZnII-
based salphen [salphen ) N,N′-phenylenebis(salicylidene-
imine)] structures have been identified as excellent, nonre-
active supramolecular components/templates. Their supra-
molecular potential has been recently used in the assembly
of new homogeneous catalysts that showed unconventional
selectivities and reactivities, as well as in noncovalently
assembled boxlike structures.5 The Lewis acidity of the ZnII
ion in these complexes allows easy formation of five-coor-
dinate, (distorted) square pyramidal surrounded ZnII in the
presence of suitable donor systems including pyridines,
furans, alcohols, nitriles, and water.6 We envisioned that the
high Lewis acid character of the ZnII metal center in the
metal (TM) acetates (M
respective TM salphen counterparts in excellent isolated yields
(80 100%). This new transmetalation procedure allows in situ
) Ni, Pd, Mn) in THF to afford their
−
−
switching between supramolecular and catalytic functions of the
metallosalphen complex.
Catalysis is at the forefront of science, playing an im-
portant role in the development of more sustainable chemical
and pharmaceutical processes. In the field of homogeneous
catalysis, salen ligands1 have been investigated for many
years. Currently these compounds are among the most widely
developed ligands in the catalysis toolbox of modern
chemistry for a range of organic transformations. Many
synthetic approaches toward metallosalen complexes are
known that start off with double Schiff base precursors.
These precursors commonly originate from a condensation
process involving 1 equiv of a diamine and 2 equiv of salicyl-
aldehyde or ketone analogue. Their respective metal salts
are generally easily obtained by treatment with acetate-based
metal precursors or via activation of the two phenolic
positions with a strong base (e.g., BuLi or ZnEt2).2 The
resultant metallosalen frameworks are thermally and kineti-
cally stable entities controlled by the presence of a tetraden-
tate ligation mode of the N2O2 pocket that is characteristic
of the salen ligand. This stabilization ability allows, for
instance, the creation of vital catalytic intermediates gener-
(3) For some illustrative examples of such catalytic intermediates see:
(a) Finney, N. S.; Pospisil, P. J.; Chang, S.; Palucki, M.; Konsler, R.
G.; Hansen, K. B.; Jacobsen, E. N. Angew. Chem., Int. Ed. Engl. 1997,
36, 1720. (b) Belokon, Y. N.; North, M.; Maleev, V. I.; Voskoboev,
N. V.; Moskalenko, M. A.; Peregudov, A. S.; Dmitriev, A.; Ikonnikov,
N. S.; Kagan, H. B. Angew. Chem., Int. Ed. 2004, 43, 4085. (c)
Feichtinger, D.; Plattner, D. A. Angew. Chem., Int. Ed. Engl. 1997,
36, 1718. (d) Zheng, X.; Jones, C. W.; Weck, M. J. Am. Chem. Soc.
2007, 129, 1105. (e) Man, W.-L.; Tang, T.-M.; Wong, T.-W.; Lau,
T.-C.; Peng, S.-M.; Wong, W.-T. J. Am. Chem. Soc. 2004, 126, 478.
(4) (a) Sun, S.-S.; Stern, C. L.; Nguyen, S. T.; Hupp, J. T. J. Am. Chem.
Soc. 2004, 126, 6314. (b) Splan, K. E.; Massari, A. M.; Morris, G.
A.; Sun, S.-S.; Reina, E.; Nguyen, S. T.; Hupp, J. T. Eur. J. Inorg.
Chem. 2003, 2348. (c) Gianneschi, N. C.; Bertin, P. A.; Nguyen, S.
T.; Mirkin, C. A.; Zakharov, L. N.; Rheingold, A. L. J. Am. Chem.
Soc. 2003, 125, 10508. (d) Gianneschi, N. C.; Cho, S.-H.; Nguyen, S.
T.; Mirkin, C. A. Angew. Chem., Int. Ed. 2004, 43, 5503. (e) Kuil,
M.; Goudriaan, P. E.; van Leeuwen, P. W. N. M.; Reek, J. N. H.
Chem. Commun. 2006, 4679. (f) Kuil, M.; Goudriaan, P. E.; Kleij, A.
W.; Tooke, D. M.; Spek, A. L.; van Leeuwen, P. W. N. M.; Reek, J.
N. H. Dalton Trans. 2007, 2311. (g) Kleij, A. W.; Kuil, M.; Tooke,
D. M.; Spek, A. L.; Reek, J. N. H. Inorg. Chem. 2007, 46, 5829.
(5) (a) Kleij, A. W.; Lutz, M.; Spek, A. L.; van Leeuwen, P. W. N. M.;
Reek, J. N. H. Chem. Commun. 2005, 3661. (b) Kleij, A. W.; Kuil,
M.; Tooke, D. M.; Lutz, M.; Spek, A. L.; Reek, J. N. H. Chem. Eur.
J. 2005, 11, 4743. (c) Kleij, A. W.; Reek, J. N. H. Chem. Eur. J.
2006, 12, 4218.
* To whom correspondence should be addressed. E-mail: akleij@iciq.es.
§ Institute of Chemical Research of Catalonia (ICIQ).
† Institucio´ Catalana de Recerca i Estudis avanc¸ats (ICREA).
(1) See for recent reviews on salen structures: (a) Leung, A. C. W.;
MacLachlan, M. J. J. Inorg. Organomet. Polym. Mater. 2007, 17, 57.
(b) Larrow, J. F.; Jacobsen, E. N. Top. Organometal. Chem. 2004, 6,
123. (c) McMarrigle, E. M.; Gilheany, D. G. Chem. ReV. 2005, 105,
1563. (d) Canali, L.; Sherrington, D. C. Chem. Soc. ReV. 1999, 28,
85. (e) Katsuka, T. AdV. Synth. Catal. 2002, 344, 131.
(2) For some representative routes toward (metallo)salens: (a) Morris,
G. A.; Zhou, H.; Stern, C. L.; Nguyen, S. B. Inorg. Chem. 2001, 40,
3222. (b) Larrow, J. F.; Jacobsen, E. N.; Gao, Y.; Hong, Y.; Nie, X.;
Zepp, C. M. J. Org. Chem. 1994, 59, 1939. (c) Holbach, M.; Zheng,
X.; Burd, C.; Jones, C. W.; Weck, M. J. Org. Chem. 2006, 71, 2903.
(6) (a) Singer, A. L.; Atwood, D. A. Inorg. Chim. Acta 1998, 277, 157.
(b) Kleij, A. W.; Kuil, M.; Lutz, M.; Tooke, D. M.; Spek, A. L.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M.; Reek, J. N. H. Inorg.
Chim. Acta 2006, 359, 1807.
10.1021/ic701245r CCC: $37.00
Published on Web 07/31/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 18, 2007 7265

COMMUNICATION
Scheme 1. Synthesis of Compounds 3-6 via Transmetalation
Scheme 2. Transmetalation of ZnII-salphens 8-11
Figure 1. X-ray molecular structure for 11 with the adopted numbering
scheme. Co-crystallized solvent molecules and H atoms are deleted for
clarity.
protons as compared to the Zn(salphens) (see Supporting
Information). For 3, the imine groups resonate at 8.74 ppm
(acetone-d6) while the parent ZnII complex showed a single
imine peak much more downfield (9.05 ppm). We then
extended the approach toward other TMs, and complexes 4
(M ) PdII, 82%) and 5 (M ) MnIIICl, 89%) were obtained
in high yield using similar preparations. The simple isolation
of complex 5 illustrates the usefulness of the transmetalation
procedure, since MnIII-centered salens have a long-standing
tradition in homogeneous catalysis.2b
Interestingly, in the case of 4, the initial transmetalation
protocol was carried out at RT and proved to be much slower
as compared to 3 and 5, and only a conversion of 8% was
obtained after 3 h. This points at some steric effect of the
3-positioned t-Bu groups in the salphen ligand upon metal
exchange with larger metals ions. This was further cor-
roborated by the synthesis of 6 (92%) which proceeded via
Zn(salphen) 2 (note the absence of 3-t-Bu groups), and an
almost immediate color change from yellow to deep red was
noted, in contrast to the synthesis of 3.9
Next, the transmetalation procedure was examined for a
range of different (non-)symmetric Zn(salphens) with either
electron-donating or -withdrawing groups located at the
phenyl side-groups to further investigate the scope of the
discovered transmetalation procedure. Zn(salphen) complexes
8-11 were prepared in good yields (62-88%) using the
previously reported monoimine 7 (Supporting Information)10
and a recently published one-pot, two-step procedure.11 All
new compounds were fully characterized (see Supporting
Information) and for the pyridine adduct of 11, the X-ray
crystal structure was also determined as a representative
model for nonsymmetric Zn(salphen) complexes (Figure 1).
Complex 11 was crystallized from bulk CH3CN containing
about 5% pyridine (v/v). As expected, the structure for 11
shows a pentacoordinated Zn center with the axial coordina-
tion site occupied by a pyridine ligand. Furthermore, the
salphen complexes may be used as a starting point for the
preparation of TM-derived complexes through a transmeta-
lation procedure7 to furnish thermodynamically more stable
structures. If successful, this could open pathways to
multipurpose systems of which the (catalytic) properties may
be controlled via in situ exchange of the metal ion, or simply
lead to the selective exchange of Zn centers for other TM
ions in multimetallic salphen derivatives,8 thereby potentially
giving access to interesting heterometallic structures. Herein
we communicate a versatile synthetic approach aimed at
metal exchange within ZnII-salphen structures and the main
factors governing the rate of metal exchange in the salphen
framework. To our knowledge, this is the first example of a
transmetalation process within a tetradentate salen framework
with an ample scope.
Our first attempt at metal exchange was carried out with
ZnII-salphen complex 1 and 1 equiv of Ni(OAc)2‚4H2O at
RT using THF as the solvent. Although THF will occupy
the fifth, apical coordination site and therefore may stabilize
the ZnII-salphen in solution,6a a gradual change in color from
yellow to red of the reaction mixture was noted in time which
was attributed to NiII-salphen formation. The drastic color
1
change and H NMR spectroscopy proved to be both very
helpful tools to determine the time-dependency of the desired
conversion. Under the applied conditions, a conversion of
12% was observed after 15 min, and complex 3 was isolated
in 95% yield after 16 h and appropriate workup. Since the
metal center in the NiII-salphen complex is much less Lewis
acidic, diagnostic shifts are generally observed for the imine
(7) For a transmetalation process carried out with oxime-based salen
derivatives: Akine, S.; Taniguchi, T.; Nabeshima, T. Angew. Chem.
Int. Ed. 2002, 41, 4670. However, in these structures only the central
all-oxygen-coordinated lanthanide metal ion is replaced whereas the
‘salen’-ligated metal centers (M ) Zn) remain unaffected.
(8) For a few recent examples of multimetallic salen-type systems see:
(a) Gallant, A. J.; Chong, J. H.; MacLachlan, M. J. Inorg. Chem. 2006,
45, 5248. (b) Hui, J. K.-H.; MacLachlan, M. J. Chem. Commun. 2006,
2480. (c) Ma, C. T. L.; MacLachlan, M. J. Angew. Chem., Int. Ed.
2005, 44, 4178. (d) Gallant, A. J.; MacLachlan, M. J. Angew. Chem.,
Int. Ed. 2003, 42, 5307. (e) Akine, S.; Taniguchi, T.; Nabeshima, T.
J. Am. Chem. Soc. 2006, 128, 15765. (f) Akine, S.; Sunaga, S.;
Taniguchi, T.; Miyazaki, H.; Nabeshima, T. Inorg. Chem. 2007, 46,
2959.
(9) We also tested the transmetalation of related Zn(salphen) complex 10
with Pd(OAc)2 under similar conditions and compared the conversion
data with those obtained in the syntheses for 4 and 14. The results
(see Supporting Information) revealed both an electronic and a steric
influence in the transmetalation protocol.
(10) Mun˜oz-Herna´ndez, M.-A.; Keizer, T. S.; Parkin, S.; Patrick, B.;
Atwood, D. A. Organometallics 2000, 19, 4416.
(11) Kleij, A. W.; Tooke, D. M.; Spek, A. L.; Reek, J. N. H. Eur. J. Inorg.
Chem. 2005, 4626.
7266 Inorganic Chemistry, Vol. 46, No. 18, 2007

COMMUNICATION
Scheme 3. In Situ Transmetalation of bis-salphen-Zn2 Complex 17
central N2O2 pocket of the salphen ligand seems readily
accessible for suitable electrophiles (cf. metal reagent) since
one of the two phenyl side-groups is not substituted at the
3-position. Interestingly, the crystal enclosed two distinct
conformations for 11 for which a number of bond distances
and angles around the central N2O2 pocket were clearly
different.12,13 The axial pyridine ligand in both conformations
showed no disorder but is nonsymmetrically situated above
the plane defined by the N and O atoms of the salphen
ligand.11
The transmetalation procedures that involved 8-11 af-
fording Ni(salphens) 12-15 were simply visually followed,
and full conversions were supported in all cases by separate
1H NMR analyses. It should be noted that the color change
(from yellow-orange to deep red) in the case of 12-13 was
significantly faster than observed for both 14 and 15, which
points at an electronic influence of the pendant groups on
the salphen framework. Complexes 12-15 were isolated in
excellent yield (90-100%) and fully analyzed by NMR, MS,
and elemental analyses (Supporting Information). In par-
ticular, Ni(salphen) derivatives 12 and 13 proved to be
sparingly soluble in the common organic solvents, which is
ascribed to the presence of nitro-aryl groups; this causes a
fast crystallization, even from DMSO.
Additionally, the transmetalation of the known bis-ZnII
complex of tetraimine 1614 was considered as a probe for
the introduction of multiple TM metal centers in multi-
metallic Zn(salphen) species. The subject of multimetallic
salen synthons is currently attracting profound interest in the
field of functional materials.8 The bis-ZnII-salphen complex
175b was generated in situ and directly submitted to the
transmetalation protocol by changing the solvent from MeOH
to THF, but essentially without isolation of intermediate 17.
After the addition of the NiII reagent, the evident change in
color from orange to deep purple-red unambiguously revealed
the formation of bis-NiII-salphen 18, and this compound
was isolated in 88% yield. MS analysis provided unambigu-
ous evidence for the formation of 18 since the presence of
two Ni centers is easily recognized in the observed isotope
pattern.
In conclusion, we here present a facile and effective
method for the transmetalation of readily available sym-
metrical and nonsymmetrical ZnII-salphen complexes. ZnII-
salphen derivatives have recently proven to be excellent
supramolecular building blocks.4g,5,6b An appropriate trans-
metalation (e.g, a ZnII-for-MnIII exchange, cf. compound 5)
will allow a predetermined change in the properties of the
metallosalphen unit. This approach will be beneficiary in
cases where it is desired to change the catalytic/supramo-
lecular properties of the salphen complex in situ (cf., complex
18). Besides, selective exchange of one of the ZnII ions in
17 affords interesting heterobimetallic metallosalphens that
will have significance as multifunctional materials and/or
catalysts. Multistage, one-pot catalytic conversions based on
this expanded salen technology may now be designed to
produce more efficient chemical processes. Studies directed
toward this challenging goal are presently ongoing in our
laboratory.
(12) Some crystallographic details for 11: C39H39N5O2Zn, Mw ) 675.12,
triclinic P1h, a ) 7.3337(2) Å, b ) 10.8421(2) Å, c ) 22.8755(7) Å,
R ) 100.664(2)°, â ) 96.051(2)°, γ ) 104.0700(10)°, Z ) 2, F )
1.310 Mg/M3, reflections collected 33 881, independent 11 004, GOF
on F2 ) 0.968, R1 ) 0.0482, wR2 ) 0.1267.
(13) For 11 two different disordered orientations in the crystal were found
in a 56:44 ratio, which is the consequence of two distinct conforma-
tional positions of the phenylazo unit. Only the Zn(1), O(1), O(2),
and pyridine atoms are located on unique positions in the crystal lattice.
Some small deviations are found as compared to a similar nonsym-
metric Zn(salphen) analogue (see ref 10). Selected bond distances-
(Å)/angles(deg) for 12 with esd’s: Zn(1)-O(1) ) 1.9760(11), Zn(1)-
O(2) ) 1.9972(11), Zn(1)-N(2′) ) 2.016(16), Zn(1)-N(1) )
2.073(11), Zn(1)-N(5) ) 2.1038(17), Zn(1)-N(2) ) 2.109(11),
Zn(1)-N(1′) ) 2.194(13), O(1)-Zn(1)-(O2) ) 96.96(4), O(1)-
Zn(1)-N(2′) ) 153.8(5), O(2)-Zn(1)-N(2′) ) 92.9(4), O(1)-
Zn(1)-N(1) ) 91.94(17), O(2)-Zn(1)-N(1) ) 156.3(2), N(2′)-
Zn(1)-N(1) ) 70.6(4), O(1)-Zn(1)-N(5) ) 95.62(6), O(2)-Zn(1)-
N(5) ) 93.80(6), N(2′)-Zn(1)-N(5) ) 107.9(5), N(1)-Zn(1)-N(5)
) 107.2(3), O(1)-Zn(1)-N(2) ) 160.3(4), O(2)-Zn(1)-N(2) ) 86.2-
(3), N(2′)-Zn(1)-N(2) ) 8.2(6), N(1)-Zn(1)-N(2) ) 78.4(3), N(5)-
Zn(1)-N(2) ) 103.5(4), O(1)-Zn(1)-N(1′) ) 83.8(2), O(2)-Zn(1)-
N(1′) ) 160.4(4), N(2′)-Zn(1)-N(1′) ) 79.1(4), N(1)-Zn(1)-N(1′)
) 8.5(3), N(5)-Zn(1)-N(1′) ) 105.6(4), N(2)-Zn(1)-N(1′) )
86.9(3).
Acknowledgment. A.W.K. is an ICREA fellow, and
ICREA and ICIQ are thanked for financial support. Dr.
Jonathan Barr, J. Salles, and L. Farjon are acknowledged
for the mass spectrometric studies
Supporting Information Available: Crystallographic details
for the structure of compound 11 (in CIF format), detailed
experimental and analytical details for all new derivatives, and
known compounds 5-6. This material is available free of charge
via the Internet at http://pubs.acs.org.
(14) Chichak, K.; Jacquemard, U.; Branda, N. R. Eur. J. Inorg. Chem. 2002,
357.
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Inorganic Chemistry, Vol. 46, No. 18, 2007 7267