Flexible cofacial binuclear metal complexes derived from α,α-bis(salicylimino)-m-xylene

Article abstract of DOI:10.1039/b412666a

The tetradentate Schiff-base ligand SIXH₂ (α,α-bis(salicylimino)-m-xylene), prepared from salicylaldehyde and m-xylylenediamine, forms cofacial binuclear complexes with Pd and Cu. Of the two isomers possible (trans-syn and trans-anti) for M₂(SIX) ₂, these complexes crystallize exclusively as the trans-anti isomer. In ansolvous Pd₂(SIX)₂, the metal-containing planes are approximately parallel, with Pd ... Pd 4.416(1) A. Pd ₂(SIX)₂ also forms a crystalline solvate, in which the molecules adopt a more open conformation with longer metal-metal distances (5.109(1) and 5.112(1) A). The M ... M distance is significantly longer in Cu₂(SIX)₂ (6.653(1) A), because of conformational changes in the m-xylylene moieties and substantial tetrahedral distortion about Cu.

Full text of DOI:10.1039/b412666a

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F U L L P A P E R
Flexible cofacial binuclear metal complexes derived from
a,a-bis(salicylimino)-m-xylene†
Andrew W. Maverick,* Ravi K. Laxman, Mark A. Hawkins, Daniel P. Martone and
Frank R. Fronczek
Department of Chemistry, Louisiana State University, Baton Rouge, LA, 70803-1804, USA
Received 17th August 2004, Accepted 29th October 2004
First published as an Advance Article on the web 29th November 2004
The tetradentate Schiff-base ligand SIXH
2
(a,a-bis(salicylimino)-m-xylene), prepared from salicylaldehyde and
m-xylylenediamine, forms cofacial binuclear complexes with Pd and Cu. Of the two isomers possible (trans-syn and
trans-anti) for M
2
(SIX)
2
, these complexes crystallize exclusively as the trans-anti isomer. In ansolvous Pd
2
(SIX) , the
2
˚
metal-containing planes are approximately parallel, with Pd · · · Pd 4.416(1) A. Pd
2
(SIX) also forms a crystalline
2
solvate, in which the molecules adopt a more open conformation with longer metal–metal distances (5.109(1) and
˚
˚
(SIX) (6.653(1) A), because of conformational
5
.112(1) A). The M · · · M distance is significantly longer in Cu
2
2
changes in the m-xylylene moieties and substantial tetrahedral distortion about Cu.
Introduction
Experimental
We have been investigating the chemistry of cofacial binuclear
Materials and procedures
1
transition-metal complexes derived from bis(b-diketone) and
2
Chemicals and solvents were reagent or spectrophotometric
bis(b-ketoenamine) ligands (see Fig. 1). One objective of this
1
13
grade and were used as received. H and C NMR spectra were
recorded on Bruker 200- and 400-MHz spectrometers. Cyclic
voltammograms were recorded by using either a PAR model
work has been to achieve the geometric features of the cofacial
3
diporphyrins with binucleating ligands that are more synthet-
ically versatile. The bis(b-ketoenamine) complexes Cu
2
(BBI)
2
1
74A Polarographic Analyzer or a Wenking TS70/1 potentiostat
and Ni (BBI) , for example (Fig. 1(b)), undergo quasireversible
2
2
with a home-built microcomputer interface. The cell contained
Pt working and counter electrodes and an aqueous Ag/AgCl
two-electron electrochemical oxidation, whereas the analogous
mononuclear complexes are oxidized only irreversibly by one
2
(3 M NaCl) reference electrode. The half-wave potential for
electron. Although we were able to characterize several of
+
the Fc/Fc (Fc = ferrocene) reference redox couple in CH
3
CN
these bis(b-ketoenamine) complexes, other M
2
(BBI) species
2
8
(
0.1 M (Bu
4
N)(O
3
SCF
3
) ) under these conditions was 0.47 V. Mi-
have proved more difficult to purify. This difficulty may occur
because the prepared complexes consist of several isomers:
the possibility of cis and trans arrangements of O and NH
groups at each metal leads to a total of five possible isomers
croanalyses were performed by Desert Analytics (Tucson, AZ).
We prepared mononuclear M(salim-Bz) complexes (Fig. 1(d),
2
R = benzyl; M = Pd, Cu; the free ligand Hsalim-Bz is also
9
known as saddamine) for electrochemical comparisons.
for M
2
(BBI)
2
. We now report the use of the binucleating Schiff
, in the
base a,a-bis(salicylimino)-m-xylene, which we call SIXH
2
ꢀ
a,a -Bis(salicylimino)-m-xylene (or
preparation of cofacial binuclear complexes of palladium and
copper; see sketches in Fig. 2. This work has two important
advantages over our previous studies of bis(b-ketoenamine)
m-xylylenebis(salicylaldimine)), SIXH
To a benzene solution (250 mL) of m-xylylenediamine (5.0 g,
.035 mol), salicylaldehyde (9.0 g, 0.070 mol) was added
2
complexes. First, the ligand SIXH is easier to prepare, in that
2
0
a single condensation reaction introduces both the m-xylylene
bridging group and the coordinating N atoms. Second, the
dropwise over a period of 45 min. The yellow reaction mixture
was stirred for an additional 45 min and then allowed to
stand overnight. The precipitate that formed was collected,
redissolved in dichloromethane and the resulting solution dried
binuclear complexes M
two isomers, show substantially greater flexibility in metal–metal
distance and conformation than the M (BBI) species.
The binuclear complex most closely related to the present
work is Cu (p-SIX) , recently prepared by McNelis et al.
Fig. 2(b)). As might be expected from the para geometry of the
bridging groups, this ligand leads to somewhat longer Cu · · · Cu
2
(SIX)
2
, which can be formed in only
2
2
over MgSO , filtered and concentrated. Yellow plate-like crystals
4
formed on standing; yield 11 g (90%). This material was recrys-
tallized from hexane; mp 62 C. Anal. Calc. for C22
C, 76.72; H, 8.13; N, 5.85. Found: C, 75.71; H, 8.13; N, 5.64%.
Although the recrystallized ligand was not analytically pure, no
impurities were evident in its NMR or mass spectra (parent ion,
m/z 344). NMR (in CDCl
represent positions shown in Fig. 2): H, d 4.82 (s, 4H, h); 7.3
◦
2
2
H
20
N
2
O :
2
4
(
˚
distances (ca. 7.3 A) than we find with the meta isomer. The
2
−
two SIX ligands have also been used to prepare binuclear
complexes with a single bridging ligand: a dimanganese complex
bridged by the meta isomer, and diruthenium complexes
3
, d/ppm vs. TMS; lower-case letters
1
5
13
(m, 12H, aromatic); 8.46 (s, 2H, g); 13.4 (s, 2H, l). C, d63.1 (h);
2
−
6
bridged by one p-SIX and related ligands. Wayland and co-
workers have used a single m-xylylene linker to join two Rh(II)
118.8 (f); 117.0, 118.6 (c, e); 126.8, 127.1, 129.0, 131.4, 132.3 (b,
d, j, k, m); 138.6 (i); 161.0 (a); 165.7 (g). (The preparation and
7
porphyrin moieties; this species reacts with methanol to yield a
1
13
H and C NMR assignments for this compound are similar to
H–Rh–Rh–CH
2
OH product.
10
those for p-SIXH
2
. )
Pd
A solution of (CH
0.51 g, 2.87 mmol) in 250 mL of acetonitrile was added
dropwise to a well stirred solution of SIXH (0.996 g, 2.9 mmol)
2
(SIX)
2
(1)
3
CN)
2
PdCl
2
prepared by dissolving PdCl
2
†
Electronic supplementary information (ESI) available: A summary of
the structure of solvated Pd (SIX) , and experimental and simulated
(SIX) . See http://www.rsc.org/suppdata/
(
2
2
1
H NMR spectra for Pd
dt/b4/b412666a/
2
2
2
and triethylamine (0.625 g, 6.18 mmol) in CH CN. After ca.
3
2
0 0
D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5

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Fig. 1 Schiff base and related metal complexes: (a) binuclear bis(b-diketone) complex M
2
(XBA)
2
, (b) binuclear bis(b-ketoenamine) com-
plex M (BBI) , (c) M(salen), (d) M(salim-R) , (e) and (f) chelating and macrocyclic binuclear complexes derived from 2,6-diformylphenol.
2
2
2
Fig. 2 Binucleating Schiff bases and their complexes: (a) SIXH
2
and the two possible isomers of M
2
(SIX)
2
; the trans-anti isomer predominates in
and
. (c) The m-phenylenediamine
the present work. Letters a–m denote positions for NMR assignments (see Table 1 and Fig. S1†); a and b are the torsion angles about the N–CH
2
4
CH
2
–arene bonds, respectively. (b) p-SIXH
and its Cu complex.
2
, as studied by McNelis et al.; they also isolated only trans-anti-Cu
2
(p-SIX)
2
derivative sal-PDAH
2
3
0 min, a small amount of yellow precipitate was removed by
filtration and the yellow–orange filtrate evaporated to dryness.
The residue was stirred with CH Cl , the resulting mixture
filtered again, and the filtrate evaporated to produce crude
Pd (SIX) . The complex was obtained in analytically pure,
crystalline form by layering a CH Cl solution of the crude
material with CH OH. Anal. Calc. for Pd : C, 58.88;
H, 4.04; N, 6.24. Found: C, 58.51, H, 3.88, N, 6.30%. H and
has been shown (by X-ray analysis; see below) to have the trans-
anti geometry, are presented in Table 1. (Additional resonances
with similar coupling patterns, which may be due to small
amounts of the trans-syn isomer, were visible in some spectra
2
2
2
2
of partially purified Pd
2
(SIX) . However, these disappeared
2
2
2
on repeated recrystallization; all further experiments reported
3
2
C
44
H
36
N
4
O
4
herein were performed on fully purified trans-anti material.)
1
1
The H chemical shifts and coupling constants for resonances
1
3
C NMR spectral data for purified crystalline Pd
2
(SIX)
2
, which
b–f, g and h were identical (within ± 0.01 ppm and ± 0.1 Hz,
D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6
2 0 1

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1
13
a
2 2
Table 1 H and C NMR parameters for trans-anti-Pd (SIX)
a
b
c
d
e
f
g
h
i
j
k
m
1
H d/ppm
6.63
dd
6.25
ddd
6.96
ddd
6.35
d (br)
7.44
s (br)
4.00, 5.70
dd (AB)
13.4
7.15
dd
7.29
dt
8.02
s (br)
Pattern
2
J/Hz
3
J/Hz
7.9
1.8
6.8, 7.8
1.0
6.8, 8.6
1.8
8b .0
7
3
7
4
b
b
J/Hz
5
J/Hz
1.5
127.6
13
C d/ppm
162.8
134.4
114.7
133.9
119.3
119.6
164.0
63.2
148.6
125.6
126.8
a
b
In CD
resolved.
2
Cl
2
; chemical shifts vs. TMS. Lower-case letters a–m refer to the carbon atoms labeled in Fig. 2 or to their attached hydrogen atoms. Not
◦
Table 2 Selected interatomic distances (A
˚
) and angles ( ) for M (SIX)
2
2
respectively) in spectra recorded at 200 and 400 MHz. Therefore,
second-order effects were judged to be negligible in this region.
Chemical shifts (± 0.02 ppm) and coupling constants (± 0.5 Hz)
for resonances j–m were determined from a homonuclear 2D J-
M = Pd (1)
M = Cu (2)
1
M1–O1A
M1–O1B
M1–N1A
M1–N1B
M2–O2A
M2–O2B
M2–N2A
M2–N2B
1.9764(9)
1.9749(8)
2.0190(11)
2.0206(10)
1.9757(8)
1.9843(9)
2.0187(9)
2.0225(10)
1.916(2)
1.901(2)
1.964(3)
1.977(3)
1.8991(19)
1.9077(19)
1.953(3)
1.958(3)
resolved spectrum. Observed and simulated 200 MHz H spectra
for Pd
2
(SIX)
2
are included as supplementary data.
Cu
A solution of SIXH
was shaken vigorously with Cu(NH
pared from CuSO O and NH (aq)) in a separatory funnel
·5H
over a 30-min period. The dark violet–brown organic layer was
separated, dried over Na SO , filtered and evaporated to dryness.
2
(SIX)
2
·CH
2
Cl
2
(2)
2
(0.904 g, 2.62 mmol) in dichloromethane
2
+
3
)
4
(aq) (26.2 mmol, pre-
4
2
3
O1A–M1–O1B
O1A–M1–N1A
O1A–M1–N1B
O1B–M1–N1A
O1B–M1–N1B
N1A–M1–N1B
O2A–M2–O2B
O2A–M2–N2A
O2A–M2–N2B
O2B–M2–N2A
O2B–M2–N2B
N2A–M2–N2B
178.77(4)
92.64(4)
87.79(4)
87.55(3)
92.05(4)
178.66(4)
177.31(4)
92.42(4)
87.59(4)
88.14(3)
91.91(4)
178.53(4)
149.58(10)
93.70(11)
92.84(10)
92.27(10)
93.97(10)
155.47(10)
151.20(9)
93.16(9)
2
4
The dark residue (ca. 1.0 g) was dissolved in dichloromethane
and the solution dried, filtered, and layered with ethanol.
Dark violet crystals (yield 0.75 g, 70%) were deposited over
a period of several days. Anal. Calc. for Cu
2
C
45
H
38
N
4
O
4
Cl
2
(
Cu (SIX) Cl ): C, 60.27; H, 4.27, N, 6.25. Found: C, 60.43,
2
2
·CH
2
2
91.55(10)
92.50(10)
93.83(9)
H, 4.10, N, 6.10%.
X-Ray analyses
157.65(11)
Crystals obtained as described above were suitable for X-ray
analysis. Because of difficulties encountered initially in solving
the structure of Pd
other solvent combinations that might yield the complex in a
different crystalline form. We found that a solution of Pd (SIX)
in CH Cl –CH OH (9 : 1 v/v) yielded triclinic solvated crystals
as well as monoclinic Pd (SIX) (1) on layering with pentane. The
solvated crystals were extensively twinned; this led to poorer X-
ray data and refined parameters than in the other two structures.
The structures of 1 and 2 are presented in full; a summary of the
2
(SIX)
2
(see below), we experimented with
and the resulting structure showed a surprisingly large number
of unreasonable bond distances and angles. Palenik et al.
have discussed this type of structure, in which the heavy atoms
11
2
2
2
2
3
are located near y = 0.25 in space group P2
1
/c. Under these
2
2
conditions, a two-fold ambiguity (differing by 0.25 in z) exists
in coordinates for the heavy atoms derived from the Patterson
function. Indeed, when we shifted the starting z coordinates for
the two Pd atoms by 0.25, the much more reasonable solution
described here was quickly reached.
structure of solvated Pd
2
(SIX) is available as ESI.†
2
Diffraction data were collected on a Nonius KappaCCD
diffractometer fitted with a Mo-Ka source, a graphite mono-
chromator, and an Oxford Cryostream low-temperature device.
Cu
2
(SIX)
2
·CH
2
2
Cl (2)
Crystal data: Pd
monoclinic, space group P2
2
(SIX)
2
(1), C44
H
36
N
4
O
4
Pd
2
, M = 897.63,
Refinement of this structure was straightforward.
1
/c (no. 14), a = 10.6000(10), b =
◦
−
3
˚
˚
CCDC reference numbers 247674 (1) and 247675 (2).
See http://www.rsc.org/suppdata/dt/b4/b412666a/ for cry-
stallographic data in CIF or other electronic format.
2
2.796(3), c = 15.486(2) A, b = 102.341(6) , U = 3655.5(8) A ,
1
T = 100 K, Z = 4, l(Mo-Ka) = 1.03 mm , 93055 reflections
◦
measured (with h < 40.3 ), 22828 unique (Rint = 0.048), which
2
were used in all calculations. Final R = 0.033, wR(F ) = 0.086.
Cu
2
(SIX)
2
·CH
2
Cl
2
(2), C45
H
38Cl
2
Cu
2
N
4
O
4
, M = 896.82, mon-
Results
oclinic, space group P2
1
/c (no. 14), a = 9.931(5), b = 10.021(5),
◦
3
˚
˚
Synthesis of ligand and complexes
c = 40.358(19) A, b = 93.892(16) , U = 4007(3) A , T = 120 K,
−
1
Z = 4, l(Mo-Ka) = 1.24 mm , 24789 reflections measured
The preparation of a,a-bis(salicylimino)xylene, as a mixture
◦
(
with h < 26.0 ), 7110 unique (Rint = 0.042), which were used in
12
of meta and para isomers, was first reported in 1973.
A
2
all calculations. Final R = 0.037, wR(F ) = 0.073.
Selected bond distances and angles are given for 1 and 2 in
Table 2.
13
photochemical study of the pure meta isomer was published,
but without preparative details. Our procedure for the pure
meta isomer, from m-xylylenediamine and salicylaldehyde in
1
: 2 ratio, is similar to that recently reported by Ha and
Pd
2
(SIX) (1)
2
14
co-workers Reaction of SIXH
triethylamine, or with aqueous Cu(NH
clear complexes M (SIX) . Small quantities of oligomeric or
polymeric materials formed in these reactions were much less
2
with PdCl
2
(CH
3
CN)
2
and
2
+
Initial attempts to solve this structure gave coordinates for Pd1
3 4
)
, affords the binu-
and Pd2 of approximately (0.17, 0.25, −0.03) and (0.12, 0.25,
2
2
−0.32) respectively. However, this model was difficult to refine,
2
0 2
D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6

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soluble and readily removed by filtration. (In contrast, for
example, our attempts to prepare Ni (SIX) by similar methods
gave only insoluble, probably polymeric, powders.) Pd (SIX)
2
2
2
2
ordinarily crystallizes in ansolvous monoclinic needles (1);
however, if the solvent mixture contains methanol, some of the
material crystallizes in a solvated triclinic form (see ESI†).
Crystal structure analyses
The M
plexes joined at the N atoms by m-xylylene bridges. Structures
for the two possible isomers for M (SIX) , trans-syn (idealized
symmetry C2h) and trans-anti (idealized symmetry D ), are
2
(SIX)
2
complexes consist of two trans-M(salim)
2
com-
2
2
2
shown in Fig. 2. The structures reported here contain only the
trans-anti isomer. NMR spectral data for purified trans-anti-
Pd
Individual trans-anti-M
rality, and the two enantiomers can be designated P and M
2
(SIX)
2
are discussed below.
2
(SIX)
2
molecules have helical chi-
15
according to the screw sense of the m-xylylene bridging groups.
Since the structures are centrosymmetric, they contain equal
numbers of the two enantiomers. The P enantiomer is shown in
the ORTEP drawings in Figs. 3 (Pd) and 4 (Cu).
Fig. 4 ORTEP drawing of Cu
0% probability level.
2
(SIX)
2
·CH
2
Cl
2
, 2, with ellipsoids at the
5
slightly greater deviation from planarity is evident in the larger
Pd(salim)
2
moieties: the two (salim)Pd least-squares planes make
◦
angles of 18.34(2) and 8.61(4) at Pd1 and Pd2, respectively. The
larger of these slight distortions is noticeable as a slight saddle
shape at Pd1 (see Fig. 3).
The Pd
2
(SIX)
2
molecules in 1 are arranged in stacks along
˚
c, with short intermolecular contacts (Pd1 · · · Pd2N 3.412(1) A)
suggesting weak p interactions between adjacent molecules. It is
noteworthy that this intermolecular Pd · · · Pd distance is approx.
˚
A shorter than the intramolecular distance.
1
(
b) Cu
2
(SIX)
2
·CH
2
2
Cl (2)
˚
The metal–metal distance in this structure (6.653(1) A; see
Fig. 4) is much larger than that in the Pd complex. There are
also several other significant changes in geometry in Cu
2
(SIX) .
2
First, the environment about the Cu atoms is distorted away
from square-planar toward tetrahedral: the dihedral angles
between the Cu1–O1A–N1A and Cu1–O1B–N1B planes, and
between the Cu2–O2A–N2A and Cu2–O2B–N2B planes, are
◦
3
8.2(1) and 35.7(1) respectively. Second, the departure of the
two Cu(salim) moieties from parallelism is much greater: the
2
dihedral angle between the Cu1–O1A–O1B–N1A–N1B and
Cu2–O2A–O2B–N2A–N2B planes is 62.57(6) . And finally, the
◦
2 2
Fig. 3 ORTEP drawings of Pd (SIX) (1), with ellipsoids at the 50%
probability level. (a) Top view, showing orientation of m-xylylene
m-xylylene bridges in Cu
2
(SIX) are turned still farther inward,
2
bridging groups. (b) Side view, with complete atom numbering scheme.
so that they are almost touching: the closest nonbonded contact
˚
between these groups is C11A · · · C11B 3.663(5) A. This close
approach may reflect a weak p interaction between the m-
(
a) Pd
The Pd
are very close to the idealized trans-anti-M
2
(SIX)
2
(1)
molecules in 1 (Pd · · · Pd 4.416(1) A; see Fig. 3)
(SIX) shape of
xylylene groups. (There are also short intermolecular contacts,
˚
˚
2
(SIX)
2
e.g. 3.381(4) A between N1A of one molecule and C4A of an
2
2
adjacent molecule.)
Fig. 2. The immediate coordination environments of the Pd
atoms are planar within 0.038(1) A, and nearly parallel: the
The close intermolecular M · · · M contacts observed in 1
are absent in 2. This is most likely because the nonplanar
coordination geometry about the Cu atoms in 2 prevents close
approach of adjacent metal atoms.
˚
dihedral angle between the Pd1–O1A–O1B–N1A–N1B and
◦
Pd2–O2A–O2B–N2A–N2B least-squares planes is 4.62(7) . A
D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6
2 0 3

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a
Table 3 Electrochemical data
was this requirement that we wished to take advantage of in the
preparation of M
2
(SIX)
2
.
Complex
Reduction
Oxidation
Comparison with other binuclear complexes
Cu
Cu(salim-Bz)
Pd (SIX)
Pd(salim-Bz)
2
(SIX)
2
− 0.91
− 1.37
− 1.04
− 0.90
1.07
0.76
0.98
20
2
We have studied a variety of bis(b-diketones) and bis(b-
2
2
2
ketoenamines) as ligands for cofacial binuclear metal com-
2
> 1.5
plexes. The SIXH ligand introduced here provides three ad-
2
a
−1
vantages over the others we have examined. First, SIXH is
easy to prepare, requiring only one step from readily available
Peak potentials (cyclic voltammetry, vs. Ag/AgCl, in CH
for irreversible reduction and oxidation.
3
CN; 1 V s
)
2
starting materials; it is air-stable and does not hydrolyze readily.
Second, the number of isomers possible for the binuclear
NMR spectra
complexes M
2
(SIX)
2
is more limited. And third, the position
(SIX)
and orientation of the m-xylylene bridging groups in M
leads to greater conformational flexibility than is observed in
the other binuclear complexes.
2
2
1
The H NMR spectrum for crystalline trans-anti-Pd
2
(SIX) (1;
2
1
see Fig. S1, ESI†) is well resolved. Assignments of the H and
1
3
C NMR data were made with use of two-dimensional spectra
Ligands closely related to SIXH
2
have been used previously
(p-
shows helical chirality similar to that of the Cu and
and homonuclear decoupling experiments. Protons c and d (see
labeling scheme in Fig. 2) appear as partially overlapping eight-
line patterns (ddd), and proton b as a doublet of doublets, due to
to prepare binuclear metal complexes. The structure of Cu
2
4
SIX)
2
Pd complexes described here. Although this type of helicity
has been exploited more extensively with polypyridine and
related ligands, it is also possible with Schiff bases, even when
3
1
1
4
1
1
J( H– H) and J( H– H) coupling. The e, g and h resonances
are all noticeably broadened, probably due to weak coupling
among them or to N.
15
the local coordination environments approach planarity (as in
Pd
2
(SIX) ).
2
Electrochemistry
Other binucleating ligands lead to a coplanar environment for
the two metal atoms, because they contain OH groups that can
bridge two metal atoms on deprotonation. Binuclear complexes
of this type, with the general structure shown in Fig. 1(e), have
been prepared with various bridging ligands L and substituents
Like many of the other binuclear complexes we have studied, the
new species M
carbon solvents; they are also slightly soluble in CH
voltammetry reveals irreversible oxidation and reduction waves
for both Cu (SIX) and Pd (SIX) , which are better defined
in CH CN than in CH Cl . (We found no sign of chemical
reversibility in these waves at scan rates up to 20 V s , in
2
(SIX)
2
are most soluble in halogenated hydro-
3
CN. Cyclic
21
22
X. A similar macrocyclic coplanar binuclear complex is
shown in Fig. 1(f). SIXH , on the other hand, cannot chelate
2
2
2
2
2
3
2
2
to one metal atom, and cannot bridge two close-lying metal
−
1
2−
atoms effectively; instead, two SIX ligands can bridge two
either solvent.) Table 3 lists peak potentials for the M
complexes and for the mononuclear analogs M(salim-Bz)
Fig. 1(d), R = PhCH
binuclear complexes is the oxidation of Pd
least 0.5 V easier than that for Pd(salim-Bz)
some stabilization of the higher oxidation states (Pd ), as we
2
(SIX)
2
metal atoms in the desired cofacial manner.
2
A macrocyclic ligand based on o-phenylenebis(salicylimine),
with polyether bridging groups, has been shown to form trinu-
(
2
). The best-defined new feature in the
(SIX) , which is at
. This suggests
2
2
23
24
25
clear complexes. Nickel(II) and cobalt(II) complexes of the
bis(Schiff base) ligand 2,6-bis(salicyliminomethyl)pyridine have
been reported. This ligand is likely to yield mononuclear com-
plexes by using the central pyridine N atom for coordination,
2
III
2
have observed previously in the M
were unable to isolate the oxidized Pd
chemical or electrochemical methods.
2
(BBI)
2
systems. However, we
2
(SIX) species by either
2
though cofacial binuclear complexes similar to M
also be possible. Ha and co-workers prepared Be, Mg and Zn
complexes of SIXH and related ligands for electroluminescence
2
(SIX)
2
may
The electrochemical waves for Cu
2
(SIX)
2
, both in CH CN and
3
2
CH Cl , were also complicated by adsorption. Successive anodic
2
2
14
applications, but they did not determine the structures of these
complexes.
scans, for example, showed gradually shifting peak potentials
and currents. After several such scans, the initial voltammogram
shape could be restored by polishing the electrode.
The copper(II) complex of the m-phenylenediamine derivative
26,27
sal-m-pdaH
2
(see Fig. 2(c)) is known to be binuclear.
This
We also recorded the cyclic voltammograms of Pd
2
(SIX)
the presence of Cl and Br . These experiments were suggested
by the work of Ooi and co-workers with Pd (pyt)
2
in
complex exhibits helical chirality and a coordination environ-
−
−
ment about Cu similar to that in Cu
between the Cu(salim) planes in Cu
the m-phenylene bridging groups inCu
2
(SIX)
(sal-m-pda)
2
(the dihedral angle
2
4
(pytH = 2-
◦
2
2
is 43.6 ), but
16
pyridinethiol). They found more facile oxidation of Pd
2
(pyt)
4
2
(sal-m-pda)
2
force the Cu
−
in the presence of halide ions X , due to formation of the
˚
atoms considerably farther apart (Cu · · · Cu 7.435(2) A). Also,
III
metal–metal bonded species Pd
ions have no significant effect on the electrochemical oxidation
of Pd (SIX) . This difference may be due to the fact that the
relatively large Pd · · · Pd distance in Pd (SIX) (compared to
) makes the formation of Pd–Pd bonds
2
(pyt)
4
X
2
. In contrast, halide
the Cu(salim) moieties in the molecule are approximately copla-
2
nar rather than cofacial, so that the bridging groups occupy the
central portion of the complex between the two Cu atoms. Still
larger metal–metal distances, but with similar environments at
2
2
2
2
16
˚
2
.677(1) A for Pd
2
(pyt)
4
ꢀ
the two metal atoms, are obtained with the salicylidenes of 4,4 -
difficult.
28
ꢀ
29
diaminodiphenyl ether and 4,4 -diaminodiphenylmethane.
Bis(salicylidene) ligands with hydrocarbon bridging groups
30
have been prepared by Hiratani and co-workers and used
to synthesize binuclear Ti(IV) oxo complexes. Diplatinum(II)
metalloreceptors recently reported by Hanan and co-workers
Discussion
31
Isomerism in M
2
(SIX)
2
and related complexes
are based on pyridine ligands, but they provide similar internal
cavities and are able to bind anions as internal guests.
Mononuclear planar Schiff-base complexes M(salim-R) (see
2
Fig. 1(d)) can form both cis and trans isomers when R =
H. Although the trans isomer has been assumed for most such
Conformations of M
The substantial differences in geometry between Cu
Pd (SIX) complexes (see Figs. 3 and 4) are not due to changes
2
(SIX)
2
1
7,18
complexes,
we have shown that the configuration at the metal
(BBI)
(M = Ni,
Pd) is cis. Howie and Fay have also separated the cis and trans
atoms in the closely related binuclear species M
2
2
2
(SIX) and
2
2
2
2
19
isomers of a similar mononuclear Pd complex. On the other
in bond distances, which differ only slightly between Cu and
Pd. (The largest differences are observed in the M–N and M–O
hand, for larger substituents R, M(salim-R) must be trans; it
2
2
0 4
D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6

View Article Online
Table 4
M
2
(SIX)
2
torsion angles
Type
can lead to much larger displacements in the M(salim) moieties
2
of M
2
(SIX)
2
than are possible for M
2
(BBI) .
2
a
M = Pd (1)
M = Cu (2)
Coordination environments
C7A–N1A–C8A–C9A
N1A–C8A–C9A–C10A
C13A–C15A–N2A–C16A
C12A–C13A–C15A–N2A
C7B–N1B–C8B–C9B
N1B–C8B–C9B–C10B
C13B–C15B–N2B–C16B
C12B–C13B–C15B–N2B
a
− 109.30(12)
− 172.94(10)
− 88.49(12)
127.23(10)
− 101.65(12)
− 174.75(11)
− 79.62(12)
128.81(11)
− 110.8(3)
8
b
a
b
a
b
a
b
47.8(4)
The d Pd complex 1 shows the expected square-planar ge-
− 102.2(3)
− 75.9(3)
− 111.2(3)
− 102.3(4)
− 109.1(3)
74.5(4)
ometry, whereas Cu
tetrahedral coordination. Holm and O’Connor
2
(SIX)
2
is significantly distorted toward
18b
used the
dihedral angle h between the two O–M–N planes as an indication
of the square planar-tetrahedral geometry change. The values of
◦
h at Cu1 and Cu2 in Cu
2
(SIX)
2
, 38.2(1) and 35.7(1) , are similar
◦
to that observed (35.6 ) in one of the polymorphs of Cu(salim-
a
Torsion angle about CH
in Fig. 2.
2
–N (a) or CH
2
-C
6
H
4
(b) bond; see illustration
32
C
2
H
5
)
2
.
Solid-state geometries of other Cu(salim-R)
range from planar (R = H, CH , CH CH OH, C , C
to more highly distorted (R = CH(CH , 59.7 ; R = C(CH
3.6 ). This variability in the solid state, even for R groups of
similar sizes in Cu(salim-R) , suggests that the planar and non-
planar structures are very close in energy. The same is likely to be
true in the present case; thus, in solution, Cu (SIX) molecules
2
species
3
2
2
3
H
33
7
4
H
9
, Ph)
◦
3
)
2
3
) ,
3
◦
34
5
2
˚
distances, which are ca. 0.05–0.10 A shorter for Cu than for Pd.)
Changes in bond angles at the metal atoms are more important:
the tetrahedral distortion in Cu
be considerably farther apart than the N atoms.
2
2
2
(SIX) allows the Cu atoms to
2
may interconvert among a variety of conformations.
The largest changes are found in the conformations of the
Redox reactions
N–CH
2
-C
6
H
4
-CH
2
-N bridging groups. The m-xylylene moiety
groups:
provides an essentially fixed distance between the CH
2
We observed only irreversible waves in cyclic voltammograms
of M (SIX) and the mononuclear analogs M(salim-Bz)
the C8A · · · C15A and C8B · · · C15B intramolecular distances
2
2
2
(M =
˚
Pd, Cu; see Table 3); thus, it is difficult to draw quantita-
tive conclusions. However, comparisons of the oxidative peak
potentials may be chemically useful. First, Pd (SIX) appears
are between 5.02 and 5.08 A in 1 and 2. However, the
N · · · N distances vary considerably more (N1A · · · N2A and
˚
˚
N1B · · · N2B range from 5.20 A in 1 to 6.37 A in 2), along with
2
2
changes in the C
6
H
4
-CH
2
torsion angle b; see Table 4 and Fig. 2.
to be significantly easier to oxidize than Pd(salim-Bz)
have previously noted a similar trend in Pd (BBI) and related
complexes; both of these may indicate stabilization of the
2
. We
We have previously discussed the effects of ligand confor-
2
2
2
mation on the ability of m-xylylenebis(b-diketones) to form
20
II,III
+
cofacial binuclear complexes. A similar analysis, based on
the torsion angles at the N–CH (i.e. atoms N1–C8 and C15–
N2; designated a) and C –CH bonds (atoms C8–C9 and
C13–C15; designated b) applies here; see labels in Fig. 2. The
oxidized binuclear complex [Pd
interaction. Second, the opposite shift occurs in the Cu system,
with the binuclear species more difficult to oxidize. This shift
may be connected with the significant distortion of Cu
toward tetrahedral geometry: Cu(salim-Bz) is very flexible, so
that oxidation to Cu (which would strongly favor square-
planar coordination ) is expected to occur readily; in contrast,
oxidizing the less flexible Cu
2
]
by weak metal–metal
2
6
H
4
2
2
(SIX)
2
◦
◦
symmetrical conformation represented by a = 90 (or −90 )
2
◦
◦
III
and b = 0 (or 180 ) is readily accessible for our previous
bis(b-diketone) and bis(b-ketoenamine) complexes (see, for
example, M (XBA) , Fig. 1(a)); it would probably also be
favorable for the trans-syn isomer of M (SIX) . The trans-
1
2
35
III
2
2
2
(SIX) to a square-planar Cu
2
complex would require substantial structural rearrangement.
2
2
anti isomer requires a more twisted orientation of the m-
xylylene bridging groups. Molecular models suggest that the
Summary
most symmetrical arrangement of M(salim)
2
planes and m-
point-
New cofacial binuclear metal complexes have been prepared
using the a,a-bis(salicylimino)-m-xylene (SIXH
geometry of the M (SIX) complexes is similar to that of the
cofacial bis(b-diketone) and bis(b-ketoenamine) species we have
studied previously. However, the new complexes are easier to
prepare in isomerically pure form. Also, the orientation of
the m-xylylene bridging groups allows for substantially greater
conformational flexibility in the complexes. Future work in this
area includes related complexes with N
metal atoms, which should undergo more facile redox reactions;
and multifunctional analogs of SIXH
supramolecular species.
xylylene bridges in (P)-trans-anti-M (SIX) , with full D
2
2
2
2
) ligand. The
group symmetry, would give a and b of approximately −80
◦
2
2
and 165 , respectively. These values are approached closely in
Pd
2
(SIX)
2
(1). (The molecules in the crystalline solvated form
of Pd
2
(SIX)
2
have a somewhat more open conformation, with
˚
greater variations in a and b, and Pd · · · Pd distance ca. 5.11 A;
see ESI.†) For Cu
2
(SIX) (2), the variations in both a and b
2
are still more severe, with the m-xylylene moieties turned so far
toward the open side of the complex that they nearly touch each
other.
4
coordination about the
2
, for construction of larger
Thus, the M
2
(SIX) complexes studied here exhibit substantial
2
conformational flexibility. Some of this flexibility is associated
with distortions of the metal coordination environment to-
Acknowledgements
ward tetrahedral in Cu
2
(SIX)
2
. However, even the Pd
2
(SIX)
2
molecules in 1 and its solvate (see ESI†), all of which are
essentially square-planar at Pd, are noticeably different in
conformation and show Pd · · · Pd distances that vary by nearly
Acknowledgment is made to the Donors of the Petroleum Re-
search Fund, administered by the American Chemical Society,
and to the U. S. Department of Energy (DE-FG02-01ER15267)
for support of this research. M. A. H. acknowledges additional
support through ACS Project SEED.
˚
0
.7 A.
This greater flexibility, compared to the m-xylylene-bridged
complexes we have studied previously, is due largely to the
orientation of the bridges. In M (NBA) and M (BBI) , for
2
2
2
2
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D a l t o n T r a n s . , 2 0 0 5 , 2 0 0 – 2 0 6
2 0 5

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