Supramolecular self-assembled polynuclear complexes from tritopic, tetratopic, and pentatopic ligands: Structural, magnetic and surface studies

Inorg. Chem. 2007, 46, 7767−7781

Supramolecular Self-Assembled Polynuclear Complexes from Tritopic,

Tetratopic, and Pentatopic Ligands: Structural, Magnetic and Surface

Studies

Subrata K. Dey,^†Tareque S. M. Abedin,^†Louise N. Dawe,^†Santokh S. Tandon,^‡Julie L. Collins,^†

Laurence K. Thompson,*^,†Andrei V. Postnikov,^§Mohammad S. Alam,^|and Paul Muller*^,|

1

Department of Chemistry, Memorial UniVersity, St. John’s, Newfoundland, A1B 3X7, Canada,

Department of Chemistry, Kent State UniVersity, Salem, Ohio 44460, Institute de Physique

Electronique et Chimie Laboratoire de Physique des Milieux Denses, Paul Verlaine UniVersity,

1 Bd Arago F-57078, Metz, France, and Physikalisches Institut III, UniVersita¨t

Erlangen-Nu¨rnberg, Erwin-Rommel-Strasse 1, 91058 Erlangen, Germany

Received February 21, 2007

Polymetallic, highly organized molecular architectures can be created by “bottom-up” self-assembly methods using

ligands with appropriately programmed coordination information. Ligands based on 2,6-picolyldihydrazone (tritopic

and pentatopic) and 3,6-pyridazinedihydrazone (tetratopic) cores, with tridentate coordination pockets, are highly

specific and lead to the efficient self-assembly of square [3

grids. Subtle changes in the tritopic ligand composition to include bulky end groups can lead to a rectangular 3

[1 3] Mn₉grid, while changing the central pyridazine to a more sterically demanding pyrazole leads to simple

× 3] Mn₉, [4 × 4] Mn₁₆, and [5 × 5] Mn₂₅nanoscale

×

dinuclear copper complexes, despite the potential for binding four metal ions. The creation of all bidentate sites in

a tetratopic pyridazine ligand leads to a dramatically different spiral Mn₄strand. Single-crystal X-ray structural data

show metallic connectivity through both

µ-O and µ-NN bridges, which leads to dominant intramolecular

antiferromagnetic spin exchange in all cases. Surface depositions of the Mn₉, Mn₁₆, and Mn₂₅square grid molecules

on graphite (HOPG) have been examined using STM/CITS imagery (scanning tunneling microscopy/current imaging

tunneling spectroscopy), where tunneling through the metal d-orbital-based HOMO levels reveals the metal ion

positions. CITS imagery of the grids clearly shows the presence of 9, 16, and 25 manganese ions in the expected

square grid arrangements, highlighting the importance and power of this technique in establishing the molecular

nature of the surface adsorbed species. Nanoscale, electronically functional, polymetallic assemblies of this sort,

created by such a bottom-up synthetic approach, constitute important components for advanced molecule-based

materials.

Introduction

Co(II), Zn(II)),¹M₉[3 × 3] (Scheme 1; tritopic ligand self-

assembly; Ag(I)),²and M₁₆[4 × 4] (Pb(II))^3,4square grids

have been reported. A pentatopic pyridazine ligand, synthe-

sized by Lehn did not produce the expected [5 × 5] grid,

but instead, an icosanuclear Ag(I)₂₀partial grid formed.⁵

In the area of “controlled” molecular self-assembly of high

nuclearity coordination complexes, ligand design is crucial,

and successful approaches with “linear” heterocyclic ditopic,

tritopic, and tetratopic pyrimidine- and pyridazine-based

ligands have led to the synthesis of a number of square-

based polymetallic grids. Examples of M₄[2 × 2] (Fe(II),

(1) Patroniak, V.; Baxter, P. N. W.; Lehn, L.-M.; Kubicki, M.; Nissinen,

M.; Rissanen, K. Eur. J. Inorg. Chem. 2003, 4001.

(2) Baxter, P. N. W.; Lehn, J.-M.; Fischer, J.; Youinou, M. T. Angew.

Chem., Int. Ed. 1994, 33, 2284.

* To whom correspondence should be addressed. E-mail: lthomp@mun.ca

(L.K.T); phm@physik.uni-erlangen.de (P.M). Fax: 709-737-3702 (L.K.T).

^†Memorial University.

(3) Garcia, A. M.; Romero-Salguero, F. J.; Bassani, D. M.; Lehn, J-M.;

Baum, G.; Fenske, D. Chem.sEur. J. 1999, 5, 1803.

(4) Onions, S. T; Frankin, A. M.; Horton, P. N.; Hursthouse, M. B.;

Matthews, C. J. Chem. Commun. 2003, 2864.

(5) Baxter, P. N. W.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.sEur.

J. 2000, 6, 4510.

^‡Kent State University Salem Campus.

^§Paul Verlaine University.

^|Universita¨t Erlangen-Nu¨rnberg.

10.1021/ic070336a CCC: $37.00

Published on Web 08/14/2007

Inorganic Chemistry, Vol. 46, No. 19, 2007 7767

Dey et al.

Scheme 1

Chart 1

Figure 1. POVRAY structural representaion of the cation in 1: magenta

) Mn, green ) Cl, blue ) N, red ) O, black ) C.

) Mn(II), Co(II), Ni(II), Cu(II), Zn(II)) in high yield.^6-8[3

× 3] M₉(M ) Mn(II), Fe(III), Cu(II), Zn(II)) grids, based

on closely related tritopic 2,6-picolinic-dihydrazone ligands

(e.g., 2poap and analogues, Chart 1), also form in high yield

in the self-assembly reactions (Scheme 1).^9-13[Mn₉(Cl2poap)₆]-

(ClO₄)₆(1)¹⁰(Figure 1, Chart 1, Cl2poap) is a typical

example, with the nine metal ions held in a [3 × 3] grid

array within the assembly of six ligands. The Mn(II) centers

are antiferromagnetically coupled because of superexchange

through the twelve hydrazone oxygen bridges, resulting in

a ground-state spin of S′ ) 5/2.

Some Mn(II)₉grids exhibit unprecedented redox proper-

ties, with an eight-electron reversible redox window between

0.5 and 1.6 V (vs Ag/AgCl),^9,10,12,13associated with the

oxidation of the eight Mn(II) centers in the outer ring of the

grid to Mn(III). The magnetic properties are modulated when

oxidation occurs, leading to controlled changes in the ground-

state spin.^9,13Suitably modified grids with sulfur- and

(6) Matthews, C. J.; Avery, K.; Xu, Z.; Thompson, L. K.; Zhao, L.;

Miller, D. O.; Biradha, K.; Poirier, K.; Zaworotko, M. J.; Wilson, C.;

Goeta, A. E.; Howard, J. A. K. Inorg. Chem. 1999, 38, 5266-5276.

(7) Thompson, L. K.; Matthews, C. J.; Zhao, L.; Xu, Z.; Miller, D. O.;

Wilson, C.; Leech, M. A.; Howard, J. A. K.; Heath, S. L.; Whittaker,

A. G.; Winpenny, R. E. P. J. Solid State Chem. 2001, 159, 308.

(8) Dawe, L. N.; Abedin, T. S. M.; Kelly, T. L.; Thompson, L. K.; Miller,

D. O.; Zhao, L.; Wilson, C.; Leech, M. A.; Howard, J. A. K. J. Mater.

Chem. 2006, 2645.

(9) Zhao, L.; Xu, Z.; Grove, H.; Milway, V. A.; Dawe, L. N.; Abedin,

T. S. M.; Thompson, L. K.; Kelly, T. L.; Harvey, R. G.; Miller, D.

O.; Weeks, L.; Shapter, J. G.; Pope, K. J. Inorg. Chem. 2004, 43,

3812.

(10) Thompson, L. K.; Zhao, L.; Xu, Z.; Miller, D. O.; Reiff, W. M.

Inorg. Chem. 2003, 42, 128.

(11) Milway, V. A.; Niel, V.; Abedin, T. S. M.; Xu, Z.; Thompson, L.

K.; Grove, H.; Miller, D. O.; Parsons, S. R. Inorg. Chem. 2004, 43,

1874.

(12) Milway, V. A.; Abedin, T. S. M.; Niel, V.; Kelly, T. L.; Dawe, L.

N.; Dey, S. K.; Thompson, D. W.; Miller, D. O.; Alam, M. S.; Mu¨ller,

P.; Thompson, L. K. J. Chem. Soc., Dalton Trans. 2006, 2835.

(13) Thompson, L. K.; Kelly, T. L.; Dawe, L. N.; Grove, H.; Lemaire,

M. T.; Howard, J. A. K.; Spencer, E.; Matthews, C. J.; Onions, S. T.;

Coles, S. J.; Horton, P. N.; Hursthouse, M. B.; Light, M. E. Inorg.

Chem. 2004, 43, 7605.

Ligand rotational flexibility, and a preferred cisoid confor-

mation, appear to have prevented full [5 × 5] grid assembly.

Ditopic picolinic hydrazone ligands (e.g., poap, Chart 1)

form [2 × 2] self-assembled M₄square-grid complexes (M

7768 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

obtained using a Quantum Design MPMS5S SQUID magnetometer

using field strengths in the range of 0.1-5 T. Background

corrections for the sample holder assembly and diamagnetic

components of the complexes were applied. NMR measure-

ments were taken with a Bruker AVANCE 500 MHz spectrometer.

LCMS measurements were taken using an Agilent 1100 Series

LC/MSD in APCI mode with methanol and acetonitrile as the

solvent.

chlorine-appended ligands (Chart 1; e.g., S2poap, Cl2poap;

e.g., 1) adhere to Au(111) surfaces and also form SAMS

(self-assembled monolayers), as shown by STM (scanning

tunneling microscopy) measurements.^9,14The flat molecular

orientations on the surface are associated with the prominent

external positions of the sulfur and chlorine sites on the grid

extremities, which provide convenient gold surface-atom

contacts. Ordered metallosupramolecular arrays of function-

ally active molecules of this sort are considered to be

important targets for nanoscale device assemblies.^15-18

Surface studies on 1 deposited on HOPG (highly ordered

pyrollytic graphite) have also been carried out and reveal

not only the individual grid cations in STM topography but

also the spatial arrangement of the individual metal centers

using the CITS (current imaging tunneling spectroscopy)

technique.¹²This powerful surface structural approach probes

the density of states close to the Fermi level of the system,

allows discrimination between metal 3d-based HOMO (high-

est-occupied molecular orbital) levels and ligand orbitals,

which occur at quite different energies, and consequently,

clearly reveals the metal ion positions. Grid core dimensions

obtained by this method closely match those revealed by

single-crystal structural methods, even to the details of the

canted sideways orientation of the grid on the surface. Similar

observations have recently been made with a Co(II)₄[2 ×

2] square-grid system,¹⁵a Cu(II)₂₀wheel-shaped polyanion,¹⁶

and a Fe(III)₄star-shaped cluster.¹⁷Recent reviews highlight

the importance of these techniques for addressing and

probing metal centers in supramolecular assemblies applied

to surfaces.^18a,b

STM Measurements. All measurements were carried out with

a home-built low-drift STM head equipped with commercially

available low-current control electronics (RHK technology, with

ITHACO preamplifier), under ambient conditions. For high-

resolution STM studies, the highly oriented pyrolytic graphite

(HOPG) was freshly cleaved. The graphite surface was then imaged

by STM to confirm the high resolution of the tip. Mechanically

cut Pt-Ir (90/10) tips of diameter 0.25 mm were used. Distances

were calibrated in the STM images by observation of the atomic

spacing on the HOPG surface. After the graphite surface was

successfully imaged, a droplet of the solution in CH₃CN/CH₃OH

was deposited (concentration 10^-9M) and allowed to run down

the graphite surface. Typically, tunneling currents between 5 and

200 pA were employed. The bias voltage was (100 to (200 mV.

The scan frequency was varied between 2 and 5 Hz. The resolution

was 256 × 256 points for topography and 128 × 128 in the CITS

measurements. In CITS mode, current-voltage (I-V) curves were

taken simultaneously with a constant-current STM image with the

interrupted-feedback loop technique. The spectroscopic measure-

ments were confined to negative sample-to-tip bias voltages, that

is, to the tunneling spectroscopy of occupied molecular energy levels

to avoid redox processes.

DFT Calculations. Density functional calculations were per-

formed using the SIESTA method¹⁹and computer code.²⁰Norm-

conserving pseudopotentials were generated in the Troullier-

Martins scheme²¹and include the states from Mn-3p upward as

valence contributors. Atom-centered, strictly confined basis func-

tions (see ref 22 for a detailed discussion on basis functions in

SIESTA) were of triple-ú quality for Mn and double-ú for other

elements. Exchange-correlation was treated in the generalized

gradient approximation, according to the formulation by Perdew,

Burke, and Ernzerhof.²³The present results correspond to a single-

point calculation using the experimentally determined structure, with

the spins of the individual Mn atoms set alternately (i.e., up and

down) in the [3 × 3] grid arrangement. The (uncompensated)

calculated total magnetic moment of the [3 × 3] grid is 5 µ_B,

associated with a spin of S ) 5/2 at each Mn atom.

The present report describes new manganese and copper

complexes of a series of extended polytopic hydrazone

ligands and their structural and magnetic properties. Novel

extended [4 × 4] and [5 × 5] square Mn(II)₁₆and Mn(II)₂₅

grid arrays are discussed, involving tetratopic and pentatopic

hydrazone ligands, based on 3,6-pyridazine and 2,6-pyridine

cores (Chart 1, L2 and L6). CITS measurements on the new

grids clearly reveal the Mn₁₆and Mn₂₅square [n x n]

structural motifs. DFT studies on Mn₉are now introduced,

showing an exact equivalence with the observed d-orbital

MO picture obtained through tunneling experiments.

Synthesis of Ligands. L1. 2-Quinolinecarboxaldehyde (2.05 g,

1.30 × 10^-2mol) was added to a slurry of pyridine-2,6-dicarbo-

hydrazide (1.29 g, 6.61 × 10^-3mol) in 300 mL of methanol. The

resulting mixture was refluxed for 18 h to produce a yellow powder,

which was filtered off and washed with diethyl ether to give 2.32

g (4.90 × 10^-3mol, 75% yield) of L1. mp: 230-232 °C. Mass

spectrum (m/z APCI+): 474. IR (cm^-1): ν_CO1670; ν_CN1554. Anal.

Calcd (%) for C₂₇H₁₇N₇O₂‚2H₂O (bulk sample): C, 61.48; H, 4.74;

N, 18.60. Found: C, 61.05; H, 3.85; N, 19.30. This ligand was

used without further purification.

Experimental Section

Physical Measurements. Infrared spectra were recorded as Nujol

mulls using a Mattson Polaris FT-IR instrument, and UV-vis

spectra were obtained using a Cary 5E spectrometer. Microanalyses

were carried out by Canadian Microanalytical Service, Delta,

Canada. Variable-temperature magnetic data (2-300 K) were

(14) Shapter, J. G.; Weeks, L.; Thompson, L. K.; Pope, K. J.; Xu, Z.;

Johnston, M. R. Smart Mater. Struct. 2006, 15, S171.

(15) Alam, M. S.; Stro¨msdo¨rfer, S.; Dremov, V.; Mu¨ller, P.; Kortus, J.;

Ruben, M.; Lehn, J-M. Angew. Chem., Int. Ed. 2005, 44, 7896.

(16) Alam, M. S.; Dremov, V.; Mu¨ller, P.; Postnikov, A. V.; Mal, S. S.;

Hussain, F.; Kortz, U. Inorg. Chem. 2006, 45, 2866.

(17) Saalfrank, R. W.; Scheurer, A.; Bernt, I.; Heinemann, F. W.;

Postnikov, A. V.; Schu¨nemann, V.; Trautwein, A. X.; Alam, M. S.;

Rupp, H.; Mu¨ller, P. J. Chem. Soc., Dalton Trans. 2006, 2865-2874.

(18) (a) Ruben, M.; Lehn, J.-M.; Mu¨ller, P. Chem. Soc. ReV. 2006, 35,

1056 and references therein. (b) Ruben, M.; Rojo, J.; Romero-Salguero,

F. J.; Uppadine, L. H.; Lehn. J.-M. Angew. Chem., Int. Ed. 2004, 43,

3644 and references therein.

(19) Soler, J. M.; Artacho, E.; Gale, J. D.; Garc´ıa, A.; Junquera, J.;

Sa´nchez-Portal, D.; Ordejo´n, P. J. Phys. Condens. Matter 2002, 14,

2745.

(20) http://www.uam.es/siesta.

(21) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993.

(22) Junquera, J.; Paz, OÄ .; Sa´nchez-Portal, D.; Artacho, E. Phys. ReV. B

2001, 64, 235111.

(23) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,

3865; (Erratum) Phys. ReV. Lett. 1997, 78, 1396.

Inorganic Chemistry, Vol. 46, No. 19, 2007 7769

Dey et al.

L4. Hydrazine hydrate (1.06 g, 2.12 × 10^-2mol) was added to

a solution of diethyl 3,5-pyrazoledicarboxylate (1.97 g, 9.28 × 10^-3

mol) in 175 mL of methanol, and the resulting clear, colorless

solution was refluxed for 48 h to produce a white slurry. The white

powder (1.54 g, 8.36 × 10^-3mol), 1H-pyrazole-3,5-dicarbohy-

drazide, was collected by suction filtration, washed three times with

diethyl ether, and added to a neutral solution of methyl pyrimidine-

2-carboximidate, produced by the action of sodium in methanol

on 1.76 g (1.67 × 10^-2mol) of 2-cyanopyrimidine in 200 mL of

methanol. A white slurry formed, and the mixture was refluxed

for 24 h resulting in the formation of a yellow solid. L4 (3.29 g,

8.34 × 10^-3mol, 82% yield overall) was collected by suction

filtration and washed three times with diethyl ether. mp: >360 °C.

Mass spectrum (m/z APCI+): 391.4. IR (cm^-1): ν_NH3313; ν_CO

1701; ν_CN1631, 1566. Anal. Calcd (%) for C₁₅H₁₄N₁₂O₂‚0.5H₂O

(bulk sample): C, 44.67; H, 3.72; N, 41.69. Found: C, 44.83; H,

3.43; N, 41.95.

Very dry conditions appear to be necessary for the successful

synthesis of L6. We have identified the starting bishydrazone, the

hydrazone of methyl benzoyl formate, and even the double

amidrazone, 2poap (Chart 1), in reactions where dry conditions were

not employed.

Synthesis of Complexes. [Mn₉(L1)₆](ClO₄)₆‚8H₂O (3). L1 (0.12

g, 0.25 mmol) was added to a warm solution of Mn(ClO₄)₂‚6H₂O

(0.14 g, 0.55 mmol) in methanol/acetonitrile (10/10 mL). A cloudy,

white suspension formed. The addition of 2 drops of Et₃N produced

a dark orange solution, which was stirred for 2 h and kept after

filtration for crystallization. Red prismatic crystals, suitable for

X-ray analysis, formed after the mixture was left standing for two

weeks (yield 0.3 g, 29.5%). Anal. Calcd (%) for [(C₂₇H₁₇N₇O₂)₆-

Mn₉](ClO₄)₆·8H₂O (bulk sample): C, 47.89; H, 2.91; N, 14.89.

Found (%): C, 47.87; H, 2.85; N, 14.41.

[Mn₁₆(L2)₈(OH)₈](NO₃)₈‚15H₂O (4) and [Mn₁₆(L2)₈(OH)₈]-

(ClO₄)₈‚15H₂O (5). Compound 4 was re-prepared by a slightly

different procedure from that described previously.²⁴L2 (0.37 g;

1.0 mmol) was added to a solution of Mn(NO₃)₂‚6H₂O (0.57 g,

2.0 mmol) in an ethanol/dichloromethane mixture (40/20 mL).

Triethylamine (6 drops) was added dropwise, and the mixture was

heated for 35 min, with the formation of an orange precipitate.

Water (2 mL) was added, resulting in a clear orange solution. Red-

orange crystals, suitable for an X-ray structure, formed upon

standing (Yield 30%). The structure obtained from this sample

represents a significant improvement over that previously reported.

Mn(ClO₄)₂·6H₂O (1.45 g, 4.00 mmol) was added as a solid to a

stirred white suspension of ligand L2²⁴(0.75 g, 2.0 mmol) in

methanol/acetonitrile (1:1) (30 mL). The addition of a few drops

of triethylamine led to the formation of an orange solution, which

was stirred for 30 min at ∼50 °C and then kept at room temperature.

Rectangular orange crystals (5) were obtained after two weeks.

Anal. calcd (%) for Mn₁₆(C₁₈H₁₂N₈O₂)₈(OH)₈(ClO₄)₈(H₂O)₁₅: C,

34.18; H, 2.67; N, 17.72. Found: C, 34.27; H, 2.54; N, 17.42. This

sample was used for STS/CITS imagery on HOPG.

L3 was produced in a similar fashion by the reaction of 1H-

pyrazole-3,5-dicarbohydrazide with pyridine carboxaldehyde and

was obtained as a white solid (yield 80%). mp: 239-245 °C. Anal.

Calcd (%) for C₁₇H₁₄N₈O₂‚2.5CH₃OH (bulk sample): C, 52.94;

H, 5.32; N, 25.32. Found: C, 53.49; H, 4.51; N, 24.94. L3 and L4

were used without further purification.

L6. Methyl benzoyl formate (10.7 g, 65 mmol) was added to

2,6-picolinic acid dihydrazide (6.00 g, 31.0 mmol) suspended in a

chloroform/methanol mixture (30/20 mL). The mixture was refluxed

overnight, and the volume of the resulting pale yellow solution was

reduced, forming the corresponding extended dibenzoyl ester (2)

as a white powder after 2 days (yield >95%). mp: 190-192 °C.

Mass spectrum (m/z): 488 (base peak, M + 1)⁺, 428, 368, 282,

222, 105, 77. IR (Nujol, cm^-1): ν_NH3326, 3240; ν_CO1743, 1713;

ν

_pyr999. ¹H NMR (500 MHz, CDCl₃): δ 3.91 (s, 6H, OCH₃), 7.45

(m, 6H, Ar-H), 7.79 (d, 4H, Ar-H, J ) 7 Hz), 8.18 (t, 1H, Ar-

H, J ) 8.1 Hz), 8.53 (d, 2H, Ar-H, J ) 7.2 Hz), 13.49 (s, 2H,

OH).

The extended diester (2) (4.0 g, 8.20 mmol) was dissolved in 50

mL of carefully dried THF in a 250 mL three-necked flask under

a N₂atmosphere and cooled in an ice/water bath; 19.0 mL (18.45

mmol) of a 1.0 M solution of hydrazine in THF was added dropwise

via syringe, forming a golden yellow solution, which was allowed

to slowly come to room temperature and was stirred for 2 days. A

pale yellow precipitate of the extended bishydrazone formed, which

was filtered off and dried in air (yield 70%). mp: 180-195 °C.

Mass spectrum (m/z): 488 (M + 1)⁺, 470 (M - H₂O), 342 (base

peak). IR (Nujol, cm^-1): ν_NH3428, 3316, 3224; ν_CO,CN1685, 1666,

1619; ν_py998. ¹H NMR (500 MHz, DMSO-d₆): δ δ 3.16 (s, NH₂),

7.50 (s, 6H), 7.73 (t, 4H), 8.24 (m, 3H), 9.94 (s, OH).

[(L3-H)Cu₂(OH)(NO₃)(H₂O)](NO₃)‚2H₂O (6) and [(L3-H)Cu₂-

(OH)(H₂O)₂](ClO₄)₂‚H₂O (7). L3 (0.090 g, 0.25 mmol) was added

to a solution of Cu(NO₃)₂‚3H₂O (0.25 g, 1.0 mmol) in a methanol/

water mixture (20/5 mL) and warmed, forming a light green

solution. Triethylamine (3 drops) was added with stirring, resulting

in the formation of a light blue solid. Five milliliters of water was

added, and the mixture was heated with stirring at ∼90 °C, forming

a bright green solution, which was filtered hot, and the solution

was allowed to stand at room temperature. Blue crystals formed

after several days, suitable for structural study (yield 0.12 g, 70%).

Anal. Calcd (%) for (C₁₇H₁₂N₈O₂)Cu₂(OH)(NO₃)₂(H₂O)₃: C, 29.96;

H, 2.79; N, 20.56. Found: C, 30.26; H, 2.92; N, 20.56. Compound

7 was synthesized in a similar fashion, using copper perchlorate,

and was obtained as blue crystals suitable for structural determi-

nation. Anal. Calcd (%) for (C₁₇H₁₂N₈O₂)Cu₂(OH)(ClO₄)₂(H₂O)₃:

C, 27.02; H, 2.52; N, 14.83. Found: C, 27.04; H, 2.53; N, 14.72.

[(L4-H)Cu₂(OH)(H₂O)][(L4-H)Cu₂(OH)(ClO₄)](ClO₄)₃‚

4H₂O (8). L4 (0.12 g, 0.30 mmol) was added to a solution of Cu-

(ClO₄)₂‚6H₂O (0.22 g, 0.59 mmol) in methanol/acetonitrile/water

(10/20/10 mL) producing a clear, green solution. The addition of 5

drops of Et₃N produced a brown solution, which was stirred and

heated gently for 18 h. Upon filtration, a small amount of brown

solid was collected and discarded, while the clear green filtrate was

kept for crystallization. Blue-green prismatic crystals, suitable for

X-ray analysis, formed after the mixture was left standing for 35

The methyl ester of iminopicolinic acid was prepared in situ by

reaction of 2-cyanopyridine (0.3 g, 2.88 mmol) with sodium

methoxide solution, produced by dissolving sodium metal (0.3 g,

13.0 mmol) in very dry methanol (50 mL) under N₂. The crude

extended dihydrazone (0.5 g, 1.0 mmol) was added to the above

solution, followed by a few drops of glacial acetic acid to neutralize

the excess methoxide. The mixture was stirred at room tempera-

ture overnight. A light yellow powder (L6) was obtained which

was filtered off and air-dried (yield 85%). mp: 228-230 °C. Mass

spectrum (APCI, m/z): 696 (M + 1)⁺, 678 (M - H₂O), 488,

446 (base peak). IR (cm^-1): ν_NH3413, 3324, 3243; ν_CO3158,

1

1697, 1666; ν_py998. H NMR (500 MHz, DMSO-d₆): δ 6.84

(s, 4H, NH₂), 7.47 (m, 6H, Ar-H), 7.61 (m, 2H, Ar-H), 7.85

(m, 4H, Ar-H), 8.01 (m, 2H, Ar-H), 8.23 (m, 5H, Ar-H),

8.45 (d, 2H, Ar-H, J ) 4.8 Hz), 9.94 (s, 2H, OH), 11.82 (s, 2H,

OH).

(24) Dey, S. K.; Thompson, L. K.; Dawe, L. N. Chem. Commun. 2006,

4967.

7770 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

days (yield 0.05 g, 14%). Anal. Calcd (%) for (C₁₅H₁₂N₁₂O₂)Cu₂-

(ClO₄)₂(H₂O) (bulk sample): C, 23.97; H, 2.00; N, 22.37. Found

(%): C, 24.26; H, 2.03; N, 22.07.

the values for ∆f′ and ∆f′′ were those of Creagh and McAuley.³¹

The values for the mass attenuation coefficients are those of Creagh

and Hubbell.³²All calculations were performed using the Crystal-

Structure^33,34crystallographic software package, except for refine-

ment, which was performed using SHELXL-97.³⁵

[(L5)₃Mn₄](ClO₄)₈‚21H₂O (9). L5²⁵(0.10 g, 0.25 mmol) was

added to a solution of Mn(ClO₄)₂‚6H₂O (0.20 g, 0.053 mmol) in

methanol/acetronitrile (20/20 mL) with heating and stirring. A clear

orange solution formed, which was filtered, reduced in volume,

and left to crystallize. Orange crystals of 9 suitable for structural

study formed (yield 0.05 g, 20%). Anal. Calcd (%) for (C₁₈H₁₈N₁₂)₃-

Mn₄(ClO₄)₈(H₂O)₂₁: C, 24.94; H, 3.72; N, 19.39. Found (%): C,

24.97; H, 2.90; N, 19.20.

Diffraction intensities for 3-7, 9, 10, and L5 were collected

similarly, and structural solutions were achieved using the same

methods (see Table 1 and Supporting Information). For 3, the

Platon³⁶Squeeze procedure was applied to recover 396.7 electrons

per unit cell in four voids (3203.8 Å³), that is, 99.2 electrons per

asymmetric unit. Disordered solvent water and acetonitrile mol-

ecules appeared to be present prior to the application of Squeeze,

although a good point atom model could not be achieved. The

application of Squeeze gave a good improvement in the data

statistics. For 9, apparent twinning effects were dealt with using

Rigaku’s Twin Solve²⁶software to generate a single-component

reflection data file, which was then treated normally.

[Mn(L6)] (10). A suspension of L6 (0.10 g, 0.14 mmol) in

MeOH/CHCl₃(3:1) was added to a solution of Mn(CF₃SO₃)₂‚xH₂O

(0.3 g, ∼0.8 mmol) in MeOH/H₂O (2:1, 20 mL), and the mixture

stirred for 5 h at room temperature. A deep-orange solution formed,

which was filtered and kept for crystallization by slow evaporation.

A red-yellow polycrystalline solid formed after two weeks (yield

11%). X-ray quality crystals were obtained by slow diffusion of

diethyl ether into a solution of the complex in MeOH/CH₃CN/

CHCl₃(1:1:1).

Results and Discussion

[Mn₂₅(L6)₁₀](ClO₄)₂₀‚65H₂O (11). L6 (0.10 g, 0.14 mmol) was

added to a solution of Mn(ClO₄)₂·6H₂O (0.12 g, 0.33 mmol) in

ethanol, and the mixture was stirred for 5 h resulting in a dark

yellow fine polycrystalline powder (yield 62.5%). IR (Nujol, cm^-1):

Structural Studies. [Mn₉(L1)₆](ClO₄)₆‚8H₂O (3). The full

and core cationic structures of 3 are shown in Figure 2, and

important bond distances and angles are listed in Table 2.

The self-assembled grid has a remarkable rectangular

structure, representing the first example in this class. Six

ligands encompass nine Mn(II) ions with three in a µ-O

bridging conformation and three in a µ-NN bridging con-

formation. The “NN” bridging ligands span the long rectangle

dimensions, linking the Mn3-Mn6-Mn9, Mn2-Mn5-

Mn8, and Mn1-Mn4-Mn7 metal groupings and are ar-

ranged on the same side of the rectangle in a roughly parallel

fashion. The “O” bridging ligands span the short dimensions

(Mn1-Mn2-Mn3, Mn4-Mn5-Mn6, Mn7-Mn8-Mn9),

but the end ligands project on one side of the rectangle, while

the central ligand projects on the other side. The normal Mn₉

square grids have all µ-O bridges, but the µ-NN mode has

been observed before in octanuclear Cu₈complexes with

related ligands.^37,38The short Mn-Mn (µ-O) distances fall

in the range of 4.142-4.182 Å, while the longer Mn-Mn

(µ-NN) distances fall in the range of 5.216-5.616 Å. These

distances are consistent with the bridge group sizes. Mn-

O-Mn angles fall in the range of 132.2-134.4 °, and the

Mn-N-N-Mn torsional angles fall in the range of 160.6-

179.1 °. These large µ-O and µ-NN bridge angles are

-

ν_{H O}3548, 3455; ν_NH3355; ν_CO,CN1646; ν_ClO1099 (br). Anal.

2

4

Calcd (%) for Mn₂₅(C₃₅H₂₇N₁₃O₄)₁₀(ClO₄)₂₀(H₂O)₆₅: C, 36.65; H,

3.51; N, 15.87; Cl, 6.18. Found: C, 36.52; H, 2.68; N, 15.45; Cl,

6.29. The addition of a small amount of Et₃N to the powder in a

mixture of CH₃CN/MeOH/CHCl₃gave a dark orange solution, from

which dark red crystals of 12 were obtained upon addition of ether

(yield 50%). This sample was used for STS/CITS imagery on

HOPG. Repeated syntheses and recrystallizations of this compound

have not yet produced crystals which diffract well enough for a

successful structural solution. Solvent inclusions in the crystals

appear to cause significant mosaic spread.

X-ray Crystallography. A light blue prism crystal of 8 having

approximate dimensions of 0.36 × 0.17 × 0.14 mm was mounted

on a glass fiber. All measurements were made on a Rigaku Saturn

CCD area detector diffractometer with graphite-monochromated Mo

KR radiation. The data were collected at a temperature of -160 (

1 °C to a maximum 2θ value of 62.5°. Of the 21 786 reflections

that were collected, 9766 were unique (R_int) 0.0789); equivalent

reflections were merged. Data were collected and processed using

CrystalClear (Rigaku).²⁶The structure was solved by direct

methods²⁷and expanded using Fourier techniques.²⁸Some non-

hydrogen atoms were refined anisotropically, while the rest were

refined isotropically. Hydrogen atoms were refined using the riding

model.

Neutral atom scattering factors were taken from Cromer and

(31) Creagh, D. C.; McAuley, W. J. International Tables for Crystal-

lography, Vol. C; Wilson, A. J. C., Ed.; Kluwer Academic Publish-

ers: Boston, 1992; Table 4.2.6.8, pp 219-222.

30

Waber.²⁹Anomalous dispersion effects were included in F_calcd

;

(25) Matthews, C. J.; Onions, S. T.; Morata, G.; Davis, L. J.; Heath, S.

L.; Price, D. J. Angew. Chem. 2003, 42, 3166.

(32) Creagh, D. C.; Hubbell, J. H. International Tables for Crystal-

lography, Vol. C; Wilson, A. J. C., Ed.; Kluwer Academic Publish-

ers: Boston, 1992; Table 4.2.4.3, pp 200-206.

(26) (a) CrystalClear; Rigaku Corporation: The Woodlands, TX, 1999.

(b) CrystalClear Software User’s Guide; Molecular Structure Corpora-

tion: The Woodlands, TX, 2000. (c) Pflugrath, J. W. Acta Crystallogr.

1999, D55, 1718-1725.

(27) Sheldrick, G. M. SHELX97; University of Go¨ttingen: Go¨ttingen,

Germany, 1997.

(33) CrystalStructure 3.7.0: Crystal Structure Analysis Package; Rigaku

and Rigaku/MSC: The Woodlands, TX, 2000-2005.

(34) Watkin, D. J.; Prout, C. K.; Carruthers, J. R.; Betteridge, P. W.

CRYSTALS, issue 10; Chemical Crystallography Laboratory: Oxford,

U.K., 1996.

(28) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de

Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 Program

System; Technical Report of the Crystallography Laboratory; Univer-

sity of Nijmegen: Nijmegen: The Netherlands, 1999.

(29) Cromer, D. T.; Waber, J. T. International Tables for X-ray

Crystallography, Vol. IV; The Kynoch Press: Birmingham, U.K.,

1974; Table 2.2A.

(35) Sheldrick, G. M. SHELX97; University of Go¨ttingen: Go¨ttingen,

Germany, 1997.

(36) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7-13.

(37) Milway, V. A.; Niel, V.; Abedin, T. S. M.; Xu, Z.; Thompson, L.

K.; Grove, H.; Miller, D. O.; Parsons, S. R. Inorg. Chem. 2004, 43,

1874.

(38) Milway, V. A.; Abedin, T. S. M.; Thompson, L. K.; Miller, D. O.

Inorg. Chim. Acta 2006, 359, 2700.

(30) Ibers, J. A.; Hamilton, W. C. Acta Crystallogr. 1964, 17, 781.

Inorganic Chemistry, Vol. 46, No. 19, 2007 7771

Dey et al.

Table 1. Summary of Crystallographic Data for 3, 4, 6-9, and L5

3

4

6

7

empirical formula

M

C

₁₆₂H₁₀₂N₄₂Mn₉O₃₆Cl₆

C

₁₄₄H₁₃₄N₇₂O₆₃Mn₁₆

C₁₇H₂₀N₁₀O₁₂Cu₂

683.51

C₁₇H_23.6N₈O_15.8Cl₂Cu₂

790.82

3920.01

4760.1

cryst syst

space group

a (Å)

b (Å)

c (Å)

monoclinic

P2₁/n

20.908(4)

27.102(4)

32.492(5)

orthorhombic

I₂₂₂

13.27(2)

21.36(3)

33.87(5)

monoclinic

C2/c

25.238(3)

13.5859(16)

14.2647(17)

monoclinic

P2₁

8.1242(5)

20.8794(14)

8.3519(6)

R (deg)

â (deg)

γ (deg)

V (Å³)

98.239(4)

93.864(3)

103.6728(12)

18222(5)

1.429

153(1)

4

9603(26)

1.646

153(1)

2

4880.9(10)

1.860

153(1)

8

1376.57(16)

1.908

153(1)

2

F

_calcd(g cm^-3

)

T (K)

Z

µ (cm^-1

)

7.72

11.14

18.28

18.30

reflns collected

total

unique

137 797

37 263

19 686

8536

22 308

5617

13 809

7058

R_int

0.0513

37 263

0-55.00

2297

0.051

8536

0-51.00

677

0.036

5617

0-55.00

372

0.0242

7058

0-61.50

435

obsd (I > 2.00σ(I))

2θ range (deg)

params

final R1, wR2^a

0.0936, 0.2597

0.1229,0.3768

0.0524,0.1429

0.0362,0.0961

8

9

10

L5

empirical formula

M

C₁₅H_16.6N₁₂Cu₂O_12.8Cl₂

767.77

C₆₆H₇₃N₄₂O_32.5Mn₄Cl₈

2478.02

C₃₅H₂₇N₁₃O₄Mn

748.62

C₁₈H₁₈N₁₂

402.4

cryst syst

space group

a (Å)

b (Å)

c (Å)

triclinic

P

monoclinic

C2/c

14.508(2)

19.241(3)

36.547(5)

monoclinic

Cc

15.140(10)

10.791(7)

21.138(13)

monoclinic

P2₁/n

7.0375(9)

27.752(4)

9.9941(14)

12.180(6)

14.450(6)

16.120(7)

68.36(5)

77.37(4)

75.81(5)

2530.7(20)

2.015

R (deg)

â (deg)

γ (deg)

V (Å³)

90.801(9)

97.04(2)

97.328(4)

10201(3)

1.614

153(1)

4

3427(4)

1.451

153(1)

4

1936.0(4)

1.381

153(1)

4

F

_calcd(g cm^-3

T (K)

Z

)

113(2)

4

19.83

µ (cm^-1

)

7.91

4.462

0.94

reflns collected

total

unique

21 786

9766

13 026

8731

14 455

6893

17 298

4398

R_int

0.0789

9766

0-52.00

829

0.049

8731

0-52.00

713

0.050

6893

0-61.50

486

0.0227

4398

0-55.00

272

obsd (I > 2.00σ(I))

2θ range (deg)

params

final R1, wR2^a

0.1203, 0.3187

0.0918,0.2390

0.0883,0.2174

0.0453,0.1210

^aR1 ) ∑2F₀* - *F_c2/∑*F₀*, wR2 ) [∑[w(*F₀*²- *F_c*²)²]/∑[w(*F₀*²)²]]^1/2

.

suggestive of intramolecular antiferromagnetic exchange

(vide infra).^9-13,39

interactions are considerably greater. In the N-N bridging

mode, the metal ions will, of necessity, be further apart and,

by default, will then allow the π-rich ligand end pieces to

get further apart, thus creating the opportunity for the

formation of a rectangle rather than a square.

One of the molecular features, which appears to character-

ize the [3 × 3] square grids in general, is the facility with

which the ligands align themselves in the two roughly parallel

groups of three above and below the grid metal pseudoplane

(see Figure 1). Separation between the ligands in this case

is 4 Å or less, suggesting that π interactions between the

aromatic rings are important. The slight offset between

adjacent parallel pyridine aromatic rings indicates the normal

tendency of such rings to minimize π repulsions. The

quinoline end pieces on L1 are clearly much larger aromatic

components than the pyridine rings normally present in this

ligand type, and it is reasonable to assume that, as the ligands

approach each other in the typical grid arrangement, the π

[Mn₁₆(L2)₈(OH)₈](NO₃)₈‚15H₂O (4). The structure of the

hexadecanuclear cation in 4 is shown in Figure 3a, and a

labeled core structural representation is shown in Figure 3b.

Important bond distances and angles are listed in Table 3.

A brief report of an essentially identical structure appeared

recently but with a different space group.²⁴However, in the

present structure, all the required nitrate anions were located,

which was not possible previously. Consequently, more

complete structural details are now considered. The overall

grid structure is really a composite of four [2 × 2] [Mn₄-

(µ-O)₄] corner grid subunits with hydrazone oxygen bridges,

connected in the center by a combination of eight pyridazine

(39) Xu, Z.; Thompson, L. K.; Miller, D. O.; Clase, H. J.; Howard, J. A.

K.; Goeta, A. E. Inorg. Chem. 1998, 37, 3620 and references therein.

7772 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

Table 2. Bond Distances (Å) and Angles (deg) for 3

Mn1

Mn2

Mn3

Mn4

Mn5

N2

2.135(5)

2.167(4)

2.228(5)

2.228(3)

2.231(4)

2.441(4)

2.147(4)

2.162(4)

2.259(4)

2.262(4)

2.304(3)

2.327(3)

2.136(4)

2.178(4)

2.187(4)

2.242(4)

2.248(4)

2.417(4)

2.124(4)

2.185(4)

2.200(4)

2.208(4)

2.244(3)

2.435(4)

2.176(4)

2.200(4)

2.266(3)

Mn5

Mn6

Mn7

Mn8

Mn9

O3

2.272(3)

2.373(4)

2.441(4)

2.134(4)

2.184(4)

2.200(4)

2.253(4)

2.262(3)

2.378(4)

2.126(5)

2.165(4)

2.221(4)

2.227(4)

2.259(5)

2.397(5)

2.156(3)

2.184(4)

2.254(4)

2.257(4)

2.283(3)

2.321(4)

2.142(5)

2.161(4)

2.224(4)

2.240(4)

2.258(5)

2.449(4)

N37

N36

O1

O11

N1

O9

N4

N30

N29

O1

O2

N6

N23

O7

O2

N22

N7

N9

N39

N38

N40

O3

N33

N31

N13

N25

N26

N24

O4

N14

N16

N41

O5

O12

N42

N15

O10

N18

N35

N34

O6

O5

N20

N27

O8

O6

N28

N21

N8

N11

N32

O4

Mn1

Mn3

Mn4

Mn6

Mn7

Mn9

O1

O2

O3

O4

O5

O6

Mn2

Mn5

Mn8

134.37(18)

132.49(16)

133.30(15)

132.33(15)

133.11(16)

133.56(15)

X-ray structural study on 5 indicates the same cationic

fragment as in 4.

[(L3-H)Cu₂(OH)(NO₃)(H₂O)](NO₃)‚2H₂O (6). The struc-

ture of 6 is shown in Figure 4, and important distances and

angle are listed in Table 4. Surprisingly, the structure consists

of a dinuclear cation, with the two copper ions bridged by

the central pyrazole group, and the incorporation of a second,

adventitious hydroxide bridge. The ligand adopts an unusual

cis conformation, in which the hydazone oxygen and diazine

NN groups are not involved in bridging, and the ligand end

pieces form a tridentate N₃coordinating pocket, with a

mixture of five- and six-membered chelate rings. The net

result is a near perfect fit of the metal ions in the pockets

forming an almost planar dinuclear entity. The Cu-Cu

separation is 3.282 Å, and the Cu-OH-Cu angle 117.9 °.

The two copper ions are square pyramidal, with an axial

nitrate bound to Cu(1) and an axial water molecule bound

to Cu(2). The fortuitous arrangement of these two groups

allows a hydrogen-bonding connection between O(10) and

nitrate O(4), which may help to stabilize the Cu₂subunit

(Vide infra). The O(4)-O(10) distance is 2.812 Å, and the

O-H-O angle 169.8 °.

Figure 2. POVRAY structural representations of the cation in 3: magenta

) Mn, blue ) N, red ) O, black ) C.

(N-N) and eight hydroxide (µ-OH) bridges. The ligands are

essentially flat and have a cis conformation, which ensures

that the metals all bind on one side of each ligand. This,

combined with the formation of five-membered chelate rings

throughout, ensures that the ligand components are aligned

appropriately for efficient grid self-assembly.

Hydrazone Mn-O-Mn bridge angles fall in the range of

119-129°, while the Mn-OH-Mn angles fall in the range

of 118-127°. Mn-Mn distances for pyridazine-bridged

metal pairs fall in the range of 3.6-3.7 Å, while Mn-Mn

distances for those pairs bridged by hydrazone oxygen atoms

fall in the range of 3.8-4.0 Å, more typical of the [2 × 2]

and [3 × 3] Mn grids.^8-10,12,13The grid has a roughly flat,

square overall molecular footprint with edge lengths of ∼21

Å (2.1 nm). However, closer examination of the core (Figure

3c) reveals that the metals are far from coplanar and that

there is considerable puckering of the metallic core itself.

This would be expected on the basis of the projected natural

bend in L2 (Chart 1), which would not occur in ligands based

on a central 2,6-picolinic hydrazone core. A preliminary

[(L3-H)Cu₂(OH)(H₂O)₂](ClO₄)₂‚H₂O (7). The structural

representation of the dinuclear cation in 7 is shown in Figure

5, and important bond distances and angles are listed in Table

5. The structure is very similar to that of 6, with two water

molecules in a trans arrangement occupying axial positions

at the square pyramidal copper centers. The Cu-Cu distance

is 3.308 Å, and the Cu-OH-Cu angle 117.5 °. The absence

of any intramolecular hydrogen bonding interaction, present

Inorganic Chemistry, Vol. 46, No. 19, 2007 7773

Dey et al.

Figure 4. POVRAY structural representation of the cation in 6.

Figure 5. POVRAY structural representation of the cation in 7.

Figure 6. POVRAY structural representation of the cations in 8.

Table 4. Bond Distances (Å) and Angles (deg) for 6

Figure 3. POVRAY structural representations of the cation in 4: green

) Mn, blue ) N, red ) O, black ) C.

Cu1

O3

N4

N1

N2

O15

1.927(2)

1.935(3)

1.990(3)

2.035(3)

2.368(2)

Cu2

N5

O3

N8

N7

1.933(2)

1.942(2)

1.978(2)

2.042(2)

Table 3. Bond Distances (Å) and Angles (deg) for 4

Mn1

Mn2

Mn3

N2

O1

N1

O5

O1

N10

O3

N4

N9

O5

N15

O4

O2

N5

N16

2.102(10)

2.164(8)

2.295(9)

2.032(8)

2.194(9)

2.203(10)

2.232(8)

2.255(11)

2.261(9)

2.022(8)

2.166(11)

2.222(8)

2.223(9)

2.233(11)

2.260(12)

Mn4

Mn5

Mn6

N7

O2

N8

O6

O3

N12

O6

O4

N13

2.161(10)

2.196(8)

2.316(11)

2.061(10)

2.194(8)

2.302(9)

2.047(10)

2.184(8)

2.288(9)

Cu1

O3

Cu2

117.52(10)

Table 5. Bond Distances (Å) and Angles (deg) for 7

Cu1

O3

N4

N1

N2

O15

1.927(2)

1.935(3)

1.990(3)

2.035(3)

2.368(2)

Cu2

N5

O3

N8

N7

1.933(2)

1.942(2)

1.978(2)

2.042(2)

Cu1

O3

Cu2

117.52(10)

[(L4-H)Cu₂(OH)(H₂O)][(L4-H)Cu₂(OH)(ClO₄)](ClO₄)₃‚

4H₂O (8). The structure of 8 is shown in Figure 6, and

important bond distances and angles are listed in Table 6.

Compound 8 has a structure similar to that of 6 and 7, but

with two slightly different hydroxo-bridged dinuclear copper

complex ions in the asymmetric unit. In one, a water

molecule (O(29)) is bonded to Cu(3), while in the other a

perchlorate is bonded via O(7) to Cu(1), creating a combina-

tion of square and square pyramidal centers in both subunits.

Cu-Cu distances are 3.244 Å (Cu(1)-Cu(2)) and 3.254 Å

(Cu(2)-Cu(3)), with corresponding Cu-OH-Cu angles of

Mn1

Mn4

Mn5

Mn6

Mn3

Mn6

O1

O2

O3

O4

O5

O6

Mn2

Mn3

Mn2

Mn3

Mn2

Mn5

126.1(4)

120.5(3)

121.9(4)

128.3(4)

123.7(7)

in 6, indicates the inherent stability of the dinuclear unit and

the apparently preferred dinucleating ligand bonding mode.

7774 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

Table 6. Bond Distances (Å) and Angles (deg) for 8

Cu1

Cu2

Cu3

O5

N6

N1

N4

O7

O5

N7

N12

N9

O6

1.903(7)

1.918(8)

1.947(8)

1.963(8)

2.263(8)

1.883(7)

1.922(8)

1.950(8)

1.960(9)

1.879(7)

Cu3

Cu4

N18

N13

N16

O29

O6

N19

N24

N21

O18

1.911(8)

1.954(8)

1.969(8)

2.292(8)

1.903(7)

1.923(8)

1.951(8)

1.956(8)

2.420(9)

Figure 7. POVRAY structural representation of the ligand L5.

Cu2

Cu3

O5

O6

Cu1

Cu4

117.9(3)

118.7(3)

Table 7. Bond Distances (Å) and Angles (deg) for L5

N1

N2

N3

N4

N5

N6

C5

C1

C6

N4

C7

C8

N7

1.3397(17)

1.3436(18)

1.3425(17)

1.2946(17)

1.4105(14)

1.2985(17)

1.3491(16)

1.3360(16)

1.3379(15)

N7

N8

N9

N10

N11

N12

C11

C12

N10

C13

C14

C18

1.3322(16)

1.3453(16)

1.3054(16)

1.4070(14)

1.2978(16)

1.3488(16)

1.3345(17)

1.3402(18)

C5

C6

C7

C8

C11

C12

C13

C14

N1

N3

N4

N6

N7

N9

N10

N12

C1

117.17(12)

113.73(10)

109.00(10)

119.39(10)

120.39(10)

110.78(10)

111.92(10)

117.78(12)

N4

N3

N7

N6

N10

N9

C18

Figure 8. POVRAY structural representations of the cation in 9.

Table 8. Bond Distances (Å) and Angles (deg) for 9

Mn1

N15

N9

N1

N7

N13

N3

2.216(5)

2.226(5)

2.228(5)

2.230(6)

2.235(6)

2.243(5)

3.802(3)

Mn2

Mn2′

N10

N16

N4

N6

N12

N18

3.850(3)

2.155(5)

2.158(5)

2.190(5)

2.237(5)

2.256(5)

117.9 and 118.7 ° respectively. One novel and fascinating

feature of the structure of 8 surrounds the well-matched

docking of the two dinuclear subunits via a concerted

hydrogen-bonding network involving the external O-N-N

groups. N-O distances fall in the range of 2.765-2.886 Å,

with four such connections between each dinuclear center.

Figure 6 just shows the association between Cu(1) and Cu-

(3), but it extends via similar connections between Cu(2)

and Cu(4) and creates a flat bifurcated extended chain of

Cu₂subunits. A molecular view of a segment of this extended

structure is shown in Figure S1 (see Supporting Information).

Interestingly, this extended hydrogen-bonded structure is

reminiscent of the cytosine-guanine base-pair interaction

in DNA.

L5 and [(L5)₃Mn₄](ClO₄)₈‚21H₂O (9). The structure of

ligand L5 is shown in Figure 7, and some distances and

angles are listed in Table 7. Protons were located on N(2),

N(5), N(8), and N(11) from difference maps, indicating that

the ligand exists in a tautomeric form in which all the diazine

CN bonds have essentially double bond character (1.295-

1.305 Å). The ligand adopts a trans arrangement at its ends

with respect to the NH₂groups, with C-N-N-C torsional

angles of 154.9 and 168.3°, indicating a slight out-of-plane

rotation about the N-N bonds.

Mn2

Mn1

Mn2

N15

N3

N9

N6

N12

N16

N4

N10

N18′

N12′

Mn2

Mn2′

31.9

26.9

34.1

21.7

32.9

Table 9. Bond Distances (Å) and Angles (deg) for 10

Mn1

N11

O4

N3

2.129(6)

2.144(6)

2.146(6)

Mn1

O3

N1

N13

2.158(4)

2.298(6)

2.385(6)

N11

O4

N11

O4

N3

N11

O4

N3

O3

N11

O4

N3

Mn1

O4

N3

O3

N1

107.3(2)

159.16(19)

72.8(2)

73.06(19)

90.68(19)

127.56(19)

112.2(2)

140.3(2)

71.8(2)

96.8(2)

70.30(19)

93.1(2)

88.85(19)

142.59(19)

103.6(2)

N1

N13

O3

N1

The structure of the tetranuclear cation in 9 is shown in

Figure 8a, and a core structure is shown in Figure 8b.

Important distances and angles are listed in Table 8. This

complex has an unusual linear spiral structure with four six-

coordinate Mn(II) ions encompassed by three ligands, which

all have a pronounced twist, and bridge the metal centers

by open-chain diazine and pyridazine NN groups. Mn-Mn

distances along the chain are 3.802 Å (Mn(1)-Mn(2)),

involving the open chain diazine bridge, and 3.850 Å,

involving the pyridazine bridge. The twisting about the NN

bridges can be quantified via the Mn-N-N-Mn torsional

angles which fall in the range of 26.9-34.1 ° for Mn(1) and

Mn(2) and 21.7-32.9 ° for Mn(2) and Mn(2)′. These are of

Inorganic Chemistry, Vol. 46, No. 19, 2007 7775

Dey et al.

Surface Studies and DFT Calculations. The use of single

molecules in a device capacity requires that they can be

probed and appropriately activated via some suitable mo-

lecular property. Fixing their positional location can be

achieved by application to a surface, with a monolayer

assembly as a reasonable goal. The [3 × 3] Mn grids have

been applied to both Au(III) and HOPG surfaces and imaged

using STM and CITS techniques.^9,12,14The power of CITS

imagery rests not only with the ability to actually tunnel

current through an individual molecule but also with the

ability to tunnel though individual metal atoms with ap-

propriate tuning of tunneling energies.¹²This probes exclu-

sively the metal d HOMO states, thus creating an image of

the surface-applied grid molecule based on its metal ion

components only. STM and CITS images of 1 adsorbed on

HOPG are shown in Figure 10a and b, respectively.¹²

To substantiate the role of the metal 3d HOMO states in

the CITS imagery, first-principle calculations of the electronic

structure of the Mn₉grid 1 have now been carried out using

density functional theory (DFT).^19-23Figure S2 (Supporting

Information) shows the spin-resolved local density of states

of the Mn atoms at three different sites (Mn1 corner, Mn2

side, Mn5 center). The DFT calculations demonstrate that

the HOMOs are constructed from the 3d-states of Mn(II)

and that the energy gap between the HOMO and the LUMO

is 0.44 eV. It follows that the molecular orbitals within an

energy window between the Fermi level (E_F) and -0.9 eV

would give a significant contribution exclusively from the

3d states of the Mn1, Mn2, and Mn5 centers. Therefore, no

noticeable positional contrast would be expected for the

oxygen and nitrogen atoms. This is consistent with the CITS

data (Figure 10b), which means that the highest peak of the

density of states is located at 0.9 eV below the Fermi level,

and so maximum contrast in the measurements would be

expected at bias voltages below -0.9 V. On the basis of the

calculated energies, decreasing the bias voltage from zero,

the corner Mn atoms should contribute to the signal first,

followed by the central Mn and then the side Mn atoms.

This is indeed observed in the STS experiments (Figure 10c).

Also no significant O- and N-related states are observed

down to -0.9 V, which is also in good agreement with DFT

calculations. A 3D representation of the calculated electron

density map summing up all occupied states down to -0.9

eV is shown in Figure 10d. The experimental observation at

-0.879 V (Figure 10e) is quite consistent with the predicted

DFT data

Figure 9. ORTEP structural representation of the complex 10.

relevance for the magnetic exchange analysis (vide infra).

There appears to be only one other related tetranuclear

example of a spiral chain of this sort with Cu(II) and the

same ligand.²⁵However, smaller dinuclear spiral complexes

with the simpler, closely related ligand PAHAP (picolina-

mide azine) and Mn(II), Fe(II), Co(III), and Ni(II) are well

documented.³⁹These involve just open-chain diazine bridges.

[Mn(L6)] (10). The ligand L6 has the appropriate design

elements to bind five metals in its five coordination pockets

in a roughly linear fashion (Chart 1), as a result of the

contiguous linear arrangement of ten potentially five-

membered chelate rings. It is therefore encoded with the

coordination information necessary to possibly generate a

[5 × 5] grid upon self-assembly with a six-coordinate metal

ion. Reactions of L6 with Mn(II) salts produced orange

crystalline solids (see experimental), but in the case of Mn-

(CF₃SO₃)₂, the mononuclear complex 10 was obtained in low

yield. The structure reveals a simple neutral mononuclear

complex (Figure 9), with the manganese ion bonded to two

deprotonated ligand end pockets, which bend around and

encompass the metal creating a cis-N₄O₂pseudo-octahedral

coordination sphere. Important distances and angles are listed

in Table 9. Mn-N distances fall in the range of 2.129-

2.385 Å, and the Mn-O distances are 2.144 and 2.158 Å.

Oligomeric species have been observed with tritopic

picolinic hydrazone ligands, although not frequently,^10,40and

indicate clearly that while the [n × n] grids are favored

thermodynamically, other polynuclear and mononuclear

species are also possible and appear to depend in part on

the metal ion present. In the case of Mn(II) complexes of

L6, significant difficulties have been experienced with poor

crystal diffraction, and despite other positive evidence for

full grid formation, no structural solution has been forthcom-

ing. However indirect structural evidence through surface

studies on 12 does show a [5 × 5] grid formation (vide infra).

Mn₁₆and Mn₂₅. STM measurements on 5 (Mn₁₆) applied

as a dilute solution to a freshly cleaved HOPG surface show

that the molecules self-organize into well-ordered arrays

along the monatomic step edges of the HOPG surface (Figure

11a-b). Simultaneously recorded STM topography and CITS

current images on a singly deposited molecule are shown in

Figure 11c and d, respectively. STM topography (11c) shows

a featureless blob with an outer diameter of approximately

2.8 nm, which is roughly consistent with the dimensions of

a single molecular cation, as determined from the X-ray

crystallographic data. The CITS technique is again capable

of selectively probing the manganese HOMO 3d states at

(40) Kelly, T. L.; Milway, V. A.; Grove, H.; Niel, V.; Abedin, T. S. M.;

Thompson, L. K.; Zhao, L.; Harvey, R. G.; Miller, D. O.; Leech. M.;

Goeta, A. E.; Howard, J. A. K. Polyhedron 2005, 24, 807.

7776 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

Figure 10. STM/CITS images of Mn [3 × 3] grid complex 1 deposited onto a HOPG surface. (a) STM image showing the underlying HOPG lattice and

an isolated Mn complex simultaneously (V ) 100 mV, I ) 50 pA). (b) CITS current image at bias voltage -0.879 V. The grid structure appears to be

slightly distorted because of thermal drift of the scanning piezo tip. (c) 3D representation of a set of I-V characteristics recorded at 38 equidistant positions

between the two arrows in Figure 10b. (d) 3D representation of DFT-calculated electron density maps within an energy window between E_Fand -0.9 eV.

(e) FFT filtered view of Figure 10b on an enlarged scale, measured at -0.879 V. The 3D picture is rotated to match the DFT-calculated image (Figure 10d).

arrangement, again in direct agreement with the X-ray

structural results.

The putative Mn₂₅grid sample 12 was deposited on HOPG

from a very dilute solution in CH₃CN/MeOH and examined

using STM/CITS experiments. Figure 12a shows the STM

topography of a single molecular line arranged on an HOPG

surface, with the molecules residing predominantly at the

step edges of the HOPG surface. Figure 12b shows a high-

resolution STM image of a single complex ion, with

background regions of the frame showing atomic resolution

of the HOPG substrate. The roughly square-shaped feature

has an outer diameter of approximately 3.2 nm, close to the

expected diameter of 2.8 nm of a single Mn₂₅grid molecule,

based on a simple extension of the molecular dimensions of

known Mn₉and Mn₁₆grids. CITS spectroscopy was also

performed, and the topography of a single molecule (Figure

12c) mapped simultaneously with CITS again shows a rather

featureless entity. Estimates of its size are ∼2.8 × 3.2 Å,

consistent with the expected dimensions of a Mn₂₅grid.

Figure 11. STM/CITS images of the Mn₁₆[4 × 4] complex 5 on an HOPG

CITS measurements were carried out at ramping voltages

substrate representing (a) a large-scale STM image (V ) 100 mV, I ) 5

comparable to those used for the Mn₉and Mn₁₆grids.

Remarkably, the metal atomic positions are clearly revealed

in a regular [5 × 5] gridlike array (Figure 12d). The estimated

metallic grid core dimensions from this image give a square

with an edge length of ∼17 Å (1.7 nm), in complete

agreement with Mn-Mn separations in the Mn₄, Mn₉, and

Mn₁₆grids, with hydrazone oxygen bridging connections,

which are close to 4 Å. A segment of the Mn(II)₂₅grid

showing just the middle row of metal centers is illustrated

in relief form in Figure 13, by plotting current isopols as a

function of voltage. The 3D peaks represent the metal ion

pA); (b) isolated single molecules, under the same imaging conditions as

in panel a; (c) constant current STM topography; and (d) CITS current

image of the same area at a bias of -0.4 V recorded simultaneously with

topography. Figure 11d is averaged two times using Gaussian smoothing

to increase the signal-to-noise ratio.

negative sample bias, and Figure 11d reveals a group of

image spots in a gridlike arrangement, which clearly cor-

respond to the sixteen Mn(II) centers. Estimated distances

of separation are entirely consistent with those observed in

X-ray crystallography. Close inspection of the image indi-

cates that the metal sites are in a nonlinear puckered

Inorganic Chemistry, Vol. 46, No. 19, 2007 7777

Dey et al.

Figure 14. Variable-temperature magnetic data for 3 (see text for fitted

parameters).

principles is not possible because of the immensity of the

matrix calculations involved. Even the application of point-

group symmetry-reduction methods would still involve an

enormous calculation.^41,42Because the rectangular grid

comprises three [M₃-(µ-O)₂] subunits separated by µ-NN

bridges, an approximate data fit was approached by consider-

ing 3 as a composite of three linear trinuclear subunits. The

total spin quantum number combinations and their energies

for the appropriate exchange Hamiltonian (H_ex) -J{S₁S₂

+ S₂S₃}) were calculated and substituted into the van Vleck

equation (eq 1) within the software package MAG-

MUN4.11,⁴³to give a reasonable data fit. The solid line in

Figure 14 was obtained in a best fit analysis for g ) 2.01, J

) -2.32 cm^-1, TIP ) 0 cm³mol^-1, F ) 0.007, and θ )

-3.6 K (10²R ) 4.9; R ) [∑(ø_obsd- ø_calcd)²/∑ø_obsd²]^1/2; F )

fraction of paramagnetic impurity, TIP ) temperature-

independent paramagnetism, θ ) Weiss correction), with

magnetic data scaled for Mn(II)₃subunits. The J value is

consistent with related square Mn(II)₉grids with µ-O bridges.

The substantial negative θ value can be interpreted in terms

of an antiferromagnetic exchange interaction between the

Mn₃subunits.

Figure 12. STM/CITS measurements of putative Mn [5 × 5] complex

showing (a) constant current topography of a linear chain of single molecules

(I ) 5 pA, V ) 100 mV); (b) an isolated single molecule with atomic scale

resolution of HOPG (I ) 50 pA and V ) 200 mV); and (c and d)

simultaneously recorded constant current topography and CITS current

image recorded at -0.732 V. The topographic parameters were 100 mV

and 30 pA. Figure 12d is averaged with the same procedure used in Figure

11d.

Figure 13. 3D representation of a set of I-V characteristics recorded at

39 equidistant positions along the middle row of peaks of a CITS map of

the Mn [5 × 5]. The background current, arising from the HOPG substrate,

has been subtracted. Five peaks are clearly visible. The average distance

between neighboring peaks is 0.4 nm.

S′(S′ + 1)(2S′ + 1)e^-E(S′)/kΤ

2

Nâ g ∑

3k(T - θ)

ø_M)

(1 - F) +

(2S′ + 1)e^-E(S′)/kΤ

∑

Nâ²g²S(S + 1)F

3kT

positions, with the expected Mn-Mn peak-peak separations

of ∼0.4 nm. This remarkable result confirms the fact that

there are twenty five metals in the grid core, separated by

distances consistent with the expected hydrazone oxygen

bridges, but absolute proof of the presence of the expected

ten ligands has not been obtained directly, since with the

experimental CITS conditions chosen, only the metal ion

electronic states are probed. However, it is reasonable to

assume that such an arrangement exists, based on the

elemental analytical data, and because it is highly unlikely

that all the metals would be seen in this expected grid

arrangement without the organizing effect of the ligands.

Magnetic Properties. Variable-temperature magnetic data

for 3 are shown in Figure 14, with a drop in moment (per

mole) from 17.7 µ_Bat 300 K to 6.4 µ_Bat 2 K. This is

consistent with the presence of nine Mn(II) centers within

an antiferromagnetically coupled grid. Dealing with a full

matrix calculation on this 45-electron problem from first

+ TIP (1)

The variable-temperature magnetic data for 4 and 5 are

essentially identical and show a moment at 300 K of ∼22

µ_Bconsistent with the presence of sixteen Mn(II) ions (5.5

µ_Bper metal), dropping to ∼4 µ_Bat 2 K, indicating

significant intramolecular antiferromagnetic exchange. The

profile for 4 is shown in Figure 15. Once again, the

(41) Waldmann, O.; Gu¨del, H. U.; Kelly, T. L.; Thompson, L. K. Inorg.

Chem. 2006, 45, 3295.

(42) Thompson, L. K.; Waldmann, O.; Xu, Z. Coord. Chem. ReVs. 2005,

249, 2677.

(43) MAGMUN4.11/OW01.exe is available as a combined package free

of charge from the authors (http://www.ucs.mun.ca/∼lthomp/magmun).

MAGMUN was developed by Dr. Zhiqiang Xu (Memorial University),

and OW01.exe was developed by Dr. O. Waldmann. We do not

distribute the source codes. The programs may be used only for

scientific purposes, and economic utilization is not allowed. If either

routine is used to obtain scientific results, which are published, the

origin of the programs should be quoted.

7778 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

Figure 17. Variable-temperature magnetic data for 8 (see text for fitted

parameters).

Figure 15. Variable-temperature magnetic data for 4.

pyrazole and hydroxide, and the large Cu-OH-Cu bridge

angle (117.9 °) suggests that hydroxide would provide a

significant if not dominant contribution to the total exhange.

It is of interest to note that in a series of related dinuclear

copper(II) complexes with equivalent combinations of py-

ridazine or phthalazine and hydroxide bridges, a magneto-

structural correlation indicates an exchange integral of -J

) ∼580 cm^-1for this OH bridge angle.⁴⁶These combina-

tions appear therefore to be more effective at propagating

antiferromagnetic exchange than the present pyrazole/OH

combination. Compound 7 has an almost identical magnetic

profile, and fitting to the same exchange Hamiltonian gave

g ) 2.11(2), J ) -398(5) cm^-1, TIP ) 195 × 10^-6cm³

mol^-1, F ) 0.014 (10²R ) 1.3). The J value is consistent

with the double bridge, and the large Cu-OH-Cu bridge

angle (117.5 °).

Compound 8 has a similar magnetic profile in general to

that of 6 and 7, with moment per mole dropping from 1.81

µ_Bat 300 K to 0.46 µ_Bat 2 K, indicating substantial residual

spin in what is clearly a strongly coupled dinuclear copper-

(II) system (Figure 17). Data treatment using a simple

dinuclear model (H_ex) -J{S₁S₂}) did not produce a very

good fit below 120 K, and the best fit gave g ) 2.02(4), J

) -357(12) cm^-1, TIP ) 100 × 10^-6cm³mol^-1, F ) 0.05,

and θ ) -3.8 K (10²R ) 3.8). The J value is consistent

with 6 and 7, but a low g value prompted us to examine an

alternate exchange model in which the hydrogen-bonded

chain structure of 8 is considered to possibly contribute to

the overall exchange process. An alternating chain model^47,48

was therefore tested (H_ex) -2[J{S_iS_i+1} + RJ{S_i+1S_i+2}])

and found to give a significant improvement in fitting,

particularly in the lower-temperature regime. The solid line

in Figure 17 is calculated for g ) 2.07, J ) -187 cm^-1, θ

) -4 K, F ) 0.04, R ) 0.2, and TIP ) 50 × 10^-6cm³

mol^-1(10²R ) 1.6). Varying R above and below this value

led to significantly worse fits. While this cannot be consid-

ered a rigorous approach to the exchange model for 8,

particularly given the large number of fitted parameters and

the poor fit below 40 K, the improvement of the fitting with

the alternating chain equation is significant. It suggests that

Figure 16. Variable-temperature magnetic data for 6 (see text for fitted

parameters).

magnitude of the spin-state matrix calculation prevents any

direct assessment of the exchange picture in this system. In

keeping with the J values for 3 and other related Mn(II) [2

× 2] and [3 × 3] grids (e.g., 1) with comparable Mn-O-

Mn angles, exchange integrals of -2 to -5 cm^-1would be

expected.^7,9,10,24A ground state spin of S′ ) 0 would be

anticipated for this even-numbered metal grid, but magne-

tization versus field data at 2 K (Figure S3) indicate

significant residual spin, consistent with the nonzero moment

at this temperature.

Variable-temperature magnetic data for 6 are shown in

Figure 16, with susceptibility rising to a maximum above

300 K, consistent with intramolecular antiferromagnetic

exchange between the two copper centers via both the

pyrazole and OH bridges. The data were fitted to a simple

dinuclear exchange expression (H_ex) -J{S₁S₂} within

MAGMU4.11 to give g ) 2.26(2), J ) -443(4) cm^-1, TIP

) 230 × 10^-6cm³mol^-1, F ) 0.0002, and 10²R ) 1.8 (eq

1). The solid line in Figure 16 was calculated with these

parameters. Pyrazole bridges have been shown to propagate

antiferromagnetic exchange. A bis-µ-pyrazolato-bridged di-

copper(II) complex with trigonal-bipyramidal metal geom-

etries was shown to have J ) -188 cm^-1,44while in other

bis-µ-pyrazolato-bridged dicopper(II) complexes involving

square pyramidal geometries, larger J values (-330, -426

cm^-1) were observed.⁴⁵The bridge combination in 6 involves

(44) Tanase, S.; Koval, I. A.; Bouwman, E.; de Gelder, R.; Reedijk, J.

Inorg. Chem. 2005, 44, 7860.

(45) de Geest, D. J.; Noble, A.; Moubaraki, B.; Murray. K. S.; Larsen,

D. S.; Brooker, S. Dalton Trans. 2007, 467.

(46) Thompson, L. K.; Lee, F. L.; Gabe, E. J. Inorg. Chem. 1988, 27, 39.

(47) Duffy, W.; Barr, K. P. Phys. ReV. 1968, 165, 647.

(48) Hall, J. W.; Marsh, W. E.; Weller, R. R.; Hatfield, W. E. Inorg.

Chem. 1981, 20, 1033.

Inorganic Chemistry, Vol. 46, No. 19, 2007 7779

Dey et al.

Figure 18. Variable-temperature magnetic data for 9 (see text for fitted

parameters).

Figure 19. Variable-temperature magnetic data for 12.

consistent with previously reported open-chain diazine

spirally bridged systems. The closely related Cu₄spiral chain

complex [Cu₄(L5)₃](ClO₄)₈shows a very slight drop in

moment from 280 to 5 K, associated with a possible

combination of ferromagnetic and antiferromagnetic ex-

change terms, but no analysis of the exchange situation was

carried out.

the extended hydrogen-bonded structure is influencing the

overall exchange and provides a conduit for weak magnetic

communication between the dinuclear centers.

25

Variable-temperature magnetic data for the Mn₄strand 9

(Figure 18) show a moment per mole dropping from 12.1

µ_Bat 300 K to 6.5 µ_Bat 2 K, indicating overall dominant

intramolecular antiferromagnetic exchange. Each pair of

external adjacent Mn(II) ions is bridged by three open-chain

diazine groups in a twisted spiral arrangement, while the

inner pair is bridged by three twisted pyridazine NN groups.

It is constructive, to assess the possible magnetic nature of

the bridging interactions, to compare 9 with the closely

related complex [Mn₂(pahap)₃](ClO₄)₄,³⁹which has a spiral

arrangement of three equivalent open-chain diazine bridges

linking two Mn(II) centers. Mn-N-N-Mn torsion angles

in this compound are 44.2° on average and lead to weak

intramolecular ferromagnetic exchange (2J ) 2.1 cm^-1, H_ex

) -2J{S₁S₂}). The average Mn-N-N-Mn angle for 9 is

31.0 °. Such a small angle would reasonably lead to a

ferromagnetic exchange term as well.³⁹Given the general

trend of antiferromagnetic exchange in pyridazine-bridged

dicopper,^49,50dimanganese, and other dimetal systems,⁵¹an

antiferromagnetic exchange term would reasonably be an-

ticipated for the central triple pyridazine bridge in 9. With

these considerations in mind, the fitting for 9 was approached

using the exchange Hamiltonian H_ex) -J₁{S₁S₂+ S₃S₄} -

J₂{S₂S₃}, but limitations were set on J₁and J₂to avoid

possibly “overfitting” the data in a free-form regression.

Preliminary fitting was carried out using a range of J₁/J₂

ratios, and the range which appeared to approach the best

fit region was J₁/J₂) -0.5. A basis set of theoretical S′/

energy data for all spin vector couplings in the model were

then calculated assuming J₁/J₂) -0.5, and the experimental

data fitted accordingly using MAGMUN4.11. A close fit

gave g ) 2.015(10), J₁) + 1.6(1) cm^-1, J₂) -3.2(1) cm^-1,

and TIP ) 0 cm³mol^-1. The solid line in Figure 18 was

calculated using these parameters. The fitted J₁value is

Magnetic data on complexes 11 and 12 show room-

temperature moments of ∼30 µ_B, consistent with the presence

of 25 Mn(II) centers (µ_SO) 29.6 µ_B), dropping to ∼ 19 µ_B

at 2 K, again indicating the presence of intramolecular

antiferromagnetic exchange (see Figure 19 for data on 12).

A comparison of both 11 and 12 with the well-understood

Mn(II)₉[3 × 3] grids^8-12might suggest that the ground state

for the Mn₂₅grid would approach S′ ) 5/2, because of the

presence of an odd number of spin centers, and the expected

presence of an interconnected [5 × 5] grid network of Mn-

(II) ions, with putative µ-O-type bridging linkages. The

moments at 2 K for are much higher than expected and

suggest that the exchange coupling situation is different from

that apparent in the lower-order antiferromagnetic [n × n]

Mn(II) grids, and perhaps weaker than expected. Magnetiza-

tion data for 12, measured as a function of field at 2 K (see

Supporting Information Figure S3), show a rise to very large

Nâ values as field increases, approaching 55 Nâ at 5 T,

consistent with a large number of residual spins in the grid

at this temperature. The M/H profile shows no saturation

and is best interpreted in terms of a large collection of S )

5/2 spin subunits in the grid, with weak overall antiferro-

magnetic exchange. This in itself is most interesting, given

the confined space within which such a large pool of

electrons (125) would reside. Efforts are underway to

understand more thoroughly the nature of the spin coupling

in these highly electron-rich nanometer scale grids and to

obtain direct structural information through X-ray crystal-

lography for Mn(II)₂₅.

Conclusions

Linear polytopic hydrazone ligands with coordination

pockets connected by small flexible bridges (e.g., O, N-N)

self-assemble in the presence of metal ions to give square

and rectangular grids and also chains. The grids clearly have

a high degree of thermodynamic stability, since other smaller

oligomeric entities are uncommon, although in the case of a

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7780 Inorganic Chemistry, Vol. 46, No. 19, 2007

Supramolecular Self-Assembled Polynuclear Complexes

tetratopic pyrazole ligand, copper prefers to form dinuclear

complexes. The short bridging connections between metal

ions encourage intramolecular spin exchange and electronic

communication between the metal centers. Examples of both

antiferromagnetic and ferromagnetic exchange have been

discussed, with ferromagnetic coupling between Mn(II)

centers resulting from an acute twist of the metal-ligand

bonds around the N-N diazine bridges.

Surface imaging techniques (STM/CITS) show the core

metallic nature of the individual Mn₂₅grid cations, with 25

manganese centers showing up clearly, disposed at the

expected distances of separation in a [5 × 5] square grid

arrangement, despite the absence of a crystal structure. This

is based convincingly on structurally documented [3 × 3]

and [4 × 4] examples, and their CITS measurements. It also

highlights the novel and potentially important use of this new

structural probe, which, while not establishing the complete

structural features of a complex ion, determines the nucle-

arity, and the metal ion core arrangement.

redox changes occur, could allow an individual molecule to

store binary information, under the simple influence of an

applied electrical potential, and the selective probing of

individual metal atoms within the grid using the CITS

technique suggests that “multistability” within a single

molecule may also be possible. This, coupled with the

formation of a monolayer assembly of such a system on a

suitable surface, could lead to the ultraminiaturization of

information storage subunits by a bottom-up molecular

construction technique, rather than a top-down approach,

which is currently used. This could then lead to the creation

of very high density information storage media. Studies to

further examine and exploit these systems are ongoing.

Acknowledgment. We thank the Natural Sciences and

Engineering Research Council of Canada (L.K.T.) and SFB

583, Deutsche Forschungsgemeinschaft (P.M.), for funding

to support these studies. Figures 10-12 were generated using

the program WSxM [Nanotec Electro´nica, Madrid].⁵²

Supporting Information Available: Crystallographic data in

CIF format and figures showing extended structural features of 8,

the local density if states at three Mn sites in 3, magnetization versus

field data for 4, and magnetization versus field data for 12. This

materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.

The ability to apply these grid molecules to surfaces and

to probe the molecules and individual metal atoms using

tunneling techniques sets the stage for the possible use of

this type of nanoscale molecular entity in a device context.

The rich electrochemistry and redox “bistability”¹²observed

with the Mn(II)₉grids, in which controlled and reversible

IC070336A

Inorganic Chemistry, Vol. 46, No. 19, 2007 7781

Article Doi

Supramolecular self-assembled polynuclear complexes from tritopic, tetratopic, and pentatopic ligands: Structural, magnetic and surface studies

DOI: 10.1021/ic070336a

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Article abstract of DOI:10.1021/ic070336a

Full text of DOI:10.1021/ic070336a

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