Two new one-dimensional compounds with end-to-end dicyanamide as a bridging ligand: Syntheses and structural characterization of trans-[Mn(4-bzpy)2(N(CN)2)2](n) and cis-[Mn(Bpy)(N(CN)2)2](n), (4-bzpy = 4-benzoylpyridine; bpy = 2,2'-bipyridyl)

Article abstract of DOI:10.1021/ic990905h

Two new one-dimensional compounds, trans-[Mn(4-bzpy)₂(N(CN)₂)₂](n) (1) and cis-[Mn(bpy)(N(CN)₂)₂](n) (2), have been synthesized and studied from a magnetic point of view (4-bzpy = 4-benzoylpyridine; bpy = 2,2'-bipyridyl). The crystal structures of 1 and 2 have been solved. Compound 1 crystallizes in the monoclinic system, P2₁/n group, a = 6.374(2) A, b = 7.584(2) A, c = 26.766(5) A, β = 91.87°, and Z = 2, whereas compound 2 crystallizes in the monoclinic system, C2/c group, a = 6.707(2) A, b = 17.188(5) A, c = 13.096(5) A, β = 90.54°, and Z = 4. The two compounds consist of chains with double μ_1,5-dicyanamide bridges between neighboring manganese(II) atoms. The weak antiferromagnetic coupling found for the two compounds (J = -0.3 cm^-1 for 1 and -0.4 cm^-1 for 2) has been studied by MO analysis, and the superexchange pathway through the μ_1,5-(NCNCN^-) bridge has been compared with the shorter μ_1,3-(NNN^-).

Full text of DOI:10.1021/ic990905h

1668
Inorg. Chem. 2000, 39, 1668-1673
Two New One-Dimensional Compounds with End-to-End Dicyanamide as a Bridging
Ligand: Syntheses and Structural Characterization of trans-[Mn(4-bzpy)2(N(CN)2)2]n and
cis-[Mn(Bpy)(N(CN)2)2]n, (4-bzpy ) 4-benzoylpyridine; bpy ) 2,2′-bipyridyl)
Albert Escuer,*,† Franz A. Mautner,‡ Nu´ria Sanz,† and Ramon Vicente†
Departament de Qu´ımica Inorga`nica, Universitat de Barcelona, Diagonal 647, 08028 Barcelona, Spain,
and Institut fu¨r Physikalische und Theoretische Chemie, Technische Universita¨t Graz,
A-8010 Graz, Austria
ReceiVed July 29, 1999
Two new one-dimensional compounds, trans-[Mn(4-bzpy)2(N(CN)2)2]n (1) and cis-[Mn(bpy)(N(CN)2)2]n (2), have
been synthesized and studied from a magnetic point of view (4-bzpy ) 4-benzoylpyridine; bpy ) 2,2′-bipyridyl).
The crystal structures of 1 and 2 have been solved. Compound 1 crystallizes in the monoclinic system, P21/n
group, a ) 6.374(2) Å, b ) 7.584(2) Å, c ) 26.766(5) Å, â ) 91.87°, and Z ) 2, whereas compound 2 crystallizes
in the monoclinic system, C2/c group, a ) 6.707(2) Å, b ) 17.188(5) Å, c ) 13.096(5) Å, â ) 90.54°, and Z
) 4. The two compounds consist of chains with double µ1,5-dicyanamide bridges between neighboring
manganese(II) atoms. The weak antiferromagnetic coupling found for the two compounds (J ) -0.3 cm-1 for 1
and -0.4 cm-1 for 2) has been studied by MO analysis, and the superexchange pathway through the µ1,5-(NCNCN-)
bridge has been compared with the shorter µ1,3-(NNN-).
Introduction
Chart 1
Weak ferromagnets of general formula [M(N(CN)2)2] with a
3-D rutile-type structure in which N(CN)2- is the dicyanamide
ligand (dca) have recently been the object of considerable
attention because of the ability of the dca ligand to act as a
molecular-based magnet precursor with several first-row transi-
tion ions such as Cu(II), Ni(II), Co(II), Mn(II), and Cr(II).1 In
addition to compounds of this kind, other high-dimensional
systems based on the related tricyanomethanide ligand2 or based
on the mixing of different bridging ligands such as dicyanamide
and pyrazine bridges3 have been characterized and studied in
the search for interesting magnetic properties. When the M/dca
ratio is 1:2 and M is hexacoordinated, neutral three-dimensional
compounds are formed as a consequence of the ability of the
dca ligand to act as a tridentate ligand with the three nitrogen
atoms (six coordination sites to six donor atoms, Chart 1A). If
the coordination number is 4 instead of 6, the M/dca ratio of
1:2 allows the formation of a bidentate end-to-end µ1,5-dca
ligand (four coordination sites to four donor atoms), as was
found for [Zn(dca)2] 4 (Chart 1B). For hexacoordinated metallic
centers with two coordination sites occupied by additional
blocking ligands, the dca should also act as bidentate µ1,5-dca
bridging ligand as was reported3 for [Mn(pyz)(dca)2] (pyz )
pyrazine) or may even act as a monodentate ligand through only
one N-terminal atom for the [ML4(dca)2] general formula (Chart
1C), as occurs in compounds such as [Cu(phen)2(dca)2] (phen
) 1,10-phenanthroline)5 and [Ni(4Meim)4(dca)2] (4Meim )
4-methylimidazole),6 as a result of which the 1:2 ratio is
maintained.
† Universitat de Barcelona.
‡ Technische Universita¨t Graz.
(1) (a) Batten, S. R.; Jensen, P.; Moubaraki, B.; Murray, K. S.; Robson,
R. J. Chem. Soc., Chem. Commun. 1998, 439. (b) Kurmoo, M.; Kepert,
C. J. New J. Chem. 1998, 22, 1515. (c) Manson, J. L.; Kmety, C. R.;
Huang, Q. Z.; Lynn, J. W.; Bendele, G. M.; Pagola, S.; Stephens, P.
W.; Liablesands, L. M.; Rheingold, A. L.; Epstein, A. J.; Miller, J. S.
Chem. Mater. 1998, 10, 2552. (d) Jensen, P.; Batten, S. R.; Fallon, G.
D.; Moubaraki, B.; Murray, K. S.; Price, D. J. J. Chem. Soc., Chem.
Commun. 1999, 177. (e) Manson, J. L.; Kmety, C. R.; Epstein, A. J.;
Miller, J. S. Inorg. Chem. 1999, 38, 2552.
(2) (a) Hvastijova, M.; Kozisek, J.; Kohout, J.; Jager, L.; Fuess, H.
Transition Met. Chem. 1995, 20, 276. (b) Manson, J. L.; Campana,
C.; Miller, J. S. J. Chem. Soc., Chem. Commun. 1998, 251. (c) Batten,
S. R.; Hoskins, B. F.; Robson, R. Inorg. Chem. 1998, 37, 3432. (d)
Hvastijova, M.; Kohout, J.; Kozisek, J.; Diaz, J. G.; Jager, L.;
Mrozinski, J. Z. Anorg. Allg. Chem. 1998, 624, 349. (e) Triki, S.; Sala
Pala, J.; Decoster, M.; Molinie, P.; Toupet, L. Angew. Chem., Int. Ed.
Engl. 1999, 38, 113.
(4) Manson, J. L.; Lee, D. W.; Rheingold, A. L.; Miller, J. S. Inorg. Chem.
1998, 37, 5966.
(5) Potocnak, I.; Dunajjurco, M.; Miklos, D.; Kabesova, M.; Jager, L.
Acta Crystallogr. C 1995, 51, 600.
(3) Manson, J. L.; Incarvito, C. D.; Rheingold, A. L.; Miller, J. S. J. Chem.
Soc., Dalton Trans. 1998, 3705.
(6) Kozisek, J.; Paulus, H.; Dankova, M.; Hvastijova, M. Acta Crystallogr.
C 1996, 52, 3019.
10.1021/ic990905h CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/25/2000

Compounds with Bridging Dicyanamide
Inorganic Chemistry, Vol. 39, No. 8, 2000 1669
Table 1. Crystal Data and Structure Refinement for
The stoichiometric compound [ML2(dca)2], in which L may
be two monodentate ligands or one bidentate ligand (four free
coordination sites remaining), should afford compounds with
bidentate dca ligands as in the [Zn(dca)2] case. Following this
strategy we have synthesized two new one-dimensional com-
pounds in which M is manganese(II) and L2 are two 4-benzoyl-
pyridine or one 2,2′-bipyridyl, with the formula trans-[Mn(4-
bzpy)2(N(CN)2)2]n (1) or cis-[Mn(bpy)(N(CN)2)2]n (2). In both
cases the dca ligand was found to be coordinated in the expected
[Mn(4-bzpy)2(N(CN)2)2]n (1) and [Mn(bpy)(N(CN)2)2]n (2)
1
2
chemical formula
fw
space group
a, Å
b, Å
C28H18MnN8O2
553.44
P21/n
6.374(2)
7.584(2)
26.766(5)
91.87(2)
1293.3(6)
2
C14H8MnN8
343.22
C2/c
6.707(2)
17.188(5)
13.096(5)
90.54(2)
1509.6(9)
4
c, Å
â, deg
V, Å3
Z
µ
1,5 bidentate mode. Susceptibility measurements indicate weak
antiferromagnetic coupling with J ) -0.3 and -0.4 cm-1
,
T, °C
25(2)
0.710 69
1.421
25(2)
0.710 69
1.510
respectively.
λ(Mo KR), Å
Compounds 1 and 2 have structures similar to those of the
recently reported µ1,3-azide derivatives with formula [Mn(L)2-
(N3)2] (L ) 2-hydroxypyridine or 3-ethylpyridine),7 which also
consist of chains with double end-to-end bridges. The most
interesting difference lies in the moderately strong coupling (J
ranging from -7 to -13 cm-1) found for the azido derivatives.
The different behavior as superexchange mediator between these
two bridges, on the basis of the topology of the bridging ligands
d
calc, g cm-3
µ(Mo KR), mm-1
0.553
0.886
Ra
0.0440
0.0452
R2ωb
0.1024b
0.0979b
a R(Fo) ) ∑(Fo - Fc)/∑Fo. b Rω(Fo)2 ) {∑[ω((Fo)2 - (Fc)2)2]/
∑[ω((Fo)2)2]}1/2
.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
[Mn(4-bzpy)2(N(CN)2)2]n (1)
µ
1,3-azido and µ1,5-dca, has been compared by means of MO
calculations.
Mn(1)-N(1)
Mn(1)-N(3B)
Mn(1)-N(4)
N(1)-C(1)
2.212(2)
2.221(3)
2.291(2)
1.130(3)
1.295(4)
Mn(1)-N(1A)
Mn(1)-N(3C)
Mn(1)-N(4A)
C(1)-N(2)
2.212(2)
2.221(3)
2.291(2)
1.291(4)
1.135(4)
Experimental Section
Synthesis. The two compounds were prepared in a similar form.
An amount of 5 mL of an aqueous solution of sodium dicyanamide
(0.18 g, 2 mmol) was added dropwise to 50 mL of a methanolic solution
of manganese nitrate hexahydrate (0.29 g, 1 mmol) and 2,2′bipyridyl
(0.16 g, 1 mmol) or manganese nitrate hexahydrate (0.29 g, 1 mmol)
and 4-benzoylpyridine (0.37 g, 2 mmol). The final clear yellow solution
was left to evaporate. Good quality crystals were obtained after 2 weeks.
The yield of the first crop was around 40% for both compounds.
Analytical data for 1 found (calcd for C28H18N8MnO2): C, 61.2 (60.76);
H, 3.3 (3.28); N, 20.9 (20.3). For 2 found (calcd for C14H8N8Mn): C,
48.6 (48.95); H, 2.3 (2.34); N, 32.7 (32.63). For IR spectra, in addition
to the characteristic bands of the corresponding pyridyl derivatives, a
set of very intense bands of the dicyanamide groups appeared at 2308,
2233, and 2176 cm-1 for 1 and at 2302, 2232, 2206, and 2168 cm-1
for 2.
Spectral and Magnetic Measurements. Infrared spectra (4000-
400 cm-1) were recorded from KBr pellets on a Nicolet 520 FTIR
spectrophotometer. Magnetic measurements were carried out with a
Quantum Design instrument with a SQUID detector in the temperature
range 2-300 K under a 500 G field. Magnetization measurements were
recorded up to 5 T of external magnetic field. Diamagnetic corrections
were estimated using Pascal tables. EPR spectra were recorded with a
Bruker ES200 spectrometer at X-band frequency equipped with an
Oxford liquid-helium cryostat for variable temperature work.
Crystallographic Data Collection and Refinement of the Struc-
tures. [Mn(4-bzpy)2(N(CN)2)2]n (1) and [Mn(bpy)(N(CN)2)2]n (2). The
X-ray single-crystal data for both compounds were collected on a
modified STOE four-circle diffractometer. Crystal size: 0.55 mm ×
0.40 mm × 0.22 mm for 1 and 0.42 mm × 0.18 mm × 0.18 mm for
2. The crystallographic data, the conditions for the intensity data
collection, and some features of the structure refinements are listed in
Table 1. Graphite-monochromatized Mo KR radiation (λ ) 0.710 69
Å) and the ω-scan technique were used to collect the data sets. Accurate
unit cell parameters were determined from automatic centering of 50
reflections (10.2° < θ < 16.4°) for 1 and 47 reflections (4.0° < θ <
13.9°) for 2 and refined by least-squares methods. A total of 2788
reflections (2278 independent reflections; Rint ) 0.0237) were collected
for 1 in the range 2.79° < θ < 25.00° and 1595 reflections (1321
independent reflections; Rint ) 0.0269) for 2 in the range 2.83° < θ <
24.99. Corrections were applied for Lorentz polarization effects and
for absorption using the DIFABS8 computer program (range of
N(2)-C(2)
C(2)-N(3)
N(1)-Mn(1)-N(1A) 180.0
N(1)-Mn(1)-N(3B)
86.49(9)
93.51(9)
N(1A)-Mn(1)-N(3B) 93.51(9) N(1)-Mn(1)-N(3C)
N(1A)-Mn(1)-N(3C) 86.49(9) N(3B)-Mn(1)-N(3C) 180.0
N(1)-Mn(1)-N(4)
N(3B)-Mn(1)-N(4)
N(1)-Mn(1)-N(4A)
89.20(9) N(1A)-Mn(1)-N(4)
88.82(9) N(3C)-Mn(1)-N(4)
90.80(9) N(1A)-Mn(1)-N(4A) 89.20(9)
90.80(9)
91.18(9)
N(3B)-Mn(1)-N(4A) 91.18(9) N(3C)-Mn(1)-N(4A) 88.82(9)
N(4)-Mn(1)-N(4A) 180.0
C(1)-N(1)-Mn(1)
155.0(2)
122.3(3)
N(1)-C(1)-N(2)
N(3)-C(2)-N(2)
173.5(3) C(1)-N(2)-C(2)
171.7(3) C(2)-N(3)-Mn(1D) 158.8(2)
normalized transmission factors: 0.385-1.000 for 1 and 0.253-1.000
for 2). The structures were solved by direct methods using the SHELXS-
869 computer program and refined by full-matrix least-squares methods
on F2 for 2, using the SHELXL-9310 program incorporated in the
SHELXTL/PC, version 5.03,11 program library and the graphics
program PLUTON.12 All non-hydrogen atoms were refined anisotro-
pically. The hydrogen atoms were located at calculated positions (C-H
bond lengths of 0.930 Å), and their isotropic displacement factors were
set at 1.2 times the value of the equivalent isotropic displacement
parameter of the corresponding parent carbon atom. The final R indices
were 0.0440 and 0.0452, respectively, for all observed reflections.
Numbers of refined parameters were 178 (1) and 105 (2). Maximum
and minimum peaks in the final difference Fourier syntheses were 0.417
and -0.386 e Å-3 for 1 and 0.235 and -0.172 e Å-3 for 2. Significant
bond parameters for 1 and 2 are given in Tables 2 and 3.
Results and Discussion
Description of the Crystal Structure of [Mn(4-bzpy)2-
(N(CN)2)2]n (1). The labeled diagram for 1 is shown in Figure
1. The structure consists of octahedrally coordinated manganese
atoms in which the coordination sites are occupied by two
(8) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(9) Sheldrick, G. M. SHELXS-86, Program for the Solution of Crystal
Structure; University of Gottingen, Germany, 1986.
(10) Sheldrick, G. M. SHELXL-93, Program for the Refinement of Crystal
Structure; University of Gottingen, Germany, 1993.
(11) SHELXTL, Program Library for the Solution and Molecular Graphics,
Version 5.03 (PC Version); Siemens Analytical Instruments Division:
Madison, WI, 1995.
(7) Escuer, A.; Vicente, R.; Goher, M. A. S.; Mautner, F. A. Inorg. Chem.
1998, 37, 782.
(12) Spek, A. L. PLUTON-92; University of Utrecht: Utrecht, The
Netherlands, 1992.

1670 Inorganic Chemistry, Vol. 39, No. 8, 2000
Escuer et al.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for
[Mn(bpy)(N(CN)2)2]n (2)
50 K, and tends to zero at low temperature. The øM value
increases continuously on cooling. The magnetic data were
analyzed by means of the analytical expression13 for an infinite
chain of classical spins derived by Fisher:
Mn(1)-N(4)
Mn(1)-N(2A)
Mn(1)-N(1A)
N(2)-C(6)
2.187(4)
2.223(4)
2.255(3)
1.131(5)
1.295(5)
Mn(1)-N(4A)
Mn(1)-N(2)
Mn(1)-N(1)
C(6)-N(3)
2.187(4)
2.223(4)
2.255(3)
1.275(5)
1.128(5)
Ng2â2S(S + 1)
1 + u
N(3)-C(7B)
C(7)-N(4)
ø )
[
][
]
3kT
1 - u
N(4)-Mn(1)-N(4A)
102.1(2)
N(4)-Mn(1)-N(2A)
86.45(14)
90.72(14)
175.5(2)
N(4A)-Mn(1)-N(2A) 90.72(14) N(4)-Mn(1)-N(2)
with
N(4A)-Mn(1)-N(2)
N(4)-Mn(1)-N(1A)
N(2A)-Mn(1)-N(1A) 89.36(13) N(2)-Mn(1)-N(1A)
86.45(14) N(2A)-Mn(1)-N(2)
164.35(12) N(4A)-Mn(1)-N(1A) 93.04(12)
JS(S + 1)
94.28(12)
164.35(12)
89.36(13)
158.7(4)
120.9(4)
161.9(4)
kT
JS(S + 1)
u ) coth
-
N(4)-Mn(1)-N(1)
N(2A)-Mn(1)-N(1)
N(1A)-Mn(1)-N(1)
N(2)-C(6)-N(3)
93.04(12) N(4A)-Mn(1)-N(1)
94.28(12) N(2)-Mn(1)-N(1)
72.2(2)
173.2(5)
174.4(5)
[
]
kT
C(6)-N(2)-Mn(1)
C(6)-N(3)-C(7B)
C(7)-N(4)-Mn(1)
Best-fit parameters were J ) -0.3 cm-1, g ) 1.98; J ) -0.4
cm-1, g ) 2.00 for 1 and 2, respectively. The weak antiferro-
magnetic interaction was confirmed by magnetization measure-
ments at 2 K up to an external field of 5 T. At higher field, the
magnetization in M/Nâ units indicates a quasi-saturated S )
5/2 system for both compounds (Figure 5). Comparison of the
overall shape of the plots with the Brillouin plot for a fully
isolated S ) 5/2 system indicates slower magnetization consistent
with a weak AF interaction. These low J values compare well
with the weak coupling found for the few examples of
dicyanamide or tricyanomethanide compounds measured to date.
EPR spectra recorded on powdered samples at room tem-
perature show an isotropic signal centered at g ) 2.00, with a
peak-to-peak line width of 90 G for 1 and 121 G for 2. In
addition, a weak half-field signal centered at g ) 4.00 was found
for both complexes. The g ) 2.00 signal shows the same line
width at 77 K, increasing slightly at 4 K (∆Hpp ) 200 G). The
room-temperature line width is between those of the sharp
signals (∆Hpp ) 35 G at room temperature) recently reported
for the structurally related compounds with end-to-end azide
bridges,7,14 [Mn(pyOH)2(µ-N3)2]n, [Mn(3-etpy)2(µ-N3)2]n, and
[Mn(3,5lutidine)2(N3)2], and those of the very broad signal
(∆Hpp ) 750 G) reported for the end-on azide-bridged system
[Mn(2-bzpy)(N3)2]n (2-bzpy ) 2-benzoylpyridine).14
N(4)-C(7)-N(3C)
N-pyridine atoms of two 4-benzoylpyridine ligands in trans
arrangement and four N-terminal atoms of dicyanamide ligands.
Two of the dicyanamide ligands act as end-to-end bridging
ligands with one neighboring manganese atom, and the other
two also act as end-to-end bridging ligand with the other
neighboring manganese atom, giving a uniform one-dimensional
system (Figure 1). The bond parameters related to the bridges
are Mn(1)-N(1) ) 2.212(2) Å, Mn(1)-N(3B) ) 2.221(3) Å,
Mn(1)-N(1)-C(1) ) 155.0(2)°, and Mn(1)-N(3B)-C(2B) )
158.8(2)°. The dicyanamide ligand is angular with a C(1)-
N(2)-C(2) angle of 122.3(3)° with two nearly linear N-C-N
units with angles of 173.5(3)° and 171.7(3)°. The shortest
intrachain Mn-Mn distance is 7.584(2) Å (b axis of the cell)
as consequence of the large bridging ligand, whereas the
minimum interchain Mn-Mn is only 6.374(2) Å (a axis of the
cell). Along the c direction the minimum Mn-Mn distance is
14.172(2) Å (Figure 2). The Mn(NCNCN)2Mn ring shows a
slight chair distortion from planarity, with a maximum deviation
of the manganese atoms of 0.212 Å over the plane defined by
the two dicyanamide ligands (angle between the mean plane of
the dicyanamide ligands and the N(1)-Mn(1)-N(3D) plane is
8.0(1)°).
This behavior may be explained by attending to the main
factors15 that influence the line width for an isotropic Heisenberg
one-dimensional system: exchange narrowing, due to the
superexchange interactions along the chain, and intra- or
interchain dipolar interactions that broaden the signal. For the
two [Mn(L)2(µ-N3)2]n complexes, the chains were neutral and
well isolated by the large pyridinic ligands (Mn‚‚‚Mn interchain
distance greater than 9 Å) and the intrachain Mn‚‚‚Mn distances
were around 5.3 Å.7 From these structural parameters, dipolar
intra- or interchain interactions are negligible, and so the
narrower signals observed for the azido chains may be attributed
to the exchange narrowing: very weak for 1 and 2 (|J| < 0.5
cm-1) and strong for the two [Mn(L)2(µ-N3)2]n compounds (|J|
values close to 10 cm-1). In contrast, dipolar interactions explain
the large line width found in the EPR signal (750 G) of the
end-on azide-bridged chain [Mn(2-bzpy)(N3)2]n, which shows
comparable J superexchange parameters (J ) 0.8 cm-1) and
interchain Mn‚‚‚Mn distances but strong dipolar interactions due
to the short Mn‚‚‚Mn intrachain distances,14 close to 3.5 Å.
Superexchange Mechanism. A priori, weak interactions may
be expected for the end-to-end dicyanamide bridges, present
because of the large Mn-Mn interaction pathway through a
Description of the Crystal Structure of cis-[Mn(bpy)-
(N(CN)2)2]n. The labeled diagram for 2 is shown in Figure 3.
This compound has the same double dicyanamide bridges in a
one-dimensional arrangement, similar to the above compound
with 4-bzpy, but the chelate character of 2,2′-bpy leads to the
cis 1D compound. The bond parameters of the bridging region
are also quite similar to those of 1: Mn(1)-N(2) ) 2.223(4)
Å, Mn(1)-N(4) ) 2.187(4) Å, Mn(1)-N(2)-C(6) ) 158.7(4)°,
and Mn(1)-N(4)-C(7) ) 161.9(4)°. The dicyanamide ligand
is bent with a C(6)-N(3)-C(7B) angle of 120.9(4)° and two
N-C-N straight linear units with angles of 173.2(5)° and
174.4(5)° for N(2)-C(6)-N(3) and N(4)-C(7)-N(3C), re-
spectively.
The shortest intrachain Mn-Mn distance is 7.512(3) Å (b
axis of the cell) as consequence of the large bridging ligand,
whereas the minimum interchain Mn-Mn is 6.707(3) Å with
neighboring chains in the (100) direction (Figure 2). In this case
the Mn(NCNCN)2Mn rings are roughly planar, with deviation
from the mean plane of less than 0.1 Å for all atoms.
Magnetic Data and Coupling Constants Evaluation. The
øMT product and the molar magnetic susceptibilities vs T in
the 300-2 K range of temperature for [Mn(4-bzpy)2(N(CN)2)2]n
(1) and [Mn(bpy)(N(CN)2)2]n (2) are plotted in Figure 4. The
overall behavior of 1 and 2 corresponds to weak antiferromag-
netically coupled systems. When the samples are cooled, the
øMT product decreases slowly from 4.20 and 4.36 cm3 K mol-1
at 300 K for 1 and 2, respectively, decreases more quickly below
(13) Fisher, M. E. Am. J. Phys. 1964, 32, 343.
(14) Abu-Youssef, M. A. M.; Escuer, A.; Gatteschi, D.; Goher, M. A. S.;
Mautner, F. M.; Vicente, R. Inorg. Chem. 1999, 38, 5716.
(15) Bencini, A.; Gatteschi, D. EPR of Exchange Coupled Systems;
Springer-Verlag: Berlin, Heidelberg, 1990; Chapter 10 and references
therein.

Compounds with Bridging Dicyanamide
Inorganic Chemistry, Vol. 39, No. 8, 2000 1671
Figure 1. Labeled ORTEP drawing for trans-[Mn(4-bzpy)2(N(CN)2)2]n (1). Ellipsoids are shown at the 40% probability level together with a view
of the chain arrangement.
Figure 2. View along (001) direction of the unit cell of trans-[Mn(4-bzpy)2(N(CN)2)2]n (1) and cis-[Mn(bpy)(N(CN)2)2]n (2) compounds showing
the packing of the chains.
bridge of five atoms. This is important, but it is not the only
factor that promotes very weak coupling between the paramag-
netic centers. MO calculations16 performed on the dicyanamide
ligand and also on a modeled dimeric unit show some
determinant topological factors that may be compared with the
very effective end-to-end azide bridge. The model used for the
calculation was a [(NH3)4Mn-(µ1,5-dca)2-Mn(NH3)4]2+ unit
with the bond parameters Mn-N 2.150 Å, N-C 1.130 and
1.290 Å, Mn-N-C 164.0°, N-C-N 180°, and C-N-C
122.0°.
The main superexchange pathways for the azide ligand consist
of two degenerate πx and πy nonbonding MOs that may overlap
with the appropriate atomic orbitals of the paramagnetic center.17
For the angular dicyanamide ligand, the two HOMOs correspond
to one π nonbonding and one σ orbital, not strictly degenerate
but with similar energy. For both azide and dicyanamide ligands,
one of the HOMOs of the bridge corresponds to one π
(16) Mealli, C.; Proserpio, D. M. CACAO program (Computed Aided
(17) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993
Composition of Atomic Orbitals). J. Chem. Educ. 1990, 67, 399.
and references therein.

1672 Inorganic Chemistry, Vol. 39, No. 8, 2000
Escuer et al.
Figure 5. Plot of the magnetization in 2S units measured at 2 K for
the compounds trans-[Mn(4-bzpy)2(N(CN)2)2]n (1) (solid diamonds) and
cis-[Mn(bpy)(N(CN)2)2]n (2) (solid circles). Continuous line corresponds
5
to the Brillouin function for an isolated S ) /2. Lower magnetization
for 2 confirms a J value slightly greater than that of compound 1.
Figure 3. Labeled ORTEP drawing for cis-[Mn(bpy)(N(CN)2)2]n (2).
Ellipsoids are at 30% probability level, and a view of the chain
arrangement is shown.
Figure 6. Left: schematic drawing of the main superexchange
pathways for the azido ligand. Right: the corresponding πy and σ MOs
for the dicyanamide ligand together the schematic superexchange
pathways. The efficiency of the t2g-πy-t2g interaction is reduced for
the dicyanamide ligand because of the lower density of the bridge on
the terminal N atoms (25% vs 50% for the azido bridge). For the eg-
σ-eg interaction, the N-Mn bond direction is close to the nodal plane,
giving a negligible net overlap, in contrast with the very efficient eg-
πx-eg interaction found for the azido bridge.
Figure 4. Plot of the øMT product vs T for compounds trans-[Mn(4-
bzpy)2(N(CN)2)2]n (1) (dot-centered diamonds) and cis-[Mn(bpy)-
(N(CN)2)2]n (2) (dot-centered circles). Solid line shows the best fit of
the data (see text).
azido system lies in the lower electronic density on the terminal
-
N atoms. For the N3 ion the electronic probability on the
terminal N atoms is 50%, whereas for the dicyanamide ligand
the maximum of probability resides on the central nitrogen and
only 25% lies on the terminal N atoms. Consequently, the
overlap is drastically reduced, and this superexchange pathway
thus becomes less efficient for the dicyanamide ligand than the
azide bridge.
Chart 2
For the azido bridge, the most effective pathway that gives
strong antiferromagnetism for moderate or medium M-N-N
bond angles is the second π orbital, which may interact with
the eg orbitals of the paramagnetic centers, giving an eg-π-eg
pathway with good overlap.7,17 For the dicyanamide ligand, the
equivalent pathway consists of an eg-σ-eg interaction, which,
for the experimental M-N-C bond angles in all cases close to
160°, shows a net overlap close to zero, as may be observed in
the Figure 6 in addition to the lower density in the terminal N
atoms.
nonbonding orbital (Figure 6) with a similar difference in energy
with the d orbitals of the metal. These π MOs may overlap
appropriately with the corresponding t2g atomic orbitals of a
paramagnetic center such as Mn(II) (d5 ion). The poor efficiency
of this pathway compared with the π nonbonding orbital of the

Compounds with Bridging Dicyanamide
Inorganic Chemistry, Vol. 39, No. 8, 2000 1673
The immediate conclusion is that the µ1,5-dicyanamide bridge
should be assumed to be a very poor superexchange mediator,
and practically noncoupled systems should be expected. The
above conclusions can be easily extrapolated to the µ1,3-µ3,5
bridges recently found in the [M(dca)2] compounds; the σ
pathway for M-N-C bond angles close to 160° should always
allow reduced overlap and low magnetic interaction, and the
main (and experimentally low1-3) interactions may be related
to the moderately effective π pathway. The most important
consequence of these results is the predictable low interaction
for all coordination modes of the dicyanamide bridge, which
should lead to low Tc in extended systems.18
of Graz) for use of experimental equipment and the OENB for
partial financial support (Project 7967).
Supporting Information Available: A complete listing of data
collection and processing parameters, bond lengths and bond angles,
atomic coordinates, anisotropic displacement coefficients, and hydrogen
atom coordinates for 1 and 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC990905H
(18) During the refereeing process of this paper, some related chains have
been reported by other authors, showing always very low coupling
constants. Manson, J. L.; Arif, A. M.; Miller, J. S. J. Mater. Chem.
1999, 9, 979. Batten, S. R.; Jensen, P.; Kepert, C. J.; Kurmoo, M.;
Moubaraki, B.; Murray, K. S.; Price, D. J. J. Chem. Soc., Dalton Trans.
1999, 2987. Manson, J. L.; Arif, A. M.; Incarvito, C. D.; Liable-Sands,
L. M.; Rheingold, A. L.; Miller, J. S. J. Solid State Chem. 1999, 145,
369.
Acknowledgment. A. Escuer and R. Vicente thank the
CICYT (Grant PB96/0163) for support of this research. F. A.
Mautner thanks Prof. C. Kratky and Prof. F. Belaj (University