N–N Bridged dinuclear complexes with Mn(II), Ni(II), Cu(II),
Zn(II) and Cd(II); examples with antiferromagnetic and
ferromagnetic coupling
Zhiqiang Xu,
a
Laurence K. Thompson,*
a
Daniel A. Black,
a
Corbin Ralph,
a
David O. Miller,
a
Michael A. Leech
b
and Judith A. K. Howard
b
a
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland,
A1B 3X7, Canada
b
Department of Chemistry, University of Durham, Durham, UK DH1 3LE
Received 14th February 2001, Accepted 11th May 2001
First published as an Advance Article on the web 11th June 2001
A series of new complexes of some bis-bidentate N
4
ligands [picolinamide azine (pahap), 2-pyrazinecarboxamide
azine (pzhpz) and butanedione-monoxime picolinamide hydrazone (pahox)], based on a rotationally flexible N–N
bridging unit, with Mn(), Ni(), Cu(), Zn() and Cd() is reported. 2 : 2 (M : L) ratio complexes with Mn() and
Cd(), in which anionic ligands (Cl
-
, NO
3
-
) are bonded to the metals, have large >80° M–N–N–M torsional angles,
indicating ‘open’ dinuclear structures. With weakly or non-coordinating anions (e.g. ClO
4
-
, ZnBr
4
2-
) 2 : 3 complexes
are formed with smaller torsional angles (39–43°) in keeping with the presence of three N–N bridges and ‘closed’
spiral structures. Antiferromagnetic exchange is observed bewteen Mn() and Ni() centres in both types of complex,
whereas with Cu() small Cu–N–N–Cu angles lead to dominant ferromagnetic exchange coupling. Structural and
magnetic data are discussed.
Introduction
The bis-bidentate ligands pahap (picolinamide azine, see
Scheme 1) and pahox (butanedione-monoxime picolinamide
hydrazone) exhibit rotational flexibility around the N–N bond,
and have been shown to produce twisted mono-ligand and di-
ligand dinuclear complexes with Cu() with N–N single bond
bridges between the metal centres. Magnetic properties depend
on the angle of rotation of the copper magnetic planes around
the N–N bond, and relative orientation of the nitrogen p
orbitals.
1–3
Acute angles (< 80°) lead to ferromagnetic coupling,
while at larger angles antiferromagnetic coupling is observed,
with a linear relationship between exchange integral and angle.
The related dipyrazine ligand pzhpz (2-pyrazinecarboxamide
azine) behaves in a similar manner. Pahap and pzhpz also form
spiral dinuclear complexes [M
2
L
3
]X
4
(L = pahap; M = Mn(),
X = ClO
4
-
, M = Fe(), X = NO
3
-
, M = Co(), X = NO
3
-
,
M = Ni(), X = NO
3
-
; L = pzhpz; M = Fe(), X = NO
3
-
) in
which three ligands wrap themselves around the dinuclear
center with the N–N diazine fragments acting as the bridges.
The spiral conformations of these tris-ligand complexes are
Scheme 1
effectively locked in place, with small M–N–N–M torsional
angles, leading either to weak antiferromagnetic or ferro-
magnetic coupling.
4
The present study describes a series of new
dinuclear di-ligand and tri-ligand complexes with the ligands
pahap, pzhpz and pahox, again involving just single N–N
bridges between the metal centres (metal = Mn(), Ni(),
Cu(), Zn(), Cd()). Magnetic properties indicate weak anti-
ferromagnetic coupling or no coupling for the manganese and
nickel complexes, and ferromagnetic coupling for the copper
derivatives.
Experimental
Materials
Commercially available solvents and chemicals were used with-
out further purification.
Physical measurements
Electronic spectra were recorded as Nujol mulls and in solution
using a Cary 5E spectrometer. Infrared spectra were recorded as
Nujol mulls using a Mattson Polaris FTIR instrument. Mass
spectra were obtained using a VG micromass 7070HS spec-
trometer. C, H, N analyses on vacuum dried samples (24 h)
were performed by the Canadian Microanalytical Service,
Delta, BC, Canada. Variable temperature magnetic data (2–300
K) were obtained with a Quantum Design MPMS5S Squid
magnetometer operating at 0.1–0.5 T. Calibrations were carried
out with a palladium standard cylinder, and temperature
errors were determined with [H
2
tmen][CuCl
4
] (H
2
tmen =
(CH
3
)
2
HNCH
2
CH
2
NH(CH
3
)
2
2+
).
5
Preparations
Pahap, pzhpz and pahox were prepared by procedures
described in previous reports.
1,2,4
[Mn
2
(pahox)
2
Cl
4
]H
2
O (1H
2
O). Pahox (0.22 g, 1.0 mmol)
was added to a warm aqueous methanol (4 : 1) solution (25
DALTON
FULL PAPER
2042 J. Chem. Soc., Dalton Trans., 2001, 2042–2048 DOI: 10.1039/b101445p
This journal is © The Royal Society of Chemistry 2001
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