Introduction

Dinuclear metal complexes have been employed for a long time to mimic the mechanistic pathways for metalloenzymes, catecholase oxidases, 1,2 metallo-b-lactamases (MbL), 3 hemocyanin, 4 DNA and RNA cleavage, 5-7 hydrolysis of phosphodiesters 8 and purple acid phosphatases 9 and as anticancer agents. 10 These molecules have been widely used to monitor and elucidate the spectroscopic structural features in many biological systems. Some dinuclear dioxido-bridged complexes of Mn, Fe, Cu and Ni were reported to activate oxygen insertion in the aliphatic and aromatic compounds' C-H bonds reactions (monooxygenase). 11,12 Moreover, di-nuclear 3d metal complexes have potential applications as catalysts, oxidizing agents, 13,14 and electrochemical and photochemical catalysts for the oxidation of alcohol, ether and water. [14][15][16] Another feature of these complexes is their use in the field of materials science for molecular magnetism, [17][18][19][20] where some of these compounds were shown to behave as single molecule magnets (SMMs). 16,21,22 Also, heteronuclear transition metal-lanthanides, 3d-4f complexes derived from Schiff bases containing phenolic and alkoxy groups, have the capability of fixing atmospheric CO 2 . 21 Binucleating compounds bearing bis(pyridyl) groups assembled through m-or p-xylyl, pyridyl, 20,23,24 1,3,5-triazine 6 and in particular mono-phenolate spacers 2,7,8, 10,12,[17][18][19]25 have been reported (Scheme 1). Obviously, each of the pendant pyridyl arms in these molecules binds a metal ion (M 2+ or M 3+ ) leading to the formation of dinuclear species, which in some ligands may have an extra coordination donor atom (Scheme 1: L-py, bdpaT Cl , and HL R ). This property is well pronounced in binucleating compounds containing phenolate as a spacer group (Scheme 1: HL R ). In this class of phenolate derivatives, dinuclear bridging bimetallic complexes are obtained and in most cases through deprotonated phenolate (phenoxido groups) or neutral phenolate like m-phenoxido groups. 2,7,8, 10,12,[17][18][19]25 However, in the presence of other small bridging groups, such as hydroxide, oxide, peroxide, pseudohalides and acetate, and due to the close proximity between the two metal centers, doubly homoletically bridged metal complexes like di-m-phenoxido, di-m-hydroxido and di-m-oxido, or doubly heterolytically bridged metal complexes like m-phenoxido-mpseduohalide are produced. 5b,7,12d,19,25-27 Herein we report the synthesis of a novel bicompartmental bis(phenolato) compound containing two pendant bis(pyridyl)amine arms, namely 6,6 0 -methylenebis(2-((bis(pyridin-2-ylmethyl)amino)-methyl)-4-chlorophenol)hemihydrate, H 2 LÁ 1 2 H 2 O, where the two bis(pyridyl)phenolate moieties are separated from each other by the CH 2 group (Scheme 1). As a result, the two phenolates exhibit magnetically independent behavior, except in the presence of bridging groups. The coordination chemistry of the ligand is investigated with various divalent metal ions: Mn 2+ , Cu 2+ , Zn 2+ and Cd 2+ . The complexes are spectroscopically and structurally characterized and their magnetic properties are examined when coordinated to paramagnetic ions.

Results and discussion

Synthetic aspects

The synthesis of bicompartmental bis(phenolato) compound 6,6 0 -methylenebis(2-((bis(pyridin-2-ylmethyl)amino)methyl)-4chlorophenol)hemihydrate (H 2 LÁ 1 2 H 2 O, Scheme 1) was straight base were employed in all reactions to assist the deprotonation of the phenolate groups, only one was deprotonated in most products (1, 2, 4-8); probably this may be attributed to the relatively strong hydrogen bonds generated from the deprotonated and protonated phenolates phOÁ Á ÁHÁ Á ÁOph. This intra O-HÁ Á ÁO hydrogen bonds were demonstrated in complexes 1, and 4-6 (see the X-ray section). However, we should mention that in complex 3, the bis(phenolate) ligand reacted in the fully protonated neutral form, H 2 L.

The structural features of the isolated complexes were established by elemental microanalyses, ESI-MS, IR and UV-Vis spectroscopy, conductivity measurements and single crystal X-ray crystallography for 1, 4-6 compounds.

IR spectra

The IR spectra of the investigated complexes and their parent ligand revealed some general characteristic features including broad weak to very weak intense bands over the range 3610-3380 cm À1 , characteristic for the n(O-H) stretching frequency of the coordinated aqua or lattice water molecules in 2-6 and 8 or coordinated CH 3 OH in 7. The bis(phenolate) ligand, H 2 L and all its complexes 1-8 exhibit a series of very weak vibrations over the 3040-2900 cm À1 region assigned to the n(C-H) stretching frequencies of the aromatic and aliphatic groups. In addition, all compounds display a symmetrical series of four bands of strong to medium intensity in the 1600-1415 cm À1 region, characteristic to the bis(pyridyl) group. 29,30 . These bands can be assigned to the asymmetric stretching frequencies n as (N 3 À ) and n as (NCS À ), respectively. The split bands in the azido compound are mainly due to the presence of the azido groups in two different bonding modes, m 1,1 -N 3 and m 1,3 -N 3 . Taking this into consideration, the high preference of Cu 2+ binds the nitrogen site in the vast majority of mononuclear Cu-thiocyanato complexes, which in most cases show an absorbance band at a frequency lower than 2000 cm À1 , whereas the corresponding S-NCS bonding, and in some cases, bridged m N,S -NCS, shows a frequency higher than 2000 cm À1 . 31,32 Thus, based on this criterion and the observed n as (NCS À ) for complex 8 at 2083 cm À1 , one can consider that the thiocyanates undergo N-NCS Cu 2+ bonding. 31,32 The very close IR spectral pattern of the complex cation of [Cu 2 (HL)(OAc)(CH 3 OH)](PF 6 ) 2 (7) to the corresponding structurally determined acetato complexes 1 and 5 may suggest a similar acetate bonding mode, therefore, we tentatively suggest the structural formula of 7 as

Conductivity measurements

In order to explore the electrolytic nature of the complexes under investigation, their molar conductivities L M were measured whenever their solubility permits, and the values are collected in

UV-Vis spectra

The electronic spectra of Cu(II)-H 2 L complexes, 2-4, 7 and 8 obtained in CH 3 CN or CH 3 OH tabulated in Table 1 exhibit a general spectral pattern for the presence of a low energy single broad band over the wavelength region 640-720 nm. Although this spectral feature may indicate five-coordinate Cu(II) complexes in a distorted square pyramidal geometry (SP), 30,34,35 we still cannot exclude the possibility of a distorted octahedral environment. 36 This band, which is insensitive to the solvent nature (complexes 3 and 8), can be assigned to ligand field d-d transition. The strong intense band observed over the region 370-450 nm results from ligand phenolate-oxygen to the Cu(II) ion charge transfer (L -M, CT). 18,26,37,38 The electronic spectra of [Mn 2 (HL)(m 1,2 -OAc) 2 ]PF 6 (1) in MeOH revealed a series of very weak bands at 510, 565 and B650 nm due to Laporte and spinforbidden transitions. 36 In addition, all complexes and specifically 1, 5 and 6 showed very strong bands around 260 and 310 nm. These bands were located at comparable positions to those detected in the free ligand, H 2 L and attributed to p-p* and n-p* transitions, respectively. Electronic spectra and molar conductivity of complexes are compiled in Table 1.

Mass spectra of the complexes

The Description of the structures [Mn 2 (HL)(l 1,2 -OAc) 2 ]PF 6 (1). A perspective view of title compound 1 is given in Fig. 2 and selected bond parameters are listed in Table S1 (ESI †). In 1, the three N donor atoms and the phenolato O donor atom of each arm of the bicompartmental HL À anion are ligated to the Mn(II) center to form a dinuclear complex cation, with a MnÁ Á ÁMn separation of 4.2381(9) Å. The MnN 3 O 3 distorted octahedron around the two metal centers is completed by two oxygen atoms of two bridging acetate anions. The Mn-O and Mn-N bond distances vary from 2.073(3) to 2.361(3) Å. The Mn-O-C bond angles of the bridging acetate groups are 140.2(3) and 147.9(3)1. The two phenolato moieties act in protonated and deprotonated forms to create an internal hydrogen bond [O2Á Á ÁO1 = 2.398(4) Å; O2-HÁ Á ÁO1 = 172(6)1] (Fig. S10, ESI †). The complex cation of 1 co-crystallizes with a PF 6

À counter anion.

Catena-[Cu 4 (HL) 2 (l 1,1 -N 3 ) 2 (l 1,3 -N 3 ) 2 ](NO 3 ) 2 Á5H 2 O (4). Each arm of the two bicompartmental ligands HL in 4 is ligated via their three N-donors and one phenolato O-donor atom to a copper(II) center to form two dinuclear subunits. The elongated octahedra around each of the four metal centers are completed by two N atoms of a pair of bridging azide anions to link the subunits to a 1D polymeric chain (Fig. 3). Within the polymeric S1, ESI †). The two phenolato moieties act in protonated and deprotonated forms to create an internal hydrogen bond [O2Á Á ÁO1 = 2.460(6) Å; O2-H2Á Á ÁO1 = 170(5)1]. The complex cation of 5 co-crystallizes with partially disordered PF 6

À counter anions and a non-coordinated water molecule with an occupancy of 0.453(18).

[Cd 2 (HL)(l 1,1,2 -CH 3 COO)(CH 3 COO)(H 2 O)]PF 6 ÁH 2 O (6). This complex behaves in a similar way to 1 and 5, where the three N donor atoms and the phenolato O donor atom of each arm of the bicompartmental ligand HL À are ligated to a Cd(II) center to form a dinuclear complex cation, with a CdÁ Á ÁCd separation of 4.5690(6) Å (Fig. 5). Each Cd center is further ligated by two oxygen atoms of a chelating acetate anion. In addition, acetato oxygen atom O3 is also linked to the Cd2 center.

Magnetic measurements

[Mn 2 (HL)(l 1,2 -OAc) 2 ]PF 6 (1). The magnetic susceptibility data of 1 were measured in the temperature range 2-300 K. Fig. 6 shows the temperature dependence of molar magnetic susceptibility and molar magnetic moment per dinuclear Mn 2 unit. The magnetic moment at 300 K is 8.34m B , which is close to non-interacting two S = 5/2 spins, corresponding to the spinonly value 5.92m B for high-spin d 5 Mn(II). The magnetic moment gradually decreases with the lowering of temperature until 50 K and then decreases abruptly to 1.55m B at 2 K. This magnetic behavior is characteristic of a pair of weakly antiferromagnetic-coupled manganese(II) ions. The magnetic data were analyzed by the van Vleck equation for two interacting S = 5/2 spins (eqn (1) based on the Heisenberg model, h = À2JS 1 ÁS 2 : w M = (2Ng 2 m B 2 /kT){(55 + 30x 10 + 14x 18 + 5x 24 + x 28 )/ (11 + 9x 10 + 7x 18 + 5x 24 + 3x 28 + x 30 ) (1) where x = exp(ÀJ/kT) and the other symbols have their usual meanings. 40 The solid lines in Fig. 6 represent the best fits and produced the parameters: g = 2.04 and J = À1.64 cm À1 . The obtained J value is comparable to those of di-m-acetato-m-phenoxido-bridged dimanganese(II) complexes (|J| = 2.5-6 cm À1 ), [40][41][42] di-m-acetato-m-aqua-bridged dimanganese(II) complexes (| J| = 1-3 cm À1 ), [43][44][45] and tri-m-acetato-bridged dimanganese(II) complex ( J = À1.3 cm À1 ). 46 [Cu 4 (HL) 2 (ClO 4 ) 3 (H

The crystal structure of this complex showed that there are two crystallographically independent dinuclear copper(II) units composed of [Cu 2 (HL)(ClO 4 ) 2 (H 2 O) 2 ]ClO 4 and [Cu 2 (HL)(ClO 4 )-(H 2 O) 3 ](ClO 4 ) 2 , which are similar to each other in the crystal. Therefore, the complex can be regarded as an essentially dinuclear copper(II) species. The temperature dependence of molar magnetic susceptibility and molar magnetic moment of 2 are illustrated in Fig. 7. The magnetic moment at 300 K is 2.71m B , which is close to the non-interacting two S = 1/2 spins, corresponding to the spin-only value 1.73m B for d 9 Cu(II). The magnetic moment is almost constant and decreases slightly reaching 2.51m B at 2 K upon cooling the sample. The magnetic data were analyzed by the molecular field approximation (eqn (2) for the Bleaney-Bowers equation (eqn (3), taking into consideration the magnetic interaction between the neighboring dinuclear molecules as zJ 0 (z = number of interacting neighbors): 47a

where J is the exchange coupling constant for the dicopper(II) molecule and Na is the temperature independent paramagnetism. The best-fitting parameters of solid lines shown in Fig. 7 resulted in g = 2.15(1), J = 0(3) cm À1 , Na = 60 Â 10 À6 cm 3 mol À1 (fixed) and zJ 0 = À0.2(13) cm À1 . This result shows a very weak magnetic coupling between the two copper(II) ions in accordance with the lack of the bridging group between the metal ions. The best-fitting parameters of solid lines shown in Fig. 7 resulted in g = 2.15, J = 0.02 cm À1 , Na = 60 Â 10 À6 cm 3 mol À1 (fixed) and zJ 0 = À0.20 cm À1 . This result shows a very weak magnetic coupling between the two copper(II) ions in accordance with the lack of the bridging group between the metal ions.

Catena-[Cu 4 (HL) 2 (l 1,1 -N 3 ) 2 (l 1,3 -N 3 ) 2 ](NO 3 ) 2 Á5H 2 O (4). The crystal structure of 4 revealed two kinds of dinuclear molecules: the di-m 1,1 -N 3 -bridged dicopper(II) unit, which exhibits more tendency to ferromagnetic or less likely weak antiferromagnetic coupling, and the di-m 1,3 -N 3 -bridged dicopper(II) unit, which predominantly shows antiferromagnetic coupling. The two dinuclear Cu(II) units are connected by the HL À ligands to form a zigzag chain of tetranuclear units. This structure may be described as the coexistence of two magnetically different dinuclear copper(II) species in the crystal. Taking into consideration the fact that longer axial coordination is in the range 2.219(5) to 2.725(5) Å, one can ignore the magnetic interaction within the di-m 1,1 -N 3 -bridged dicopper(II) unit. The temperature dependence of molar magnetic susceptibility and molar magnetic moment per tetranuclear Cu 4 unit (Fig. 8) reveals molar magnetic moment of 3.86m B at 300 K, which corresponds to 1.93m B /Cu and close to non-interacting four S = 1/2 spins with the spin-only value of 1.73m B for d 9 Cu(II). The magnetic moment decreases smoothly with lowering temperature and reaches to 2.61m B at 2 K. The magnetic data were analyzed by the molecular field approximation (eqn (4), for the modified Bleaney-Bowers eqn (5) consisting of magnetic interaction in the di-m 1,3 -N 3 -bridged dicopper(II) unit and non-interaction between the two copper(II) ions of the di-m 1,1 -N 3 -bridged dicopper(II) unit, 47 considering the magnetic interaction between the neighboring copper(II) moieties as zJ 0 (z = number of interacting neighbors):

where J is the exchange coupling constant for the di-m 1,3 -N 3 -bridged dicopper(II) unit. The best-fitting parameters (Fig. 8) resulted in g = 2.39(1), J = À133(3) cm À1 , Na = 60 Â 10 À6 cm 3 mol À1 (fixed), and zJ 0 = À0.44(5) cm À1 . The obtained J value agrees with observed magnetic coupling via the di-m 1,3 -N 3bridged dicopper(II) unit. 7 No attempts were made to determine the magnetic properties of the complexes [Cu 2 (H 2 L)(NO 3 ) 2 (H 2 O) 2 ](NO 3 ) 2 Á5H 2 O (3), [Cu 2 (HL)(OAc)(CH 3 OH)](PF 6 ) 2 (7) and [Cu 2 (HL)(NCS) 2 ]NO 3 Á2H 2 O (8) due to the expected weak magnetic coupling between the Cu(II) centers as demonstrated in 2, and lack of structural information.

Experimental

Materials and physical measurements 2,2 0 -Methylenebis(4-chlorophenol) was purchased from Alfa Aesar and bis(pyridine-2-ylmethyl)amine (DPA) from TCI America. All other chemicals used in this study were of reagent grade. NMR spectra were collected on a Bruker AvNeo 700 MHz spectrometer at room temperature, whereas 13 C NMR spectra were obtained on a Varian 400 NMR spectrometer operating at 100 MHz ( 13 C). Chemical shifts (d) [Cu 2 (HL)(CH 3 COO)(CH 3 OH)](PF 6 ) 2 (7). The complex was isolated as light navy-blue crystals and synthesized using a procedure similar to that described for complex 1 (overall yield: 55.6%

Crystallography

The X-ray single-crystal data of title compounds 1, 4, 5 and 6 were collected on a Bruker-AXS APEX II CCD diffractometer at 100(2) K. The crystallographic data, conditions retained for the