The one pot synthesis of heterobimetallic complexes from a homoditopic pyrimidine – hydrazone ligand †

The symmetrical, homoditopic, pyrimidine-hydrazone (pym-hyz) ligand L1 was used to synthesise three new heterobimetallic complexes, CuPb L1 (ClO 4 ) 4 , CuAg L1 (SO 3 CF 3 ) 3 , and CuZn L1 (SO 3 CF 3 ) 4 . Each of the complexes was produced in a one-pot reaction in CH 3 CN, and was isolated in high yield and purity simply by precipitation through the addition of diethyl ether. Analysis was carried out by IR, UV-Vis and ESMS spectroscopy, as well as microanalysis. Crystals were also grown for the purposes of X-ray di ﬀ raction studies, which yielded the structures [CuPb L1 (ClO 4 )(CH 3 CN) 2 (H 2 O)](ClO 4 ) 3 ( 1 ), [CuAg L1 (SO 3 CF 3 )(CH 3 CN) 2 ](SO 3 CF 3 ) 2 $ CH 3 CN ( 2 ), and CuZn L1 (SO 3 CF 3 ) 2 (CH 3 CN)(H 2 O)](SO 3 CF 3 ) 2 $ CH 3 CN ( 3 ), all of which were linear complexes containing a Cu( II ) ion in one of the pym-hyz-py coordination sites, and either a Pb( II ), Ag( I ), or Zn( II ) ion in the other.


Introduction
The cooperation of two different metal centres imbues heterobimetallic complexes with distinctly different physical and chemical properties from their mono-and homobimetallic analogues. 1 As a result, synthetic heterobimetallics displaying novel properties have been extensively studied and exploited in the elds of catalysis, 2 mixed spin magnetic systems, 3 and molecular sensing and imaging. 4 Additionally, they act as model compounds for studying the mechanisms and cooperative effects seen in important metalloenyzmes which utilise mixed metal ion active sites. 5 Supramolecular self-assembly can be employed to efficiently synthesise heterobimetallic complexes through the use of ditopic ligands with disparate metal binding sites which are selective towards different metal ions. 6 The desired heterobimetallic complex can then be achieved by reacting the heteroditopic ligand with the metal ions in a sequential fashion. However, even with carefully designed heteroditopic ligands, these reactions oen result in the formation of multiple complexes, necessitating yield limiting purication steps. 7 Evidently, the synthesis of heterobimetallic complexes is not trivial and is aided by an adept understanding of molecular recognition.
We have previously reported that the addition of Cu(II) ions to a homoditopic pyrimidine-hydrazone (pym-hyz) ligand (L1) in a 1 : 1 metal to ligand ratio resulted in discrete Cu(L1H)-(ClO 4 ) 3 and CuL1(SO 3 CF 3 ) 2 complexes in which one of the coordination sites was occupied by a Cu(II) ion while the other one remained vacant (Fig. 1). 8 Usually, ditopic pym-hyz ligands self-assemble into [2 Â 2] grids when reacted with Cu(II) ions in a 1 : 1 metal to ligand 9 ratio as the terpyridine like pym-hyz-py coordination pockets are well suited to binding octahedral metal ions in a coplanar mer fashion. However, the addition of hydroxymethyl arms to the terminal py rings of L1 results in tetradentate coordination to the Cu(II) ion, which prevents the perpendicular arrangement of ligand molecules required to form a grid complex. 10 At the time of publication we envisioned that these monocopper L1 complexes could be useful precursors in the formation of heterobimetallic complexes. 8 The facile synthesis of L1 and the high purity and yield at which the monocopper complexes are synthesised makes them readily accessible. Herein we report that reacting the Cu(L1H)(ClO 4 ) 3 and CuL1-(SO 3 CF 3 ) 2 complexes with either Pb(II), Ag(I) or Zn(II) ions resulted in the formation of heterobimetallic CuM n+ L1A (2+n) complexes (where A ¼ ClO 4 À or SO 3 CF 3 À ; Fig. 2). Each of the complexes was formed in a one pot reaction in which L1 was rst mixed with a solution of Cu(II) ions, before a solution of the other metal ion was added. The heterobimetallic complexes were isolated in high yield without the need for extensive purication steps. They were characterised by microanalysis, mass spectrometry, IR and UV-Vis spectroscopy, and X-ray crystallography.

Synthesis and analysis of complexes
The CuM n+ L1A (2+n) heterobimetallic complexes were all produced in high yields and purity by rst synthesising the monocopper Cu(L1H)(ClO 4 ) 3 or CuL1(SO 3 CF 3 ) 2 complexes, then lling the vacant coordination site with either a Pb(II), Ag(I) or Zn(II) ion (Fig. 2). As previously described, adding a CH 3 CN solution of either Cu(ClO 4 ) 2 $6H 2 O or Cu(SO 3 CF 3 ) 2 $4H 2 O to L1 in a 1 : 1 metal to ligand ratio resulted in dark green solutions of the monocopper complexes. 8 Solutions of either Pb(ClO 4 ) 2 $3H 2 O, AgSO 3 CF 3 , or Zn(SO 3 CF 3 ) 2 in CH 3 CN in a 1 : 1 metal to ligand ratio were then added, and the heterobimetallic complexes were isolated as green precipitates simply by adding diethyl ether to the resulting solutions.
Samples of the complexes were dissolved in CH 3 CN and analysed by UV-Vis spectroscopy. Each of the complexes showed a single, broad, featureless d-d transition, in a similar fashion to the mono-and homobicopper L1 complexes. 8 The d-d transitions of the CuPbL1(ClO 4 ) 4 , CuAgL1(SO 3 CF 3 ) 3 and CuZnL1-(SO 3 CF 3 ) 4 complexes had l max values of either 697, 670, or 699 nm, and extinction coefficients of either 147, 144, or 135 L mol À1 cm À1 , respectively. By comparison the transitions of the mono-and homobicopper complexes of L1 had l max values of 656 nm and 699 nm, respectively, with extinction coefficients of 170 and 250 L mol À1 cm À1 , respectively. 8

X-ray crystallography
Crystals of the CuM n+ L1A (2+n) complexes were also produced through the diffusion of diethyl ether vapour into the solutions, resulting in the solid state structures of [CuPbL1- Fig. 3-5). The overall structures of  these complexes were similar to the previously reported Cu 2 L1A 4 , Pb 2 L1A 4 , Ag 2 L1A 2 and Zn 2 L1A 4 structures, as well as other pym-hyz ditopic complexes, in that they were linear with both pym-hyz-py bonds in the cisoid-cisoid conformation. 8,[11][12][13] Additionally, in complexes 1 and 2, the halves of L1 which were bound to the Cu(II) ions were slightly curved in the mean plane of the central pym ring, while the halves bound to either Pb(II) or Ag(I) were straighter, as seen previously in the homobimetallic L1 complexes. 8,11,12 As a result the lengths of complexes 1 and 2, 13.45 and 13.19Å, respectively, were longer than the curved Cu 2 L1A 4 8 complexes but shorter than the straight Pb 2 L1A 4 11 and Ag 2 L1A 2 12 complexes. Both halves of complex 3 were slightly curved in the plane of the central pym ring, resulting in a length of 12.82Å, which was slightly longer than the Cu 2 L1A 4 complexes and similar in length to the curved Zn 2 L1(SO 3 CF 3 ) 4 complex. The intermetallic distances of complexes 1, 2 and 3 were 6.55, 6.36 and 6.11Å, respectively, which were longer than the Cu-Cu distances previously reported for L1 homobimetallic complexes, but shorter than the Pb-Pb, Ag-Ag and Zn-Zn distances. 8,11,12 The crystal structures of the monocopper complexes of L1 all contained a Cu(II) ion bound to the hydroxymethyl arm and N donors of L1 in a tetradentate fashion. 8 However, when comparing complexes 1-3 it appeared that the tendency of the hydroxymethyl arm to bind to the Cu(II) ion was controlled by whether the other hydroxymethyl arm was bound to the other metal ion. In complex 1 the Cu(II) ion was not bound to the hydroxymethyl arm, while the Pb(II) ion was, as has been consistently observed in other Pb(II) L1 complexes. 11 Conversely, in complex 2 the hydroxymethyl arm was coordinated to the Cu(II) ion, but not to the Ag(I) ion, which commonly does not bind to the hydroxymethyl arms of L1. 12 The asymmetric binding behaviour of the hydroxymethyl arms of L1 was previously observed in the Cu 2 L1A 4 complexes in which only one of the Cu(II) ions in each complex was bound to the hydroxymethyl arms. 8 In contrast, complex 3 had the hydroxymethyl arms bound to both the Cu(II) and Zn(II) ions.
In addition to the hydroxymethyl arms, each of the metals in 1-3 was bound to the three N donors of the pym-hyz-py sites, CH 3 CN solvent molecules, H 2 O molecules and either ClO 4 À or SO 3 CF 3 À anions. The Cu(II) ions in 1 and 2 adopted either a distorted trigonal bipyrimidal (s 5 ¼ 0.59) 14 or perfect square pyramidal (s 5 ¼ 0.01) 14 geometry, respectively. The Cu(II) ion in complex 3 had an octahedral geometry which was tetragonally distorted, with elongated bonds to the axially positioned SO 3 CF 3 À anions, as is typical of Jahn-Teller distorted octahedral Cu(II) ions. 15 The geometries of the Pb(II) and Zn(II) ions in 1-3 were typical of other Pb(II), and Zn(II) complexes of L1. The Pb(II) ion in 1 adopted a six coordinate geometry, which resembled a distorted pentagonal bipyrimid due to the presence of a stereochemically active lone pair of electrons, 16 while the Zn(II) ion in 3 was present in a distorted octahedral geometry. The Ag(I) ion in 2 occupied a distorted square pyramidal environment (s 5 ¼ 0.25), 14 which is a coordination geometry rarely displayed by Ag(I) ions. 17 Both complexes 1 and 3 displayed H-bonding networks which arranged them into one dimensional chains ( Fig. 6 and 7) while complex 2 was H-bonded into dimers (Fig. 8)  was an intermolecular anion-p interaction 18 in complex 1 between the ClO 4 À anion bound to Pb1 and the central pym ring of L1. The distance from O14 to the centroid of the pym ring was 3.159(7)Å (Fig. 6) were paired into dimers by H-bonding between the hydroxymethyl arm coordinated to Cu(II) and the SO 3 CF 3 À anion coordinated to Ag(I) (Fig. 8).

General
All metal salts were purchased from commercial sources and were used as received without further purication with the exception of Cu(SO 3 CF 3 ) 2 $4H 2 O, which was produced by the treatment of CuCO 3 $Cu(OH) 2 $H 2 O with aqueous triic acid. Ligand L1 was synthesised according to its literature method. 8 All solvents were used as received, and were of LR grade or better.
Microanalyses were carried out in the Campbell Microanalytical Laboratory, University of Otago. All measured microanalysis results had an uncertainty of AE0.4. Electrospray mass spectrometry (ESMS) was carried out on a Bruker microTOFQ instrument (Bruker Daltronics, Bremen, Germany) by employing direct infusion into an ESI source in positive mode. Infrared (IR) spectra were recorded on a Bruker Alpha-P ATR-IR spectrometer. UV-Vis spectra were recorded on an Agilent 8453 spectrophotometer against a CH 3 CN background using quartz cells with a 1 cm path length.

CuPbL1(ClO 4 ) 4
Cu(ClO 4 ) 2 $6H 2 O (28.4 mg, 0.0768 mmol) was dissolved in CH 3 CN (2.00 mL) and added to a suspension of L1 (32.0 mg, 0.0762 mmol) in CH 3 CN (2.00 mL) stirring at 75 C. This resulted in complete dissolution of the ligand material and the formation of a clear, green solution. Pb(ClO 4 ) 2 $3H 2 O (37.1 mg, 0.0807 mmol) in CH 3 CN (2.00 mL) was then added to the stirring solution at 75 C. The solution was cooled to rt and diethyl ether (20.0 mL) was added, resulting in the precipitation of a green solid. The solid was washed with diethyl ether and dried in vacuo (63.9 mg, 78%): Anal. found: C 23.42;H 2.76;N 10.41. Calc. for C 21 H 24 N 8 O 18   The H-bonding interaction between O1 and a CH 3 CN molecule is also shown (hydrogens have been omitted for clarity; symmetry codes A: Àx, Ày, 1 À z; B: 1 À x, Ày, 1 À z).