Molecular Architectures Derived from Metal Ions and the Flexible 3,3′Bipyridine Ligand: Unexpected Dimer with Hg(II

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  Molecular Architectures Derived from Metal Ions and the Flexible 3,3′Bipyridine Ligand: Unexpected Dimer with Hg(II
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  Hindawi Publishing CorporationBioinorganic Chemistry and ApplicationsVolume 2010, Article ID 169054, 8 pagesdoi:10.1155/2010/169054 Research Article MolecularArchitecturesDerivedfromMetal Ions andthe Flexible3,3  -BipyridineLigand:UnexpectedDimerwithHg(II)  AnupamKhutia,Pablo J. SanzMiguel,andBernhard Lippert Fakult ¨at Chemie, Technische Universit ¨at Dortmund, 44221 Dortmund, Germany  Correspondence should be addressed to Pablo J. Sanz Miguel, pablo.sanz@tu-dortmund.de andBernhard Lippert, bernhard.lippert@tu-dortmund.deReceived 4 March 2010; Accepted 12 March 2010Academic Editor: Spyros PerlepesCopyright © 2010 Anupam Khutia et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the srcinal work is properly cited.The flexible ditopic ligand 3,3  -bipyridine (3,3  -bpy) has been reacted with a series of transition metal species (Ag + , Hg 2+ , cis-a 2 M 2+ (a  =  NH 3  or a 2  =  en; M  =  Pt,Pd), trans-a 2 Pt 2+ (a  =  NH 3 )) in an attempt to produce discrete cyclic constructs. WhileAg + gave a polymeric structure  { [Ag(3,3  -bpy)](ClO 4 ) · H 2 O } n  ( 1 ), with all other metal entities cyclic structures were formed.Interestingly, Hg(CH 3 COO) 2  produced a dinuclear complex [Hg(3,3  -bpy)(CH 3 COO) 2 ] 2 · 3H 2 O ( 2 ), in which the two 3,3  -bpy ligands adopt a cis-orientation of the coordinating pyridyl entities. With cis-(NH 3 ) 2 Pt 2+ , a cyclic complex   4  was isolatedin crystalline form which, according to HRMS, is a trimer. With trans-(NH 3 ) 2 Pt 2+ , di ff  erent species are formed according to  1 HNMR spectroscopy, the nature of which was not established. 1.Introduction The “molecular library” concept has proven highly e ffi cientin designing discrete supramolecular metal complexes by combining di- or multitopic metal entities with rigid di-or multitopic ligands [1, 2]. It is less straightforward if  ligands are flexible and can adopt, in principle, di ff  erentrotamer states. In its simplest form, this is the case whentwo N-heterocyclic ligands are connected via a C–C bond.Examples are, among others, 2,2  -bipyridine (2,2  -bpy),3,3  -bipyridine (3,3  -bpy) and, 2,2  -bipyrazine (2,2  -bpz)(Scheme 1). While 2,2  -bpy, in the overwhelming numberof structures, acts as a chelating ligand with the two ringN atoms in a cis-orientation, there are also rare casesof 2,2  -bpy adopting a bridging mode, hence being ina transconfiguration or half-way between cis and trans-[3]. It depends on the conformation of the ligand andthe geometry of the metal, what kind of construct/s is/areformed. With 2,2  -bpz, we have studied this question inmore detail and have characterized a number of discretemolecular entities, which include a flat triangular struc-ture, 3D triangular entities of di ff  erent shapes (prism,vase), as well as a tetranuclear open box [4, 5]. In all these cases the N4  /  N4  positions are involved in metalcoordination, occasionally complemented by addition of metal chelation via N1  /  N1  , and influenced by counteranions.In principle, 3,3  -bipyridine (3,3  -bpy) metal complexesshould be able to reveal analogous topologies as 2,2  -bpz,with the advantage of higher basicities of the N donor atoms(Figure 1). There are several reports in the literature onpolymeric structures containing cis- [6] and in particulartrans-arranged 3,3  -bpy ligands [7], yet none with a discretemolecular metallacycle. The only related examples are thoseof trinuclear cycles containing three cis-a 2 M II units (a 2 =  diamine; M  =  Pd or Pt) and three 4,7-phenanthrolineligands, which can be considered rigid analogous of 3,3  -bpy ligands with the two pyridine entities fixed in a cis-orientation [8, 9]. Our interest in discrete cationic metallacy- cles stems, among others, from their potential of interactingnoncovalently with DNA [10] or particular DNA secondary structures such as DNA quadruplexes [11], as well as theirability to act as hosts for anions [12, 13]. In the present study, we have employed di ff  erent transition metal ions and metalentities which previously have been shown by others andourselves to produce discrete cyclic complexes, namely Ag(I),Hg(II),enPd(II),cis-(NH 3 ) 2 Pt(II)as wellastrans-a 2 Pt(II)(a =  NH 3 ) [14–16].  2 Bioinorganic Chemistry and Applications NNNN N NN NNNNNNNNN MM M M MMMMMMM1 2345 4  5  3  6  2   1  6 cistrans2,2  -bpy 3,3  -bpy 2,2  -bpz Scheme  1: cis- and trans-orientation of pyridine and pyrazine rings in 2,2  -bpy, 3,3  -bpy, and 2,2  -bpz. NNNN NNNNNNNN MMM MM MM NN NN NNNNNNNNNNNN M M MM M MMMMMMMMMcis transL: cis; M: trans(I)L: cis; M: cis(II)L: trans; M: cis(III)L: trans; M: cis(IV)L: cis; M: trans(V)L: trans; M: trans(VI)L: cis; M: tetrahedral(VII) Figure 1:Feasiblediscrete(I–IV)andpolymeric(V,VI)structuresof3,3  -bpymetalcomplexes,andnoveldinuclearcomplex(VII)observedin the Hg II complex   2 .  Bioinorganic Chemistry and Applications 3 2.Experimental  2.1. Synthesis Procedures.  AgClO 4  and Hg(CH 3 COO) 2  wereof commercial srcin. 3,3  -bpy [17], PdCl 2 (en) [18], cis-PtCl 2 (NH 3 ) 2 [19], and trans-PtCl 2 (NH 3 ) 2  [20] were pre-pared according to known literature procedures. { [Ag(3, 3  -bpy)] (ClO 4 ) · H  2 O } n  (1) .  To a solution of 3,3  -bpy (15.6mg, 0.1mmoL) in water (3mL), an aqueous solu-tion (2mL) of AgClO 4  (20.7mg, 0.1mmoL) was added. Thewhite precipitate which formed immediately was centrifugedo ff   and recrystallized from water (4mL, 40 ◦ C). Colorlesscrystals were obtained after 2d at room temperature. Yield:30.5mg (80%). Anal. Calcd (%) for C 10 H 10 AgClN 2 O 5 : C,31.5; H, 2.6; N, 7.3. Found: C, 31.4; H, 2.6; N, 7.5. [Hg(3, 3  -bpy)(CH  3 COO) 2 ] 2 · 3H  2 O  (2) .  An aqueous so-lution (4mL) of 3,3  -bpy (15.6mg, 0.1mmoL) andHg(CH 3 COO) 2  (31.9mg, 0.1mmoL) was stirred at roomtemperature for 12h. The solution is filtered and kept atroomtemperature.After3d,colorlesscrystalswereobtained.Yield: 37.1mg (74%).  2  was characterized by X-ray analysis. [  { Pd(en)(3, 3  -bpy) } (NO 3 ) 2 · H  2 O ] n  (3) .  An aqueoussuspen-sion (15mL) of PdCl 2 (en) (47.4mg, 0.2mmoL) and AgNO 3 (68mg, 0.4mmoL) was stirred in dark for 12h. The resultantAgCl precipitate was filtered o ff   and 3,3  -bpy (31.2mg,0.2mmoL) was added to the filtrate. The solution was stirredat 40 ◦ C for 1 day and then concentrated to a volume of 4mL by rotary evaporator. The solution was filtered andkept at room temperature. After 4d, light yellow powderwas recovered. Yield: 61mg (66%). Anal. Calcd (%) for(C 12 H 18 N 6 O 7 Pd) n  (1-hydrate): C, 31.0; H, 3.9; N, 18.1.Found: C, 30.8; H, 4.0; N, 18.0. cis-[  { Pt(NH  3 ) 2 (3, 3  -bpy) } (PF  6 ) 2 · H  2 O ] n  (4) .  An aqueoussuspension (20mL) of cis-PtCl 2 (NH 3 ) 2  (60mg, 0.2mmoL)and AgNO 3  (68mg, 0.4mmoL) was stirred in dark for 12h.The resultant AgCl precipitate was filtered o ff   and 3,3  -bpy (31.2mg, 0.2mmoL) was added to the filtrate. The solutionwas stirred at 60 ◦ C for 3d, then solid NH 4 PF 6  (65.2mg,0.4mmoL) was added to it and the solution was stirred at60 ◦ C for another day. The solution was concentrated to avolume of 5mL (pD  =  3.20) and kept in an open beaker at4 ◦ C. After 5d, colorless crystals were obtained. According toMS,  4  represents a cyclic trimer, hence  n  =  3 .  Yield: 73mg(54%). Anal. Calcd (%) for C 30 H 48 N 12 O 3 P 6 F 36 Pt 3 : C, 17.3;H, 2.3; N, 8.1. Found: C, 17.5; H, 2.6; N, 7.9.  2.2. X-Ray Crystal Structure Determination.  X-ray crystaldata for  1  and  2  (Table 1) were recorded at 150K withan Xcalibur di ff  ractometer equipped with an area detectorand graphite monochromated Mo K α  radiation (0.71073 ˚A).Data reduction was done with the CrysAlisPro software[21]. Both structures were solved by direct methods andrefined by full-matrix least-squares methods based on F  2 using SHELXL-97 [22]. All nonhydrogen atoms wererefined anisotropically. Hydrogen atoms (including watermolecules) were positioned geometrically and refined withisotropic displacement parameters according to the ridingmodel. All calculations were performed using the SHELXL-97andWinGXprograms[22,23].CCDC763713and763714 contain the crystallographic data for compounds  1  and  2 .  2.3. Instruments.  Elemental (C, H, N) analysis data wereobtained on a Leco CHNS-932 instrument. The  1 H NMR spectra were recorded in D 2 O with tetramethylammo-nium chloride (TMA) and sodium-3-(trimethylsilyl)-1-propanesulfonate (TSP) as internal reference, on Bruker AC200 and Bruker AC 300 spectrometers.  2.4. Electrospray Mass Spectrometry.  The mass spectrum of  4  was recorded with an LTQ orbitrap (high resolutionmass spectrometer) coupled to an Accela HPLC-system(consisting of Accela pump, Accela autosampler, and AccelaPDA detector), from Thermo Electron. The parameters forHPLC were as follows: (i) Eluent A (0.1% formic acid inH 2 O) and eluent B (0.1% formic acid in acetonitrile) withmobile phase consisting of 50% A and 50% B, (ii) Flow rate 250 µ L/min, (iii) injection volume 5 µ L, (iv) scan of wavelength range from 200 to 600nm. The parameters forMS were as follows: (i) ionisation mode ESI (electrospray ionization), (ii) source voltage 3.8kV, Capillary voltage 41V,Capillary temperature 275 ◦ C, tube lens voltage 140V, (iii)scanned mass range 150m/z to 2000m/z with resolution setto 60000. Analysis was done by flow injection (without any column).  2.5. Determination of pK  a  Values.  The pK a  values of 3,3  -bpy ligand were determined by evaluating the changesin chemical shifts of bipyridine protons at di ff  erent pDvalues. pD values were measured by use of a glass electrodeand addition of 0.4 units to the uncorrected pH meterreading (pH ∗ ). The graphs (chemical shifts versus pD)were evaluated with a nonlinear least-squares fit accordingto Newton-Gauss method [24] and the acidity constants(calculated for D 2 O) were converted to values valid for H 2 O[25]. 3.Results and Discussion 1 H NMR Spectra of   3,3  -Bipyridine.  Figure 2 displays atypical  1 H NMR spectrum of the free ligand at pD 6.8. Theindividual resonances show the expected coupling patterns[17]. In the D 2 O spectrum, all resonances show splittingdue to long-range coupling. For example, the H2 signal issplit into a doublet due to coupling with H4 (1.5Hz) andadditionallydisplayscouplingwithH5(0.7Hz).Uponproto-nation, all resonances are downfield shifted, with H6 a ff  ectedmost. pK a  values for [3,3  -bpyH] + and [3,3  -bpyH 2 ] 2+ , asdeterminedbypDdependent 1 HNMRspectroscopy,are4.58 ±  0.1 and 2.71  ±  0.1 (values converted to H 2 O), respectively.These values compare with 4.3 and  ca.  0.3 for 2,2  -bpy, and0.45 and –1.35 for 2,2  -bpz, and reflect the higher basicity of 3,3  -bpy as compared to two other ligands.  4 Bioinorganic Chemistry and Applications Table  1: Crystallographic data for compounds  { [Ag(3,3  -bpy)](ClO 4 )  ·  H 2 O } n (1)  and [Hg(3,3  -bpy)(CH 3 COO) 2 ] 2 ·  3H 2 O  (2) . 1 2 Formula C 10 H 10 Ag 1 Cl 1 N 2 O 5  C 28 H 34 Hg 2 N 4 O 11 Formula weight (g moL − 1 ) 381.52 1003.77Crystal color and habit colorless prisms colorless prismsCrystal size (mm) 0 . 20 × 0 . 20 × 0 . 10 0 . 15 × 0 . 10 × 0 . 05Crystal system monoclinic triclinicSpace group  P  2 1 /c  P  -1 a  (˚A) 9.7606(10) 8.5635(5) b  (˚A) 7.3145(8) 9.2096(6) c  (˚A) 19.572(2) 11.3262(6) α  ( ◦ ) 90 74.746(5)  β  ( ◦ ) 119.148(9) 84.183(4) γ  ( ◦ ) 90 63.221(6) V   (˚A 3 ) 1220.4(2) 769.21(8) Z   4 1 D calcd .  (g cm − 3 ) 2.077 2.167 F   (000) 752 478 µ  (mm − 1 ) 1.888 10.034No. reflections collected 2333 3572No. reflections observed 1557 2969 R int  0.0317 0.0359No. parameters refined 172 208 R  [ I >  2 σ  ( I  )] 0.0331 0.0270 wR  (all reflections) 0.0572 0.0453Goodness-of-fit (GOF) 1.041 0.911 ∆  ρ max   and ∆  ρ min  ( e  ˚A − 3 ) 0.941 and –0.517 1.078 and –1.283 GOF  =  [ Σ w  ( F  2 o  − F  2 c  ) 2 /(  N  o −  N  ν )] 1  /  2 ;  R  = Σ  F  o |−| F  c   /  Σ | F  o | ;  wR  =  [ Σ ( w  ( F  2 o  − F  2 c  ) 2 )/ Σ w  ( F  2 o ) 2 ] 1  /  2 . 1 H NMR resonances of 3,3  -bpy in D 2 O display amoderate sensitivity on concentration, which is consistentwith intermolecular stacking. For example, when going from0.0125M to 0.125M, upfield shifts are 0.06ppm (H2),0.03ppm (H4), 0.05ppm (H6), and 0.04ppm (H5).  Ag  + and Hg  2+ Coordination.  Addition of Ag + ions to anaqueous solution of 3,3  -bpy in D 2 O expectedly does notreveal resonances due to individual species, but rather givesonly averaged signals of the free ligand and the various Agcomplexes as a consequence of fast exchange.A similar situation applies to mixtures of 3,3  -bpy and Hg(II) acetate. The spectrum of the dinuclear Hg(II)complex   2  has its  1 H resonances ( δ  , ppm; D 2 O, pD 5.2)at 8.97, 8.73, 8.44, and 7.88 as well as 2.02 (acetate). Nocoupling of any of the 3,3  -bpy resonances with the  199 Hgisotope is observed as in a previously reported case [26],and a comparison of the shifts of   2  with those of the freeligand at the same pD (downfields shifts of H2, 0.06ppm;H4, 0.06ppm; H6, 0.09ppm; H5, 0.13ppm) does not permitany conclusions regarding the bonding situation in solution.The crystal structure of   { [Ag(3,3  -bpy)](ClO 4 )  ·  H 2 O } n ( 1 ) reveals a polymeric structure rather than a discrete cyclicstructure as we had hoped for. The silver atom (Ag1) showsa distorted octahedral coordination sphere (Figure 3(a)),with two 3,3  -bpy ligands at the apical positions (Ag1-N1, 2.181(3) ˚A; Ag1-N11, 2.189(3) ˚A). The equatorial coor-dination is completed by a water molecule (Ag1-O1w,2.722(3) ˚A), two perchlorate counter anions (Ag1-O13,2.773(4) ˚A; Ag1-O13  , 2.861(4) ˚A), and an argentophilicinteraction [27, 28] with a neighbor silver atom (Ag ··· Ag,3.3751(8) ˚A). Angles and distances involving the coordina-tion sphere of Ag1 are listed in Table 2. The polymeric struc-ture is assembled by coordination of additional silver unitsto the bridging 3,3  -bpy ligands of the apical positions, witha –Ag–[N11-3,3  -bpy-N21]–Ag– basic motif, which extendsalong the [1 0 1] direction (Figure 3(b)). The 3,3  -bpy lig-ands adopt transconformations with a twist angle of 27.9(1) ◦ between pyridine halves. The dihedral angle between twopyridyl rings coordinated to Ag1 is 7.2(1) ◦ . The crystal pack-ing is based on  π   − π   stacking and argentophilic interactionsbetween polymer strands. An upper view of the  ac   planeevidencesthepresenceof voids in thestructure(Figure 4(a)).They are essentially rectangular tunnels along the  b  axis,which house two sets of hydrogen bond-based perchlorate-water polymers. These polymers are built by connectingwater molecules of crystallization and perchlorate anions.  Bioinorganic Chemistry and Applications 5 Table  2: Selected bond distances (˚A) and angles ( o ) for compound  1 .Ag1-N1, 2.181(3) N1-Ag1-N11, 174.37(13) N11-Ag1-Ag1  , 76.06(9)Ag1-N11, 2.189(3) O13-Ag1-Ag1, 157.52(8) N11-Ag1-O13, 87.31(11)Ag1-Ag1, 3.3751(8) O1w-Ag1-O13, 168.17(10) N11-Ag1-O13  , 92.86(11)Ag1-O1w, 2.722(3) N1-Ag1-Ag1  , 108.03(9) N11-Ag1 O1W 95.33(11)Ag1-O13, 2.773(4) N1-Ag1-O13, 84.58(11) Ag1  -Ag1-O1w, 78.60(7)Ag1-O13  , 2.861(4) N1-Ag1-O13  , 87.48(11) O1w-Ag1-O13, 83.13(10)Bpy-rings, 27.86(7) N1-Ag1-O1w, 89.35(11) O13-Ag1-O13  , 85.22(10)py-Ag-py   , 7.19(10) O13-Ag1-Ag1  , 113.22(7) N N1 234569 8 . 5 8 7 . 5 δ   (ppm)H2H2H4H6H5(a)1 2 3 4 5 6 7 8pDH6H5H4H27 . 688 . 48 . 89 . 2         δ      (   p   p   m     ) (b) Figure  2: (a) Low field section of   1 H NMR spectrum of 3,3  -bpy (D 2 O, pD  =  6.8) and pD dependence of individual resonances,focusing splitting of the H2 resonances. (b) pD dependence of H2,H4, H6, and H8 resonances of free 3,3  -bipyridine. Each O1w forms two hydrogen bonds with two perchlorateanions:  ··· O14-Cl1-O12 ···  (H1w)O1w(H2w)  ··· O14-Cl1-O12 ···  (Figure 4(b)). Distances and angles involvingO1w are O1w  ··· O12, 2.969(5) ˚A; O1w  ··· O14, 2.886(5) ˚A;O12 ··· O1w  ··· O14, 119.5(2) ◦ .The crystal structure of the dinuclear species [Hg(3,3’-bpy)(CH 3 COO) 2 ] 2 ·  3H 2 O ( 2 ) is given in Figure 5. Unlikein  1 , in  2  the 3,3  -bpy ligands adopt a cis-conformation of the two pyridyl rings, with a twist angle of 30.4(2) ◦ , and actas bridges between two mercury centers. The coordinationgeometry of the Hg ion (Table 3) is distorted tetrahedral,enclosing two 3,3  -bpy entities (Hg1-N1a, 2.274(3) ˚A; Hg1-N1b, 2.263(3) ˚A), and two chelating/semichelating acetates(Hg1-O11, 2.490(3) ˚A; Hg1-O12, 2.392(3) ˚A; and Hg1-O21,2.286(3) ˚A; Hg1-O22, 2.762(3) ˚A). Selected distances andangles around mercury are listed in Table 3. Both 3,3  -bpy ligands and their bonded mercury atoms are almost coplanarwith a tendency towards a boat conformation (distance fromHg1 to the plane defined by N1a, N1b,N1a  ,N1b  is 0.58 ˚A).The disposition of the acetate ligands is worthy tobe discussed in more detail. Both ligands form a dihe-dral angle of 79.23(16) ◦ with each other. The ligandcontaining O11,O12 is roughly coplanar with the pyridylrings (7.27(27) ◦ , 23.28(23) ◦ ), whereas the ligand A2 (withO21,O22) is roughly perpendicular (78.33(15), 72.81(14) ◦ ).Both are asymmetrically coordinated to Hg1, displayingsignificant longer bond distances of those oxygen atomsinvolved in hydrogen bonding: O1w  ··· O11, 2.802(5) ˚A(Hg1-O11, 2.490(3) ˚A versus Hg1-O12, 2.392(3) ˚A) andO1w  ··· O22, 2.757(4) ˚A (Hg1-O21, 2.286(3) ˚A versus Hg1-O22, 2.762(3) ˚A). Further hydrogen bonding includes atwofold O1w  ··· O2w (2.785(10) ˚A) connection. Besideshydrogen bonding, the crystal packing includes  π   −  π  -and anion– π  -interactions. N1a-pyridyl rings are pairwise π   −  π   stacked (3.5 ˚A), and both rings are involved in anadditional anion– π  -interaction with O11 (O11 ··· centroid,3.47 ˚A). Considering the latter, the formation of staggeredrows is observed, in which each molecule displays fouranion– π  -interactions with neighbor molecules. Rows areinterconnected by   π   − π  -stacking and hydrogen bonding. Complexes with enPd  II  and cis- (  NH  3 ) 2 Pt  II  .  Reactionsof 3,3  -bpy with [Pd(en)(H 2 O) 2 ](NO 3 ) 2  and cis-[Pt(NH 3 ) 2 (H 2 O) 2 ](NO 3 ) 2  (1 : 1 ratio) give products of 1 : 1 stoichiometry [ { Pd(en)(3,3  -bpy) } (NO 3 ) 2 ] n  ( 3 ) andcis-[ { Pt(NH 3 ) 2 (3,3  -bpy) } (PF 6 ) 2 ] n  ( 4 ) which, according to 1 H NMR spectroscopy, are pure materials. Only single setsof 3,3  -bpy resonances are observed in both compounds,indicating that both compounds must be cyclic. Chemicalshifts ( δ  , ppm; D 2 O, TMA as internal reference) are asfollows:  3 , 9.15, 8.84, 8.27, 7.69 ppm (3,3  -bpy) and 2.98(en);  4 , 8.99, 8.95, 8.23, 7.67 (3,3  -bpy). When TSP was used
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