Inorganic Chemistry Communications 14 (2011) 1039–1042

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Inorganic Chemistry Communications

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Ag3 triangle and Ag4 rectangle supported by deprotonated aminopyrimidylderivatives and bis(diphenylphosphino)methane

Di Sun, Na Zhang, Rong-Bin Huang ⁎, Lan-Sun ZhengDepartment of Chemistry, College of Chemistry and Chemical Engineering and State key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005,People's Republic of China

⁎ Corresponding author. Fax: +86 592 2183047.E-mail address: rbhuang@xmu.edu.cn (R.-B. Huang)

1387-7003/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.inoche.2011.03.067

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 March 2011Accepted 31 March 2011Available online 12 April 2011

Keywords:Silver(I)2-Amino-4, 6-dimethylpyrimidine2-AminopyrimidineBis(diphenylphosphino)methaneAg···Ag interactionPhotoluminescence

Two new coordination compounds [Ag6(NHdmpym)2(dppm)4] (1·4ClO4) and [Ag4(NHpym)2(dppm)2](2·2ClO4·2DMF) (NH2dmpym=2-amino-4, 6-dimethylpyrimidine, NH2pym=2-aminopyrimidine, dppm =bis(diphenylphosphino)methane, DMF=N, N-dimethylformamide) have been synthesized under the ammo-niacal conditions and structurally characterized. In 1 and 2, NH2dmpym and NH2pym ligands are deprotonatedand show rare μ4-N1:N1:N2 and μ3-N1:N2 coordination modes, respectively. The bridging dppm ligand combinesthe N-donor ligands to give 1 and 2 triangle Ag3 and rectangle Ag4 cores, respectively. The resulting Ag3metallacycle is a scalene triangle with an average Ag···Ag distance of 3.1043(9) Å. The Ag4 metallacycle is anirregular rectangle with a shorter and a longer Ag···Ag distances of 2.8852(15) and 3.3202(13) Å, respectively.The structural variances between 1 and 2 are caused by the ratios of metal and ligands and substituent groupeffect. In addition, 1 and 2 exhibit blue photoluminescence which may be assigned to ligand-to-metal chargetransfer (LMCT) transition.

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Coordination complexes (CCs) formed by simple Ag(I) salts withmixed group 15 N- and P-donors have been widely investigated [1].Amino-containing heterocyclic N-donors, such as 2-aminopyridine(NH2py), are versatile ligands exhibiting diversemetal binding patternssuch as the endocyclic N atom [2] or the exocyclic N atom of aminogroup [3], occasionally in equilibrium [4], and simultaneously bothpositions, either in a chelatingor bridging fashion [5]. On theother hand,P-donor ligand has stronger coordination ability to Ag(I) than N-donorligand according to the hard and soft acids and bases (HSAB) theorywhich makes Ag(I)/N/P mixed ligands CCs hard to access [6]. Eventhough the formation of multinuclear Pt (n=2, 3) complexes withNH2py following deprotonation of the exocyclic amino group appears tobe common and is of interest to the chemistry of small clusters aswell astometal···metal bond formation [7], NH2pym, isoelectronicity ofNH2py,has received rare concern and no precedent of deprotonated NH2pymappeared due to its inherently high pKa value of the exocyclic aminogroup [8]. The deprotonated similar N-donor ligand has been exclusivelyobserved by us in two coordination polymers, [Ag7(NHpyz)6(ClO4)]nand [Ag4(NHpyz)2(NH2pyz)3(PF6)2]n (NH2pyz=2-aminopyrazine) [9].Based on our previouswork in Ag/N-/O-donor system [10], we exploitedthe Ag(I) with mixed N- and P-donor system and successfully obtainedtwo novel CCs with different Ag(I) cores, namely, [Ag6(NHdmpym)2(dppm)4] (1·4ClO4) and [Ag4(NHpym)2(dppm)2] (2·2ClO4·2DMF)(NH2dmpym=2-amino-4, 6-dimethylpyrimidine, NH2pym=2-amino-

pyrimidine, dppm = bis(diphenylphosphino)methane, DMF=N,N-dimethylformamide) (Scheme 1).

Reaction of amino-substituted N-heterocyclic ligand and dppm withAg2O in the presence of aqueous ammonia under ultrasonic treatmentgave pale yellow clear solution. We found that fine yellow crystals of 1and 2 could be obtained from the crystallization of filtrate under N2

atmosphere after 1 week [11]. Ammonia here may serve as (i) a base todeprotonate the NH2dmpym or NH2pym and enhance the solubility ofthe mixture and (ii) a ligand favoring to form [Ag(NH3)2]+, which mayreduce the reaction rate and facilitate to the formation of the satisfactorysingle crystals. The IR spectra (Fig. S1) showcharacteristic peaks at ~1100and ~620 cm−1 for ClO4

− counteranion in 1 and 2, and at 2960 cm−1 formethyl groups in 1 [12]. The above results are consistent with the X-raysingle-crystal diffraction studies. Powder X-ray diffraction (PXRD) hasbeen used to check the phase purity of the bulky samples in the solidstate. For complexes 1 and 2, themeasured PXRD patterns closelymatchthe simulated patterns generated from the results of single-crystaldiffraction data, indicative of pure products (Fig. S2).

When the molar ratio of reactants is 3:2:2 (Ag2O:NH2dmpym:dppm), we obtained single crystals of complex 1 that crystallizes inthe monoclinic space group P21/c [13]. Selected bond lengths andangles are compiled in Table S1. As shown in Fig. 1, complex 1 is acentrosymmetric hexanuclear Ag(I) cluster comprised of double[Ag3(NHdmpym)(dppm)2]2+ secondary building units (SBUs). Ineach SBU (Fig. 1b), three Ag(I) ions are arranged in a scalene trianglewith the Ag···Ag distances ranging from 3.0362(9) to 3.1721(8) Åwhich are obviously shorter than twice the van der Waals radius forsilver atoms (3.44 Å) [14]. The Ag3 metallacycle is clamped by two

Scheme 1. Preparation route of Ag(I) CCs from mixed N- and P-donors.

1040 D. Sun et al. / Inorganic Chemistry Communications 14 (2011) 1039–1042

dppm ligands and one NHdmpym anion with three interior angles of57.85(2)°, 59.96(2)° and 62.19(2)°, respectively (Fig. 1b). The Ag3triangle and phenyl ring of NHdmpym are approximately perpendic-ular to each other with a dihedral angle of 85.16(3)°. Two SBUs

Fig. 1. (a) The molecular structure of 1, showing the coordination environments of theAg(I) centers. All hydrogen atoms and ClO4

− anion are omitted for clarity. (b) Simplified[Ag3(NHdmpym)(dppm)2]2+ SBU. (Symmetry code: (i) −x+1, −y+1, −z+1.)

interlink to form the hexanuclear Ag(I) cluster by two μ4-NHdmpymwith the longer Ag–N bonds. In 1, the Ag–N bond lengths are in therange of 2.161(5)–2.439(5) Å which are comparable to the relatedcomplexes [15]. As we know, the Ag–Npyrimidyl distances should beshorter than that of Ag–Namino which is partially due to the differenceof electric effect between amino and pyrimidyl N atoms [16]. But inthis case, each amino group in 1 loses one H atom to form NHdmpymanionwhich causes the Nimino to be a stronger electronic donor to Ag(I)thanNpyrimidyl. It is noteworthy that the configurationofNimino atomalsochanges from original triangle to tetrahedron geometry in which thelargest C31–N3–Ag2 angle opens up to 122.3(4)° from the ideal tetra-hedral angle while the remaining angles are in the range of 90.9(2)–116.0(4)°.

As shown in Fig. S3, there are abundant noncovalent interactionsin 1 including anion···π (O5···Cg1i=3.307(6) Å), C–H···π (C55–H55C···Cg4=3.528(10) Å), π···π (dmin=3.760(6) Å, (Cg2···Cg3)]and hydrogen bonds (Table S1) which attribute to the stability ofmolecule packing in the solid state (Fig. S4). (Cg1–Cg4 are thecentroids of six-membered rings N1/C51/N2/C54/C53/C52, C32–C37,C38–C43 and C44–C49, respectively.)

Complex 2 was obtained by changing molar ratio of reactants to1:2:1 (Ag2O:NH2pym:dppm) in the same preparative procedure.Single-crystal X-ray analysis suggests that complex 2 crystallizes inthe same space group to that of 1. It is a discrete centrosymmetrictetranuclear Ag(I) cluster (Fig. 2a). One dppm and NHpym ligandbridge two Ag(I) ions to form a [Ag2(NHpym)(dppm)]+ SBU. TwoSBUs form a cationic unit of 2 (Fig. 2b) through two symmetry-relatedAg–Nimino bonds (Ag2–N1i=2.534(5) Å) which is obviously longerthan the Ag–Npyrimidyl bond (Ag1–N1=2.113(5) Å). The geometry ofNimino in 2 is a triangle and the sum of angles around it is 343° (withAg2 ignored), and so the hybridization of Nimino is nearly sp2. TheNimino atom affords a lone pair electron from its p-orbit and a negativecharge to Ag1 and Ag2 respectively to form two coordination bondswith different lengths. In the core of 2, four Ag(I) ions are arranged inan irregular rectangular with two interior angles of 85.46(5)° and94.54(5)°, respectively. Bridging dppm and NHpym simultaneouslyclamp a pair of Ag(I) ions to generate a short Ag···Ag distance(Ag1···Ag2=2.8852(15) Å) which is comparable to that in metallicsilver (2.886 Å) [14]. Another Ag···Ag distance (Ag2–Ag1i=3.3202(13) Å) supported by μ2-Nimino is obviously longer than the formerone. Both of them indicate important argentophilic interaction whichhas been proved to influence the structures of supramolecularassemblies as well as related properties such as photoluminescenceand conduction, and their importance has been crystallographically

Fig. 2. (a) The molecular structure of 2, showing the coordination environments of theAg(I) centers. All hydrogen atoms, DMF and ClO4

− anion are omitted for clarity.(b) Simplified molecule structure of 2. (Symmetry code: (i) −x+1, −y+1, −z+1.)

Fig. 3. The photoluminescence of complexes 1 and 2 in solid state.

1041D. Sun et al. / Inorganic Chemistry Communications 14 (2011) 1039–1042

and theoretically documented [14,17]. (Symmetry code: (i) −x+1,−y+1, −z+1.)

The ClO4− anions balance the charge neutrality and interact with Ag

(I) through weak Ag···O interactions (Ag1···O2ii=2.948(9);Ag2···O4iv=2.897(5) Å) [18]. In addition, the non-classic C–H···Ohydrogen bonds (Table S2) involving dppm ligand, counteranions andsolvent DMF molecules attribute to the stability of molecules packing(Fig. S5). (Symmetry codes: (ii)−x+1, y+1/2,−z+1/2; (iv) x,−y+1/2, z+1/2).

Comparing the geometries of Nimino, we found that Nimino atoms in1 and 2 should be sp3 and sp2 hybridized, respectively. In 1, both Ag–Nimino bonds are shorter ones (2.194(6) and 2.161(5) Å) whichindicates lone pair electron and negative charge of Nimino have thenearly equal ability of binding Ag(I) ions and are delocalized to someextent. But in 2, Ag–Nimino bonds are a shorter and a longer one,respectively (2.113(5) and 2.534(5) Å), which reflects unequalcoordination behaviors between the lone pair electron and negativecharge of Nimino on NHpym. Although the pKa value of the exocyclicamino group is high (20.5) [8], metal binding effect can cause adramatic increase in acidity of these protons. As we know, the electronon heterocyclic aromatic system is asymmetrically distributed whichcan be further perturbed by coordination to a positively charged Ag(I)center [19]; as a consequence, a more acidic amino group will resultwhich makes the formation of imine anion to be facile and enables asecond metal to bind at this site [20]. In combination with theendocyclic N atoms, rare μ3- or μ4-binding modes of the aminopyr-imidyl ligands are in situ generated in this system.

It has been shown that the diverse structures of the complexes 1and 2 are indubitably related to reactant molar ratio as well ascoordination modes of N-donor [21]. When the Ag2O/N-/P-donormolar ratio is 3:2:2, Ag6 cluster in 1was obtained, while Ag4 cluster in2 was obtained when the Ag2O/N-/P-donor molar ratio changes to1:2:1. The analogue of 2, [Ag4(NHdmpym)2(dppm)2] (3·2ClO4), hasbeen reported elsewhere by us [22]. Complex 3 was synthesized usingthe same reactants to that of 1 and the same Ag/N-/P-donor molar ratioto that of 2. Structural similarity between 2 and 3 indicates substituenteffect has neglectable influence on the resultant structures. In 1 and 2,aminopyrimidyl ligands show two kinds of rare coordination modes:μ4-N1:N1:N2 and μ3-N1:N2 (Scheme S1). The μ4-N1:N1:N2 NHdmpymplays a key role in linking twosymmetry-relatedAg3 triangle SBUs into ahexanuclear motif, while μ3-N1:N2 NHpym just combines with dppm tosupport the Ag4 rectangle motif. In all, the reactant molar ratio plays amore important role in the formation of resultant structures comparedto the substituent effect in this system.

The solid-state photoluminescence properties of 1 and 2 as well asfree ligands were measured at room temperature. The 1 and 2 exhibitemission peaks at 420 and 464 nm (λex=330 nm), respectively(Fig. 3). The distinct emission behaviors of 1 and 2 should be takeninto account. The emission of 1 occurs at a higher energy than that of 2(Δem=44 nm) which can be partially explained by existence of ashorter Ag···Ag distance (2.8852(15) Å) in 2 and hence strongerAg···Ag interaction which has important influence on the photo-luminescence properties of this kind of complexes [23]. Furthermore,we analyzed the photoluminescence properties of the correspondingfree ligands including NH2dmpym (λem=340 nm), NH2pym(λem=360 nm) and dppm (λem=418 nm) as shown in Fig. S6.Structurally, both of the CCs are constructed from mixed ligands withdifferent arrangements and incorporate the Ag···Ag interactionswith different distances. Therefore, we tentatively assigned theemitting states of 1 and 2 to be the ligand-to-metal charge transfer(LMCT) transition which also is probably mixed with a metal-centered transition state modified by Ag···Ag interaction [24].

In summary, we successfully synthesized two new Ag(I) CCs withdifferent Agn (n=3, 4) metallacycle topological structures under theammoniacal condition. The solid-state structures of complexes 1 and2 provide a wealth of interesting features. They demonstrate that the2-aminopyrimidyl derivative ligands, following single deprotonationat the exocyclic amino group, hence formation of an imine anionligands, can act as more versatile tridentate or tetradentate ligand. Inaddition, the photoluminescence properties of complexes 1 and 2were also discussed.

1042 D. Sun et al. / Inorganic Chemistry Communications 14 (2011) 1039–1042

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (Nos. 21021061 and 21071118), 973Project (Grant 2007CB815301) from MSTC.

Appendix A. Supplementary material

CCDC 800281 and 800282 contain the supplementary crystallo-graphic data for this paper. These data can be obtained free of chargevia http://www.ccdc.cam.ac.uk/conts/ retrieving.html, or from theCambridge Crystallographic Data Centre, 12 Union Road, CambridgeCB 21EZ, UK; fax: +44 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.inoche.2011.03.067.

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