Synthesis and Crystal Packing of trans-Bis(2-aminotroponato)palladium(II) Complexes Bearing Linear Alkyl Chains – Hard Lamellar Structures Self-Locked by Cross-Shaped Molecular Units

Synthesis and Crystal Packing of trans-Bis(2-aminotroponato)palladium(II) Complexes Bearing Linear Alkyl Chains – Hard Lamellar Structures Self-Locked by Cross-Shaped Molecular Units

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  FULL PAPER DOI:10.1002/ejic.201300914 Synthesis and Crystal Packing of   trans -Bis(2-aminotroponato)palladium(II) Complexes Bearing LinearAlkyl Chains – Hard Lamellar Structures Self-Locked byCross-Shaped Molecular Units Naruyoshi Komiya, [a] Takao Hori, [a] Masaya Naito, [a] andTakeshi Naota* [a] Keywords:  Palladium / Aminotroponato complexes / N,O ligands / Lamellar structures / Crystal engineering The synthesis, structure, and three-dimensional lamellararray of a series of  trans -bis(2-aminotroponato- κ N  , κ O )palla-dium(II) complexes bearing linear alkyl chains ( 1a:  n  = 5;  1b: n  = 8;  1c:  n  = 14;  1d:  n  = 16;  1e:  n  = 18, where  n  is the numberof carbon atoms in the chain) attached to  trans  nitrogen do-nor atoms are described. The  trans -coordination of the li-gands, the cross-shaped molecular structures, and the crystalpacking of  1b  and  1c  have been unequivocally establishedfrom single-crystal XRD studies. Highly regulated multilay-ered lamellar structures are observed for  1b  and  1c , whereevery lamellar layer in the crystallographic  ab  plane, formedby  π -stacking interactions of metal cores and van der Waals Introduction The elucidation and application of lamellar arrange-ments of linear chain molecules are important for the devel-opment of new materials whose functions rely on molecularaggregation. Lamellar crystals have been studied in orderto investigate the higher order structures of various poly-mers such as paraffins, [1] polyolefins, [2] polyamides, [3] poly-urethanes, [4] and polyesters. [5] In the fields of inorganic andorganometallic chemistry, similar layered structures havebeen extensively studied using transition metal complexesbearing long alkyl chains, with the aim of achieving liquidcrystalline properties. A variety of transition metal com-plexes bearing square-planar coordination sites to whichlong alkyl chains are appended have been reported to actas efficient mesogens. [6] These complexes typically havelong, rod-like molecular shapes due to pairs of   trans -dis-posed linear alkyl chains along the axial direction of themolecule. This characteristic molecular shape generates amoderate balance of molecular mobility and rigidity in the [a] Department of Chemistry, Graduate School of EngineeringScience, Osaka University,Machikaneyama, Toyonaka, Osaka 560-8531, JapanE-mail: information for this article is available on theWWW under Eur. J. Inorg. Chem.  2014 , 156–163 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 156 interactions between alkyl chains, is laminated alternately onthe  c  axis with an orthogonal lamellar array of the identicalunit. Powder XRD analysis showed that  1b – 1e  exhibit diffrac-tion peaks attributed to periodic ( n 00) reflections of a typicallamellar structure. A linear correlation between the  d -spac-ing and chain lengths in these compounds indicates that theyform the same type of three-dimensional layer-by-layerstructure of lamellar aggregates. The correlation between thecross-shaped molecular structures of  1b – 1e  and the dimen-sionality of molecular constraint in the crystal packing is dis-cussed by comparison with those of rod-shaped analoguesthat exhibit high liquid crystallinity. condensed state, due to the low-dimensional aggregationproperties. In contrast to the soft lamellar structures of theabove complexes, hard lamellar crystals have been reportedto form in the limited cases of less planar, long-chainedcomplexes such as ferrocene, [7] [Cu(OSO 2 R) 2 ], [8] [Cu 2 -(OCOR) 4 ], [9] [Zn 2 (OAc) 4 (tpy–OR) 2 ], [10] and [AgBr 2 -(NHCR) 2 ] (NHC: N-heterocyclic carbene). [11] Hard lamellar crystals of diads comprising transitionmetals and alkyl chains could potentially express a varietyof photoelectric properties upon morphological control of the functional metal cores, although, in contrast to exten-sive studies on the liquid crystalline properties of soft lam-ellar structures, this has not yet been investigated. [6] As partof our program aimed at developing new functionalities of square-planar d 8 transition metal complexes, [12] we recentlyshowed that typically nonemissive crystals of   trans -bis(sali-cylaldiminato)platinum(II) complexes bearing long alkylchains can be changed to highly emissive crystals by mor-phological control of the lamellar structure through crystalengineering. [13] This result indicates that an understandingof the correlation between lamellar structures and the shapeof molecular units can aid in the design of future photoelec-tric devices. In this paper, we investigate the molecular ar-rays formed by  trans -bis(2-aminotroponato- κ N  , κ O )palladi-um(II) complexes ( 1 ) bearing long alkyl chains attached to trans  nitrogen donor atoms in the crystalline state FULL PAPER (Scheme 1). [14] Unlike the rod-shaped (alkoxytroponato)-metal analogues, [14f,15] cross-shaped complexes  1  with“horizontal” alkyl chains form hard lamellar crystals withnonmesogenic properties. Single-crystal XRD analyses of  1b  and  1c , and powder XRD analyses of   1a  –  1e , show that 1b  –  1e  consist of characteristic multilayered lamellar struc-tures, where each lamellar layer is laminated alternatelywith another identical lamellar layer rotated by 180°. Scheme 1. Both the structure and crystal packing show that highdimensionality in self-constrained molecular units can beattributed to the specific anisotropy of the intermolecularinteractions, which results from the cross-shaped structuresof   1b  –  1e . Recently we reported that the self-constraint of molecular units in the condensed state is one of the majorcontrolling factors in the intense solid-state phosphores-cence of platinum complexes. [12b–12e,13] The present resultsprovide significant information regarding the correlationbetween molecular shape and self-constraint of the molecu-lar units in crystals, which will aid in the design of hardand soft matter with excellent photoelectric properties. Results and Discussion Synthesis and Characterization of 1 The  trans -bis(aminotroponato- κ N  , κ O )palladium(II)complexes  1a  –  1e , bearing linear alkyl chains ( 1a:  n  = 5;  1b: n  = 8;  1c:  n  = 14;  1d:  n  = 16;  1e:  n  = 18, where  n  is thenumber of carbon atoms in the chain) were synthesized bythe reaction of [Pd(OAc) 2 ] with the corresponding 2-(alk-ylamino)tropone in the presence of   N  , N  , N   , N   -tetramethyl-ethylenediamine in refluxing toluene at reflux. The 2-(alk-ylamino)tropones were prepared by reaction of tropolonewith SOCl 2  and subsequent amination of the resultingchloride. [16] Complexes  1a  –  1e  were characterized using  1 HNMR,  13 C NMR (Figures S1–S5, Supporting Infor-mation), and FTIR spectroscopy, and high-resolution massspectrometry. The  trans  disposition of the alkyl groups in  1b and  1c  has been unequivocally established by single-crystal Eur. J. Inorg. Chem.  2014 , 156–163 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 157 XRD (Table 1). ORTEP drawings of   1b  and  1c  are shownin Figures 1 and 2. The tropone rings, ligand donor atoms,and metal ion are essentially coplanar, as indicated by thesmall dihedral angles between the coordination plane [Pd1–  Table 1. Crystal data and structure refinement details for com-pounds  1b  and  1c . 1b 1c Formula C 30 H 44 N 2 O 2 Pd C 42 H 68 N 2 O 2 PdFormula weight 571.09 739.41Temperature /K 113 113Crystal color, habit red, platelets red, plateletsCrystal size /mm 0.60  0.20  0.04 0.24  0.12  0.04Crystal system monoclinic monoclinicSpace group  P 2 1 / c  (#14)  P 2 1 / c  (#14) a  /Å 14.963(3) 22.083(2) b  /Å 5.0165(8) 4.9812(3) c  /Å 18.774(4) 17.903(1) α  /° 90 90  β   /° 96.808(5) 95.354(2) γ  /° 90 90 V   /Å 3 1399.2(4) 1960.7(2) Z   2 2 D calcd.  /gcm  –3 1.355 1.252  µ (Mo- K  α ) /cm  –1 6.918 5.093 F  (000) 600.00 792.002 θ max  /° 55.0 54.9No. of reflns. measd. 24278 38387No. of obsd. reflns. 3195 4467No. of variables 248 215 R 1  [ I   2 σ ( I  )] [a] 0.055 0.064 wR 2  (all reflns.) [b] 0.168 0.168Goodness of fit 1.19 1.15[a]  R 1  =  Σ (| F  o | – | F  c |)/ Σ (| F  o |). [b]  wR 2  = { Σ [ w ( F  o2  –   F  c2 ) 2 ]/ Σ w ( F  o2 ) 2 } 1/2 .Figure 1. ORTEP drawing of   1b . (a) Top and (b) side views [C5– C5   projection]. Thermal ellipsoids are shown at the 50% prob-ability level. Hydrogen atoms are omitted for clarity. Selected bondlengths (in Å) and angles (in °): Pd1–O1 1.986(3), Pd1–N1 1.977(4),O1–C1 1.305(6), N1–C2 1.324(6); O1–Pd1–N1 80.40(13), O1–Pd1– N1  99.60(13), O1–Pd1–N1–C2 1.7(3), N1–Pd1–O1–C1 0.3(2). FULL PAPER Figure 2. ORTEP drawing of   1c . (a) Top and (b) side views [C5–C5   projection]. Thermal ellipsoids are shown at the 50% probabilitylevel. Hydrogen atoms are omitted for clarity. Selected bond lengths (in Å) and angles (in °): Pd1–O1 1.985(3), Pd1–N1 1.979(4), O1–C11.301(6), N1–C2 1.334(6); O1–Pd1–N1 80.55(14), O1–Pd1–N1  99.45(14), O1–Pd1–N1–C2 1.6(2), N1–Pd1–O1–C1 1.9(2). O1–N1] and the tropone ring [C1–C3–C5] (2.50°  1b , 1.22° 1c ) as well as the small [C1–C2–N1–Pd1] (2.69°  1b , 1.21° 1c ) and [C2–C1–O1–Pd1] (1.10°  1b , 1.76°  1c ) torsion angles.Intramolecular H-bonding interactions between O1 andH8   are observed, with distances of 2.54 (for  1b ) and 2.58(for  1c ) Å. The characteristic conformation observed forboth  1b  and  1c , where horizontal alkyl chains extend aboveand below the square plane, (Figures 1b and 2b), is note-worthy. As discussed later, such a conformation induceshigh dimensionality with significant intermolecular con-straint of the crystals. Complexes  1b  –  1e  immediately formplatelet crystals when hot solutions in various solvents suchas cyclohexane, benzene, toluene, EtOAc, and THF are co-oled to room temperature. Scanning electron microscopy(SEM) images of microcrystalline  1c  and  1e  obtained fromEtOAc are shown in Figure 3. Differential scanning calo-rimetry (DSC) analysis (Figure S6, Supporting Infor-mation) and polarized microscopy observations indicatethat complexes  1a  –  1e  do not exhibit mesophases. Figure 3. SEM images of microcrystalline (a)  1c  and (b)  1e  ob-tained by crystallization from hot EtOAc solution. Eur. J. Inorg. Chem.  2014 , 156–163 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 158 XRD Studies on Lamellar Crystals 1 Packing diagrams of   1b  and  1c  are shown in Figure 4.The most important features evident in these diagrams arethe multilayered lamellar motifs, which are supported by π -stacking interactions between the planar regions of thecomplexes, and van der Waals forces between the linearalkyl chains. Single-layered lamellar structures viewed downthe  c  axis are shown in Figure 4a and 4d, where the  trans -bis(2-aminotroponato)Pd unit of each molecule is alignedalong the crystallographic  b  axis on the  ab  plane, giving  π -stacking interactions with distances of 3.50 Å (for  1b ) and3.48 Å (for  1c ). Alkyl chains are aligned in a highly linearmanner with a typical zigzag conformation, assisted byvan der Waals interactions between neighboring cofacialmolecular units. Note that every lamellar layer in the  ab plane is laminated alternately along the  c  axis with anotherlamellar layer, where each molecular unit is identical, butrotated 180° about the  c  axis [symmetry code: (-x, 1/2 +  y ,1/2 –   z ), ( x , 1/2 –   y , 1/2 +  z )]. Such a characteristic layer-by-layer structure, which consists of alternate stacking of orthogonal arrays of each unit, can be visualized by viewingdown the  c  axis (Figure 4b, e) or  b  axis (Figure 4c, f). Close-up views of adjacent stacked pairs of   1b  and  1c  molecules(Figure 5a, b) indicate that each molecular unit undergoestypical offset stacking between O1 and the C3–C4–C5 planeof the tropone ring. The contribution of the “2-aminotro-pone” form is greater than that of the “2-hydroxytroponeimine” form in (2-aminotroponato)metal complexes, [14d] sothat the offset stacking shown in Figure 5 is attributed to adonor–acceptor interaction between the electron-deficient FULL PAPER Figure 4. Packing of (a–c)  1b  and (d–f)  1c . (a,d) Single-layered lamellar structures in the  ab  plane ( c  axis projection). (b,e) Multilayeredlamellar structures in the  ab  plane ( c  axis projection), and (c,f) side views in the  ac  plane ( b  axis projection). Molecular units in the upperand lower layers of the  ab  plane are colored orange. Hydrogen atoms are omitted for clarity. carbonyl [C1–O1] and electron-rich 1,3-dienamine [N1–C2– C3–C4–C5] moieties of the tropone rings. The coordinationplanes on each layer are hydrogen-bonded to each otherthrough the O1 and H6 atoms of the tropone ring, as shownin Figure 5c and 5d, which means that each lamellar layerexperiences both intermolecular hydrogen-bonding andvan der Waals interactions to form the multilayered lam-ellar structure.Powder XRD patterns of   1b  –  1e  ( n  = 8–18) show a seriesof sharp diffraction peaks attributable to the typical lam-ellar reflection of a multilayered structure, while the patternof   1a , which contains shorter chains, ( n  = 5) is different(Figure 6). Layer spacings ( d  -spacings) for  1b  (15.2 Å) and 1c  (22.5 Å) are in accordance with the  a  axis lengths of eachunit cell (15.0 Å, 22.1 Å), which indicates that the majorpeaks in these XRD patterns are due to the (100), (200),and (300) reflections. Figure 7 shows the correlation be-tween chain length ( n ) and  d  -spacing estimated by indexingthe diffraction peaks of the (100) reflections. The  d  -spacingsof   1b  –  1e  increase essentially proportionally to the chainlengths ( R 2 = 0.999); however, that of   1a  is much longerthan that expected from this relationship. These results indi-cate that  1b  –  1e , which contain long alkyl chains, have sim-ilar multilayered lamellar structures, while  1a  forms a non-lamellar packing motif. Eur. J. Inorg. Chem.  2014 , 156–163 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 159 Figure 5. Major interactions between the coordination planes incrystals of (a,c)  1b  and (b,d)  1c . (a,b) Offset  π -stacking interactionin the single lamellar layer. (c,d) Interlayer hydrogen-bonding inter-action. Molecular units in the upper layer of the  ab  plane are col-ored orange. FULL PAPER Figure 6. XRD patterns for crystals of (a)  1a , (b)  1b , (c)  1c , (d)  1d , and (e)  1e . The calculated interlayer spacings from the (100) reflectionsare listed in each pattern.Figure 7. Correlation between chain length ( n ) and interlayer spac-ings ( d  -spacings) in  1a  –  1e . Molecular Mobility of Cross- and Rod-Shaped Molecules inthe Lamellar Arrangement The correlation between the molecular shape and lamellarstructure for typical cross- and rod-shaped molecules isshown in Figure 8. Cross-shaped complexes  1b  –  1e  formhard lamellar structures that are not liquid crystals. Thisis in contrast to the rod-shaped, square-planar complexesbearing  trans -bis(5-alkoxy-2-aminotroponato) [14f] and bis(4-alkoxytroponato) [15] ligands, which form low-dimensional Eur. J. Inorg. Chem.  2014 , 156–163 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 160 soft lamellar structures that exhibit high mesogenic proper-ties. This can be rationalized by assuming anisotropy inboth the  π -stacking and van der Waals interactions of thesemolecular units in lamellar aggregates. Both of the cross-and rod-shaped units form layer-by-layer lamellar struc-tures upon molecular aggregation. Each molecular unit ex-periences consecutive  π -stacking interactions between thecoordination planes and van der Waals forces between longalkyl chains to form a single-layered lamellar structure,which becomes increasingly multistratified, mainly as a re-sult of the van der Waals forces between the alkyl chains.Assuming that the most efficient direction for molecularmobility in the crystals is perpendicular to the direction of the axis for consecutive stacking and van der Waals interac-tions, then the direction of molecular mobility to break the π -stacking interactions in  1b  –  1e  is perpendicular to thatneeded for breaking the van der Waals interactions in sin-gle-layered lamellar arrays (Figure 8a). As shown in thepacking of   1b  and  1c  (Figure 4b, e), this single layer is lami-nated to afford the multilayered lamellar structure in analternate manner, where the upper identical lamellar layerof   1b  and  1c  units is situated orthogonally over the srcinallayer to maximize the interlayer interactions. Thus, the di-rection of molecular mobility to unlock the  π -stacking in-teraction between molecular units on the lower horizontalsingle lamellar layer should be perpendicular to that on theupper layer. This means that three-dimensional molecularmobility is required to unlock the overall self-constraint inthe crystal packing of the cross-shaped molecules  1b  –  1e .In contrast to complexes  1b  –  1e , the molecular constraintof rod-shaped complexes with high mesogenic properties is
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