Received 23rd February 2010, Accepted 18th June 2010DOI: 10.1039/c0dt00018c
Two types of Pd(II) azobenzene/bipyridine complexes with unusual coordination mode of azobenzenes,PdCl{(m-Cl)(m-R1C6H3N NC6H3R2)}Pd(bpy) 1a–4a and [(bpy)PdCl(m-NH2C6H3N NC6H4)-Pd(bpy)]Cl 3b were formed by the reaction of dicyclopalladated azobenzenes (DMF)PdCl(m-R1C6H3N NC6H3R2)PdCl(DMF) with excess of bpy, where bpy=2,2¢-bipyridine; R2 H and R1 H(1), CH3 (2), NH2 (3) or R1 N(CH3)2 and R2 NO2 (4). Neutral species 1a–4a were obtained inacetone, while in DMSO or MeOH the ionic complex 3b was produced. When dissolved, 3bdecomposes to 3a and free bpy; however in DMSO upon addition of bpy 3b crystallizes again. X-raystructures of all complexes confirmed breaking of one Pd–N bond in the initial precursors, thusallowing rotation of one phenyl ring and positioning of both Pd atoms on the same side of theazobenzene ligand. Two Pd atoms are connected by the azobenzene ligand and in neutral complexesadditionally by the Cl-bridge. In all complexes in the solid-state azobenzenes act simultaneously asmonodentate C- and bidentate C,N-donors while bpy acts as bidentate donor. Variable-temperature 1HNMR experiments established that structures of 1a–4a in DMF and DMSO at ambient temperature arenot consistent with solid-state structures due to the fast exchange of one of the bpy nitrogen atomsbound to the Pd atom with solvent molecules. Theoretical studies confirmed the experimentalstructures as the most stable isomers. Photoabsorption and photoemission properties of the newcomplexes have been measured and photoabsorption is rationalized by time dependent DFTcalculations. The presence of bpy significantly increases the intensity of fluorescence either in thesolution (4a) or in the solid state (3a, 4a, 3b) at ambient temperature.
Introduction
Recently, considerable effort has been focused to the designand synthesis of cyclopalladated complexes containing N-donorligands because of their potential application as model compoundsfor photoactive units in different optical devices,1 as differentialchromogenic and fluorescent chemosensors2 as well as liquidcrystalline materials and catalysts.3 In this regard, particularlyinteresting are complexes with azobenzenes, due to their easyortho-metallation by Pd(II) salts as well as to their structuraland photophysical properties.2e,4 Azobenzene and its derivativesreadily undergo reaction with Pd(II) convenient precursors toproduce single or double cyclopalladated complexes containingone5 or two metalacycles.2b,6 Such compounds, with metalacycles
aDivision of Physical Chemistry, “Ruąer Boskovic” Institute, Bijenicka 54,HR-10002, Zagreb, CroatiabDepartment of Chemistry, Faculty of Science, University of Zagreb,Horvatovac 102, HR-10002, Zagreb, Croatia† Electronic supplementary information (ESI) available: Crystal structuresof 1a–4a, 3b1 and 3b2 (CIF files); 1H and 13C NMR spectra of 1a–4a indifferent solvents at ambient temperatures; crystallographic data collectionand structure refinement data; analysis of molecular geometric and crystalpacking features; calculated geometries and energies of alpha and betaisomers in vacuum and in the solution; complete list of the computedexcited states and experimental UV-vis spectra of 1a–4a; graphics of chargedensity changes for selected electronic excitations of 4a; absorption andemission spectra of 4a in CHCl3 and solid state fluorescence photographsof 4a, 3a and 3b1. CCDC reference numbers 746371–746376. For ESIand crystallographic data in CIF or other electronic format see DOI:10.1039/c0dt00018c
including conjugated C C and N N bonds show interestingoptical properties.7
We have previously described synthesis and full characterizationof a novel class of double cyclopalladated complexes withazobenzenes, which absorb strongly in the visible region and,according to time dependent DFT calculations, most of theabsorption bands were attributed to intraligand p* ← p or MLCT(metal-to-ligand charge transfer) transitions.6a Furthermore,weak luminescence has been observed at ambient temperatureseither in the solution or in the solid state for complexes with4-aminoazobenzene or 4-(dimethylamino)-4¢-nitroazobenzene. Inorder to enhance the optical properties of double cyclopalladatedazobenzenes we have prepared and studied their complexeswith bipyridine as an ancillary ligand since d8 transition metalcomplexes with bipyridyl ligands exhibit strong luminescence inthe solid-state and/or in the solutions.8
Herein, we report on the synthesis and structural, dynamicaland photophysical studies of a series of azobenzene/bipyridinecomplexes as well as on the rationalization of their structural andspectroscopic properties by quantum-chemical calculations. Wealso report on the metal–metal interactions in these systems.
Results and discussion
Synthesis and general characterization of complexes
Compounds and reactions are shown in Scheme 1. Initialcomplexes (DMF)PdCl(m-R1C6H3N NC6H3R2)PdCl(DMF) 1–4 were prepared as previously described.6a,c
Scheme 1 Synthesis of the azobenzene/bipyridine complexes and theatom numbering of azobenzenes and bpy used in the assignment of NMRresonances.
The neutral complexes 1a–4a were prepared in acetone byreaction of 1–4 and bpy in 1 : 2 molar ratio. The ionic complex 3bcan be obtained either directly from the reaction of 3 with excessof bpy in DMSO or MeOH (molar ratio of 3/bpy 1 : 4 and 1 : 2.5,respectively), or by a two-step process through neutral complex 3awith excess of bpy in DMSO (molar ratio of 3/bpy 1 : 3). Complex3b decomposes very fast by dissolving in coordinative (DMSO,DMF) and in non-coordinative (CH2Cl2, CHCl3) solvents to theneutral complex 3a and free bpy as detected by NMR spectroscopy(ESI, Fig. S1).† Addition of bpy to the DMSO solution leads backto crystallization of 3b. All attempts to obtain ionic complexesfrom 1, 2 and 4 by using the same procedures as described for 3bresulted in neutral complexes 1a, 2a and 4a. The results of chemicalanalysis support formulations of new complexes as shown by theformulas PdCl{(m-Cl)(m-R1C6H3N NC6H3R2)}Pd(bpy) (1a–4a)and [(bpy)PdCl(m-NH2C6H3N NC6H4)Pd(bpy)]Cl (3b).
NMR spectra
1H and 13C NMR data are given in the Experimental sectionand in the ESI, Tables S1 and S2.† 1H NMR spectra revealed
that the structures of complexes 1a–4a in solvents with strongcoordinating ability i.e. DMSO-d6 or DMF-d7 are not consistentwith observed solid state structures (see next section) in contrastto their structures in solvents with poor coordinating ability i.e.CD2Cl2 or CDCl3.
The proton signals of both ligands in the spectra of complexesrecorded in CD2Cl2 (or CDCl3) at ambient temperature are sharpand their integration is in good agreement with the number ofprotons (ESI, Fig. S1).† However, in DMSO-d6 and DMF-d7,the signals corresponding to the azobenzene ligand are very wellresolved while the most signals of bpy are rather broad and theirintegration is in disagreement with the number of protons, thusindicating that DMSO-d6 and DMF-d7 molecules exchange fast(on the NMR scale) with one of the bpy nitrogen atoms bound tothe Pd atom (Fig. 1 and ESI, Fig. S2).† Therefore, in DMSO-d6 andDMF-d7 solutions of 1a–4a, we assumed a coexistence of speciesin which bpy acts as a monodentate and as a bidentate ligand.This is additionally supported by variable-temperature 1H NMRexperiments in DMF-d7 (Fig. 1). The spectra recorded in DMF-d7
at -25 ◦C contain eight resolved signals of bpy, whose integrationis in good agreement with the number of protons. These signalsshow a similar shifts pattern as in spectra recorded in CD2Cl2 atambient temperature (Fig. 1).9 In contrast to CD2Cl2 (or CDCl3)solutions of those complexes, in DMF only at -25 ◦C the speciesin which bpy acts as a bidentate ligand are predominant since atthis temperature the exchange between solvent molecules and bpynitrogen is significantly reduced.
Furthermore, NMR results also suggested the cleavage of onlyone Pd–N bond of azobenzene precursors in the reactions withbpy since the 1H NMR spectra of 1a show eight signals ofazobenzene protons while spectra of its initial complex, withtwo equivalent phenyl rings, show only four different aromaticprotons (Fig. 1 and ESI, Fig. S2,† Scheme 1). This assumptionwas additionally confirmed by double set of signals observed in1H and 13C NMR spectra of complex 2a with 4-methylazobenzene,due to two isomers (alpha and beta) formed by cleaving either Pd–Na or Pd–Nb bond. Alpha and beta isomers occur in the ratio 3 : 2(ESI, Fig. S2, Tables S1 and S2).† The presence of alpha and betaisomers is in agreement with results reported for monocyclopal-ladated compounds with 4-methyl- or 4-methoxy-azobenzenes
Fig. 1 1H NMR spectra of complex 1a recorded in DMF-d7 at -25 ◦C, 0 ◦C and 25 ◦C.
whose 1H NMR spectra also contain signals of alpha and betaisomers.2e,5d
Crystal structures
Crystal structures of complexes 1a–4a and 3b are shown in Fig. 2–7, and the selected bonds and angles are presented in Table 1.
Fig. 2 Molecular structure of 1a. Displacement ellipsoids are shown at50% probability and hydrogen atoms are depicted as spheres of arbitraryradii.
Fig. 3 Molecular structure of 2a. Displacement ellipsoids are shown at50% probability and hydrogen atoms are depicted as spheres of arbitraryradii.
X-Ray structure analysis of complexes has definitively con-firmed breaking of only one Pd–N bond in initial complexes 1–4 and demonstrated for the first time that azobenzenes can actsimultaneously as monodentate C- and chelating C, N-donors.In all complexes, the Pd centers are connected by the bridgingazobenzene ligand, and in neutral complexes additionally by theCl-bridge. The majority of the values of bond angles and distancesrelated to the Pd environments are similar to those found in otherPd complexes containing azobenzene and bpy ligands and, as
Fig. 4 Molecular structure of 3a. Hydrogen bonds are depicted by dotted lines: N(5)–H ◊ ◊ ◊ O(1) 2.886, 1.999 A, 168◦, N(5)–H ◊ ◊ ◊ O(2) 2.859, 1.985 A,175◦. Displacement ellipsoids are shown at 50% probability and hydrogen atoms are depicted as spheres of arbitrary radii.
Fig. 5 Molecular structure of 4a. Displacement ellipsoids are shown at50% probability and hydrogen atoms are depicted as spheres of arbitraryradii.
expected, Pd–N and/or Pd–Cl bonds oriented trans to the Pd–C bond are significantly longer than values predicted from theircovalent radii due to a trans influence of C-donors (Table 1).6a,c,10,11
The most notable structural feature of neutral complexes 1a–4aare Pd(m-Cl)Pd units with the most unsymmetric Cl-bridges of thistype known, (Table 1). The Pd(1)–Cl(1) bridging bond is in 1a–4alonger by an average of 0.19 A than the other Pd(2)–Cl(1) bond.The Cl-bridge is included in the six-membered ring, Pd(1)–Cl(1)–Pd(2)–C(8)–C(7)–N(1), with a distorted screw boat conformation
in 1a–3a, whereas in 4a it is between the envelope and screw boatconformation (Fig. 2–5 and ESI, Table S4).†12
Depending on the crystallization conditions, the ionic complex3b crystallizes in two pseudopolymorphic forms 3b1 or 3b2 (Fig. 6and 7, respectively). The difference between these two structuresis in solvate molecules, DMSO and H2O molecules in 3b1, andbipyridine and H2O molecules in 3b2. Two Pd atoms in 3b arebridged only by the azobenzene ligand. Each Pd atom is chelatedalso by one bpy molecule.
The structures of 1a–4a, 3b1 and 3b2 are characterized byintramolecular metal-metal interactions with the Pd ◊ ◊ ◊ Pd dis-tances ranging from 2.9760(4) to 3.1942(9) A. The presenceof intramolecular p ◊ ◊ ◊ p interactions between two chelate ringsin 3b1 and 3b2 and, in the latter structure also between thetwo bipyridyl ligands, resulted in shorter Pd ◊ ◊ ◊ Pd distancesin these crystal structures than in 1a, 2a and 3a complexes(Tables 1 and ESI, Fig. S6).† However, in complex 4a, which iswithout intramolecular p ◊ ◊ ◊ p interactions, the Pd ◊ ◊ ◊ Pd distanceis significantly shorter (2.9760(3) A) than in all other complexes.The reduction of the Pd ◊ ◊ ◊ Pd distance in complex 4a could be aconsequence of the presence of the strongly polarized 4-nitro-4¢-(dimethylamino)azobenzene ligand.
In the crystal structures of 1a–4a there are only weak inter-molecular contacts, mostly C–H ◊ ◊ ◊ X hydrogen bonds and p ◊ ◊ ◊ pinteractions (ESI, Tables S5 and S6)† and in 3a there are stronghydrogen bonds of the N–H ◊ ◊ ◊ O type connecting the amino groupwith neighbouring DMSO molecules (Fig. 4). The structure ismade up of alternating layers of complex and solvent molecules(ESI, Fig. S5 and S6).†
Pseudopolymorphs 3b1 and 3b2 are also layered structures, withchloride ions, H2O molecules and DMSO (bipyridine) moleculesin the solvent layer. The chloride ions and water molecules inthe layer are connected by hydrogen bonds (ESI, Fig. S5).† Thecomplex cation layer is dominated by p ◊ ◊ ◊ p interactions and it is
Fig. 6 Molecular structure of 3b1. Displacement ellipsoids are shown at 50% probability and hydrogen atoms are depicted as spheres of arbitrary radii.
a The Pd ◊ ◊ ◊ Pd separation distances in all complexes.
Fig. 7 Molecular structure of 3b2. Displacement ellipsoids are shown at 30% probability and hydrogen atoms are depicted as spheres of arbitrary radiiwithout solvent molecules.
linked to the solvent layer by hydrogen bonds through the aminogroups and by other weak hydrogen bonds of the C–H ◊ ◊ ◊ X type(ESI, Table S6, Fig. S7 and S8).†
Quantum-chemical calculations
The reactions of the double cyclopalladated azobenzenes 1–4with bpy resulted in the cleavage of only one Pd–N bond in
initial complexes and formation of complexes 1a–4a and 3b. Sincetwo nitrogen atoms, denoted as Na and Nb, in unsymmetricalsubstituted azobenzenes are not equivalent, two isomers (alphaand beta) of complexes 2a–4a and 3b could be produced. However,the presence of alpha and beta isomers formed by cleaving eitherPd–Na or Pd–Nb bond, was detected by NMR spectroscopy onlyfor complex 2a. In order to rationalize the formation of the
Table 2 Calculated free energies (in kcal mol-1) of the beta isomers of2a–4a and 3b relative to their alpha isomers (DG) in the gas phase and inDMSO or acetone solution
preferred isomer in isolated complexes 3a, 4a and 3b, the moleculargeometries and energies were calculated for both isomers of allcomplexes.
Experimental and calculated bond distances and bond anglesaround the Pd atoms are in reasonable agreement (ESI, Table S7).†Quantum-chemical calculations confirmed that the experimentallypreferred isomers are also the most stable isomers in the gas phaseas well as in DMSO and acetone. Alpha isomers of complexes 3a,4a and 3b are more stable than their respective beta isomers by2–5 kcal mol-1 (Table 2). The free energy differences between alphaand beta isomers of complex 2a are well below 1 kcal mol-1 in thegas phase, DMSO and acetone (Table 2). This is in agreement withexperimental results since alpha/beta isomeric ratio is quite low,i.e. only 3 : 2. These results clearly show that the five-memberedring fused to phenyl ring with electron-releasing substituentof dicyclopalladated complexes 2–4 is thermodynamically morestable than the five-membered ring fused to phenyl ring with (orwithout) electron-acceptor substituent (Scheme 1).
Furthermore, quantum-chemical calculations also confirmedtrans influence.11 In all cases Pd–Cl and Pd–N bonds in transpositions to Pd–C bond were by 0.07–0.21 A longer than thecorresponding bonds in the cis position.
Absorption and emission spectra
The electronic absorption spectra of complexes 1a–4a, recorded atroom temperature in CHCl3, are summarized in the Experimental.The assignment of absorption bands (between 300 and 600 nm)was made on the basis of TD-DFT calculations of the singletexcited states that are shown in Fig. 8 and ESI, Fig. S10† alongwith the experimental spectra. The calculated excited states aredescribed in terms of electronic transitions between occupied and
unoccupied molecular orbitals. The electronic transitions with thelargest contribution to the corresponding excited states are givenin Table 3 and in the ESI (Table S17).†
According to calculations, the most intense transitions incomplexes 1a–4a are IL p* ← p transitions localized on theazobenzene ligands. These transitions are shifted to lower energiesin the following order ((1a, 2a) > 3a > 4a) when compared with thespectra of ligands (Table 3 and ESI, Table S17).† The calculatedwavelengths of p* ← p transitions localized on the azobenzeneligands in all complexes are slightly blue shifted relative to theexperimental values while the intensity of these transitions is wellreproduced.
The calculations show that the majority of the bands in elec-tronic absorption spectra of complexes 1a–4a have MML(L’)CT,XML(L’)CT and LL’CT character, where L = azobenzene, L’ =bpy, and X = terminal chlorine (Table 3 and ESI Table S17).† TheMML(L’)CT transitions involve transfer of the electronic chargefrom both Pd atoms to L or L’, while XML(L’)CT transitionsinvolve transfer of the electronic charge from X and Pd atoms alsoto L or L’. These two transitions dominate in the region between300 and 520 nm as well as the transition involving transfer of theelectronic charge from azobenzene to the bipyridine ligand (long-range LL’CT transition). In contrast to the LL’CT transitions,the L’LCT transitions are shifted to higher energies (Table 3 andESI Table S17).† The absorption bands corresponding to p* ←p transitions localized on the bpy ligand are not detected byTD-DFT calculations in the region between 300 and 600 nm.According to the literature13 these transitions could be foundbetween 250 and 300 nm in complexes with bpy ligand.
Only complexes 3a and 4a exhibit fluorescence in the CHCl3
solution at ambient-temperature with lmax at 590 and 645 nmupon excitation at 510 and 590 nm, respectively. Thus, the 4,4¢-substituents (electron-releasing in 3a and 4a, and additionallyelectron-withdrawing in 4a) and an extended conjugation ofazobenzenes play the important role in fluorescence of 3a and4a and their initial complexes 3 and 4. In contrast to complex 3a,and initial complexes 3 and 4, that show weak fluorescence, thepresence of bpy in complex 4a significantly increases the intensityof fluorescence in the solution at ambient-temperatures (ESI,Fig. S9).† According to the literature1f,14 fluorescence observedfor Pd species is generated by IL p* → p, MLCT or MMLCTtransitions. Since the crystal structure of 4a revealed much stronger
Fig. 8 UV-vis spectrum of 3a and 4a (recorded in CHCl3). Bold dots on the abscissa denote calculated transitions from the ground to excited singletstates; heights of vertical lines correspond to oscillator strengths.
Table 3 Wavelengths (l/nm), oscillatory strengths (f ) and character of electronic transitions from ground to the lowest ten excited states of 1a–4a,calculated by TD-DFT in CHCl3.a
f 0.06 0.06 0.05 0.02 0.01XML(L’)CT, LL’CT, MML’CT,LMMCT
LL’CT, MML’CT XML’CT, LL’CT,LMMCT
LL’CT, XML(L’)CT MML’CT
a MML(L’)CT (metal-metal to ligand charge transfer, XML(L’)CT (chlorine-metal to ligand charge transfer, IL (intra-ligand p* ← p), LMMCT (ligandto metal-metal charge transfer), LL’CT or L’LCT (ligand to ligand charge transfer). L = azobenzene, L’ = bpy, and X = terminal chlorine.
Pd–Pd interactions compared to 3a, the contribution of MMLCTtransitions to the fluorescence of 4a seems to be the most probableexplanation of its stronger fluorescence intensity. Furthermore, thecomplexes 3a, 3b, 3b1 and 4a as well as their initial complexes 3 and4 also exhibit fluorescence in solid-state at ambient-temperature(Table 4 and ESI, Fig. S12).† Complexes 4a and 3b1 display moreintense solid-state emission than the complexes 3a, 3b, 3 and 4.Much stronger fluorescence intensity of 3b1 than 3b could beattributed to differences in their crystal packing.2c
Experimental
General measurements
All chemicals were obtained commercially and used as supplied.The reactions were done under normal atmosphere. Complexes1–4 were prepared as described previously.6a,c Elemental analyseswere carried out with a Perkin-Elmer Series II 2400 CHNS/Oanalyzer.
The one- and two-dimensional 1H and 13C NMR spectra wererecorded at 25 ◦C in DMSO-d6 and/or in CD2Cl2 and CDCl3 withBruker AV-600 spectrometer operating at 600.13 MHz for the1H and 150.90 MHz for the 13C resonances. 1H and 1H–1H COSY
Table 4 Fluorescence data for compounds 3a, 3b, 3b1 and 4a
spectra were recorded at 25, 0 and -25 ◦C in DMF-d7 with the sameinstruments. The signal assignment was based on the chemicalshifts and spin–spin couplings, two-dimensional experiments andquantum chemical calculations of the chemical shifts. The 1H–1H COSY spectra were obtained in the magnitude mode with2048 points in the F2 dimension and 512 increments in the F1
dimension. The 1H–13C HMQC spectra were measured with one-bond C, H coupling value set to 145 Hz, using 2048 points in theF2 dimension and 256 increments in the F1 dimension. The 1H-13C HMBC spectra were measured with C, H coupling value setto 145 and 8 Hz using 2048 points in the F2 dimension and 256increments in the F1 dimension. All two-dimensional experimentswere performed by standard pulse sequences using Bruker XWIN-NMR software Version 3.5. Waltz 16 modulation was used forproton decoupling. Electronic absorption spectra were recordedon a Hewlett-Packard 8425 spectrophotometer with thermostatedcell compartment at 25 ◦C. Fluorescence spectra were taken ona Varian Eclipse spectrophotometer in the CHCl3 solution at25 ◦C. Fluorescence in solid-state was detected at 25 ◦C by confocalmicroscopy LEICA/TCS/SP2 using nujol mull of samples withappropriate excitation wavelengths.
Synthesis of PdCl{(l-Cl)(l-C6H3R1N NC6H4)}Pd(bpy), {R1 =H (1a) or NH2 (3a)}. To a suspension of 1 or 3 (80 mg, 0.13 mmol)in acetone (10 mL) bpy (41 mg, 0.26 mmol) was added and thereaction mixture was stirred for 4 h. The orange (1a) or red (3a)crystals were filtered off and dried under vacuum. (1a: 64.1 mg,64%; 3a: 58.3 mg, 58%). 1a found: C 42.52, H 2.64, N 8.98. Calcdfor C22H16N4Pd2Cl2: C 42.61, H 2.60, N 9.03; 3a found: C 42.08,H 3.16, N 10.58. Calcd for C22H17N5Pd2Cl2: C 41.60, H 2.70, N11.03.
1a: 1H NMR (DMF-d7) at -25 ◦C (d (ppm), J/Hz): 8.02 d (H-3, 3J(HH) = 7.7 Hz), 7.36 t (H-4, 3J(HH) = 7.6 Hz), 7.27 t (H-5,3J(HH) = 7.5 Hz), 7.86 d (H-6,9, 3J(HH) = 7.8 Hz), 7.43 t (H-10,3J(HH) = 7.5 Hz), 7.34 t (H-11, 3J(HH) = 7.6 Hz), 7.57 d (H-12,3J(HH) = 7.8 Hz), 8.82, 8.80 d (H-b,b’, 3J(HH) = 8.0 Hz), 8.40,8.46 t (H-c,c’, 3J(HH) = 7.9, 7.7 Hz), 7.74, 8.80 t (H-d,d’, 3J(HH) =6.5, 6.5 Hz), 7.95, 9.12 d (H-e,e’, 3J(HH) = 5.9, 5.7 Hz). UV-vis(CHCl3) lmax/nm (e/104 M-1cm-1): 306 sh (1.82), 315 (1.98), 340(1.27), 378 (1.30), 469 (0.17), 498 (0.15).
3a: 1H NMR (DMF-d7) at -25 ◦C (d (ppm), J/Hz): 7.10 s (H-3), 6.51 d (H-5, 3J(HH) = 8.7 Hz), 7.63 d (H-6, 3J(HH) = 8.7 Hz),7.67 d (H-9, 3J(HH) = 9.1 Hz), 7.20 m (H-10,11), 7.41 d (H-12,3J(HH) = 9.1 Hz), 7.18 s (-NH2), 8.79, 8.75 d (H-b,b’, 3J(HH) =8.0, 8.2 Hz), 8.22, 8.33 t, (H-c,c’, 3J(HH) = 7.9, 8.0 Hz), H-doverlapped with H-6, 7.87 m (H-e,d’), 9.08 d-unresolved (H-e’).UV-vis (CHCl3) lmax/nm (e/104 M-1cm-1): 304 (1.88), 315 (1.72),335 (0.92), 402 (1.12), 505 (1.20).
Synthesis of PdCl{(l-Cl)(l-C6H3R1N NC6H3R2)}Pd(bpy),{R1 = CH3; R2 = H (2a) or R1 = N(CH3)2, R2 = NO2 (4a)}. Amixture of 2 (80 mg, 0.13 mmol) and bpy (41 mg, 0.26 mmol) or 4(80 mg, 0.11 mmol) and bpy (34 mg, 0.22 mmol) in acetone (10 mL)was heated under reflux for 4 h. After cooling, the orange (2a) ordark violet (4a) crystals were filtered off and dried under vacuum.(2a: 63.2 mg, 63%; 4a: 61.0 mg, 52%). 2a found: C 44.02, H 2.94,N 8.44. Calcd. for C23H18N4Pd2Cl2: C 43.56, H 2.86, N 8.83. 4afound: C 40.28, H 3.12, N 11.58. Calcd. for C24H20N6O2Pd2Cl2: C40.70, H 2.85, N 11.87.
3J(HH) = 8.0 Hz),H-12b overlapped with H-d’,e, 2.33, 2.43 s (-CH3 a,b), 8.82 m (H-b,b’a,b), 8,42, 8.50 m (H-c,c’a,b), 7.75 m (H-d a,b), 7.99 m (H-d’a,b
and H-ea,b), 9.13, 9.12 d (H-e’a,b, 3J(HH) = 6.0, 6.0 Hz). UV-vis(CHCl3) lmax/nm (e/104 M-1cm-1): 306 sh (1.97), 316 (2.08), 351(1.41), 382 (1.55), 462 (0.29), 492 (0.26).
4a: 1H NMR (DMF-d7) at -25 ◦C (d (ppm), J/Hz) 7.16 s (H-3),6.71 d (H-5, 3J(HH) = 9.1 Hz), 7.76 d (H-6, 3J(HH) = 9.0 Hz), 8.57 s(H-9), 8.15 d (H-11, 3J(HH) = 8.9 Hz), 7.63 d (H-12, 3J(HH) =9.0 Hz), 3.21 s (-CH3)2. 8.83 d (H-b,b’, 3J(HH) = 8.0 Hz), 8.40,8.48 s, br, (H-c,c’), 7.67 s, br (H-d), 7.99 s, br (H-e,d’), 9.15 s, br(H-e’). UV-vis (CHCl3) lmax/nm (e/104 M-1cm-1): 304 (1.01), 312(0.91), 328 (0.48), 425 (0.33), 574 (1.49).
Complexes 1a, 2a and 4a were also prepared by the reactionof complexes 1, 2 and 4 with an excess of bpy in methanolor DMSO. The methanol solutions were refluxed for 4 h. Aftercooling, the crystals of 1a, 2a and 4a were filtered off and driedunder vacuum. DMSO reaction solutions were allowed to stand atambient temperature until the crystals of 1a, 2a and 4a appeared.The yields of complexes 1a, 2a and 4a prepared by both methodswere similar to those obtained from acetone.
Synthesis of [(bpy)PdCl(l-NH2C6H3N NC6H4)Pd(bpy)]Clx-0.5H2O, (3b). A mixture of 3 (50 mg, 0.08 mmol) and bpy (30 mg,0.20 mmol) in methanol (10 mL) was heated under reflux for4 h. The resulting solution was concentrated to 5 mL and aftercooling, the red crystals were filtered off and dried under vacuum.(24.8 mg, 33%). (Found: C 48.10, H 3.59, N 11.91. Calcd forC32H26N7O0.5Pd2Cl2: C 48.02, H 3.27, N 12.25).
Synthesis of [(bpy)PdCl(l-NH2C6H3N NC6H4)Pd(bpy)]Clx2-(DMSO)xH2O, (3b1). Method a. To a solution of bpy (60 mg,0.4 mmol) in DMSO (8 mL) 3 was added (60 mg, 0.10 mmol). Thedark-blue solution changed immediately into a blood-red color.It was left at ambient temperature until the red crystals of 3b1appeared (36 mg of 40%). Method b. To a solution of bpy (30 mg,0.20 mmol) in DMSO (6 mL) 3a was added (40 mg, 0.06 mmol).The resulting solution was left at ambient temperature until thered crystals of 3b1 appeared. (39.6 mg, 44%). (Found: C 44.51, H3.99, N 10.35. Calcd. for C36H39N7O3S2Pd2Cl2: C 44.78, H 4.07, N10.15).
Another crystal form of 3b was obtained from a DMSO solutionof 3 with a large excess of bpy (molar ratio of 3/bpy 1 : 6) byslow evaporation, almost to dryness. The red crystals with formu-lation [(bpy)PdCl(m-NH2C6H3N NC6H4)Pd(bpy)]Clx0.5(2,2¢-bpy)x2.5H2O, (3b2) were formed along with single crystals of bpyand 3b1.
X-Ray structure analysis
Single crystals of 1a–4a, 3b1 and 3b2 suitable for an X-raydiffraction study were obtained from DMSO solution. The crystalsof 2a, 4a, 3b1 and 3b2 were measured on an Oxford DiffractionXcalibur Nova R diffractometer (microfocus Cu tube, CCDdetector), 3a was measured on an Oxford Diffraction Xcalibur(Mo tube, CCD detector) and 1a was measured on an Enraf-Nonius CAD-4 (conventional Cu tube, point detector; threestandard reflection measured every 120 min as intensity control).Program packages CrysAlis PRO15 and XCAD-416 were usedfor data reduction. The structures were solved using SHELXS-9717and refined with SHELXL-9717 The models were refined usingthe full-matrix least squares refinement; all non-hydrogen atomswere refined anisotropically. Hydrogen atoms were treated as
constrained entities, using the command AFIX in SHELXL-97.17
Molecular geometry calculations were performed by PLATON18
and the molecular graphics were prepared using ORTEP-319 andCCDC-Mercury.20 Crystallographic and refinement data for thestructures reported in this paper are shown in the ESI, Table S3.†
Computational methods
Electronic structure was calculated by DFT method, with theB3LYP functional21 and 6-31G(d,p) basis set on nonmetal atoms.For the Pd atoms Stuttgart-Dresden pseudopotential and theaccompanying basis set (SDD) were used.22 Calculations wereperformed with the program package Gaussian 0323 and Jaguar.24
The latter package was used only for estimation of solvationenergy by the Poisson–Boltzman model.25 Dielectric constantsand solvent radii were for DMSO: e = 46.7, r = 2.455 A andfor acetone: e = 20.7, r = 2.063 A. Molecular geometries wereoptimized with tight convergence criteria in the gas phase only.Harmonic frequency calculations were performed to confirmstationary points as minima on the potential energy surfaces andto estimate vibrational contributions to the free energy. Verticallyexcited states were calculated by the time dependent (TD) DFTmethod (B3LYP) with the 6-311+G(d,p) basis on nonmetal atomsand with the SDD basis on Pd extended by the additional s, p,d and f functions.26 NMR shifts were calculated by the GIAOmethod with the basis set including diffuse functions on thehydrogen atoms (6-311++G(d,p)). The solvation effect on theelectronic structure of the excited states and on the NMR shiftswas accounted by the PCM model with UA0 atomic radii.23
Conclusion
We have synthesized and fully characterized the first examplesof azobenzene/bipyridine palladacycles in which azobenzenesact simultaneously as monodentate C- and bidentate C, N-donors. The cleavage of one Pd–N bond in double cyclopalladatedazobenzenes in reactions with bpy enables positioning of both Pdatoms onto the same side of azobenzene ligand and significantPd ◊ ◊ ◊ Pd interactions. These interactions have important rolein luminescence of azobenzene/bipyridine palladacycles in thesolution and in the solid state at the ambient-temperature. As mostPd(II) complexes exhibit luminescence only at low temperatures,those complexes represent rare examples of Pd(II) emitters in thesolution and in the solid state at ambient temperatures. For thisreason they could have application in design of new biosensors.2c,27
Acknowledgements
The Ministry of Science, Education and Sports of The Republic ofCroatia (grant no. 098-0982915-2950 and 098-1191344-2943 and119-1193079-1084) provided financial support for this research.Computations were done on the Isabella cluster at SRCE and onthe CRO-NGI grid, Zagreb. Discussions with Dr Mladen Andreisand Dr Ivan Ljubic are gratefully acknowledged. The crystaland molecular structure of 3a was determined during the ZurichCrystallography School 2008. The help of Dr Markus Neuburgerand all other tutors is acknowledged with gratitude.
Notes and references1 (a) J. Dupont, C. S. Consorti and J. Spencer, Chem. Rev., 2005, 105,
2527–2572; (b) Q.-D. Liu, W.-L. Jia and S. Wang, Inorg. Chem., 2005,44, 1332–1343; (c) T.-H. Kwon, H. S. Cho, M. K. Kim, J.-W. Kim, J.-J.Kim, K.-H. Lee, S. J. Park, I.-K. Shin, H. Kim, D. M. Shin, Y. K. Chungand J.-I. Hong, Organometallics, 2005, 24, 1578–1585; (d) M. Ghedini,I. Aiello, A. Crispini and M. La Deda, Dalton Trans., 2004, 1386–1392;(e) I. Aiello, D. Dattilo, M. Ghedini and A. Golemme, J. Am. Chem.Soc., 2001, 123, 5598–5599; (f) Q. Wu, A. Hook and S. Wang, Angew.Chem., Int. Ed., 2000, 39, 3933–3935; (g) A. El-ghayoury, L. Douce,A. Skoulios and R. Ziessel, Angew. Chem., Int. Ed., 1998, 37, 1255–1258.
2 (a) T. Pugliese, N. Godbert, I. Aiello, M. La Deda, M. Ghedini, M.Amati, S. Belviso and F. Lelj, Dalton Trans., 2008, 6563–6572; (b) S.-H. Li, C.-W. Yu and J.-G. Xu, Chem. Commun., 2005, 450–452; (c) M.Ghedini, I. Aiello, M. La Deda and A. Grisolia, Chem. Commun., 2003,2198–2199; (d) F. Neve, A. Crispini, C. Di Petrio and S. Campagna,Organometallics, 2002, 21, 3511–3518; (e) Y. Wakatsuki, H. Yamazaki,P. A. Grutsch, M. Santhanam and C. Kutal, J. Am. Chem. Soc., 1985,107, 8153–8159.
3 C. Najera and D. A. Alonso, Application of cyclopalladated compoundsas catalysts for Heck and Sonogashira reactions, pp. 155–207; R. B.Bedford, Palladacyclic pre-catalysts for Suzuki coupling, Buchwald–Hartwig amination and related reactions, pp. 209–225; B. Donnio andD. W. Bruce, Liquid crystalline ortho-palladated complexes, pp. 239–283, in Palladacycles: Synthesis, Characterization and Applications, ed.J. Dupont and M. Pfeffer, Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim, 2008.
4 A. C. Cope and R. W. Siekman, J. Am. Chem. Soc., 1965, 87, 3272–3273.
5 (a) M. Ghedini, I. Aiello, A. Crispini, A. Golemme, M. La Dedaand D. Pucci, Coord. Chem. Rev., 2006, 250, 1373–1390; (b) I. Omae,Coord. Chem. Rev., 2004, 248, 995–1023; (c) M. Curic, D. Babic,Z. Marinic, Lj. Pasa-Tolic, V. Butkovic, J. Plavec and Lj. Tusek-Bozic, J. Organomet. Chem., 2003, 687, 85–99; (d) M. Ghedini, D.Pucci, A. Crispini, G. Barberio, I. Aiello, F. Barigelletti, A. Gessiand O. Francescangeli, Appl. Organomet. Chem., 1999, 13, 565–581;(e) C. Sinha, D. Bandyopadhyay and A. Chakravorty, Inorg. Chem.,1988, 27, 1173–1178; (f) A. K. Mahapatra, D. Bandyopadhyay, P.Bandyopadhyay and A. Chakravorty, Inorg. Chem., 1986, 25, 2214–2221; (g) A. D. Ryabov, Synthesis, 1985, 233–252; (h) I. Omae, Chem.Rev., 1979, 79, 287–321; (i) M. I. Bruce, B. Goodall and G. A. Stone,J. Chem. Soc., Dalton Trans., 1978, 687–694; (j) M. I. Bruce, Angew.Chem., Int. Ed. Engl., 1977, 16, 73–86; (k) J. Dehand and M. Pfeffer,Coord. Chem. Rev., 1976, 18, 327–352.
6 (a) D. Babic, M. Curic, K. Molcanov, G. Ilc and J. Plavec,, Inorg.Chem., 2008, 47, 10446–10454; (b) K. Molcanov, M. Curic, D. Babicand B. Kojic-Prodic, J. Organomet. Chem., 2007, 692, 3874–3881; (c) M.Curic, D. Babic, A. Visnjevac and K. Molcanov, Inorg. Chem., 2005,44, 5975–5977; (d) A. R. Siedle, J. Organomet. Chem., 1981, 208, 115–123.
7 P. Jolliet, M. Gianini, A. von Zalewsky, G. Bernardinelli and H.Stoeckli-Evans, Inorg. Chem., 1996, 35, 4883–4888.
8 (a) S. Campagna, F. Puntoriero, F. Nastasi, G. Bergamini and V.Balzani, Top. Curr. Chem., 2007, 280, 117–214; (b) J. A. G. Williams,Top. Curr. Chem., 2007, 281, 205–268 and ref. therein.
9 The spectra recorded either in CD2Cl2 (or CDCl3) at -25◦ and 25 ◦C orin DMF at -25 ◦C are characterized by downfield shift of H-e’ signaland upfield shift of its analogous H-e relative to the free ligand signals.Upfield shift of H-e signal could be assigned to the shielding effect ofNa-bonded phenyl ring on this proton since in all neutral complexes itis located in proximity of Na-bonded phenyl ring.
10 (a) S. Roy, I. Hartenbach and B. Sarkar, Eur. J. Inorg. Chem., 2009,2553–2558; (b) A. Zucca, G. L. Petretto, S. Stoccoro, M. A. Cinellu, M.Manassero, C. Manassero and G. Minghetti, Organometallics, 2009,28, 2150–2159; (c) R. A. Adrian, S. Zhu, K. K. Klausmeyer and J. A.Walmsley, Inorg. Chem. Commun., 2007, 10, 1527–1530; (d) S.-Y. Yu,M. Fujita and K. Yamaguchi, J. Chem. Soc., Dalton Trans., 2001, 3415–3416.
11 (a) J. Vicente, A. Areas, D. Bautista and P. G. Jones, Organometallics,1997, 16, 2127–2138; (b) J. Vicente, M. T. Chichote, M. C. Lagunas,P. G. Jones and E. Bembenek, Organometallics, 1994, 13, 1243–1250;(c) S. Ruttmann, A. F. Williams and G. Bernardinelli, Angew. Chem.,Int. Ed. Engl., 1993, 32, 392–394.
12 (a) D. Cremer and J. A. Pople, J. Am. Chem. Soc., 1975, 97, 1354–1358; (b) D. Cremer, Acta Crystallogr., Sect. B: Struct. Sci., 1984, 40,498–500.
13 T. Pugliese, N. Godbert, I. Aiello, M. Ghedini and M. La Deda, Inorg.Chem. Commun., 2006, 9, 93–95.
14 (a) S. W. Lai, T. C. Cheung, M. C. W. Chan, K. K. Cheung, S. M. Pengand C. M. Che, Inorg. Chem., 2000, 39, 255–262; (b) K. A. Van Houten,D. C. Heath, C. A. Barringer, A. L. Rheingold and R. S. Pilato, Inorg.Chem., 1998, 37, 4647–4653; (c) F. Neve, A. Crispini and S. Campagna,,Inorg. Chem., 1997, 36, 6150–6156.
15 CrysAlis PRO, Oxford Diffraction Ltd., Oxford, U. K., 2007.16 K. S. Harms, Wocadlo, XCAD-4, Program for Processing CAD4
Diffractometer Data, University of Marburg, Germany, 1995.17 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112–122.18 A. L. Spek, J. Appl. Crystallogr., 2003, 36, 7–13.19 L. J. Farrugia, J. Appl. Crystallogr., 1997, 565–565.20 P. McCabe, E. Pidcock, G. P. Shields, R. Taylor, M. Towler, C. F.
Macrae, P. R. Edgington and J. Van de Streek, J. Appl. Crystallogr.,2006, 39, 453–457.
21 P. J. Stephens, F. J. Devlin, C. F. Chabalowski and M. J. Frisch, J. Phys.Chem., 1994, 98, 11623–11627.
22 D. Andrae, U. Haussermann, M. Dolg, H. Stoll and H. Preuss, Theor.Chim. Acta, 1990, 77, 123–141.
23 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,J. R. Cheeseman, J. A. Montgomery, Jr., T. Vreven, K. N. Kudin, J. C.
Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M.Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T.Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E.Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,C. Pomelli, J. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D.Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K.Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S.Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. G. Johnson,W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN03 (Revisions D.02 and E.01), Gaussian, Inc., Wallingford, CT,2004.
24 Jaguar, version 6.5, Schrodinger, LLC, New York, NY,2005.
25 D. J. Tannor, B. Marten, R. Murphy, R. A. Friesner, D. Sitkoff, A.Nicholls, M. Ringnalda, W. A. III Goddard, B. Honig, J. Am. Chem.Soc., 1994, 116, 11875-11882.
26 M. J. Zeizinger, V. Burda, J. Sponer, V. Kapsa and J. Leszczynski, J. Phys.Chem. A, 2001, 105, 8086–8092.
27 A. Diez, J. Fornies, S. Fuertes, E. Lalinde, C. Larraz, J. A. Lopez,A. Martin, M. T. Moreno and V. Sicilia, Organometallics, 2009, 28,1705–1718.
