Oxidative Coupling of Imino, Amide Platinum(II) Complexes Yields

Article pubs.acs.org/Organometallics

Oxidative Coupling of Imino, Amide Platinum(II) Complexes Yields Highly Conjugated Blue Dimers Roberto Esposito,†,‡ Luisa Calvanese,§ Maria Elena Cucciolito,†,‡ Gabriella D’Auria,∥,⊥ Lucia Falcigno,∥,⊥ Valentina Fiorini,# Prisco Pezzella,† Giuseppina Roviello,¶ Stefano Stagni,# Giovanni Talarico,† and Francesco Ruffo*,†,‡ †

Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Complesso Universitario di Monte S. Angelo, Via Cintia 21, 80126 Napoli, Italy ‡ Consorzio Interuniversitario di Reattività Chimica e Catalisi (CIRCC), Via Celso Ulpiani 27, 70126 Bari, Italy § CIRPeB, and ∥Dipartimento di Farmacia, Università di Napoli Federico II, Via Mezzocannone 16, 80134 Napoli, Italy ⊥ Istituto di Biostrutture e Bioimmagini (IBB) - CNR, Via Mezzocannone 16, 80134 Napoli, Italy # Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale del Risorgimento 4, 40126 Bologna, Italy ¶ Dipartimento di Ingegneria, Università di Napoli Parthenope, Centro Direzionale Isola C4, 80143 Napoli, Italy S Supporting Information *

ABSTRACT: Platinum(II) olefin compounds [PtMe(N,N′-imino, amide)(η2-olefin)] have been found to dimerize spontaneously in solution, yielding dinuclear complexes through an oxidative coupling. The extended conjugation within the molecules produces an intense blue color, and the properties of the compounds have been investigated through NMR spectroscopy, optical measurements, and theoretical calculations. The solid-state structure of a representative compound has been solved.

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In particular, Pt(II) complexes containing an alkyl and an alkene group (type 1 complexes, see Scheme 1) are stable2 and display migratory insertions3 only in the presence of electronpoor alkenes3a or under severe conditions.3b Conversely, the presence of an aryl moiety (type 2 complexes, see Scheme 1) enhances the reactivity of these species4 by favoring fast rearrangements that lead to the β-phenethyl derivative 2′.4b In this regard, the reactivity that is displayed by Pt(II) complexes bearing an alkyl and an alkyne features (type 3) appears as being essentially ruled by the nature of the coordinated alkyne.5 For instance, terminal alkynes are involved in a remarkable intramolecular reaction that results in the formation of the π-allyl derivative 3′,5j while the introduction of an aryl group in place of an alkyl moiety (type 4 complexes, Scheme 1)6 led to unprecedented formation of an indene.6c Taking advantage of the great stability exhibited by complexes of type 1, which can be considered as models of the active intermediates in important catalytic processes (i.e., olefin polymerization),7 studies dealing with the elucidation of their structure and the occurrence of dynamic processes were

ver the past decades, cationic platinum(II) complexes containing one hydrocarbyl and one unsaturated ligand in mutual cis arrangement (complexes 1−4 in Scheme 1) have attracted increasing attention, as their stability and reactivity are heavily influenced by the nature of these two ligands.1

Scheme 1. Formula and Reactivity of Pt(II) Complexes

Received: October 18, 2016

© XXXX American Chemical Society

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DOI: 10.1021/acs.organomet.6b00798 Organometallics XXXX, XXX, XXX−XXX

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Organometallics performed.2d In particular, ligands derived from N,N-bis(aryl)butane-2,3-diimine (Figure 1) can tune or modulate the steric

calculations. The X-ray crystal structure of the ethylene complex (R = H) has also been determined.

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RESULTS AND DISCUSSION Synthesis and NMR Characterization of the Complexes. The formation of type 6 complexes was accomplished by following the synthetic pathway depicted in Scheme 3, in Scheme 3. Synthesis of the Complexes of Type 6 Figure 1. General formula of complexes of type 1 with N,Nbis(aryl)butane-2,3-diimine ligands.

hindrance above and below the coordination plane, a parameter that plays a fundamental role in determining the extent to which dynamic processes involving the alkene moiety take place. When compared to the NMR time scale, exchange and rotation processes occur faster when R′ = H, while the presence of more sterically demanding substituents (R′ = Me, Et, or i-Pr) inhibits the same processes even at high temperatures.2d By taking advantage of this behavior, we decided to study the stereochemistry and the mechanism of nucleophilic additions on the coordinated alkenes. The results that were obtained were quite unexpected.8 In fact, the reactions of type 1 complexes with OH− did not involve any nucleophilic addition to the alkene, but resulted in the deprotonation of the bidentate ligand, leading to the formation of neutral complexes (5a−e) containing imino-amide ligands (Scheme 2). Scheme 2. Addition of OH− Ions to Complexes of Type 1

which are reported also the alkenes used within this work. The bidentate ligand dameph (R′ = Me, Figure 1) was selected according to its ability to control the dynamic processes of the alkene. However, the same moderate steric motif did not prevent the relatively fast evolution of type 5 complexes, which was found to be less selective when R′ = Et or i-Pr. Electronrich alkenes (ethylene, propylene, and styrenes) were chosen due to their well-known stabilizing ability of the corresponding square-planar Pt(II) complexes. Upon addition of one equivalent of KOH to a methanolic suspension of 1a−e (step i in Scheme 3), the formation of products 5a−e was complete within a few seconds. The NMR features of the crude yellowish products were in agreement with those already discussed for related compounds.8 The presence of a single pattern of resonances in the case of α-olefins was attributed to the presence of the isomer in which the olefin substituent R is oriented toward the Pt−Me vector. As discussed thoroughly elsewhere,2d,8 this relative orientation is due to steric factors. Worth mentioning is the presence of two singlets at ca. 4− 4.5 ppm, whose occurrence is ascribable to the nonequivalent CH2 nuclei. The olefin protons appear with the expected multiplicities at low frequency with respect to the free alkenes (2JPt−H = 60−85 Hz), while the methyl located on the Pt metal center was found at low frequencies (below 0 ppm). However, the NMR spectra of freshly prepared solutions of complexes 5a−e revealed the presence of an evolution product. In order to investigate this phenomena, it was found more convenient to age the reaction mixture that resulted from step i, avoiding the isolation of the neutral species 5a−e. Within a couple of days the color of the precipitates turned from yellow to green and eventually deep blue (step ii). The blue

While the identity of type 5 products was unambiguously confirmed in a previous investigation,8 still unknown are their stability and behavior in solution. We aim in this work to extend our studies on the reactivity of type 5 species, and, in particular, we report on their spontaneous dimerization, which yields highly conjugated dinuclear blue complexes 6a−e (Figure 2, R′ = Me) through a rare oxidative coupling. The products 6a−e have been thoroughly characterized through 2D NMR spectra, optical measurements, and DFT

Figure 2. Structure of complexes of type 6. B


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Organometallics microcrystalline products were then isolated and washed with methanol. When the same treatment was accomplished with R′ = Et or i-Pr, type 6 products were obtained only in the case of the ethylene derivative. The reactions carried out with all the other olefins gave rise to a complicated mixture of products, due to the overall steric hindrance that makes the transformation poorly selective. Although their binuclear nature was unambiguously ascertained by the analysis of the crystal structure of complex 6a (see below), the NMR characterization gave equally clear indications about the outcome of this unexpected product (e.g., Figure 3 shows the spectrum of the styrene complex 6c).

Scheme 4. Plausible Intermediate for the Conversion of 5 to 6

significantly inhibits the reaction. Further studies will be complete to address a deeper understanding of the mechanism. In the case of α-olefins, the observation of a unique isomer displaying olefin 2J scalar couplings to 195Pt reflects the prevalence of the rotamer with the olefin substituent toward the Pt−Me vector. The geometry was confirmed also by the relative NOE intensity between the alkene RCHCH2 proton and one methyl group of the ligand with respect to the NOE intensity detected for the −CH3 directly linked on the Pt metal center (see Figure S6). On the other hand, the ethylene resonances in complex 6a are broad, and the methyl groups of both aryl ligands become magnetically equivalent (signals at 1.87 and 2.45 ppm, respectively), revealing a fast propeller-like rotation, as already observed for unhindered alkene.2d Variation of the electronic properties as well as steric hindrance of the olefin ligands produced some differences among the corresponding NMR spectra. The Pt−CH 3 resonance is the most sensitive toward variation of the alkene nature, as in all the reported complexes it was found below 0 ppm, as a direct consequence of the diamagnetic shielding of the aryl group of the bidentate ligand. An increasing upfield shift to −0.6 ppm was observed with the styrene moiety, due to an additional shielding prompted by the aryl substituent on the alkene. The imino methyl group, sandwiched between the π electronic clouds of two aromatic rings, exhibited a signal significantly shifted. The overall lack of symmetry of the metal environment is confirmed by the chemical shift values of the four methyl groups linked to the aromatic rings. In the series, the two methyl groups of the aromatic ring cis-oriented to the alkene displayed δ values lower than those measured for the methyl groups of the trans ones. Moving through the series, from higher to lower steric hindrance of the olefin ligand, the asymmetry of the metal environment progressively relaxes. Crystal Structure of 6a. X-ray diffraction of a suitable single crystal of 6a confirmed the binuclear nature of the complex (Figure 4). The dimer crystallizes in a monoclinic system (P21/c space group) with a square-planar coordination around each Pt atom. The coordination plane is defined by the C atom of methyl group, the nitrogen atoms of the ligand, and the midpoint of the olefin bond that lies perpendicular to the plane. A slight distortion of the coordination geometry, attributable to the steric constraints of the nitrogen bidentate ligand, is observed (see N−Pt−N angles). Within the coordination sphere of Pt1 and Pt2, bond lengths and angles are substantially equivalent and fall in the ranges reported in the literature for typical olefin square-planar Pt(II) complexes. Also regarding the coordinated olefins, the values observed for the bond lengths are similar and comparable with those found in complexes of similar geometry. An extended conjugation within the molecule is evident from the trans planar disposition of the bonds connecting the two imido nitrogen atoms and from the bond distances observed.9a

Figure 3. 1H NMR spectrum of 6c (C6D6).

The complete assignments of the proton resonances were performed by 2D NOESY experiments together with NOE intensities for the bonding scheme (interproton distances) of the binuclear complexes. For instance, the disappearance of CH2 signals together with the advent of a singlet in the olefin region (around 6 ppm) was diagnostic for the effective makeup of the newly formed bridge, whose resonance was found at 110 ppm in the 13C NMR spectra. Several lines of evidence suggest that complexes 6a−e result from the oxidative coupling of the corresponding type 5 monomers with the formal loss of hydrogen. The presence of dark byproducts in the mother liquor in conjunction with moderate yields (around 50%) suggests that the reaction mechanism is much more complicated with respect to what is suggested by its stoichiometry. It is worth noting that the formation of similar −CH2−CH2− bridges has been reported9a for cobalt complexes, in which the mechanism seems to proceed through a radical intermediate. In the case of similar germanium complexes,9b this kind of oxidative coupling has been described, and the N,N′-ligand acts as a hydrogen-removing agent. Accordingly, we suggest that the dimerization reaction might be initiated by species with radical character on the enamide methylene position (Scheme 4). The subsequent darkening of the mother liquor and the moderate yields in complexes 6a−e could be explained by the simultaneous formation of other decomposition products with formal release of hydrogen assisted by the ligand.9b A preliminary experiment suggested the fundamental role of the radical scavenger 3-tert-butyl-4-hydroxyanisole, which C


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Organometallics

the former band can be attributed to π → π* intraligand (IL) transitions, while the latter onewhich is responsible for the yellow color displayed by complex 5ais assigned to the intervention of the Pt(5d) → π* MLCT (metal to ligand charge transfer) processes. In this regard, even though the MLCT features of complex 5a appear as a broad and structureless band, the analysis of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) contour plots that were obtained upon performing TD-DFT calculations (vide inf ra) suggests how both the Schiff base and the substituted olefin effectively contribute to the composition of MLCT transitions. On passing from the mononuclear species to dinuclear complexes 6a−e, a dramatic change in the absorption profiles has been detected (Figure 6 and Table 1).

Figure 4. Crystal structure of complex 6a. Thermal ellipsoids are shown at the 30% probability level. Selected bond distances and angles (Å, deg): Pt1−N1 = 2.110(4), Pt1−N2 = 1.996(4), Pt1−C1 = 2.041(5), C2−C3 = 1.381(8), N2−C8 = 1.362(6), Pt2−N3 = 1.994(4), Pt2−N4 = 2.105(4), Pt2−C16 = 2.042(5), C17−C18 = 1.393(8), N3−C11 = 1.385(6), N1−C7 = 1.286(6), C7−C8 = 1.474(6), C8−C9 = 1.378(7), C9−C10 = 1.413(7), C10−C11 = 1.363(7), C11−C12 = 1.454(7), C12−N4 = 1.297(6), N1−Pt1−N2 = 78.17(2), N3−Pt2−N4 = 78.13(2).

Table 1. Absorption Maxima for Complexes 6a−e complex, THF as solvent, rt 5a 6a 6b 6c 6d 6e

The phenyl rings lie orthogonal to each coordination plane of the metal.2d,3a Optical Measurements and DFT Calculations. The absorption spectra of the exemplar mononuclear complex 5a and the dinuclear species 6a−e were recorded from the corresponding dilute (10−5 M) THF solutions at room temperature (Figures 5 and 6).

λ (nm) [(10−4 ε) (M−1 cm−1)] 411 652 661 672 676 671

[0.61], [1.56], [2.16], [3.57], [1.73], [2.18],

319 601 605 613 621 615

[0.24] [0.67] [1.01] [1.52] [0.71] [0.93]

When compared to the exemplar mononuclear complex 5a, the absorption profiles of all the dinuclear complexes 6a−e display an extremely evident bathochromic shift (ca. 200 nm) of the MLCT features, enlightening a trend that is in complete agreement with the transformation of the yellow-colored mononuclear complexes into their corresponding blue dimers 6a−e. In particular, each of the dinuclear complexes 6a−e displays an equally shaped MLCT band peaking in a wavelength range between 652 nm (complex 6a) and 676 nm (complex 6d), with the variation of the absorption maxima that appear to be dictated by the electron-withdrawing character of the substituents to the Pt(II)-coordinated olefin. Taken together, these results suggest that the dimerization of the mononuclear Pt(II) compounds 5a−e leads to dimeric species (complexes 6a−e) that show an impressively extended conjugation. The involvement of almost the whole molecule in the occurrence of this effect was corroborated by the time-dependent density theory (TD-DFT) calculations that were performed on the exemplar dinuclear complex 6a, whose results displayed significant contributions in determining the composition of both the HOMO and the LUMO levels coming from the salentype bridging ligand, the Pt(II) centers, and the coordinated olefins. As expected11 for Pt(II) complexes, the absence of aromatic cyclometalated ligands in the bimetallic systems 6a−e determines the lack of luminescence.12 To elucidate the relationship between the absorption properties and the extended conjugation of the dinuclear complexes, systems 5a, 5c, 5e and 6a, 6c, 6e were examined by performing theoretical calculations. After optimization of the geometric structures (see for example 5a and 6a in Figure 7), TD-DFT calculations were performed in the Gaussian 09 program suite (see Experimental Section and Supporting Information for details). By comparing the HOMO and the LUMO of system 5a (Figure 8) with those of system 6a (Figure 9), it appears that

Figure 5. Normalized absorption profile of 5a (orange line) and 6a (blue line), 10−5 M THF solutions, rt.

Figure 6. Normalized absorption profiles of 6a−e, 10−5 M THF solutions, rt.

The UV−vis absorption profile of the monometallic complex 5a displayed two intense bands, which are centered at 319 and 411 nm, respectively. In agreement with previously reported studies dealing with Pt(II) salen-type Schiff base complexes,10a,b D


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Organometallics

CDCl3, δ 77.0, as internal standards) and C6D6 (C6D5H, δ 7.15, and C6D6, δ 127.86, as internal standards). The following abbreviations were used for describing NMR multiciplities: s, singlet; d, doublet; dd, double doublet; t, triplet; m, multiplet; br, broad. Two-dimensional (2D) experiments, such DQFCOSY, NOESY, and ROESY, were recorded with standard pulse sequences. NOE intensities were evaluated by integration of cross-peaks in the 300, 500, and 700 ms NOESY spectra using the CARA program.13 They were then converted into interproton distances by a distance calibration method.14 Complexes [PtClMe(dameph)]2d and 1b2f were prepared according to literature methods. Purity of the new compounds of type 6 was established by X-ray diffraction (6a), mono- and bidimensional NMR spectroscopy, and elemental analysis. Synthesis of Complexes 6a−e. A solution of KOH (0.028 g, 0.50 mmol) in methanol (2 mL) was added to a magnetically stirred suspension of the appropriate precursor of type 1 (0.50 mmol) in methanol (4 mL) kept in an ice bath. The color of the precipitate rapidly turned greenish and gradually deep blue. After 48 h the complex of type 6 was separated, washed with cold methanol, and dried under vacuum. Yields: 45−50%. Separation of the yellowish solid after only 5 min of stirring allowed the isolation of substantially pure complexes of type 5. 1 H NMR (δ, 298 K): 5a (400 MHz, CDCl3) 6.90−7.20 (m, 6H), 4.30 (s, 1H), 3.81 (s, 1H), 2.50 (br, 4H), 2.25 (s, 6H), 2.18 (s, 6H), 1.86 (s, 3H), −0.37 (s, 3H, 2JPt−H = 76.0 Hz); 5b (400 MHz, CDCl3) 7.03−7.23 (m, 5H), 6.95 (t, 1H), 4.27 (s, 1H), 3.75 (s, 1H), 2.80 (br, 1H), 2.58 (br, 1H), 2.47 (br, 1H), 2.27 (s, 3H), 2.23 (s, 3H), 2.18 (s, 6H), 1.84 (s, 3H), 1.37 (br, 3H), −0.33 (s, 3H, 2JPt−H = 78.4 Hz); 5c (400 MHz, CDCl3) 7.00−7.25 (m, 10H), 6.88 (t, 1H), 4.34 (s, 1H), 4.11 (m, 1H, 2JPt−H = 69.6 Hz), 3.83 (s, 1H), 3.08 (d, 1H, 3Jtrans = 13.6 Hz, 2JPt−H = 46.0 Hz), 2.38 (s, 3H), 2.26 (m, 4H), 2.20 (s, 3H), 2.16 (s, 3H), 1.92 (s, 3H), −0.79 (s, 3H, 2JPt−H = 79.6 Hz); 5d (400 MHz, CDCl3) 7.15−7.25 (m, 5H), 7.00−7.13 (m, 4H), 6.91 (t, 1H), 6.70 (d, 2H), 4.35 (s, 1H), 4.09 (m, 1H, 2JPt−H = 68.0 Hz), 3.83 (s, 1H), 3.76 (s, 3H), 3.07 (d, 1H, 3Jtrans = 12.8 Hz, 2JPt−H = 46.0 Hz), 2.40 (s, 3H), 2.28 (m, 4H), 2.22 (s, 3H), 2.18 (s, 3H), 1.93 (s, 3H), −0.76 (s, 3H, 2 JPt−H = 78.0 Hz); 5e (selected signals, 400 MHz, CDCl3) 4.41 (s, 1H), 4.08 (dd, 1H, 2JPt−H = 65.0 Hz), 3.91 (s, 1H), 3.09 (d, 1H, 3Jtrans = 11.6 Hz), −0.79 (s, 3H, 2JPt−H = 79.6 Hz); 6a (500 MHz, C6D6) 7.07 (d, 4H), 6.75−6.85 (m, 8H), 5.87 (s, 2H), 2.80−2.20 (br, 8H), 2.51 (s, 12H), 1.93 (s, 12H), 1.17 (s, 6H), −0.066 (s, 6H, 2JPt−H = 72.5 Hz); 6b (400 MHz, C6D6) 7.08 (m, 4H), 6.75−6.90 (m, 8H), 5.87 (s, 2H), 2.99 (br, 2H), 2.73 (br, 2H), 2.61 (br, 2H), 2.56 (s, 6H), 2.50 (s, 6H), 1.97 (s, 12H), 1.33 (br, 6H), 1.20 (s, 6H), −0.040 (s, 6H, 2JPt−H = 72.0 Hz); 6c (400 MHz, C6D6) 7.30 (d, 4H), 6.70−7.10 (m, 18H), 5.90 (s, 2H), 4.38 (dd, 2H, 3Jcis = 8.4 Hz, 3Jtrans = 12.8 Hz, 2JPt−H = 68.0 Hz), 3.20 (d, 2H, 2JPt−H = 48.0 Hz), 2.43 (m, 14H), 2.14 (s, 6H), 2.06 (s, 6H), 1.23 (s, 6H), −0.41 (s, 6H, 2JPt−H = 73.6 Hz); 6d (500 MHz, C6D6) 7.20 (d, 4H), 7.02 (m, 2H), 6.80−6.95 (m, 8H), 6.71 (m, 2H), 6.60 (d, 4H), 5.88 (s, 2H), 4.36 (dd, 2H, 3Jcis = 7.5 Hz, 3Jtrans = 12.5 Hz, 2JPt−H = 70.0 Hz), 3.18 (m, 8H), 2.46 (s, 12H), 2.42 (d, 2H), 2.13 (s, 6H), 2.04 (s, 6H), 1.21 (s, 6H), −0.39 (s, 6H, 2JPt−H = 68.0 Hz); 6e (400 MHz, C6D6) 6.80−7.20 (m, 18H), 6.72 (t, 2H), 5.86 (s, 2H), 4.15 (dd, 2H, 3Jcis = 8.4 Hz, 3Jtrans = 13.2 Hz, 2JPt−H = 72.0 Hz), 3.03 (d, 2H, 2JPt−H = 45.6 Hz), 2.43 (m, 8H), 2.38 (s, 6H), 2.08 (s, 6H), 2.00 (s, 6H), 1.20 (s, 6H), −0.55 (s, 6H, 2JPt−H = 72.0 Hz). Selected 13 C NMR data (δ, C6D6, 298 K, 100 MHz) 6a, 110.3, 57.2 (1JPt−C = 193 Hz), − 5.27; 6b, 109.7, 73.8 (1JPt−C = 203 Hz), 57.3 (1JPt−C = 180 Hz), −4.26 (1JPt−C = 757 Hz); 6c, 109.9, 73.7, 48.8, −1.32; 6d, 109.8, 74.5, 48.5, −1.36; 6e, 109.9, 70.6, 48.7, −1.27. Anal. Calcd (found) for 6a (C46H58N4Pt2): C, 52.26 (52.41); H, 5.53 (5.60); N, 5.30 (5.19); 6b (C48H62N4Pt2): C, 53.13 (53.27); H, 5.76 (5.66); N, 5.16 (5.15); 6c (C58H66N4Pt2): C, 57.60 (57.78); H, 5.50 (5.42); N, 4.63 (4.75); 6d (C60H70N4O2Pt2): C, 56.77 (56.61); H, 5.56 (5.58); N, 4.41 (4.34); 6e (C60H64F6N4Pt2): C, 53.57 (53.50); H, 4.79 (4.69); N, 4.16 (4.23). Details of Absorption Spectroscopy and Computational Methods. Absorption spectra of complexes 5a and 6a−e were recorded in diluted (10−5 M) THF solutions at room temperature using a PerkinElmer Lambda 35 UV/vis spectrometer. All the 13 13

Figure 7. Geometry optimization of the structures 5a and 6a at the B3LYP/SVP level; see text.

Figure 8. Molecular orbital analysis for system 5a with the HOMO (A) and LUMO (B) levels.

Figure 9. Molecular orbital analysis for system 6a with the HOMO (A) and LUMO (B) levels.

the extended conjugation is responsible for the modification of the HOMO level in system 6a. Similar results are obtained for the systems 5c, 6c and 5e, 6e, respectively (reported in Figures S7−S10). The absorption maxima calculated at the TD-DFT level in THF solution are 375.96 nm for 5a and 610.15 nm for 6a, in good agreement with the experimental results (Table 1). Similarly, the variation of absorption maxima due to olefin substituents are well reproduced at the TD-DFT level, with 6c and 6e showing maxima at 625.93 and 625.26 nm, respectively (see Table 1).

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CONCLUSION This study has revealed a rare behavior of imino, amide complexes of platinum(II), which undergo an oxidative coupling. The resulting family of binuclear complexes was characterized by NMR spectroscopy, X-ray diffraction, optical measurements, and DFT calculations. The significant bathochromic shift of the absorption maxima and the intense blue color derive from an extended conjugation within the compounds.

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EXPERIMENTAL SECTION

General Procedures. NMR spectra were acquired on 400 and 600 Varian Inova spectrometers operating at proton frequencies of 400 and 600 MHz, located at the Istituto di Biostrutture e Bioimmagini (IBB) of CNR, Napoli (Italy), and on a 500 Varian Inova spectrometer operating at a proton frequency of 500 MHz, located at the Dipartimento di Scienze Chimiche, Università di Napoli Federico II, Napoli (Italy). The solvents were CDCl3 (CHCl3, δ 7.26, and E


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geometry optimizations by DFT calculations have been performed with the Gaussian09 set of programs,15 using the B3LYP functional of Becke and Perdew.16 The electronic configuration has been described with the standard split-valence basis set with a polarization function of Ahlrichs and co-workers for H, C, N, and O (SVP)17 and with the SDD basis and pseudopotential18 at the metal. Stationary points were characterized using vibrational analysis, and this analysis has been also used to calculate zero-point energies and thermal (enthalpy and entropy) corrections (298.15 K, 1 bar). For TD-DFT calculations we used a range-separated version CAM-B3LYP19 with a TZVP basis set20 on the main atoms, a solvation contribution (PCM model,21 THF). Single-Crystal X-ray Crystallography. Single crystals of 6a suitable for X-ray analysis were obtained from a chloroform solution by slow evaporation. Data collection was performed in flowing N2 at 173 K on a Bruker-Nonius kappa CCD diffractometer (Mo Kα radiation, CCD rotation images, thick slices, φ scans + ω scans to fill the asymmetric unit). Cell parameters were determined from 186 reflections in the range 3.187° ≤ θ ≤ 21.225°. Semiempirical absorption corrections (multiscan SADABS)22 were applied. The structure was solved by direct methods (SIR 97 package)23 and refined by the full matrix least-squares method (SHELXL program of SHELX97 package)24 on F2 against all independent measured reflections, using anisotropic thermal parameters for all non-hydrogen atoms. H atoms were placed in calculated positions with Ueq equal to those of the carrier atom and refined by the riding method. All the H olefin atoms were found in difference Fourier maps, and their coordinates were refined without restraints. R1 = 0.0350; wR2 = 0.0526 (on reflections with I > 2σ(I)) and R1 = 0.0733, wR2 = 0.0599 on all reflections. Max and min residual electron density (e·Å−3): +0.888 and −0.947.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00798. Synthesis and characterization of complexes 1a, 1c−e; H NMR spectra of complexes 6a−e; schemes of the relevant NOEs of 6b−e; molecular orbital analysis for the structures 5c, 5e, 6c, 6e (PDF) Optimized DFT structures discussed in the text (XYZ) Crystallographic data, bond lengths and angles for 6a (CIF) 1

AUTHOR INFORMATION

Corresponding Author

*E-mail (F. Ruffo): ruff[email protected]. ORCID

Francesco Ruffo: 0000-0003-1624-079X Notes

The authors declare no competing financial interest. Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 1503150.

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REFERENCES

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ACKNOWLEDGMENTS

The authors wish to thank the Italian Ministry of Education, University and Research (MIUR) for financial support (PRIN project: Towards a Sustainable Chemistry: Design of Innovative Metal-Ligand Systems for Catalysis and Energy Applications). F


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