Synthesis and Reactivity of Dipalladated Derivatives of Terephthalaldehyde

The polynuclear complex [{μ-C1,C4,N,N″-C6H2{C(H)=N(nBu)}2-2,5}{Pd(μ-OAc)}]2 (I) reacts with tbbpy (4,4′-di-tert-butyl-2,2′-bipyridine) and TlOTf to form the dinuclear complex [{μ-C1,C4,N,N″-C6H2{C(H)=N(nBu)}2-2,5}{Pd(tbbpy)}2] (1). The hydrolysis of I with acetic acid in a 5:1 acetone/water mixture, in the presence of two equivalents of tbbpy and excess NaX (X = Br, I), yields the dipalladated terephthalaldehyde complexes [C6H2{PdX(tbbpy)}2-1,4-(CHO)2-2,5] [X = Br (2a), X = I (2b)], which are the first fully characterized complexes of this type. The reaction of 2a,b with CO results in the insertion of CO into both aryl–Pd bonds, forming [C6H2{C(O){PdX(tbbpy)}}2-1,4-(CHO)2-2,5] [X = Br (3a), X = I (3b)], which are the first examples of complexes with CO inserted into two separate aryl–metal bonds involving the same ligand. The bromo complex 2a reacts with excess XylNC in acetone, causing the precipitation of the dinuclear complex 2,3,6,7-tetrahydrobenzo[1,2-c:4,5-c′]dipyrrole-1,5-dione-2,6-dixylyl-3,7-bis{=C(NHXyl)-C(=NXyl)-[PdBr(CNXyl)2]} (4), which is the result of the insertion of three molecules of the isocyanide into each aryl–Pd bond and the nucleophilic attack of one of them at each formyl group. When complex 4 reacts with TlOTf and residual water in 1,2-dichloroethane at 70 °C, depalladation occurs, and the organic compound 2,3,6,7-tetrahydrobenzo[1,2-c:4,5-c′]dipyrrole-1,5-dione-2,6-dixylyl-3,7-bis{=C(NHXyl)–C(O)NHXyl} (5) can be isolated. The crystal structures of 1·4CHCl3, 4·2CH2Cl2·3hexane, and 5·2CDCl3 have been determined by X-ray crystallography.

−57 We report now the first fully characterized dipalladated derivatives of terephthalaldehyde (R�CHO in Chart 1B) and a preliminary investigation of their reactivity toward unsaturated molecules (CO and XylNC).These reactions have resulted in the first complexes resulting from the double insertion of CO into two separate aryl−metal bonds of the same aryl ring, as well as the synthesis of a novel dinuclear Pd(II) complex derived from a multiple XylNC insertion, which can be depalladated to yield an interesting polycyclic organic compound containing a benzodipyrrole-1,5-dione core.
■ RESULTS AND DISCUSSION Synthesis.The polynuclear complex [{μ-C1,C4,N,N″-C 6 H 2 {C(H)�N( n Bu)} 2 -2,5}{Pd(μ-OAc)}] 2 (I, Scheme 1) had been previously prepared in our research group by palladation of the diimine C 6 H 4 (CH�N n Bu) 2 -1,4 with [Pd(OAc) 2 ]. 58 Complex I is soluble in common solvents, in contrast to a similar complex with Tol instead of n Bu, also prepared in our research group. 51The reaction of I with tbbpy (4,4′-di-tert-butyl-2,2′-bipyridine) and TlOTf results in the dinuclear complex [{μ-C1,C4,N,N″-C 6 H 2 {C(H)�N( n Bu)} 2 -2,5}{Pd(tbbpy)} 2 ] (1, Scheme 1).Complexes I and 1 are of interest because they contain an aryl ligand capable of binding two different metal centers simultaneously in a tetradentate fashion, resulting in two independent palladacycles on the same aryl ring.−57 The examples most closely related to our work are the dipalladated Schiff bases reported by Vila and co-workers, prepared by palladation or oxidative addition reactions, followed by ligand exchange. 55,56These dinuclear square-planar palladium(II) complexes with two blocked cis-coordination sites can be very useful as building blocks in supramolecular chemistry. 55,59he hydrolysis of I with acetic acid in a 5:1 acetone/water mixture, in the presence of two equivalents of tbbpy and excess NaX (X = Br, I), yields the dipalladated terephthalaldehyde complexes [C 6 H 2 {PdX(tbbpy)} 2 -1,4-(CHO) 2 -2,5] [X = Br (2a), X = I (2b), Scheme 2].These are the first such dinuclear Pd complexes to be fully characterized, as in a previous attempt by our research group, 51 starting from a complex similar to I with Tol instead of n Bu, and using bpy instead of tbbpy as the ligand, the formation of a similar complex [C 6 H 2 {PdBr-(bpy)} 2 -1,4-(CHO) 2 -2,5] was proposed, but it was too insoluble to be purified and characterized.Similar reactions with tmeda instead of tbbpy have resulted in mixtures of compounds that could not be separated.
The reaction of 2a,b with CO results in the complexes [C 6 H 2 {C(O){PdX(tbbpy)}} 2 -1,4-(CHO) 2 -2,5] [X = Br (3a), X = I (3b), Scheme 3], resulting from the insertion of CO into both aryl−Pd bonds of 2a,b (Scheme 3).These are the first reported complexes resulting from the insertion of CO into two separate aryl−metal bonds involving the same aryl ligand.The infrared (IR) and nuclear magnetic resonance (NMR) spectra of 3a,b confirm the insertion of CO (see below).Additionally, the NMR spectra suggest that one of the inserted CO groups forms a hydrogen bond with the adjacent aromatic hydrogen, while the other inserted CO does not behave similarly.The syntheses of 3a,b are best carried out in distilled When bromo complex 2a reacts with a 2-fold excess of XylNC in acetone, singular dinuclear complex 2,3,6,7tetrahydrobenzo[1,2-c:4,5-c′]dipyrrole-1,5-dione-2,6-dixylyl-3,7-bis{�C(NHXyl)-C(�NXyl)-[PdBr(CNXyl) 2 ]} (4) precipitates as a red solid (Scheme 4).Complex 4 is the result of the insertion of three molecules of the isocyanide into each aryl−Pd bond and the nucleophilic attack of one of them at each formyl group, followed by an intramolecular proton migration (Scheme 5).The tbbpy ligands on the Pd atoms are displaced by two other XylNC molecules, which, as usual, adopt a trans disposition. 23,24,27,43It would seem that the insolubility of 4 plays an important role in its formation as similar reactions with iodo complex 2b, or with t BuNC instead of XylNC, result in mixtures of compounds.Complex 4 decomposes slowly in solution to give [PdBr 2 (XylNC) 4 ], which is easily identified by its 1 H NMR resonance at 2.52 ppm. 23hen complex 4 reacts with TlOTf in 1,2-dichloroethane at 70 °C, depalladation occurs, forming the new organic compound 2,3,6,7-tetrahydrobenzo[1,2-c:4,5-c′]dipyrrole-1,5dione-2,6-dixylyl-3,7-bis{�C(NHXyl)−C(O)NHXyl} (5,  Scheme 6).In this reaction, the precipitation of TlBr promotes the substitution of both [PdBr(XylNC) 2 ] moieties by hydroxyl groups from residual water molecules, followed by a tautomeric equilibrium, resulting in a benzodipyrrole-1,5-dione core with two alkylidene substituents at positions 2 and 6.There is no other synthetic route described for the synthesis of a compound such as 5, and even for related, although more simple, benzodipyrrolediones, we have only found two precedents in the literature, none of them involving Pd: a report on cobalt-catalyzed carbonylation of Schiff bases as a step for the synthesis of benzenetetracarboxylic acids 60 and the organic synthesis of bis(hydroxy-isoindolinones) as building blocks for supramolecular assembly. 61hows fluxional behavior, resulting in broad NMR resonances for the tbbpy ligand.This dynamic process might be promoted by the coordination of the OTf anion to the Pd atom, leading to a five-coordinated intermediate where the two halves of the tbbpy ligand would exchange by dissociation of one of the N atoms, rotation around the remaining Pd−N bond, and recoordination to Pd.We have described a similar process in anionic dinuclear indacenediide palladium complexes, also featuring a tbbpy ligand and an OTf anion. 43omplexes 2a,b show the expected IR bands for the formyl C�O bonds at 1672 and 1662 cm −1 , respectively, while for 3a,b, we observe a broad band at 1682 cm −1 (3a) or two bands at 1662 and 1678 cm −1 (3b), confirming the presence of additional C�O groups resulting from the insertion of CO into both aryl−Pd bonds.1D and 2D NMR spectra of 2a,b and 3a,b have allowed full assignment of their 1 H and 13 C resonances (see Experimental Section).2a,b complexes show a single set of 1 H and 13 C resonances for the two halves of the molecule, which are made equivalent by an inversion center in solution.For 3a,b, interestingly, there is no such symmetry, and two well-separated 1 H and 13 C resonances for the two CH groups of the aryl ring are observed, while all of the other resonances of the two halves of the molecule coincide (see Table 1).This inequivalence of the two CH groups in 3a,b could be explained by the formation of a hydrogen bond between one of the inserted CO groups and the adjacent aryl proton, while the same would not happen for the other CO group, as a result of steric or electronic reasons.The aryl hydrogen involved in the hydrogen bond (H3″) in 3a,b would be shifted to higher frequencies (δ 8.48 ppm) with respect to the other aryl hydrogen, which would resonate at a similar frequency (δ 8.1 ppm) as in 2a,b (data in blue in Table 1).
The 13 C resonances of CH3 and CH3″ would also be affected, while for the other carbons in 3a,b, the difference in chemical shift would be so small that it would not be observable.a As also shown in Table 1, 2a,b and 3a,b show the expected 1 H and 13 C NMR resonances for the CHO groups.However, the 13 C resonances of the inserted CO groups were not observed in 3a,b, possibly because of the long relaxation times.Nonetheless, the insertion of the CO molecules is confirmed by the elemental analyses and also by the IR spectra (as mentioned above).

Organometallics
The IR spectrum of complex 4 shows the expected bands for the N−H bond (3376 cm −1 ), the C�N bonds of the coordinated isocyanides (2182 cm −1 ), the carbonyl C�O bonds (1682 cm −1 ), and the C�N bond of the inserted isocyanide (1614 cm −1 ).For the organic compound 5, we observe an N−H band at 3369 cm −1 and a broad C�O band at 1674 cm −1 .The 1 H and 13 C NMR resonances of both 4 and 5 were also assigned with the help of 1D and 2D spectra.Both complexes feature an inversion center in solution (as in the solid state) so that the halves of the molecules are equivalent, and only one set of 1 H and 13 C NMR resonances is observed.The NMR data also indicate that there is free rotation around all the N−Xyl bonds, making both Me groups on each ring equivalent.For complex 4, the two XylNC ligands on each Pd are also equivalent, confirming the trans geometry proposed for this complex.
1 Indeed, the APT spectrum of 2b, when processed without window function, shows a significant broadening of the 13 C resonances of the CHO/CHO″, C1/C1″, and C2/C2″ carbons but not of those of the tbbpy ligand.
The crystal structures of 1•4CHCl 3 (Figure 1), 4•2CH 2 Cl 2 • 3hexane (Figures 2 and 3), and 5•2CDCl 3 (Figures 4 and 5) have been determined by X-ray diffraction studies (Table 1 in Supporting Information).The structure of 1•4CHCl 3 (Figure 1) shows the crystallographic inversion symmetry of 1 and confirms the doubly chelating nature of the diimine ligand.The chelate ring is to a good approximation planar, with a mean deviation of 0.014 Å.The coordination of the iminic nitrogen to Pd leads to a slight lengthening of the C�N bond [1.291( 7) Å] with respect to the mean value in imines (1.279 Å). 62 The coordination around the Pd atoms is square planar but is markedly distorted to avoid a close contact between H3 and H26 (for which the observed distance is 2.12 Å); the atoms Pd, N11, N21, and N1 are coplanar (mean deviation 0.04 Å), but C2 lies 0.64 Å outside the plane thus defined.The Pd−C bond distance is 2.002(5) Å, similar to the values reported by some of us for other aryl palladium complexes with an aryl ligand trans to bpy or tbbpy (ca.1.97−2.00Å). 20,27,46,63 The three Pd−N bond distances follow the expected order of the trans influence: Pd−N trans to aryl [2.160(4) Å] > Pd−N trans to N [2.039(5) and 2.058(5) Å].
The structure of complex 4•2CH 2 Cl 2 •3hexane is also confirmed by its X-ray diffraction study (Figure 2   the Pd−Br bond distance of 2.5338(10) Å is similar to those observed in other complexes where the Br ligand is trans to an iminoacyl ligand. 23,26,68t first sight, the NH group does not seem to be involved in hydrogen bonding.However, closer inspection reveals a close intramolecular approach of the NH hydrogen atom to the atoms N3, C31, C32, and C36 of a neighboring ring, with distances of 2.65(7), 2.24(5), 2.49(4), and 2.85(5) Å, respectively (Figure 3).This may reasonably be regarded as a C−HΛπ interaction.The dichloromethane molecule, which is ordered, makes a very short contact (a "weak" hydrogen bond) with C−HΛO1 of only 2.20 Å.
The structure of organic compound 5•2CDCl 3 has also been confirmed by its X-ray diffraction study (Figure 4 2) Å] are as expected for C sp2 �O amides. 62imilarly to 4•2CH 2 Cl 2 •3hexane, the hydrogen atom of one NH group makes short intramolecular contacts to a neighboring ring, with H02ΛC11 2.32(2), H02ΛC12 2.75(2), H02ΛC16 2.72(2), and H02ΛN1 2.69(2) Å.The other NH group is hydrogen-bonded to a solvent chlorine atom [H03ΛCl2 2.94(2) Å, symmetry operator 1−x, 1−y, and 1−z], and the solvent CD group is hydrogen-bonded to O1, with a DΛO distance of only 2.05 Å.The hydrogen bonds combine to form broad ribbons of residues parallel to the a axis (Figure 5).

■ CONCLUSIONS
We have described the first fully characterized dipalladated derivatives of terephthalaldehyde and also their reactivity toward CO and XylNC, which has resulted in the first reported complexes featuring CO insertion into two separate aryl−metal bonds on the same ligand, as well as the double 3-fold insertion of XylNC, leading to the synthesis of singular dipalladated benzodipyrrole-1,5-dione derivative, which can be depalladated to yield the free polycyclic organic ligand.This chemistry showcases the great potential of polypalladated arenes for the synthesis of complex organic compounds.

■ EXPERIMENTAL SECTION
NMR spectra ( 1 H and 13 C) were recorded on 400 and 600 MHz Bruker AVANCE spectrometers at room temperature.Chemical shifts are given in parts per million (δ) relative to TMS ( 1 H, 13 C).IR spectra were recorded on a PerkinElmer 16F-PC-FT spectrometer with Nujol mulls between polyethylene sheets.Melting points were determined on a Reichert apparatus and are uncorrected.Elemental analyses were carried out with a Carlo Erba 1106 microanalyzer.Experiments under a N 2 atmosphere were conducted using standard Schlenk techniques.THF, CH 2 Cl 2 , and Et 2 O were degassed and dried by using a Pure Solv MD-5 solvent purification system from Innovative Technology Inc. [{μ-C1,C4,N,N″-C 6 H 2 {C(H)�N-( n Bu)} 2 -2,5}{Pd(μ-OAc)}] 2 (I) was prepared from the diimine C 6 H 4 {C(H)�N( n Bu)} 2 -1,4 (generated in situ from terephthalaldehyde and n BuNH 2 ) by palladation with [Pd(OAc) 2 ], as previously described 58 (see Supporting Information).TlOTf was prepared by the reaction of Tl 2 CO 3 and triflic acid (1:2) in water and recrystallized from acetone/Et 2 O. [Pd(dba) 2 ] was prepared according to a literature procedure. 69,70All other reagents were obtained from commercial sources and used as received.
) and, similarly to 1•4CHCl 3 , shows the crystallographic inversion symmetry of 4. Unfortunately, the large amount of included solvent (with high U values for the hexanes) leads to data with a relatively low resolution.The Pd atoms in 4•2CH 2 Cl 2 • 3hexane show square planar coordination to a reasonable approximation with a mean deviation from the best plane through Pd and the four donor atoms of 0.07 Å.The Pd− C(10) bond distance of the iminoacyl ligand is 2.041(7) Å, significantly longer than that in the aryl palladium complex 1• 4CHCl 3 [2.002(5)Å, see above].Pd−C bond distances in iminoacyl ligands are known to vary considerably, depending on the influence of the trans ligand. 23,64−66 For the isocyanide ligands, the Pd−C(40) and Pd−C(50) bond distances are 1.969(8) and 1.970(8) Å, similar to the values found in other Pd complexes with two mutually trans XylNC ligands (ca.1.96−1.99Å in those reported by some of us). 23,64−67 Finally,

Figure 5 .
Figure 5. Hydrogen bonding system in 5•2CDCl 3 .Thick dashed lines hydrogen bonds.Hydrogen atoms not involved in hydrogen bonding have been omitted for clarity.Only the major component (83%) of the disordered solvent is shown.