New phospha-palladacycles: efficient catalysts in the formylation of aryl chlorides

Abstract

The direct reaction between [{Pd(μ-AcO)(CˆP)}₂] 1 and cyclic imides to yield new dinuclear cyclometallated palladium(II) complexes containing –NCO– bridging imidates of general formula [{Pd(μ-NCO)(CˆP)}₂] (CˆP = CH₂C₆H₄P(o-tolyl)₂; –NCO– = succinimidate (suc) 2, maleimidate (mal) 3, phthalimidate (phthal) 4 or saccharinate (sacc) 5 is described. Spectroscopic techniques and structural characterisation by X-ray diffraction of complex 2 confirmed the proposed formulae. The new dimeric complexes are shown to be excellent catalysts for the selective formylation of aryl halides under CO-free conditions to obtain synthetically important aromatic aldehydes. Yields are diminished by steric hindrance around the aryl halides.

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Introduction

Since the first descriptions of cyclopalladation in the 1960s¹ to date several reviews have been dedicated to those different aspects of chemistry of palladacycles that have brought them to a prominent place in organometallic palladium chemistry. Thus, their synthesis and structural characterization, their applications in organic synthesis or organometallic catalysis, and their presence in medicinal and biological chemistry or use as chiral auxiliaries, mesogenic and luminescent agents have been widely explored.² Probably the most striking application comes from their use as catalysts in C–C and C–heteroatom bond forming reactions, a field in which palladacycles changed from being perceived as dead-end deactivation products to be considered among the most useful reagents, and therefore the object of considerable attention since then.³ Complexes containing a cyclopalladated (di-o-tolylphoshino)benzyl fragment such as 1 illustrate like no others this revaluation experienced after the original articles of Herrmann, Beller and co-workers,⁴ that has kept since 1995 a permanent interest in phosphapalladacycles mainly focused in the design of new CˆP systems.^2,3,5 However, in contrast with complexes that contain CˆN cyclopalladated backbones whose chemistry has been widely expanded from the dinuclear acetate or chloride bridged precursors by reacting them with neutral or anionic ligands to yield di- or mononuclear derivatives, that in some cases improved the catalytic performance of the starting materials,^3b only a few reports exploiting the reactivity of acetate bridges in 1 can be found.^3a,4c,6

In this sense, during the last few years we have taken advantage of binuclear acetate/hydroxo complexes of palladium, some of them with a cyclometalated backbone,⁷ in the preparation of a wide range of new compounds by means of a simple acid–base reaction without addition of other basic reagents.⁸ Regarding the application in catalysis of these new derivatives, we particularly succeeded with those containing imidate –CNO– bridges such as reported here. Thus, binuclear imidate cyclometalated palladium(II) complexes and mononuclear phosphine adducts have been tested in Stille,^8a Suzuki and Sonogashira cross-couplings,^7a,9 and phosphine-free anionic dinuclear imidate complexes with a palladacyclopentadiene backbone have displayed impressive performances in Stille couplings.¹⁰ Despite the potential of imidato complexes not only in catalysis, but also in medicinal¹¹ or supramolecular chemistry,¹² their coordination chemistry has not been much explored, with saccharinate complexes being most fully characterized, mainly by Yilmaz and co-workers.¹³

On the other hand, aromatic aldehydes have found a prominent place in synthesis due to the highly reactive nature of the aldehyde functional group. These are readily transformed into different functional groups that can provide a handle towards achieving the synthesis of complex biologically active molecules. Given the importance of aromatic aldehydes over the years, several novel protocols have been developed that have tried to replace the more traditional approaches¹⁴ which have suffered from the formation of undesired side products and waste. Several formylating reagents have been employed such as formate salts, carbon monoxide under reductive formylation conditions^15,16 as well as the environmentally benign formylation source CO/H₂ (synthesis gas).¹⁷ Although in all these cases high reactivity could be achieved, the use of relatively hazardous chemical reagents or gases at elevated temperatures and pressures are far from ideal.

Like in the usual C–C, C–O and C–N bond-forming technologies,¹⁸ aryl chlorides are among the most desirable substrates due to their lower costs. However, the reduced reactivity associated with aryl chlorides because of their inherently high C–Cl bond strength makes these substrates synthetically more challenging towards different reactions.^19,5a To the best of our knowledge, efficient palladium-catalyzed formylation of aryl chlorides can be accomplished via hydroformylation using either electron-rich and bulky phosphines or highly elaborate palladium complexes.¹⁵ In this paper we report the synthesis and characterization of new dinuclear imidate complexes that contain the cyclometalated (di-o-tolylphoshino)benzyl fragment by means of one-step reaction with the well known di-μ-acetate precursor. We also present here a mild and efficient CO-free protocol for the synthesis of aromatic aldehydes from aryl chlorides using these novel palladacyclic complexes, which display excellent selectivity of formylation vs. cyanation.

Results and discussion

Synthesis and characterization

In 1999 Herrmann and co-workers published the first review after the revival experienced by phosphapalladacycles a few years before.^3a Although, mainly focused in catalytic applications, this article also collected the previous reports on CˆP-palladacycles²⁰ and interestingly it gathered together most of the chemistry done on the acetate groups of the complex trans-di(μ-acetato)-bis[o-(di-o-tolylphosphino)benzyl]dipalladium(II) 1 and related complexes to date. Thus anion interchange with tetrabutylammonium halides produces poorly soluble di-μ-halide dimers^4c and bridge-splitting with coordinating solvents or neutral donor ligands such as pyridine or PPh₃ is reported to form soluble monomeric complexes,^3a while acetylacetonate or 2-acylphenolate complexes are obtained by reacting 1 with 2,4-pentanodione (Hacac)^6b,c or 2-acylphenoles, respectively.^6a Whereas the presence of a strong base is needed for the success of this last reaction, direct treatment of 1 with Hacac produced the expected monomeric derivatives.

This was found in our related studies and di-μ-acetate-complex 1 reacted with cyclic imides to give binuclear species of general formula [{Pd(μ-NCO)(CˆP)}₂] (CˆP = CH₂C₆H₄P(o-tolyl)₂; –NCO– = imidate). The structures of the protonated imidate ligands with their abbreviations are shown in Scheme 1.

Scheme 1

The specific conditions followed to prepare complexes 2–5 are given in the Experimental section. The new palladium complexes 2–5 are yellow air-stable solids and their IR spectra show absorptions assignable to the cyclometallated CˆP backbone around 500 cm⁻¹,^4c,21 with absence of relevant bands in the region 3000–3500 cm⁻¹ and important variations in the carbonyl region indicating that acetate–imidate interchange has taken place. Two strong bands in the range 1728–1719 and 1632–1600 cm⁻¹ appear now instead of the very strong band at 1578 cm⁻¹ observed in 1. This suggests coordination of one imidate-carbonyl group to the metal as a part of an –NCO– bridging unit. According to previously reported data,²² a bridging structure would remove the two-fold symmetry of the free or N-bonded imidato ligand, increasing the intensity of the symmetric absorption. This higher frequency mode ν_sym(CO) is normally weak in cyclic imides and has negligible intensity compared with the antisymmetric ν(CO) absorption.

The dinuclearity of complexes 2–5 was confirmed by means of FAB mass spectrometry, as can be implied by the m/z values for the observed fragments. Spectra of the new bridged compounds show a similar fragmentation pattern which includes the peaks corresponding to M⁺ − imidate and M⁺/2. The abundances of the signals are consistent with the natural isotopic abundances.

Typical broad NMR resonances are observed, that in related compounds have been justified in terms of equilibria in solution caused by the interaction of the labile acetate bridges and deuterated solvents.^3a,4c,6a In spite of this the spectra show the corresponding signals of the incoming imidate ligands, overlapped in the aromatic region with those of the cyclometallated backbone.

In the case of complex 2, its dinuclear nature has also been confirmed by single-crystal X-ray analysis. The corresponding ORTEP drawing is shown in Fig. 1 with relevant bond lengths and angles provided in the figure caption.

Fig. 1 ORTEP diagram of complex 2 with atom numbering scheme; thermal ellipsoids are drawn at the 50% probability level. (CH₃)₂CO found in the unit cell is not shown. Selected bond lengths (Å) and angles (°): Pd(1)–C(1) 2.024(4), Pd(1)–N(1) 2.099(3), Pd(1)–O(1) 2.151(3), Pd(1)–P(1) 2.2330(10), Pd(2)–C(30) 2.028(4), Pd(2)–N(2) 2.101(3), Pd(2)–O(2) 2.158(3), Pd(2)–P(2) 2.2284(10), Pd(1)–Pd(2) 3.2338(4); C(1)–Pd(1)–N(1) 88.40(15), C(1)–Pd(1)–O(1) 172.92(15), C(1)–Pd(1)–P(1) 84.40(12), N(1)–Pd(1)–O(1) 87.58(13), N(1)–Pd(1)–P(1) 171.13(10), O(1)–Pd(1)–P(1) 99.03(8), C(30)–Pd(2)–N(2) 87.54(17), C(30)–Pd(2)–O(2) 173.34(16), C(30)–Pd(2)–P(2) 84.03(14), N(2)–Pd(2)–O(2) 88.37(13), N(2)–Pd(2)–P(2) 168.53(10), O(2)–Pd(2)–P(2) 99.27(9).

The X-ray analysis has also established a head-to tail arrangement, the most common in this type of complexes. Regarding the cyclometallated CˆP ligands, an anti-arrangement of identical donor atoms is found. The structure displays a so-called open-book geometry accompanied by a Pd(1)–Pd(2) distance of 3.2338(4) Å. We have recently developed methods for the conformational classification of eight-membered rings.²³ From this perspective the Pd(1)–N(1)–C(8)–O(2)–Pd(2)–N(2)–C(12)–O(1) ring displays a twist-boat conformation distorted by 38° (TB = 0.7719; BB = 0.2281 for σ = 10).The structure around the two palladium atoms may be described as nearly planar, and their deviation from the planar coordination has been quantified by measures of improper torsion angles: 4.10 and −3.94° for Pd(1); −3.74 and 5.77° for Pd(2) (both square pyramidal).²⁴

Bond distances and angles for 2 are close to those displayed by related complexes containing a di-o-tolylphosphino)benzyl backbone that have been previously reported.^25,3a,6a Following the classification of Dance and Scudder²⁶ for PPh₃ based on measures of torsion angles M–P–C_ipso–C, the conformation of PdP(o-tolyl)₃ groups is described as no rotor for this complex.

C,P-Palladacyclic imidate complexes for the conversion of aryl chlorides to aldehydes

We explore here the application the new C,P-palladacyclic imidate complexes in palladium catalyzed reactions, specifically in the selective formylation of aryl halides under CO-free conditions to obtain synthetically important aromatic aldehydes.

Initially, our aim was to synthesize aryl cyanides from aryl chlorides based on a recent protocol employing mild cyanide-free conditions.²⁷ Accordingly, we submitted 4-nitrochlorobenzene to the cyanation conditions using formamide/POCl₃ as the formylating reagent in the presence of different catalytic systems. Under the given conditions three types of products could form (cyanide, the amide or the aldehyde). A competing side reaction could also occur through Vilsmeier–Haack formylation of the aromatic ring at positions other than C–Cl. Our aim therefore was to selectively obtain the cyanide product over other competing side reactions in order to make the protocol synthetically useful.

At the outset of our studies the absence of palladium precursor failed to furnish any product (Table 1, entry 1), suggesting the requirement of a metal catalyst for such a transformation to take place, which was confirmed when Pd(OAc)₂ was used in the absence of any activating ligand (Table 1, entry 2). Nevertheless, poor yields of the product were obtained and selectivity observed with respect to different possibilities was also low (GC-MS analysis of reaction mixture showed formation of aldehyde as the major product with small amount of cyanide and amide). Slight improvement in yield and selectivity was observed when activating phosphine ligands such as PPh₃ and P(o-Tolyl)₃ were employed in combination with Pd(OAc)₂ as the palladium(II) precursor (Table 1, entries 3 and 4). Change in the metal to ligand ratio also had little effect on the outcome of the reaction. To ascertain the importance of POCl₃ (primary reagent for the formation of Vilsmeier–Haack adduct), its exclusion from the reaction led to formation of nitrobenzene (hydro-dehalogenated product formed on hydrolysis of the oxidative addition of palladium with 4-nitrochlorobenzene). The employment of more electron-rich ligands such as SIPrHCl and X-Phos resulted in poor product yields and selectivity as compared to P(o-Tol)₃ (Table 1, entries 8 and 9).

Table 1 Screening studies for the formylation of aryl chlorides^a

Entry	Catalyst system (mol%)	Yield^b (%)
a General procedure: 6a (1 mmol), 7 (10 mL mmol^−1), Pd complex, POCl3 (2.0 mmol), 140 °C, 48 h under nitrogen.b Isolated yields (product aldehydes confirmed by ^1H and ^13C NMR).c No POCl3 used in the reaction.d 20 mmol of POCl3 used.e Large amount of hydro-dehalogenated nitrobenzene was obtained.
1	—	0
2	Pd(OAc)2 (5)	21
3	Pd(OAc)2 (5)/PPh3 (10)	28
4	Pd(OAc)2 (5)/P(o-tol)3 (5)	35
5	Pd(OAc)2 (5)/P(o-tol)3 (10)	40
6	Pd(OAc)2 (5)/P(o-tol)3 (10)	0^c
7	Pd(OAc)2 (5)/P(o-tol)3 (10)	42^d
8	Pd(OAc)2 (5)/SIPrHCl (5)	15
9	Pd(OAc)2 (5)/X-Phos (5)	12
10	2 (2.5)	52
11	3 (2.5)	68
12	4 (2.5)	30
13	5 (2.5)	27
14	HB complex (1) (2.5)	54
15	POPD1 (2.5)	36
16	Pd(OAc)2 (5)/CM-Phos (5)	16^e

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We then employed our novel dimeric palladium complexes 2–5 and to our surprise there was a drastic improvement in the yields. On closer inspection of the products it was observed that rather than obtaining the desired cyanated product the dimeric palladium complexes led to the selective formation of the formylated 4-nitrobenzaldehyde (GCMS analysis showed exclusive formation of aldehyde product). Surprisingly, the formylation had occurred only at the C–Cl bond rather than any other position of the aromatic ring. Although the dimeric complexes have similar backbones i.e. cyclometallated phosphine coordination to the palladium centre, they differ with respect to the bridging ligand that has been employed. Next experiments were then directed to ascertain whether changes in the bridging ligand could lead to any improvement in the yields or selectivity for the above-mentioned transformation. Notable differences were observed between the imide ligands (for 2, 52%; 3, 68%; 4, 30%; 5, 27%) with the maleimide bridged palladacyclic complex 3 outperforming all others.

The commercially available Herrmann–Beller complex 1 also shows slightly reduced reactivity than 3 with the formation of the aromatic aldehyde as the major product. Commercially available CombiPhos® POPD1 dimer complex when employed also showed poor yield of the aldehyde product. Recently, CM-Phos has been shown to undergo in situ formation of a C,P-palladacycle on complexation with Pd(OAc)₂.^5j,28 Accordingly, CM-Phos was also subjected to the formylation conditions, however poor yields of the desired product was obtained (the major product obtained was hydro-dehalogenated nitrobenzene, Table 1, entry 16).

Based on these results we further investigated the effect of temperature on the reactivity of the palladacyclic complex 3 to catalyse the above transformation (Table 2). Initially, at a lower temperature of 100 °C complete recovery of starting material was observed (Table 2, entry 1). This suggests that the energy required for the activation of the palladacyclic complexes to efficiently catalyse the reaction cannot be achieved at this temperature. Increasing the temperature to either 150 or 170 °C resulted in drastic reduction in yields of the desired product, although this might be due to the degradation of the product formed under elevated temperature conditions (Table 2, entries 3 and 4, respectively). It is therefore evident that the optimum temperature at which the palladacyclic complex 3 efficiently catalyzes the formation of aromatic aldehydes is 140 °C.

Table 2 Effect of temperature on formylation of aryl chlorides^a

Entry	T/°C	Yield^b (%)
a General procedure: aryl chloride 6a (1 mmol), 7 (10 mL mmol^−1), complex 3 (2.5 mol%), POCl3 (2.0 mmol), 48 h under nitrogen.b Isolated yields (product aldehydes confirmed by ^1H and ^13C NMR).
1	100	0
2	140	68
3	150	42
4	170	34

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Next we turned our attention towards understanding the effect of catalyst concentration on the catalytic efficiency of palladacyclic complex 3. Catalyst loading experiments were performed to determine the overall reactivity of the complex by varying its concentration in the reaction and is generally expressed in terms of turnover number (TON = mol product/mol catalyst). Initial studies were carried out at 2.5 mol% catalyst concentration (5.0 mol% Pd) furnishing selectively the aldehyde product in 68% yield (Table 3, entry 1, TON = 27.2). A higher concentration of Pd (10.0 mol% Pd, Table 3, entry 2) led to lower yields of the formylated product (58%, TON = 11.6). Lowering of catalyst concentration to 1.25 mol% (2.5 mol% Pd, Table 3, entry 3, 39%, TON = 31.2) or 1.00 mol% (2.0 mol% Pd, Table 3, entry 4, 32%, TON = 32) resulted in drastic reduction in the product yield and selectivity. Although the turnover numbers for lower catalyst loading values (1.0 mol% and 1.25 mol%) are slightly higher than for 2.5 mol% (entry 1) the overall yield of the product is better at 2.5 mol%. Accordingly, 2.5 mol% catalyst concentration of the palladacycle 3 was selected as the optimum concentration for obtaining higher selectivity towards the formation of the aldehyde product.

Table 3 Catalyst loading experiments^a

Entry	Complex 3 (mol%)	Yield^b (%)	TON^c
a General procedure: aryl chloride 6a (1 mmol), 7 (10 mL mmol^−1), complex 3, POCl3 (2.0 mmol), 140 °C, 48 h under nitrogen.b Isolated yields (product aldehydes confirmed by ^1H and ^13C NMR).c TON = mol product/mol catalyst.
1	2.5	68	27.2
2	5.0	58	11.6
3	1.25	39	31.2
4	1.0	32	32

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With the optimized conditions in hand, we probed the most active palladacyclic complex 3 for the formylation of further activated and unactivated aryl chlorides and bromides. Electron-withdrawing substituents such as 4-F (Table 4, entry 2), 4-CF₃ (Table 4, entry 3) or 3-NO₂ (Table 4, entry 4) on the aryl chloride moiety exhibited improved product yields while the unactivated chlorobenzene (Table 4, entry 5) also furnished good yield of the corresponding aldehyde. Although, enhancement in reactivity was observed for different activated (electron-attracting substituents) aryl chlorides, the same was not true for a deactivated aryl chloride such as 4-chloroanisole that gave poor yield of the desired product (Table 4, entry 6). When 1,4-dichlorobenzene was employed as substrate, a regioselectivity of 3 : 1 was observed for the formation of 4-chlorobenzaldehyde as compared to 1,4-benzene dicarbaldehyde (Table 4, entry 7).

Table 4 Palladium-catalyzed synthesis of substituted benzaldehydes from aryl chlorides^a

Entry	Aryl halide	Product	Yield^b (%)
a General procedure: aryl halide 6 (1 mmol), 7 (10 mL mmol^−1), complex 3 (2.5 mol%), POCl3 (2.0 mmol), 140 °C, 48 h under nitrogen.b Isolated yields (product aldehydes confirmed by ^1H and ^13C NMR).c 16% of 1,4-benzene dicarbaldehyde also obtained.
1			68
2			64
3			69
4			74
5			72
6			31
7			43^c
8			82
9			—

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Aryl bromides when employed as substrates also gave high yield of the selective aromatic aldehyde exhibiting the generality of procedure towards the formylation of different aryl halides. To increase the viability of the developed protocol a more challenging ortho-substituted aryl bromide was subjected to formylation conditions. Surprisingly, the formylation reactions did not furnish any product suggesting a limitation towards sterically hindered substrates.

Suggested mechanism for palladium-catalyzed formylation of activated aryl chlorides

In case of the C,P-palladacyclic complexes the catalytically active species has been suggested to be a monoligated Pd(0) species [PdP(o-Tol)₃].²⁹ This could be generated under our specific conditions via reduction with solvent, in this case formamide.^19,30 The C,P-palladacycles, as lately accepted also for other palladacycles, could be therefore considered as reservoirs of monoligated Pd(0) species and are expected to be in equilibrium with the monomeric I. Preliminary electrospray ionization studies carried out on the palladacyclic complex 3 in formamide solution also suggest the formation of the monoligated [PdP(o-Tol)₃] I as the major species, although the fact that 2–5 produce such a fragment at 409 m/z on its own (see Experimental section) indicates that further experiments are needed to ascertain this observation. Additional peaks at m/z 444 and 556 detected once the reaction takes place makes the following mechanism plausible. The first step of the suggested catalytic cycle (Fig. 2) would involve the oxidative addition of the aryl chloride (6e) with the monomeric palladium(0) species I resulting in the formation of a 14-electron monoligated species II. Transmetallation with the in situ generated Vilsmeier reagent A would give intermediate III. Reductive elimination will then generate the imine product which on hydrolysis would give the aldehyde 9e. The catalytically active monoligated species I would then be regenerated by the loss of HCl from species IV, thus re-entering the catalytic cycle.

Fig. 2 Proposed mechanistic pathway for the palladium-catalyzed formylation of activated aryl chlorides.

Conclusions

By means of simple acid–base reaction with imidate ligands new phosphapalladacyclic complexes with interesting catalytic properties have been prepared. Extensive characterization has been carried out, including X-ray single-crystal analysis. Their application in the efficient and selective formylation of activated aryl chlorides has been explored and presented in this report.

Experimental section

General remarks

C, H and N analyses were carried out with a Carlo Erba instrument. IR spectra were recorded on a Perkin-Elmer spectrophotometer 16F PC FT-IR, using Nujol mulls between polyethylene sheets. NMR data (¹H) were recorded on Bruker 200, 300 or 400 spectrometers. Mass spectrometric analyses were performed on a Fisons VG Autospec double-focusing spectrometer, operated in positive mode. Ions were produced by fast atom bombardment (FAB) with a beam of 25-keV Cs atoms. The mass spectrometer was operated with an accelerating voltage of 8 kV and a resolution of at least 1000. The cyclometallated precursor [{Pd(μ-AcO)(CˆP)}₂] 1 was prepared as described previously.^4c The commercially available chemicals were purchased from Aldrich Chemical Co. and were used without further purification, and all the solvents were dried by standard methods before use.

Synthesis

Preparation of complexes [{Pd(μ-NCO)(CˆP)}₂] (CˆP = CH₂C₆H₄P(o-tolyl)₂; –NCO– = succinimidate (suc) 2, maleimidate (mal) 3, phthalimidate (phthal) 4 or saccharinate (sacc) 5. The new complexes were obtained by treating an acetone suspension (20 mL) of the precursor complex 1 (0.300 g) with the corresponding protic ligand (molar ratio 1 : 2). A clear solution was immediately obtained, and it was stirred at reflux temperature for 1 h. Then it was concentrated under reduced pressure until ca. one fifth of the initial volume. Slow addition of diethyl ether caused the formation of the complexes, which were filtered off, washed with diethyl ether and air-dried. The compounds were recrystallised from dichloromethane-hexane.
[{Pd(μ-suc)(CˆP)}₂] 2. (0.243 g, 75%). Anal. Calc. for C₅₀H₄₈N₂O₄P₂Pd₂: C 59.1; H 4.7; N 2.7. Found: C 59.3; H 4.9; N 2.8%. IR (cm⁻¹): 1724s, 1600 vs, 758 s, 584 s, 526 s, 466 s, 456 s. ¹H NMR (300 MHz; CD₃CN, 298 K, δ): 7.20–6.70 (m, br, 24 H, arom), 3.32 (s, br, 4 H, CH₂ benzyl), 2.67 (s, br, 12 H, CH₃ benzyl), 2.45 (s, br, 8 H, CH₂ suc). ³¹P NMR (300 MHz; CD₃CN, 298 K, δ): 34.9. FAB-MS (positive mode) m/z: 918 [{Pd(CˆP)}₂(suc)], 713 [Pd(CˆP)₂]⁺, 508 [{Pd(suc)(CˆP)}]⁺, 409 [Pd(CˆP)]⁺.
[{Pd(μ-mal)(CˆP)}₂] 3. (0.223 g, 69%). Anal. Calc. for C₅₀H₄₄N₂O₄P₂Pd₂: C 59.3; H 4.4; N 2.7. Found: C 59.4; H 4.5; N 2.9%. IR (cm⁻¹): 1724 s, 1614 vs, 1586 s, 756 s, 584 s, 526 s, 468 s, 460 s. ¹H NMR (300 MHz; CD₃CN, 298 K, δ): 7.00–6.64 (m, br, 24 H, arom), 6.49 (s, 4 H, mal), 3.41 (s, br, 4 H, CH₂ benzyl), 2.68 (s, br, 12 H, CH₃ benzyl). ³¹P NMR (300 MHz; CD₃CN, 298 K, δ): 35.5. FAB-MS (positive mode) m/z: 916 [{Pd(CˆP)}₂(mal)], 504 [{Pd(mal)(CˆP)}]⁺, 409 [Pd(CˆP)]⁺.
[{Pd(μ-phthal)(CˆP)}₂] 4. (0.249 g, 70%). Anal. Calc. for C₅₈H₄₈N₂O₄P₂Pd₂: C 62.7; H 4.3; N 2.5. Found: C 62.8; H 4.4; N 2.7%. IR (cm⁻¹): 1728 s, 1632 vs, 1620 s, 758 s, 586 s, 526 s, 480 s, 448 s. ¹H NMR (300 MHz; CD₃CN, 298 K, δ): 7.31–6.76 (m, br, 32 H, arom), 3.41 (s, br, 4 H, CH₂ benzyl), 2.73 (s, br, 12 H, CH₃ benzyl). ³¹P NMR (300 MHz; CD₃CN, 298 K, δ): 35.5. FAB-MS (positive mode) m/z: 966 [{Pd(CˆP)}₂(phthal)], 556 [{Pd(phthal)(CˆP)}]⁺, 409 [Pd(CˆP)]⁺.
[{Pd(μ-sacc)(CˆP)}₂] 5. (0.306 g, 81%). Anal. Calc. for C₅₆H₄₈N₂O₆P₂S₂Pd₂: C 56.8; H 4.1; N 2.4. Found: C 57.0; H 4.3; N 2.5%. IR (cm⁻¹): 1719 s, 1627 vs, 1583 s, 755 s, 594 s, 528 s, 470 s, 453 s.¹H NMR (300 MHz; CD₃CN, 298 K, δ): 7.30–6.61 (m, br, 32 H, arom), 3.50 (s, br, 4 H, CH₂ benzyl), 2.73 (s, br, 12 H, CH₃ benzyl). ³¹P NMR (300 MHz; CD₃CN, 298 K, δ): 35.3. FAB-MS (positive mode) m/z: 1000 [{Pd(CˆP)}₂(sacc)], 591 [{Pd(sacc)(CˆP)}]⁺, 409 [Pd(CˆP)]⁺.

Representative procedure for the palladium(II)-catalyzed formylation of activated aryl chlorides. A mixture of p-chloronitrobenzene (1 mmol) and Pd-dimer 3 (2.5 mol%) was placed in a Schlenk tube. 10 mL mmol⁻¹ (aryl chloride) of formamide was injected into the Schlenk tube using a 10 mL syringe following which was injected POCl₃ (2 mmol) via a syringe. The resultant solution was then stirred at 140 °C for 48 h. After completion, the reaction mixture was cooled to room temperature (checked by TLC) and was poured into a saturated solution of NaHCO₃ (50 mL). The product was extracted with dichloromethane (3 × 10 mL). The combined organic layers were dried over Na₂SO₄ and evaporated to afford the crude product which was then purified by column chromatography on silica gel (using petroleum ether/ethyl acetate combination) to afford the pure product in a pale yellow solid.
4-Nitrobenzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.20 (s, 1 H), 8.48 (d, J = 7.6 Hz, 2 H), 8.18 (d, J = 7.6 Hz, 2 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 192.3, 140.0, 130.6, 124.2, 122.3; MS (EI) m/z (relative intensity) 151 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
4-Fluorobenzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.00 (s, 1 H), 7.94 (td, J = 5.7, 2.1 Hz, 2 H), 7.19 (t, J = 8.3 Hz, 2 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 191.2, 167.4, 133.4, 132.2, 116.6; MS (EI) m/z (relative intensity) 125 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
4-Trifluoromethylbenzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.13 (s, 1 H), 8.08 (d, J = 8.1 Hz, 2 H), 7.72 (d, J = 7.9 Hz, 2 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 191.7, 138.7, 133.8, 129.8, 126.0, 125.8; MS (EI) m/z (relative intensity) 174 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
3-Nitrobenzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.26 (s, 1 H), 8.14–8.09 (m, 1 H), 7.88–7.83 (m, 3 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 190.3, 149.6, 134.7, 134.6, 131.0, 130.3, 124.8; MS (EI) m/z (relative intensity) 151 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
Benzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.03 (s, 1 H), 7.98 (d, J = 7.0 Hz, 2 H), 7.65 (d, J = 7.3 Hz, 1 H), 7.56 (t, J = 7.5 Hz, 2 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 193.2, 136.5, 136.1, 129.7, 129.2; MS (EI) m/z (relative intensity) 106 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
4-Methoxybenzaldehyde. ¹H NMR (300 MHz, CDCl₃): δ 10.06 (s, 1 H), 8.01 (d, J = 8.2 Hz, 2 H), 7.12 (d, J = 8.2 Hz, 2 H), 4.03 (s, 3 H); ¹³C NMR (75 MHz, CDCl₃): δ 190.8, 164.6, 132.0, 130.0, 114.3, 55.6; MS (EI) m/z (relative intensity) 136 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³¹
4-Chlorobenzaldehyde. ¹H NMR (300 MHz, DMSO-d₆): δ 10.14 (s, 1 H), 8.11 (d, J = 8.0 Hz, 2 H), 7.69 (d, J = 8.0 Hz, 2 H); ¹³C NMR (75 MHz, DMSO-d₆): δ 192.6, 139.8, 135.3, 130.8, 129.2; MS (EI) m/z (relative intensity) 140 (100) [M⁺]. The spectral data were in accordance with those reported in the literature.³²

Crystal structure determination of 2. Data collection for 2 were obtained at 100(2) K on a Bruker Smart CCD diffractometer with a nominal crystal to detector distance of 4.5 cm. Diffraction data were collected based on a ω scan run. A total of 2524 frames were collected at 0.3° intervals and 10 s per frame. The diffraction frames were integrated using the SAINT package³³ and corrected for absorption with SADABS.³⁴ The structures were solved by direct methods³⁵ and refined by full-matrix least-squares techniques using anisotropic thermal parameters for non-H atoms (Table 5).

Table 5 Crystal data and structure refinement for complex 2·(CH₃)₂CO

Empirical formula	C53H54N2O5P2Pd2
M	1073.72
T/K	100(2)
λ/Å	0.71073
Crystal system	Triclinic
Space group	P1̄
a/Å	13.2632(10)
b/Å	13.7584(10)
c/Å	15.9030(12)
α/°	102.9940(10)
β/°	92.4610(10)
γ/°	113.4320(10)
V/Å^3	2565.9(3)
Z	2
Dc/Mg m^−3	1.390
μ/mm^−1	0.809
F(000)	1096
θ range for data collection/°	1.33–28.63
Reflections collected	31 383
Independent reflections	12 002
Data/parameters/restrains	12002/0/562
Goodness-of-fit on F^2	1.082
Final R indices [I > 2σ(I)]	R1 = 0.0518, wR2 = 0.1702
R indices (all data)	R1 = 0.0562, wR2 = 0.1764
Max./min. Δρ/e Å^−3	3.165, −1.019

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Acknowledgements

We thank Fundación Séneca (project 08670/PI/08) for financial support and Department of Science and Technology-India for a Fast Track fellowship to A. R. K. (SR/FT/CS-58/2010). This work was partially supported by CICYT-MICINN (CTQ 2011-28903-C02).

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Footnote

† CCDC reference number 895458. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2ra21770h

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