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Chelating and bridging bis(diphenylphosphino)aniline complexes of copper(I)
Inorganica Chimica Acta 372 (2011) 220–226
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Chelating and bridging bis(diphenylphosphino)aniline complexes of copper(I) Ritu Ahuja, Munirathinam Nethaji, Ashoka G. Samuelson ⇑ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Available online 16 March 2011 Dedicated to S.S. Krishnamurthy. Keywords: Binding mode Copper(I) Diphosphinoamine Nuclearity Short bite ligands
a b s t r a c t The ligand bis(diphenylphosphino)aniline (dppan) has been shown to be a versatile ligand sporting different coordination modes and geometries as dictated by copper(I) and the counter ion. The molecular structures of its Cu(I) complexes were characterized by X-ray crystallography. The ligand was found in a chelating mode and monomeric complexes were formed when the ligand to copper ratio was 2:1 and the anion was non-coordinating. However, with thiocyanate as the counter anion, the ligand was found to adopt two different modes, with one ligand chelating and the other acting as a monodentate ligand. With CuX (X = Cl, Br), dppan formed a tetrameric complex when the ligand and metal were reacted in the ratio of 1:1. But reactions containing ligand and metal in the ratios of 1:2 or 2:1, resulted in the formation of a mixture of species in solution. Crystallization however, led to the isolation of the tetrameric complex. Variable temperature 31P{1H} NMR spectra of the isolated tetramers did not show the presence of chelated structures in solution. Tetra-alkylammonium salts were added to solutions of various complexes of dppan and studied by 31P{1H} NMR to probe the effect of anions on the stability of complexes in solution. The Cu–dppan complexes were robust and did not interconvert with other structures in solution unlike the bis(diphenylphosphino)isopropylamine complexes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tertiary phosphines stabilize low valent transition metal complexes and are of great value in catalysis due to their tunable steric and p-acceptor properties [1]. Although organometallic compounds with P–C–P ligands are extremely well studied in the last three decades [2], interest in analogous diphosphinoamine ligands (Fig. 1) has picked up only recently [3]. Diphosphinoamine ligands are as versatile as P–C–P ligands as the substitutents on phosphorus and nitrogen can be readily varied resulting in significant changes in P–N–P bond angles [4]. Dramatic changes in their coordination behavior, structural features of the resulting complexes, and catalytic properties have been realized [5–7]. The coordination behavior of (PX2)2NR is predominantly bidentate ligation either through chelation or bridging of metal atoms, though the P lone pairs. Unlike the central carbon in P–C–P ligands, a planar N is at the centre of the P–N–P ligand. The PF2 derivatives of these ligands have been extensively studied by King [8], Johnson and Nixon [9], and Cotton et al. [10] due to the excellent p accepting property of P in these ligands. Diphosphazane ligands have been studied extensively for other reasons as well: they have a short bite resulting in multinuclear transition metal complexes which have very short metal-metal contacts [11,12]. Studies on dppan with 3d, 4d and 5d transition metal ions are available [13–15] but with copper(I), only NMR studies have been ⇑ Corresponding author. Tel.: +91 080 2293 2663; fax: +91 080 2360 1552. E-mail address: [email protected] (A.G. Samuelson). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.03.035
reported [16] and structural studies are lacking. The utility of these complexes as catalysts in the cyclopropanation of olefins was the subject of a recent article [17]. The present report discusses the coordination behavior of (PPh2)2NPh (dppan) with copper(I) in solution and in the solid state through NMR and X-ray crystallographic studies, respectively. Unlike silver(I) which prefers low coordination numbers [18], copper(I) prefers four coordination. A combination of two factors, the diphosphinoamine to copper ratio, and the nature of the substituent on N, control the nuclearity and structure of the copper complexes in the solid state. The solid state and solution behavior for the dppan ligand system has also been compared with that of the recently reported bis(diphenylphosphino)isopropylamine (dppipa) complexes of copper(I) [19]. Fig. 2 summarizes the different complexes isolated in this study. 2. Results and discussion 2.1. Copper(I) complexes of dppan Complexes of dppan were readily synthesized by reacting it with various Cu(I) precursors in different ratios. The following subsections describe the complexes obtained and their molecular structure. 2.2. Complex 1: [Cu(dppan)2]ClO4 The reaction of dppan and [Cu(CH3CN)4]ClO4 in the ratio of 2:1, resulted in a bis chelated monomer which was isostructural to the
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of binding [20]. Copper was found to be in a distorted tetrahedral environment with four coordination sites occupied by three phosphorus donors and one hard N donor from the thiocyanate ion. The Cu–N distance was 1.980(3) Å. This structure was very different from what was obtained in the case of the dppipa ligand which was a dimer with the thiocyanate bound via nitrogen. The monomeric complex here behaves differently probably due to the higher p-acceptance of the dppan ligand that stabilizes the CuP3N core. As a result the other ligand is bound through only one of its arms and remains monodentate. A structural motif of this kind with monodentate thiocyanate ion is also found in other phosphine–copper(I) complexes, bearing triphenylphosphine-phenanthroline [21], 4,6-dimethylpyrimidine2(1H)-thione-S [22], bis(diphenylphosphino)methane (dppm) [23] and bis(diphenylphosphino)ethane (dppe) [24].
Fig. 1. P–N–P framework.
analogous dppipa complex published recently [19]. The ORTEP plot and the selected bond lengths and bond angles are given in the Fig. 3 and Table 1, respectively. The copper ion, bound to two chelating ligands result in a distorted tetrahedral geometry. As a result, the P–N–P angle was reduced to 106° on chelation with the metal, whereas it was 122.8(3)° in the free ligand. But interestingly, there was no significant change in the P–N bond length on complexation (P–N bond length in free ligand are 1.706(4) Å and 1.711(4) Å).
2.4. Complex 3: [Cu4(dppan)2Cl4] Reactions with CuCl were attempted with L:M ratios of 1:2 or 2:1, and the 31P{1H} NMR spectrum in CDCl3 indicated the formation of a mixture of complexes. Strangely, another reaction attempted with a L:M ratio of 1:1 showed a single peak in the 31 1 P{ H} NMR and the complex obtained on crystallization was found to be a tetramer. Two of the ligands were bound to four Cu centers in a bridging fashion and four chloride bridges completed the core structure. The complex crystallized in a centrosymmetric space group and had all four copper atoms in the same plane with two l3-chloride bridges. This complex was isostructural to the analogous dppipa complex. Although the Cu–Cu distances in the dppipa complex were shorter (Cu–Cu distance ranges from 2.641(2) to 2.784(2) Å) than what was found in 3. Fig. 5 and Table 3 give the ORTEP plot of the complex and selected metric parameters, respectively.
2.3. Complex 2: [Cu(dppan)(1-dppan)(NCS)] The reaction between the ligand and copper(I) thiocyanate in the ratio of 2:1 in dichloromethane resulted in an insoluble material. But when the same reaction was attempted under similar conditions in the solvent THF, a clear yellow solution was formed. Slow diffusion of petroleum ether into the THF solution of the complex afforded bright yellow crystals that were identified to be a monomeric complex. There was one ligand bound to copper in a chelating mode and the other was bound in a g1-fashion. The ORTEP plot is given in Fig. 4 and selected bond lengths and bond angles in Table 2. A half sandwich nickel complex of the same ligand had been reported by Kuhn and Winter to have a monodentate mode
3
4
S
C
PPh2 N
Cu P Ph2
P Ph2
P Ph2
N
N N
Cu
N
Ph2 P
Ph2 P
Ph2 P
P Ph2
4
Ph2P
N
PPh2
Cl Ph2P
Br Cu
Cu Cl Cl
N P Ph2
Ph2P
N Cu
Cu
PPh2
Cu
N
PPh2
P Ph2
Cl
+ Ph2 P
I-
Ph2 P N
N Cu I Ph2P
I
Cu
PPh2
Cu
Ph2P
PPh2 N
Fig. 2. Reactivity of dppan with Cu(X).
Cu Br
N
Br Cu
Cu Br
PPh2
PPh2
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Fig. 3. ORTEP view of complex 1 at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 1 Selected bond lengths (Å) and bond angles (°) of complex 1. Bond lengths (Å) Cu1–P1 Cu1–P2 Cu1–P3 Cu1–P4
2.2853(19) 2.291(2) 2.3040(19) 2.2886(19)
P1–N1 P2–N1 P3–N2 P4–N2
1.707(5) 1.708(5) 1.701(5) 1.726(5)
Bond angles (°) P1–Cu1–P2 P1–Cu1–P4 P2–Cu1–P3
73.28(7) 132.72(8) 131.12(7)
P2–Cu1–P4 P1–N1–P2 P3–N2–P4
128.24(7) 106.2(3) 106.8(3)
Fig. 4. ORTEP view of complex 2; [Cu(dppan)(g1-dppan)(NCS)] at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 2 Selected bond lengths and bond angles in complex 2. Bond lengths (Å) Cu1–P1 Cu1–P2 Cu1–P3 Cu1–N3
2.336(1) 2.354(10) 2.265(1) 1.980(3)
P1–N1 P2–N1 P3–N2 P4–N2
1.718(3) 1.713(3) 1.712(3) 1.740(3)
Bond angles (°) P1–Cu1–P2 N3–Cu1–P1 N3–Cu1–P3
72.45(3) 114.83(9) 107.25(9)
P2–Cu1–P3 P1–N1–P2 P3–N2–P4
120.39(3) 107.76(15) 119.00(16)
The role of halide bridges in tuning the Cu–Cu distances has been elegantly demonstrated in a recent paper by Knight et al. [25]. Obviously, the copper-copper distances are affected by both ligands and this is amply illustrated by the work of Ellermann et al. [26] and Bera et al. [27]. It has also been shown that shorter distances can arise when there is unsymmetric coordination [28].
2.5. Complex 4: [Cu4(dppan)2Br4] Analogous to CuCl, CuBr also formed a tetrameric core (Fig. 6). But there were differences in the Cu–Br distances observed within
the complex (Table 4). In the case of the chloride complex, two chloride ions were found to be l2-bridges and two were in l3 mode. In the present case, two of the bromide ions form l2-bridges with copper, but two of them were found to be in l4-mode. However, the Cu–Br distances were quite unsymmetrical and only the shorter distances are shown bonded in the ORTEP plot. The analogous bromide tetramer of dppm ligand had a Cu–Cu distance of 3.189 Å for dppm bridged copper atoms [29,30] which was longer by almost 0.41 Å compared to the Cu–Cu distance in the PNP bridged copper centers of the diphosphinoamine complex. There was an increase of almost 4.4° in the P–N–P bond angle (120.51(13)°) on forming a bridge with two metal centers in the case of the dppan ligand. In the case of the dppan ligand, the two PPh2 units could probably bend back further due to the orthogonal disposition of the phenyl ring on the nitrogen resulting in widening of the PNP bond angle. In spite of this, the ligand has a short bite with a short Cu–Cu distance of 2.786(1) Å for the phosphine bridged centers. In the analogous dppm complex, [19] the phosphine bridged Cu-Cu distance was longer by almost 0.12 Å (2.905 Å). When chloride and bromide complexes are compared, one finds that there is a reversal of ordering of phosphine bridged and halide bridged Cu–Cu distances. The iodo complex can be synthesized in a similar fashion and is a trimeric species. The complex did not give satisfactory elemental analysis, but structural details obtained from X-ray diffraction study has been included in the Supplementary material. In the case of chloride, in both dppm and in PNP ligands, Cu–Cu distances bridged by phosphine were shorter than the Cu–Cu distances bridged by chloride. However, in the case of the bromide complex, the Cu–Cu distance was longer when it was phosphine bridged [30]. This was clearly a counterintuitive result and reminiscent of anion control in Cu–Cu distances [22]. Recently Knight et al have however reported a system where the Cu– Cu distances follow the sizes of the bridging atoms and forms an interesting contrast. In all the dppan complexes described above, the phenyl ring on the central nitrogen of the dppan ligand was found to lie orthogonal to the P–N–P plane. For instance, in complex 1, the dihedral angle between P1–N1–P2 plane and the plane defined by C49–C54 carbon atoms (phenyl ring) was around 80.6°. The orthogonal disposition of the phenyl ring permits a larger P–P distance and better packing due to interdigitization of phenyl rings in the lattice (Fig. S1 in Supplementary information).
2.6. Variable temperature
31
P{1H} NMR studies
The relative stability of chelating and bridging modes could be gauged by examining the nature of these complexes in solution using variable temperature 31P{1H} NMR experiments. This is important since the structures seen in the solid state could be attributed to selective crystallization of one of the forms. Notably, the Cu–P bond was labile in the case of previously examined dppipa complexes and in solution it generated a set of two doublets similar to that observed for the Cs conformer of dppipa. Invariably the complexes having a chelating phosphine, were unstable in solution and tended to go to the more stable bridging geometry. However complexes with a bridging ligand retained their structure in solution and did not equilibrate with structures having a chelated ligand so easily. In the case of dppan complexes, one could not find analogous equilibria in solution, as there were no significant changes in the 31 P NMR when the temperature was varied. These subtle differences between the two ligands suggests, that in solution, the Cu(I) dppan complexes are more stable than the corresponding dppipa complexes. This could be attributed to higher stability of
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Fig. 5.
ORTEP
view of complex 3 at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 3 Selected bond lengths (Å) and bond angles (°) in complex 3.
a b
Table 4 Selected bond lengths and bond angles in complex 4.
Bond lengths (Å) Cu1–Cu2 Cu1–Cu2_3 Cu1–P1 Cu1–Cl1 Cu1–Cl2 Cu2–P2
2.7861(9)a 2.8892(8)b 2.1727(9) 2.3826(9)a 2.2572(9)b 2.1797(10)
Cu2–Cl1 Cu2–Cl1_3 Cu2–Cl2_3 P1–N1 P2–N1
2.3871(9) 2.5611(10) 2.3439(10) 1.706(2) 1.715(2)
Bond angles (°) P1–Cu1–Cl1 P1–Cu1–Cl2 P1–Cu1–Cu2 Cl1–Cu1–Cl2 P2–Cu2–Cl1
120.32(3) 136.11(3) 92.52(3) 100.67(4) 132.37(3)
P2–Cu2–Cl2_3 Cl1–Cu2–P2 P2–Cu2–Cu1 Cl1–Cu2–Cl2_3 P1–N1–P2
120.60(4) 132.37(3) 89.69(3) 93.39(3) 120.56(13)
Chloride bridged. Phosphine bridged.
Cu–P bond in the case of dppan complexes due to the greater paccepting character of the dppan ligand. 2.7. Interconversion of complexes In order to find the possibility of inter-converting complexes having the same ratio of ligand/metal, we carried out 31P{1H} NMR experiments with different counterions. A known concentration of the complex was titrated with a different anion using the appropriate tetra-alkylammonium salt in CDCl3. Unlike dppipa complexes reported earlier, the exchange of counter-ions was quite sluggish as seen in the 31P NMR in case of Cu–dppan complexes. The perchlorate complex 1 reacted with the halide ion to form the corresponding halide tetramers. With cyanide, as seen in the case of dppipa complex, free dppan was observed (Figs. S2 and S3). The halide tetramers 3 and 4 on addition of a different halide form mixed clusters in very small amounts.
Fig. 6.
ORTEP
a b
Bond lengths (Å) Cu1–Cu2 Cu1–Cu2_3 Cu1–P1 Cu1–Br1 Cu1–Br2 Cu1–Br2_3
2.677(2)a 2.747(2)b 2.176(2) 2.364(2) 2.744(2) 2.760(2)
Cu2–P2 Cu2–Br1 Cu2–Br2 Cu2–Br2_3 P1–N1 P2_3–N1
2.179(3) 2.442(2) 2.516(2) 2.720(2) 1.721(7) 1.691(7)
Bond Angles (°) P1–Cu1–Br1 P1–Cu1–Cu2_3 P2–Cu2–Br1 P2–Cu2–Br2 Br1–Cu2–Br2
126.12(10) 91.21(8) 116.57(9) 130.28(9) 100.24(6)
Br1–Cu2–Cu1_3 Br2–Cu2–Cu1_3 Cu1_3–Cu2–P2 P1–N1–P2_3
150.21(6) 63.07(5) 91.99(8) 120.1(4)
Bromide bridged. Phosphine bridged.
But with addition of cyanide, the free PNP ligand was obtained. unlike dppipa complexes wherein a similar reaction gives the bis chelated complex. Slow exchange with dppan complexes indicates higher stability of Cu–P bonds in these clusters in solution in comparison with the analogous dppipa complexes.
3. Summary and conclusions Dppan ligand like many diphosphinoamines is quite a flexible ligand and capable of bridging and chelating copper with ease. The dppan ligand formed a totally different structural core in the presence of the thiocyanate ion, while in the presence of oxyanion and halide ions, the structures were similar to those obtained in the case of dppipa. The variable temperature 31P{1H} NMR showed no significant changes in the case of dppan complexes. This clearly proved that Cu(I) dppan complexes were more stable in solution in
view of complex 4; [Cu4(dppan)2(Br)4] at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
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comparison with the analogous dppipa complexes suggestive of better Cu–P p-bonding in dppan complexes. The present work has brought out the ability of diphosphinoamines to adopt different binding modes with copper(I) in the presence of different anions. By tuning the ligand to metal ratio, a variety of Cu(I) complexes with different nuclearity could be synthesized. With electronically different substituents (iPr and Ph) on the nitrogen atom of PNP ligand, very similar structural motifs were obtained in most cases. However in solution, the electronic differences were clearly manifested in the lability of dppipa relative to dppan complexes as the latter is a better p acceptor. 4. Experimental section 4.1. General Dichloromethane, chloroform, petroleum ether (b.p. 60–80 °C), acetonitrile, tetrahydrofuran, methanol and benzene were purified and dried under nitrogen atmosphere by conventional methods [31]. All manipulations were carried out under an atmosphere of purified N2 using a standard double manifold and Schlenk ware. Bis(diphenylphosphino)aniline was synthesized by a reported procedure [32] with slight modifications. [Cu(CH3CN)4]ClO4, [33] CuSCN [34] and CuCl [35] and CuBr [36] were prepared by reported methods. CuI was purchased from Aldrich and used as supplied.
7 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain colorless crystals. (Yield: 86%). Anal. Calc. for CuC60H50N2P4ClO4.2CH2Cl2: C, 65.98; H, 4.58; N, 2.12. Found: C, 66.36; H, 4.61; N, 2.58%. 1H NMR: d 6.29 (2H, d, NPh), 6.92 (2H, t, NPh), 7.15 (1H, t, NPh), 7.18–7.52 (20H, m, PPh2). 31P{1H} NMR: d 87.7 (s). IR data (cm 1): ClO4 : 1096 (vs, br). 4.4.2. Reaction with CuSCN; Complex 2: [Cu(dppan)(1-dppan)(NCS)] Dppan (0.23 g, 0.5 mmol) was added to a solution of CuSCN (0.03 g, 0.25 mmol) in THF (15 ml) and stirred for 1 h at RT. Subsequently the resultant yellow colored solution was concentrated to about 7 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain yellow colored crystals. (Yield: 72%). Anal. Calc. for CuC61H50N3P4STHF3H2O: C, 66.24; H, 4.93; N, 3.11. Found: C, 66.69; H, 5.47; N, 3.59%. 1H NMR: d 6.23 (2H, d, NPh), 6.80 (2H, t, NPh), 6.98 (1H, t, NPh), 7.21 7.38 (20H, m, PPh2). 31P{1H} NMR: d 24.9 d (br), 23.5, 16.5. IR data (cm 1): SCN : 2055 (vs, sharp). 4.4.3. Reactions with CuCl; Complex 3: [Cu4(dppan)2Cl4] Synthesis of complex 3 was carried out using a similar procedure as for complex 1 starting with CuCl (0.025 g, 0.25 mmol) and varying the amount of dppan according to the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the pure complex.
4.2. Physical measurements 1
H NMR spectra were recorded either on a Bruker ACF 200 MHz or AMX 400 MHz spectrometer with tetramethylsilane (TMS) as the internal reference. 31P{1H} NMR spectra were recorded either on a Bruker AMX 400 MHz spectrometer operating at 162.2 MHz or a Bruker ACF 200 MHz operating at 81.1 MHz with H3PO4 (85%) as the external reference. Variable temperature 31P{1H} NMR were recorded in the temperature range 20 °C and 60 °C. The complex was dissolved in CH2Cl2 and 31P NMR was recorded with acetone-d6 as the external lock. IR spectra were recorded in the solid state as KBr pellets on a Bruker Equinox 55 spectrometer. Elemental analyses were done with Carlo Erba Elemental Analyzer model 1106. Anion challenge experiments were carried out by titrating a known concentration of the complex in a NMR tube in CDCl3 with a CDCl3 solution of the tetraalkylammonium salt of the anion and followed by 31P{1H} NMR. 4.3. Preparation of dppan The procedure involved a dropwise addition of PPh2Cl (20 mmol, 3.59 ml) to aniline (0.91 ml, 10 mmol) and triethylamine (2.78 ml, 20 mmol) added to 30 ml of diethylether in a 100 ml double necked round bottom flask at 10 °C and stirring at RT for 4 h. Subsequently the solvent was evaporated and 30 ml of benzene was added. Then the resultant solution was filtered to remove the amine hydrochloride. The insoluble fraction was thoroughly washed with benzene and the filtrate evacuated under vacuum. The crude product was repeatedly washed with distilled methanol to get the pure ligand. (Yield: 85%). 1H NMR: d 6.64 (2H, m, NPh), 6.95 (3H, d, NPh), 7.30–7.38 (20H, m, PPh2). 31 1 P{ H} NMR: d 68.1 (s).
4.4.3.1. L:M = 1:1. Dppan (0.23 g, 0.5 mmol) was added to a CH2Cl2 solution (15 ml) containing CuCl (0.05 g, 0.5 mmol) and stirred for 1 h at RT. Subsequently the solution was concentrated to about 8 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain colorless crystals. (Yield: 85%). Anal. Calc. for Cu4Cl4C60H50N2P4.CH2Cl2: C, 52.45; H, 3.91; N, 2.35%. Found: C, 52.19); H, 3.73; N, 1.99%. 1H NMR: d 5.43 (2H, d, NPh), 6.46 (2H, t, NPh), 6.78 (1H, t, NPh), 6.90–7.24 (20H, PPh2). 31P{1H} NMR: d 59.05 (s). 4.4.3.2. L:M = 2:1. 1H NMR: d 5.43 (2H, d, NPh), 6.27 (4H, d, NPh), 6.45 (2H, t, NPh), 6.75 (1H, t, NPh), 6.92-7.46 (41H, m, NPh + PPh2). 31 1 P{ H} NMR: d 84.6 (br), 68.0, 58.9 (s). 4.4.3.3. L:M = 1:2. The reaction of CuCl and dppan in the ratio 2:1 resulted in a white colored powdery material, which was only sparingly soluble in chlorinated solvents. 1H NMR: d 5.44 (2H, d, NPh), 5.95 (1.5H, d, NPh), 6.45 (2H, t, NPh), 6.66 (1.5H, t, NPh), 6.78 (1H, t, NPh), 6.95-7.02 (15H, m, NPh + PPh2), 7.25–7.50 (20H, m, PPh2). 31P{1H} NMR: d 67.6 (br), 58.9 (s). This insoluble material was treated with excess ligand in CH2Cl2 to obtain a clear colorless solution. The complex crystallized from the above solution was confirmed to be the tetramer i.e. complex 3 through 31P NMR. 4.4.4. Reactions with CuBr Synthesis of complexes was carried out using a similar procedure as for complex 1 starting with CuBr (0.036 g, 0.25 mmol) and varying the amount of dppan according to the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the pure complex.
4.4. Synthesis of copper(I) complexes of dppan 4.4.1. Reaction with [Cu(CH3CN)4]ClO4; Complex 1: [Cu(dppan)2]ClO4 Dppan (0.23 g, 0.5 mmol) was added to a solution of [Cu(CH3CN)4]ClO4 (0.08 g, 0.25 mmol) in CH2Cl2 (15 ml) and stirred for 1 h at RT. Subsequently the solution was concentrated to about
4.4.4.1. L:M = 1:1. Complex 4: [Cu4(dppan)2Br4]: Faint yellow crystals. (Yield: 88%). Anal. Calc. for Cu4Br4C60H50N2P4: C, 48.37; H, 3.56; N, 2.12. Found: C, 48.15; H, 3.34; N, 1.87%. 1H NMR: d 5.43 (2H, d, NPh), 6.45 (2H, t, NPh), 6.80 (1H, t, NPh), 6.90–7.30 (m, 20H, PPh2). 31P{1H} NMR: d 59.0 (s).
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R. Ahuja et al. / Inorganica Chimica Acta 372 (2011) 220–226 Table 5 Crystallographic data for copper(I) dppan complexes.
Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) Crystal size (mm3) Reflections collected Independent reflections (Rint) Final R1a, wR2b [I > 2r(I)] Final R1a, wR2b (all data) Goodness-of-fit (GOF) on F2c a b c
1
2
3
4
C62H54Cl5CuN2O4P4 1255.74 monoclinic P21/n 10.832(3) 38.525(4) 14.440(3) 90 94.15(4) 90 6009.9(4) 4 1.388 0.52 0.32 0.26 7866 7866 (0.0421) R1 = 0.0687, wR2 = 0.1944 R1 = 0.1083, wR2 = 0.2185 1.056
C67H62CuN3P4S 1128.68 triclinic P1 10.311(2) 11.467(3) 13.644(3) 76.603(4) 74.800(4) 69.606(4) 1441.6(6) 1 1.300 0.59 0.31 0.27 16863 12683 (0.0235) R1 = 0.0455, wR2 = 0.0921 R1 = 0.0596, wR2 = 0.0967 0.934
C31H27Cl4Cu2NP2 744.36 monoclinic P21/c 10.034(3) 14.971(4) 21.369(6) 90 100.132(5) 90 3160.2(15) 4 1.565 0.4 0.35 0.3 27023 7433 (0.0519) R1 = 0.0448, wR2 = 0.1187 R1 = 0.0655, wR2 = 0.1276 1.022
C31H27Br2Cl2Cu2N1P2 833.28 monoclinic P2(1)/c 10.260(2) 21.185(4) 15.106(3) 90 95.38(3) 90 3269.0(11) 4 1.693 0.18 0.14 0.12 27388 7564 (0.1314) R1 = 0.0815, wR2 = 0.1916 R1 = 0.1975, wR2 = 0.2317 1.147
P P R1 = ( ||Fo| |Fc||)/( |Fo|). P P wR2 = [ (w|Fo|2 |Fc|2)2/ w|Fo|2)2]1/2. GOF = [w(F 2o F 2c )2]/(n p)1/2.
The reactions with other M:L ratios gave mixture of compounds as seen in the case of reactions with CuCl. 4.4.5. Reactions with CuI Synthesis of complex 5 was carried out using a similar procedure as for complex 1 starting with CuI (0.048 g, 0.25 mmol) and varying the amount of dppan:Cu in the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the complex. Although a spectroscopically, and crystallographically characterizable complex was obtained in the 1:1 ratio, satisfactory elemental analysis could not be obtained and structural details might be obtained in the Supplementary information.
Fourier map using SHELXL-97 [42]. Hydrogen atoms were geometrically fixed in all the complexes. The crystallographic data of complexes 1–4 are summarized in Tables 1–4 and Table 5, respectively in the main article. In the case of complex 5, the quality of the data collected was not so good, as the crystal was very unstable. The solvent atoms that appeared in the difference Fourier map were highly disordered and were isotropically refined with constraints. After the final refinement in the case of complex 4, two residual peaks of electron density 1.77 and 1.68 e Å3 were located at 1.20 Å and 1.14 Å from Cu4 atom, respectively. In case of complex 5, the data was complete only upto 82% as the crystal was highly unstable and there was a constant decay in intensities acquired from the crystal. The details are given in the Supplementary information Table S1 And S2.
4.5. X-ray data collection, structure solution and refinement Crystals of complexes 3 and 4 were mounted with paraffin oil in Lindemann capillaries for data collection. Whereas a crystal of complex 2 was separately glued to the tip of a glass fiber along the largest dimension for indexing. Reasonably good crystals of complex 1 and 5 were mounted along with the mother liquor in Lindemann capillaries. Data for complexes 2, 3, 4 and 5 were collected on a Bruker AXS single crystal diffractometer equipped with SMART APEX CCD detector and a sealed Mo Ka source working at 1.75 kW. For complex 1, the data were collected on an Enraf-Nonius CAD4 single crystal diffractometer equipped with graphitemonochromatized Mo K radiation. Intensity data were collected at 293 K for all complexes except for complex 5, which was collected at 160 K. For the CAD4 data, accurate unit cell parameters and orientation matrices were determined by least-squares refinement of 25 well centered reflections in the range 9° 6 h 6 13°. Three periodically measured reference reflections showed no significant decay during the time of data collection for complex 2 and 3. Whereas for complex 1, the crystal was not stable during the data collection. Hence another suitable crystal was mounted to complete the data. All computations were performed using the WINGX package. [37] The data were corrected for Lorentz and polarization effects. Absorption correction for complex 1 was applied using PSI-SCAN [38] and for complex 1, DIFABS [39] was used. For complexes 2, 3 and 4, SADABS [40] was used for the absorption correction. The positions of heavy atoms were determined by SHELXS-86 [41]. The remaining atoms were located from the difference
Acknowledgements We wish to thank DST, New Delhi for the financial support and for funding the CCD facility at IISc, Bangalore. The assistance of M. Thenraj and R. Mitra in preparing the final manuscript is gratefully acknowledged. Appendix A. Supplementary material CCDC 252102, 252103, 252104, 252105 and 252106 contain the supplementary crystallographic data for complexes 1: [Cu(dppan)2]ClO4, 2: [Cu(dppan)2(NCS)], 3: [Cu4(dppan)2Cl4], 4: [Cu4(dppan)2Br4] and 5: [Cu3(dppan)3I2]I, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.03.035. References [1] L.H. Pignolet (Ed.), Homogenous Catalysis with Metal Phosphine Complexes, Plenum Press, New York, 1983. [2] B. Chaudret, B. Delavaux, R. Poilblane, Coord. Chem. Rev. 86 (1988) 191;. [3] M. Aydemir, A. Baysal, G. Ozturk, B. Gumgum, Appl. Organomet. Chem. 23 (2009) 108. [4] (a) G. Calabro, D. Drommi, G. Bruno, F. Faraone, Dalton Trans. (2004) 81; (b) H.-J. Chen, J.F. Barendt, R.C. Haltiwanger, T.G. Hill, A.D. Norman, Phosphorus, Sulfur Silicon Relat. Elem. 26 (1986) 155;
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[19] R. Ahuja, A.G. Samuelson, J. Organomet. Chem. 694 (2009) 1144. [20] N. Kuhn, M. Winter, J. Organomet. Chem. 238 (1982) C53. [21] A.E. Mauro, C.C. Porta, S.R. Ananias, V. Sargentelli, R.H.D.A. Santos, M.T. do P. Gambardella, J. Coord. Chem. 49 (1999) 9. [22] S.S. Lemos, M.A. Camargo, Z.Z. Cardoso, V.M. Deflon, F.H. Forsterling, A. Hagenbach, Polyhedron 20 (2001) 849. [23] A. Lorenz, D. Fenske, J. Anorg. Allg. Chem. 627 (2001) 2232. [24] D. Saravanabharathi, M.P. Venugopalan, A.G. Samuelson, Polyhedron 21 (2002) 2433. [25] F.R. Knight, A.L. Fuller, A.M.Z. Slawin, J.D. Woollins, Dalton Trans. (2009) 8476. [26] J. Ellermann, F.A. Knoch, K.J. Meier, Z. Naturforsch. Teil B. 45 (1990) 1657. [27] J.K. Bera, M. Nethaji, A.G. Samuelson, Inorg. Chem. 38 (1999) 218. [28] R. Ahuja, M. Nethaji, A.G. Samuelson, Polyhedron 26 (2007) 142. [29] G. Camus, L. Nardin, Randaccio, Inorg. Chim. Acta 12 (1975) 23. [30] S. Ramaprabhu, N. Amstutz, E.A.C. Lucken, G. Bernardinelli, J. Chem. Soc., Dalton Trans. (1993) 871. [31] D. Perin, W.L.F. Armarego, Purification of Laboratory Reagents, third ed., Pergamon Press, London, 1988. [32] M.S. Balakrishna, T.K. Prakasha, S.S. Krishnamurthy, U. Siriwardane, S.N. Hosmane, J. Organomet. Chem. 390 (1990) 203. [33] J. Hathaway, D.G. Holah, J.D. Postlethwaite, J. Chem. Soc., Dalton Trans. (1961) 3215. [34] G.H. Jeffery, J. Bassett, J. Mendham, R.C. Denney, Vogel’s Textbook of Quantitative Chemical Analysis, fifth ed., Longman UK Ltd., 1989, p. 495. [35] R.N. Keller, H.D. Wycoff, Inorg. Synth. 2 (1946) 1. [36] R.N. Keller, H.D. Wycoff, Inorg. Synth. 2 (1946) 3. [37] L.J. Farrugia, J. Appl. Crystallogr. 32 (1999) 837. [38] C.T. North, D.C. Phillips, F.S. Mathews, Acta. Crystallogr., Sect. A 24 (1968) 351. [39] N. Walker, D. Stuart, Acta Crystallogr., Sect. A 39 (1983) 158. [40] R.H. Blessing, Acta Crystallogr., Sect. A 51 (1995) 33. [41] G.M. Sheldrick, SHELXS86, A Computer Program for Crystal Structure Determination, University of Cambridge, Cambridge, England, 1986. [42] G.M. Sheldrick, SHELXL-97, A Computer Program for Crystal Structure Solution and Refinement, Universität of Göttingen, Göttingen, Germany, 1997.
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Chelating and bridging bis(diphenylphosphino)aniline complexes of copper(I) Ritu Ahuja, Munirathinam Nethaji, Ashoka G. Samuelson ⇑ Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Available online 16 March 2011 Dedicated to S.S. Krishnamurthy. Keywords: Binding mode Copper(I) Diphosphinoamine Nuclearity Short bite ligands
a b s t r a c t The ligand bis(diphenylphosphino)aniline (dppan) has been shown to be a versatile ligand sporting different coordination modes and geometries as dictated by copper(I) and the counter ion. The molecular structures of its Cu(I) complexes were characterized by X-ray crystallography. The ligand was found in a chelating mode and monomeric complexes were formed when the ligand to copper ratio was 2:1 and the anion was non-coordinating. However, with thiocyanate as the counter anion, the ligand was found to adopt two different modes, with one ligand chelating and the other acting as a monodentate ligand. With CuX (X = Cl, Br), dppan formed a tetrameric complex when the ligand and metal were reacted in the ratio of 1:1. But reactions containing ligand and metal in the ratios of 1:2 or 2:1, resulted in the formation of a mixture of species in solution. Crystallization however, led to the isolation of the tetrameric complex. Variable temperature 31P{1H} NMR spectra of the isolated tetramers did not show the presence of chelated structures in solution. Tetra-alkylammonium salts were added to solutions of various complexes of dppan and studied by 31P{1H} NMR to probe the effect of anions on the stability of complexes in solution. The Cu–dppan complexes were robust and did not interconvert with other structures in solution unlike the bis(diphenylphosphino)isopropylamine complexes. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Tertiary phosphines stabilize low valent transition metal complexes and are of great value in catalysis due to their tunable steric and p-acceptor properties [1]. Although organometallic compounds with P–C–P ligands are extremely well studied in the last three decades [2], interest in analogous diphosphinoamine ligands (Fig. 1) has picked up only recently [3]. Diphosphinoamine ligands are as versatile as P–C–P ligands as the substitutents on phosphorus and nitrogen can be readily varied resulting in significant changes in P–N–P bond angles [4]. Dramatic changes in their coordination behavior, structural features of the resulting complexes, and catalytic properties have been realized [5–7]. The coordination behavior of (PX2)2NR is predominantly bidentate ligation either through chelation or bridging of metal atoms, though the P lone pairs. Unlike the central carbon in P–C–P ligands, a planar N is at the centre of the P–N–P ligand. The PF2 derivatives of these ligands have been extensively studied by King [8], Johnson and Nixon [9], and Cotton et al. [10] due to the excellent p accepting property of P in these ligands. Diphosphazane ligands have been studied extensively for other reasons as well: they have a short bite resulting in multinuclear transition metal complexes which have very short metal-metal contacts [11,12]. Studies on dppan with 3d, 4d and 5d transition metal ions are available [13–15] but with copper(I), only NMR studies have been ⇑ Corresponding author. Tel.: +91 080 2293 2663; fax: +91 080 2360 1552. E-mail address: [email protected] (A.G. Samuelson). 0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2011.03.035
reported [16] and structural studies are lacking. The utility of these complexes as catalysts in the cyclopropanation of olefins was the subject of a recent article [17]. The present report discusses the coordination behavior of (PPh2)2NPh (dppan) with copper(I) in solution and in the solid state through NMR and X-ray crystallographic studies, respectively. Unlike silver(I) which prefers low coordination numbers [18], copper(I) prefers four coordination. A combination of two factors, the diphosphinoamine to copper ratio, and the nature of the substituent on N, control the nuclearity and structure of the copper complexes in the solid state. The solid state and solution behavior for the dppan ligand system has also been compared with that of the recently reported bis(diphenylphosphino)isopropylamine (dppipa) complexes of copper(I) [19]. Fig. 2 summarizes the different complexes isolated in this study. 2. Results and discussion 2.1. Copper(I) complexes of dppan Complexes of dppan were readily synthesized by reacting it with various Cu(I) precursors in different ratios. The following subsections describe the complexes obtained and their molecular structure. 2.2. Complex 1: [Cu(dppan)2]ClO4 The reaction of dppan and [Cu(CH3CN)4]ClO4 in the ratio of 2:1, resulted in a bis chelated monomer which was isostructural to the
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of binding [20]. Copper was found to be in a distorted tetrahedral environment with four coordination sites occupied by three phosphorus donors and one hard N donor from the thiocyanate ion. The Cu–N distance was 1.980(3) Å. This structure was very different from what was obtained in the case of the dppipa ligand which was a dimer with the thiocyanate bound via nitrogen. The monomeric complex here behaves differently probably due to the higher p-acceptance of the dppan ligand that stabilizes the CuP3N core. As a result the other ligand is bound through only one of its arms and remains monodentate. A structural motif of this kind with monodentate thiocyanate ion is also found in other phosphine–copper(I) complexes, bearing triphenylphosphine-phenanthroline [21], 4,6-dimethylpyrimidine2(1H)-thione-S [22], bis(diphenylphosphino)methane (dppm) [23] and bis(diphenylphosphino)ethane (dppe) [24].
Fig. 1. P–N–P framework.
analogous dppipa complex published recently [19]. The ORTEP plot and the selected bond lengths and bond angles are given in the Fig. 3 and Table 1, respectively. The copper ion, bound to two chelating ligands result in a distorted tetrahedral geometry. As a result, the P–N–P angle was reduced to 106° on chelation with the metal, whereas it was 122.8(3)° in the free ligand. But interestingly, there was no significant change in the P–N bond length on complexation (P–N bond length in free ligand are 1.706(4) Å and 1.711(4) Å).
2.4. Complex 3: [Cu4(dppan)2Cl4] Reactions with CuCl were attempted with L:M ratios of 1:2 or 2:1, and the 31P{1H} NMR spectrum in CDCl3 indicated the formation of a mixture of complexes. Strangely, another reaction attempted with a L:M ratio of 1:1 showed a single peak in the 31 1 P{ H} NMR and the complex obtained on crystallization was found to be a tetramer. Two of the ligands were bound to four Cu centers in a bridging fashion and four chloride bridges completed the core structure. The complex crystallized in a centrosymmetric space group and had all four copper atoms in the same plane with two l3-chloride bridges. This complex was isostructural to the analogous dppipa complex. Although the Cu–Cu distances in the dppipa complex were shorter (Cu–Cu distance ranges from 2.641(2) to 2.784(2) Å) than what was found in 3. Fig. 5 and Table 3 give the ORTEP plot of the complex and selected metric parameters, respectively.
2.3. Complex 2: [Cu(dppan)(1-dppan)(NCS)] The reaction between the ligand and copper(I) thiocyanate in the ratio of 2:1 in dichloromethane resulted in an insoluble material. But when the same reaction was attempted under similar conditions in the solvent THF, a clear yellow solution was formed. Slow diffusion of petroleum ether into the THF solution of the complex afforded bright yellow crystals that were identified to be a monomeric complex. There was one ligand bound to copper in a chelating mode and the other was bound in a g1-fashion. The ORTEP plot is given in Fig. 4 and selected bond lengths and bond angles in Table 2. A half sandwich nickel complex of the same ligand had been reported by Kuhn and Winter to have a monodentate mode
3
4
S
C
PPh2 N
Cu P Ph2
P Ph2
P Ph2
N
N N
Cu
N
Ph2 P
Ph2 P
Ph2 P
P Ph2
4
Ph2P
N
PPh2
Cl Ph2P
Br Cu
Cu Cl Cl
N P Ph2
Ph2P
N Cu
Cu
PPh2
Cu
N
PPh2
P Ph2
Cl
+ Ph2 P
I-
Ph2 P N
N Cu I Ph2P
I
Cu
PPh2
Cu
Ph2P
PPh2 N
Fig. 2. Reactivity of dppan with Cu(X).
Cu Br
N
Br Cu
Cu Br
PPh2
PPh2
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Fig. 3. ORTEP view of complex 1 at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 1 Selected bond lengths (Å) and bond angles (°) of complex 1. Bond lengths (Å) Cu1–P1 Cu1–P2 Cu1–P3 Cu1–P4
2.2853(19) 2.291(2) 2.3040(19) 2.2886(19)
P1–N1 P2–N1 P3–N2 P4–N2
1.707(5) 1.708(5) 1.701(5) 1.726(5)
Bond angles (°) P1–Cu1–P2 P1–Cu1–P4 P2–Cu1–P3
73.28(7) 132.72(8) 131.12(7)
P2–Cu1–P4 P1–N1–P2 P3–N2–P4
128.24(7) 106.2(3) 106.8(3)
Fig. 4. ORTEP view of complex 2; [Cu(dppan)(g1-dppan)(NCS)] at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 2 Selected bond lengths and bond angles in complex 2. Bond lengths (Å) Cu1–P1 Cu1–P2 Cu1–P3 Cu1–N3
2.336(1) 2.354(10) 2.265(1) 1.980(3)
P1–N1 P2–N1 P3–N2 P4–N2
1.718(3) 1.713(3) 1.712(3) 1.740(3)
Bond angles (°) P1–Cu1–P2 N3–Cu1–P1 N3–Cu1–P3
72.45(3) 114.83(9) 107.25(9)
P2–Cu1–P3 P1–N1–P2 P3–N2–P4
120.39(3) 107.76(15) 119.00(16)
The role of halide bridges in tuning the Cu–Cu distances has been elegantly demonstrated in a recent paper by Knight et al. [25]. Obviously, the copper-copper distances are affected by both ligands and this is amply illustrated by the work of Ellermann et al. [26] and Bera et al. [27]. It has also been shown that shorter distances can arise when there is unsymmetric coordination [28].
2.5. Complex 4: [Cu4(dppan)2Br4] Analogous to CuCl, CuBr also formed a tetrameric core (Fig. 6). But there were differences in the Cu–Br distances observed within
the complex (Table 4). In the case of the chloride complex, two chloride ions were found to be l2-bridges and two were in l3 mode. In the present case, two of the bromide ions form l2-bridges with copper, but two of them were found to be in l4-mode. However, the Cu–Br distances were quite unsymmetrical and only the shorter distances are shown bonded in the ORTEP plot. The analogous bromide tetramer of dppm ligand had a Cu–Cu distance of 3.189 Å for dppm bridged copper atoms [29,30] which was longer by almost 0.41 Å compared to the Cu–Cu distance in the PNP bridged copper centers of the diphosphinoamine complex. There was an increase of almost 4.4° in the P–N–P bond angle (120.51(13)°) on forming a bridge with two metal centers in the case of the dppan ligand. In the case of the dppan ligand, the two PPh2 units could probably bend back further due to the orthogonal disposition of the phenyl ring on the nitrogen resulting in widening of the PNP bond angle. In spite of this, the ligand has a short bite with a short Cu–Cu distance of 2.786(1) Å for the phosphine bridged centers. In the analogous dppm complex, [19] the phosphine bridged Cu-Cu distance was longer by almost 0.12 Å (2.905 Å). When chloride and bromide complexes are compared, one finds that there is a reversal of ordering of phosphine bridged and halide bridged Cu–Cu distances. The iodo complex can be synthesized in a similar fashion and is a trimeric species. The complex did not give satisfactory elemental analysis, but structural details obtained from X-ray diffraction study has been included in the Supplementary material. In the case of chloride, in both dppm and in PNP ligands, Cu–Cu distances bridged by phosphine were shorter than the Cu–Cu distances bridged by chloride. However, in the case of the bromide complex, the Cu–Cu distance was longer when it was phosphine bridged [30]. This was clearly a counterintuitive result and reminiscent of anion control in Cu–Cu distances [22]. Recently Knight et al have however reported a system where the Cu– Cu distances follow the sizes of the bridging atoms and forms an interesting contrast. In all the dppan complexes described above, the phenyl ring on the central nitrogen of the dppan ligand was found to lie orthogonal to the P–N–P plane. For instance, in complex 1, the dihedral angle between P1–N1–P2 plane and the plane defined by C49–C54 carbon atoms (phenyl ring) was around 80.6°. The orthogonal disposition of the phenyl ring permits a larger P–P distance and better packing due to interdigitization of phenyl rings in the lattice (Fig. S1 in Supplementary information).
2.6. Variable temperature
31
P{1H} NMR studies
The relative stability of chelating and bridging modes could be gauged by examining the nature of these complexes in solution using variable temperature 31P{1H} NMR experiments. This is important since the structures seen in the solid state could be attributed to selective crystallization of one of the forms. Notably, the Cu–P bond was labile in the case of previously examined dppipa complexes and in solution it generated a set of two doublets similar to that observed for the Cs conformer of dppipa. Invariably the complexes having a chelating phosphine, were unstable in solution and tended to go to the more stable bridging geometry. However complexes with a bridging ligand retained their structure in solution and did not equilibrate with structures having a chelated ligand so easily. In the case of dppan complexes, one could not find analogous equilibria in solution, as there were no significant changes in the 31 P NMR when the temperature was varied. These subtle differences between the two ligands suggests, that in solution, the Cu(I) dppan complexes are more stable than the corresponding dppipa complexes. This could be attributed to higher stability of
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Fig. 5.
ORTEP
view of complex 3 at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
Table 3 Selected bond lengths (Å) and bond angles (°) in complex 3.
a b
Table 4 Selected bond lengths and bond angles in complex 4.
Bond lengths (Å) Cu1–Cu2 Cu1–Cu2_3 Cu1–P1 Cu1–Cl1 Cu1–Cl2 Cu2–P2
2.7861(9)a 2.8892(8)b 2.1727(9) 2.3826(9)a 2.2572(9)b 2.1797(10)
Cu2–Cl1 Cu2–Cl1_3 Cu2–Cl2_3 P1–N1 P2–N1
2.3871(9) 2.5611(10) 2.3439(10) 1.706(2) 1.715(2)
Bond angles (°) P1–Cu1–Cl1 P1–Cu1–Cl2 P1–Cu1–Cu2 Cl1–Cu1–Cl2 P2–Cu2–Cl1
120.32(3) 136.11(3) 92.52(3) 100.67(4) 132.37(3)
P2–Cu2–Cl2_3 Cl1–Cu2–P2 P2–Cu2–Cu1 Cl1–Cu2–Cl2_3 P1–N1–P2
120.60(4) 132.37(3) 89.69(3) 93.39(3) 120.56(13)
Chloride bridged. Phosphine bridged.
Cu–P bond in the case of dppan complexes due to the greater paccepting character of the dppan ligand. 2.7. Interconversion of complexes In order to find the possibility of inter-converting complexes having the same ratio of ligand/metal, we carried out 31P{1H} NMR experiments with different counterions. A known concentration of the complex was titrated with a different anion using the appropriate tetra-alkylammonium salt in CDCl3. Unlike dppipa complexes reported earlier, the exchange of counter-ions was quite sluggish as seen in the 31P NMR in case of Cu–dppan complexes. The perchlorate complex 1 reacted with the halide ion to form the corresponding halide tetramers. With cyanide, as seen in the case of dppipa complex, free dppan was observed (Figs. S2 and S3). The halide tetramers 3 and 4 on addition of a different halide form mixed clusters in very small amounts.
Fig. 6.
ORTEP
a b
Bond lengths (Å) Cu1–Cu2 Cu1–Cu2_3 Cu1–P1 Cu1–Br1 Cu1–Br2 Cu1–Br2_3
2.677(2)a 2.747(2)b 2.176(2) 2.364(2) 2.744(2) 2.760(2)
Cu2–P2 Cu2–Br1 Cu2–Br2 Cu2–Br2_3 P1–N1 P2_3–N1
2.179(3) 2.442(2) 2.516(2) 2.720(2) 1.721(7) 1.691(7)
Bond Angles (°) P1–Cu1–Br1 P1–Cu1–Cu2_3 P2–Cu2–Br1 P2–Cu2–Br2 Br1–Cu2–Br2
126.12(10) 91.21(8) 116.57(9) 130.28(9) 100.24(6)
Br1–Cu2–Cu1_3 Br2–Cu2–Cu1_3 Cu1_3–Cu2–P2 P1–N1–P2_3
150.21(6) 63.07(5) 91.99(8) 120.1(4)
Bromide bridged. Phosphine bridged.
But with addition of cyanide, the free PNP ligand was obtained. unlike dppipa complexes wherein a similar reaction gives the bis chelated complex. Slow exchange with dppan complexes indicates higher stability of Cu–P bonds in these clusters in solution in comparison with the analogous dppipa complexes.
3. Summary and conclusions Dppan ligand like many diphosphinoamines is quite a flexible ligand and capable of bridging and chelating copper with ease. The dppan ligand formed a totally different structural core in the presence of the thiocyanate ion, while in the presence of oxyanion and halide ions, the structures were similar to those obtained in the case of dppipa. The variable temperature 31P{1H} NMR showed no significant changes in the case of dppan complexes. This clearly proved that Cu(I) dppan complexes were more stable in solution in
view of complex 4; [Cu4(dppan)2(Br)4] at 50% thermal ellipsoid probability. Phenyl rings on phosphorus and hydrogen atoms omitted for clarity.
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comparison with the analogous dppipa complexes suggestive of better Cu–P p-bonding in dppan complexes. The present work has brought out the ability of diphosphinoamines to adopt different binding modes with copper(I) in the presence of different anions. By tuning the ligand to metal ratio, a variety of Cu(I) complexes with different nuclearity could be synthesized. With electronically different substituents (iPr and Ph) on the nitrogen atom of PNP ligand, very similar structural motifs were obtained in most cases. However in solution, the electronic differences were clearly manifested in the lability of dppipa relative to dppan complexes as the latter is a better p acceptor. 4. Experimental section 4.1. General Dichloromethane, chloroform, petroleum ether (b.p. 60–80 °C), acetonitrile, tetrahydrofuran, methanol and benzene were purified and dried under nitrogen atmosphere by conventional methods [31]. All manipulations were carried out under an atmosphere of purified N2 using a standard double manifold and Schlenk ware. Bis(diphenylphosphino)aniline was synthesized by a reported procedure [32] with slight modifications. [Cu(CH3CN)4]ClO4, [33] CuSCN [34] and CuCl [35] and CuBr [36] were prepared by reported methods. CuI was purchased from Aldrich and used as supplied.
7 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain colorless crystals. (Yield: 86%). Anal. Calc. for CuC60H50N2P4ClO4.2CH2Cl2: C, 65.98; H, 4.58; N, 2.12. Found: C, 66.36; H, 4.61; N, 2.58%. 1H NMR: d 6.29 (2H, d, NPh), 6.92 (2H, t, NPh), 7.15 (1H, t, NPh), 7.18–7.52 (20H, m, PPh2). 31P{1H} NMR: d 87.7 (s). IR data (cm 1): ClO4 : 1096 (vs, br). 4.4.2. Reaction with CuSCN; Complex 2: [Cu(dppan)(1-dppan)(NCS)] Dppan (0.23 g, 0.5 mmol) was added to a solution of CuSCN (0.03 g, 0.25 mmol) in THF (15 ml) and stirred for 1 h at RT. Subsequently the resultant yellow colored solution was concentrated to about 7 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain yellow colored crystals. (Yield: 72%). Anal. Calc. for CuC61H50N3P4STHF3H2O: C, 66.24; H, 4.93; N, 3.11. Found: C, 66.69; H, 5.47; N, 3.59%. 1H NMR: d 6.23 (2H, d, NPh), 6.80 (2H, t, NPh), 6.98 (1H, t, NPh), 7.21 7.38 (20H, m, PPh2). 31P{1H} NMR: d 24.9 d (br), 23.5, 16.5. IR data (cm 1): SCN : 2055 (vs, sharp). 4.4.3. Reactions with CuCl; Complex 3: [Cu4(dppan)2Cl4] Synthesis of complex 3 was carried out using a similar procedure as for complex 1 starting with CuCl (0.025 g, 0.25 mmol) and varying the amount of dppan according to the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the pure complex.
4.2. Physical measurements 1
H NMR spectra were recorded either on a Bruker ACF 200 MHz or AMX 400 MHz spectrometer with tetramethylsilane (TMS) as the internal reference. 31P{1H} NMR spectra were recorded either on a Bruker AMX 400 MHz spectrometer operating at 162.2 MHz or a Bruker ACF 200 MHz operating at 81.1 MHz with H3PO4 (85%) as the external reference. Variable temperature 31P{1H} NMR were recorded in the temperature range 20 °C and 60 °C. The complex was dissolved in CH2Cl2 and 31P NMR was recorded with acetone-d6 as the external lock. IR spectra were recorded in the solid state as KBr pellets on a Bruker Equinox 55 spectrometer. Elemental analyses were done with Carlo Erba Elemental Analyzer model 1106. Anion challenge experiments were carried out by titrating a known concentration of the complex in a NMR tube in CDCl3 with a CDCl3 solution of the tetraalkylammonium salt of the anion and followed by 31P{1H} NMR. 4.3. Preparation of dppan The procedure involved a dropwise addition of PPh2Cl (20 mmol, 3.59 ml) to aniline (0.91 ml, 10 mmol) and triethylamine (2.78 ml, 20 mmol) added to 30 ml of diethylether in a 100 ml double necked round bottom flask at 10 °C and stirring at RT for 4 h. Subsequently the solvent was evaporated and 30 ml of benzene was added. Then the resultant solution was filtered to remove the amine hydrochloride. The insoluble fraction was thoroughly washed with benzene and the filtrate evacuated under vacuum. The crude product was repeatedly washed with distilled methanol to get the pure ligand. (Yield: 85%). 1H NMR: d 6.64 (2H, m, NPh), 6.95 (3H, d, NPh), 7.30–7.38 (20H, m, PPh2). 31 1 P{ H} NMR: d 68.1 (s).
4.4.3.1. L:M = 1:1. Dppan (0.23 g, 0.5 mmol) was added to a CH2Cl2 solution (15 ml) containing CuCl (0.05 g, 0.5 mmol) and stirred for 1 h at RT. Subsequently the solution was concentrated to about 8 ml and 15 ml of petroleum ether was allowed to diffuse through the solution to obtain colorless crystals. (Yield: 85%). Anal. Calc. for Cu4Cl4C60H50N2P4.CH2Cl2: C, 52.45; H, 3.91; N, 2.35%. Found: C, 52.19); H, 3.73; N, 1.99%. 1H NMR: d 5.43 (2H, d, NPh), 6.46 (2H, t, NPh), 6.78 (1H, t, NPh), 6.90–7.24 (20H, PPh2). 31P{1H} NMR: d 59.05 (s). 4.4.3.2. L:M = 2:1. 1H NMR: d 5.43 (2H, d, NPh), 6.27 (4H, d, NPh), 6.45 (2H, t, NPh), 6.75 (1H, t, NPh), 6.92-7.46 (41H, m, NPh + PPh2). 31 1 P{ H} NMR: d 84.6 (br), 68.0, 58.9 (s). 4.4.3.3. L:M = 1:2. The reaction of CuCl and dppan in the ratio 2:1 resulted in a white colored powdery material, which was only sparingly soluble in chlorinated solvents. 1H NMR: d 5.44 (2H, d, NPh), 5.95 (1.5H, d, NPh), 6.45 (2H, t, NPh), 6.66 (1.5H, t, NPh), 6.78 (1H, t, NPh), 6.95-7.02 (15H, m, NPh + PPh2), 7.25–7.50 (20H, m, PPh2). 31P{1H} NMR: d 67.6 (br), 58.9 (s). This insoluble material was treated with excess ligand in CH2Cl2 to obtain a clear colorless solution. The complex crystallized from the above solution was confirmed to be the tetramer i.e. complex 3 through 31P NMR. 4.4.4. Reactions with CuBr Synthesis of complexes was carried out using a similar procedure as for complex 1 starting with CuBr (0.036 g, 0.25 mmol) and varying the amount of dppan according to the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the pure complex.
4.4. Synthesis of copper(I) complexes of dppan 4.4.1. Reaction with [Cu(CH3CN)4]ClO4; Complex 1: [Cu(dppan)2]ClO4 Dppan (0.23 g, 0.5 mmol) was added to a solution of [Cu(CH3CN)4]ClO4 (0.08 g, 0.25 mmol) in CH2Cl2 (15 ml) and stirred for 1 h at RT. Subsequently the solution was concentrated to about
4.4.4.1. L:M = 1:1. Complex 4: [Cu4(dppan)2Br4]: Faint yellow crystals. (Yield: 88%). Anal. Calc. for Cu4Br4C60H50N2P4: C, 48.37; H, 3.56; N, 2.12. Found: C, 48.15; H, 3.34; N, 1.87%. 1H NMR: d 5.43 (2H, d, NPh), 6.45 (2H, t, NPh), 6.80 (1H, t, NPh), 6.90–7.30 (m, 20H, PPh2). 31P{1H} NMR: d 59.0 (s).
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R. Ahuja et al. / Inorganica Chimica Acta 372 (2011) 220–226 Table 5 Crystallographic data for copper(I) dppan complexes.
Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (Mg/m3) Crystal size (mm3) Reflections collected Independent reflections (Rint) Final R1a, wR2b [I > 2r(I)] Final R1a, wR2b (all data) Goodness-of-fit (GOF) on F2c a b c
1
2
3
4
C62H54Cl5CuN2O4P4 1255.74 monoclinic P21/n 10.832(3) 38.525(4) 14.440(3) 90 94.15(4) 90 6009.9(4) 4 1.388 0.52 0.32 0.26 7866 7866 (0.0421) R1 = 0.0687, wR2 = 0.1944 R1 = 0.1083, wR2 = 0.2185 1.056
C67H62CuN3P4S 1128.68 triclinic P1 10.311(2) 11.467(3) 13.644(3) 76.603(4) 74.800(4) 69.606(4) 1441.6(6) 1 1.300 0.59 0.31 0.27 16863 12683 (0.0235) R1 = 0.0455, wR2 = 0.0921 R1 = 0.0596, wR2 = 0.0967 0.934
C31H27Cl4Cu2NP2 744.36 monoclinic P21/c 10.034(3) 14.971(4) 21.369(6) 90 100.132(5) 90 3160.2(15) 4 1.565 0.4 0.35 0.3 27023 7433 (0.0519) R1 = 0.0448, wR2 = 0.1187 R1 = 0.0655, wR2 = 0.1276 1.022
C31H27Br2Cl2Cu2N1P2 833.28 monoclinic P2(1)/c 10.260(2) 21.185(4) 15.106(3) 90 95.38(3) 90 3269.0(11) 4 1.693 0.18 0.14 0.12 27388 7564 (0.1314) R1 = 0.0815, wR2 = 0.1916 R1 = 0.1975, wR2 = 0.2317 1.147
P P R1 = ( ||Fo| |Fc||)/( |Fo|). P P wR2 = [ (w|Fo|2 |Fc|2)2/ w|Fo|2)2]1/2. GOF = [w(F 2o F 2c )2]/(n p)1/2.
The reactions with other M:L ratios gave mixture of compounds as seen in the case of reactions with CuCl. 4.4.5. Reactions with CuI Synthesis of complex 5 was carried out using a similar procedure as for complex 1 starting with CuI (0.048 g, 0.25 mmol) and varying the amount of dppan:Cu in the ratios 1:0.5, 1:1 and 1:2. All the reactions were carried out at room temperature for an hour. Petroleum ether was added to the concentrated solution of CH2Cl2 to yield the complex. Although a spectroscopically, and crystallographically characterizable complex was obtained in the 1:1 ratio, satisfactory elemental analysis could not be obtained and structural details might be obtained in the Supplementary information.
Fourier map using SHELXL-97 [42]. Hydrogen atoms were geometrically fixed in all the complexes. The crystallographic data of complexes 1–4 are summarized in Tables 1–4 and Table 5, respectively in the main article. In the case of complex 5, the quality of the data collected was not so good, as the crystal was very unstable. The solvent atoms that appeared in the difference Fourier map were highly disordered and were isotropically refined with constraints. After the final refinement in the case of complex 4, two residual peaks of electron density 1.77 and 1.68 e Å3 were located at 1.20 Å and 1.14 Å from Cu4 atom, respectively. In case of complex 5, the data was complete only upto 82% as the crystal was highly unstable and there was a constant decay in intensities acquired from the crystal. The details are given in the Supplementary information Table S1 And S2.
4.5. X-ray data collection, structure solution and refinement Crystals of complexes 3 and 4 were mounted with paraffin oil in Lindemann capillaries for data collection. Whereas a crystal of complex 2 was separately glued to the tip of a glass fiber along the largest dimension for indexing. Reasonably good crystals of complex 1 and 5 were mounted along with the mother liquor in Lindemann capillaries. Data for complexes 2, 3, 4 and 5 were collected on a Bruker AXS single crystal diffractometer equipped with SMART APEX CCD detector and a sealed Mo Ka source working at 1.75 kW. For complex 1, the data were collected on an Enraf-Nonius CAD4 single crystal diffractometer equipped with graphitemonochromatized Mo K radiation. Intensity data were collected at 293 K for all complexes except for complex 5, which was collected at 160 K. For the CAD4 data, accurate unit cell parameters and orientation matrices were determined by least-squares refinement of 25 well centered reflections in the range 9° 6 h 6 13°. Three periodically measured reference reflections showed no significant decay during the time of data collection for complex 2 and 3. Whereas for complex 1, the crystal was not stable during the data collection. Hence another suitable crystal was mounted to complete the data. All computations were performed using the WINGX package. [37] The data were corrected for Lorentz and polarization effects. Absorption correction for complex 1 was applied using PSI-SCAN [38] and for complex 1, DIFABS [39] was used. For complexes 2, 3 and 4, SADABS [40] was used for the absorption correction. The positions of heavy atoms were determined by SHELXS-86 [41]. The remaining atoms were located from the difference
Acknowledgements We wish to thank DST, New Delhi for the financial support and for funding the CCD facility at IISc, Bangalore. The assistance of M. Thenraj and R. Mitra in preparing the final manuscript is gratefully acknowledged. Appendix A. Supplementary material CCDC 252102, 252103, 252104, 252105 and 252106 contain the supplementary crystallographic data for complexes 1: [Cu(dppan)2]ClO4, 2: [Cu(dppan)2(NCS)], 3: [Cu4(dppan)2Cl4], 4: [Cu4(dppan)2Br4] and 5: [Cu3(dppan)3I2]I, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2011.03.035. References [1] L.H. Pignolet (Ed.), Homogenous Catalysis with Metal Phosphine Complexes, Plenum Press, New York, 1983. [2] B. Chaudret, B. Delavaux, R. Poilblane, Coord. Chem. Rev. 86 (1988) 191;. [3] M. Aydemir, A. Baysal, G. Ozturk, B. Gumgum, Appl. Organomet. Chem. 23 (2009) 108. [4] (a) G. Calabro, D. Drommi, G. Bruno, F. Faraone, Dalton Trans. (2004) 81; (b) H.-J. Chen, J.F. Barendt, R.C. Haltiwanger, T.G. Hill, A.D. Norman, Phosphorus, Sulfur Silicon Relat. Elem. 26 (1986) 155;
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