[Bis­(diphenyl­phosphino)methane-κ2 P,P′]dichloridopalladium(II)

Muhammad Shahid; Imtiaz-ud-Din; Muhammad Mazhar; Matthias Zeller; Allen D Hunter

doi:10.1107/S1600536808042980

. 2009 Jan 8;65(Pt 2):m158–m159. doi: 10.1107/S1600536808042980

[Bis(diphenylphosphino)methane-κ² P,P′]dichloridopalladium(II)

Muhammad Shahid ^a, Imtiaz-ud-Din ^a, Muhammad Mazhar ^a,^*, Matthias Zeller ^b, Allen D Hunter ^b

PMCID: PMC2968394 PMID: 21581770

Abstract

The title complex, [PdCl₂(C₂₅H₂₂P₂)], is a slightly distorted square-planar bis(diphenylphosphino)methane cis-complex of PdCl₂. The structure of a polymorph of the title compound has been described earlier, but the arrangement of the molecules observed in the current structure is distinctively different from that previously reported [Steffen & Palenik (1976 ▶). Inorg. Chem. 15, 2432–2439]. The earlier report describes a structure with individual well separated molecules crystallizing in space group P2₁/n. The polymorph described here, which is isostructrural to its Pt analogue [Babai et al. (2006 ▶). Z. Anorg. Allg. Chem. 632, 639–644], crystallizes in C2/c with chains of C2-symmetric molecules stretching parallel to the b axis. The Pd atoms and the bisphosphinomethane units are located on two different positions created by a non-crystallographic mirror operation with an occupancy of 0.6677 (11) for the major (PCH₂P)Pd moiety. The positions of the Cl atoms of the minor moiety do coincide perfectly with those of the next molecule along the chain parallel to b, and they are thus not included in the disorder. The phenyl rings also do not take part in the disorder and are common to both the major and minor moieties of the (PCH₂P)PdCl₂ units. Assuming no defects, molecules in each chain will thus have to be oriented the same way and the effect of the disorder of the (PCH₂P)Pd unit is thus a reversal in direction of the chains parallel to b. The presence of light streaks of intensity between actual Bragg peaks indicates that a somehow ordered arrangement not resolved in the Bragg diffraction data may be present (i.e. an incommensurate superstructure) rather than a random or domain arrangement of the chains.

Related literature

For a different polymorph, see: Steffen & Palenik (1976 ▶). For a related structure, see: Babai et al. (2006 ▶). For background literature, see: Braun et al. (2007 ▶; Puddephatt (1983 ▶); Farina et al. (1997 ▶); Chaudret et al. (1988 ▶); Mitchell (1992 ▶); Witt & Roesky (1994 ▶); Balakrishna et al. (1994 ▶); Tsuji (1996 ▶); Miyaura & Suzuki (1995 ▶); Suzuki (1991 ▶); Ozawa (1997 ▶).

Experimental

Crystal data

[PdCl₂(C₂₅H₂₂P₂)]
M _r = 561.67
Monoclinic,
a = 16.137 (2) Å
b = 7.7836 (9) Å
c = 19.217 (2) Å
β = 99.029 (3)°
V = 2383.8 (5) Å³
Z = 4
Mo Kα radiation
μ = 1.15 mm⁻¹
T = 100 (2) K
0.60 × 0.40 × 0.20 mm

Data collection

Bruker SMART APEX CCD diffractometer
Absorption correction: multi-scan (SADABS in SAINT-Plus; Bruker, 2003 ▶) T _min = 0.635, T _max = 0.795
11903 measured reflections
2951 independent reflections
2889 reflections with I > 2σ(I)
R _int = 0.022

Refinement

R[F ² > 2σ(F ²)] = 0.035
wR(F ²) = 0.078
S = 1.34
2951 reflections
159 parameters
H-atom parameters constrained
Δρ_max = 0.43 e Å⁻³
Δρ_min = −0.47 e Å⁻³

Data collection: SMART (Bruker, 2002 ▶); cell refinement: SAINT-Plus (Bruker, 2003 ▶); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablocks I, global. DOI: 10.1107/S1600536808042980/pv2124sup1.cif

e-65-0m158-sup1.cif^{(20.8KB, cif)}

Structure factors: contains datablocks I. DOI: 10.1107/S1600536808042980/pv2124Isup2.hkl

e-65-0m158-Isup2.hkl^{(144.3KB, hkl)}

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Acknowledgments

MS is grateful to the Higher Education Commission of Pakistan and the Pakistan Science Foundation, Islamabad, Pakistan, for financial support of the PhD programme. The X-ray diffractometer at Youngstown State University was funded by NSF grant 0087210, Ohio Board of Regents grant CAP-491 and by Youngstown State University.

supplementary crystallographic information

Comment

Stabilization of organotransition metal complexes is commonly achieved through bidentante-chelating bisphosphine ligands of the type X(PPh)₂ and these kinds of ligands were e.g. used to gain insight into several binuclear bond activation processes (Braun et al., 2007; Puddephatt, 1983). If X is a one-atom-spacer such as CH₂ (diphenylphosphinomethane or dppm) or NH (diphenylphosphinoamine or dppa), their chelate complexes result in strained 4-membered rings, and the ligands most commonly act as bridging ligands. They can, however, also act as bidentate chelating ligands and form 4-membered rings. The applications of dppm (Puddephatt, 1983; Chaudret et al., 1988) and dppa (Witt & Roesky, 1994; Balakrishna et al., 1994) in binuclear organometallic and coordination chemistry have been reviewed. There has been considerable interest in the properties of palladium complexes because of their frequent use as catalysts for carbon-carbon coupling reactions (Tsuji, 1996). These reactions play a key role in the synthesis of many organic chemicals, natural products and also in a variety of industrial processes. The most important examples for such a type of catalysis are the Suzuki (Miyaura & Suzuki, 1995; Suzuki, 1991) and the Stille (Farina et al., 1997; Mitchell, 1992) cross-coupling reactions. Phosphine complexes of palladium (II) are well known and are prepared from palladium (II) salts with an excess of phosphine ligand quite easily (Ozawa, 1997). The complexes have been used for the establishment of catalytic activity of palladium in various reactions including CC-coupling (Tsuji, 1996).

The title compound, (I), a complex of dppm with PdCl₂, is such a complex (Fig. 1). The complex has a slightly distorted square planar coordination environment around the Pd atom and the two chlorine atoms are, due to the restraints of the bidentate chelating bis-phosphine ligand, in cis position to each other. The structure of a polymorph of the title compound was described earlier, but the arrangement of the molecules observed in the current structure is distinctively different from that of previously reported (Steffen & Palenik, 1976). The earlier report describes a structure with individual well separated molecules crystallized in the monoclinic space group P2₁/n. The molecules are arranged in a pattern typical for close packed structure, and the chlorine atoms show several weak C—H···Cl interactions with hydrogen atoms of the symmetry related molecules. The coordination environment of the Pd atom is quite asymmetric and especially the four membered PdP₂C ring is not planar in this structure. Instead the CH₂ unit is located above the mean plane of the Cl₂PdP₂ unit. The phenyl rings follow the CH₂ group and are rotated towards each other on one side of the molecule, away from each other on the other.

The title compound is isostructrural to its Pt analogue (Babai et al. 2006) and crystallizes in a different space group. The molecules of (I) lie on a crystallographic two fold axis that stretches through the Pd atom and the CH₂ group of the ligand, and the molecules are C2-symmetric (Fig. 1). The Pd atoms in (I) exhibit a much regular arrangement than in its Pt analogue and the PdCl₂P₂ units are essentially planar; especially the CH₂ unit is located within the mean plane of the PdCl₂P₂ fragment, and the positions of the phenyl groups are the same on both sides of the PdCl₂P₂ plane. The pronounced differences in the geometries of the two polymorphs can be easily seen in an overlay of the molecules as shown in Figure 2. In (I), the Pd—Cl and Pd—P bond distances are 2.3933 (7) and 2.2288 (11) Å, respectively; these distances vary by 0.10 and 0.14 Å, respectively, in the other polymorph.

The individual molecules in (I) are arranged in a head to tail fashion in chains parallel to the b-axis of the unit cell with the Cl atoms pointing towards the P—CH₂—P end of a symmetry related molecule. This arrangement does allow an interesting type of disorder to manifest itself. The palladium atoms and the bisphosphinomethane units are disordered over two different positions created by a non-crystallographic mirror operation. Figure 3 shows the disorder for one individual molecule. The occupancy of the major (PCH₂P)Pd moiety refined to 0.6677 (10) (a similar disorder, but to a lesser extent, was observed in the Pt analogue). The positions of the chlorine atoms of the minor component do however coincide perfectly with those of the next molecule along the chain parallel to b, and they are thus not part of the disorder: the displacement parameters of the Cl atoms are essentially isotropic and the largest residual electron densities are located in the C—C bonds of the phenyl rings, thus excluding disorder of the chlorine atom positions. The phenyl rings are also not disordered and are common to both the major and minor components of the molecule with only some slightly pronounced anisotropic displacement parameters of the carbon atoms hinting towards a small adjustment of the C atom positions within the two moieties. Assuming no defect molecules in each chain will thus have to be oriented the same way and the effect of the disorder of the (PCH₂P)Pd unit is thus a reversal in direction of the chains parallel to the b-axis as depicted in Figure 4.

The Pd—P distances in the minor fraction of the (I) differ only slightly compared to those in the major one (2.231 (2) versus 2.2288 (11) Å). The Pd—Cl distances, however, do show significant differences, at 2.3933 (7) and 2.4591 (8) Å for the major and the monor fractions, respectively. The Pd—Cl distances in the previously described polymorph are shorter than both of these values (2.362 (1) and 2.352 (1) Å, respectively). It has to be pointed out, however, that light streaks of itensity are found between the positions of the Bragg peaks, thus pointing towards the possible presence of an incommensurate superstructure rather than a random or domain arrangement.

Experimental

(1.08 g, 2.82 mmol) dppm was added to a stirred suspension of PdCl₂ (0.50 g, 2.82 mmol) in 20 ml of methanol in a Schlenk tube under inert atmosphere. After two hours stirring, the mixture was evaporated under vacuum to dryness and the solid was recrystallized from DMSO to obtain yellowish block-shaped crystals at room temperature after several days.