Special effects of ortho-isopropylphenyl groups. Diastereoisomerism in platinum(ii) and palladium(ii) complexes of helically chiral PAr3 ligands

Abstract

The coordination chemistry of the four phosphines, P{C6H3(o-CH3)(p-Z)}3 where Z = H (1a) or OMe (1b) and P{C6H3(o-CHMe2)(p-Z)}3 Z = H (1c) or OMe (1d) with platinum(II) and palladium(II) is reported. Mononuclear complexes trans-[PdCl2L2](L = 1a,b) and trans-[PtCl2L2](L = 1a-c) have been prepared and the crystal structures of trans-[PdCl2(1b)2] and trans-[PtCl2(1c)2] as their dichloromethane solvates have been determined. The structures show that in these complexes, the ligands adopt g+ g+ a conformations. Examination of the Cambridge Structural Database confirms this to be one of only two conformer types that tri-o-tolylphosphines adopt and the only viable conformer in 4 and 6 coordinate complexes. The binuclear complexes trans-[Pd2Cl4L2](L = 1c,d) are formed even when an excess of the bulky 1c,d is used in the synthesis and the crystal structure of trans-[Pd2Cl4(1c)2] as its chloroform solvate is reported. Reaction of [PtCl2(NCBu(t))2] with 1b-d in refluxing toluene gave the cycloplatinated species [Pt2Cl2(L - H)2] where L - H is phosphine 1b-d deprotonated at one of the ortho-methyl carbon atoms. Variable temperature 31P and 1H NMR spectroscopy reveals that all the complexes reported are fluxional. The processes are analysed in terms of restricted P-C and P-M rotations that give rise to diastereoisomeric rotamers because of the helically chiral orientations of the aryl substituents. For the complexes of the bulky ligands 1c,d, rotation about the P-C bond is slow on the NMR timescale even up to 75 degrees C. The crystal structure of the cyclometallated complex [Pt2Cl2(1d - H)2] has been determined.

Mono and dinuclear palladium complexes of o-alkyl substituted arylphosphane ligands: Solvent-dependent syntheses, NMR-spectroscopic characterization and X-ray crystallographic studies

“…For ligands 2 and 3, the results were similar as before, combinations of mono and dinuclear complexes were formed regardless of the Pd:P ratio. Similar results were reported previously [15,16].…”

“…The X-ray structures of 6, 8-12 and 14 were also obtained. The lack of control for the formation of complexes with the desired nuclearity has been recognized as a problem in various papers [9,10,[12][13][14][15]. We tested a variety of solvents in order to clarify the role of solvents and to develop a method to produce palladium complexes of o-alkyl substituted arylphosphanes with the desired nuclearity.…”

“…The following dinuclear palladium complexes were prepared: di-l-chlorodichlorobis(o-methylphenyldiphenylphosphane)di-palladium (6), di-l-chlorodichlorobis(o-ethylphenyldiphenylphosphane)dipalladium (7), di-l-chlorodichlorobis(o-isopropylphenyldiphenylphosphane)di-palladium (8), di-l-chlorodichlorobis-(o-cyclohexylphenyldiphenylphosphane)di-palladium (9) and di-l-chlorodichlorobis(o-phenylphenyldiphenylphosphane)di-palladium (10). We also prepared the following mononuclear complexes: dichlorobis(o-methylphenyldiphenylphosphane) palladium (11), dichlorobis(o-ethylphenyldiphenylphosphane) palladium (12), dichlorobis(o-isopropylphenyldiphenylphosphane) palladium (13), dichlorobis(ocyclohexylphenyldiphenylphosphane) palladium (14) and dichlorobis(o-phenylphenyldiphenylphosphane) palladium (15). The X-ray structures of 6, 8-12 and 14 were also obtained.…”

Development of a Ligand Knowledge Base, Part 1: Computational Descriptors for Phosphorus Donor Ligands

“…Even though the ligand geometry in the He 8 ·L species was re-optimized, significant conformational relaxation did not always occur, substituent rotation often being restricted. A survey of the o-tolyl ring conformations in crystal structures involving ligand 24 [34] confirms that two conformer types (ggg/exo3 and gga/exo2, see reference [34] and references therein) are adopted in different coordination environments (see Table S2). The He 8 _steric parameter was calculated for both conformers.…”

“…The He 8 _steric parameter was calculated for both conformers. The value for the conformer most commonly observed in sterically hindered complexes (gga/exo2, open circle, Figure 3, see Table S2 and reference [34]), is substantially lower.…”