Rhodium(I) iminophosphine poly(propyleneimine) dendrimers: Synthesis, characterization and molecular structure of a mononuclear analogue

Abstract

a b s t r a c tFirst (G1)-and second-generation (G2) poly(propyleneimine) (PPI) dendrimers were reacted with 2-(diphenylphosphino)benzaldehyde, to form iminophosphine-functionalised dendritic ligands (1, 2). The reaction of the dimeric precursor [RhCl(CO) 2 ] 2 with the heterobidentate dendritic ligands 1, 2 gave the corresponding rhodium(I) complexes (3, 4). There has been recent interest in the chemistry of polydentate ligands, especially those which combine ''soft" and ''hard" donor atoms [11]. The most used hemilabile ligands are the P,N-donor bidentate ligands, combining the distinct behaviour of a soft phosphorus atom which coordinates very strongly to a soft metal centre (e.g. palladium) and a hard nitrogen donor which is weakly bound. Non-rigid ligands, such as the widely studied iminophosphines, often act as chelating ligands that easily form five-or six-membered chelate rings. Iminophosphines display versatile coordination behaviour and are potentially hemilabile, making them useful for a variety of homogeneous processes, such as, C-C coupling reac- [16]. The steric and electronic properties of such ligands can also be fine-tuned and this can have a huge effect on the kinetics and thermodynamics of a chemical reaction [17].Examples of metallodendritic chelating heterobidentate complexes are rare. For metallodendrimers containing P and N donor atoms, many transition metals are coordinated in a P-monodentate or N-monodentate fashion. In this communication, we report on the synthesis of first-and second-generation chelating, heterobidentate [P, N] iminophosphine dendritic ligands and their coordination chemistry with a Rh(I) precursor. The molecular structure of a mononuclear analogue is also presented and discussed.The synthesis of the first-and second-generation iminophosphine-functionalised dendritic ligands 1 and 2 ( Fig. 1) [19] was achieved by a Schiff-base condensation reaction of the commercially available DAB-dendr-(NH 2 ) n with 2-diphenylphosphinobenzaldehyde. Screttas et al. first reported the synthesis of a third generation iminophosphine dendritic ligand based on a poly(propyleneimine) scaffold [18]. These dendritic P,N ligands were used in situ in the presence of palladium acetate in the Heck cross-coupling of p-anisyl bromide with styrene.The dendritic ligands (1, 2) were isolated as pale yellow solids in moderate yields (65-68%). These compounds were characterized by elemental analysis (C, H, N) Published in Inorganic Chemistry Communications 12, issue 8, 716-719, 2009 which should be used for any reference to this work 1imine moiety are observed as a triplet around 3.69 ppm, for 1 and 2. The appearance of a singlet at ca. 8.37 ppm, assigned to the imine proton (-CH 2 N@CH-CH 2 -) establishes that the condensation reaction has occurred. As was the case for Screttas and co-workers, it is important to consider syn and anti isomerism about the imine double bond [18]. One resonance is noted for the imine proton in both ligands, assigned to the trans isomer for steric reasons. The 31 P{ 1 H} ...

Imino-phosphine palladium(II) and platinum(II) complexes: Synthesis, molecular structures and evaluation as antitumor agents

“…In the IR spectra of the ligands, imine formation was confirmed by strong absorption bands at 1622 and 1632 cm − 1 for L1 and L2 respectively. These absorption frequencies are consistent with those of related imino-phosphine compounds [22][23][24][25]. Coordination of the ligands was further confirmed by lower absorption bands between 1603 and 1614 cm − 1 , which is a typical characteristic of metal coordinated imines [22][23][24][25].…”

Rh(i) complexes supported on a biopolymer as recyclable and selective hydroformylation catalysts

“…This is consistent with Rh-P coupling. 23 For complex (6), a shifting of the methine proton resonance from d 8.51 ppm in (6a) to d 8.35 ppm in (6) was seen as well as a downfield shift of the protons on the carbon adjacent to the imine nitrogen from d 3.41 ppm to d 4.53 ppm. ESI-mass spectrometry further confirmed the integrity of the complexes by displaying base peaks at m/z 502.1 and 318.92 representing the [M-Cl] + ions for complexes 5 and 6 respectively.…”

“…The geometric parameter around the Rh atom are comparable with those found in similar complexes [RhCl(PyP)(CO)] (PyP = 1-(2-diphenylphosphino)ethyl parazole) 25b and [RhCl(P-N)(CO)] (P-N = 2-(diphenylphosphino)propylimine. 23 The angle P(1)-Rh(1)-Cl(1) (171.327(1) • ) is not the expected 180 • indicating some distortion, possibly imposed by the 6-membered chelate ring about the Rh atom. This effect is also seen to cause deviation of the angle N(1)-Rh(1)-P(1) (84.91(4) • ) from 90 • .…”

Hydroformylation activity of multinuclear rhodium complexes coordinated to dendritic iminopyridyl and iminophosphine scaffolds

“…[RhCl(CO) 2 ] 2 was purchased from Strem Chemicals. Ligands L1 and L2 [27], [RhCl(COD)] 2 [28], (acetylacetonato)dicarbonylrhodium(I) [29] and complexes 4e6 [30] were prepared according to the literature methods. The NMR spectra were recorded on a Varian Unity 400 spectrometer ( 1 H: 400 MHz, 13 C: 100 MHz) or Varian Mercury 300 ( 1 H: 300 MHz, 13 C: 75 MHz) at ambient temperature unless stated otherwise.…”

“…2). The synthesis and characterisation of these rhodium(I) metallodendrimers (4,5) and the mononuclear complex (6) were reported previously [30].…”