Abstract
It was studied nucleophilic attack of diethylamine upon some coordination compounds of platinum(II) containing two different unsaturated ligands, [PtCl2L’L’’]. The experimental work was
organized in three parts: (1) the synthesis of the compounds, improving literature pathways, (2) the study of the competition between two of the unsaturated ligands and (3) the reactivity towards diethylamine of the synthetized compounds.
Unsaturated ligands studied were ethylene, 1-octene, cycloexene, propionitrile, acetonitrile and carbon monoxyde. Products were characterized with IR spectra, 1H, 13 C and 195 Pt NMR spectra and, in some case, with elementary analysis; all the reaction were performed under nitrogen or argon atmosphere, using Schlenk techniques.
About olefin compounds, equilibrium between dimeric [PtCl2(olefin)]2 and monomeric
[PtCl2(olefin)2] was also investigated: at room temperature and in large olefin excess, it was
possible to observe the monomeric species of ethylene, and 1-octene, but no monomer was observed for cyclohexene.
Competition on the coordination to Pt(II) was studied for propionitrile VS ethylene: trans-[PtCl2(C2H4)]2 was treated with propionitrile as solvent; after 8 days, no substitution of ethylene
was observed, only bridge cleavage. On the other hand, solutions of [PtCl2(CH3CH2CN)2] CH2Cl2
were maintained under ethylene atmosphere for about 50 days: 195Pt NMR spectra showed the presence of cis-[PtCl(CH3CH2CN)(C2H4)] as the main product, with low quantities of
[PtCl2(C2H4)2].
About [PtCl2(RCN)(C2H4)] , with R= Me, Et, cis/trans stereoselective synthesis was investigated.
In the reaction of bridge cleavage of [PtCl2(C2H4)]2 with RCN, trans-[PtCl2(RCN)(C2H4)] is the
kinetic product, and cis-[PtCl2(RCN)(C2H4)] the thermodynamic one. The nitrile ligand shows a
very different lability in the two isomers: under vacuum treatments, NMR data and different reactivity toward diethylamine testify a really higher lability of the nitrile ligand in trans isomer than in cis one.
Treating trans-[PtCl2(RCN)(C2H4)] with diethylamine in molar ratio 1 : 1, substitution of the nitrile
is observed (see pic1-a) ; an excess of diethylamine attacks the coordinated ethylene: the π :σ reorganization brings to a zwitterionic ligand, as showed in pic 1-b.
Treating cis-[PtCl2(CH3CN)(C2H4)] with diethylamine in excess, attack on the coordinated
ethylene is observed, with the formation of the zwitterionic ligand (see pic 2-a); after 2-3 days, also the nitrile ligand is attacked, with formation of an amidine (see pic 2-b).
Under vacuum treatment causes the issue of the dietylamine from the zwitterionic ligand, with the restoration of the η2-ethylene; the amidine ligand shows to be stable under vacuum.
All the obtained products are stable under nitrogen or argon atmosphere; if exposed to the free air, they decompose in about 24-48 h.
Behaviour of [PtCl2(CO)(olefin)] and [PtCl2(CO)(NHEt2)] species toward diethylamine were
investigated, but results may be considered only as preliminary. In the case of [PtCl2(CO)(olefin)],
substitution of the olefin was observed, but it is quite probable that this behaviour was originated by an accidental contamination by atmospheric water. In the case of [PtCl2(CO)(NHEt2)] , carbamoylic
bands were observed in IR spectra, but more experimental work is needed for attempt to a clear and undutiful result.
(1 -a) (1-b)
Picture 1: products of attack of diethylammine upon trans-[PtCl2(RCN)(C2H4)]
(1-a) (1-b)
