New chemistry of olefin complexes of platinum(ii) unravelled by basic conditions: synthesis and properties of elusive cationic species

Abstract

The evolution in basic medium ([RO-] = 1 M in methanol, R = H or Me) of five-coordinate platinum(II) compounds, [PtCl2(eta2-C2H4)(N-N)], 2a-c, (N-N = N,N,N',N'-tetramethyl-1,2-ethanediamine, a; 2,2'-bipyridyl, b; 1,10-phenanthroline, c) leads to the formation of [PtCl(eta1-CH2CH2-OCH3)(N-N)], 5a-c. The analogous compound 5d (N-N = 2,9-dimethyl-1,10-phenanthroline, d) can also be prepared, but not via transformation of the five-coordinate species 2d in basic medium where it is quite stable. 5d can instead be prepared by reaction of d with a strongly basic methanol solution of Zeise's anion [PtCl3(eta2-C2H4)](-), 1. In such a medium the di-anionic trans-[PtCl2(OR)(eta1-CH2CH2-OCH3)](2-) species (1") reacts with to form exclusively 5d. Hydrolysis of with acids bearing weakly coordinating anions leads to [PtCl(eta2-C2H4)(N-N)]+, 3a-c, as stable cations; upon the same treatment 5d does not generate 3d, but it reacts with HCl to give 2d in almost quantitative yield. Cationic complexes 3b, 3c, here reported for the first time, were reacted with some nucleophiles and their behaviour compared with that of the already known 3a. In 3b, 3c the metal centre competes with the coordinated ethene for binding to nucleophiles; therefore the acetylacetonate anion can either add to the olefin (affording compounds 6b, 6c ) or to the metal ion replacing the ethene ligand (yielding compounds 7b, 7c). Under similar conditions, 3a gives exclusively 6a. Secondary amines readily add to ethene in 3b, 3c, affording the addition products 8b, 8c, which undergo a ready cyclization to an azaplatinacyclobutane ring (9b, 9c). The remarkable ease of the four-membered ring formation has been related to the high electrophilic character of the metal core in 3b, 3c.

A new polymorph of dichlorido(1,10-phenanthroline)platinum(II)

“…Traces of HCl, released from the solvent [33], reverse the reaction sequence that, starting from the cationic complex [PtCl(g 2 -C 2 H 4 )(1,10-phenanthroline)] + , C, and the secondary amine NHEt 2 leads to A [23]. Thus HCl promotes the opening of the azaplatinacyclobutane ring in A and favors the regression of the open chain complex B (through protonation of the freed amine) to the g 2 -C 2 H 4 complex C [34].…”

Pentacoordinate [PtCl2(η2-C2H4)(N–N′)] complexes with asymmetrically hindered nitrogen donor chelates. Stereospecific synthesis of syn- and anti-[PtCl(η1-CH2CH2OMe)(Mebpy)]

“…This special feature, which clearly indicates the unstable intermediate character of the cationic Pt olefin complexes with hindered chelates, has been already observed for [PtCl(h 1 -CH 2 CH 2 OMe)(-Me 2 phen)], [34]. The present work demonstrate that sterical hindrance at a single N-donor of a chelate system is enough to destabilize the cationic species [PtCl(h 2 -C 2 H 4 )(NeN 0 )] þ .…”

“…MeOH, before use was partially dried over molecular sieves of type 4A (activated by heating at 120 C for z 48 h). Zeise's salt was prepared by a modified procedure [34] of the method reported by Chock et.al. [50].…”

“…Last process is similar to that described for the formation of trans-[PdCl 2 (OR)(h 2 -C 2 H 4 )] À and trans-[PdCl 2 (OR)(h 1 -CH 2 CH 2 OR)] 2À anionic complexes, recognized as key steps in the Wacker process mechanism [30e33]. By using the reactivity exhibited by trans-[PtCl 2 (OR)(h 2 -C 2 H 4 )] À and trans-[PtCl 2 (OR)(h 1 -CH 2 CH 2 OR)] 2À [34].…”

Hindrance, Donor Ability of Men∩NN Chelates and Overall Stability of Pentacoordinate [PtCl2(η2‐CH2=CH2)(Men∩NN)] Complexes as Observed by η2‐Olefin 1JPt,C Modulation: An NMR Study

“…Zeise's salt was prepared from K 2 [PtCl 4 ] and ethene, through a modification [9] of the method proposed by Chock et al [50] The synthesis of R,R-and S,S-Me 4 chxn ligands was performed by using a previously reported method. [51,52] The ESI-MS spectra were recorded with an Agilent 1100 Series LC-MSD Trap System VL.…”