In order to achieve a deeper understanding of the behaviour of various platinum species in solution we further studied coordination chemistry of Zeise’s dimer (Scheme 14). We found that this complex is highly reactive toward ligand exchange in solution. Thus, it reacts immediately with hexyne, Me2S, PPh3, PrSp and MeCN. In reactions with Me2S or PPh3 the exchange goes till the final PtL2Cl2 (L = Me2S or PPh3), no monosubstitution product PtL(C2H4)Cl2 could be detected as intermediate. In contrast, the reaction with MeCN stopped rigorously at the stage of
no ethylene exchange occurs. Trans-Pt(C2H4)(MeCN)Cl2 slowly isomerizes to cis-Pt(C2H4)(MeCN)Cl2 in solution (56% conversion over 4 days at 40 °C in CDCl3). Notably, cis- Pt(C2H4)(MeCN)Cl2 was inert toward ligand exchange. For example, it does not liberate MeCN when dissolved in CD3CN, unlike trans-Pt(C2H4)(MeCN)Cl2.
Next, an NMR spectrum of a Zeise’s dimer/hexyne mixture was recorded to observe dynamic situation, that also would indicate fast ligand exchange processes. The exact composition of this mixture remained unknown. However we did obtain useful information from the reactivity of the mixture. We found that in the presence of a sufficient excess of hexyne the ethylene gets almost completely liberated (as evidenced by the common ethylene signal approaching the chemical shift value of pure ethylene at 5.39 ppm in CDCl3). Passing fresh air into Zeise’s dimer/hexyne mixtures led to complete elimination of ethylene from the solution, leaving only alkyne complexes. Initially there are at least two compounds, but in the presence of a large excess of hexyne only one compound remains. Based on the recently reported data, this was tentatively identified as [Pt(hex)2Cl2], while the former temporary intermediate is suggested to be a Zeise’s dimer analogue [Pt(hex)Cl2]2.14 Both these complexes exhibit slow mode of hexyne exchange as evidenced by separate sets of free/bound hexyne signals (but the hexyne signals remain slightly broadened). When Et4N+ Cl– was added to this mixture, immediate reaction followed to form a single platinum complex, which was identified as [Pt(hex)Cl3]–.15 All the dynamic ligand exchanges got frozen, sharp signals of hexyne excess were completely restored.
These facts points to the importance of trans-effects in platinum chemistry, which means that any ligand in a square platinum(II) complex directly influences the rate of ligand exchange specifically at the opposite position.16 Simply MeCN or Cl–
, have a much lower trans-effect than ethylene or alkyne and this makes the ligand exchange at the trans- position to MeCN or Cl– rather slow. This conclusion was additionally verified in experiments with a simple complex Pt(MeCN)2Cl2 (available as a mixture of cis- and trans- isomers).17 Indeed, this complex was completely unreactive towards hexyne. Pt(MeCN)2Cl2/MeCN mixtures exhibited separate signals for bound and free MeCN, indicating slow ligand exchange. In contrast to Pt(C2H4)(MeCN)Cl2, the complex reacted slowly with Me2S but quickly with PPh3 to provide the corresponding PtL2Cl2. It demonstrates that ligand exchange with sufficiently strong ligands may occur smoothly regardless of the trans-effects. In contrast, the presence of a suitable trans-ligand (e.g. ethylene) becomes crucial when weak ligands (MeCN, alkyne, alkene) come to play! Typical substrates and products in platinum catalyzed alkyne chemistry would qualify as weak ligands, therefore we suggest that the huge trans-effects in ligand exchange at Pt is a very important factor controlling catalytic activity of Pt compounds. Due to the knowledge of the trans-effects, we may suggest that simple PtCl2, the most often used catalyst for alkyne reactions, simply does not allow the best exploitation of intrinsic Pt abilities. Solid PtCl2 has a polymeric structure, where Pt is surrounded only by the Cl ligands with their poor trans-effects, making PtCl2 to be reluctant in ligand exchange. It makes sense because PtCl2 requires boiling in MeCN for several minutes to
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become Pt(MeCN)2Cl2.17 And it does not give Pt(alkene)2Cl2 upon reaction with alkenes.
Now it is easier to understand what happens in Zeise’s dimer/hexyne mixtures. The NMR line broadening of all components in the system indicates that both ethylene and hexyne should have comparable binding affinities and a high trans-effect to
enable fast ligand exchange. As soon as a third strong ligand is added (Cl–), it quickly substitutes only one alkyne or alkene from one side of platinum. Because of the low trans-effect of Cl– the second ligand is left without ability to undergo fast exchange.
Scheme14. Coordination Chemistry of Platinum Compounds.a
aAll reactions in CDCl
3 at r.t. All reactions are immediately fast except otherwise noted Encouraged by the fast Zeise’s
dimer/hexyne reaction we attempted to isolate the possible hexyne complexes by evaporation of the reaction mixture. However, this led to complete formation of the known cyclobutadiene complex [Pt(C4Et4)Cl2] (shown on Scheme 14).18 It appears that formation of this complex is slow in a diluted solution, but becomes fast upon concentration. Indeed, if the Zeise’s dimer/hexyne solution is left to stay overnight, all initial dynamic processes disappear and [Pt(C4Et4)Cl2] is formed as a sole product. This complex was inert toward weak ligands like hexyne-3 or MeCN, but it did react with Me2S to give [Pt(C4Et4)(Me2S)Cl2]. The mixture [Pt(C4Et4)(Me2S)Cl2]/Me2S exhibited fast mode of ligand exchange as evidenced by a single common Me2S peak. Based on this result it was concluded, that [Pt(C4Et4)Cl2] may additionally coordinate only sufficiently strong ligands, while binding of weak alkyne or MeCN is strongly disfavored.
Since this dimerization of an alkyne could be a competitive side process during platinum catalyzed transformations in general, we investigated if [Pt(C4Et4)Cl2] possesses any catalytic activity. Control experiments indicated that it possesses very poor catalytic activity. For instance, after 19 h only 37 % conversion of 3- pentynol-1 (a very reactive substrate) was achieved in the presence of [Pt(C4Et4)Cl2] (1.7 %) in CDCl3. This complex was unable to catalyze intermolecular hydroalkoxylation of hexyne in MeOH. In light of the aforementioned inertness of [Pt(C4Et4)Cl2] to accept additional weak ligands it is not surprising that its catalytic activity is drastically decreased. Therefore, formation of Pt cyclobutadiene complexes can be regarded as catalyst deactivation. In order to check the occurrence of this undesired process under catalytic conditions, Zeise’s dimer was mixed with a large excess of hexyne in acetonitrile and ethylene was eliminated by passing fresh air through the solution. Immediate formation of trans- Pt(hexyne)(MeCN)Cl2 occurred as evidenced by NMR. When the reaction mixture was allowed to stay overnight, a high conversion of hexyne to hexanone occurred as a result of hydration by the
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alkyne dimerization should be generally slow and should not compete if the desired catalytic reaction is fast (especially in case of more complicated substrates).