Platinum Metals in Organometallic Chemistry
Platinum Metals in Organometallic Chemistry
The International Conference in Moscow
At the Fifth International Conference on Organometallic Chemistry, held in Moscow in August 1971, no fewer than 463 papers were presented, of which 189 were actually delivered orally, together with 11 section lectures, in three parallel sessions. This review can therefore give only a brief account of such a conference and of the many contributions that concerned the platinum metals.
A highlight of the Conference came on the first day when Professor Geoffrey Wilkinson (Imperial College, London) cast aside many time-honoured ideas about the factors responsible for the formation of strong metal-carbon σ-bonds. Wilkinson emphasised that, before these factors can be meaningfully discussed, it is necessary to have criteria for deciding how to measure bond strength. The ideal source of data, namely thermodynamic measurements, is at present insufficient to be of any value in comparing bonds. In addition, relatively few metal-carbon σ-bond lengths are known with sufficient accuracy for them to be used. One possible way of comparing bond strengths is to compare the thermal stabilities of complexes, although for this to be used it is essential to understand the mode of decomposition of the complex which Wilkinson suggested was by an “alkene-elimination hydride transfer” mechanism:
The formation of a stable metal-carbon σ-bond requires the formation of a complex stabilised against such a reaction. This may be achieved by:
the formation of complexes with alkyl ligands of the type −CH2Y where Y is an atom that cannot form a double-bond to carbon (e.g., Y = H, CMe3, SiMe3 or Ph),
blocking all the coordination sites around the metal atom with strong ligands such as triphenylphosphine, pyridine, or even carbon-bonded ligands themselves, thus preventing a second coordination site being available for the hydride to transfer to. This new approach, which satisfactorily explains many of the anomalies that have recently arisen in connection with Chatt’s electronic theory of the stabilisation of metal-carbon σ-bonds by tertiary phosphine ligands has enabled Wilkinson to prepare a number of binary alkyl complexes of such species as VIV, CrIV, MoIV, WIV, etc., with ligands such as −CH2SiMe3. The stability of these complexes dispels the suggestion that the presence of a tertiary phosphine or other similar ligand is necessary for the formation of stable metal-carbon σ-bonds.
F. J. McQuillin (University of Newcastle upon Tyne) reported on the reaction of substituted cyclopropanes with platinum(II) complexes to form analogues of Tipper’s Compound:
It was found that the reactivity of the cyclopropane increased with increasing electron density in the ring (with electron withdrawing groups such as CN or OCH3 no reaction occurred). When R was H, n -C6H13, Ph, o -NO2C6H4, PhCH2, the resulting product
indicated that ring opening had occurred at the least substituted bond, probably due to electronic rather than steric effects since the bulky p -tolyl group gave
Although with trans- 1,2-disubstituted cyclo-propanes ring opening occurred ring opening occurred to give
with cis -1,2-disubstituted cyclopropanes, the cyclopropane was isomerised to olefin. No platinocyclopropane ring complex was formed.
H. M. Walborsky (Florida State University) reported a study of the decarbonylation of acyl complexes of the “Wilkinson Catalyst”. The reaction was stereoselective but not stereospecific, and consistent with the formation of a radical pair intermediate:
Crociani (CNR, Bologna) reported a study of the kinetics of formation of carbene complexes of palladium(II) according to the above reaction.
It was found that the rate increased with both increasing σ-donor ability of the amine nitrogen and increasing electron-withdrawing ability of the substituent Y on the isocyanide ligand indicating that the reaction proceeds by nucleophilic attack of the amine on the coordinated isocyanide.
R. J. Angelici and his colleagues (Iowa State University) reported two routes for the preparation of carbene complexes of platinum(II) and palladium(II):
(i) Nucleophilic attack of amines on cationic carbonyl complexes.
This reaction, which can be reversed by addition of hydrochloric acid, is only applicable to cationic carbonyl complexes, since in neutral carbonyls the amine substitutes the carbonyl ligand.
(ii) Oxidative addition of and to platinum(O) and palladium(O) complexes.
The triphenylphosphine complex shows no rotation about the C-N bond even on heating to 200°C, although with less bulky tertiary phosphine such rotation is observed.
Professor Abel read a paper by M. F. Lappert and his co-workers (University of Sussex) on the use of electron-rich olefins in the synthesis of carbene complexes in which the published range of olefins that can be used in the reaction
was extended to include
The bis(dibenzylideneacetone) complexes of both platinum and palladium were described by two groups of workers, Professor Maitlis of McMaster University, and Ito and his colleagues from Nagoya University, Japan, as useful air-stable starting materials for the preparation of other complexes of these metals. The complexes are prepared by treating Na2PdCl4 or Na2PtCl4 with dibenzylideneacetone (DBA) in an alcoholic solvent in the presence of sodium acetate. The structures of the complexes [M(DBA)2] are unknown, the Canadian group favouring bonding of the carbonyl group to the metal and the Japanese bonding of the olefin group to the metal.
Professor Maitlis, in a masterly section lecture, reviewed his work on the cyclooligomerisation of acetylenes by palladium salts and emphasised the importance of the oxidation state of the metal on the mechanism of the cyclotrimerisation of acetylenes. This is not unexpected, since acetylene complexes of palladium(O) involve strong π-back donation of electron density from the metal to the acetylene so that the acetylene is susceptible to electrophilic attack whereas, in the acetylene complexes of palladium(II), σ-donation of charge from the acetylene to the metal is more important than the reverse π-back donation so that the acetylene is susceptible to nucleophilic attack. These and other differences in the properties of olefin and acetylene complexes of transition metals were considered by the present author, who suggested that olefin complexes fell into one of two classes, whose similarities and differences are shown in the table.
In other papers on olefin complexes Panunzi (University of Naples) reported extensions of his work on the attack of nucleophiles on olefin complexes to include both cis- and trans -[Pt(olefin)LCl2] and cationic [Pt(olefin)ClLL′]+ complexes (L is a ligand that enables the initial platinum(II)alkyl complex to be isolated). It was found that the stereochemical route of the reaction of the trans -complexes was less dependent on the nature of L than in the cis -complexes.
H. Thyret of Munich described evidence which suggested that an earlier report of a vinyl alcohol complex of platinum(II), [(CH2=CHOH)PtCl2]2, was incorrect and described the preparation of complexes of Fe(CO)4 with vinyl alcohol and vinyl alcohol derivatives. H. A. Tayim (American University of Beirut) has prepared complexes of 1,4-cyclooctadiene (1,4-COD) with PtII, PdII, RhI, AgI, and Cu1 and found that the properties of 1,4-COD are intermediate between 1,3-COD and 1,5-COD.
Y. Fujiwara of Osaka University reported a detailed study of the reaction of styrene and benzene in the presence of palladium(II) acetate to give trans -stilbene. The electronic effects of substituents in the aromatic rings (electron-donating substituents in the benzene give ortho, para -substitution; electron-withdrawing give meta -substitution; electron-withdrawing substituents on the styrene make it more reactive), the steric effect of groups on the olefin, which lead to the aryl group being introduced on to the least substituted carbon atom of the olefin, and the fact that no hydride shift occurs during the reaction are consistent with the mechanism shown here:
Professor Stone of Bristol discussed the ability of unsaturated fluorocompounds to form 3- and 5-membered rings with zerovalent platinum, palladium, and nickel. Thus, for example, while most unsaturated fluoro-compounds such as CF2=CF2, (CF3)2C=O, (CF3)2C=NCH3 form 3-membered ring compounds of the type
some of these compounds can react further to give 5-membered ring compounds:
Stone showed that such ring expansion depended in a complex way on the electronic properties of the phosphine ligand, which should be a strong, but not too powerful, σ-donor. Nickel(O) appears to give ring expansion more readily than platinum(O).
A. P. Belov (Kurnakov Institute, Moscow) described the chemical oxidation of π-allyl palladium halides in aqueous solution and showed that the redox potential decreases as the halide is altered in the order chloride>bromide>iodide.
F. Glockling (Queen’s University, Belfast) reported the preparation of platinum(IV) complexes of trimethylstannane which, although widely implicated as intermediates
in the reactions of platinum(II) complexes of trimethylstannane, had not been isolated until recently. Two main routes both involving oxidative-addition reactions were reported for the preparation of these complexes:
M. I. Bruce (Bristol University), described a number of interesting complexes in which iron, ruthenium, or osmium were co-ordinated directly to platinum(II).
J. Dehand (Institute of Chemistry, Strasbourg) discussed the vibrational spectra of the related compounds [(pyr)2M(M′(CO)n)2], where M=Pd or Pt, M′(CO)n = Mn(CO)5, Co(CO)4 or Mo(CO)3(π-C5H5) and pyr = pyridine, 3-methylpyridine or 4-methyl-pyridine.