Studies In The Behavior Of Certain Cyclic Olefins Toward Divalent Platinum And Palladium

The initial program of study for consideration was the extension of cyclooctatetraene complexation to other metals of the transition group. Compounds of (COT) with iron, cobalt, chromium, molybdenum, and tungsten which have been reported in the literature involve carbonyl ligands simultaneously as (COT)M(CO)x with x = 2,3----; only Ag(I) as AgN03 adducts are known. Rhodium (I) is said to form an unstable dimer while ruthenium (I) forms a polymer. (COT)Pt(II) chloride, bromide and iodide have been reported but only the iodide has been studied; only (COT)PdCl2 has been reported recently. It was further conceived that substituted (COT) would present a good comparison a ligand especially if steric inhibition of pi-bonding were to be observed. The second phase of the study was limited to platinum (II) and palladium (II) chloride and bromide, owing to the known greater stability of the platinum and palladium complexes; attempted investigation with other metals proved fruitless. The following olefins were chosen for their availability and for the fact that each of these is an eight carbon olefinic system and hence can be considered as an extension of the (COT) system. a. 1,5-cyclooctadiene 1,5-(COD) b. 1,3-cyclooctadiene 1,3-(COD) c. 4-vinylcyclohexene 4-(VCR) d. cyclooctene (COC) Though 1,5-(COD) had been reported as a ligand, the literature records few data on its complexes. The choice of 1,3-(COD)--the isomer of 1 ,5-(COD)--as a ligand posed interesting stereochemical problems in that if both double bonds of this system were employed in pi-bonding to the same metal atom it would produce a distortion of the perpendicularity requirement. On the other hand, 1,3-(COD) offers a conjugated system which might conceivably enhance stability of an over-all pi-bonded system. The other possibilities, of course, would be the use of only one ring olefin per metal atom as: [see PDF file for the chemical bond] Finally, each ring might form a bridge system between two metal atoms. This would be possible if the -MX2 planes were perpendicular rather than coplanar [see PDF file for the chemical bond] In any event there is no mention in the literature on the use of 1,3-(COD) as a ligand except by Cope, Stevens and Hochstein (1950) who prepared a silver nitrate adduct 1 ,3-(COD)· 2AgNO.3 4-vinylcyclohexene provides two olefin systems. Would this hydrocarbon behave as a diolefin or would there be exhibited a preference in pi-bonding of one olefin site over the other? Many possible combinations could be conceived; some of which are shown below. [see PDF file for the chemical bond] The inclusion of cyclooctene in the study was made because the only other cycloalkene reported in the earlier literature was cyclohexene. Only an unstable adduct AgNO3• (C8H14)2 was reported by Jones (1954). Recently a compound with the trans form of cyclooctene was made by Cope (1963). [See PDF file for the chemical bond] This compound exists as diastereoisomers and subsequently was resolved into two optically active forms. In the program of study carried out in this research ciscyclooctene was used exclusively. Not only was it the object of this research to attempt the preparation of the olefin complexes, but also to determine something of their chemistry, such as, thermal stability, displacement reactions and the character of their absorption spectra in the visible, ultraviolet and infrared regions. It was intended to extend the often made comparison of platinum chemistry with that of palladium. Toward the end of this research, one other olefin 1,5,9-cyclododecatriene was employed in a limited investigation because of its triple unsaturation; inspection reveals its relation to 1,5-(COD) by the appearance of two condensed rings with 4 fewer carbon atoms. Yet, the olefins are each 4 carbons removed from the other as in the case of 1,5-(COD)