Palladium Complexes with 3-Alkyl-Substituted-1,10-Phenanthrolines:  Effect of the Remote Alkyl Substituent on the CO/Olefin Copolymerization Reactions

Abstract

The coordination chemistry of a series of 3-alkyl-substituted-1,10-phenanthrolines (3-Rphen) to palladium as well as the catalytic behavior of the corresponding bischelated derivatives, [Pd(3-R-phen) 2 ][PF 6 ] 2 , in the CO/vinyl arenes copolymerization reaction has been investigated in detail. The alkyl substituents differ in length and steric hindrance. The crystal structure characterization reveals that the two molecules of 3-R-phen are bound to palladium in a syn arrangement with the alkyl groups on the same side of the square planar geometry. In solution a dynamic process involving the equilibrium between syn and anti isomers is evidenced by NMR spectroscopic analysis. This is in agreement with the results of DFT calculations, which indicate similar stabilities for the two isomers. The severe distortions from the ideal square planar coordination geometry observed in the solid state are rationalized, through the DFT analysis, in terms of the HOMO orbitals responsible for the Pd-N bonds. The [Pd(3-R-phen) 2 ][PF 6 ] 2 complexes efficiently promote the CO/styrene and CO/p-Me-styrene copolymerizations to the corresponding syndiotactic polyketones. Yields and molecular weights show an increasing trend on increasing the steric demand of the R substituent, and the values recorded are the best ones ever reported for copolymerization reactions of this kind in the absence of the oxidant. From the TON numbers this result seems related to an increase of the olefin insertion rate, which proceeds faster when 3-Rphen are used as ligands. A change of the physical nature of the active species, from homogeneous to heterogeneous, occurs during the polymerization process, and the time at which this variation takes place depends on the nature of the olefin. The positive effect of the alkyl substitution is less evident in the CO/ethylene copolymerization.

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…In a previous paper in which some of the same ligands employed here had been tested, it had been noted that a higher productivity was obtained with unsubstituted styrene, 17 contrary to what was generally reported in the literature. 39,[40][41][42][43] The new, more extensive, series of data supports this observation, but also shows that the higher activity is also associated with a faster deactivation, since not only k 1 , but also k 3 values are consistently higher in the case of styrene. To the best of our knowledge, an active role of the olefin in the deactivation process has not been proposed before.…”

“…This trend is in agreement with the productivity data obtained in the batch copolymerization reactions, 17 but is the opposite of that found in the case of the phenanthroline-containing catalysts. 21,43 The kinetic analysis shows that the higher activity of the present catalysts in the CO/styrene copolymerization is also associated with a faster deactivation, since not only k 1 , but also k 3 values are consistently higher than in the case of 4-Me-styrene (Tables 2 and 3). The values of the kinetic constants highlight that in the series of the catalysts with the symmetric Ar-BIANs, those with electron-donating substituents show the lowest propagation and decomposition rate, while the ligands symmetrically substituted with CF 3 or F yield the most active catalysts, but also show the highest deactivation rate, in particular in the CO/4-Mestyrene copolymerization.…”

“…This phenomenon was previously pointed out in both the CO/ethylene 47 and CO/vinyl arene copolymerizations. 43 Thus, this decrease cannot be described by our models and we have All constant values are reported in Table 5. A good agreement exists between the two calculation approaches, except for the k 3 ' value for 3,5-(CH 3 ) 2 C 6 H 3 ;3,5-(CF 3 ) 2 C 6 H 3 -BIAN, which appears to be overestimated.…”

Unprecedented Comonomer Dependence of the Stereochemistry Control in Pd‐Catalyzed CO/Vinyl Arene Polyketone Synthesis

“…The latter trend is in agreement with mosto ft he catalytic systems described in the literature, for whicht he introductiono fa na lkyl group in para position on the vinyl arene comonomer resulted in an enhanced reactivity. [16,51] The opposite trend, analogous to that observedh erefor 1b,was,however,reported forpalladium catalysts containing bis(aryl-imino)acenaphthenes,f or which productivities in the CO/MSc opolymerization were similaro rl ower compared to those obtained in the CO/S counterpart. [22,49] The literature catalytic systemsw ith iminopyridines showedr emarkably lower productivities than those reported here, that is, from 0.097 to 0.541 kg CP g Pd…”

“…For all the three vinyl arenes, moving from the aldimine to the ketimine catalyst, as teep improvement of the Mw values was achieved,y ielding polyketones with Mw data ranging from 250 000 to almost 500 000 gmol À1 ,w hich are among the highest reported so far. [16,51] Thei ncreasei nt he molecular weight indicated that the increase in the productivity obtained with 2b was due to an increase in the length of the polymer chains rather than in their number,a sa lso revealed by the differences in the turnover numbers (TON, Ta ble 1). This suggests that, in the case of the ketimine catalyst, the increase in the ratio between the propagation and the terminationr ate might be due to both an increaseinthe propagation rate and adecreaseinthe termination rate.…”