Catalytic Effect of Sulfoxides on the Fluxional Motion of 2,9-Dimethyl-1,10-phenanthroline in a Platinum(II) Complex:  Evaluation of Steric and Electronic Contributions

Abstract

Addition of external weak nucleophiles to a chloroform solution of the cationic complex ion [Pt(Me)(dmphen)(PPh(3))](+) (1) accelerates the fluxional motion of the symmetric chelating ligand 2,9-dimethyl-1,10-phenanthroline (dmphen) between nonequivalent exchanging sites. The rates of the dynamic process can be measured by line-shape analysis of the (1)H NMR spectra. Concentration-dependent measurements were carried out with the ligands SOMe(2), SO(CH(2))(4), SO(n-Bu)(2), SO(sec-Bu)(2), SO(i-Pr)(2), SOEt(Ph), SOPr(Ph), SO(Bz)(2), SO(p-MeC(6)H(4))(2), SOPh(2), SO(p-ClC(6)H(4))(2), SOMe(p-MeOC(6)H(4)), SOMe(p-MeC(6)H(4)), SOMe(Ph), SOMe(p-BrC(6)H(4)), and SOMe(p-ClC(6)H(4)). The rate constants k(obsd), when plotted against the concentration of the added ligands SOR(R'), give a family of straight lines with a common intercept, indicating that the two-term rate law k(obsd) = k(1) + k(2)[SOR(R')] is obeyed. The same rate law applies to the displacement of SOMe(2) from [Pt(Me)(phen)(SOMe(2))](+) (2) (phen = 1,10-phenanthroline) by sulfoxides (SOMe(2), SO(i-Pr)(2), SOMe(p-MeOC(6)H(4)), SO(p-MeC(6)H(4))(2), SOPh(2), SO(p-ClC(6)H(4))(2), and SO(sec-Bu)(2)). The fluxional rates in 1 are 6-7 orders of magnitude higher than the substitution rates in 2. The values of the rate constants for the two processes were resolved quantitatively into steric and electronic contributions by use of quantitative analysis of ligand effects (QALE). Inhibitory steric effects are linearly operative for the entire set of ligands, the rates of the reactions are enhanced with increasing electron donor capacity of the sulfoxides, and there is a small but significant E(ar) effect that enhances the reactivity of the aryl sulfoxides. The strict similarity of the patterns of the two processes and of their dependence upon the stereoelectronic properties of the ligands, combined with the intrinsic lability of the platinum-nitrogen bonds, would suggest the operation of stereospecific consecutive ring-opening and ring-closure steps for the fluxional motion of dmphen in 1. However, the available evidence does not allow alternative mechanisms involving intramolecular rearrangements of the five-coordinate intermediate to be ruled out.

Overcrowded Organometallic Platinum(II) Complexes That Behave as Molecular Gears

“…No distinction can be made among the possible reaction pathways on the basis of the kinetic data alone. However, we observe that 1) the flipping motion of dmphen in these complexes features an uncatalyzed intramolecular geometrical isomerization that, in platinum(II) complexes, is known to proceed with a dissociative mechanism very similar to that described in Scheme ,16 2) if the addition of a fifth ligand is a prerequisite for the occurrence of the process we would have expected activation of fluxional motion of phen in 2 and an acceleration and a catalytic effect in 1 , 3 , and 4 by the addition of external nucleophiles that are much stronger than the oxygen of the methoxy group,8 and 3) an associative attack should be facilitated by the number of oxygen atoms on the phosphane ligand, while the rates increase sharply along the series of complexes in the order 1 < 3 < 4 . Therefore, we conclude that release of steric congestion in these compounds confers a further impetus to ring opening, as expected from the results of previous studies, but ring closing is impeded by the contemporary slow rotation about the PtP bond.…”

“…When addressing the question of how the connection between the two dynamic motions in compound 1 is generated, we must take into account two main observations: 1) the rate of phosphane rotation is not greatly influenced by the coordination environment or whether the phenanthroline is flat or distorted; rather, the contrary seems to be true, namely the fluxional motion of dmphen in 1 is controlled by the rotation rate of the crowded phosphane; 2) the fluxional motion of dmphen in compound 1 is not affected by the nature of the counterion, the solvent (including CD 3 OD), or by deliberate addition of weak nucleophiles (sulfoxides or sulfides) to the solution;8 no traces of free PR 3 were observed by 31 P{ 1 H} NMR spectroscopy. These observations are perfectly in keeping with the dissociative mechanism established for these compounds 7–9.…”

The Control of the Stereochemistry in the Palladium-Catalyzed Alternating Styrene/Carbon Monoxide Copolymerization: Effect of the Chirality of the Ligand and of the Ligand-to-Palladium Ratio, Preliminary Communication

“…of the meso-ligand 3 is added to a solution of the Pd- [16]. These experiments clearly indicate that association of a second molecule of the ligand takes place, yielding a five-coordinated species, which is involved in fluxional processes [15]. Complete exchange between the bound and the free ligand was found to occur in the aforementioned Pd-bioxazoline [10].…”

“…Indeed, exchange of the meso-ligand by itself cannot change the nature of the catalytic species. The isomerization of the coordination site might be caused by association of the free ligand with the formation of sterically labile five-coordinated intermediates [15]. The chemical behavior of the related species in solution was investigated by means of NMR spectroscopy.…”

N′-(3-Bromo-2-Hydroxybenzylidene)-4-nitrobenzohydrazide and Its Oxovanadium(V) Complex: Synthesis, Crystal Structures, and Catalytic Property

“…[8,9] Sulfoxidation catalyzed by complexes of transition metals such as titanium, manganese, iron, molybdenum, and vanadium have produced interesting results in the synthesis of various sulfoxides. [10][11][12] As a continuation to explore effective catalysts used in sulfoxidation, in the present paper, a new benzohydrazone compound, N -(3-bromo-2-hydroxybenzylidene)-4-nitrobenzohydrazide (H 2 L; Scheme 1), and its oxovanadium(V) complex, 2[VOL(BHA)]·CH 3 CN, where HBHA is benzohydroxamic acid, were prepared and investigated for their peroxidic oxidation on sulfides. …”