Complexation of Diphenyl(phenylacetenyl)phosphine to Rhodium(III) Tetraphenyl Porphyrins:  Synthesis and Structural, Spectroscopic, and Thermodynamic Studies

Abstract

The coordination of diphenyl(phenylacetenyl)phosphine (DPAP, 1) to (X)Rh(III)TPP (X = I (2) or Me (3); TPP = tetraphenyl porphyrin) was studied in solution and in the solid state. The iodide is readily displaced by the phosphine, leading to the bis-phosphine complex [(DPAP)(2)Rh(TPP)](I) (4). The methylide on rhodium in 3 is not displaced, leading selectively to the mono-phosphine complex (DPAP)(Me)Rh(TPP) (5). The first and second association constants, as determined by isothermal titration calorimetry and UV-vis titrations, are in the range 10(4)-10(7) M(-1) (in CH(2)Cl(2)). Using LDI-TOF mass spectrometry, the mono-phosphine complexes can be detected but not the bis-phosphine complexes. The electronic spectrum of 4 is similar to those previously reported with other tertiary phosphine ligands, whereas (DPAP)(I)Rh(TPP) (6) displays a low energy B-band absorption and a high energy Q-band absorption. In contrast to earlier reports, displacement of the methylide on rhodium in 5 could not be observed at any concentration, and the electronic spectra of 4 and 5 are almost identical. Isothermal titration calorimetry experiments showed that all binding events are exothermic, and all are enthalpy driven. The largest values of DeltaG degrees are found for 6. The thermodynamic and UV-vis data reveal that the methylide and the phosphine ligand have an almost identical electronic trans-influence on the sixth ligand.

Selection and Amplification of Mixed‐Metal Porphyrin Cages from Dynamic Combinatorial Libraries

“…Similar shifts are observed for model complexes of the Ru and Rh porphyrins with DPAP. [6,7,10] H a occurs in the most upfield position in cage 2, whereas in the cages 1 and 3 the downfield shifts are slightly smaller. H b shows a similar trend but with a smaller shift differences between the various cages, and H c resonates at approximately the same value for all cages.…”

“…Since the chemical shift of the meso protons of Zn1 are mainly influenced by the presence or absence of any rhodium porphyrin, the upfield shift suggests that the intermediate complexes are composed of various mono-and bisphosphine complexes of the composition [Zn1/Rh2], [Rh2/Zn1/Rh2] and [Zn1/Rh2/Zn1], with or without a bound bpy group. Given the binding constants of K 1 = 3 î 10 7 m À1 for the first binding of phosphine to Rh2 and K 2 = 4 î 10 4 m À1 for the second binding, [6] we can determine that over 99 % of the first binding site has phosphine bound, while 88 % of the second binding site is occupied under the experimental conditions. The depletion of Zn1 is therefore considered to follow pseudo-first-order kinetics, and the rate constant for the process was determined to be (2.1 AE 0.1) î 10 À4 s…”

“…This study is a continuation of our earlier work, [5] in which we prepared a tetraporphyrin cage composed of two bisphosphine-substituted Zn II porphyrins as donors, two Rh III TPPs as phosphine acceptors, and one 4,4'-bpy as a template (ligand), the cage being stabilized through orthogonal Zn II ±nitrogen and Rh III ±phosphorus coordination modes (Scheme 1). We now extend the study to include several different Rh III /Ru II porphyrins and different templates, and also to combine Rh III or Ru II porphyrin building blocks in complex mixtures to examine the prospects for selective templating of new mixed-metal porphyrin cages (Table 1) From previous studies on phosphorus [6,7] and nitrogen [5,8] complexes with metallo porphyrins, we know that the Ru/ RhÀP bond is kinetically more inert than the ZnÀN bond. This is manifested in, for example, the 1 H NMR spectrum for bpy bound to zinc porphyrin: the resonances appear broad, and the chemical shift is concentration dependent, which indicates fast exchange between free and bound species.…”

“…[5] For phosphines bound to rhodium or ruthenium porphyrins with a binding constant in the range of 10 6 to 10 7 m À1 , [6,7] the resonances are generally sharp and well resolved, which indicates slow ligand exchange. These values were determined by using a model ligand, diphenyl(phenylacetenyl)phosphine (DPAP), that has an identical substitution pattern on the phosphorus but does not possess the porphyrin moiety.…”

“…This substantial distortion of the cage geometry is in accordance with our previous suggestions, derived from 1 H NMR data. [6] Projection approximately along the Rh¥¥¥Rh vector (Figure 7b) shows that the least-squares planes through the Rh3 porphyrins are tilted by 60.1(1)8 with respect to the planes through the Zn1 porphyrins. The perpendicular separation between the leastsquares planes of the Zn1 porphyrins is about 11.55 ä, and the two porphyrins are offset from each other, such that the bpy ligand is tilted by about 178 from the normal to the Zn1 porphyrin planes.…”