Single-chain polybutadiene organometallic nanoparticles: an experimental and theoretical study

High molecular weight polybutadienes and rhodium complexes were used to produce single chain organometallic nanoparticles. A relationship was found between the cis double bond content of the polymer and metal binding kinetics.

introduced separately into 5 ml syringes. The formation of Rh(I)-ONPs was monitored at 360 nm. Rate constants were fitted to a single exponent.

Kinetic study of ligand exchange of Rh(I)-ONPs with PCy 3 . Rh(I)-ONPs (10 mol% Rh(I))
were prepared from each PBD/PCOD (1.4 μM in THF) according to the synthetic procedure described above. Then, 5 ml of each Rh(I)-ONPs solution and PCy 3 (0.014 M in THF) were introduced separately into 5 ml syringes. The decrease in the Rh-olefin absorption peak was monitored at 360 nm. Rate constants were fitted to a single exponent. 3 Potassium tert-butoxide (0.7 g, 6.25 mmol) was added to a suspension of butane-1,4-diylbis(triphenylphosphonium) bromide (2.0 g, 2.70 mmol) in diethyl ether (40 mL) at 0 C. The yellow suspension was stirred at 0 °C for 30 min.

Preparation of cis,cis-4,8-dodecadiene.
Butyraldehyde (0.6 mL, 6.67 mmol) was added drop-wise at 0 C. The resulting mixture was stirred at room temperature for 60 h. The reaction mixture was quenched by the addition of water (5 mL) and diluted with hexane (50 mL). The organic phase was separated, washed with water (30 mL), brine (20 mL) and dried over anhydrous MgSO4, and then the solvent was evaporated in vacuo. The crude product was purified by silica-gel column chromatography using hexane as eluent to afford cis,cis-4, 8  of 95% cis-PBD was weighed and dissolved 2 ml of THF in two separate vials. A solution of 2 ml of 1 in THF was added to each vial in one portion (5%, 3.5 mM and 10%, 7.0 mM) and allowed to stand at room temperature. After 16 hours, the samples were evaporated in vacuo and DSC measurements were done.                      TGA analysis has shown that 95% cis-PBD has a single degradation step with a midpoint of 459.1 C. As more rhodium was added to the polymer, the midpoint of this step was shifted to lower temperatures (431.7 C and 412.4 C for 5% and 10%, respectively). In addition, the added rhodium led to another degradation step at even lower temperatures -in the 5% Rh(I) sample, 7.5% of the sample was degraded at 213.4 C and in the 10% Rh(I) sample 18.7% of the sample was degraded at 196.7 C

Computational calculations of rhodium binding and release Theoretical Method:
The B97D3BJ method of Grimme was selected for geometry optimizations 5,6 , since it provides fast but also reliable geometries for transition metal complexes. It includes the most advanced dispersion correction of Grimme's group, and being a pure GGA, density fitting correction can be used to significantly accelerate the computations. The Def2-SVP basis set is not always accurate enough for energies, but it is adequate and inexpensive for geometries, and therefore it was used for all the optimizations. This method provides good solvation Gibbs energies, and thus it is more adequate for Gibbs energies (the values expressed in the article) than for internal energies.
Nevertheless, as can be seen from table S8, the relative kinetics of the cis and trans butene ligand substitution are almost unaffected by the solvent, as the solvation energy is practically identical for both reactants.
In the paper it is argued that the association energy for the double cis octadiene is less exothermic than for the double trans conformer. It must be noted that there is another cis-diene complex conformation ("conf. II" in table S8, see its xyz geometry below) with stronger association energy than the one depicted in Fig. 6 of the main text. This conformer is still less stable than the trans case, so the postulated explanation of the faster cis diene dissociation still stands when considering this alternative cis conformation.
Nonetheless, it is much harder to obtain the conf. II through a stepwise substitution of the ethene groups, and therefore we did not consider it further.
As explained in the main text, breaking the verticality of the olefin ligand diminishes the back-bonding ability of the metal to the ligand, generating a more labile bond. To quantitatively analyse this effect, we studied the energy required to twist the dihedral angle of the double bond in a model trans-Rh(PH 3 ) 2 Cl(H 2 C=CH 2 ) complex with respect to the complex plane. By twisting the ligand by 10 o and 20 o , the energy of the complex rises by 5.6 and 11.0 kJ/mol, respectively. This shows that the stronger steric impediment felt by the trans-butene indeed generates a significant effect on the stability and ligand substitution kinetics of the Rh complex. Table S8. Absolute and relative energies of the computed systems (see geometries below). An "Inverted" conformation corresponds to the systems with an inversion of the angle between the coordination planes of the two metal atoms. For the transbutene system the olefin can be coordinated in two ways, as can be seen in Fig. 4 of the main text; "conf II" has a slightly higher TS cd , and therefore corresponds to the less crossed pathway (hence not discussed in the article).