Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid ...

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B. Pulsed Gradient Spin-Echo Diffusion NMR Studies on 2-E 2.
1 H NMR pulsed gradient spin-echo (PGSE) NMR measurements were performed using the
standard echo pulse sequence on a Varian INOVA 500 MHz spectrometer at 298.15 K without
spinning. The gradient shape was rectangular, with gradient length 2-4.2 ms, and the gradient
strength was varied during the experiment. The spectra were processed with 2.0 Hz line
broadening, and single exponential fits of selected resonances provided values for the diffusion
coefficient D t. The experimental diffusion coefficient was used to obtain values of hydrodynamic
radii and volumes using Chen’s modification 14 of the Stokes-Einstein equation:
D t = kT
cπηr H
6
c =
1+ 0.695 r ⎛⎛ ⎞⎞
solv
⎜⎜ ⎟⎟
⎝⎝ rH ⎠⎠
2.234
(S1)
c = 6 is the Stokes-Einstein equation.
The measurements were carried out in C 6D 5Cl (an atypical solvent for diffusion studies),
due to low solubility of 2-E 2 in nonpolar solvents, reactivity with halogenated alkane solvents
such as CD 2Cl 2, and interaction of the borane groups with strong donor solvents such as CD 3CN.
The value for r solv (2.69 x 10 -10 m) 15 in equation S1 is that of C 6H 5Cl, a reasonably good
approximation. The viscosity, η (0.008 Pa s -1 ) 16 is that of C 6D 5Cl. The classical Stokes-Einstein
equation has c = 6, whereas the Chen modification corrects for molecules that are not much
larger than the solvent they are in. As expected for a moderately sized organometallic molecule,
S69