Supporting information Structure and Dynamics of Catalytically Competent but Labile Paramagnetic Metal-hydrides: the Ti(III)-H in Homogeneous Olefin Polymerization

Metal hydride complexes find widespread application in catalysis and their properties are often understood on the basis of the available crystal structures. However, some catalytically relevant metal hydrides are only spontaneously formed in situ, cannot be isolated in large quantities or crystallised and their structure is therefore ill defined. One such example is the paramagnetic Ti(iii)-hydride involved in homogeneous Ziegler–Natta catalysis, formed upon activation of CpTi(iv)Cl3 with modified methylalumoxane (MMAO). In this contribution, through a combined use of electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies we identify the nature of the ligands, their bonding interaction and the extent of the spin distribution. From the data, an atomistic and electronic model is proposed, which supports the presence of a self-assembled ion pair between a cationic terminal Ti-hydride and an aluminate anion, with a hydrodynamic radius of ca. 16 Å.


Table of contents
In modified-MAO (MMAO) some isobutyl groups replace methyl groups in the structure giving an overall composition of [(CH 3 ) 0.7 (i-C 4 H 9 ) 0.3 AlO] n . In turn, this modification increases stability and solubility in aliphatic hydrocarbon solvents, but without affecting the three-dimensional structure or reactivity.

S3
Calculation of s-spin density from the isotropic hyperfine constant From the isotropic (a iso ) hyperfine couplings, the s-contribution of each nucleus to the SOMO in a molecular fragment can be obtained. Assuming that the a iso hyperfine coupling interaction at a given nucleus is proportional to the electron spin density at that nucleus, one can obtain the spin population in s-type orbitals  s . For an unpaired electron (free electron, g e =2.0023) with a unitary spin population ( s =1) in an s-type orbital, one would observe an isotropic hyperfine coupling constant of a 0 1H = 1420 MHz for a 1 H nucleus and a 0 27Al = 3367.76 MHz for a 27 Al nucleus. Including a correction for the difference in the g values, the spin populations in s-type can thus be estimated as: where g e is the free electron g-value, g e = 2.00231930436256(35).

Individual contributions of Ti-H and Ti-R
The simulations of the CW spectra reported in Figures 1b and 2a (main text) contain both the contribution of Ti-H and Ti-R. In Figure S2 we report the individual spectral components for the X-band spectrum at RT and the Q-band spectrum at 50 K. The relative spin concentrations [Ti-H]:[Ti-R] vary from 30:70 at RT to 35:65 at 50 K as result of the different relaxation properties of the two species. The simulation parameters are those reported in Table 1 (main text).

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ENDOR simulation inclusive of weakly coupled protons Figure 3 in the main text reports the orientation selective simulations for the hydride proton. A reasonable simulation of the whole ENDOR spectrum (obtained as the sum of the three spectra presented in Figure 3, main text) can be accomplished considering three distinct contributions: i) the aforementioned hydride protons; ii) typical values for Cp protons B ; and iii) an additional coupling compatible with those measured for CpTi(III)(CH 3 ) 2 C and is reported in Figure S4. Given the low signal-to-noise ratio of the ENDOR spectra and to simplify the determination of the principal components of the hyperfine tensor by eliminating the orientation selectivity, the sum of the individual ENDOR spectra were simulated. A similar procedure was described by Enemark D and is justified by : 1) the small g anisotropy so that its effects on the anisotropy of the hyperfine interaction can be neglected; and 2) the negligible change of ν I across the EPR spectrum (this also depends on condition 1).
The hyperfine parameters (a iso and T) for each set of protons are reported in the table below. The sum simulation does not include any contribution for remote protons (a ~ 0 MHz), therefore the peak centered at ν RF -ν H = 0 MHz is not accounted for. This data supports the idea that the interaction between Ti and Al is mediated by the methyl groups of MAO and provide an estimate of the hyperfine couplings as well as corroborating the fit of the hydride proton presented in the main text.   Table S1.

S6
Additional HYSCORE spectra At each field position, HYSCORE spectra (π/2-τ-π/2-t 1 -π inv -t 2 -π/2-τ-echo) were recorded with two different τ values to minimise blind spot effects (g x = 110 and 148 ns, g y = 100 and 148 ns, g z = 100 and 150 ns). Here we report the full spectra -both (+,+) and (-,+) quadrants -where the spectra recorded at different τ values were summed together after Fourier transform. The supplementary spectra do not provide any additional constrains for the determination of 27 Al parameters, but also do not show any 35,37 Cl signals further supporting the notion that the Clligands present in the parent CpTi(IV)Cl 3 are completely lost upon activation with MMAO in toluene. Figure S4. HYSCORE spectra recorded at a) 1205.4 mT (g z ), b) 1210.0 mT (g y ) and c) 1216.0 mT (g x ). All spectra were recorded at 50 K. For each magnetic field setting, two spectra were recorded and added together after Fourier transform.

Dynamic properties of Ti-H
The A 1 H tensor derived from Q-band ENDOR measurements (Figure 3 main text), along with the g tensor obtained from the simulations of Q-band CW of the frozen solution spectra (Figure 2a main text), was used to simulate the motionally averaged (fluid solution) CW spectrum at X-band, assuming an isotropic tumbling. Considering an intrinsic linewidth 0.3 mT (≈ 8.4 MHz), which is compatible with the hyperfine values derived in this work, a series of simulations has been performed. Figure S5A reports the whole set of simulated EPR spectra for Ti-H at various correlation times, from τ corr = 110 -12 s (fast motion limit) to τ corr = 510 -09 s. Figure  S5B, C, and D reports a direct comparison of the experimental spectrum with the simulated one corresponding to the best fit (D, τ corr = 410 -10 s) and the lower un upper limits (B, τ corr = 210 -10 s and C, τ corr = 610 -10 s, respectively). As a point of reference a direct comparison in the fast motion limit (τ corr = 110 -12 s) is reported in Figure S5E.
The estimated rotational correlation time can be translated into a hydrodynamic radius (r h ) through the Stoke-Einstein-Debye E,F equation where η is the dynamic viscosity (for toluene η = 0.00055 N s m -2 ), k B is the Boltzmann constant (J K -1 ), T is the absolute temperature (K) and D corr is related to τ corr derived from spectral simulation through the relation D corr = 1/6*τ corr . Using the τ corr values derived from the analysis of the motionally averaged spectrum, the hydrodynamic radius for the Ti-H/MMAO ion pair in toluene solution is found to be within 1.28 < r h < 1.85 nm, with a most likely value of 1.61 nm. This value is somewhat larger than what reported for Zr complexes ion-paired to MAO measured through NMR. G and consistent with values for Ti complexes forming ion pairs with modified MAO H . Figure S5: Panel A reports the simulated spectra for Ti-H at various rotational correlation times in the interval 110 -12 s < τ corr < 510 -09 s. Simulations were performed at mw frequency = 9. 5 GHz and considering the full g and A 1 H tensors, as reported in Table 1 (main text). Panels B to D report a direct comparison of the experimental spectrum of Ti-H at RT with simulated spectra at various correlation times, namely τ corr = 210 -10 s (lower limit, B), 410 -10 s (best fit, D), 610 10 s (upper limit, C). The corresponding hydrodynamic radii are also reported (1.28 < r h < 1.85 nm). For reference, panel E shows the comparison of the experimental spectrum for Ti-H with the spectrum simulated in the fast motion regime (τ corr = 110 -12 s). The Easyspin routine Garlic was used to simulate all spectra reported in Figure