The Union Carbide (UC) ethylene polymerization catalysts, based on chromocene dispersed on silica, show distinct features from the Phillips catalysts, but share the same heated debate regarding the structure of its active sites. Based on a combination of IR, EPR spectroscopies, labelling experiments, and DFT modelling, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These sites are formed in the presence of grafted and adsorbed chromocene as well as residual surface OH groups, only possible at high Cr loading, and involves a C-H activation of the Cp ring. These Cr-hydrides initiate polymerization, yielding Cr(III) alkyl species that insert ethylene through a Cossee-Arlman-type mechanism, as evidenced by spectroscopic studies. These insights inspired the design of a well-defined analogue, CpCr(CH(SiMe3)2)2 grafted on partially dehydroxylated silica, that shows similar spectroscopic and polymer structure as the UC catalyst, further supporting the proposed active site structure.

X-ray absorption near edge structure (XANES) spectroscopy is a powerful method to probe the oxidation state and local structure of metals in catalytic materials. However, it suffers from the lack of unbiased data analysis protocols. Machine learning (ML) overcomes human-related factors by uncovering relevant spectrum-structure relationships and subsequent cross-validation analysis. The bottlenecks in the automatic processing of experimental data are the lack of chemically diverse XANES reference libraries and systematic differences between theory and experiment. Therefore, compiling experimental reference libraries across the periodic table and rational application of ML methodology to small (in terms of data science) training datasets becomes increasingly important. This work revises the classical XANES fingerprint analysis by database augmentation, feature extraction, cross-validation, and uncertainty analysis. We apply the developed methodology to decipher the oxidation state and local coordination of supported vanadium-oxo species (VOx), which change their structure participating in oxidative dehydrogenation catalysis. The developed library and instruments for analysis may serve as a starting point for a unified platform of fingerprint XANES data analysis.

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry (SOMC), EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls, and identified the presence of several surface species from high to low spin Cr(III), associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

The Union Carbide (UC) ethylene polymerization catalyst, based on silica-supported chromocene, is one of the first industrial catalysts prepared by surface organometallic chemistry, though the structure of the surface sites remains elusive. Recently, our group reported that monomeric and dimeric Cr(ii) sites, as well as Cr(iii) hydride sites, are present and that their proportion varies as a function of the Cr loading. While 1H chemical shifts extracted from solid-state 1H NMR spectra should be diagnostic of the structure of such surface sites, unpaired electrons centered on Cr atoms induce large paramagnetic 1H shifts that complicate their NMR analysis. Here, we implement a cost-efficient DFT methodology to calculate 1H chemical shifts for antiferromagnetically coupled metal dimeric sites using a Boltzmann-averaged Fermi contact term over the population of the different spin states. This method allowed us to assign the 1H chemical shifts observed for the industrial-like UC catalyst. The presence of monomeric and dimeric Cr(ii) sites, as well as a dimeric Cr(iii)-hydride sites, was confirmed and their structure was clarified.

Zr imido chemistry: well-defined silica-supported Zr imido complexes via Surface Organometallic Chemistry, oxo/imido heterometathesis and comparison with Ti analogs.

The Union Carbide(UC) catalyst, one of the first ethylene polymerization (EP) catalysts based on supported organometallic compounds, is based on chromocene dispersed on silica; it is distinct from the corresponding Phillips catalysts, based on supported chromium oxide, because it is responsive to H2 enabling tuning the resulting polymer. Yet, despite 50 years of research, the structure of its active sites has also been controversially discussed and remains elusive. Based on a combination of IR, EPR spectroscopies, labeling experiments, and modeling at the DFT level, we identified monomeric surface-supported Cr(III) hydrides, (≡SiO)Cr(Cp)-H, as the active sites of the UC catalyst. These active sites, with distinct EPR signatures, are exclusively formed at high Cr loadings because their formation requires a combination of congruent factors, namely the presence of grafted as well as adsorbed chromocene together with residual surface OH groups, and involves a C-H bond activation step of the Cp ring. These Cr hydrides readily initiate polymerization, yielding Cr(III) alkyl species that insert ethylene monomers through a Cossee Arlman-type mechanism, as evidenced by labeling and spectroscopic studies. This information inspired the design of a well-defined analog, that was prepared and characterized using SOMC by grafting CpCr(CH(SiMe3)2)2 on partially dehydroxylated silica. The resulting surface Cr sites show similar EPR signatures and produce similar polyethylene as the UC catalyst, further supporting the presence of similar active sites and opening the way to generating new classes of catalysts.

Unveiling the nature and the distribution of surface sites in heterogeneous catalysts, and for the Phillips catalyst (CrO3/SiO2) in particular, is still a grand challenge despite more than 60 years of research. Commonly used references in Cr K-edge XANES spectral analysis rely on bulk materials (Cr-foil, Cr2O3) or molecules (CrCl3) that significantly differ from actual surface sites. In this work, we built a library of Cr K-edge XANES spectra for a series of tailored molecular Cr complexes, varying in oxidation state, local coordination environment, and ligand strength. Quantitative analysis of the pre-edge region revealed the origin of the pre-edge shape and intensity distribution. In particular, the characteristic pre-edge splitting observed for Cr(III) and Cr(IV) molecular complexes is directly related to the electronic exchange interactions in the frontier orbitals (spin-up and -down transitions). The series of experimental references was extended by theoretical spectra for potential active site structures and used for training the Extra Trees machine learning algorithm. The most informative features of the spectra (descriptors) were selected for the prediction of Cr oxidation states, mean interatomic distances in the first coordination sphere, and type of ligands. This set of descriptors was applied to uncover the site distribution in the Phillips catalyst at three different stages of the process. The freshly calcined catalyst consists of mainly Cr(VI) sites. The CO-exposed catalyst contains mainly Cr(II) silicates with a minor fraction of Cr(III) sites. The Phillips catalyst exposed to ethylene contains mainly highly coordinated Cr(III) silicates along with unreduced Cr(VI) sites.

Metallocene alkyl complexes with d(0) electron configuration and d(8)-configured square planar alpha-diimine late transition metal alkyl complexes show activity in both C-H activation through sigma-bond metathesis and olefin insertion. Herein, we show by analysis of their M-CH3 C-13 chemical shift tensors that these reactions involve a pi(M-C) interaction in the horizontal plane of the complex for both d(0) and d(8) systems. While in the case of d(0) systems the interaction of an empty metal d-orbital and a filled carbon p-orbital causes partial alkylidene character of the M-C bond, the corresponding metal d-orbital is filled in d(8) systems, thus generating a filled pi*(M-C) orbital that increases the anionic character of the methyl group. This entails fundamentally different reaction mechanisms for d(0) and d(8) systems, which are reflected in the structures of the transition states: While d(0) olefin insertion can be viewed as a [2+2] cycloaddition reaction, d(8) olefin insertion rather resembles methyl group migration onto a positively polarized olefin, thus explaining the observed differences in regioselectivity. These findings are translated to sigma-bond metathesis, a reaction which is isolobal to olefin insertion for both early and late transition metals.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.

Silica nanoparticles of 12 nm diameter were surface-doped with ca. 350 (TTF-dppz)Yb(III) surface species, containing bis (propylthio) tetrathiafulvenyl[i]dipyrido-[3,2-a:2',3-c]phenazine (TTF-dppz) as an antenna ligand through a surface organometallic chemistry approach. These nanoparticles absorb and emit in the NIR (lambda(abs) = 750 nm, lambda(em) = 983 and 1050 nm) with a lifetime (tau(1)) of 2.8 mu s, similarly to the corresponding Yb(III) molecular complex (lambda(abs) = 750 nm, lambda(em) = 975, 986, 1009, and 1020 nm with tau(1) = 6.93 mu s). The silica materials were fully characterized using combined spectroscopic techniques (IR, NMR, UV vis, luminescence and lifetime), molecular models and isostructural diamagnetic yttrium-containing materials for easier characterization by NMR spectroscopy. Having established the surface structures and photophysical properties of these nanopartides, we transposed this methodology to larger silica particles with a diameter of ca. 100 nm. These larger nanopartides have similar photophysical properties and contain ca. 30 000 chromophores, making possible one-photon NIR-to-NIR. emission optical microscopy imaging of single nanoparticles.