Synthesis, Characterization, and Reactivity of Heterometallic Trinuclear Clusters

Abstract

The prevalence of heterometallic active sites in biological and industrial catalysis has inspired researchers to target synthetic heterometallic systems to probe their electronic properties and reactivity. While many of the catalytically active heterometallic species are high spin in nature, the majority of the synthetic systems to date are low spin. Using a weak field hexaanilide ligand platform, our group has showcased the synthesis of high spin homo- and heterotrinuclear clusters. The work of this thesis aims to extend our previously reported methodology for synthesizing bimetallic, trinuclear clusters to incorporate redox inactive metals. In the first step of our synthesis, metalation is arrested at dinuclear species (tbsLH2)Fe2 in the absence of coordinating solvent. Zinc is incorporated into the third ligand binding site via stepwise deprotonation with Na(N(SiMe3)2) and subsequent transmetalation with ZnCl2(py)n (n = 0,2) to afford [Fe2Zn] clusters. With these open-shell clusters in hand, we sought to probe the dynamic behavior of the clusters undergoing simple reaction types: ligand exchange, oxidative group transfer, and outer-sphere electron transfer. (I) Cluster anation: Anation of (tbsL)Fe2Zn(thf) resulted in a mixture of species, where the Fe:Zn of the reaction mixture was maintained, but the composition of the individual species varied, and both [NBu4][(tbsL)Fe3(μ3–Cl)] and [NBu4][(tbsL)Zn3(μ3–Cl)] were observed in the reaction mixture in addition to at least one new paramagnetic complex. (II) Oxidative group transfer: Addition of aryl azides to (tbsL)Fe2Zn(thf) or (tbsL)Fe2Zn(py) yielded a mixture of two products: the triiron imido product (tbsL)Fe3(μ3–NAr) and a mixed metal imido cluster (tbsL)Fe2Zn(μ3–NAr)(py)n (n = 0, 1 for (tbsL)Fe2Zn(thf) and (tbsL)Fe2Zn(py) starting clusters, respectively). The pyridine-bound imido products were separated via crystallization to afford a clean preparative route to mixed-metal, diferric (tbsL)Fe2Zn(μ3–NAr)(py) imidoes. (III) Outer-sphere electron transfer: Outer sphere reduction of the py-bound, imido-capped cluster (tbsL)Fe2Zn(μ3–NAr)(py) resulted in the formation of a single product [2,2,2-crypt(K)][(tbsL)Fe2Zn(μ3–NAr)] with no evidence of metal atom lability from the cluster core. To confirm site-specific metal identity within our heterometallic clusters, we began to explore both neutron and anomalous X-ray diffraction to quantify the distribution of metals in our [Fe2Zn] clusters. Through our anomalous X-ray diffraction studies, we established that the use of theoretical dispersion correction values in occupancy determination per convention leads to large errors in occupancies obtained. However, minimization of the error in occupancy determination can be effected by substitution of the theoretical dispersion correction terms with experimentally determined correction terms from molecular control samples, providing a ± 5% resolution for occupancy values obtained. The established protocol was applied to our family of heterometallic clusters and the site specific elemental analysis was performed to reveal overall compositional analysis. We then sought to probe one of these clusters [Cp*2Co][(tbsL)Fe2Zn(μ3–NAr)] by neutron diffraction and obtained comparable results to the anomalous scattering measurements suggesting viability of neutron diffraction for metal occupancy determination in molecular clusters. Having (1) developed an understanding of how the distribution of metals in our heterometallic clusters can change upon chemical and electronic perturbations and (2) established a method to confirm metal site occupancy within our clusters and their resulting reaction products, we began to target novel reactive species. Building a heteronuclear cluster containing a single redox active Ni site supported by a dizinc unit, we sought to probe whether we could use this platform to access oxidation states that are unattainable by other means. While we demonstrated the potential for two-electron reaction chemistry with sulfur- and oxygen-based transfer reagents, the thermodynamic products of these reactions limited the oxidation state at the Ni site to 2+. Conversely, the use of nitrene-transfer reagents to form a stable imido product [K(thf)3][(FtbsL)Zn2Ni(μ3–NAd)] suggests the possibility of a two-electron oxidation localized at the metal ion and accessibility of Ni in a formally 3+ oxidation state.