Composition Determination\nof Heterometallic Trinuclear\nClusters via Anomalous X‑ray and Neutron Diffraction

Anomalous X-ray diffraction (AXD) and neutron diffraction\ncan be\nused to crystallographically distinguish between metals of similar\nelectron density. Despite the use of AXD for structural characterization\nin mixed metal clusters, there are no benchmark studies evaluating\nthe accuracy of AXD toward assessing elemental occupancy in molecules\nwith comparisons with what is determined via neutron diffraction.\nWe collected resonant diffraction data on several homo and heterometallic\nclusters and refined their anomalous scattering components to determine\nmetal site occupancies. Theoretical resonant scattering terms for\nFe<sup>0</sup>, Co<sup>0</sup>, and Zn<sup>0</sup> were compared against\nexperimental values, revealing theoretical values are ill-suited to\nserve as references for occupancy determination. The cluster featuring\ndistinct cation and anion metal compositions [CoCp<sub>2</sub>*][(<sup>tbs</sup>L)Fe<sub>3</sub>(μ<sup>3</sup>–NAr)] was used\nto assess the accuracy of different <i>f</i>′ references\nfor occupancy determination (<i>f</i>′<sub>theoretical</sub> ± 15–17%; <i>f</i>′<sub>experimental</sub> ± 10%). This methodology was applied toward calculating the\noccupancy of three different clusters: (<sup>tbs</sup>L)Fe<sub>2</sub>Zn(py) (<b>6</b>), (<sup>tbs</sup>L)Fe<sub>2</sub>Zn(μ<sup>3</sup>–NAr)(py) (<b>7</b>), and [CoCp*<sub>2</sub>][(<sup>tbs</sup>L)Fe<sub>2</sub>Zn(μ<sup>3</sup>–NAr)] (<b>8</b>). The first two clusters maintain 100% Fe/Zn site isolation,\nwhereas <b>8</b> showed metal mixing within the sites. The large\ncrystal size of <b>8</b> enabled collection of neutron diffraction\ndata which was compared against the results found with AXD. The ability\nof AXD to replicate the metal occupancies as determined by neutron\ndiffraction supports the AXD occupancy methodology developed herein.\nFurthermore, the advantages innate to AXD (e.g., smaller crystal sizes,\nshorter collection times, and greater availability of synchrotron\nresources) versus neutron diffraction further support the need for\nits development as a standard technique.