Synthesis and characterization of a new and electronically unusual uranium metallacyclocumulene, (C5Me5)2U(η4-1,2,3,4-PhC4Ph)

Abstract

A new uranium metallacyclocumulene, (C 5 Me 5) 2 U(η 4-PhC 4 Ph), was synthesized by both reaction of (C 5 Me 5) 2 UCl 2 with 1,4-diphenylbutadiyne in the presence of KC 8 and by ligand exchange between (C 5 Me 5) 2 U(η 2-Me 3 SiC 2 SiMe 3) and 1,4-diphenylbutadiyne. Full characterization of (C 5 Me 5) 2 U(η 4-PhC 4 Ph) is reported, including the solid-state structure. (C 5 Me 5) 2 U(η 4-PhC 4 Ph) displays an unusually detailed UV-visible spectrum, which is rare for uranium(IV) metallocene complexes. t Bu 3 H 2) 2 Th(η 2-1,2-PhC 2 Ph) and (C 5 Me 5) 2 U(η 2-1,2-Me 3 SiC 2 SiMe 3).[20-23] In related work, the Meyer group reported that the uranium(III) complex [((Ad ArO) 3 N)U III (DME)] reacts with the terminal alkynes 1-hexyne or 4-t butyl-phenylacetylene,

δ and φ back-donation in AnIV metallacycles

“…Our DFT optimized geometries for the metallacyclopropene and metallacyclocumulene complexes (henceforth referred to as 'propene' and 'cumulene', respectively, for brevity) match published crystal structures, with bond distances and angles generally within experimental uncertainty for both Group 4 transition metals [33][34][35][36][37][38][39] and actinides (Th and U); [27][28][29][30][31][32] see Supplementary Tables 1, 2. The calculated IR spectrum matches the measured spectra of U-cumulene, including the strong C α -C β stretch peak at 1590 cm -1 discussed in the literature 32 . Calculated IR spectra are given in Supplementary Fig.…”

“…3a), which-apart from the drop between Th and Pa-remain fairly constant across the series, thus suggesting that it is the M-C α interactions that dominate the M-C bonding. Similar to the propene complexes, the trends between Th and U are reflected in published crystal structures (Supplementary Table 2) [29][30][31][32] . Comparing experimental crystal structures one can see that there is an overall increase in the M-C distances by approximately 0.2 Å in the cumulene complexes relative to their propene counterparts.…”

“…Based on the two points available in the literature for actinides (Th and U) [27][28][29][30][31][32] , it is difficult to predict a priori the unusual trend of the M-C bond distances as a function of the metal ion (Th-Pu) in both the propene and cumulene series. One may assume that the specificity of the M-C bond distances is due to the possible delocalized M-L bonding interactions, which occur due to the presence of π electrons in the C 2 R 2 (C=C π system) and C 4 R 2 (C=C=C=C π system) ligands.…”

“…UV-visible and near-IR spectroscopy are among the most used experimental tools to probe chemical bonding interactions 3,4,9,17,18,22,[24][25][26]34,39,[51][52][53] . The UV-visible-NIR spectra for the cumulene complex (C 5 Me 5 ) 2 U[η 4 -1,2,3,4-PhC 4 Ph] has been recently reported 32 , providing an opportunity for experimental validation of the orbital analysis discussed above. In particular, the appearance of transitions involving the φ bonding orbital would allow this interaction to be measured experimentally.…”

“…Based on recent syntheses reports [27][28][29][30][31][32] and our chemical bonding models for Th and U complexes of metallacyclopropenes and metallacyclocumulenes, we predict three more actinide compounds of Pa, Np, and Pu, which also help to fully understand the electronic structures of the Th and U complexes. Specifically, we demonstrate that the direct L-M σ and π donations combined with the M-L δ or φ back-donation are crucial in explaining the non-classical trend of the M-L bond lengths in both series, thus underscoring the significance of these δ and φ interactions.…”