¹³C NMR Shifts as an Indicator of U–C Bond Covalency in Uranium(VI) Acetylide Complexes: An Experimental and Computational Study

Abstract

A series of uranium(VI)-acetylide complexes of the general formula, U VI (O)(C≡C-C6H4-R)[N(SiMe3)2]3, with variation of the para substituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, were prepared. These compounds were analyzed by 13 C NMR spectroscopy, which showed that the acetylide carbon bound to the uranium(VI) center, U-C≡C-Ar, was shifted strongly downfield, with δ(13 C) values ranging from 392.1 to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. These extreme high-frequency 13 C resonances are attributed to large negative paramagnetic (σ para) and relativistic spin-orbit (σ SO) shielding contributions, associated with extensive U(5f) and C(2s) orbital contributions to the U−C bonding in title complexes. The trend in the 13 C chemical shift of the terminal acetylide carbon is opposite of that observed in the series of parent (aryl)acetylenes, due to shielding effects of the para substituent. The 13 C chemical shifts of the acetylide carbon instead correlate with DFT computed U-C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibility measurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes, suggesting a magnetic fieldinduced mixing of the singlet ground-state (f 0) of the U(VI) ion with low-lying (thermally inaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually large paramagnetic and SO contributions to the 13 C shifts). Thus, together with reported data, we demonstrate the sensitive 13 C NMR shifts serve as a direct, simple and accessible measure of uranium(VI)-carbon bond covalency.

Isolation and characterization of a covalent CeIV-Aryl complex with an anomalous 13C chemical shift

“…This 13 C{ 1 H} shift is further downfield than observed for other characterized, diamagnetic M IV − C aryl interactions; the highest being Th(2-C 6 H 4 CH 2 NMe 2 ) 4 , which exhibits a Th IV − C 13 C{ 1 H} signal at 230.8 ppm 38,39 . The shift of the 13 C{ 1 H} signal for the carbon atom bound to metal cations has been implicated as a reporter for the degree of covalency in felement cation-carbon bonds [7][8][9] . In this light, 3-THF and 3-DME have an anomalously high covalency for a M IV -aryl interaction.…”

“…Previously reported Ce IV − C bonds contain 8-13% Ce contribution 25 . Ce tends to be less covalent than U VI (U VI − C bonds 22-29% U) but similar to Th IV (Th IV − C aryl bonds 10-15%) 9,25,38,40 . In the remaining σ and π bonds with the N and O atoms, Ce contributes about 3-4% weight, suggesting that these bonds are mainly ionic.…”

“…However, modern spectroscopic and computational techniques have challenged this simple assumption [3][4][5][6] . The covalency in M − X interactions can result in anomalous and diagnostic nuclear magnetic resonance shifts, X = 13 C, 15 N, 19 F, 77 Se, 125 Te, resulting from participation of f-element orbital angular momentum [7][8][9][10][11][12][13] . Another important aspect of detailed f-element electronic structure is multiconfigurational character, as described in the model example of cerocene, Ce(COT) 2 , COT = cyclooctatetraene ligand 14 .…”

Use of 15N NMR spectroscopy to probe covalency in a thorium nitride

“…Previously, 13 C, 77 Se, and 125 Te NMR spectroscopies had been used to evaluate covalency in An-E bonds. 39,46,[48][49][50][51] In the case of An-C bonding, large downeld 13 C shis have been consistently observed for the 13 C nuclei bonded directly to an actinide center. More signicantly, the degree of deshielding was found to correlate with the amount of 5f covalency within the An-C bond.…”

“…For example, the 13 C NMR shi of acetylide carbon in the U(VI) acetylide complexes, U VI (O)(C^CC 6 H 4 -p-R)(NR 2 ) 3 (R ¼ NMe 2 , OMe, Me, Ph, H, Cl), correlated well with two measures of covalency, the QTAIM delocalization index and the Wiberg bond order of the U-C bond. 48 Highly deshielded 13 C resonances are also observed for the carbene resonance in [Th(CHPPh 3 )(NR 2 ) 3 ] and the methylene resonances in [UO 2 (-CH 2 SiMe 3 ) 4 ] 2À and [U(CH 2 SiMe 3 ) 6 ] À . 46,52 Our results demonstrate that 15 N NMR spectroscopy can also be used to evaluate covalency in actinide-ligand bonding, and like 13 C NMR spectroscopy, the magnitude of the downeld shi correlates with the degree of 5f character in the An-N bond.…”