Accessing Lanthanide Metallocene Two-Electron Reduction Chemistry Using 2,2′-Bipyridine

Abstract

The feasibility of using redox-active 2,2′-bipyridine (bipy) as a synthetically convenient electron carrier to enable lanthanide metallocenes to effect two-electron reduction chemistry has been examined. The Ln­(III) precursor complexes, Cp*2LnCl­(bipy), 1-Ln (Ln = Nd, Gd; Cp* = C5Me5), were readily synthesized in 75–81% yield in one pot from LnCl3, KCp*, and bipy and were identified by X-ray crystallography. Treatment of the 1-Ln compounds with K/KI (4.3 wt %) afforded in 74–77% yield the products, Cp*2Ln­(bipy), 2-Ln, which contain (bipy)1– ligands based on their spectroscopic characteristics and X-ray crystal structures. The molecular structure of the previously reported 2-Eu was also determined, which showed structural features consistent with a neutral bipy ligand. Further reduction of 2-Nd or 2-Gd, or the previously reported 2-Sm, 2-Eu, and 2-Yb with K/KI (4.3 wt %) in the presence of 2.2.2-cryptand (crypt) afforded in 58–88% yield the salts, [K­(crypt)]­[Cp*2Ln­(bipy)], 3-Ln, which show metrical parameters identified by X-ray crystallography that are consistent with a (bipy)1– ligand for 3-Eu and 3-Yb and a (bipy)2– ligand for 3-Nd, 3-Sm, and 3-Gd. The two-electron reduction of azobenzene (PhN=NPh) to (PhN−NPh)2– using the 3-Ln complexes was examined as a proof of concept. The 1:1 reactions of 3-Nd, 3-Sm, and 3-Yb with azobenzene afforded in 36–79% yield the [K­(crypt)]­[Cp*2Ln­(N2Ph2)], 4-Ln (Ln = Nd, Sm, Yb) complexes, each of which were crystallographically characterized. In another example of two-electron reduction by 3-Ln, treatment of 3-Nd or 3-Sm with elemental sulfur afforded in 70–75% yield [K­(crypt)]­[Cp*2Ln­(S5)], 5-Ln (Ln = Nd, Sm), which were shown by crystallography to be rare examples of f-element organometallic complexes that contain the (S5)2– ligand.

Reduced 2,2’‐Bipyridine Lanthanide Metallocenes Provide Access to Mono‐C5Me5 and Polyazide Complexes

“…[38] Selected metrical data on complexes 1 and 2 are compared with related complexes in Table 1. [23] A critical metric in the structural analysis of bipy complexes is the C bipy À C' bipy distance, which has been shown previously to be a useful indicator of the oxidation state of the bipyridine ligand (neutral, radical anionic, or dianionic). Thus, the C bipy À C' bipy distances of ca.…”

“…In further support of this, the infrared spectrum of complex 3 displays two sharp absorbances at 722 and 680 cm 1À , which we previously noted to be indicative of the presence of (bipy) 1À and (bipy) 2À ligands, respectively, in the [Cp* 2 Ln(bipy)] n series (n = 0, À 1). [23] It is also worth noting that the dimeric nature of 3 may be at least partially preserved in solution.…”

“…A similar distortion is also present in the actinide species U(bipy) 4 and [U(bipy) 4 ] 1À , wherein one pair of bipy ligands in each of the complexes displays this distortion. [66] A Cp* Inverse Sandwich Salt [K 2 (18- Since previous attempts to further reduce the Sm(III) (bipy) 2À complex, [K(crypt)][Cp* 2 Sm(bipy)], with K/crypt afforded no crystalline material, [23] we examined the reduction chemistry of Cp* 2 Sm(bipy) with K and 18-crown-6 (18-c-6) as the chelate. When Cp* 2 Sm(bipy) was treated with 2 equiv.…”

“…These values are similar to those previously reported for [K(crypt)][Cp* 2 Sm(bipy)] (C bipy À C' bipy : 1.383(3) Å; SmÀ N: 2.3568(19), 2.3600(16) Å) and therefore are consistent with a Sm III /(bipy 2À ) formulation for 10. [23] Thus, while the structure of the anion of 10 is essentially the same as the previously reported [Cp* 2 Sm(bipy)] 1À anion, the presence of [K 2 (18-c-6) 2 Cp*] + cations and the neutral K(18-c-6)Cp* unit in its structure indicate that other samarium-containing products with less than two Cp* ligands per metal are probably formed in this reaction. This is consistent with the formation of the several mono-Cp* complexes described earlier.…”

“…[14][15][16][17][18][19][20][21][22] We have recently found that bipy provides a facile high yield synthetic route, Scheme 1, to organometallic lanthanide metallocenes that function as monometallic twoelectron reducing agents. [23] Specifically, one-pot reactions of LnCl 3 , KCp* (Cp* = C 5 Me 5 ) and bipy were shown to form Cp* 2 LnCl(bipy). These complexes can be reduced by either one electron to afford Cp* 2 Ln(bipy) or by two electrons to give [Cp* 2 Ln(bipy)] 1À where the oxidation state of the bipy ligand in these complexes depends upon the specific lanthanide.…”