Reductive Reactivity of the 4f75d1 Gd(II) Ion in {GdII[N(SiMe3)2]3}-: Structural Characterization of Products of Coupling, Bond Cleavage, Insertion, and Radical Reactions.

The reductive reactivity of a Ln(II) ion with a nontraditional 4fn5d1 electron configuration has been investigated by studying reactions of the {GdII(N(SiMe3)2)3]}- anion with a variety of reagents that survey the many reaction pathways available to this ion. The chemistry of both [K(18-c-6)2]+ and [K(crypt)]+ salts (18-c-6 = 18-crown-6; crypt = 2.2.2-cryptand) was examined to study the effect of the countercation. CS2 reacts with the crown salt [K(18-c-6)2][Gd(NR2)3] (1) to generate the bimetallic (CS3)2- complex {[K(18-c-6)](µ3-CS3-κS,κ2S',S'')Gd(NR2)2]}2, which contains two trithiocarbonate dianions that bridge Gd(III) centers and a potassium ion coordinated by 18-c-6. In contrast, the only crystalline product isolated from the reaction of CS2 with the crypt salt [K(crypt)][Gd(NR2)3] (2) is [K(crypt)]{(R2N)2Gd[SCS(CH2)Si(Me2)N(SiMe3)-κN,κS]}, which has a CS2 unit inserted into a cyclometalated amide ligand. Complexes 1 and 2 reductively couple pyridine to form bridging dipyridyl moieties, (NC5H4-C5H4N)2-, that generate bimetallic complexes differing only in the countercation, {[K(18-c-6)(C5H5N)2]}2{[(R2N)3Gd]2[µ-(NC5H4-C5H4N)2]} and [K(crypt)]2{[(R2N)3Gd]2[µ-(NC5H4-C5H4N)2]}. Complexes 1 and 2 also show similar reactivity with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to form the (TEMPO)- complexes [K(18-c-6)][(R2N)3Gd(η1-ONC5H6Me4)] and [K(crypt)][(R2N)3Gd(η1-ONC5H6Me4)], respectively. The first example of a bimetallic coordination complex containing a Bi-Gd bond, [K(crypt)][(R2N)3Gd(BiPh2)], was obtained by treating 2 with BiPh3.

Formation of the End-on Bound Lanthanide Dinitrogen Complexes [(R2N)3Ln-N═N-Ln(NR2)3]2- from Divalent [(R2N)3Ln]1- Salts (R = SiMe3).

Lanthanide-based dinitrogen reduction chemistry has been expanded by the discovery of the first end-on Ln2(µ-η1:η1-N2) complexes, whose synthesis and reactivity help explain the reduction of N2 by the combination of trivalent Ln(NR2)3 complexes (R = SiMe3) and potassium. The formation of end-on versus the more common side-on Ln2(µ-η2:η2-N2) complexes is possible by using recently discovered Ln(II) complexes ligated by three NR2 amide ligands (R = SiMe3). The isolated Ln(II) tris(amide) complex [K(crypt)][Tb(NR2)3] (crypt = 2.2.2-cryptand), 1-Tb, reacts with dinitrogen in Et2O at -35 °C to form the end-on bridging dinitrogen complex [K(crypt)]2{[(R2N)3Tb]2[µ-η1:η1-N2]}, 2-Tb. The 18-crown-6 (18-c-6) Tb(II) analogue, [K(18-c-6)2][Tb(NR2)3], 3-Tb, also reacts with N2 to form an end-on product, [K2(18-c-6)3]{[(R2N)3Tb]2[µ-η1:η1-N2]}, 4-Tb. The reaction of 1-Gd with dinitrogen forms a complex with the same composition as 2-Tb but with both side-on and end-on bonding of the N2 unit in the same crystal, [K(crypt)]2{[(R2N)3Gd]2[µ-ηx:ηx-N2]} (x = 1 and 2), 5-Gd. Similarly, the 18-c-6 Gd(II) complex, 3-Gd, generates a product with both binding modes: [K2(18-c-6)3]{[(R2N)3Gd]2[µ-ηx:ηx-N2]} (x = 1, 2), 6-Gd. All of these new reduced dinitrogen complexes, 2-Tb, 4-Tb, 5-Gd, and 6-Gd, have three ancillary amide ligands per metal. In contrast, the side-on bound complexes, [(THF)(R2N)2Ln]2[µ-η2:η2-N2], 7-Ln, observed previously in Ln(NR2)3/K/N2 reactions, have only two amides per metal. A connection between these systems related to their formation was observed in the structure of the bimetallic penta-amide complex, [K(THF)6]{[(THF)(R2N)2Gd][µ-η2:η2-N2][Gd(NR2)3]}, 8-Gd, synthesized at -196 °C. Reaction conditions are crucial in this dinitrogen reaction system. When 5-Gd and 6-Gd are warmed above -15 °C, they reform Gd(II) complexes. If 1-Gd is dissolved in THF instead of Et2O under N2, the irreversible formation of an (N2)3- complex [K(crypt)][(THF)(R2N)2Gd]2[µ-η2:η2-N2], 9-Gd, is observed.

Synthesis, Structure, and Magnetism of Tris(amide) [Ln{N(SiMe3 )2 }3 ]1- Complexes of the Non-traditional +2 Lanthanide Ions.

A new series of Ln2+ complexes has been synthesized that overturns two previous generalizations in rare-earth metal reduction chemistry: that amide ligands do not form isolable complexes of the highly reducing non-traditional Ln2+ ions, and that yttrium is a good model for the late lanthanides in these reductive reactions. Reduction of Ln(NR2 )3 (R=SiMe3 ) complexes in THF under Ar with M=K or Rb in the presence of 2.2.2-cryptand (crypt) forms crystallographically characterizable [M(crypt)][Ln(NR2 )3 ] complexes not only for the traditional Tm2+ ion and the configurational crossover ions, Nd2+ and Dy2+ , but also for the non-traditional Gd2+ , Tb2+ , Ho2+ , and Er2+ ions. Crystallographic data as well as UV/Vis, magnetic susceptibility, and density functional theory studies are consistent with the accessibility of 4fn 5d1 configurations for Ln2+ ions in this tris(silylamide) ligand environment. The Dy2+ complex, [K(crypt)][Dy(NR2 )3 ], has a higher magnetic moment than previously observed for any monometallic complex: 11.67 µB .

Redox Potential and Electronic Structure Effects of Proximal Nonredox Active Cations in Cobalt Schiff Base Complexes.

Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na+, K+, Ca2+, Sr2+, and Ba2+) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na+ and K+ and 230-270 mV more positive for M = Ca2+, Sr2+, and Ba2+compared to Co(salen-OMe) (salen-OMe = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge.

The importance of the counter-cation in reductive rare-earth metal chemistry: 18-crown-6 instead of 2,2,2-cryptand allows isolation of [YII(NR2)3]1- and ynediolate and enediolate complexes from CO reactions.

The use of 18-crown-6 (18-c-6) in place of 2.2.2-cryptand (crypt) in rare earth amide reduction reactions involving potassium has proven to be crucial in the synthesis of Ln(ii) complexes and isolation of their CO reduction products. The faster speed of crystallization with 18-c-6 appears to be important. Previous studies have shown that reduction of the trivalent amide complexes Ln(NR2)3 (R = SiMe3) with potassium in the presence of 2.2.2-cryptand (crypt) forms the divalent [K(crypt)][LnII(NR2)3] complexes for Ln = Gd, Tb, Dy, and Tm. However, for Ho and Er, the [Ln(NR2)3]1- anions were only isolable with [Rb(crypt)]1+ counter-cations and isolation of the [YII(NR2)3]1- anion was not possible under any of these conditions. We now report that by changing the potassium chelator from crypt to 18-crown-6 (18-c-6), the [Ln(NR2)3]1- anions can be isolated not only for Ln = Gd, Tb, Dy, and Tm, but also for Ho, Er, and Y. Specifically, these anions are isolated as salts of a 1 : 2 potassium : crown sandwich cation, [K(18-c-6)2]1+, i.e. [K(18-c-6)2][Ln(NR2)3]. The [K(18-c-6)2]1+ counter-cation was superior not only in the synthesis, but it also allowed the isolation of crystallographically-characterizable products from reactions of CO with the [Ln(NR2)3]1- anions that were not obtainable from the [K(crypt)]1+ analogs. Reaction of CO with [K(18-c-6)2][Ln(NR2)3], generated in situ, yielded crystals of the ynediolate products, {[(R2N)3Ln]2(µ-OC[triple bond, length as m-dash]CO)}2-, which crystallized with counter-cations possessing 2 : 3 potassium : crown ratios, i.e.{[K2(18-c-6)3]}2+, for Gd, Dy, Ho. In contrast, reaction of CO with a solution of isolated [K(18-c-6)2][Gd(NR2)3], produced crystals of an enediolate complex isolated with a counter-cation with a 2 : 2 potassium : crown ratio namely [K(18-c-6)]2 2+ in the complex [K(18-c-6)]2{[(R2N)2Gd2(µ-OCH[double bond, length as m-dash]CHO)2]}.

Isolation of U(ii) compounds using strong donor ligands, C5Me4H and N(SiMe3)2, including a three-coordinate U(ii) complex.

New examples of uranium in the formal +2 oxidation state have been isolated by reduction of Cptet3U (Cptet = C5Me4H) and U(NR2)3 (R = SiMe3) in the presence of 2.2.2-cryptand (crypt) to produce [K(crypt)][Cptet3U] and [K(crypt)][U(NR2)3], respectively. Both complexes have properties consistent with 5f36d1 electron configurations and demonstrate that the U(ii) ion can be isolated with electron donating ligands.

Evaluating the electronic structure of formal LnII ions in LnII(C5H4SiMe3)31- using XANES spectroscopy and DFT calculations.

The isolation of [K(2.2.2-cryptand)][Ln(C5H4SiMe3)3], formally containing LnII, for all lanthanides (excluding Pm) was surprising given that +2 oxidation states are typically regarded as inaccessible for most 4f-elements. Herein, X-ray absorption near-edge spectroscopy (XANES), ground-state density functional theory (DFT), and transition dipole moment calculations are used to investigate the possibility that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb and Lu) compounds represented molecular LnII complexes. Results from the ground-state DFT calculations were supported by additional calculations that utilized complete-active-space multi-configuration approach with second-order perturbation theoretical correction (CASPT2). Through comparisons with standards, Ln(C5H4SiMe3)31- (Ln = Sm, Tm, Yb, Lu, Y) are determined to contain 4f6 5d0 (SmII), 4f13 5d0 (TmII), 4f14 5d0 (YbII), 4f14 5d1 (LuII), and 4d1 (YII) electronic configurations. Additionally, our results suggest that Ln(C5H4SiMe3)31- (Ln = Pr, Nd, Gd, Tb, Dy, Ho, and Er) also contain LnII ions, but with 4f n 5d1 configurations (not 4f n+1 5d0). In these 4f n 5d1 complexes, the C3h-symmetric ligand environment provides a highly shielded 5d-orbital of a' symmetry that made the 4f n 5d1 electronic configurations lower in energy than the more typical 4f n+1 5d0 configuration.