Synthesis and Characterization of Two "Tied-Back" Lithium Ketimides and Isolation of a Ketimide-Bridged [Cr2]6+ Dimer with Strong Antiferromagnetic Coupling

Reaction of 1 equiv of KN(SiMe₃)₂ with 9-fluorenone results in the formation of (Me₃Si)N═C₁₃H₈ (1) in high yield after work-up. Addition of 1 equiv of phenol to 1 results in rapid desilylation and formation of 9-fluorenone imine, HN═C₁₃H₈ (2). Subsequent reaction of 2 with 1 equiv of LiNⁱPr₂ results in deprotonation and formation of [Li(Et₂O)]₄[N═C₁₃H₈]₄ (3) in good yield. Reaction of 1 equiv of KN(SiMe₃)₂ with 2-adamantanone for 7 days at room temperature results in the formation of (Me₃Si)N═C₁₀H₁₄ (4) in good yield. Dissolution of 4 in neat MeOH results in rapid desilylation concomitant with formation of 2-adamantanone imine, HN═C₁₀H₁₄ (5). Subsequent reaction of 5 with 1 equiv of LiNⁱPr₂ results in formation of [Li(THF)]₄[N═C₁₀H₁₄]₄ (6). Both 3 and 6 were characterized by X-ray crystallography. Finally, reaction of CrCl₃ with 3.5 equiv of 6 results in formation of the [Cr₂]⁶⁺ dimer, [Li][Cr₂(N═C₁₀H₁₄)₇] (7), which can be isolated in modest yield after work-up. Complex 7 features a Cr-Cr bond length of 2.653(2) Å. Additionally, solid-state magnetic susceptibility measurements reveal strong antiferromagnetic coupling between the two Cr centers, with J = -200 cm^-1.

Synthesis and Characterization of Two Uranyl-Aryl "Ate" Complexes

Reaction of [UO₂ Cl₂ (THF)₃ ] with 3 equivalents of LiC₆ Cl₅ in Et₂ O resulted in the formation of first uranyl aryl complex [Li(Et₂ O)₂ (THF)][UO₂ (C₆ Cl₅ )₃ ] ([Li][1]) in good yields. Subsequent dissolution of [Li][1] in THF resulted in conversion into [Li(THF)₄ ][UO₂ (C₆ Cl₅ )₃ (THF)] ([Li][2]), also in good yields. DFT calculations reveal that the U-C bonds in [Li][1] and [Li][2] exhibit appreciable covalency. Additionally, the ¹³ C NMR chemical shifts for their C_ipso environments are strongly affected by spin-orbit coupling-a consequence of 5f orbital participation in the U-C bonds.

Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)

Reaction of 3 equiv of NaNR₂ (R = SiMe₃) with NpCl₄(DME)₂ in THF afforded the Np(IV) silylamide complex, [Np(NR₂)₃Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC₈ in THF, in the presence of 1 equiv of dibenzo-18-crown-6, resulted in formation of [{K(DB-18-C-6)(THF)}₃(μ₃-Cl)][Np(NR₂)₃Cl]₂ (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl₄(DME)₂ with 5 equiv of NaNR₂ and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et₂O)_0.62(κ¹-DME)_0.38}₂(μ-DME)][Np{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (8), in moderate yield. Complex 8 was characterized by ¹H NMR spectroscopy and X-ray crystallography and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, we also synthesized the uranium analogues of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR₂)₃Cl] (2), [K(DB-18-C-6)(THF)₂][U(NR₂)₃Cl] (3), [Na(DME)₃][U{N(R)(SiMe₂CH₂)}₂(NR₂)] (6), and [{Na(DB-18-C-6)(Et₂O)_0.5(κ¹-DME)_0.5}₂(μ-DME)][U{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (7). Complexes 2, 3, 6, and 7 were characterized by a number of techniques, including NMR spectroscopy and X-ray crystallography.

Coordination of Uranyl to the Redox-Active Calix[4]pyrrole Ligand

Reaction of [Li(THF)]₄[L] (L = Me₈-calix[4]pyrrole]) with 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ results in formation of the oxidized calix[4]pyrrole product, [Li(THF)]₂[L^Δ] (1), concomitant with formation of reduced uranium oxide byproducts. Complex 1 can also be generated by reaction of [Li(THF)]₄[L] with 1 equiv of I₂. We hypothesize that formation of 1 proceeds via formation of a highly oxidizing cis-uranyl intermediate, [Li]₂[cis-U^VIO₂(calix[4]pyrrole)]. To test this hypothesis, we explored the reaction of 1 with either 0.5 equiv of [U^VIO₂Cl₂(THF)₂]₂ or 1 equiv of [U^VIO₂(OTf)₂(THF)₃], which affords the isostructural uranyl complexes, [Li(THF)][U^VIO₂(L^Δ)Cl(THF)] (2) and [Li(THF)][U^VIO₂(L^Δ)(OTf)(THF)] (3), respectively. In the solid state, 2 and 3 feature unprecedented uranyl-η⁵-pyrrole interactions, making them rare examples of uranyl organometallic complexes. In addition, 2 and 3 exhibit some of the smallest O-U-O angles reported to date (2: 162.0(7) and 162.7(7)°; 3: 164.5(5)°). Importantly, the O-U-O bending observed in these complexes suggests that the oxidation of [Li(THF)]₄[L] does indeed occur via an unobserved cis-uranyl intermediate.

Synthesis of a "Masked" Terminal Zinc Sulfide and Its Reactivity with Brønsted and Lewis Acids

The "masked" terminal Zn sulfide, [K(2.2.2-cryptand)][^Me LZn(S)] (2) (^Me L={(2,6-ⁱ Pr₂ C₆ H₃ )NC(Me)}₂ CH), was isolated via reaction of [^Me LZnSCPh₃ ] (1) with 2.3 equivalents of KC₈ in THF, in the presence of 2.2.2-cryptand, at -78 °C. Complex 2 reacts readily with PhCCH and N₂ O to form [K(2.2.2-cryptand)][^Me LZn(SH)(CCPh)] (4) and [K(2.2.2-cryptand)][^Me LZn(SNNO)] (5), respectively, displaying both Brønsted and Lewis basicity. In addition, the electronic structure of 2 was examined computationally and compared with the previously reported Ni congener, [K(2.2.2-cryptand)][^tBu LNi(S)] (^tBu L={(2,6-ⁱ Pr₂ C₆ H₃ )NC(^t Bu)}₂ CH).

An iron ketimide single-molecule magnet [Fe4(N[double bond, length as m-dash]CPh2)6] with suppressed through-barrier relaxation

Reaction of FeBr₂ with 1.5 equiv. of LiN[double bond, length as m-dash]CPh₂ and 2 equiv. of Zn, in THF, results in the formation of the tetrametallic iron ketimide cluster [Fe₄(N[double bond, length as m-dash]CPh₂)₆] (1) in moderate yield. Formally, two Fe centers in 1 are Fe(i) and two are Fe(ii); however, Mössbauer spectroscopy and SQUID magnetometry suggests that the [Fe₄]⁶⁺ core of 1 exhibits complete valence electron delocalization, with a thermally-persistent spin ground state of S = 7. AC and DC SQUID magnetometry reveals the presence of slow magnetic relaxation in 1, indicative of single-molecule magnetic (SMM) behaviour with a relaxation barrier of U _eff = 29 cm^-1. Remarkably, very little quantum tunnelling or Raman relaxation is observed down to 1.8 K, which leads to an open hysteresis loop and long relaxation times (up to 34 s at 1.8 K and zero field and 440 s at 1.67 kOe). These results suggest that transition metal ketimide clusters represent a promising avenue to create long-lifetime single molecule magnets.

Enantioselective Alkylation of 2-Alkylpyridines Controlled by Organolithium Aggregation

Direct enantioselective α-alkylation of 2-alkylpyridines provides access to chiral pyridines via an operationally simple protocol that obviates the need for prefunctionalization or preactivation of the substrate. The alkylation is accomplished using chiral lithium amides as noncovalent stereodirecting auxiliaries. Crystallographic and solution NMR studies provide insight into the structure of well-defined chiral aggregates in which a lithium amide reagent directs asymmetric alkylation.

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

The first isolable molecular thorium nitride, [(NR₂)₃Th(μ-N)Th(NR₂)₃]^–, was synthesized by reaction of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] with NaNH₂ and characterized by X-ray crystallography, ¹⁵N NMR spectroscopy, and DFT calculations.

Reaction of the thorium metallacycle, [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] (R = SiMe₃) with 1 equiv. of NaNH₂ in THF, in the presence of 18-crown-6, results in formation of the bridged thorium nitride complex, [Na(18-crown-6)(Et₂O)][(R₂N)₃Th(μ-N)(Th(NR₂)₃] ([Na][1]), which can be isolated in 66% yield after work-up. Complex [Na][1] is the first isolable molecular thorium nitride complex. Mechanistic studies suggest that the first step of the reaction is deprotonation of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] by NaNH₂, which results in formation of the thorium bis(metallacycle) complex, [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)], and NH₃. NH₃ then reacts with unreacted [Th{N(R)(SiMe₂)CH₂}(NR₂)₂], forming [Th(NR₂)₃(NH₂)] (2), which protonates [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] to give [Na][1]. Consistent with hypothesis, addition of excess NH₃ to a THF solution of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] results in formation of [Th(NR₂)₃(NH₂)] (2), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] with 2, in THF-d₈, results in clean formation of [K][1], according to ¹H NMR spectroscopy. The electronic structures of [1]^– and 2 were investigated by ¹⁵N NMR spectroscopy and DFT calculations. This analysis reveals that the Th–N_nitride bond in [1]^– features more covalency and a greater degree of bond multiplicity than the Th–NH₂ bond in 2. Similarly, our analysis indicates a greater degree of covalency in [1]^–vs. comparable thorium imido and oxo complexes.

Case Studies in Nanocluster Synthesis and Characterization: Challenges and Opportunities

Atomically precise nanoclusters (APNCs) are an emerging area of nanoscience. Their monodispersity and well-defined arrangement of capping ligands facilitates the interrogation of their fundamental physical properties, allowing for the development of structure-function relationships, as well as their optimization for a variety of applications, including quantum computing, solid-state memory, catalysis, sensing, and imaging. However, APNCs present several unique synthetic and characterization challenges. For example, nanocluster syntheses are infamously low yielding and often generate complicated mixtures. This combination of factors makes nanocluster purification and characterization more difficult than that of typical inorganic or organometallic complexes. Yet, while this fact is undoubtedly true, the past lessons learned from the characterization of inorganic complexes are still useful today. In this Account, we discuss six case studies taken from the recent literature in an attempt to identify common challenges and pitfalls encountered in APNC synthesis and characterization. For example, we show that several reducing agents employed in APNC synthesis, including the commonly used reagent NaBH₄, do not always behave as anticipated. Indeed, we highlight one case where NaBH₄ reduces the ligand and not the metal center, and other cases where NaBH₄ acts as a Brønstead base instead of a reducing agent. In addition, we have identified several instances where the use of phase transfer agents, which were added to mediate APNC formation, played no role in the nanocluster synthesis, and likely made the isolation of pure material more difficult. We have also identified several cases of cluster misidentification driven by spurious or ambiguous characterization data, most commonly collected by mass spectrometry. To address these challenges, we propose that the nanocluster community adopt a standard protocol of characterization, similar to those used by the organometallic and coordination chemistry communities. This protocol requires that many complementary techniques be used in concert to confirm formulation, structure, and analytical purity of APNC samples. Two techniques that are underutilized in this regard are combustion analysis and NMR spectroscopy. NMR spectroscopy, in particular, can provide information on purity and formulation that are difficult to collect with any other technique. X-ray absorption spectroscopy is another powerful method of nanocluster characterization, especially in cases where single crystals for X-ray diffraction are not forthcoming. Chromatographic techniques can also be extremely valuable for assessing purity, but are rarely used during APNC characterization. Our goal with this Account is to begin a discussion with respect to the best protocols for nanocluster synthesis and characterization. We believe that embracing a standard characterization protocol would make APNC synthesis more reliable, thereby accelerating their integration into a variety of technologies.

Synthesis and reactivity of a nickel(ii) thioperoxide complex: demonstration of sulfide-mediated N2O reduction

The “masked” terminal nickel sulfide [K(18-crown-6)][L^tBuNi^II(S)] mediates the reduction of N₂O by CO, via the thioperoxide complex [K(18-crown-6)][L^tBuNi^II(η²-SO)].

The thiohyponitrite ([SNNO]^2–) complex, [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] (L^tBu = {(2,6-ⁱPr₂C₆H₃)NC(^tBu)}₂CH), extrudes N₂ under mild heating to yield [K(18-crown-6)][L^tBuNi^II(η²-SO)] (1), along with minor products [K(18-crown-6)][L^tBuNi^II(η²-OSSO)] (2) and [K(18-crown-6)][L^tBuNi^II(η²-S₂)] (3). Subsequent reaction of 1 with carbon monoxide (CO) results in the formation of [K(18-crown-6)][L^tBuNi^II(η²-SCO)] (4), [K(18-crown-6)][L^tBuNi^II(S,O:κ²-SCO₂)] (5), [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6), carbonyl sulfide (COS) (7), and [K(18-crown-6)][L^tBuNi^II(S₂CO)] (8). To rationalize the formation of these products we propose that 1 first reacts with CO to form [K(18-crown-6)][L^tBuNi^II(S)] (I) and CO₂, via O-atom abstraction. Subsequently, complex I reacts with CO or CO₂ to form 4 and 5, respectively. Similarly, the formation of complex 6 and COS can be rationalized by the reaction of 1 with CO₂ to form a putative Ni(ii) monothiopercarbonate, [K(18-crown-6)][L^tBuNi^II(κ²-SOCO₂)] (11). The Ni(ii) monothiopercarbonate subsequently transfers a S-atom to CO to form COS and [K(18-crown-6)][L^tBuNi^II(κ²-CO₃)] (6). Finally, the formation of 8 can be rationalized by the reaction of COS with I. Critically, the observation of complexes 4 and 5 in the reaction mixture reveals the stepwise conversion of [K(18-crown-6)][L^tBuNi^II(κ²-SNNO)] to 1 and then I, which represents the formal reduction of N₂O by CO.

A Re-examination of the Synthesis of Monolayer-Protected Co x(SCH2CH2Ph) m Nanoclusters: Unexpected Formation of a Thiolate-Protected Co(II) T3 Supertetrahedron

Herein, we report a re-examination of the synthesis and characterization of monolayer-protected Co _x(SCH₂CH₂Ph) _m nanoclusters. These clusters were reportedly formed by the reaction of CoCl₂ with NaBH₄ in the presence of HSCH₂CH₂Ph and were suggested to contain between 25 and 30 Co atoms. In our hands, however, we found no experimental evidence to support the existence of these large clusters in the reaction mixture. Instead, this reaction results in the relatively clean formation of the cobalt(II) coordination complex [Co₁₀(SCH₂CH₂Ph)₁₆Cl₄] (1). Complex 1 has been fully characterized using a wide variety of techniques, including single-crystal X-ray crystallography, NMR spectroscopy, mass spectrometry, and magnetometry. This complex represents the first example of a thiolate-protected Co(II) T3 supertetrahedral cluster.