Metal-Metal-Bonded Fe4 Clusters with Slow Magnetic Relaxation.

Reaction of FeBr2 with Li(N═CtBu2) (0.5 equiv) and Zn0 (2 equiv) results in the formation of the formally mixed-valent cluster [Fe4Br2(N═CtBu2)4] (1) in moderate yield. The subsequent reaction of 1 with Na(N═CtBu2) results in formation of [Fe4Br(N═CtBu2)5] (2), also in moderate yield. Both 1 and 2 were characterized by zero-field 57Fe Mössbauer spectroscopy, X-ray crystallography, and superconducting quantum interference device magnetometry. Their tetrahedral [Fe4]6+ cores feature short Fe-Fe interactions (ca. 2.50 Å). Additionally, both 1 and 2 display S = 7 ground states at room temperature and slow magnetic relaxation with zero-field relaxation barriers of Ueff = 14.7(4) and 15.6(7) cm-1, respectively. Moreover, AC magnetic susceptibility measurements were well modeled by assuming an Orbach relaxation process.

Understanding the Early Stages of Nickel Sulfide Nanocluster Growth: Isolation of Ni3 , Ni4 , Ni5 , and Ni8 Intermediates.

Addition of sub-stoichiometric quantities of PEt3 and diphenyl disulfide to a solution of [Ni(1,5-cod)2 ] generates a mixture of [Ni3 (SPh)4 (PEt3 )3 ] (1), unreacted [Ni(1,5-cod)2 ], and [(1,5-cod)Ni(PEt3 )2 ], according to 1 H and 31 P{1 H} NMR spectroscopic monitoring of the in situ reaction mixture. On standing, complex 1 converts into [Ni4 (S)(Ph)(SPh)3 (PEt3 )3 ] (2), via formal addition of a "Ni(0)" equivalent, coupled with a CS oxidative addition step, which simultaneously generates the Ni-bound phenyl ligand and the µ3 -sulfide ligand. Upon gentle heating, complex 2 converts into a mixture of [Ni5 (S)2 (SPh)2 (PEt3 )5 ] (3) and [Ni8 (S)5 (PEt3 )7 ] (4), via further addition of "Ni(0)" equivalents, in combination with a series of C-S oxidative addition and CC reductive elimination steps, which serve to convert thiophenolate ligands into sulfide ligands and biphenyl. The presence of 1-4 in the reaction mixture is confirmed by their independent syntheses and subsequent spectroscopic characterization. Overall, this work provides an unprecedented level of detail of the early stages of Ni nanocluster growth and highlights the fundamental reaction steps (i.e., metal atom addition, CS oxidative addition, and CC reductive elimination) that are required to grow an individual cluster.

[Ni23Se12(PEt3)13] Revisited: Isolation and Characterization of [Ni23Se12Cl3(PEt3)10].

The reaction of [Ni(1,5-COD)2] (1.0 equiv), PEt3 (0.04 equiv), SePEt3 (0.52 equiv), and [NiCl2(PEt3)2] (0.07 equiv) in a mixture of toluene and THF results in the formation of [Ni23Se12Cl3(PEt3)10] (1), which can be isolated in moderate yield after workup. Complex 1 was characterized by NMR spectroscopy, ESI-MS, and X-ray crystallography. This open-shell nanocluster features a central [Ni13]7+ anticuboctahedral kernel, which is encapsulated by a [Ni10(µ-Se)9Cl3]- shell, along with ten PEt3 ligands and three (µ4-Se)2- ligands. On the basis of our spectroscopic and crystallographic analysis, coupled with in situ spectroscopic monitoring, we believe that the previously reported nanocluster, [Ni23Se12(PEt3)13], is actually better formulated as [Ni23Se12Cl3(PEt3)10].

[Ni8(CNtBu)12][Cl]: A nickel isocyanide nanocluster with a folded nanosheet structure.

The reaction of 1.75 equiv of tBuNC with Ni(1,5-COD)2, followed by crystallization from benzene/pentane, resulted in the isolation of [Ni8(CNtBu)12][Cl] (2) in low yields. Similarly, the reaction of Ni(1,5-COD)2 with 0.6 equiv of [Ni(CNtBu)4], followed by addition of 0.08 equiv of I2, resulted in the formation of [Ni8(CNtBu)12][I] (3), which could be isolated in 52% yield after work-up. Both 2 and 3 adopt folded nanosheet structures in the solid state, characterized by two symmetry-related planar Ni4 arrays, six terminally bound tBuNC ligands, and six tBuNC ligands that adopt bridging coordination modes. The metrical parameters of the six bridging tBuNC ligands suggest that they have been reduced to their [tBuNC]2- form. In contrast to the nanosheet structures observed for 2 and 3, gas phase Ni8 is predicted to feature a compact bisdisphenoid ground state structure. The strikingly different structural outcomes reveal the profound structural changes that can occur upon addition of ligands to bare metal clusters. Ultimately, the characterization of 2 and 3 will enable more accurate structural predictions of ligand-protected nanoclusters in the future.

Synthesis and Characterization of Tungsten Nitrido Amido Guanidinato Complexes as Precursors for Chemical Vapor Deposition of WNxCy Thin Films.

Tungsten nitrido amido guanidinato complexes of the type WN(NR2)[(NR')2C(NR2)]2 (R = Me, Et; R' = i Pr, Cy) were synthesized as precursors for aerosol-assisted chemical vapor deposition (AACVD) of WNxCy thin films. The reaction of tungsten nitrido amido complexes of the type WN(NR2)3 (R = Me, Et) with two equivalents of a carbodiimide R'N=C=NR' (R' = i Pr, Cy) resulted in two insertions of a carbodiimide into W-N(amido) bonds, affording bis(guanidinato) amido nitrido tungsten complexes. These compounds were characterized by 14N NMR, indicating distinctive chemical shifts for each type of N-bound ligand. Crystallographic structure determination of WN(NMe2)[(N i Pr)2C(NMe2)]2 showed the guanidinato ligands to be non-equivalent. The complex WN(NMe2)[(N i Pr)2C(NMe2)]2 was demonstrated to serve as a precursor for AACVD of WNxCy thin films, resulting in featureless, X-ray amorphous thin films for growth temperatures 200 - 400 °C.

[Ni30S16(PEt3)11]: an open-shell nickel sulfide nanocluster with a "metal-like" core.

Reaction of [Ni(1,5-cod)2] (30 equiv.) with PEt3 (46 equiv.) and S8 (1.9 equiv.) in toluene, followed by heating at 115 °C for 16 h, results in the formation of the atomically precise nanocluster (APNC), [Ni30S16(PEt3)11] (1), in 14% isolated yield. Complex 1 represents the largest open-shell Ni APNC yet isolated. In the solid state, 1 features a compact "metal-like" core indicative of a high degree of Ni-Ni bonding. Additionally, SQUID magnetometry suggests that 1 possesses a manifold of closely-spaced electronic states near the HOMO-LUMO gap. In situ monitoring by ESI-MS and 31P{1H} NMR spectroscopy reveal that 1 forms via the intermediacy of smaller APNCs, including [Ni8S5(PEt3)7] and [Ni26S14(PEt3)10] (2). The latter APNC was also characterized by X-ray crystallography and features a nearly identical core structure to that found in 1. This work demonstrates that large APNCs with a high degree of metal-metal bonding are isolable for nickel, and not just the noble metals.

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.