Isocyanate diinsertion into the N-H bond of the 2-pyridylamino ligand of organolanthanides

Synthesis, Structure, and Antiproliferative Action of 2-Pyridyl Urea-Based Cu(II) Complexes

Relying on a recently suggested protocol that furnishes convenient access to variously substituted 2-pyridyl ureas, twelve hitherto unknown Cu(II) complexes have been synthesized in the present work and their structures were evaluated by elemental analysis, HRMS, IR spectroscopy, and X-ray diffraction study. Two structural motifs ([Cu(L)2Cl]+[Cl]− or (Cu(L)2Cl2) depending on the substitution pattern on the 2-pyridine fragment were revealed. In addition, antiproliferative action of the obtained compounds have been investigated against lung cancer cell lines (A549, NCI-H460, NCI-H1975), and healthy WI-26 VA4 cells were used to monitor non-specific cytotoxicity. Two nitro-group substituted complexes Cu(U3)2Cl2 (IC50 = 39.6 ± 4.5 μM) and Cu(U11)2Cl2 (IC50 = 33.4 ± 3.8 μM) demonstrate enhanced activity against the drug resistant NCI-H1975 cells with moderate selectivity toward normal WI-26 VA4 cells. The antiproliferative mechanism of cell death underlying the growth inhibitory effect of the synthesized complexes was studied via additional experiments, including the cell cycle analysis and the apoptosis induction test. Reassuringly, certain 2-pyridyl urea-based Cu(II) complexes exerted cell line-specific antiproliferative effect which renders them valuable starting points for further unveiling the anticancer potential of this class of coordination compounds.

Low-coordinate first-row transition metal complexes in catalysis and small molecule activation

In this Perspective, we will highlight selected examples of transition metal complexes with low coordination numbers whose high reactivity has been exploited in catalysis and the activation of small molecules featuring strong bonds (N₂, CO₂, and CO).

Open-shell lanthanide(II+) or -(III+) complexes bearing σ-silyl and silylene ligands: synthesis, structure, and bonding analysis

Complexes featuring lanthanide (Ln)–Si bonds represent a highly neglected research area. Herein, we report a series of open-shell Ln^II+ and Ln^III+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the Ln^III+ complexes Cp₃Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp₂Ln({Si(SiMe₃)₂SiMe₂}₂)]⁻[K₂(18-cr-6)₂Cp]⁺ [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a–2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln–Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp₃Ce affords the acyclic product [Cp₃CeSi(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂-Cp₃Ce]^2–2[K(18-cr-6)]⁺ (3) as the first example of a complex featuring a Ce–Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC₆H₄-2-tBu){(NtBu)₂CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)₂CPh} (4b) were reacted with Cp*₂Sm(OEt₂), affording, by OEt₂ elimination, the corresponding silylene complexes, both featuring Sm^II+ centers: Cp*₂Sm ← :Si(O–C₆H₄-2-tBu){(NtBu)₂CPh} (6) and Cp*₂Sm ← :Si(OtBu){(NtBu)₂CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a–2d, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm–Si bonds are observed, in accordance with a donor–acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a–2d, 5, and 6 to elucidate the bonding situation between the Ln^II+ or Ln^III+ centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln–Si bond is observed in the series 2a–2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln–Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .

A series of open-shell Ln^II+ and Ln^III+ complexes bearing σ-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands are reported. By reactions of Cp₃Ln (Ln = Tm, Ho, Tb, Gd) with a 1,4-oligosilanyl dianion, the metallacyclopentasilane salts [Cp₂Ln({Si(SiMe₃)₂SiMe₂}₂)]⁻[K₂(18-cr-6)₂Cp]⁺ were obtained. Alternatively, two NHSi ligands were reacted with Cp*₂Sm(OEt₂), affording the corresponding silylene complexes, both featuring Sm^II+ centers. Density functional theory calculations were conducted for all complexes to study the bonding situation.