The incorporation of subvalent Group 13 ligands into the coordination sphere of transition metals has always been a challenging task, particularly in the formation of homoleptic complexes. Although metastable or sterically protected subvalent E I (E = Al, Ga, In) precursors have become accessible during the last few decades, [1] for example, E I halides, [1, 2] [{Cp*E} n ] (Cp* = h 5 -C 5 Me 5 ), [1, 3] and [{EC(SiMe 3 ) 3 } n ], [1, 4] stable and isolable boron congeners have still not been prepared. For this reason, only homoleptic transition-metal complexes with Al-, Ga-, and In-based ligands have so far been realized, for example, mononuclear [Ni(ECp*) 4 ] (A; E = Al, Ga) [5,6] and [Ni{EC(SiMe 3 ) 3 } 4 ] (B; E = Ga, In), [7,8] as well as numerous heteroleptic examples with two or more E I ligands. [9] The corresponding subvalent boron ligands, that is, borylenes, have only been generated directly in the coordination sphere of transition metals [10] to form species such as [(OC) 5 M=BN(SiMe 3 ) 2 ] (C; M = Cr, Mo, W).[11] To date, both mononuclear borylene complexes containing more than one borylene substituent as well as homoleptic borylene species have continuously resisted isolation. Nonetheless, borylene complexes have sparked increasing interest in fundamental organometallic research because of their close relationship with important organometallic compounds such as carbene, carbyne, and vinylidene complexes, and the similarity of the bonding properties of borylene and carbonyl ligands. Numerous experimental and computational studies have previously examined these bonding properties in detail.[10] It was thus shown that BR has stronger s-donor and p-acceptor properties than CO, which makes the M À BR bond even more stable with respect to homolytic dissociation than the M À CO bond, but in turn it is kinetically labile due to the high polarity. Getting back to the series of related ligands mentioned above, the predominance of the CO and carbene ligands in organometallic chemistry is also manifested by the fact that homoleptic complexes have only been accessible with these two ligands. By contrast, the incorporation of two borylene or carbyne ligands into a mononuclear transition-metal complex has always proven problematic. While in the former case a suitable synthetic approach is still lacking, [12] the synthesis of bis(carbyne) species is further hampered by reductive coupling to form acetylenes, particularly with alkyl-substituted carbynes.[13] It has not yet been elucidated whether bis-(borylene) complexes are resistant towards reductive coupling. In any case, it would require the availability of experimental data before this question could be clarified. We have been studying borylene complexes for more than a decade now, but all our efforts to synthesize a complex with more than one terminal borylene have so far failed.Herein, we describe the successful generation and isolation of a long-sought after mononuclear, terminal bis-(borylene) complex derived from the [Cp*Ir] half-sandwich fragment,...

Herein we report for the first time full details on the synthesis and structural characterization of novel homodinuclear bridging cobalt and nickel borylene complexes containing bridging carbonyl ligands, an unusual coordination motif rarely before observed for homodinuclear borylene complexes. Furthermore, the homodinuclear nickel complex represents the first instance of a nickel borylene complex. Quantum chemical analyses of charge-density topology, electron localization function (ELF) and natural charges indicate the absence of direct metal-metal bonds in both the cobalt and nickel systems, in contradiction with electron counting. The topology of the Laplacian of the electron density and of the ELF around the bridging boron atom is consistent with a bis-metallo-substituted borane situation for the dicobalt system, but with a three-center-bonding borylene for the dinickel complex.

New multifunctional nanoparticles (NPs) that can be used as contrast agents (CA) in different imaging techniques, such as photoluminescence (PL) microscopy and magnetic resonance imaging (MRI), open new possibilities for medical imaging, e.g., in the fields of diagnostics or tissue characterization in regenerative medicine. The focus of this study is on the synthesis and characterization of CaF2:(Tb3+,Gd3+) NPs. Fabricated in a wet-chemical procedure, the spherical NPs with a diameter of 5–10 nm show a crystalline structure. Simultaneous doping of the NPs with different lanthanide ions, leading to paramagnetism and fluorescence, makes them suitable for MR and PL imaging. Owing to the Gd3+ ions on the surface, the NPs reduce the MR T 1 relaxation time constant as a function of their concentration. Thus, the NPs can be used as a MRI CA with a mean relaxivity of about r = 0.471 mL·mg−1·s−1. Repeated MRI examinations of four different batches prove the reproducibility of the NP synthesis and determine the long-term stability of the CAs. No cytotoxicity of NP concentrations between 0.5 and 1 mg·mL−1 was observed after exposure to human dermal fibroblasts over 24 h. Overall this study shows, that the CaF2:(Tb3+,Gd3+) NPs are suitable for medical imaging.

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