Thiopyridazine-Based Palladium and Platinum Boratrane Complexes.

Palladium and platinum boratrane complexes of the type [M{B(PnMe, tBu)3}(PPh3)] (M = Pd 1, Pt 2b) have been prepared via the reaction of the soft scorpionate ligand potassium tris(4-methyl-6- tert-butyl-3-thiopyridazinyl)borate KTnMe, tBu with bis(triphenylphosphine)metal(II) dichloride. While reaction with the Pd precursor allowed direct isolation of a symmetric boratrane complex, the Pt analogue led to the hydrido compound [Pt{B(PnMe, tBu)3}(PPh3)H]Cl (2a), which after reaction with a base gave 2b. Subsequent oxidation with Br2 and I2, respectively, led to the dihalide compounds of the molecular formula [M{B(PnMe, tBu)3}X2] (3a,b-4a,b). Halide abstraction with Ag(SbF6) further gave interesting cationic compounds of either dimeric [Pd{B(PnMe, tBu)3}X]2(SbF6)2 (5a,b) or monomeric [Pd{B(PnMe, tBu)3}(NCMe)2](SbF6) (6) nature. All compounds were spectroscopically and X-ray crystallographically characterized revealing strong metal to boron interactions. DFT calculations of 1, 2a, and 2b confirm the strong M-B interaction and a high positive charge on the metal centers.

Efficient CO2 Insertion and Reduction Catalyzed by a Terminal Zinc Hydride Complex.

The terminal zinc hydride complex [Tntm]ZnH (2; Tntm=tris(6-tert-butyl-3-thiopyridazinyl)methanide) is an efficient hydrosilylation catalyst of CO2 at room temperature without the need of Lewis acidic additives. The inherent electrophilicity of the system leads to selective formation of the monosilylated product (MeO)3 SiO2 CH (at room temperature with a TOF of 22.2 h-1 and at 45 °C with a TOF of 66.7 h-1 ). In absence of silanes, the intermediate formate complex [Tntm]Zn(O2 CH) (3) is quantitatively formed within 5 min. All complexes were fully characterized by 1 H and 13 C NMR spectroscopy and single-crystal X-ray diffraction analyses. Density functional theory (DFT) calculations reveal a high positive charge on zinc and the increased preference of the ligand to adopt a κ3 -coordination mode.

Three-Fold-Symmetric Selenium-Donor Metallaboratranes of Cobalt and Nickel.

A novel selenium-containing pyridazinyl-based soft scorpionate ligand (KTnse) was synthesized. It reacts with CoCl2 and NiCl2, yielding the first metallaboratrane complexes with selenium in their donor positions. Further substitution with Ag(OTf) or NaN3 allows isolation of the respective triflate or azide complexes. Reaction with Ag(OTf) leads in the case of nickel to a dinuclear, dicationic complex with a short Ni-Ni distance, while cobalt gave a mononuclear cationic species. Substitution of the chloride by azide yields with both metals the respective azide complexes. All compounds were characterized via single-crystal X-ray diffraction analysis. Density functional theory calculations on the chloride species point to oxidized cobalt(III) and nickel(III) centers.

Unusual C-N Coupling Reactivity of Thiopyridazines: Efficient Synthesis of Iron Diorganotrisulfide Complexes.

The reaction of iron(II) triflate with 6-tert-butyl-3-thiopyridazine (PnH) and 4-methyl-6-tert-butyl-3-thiopyridazine (MePnH) respectively led to iron bis(diorganotrisulfide) complexes [Fe(RPnS3PnR)2](OTf)2 [R = H (1a) and Me (2a)]. The corresponding perchlorate complexes were prepared by using the iron(II) chloride precursor and the subsequent addition of 2 equiv of NaClO4, giving [Fe(RPnS3PnR)2](ClO4)2 [R = H (1b) and Me (2b)]. The compounds were fully characterized including single-crystal X-ray diffraction analysis. All four compounds exhibit nearly perfect octahedral geometries with an iron center coordinated by four nitrogen atoms from two RPnS3PnR ligands and by two sulfur atoms of the central atom in the S3 unit. The diamagnetic complexes exhibit unusually high redox potentials for the Fe2+/3+ couple at E1/2 = 1.15 V (for 1a and 1b) and 1.08 V (for 2a and 2b) versus Fc/Fc+, respectively, as determined by cyclic voltammetry. Furthermore, the source of the extra sulfur atom within the S3 unit was elucidated by isolation of C-N-coupled pyridazinylthiopyridazine products.

Thiopyridazine-Based Copper Boratrane Complexes Demonstrating the Z-type Nature of the Ligand.

In Z-type ligands the electrons for the coordination bond are formally provided by the metal. They represent an important addition to the much more extensively used L- and X-type σ-donor ligands for the development of transition metal complexes with new reactivities. We report here a new boron Z-type ligand with three tethering thiopyridazinyl donors forming exclusively complexes that feature a metal boron bond. Rational substitution pattern in the backbone of the pyridazinyl heterocycle led to a well-behaved ligand system that allowed preparation of a series of copper boratrane complexes in high yields. They are found to be more soluble in common organic solvents allowing reactivity studies in contrast to previous complexes with this type of ligand. Thus, copper complexes [Cu{B(Pn(Me,tBu))3}X] with X = Cl, OTf, N3, and κN-NCS are reported. Solution behavior was explored, and the molecular structures were determined by single-crystal X-ray diffraction analyses. The thiocyanate ligand is found to coordinate via its nitrogen atom pointing to a high oxidation state of the copper. Density functional theory calculations indicate a high positive charge on copper and a strong copper-boron interaction. Thus, here reported complexes deliver synthetic evidence for the Z-type nature of the ligand. These findings are important for further dissemination of these types of ligands in coordination chemistry.

Photoinduced Reactivity of the Soft Hydrotris(6-tert-butyl-3-thiopyridazinyl)borate Scorpionate Ligand in Sodium, Potassium, and Thallium Salts.

The soft scorpionate ligand hydrotris(6-tert-butyl-3-thiopyridazinyl)borate (Tn) was found to exhibit pronounced photoreactivity. Full elucidation of this process revealed the formation of 6-tert-butylpyridazine-3-thione (PnH) and 4,5-dihydro-6-tert-butylpyridazine-3-thione (H2PnH). Under exclusion of light, no solvolytic reactions occur, allowing the development of high-yield preparation protocols for the sodium, potassium, and thallium salts and improving the yield for their derived copper boratrane complex. The photoreactivity is relevant for all future studies with electron-deficient scorpionate ligands.