Amido rotation in the chromium(vi), d(0)-system NCr(NPr(i)2)2X is under investigation as a method for the parameterization of ligands for their donor properties toward high valent metals. In this study, two new series were prepared and studied based on chalcogenide ligands, X = EBu(t) and EPh and where E = O, S, Se, Te; the OPh and SPh compounds were previously reported. The ligand donor parameters for these ligands correlate with the Cr-E-C angles in these chalcogenide series. In addition, it was found that NBO calculated overlaps and DFT calculated bond dissociation enthalpies correlate within X = halide-, EBu(t)- and EPh-series. All of the new complexes were characterized by X-ray diffraction.

“Weakly coordinating anions” such as tetraarylborates are ubiquitous in applications of inorganic and organometallic chemistry, with great industrial importance. In this work, we probe the ion-pairing ability of these weakly coordinating anions using the highly sensitive chromium­(VI) nitrido bis­(diisopropylamido) system NCr­(N-i-Pr2)2X, with one variable coordination site (X). This system is being used in the quantification of ligand donor ability to high-valent metal centers and has simply been called the ligand donor parameter (LDP). The donor ability of the variable ligand can be measured by solution-state rotational barrier studies via NMR spectroscopy. If the variable ligand is neutral, the chromium complex is cationic, {NCr­(N-i-Pr2)2L}+, with its pendant anion. Despite the weakly coordinating nature of the counteranions employed, a significant amount of ion pairing has been noted in solution, the result of which is substantial enough to perturb the sensitive LDP measurement. These effects have been noted for many commonly used counteranions, including hexafluoroantimonate­(V), hexafluorophosphate­(V), tetraphenylborate, and tetrakis­(bis­(3,5-trifluoromethyl)­phenyl)­borate (BArF24). Using diffusion ordered (DOSY) and rotating-frame Overhauser effect (ROESY) NMR spectroscopy and LDP values, we have shown, predictably, that the extent of ion pairing is solvent dependent and appears to be minimized by increasing the dielectric constant of the NMR solvent utilized. Additionally, we have gained insight into differences in the nature of ion pairing dependent upon the identity of the weakly coordinating anion employed. It was found that the tetraarylborate anions appear to be fully ion paired in CDCl3 but affect amido rotation less in comparison to other anions. We postulate that the smaller effect on the internal rearrangement by these fluorinated tetraarylborate anions is due to a lack of specificity in the interaction with the cation rather than a lack of ion pairing, which may be a general feature of these anions.

A series of rare nitrido cyclopentadienyl complexes have been prepared of chromium(VI). An improved synthesis of NCr(I)(NPr i 2 ) 2 is provided, which reacts with NaCp to give NCr(η 1 -Cp)(NPr i 2 ) 2 (1) in 82% isolated yield. The η 1 -indenyl complex 2 can be prepared using a similar procedure. Replacement of one diisopropylamido ligand was accomplished to prepare NCr(NPr i 2 )Cp(O 2 CPh) (3), NCr-(NPr i 2 )Cp(Cl) (4), and [NCr(NPr i 2 )Cp(NCMe)] + [SbF 6 ] − (5). In the series, the Cr−C interactions increase in bond order, 3 < 4 < 5, as the cyclopentadienyl compensates for the decreasing donor ability of the other ligands on chromium. The Ligand Donor Parameters (LDPs) for the η 1 -cyclopentadienyl and η 1 -indenyl ligands were measured as 13.73 and 13.76 kcal/ mol, respectively. Compounds 1−5 were characterized by single-crystal X-ray diffraction.

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2H-pyrimido[1,2-a]pyrimidine, hppH) are readily formed in good to excellent yields by addition of (hpp)SiMe 3 to TaCl 5 , Ta(hpp)Cl 4 , or (η-C 5 Me 4 R)TaCl 4 , respectively, in dichloromethane. Ta(hpp)Cl 4 in solution exhibits three CH 2 multiplets in the proton NMR spectrum, consistent with a mirror plane that bisects the hpp ligand. The six-coordinate, yellow Ta(hpp)Cl 4 possesses a distorted octahedral solid-state structure with chelating hpp and Ta−N distances of 2.038(1) and 2.059(1) Å. Ta(hpp) 2 Cl 3 is fluxional in solution by proton NMR spectroscopy, with three CH 2 multiplets at room temperature that split into additional multiplets at −50 °C, consistent with the six multiplets expected from the solid-state structure. In the solid state, bright yellow Ta(hpp) 2 Cl 3 has an approximately pentagonal bipyramidal structure with both hpp chelates coordinated in the pentagonal plane. Ta−N distances are 2.114(3) and 2.110(3) Å for one hpp ligand and 2.112(3) and 2.117(3) Å for the other. The proton and carbon-13 solution NMR spectra of (η-C 5 Me 4 R)Ta(hpp)Cl 3 are consistent with a static solution structure at room temperature, with the two coordinated nitrogens in different chemical environments as shown by six different methylene resonances. The solid-state structure of bright pinkish-red (η-C 5 Me 5 )Ta(hpp)Cl 3 is a four-legged piano stool with axial ligation, with one hpp nitrogen coordinated in the square plane and one nitrogen in the axial position opposite the cyclopentadienyl centroid. Addition of (hpp)SiMe 3 to the organoditantalum(III) complex (η-C 5 Me 4 R) 2 Ta 2 (μ-Cl) 4 or Na/Hg reduction of (η-C 5 Me 4 R)Ta(hpp)Cl 3 in benzene yields the first mid-valent tantalum hpp adduct, paramagnetic red (η-C 5 Me 4 R)Ta(hpp)Cl 2 . This Ta(IV) four-legged piano stool structure in the solid state has an hpp chelate with Ta−N distances of 2.118(3) and 2.091(3) Å. Intraligand bond distances and angles for the hpp ligands, proton and carbon-13 NMR solution spectra, EPR spectra, and UV−vis spectra for the new Ta(V) and Ta(IV) hpp complexes are discussed.