Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V) Complexes.

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe2)5 (M = Nb, Ta), for example, yields [M(µ3-O)(NMe2)3]4 (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H2O with amide amidinates M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(µ3-O)(NMe2)3]4 (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H2O, leaving the amidinate ligand intact. [Ta(µ3-O)(NMe2)3]4 (2), the analogue of 1, was not observed as a product in the reaction of 4 with H2O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe2)4[MeC(NiPr)2] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe2)5 (M = Nb, Ta) with iPrN(H)C(Me)=NiPr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in iPrN(H)C(Me)=NiPr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s-1 (3), 0.033(6) s-1 (4), and 2.23(7) s-1 (5), showing that the rate of the Nb complex Nb(NMe2)4[MeC(NiPr)2] (3) is 13 times faster than that of its Ta analogue 4.

Spin-phonon couplings in transition metal complexes with slow magnetic relaxation.

Spin-phonon coupling plays an important role in single-molecule magnets and molecular qubits. However, there have been few detailed studies of its nature. Here, we show for the first time distinct couplings of g phonons of CoII(acac)2(H2O)2 (acac = acetylacetonate) and its deuterated analogs with zero-field-split, excited magnetic/spin levels (Kramers doublet (KD)) of the S = 3/2 electronic ground state. The couplings are observed as avoided crossings in magnetic-field-dependent Raman spectra with coupling constants of 1-2 cm-1. Far-IR spectra reveal the magnetic-dipole-allowed, inter-KD transition, shifting to higher energy with increasing field. Density functional theory calculations are used to rationalize energies and symmetries of the phonons. A vibronic coupling model, supported by electronic structure calculations, is proposed to rationalize the behavior of the coupled Raman peaks. This work spectroscopically reveals and quantitates the spin-phonon couplings in typical transition metal complexes and sheds light on the origin of the spin-phonon entanglement.

Product in indole detection by Ehrlich's reagent.

Ehrlich's reagent (p-dimethylaminobenzaldehyde [DMAB, 1] in 95% EtOH with HCl as catalyst) was employed in spot tests of indoles, providing a diagnosis of, for example, liver diseases, hemolytic processes, occlusion of the common bile duct, and carcinoid syndrome. Although the reagent has been widely used for more than a century, it is not clear how many indole molecules react with a DMAB molecule and whether the reaction takes place at the α- or ß-position of the indole molecule. Research here shows that the reaction of DMAB (1) with indole (2) in a 1:2 ratio gives ß-bis(indolyl)methane (3). The reaction occurs at the ß-position of indole under the conditions of the Ehrlich test, as confirmed by the crystal structure of 3.

Reactions of zirconium amide amidinates with dioxygen. Observation of an unusual peroxo intermediate in the formation of oxo compounds.

Reaction of d(0) Zr(NMe2)2[MeC(N(i)Pr)2]2 (1) with O2 at -30 °C gives three Zr containing products: a peroxo trimer {(µ-η(2):η(2)-O2)Zr[MeC(N(i)Pr)2]2}3 (2), an oxo dimer {(µ-O)Zr[MeC(N(i)Pr)2]2}2 (3), and an oxo polymer {(µ-O)Zr[MeC(N(i)Pr)2]2}n (4). is a rarely observed peroxo complex from the reaction of a d(0) complex with O2.

Reactions of Group 4 amide guanidinates with dioxygen or water. Studies of the formation of oxo products.

Reactions of the zirconium amide guanidinates (R2N)2M[(i)PrNC(NR2)N(i)Pr]2 (R = Me, M = Zr, 1; M = Hf, 2; R = Et, M = Zr, 3) with O2 or H2O give products that are consistent with the oxo dimers {M(µ-O)[(i)PrNC(NR2)N(i)Pr]2}2 (R = Me, M = Zr, 4; M = Hf, 5; R = Et, M = Zr, 6) and polymers {M(µ-O)[(i)PrNC(NR2)N(i)Pr]2}n (R = Me, M = Zr, 7; M = Hf, 8; R = Et, M = Zr, 9). Mass spectrometric (MS) analyses of the reactions of water in air with 1 and 2 show formation of the Zr monomer Zr(═O)[(i)PrNC(NMe2)N(i)Pr]2 (10), oxo dimers 4 and 5, and dihydroxyl complexes M(OH)2[(i)PrNC(NMe2)N(i)Pr]2 (M = Zr, 11; Hf, 12). Similar MS analyses of the reaction of diethylamide guanidinate 3 with water in air show the formation of Zr(═O)[(i)PrNC(NEt2)N(i)Pr]2 (13), Zr(OH)2[(i)PrNC(NEt2)N(i)Pr]2 (14), 6, and {(Et2N)Zr[(i)PrNC(NEt2)N(i)Pr]2}(+) (15). Kinetic studies of the reaction between 1 and a continuous flow of 1.0 atm of O2 at 80-105 °C indicate that it follows pseudo-first-order kinetics with ΔH(‡) = 8.7(1.1) kcal/mol, ΔS(‡) = -54(3) eu, ΔG(‡)(358 K) = 28(2) kcal/mol, and a half-life of 213(1) min at 85 °C.