Metalloradical Compounds with 1,2‐Dipivaloylhydrazido Ligands: Electron Transfer and Alkylation/Protonation Effects

Abstract

Under oxidative conditions the complexes M(bpy) 2 Cl 2 , M = Ru or Os, react with 1,2-dipivaloylhydrazine H 2 L = tBuC(O)-NH-NH-C(O)-tBu to yield the paramagnetic compounds [M(L)(bpy) 2 ](PF 6 ) {1(PF 6 ), M = Ru and 2(PF 6 ), M = Os}.Crystal structures of 1(ClO 4 ) and 2(PF 6 ) reveal an NNCO chelate coordination of L with d(NN) ≈ 1.39 Å. EPR spectroscopy suggests a metal/ligand mixed situation for the unpaired electron, with 29 % (Ru, 1 + ) or 46 % (Os, 2 + ) metal spin contribution according to DFT calculations. Both complex cations exhibit reversible one-electron oxidation and reduction (K c ≈ 10 19 for 1 + and 10 12 for 2 + ) which could also be monitored by IR (ν CO ) and UV/Vis-NIR spectroelectrochemistry. These data confirm a [a] Scheme 3. Chemical structures of singly charged mononuclear complexes. Scheme 4. Representation of differently charged species [M(bpy) 2 (L)] 0/+/2+ incorporating different ligand redox states (L 2-/·-/0 ). Results and Discussion Synthesis and CharacterizationScheme 5 illustrates the reactions leading to the compounds from Scheme 3. Deprotonation of the 1,2-dipivaloylhydrazine Scheme 5. Reaction scheme for mononuclear ruthenium complexes (top) and for the mononuclear osmium complex (bottom).

Synthesis of N,O-Chelating Hydrazidopalladium Complexes from 1,2-Bis(trifluoroacetyl)hydrazine

“…After refluxing a mixture of 1, [Pd(dba) 2 ], and two molar amounts of PPh 3 in THF for 19 h, an orange-red solid of bis(triphenylphosphine)palladium 4 was isolated in 27% yield. Inequivalent N,O-coordination of 1,2-bis(trifluoroacetyl)hydrazido in 4 was corroborated by two independent 19 F NMR signals at −69.2 and −69.5 ppm in CD 2 Cl 2 . Interestingly, the 31 P{ 1 H} NMR spectrum of 3 displayed only a singlet signal at 31.2 ppm at room temperature, distinct from that of the DPPP-coordinated complex 2.…”

“…Pd 2 (dba) 4 [30] and 1,2-bis(trifluoroacetyl)hydrazine [29] were prepared according to the procedures described in the literature with modifications. 1 H, 19 F, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra were recorded on JEOL JNM-LA300 and JNM-ECX400 spectrometers (JEOL Ltd., Tokyo, Japan) at around 25 • C unless otherwise noted. The chemical shifts were referenced to an external tetramethylsilane signal (0.0 ppm) by using the signals of residual proton impurities in the deuterated solvents for 1 H NMR and by using the solvent resonance for 13 C{ 1 H} NMR.…”

“…Mp. 246-248 19 F NMR (376.2 MHz, CD 2 Cl 2 , RT): δ -68.9 (s) -68.0 (s). 13 C{ 1 H} NMR (100.5 MHz, CD 2 Cl 2 , RT): δ 51.2, 51.5, 61.1, 64.9, 117.0 (q, 1 J CF = 276 Hz, CF 3 ), 117.4 (q, 1 J CF = 291 Hz, CF 3 ), 152.9 (q, 2 J CF = 35 Hz), 162.4 (q, 2 J CF = 30 Hz).…”

“…The autoclave was flushed with H 2 and then pressurized to 50 atm. The solution was stirred at 80 • C for 26.5 h. After carefully venting hydrogen, a sample of the reaction mixture was diluted in CD 2 Cl 2 under argon and 19 F NMR was recorded. Subsequently, the resulting mixture was transferred to a 20 mL Schlenk tube and stirred at 65 • C for 16 h. The recovery of 2 from the hydrogenolysis products was confirmed by 19 F NMR.…”

“…Introduction of acyl substituents into hydrazine also exerts a stimulatory influence on the access to hydrazides [11], as typified by the platinum-catalyzed intramolecular hydrohydrazination of alkenes [12]. Enhanced acidity of hydrazines attached to the electron withdrawing groups facilitates the formation of hydrazido complexes, as seen in widespread studies on the deprotonative conversion of 1,2-diacylhydrazines [13][14][15][16][17][18][19][20]. Dilworth treated 1,2-dibenzoylhydrazine with cis-[PtCl 2 (PPh 3 ) 2 ] in the presence of NaHCO 3 to obtain a mononuclear dibenzoylhydrazido(2-) complex [13].…”