Niobium−Mercury Heterometallic Compounds as Sources of Niobium(II):  Radical Paths to Organoniobium Species

Abstract

The niobium−mercury compounds [Cp‘2Nb(L)]2Hg (Cp‘ = η5-C5H4SiMe3, L = CO (4), PMe3 (5), or CNtBu (6)) serve as stable precursors to the short-lived Nb(II) radicals of general formula [Cp‘2Nb−L]. The homolysis of the Nb−Hg bond may result from a slow thermal reaction (generating low concentrations of radicals) or from a photochemical process in which mercury extrusion is more rapid. Since the Nb(II) species show no evidence for Nb−Nb bond formation, they are useful in the synthesis of a variety of Nb(III) and Nb(IV) species. Reactions of 4 with dimeric species such as [CpFe(CO)2]2, [CpNi(CO)]2, Co2(CO)8, or RSe−SeR give rise to new Nb−M or Nb−Se compounds, while reactions with potential π donors such as formaldehyde or azobenzene lead to displacement of the ligand L and formation of the Nb(IV) complexes. Crystallographic and variable-temperature NMR studies on the Nb−Fe compound Cp‘2Nb(μ-CO)2Fe(CO)Cp (10) are consistent with a low-energy fluxional process involving exchange of bridging and terminal carbonyl ligands.

Heterobimetallic Complexes Comprised of Nb and Fe: Isolation of a Coordinatively Unsaturated NbIII/Fe0 Bimetallic Complex Featuring a Nb≡Fe Triple Bond

“…This distance is 0.4 Å shorter than the shortest known Nb–Fe distance reported (2.8156(6) Å in Cp′ 2 Nb­(μ-CO) 2 Fe­(CO)­Cp, where Cp′ = η 5 -C 5 H 4 SiMe 3 , Chart ). To compare metal–metal distances among a series of different bimetallic combinations, Cotton’s “formal shortness ratio” (FSR) is used to normalize interatomic distances by representing the ratio of the metal–metal distance to the sum of the single bond atomic radii ( R 1 , Pauling) of the two metal ions . The FSR for 2 was calculated to be 0.97, suggesting a Nb/Fe interaction stronger than the V/Fe interaction in F–V­( i PrNPPh 2 ) 3 Fe–I (2.4571(5) Å, FSR = 1.02) and stronger than the Zr/Co interaction in Cl–Zr­( i PrNPPh 2 ) 3 Co–I (2.7315­(5) Å, FSR = 1.04) .…”

“…The solution was then allowed to warm to room temperature and stirred for 2 h. The resulting solution was filtered through Celite, concentrated to half its volume, and left at −35 °C for 12 h to obtain analytically pure colorless crystals of 7 (319 mg, 74%). X-ray quality crystals were grown from a concentrated solution of 7 in Et i PrNNb( i PrNPPh 2 ) 3 Fe-Br (8). Zinc dust (4 mg, 0.06 mmol) and FeBr 2 (27 mg, 0.12 mmol) were combined as solids in a 20 mL vial equipped with a stir bar.…”

Derivatization of Niobium Complexes Bearing Imido and Acetophenone Imine Ligands

“…Previous studies on the chemistry of amide ligands with niobium(III) and tantalum(III) suggest that amides can be successfully utilized to support group V metals in that oxidation state . On the other hand, the use of amide ligands with a lower oxidation state group V metal such as niobium(II) results in alternate reaction pathways and the formation of highly reactive species .…”

Assessing the Metal–Metal Interactions in a Series of Heterobimetallic Nb/M Complexes (M = Fe, Co, Ni, Cu) and Their Effect on Multielectron Redox Properties

“…Cp 2 Nb­(CO)­HZn­(BH 4 ) 2 and Cp 2 NbH 2 ZnCp were found to have Nb–Zn distances of 2.829 Å and 2.5407(7) Å, indicative of dative and normal covalent Nb–Zn bonds, respectively . More recently, Bruno and co-workers reported the isolation of the Nb-M single-bonded complexes (C 5 H 4 SiMe 3 ) 2 Nb­(μ-CO) 2 Fe­(CO)­Cp, (C 5 H 4 SiMe 3 ) 2 (CO)­NbCo­(CO) 4 , and (C 5 H 4 SiMe 3 ) 2 Nb­(μ-CO) 2 NiCp, and structural characterization of the Nb/Fe complex revealed a Nb–Fe separation of 2.8156(6) Å . A Nb–Cr single bond was also reported in Cp 2 Nb­(μ-PPh 2 )­(μ-CO)­Cr­(CO) 3 (PMe 2 H) .…”