Synthesis, Structures, and Redox Properties of Mixed-Sandwich Complexes of Cyclopentadienyl and Hydrotris(pyrazolyl)borate Ligands with First-Row Transition Metals

Abstract

A series of [MCpRTp] n + (CpR = Cp*(pentamethylcyclopentadienyl) or Cp (cyclopentadienyl), Tp = hydrotris(pyrazolyl)borate) complexes have been synthesized by reaction of a suitable MCpR precursor with KTp (CpR = Cp*; n = 0, M = Cr, Fe, Co, Ni; n = 1, M = Cr, Co, Ni; CpR = Cp; n = 0, M = V, Co, Ni; n = 1, M = V, Co). Oxidation with [FeCp2]+ salts or reduction with CoCp2 where appropriate provided easy access to the corresponding M(III) or M(II) species. All of the complexes studied showed reversible M(III)/M(II) redox couples. Similarly [MCpRTpm] n + complexes have also been isolated (Tpm = hydrotris(pyrazolyl)methane; CpR = Cp*, n = 1, M = Fe; CpR = Cp, n = 1 or 2, M = Co). Analytical, NMR, IR, and mass spectroscopic data are consistent with the formulation of these species as mixed-sandwich complexes. Oxidation of VCpTp in MeCN solution yields [VCpTp(MeCN)]+, whereas similar reaction in CH2Cl2 solution yields [VCpTp]+. [VCpTp]+ reacts with σ-donor ligands L, where L = CNtBu and PMe3, to form [VCpTpL]+ species, but is unreactive when L = CO, indicating little π-base character to the V center. Crystal structure determinations were performed for various complexes: CoCp*Tp is unique in displaying κ2-Tp and η5-Cp* coordination, whereas [VCpTp]+, [CoCp*Tp]+, NiCp*Tp, and [CoCpTpm]2+ all display mixed-sandwich structures with κ3-Tp binding. The factors determining the relative conformations of the CpR and Tp ligands are discussed. The structures of [VCpTp(MeCN)]+ and [VCpTp(PMe3)]+ were also determined and show that considerable distortion of the sandwich moiety has occurred to accommodate coordination of the extra ligand. For the MCpRTp complexes a dependence of νB - H on Tp hapticity, ancillary ligand, and oxidation state is observed from IR spectroscopic data in the solid state. IR data obtained in solution suggest that CoCp*Tp is in conformational equilibrium, with a κ3-Tp structure also present. Some of the factors affecting 1H and 13C NMR shifts are discussed and compared with relevant homoleptic analogues. Electrochemical data reveal, in general, that MCp2 and MCp*2 species are more electron rich, although comparisons are best confined to a per metal basis.

(Allenylidene)ruthenium Complexes with Redox‐Active Substituents and Ligands

“…Thus, O'Hare and co-workers have just published a series of mixed-sandwich complexes containing both the Cp and the Tp ligand or the 3,5-dimethyl derivative thereof, several of them in the adjacent M II and M III oxidation states (M ϭ V, Cr, Co, Ni). [55] In all these cases oxidation of the metal atom was found to induce a shift of the BϪH band to higher energies, though of a strongly varying magnitude (7Ϫ72 cm Ϫ1 ). We have made the same observations with the complexes reported here.…”

“…The position of the BϪH stretch in infrared spectra has been recognized as an indicator for the coordination mode of the Tp ligand (i.e., κ 3 versus κ 2 coordination). [54,55] It has, however, only very recently been realized that the frequency of this band also corresponds to the electron density on the metal ion to which it is coordinated and, more specifically, to its oxidation state. Thus, O'Hare and co-workers have just published a series of mixed-sandwich complexes containing both the Cp and the Tp ligand or the 3,5-dimethyl derivative thereof, several of them in the adjacent M II and M III oxidation states (M ϭ V, Cr, Co, Ni).…”

C-Homoscorpionate Oxidation Catalysts—Electrochemical and Catalytic Activity

“…It is commonly agreed to compare [2,3,30,[36][37][38] the main characteristics of tris(pyrazol-1-yl) type scorpionate ligands with other face-capping ligands. In particular, the parallel between Tp and cyclopendadienyl (Cp, Figure 2c) ligands is established in that both are mononegative, six-electron (ionic model) or five-electron donor (covalent model) ligands.…”

“…They are also formally isolobal [2,3]. The former are weak-field hard σ-N donors which tend to behave as fac-capping chelating ligands (i.e., occupy three coordination positions), while Cp are typically 5-fold π-donors and tend to form tetrahedral complexes [3,38].…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…22 The synthesis, structure, and redox properties of mixed-sandwich first row transition metal complexes of Cp, Cp* and Tp has also been reported. 23 A brief history and prospects of C-H bond activation 24 and a theoretical investigation of methane activation by first row transition metals have been published. 25 Reactions and mechanistic aspects of late transition metal catalysed C-C bond formation involving C-H activation 26 and the addition of aromatic ortho C-H bonds to olefins and acetylenes have also been reviewed.…”