Tetrahedral, Highly Coordinatively Unsaturated 14e (Fe) and 15e (Co) Hydrocarbyl Complexes Bearing Hydrotris(pyrazolyl)borato Ligands (Tp^R‘), Tp^R‘M−R (M = Fe, Co, Ni)

A series of hydrocarbyl complexes supported only by hydrotris(pyrazolyl)borato ligands (TpR‘ = 3,5-diisopropylpyrazolyl (TpiPr2) and 3,4,5-trimethylpyrazolyl derivatives (TpMe3)), TpR‘M−R (M/R/R‘ = Ni/η3-allyl/iPr2 (2iPr2Ni), Co/η3-allyl/iPr2 (2iPr2Co), Fe/η1-allyl/iPr2 (2iPr2Fe), Ni/η3-prenyl/iPr2 (3iPr2Ni), Co/η1-p-methylbenzyl/iPr2 (4iPr2Co), Fe/η1-p-methylbenzyl/iPr2 (4iPr2Fe), Co/η1-p-methylbenzyl/Me3 (4Me3Co), Fe/η1-p-methylbenzyl/Me3 (4Me3Fe), Co/η1-α-naphthylmethyl/iPr2 (5iPr2Co), Co/η1-ethyl/iPr2 (6iPr2Co), Fe/η1-ethyl/iPr2 (6iPr2Fe)) is prepared by reaction of the corresponding precursors, TpiPr2M−Cl (M = Fe (1iPr2Fe), Co (1iPr2Co), Ni (1iPr2Ni)) or TpMe3M(κ2-X) (M/X = Co/NO3 (1‘Me3Co), Fe/OAc (1‘Me3Fe)), with appropriate Grignard reagents. η1-Hydrocarbyl complexes are obtained when M = Fe, Co, and the thermally unstable η1-ethylnickel derivative 6iPr2Ni is characterized via conversion to the acyl derivative TpiPr2Ni(CO)C(O)Et (12). Characterization by a combination of various spectroscopic methods (IR, NMR, UV, and ESR), magnetic susceptibility, X-ray crystallography, and chemical reactions reveals that the η1-hydrocarbyl complexes 2iPr2Fe and 4−6 are tetrahedral, highly coordinatively unsaturated species with 14 (M = Fe) and 15 (M = Co) valence electrons. It is remarkable that the ethyl complexes 6 are thermally stable even at higher temperatures (70 °C (Co); 110 °C (Fe)), although they contain β-hydrogen atoms. The presence of a metal−carbon bond in 4−6 has been confirmed by protonation and hydrogenolysis, giving alkane (R−H) and carbonylation leading to acyl−carbonyl species (TpR‘M(CO)nC(O)R: M/n = Fe/2 (7), Co/1 (8)). Despite the coordinative unsaturation at the metal centers complexes 4−6 turn out to be sluggish toward unsaturated organic compounds such as olefin, internal acetylene, ketone, and nitrile. Only phenylacetylene reacted with the ethyl complexes 6iPr2 to result in insertion of the C⋮C bond into the M−C bond (6iPr2Co) or protonolysis of the M−C bond to afford the acetylide complex TpiPr2FeC⋮CPh, which was characterized after carbonylation, giving TpiPr2Fe(CO)2C⋮CPh (12). The magnetic susceptibility of η1-hydrocarbyl complexes combined with the results of EHMO calculations for the model complexes TpHM−CH3 (M = Fe, Co) reveals the high-spin configuration of d electrons, which leads to occupation of all five frontier orbitals by electron pairs or unpaired electrons. The lack of a vacant d orbital is concluded to be the origin of the thermal stability of the electron-deficient hydrocarbyl complexes.