Variable Coordination Modes of Hydrotris(3-isopropyl-4-bromopyrazolyl)borate (Tp‘) in Fe(II), Mn(II), Cr(II), and Cr(III) Complexes:  Formation of MTp‘Cl (M = Fe and Mn), Structural Isomerism in CrTp‘₂, and the Observation of Tp‘ ^- as an Uncoordinated Anion

The syntheses of the 4-coordinate Tp‘MCl complexes (where M = Fe (1), Mn (2); and Tp‘ = hydrotris(3-isopropyl-4-bromopyrazolyl)borate) are described. The single-crystal X-ray structures show that the metal centers have distorted tetrahedral coordination. Analogous reaction of CrCl₂(MeCN)₂ with TlTp‘ gave Cr(κ³-Tp‘)(κ²-Tp‘) (3) as the initial product. The 5-coordinate structure was assigned by single-crystal X-ray crystallography, and it was found that the κ³ ligand had isomerized to hydro(3-isopropyl-4-bromopyrazolyl)₂(5-isopropyl-4-bromopyrazolyl)borate). 3 is labile in solution:  in pentane it slowly converts to the 6-coordinate isomer Cr(κ³-Tp‘)₂ (4), whose structure was determined by X-ray crystallography. In 4 both ligands are isomerized. Both 3 and 4 display Jahn−Teller distorted structures expected for high-spin d⁴ configurations. Variable temperature magnetic susceptibility measurements confirm that 1, 2, and 3 all have high-spin electronic configurations in the range 5−300 K. In benzene solution 3 decomposes; one product [Cr(κ³-Tp‘)₂]⁺[Tp‘]^- (5), was identified by X-ray crystallography. 5 contains a pseudooctahedral Cr(III) cation with both ligands in the isomerized form and an uncoordinated Tp‘ ligand as counterion. Mechanistic studies reveal that this reaction is light rather than heat induced. IR spectroscopy is utilized to confirm the ligand hapticity in all complexes from the value of ν_B-H, and comparison is made with similar compounds.