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

Abstract

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 CrCl(2)(MeCN)(2) with TlTp' gave Cr(kappa(3)-Tp')(kappa(2)-Tp') (3) as the initial product. The 5-coordinate structure was assigned by single-crystal X-ray crystallography, and it was found that the kappa(3) 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(kappa(3)-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 d(4) 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(kappa(3)-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 nu(B)(-)(H), and comparison is made with similar compounds.

A Family of Four‐Coordinate Iron(II) Complexes Bearing the Sterically Hindered Tris(pyrazolyl)borato Ligand TptBu,Me

“…Subtracting the covalent radii of chlorine (0.99 ), bromine (1.14 ), iodine (1.33 ), and fluorine (0.64 ) [15] from the respective Fe À X bond length in complexes 1-4, gives four values for the covalent radius of iron in these complexes. The results, shown in Table 2, are internally consistent and comparable to those featured by other [12] and [tBuTp iPr FeCl] [13] ), which have previously been structurally characterized. Entries 1-3 show that the Fe À Cl and Fe À N bond lengths are identical for those complexes bearing a tBu group in the 3-position of the pyrazole ring.…”

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

“…The reversibility of the redox couple was judged by comparison with the behavior of the [FeCp 2 ] + /FeCp 2 couple under the same conditions. Magnetic measurements were performed as described previously . KTp and KTp*, [CrCp*Cl] 2 , [FeCp 2 ] + [PF 6 ] - , [FeCp*(MeCN) 3 ] + [PF 6 ] - , [CoCp*Cl] 2 , NiCp*(acac), VCp 2 Cl, [FeCp 2 ] + [BAr‘ 4 ] - , CoCp(CO)I 2 , CoCp 2 , [NiCp(COD)] + [BF 4 ] - , and Tpm were prepared by published procedures.…”

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“…1 The first of these simple, symmetric species date back to Trofimenko's disclosure of the ligand class, 2 and neutral Tp 2 M complexes of divalent metal ions (or equivalently substituted pyrazole homologues) are now known for almost all of the 3d (V to Zn) and three 4, 5d elements (Ru, Cd, Hg), as well as for a number of alkaline earths (Be-Ba), rare earths (Sm, Eu, Yb) and metalloids (Sn, Pb). 1,3 We report here the extension of this chemical series to the uncommon, reactive divalent oxidation state of titanium. †…”