Oxo-bridged chromium(III) dimers derived from the reactions of anions (thiocyanate, cyanate, azide, cyanide, chloride) with the bis(.mu.-hydroxo)bis{tris(2-pyridylmethyl)amine)chromium(III)} ion

Abstract

Five linear, oxo-bridged dimers of the form [L(tmpa)CrOCr(tmpa)LI2+ (L = NCS-, NCO-, N3-, CN-, C1-) have been prepared through the nucleophilic displacement of a hydroxo bridge from [(t~npa)Cr(OH)~Cr(tmpa)]~+ by L-. A mechanism involving the migration of a bridging oxo function to a cis coordination position is proposed to account for the change in stereochemistry that accompanies the conversion of the chromium diol to [(tmpa)(SCN)CrOCr(NCS)(tmpa)]2+. Electronic spectra, magnetic susceptibilities, and one-electron-oxidation half-wave potentials of the dimers are reported along with acid ionization constants of the corresponding hydroxo-bridged [L(tmpa)Cr(0H)Cr(tmpa)Ll3+ complexes. Large singlet-triplet energy gaps determined from the temperature dependence of the magnetic susceptibilities of the oxo-bridged dimers with L = NCS-(509 cm-I) and CN-(580 cm-I) support the formulation of diamagnetic ground states with the R electronic configuration (e,b)4(e,)4(b2,)2, where cub, e,, and bzs represent the *-bonding level, a nonbonding Cr-centered R MO and a weakly &bonding orbital in the approximate D, , , symmetry of the NSCrOCrNS coordination sphere, respectively. The assignment of two strong [L(tmpa)CrOCr(tmpa)LI2+ dimer absorptions near 28 500 and 24000 cm-I to e,%,* and b2,-eu* transitions (eu* designates the r-antibonding level) is supported in part by a study of the e,-e,* band energy dependence on solvent dielectric constant for L = SCN-. Consistent with expectations from the R molecular orbital model, pK, values of the hydroxo-bridged complexes fall off sharply with increasing lODq while EIl2 for the [L(tmpa)CrOCr(tmpa)LI2+/'+ couple follows an increasing trend as the L-substituent moves from weaker to stronger fields within the spectrochemical series.

Magnetic Susceptibility Trends in Oxo-Bridged, Dinuclear Chromium(III) Complexes. Crystal Structure of [(tmpa)Cr(μ-O)(μ-CO3)Cr(tmpa)](ClO4)2·2H2O

“…Dinuclear products with both O- and N-donor bridging ligands could not be prepared through reactions of the diol precursor with amido or imido anions; NaNH 2 and potassium phthalimide afforded dimer cleavage products and [(tmpa)Cr(O)(OH)Cr(tmpa)](ClO 4 ) 3 ·H 2 O, respectively. In the the case of N(CN) 2 - , a pseudohalide ligand, the product was [Cr(tmpa)(N(CN) 2 )] 2 O(ClO 4 ) 2 ·3H 2 O, an unsupported CrOCr complex whose spectroscopic, electrochemical, and acid−base properties strongly resemble those of related species. , Thus, [Cr(tmpa)(N(CN) 2 )] 2 O 2+ exhibits an asymmetric CrOCr stretching mode at 858 cm -1 , intense e g → e u * (28 500 cm -1 ) and b 2g → e u * (24 200 cm -1 ) electronic transitions, and E 1/2 (Cr(III,IV/III,III) and p K a (Cr(OH)Cr) values similar to those of [Cr(tmpa)(NCS)] 2 O 2+ ( E 1/2 = 1.17 V; p K a = 2.05) . We note, however, that the dicyanamide complex exhibits splittings of pyridyl o - and m -H NMR resonances that were not observed for any of the other pseudohalide complexes, although ratios of integrated peak intensities are still consistent with a geometry in which both apical tmpa N atoms are trans to N(CN) 2 - ligands.…”

“…UV−vis and IR (KBr pellet) spectra were acquired on Shimadzu UV-260 and Perkin-Elmer Model 1600 instruments, respectively. Spectrophotometric titrations, cyclic voltammetric scans, and magnetic susceptibility measurements (4−300 K) were carried out and quantitatively interpreted as previously described. ,, 1 H NMR spectra were recorded on a Bruker 200-MHz spectrometer; chemical shifts are reported in parts per million (δ) downfield from TMS. Microanalyses were performed by Desert Analytics; chromium was assayed by the basic peroxide method …”

“…Oxo-bridged transition metal complexes with complementary bridging ligands are of interest from both bioinorganic and theoretical − perspectives. While most recent work in this area deals with the bioinorganic chemistry of Fe(III) dinuclear complexes, we have concentrated on Cr(III) dimers, for which relationships among electronic spectra, oxidation potentials and oxo-bridge basicities are particularly informative with regard to electronic structure. − In this concluding paper of our synthetic series, we describe [(tmpa)Cr(μ-O)(X)Cr(tmpa)] n + dinuclear compounds (tmpa = tris(2-pyridylmethyl)amine) with X = CO 3 2- , PhPO 4 2- , and HS - . Structures of hydroxo-bridged , and unsupported μ-CO 3 2- Cr(III) dinuclear complexes are known, but the instability of Cr(μ-O)(μ-CO 3 )Cr species with aliphatic amine chelating ligands prevents their characterization in the solid state.…”

Synthesis and Structure of [Fe(TPA)Cl2](ClO4) and [{Fe(TPA)Cl}2O](ClO4)2 Where TPA = Tris-(2-pyridylmethyl)amine

“…It can simultaneously provide four nitrogen donors (three pyridyl nitrogen atoms and a tertiary amine nitrogen) to a single metal center, thus leaving one or two available coordination sites. TPA complexes with most of the 3d metals are known including V(III), V(IV), V(V) [2], Cr(II) [3], Cr(III) [3][4][5], Mn(II) [6], Mn(III) [7], Mn(IV) [7], Fe(II) [8][9][10], Fe(III) [11][12][13][14][15][16], Co(III) [17], Ni(II) [18,19], Cu(I) [20], Cu(II) [20][21][22][23], and Zn(II) [24,25].…”

Synthesis, structure and magnetic property of a dihydroxo-bridged dichromium(III) complex with N-(2-pyridylmethyl)iminodiacetate

“…The major structural factors affecting exchange coupling in the complex are the angles at the bridging oxygen atom and the chromium(III)-oxygen bridge bond distances [8,10,11]. As expected, in the title complex, the magnetic interaction between the two Cr(III) ions through the hydroxo bridges is antiferromagnetic (Cr-O-Cr angles, 98.91(12)°; Cr-O bond lengths, 1.958(3) and 1.964(3) Å ).…”