“…Electronic spectra, oxo-bridge basicities, antiferromagnetic coupling constants and half-wave, one-electron oxidation potentials are presented in Table . Nearultraviolet spectra resemble those of [Cr 2 (tmpa) 2 (μ-RCO 2 )(μ-O)] 3+ complexes, , for which the strong, e g → e u * charge-transfer band near 28 600 cm -1 in linear CrOCr species splits into two components of lesser intensity, at lower (b 2 → a 1 ; ca. 27 000 cm -1 ) and higher (a 2 → b 1 ; ca.…”
Section: Resultsmentioning
confidence: 86%
“… 2 Correlation between antiferromagnetic coupling constants and oxo-bridge basicity for [(tmpa)Cr(μ-O)(μ-X)Cr(tmpa)] n + dimers, including X = HS - , CO 3 2- , PhPO 4 2- , SO 4 2- , and RCO 2 - (R = H, CH 3 , CH 2 Cl, CHCl 2 , Ph, Ph-4-CH 3 , Ph-4-CF 3 , Ph-4-OCH 3 , Ph-4-F, Ph-4-Cl). Data for RCO 2 - complexes was taken from refs and .…”
Section: Resultsmentioning
confidence: 99%
“…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.…”
Section: Introductionmentioning
confidence: 99%
“…A correlation between HOMO energy and π-donating ability of bridging and non-bridging ligands has been demonstrated for both singly- and doubly-bridged CrOCr dimers through a linear free energy relationship between E 1/2 (Cr(III,IV/III,III) and p K a (Cr(μ-OH)Cr). , Our objective in this paper is to explore the factors underlying variations in the antiferromagnetic coupling constant J as a function of the X group in [(tmpa)Cr(μ-O)(X)Cr(tmpa)] n + complexes. The Glerup-Hodgson-Pedersen model successfully expresses the singlet-triplet gap (−2 J ) as a function of Cr−O−Cr and dihedral angles in hydroxo- bridged complexes …”
The synthesis and physical characterization of oxo-bridged [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) complexes (tmpa = tris(2-pyridylmethyl)amine) containing a variety of complementary ligands (X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-)) are described, with the objective of understanding factors underlying variations in the antiferromagnetic coupling constant J. We also present the crystal structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O, for comparison with previous findings on [(tmpa)Cr(&mgr;-O)(&mgr;-CH(3)CO(2))Cr(tmpa)](ClO(4))(3). The carbonate-bridged complex crystallizes in the monoclinic space group P2(1)/c with a = 11.286(10) Å, b = 18.12(2) Å, c = 20.592(12) Å, beta = 95.99(5) degrees, and V = 4190 Å(3) and Z = 4. Asymmetric tmpa ligation pertains, with apical N atoms situated trans to bridging oxo and acido O atoms. Key structural parameters include Cr-O(b) bond lengths of 1.818(6) and 1.838(6) Å, Cr-OCO(2) distances of 1.924(7) and 1.934(7) Å, and a bridging bond angle of 128.3(3) degrees. Several attempts to prepare oxo, amido-bridged dimers were unsuccessful, but the nearlinear [Cr(tmpa)(N(CN)(2))](2)O(ClO(4))(2).3H(2)O complex was isolated from the reaction of dicyanamide ion with [Cr(tmpa)(OH)](2)(4+). In contrast to the behavior of analogous diiron(III) complexes, antiferromagnetic coupling constants of [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) dinuclear species are highly responsive to the X group. Considering the complexes with X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-), SO(4)(2)(-), and RCO(2)(-) (10 R substituents), we find a reasonably linear, empirical relationship between J and oxo bridge basicity, as measured by pK(a) (Cr(OH)Cr) values in aqueous solution. While there is no theoretical basis for such a correlation between solid-state and solution-phase properties, this relationship demonstrates that CrOCr pi-bonding contributes significantly to antiferromagnetic exchange. Thus, J tends to become less negative with increasing &mgr;-O(2)(-) basicity, showing that greater availability of a bridging oxo group lone pair toward the proton, with decreasing CrOCr pi-interaction, reduces the singlet-triplet gap.
“…Electronic spectra, oxo-bridge basicities, antiferromagnetic coupling constants and half-wave, one-electron oxidation potentials are presented in Table . Nearultraviolet spectra resemble those of [Cr 2 (tmpa) 2 (μ-RCO 2 )(μ-O)] 3+ complexes, , for which the strong, e g → e u * charge-transfer band near 28 600 cm -1 in linear CrOCr species splits into two components of lesser intensity, at lower (b 2 → a 1 ; ca. 27 000 cm -1 ) and higher (a 2 → b 1 ; ca.…”
Section: Resultsmentioning
confidence: 86%
“… 2 Correlation between antiferromagnetic coupling constants and oxo-bridge basicity for [(tmpa)Cr(μ-O)(μ-X)Cr(tmpa)] n + dimers, including X = HS - , CO 3 2- , PhPO 4 2- , SO 4 2- , and RCO 2 - (R = H, CH 3 , CH 2 Cl, CHCl 2 , Ph, Ph-4-CH 3 , Ph-4-CF 3 , Ph-4-OCH 3 , Ph-4-F, Ph-4-Cl). Data for RCO 2 - complexes was taken from refs and .…”
Section: Resultsmentioning
confidence: 99%
“…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.…”
Section: Introductionmentioning
confidence: 99%
“…A correlation between HOMO energy and π-donating ability of bridging and non-bridging ligands has been demonstrated for both singly- and doubly-bridged CrOCr dimers through a linear free energy relationship between E 1/2 (Cr(III,IV/III,III) and p K a (Cr(μ-OH)Cr). , Our objective in this paper is to explore the factors underlying variations in the antiferromagnetic coupling constant J as a function of the X group in [(tmpa)Cr(μ-O)(X)Cr(tmpa)] n + complexes. The Glerup-Hodgson-Pedersen model successfully expresses the singlet-triplet gap (−2 J ) as a function of Cr−O−Cr and dihedral angles in hydroxo- bridged complexes …”
The synthesis and physical characterization of oxo-bridged [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) complexes (tmpa = tris(2-pyridylmethyl)amine) containing a variety of complementary ligands (X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-)) are described, with the objective of understanding factors underlying variations in the antiferromagnetic coupling constant J. We also present the crystal structure of [(tmpa)Cr(&mgr;-O)(&mgr;-CO(3))Cr(tmpa)](ClO(4))(2).2H(2)O, for comparison with previous findings on [(tmpa)Cr(&mgr;-O)(&mgr;-CH(3)CO(2))Cr(tmpa)](ClO(4))(3). The carbonate-bridged complex crystallizes in the monoclinic space group P2(1)/c with a = 11.286(10) Å, b = 18.12(2) Å, c = 20.592(12) Å, beta = 95.99(5) degrees, and V = 4190 Å(3) and Z = 4. Asymmetric tmpa ligation pertains, with apical N atoms situated trans to bridging oxo and acido O atoms. Key structural parameters include Cr-O(b) bond lengths of 1.818(6) and 1.838(6) Å, Cr-OCO(2) distances of 1.924(7) and 1.934(7) Å, and a bridging bond angle of 128.3(3) degrees. Several attempts to prepare oxo, amido-bridged dimers were unsuccessful, but the nearlinear [Cr(tmpa)(N(CN)(2))](2)O(ClO(4))(2).3H(2)O complex was isolated from the reaction of dicyanamide ion with [Cr(tmpa)(OH)](2)(4+). In contrast to the behavior of analogous diiron(III) complexes, antiferromagnetic coupling constants of [Cr(2)(tmpa)(2)(&mgr;-O)(X)](n)()(+) dinuclear species are highly responsive to the X group. Considering the complexes with X = CO(3)(2)(-), PhPO(4)(2)(-), HS(-), SO(4)(2)(-), and RCO(2)(-) (10 R substituents), we find a reasonably linear, empirical relationship between J and oxo bridge basicity, as measured by pK(a) (Cr(OH)Cr) values in aqueous solution. While there is no theoretical basis for such a correlation between solid-state and solution-phase properties, this relationship demonstrates that CrOCr pi-bonding contributes significantly to antiferromagnetic exchange. Thus, J tends to become less negative with increasing &mgr;-O(2)(-) basicity, showing that greater availability of a bridging oxo group lone pair toward the proton, with decreasing CrOCr pi-interaction, reduces the singlet-triplet gap.
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