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

Abstract

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.

Effective Hamiltonian Crystal Field As Applied to Magnetic Exchange Parameters in μ-Oxo-Bridged Cr(III) Dimers

“…f VIDTIL = [{(tmpa)Cr} 2 (O)(CH 3 COO)] 3+ ; for geometry see ref . g ZUVMIM = [{(tmpa)Cr} 2 (O)(CO 3 )] 2+ ; for geometry see ref . h SIMKAA = [{(Me 3 tacn)Cr} 2 (O)(CH 3 COO) 2 ] 2+ , tacn stands for 1,4,7-trimethyl-1,4,7-triazacyclononane- N , N ′, N ″; for geometry see ref .…”

Acidities of Platinum(II) μ-Hydroxo Complexes Bearing Diphosphine Ligands

“…pK a values for several other μ-hydroxo complexes have been measured in water. ,− Unfortunately, comparison of these pK a values with DMSO results are problematic since variations in hydrogen bonding can dramatically shift relative values . For example, pK a for PhNH 3 + in water and DMSO differ by only one unit whereas acetic acid is a markedly weaker acid in DMSO with a pK a of 12.3 versus 4.75 in water …”

Average One-Center Two-Electron Exchange Integrals and Exchange Interactions

“…The metal-oxo distance -see Tables 1 and 2 -is significantly shorter than the other metal-ligand distances which are found in the range 2.03-2.21 Å [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”