Rosemary C. Hynes

Electron paramagnetic resonance study of isolated free radical pairs in M+ 18-crown-6 TCNQ–. (TCNQ = 7,7′,8,8′-tetracyano-p-quinodimethane; M = K, Rb)

Intense EPR spectra observed in M+ 18-crown-6 TCNQ–(M = K, Rb) at ambient temperature and above, in both powder and single-crystal samples, are typical of electronically excited triplet species (S= 1). From measurements of the fine-structure splitting in single-crystal specimens as a function of angle in three orthogonal crystal planes it has been established that the g-tensor is nearly isotropic, with gk= 2.0031 and gRb= 2.0023, while the spin–spin interaction tensors are rhombic, with Dk=(∓305.5, ±221.5, ±88.7 MHz) and DRb=(∓295.1, ±214.9, ±84.7 MHz). The zero-field splitting parameters, in MHz, are thus |Dk|= 458.3, |Ek|= 66.4, and |DRb|= 442.7, |ERb|= 65.1. Comparison of the D eigenvectors with directions determined crystallographically within the crystals shows unequivocally that the triplet spectra are due to the intrinsic localized pairs of TCNQ– radical anions. The principal values and directions of D in these radical pairs are described very well in terms of classical pairwise dipolar interactions based on literature values of the unpaired spin distribution in TCNQ–.

Phenoxo-bridged dinickel(II) complexes of a macrocyclic ligand: synthesis, stereochemical equilibria and structure

Dinuclear nickel(II) complexes of a tetraaminediphenol macrocyclic ligand (H2L) of composition [Ni2L(B)2][ClO4]2(B = NH3, imidazole, pyridine, or pyrazine) have been synthesised and characterized. With pyrazole (Hpz)[Ni2L(pz)(H2O)2][ClO4] and [Ni2L(pz)(Hpz)(H2O)][ClO4] complexes have been obtained in which pz– appears to act as a bridging ligand. The equilibrium constants for the reaction [Ni2L(MeOH)4]2++ 2B ⇌[Ni2L(B)2]2++ 4MeOH [B = pyridine, 2-methylpyridine (2Me-py), 3Me-py, 4Me-py, imidazole, or pyrazine] in methanol solutions have been determined spectrophotometrically at room temperature. The X-ray structure of [Ni2L(py)2][ClO4]2 has been determined: monoclinic, space group P21/n, with a= 9.251(1), b= 11.457(2), c= 18.555(2)A, β= 100.21(1)°, and Z= 2. The structure was determined by direct methods and refined to R= 0.053 and R′= 0.039. In the complex cation the two nickel(II) centres are in square-pyramidal configuration with N2O2 in-plane donor sets, involving two phenoxide bridges and two trans-axially disposed pyridines. The distance between the two nickel centres is 3.206(5)A.

Electron paramagnetic resonance studies of radical pairs [M(CO)–5]2(M = Cr, Mo, W) trapped in single crystals of PPh+4HM(CO)–5

γ-Irradiated single crystals of PPh+4HM(CO)–5(M = Cr, Mo, W) exhibit powerful EPR spectra due to two distinct interacting pairs of M(CO)–5 radicals. g- and fine-structure (D) matrices were determined from measurements at 77 K of EPR spectra obtained as a function of angle in the ac and aa′ planes of crystallographically aligned single crystals of the Mo and W compounds. The data are compared with those obtained previously for the Cr compound. In all three cases, g and one of the D-tensors are aligned with the tetragonal c-axis of the crystal, and the principal g-values are appropriate for d7 square-pyramidal M(CO)–5 radicals. Classical dipolar calculations show that the two radical pairs correspond to the closest and next-closest pair of undamaged anions in the crystals, the former lying along c and the latter along a skew direction. The D-values for the Mo and W closest pairs indicate a substantial contribution from anisotropic exchange to the coupling of the two unpaired electrons.

Electron paramagnetic resonance study of the [W(CO)4{P(OMe)3}]− radical anion trapped in a single crystal of [N(PPh3)2][W(CO)4H{P(OMe)3}]

Gamma-irradiated single crystals of [N(PPh3)2][W(CO)4H{P(OMe)3}] contain an anisotropic e.p.r. signal which exhibits hyperfine coupling to a single 31P nucleus. X-Ray crystallography showed that the crystals belonged to the orthorhombic system (Pbca), with a= 15.562 6(21), b= 19.980(3), c= 27.161(4)A, and Z= 8; the undamaged anion had an approximately octahedral geometry in which the H atom was cis to the phosphite ligand. Crystallographically aligned specimens were used to determine both the g and the 31P hyperfine interaction tensors [g= 2.0708, 2.0740, 1.9914; a(31P)= 75.1, 68.4, 45.5 MHz] and their direction cosines in the crystal-axis system. The form of the g tensor and the alignment of gmin. with the H–W–Cax. direction of the undamaged anion point to considerable unpaired spin density in the W 5dz2 orbital. Alignment of the 31P tensor along gmin. and the W–P bond is attributed to direct overlap of the P 3pz orbital with the metal atomic orbital. Comparison with other 31P hyperfine measurements suggests that e.p.r. provides a good indication of the efficiency of back-bonding in such organometallic radicals.