QUANTITATIVE-DETERMINATION OF THE SINGLET-TRIPLET ENERGY-GAP IN PSEUDO-OCTAHEDRAL COMPLEXES OF W(IV) - MEASUREMENT OF THE RELATIVE PI-ACIDITIES OF N-HETEROCYCLIC LIGANDS

The diamagnetic bis-phosphine complex [W(OC(6)H(3)Ph-C6H4)(2)(PMePh(2))(2)] (1) (OC(6)H(3)Ph.C6H4 = cyclo-metallated 2,6-diphenylphenoxide) reacts rapidly in hydrocarbon solvent with nitrogen heterocyclic ligands to produce a new series of complexes [W(OC(6)H(3)Ph-C6H4)(2)(L)(2)] (2: L = substituted pyridine, 3: (L)(2) = substituted bipyridines; 4: (L)(2) = phenanthrolines). Complexes 2, 3, and 4 exhibit sharp, contact-shifted H-1 NMR spectra at 25 degrees C, the amount of contact shifting being strongly dependent on the nature of the nitrogen donor ligand. Assignment of the signals in the H-1 NMR spectra as well as their temperature dependence indicates that a thermal equilibrium exists between a singlet ground state for the d(2)-W(IV) metal center and a triplet excited state which undergoes through-bond coupling to the ligand protons. The temperature dependence of the chemical shifts of the four protons (H2, H3, H4, and H5) attached to the metalated arene rings has been used to calculate the hyperfine coupling constants and single-triplet energy gap (E) for the ligands (L) 4-tert-butylpyridine (E = 1138 cm(-1)), 4-pyrrolidinopyridine (714 cm(-1)), 4,4'-dimethyl-2,2'-bipyridine (528 cm(-1)), and 4,4'-diphenyl-2,2'-bipyridine (362 cm(-1)). The relative singlet-triplet energy gap for different ligands can also be estimated by measuring the amount of contact shifting of these aryl protons at 25 degrees C. It is found that substituents on pyridine that decrease the value of E in compounds 2 increase the value of E in complexes 3 and 4 when introduced onto the bipyridine or phenanthroline nucleus. It is argued that stronger pi-acid pyridine ligands stabilize the singlet ground state by interacting with the formally d(xy) (HOMO) orbital on the metal while stronger pi-acid bipyridine and phenanthroline ligands will stabilize the d(xz), d(yz) (LUMO) pair of orbitals. The fact that the cis-pyridine ligands in [W(OC(6)H(3)Ph-C6H3)(2)(py)(2)] (2a) lie along the O-W-O (Z) axis has been confirmed by a single-crystal X-ray diffraction analysis. Crystal data at 20 degrees C for WO2N2C46H34: a = 26.299(4) Angstrom,b = 14.802(3)Angstrom,c 10.1339(9) Angstrom,beta = 103.17(1)degrees, Z = 4 in space group C2,