Electronic structure of bispidine Iron(IV) oxo complexes

The electronic structure, based on DFT calculations, of a range of FeIV=O complexes with two tetra- (L1 and L2) and two isomeric pentadentate bispidine ligands (L3 and L4) is discussed with special emphasis on the relative stability of the two possible spin states (S = 1, triplet, intermediate-spin, and S = 2, quintet, high-spin; bispidines are very rigid diazaadamantane-derived 3,7-diazabicyclo[3.3.1]nonane ligands with two tertiary amine and two or three pyridine donors, leading to cis-octahedral [(X)(L)FeIV=O]2+ complexes, where X = NCCH3, OH2, OH-, and pyridine, and where X = pyridine is tethered to the bispidine backbone in L3, L4). The two main structural effects are a strong trans influence, exerted by the oxo group in both the triplet and the quintet spin states, and a Jahn-Teller-type distortion in the plane perpendicular to the oxo group in the quintet state. Due to the ligand architecture the two sites for substrate coordination in complexes with the tetradentate ligands L1 and L2 are electronically very different, and with the pentadentate ligands L3 and L4, a single isomer is enforced in each case. Because of the rigidity of the bispidine ligands and the orientation of the "Jahn-Teller axis", which is controlled by the sixth donor X, the Jahn-Teller-type distortion in the high-spin state of the two isomers is quite different. It is shown how this can be used as a design principle to tune the relative stability of the two spin states

Zinc(II) complexes of putative obligate tetrahedrally co-ordinating pro-ligands

An attempt to impose tetrahedral geometry on zinc() through use of bis(salicylideneamino)biphenyl pro-ligands has produced hydrolytically sensitive complexes. The single-crystal structures of ZnL(EtOH)HL = 6{,}6prime or minute]-dimethyl-2,2prime or minute-bis(salicylideneamino)biphenylmonoclinic{,} space group 2/{,} = 11.654(2){,} = 11.753(2){,} = 18.878(3)A{,} small beta= 98.97(2)degree{,} = 4 and its pro-ligand HL monoclinic{,} space group 2/{,} = 13.911(2){,} = 13.759(2){,} = 12.015(2)A{,} small beta= 94.836(10)degree{,} = 4 have been determined. The co-ordination geometry at zinc is slightly distorted trigonal bipyramidal

Coordination chemistry of a new rigid, hexadentate bispidine-based bis(amine)tetrakis(pyridine) ligand

The hexadentate bispidine-based ligand 2,4-bis(2-pyridyl)-3,7-bis(2-methylenepyridine)-3,7-diazabicyclo[3.3.1]n onane-9-on-1,5-bis(carbonic acid methyl ester), L-6m, with four pyridine and two tertiary amine donors, based on a very rigid diazaadamantane-derived backbone, is coordinated to a range of metal ions. On the basis of experimental and computed structural data, the ligand is predicted to form very stable complexes. Force field calculations indicate that short metal-donor distances lead to a buildup of strain in the ligand; that is, the coordination of large metal ions is preferred. This is confirmed by experimentally determined stability constants, which indicate that, in general, stabilities comparable to those with macrocyclic ligands are obtained with the relative order Cu2+ > Zn2+ >> Ni2+ < Co2+, which is not the typical Irving-Williams behavior. The preference for large M-N distances also emerges from relatively high redox potentials (the higher oxidation states, that is, the smaller metal ions, are destabilized) and from relatively weak ligand fields (dd-transition, high-spin electronic ground states). The potentiometric titrations confirm the efficient encapsulation of the metal ions since only 1:1 complexes are observed, and, over a large pH range, ML is generally the only species present in solution