p-Hydroquinones along with their oxidation products,p-semiquinones andp-quinones, comprise perhaps the quintessential organic electron and hydrogen transfer systems.1For example, electron transfer reactions between transition metal centers andp-quinone cofactors are vital for all life, occurring in key biological processes as diverse as the oxidative maintenance of biological amine levels,2,3tissue (collagen and elastin) formation,3,4photosynthesis5and aerobic (mitochondrial) respiration.6The interaction ofp-hydroquinones with vanadium, in high-oxidation states, presents additional interest due to the participation of vanadium in redox reactions in biological systems7such as the reduction of vanadium(v), present in sea water, to vanadium(iii) in the blood cells of tunicates.8,9In marked contrast to the extensive structural chemistry for chelate stabilizedo-(hydro/semi)quinone metal compounds,10examples of structurally characterized non-polymeric11σ-bondedp-hydroquinone–metal compounds are surprisingly rare12and there are no examples of σ-bondedp-semiquinone–metal complexes. A strategy to prepare such species is to synthesize substituted, in theo-position,p-hydroquinones with substituents containing one or more donor atoms, thus enabling the metal atom to form chelate rings. In this communication, we wish to report the synthesis of the potentially μ-bridging chelateo-disubstituted ligand 2,5-bis[N,N-bis(carboxymethyl)aminomethyl]hydroquinone (H6bicah) as well as its tetranuclear VIVO2+and dinuclear VVO2+compounds.The sodium salt of H6bicah, Na4H2bicah·4H2O, was synthesized by condensingp-hydroquinone with iminodiacetic acid in the presence of formaldehyde (Mannich reaction) in alkaline water/ethyl alcohol solution under argon.1·Na2SO4·20H2O: The deep blue crystals analysed satisfactorily (C,H,N,Na) as1(Na2SO4·20H2O. UV-Vis of1·Na2SO4·20H2O [λ(nm),ϵ(M−1cm−1)] at pH ∼ 4, 856 (8800), 642 (10 300), 195 (70 000) and 294(sh) (10 300). DC cyclic voltammetry, rotating disk voltammetry and coulometry of1·Na2SO4·20H2O in H2O at pH 4.19 gave a reversible and a quasi-reversible one-electron metal centered redox processes at 0.48 ([VIV3VV+ e−⇌ VIV4]) and 0.10 V ([VIV4+ e−⇌ VIV3VIII]) (vs. NHE, normal hydrogen electrode) respectively. Compound1does not possess observable NMR spectra (1H,13C,51V).2·11H2O: The brown red crystals analyzed satisfactorily (C,H,N,Na) as2·12H2O. UV-Vis of2·11H2O [λ(nm),ϵ(M−1cm−1)] at pH = 7.00, 396 (with a tail up to 500 nm) (4000), 304 (8000), 243(sh) (15 000).51V NMRδ(D2O, pH = 8.0) 492.1H NMRδ(D2O, 0 °C) 6.03 (2H, aromatic), 4.37(2H, broad, Ar-CH2-N), 3.68 (2H, broad, N-CH2-COO), 3.47 (2H, broad, Ar-CH2-N), 3.13 (4H, broad, N-CH2-COO), 2.75 (2H, broad, N-CH2-COO).13C{1H}δ(D2O, 0 °C) 60.63 (C2,C3), 64.65 (C5), 119.13 (C8), 123.78 (C6), 155.78 (C7), 178.34 (C4), 180.41 (C1) numbering is shown inFig. 1.Compound1·Na2SO4·20H2O was prepared by reacting VIVOSO4·3H2O (4 mmol) and NaVVO3(4 mmol) with Na4H2bicah·4H2O (4 mmol) in water (5 mL) under argon (pH ≈ 4), followed by vapor diffusion of methyl alcohol at 4 °C into this solution (yield 45%). Compound2·11H2O was prepared by reacting NaVO3(2 mmol) with Na4H2bicah·4H2O (1 mmol) in water (5 mL) (pH ≈ 8) under argon followed by vapor diffusion of ethyl alcohol into this solution (yield 60%).ORTEP drawing of the anions of (a)1and (b)2at 50% probability ellipsoids giving atomic numbering. For clarity, hydrogen atoms are omitted. Selected bond lengths (Å) for1·Na2SO4·20H2O and2·11H2O (in square brackets): V–O(1) 2.022(4) [2.019(3)], V–N(1) 2.289(4) [2.280(4)], V–O(3) 2.068(4) [2.209(3)], V–O(5) 1.887(3) [1.864(3)], V–O(6) 1.620(3) [1.652(3)], V–O(7) 1.807(1) [1.620(4)].The molecular structuresCrystal datafor C32H68N4Na8O50SV41·Na2SO4·20H2O:M= 1728.76, space groupP4/ncc, tetragonal,a= 23.307(9),b= 23.307(9),c= 12.233(5) Å,V= 6645(5) Å3,Z= 4,ρcalc=1.727 g cm−3,T= 298 K. 3748 reflections measured, 2924 unique (Rint= 0.0131),R1 andwR2 are 0.0605 and 0.1533 respectively, for 2193 reflections withI>2σ(I).Crystal datafor C16H36N2Na4O25V22·11H2O:M= 850.31, space groupP1&cmb.macr;, triclinic,a= 9.664(6),b= 12.009(4),c= 14.629(4) Å,α= 97.69(2),β= 101.01(4),γ= 99.78(4)°,V= 1617.9(12) Å3,Z= 2,ρcalc= 1.745 g cm−3,μ= 0.732 mm−1,T= 293 K. 3283 reflections measured, 3039 independent (Rint= 0.013),R1 = 0.0566,wR2 = 0.1724. The positions of the hydrogen atoms of1and2were calculated from stereochemical considerations and kept fixed isotropic during refinement; the hydrogen atoms for the water molecules were not located.CCDC reference numbers 136227 (1·Na2SO4·20H2O) and 190302 (2·11H2O). Seehttp://www.rsc.org/suppdata/cc/b2/b207330g/for crystallographic data in CIF or other electronic format.of the anions of1and2are shown inFig. 1. The anion of2(Fig. 1B) contains a bridging (μ-)N,O,O,Oligatedp-hydroquinonate(−6) ligand (Scheme 1,a), as is clearly borne out by the observation,13that (i) the C–C bonds of the six-membered ring are equidistant at 1.386 ± 0.004 Å and (ii) the C–O bond length at 1.354(6) Å is long and typical forp-hydroquinonates (Scheme 1,a). In contrast, the anion of1(Fig. 1A) contains two bridging (μ-)N,O,O,Ocoordinatedp-semiquinonate(−5) π radical ligands (Scheme 1,c);13the C–O bond length [1.322(5) Å] is shorter than in2and longer than the bond length expected for quinone [∼1.21 Å]12and this denotes a partial double bond character (Scheme 1,c). In addition, the C–C bonds of the six-membered ring, 1.399(7), 1.371(7) and 1.427(7) Å in a long-short-long pattern (Scheme 1,c), are expected forp-semiquinonates. Although the six-membered ring of the ligatedp-quinone ligands shows a similar pattern top-semiquinonates; different bond lengths are expected [∼1.46, ∼1.33 and ∼1.46 Å].12These data unambiguously define the oxidation levels of the organic ligands in1and2, which render the central metal ions as VIV(d1) and VV(d°) for1and2respectively. The structure of the anion of1contains two [V2IV(μ-bicas)] edges linked together with two μ-oxo-bridged ligands to get a twisted rectangular structure. The four vanadium atoms in the anion of1are indistinguishable and each of them resides in a distorted octahedral NO5coordination environment. The unit cell of2·11H2O contains two discrete molecules of2. Each of them has two indistinguishable six-coordinate vanadium atoms bridged with a μ-bicah−6ligand. The coordination environments of the vanadium atoms and the bond lengths in2are similar to those found in complex1, except that the two μ-O2−bridged atoms in1have been replaced by two terminal oxo groupsantito each other in2.A full geometry optimization of [V4O4(μ-O)2(μ-bicas)2]6−(1) and [(VVO2)2(μ-bicah)]4−(2) was carried at the UHF level and HF level under the symmetry constrains of andD2andCipoint group respectively, using the effective core potential approximation of Hay and Wadt with a valence double-ζ basis set for the V atom14and the STO-3G basis set for all other atoms.15There is an overall agreement between the calculated and experimental data in both cases. The optimized structures remain unchanged upon distortion and reoptimization with no symmetry constraints. Bond distances agree within 0.06 Å, while the largest deviation of bond angles appears to be about 7°. The calculated Mulliken and Lowdin atomic spin densities for the four vanadium atoms in1found equal to 0.8458 and 0.8345 respectively, confirm further the characterization of all vanadium atoms as VIV. The bond distances within the aromatic rings of1were 1.450, 1.369 and 1.454 Å revealing that the organic ligand has a semiquinonate structure (bicas5−), whereas those in2were almost equal (1.405, 1.412 and 1.408 Å) as expected for ap-hydroquinonate. Each of the four highest single occupied molecular orbitals (SOMOs), shown in Fig. S1 (ESI), is localized on the d orbitals of the four VIVatoms with zero participations from orbitals of the intervening π-system of the bicas5−·ligand and a significant participation from a p-orbital of the two oxygen atoms bridging the two parallel slices of the complex. Thus, whereas a magnetic coupling of the two VIVcenters is expected due to both the spatial proximity and the high overlap between the orbitals within the oxygen bridge, no exchange interaction is expected through the bridged bicas5−&z.rad; ligand. This is quite in line with the experimentally observed value of theμeffbeing 1.9μBat room temperature, indicating that the vanadium(iv) (S= 1/2) centers are strongly coupled.In summary, the dinuclear VV–p-hydroquinonate and the tetranuclear VIV–p-semiquinonate compounds were synthesized by taking advantage of the twoorthosubstituents present in thep-hydroquinone moiety and structurally characterized.Ab initiotheoretical calculations for1·Na2SO4·20H2O and2·11H2O are nicely corroborated with the crystallographic data and confirm the semiquinonate and hydroquinonate redox levels of the organic ligand respectively. The isolation of a stable metal-p-semiquinonate species is the first one to be reported. In order to understand the reasons for the stability of such an actually unstable species, variable-temperature magnetic measurements and EPR studies are under way.