Molybdenum(VI) Coordination
Chemistry of the N,N-Disubstituted
Bis(hydroxylamido)-1,3,5-triazine Ligand, H<sub>2</sub>bihyat. Water-Assisted
Activation of the Mo<sup>VI</sup>O Bond and Reversible Dimerization
of <i>cis</i>-[Mo<sup>VI</sup>O<sub>2</sub>(bihyat)] to
[Mo<sup>VI</sup><sub>2</sub>O<sub>4</sub>(bihyat)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>]
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Abstract
Reaction of the N,N-disubstituted bis(hydroxylamino)
ligand 2,6-bis[hydroxy(methyl)amino]-4-morpholino-1,3,5-triazine
(H<sub>2</sub>bihyat) with <i>cis</i>-[Mo<sup>VI</sup>O<sub>2</sub>(acac)<sub>2</sub>] in tetrahydrofuran resulted in isolation
of the mononuclear compound <i>cis</i>-[Mo<sup>VI</sup>O<sub>2</sub>(bihyat)] (<b>1</b>). The treatment of Na<sub>2</sub>Mo<sup>VI</sup>O<sub>4</sub>·2H<sub>2</sub>O with the ligand
H<sub>2</sub>bihyat in aqueous solution gave the dinuclear compounds <i>cis</i>-[Mo<sup>VI</sup><sub>2</sub>O<sub>4</sub>(bihyat)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>] (<b>2</b>) and <i>trans</i>-[Mo<sup>VI</sup><sub>2</sub>O<sub>4</sub>(bihyat)<sub>2</sub>(H<sub>2</sub>O)<sub>2</sub>] (<b>3</b>) at pH values
of 3.5 and 5.5, respectively. The structures for the three molybdenum(VI)
compounds were determined by X-ray crystallography. Compound <b>1</b> has a square-pyramidal arrangement around molybdenum, while
in the two dinuclear compounds, each molybdenum atom is in a distorted
pentagonal-bipyramidal environment of two bridging and one terminal
oxido groups, a tridentate (O,N,O) bihyat<sup>2–</sup> ligand
that forms two five-membered chelate rings, and a water molecule trans
to the terminal oxido group. The dinuclear compounds constitute rare
examples containing the {Mo<sub>2</sub><sup>VI</sup>O<sub>2</sub>(μ<sub>2</sub>-O<sub>2</sub>)}<sup>4+</sup> moiety. The potentiometry revealed
that the Mo<sup>VI</sup>bihyat<sup>2–</sup> species exhibit
high hydrolytic stability in aqueous solution at a narrow range of
pH values, 3–5. A subtle change in the coordination environment
of the five-coordinate compound <b>1</b> with ligation of a
weakly bound water molecule trans to the oxido ligand (<b>1w</b>) renders the equatorial oxido group in <b>1w</b> more nucleophilic
than that in <b>1</b>, and this oxido group attacks a molybdenum
atom and thus the dinuclear compounds <b>2</b> and <b>3</b> are formed. This process might be considered as the first step of
the oxido group nucleophilic attack on organic substrates, resulting
in oxidation of the substrate, in the active site of molybdenum enzymes
such as xanthine oxidase. Theoretical calculations in the gas phase
were performed to examine the influence of water on the dimerization
process (<b>1</b> → <b>2</b>/<b>3</b>). In
addition, the molecular structures, cis/trans geometrical isomerism
for the dinuclear molybdenum(VI) species, vibrational spectra, and
energetics of the metal–ligand interaction for the three molybdenum(VI)
compounds <b>1</b>–<b>3</b> have been studied by
means of density functional theory calculations
