Molybdenum(VI) Coordination Chemistry of the N,N-Disubstituted Bis(hydroxylamido)-1,3,5-triazine Ligand, H2bihyat. Water-Assisted Activation of the MoVIO Bond and Reversible Dimerization of cis-[MoVIO2(bihyat)] to [MoVI2O4(bihyat)2(H2O)2]

Reaction of the N,N-disubstituted bis­(hydroxylamino) ligand 2,6-bis­[hydroxy­(methyl)­amino]-4-morpholino-1,3,5-triazine (H2bihyat) with cis-[MoVIO2(acac)2] in tetrahydrofuran resulted in isolation of the mononuclear compound cis-[MoVIO2(bihyat)] (1). The treatment of Na2MoVIO4·2H2O with the ligand H2bihyat in aqueous solution gave the dinuclear compounds cis-[MoVI2O4(bihyat)2(H2O)2] (2) and trans-[MoVI2O4(bihyat)2(H2O)2] (3) 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 1 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) bihyat2– 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 {Mo2VIO2(μ2-O2)}4+ moiety. The potentiometry revealed that the MoVIbihyat2– 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 1 with ligation of a weakly bound water molecule trans to the oxido ligand (1w) renders the equatorial oxido group in 1w more nucleophilic than that in 1, and this oxido group attacks a molybdenum atom and thus the dinuclear compounds 2 and 3 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 (1 → 2/3). 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 1–3 have been studied by means of density functional theory calculations.

将N,N-二取代双(羟氨基)配体2,6-双[羟基(甲基)氨基]-4-吗啉基-1,3,5-三嗪（H2bihyat）与顺式-[MoVIO2(acac)2]在四氢呋喃中反应，成功分离得到单核化合物顺式-[MoVIO2(bihyat)]（1）。将Na2MoVIO4·2H2O与配体H2bihyat在水溶液中反应，分别在pH为3.5和5.5的条件下得到双核化合物顺式-[MoVI2O4(bihyat)2(H2O)2]（2）与反式-[MoVI2O4(bihyat)2(H2O)2]（3）。通过X射线晶体学测定了这三种钼(VI)化合物的晶体结构。化合物1的钼中心呈现四方锥配位构型；而在两种双核化合物中，每个钼原子均处于扭曲的五角双锥配位环境，该环境包含两个桥联氧原子、一个端基氧原子、一个三齿(O,N,O)型的bihyat2–配体（该配体形成两个五元螯合环），以及一个与端基氧反式配位的水分子。这类双核化合物是含有{Mo2VIO2(μ2-O2)}4+结构单元的罕见案例。电位滴定结果显示，MoVIbihyat2–物种在pH 3~5的窄范围内于水溶液中具有较高的水解稳定性。五配位化合物1与一个弱结合的水分子在端基氧的反式位点配位（生成物种1w）后，1w中赤道面的氧原子相较于1中的对应氧原子亲核性更强，该氧原子可进攻另一钼原子，进而形成双核化合物2和3。该过程可被视作黄嘌呤氧化酶等钼酶活性中心中，氧原子亲核进攻有机底物以实现底物氧化的第一步。在气相条件下开展了理论计算，以考察水对二聚过程（1→2/3）的影响。此外，通过密度泛函理论计算，研究了这三种钼(VI)化合物1~3的分子结构、双核钼(VI)物种的顺/反几何异构现象、振动光谱以及金属-配体相互作用的能量学。