Synthesis, Structural Elucidation, and Application of a Pyrazolylpyridine–Molybdenum Oxide Composite as a Heterogeneous Catalyst for Olefin Epoxidation

The reaction of [MoO2Cl2(pypzEA)]
(1) (pypzEA = ethyl­[3-(pyridin-2-yl)-1H-pyrazol-1-yl]­acetate)
with water in a Teflon-lined stainless steel autoclave (100 °C)
or in an open reflux system leads to the isolation of the molybdenum
oxide/pyrazolylpyridine composite material [Mo2O6(HpypzA)] (2; HpypzA = [3-(pyridinium-2-yl)-1H-pyrazol-1-yl]­acetate). The solid state structure of 2 was solved through single crystal and powder X-ray diffraction
analyses in conjunction with information derived from FT-IR and 13C CP MAS NMR spectroscopies and elemental analyses. In the
asymmetric unit of 2, two crystallographically distinct
Mo6+ centers are bridged by a syn,syn-carboxylate group of HpypzA. The periodic repetition
of these units along the a axis of the unit cell
leads to the formation of a one-dimensional composite polymer, ∞1[Mo2O6(HpypzA)].
The outstretched pyrazolylpyridine groups of adjacent polymers interdigitate
to form a zipper-like motif, generating strong onset π–π
contacts between adjacent rings of coordinated HpypzA molecules. The
composite oxide 2 is a stable heterogeneous catalyst
for liquid-phase olefin epoxidation.

将配合物[MoO₂Cl₂(pypzEA)]（1，其中pypzEA = 乙基[3-(吡啶-2-基)-1H-吡唑-1-基]乙酸酯）在聚四氟乙烯内衬不锈钢高压釜（100 ℃）中或开放回流体系内与水反应，可分离得到钼氧化物/吡唑基吡啶复合材料[Mo₂O₆(HpypzA)]（2；HpypzA = [3-(吡啶鎓-2-基)-1H-吡唑-1-基]乙酸酯）。配合物2的固态结构通过单晶X射线衍射（single crystal X-ray diffraction）与粉末X射线衍射（powder X-ray diffraction）分析，并结合傅里叶变换红外光谱（FT-IR）、碳13交叉极化魔角旋转核磁共振波谱（13C CP MAS NMR）以及元素分析所获信息得以解析确定。在配合物2的不对称单元中，两个晶体学独立的六价钼（Mo⁶⁺）中心被HpypzA的顺式-顺式羧酸根基桥联。这些结构单元沿晶胞a轴周期性重复，形成一维复合聚合物∞¹[Mo₂O₆(HpypzA)]。相邻聚合物链上伸展的吡唑基吡啶基团相互穿插，形成类拉链结构，使配位的HpypzA分子的相邻环之间产生强π-π堆积相互作用。该复合氧化物2是一种性能稳定的液相烯烃环氧化多相催化剂。