Steric and Electronic Effects in the First Homoleptic Imino Ether Complex: Synthesis and X-ray Crystallographic Determination of [Pt(NHC(OEt)Et)₄](CF₃SO₃)₂

Imino ethers, NHC(OR‘)R, represent an interesting alkoxy-substituted class of otherwise elusive monodentate imine ligands. We have discovered that 4-fold addition of EtOH to [Pt(N⋮CEt)4]2+ proceeds smoothly under basic conditions to yield stable, colorless [Pt(NHC(OEt)Et)4]2+(CF3SO3-)2 (1), which is the first homoleptic imino ether complex reported for any metal ion. The monodentate imino ether ligands are all equivalent in solution, according to 1H NMR, and prove to have the E isomeric form with cis NH/COEt groups. The IR-active C−N stretching vibration moves from 2330 cm-1 in the nitrile complex to 1633 cm-1 in 1, and νNH emerges at 3250 cm-1. The 195Pt NMR shift for 1 is found to be −2450 ppm, lying between that for [Pt(NCEt)4]2+ at −2409 ppm and that for [Pt(NH3)4]2+ at −2580 ppm (all relative to [PtCl6]2-). Single-crystal X-ray diffraction establishes that 1 crystallizes in the monoclinic space group P21/c, with a = 9.026(2) Å, b = 14.838 (1) Å, c = 13.946 (1) Å, β = 106.328(2)°, and Z = 2. The metal ion lies at a center of inversion and displays almost ideal square-planar PtN4 coordination with mean Pt−N distances of 2.016(4) and N−Pt−N angles of 90 ± 0.6°. Within the two crystallographically independent NHC(OEt)Et ligands, the mean NC distance is 1.283 Å and there is structural evidence of partial π-delocalization of O lone pairs into the NCO moiety. Nonetheless, according to both structural and 195Pt NMR data, the imino ether ligand behaves as innocent Nσ donor toward Pt2+, with a trans influence similar to that of Cl-. Due to their extended, asymmetric planar nature, such E-NHC(OR‘)R ligands have exceptional steric requirements which control the coordination geometry and prevent unhindered rotation. These requirements are dominated by alkyl tail-to-tail (R/R) clashes that can be accommodated only by systematic twisting of the ligands about their respective Pt−N bonds. The present study reveals that 1 has the Pt(HHTT) C2h conformation, where H (head up) and T (tail up) represent mutual NH-up and NH-down orientations of the imino ether ligands, with trans-ligand pairs cooperatively rotated by about 20° with respect to the PtN4 plane. The alternative Pt(HTHT) conformer of 1, with counterrotated trans ligands, is predicted to exist independently, possibly in the chiral D2 form, in contrast to unfavored Pt(HHHH).亚胺醚（imino ethers，通式为NH=C(OR')R）是一类极具研究价值的烷氧基取代型单齿亚胺配体，此类配体通常难以分离获取。我们发现，在碱性条件下，将乙醇（EtOH）以四分子加成至四(丙腈)合铂(II)阳离子([Pt(N≡CEt)₄]²⁺)中，可顺利得到稳定无色的配合物1：[Pt(NH=C(OEt)Et)₄]²⁺(CF₃SO₃⁻)₂，这是首例报道的金属离子同配体亚胺醚配合物。 氢核磁共振谱（¹H NMR）表征显示，溶液中所有单齿亚胺醚配体均等价，且该配体以E型异构体形式存在，其中NH与COEt基团呈顺式排布。红外活性的C-N伸缩振动峰从初始腈基配合物中的2330 cm⁻¹位移至配合物1的1633 cm⁻¹，同时NH伸缩振动特征峰（νNH）出现在3250 cm⁻¹处。配合物1的铂-195核磁共振谱（¹⁹⁵Pt NMR）化学位移为-2450 ppm，介于四(丙腈)合铂(II)阳离子([Pt(NCEt)₄]²⁺，-2409 ppm)与四氨合铂(II)阳离子([Pt(NH₃)₄]²⁺，-2580 ppm)的化学位移之间，所有化学位移均以六氯合铂(IV)酸根([PtCl₆]²⁻)为参比标定。 单晶X射线衍射（single-crystal X-ray diffraction）分析表明，配合物1结晶于单斜晶系空间群P2₁/c，晶胞参数为：a=9.026(2) Å，b=14.838(1) Å，c=13.946(1) Å，β=106.328(2)°，晶胞内分子数Z=2。中心金属离子位于反演对称中心，配位结构为近乎理想的平面四方形PtN₄构型，平均Pt-N键长为2.016(4) Å，N-Pt-N键角为90±0.6°。在两个晶体学独立的NH=C(OEt)Et配体中，平均N=C键长为1.283 Å，结构数据表明氧原子孤对电子向NCO基团存在部分π离域现象。尽管存在上述π离域现象，结合结构与铂-195核磁共振谱数据可知，该亚胺醚配体作为纯σ型氮给体配体（innocent Nσ donor）与Pt²⁺配位，其反位效应与氯离子(Cl⁻)相近。 由于这类E-NH=C(OR')R配体具有扩展的不对称平面结构，其空间位阻需求极高，这不仅决定了配位几何构型，还抑制了配体的自由旋转。该空间位阻主要源于烷基链间的尾-尾(R/R)空间排斥，仅能通过配体围绕各自Pt-N键进行系统性扭转来缓解。本研究表明，配合物1采用Pt(HHTT) C₂h构象：其中H（头朝上）与T（尾朝上）分别代表亚胺醚配体的NH朝上与NH朝下取向，反位配体对相对于PtN₄配位平面协同扭转约20°。配合物1的另一种Pt(HTHT)构型中，反位配体呈反向扭转，理论上可独立稳定存在，可能以手性D₂点群形式存在，而不利于稳定存在的Pt(HHHH)构型则难以形成。