Prenzler Paul D., Hockless David C. R., Heath Graham A.
Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia.
Inorg Chem. 1997 Dec 3;36(25):5845-5849. doi: 10.1021/ic970312b.
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&tbd1;CEt)(4) proceeds smoothly under basic conditions to yield stable, colorless Pt(NH=C(OEt)Et)(4)(CF(3)SO(3)(-))(2) (1), which is the firsthomoleptic imino ether complex reported for any metal ion. The monodentate imino ether ligands are all equivalent in solution, according to (1)H 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 nu(NH) emerges at 3250 cm(-1). The (195)Pt NMR shift for 1 is found to be -2450 ppm, lying between that for Pt(NCEt)(4) at -2409 ppm and that for Pt(NH(3))(4) at -2580 ppm (all relative to PtCl(6)). Single-crystal X-ray diffraction establishes that 1 crystallizes in the monoclinic space group P2(1)/c, with a = 9.026(2) Å, b = 14.838 (1) Å, c = 13.946 (1) Å, beta = 106.328(2) degrees, and Z = 2. The metal ion lies at a center of inversion and displays almost ideal square-planar PtN(4) coordination with mean Pt-N distances of 2.016(4) and N-Pt-N angles of 90 +/- 0.6 degrees. 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 pi-delocalization of O lone pairs into the NCO moiety. Nonetheless, according to both structural and (195)Pt NMR data, the imino ether ligand behaves as innocent Nsigma donor toward Pt(2+), 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) C(2)(h)() 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 degrees with respect to the PtN(4) plane. The alternative Pt(HTHT) conformer of 1, with counterrotated trans ligands, is predicted to exist independently, possibly in the chiral D(2) form, in contrast to unfavored Pt(HHHH).
亚氨基醚（NH = C(OR')R）是一类有趣的烷氧基取代的单齿亚胺配体，这类配体在其他情况下难以捉摸。我们发现，在碱性条件下，向[Pt(N≡CEt)₄]²⁺中四次加入乙醇能顺利进行，生成稳定的无色[Pt(NH = C(OEt)Et)₄]²⁺(CF₃SO₃⁻)₂（1），这是首次报道的任何金属离子的均配亚氨基醚配合物。根据¹H NMR，单齿亚氨基醚配体在溶液中都是等价的，且证明具有顺式NH/COEt基团的E异构体形式。红外活性的C - N伸缩振动从腈配合物中的2330 cm⁻¹移至1中的1633 cm⁻¹，ν(NH)出现在3250 cm⁻¹处。发现1的¹⁹⁵Pt NMR位移为 - 2450 ppm，介于[Pt(NCEt)₄]²⁺的 - 2409 ppm和[Pt(NH₃)₄]²⁺的 - 2580 ppm之间（均相对于[PtCl₆]²⁻）。单晶X射线衍射表明1结晶于单斜空间群P2₁/c，a = 9.026(2) Å，b = 14.838(1) Å，c = 13.946(1) Å，β = 106.328(2)°，Z = 2。金属离子位于对称中心，呈现几乎理想的平面正方形PtN₄配位，平均Pt - N距离为2.016(4) Å，N - Pt - N角度为90 ± 0.6°。在两个晶体学独立的NH = C(OEt)Et配体中，平均N = C距离为1.283 Å，并且有结构证据表明O孤对电子部分π离域到NCO部分。尽管如此，根据结构和¹⁹⁵Pt NMR数据，亚氨基醚配体对Pt²⁺表现为无害的Nσ供体，其反位影响与Cl⁻相似。由于其延伸的、不对称的平面性质，这种E - NH = C(OR')R配体具有特殊的空间要求，这些要求控制着配位几何结构并阻止无阻碍的旋转。这些要求主要由烷基尾对尾（R/R）冲突主导，只有通过配体围绕各自的Pt - N键进行系统扭曲才能适应。本研究表明1具有Pt(HHTT) C₂(h)对称性，其中H（头朝上）和T（尾朝上）表示亚氨基醚配体的相互NH朝上和NH朝下取向，反位配体对相对于PtN₄平面协同旋转约20°。预计1的另一种Pt(HTHT)构象体，其反位配体反向旋转，可能以手性D₂形式独立存在，与不受欢迎的Pt(HHHH)相反。
