Steric and Electronic Effects in the First Homoleptic Imino Ether Complex: Synthesis and X-ray Crystallographic Determination of [Pt(NHC(OEt)Et)(4)](CF(3)SO(3))(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&tbd1;CEt)(4)](2+) proceeds smoothly under basic conditions to yield stable, colorless [Pt(NH=C(OEt)Et)(4)](2+)(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)](2+) at -2409 ppm and that for [Pt(NH(3))(4)](2+) at -2580 ppm (all relative to [PtCl(6)](2-)). 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).
Prenzler
Paul D.
PD
Research School of Chemistry, The Australian National University, Canberra, ACT 0200, Australia.
Hockless
David C. R.
DC
Heath
Graham A.
GA