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
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- 作者：Paul D. Prenzler ; David C. R. Hockless ; and Graham A. Heath
- 刊名：Inorganic Chemistry
- 出版年：1997
- 出版时间：December 3, 1997
- 年：1997
- 卷：36
- 期：25
- 页码：5845 - 5849
- 全文大小：115K
- 年卷期：v.36,no.25(December 3, 1997)
- ISSN：1520-510X
文摘
Imino ethers, NH=C(OR')R, represent aninteresting alkoxy-substituted class of otherwise elusivemonodentateimine ligands. We have discovered that 4-fold addition of EtOH to[Pt(N
CEt)4]2+ proceeds smoothlyunderbasic conditions to yield stable, colorless[Pt(NH=C(OEt)Et)4]2+(CF3SO3-)2(1), which is the firsthomolepticimino ether complex reported for any metal ion.The monodentate imino ether ligands are all equivalentinsolution, according to 1H NMR, and prove to have theE isomeric form with cisNH/COEt groups. The IR-active C-N stretching vibration moves from 2330 cm-1 inthe nitrile complex to 1633 cm-1 in 1, and
NH emergesat 3250 cm-1. The 195Pt NMR shiftfor 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 -2580ppm (all relative to [PtCl6]2-).Single-crystal X-ray diffractionestablishes that 1 crystallizes in the monoclinic spacegroup P21/c, with a =9.026(2) Å, b = 14.838 (1) Å, c=13.946 (1) Å, 
= 106.328(2)
, and Z = 2.The metal ion lies at a center of inversion and displays almostidealsquare-planar PtN4 coordination with mean Pt-N distancesof 2.016(4) and N-Pt-N angles of 90 ± 0.6.Withinthe two crystallographically independent NH=C(OEt)Etligands, the mean N=C distance is 1.283 Å and there isstructural evidence of partial 
-delocalization of O lone pairs intothe NCO moiety. Nonetheless, according toboth structural and 195Pt NMR data, the imino etherligand behaves as innocent N
donor toward Pt2+, withatrans influence similar to that of Cl-.Due to their extended, asymmetric planar nature, suchE-NH=C(OR')Rligands have exceptional steric requirements which control thecoordination geometry and prevent unhinderedrotation. These requirements are dominated by alkyl tail-to-tail(R/R) clashes that can be accommodated only bysystematic twisting of the ligands about their respective Pt-N bonds.The present study reveals that 1 has thePt(HHTT) C2hconformation, where H (head up) and T (tail up) represent mutual NH-upand NH-down orientationsof the imino ether ligands, with trans-ligand pairscooperatively rotated by about 20 with respect to thePtN4plane. The alternative Pt(HTHT) conformer of 1,with counterrotated trans ligands, is predicted toexistindependently, possibly in the chiralD2 form, in contrast to unfavoredPt(HHHH).
CEt)4]2+ proceeds smoothlyunderbasic conditions to yield stable, colorless[Pt(NH=C(OEt)Et)4]2+(CF3SO3-)2(1), which is the firsthomolepticimino ether complex reported for any metal ion.The monodentate imino ether ligands are all equivalentinsolution, according to 1H NMR, and prove to have theE isomeric form with cisNH/COEt groups. The IR-active C-N stretching vibration moves from 2330 cm-1 inthe nitrile complex to 1633 cm-1 in 1, andNH emergesat 3250 cm-1. The 195Pt NMR shiftfor 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 -2580ppm (all relative to [PtCl6]2-).Single-crystal X-ray diffractionestablishes that 1 crystallizes in the monoclinic spacegroup P21/c, with a =9.026(2) Å, b = 14.838 (1) Å, c=13.946 (1) Å, = 106.328(2), and Z = 2.The metal ion lies at a center of inversion and displays almostidealsquare-planar PtN4 coordination with mean Pt-N distancesof 2.016(4) and N-Pt-N angles of 90 ± 0.6.Withinthe two crystallographically independent NH=C(OEt)Etligands, the mean N=C distance is 1.283 Å and there isstructural evidence of partial -delocalization of O lone pairs intothe NCO moiety. Nonetheless, according toboth structural and 195Pt NMR data, the imino etherligand behaves as innocent N donor toward Pt2+, withatrans influence similar to that of Cl-.Due to their extended, asymmetric planar nature, suchE-NH=C(OR')Rligands have exceptional steric requirements which control thecoordination geometry and prevent unhinderedrotation. These requirements are dominated by alkyl tail-to-tail(R/R) clashes that can be accommodated only bysystematic twisting of the ligands about their respective Pt-N bonds.The present study reveals that 1 has thePt(HHTT) C2hconformation, where H (head up) and T (tail up) represent mutual NH-upand NH-down orientationsof the imino ether ligands, with trans-ligand pairscooperatively rotated by about 20 with respect to thePtN4plane. The alternative Pt(HTHT) conformer of 1,with counterrotated trans ligands, is predicted toexistindependently, possibly in the chiralD2 form, in contrast to unfavoredPt(HHHH).