The origin of hindered rotation around the Pt-N bond in platinum amides.

Abstract

Several platinum amides of formula trans-[PtCl(NHAr)(PEt(3))(2)] (Ar = 3-FC(6)H(4), 2; 4-FC(6)H(4), 3; 4-ClC(6)H(4), 4; 4-IC(6)H(4), 5; 4-Cl,3-NO(2)-C(6)H(3), 6) have been synthesized by reaction of [PtHCl(PEt(3))(2)] with aryl azides. All the complexes feature planar arylamido moieties and hindered rotation around the N-aryl and Pt-N bonds have been detected and separately studied. The X-ray crystal structures of complexes 5 and 6 have been determined. Complex 5 crystallizes in the orthorhombic space group Pnma, with a = 23.806(4) A, b = 15.099(2) A, c = 6.7593(10) A, alpha = beta = gamma = 90 degrees, and Z = 4. Compound 6 shows an N-H...O(NO) hydrogen bond and it crystallizes in the monoclinic space group P2(1)/n, with a = 12.215(3) A, b = 8.078(2) A, c = 13.052(4) A, alpha = gamma = 90 degrees, beta = 90.057(6) degrees, and Z = 2. Except for Ar = 4-Cl,3-NO(2)-C(6)H(3), the activation energies obtained for the complexes indicate that both dynamic processes occur simultaneously with a common barrier which originates in the multiple bond character of the N-aryl bond due to a strong pi-donor behavior of the N atom in the N-aryl bond. The rotation about the Pt-N bond is unfavorable because of steric congestion with the planar amide, which can be overcome only when the aromatic ring can rotate. For the complex trans-[PtCl[NH(4-Cl,3-NO(2)-C(6)H(3))](PEt(3))(2)] the barrier to rotation is mostly due to hydrogen bond interaction between the NO(2) ortho substituent and the amide H atom.