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

Several platinum amides of formula trans-[PtCl(NHAr)(PEt3)(2)] (Ar = 3-FC6H4, 2; 4-FC6H4, 3; 4-ClC6H4., 4; 4-IC6H4, 5; 4-Cl,3-NO2-C6H3, 6) have been synthesized by reaction of [PtHCl(PEt3)(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) Angstrom, b = 15. 099(2) Angstrom, c = 6.7593(10) Angstrom, 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) Angstrom, b = 8.078(2) Angstrom, c = 13.052(4) Angstrom, alpha = gamma = 90 degrees, beta = 90.057(6)degrees, and Z = 2. Except for Ar = 4-Cl,3-NO2-C6H3, 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 which can be rotation about the Pt-N bond is unfavorable because of steric congestion with the, planar amide overcome only when the aromatic ring can rotate. For the complex trans-[PtCl{NH(4-Cl,3-NO2-C6H3)}(PEt3)(2)] the barrier to rotation is mostly due to hydrogen bond interaction between the,NO ortho substituent and the amide H atom.