Preparation of platinum(IV) complexes with dipeptide and diimine. X-ray crystal structure and 195Pt NMR spectra

We prepared platinum(IV) complexes containing dipeptide and diimine or diamine, the [PtCl(dipeptide-N,N,O)(diimine or diamine)]Cl complex, where -N,N,O means dipeptide coordinated as a tridentate chelate, dipeptide = glycylglycine (NH2CH2CON−CH2COO−, digly, where two protons of dipeptide are detached when the dipeptide coordinates to metal ion as a tridentate chelate), glycyl-l-alanine (NH2CH2CON−CHCH3COO−, gly-l-ala), l-alanylglycine (NH2CH CH3CON−CH2COO−, l-alagly), or l-alanyl-l-alanine (NH2CHCH3CON−CHCH3COO−, dil-ala), and diimine or diamine = bipyridine (bpy), ethylenediamine (en), N-methylethylenediamine (N-Me-en), or N,N′-dimethylethylenediamine (N,N′-diMe-en). In the complexes containing gly-l-ala or dil-ala, two separate peaks of the 195Pt NMR spectra of the [PtCl(dipeptide-N,N,O)(diimine or diamine)]Cl complexes appeared in, but in the complexes containing digly or l-alagly, one peak which contained two overlapped signals appeared. One of the two complexes containing gly-l-ala and bpy, [PtCl(gly-l-ala-N,N,O)(bpy)]NO3, crystallized and was analyzed. This complex has the monoclinic space group P212121 with unit cell dimensions of a = 9.7906(3) Å, b = 11.1847(2) Å, c = 16.6796(2) Å, Z = 4. The crystal data revealed that this [PtCl(gly-l-ala-N,N,O)(bpy)]NO3 complex has the near- (Cl, CH3) configuration of two possible isomers. Based on elemental analysis, the other complex must have the near- (Cl, CH3)-[PtCl(gly-l-ala-N,N,O)(bpy)]NO3 configuration. The 195Pt NMR chemical shifts of the near- (Cl, CH3)-[PtCl(gly-l-ala-N,N,O)(bpy)]NO3 complex and the far- (Cl, CH3)-[PtCl(gly-l-ala-N,N,O)(bpy)]NO3 complex are 0 ppm and −19 ppm, respectively (0 ppm for the Na2[PtCl6] signal). The additive property of the 195Pt NMR chemical shift is discussed. The 195Pt NMR chemical shifts of [PtCl(dipeptide-N,N,O)(bpy)]Cl appeared at a higher field when the H attached to the dipeptide carbon atom was replaced with a methyl group. On the other hand, the 195Pt NMR chemicals shifts of [PtCl(dipeptide-N,N,O)(diamine)] appeared at a lower field when the H attached to the diamine nitrogen atom was replaced with a methyl group, in the order of [PtCl(digly-N,N,O)(en)]Cl, [PtCl(digly-N,N,O)(N-Me-en)]Cl, and [PtCl(digly-N,N,O)(N,N′-diMe-en)]Cl.