Kinetics and mechanism of solvent-exchange and anation reactions of sterically hindered diethylenetriamine complexes of palladium(II) in aqueous solution

Abstract

charge-transfer band, not observed experimentally. Nevertheless the problem is further complicated in that the position of the alg ( d t ) level depends critically upon whether the molecule is truly square planar with no near neighbors along the z axis or whether it is tetragonal with weak interactions along the z axis. Since the 1 :2 nickel complex, according to X-ray data,37v38 is N,N-coordinated forming five-membered rings, we could suggest a similar coordination for the ML species via the N atom of the amino group and the nitrogen atom of the deprotonated -NHO-group, as previously reported for nickel complexes of glycyl hydroxamic acid (aha)15*39 and seryl hydroxamic acid (adhp).30 The NiL2 complex being square planar, the small difference in log K for NiL and NiLz species [A log K (6.824 -6.476) = 0.3481 could be explained if we considered that it corresponds to an octahedral (NiL) to square-planar (NiLz) transformation. In addition, the logarithm of the second stepwise formation constant (log Kz) for the Ni2+-adhb system is appreciably greater than that of the first, log flIol. This is contrary to earlier findingsm3 using a method of calculation based on the erroneous assumption that the formation of NiL is effectively complete before NiL2 begins to be formed. The explanation of this behavior may lie in the ability of NiZ+ to form square-planar complexes; when a second ligand is bonding facially in a bidentate manner, the two five-membered (40) Braibanti, A.; Dallavalle, F.; Leporati, E.; Mori, G. J. Chem. SOC., Dalton Trans. 1973, 2539. (41) Braibanti, A.; Dallavalle, F.; Leporati, E.; Mori, G. J. Chem. Soc., Dalton Trans. 1973, 323. (42) Braibanti, A.; Mori, G.; Dallavalle, F.; Leporati, E. Inorg. Chim. Acta 1972, 6, 106. (43) Braibanti, A.; Dallavalle, F.; Leporati, E. Inorg. Chim. Acta 1969, 3, 459.Inorg. Chem. 1989, 28, 3759-3765 3759 chelate rings in the complex [NiL2] are much more stable than one five-membered ring in the [NiL] species. In conclusion the X-ray crystal structures of two complexes of n i~k e l ( I I )~~-~* and one of copper(II)eO containing two glycyl hydroxamate anions (L-= HzN-CH2-CONHO-) provide evidence of coordination by the nitrogen amino and the nitrogen of the hydroxamate moiety for adhb and ahpp, respectively. Additional evidence for this type of coordination, which persists in solution, was given by potentiometric and spectrophotometric studies on n i~k e l ( I 1 )~~~~~ and ~o p p e r ( I I ) .~~ We conclude therefore that M(I1)adhb and M-(11)-ahpp systems can probably satisfy different criteria for the biological activities and the analytical role proposed, giving a strong indication for the complexes investigated as being suitable sources of metal ions as trace elements that are essential in animal nutrition. Our results show clearly that, at physiological pH values, the assumption of an uncoordinated a-amino group in the M-(11)-adhb and M(I1)-ahpp systems is probably incorrect since the major species in this pH range is MLz, which involves coordination of the a-amino grou...

Synthesis, Characterization and Cytotoxicity Evaluation of New Biimidazole Palladium(II) Complexes with Thioureas

“…This finding could be probably related to the expected decrease of the rate of substitution reactions in square planar complexes due to the presence of more sterically demanding N-phenylthiourea ligands, increasing the difficulty encountered by the entering ligand in binding to the metal center during an associative substitution process. 38 In 5, two signals groups were observed. The first group show one signal in 2.23 ppm, assigned to chemically equivalent tmbiimH 2 ring methyl groups (Me 4,4' , Me 5,5' ) for free ligand in solution and two singlet resonances of equal integrated area in 2.11 and 1.20 ppm, assigned to chemically inequivalent tmbiimH 2 ring methyl groups (Me 4,4' , Me 5,5' ) for coordinated ligand.…”

Ligand Substitution Dynamics in Metal Complexes

“…Indeed, as we go from Pd(dien)OH 2 2þ to Pd(Me 5 dien)OH 2 2þ to Pd(Et 5 dien)OH 2 2þ , the rate constant k 298 ex for water exchange falls from 5100 to 187 to 2.9 s À1 , and a similar pattern is found for the corresponding ligation reactions, but values of DV ‡ ex for the water exchange reactions are À3, À7, and À8 cm 3 /mol, respectively. 163 Thus, the water exchange reaction remains associatively activated as steric hindrance increases, evidently because for Pd II , like Pt II , dissociatively activated pathways are energetically inaccessible in the absence of electronic effects involving strong s donor ligands such as alkyl or aryl ligands (Section 8.3.3.1). This stands in sharp contrast to the question of steric hindrance in interchange reactions of octahedral complexes M III (RNH 2 ) 5 X, in which there is a move toward more dissociative character in the I a /I d continuum as we go from H to Me and beyond (Section 8.5.2).…”

Kinetic and mechanistic studies on the Pd–Cl cleavage of dichloro‐ [1‐alkyl‐2‐(naphthylazo)imidazole]palladium(II) complexes by 2,2′‐bipyridine

“…Naphthylazoimidazoles (ii) are chemical analogues to phenylazoimidazoles (i) but with a higher degree of steric-crowding and electron-donating ability. The kinetics and mechanism of the reactions involving Pd(II) complexes [23][24][25][26][27][28] stimulates us to study the kinetic and mechanistic aspects of palladium(II) complexes of cisplatin analogues. Heterocyclic compounds are very widely distributed in nature and are very much essential to many biochemical processes.…”