A Unique Mechanism for Base Catalyzed Hydrolysis of Pentaaminecobalt(III) Complexes Containing Picolyl Residues

Abstract

A novel [Co(pentaamine)Cl](2+) complex having all tertiary amine or pyridine donors has been synthesized (pentaamine = 1,4-bis(2'-pyridyl)-7-methyl-1,4,7-triazacyclononane). This asym-[Co(dmpmetacn)Cl](2+) species has been completely characterized through 1D and 2D NMR studies, and through the X-ray structure for the ZnCl(4)(2)(-) salt. Despite the lack of an activating NH center, remarkably its hydrolysis to [Co(pentaamine)OH](2+) is base catalyzed (k(OH) 0.70 M(-)(1) s(-)(1), 25 degrees C, I = 1.0 M, NaCl). Detailed NMR studies reveal that the base catalyzed substitution leads to the exchange of just one deuterium in one of the two -CH(2)- pyridyl arms, that is approximately trans to the leaving group, and this occurs during and not after base hydrolysis. Quenching experiments for the reaction of asym-[Co(dmpmetacn)Cl](2+) and control experiments on H/D exchange for the product asym-[Co(dmpmetacn)OD](2+) in OD(-) show that each act of deprotonation at the acidic methylene leads to loss of Cl(-). This is the first established case of base catalyzed substitution for a complex where the effective site of deprotonation is at a pyridyl group. A pronounced kinetic isotope effect is observed for the species perdeuterated at the pyridyl methylenes (k(H)/k(D) = 5.0), consistent with rate limiting deprotonation which is a rare event in Co(III) substitution chemistry. The activation afforded by the carbanion is discussed in terms of a new process coined the pseudo-aminate mechanism.

Synthesis, structure, semiconductive and photoluminescent properties of [{Eu(NC5H4COOH)3(H2O)2}(1.5ZnCl4)·(2H2O)]n

“…The Zn2 atom is positionally disordered and the occupancy of Zn2 had to be set to 0.5 in order to obtain a rational structure model and thermal displacement parameters. The bond lengths of Zn-Cl range from 2.205(2) to 2.396(1) Å with an average value of 2.297(2) Å, which are normal and similar to the counterparts found in the literature [18][19][20][21]. The europium atom is coordinated to eight oxygen atoms, of which two are from the two water molecules and six are from the six isonicotinic acid ligands, yielding a distorted square anti-prism with the top and bottom planes defined by O(CC) SCS _______________________________________________________________________________________________________________________________________________ Available online at www.shd.org.rs/JSCS NOVEL 4f-3d METAL-ISONICOTINIC ACID HYBRID COMPLEX 759 -z) atoms, respectively.…”

Significant relaxivity gap between a low-spin and a high-spin iron(ii) complex of structural similarity: an attractive off–on system for the potential design of responsive MRI probes

“…The last tosyl group is then removed by treatment with refluxing concentrated sulfuric acid for 34 h (see for example ref. 46).…”

A New Member of the Growing Family of Interconvertible {RuNO}6,7,8 Species. Redox and Acid‐Base Characterization of [Ru((CH2py)2Me[9]aneN3)(NO)]n+

“…The precursor complex Ru(dmso) 4 Cl 2 and the ligand (CH 2 py) 2 Me[9]aneN 3 were prepared following published procedures. [21,60] Synthesis of [Ru((CH 2 py) 2 Me [9]aneN 3 )(NO)](PF 6 ) 3 ([1-NO](PF 6 ) 3 ). An orange suspension of 400 mg of Ru(dmso) 4 Cl 2 (0.826 mmol) and 880 mg of (CH 2 py) 2 Me[9]aneN 3 (2.708 mmol) in 15 mL of ethanol was refluxed under argon.…”

“…A thorough search of the Cambridge Crystallographic Database shows examples of hexacoordinate species of (CH 2 py) 2 Me- [9]aneN 3 based on Co(III), [21] Fe(II)/Fe(III) [22][23][24] and Ru(II)/Ru(III), [25] which in all cases adopt the asymm-configuration. A similar picture emerges when evaluating structures based on the related (CH 2 py) 2 [9]aneN 3 , with several examples of asymmisomers in octahedral Cr(III), [26][27] Mn(II), [28][29] Fe(II)/Fe(III), [30][31] Co(III) [32] and Zn(II) [33][34] species.…”