Effect of ligand denticity on the nitric oxide reactivity of cobalt(ii) complexes

Abstract

The activation of nitric oxide (NO) by transition metal complexes has attracted a wide range of research activity. To study the role of ligand denticity on the NO reactivity of Co(ii) complexes, three complexes (, and ) were prepared with ligands , and [ = N(1),N(2)-bis(2,4,6-trimethylbenzyl)ethane-1,2-diamine; = N(1)-(2,4,6-trimethylbenzyl)-N(2)-(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine] and = N(1)-(2,4,6-trimethylbenzyl)-N(2),N(2)-bis(2-((2,4,6-trimethylbenzyl)amino)ethyl)ethane-1,2-diamine], respectively. The complexes differ from each other in terms of denticity and flexibility of the ligand frameworks. In degassed methanol solution, they were exposed to NO gas and their reactivity was studied using various spectroscopic techniques. In the case of complex with a bidentate ligand, reductive nitrosylation of the metal ion with concomitant dinitrosation of the ligand framework was observed. Complex with a tridentate ligand did not undergo reductive nitrosylation; rather, the formation of [Co(III)(NO(-))] was observed. The nitrosyl complexes were isolated and structurally characterized. On the other hand, complex with a tetradentate tripodal ligand did not react with NO. This can be attributed to the geometry of the complex as well as due to the accessibility of the corresponding redox potential.

Bent and Linear {CoNO}8 Entities: Structure and Bonding in a Prototypic Class of Nitrosyls

“…If we thus restrict ourselves to well-solved structures, about 20 {CoNO} 8 cases have been published with a clear focus on bent CoNO bonding with an overall pentacoordination of the cobalt center. For various donor sets of the coligands (N 4 , N 3 P, HNP 2 , N 3 Cl, N 2 S 2 , and O 4 ), the Co–N–O angle was found in the range of 118–129°. − Two hexacoordinate structures with trans -chlorido ligands were reported with the same degree of CoNO bending (122 and 124°). , Outside this region, we find pentacoordinate centers with a carbon atom in the donor set (CClP 2 , 140°; CHP 2 , 164°) and a linear CoNO function (177°) in a tetracoordinate species with a CP 2 donor set. , When the quality criteria mentioned below are applied, a disappointing result is obtained in that no unambiguously solved crystal structure of a pentacoordinate CoNO center of the linear subclass is available to date, including early reports such as the structure analyses on [Co­(das) 2 (NO)]­(ClO 4 ) 2 [das = 1,2-bis­(dimethylarsino)­benzene], which shows an unreliable ellipsoid of the N NO atom stretched along the CoNO axis and [CoI 2 (NO)­(PMe 3 ) 2 ] with a large O NO ellipsoid through a crystallographic mirror plane and an unreliably short N–O distance. − …”

Dioxygenation Reaction of a Cobalt-Nitrosyl: Putative Formation of a Cobalt–Peroxynitrite via a {CoIII(NO)(O2–)} Intermediate

“…In the cases of recently structurally characterized Co-nitrosyl complexes of 12- and 13-membered N-tetramethylated cyclam ligands, [(12-TMC)­Co­(NO)] 2+ and [(13-TMC)­Co­(NO)] 2+ , the Co–N–O angles are 128.5 and 124.4°; whereas, the N–O distances are 1.155 and 1.159 Å, respectively . In Co­(II)-nitrosyl complex of N 1 -(2,4,6-trimethylbenzyl)- N 2 -(2-((2,4,6-trimethylbenzyl)­amino)­ethyl-1,2-diamine ligand having {CoNO} 8 description, the Co–NO distance was reported to be 1.765(10) Å; whereas, the N–O distance and Co–N–O angles were 1.103(9)­Å and 124.6(10)° . In this 5-coordinated square pyramidal Co­(II)-nitrosyl, the NO stretching frequency appeared at 1659 cm –1 .…”

“…In Co­(II)-nitrosyl complex of N 1 -(2,4,6-trimethylbenzyl)- N 2 -(2-((2,4,6-trimethylbenzyl)­amino)­ethyl-1,2-diamine ligand having {CoNO} 8 description, the Co–NO distance was reported to be 1.765(10) Å; whereas, the N–O distance and Co–N–O angles were 1.103(9)­Å and 124.6(10)° . In this 5-coordinated square pyramidal Co­(II)-nitrosyl, the NO stretching frequency appeared at 1659 cm –1 . In FT-IR spectrum of complex 1 , the NO stretching frequency appeared at 1673 cm –1 .…”

An Integrated View of Nitrogen Oxyanion Deoxygenation in Solution Chemistry and Electrospray Ion Production

“…After reporting different reduced products for bis­(oxyanion) complexes coordinated to iron and nickel, we were interested in the reductive deoxygenation of a cobalt complex bearing two nitrogen oxyanion ligands. We hypothesized that the formation of a cobalt dinitrosyl complex was possible; however, most reported Co­(NO) 2 complexes with neutral ancillary donors are monocationic to invoke {Co­(NO) 2 } in the Enemark–Feltham notation. − …”