NOx Linkage Isomerization in Metal Complexes

Dennis Awasabisah; George B. Richter-Addo    Department of Chemistry and Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, Norman, Oklahoma, USA

Abstract

The binding of small molecules to metals often imparts varied chemistry to the small molecules. Such chemistry is dependent on the coordination mode of the small molecule ligands, as the coordination mode affects the electronic distributions along the ligand atoms. In this review, we outline the current knowledge of the linkage isomerization of NOx ligands in their metal complexes for both non-porphyrin and porphyrin systems. We present their modes of preparation and detection and speculate on the consequences of such linkage isomerization on the resultant chemistry.

Keywords

Heme

Hyponitrite

Isomerization

Isonitrosyl

Linkage

Nitrate

Nitric oxide

Nitrite

Nitrosyl

Porphyrin

1 Introduction

The interactions of ambidentate ligands with transitions metals have often resulted in complexes with very interesting chemistry. For example, the complex [(NH3)5Co(NO2)]Cl2, first prepared by Jörgensen (1) in 1894, contains the ambidentate ligand NO2 and the complex exists in two forms. Crystalline solids obtained for this compound showed a mixture of two different colored species: yellow and red, which were readily isolated with a pair of tweezers (1). Later, Werner identified these two species as isomers arising from the different modes of binding of the NO2 ligand to Co, either via the O or via the N atoms. This resulted in the birth of the concept of linkage isomerization in 1907 (2). About five decades later, Penland provided infrared spectroscopic data to show that the yellow [(NH3)5Co(NO2)]Cl2 complex had NO2 bound to Co via its N atom, and the red isomer had NO2 bonded to Co via the O atom (3). By way of definition, linkage isomerization may be defined as the existence of two or more species that have the same molecular formula, and the same bonding ligands, but differ in the mode of attachment of at least one of the ligands (usually ambidentate) to the central metal atom.

Linkage isomerization in complexes containing several other ambidentate ligands including those of SCN-, SeCN-, CN-(4-6), and NO (5,7) have been reported. We wish to limit this review to linkage isomerization in NOx complexes and to provide current knowledge in the area of linkage isomerization partly because of the myriad of applications and relevance of NOx complexes. There are only a handful of recent reviews in the literature on linkage isomerization in NOx complexes, including a review by Coppens and Novozhilova on photoinduced isomerization (7), and a more recent forum paper on NOx linkage isomerization in porphyrin complexes (8). This review covers linkage isomerization deriving from isolable metal complex precursors. Thus, we will not cover the systems involving laser ablated atomic systems (9).

The importance of linkage isomerization has been highlighted in a number of reviews (8,10,11). A good understanding of the various modes of binding of an ambidentate ligand, and factors that influence these modes of binding will provide more insight in the kind of chemistry they present. For instance, nitric oxide (NO) is known to bind to the iron center of a heme enzyme to carry out its function as a hypotensive agent (12-14). An increased knowledge of Fe-NO coordination has helped in designing better NO-releasing drugs, and understanding their use in the treatment of hypertension as in the case of sodium nitroprusside (SNP) (15,16). Recently, a book chapter was dedicated to a review on medical applications of solid NO complexes (17). Also, the chemistry of NOx complexes is relevant in understanding the mechanism of the denitrification process that forms part of the global nitrogen cycle (18-21), and in understanding the action of the metal-dependent reduction of nitrite (22).

NOx species are generated by combustion processes in industries and automobiles, and may be produced naturally by lightning strikes. This has led to a rising interest in finding improved catalysts for removal of these toxic gases from the atmosphere (23-25). In addition, and more recently, metastable linkage isomers of NOx complexes have been generated to produce photoswitchable complexes which may be applied in ultrafast optical switching and storage devices (26-31). Recent work by Schuy (32), Cervellino (33), and Tahri (34) have shown how the nitroprusside anion [(CN)5Fe(NO)]2 - could be incorporated into silica gel pores to generate its corresponding linkage isomer for potential use in optical devices. Photoinduced linkage isomerism, Schaniel et al. have noted, is known to modify the polarizability of [(CN)5Fe(NO)]2 - so as to cause a macroscopic change of single-crystal refractive index according to the Lorentz-Lorenz equation (27).

1.1 Modes of binding of NOx moieties in monometallic complexes

1.1.1 Nitric oxide complexes

NO is a colorless monomeric gas which is biosynthesized by the enzyme nitric oxide synthase (NOS) (35). NO is known to bind to transition metals in three main ways. The first is via the N end of the molecule to form the linear (Figure 1 Ia) and bent (Figure 1 Ib) nitrosyl (?1-NO) modes, or via the O end to produce the isonitrosyl (?1-ON) linkage isomer (Figure 1 Ic) (36). Isonitrosyl complexes of SNP (37), and some ruthenium nitrosyl complexes were detected in the solid state as metastable species just less than two decades ago by Coppens and coworkers (38). The third mode of binding is the side-on NO (or the ?2-NO) binding mode to a metal as shown in Figure 1 Id. Complexes containing this mode of binding were first demonstrated by Coppens and coworkers for their metastable SNP species (37). Side-on NO species were obtained as short-lived species from photolysis of (OEP)Ru(NO)(O-i-C5H11) and (OEP)Ru(NO)(SCH2CF3) porphyrin complexes (39). Theoretical evidence for the existence of the metastable modes of binding have been demonstrated for SNP (40-42) and for some (por)Fe(NO) models (43).

1.1.2 NO2 complexes

The binding modes of NO2 have been reviewed by Hitchman and Rowbottom (44). Relevant to us in this review are the three nitrite binding modes shown in Figure 1 IIa-c. These are the N-nitro, O-nitrito, and the O,O-bidentate modes. The N-nitro mode has the nitrite ligand bound to the metal via the N atom (Figure 1 IIa). This appears to be the most common binding mode of NO2 in its complexes, thus this binding mode is usually referred to as the ground state binding mode for nitrite, although clearly this is an oversimplification. In the nitrito binding mode, NO2 is bound to the metal via the O atom as shown in Figure 1 IIb. Finally, in the O,O-binding mode, both oxygen atoms of nitrite are bound to the same metal to give an ?2-NO2 configuration as shown in Figure 1 IIc.

1.1.3 NO3 complexes

There are two common binding modes of the nitrate 3- ligand. The first is binding via one oxygen atom to give the O-nitrato form (Figure 1 IIIa) and the second is binding through two NO3 oxygens to give the O,O-bidentate configuration (Figure 1 IIIb). The monodentate mode of binding has been observed in some metalloporphyrin complexes including (OEP)Fe(NO3) (45), (F8TPP)Fe(NO3) (46), (TpivPP)Fe(NO3)-(47), and (TPP)Mn(NO3) (48). Some examples of the O,O-bidentate binding mode in NO3-coordinated metalloporphyrins include (TPP)Fe(NO3) (49,50) and (TpivPP)Fe(NO3) (51). A review article on the coordination chemistry of the nitrate ligand was published in 1971 by Addison and Garner (52).

1.2 Methods that induce linkage...