Second sphere interaction

In the case of the Na+ and K+ complexes of N9-ethyladenine-aza-18-crown-6, the metal ions exhibit different types of interaction at the minor groove site N3 (Fig, 14) (52). The Na+ complex, 6, shows a second-sphere interaction involving the coordinated H20 hydrogen bonding to N3 [Na-OH2 2.327, H0 N3 2.836 A], In contrast, the K+ complex, 7,... 

Fig. 14. Structures of (6), illustrating the second-sphere interaction with A-N3, and (7), showing the K1 A N3 binding. Reproduced with permission from Ref. (52). Copyright 2001, Royal Society of Chemistry.

A second sphere interaction may result in quenching of the CT excited states by energy transfer processes, leading to luminescence quenching, eg the MLC I Re bpy luminescence of Re1 monomeric and polymeric complexes is quenched by Cu11 species and the sacrificial electron donor 2,2, 2"-nitrilotriethanol (TEOA) [95]. 

Bowman SEJ, Bren KL (2010) Variation and analysis of second-sphere interactions and axial histidinate character in c-type cytochromes. Inorg Chem 49 7890-7897... 

Second-sphere interactions to the metal ion should not be strained, but should lie within normally encountered limits. For example, the interactions of secondary shell hydrogen bonds around the metal ion need to be taken into account in addition to the ligands that form direct bonds to the metal ion. This will ensure that there are no unsatisfied bonds in the structure. 

Exciplexes and Second Sphere Interactions The concept of exdplex formation in inorganic systems has received considerable attention in recent years. Exciplexes can be observed when ground state complex formation is forbidden but the excited state complex has a shallow energy minimum that can radiatively decay to the ground state (Equation (6) and (7)). McMillin and co-workers postulated exdplex contributions to nonradiative relaxation of Cu phenanthroline... 

Another example of the photoassisted substrate conversion due to a short-lived intermediate in the ground state is shown in figure 6. Chromic acid esters form chromium(V)/alkoxy radical pairs within the photochemical primary reaction. In the presence of such iron(III), cobalt(III), or copper(II) complexes which are able to interact coordinatively or by second sphere interactions with chromium(V) within the radical pair cage reoxidation to chromate(VI) occurs under simultaneous reduction of the metal complexes to corresponding iron(II), cobalt(II), and copper(I) species, respectively. Unfortunately, the efficiency of this photoassisted reaction is limited by... 

Siderophore-ionophore supramolecular assembly formation via host-guest complexation of the pendant protonated amine arm of ferrioxamine B has been confirmed by X-ray crystallography (Fig. 28) (203). The stability and selectivity of this interaction as a function of ionophore structure, metal ion identity, and counter anion identity were determined by liquid-liquid extraction, isothermal calorimetry, and MS (204 -211). Second-sphere host-guest complexation constants fall in the range 103— 106M-1 in CHC13 and methanol depending on ionophore structure. 

Most network structures involving crown ethers are simple hydrogen bonded chains where the crown forms second sphere coordination interactions with a complex ion. These are known for [18]crown-6, [15]crown-5 and [12]crown-4 hosts with a variety of metal complexes [17-25]. For instance when combined with the small [12] crown-4, the perchlorate salts of Mn(II), Ni(II) and Zn(II) form polymeric chain structures with alternating crown ethers and [M(H20)6]2+ cations [19]. Similarly the larger [18]crown-6 forms simple linear chains with metal complexes and cations such as fra s-[Pt(NH3)2Cl2] [20], [Cu(NH3)4(H20)]2+ (Fig.2) [21],and [Mg(H20)5(N03)] + [22],... 

The smaller contribution to solvent proton relaxation due to the slow exchanging regime also allows detection of second and outer sphere contributions (62). In fact outer-sphere and/or second sphere protons contribute less than 5% of proton relaxivity for the highest temperature profile, and to about 30% for the lowest temperature profile. The fact that they affect differently the profiles acquired at different temperature influences the best-fit values of all parameters with respect to the values obtained without including outer and second sphere contributions, and not only the value of the first sphere proton-metal ion distance (as it usually happens for the other metal aqua ions). A simultaneous fit of longitudinal and transverse relaxation rates provides the values of the distance of the 12 water protons from the metal ion (2.71 A), of the transient ZFS (0.11 cm ), of the correlation time for electron relaxation (about 2 x 10 s at room temperature), of the reorienta-tional time (about 70 x 10 s at room temperature), of the lifetime (about 7 x 10 s at room temperature), of the constant of contact interaction (2.1 MHz). A second coordination sphere was considered with 26 fast exchanging water protons at 4.5 A from the metal ion (99), and the distance of closest approach was fixed in the range between 5.5 and 6.5 A. 

Both the inner- and the second-sphere contributions to the overall relaxivity are directly dependent on the molar fraction of water protons interacting with the paramagnetic center (see Eq. (3)). Therefore, a relaxivity enhancement might be simply obtained by increasing the number of water protons in the coordination shells (inner- and second-) of the Gd(III) ion. 

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