Cluster monomers

Fig. 5 Cyclic and differential voltammograms of RU3O cluster monomer 36 and dimer 37 with orf/zometallated 2,2/-bipyrimidine, recorded in 0.1 M dichloromethane solution of (Bu4N)(PF6). The scan rate is lOOmVs-1 for CV and 20mVs-1 for DPV...

The 2,2 -bipyrazine (bpz)-substituted RU3 cluster monomer 42, dimer 43, and trimer 44 could be accessed by reaction of triruthenium precursor 2 with different amounts of 2,2/-bipyrazine [30]. The trimeric species 44 containing two parent Ru30(0Ac)6(py)2 n m and one derivate Ru30(0Ac)5(py)2II m m units could be directly prepared by reaction of 3.8 equivalents of 2 with 2,2/-bipyrizine. It is also accessible by reaction of dimeric species 43 with 1.8 equivalents of 2. The bpz adopts ri1 (N),(x-r 1 (N),r(1 (N) and p,4-r 1(N),r 1(N),r 1(C),r 2(N,N) bonding modes in 42, 43, and 44, respectively. Reduction of 3+ trimer 44 by addition of aqueous hydrazine allowed isolation of neutral intracluster mixed-valence species 44b containing three Ru 11,111,11 units. Oxidation of 44b with two... 

The v(CO) band can be observed in each of the five available redox states, and it shifts to progressively lower frequencies upon reduction from the (+4) state to the (—2) state due to the increasing 7i-backbonding ability of the metal centre. All of the redox states can be considered symmetric in that both cluster monomers are in the same oxidation state, with the exception of the singly reduced (—1) mixed-valence state. As such, the v(CO) band in each of the symmetric states is sharp (FWHM = 16 cm ), while the v(CO) band in the (—1) state is broad (FWHM 50-60 cm ) and is midway (with half-intensity) between the peaks for the (0) and (—2) states (Figure 5.4). 

This phenomenon was originally thought to be indicative of a localised mixed-valence state, i.e. that the appearance of this mode is proof of electronic asymmetry on the infrared timescale. Flowever, control experiments with the cluster monomers that have pronounced structural asymmetry, were carried... 

Figure 13. Extinction and scattering spectra of bbc-clusters with the number of conjugates iV=701 (1, maximal volume density), 561(2), 421 (3), and 281 (4). Diluted clusters were obtained by random elimination of cluster monomers. The volume fractions of conjugates equal 0.53 (maximal density), 0.42, 0.316, and 0.21, respectively. Calculations were carried out by GMM method [66] using the following parameters of conjugates gold core = 15 nm, polymer shell s = 1,0, n, =1.40. ...

Figure 8.1. Sketch of linear and cyclic (HCN) and (HCjN) clusters. Monomers are represented by arrows.

Copolymerization of the cluster monomer 90 with styrene Radical copolymerization of 90 with styrene was carried out in bulk or in toluene in the presence of AIBN (0.1 to 0.6 mol. %). Rh6(CO)i5(4-VPy) (0.06 g, 0.052 mmol), AIBN (1 wt. %), styrene (1.4 g, 10 mmol), benzene (18 mL) were placed in a glass ampoule. The mixture was degassed by freeze-thaw cycles to remove the oxygen from the reaction mixture completely. The ampoule was then sealed in vacuum and heated at 70 °C for 6 h. The copolymer formed was precipitated with ethanol from benzene solution, and dried. The yield 0.57 g of the copolymer (3.27% Rh, Af = 180,000 Da). 

Experiment 4-10 Synthesis and Copolymerization of the Cluster Monomer 89 (p-H)Os3(p-OCNMe2)(CO)9P(C6Hs)2CH2CH=CH2 (Section 4.2.8) [137]... 

In paper [102] the theoretical treatment of a cluster-cluster aggregation accounting for the existence of coalescing of particles or of clusters (monomers or macromolecular coils) to aggregate in real polymerization processes, and their disconnection (destruction) was reported. Macromolecules are in a random environment, influencing the processes of aggregation or destmction in dilnte polymeric solution that is described by the stochastic equation [101] ... 

Figure 5-22. a) Projection of the tetragonal structure of NaMo40 along [001]. The chains of Moe octahedra are outlined, b) Comparison of a discrete [Mo Ou] cluster ( monomer ) to the infinite Mo40g chain ( polymer ). 

Starting with very dilute solutions, in which only cavities e and monomer cluster ions M+ are present, there would be an increase in the number of cluster monomers M as the concentration increases. With further increase of concentration, dimer clusters Mj are produced, and their number increases with concentration. One would therefore expect to find two dissociation energies for the two reactions (34) and (35). Only one dissociation energy, 0.2 0.05 ev, has been obtained from the concentration dependence of heats of solution and is ascribed to the second reaction. A possible explanation for the failure to obtain any evidence for the heat of dissociation for process (35) could be that the heats of solution for the monomer and cavity are almost equal. This would seem rather surprising, and evidently more accurate experimental data and a careful theoretical calculation of the heat of solution for the electron in the monomer are necessary to clear up the situation. 

The available data on temperature coefficients of conductance of the solutions can also receive adequate qualitative explanation from the unified model. The temperature coefficient of 2 per cent per degree in dilute solutions of sodium in ammonia is larger than the temperature coefficient of the viscosity which is 1.1 per cent per degree. In addition, for dilute potassium-ammonia solutions the temperature coefficient of the conductance is larger than in sodium-ammonia solutions and is found to be 2.9 per cent per degree. These two observations quite definitely indicate that some other factor than the decrease in viscosity with temperature is responsible for the observed temperature coefficient. For dilute solutions this factor is the expected increased dissociation of cluster monomers with... 

Explanation of Nuclear Magnetic Resonance Data. The observation of a finite Na resonance shift by McConnell and Holm is one of the strongest points proving the existence of cluster monomers. The shift can on the other hand be explained by both the cavity and cluster models. The lack of any appreciable shift in the proton resonance has been explained quantitatively using the cluster model alone. No attempt has yet been made to explain the proton resonance shift with the cavity model. We shall now consider these explanations in detail. 

For the cavity model, the Na+ ions are completely separated from the electron cavities, and so there should not be any unpaired electron density at the Na nuclei. So no shift in the Na resonance is to be expected. In the cluster monomer the electron moves around the Na+ ion albeit in a more expanded orbital - > than the sodium atom 3s orbital. An earlier semi-quantitative explanation of the Na shift was attempted at by McConnell and Holm using ex-... 

It should be noted extra-coordination processes are also important in controlling the properties of metallopolymers. Thus mechanical parameters and performance of many metal-containing polymers are determined by the ability of the metals to form ionic or coordination crosslinks (i.e., additional interchain interaction), and to exhibit cohesion and adhesion properties. Formally, unit variability in these cases is determined by the presence of metals in the chain with different coordination numbers. The incorporation of cluster-containing Os3-monomers into a polystyrene or poly(acrylonitrile) chain results in a mutual thermal stabilization of both the polymers and the clusters incorporated into the chains. These effects are observed only in cases where the cluster monomers are chemically bound to the polymeric chain. The influence of the chain may be manifested as the transfer of energy from the rotation-vibration degrees of freedom of the cluster to the translational degrees of freedom of the polymer chain segments at elevated temperatures. 

---