Dimeric heterocyclic ligands

L = C3H3, C H ) and then [Rh(acac)(CO),] to yield the tetranuclear species 180 (85ICA(i00)L5), where the heterocyclic ligands are tridentate. The product reacts with the rhodium(I) dimer [Rh(CO)2Cl]3 to give the trinuclear complex 181. In the solid state, the molecules of this complex form the intermolecular stacks along the z-axis. 

An example of thione coordination is demonstrated with the thiocarbonyl donor ligand 2,6-dimethyl-4H-pyran-4-thione, that gives a complex with zinc chloride of the form ZnL2Cl2-The crystal structure shows the zinc is tetrahedrally coordinated by two chloride and two sulfur donors and the compound was further studied by 3H NMR and IR spectroscopy.573 Both monomeric and dimeric heterocyclic thione complexes have been formed with zinc.5 4,575... 

The pyrazine-bridged dimer (330) is prepared as shown in Scheme 67. Detailed cyclic vol-tammetric studies on this and related N-heterocyclic ligand bridged dimers indicate a weak cluster-cluster interaction across the bridge at anodic potentials but significant interactions at cathodic potentials as the electron content of the clusters increases. Electronic spectral measurements support... 

With heterocyclic ligands ZrCl3-2py room 1.29 Gouy probably halogen-bridged, dimeric 68F20, 67F17... 

Ray, L., Shaikh, M.M. and Ghosh, P. (2008) Shorter Argentophilic Interaction than Aurophilic Interaction in a Pair of Dimeric (NHC)MC1 2 (M=Ag, Au) Complexes Supported over a N/O-Functionalized N-Heterocyclic Carbene (NHC) Ligand. Inorganic Chemistry, 47, 230-240. 

Complexes of this type cannot be prepared from the parent ligand, the bivalent anion, [(CF3)2C2S2]2, which has not yet been isolated as a simple salt. Attempts to prepare it from the novel heterocycle bis(trifluoromethyl)-l,2-dithiete19 (CF3)2C2S2 cause dimerization or decomposition. The preparations are achieved by treating the heterocycle with a low-valent transition metal... 

The two most common methods for the synthesis of complexes with NHC ligands are the reaction of a free carbene (a) or its enetetramine dimer (b) with a suitable metal precursor or the in situ deprotonation of an azolium salt (c) depicted in Fig. 8 using diaminocarbenes with five-membered heterocycles as examples. 

Metal complexes with M-heterocyclic carbene ligands were known long before the first stable NHCs were isolated. Wanzlick [5] and Ofele [6] demonstrated as early as 1968 that NHC complexes can be obtained by in situ deprotonation of azolium salts in the presence of a suitable metal complex without prior isolation of the free NHC ligand (Fig. 1). In these cases a ligand of the metal complex precursor (acetate or hydride) acted as a base for the deprotonation of the imidazolium cation. This method has been successfully transferred to other metal precursors containing basic ligands like [Pd(OAc)2] [97] and [(cod)lr(p-OR)2lr(cod)] [98, 99]. Alternatively, an external base such as NaOAc, KOf-Bu or MHMDS (M = Li, Na, K) can be added for the deprotonation of the azolium salt [100]. In general, the in situ deprotonation of azolium salts appears as the most attractive method for the preparation of NHC complexes as it does not require the isolation of the reactive free carbene or its enetetramine dimer. 

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