The bis-guanidyl compound H2C{hpp}2 (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]- pyrimidine) coordinates to palladium(II) as a neutral, chelating N,N-bidentate ligand. Structural analysis of PdMeCl(H2C{hpp}2) shows a non-planar metallacycle that is twisted relative to the square plane of the metal and an anagostic interaction with a CH from the bridging methylene group. Generation of the cationic palladium complex, [PdCl(H2C{hpp}2)][OTf] was achieved by halide abstraction from the dichloride PdCl2(H2C{hpp}2) using NaOTf. The product was identified as a mixture of different species in solution; in the solid-state, the molecular structure is dimeric, consisting of the m,m-dichlorobridged dication. The new bis-guanidyl compound H2C{tbo}2 (Htbo = 1,4,6-triaza-bicyclo[3.3.0]oct-4-ene) was synthesized, and structurally characterized. Coordination of this compound at palladium dichloride was accompanied by ionization of the PdCl bond, and formation of the dication [Pd(H2C{tbo}2)2][Cl]2. Structural analysis shows a significant reduction in the twisting of this ligand.

Partial deprotonation of the bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) is achieved using nBuLi. Isolation of the resulting lithium salts has resulted in the structural characterization of the mixed anion complex {[Li(tbo)( VIII)(tboH)]2} 1a (where VIII-H = 1-(2-aminoethyl)-2-imidazolidinethione) and the partially deprotonated salt Li6(tbo)6(Htbo)3, 1b. The neutral guanidine Htbo reacts cleanly with AlMe3 and ZnMe2 to afford the organometallic complexes [Al(tbo)Me2]2 [ 2]2, and Zn3(tbo)4Me2 ( 3). Structural characterization of these compounds enables comparison between the {5:5}-bicyclic system, [tbo]-, and the previously reported {6:6}-bicyclic system, [hpp]- (where hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). Results indicate that delocalization within the [tbo]- anion is restricted to the CN2 amidinate component, with retention of electron density in the non-bonding nitrogen lone-pair. These conclusions are supported by a DFT analysis of the neutral guanidines, Htbo and hppH.

In this paper, we report an example of intermolecular solid-state proton transfer in the bicyclic guanidine, hppH. A combination of X-ray crystallography, CPMAS NMR (C-13 and N-15) and theoretical calculations allows us to determine that a double proton transfer takes place in the (hppH)(2) dimer with an activation energy of about 50 kJ mol(-1). According to the B3LYP/6-311++G(d,p) calculations, the double proton transfer occurs non-symmetrically through a zwitterion.

Protonation of the {6,6}- and {5,5}-bicyclic guanidines 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine (hppH) and 1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo), respectively, to afford the hydrochloride salts [hppH2][Cl] (1a) and [HtboH][Cl] (2) was achieved using [NEt3H][Cl]. Anion exchange involving 1a and NaBPh4 generated the borate salt [hppH2][BPh4] (1c). Crystal structure analysis of 1a and the analogous hydrobromide salt [hppH2][Br] (1b) showed two NHX hydrogen-bonds between the cation and the anion, and association of the [hppH2][X] ion-pairs into dimeric units. In salt 1c the [hppH2]+ cation was shown to be located within a cavity defined by phenyl substituents from the [BPh4] anion, with no hydrogen bonding present. Reducing the size of the heterocyclic rings from {6,6}- to {5,5}- in the [HtboH]+ salt promotes formation of extended structures due to a wider angle between the projected hydrogen-bonds to the halide anion. Results from analysis of the bond parameters within the guanidinium cations are used to explain the distribution of -electron density throughout the bicyclic framework

A [5:5] ligand based on the bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene [Htbo] has been synthesised. Two of these tbo units have been linked via a carbon bridgehead to form a novel bidentate ligand. Based on the knowledge that the electronic properties and specific steric interactions at a metal can lead to new insights into the relationship between crystal structure and ligand activity, the synthesised compounds have been investigated as polydentate ligands on the assumption that that they could be used to precisely control coordination environment. The corresponding [6:6] system which is based on the bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) [hppH] has also been synthesised and its coordination to silyl groups investigated.
1,4,6-triazabicyclo[3.3.0]oct-4-ene (Htbo) has been deprotonated to form the [tbo]- anion, which has been assessed as a ligand at Li, Al and Zn centres, allowing direct structural comparison to be made with the [hpp]- analogues. The coordination chemistry of Htbo and its derivatives has been explored through formation of a variety of transition metal complexes. Additionally, crystal structures showed that Htbo coordinates to the metal through the N-imine atom as a monodentate ligand to form trigonal planar, tetrahedral, square planar and octahedral complexes.
The bis (tbo) methane H2C{tbo}2 ligand was found to support a range of coordination geometries and upon coordination, ionic structures such as [Pd(H2C{tbo}2)2][Cl]2 were formed. The synthesis of the monosubstituted compound hppSiCl3 afforded a series of (hpp) silanes which were isolated and reacted with LiOAr (Ar= 2,6-tBu2C6H3), providing a proof of N-Si bond stabilisation using a metal. Five examples of pentacoordinated silicon species have been synthesised via the reaction of [hpp]- and R2SiCl2 (R= Ph, Me).