This special issue of the Journal is dedicated to the chemistry of compounds derived from the guanidine functionality. It describes compounds containing this group in its neutral (guanidine), cationic (guanidinium) and anionic (guanidinate) forms. The diversity associated with this functional group is reflected in the contributions from all of the traditional areas of chemical research, including organic, inorganic, physical, and medicinal chemistry.

Guanidine is a structural motif present in a large number of biologically active compounds such as peptides or peptide mimetics that exhibit appealing properties and play important roles in medicinal chemistry.

An account is provided of how the destabilization of a delta bond in quadruple bonded species by bicyclic guanidinate ligands leads to record low ionization energies of thermally stable compounds and concomitant production of very strong reducing agents. This paper commemorates the 50th anniversary of the seminal paper published on September 18, 1964 in Science describing how interactions between d-orbitals lead to a quadruple bond in species with idealized D_4h symmetry.

The latest advances on CO₂ capture using guanidine and their derivatives as absorbents are summarized. In particular, guanidine-catalyzed transformation of CO₂ to a series of value-added chemicals with mechanistic consideration on a molecular level will be particularly elaborated in this article.

The use of guanidinate ligands for chemical vapour deposition (CVD) and atomic layer deposition (ALD) of thin films over the past 5 years is reviewed. Design trends for metal complexes, promising precursor candidates, and the future outlook for guanidinates in ALD and CVD are reported.

Two N-acetylated α-fluoro and α,α-difluoro-β³-arginine esters, as well as their non-fluorinated analogue, were prepared in enantiomerically pure form from a common aldehyde precursor. The ability of these β³-arginine derivatives to inhibit bovine trypsin was examined.

Competitive crystallisation of guanidinium and sodium sulfonates may be done with high selectivity.

Reactivity of a Fe^I–Fe^I multiply bonded species towards a range of unsaturated small molecule substrates was investigated. Depending on the substrate involved, either iron coordination, reductive disproportionation, reductive cleavage, or reductive coupling processes occurred.

A series of guanidine-bisurea bifunctional organocatalysts, in which the chiral centres are located outside the urea groups, catalyzed the α-hydroxylation of tetralone-derived β-keto esters in up to 99 % yield with up to 51 % enantiomeric excess.

One equivalent of lithium 2,2-dialkyl-1,3-dicyclohexylguanidinate was reacted with one equivalent of Group 11 halide (CuCl, AgBr, and AuCl) to generate oligonuclear complexes with the general formula {M[CyNC(NR₂)NCy]}_n where {M, n} = {Cu, 2}, {Ag, 3}, and {Au, 2}.

An unexpected new guanidinate expansion reaction was uncovered in which a Ga^III gaunidinate 2 adds to the carbodiimide, ⁱPrN=C=NⁱPr, to give [ⁱPrN{C(NⁱPr)=NⁱPr}{C(=NⁱPr)NⁱPr₂}GaCl₂] (4), containing an unusual bis(diguanidinate) ligand.

A series of new energetic salts based on 1,2-dinitroguanidine were successfully synthesised and fully characterised.

The redox-active guanidine-functionalized 1,2,4,5-tetrakis(N,N’-diisopropylguanidino)benzene was recently shown to be a redox-active switch, and reversibly forms hydrogen-bond aggregates upon two-electron oxidation. In this work, the hydrogen-bond motifs in the first coordination compounds with the neutral and oxidized guanidine as ligand are reported.

Predicting acid/base properties of unknown molecules using data which can be tabulated is an everlasting goal in chemistry. In this sense, molecules containing intramolecular hydrogen bond are especially problematic. Our results show very good correlation of GB against pK_HB and newly introduced σ^4B parameter for a series of guanidine bases with intramolecular hydrogen bond.

Asymmetric guanidinate-supported zirconium complexes with different degrees of substitution can be easily prepared by reaction of the simple precursors ZrCl₄ or [ZrCl₂(η⁵-C₅H₅)₂] with the corresponding lithium guanidinate salt, or by reaction of [Zr{κ²N,N′-(NEt)(N-t-Bu)CNMe₂}₂Cl₂], obtained by insertion of carbodiimide into a Zr–amido bond, with NaC₅H₅.

The structures of four compounds containing the bidentate ligand C(SiMe₃)₂(SiMe₂{hpp}) (R, where hppH = 1,3,4,6,7,8,-hexahydro-2H-pyrimido[1,2-a]pyrimidine) are described. The lithium derivative LiR (1), which shows Li···H₃C interactions in the solid-state, may be converted into the mercury compound HgRCl (2) and the tin derivatives SnRCl (3), and SnR(P{H}Ar*) (4, Ar* = 2,4,6-tBu₃C₆H₂) with a rare terminal-phosphanide ligand.

The reaction of monolithiated amidines and guanidines with Cp₂M (Cp = cyclopentadienyl; M = Mn, Ni) yields a variety of new organometallic complexes; for the use of Cp₂Ni, kinetically unprotected paddle-wheel homodimers of formulation Ni₂L₄ (L = amidinate, guanidinate) are obtained.

Decyloxybiphenyl guanidinium ionic liquid crystals self-assemble into smectic A (SmA) phases regardless of the substitution pattern of the phenylguanidinium head group, but a minimum of C6 alkyl spacer length is required. The cyanobiphenyl analogues are surprisingly non-mesogenic.

The general principle of asymmetric organocatalysis using chiral bicyclic guanidine as a bifunctional catalyst is illustrated in a density functional theory study of [5,5]-bicyclic guanidine-catalyzed Michael addition of dimethyl malonate to cyclopentenone. Two types of bifunctional activation modes were examined. The calculated enantioselectivity is in excellent accord with experiment result.

The first lithium-aziridinylamidinates, Li[(C₂H₄N)C(NR)₂]·THF (R = ⁱPr, Cy; ⁱPr = isopropyl, Cy = cyclohexyl), have been prepared by addition of N-aziridinyllithium, C₂H₄NLi, to either N,N′-diisopropylcarbodiimide or N,N′-dicyclohexylcarbodiimide. In the solid state, these lithium-aziridinylamidinates comprise ladder-type dimeric molecular structures.

A novel tartrate-based guanidine molecule was identified as a suitable promoter for the enantioselective fluorination of 1,3-dicarbonyl and α-cyano carbonyl compounds to furnish the fluorinated product with up to 84 % ee and 99 % yield using N-fluorobenzenesulfonimide as the fluorinating agent.

By utilising a chiral bicyclic guanidine as catalyst, the first asymmetric Michael addition of α-fluoro-β-ketoesters to various cyclic enones has been successfully developed, affording a variety of Michael adducts with potential synthetic utilities with satisfactory stereoselectivity (up to 94 % ee and 4.3 : 1 dr).

Uncharged chiral bis(guanidino)iminophosphorane organosuperbase catalysts substituted by bulky aryl groups were synthesized and evaluated as chiral organosuperbase catalysts in the enantioselective amination of 2-methyltetralone.

A synthetic approach towards peralkylated triguanide superbases based on a coupling reaction between a Vilsmeier salt and monosubstituted guanidines is described. Single crystal X-ray diffraction enabled the first structural characterization of benzyloctamethyl derivative, which was also evaluated as a potential organocatalyst in the transesterification reaction of vegetable oil.

The reaction of N-arylcyanamides, obtained from corresponding nitriles via Tiemann rearrangement, with various primary and secondary alkylamines under the catalysis of copper(I) iodide and Xantphos in DMF afforded a wide variety of N-alkyl-N′-arylguanidine derivatives.