A new series of iron(II) piano stool complexes was synthesized that contain monodentate triazolylidene ligands with different aryl and alkyl substituents as well as an example of a C,N-chelating pyridine-substituted triazolylidene iron complex. The electronic and steric effect of wingtip modification was assessed by electrochemical, infrared spectroscopic, and X-ray diffraction analysis. All complexes were active in the catalytic hydrosilylation of aldehydes and ketones. The monodentate systems outperform the chelating triazolylidene analogue by far, reaching turnover frequencies TOF max as high as 14400 h −1 at 0.1 mol % catalyst loading. Mechanistic investigations indicate a radical mechanism for the catalytic H−Si bond activation.

Two new Rdpt ligands featuring long "tails", padpt (N-4H-1,2,4-triazole-3,5-di(2-pyridyl)palmitamide) and hpdpt (4-(4-heptadecafluoroctylphenyl)-3,5-bis(2-pyridyl)-4H-1,2,4-triazole), were made and reacted with [Fe(II)(py)4(NCS)2] to give pinkish-red [Fe(II)(padpt)2(SCN)2] (1) and purple-red [Fe(II)(hpdpt)2(SCN)2] (2) as solvent-free crystals. Magnetic measurements reveal that both 1 and 2 exhibit complete and reproducible spin crossovers, with a far lower T1/2 for the amide-alkyl tailed 1 (182 K) than for the fluorocarbon tailed 2 (248 K), which in turn is far lower than the T1/2 of 290 K previously reported for the nonamide-alkyl tailed analogue [Fe(II)(C16dpt)2(SCN)2]·(2)/3H2O (3). Structure determinations for 1 and 2 in both the high spin (HS) and low spin (LS) states confirm the expected trans-NCS conformation and reveal that (a) the "tails" interdigitate and (b) the LS forms are less distorted than the HS forms (Σ = 58-70° vs 47-54°). DSC and Raman spectroscopy confirmed the high tail-dependence of the SCO events in 1 and 2, as well as in 3, with the Raman data giving T1/2 values of 190, 243, and 285 K, respectively. Bright orange single crystals of the solvatomorph [Fe(II)(hpdpt)2(SCN)2]·MeOH·H2O (2solv) were also structurally and magnetically characterized and, in contrast to 2, found to remain HS down to 4 K, providing further evidence of the huge impact of crystal packing on SCO. Both 1 and 2 form stable Langmuir films at an air-water interface, a single layer of which can be transferred to a solid support.

This review summarises the advances of N-heterocyclic carbenes (NHCs) as ligands to iron half-sandwich complexes, often referred to as piano-stool complexes because of their structural motif. The review introduces various synthetic routes towards this sub-class of NHC iron-complexes including in particular direct and transmetallation strategies as well as the synthesis of the carbene directly within the iron coordination sphere. Many of these complexes have demonstrated promising reactivity that is relevant to the arenas of catalysis and materials chemistry. In particular, bond activation (e.g. C-H, Si-H, H-H) processes provide exciting new opportunities to exploit these systems for stoichiometric and catalytic transformations. Substantial advances have been achieved specifically in the catalytic reduction of unsaturated substrates, and this topic is emphasised in particular.

Piano-stool NHC iron complexes catalyze the selective semi-hydrogenation of alkynes without any over-reduction and with high selectivity towards Z-alkynes when starting from disubstituted alkynes.

A new series of amphiphilic alkylurea functionalised iron(iii) sal2trien complexes were prepared by substitution of phenolic ligand site with OCnH2n–NHC(O)NHCmH2m+1 tails (n = 5, 9, m = 4, 12, 14, 16).