'Fensic' Tris[bis(trimethylsilyl)amido]iron(III) or [Fe(N(SiMe3)2)3] A pioneer of three-coordination Simon Cotton University of Birmingham Molecule of the Month September 2025 Also available: HTML version.

What’s special about that?

It’s got three-coordinate iron.

Well, that’s nothing special, just think of BF₃.

An atom bound to three other atoms is found often with non-metals, like the BF₃ you mention, or ammonia, NH₃ (MOTM June 2013). The ‘coinage metals’ like Cu, Ag and Au in the (+1) oxidation state, and the succeeding Group IIB metals such as Cd and Hg in the (+2) state, have a few examples of such low coordination numbers.

Such as?

Examples include [Au(CN)₂]^- (a species used in gold extraction), [AuCl(PPh₃)₂], Hg(CH₃)₂ (MOTM October 2003) and [HgI₃]^-.

Various species with low coordination numbers.

So?

The metals I mentioned have the d¹⁰ electron configuration, where such species with two- and three-coordination are known.

And?

But such behaviour was unknown among the typical transition metals until half a century ago. Six coordination, and to a lesser extent four and five coordination, were the norm among compounds of these metals.

So what changed? Professor Don Bradley (1924-2014), of Queen Mary College, London, had made a speciality of studying transition metal alkoxides, [M(OR)n] since the days of his PhD; this remained an area of active interest throughout his research career. In the 1960s he extended this to the study of transition metal alkylamides, initially of the early transition metals like tungsten. In 1963, two German chemists named Bürger and Wannagat reported the synthesis of the very dark-green, almost black, air and moisture-sensitive [Fe(N(SiMe3)2)3]. Some years later, Don’s research group undertook the study of this compound and also made compounds of the bis(trimethyl)silylamide ligand with other 3d metals, also obtaining [M(N(SiMe3)2)3] (M = Sc, Ti, V, Cr); the Mn and Co compounds, completing the series for metals with d0-d6 electron configurations, were made by others some years later. What did they find? Crystal structures of these compounds, first obtained for [Fe(N(SiMe3)2)3] and reported in 1969, showed that the metal was bound to three nitrogen atoms, in a trigonal planar geometry. Evidently the very bulky SiMe3 groups were occupying so much space round the iron atom that no other ligands could approach.

Professor Don Bradley

Was the compound easy to make?

The synthesis employs the reaction between FeCl₃ and LiN(SiMe₃)₂, in a ‘salt-elimination reaction’ (in THF or benzene-ether), as you might expect. The volatile product of the reaction can be purified by sublimation in vacuo at around 100°C; its volatility is in accord with its simple molecular structure.

But things are not as simple as the expected equation (below) would suggest:

FeCl₃ + 3 LiN(SiMe₃)₂ [Fe(N(SiMe₃)₂)₃] + 3 LiCl

How is that?

The reaction by-product, expected to be just LiCl, contains iron(II) impurities. The most recent study of the synthesis has identified Li₂FeCl₄ and Li[Fe{N(SiMe₃)₂}₃] among the by-products. On reflection, this is not too surprising, as FeCl₃ is an oxidant, whilst LiN(SiMe₃)₂ is a reductant.

What else is special about the compound?

The electronic and magnetic properties are interesting. The magnetic moment is 5.91 μB, as expected for high-spin iron(III). The EPR spectrum shows that it is a high-spin iron(III) compound with a strong trigonal distortion, as the structure indicates. It is also a single-molecule magnet (SMM) at low temperatures. But I think of this compound as 'Fensic'.

How so?

Well, the crystallographers at Queen Mary College, London, who first determined the structure of the compound, used six-letter labels to identify the compounds that they were studying. They called it 'Fensic'. This reflected the atomic linkage Fe-N-Si-C, but was also a play on the name of Phensic, a popular painkilling medicine, which was used by James Bond in Ian Fleming’s Moonraker and Thunderball.

What did Phensic contain?

For many years it used phenacetin as the active ingredient, which was synthesised about the same time as paracetamol (MOTM September 2020).

Phenacetin		Phensic painiller advert

Phenacetin probably owed its action to its decomposition in the body to paracetamol, so that it acted as a prodrug. However, it could also lead to the formation of the carcinogen p-phenetidine, and around 1970 this formulation of Phensic was withdrawn from use.

It has also been suggested that phenacetin could have been responsible for the premature death of Howard Hughes, the film magnate (and flying-boat designer).

And is it anything to do with the ska/reggae group The Phensic?

No, but that is not all that should be said about the significance of [Fe(N(SiMe₃)₂)₃].

How's that?

The discovery that the bulky –N(SiMe₃)₂ ligand could congest the environment around a metal so much that three coordination was possible, led Don Bradley to look into the possibilities with other metals. Within two years, Joginder Singh Ghotra, a talented postdoctoral worker, had synthesised [Ln(N(SiMe₃)₂)₃] for yttrium and all the lanthanides except for the short-lived promethium.

The slightly larger lanthanide ions in these compounds can form a four-coordinate phosphine-oxide adduct [Ln(N(SiMe₃)₂)₃.Ph₃PO], something that the 3d metals cannot do in their silylamides.

The compounds [Ln(N(SiMe₃)₂)₃] differ in that in the solid state (but not in solution) they have a slightly pyramidal geometry at the lanthanide (though still being three coordinate). Calculations indicate that it is β-Si-C agostic interactions with the lanthanide that is responsible for this small pyramidal distortion, though others think that it is down to steric congestion. But that is another story.

[Ln(N(SiMe₃)₂)₃], where Ln is a lanthanide metal.

Richard Andersen was later to extend three-coordination to [U(N(SiMe₃)₂)₃]. So this iron compound links with the burgeoning research in low-coordination in lanthanide and actinide chemistry.

In memory of Don Bradley FRS and of Dick Copperthwaite.

Bibliography

• Chapman and Hall Combined Chemical Dictionary code number: JQF96-G

Don Bradley

• P. O’Brien, Coordination Chemistry Reviews, 2000, 209, 35–47. (‘Celebration of Inorganic Lives: Interview with Donald C. Bradley’)
• A. C. Sullivan and J. M. Thomas, Biogr. Mems. Fell. R. Soc., 2019, 67, 119–141.
• https://en.wikipedia.org/wiki/Donald_Charlton_Bradley

Synthesis

• H. Bürger and U. Wannagat, Monatsh. Chem., 1963, 94, 1007–1012.
• E.C. Alyea, D.C. Bradley and R.G. Copperthwaite, J. Chem. Soc. Dalton Trans., 1972, 1580–1584.
• D.C. Bradley and R.G. Copperthwaite, Inorg. Synth., 1978, 18, 112–120.
• J.J. Ellison, P.P. Power and S.C. Shoner, J. Am. Chem. Soc., 1989, 111, 8044-8046 (Mn and Co analogues).

Crystal structure

• D.C. Bradley, M.B. Hursthouse and R.F. Rodesiler, J. Chem. Soc. Chem. Commun., 1969, 14–15.
• M.B. Hursthouse and P.F. Rodesiler, J. Chem. Soc. Dalton Trans. 1972, 2100–2102.
• S. Indris, M. Knapp, B. Schwarz and A. Eichhöfer, Eur. J. Inorg. Chem., 2021, 951–959.

Magnetism and EPR

• E.C. Alyea, D.C. Bradley, R.G. Copperthwaite and K.D. Sales, J. Chem. Soc. Dalton Trans. 1973, 185–191.
• D.C. Bradley, R.G. Copperthwaite, S.A. Cotton, J.F. Gibson and K.D. Sales, J. Chem. Soc. Dalton Trans. 1973, 191–194 (EPR)
• N. Ge, Y-Q. Zhai, Y-F. Deng, Y-S. Ding, T. Wu, Z-X. Wang, Z. Ouyang, H. Nojiric and Y-Z. Zheng, Inorg. Chem. Front., 2018, 5, 2486 (crystal structure, EPR and SMM behaviour).

Lanthanide and actinide silylamides

• J.S. Ghotra, M.B. Hursthouse and A.J. Welch, J. Chem. Soc. Chem. Commun. 1973, 669 (structures of [M(N(SiMe₃)₂)₃] (M = Sc, Eu, Yb))
• D.C. Bradley, J.S. Ghotra and F A. Hart, J. Chem. Soc., Dalton Trans., 1973, 1021-1023 (syntheses of [Ln(N(SiMe₃)₂)₃] (Ln = La-Lu except Pm)
• D.C. Bradley, J.S. Ghotra, F.A. Hart, M.B. Hursthouse and P.R. Raithby, J. Chem. Soc., Dalton Trans., 1977, 1166-1172 ([Ln(N(SiMe₃)₂)₃. Ph₃PO]
• R. A. Andersen, Inorg. Chem., 1979, 18, 1507-1509 (synthesis of [U(N(SiMe₃)₂)₃])
• J.L. Stewart and R.A. Andersen, Polyhedron, 1998, 17, 953–958 (structure of [U(N(SiMe₃)₂)₃])

Coordination numbers in d- and f-block metal compounds

• P. G. Eller, D.C. Bradley, M.B. Hursthouse and D.W. Meek, Coord. Chem. Rev., 1977, 24, 1–95 (three coordination in metal complexes)
• M.A. Carvajal, J.J. Novoa and S. Alvarez, J. Am. Chem. Soc., 2004, 126, 1465–1477 (choice of coordination number in d¹⁰ complexes of Group-11 metals)
• S.A. Cotton, C.R. Chimie, 2005, 8, 129–145 (‘Establishing coordination numbers for the lanthanides in simple complexes’)
• P.P. Power, Chem. Rev., 2012, 112, 3482–3507 (two coordination in open-shell (d¹–d⁹) transition metal complexes)
• F. Ortu and D.P. Mills, Handbook on the Physics and Chemistry of Rare Earths. 2019, 55, 1-87 (review on low coordinate rare earth and actinide complexes)

Phensic, phenacetin and Howard Hughes

• Wikipedia - Phenacetin
• Wikipedia - Howard Hughes

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