The ferraboratrane [κ4-B(mim^But)3]Fe(CO)2 (mim^But = 2-mercapto-1-tert-butylimidazolyl) contains a crystallographically verified iron-to-boron retrodative σ-bond, and its discovery revealed that reagents can add formally across this bond: one fragment is delivered to iron and the other to boron, the Fe→B contact is eradicated, and B-functionalized tris(mercaptoimidazolyl)borate iron complexes [XTm^But]FeY result. The mechanisms of these formal 1,2-additions (dibenzoyl peroxide, haloforms CHCl3/CHBr3, and I2 in CHCl3) are explicitly unknown — the mixed-addition product [ClTm^But]FeI argues against a single concerted pathway. The source also reports that the ferraboratrane forms via alkylation, CO-induced ligand rearrangement, and B–H cleavage, with the organoiron intermediate [Tm^But]FeCH2SiMe3 never isolated; these formation details, and the range of boron functionalization the additions provide, are context for the mechanistic question rather than its target. No published work establishes the mechanisms of these additions for this system. The mechanistic problem remains open.

Background

Tris(2-mercapto-1-R-imidazolyl)hydroborato ligands, abbreviated [Tm^R], are 'soft scorpionate' tripods: three imidazole-2-thione sulfur donors bind a metal in a facial arrangement, while the substituent on the apical boron (normally hydrogen or a hydrocarbyl group) points along the threefold axis. When the coordinated metal is sufficiently electron-rich, it can donate a filled d-orbital pair into the empty orbital of the boron, converting the ligand into a so-called metallaboratrane: a cage in which boron caps the metal–sulfur face through a metal-to-boron dative ('retrodative') σ-bond. In the covalent bond classification used by organometallic chemists, boron here acts as a Z-type ligand — a Lewis acid accepting electron density — so the M→B bond is the inverse of the familiar dative ligand→metal donation. Metallaboratranes are therefore model systems for asking what chemistry a dative bond from a transition metal to a main-group acceptor can support.

Figueroa, Melnick, and Parkin (Inorg. Chem. 2006, 45, 7056) reported the first iron example, the ferraboratrane [κ4-B(mim^But)3]Fe(CO)2, where κ4 indicates coordination of the three sulfur donors plus the boron acceptor. It is prepared from [Tm^But]FeCl by alkylation with LiCH2SiMe3 followed by exposure to carbon monoxide; crystallography gives an Fe–B separation of 2.108(6) Å, taken as a definitive Fe→B σ-interaction. The proposed formation sequence passes through the organoiron intermediate [Tm^But]FeCH2SiMe3, which has never been isolated (concentrating the mixture causes ligand redistribution); its transient existence is supported by a trapping experiment, in which adding a bulky phenol affords [Tm^But]FeOAr and tetramethylsilane, consistent with protonolysis of an Fe–CH2SiMe3 bond. A pyridine adduct, [κ3-H,S,S-Tm^But]FeCl(py), was structurally characterized with one imidazolyl arm dissociated from boron and a three-center two-electron B–H–Fe interaction (a bond in which one hydrogen bridges boron and iron and is bound by a shared electron pair); this adduct serves as a model for how CO coordination might promote B–H bond cleavage en route to the ferraboratrane.

The central discovery is that reagents add formally across the Fe→B bond, in close analogy to 1,2-addition across a polarized metal–ligand multiple bond: one fragment is delivered to iron, the other to boron, and the Fe→B contact is eradicated. Dibenzoyl peroxide gives [PhC(O)OTm^But]Fe[κ2-OC(O)Ph], with benzoate installed at both boron and iron; CHCl3 and CHBr3 give [XTm^But]FeX (X = Cl, Br); and 0.5 equivalents of I2 in CHCl3 give the mixed-addition product [ClTm^But]FeI, in which chlorine (attributed to the solvent, though the source was not experimentally determined) resides on boron and iodine on iron. These reactions constitute a route to B-functionalized [XTm^But] ligands — previously the boron substituent in tris(mercaptoimidazolyl)borate chemistry was essentially limited to hydrogen, methyl, and phenyl — while preserving the synthetically valuable tripodal [S3] donor set. The authors state that the mechanisms of these additions are not known, and the mixed-addition product argues against a single concerted pathway; plausible manifolds include discrete radical intermediates, stepwise heterolytic transfer, and other stepwise routes.

The question sits inside a broader effort to exploit Lewis-acidic ligand functions in cooperative bond activation. In other ligand platforms — a nickel–borane pincer studied by Harman and Peters, the tris(phosphino)borane iron systems of Peters and co-workers, the phosphine-tethered iridaboratranes of Kameo and Nakazawa, and the platinum (boryl)iminomethane system of Barnett and Figueroa — H2 addition, hydride migration, and oxidative insertion across M→B or M–B units have been observed and, in some cases, mechanistically characterized. Braunschweig and co-workers have likewise described 1,2-additions across transition metal–boron double bonds of borylene complexes. None of these systems, however, uses the ferraboratrane framework, and none addresses the peroxide, haloform, or iodine additions discovered by Figueroa, Melnick, and Parkin.

Problem Statement

Determine, with direct evidence, how reagents effect formal 1,2-addition across the Fe→B dative σ-bond of the ferraboratrane [κ4-B(mim^But)3]Fe(CO)2 (mim^But = 2-mercapto-1-tert-butylimidazolyl; [Tm^But] = tris(2-mercapto-1-tert-butylimidazolyl)borate, the hydroborato precursor of the ferraboratrane cage). For each demonstrated reagent class — diacyl peroxides (dibenzoyl peroxide giving [PhC(O)OTm^But]Fe[κ2-OC(O)Ph]), haloforms (CHCl3 and CHBr3 giving [XTm^But]FeX, X = Cl, Br), and I2 in CHCl3 (giving the mixed-addition product [ClTm^But]FeI) — identify the operative pathway among discrete radical intermediates, stepwise heterolytic transfer, concerted addition, and other mechanistic manifolds. This includes establishing the origin of each atom installed at boron and at iron (in particular whether the chlorine in [ClTm^But]FeI derives from the solvent, as assumed but never tested), and the factors that decide whether addition is symmetric (identical groups at B and Fe) or mixed (different groups at B and Fe).

The formation sequence of the ferraboratrane itself (alkylation of [Tm^But]FeCl, CO coordination, ligand rearrangement, and B–H cleavage, including the unisolated intermediate [Tm^But]FeCH2SiMe3) and the range of substituents installable at boron are described in the Background as reported results and context from the source; they are not mandatory completion requirements of this problem.

An answer suffices when each demonstrated reagent class (dibenzoyl peroxide; CHCl3 and CHBr3; I2 in CHCl3) has a mechanistic assignment for the formal 1,2-addition across the Fe→B bond, the origin of each atom installed at boron and at iron is established, and the factors that decide whether addition is symmetric or mixed are identified, each supported by discriminating evidence — for example, kinetics, intermediate observation or trapping, isotopic or elemental labeling, and/or located transition states.

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