C₅H₄BR₂ Bending in Ferrocenylboranes: A Delocalized Through‐Space Interaction Between Iron and Boron

Abstract

A comparison of the molecular structures of mono-, di- and tetraborylated ferrocenes [Fc{B(R(1))(R(2))}] (R(1)/R(2)=Br/Br, Br/Fc, Br/Me, Me/Me, Me/OH, OMe/OMe), 1,1'-[fc{B(R(1))(R(2))}(2)] (R(1)/R(2)=Br/Br, Br/Me, OMe/OMe), and 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)] revealed the boryl substituent(s) to be bent out of the Cp ring plane towards the iron center. The corresponding dip angle alpha* decreases with decreasing Lewis acidity of the boron atom and with increasing degree of borylation at the ferrocene core. This trend is well reproduced by DFT calculations (including [FcBH(2)], not yet accessible experimentally). A Bader analysis of the electron density topology of [FcBH(2)] (alpha*=26.5 degrees ; BP86/TZVP) clearly showed that there is no direct iron-boron bonding in this compound. Instead, strongly delocalized orbital interactions have been identified that involve the boron p orbital, C(ipso) of the adjacent Cp ring, d orbitals at iron, and a through-space interaction with the second Cp ring. A second important factor is attractive electrostatic interactions, which are enhanced upon ligand bending. Cyclic voltammetric measurements on the series [FcBMe(2)], 1,1'-[fc(BMe(2))(2)], and 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe(2) substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe(II)/Fe(III) redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)], to 1,1'-[fc(BMe(2))(2)], and to 1,1',3,3'-[Fe{C(5)H(3)(BMe(2))(2)}(2)]. In contrast, no significant differences are found between the QS values of ferrocene, [Fc(BMe(2)-DMAP)], and 1,1'-[fc(BMe(2)-DMAP)(2)].

Synthesis and oxidation of hexafluoroisopropoxyborylferrocenes

“…The electron-withdrawing effect of the polyfluorinated groups on the B-O π-bonding is probably responsible for the increase of the dip angle α* of 2.2º if compared to FcB(OMe) 2 . The same trend is observed when studying the Fe-B distance of 3.126(1) Å (FcB(Me) 2 = 3.008(3) Å, FcB(OH)(Me) = 3.056(3) Å, and FcB(OMe) 2 = 3.179(3) Å) [28]. The B atom is coordinated trigonal planar, the sum of angles about B1 is 360º, but both angles around the C ipso -C are significantly different: the C1-B1-O1 angle is compressed with 115.2(1)º compared to the wide C1-B1-O2 angle of 129.3(1)º.…”

“…The dip angle α* of 7.8º is found in the expected range (FcB(Me) 2 = 13.0º, FcB(OH)(Me) = 10.8º, and FcB(OMe) 2 = 5.6º) [28]. The electron-withdrawing effect of the polyfluorinated groups on the B-O π-bonding is probably responsible for the increase of the dip angle α* of 2.2º if compared to FcB(OMe) 2 .…”

“…The B atom is coordinated trigonal planar, the sum of angles about B1 is 360º, but both angles around the C ipso -C are significantly different: the C1-B1-O1 angle is compressed with 115.2(1)º compared to the wide C1-B1-O2 angle of 129.3(1)º. This effect occurs also in FcB(OH) 2 (118.2º and 123.9º), but is more pronounced in ferrocenyl alkoxides like FcB(OMe) 2 (e.g., 114.9º and 127.7º) [28,38]. The Ohfip moieties are twisted out of the plane defined by the five Cp carbons by 15.8º, respectively, as previously observed for ferrocenylboranes [39].…”

“…Another intriguing aspect of ferrocenylboranebased chemistry is the still not well understood through-space B-Fe interaction, which was part of detailed investigations [27,28]. This interaction is linked to the bending of the boryl residue toward the Fe center: the α* dip angle (α* = 180º -α, with α being the angle Cp(centroid)-C ipso -B) is used as an indicator for the Lewis acidity of the boryl moiety.…”

A theoretical study of the aromaticity in neutral and anionic borole compounds

“…This is further reflected in the bending of the borolyl unit toward the metal center in the molecular structures. [46][47][48] While the gasphase optimized geometry matches the experimentally observed structure well (the dip angle α*, defined between the cyclopentadienyl ring plane and the borolyl unit, is 29.4° (exp.) [5] vs. 19.2° (calc.…”