Borane, boryl and borylene complexes of electron rich metal centres
Keywords: Borane
Boryl
Borylene

The synthesis and characterisation of a series of novel borane, boryl and borylene complexes of electron rich group 8 and 9 metal centres are described in this thesis.
Chapter 3 reports on the properties of a highly nucleophile tolerant borylene system, [CpFe(dmpe)(BNMe₂)]⁺, together with its surprising formation via an unprecedented spontaneous halide ejection process. The incorporation of strongly electron releasing ancillary phosphine ligands is reflected by an Fe-B distance (ca. 1.80 Å) which is more akin to alkyl/aryl substituted borylene complexes, and perhaps more strikingly, by the very low exothermicity associated with the binding of pyridine to the two-coordinate boron centre (∆H = -7.4 kcal mol⁻¹ cf. -40.7 kcal mol⁻¹ for BCl₃). Despite the strong π electron release from the metal fragment implied by this suppressed reactivity and short Fe-B bond, the barrier to rotation about the Fe=B bond in the asymmetric variant [CpFe(dmpe){BN(C₆H₄OMe-4)Me}]⁺ is very small (ca. 2.9 kcal mol⁻¹). This apparent contradiction is rationalised by the orthogonal orientations of the HOMO and HOMO-2 orbitals of the [CpML2]⁺ fragment, which mean that the M-B π interaction does not fall to zero even in the highest energy conformation.
The reactivities of the aminoboryl complexes, CpFe(CO)₂B(NR₂)Cl (R = Me, Cy), towards electrophiles (H⁺, Me⁺) are discussed in Chapter 4, with a view to probing potential modification of the boryl ligand substituents. The reaction of CpFe(CO)₂B(NCy₂)Cl with [Me₃O][BF₄] leads to the formation of CpFe(CO)₂B(NCy₂)F. Subsequent reactivity with Brookhart’s acid results in the formation of the known difluoroboryl system CpFe(CO)₂BF₂. Reaction of the dimethylaminoboryl complex CpFe(CO)₂B(NMe₂)Cl with [Me₃O][BF₄] generates CpFe(CO)₂BF₂ directly; however, reaction of CpFe(dmpe)B(NMe₂)Cl with [Me₃O][BF₄] is limited to the formation of CpFe(dmpe)B(NMe₂)F, presumably on steric grounds. Additionally, given the enhanced stability of the bis(phosphine) ligated systems, [CpM(PR₃)₂(BNR2)]⁺ compared to related dicarbonyl ligated complexes, it has also proved possible to synthesise other borylene complexes e.g. [CpFe(dmpe)(BOMes)]⁺ which are otherwise unstable under ambient conditions.
Chapter 5 reports the coordination and B-H bond activation of aminoboranes at ruthenium and iridium metal centres. Reaction of aminoboranes, H₂BNR₂, with 14-electron fragments of the type [Cp*RuL]⁺, leads to κ² coordination. The interaction with 16- electron fragments, [CpRu(PR₃)₂]⁺, has also been probed. In contrast to side on-binding of isoelectronic alkene donors, an alternative κ¹-(σ-BH) mode of aminoborane ligation has been established, albeit with binding energies only ~ 8 kcal mol⁻¹ greater than for those for analogous dinitrogen complexes. Variations in ground-state structure and exchange dynamics as a function of the phosphine ancillary ligand set are consistent with chemically significant back-bonding into an orbital of B-H σ* character. By contrast, simple borane coordination compounds prove difficult to isolate on addition of aminoboranes, H₂BNR₂, to in situ generated sources of [(p-cymene)Ru(PR₃)Cl]⁺; spontaneous loss of HCl to generate a rare class of primary hydridoboryl complexes is witnessed instead. Attempts to synthesise boryl complexes via simple oxidative addition of monomeric aminoboranes have also proved successful, through the use of electron rich iridium precursors containing the [Ir(PMe₃)₃] fragment. This step results in the synthesis of novel amino(hydrido)boryl complexes, Ln(H)M{B(H)NR₂}; subsequent conversion (on loss of an ancillary ligand) to a borylene dihydride system proceeds via a novel B-to-M α hydride migration. The latter step is unprecedented for group 13 ligand systems and is remarkable in offering α-substituent migration from a Lewis acidic centre as a route to a two-coordinate ligand system.