Dissertation/Thesis Abstract

Reaktivitätsstudien an Metalloborylenkomplexen und Eisen-substituierten Borirenen
by Ferkinghoff, Katharina, Ph.D., Bayerische Julius-Maximilians-Universitaet Wuerzburg (Germany), 2015, 182; 27766712
Abstract (Summary)

E) Summary This work aims to investigate the reactivity of the metalloborylene complex [{(η5 C5Me5)Fe(CO)2}(μ-B){Cr(CO)5}] (43) towards other transition metal complexes, a variety of mono- and dialkynes, and several isonitriles. Thereby, discussion of spectroscopical and structural data of the prepared compounds confirms known facts and helps to determine the nature of the metal–boron bond. According to a well-established synthetic protocol, the borido complex [{(η5 C5Me5)Fe(CO)2}(μ-B){W(CO)5}] (73) could be prepared by a double salt-elimination reaction of the dichloroboryl compound 11 and the corresponding metal carbonylate Na2[W(CO)5] in isolated yields of 46% (Figure 107). Figure 107: Synthesis of the borido complex 73. As observed for related metalloborylene species, the borido complex 73 shows the typical linear Fe–B–W moiety (178.5(2)°) as well as a low-field-shifted 11B{1H} NMR (δ = 204.6 ppm) resonance. The borido complex 43 can be employed as a synthetic source of the metalloborylene fragment {(η5 C5Me5)Fe(CO)2(B:)}. The metalloborylene complex is known for its ability to transfer the borylene moiety to both organic and organometallic fragments. Thus, reaction of 43 with the tungsten hydride complex [(η5 C5H5)(H)W(CO)3] leads to the dinuclear hydridoborylene complex [{(η5-C5Me5)(CO)2Fe}(µ-B)(µ-H){CpW(CO)2}] (74) in yields of 52% (Figure 108). This intermetallic borylene transfer offers an alternative synthesis route to the well-known salt-elimination reaction. X-ray crystallographic studies and NMR spectroscopic data confirmed the bridging position of the hydride between the tungsten and the boron center. Figure 108: Synthesis of the borido complex 74. The addition of one equivalent of the zerovalent platinum fragment [Pt(PCy3)2] to the hydridoborylene complexes [{(η5-C5Me5)(CO)2Fe}(µ B)(µ-H){CpM(CO)2}] (M = W (74), M = Mo (75)) results in B–H-bond breakage and the formation of the T-shaped trinuclear borido complexes [{(η5-C5Me5)Fe(CO)}(µ-CO){Pt(PCy3)}{CpM(CO)2}(µ3-B)] (M = W (76), Mo (77)) (Figure 109). The hydrido ligand occupies a bridging position between the M–Pt bond and single crystal X-ray analysis confirmed the absence of a B–H interaction. Single crystal X ray diffraction studies of 76 and 77 revealed M–B bond distances comparable to those found for other borido complexes. In addition, the Pt–B distances are significantly elongated and resemble those found in metal base adducts of other boron species. With an additional equivalent of the [Pt(PCy3)2] fragment it was possible to occupy the last free coordination site at the central boron atom. Thus, the tetranuclear species [{(Cp*)Fe(CO)}(µ-CO){Pt(PCy3)2}{CpM(CO)2}(µ4-B)] (M = W (78), M = Mo (79)) (Figure 109) were obtained in isolated yields of 44% and 30%. As was determined by the solid-state structure, the complex 79 retains the bridging hydride and carbonyl ligands of its precursor 77, adding a [Pt(PCy3)2] fragment to the boron atom held in place by two further bridging carbonyl ligands. The boron atom in 79 is even more distorted from planarity (angular sum around boron: 335.7(7)°) than the two published examples of planar tetracoordinate boron complexes (362.4° and 364.6°). Consequently the four metal atoms and the boron (Fe–B–Mo: 162.5(3)°, Pt–B–Pt: 91.1(2)°) form a saw-horse geometry and not the expected nearly-square-planar coordination mode (Anti-van`t Hoff-Le Bel-compounds). The 31P{1H} NMR spectra shows only one signal in solution, thus the bridging hydride and carbonyl ligands are fluxional at room temperature. Figure 109: Synthesis of the tri- and tetranuclear borido complexes 76-79. Furthermore, the metalloborylene moiety {(η5-C5Me5)Fe(CO)2(B:)} of 43 can also be transferred successfully to alkynes. The thermal borylene transfer has turned out to be applicable to a set of alkynes with different functional groups, expanding the class of ferroborirenes to 81-86 (Figure 110), which were obtained in yields of 24-61%. Figure 110: Synthesis of the ferroborirenes 81-86. The characteristic structural feature of these compounds is a three-membered BCC-ring. The short B–C bonds as well as the long C–C bonds suggests a delocalisation of the two π electrons over a three centered bonding molecular orbital comprised of the pz atomic orbitals of boron and carbon. The first complete characterization of a ferro(bis)borirene (87) was carried out by thermal metalloborylene transfer and drastic reaction conditions. Thus, the reaction of 43 with different diynes leads to the formation of the ferro(bis)borirenes 87-89 (Figure 111). Figure 111: Synthesis of the ferro(bis)borirenes 87-89. Due to the shortening of the C–C single bond between the two boracycles (1.411(3) Å) it can be assumed that there is a strong electronic interaction between the two boracyclopropene rings. Numerous attempts to cleave the Fe–B bond of the ferroborirene 63 with H2, Br2 or HCl to gain access to borirenes with modified properties failed. Additionally, several quaternization attempts of the ring boron atom from 63 with less basic pyridine derivates (3,5-lutidine, 4-(dimethylamino)-pyridine) were unsuccessful. The 11B{1H} NMR spectra showed in all cases only the reactant signal of 63 at δ = 63.4 ppm. The implementation of 63 with a cyclic (alkyl)(amino) carbene also yielded no reaction. Further investigations on the reactivity of 63 showed that it is possible to cleave the Fe–B bond. Treatment of 63 with two equivalents of the N-heterocyclic carbenes IMe, IMeMe and IiPr results in heterolytic Fe–B bond cleavage, yielding the boronium cations 90-92 (Figure 112). In this way, the first borironium salts of a borirene could be obtained. Figure 112: Synthesis of the boronium cations 90-92. Because of the quaternization of the boron atom the structural findings for the boronium cations are interpreted as indicative for annihilation of delocalization of the two π electrons over the three-centered bonding molecular orbital comprised of the pz atomic orbitals of boron and carbon. A further topic of this thesis focussed on the reactivity of manganese boryl complexes towards isonitriles. It turns out that the reaction of the dibromoboryl complex 94 with cyclohexyl- or tert-butylisonitrile leads to the formation of the Lewis-base adducts 95 and 96 (Figure 113). Figure 113: Synthesis of the base adducts 95 and 96. In contrast to the afore-mentioned reactions, treatment of the phosphine-substituted manganese dichloroboryl complex 98 with cyclohexyl- or tert-butylisonitrile leads not to the Lewis adduct formation, but insertion of the isonitriles into the Mn–B bond (Figure 114). In these complexes the former boryl unit is coordinated by the carbon and nitrogen atom of one isonitrile and by the carbon of the second isonitrile, forming a four-membered ring. This compound might be best described as a manganese carbene-like complex. Furthermore, two carbonyl ligands at the manganese were replaced by two isonitriles. Figure 114: Synthesis of insertion complexes 99 and 100. The last objective of this work was the exploration of the reactivity of the borido complex 43 with different isonitriles. While the treatment of the borido complex 43 with tert-butyl- or mesitylisonitrile does not lead to a selective reaction, the reaction of 43 with three equivalents of cyclohexylisonitrile leads to insertion of the isonitriles into the M–B bonds, revealing a [2.3] spiro species 103 (Figure 115). In 103, the boron atom is coordinated to three separate isonitrile units, two of which have been coupled head-to-head. In this case, insertion into the Fe–B single bond shows reactivity similar to that observed with the iron boryl complex 11, while the double insertion into the Cr=B bond is analogous to the reactivity observed for the chromium aminoborylene complex 17. This reactivity, leading to 103, allows an interesting internal comparison of the boryl and borylene functionalities, which show distinct reactivity even within the same molecule. Figure 115: Synthesis of the spiro compound 103. X-ray diffraction of suitable crystals from the treatment of 43 with supermesitylisonitrile confirmed the formation of [(OC)4(Mes*NC)2Cr] (109). This finding, as well as quantum chemical calculations support the formation of 115 (Figure 116). Figure 116: Reaction of 43 with Mes*NC. As we were interested in the nature of the bonding within the three- and four-membered rings of the isonitrile-inserted [2.3] spiro complex 103, further investigations with the strong Lewis acid tris(pentafluorophenyl)borane revealed the formation of the Lewis adduct (C5F5)3B−CNtBu. Furthermore the possibility to selectively add HCl to the B−N bond of the three-membered ring without decomposition of the compound allowed the characterization of 118 (Figure 117). This finding suggests that the B–N bond in 103 can be described as a dative N→B interaction, thus being easier to cleave than the B–C single bond of the three-membered ring.

Indexing (document details)
Advisor: Braunschweig , Holger
Commitee:
School: Bayerische Julius-Maximilians-Universitaet Wuerzburg (Germany)
School Location: Germany
Source: DAI-C 81/7(E), Dissertation Abstracts International
Source Type: DISSERTATION
Subjects: Chemistry
Keywords: Transition metal complexes
Publication Number: 27766712
ISBN: 9781392667248
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