Bhambri, Sameer; (1998) Ruthenium(arene){poly(pyrazolyl)borates}: Synthesis, reactivity and molecular orbital calculations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is concerned with the exploration of the syntheses and reactivities of a class of mixed sandwich complexes: ruthenium(η6-arene){poly(pyrazolyl)borates} and methanes. In Chapter One background to the previously explored chemistry of poly(pyrazolyl)borates as a ligand class is presented along with reports associated with ruthenium derivatives. Chapter Two details the synthesis and characterisation of a range of mixed sandwich complexes of the formula [Ru(η6-arene){HX(Pz)3}], X=B, n=l, arene = benzene [1], para-cymene [2], para-xylene [3], para-diisopropylbenzene [4], mesitylene [5], durene [6], hexamethylbenzene [7]; X = C, n=2, arene = benzene [9], para-cymene [10], para-xylene [11], mesitylene [12], The moderate reaction conditions employed not only allow the successful isolation of the previously elusive [Ru(η6 - C6H6){κ3-HB(3,5-Me2Pz)3}][PF6] [8] but also permit the formation of a range of half sandwich complexes [Ru(η6-l,4-Me2C6H4){κ2-HB(Pz)3}Cl] [13], [Ru(η6-1,3,5-Me3C6H3){κ2-HB(Pz)3}Cl] [14], [Ru(η6-C6Me6){κ2-HB(Pz)3}Cl] [15], [Ru(η6-C6H6){κ2-HC(Pz)3}Cl][PF6] [16], [Ru(η6-C6H6){κ2-HC(3,5-Me2Pz)3}Cl][PF6] [17], [Ru(η6-1,4-Me2C6H4){κ2-HC(3,5-Me2Pz)3}Cl][PF6] [18], which can be converted into the tris-chelated derivatives by treatment with NH4PF6 or AgPF6. Details of the X-ray structures of [2], [8], [10], [15] and [17] are presented. In Chapter Three compounds resulting from degradation of the mixed sandwich complexes are reported, namely [Ru(η6-C6H6{κ3-N,N,O-HB(3,5-Me2Pz)2(OEt)}][PF6] [20], [Ru(η6-1,2,4,5-Me4C6H2)(3,5-Me2PzH){κ2-HB(3,5-Me2Pz)2(OMe)}][PF6] [21] and [Ru(η6-C6H6){κ2-HN-C(Me)=(3,5-Me2PzH)}][PF6] [22]. While [20] and [22] have been comprehensively characterised [21] was only characterised through X-ray diffraction experiments. The tendency towards degradation is attributed to the large interligand repulsions between the methyl substituents on the pyrazolyl groups and the hydrogen atoms on the arene ligands. A mechanism is proposed for the speculative origin of these products. In Chapter Four when complex [1] is subjected to nucleophihc attack with Na[BH4], NaCN, KOH and Na[BD4], neutral compounds of the type [Ru(η5-C6H6Y){κ3-HB(Pz)3}], Y=H [23], CN [24], OH [25] and D [26] are formed. The dynamic NMR behaviour of these compounds is attributed to restricted rotation of the carbocycle. When nucleophiles are added to compounds bearing alkylated arenes higher rotational barriers are observed for the derived cyclohexadienyl compounds, [Ru(η5-CHD){κ3-HB(Pz)3}], CHD = [1-iPr-4MeC6H5] [27], [l-iPr-4-MeC6H4CN] [28], [l-iPr-4-MeC6H4OH] [29], [1,4-Me2C6H5] [30], [1,4-iPr2C6H5] [31], [1,3,5-Me3C6H4] [32] and [1,3,5-Me3C6H3CN] [33]. Similar observations are also made when the complexes [8]-[12] are treated with KCN and Na[BH4] to yield, [Ru(η5-CHD){κ3-HC(Pz)3}][PF6], CHD= [C6H6CN] [34], [iPr-4MeC6H4CN] [35], [1,4Me2C6H4CN] [36] and [Ru(n5-C6H7){κ3-HB(3,5-Me2Pz)3}] [37]. Details of X-ray structures of [24] and [34] are presented. Chapter Five attempts to substantiate the plausibility of restricted rotation of the cyclohexadienyl ligands being responsible for the dynamic NMR behaviour. This is carried out through Extended Huckel molecular orbital treatment of a series of ruthenium(arene)/(cyclohexadienyl){poly(pyrazolyl)borate} compounds. A variety of arenes and cyclohexadienyl ligands are subjected to rotational motion while attached to a static ruthenium {tris(pyrazolyl)borate} fragment, and their overall energies monitored. Although in general the extent of the rotational barrier increase as the number and size of the alkyl substituents increases, the actual differences within a series of mono, bis, or tris-alkylated cyclohexadienyls is very much dependent upon the positioning of the substituents on the carbocycle.

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