Tripodal Inorganic 2-Pyridyl-phosphine Ligands and Unsymmetrical Analogues: Towards New Applications in Catalysis
Creators: name: Hanf, Schirin
Publisher: name: Apollo - University of Cambridge Repository
Year: 2019
Subjects: pyridyl-phosphines
catalysis
inorganic chemistry
coordination chemistry

description: Although a range of phosphorus-based ligands are used extensively in homogeneous catalysis, 2-pyridyl-phosphines have been largely overlooked in this field. However, such ligands offer great potential for catalytic applications through the combination of two ligand sites with different donating and accepting properties. In Chapter 1 previous studies of the structural and coordination chemistry of 2-pyridyl main group ligands are discussed, together with the applications of phosphine and tris-2-pyridyl ligands in catalysis which are relevant to the studies undertaken in the thesis. This chapter finishes with a statement of the aims of the thesis. Chapter 2 focuses on sterically-constrained C3-symmetric tris-2-pyridyl ligands and the effects of the substitution of the pyridyl rings at the 6-position on coordination to transition metal ions. The impact of such ligand modification is illustrated directly from the coordination studies of P(6-R-2-py)3 (R = H, Me) to iron(II). Whereas P(2-py)3 readily forms the cationic sandwich complex [{P(2-py)3}2Fe]2+, P(6-Me-2-py)3 forms only half-sandwich complexes of the type [{P(6-Me-2-py)3}FeX2] (X = Cl, OTf). In addition to C3-symmetric ligands, a range of unsymmetrical multidentate 2-pyridyl-phosphine ligands with tuneable electronic and steric character have been synthesised and are reported in Chapter 3. The stoichiometric reactions of (R2N)xP(2-py)3−x (R = Me, Et, x = 1, 2) with alcohols result in the formation of (alkoxy)-2-pyridyl-phosphines (RO)xP(2-py)3−x (R = Me, 2-Bu, Ph, x = 1, 2). This synthetic procedure also allows the introduction of enantiomerically pure alcohols, and as such provides a very convenient two-step route to chiral 2-pyridyl-phosphine ligands. Coordination studies towards CuI, NiII and RhI have shown that both the pyridyl-N and bridgehead-P atoms can be involved in coordination to the metal centres. In the case of nickel, an interesting in-situ reduction of the metal centre was observed. Different synthetic and computational approaches to quantify the donor properties of various 2-pyridyl-phosphines are presented in Chapter 4. The calculated Tolman electronic parameters give an overview of the donor properties of all investigated 2-pyridyl-phosphines with respect to PPh3 and P(tBu)3. In Chapter 5, the modification of the phosphorus bridgehead is reported. Air- and moisture-stable phosphorus(V) chalcogenides and fluorinated phosphines were synthesised. Also, a novel one-step procedure for the synthesis of fluorophosphonium salts was developed. Treatment of (Et2N)2P(2-py) with the bench-stable fluorinating reagent NFSI (N-fluorobenzenesulfonimide) leads to the selective formation [FP(NEt2)2(2-py)][N(SO2Ph)2]. Investigations of the applications of tris-2-pyridyl-phosphine complexes for hydrofunctionalisation reactions are explored in Chapter 6. Central to the study are the iron(II) complexes [{P(6-Me-2-py)3}FeCl2] and [{P(6-Me-2-py)3}FeCl(OTf)]. In-situ reduction of the iron complex [{P(6-Me-2-py)3}FeCl(OTf)] potentially results in the formation of Fe0 species, which are active in hydrogenation reactions of minimally functionalised alkenes.