Synthesis and Coordination of Bicyclic Phosphorus-Nitrogen Ligands
Authors:   , Zayya Almas Ismaeil 
Published: November 2021

<p>This thesis describes the synthesis and coordination chemistry of bicyclic phosphorus-nitrogen (PN) ligands containing the rigid and preorganised bicyclo[3.3.1]nonan-9-one framework. The PN ligands were prepared via the Mannich condensation reaction of four different phosphorinanone classes with amines and aldehydes. The phosphorinanone compounds, 2,6-dimethyl-3,5-diphenyl-4-phenyl-4- phosphacyclohexanone (isomers 50 and 51), 3,5-diphenyl-4-phenyl-4- phosphacyclohexanone (44, 45) and 4-phenyl-4-phosphacyclohexanone (42) were prepared by literature methods, whereas the isomers of 4-t-butyl-2,6- di(carbomethoxy) - 3,5 - bis(p - dimethylaminophenyl) - 4 - phosphacyclohexanone (53, 54) were synthesised by the reaction of ButPH2 with 2,4-di(carbomethoxy)- 1,5 - bis(p - dimethylaminophenyl)penta - 1,4 - dien - 3 - one (38). The Mannich reactions of phosphorinanones 50 and 51 were not successful, whereas the reactions of 44, 45 and 42 produced unidentifiable products. The reaction of phosphorinanone 53 with methylamine and formaldehyde produced the bicyclic PN compound 7-t-butyl-1,5-di(carbomethoxy)-6,8-bis(p-dimethylaminophenyl)- 3 - methyl - 3 - aza - 7 - phosphabicyclo[3.3.1]nonan - 9 - one (65). The identical Mannich reaction of phosphorinanone 54 also yielded 65, as well as the PN compound 4-t-butyl-6-carbomethoxy-5-(p-dimethylaminophenyl)- 2-methyl-2-aza-4-phosphacyclohexanone (66) and the E/Z isomers of 3-(p-dimethylaminophenyl)methyl-2-propenoate (67). The bicyclic PN ligand 65 adopts a chair-chair conformation in solution and the solid state as confirmed by X-ray crystallography. The coordination chemistry of this ligand was comprehensively explored with rhodium, palladium and platinum, and a wide range of complexes were synthesised including [ML2(65)] (M = Pd, Pt; L = Cl, Me), [ML(65)] (M = Rh, Pd, Pt; L = C2H4, cod, dba, norb) (cod = cycloocta-1,5-diene, dba = trans,trans- dibenzylideneacetone, norb = norborn-2-ene), [Pd(n3 -C3H5)(65)]X (X = Cl, SbF6) and [PtL(65)]CH(SO2CF3)2 (L = 1-o,4-5-n-C8H13, 1-3-n-C8H13). Cycloplatination at the ortho-position of the 6,8-dimethylaminophenyl sub- stituents was an interesting feature of the coordination chemistry of PN ligand 65. Ortho-metallation at both dimethylaminophenyl groups led to the formation of complex [Pt(C2H4)(65-2H)] (76), whereas metallation of only one aryl group produced the complex [Pt(C8H13)(65-H)] (87). Further reaction of complex 76 yielded the trans- and cis-hydroxo-bridged dimers [Pt2(u-OH)2(65-H)2] (98, 101). The nitrogen donor atom is not coordinated to the platinum metal centres in the cyclometallated PN complexes. Protonation of [Pt(C2H4)(65)] (75) with CH2(SO2CF3)2 produced the hydride complex [PtH{CH(SO2CF3)2}(65)] (92) and the agostic ethyl complex [Pt(C2H5)(65)]CH(SO2CF3)2 (93). Similarly, protonation of [Pt(norb)(65)] (74) with CHPh(SO2CF3)2 gave the norbornyl agostic complex [Pt(C7H11)(65)]CPh(SO2CF3)2 (94) as confirmed by X-ray crystallography.  In addition, hydrated analogues of some of the coordination complexes of PN ligand 65 mentioned previously were also observed. In such complexes, the central carbonyl group at position 9 was hydrated to form a geminal diol. The hydrated complexes exhibited similar chemical characteristics to their ketone counterparts. The 15N NMR chemical shifts of the nitrogen donor atom in PN ligand 65 and its various metal complexes were obtained from inversely-detected 1H- 15N HMBC experiments. The NMR data showed no explicit relationship between the coordination mode of the nitrogen group and the 15N chemical shift.</p>