Aerobic Oxidation-Functionalization of the Aryl Moiety in van Koten’s Pincer Complex (NCN)Ni(II)Br: Relevance to Carbon-Heteroatom Coupling Reactions Promoted by High-Valent Nickel Species

The growing popularity of metal-promoted C-H functionalization methodologies has led to increased efforts aimed at improving our understanding of the mechanistic underpinnings of C-C or C-heteroatom bond forming steps. One...

Manganese Catalysed Dehydrogenative Synthesis of Urea Derivatives and Polyureas

Urea derivatives are prevalent intermediates in the synthesis of resin precursors, dyes, agrochemicals, and pharmaceutical drugs. Furthermore, polyureas are useful plastics with applications in coating, adhesive, and biomedical industries and have a current annual market of USD 885 million. However, the conventional methods for the synthesis of urea derivatives and polyureas involve toxic reagents such as (di)isocyanates, phosgene, CO, and azides. We present here the synthesis of (poly)ureas using much less toxic reagents - (di)amines, and methanol via a catalytic dehydrogenative coupling process. The reaction is catalyzed by a pincer complex of an earth-abundant metal, manganese, and liberates H2 gas, valuable by itself, as the only by-product making the overall process atom-economic, and sustainable. A broad variety of symmetrical, and unsymmetrical urea derivatives and polyureas have been synthesized in moderate to quantitative yields using this catalytic protocol. Mechanistic insights have also been provided using experiments and DFT computation suggesting that the reaction proceeds via an isocyanate intermediate.

Late Transition Metal Complexes of Pyridyldiphosphines

<p>This thesis provides an account of research into the properties of pyridyldiphosphines with o-xylene and m-xylene backbones. The coordination behaviour of the o-xylene based ligand with platinum, palladium, silver, rhodium and iridium metal centres has been studied, with an emphasis on whether the presence of the pyridyl rings affects the products formed. Platinum and palladium pincer complexes have been synthesised and the intermediates investigated. The formation of trimetallic complexes with these ligands acting as bridging ligands has also been explored. Two new pyridyldiphosphines, o-C₆H₄(CH₂PPy₂)₂ (3) and m-C₆H₄(CH₂PPy₂)₂ (4), and one known pyridyldiphosphine, PPy₂(CH₂)₃PPy₂ (5), have been synthesised via an improved method. Tris(2-pyridyl)phopshine was reacted with a lithium dispersion to give LiPPy₂, which was then reacted with the appropriate dichloride or dibromide compound to yield the desired ligand. The phosphine selenides of 3 and 4 were synthesised and the ¹J PSe values of 738 and 742 Hz indicated these ligands were less basic than PPh₃. While the ligands themselves were not water-soluble, protonation by a strong acid, such as HCl or H₂C(SO₂CF₃)₃, rendered them soluble in water. A series of [MX₂(PP)] complexes (where M = Pt, X = Cl, I, Me, Et, PP = 3, 5; M = Pd, X = Cl, Me PP = 3, 5) were synthesised. Complexes of 3 displayed dynamic behaviour in solution which was attributed to the backbone of the ligand inverting. When [PtMeCl(PP)] (27) was reacted with NaCH(SO₂CF₃)₂ no evidence for the coordination of the pyridyl nitrogens was observed. The synthesis of a series of unsymetrical [PtMeL(PP)]⁺ complexes enabled the comparison of the cis and trans influences of a range of ligands. The following cis influence series was compiled based on ³¹P NMR data of these complexes: Py ≈ Cl > SEt₂ > PTA > PPh₃. Reaction of 27 with NaCH(SO₂CF₃)₂ and carbon monoxide slowly formed an acyl complex, where the CO had inserted in the Pt–Me bond. The bis-chelated complexes [M(PP)₂] where M = Pt, Pd, and [Ag(PP)₂]⁺ were formed. In these complexes 3 acted as a diphosphine ligand and there was no evidence for any interaction between the pyridyl nitrogen atoms and the metal centre. Reaction of 3 with [Ir(COD)(μ-Cl)]₂ formed [IrCl(PP)(COD)] (42). When the chloride ligand in 42 was abstracted, the pyridyl nitrogens were able to interact with the iridium centre faciliating the isomerisation of the 1,2,5,6-ƞ⁴-COD ligand to a 1-к-4,5,6-ƞ³-C₈H₁₂ ligand. The X-ray crystal structure of [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(PPN)]BPh₄ (43) confirmed the P,P,N chelation mode of the ligand. In solution, 43 displayed hemilabile behaviour, with the pyridyl nitrogens exchanging at a rate faster than the NMR time scale at room temperature. The coordinated pyridyl nitrogen was able to be displaced by carbon monoxide to form [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(CO)(PP)]⁺. A series of [PtXY(μ-PP)]₂ complexes, where X = Y = Cl, Me, X = Cl, Y = Me and PP = 4, were formed initially when 4 was reacted with platinum(II) complexes. When heated, the dimers containing methyl ligands eliminated methane to form [PtX(PCP)] pincer complexes, X = Cl (49), Me (51). When the chloride ligand in 49 was abstracted no evidence of pyridyl nitrogen coordination was observed. Protonation of 49 did not yield a water-soluble pincer complex. The [PdCl₂(μ-PP)]₂ complex readily metallated when heated to give the pincer complex [PdCl(PCP)]. Given pyridyl nitrogen atoms are known to be good ligands for “hard” metal centres, the ability of the pyridyl nitrogens in 3 and 4 to coordinate to metal centres was investigated. While complexes with chloride ligands were found to form insoluble products, the synthesis of [(PtMe₂)₃(PP)], from the reaction of either 3 or [PtMe₂(PP)] (17) with dimethyl(hexa-1,5-diene)platinum, proceeded smoothly through a dimetallic intermediate. The same reactivity was observed in the synthesis of [(PtMe₂)₂PtMe(PCP)]. In contrast, the cationic heterotrimetallic complexes [{M(COD)}₂PtMe(PP)]²⁺ and [{M(COD)}₂PtMe(PCP)]²⁺, where M = Rh or Ir, were synthesised without the detection of any intermediates. However, dimetallic complexes were formed as part of a mixture when 17 or 51 was reacted with one equivalent of the appropriate metal complex.</p>

Late Transition Metal Complexes of Pyridyldiphosphines

<p>This thesis provides an account of research into the properties of pyridyldiphosphines with o-xylene and m-xylene backbones. The coordination behaviour of the o-xylene based ligand with platinum, palladium, silver, rhodium and iridium metal centres has been studied, with an emphasis on whether the presence of the pyridyl rings affects the products formed. Platinum and palladium pincer complexes have been synthesised and the intermediates investigated. The formation of trimetallic complexes with these ligands acting as bridging ligands has also been explored. Two new pyridyldiphosphines, o-C₆H₄(CH₂PPy₂)₂ (3) and m-C₆H₄(CH₂PPy₂)₂ (4), and one known pyridyldiphosphine, PPy₂(CH₂)₃PPy₂ (5), have been synthesised via an improved method. Tris(2-pyridyl)phopshine was reacted with a lithium dispersion to give LiPPy₂, which was then reacted with the appropriate dichloride or dibromide compound to yield the desired ligand. The phosphine selenides of 3 and 4 were synthesised and the ¹J PSe values of 738 and 742 Hz indicated these ligands were less basic than PPh₃. While the ligands themselves were not water-soluble, protonation by a strong acid, such as HCl or H₂C(SO₂CF₃)₃, rendered them soluble in water. A series of [MX₂(PP)] complexes (where M = Pt, X = Cl, I, Me, Et, PP = 3, 5; M = Pd, X = Cl, Me PP = 3, 5) were synthesised. Complexes of 3 displayed dynamic behaviour in solution which was attributed to the backbone of the ligand inverting. When [PtMeCl(PP)] (27) was reacted with NaCH(SO₂CF₃)₂ no evidence for the coordination of the pyridyl nitrogens was observed. The synthesis of a series of unsymetrical [PtMeL(PP)]⁺ complexes enabled the comparison of the cis and trans influences of a range of ligands. The following cis influence series was compiled based on ³¹P NMR data of these complexes: Py ≈ Cl > SEt₂ > PTA > PPh₃. Reaction of 27 with NaCH(SO₂CF₃)₂ and carbon monoxide slowly formed an acyl complex, where the CO had inserted in the Pt–Me bond. The bis-chelated complexes [M(PP)₂] where M = Pt, Pd, and [Ag(PP)₂]⁺ were formed. In these complexes 3 acted as a diphosphine ligand and there was no evidence for any interaction between the pyridyl nitrogen atoms and the metal centre. Reaction of 3 with [Ir(COD)(μ-Cl)]₂ formed [IrCl(PP)(COD)] (42). When the chloride ligand in 42 was abstracted, the pyridyl nitrogens were able to interact with the iridium centre faciliating the isomerisation of the 1,2,5,6-ƞ⁴-COD ligand to a 1-к-4,5,6-ƞ³-C₈H₁₂ ligand. The X-ray crystal structure of [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(PPN)]BPh₄ (43) confirmed the P,P,N chelation mode of the ligand. In solution, 43 displayed hemilabile behaviour, with the pyridyl nitrogens exchanging at a rate faster than the NMR time scale at room temperature. The coordinated pyridyl nitrogen was able to be displaced by carbon monoxide to form [Ir(1-к-4,5,6-ƞ³-C₈H₁₂)(CO)(PP)]⁺. A series of [PtXY(μ-PP)]₂ complexes, where X = Y = Cl, Me, X = Cl, Y = Me and PP = 4, were formed initially when 4 was reacted with platinum(II) complexes. When heated, the dimers containing methyl ligands eliminated methane to form [PtX(PCP)] pincer complexes, X = Cl (49), Me (51). When the chloride ligand in 49 was abstracted no evidence of pyridyl nitrogen coordination was observed. Protonation of 49 did not yield a water-soluble pincer complex. The [PdCl₂(μ-PP)]₂ complex readily metallated when heated to give the pincer complex [PdCl(PCP)]. Given pyridyl nitrogen atoms are known to be good ligands for “hard” metal centres, the ability of the pyridyl nitrogens in 3 and 4 to coordinate to metal centres was investigated. While complexes with chloride ligands were found to form insoluble products, the synthesis of [(PtMe₂)₃(PP)], from the reaction of either 3 or [PtMe₂(PP)] (17) with dimethyl(hexa-1,5-diene)platinum, proceeded smoothly through a dimetallic intermediate. The same reactivity was observed in the synthesis of [(PtMe₂)₂PtMe(PCP)]. In contrast, the cationic heterotrimetallic complexes [{M(COD)}₂PtMe(PP)]²⁺ and [{M(COD)}₂PtMe(PCP)]²⁺, where M = Rh or Ir, were synthesised without the detection of any intermediates. However, dimetallic complexes were formed as part of a mixture when 17 or 51 was reacted with one equivalent of the appropriate metal complex.</p>

Late Transition Metal Complexes of Pentafluorophenylphosphino-Pincer Ligands

<p>This thesis details the synthesis of new examples of electron-poor pincer ligands, featuring bis(pentafluorophenyl)phosphine donors attached to 1,3-substituted phenylene or 2,6-substituted pyridine backbones, to create tridentate PCP and PNP ligands. The effect of the ligands’ electronic nature on the coordination chemistry and ease of pincer complex synthesis with late transition metals is discussed, as is the catalytic activity of the resultant palladium pincer complexes in the Heck and Suzuki reactions. Symmetric PCP and PNP ligands possessing bis(pentafluorophenyl)phosphinite and bis(pentafluorophenyl)phosphoramine functionalities were synthesised by reaction of bis(pentafluorophenyl)phosphine bromide with resorcinol, 3-hydroxybenzyl-di- tert -butylphosphine, 2,6-diaminopyridine, or 2,6-dihydroxypyridine, affording 1,3- [(C6F5)2PO]2C6H4 (POCOPH, 1), 1-[(C6F5)2PO]-3-(tBu2PCH2)2C6H4 (POCCPH, 3), 2,6-[(C6F5)2PNH]2C6H3N (PNNNP, 10), and 2,6-[(C6F5)2PO]2C6H3N (PONOP, 11) respectively. The previously reported 1,3-[(C6F5)2PCH2]2C6H4 (PCCCPH, 2) was also synthesised, with the literature yield improved upon by the use of magnesium-anthracene to generate the required Grignard reagent. The coordination chemistry of the POCOPH ligand 1 with platinum(0) alkene and platinum(II) dimethyl precursors revealed an affinity for the formation of cis-bridged oligomeric structures. The dimer [(POCOPH)Pt(nb)]2 (14, nb = norbornene) was isolated and crystallographically characterised from the reaction between 1 and [Pt(nb)3]. The solid state structure revealed the presence of stabilising - interactions between the aromatic ligand backbones, which were also observed in solution by 1H NMR spectroscopy. Reactions of ligand 1 with platinum and palladium dichloride or chloromethyl starting materials led to rare examples of cis,trans-dimers of the type cis,trans-[(POCOPH)MClX]2 (M = Pd, Pt; X = Cl, Me). In part due to facile dimer formation with 1, metallation of the ligand backbone to form the tridentate pincer complex [(POCOP)PtCl] (25) required long reaction times and high temperatures. It was observed that platinum dichloride starting materials with more strongly binding ancillary ligands were less prone to oligomer formation, and could facilitate more rapid metallation to from 25. More facile pincer complex formation was also observed for more electron-rich ligands with both PCP and PNP pincer ligands. The electron poor platinum and palladium POCOP, PCCCP, and POCCP pincer complexes (where the free ligand had been deprotonated upon metallation) were synthesised and subsequently converted into the metal carbonyl species [(PCP)M(CO)]+. Analysis of C−O stretching frequencies by infrared spectroscopy confirmed complexes of POCOP ligand 1 were the most electron poor, while those of POCCP ligand 3 were the most electron rich. Decarbonylation of the palladium pincer complexes was observed in solution and in the solid state, and was more facile for complexes with a higher wavenumber C−O stretch. Reaction of the [(PCP)PtCl] pincer complexes with methyl nucleophiles revealed that treatment with methylmagnesium iodide resulted in halide exchange, while methyllithium promoted nucleophilic attack at phosphorus. Spectroscopic data indicated that in one instance this led to pentafluorophenyl migration to the metal centre to form a [(PCP)Pt(C6F5)] complex. Dimethylzinc was successful in methylating the platinum PCP complexes; however, it was observed to degrade the palladium PCP pincer complexes. Treatment of the rhodium PNP pincer complex [(PNNNP)RhCl] (49) with dimethylzinc also resulted in degradation, which spectroscopic evidence indicated proceeded via ligand deprotonation and the formation of a zinc adduct of 49. Low temperature protonolysis of the [(PCP)PtMe] species did not reveal any information about possible interactions between the metal and liberated methane. The catalytic activity of the electron-poor [(PCP)PdCl] complexes were assessed in the Heck and Suzuki cross-coupling reactions. The complexes of 1, 2, and 3 were all found to possess only modest activity in the Heck reaction, functioning as precatalysts which decomposed to give catalytically-active Pd(0) colloids. Under milder Suzuki reaction conditions, the most electron-poor complex, [(POCOP)PdCl] (28) proved to be one of the most active pincer catalysts known for this reaction, able to achieve a turnover number of 176,000 for the coupling of electronically-deactivated aryl bromides and phenylboronic acid. Mercury poisoning tests revealed that Suzuki reactions catalysed by 28 proceeded via a homogeneous active species.</p>

Late Transition Metal Complexes of Pentafluorophenylphosphino-Pincer Ligands

<p>This thesis details the synthesis of new examples of electron-poor pincer ligands, featuring bis(pentafluorophenyl)phosphine donors attached to 1,3-substituted phenylene or 2,6-substituted pyridine backbones, to create tridentate PCP and PNP ligands. The effect of the ligands’ electronic nature on the coordination chemistry and ease of pincer complex synthesis with late transition metals is discussed, as is the catalytic activity of the resultant palladium pincer complexes in the Heck and Suzuki reactions. Symmetric PCP and PNP ligands possessing bis(pentafluorophenyl)phosphinite and bis(pentafluorophenyl)phosphoramine functionalities were synthesised by reaction of bis(pentafluorophenyl)phosphine bromide with resorcinol, 3-hydroxybenzyl-di- tert -butylphosphine, 2,6-diaminopyridine, or 2,6-dihydroxypyridine, affording 1,3- [(C6F5)2PO]2C6H4 (POCOPH, 1), 1-[(C6F5)2PO]-3-(tBu2PCH2)2C6H4 (POCCPH, 3), 2,6-[(C6F5)2PNH]2C6H3N (PNNNP, 10), and 2,6-[(C6F5)2PO]2C6H3N (PONOP, 11) respectively. The previously reported 1,3-[(C6F5)2PCH2]2C6H4 (PCCCPH, 2) was also synthesised, with the literature yield improved upon by the use of magnesium-anthracene to generate the required Grignard reagent. The coordination chemistry of the POCOPH ligand 1 with platinum(0) alkene and platinum(II) dimethyl precursors revealed an affinity for the formation of cis-bridged oligomeric structures. The dimer [(POCOPH)Pt(nb)]2 (14, nb = norbornene) was isolated and crystallographically characterised from the reaction between 1 and [Pt(nb)3]. The solid state structure revealed the presence of stabilising - interactions between the aromatic ligand backbones, which were also observed in solution by 1H NMR spectroscopy. Reactions of ligand 1 with platinum and palladium dichloride or chloromethyl starting materials led to rare examples of cis,trans-dimers of the type cis,trans-[(POCOPH)MClX]2 (M = Pd, Pt; X = Cl, Me). In part due to facile dimer formation with 1, metallation of the ligand backbone to form the tridentate pincer complex [(POCOP)PtCl] (25) required long reaction times and high temperatures. It was observed that platinum dichloride starting materials with more strongly binding ancillary ligands were less prone to oligomer formation, and could facilitate more rapid metallation to from 25. More facile pincer complex formation was also observed for more electron-rich ligands with both PCP and PNP pincer ligands. The electron poor platinum and palladium POCOP, PCCCP, and POCCP pincer complexes (where the free ligand had been deprotonated upon metallation) were synthesised and subsequently converted into the metal carbonyl species [(PCP)M(CO)]+. Analysis of C−O stretching frequencies by infrared spectroscopy confirmed complexes of POCOP ligand 1 were the most electron poor, while those of POCCP ligand 3 were the most electron rich. Decarbonylation of the palladium pincer complexes was observed in solution and in the solid state, and was more facile for complexes with a higher wavenumber C−O stretch. Reaction of the [(PCP)PtCl] pincer complexes with methyl nucleophiles revealed that treatment with methylmagnesium iodide resulted in halide exchange, while methyllithium promoted nucleophilic attack at phosphorus. Spectroscopic data indicated that in one instance this led to pentafluorophenyl migration to the metal centre to form a [(PCP)Pt(C6F5)] complex. Dimethylzinc was successful in methylating the platinum PCP complexes; however, it was observed to degrade the palladium PCP pincer complexes. Treatment of the rhodium PNP pincer complex [(PNNNP)RhCl] (49) with dimethylzinc also resulted in degradation, which spectroscopic evidence indicated proceeded via ligand deprotonation and the formation of a zinc adduct of 49. Low temperature protonolysis of the [(PCP)PtMe] species did not reveal any information about possible interactions between the metal and liberated methane. The catalytic activity of the electron-poor [(PCP)PdCl] complexes were assessed in the Heck and Suzuki cross-coupling reactions. The complexes of 1, 2, and 3 were all found to possess only modest activity in the Heck reaction, functioning as precatalysts which decomposed to give catalytically-active Pd(0) colloids. Under milder Suzuki reaction conditions, the most electron-poor complex, [(POCOP)PdCl] (28) proved to be one of the most active pincer catalysts known for this reaction, able to achieve a turnover number of 176,000 for the coupling of electronically-deactivated aryl bromides and phenylboronic acid. Mercury poisoning tests revealed that Suzuki reactions catalysed by 28 proceeded via a homogeneous active species.</p>