Synthesis protocol of several forms of pincer complexes

CERTIFICATE

Scheme 1.6: Schematic representation of organometallic complex catalyzed hydrogenation of various unsaturated substrates

1.2 Pincer complexes

1.2.2 Synthesis protocol of several forms of pincer complexes

In the development of pincer ligand based transition metal complexes, the Milstein group demonstrated the synthetic protocol of PNP pincer-iron complex (Scheme 1.8).⁹⁰ In the initial step, an equivalent amount of PNP pincer ligand and FeBr2 in the presence of four equivalent of NaBH4 were stirred in acetonitrile/ethanol medium which resulted in a monohydride cationic complex 1.67 having one molecule of solvent (acetonitrile) coordinated to the metal center.

Treating the equivalent amount of PNP pincer ligand and FeBr2 in the presence of CO provided PNP pincer-iron dibromo complex 1.70.³⁵ The complex 1.67, upon applying vacuum loses the coordinated solvent molecule and resulted into a tri-hydrido borohydride iron (II) complex 1.68. On the other hand, the complex 1.67, when subjected to a CO atmosphere yields a trans CO complex 1.69 (with respect to hydride), where acetonitrile remained coordinated to the metal center. This result also proved the strong trans effect of the hydride ligand, where acetonitrile gets substituted by carbon monoxide. Similarly, treating the complex 1.69 under the vacuum, the solvent molecule is lost resulting in a trans dihydride carbon monoxide iron PNP pincer complex 1.60.

Scheme 1.8: Synthetic routes to different forms of the PNP pincer-iron complexes.^{35, 90}

The PNP pincer-iron dihydride complex 1.60 showed remarkably very good reactivity towards CO2 activation to generate formate salt (HCOO^-Na⁺) under very low pressure and obtained 788 TON.⁹⁰ The pincer-iron dibromo complex 1.70 in the presence of one equivalent of NaHBEt3

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Kanu Das, Ph.D Thesis, IIT Guwahati 10 provided mono hydrido iron complex 1.71 (Scheme 1.8), which showed good hydrogenation activity. In addition, hydrogenation of ketone derivative at low pressure of H2 (4.1 atm) was achieved at 1880 TON.³⁵

The trans pincer dihydride complex is uncommon mainly as it has trans influence by the hydride ligand. These electronic factors are slightly different from the cis isomer, which is stabilized by the bulky ligands.^91-92 Milstein group designed a synthetic protocol for the various type of PONOP pincer-ruthenium complexes and demonstrated its stability towards various additives.⁹³

Scheme 1.9: Synthetic protocol of PONOP pincer-Ru mono/di-hydride complex.⁹³

The treatment of ^tBu4PONOP ligand with HRuCl(CO)(PPh3)3 afforded the mono hydride complex 1.73 (Scheme 1.9).⁹⁴ The IR stretching frequency of the CO bond in complex 1.73 appears at 1932 cm^-1 whereas CO group of the analogous PNP-ruthenium system stretches at 1906 cm^-1.⁹⁴ The IR analysis of both ^tBu4PONOPRuHCl(CO) (1.73) and ^tBu4PNPRuHCl(CO) (1.62) demonstrated that the replacement of methylene group by oxygen atoms increased the electron density at the metal center, which thereby increased the  back bonding to * of the CO group, lowering the stretching frequency by  = 26 cm^-1.⁹³ Interestingly, the treatment of one equivalent of NaHBEt3 resulted in dihydrido complex 1.74. Alternatively, the complex 1.74 could also be synthesized by treating ligand 1.72 with H2Ru(CO)(PPh3)3 (Scheme 1.9).

The active pincer-Ru dihydrido complex 1.74 in the presence of CO atmosphere in THF solution resulted in a 5-coordinated 18-electron bis-carbonyl pincer-Ru(0) complex 1.75 (Scheme 1.10), where the elimination of H2 was confirmed by GC analysis. Alternatively, the complex 1.75 could be obtained either by treating one equivalent of KO^tBu under CO

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Kanu Das, Ph.D Thesis, IIT Guwahati 11 atmosphere with monohydrido complex 1.73 or by adding one equivalent of CO with complex 1.77 (Scheme 1.10).⁹³ They demonstrated that the unstable complex 1.77 could be synthesized by reacting 1.73 with one equivalent of KO^tBu in benzene or THF. The presence of 4- coordinated complex 1.77 (16-electron) was confirmed by the absence of hydride signal in ¹H NMR, ³¹P NMR (a singlet at  = 233.45 ppm), and appearance of a carbonyl group at 1928 cm^-1 in the IR spectra. The 5-coordinated bis-carbonyl Ru(0) complex 1.75 could react with water via the cleavage of one of the P−O bond (scheme 1.10). The zwitterionic complex 1.76 exhibits a doublet of doublet ( = 214.01 and 154.59 ppm) in the ³¹P NMR and two hydride signals as a doublet ( = -5.9 ppm) in the ¹H NMR. The IR spectra (CO = 1992, 1948 cm^-1 and

Ru−H = 2061cm^-1) suggested strong trans influence nature of the CO ligand in complex 1.76.

Scheme 1.10: Synthetic route to various PONOP pincer-Ru arbonyl complexes.⁹³

The 4-coordinated complex 1.77 (16-electron) in the presence of H2 atmosphere hastwo broad singlets  = -6.41 and -11.46 ppm in ¹H NMR with a ratio 1:1, which proves the presence of cis-dihydrido complex 1.74. Furthermore, the complex 1.74 gets converted into the thermodynamically more stable isomer trans-dihydrido complex 1.74 (Scheme 1.11).

Scheme 1.11: The 16e to18e cis-trans isomerization of PONOP-Ru dihydride complex.⁹³ The mono hydrido-chloro complex of both 1.73 and 1.78 leads to chloride ligand abstraction on treatment with MeOTf and MeI (Scheme 1.12).⁹³ Interestingly, they observed that the presence of hydride signal in ¹H NMR reflects that MeX is not attacking as a electrophile to the metal center. In contrast to the reactivity exhibited by 1.78, the treatment of MeI with

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Kanu Das, Ph.D Thesis, IIT Guwahati 12 complex 1.73 resulted in a prolonged reaction even at a high loading of MeI compared to the corresponding reaction with MeOTf. This result indicates both the electrophilicity of MeI and the steric effect of the ^tBu group (compared with the ⁱPr group) on the pincer-Ru complex have a pivotal role to play.

Scheme 1.12: Reaction of hydrido chloride complex with reactive methyl species.⁹³

The treatment of ligand 1.81 with RuCl2(PPh3)3 at 80 C resulted in a trans dichloro pincer-Ru complex 1.82 where PPh3 is coordinated to the metal center (Scheme 1.13). The PPh3 complex 1.82 formation was confirmed by ³¹P NMR, which has two phosphorus signals observed as a triplet ( = 194.30 ppm) and doublet ( = 3.95 ppm). On the other hand, treating 1.72 with RuCl2(PPh3)3 at room temperature resulted in a 5-coordinated complex 1.83 (Scheme 1.13).⁹³

Scheme 1.13: Synthesis of the dichloro pincer-ruthenium complex.⁹³