Ligand Modifications on Transition Metals to Impact Redox Reactions: Capping Arene-Ligated Rh, Ir and Pt Complexes and Z-Type-Ligated Cu Complex 8 views

Many current processes for industrial production of chemicals are indirect and energy intensive. Advancements in catalysis could lead to lower energy requirements and fewer byproducts in reactions by introducing reaction pathways that optimize selectivity for the desired commercial product. Much research has focused on developing transition metal complexes to optimize for redox reactions relevant to catalytic cycles such as oxidations and reductive eliminations. Our group has studied a set of ligands deemed “capping arene” (FP) ligands: 5-FP (1,2-bis(N-7-azaindolyl)benzene) (first synthesized by the Wang group), 6-FP (8,8′-(1,2-phenylene)diquinoline), 5-NPFP (1,2-bis(N-7-azaindolyl)naphthalene) and 6-NPFP (8,8′-(1,2-naphthalene)diquinoline). The capping arene-ligated PtII complexes (FP)Pt(Ph)2 (FP = 5-FP, 6-FP, 5-NPFP or 6-NPFP) complexes were synthesized and solid-state structures were obtained using single crystal X-ray diffraction. Oxidatively induced reductive elimination of biphenyl from (FP)Pt(Ph)2 was investigated using CuII {Cu(OAc)2, Cu(OPiv)2 such that OPiv = pivalate} and AgI {AgTFA, AgOTf and AgOAc such that TFA = trifluoroacetate and OTf = trifluoromethanesulfonate} oxidants. Production of biphenyl from (FP)Pt(Ph)2 was monitored by 1H NMR spectroscopy. In CD3CN at 90 ºC with all oxidants, the highest yields of biphenyl were observed from (6-FP)Pt(Ph)2 and (6-NPFP)Pt(Ph)2. Using (6-FP)Pt(Ph)2 60(12)% yield of biphenyl was achieved using 2 eq. Cu(OPiv)2 and 67(8)% yield of biphenyl using 2 eq. AgTFA. Using 2 eq. of Cu(OPiv)2 or AgTFA with (6-NPFP)Pt(Ph)2 resulted in 51(9)% and 79(3)% yield of biphenyl. In contrast the yields of biphenyl from (FP)Pt(Ph)2 were lowest when FP = 5-FP (32(5)% yield of biphenyl was achieved using 2 eq. of Cu(OPiv)2 and 46(3)% yield of biphenyl was achieved using 2 eq. of AgTFA) or FP = 5-NPFP (33(1)% yield of biphenyl using Cu(OPiv)2 and 34(1)% yield of biphenyl using AgTFA). The higher biphenyl yield from (FP)Pt(Ph)2 when FP = 6-FP or 6-NPFP than when FP = 5-FP or 5-NPFP suggests the ability of 6-FP and 6-NPFP ligated complexes to undergo oxidation may be advantageous for the oxidatively induced reductive elimination of biphenyl from (FP)Pt(Ph)2. The use of the Z-type SbV ligand, SbQ3(o-chloranil), in the previously reported complex {SbQ3(o-chloranil)}Cu(OTf) was used in nitrene transfer reactions. The use of Z-type ligands has been employed to modulate reactivity at transition metal centers by accepting electron density from transition metal. We hypothesized the Sb-based Z-type ligand SbQ3(o-chloranil) would result in reactivity at Cu for {SbQ3(o-chloranil)}Cu(OTf) such that Q = 8-quinolinyl. Catalytic aziridination reactions of styrene were analyzed using nitrene transfer reagents {PhINTs (N-(p-toluenesulfonyl)imino]phenyliodinanes) and PhINTs derivatives}. The aziridination of styrene with up to 13(1) TOs and 65(4)% yield of aziridine product proceeds using 1.73 mM {SbQ3(o-chloranil)}Cu(OTf) with 50 equiv. of styrene and 20 equiv. of PhINTs. The reaction is believed to proceed via a Cu-nitrene intermediate. Attempts to functionalize ethylene using capping arene-ligated Rh or Ir complexes were unsuccessful. The reductive elimination of bis(trifluoroacetyl)ethane was accomplished in less than 5% yield from proposed (6-FP)Ir(TFA)2(CH2CH2TFA) or [(6-FP)Ir(η2-C2H4)(TFA)2][TFA] in CD3CN at 90 ºC. For the capping arene-ligated Rh complexes addition of 3 equiv. of AgTFA to (FP)Rh(η2-C2H4)(Cl) (FP = 5-FP, 5-NPFP, 6-FP or 6-NPFP) resulted in dissociation of ethylene and formation of (FP)Rh(TFA)3 The previously reported (FP)Ir(η2-C2H4)(Cl) (FP = 5-NPFP, 6-FP or 6-NPFP) complexes were used as catalyst precursors in the hydrogenation of styrene which was monitored by 1H NMR spectroscopy. The TOF for 1 mM (FP)Ir(η2-C2H4)(Cl) in CD2Cl2 at 40 ºC with 30 psi H2 and 50 mM styrene was 0.04 µM/s (FP = 5-NPFP), 0.01 µM/s (FP = 6-FP) and 0.009 µM/s (FP = 6-NPFP). The increased rate of styrene hydrogenation for (5-NPFP)Ir(η2-C2H4)(Cl) over (6-FP)Ir(η2-C2H4)(Cl) and (6-NPFP)Ir(η2-C2H4)(Cl) may be attributed to the weaker bond between ethylene and Ir for (5-NPFP)Ir(η2-C2H4)(Cl). Ethylene dissociation is a necessary step in the hydrogenation of styrene to bind styrene to the Ir metal center.

PHD (Doctor of Philosophy)

catalysis; ligands; oxidation; reductive elimination; organometallic

English

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2025-12-18