Redox Non-Innocent Ligands as Electron Reservoirs in Catalysis

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1 edox on-innocent Ligands as Electron eservoirs in Catalysis Carina Jette toltz/eisman Literature etings 3/9/2018

2 Jørgenson, 1966: edox-active Ligands: ome Definitions Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." xidation state ambiguity is the central aspect to metal complexes involving redox non-innocent ligands Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50,

3 Jørgenson, 1966: edox-active Ligands: ome Definitions Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." Innocent Ligands: Ti Fe 3 3 Pt 2 Cp = 2 L 2 2 = 2 Ti IV 2 Cp = 2 L 2 Fe II 2 3 = 2 L 2 = 2 Pt II Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50, Inorg. Chem. 2011, 50,

4 edox-active Ligands: ome Definitions Jørgenson, 1966: Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." edox on-innocent Ligands: arry Gray, 1960's: i i III, 4 - i i II, 1L, 3 - which resonance structure is the main contributor? Is the M primarily ligand or metal-based? Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50, Inorg. Chem. 2011, 50,

5 edox-active Ligands: ome Definitions Jørgenson, 1966: Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." edox on-innocent Ligands: arry Gray, 1960's: i i III, 4 - i i II, 1L, 3 - which resonance structure is the main contributor? Is the M primarily ligand or metal-based? Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50, Inorg. Chem. 2011, 50,

6 Jørgenson, 1966: edox-active Ligands: ome Definitions Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." edox on-innocent Ligands: arry Gray, 1960's: i e - - 1e i - i i II, 4 i II, 1L, 3 i II, 2, 2L The ligand, rather than the metal, gets reduced! Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50, Inorg. Chem. 2011, 50,

7 Jørgenson, 1966: edox-active Ligands: ome Definitions Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." edox on-innocent Ligands: arry Gray, 1960's: i e - + 1e i - i i II, 4 i II, 1L, 3 i II, 2, 2L "Although we loved these ligands, by the end of the 1960s, we knew that they were guilty as charged!"(.b.g., 2011) Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50, Inorg. Chem. 2011, 50,

8 Jørgenson, 1966: edox-active Ligands: ome Definitions Chirik, 2010: "Ligands are innocent when they allow the oxidation state of the central atoms to be defined" therefore: non-innocent ligands: formal and spectroscopically determined oxidation state of the metalligand system is ambiguous "edox-active, or 'noninnocent,' ligands have more energetically accessible levels that allow redox reactions to change their charge state." edox -active ligands can be: on-innocent ligands: not all non-innocent ligands are redox-active today! This term is also used to indicate the perturbation of metal coordination due to unexpected reactivity at the ligand in response to external stimuli (i.e. Protonation/deprotonation) Cooperating ligands: a ligand in a metal complex tht actively participates in substrate activation Bifunctional catalysts: a catalyst comprised of two different functionalities that activate both substrates within the same scaffold BUT: not all non-innocent ligands, cooperating ligands, and bifunctional catalysts are redox active ligands! Coord. Chem. ev. 1966, 1, 164. Inorg. Chem. 2011, 50,

9 Types of edox-active Ligands i.modulation of reactivity of metal (LA): L M +n -e - M +n L iii. involved in substrate bond breaking/making - M L M L pectator L M reactant L M M L Actor: reactant ii. electron reservoir: iv. reactive ligand radical L n M - L n+2 M L M +n L M (n+1) AC Catal. 2012, 2,

10 Types of edox-active Ligands i.modulation of reactivity of metal (LA): L M +n -e - M +n L iii. involved in substrate bond breaking/making - M L M L pectator L M reactant L M M L Actor: reactant ii. electron reservoir: iv. reactive ligand radical L n M - L n+2 M L M +n L M (n+1) AC Catal. 2012, 2,

11 edox-active Ligands as Electron eservoirs xidative Addition with Innocent Ligands - L M L M n+2 xidative Addition with edox-active Ligands L M - L n+2 M eductive Elimination from Zr(IV): 2- TF Zr TF TF Zr Zr IV, 2L, 2 Zr IV, 4 - xidation state of metal remains unchanged - Ligands are not directly involved in the bond-forming process - tore reducing equivalents in the ligand, rather than the metal - ature's enzymes operate close to the thermodynamic potential - Impart novel reactivity patterns to certain transition metals -Chirik: give noble character to first row transition metals Chem. Eur. J. 2015, 21,

12 Aminophenols: a well-established class of redox-active ligands - inspired by galactose oxidase: (catalyzes the oxidation of alcohols to aldehydes) is is Tyr Cu II Tyr hydrogen abstraction (rds) Tyr is is Cu II Tyr Proton transfer Tyr is is Cu II Tyr - aminophenols shuttle between 3 distinct states: e - e aminophenol amidophenolate iminosemiquinonate iminoquinone - related to quinones ( adds another functional handle, more coordinating) - iminoquinones have lower reduction potential - fine tune M-L complex (electrophilicity, bond lengths) - complex synthesis varies - Cu, i, Fe, Pt, Pd, u, s Coord. Chem. ev. 2009, 253,

13 Copper and Zinc-catalyzed Aerobic oxidation of alcohols catalysts: mol % catalyst =, Et Et 3, TF, rt, 2 40 h M II confirmed by EP, Magnetic and electrochemical data stoichiometric (anaerobic conditions), catalytic (aerobic) M = Cu II, TF = 0.03 s -1 (5000 turnovers in 50 h) M = Zn II, TF = s -1 can access 5 different ligand-centered oxidation states (both Zn and Cu) Active Cu catalyst generated from: Cu M II J. Am. Chem. oc. 1999, 121,

14 chanism for the aerobic oxidation of alcohols M II C 2 coordination M II M II electron transfer M = Cu II, Zn II M II intramolecular -atom abstraction (r.d.s) J. Am. Chem. oc. 1999, 121,

15 A to Zr(IV): disproportionation of hydrazine eyduk and coworkers, mol % catalyst 24 h, rt 2 2 L Zr L L educed Ligand Zr IV - d 0 metal catalyzing a multielectron reaction through the use of ligand-based valence changes - disproportionation is thermodynamically favorable - electrophilic metal coupled with redox properties of the ligand - control reactions with Zr( 4 (TF) 2, Zr() 4, Zr() resulted in J. Am. Chem. oc. 2008, 130, Inorg. Chem. 2011, 50,

16 A to Zr(IV): disproportionation of hydrazine elimination 2 L Zr L L educed Ligand Zr IV L coordination L Zr L 2 educed Ligand Zr IV L Zr L educed Ligand Zr IV oxidation of hydrazine L Zr L xidized Ligand Zr IV (detected by UV-vis absorption) 2 elimination "A" J. Am. Chem. oc. 2008, 130, Inorg. Chem. 2011, 50,

17 egishi couplings with Co III oper and coworkers, 2010: ZnBr Co III catalyst 15% yield chanism: Ligand centered radical pseudo oxidative addition - - Co III nucleophilic, triplet ground state Co III e - for A stem from ligand EI-M, deuterium labeling, -ray structure mech studies- similar trends to 2 displacement no follow up reports -' 'Zn J. Am. Chem. oc. 2010, 132,

18 Fe(III)-catalyzed synthesis of -eterocycles 3 Boc 2 LFe III (5 mol %) C 6 6, 100 C, 24 h Boc Fe III LFe III Catalyst synthesis: 1. Fe 3, air, -80 C 2. Et 3, air, -80 C-rt Fe III Characterized by: - UV-vis spectroscopy (semiquinone) - Magnetic susceptibility ( = 2 ground state) - confirmed spin by QUID and Mössbauer - -ray structure determination (geometry, BL's) - DFT calculations (5.3 kcals to = 3) - Löwdin population analysis (4 unpaired electrons) - CV for 1e - reduction and oxidation potentials LFe III air stable, recyclable catalyst - linear, Boc protected amine was the main byproduct - for Pd version see: J. Am. Chem. oc. 2014, 136, J. Am. Chem. oc. 2017, 139,

19 Fe(III)-catalyzed synthesis of -eterocycles Van der Vlugt et. al, Boc 2 LFe III (5 mol %) C 6 6, 100 C, 24 h Boc Fe III LFe III Boc Boc Bn Boc Et Boc 96% yield 51% yield 39% yield 90% yield Boc Boc Boc Boc 48% yield 46% yield 44% yield 95% yield air stable, recyclable catalyst - linear, Boc protected amine was the main byproduct - for Pd version see: J. Am. Chem. oc. 2014, 136, J. Am. Chem. oc. 2017, 139,

20 Fe(III)-catalyzed synthesis of -eterocycles Boc 2 Fe III Boc 2 catalyst activation zero-order in substrate: activation of azide is not rds no Boc 2, no azide-fe complex [L Fe III ]* [LFe III ]* activated catalyst 3 nitrene insertion azide coordination [L Fe IV ]* [L 0 Fe III ]* nitrene stabilization [L Fe III ]* 2 [L Fe III ]* 2 loss 2 J. Am. Chem. oc. 2017, 139,

21 Fe(III)-catalyzed synthesis of -eterocycles Boc 2 Fe III Boc 2 catalyst activation zero-order in substrate: activation of azide is not rds no Boc 2, no azide-fe complex [L Fe III ]* [LFe III ]* activated catalyst 3 nitrene insertion azide coordination [L Fe IV ]* [L 0 Fe III ]* nitrene stabilization [L Fe III ]* 2 [L Fe III ]* 2 loss 2 J. Am. Chem. oc. 2017, 139,

22 Fe(III)-catalyzed synthesis of -eterocycles: Possible modes of catalyst activation Chloride Dissociation Boc 2 Fe III, C 2, - Fe III LFe III eaction with the Ligand Fe III Boc 2 C 2, - Fe III LFe III J. Am. Chem. oc. 2017, 139,

23 Fe(III)-catalyzed synthesis of -eterocycles Boc 2 Fe III Boc 2 catalyst activation zero-order in substrate: activation of azide is not rds no Boc 2, no azide-fe complex [L Fe III ]* [LFe III ]* activated catalyst 3 nitrene insertion azide coordination [L Fe IV ]* [L 0 Fe III ]* nitrene stabilization [L Fe III ]* 2 [L Fe III ]* 2 loss 2 J. Am. Chem. oc. 2017, 139,

24 Copper-Catalyzed Aziridination Ts complex 1 (1 mol%) I C 2 2, rt, 20 min 5 equiv 1 equiv 99% yield Ts Cu II complex 1 F Ts 97% 65%, 2h Ts Ts Ts 86%, 2h 29%, overnight Ts Ts Ts Ts 44%, overnight 86%, 3h 73%, overnight 23%, overnight Ts Ts Ts Ts 36%, overnight 12%, overnight 81%, 5 h 47%, overnight * reaction is not air sensitive, all run under air Chem. Eur. J. 2018, 24, 1-6.

25 Copper-Catalyzed Aziridination: edox Active ligand for spin fluxionality Cu II I Ts = 1/2 rt = 3/2 I ITs I Ts Ts Cu II Ts = 3/2 Cu II Ts I Ts Cu II Ts = 1/2 - Cu (II) is maintained throughout - UV-Vis, YCE, time dpt DFT, variable temp EP, broken symmetry calculations redox active ligand allows for access to different spin states at rt (energy diff is low) Chem. Eur. J. 2018, 24, 1-6.

26 2,6-Diiminepyridine Ligand as an Electron eservoir Chirik and coworkers, 2006: 10 mol% catalyst 23 C, 2-5 h = C 2, Alk, C(C 2 Et) 2 up to 95% yield 2 Fe 2 Fe II reduced ligand Catalyst ynthesis: Fe Fe II oxidized ligand 0.5% a(g) 2 atmosphere toluene Fe 2 2 or: Fe 2 2 Fe II reduced ligand The ligand stores two reducing equivalents in an iron(ii) complex! -ay crystal structure, Mössbauer, QUID, DFT J. Am. Chem. oc. 2006, 128,

27 2,6-Diiminepyridine Ligand as an Electron eservoir - energetically favorable Fe II 10 mol% catalyst 23 C, 2-5 h = C 2, Alk, C(C 2 Et) 2 up to 95% yield -electrons for A/ E stem from ligand L Fe L Fe II reduced ligand - DFT, experimental, spectroscopic, crystal structure Ar Fe II oxidized ligand Fe Ar Ar Fe Ar Fe II reduced ligand J. Am. Chem. oc. 2012, 134, J. Am. Chem. oc. 2006, 128,

28 2,6-Diiminepyridine Ligand as an Electron eservoir 1,6- enyne cycloaddition E 5 mol% 1-( 2 ) 2 4 atm 2, 23 C 1-3h =, TM E = Ts, Bn, (EtC 2 ) 2 C E 70-95% yield 2 Fe 2 ydrogenation of olefins: 1-( 2 ) 2 =, C(), C 2 Et, mol% 1-( 2 ) 2 4 atm 2, 23 C 15 min-24 h up to 95% yield ydrosilylation: TM TM i 200 ppm 1-( 2 ) 2 < 15 min, 23 C TM i TM ydroboration: B 1 mol% 1-( 2 ) 2 rt, 15 min-24 h B > 98 % yield in most examples J. Am. Chem. oc. 2009, 131, rganometallics, 2008, 27, cience, 2012, 335, AC Catal. 2012, 2, rg. Lett. 2013, 15,

29 2,6-Diiminepyridine Ligand as an Electron eservoir: The eal tory 10 mol% catalyst -Ligand helps adopt Fe III - - C-C E over β- elim. - Ligand prevents Fe(0) 23 C, 2-5 h = C 2, Alk, C(C 2 Et) 2 up to 95% yield Fe L Fe I Ar Fe Ar Ar Fe Ar Fe III -Isolated metallocyclic intermediates -magnetochemistry, Mössbauer, DFT Fe I J. Am. Chem. oc. 2012, 134, J. Am. Chem. oc. 2006, 128, J. Am. Chem. oc. 2013, 135,

30 2,6-Diiminepyridine Ligand as an Electron eservoir: The eal tory -1 e - at oxidation of both L and Fe 5 mol % 1-( 2 ) 2 4 atm 2, 23 C 1-3 h Fe L Fe II Ar Fe Ar Ar Fe Ar Fe III -Isolated metallocyclic intermediates -magnetochemistry, Mössbauer, DFT 2 Fe III J. Am. Chem. oc. 2012, 134, J. Am. Chem. oc. 2006, 128, J. Am. Chem. oc. 2013, 135,

31 2,6-Diiminepyridine Ligand as an Electron eservoir: Intermolecular 2+2 omo-dimerization: 2 = alkyl, benzyl 1 mol% Fe cat. 1 neat, 23 C, 48 h 14-85% yield ' ' Fe 2 ' ' ' = C 5 9 Fe cat. 1 ' ' etero-coupling: 5 mol% Fe cat. 2 neat, 23 C, 48 h ipr Fe 2 ipr 5 equiv = alkyl 80-98% yield 95:5 dr in most cases ipr ipr Fe cat. 2 no homo-dimerization no 4+2 cycloaddition products With Cobalt: J. Am. Chem. oc. 2015, 137, AC Catal., 2016, 6, cience, 2015, 349,

32 Fe-catalyzed C- activation/arylation Fensterbank et al, 2014 Br L-FeBr 2 (15 mol%) KMD, 80 C, 18 h L Et TB 53% yield 20% yield 35% yield 35% yield Bn TM 7% yield 16 % yield 50% yield 26% yield 2 F CF 3 55% yield 20% yield 41% yield Chem. Eur. J. 2014, 20,

33 Fe-catalyzed C- activation/arylation Fensterbank et al, 2014 Br L-FeBr 2 (15 mol%) KMD, 80 C, 18 h L May proceed through a metal-based inner-spere C- activation TM ET from KMD Fe Br TM chanistic studies show: - structure of the base is more important than basicity - not a homolytic aromatic substitution - spectroscopy- ligand centered radical - DFT calculations- - electron transfer from base to starting complex (ligand-centered radical) - -abstraction to form Fe-C bond Putative transition state for C- activation Chem. Eur. J. 2014, 20, Chem. Cat. Chem. 2016, 8,

34 Vicic and coworkers, 2006: i-catalyzed C(sp 3 )-C(sp 3 ) cross couplings I C 5 11 ZnBr i(cd) 2 (5 mol%) ligand (5 mol%) TF, 23 h, rt 98% yield C 5 11 chanism: i II -alkyl cation w/reduced ligand Zn i II ' I DFT/EP - evidence for organic π system-based radical ligand-based alkyl halide reduction I i I i II ' I otes: - o ligand, - no E from a 2 i II species - imine-type ligand is critical to reactivity - thermodynamically favorable - reduction ' i III I J. Am. Chem. oc. 2006, 128, Chem. Commun. 2005, Chem. Cat. Chem. 2016, 8, J. Am. Chem. oc. 2004, 126,

35 Catalytic ydrogenation with Bis(arylimidazol-2-ylidene)pyridine Co complexes Chirik and coworkers: catalyst: ' = alkyl, aryl ' = C 2 Et, alkyl, aryl 5 mol% cat. 4 atm 2 benzene, h ' > 95% yield Co 2 Evidence for Pyridyl-based adical intermediate: Ar Co Ar 1 atm 2 minutes, 22 C Co 2 Ar Ar confirmed by -ray diffraction Co 2 - M analysis of intermediates, - EP: ligand-centered radical Chiral cobalt hydrogenation catalysts: - -ray crystal structure J. Am. Chem. oc. 2012, 134, spin density calculations - DFT J. Am. Chem. oc. 2013, 135,

36 What about more common ligands such as B/ PyBox?

37 What about more common ligands such as B/ PyBox? eduction Potentials of free ligands: Et Et Et Et V V V V V V Cyclic voltammograms were recorded in TF solution (~ 1 mm in compound) with [nbu4]pf6 electrolyte (~ 0.1 M), using a 3 mm glassy carbon working electrode, Pt wire counter electrode, and Ag/Ag+ reference electrode. Values are reported versus ferrocene/ferrocinium emember: ΔG = -nfε, the more positive the P, the greater the affinity for electrons or tendency to be reduced - more electron rich ligands, lower P - PyBox ligands fall within the range of the bis(imino)pyridine ligands rganomet. 2009, 28,

38 What about more common ligands such as B/ PyBox? ynthesis of the reduced Fe(PyB) complexes: Fe 0.5% a(g) 2 atmosphere toluene Fe L L Chirik and coworkers: synthesis of active Fe catalyst for 2+2, alkene hydrogenation, etc. Fe 0.5% a(g) 2 atmosphere toluene Fe only bis(chelate) was isolated under a wide variety of reduction conditions Fe 0.5% a(g) C atmosphere toluene Fe C C compared I-stretching frequency - ligands are more electron donating than bis(imino)pyridine, less likely to be reduced - PyBox and B exhibited decent reactivity but low enantioselectivity in hydrosilylation of ketones - not as efficient as bis(imino)pyridines, more info is needed to confirm similar reaction pathway - bis(chelate) formation is principal catalyst deactivation pathway rganomet. 2009, 28,

39 What About more common ligands such as PyBox/ Box? Greg Fu (2014): Br n-bu TM racemic 2 Zn 1.6 equiv 3.0 mol % ibr 2 DME 3.9 mol % (-)--pybox DME, -20 C, 17h TM 77% yield 82% ee n-bu EP spectrum of proposed intermediate: i I ipr revised mechanism doesnt involve this intermediate EP spectrum of (--pybox)i I (2; black) and corresponding fit (red). Fit parameters: g1 = , g2 = , g3 = , 14 coupling (Mz) = , , , line width = band EP spectra were collected at 77 K in a toluene glass at œö = Gz at 2 mw power and a modulation amplitude of 2 G. (reproduced from JAC, 2014, 136, ) axial signal at g = 2.00, indicative of coupling to a single 14 in one g component, which is consistent with a largely ligand-centered radical J. Am. Chem. oc. 2014, 136,

40 utlook Ligand Characteristics: Ar Ar Ar - high degree of delocalization - itrogen is key! verall conclusions: - Limited scope- - ow should they be studied? - sterics: Tolman angle, % buried volume, etc - electronics: Crabtree: CV of free ligand, ligand bound to redox-inactive metal - establish, ID radical ligands, physical/chemical consequences - ole of tal in the reaction- structural stability or crucial role? - Versatile new tools Chem. oc. ev. 2013, 42,

41 ome eviews: Fensterbank, Chem. Cat. Chem. 2016, 8, Crabtree, Chem. oc. ev. 2013, 42, Caulton, Eur. J. Inorg. Chem. 2012, Kaim. Inorg. Chem. 2011, 50, Ward, Angew. Chem. Int. Ed. 2012, 51, van der Vlugt, Chem. oc. ev., 2015, 44, de Bruin, AC Catal. 2012, 2,

Nitrogen Centered Radical Ligands Nagashima Nozomu

1 Nitrogen Centered Radical Ligands 2015. 7. 4. Nagashima Nozomu 1. Introduction 2 3 Aminyl radical 1) D. E. Wiliams, JACS, 1966, 88, 5665 2) Y. Teki et al. JOC, 2000, 65, 7889 Sterically protected aminyl