Thermochemistry of Transition Metal Hydrides: Bond Strength, Acidity, and Hydricity

tBu C 3 Rh tBu CH Hydrogen Atom + N 3 Transfer THF, rt 3 -1 -1 kH• = 1.2 × 10 M s FAST! BDFE = 52.3 kcal/mol BDFE ~ 70 kcal/mol

tBu N

P N N N tBu H H Proton N Transfer Rh Rh P N N N + N N CD3CN, rt -4 -1 -1 kH+ = 5 × 10 M s Rh(III)–H SLOW! pKa = 30.3 pKa = 28.4

N BF4 Me

NCCH3 Hydride N Rh BF4 + Transfer N Me CH3CN, rt 5 -1 -1 kH- = 3.5 × 10 M s FAST! ∆GºH- = 49.5 kcal/mol ∆GºH- > 90 kcal/mol

Hu, Y.; Norton, J. R., J. Am. Chem. Soc. 2014, 136, 5938-5948. Thermochemistry of Transition Metal Hydrides: Bond Strength, Acidity, and Hydricity

Nick Shin The Knowles Group Princeton University

Group Meeting September 28, 2019

Hieber, W.; Leutert, F., Naturwissenschaften 1931, 19, 360-361. Outline

1. Brief Overview of Metal Hydrides

2. Bond Strength of Transition Metal Hydrides

3. Acidity of Transition Metal Hydrides

4. Hydricity of Transition Metal Hydrides Types of Metal Hydrides

What is a Hydride?

• The anion of hydrogen, H- • A compound in which one or more hydrogen centers have nucleophilic, reducing, or basic properties

1. Ionic Hydrides

• Hydrides bound to alkali metal or alkaline earth metal

• NaH, KH, CaH2, … • Applications: strong base, reducing reagent, dessicant. CaH2

2. Interstitial (Metallic) Hydrides

• Hydrides within metals or alloys • Non-stoichiometric adsorption of H atoms in the lattice • Examples: Ni–H for nickel-metal hydride battery (NiMH), Pd–H for fuel cells, heterogenous catalysis

Mcgrady, G. S.; Guilera, G., Chem. Soc. Rev. 2003, 32, 383-392. Types of Metal Hydrides

3. Molecular (Covalent) Hydrides • All other metal hydrides • p-block elements: common reducing reagents

“Superhydride” NaBH4 LiAlH4 DIBAL TTMS

• and… transition metal hydrides!

Re H

The first molecular hydride The first “organometallic TM hydride” Hieber (1931) Wilkinson (1955)

Mcgrady, G. S.; Guilera, G., Chem. Soc. Rev. 2003, 32, 383-392. Norton, J. R.; Sowa, J., Chem. Rev. 2016, 116, 8315-8317. Wilkinson, G.; Birmingham, J. M., J. Am. Chem. Soc. 1955, 77, 3421-3422. Transition Metal Hydrides in Catalysis

Hydrogenation (Food, Petrochemical, Pharmaceutical, and Agrochemical Industries)

MeO MeO MeO Ir/JosiPhos O 50 ºC, H2 (80 bar) Cl Me N Me NH Me N Me yield > 90% Me Me Me Me Me ee = 80% TON = 2,000,000 TOF = 400,000 h–1 (S)-metolachlor Herbicide (Dual Magnum®) > 10,000 t / year

Hydroformylation (Detergent, Fragrance, Petrochemical, and Pharmaceutical Industries)

H O H O HCo(CO)4, H2, CO Production Scale: R + H 6.6 million tons (1995) R H R 120–180 ºC, 200–300 atm 10.4 million tons (2008) 3-4 : 1

Chemical Processes: Hydrosilylation, hydrodesulfurization, C–H functionalization, olefin isomerization

Energy conversion: CO2 reduction, H2 oxidation, reversible H2 storage and more

H. U. Blaser, F. Spindler and M. Studer, App. Cat. A: Gen. 2001, 221, 119-143. Hartwig, J. F., Organotransition Metal Chemistry: From Bonding to Catalysis, University Science Books, 2010. Versatile Reactivity of Transition Metal Hydrides

The three thermochemical properties: pKa, BDFE, and ∆GºH-

What is covered: • Definition • Experimental methods for measurement of each thermochemical property • General trends with a focus on late transition metals • Relevant examples in catalysis

Today’s goal: Have a ballpark figure for acidity, BDFE, and hydricity of any transition metal hydride complex

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Bond Strength of Transition Metal Hydrides

Definition

LnM–H LnM • + H • ∆Gº = Bond Dissociation Free Energy (BDFE)

Experimental Method of Determining M–H BDFE: Thermochemical Cycle

BDFE (M–H) LnM–H LnM• + H•

2.3 RT pKa(M–H) 0.5 BDFE (H2)

– + LnM + H LnM• + 0.5 H2 – + F [Eº(M /M) - Eº(H /H2)]

– – BDFE (M–H) = 1.37 pKa + 23.06 Eº(M /M) + 53.6 kcal/mol Measure pKa (explained later) & Eº(M /M) (via CV) (CH3CN, rt)

Other methods include calorimetric measurement and kinetic measurement using FT-ICR-MS, but those require well-understood decomposition pathways.

FT-ICR-MS (left), “the most complex method of mass analysis and detection.”

Tilset, M.; Parker, V. D., J. Am. Chem. Soc. 1989, 111, 6711-6717. Mass Spectrometer Facility at the University of Bristol (accessed September 24, 2019). http://www.chm.bris.ac.uk/ms/ Trends in M–H Bond Strength

1. Bond Dissociation Enthalpy (gas) ~ Bond Dissociation Free Energy (sol) + 4.8 kcal/mol

LnM–H (g) LnM • (g) + H • (g) ∆Hº = BDE (M–H)gas

LnM–H (sol) LnM • (sol) + H • (sol) ∆Gº = BDFE (M–H)sol

2. Most metal hydrides have M–H BDFE’s between 55 and 65 kcal/mol. 3. M–H bond strengths: first < second < third row 4. Electronic properties of ligands have little effect on the M–H bond strength. ← actually BDFE in our definition

Wayner, D. D. M.; Parker, V. D., Acc. Chem. Res. 1993, 26, 287-294. Tilset, M.; Parker, V. D., J. Am. Chem. Soc. 1989, 111, 6711-6717. Simoes, J. a. M.; Beauchamp, J. L., Chem. Rev. 1990, 90, 629-688. Trends in M–H Bond Strength

4. BDE (M–H) is almost always greater than BDE (M–C).

5. Rates of H• transfer are substantially influenced by sterics and bond strength.

M–H + tBu CH M • + tBu C 3 3 BDE ~ 75 kcal/mol

Simoes, J. a. M.; Beauchamp, J. L., Chem. Rev. 1990, 90, 629-688. Eisenberg, D. C.; Lawrie, C. J. C.; Moody, A. E.; Norton, J. R., J. Am. Chem. Soc. 1991, 113, 4888-4895. Selected Example: Radical Hydrofunctionalization of Olefins

Initial Mechanistic Observation: Halpern (1975)

R HCo(CO)4 H R HCo(CO)4 H R

R Co(CO)4 R Co(CO)4 R H

2 Co(CO)4 Co2(CO)4

Observations: 1) When using DCo(CO)4, H/D exchange at 9/10 positions is faster than hydrogenation. 2) Introduction of alkyl groups R increases the rate. 3) Equal mixture of cis-/trans-hydrogenation.

H• Addition to Olefin: Halpern (1977)

HMn(CO) • Mn(CO)4 HMn(CO) 4 Me 4 Me Ph Ph Me Ph Me • Mn(CO)4 Ph Me Me Me • Mn(CO) radical cage 4

Observations: 1) 1st order kinetics in both alkene and hydride. 2) First HAT is reversible.

3) When reacted with DMn(CO)4, D incorporation in both reactant and product. 4) Inversie isotope effect of 0.4 observed with H/DMn(CO)4.

Feder, H. M.; Halpern, J., J. Am. Chem. Soc. 1975, 97, 7186-7188. Sweany, R. L.; Halpern, J., J. Am. Chem. Soc. 1977, 99, 8335-8337. Selected Example: Radical Hydrofunctionalization of Olefins

Challenge: Thermodynamically Unfavorable H• Addition

Me Ph • Mn(CO)4

H BDFE(βC–H) ~ 35–45 kcal/mol

+ H–Mn(CO)4 Ph BDFE(M–H) ~ 55–65 kcal/mol

Weak M–H bond & stable product radical required.

Tang, L.; Papish, E. T.; Abramo, G. P.; Norton, J. R.; Baik, M.-H.; Friesner, R. A.; Rappé, A., J. Am. Chem. Soc. 2003, 125, 10093-10102. Selected Example: Radical Hydrofunctionalization of Olefins

Huge Advantage: Better Chemoselectivity Compared to Bronsted Acid Reactions

Initial Inspiration from Biology

General Scheme for Reductive-Oxidative Cycle Toward Alkene Hydrofunctionalization

Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A., Chem. Rev. 2016, 116, 8912-9000. Selected Example: Mukaiyama Hydration

Isayama and Mukaiyama of Mitsui Petrochemical Industries (1989): Co(acac) (20 mol%) 2 O OH O2 Ph Ph Me + Ph Me + Ph Me iPrOH (0.2 M) 8% 46% 17% 75 ºC, 1 hr.

Mukaiyama’s Proposed Mechanism: Nojima (2002):

II OOSiEt3 L2Co II iPrOH L2Co O2 R Me R L CoIII–H Et3SiH 2 III L2Co –O• III L2Co –OO• -2.8 ppm OH III 1 Co L2 ( H NMR) L CoIII O 2 R Me O iPrOH R Me III H Co L2 R O O III L CoII L2Co –OOH II OO 2 H L2Co R R Me R Me R

O2 Note: Halpern and Norton’s reports suggest HAT instead of hydrometalation

Teruaki, M.; Shigeru, I.; Satoshi, I.; Koji, K.; Tohru, Y.; Toshihiro, T., Chem. Lett. 1989, 18, 449-452. Tokuyasu, T.; Kunikawa, S.; Masuyama, A.; Nojima, M., Org. Lett. 2002, 4, 3595-3598. Crossley, S. W. M.; Obradors, C.; Martinez, R. M.; Shenvi, R. A., Chem. Rev. 2016, 116, 8912-9000. Selected Example: Dualcatalytic Hydrofunctionalizaiton

Hydroalkylation (Shenvi, 2019):

O O Mn(dpm)3 (20 mol%), Ni(acac)2 (2.5 mol%) O O PhSiH3 (3 equiv), HFIP (2 equiv) PMBO Me PMBO + Me tBu tBu I TBSO K2CO3 (1 equiv) TBSO Me DCE:Propylene Carbonate = 9:1 dpm rt, air 77% … and many substrates

Proposed Mechanism:

Green, S. A.; Huffman, T. R.; Mccourt, R. O.; Van Der Puyl, V.; Shenvi, R. A., J. Am. Chem. Soc. 2019, 141, 7709-7714. Selected Example: Dualcatalytic Hydrofunctionalizaiton

Electrocatalytic Enantioselective Hydrocyanation (Lin, 2019): Proposed Mechanism:

[Co] (0.5 mol%) CN Cu(OTf) (5 mol%) 2 H L (10 mol%)

tBu tBu PhSiH3 (1.1 equiv) TMSCN (2.0 equiv) 79%, 91% ee TBABF4 (2.0 equiv) HOAc (5.0 equiv) DMF, 0 ºC, 10 h

C(+)/Pt(–), Ucell = 2.3 V

N N O O Co N N tBu O O tBu O O OtBu tBuO tBu tBu

[Co] L

… and many substrates

Lu, S.; Niankai, F.; Brian G., E.; Wai-Hang, L.; Michael O., F.; Robert A., D. J.; Song, L., Chemrxiv, 2019. Selected Example: Epoxide Hydrogenation

Anti-Markovnikov Alcohol Synthesis via Epoxide Hydrogenation (Norton, 2019):

Cp2Ti(OMs)2 (10 mol%), NaCpCr(CO)3 (10 mol%) OH O HCpCr(CO)3 (10 mol%) Ph Me Ph Me H2 (7 atm), C6H6, 75 ºC, 72 hr 95% 11 examples Proposed Mechanism: (all thermochemical values are for MeCN)

Eº ~ -0.67 V (vs. Fc/Fc+) for both

pKa = 13.3 BDFE (M–H) = 57 kcal/mol

BDFE (C–H) BDFE (H–H) ~ 97 kcal/mol = 103.6 kcal/mol

Yao, C.; Dahmen, T.; Gansäuer, A.; Norton, J., Science 2019, 364, 764-767. Acidity of Transition Metal Hydrides

Definition of Solution Acidity M–H Ka

LnM–H (sol) LnM (sol) + H (solvent)

Complications in measurement: ion pairing, aggregates, H–bonding with solvents, … “The use of MeCN is strongly recommended whenever possible for pKa determinations of hydride complexes.” – Morris, R. H. (2016)

Experimental Methods of Determining M–H pKa: Equilibria with Known Standards

Keq pK M–H = pK B–H – pK LnM–H + B LnM + B–H a a eq

• Equilibria measured commonly by UV-Vis, IR, and NMR• • Another popular method: calorimetric measurement of protonation enthalpy

Selected pKa Values of Organic Acids in MeCN

MeCN MeCN acid pKa acid pKa

i + HP(N PrCH2CH2)3N 33.5 H2 (50) DBU-H+ 24.3 HOPh 27.2

HNEt3 18.5 HOAc 22.3

+ Py-H 12.3 CF3COOH 12.7 HPPh3 8 HCl 10.4 H–acetone+ -0.1 HOTf 2.6

Morris, R. H., Chem. Rev. 2016, 116, 8588-8654. Trends in M–H Acidity

1. Data for early transition metal hydrides is limited. Trends can be found from Group 6 to 10.

2. The solvent pKa of transition metal hydrides ranges from –20 to 50.

3. Additive Ligand Acidity Constant (LAC) Method for Predicting Acid Strengths (Morris)

LAC THF MeCN • Diamagnetic group 6–10 hydrides • pKa ~ pKa ~ pKa (neutral) MeCN THF/LAC pKa (cation) = 1.13 pKa + 3.2

LAC pKa = ∑ AL + Ccharge + Cnd + Cd6

AL: Electronic property of each ligand. Cnd: 0 (3d and 4d metals) Ranges from –12 to 6 2 (5d metals)

Electon-donating ligand = large AL

6 Ccharge: charge x of the conjugate base Cd6: 6 (if loses d octahedral configuration) 0 (all other cases) x Ccharge +1 –15 0 0 –1 30

Morris, R. H., Chem. Rev. 2016, 116, 8588-8654. Additive Ligand Acidity Constant (LAC) Method

Ligand Acidity Constants:

Examples: + CO H Et Et OC CO Re – P Cr H [B(C6F5)4] P Rh OC H OC P OC CO CO H P Et Et

CrH(CO)3Cp* [Re(H2)(CO)5][B(C6F5)4] RhH(depx)2

pK LAC = (–4)•3 + 0.2 + 0.9•3 + 30 + 0 + 0 = 21 LAC LAC a pKa = –18 pKa = 50 CO H- Cp* Charge

pK MeCN = 17.1 (exp) Protonation of monohydride MeCN a pKa = 51.0 (exp) observed in HOTf in PhF MeCN (tends to give errors with multiple π acids) (pKa of toluene ~ 50) Morris, R. H., Chem. Rev. 2016, 116, 8588-8654. Raebiger, J. W.; Dubois, D. L., Organometallics 2005, 24, 110-118. Acidity of Selected Cationic Metal Hydrides

Morris, R. H., Chem. Rev. 2016, 116, 8588-8654. Acidity of Selected Neutral Metal Hydrides

Morris, R. H., Chem. Rev. 2016, 116, 8588-8654. Selected Example: Heterolytic Splitting of Dihydrogen

Thermochemical Analysis of (Reversible) Heterolytic H2 Splitting

pKa(M–H2) << 50 B: H H LnM + H2 + B: LnM–H + B–H LnM pKa(H2) = 50 ∆GH–º ∆Gº(H2 heterolysis) = 76 kcal/mol

1. pK (B–H) ~ pK (M–H ) Requires: a a 2 2. ∆Gheteroº = 76 – ∆GH–º – 1.37 pKa(B–H)

Reversible Heterolytic Cleavage of H2 Using a Pendant Amine (Bullock, 2013)

LAC DCM pKa = 10 pKa ~ 12 Rate > 107 s-1 at 25 ºC

CH2Cl2

∆Gº = –2.1(3) kcal/mol (exp)

∆GH–º ~ 61 kcal/mol (calculated from above eq.) Hulley, E. B.; Welch, K. D.; Appel, A. M.; Dubois, D. L.; Bullock, R. M., J. Am. Chem. Soc. 2013, 135, 11736-11739. Selected Example: Heterolytic Splitting of Dihydrogen

H2 Splitting Using Remote Carbanion (Song, 2010) THF pKa ~ 35 H H H H

H2 (1 atm) N N N N N N

Ph3P Ru PPh3 THF, 60 ºC Ph3P Ru PPh3 Ph3P Ru PPh3 H H N2 H H H H

LAC pKa (M–H2) ~ 14 – 44

Ionic Hydrogenation of Ketone (Bullock, 2000) Proposed Mechanism: pK LAC ~ 1 a O O H Et 2 F L MH Et Et W BAr 4 Et Et n 2 OC O Et OC PCy resting O 3 OH LAC state pKa ~ –6 Et LnM OH Et Et Et Et O H2 (< 4 atm) LnM–H + Et Et Et CH2Cl2, 23 ºC 2 turnovers/h upto 11 turnovers O OH LnM Et Et Et Et

Stepowska, E.; Jiang, H.; Song, D., Chem. Commun. 2010, 46, 556-558. Bullock, R. M.; Voges, M. H., J. Am. Chem. Soc. 2000, 122, 12594-12595. Selected Example: Electrocatalytic Hydrogen Generation

Bullock (2013): Condition: 1:1 HOTf:DMF -1 H2 production with TOF of 980 s and overpotential at Ecat/2 of 930 mV

MeCN pKa = 2.0 Eº = –0.82 V ∆G º = 58.6 kcal/mol H– ∆Gheteroº = 76 – ∆GH–º – 1.37 pKa(B–H) = 14.7 kcal/mol

H2 evolution: ∆Gº = -14.7 kcal/mol

kinetic product

MeCN pKa = 33 MeCN pKa = 2.0

Eº = –1.55 V thermodynamic product

MeCN pKa = 16 Wiedner, E. S.; Roberts, J. a. S.; Dougherty, W. G.; Kassel, W. S.; Dubois, D. L.; Bullock, R. M., Inorg. Chem. 2013, 52, 9975-9988. Hydricity of Transition Metal Hydrides

Hydride Hydridic Hydricity Acid Acidic Acidity Base Basic Basicity

Miller and Appel (2016):

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Hydricity of Transition Metal Hydrides

Definition of Hydricity

– LnM–H (sol) LnM (sol) + H (sol) ∆GºH–

“the free energy required to remove a hydride anion (H–) from a species”

Small ∆GºH– = Less energy required for heterolytic bond cleavage = Strong hydride donor = More hydridic

Complication in the measurement: H– transfer leaves a vacant coordination site

Solution 1: Compare five-coordinate, 18e–, d8 metal hydrides

H ∆Gº H– L L – 8 L M L M + H– (sol) forms stable, squiare-planar, 16e , d metal complex L L L L

Solution 2: Effective Hydricity

–H Y LnM–H LnM LnM–Y Measure ∆Gº separately. ∆Gº assoc. ∆GºH– assoc.

∆GºH–(Y), effective hydricity

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Hydricity of Transition Metal Hydrides

Experimental Measurement 1: Hydride Transfer Method

Keq 1.37pK = ∆Gº (M–H) – ∆Gº (A–H) LnM–H + A LnM + A–H eq H– H–

• Equilibria measured commonly by UV-Vis and NMR

• Requires a known hydride acceptor A with ∆∆GºH– < 3 kcal/mol

Experimental Measurement 2: H2 Heterolysis Method

Keq 1.37pK = 76 – ∆G º – 1.37 pK (B–H) LnM + H2 + B: LnM–H + B–H eq H– a

• Equilibria measured commonly by NMR

Experimental Measurement 3: Potential-pKa method

∆GºH– + – ∆Gº = 1.37 pK – 46.12 Eº(M–/M+) + 79.6 kcal/mol LnM–H LnM + H H– a (MeCN, rt) + – 1.37 pKa 2FEº(H /H ) Stable L M– with reversible oxidation potentials – + + + n LnM + H LnM + H is required. –F [Eº(M–/M) + Eº(M/M+)]

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Hydricity Values of Selected Organic Hydride Donors

* All values are measured in MeCN. Me

113 kcal/mol 109 kcal/mol 99 kcal/mol H Si Ph Ph Ph

84.9 kcal/mol

H O H2 B H H H H O 70 kcal/mol 50 kcal/mol 44 kcal/mol H Me Me N Me N Me Me EtO2C CO2Et H B Et O O Et Et

70 kcal/mol 61 kcal/mol 26 kcal/mol 22 kcal/mol

Ilic, S.; Alherz, A.; Musgrave, C. B.; Glusac, K. D., Chem. Soc. Rev. 2018, 47, 2809-2836. Trends in M–H Hydricity

1. ∆GºH– generally ranges from 30 to 80 kcal/mol (Superhydride ~ AcrH2). – – 2. When BDFE(M–H) is similar: Low Eº(M/M ) = Low ∆GºH– = Strong H donor. + Study on [Ni(diphosphine)2H] (Dubois, 2001): ∆GºH– + – LnM–H LnM + H

BDFE (M–H) FEº(H•/H–)

• • + • LnM + H LnM + H F [Eº(M•/M+)]

• + ∆GºH– = BDFE (M–H) + 23.06 Eº(M /M ) + 26.0 kcal/mol (MeCN, rt)

2+ Ni(II/I) couple of [Ni(P–P)2]

3. In isostructual complexes, ∆GºH– follows first row >> second row ~ third row. + Comparing group 10 HM(PNP)2 complexes in MeCN (Dubois, 2004):

PEt2 ∆GºH– (kcal/mol): Ni (66.0) >> Pt (54.7) ~ Pd (51.1) PNP: Me N Weak H– donor Strong H– donor PEt2

pKa: Pt (27.6) > Ni (22.2) ~ Pd (22.1)

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Berning, D. E.; Miedaner, A.; Curtis, C. J.; Noll, B. C.; Rakowski Dubois, M. C.; Dubois, D. L., Organometallics 2001, 20, 1832-1839. Curtis, C. J.; Miedaner, A.; Raebiger, J. W.; Dubois, D. L., Organometallics 2004, 23, 511-516. Selected Example:

4. Lateral Moves between Isoelectronic Metal Hydrides: More Negative Charge = 5–30 kcal/mol More Hydridic

+ + Ph2 Ph2 Ph2 Ph2 Et2 Et2 Et2 Et2 P H P P H P P H P P H P Ni Co Pd Rh P P P P P P P P Ph2 Ph2 Ph2 Ph2 Et2 Et2 Et2 Et2

+ + [Ni(dppe)2H] Co(dppe)2H [Pd(depx)2H] Rh(depx)2H ∆GºH– = 62.8 kcal/mol ∆GºH– = 49.9 kcal/mol ∆GºH– = 61.8 kcal/mol ∆GºH– = 46.5 kcal/mol

Selected Examples on the Extremes: – Et Et + Et Et P P H N Me Me Pt N N O O Ru Me Me N H P P N Et Et Et Et

+ [Pt(EtXantphos)2H] [Ru(terpy)(bpy)H]– ∆GºH– = 77.2 kcal/mol (BDFE = 69 kcal/mol) ∆GºH– = 26.6 kcal/mol

Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Miedaner, A.; Raebiger, J. W.; Curtis, C. J.; Miller, S. M.; Dubois, D. L., Organometallics 2004, 23, 2670-2679. Matsubara, Y.; Fujita, E.; Doherty, M. D.; Muckerman, J. T.; Creutz, C., J. Am. Chem. Soc. 2012, 134, 15743-15757. Selected Example: Imine Hydrogenation

Hydride Transfer Using Ligands as Hydride Shuttles (Colbran, 2013)

nBu nBu N NaHCO2 (1.1 equiv) / HCO2H (1.1 equiv) HN [Rh]Cl2 (1 mol%), AgOTf (1 mol%) H H MeOH, rt, 16 h MeO MeO 9 examples 94%

Proposed Mechanism:

Mcskimming, A.; Bhadbhade, M. M.; Colbran, S. B., Angew. Chem. Int. Ed. 2013, 52, 3411-3416. Selected Example: Carbon Dioxide Reduction

Using Thermochemical Analysis for Carbon Dioxide Reduction (Linehan, 2013):

Condition: Co(dmpe)2H (0.28 mM), Verkade’s base (570 mM), 1:1 CO2:H2 (1 atm), THF, rt. Result: Production of formate, TOF = 3400 h-1, TON = 2000

∆GºH– ~ 36 kcal/mol (DFT) ∆GºH– = 44 kcal/mol

Verkade’s base (R = iPr)

pKa = 33.6 (DFT)

Not hydridic

enough for CO2

pKa = 33.7 (DFT) (all values are for MeCN) Jeletic, M. S.; Mock, M. T.; Appel, A. M.; Linehan, J. C., J. Am. Chem. Soc. 2013, 135, 11533-11536. Selected Example: Light-Driven Hydride Transfer

Ground-State Reactivity toward H2 Evolution: + ∆GºH– = 62 kcal/mol (MeCN)

Ir N ∆Gº (H2 evolution) = ∆GH–º + 1.37 pKa(B–H) – 76 H N pKa(B–H) needs to be less than 10.2.

Dark: H2 evolution observed with HOMs (pKa = 10), but not with pyH+ (pKa = 12.5) or HOAc (pKa = 23.5). [Ir(Cp*)(bpy)H]+

H2 Evolution via Excited-State Hydride Transfer (Miller, 2014):

∆GºH–* < 43 kcal/mol (MeCN) Further measurement limited by its own pKa (23.3)

Barrett, S. M.; Pitman, C. L.; Walden, A. G.; Miller, A. J. M., J. Am. Chem. Soc. 2014, 136, 14718-14721. Selected Example: Light-Driven Hydride Transfer

Photohydride Transfer to Organic Molecule:

Me Me Me H H Me N N N H H N + + H – H2N H2N H2N I H H2N O H H O O H H O 460 nm LED 59% 10% 7% + MNA ∆GºH– = 56 kcal/mol CD CN, rt + 3 + OTf– 20 min

Ir Ir N N H L N N

– L = I , NCCD3, or MNA

Barrett, S. M.; Pitman, C. L.; Walden, A. G.; Miller, A. J. M., J. Am. Chem. Soc. 2014, 136, 14718-14721. Wiedner, E. S.; Chambers, M. B.; Pitman, C. L.; Bullock, R. M.; Miller, A. J. M.; Appel, A. M., Chem. Rev. 2016, 116, 8655-8692. Selected Example: Photohydride? Photoacid?

14 kcal/mol

48.3 kcal/mol

–12 62 kcal/mol

Not Extensively Covered:

• Kinetics of Proton-, Hydrogen Atom-, and Hydride-Transfer • Photochemistry of Transition Metal Hydrides • Coinage Metal Hydrides (Cu, Ag, Au) • Transition-Metal Hydride Radical Cations • Surface Hydrides • Hydrogen and Dihydrogen Bonds of Metal Hydrides • Methyl Hydrides in Metalloenzymes • Hydrogenation Reactions • Polyhydrides and Bond Activation