!

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

I

+

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

II

+

- PPh2Me

N N

N

Ph

Mo

PPh2Me

NH3

I

+

N N

N

Ph

Mo

PPh2Me

NH2

III

+H

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH

III

+

- H2

+ PPh2Me

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH

III

+

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

I

+

+ 2PCET

∆G° = -1 kcal/mol (DFT)

BDFEN–H (DFT) = 47 kcal mol-1 BDFEN–H (DFT) = 47 kcal mol-1 BDFEMo–H (DFT) = 48 kcal mol-1

∆G° = - 21 kcal mol-1 (DFT)

BDFEN–H (DFT) = 68 kcal mol-1

PtH3N ClH3N

Cl

Coordination-Induced Weakening of Ammonia, Water, and Hydrazine X–H Bonds in a Molybdenum ComplexMáté J. Bezdek, Sheng Guo and Paul J. Chirik* Science 2016, 354, 730-733.

Department of Chemistry, Princeton University, Princeton, New Jersey 08544

Ammonia Activation by Coordination-Induced N–H Bond WeakeningA Fundamental Question: What is the N-H Bond Dissociation Free Energy (BDFE) of Coordinated Ammonia?

DFT Studies on Classical Ammine Complexes: CalibrationA. Werner (1893)1

105 kcal/mol (DFT) 82 kcal/mol (DFT) 77 kcal/mol (DFT)

M. Peyrone (1844)2 H. Taube (1968)3

Determination of X–H BDFEs: Thermochemical Considerations4

XH

LnMn XLnMn+1 H+BDFEX–H = ΔG°

+ e- - e-

pKa

- H+

+ H+

BDFEX–H E°

X–LnMn+ H++

- e- + e-CG

XLnMn+1 H+

XH

LnMn

Coordination-Induced Bond Weakening: Background

2+

N

NRu

NO

NN

H H

NFe

O

N

NH3CO OCH3

NN

H CH32+

Fe

2+

N

NH

NH

N

N

NHHN

N

NHN

HN N

NH3

CoH3N NH3

NH3H3N

NH33+ NH3

RuH3N NH3

pyH3N

NH32+

NH

HH

Ln[M] N

H

H

H+ Ln[M]

BDFEN–H = 100 kcal/mol BDFEN–H = ? kcal/mol

Why are some N–H bonds weaker than others? Can we take advantage of bond weakening?

Select Examples of Experimentally Measured X–H BDEs/BDFEs

NH

HH

M N

H

H

H

BDFEN–H99.5 kcal/mol

BDFEN–H< 48.6 kcal/mol

1/2 H2

- [M]-NH2H

– or –

NH3Carbon Neutral Cycle

PCET

R R

+ Ln[M]

Despite > 100 years of ammonia as a ligand in coordination chemistry, no systematic answer in literature.

Targeting “Nonclassical” Ammine Complexes

Synthesis: Addition of H-atom equivalents to unsaturated substrates by proton-coupled electron transfer.

Energy research: H2 Evolution from NH3 in a carbon-neutral fuel cycle.

BDFEX–H = 23.06(E°) + 1.37(pKa) + CG

Synthetic Applications of Complexes with Weak X–H Bonds

References Funding

Cuerva,9 Gansäuer10

TiOH2OH

H

(THF)n

R

O

RHO

+

Flowers,11 Mayer12

Norton8Mayer7

Meyer5 Stack6

SmI2 OH

H

(H2O)n

R’or

or

R”

NR2

R’ R”

NR2

CrHOC

COOC

82 kcal/mol 84 kcal/mol

72 kcal/mol 57 kcal/mol

BDE ≈ 54 kcal/mol BDFE = 26 kcal/mol

Coordination-Induced Bond Weakening of NH3 and H2Evolution in a Non-Classical Molybdenum Complex

Synthesis of a Nonclassical Molybdenum(I) Ammine Complex

Solid-State Structure of [1-NH3]+

giso = 1.988

EPR Spectrum of [1-NH3]+

Aiso(95/97Mo) = 80 MHzAiso(31P) = 33 MHzBenzene, 296 K

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

I

+

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

II

2+

E°ox

- e-

+ e-

pKa - H++ H+

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

+

II

BDFEN-H

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

+

IIN N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

II

2+N OMe

+

-HN OMe

[1-NH3]+

δ(15N) = 235 (t, NH2, 1JNH = 68.5 Hz)

δ(31P) = 15.6 (s, PPh2Me)

μeff = 1.7 μBμeff = 1.7 μBμeff = 3.6 μB

Ellipsoids at 30% probability,[BArF24]- anion omitted for clarity.

Cyclic Voltammetry (THF, 296 K)

pKa Determination (THF, 296 K)

Mo(I)/Mo(II)

Thermochemical Square Scheme to Quantify Coordination-Induced Bond Weakening

Mechanism of H2 Evolution from Coordinated NH3

Extending Coordination-Induced Bond Weakening to Hydrazine and Water

N–H BDFE (Expt): 45 kcal/molN–H BDFE (DFT) : 46 kcal/mol

E° = -1.09 V

pKa (THF) = 3.6

• Extraordinary N–H bond weakening in NH3 upon coordination.

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH3

I

+

C6D660°C, 6 h

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

II

+

- H2, HD, D2

+N N

N

Ph

Mo

PPh2Me

PPh2Me

ND2

II

+

+

N N

N

Ph

Mo

PPh2Me

PPh2Me

NHD

II

+

+N N

N

Ph

Mo

PPh2Me

PPh2Me

ND3

I

+

25% 25%

50%

H2 : HD, 9:1

14.214.414.614.815.015.215.415.615.816.016.216.416.616.8f1 (ppm)

0

50

100

150

200

250

300

350

400

450

500

550

600MB-2-201_4h.11.fid

15

.58

15

.61

15

.65

Crossover Experiment to Probe H2 Evolution Molecularity

Proposed Unimolecular H2 Evolution Pathway and DFT-Computed Thermodynamics

Nonstatistical ratio of H2isotopologs

Enabled by weak ammine N–H bond

H2 Evolution either: a) Unimolecularor

b) Bimolecular with a significant KIE

N N

N

Ph

Mo

PPh2Me

PPh2Me

Cl

I

- NaCl

1 eq. N2H4

Na[BArF24]

- 1/2 H2

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

I

+

NH2

N N

N

Ph

Mo

PPh2Me

PPh2Me

OH2

I

+N–H BDFE (DFT) = 35 kcal/mol

N N

N

Ph

Mo

PPh2Me

PPh2Me

NH2

II

+

NH

O–H BDFE (DFT) = 34 kcal/mol

N N

N

Ph

Mo

PPh2Me

PPh2Me

OH

II

+

- NaCl

1 eq. H2ONa[BArF24]

- 1/2 H2

1. Werner, A. Anorg. Chem. 1893, 3, 267–330.2. Peyrone, M. Ann. Chem. Pharm. 1844, 51, 1–29.3. Ford, P. et al. J. Am. Chem. Soc. 1968, 90, 1187–1194.4. Warren, J. J. et al. Chem. Rev. 2010, 110, 6961–7001.5. Binstead, R. A. et al. J. Am. Chem. Soc. 1981, 103, 2897− 2899.

6. Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc. 1997, 119, 8566−8567.7. Roth, J. P.; Mayer, J. M. Inorg. Chem. 1999, 38, 2760–2761.8. Jordan, R. F.; Norton, J. R. J. Am. Chem. Soc. 1982, 104, 1255–1263.9. Cuerva, J. M. et al. Angew. Chem., Int. Ed. 2006, 45, 5522−5526.10. Gansäuer, A. et al. Angew. Chem., Int. Ed. 2012, 51, 3266−3270.11. Chciuk, T. V.; Flowers, R. A. J. Am. Chem. Soc. 2015, 137, 11526−11531.12. Kolmar, S. S.; Mayer, J. M. J. Am. Chem. Soc. 2017, 139, 10687−10692. Predoctoral Fellowship (PGS-D) Jacobus Honorific FellowshipOffice of Science, Basic Energy Science (DESC0006498)

31P-NMR Spectrum, C6D6

1H-NMR Spectrum