coordination-induced weakening of ammonia, water, and … · 2019-08-28 · coordination-induced...
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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