Post on 14-Apr-2018
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Agostic Interaction in d0 TransitionMetal Alkyl Complexes
Bani Kanta Sarma
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Introduction:
Definition,Historical perspective and Importance.
Characterization of Agostic Interactions.
Theoretical Treatment of the Agostic Interaction.
Revised Bonding Model for d0 System and Evidences.
Driving force and Bond Ellipticity
Manipulation of Agostic Interactions:
Metal Polarization and the Extent of Delocalization.
A Note on dn (n 0) Agostic Complexes.
Conclusion
Plan of Talk
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Original definition: to refer specifically to situations in which a hydrogen
atom is covalent lybonded to both a carbon and to a transition metal atom.
Brookhart, M.; Green, M. L. H. J. Organometall. Chem. 1983, 250, 395.
Generalized definition : distortion of an organometallic moietywhich brings
an appended C-H bondinto close proximitywith the metal center.
Scherer, W.; McGrady, G. S.Angew. Chem., Int. Ed. Engl. 2004, 43, 1782.
Such a definition accommodates most of the reported examples, and separates
the nature of the phenomenon and the driving force behind it from its observable
chemical consequences.
Definition
LnM
H2CCH2
H
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Development of the Concept
La Placa S. J.; Ibers, J. A. Inorg. Chem. 1965, 4, 778.
- is about what is expected from van der Waals radii-
Considered it to be a true five coordinated species.
Bailey, N. A.; Jenkins, J. M.; Mason, R.; Shaw, B. L. J.
Chem. Soc. Chem. Commun. 1965, 237.
-a distorted octahedron, the sixth coordination
being occupied by a hydrogen
Sum of rvdw = 3.1
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MountingEvidence
Trofimenko, S. J. Am. Chem. Soc. 1968, 90, 4754.
Inorg. Chem.1970, 9, 2493.
-Observation Hydridic(-2.6 ppm) character in the 1H NMR
hydrogens are intruding into a suitable empty metal orbital.
Cotton, F. A.; LaCour, T.; Stanislowski, A. G.J. Am. Chem. Soc. 1974, 96, 754.
- studied by single-crystal X-ray diffraction
a three center, two-electron bond encompassing the CHMo atoms
- analogy to bonding concept in borane chemistry.
Mo
OC
N N
CO
N NB
H2CCH3
Ph
CH3
H
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Mounting Evidence
PdH distance isLess than the sum of the van der Waals radii
Roe, D. M.; Bailey, P. M.; Moseley, K.; Maitlis, P.M. J.Chem.Soc.Chem. Commun. 1972, 1237.
This type of interaction has not been observed before, though Trofimenko noted that
the methylene hydrogens of the ethyl groups in [Pd{Et2B(pz)2}(3-C3H4Ph)(CO)2] and
[Ni{Et2B(pz)2}2] are shifted upfield.
Sum of rvdw = 3.1
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Conclusive Evidence: X-ray and Neutron diffraction Studies
Brown, R. K.; Williams, J. M.; Schultz, A. J.; Stucky, G.
D.; Ittel, S. D.; Harlow, R. L. J. Am. Chem. Soc. 1980,
102, 981.1.874
a very short FeH distance of 1.874 at 30 K.
stretchedaliphatic C-H bond (1.164 ), the longest ever observed in a
crystal.
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In 1982, Green et al. reported what are now the textbook examples of MHC -
and -agostic interactions in the transition metal alkyl complexes [RTiCl3(dmpe)]
(dmpe = Me2PCH2CH2PMe2; R = Me or Et).
Green et al coined the term agostic to describe the phenomenon. They also
introduced a halfarrow convention to represent the interaction.
The term agostic comes from the Greek word which occurs in
Homer and translates as to clasp or hold to oneself.
Growing Evidences
a) Green et al J.Chem. Soc. Chem. Commun.1982, 1410.
b) Green et al, J. Chem. Soc. Dalton Trans.1986, 1629.
C
TiP
P Cl
H
H
H
Cl
ClH2C
Ti
HH
H
P
PClCl
Cl
C-H M
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Reflection of Growing Importance
Fig 1. Number of publications N with the keyword
agostic which appeared in the chemical literature
between 1980 and 2000.
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Why do we care?1. Olefin elimination from transition metal alkyls and the reverse reaction-olefin
insertion into M-H bond - are both believed to proceed through -agostic
interaction.
2. ZieglerNatta Polymerization:
LnM
H2C
CH3
LnM
H2CCH2
H
Ln
M
H2C CH2
H
Conversion of -agostic interaction to -agostic interaction in a step wise manner.
C
Ti
H CH2
CH2
P
C
Ti
H CH2
CH2
P
C
Ti
H CH2
CH2
P
C
Ti
H
P
(i)
(ii)
(iii)
(iv) C C+
-agostic-agosticGreen et al. J. Organometall. Chem. 1983, 250, 395 9
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3. Hydroformylation:
Why do we care?
C Co
CO
CO
CO
O
H2C
H2
C
H
H2C Co
CO
CO
CO
H2C H
HCo(CO)4
a
HCo(CO)3
Co(CO)3
H
RCo(CO)4
RCO-Co(CO)3
H2
RCHO
CO
b
cd
e
+ CO
II
IIIIIIII
HCo(CO)4 + CO + H2 CH3CH2CHOH2C CH2
Ziegler, T. Can. J. Chem.1995, 73, 743.
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4. Activation of C-H bonds: Functionalization of inert C-H bond.
Why do we care?
O
B
O
B
O
O
B
O
O
+ H2
BR2
RhR2B
C H
66
Rh
R2B H
R2B C
RhR2B H
R2B C
RhR2B C H+
H
RhR2B+CR2B
+
BR2
Catalyst
= Cp*Rh(4-C6Me6)
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X-ray diffraction study of a single crystal represents by far the most commonly
used structural technique to reveal agostic interactions.
Characterization of Agostic Interaction
X-ray Diffraction
Low scattering factor of the hydrogen atom
The relative X-ray scattering ratio of Z(M)/1 is unfavorable.
Discrepancies of more than 0.1 in the MH separations.
Problem arises with the accurate location of H atoms.
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More accurate location of the hydrogen atoms
The neutron scattering ratio M versus H is more favorable than in the X-
ray case.
Neutron Diffraction
Only a handful of agostic alkyl systems has been characterized to date by
neutron diffraction studies.
Long collection times and large and high quality single crystals required.
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Study of isolated molecule free from intermolecular forces
No perturbation of the equilibrium structure
Gas Electron Diffraction
To date, no compound has been shown unambiguously to
possess an agostic structure by GED.
Problems of Thermal stability, involatility.
The complicated diffraction pattern
Does not discriminate well between comparable interatomic distances
The relatively weak scattering of H atoms as in X-ray diffraction.
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1H chemical shifts range ( typically 0 to -16 ppm for dn ; n 0 systems)
0 to 3 ppm for d0
systems
Coupling Constants for the J(13C-1H); (typically 75 -100 Hz)
NMR Spectroscopy
Long timescale of the NMR experiment (ms or longer).
Weak nature of the interaction (1-10 kcal/mol)
Fluxionality averages the changes between two or more C-H bonds.
Static spectra are in the best case observable at low temperatures
(e.g. 80 to -100 C). 15
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A striking -agostic complex [EtNi(dbpe)]+ (dbpe=
tBu2PCH2CH2PtBu2) reported by Spencer et al.
Ambient temperature
1H (NMR); = - 1.24 ppm
Characterization by Low Temperature NMR
At -100 C this splits into normal methylene and agostic resonances
at = +1.1 and - 5.8 ppm, respectively.Coupling constant 1J(C-H) = 68 Hz (agostic H)
= 151 Hz (terminal H)
a) Spencer et al J.Chem. Soc. Chem. Commun.1991, 1601.
b) Spencer et al Organometallics1991, 10, 49.16
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J(C- H)ave
= 120-130 Hz
J(C H)ave
= 80 - 90 Hz
J(C- H)ave
= 120-130 Hz
Fluxional System
C
H
H
M
H
C
C
H
H
M
B
C
H
H
M
A
HH
J(C-Hbridging) 80-100 Hz.J(C-Hterminal) 140 Hz.
This arises from an increase in s-characterin the terminal C-H bonds.
value in 1H NMR - a poor criterion for distinguishing between A, B, and C.
J(C- H)
ave= 120 Hz
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Isotopic Perturbation of Resonance (IPR) Studies
J( 13C-1H) 110 Hz rules out C
Isotopic Perturbation of Resonance (IPR) found rule out A
1. 1H chemical shift (CH3) > (CH2D > (CHD2).
2. 13C-H coupling J(13C-H) values
J(C-H) for CH3 > J(C-H) for CH2D > J(C-H) for CHD2
C-H-M and C-D-M small zero point energy difference
C-H and C-D large zero point energy difference
This arises because there is a thermodynamic preference forhydrogen rather than deuterium to occupy the bridging position.
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The C-H stretching vibration, (C-H) of the agostic C-H bond should
move to lower frequency.
The lower energy vibrations involving C-H deformation and rocking
motions should likewise be significantly perturbed.
Vibrational Spectroscopy
Advantage is the short timescale of vibrational transitions.
(C-H)28002350 cm-1
has in many instances been interpretedas diagnostic evidence of a MHC interaction.
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Complications with Vibrational Spectroscopy
The individual C-H oscillators on a carbon atom couple to give symmetric and
antisymmetric combinations.
Overtones of deformation modes around 1400 cm-1 fall close to the region of
interest.
Fermi resonance of overtones with the (C-H) fundamentals, leads to hybrid
features which are difficult to deconvolute.
All but one C-H bond in an alkyl group is substituted with deuterium.
The (C-H) fundamentals are thus decoupled from all other vibrations.
Partial Deuteriation
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Theoretical Studies
Hypothetical model [MeTaH4]n- (n=2 or 4) using the Extended Huckel (EHMO)
Methyl group could in principle distort like a carbene group.
According to this study secondary interaction weakens theC-H bond and attracts the -hydrogen to the metal.
R. J. Goddard, R. Hoffmann, E. D. Jemmis, J. Am. Chem. Soc.1980, 102, 7667.
Ti
H
H
H
C
H
H
HH
[MeTiH5]2-MeTiCl3(dmpe)
O. Eisenstein, Y. Jean, J. Am. Chem. Soc.1985, 107,1177.
Ta
H
CH3
H
H
Hn-
n = 2 or 4
(CH3) acceptor orbitals of the metal
multiple bonding between Ti (d0) centerand the Me group.
(C-H) acceptor orbitals of Ti
C
TiP
PCl
H
H
H
Cl
Cl
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The Valence-Bond Approach
A valence-bond approach
1) The valence electron (VE) count ( 16 VE) of the transition metal M.
2) The Lewis acidity and charge of M.
3) The degree of steric congestion at the M, measured primarily by itscoordination number (CN); and
4) The presence of an available acceptor orbital of suitable symmetry at M.
Valence-bond approach suffers from poor predictive power.
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Experimental and theoretical studieshave unambiguously shown topossess no agostic interactions of any
significance.
Fig 1. Molecular structures of MeTiCl3(gas electron diffraction)
Fig 2. molecular structure of MeTiCl3(dmpe)(neutron diffraction study)
Agostic methyl configuration wasconfirmed by a neutron diffraction studyon a single crystal.
1. VE count 8.
2. CN 4.
1. VE count 12.
2. CN 6.
Limitations of Valence Bond Approach
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A Revised Bonding Model for d0 System
1. The agostic interaction is a phenomenon driven by electron delocalization.
2. Hyperconjugative delocalization of the M-C bonding electrons.
3. This delocalization causes a global bonding redistribution within the metal
alkyl fragment.
geometrical distortions and significant changes in force constants.
The development of an agostic interaction is assisted by:
A softening of the M-C-X (X = C or H) bending potential and a flexible MLn
coordination geometry, each of which facilitates the canting of the alkyl group.
The presence of sites of locally enhanced Lewis acidity.
Scherer, W.; McGrady, G. S.Angew. Chem. Int. Ed.2004, 43, 1782. 24
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1. Agostic interactions in d0 transition-metal complexes do not rely on the
presence of 3c-2e interactions between the metal atom and C-H bonds.
Contrasts between New and Previous models
3. It is not the total but the local Lewis acidity at the metal center which controlsthese agostic interactions.
2. The bonding between the metal and an agostic alkyl group is accomplished
mainly by one bonding orbital and its associated pair of electrons.
Scherer, W.; McGrady, G. S.Angew. Chem. Int. Ed.2004, 43, 1782.
No C-H activation
VE count 16 is not important
Ligand environment is important
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Examples as Evidence of Revised Bonding Model
1. -agostic interactions are generally stronger than their- or-counterparts; and
2. An acute M-C-C angle -agostic interaction
The gas electron diffraction structure of
CH3CH2TiCl3 and both X-ray and neutron diffraction
data for CH3CH2TiCl3(dmpe) are now well
established.
How important is VE count and CN around the metal centre?
V. E. = 8 V. E. = 12
no dependence on the precise location of H atoms.
H2C
TiCl
Cl
Cl
H
H
HH2C
Ti
HH
H
P
PCl
Cl
Cl
anagostic agostic
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Fig. A) Gas electron diffraction (at 293 K) [EtTiCl3]
B) The molecular structures of [EtTiCl3(dmpe)] based on a high
resolution charge density study (sin/l=1.097 -1; T=105 K)
Calculated values at the B3LYP/6-311G(d,p) level with
additional f-polarization function for Ti.
A B
Structural Studies of CH3CH2TiCl3 and CH3CH2TiCl3(dmpe)
TiH
= 2.23
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Vibrational Spectroscopic Studies
The IR spectrum of the gaseous [CH3CH2TiCl3] [Fig. (a)]
(CH) 28003100 cm-1
.
The spectrum of the CHD2CD2 isotopomer, [Fig. (b)]
is (CH) 2953 cm-1 (A) and
is (CH) 2927 cm-1 (B)
D2C
TiCl
Cl
Cl
H'
D''
D''
D2C
TiCl
Cl
Cl
D'
H''
D''
Fig. IR spectra of (a) [CH3CH2TiCl3]
and (b) [CH3CH2TiCl3]
Normal or anagostic structure for [CH3CH2TiCl3]
G. S. McGrady et al. Chem. Commun.1997, 1547. 28
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The IR spectrum of solid [CH3CH2TiCl3(dmpe)] [Fig.
(CHterminal) 28003100 cm-1
(CHagostic) 2615 cm-1 (C).
The spectrum of the CHD2CD2 isotopomer, [Fig. (d)] -
is (CHterminal) 2933 cm-1 (D) and
is (CHagostic) 2585 cm-1 (E)
The IR spectral pattern clearly shows an agostic structure for [CH3CH2TiCl3(dmpe)]
Vibrational Spectroscopic Studies
Fig. IR spectra of (c) [CH3CH2TiCl3(dmpe)]and (d) [CHD2CD2TiCl3(dmpe)]
2615 cm-1
2585 cm-1
G. S. McGrady et al. Chem. Commun.1997, 1547.
D2C
Ti
D''D''
H'
P
PCl
D2C
Ti
H''D''
D'
P
PClCl
Cl
Cl
Cl
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NMR Spectroscopic Studies
/C (CH3)
ppm
(CH2)
ppm
J(C-H)
Hz
J(C-H)
Hz
CH3CH2TiCl3 0 2.0 3.3 134.3 130.0CH3CH2TiCl3(dmpe] 0 2.65 2.61 147.3 125.7
[CH3CH2TiCl3(dmpe)]
/C (CH3)
ppm
(CH2)
ppm
-90 2.70 2.53
-70 2.70 2.55
-50 2.70 2.58
-30 2.67 2.61
0 2.65 2.65
20 2.61 2.69
40 2.57 2.76
[CH3CH2TiCl3(dmpe)]CH3CH2TiCl3
Swang et al. J. Am. Chem. Soc.1998, 120, 3762.
D2C
Ti
H''H''
H
P
PClCl
ClH2C
TiCl
Cl
Cl
H'
H''
H''
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CH3CH2TiCl3 and CDH2CH2TiCl3
= (CH3) - (CH2D)0.019
And varies slightly with temperature
CH3CH2TiCl3(dmpe) and CDH2CH2TiCl3(dmpe)
= (CH3) - (CH2D)
varies from0.05 to0.01 in the temperature
range 193-313 K.
No significant differences are observed in eitherH or1JC-H for the -methyl group
of CH3CH2TiCl3(dmpe) compared with CH3CH2TiCl3.
Isotopic Perturbation of Resonance (IPR) Studies
H2C
TiCl
Cl
Cl
H'
H''
D''H2C
TiCl
Cl
Cl
H
H''
H''
H2C
Ti
H''D''
H
P
PClCl
ClH2C
Ti
H''H''
H
P
PClCl
Cl
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DFT Calculations and Frontier MO Analysis
The LUMO of [TiCl3]+ corresponds to a nearly pure dz
2 metal atom orbital.
The HOMO of [C2H5]- resembles an sp3 lone pair orbital on C, considerablydelocalized.
The main bonding interaction is between Ti and C, with a slight antibonding
interaction between the metal atom and the C-H moiety.
W. Scherer et al. Chem. Eur. J.2002, 8, 2324.
Fig. Frontier orbital interaction diagram and
shape of the HOMO and LUMO for [EtTiCl3]
(KohnSham orbitals based on B3LYP/6-311g(d,p) calculations).
32
DFT C l l ti d F ti MO A l i
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Fig. Frontier orbital interaction diagram and shape of the HOMO and LUMO for
[EtTiCl3(dmpe)], espectively (KohnSham orbitals based on B3LYP/6-311G(d,p) level.
The binding of dmpe in CH3CH2TiCl3(dmpe), lengthens the corresponding
Ti-C and Ti-Cl bonds (by ca. 0.06 and 0.2 , respectively),
This canting of the ethyl group is then driven by a positive bonding
overlap between the torus of the Ti dz2 orbital and the C atom.
DFT Calculations and Frontier MO Analysis
HOMO LUMO
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1. The geometry of the metalethyl group is such that bonding to both
Cand C is effected by the same orbital on M.
2. The -agostic interaction can be understood only in terms of a Ti-C
bonding orbital that is delocalized over the entire alkyl group.
3. Thus, electron delocalization, rather than MHC donation, appears
to be the primary driving force and the VE count is not a controlling
factor.
Summary of DFT Calculations and Frontier MO Analysis
W. Scherer et al. Chem. Eur. J.2002, 8, 2324.
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Fig a) Theoretical bond paths and 2(r) contour map in the Ti-C-C-H planefor [CH3CH2TiCl3(dmpe)] (B3LYP/6-311G(d,p) ) b) Experimental bondpaths and 2(r) contour map for [CH3CH2TiCl3(dmpe)].
a b
Atoms In Molecules (AIM) Approach
No bond CP is evident between Ti and Hatom.
TiH interacrtion plays a minor role in the overall agostic interaction.
The existence of a MHbond CP is not a reliable criteria.
W. Scherer et al. Chem. Eur. J.2003, 9, 6057.
H2C
Ti
HH
H
P
PCl
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M-C bonding region:
CH3CH2TiCl3 CH3CH2TiCl3(dmpe)
CP (r) (e-3) 0.84 0.66
Bond length (Exp.) () 2.090 2.154
C-C bonding region:
Analysis of the Laplacian 2 (r)and Local Energy Density H(r)
CH3CH2TiCl3 CH3CH2TiCl3(dmpe)
CP (r) (e-3) 1.576 1.607
Bond length (Exp.) () 1.526 1.513
W. Scherer et al. Chem. Eur. J.2002, 8, 2324.
H2C
TiCl
Cl
Cl
H
H
H
H2C
Ti
HH
H
P
PClCl
Cl
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Driving Force Delocalization of M-C bonding electrons
Delocalization is evident if bond ellipticity is traced along the full C-C bond path.
Bond ellipticity 0partial character in a bond, or electronic distortion away from symmetry along
the bond path.
1. The ellipticity profile of [CH3CH2TiCl3(dmpe)]along C-C bond path reveals significant
character.
2. On evidence of its ellipticity profile system
is very distorted.
3. Two maximums with
max = 0.17 and max = 0.03Figure: Calculated ellipticity profile
W. Scherer et al. Chem. Eur. J.2002, 8, 2324. 37
Ellipticity Profile
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Ellipticity Profile
The maximum max= 0.17 is located
closer to the -carbon atom
At the bond CP = 0.10.
Second maximum close to C (max = 0.03).
This second maximum
hypervalent character induced by an additional TiC interaction.
The ellipticity profiles of C-C bonds may be used in a general manner to reveal
delocalization of M-C bonding electron density into the C-C bonding region.
Fig: Calculated ellipticity profile
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1. The existence of a very weak TiH interaction,
2. An increase in the C-C bond order, and
3. Weakening of the M-C and C-H bonds by hyperconjugative
electron delocalization.
The analysis of agostic complex [CH3CH2TiCl3(dmpe)] relative to anagosticcomplex CH3CH2TiCl3, reveals
Summary of the AIM analysis
39
M i l ti f A ti I t ti
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Manipulation of Agostic Interactions:
Metal Polarization and the Extent of Delocalization
Analysis of the electronic structures of a series of complexes
[EtTiCl2(L)]+ where L is
H2C
Ti
C
L
Cl
H
HH
Cl
+
1. a strong -acceptor (CO or PF3).
2. a weak -acceptor (PMe3).
3. a -donor (H, CH3 or NMe3) or
4. a -donor ligand (Cl, F or OMe2).
W. Scherer et al. Chem. Eur. J.2002, 8, 2324.
[EtTiCl2]+ have the electronic features of metal-ethyl bonding in EtTiCl3(dmpe)
avoids complicaions from steric factors and
shows more pronounced -agostic interaction than does EtTiCl3(dmpe)
40
M i l ti f ti i t ti
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Manipulation of agostic interaction
1. All -acceptor ligands display acute C-Ti-L.
2. Thus, the -acceptor ligands face directly cis-LICC(2), which corresponds to a
site of locally reduced Lewis acidity.
3. This site is avoided by all -donor or-donor ligands, which prefer instead to
coordinate between cis-LICC(2) and trans-LICC - a site of locally enhanced
Lewis acidity at Ti.
41
Differences for accepting and donor ligands
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Differences for-accepting, - and -donor ligandsThe magnitude of the charge
concentration LICC(1) decreases with
increasing -acceptor character of L.
cis-LICC(1) = 309 e-5(L=PMe3)
cis-LICC(1) = 282 e-5(L=PF3)
cis-LICC(1) = 299 e-5(L=CO)
shift of LICC(1) towards BCC.
L = NMe3
L = OMe2
Agostic interaction is no longer favored in - and -donor ligands
BCC
trans-LICC
C C
H
Shift direction
cis-LICC(1)L = -acceptor
L = and-donor
cis-LICC(2) TiAtomic centre
X
~ 0.4
Fig. Ligand-Induced polarization of Ti atom
in [EtTiCl2 L]+
82.0
80.1
The pronounced local Lewis acidity of the Ti site facing C is reduced.
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1. -acceptor ligands trans to the -C-H favors agostic interaction.
- and -donor ligands trans to the -C-H disfavor the interaction.
2. Not global but the locally induced sites of increased Lewis acidity
3. Controlled by local ligand effects the polarization of the metal center.
Manipulation of agostic interaction
Favors agostic interaction
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A Note on dn (n 0) Agostic Complexes
1. Ellipticity profiles resemble closely those of alkene complexes.
2. These systems are close to -hydride elimination, possessing a high degree of
C=C and M-H bond character.
3. The occupied C-H bonding orbital acts as a donor and the empty *(C-H) orbital
acts as an acceptor.
4. Significant C-H bond activation is found (C-H elongation > 0.1) than in d0
transition-metal alkyl complexes.
LnM
H2C
CH3
LnM
H2C
CH2
H
LnM
H2
C CH2
H
I II III
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C l i
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Conclusion
1. Agostic stabilization in early d0 transition-metal alkyl complexes has little or no
dependence on MHC electron donation, but it arises rather from thenegative hyperconjugative delocalization of the M-C bonding electrons.
2. The bonding between the metal atom and the ethyl group is effectively
stabilized by one electron pair/molecular orbital.
3. Analysis of CCs and of bond ellipticities provides a novel and general method
for quantifying the extent of electron delocalization.
4. An understanding of the way in which the ancillary ligands induce polarizationat the metal center, affords the possibility ofpredictingand hence controlling
and manipulating the development of an agostic interaction.
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Thank you