Characterization of an iron-ruthenium interaction in a ferrocene diamide complex

Article abstract of DOI:10.1021/ic400773s

Reaction of [fc(NH₂)₂]RuCl₂(PPh ₃)₂ (fc = 1,1′-ferrocenylene) with 2 equiv of KO^tBu led to the formation of a diamido ruthenium complex, [fc(NH)₂]Ru(PPh₃)₂, whose solid-state molecular structure revealed a short Fe-Ru distance. A metal-to-metal charge transfer band was observed in the electronic absorption spectrum of [fc(NH) ₂]Ru(PPh₃)₂. The Fe-Ru interaction was characterized by resonance Raman spectroscopy for the first time and also by ¹H NMR, UV-vis, NIR, Moessbauer spectroscopy, and X-ray crystallography. Density functional theory (DFT) calculations including natural bond order analysis, Bader's atom in molecules method, and time-dependent DFT (TDDFT) provided further support that the iron-ruthenium bond is a weak donor-acceptor interaction with iron acting as the Lewis base.

Full text of DOI:10.1021/ic400773s

Article
pubs.acs.org/IC
Characterization of an Iron−Ruthenium Interaction in a Ferrocene
Diamide Complex
Aaron G. Green,† Matthew D. Kiesz,† Jeremy V. Oria,† Andrew G. Elliott,† Andrew K. Buechler,†
Johannes Hohenberger,‡ Karsten Meyer,‡ Jeffrey I. Zink,*,† and Paula L. Diaconescu*,†
†Department of Chemistry & Biochemistry, University of California, Los Angeles, California 90095, United States
‡Department of Chemistry and Pharmacy, Friedrich-Alexander-University Erlangen-Nurnberg, 91058 Erlangen, Germany
̈
S
* Supporting Information
ABSTRACT: Reaction of [fc(NH2)2]RuCl2(PPh3)2 (fc = 1,1′-ferrocenylene) with 2
equiv of KOtBu led to the formation of a diamido ruthenium complex,
[fc(NH)2]Ru(PPh3)2, whose solid-state molecular structure revealed a short Fe−
Ru distance. A metal-to-metal charge transfer band was observed in the electronic
absorption spectrum of [fc(NH)2]Ru(PPh3)2. The Fe−Ru interaction was
1
characterized by resonance Raman spectroscopy for the first time and also by H
NMR, UV−vis, NIR, Mossbauer spectroscopy, and X-ray crystallography. Density
̈
functional theory (DFT) calculations including natural bond order analysis, Bader’s
atom in molecules method, and time-dependent DFT (TDDFT) provided further support that the iron−ruthenium bond is a
weak donor−acceptor interaction with iron acting as the Lewis base.
Weak metal−metal bonding was reviewed by Braunschweig
1. INTRODUCTION
et al. from the perspective of Lewis pairs.40 X-ray crystallog-
Metal−metal bonds have been the subject of intense research
and discussion.1 Specifically, interactions between metal centers
in bimetallic complexes can give rise to interesting electronic
and magnetic properties2−13 and novel reactivity.14−20
Ferrocene derivatives21 have seen widespread use as ancillary
ligands for metal complexes,21−24 largely because of the unique
steric and electronic properties of the flexible and redox-
active25,26 ferrocene backbone. Metal complexes supported by
chelating 1,1′-ferrocenediyl derivatives are particularly interest-
ing and may allow a direct interaction between iron and the
metal center.27−29
Weak interactions between the ferrocene iron with both early
and late transition metals have been reported;30−37 however,
the nature of these interactions has been controversial. For
example, the Seyferth group found that upon the oxidative
addition of 1,2,3-trithia[3]ferrocene to Pd(PPh3)4, a four-
coordinate palladium complex was formed, featuring an iron−
palladium bond.32 Sato and co-workers also synthesized a
variety of late transition metal complexes that showed an
interaction between the iron and the metal; that interaction was
characterized by X-ray crystallography, NMR, and absorption
spectroscopy.38,39
For group 4 metals, complexes supported by 1,1′-ferrocene
diamide ligands were initially reported by Arnold et al., who
showed that a highly reactive cationic titanium(IV) species was
stabilized by an interaction with iron.31 Recently, the Stephan
group reported that ferrocene can stabilize cyclopentadienyl
zirconium alkyl cations through an iron−zirconium inter-
action.30 In both cases, the iron−metal interactions were
established based on structural parameters obtained from X-ray
crystallography.
raphy, Mossbauer, X-ray absorption spectroscopy, and
̈
computational methods were listed as methods used to
determine the nature of weak metal−metal interactions. The
authors also noted that it was especially difficult to assign the
Lewis base and the Lewis acid in the case of 1,1′-ferrocenediyl
(fc) complexes of middle-to-late transition metals. Such an
assignment is important in expanding the concept of transition
metal Lewis basicity, which is of fundamental interest in
understanding the reactivity of the corresponding metal
complexes.40 This concept is difficult to grasp since most
transition metals are viewed as Lewis acids. Herein, we report
the characterization of [fc(NH)2]Ru(PPh3)2 (fc = 1,1′-
ferrocenylene),41 a complex that features a short Fe−Ru
distance, consistent with an iron−ruthenium donor−acceptor
interaction. This interaction was characterized by electronic
absorption, Mossbauer, and, for the first time, resonance
̈
Raman spectroscopy in conjunction with density functional
theory (DFT) calculations. Resonance Raman studies were also
carried out on previously published palladium complexes that
feature an iron−palladium interaction to validate the method.
Aside from the importance of understanding the role played by
the Lewis basicity of metal ions in organic reactions (for
example, oxidative addition processes),40 the present article
shows that it is possible to characterize donor−acceptor
interactions between two metals with similar Lewis acid/base
character, even when these properties are dependent on their
chemical environment.
Received: March 28, 2013
© XXXX American Chemical Society
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2. RESULTS AND DISCUSSION
Synthesis and Structural Characterization. We have
been interested in the chemistry of substituted 1,1′-ferrocene
diamides as ancillary ligands for group 3 metals, lanthanides,
and uranium.27,28 In some instances, we observed short iron−
metal distances and proposed that the ferrocene diamide
supporting ligand was instrumental in stabilizing certain
bonding motifs.42,43 To differentiate between the influence of
the nitrogen substituent and that of the ferrocene diamide core,
we chose the parent 1,1′-ferrocene diamine, (fc(NH2)2), as a
platform to study metal−metal bonding. Inspired by Seyferth’s
and Sato’s pioneering work with late transition metals, and
because of numerous reports of stable amine44−50 and
amide50−53 ruthenium compounds, we chose to investigate
ruthenium complexes. Additionally, the syntheses of the
dichloride [fc(NH2)2]RuCl2(PPh3)2 (1) and of the diamide
[fc(NH)2]Ru(PPh3)2 (2) species were recently reported by
us.41
Figure 1. Thermal-ellipsoid (50% probability) representation of 1.
Hydrogen atoms were omitted for clarity. Selected distances [Å] and
angles [deg]: Ru1−Cl1 2.3948(5), Ru1−Cl2 2.4427(4), Ru1−P1
2.3150(5), Ru1−P2 2.3205(4), Ru1−N1 2.2669(11), Ru1−N2
2.2407(12); Cl1−Ru1−Cl2 164.96(1), Cl1−Ru1−P1 91.71(1),
Cl1−Ru1−P2 93.60(2), Cl1−Ru1−N1 92.36(3), Cl1−Ru1−N2
89.36(3), Cl2−Ru1−P1 100.57(1), Cl2−Ru1−P2 93.91(2), Cl2−
Ru1−N1 79.43(3), Cl2−Ru1−N2 77.69(3), P1−Ru1−P2 95.13(1),
P1−Ru1−N1 88.51(3), P1−Ru1−N2 175.04(3), P2−Ru1−N1
172.93(3), P2−Ru1−N2 89.64(3), N1−Ru1−N2 86.61(4), Fe1−
C1−N1 127.48(8), Fe1−C6−N2 126.78(9), twist 1.66, tilt 102.90.
Compound 2 was prepared by the reaction of 1 with KOtBu
(Scheme 1).41 Interestingly, 2 was found to be remarkably
Scheme 1. Syntheses of Complexes 1−4
ring twist angle was 1.7° and the Cp-Cp tilt angle was 102.9°
(see Figure 2 for an explanation of twist and tilt).
Figure 2. Description of twist and tilt angle for 1,1′-ferrocenediyl
ligands. Twist angles were calculated by measuring the angle between
the two planes formed by C1 and the centroids of the two Cp rings,
and C1′ and the centroids of the two Cp rings. The Cp tilt angle is
C1−Fe-C1′; the Cp-Cp tilt angle in ferrocene is 107.4°.
resistant to σ donors. No reactions were observed in the
presence of tetrahydrofuran (THF), MeCN, or aniline. When 2
equiv of PMe2Ph were added to 2, however, a new
bisphosphine complex [fc(NH)2]Ru(PMe2Ph)2, 3, was formed
(Scheme 1). This complex was isolated as black crystals, which
decomposed slowly at room temperature in the absence of
PMe2Ph to form 1,1′-ferrocene diamine. Addition of a third
equivalent of phosphine to this complex resulted in the
formation of the trisphosphine species [fc(NH)2]Ru-
The X-ray crystal structure of 2 (Figure 3) shows the
ruthenium center in a distorted trigonal bipyramidal geometry
with an N1−Ru−N2 angle of 159° and a Ru−N distance of
2.16 Å. The Ru−P distances decreased to 2.20 Å. More
importantly, a drastic decrease in the iron−ruthenium distance
in 2 (2.80 Å) compared to 1 (4.20 Å) was observed. The Fe−
Ru distance in 2 is only 0.02 Å greater than the sum of the
covalent radii (2.78 Å),55 indicating that a metal−metal
interaction may be present. The Cp-ring twist angle was
10.61° and the Cp-Cp tilt angle was 121.63° (for comparison,
the Cp-Cp tilt angle in ferrocene is 107.4°) indicating
appreciable distortion of the ferrocene ligand relative to 1.
The solid-state molecular structure of 3 (Figure 4) exhibits
similar features to complex 2 with a notably short Fe−Ru
distance of 2.77 Å, a Cp-ring twist angle of 0.1° and a Cp-Cp
tilt angle of 118.7°.
1
(PMe2Ph)3, 4, as determined by H and 31P NMR spectros-
copy. The formation of 4 could also be observed by the direct
addition of 6 equiv of PPhMe2 to 2, or by the addition of 1
equiv of fc(NH2)2 to RuCl2(PPhMe2)454 at −78 °C followed by
the addition of 2.1 equiv of KOtBu (see the Supporting
Information for details). Compound 4 could not be isolated
since it rapidly decomposed to form fc(NH2)2 and an
intractable mixture containing phosphorus species.
Compounds 1, 2, and 3 were characterized by X-ray
crystallography. The solid-state molecular structure of 1 (Figure
1) exhibits a distorted octahedral ruthenium complex with
chloride ligands in the axial positions and a Cl−Ru−Cl angle of
165°. The Ru−N distances were 2.24 Å and 2.27 Å, while the
Ru−P distances were 2.31 Å and 2.32 Å. In addition, the Cp-
1
The H NMR spectra of 2 and 3 in C6D6 exhibit a large
splitting between the signals for the α (4.10 and 4.21 ppm,
respectively) and β (2.20 and 2.08 ppm, respectively) Cp
protons (Δδ = 1.9 and 2.1 ppm, respectively). Splitting of the α
and β proton peaks of the Cp rings has been previously
explained by the anisotropic effect of the iron−metal bond
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Figure 3. Thermal-ellipsoid (50% probability) representation of one of
the two crystallographically independent molecules of 2. Hydrogen
atoms were omitted for clarity. Selected distances [Å] and angles
[deg]: Ru1−Fe1 2.7994(5), Ru1−P1 2.2033(7), Ru1−P2 2.2026(7),
Ru1−N1 2.1586(22), Ru1−N2 2.1596(22); Fe1−Ru1−P1 130.02(2),
Fe1−Ru1−P2 131.61(2), Fe1−Ru1−N1 79.67(6), Fe1−Ru1−N2
79.70(6), P1−Ru1−P2 98.37(3), P1−Ru1−N1 93.63(7), P1−Ru1−
N2 100.06(7), P2−Ru1−N1 99.56(7), P2−Ru1−N2 93.67(7), N1−
Ru1−N2 159.36(9), Cp-Fe1-Cp 170.2, twist 10.31, tilt 121.63.
Figure 5. (A) Overlaid Mossbauer spectra of 1 and 2. (B) Overlaid
̈
Mossbauer spectra of 5 and 6.
̈
DFT Calculations. DFT geometry optimizations were
carried out with ADF2012.01 on the full molecules of 1, 2,
and 3. For 2, calculations were performed at the LDA64 and
PW9165 theory levels, with full electron (no frozen cores)
triple-ζ-potential (TZP) basis sets, and using the relativistic
scalar ZORA approximation. The values for the calculated Fe−
Ru distance in 2, 2.76 Å (LDA) and 2.87 Å (PW91, see the
Supporting Information for other parameters), match well the
experimental distance determined by X-ray crystallography
(2.80 Å, see above). In accord with the short Fe−Ru distance,
HOMO-6 of 2 shows a σ interaction between the two metals
(Figure 6). The calculated Mayer bond order69,70 for the iron−
Figure 4. Thermal-ellipsoid (50% probability) representation of 3.
Hydrogen atoms were omitted for clarity. Selected distances [Å] and
angles [deg]: Ru1−Fe1 2.768(2), Ru1−P1 2.192(1), Ru1−N1
2.163(4); Fe1−Ru1−P1 133.40(3), Fe1−Ru1−N1 82.20(10), P1−
Ru1−P2 93.20(6), P1−Ru1−N1 93.07(11), N1−Ru1−N1
164.39(20), Ru1−N1−C9 87.75(26), Cp-Fe1-Cp 173.09, twist 0.06,
tilt 118.71.
providing a shielding effect on the α protons, while the β
protons are less affected because they are further away.35−39
̈
Mossbauer Spectroscopy. To probe the electronic
Figure 6. HOMO-6 for 2. Orbitals are similar for the LDA and PW91
calculations.
structure of iron in complexes 1 and 2, Mossbauer spectroscopy
̈
studies were carried out. The Mossbauer spectrum of 1 (Figure
̈
5) exhibits a doublet at 0.53(1) mm s−1, consistent with an
Fe(II) center. The Mossbauer spectrum obtained for 2 (Figure
5) also exhibits a doublet, at δ = 0.56(1), indicating an Fe(II)
ruthenium interaction for 2 and 3 is 0.28 (PW91) and 0.26,
respectively. For comparison, the Mayer bond orders for the
single Ru−Ru bond in [Ru(CO)4]8 and Ru3(CO)12 were found
to be 0.70 and 0.62, respectively,71 indicating that the Fe−Ru
bonds of 2 and 3 are weaker than metal−metal single bonds.
The optimized coordinates were used for further analysis
with NBO 5.066 and Bader’s atoms in molecules (AIM)
methods.67,68 Analysis of the generated natural bond orbitals
(NBOs) shows overlap from two occupied valence orbitals
̈
center. For comparison, Mossbauer spectroscopic studies were
̈
also carried out (Figure 5) on [(dppf)Pd(PPh3)][BF4]2 (dppf =
1,1′-bis(diphenylphosphino)ferrocene), 5, and [(dppf)Pd(4-
picoline)2][BF4]2, 6.39 These measurements resulted in data
that are consistent with Fe(II) centers. The results obtained for
1, 2, 5, and 6 are expected since the 1H NMR spectra for these
complexes exhibit diamagnetic characteristics.
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centered on Fe and Ru (Figure 7). Natural localized molecular
orbitals (NLMOs) generated from these NBOs show a
Figure 7. Left: Overlap between NBOs 209 (Ru) and 211 (Fe), right:
NLMO 211 for 2.
Figure 8. Contour plot of the Laplacian ∇2 in the plane of iron,
ρ
ruthenium, and one of the amide nitrogen atoms for 2. Red lines
significant contribution from Fe and a small total (7.15%)
contribution from Ru (Figure 7 and Supporting Information,
Figure S8). Conversely, three NLMOs centered on Ru only
have a 0.39% contribution from Fe (see the Supporting
Information for details). These data indicate that there is
donation from iron to ruthenium. Natural population analysis
shows that there is a net linear overlap of 0.29 (both LDA and
PW91 calculations gave the same value) between the metal
centers.
indicate areas of high charge ∇2 < 0, blue lines indicate depletion of
ρ
charge ∇2 > 0.
ρ
Additionally, an NBO second order perturbation theory
analysis of the donor−acceptor data shows stabilizing
interactions (10.1 kcal/mol total energy) from occupied
valence orbitals on iron to empty orbitals on ruthenium. The
reverse, that is, stabilizing interactions from occupied valence
orbitals on ruthenium to empty orbitals on iron, was not found,
consistent with a donor−acceptor interaction between iron and
ruthenium with iron functioning as the donor.
Topological analysis of electron density was carried out using
Bader’s atoms in molecule (AIM) theory. This module is
provided as a subroutine in ADF2012. AIM identifies any type
of bond by calculating a (3, −1) critical point, and it
differentiates between covalent bonds, on one hand, and
weak interactions such as hydrogen bonds, van der Waals, and
donor−acceptor, on the other, by the value of the Laplacian
(∇2 ). If ∇2ρ < 0, the interaction is considered covalent. If ∇2
> 0, the interaction is a weak interaction. AIM has been used to
calculate bond critical points between two metal centers72 and
in ferrocene complexes.73 A (3, −1) critical point was found in
2 at 1.43 Å (PW91), the midpoint of the Fe−Ru distance,
supporting the presence of a weak interaction between the two
metal centers. Calculation of the Laplacian for 2 shows greater
electron density surrounding iron than ruthenium, consistent
with the NBO findings of electron donation from iron to
ruthenium. The contour plot of the Laplacian in the plane of
iron, ruthenium, and one of the amide nitrogen atoms (Figure
8) shows low electron density between the two metal centers,
with the gradient consistent with a weak interaction. For
comparison, a significant gradient is found between the amide
nitrogens and carbons, indicative of a covalent bond. Combined
with the short distances determined experimentally and the
spectroscopic data discussed above, the results of DFT
calculations support the existence of a weak interaction
between iron and ruthenium in 2, consistent with electron
donation from iron to ruthenium.
Figure 9. Near-infrared spectra in THF of 1 (2.5 mM), 2 (0.8 mM),
and 3 (0.8 mM).
dichloride complex exhibits peaks at 320 nm (ε = 5784 M−1
cm−1) and 500 nm (ε = 578 M−1 cm−1), which were assigned
to Fe-Cp charge transfer and Fe d-d transitions. Similarly, the
diamide complexes 2 and 3 (Figure 9) exhibit peaks at 360 (ε =
9647 M−1 cm−1) and 352 (ε = 6258 M−1 cm−1) nm, 530 (ε =
1214 M−1 cm−1) and 454 (ε = 1427 M−1 cm−1) nm. In addition
to those peaks, an absorption at 798 (ε = 236 M−1 cm−1) and
663 (ε = 261 M−1 cm−1) nm, respectively, is observed for these
two complexes. As for 1, the first two peaks in the UV−vis
spectra of 2 and 3 were assigned to Fe-Cp and d-d transitions
(see also the Supporting Information for calculated transitions).
By comparison to Sato’s previous observations, the third,
weakly absorbing peak, was assigned to an iron−ruthenium
charge transfer band.39 The increase in energy of the iron−
ruthenium charge transfer band from 2 to 3 can be expected
since PMe2Ph is a stronger σ-donor than PPh3.
The time-dependent DFT calculated excitation spectrum of
2 matches the UV−vis and NIR spectra obtained exper-
imentally, with an offset of −350 nm (see the Supporting
Information for details). These calculations show a low energy
transition corresponding to the IVCT band, calculated at 768
nm (corresponding to the 798 nm absorption found
experimentally); this transition arises from promoting an
electron from the highest occupied molecular orbital
(HOMO) to the lowest unoccupied molecular orbital
ρ
ρ
Electronic Absorption Spectroscopy. The UV−vis and
NIR (near-infrared) spectra of 1 and 2 (Figure 9 and
Supporting Information) show remarkable differences. The
D
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(LUMO). The HOMO of 2 (Figure 10) was found to consist
(see the Supporting Information for details) of mainly
Figure 11. Displacement vectors for the normal metal−metal
stretching mode of 5 (left) and 2 (right). Displacement involves
primarily elongation along the metal−metal axis and contraction along
the Pd−P or Ru−N axis.
(diphenylphosphino)ferrocene), 5, which exhibits metal-to-
metal charge transfer, and [(dppf)Pd(4-picoline)2][BF4]2, 6,
which does not.39 As expected, 5 showed enhancement of a
vibration at 330 cm−1 with displacement along the iron−
palladium axis as the excitation wavelength passed the metal-to-
metal charge transfer (Figure 12). In contrast, 6 showed no
Figure 10. HOMO (left) and LUMO (right) of 2 (isosurface value =
0.03).
ruthenium d orbitals (31% total contribution) and iron d
orbitals (22% total contribution) in addition to contributions
from nitrogen p orbitals (19% total contribution). In contrast,
the LUMO of 2 (Figure 10) is primarily ruthenium based with
some contribution from the phosphorus atoms (see the
Supporting Information for details). Neither molecular orbital
shows contributions from the phosphine ligands or the
ferrocene Cp rings. The HOMO shows antibonding
interactions between the ruthenium and the nitrogen, with a
nonbonding interaction between the ruthenium and iron. The
LUMO shows no contribution from the nitrogen, with an
antibonding interaction between ruthenium and iron; this
indicates that promotion of an electron from HOMO to
LUMO weakens the Ru−Fe bond while strengthening the Ru−
N bond. Therefore, the change in bond length of the iron−
ruthenium interaction may be probed by resonance Raman
spectroscopy (see below).
Resonance Raman Spectroscopy. Resonance Raman
spectroscopy utilizes photons that are in resonance with an
electronic transition of interest. When an electronic transition
occurs, the electron density of the molecule shifts relative to the
ground state. Normal modes with nuclear motion correspond-
ing to the changes in electron density in the excited state are
displaced relative to the ground state, enhancing the Raman
signal. Symmetric transitions are resonantly enhanced most
strongly because the slope of the excited state potential energy
surface is nonzero at the Franck−Condon region; the intensity
of scattering scales with the square of the distortion.56,57 The
ability to amplify the intensity of vibrations when in resonance
with an electronic transition enables resonance Raman
spectroscopy to assist in the assignment of these specific
electronic transitions.57 This technique,58,59 as well as regular
Raman spectroscopy,60,61 has been used previously to establish
electronic communication within bimetallic complexes. How-
ever, the present study represents the first instance when
resonance Raman spectroscopy is used to investigate the nature
of weak metal−metal interactions. Here we will demonstrate
the resonance enhancement of a Raman active iron−metal (Pd
or Ru) stretch when in resonance with the charge transfer
electronic transition (Figure 11). This resonance enhancement,
in conjunction with the results of DFT calculations, supports
the assignment of weak metal−metal interactions.
Figure 12. Raman intensity for 347 cm−1 mode of 6 (top) and 330
cm−1 mode of 5 (bottom) as a function of excitation wavelength
superimposed with absorption spectra. Raman intensities for both
compounds are scaled relative to 750 cm−1 vibration of PF6− (internal
standard). Intensities are calculated by fitting vibrations with Gaussian
functions. Error bars are derived from the error in fitting.
significant vibrational enhancement upon red excitation (Figure
12). Other vibrations of 6 displayed a similar trend of lowered
relative intensity at 568 nm. That loss was attributed to Raman
de-enhancement and was only observed at 568 nm for 6
because of the red shift of the Fe d-d transitions.
The Raman spectra of 1 and 2 were obtained at excitation
wavelengths of 407, 476, 530, 568, 647, and 676 nm (Figure 13,
see the Supporting Information for all spectra), using a Kr+ ion
To validate this method, we first undertook resonance
Raman spectroscopy studies on the pair of previously reported
complexes, [(dppf)Pd(PPh3)][BF4]2 (dppf = 1,1′-bis-
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Figure 13. Raman spectra of 1 and 2 upon 676.4 nm excitation.
Asterisks indicate the vibration (715 cm−1) of the internal standard,
KNO3.
laser. As expected, the Raman spectra for 1 showed no trend of
enhancement in any of the vibrational modes as the excitation
wavelength increased (Figure 14). In the case of 2, however,
significant enhancement of the 342 cm−1 mode was observed as
the excitation wavelength of the laser was increased (Figure
14). Examination of the resonance Raman spectra of 1 and 2
upon higher excitation energies reveals de-enhancement trends
typical of cyclopentadienyl sandwich compounds.62,63
The vibrational modes that appear in the resonance Raman
spectra were assigned using DFT calculations at the PW91/
TZP level. The vibration at 342 cm−1 was assigned to a normal
mode consisting primarily of an iron−ruthenium stretch in
conjunction with nitrogen−ruthenium stretches similar to the
normal mode previously described for 5; as the iron−
ruthenium distance expands the nitrogen−ruthenium bond
contracts. The calculated frequency for this vibration is 256
cm−1.
Additionally, the changes in electron density between the
HOMO and LUMO (Figure 10) support this assignment.
Because the 798 nm absorption is assigned to the HOMO to
LUMO transition, normal modes with displacement coincident
with the changes in electron density will be enhanced in the
resonance Raman spectrum. Specifically, the NIR metal-to-
metal charge transfer weakens the iron−ruthenium bond,
thereby elongating it and producing a stretch along the metal−
metal axis. In addition to the increase of the Ru−Fe distance,
this mode shows contraction of the Ru−N bonds. The
resonance enhancement of this mode increases as the excitation
wavelength nears the maximum of the charge transfer band.
Thus, the change in the Ru−Fe distance results primarily from
the interaction of the two metal centers.
Figure 14. Raman intensity for 300 cm−1 mode of 1 (top) and 342
cm−1 mode of 2 (bottom) as a function of excitation wavelength
superimposed with the corresponding NIR spectrum. Raman
intensities for both compounds are scaled relative to 750 cm−1
vibration of PF6 (internal standard). Intensities are calculated by
fitting vibrations with Gaussian functions. Error bars are derived from
the error in fitting.
−
acting as the Lewis base. The present work shows that an in-
depth, combined spectroscopic and computational study is
powerful in investigating weak metal−metal interactions with
complexes 2, 3, and 5 representing well characterized examples.
ASSOCIATED CONTENT
■
3. CONCLUSIONS
S
* Supporting Information
X-ray crystallographic data in CIF format for complexes 1, 2,
and 3, spectroscopic and DFT data. This material is available
free of charge via the Internet at http://pubs.acs.org.
In summary, a weak, donor−acceptor interaction between iron
and ruthenium in complexes 2 and 3 was described. This
interaction was characterized by 1H NMR, UV−vis, NIR,
Mossbauer, and Raman spectroscopy, X-ray crystallography,
̈
and DFT calculations. The Raman spectra of 2 and 5 exhibit
similar features indicating that resonance Raman spectroscopy,
in conjunction with UV−vis/NIR spectroscopy, is a useful tool
for determining whether or not a weak interaction between two
metal centers exists in such complexes. Furthermore, DFT
calculations indicate that the metal−metal interaction is
primarily an Fe−Ru donor−acceptor interaction with iron
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zink@chem.ucla.edu (J.I.Z.), pld@chem.ucla.edu
(P.L.D.).
Notes
The authors declare no competing financial interest.
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(32) Seyferth, D.; Hames, B. W.; Rucker, T. G.; Cowie, M.; Dickson,
R. S. Organometallics 1983, 2, 472.
(33) Mann, G.; Shelby, Q.; Roy, A. H.; Hartwig, J. F. Organometallics
ACKNOWLEDGMENTS
■
This material is based upon work supported by the National
Science Foundation (CAREER Grant to P.L.D. and Graduate
Research Fellowship DGE-0707424 to A.G.G.), Sloan
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