Measurement of Diphosphine σ-Donor and π-Acceptor Properties in d0 Titanium Complexes Using Ligand K-Edge XAS and TDDFT

Article abstract of DOI:10.1021/acs.inorgchem.8b01511

Diphosphines are highly versatile ancillary ligands in coordination chemistry and catalysis because their structures and donor-acceptor properties can vary widely depending on the substituents attached to phosphorus. Experimental and theoretical methods have been developed to quantify differences in phosphine and diphosphine ligand field strength, but experimentally measuring individual σ-donor and π-acceptor contributions to metal-phosphorus bonding remains a formidable challenge. Here we report P and Cl K-edge X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-dependent density functional theory (TDDFT) studies of a series of [Ph₂P(CH₂)_nPPh₂]TiCl₄ complexes, where n = 1, 2, or 3. The d⁰ metal complexes (Ti⁴⁺) revealed both P 1s → Ti-P π and P 1s → Ti-P σ? transitions in the P K-edge XAS spectra, which allowed spectral changes associated with Ti-P σ-bonding and π-backbonding to be evaluated as a function of diphosphine alkane length. DFT and TDDFT calculations were used to assign and quantify changes in Ti-P σ-bonding and π-backbonding. The calculated results for [Ph₂P(CH₂)₂PPh₂]TiCl₄ were subsequently compared to electronic structure calculations and simulated spectra for [R₂P(CH₂)₂PR₂]TiCl₄, where R = cyclohexyl or CF₃, to evaluate spectral changes as a function of diphosphine ligand field strength. Collectively, our results demonstrate how P K-edge XAS can be used to experimentally measure M-P π-backbonding with a d⁰ metal and corroborate earlier studies showing that relative changes in covalent M-P σ bonding do not depend solely on changes in diphosphine bite angle.

Full text of DOI:10.1021/acs.inorgchem.8b01511

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Measurement of Diphosphine σ‑Donor and π-Acceptor Properties
in d0 Titanium Complexes Using Ligand K‑Edge XAS and TDDFT
Kyounghoon Lee,† Haochuan Wei,§ Anastasia V. Blake,† Courtney M. Donahue,† Jason M. Keith,*,§
and Scott R. Daly*,†
†Department of Chemistry, The University of Iowa, E331 Chemistry Building, Iowa City, Iowa 52242-1294, United States
§Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
S
* Supporting Information
ABSTRACT: Diphosphines are highly versatile ancillary ligands in
coordination chemistry and catalysis because their structures and
donor−acceptor properties can vary widely depending on the
substituents attached to phosphorus. Experimental and theoretical
methods have been developed to quantify differences in phosphine
and diphosphine ligand field strength, but experimentally measuring
individual σ-donor and π-acceptor contributions to metal−phosphorus
bonding remains a formidable challenge. Here we report P and Cl
K-edge X-ray absorption spectroscopy (XAS), density functional theory
(DFT), and time-dependent density functional theory (TDDFT)
studies of a series of [Ph2P(CH2)nPPh2]TiCl4 complexes, where n = 1, 2, or 3. The d0 metal complexes (Ti4+) revealed both
P 1s → Ti−P π and P 1s → Ti−P σ* transitions in the P K-edge XAS spectra, which allowed spectral changes associated with
Ti−P σ-bonding and π-backbonding to be evaluated as a function of diphosphine alkane length. DFT and TDDFT calculations
were used to assign and quantify changes in Ti−P σ-bonding and π-backbonding. The calculated results for [Ph2P(CH2)2PPh2]-
TiCl4 were subsequently compared to electronic structure calculations and simulated spectra for [R2P(CH2)2PR2]TiCl4, where R =
cyclohexyl or CF3, to evaluate spectral changes as a function of diphosphine ligand field strength. Collectively, our results
demonstrate how P K-edge XAS can be used to experimentally measure M-P π-backbonding with a d0 metal and corroborate
earlier studies showing that relative changes in covalent M-P σ bonding do not depend solely on changes in diphosphine bite
angle.
results to establish predictive models. For example, it was
recently shown by Rampino and co-workers that experimental
CO stretching frequencies in square planar Rh diphosphine
complexes could be combined with calculated charge distri-
bution profiles to tease out σ and π contributions using spec-
troscopic data.42 In an earlier study, Carlton et al. showed that
comparison of 57Fe NMR shifts and CO stretches in Fe(η5-
Cp)(SnPh3)(CO)(PR3) complexes could be used to exper-
imentally distinguish between electronic σ and π effects in
phosphines and phosphites.43 As exemplified in these reports,
as well as the influential Tolman studies, CO stretching fre-
quencies are often used as indirect reporters of phosphine field
strength. However, great care must be taken when using CO as
a reporter because CO stretching frequencies are influenced
by electrostatic effects and coupling of molecular vibrations,
which can lead to erroneous conclusions about phosphine field
strength.44−47 In this context, a direct measure of orbital-mixing
in M-P bonds would be more reliable.
INTRODUCTION
■
Phosphines and diphosphines are important ancillary ligands in
inorganic and organometallic chemistry.1 They form coordi-
nation complexes with all transition metals (as well as some
lanthanides and actinides)2−19 and show remarkable versatility
due to the large number of ways in which their substituents and
structures can be modified. As such, these ligands figure promi-
nently in homogeneous catalysis,20−22 and there have been
many experimental and theoretical studies aimed at determining
how changes to phosphorus substituents give rise to differences
in ligand field strength.23−36 Phosphines can range from strong
σ-donors to strong π-acceptors depending on the attached sub-
stituents, and changes in donor−acceptor properties have been
empirically ranked using a variety of experimental methods.
Among the earliest and most prominent of these studies were
Tolman’s formative investigations of Ni(CO)3L complexes
(where L = phosphine), which used CO stretching frequencies as
a spectroscopic reporter of changes in phosphine field strength.37
Although methods to rank phosphine and diphosphine field
strength are known, experimentally isolating and measuring
individual σ and π contributions to M-P bonding remains a sig-
nificant challenge.38 Theoretical methods have shown the most
success in quantifying individual σ and π contributions,26,36,39−41
and in some cases have been combined with experimental
We postulated that an experimental method known as ligand
K-edge X-ray absorption spectroscopy (XAS) could selectively
quantify σ and π contributions to M-P bonding. Ligand K-edge
Received: May 31, 2018
© XXXX American Chemical Society
A
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XAS − formally an X-ray absorption near edge structure (XANES)
spectroscopy where ligand refers to the atom bound or in close
proximity to the metal−uses synchrotron-generated X-rays to
excite 1s electrons localized on the ligand into unoccupied
molecular orbitals (MOs) containing ligand p and metal d char-
acter.48−50 As a consequence of metal−ligand orbital-mixing,
forbidden 1s → nd transitions become dipole-allowed (i.e.,
1s → np) and the corresponding XAS transition intensity is
governed by the amount of ligand p-character present in the
antibonding wave function. As a result, transition intensity can
be used to quantify metal−ligand orbital-mixing in unoccu-
pied molecular orbitals. Ligand K-edge XAS was developed
and has been used most extensively for third-row ligand
elements Cl and S, but has emerged more recently for analysis
of M-P bonding. A review of ligand K-edge XAS studies of M-P
bonding, along with opportunities and challenges, was recently
reported by some of the authors.51
studied previously despite having the same diphosphine ligands.
These differences arise because the Ti4+ complexes have “non-
VSEPR” structures that distort from idealized octahedral geom-
etry, which is a phenomenon commonly observed for d0 metal
complexes.67 As previously described, DFT and TDDFT
calculations were performed on 1 − 3, as well as Ti(dcpe)Cl4
(4) and Ti(dtfpe)Cl4 (5) (where dtfpe = 1,2-bis[bis-
(trifluoromethylphosphino)]ethane) for comparison to the
XAS results and to determine how diphosphines with different
field strengths affect Ti−P σ and π bonding.
RESULTS AND DISCUSSION
■
Synthesis and Structural Characterization. The Ti
complexes 1 − 3 were prepared as described previously by
treating TiCl4 with the corresponding diphosphine (dppm,
dppe, or dppp, respectively) in toluene.68 Ti(dcpe)Cl4 (4),
where dcpe = bis(dicyclohexylphosphino)ethane, was prepared
similarly and is described for the first time. The molecular
structures of 1 and 3 have yet to be reported, so we collected
their single-crystal X-ray diffraction data so that their exper-
imental structures could be compared to the XAS and DFT
results (Figure 1). Selected bond distances and angles are
In this report, we describe ligand K-edge XAS, DFT, and
TDDFT studies of [Ph2P(CH2)nPPh2]TiCl4, where n = 1
(dppm), 2 (dppe), or 3 (dppp) (Chart 1). Ti4+ complexes were
Chart 1. Ti(IV) Compounds Studied with Ligand K-Edge
XAS and DFT (left) and a Simplified Orbital Depiction of
M-P σ and π Bonding (right)
Figure 1. Molecular structures of Ti(dppm)Cl4 (1; left) and
Ti(dppp)Cl4 (3; right). Ellipsoids are drawn at a 35% probability
level. The second molecule of Ti(dppm)Cl4 in the asymmetric unit
cell, cocrystallized solvent molecules, and hydrogen atoms have been
omitted from the figure.
targeted to investigate σ and π contributions to M-P bonding
using P K-edge XAS because we needed d0 metal complexes
with unoccupied M-P σ and π molecular orbitals available for
P K-edge transitions. This requirement appears at odds with the
function of π-acceptor orbitals in metal phosphine complexes,
which typically participate in M → P π-backbonding with
electron-rich metals. M → P π-backbonding requires d-electrons
so that the metal can donate electron density into low-lying σ*
orbitals on phosphorus that arise from bonding between phos-
phorus and the attached substituents. However, virtual M → P
MOs involved in π-backbonding are still present even when no
d-electrons are available on the metal. Although less relevant to
our investigation here, it should be noted that positioning
π-donor ligands (such as chloride) trans to π-acceptor ligands
is proposed to result in appreciable π-backdonation in d0 metal
complexes.52−55
In addition to our desire to explore M-P donor−acceptor
properties, the bis(diphenylphosphino)alkanes shown in Chart 1
were selected because we were interested in comparing the
P K-edge XAS results to those collected on square planar
PdCl2 complexes containing the same diphosphine ligands. We
previously showed that Pd−P σ covalency did not track as
expected with the diphosphine bite angle (β; P-M-P),56,57
which is notable because β is a structural parameter that is often
used in structure/reactivity correlations in transition metal
catalysis with diphosphine ligands.58−66 The Ti complexes 1 − 3
have very different bite angles compared to the PdCl2 complexes
provided in Table 1 along with those obtained from DFT cal-
culations (vide infra). Not surprisingly, the biggest structural
change across the series is observed in the P−Ti−P and Ti−
P−C angles. The diphosphine bite angle increased sequentially
in a stepwise fashion from 65.09(3) in 1 to 76.66(5)° in 2 to
80.66(2)° in 3 as the number of methylene units in the dip-
hosphine backbone increased. These are significantly smaller
than those in [Ph2P(CH2)nPPh2]PdCl2 complexes with the
same diphosphine ligands at 72.68(3)° (dppm), 85.82(7)°
(dppe), and 90.58(5)° (dppp) due to the d0 configuration and
the associated distortions from octahedral geometry.53,67 Like-
wise, the Ti−P−C angles increased from 95.1(1)° in 1 to
111.76(7)° in 3. Only subtle differences were observed in the
experimental Ti−P and Ti−Cl bond distances and the Cl−Ti−Cl
angles. XRD data collected on twinned crystals of 4 confirmed
chelation of dcpe to TiCl4 (Figure S1), but the twinning could
not be modeled to produce an acceptable R-factor for more
thorough structural comparisons.
P and Cl K-Edge XAS Studies. To investigate the elec-
tronic structure of the Ti complexes, we collected their P and
Cl K-edge XAS data at the Stanford Synchrotron Radiation
Lightsource (SSRL) in Menlo Park, CA. The Ti complexes are
all highly air-sensitive, which required special measures to
avoid exposing the samples to air during loading. In past exper-
iments, we and others have shown that moderately air-sensitive
samples can be protected from air during sample loading by
B
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a
Table 1. Experimental (XRD) and Calculated (DFT) Bond Distances and Angles for 1−3
Ti−P (Å)
expt
Ti−Cl (Å)
expt
P−Ti−P (deg)
Ti−P−C (deg)
Cleq−Ti-Cleq (deg)
compd
calcd
calcd
expt
calcd
expt
calcd
expt
calcd
110.1
Ti(dppm)Cl4
(1)
(molecule 1)
2.662(1)
2.664(1)
2.690
2.690
2.2749(8)
2.2895(8)
2.249(1)
2.244(1)
2.2723(9)
2.294(1)
2.2560(8)
2.262(1)
2.265(2)
2.296(2)
2.265(2)
2.251(2)
2.3013(6)
2.2799(6)
2.2619(7)
2.2324(7)
2.306
2.306
2.246
2.246
65.09(3)
65.2
95.1(1)
95.3(1)
96.6
96.6
106.56(4)
Ti(dppm)Cl4
(1)
(molecule 2)
2.6435(9)
2.646(1)
65.27(3)
76.66(5)
80.66(2)
95.2(1)
95.5(1)
108.51(4)
107.74(7)
105.19(2)
Ti(dppe)Cl4
2.660(2)
2.640(2)
2.715
2.712
2.307
2.307
2.251
2.251
2.313
2.312
2.242
2.242
76.7
80.8
106.8(2)
106.1(2)
106.2
106.2
105.8
106.6
(2)
Ti(dppp)Cl4
2.6832(6)
2.6888(7)
2.733
2.733
111.09(7)
111.76(7)
110.1
110.0
(3)
a
Bond distances and angles for 2 are reported in ref 68. A polymorph of 2 with similar structural metrics is reported in the Supporting Information.
(1) sealing them behind a layer of thin polypropylene in a
glovebox, (2) transporting them to the beamline in a sealed
container, and (3) quickly transferring them to the sample
chamber, sometimes using a glovebag for additional protec-
tion.69 However, this method did not work for the more air-
sensitive Ti complexes 1−3. The problem is that the protecting
layer of polypropylene must be thin to avoid attenuation of the
X-rays at the P K-edges (the attenuation length of polypro-
pylene is ca. 53 μm at 2150 eV),70 but such thin barriers allow
small amounts of air to effuse through the material fast enough
to decompose the complexes described here. To overcome this
challenge, we prepared a custom gas flow cell designed to keep
the samples under a continuous flow of UHP He during trans-
fer to the sample chamber and data collection (Figure S4). The
continuous flow of He quickly removes any air that passes
through the polypropylene window before it can decompose
the sample. This allowed us to collect reproducible data for
1−3. In contrast, repeated attempts to collect XAS data on the
cyclohexyl-substituted 4 were unsuccessful; only peaks attrib-
uted to decomposition were observed. We were unable to deter-
mine if the decomposition was photoinduced or due to small
amounts gas impurities during handling at SSRL. However, we
note that 4 is more air-sensitive than 1−3, consistent with the
well-known increased air-sensitivity of alkyl vs phenyl substi-
tuted phosphines.
The Cl K-edge XAS spectra for 1−3 each revealed two
resolved pre-edge features that closely match those reported
for the octahedral dianion in (PPh4)2TiCl6 (Figure 2).71 Anal-
ysis of the second derivative traces revealed that the two peak
positions were almost identical for all three complexes and
shifted ca. 0.4−0.5 higher energy relative to those observed in
the Cl K-edge XAS spectrum of (PPh4)2TiCl6 at 2820.80 and
2822.25 eV (Table 2).71 The first pre-edge features in the
spectra of 1−3 were located at 2821.2 eV, whereas the second
features were located at 2822.7−2822.8 eV. On the basis of peak
assignments made for (PPh4)2TiCl6 and our TDDFT calcu-
lations presented in the following section,71 the first pre-edge
feature in each spectrum was assigned as 1s → Ti−Cl π*, and
the second higher energy feature was assigned as 1s → Ti−Cl σ*.
The Cl K-edge pre-edge peaks were modeled with curves
containing a 1:1 mixture of Lorentzian and Gaussian lineshapes
Figure 2. Normalized and background-subtracted fluorescence yield P
K-edge (top) and Cl K-edge (bottom) XAS data for Ti(dppm)Cl4
(1; black trace), Ti(dppe)Cl4 (2; red trace), and Ti(dppp)Cl4 (3; blue
trace).
(as described previously),49,56,57,69,72 to quantify their relative
intensities (Figure 3). The postedge was modeled with a step
function containing a 1:1 mixture of arctan and error functions.
The relative intensities of the first peak assigned to 1s → Ti−
Cl π* were 1.07(5) (1), 1.21(6) (2), and 1.09(5) (3), whereas
those for the 1s → Ti−Cl σ* transitions at higher energy were
0.94(5), 0.83(4), and 0.91(5) for 1−3, respectively. Although
there appears to be a statistically significant intensity difference
between the σ and π transitions for 2 compared to 1 and 3, we
attribute this to the decreased resolution in the spectra of 1 and 3.
C
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Table 2. Experimental and Calculated P and Cl K-Edge XAS Peak Positions (eV), Calculated Oscillator Strengths
(f; Uncorrected from TDDFT Calculations), and Pre-edge Peak Assignments for Ti(dppm)Cl4 (1), Ti(dppe)Cl4 (2),
and Ti(dppp)Cl4 (3)
Ti(dppm)Cl4 (1)
Ti(dppe)Cl4 (2)
Ti(dppp)Cl4 (3)
expt
calcd
f
assignment
expt
calcd
f
assignment
expt
calcd
f
assignment
P K
2144.1
2145.2
2145.7
2146.8
2147.5
2147.8
2149.3
2821.2
2822.8
2824.6
2144.1
2145.3
2145.6
2146.1
2147.5
0.0007
0.0013
1s → Ti−P π
1s → Ti−P σ*
2143.9
2145.1
2146
2147
2147.8
2148.8
2144.0
2145.2
2145.7
2146.2
2147.4
0.0004
0.0016
1s → Ti−P π
1s → Ti−P σ*
2144.0
2145.2
2145.9
2146.8
2147.5
2148.8
2149.2
2821.2
2822.7
2824.8
2144.0
2145.2
2145.8
2146.3
2146.6
0.0003
0.0013
1s → Ti−P π
1s → Ti−P σ*
Cl K
2821.3
2822.7
0.0067
0.0039
1s→Ti−Cl π*
1s→Ti−Cl σ*
2821.2
2822.8
2824.6
2821.2
2822.7
0.0065
0.0041
1s→Ti−Cl π*
1s→Ti−Cl σ*
2821.2
2822.7
0.0064
0.0041
1s→Ti−Cl π*
1s→Ti−Cl σ*
intensities were converted into percent Cl 3p character per
bond using the D2d-Cs2CuCl4 intensity standard developed by
Solomon and co-workers.49 The three values derived for 1−3
were identical within error at 28(1)% Cl 3p per Ti−Cl bond.
These fall within the range of summed values reported pre-
viously for (PPh4)2TiCl6 (22%),71 (C5H5)2TiCl2 (25%),73,74
(C5Me5)2TiCl2 (25%),75 (C5H5)TiCl3 (32%),73 and TiCl4
(37%).73
The P K-edge XAS data collected for 1−3 all revealed
spectra with a small pre-edge shoulder at ca. 2144 eV and
unresolved features merged into the rising edge. Analysis of the
second derivative trace for 1 allowed us to identify several
minima at 2145.2, 2145.7, and 2146.8 eV prior to the white line
(the most intense peak), and similar features were observed in
the P K-edge spectra of 2 and 3. The P K-edge data was
modeled with curve fits, as described for the Cl K-edge data,
but the low peak-to-peak resolution led to relatively large uncer-
tainties in the pre-edge intensities (>10%; Figure 3). Also, unlike
the Cl K-edge XAS data, which are relatively well-understood
and more easily interpreted for octahedral Group IV metal com-
plexes,71 interpretation of the P K-edge data for 1−3 required
DFT and time-dependent density functional theory TDDFT
calculations to help assign and corroborate our analysis of the
spectral features.
DFT and TDDFT Studies. Dispersion-corrected ground-
state DFT calculations were performed as previously described
by us on optimized gas-phase structures of 1−3 (see the
Supporting Information for details). The B3LYP functional
was used because it has been shown to be remarkably accurate
in the simulation of experimental XAS spectra via TDDFT for
transition metal,56,69,71,74−76 main group,72,77,78 lanthanide,79
and actinide complexes.17,71,75,76 Although some of the bond
distances are slightly overestimated, which is common for DFT
even when correcting for dispersion, the calculated structural
metrics for 1−3 were in good agreement with the experimental
data (Table 1). Even though we were unable to collect XAS
data on 4, we performed calculations on 4 and the hypothetical
complex 5 with CF3 substituents attached to P so that their
electronic structures could be compared to 1−3.
Data obtained from the DFT calculations were used to
construct the MO correlation diagram provided in Figure 4.
Labels and Mulliken symbols were assigned by analyzing the
individual virtual Kohn−Sham orbitals for 1 in C2v point group
symmetry, whereas 2 and 3 were assigned using C2 symmetry
due to the staggered ethylene and propylene phosphine linkers
that eliminate the C2v mirror planes. As expected, all three Ti
Figure 3. P K-edge (left) and Cl K-edge (right) XAS curve fit models
for Ti(dppm)Cl4 (1), Ti(dppe)Cl4 (2), and Ti(dppp)Cl4 (3). Experi-
mental data points are depicted as open circles and the model fit is
shown as a red trace. Total residual data equal to the experimental
data minus the curve fit is offset below the trace and depicted in
orange.
The lower resolution allows the second curve in each spectra
to broaden and capture some of the area under the first curve.
We were able to fit both curves reproducibly, as evident from
the small standard deviations in the peak areas shown in paren-
theses in Figure 3, but summing the two peak areas yielded
practically identical values for 1−3 (2.01, 2.04, and 2.00,
respectively). The calculated oscillator strengths from our
TDDFT calculations also suggest only small differences in the
intensity of Ti−Cl σ and π transitions for 1−3 (vide infra).
Although the overlapping pre-edge features limited analysis
of separate σ and π contributions to covalent Ti−Cl bonds, we
were able to quantify how they combined for comparison to
other Ti complexes. The sum of the two Cl pre-edge peak
D
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or 3a MOs derived from in-phase P lone pair mixing with the
torus of the Ti 3dz2 orbital, as shown in Figure 6. In contrast,
P 3p character in the Ti−P σ* MO derived from out-of-phase
P lone pair mixing with the lobes of the Ti 3dxy (2b1 or 3b)
was slightly higher in 2 (10.4%) vs 1 (9.0%) and 3 (8.8%).
This is notable because 2b1 for 1 and 3b for 2 and 3 are the
same MOs that were previously interrogated in P K-edge XAS
and DFT studies of d8 square-planar PdCl2 complexes with
bis(diphenylphosphino)alkanes and bis(dicyclohexylphosphino)-
alkanes.56,57 Remarkably, despite significant differences in metal-
type, P-M-P bite angles, coordination geometry, and oxidation
state, the Ti complexes 1−3 show the same relative trend in
P 3p orbital mixing (Figure 6), albeit with lower amounts of
P 3p character overall compared to the Pd complexes.
TDDFT calculations were in excellent agreement with the
ligand K-edge XAS data collected for 1−3 and corroborated the
influence of Cl and P 3p mixing in the MOs on the observed
transition intensities (Figure 7). The simulated Cl K-edge
spectra each revealed two pre-edge peaks arising from collec-
tions of clustered Cl 1s → Ti−Cl π* and Cl 1s → Ti−Cl σ*
transitions, respectively. The sum of the calculated oscillator
strengths for the first pre-edge peak were larger than the second
pre-edge peak, consistent with the modeled Cl K-edge peak
intensities (Table 2). Only small differences in the calculated
oscillator strengths were observed for 1−3, consistent with our
assessment based on curve fits of the experimental data (vide
supra).
In contrast to the Cl K-edge results, the simulated P K-edge
XAS spectra revealed a broader distribution of transitions,
which accounted for the overlapping peaks and lower peak res-
olution in the experimental P K-edge XAS data. The simulated
P K-edge spectra for 1−3 from TDDFT calculations were also
remarkably accurate and allowed us to assign the spectral fea-
tures. The lowest energy pre-edge feature was P 1s → LUMO
(Ti−P π), whereas closely spaced transitions assigned to P 1s →
Ti−P σ* and P 1s → ligand were observed at higher energy.
The calculated oscillator strengths from TDDFT closely mir-
rored the relative changes in P 3p orbital-mixing in the associ-
ated MOs mixing in the excited state (as provided in Table 3
and Figure 6). The calculated spectra also allowed us to
determine the origin of the white line feature, which is assigned
to Ti−P π* transitions involving the higher energy Ti 4p
orbitals. These Ti−P π* orbitals have small amounts of Cl p
character that give rise to higher energy features in the Cl K-edge
XAS spectra (>2824.5 eV), as captured in the experimental curve
fit models (Figure 3) and simulated spectra from TDDFT (see
below).
Figure 4. Calculated molecular orbital correlation diagram for 1−5.
Symmetry labels correspond to those used in Table 3.
complexes have five unoccupied MOs derived from the Ti 3d-
orbitals mixing with frontier orbitals on P and Cl. MOs
assigned as 1a1, 1a2, and 1b2 are π-type MOs derived from the
2
2
nondegenerate 3dx −y , 3dxz, and 3dyz orbitals and the comple-
mentary σ-type MOs 2b1 and 2a1 derived from the 3dxy and
2
3dz are higher in energy. Analysis of the virtual Kohn−Sham
orbitals revealed that the π-type orbitals are best described as
Ti−P π and Ti−Cl π*, whereas the σ-type orbitals are anti-
bonding Ti−P and Ti−Cl σ* (Figure 5).
DFT and TDDFT calculations performed on 4 revealed how
changing the phosphorus substituents from phenyl to alkyl
affected the metal−ligand bonding and the simulated XAS
spectra. Calculations on the hypothetical molecule Ti(dtfpe)-
Cl4 (5), where dtfpe = 1,2-bis[bis(trifluoromethylphosphino)]-
ethane, were also performed to investigate the nature of the
Ti−P σ and π bonding with a stronger-field diphosphine
ligand. It is well-known that electron-withdrawing fluorine
substituents increase metal-phosphine π-backbonding by
lowering the energy of the π-acceptor orbitals. Indeed, the
calculated MOs in 5 are shifted lower in energy relative to
those in 1 − 4 due to the electron-withdrawing CF3 groups.
The calculated MO diagrams for 4 and 5 show the same
d-orbital splitting and assignments as 1−3. One other notable
distinction for 4 is the lack of low-lying phenyl π orbitals
Figure 5. Representative virtual Kohn−Sham orbitals calculated for 1.
C2v and C2 point group symmetry labels correspond to those used in
Table 3. The y-axis was used as the principal C2 axis to keep the
2
calculated dz MOs (2a1 or 3a) aligned with the z-axis.
Analysis of the calculated MO compositions revealed that
the first five MOs in 1−3 have significant Cl 3p character with
values ranging from 15.6−25.4% (Table 3). In contrast, only
the LUMO and σ-type MOs (2b1 and 2a1 for 1; 3b and 3a for
2 and 3) have appreciable amounts of P 3p character. The
LUMO in 1 (Ti−P π) has more P 3p character (3.6%) than
the LUMO in 2 and 3 (1.6 and 1.4%, respectively). A similar
reduction in Ti−P σ* orbital mixing is observed in the 2a1
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Table 3. Calculated MO Energies, Compositions, And Symmetry Labels for Ti(dppm)Cl4 (1), Ti(dppe)Cl4 (2), Ti(dppp)Cl4
(3), Ti(dcpe)Cl4 (4), and Ti(dtfpe)Cl4 (5)
MO Compositions (%)
a
a
Ti
P
p
Cl
compd
MO # and assignment
energies (eV)
p
d
s
d
p
d
1-C2v
146 (2a1)
145 (2b1)
144 (1b2)
143 (1a2)
142 (1a1)
141 (1b1)
150 (3a)
149 (3b)
148 (2b)
147 (2a)
146 (1a)
145 (1b)
154 (3a)
153 (3b)
152 (2b)
151 (2a)
150 (1a)
149 (1b)
162 (3a)
161 (3b)
160 (2b)
159 (2a)
158 (1a)
157 (1b)
134 (3a)
133 (3b)
132 (2b)
131 (2a)
130 (1a)
129 (1b)
−1.22
−1.68
−2.78
−2.79
−2.88
−6.87
−1.24
−1.60
−2.76
−2.82
−2.95
−6.66
−1.21
−1.58
−2.70
−2.73
−2.85
−6.53
−1.19
−1.36
−2.72
−2.75
−2.84
−6.56
−2.20
−2.67
−3.62
−3.67
−3.89
−8.09
0.9
1.8
0.7
43.4
54.8
80.8
80.8
76.8
4.9
60.6
56.0
81.2
80.8
78.5
5.6
56.4
52.8
81.4
80.4
78.2
7.4
64.0
59.5
81.8
81.2
79.0
7.6
0.6
3.2
10.4
9.0
1.2
0.8
0.2
0.2
0.6
0.6
0.4
0.8
0.2
0.2
0.6
0.4
0.6
0.8
0.2
0.2
0.4
0.4
0.2
0.4
0.2
0.2
0.4
0.2
0.5
0.8
18.8
14.0
17.6
17.5
15.6
28.6
25.4
15.0
17.4
17.4
15.8
20.6
24.0
14.8
16.6
17.2
15.6
15.0
26.0
15.6
16.1
15.9
15.8
35.2
27.8
20.5
19.5
19.4
17.5
57.4
0.8
0.6
0.6
0.8
0.6
LUMO
HOMO
2-C2
1.0
5.2
1.4
3.3
3.6
22.6
3.6
0.1
1.5
1.7
0.7
1.2
0.6
0.6
0.8
0.6
10.4
LUMO
HOMO
3-C2
0.2
0.2
1.4
1.8
0.9
0.8
2.8
1.0
2.6
1.6
16.4
3.0
1.0
0.6
0.6
0.8
0.6
8.8
0.2
0.4
3.4
1.8
4.2
0.4
1.4
17.6
3.8
LUMO
HOMO
4-C2
0.4
0.1
1.5
1.8
0.9
1.2
0.6
0.6
0.8
0.8
12.4
0.2
0.4
3.4
1.6
2.9
0.6
1.4
31.8
2.5
LUMO
HOMO
5-C2
0.4
0.3
1.0
1.1
0.6
63.2
64.2
78.9
79.3
75.3
1.2
1.1
0.8
0.8
0.8
0.6
5.3
LUMO
HOMO
0.4
0.5
1.0
4.9
2.1
24.9
0.7
a
Ti and Cl s-orbital character ≤0.5% in the MOs provided.
2
2
Figure 6. Left, comparison of calculated %P 3p character in the Ti−P σ* and Ti−P π derived from the Ti 3dz2 (2a1 or 3a) and 3dx −y (1a1 or 1a),
respectively, for 1−3. Right, comparison of %P 3p character in the M-P σ* derived from the dxy (2b1 or 3b) in 1−3 and PdCl2
bis(diphosphino)alkane complexes as a function of methylene units in the diphosphine backbone. The P-M-P angles for each series of
compounds are provided in the figure for reference.56,57
that extend just beyond the d-manifold (as observed in 1−3;
Figure 4).
We compared the P 3p character mixing in the Ti−P π and
σ MOs as a function of ligand field strength. A plot of the %P
3p character revealed that Ti−P π orbital mixing increases as
expected with increasing ligand field strength in the order of
4 (alkyl) < 2 (phenyl) < 5 (CF3), whereas P 3p character in the
Ti−P σ* MO (3b) decreases (Figure 8). Simulated P K-edge
XAS spectra from TDDFT calculations on 2, 4, and 5 show the
same trend (Figure 9). The calculated oscillator strength for
P 1s → 1a (Ti−P π) increases only slightly in the order 4 ∼ 2
< 5 (f = 0.0004−0.0005), whereas P 1s → 3b (Ti−P σ*)
F
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Figure 7. Comparison of experimental (solid line) and calculated
(dashed line) P and Cl K-edge XAS spectra for Ti(dppm)Cl4 (1),
Ti(dppe)Cl4 (2), and Ti(dppp)Cl4 (3). Calculated transitions are
represented as red bars and their heights represent relative differences
in oscillator strength. Labels indicate the dominant MO mixing in the
excited state and correspond to those provided in Table 3 and Figure 4.
Figure 9. Simulated P K-edge XAS data for Ti(dcpe)Cl4 (4; top),
Ti(dppe)Cl4 (2; middle), and Ti(dtfpe)Cl4 (5; bottom). Individual
TDDFT transitions are represented as red bars. Calculated transitions
are represented as red bars and their heights represent relative
differences in oscillator strength. Labels indicate the dominant MO
mixing in the excited state and correspond to those provided in Table 3
and Figure 4.
measurable Ti−P π-bonding in all three phenyl-substituted
Ti diphosphine complexes. Simulated spectra obtained from
TDDFT calculations reproduced the experimental spectra and
allowed us to determine how Ti−P σ-bonding changes as a
function of alkane length in the diphosphine backbone. Our
analysis revealed that orbital-mixing in the Ti−P π and Ti−P
2
2
2
σ* derived from the Ti 3dx −y and 3dz , decreases as the
diphosphine alkane linker length is increased. In contrast, a
unique increase in Ti−P σ* orbital-mixing between the Ti 3dxy
and the phosphine lone pairs for the dppe complex 2 com-
pared to 1 and 3 with dppm and dppp, respectively. Despite
large differences in metal-type (early vs late transition metal),
oxidation state, coordination geometry, and P-M-P bite angle,
the relative differences in Ti−P σ bonding are identical to the
10% relative increase in Pd−P σ* orbital-mixing previously
reported for Pd(dppe)Cl2 and Pd(dcpe)Cl2 vs PdCl2 com-
plexes with one or three methylene groups in the diphosphine
backbone.56,57 Collectively, these results corroborate our previ-
ous reports that changes in M-P σ bonding cannot be explained
by only accounting for changes in diphosphine bite angle.
Comparison of DFT and TDDFT calculations on bis-
(diphosphino)ethane complexes with cyclohexyl and trifluor-
omethyl substituents (dcpe and dtfpe, respectively) with
Ti(dppe)Cl4 revealed the expected changes in diphosphine
σ donor and π acceptor strength as a function of ligand field
strength. Notably, Ti−P π-bonding increased slightly in the
order dcpe < dppe < dtfpe, whereas phosphine σ-donor strength
Figure 8. Comparison of %P 3p character in the 1a (Ti−P π; blue)
and 3b (Ti−P σ*; red) MOs of phosphorus substituents in
Ti(dcpe)Cl4 (4), Ti(dppe)Cl4 (2), and Ti(dtfpe)Cl4 (5).
decreases across the same series (f = 0.0019, 0.0016, and 0.0010
for 4, 2, and 5, respectively). Collectively, these results demon-
strate the expected changes in diphosphine σ-donor and
π-acceptor ability as a function of ligand field strength, as reported
previously in DFT studies of other diphosphine complexes.26
CONCLUSIONS
■
In summary, we have demonstrated how analyzing diphos-
phine complexes containing a d0 metal ion such as Ti4+ can be
used to investigate orbital mixing in M-P σ and π bonds. Our P
K-edge XAS results revealed small, but spectroscopically
G
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decreased across the same series. Even though the Ti complexes
are d0, the results revealed that the vacant Ti−P π-backbonding
MOs are present and ready to participate in π-backbonding once
d-electrons are provided. Notable in this respect is the fact that
formally reduced Ti2+, Ti0, and Ti2− complexes are known to
form with strong π-acceptor ligands such as CO and PF3.80−83
Although our P K-edge XAS studies demonstrated how
σ and π contributions to M-P bonds can be distinguished, the
pre-edge features lacked sufficient resolution to experimentally
quantify differences in M-P orbital-mixing using curve fit
models. This is a common issue in P K-edge data of transition
metal complexes with phosphorus ligands,51 but as shown here,
TDDFT calculations can be used in tandem to help extract
rich electronic structure information from the spectra. We anti-
cipate that the calculations can be used to more rapidly identify
metal−ligand combinations that yield highly resolved pre-edge
features for comparative analysis of phosphine and diphos-
phine donor−acceptor properties using P K-edge XAS.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.8b01511.
■
S
Experimental details and NMR spectra for 4, photos of
XAS cell, complete XAS spectra, atomic coordinates, and
Mulliken charges from DFT (PDF)
Accession Codes
CCDC 1846146−1846148 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: scott-daly@uiowa.edu.
*E-mail: jkeith@colgate.edu.
■
ORCID
Scott R. Daly: 0000-0001-6229-0822
Author Contributions
All authors have given approval to the final version of the
manuscript.
Funding
The American Chemical Society’s Petroleum Research Fund
(55989-DNI3).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.R.D. is grateful to The American Chemical Society’s
Petroleum Research Fund for their support, and J.M.K. thanks
Colgate University for computational resources. C.M.D. would
like to thank the U.S. Department of Education for a Graduate
Assistance in Areas of National Need (GAANN) fellowship.
We thank Dale Swenson for collecting the single-crystal XRD
data. Portions of this research were carried out at the Stanford
Synchrotron Radiation Laboratory (SSRL), which is a national
user facility supported by the US Department of Energy, Office
of Science, Office of Basic Energy Sciences, under Contract
DE-AC02-76SF00515.
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