Bimetallic nickel-lutetium complexes: tuning the properties and catalytic hydrogenation activity of the Ni site by varying the Lu coordination environment

Article abstract of DOI:10.1039/C8SC04712J

We present three heterobimetallic complexes containing an isostructural nickel center and a lutetium ion in varying coordination environments. The bidentate ⁱPr₂PCH₂NHPh and nonadentate (ⁱPr₂PCH₂NHAr)₃tacn ligands were used to prepare the Lu metalloligands, Lu(ⁱPr₂PCH₂NPh)₃ (1) and Lu{(ⁱPr₂PCH₂NAr)₃tacn} (2), respectively. Reaction of Ni(COD)₂ (where COD is 1,5-cyclooctadiene) and 1 afforded NiLu(ⁱPr₂PCH₂NPh)₃ (3), with a Lu coordination number (CN) of 4 and a Ni-Lu distance, d(Ni-Lu), of 2.4644(2) ?. Complex 3 can further bind THF to form 3-THF, increasing both the Lu CN to 5 and d(Ni-Lu) to 2.5989(4) ?. On the other hand, incorporation of Ni(0) into 2 provides NiLu{(ⁱPr₂PCH₂NAr)₃tacn} (4), in which the Lu coordination environment is more saturated (CN = 6), and the d(Ni-Lu) is substantially elongated at 2.9771(5) ?. Cyclic voltammetry of the three Ni-Lu complexes shows an overall ～410 mV shift in the Ni(0/I) redox couple, suggesting tunability of the Ni electronics across the series. Computational studies reveal polarized bonding interactions between the Ni 3d_z² (major) and the Lu 5d_z² (minor) orbitals, where the percentage of Lu character increases in the order: 4 (6.0% Lu 5d_z²) < 3-THF (8.5%) < 3 (9.3%). All three Ni-Lu complexes bind H₂ at low temperatures (?30 to ?80 °C) and are competent catalysts for styrene hydrogenation. Complex 3 outperforms 4 with a four-fold faster rate. Additionally, adding increasing THF equivalents to 3, which would favor build-up of 3-THF, decreases the rate. We propose that altering the coordination sphere of the Lu support can influence the resulting properties and catalytic activity of the active Ni(0) metal center.

Full text of DOI:10.1039/C8SC04712J

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Bimetallic nickel-lutetium complexes: Tuning the properties and catalytic hydrogenation
activity of the Ni site by varying the Lu coordination environment
Bianca L. Ramirez†, Prachi Sharma†,‡; Reed J. Eisenhart†; Laura Gagliardi†,‡; Connie C. Lu†
†Department of Chemistry and ‡Minnesota Supercomputing Institute & Chemical Theory Center,
University of Minnesota, Minneapolis, Minnesota 55455-0431, United States
Abstract
We present three heterobimetallic complexes containing an isostructural nickel center and a lutetium ion in
varying coordination environments. The bidentate iPr2PCH2NHPh and nonadentate (iPr2PCH2NHAr)3tacn
ligands were used to prepare the Lu metalloligands, Lu(iPr2PCH2NPh)3 (1) and Lu{(iPr2PCH2NAr)3tacn}
(2), respectively. Reaction of Ni(COD)2 (where COD is 1,5-cyclooctadiene) and 1 afforded
NiLu(iPr2PCH2NPh)3 (3), with a Lu coordination number (CN) of 4 and a Ni–Lu distance, d(Ni–Lu), of
2.4644(2) Å. Complex 3 can further bind THF to form 3−THF, increasing both the Lu CN to 5 and d(Ni–
Lu) to 2.5997(6) Å. On the other hand, incorporation of Ni(0) into 2 provides NiLu{(iPr2PCH2NAr)3tacn}
(4), in which the Lucoordination environment is more saturated (CN = 6), and the d(Ni–Lu) is substantially
elongated at 2.9771(5) Å. Cyclic voltammetry of the three Ni–Lu complexes show an overall ~410 mV
shift in the Ni(0/I) redox couple, suggesting tunability of the Ni electronics across the series. Computational
studies reveal polarized bonding interactions between the Ni 3dz2 (major) and the Lu 5dz2 (minor) orbitals,
where the percentage of Lu character increases in the order: 4 (6.0% Lu 5dz2) < 3−THF (8.5%) < 3 (9.3%).
All three Ni–Lu complexes bind H2 at low temperatures (−30 to −80 °C) and are competent catalysts for
styrene hydrogenation. Complex 3 outperforms 4 with a four-fold faster rate. Additionally, adding
increasing THF equivalents to 3, which would favor build-up of 3−THF, decreases the rate. We propose
that altering the coordination sphere of the Lu support can influence the resulting properties and catalytic
activity of the active Ni(0) metal center.

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Introduction
Despite a growing understanding of the chemical bonding between transition metals and 4f elements,
the application of d−4f metal interactions in homogenous catalysis has rarely been studied.1-3 In contrast,
utilization of 4f-and d-block metal cooperativity have proven beneficial in heterogeneous catalysis. For
example, lanthanide-based oxide supports have been shown to modify the electronic properties of bulk
transition metals in what has been termed as electronic metal–support interactions.4, 5 These electronic
perturbations have important ramifications in catalysis, where for example, Pt nanoparticles deposited on a
ceria support showed a 20-fold rate enhancement in the water-gas shift reaction compared to Pt(111).6
On a related note, 4f-block metal ions that have been incorporated into transition-metal oxide clusters
can significantly alter the overall redox potentials and reactivity. For example, a study of {LnMn3O4}
cubanes illustrates that the single Ln ion electronically modulates the Mn3O4 cores, where the
{LnIIIMnIV3O4}/{LnMnIV2MnIIIO4} redox couple increases linearly with the pKa of the {LnIII(OH2)6} ion, a
parameter of Lewis acidity.7 In a subsequent study on {LnIIICoII (OAc)4} cubanes, the single Ln ion serves
3
as an electronic modulator for the cluster and exerts a beneficial effect on the overall photocatalytic water
oxidation. The Ln ion boosts the water oxidation activity of the cluster by: (1) increasing the
electrochemical driving force, and (2) lowering the energy for acetate-water ligand exchange at the cluster.
These effects result in a large increase in the initial rate by two orders of magnitude for {LnCo3(OAc)4}
compared to the tetracobalt cubane, {CoII (OAc)4}.8 Of note, in both of these systems, the d and 4f metal
4
centers are separated by bridging oxygen atoms; and hence, these cubanes do not involve any direct d−4f
metal interactions. As an aside, a heterobimetallic Ni-NdIII complex was recently reported where the metal
centers are separated via bridging oxygen atoms at a long intermetal distance of 3.505(1) Å, which precludes
a direct d-4f interaction.9
Expanding on previous work in using direct Ni−group 13 interactions for promoting Ni-mediated
H2/CO2 catalysis,10-13 we hypothesized that a direct d−4f metal interaction would allow for a large electronic
perturbation of the transition metal, and potentially offer a greater degree of tunability with respect to
reactivity. Even so, structural examples of d−4f bonding interactions remain uncommon.14-23 Selected
examples are shown in Figure 1. To the best of our knowledge, no examples of catalytic reactivity have
been reported for any coordination compounds containing a direct d−4f metal interaction. Considering the
recent progress in using heterobimetallic metal-metal bonded complexes in catalysis,24-30 the pursuit of d−4f
complexes seemed ripe for exploration. Furthermore, the ability of lanthanides to support a larger range of
coordination numbers (CN = 3 to 12)31 may be advantageous as a new paradigm for tuning catalytic activity.
In this case, controlling the supporting Ln ion’s coordination environment affords another lever for catalyst
tuning.

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Figure 1. Selected examples of d−4f heterobimetallic complexes featuring the two metals in close
proximity.
Two new ligand frameworks were employed to make a triad of heterobimetallic nickel(0)−lutetium(III)
complexes allowing for the first study of nickel-lutetium bonding interactions. The choice of Lu was
motivated by the fact that LuIII is a diamagnetic ion, which allows for facile characterization by NMR
spectroscopy, and that LuIII is the most Lewis acidic of the Ln ions, with a pKa of the {LuIII(OH2)6} ion of
6.6 (c.f. pKa of {LaIII(OH2)6} is 9.9).32 Additionally, we show that the electronic properties at Ni are strongly
휎
influenced by alteration of the coordination sphere at Lu. The lutetium ion, which acts as a –acceptor to
Ni, is critical for initiating H2 binding at the nickel(0) center and its subsequent olefin hydrogenation
catalysis. In general, this study probes the effect of tuning the active transition metal beyond its first
coordination sphere by altering the coordination environment of the supporting metal.
Results and Discussion
Preparation of monometallic Lu(III) and bimetallic Ni-Lu compounds
We introduce two ligands, bidentate iPr2PCH2NHPh33 and nonadentate (iPr2PCH2NHAr)3tacn, which
are shown in Scheme 1. Both ligands comprise hard amido and soft phosphine donors. The key step in their
syntheses is the condensation reaction of aniline or 1,4,7-tris(2-aminophenyl)-1,4,7-triazacyclononane34
(abbreviated as tacn) with either 1 or 3 equiv of diisopropylphosphinomethanol, to afford iPr2PCH2NHPh
or (iPr2PCH2NHAr)3tacn, respectively.
Deprotonation of iPr2PCH2NHPh (3 equiv) or (iPr2PCH2NHAr)3tacn (1 equiv) with 3 equiv nBuLi
and subsequent addition of LuCl3 afforded the Lu(III) metalloligands, Lu(iPr2PCH2NPh)3 (1) or
Lu{(iPr2PCH2NAr)3tacn} (2), respectively, as white powders (Scheme 1). Complexes 1 and 2 display a
single 31P NMR resonance at −9.4 and −7.1 ppm, respectively, in C6D6. These resonances are both shifted
upfield from the free ligands, iPr2PCH2NHPh (4.2 ppm) and (iPr2PCH2NHAr)3tacn (3.3 ppm). The 1H-

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NMR spectrum of 1 shows a single sharp methylene resonance and two nearly coalesced methine
resonances, which suggests a nearly ideal C3v solution-state geometry for 1 (Figure S7). On the other hand,
1
the H-NMR spectrum of 2 has two distinct methine peaks and diastereotopic methylene protons in both
the PCH2N and the tacn moieties, which is consistent with C3 solution-state geometry for 2 (Figure S8).
Scheme 1. Synthesis of Lu(III) metalloligands (1 and 2) and the corresponding Ni-Lu bimetallic
complexes (3, 3−THF, and 4).
The heterobimetallic Ni-Lu compounds, NiLu(iPr2PCH2NPh)3 (3) and NiLu{(iPr2PCH2NAr)3tacn} (4),
were isolated from the reaction of Ni(COD)2, where COD = 1,5-cyclooctadiene, with 1 and 2, respectively
(Scheme 1). One interesting difference is that the metalation of 1 with Ni(COD)2 gave an immediate color
change to dark red, whereas the color of the corresponding reaction with 2 deepened more gradually over
several hours to a dark purple-red. Complexes 3 and 4 exhibit a single 31PNMR resonance at −0.8 and 15.0
ppm, respectively, when dissolved in C6D6.
During the NMR studies, we uncovered a pronounced solvent effect on the speciation of 3. Upon
changing the solvent to THF-d8, the 31P resonance shifts downfield by 11 ppm. We hypothesized that the
THF solvent molecule can coordinate the unsaturated Lu center in 3 to form 3−THF. To interrogate this
hypothesis, we sought to first understand the THF-binding equilibrium between 3 and 3−THF. Titrating
THF into a toluene-d8 solution of 3 resulted in the broadening and shifting of a single 31P NMR resonance
(Figure S13, Δδmax = 11.3). This behavior is consistent with a rapid equilibrium process, where the two
species are rapidly interconverting such that only an average signal is observed.35 Plotting the change in the
31P chemical shift versus THF equivalents yields a hyperbolic binding isotherm that is consistent with a
simple equilibrium of 1:1 binding.36 At room temperature, saturation was observed at 80 equiv of THF
(Figure S12-S14), and the fitted binding equilibrium constant (Ka) of 59 ± 2 M−1,37, 38 which corresponds to
ΔG298K = −2.4 kcal/mol, signifies weak binding of THF to 3.39, 40

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On the other hand, 4 showed no notable solvent-dependence of its 31P chemical shift, which is consistent
with the Lu site being more fully coordinated and/or sterically protected within the triamido-tacn binding
1
pocket. The H-NMR spectra of 3 and 3−THF are both consistent with an average C3v solution-state
geometry, whereas the 1H-NMR spectrum of 4 is indicative of a locked C3 geometry (Figure S9-S11).
Molecular structures of monometallic Lu(III) and bimetallic Ni-Lu compounds
Single-crystal X-ray diffraction studies were performed on 1, 2, 3, 3−THF, and 4. The molecular
structures are shown in Figure 2 with average bond distances. Individual bond distances, bond angles, and
other relevant geometrical parameters are provided in Table 1. As designed, the two ligand platforms
provide different coordination environments for the Lu ion. The molecular structure of 1 reveals a six-
coordinate Lu in an N3P3-donor environment. The average twist angle (θ) between the P3- and N3-triangular
faces is 32.6 deg, which is close to the midpoint between an ideal octahedron (θ = 60 deg) and trigonal
prism (θ = 0 deg).41, 42
Figure 2. Molecular structures of 1−4 shown at 50% thermal ellipsoid probability. Hydrogen atoms and
non-coordinating solvent molecules have been omitted for clarity. The average bond lengths (Å) are shown.
Atom colors: Lu, green; Ni, pink; P, orange; N, blue; O, red; C, gray.

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Table 1. Geometrical Parameters, Including Bond Lengths (Å) and Angles (deg), for 1−4.a
1
2
3
3−THF
4
Ni−Lu
-
-
-
-
-
-
2.4644(2)
2.2078(4)
2.2211(4)
2.2275(4)
2.2188(7)
2.5989(6)
2.9771(5)
2.1576(8)
2.1642(9)
2.1683(15)
2.1634(19)
2.1834(8)
2.2046(9)
2.2121(8)
2.2000(14)
Ni−P
avg. Ni−P
Lu−P
2.8873(6)
2.9398(6)
2.9536(6)
2.9269(10)
2.2207(17)
2.2233(19)
2.2251(17)
2.223(3)
3.2451(9)
-
-
-
avg. Lu−P
Lu−Namide
avg. Lu−Namide
P−Ni−P
-
-
-
-
2.210(3)
2.251(3)
2.252(3)
2.238(5)
2.2037(12)
2.2132(11)
2.2211(11)
2.213(2)
121.133(15)
118.340(15)
119.928(15)
359.401(3)
115.88(4)
118.47(4)
118.04(5)
352.39(8)
2.1834(8)
2.2046(9)
2.2121(8)
2.235(1)
121.99(3)
118.62(3)
118.58(3)
359.19(5)
112.32(9)
114.48(9)
133.20(9)
360.00(16)
2.298(2)
2.299(3)
2.324(2)
2.307(4)
120.26(3)
116.03(5)
122.35(5)
358.64(8)
116.30(8)
119.30(9)
113.09(9)
348.69(15)
2.559(2)
2.563(2)
2.563(2)
2.562(3)
−0.4533(14)
0.1464(8)
-
-
Σ(P−Ni−P)
Namide−Lu−Namide
Σ(Namide−Lu−Namide
Lu−Ntacn
-
-
109.79(7)
108.75(6)
105.39(6)
323.93(11)
103.37(10)
106.64(10)
126.55(9)
336.56(17)
2.445(3)
2.495(3)
2.502(3)
2.481(5)
−0.6281(16)
-
)
-
-
-
avg. Lu−Ntacn
Lu to N3-plane
Ni to P3-plane
0.7935(10)
-
0.3559(7)
0.0995(3)
0.0090(13)
0.1150(6)
aEstimated standard deviations (esd’s) are provided in parentheses.
In 2, the Lu center is coordinated by six N-donors: three relatively short Lu−Namide bonds of 2.238(5)
Å and three slightly longer Lu−Ntacn bonds of 2.481(2) Å. The average twist angle of 34.0 deg is also
indicative of an intermediate geometry between octahedral and trigonal prismatic. Further, owing to the
favoring of high coordination numbers, an additional weak Lu−P interaction with a distance of 3.2451(9)
Å was observed in the molecular structure of 2. The two Lu metalloligands also differ in the position of the
Lu center relative to the triamido donor set, which will be referred to as forming the N3-plane. In 1, Lu
resides in-between the P3- and N3-planes at ~0.8 Å “above” the N3-plane. On the other hand, Lu resides
~0.6 Å “below” the N3-plane in 2.
The three Ni-Lu bimetallic complexes, 3, 3–THF, and 4, each have a common NiP3 site, but a different
Lu geometry and coordination number (CN). In 3 and 3−THF, the Lu geometry is trigonal monopyramidal
(CN = 4) and trigonal bipyramidal (CN = 5, τ5 = 0.88), respectively.43 In 4, the Lu geometry is intermediate
between octahedral and trigonal prismatic (CN = 6, θ = 35.6 deg). The Ni−Lu bond distance also varies
significantly across the triad. Interestingly, the coordination environment of the Lu appears to dictate the
proximity between Ni and Lu. For example, complex 3 possesses the lowest coordinate Lu in this series

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and has the shortest Ni−Lu bond distance of 2.4644(2) Å. In 3−THF, the addition of a single THF donor
along the metal-metal axis increases the CN of Lu by 1 and elongates the Ni−Lu bond distance by 0.14 Å
to 2.5989(6) Å. In 4, the 6-coordinate Lu center is either non- or only weakly bonding to Ni with a long
Ni−Lu distance of 2.9771(5) Å, which is longer than that in 3 by over 0.5 Å.
Complexes 3, 3−THF, and 4 join a handful of crystallographically characterized compounds containing
both Ni and Lu metals. Among these examples, the intermetal distances are too large to allow for any
significant metal-metal interactions.44-50 Hence, without sufficient experimental comparisons to evaluate
and/or interpret our Ni−Lu bond distances, we considered several different approaches for estimating a
single bond length that is based on summing the two atoms’ radii. Depending on the radii values, predictions
of a single bond length can vary. Pyykkö and Atsumi derived a self-consistent system of single-bond
covalent radii based on both experimental and theoretical data.51, 52 Covalent radii have also been tabulated
by Cordero and co-workers using a large data set obtained from the Cambridge Structural Database.53
Another complementary set of values are Pauling’s single-bond metallic radii.54 Using the above
approaches, the predicted distances of a single Ni−Lu covalent bond are: 2.72 Å (Pyykkö and Atsumi), 3.11
Å (Cordero et al.), and 2.706 Å (Pauling). Compared with these estimates, the Ni−Lu bond lengths in 3
(2.46 Å) and 3−THF (2.60 Å) are significantly shorter. Of note, the shortest Ni-Lu bond distances that were
previously reported are in the range of 2.92 to 3.15 Å (Table S2).44, 48 Hence, we conclude that the Ni−Lu
bond lengths in both 3 and 3−THF are consistent with direct Ni−Lu bonding interactions.
Notably, the intermetallic bond distance in 3 is significantly shorter than that of any other d-f bimetallic
compound that has been crystallographically characterized (Table S2). Prior to this work, the shortest d-f
bond length of 2.520(5) Å was reported for NiU(2-PPh2-4-Me-6-tBu(C6H2O))3.55 If one accounts for the
single-bond covalent radius difference between Lu (1.62 Å) and U (1.70 Å), then 3 and NiU(2-PPh2-4-Me-
6-tBu(C6H2O))3 have similar r values of 0.91 and 0.90, respectively, where r is the ratio of their metal-
metal bond distance to the corresponding sum of the metals’ single-bond radii.51 Also, only a handful of
Lu−group 10 compounds have been reported that have intermetal distances < 3 Å.15, 44, 56, 57 These limited
examples include (C5Me4SiMe2CH2PPh2)Lu(μ-CH2SiMe2CH2)(OC4H8)PtMe2 and [(Ph2PNHPh)M{μ-
(Ph2PNPh)}3Lu(μ-Cl)Li(THF)3] (M = Pd or Pt), where the intermetallic distances are longer at 2.7668(5)
Å, 2.9031(11), and 2.9523(9), respectively.
Comparing 3, 3−THF, and 4, the Lu−Namide bond length elongates with increasing CN. In both ligand
systems, the Lu ion becomes increasingly co-planar with the triamido donors upon incorporating Ni into
the metalloligand. The distance between Lu and the N3-plane also correlates well with the Ni−Lu distance.
In 3, Lu is 0.4 Å above the N3-plane and closest to Ni. In 3−THF, Lu is nearly co-planar with a slightly
longer Ni−Lu distance. In contrast, Lu is positioned below the N3-plane by 0.5 Å in 4, which is consistent
with little or no interaction with Ni. On the other hand, the Ni site is relatively invariant across 3, 3−THF,

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and 4, where the distance between Ni and the P3-plane only changes slightly, from 0.10 to 0.15 Å. The only
notable difference in the Ni coordination sphere is the contraction of the Ni−P bonds from 3 (avg. 2.22 Å)
to 3−THF (2.20 Å) to 4 (2.16 Å). This trend is consistent with increased π-back-bonding from a more
electron–rich Ni center in 4 (relative to 3 and 3–THF) to the phosphine ligands. The greater Ni electron
density in 4 further suggests diminished Lewis acidity of Lu(III), which can be rationalized by the longer
Ni–Lu distance and the increase in the Lu(III) coordination environment.
Electrochemistry
To probe the influence of the Lu(III) metalloligand on the electronics at the Ni center, cyclic
voltammetry experiments were performed. Figure 3 is an overlay of the cyclic voltammograms (CVs) of 3,
3−THF, and 4. The CVs were collected in 0.1 M [nPr4N][BArF ] electrolyte solutions (ArF = 3,5-
4
bis(trifluoromethyl)phenyl) and internally referenced to the [FeCp2]+/0 potential. Because donating solvents
can bind an unsaturated Lu(III) center, the electrochemical study of 3 was performed in 1,2-difluorobenzene
(DFB).58 To minimize any shifts in the [FeCp2]+/0 reference potential due to solvent effects,59 the CV of 4
was also measured in DFB. For the CV study of 3–THF, the sample was prepared by adding 320 equiv
THF to 3 in DFB. For both 3–THF and 4, the CVs were also collected in THF.
4
3-THF
3
-0.4 -0.6 -0.8
-1
-1.2 -1.4 -1.6 -1.8
+/0
V (vs FeCp2
)
Figure 3. CVs of 3, 3−THF and 4 with 0.1 M [nPr4N][BArF ] electrolyte in DFB (scan rate of 100 mV/s;
4
collected under Ar).
In DFB, 4 displayed a reversible Ni(0/I) oxidation at E = −1.41 V vs [FeCp2]+/0 (Figure 3).12 Complex
½
3 showed an irreversible oxidation at Epa = −1.00 V in DFB, which is ~410 mV more positive than that of
4. In situ generation of 3–THF results in a ~50 mV cathodic shift in the irreversible oxidation to Epa = –1.05
V (Figures 3 and S42). In THF, the Ni(0/I) oxidation for 3−THF becomes quasi-reversible at Epa = −0.97
V (ipc/ipa = 0.6 at 250 mV/s, Figure S43). The Ni(0/I) redox couple for 4 remains reversible in THF, with

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E = –1.44 V (Figure S45). Of note, the Ni(0/I) redox potential for 3−THF is 470 mV more positive than
½
that of 4 in THF, whereas the difference in their redox potentials decreases to ~360 mV in DFB.
Overall, the Ni(0/I) oxidation potential becomes increasingly positive in moving from 4 to 3−THF to
3. This trend correlates with the increasing strength of the Ni−Lu interaction, as reflected by the intermetal
distances. Hence, the Ni(0) center in 3 shows the greatest withdrawal of electron density, or alternatively,
the Lu(III) support in 3 exhibits the greatest Lewis acidity in this series. This supports the hypothesis that
the less coordinatively saturated Lu(III) supporting ion more greatly perturbs the Ni electronics, presumably
via better bonding overlap with the soft Ni(0) Lewis base. Because the Ni-Lu interaction is greatly
attenuated in 4, the Ni electronics may be expected to resemble that of the mononickel complex, Ni{N(o-
(NCH2PiPr2)C6H4)3},60 which has an isostructural Ni(0) center within a tris(diisopropylphosphine)
coordination environment. The mononickel complex displays a reversible Ni(0/I) redox couple at E1/2 = –
1.26 V in 0.1 M [nPr4N][BArF ]/DFB (Figure S46), signifying that 4 is slightly more electron-rich than
4
Ni{N(o-(NCH2PiPr2)C6H4)3}.
As an aside, an irreversible reduction at Epc ~ −3 V was also observed for 3−THF in 0.1 M
[nBu4N][PF6]/THF, whereas no reduction events are observed for 4 in the same electrolyte solution (Figure
S47). So far, no reduction process has been observed for 3. However, this may be due to the more limited
electrochemical window of DFB, for which we measured a lower limit of −2.8 V vs [FeCp2]+/0 (Figure
S52).
Electronic Absorption Spectroscopy
The colors of the Ni-Lu complexes are varying shades of red: bright red for 3, red orange for 3−THF,
and purple red for 4. Figure 4 shows an overlay of the UV-Vis spectra for the Ni-Lu complexes. Compound
4 displays an intense band at 504 nm (ε = 4700 M−1 cm−1). A similar absorption was reported for the
mononickel compound, Ni{N(o-(NCH2PiPr2)C6H4)3} (cf. 491 nm, ε = 4300 M−1 cm−1).60 The striking
similarity between 4 and Ni{N(o-(NHCH2PiPr2)C6H4)3} also suggests that the Lu(III) ion minimally
perturbs the Ni electronics in 4.
6
3
3-THF
4
4
2
0
350
450
550
650
750
Wavenumber (nm)

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Figure 4. UV-Vis spectra of complexes 3 (red) and 4 (green) in DFB and 3−THF (blue) in THF at 298 K.
Complexes 3 and 3–THF each display two overlapping bands in the region from 370 to 420 nm, and a
third low-intensity absorption at higher wavelengths of 515 and 550 nm, respectively (Figure 4, S50, and
S51). Both spectra qualitatively resemble that reported for NiAl{N(o-(NCH2PiPr2)C6H4)3}, which contains
a dative Ni→Al bonding interaction.61 Hence, we propose that the stark change in the UV-vis spectrum of
4 and that of 3 or 3−THF is consistent with the presence of Ni→Lu bonding interactions in both 3 and 3–
THF.
Computational Investigation of 3, 3–THF, and 4
To investigate the electronic structures of 3, 3–THF, and 4 and to better understand the nature of their
metal−metal interactions, we performed quantum-chemical calculations on the full structures. Geometry
optimizations were conducted using density functional theory (DFT) at PBE-D362-64 level of theory, and
the optimized ground-state geometries compare well to the experimental structures (Table S7). For further
studies, complete active-space self-consistent field (CASSCF)65 calculations were performed. For each Ni-
Lu complex, the natural orbitals included five Ni 3d orbitals that are each doubly occupied. Notably, one
of the orbitals, though heavily Ni based (ca. 90% or greater), showed non-negligible contributions from Lu
(Figure 5, S52-54). This natural orbital revealed a polarized bonding interaction between the Ni 3dz2 and
the Lu 5dz2, where the percentage of Lu character increases slightly in the order: 4 (6.0% Lu 5dz2) < 3–THF
(8.5%) < 3 (9.3%) (Table S9). The trend is consistent with the increasing proximity of the Ni and Lu centers
and with a lower coordination number for Lu, both of which would result in better bonding overlap between
these two metals. The large degree of polarization in this natural orbital further suggests that the Ni-Lu
bonding is best described as a dative interaction of the Ni 3dz2 electrons into the empty Lu 5dz2 orbital.

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Figure 5. CASSCF-derived natural orbitals for 4, shown with occupation numbers. Similar natural orbitals
were observed for 3 and 3−THF. Inset shows the Ni 3dz2 natural orbitals for 3 and 3−THF, where a minor
contribution from Lu 5dz2 is visible.
The MO diagrams for 3, 3−THF, and 4, which were obtained from DFT calculations, are shown in
Figure 6 (Fig. S55-S57). In comparing 3 and 4, the different Lu supports have a profound effect on the
relative energy of the Ni 3dz2 orbital. For 4, the Ni 3dz2 orbital is more destabilized than the Ni 3dxz/3dyz
2
2
orbitals and is stabilized relative to the Ni 3
/3dxy orbitals, as one would expect for a metal center
dx ―y
with trigonal donors. For both 3 and 3−THF, the Ni 3dz2 orbital is the most energy-stabilized Ni d-orbital,
presumably due to Ni 3dz2→Lu 5dz2 interaction. Also of note, the LUMO for all the Ni-Lu bimetallic
complexes is primarily comprised of the Ni 4pZ and Lu 6s/5dz2 orbitals, with additional contribution from
the P 3pZ orbitals (Table S13). The presence of an energetically accessible metal-based 4pZ LUMO has also
been invoked in other transition metal-group 13 coordination complexes.28, 66, 67 Binding of weak sigma
11, 12, 68, 69
donors, ranging from solvent donors67 to H2
have been reported, which can be attributed to the
energetically low-lying, metal-based 4pZ acceptor orbital.70 Hence, the prediction of similar LUMOs in
each of the Ni-Lu bimetallic complexes may also indicate that their respective Ni sites are primed to bind
H2.
Figure 6. DFT-derived qualitative MO diagrams across the Ni-Lu series.
H2 Reactivity and Catalysis
Compounds 3, 3−THF, and 4 showcase a range of H2 binding reactivity. At ambient temperature and 1
atm H2, none of these complexes showed any formation of any Ni(η2-H2) adducts.12 However, the 31P peak

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1
of the bimetallic compounds and the H signal for free H2 both shifted slightly, which could be a hint of
weak binding (Figure S16-S23). Hence, to maximize H2 binding, samples of 3−THF and 4 in THF−d8 were
subjected to 4 atm H2 (at room temperature) and characterized in situ by low-temperature NMR
spectroscopy. To study H2 binding to 3, the same protocol was applied except that toluene−d8 was used as
the solvent. The low T NMR spectra for 3 have notably weaker signal intensities due to its generally poor
solubility in non-coordinating solvents.
At −80 °C and 4 atm H2, an equilibrium between 3 and a new species was observed in an approximate
31
1:0.4 ratio based on the appearance of two P peaks at −1.7 and 9.6 ppm, respectively (Figures S24 and
S25). The assignment of the new species as the η2-H2 adduct, 3−(H2), is based on the appearance of a broad
1H resonance at −1.4 ppm (Figure S26). T1(min) relaxation time measurements, however, could not be
obtained due to the broadness of the resonance. At −80 °C and 4 atm H2 in THF−d8, a similar equilibrium
between 3−THF and a new species was observed in an approximate 1:2.5 ratio based on the appearance of
two 31P peaks at 8.9 and 22.5 ppm, respectively (Figures S29 and S30). The assignment of the new species
as the η2-H2 adduct, 3(H2)−THF, is based on the appearance of a broad 1H resonance at −1.3 ppm (Figure
S31), whose short T1(min) relaxation time of 20(1) ms (400 MHz) is consistent with an intact H2 ligand
(Figure S33).71 On the other hand, exposing 4 to 4 atm H2 at −80 °C (in either THF−d8 or toluene−d8 ) did
not generate an observable H2 adduct, though broadening in the 31P resonance and the disappearance of the
free H2 resonance both suggest that 4 does interact with H2, even if weakly (Figures S34 and S35). Hence,
the strength of the H2 interaction with the Ni(0) center decreases in the order, 3-THF > 3 >> 4. Moreover,
the in situ characterization of 3−(H2)THF adds to the few (η2-H2)Ni(0) examples in the literature.11,12 Since
the Ni center is more electron-deficient in both 3−THF and 3 than in 4, the Ni-Lu compounds roughly
follow the same trend that was observed previously for bimetallic Ni-group 13 complexes.12, 28 Namely, the
more Lewis acidic metalloligands lead to more stable Ni(η2-H2) adducts.
Following the H2 binding studies, we investigated the propensity of 3 and 4 to mediate catalytic
olefin hydrogenation, a process which is typically challenging for a single Ni center to perform.10, 12, 68, 72-79
In general, the greater lack of molecular Ni hydrogenation catalysts compared to related first-row transitions
metals such as Fe and Co may be attributed to the greater electronegativity of Ni, which would hinder π-
backbonding and consequently, H2 activation.80 Using a loading of 2.5 mol %, 3 catalyzes the hydrogenation
of styrene to ethyl benzene in high yield under 4 atm H2 and heating at 100 °C in toluene-d8 for 2 h (Table
2, entry 1). Under these standard conditions, 4 also performs the catalysis, albeit more sluggishly and in
low yield (entry 2). The importance of the Lu supporting ion can be inferred from the monometallic Ni
control reactions (entries 3−5), where neither the mononickel complex, Ni{N(o-(NCH2PiPr2)C6H4)3}, nor
the catalyst mixtures of Ni(COD)2 with either of the current ligands gave any significant product. Further,
the Lu metalloligands (1 and 2) by themselves do not mediate this catalysis (Table S4). Finally, the presence

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of excess Hg during catalysis did not affect the turnovers achieved by either 3 or 4, which supports their
homogeneous nature (see ESI).

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Table 2. Hydrogenation of Styrene to Ethylbenzene Mediated by 3 and 4a
entry
catalyst
T (°C)
% conversion
overall rate (h−1)
1
2
3
4
100
100
100
100
100
63
94(4)c
24(3)c
<1c
18.8(9)
4.7(2)
0
3
Ni{N(o-(NCH2PiPr2)C6H4)3}
Ni(COD)2 + 3 equiv iPr2PCH2NHPh
Ni(COD)2 + (iPr2PCH2NHAr)3tacn
3
4
8(1)c
1.6(2)
0
5
<1c
6
>99d
4.1(1)
1.4(1)
3.9(1)
3.5(1)
3.1(1)
2.7(1)
7b
8
3–THF
63
35(2)d
96(1)d
86(2)d
77(1)d
68(3)d
3 + 20 equiv THF
3 + 40 equiv THF
3 + 110 equiv THF
3 + 660 equiv THF
63
9
63
10
11
63
63
aCatalytic conditions: 2.5 mol % catalyst, 0.37 M olefin in ca. 600 μL of d8-toluene, 4 atm H2. Conversion are based
on triplicate runs using 1H NMR integration. bIn ca. 600 μL of d8-THF. ct = 2 h. dt = 10 h.
We also sought to investigate the effect of THF binding to the remote Lu site on the hydrogenation of
styrene. If the reaction solvent is changed to THF (and consequently, a lower reaction temperature of 63
°C), then the overall rate of catalysis diminishes by nearly three-fold between 3 and 3−THF (entries 5 and
6). However, the addition of less than 40 equiv THF has no observable effects on the rate. Above 40 equiv
THF, the rate perceptibly decreases with increasing THF equivalents. Presumably, the negligible changes
in rate at lower concentrations of THF is due to its weak reversible binding at the LuIII center of 3 such that
solvent effects are only manifested at higher concentrations. Indeed, the effect of THF on the rate of
catalysis highlights the remote binding effect of THF on 3 and, in turn, the importance of the lanthanide
coordination environment in the design of future d–f-bonded heterometallic catalysts.
Lastly, the substrate scope was further investigated for catalyst 3 (Table S5, % conversion at 24 h).
Under the standard catalytic conditions, 3 readily hydrogenated unhindered alkenes (>99%), including
terminal and cyclic olefins: 1-octene, allylbenzene, and cis-cyclooctene. Linear internal olefins were either
hydrogenated more sluggishly, e.g. trans-2-octene (65%), or were unreactive (<2%), e.g. trans-4-octene
and trans-stilbene. For cis-stilbene, facile isomerization to the thermodynamically favored trans-stilbene
was observed (73%) with some bibenzyl formation (7%). In the absence of H2, only a small amount of
isomerization (7%) was observed even after 26 h, which demonstrates the importance of H2 for the
isomerization reaction. Overall, the substrate scope for 3 is similar to that reported for a similar Ni-Ga
complex.12 One surprising difference is the different outcomes with allylbenzene: 3 generated
propylbenzene in nearly quantitative yield in 2 h, whereas reaction with the Ni-Ga catalyst only showed

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3% yield at 24 h. Perhaps, the greater flexibility of the non-tethered ligand of 3 allows for its greater
reactivity with this substrate.
Conclusions
This work studies the effect of tuning an active metal (Ni) beyond its primary coordination sphere,
specifically by tuning the ligand environment of a supporting Lewis acidic ion (Lu). Through the
employment of two new phosphinoamido ligand frameworks, 1 and 2, three new heterobimetallic Ni(0)-
Lu(III) complexes were synthesized. The ligand [iPr2PCH2NPh]– stabilizes the Ni–Lu complex, 3, with a
low-coordinate Lu ion (CN = 4), and the Ni−Lu interaction exists without any competing trans-ligand(s).
The Ni−Lu interaction can be perturbed simply by the addition of THF, which binds reversibly to the Lu
center in 3–THF (CN = 5). At the other extreme, the [(iPr2PCH2NAr)3tacn]3– ligand enforces a more
saturated Lu environment (CN = 6), resulting in a diminished Ni-Lu interaction in 4. This work showcases
the first two examples of coordination complexes with Ni−Lu bonding interactions, which also collectively
add to the few examples of complexes with direct 3d-4f interactions.
Varying the coordination environment of the distal Lu ion significantly impacts the ability of Lu to act
as a Lewis acidic acceptor for the Ni metal, which consequently, impacts the Ni-Lu bonding interaction.
Changes in the latter are reflected in the Ni-Lu distances differing by over 0.5 Å, as well as the variable
mixing of the Lu and Ni dz2 orbitals in the σ-bonding MO. We propose that the different Ni–Lu bonding
interactions are the underlying reason for the property and reactivity differences among these otherwise
isostructural Ni sites. For example, tunability of the Ni electronics across the series is reflected in the ~410
mV shift in the Ni(0/I) redox couple. Large variability in H2 binding is also evident: while barely detectable
for 4, Ni(η2-H2) adducts are spectroscopically characterized at low T for both 3 and 3−THF. To our
knowledge, the latter are the first reported non-classical H2 adducts for any d-f heterobimetallic compounds,
and they represent the first demonstration of using Ln supports to induce H2 binding at a single Ni center.
The accumulation of these remote coordination effects on the Ni electronics further influences their
catalytic activity. Complex 3 outperforms 4 in catalytic styrene hydrogenation by a factor of 4. Further, the
presence of an open coordination site at Lu in 3 presents the unique opportunity to tune catalytic activity
via external ligand binding at this remote site. Along these lines, adding increasing equivalents of THF to
3 does decrease the overall rate of hydrogenation, albeit more than 40 equiv of THF are necessary to impede
the rate.
In closing, d–f bonding interactions appear promising for promoting reactivity at a base transition metal
center. From the perspective of catalyst design, lanthanide supporting ions may offer key advantages. The
full Ln series is synthetically accessible for fine tuning of both the Lewis acidity and ionic size of the
supporting 4f ion. Developing low-coordinate Ln metalloligands that allow for dynamic binding of external

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ligands may also be potentially useful for switchable catalysis applications, and as a general design strategy
for tuning beyond the binding site. We are currently exploring these research avenues.
Electronic Supplementary Information
*Electronic supplementary information (ESI) available: Synthetic and computational details,
spectroscopic data, and detailed crystallographic information for 1–4 (CCDC 1870303-1870307).
Conflicts of Interest
There are no conflicts of interest.
Acknowledgements
The authors thank Dr. Laura J. Clouston and Dr. Dale R. Pahls for preliminary results and helpful
discussions. BLR is grateful to 3M for a research fellowship. The experimental work was supported by the
American Chemical Society Petroleum Research Fund (57192-ND3). C.C.L. acknowledges the National
Science Foundation (CHE-1665010) for support. The computational work was supported by the Division
of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S.
Department of Energy under Grant USDOE/DESC002183. X-ray diffraction experiments were performed
using a crystal diffractometer acquired through an NSF−MRI award (CHE-1229400) in the X-ray
laboratory supervised by Dr. Victor G. Young, Jr.

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Chemical Science
Page 20 of 20
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DOI: 10.1039/C8SC04712J
TOC graphic:
Varying the Lu coordination environment tunes the Ni-Lu interaction and the catalytic activity of
the Ni site in styrene hydrogenation.