Synthesis of chromium(II) complexes with chelating bis(alkoxide) ligand and their reactions with organoazides and diazoalkanes

Article abstract of DOI:10.3390/molecules25020273

Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]^Ph (H₂[OO]^Ph = [1,1⁰:4⁰,1⁰’-terphenyl]-2,2⁰’-diylbis(diphenylmethanol)) and their subsequent react

Full text of DOI:10.3390/molecules25020273

molecules
Article
Synthesis of Chromium(II) Complexes with
Chelating Bis(alkoxide) Ligand and Their Reactions
with Organoazides and Diazoalkanes
Sudheer S. Kurup 1, Richard J. Staples 2 , Richard L. Lord 3,
*
and Stanislav Groysman 1,
*
1
Department of Chemistry, Wayne State University, 5101 Cass Ave., Detroit, MI 48202, USA;
sudheer.kurup@wayne.edu
Department of Chemistry, Michigan State University, 578 S Shaw Ln, East Lansing, MI 48824, USA;
staples@chemistry.msu.edu
Department of Chemistry, Grand Valley State University, 1 Campus Dr, Allendale, MI 49401, USA
Correspondence: lordri@gvsu.edu (R.L.L.); groysman@wayne.edu (S.G.)
2
3
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Klaus Banert
Received: 12 December 2019; Accepted: 7 January 2020; Published: 9 January 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]Ph
(H2[OO]Ph = [1,10:40,10’-terphenyl]-2,20’-diylbis(diphenylmethanol)) and their subsequent reactivity
in the context of catalytic production of carbodiimides from azides and isocyanides are described.
Two different Cr(II) complexes are obtained, as a function of the crystallization solvent:
mononuclear Cr[OO]Ph(THF)2 (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr2([OO]Ph
)
2
(in CH2Cl2/THF). The electronic structure and bonding in Cr[OO]Ph(THF)2 were probed by
density functional theory calculations. Isolated Cr2([OO]Ph)2 undergoes facile reaction with 4-
MeC6H4N3, 4-MeOC6H4N3, or 3,5-Me2C6H3N3 to yield diamagnetic Cr(VI) bis(imido) complexes;
a structure of Cr[OO]Ph(N(4-MeC6H4))2 was confirmed by X-ray crystallography. The reaction of
Cr2([OO]Ph)2 with bulkier azides N3R (MesN3, AdN3) forms paramagnetic products, formulated
as Cr[OO]Ph(NR). The attempted formation of a Cr–alkylidene complex (using N2CPh2) instead
forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]Ph(NNCPh2)2. Catalytic
formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various
aryl azides and isocyanides. The formation of carbodiimides was found to depend on the
nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides
with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or
4-methylphenylazide. Treatment of Cr2([OO]Ph 2 or H2[OO]Ph with NO+ leads to the formation of
)
[1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand.
Keywords: alkoxides; azides; carbodiimides
1. Introduction
Organoazides constitute efficient and sustainable precursors for the formation of reactive
nitrene functionality, which engenders many useful transformations, including C–H bond amination,
aziridination, formation of azoarenes and carbodiimides, and others [1–7]. Transition metals generally
enable milder conditions for the formation of nitrenes and higher selectivity in their subsequent
transfer. The majority of nitrene-transfer studies are focused on middle and late transition metals,
which generally feature weaker metal–imido bonds and therefore exhibit more reactive nitrene
functionalities [
reactive nitrene functionalities due to the stability of multiple metal-nitrogen bonds [22
recent studies demonstrated the ability of coordinatively unsaturated chromium(IV)–imido to transfer
8–
21]. In contrast, early transition metals (particularly chromium) typically exhibit less
–28]. However,
Molecules 2020, 25, 273; doi:10.3390/molecules25020273
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nitrene to olefins or isocyanides to form aziridines and carbodiimides, respectively [29
–
31]. C–C
and C–H bond activation reactions featuring reactive chromium(IV)–imido were also reported [32].
As part of our ongoing investigation of nitrene- and carbene-transfer chemistry mediated by 3d metal
complexes in bulky alkoxide ligand environments [33,34], we have reported the synthesis and reactions
of chromium–imido complexes featuring two bulky monodentate alkoxides [OCtBu2Ph] [31]. We have
demonstrated that CrIV(OCtBu2Ph)2(NR’) complexes, which were formed for bulky R’ groups (R’ =
Mes, 2,6-Et2C6H3, 1-adamantyl), enabled catalytic transfer of the nitrene group [NR’] to isocyanides
to form carbodiimides. In contrast, the reaction of less bulky para-substituted aryl azides N3R’ (R’ =
4-MeC6H4, 4-MeOC6H4, 4-CF3C6H4) formed Cr(VI) complexes CrVI(OR)2(NR’)2 that were catalytically
inactive. Recently, we have reported a new chelating bis(alkoxide) ligand [OO]Ph
[35] that created a
similar bis(alkoxide) ligand environment but warranted a higher degree of stability of the resulting
iron complex Fe[OO]Ph, and consequently led to a broader range of catalytic nitrene transfer compared
with related Fe(OR)2 systems [36,37]. Herein, we report our synthetic, structural, and catalytic studies
on the chemistry of CrII[OO]Ph with organoazides and diazoalkanes and compare this reactivity with
the previously reported CrII(OCtBu2Ph)2 system.
2. Results and Discussion
2.1. Synthesis and Structures of Cr(II) Precursors
Treatment of the chromium(II) amide precursor with one equivalent of H2[OO]Ph in THF produces
a green solid. Recrystallization of the product from the toluene/THF mixture yields mononuclear
pale green Cr[OO]Ph(THF)2 ( ) related to the mononuclear Cr(OCtBu2(3,5-Ph2C6H3))2(THF)2 [38].
1
In contrast, recrystallization of the product from a mixture of two polar solvents (CH2Cl2 and
THF) forms pale green–blue homodinuclear complex Cr2[OO]Ph
, Scheme 1) isolated in 79%
is structurally related to the previously reported alkoxide-bridged
Cr2(OCtBu2Ph)4 [31], Cr2(OCtBu3)2( 2-OCtBu2H)2 [39], and siloxide-bridged Cr2(OSitBu3)4 [40].
We note that monodentate alkoxides [OCtBu2Ph] and [OCtBu2(3,5-Ph2C6H3)] formed dinuclear or
mononuclear complexes selectively [31 38], whereas the chelating ligand appears to support both types
(
2
2
yield. Homodinuclear complex
2
µ
,
of geometries in the solid state, depending on the nature of the crystallization medium.
Scheme 1. Synthesis and reactivity of Cr(II) complexes in the chelating bis(alkoxide) ligand environment.

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The structures of complexes
1
and
2
are given in Figure 1. The structure of
1
is similar to the
previously reported structure of Fe[OO]Ph(THF)2 [35]. However, the structure of Fe[OO]Ph(THF)2
exhibited a very long distance between the metal and the central phenyl (>3 Å), indicating no
interaction [35]. In contrast, the distance between the chromium center and the central phenyl in
1
is 2.49(1) Å, which suggests bonding between the metal and the phenyl ring. The inter-alkoxide
RO-Cr-OR and THF-Cr-THF angles are 173.6(2)◦ and 88.1(2)◦, respectively. Two independent molecules
of homodinuclear complex were found in the asymmetric unit exhibiting similar structural parameters;
only one is shown. Each [OO]Ph ligand provides one terminal and one bridging alkoxide donor; no
coordination to the central phenyl is observed in this case (Cr—Ph 3.0 Å). While the overall geometry
and metrics in 39 40], the metal
are similar to those of Cr2(OCtBu2Ph)4 and related complexes [32
centers in exhibit significant distortion of trigonal planar geometry, as indicated by the considerable
2
≥
2
, ,
2
difference in O-Cr-O angles (Figure 2) and displacement of Cr from the trigonal plane (varies between
0.105 and 0.348 Å).
Figure 1. X-ray structures of
1
(left) and
2
(right), 50% probability ellipsoids. H atoms and co-crystallized
: Cr O1,
solvent molecules were omitted for clarity. Selected bond distances (Å) and angles (◦) for
1
1.924(6), Cr O2, 1.935(6), Cr O3, 2.156(7), Cr O4, 2.277(6), Cr — Ph 2.49(1), O1 Cr1 O2, 173.6(2), O1 Cr1
O3, 86.7(2), O2 Cr1 O3, 90.0(3), O1 Cr1 O4, 85.9(2), O2 Cr1 O4, 88.5(2), O3 Cr1 O4, 88.1(2). Selected
bond distances (Å) and angles (◦) for
2: Cr1 O1, 1.788(5), Cr2 O4, 1.799(5), Cr1 O2, 1.976(5), Cr1 O3,
2.013(6), Cr2 O3, 1.981(5), Cr2 O2, 2.049(5), Cr1 — Cr2, 2.896(2), O2 Cr1 O3, 81.2(2), O1 Cr1 O2, 150.2(3),
O1 Cr1 O3, 120.1(2).
Figure 2. Corresponding orbital diagrams for 1Q (left) and 1T (right) at the BP86-D3/def2-SVP level of
theory. Orbitals are plotted with an isosurface value of 0.05 au.

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To better understand the electronic structure and bonding in
1
, density functional theory (DFT)
calculations were performed. Singlet ( S), triplet ( T), and quintet (1Q) states were optimized. 1Q is
1
1
lowest in free energy, followed by 1T at 19.7 kcal/mol, and 1S at 28.6 kcal/mol. The optimized structure
of 1Q agrees well with the crystallographically determined structure and has bond lengths of Cr–O1 =
1.915 Å, Cr–O2 = 1.928 Å, Cr–O3 = 2.203 Å, Cr–O4 = 2.281 Å, Cr–C1 = 2.475 Å, and Cr–C2 = 2.525 Å;
both
Cr, 1Q is best described as a high-spin Cr(II) species. Similar to what was observed in the optimization
of different spin states of Fe[OO]Ph(THF)2 [35], the higher-energy, lower spin states 1T
S exhibit much
shorter Cr–phenyl contacts of 2.1–2.2 Å. Mayer bond orders for Cr–C1/C2 are calculated to be 0.25 and
0.23 in
Q. In comparison, quintet Fe[OO]Ph(THF)2 (with Fe–C distances of 2.8–2.9 Å) has smaller bond
1T and 1S are inconsistent with the x-ray data (Table S1, SI). Based on the spin density localized at
/
1
1
orders of 0.06 and 0.09, whereas 1T has larger bond orders of 0.64 and 0.65. For perspective, M–OTHF
bond orders are 0.2–0.3 and the M–Oalkoxide bond orders are 0.6–0.9. Frontier orbital diagrams of 1Q
and 1T (Figure 2) illustrate the Cr–phenyl bonding in both spin states. The much stronger bonding in
1T is due to the d-π backbonding that is possible with intermediate-spin Cr(II) ion. This back-bonding
is not significant in 1Q due to half-occupation of that d-orbital in the high-spin Cr(II) ion. Collectively,
these computational data suggest a weak covalent interaction between Cr and the bridging phenyl
moiety in 1.
We have also investigated the structures of
paramagnetic NMR spectra, as anticipated. The UV–Vis spectra of
similar, exhibiting a peak at approximately 710 nm, and a shoulder at approximately 510 nm (Figures
S39 and S40, SI). The solution magnetic moment of (3.0(3) B, C6D6) was found to be significantly
lower than the expected spin-only value of 4.9
B. Recalculated for the structure of Cr2([OO]Ph)2, the
B (per Cr), which is close to the observed value of 2.5(3)
1
and
2
in solution. Both compounds give rise to
1
and (collected in THF) are very
2
1
µ
µ
observed magnetic moment is 2.6(2)
µ
µB
reported for Cr2(OCtBu2Ph)2 [31]. These data suggest that
1 undergoes dimerization in solution to
form 2.
2.2. Reactions of Chromium(II) Complexes with Organoazides, Azoarenes and Diazoalkanes
Following the synthesis of the chromium(II) precursor, its reactivity with organoazides was
investigated. Complex was utilized as a precursor in group-transfer reactions. Similar to
Cr2(OCtBu2Ph)4 [31],
was found to exhibit profoundly distinct reactivity with non-bulky (4-tolyl,
4-methoxyphenyl, and 3,5-dimethylphenyl) vs. bulky (mesityl, 1-adamantyl) organoazides. Treating
a green solution of with two equivalents of para-tolylazide in THF leads to a dark-brown solution,
from which diamagnetic is isolated in 69% yield by recrystallization from THF/ether. Similarly,
treating with two equivalents of 4-MeOC6H4N3 or 3,5-Me2C6H3N3 forms brown solutions containing
diamagnetic products and in 54% and 59%, respectively. We were not able to obtain X-ray quality
crystals of and , however, their NMR and UV–Vis data (see SI) suggests similarity to . The overall
connectivity of was confirmed by X-ray structure determination (Figure 3). The structure reveals a
2
2
2
3
2
4
5
4
5
3
3
distorted tetrahedral geometry, consistent with Cr(VI) bis(imido) formulation. No chromium-phenyl
interaction is observed in this case, with the chromium-phenyl distance exceeding 3.5 Å. As the
structure was of relatively low quality, its metric parameters will not be further discussed.
In contrast, the reaction of the chromium(II) precursor with two equivalents of bulkier mesityl azide
(per Cr) produces a paramagnetic brown solution, containing approximately one equivalent of unreacted
azide. Similarly, treating chromium(II) precursor with two equivalents of 1-adamantyl azide also
produces paramagnetic product, along with unreacted azide. We were not able to obtain X-ray quality
crystals of the products. The formation of trigonal Cr(IV) mono(imido) complexes is proposed, based on
similar formation of brown paramagnetic CrIV(OCtBu2Ph)2(NMes) and CrIV(OCtBu2Ph)2(NAd) upon
reaction of [Cr(OCtBu2Ph)2] with excess azide. We note that selective formation of Cr(IV) mono(imido)
complexes upon reaction with bulkier organoazides was also observed for the chromium(II) system
ligated by two monodentate siloxides, [Cr(OSitBu3)2] [40].

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Figure 3. X-ray structure of
solvent molecules were omitted for clarity. Selected bond distances (Å) for
3
(left) and
6
(right), 50% probability ellipsoids. H atoms and co-crystallized
: Cr O1 1.807(2), Cr O2
6
1.777(2), Cr N1 1.645(3), Cr N2 1.716(3), N1 N3 1.305(4), N2 N4 1.301(4). Selected bond angles (◦): O2
Cr O1 110.0(1), N1 Cr N3 99.7(2), N1 Cr O2 110.5(1), N2 Cr O2 109.2(1), N1 Cr O1 113.1(1), Cr N1 N3
159.8(2), Cr N2 N4 131.2(2).
In addition to the reactions of Cr(II) precursor with organoazides, we have also studied its
reactivity with azobenzene, with the goal to produce Cr(VI) bis(imido) via azobenzene cleavage.
Cleavage of the N=N double bond in azoarenes at a single metal center is rare [41
–
44]. Heating the
◦
mixture of complex
2
with azobenzene to 60 C in C6D6 for 24 h did not lead to the formation of
CrVI[OO]Ph(NPh)2 as indicated by 1H NMR spectroscopy.
Next, the reactivity of the Cr(II) bis(alkoxide) precursor with diazoalkanes was investigated,
targeting formation of Cr(IV)/Cr(VI)-alkylidene complexes. Cr–alkylidene complexes are rare [26 45 46],
The related cobalt(II) bis(alkoxide) complex
,
,
and their reactivity is relatively unexplored.
Co(OCtBu2Ph)2(THF)2 formed high-valent cobalt-alkylidene upon reaction with N2CPh2 and
N2C(Ph)(CO2Me) [47], whereas the corresponding iron complex Fe(OCtBu2Ph)2(THF)2 reductively
coupled diazoalkanes/diazoesters via terminal nitrogens to form bridging tetrazene species [48].
A different reactivity mode was observed for the chromium bis(alkoxide). Treatment of
2 with
diphenyldiazomethane results in the formation of brown Cr[OO]Ph(NNCPh2)2 species (
6
), which
was characterized by X-ray crystallography. The complex appears unstable in both solid state and
solution. Our repeated attempts to collect the NMR spectrum of the X-ray quality crystals produced
spectra with varying but significant decomposition products (benzophenone azine, free ligand); peaks
attributable to the complex were broad and not informative. Wolczanski and coworkers have reported
similar reactivity of Cr2(OSitBu3)4 with N2CPh2 to yield Cr(OSitBu3)2(N2CPh2)2 complex [40] and
their complex also demonstrated limited stability.
The structure of
6 and selected metrics are given in Figure 3. The chromium center exhibits
distorted tetrahedral geometry. Interestingly, the Cr–N bonds are different, one being significantly
shorter (1.645(3) Å) than the other (1.716(3) Å). The shorter bond could be indicative of a higher
metal-nitrogen bond order (triple), while the longer one suggests a metal-nitrogen double bond.
The Cr–N–N bond angles of the [NNCPh2] functionality are consistent with these formulations: for the
triply bonded [Cr
N1-N3] hydrazido, the angle is closer to linear (159.8(2) ◦), whereas for the doubly
≡
bonded [Cr=N2-N4] the angle is smaller (131.2(2) ◦). The N1-N3 and N2-N4 bonds are similar (1.304(4)
vs. 1.301(4) Å), being intermediate between single and double. Similar dependence of the Cr–N–N
angle within diphenylmethylenehydrazido functionalities on the chromium-nitrogen bond order was
reported by Wolczanski, Cundari, and coworkers for the related Cr(OSitBu3)2(=N2CPh2)2 [40]. The
major difference between the present structure and the structure reported by Wolczanski is that in

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Cr(OSitBu3)2(=N2CPh2)2 the Cr–N bonds were similar (1.664(2) and 1.666(2) Å), leading to similar
Cr–N–N bond angles (150.2(2) and 150.4(3) ◦).
DFT calculations on
6
were performed to analyze the electronic structure of this species. Singlet
Q) states were optimized and the singlet was found to be lowest
(6
S), triplet ( T), and quintet (6
6
in free energy followed by the triplet (+15.1 kcal/mol) then the quintet (+33.9 kcal/mol). Only the
optimized structure of 6S is consistent with the experimental X-ray structure (Table S3, SI) and has no
unpaired electrons. As the localized frontier orbitals in Figure 4 demonstrate, there are two -π-bonding
interactions with the more linear diazoalkane ligand (HOMO–1/LUMO+3, HOMO–3/LUMO) compared
to one with the less linear diazoalkane ligand (HOMO/LUMO+4), though the HOMO–1/LUMO+3 pair
shows interaction with both diazoalkane ligands. The modest difference in Mayer bond orders of 1.61
vs. 1.52 for Cr–N1 vs. Cr–N2 suggests that both Cr–N bonds are between doubly and triply bonded,
but that bonding to N1 is stronger.
Figure 4. Frontier orbital diagram for 6S at the BP86-D3/def2-SVP level of theory. Orbitals are plotted
with an isosurface value of 0.05 au.
2.3. Reactivity of Cr(II) with CO, CNR and NO+
Following the investigation of the reactivity of Cr(II) bis(alkoxide) precursor with organoazides
and diazoalkanes, its reactivity with isocyanides, carbon monoxide, and nitrosonium was explored next.
It was previously shown that Cr2(OCtBu2Ph)4 reacted with isocyanides to form CrII(OCtBu2Ph)2(CNR)4
(R = 2,6-Me2C6H3); formation of CrII(OCtBu2Ph)2(CNR)4 enabled subsequent development of the
catalytic cycle for the production of carbodiimides. Hoping that the more stable and sterically
demanding chelating bis(alkoxide) system will enable better catalytic reactivity in CNR and perhaps
CO transfer, reactions of Cr[OO]Ph with xylyl isocyanide and carbon monoxide were investigated.

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The reactivity of this species with nitrosonium ((NO)(BF4)) was also studied, in light of the isoelectronic
nature between NO+ and CO/CNR.
Addition of excess xylyl isocyanide to the light-green solution of
2 (dissolved in THF) forms a
brown solution immediately. 1H NMR spectrum of the product indicates its paramagnetic nature.
Although our multiple attempts to obtain X-ray quality crystals of the isocyanide product did not lead
to fruition, the UV–Vis spectrum of the crude product is similar to the UV–Vis spectrum of isolated
CrII(OCtBu2Ph)2(CN(2,6-Me2C6H3))4 (peaks at approximately 400 and 460 nm, Figure S44) suggesting
similarity of the products. In contrast, no color change was observed after treating the solution of
2 in
THF with CO. Subsequent recrystallization from a toluene/THF mixture formed instead the crystals of
compound 1, further confirming the lack of reactivity with CO.
The reaction of
(THF) led to the development of cloudy yellow solution over the course of several
hours. Subsequent recrystallization from THF yields colorless crystals of 6,6,12,12-tetraphenyl-6,12-
dihydroindeno[1,2-b]fluorene (Figure 5) isolated in 92% yield. The reaction course does not depend
on the presence of Cr: treatment of the ligands precursor H2[OO]Ph with (NO)(BF4) yields the same
product . Cyclization of terphenyl bis(carbinol)s via carbocation intermediates to give various
substituted dihydroindenofluorenes had been previously reported [49 50]. We surmise that the reaction
2
with NO+ took an unexpected turn. Addition of (NO)(BF4) to the green solution
of
2
7
7
,
proceeds via nitrosonium abstraction of hydroxyl (to yield nitrous acid) followed by cyclization,
as described in Scheme 2.
Figure 5. X-ray structure of 7, 50% probability ellipsoids. H atoms and co-crystallized solvent molecules
were omitted for clarity.
Scheme 2. Proposed cyclization mechanism of H2[OO]Ph
2.4. Catalytic Reactivity in the Formation of Carbodiimides
.
Given the ability of the CrII[OO]Ph precursor to undergo stoichiometric reactions with organoazides
and isocyanides, we investigated its catalytic reactivity in formation of carbodiimides. A matrix of
four representative aryl azides of varying steric bulk (mesityl, 2-methylphenyl, 3,5-dimethylphenyl,
and 4-methylphenyl) and four representative isocyanides (xylyl, 4-methoxyphenyl, 1-adamantyl, and
cyclohexyl) was investigated (Scheme 3). All reactions were carried out in C6D6 at 60 ◦C for 24 h; the
nature of the reaction products and their yields were determined by 1H NMR spectroscopy. The outcome
of the catalytic reaction appears to be a primary function of the organoazide. Catalytic formation
of carbodiimides is observed for the bulkier mesityl azide. The reaction of mesityl azide with xylyl
isocyanide gives nearly quantitative yield; good to moderate yields (72–50%) are observed for other

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isocyanides. For the ortho-monosubstituted 2-methylphenyl azide, the reaction with xylyl isocyanide
produces carbodiimide in a good yield, whereas other isocyanides give low yields of the corresponding
products. No carbodiimide formation is observed for 3,5-dimethylphenyl azide and 4-methylphenyl
azide. These results are rationalized by the nature of the chromium–imido intermediate, as was
described in our previous study [31]. Whereas bulkier azide forms a chromium(IV)–imido, which is
capable of isocyanide coordination and subsequent N-C bond formation, tetrahedral chromium(VI)
bis(imido) (formed with non-bulky aryl azides) does not coordinate an isocyanide and therefore does
not catalyze N-C bond formation.
Scheme 3. Catalytic formation of carbodiimides mediated by 2.
3. Materials and Methods
3.1. General
CrCl2 and LiN(Si(CH3)3)2 were purchased from Strem chemicals and Sigma-Aldrich, respectively,
and used as received. Previously reported methods were used to synthesize Cr(N(SiMe3)2)2(THF)2 and
the ligand precursor H2[OO]Ph
[35,51]. Deuterated solvents were purchased from Cambridge Isotope
Laboratories and stored over 3 Å molecular sieves. Non-deuterated solvents were purchased from
Sigma-Aldrich chemicals and purified using MBraun solvent purification system. Characterization of
compounds was carried out using 1H and 13C NMR spectroscopy, high-resolution mass spectrometry
and elemental analysis. Selected chemicals were characterized by X-ray crystallography. NMR spectra
of carbodiimides and complexes were recorded using Agilent 400 MHz and Agilent DD2 600 MHz
spectrometers, respectively, in C6D6 at room temperature at the Lumigen Instrument Center. Chemical
shifts and coupling constants (J) are reported in parts per million and Hertz, respectively. Elemental
analyses were carried out by Midwest Microlab LLC under air-free conditions. A Thermo Fisher
Scientific LTQ Orbittrap XL mass spectrometer at the Lumigen Instrument Center was used for high
resolution mass spectra. IR spectra of powdered samples were recorded on a Shimadzu IR Affinity-1
FT-IR spectrometer outfitted with a MIRacle10 attenuated total reflectance accessory with a monolithic
diamond crystal stage and pressure clamp. A Shimadzu UV-1800 spectrometer was used to collect
UV–Vis spectra. All air-sensitive reactions were carried out in a nitrogen-filled glovebox.

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3.2. Synthesis of Cr Complexes 1–6 and Compound 7
Cr2([OO]Ph)2 and Cr[OO]Ph(THF)2 (1 and 2). A solution of ligand precursor H2[OO]Ph (0.034 g,
0.057 mmol) in THF (5 mL) was added to a solution of Cr(N(SiMe3)2)2(THF)2 (0.030 g, 0.057 mmol) in
THF (5 mL). The color of the reaction changed from purple to light green over a course of 4 h. The
solvent◦was removed under vacuum and the resulting residue recrystallized in DCM-THF mixture
at 35 C to obtain light green crystals of 2 in 79% yield (0.029 g, 0.023 mmol). Anal. Calcd for
−
C88H64Cr2O4: C, 81.97, H, 5.00. Found: C, 80.82, H, 6.23.
λ
max
(
εM (L−1 cm−1 mol−1)) 706 nm (28000).
IR (cm−1): 2947 (m), 2337 (m), 1334 (w), 1151 (w), 987 (m), 840 (m), 756 (s), 702 (s). A similar procedure
was used to synthesize Cr[OO]Ph (THF)2 (
1) but recrystallization was done in toluene with small
amount of THF. Calcd for C52H46CrO4: C, 79.17, H, 6.13. Found C, 78.37, H, 5.39. IR (cm−1) 2916 (m),
2353 (m), 1458 (m), 1365 (w), 1319 (w), 1149 (w), 1026 (m), 995 (w), 833 (m), 779 (s), 740 (m), 709 (m),
686 (m).
Synthesis of Cr[OO]Ph(N(4-CH3C6H4))2 (3, Figure 6). A solution of 4-azidotoluene (0.020 g,
0.15 mmol) in THF was added to a 5 mL solution of Cr2([OO]Ph)2 (0.050 g, 0.038 mmol). Upon addition,
the solution color changed from light green to brown; the release of N2 was also observed. The reaction
was stirred for 24 h. The solvent was removed under vacuum and the solid residue was recrystallized
1
from ether-THF mixture at
NMR (600 MHz, C6D6, 298 K):
He), 7.10-7.12 (m, 2H, Hb), 7.03-7.07 (m, 12H, Hg & Hh), 6.93-6.96 (m, 4H, Hc & Hd), 6.53 (d, 4H, JHH
7.6 Hz, Hj, 6.15 (d, 4H, JHH = 8.2 Hz, Hi), 1.92 (s, 6H, Hk). 13C NMR (150 MHz, C6D6, 298 K):
160.69,
−
35 ◦C to afford dark brown crystals in 69% yield (0.046 g, 0.054 mmol). H
δ
7.58 (d, 2H, JHH = 7.6 Hz, Ha), 7.6 (d, 8H, JHH = 6.1 Hz, Hf), 7.41 (s, 4H,
=
δ
151.19, 146.27, 142.97, 142.93, 137.30, 131.11, 130.92, 129.17, 128.22, 127.92, 127.55, 127.52, 127.37, 126.39,
125.57, 124.26, 95.51, 20.72.
Figure 6. Assignment of 1H NMR signals for 3 (see previous paragraph for details).
Cr[OO]Ph(N(4-CH3OC6H4))2 (4, Figure 7). A solution of 4-azidoanisole (0.023 g, 0.15 mmol in
THF) was added to a 5 mL THF solution of Cr2([OO]Ph)2 (0.050 g, 0.038 mmol). Color changed from
light green to brown with release of N2 and the reaction was stirred for 24 h. The solvent was removed
under vacuum and the resulting residue was recrystallized from ether/THF mixture at
brown solid, which was washed with ether to afford the product in 54% yield (0.54 g, 0.043 mmol). H
−
35 ◦C to give a
1
NMR (600 MHz, C6D6, 298 K):
δ 7.57–7.61 (m, 10H, Ha and Hf), 7.44 (s, 4H, He), 7.10–7.12 (m, 2H, Hb),
7.02–7.08 (t, 8H, JHH = 7.0 Hz, Hg), 7.01–7.03 (t, 4H, JHH = 7.0 Hz, Hh), 6.97 (m, 4H, Hc and Hd), 6.31 (d,
4H, JHH = 8.8 Hz, Hj), 6.27 (d, 4H, JHH = 8.8 Hz, Hi), 3.07 (s, 6H, Hk). 13C NMR (150 MHz, C6D6, 298
K):
δ 158.54, 157.54, 151.30, 146.41, 143.00, 131.08, 130.95, 129.21, 127.92, 127.71, 127.54, 127.52, 127.31,
126.32, 126.13, 125.54, 112.79, 95.07, 54.91.

Molecules 2020, 25, 273
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Figure 7. Assignment of 1H NMR signals for 4 (see previous paragraph for details).
Cr[OO]Ph(N(3,5-Me2C6H3))2 (5, Figure 8). A solution of 3,5-dimethylphenyl azide (0.022 g,
0.15 mmol) in THF) was added to THF solution (5 mL) of Cr2([OO]Ph)2 (0.050 g, 0.038 mmol). Color
changed from light green to brown with release of N2 and the reaction was stirred for 24 h. The solvent
was removed under vacuum and the residue was recrystallized from ether-THF at
brown solid, which was washed with ether to afford the product in 57% (0.038 g, 0.043 mmol). H
−
35 ◦C to form a
1
NMR (600 MHz, C6D6, 298 K)
6.4 Hz, Hb), 7.00–7.08 (m, 12H, Hg and Hh), 6.93 (t, 2H, JHH = 7.6 Hz, Hc), 6.90 (d, 2H, JHH
Hd), 6.31 (s, 2H, Hk), 5.84 (s, 4H, Hi), 1.82 (s, 12H, Hj). 13C NMR (150 MHz, C6D6, 298K):
162.5, 151.20,
δ
7.57–7.61 (m, 10H, Ha and Hf), 7.42 (s, 4H, He), 7.09–7.11, (2H, t, JHH
=
=
8.8 Hz
,
δ
146.26, 142.97, 136.83, 131.11, 130.94, 129.21, 129.14, 128.98, 128.19, 127.92, 126.96, 126.75, 126.44, 126.20,
125.55, 121.99, 95.52. 20.37.
Figure 8. Assignment of 1H NMR signals for 5 (see previous paragraph for details).
Cr[OO]Ph(N2CPh2)2 (6). A (5 mL) THF solution of Cr2([OO]Ph)2 (0.030 g, 0.023 mmol) was
reacted with diphenyldiazomethane (0.018 g, 0.092 mmol) for 4 h. The light green color of the complex
changed immediately to purple. The solution was stirred for 6 h at RT. The solvent was removed
under vacuum and the residue was recrystallized from ether-THF mixture at
crystals were isolated in 52% yield (0.025 g, 0.024 mmol). Calcd for C70H52CrN4O2
C, 80.60, H, 5.80. Found C, 79.52, H, 5.57.
−
35 ◦C. Brown color
C4H8O 0.5C6H12:
×
×
6,6,12,12-tetraphenyl-6,12-dihydroindeno[1,2-b]fluorene (7, Figure 9). 1. Reaction of Cr2-
([OO]Ph)2 with (NO)(BF4): A solution of (NO)(BF4) (0.010 g, 0.093 mmol) in THF (5 mL) was
added to a THF solution (5 mL) of Cr2([OO]Ph)2 (0.030 g, 0.023 mmol). The green color solution became
cloudy and turned yellow over the course of 6 h. The solvent was removed under vacuum and the
residue recrystallized with ether-THF mixture. Colorless crystals were obtained in 92% yield (0.023 g,
0.040 mmol). 2. Reaction of H2[OO]Ph with (NO)(BF4): A solution of (NO)(BF4) (0.012 g, 0.1 mmol) in
THF (5 mL) was added to a THF solution (5 mL) of H2[OO]Ph (0.030 g, 0.051 mmol). The colorless
solution turned light brown over a period of 6 h. The solvent was removed under vacuum and the

Molecules 2020, 25, 273
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residue recrystallized in CH2Cl2. Colorless crystals of
7
were obtained in 77% yield (0.021 g, 0.039
1
mmol). H NMR (600 MHz, CD2Cl2, 298 K):
δ 7.81 (s, 2H, He), 7.71 (d, JHH = 7.6 Hz, 2H, Ha), 7.38 (d,
2H, JHH = 7.6, Hd), 7.32 (t, 2H, JHH = 7.6 Hz, Hb), 7.26 (m, 22H, Hc, Hf, Hg, Hh). Related compound
(lacking the phenyl substituents) and its NMR characterization were previously reported [50]. 13C
NMR (125 MHz, CD2Cl2, 298K):
δ 151.48, 151.12, 145.92, 140.21, 139.93, 128.27, 128.13, 127.58, 127.48,
126.69, 126.07, 120.16, 117.81, 65.24. HRMS (m/z): Calcd, [C44H30]+ 558.2347, found 558.2333.
Figure 9. Assignment of 1H NMR signals for 7 (see previous paragraph for details).
General Procedure for Catalytic Studies. C6D6 solution containing 20 equivalents of an
organoazide, 24 equivalents of an isocyanide and 2 equivalents of 1,3,5-trimethoxybenzene (internal
standard) was mixed with 2.5 mol% catalyst in C6D6. The reaction was heated at 60 ◦C for 24 h. The
products were identified, and the yields were calculated using 1H NMR spectroscopy.
3.3. X-ray Crystallographic Details
Structures of complexes
fluorene ( ) were confirmed by X-ray structure determination; data collection and refinement details
are given in Table 1. The crystals were mounted on a Bruker APEXII/Kappa three circle goniometer
platform diffractometer equipped with an APEX-2 detector. The structures of , and were
collected using MoK = 0.71073 Å) at Wayne State University. The structure of was collected using
CuK radiation (1.54178 Å) in the Center for Crystallographic Research in the Department of Chemistry,
1, 2, 3, and 6, and 6,6,12,12-tetraphenyl-6,12-dihydroindeno-[1,2-b]
7
1,
3
,
6
7
α
(λ
2
α
Michigan State University. The data were processed and refined using the APEX2 software. Structures
were solved by direct methods in SHELXS and refined by standard difference Fourier techniques
in the SHELXTL program suite (6.10 v., Sheldrick G. M., and Siemens Industrial Automation, 2000).
Hydrogen atoms were placed in calculated positions using the standard riding model and refined
isotropically; all other atoms were refined anisotropically. The structure of
of cyclohexane and a molecule of THF in the asymmetric unit. The structure of
CH2Cl2 which was fully refined. The structure of contained multiple severely disordered THF solvent
6
contained half a molecule
7
co-crystallized with
3
molecules in the asymmetric unit, leading to the overall poor quality of the diffraction data. We had
only limited success in modelling the disorder in THF solvent, thus, SQUEEZE program was applied
to remove the disordered solvent. One of the para-tolyl groups in the structure of
disordered; the disorder was successfully modeled by finding two alternative conformations. The
structure of was also of relatively poor quality, due to the very small size of the crystals. Despite data
collection at 60 s per frame, little diffraction was observed beyond ~1 Å. The structure of contained
several toluene molecules per asymmetric unit, one of which was found to be disordered over two
conformations. The structure of contained significant amount of highly disordered THF solvent,
which was removed using SQUEEZE.
3 was found to be
1
1
2

Molecules 2020, 25, 273
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Table 1. X-ray crystallographic details for complexes 1–3, 6, and compound 7.
Complex
1
2
3
6
7
C52H46CrO4 ×
2.5C7H8
1020.25
Triclinic
P-1
9.946 (11)
14.298 (16))
19.19 (2)
83.38 (3)
82.55 (2)
82.44 (2)
2668 (5)
2
C70H52CrN4O2 ×
0.5C6H12 × C4H8O
1147.34
C22H15
CH2Cl2
364.27
Triclinic
P-1
8.7464 (5)
8.7610 (5)
12.4072 (7)
76.995 (2)
78.928 (2)
70.505 (2)
866.13 (9)
2
1.397
0.377
100 (2)
55.02
0.0388
0.0803
1.041
×
formula
2C88H64Cr2O4 C58H46CrN2O2
fw, g/mol
crystal system
space group
a (Å)
2578.80
Triclinic
P-1
854.97
Monoclinic
P21/c
Triclinic
P-1
13.9851 (4)
19.4602 (6)
30.4287 (10)
81.683 (2)
77.609 (2)
74.125 (2)
7747.2 (4)
4
17.38 (15)
13.03 (11)
28.58 (19)
90.00
127.4 (4)
90.00
10.1831 (6)
13.0107 (7)
23.0961 (13)
100.964 (3)
101.580 (3)
99.999 (3)
2870.7 (3)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
5142 (71)
4
Z
1.270
0.266
100(2)
49.00
0.0981
0.2070
1.1055
1.104
1.327
0.257
100 (2)
52.96
0.0730
0.1710
d
calcd, g/cm3
µ, mm−1
T (K)
2.673
0.263
172.99
128.14
100 (2)
47.44
2θ, deg
a
R1 [(I > 2σ)]
0.0827
0.1875
0.7573
2
0.0727
0.1512
0.881
b
wR2 [(I > 2σ)]
GOFc (F2)
0.973
1.037
P
P
P
P
P
a
b
c
2
R1 = ||Fo – |Fc||/ |Fo|. wR2 = ( (w(Fo – Fc2)2)/ (w(Fo2)2))1/2
.
GOF = ( w(Fo – Fc2)2/(n – p))1/2, where n is
the number of data and P is the number of parameters refined.
3.4. Computational Details
DFT calculations were performed using Gaussian 09 [52]. All calculations were performed at
the BP86-D3/def2-SVP level of theory [53 57], employing density fitting, ultrafine grids [58], and
–
Becke–Johnson damping for the dispersion corrections. Optimized structures were confirmed to be
minima by ensuring all harmonic frequencies were real. All wavefunctions were verified to be stable.
Orbital analyses were performed in GaussView 6.0.16 [59], and Mayer bond orders were calculated
using Multiwfn 3.5 [60].
4. Conclusions
We have demonstrated that chelating bis(alkoxide) ligand [OO]Ph, featuring a terphenyl spacer
between the alkoxide donors, forms mononuclear (Cr[OO]Ph(THF)2) or dinuclear (Cr2[OO]Ph
)
2
Cr(II) complexes, depending on the crystallization conditions. These results suggest that
[OO]Ph exhibits a larger steric profile compared with two monodentate alkoxides [OCtBu2Ph]
that formed a Cr2(OCtBu2Ph)4 dimer invariably. Reactions of this complex with organoazides
form diamagnetic Cr(VI) bis(imido) complexes or paramagnetic Cr(IV) mono(imido) complexes.
The outcome of the reaction depends on the sterics of the organoazide: mesityl azide formed Cr(IV)
mono(imido) but 3,5-dimethylphenyl/4-methylphenyl azides formed Cr(VI) bis(imido) complexes.
With diphenyldiazomethane, a chromium bis(diphenylmethylenehydrazido) product forms. Treatment
of the Cr(II) precursor with isocyanide likely forms a Cr(II) isocyanide adduct, whereas no
reaction is observed with CO. Isoelectronic nitrosonium abstracts hydroxyl to induce cyclization,
producing 6,6,12,12-tetraphenyl-6,12-dihydroindeno[1,2-b]fluorene. Catalytic studies combining
mixtures of isocyanides with organozides enable carbodiimide formation only for bulkier (mono-
or di-orthosubstituted) aryl azides. This finding is consistent with our previous report, which
demonstrated that the C–N bond formation in carbodiimide is preceded by isocyanide coordination to
the metal and thus occurs at the Cr(IV) mono(imido) intermediate only. Our future studies will focus
on the development of new chelating bis(alkoxide) ligands, which could enable catalytic reactivity for
a wider range of substrates.

Molecules 2020, 25, 273
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Supplementary Materials: The following are available online: NMR, MS, UV–Vis and IR spectra, cif files, and
computational details. Cif files of the structures of
numbers: 1970958–1970962.
1–3, 6, and 7 were deposited at CCDC under the following
Author Contributions: Conceptualization: S.S.K., R.L.L., and S.G.; methodology: S.S.K., R.J.S., R.L.L., and S.G.;
writing—original draft preparation, S.S.K., R.L.L., and S.G.; writing—review and editing, S.S.K. and R.L.L.;
supervision: R.L.L. and S.G.; funding acquisition: R.L.L., S.G., and R.J.S. All authors have read and agreed to the
published version of the manuscript.
Funding: This research was funded by National Science Foundation, Division of Chemistry, grant number CHE
1855681. Grand Valley State University Libraries are acknowledged for their financial support of this open
access publication.
Acknowledgments: Compounds characterization was carried out at Lumigen Instrument Center, Wayne State
University. The CCD based X-ray diffractometer at Michigan State University were upgraded and/or replaced by
departmental funds. We thank Amanda Grass and Duleeka Wannipurage for technical assistance.
Conflicts of Interest: The authors declare no conflict of interest.
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