Iron and Cobalt Diazoalkane Complexes Supported by β-Diketiminate Ligands: A Synthetic, Spectroscopic, and Computational Investigation

Article abstract of DOI:10.1021/acs.inorgchem.8b00468

Diazoalkanes are interesting redox-active ligands and also precursors to carbene fragments. We describe a systematic study of the binding and electronic structure of diphenyldiazomethane complexes of β-diketiminate supported iron and cobalt, which span a

Full text of DOI:10.1021/acs.inorgchem.8b00468

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Iron and Cobalt Diazoalkane Complexes Supported by
β‑Diketiminate Ligands: A Synthetic, Spectroscopic, and
Computational Investigation
†
†
‡,§
David J. Vinyard, Ryan E. Cowley,‡
†,∥
Simon J. Bonyhady, Daniel E. DeRosha, Javier Vela,
†
‡
,†
Brandon Q. Mercado, William W. Brennessel, and Patrick L. Holland*
†
Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
‡
Department of Chemistry, University of Rochester, 120 Trustee Road, Rochester, New York 14627, United States
*
S Supporting Information
ABSTRACT: Diazoalkanes are interesting redox-active ligands and also precursors to carbene
fragments. We describe a systematic study of the binding and electronic structure of
diphenyldiazomethane complexes of β-diketiminate supported iron and cobalt, which span a
range of formal d-electron counts of 7−9. In end-on diazoalkane complexes of formally
monovalent three-coordinate transition metals, the electronic structures are best described as
having the metal in the +2 oxidation state with an antiferromagnetically coupled radical anion
diazoalkane as shown by crystallography, spectroscopy, and computations. A formally zerovalent
cobalt complex has different structures depending on whether potassium binds; potassium
2
binding gives transfer of two electrons into the η -diazoalkane, but the removal of the potassium
1
with crown ether leads to a form with only one electron transferred into an η -diazoalkane. These
results demonstrate the influence of potassium binding and metal oxidation state on the charge
localization in the diazoalkane complexes. Interestingly, none of these reduced complexes yield
tBu
tBu
carbene fragments, but the new cobalt(II) complex L CoPF (L = bulky β-diketiminate) does
6
catalyze the formation of an azine from its cognate diazoalkane, suggesting N loss and transient carbene formation.
2
INTRODUCTION
■
Transition metal diazoalkane complexes have rich coordination
1
,2
chemistry and reactivity. Continued interest is based on the
redox-active nature of the diazoalkane which can accept
electrons from the metal, and also their utility as atom
economical precursors to carbene complexes that are potential
Figure 1. Lewis structures of transition metal diazoalkane complexes:
neutral adduct I; hydrazonido ligands II, III, and IV; side-bound
hydrazonido ligand V.
3
catalysts for olefin metathesis or alkene cyclopropanation. The
catalytic potential of inexpensive first row transition metal
complexes makes them especially attractive for catalysis, but
there are relatively few structurally characterized carbene
complexes of the electrophilic late first-row transition metals
suggest that no reduction of the diazoalkane has occurred
upon coordination. Second, formal hydrazonido(2-) ligands, II-
V, in which the diazoalkane has been reduced by two electrons,
feature M−N−N and/or N−N−C angles near 120°. The
4
Mn, Fe, Co, Ni, and Cu. There are a few systems for which
both the diazoalkane adduct and the corresponding carbene5
complex have been synthesized and structurally characterized,
and conversion from the former to the latter may be brought
7
3
further reactivity of these systems, such as N−N and C−N
cleavage, is inextricably linked with charge movement within
5d
5a,c
5b
the N C moiety. Therefore, it is of interest to identify the
about by photolysis, thermolysis or Lewis acid catalysis.
2
systems in which this change of electronic structure can be
modulated rationally.
There are also systems in which carbene-like reactivity has been
observed from the diazoalkane adduct when it is exposed to the
aforementioned conditions, although the reactive species was
More recently an intermediate bonding situation has been
discovered, in which the diazoalkane accepts only one electron.
Although single electron reduction of diazofluorene with
6
not isolated. However, there is a lack of guiding principles for
predicting whether carbene formation will be possible from a
given diazoalkane complex. This situation motivates the study
of the electronic structure of diazoalkane complexes as a guide
to their reactivity.
8
sodium metal was reported as early as the 1960s, ligand
radical character in transition metal or f-block metal
9
diazoalkane complexes is rarely identified. We have reported
Two descriptions of diazoalkane complexes are predominant
1
in the literature (Figure 1). First, diazoalkane adducts, I,
Received: February 21, 2018
featuring linear M−N−N and N−N−C angles, generally
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00468
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
the cobalt diazoalkane complex L Co(η -N CPh ), 1-Co (L
Article
tBu
1
tBu
9.40 GHz, power = 0.0016 W, conversion time = 29 ms, modulation
amplitude = 8.0 G, modulation frequency = 100 kHz, time constant =
2 ms, temperature = 12 K. EPR data were simulated using EasySpin
2
2
−
10
=
[{(Dipp)NC(tBu)} CH] where Dipp = 2,6-iPr C H ),
2 2 6 3
8
which X-ray crystallographic and computational studies showed
was best described as high-spin cobalt(II) antiferromagnetically
coupled to the radical anion of diphenyldiazomethane.
16
version 5.0.5.
CAUTION! KC is a powerful reducing agent, which ignites on
contact with air and moisture. Extreme care must be taken to ensure
8
Given the paucity of diazoalkane complexes in which single
electron reduction of the diazoalkane moiety has been studied,
the effect of single electron transfers on electronic structure and
reactivity is poorly understood. Due to the accessibility of late
transition metal complexes with a variety of metals and
KC is handled under an inert atmosphere.
8
CAUTION! Diazo compounds and transition metal complexes are
known to be potentially explosive. Although no problems were
encountered while conducting the work described herein, it is prudent
to handle these compounds behind a blast shield.
11
Synthesis of L FeN CPh , 1-Fe. L FeNNFeLtBu (498 mg,
tBu
tBu
oxidation states within β-diketiminate frameworks, these
ligands provide a useful platform for an investigation into the
effects of transition metal electronic configuration on the
coordinated diazoalkane moiety. Herein, we report the effect of
metal oxidation state on diazoalkane binding and reactivity,
while limiting the supporting ligand framework to a single β-
diketiminate. This study spans a range of total electron counts,
and with varied Lewis acid (potassium) coordination. We show
that the number of electrons in the frontier orbitals has a
marked influence on the diazoalkane binding mode, ligand
redox level, and reactivity.
2
2
0
.422 mmol) was dissolved in diethyl ether (40 mL). The resulting
magenta solution was cooled in a liquid nitrogen cold well until it
started to freeze. The solution was removed from the cold well and
diphenyldiazomethane (170 mg, 0.874 mmol) was added with
vigorous stirring. The solution was allowed to warm to room
temperature and the color changed to yellow/brown within 30 min.
Stirring was continued for another 30 min. The solution was filtered
through Celite and concentrated to 15 mL under reduced pressure.
Storing the solution at −40 °C led to the formation of yellow/brown
crystals of 1-Fe, suitable for single crystal X-ray diffraction (292.9 mg,
4
6%). Concentrating the supernatant and subsequent cooling led to
further crops of 1-Fe; however, a small amount of an unidentified
1
1
impurity could be detected by H NMR spectroscopy. H NMR (400
MHz, 298 K, C ): δ 83.9 (1H; γ-H), 53.1 (4H; Ph o-H, Dipp m-H,
Ph m-H or i-Pr CH), 28.5 (18H; t-Bu), −4.3 (4H; Ph o-H, Dipp m-H,
Ph m-H or i-Pr CH), −20.1 (12H; i-Pr CH ), −54.2 (4H; Ph o-H,
Dipp m-H, Ph m-H or i-Pr CH), −55.7 (2H; p-Ar), −76.6 (12H; i-Pr
CH ), −90.9 (2H; p-Ar), −91.7 (4H; Ph o-H, Dipp m-H, Ph m-H or i-
Pr CH) ppm. Solution magnetic moment (296 K, C ): μeff = 4.6(1)
. UV−vis (hexanes solution): λ = 310 nm (ε = 19.4(7) mM
EXPERIMENTAL SECTION
■
D
6 6
General. All manipulations were performed under a nitrogen
atmosphere (or argon atmosphere where specified) by Schlenk
techniques or in an M. Braun glovebox maintained at or below 1
3
ppm of O . Glassware was dried at 150 °C overnight, and Celite was
3
2
dried overnight at 200 °C under vacuum. Pentane, hexane, benzene,
diethyl ether, and toluene were purified by passage through activated
alumina and Q5 columns from Glass Contour Co, under Ar. THF was
distilled under argon from a potassium benzophenone ketyl solution.
All solvents were stored over 4 Å molecular sieves and passed through
D
6 6
−
1
μ
B
−
1
−1
−1
−1
−1
cm ), 336 nm (ε = 22(1) mM cm ), 386 nm (ε = 13.2(6) mM
−
1
−1
−1
cm ), 431 nm (ε = 11.3(6) mM cm ), 562 nm (ε = 0.81(6) mM
−
1
−1
−1
−1
cm ), 692 nm (ε = 1.0(1) mM cm ). IR (neat solid ATR, cm ):
2959(m), 2901(w), 2868(w), 1590(w), 1529(w), 1486(m), 1460(w),
1430(w), 1373(m), 1354(m), 1315(m), 1253(m), 1216(w), 1194(w),
1177(w), 1151(w), 1130(w), 1107(w), 1095(w), 1076(w), 1055(w),
1028(w), 961(w), 930(w), 901(w), 889(w), 807(w), 797(w), 778(w),
761(m), 752(m), 714(w), 690(m), 660(m), 620(w), 588(w), 528(w),
a plug of activated alumina immediately prior to use. Benzene-d was
6
dried over activated alumina and stored over 4 Å molecular sieves.
Silver hexafluorophosphate (99%) was purchased from Strem
Chemicals and used as received.
The following starting materials were prepared according to
literature procedures and stored at −40 °C under an inert atmosphere:
478(w), 430(w), 408(w). Elem. Anal. Calculated for C48H N Fe (%):
63 4
1
2
13
tBu
tBu 13
tBu
57
diphenyldiazomethane, KC ,
L
FeNNFeL
,
L
CoN CPh
2
C, 76.68; H, 8.45; N, 7.45; Found (%): C, 76.39; H, 8.49; N, 7.35. Fe
8
2
1-Co), K [L CoNNCoL ], LtBuCoCl, and L Co.
10
tBu
tBu 14
11a
tBu
11c
−1
−1
(
Mo
̈
ssbauer (80 K): δ = 0.58 mm s ; |ΔE
Q
| = 1.81 mm s .
)·K(OEt )] , 2-Fe. A yellow/
O (10 mL) was
cooled in a liquid nitrogen bath until the solution began to freeze. KC
2
tBu
2
1
NMR data were collected on an Agilent DD2 400 MHz
Synthesis of [L Fe(μ-η :η -N
2
CPh
2
2 2
1
spectrometer. Chemical shifts in H NMR spectra are referenced to
brown solution of 1-Fe (97.7 mg, 0.130 mmol) in Et
2
external SiMe using the residual protiated solvent peaks as internal
8
4
standards: C D H (δ 7.16 ppm). Solution magnetic susceptibilities
(21.9 mg, 0.162 mmol) was added and the resulting purple slurry was
allowed to warm, with stirring, to room temperature for 1 h. The
volatile components were removed under reduced pressure and the
6
5
15
were determined by the Evans method. Elemental analyses were
performed at the CENTC Elemental Analysis Facility at the University
of Rochester. IR spectroscopy was run on an Alpha Platinum ATR IR
spectrometer. UV−vis spectra were recorded on a Cary 50
spectrometer using Schlenk-adapted quartz cuvettes with a 1 mm
path length. Cyclic voltammetry was performed under a nitrogen
atmosphere using a platinum working electrode, a silver reference
electrode, and a platinum wire auxiliary electrode. Tetrabutylammo-
nium electrolyte was recrystallized three times from ethanol prior to
resulting purple/black solid was extracted with Et O (5 mL) and
2
filtered through Celite. Volatile components were removed under
reduced pressure. Crystals of 2-Fe, suitable for single crystal X-ray
diffraction, could be grown by layering concentrated Et O solutions of
2
the purple solid with pentane and allowing the solvents to diffuse at
room temperature, although other products cocrystallized. Further
attempts to purify 2-Fe were unsuccessful and therefore spectroscopic
1
use. Mo
̈
ssbauer data were recorded on a SeeCo spectrometer with
characterization was not possible. Crude H NMR and IR spectra are
alternating constant acceleration; isomer shifts are relative to iron
metal at 298 K. The sample temperature was held constant in a Janis
Research Company Inc. cryostat. Zero-field spectra were simulated
using Lorentzian doublets with γ representing the line fitting
parameter. Electron paramagnetic resonance (EPR) spectroscopy of
included in the Supporting Information. The crude yield was 22.5 mg
(20%).
Synthesis of [LtBuCo(μ-η :η -N CPh )·K(OEt )] , 2-Co, Method
2
1
2
2
2 2
tBu
A. A brown/green solution of L CoN
mmol), in Et O (8 mL) was cooled for 5 min in a dry ice/acetone cold
bath. KC (17.6 mg, 0.130 mmol) was added and the resulting purple
CPh , 1-Co (89.2 mg, 0.118
2
2
2
2
-Co was conducted using a continuous-wave Bruker EleXsys E500
8
EPR Spectrometer with an SHQ resonator and an Oxford Instruments
ESR-900 helium-flow cryostat. Acquisition parameters were as follows:
frequency = 9.39 GHz, power = 0.020 mW, conversion time = 20.5
ms, modulation amplitude = 10 G, modulation frequency = 100 kHz,
time constant = 20.5 ms, temperature = 7.6 K. EPR spectroscopy of 3-
Co was conducted using a continuous-wave Bruker EleXsys-II
spectrometer. Acquisition parameters were as follows: frequency =
slurry was allowed to warm, with stirring, to room temperature for 1 h.
The volatile components were removed under vacuum and the
resulting purple/black solid was extracted with Et O (10 mL) and
2
filtered through Celite. The filter cake was washed with Et O (25 mL)
2
until the filtrate was colorless. Volatile components were removed
under reduced pressure and the resulting purple solid contained 2-Co
1
and <10% impurities as judged by H NMR spectroscopy (96.3 mg,
B
DOI: 10.1021/acs.inorgchem.8b00468
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
crude yield 74%). Layering a concentrated solution of crude product in
1054(w), 1029(w), 991(w), 961(m), 927(w), 892(w), 836(w),
795(w), 758(w), 724(w), 693(w), 673(w), 579(w), 557(w), 541(w),
530(w), 506(w), 489(w), 449(w), 419(w). Elem. Anal. Calculated for
C H N Co·K(C H O ) (%): C, 68.25; H, 8.45; N, 5.22; Found: C,
Et O (10 mL) with pentane (4 mL) and allowing solvents to diffuse at
2
−
40 °C for 5 d afforded 16.4 mg of dark purple crystals of pure 2-Co.
Concentrating the mother liquor to 4 mL and storing at −40 °C for 1
4
9
66
4
12 24
6
week afforded an additional 18.1 mg (combined yield of 34.5 mg,
68.49; H, 8.24; N, 5.11.
1
tBu
3
4%). H NMR (400 MHz, 298 K, C D ) δ 15.2, 11.8, 9.9, 8.6, 5.7,
Synthesis of L CoPF , 4-Co-PF . Under an argon atmosphere,
6
6
6 6
2
.5, 1.8, 1.4, −1.9, −8.7 ppm. Due to the extent of paramagnetic
LtBuCo (148 mg, 0.264 mmol) was dissolved in diethyl ether (10 mL).
The resulting brown-yellow solution was added to a stirring slurry of
silver hexafluorophosphate (82.7 mg, 0.327 mmol) in diethyl ether (4
mL), giving a green reaction with dark precipitate. The heterogeneous
reaction was allowed to stir for 6 h and then passed through Celite to
remove a dark gray precipitate. Volatile components were removed
under vacuum to afford a yellow-green powder, which was extracted
into pentane (6 mL) and filtered through Celite. The resulting green
filtrate was concentrated to 5 mL under reduced pressure and stored at
−40 °C for 3 d to afford dark green crystals (54.4 mg) that contained
minor impurities as judged by proton NMR spectroscopy. Analytically
pure 4-Co-PF6 was obtained by dissolving the green crystals in
pentane (5 mL), concentrating the solution to 3 mL under reduced
pressure, filtering through Celite, and storing the green filtrate at −40
°C for 5 d to give dark green rectangular crystals suitable for X-ray
broadening of the NMR spectrum of 2-Co, reliable peak integrations
could not be obtained. Solution magnetic moment (296 K, THF-d ):
μeff = 1.7(1) μ . UV−vis (Et O solution): λ = 335 nm (ε = 109(6)
mM cm ), 392 nm (shoulder; ε = 53(2) mM cm ), 451 nm
8
B
2
−1
−1
−1
−1
−1
−1
−1
−1
(
shoulder; ε = 38(3) mM cm ), 563 nm (ε = 49(5) mM cm ).
−1
IR (neat solid ATR, cm ): 2953(m), 2927(w), 2903(w), 2865(w),
1
1
1
9
7
4
7
580(m), 1530(w), 1484(m), 1462(w), 1432(m), 1383(s), 1360(s),
339(s), 1317(s), 1292(s), 1247(m), 1216(m), 1188(m), 1173(m),
160(m), 1098(m), 1055(w), 1027(w), 992(w), 964(w), 946(w),
35(w), 922(w), 897(w), 876(w), 847(w), 807(w), 781(m), 746(s),
06(m), 693(m), 680(m), 633(w), 575(w), 552(w), 492(w), 451(w),
22(w). Elem. Anal. Calculated for C104H146N O K Co (%): C,
8
2
2
2
1.94; H, 8.48; N, 6.45; Found (%): C, 71.98; H, 8.47; N, 5.96.
tBu
2
Synthesis of [L Co(η -N CPh )·K(OEt )] , 2-Co, Method B.
2
2
2 2
1
Diphenyldiazomethane (62 mg, 0.32 mmol) was added to a black
solution of K [L CoNNCoL ] (249 mg, 0.151 mmol) in THF (10
mL). The resulting deep purple solution was stirred for 14 h. The
volatile components were removed under reduced pressure and the
resulting purple/black solid was extracted with Et O (35 mL). The
deep purple solution was concentrated to 15 mL under reduced
pressure and crystals of 2-Co started to form. These crystals were
isolated (54 mg) and the supernatant was cooled to −40 °C. This
yielded a further crop of 2-Co (85 mg; total yield = 53%).
diffraction (21.2 mg, 11%). H NMR (400 MHz, 298 K, C D ) δ 34.2
6
6
tBu
tBu
(4H, i-Pr CH or aryl m-H), 27.0 (18H, t-Bu), −7.7 (12H, i-Pr CH ),
2
3
−44.2 (1H, γ-H), −50.8 (12H, i-Pr CH ), −66.0 (4H, i-Pr CH or aryl
3
m-H), −73.4 (2H, aryl p-H) ppm. Solution magnetic moment (298 K,
−
1
C D ): μ = 4.6(1). UV−vis (toluene): λ = 340 nm (ε = 23.3 mM
2
6
6
eff
−1 −1 −1
cm ), 430 nm (shoulder; ε = 0.704 mM cm ), 510 nm (shoulder;
ε = 0.249 mM cm ), 620 nm (ε = 0.209 mM cm ).). IR (solid
−
1
−1
−1
−1
−
1
ATR, cm ): 3061(w), 2962(m), 2931(m), 2871(w), 1538(w),
1505(m), 1492(m), 1464(m), 1431(m), 1403(w), 1377(m),
1347(s), 1315(s), 1281(m), 1253(w), 1219(m), 1194(m), 1155(w),
1135(w), 1098(m), 1055(w), 1029(m), 915(s), 856(vs), 799(m),
781(m), 753(m), 724(s), 679(w), 546(s), 483(m), 434(m), 412(w)
Elem. Anal. Calculated for C H CoF N P (%): C, 59.57; H, 7.57; N,
tBu
2
Synthesis of [L Co(η -N CPh )·K(OEt )] , 2-Co, Method C. A
2
2
2 2
tBu
red/brown solution of L CoCl (567 mg, 0.951 mmol) in Et O (30
mL) was cooled in a liquid nitrogen bath until the solution began to
freeze. KC (142 mg, 1.05 mmol) was added and the resulting slurry
2
8
35 53
6
2
was allowed to warm, with stirring, for 17 h. The brown slurry was
again cooled in a liquid nitrogen bath until it began to freeze.
Diphenyldiazomethane (203 mg, 1.05 mmol) was added and the slurry
was stirred as the solution warmed to room temperature over the
course of 1 h. No color change was observed, and the slurry was
cooled a final time in a liquid nitrogen bath until it began to freeze.
3.97 Found (%): C, 59.73; H, 7.46; N, 3.89.
Computational Studies. Density Functional Theory (DFT)
calculations were performed using the Gaussian 09 suite of
1
7
programs. Natural Bond Order (NBO) studies were performed
1
8
using NBO 6.0 through the Gaussian 09 interface. These calculations
were performed at the Yale University Faculty of Arts and Sciences
High Performance Computing Center. Representative input files are
included in the Supporting Information.
KC (142 mg, 1.05 mmol) was added and the slurry instantly changed
8
to a purple color with formation of a black solid. The slurry was
warmed to room temperature, with stirring, over the course of 3.5 h.
Volatile components were removed under vacuum. The resulting dark
Due to the size of 1−3 and the requirements of the NBO 6.0
program, three basis sets were used at different stages of the
computational studies. B1 employed the SDD basis set with an
effective core potential for the transition metal atom, the 6-31+
+G(d,p) double-ζ basis set for the nitrogen atoms and the central
carbon atom on the diazoalkane moiety and the 6-31G double-ζ basis
set was used for all remaining atoms. B2 employed the SDD basis set
with an effective core potential for the transition metal atom and the 6-
311++G(d,p) triple-ζ basis set for all remaining atoms. B3 employed
the SDD basis set with an effective core potential for the transition
metal atom and the 6-311G(d,p) triple-ζ basis set for all remaining
atoms. For 2-Co, potassium was treated with the SDD basis set with
an effective core potential for B1−B3 (for further clarification see
Supporting Information, Figure S24).
solid was extracted with Et O (120 mL) and filtered to yield an
2
intensely colored purple solution. Volatile components were removed
under vacuum and the resulting purple solid was washed with pentane
1
(
30 mL). This purple solid was pure as judged by H NMR
spectroscopy (500 mg, yield = 61%).
tBu
Synthesis of [L CoN CPh ][K(18-c-6)], 3-Co. To a dark purple
2
2
solution of 2-Co (201 mg, 0.116 mmol) in THF (10 mL) was added
8-crown-6 (18-c-6; 66.5 mg, 0.252 mmol). The resulting purple
1
solution was stirred for 1.25 h. The volatile components were removed
under reduced pressure and the resulting waxy purple solid was
washed with pentane (5 mL). A purple powder remained and excess
pentane was removed under reduced pressure. This solid was extracted
with toluene (4 mL) and filtered through Celite. Layering the toluene
solution with pentane and allowing the solvents to diffuse at room
temperature resulted in the formation of crystals of 3-Co, suitable for
Geometry optimizations started from the atomic coordinates of the
respective crystal structures. Spin states for optimizations of 1-Fe, 2-
Co and 3-Co were selected based on those experimentally determined.
The full molecule of 1-Fe (neutral quartet) was optimized to give a
geometry 1-Fe-DFT. In order to save computational resources only
the monomeric unit of 2-Co (neutral doublet) and the anionic
component of 3-Co (anionic doublet), without the corresponding
potassium 18-c-6 cation, were optimized, yielding the corresponding
geometries 2-Co-DFT and 3-Co-DFT.
1
single crystal X-ray diffraction (118 mg, yield = 47%). H NMR (400
MHz, 298 K, C D ) δ 16.3, 11.1, 5.6, 5.1, 2.5, 1.9, 1.4, 0.9, −0.5 ppm.
6
6
Due to the extent of paramagnetic broadening of the NMR spectrum
of 3-Co, reliable peak integrations could not be obtained. Solution
magnetic moment (296 K, C D ): μ = 1.5(1) μ . UV−vis (hexanes):
6
6
eff
−1
B
−1
λ = 333 nm (ε = 23.7(2) mM cm ), 401 nm (shoulder; ε = 14.2(2)
mM cm ), 452 nm (shoulder; ε = 10.92(6) mM cm ), 569 nm
ε = 10.72(9) mM cm ), 614 nm (shoulder; ε = 7.13(6) mM
cm ). IR (neat solid ATR, cm ): 2952(m), 2895(m), 2858(m),
−
1
−1
−1
−1
10
As with our previous study on 1-Co, we found that optimization
−1
−1
−1
(
with M06L/B1 consistently gave geometries that were in better
agreement with the experimentally determined geometries than either
PBE0/B1 or ωB97xD/B1 (based on comparison of the metrical
parameters, as well as calculating the RMS error for the distance
−1
−1
1580(w), 1485(w), 1471(w), 1429(m), 1376(m), 1351(m), 1316(m),
1285(w), 1244(m), 1219(w), 1182(w), 1149(w), 1131(w), 1101(s),
C
DOI: 10.1021/acs.inorgchem.8b00468
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
between each atom in the crystal structure with the corresponding
atom in the calculated geometry when the two structures were
overlaid). For all optimizations, the D3 dispersion scheme, developed
19
by Grimme and co-workers, was applied for the M06L and PBE0
functionals, while the inbuilt dispersion scheme was used for the
20
ωB97xD functional. Furthermore, a pruned 99,590 point grid was
used for the integration (ultrafine) and the SCF convergence criteria
−9
were increased to 10 . Final geometries of 1-Fe-DFT, 2-Co-DFT,
and 3-Co-DFT were determined by increasing the optimization
convergence criteria to very tight and reoptimizing with M06L/B1
(
coordinates for the optimized geometries are included in the
Supporting Information).
The wave functions of 1-Fe-DFT, 2-Co-DFT, and 3-Co-DFT were
stable (M06L/B1) with respect to relaxed constraints using the inbuilt
function of Gaussian 09. The geometries were also determined to be
minima on the potential energy surfaces as no imaginary frequencies
were calculated for 1-Fe-DFT, 2-Co-DFT, or 3-Co-DFT.
The electronic structures of 1-Fe-DFT, 2-Co-DFT, and 3-Co-DFT
were calculated using the hybrid functional M06/B2, though M06L
21
gave qualitatively similar results. The α- and β-spin orbitals were
maximally aligned using the guess = BiOrth option in Gaussian 09.
Subsequent electronic analysis of 1-Fe-DFT, 2-Co-DFT, and 3-Co-
DFT was performed using NBO 6.0 and M06/B3 on unrestricted
wave functions.
Figure 2. Molecular structure of LtBuFeN CPh , 1-Fe. Thermal
ellipsoids displayed at the 50% probability level and hydrogen atoms
are omitted for clarity. For selected bond lengths and angles, see Table
2
2
1
.
RESULTS
■
tBu
tBu11b
Treating the iron(I) dinitrogen complex L FeNNFeL
with diphenyldiazomethane led to yellow-brown crystals of
studies suggest that charge transfer occurs from the formally
iron(I) centers to the bound N ligand, resulting in two iron(II)
centers, antiferromagnetically coupled to triplet N
tBu
2
L FeN CPh (1-Fe) in moderate yield (46%; Scheme 1). The
2−
24
2
2
.
The
2
2
tBu
tBu
Me
Me
Fe−N bonds in L FeNNFeL and L FeNNFeL (Fe−N :
tBu
Scheme 1. Synthesis of L FeN CPh , 1-Fe, and
2
2
1
.746−1.775 Å; Fe−N
: 1.945−1.984 Å) have similar
tBu
10
nacnac
L
CoN CPh , 1-Co.
2 2
lengths as those in 1-Fe (Fe1−N14:1.752(1) Å; Fe1−
N11:1.952(1) Å; Fe1−N21:1.942(1) Å). Finally, the Fe−
N
bond lengths in 1-Fe are shorter than the range of
nacnac
previously reported Fe−N
bond lengths in the three-
nacnac
tBu
coordinate iron(II) amido complexes L FeNHR (1.961−
22
2
.018 Å; R = Ph, Dipp, tBu, p-tolyl, mesityl), further
supporting diazoalkane reduction by a single electron to give a
2+
−
Fe -diazoalkane structure.
Further corroborating evidence for the presence of a
diazoalkane radical anion comes from the doubly bent
10
molecular structure of 1-Fe is very similar to that of 1-Co, and
features a doubly bent diazoalkane moiety terminally bound to
geometry of the Fe−N−N-C unit in 1-Fe. As in 1-Co, the
M−N-N and N−N−C angles (Fe1−N14−N24 154.3(1)°,
N14−N24−C14 133.2(1)°) do not tend to 120° or 180°, but
are intermediate between the two extremes usually observed for
transition metal diazoalkane complexes. Compound 1-Fe also
features an N14N24 double bond (1.243(2) Å) and a long
a trigonal-planar iron center (Σ = 360.0°, Figure 2). As in 1-
Fe
Co, there is evidence for multiple bond character between the
transition metal and the coordinated diazoalkane, because the
Fe1−N14 bond length in 1-Fe (1.752(1) Å) is shorter than the
Fe−N single bond in the three-coordinate iron(II) amido
25
N24C14 double bond (1.319(2) Å), which are again in
good agreement with those in 1-Co. This geometry remains
rare in the literature, as there are only three other complexes,
apart from 1-Co and [tpeFeN CPh ][Li(THF)], that feature
tBu
complexes L FeN(H)R (1.787−1.902 Å; R = Ph, Dipp, tBu,
2
2
p-tolyl, mesityl). However, the distance is longer than the
FeN double bond in the three-coordinate iron imido
2
2
Me
Ph
complexes L FeNAd and L FeNAd (1.670 and 1.700
similar bond lengths and angles and the diazoalkane geometry
Me
−
Ph
2f,26
Å, respectively; L
= [{(Dipp)NC(Me)} CH] , L
=
is not constrained as part of a chelate.
2
−
23
[
{(2,4,6-Ph C H )NC(Me)} CH] ). (For a summary of
Spectroscopic characterization of 1-Fe contains many
3
6
2
2
10
selected bond lengths and angles, see Table 1.) Furthermore,
the Fe1−N14 bond in 1-Fe is shorter than the Fe−N bond
parallels with that for 1-Co. Compound 1-Fe displays
averaged C2v symmetry in solution, as evidenced by the 10-
diazo
1
in the four-coordinate iron diazoalkane complex
tpeFeN CPh ][Li(THF)] (1.781(9) Å; tpe = tris(5-
line pattern in its H NMR spectrum. No IR stretching modes
[
could be definitively assigned to the N−N stretching vibration,
presumably because these are obscured by aromatic and C−N
stretching bands from the supporting ligand. The solution
2
2
mesitylpyrrolyl)ethane), the only other reported example of
an iron diazoalkane complex with significant diazoalkane radical
9
b
anion character.
The previously reported iron(I) dinitrogen complexes
magnetic moment (4.6(1) μ ) was larger than the spin-only
B
value for an S = 3/2 ground state (3.87 μ ), which is likely to
B
tBu
tBu
Me
Me
L FeNNFeL and L FeNNFeL feature three-coordinate
arise from orbital contributions to the magnetic moment as
observed in other diketiminate-supported iron(I) and iron(II)
iron centers coordinated to three N-donor ligands, as in 1-
1
1b,13
24
Fe.
Significantly, detailed spectroscopic and computational
complexes. However, these magnetic data are consistent with
D
DOI: 10.1021/acs.inorgchem.8b00468
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 1. Selected Bond Lengths and Angles for the Compounds That Are Reported in This Article
b
b
1
-Fe/1-Fe-DFT
1-Co /1-Co-DFT
2-Fe
2-Co/2-Co-DFT
3-Co/3-Co-DFT
M−N11 (Å)
N−N21 (Å)
M−N14 (Å)
M−N24 (Å)
N14−N24 (Å)
N24−C14 (Å)
M−N14−N24 (deg)
N14−N24−C14 (deg)
1.952(1)/1.980
1.942(1)/1.953
1.752(1)/1.750
1.902(2)/1.949
1.916(2)/1.910
1.719(2)/1.724
1.957(1)
1.936(1)
1.839(2)
1.937(2)
1.288(2)
1.357(2)
1.896(3)/1.904
1.910(4)/1.938
1.804(4)/1.816
1.929(4)/1.912
1.271(5)/1.290
1.386(6)/1.340
1.887(3)/1.937
1.885(3)/1.920
1.703(3)/1.742
1.243(2)/1.238
1.319(2)/1.323
154.3(1)/154.9
133.2(1)/133.6
1.216(2)/1.218
1.322(3)/1.317
159.4(2)/159.6
134.2(2)/139.2
1.264(4)/1.235
1.341(5)/1.342
155.3(3)/154.0
131.4(4)/133.2
125.2(2)
125.0(4)/129.4
a
b
The crystallographically determined geometrical parameter appears first, followed by the one calculated by DFT. This compound and the
corresponding geometry-optimized structure were previously reported.
1
0
+
either a high-spin iron(I) diazoalkane adduct or high-spin
iron(II) antiferromagnetically coupled to a diazoalkane radical
anion, because both have S = 3/2 ground states. Multiple
attempts to collect EPR data on 1-Fe revealed no signals, both
in a toluene glass and as solid. There are precedents for large
zero-field splitting in S = 3/2 cobalt(II) complexes, which can
electron reduction event (1-Fe: −2.26 V versus Fc /Fc, 1-Co:
+
−1.84 V versus Fc /Fc). An irreversible single-electron
oxidation event is also evident in the voltammogram of each
compound, which we assign to loss of diazoalkane upon
tBu 2+
·−
oxidation of L M N CPh (1-Fe: −0.75 V; 1-Co: −0.28 V;
2
2
see below for chemical oxidation of 1-Fe and 1-Co; see
Supporting Information, Figures S5 and S1 for full voltammo-
grams).
27
render them EPR-silent at X-band frequencies.
An advantage of the iron complex 1-Fe over the earlier
5
7
̈
reported cobalt complex 1-Co is that Fe Mossbauer
In order to understand the nature of the reduced product,
compounds 1-Fe and 1-Co were each treated with 1 equiv of
KC in diethyl ether. In each case, this resulted in an intensely
colored purple solution. Following workup, the reduced species
)K(OEt )] , 2-Fe, and [L Co-
(μ−η :η -N CPh )K(OEt )] , 2-Co, were isolated (20% and
74%, respectively; Scheme 2). Crystals of each compound were
grown from concentrated diethyl ether solutions when layered
with pentane, although other unidentified products cocrystal-
lized with 2-Fe. These crystals were suitable for single crystal X-
ray diffraction studies using either Mo Kα radiation (2-Co) or
synchrotron radiation (λ = 0.7749 Å; 2-Fe). Compounds 2-Fe
and 2-Co are nearly isostructural, and reveal that the binding
mode of the diazoalkane changed upon reduction from terminal
spectroscopy can provide further insight into the electronic
environment of the iron center. The quadrupole doublet of 1-
−
1
−1
8
Fe (δ = 0.58 mm s , |ΔE | = 1.81 mm s ; see Figure S6) is
Q
quite similar to that for the iron dinitrogen complex
(δ = 0.58(1) mm s , |ΔE | = 1.41(1)
mm s ). As discussed above, the proposed iron environment
in 1-Fe is very similar to that in L FeNNFeL , where
detailed computational and Mossbauer studies supported an
tBu
2
1
tBu
Me
Me
−1
[L Fe(μ−η :η -N
2
CPh
2 2 2
L FeNNFeL
Q
2
1
−
1
24
2
2
2
2
Me
Me
̈
iron(II)-ligand radical formulation. Based on this spectroscopic
characterization of 1-Fe, we therefore favor an assignment of
high-spin iron(II) antiferromagnetically coupled to diphenyl-
diazomethane radical anion and this is supported by computa-
tional studies (see below).
In order to further explore electronic effects on diazoalkane
ligands, we performed cyclic voltammetry (ca. 1 mM solutions
in THF, Figure 3) on complexes 1-Fe and 1-Co. The cyclic
voltammogram of each compound features a reversible single-
2
N-bound to η -N,N′ (for 2-Co see Figure 4, for 2-Fe see
Supporting Information, Figure S7).
Figure 4. Molecular structure of [LtBuCo(μ−η :η -N CPh )K(OEt )] ,
2
1
2
2
2
2
2
-Co. Thermal ellipsoids are displayed at the 50% probability level.
Figure 3. Cyclic voltammograms of 1 mM 1-Co and 1 mM 1-Fe in
THF using 0.1 M tetrabutylammonium hexafluorophosphate electro-
lyte and platinum working electrode. Scan rate: 500 mV/s.
Hydrogen atoms are omitted and Dipp groups are displayed in
wireframe format for clarity. For selected bond lengths and angles, see
Table 1.
E
DOI: 10.1021/acs.inorgchem.8b00468
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Scheme 2. Synthesis of [L Fe(μ−η :η -N CPh )K(OEt )] ,
Article
tBu
2
1
2
2
2
2
tBu
2
1
2
-Fe, and [L Co(μ−η :η -N CPh )K(OEt )] , 2-Co
2
2
2
2
The formally iron(0) and cobalt(0) centers in 2-Fe and 2-Co,
respectively, adopt distorted square planar geometries to
accommodate the change in ligand binding mode (Σ
=
Nacnac
Fe
3
61.0°; Σ = 361.1°). There is little change in the M-N
Co
bond lengths upon reduction from 1-Fe to 2-Fe or 1-Co to 2-
Co (Table 1). However, due to the change in coordination
number, it is difficult to draw definitive conclusions with
respect to the oxidation state of the metal centers in 2-Fe and
Figure 5. X-band EPR spectrum of 2-Co in toluene/THF at 7.6 K.
The experimental spectrum (black) was simulated using a linear
combination (green) of 50% component A (red) and 50% component
B (blue) as described in the text.
2
-Co based on crystallographic evidence alone.
Crystallography does, however, help to evaluate the oxidation
nearly axial S = 1/2 species with g-values of 2.555, 2.015, and
59
level of the diazoalkane moieties in 2-Fe and 2-Co, and the
oxidation states of the metal centers can be inferred from these.
In the molecular structures of the reduced compounds 2-Fe
and 2-Co, the diazoalkane moieties are bent at N14−N24−
C14. Lengthening of the N24−C14 bonds to short single
2.001, and has clearly defined Co hyperfine features with A =
17.0, 46.2, and 137 MHz. Anisotropic line broadening of
Component A was simulated using H-strain values of 462, 186,
and 99.3 MHz. Component B is a nearly axial S = 1/2 species
with g-values of 2.696, 2.044, and 2.073, and does not have
2
5
59
bonds in 2-Fe (1.357(2) Å) and 2-Co (1.386(6) Å) suggests
further reduction of the diazoalkane ligand by population of
ligand based π* antibonding orbitals. The N14−N24 bonds in
defined Co features. Anisotropic line broadening of
Component B was simulated using H-strain values of 242,
511, and 143 MHz. The most precise fit of the experimental
data was a linear combination of 50% of Component A and
50% of Component B. The appearance of two components
could be due to an equilibrium involving a change in the
diazoalkane coordination mode; it is unlikely that it is a solvent-
based equilibrium, because monitoring 2-Co by variable-
2
-Fe and 2-Co remain consistent with NN double bonds
25
(
1.288(2) Å and 1.271(5) Å, respectively). The shorter M-
N14 bonds (Fe1−N14 1.839(2) Å and Co1−N14 1.804(4) Å)
relative to the M-N24 bonds (Fe1−N24 1.937(2) Å and Co1−
N24 1.929(4) Å) are consistent with an assignment of the
former as single bonds, and the latter as dative interactions
between an N-centered lone-pair and the transition metal
center. These metrical parameters are consistent with
previously reported side-on bound diazoalkane complexes
which also feature N−N double bonds (1.236−1.280 Å), C−
N single bonds (1.331−1.367 Å), one M-N covalent bond and
one M-N dative bond.
reduction of the diazoalkane ligand, and the only prior
example of an η -diazoalkane ligand assigned as a radical anion
featured a longer N−N bond (1.338(5) Å).
1
temperature H NMR between −80 and 20 °C showed no
changes in solution speciation (Supporting Information, Figure
S10b). Additionally, the EPR spectrum of 2-Co measured in
neat toluene shows only a slight change in the ratio of
Component A to Component B from 50:50 in toluene/THF to
1
61:39 (Supporting Information, Figure S14). The H NMR
4h,5d,28
−
This is usually assigned as a 2e
spectrum of 2-Co has few peaks because of severe paramagnetic
broadening, as is typical for compounds with a S = 1/2 ground
state; however, it contains one diagnostic resonance (δ 15.2
ppm) that can be used for identification. Based on the spin state
and structural parameters determined by X-ray crystallography,
2-Co could be assigned as having cobalt(I) antiferromagneti-
cally coupled to a diazoalkane radical anion, or a cobalt(II)
hydrazonido complex. Computational studies (see below) favor
the latter description.
1d
2
9a
The potassium ions in 2-Fe and 2-Co are bound to N14 of
one diazomethane and coordinated diethyl ether, and also have
three η -aryl interactions (one to L
diphenyldiazomethane). The K1−N14 bond length (2.760(4)
Å) is shorter than the K−N dative interaction in K(18-crown-
3
tBu
and one to each
29
6
)N ·H O (2.896 Å) but longer than the sum of the covalent
No N−N stretching modes could be definitively assigned in
3
2
25
single bond radii (2.67 Å). As 2-Fe could not be isolated as a
pure solid, and the metrical parameters from the single crystal
X-ray diffraction studies are similar for both species, further
discussion of characterization data will be limited to 2-Co.
Compound 2-Co has an S = 1/2 ground state, as indicated
the IR spectrum of 2-Co. 2-Co is stable in solution for 24 h at
1
elevated temperatures (80 °C), as judged by H NMR
spectroscopy. Treating 2-Co with Lewis acids (Sm(OTf)3,
La(OTf) , BPh , B(C F ) ) either led to no reaction or a
3
3
6 5 3
mixture of 1-Co and other (unidentified) products. Attempts to
by the solution magnetic moment (1.7(1) μ ). The EPR
reduce 2-Co with a further molar equivalent of KC did not
result in any reaction, as judged by H NMR spectroscopy.
B
8
1
spectrum of 2-Co in a 1:1 toluene/THF glass (Figure 5) is
consistent with this spin state. However, despite the purity as
judged by microanalysis, the experimental EPR spectrum of 2-
Co could only be adequately simulated using a combination of
two signals. The first species (notated Component A) is a
Compound 2-Co can also be synthesized from the formally
tBu
tBu 14
cobalt(0) dinitrogen complex K [L CoNNCoL ]. Treat-
2
ing the cobalt(0) synthon with two molar equivalents of
diphenyldiazomethane led directly to 2-Co, which could be
F
DOI: 10.1021/acs.inorgchem.8b00468
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Inorganic Chemistry
Article
isolated in moderate yield (53%; Scheme 3). Furthermore, this
extrusion. Relative to 2-Co in which coordination of the
potassium cation is observed, the structural change in 3-Co to
terminal diazoalkane binding is significant, as it demonstrates
how a Lewis acid can alter the geometry of a diazoalkane
complex.
compound could be synthesized in a “one-pot” procedure from
tBu
the cobalt(II) halide complex L CoCl using two molar
equivalents of KC (which forms the dinitrogen complex in
8
situ) and subsequently a single molar equivalent of N CPh , in
2
2
an improved yield (61%; Scheme 3).
Scheme 4. Synthesis of [LtBuCoN CPh ][K(18-c-6)], 3-Co
2
2
Scheme 3. Alternative syntheses of [LtBuCo(μ−η :η -
2
1
a
N CPh )K(OEt )] , 2-Co
2
2
2
2
a
Reaction conditions: (i) 2 KC , 1 N CPh , Et O; (ii) 1 N CPh ,
8
2
2
2
2
2
THF.
Although there is no formal oxidation state change upon
potassium sequestration from 2-Co to 3-Co, the anionic
component of 3-Co has a structure that is reminiscent of 1-Fe
and 1-Co. The Co1−N11/N21 bond lengths (1.887(3) Å and
1.885(3) Å, respectively) remain similar to those in 1-Co and
2-Co (Table 1). Surprisingly, and Co1−N14 bond length is
slightly shorter in 3-Co than 1-Co (1.703(3) Å and 1.719(2) Å,
respectively), despite a decrease in formal oxidation state.
However, the elongated N14−N24 (1.264(4) Å) and N24−
C14 (1.341(5) Å) bonds, relative to those in 1-Co, suggest an
increase in charge buildup on the diazoalkane. Despite this
increase in reduction of the diazoalkane, the intermediate
Co1−N14−N24 (155.3(3)°) and N14−N24−C14 (131.4(4)°)
angles, and shorter N24−C14 bond (1.341(5) Å) relative to
the hydrazonido ligand in 2-Co (1.386(6) Å), suggest that the
diazoalkane in 3-Co is best described as a radical anion.
The EPR spectrum of 3-Co is consistent with a nearly axial S
Given the coordination of the terminal diazoalkane N atom
to potassium, we were interested to learn whether the
potassium ion induced the side-on coordination mode in 2-
Co. We therefore added 18-crown-6 (18-c-6) to a purple
solution of 2-Co in THF, which gave a 47% yield of the
tBu
cation−anion pair [L CoN CPh ][K(18-c-6)], 3-Co (Scheme
2
2
4
). The molecular structure shows that the diazoalkane ligand
reverts to a terminal binding mode (Figure 6) after potassium
5
9
=
1/2 species, with g-values of 2.545, 2.095, and 2.006 and Co
hyperfine features with A = 100, 10.0, and 137 MHz (Figure 7).
The solution magnetic moment is also consistent with this spin
1
state (1.5(1) μ ). As with 2-Co, the H NMR spectrum of 3-
B
Co is paramagnetically broadened. However, there is a shift of
the diagnostic resonance of 2-Co (δ 15.2 ppm) upon
sequestration of the potassium cation with crown ether to
form 3-Co (δ 16.3 ppm). As with the previously discussed
compounds, no N−N stretching mode was identified in the IR
spectrum of 3-Co. This spectroscopic characterization is
consistent with high-spin cobalt(I) antiferromagnetically
coupled to a diazoalkane radical anion with an overall
S = 1/2 ground state.
Although stable in solution at elevated temperature (60 °C)
for 36 h, 3-Co decomposes within 12 h at 80 °C as determined
by proton NMR spectroscopy, as compared to 2-Co which is
stable for more than a day at 80 °C. Thus, the presence of the
alkali metal within the structure of 2-Co is stabilizing. As with
2-Co, the addition of Lewis acids to 3-Co gave either no
reaction or product mixtures containing 1-Co.
Figure 6. Molecular structure of [LtBuCoN CPh ][K(18-c-6)], 3-Co.
Thermal ellipsoids are displayed at the 50% probability level.
Hydrogen atoms are omitted for clarity. For selected bond lengths
and angles, see Table 1.
2
2
G
DOI: 10.1021/acs.inorgchem.8b00468
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Inorganic Chemistry
Article
31
hexafluorophosphate to a transition metal. The solution
magnetic moment of 4.6(1) μ for 4-Co-PF is consistent with
B
6
high spin cobalt(II), falling within error of the solution
magnetic moment of the three-coordinate cobalt(II) (S = 3/2
tBu
11a
ground state) complex L CoCl. The coordination of the
PF anion is fluxional, as a second structure measured of
6
crystals grown from diethyl ether revealed the PF anion
6
binding in a monodentate fashion under these conditions, with
a fourth coordination site occupied by diethyl ether (4-Co-
PF (OEt ); see Supporting Information, Figure S22).
6
2
Intrigued by our observation of benzophenone azine
formation in oxidations of 1-Fe and 1-Co, we subsequently
tBu
explored the catalytic activity of L FeOTf and 4-Co-PF
6
toward the conversion of diphenyldiazomethane to benzophe-
none azine. While preliminary investigations into the iron
system gave multiple unidentified products in addition to
benzophenone azine, 10 mol % loading of 4-Co-PF in benzene
6
catalyzed the conversion of diphenyldiazomethane to benzo-
phenone azine as the only organic product in 54% yield after 4
h at room temperature. In the absence of added catalyst,
diphenyldiazomethane yielded benzophenone azine in <5%
yield under the same conditions. In order for catalysis to occur,
a vacant coordination site is necessary at the metal center and
Figure 7. X-band EPR spectrum of 3-Co in toluene at 12 K. The
experimental spectrum (black) was simulated using H-Strain values of
900, 196, and 60.0 MHz.
the PF anion is presumably sufficiently labile for this to occur.
6
Consistent with the irreversible oxidation peak that was
evident in the cyclic voltammograms of 1-Fe and 1-Co,
Catalytic decomposition of diphenyldiazomethane to benzo-
phenone azine by the Lewis acidic complex [CpFe-
32
attempts to oxidize each compound with AgOTf or AgPF did
(CO) (THF)][BF ] has been reported. Given the availability
6
2 4
not lead to diazoalkane containing complexes. Oxidation of 1-
of an open coordination site in 4-Co-PF following dissociation
6
Fe with AgOTf instead afforded the previously reported
of a coordinated F, it is reasonable to suggest that 4-Co-PF6
catalyzes diazoalkane decomposition by a mechanism that
proceeds via a cobalt carbene intermediate. This reactivity
tBu
30
compound L FeOTf, in addition to benzophenone azine as
1
identified by H NMR spectroscopy and GC-MS. No other
contrasts with the previously reported reaction of 4-Fe-BArF
organic products (such as tetraphenylethylene) were identified.
Likewise, oxidation of 1-Co with AgPF6 formed a new
paramagnetic product and benzophenone azine. This para-
magnetic product can be independently synthesized by
4
with ethyl diazoacetate, which resulted in electrophilic attack of
the central position of the β-diketiminate by the diazo nitrogen
33
after coordination by the carbonyl oxygen to the metal center.
tBu
11c
oxidation of L Co with AgPF , and X-ray diffraction of a
6
tBu
single crystal revealed the product to be L CoPF , 4-Co-PF .
COMPUTATIONAL STUDIES
6
6
■
The solid state molecular structure of 4-Co-PF (Figure 8)
6
To complement the computational studies that had been
performed previously on 1-Co, the new compounds 1-Fe, 2-Co
and 3-Co were investigated using Density Functional Theory
features only the second example of bidentate coordination of
(
DFT). In accordance with the experimentally determined
magnetic moment of 1-Fe, optimization of 1-Fe as a high-spin
S = 3/2) system resulted in a lower energy and an optimized
(
structure that agreed better with the experimentally determined
solid-state structure (see Table 1) than the corresponding low
spin (S = 1/2) optimization. Compounds 2-Co (S = 1/2) and
3-Co (S = 1/2) were also optimized using the indicated spin
states, based on the experimentally determined magnetic
moments, and yielded geometries that were in reasonable
agreement with the structure determined by X-ray crystallog-
raphy. As with the previous studies on 1-Co, the M06L/B1
functional and basis set (see Experimental Section for
description of the basis sets) consistently gave geometries
that were in better agreement with the experimentally
determined structures than either ωB97xD/B1 or PBE0/B1
2
0
(
see Table 1 for comparison of select metrical parameters).
There is a tendency for the M−N bond lengths to be slightly
overestimated in the calculated structures of 1−3, as is often
34
Figure 8. Molecular structure of LtBuCoPF , 4-Co-PF . Thermal
observed in transition metal systems. Though this is most
pronounced in 3-Co-DFT, the predicted bond lengths are
within good agreement of those determined experimentally.
There is no evidence of systematic over/underestimation of the
bond lengths within the ligands, and all are within good
6
6
ellipsoids are displayed at the 50% probability level. Hydrogen atoms
are omitted for clarity. Selected bond lengths (Å) and angles (deg):
Co1−N11 1.914(1), Co1−N21 1.913(1), Co1−F1 2.0794(8), Co1−
F2 2.0843(8); N11−Co1−N21 99.96(4), F1−Co1−F2 65.16(3).
H
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Inorganic Chemistry
Article
agreement of the experimentally determined values. The
experimentally validated geometries, 1-Fe-DFT, 2-Co-DFT
and 3-Co-DFT, were therefore used for all further computa-
tional studies (we distinguish the calculated geometries from
those determined experimentally with the DFT suffix).
The electronic structures of 1-Fe-DFT, 2-Co-DFT, and 3-
Co-DFT were subsequently investigated using the hybrid
functional M06/B2 (see Experimental Section for description).
Frontier molecular orbital diagrams for 1-Fe-DFT (Figure
S25), 2-Co-DFT (Figure S26) and 3-Co-DFT (Figure S27) are
included in the Supporting Information. Maximally aligning the
α- and β-spin electrons indicated that 1-Fe-DFT and 3-Co-
DFT contain ligand based radicals (overlap between corre-
sponding orbital pair 62% and 66%, respectively; see Figure 9).
Figure 10. Spin-density plot of 1-Fe-DFT. Surfaces depicted with an
isovalue of 0.01 au. Color coded: α-spin, blue; β-spin, green.
Figure 11. Resonance structures that correspond to the NBO-
determined single electron α- and β-spin orbitals for 1-Fe-DFT, 2-Co-
DFT and 3-Co-DFT. Two-electron Lewis structures are depicted for
clarity, though these are meant to indicate the locations of single
electrons. See Figure 1 for common resonance structures ascribed to
transition metal diazoalkane complexes.
Figure 9. Spin polarized orbitals in the qualitative MO diagram of 1-
Fe-DFT and 3-Co-DFT that come from maximally aligning the α- and
β-spin orbitals. Surfaces are depicted with an isovalue of 0.04 au S
represents the overlap integral between the correlated α- and β-spin
electrons; all other overlaps were greater than 0.97. Other frontier
orbitals are shown in the Supporting Information.
based radical that is antiferromagnetically coupled to the
transition metal center (Figure 11). This is evidenced by the
extra β-spin electron on the diazoalkane moiety, and one fewer
transition metal centered β-spin electron than would be
expected for the formal d-electron count and spin state. In
the case of 2-Co-DFT, however, NBO analysis suggests that
two-electron reduction of the diazoalkane has occurred, and
that it is now best described as a dianionic ligand (Figure 11).
This assignment is consistent with the longer N−N and N−C
bonds in the molecular structure of 2-Co, relative to the
terminally bound compounds 1-Fe and 1-Co. WBIs are
consistent with the description of all compounds reported
herein based on spectroscopic and computational studies (see
Supporting Information, Figure S30).
The AO contributions to the corresponding pairs in 1-Fe-DFT
and 3-Co-DFT clearly show that the α-spin orbital is metal
based (83% and 84%, respectively) while the β-spin orbital is
located on the diazoalkane moiety (76% and 77%, respectively).
35
Broken symmetry calculations, as developed by Noodleman,
were performed on these two compounds, and converged to
match the unrestricted calculation. Spin density plots of 1-Fe-
DFT, 1-Co-DFT and 3-Co-DFT are almost indistinguishable
from each other and feature significant β-spin density on the
NNC unit of the diazoalkane ligand (for 1-Fe-DFT see Figure
1
0; for 1-Co-DFT and 3-Co-DFT see Supporting Information
DISCUSSION
Figures S28 and S29, respectively).
■
Natural Bond Order (NBO) studies continue to grow in
popularity, because they provide a way to correlate canonical
molecular orbitals with Lewis structures, a description of
bonding that is more familiar to chemists. Furthermore, they
provide an impartial assessment of bond order, both through
the Lewis structure description they generate, and Wiberg Bond
Indices (WBIs). For the unrestricted calculations described
here, the different Lewis structures for different spins approach
Periodic Trends in Formation and Structures of
Diazoalkane Complexes. Diazoalkane complexes 1-3 cover
formal oxidation states 0-I and d-electron counts of 7−9 at the
36
6
7
iron and cobalt atoms. The iron(II) and cobalt(II) d and d
tBu
complexes, L Fe(OTf) and 4-Co-PF , which result from
6
oxidation of the formally iron(I) and cobalt(I) precursors 1-Co
and 1-Fe, did not form stable complexes with diphenyldia-
zoalkane. All complexes have been characterized structurally,
spectroscopically and by computational methods either in this
study or previously.
studies, magnetic moment, EPR spectroscopy, and DFT
37
(
DLDS) was used. Due to the requirements of the NBO
program, a basis set without diffuse functionals, B3, was used
see experimental details). The NBO analysis of 1-Fe-DFT and
-Co-DFT is consistent with an anionic diazoalkane ligand
10,29
On the basis of X-ray diffraction
(
3
studies, 1-Fe, 1-Co and 3-Co feature a diazoalkane radical
I
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
anion antiferromagnetically coupled to the transition metal
center. Ligand noninnocence of this type can be difficult to
distinguish spectroscopically, but the nonlinear geometry of the
diazoalkane ligand in all complexes strongly implicates anion
character on the diazoalkane. The magnetic moment indicates
that any radical character on the ligand is antiferromagentically
coupled to the metal center, and this is supported by the spin-
unrestricted DFT calculations. Furthermore, these computa-
tional studies suggest that resonance structures I and IV best
describe the α- and β-spin electrons, respectively (Figure 1), as
part of the different Lewis structures for different spins
36
approach (DLDS). The extra β-spin on the diazoalkane in
this valence-bond description corresponds to the radical anion
character in the molecular-orbital picture. Although these
systems are charge separated, the DFT calculations indicate
strong coupling between the corresponding pair of α- and β-
Figure 12. Possible mechanisms of carbene formation.
−1
−1
electrons (calculated J for 1-Fe, −629 cm ; 1-Co, −909 cm ;
−
1
3
-Co, −806 cm ). Radical character of this type is rarely
also isolated (26%). They used the isolated products to suggest
a mechanism that has an iron(IV) carbene intermediate. It is
reasonable that an analogous mechanism is effective in the
diketiminate system described here, and, given the identical
ligand environments used in the current study, clearly shows
that steric effects alone do not prevent carbene formation in 1−
9
assigned in diazoalkane complexes, but not unprecedented,
and a reversible diazoalkane radical coupling reaction has been
3
8
reported.
The electronic flexibility of the diazoalkane is demonstrated
2
by 2-Fe and 2-Co, in which the diazoalkane isomerizes to an η -
N,N′ mode that is reduced by two electrons as shown by
structural, spectroscopic, and for 2-Co, DFT studies. DFT
studies suggest that resonance structure V best describes both
3
.
The mechanism of azine formation in the current system is
2
unclear, though it presumably proceeds through a carbene
intermediate. Reasonable monometallic mechanisms for
carbene formation include the following (Figure 12). (1) Initial
N-bound adduct formation by the diazoalkane that then “slips”
the α- and β-spin electrons of the η -N,N′ bound diazoalkane
(
Figure 1). This is consistent with previously reported side-on
1d
bound diazoalkane complexes. The increase in diazoalkane
reduction is reasonable given the greater overlap between the d-
orbitals of the metal centers and the π* antibonding orbitals of
the diazoalkane ligand. Comparison of 2-Co and 3-Co, in
which the formal oxidation state of the cobalt atom is the same,
shows that the change in geometry correlates with the increased
charge movement to the diazoalkane.
to C-bound, followed by N loss. (2) Initial N-bound adduct
2
39
formation that subsequently forms a heterocycle following the
formation of a M-C bond, which then leads to N
analogous mechanism is believed to be active in the conversion
loss (the
2
41
of organic azides to imido complexes). (3) Direct formation
The coordination of the potassium ion in 2-Fe and 2-Co to
the terminal nitrogen atom of the diazoalkane warrants further
discussion. With the exception of transition metal complexes in
which the diazoalkane ligand bridges between identical metal
of a M-C bond, followed by N elimination.
A detailed study by Milstein and co-workers on a rhodium
2
system elucidated the importance of steric effects on carbene
42
1
formation. They favored mechanism 3 (above), in which η -C
binding leads to a carbene containing compound, whereas
initial N-bound complex formation (which is the kinetic
product in sterically encumbered systems) was deleterious.
Chirik and co-workers have also demonstrated a similar effect
where decreasing the steric bulk of the ligands enabled carbene
2
1
centers (e.g., Cp(Cl)Ti(μ−η :η -N CPh ) Ti(Cl)Cp,
2
2 2
5
5
2
1
28b,40
(
μ−η :η -C H )Zr (μ−η :η -N CPh )Cl Cp ),
there is
10
8
2
2
2
2
2
only one example in the literature in which a diazoalkane
2
1
ligand is also coordinated to a Lewis acid, Cp* Ti{μ−η :η -
N C(H)(p-tol)}·AlNp (Np = neopentyl). However, this
2
2
3
2
titanium diazoalkane complex adopted η -N,N′ binding
modes with or without the Lewis acid. Our system is thus
the first crystallographically and spectroscopically characterized
example of a Lewis acid induced change in coordination mode
of a diazoalkane transition metal complex. The fact that this
change is caused by the potassium cation, not by a change in
oxidation state, is shown by reversion to the terminally bound
diazoalkane structure after the potassium is sequestered by a
crown ether. Potassium sequestration by crown ether in 3-Co
4h
formation. As all of the complexes discussed in this article
have the same supporting ligand, the tendency of the lower
oxidation state complexes to form stable adducts (1-Fe, 1-Co,
and 3-Co) relative to the iron(II) and cobalt(II) compounds
tBu
(
L Fe(OTf) and 4-Co-PF ) suggests that electronic factors
6
also play an important role.
In our system, it may seem surprising that the more highly
reduced complexes are less prone to generate carbene
intermediates with formal oxidation at the metal center, while
the cobalt complex with the lowest d-electron count is the one
that gives evidence for carbene formation. We surmise that in
the low-coordinate, low-valent systems used here, end-on
binding gives strong overlap between high-lying, filled metal d-
orbitals and diazoalkane π*-orbitals, leading to charge buildup
on the diazoalkane and strengthening of the M−N and C−N
(
in which there is no change in formal oxidation state from 2-
Co) induces the diazoalkane to revert back to radical anion
character.
Catalytic Formation of Azine and a Cobalt Carbene.
7
The cobalt(II), d complex 4-Co-PF catalyzes the conversion
6
of diphenyldiazomethane to benzophenone azine. The iron(II)
complex [CpFe(CO) (THF)][BF ], reported by Bennett and
2
4
co-workers, has previously been shown to afford the same
bonds that prevents N loss. The higher oxidation state
2
32
transformation with 10 mol % catalyst loading. In addition to
benzophenone azine (yield = 39%), tetraphenylacetylene was
compounds, on the other hand, do not have analogous charge
transfer and can lose N2.
J
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
CONCLUSIONS
Boris Dzikovski for assistance with EPR data collection. We
also thank Gary Brudvig for the use of an EPR spectrometer
and for helpful discussions. Some crystallography was
performed at the Advanced Light Source (ALS), which is
supported by the Director, Office of Science, Office of Basic
Energy Sciences, of the U.S. Department of Energy under
Contract No. DE-AC02-05CH11231.
■
Structural and computational characterization of new diazo-
alkane complexes of iron and cobalt demonstrates the
noninnocent nature of the diazoalkane. This easily reduced
ligand can accommodate one or two electrons from the metal,
and the extent of charge transfer depends not only on the
oxidation level of the metal, but also on the presence of
2
potassium ions that encourage an η binding mode. These
insights can potentially be used to control the binding mode
and charge transfer to other diazoalkane adducts that are
precursors to carbene complexes.
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AUTHOR INFORMATION
Corresponding Author
E-mail: patrick.holland@yale.edu.
■
*
ORCID
Javier Vela: 0000-0001-5124-6893
Ryan E. Cowley: 0000-0003-1791-4266
William W. Brennessel: 0000-0001-5461-1825
Patrick L. Holland: 0000-0002-2883-2031
Present Addresses
J.V.) Department of Chemistry, Iowa State University, Ames,
Iowa 50010, United States.
D.J.V.) Department of Biological Sciences, Louisiana State
University, Baton Rouge, Louisiana 70803, United States.
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§
(
∥
(
Funding
Funding was provided by the American Australian Association
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Energy Sciences, Grant DE-FG02−09ER16089 (P.L.H.).
Analytical data were from the CENTC Elemental Analysis
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■
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K
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