Metal-amidato complexes: Synthesis, characterization, and reactivity of a diamidato-bis(phosphine) nickel(II) complex

Article abstract of DOI:10.1016/j.ica.2014.08.033

Reaction of 1,2-bis-N-[20-(diphenyl-phosphanyl)benzoyl]diaminobenzene (H₂N₂P₂: H₂1) with Ni(ClO₄)₂ produced the corresponding tetradentate diamidato-bis(phosphine) nickel(II) complex Ni(N₂P₂) (2). Reactivity studies of 2 with reductants yielded the orthometallated product [M+][Ni(N₂PC)] ([M+][3]), where M = Cp/ ₂CoIII or NBu₄ and N2PC = N-((2-diphenylphosphino)-benzoyl)-N0-((1-phen-2-idyl)oxomethyl)- 1,2-diaminobenzene, via reductive cleavage of one of the PPh₂ groups of H₂1. The synthesis and characterization of the complexes are described.

Full text of DOI:10.1016/j.ica.2014.08.033

Inorganica Chimica Acta 423 (2014) 290–297
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Metal-amidato complexes: Synthesis, characterization, and reactivity
of a diamidato-bis(phosphine) nickel(II) complex
Daniel N. Huh d, Joseph B. Gibbons c, Ryan S. Haywood d, Curtis E. Moore a, Arnold L. Rheingold a,
Michael J. Ferguson b, Christopher J.A. Daley d,
⇑
a Department of Chemistry and Biochemistry, University of California – San Diego, 9500 Gillman Drive, La Jolla, CA 92093, United States
b Chemistry Department, University of Alberta, 11227 Saskatchewan Dr. NW, Edmonton, AB T6G 2G2, Canada
c Department of Chemistry, University of Utah, 315 S. 1400 E, Salt Lake City, UT 84112, United States
d Department of Chemistry and Biochemistry, University of San Diego, 5998 Alcala Park, San Diego, CA 92110, United States
a r t i c l e i n f o
a b s t r a c t
Reaction of 1,2-bis-N-[20-(diphenyl-phosphanyl)benzoyl]diaminobenzene (H2N2P2: H21) with Ni(ClO4)2
produced the corresponding tetradentate diamidato-bis(phosphine) nickel(II) complex Ni(N2P2) (2).
Reactivity studies of 2 with reductants yielded the orthometallated product [M+][Ni(N2PC)] ([M+][3]),
where M = Cp⁄2CoIII or NBu4 and N2PC = N-((2-diphenylphosphino)-benzoyl)-N0-((1-phen-2-idyl)oxometh-
yl)-1,2-diaminobenzene, via reductive cleavage of one of the PPh2 groups of H21. The synthesis and
characterization of the complexes are described.
Article history:
Received 17 May 2014
Received in revised form 22 August 2014
Accepted 24 August 2014
Available online 2 September 2014
Keywords:
Ó 2014 Elsevier B.V. All rights reserved.
Nickel-amidato complex
X-ray crystallography
Reductive cleavage
Cyclic voltammetry
Orthometallation
1. Introduction
amidato-Fe, -Co, and -Ni complexes that contain amidato-sulfur-,
amidato-oxygen-, and amidato-nitrogen-based ligands, very little
Metal-amidato complexes have been the subject of significant
investigation over the last few decades mainly owing to their
importance in the areas of biochemistry and medicinal chemistry.
Notable examples include their (i) occurrence in important metal-
loenzymes including nitrogenase [1], nitrile hydratase (NHase) [2],
and carbon monoxide dehydrogenase/acetyl Co-A synthase
(CODH/ACS) [3], (ii) implication in disease states, most signifi-
cantly in prion proteins where their complexes are potentially
responsible for the neurodegenerative diseases called transmissi-
ble spongiform encephalopathies (e.g. mad cow, kuru, and
Creutzfeldt–Jakob diseases) [4], and (iii) potential as site specific
NO delivery systems for biologically relevant targets [5]. Numerous
studies [6,7], including work from our group [8], have focused on
polydentate, diamidato-containing, ligand complexes of Fe, Co,
and Ni in order to understand their fundamental properties and
their possible use as synthetic analogues to the active sites of
NHase and CODH/ACS (A-cluster). The other metal ligating atoms
of these active sites are sulfurs derived from cysteine side chains.
While significant work has been reported on polydentate,
has been reported for amidato-phosphine-based ligands. Of the
few of the phosphorus-for-sulfur exchanged systems reported,
key discoveries were made, including the preparation of previously
inaccessible reduced iron-sulfur-based clusters [9] and enhanced
complex stability of potentially relevant Tc and Re radiopharmceu-
tical complexes [10]. Thus, there is interest in uncovering the fun-
damental chemistry of metal-amidato systems; particularly with
phosphorus-for-sulfur exchanged systems. Herein we report our
findings on the synthesis and characterization of a diamidato-
bis(phosphine) nickel(II) complex and its reactivity with
reductants to form orthometallated complexes via reductive cleav-
age of a ligand C–P bond, releasing PPh2, and formation of a C–Ni
bond. Characterization of the latter complexes is also included.
2. Materials and methods
2.1. Reagents and techniques
All reagents were purchased from commercial sources and used
as received. Solvents (Et2O, DMF, MeCN, and CH2Cl2) were dried by
passing through an Innovative Technologies solvent purification
system and collected just prior to use and further deoxygenated
⇑
Corresponding author. Fax: +1 (619) 260 2211.
E-mail address: cjdaley@sandiego.edu (C.J.A. Daley).
http://dx.doi.org/10.1016/j.ica.2014.08.033
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
291
by bubbling with a stream of N2 for 30 min prior to use. The com-
pounds Ni(ClO4)2ꢀ6H2O (Aldrich), 2-diphenylphosphanyl benzoic
acid (Aldrich), acenaphthylene (TCI America), 1,2-diaminobenzene
(ACROS), N,N0-dicyclohexylcarbodiimide (Alfa Aesar), N,N-diisopro-
pylethylamine (Aldrich), and 4-(dimethylamino)pyridine (Aldrich)
were purchased from commercial sources and used as received.
NMR spectra were collected on a 400 MHz or 500 MHz Varian
NMR spectrometer. 1H NMR chemical shifts are referenced to
TMS. 31P{1H} NMR chemical shifts are uncorrected. Elemental anal-
ysis performed by Micro-Analaysis, Inc. (Wilmington, DE, 19808).
dark, opaque red. To a 100-mL side-arm flask was added 2 (50 mg,
6.7 ꢂ 10ꢁ5 mol), [NBu4][Cl] (38 mg, 1.4 ꢂ 10ꢁ4 mol), THF (20 mL),
MeCN (5 mL), and a stirbar. The mixture was stirred for 10 min
to dissolve all the contents producing a dark green colored solu-
tion. To the 100-mL flask was added the reductant solution
(5 mL) via pipette over 5 min. The mixture was stirred for 16 h
and the solution colored changed to a deep orange. The solvent
was removed under vacuum and the remaining reddish solid resi-
due was redissolved in MeCN (5 mL) and layered with diethyl ether
(15 mL) for crystallization. Red crystals of [NBu4][3] (30 mg,
3.8 ꢂ 10ꢁ5 mol) were isolated via filtration and dried under high
vacuum in a 56% yield. 1H NMR (500 MHz, CDCl3, rt) d 8.75 (dd,
1H), 8.44 (ddd, 1H), 7.94 (dd, 1H), 7.79–7.72 (m, 4H), 7.48–7.39
(m, 3H), 7.36–7.31 (m, 4H), 7.31–7.28 (m, 1H), 7.23 (t, 1H), 6.96–
6.89 (m, 1H), 6.77 (td, 1H), 6.67 (pd, 2H), 6.40 (td, 1H), 6.18 (d,
1H). 31P{1H} (202 MHz, CDCl3, rt) d 33.0 (s, 1P).
2.2. Syntheses
2.2.1. Synthesis of 1,2-bis-N-[20-
(diphenylphosphanyl)benzoyl]diaminobenzene (H21)
The ligand was prepared following the protocol reported by
Süss-Fink [11] using slight modifications. To a 500 mL side-arm
flask was dissolved 2-(diphenylphoshanyl)benzoic acid (6.012 g,
19.63 mmol) in CH2Cl2 (200 mL) on stirring. To this was added
1,2-diaminobenzene (1.053 g, 9.741 mmol) followed by N, N0-dicy-
clohexylcarbodiimide (4.872 g, 23.61 mmol) and 4-dimethylami-
nopyridine (DMAP: 0.1197 g, 0.9798 mmol). The mixture was
stirred for 14 h, during this time a cloudy white precipitate formed
and the solution was light brown in color. The mixture was filtered
through Celite to yield a yellow-gold solution. The solvent was
removed under reduced pressure on a rotary evaporator leaving
a yellow light-brown colored solid. The solid was transferred to a
500-mL round bottom flask along with EtOAc (200 mL) and a stir-
bar. The heterogeneous mixture was stirred for 4 h to dissolve
impurities. The mixture was filtered through a fine glass-frit
funnel, washed with minimal EtOAc, and dried under vacuum for
1 h. White solid H21 (9.795 g, 14.31 mol) was collected in a 70.5%
yield. 1H NMR (500 MHz, CDCl3): d 8.45 (br s, 2H), 7.75 (dd, 2H),
7.40–7.17 (m, 26H), 7.12 (dd, 2H), 6.99 (dd, 2H). 31P{1H} NMR
(500 MHz, CDCl3): d ꢁ9.03 (s). Product identity was confirmed on
comparison to literature NMR data.
2.2.4. Synthesis of [Cp⁄2CoIII][Ni(N2PC)] ([Cp2⁄CoIII][3])
In the glovebox, 2 (200 mg, 2.69 ꢂ 10ꢁ4 mol) was dissolved
with DMF (30 mL) on stirring in a 250-mL side-arm flask to pro-
duce a dark green colored solution. To this was added Co(C5Me5)2
(Cp⁄2Co: 178 mg, 5.40ꢁ4 mol) directly. The solution was left stirring
for 14 h. The solution turned dark orange-red in color. The solution
was filtered via inverse-filtration into a 500-mL side-arm flask to
remove trace black insoluble solid. The DMF filtrate was layered with
Et2O (350 mL) and left to crystallize for 72 h. The solid [Cp⁄2CoIII][3]
(0.1468 g, 1.658 ꢂ 10ꢁ4) was collected via inverse-filtration, washed
with Et2O (3 ꢂ 15 mL), and dried under high vacuum in a 61.5%
yield. Combustion analysis as [Cp⁄2CoIII][3] (calculated, observed): C
70.5, 69.65; H 5.9, 5.90; N 3.2, 3.23. 1H NMR (500 MHz, DMSO-d6,
rt) d 8.63 (dt, 1H), 8.14 (dd, 1H), 7.91 (dt, 1H), 7.80 (ddt, 4H), 7.50
(m, 7H), 7.35 (t, 1H), 6.99 (d, 1H), 6.91 (dd, 1H), 6.64 (tt, 1H), 6.54
(m, 2H), 6.27 (ddd, 1H), 6.16 (d, 1H), 1.72 (s, 30H). 31P{1H}
(202 MHz, DMSO-d6, rt) d 32.7 (s, 1P).
2.3. Crystallography measurements
2.2.2. Synthesis of Ni(N2P2) (2)
X-ray quality crystals of 2 were grown via vapor diffusion of
Et2O into a concentrated DMF solution of 2, crystals of [NBu4][3]
were grown via vapor diffusion of Et2O into a concentrated MeCN
solution of [NBu4][3], and crystals of [Cp⁄2CoIII][3] were grown via
To
a
200-mL side-arm flask containing H21 (1.036 g,
1.513 mmol) and a stirbar was added N, N-dimethylformamide
(DMF: 50 mL) via cannula. The flask was placed in a 45 °C water
bath and N,N-diisopropylethylamine (DIPEA: 1.00 mL, 0.742 g,
5.74 mmol) was added to the flask via syringe. In the glovebox,
vapor diffusion of Et2O into
a concentrated DMF solution of
[Cp⁄2CoIII][3]. Data for 2 was collected at the University of Alberta
in
a
separate 20-mL sample vial, Ni(ClO4)2ꢀ6H2O (0.5320 g,
on a Bruker PLATFORM/SMART 1000 CCD diffractometer equipped
1.455 mmol) was added and dissolved in DMF (10 mL) with
stirring. The Ni(II) solution was added dropwise via cannula to
the 200-mL side-arm flask containing H21. The reaction mixture
was stirred for 3 h and turned dark green in color. The solution vol-
ume was reduced (10 mL) under high vacuum and precipitation of
2 was completed by layering the reaction mixture with Et2O
(70 mL) and allowing it to stand for 28 h. The solution was
inverse-filtered and the remaining forest green solid was dried
under high vacuum. The solid was washed quickly with MeCN
(1 ꢂ 5 mL), filtered, washed again with Et2O (4 ꢂ 5 mL), filtered
and dried under high vacuum to give dark green 2 (0.8445 g,
1.139 mmol) in a 75.3% yield. Combustion analysis as 2 (calculated,
observed): C 71.3, 70.5; H 4.35, 4.30; N 3.8, 3.99. 1H NMR
(500 MHz, CDCl3, rt) d 8.49 (d, 2H), 7.57 (t, 2H), 7.42–7.33 (m,
6H), 7.25–7.09 (m, 18H), 6.75 (dddd, 2H), 6.64 (t, 2H). 31P{1H}
(202 MHz, CDCl3, rt) d 20.8 (s, 2P).
with Mo
[Cp⁄2CoIII][3] were collected at the University of California San Diego
on a Bruker Kappa APEX II CCD diffractometer equipped with Mo K
Ka radiation (k = 0.71073). Data for [NBu4][3] and
a
radiation (k = 0.71073). The crystals were mounted on a Cryoloop
with Paratone oil and data were collected in a nitrogen gas stream
at 173(2) K (2) or 100(2) K ([NBu4][3] and [Cp⁄2CoIII][3]). The data
were integrated using the Bruker SAINT [12] software program and
scaled using the SADABS [13] software program. The structures were
solved using direct methods (SHELXS).[13] All non-hydrogen atoms
were refined anisotropically by full-matrix least-squares (SHELXL).
[13,14] All hydrogen atoms were placed using a riding model. Their
positions were constrained relative to their parent atom using the
appropriate HFIX command in SHELXL. Crystallographic data are
summarized in Table 1.
2.4. Cyclic voltammetry measurements
2.2.3. Synthesis of [NBu4][Ni(N2PC)] ([NBu4][3])
The electrochemical analyses of 2 and [Cp⁄2CoIII][3] via cyclic vol-
tammetry were performed on a Pine Instrument bipotentiostat using
a platinum (2) or glassy carbon ([Cp⁄2CoIII][3]) working and platinum
counter electrodes. The analysis of 2 was performed in MeCN con-
taining 0.3 M [NBu4][PF6] as supporting electrolyte and [Cp⁄2CoIII][3]
In the glovebox, acenaphthylene (54 mg, 3.5 ꢂ 10ꢁ4 mol), Na(s)
(9 mg, 4 ꢂ 10ꢁ4 mol), and THF (10 mL) were added to a 20-mL vial
to prepare the reductant. The reductant solution was stirred for 3 h
and the solution color changed from yellow, to red, to a final deep,

292
D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
Table 1
Crystallographic data of compounds 2, [NBu4][3], and [Cp⁄2CoIII][3].
2
[NBu4][3]ꢀ0.5 Et2O
[Cp⁄2CoIII][3]
Formula
C44H32N2NiO2P2
741.37
C50H63N3NiO2.5P
835.71
0.49 ꢂ 0.20 ꢂ 0.08
orthorhombic
Pbca
C52H52CoN2NiO2P
885.56
0.30 ꢂ 0.28 ꢂ 0.26
monoclinic
P21/c
Formula weight
Crystal dimensions (mm)
Crystal system
Space group
Unit cell parametersa
a (Å)
0.33 ꢂ 0.31 ꢂ 0.27
triclinic
ꢀ
P 1 (No. 2)
10.7479 (8)
11.3711 (8)
16.2272 (12)
74.9894 (9)
79.8647 (9)
64.1492 (8)
1719.3 (2)
2
23.032(3)
16.617(2)
23.698(3)
90
90
90
9070 (2)
8
1.224
12.0721(8)
10.7360(7)
32.667(2)
90
90.4960 (10)
90
4233.7 (5)
4
1.389
b (Å)
c (Å)
a
(°)
b (°)
k (°)
V (Å3)
Z
Dcalc (g cmꢁ3
)
1.432
0.700
l
(mmꢁ1
)
0.506
0.919
Radiation (k [Å]) Mo K
T (°C)
a
0.71073
173 (2)
0.71073
100 (2)
3.44–56.16
35870
10127 (0.0977)
10127/0/533
1.005
0.71073
100 (2)
3.374–50.864
43998
7820 (0.0458)
7820/0/542
1.046
2h range data collection (deg)
No. reflections collected
No. independent reflections (Rint
Data/restraints/parameters
Goodness-of-fit (GoF)
Final R indices
4.50–54.88
14556
7743 (0.0125)
7743/0/460
1.059
)
R1
0.0340
0.0586
0.0318
wR2
Largest
0.0865
0.49, ꢁ0.24
0.1100
0.58, ꢁ0.66
0.0756
0.50, ꢁ0.31
D
peak and hole (e Åꢁ3
)
P
P
P
P
P
GoF = [ w(Fo ꢁ Fc2)2/(Ndata ꢁ Nparams)]1/2; R1
=
||Fo| ꢁ |Fc||/ |Fo|; wR2 = [ w(Fo ꢁ Fc2)2/ w(Fo4)]1/2
.
2
2
was performed in DMF containing 0.3 M [NBu4][PF6] as supporting
electrolyte. The reference electrode was a silver wire and potentials
were referenced to internal ferrocene standard. The voltammograms
were swept between +0.8 and ꢁ2.2 V for 2 and +1.3 and ꢁ2.8 V for
[Cp⁄2Co][3] at multiple scan rates.
salt with H21 in a dry DMF solution for 3 h. The emerald green
product 2 was isolated in 75% yield by filtration after DMF-Et2O
crystallization at room temperature. It was subsequently charac-
terized in both the solution and solid states.
3.2. Characterization of Ni(N2P2) (2)
3. Results and discussion
NMR analysis was used to characterize 2 in solution (Fig. 1). 1H
NMR in CDCl3 showed the presence of a key signal at 8.5 ppm,
which we noted to be an indicator for tetradentate coordination
in an analogous Trost-ligand series of complexes [16]. The signal
corresponds to the H4 proton of 12ꢁ, which was confirmed by
the observed correlation to the carbonyl carbon (13C{1H}: d
165.2) via HMBC analyses. The assignment of the remaining
backbone H-atoms were determined via COSY, HSQC, and HMBC
analyses (Fig. 1). The number of signals in the 1H NMR and the
31P{1H} NMR spectrum having only a single peak at d 20.8 (s) indi-
cate that 2 exhibits C2-symmetry and, along with being diamag-
3.1. Synthesis of Ni(N2P2) (2) using Süss-Fink’s diamidato-
bis(phosphine) ligand (H2N2P2: H21)
Ligand H21, developed by Süss-Fink [11], has been shown to
have several useful features. Süss-Fink showed, with Pt and Pd
complexes, that it can act as a two- or four-coordinate ligand bind-
ing to the amide nitrogens and/or the phosphines depending on
the metal and reaction conditions used. More recently, Mascharak
reported the use of H21 in attempts to prepare Ru2+ nitrosyl
(Ru–NO) complexes [15]. No Ru-complexes of H21 were isolated;
H21 underwent oxidation (ꢁ4H+, ꢁ4e–) to its diimine form and
formed a tetracoordinate diimino-bis(phosphine) Ru2+ complex.
Interestingly, attempted nitrosylation of the complex resulted not
in the formation of a Ru–NO complex but rather nitrosylation of
the diimine ligand backbone. Our interest in H21 stems from the
structural framework similarities to the Trost series of ligands
(e.g. N,N0-bis(2-diphenylphosphino-1-benzoyl)-1,2-diaminocyclo-
hexane), which we have investigated to further understand the
chemistry of metal-amidato systems [16]. Furthermore, as with
the Trost series of ligands, H21 is of interest owing to its structural
similarity to the active sites in NHase and, more relevant to this
report, the distal nickel center in the A-cluster of CODH/ACS, where
two of the thiol groups are replaced with phosphine groups.
Using the strategy we reported for the formation of a series of
Pd-, and Pt-Trost-based complexes [16], H21 was reacted with
Ni(ClO4)2ꢀ6H2O to prepare the tetradentate diamidato-bis(phos-
phine) nickel(II) complex 2 (Scheme 1), completing the Group 10
series of 1-based complexes (Pd and Pt complexes reported) [11].
The complex was prepared on stirring equimolar amounts of nickel
netic, is best described as
a
square-planar tetradentate
diamidato-bis(phosphine) NiII complex in solution [17]. The solu-
tion structure of 2 is consistent with Pd(1) [11], Pt(1) [11],
Pd(Trost) [16b], and Pt(Trost) [16a], the other Group 10 Süss-
Fink-based as well as Trost-based complexes; each of which dis-
play single 31P signals and a corresponding reduction in 1H signals
owing to C2-symmetrization.
Needle crystals of 2, grown via a vapor diffusion of Et2O into a
concentrated solution of 2 in DMF, were analyzed by X-ray diffrac-
tion (Table 1 and Fig. 2). The structure is best described as square
planar about the NiII center but not C2-symmetric as noted by the
puckering of the backbone out of the plane. In comparison to the
Süss-Fink reported Pd(1) and Pt(1) complexes, the nickel-ligating
atom bond lengths are slightly shorter (0.13 and 0.08 Å shorter
M–N and M–P bonds, respectively). The shortening is expected
based on the smaller size Ni(II) ion versus the similarly sized Pd(II)
and Pt(II) ions. With respect to the bond angles between the metal
and ligating atoms, they were very similar being less than ꢃ2° dif-
ferent for the cis-N-M-P, trans-N-M-P, and P–M–P angles with only

D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
293
Scheme 1. Synthesis of Ni(N2P2) (2), where N2P2 (1) is the dianion of 1,2-bis-N-[20-(diphenylphosphanyl)benzoyl]diaminobenzene (H2N2P2: H21).
Fig. 1. 1H NMR of complex 2 in CDCl3 with 31P{1H} NMR inset at top right. Backbone signals are assigned and solvent impurities are denoted with ‘‘x’’.
Pd(1) and Pt(1) complexes with the metal-P bonds being similarly
shorter (ꢃ0.07–0.09 Å shorter). While the 1-based complexes form
more bowled type structures with more noticeable deformation
from the ideal square planar structure about the metal center,
the Trost systems are less distorted, adhering more closely to the
ideal square planar structure (cis-ligation and trans-ligation angles
closer to 90° and 180°, respectively). The distortion in the 1-based
systems is likely owing to the more rigid nature of 12ꢁ as compared
to the Trost ligands, specifically with the C-C bridge between the
amide nitrogens which is olefinic in 12ꢁ as opposed to aliphatic
in Trost. That the latter system has more rotational freedom about
the N–C–C–N backbone, allowing for more conformation fluxional-
ity/fluidity, the coordinating atoms can more easily adopt a square
planar structure [16]. In comparison to the diamidato-bis(phos-
phine) Ni(II) complex, cis-Ni{2-Ph2PC6H4NC(O)CH2NHCO2Bz}2
(Fig. 3: 4), reported by the Smith group [18], the Ni–P bond lengths
(2.16 Å) and Ni–N(CO) bond lengths (1.93–1.96 Å) are very nearly
equivalent. In contrast, the bond angles around the metal center
are significantly different. The N–Ni–N bond angle of 4 (97.05°) is
significantly larger than that of 2 (DN–Ni–N = 12°), the P–Ni–P bond
angle (97.93°) is significantly smaller (DP–Ni–P = 7°), and the N–Ni–
P bond angles (83.2–84.6°) are median between those of 2 (81.1°
and 90.3°). The bond angle deviations can be explained based on
the more rigid nature of 1–, specifically the tetradentate chelation
that forces the two amidato-N atoms to remain closer together as
compared to the bidentate ligation in 4. The former forces the N–
Ni–N bond angle to be smaller but it allows for the P–Ni–P bond
angle to expand, accounting for the observed bond angle differ-
ences in 2 as compared to 4.
Fig. 2. X-ray crystal structure of
2 with ellipsoids drawn at 50% probability.
Selected bond lengths (Å) and angles (°): Ni–P1: 2.176; Ni–P2: 2.158; Ni–N1: 1.911;
Ni–N2: 1.926; N1–C10: 1.355; N2–C20: 1.348; N1–C1: 1.422; N2–C2: 1.437; N1–
Ni1–N2: 85.0; N1–Ni1–P1: 81.1; N2–Ni1–P2: 90.3; P1–Ni1–P2: 104.9.
the N–M–N angles being slightly different (N–Ni–N: 2.8° and 4.7°
larger than N–Pd–N and N–Pt–N, respectively). Overall, the Group
10 series of M(1) complexes are structurally very similar. In com-
parison to their Pd- and Pt-Trost-based analogues, the metal-N
bond lengths are shorter still (ꢃ0.14–0.16 Å shorter) than the

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D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
diimine form [15], which may account for irreversible oxidation
of 2. With respect to reductive stability, the reduction potential
of 2, as compared to reported bis(phosphine) nickel(II) complexes
[21], corroborates the theory that coordination of deprotonated
amide nitrogen (amidato) atoms to metal centers increases the
metal’s reductive stability, owing to the highly sigma donating nat-
ure of the amidato group [2,3]. Mascharak reported [22] (Et4N)2
[Ni(PhPepS)], the diamidato-dithiolate analogue of 2, where the
PPh2 moieties are replaced by Sꢁ moieties, had no observable
reduction wave up to ꢁ1.8 V and indicates that the PPh2-for-S
exchange has made the complex more readily reducible. Of note,
it has been shown that on methylating the thiolate groups of
diamidato-dithiolate nickel(II) complexes, generating neutral
bound thioethers,
a
reversible NiII/NiI reduction potential is
observed [23]. The diamidato-dithioether nickel(II) complex reduc-
tion potential (quasi-reversible: ꢁ1.24 V versus NHE) is shifted sig-
nificantly, to a more positive potential, as compared to the thiolate
complex (not observed,>-2 V). As 2 contains the two neutral coor-
dinated phosphine donor atoms, the observed reduction potential,
proposed to be the NiII/NiI couple (confirmation required but all
current data support it), is more similar to diamidato-dithioether
nickel(II) systems as compared to the latter’s dithiolate analogues,
as expected.
Fig. 3. ChemDraw representation of the diamidato-bis(phosphine) nickel(II) com-
plex reported by the Smith [18] group (top) and the A-cluster in CODH/ACS
(bottom).
Finally, in comparison to the coordination sphere about the dis-
tal Ni(II) center in the A-cluster of CODH/ACS (Fig. 3), the struc-
tures are again similar. In particular, the Ni–N bond lengths are
quite similar to the reported EXAFS findings (1.89 Å) [19] and sin-
gle crystal diffraction studies (XRD: 1.83–1.96 Å) [3a,3b,20] on
CODH/ACS, as are the N–Ni–N bond angles reported in the latter
studies (85.6–91.6°). Even the Ni–P bond lengths (2.16–2.18 Å)
are similar to the Ni–S bond lengths (EXAFS: 2.19 Å; XRD: 2.09–
2.24 Å) of the A-Cluster. The most marked difference is the P–Ni–
P bond angle (104.5°) versus the native S–Ni–S bond angle (82.4–
87.0°). However, the bond angle discrepancy is expected as the
coordinated cysteine sulfur atoms of the distal nickel center are
bridged to the proximal nickel center and is thus enforced to be
smaller. Overall, 2 can be thought of as a phosphorus-for-sulfur
exchanged structural synthetic analogue of the distal nickel center
in the A-cluster of CODH/ACS.
3.4. Reduction of Ni(N2P2) (2) to [Ni(N2PC)]– ([3]ꢁ)
To further probe the properties of 2, it was reacted with a reduc-
tant prepared by the reaction of Na(s) with acenapthylene
(Scheme 2) where 2 equiv of [NBu4][Cl] was also added for coun-
terion exchange purposes. After stirring for 1 h, solvent was
removed and 1H and 31P NMR analyses were conducted. 1H NMR
analysis indicated a mixture of species. The 31P NMR was more
informative, showing only two phosphorus containing species in
solution with signals at d 32.8 (s) and ꢁ3.0 (s). The compound cor-
responding to the signal at d ꢁ3.0 could not be isolated but the
main phosphorus containing species (31P: d 32.8) was isolated via
crystallization from a MeCN:Et2O solution. The species was found
to be [NBu4][Ni(N2PC)] ([NBu4][3]), where N2PC is N-((2-diphenyl-
phosphino)benzoyl)-N0-((1-phen-2-idyl)oxomethyl)-1,2-diamino-
benzene (Scheme 2).
3.3. Electrochemistry of Ni(N2P2) (2)
Cyclic voltammetry studies, in MeCN (0.3 M [NBu4][PF6]) solu-
tion (Fig. 4), were performed to examine the oxidative/reductive
stability of 2. Complex 2 exhibited an irreversible oxidation at
Eox = 0.15 V and a pseudo-reversible reduction at E1/2 = ꢁ1.21 V
(versus SCE). While other diamidato-bound NiII complexes [7] have
shown a reversible oxidation to NiIII, 2 does not. However H21 has
been reported to undergo irreversible ligand oxidation to its
While the original goal was to have the reductant reduce the NiII
metal center to NiI, the strong reductant used resulted instead in
the reductive cleavage of a ligand backbone C–P bond. Cleavage
of P–Ph bonds with strong reductants is well-documented, being
particularly well-known for the preparation of Ph2Pꢁ from PPh3
[24]. The carbon of the cleaved C–P bond of 2 did not acquire a
hydrogen atom but rather underwent orthometallation to the
nickel center resulting in the formation of [NBu4][3]. The structure
of [NBu4][3] was supported via NMR (Fig. S1: supplementary mate-
rial) analysis and confirmed via X-ray diffraction studies (Table 1
and supplementary material). 1H NMR analysis indicated the com-
plex was asymmetric owing to the greater number of peaks in the
aromatic region of the spectrum that would be expected for a sym-
metric complex. In combination with the asymmetry noted from
1H NMR analysis, 31P{1H} NMR analysis showed one signal (d
32.8 (s)), providing conclusive evidence that only one phosphine
was present in the complex. Taken together with the fact that
[NBu4][3] is diamagnetic (based on NMR analysis), the structure
is best described as being square-planar about the NiII center in
solution [17].
Further efforts to reduce the NiII center via 1-electron were
attempted using Cp⁄2CoII, a milder reducing agent that is more easily
measured to ensure accurate stiochiometric addition. Performed
under similar conditions as with Na/acenaphthylene, excluding
added [NBu4][Cl], the reaction proceeded to again yield [3]ꢁ with
Fig. 4. Cyclic voltammogram of 2 in MeCN with 0.3 M [NBu4][PF6] as supporting
electrolyte. Arrow indicates origin and direction of scan (scan rate of 500 mV/s).

D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
295
Scheme 2. Synthesis of orthometallated [NBu4][Ni(N2PC)] ([NBu4][3]) via reductive cleavage of a backbone P–C bond of 2.
Fig. 5. 1H NMR of [Cp2⁄CoIII][3] in DMSO-d6 with 31P{1H} NMR inset (middle). Backbone signals are assigned.
Cp⁄2CoIII as the counterion. The structure was confirmed both by mul-
tinuclear NMR (Fig. 5) and X-ray diffraction analyses (Fig. 6).
3.5. Characterization of [Cp⁄2CoIII][3]
NMR analysis was used to characterize [Cp⁄2CoIII][3] in solution
(Fig. 5). The 1H NMR of [Cp2⁄CoIII][3] in DMSO-d6 showed the pres-
ence of a 12 peaks in the aromatic region. Of those signals, 9 are
unique, corresponding to individual protons (i.e. integration = 1),
indicating the asymmetric nature of the complex as a maximum of
6 (no overlaps) similar signals are expected for a symmetric system.
The proton signals for individual ring spin systems were first deter-
mined based on COSY analysis and the final assignment of each sig-
nal to the exact ring system was completed via 1H–31P coupling and
HMBC analyses. The protons of ring system H9–H12 were assigned
first based on the observed cross peak of H9 to the carbonyl carbon
(
13C{1H: d 171.4) in the HMBC spectrum as well as from the observa-
tion of the coupling of the H12 signal as a triplet owing to interaction
with both H11 and the P atom of the PPh2 moiety. The assignment of
the H1–H4 ring system was completed next based on the observed
cross peak of H4 to the carbonyl carbon (13C{1H: d 175.7) in the
HMBC spectrum. The remaining H5–H8 system was then assigned
to the last backbone spin system. It must be noted that H5 and H8
could not be uniquely assigned owing to a lack of evidence (i.e. no
unique cross peaks observed in HMBC or other analyses). The assign-
ment of the PPh2 proton signals was made based on integration and
COSY analyses. As expected, the 31P{1H} NMR spectrum had only a
single peak (d 32.7 (s)). As the sample was diamagnetic, it is best
Fig. 6. X-ray crystal structure of [Cp⁄2CoIII][3] with ellipsoids drawn at 50%
probability. Selected bond lengths (Å) and angles (°): Ni1–C21: 1.922; Ni–P1:
2.135; Ni1–N1: 1.952; Ni1–N2: 1.876; N11–C34: 1.362; N2–C27: 1.358; N1–C33:
1.428; N2–C28: 1.398; N1–Ni1–N2: 84.0; N1–Ni1–P1: 95.8; N2–Ni1–C21: 84.5; C21–
Ni1–P1: 96.0.
described as a square-planar tetradentate diamidato-mono(phos-
phine)-orthometalated NiII complex in solution [17]. The 31P{1H}
NMR analysis of the filtrate solution revealed a mixture of 3 other

296
D.N. Huh et al. / Inorganica Chimica Acta 423 (2014) 290–297
coefficient (in cm2/s), C is the concentration of the redox species (in
mol/cm3), and
v is the scan rate (in V/s). The diffusion coefficient
(1.19 ꢂ 10ꢁ3 cm2/s in a 0.3 M [NBu4][PF6] DMF solution) was then
used to determine that the quasi-reversible reduction wave at
ꢁ1.52 V is a 1eꢁ (calculated 0.87eꢁ) transfer supporting its identifi-
cation as a NiII/NiI couple.
4. Conclusions
Reaction of H21 with Ni(ClO4)2 has allowed for the synthesis
and characterization of an interesting diamidato-bis(phosphine)
Ni(II) complex (2), the first characterized in both the solution
and solid state. NMR and X-ray crystallographic studies have con-
firmed that the complex has structural similarities to the Pd(II) and
Pt(II) complexes of 12ꢁ and those of the Trost series of ligands. Cyc-
lic voltammetry studies of 2 indicate a quasi-reversible NiII/NiI
reduction at high potential that is consistent with metal-amidato
complexes where the coordinated deprotonated carboxamido-N
atom has been implicated as a strong sigma donor, stabilizing
the metal center from reduction. Using strong chemical reductants,
NiII/NiI reduction was not observed but rather orthometallation of
Fig. 7. Cyclic voltammograms of [Cp⁄2CoIII][3] in DMF with 0.3 M [NBu4][PF6] as
supporting electrolyte: (a) cyclic sweep from ꢁ1.8 to ꢁ2.8 V, (b) cyclic sweep from
ꢁ0.5 to ꢁ1.8 V, and (c) cyclic sweep from ꢁ0.5 to +1.3 V. Arrows on voltammograms
indicate direction of scan (scan rate of 500 mV/s).
species present; including a major species (d 9.4 (s)) and two other
significant species (d 29.9 (s) and ꢁ17.6 (s)). As with the
Na/acenapthylene reaction, efforts to separate, isolate and identify
the byproducts of reaction were unsuccessful.
1
2ꢁ to the nickel(II) was achieved by the reductive cleavage of one
of the Cnaphthyl-P bonds forming [Ni(N2PC)]– ([3]ꢁ). Cyclic voltam-
metry studies of [Cp⁄2CoIII][3] show the expected quasi-reversible
Cp2⁄CoIII/Cp⁄2CoII couple along with a quasi-reversible NiII/NiI reduc-
tion, which is at higher potential than that of 2. Efforts to synthesize
and characterize the 1-electron reduced [NiI(N2P2)]– analogue of 2
and [NiI(N2CP)]2– analogue of [Cp⁄2CoIII][3] are currently under inves-
tigation. Finally, we note that complex 2 is structurally similar to the
distal nickel center in the A-cluster of CODH/ACS thus representing
the first example of a tetradentate phosphorus-for-sulfur exchanged
ligand structural synthetic analogue characterized in both the solid
and solution states.
The X-ray structure of [Cp2⁄CoIII][3] (Table 1 and Fig. 6) shows the
complex to be square planar about the NiII center. The structure is very
similar (bond distances and angles) to that of [NBu4][3], with respect
to the [3]ꢁ moiety. As compared to 2 (
D
Ni–N: 0.015 Å), the Ni–N bond
distances of [3]ꢁ
(
D
Ni–N: ꢃ0.08 Å) are more distorted with respect to
each other likely owing to the smaller bite angle and decreased sterics
of the orthometallated ring as compared to the P-coordinated ring.
The Ni–P bond distance in [3]ꢁ is also slightly shorter than those in
2 (0.02–0.04 Å shorter), likely because of the decreased steric interac-
tions about the P atom in [3]ꢁ owing to the loss of a coordinated bulky
PPh2 moiety. Overall, the bond angles and distances are consistent
with similar coordination complexes reported.
Acknowledgments
Financial support from the NSF (NSF-RUI: CHE-0809266;
NSF-MRI: 1126585) and the University of San Diego is gratefully
acknowledged.
3.6. Electrochemistry of [Cp⁄2Co][Ni(N2PC)] ([Cp2⁄CoIII][3])
Cyclic voltammetry studies, in a 0.3 M [NBu4][PF6] DMF solution
(Fig. 7), were performed to examine the oxidative/reductive stabil-
ity of [Cp⁄2CoIII][3]. Complex [Cp⁄2CoIII][3] exhibited an irreversible
oxidation at Eox = 0.39 V (Fig. 7(c)), an irreversible reduction at
Ered = ꢁ2.40 V (Fig. 7(a)), and two quasi-reversible reductions at
E1/2 = ꢁ1.51 V (Fig. 7(b)) and E1/2 = ꢁ2.51 V (Fig. 7(a)) versus SCE.
As with 2, no reversible oxidation to NiIII is observed but a similar,
potentially ligand-based to the diimine form [15], irreversible oxida-
tion is observed. The reductive stability of [3]ꢁ shows similar
features to that of 2, including a high reduction potential as com-
pared to reported bis(phosphine) nickel(II) complexes [21], again
corroborating the theory that the strongly sigma donating nature
of the coordinated amide nitrogen (amidato) atoms increases the
metal’s reductive stability [2,3]. The quasi-reversible proposed NiII/
NiI reduction of [Cp⁄2CoIII][3] is shifted by approximately ꢁ0.3 V as
compared to that of 2 indicating that it is more reductively stable
but still not as stable as the diamidato-dithiolate complex, (Et4N)2[-
Ni(PhPepS)], reported by Mascharak [22]. The quasi-reversible
reduction wave at ꢁ2.51 V corresponds to the Cp⁄2CoIII to Cp2⁄CoII
couple. By examining the transition at 5 different scan rates, we
were able to calculate a diffusion coefficient for [Cp⁄2CoIII][3] using
the Randles–Sevcik [25] equation for a room temperature analysis,
which assumes that mass transport occurs only by a diffusion
Appendix A. Supplementary material
CCDC 1001890, 1001891 and 1001892 contain the supplemen-
tary crystallographic data for compounds 2, [NBu4][3] and
[Cp⁄2CoIII][3], respectively. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.ica.2014.08.033.
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