Structural and spectroscopic characterization of five coordinate iron and cobalt bis(dithiolene)-trimethylphosphine complexes

Article abstract of DOI:10.1016/j.molstruc.2017.03.119

Heteroleptic bis(dithiolene)-phosphine iron and cobalt complexes [Fe(adt)₂(PMe₃)] (1) and [Co(adt)₂(PMe₃)] (2) (adt?=?para-anisyldithiolene) have been synthesized from corresponding bis(dithiolene) dimers [M₂(adt)₄]₂ (M?=?Fe, Co) by reacting with excess PMe₃ in dichloromethane. Solid-state structures of 1 and 2 have been determined by single crystal X-ray crystallography, and the dithiolene ligands C[sbnd]S (≈1.72??) and C[sbnd]C (≈1.37??) bond distances reveal the coordination of π-radical monoanionic dithiolene (adt?^1?) ligands to metal centers. Intense low energy ligand-to-ligand-charge transfer (LLCT) absorption band (743?nm for 1; 905?nm for 2) in UV–vis spectra and characteristic ν(C[dbnd]S?) (1168?cm^?1 for 1; 1170?cm^?1 for 2) in IR spectra affirms coordination of π-radical monoanionic dithiolenes to divalent metal ions. The cyclic voltammogram of 1 and 2 shows reversible oxidation and reduction waves attributed to M^II?→?M^III?+?e^? (+0.52?V for 1;?+0.29?V for 2) and adt?^1??+?e^??→?adt^2? (?0.63?V for 1;??0.46?V for 2) redox process respectively. Comprehensive structural and spectroscopic investigations conclude, [M₂^III(adt^2?)₂(adt?^1-)₂]?→?2 [M^II(adt?^1-)₂(PMe₃)] intramolecular redox interplay during phosphine coordination induced cleavage of homoleptic bis(dithiolene) dimers to produce square pyramidal complexes.

Full text of DOI:10.1016/j.molstruc.2017.03.119

Accepted Manuscript
Structural and spectroscopic characterization of five coordinate iron and cobalt
bis(dithiolene)-trimethylphosphine complexes
Troy Selby-Karney, David A. Grossie, Kuppuswamy Arumugam, Eyglo Wright, P.
Chandrasekaran
PII:
S0022-2860(17)30424-6
10.1016/j.molstruc.2017.03.119
MOLSTR 23615
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 4 January 2017
Revised Date: 21 March 2017
Accepted Date: 31 March 2017
Please cite this article as: T. Selby-Karney, D.A. Grossie, K. Arumugam, E. Wright, P.
Chandrasekaran, Structural and spectroscopic characterization of five coordinate iron and cobalt
bis(dithiolene)-trimethylphosphine complexes, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.03.119.
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ACCEPTED MANUSCRIPT
Structural and Spectroscopic Characterization of Five Coordinate Iron and
Cobalt Bis(dithiolene)-Trimethylphosphine Complexes
Troy Selby-Karney,a David A. Grossie,b Kuppuswamy Arumugam,b Eyglo Wrightb and
P. Chandrasekaran*a
aDepartment of Chemistry and Biochemistry, Lamar University, Beaumont, TX 77710,
USA. E-mail: chandru@lamar.edu; Tel: 1-4098807514
bDepartment of Chemistry, Wright State University, 3640 Colonel Glenn Hwy, Dayton,
OH 45435, USA.
Keywords: Dithiolene; Redox-active; Noninnocent ligand; Phosphine; Iron complex;
Cobalt complex
1

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Graphical Abstract:
Structural and Spectroscopic Characterization of Five Coordinate Iron and
Cobalt Bis(dithiolene)-Trimethylphosphine Complexes
Troy Selby-Karney, David A. Grossie, Kuppuswamy Arumugam, Eyglo Wright and P.
Chandrasekaran
2

ACCEPTED MANUSCRIPT
Abstract: Heteroleptic bis(dithiolene)-phosphine iron and cobalt complexes
[Fe(adt)2(PMe3)] (1) and [Co(adt)2(PMe3)] (2) (adt = para-anisyldithiolene) have
been synthesized from corresponding bis(dithiolene) dimers [M2(adt)4]2 (M = Fe,
Co) by reacting with excess PMe3 in dichloromethane. Solid-state structures of 1 and
2 have been determined by single crystal X-ray crystallography, and the dithiolene
ligands C-S (≈1.72 Å) and C-C (≈1.37 Å) bond distances reveal the coordination of π-
radical monoanionic dithiolene (adtꢀ1−) ligands to metal centers. Intense low energy
ligand-to-ligand-charge transfer (LLCT) absorption band (743 nm for 1; 905 nm for
2) in UV-vis spectra and characteristic ν(C=Sꢀ
) (1168 cm−1 for 1; 1170 cm−1 for 2)in
IR spectra affirms coordination of π-radical monoanionic dithiolenes to divalent
metal ions. The cyclic voltammogram of 1 and 2 shows reversible oxidation and
reduction waves attributed to MII
adtꢀ1− + e− adt2− (-0.63 V for 1; -0.46 V for 2) redox process respectively.
Comprehensive structural and spectroscopic investigations conclude, [M2III(adt2-
)2(adtꢀ1-)2] 2 [MII(adtꢀ1-)2(PMe3)] intramolecular redox interplay during
→
MIII + e− (+0.52 V for 1; +0.29 V for 2) and
→
→
phosphine coordination induced cleavage of homoleptic bis(dithiolene) dimers to
produce square pyramidal complexes.
3

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1. Introduction: Coordination complexes demonstrating metal-ligand redox
interplay have captured the attention of the chemical community due to their
significance in catalysis and molecular electronics [1-8]. In this context, metal-
dithiolenes are one of the most studied systems, and well-known to undergo
interesting redox chemistry to form several electron transfer series complexes [9-
12]. Furthermore, homoleptic metal-dithiolene complexes upon incorporation of
ligands such as phosphines, amines, carbenes, isonitriles and halides have been
reported to undergo metal-dithiolene redox alterations to produce corresponding
heteroleptic complexes [13-15].
Iron and cobalt homoleptic bis(dithiolene) dimers [M2(S2C2R2)4]z (R = CN,
CF3; z = 0, 1-, 2-) are well known to undergo cleavage reactions in the presence of
phosphorus or nitrogen donor ligands (L) to yield mononuclear square pyramidal
complexes of the type [M(S2C2R2)2(L)]z [16-19]. Sellmann et al. reported interesting
six coordinated high-valent iron(IV) bis(dithiolene)-phosphine complex
[Fe(S2C6H2)2(PMe3)2], synthesized directly from FeCl2, 1,2-benzenedithiol and PMe3
in one pot reaction [20]. Later Wieghardt and his coworkers concluded the presence
of Fe(III) center in [Fe(S2C6H2)2(PMe3)2] through comprehensive structural,
spectroscopic and computational studies, and also assigned proper electronic
description to several homo- and hetero-leptic iron bis(dithiolene) complexes [21-
23]. Recently Donahue and his coworkers reported five coordinated complexes
[M(adt)2(PPh3)] (M = Fe, Co), synthesized from corresponding bis(dithiolene)
dimers by reacting with triphenylphosphine, and demonstrated electrochemically
controlled reversible release of PPh3 from iron center [24]. Although,
trimethylphosphine (PMe3) is a highly reactive ligand known to facilitate significant
redox alternations in dithiolene complexes, direct cleavage reactivity of PMe3 with
iron and cobalt bis(dithiolene) dimers have never been investigated. In this context,
we are interested to examine reactivity of PMe3 towards iron and cobalt
bis(dithiolene) dimers.
To understand the role of strong σ-donor trimethylphosphine (PMe3) in
metal-ligand redox interplay in iron and cobalt bis(dithiolenes), we have prepared
five coordinated heteroleptic bis(dithiolene)-phosphine adducts [M(adt)2(PMe3)]
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(M = Fe and Co; adt =
p-anisyldithiolene), and comprehensive structural and
spectroscopic investigation have been presented.
2. Experimental
2.1 Instrumentation and Materials
Air- and moisture-sensitive compounds were prepared and handled under a N2
atmosphere using a drybox or standard Schlenk techniques. Reported procedures
were followed for the syntheses of [Fe2(adt)4] (1a) and [Co2(adt)4] (2a) (adt = p-
anisyldithiolene) with minor modification [41], whereas PMe3 (1 M in toluene) was
obtained from commercial source and used as received. All the solvents used herein
are purified using standard purification procedures [25]. NMR spectra were
recorded at 25 °C with a Bruker 400 MHz NMR spectrometer and were referenced
to the solvent residual. UV-vis spectra were obtained at ambient temperature with a
Thermo Scientific Evolution 260 Bio dual silicon photodiodes spectrophotometer,
while IR spectra were recorded using Thermo Scientific Nicolet iS10 instrument.
Electrochemical measurements were made with a CHI600E electroanalyzer
workstation using an Ag/AgCl reference electrode a platinum disk working
electrode, a platinum wire auxiliary electrode, and [nBu4N][PF6] as the supporting
electrolyte in CH2Cl2. Under these conditions, the [Cp2Fe]+/Cp2Fe couple
consistently occurred at +440 mV. Elemental analyses were performed by Atlantic
Microlab, Inc. Norcross, GA, USA.
2.2.
Synthesis of [Fe2(adt)4] (1a)
In 500 mL Schlenk flask containing magnetic bar was charged with anisoine (10 g,
36.7 mmol), P4S10 (16.3 g, 36.7 mmol) and dry dioxane (200 mL). Resulted reaction
mixture was refluxed under nitrogen atmosphere for 6 h. Cooled to room
.
temperature, and FeCl3 6H2O (4.5 g, 16.6 mmol) was added into the reaction mixture
and further refluxed for 2 h. Cooled the reaction mixture in ice bath for an hour to
get black solid, which was collected by filtration, and washed with water, ethanol,
and diethyl ether, then dried in vacuo. Yield: 63% (13.8 g). UV-visible in CH2Cl2
[λmax/nm (ε/M−1cm−1)]: 269 nm (59300), 643 nm (14100). IR (cm-1): 3001(w),
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2962(w), 2838(w), 1599(s), 1506(s), 1458(m), 1377(s), 1292(s), 1248(s), 1170(s),
1119(m), 1028(s), 873(m), 831(w).
2.3.
Synthesis of [Fe(adt)2(PMe3)] (1)
A 50 mL Schlenk flask with stir bar was charged with [Fe2(adt)4] (0.20 g; 0.15 mmol)
under N2 atmosphere. Then 20 mL of CH2Cl2 was added and stirred for 10 min. Over
this 1
M toluene solution of PMe3 (0.5 mL) was injected using syringe. Resulted
reaction mixture was stirred at room temperature for 4 h. Solvent was removed
under reduced pressure and resulted dark green solid was washed with 3 × 5 mL of
Et2O and then dried in vacuo. Yield: 82% (0.18 g). M.p.: 159 °C (dec). Analysis:
Calculated for C35H37O4PS4Fe: C, 57.06; H, 5.06; S, 17.41. Found: C, 56.88; H, 4.98; S,
1
17.21%. H NMR (400 MHz, CDCl3): δ 7.46 (d,
J
= 8.8 Hz, 8H), 6.85(d, 8H) 3.84 (s,
12H), 1.58 (br s, 9H). 13C NMR (100 MHz, CDCl3): δ 189.1, 159.7, 137.1, 130.7, 113.9,
55.6, 0.7. 31P NMR (161 MHz, CDCl3): δ 43.1 (s). IR (cm−1): 2956w, 2831w, 1598s,
1512s, 1374m, 1290m, 1254s, 1168s, 1031s, 939m, 817m, 708w (vs, very strong; s,
strong; m, medium; w, weak). UV-visible in CH2Cl2 [λmax/nm (ε/M−1cm−1)]: 270
(61833), 393 (13611), 743 (25667).
2.4.
Synthesis of [Co2(adt)4] (2a)
.
Synthetic procedure is similar to 1a, except using CoCl2 6H2O (4.0 g, 16.8 mmol) in
.
place of FeCl3 6H2O. Yield: 52% (11.6 g). UV-visible in CH2Cl2 [λmax/nm (ε/M−1cm−1)]:
280 nm (61200), 739 nm (15700). IR (cm-1): 3000(w), 2962(w), 2841(w), 1597(s),
1506(s), 1436(m), 1357(s), 1291(s), 1247(s), 1168(s), 1114(m), 1024(s), 873(m),
818(w).
2.5.
Synthesis of [Co(adt)2(PMe3)] (2)
Synthetic procedure is similar to 1, using [Co2(adt)4] (0.20 g; 0.15 mmol). Yield: 73%
(0.16 g). M.p.: 162 °C (dec). Analysis: Calculated for C35H37O4PS4Co: C, 56.82; H, 5.04;
S, 17.34. Found: C, 56.77; H, 5.01; S 17.85%. IR (cm−1): 2912w, 2834w, 1598s, 1508s,
1422m, 1292m, 1245s, 1170s, 1109m, 1021s, 944s, 816m. UV-visible in CH2Cl2
6

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[λmax/nm (ε/M−1cm−1)]: 255 (60472), 302 (66889), 453 (10305), 748 (7194), 905
(21194).
2.6.
X-ray Crystallography
Single crystal X-ray diffraction data were collected using a Bruker Smart X2S
diffractometer controlled by APEX2 software suite [26]. Collected images were
integrated using SAINT and scaled and corrected using SADABS [27]. SHELXT and
SHELXL were used to solve and refine the structures [28], and hydrogen atom
positions were calculated using geometric parameters and allowed to refined as a
"riding" atom with a isotropic thermal factor equal to 1.5 Ueq of the attached atoms
with sp3 hybridization and 1.2 Ueq of the attached atoms with sp2 hybridization
positions. Each structure contained a 1,2-dichloroethane molecule, which was badly
disordered, so SQUEEZE was used to determine the volume of space containing the
electron density attributable to the solvent molecule [29]. The void found for
compound 1 had a volume of 164 Å and contained 70 electrons, whereas the void in
compound 2 had a volume of 161 Å and contained 61 electrons. In each case the
void was at fractional coordinates 0.5, 1, 1. OLEX2 was used to manage the solution
and refinement process [30]. Crystallographic data for the structures reported in
this article have been deposited with the Cambridge Crystallographic Data Centre
(CCDC), and the reference numbers are 1510569 (1) and 1510570 (2).
2.7.
Computational Details
Density functional theory (DFT) calculations were performed with the ORCA 3.0.3
electronic structure program package developed by Neese and co-workers [31].
Geometry optimizations were performed using the BP86 functional [32,33], in
combination with the def2-TZVP(-f) basis set and the def2-TZVP/J auxiliary basis set
[34]. Scalar relativistic effects were introduced using the zeroth-order regular
approximation (ZORA) [35]. All the calculations used a dense integration grid (ORCA
Grid 4) with the total energy convergence tolerances of 10−6 Eh within tight
optimization criteria. Chimera was used for the visualization of molecular orbital
contour plots [36].
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3. Results and Discussion
Homoleptic bis(dithiolene) dimers [Fe2(adt)4] and [Co2(adt)4] have been prepared
following reported procedures [41]. Addition of excess PMe3 in toluene over
dichloromethane solution of [Fe2(adt)4] (adt = para-anisyldithiolene) at ambient
temperature cleanly afforded five coordinated bis(dithiolene)-phosphine adduct
[Fe(adt)2(PMe3)] (1) as a dark green solid (Scheme 1). Analogous reaction between
[Co2(adt)4] and PMe3 produced dark red complex [Co(adt)2(PMe3)] (2)
quantitatively. Composition and solid-state structure of 1 and 2 were confirmed by
elemental analysis and X-ray crystallography.
Scheme 1. Synthesis of iron and cobalt dithiolene-phosphine complexes.
X-ray diffraction quality single crystals of 1 and 2 were grown by diffusion of
hexanes into 1,2-dichloroethane saturated solutions of respective compounds.
Molecular structures of 1 and 2 with their atom numbering schemes are shown in
Fig. 1 and 2, respectively. Crystal parameters and refinement details for 1 and 2 are
presented in Table 1, while selected bond distances and bond angles are listed in
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Table 2.
Table 1. Crystal parameters and refinement details for 1 and 2.
1
2
formula
C35H37FeO4PS4
784.16
Triclinic
C35H37CoO4PS4
739.78
Triclinic
fw, g mol-1
crystal system
space group
P1
P1
a
, Å
10.3129(11)
12.3541(12)
15.5395(18)
86.541(4)
76.046(4)
78.133(4)
1880.1(4)
2
10.2718(16)
12.3618(17)
15.552(3)
86.701(5)
76.113(5)
78.186(5)
1876.36
2
b
, Å
c, Å
α
β
, deg
, deg
γ, deg
V
Z
, Å3
ρcalc, g cm-3
1.301
0.450
768
1.309
0.71073
770
μ
F
(MoKa), mm-1
(000)
crystal size(mm)
(K)
2θ range, deg
0.64 × 0.32 × 0.08
300.15
1.35–25.04
0.5 × 0.5 × 0.1
163
2.16–29.63
T
Total no. reflns
No. of indep reflns
2725 [Rint = 0.0615]
1.064
6569 [Rint = 0.0373]
1.096
0.0351, 0.111
GOF (F2
)
R1,a wR2b [I > 2σ(I)] 0.0439, 0.129
R1, wR2 [all data]
0.0666
0.0489
aR
bwR2 = {[
=
Σ||Fo|
-
|Fc||
/
Σ|Fo|
.
2
2
2 2
2
2
2
Σ
w(Fo -Fc )/
Σ
w(Fo ) ]}1/2; w = 1/[σ2(Fo )+(xP)2
]
, where P = (Fo +2Fc )/3.
Table 2. Selected Bond Distance (Å) and Bond Angle (°) for 1 and 2.
Bond Distances (Å) Bond Angles (°)
[Fe(adt)2(PMe3)] (1)
Fe1-P1
Fe1-S1
Fe1-S2
Fe1-S3
Fe1-S4
C1-S1
2.198(1)
2.169(1)
2.156(1)
2.164(1)
2.157(1)
1.729(3)
1.715(4)
1.720(3)
S1-Fe1-S2
88.57(4)
88.47(4)
90.74(4)
98.70(4)
97.81(4)
93.19(4)
91.17(4)
90.03(4)
S3-Fe1-S4
S1-Fe1-P1
S2-Fe1-P1
S3-Fe1-P1
S4-Fe1-P1
S1-Fe1-S4
S2-Fe1-S3
C2-S2
C3-S3
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C4-S4
C1-C2
C3-C4
1.725(4)
1.387(5)
1.385(5)
S1-Fe1-S3
S2-Fe1-S4
171.45(4)
168.12(4)
[Co(adt)2(PMe3)] (2)
Co1-P1
Co1-S1
Co1-S2
Co1-S3
Co1-S4
C1-S1
C2-S2
C3-S3
C4-S4
C1-C2
C3-C4
2.163(1)
S1-Co1-S2
2.1762(9) S3-Co1-S4
2.1664(9) S1-Co1-P1
2.1765(9) S2-Co1-P1
2.1659(9) S3-Co1-P1
89.09(3)
88.79(3)
91.19(3)
98.62(3)
99.65(3)
94.20(3)
90.43(3)
89.28(3)
169.16(3)
167.18(3)
1.738(3)
1.730(3)
1.742(3)
1.731(3)
1.380(4)
1.379(4)
S4-Co1-P1
S1-Co1-S4
S2-Co1-S3
S1-Co1-S3
S2-Co1-S4
In solid-state compound 1 adopted square pyramidal geometry with an iron
center coordinated to two dithiolene and PMe3 ligands. Dithiolene ligands forms
base for the square pyramidal geometry with Fe ion is 0.19 Å above the S4 plane,
while PMe3 occupies the apex position. In reported crystal structure of similar
complexes [Fe(adt)2(PPh3)] and [Fe(S2C2Ph2)2(P(OPh)3)] Fe ion is 0.39 Å and 0.36 Å
above the S4 plane, respectively [24,37]. The average C-Save = 1.721(3) Å bond
distance in 1 is shorter than typical C-S bond distance (1.761(4) Å) and longer than
C=S bond distance (1.688(8) Å) [38]. The C-Cave length of 1.385(5) Å in compound 1
is longer compared to standard olefin bond distance (1.337(7) Å). These bond
distances reveal that two π-radical monoanionic ligands (adtꢀ1-) are coordinated to
the metal, rendering divalent iron center in complex 1 (Scheme 2). Furthermore, the
C-S and C-C bond distances suggests that the dithiolene ligands in complex 1 are
somewhat
further
reduced
compared
to
[Fe(adt)2(PPh3)]
and
[Fe(S2C2Ph2)2(P(OPh)3)], however comparable with reported isoelectronic complex
t
[Fe(L)2(CN)]- (L = S2C2(C6H4 Bu-
p)) [37]. The Fe1-P1 distance is 2.199(1) Å, as
expected shorter compared to reported Fe-P distances in similar compounds, which
indicates existence of strong interaction between PMe3 and iron center [39].
10

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Fig 1. Molecular structure of 1 shown at 50% thermal ellipsoid probability. All
hydrogen atoms have been omitted for clarity.
Fig 2. Molecular structure of 2 shown at 50% thermal ellipsoid probability. All
hydrogen atoms have been omitted for clarity.
S-
S
S
R
S
R
R
R
R
R
S
.
- e-
+ e-
- e-
+ e-
.
R
S-
S
S
R
ene-1,2-dithiolate
α-dithione
π-radical monoanion
C-S = 1.761(4) Å
C-C = 1.337(9) Å
C-S = 1.668(8) Å
C-C 1.477(12) Å
C-S = 1.714(1) Å
C-C = 1.355(9) Å
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Scheme 2. Redox levels of dithiolene ligands with typical C-S and C-C bond lengths.
The molecular structure of cobalt complex 2 is analogues to iron complex 1.
In complex 2 the cobalt ion occupies the center of square pyramidal coordination
polyhedron, and located 0.22 Å above the S4 plane. The similar parameter in
reported complex [Co(adt)2(PPh3)] is 0.35 Å, which indicates greater bonding
interaction between Co and S atoms [24]. The Co1-P1 bond distance in 2 is 2.163(1)
Å, which is slightly shorter compared to Co-P present in [Co(adt)2(PPh3)] (2.192(1)
Å) due to strong σ-donor PMe3 ligand. The average C-S and C-C bond distances in
complex 2 are 1.735(3) Å and 1.383(3) Å respectively, and these distances are
identical to similar bond present in iron complex 1 within experimental error. The
dithiolene C-S and long C-C bond distances confirms presence of π-radical
monoanionic ligands in both iron and cobalt complexes.
The UV−vis absorption spectra of complexes 1 and 2 are presented in Fig. 3.
The arresting feature in the spectrum of 1 is the low-energy intense absorption
band at 743 nm, which is due to a ligand-to-ligand charge-transfer (LLCT) transition
and well-known spectroscopic marker for the presence of π-radical monoanionic
dithiolene ligands (Scheme 2). In cobalt bis(dithiolene)-phosphine complex 2
exhibits intense LLCT band at 905 nm, which is considerable red shifted compared
to iron complex 1. The LLCT band absorption maxima of varies with metal ions and
ligand substituents due to change in singlet-triplet gap of metal bis(dithiolene)
complexes [40]. The LLCT band absorption maxima changes with change in
electronic environment of innocent ancillary ligands as well, example complex
[Fe(adt)2(PPh3)]
[Fe(S2C2Ph2)2(P(OPh)3)] shows at 687 nm. IR spectra of 1 and 2 shows strong
characteristic ν(C=S
) peak respectively at 1168 cm−1 and 1170 cm−1, which
supports the coordination of π-radical monoanionic dithiolenes to metals [37].
the
LLCT
band
appears
at
726
nm,
whereas
ꢀ
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Fig 3. UV−vis absorption spectra of 1 and 2 in CH2Cl2 solution.
The cyclic voltammogram (CV) of complex 1 and 2 recorded in
dichloromethane solutions are presented in Fig. 4, and clearly evident that both of
the complexes displays two reversible redox features. Iron complex 1 show
reversible wave at E1/2 = +0.52 V corresponds to one electron oxidation to form
[1]+. The second reversible feature at E1/2 = −0.63 V due to one electron reduction to
produce [1]-. The redox processes in complex 1 are very similar to well-studied
t
isoelectronic complex [Fe(L)2(CN)]– (L = S2C2(C6H4 Bu-
p)) reported by Weighardt
and his coworkers. Systematic electronic structure investigation of isolated electron
transfer series [Fe(L)2(CN)]z (Z = 0, -1, -2) concluded that the oxidation is due to
FeII/FeIII couple, whereas the reduction occurs at dithiolene ligand [37]. Cobalt
complex 2 exhibit reversible oxidation wave at +0.29 V, which is 0.23 V lower
compared to iron complex 1, and supports the metal based reversible oxidation
process [MII(adtꢀ1-)2(PMe3)]
→
[MIII(adtꢀ1-)2(PMe3)] + e–. While the reduction wave
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for 2 appears at -0.46 V, which is 0.17 V lower compared to complex 1 and plausible
due to [MII(adtꢀ1-)2(PMe3)] + e–
→
[MII(adtꢀ1-)(adt2-)(PMe3)]–, where electron added
into dithiolene based orbital.
Fig 4. Cyclic voltammetry for 1 and 2 in CH2Cl2 recorded with [nBu4N][PF6] as the
supporting electrolyte at 25 °C with a scan rate of 100 mV·s−1.
The CV of related iron bis(dithiolene)-phosphine complexes
[Fe(S2C2Ph2)2(PR3)] (PR3 = P(OPh)3 and P(OMe)3) were reported to exhibit
reversible oxidation wave [23], and an irreversible wave at negative potential
attributed to phosphine dissociation to produce iron bis(dithiolene) dimer. The
phosphine dissociation was suppressed in the presence of excess phosphine, and CV
exhibits reversible reduction wave. Donahue and his coworkers reported
triphenylphosphine containing heteroleptic complexes [M(adt)2(PPh3)] (M = Fe, Co),
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and demonstrated electrochemically controlled dissociation and association of
triphenylphosphine from iron center [24]. Whereas CV of complexes 1 and 2, did
not support trimethylphosphine dissociation at negative potential, presumably due
to stronger coordination PMe3 ligand to the metal center. Similar electrochemical
behaviors are reported in PMe3 adduct of iron bis(dithiolene) complexes [20,22].
[Fe(adt)2(PMe3)] (1)
[Co(adt)2(PMe3)] (2)
LUMO: 21% Co d orbitals
LUMO: 31% Fe d orbitals
61% dithiolene π* orbitals
53% dithiolene π* orbitals
HOMO: 42% Co d orbitals (dxy)
HOMO: 60% Fe d orbitals (dxy)
27% S p orbitals
20% S p orbitals
Fig 5. DFT calculated HOMO and LUMO orbital contour plots for 1 and 2. Orbitals
are drawn at 0.035 contour levels (red-positive; grey-negative).
In order to further support electrochemical redox assessments, complexes 1
and 2 were geometry optimized (BP86 functional) in gas phase beginning with
experimental crystallographic coordinates. Geometry optimized structure bond
distances agree well with experimental values (Table S3). A Loewdin population
analysis of the frontier MOs in the optimized structures reveals their composition
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and supports the structural and spectroscopic data. Contour plots of HOMO and
LUMO for 1 and 2 are presented in Fig. 5. Iron complex 1, the HOMO composition
consists of predominantly iron d orbitals (60%), whereas the LUMO comprised of
dithiolene π* orbital (53%) mixed with 31% of iron d character. The HOMO cobalt
complex 2 consists of 42% Co d character mixed with 27% S p orbitals. This
indicates that, cobalt complex has greater degree metal and ligands (covalence)
interaction compared to iron complex 1. The LUMO of complex 2, is comprised of
dithiolene π* orbital and cobalt d orbital in a ∼ 61:21 ratio. The frontier molecular
orbitals (MOs) from single point DFT calculations affirmed metal oxidation and
ligand centered reduction interpretation in CV for complexes 1 and 2.
4. Conclusions
In summary, five coordinated heteroleptic bis(dithiolene)-phosphine iron and
cobalt complexes containing strong δ-donor phosphine ligand (PMe3) have been
prepared and structurally as well as spectroscopically characterized. Dithiolene C-S
and C-C bond distances and intense LLCT bands in UV-vis spectroscopy confirm the
coordination of π-radical monoanionic dithiolenes to divalent metal ions.
Comprehensive structural and spectroscopic investigation presented here
concludes, [M2III(adtꢀ1-)2(adt2-)2]
→
2 [MII(adtꢀ1-)2(PMe3)] intramolecular redox
interplay between metal and ligands upon cleavage of homoleptic bis(dithiolene)
dimer by PMe3 to produce mononuclear square pyramidal complexes.
Electrocatalytic activity of these complexes for proton reduction reaction is
currently under investigation.
Acknowledgements
This work was supported in part by Center for Advances in Water and Air Quality,
and Welch Foundation (V-0004), which are gratefully acknowledged.
Appendix A. Supplementary material
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Crystallographic data and full lists of bond lengths and angles have been deposited
with the Cambridge Crystallographic Data Center, CCDC No. 1510569 (1) and
1510570 (2). Copies of this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ UK (fax: +44-1223-336-033;
email: deposite@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk). Elemental analysis
report, NMR, IR spectra, DFT optimized coordinates for complexes 1 and 2 are
available supplementary information.
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Research highlights
•
•
Iron and cobalt bis(dithiolene)-PMe3 complexes have been synthesized.
Comprehensive structural and spectroscopic investigations conclude divalent metal
centers.
•
In CV, these complexes displays reversible oxidation and reduction.