Reactivity of bulky formamidinatosamarium(II or III) complexes with C=O and C=S bonds

Article abstract of DOI:10.1021/ic502181g

The preparation of a new heterobimetallic samarium(II) formamidinate complex and selected reactions of samarium(II) complexes and one samarium(III) formamidinate complex with benzophenone or CS₂ are discussed. Treatment of the tris(formamidinato)samarium(III) complex [Sm(DippForm)₃] 1 (DippForm = N,N′-bis(2,6-diisopropylphenyl)formamidinate, (CH(NC₆H₃-ⁱPr₂-2,6)₂) with potassium graphite in toluene yielded the dark brown heterobimetallic formamidinatosamarium(II)/potassium complex [KSm(DippForm)₃]_n 2. Divalent 2, a Lewis base solvent free homoleptic species, differs significantly from the related heteroleptic formamidinatosamarium(II) complex [Sm(DippForm)₂(thf)₂] 3 with respect to its constitution, structure, and reactivity toward benzophenone. While 2 reacts giving complex 1, the reaction of 3 with benzophenone generates the highly unusual [Sm(DippForm)₂(thf){μ-OC(Ph)=(C₆H₅)C(Ph)₂O}Sm(DippForm)₂] (C₆H₅ = 1,4-cyclohexadiene-3-yl-6-ylidene) 4. The formation of 4 highlights a rare C-C coupling between a carbonyl carbon and the carbon at the para position of a phenyl group of the OCPh₂ fragment. An analogous reaction of [Yb(DippForm)₂(thf)₂] gives an isostructural complex 4Yb. 3 reacts with carbon disulfide forming a light green dinuclear formamidinatosamarium(III) complex [{Sm(DippForm)₂(thf)}₂(μ-²(C,S):(S′,S″)-SCSCS₂)] 5 through an unusual C-S coupling induced by an amidinatolanthanoid species giving the thioformylcarbonotrithioate ligand. The trivalent organometallic [Sm(DippForm)₂(CCPh)(thf)] complex activates the C=O bond of benzophenone by an insertion reaction, forming the light yellow [Sm(DippForm)₂{OC(Ph)₂C₂Ph}(thf)] 6 as a major product and light yellow unsolvated [Sm(DippForm)₂{OC(Ph)₂C₂Ph}] 7 as a minor product. Molecular structures of complexes (2, 4-7) show that (N,N′) bonding between a DippForm and samarium atom exists in all compounds, but in 2, DippForm also bridges K and Sm by 1(N):2(N′) bonding and two 2,6-diisopropylphenyl groups are ⁶-bonded to potassium.

Full text of DOI:10.1021/ic502181g

Article
pubs.acs.org/IC
Reactivity of Bulky Formamidinatosamarium(II or III) Complexes with
CO and CS Bonds#
Glen B. Deacon,*,† Peter C. Junk,*,‡ Jun Wang,†,‡ and Daniel Werner†
†School of Chemistry, Monash University, Victoria, 3800, Australia
‡College of Science, Technology & Engineering, James Cook University, Townsville, Queensland 4811, Australia
S
* Supporting Information
ABSTRACT: The preparation of a new heterobimetallic samarium-
(II) formamidinate complex and selected reactions of samarium(II)
complexes and one samarium(III) formamidinate complex with
benzophenone or CS2 are discussed. Treatment of the tris-
(formamidinato)samarium(III) complex [Sm(DippForm)3] 1
(DippForm = N,N′-bis(2,6-diisopropylphenyl)formamidinate, (CH-
(NC6H3-iPr2-2,6)2) with potassium graphite in toluene yielded the
dark brown heterobimetallic formamidinatosamarium(II)/potassi-
um complex [KSm(DippForm)3]n 2. Divalent 2, a Lewis base
solvent free homoleptic species, differs significantly from the related
heteroleptic formamidinatosamarium(II) complex [Sm-
(DippForm)2(thf)2] 3 with respect to its constitution, structure, and reactivity toward benzophenone. While 2 reacts giving
complex 1, the reaction of 3 with benzophenone generates the highly unusual [Sm(DippForm)2(thf){μ-OC(Ph)(C6H5)-
C(Ph)2O}Sm(DippForm)2] (C6H5 = 1,4-cyclohexadiene-3-yl-6-ylidene) 4. The formation of 4 highlights a rare C−C coupling
between a carbonyl carbon and the carbon at the para position of a phenyl group of the OCPh2 fragment. An analogous reaction
of [Yb(DippForm)2(thf)2] gives an isostructural complex 4Yb. 3 reacts with carbon disulfide forming a light green dinuclear
formamidinatosamarium(III) complex [{Sm(DippForm)2(thf)}2(μ-η2(C,S):κ(S′,S″)-SCSCS2)] 5 through an unusual C−S
coupling induced by an amidinatolanthanoid species giving the thioformylcarbonotrithioate ligand. The trivalent organometallic
[Sm(DippForm)2(CCPh)(thf)] complex activates the CO bond of benzophenone by an insertion reaction, forming the light
yellow [Sm(DippForm)2{OC(Ph)2C2Ph}(thf)] 6 as a major product and light yellow unsolvated [Sm(DippForm)2{OC-
(Ph)2C2Ph}] 7 as a minor product. Molecular structures of complexes (2, 4−7) show that κ(N,N′) bonding between a
DippForm and samarium atom exists in all compounds, but in 2, DippForm also bridges K and Sm by 1κ(N):2κ(N′) bonding
and two 2,6-diisopropylphenyl groups are η6-bonded to potassium.
[Sm(DippForm)2(thf)2] (DippForm = N,N′-bis(2,6-
diisopropylphenyl)formamidinate, CH(NC6H3-iPr2-2,6)2.6
These processes demonstrated that the size of the carbodiimide
substrate and the ligation of samarium(II) fundamentally alter
the outcome of related reduction reactions. We have also
investigated the contrasting reaction type of phenylethyn-
ylsamarium(III) complexes with carbodiimides, where CN
bonds were activated in insertion reactions.6
Targeting a CO bond in a molecule such as fluorenone,
divalent samarium complexes induce reduction and usually
form pinacolate complexes through C−C coupling (Scheme
1iv).7 The process involves ketyl intermediates which are
isolable when well stabilized.7−9 In contrast, the parallel
reactions with benzophenone were attempted but mainly led
to an unisolable product mixture.10 However, the successful
transformation by reductive C−C coupling to form isolable
pinacolate complexes was induced by [Yb(NCS)2(thf)2]11 and
a magnesium(II) 1,2-bis{(2,6-diisopropylphenyl)imino}-
INTRODUCTION
■
The reactivity of divalent samarium complexes has captured the
interest of researchers from various chemistry disciplines.1
Commercially available samarium(II) diiodide, widely used as a
single electron reducing agent in organic reaction processes,2
underpins the applications of reactive samarium(II) com-
pounds. Upon coordination by organic moieties, samarium(II)
species can have enhanced reduction potential (cf. Sm3+/Sm2+,
3
E1/2 = −1.55 V, and SmIIICp3/SmIICp3 , −2.50 V). Divalent
−
organosamarium complexes have been found to possess high
reactivity toward unsaturated chemical bonds including some of
the most unreactive such as the NN bond.4
Previously we have reported reduction reactions of the
divalent samarium complex [Sm(N(SiMe3)2)2)(thf)2] with
CN bonds in carbodiimides to form either a dinuclear
oxalamidinate ((RN)2CC(NR)2) (Scheme 1i) through C−C
coupling of the central diimide carbon atoms of RNCNR
fragments,5 a dinuclear diformamidinate complex (Scheme 1ii)
i
formed through coupling of two methine carbon atoms of Pr
groups,5 or mononuclear formamidinate complexes (Scheme
Received: September 7, 2014
1iii), formed by C−H activation of solvent toluene by
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
Scheme 1. Reactions between Divalent Samarium Complexes and Carbodiimides (RNCNR, R = Cyclohexyl (Cy)), 2,6-
a
Diisopropylphenyl (Dipp), or 9-Fluorenone (FN)
a
OAr = 2,6-di-tert-butyl-4-methylphenolate.
acenaphthenate.12 When CO2 was treated with [SmCp*2(thf)2]
(Cp* = pentamethylcyclopentadienyl), reductive coupling gave
an oxalate complex,13 but a different pathway was identified
when utilizing a macrocyclic N,N′-dimethyl-meso-octaethylcalix-
[4]pyrrolate−samarium(II) complex. In the latter, CO
bonds cleave, giving a carbonate complex and CO as
coproduct.13b,14 Targeting a CS bond, [SmCp*2(thf)2]
reacts with COS, yielding a disproportionation product
[Cp*2Sm(μ-1κ(S,S′):2κ(O)-S2CO)SmCp*2(thf)] and CO.13
The transformation of CS2 by organolanthanoids has only
recently been explored in the reaction with [Sm(II)(Giso)2]
(Giso = [(ArN)2CNCy2], Ar = 2,6-diisopropylphenyl, Cy =
cyclohexyl), which was observed to form [(Giso)2Sm(μ-
η2(C,S):κ(S′,S″)-SCSCS2)Sm(Giso)2].15a The product was
shown to be formed through C−S coupling, but some
uncertainty remained about the bonding modes of the central
(SCSCS2) moiety due to the limited quality of crystallography
data. An entirely different outcome results from the reaction of
[K2Ln(OSi(O-tBu)3)4] (Ln = Yb, Eu) with CS2, which process
yields K2CS3 (+CS), K2C2S4 and K2C3S5 as the sulfur
containing products.15b The transformations of CS2 by d-
metal complexes mainly result in C−C coupling (yielding
tetrathiooxalate [S2CCS2]2−),16 but C−S couplings have also
occasionally been observed.17
Extending our studies of N-donor alternatives to the
cyclopentadienyl family of ligands,18 we now report the
synthesis of a rare Lewis base free heterobimetallic
formamidinatosamarium(II) complex [KSm(DippForm)3]n 2,
and the reactivity of divalent samarium complexes 2 and
[Sm(DippForm)2(thf)2] 3 with benzophenone as well as the
analogous reaction of [Yb(DippForm)2(thf)2]. The reaction of
3 with CS2 is also described. In addition we report the
activation of the CO bond in benzophenone by a
phenylethynylsamarium(III) complex, thereby providing a
contrast to reductive activation of 2 and 3.
EXPERIMENTAL SECTION
■
The compounds described herein were prepared and handled using
conventional inert atmosphere techniques with dinitrogen gas. IR
spectra were recorded as Nujol mulls between NaCl plates using either
a PerkinElmer 1600 series FTIR instrument or a PerkinElmer
Spectrum RX I FTIR spectrometer within the range 4000−600
cm−1. 1H NMR spectra were recorded on a Bruker DPX 300
1
spectrometer, and chemical shifts were referenced to the residual H
resonances of the deuterated solvents. Melting points were determined
in sealed glass capillaries under nitrogen and are uncalibrated.
B
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Article
Microanalyses were determined by the Campbell Microanalytical
Service, University of Otago (New Zealand), or Elemental Analysis
Service of London Metropolitan University (U.K.). Solvents used for
reactions were predried over sodium metal and distilled over sodium
for toluene or sodium benzophenone ketyl for thf and hexane. NMR
solvent C6D6 was degassed and dried over sodium with stirring
overnight and filtered before use. Benzophenone and carbon disulfide
were purchased from Merck, potassium was purchased from Chem
Supply, and they were used as supplied. [Sm(DippForm)3] 1,6
potassium graphite,19 H(DippForm),20 SmI2 (0.1 M solution in thf),21
[Yb(DippForm)2(thf)2],22 [Sm(DippForm)2Cl(thf)],23 and [Sm-
(DippForm)2(CCPh)(thf)]24 were prepared by literature methods.
Synthesis of [KSm(DippForm)3] 2. Toluene (20 mL) was added
to a Schlenk flask charged with tris(formamidinato)samarium complex
[Sm(DippForm)3] 1 (0.62 g, 0.50 mmol) and potassium graphite
(0.10 g, 0.75 mmol). The mixture was stirred at 90 °C for 0.5 h and
kept at this temperature for 0.5 h. The mixture was filtered at this
temperature, and the volume of the brown colored solution was
reduced to 5 mL under vacuum. On storage at ambient temperature
for 2 days, dark brown crystalline 2·1.25PhMe precipitated and was
collected after decanting the solution (yield: 0.42 g, 61%), mp 302−
306 °C. IR (Nujol)/cm−1: 1593 m, 1530 s, 1358 m, 1311 s, 1253 m,
1228 w, 1180 m, 1157 w, 1109 w, 1055 w, 1039 w, 991 w, 922 m, 882
w, 820 w, 801 w, 779 w, 772 w, 754 m, 694 w, 674 w. Anal. Calcd (%)
for C83.75H115N6KSm (1395.27): C, 72.09; H, 8.31; N, 6.02. Found: C,
71.55; H, 8.19; N, 6.04.
vacuum, hexane (5 mL) was added, and the mixture was stored at
ambient temperature for 2 days, during which time a pale solid
precipitated and was collected by decanting of the solution. The solid
was dried under vacuum and crystallized from 1 mL of toluene, giving
crystalline colorless 4·2.5PhMe (yield: 0.33 g, 54%), mp 142 °C (dec).
IR (Nujol)/cm−1: 1666 m, 1593 w, 1515 m, 1316 w, 1276 s, 1188 m,
1159 w, 1140 m, 1112 w, 1056 w, 1042 w, 932 w, 898 w, 867 w, 798
m, 766 m, 752 m, 700 m, 642 w, 625 w. Anal. Calcd (%) for
C147.5H188N8O3Sm2 (2421.76): C, 73.15; H, 7.82; N, 4.63. Found: C,
72.99; H, 6.91; N, 4.35.
Synthesis of [(DippForm)2(thf)Yb{μ-OC(Ph)(C6H5)C(Ph)2O}-
Yb(DippForm)2] 4Yb. [Yb(DippForm)2(thf)2] (0.75 g, 0.71 mmol)
and benzophenone (0.13 g, 0.71 mmol) were dissolved in thf (3 mL)
in a Schlenk flask. After 5 min, hexane was added (2 mL) to the deep
purple solution and the flask was heated to 40 °C. Upon concentration
in vacuo (1 mL) and cooling to room temperature, small dichroic
yellow/red crystals of 4Yb·2.25thf were isolated suitable for single
crystal X-ray crystallography analysis. The sample was further dried
under vacuum, leaving a light yellow powder (yield: 0.57 g, 67%), mp
220−226 °C. IR (Nujol): 1665 m, 1638 w, 1595 m, 1579 m, 1560 m,
1519 vs, 1501 s, 1401 w, 1318 s, 1276 vs, 1235 w, 1193 w, 1160 w,
1142 m, 1093 vs, 1066 vs, 1046 s, 1020 s, 946 vw, 933 vw, 899 vw, 801
s, 766 s, 755 s, 723 vw, 701 m. Anal. Calcd (%) for C139H186N8O5.25Yb2
(2399.04): C, 69.59; H, 7.81; N, 4.67. Found: C, 69.61; H, 7.62; N,
4.85.
Synthesis of [{Sm(DippForm)2(thf)}2(μ-η2(C,S):κ(S′,S″)-
SCSCS2)] 5. Carbon disulfide (one drop) was added at ambient
temperature to a Schlenk tube charged with a dark green solution of
[Sm(DippForm)2(thf)2], 3 (51 mg, 0.05 mmol), in deuterated
benzene (0.8 mL). The solution was shaken gently for 1 min, and
the color of the solution gradually turned bright green. The solution
was then stored at ambient temperature for 2 days, during which time
light green crystalline 5·6C6D6 was collected (yield: 30 mg, 47%), mp
160 °C (dec). IR (Nujol)/cm−1: 1588 w, 1520 s, 1460 s, 1316 m, 1286
m, 1253 w, 1234 w, 1186 m, 1099 w, 1056 w, 1044 w, 1021 w, 935 m,
822 w, 800 m, 770 w, 668 w. Anal. Calcd (%) for C102H140N8S4Sm2
(loss of ligated thf and C6D6 of solvation, 1907.24): C, 64.23; H, 7.40;
N, 5.88. Found: C, 63.21; H, 7.70; N, 5.52.
Reaction of [KSm(DippForm)3]n 2 with benzophenone.
Toluene (1 mL) was added to a Schlenk flask charged with
[KSm(DippForm)3] 2 (0.14 g, 0.1 mmol) and benzophenone (18
mg, 0.1 mmol). The mixture was warmed to dissolve both reagents,
and the mixture was stored for 2 weeks at ambient temperature, during
which time light yellow needle crystals precipitated and were collected
(yield: 30 mg, 22%). The unit cell measurement for the crystals
showed that they were [Sm(DippForm)3]25 space group P21/n, a =
13.136(2), b = 37.816(7), c = 16.395(3) Å, β = 96.386(30)°, V =
8093.70 Å3.
Synthesis of [Sm(DippForm)2(thf)2] 3. The compound was
synthesized by a modified literature procedure:25 thf (50 mL) was
added to a Schlenk flask charged with potassium metal (0.16 g, 4.1
mmol) and H(DippForm) (1.46 g, 4.0 mmol). The mixture was
heated to reflux for 3 h with stirring and then cooled to ambient
temperature. SmI2 (0.10 M solution in thf, 20 mL, 2.0 mmol) was
added dropwise with stirring, and the mixture was stirred overnight.
Thf was removed in vacuo and replaced by toluene (50 mL). The
resulting slurry was stirred for half an hour. After standing overnight,
the mixture was filtered. The dark green filtrate was concentrated in
vacuo to ca. 20 mL and stored at −10 °C for 1 week, during which
time dark green crystals precipitated and were collected (yield: 1.60 g,
78%). 1H NMR (300 MHz, C6D6, 303 K): −12.78 (br s, 2H, NCHN),
1.17 (br s, 8H, CH(CH3)2), 3.09 (br s, 48H, CH(CH3)2), 3.78 (s, 8H,
CH2(thf)), 4.47 (d, J = 7.2 Hz, 8H, Ar-m-H), 6.17 (t, J = 7.1 Hz, 4H,
Synthesis of [Sm(DippForm)2{OC(Ph)2C2Ph}(thf)] 6. Toluene
(2 mL) was added to a Schlenk flask charged with [Sm-
(DippForm)2(CCPh)(thf)] (0.54 g, 0.50 mmol) and benzophenone
(0.09 g, 0.50 mmol). The mixture was warmed until all reactants were
dissolved. The mixture was stored for 1 week, during which time light
yellow crystalline 6 precipitated and was collected (yield: 0.40 g, 65%),
mp 258−262 °C. IR (Nujol)/cm−1: 1666 m, 1594 w, 1520 m, 1314 w,
1278 m, 1183 w, 1108 w, 1068 w, 1024 w, 1010 w, 939 w, 868 w, 800
1
m, 770 w, 754 m, 702 w, 693 w, 656 w, 646 w. H NMR (C6D6, 303
K)/ppm: 14.72 (s, 2H, NC(H)N), 6.54−9.91 (m, 27H, C6H3 +
i
C6H5), 3.40 (m, 4H, OCH2, thf), 2.09 (s, br, 8H, CH, Pr), 1.34 (m,
4H, CH2, thf), 0.09 (s, 48H, CH3, iPr). Anal. Calcd (%) for
C75H93N4O2Sm (1232.88): C, 73.06; H, 7.60; N, 4.54. Found: C,
73.03; H, 7.56; N, 4.58.
1
Ar-p-H), 11.42 (br s, 8H, OCH2(thf)). The H NMR spectrum was
similar to that reported.6
Two large rod-shaped light yellow crystals grew from the solution
having different morphology to the rhombic-shaped crystals of 6. They
were picked up by hand and identified by X-ray crystal structure
determination to be [Sm(DippForm)2{OC(Ph)2C2Ph}]·0.5PhMe (7·
0.5PhMe).
Transformation of [Sm(DippForm)2Cl(thf)] into 3. C6D6 (0.7
mL) was added to a J. Young valve equipped NMR tube charged with
[Sm(DippForm)2Cl(thf)] (0.030 g) and potassium graphite (0.030 g).
The mixture was shaken by hand at ambient temperature for 1 min. 1H
NMR spectroscopy identified that 3 was formed.
X-ray Crystal Structure Determinations. Crystals in viscous
paraffin oil were mounted on glass fibers or cryoloops. Intensity data
were collected on the Australian Synchrotron MX1 beamline (2·
1.25PhMe, 4·1.5PhMe, and 4Yb·2.25thf) at 100 K with wavelength λ =
0.712 Å or a Bruker X8 APEX II CCD diffractometer (5·6C6D6) or an
Enraf-Nonius Kappa CCD diffractometer (6 and 7·0.5PhMe) using
Mo Kα radiation at 123 K (λ = 0.71073 Å). With the MX1 beamline,
data were collected using the BlueIce26a GUI and processed with the
XDS26b software package. Data sets collected from other diffrac-
tometers were empirically corrected for absorption with SADABS27 or
SORTAV.28 The structures were solved by conventional methods and
refined by full-matrix least-squares on all F2 data using SHELX9729 or
SHELX2014 in conjunction with the X-Seed30 or Olex231 graphical
Transformation of [KSm(DippForm)3] 2 into 3. C6D6 (0.7 mL)
was added to a J. Young valve equipped NMR tube charged with
[KSm(DippForm)3] 2 (0.030 g) and one drop of thf. The mixture was
1
shaken by hand and warmed to complete the transformation of 2. H
NMR spectroscopy identified that 3 was formed.
Synthesis of [(DippForm)2(thf)Sm{μ-OC(Ph)(C6H5)C-
(Ph)2O}Sm(DippForm)2] 4. A solution of benzophenone (0.10 g,
0.50 mmol) in toluene (5 mL) was added with stirring to a Schlenk
flask charged with a green solution of [Sm(DippForm)2(thf)2], 3 (0.50
g, 0.50 mmol), in toluene (10 mL) at ambient temperature. The color
of the solution turned purple, and the mixture was stirred continuously
for half an hour. The volume of solution was reduced to 2 mL under
C
dx.doi.org/10.1021/ic502181g | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 1. Crystal Data and Structure Refinement for Complexes 2 and 4−7
2·1.25PhMe
4·2.5PhMe
4Yb·2.25thf
formula
Mr
C
83.75H115N6KSm
C147.50H188N8O3Sm2
2421.76
C139H186N8O5.25Yb2
2399.04
1395.27
P21/c
space group
a (Å)
P1
̅
P1
̅
24.734(5)
26.888(5)
24.902(5)
90
24.439(5)
25.688(5)
25.726(5)
102.65(3)
108.63(3)
112.47(3)
13026(4)
4
13.792(3)
22.820(5)
23.442(5)
117.65(3)
103.71(3)
91.51(3)
6269(2)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
111.88(5)
90
15368(5)
8
Z
2
μ, mm−1
ρcalc, g cm−3
Nτ
0.864
0.948
1.539
1.206
1.235
1.271
198965
174813
28645
N (Rint)
R1/wR2 (I > 2σ(I))
R1/wR2 (all data)
GOF
26829 (0.347)
0.0326/0.0831
0.0354/0.0847
1.063
45749(0.0286)
0.0620/0.1678
0.0667/0.1722
1.056
28645(0.0142)
0.0327/0.0788
0.0339/0.0794
1.106
max/min Δe [e·Å−3
]
1.304/−1.076
5·6C6D6
2.760/−2.174
6
1.677/−2.081
7·0.5PhMe
a
formula
Mr
C146H192N8O2S4Sm2
2520.04
C75H93N4O2Sm
1232.88
C74.5H89N4OSm
1206.84
space group
a (Å)
P21/n
P1
P1
̅
̅
15.8749(3)
23.1663(6)
19.3292(3)
90
12.937(3)
13.607(3)
20.847(4)
94.09(3)
93.12(3)
116.75(3)
3252.1(11)
2
14.276(3)
14.806(3)
18.060(4)
97.15(3)
98.88(3)
118.30(3)
3233.7(11)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
107.876(1)
90
6765.4(2)
2
μ, mm−1
ρcalc, g cm−3
Nτ
0.974
0.951
0.954
1.255
1.259
1.237
55165
35717
54056
N (Rint)
15525(0.0550)
0.0403/0.0720
0.0533/0.0787
1.073
11037(0.0681)
0.0444/0.1092
0.0536/0.1157
1.074
11382(0.0648)
0.0377/0.0956
0.0410/0.0989
1.067
R1/wR2 (I > 2σ(I))
R1/wR2 (all data)
GOF
max/min Δe [e·Å−3
]
0.777/−0.850
2.120/−0.959
1.696/−1.015
a
D modeled as H.
user interface. Anisotropic thermal parameters were refined for non-
hydrogen atoms, and hydrogen atoms were calculated and refined with
a riding model. Crystal data, data collection, and refinement details are
given in Table 1.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication numbers
CCDC 1023027−1023033. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, U.K. (fax, (+44) 1223 336-033; e-mail, deposit@ccdc.cam.ac.uk.
solvents and the paramagnetic nature of the compound
prevented an informative NMR spectrum. Previously reported
[KLnIIL3] complexes include L = N(SiMe3)2, Ln = Yb,32a
Sm;32b L = C(SiMe3)3, Ln = Yb;32c L = OC6H3Ph2-2,6, Ln =
Eu.32d
Complex 2 and the previously reported6,25 dark green SmII
complex [Sm(DippForm)2(thf)2] 3 are the only known
divalent formamidinatosamarium species so far reported, and
amidinato analogues are rare.18 2 is a polymeric heteronuclear
samarium(II) species free of Lewis base solvent coordination in
contrast to the mononuclear heteroleptic samarium(II)
complex 3. Both formamidinatosamarium(II) complexes can
be directly (Scheme 2ii) or indirectly (Scheme 2iv+i)
transformed into each other, with the key factor being the
presence or absence of thf. The addition of thf to 2 leads to
breakup of the polymer and the transformation of 2 into 3
(Scheme 2ii). The reduction of the related [Sm-
(DippForm)2Cl(thf)]23 complex by potassium graphite yielded
the Lewis base coordinated samarium(II) complex 3, thf being
RESULTS AND DISCUSSION
■
Treatment of the homoleptic formamidinatosamarium(III)
complex [Sm(DippForm)3] 1 with potassium graphite at 90
° C y i e l d e d
a d a r k b r o w n h e t e r o b i m e t a l l i c
formamidinatosamarium(II)/potassium complex [KSm-
(DippForm)3] 2 in moderate yield (Scheme 2i). C, H, and N
elemental analysis for 2 was consistent with the proposed
formula, and the molecular structure was confirmed by X-ray
crystallography. However, the poor solubility in hydrocarbon
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Scheme 2. Synthesis of [KSm(DippForm)3]n 2 and the Transformation of 2 or [Sm(DippForm)2Cl(thf)] into 3
Figure 1. Molecular structure of 2·1.25PhMe shown with 50% thermal ellipsoids. Lattice toluene molecules and hydrogen atoms omitted for clarity.
Selected bond distances (Å) and angles (deg): Sm1−N1 = 2.575(2); Sm1−N2 = 2.641(2); Sm1−N3 = 2.587(2); Sm1−N4 = 2.559(2); Sm1−N5 =
2.557(2); K1−N6 = 2.677(2); K1−C14# = 3.294(3); K1−C15# = 3.307(3); K1−C16# = 3.242(3); K1−C17# = 3.182(3); K1−C18# = 3.151(3);
K1−C19# = 3.204(3); K1−C52 = 3.147(3); K1−C53 = 3.231(3); K1−C54 = 3.276(3); K1−C55 = 3.255(3); K1−C56 = 3.184(3); K1−C57 =
3.130(3); K1−centroidη6(C52−C57) = 2.88; K1−centroidη6(C14#−C19#) = 2.91; centroidη6(C52−C57)−K1−N6 = 81.1°; centroidη6(C14#−
C19#)−K1−N6 = 145.6°; centroidη6(C52−C57)−K1−centroidη6(C14#−C19#) = 123.5. [Symmetry code for C14#−C19#: x, 5/2 − y, z − 1/2].
sourced from the reactant (Scheme 2iii). Complex 1 was
o r i g i n a l l y m a d e b y t h e o x i d a t i o n o f t h e
formamidinatosamarium(II) complex 3 by N,N′-bis(2,6-
diisopropylphenyl)carbodiimide (Scheme 2iv).6 Ligation by
the three bulky formamidinates prevents additional Lewis base
solvent coordination in 1, from which the Lewis base solvent
free complex 2 can be prepared (Scheme 2i).
2 crystallized from toluene in the P21/c space group (Table
1). The crystal structure of 2 contains two similar but
crystallographically independent molecules, and only one is
chosen as representative to describe the structure (Figure 1).
The homoleptic and heteronuclear (Sm/K) complex adopts a
one-dimensional polymeric form with alternating Sm and K
atoms. Two formamidinate ligands each chelate a samarium
atom in a κ(N,N′) manner, and one also binds to potassium by
an η6-2,6-diisopropylphenyl group. A third formamidinate
bridges Sm and K in a μ-1κ(N):2κ(N′) manner, and the 2,6-
diisopropylphenyl group on the nitrogen bound to Sm is η6-
coordinated to potassium. The samarium atom is thus a low
five coordinate, and the coordination geometry can be
described as a distorted square pyramid with the unidentate
formamidinate nitrogen atom in an apical position. The Sm−N
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Scheme 3. Synthesis of [(DippForm)2(thf)Ln{μ-
OC(Ph)(C6H5)C(Ph)2O}Ln(DippForm)2] 4 (Ln = Sm)
and 4Yb (Ln = Yb)
Scheme 4. Synthesis of [{Sm(DippForm)2(thf)}2(μ-
η2(C,S):κ(S′,S″)-SCSCS2)] 5
the effect of the lower coordination number. The Sm−N bond
lengths, as expected,33 are longer than those of trivalent
samarium complexes (e.g., [Sm(DippForm)2Cl(thf)], mean
2.44 Å).23 The difference between the C−N or CN bond
lengths of the diaza-allyl moiety of each formamidinate ligand
increases from κ(N,N′) bonding (typically ≤0.01 Å) to bridging
1κ(N):2κ(N′) (ca. 0.03 Å). Samarium and potassium metal
atoms are bridged by formamidinate ligands through 1κ(N):2κ-
(N′) ligation and η6-bonding of aryl substituents. The latter
architects a metallocene arrangement around the potassium
atom (centroidη6(C52−C57)−K1−centroidη6(C14#−C19#) =
123.5°). The interactions between the seven coordinate
potassium atom and arene rings (K1−centroidη6(C52−C57)
= 2.88 Å and K1−centroidη6(C14#−C19#) = 2.91 Å) are
shorter than those of η6-PhMe of seven coordinate potassium in
[Lu{CH(SiMe3)2}3(μ-Cl)K(PhMe)2] (3.00−3.13 Å).34 The
K−C bond lengths (3.130(3)−3.307(3) Å) are within the
range of the η6-Ar−K interactions in the seven coordinate
potassium/samarium complex [{Ph2Si(NAr)2}2SmII{K-
(OEt2)}2] (3.032(3)−3.456(3) Å).35 The K−N bond caps
the sandwich unit asymmetrically with centroidη6(C52-C57)−
bond length (2.577(2) Å) of the unidentate formamidinate is
close to the mean Sm−N value of the chelating formamidinates
(2.59 Å) (Figure 1). These bond lengths are comparable to
those in the six coordinate samarium(II) complex [Sm-
(DippForm)2(thf)2] 3 (mean, 2.57 Å)25 despite the difference
in coordination number between 2 and 3. Evidently,
formamidinate bridging between two metals (Sm/K) offsets
Figure 2. Molecular structure of 4·2.5PhMe shown with 30% thermal ellipsoids. Hydrogen atoms, lattice solvent, and half of the disordered site
omitted for clarity. Selected bond distances (Å) and angles (deg): Sm1−O1 = 2.075(3); Sm1−N1 = 2.381(4); Sm1−N2 = 2.409(4); Sm1−N3 =
2.424(4); Sm1−N4 = 2.434(4); Sm2−O2 = 2.142(3); Sm2−N5 = 2.431(3); Sm2−N6 = 2.501(4); Sm2−N7 = 2.447(4); Sm2−N8 = 2.435(4);
C64−C65 = 1.505(6); C64−C69 = 1.506(6); C65−C66 = 1.342(6); C68−C69 = 1.344(6); C66−C67 = 1.455(6); C67−C68 = 1.461(6); C67−
C70 = 1.379(6); C1···Sm1···C26 =128.56(14); C1···Sm1−O1 = 113.83(13); C26···Sm1−O1 = 116.81(13); O2−Sm2−O3 = 86.77(16); C77···
Sm2···C102 = 123.94(13). [Relevant bond distances (Å) and angles (deg) for 4Yb·2.25thf: Yb1−O1 = 2.010(2); Yb1−N1 = 2.317(2); Yb1−N2 =
2.285(2); Yb1−N3 = 2.318(2); Yb1−N4 = 2.351(2); Yb2−O2 = 2.0722(19); Yb2−N5 = 2.391(2); Yb2−N6 = 2.347(3); Yb2−N7 = 2.372(2);
Yb2−N8 = 2.356(2); C64−C65 = 1.505(4); C64−C69 = 1.498(4); C65−C66 = 1.335(4); C68−C69 = 1.336(4); C66−C67 = 1.457(4); C67−C68
= 1.458(4); C67−C70 = 1.370(4); C1···Yb1···C26 = 125.92(8); C1···Yb1−O1 = 115.79(9); C26···Yb1−O1 = 118.26(9); O2−Yb2−O3 = 82.97(7);
C77···Yb2···C102 = 121.56(8).]
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Figure 3. (a) Molecular structure of 5·6C6D6 shown with 50% thermal ellipsoids. Hydrogen atoms, lattice solvent, and half of the disordered site
omitted for clarity. Selected bond distances (Å) and angles (deg): Sm1−N1 = 2.457(2); Sm1−N2 = 2.482(2); Sm1−N3 = 2.424(2); Sm1−N4 =
2.479(2); Sm1−O1 = 2.4604(18); Sm1−C1 = 2.476(7); Sm1−S1 = 2.7724(18); Sm1#−S3 = 3.0051(18); Sm1#−S4 = 2.9484(19); C1−S1 =
1.641(8); C1−S2 = 1.682(7); C2−S2 = 1.803(6); C2−S3 = 1.659(6); C2−S4 = 1.697(6); S3−Sm1#−S4 = 60.05(6). [Symmetry code for Sm1#: 2 −
x, −y, 2 − z.] (b) Central part of the molecular structure of 5·6C6D6: Sm1(μ-η2(C,S):κ(S′,S″)-SCSCS2)Sm1# core. (c) Central part of the molecular
structure in ChemDraw.
K1−N6 = 81.1° and centroidη6(C14#-C19#)−K1−N6 =
145.6°.
reaction.7 The equilibrium process in solution, along with the
paramagnetic effect of the samarium(III) atom, prevents an
informative NMR spectrum. However, the formulation of the
solid compound was supported by microanalysis, and the
structure was clarified by single crystal X-ray structure
determination (see below).
The different constitution of the samarium(II) complexes 2
and 3 is expected to diversify their reactivity. This is observed in
their behavior with benzophenone. 2 reacts with benzophenone
giving 1 with the plausible coproduct [KOCPh2]• (eq 1), which
can abstract a hydrogen atom to give KOCHPh2 or dimerize to
the pinacolate [K2(OCPh2)2]. However, the reduction of
benzophenone by 3 led to the isolation of the novel
heteroleptic dinuclear samarium(III) complex 4 (Scheme 3i).
4 has a pale color in the solid state, but exhibits a purple color
when dissolved in toluene. Supposedly, an equilibrium occurs
in solution forming a ketyl species (Scheme 3iii), the
phenomenon having been observed previously in a related
(1/n)[KSm(DippForm)3]n + OCPh2
.
→ [Sm(DippForm)3] + [KOCPh2]
(1)
The formation of 4 highlights an unusual C−C coupling of
the para position of an arene ring with a carbonyl carbon. To
examine whether or not this phenomenon is unique to
samarium metal, we extended the study to the analogous
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recent example in d-block chemistry involving a Co-Zr-
bimetallic.36b These reactions differ from most other reductive
C−C coupling reactions of benzophenone, which usually occur
at two carbonyl carbon atoms forming pinacolate ligands.11,12
The current unusual coupling behavior is an extension of
previously observed C−C coupling of ArNCNAr (Ar =
2,6-diisopropylphenyl) fragments at two methine carbon atoms
Scheme 5. Synthesis of
[Sm(DippForm)2{OC(Ph)2C2Ph}(thf)] 6 and
[Sm(DippForm)2{OC(Ph)2C2Ph}] 7
5
i
of the Pr group (Scheme 1ii). Both processes probably result
from the very bulky nature of the N,N′-bis(2,6-
diisopropylphenyl)formamidinate) ligand (or N,N′-bis(2,6-
diisopropylphenyl)carbodiimide). A different loss of aromatic-
ity of benzophenone caused by lanthanoid metals is observed in
formation of enolate complexes,37 and dearomatization of
pyridine was observed recently in a reaction of [Sm(Cp*2)-
(thf)2] with pyridine forming [{Cp*2Sm(C5H5N)}2{μ-
(NC5H5−C5H5N)}].38
The redox reaction of 3 with carbon disulfide yielded a
samarium(III) complex 5 with a rare thioformyl carbon-
otrithioate ((SCSCS2)2−) bridging ligand (rather than a
tetrathiooxalate ligand) in moderate yield (Scheme 4). The
poor solubility of the compound in hydrocarbon solvent
prevented NMR characterization. However, its constitution was
supported by microanalysis (after loss of coordinated thf and
C6D6 of solvation under vacuum). The molecular structure has
been determined by single crystal X-ray crystallography.
Dinuclear compound 5 crystallized from C6D6 in the P21/n
space group (Table 1), and the molecular structure is shown in
Figure 3a. The formamidinate ligands chelate (N,N′) seven
coordinate samarium atoms with a mean Sm−N bond length of
2.46 Å, which is close to the average Sm−N (2.45 Å) of the six
coordinate samarium atom in the dinuclear samarium(III)
complex 4, the combined steric/electronic effect of the less
bulky chelating (SCSCS2)2− in 5 contrasting that of the bulky
unidentate dialkoxide ligand in 4. If the amidinate ligand is
treated as a single donor located on the backbone carbon
(NCN), the coordination geometry of the samarium atoms can
be described as capped distorted tetrahedral
(C15C40O1C1Sm1 or C15#C40#O1#S4Sm1#) with Sm−S1
or Sm−S3 bonds capping one face of each tetrahedron. The
asymmetric (SCSCS2)2− ligand is disordered evenly over two
sites (SCSCS2) and (S2CSCS). As a result, the molecule has
crystallographic symmetry with an inversion center in the
midpoint of the two samarium array. The bonding modes
related to the central disamarium thioformylcarbonotrithioate
fragment Sm1(SCSCS2)Sm1# are shown in Figure 3b and
Figure 3c. One CS group has an η2-interaction with one
samarium atom while the CS2 fragment chelates the other
samarium atom through two sulfur donors. Two CS2 moieties
are coupled through a single bond (C2−S2 = 1.803(6) Å), and
the bond length is comparable with that (1.778(12) Å) of
[(Giso)2Sm(μ-η2(C,S):κ(S′,S″)-SCSCS2)Sm(Giso)2].15a Elec-
trons of the SC bonds are delocalized over the CS2 or
SCS units since their C−S bond lengths (1.640(7)−1.698(6)
Å) are in a range between single and double bonds, though
C1−S1 approaches a double bond value (∼1.60 Å). These
values are somewhat shorter than those of related [Cp*2Sm-
(κ(S,S′)-S2CCHCHCH3)] (S−C bond lengths, 1.693(13)
and 1.703(12) Å).39 As obtained from XRD data of good
quality (Table 1), the current bond length values of the
structure of 5 are more precise (with lower esd values) and
assist more in determining the bonding mode of the central
carbon−sulfide fragment than those of the recently reported
[(Giso)2Sm(μ-η2(C,S):κ(S′,S″)-SCSCS2)Sm(Giso)2].15a In the
complex [Yb(DippForm)2(thf)2]. Interestingly, the reaction
with benzophenone gave a similar product 4Yb. An equilibrium
in solution also occurs forming a purple ketyl species for 4Yb
(Scheme 3iii). The identification of 4Yb was made by X-ray
crystallography, and its structure is very similar to that of 4
(below). A satisfactory elemental microanalysis of 4Yb was also
obtained.
Both compounds 4 and 4Yb crystallized in the P1 space
̅
groups, and they have similar structures though they are not
isomorphous (Table 1 and Figure 2). They adopt dinuclear
forms with formamidinate ligands chelating samarium atoms in
a κ(N,N′) manner. The two samarium atoms are joined
through two benzophenone molecules coupled from the para
position of a benzene ring of one molecule to a carbonyl carbon
of the other. The two metal atoms have different coordination
numbers: one is five coordinate (Sm1), and the other is six
coordinate (Sm2). Consequently, the mean Sm1−N bond
length for Sm1 (2.41 Å) is slightly shorter than that of Sm2
(2.45 Å). These Sm−N bonds are longer than Yb−N bonds
(mean Yb1−N, 2.32 Å; Yb2−N, 2.37 Å), corresponding to the
ionic radius difference between the two metals.33 If the
amidinate ligand is viewed from the position of the backbone
carbon (NCN), the coordination geometry of Sm1 or Yb1 can
be described as trigonal planar and of Sm2 or Yb2 as distorted
tetrahedral. The structure of the new organic moiety in the two
complexes is the same (Figure 2). The C−C coupling of the
two benzophenone units between one carbonyl carbon and one
para position of one arene ring involves formation of a single
bond (e.g., C51−C64 = 1.601(6) Å in 4). There is a loss of
aromaticity of arene ring C64−C69 with two adjacent single
bonds (C64−C65, 1.505(6); C64−C69, 1.506(6) Å in 4), two
further adjacent near single bonds (C66−C67, 1.454(6); C67−
C68, 1.461(6) Å in 4) and two cross ring near double bonds
(C65−C66, 1.342(6); C68−C69, 1.344(6) Å in 4). There is a
partial double bond from the ring C67 to exocyclic C70
(1.379(6) Å). The nonaromatic nature of six membered ring
C(64−69) is further shown by the location of a substituent C51
displaced from the ring plane by 1.284 Å (in 4). Compound
4Yb shows similar structural features. This type of C−C
coupling is extremely rare and has been observed only once in
f-block chemistry benzophenone was reduced by a UIII
complex,36a where the coupled species was only a minor
product, unlike the major products, 4, 4Yb. There is also a very
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Figure 4. (a) Molecular structure of 6 shown with 50% thermal ellipsoids. Hydrogen atoms omitted for clarity. Selected bond distances (Å) and
angles (deg): Sm1−O1 = 2.100(2); Sm1−O2 = 2.473(3); Sm1−N1 = 2.459(3); Sm1−N2 = 2.553(3); Sm1−N3 = 2.491(3); Sm1−N4 = 2.497(3);
C1···Sm1···C26 = 128.18(10); C1···Sm1−O1 = 114.04(12); C26···Sm1−O1 = 113.09(11); O1−Sm1−O2 = 84.65(10). (b) Molecular structure of
7·0.5PhMe shown with 50% thermal ellipsoids. Lattice toluene molecules and hydrogen atoms omitted for clarity. Selected bond distances (Å) and
angles (deg): Sm1−O1 = 2.089(2); Sm1−N1 = 2.434(3); Sm1−N2 = 2.464(2); Sm1−N3 = 2.392(2); Sm1−N4 = 2.440(2); C1···Sm1···C26
=132.08(8); C1···Sm1−O1= 109.80(9); C26···Sm1−O1 = 118.10(8).
present structure, electron delocalization in the S1C1S2 and
S3C2S4 units is clear and S2−C2 is a single bond. However,
the overall pattern of bond lengths in both structures is the
same, and in particular the Sm−S distance in the Sm−C(S)
chelate is shorter than those of the Sm−S2 chelate (Figure 3).
The reaction of 3 with CS2 (Scheme 4) contrasts with a recent
report of the corresponding reaction of [K2LnII(OSi(OtBu)3)4]
(Ln = Eu, Yb, see Introduction). Preliminary results indicate
that [Yb(DippForm)2(thf)2] reacts differently from 3, and this
is an ongoing investigation.40
In contrast to the redox reactions between samarium(II)
complexes such as 2 and 3 with benzophenone,
phenylethynylbis(N,N′-bis(2,6-diisopropylphenyl)-
formamidinato)tetrahydrofuransamarium(III) activates the
CO bond by insertion (Scheme 5). Thus, it reacts with
benzophenone in toluene at ambient temperature, yielding the
light yellow alkoxide [Sm(DippForm)2(OC(Ph)2C2Ph)(thf)] 6
in good yield. A minor unsolvated product, [Sm-
(DippForm)2(OC(Ph)2C2Ph)] 7, was isolated by hand picking
from the bulk crystalline material. Compound 6 is very soluble
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Notes
in hydrocarbon solvents such as toluene and benzene. The
constitution of the compound was supported by NMR
characterization and C, H, and N elemental analysis. The
molecular structures of both 6 and the unsolvated coproduct 7
were determined by single crystal crystallography (see below).
The low yield of 7 prevented any further characterization.
Both bis(formamidinato)samarium complexes crystallized
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Australian Research Council
(Grant DP130100152) and Australian Postgraduate Award (to
D.W.). Some aspects of this research were undertaken on the
MX1 beamline at the Australian Synchrotron, Victoria,
Australia.
from toluene in the P1 space group (Table 1), and their
̅
molecular structures are shown in Figure 4a (6) and Figure 4b
(7). In these monomeric and heteroleptic species, the
stereochemistry of the coordination of samarium metals can
be described as distorted tetrahedral (for six coordinate 6) and
trigonal planar (for five coordinate 7) if the amidinate ligand is
treated as a single donor ligand on the NCN carbon. The
formamidinate ligands are κ(N,N′) bonded to the samarium
metal as in 4 and 5, with the average Sm−N bond length (2.50
Å) in 6 somewhat longer than for the six coordinate Sm2 atom
in 4 (2.45 Å) and in [Sm(DippForm)2Cl(thf)] (2.44 Å),23
owing to the bulky alkoxy ligand. The molecular structure of
complex 7 is similar to that of the major product 6, except for
an absence of Lewis base thf coordination. As a consequence,
the mean Sm−N bond length (2.43 Å) and Sm−O (alkoxy)
(2.089(2)A) bond in 7 are shorter or slightly shorter than those
of 6 (Sm−N, 2.50 Å and Sm−O (alkoxy), 2.100(2) Å). The
variation in the structures reflects the combined bulk of the
alkoxy group (OCPh2(CCPh)) and the two formamidinate
ligands, which leads to labile Lewis base coordination. Related
insertion reactions of alkylsamarium complexes with benzo-
phenone are known, and the bulkiness of resulting alkoxide
ligand can exclude the coordination of Lewis base solvent, e.g.,
[Cp*2SmOCPh2(SiH3)].41
DEDICATION
■
#This paper is dedicated to the memory of Professor John
Corbett, a solid state chemist of the highest caliber.
REFERENCES
■
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CONCLUSION
■
The formation of a novel heteronuclear and homoleptic
samarium(II) complex [KSm(DippForm)3]n 2 shows that the
bulky formamidinates are able to stabilize both samarium and
potassium metals in a heteronuclear species without the need
for coligands. The varied constitution of divalent samarium(II)
complexes 2 and 3 diversifies their redox reactivity, as
demonstrated by the reactions with benzophenone. The
redox reactivity of [Sm(DippForm)2(thf)2] 3 with CO/
CS bonds instigates C−C or C−S coupling. The unusual C−
C coupling forming 4 is not unique to samarium metal since a
parallel reaction occurs for ytterbium forming 4Yb. Both 4 and
4Yb feature an isomer of the phenyl group in the bridging
ligand. Reaction of 3 with CS2 leads to the unusual [SCSCS2]−
bridging ligand. The organometallic samarium(III) complex,
[Sm(DippForm)2(CCPh)(thf)], activates the CO bonds of
Ph2CO in an insertion reaction giving new heteroleptic
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ASSOCIATED CONTENT
* Supporting Information
Selected X-ray crystal data information in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(G.B.D.) E-mail: glen.deacon@monash.edu. Fax: (+61)
399054597.
■
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