Formation of [NiIII(κ1-S2CH)(P(o- C6H3-3-SiMe3-2-S)3)]- via CS2 insertion into nickel(III) hydride containing [Ni III(H)(P(o-C6H3-3-SiMe3-2-S) 3)]-

Article abstract of DOI:10.1021/ic400293k

Insertion of CS₂ into the thermally unstable nickel(III) hydride [PPN][Ni(H)(P(o-C₆H₃-3-SiMe₃-2-S) ₃)] (1), freshly prepared from the reaction of [PPN][Ni(OC ₆H₅)P(C₆H₃-3-SiMe ₃-2-S)₃] and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin; pin = OCMe₂CMe₂O) in tetrahydrofuran at -80 C via a metathesis reaction, readily affords [PPN] [Ni^III(κ¹-S₂CH)(P(o-C₆H ₃-3-SiMe₃-2-S)₃)] (2) featuring a κ¹-S₂CH moiety.

Full text of DOI:10.1021/ic400293k

Communication
pubs.acs.org/IC
Formation of [NiIII(κ1‑S2CH)(P(o‑C6H3‑3-SiMe3‑2-S)3)]− via CS2 Insertion
into Nickel(III) Hydride Containing [NiIII(H)(P(o‑C6H3‑3-SiMe3‑2-S)3)]−
Kuan-Ting Lai, Wei-Chieh Ho, Tzung-Wen Chiou, and Wen-Feng Liaw*
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
S
* Supporting Information
donor 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (HBpin; pin =
ABSTRACT: Insertion of CS2 into the thermally unstable
OCMe2CMe2O) in tetrahydrofuran (THF) at −80 °C.
nickel(III) hydride [PPN][Ni(H)(P(o-C6H3-3-SiMe3-2-
Attempts to prepare complex 1 from [PPN][Ni(L)P(C6H3-3-
S)3)] (1), freshly prepared from the reaction of [PPN]-
[Ni(OC6H5)P(C6H3-3-SiMe3-2-S)3] and 4,4,5,5-tetra-
such as NaBH4, LiB(Et)3H, or LiAlH4 yielded the well-known
methyl-1,3,2-dioxaborolane (HBpin; pin = OC-
Me2CMe2O) in tetrahydrofuran at −80 °C via a
metathesis reaction, readily affords [PPN][NiIII(κ1-S2CH)-
(P(o-C6H3-3-SiMe3-2-S)3)] (2) featuring a κ1-S2CH
moiety.
SiMe3-2-S)3] (L = OC6H5, Cl) with a variety of hydride reagents
8
[Ni2 (P(C6H3-3-SiMe3-2-S)3)2]2− as an identifiable product. In
contrast to the inertness of [Ni(SC6H5)P(C6H3-3-SiMe3-2-S)3]−
toward HBpin in THF at −80 °C, complex [Ni(OC6H5)P-
(C6H3-3-SiMe3-2-S)3]− and 1.3-fold excess of HBpin dissolved
in THF were stirred at −80 °C to yield the mononuclear 1 and
II
Scheme 1
[NiFe] hydrogenase ([NiFe]H2ase) containing a bimetallic
[NiFe] core that is presumed to be the catalytic site for hydrogen
activation functions more effectively in the direction of H2
oxidation.1−4 The redox-active Ni site was proposed to change
between NiIII and NiII, while the Fe site remains as FeII in all
spectrally defined redox states of the enzyme.2 The active form
Ni−C, an intermediate in the catalytic cycle, of [NiFe]H2ase was
suggested to exist as the [(ScysH)NiIII(μ-SR)2(μ-H)Fe] inter-
mediate.2 Also, X-ray absorption spectroscopy shows that the Ni
site of the regulatory hydrogenase (RH) in the presence of
hydrogen (RH+H ), proposed as the Ni−C state, isolated from
2
Ralstonia eutropha is a six-coordinate [NiIIIS2(O/N)3(H)].3 In
the model study, the hydride-containing heterobimetallic
[NiII(μ-H)FeII] ([Ni(μ-S(CH2)3S)Fe]) active-site models and
hydride-containing bimetallic [Fe(μ-H)(μ-pdt)Fe] (pdt = 1,3-
propanedithiolate) active-site models for the [NiFe] enzymes
and [Fe]H2ase were reported, respectively, recently.4 Although
diverse nickel(II) hydride complexes relevant to the research
areas of coordination chemistry and homogeneous catalysis have
been well documented,5 there have been a few reports about the
preparation of a [NiIIIH]-containing complex and no examples of
the insertion reaction of small molecule (CS2) into the NiIII−H
bond reported. We are aware that an electrochemical study
provided evidence for such a NiIIIH species generated by one-
electron reduction of a nickel(II) macrocyclic complex
accompanied by protonation.6 Complex [Ni(psnet)]+ {psnet =
bis[5-(diphenylphosphino)-3-thiapentanyl]amine} of known
structure can stoichiometrically evolve H2 via the proposed
protic oxidation addition to NiI, generating NiIIIH.7 In this
contribution, the reaction of nickel(III) hydride [PPN][Ni(H)-
(P(o-C6H3-3-SiMe3-2-S)3)] (1) and CS2, generating [PPN]-
[NiIII(κ1-S2CH)(P(o-C6H3-3-SiMe3-2-S)3)] [2; PPN = bis-
(triphenylphosphoranylidene)ammonium], was described,
where complex 1 was prepared by the reaction of [PPN][Ni-
(OC6H5)P(C6H3-3-SiMe3-2-S)3] and the appropriate hydride
byproduct PhO-Bpin within less than 30 min (Scheme 1a,b).9
11B NMR spectroscopic experiments revealed a broad boron
resonance [δ 22.85 (C6D6) vs BF3·OEt2; Supporting Informa-
tion, Figure S1)] that could identify the byproduct PhO-Bpin
formed in the [Ni(OC6H5)P(C6H3-3-SiMe3-2-S)3]− → complex
1 conversion.9
Complex 1 is extremely thermally sensitive in solution. At 4 K,
complex 1 exhibits a rhombic electron paramagnetic resonance
(EPR) spectrum (g = 2.46, 1.96, and 1.92) similar to that (g =
2.44, 2.00, and 1.96) of [NiIII(CH3)(P(o-C6H3-3-SiMe3-2-
S)3)]−,10 deviating from complexes [NiIII(L)(P(o-C6H3-3-
SiMe3-2-S)3)]−, displaying EPR signals at g = 2.31, 2.04, and
Received: February 4, 2013
Published: March 29, 2013
© 2013 American Chemical Society
4151
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Inorganic Chemistry
Communication
1.99 for L = OPh and g = 2.31, 2.09, and 2.00 for L = SEt (77
K),11 respectively, suggestive of the d7 NiIII electronic structure
(Figure 1a). Monitoring the mixture of [Ni(OC6H5)P(C6H3-3-
known [NiIII(Cl)(P(o-C6H3-3-SiMe3-2-S)3)]− characterized by
UV−vis spectroscopy also supports the existence of complex 1 at
−80 °C.11b In the present study, the well-characterized complex
1 provides a suitable benchmark toward the synthesis of [NiIIIH]-
containing complexes by means of the metathesis of nickel(III)
phenoxide with pinacolborane (HBpin; pin = OCMe2CMe2O).
In contrast to NaBH4, LiB(Et)3H or LiAlH4, the trans-
formation of [Ni(OC6H5)P(C6H3-3-SiMe3-2-S)3]− into com-
plex 1 triggered by HBpin is presumably promoted by the
formation of a thermodynamically stable PhO-Bpin bond via the
four-center transition state of the metathesis reaction (Scheme
1a,b).9 That is, PhO-Bpin is generated from the interaction of the
lone pair on the phenoxide ligand with the empty p orbital of the
B atom of HBpin.9 A nucleophilic attack of NiIII−H on CS2 is
used to rationalize the preference for the NiIII−H bond to
undergo CS2 insertion, which may also involve a four-center
transition state. Despite the high thermodynamic stability of
CS2,13 it is found that the reaction of complex 1 with CS2 in THF
at −80 °C proceeds to yield green complex 2, characterized by IR
νC=S, UV−vis, EPR, 1H NMR, and single-crystal X-ray structure
(Scheme 1c). Insertion of CS2 into the NiIII−H bond of complex
1 yields [NiIII-κ1-S2CH]-containing complex 2, presumably due
to the preferential coordination of dithioformate [S2CH]− to the
NiIII center via the S atom.11 Consistent with the characteristic g
value of [PPN][Ni(SEt)(P(o-C6H3-3-SiMe3-2-S)3)] (g = 2.31,
2.09, and 2.00),11 complex 2 displays a rhombic EPR spectrum
with g1 = 2.28, g2 = 2.03, and g3 = 1.99 at 77 K (Figure 1b).
Figure 1. (a) X-band EPR spectrum (g values 2.46, 1.96, and 1.92) of
complex 1 in THF at 4 K. (b) X-band EPR spectrum (g values 2.28, 2.03,
and 1.99) of complex 2 in THF−CH3CN (4:1 volume ratio) at 77 K.
SiMe3-2-S)3]− and HBpin in THF by UV−vis spectroscopy
revealed a relatively clean conversion of [Ni(OC6H5)P(C6H3-3-
SiMe3-2-S)3]− to complex 1 within 30 min at −80 °C (Figure 2).
Further confirmation comes from the presence of the IR νas(CS )
2
absorption at 1007 cm−1 in KBr, assigned to the asymmetric C
S vibration.13 In comparison with complexes [PPN][Ni(L)(P(o-
C6H3-3-SiMe3-2-S)3)] (L = SEt, SPh) dominated by two intense
absorption bands at 577 and 920 nm and 592 and 949 nm,
respectively,11 the electronic spectrum of complex 2 displays a
red shift to 604 and 1028 nm. The proton resonances δ 15.7 (br),
10.8 (br), −6.9 (br) (acetone-d6) were assigned to the [P(C6H3-
3-SiMe3-2-S)3]3− ligand (SI, Figure S3). This result is consistent
with the central NiIII possessing a d7 electronic configuration in a
trignoal-bipyramidal ligand field.
Figure 2. Reaction of [PPN][Ni(OC6H5)(P(o-C6H3-3-SiMe3-2-S)3)]
and 4,4,5,5-tetramethyl-1,3,2-dioxaborolane (excess) in THF at −80 °C
monitored by UV−vis.
As is evidenced by monitoring of the reaction by UV−vis
spectroscopy, excess HBpin is required to ensure completeness
of the reaction. The increase of the intensity of the absorption
bands (505 and 864 nm) and the concomitant decay of the
absorption bands (605, 756, and 932 nm) with the presence of
isosbestic points indicate the absence of an intermediate during
the transformation of [Ni(OC6H5)P(C6H3-3-SiMe3-2-S)3]− into
complex 1. In comparison with [NiIII(CH3)(P(o-C6H3-3-SiMe3-
2-S)3)]− dominated by intense absorption bands at 500 and 825
nm (4:1 volume ratio of THF−CH3CN),10 the electronic
spectrum of complex 1 coordinated by hydride displays red shifts
to 505 and 864 nm (THF). Attempts to detect the νNi−H
stretching frequency at −80 °C were unsuccessful. Complex 1
To further examine the reactivity of [NiIIIH]-containing
complex 1, the reaction of 1 and diphenyldisulfide was
investigated. The transformation of complex 1 into the known
[NiIII(SPh)(P(o-C6H3-3-SiMe3-2-S)3)]−, characterized by UV−
vis and single-crystal X-ray diffraction, was observed when 1
equiv of diphenyldisulfide was added into the THF solution of
complex 1.11a In addition, upon the addition of CO gas into the
THF solution of complex 1 for 5 h, complex 1 completely
transformed into the known [NiII(CO)(P(o-C6H3-3-SiMe3-2-
S)3)]−, as monitored by UV−vis and IR with a νCO stretching
band [2033 cm−1 (KBr)],11b,14 accompanied by the release of H2
via reductive elimination.
1
exhibits a diagnostic H NMR spectrum with phenyl proton
resonances [δ 18.6 (br), 14.5 (br), −10.0 (br)] well-removed
from the diamagnetic region at −80 °C. Detection of the signal at
a negative chemical shift, which has been interpreted as nickel
hydride, was not successful.5 When a THF solution of complex 1
was stirred above −60 °C, the thermally unstable complex 1
undergoes reductive elimination of the coordinated hydride to
The structure of the [NiIII(κ1-S2CH)(P(o-C6H3-3-SiMe3-2-
S)3)]− unit for complex 2 in [PPN]+ salt is shown in Figure 3. It is
noticed that the S5−C64−S4 bond angle of 127.8(2)° is
comparable to those [126.4(2)° and 126.8(2)°] of [Al-
(OCMeCHCMeNAr)2(κ1-S2CH)] and [TpPh,MeZn(κ1-S2CH)],
respectively.15 Compared to the C−S bond distance of 1.56 Å in
́
II
yield the well-known [Ni2 (P(C6H3-3-SiMe3-2-S)3)2]2− charac-
free CS2,13 the significantly longer S4−C64 [1.703(2) Å] and
́
terized by UV−vis,8 with concomitant release of H2 gas identified
by milli-Whistle/gas chromatography.12 With the aid of isotopic
experiments, the reaction of a deuterated THF solution of
complex 1 and CDCl3 yields the very characteristic CHDCl2
identified by 1H NMR [δ 5.46 (t, C4D8O); SI, Figure S2], and the
́
S5−C64 [1.645(2) Å] bond lengths reflect that the [S2CH]
ligand of complex 2 acts as a κ1-S2CH coordination ligand, as
1
observed in [TpPh,MeZn(κ -S2CH)] [S1−C1 1.694(3) Å and
́
15b
́
́
S2−C1 1.623(3) Å]. The Ni1−S4 bond length of 2.261(1) Å
4152
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Inorganic Chemistry
Communication
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wfliaw@mx.nthu.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Science Council of Taiwan. The authors thank Pei-Lin Chen for
single-crystal X-ray structural determinations and Dr. Cheng-
Huang Lin and Chien-Hung Lin (National Taiwan Normal
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