Unique stepwise substitution reaction of a mono(guanidinate)tetraplatinum complex with amidines, giving mono(amidinate)tetraplatinum complexes through mixed-ligand intermediate complexes

Article abstract of DOI:10.1039/c2dt32136j

A mono(guanidinate)tetraplatinum complex [Pt₄(μ-OCOCH ₃)₇(μ-(TolN)₂CNⁱPr₂)] (5: Tol = C₆H₄Me-4) was prepared by treating [Pt ₄(μ-OCOCH₃)₈] (1) with excess amounts of N,N′-bis(p-tolyl)-N″-diisopropylguanidine (4a). A substitution reaction of the guanidinate moiety of 5 with N,N′-bis(aryl)formamidine (10: aryl = C₆H₄Me-4, C₆H₄COMe-4, and C₆H₄OMe-4) via mixed-ligand intermediates, trans-[Pt₄(μ-OCOCH₃)₆(μ-(TolN) ₂CNⁱPr₂)(μ-ArNCHNAr)] (11: Tol = C ₆H₄Me-4; Ar = C₆H₄Me-4, C ₆H₄COMe-4, and C₆H₄OMe-4), as a stepwise substitution mechanism, afforded the corresponding mono(amidinate) tetraplatinum complexes [Pt₄(μ-OCOCH₃) ₇(μ-ArNCHNAr)] (2a: Ar = C₆H₄Me-4; 2b: Ar = C₆H₄COMe-4; 2c: Ar = C₆H₄OMe-4) along with trans-bis(amidinate)tetraplatinum complexes trans-[Pt ₄(μ-OCOCH₃)₆(μ-ArNCHNAr)₂] (3a: Ar = C₆H₄Me-4; 3b: Ar = C₆H ₄COMe-4; 3c: Ar = C₆H₄OMe-4) as minor products. A Pt₄ dimer complex [Pt₄(μ-OCOCH₃) ₇]₂-{μ-(ArN)₂C(C₆H ₄)₂C(NAr)₂} (13: Ar = C₆H ₄^tBu-4) was selectively obtained upon treatment of 5 with a bis(amidine) linker ligand, N⁴,N⁴,N⁴′, N⁴′-tetrakis(p-(tert-butyl)phenyl)-[1,1′-biphenyl]-4, 4′-dicarboxamidine (12). All in-plane acetates of the two Pt₄ cores in 13 were fully substituted by 2,6-dimethylbenzoic acid, 2,4,6-triisopropylbenzoic acid, and ferrocenecarboxylic acid while maintaining its dimer structure to give the corresponding derivatives [Pt ₄(μ-OCOCH₃)₄(μ-OCOC₆H ₃Me₂-2,6)₃]₂-{μ-(ArN) ₂C(C₆H₄)₂C(NAr)₂} (14: Ar = C₆H₄^tBu-4), [Pt₄(μ-OCOCH ₃)₄(μ-OCOC₆H₂ⁱPr ₃-2,4,6)₃]₂-{μ-(ArN)₂C(C ₆H₄)₂C(NAr)₂} (15: Ar = C ₆H₄^tBu-4), and [Pt₄(μ-OCOCH ₃)₄(μ-OCOC₅H₄FeCp) ₃]₂-{μ-(ArN)₂C(C₆H ₄)₂C(NAr)₂} (16: Ar = C₆H ₄^tBu-4). The electrochemistry of the four Pt₄ dimer complexes 13-16 displayed a one-electron process attributed to Pt ₄⁹⁺/Pt₄⁸⁺, and another one-electron process due to Fe³⁺/Fe²⁺ was observed for the hexaferrocenyl derivative 16.

Full text of DOI:10.1039/c2dt32136j

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Unique stepwise substitution reaction of a mono-
(guanidinate)tetraplatinum complex with amidines,
giving mono(amidinate)tetraplatinum complexes
through mixed-ligand intermediate complexes†
Cite this: Dalton Trans., 2013, 42, 2831
Shinji Tanaka and Kazushi Mashima*
A mono(guanidinate)tetraplatinum complex [Pt4(μ-OCOCH3)7(μ-(TolN)2CNiPr2)] (5: Tol = C6H4Me-4) was
prepared by treating [Pt4(μ-OCOCH3)8] (1) with excess amounts of N,N’-bis(p-tolyl)-N’’-diisopropylguan-
idine (4a). A substitution reaction of the guanidinate moiety of 5 with N,N’-bis(aryl)formamidine (10: aryl =
C6H4Me-4, C6H4COMe-4, and C6H4OMe-4) via mixed-ligand intermediates, trans-[Pt4(μ-OCOCH3)6-
(μ-(TolN)2CNiPr2)(μ-ArNCHNAr)] (11: Tol = C6H4Me-4; Ar = C6H4Me-4, C6H4COMe-4, and C6H4OMe-4), as a
stepwise substitution mechanism, afforded the corresponding mono(amidinate)tetraplatinum complexes
[Pt4(μ-OCOCH3)7(μ-ArNCHNAr)] (2a: Ar = C6H4Me-4; 2b: Ar = C6H4COMe-4; 2c: Ar = C6H4OMe-4)
along with trans-bis(amidinate)tetraplatinum complexes trans-[Pt4(μ-OCOCH3)6(μ-ArNCHNAr)2] (3a: Ar =
C6H4Me-4; 3b: Ar = C6H4COMe-4; 3c: Ar = C6H4OMe-4) as minor products. A Pt4 dimer complex [Pt4-
t
(μ-OCOCH3)7]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (13: Ar = C6H4 Bu-4) was selectively obtained upon treatment
of
5 with a
bis(amidine) linker ligand, N4,N4,N4’,N4’-tetrakis(p-(tert-butyl)phenyl)-[1,1’-biphenyl]-
4,4’-dicarboxamidine (12). All in-plane acetates of the two Pt4 cores in 13 were fully substituted by
2,6-dimethylbenzoic acid, 2,4,6-triisopropylbenzoic acid, and ferrocenecarboxylic acid while maintaining
its dimer structure to give the corresponding derivatives [Pt4(μ-OCOCH3)4(μ-OCOC6H3Me2-2,6)3]2-
t
i
{μ-(ArN)2C(C6H4)2C(NAr)2} (14: Ar
= C6H4 Bu-4), [Pt4(μ-OCOCH3)4(μ-OCOC6H2 Pr3-2,4,6)3]2-{μ-(ArN)2C-
Received 15th September 2012,
Accepted 28th November 2012
t
(C6H4)2C(NAr)2} (15: Ar = C6H4 Bu-4), and [Pt4(μ-OCOCH3)4(μ-OCOC5H4FeCp)3]2-{μ-(ArN)2C(C6H4)2C(NAr)2}
(16: Ar = C6H4 Bu-4). The electrochemistry of the four Pt4 dimer complexes 13–16 displayed a one-
electron process attributed to Pt49+/Pt48+, and another one-electron process due to Fe3+/Fe2+ was
t
DOI: 10.1039/c2dt32136j
www.rsc.org/dalton
observed for the hexaferrocenyl derivative 16.
chelate diamine ligands and a mononuclear Pt(II) complex
bearing chelate diphosphine ligands with the appropriate multi-
Introduction
Supramolecular chemistry continues to attract high interest in dentate linker ligands, mainly mononuclear metal entities
the field of inorganic chemistry.1,2 Well-controlled ligand sub- have been used. On the other hand, to construct supramole-
stitution reactions of metal complexes play a major role in the cules, metal cluster units are rarely applied except for some
rational construction of supramolecular complexes via a self- dinuclear paddle-wheel complexes,9–13 trinuclear complexes,14
assembly process of metal units with the appropriate linker and octahedral hexanuclear complexes,15,16 because of their
ligands.2–8 Since Fujita et al.3 and Stang et al.4 independently complicated substitution reactions on a multinuclear core,
developed the syntheses of supramolecular assemblies by sub- despite being highly attractive entities due to their redox and
stitution reactions of a mononuclear Pd(II) complex bearing magnetic properties.17–22
A square planar tetrametal entity, [Pt4(μ-OCOCH3)8] (1), has
four acetate ligands coordinated in-plane of the square Pt4
core and four other acetate ligands coordinated out-of-
Department of Chemistry, Graduate School of Engineering Science, Osaka University,
plane,23,24 and the in-plane acetate ligands were selectively
and CREST, JST, Toyonaka, Osaka 560-8531, Japan.
E-mail: mashima@chem.es.osaka-u.ac.jp
†Electronic supplementary information (ESI) available: X-ray crystallographic
replaced by some bridging or chelating ligands due to the
trans-effect of the platinum ion.25 We previously reported the
data in CIF format, VT-NMR and simulated spectra of complex 5, tables of
unique substitution reaction of 1 with amidines to give mono-
(amidinate)tetraplatinum complexes and trans-bis(amid-
crystal data and refinement parameters for 5 and 13, and 1H NMR spectra of
complexes 5, 13, 14, 15, and 16. CCDC 901346, 910750. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c2dt32136j
2
inate)tetraplatinum complex 3 (Chart 1), whose electrochemical
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shape analysis at varying temperatures (Fig. S1†). Hindered
rotation of the N–C bond of a guanidinate ligand bridged to a
Rh2 unit was reported by Turro et al.28 Fig. 1 shows the mole-
cular structure of 5, in which the guanidinate ligand coordi-
nates to the in-plane side of the Pt4 core. The mean Pt–N
distance (2.11 Å) is shorter than that (2.17 Å) of 2a due to a
stronger donative nature of the guanidinate ligand. Notably,
the sp2 nature of the nitrogen atom in the guanidinate ligand
is decreased as indicated by smaller values for the sum (ϕ) of
the angles around the nitrogen atom in 5 (355.0° and 357.8°)
than in 2a (359.4° and 358.5°),26b presumably due to the steric
hindrance between tolyl groups and a bulky N(iPr)2 group
attached to the central carbon of the NCN moiety in 5. This
deviation around the nitrogen atoms of the guanidinate ligand
leads to the distortion of the planar geometry of the Pt4 core,
resulting in a smaller value of the sum (θ) of the internal
angles of the Pt4 square in 5 (355.8°). The mean Pt–eqO bond
distance (2.23 Å) between the oxygen atom of the trans-posi-
tioned acetate and each platinum atom is longer by 0.06 Å
than that (2.17 Å) between the oxygen atom of the cis-posi-
tioned acetate and each platinum atom, due to a trans-influ-
ence of the guanidinate ligand through the Pt–Pt bond.26b
In contrast to the successful synthesis of 5, treatment of 1
Chart 1
properties were elucidated.26,27 Herein we report the synthesis
and structure of a mono(guanidinate)tetraplatinum complex
and its substitution reactions with carboxylic acids and ami-
dines. Based on the substitution pattern of guanidinate bound
to the Pt4 unit with amidines, we synthesized a Pt4 dimer
complex by treatment with a bis(amidine) linker.
with
4 equiv. of triphenylguanidine 4b resulted in a
64 : 36 mixture of monosubstituted complex 6 and trans-disub-
stituted complex 7, and no product could be separated
(Scheme 1). The second substitution of a stronger donative
guanidinate ligand to the Pt4 core did not complete, even in
the presence of an excess of 4b, presumably due to the trans-
effect of the guanidinate ligand rather than steric effects. In
fact, we previously reported that the reaction of 1 with excess
amounts of amidines selectively afforded trans-disubstituted
products 3.26b Furthermore, the reaction of 1 with a cyclic
Results and discussion
A mono(guanidinate)tetraplatinum complex, [Pt4(μ-OCOCH3)7- guanidine, 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidine
(μ-(TolN)2CNiPr2)] (5: Tol = C6H4Me-4), was prepared in (4c), which is a highly donative ligand used for paddle-wheel
67% yield by treating [Pt4(μ-OCOCH3)8] (1) with 4 equiv. of a type dinuclear complexes,29 resulted in decomposition.
guanidine, N,N′-bis(p-tolyl)-N′′-diisopropylguanidine (4a), in a
We conducted substitution reactions of 5 with 2,6-dimethyl-
mixture of dichloromethane and methanol for 16 h followed benzoic acid and N,N′-(p-aryl)formamidine 10a–c to competi-
by removal of excess amounts of 4a by washing with Et2O and tively replace the guanidinate ligand and in-plane acetate
then with hexane (Scheme 1). Excess amounts of 4a were ligands of 5 (Scheme 2). When excess amounts of 2,6-dimethyl-
essential for full conversion of 1; however, to our surprise, benzoic acid were added to 5, a trisubstituted complex 8
further substitution of the monosubstituted product 5 was not bearing the guanidinate ligand and a tetrasubstituted complex
detected even when 10 equiv. of 4a was added. Complex 5 was 9, in which four 2,6-dimethylbenzoic acids were located in-
characterized by NMR spectroscopy, ESI-MS, and combustion plane of the square Pt4 core and there was no guanidinate
1
analysis together with its single crystal X-ray analysis. The ligand, were obtained in an 87 : 13 ratio. Its H NMR spectrum
1H NMR spectrum of 5 in CDCl3 exhibited five singlets due to displayed a new set of signals centered at δ 1.68, 1.76, and 1.80
magnetically nonequivalent acetate protons at δ 1.67, 1.76, due to axOAc with a 2 : 1 : 1 integral ratio, assignable to
1.82, 2.38, and 2.40 in a 2 : 1 : 1 : 2 : 1 integral ratio. Proton complex 8, and a singlet at δ 2.06 due to axOAc of 9 together
i
signals assignable to two Pr groups bound to the nitrogen with signals due to the dissociated guanidine 4a. Because the
atom of the guanidinate ligand were observed as two doublets major product was 8, the guanidinate ligand of 5 was too inert
at δ 0.74 and 0.77 (3JHH = 6.8 Hz), indicating that rotation for a substitution reaction with 2,6-dimethylbenzoic acid
around the (iPr)2N–C(NAr)2 bond was locked in solution compared with in-plane acetates. When 1 equiv. of amidine
at 30 °C. These two doublets gradually coalesced as the temp- N,N′-(p-tolyl)formamidine (10a) was added to 5, the guanidi-
erature was increased, and a doublet at δ 0.80 was observed at nate ligand 4a was gradually dissociated to give an
60 °C. The activation energy (ΔG‡ = 16.9 kcal mol−1 at 30 °C) of 83 : 17 mixture of mono(amidinate) complex 2a and trans-
rotation of the (iPr)2N–C(NAr)2 bond was estimated by NMR bis(amidinate) complex 3a. Similarly, the reaction of 5 with
2832 | Dalton Trans., 2013, 42, 2831–2840
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Scheme 1 Reactions of 1 with some guanidines.
mechanism of the substitution reaction of 5 with amidines.
Monitoring the reaction of 5 with 1 equiv. of 10b in CDCl3 by
1H NMR spectroscopy, a mixed ligand guanidinate–amidinate
complex,
[Pt4(μ-OCOCH3)6(μ-(Ar1N)2CNiPr2)(μ-Ar2NCNAr2)]
(11: Ar1 = C6H4Me-4, Ar2 = C6H4COMe-4), was immediately gene-
1
rated based on its H NMR spectrum that displayed a new set
of signals assignable to acetate ligands at δ 1.59, 1.67, 1.72,
and 2.35 ppm in a 2 : 1 : 1 : 2 ratio together with a singlet at
δ 2.12 due to free acetic acid, and these transient signals
gradually decreased with an increase of the two sets of signals
assignable to the known mono(amidinate) complex 2b and
free guanidine 4a (Fig. 2). This observation clearly indicated
that complex 11 was a key intermediate that further reacted
with the in situ-generated acetic acid to give the mono(amid-
inate) complex 2 (Scheme 3) through the stepwise substitution
reaction mechanism. The formation of 2b was the conse-
quence of the more labile nature of the guanidinate ligand in
the intermediate 11 than that of the amidinate ligand located
on the trans-side of the Pt4 core, with the guanidinate ligand
acting as a labile group.26b
Fig. 1 Molecular structure of complex 5. Hydrogen atoms and a hexane mole-
cule are omitted for clarity.
The stepwise and selective substitution reaction of 5 with
N,N′-bis(p-acetylphenyl)formamidine (10b) afforded the corre- amidine ligands allowed us to prepare a dimer of the Pt4
sponding mono(amidinate) complex 2b and trans-bis(amid- cluster using linker bearing bis(amidinate) moieties.
inate) complex 3b in a 94 : 6 ratio, and that with N,N′-bis- Complex 5 gradually reacted with a bis(amidine) ligand, N4,N4,
(p-anisyl)formamidine (10c) gave 2c and 3c in a 76 : 24 ratio.
N4′,N4′-tetrakis(p-(tert-butyl)phenyl)-[1,1′-biphenyl]-4,4′-dicarb-
a
Notably, the reaction of 5 with N,N′-bis(p-acetylphenyl)form- oxamidine (12), which was derived from 4,4′-biphenyldicarb-
amidine 10b provided direct information about the oxylic acid according to a reported procedure,30 and a dimer
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Scheme 2 Substitution reactions of 5 with 2,6-dimethylbenzoic acid and amidines.
Fig. 2 Reaction of complex 5 with N,N’-bis(p-acetylphenyl)formamidine 10b in CDCl3 solution monitored by 1H NMR (400 MHz, 30 °C).
2834 | Dalton Trans., 2013, 42, 2831–2840
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Scheme 3 Mechanism for substitution of guanidinate by amidine.
by the Pt–Pt bond from nitrogen-based bridging ligands, being
the same tendency as observed for 2a and 5.26b
ð1Þ
Because each Pt4 core of complex 13 has three in-plane
acetates, the addition of 6 equiv. of 2,6-dimethylbenzoic acid
to the solution of 13 in a mixture of dichloromethane and
methanol under reduced pressure gave [Pt4(μ-OCOCH3)4-
(μ-OCOC6H3Me2-2,6)3]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (14: Ar
=
t
C6H4 Bu-4) quantitatively. Similarly, we prepared a 2,4,6-tri-
Fig. 3 Molecular structure of complex 13. Hydrogen atoms, CHCl3 molecules,
i
isopropylbenzoate derivative [Pt4(μ-OCOCH3)4(μ-OCOC6H2 Pr3-
and an Et2O molecule are omitted for clarity.
t
2,4,6)3]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (15: Ar = C6H4 Bu-4) and a
ferrocenyl derivative [Pt4(μ-OCOCH3)4(μ-OCOC5H4FeCp)3]2-
t
{μ-(ArN)2C(C6H4)2C(NAr)2} (16: Ar = C6H4 Bu-4) by a substi-
complex [Pt4(μ-OCOCH3)7]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (13: Ar =
C6H4 Bu-4) was obtained in 96% yield together with the quan-
t
tution reaction of 13 with 6 equiv. of 2,4,6-triisopropylbenzoic
acid and ferrocenecarboxylic acid, respectively. Complexes 14,
15, and 16 were characterized by 1H, 13C{1H} NMR, and
ESI-MS and combustion analysis for complexes 15 and 16.
titative dissociation of guanidine 4a (eqn (1)). The Pt4 dimer
complex 13 was characterized by 1H, 13C{1H} NMR, ESI-MS,
combustion analysis, and single crystal X-ray analysis. In the
1H NMR spectrum of 13, acetate groups bound to Pt4 units in
13 were observed as five nonequivalent singlets at δ 1.85, 1.87,
2.02, 2.26, and 2.42 with a 1 : 2 : 1 : 2 : 1 integral ratio, indicat-
ing that two Pt4 cluster units were equivalent in solution and
each [Pt4(OCOCH3)7]+ unit had Cs symmetry. The ESI-MS of 13
showed a peak at m/z 3091, which was assignable to [13–OAc]+,
and its isotopic distribution was in good agreement with the
simulated one. Fig. 3 shows the molecular structure of 13, in
which a bis(amidinate) ligand coordinates to the in-plane side
of the Pt4 core, and eventually the two Pt4 units are connected
by the biphenyl linker. Selected geometrical parameters of 13
are listed in Table 1. In the dimer complex 13, geometries
around the Pt4 core are nearly the same as those of 2a except
for shorter Pt–N distances and a smaller value of the sum (θ)
of internal angles of the Pt4 square. The mean Pt–Oeq distance
(2.22 Å) of trans-positioned acetates is longer than that (2.17 Å)
of cis-positioned acetates due to the trans-influence mediated
ð2Þ
The electrochemical interaction between the two Pt4 units
was elucidated by the cyclic voltammetry (CV) measurement
for complexes 5, 13, 14, 15, 16 (Fig. 4), and their E1/2 values are
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Table 1 Selected mean bond distances (Å) and sums of angles (°) in complexes 1, 2a, 5, and 13
Pt–Pt
Pt–axO
Pt–eqO
Pt–N
θa
ϕb
Ref.
1
2a
5
2.495
2.512
2.505
2.505
2.002
2.01
2.01
2.00
2.162
—
358.36
359.38
355.81
357.18
—
24b
26b
This work
This work
2.20 (cis), 2.26 (trans)
2.17 (cis), 2.23 (trans)
2.17 (cis), 2.22 (trans)
2.17
2.11
2.12
359.4, 358.5
354.9, 357.9
358.3, 359.2
13
a θ = the sum of the internal angles of the Pt4 square. b ϕ = the sum of the angles around the nitrogen atom.
process B, whose E1/2 value (720 mV) was comparable to that
(710 mV) of mono(amidinate) complex 2a. Similarly, the CV of
complex 14 displayed a reversible oxidation process B at E1/2
=
720 mV together with an irreversible reduction process A. In
the CV of complex 15, an oxidation process of Pt4 units (B) was
observed as a broader wave compared with that of 13. In fact,
the difference between the oxidation and reduction peak (ΔEp)
of process B was larger (270 mV) for 15 than that (120 mV) for
13, indicating that electrochemical reversibility of the redox
event at the Pt4 unit in 15 was decreased due to the incorpor-
ation of bulky triisopropylphenyl groups on the in-plane sides
of the Pt4 units. The CV of complex 16 displayed an oxidation
process labeled as Fc assignable to one electron oxidation of
six Fc units (Fe2+/Fe3+) along with process B. The E1/2 value
(910 mV) of process B was positively shifted by 190 mV from
the parent complex 13 due to an electron withdrawing effect of
oxidized Fc units. A notable feature of the CV of 16 was the
large reduction and oxidation peak current ratio of process Fc,
which was attributed to the deposition of multiply charged
species on the working electrode. Similar phenomena were
also observed in other poly ferrocenyl complexes.31 The elec-
tron number in process B was assigned to two electrons on the
basis of the ratio of oxidation currents of process B and
process Fc. Thus, two Pt4 units in a Pt4 dimer complex were
oxidized independently without the formation of any inter-
mediate one-electron oxidized species.
Fig. 4 CVs of 5, 13, 14, 15, and 16 recorded in 1 mM dichloromethane solu-
tion containing 0.1 M of [nBu4N][PF6] using a scan rate of 100 mV s−1
.
Table 2 E1/2 (mV vs. Fc+/Fc) and ΔEp (mV) of 2a, 5, 13, 14, 15, and 16 deter-
mined by CV analysis in 1 mM dichloromethane solution containing 0.1 M of
[nBu4N][PF6]
Complex A (Pt48+/Pt4
)
Fc (Fe3+/Fe2+
)
B (Pt49+/Pt4
)
ΔEp of B
7+
8+
b
2a26b
5
13
14
15
−1470a
—
710
570
720
720
690
910
100
80
120
140
270
130
c
b
Conclusion
—
—
c
b
—
—
b
−1490a
—
In summary, we successfully prepared the mono(guanidinate)-
tetraplatinum complex 5, in which a guanidinate ligand was
selectively substituted by amidine ligands in a unique stepwise
substitution sequence through the guanidinate–amidinate Pt4
complex 11. The Pt4 dimer complex 13 was obtained in good
yield by treating 5 with 0.5 equiv. of a bis(amidine) linker
ligand 12. In the Pt4 dimer complex 13, six in-plane acetate
ligands were found to be labile so that further substitution by
b
−1880a
—
c
16
—
210
a Irreversible wave: Ep.a. value is shown instead of E1/2 value. b Not
observed. c Irreversible wave: Ep.a. value was not determined from the
observed wave.
listed in Table 2. The CV of mono(guanidinate)tetraplatinum benzoic acid derivatives and ferrocenecarboxylic acid pro-
complex 5 exhibited an irreversible reduction process A assign- ceeded while maintaining its dimer structure, giving Pt4 dimer
7+
able to Pt48+/Pt4 and a reversible oxidation process B due to derivatives 14–16. The electrochemical measurement revealed
Pt49+/Pt48+. The E1/2 value (570 mV) of process B was negatively the reversible oxidation process of Pt4 dimer complexes 13–16
shifted by 140 mV from that of mono(amidinate)tetraplatinum due to Pt49+/Pt48+. In addition, a multielectron oxidation
complex 2a due to the stronger electron donative nature of the process due to Fe3+/Fe2+ of six ferrocenyl units was observed
guanidinate ligand. The Pt4 dimer complexes 13 also showed for hexaferrocenyl derivative 16, but there was no electronic
an irreversible reduction process A and a reversible oxidation communication between the two Pt4 centers or the Fc unit.
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3
(s, 6H, C6H4CH3), 3.85 (sept, JHH = 6.8 Hz, 2H, N(CH(CH3)2)2),
Experimental section
General procedures
3
5.27 (br s, 1H, NH), 6.77 (d, JHH = 8.0 Hz, 4H, ArH), 6.98
3
(d, JHH = 8.0 Hz, 4H, ArH). 13C{1H} NMR (100 MHz, CDCl3,
All manipulations involving air- and moisture-sensitive com- 30 °C, δ/ppm): 20.6 (C6H4CH3), 21.4 (CH(CH3)2), 47.4 (CH
pounds were carried out under argon using standard Schlenk (CH3)2), 120.6, 129.4, 130.7, 149.4 (ArC), a guanidinate carbon
techniques or in an argon-filled glovebox. [Pt4(μ-OCOCH3)8] was not detected. HRMS-ESI (m/z): [M + H]+ calcd for
(1)25a and N,N′-bis(aryl)formamidine32 were prepared accord- C21H30N3, 324.2434; found, 324.2462.
ing to literature procedures. 2,6-Dimethylbenzoic acid, 2,4,6-
Preparation of [Pt4(μ-OCOCH3)7(μ-(TolN)2CNiPr2)] (5: Tol =
triisopropylbenzoic acid, and ferrocenecarboxylic acid were C6H4Me-4). The solution of 1 (248 mg, 198 μmol) and 4a
purchased and used as received. Dehydrated hexane, Et2O, and (256 mg, 791 μmol, 4.0 equiv.) in a mixture of CH2Cl2 (5 mL)
CH2Cl2 were purchased from Kanto Chemical and further puri- and MeOH (5 mL) was stirred for 16 h at ambient temperature.
fied by passage through activated alumina under positive The resulting deep-red solution was concentrated under
argon pressure as described by Grubbs et al.33 Dehydrated vacuum and the residue was washed with a mixture of Et2O
MeOH, CD3OD, CHCl3, CDCl3, and DMSO-d6 were degassed and hexane (5 mL + 5 mL, four times), giving 5 as a reddish-
and stored under argon over activated 3 Å (MeOH and CD3OD) brown powder (200 mg, 67% yield), mp. 179–182 °C (dec.).
3
and 4 Å (CHCl3, CDCl3, and DMSO-d6) molecular sieves. 1H NMR (400 MHz, CDCl3, 30 °C, δ/ppm): 0.74 (d, JHH = 6.8
3
1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) were Hz, 6H, N(CH(CH3)2)2), 0.77 (d, JHH = 6.8 Hz, 6H, N(CH-
measured on Bruker AVANCEIII-400 spectrometers, and all (CH3)2)2), 1.67 (s, 6H, axCH3CO2), 1.76 (s, 3H, axCH3CO2), 1.82
spectra were recorded at 30 °C or 35 °C unless mentioned (s, 3H, axCH3CO2), 2.25 (s, 6H, C6H4CH3), 2.38 (s, 6H,
3
otherwise and referenced to an internal solvent. Cyclic voltammo- eqCH3CO2), 2.40 (s, 3H, eqCH3CO2), 3.91 (sept, JHH = 6.8 Hz,
3
grams were recorded using an electrochemical analyzer 1H, N(CH(CH3)2)2), 4.08 (sept, JHH = 6.8 Hz, 1H, N(CH-
(Model 610D, ALS/CH Instruments) at room temperature in a (CH3)2)2), 7.07 (br d, 4H, C6H4CH3), 7.20 (br s, 4H, C6H4CH3).
0.1 M [nBu4N][PF6] solution in CH2Cl2 with a glassy carbon 13C{1H} NMR (100 MHz, CDCl3, 35 °C, δ/ppm): 20.9
working electrode, a platinum wire auxiliary electrode, a satu- (axCH3CO2), 21.1 (C6H4CH3), 21.2 (axCH3CO2), 21.99, 22.03
rated calomel reference electrode (SCE), and a scan rate of (N(CH(CH3)2)2), 22.2 (axCH3CO2), 22.4, 22.6 (eqCH3CO2), 52.1,
100 mV s−1. Elemental analyses were recorded on a Perkin- 52.6 (N(CH(CH3)2)2) 128.6, 133.1, 146.3, 168.1 (ArC), 182.5,
Elmer 2400II microanalyzer in the Department of Chemistry, 186.9, 192.3, 192.5 (CH3CO2), one carboxylate carbon
Faculty of Engineering Science, Osaka University. Mass spectro- (CH3CO2) was overlapped with another carbon, and a guanidi-
metric data were obtained using ESI or FAB techniques (on nate carbon was not detected. MS (ESI positive, CH3CN, m/z):
a JEOL SX-102 spectrometer) or ESI techniques (on a BRUKER 1456 ([M − CH3COO]+). Elemental analysis calcd (%) for
micrOTOF spectrometer). Melting points were measured in C35H49N3O14Pt4: C 27.73, H 3.26, N 2.77; found: C 27.79,
sealed tubes and were not corrected.
Preparation of N,N′–bis(p-tolyl)-N′′-diisopropylguanidine
H 2.99, N 2.80.
Preparation of N4,N4′-bis(p-(tert-butyl)phenyl)-[1,1′-biphenyl]-
(4a). To a solution of diisopropylamine (0.950 mL, 6.72 mmol) 4,4′-dicarboxamide. To the mixture of 4,4′-biphenyldicarb-
n
in THF (10 mL) at −78 °C was dropwise added BuLi (1.57 M oxylic acid (1.33 g, 5.50 mmol), triethylamine (2 mL,
solution in hexane, 4.30 mL, 6.75 mmol, 1.0 equiv.). After stir- 0.01 mol, 3 equiv.), p-tert-butylaniline (1.80 mL, 11.4 mmol,
ring for 1 hour at ambient temperature, this solution was 2.1 equiv.) and p-N,N-dimethylaminopyridine (1.35 g,
added to a solution of N,N′-bis(p-tolyl)carbodiimide (1.57 g, 11.1 mmol, 2.0 equiv.) in DMF (100 mL) was added 1-ethyl-
6.78 mmol, 1.0 equiv.) in THF (15 mL) at −78 °C, giving a 3-(3-dimethylaminopropyl)carbodiimide hydrochloride (2.21 g,
green suspension. The cold bath was removed and the reaction 11.6 mmol, 2.1 equiv.) at 0 °C, giving a pale red solution. After
mixture was stirred at ambient temperature for 14 hours. stirring at ambient temperature for 3 days, the reaction
During this operation, precipitated solids were dissolved and mixture turned to an orange suspension, which was concen-
the reaction mixture turned to a green solution. The removal trated under reduced pressure. CH2Cl2 (50 mL) and H2O
of volatiles under vacuum afforded a yellow-green oil, which (50 mL) were added to the resulting solid, giving an orange
was washed with hexane to give the lithium salt of 4a as a light suspension, from which an insoluble white solid was separ-
yellow powder. A mixture of toluene (10 mL) and deionized ated by filtration. The white powder was washed with H2O and
water (10 mL) was added to this powder, and the resulting sus- then dried under vacuum. The title compound was obtained
pension was stirred for 30 min, followed by separation of the as a white powder (1.07 g, 39%), mp >300 °C. 1H NMR
organic layer using a separating funnel, and the water layer (400 MHz, DMSO-d6, 30 °C, δ/ppm): 1.29 (s, 18H, C(CH3)3),
3
3
was extracted with toluene (10 mL, twice). A combined toluene 7.38 (d, JHH = 8.8 Hz, 4H, ArH), 7.72 (d, JHH = 8.8 Hz, 4H,
3
3
solution was washed with deionized water (10 mL), and then ArH), 7.93 (d, JHH = 8.6 Hz, 4H, ArH), 8.10 (d, JHH = 8.6 Hz,
dried over anhydrous MgSO4. All volatiles were removed under 4H, ArH), 10.25 (s, 2H, CONH). 13C{1H} NMR (100 MHz,
vacuum, and the resulting red oil was washed with hexane DMSO-d6, 30 °C, δ/ppm): 31.2 (C6H4C(CH3)3), 34.0 (C6H4C
(5 mL, three times). 4a was obtained as a pale red powder (CH3)3), 120.2, 125.2, 126.8, 128.4, 134.3, 136.5, 141.9, 146.0
(393 mg, 18%), mp 83–84 °C. 1H NMR (400 MHz, CDCl3, (ArC), 164.8 (CONH). HRMS-FAB (m/z): [M + H]+ calcd for
30 °C, δ/ppm): 1.30 (d, 3JHH = 6.8 Hz, 12H, N(CH(CH3)2)2), 2.25 C34H37N2O2, 505.2855; found, 505.2883.
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Preparation of N4,N4,N4′,N4′-tetrakis(p-(tert-butyl)phenyl)- 21.4 (axCH3CO2), 31.2 (C(CH3)3), 34.2 (C(CH3)3), 124.7, 125.3,
[1,1′-biphenyl]-4,4′-dicarboxamidine (12). An 80 mL Schlenk 126.9, 127.0, 127.4, 127.6, 128.0, 130.7, 132.8, 133.9, 134.9,
tube charged with N4,N4′-bis(p-(tert-butyl)phenyl)-[1,1′-biphenyl]- 135.1, 137.4, 139.2, 144.9, 146.3 (ArC), 172.0 (NCRN), 181.7,
4,4′-dicarboxamide (492 mg, 975 μmol) and SOCl2 (10 mL) 184.2 (eqArCO2), 192.2, 192.4, 192.9 (axCH3CO2), one methyl
was heated at 60 °C for 6 h. After removal of volatile com- carbon (axCH3CO2) was overlapped with another carbon.
pounds in vacuo, the resulting yellow solid was treated with
Complexes 15 and 16 were prepared in a similar manner.
p-tert-butylaniline (0.320 mL, 2.01 mmol, 2.1 equiv.) in toluene 15: a dark red powder (63% yield), mp 206–210 °C (dec.).
(50 mL). The reaction mixture was refluxed for 17 h. After 1H NMR (400 MHz, CDCl3, 30 °C, δ/ppm): 0.98 (s, 36H,
3
cooling, saturated Na2CO3 aq. (20 mL) was added to the yellow –C6H4C(CH3)3), 1.13 (br d, JHH = 6.8 Hz, 48H, –C6H2(CH-
3
suspension. The yellow solid was separated by filtration and (CH3)2)3), 1.18 (d, JHH = 6.8 Hz, 12H, –C6H2(CH(CH3)2)3), 1.19
3
3
washed with CHCl3. 12 was obtained as a yellow powder (d, JHH = 6.8 Hz, 24H, –C6H2(CH(CH3)2)3), 1.22 (d, JHH = 6.8
(140 mg, 19%), mp 168–171 °C. 1H NMR (400 MHz, CD3OD, Hz, 24H, –C6H2(CH(CH3)2)3), 1.85 (s, 6H, axCH3CO2), 1.88
60 °C, δ/ppm): 1.28 (br s, 36H, C(CH3)3), 7.20 (br d, 8H, ArH), (s, 12H, axCH3CO2), 2.00 (s, 6H, axCH3CO2), 2.82 (sept, JHH
=
3
7.39 (br d, 8H, ArH), 7.90 (br s, 8H, ArH). 13C{1H} NMR 6.8 Hz, 2H, C6H2(CH(CH3)2)3), 2.84 (sept, JHH = 6.8 Hz, 4H,
3
(100 MHz, CD3OD, 60 °C, δ/ppm): 31.6 (C(CH3)3), 35.5 C6H2(CH(CH3)2)3), 2.96 (br s, 8H, C6H2(CH(CH3)2)3), 3.15 (sept,
3
(C(CH3)3), 125.8, 127.5, 128.9, 130.2, 132.0, 134.2, 145.4, 152.9 3JHH = 6.8 Hz, 4H, C6H2(CH(CH3)2)3), 6.83 (d, JHH = 8.8 Hz,
3
(ArC), 163.2 (NCRN). HRMS-ESI (m/z): [M + H]+ calcd for 8H, C6H4C(CH3)3), 6.86 (d, JHH = 8.8 Hz, 8H, C6H4C(CH3)3),
3
C54H63N4, 767.5047; found, 767.5056.
6.87 (s, 12H, C6H2(CH(CH3)2)3), 6.97 (d, JHH = 8.8 Hz, 4H,
Preparation of [Pt4(μ-OCOCH3)7]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (C6H4)2), 7.08–7.14 (m, 4H, (C6H4)2). 13C{1H} NMR (100 MHz,
(13: Ar = C6H4 Bu-4). The mixture of 5 (92.4 mg, 609 μmol) CDCl3, 30 °C, δ/ppm): 20.56, 20.62, 21.2 (axCH3CO2), 24.05,
t
and 12 (23.5 mg, 30.6 μmol, 0.50 equiv.) in CHCl3 (5 mL) was 24.06, 24.1, 24.2 (CH(CH3)2), 30.8 (CH(CH3)2), 31.1 (C(CH3)3),
stirred for 42 hours at ambient temperature. After the removal 34.37, 34.40 (CH(CH3)2), 34.1 (C(CH3)3), 119.8, 119.9, 124.6,
of volatile compounds under reduced pressure, the resulting 125.4, 127.4, 130.9, 133.1, 134.1, 139.2, 143.4, 143.7, 144.8,
dark-red powders were washed with MeOH (5 mL, twice). 145.9, 147.9, 148.1 (ArC), 171.2 (NCRN), 182.7, 185.5 (eqArCO2),
Complex 13 was obtained as a brown powder (92.2 mg, 96%), 191.8, 191.9, 192.6 (axCH3CO2), one isopropyl carbon (CH-
mp 190–193 °C (dec.). 1H NMR (400 MHz, CDCl3, 30 °C, (CH3)2) and one aryl carbon were overlapped with another
δ/ppm): 1.17 (s, 36H, –C6H4C(CH3)3), 1.85 (s, 6H, axCH3CO2), carbon. MS (ESI positive, CH3CN, m/z): 4304 ([M + Na]+), 4032
i
1.87 (s, 12H, axCH3CO2), 2.02 (s, 6H, axCH3CO2), 2.26 (s, 12H, ([M
−
O2CC6H2 Pr3]+). Elemental analysis calcd (%) for
eqCH3CO2), 2.42 (s, 6H, eqCH3CO2), 6.91 (d like, 12H, C6H4C-
(CH3)3 + (C6H4)2), 7.02 (d like, 12H, C6H4C(CH3)3 + (C6H4)2). H 5.23, N 1.33.
13C{1H} NMR (100 MHz, CDCl3, 30 °C, δ/ppm): 21.2, 21.3, 21.6
16: an orange powder (90% yield), mp 192–196 °C (dec.).
(axCH3CO2), 22.2, 22.7
C166H222N4O28Pt8: C 46.56, H 5.23, N 1.31; found: C 46.18,
(
eqCH3CO2), 31.3 (C(CH3)3), 34.3 1H NMR (400 MHz, CDCl3, 30 °C, δ/ppm): 1.33 (s, 36H,
(C(CH3)3), 124.6, 125.3, 127.6, 130.8, 133.1, 139.4, 144.7, 146.4 –C6H4C(CH3)3), 1.94 (s, 6H, axCH3CO2), 2.04 (s, 12H,
(ArC), 171.9 (NCRN), 183.8, 186.7 (eqCH3CO2), 192.3, 192.5, axCH3CO2), 2.13 (s, 6H, axCH3CO2), 4.08 (s, 20H, CpH), 4.22
193.0 (axCH3CO2). MS (ESI positive, CH3CN, m/z): 3150 ([M]+), (br s, 4H, C5H4), 4.24 (br s, 4H, C5H4), 4.26 (s, 10H, CpH), 4.36
3091 ([M − CH3COO]+). Anal. calcd for C82H102N4O28Pt8: (br s, 4H, C5H4), 4.57 (br s, 4H, C5H4), 4.81 (br s, 4H, C5H4),
3
C 31.24, H 3.26, N 1.78; found: C 31.22, H 3.48, N 1.89.
5.17 (br s, 4H, C5H4), 7.02 (br d, JHH = 8.4 Hz, 16H, C6H4C-
3
Preparation of [Pt4(μ-OCOCH3)4(μ-OCOC6H3Me2-2,6)3]2- (CH3)3, +(C6H4)2), 7.22 (d, JHH = 8.4 Hz, 8H, C6H4C(CH3)3).
t
{μ-(ArN)2C(C6H4)2C(NAr)2} (14: Ar
=
C6H4 Bu-4), [Pt4(μ-O- 13C{1H} NMR (100 MHz, CDCl3, 30 °C, δ/ppm): 21.6, 21.7, 21.9
COCH3)4(μ-OCOC6H2 Pr3-2,4,6)3]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (axCH3CO2), 31.5 (C(CH3)3), 34.4 (C(CH3)3), 70.0, 70.1 (CpC),
i
t
(15: Ar
=
C6H4 Bu-4), and [Pt4(μ-OCOCH3)4(μ-OCOC5- 70.58, 70.60, 70.7, 70.9, 71.6, 71.8, 72.3, 73.9 (C5H4), 124.8,
t
H4FeCp)3]2-{μ-(ArN)2C(C6H4)2C(NAr)2} (16: Ar = C6H4 Bu-4). The 125.2, 127.9, 130.3, 132.7, 139.2, 144.9, 146.2 (ArC), 171.6
mixture of 13 (30.8 mg, 609 μmol) and 2,6-dimethylbenzoic (NCRN), 182.8, 185.2 (FcCO2), 192.0, 192.2, 193.0 (axCH3CO2).
acid (9.2 mg, 59 μmol, 6.1 equiv.) in a mixture of CH2Cl2 MS (ESI positive, CH3CN, m/z): 4172 ([M]+), 3942
(5 mL) and MeOH (3 mL) was stirred for 6 hours at ambient ([M − O2CC5H4FeCp]+). Elemental analysis calcd (%) for
temperature under partially reduced pressure. During this
C136H138Fe6N4O28Pt8: C 39.15, H 3.33, N 1.34; found: C 39.16,
operation, a mixture of CHCl3 (5 mL) and MeOH (5 mL) was H 2.89, N 1.49.
repeatedly added before all volatiles were removed. The
X-ray crystallographic analysis
removal of all volatiles afforded dark red solids, which were
washed with Et2O. 14 was obtained as a brown powder A crystal of 5 (red, block) was grown in the saturated diethyl
(37.6 mg, quant.), mp 184–187 °C (dec.). 1H NMR (400 MHz, ether–hexane mixed solution, and a crystal of 13 (red, platelet)
CDCl3, 30 °C, δ/ppm): 1.09 (s, 36H, C6H4C(CH3)3), 1.91 (s, 6H, was obtained via diffusion of diethyl ether into the chloroform
axCH3CO2), 1.93 (s, 12H, axCH3CO2), 2.07 (s, 6H, axCH3CO2), solution. Both crystals were mounted on the CryoLoop
2.13 (s, 24H, C6H3(CH3)2), 2.36 (s, 12H, C6H3(CH3)2), 6.87–6.95 (Hampton ReseArCh Corp.) with a layer of mineral oil and
(m, 32H, ArH), 7.01–7.06 (m, 10H, ArH). 13C{1H} NMR placed in a nitrogen stream. Complexes 5 and 13 were
(100 MHz, CDCl3, 30 °C, δ/ppm): 19.5, 20.0 (C6H3(CH3)2), 21.0, measured with a Rigaku RAXIS-RAPID Imaging Plate equipped
2838 | Dalton Trans., 2013, 42, 2831–2840
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with a sealed tube X-ray generator (50 kV, 40 mA) with graphite 10 M. W. Cooke and G. S. Hanan, Chem. Soc. Rev., 2007, 36,
monochromated Mo-Kα (0.71075 Å) radiation in a nitrogen 1466–1476.
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squares refinement with the setting angles, which are listed in 12 (a) R. H. Cayton and M. H. Chisholm, J. Am. Chem. Soc.,
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direct methods on SIR97,34 after being refined on F2 by full-
matrix least-squares methods using SHELXL-97.35 Measured
nonequivalent reflections with I > 2.0σ(I) were used for the
structure determination. The hydrogen atoms were included in
1989, 111, 8921–8923; (b) R. H. Cayton, M. H. Chisholm,
J. C. Huffman and E. B. Lobkovsky, J. Am. Chem. Soc., 1991,
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the refinement on calculated positions riding on their carrier 13 M. Köberl, M. Cokoja, B. Bechlars, E. Herdtweck and
2
2
atoms. The function minimized was [Σw(Fo − Fc )] (w =
F. E. Kühn, Dalton Trans., 2011, 40, 11490–11496.
2
2
2
1/[σ2(Fo ) + (aP)2 + bP]), where P = (Max(Fo ,0) + 2 Fc )/3 with 14 (a) H. Kido, H. Nagino and T. Ito, Chem. Lett., 1996,
σ2(Fo ) from counting statistics. The functions R1 and wR2 were
745–746; (b) T. Ito, T. Hamaguchi, H. Nagino,
T. Yamaguchi, J. Washington and C. P. Kubiak, Science,
1997, 277, 660–663; (c) T. Ito, T. Hamaguchi, H. Nagino,
T. Yamaguchi, H. Kido, I. S. Zavarine, T. Richmond,
J. Washington and C. P. Kubiak, J. Am. Chem. Soc., 1999,
121, 4625–4632.
2
(Σ||Fo| − |Fc||)/Σ|Fo| and [Σw(Fo − Fc ) /Σ(wFo )]1/2, respecti-
vely. R values of 13 were relatively high because the crystal of
13 contained disordered solvent molecules. Refinement of
these molecules was unsuccessful. The ORTEP-3 program was
used to draw two molecules.36 CCDC-901346 (5) and 910750
2
2 2
4
(13) contain the supplementary crystallographic data for 15 H. D. Selby, B. K. Roland and Z. Zheng, Acc. Chem. Res.,
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19 P. Venkateswara Rao and R. H. Holm, Chem. Rev., 2004,
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