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Acta Crystallographica Section EStructure Reports
OnlineISSN 1600-5368

A head-to-head isomer of bis(-N-methylisonicotinamidato)bis[cisdiammineplatinum(II)] tetraperchlorate
Ken Sakai, Meiko Shiomi, Taro Tsubomura, Kaoru Kato, Yoshimi Yokoyama,Takashi Kajiwara and Tasuku Ito

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Acta Cryst. (2003). E59, m559–m561

Ken Sakai et al.  [Pt2 (C7 H8 N2 O)2 (NH3 )4 ](ClO4 )4metal-organic papers
Acta Crystallographica Section E

Structure Reports
OnlineISSN 1600-5368

Ken Sakai,a* Meiko Shiomi,bTaro Tsubomura,b Kaoru Kato,aYoshimi Yokoyama,a Takashi
Kajiwarac and Tasuku ItocaDepartment of Applied Chemistry, Faculty ofScience, Tokyo University of Science,
Kagurazaka 1-3, Shinjuku-ku, Tokyo 162-8601,
Japan, bDepartment of Applied Chemistry,
Faculty of Engineering, Seikei University,Kichijoji-Kitamachi 3-3-1, Musashino, Tokyo
180-8633, Japan, and cDepartment ofChemistry, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
Correspondence e-mail:ksakai@rs.kagu.tus.ac.jp

Key indicators
Single-crystal X-ray study
T = 296 K
ÊMean (C±C) = 0.013 A
Disorder in solvent or counterion
R factor = 0.035
wR factor = 0.062
Data-to-parameter ratio = 10.5
For details of how these key indicators wereautomatically derived from the article, see
http://journals.iucr.org/e.

# 2003 International Union of CrystallographyPrinted in Great Britain ± all rights reserved

Acta Cryst. (2003). E59, m559±m561

A head-to-head isomer of bis(l-N-methylisonicotinamidato)bis[cis-diammineplatinum(II)]tetraperchlorate
In the title compound, [Pt2(C7H8N2O)2(NH3)4](ClO4)4, thehead-to-head isomer of the PtII dimer has a mirror planethrough the PtÐPt bond axis. The intradimer PtÐPt distanceÊ , while the shortest interdimer Pt  Pt distanceis 3.0569 (8) A
Ê . Two adjacent dimers are associated with one
is 7.2325 (9) A
another, with a -stacking interaction between the N-methylpyridinium moieties, where the plane-to-plane separation isÊ.
3.27 (1) A

Received 23 June 2003
Accepted 30 June 2003
Online 10 July 2003

Comment
A photochemical system made up of Ru(bpy)32+ (bpy = 2,20 bipyridine) and methylviologen (usually, N,N0 -dimethyl-4,40 bipyridinium dichloride) has been thought of as one of thepromising candidates to achieve arti®cial photosynthetic
devices (Borgarello et al., 1981). In this context, we previously
reported that some amidate-bridged platinum dimers,[Pt2(NH3)4(-amidato)2]2+ (amidate = acetamidate, -pyrrolidinonate, -pyridonate, etc.), serve as effective H2-producing
catalysts in the photosystem mentioned above (Sakai &Matsumoto, 1990; Sakai et al., 1993). Since then, various
efforts have been made to develop a single molecular device inwhich visible-light-induced reduction of water to molecularhydrogen is carried out in a sophisticated manner. In order tocon®rm the validity of our approach, various amidate-bridged
platinum dimers tethered to methylviologen and derivatives
have also been prepared. Here we report the crystal structureof the title compound, (I), which is one of such platinum
dimers prepared in our laboratory. The H2-producing activityof (I) has been con®rmed to be comparable to those examinedin our original report in 1990.

The asymmetric unit of (I) consists of one half of a dinuclear
PtII cation and two perchlorate anions. Since two Pt atoms areboth located on a mirror plane, there is a crystallographic
requirement that this is a head-to-head dimer in which twoamidate NÐCÐO units are arranged in the same direction
within the dimeric unit (Fig. 1 and Table 1). However, one
might contend that this could be a mixture of both the headto-head and the head-to-tail isomers. Nevertheless, it is often
possible to determine the correct direction of NÐCÐO inleast-sqares re®nement, as carried out in the present study
DOI: 10.1107/S1600536803014521

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Ken Sakai et al.  [Pt2(C7H8N2O)2(NH3)4](ClO4)4

m559

metal-organic papers

Figure 1

The structure of the dinuclear Pt2II cation in (I), showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50%
probability level.

Figure 2

Crystal packing view along the c axis of (I). H atoms have been omitted
for clarity.

(see Experimental). Moreover, we also have a good argument
against such a contention, as discussed below. As previously
described (Sakai, Tanaka et al., 1998; Matsumoto & Sakai,1999), the intradimer PtIIÐPtII distance in this class of headÊ shorter than that for
to-head dimers is generally 0.02±0.05 A
the head-to-tail dimer having the same bridging amidate
ligands. Importantly, the donor±acceptor properties of PtII
centres are greatly affected by the electronegativities ofcoordinated atoms. In other words, the Pt atoms possess
different donor±acceptor properties owing to the effective
difference between the electronegativities of O and N. The
head-to-head isomer has a shorter intradimer PtIIÐPtIIdistance because it is an interaction between a good donor(N4-coordinated Pt) and a good acceptor (N2O2-coordinatedPt). On the other hand, the head-to-tail isomer has a longerPtÐPt distance because it is an interaction between the twoN3O-coordinated Pt atoms having an intermediate donor±
acceptor property. Such Pt  Pt interactions are considered tobe reinforced by a sort of dative bond formed between a ®lled
Pt 5 dz2 orbital and a vacant 6pz orbital (Nova et al., 1995;
Connick et al., 1997) (we call such bonds `mutual dativebonds'). These indicate that, if the two isomers coexist, thedisplacement ellipsoid of each Pt atom must show considerable distortion or elongation along the intradimer PtÐPt axis.
However, the displacement ellipsoids of the Pt centres areÊ 2 andobserved to be quite normal [Ueq(Pt1) = 0.0316 (2) A2
Ê ], supporting the conclusion that (I) isUeq(Pt2) = 0.0313 (2) Aa pure form of a head-to-head isomer.
Ê]The observed intradimer PtÐPt distance [3.0569 (8) Ashows that it is longer than those reported for the analogous
Ê for
PtII dimers bridged by other amidate ligands [2.8767 (7) Am560

Ken Sakai et al.  [Pt2(C7H8N2O)2(NH3)4](ClO4)4

the -pyridonate analogue (Hollis & Lippard, 1983);
Ê for the 5-carboxy- -pyridonate analogue (Sakai
2.9023 (8) AÊ for the -pyrrolidinonate& Takahashi, 2003); 3.029 (2) AÊ for the acetanalogue (Matsumoto et al., 1989); 2.89±2.93 A
amidate analogue (unpublished results)]. These suggest that
the PtIIÐPtII bond in the title compound is weaker than thosein the reported compounds. Our tentative interpretation isthat the positively charged N-methylpyridinium moiety in (I)plays a role as an electron-withdrawing group to diminish theelectron density at the PtII centres, leading to the weakening of`mutual dative bonds' within the diplatinum cation.
The dihedral angle between the two Pt coordination planes
within the dimeric unit () and the average torsional twist ofthem about the PtÐPt axis (!) are estimated as  = 37.7 (2)and ! = 0 . As observed thus far for related compounds, thetwo Pt atoms, Pt1 and Pt2, are displaced from their coordiÊ in such a manner
nation planes by 0.067 (5) and 0.049 (5) A
that they have an attractive interaction with one another. The
pyridinium plane (N4/C2±C6) is inclined by 33.3 (9) withrespect to the carbamoyl unit (O1/C1/N3).
Fig. 2 shows a crystal packing view of (I). The shortestÊ (see Table 1),interdimer Pt  Pt distance is 7.2325 (9) A
con®rming the lack of any intermolecular PtÐPt interaction inthe crystal. As shown in Fig. 2, the dimers form a zigzag chain
along the b axis, with -stacking interactions achieved
between the pyridinium units. Each stack is formed through aninversion centre, and the plane-to-plane separation isÊ . The crystal packing is also stabilized by hydrogen3.27 (1) Abonds formed between the ammines and the O atoms ofperchlorate ions (see Table 2).

Experimental
N-Methyl-4-carbamoylpyridinium perchlorate was prepared asfollows [this is quite similar to the synthesis of N-(carbamoylmethyl)pyridinium perchlorate (Sakai et al., 1997)]: a solution ofisonicotinamide (10 mmol) and methyl bromide (10.7 mmol) in
CH3CN (20 ml) was stirred at room temperature for 48 h. The
deposited colourless precipitate was collected by ®ltration and airdried. A solution of the bromide salt of N-methyl-4-carbamoylpyridinium (10 mmol) and an equivalent of AgClO4 (10 mmol) inwater (30 ml) was heated in the dark at 333 K for 1 h. After the AgBr
precipitate was removed by ®ltration, the ®ltrate was evaporated to atotal volume of ca 1 ml (Caution: Perchlorate salts are potentiallyexplosive and must be handled with care. Be careful not to evaporate
to dryness). A large excess of ethanol was added to the ®ltrate toreprecipitate the product. The product was recrystallized frommethanol to give the ®nal product as colourless needles (yield: 91%).
Analysis calculated for C7H9ClN2O5: C 35.53, H 3.83, N 11.84%;found: C 35.47, H 3.68, N 11.52%. 1H NMR (D2O, 296 K):  4.47 (s,3H); 8.36 (d, 2H, J = 6.1 Hz); 8.98 p.p.m. (d, 2H, J = 6.6 Hz).
Compound (I) was prepared as follows: to an aqueous solution ofcis-[Pt(NH3)2(OH2)2](ClO4)2 (0.2 mmol Pt/2 ml H2O), prepared aspreviously described (Sakai, Takeshita et al., 1998), was added
N-methyl-4-carbamoylpyridinium perchlorate (0.32 mmol). The
solution was heated at 333 K for 5 h. To the solution was added asolution of NaClO4 (6 mmol) in water (ca. 0.30 ml) while it was hot,followed by ®ltration if necessary. Allowing the ®ltrate to stand atroom temperature for a few days afforded (I) as pale yellow±green

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Acta Cryst. (2003). E59, m559±m561

metal-organic papers
plates (yield 8%). Analysis calculated for C14H28Cl4N8O18Pt2: C
14.90, H 2.50, N 9.93%; found: C 14.75, H 2.63, N 9.68%. 1H NMR(D2O, 296 K):  4.33±4.41 (m, 3H); 8.18±8.41 (m, 2H); 8.72±8.88 p.p.m.
(m, 2H).
Crystal data
[Pt2(C7H8N2O)2(NH3)4](ClO4)4
Mr = 1128.42
Orthorhombic, PnmaÊa = 17.9590 (7) AÊ
b = 17.0728 (7) AÊc = 10.1189 (4) A
Ê3
V = 3102.6 (2) A
Z=4
Dx = 2.416 Mg mÿ3

Mo K radiation
Cell parameters from 2336
re¯ections
 = 2.3±22.0
 = 9.44 mmÿ1
T = 296 (2) KPlate, pale yellow±green
0.06  0.05  0.03 mm

Data collection
Bruker SMART APEX CCDdetector diffractometer! scans
Absorption correction: multi-scan(SADABS; Sheldrick, 1996)Tmin = 0.540, Tmax = 0.753
16797 measured re¯ections

2317 independent re¯ections
1576 re¯ections with I > 2(I)Rint = 0.117
max = 23.3
h = ÿ19 ! 19
k = ÿ18 ! 18
l = ÿ11 ! 8Re®nementRe®nement on F 2
R[F 2 > 2(F 2)] = 0.035
wR(F 2) = 0.062
S = 0.88
2317 re¯ections
221 parameters

H-atom parameters constrainedw = 1/[ 2(Fo2) + (0.0194P)2]
where P = (Fo2 + 2Fc2)/3(/)max = 0.001
Ê ÿ3max = 1.43 e AÊ ÿ3
min = ÿ0.66 e ATable 1

Ê 2 and Ueq(N) = 0.022 (2) A
Ê 2 {the reliabilityUeq(O) = 0.060 (2) A
22factors were also high: R[F > 2 (F )] = 0.036 and wR(F2) = 0.065}.
On the other hand, the reported combination, judged to be correct,
Ê 2 andgave a moderate balance in these values: Ueq(O) = 0.047 (2) Aÿ2Ê
Ueq(N) = 0.032 (2) A . There are three independent ClO4 ions in theasymmetric unit; two are located on a mirror plane and one is locatedat a general position. One of the ClO4ÿ ions located on a mirror
shows an orientational disorder around the Cl3 atom. In this disordermodel, it was assumed that the Cl3 and O9 atoms on the mirror plane
are not disordered. The remaining three O atoms were considered tobe disordered over two sites (O10A and O11A) and (O10B andO11B), where both O11A and O11B are also located on the samemirror plane. These four disordered O atoms were supposed to havethe same isotropic displacement parameter. No other restraint wasapplied to this model. The occupation factors of site A and B
converged at 0.740 (9) and 0.260 (9), respectively. All H atoms wereÊ , aromatic
placed at their idealized positions (methyl CÐH = 0.96 A
ÊÊCÐH = 0.93 A and ammine NÐH = 0.89 A) and included in there®nement in the riding-motion approximation, with Uiso values
equal to 1.2±1.5 times the Ueq of the carrier atom. The highest peakÊ from O7, while the deepest hole was located
was located 2.21 A
Ê
0.56 A from O7.
Data collection: SMART (Bruker, 2001); cell re®nement: SAINT(Bruker, 2001); data reduction: SAINT; program(s) used to solve
structure: SHELXS97 (Sheldrick, 1997); program(s) used to re®nestructure: SHELXL97 (Sheldrick, 1997); molecular graphics: KENX(Sakai, 2002); software used to prepare material for publication:
SHELXL97, TEXSAN (Molecular Structure Corporation, 2001),
KENX and ORTEPII (Johnson, 1976).

Ê ,  ).
Selected geometric parameters (APt1ÐN3
Pt1ÐN1
Pt1ÐPt2

2.007 (7)2.049 (7)3.0569 (8)Pt2ÐN2Pt2ÐO1
Pt1  Pt2i

2.025 (6)2.038 (6)7.2325 (9)N3ÐPt1ÐN3ii
N3ÐPt1ÐN1ii
N3ÐPt1ÐN1N1iiÐPt1ÐN1N3ÐPt1ÐPt2N1ÐPt1ÐPt2

93.0 (4)176.2 (3)87.4 (3)91.9 (4)76.1 (2)107.7 (2)N2iiÐPt2ÐN2N2iiÐPt2ÐO1N2ÐPt2ÐO1O1ÐPt2ÐO1ii
N2ÐPt2ÐPt1O1ÐPt2ÐPt1

87.6 (4)177.0 (3)91.1 (3)90.1 (4)101.8 (2)81.15 (18)N1ÐPt1ÐPt2ÐN2

4.0 (3)N3ÐPt1ÐPt2ÐO1

ÿ2.6 (3)Symmetry codes: (i) x; y; z ÿ 1; (ii) x; 12 ÿ y; z.

Table 2

Ê ,  ).
Hydrogen-bonding geometry (ADÐH  ADÐHH  AD  ADÐH  AN1ÐH4  O4
N1ÐH4  O8
N1ÐH3  O10A
N3ÐH1  O4
N2ÐH5  O3iii
N2ÐH6  O2iv
N2ÐH5  O5iii
N2ÐH7  O11Bii

0.89
0.89
0.89
0.86
0.89
0.89
0.89
0.89

2.65
2.43
2.41
2.38
2.52
2.58
2.29
2.17

3.136 (11)3.213 (13)3.180 (12)3.118 (11)3.058 (11)2.988 (10)3.174 (10)2.96 (4)

116
146
146
143
119
109
171
148

Symmetry codes: (ii) x; 12 ÿ y; z; (iii) 12 ÿ x; ÿy; 12  z; (iv) x; y; 1  z.

The binding direction of O and NH for the bridging amidate could
be easily determined by comparing the results of least-squares
calculations performed for two possible arrangements as follows. Awrong combination gave an asymmetric feature with regard to theequivalent displacement parameters of the supposed O and N atoms;
Acta Cryst. (2003). E59, m559±m561

This work was partly supported by a Grant-in-Aid for
Scienti®c Research (No. 14340223) from the Ministry of
Education, Science, Sports, and Culture of Japan.

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Ken Sakai et al.  [Pt2(C7H8N2O)2(NH3)4](ClO4)4m561