ARTICLE DOI: 10.1002/zaac.201200256 Hydrogen Bonding Network of 4-Amidiniumpyridine Acetate and Pt II bis(triphenylphosphine) Complexes with 4-Amidinepyridine Luca Rigamonti, [a,b] Stefano Carlino, [a] Carlo Castellano, [a] Francesco Demartin,* [a] and Alessandro Pasini* [a] Keywords: 4-Amidiniumpyridine; Hydrogen bonds; Crystal structure; Platinum Abstract. The X-ray structures of 4-amidiniumpyridine acetate, (1·H)AcO, and of cis-[Pt(1) 2 (PPh 3 ) 2 ](NO 3 ) 2 (2), as well as their IR spectra, reveal intramolecular hydrogen bonding, which held together the cations and the anions. The IR spectroscopic data suggest that this Introduction The amidine moiety and the amidinium cation can form hy- drogen bonds with a host of hydrogen acceptors. The latter cation is particularly prone to form such bonds via its NH 2 + groups, a behavior, which has many important consequences in various aspects of chemistry and biochemistry. [1,2] For in- stance, hydrogen-bonded interactions of the protonated amid- ine moiety of arginine with carboxylates occur in many bio- chemical processes, such as protein conformation, [3] substrate binding [2,4] and orientation, [5] or hydrogen bonds mediated electron-transfer reactions. [6,7] Besides carboxylates, [8,9,10] the amidinium cation can be hydrogen bonded to a host of other hydrogen acceptors, such as [PbBr 4 ] 2– , [11] phosphinates, [12] phosphate, [13] chloride, [14] sulfate, [15] tetrazolate, [16] and sul- fide. [17] The hydrogen bonding network may extend over a whole crystal, forming strong and directional charge-assisted N–H···A bonds (A = hydrogen bond acceptor), therefore amid- inium cations are widely used as building blocks in crystal engineering, as evidenced by the X-ray structures of several examples. [9,10,18,19,20,21] In the course of our studies on metal complexes of methane- diamine derivatives, such as their Schiff bases with salicylalde- hyde, [22,23] we had the occasion of synthesizing 4-amidinepyri- dine (1), which was crystallized as the acetate, (1·H)AcO. Its reaction with cis-[PtX 2 (PPh 3 ) 2 ] (X = NO 3 , Cl) gave cis-[Pt(1) 2 (PPh 3 ) 2 ](NO 3 ) 2 (2), and cis-[PtCl(1)(PPh 3 ) 2 ](BF 4 ) * Prof. Dr. F. Demartin E-Mail: francesco.demartin@unimi.it * Prof. Dr. A. Pasini E-Mail: alessandro.pasini@unimi.it [a] Università degli Studi di Milano Dipartimento di Chimica Via Golgi 19 20133 Milano, Italy [b] Università degli Studi di Firenze Dipartimento di Chimica ‘U. Schiff’ Via della Lastruccia 3–13 50019 Sesto Fiorentino (FI), Italy © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2012, 638, (14), 2252–2256 2252 may be so also in cis-[PtCl(1)(PPh 3 ) 2 ](BF 4 )(3). In (1·H)AcO and in 2 extensive intermolecular hydrogen bonding networks span through the whole crystals. (3). The X-ray structures of (1·H)AcO and 2, revealed a com- plex intra and intermolecular H-bonding network; we wish to describe hereafter these results. Result and Discussion 4-Amidiniumpyridine acetate, (1·H)AcO was synthesized by a slight modification of previously described methods, [13,24] i.e. condensation of 4-cyanopyridine with methanol, and sub- sequent substitution with ammonia. Diffusion of diisopropyl ether into the methanol reaction mixture gave very small crys- tals suitable for X-ray diffraction studies, even if weakly dif- fracting. This compound is characterized in the solid state by the presence of ionic pairs made by the cationic amidinium group and the acetate anion, yielding an almost planar eight- membered ring C2–O1···H–N1–C3–N2–H···O2 (Figure 1) originated by the two parallel N–H···O bonds between the two NH 2 + groups and the two carboxylate oxygen atoms. The acid- base adduct in this compound shows, as foreseen, a complete delocalization within the amidinium fragment [C3–N1 = 1.312(3), C3–N2 = 1.312(3) Å] as well as within the carboxyl- ate group [C2–O1 = 1.252(3), C2–O2 = 1.248(3) Å]. These values are comparable with those reported for other amidinium carboxylates. [19] The pyridine ring is not coplanar with the O1, Figure 1. The asymmetric unit of 4-amidiniumpyridine acetate (1·H)AcO. Displacement ellipsoids are drawn at the 50 % probability level.