Communication www.rsc.org/chemcomm CHEMCOMM [Ni(R 2 pipdt) 2 ](BF 4 ) 2 (R 2 pipdt = 1,4-disubstituted-piperazine-3,2-dithione) as useful precursors of mixed–ligand dithiolenes of interest for non-linear optics†‡ Francesco Bigoli, a Chin-Ti Chen, b Wei-Ching Wu, b Paola Deplano,* c Maria Laura Mercuri, c Maria Angela Pellinghelli, a Luca Pilia, c Gloria Pintus, c Angela Serpe c and Emanuele F. Trogu a a Dipartimento di Chimica Generale ed Inorganica, Chimica Analitica, Chimica Fisica, CSSD CNR, Parco Area delle Scienze 17A, I-43100 Parma, Italy b Institute of Chemistry, Academia Sinica, Taipei, Taiwan 11529, ROC c Dipartimento di Chimica Inorganica ed Analitica, Università di Cagliari, Cittadella di Monserrato, I-09042 Monserrato, Cagliari, Italy. E-mail: deplano@vaxca1.unica.it Received (in Cambridge, UK) 9th July 2001, Accepted 27th September 2001 First published as an Advance Article on the web 17th October 2001 A simple method to obtain in high yields mixed-ligand nickel–dithiolene complexes, which show strong negative solvatochromism and negative first molecular hyperpolariz- ability, and the use of Raman spectroscopy to establish the extent of electronic delocalisation in these complexes, are reported. Metal–dithiolenes represent a class of compounds of interest in several fields of materials chemistry. 1 Symmetrical bis-dithio- lene nickel complexes exhibit an intense electronic transition at low energies assigned to a p?p* transition between the HOMO and the LUMO, delocalised over both the ligands (R = RA, Scheme 1). This extensive electron-delocalization, while making these complexes useful as near-infrared (NIR) dyes, is irrelevant to intramolecular charge transfer, a crucial factor in generating second order non-linear optical (NLO) properties. It is known 1c that donor substituents in the parent dithiolene [Ni(edt) 2 ] (edt = ethylenedithiolate) raise the energy of the HOMO more than that of the LUMO, with a consequent shift of the low-energy band to lower frequencies. However, very strong donor substituents may also force the HOMO to become antibonding and the loss of one or two electrons may be observed, but examples of cations are extremely rare. 1c On the other hand, electron-withdrawing substituents lower the energy both of the HOMO and the LUMO, and the latter MO may become bonding, leading to the formation of anions, as observed in several cases (mnt, 2a tdas, 2b qxd, 2c dmit, 2d dsit, 2e tkr 2f derivatives, Scheme 2). When R ≠ RA (unsymmetrical complexes) where R is an electron donating substituent and RA an electron withdrawing substituent, the resonance form A (Scheme 1) may dominate the ground state. In this case the complexes can be described as dithione–dithiolate derivatives with the RA containing ligand contributing more to the HOMO and R containing one likewise to the LUMO. Thus the electronic transition between the HOMO and the LUMO will have intramolecular CT character, making unsymmetrical complexes potential second-order NLO chromophores. 3 However [Ni(R 2 C 2 S 2 )(RA 2 C 2 S 2 )] complexes are extremely rare, 4 mainly for the difficulties encountered in the synthetic procedure and in their purification. Among them [Ni(R 2 - pipdt)(RA 2 C 2 S 2 )] [R 2 C 2 S 2 = 1,4-dialkylpiperazine-3,2-dithione (R 2 pipdt); RA 2 C 2 S 2 = maleonitriledithiolate (mnt) and tri- fluoromethyldithiolate (tfd)] nickel complexes have been prepared by reacting the free R 2 pipdt ligand with the neutral complexes [Ni(mnt)(NH 3 ) 2 ] and [Ni(tfd) 2 ], respectively. These complexes show strong negative solvatochromism and negative first molecular hyperpolarizability (b). 5 We report here a simple method to prepare several complexes belonging to this class of potential second-order NLO chromo- phores, some of them not reported before, by reacting a salt of a nickel–dithiolene dication [Ni(R 2 pipdt) 2 ](BF 4 ) 2 ] 1 with a salt of a nickel–dithiolene dianion (Bu 4 N) 2 [Ni(S 2 C 2 RA 2 ) 2 ] 2 as shown in Scheme 2. [Ni(Me 2 pipdt) 2 ](BF 4 ) 2 (R = Me, 1A) has been characterized. Structural data 6 show that the most significant bond distances and angles in the [Ni(Me 2 pipdt) 2 ] 2+ cation fall in the typical range of square-planar nickel–dithiolene complexes (Fig. 1). This cation has been briefly mentioned in the literature, 1c but data on its characterisation were missing. The cyclic voltammogram of 1A is in agreement with a dithiolene description of this cation, showing four reversible redox steps which are ascribed to mono-electronic processes leading 1A from the dicationic to the dianionic species. All the obtained mixed-ligand compounds (3a–f) shown in Scheme 2 have been characterized by analytical, spectroscopic and electrochemical methods. [Ni(Me 2 pipdt)(mnt)] (R = Me, 3aA) has been also structurally characterised 6 (Fig. 2). Com- plexes 3aA–fA exhibit a low-frequency peak, which shows negative solvatochromism, above 700 nm. Solvatochromic measurements and dipole analysis have been performed on [Ni(Pr i 2 pipdt)(dmit)] as described in ref. 5. In Table 1 a comparison of these results with those of [Ni((EH) 2 - † Electronic supplementary information (ESI) available: experimental section. See http://www.rsc.org/suppdata/cc/b1/b106064n/ ‡ Dedicated to Prof. P. Cassoux, CNRS, Toulouse, for his 60th birthday. Scheme 1 Scheme 2 This journal is © The Royal Society of Chemistry 2001 2246 Chem. Commun., 2001, 2246–2247 DOI: 10.1039/b106064n