Tetraethylammonium tricarbonylchlorido(isoquinoline-1-carboxylato-κ2 N,O)technetate(I)

The asymmetric unit of the title compound, (C8H20N)[Tc(C10H6NO2)Cl(CO)3], consists of two crystallographically independent technetium complexes related via a pseudo-inversion centre and two tetraethylammonium cations. The Tc atoms have slightly distorted octahedral coordination geometries, and they are linked with the cations by intermolecular C—H⋯O and C—H⋯Cl hydrogen-bonding contacts, forming two-dimensional columns, which lie approximately parallel to (001) in the crystal structure. The isochinolate (isoquinoline-1-carboxylate) ligands link the columns by partial π–π stacking [centroid–centroid distance 4.3733 (11) Å], forming a three-dimensional network structure.

The asymmetric unit of the title compound, (C 8 H 20 N)-[Tc(C 10 H 6 NO 2 )Cl(CO) 3 ], consists of two crystallographically independent technetium complexes related via a pseudoinversion centre and two tetraethylammonium cations. The Tc atoms have slightly distorted octahedral coordination geometries, and they are linked with the cations by intermolecular C-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen-bonding contacts, forming two-dimensional columns, which lie approximately parallel to (001) in the crystal structure. The isochinolate (isoquinoline-1-carboxylate) ligands link the columns by partial stacking [centroid-centroid distance 4.3733 (11) Å ], forming a three-dimensional network structure.

Comment
The chemistry of technetium is well established for diagnostic purposes, where tricarbonyl Tc-99m and Tc-99m dicarbonylnitrosyl complexes are employed (Alberto et al., 1996;Rattat et al., 2001). The title compound, (I), is one of several synthesized using isochinolinic acid under a variety of ligands in order to study their coordination or complexation properties (Alberto et al. 1996). Clinical or biological test results are summarized in Waibel et al. (1999).
A careful check for missed symmetry in structures that crystallize in space group P1 with Z' = 2 is always good practice, as can be judged in two out of many critical articles: in his classic review, Marsh (1995) detected about 20 structures (described in space group P1 with Z = 4) which were shown to be better described by the monoclinic space groups No. 14 and 15; and some 60 space-group corrections have been reported (Marsh et al. 2002) of which four displayed the same missed symmetry.
In (I), a pseudo-inversion centre (Fig. 1) and a pseudo-translation ( Fig. 2) were found during the checking process using the programme PLATON (Spek 2003).
Many homomolecular crystal structures were analysed with respect to non-crystallographic symmetry and superstructure effects, where only about 20 structures out of 1166 with Z = 4 showed local pseudo-centres of symmetry (Desiraju et al., 1991). The title compound belongs to a similar, heteromolecular class of structures with pseudoinversion centres, Z = 4 and with space group P1.
The pseudo-inversion related carbonyl atoms are found to point towards each other. The isochinolato ligands in both Tc complexes have bidentate coordination to the Tc atom via the aromatic amine N and carboxylate O atoms. This ligand, together with the two carbonyl ligands forms a distorted square planar environment around the Tc atom. The Cl atom and another carbonyl ligand are positioned trans to one another and complete the octahedral coordination geometry. The Tc coordination distances and angles are in comparable ranges, e. g. for Tc-CO (1.914 (7) -1.927 (6) Å, Alberto et al. 1995) and 1.894 (3) -1.912 (3) Å Alberto et al. 1996). Corresponding distances in the title complex vary between 1.905 (2) and 1.916 (2) Å, with an average value of 1.910 Å ( Table 1).
The hydrogen bonding contacts in (I) can be described with the graph set descriptors D, C and R (Etter et al. 1990;Bernstein et al. 1995), with tetraethylammonium cations N1 and N2 as donor and the Tc complexes Tc1 and Tc2 as acceptor units. The tetraethylammonium donor group N1 is linked to acceptor atoms O31 (carbonyl), Cl1 and both carboxyl atoms O41, O51 of complex Tc1 to form weak intermolecular C-H···O and C-H···Cl hydrogen bonds (Desiraju & Steiner, 1999) with the graph set pattern D (Bernstein et al. 1995). Similar C-H···O and C-H···Cl hydrogen bond parameters (Table 2) were reported for tetrakis(pyridine) platinum(II) chloride trihydrate (Steiner & Saenger, 1993). The donor group of tetraethylammonium N2 is linked to both Tc complexes: Atom Cl1 is a bifurcated acceptor in complex Tc1, and the carboxylate group in complex Tc2 is an acceptor for the three-centre donor H19C in cation N2 (Table 2, Figure 3).

Experimental
Caution! Tc-99 is a weak β-emitter with a half life of 2.12 × 10 5 years. Although radiation from low amounts of material is absorbed completely by the glass walls, reactions should only be carried out in specially equipped laboratories and under well ventilated hoods to avoid contamination or ingestion. Synthesis of the adduct [NEt 4 ] 2 [TcCl 3 (CO) 3 ] was prepared as described previously (Alberto et al. 1996). [NEt 4 ] 2 [TcCl 3 (CO) 3 ] was dissolved in methanol and 1 equivalent of isoquinolinic acid added to the solution. Stirring at room temperatur for about 4 h resulted in the quantitative formation of the title compound as observed from HPLC monitoring. The colour of the solvent turned to yellow. Methanol was evaporated and the residue taken up in THF. Slow evaporation of the THF gave yellow plates of (I) of good x-ray quality.

Refinement
The missing cusp of data (alert level A in the PLATON checkcif) is due to data collection by rotation around the spindle axis only. This was standard at the time when the data were collected (in 1998) on a Stoe IPDS1 image-plate system.
All the hydrogen atoms were geometrically placed (C-H = 0.95-0.99 Å) and refined as riding with U iso (H) = 1.2(U eq (C) or 1.5U eq (methyl C). Fig. 1. A view of the molecular structure of (I) with displacement ellipsoids for the non-hydrogen atoms drawn at the 30% probability level (H atoms omitted for clarity).  Tetraethylammonium tricarbonylchlorido(isoquinoline-1-carboxylato-κ 2 N,O)technetate(I)