[{(H3C)3NB(H)2NC}2Au][AuI2]: a linear chain polymer of gold(I) iodide with an unusual isocyanoborane ligand showing aurophilic behaviour

Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510, USA, UniversitaÈt TuÈbingen, Institut fuÈ r Anorganische Chemie, Auf der Morgenstelle 18, D-72076 TuÈbingen, Germany, Chemistry Department, Loughborough University, Loughborough, Leicestershire LE11 3TU, England, School of Chemistry, University of Southampton, Southampton SO17 1BJ, England, Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich NR4 7TJ, England, and University Chemical Laboratory, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, England


Comment
We have recently been interested in the formation of (isocyanide)gold(I) halide adducts, because of their propensity to interact aurophilically. The term aurophilicity is used to describe observed AuÁ Á ÁAu interactions. These intermolecular contacts have been shown to have bond energies and distances similar to those observed for classical hydrogen-bonding interactions (7.5±12.5 kcal mol À1 and 2.7±3.5 A Ê , respectively) (Schmidbaur, 1990(Schmidbaur, , 2000Mathieson et al., 2000). Hence, aurophilic behaviour is considered to be a major factor in determining the particular supramolecular motif which a series of monomers is observed to adopt. Our recent synthetic studies have involved the use of an unusual zwitterionic isocyanoborane species (L) (Andersen et al., 2001) (see scheme). The substitution reaction of [LAuCl], whereby chloride is replaced with iodide, has yielded (I), whose structure shows clear evidence for aurophilic effects directing the appearance of its extended structure.
Compound (I) crystallizes in the triclinic space group P1 (Z = 2). The asymmetric unit comprises one equivalent of the isocyanoborane donor species and a single iodide, each coordinated to crystallographically distinct gold cations Au1 and Au2, both of which are located on inversion centres (Fig. 1). Both Au1 and Au2 exhibit pseudo-square-planar coordination geometry, with bonding angles of 91.443 (11) (I1ÐAu1Á Á ÁAu2) and 97.1 (2) (C1ÐAu2Á Á ÁAu1 i ; symmetry code as in Table 1). Au1 is trans-coordinated by two equivalents of iodide; Au2 is also trans-coordinated, by isocyanide moieties. The coordination of each gold ion is completed by AuÁ Á ÁAu contacts with adjacent Au centres, where Au1Á Á ÁAu2 is a mere 3.0438 (7) A Ê , suggesting that signi®cant aurophilic character is present in (I). Literature values for observed AuÁ Á ÁAu contact distances suggest an approximate range of 4.1 A Ê (as often associated with the inter±dimer bonding in chains of dimers) to 2.9 A Ê for complexes similar in topology to (I).
A perfectly linear in®nite chain of gold atoms is thus formed, aligned parallel to the crystallographic a axis (Fig. 2). It can be seen that adjacent chains are displaced from each other along the b axis, thus forming a two-dimensional gridlike array of sheets. The B1ÐN1ÐC1 angle is 175.7 (7) , this portion of the coordinated isocyanoborane being almost linear. Adjacent iodide and isocyanide substituents are aligned approximately orthogonally to one another (Fig. 3). A network of classical (van der Waals) intermolecular interactions is formed primarily between methyl H atoms and adjacent I À atoms (Fig. 3).

Experimental
A solution of [LAuCl] (42 mg, 0.202 mmol) in dichloromethane (10 ml) was stirred vigorously with KI (51 mg, 0.307 mmol) in H 2 O (10 ml) over a period of 18 h. After removal of all solvent, the yellow±green residual solid was dissolved in dichloromethane (5 ml). Small light green shard-like crystals of (I) were grown from the solution by layering with heptane (1:1) and allowing slow evaporation of the solvent. For full experimental details and characterization data, see Humphrey et al. (2004).  View of (I), showing the chains of aurophilically bound gold centres running parallel to the crystallographic a axis. Aurophilic type bonds are drawn in red.

Figure 3
Projection of (I) on the bc plane, detailing the approximately orthogonal arrangement of the iodide and isocyanoborane substituents.
Methyl H (CÐH distance = 0.98 A Ê ) and BH 2 (BÐH distance = 0.99 A Ê ) atoms were placed in calculated positions using a riding model. U iso values were set to 1.2U eq of the parent atom for BH (1.5U eq for methyl H). The maximum and minimum difference map features were located 0.94 A Ê from Au1 and 0.81 A Ê from Au2, respectively.

Special details
Experimental. PLEASE NOTE cell_measurement_ fields are not relevant to area detector data, the entire data set is used to refine the cell, which is indexed from all observed reflections in a 10 degree phi range. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.