Reactivity of Mononuclear and Dinuclear Gold(i) Amidinate Complexes with Cs 2 and Csbr 3

To probe the reactivity of gold-nitrogen bonds, we have examined the insertion chemistry with carbon disulfide (CS2) as well as oxidation with cesium tribromide (CsBr3) with Au(I) amidinate complexes. The reaction of Ph3PAuCl with Na[(2,6-Me2C6H3N)2C(H)] yields the mononuclear, two-coordinate gold(I) complex, Ph3PAu[κ 1 was isolated with formation of Ph3PBr2. We also compared the reactivity of CS2 and CsBr3 with 3. Carbon disulfide insertion with 3 produces a dimeric product, Aun[CS2(2,6-Me2C6H3NC(H)=NC6H3Me2)]n, 4, featuring a dinuclear core with linking aurophilic interactions, making it appear polymeric in the solid state. When CsBr3 is reacted with 3 the Au(II,II) product is obtained, Au2[(2,6-Me2C6H3N)2C(H)]2(Br)2, 5.


Introduction
The chemistry of gold is dominated by soft donor ligands due to the propensity for gold to form stronger interactions with elements such as phosphorus and sulfur.Hence, gold-nitrogen bonds are uncommon [1] and the reactivity of these bonds is relatively unexplored [2].With the recent advances in gold amidinate chemistry [3], we sought to develop this reactivity by examining insertion and oxidative chemistry in Au(I) amidinate complexes.
In addition to soft donor ligands, gold chemistry is influenced by aurophilic interactions [16][17][18] in which the distance between two gold atoms is less than the sum of their van der Waals radii.These interactions can be quite strong and affect the coordination chemistry and chemical properties of gold [19][20][21].

Results and Discussion
The amidine ligand is deprotonated with NaN(SiMe3)2 and allowed to stir for approximately one hour before addition of Ph3PAuCl therefore we do not anticipate the formation of Ph3PAuN(SiMe3)2 [25].Reaction of Ph3PAuCl with Na[(2,6-Me2C6H3N)2C(H)] does not form the dinuclear gold product that is formed with (tht)AuCl, Equation (2).
(2) Instead, a different 1 H NMR spectrum was obtained which showed an asymmetric amidinate environment and had a 31 P NMR signal at 35.8 ppm in benzene-d6.This species is sparingly soluble in diethyl ether from which a saturated solution yielded colorless crystals suitable for X-ray diffraction analysis.The structure revealed the two-coordinate Au(I) complex, Ph3PAu[(2,6-Me2C6H3N)2C(H)], 1, Figure 1.The Au-P and Au-N bond distances of 2.2297(4) and 2.0331(4) Å, respectively, are typical for a phosphine and amide coordinated to a Au(I) center, Table 1.For example, the Au-N bond lengths in IPrAuN i Pr2 [26]   We then turned our attention to the reactivity of 1.The reaction of 1 with CS2 produced a color change from colorless to yellow, Equation (3). (3) The 1 H NMR spectrum revealed an asymmetric ligand environment with two different methyl resonances and a new 31 P NMR resonance at 36.6 ppm.Due to the decreased solubility in arene solvents, the 1 H NMR spectrum was obtained in CDCl3.The proton on the formamidinate backbone shifted downfield to 9.64 ppm, which is in the region observed when a localized imine bond is formed instead of a delocalized bond over the N-C-N backbone.Additional evidence of this was detected in the IR spectrum with a shift from 1605 cm −1 for the NCN stretch in 2, were obtained from a saturated diethyl ether solution, and a two-coordinate species is also observed with the insertion of CS2 into the Au-N bond and formation of the dithiocarbamate.Similar to 1, the Au-P bond distance is 2.2581(2) Å and the Au-S bond distance of 2.3247(2) Å is typical for Au(I) dithiocarbamates.For reference, the Au-S bond distances in (IPr)Au(S2CNEt2) and (IPr)Au[S2CN(CH2Ph)2], IPr = 1,3-bis(2,6diisopropylphenyl)imidazole-2-ylidene, are 2.314(5) and 2.2999(12) Å, respectively [27], and 2.312(3) Å in Ph3PAu(indazol-1-yldithiocarbamate) [28].With the recent use of CsBr3 to oxidize Au(I) to Au(III), we attempted a similar reaction with 1 [29].Indeed, reaction of 1 with CsBr3 is accompanied by a color change to red-brown; however, upon workup the 1 H NMR spectrum revealed the presence of the dinuclear Au(I) amidinate, Au2[(2,6-Me2C6H3N)2C(H)]2, 3, as well as resonances consistent with Ph3PBr2, Equation (4).Further, the 31 P NMR spectrum also confirmed the phosphorus-based product as Ph3PBr2 [30].The red-brown color may be due to the possible formation of Au2[(2,6-Me2C6H3N2)C(H)]2(Br)2 which has been previously reported [5].We rationalize that the products of this reaction result from the precipitation of CsBr, formation of the stronger P-Br bonds rather than Au-Br, and subsequent aurophilic interaction that arises.
(4) The reactivity of dinuclear Au(I) amidinate complex, Au2[(2,6-Me2C6H3N)2C(H)]2, 3, has not been reported with CS2 or CsBr3 and, therefore, we attempted these substrates to compare with the mononuclear species.Reaction of 3 with CS2 in THF gave a color change to yellow, and, over time, a red solid precipitated.The 1 H NMR spectrum showed an asymmetric ligand environment with two distinct methyl resonances and a downfield shift of the formamidinate backbone proton to 9.25 ppm.The IR spectrum also revealed an imine stretch at 1648 cm −1 .The product is soluble in chloroform and THF, forming a yellow solution but then slowly precipitates as a red solid.Red crystals were obtained from a heated saturated THF solution and allowed to cool to room temperature, and showed the structure, {Au2[CS2(2,6-Me2C6H3NC(H)=NC6H3Me2)]2}n, 4, Figure 3. Interestingly, 4 has a dinuclear core, Figure 4, with one CS2 inserting into one Au-N bond.The Au-Au bond distances within each molecule are 2.76154(5) and 2.75717(19) Å, and are much shorter than the 3.00745(7) Å aurophilic interaction that links the two dinuclear fragments, Table 1.Over time, aurophilic interactions, which have comparable strength to a hydrogen bond, start to form which could account for the solubility issues and precipitation from solution.The stacking of discrete dimeric units in the solid-state is not new in gold dithiocarbamate chemistry [28,[31][32][33][34][35].The reaction of 3 with CsBr3 produced a dark brown colored solution and the 1 H NMR spectrum showed the formation of the previously reported Au2[(2,6-Me2C6H3N2)C(H)]2(Br)2, 5, albeit in low yield.While the mononuclear product afforded the oxidative addition product, 3 produces a one-electron oxidation from both metal centers yielding two Au(II) centers.This difference in reactivity demonstrates the stability of the amidinate ligand framework in the dinuclear system.Substrates such as PhNCO and CO2 do not insert into the amidinate framework, presumably due to the stability of the dinuclear core, as well as the preference of gold for soft donor atoms, not hard atoms, such as oxygen.

General Considerations
Even if air-stable, the syntheses described were carried out under inert atmosphere (N2) using glove box or standard Schlenk techniques unless otherwise noted.Solvents were distilled under nitrogen and kept over drying sieves.NaN(SiMe3)2, anhydrous carbon disulfide (Aldrich, Milwaukee, WI, USA) and Ph3PAuCl (Strem, Newportbury, MA, USA) were used as received.The amidine ligand, [4], and CsBr3 [29] were prepared as previously described.Benzene-d6 and chloroform-d1 (Cambridge Isotope Laboratories, Tewksbury, MA, USA) were dried over molecular sieves and degassed with three freeze-evacuatethaw cycles.All 1 H, 13 C, and 31 P NMR data were obtained on a 300 MHz DRX or 500 MHz DRX Bruker (Madison, WI, USA) spectrometer.Infrared spectra were recorded as KBr pellets on Perkin-Elmer (Waltham, MA, USA) Spectrum One FT-IR spectrometer.

X-ray Crystallography Details.
A selected single crystal of 1, 2, and 4 was mounted on nylon cryoloops using viscous hydrocarbon oil.X-ray data collection was performed at 100 (2) or 173(2) K.The X-ray data were collected on a Bruker (Madison, WI, USA) CCD diffractometer with monochromated Mo-Kα radiation (λ = 0.71073 Å) with data collection and processing using the Bruker Apex2 suite of programs [37].The structures were solved using direct methods and refined by full-matrix least-squares methods on F 2 using Bruker SHELEX-97 program [38].All non-hydrogen atoms were refined with anisotropic displacement parameters and all hydrogen atoms were added on idealized positions and not allowed to vary.Thermal ellipsoid plots were prepared by using X-seed [39] at the 50% probability level for nonhydrogen atoms.Crystal data and detail for data collection for complex 1, 2, and 4 is provided in Table 2.

Conclusions
A rare mononuclear gold(I) amidinate complex has been synthesized from reaction of Ph3PAuCl with Na[(2,6-Me2C6H3N)2C(H)], and its reactivity explored with CS2 and CsBr3.Addition of CS2 to Ph3PAu[κ 1 -(2,6-Me2C6H3N)2C(H)] results in the facile insertion into the gold-nitrogen bond to produce the dithiocarbamate species which was also found to also be two-coordinate.The reaction of CsBr3 with Ph3PAu[κ 1 -(2,6-Me2C6H3N)2C(H)] led to the formation of Ph3PBr2 and the dinuclear amidinate complex, Au2[(2,6-Me2C6H3N)2C(H)]2. For comparison, the dinuclear gold amidinate complex was examined with CS2 and CsBr3.Carbon disulfide also inserted into the gold-nitrogen bonds of Au2[(2,6-Me2C6H3N)2C(H)]2 which maintained its dinuclear form but polymerized through aurophilic interactions between neighboring fragments in the solid-state.The CsBr3, oxidation did occur to the form the dinuclear Au(II,II) product, Au2[(2,6-Me2C6H3N)2C(H)]2(Br)2.The objective of this work was to examine the reactivity of a mononuclear gold(I) amidinate complex, which led to the comparison with the dinuclear gold(I) compound.Our results further demonstrate the tendency for gold to form aurophilic interactions as well as bonds with softer ligands, even when in a dinuclear framework.

Figure 1 .
Figure 1.Thermal ellipsoid plot of 1 shown at the 50% probability level.The hydrogen atoms have been omitted for clarity.

Figure 2 .
Figure 2. Thermal ellipsoid plot of 2 shown at the 50% probability level.The hydrogen atoms have been omitted for clarity.

Figure 3 .
Figure 3. Thermal ellipsoid plot of 4 shown at the 50% probability level.Hydrogen atoms have been omitted for clarity.

Figure 4 .
Figure 4. Thermal ellipsoid plot of two units of 4 shown at the 50% probability level.Hydrogen atoms have been omitted for clarity.