Transition metal complexes containing the S(N t Bu) 42 − tetraimidosulfate dianion †‡

Three novel metal complexes [(acac) 2 Cu 2 (N t Bu) 4 S] ( 3 ), [Li(thf) 4 ] 2 [I 4 Cd 2 (N t Bu) 4 S] ( 4 ) and [(thf) 2 Li{(SiMe 3 ) 2 N}- Zn(N t Bu) 4 S] ( 5 ) are prepared from the intended transmetalation of the dilithium complex of N , N ’ , N ’’ , N ’’’ - tetrakis( tert -butyl)tetraimidosulfate [(thf) 4 Li 2 (N t Bu) 4 S] ( 1 ). The two lithium cations are replaced by either the cationic (acac)Cu( II ) moiety, the neutral I 2 Cd( II ) residue or only a single lithium cation is substituted by the cationic (Me 3 Si) 2 NZn( II ) fragment. The complexes show two main results: ﬁ rst the S(N t Bu) 42 − tetra-hedron can serve as a ligand to transition metals from the soft Cu( II ) to the harder Zn( II ) at opposite sides and second the S – N bond distances vary only marginally in response to the various metals and the four distances constantly sum up to 6.38(2) Å. Hence the electropositive sulfur atom responds by internal shift to the metal-polarized negative charge at the outside of the S(NR) 42 − tetrahedron.


Synthetic and structural studies
The copper complex [(acac) 2 Cu 2 (NtBu) 4 S] (3) can be isolated from the reaction of copper acetylacetonate with [(thf ) 4 Li 2 (NtBu) 4 S] (1) according to Scheme 3. Two equivalents of lithium acetylacetonate precipitate and are removed by filtration.After one week in THF at −24 °C colorless blocks, suitable for X-ray structure determination, were obtained at a yield of 59%. 3 crystallizes in the monoclinic space group P2/n with half of the molecule and one THF molecule in the asymmetric unit.
Each copper(II) atom is fourfold coordinated by the two oxygen atoms of the planar chelating acetylacetonate anion and by two nitrogen atoms of two opposite sides of the S(NtBu) 4 2− tetrahedron.The fourfold coordination at the Cu(II) atom can be described as a nearly square planar environment (O1-Cu1-N1: 169.79°,O2-Cu1-N2: 169.33°).This differs considerably from the nearly tetrahedral N 2 O 2 -coordination of the lithiated starting material 1 (Fig. 1 and 2).The two crystallographically independent S-N bond lengths in 3 (1.59Å) do not differ significantly from the S-N bond lengths in 1 (1.60 Å) and are half-way between the values normally quoted for a typical S-N single bond (1.69 Å) 19 and a S-N double bond (1.52 Å). 19 However, for none of the investigated S-N bonds in methyl(diimido)sulfinic acid H(NtBu) 2 SMe (1.68 and 1.58 Å), methylene-bis(triimido)sulfonic acid H 2 C{S(NtBu) 2 (NHtBu)} 2 (1.52 to 1.65 Å), sulfurdiimide S(NtBu) 2 (1.54 and 1.53 Å), and sulfurtriimide S(NtBu) 3 (1.51Å), a classical double bond formulation could be supported from charge density investigations. 20This was further substantiated by the NBO/NRT approach.Valence expansion to more than eight electrons at the sulfur atom can definitely be excluded to explain the bonding. 17,21The same was shown recently for the sulfate anion, SO 4 2− , as well by charge density based both on experimental and theoretical methods. 22ue to the similar bond lengths, equal distribution of the two negative charges over the four nitrogen atoms of the S(NR) 4 2− -ligand is assumed.Furthermore, the N⋯M coordi- This can be explained by the similar cationic radius of Cu(II) and Li(I) (Cu 2+ : 0.71 Å, Li + : 0.73 Å). 23 In published complexes with coordinated metal acetylacetonate at the nitrogen atom the N⋯M distances are 2.17 Å on average but the Cu(acac) + cation in a fourfold coordination sphere attains N⋯M distances of 1.96 Å 24 which is in excellent agreement with this result.
In the reaction of [(thf   It is interesting to note that all three S-N bonds in all known metal complexes of the S-alkyltriimidosulfonates [RS-(NR) 3 ] − (M = Li, Ba, Al, Zn) and in the triimidosulfonic acid MeS(NtBu) 2 NHtBu constantly sum up to 4.70(2) Å.The SN 3 unit responds flexibly to different electronic requirements induced by either different metal cations or conjugated S-substituents in terms of the sulfur atom being shifted relative to an otherwise fixed N 3 environment.This seems to be valid for the S(VI)-N bonds as well and experimentally emphasizes the predominantly ionic S-N bonding rather than valence expansion and d-orbital participation in bonding. 26The four crystallographically independent S-N bond lengths average to 1.59 Å in 4 (Fig. 3

Structural comparison
From the comparison of the three compounds it is interesting to note that in 2 the Cd-N bonds are longer than the M-N bonds in the other complexes, even longer than anticipated by the higher radius.They are further widened due to the lower electrostatic interactions to the neutral CdI 2 moieties compared to the otherwise cationic parts.Nevertheless, the complex to be formed indicates a certain binding affinity of the S(NR) 4 2− ligand even to neutral residues.
As in the metal S-alkyltriimidosulfonates RS(NR) 3 − also in the metal tetraimidosulfates S(NR) 4 2− the sum of the S-N bond distances seems to be almost invariant to the metal coordination (4.70(2) in the first and 6.38(2) Å in the latter).In the rigid framework of the four electron-rich imido nitrogen atoms the electropositive sulfur atom is pulled towards the nitrogen atoms coordinated by the Li + because they remain more attractive as the Li + loses the competition for imide nitrogen density against the Zn 2+ and there remains more density at the LiN 2 site of the SN 4 tetrahedron.Hence the sulfur atom inside the N 4 -cage responds to the metal-polarized negative charge at the outside of the S(NR) 4 2− dianion.This again emphasises predominantly ionic S-N bonding, reminiscent to S-O bonding in sulfate. 22,29perimental section

General procedure
All experiments were performed either in an inert gas atmosphere of purified dry argon with standard Schlenk techniques 30,31 or in an argon glove box.The glassware was dried at 130 °C, assembled hot and cooled under reduced pressure.All solvents were dried over appropriate alkali metals, distilled and degassed prior to use.All NMR spectra were either recorded on a Bruker Avance DPX 300 MHz or Bruker Avance DRX 500 MHz spectrometer using TMS ( 1 H, 13 C and 29 Si) and LiCl ( 7 Li) as external reference and the protons of the deuterated solvents as internal standard.The spectra were measured at room temperature if not stated otherwise.Elemental analyses (C, H, N and S) were carried out at the Mikroanalytisches Labor, Institut für Anorganische Chemie, Universität Göttingen.

Single-crystal structural analysis
Single crystals were selected from a Schlenk flask under argon atmosphere and covered with perfluorated polyether oil on a microscope slide, which was cooled with a nitrogen gas flow supplied by the X-TEMP2 device. 32An appropriate crystal was selected using a polarizing microscope, fixed on the tip of a MiTeGen © MicroMount, transferred to a goniometer head, and shock cooled by the crystal cooling device.The data for 3, 4, and 5 were collected from these shock-cooled crystals at 100(2) K.The data for 3 and 4 were collected on an Incoatec Mo microfocus source 33 equipped with Helios mirror optics and an APEX II detector at a D8 goniometer.The data for 5 were measured on a Bruker TXS Mo rotating anode with Helios mirror optics and an APEX II detector at a D8 goniometer.Important data are summarized in Table 1.Both diffractometers used Mo K α radiation, λ = 0.71073 Å.The data for all structures were integrated with SAINT, 34 and an empirical absorption correction (SADABS) 35 was applied.The structures were solved by direct methods (SHELXS-97) 36 and refined by full-matrix least-squares methods against F 2 (SHELXL-97) 19,37 within the SHELXLE GUI. 38All non-hydrogen atoms were refined with anisotropic displacement parameters.The hydrogen atoms were refined isotropically on calculated positions using a riding model with their U iso values constrained to equal 1.5 times the U eq of their pivot atoms for terminal sp 3 carbon atoms and 1.2 times for all other carbon atoms.Disordered moieties were refined using bond lengths and angles restraints and anisotropic displacement parameter restraints.Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre.The CCDC numbers, crystal data and experimental details for the X-ray measurements are listed in Table 1.

Conclusion
The Thus, these previous results indicate that S(NR) 4 2− can be coordinated by main group metals such as lithium and barium, but the coordination to transition metals remained unknown.Hence we now embarked to synthesize d-block metal complexes, selecting late transition metals first as they resemble alkaline and alkaline earth metal properties best.By the preparation and isolation of [(acac) 2 Cu 2 (NtBu) 4 S] (3), [Li(thf) 4 ] 2 [I 4 Cd 2 (NtBu) 4 S] (4) and [(thf) 2 Li(N(SiMe 3 ) 2 )Zn(NtBu) 4 S]
).While [(thf ) 4 Ba 2 {N(SiMe 3 ) 2 } 2 {(NtBu)4 S}] (2) is the product of a transmetalation using barium-hexamethylsilylamide from [(thf ) 4 Li 2 (NtBu)4 S] (1) via a hydrogenation first (Scheme 2)17 we now report a transmetalation without the previous generation of a protonated species.Firstly, lithium hexamethylsilylamide and zinc chloride were reacted under elimination of lithium chloride to give the assumed intermediate Zn(Cl)N-(SiMe 3 ).Subsequently, this intermediate gives the first isolated heterobimetallic compound [(thf ) 2 Li{(SiMe 3 ) 2 N}Zn(NtBu) 4 S] (5) after addition of 1 (Scheme 5).5 crystallizes from THF-toluene within four days at −24 °C, to give colorless blocks in a 34% yield, which were suitable for X-ray structure analysis.The compound crystallizes in the monoclinic space group C2/c with half a molecule per asymmetric unit.At one site of the S(NtBu)4  2− dianion a lithium ion remains coordinated like in the starting material and at the other site the zinc atom is complexed in a trigonal planar fashion by the two chelating nitrogen atoms of the ligand and on additional N(SiMe 3 ) 2 amide group.In 5 the S1-N1 bond (1.5661(14) Å) is shorter than the S-N bond (1.59 Å) of the starting material because of the electron withdrawing effect of ZnN(SiMe 3 ) 2 + .While the harder Zn 2+ cation claims more negative charge from the two imide groups than the softer lithium cation the zinc-coordinated imide groups remain less attractive to the positively polarized sulfur atom (S1-N2 1.6312(14) Å), which compensates for its part at the lithium coordinated imide groups.The N1′-S1-N1 angle (96.80(11)) is wider than the N2′-S1-N2 angle (91.38(10)) presumably due to the higher steric demand of the (thf ) 2 Li moiety compared to the N(SiMe 3 ) 2 anion.The Li-N distance of 1.988(3) Å is typical for Li-N bonds.27Published distances between a lithium ion which is coordinated by two THF molecules and two nitrogen atoms, are on average 2.066 Å.The Zn-N(amide) distance is 1.880(2) Å which is only marginally shorter than the mean average of Zn-amide bonds in the CCDC (Fig.4).28
three metal complexes [(acac) 2 Cu 2 (NtBu) 4 S] (3), [Li(thf ) 4 ] 2 -[I 4 Cd 2 (NtBu) 4 S] (4) and [(thf ) 2 Li{(SiMe 3 ) 2 N}Zn(NtBu) 4 S] (5)show that transition metal complexes containing the tetraimidosulfate dianion are feasible.With the right metal moiety at opposite sides of the tetrahedron they are stable and not subject to ligand scrambling.4 can be envisaged as an intermediate on the metathesis reaction or a co-complex between CdI 2 and the lithium precursor.39Like in the intriguing structure of [(thf ) 2 Li{(NtBu) 3 SMe}•ZnMe 2 ] 26 the S-N bonds vary considerably in the heterobimetallic complex [(thf ) 2 Li{(SiMe 3 ) 2 N}-Zn(NtBu) 4 S] (5).The electropositive sulfur atom inside the imido nitrogen tetrahedron responds to the polarization induced by the coordinated metals.The more the N atoms lose density to the most electropositive metal at the outside the less they are attractive to the sulfur and the longer the S-N bonds get.The electropositive sulfur in the inside mirrors the electron density distribution on the outside.

Table 1
Crystal and structure refinement parameters for compounds 3, 4, and 5 Cu 2 N 4 O 6 S C 52 H 108 Cd 2 I 4 Li 2 N 4 O 9 S C 30 H 70 LiN 5 O 2 SSi 2 Zn