Synthesis and structural studies of some selenoureas and their metal complexes

The selenoureas PhC(O)NHC(Se)NPhMe, 4-MeC 6 H 4 C(O)NHC(Se)NPhMe and 4-MeC 6 H 4 C(O)- NHC(Se)NEt 2 were prepared and characterized by spectroscopic methods and single crystal X-ray diffraction. 4-MeC 6 H 4 C(O)NHC(Se)NEt 2 was reacted with some Pt(II), Pd(II) and Ru(II) precursors to give heteroleptic, cationic metal complexes which were characterized by NMR spectroscopy, electrospray mass spectrometry and, in the case of the Pd(II) complex [Pd{4-MeC 6 H 4 C(O)NC(Se)NEt 2 }( t Bu 2 bipy)]PF 6 by X-ray diffraction.


Introduction
3][4] While the acylisoselenocyanates are important intermediates in organoselenium chemistry, 5,6 the selenoureas themselves have been extensively studied by the inorganic and analytical communities.Selenoureas react with metal salts to form chelate complexes in which the deprotonated selenourea acts as a monoanionic O,Se-donor forming a six-membered metallacycle (Figure 1).Depending on the valency of the metal, one, two or three selenoureato ligands can be accommodated.
Metal complexes of this type, in particular the bis(selenoureato) derivatives have been extensively studied and have found application in analytical chemistry for chromatographic metal separation 7,8 and also for spectrophotometric determination of metals. 9More recently, main group metal selenoureato complexes have been explored as single source precursors for the preparation of metal selenide nanomaterials. 10,11While such homoleptic selenoureato complexes have been studied in great detail, heteroleptic derivatives are virtually unknown.We previously reported the first examples of heteroleptic selenoureato palladium complexes derived from a cyclometalled palladium(II) precursor. 12We have now extended this work to encompass other selenoureas and present the results of this study here.

Results and Discussion
The The compounds were characterized by 1 H NMR spectroscopy and, in the case of 1 and 2, also by single crystal X-ray diffraction.The molecular structures of compounds 1 and 2 are shown in Figures 2-3, respectively; important bond distances and angles are collected in Table 1.The two structures show very similar bond lengths and angles and the overall geometry i.e. the rotation of the C=Se and C=O units with respect to each other (ca.126 °) is also very similar.
The C=Se/O as well as the C-N distances are within the expected values for double and single bonds, respectively.
Selenourea 3 reacted with the palladium and platinum precursors cis-[MCl2(L-L)] [M = Pt, Pd; L-L = t Bu2bipy, (PPh3)2] in the presence of base to give the cationic complexes [M{4-MeC6H4C(O)NC(Se)NEt2}(L-L)] + 4-6, which were isolated as their PF6 -or BPh4 -salts (Scheme 2).Similarly, the reaction of 3 with the ruthenium precursor [RuCl2(p-cym)]2 in the presence of Ph3P and base gave the cationic ruthenium complex [Ru{4-MeC6H4C(O)N-C(Se)NEt2}(PPh3)(p-cym)] 7 as shown in Scheme 2. The cationic complexes 4-7 were characterized by 1 H and 31 P{ 1 H} NMR spectroscopy and electrospray mass spectrometry as well as elemental analysis.The compounds appear indefinitely stable in the solid state, but, in solution, decomposition occurs over a period of ca.12-18 h.The poor solubility of the complexes combined with the low sensitivity of the 77 Se and 13 C nuclei did unfortunately not allow us to obtain NMR spectra of these two nuclei.We were however able to obtain X-ray quality crystals of complex 6 (see below).Complexes 4-7 show strong parent ions due to the cations in the electrospray mass spectra; in some cases an additional signal due to loss of a Ph3P ligand is also seen.The observed isotopic distribution patterns exactly match the computed patterns.The 31 P{ 1 H} NMR spectra of the phosphine complexes 4 and 5, show two doublet resonances due to the presence of nonequivalent phosphorus atoms.In addition, Pt satellites with P-Pt coupling constants of ca.3000 Hz, typical for a cis geometry about the platinum, are observed in complex 4. In the 1 H NMR spectra of all four complexes, the signal due to the NH proton is absent, confirming that deprotonation of the selenourea has occurred.Furthermore, the doubling of all the proton signals of the t Bu2bipy moiety suggests an unsymmetrical environment about the metal, which is consistent with the presence of a chelating selenoureato ligand.The proposed structure was confirmed by an X-ray diffraction study of complex 6, shown in Figure 4.The cation consists of a palladium atom coordinated by the two nitrogen atoms of the t Bu2bipy as well as the oxygen and selenium atoms of the deprotonated selenoureato ligand in a square planar fashion.Overall, the structure is similar to the previously reported related sulfur analogue. 13 The ruthenium complex 7 is chiral at the metal centre, which results in the doubling of all the proton resonances in its 1 H NMR spectrum.Such behavior is typically observed in NMR spectra of ruthenium complexes which are chiral at the metal. 14Unfortunately, we were unable to obtain X-ray quality crystals of this compound; nevertheless, the spectral data are fully consistent with the proposed structure.
In summary, we have prepared and characterized some new selenourea derivatives and have shown that deprotonation in the presence of Pt(II), Pd(II) and Ru(II) precursors leads to the formation of cationic, heteroleptic selenoureato complexes.

Experimental Section
General. 1 H and 31 P{ 1 H} NMR spectra were recorded on a 400 MHz Bruker Avance spectrometer.Chemical shifts are quoted relative to external SiMe4 ( 1 H) and 85% H3PO4 ( 31 P).Elemental analyses were performed by staff of the microanalytical laboratory of the University of Wuppertal.Positive ion electrospray mass spectra were run as MeCN solutions on a Bruker Daltonics instrument.All reactions were carried out under aerobic conditions in standard glassware.KSeCN was prepared from Se metal and KCN as described elsewhere. 15The metal precursors cis-[MCl2(PPh3)2] (M = Pt, Pd), cis-[PdCl2( t Bu2bipy)2] and [RuCl2(p-cym)]2 were prepared as described in the literature. 16,17All other chemicals and solvents (HPLC grade) were sourced commercially and used as received.Full details of the crystallographic studies are provided in the supplementary material.The structural data have been deposited at the Cambridge Crystallographic Data Centre with the deposition codes CCDC 791347 -791349.

Preparation of the selenoureas (1, 2 and 3)
To a solution of KSeCN (1.53 g, 0.01 mmol) in acetone (25 mL) was added the appropriate acid chloride (0.01 mmol).After stirring for ca.30 min., the amine (0.01 mmol) was added and the resulting mixture was stirred a further 30 min.The reaction mixture was poured into 0.1 M HCl (ca.200 mL) and the resulting orange precipitate was filtered, washed with water and dried.The pure selenoureas were obtained from recrystallisation of the crude material with EtOH.

Figure 1 .
Figure 1.Schematic representation of a selenoureato metal chelate complex.

Figure 4 .
Figure 4. Molecular structure of the cation of complex 6.Ellipsoids show 50% probability levels.Hydrogen atoms as well as the PF6 -anion have been omitted for clarity.
The bond distances (C-Se/O and C-N) of the selenoureato ligand are considerably different than those observed in the selenoureas themselves: The C-Se and C-O bond lengths increase upon coordination [C2-Se2 1.8895(18) Å, C1-O1 1.281(2) Å], while the C-N bond distances decrease [C1-N1 1.325(2) Å, C2-N1 1.340(2) Å], consistent with the change of bond order due to delocalization which occurs upon deprotonation.While the five-membered Pd-N-C-C-N ring is planar, the O and Se atoms of the six-membered Pd-O-C-N-C-Se ring lie slightly above and below the mean plane, respectively.