1 Introduction

Cyclodiphosphazanes or diazadiphosphetidines are a major class of cyclic phosphorus-nitrogen compounds.[1,2] They find applications in various fields such as coordination chemistry,[3–8] in anti-tumour studies[9,10] and in catalytic organic transformations.[11–13] The dichlorocyclodiphosphazanes, cis-{ClP(μ-NR)}2, have been effectively explored as building blocks in the synthesis of a large range of phosphorus-nitrogen macrocycles.[14–20] Cyclodiphosphazanes can exist as cis and trans isomers in solution as well as in the solid state (chart 1, A and B).[2] The ring conformations of these isomers can be established by single crystal X-ray diffraction studies or by 31P NMR data, where the cis isomers show upfield chemical shifts compared to the trans analogues.[21] The cis/trans interconversion in solution is observed for several cyclodiphosphazanes. The interconversion can proceed through edge (N) or vertex (P) inversion.[22]

Chart 1
figure b

Cis and trans-cyclodiphosphazanes.

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The cis cyclodiphosphazanes act as versatile bridging ligands for the construction of polynuclear metalomacrocycles, or one- (1D), two- (2D), and three-dimensional (3D) coordination polymers and their transition metal chemistry has been extensively studied.[23–25] The coordination chemistry of the trans isomer has been much less studied.[26–29] Recently we explored the coordination chemistry of acyclic dimers of cyclodiphosphazanes derived from cis-{ClP(μ-N tBu)2P(NH tBu)}.[14,30,31] Hydrolysis of the P–Cl bond in cis-{ClP(μ-N tBu)2P(NH tBu)} to produce cis-{(tBuNH)P(μ-N tBu)2P(O)H} was reported by Wright and co-workers,[32] while a similar compound was accidentally isolated by Kumaraswamy and co-workers.[33] Herein we report transition metal chemistry and cis/trans isomerisation of cis-{(tBuNH)P(μ-N tBu)2P(O)H}.

2 Experimental

2.1 General procedures

All manipulations were performed using standard vacuum-line and Schlenk techniques under nitrogen atmosphere unless otherwise stated. All the solvents were purified by conventional methods[34] and distilled prior to use. The compounds, cis-{ClP(μ-N tBu)2P(NH tBu)},[35] [Pd(μ-Cl)(η 3-C3 H 5)]2,[36] [Ru(η 6-p-cymene)(μ-Cl)Cl]2,[37] Pd(COD)Cl2,[38] and Pt(COD)Cl2,[38] were prepared according to the published procedures. Other chemicals were obtained from commercial sources and purified before use.

2.2 Instrumentation

The NMR spectra were recorded at the following frequencies: 400 MHz (1H), 162 MHz (31P), 100 MHz (13C) and 107.5 MHz (195Pt) using Varian VXR 400, Bruker AV 400 or AV 500 spectrometers. The 13C, 31P and 195Pt NMR spectra were acquired using broadband proton decoupling. The spectra were recorded in CDCl3 solutions with CDCl3 as an internal lock; chemical shifts of 1H and 13C NMR spectra are reported in ppm downfield from TMS, used as internal standard. The chemical shifts of 31P and 195Pt NMR spectra are referred to 85% H3 PO 4 (in D2O) and K2PtCl6 (in D2O), respectively, used as an external standard. Microanalyses were performed using a Carlo Erba Model 1112 elemental analyzer. Mass spectra were recorded using Waters Q-Tof micro (YA-105). The melting points were observed in capillary tubes and are uncorrected.

2.3 Synthesis of cis-{(tBuNH)P(μ-N tBu)2P(O)H} (2)[32]

A solution of cis-{ClP(μ-N tBu)2P(NH tBu)} (1) (1.1 g, 3.528 mmol) in THF (20 mL) was added dropwise to a solution of water (0.064 g, 3.528 mmol) and triethylamine (0.54 mL, 0.39 g, 3.88 mmol) also in THF (20 mL) at –78 ∘C. The reaction mixture was warmed to room temperature and stirred overnight. The solvent was removed under vacuum and the residue obtained was dissolved in toluene (30 mL) and filtered. The filtrate was concentrated to 10 mL under reduced pressure and stored at –20 ∘C to yield analytically pure product of 2. Yield: 66% (0.68 g). M.p.: 161–164 ∘C. 1H NMR (400 MHz, CDCl3): δ 7.33 (dd, 1H, 1 J PH=586 Hz, 3 J PH=3.1 Hz, PH), 3.13 (d, 1H, 2 J PH=6.2 Hz, NH), 1.44 (s, 18H, μ-N tBu), 1.16 (s, 9H, tBuNH). 31P{ 1H} NMR (162 MHz, CDCl3): δ 73.6 (br s), –7.2 (d, 2 J PP=8.1 Hz, P =O). 13C{ 1H}: δ 52.1 (d, 2 J PC=7.5 Hz), 51.9 (d, 2 J PC=14.5 Hz), 32.7 (d, 3 J PC=9.7 Hz), 31.5 (t, 3 J PC=4.6 Hz).

2.4 Synthesis of cis-{(tBuNH)P(Se)(μ-N tBu)2P(O)H} (3)

A mixture of 2 (0.1 g, 0.341 mmol) and elemental selenium (0.027 g, 0.342 mmol) in toluene (20 mL) was refluxed for 6 h. The solution was cooled to room temperature, filtered through Celite and the solvent removed under vacuum to obtain 3 as an off-white crystalline solid. Analytically pure 3 was obtained by recrystallising the crude product from toluene. Yield: 98% (0.124 g). M.p.: 212–215 ∘C (dec). HRMS (Calcd. for M + H C12 H 29 N 3 OP 2Se): 374.1029. Found: 374.1045. Anal. Calcd. for C12 H 29 N 3 OP 2Se: C, 38.71; H, 7.85; N, 11.29. Found: C, 38.44; H, 7.73; N, 11.32. IR (KBr, ν/cm −1): 3165 (br, m, N–H str.), 2396 (m, P–H str.), 1262 (s), 1196 (s, P–O str.), 1071(s). 1H NMR (400 MHz, CDCl3): δ 7.61 (dd, 1 J PH=628 Hz, 3 J PH=8.5 Hz, PH, 1H), 3.66 (d, 2 J PH=15.0 Hz, NH, 1H), 1.60 (s, tBu, 18H), 1.48 (s, tBu, 9H). 31P{ 1H}NMR (162 MHz, CDCl3): δ 20.9 (s, 1 J SeP=876 Hz, P =Se), –12.4 (d, 2 J PP=22.7 Hz, P =O).

2.5 Synthesis of [(PdCl(η 3-C3 H 5 ) )2{(tBuNH)P(μ-N tBu)2 P(O)H}] (4)

A solution of [Pd(μ-Cl)(η 3-C3 H 5)]2 (0.025 g, 0.0683 mmol) in dichloromethane (10 mL) was added to a solution of 2 (0.04 g, 0.1364 mmol) in the same solvent (5 mL) at room temperature. After 4 h, the volume of the solvent was reduced to 2 mL, the solution was layered with 4 mL of petroleum ether and kept at −20∘C for 24 h to obtain 4 as an analytically pure yellow solid. Yield: 83% (0.054 g). M.p.: 245 ∘C (dec). Anal. Calcd. for C15 H 34ClN3 OP 2Pd: C, 37.83; H, 7.20; N, 8.82. Found: C, 37.19; H, 6.60; N, 8.45. IR (KBr, ν/cm −1): 3219 (br, m, N–H str.), 2352 (m, P–H str.), 1265 (s), 1202 (s, P–O str.), 1062 (s). 1H NMR (400 MHz, CDCl3): δ 7.42 (dd, 1 J PH=605.5 Hz, 3 J PH=7.5 Hz, PH, 1H), 5.55 (m, CH, 1H), 5.26 (d, 2 J PH=28.8 Hz, NH, 1H), 4.62 (t, J=8.4 Hz, CH, 1H), 3.56 (t, J=13.6 Hz, CH, 1H), 3.26 (d, J=6.0 Hz, CH, 1H), 2.74 (d, J=12.0 Hz, CH, 1H), 1.54 (s, tBu, 9H), 1.43 (d, J=13.4 tBu, 18H). 31P{ 1H}NMR (162 MHz, CDCl3): δ 54.9 (s, P–Pd), –13.4 (s, P =O). 31P NMR (162 MHz, CDCl3): δ 54.9 (s, P–Pd), –13.4 (d, 1 J PH=606 Hz, P =O). MS (EI): m/z=440.12 [M–Cl] +.

2.6 Synthesis of [((η 6-p-cymene)RuCl2)2{(tBuNH) P(μ -N tBu)2P(O)H}] (5)

A solution of [Ru(η 6-p-cymene)(μ-Cl)Cl]2 (0.024 g, 0.0392 mmol) in dichloromethane (10 mL) was added to a solution of 2 (0.023 g, 0.0784 mmol) in the same solvent (10 mL) at room temperature. The reaction mixture was stirred for 4 h, the solvent volume was reduced to 2 mL under vacuum, and the solution layered with 4 mL of diethyl ether and kept in −20∘C for 24 h to afford analytically pure red crystals of 5. Yield: 77% (0.036 g). M.p.: 198 ∘C (dec) Anal. Calcd. for C22 H 43 Cl2 N 3 OP 2Ru ⋅2CH2Cl2: C, 37.47; H, 6.16; N, 5.46. Found: C, 37.23; H, 6.10; N, 5.64. 1H NMR (400 MHz, CDCl3): δ 7.84 (dd, 1 J PH=604 Hz, 3 J PH=11.2 Hz, PH, 1H), 5.66 (d, 3 J HH=6.0 H, CH, 2H), 5.35 (s, CH2Cl2, 4H), 5.08 (d, 3 J HH=6.0 Hz, CH, 2H), 4.09 (d, 2 J PH=22.0 Hz, NH, 1H), 3.13 (p, 3 J HH=6.8 Hz, CH, 1H), 2.18 (s, CH3, 3H) 1.63 (s, tBu, 18H), 1.38 (s, tBu, 9H), 1.36 (d, 3 J HH=6.8 Hz, C(CH3)2, 6H). 31P{ 1H}NMR (162 MHz, CDCl3): δ 62.7 (d, 2 J PP=16.8 Hz, P–Pd), −6.4 (d, P =O). MS (EI): m/z=564.2 [M–Cl] +.

2.7 Synthesis of [PdCl2{(tBuNH)P(μ-N tBu)2 P(O)H}2] (6)

A solution of Pd(COD)Cl2 (0.0204 g, 0.0716 mmol) in dichloromethane (5 mL) was added to a solution of 2 (0.042 g, 0.1432 mmol) in the same solvent (5 mL) at room temperature. The resultant yellow solution was stirred for 4 h and all the volatiles were removed under reduced pressure to yield 6 as yellow crystalline solid. The complex 6 was recrystallized from a mixture of dichloromethane and petroleum ether to obtain analytically pure crystals. Yield: 84% (0.046 g). M.p.: 230–232 ∘C. Anal. Calcd. for C24 H 58Cl2 N 6 O 2 P 4Pd: C, 37.73; H, 7.65; N, 11.00. Found: C, 37.68; H, 7.34; N, 10.89. IR (KBr, ν/cm −1): 3242 (m, NH, str), 2972 (m), 2392 (s, PH, str), 1266 (m), 1200 (m, P =O, str). 1H NMR (400 MHz, CDCl3): δ 7.62 (dt, 1 J PH=651 Hz, J=4.4 Hz, PH, 2H), 7.59 (dt, 1 J PH=650 Hz, J=4.4 Hz, PH, 2H), 5.41 (t, J=14.4 Hz, NH, 2H), 5.24 (t, J=14.4 Hz, NH, 2H), 1.62 (s, tBu, 72H), 1.45 (s, tBu, 36H). 31P{ 1H}NMR (162 MHz, CDCl3): δ 40.92 (unresolved t, P–Pd), 40.86 (unresolved t, P–Pd), –6.5 (t, J=4 Hz, P =O), −6.8 (s, J=5.9 Hz, P =O). 31P NMR (162 MHz, CDCl3): δ 40.9 (s, P–Pd), –6.5 (d, 1 J PH=652 Hz, P =O), –6.8 (d, 1 J PH=651 Hz, P =O). MS (EI): m/z=765.2 [M + H] +.

2.8 Synthesis of [PtCl2{(tBuNH)P(μ -N tBu)2 P(O)H}2] (7)

A dichloromethane solution (5 mL) of Pt(COD)Cl2 (0.0191 g, 0.0511 mmol) was added to a solution of 2 (0.03 g, 0.1023 mmol) in the same solvent (5 mL) at room temperature. The reaction mixture was stirred for 4 h and all the volatiles were removed under reduced pressure to obtain an off-white residue. The residue obtained was re-dissolved in dichloromethane (2 mL), layered with petroleum ether (4 mL) and stored at −20∘C for 24 h to obtain 7 as analytically pure white crystalline solid. Yield: 81% (0.035 g). M.p.: >275 ∘C. Anal. Calcd. for C24 H 58Cl2 N 6 O 2 P 4Pt ⋅CH 2Cl2: C, 32.03; H, 6.45; N, 8.96. Found: C, 32.11; H, 6.37; N, 8.61. IR (KBr, ν/cm −1): 3254 (m, NH, str), 2971 (m), 2390 (s, PH, str), 1266 (m), 1203 (m, P =O, str). 1H NMR (400 MHz, CDCl3): δ 7.54 (dt, 1 J PH=650 Hz, 3 J PH=4.4 Hz, PH, 2H), 7.51 (dt, 1 J PH=648 Hz, 3 J PH=4.4 Hz, PH, 2H), 5.51 (s, CH2Cl2, 2H), 5.46 (t, J=13.2 Hz, NH, 2H), 5.29 (t, J=13.2 Hz, NH, 2H), 1.62 (s, tBu, 72H), 1.46 (s, tBu, 36H). 31P{ 1H}NMR (162 MHz, CDCl3): δ 36.7 (t, J=6.5 Hz, 1 J PtP=3379 Hz, P–Pt), –3.9 (t, J=8.3 Hz, 3 J PtP=77.1 Hz, P =O), −4.4 (t, J=6.2 Hz, 3 J PtP=83.3 Hz, P =O). 31P NMR (162 MHz, CDCl3): δ 36.7 (br s, 1 J PtP=3382 Hz, P–Pt), –3.9 (m, 1 J PH=648 Hz, P =O), −4.4 (m, P =O). 195Pt{ 1H}NMR (107.5 MHz, CDCl3): δ –3872 (t, 1 J PtP=3381 Hz), –3859 (t, 1 J PtP=3402 Hz). MS (EI): m/z=875.25 [M + Na] +.

2.9 X-ray Crystallography

A crystal of the compound 5 suitable for X-ray crystal analysis was mounted on a Cryoloop with a drop of Paratone oil and placed in the cold nitrogen stream of the Kryoflex attachment of the Bruker APEX CCD diffractometer. A full sphere of data was collected using 3 sets of 400 frames, each of width 0.5 ∘ in ω, collected at φ=0.00, 90.00 and 180.00 ∘ and 2 sets of 800 frames, each of width 0.45 ∘ in φ, collected at ω=−30.00 and 210.00 ∘ using the APEX2[39] program suite. A crystal of 7 was mounted on a glass fibre using epoxy glue and placed on a Rigaku Saturn724 CCD diffractometer equipped with graphite-monochromated Mo-K α radiation (λ=0.71073 Å). For 5, the raw data were reduced to F 2 values using the SAINT software,[39] while for 7, the images were processed with the CrystalClear program (Expert 2.1 b24). Multiple measurements of equivalent reflections provided the basis for an empirical absorption correction as well as a correction for any crystal deterioration during the data collection (SADABS[39]). Both the structures were solved by direct methods (SHELXS[40]) and refined by full-matrix least-squares procedures on F 2 using SHELXL[40] (SHELXTL program package.[39] Hydrogen atoms attached to carbon were placed in calculated positions and included as riding contributions with isotropic displacement parameters tied to those of the attached non-hydrogen atoms. Those attached to nitrogen were placed in locations derived from a difference map and also included as riding contributions as for the others. The isotropic thermal parameters of the hydrogen atoms were fixed at 1.2 times that of the corresponding carbon for phenyl hydrogen and 1.5 times for C(C H 3)3. Crystallographic data (excluding structures factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1000229 (compound 5) and 1000230 (compound 7).

3 Results and Discussion

Hydrolysis of cis-{ClP(μ-N tBu)2P(NH tBu)} (1) with one equivalent of degassed water in the presence of triethylamine at –78 ∘C resulted in the formation of a mixed-valent derivative cis-{(tBuNH)P(μ-N tBu)2P(O)H} (2), as shown in scheme 1. Because of the interference of adventitious water, 2 is an undesired product in many reactions involving 1. The compound 2 is air and moisture stable and obtained as an analytically pure white solid after recrystallisation from toluene. The reaction of 2 with elemental selenium in toluene resulted in the formation of the monoselenide, trans-{(tBuNH)P(Se)(μ-N tBu)2P(O)H} (3) along with a small amount of cis isomer 3a. The 31P{ 1H}NMR spectrum of 3 showed two doublets at 20.9 ppm and −12.4 ppm with a 2 J PP coupling of 22.7 Hz.[41,42] The signal at 20.9 ppm having characteristic selenium satellite peaks with a 1 J SeP coupling of 876 Hz was assigned to the P(Se) centre. The 1H NMR spectrum of 3 showed a doublet of doublets for the PH proton with a large 1 J PH coupling (628 Hz) and a relatively small 3 J PH coupling (8.5 Hz). The coupling constants are larger than those observed for 2 (1 J PH=586 Hz and 3 J PH=3.1 Hz). The large 1 J PH coupling in the 1H NMR spectrum of 3 confirms the presence of the P(O)H rather than the POH tautomer in solution. The structure and molecular composition of compound 3 was further confirmed by mass spectrometry and elemental analysis.

Scheme 1
scheme 1

Synthesis and reactivity of cis-{(tBuNH)P(μ-NtBu)2P(O)H} (2).

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The reaction of two equivalent of 2 with [Pd(μ-Cl) (η 3-C3 H 5)]2 in dichloromethane gave [(η 3-C3 H 5)PdCl {(tBuNH)P(μ-N tBu)2P(O)H}] (4). The 31P{ 1H}NMR spectrum of 4 recorded in CDCl3 showed two singlets for coordinated phosphorus and P(O) atoms, at 54.9 and −13.4 ppm, respectively. The spectral assignments for compound 4 were confirmed by a proton-coupled 31P NMR spectrum in which the signal at −13.4 ppm splits into a doublet with a 1 J PH coupling of 606 Hz. The 1H NMR spectrum confirms the presence of a coordinated allyl group. All the five protons of the allyl fragment appear distinctly in the 1H NMR spectrum of 4. The 1H NMR data indicate the allyl group (η 3-C3 H 5) is rigidly coordinated to the metal centre and all the four terminal syn and anti protons remain non-equivalent on NMR time scale.

The reaction of 2 with [Ru(η 6-p-cymene)(μ-Cl)Cl]2 afforded [((η 6-p-cymene)RuCl2){(tBuNH)P(μ-N tBu)2 P(O)H}] (5) in moderate yield. The 31P{ 1H} NMR spectrum of 5 showed two doublets centered at 62.7 and −6.4 ppm, with a 2 J PP coupling of 16.8 Hz. The mass spectrum of 5 showed base peak at 564.2 corresponding to the [M–Cl] + ion. Further, the structure of 5 is established by a single-crystal X-ray diffraction study.

Slow diffusion of petroleum ether into a chloroform solution of 5 led to the formation of crystals suitable for X-ray diffraction studies.[43] The coordination geometry around the ruthenium center in 5 is typical pseudo-octahedral (figure 1) with a three-legged piano-stool arrangement having ligand 2 acting as a monodentate ligand. The Ru–P1 bond distance is 2.3892(12) Å and Ru–C bond distances range from 2.205(2) to 2.262(2) Å with an average bond distance of 2.236 Å, which is slightly longer than that of [(η 6-p-cymene)RuCl2{{(o-MeOC6 H 4O)P(μ-N tBu)}2}AuCl].[44] The P2–O1 bond distance in 5 [1.474(2) Å] is similar to that of 2 [1.468(2) Å], whereas the P2–H2 bond distance in 5 [1.26(2) Å] is shorter than that of the free ligand 2 [1.35(2) Å].[32]

Figure 1
figure 1

The molecular structure of [(η 6-p-cymene) RuCl2{(tBuNH)P(μ-N tBu)2P(O)H}] (5). All hydrogen atoms except H(P) and lattice solvent molecules are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond distances [Å]: Ru1–Cl1, 2.4224(12); Ru1–Cl2, 2.4154(12); Ru1–P1, 2.3892(12); P2–O1, 1.4736(16); P1–N2, 1.7358(19); P1–N3, 1.6320(17); P2–N1, 1.6627(18); P2–N2, 1.6754(18). Selected bond angles [ ∘]: Cl1–Ru1–Cl2, 86.37(1); Cl1–Ru1–P1, 84.54(2); Cl2–Ru1–P1, 88.26(1); N1–P1–N2, 81.20(8); N1–P2–N2, 84.83(8).

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The steric and electronic aspects of the coordinating ligands dictate the conformations of d 8 square planar complexes. The bulky ligands generally confer trans configuration, whereas the strong σ-donor ligands impose cis conformation around the metal center due to the trans influence.[45–47] The reaction of 2 with M(COD)Cl2 (M = Pd and Pt) can give six possible conformational and geometrical isomers, as both the cyclodiphosphazane as well as the metal center can adopt cis and trans conformations (chart 2). The treatment of 2 with M(COD)Cl2 (M = Pd and Pt) in 2:1 ratio in dichloromethane at room temperature resulted in the formation of [MCl2{(tBuNH)P(μ-N tBu)2P(O)H}2] (6 M = Pd; 7 M = Pt) as shown in scheme 2. In both the complexes, the metal centres adopt trans geometry as established by the X-ray analysis of 7. The microanalytical data of both 6 and 7 support 1:2 ratio of metal to ligand in the isolated products. The evidence for the formation of complexes also comes from mass spectral data, where both 6 and 7 show the molecular ion peaks, at 765.2 [M + H]+ and 875.2 [M + Na] +, respectively. To assign the geometry of the cyclodiphosphazanes in 6 and 7, both 1D and 2D NMR studies were carried out.

Chart 2
figure c

Possible isomers of 6 and 7.

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Scheme 2
scheme 2

Pd(II) and Pt(II) complexes of cis-{(tBuNH)P(μ-NtBu)2P(O)H} (2).

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The 31P{ 1H} and 1H NMR spectra of complexes 6 and 7 recorded in CDCl3 at room temperature are similar, which confirm that the structures of both the complexes in solution are essentially the same. The 31P{ 1H}NMR spectrum of the palladium complex 6 shows two unresolved triplets centred at 40.92 and 40.86 ppm and two set of triplets centered at −6.5 and −6.8 ppm with couplings of 4.0 and 5.9 Hz in a ∼2:3 ratio, respectively, as shown in figure 2. The 31P{ 1H}NMR spectrum of platinum complex 7 showed three triplets at 36.7, −3.9 and −4.4 ppm. The signal at 36.7 ppm with a 1 J PtP coupling of 3379 Hz was assigned to the coordinated phosphorus. In the 1H coupled 31P NMR spectra of 6 and 7, only the upfield signals split into two sets of resonances with large 1 J PH values. This observation confirms that the upfield signals are due to the phosphorus atom directly attached to hydrogen. Based on 31P NMR integration, it is clear that the two products in 6 exist in ∼3:2 ratios. No interconversion was observed on keeping the solution of complex 6 in CDCl3 for 24 h at room temperature.

Figure 2
figure 2

The 31P{ 1H}NMR spectrum [PdCl2{(tBuNH)P(μ-N tBu)2P(O)H}2] (6).

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The 1H NMR spectrum of 6 showed complex splitting patterns due to the presence of two isomers. Two overlapping doublets of triplets centered at 7.62 and 7.59 ppm are observed for the PH protons (|1 J PH|=651 Hz, |J|=4.4 Hz), the NH protons appear as triplets centred at 5.41 and 5.24 ppm (∼3:2 ratio) with a coupling of |J|=14.4 Hz. The observed multiplicity is probably due to the long range coupling between NH and PH protons. The 1H– 1H COSY spectrum indicates that the triplet at 5.41 ppm (NH) is correlated with the doublet of triplets at 7.62 ppm (PH). Similarly, the triplet at 5.24 ppm (NH) is correlated with the doublet of triplets at 7.59 ppm (PH). Furthermore, in the 31P– 1H HSQC experiment (figure 3), the triplets at 7.62 and 7.59 ppm showed correlation with the phosphorus signals at −6.5 and −6.8 ppm, respectively. The 195Pt{ 1H}NMR spectrum of 7 showed two overlapping triplets with similar coupling constants, at –3872 (1 J PtP=3380 Hz) and −3859 (1 J PtP=3402 Hz). This obeservation clearly indicates the presence of two different species in solution.

Figure 3
figure 3

31P– 1H HSQC spectrum of 6 recorded in CDCl3.

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The X-ray structure of 7 confirms the trans geometry around the square planar platinum centre as shown in figure 4. Both the cyclodiphosphazane rings in 7 assume the trans conformation in the solid state. Because of the steric bulk, the vector Cl1–Pt1–Cl1 i is tilted away from exocyclic nitrogen, and the acute bond angle Cl1– Pt1–P1 is 87.10(2) ∘. The Pt1–P1 [2.3062(8) Å] and Pt1-Cl1 [2.3116(7) Å] bond distances for 7 are comparable to the Pt −P [2.2447(13) Å] and Pt −Cl [2.3254(15) Å] bond lengths of the complex [(PtCl2)2{ tBuNP(OC6 H4PPh2- o)}2.[48] The P–O bond lengths in 7 [1.467(2) Å] and 2 [1.468(2) Å] are nearly identical. The exocyclic and endocyclic P–N bond distances in 7 are shorter than that found in 2, but follow the same trend.[32] In general, the exocyclic P–N distances are shorter and comparable with typical P–N distances observed in both cyclic and acyclic diphosphazanes[49] and P–N bond distances associated with PV centers are shorter than that of P III centers in cyclodiphosphazanes.

Figure 4
figure 4

The molecular structure of [PtCl2{(tBuNH)P(μ-N tBu)2 P(O)H}2] (7). All hydrogen atoms except H(P) and H(N) and all lattice solvent molecules are omitted for clarity. Thermal ellipsoids are drawn at the 50% probability level. Selected bond distances [Å]: Pt1–P1, 2.3062(8); Pt1–Cl1, 2.3116(7); Pt1–Cl1 i, 2.3116(7); P2–O1, 1.467(2); P1–N2, 1.7006(19); P1–N3, 1.621(2); P2–N1, 1.6669(19); P2–N2, 1.672(2); P2–H1, 1.29(3). Selected bond angles [ ∘]:Cl1–Pt1–P1, 87.10(2); Cl1–Pt1–P1 i, 92.90(2N1–P1–N2, 82.39(9).

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4 Conclusions

The reactivity and transition metal chemistry of a mixed-valent cyclodiphosphazane derivative, cis-{(tBuNH)P(μ-N tBu)2P(O)H} (2), has been studied. The compound 2 exists purely as the cis isomer in both solution and solid states but exhibits cis/trans isomerisation when coordinated to metal centres. The reaction of 2 with palladium(II) and platinum(II) precursors yielded a mixture of two isomers. In both the complexes isomers, the two cyclodiphosphazane rings adopt cis/cis and trans/trans conformations, but the mixed cis/trans conformation was not observed. The presence of two conformations of the cyclodiphosphazane rings was established by NMR spectroscopy and single crystal X-ray diffraction studies.