Synthesis and Structural Characterization of [Ir 4 ( m -CO)(CO) 7 m 4 - h 3 -Ph 2 PC(H)C(Ph)PCBu t }( m -PPh 2 )]: Alkyne-Phosphaalkyne Coupling and Formation of a Novel 2-phosphabutadienylphosphine Ligand

a butterfly arrangement of iridium atoms with the new ligand interacting with the metal framework via four s bonds and the PPh 2 phosphorus lone pair.

Reaction of [Ir4(µ-H)(CO)9(Ph2PC≡CPh)(µ-PPh2)] 1 with P≡CBu t in CH2Cl2, at 35 °C, for 4 h yields the novel compound [Ir4(µ-CO)(CO)7{µ4-η 3 -Ph2PC(H)C(Ph)PCBu t }(µ-PPh2)] 2, which contains the 2-phosphabutadienylphosphine chain. Compound 2 is also formed upon thermolysis of [Ir4(CO)10(Ph2PC≡CPh)(PPh2H)] 3 in the presence of P≡CBu t in thf, at 40 °C, for 48 h. Small amounts of [Ir4(µ-CO)(CO)7(µ3-η 2 -HCCPh)(µ-PPh2)2] 4 are always obtained from both reactions, because of the competing rates of the transformations of 1 and 3 into 4 and of their reactions with P≡CBu t . Compound 2 was characterized by analytical and spectroscopic studies such as FAB ms, 1 H, 31 P, 13 C, 2D 31 P-1 H HETCOR, nOe difference and DEPT NMR experiments, which led to its formulation and established the coupling between the coordinated Ph2PC≡CPh and P≡CBu t and the migration of the hydride to the Cα of the Ph2PC≡CPh ligand. However, it was impossible to establish unambiguously if cleavage of the P-Csp bond of the Ph2PC≡CPh ligand had occurred and the mode of interaction of the organophosphorus chain. An X-ray diffraction study of compound 2 established
The reaction of 4 with P≡CBu t was also investigated. All attempts led to an immediate color change from orange to dark brown, but in situ 31 P{ 1 H} NMR showed no phosphorus signals, which indicated that the reaction had occurred, but the product underwent decomposition. Similar behavior was observed when the reaction of 4 with PR3 was investigated 11 .
Compound 2 was formulated on the basis of analytical and spectroscopic data discussed below. The coupling reaction between the Ph2PCCPh ligand and the PCBu t molecule, and the hydride migration to the resulting new phosphorus carbon chain were established by 1 H, 31 P and Scheme 1.
[Ir4(µ-CO)(CO)7{µ4-η 3 -Ph2PC(H)C(Ph)PCBu t }(µ-PPh2)] 13 C-NMR spectroscopy. In spite of the detailed spectroscopic studies undertaken, it was impossible to establish unambiguously whether the diphenylphosphino-alkyne had undergone P-Csp bond cleavage and the geometry of the metal polyhedron in 2, therefore an X-ray diffraction study had to be carried out.
The solution infrared spectrum of compound 2, between 2200-1600 cm -1 , only showed absorptions due to terminal and bridging carbonyl ligands. This result suggested that the triple bond of the Ph2PC≡CPh ligand was interacting with the metal framework, because of the absence of the νC≡C band at 2172 cm -1 which is observed in both starting materials 1 and 3. In the FAB mass spectrum of 2, a molecular ion at m/z 1568 and sequential loss of eight CO ligands were observed. The mass difference between 1 and 2 clearly indicated the incorporation of one PCBu t molecule and loss of a CO group, resulting in a complex having the formula ''Ir4H(CO)8(Ph2PCCPh)(PPh2)(PCBu t )'', with which the elemental analysis agreed perfectly.
The 1 H-NMR data for 2 were consistent with the presence of both PPh2 and Bu t groups. The absence of a hydride signal and the presence of a doublet of doublets at δ 5.4 (JH-P = 55 and 13 Hz) suggested that the hydride ligand had migrated to one of the carbon atoms of the Ph2PCCPh (Cα or Cβ) or of the PCBu t (Cγ) ligands, because migration of the hydride to one of the phosphorus atoms would have led to a much larger one-bond H-P coupling constant, typically between 300 and 500 Hz 12 . A nOe difference experiment established to which of the ligands Ph2PCCPh or PCBu t the hydride had migrated. This experiment consisted of continuous irradiation of the resonance at δ 5.4 (CH), which resulted in a nOe of some of the phenyl proton resonances, but did not affect the Bu t signal [ Fig. 1a]. Likewise, when the Bu t resonance at δ 1.1 was irradiated, only nOe of some of the phenyl proton resonances was observed [ Fig. 1b]. Thus, the spacial proximity of the CH and the phenyl protons indicated that migration of the hydride had occurred either to Cα or Cβ of the Ph2PCCPh ligand.
The 31 P{ 1 H} NMR spectrum of 2 showed three sets of pseudo-triplets at δ 16.1, 28.0 and 116.5 with JP-P = 5 Hz. The 2D 1 H-31 P shift correlation spectrum established that the lowest frequency peak could be assigned to the phosphorus atom of the PCBu t (PA) group, and the other two resonances at higher frequency were due to the PPh2 groups (PB and PC) [ Fig. 2]. This experiment also indicated that the strong P-H coupling of 55 Hz was to the PA nucleus (δ 16.1), whilst the 13 Hz P-H coupling was to the PPh2 phosphorus PB (δ 28.0). The signal at δ 116.5 (PC), was confidently attributed to the bridging phosphido nucleus, on the basis of previous work 7 . The second PPh2 (PB) appeared at δ 28.0 and it is indicative of a phosphine 12 , however the breaking of the P-Csp bond cannot be excluded, since the µ-PPh2 phosphorus nuclei have been shown to span a wide chemical shift range, depending on the distance between the metal atoms they bridge 12 .
The coupling of the diphenylphosphinoalkyne with the phosphaalkyne was strongly suggested by the 13 C{ 1 H} and 13 C-NMR spectra and a DEPT experiment. These experiments made it possible to identify the Cβ (Cquat) resonance as a doublet of doublets at δ 54.2, with JC-P = 37 and 28 Hz, and the Cα (CH), also as a dd, at δ 126.0, JC-P = 57 and 35 Hz, and 1 JC-H = 164 Hz. The chemical shifts of Cα and Cβ and the P-C and C-H coupling constants are in agreement Vol. 9, No. 6, 1998 Alkyne-Phosphaalkyne Coupling 565  with those normally observed for sp 2 hybridized carbon atoms which are coordinated to organometallic compounds 13 . In the 13 C{ 1 H} NMR spectrum of 2 it was possible to identify all eight CO groups and to assign the bridging CO resonance at δ 191.2, which is shifted to high frequency, in comparison with the terminal CO groups 14 . This result was in agreement with the IR and mass spectra data and the elemental analysis. It was also possible to identify five quaternary carbons and the CH carbon atom of the phenyl groups, in the aromatic carbon region between δ 141.7 and 128.3. The two Cquat resonances at δ 38.5 and 29.6, were assigned to Cγ and C* or vice versa.
On the basis of these results two possible structures A and B were proposed for the new 2-phosphabutadienyl chain, as shown in Scheme 2. In A the coupling between Ph2PCCPh and PCBu t would have occurred without cleavage of the Ph2P-C bond, and in B cleavage would have occurred, leading to a µ-PPh2 coordinated fragment. The trans-H to phospha-alkene PA arrangement would result in the large 3 JH-P = 55 Hz observed.

Crystal Structure of 2
The molecular structure of 2 in the solid state is shown in Fig. 3, together with the atomic labeling scheme, and confirmed the geometry shown in A. Selected bond distances (Å) and angles (°) are in Table 1  Although the X-ray diffraction study has confirmed structure A, proposed in Scheme 2 for the novel chain, it is impossible to be sure that P-C bond cleavage was not involved in the process, considering the analogous transformation of [Ru3(µ-H)(CO)8(µ3-η 2 -CCBu t )(Ph2PC≡ CPh)] into [Ru3(CO)8{µ3-η 4 -Ph2PC(Ph)C(H)CC(Bu t )}] 22 . In this case, the alkyne-alkyne condensation and migration of the hydride ligand resulted in a new organic chain, Ph2PC(Ph)C(H)CC(Bu t ). It was suggested that a P-Cα bond cleavage had initially occurred, with formation of a µ-PPh2 ligand, and after the condensation, insertion of the new carbon chain, ''C(Ph)C(H)CC(Bu t )'', into the Ru-PPh2 occurred leading to the observed P-Cβ(Ph); all these proposed steps had been previously observed.
No information regarding the detailed mechanism of the formation of compound 2 is available, but one can speculate the following steps: (i) CO loss and interaction of the acetylene moiety with an electron poor Ir center with a Ir-Ir bond cleavage, (ii) hydride migration to the Cα of the Ph2PCCPh ligand, and (iii) nucleophilic attack of the Cβ at the δ + P of the PCBu t ligand with formation of the 2-phosphabutadienylphosphine observed experimentally.
Preparative TLC was carried out in air by using ca. 2 mm thickness glass-backed silica plates (20 x 20 cm) prepared from silica gel type GF254 (Fluka) and CH2Cl2hexane (3:7) as eluent and the compounds were extracted from silica with CH2Cl2.
IR spectra were obtained on a Bomen MB series IR instrument scanning between 2200 and 1600 cm -1 , using CaF2 cells. Microanalyses were performed at the Instituto de Química, Unicamp, Brazil. Fast atom bombardment mass spectra (FAB MS) were obtained on a Kratos MS50, operating at 8 keV. Xenon was used as the source of fast atoms. 3-Nitrobenzylalcohol, purchased from Aldrich, and distilled under vacuo, was used as a matrix. CH2Cl2 was used as solvent. All m/z values are referred to 193 Ir and were obtained at the University Chemical Laboratories, University of Cambridge, UK. 1 H, 13 C and 13 P-NMR studies were carried out using CDCl3 solutions and a Bruker AMX 500 spectrometer. Standard pulse sequences were used for the NMR experiments 23 . Chemical shifts are given in ppm using deuterated solvents as lock and reference ( 1 H and 13 C, SiMe4; 31 P 85 % H3PO4, external) and coupling constants (J) are given in Hz. X-ray structure determination of 2 X-ray quality crystals were grown by slow evaporation of a very concentrated CHCl3 solution of 2. Data were collected at 293 K on an Enraf-Nonius CAD4 diffractome-ter. Crystal and refinement details are given in Table 2. Non-H atoms were located by heavy atom methods and the structure refined using SHELXS-86 24 and refined on F 2 with all reflections using SHELXS-93 24 . Hydrogen atoms were included in rigid mode.
Atomic coordinates, thermal parameters and a full list of bond lengths and angles have been deposited at the Cambridge Crystallographic Data Centre.