DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 271–272 271 An unprecedented Í2N,H bonding mode for a hydridotris(pyrazolyl)- borato ligand François Malbosc,a Philippe Kalck,*a Jean-Claude Daran b and Michel Etienne *b a Laboratoire de Catalyse, Chimie Fine et Polymères, ENSCT, 118 Route de Narbonne, 31077 Toulouse Cedex 4, France. E-mail: pkalck@ensct.fr b Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 Route de Narbonne, 31077 Toulouse Cedex 4, France. E-mail: etienne@lcc-toulouse.fr Received 30th November 1998, Accepted 11th December 1998 An unprecedented Í2N,H bonding mode for a hydridotris(pyrazolyl)borato (TpMe2,4-Cl) ligand is observed in [Rh(CO)(PMePh2)2(TpMe2,4-Cl)]; the complex, which features two dangling pyrazole rings and a B–(Ï-H)–Rh agostic bond, is highly fluxional in solution.Hydridotris(pyrazolyl)borato ligands (Tp9) have a strong preference for adopting a k3N,N9,N0 bonding mode.1 On some occasions, the electronic nature of the metal dictates a different coordination behaviour.A textbook example is the d8 configuration in square-planar Rh complexes where k2N,N9 coordination may also be observed, the two bonding modes interconverting in several cases.2 The third pendant pyrazolyl arm may then fulfill subsequent electronic deficiency: oxidation of [Rh(CO)(PPh3)(k2N,N9-TpMe2)] yields a pentacoordinated k3N,N9,N0 rhodium(II) complex [Rh(CO)(PPh3)(k3N,N9,N0- TpMe2)]1.3 Also, stable species containing k2N,N9 forms have been shown to be intermediates leading to k3N,N9,N0 coordination as exemplified by the thermal loss of phosphine in octahedral [RuH(PPh3)2(k2N,N9-Tp)] which aVords [RuH- (PPh3)(k3N,N9,N0-Tp)].4 Isomerization from a k2N,N9 to a k3N,N9,H situation has been recently observed in [RuH- (COD)Tpi-Pr2] (COD = cycloocta-1,5-diene).5 In this communication, we report yet another bonding mode for a hydridotris- (pyrazolyl)borato ligand featuring two pendant pyrazolyl rings in the complex [Rh(CO)(PMePh2)2(TpMe2,4-Cl)].Only one pyrazolyl ring is N-bound to rhodium, and the B-bound hydrogen agostically interacts with rhodium leading to an unprecedented k2N,H bonding mode. A very recent example of a bridging (m3-1k1N:2k1N9:3k1N0) Tp9 has been reported in a trinuclear silver complex.6 Treatment of [Rh(CO)2(TpMe2,4-Cl)] 1a with one equivalent of PMePh2 leads to [Rh(CO)(PMePh2)(k2N,N9-TpMe2,4-Cl)] 2a [n(CO) = 1996 cm21] in high yield.The k2 bonding mode is ascertained in the solid state by the result of an X-ray diVraction analysis † (Fig. 1). The square-planar arrangement around rhodium is similar to that observed in related k2N,N9-TpMe2 [Rh(CO)(L)(TpMe2)] (L = PMe3,2c PPh3,3 PMePh2 2b 7). A clean reaction converts 1a and excess PMePh2 into [Rh(CO)(PMePh2)2(TpMe2,4-Cl)] 3a‡ [n(CO) = 1979 cm21] at 273 K. At 233 K, a single 31P-{1H} NMR doublet [d 23.7 (d, JPRh = 125 Hz)] and a single 13C-{1H} NMR doublet of triplets (d 190.6, JCRh = 69, JCP = 16 Hz) indicate two equivalent trans phosphines and a cis carbonyl group bound to rhodium.This formulation is confirmed by the result of an X-ray structure determination § (Fig. 2). A single N-bound pyrazolyl ring, trans to the carbonyl, completes the coordination sphere of the rhodium in a slightly distorted square-planar geometry. Distances and angles within the square-plane are in the classical range. A potential axial site is occupied by the X-ray located B-bound hydrogen leading to a somewhat loose agostic B– (m-H)–Rh system.The Rh(1) ? ? ? H(1) distance of 2.35(3) Å seems long as compared to a typical Rh(I)–H bond length of ca. 1.55 Å.8 Also, the low frequency shift of n(BH) to 2350 cm21 (vbr, w) is modest as compared to a n(BH) of 2477 cm21 (sharp, m) in 2a. Thus the agostic interaction could be driven by steric interactions between the unbound pyrazolyl rings and the phenyl groups on phosphorus.9 Indeed, the most peculiar feature of the crystal structure is the presence of two unbound pyrazolyl rings.To our knowledge there is no example of such a bonding mode for a hydridotris(pyrazolyl)borato ligand. The k3N,N9,H bonding mode (i.e. one pendant pyrazolyl ring) has been observed in [RuH(COD)Tpi-Pr2] 5 and [RuMe(TpMe2)- (COD)].10 Also the dihydridobis(3,5-trifluoromethylpyrazolyl)- borato (Bp(CF3)2) is k3N,N9,H in [RuH(COD)(Bp(CF3)2)] (Ru–H 1.43(3) Å, Ru ? ? ? H 1.97 Å) and [RuH(PPh3)2(Bp(CF3)2)], a somewhat classical bonding situation in Bp9 chemistry.11 The bonding mode is k2N,H in [RuH(H2)(Bp(CF3)2)(PCy3)2] [n(BH) = 2514, 2149, 2007 cm21], the change being attributed to the diVerent cone angle of the phosphines.11 Inter- and intra-molecular dynamic processes are observed in solution.First, 3a partially dissociates into 2a and free phosphine (3a/2a ª 4 at 293 K). 31P NMR shows characteristic broadening of the signals of the three species above room temperature, and 2a is converted into 3a upon addition of excess PMe2Ph.Complex 3a is also highly fluxional. In [2H]8-toluene, the 31P NMR doublet broadens below 233 K to ultimately give a doublet of doublets (193 K, d 22.5, 33.2, 1JRhP = 125, 2JPP = 320 Hz). In the 1H NMR spectrum at 183 K, five Fig. 1 Plot of the molecular structure of [Rh(CO)(PMePh2)- (TpMe2,4-Cl)] 2a (30% probability ellipsoids). Relevant bond distances (Å) and angles (8): Rh(1)–P(2) 2.2529(5), Rh(1)–N(1) 2.092(2), Rh(1)–N(3) 2.092(2), Rh(1) ? ? ? N(5) 3.800(2), Rh(1)–C(1) 1.809(2); N(1)–Rh(1)– C(1) 174.80(9), N(3)–Rh(1)–P(2) 172.30(6), N(1)–Rh(1)–N(3) 82.58(7).272 J.Chem. Soc., Dalton Trans., 1999, 271–272 pyrazole methyl signals out of six are well resolved. Between 213 and 273 K, four peaks in a 1:1:2:2 ratio for the pyrazolyl methyls together with an unresolved multiplet for the phosphine methyls are observed which now accounts for a symmetry plane on the NMR time scale.When the temperature is further raised, pyrazolyl methyl signals broaden and merge, first within each set of 1 : 2 peaks to give two very broad peaks, then altogether. The signal of the hydrogen bound to boron remains large in the d 5 region, except below 233 K where it vanishes into the base line, questioning the presence of a strong agostic interaction in solution.5,10,11 These observations are consistent with a low temperature asymmetric structure akin to that observed in the solid state.In the intermediate temperature range, exchange of the unbound pyrazolyl rings occurs via rotation about the B–N bonds. This includes rotation about B(1)–N(2) and opening of the B–(m-H)–Rh interaction. At higher temperatures, we cannot as yet diVerentiate unambiguously mechanisms in which the three pyrazolyl rings interconvert intramolecularly in 3a or intermolecularly via the equilibrium with 2a and free phosphine. Finally the ease with which 3a is formed from 1a and PMePh2 is striking.We have observed that, under comparable conditions, neither 1b nor 2b reacts with PMePh2 to give putative [Rh(CO)(PMePh2)2(TpMe2)]. Pyrazolylborates TpMe2,4-Cl and TpMe2 have similar steric requirements but markedly diVer in their electron withdrawing properties.12 Thus although steric eVects are undoubtedly responsible for the observed structure of 3a, the reduced electron density at the rhodium allows the reaction to occur in the case of 1a only.Fig. 2 Plot of the molecular structure of [Rh(CO)(PMePh2)2- (TpMe2,4-Cl)] 3a (30% probability ellipsoids). Relevant bond distances (Å) and angles (8): Rh(1)–P(1) 2.3273(8), Rh(1)–P(2) 2.3317(8), Rh(1)–N(1) 2.105(3), Rh(1)–C(1) 1.802(2), Rh(1) ? ? ? H(1) 2.35(3); N(1)–Rh(1)– C(1) 178.0(2), P(1)–Rh(1)–P(2) 167.54(3), P(1)–Rh(1)–N(1) 89.97(7), P(2)–Rh(1)–N(1) 90.21(7), Rh(1)–H(1)–B(1) 126(2). Notes and references † Crystal data for 2a: C29H32BCl3N6OPRh, M = 731.7, triclinic, P1� , a = 9.026(1), b = 10.434(2), c = 17.290(2) Å, a = 88.91(2), b = 85.96(2), g = 77.91(2)8, U = 1556.8(3) Å3, Z = 2, m = 8.657 cm21, T = 180(2) K, reflections collected/ique/used: 15694/5810 (Rint = 0.0338)/4794 [I > 2s(I )], 384 parameters, R/Rw 0.0237/0.0273.‡ Preparation of 3a. Addition of PMePh2 (0.335 ml, 1.82 mmol) to a cooled (273 K) pentane solution (30 ml) of [Rh(CO)2(k3-TpMe2,4-Cl)] (0.505 g, 0.90 mmol) yielded an orange precipitate. Recrystallisation from pure pentane at 273 K aVorded orange crystals of the product (0.55 g, 68 mmol, 76%) (Found: C, 53.9; H, 5.0; N, 8.9.C42H45- BCl3N6OP2Rh requires C, 54.1; H, 4.9; N, 9.0%). IR (KBr): n(CO) 1979, n(BH) 2442–2405 (vbr) cm21. NMR (233 K, 400 MHz for 1H, except phenyl resonances, all s unless specified). 1H ([2H]8-toluene): d 2.63 (6 H, C3N2ClMe2), 2.55 (3 H, C3N2ClMe2), 2.24 (3 H, C3N2ClMe2), 2.13 (6 H, C3N2ClMe2), 1.84 (6 H, PPh2Me). 31P-{1H} ([2H]8-toluene): d 23.7 (d, JRhP 125 Hz). 13C-{1H} (CD2Cl2): d 190.6 (dt, RhCO, JCRh 69, JCP 16 Hz), 144.4, 143.9, 143.0, 139.6 (CN2ClC2Me2), 107.9, 106.4 (C2N2Me2CCl), 11.3 (t, MeP, JPC 14 Hz), 12.6, 11.0, 9.8, 8.5 (C3N2ClMe2). 103Rh-{1H} (CD2Cl2): d 344.3 (d, JRhP 125 Hz). § Crystal data for 3a: C42H45BCl3N6OP2Rh, M = 931.88, triclinic, P1� , a = 11.164(2), b = 12.006(2), c = 17.097(3) Å, a = 101.07(2), b = 102.28(2), g = 92.46(2)8, U = 2189.1(6) Å3, Z = 2, m = 6.785 cm21, T = 180(2) K, reflections collected/unique/used: 17587/6532 (Rint = 0.049)/5120 [I > 2s(I)], 510 parameters, R/Rw 0.0407/0.0425.CCDC reference number 186/1279. See http://www.rsc.org/suppdata/dt/1999/ 271/ for crystallographic files in .cif format. 1 (a) S. Trofimenko, Chem. Rev., 1993, 93, 943; (b) in this paper we use the nomenclature proposed by Trofimenko,1a Tp9 referring to the generic ligand. 2 Selected examples: (a) U. E. Bucher, A. Currao, R. Nesper, H. Ruegger, L. M. Venanzi and E. Younger, Inorg.Chem., 1995, 34, 66; (b) R. G. Ball, C. K. Ghosh, J. K. Hoyano, A. D. McMaster and W. A. G. Graham, J. Chem. Soc., Chem. Commun., 1989, 341; (c) V. Chauby, C. Serra Le Berre, P. Kalck, J.-C. Daran and G. Commenges, Inorg. Chem., 1996, 35, 6354. 3 N. G. Connelly, D. J. H. Emslie, B. Metz, A. G. Orpen and M. J. Quayle, Chem. Commun., 1996, 2289. 4 I. D. Burns, A. F. Hill, A. J. P. White, D. J. Williams and J. D. E. T. Wilton-Ely, Organometallics, 1998, 17, 1552. 5 Y. Takahashi, M.Akita, S. Hikichi and Y. Moro-oka, Organometallics, 1998, 17, 4884. 6 E. R. Humphrey, N. C. Harden, L. H. Rees, J. C. JeVrey, J. A. McCleverty and M. D. Ward, J. Chem. Soc., Dalton Trans., 1998, 3353. 7 F. Malbosc, unpublished work. 8 (a) Rh–H 1.51(4) Å in [RhH(PPh3){trans-(PPh2CH2)2ZrCp2}], R. Choukroun, A. Iraqui, D. Gervais, J.-C. Daran and Y. Jeannin, Organometallics, 1987, 6, 1197; (b) Rh–H 1.58(2) Å in [RhH- (PiPr3)3], T. Yoshida, D. L. Thorn, T. Okano, S. Otsuka and J. A. Ibers, J. Am. Chem. Soc., 1980, 102, 6451. 9 (a) G. Ujaque, A. C. Cooper, F. Maseras, O. Eisenstein and K. G. Caulton, J. Am. Chem. Soc., 1998, 120, 361; (b) J. JaVart, R. Mathieu, M. Etienne, J. E. McGrady, O. Eisenstein and F. Maseras, Chem. Commun., 1998, 2011. 10 A. E. Corrochano, F. A. Jalon, A. Otero, M. M. Kubicki and P. Richard, Organometallics, 1997, 16, 145. 11 V. Rodrigez, J. Full, B. Donnadieu, S. Sabo-Etienne and B. Chaudret, New J. Chem., 1997, 21, 847. 12 (a) J. JaVart, C. Nayral, R. Choukroun, R. Mathieu and M. Etienne, Eur. J. Inorg. Chem., 1998, 425; (b) F. J. Lalor, T. J. Desmond, G. M. Cotter, C. A. Shanahan, G. Ferguson, M. Parvez and B. Ruhl, J. Chem. Soc., Dalton Trans., 1995, 1709. 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