Organometallics 1983, 2, 164-165
164
available on both the position and the binding mode of the remaining three tetrahydroborate ligands. The high symmetry of the complex in solution, as suggested by the 'H SIC!, NMR spectrum, is perhaps a result of exchange of the three -BH4moieties between the two hafnium centers; this would require a bridgingI4 -BH4 ligand at some point in the exchange process. One possible formulation is I [ (Me2PCH2SiMe2)2NlHf(BH4))2(11-H)3(~-BH4);one bridging -BH4and two "terminal" -BH4groups. The broad 'H NMR resonances and complex IR spectrum of 5 make such a formulation ~peculative.'~ The formation of the dimeric hydride 5 and the borane adduct proceeds via a complicated mechanism involving stepwise removal of BH, by the Lewis base and at some point, a dimerization. Although we have been unable to 9 8 7 6 5 4 3 2 1 PPm isolate or identify any intermediates as yet, some are observable by 31P(1H)and 'H NMR and are under investiFigure 1. 80-MHz 'H NMR of (Hf[N(SiMe2CH2PMeJ2112gation at present. (H)3(BH4)3(5). The -BH4 resonances appear as a broad hump The hydrides of 5 do not exchange with D2 (4atm); in at -2 ppm. The C 6 D a peak is marked by an asterisk. addition 5 is not a hydrogenation catalyst for 1-hexene even at 100 atm of H2. This lack of reactivity may be due plicated sequence ensues. The addition of excess NEt3 to to the fact that each hafnium is formally nine-coordinate a toluene solution of 4 results in the quantitative pro(assuming bidsptate -BH4 ligation) and perhaps inaccesduction (by 'H NMR) of a dimeric hafnium hydride with sible to substrates.16 Synthetic routes to tetrahydrothe molecular formula (Hf[N(SiMe2CH2PMe2)2])2(H)3borate-free hafnium hydrides are in progress. (BH4)," ( 5 ) . Separation of 5 from the borane adduct H,B+-NEt3 is difficult; however, the use of PMe, as the Acknowledgment, Financial support for this research Lewis base provides >65% recrystallized yields of 5 by was generously provided by the Department of Chemistry fractional crystallization from hexane. Less basic amines and the Natural Sciences and Engineering Research such as pyridine and ethers such as tetrahydrofuran do not Council of Canada. We also thank Dr. G. M. Williams for remove BH3 from 4 suggesting that a minimum bascity is valuable discussions. required for this transformation. Further reaction of the Registry No. 2, 83634-64-4; 3, 83634-65-5; 4, 83634-66-6; 5, remaining -BH4 ligands of 5 is not observed even with 8363467-7; HFC14, 13499-05-3;LiBH4,16949-15-8;NE5,121-44-8; tetramethylethylenediamine and longer reaction times. PMe3, 594-09-2. The dimeric nature of 5 is evident from the 'H NMR (Figure 1) wherein the hydride resonance appears as a Supplementary Material Available: Full experimental details for the synthesis of 3, 4, and 5 and the infrared spectra binomial quintet at 8.68 ppm due to coupling with f o u r of 4 and 5 (4 pages). Ordering information is given on any current equivalent phosphorus nuclei; a solution molecular weight masthead page. confirms this dimeric formulation. That there are three 4
PMe
IHf [N(SiMe,CH,PMe,),l),(H),(BH,),
+
H,B-PMe3
5
hydrides per dimer is evident from the quartet observed in the 31PNMR when the methylene and phosphorusmethyl protons are selectively decoupled. As with 4, the structure of 5 is ambiguous solely on the basis of solution spectroscopic techniques. Once again, the 'H NMR spectrum is consistent with the hybrid ligand arranged in a meridional bonding mode on each hafnium in solution; however, the relative orientation of the planar tridentate ligands on each hafnium (parallel or skew) is unknown. In addition, the quintet observed for the hydride protons suggests fluxional,12bridging hydrides as is observed for Ta2C14(PMe3)4H2.13 Unfortunately, little information is (11) 5: mp 134-136 "C; molecular weight (Signer, C&6, 25 " c ) 980, theoretical 965; 'H NMR (C7D8,ppm) 8.68 (4, H M , J p = 8.6 Hz), 1.37 J = 3.4 Hz), 0.94 (t, PCH2S1, Jlpp= 4.9 Hz), 0.37 ( 8 , (t, P(CH Si(CH&$lP{%] NMR (C&, ppm relative to external (P(OM& at 141.0) -16.16 ( 8 ) ; IlB NMR (C6D6,ppm relative to external B(OMe)3at -18.1) -49.2 (br q, JiH = 95 Hz); E t (hexane, cm-') 2520 (w), 2425 (s), 2400 (w, sh), 2144 (s), 1545 ( 8 ) (B-H and Hf-H modes). Anal. Calcd for C&I,,N2B3Hf2P4Si,: C, 24.88; H, 7.41; N, 2.90; B, 3.36; P, 12.83. Found: C, 25.00; H, 7.08; N, 3.20; B, 3.69; P, 12.96. (12) Variable-temperature'H NMR studies on 5 have been inconclusive; broadening of all resonances as the temperature is lowered is observed, but no limiting low-temperature spectrum has been as yet obtained. The singlet observed" in the slP{lHJNMR broadens as the temperature is lowered (
