Solution and solid-state structural properties of silver(I)poly(pyrazolyl)borate compounds with bidentate diphosphines
Effendy a,b, Giancarlo Gioia Lobbia c, Maura Pellei c, Claudio Pettinari c,*1,Carlo Santini c,*2, Brian W. Skelton a, Allan H. White a
a Department of Chemistry, The Uni6ersity of Western Australia, Nedlands, WA 6907, Australiab Jurusan Kimia, FMIPA Uni6ersitas Negeri Malang, Jalan Surabaya 6, Malang 65145, Indonesia
c Dipartimento di Scienze Chimiche, Uni6ersita degli Studi di Camerino, 6ia S. Agostino 1, 62032 Camerino MC, Italy
Received 25 September 2000; accepted 8 January 2001
Poly(1H-pyrazol-1-yl)borates [HnB(pz)4−n ]− are aversatile class of anionic nitrogen-donor ligands [1],which form stable coordination compounds with sev-eral transition and main group metal ions [2]. Thesestable and flexible polydentate ligands are ideally suit-able for the preparation of mononuclear derivatives inwhich the coordination sphere about the metal can becarefully controlled [3]. The donors [HnB(pz)4−n ]− cangenerally act as bidentate (n=0 or 2) or tridentateligands (n=0 or 1), but also several examples withunidentate or bridging coordination modes have beenreported [4]. It is possible to synthesize simple modelsfor active sites of bio-organic macromolecules such asenzymes: for example the rigid N3 ligand framework of
tris- and tetrakis(pyrazol-1-yl)borates is able to mimicthe spectroscopic behavior of the blue copper proteins[5]. We have recently commenced an investigation onthe interaction of Ag(I) salts with [HnB(pz)4−n ]−, to-gether with ancillary ligands such as CNR [6] and R3P[7,8], and reported different structural types dependenton the steric and electronic properties of both anionicand ancillary ligands. To date there are no reportsdetailing any interaction between tertiary chelatingdiphosphines, Ag and [HnB(pz)4−n ]−. For this reason,and also on the basis of recent studies which haveshowed that silver(I) diphosphine complexes may ex-hibit antitumor activity against i.p P388 leukemia inmice [8], as well as antifungal and modest antibacterialproperties [9,10], we have decided to investigate system-atically the interaction between bis- and tetrakis-poly(pyrazolyl)borates, Ag(I) and diphosphines. Wereport here synthetic, spectroscopic, and structuralstudies of a series of new Ag(I) derivatives also with theadded aim of comparing, where possible, the structure
Table 1Selected molecular geometries (6, 12). Values for 6, 12 (molecules 1,2) are presented, together with, as the final entry where appropriate,counterpart values (italicized) for [(Ph3P)2Ag{(pz)2BH2}], taken fromRef. [8]
in the solid state, as obtained by single crystal X-raystructure determination, with that in solution, by 31PNMR spectroscopy.
2. Experimental
2.1. General procedures
All reactions were carried out under an atmosphereof dry oxygen-free dinitrogen, using standard Schlenktechniques and protected from light. All solvents weredried, degassed and distilled prior to use. Elementalanalyses (C, H, N, S) were performed with a FisonsInstruments 1108 CHNS-O Elemental Analyzer. IRspectra were recorded from 4000 to 100 cm−1 with aPerkin–Elmer System 2000 FT-IR instrument. 1H and31P NMR spectra were recorded on a VXR-300 Varianspectrometer operating at room temperature (300 MHzfor 1H and 121.4 MHz for 31P). 31P NMR data arereported in Table 1. The electrical resistance of acetoneand CH2Cl2 solutions was measured with a CrisonCDTM 522 conductimeter at room temperature. Se-lected IR and electrical resistance data for 1–14 areavailable as supplementary material or on request fromone of the authors.
2.2. Syntheses
Salts of the donors dihydrobis(pyrazol-1-yl)borate,[H2B(pz)2]−, bis(3,5-dimethylpyrazol-1-yl)dihydro-borate, [H2B(m2pz)2]−, tetrakis(pyrazolyl)borate,[B(pz)4]−, tetrakis(3-methylpyrazolyl)borate [B(mpz)4]−,were prepared in accordance with the procedure firstreported by Trofimenko [1]. AgNO3, bis(diphenylphos-phino)methane (dppm), 1,2-bis(diphenylphosphino)-ethane (dppe), 1,3-bis(diphenylphosphino)propane(dppp), 1,4-bis(diphenylphosphino)butane (dppb) or1,1%-bis(diphenylphosphino)ferrocene (dppf) were pur-chased from Aldrich and used without further purifica-tion. Structures of bis- and tetrakis-poly(pyrazolyl)-borates and diphosphines here employed are shown inFig. 1.
The compounds studied take the form[(L){Ag(pz)2BH2}n ]: n=1, L=dppm, 1; n=1, L=dppe,2; n=2, L=dppe, 3; n=1, L=dppp, 4; n=1, L=dppb,5; n=1, L=dppf, 6; or [(L){Ag(m2pz)2BH2}n ]: n=1,
Fig. 1. Structure of the P- and N-donor ligands employed in thiswork.
22H, CH (Ph) and 3 or 5-CH (pz)), 7.68 (d, 2H, 3 or5-CH). Anal. Found: C, 58.6; H, 4.8; N, 8.4. Calc. forC32H32AgBN4P2: C, 58.9; H, 4.9; N, 8.6%.
2.2.3. Synthesis of 3Compound 3 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppe (0.199 g, 0.5mmol) and K[H2B(pz)2] (0.185 g, 1 mmol); recrystal-lized from CHCl3–diethyl ether (0.300 g, yield 75%);m.p. (dec.): 200°C. 1H NMR (CHCl3-d, 293 K, d ppm):2.50 (sbr, 4H, CH2), 6.18 (t, 4H, 4-CH), 7.33–7.56 (m,24H, CH), 7.40 (d, 4H, 3 or 5-CH), 7.67 (d, 4H, 3 or5-CH). 31P{1H} NMR (CHCl3-d, 293 K, d ppm): 9.6(sbr). Anal. Found: C, 49.8; H, 4.6; N, 12.3. Calc. forC38H40Ag2B2N8P2: C, 50.3; H, 4.4; N, 12.3%.
2.2.4. Synthesis of 4Compound 4 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppp (0.412 g, 1mmol) and K[H2B(pz)2] (0.185 g, 1 mmol); recrystal-lized from CHCl3–Et2O (0.467 g, yield 70%); m.p.(dec.): 159°C. 1H NMR (CHCl3-d, 293 K, d ppm): 1.85(m, 2H, CH2), 2.43 (m, 4H, CH2), 6.09 (t, 2H, 4-CH),7.21–7.46 (m, 22H, CH and 3 or 5-CH), 7.65 (d, 2H, 3or 5-CH). Anal. Found: C, 59.2; H, 5.2; N, 8.6. Calc.for C33H34AgBN4P2: C, 59.4; H, 5.1; N, 8.4%.
2.2.5. Synthesis of 5Compound 5 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppb (0.426 g, 1mmol) and K[H2B(pz)2] (0.185 g, 1 mmol); recrystal-lized from CHCl3–Et2O (0.517 g, yield 76%); m.p.(dec.): 169°C. 1H NMR (CHCl3-d, 293 K, d ppm): 1.62(m, 4H, CH2), 2.19 (m, 4H, CH2), 6.12 (t, 2H, 4-CH),7.20–7.40 (m, 22H, CH and 3 or 5-CH), 7.64 (d, 2H, 3or 5-CH). Anal. Found: C, 59.5; H, 5.2; N, 8.5. Calc.for C34H36AgBN4P2; C, 59.9; H, 5.3; N, 8.2%.
2.2.6. Synthesis of 6Compound 6 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppf (0.554 g, 1mmol) and K[H2B(pz)2] (0.185 g, 1 mmol); recrystal-lized from CHCl3–diethyl ether (0.704 g, yield 87%);m.p. 174–175°C. 1H NMR (CHCl3-d, 293 K, d ppm):4.25 (sbr, 4H, CH), 4.34 (sbr, 4H, CH), 6.05 (t, 2H,4-CH), 7.15–7.57 (m, 22H, CH and 3 or 5-CH), 7.67(d, 2H, 3 or 5-CH). Anal. Found: C, 59.1; H, 4.6; N,6.8. Calc. for C40H36AgBFeN4P2: C, 59.4; H, 4.5; N,6.9%.
2.2.7. Synthesis of 7To a methanol solution (50 ml) of AgNO3 (0.170 g, 1
mmol) and dppe (0.800 g, 2 mmol), K[H2B(m2pz)2](0.242 g, 1 mmol) was added at room temperature.After the addition, the solution was stirred for 1 h andsubsequently the solvent was removed with a rotaryevaporator. Chloroform (50 ml) was added. The sus-
2.2.1. Synthesis of 1To a methanol solution (50 ml) of AgNO3 (0.170 g, 1
mmol) and dppm (0.384 g, 1 mmol) K[H2B(pz)2] (0.185g, 1 mmol) was added at room temperature. After theaddition, the solution was stirred for 1 h. The colorlessprecipitate obtained was filtered off and washed withdiethyl ether. Crystallization from CHCl3–n-hexanegave complex 1 as a microcrystalline solid in 70% yield(0.448 g); m.p. (dec.): 94°C. 1H NMR (CHCl3-d, 293 K,d ppm): 3.12 (sbr, 2H, CH2), 6.35 (t, 2H, 4-CH),6.98–7.24 (m, 22H, CH and 3 or 5-CH), 7.58 (d, 2H, 3or 5-CH). Anal. Found: C, 58.3; H, 4.6; N, 8.8. Calc.for C31H30AgBN4P2: C, 58.2; H, 4.7; N, 8.8%.
2.2.2. Synthesis of 2Compound 2 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppe (0.398 g, 1mmol) and K[H2B(pz)2] (0.185 g, 1 mmol); recrystal-lized from CHCl3–diethyl ether (0.601 g, yield 92%);m.p. (dec.): 169°C. 1H NMR (CHCl3-d, 293 K, d ppm):2.44 (sbr, 4H, CH2), 6.16 (t, 2H, 4-CH), 7.28–7.39 (m,
pension was filtered and the organic layer was dried onNa2SO4, filtered and concentrated under reduced pres-sure. A colorless precipitate was formed which wasfiltered off and washed with diethyl ether. Crystalliza-tion from CH2Cl2–petroleum ether gave complex 7 as amicrocrystalline solid in 68% yield (0.483 g); m.p.(dec.): 162°C. 1H NMR (CHCl3-d, 293 K, d ppm): 1.99(s, 6H, 3- or 5-CCH3), 2.39 (s, 6H, 3- or 5-CCH3), 2.57(sbr, 4H, CH2), 5.74 (s, 2H, 4-CH), 7.34–7.61 (m, 20H,CH). Anal. Found: C, 60.7; H, 5.5; N, 7.6. Calc. forC36H42AgBN4P2: C, 60.9; H, 5.7; N, 7.9%.
2.2.8. Synthesis of 8Compound 8 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppe (0.279 g, 0.5mmol) and K[H2B(m2pz)2] (0.242 g, 1 mmol); recrystal-lized from CHCl3–diethyl ether (0.331 g, 65% yield);m.p. (dec.): 173°C. 1H NMR (CHCl3-d, 293 K, d ppm):1.95 (s, 12H, 3- or 5-CCH3), 2.37 (s, 12H, 3- or5-CCH3), 2.50 (sbr, 4H, CH2), 5.71 (s, 4H, 4-CH),7.24–7.61 (m, 20H, CH). Anal. Found: C, 53.9; H, 5.6;N, 10.8. Calc. for C46H56Ag2B2N8P2: C, 54.1; H, 5.5; N,11.0%.
2.2.9. Synthesis of 9Compound 9 was prepared similarly to compound 1,
by using AgNO3 (0.170 g, 1 mmol), dppf (0.554 g, 1mmol) and K[H2B(m2pz)2] (0.242 g, 1 mmol); it wasrecrystallized from CHCl3–diethyl ether (0.744 g, yield92%); m.p. (dec.): 178°C. 1H NMR (CHCl3-d, 293 K, d
2.2.10. Synthesis of 10Compound 10 was prepared similarly to compound
8, by using AgNO3 (0.170 g, 1 mmol), dppe (0.398 g, 1mmol) and K[B(pz)4] (0.320 g, 1 mmol). Recrystallizedfrom CHCl3–Et2O (0.550g, yield 70%); m.p. 184–185°C. 1H NMR (CHCl3-d, 293 K, d ppm): 2.30 (sbr,4H, CH2), 6.12 (t, 4H, 4-CH), 7.11 (d, 4H, 3 or 5-CH),7.34–7.42 (m, 20H, CH), 7.54 (d, 4H, 3 or 5-CH).Anal. Found: C, 58.2; H, 4.7; N, 14.1. Calc. forC38H36AgBN8P2: C, 58.1; H, 4.6; N, 14.3%.
2.2.11. Synthesis of 11Compound 11 was prepared similarly to compound
8, by using AgNO3 (0.340 g, 2 mmol), dppe (0.398 g, 1mmol) and K[B(pz)4] (0.640 g, 2 mmol). Recrystallizedfrom CHCl3–Et2O (0.939 g, yield 70%); m.p. (dec.):207°C. 1H NMR (CHCl3-d, 293 K, d ppm): d 2.19 (sbr,4H, CH2), 6.16 (t, 8H, 4-CH), 7.16 (d, 8H, 3 or 5-CH),7.34–7.45 (m, 20H, CH), 7.58 (d, 8H, 3 or 5-CH).Anal. Found: C, 51.3; H, 4.2; N, 18.9. Calc. forC50H48Ag2B2N16P2: C, 51.2; H, 4.1; N, 19.1%.
2.2.12. Synthesis of 12Compound 12 was prepared similarly to compound
1, by using AgNO3 (0.170 g, 1 mmol), dppf (0.554 g, 1mmol) and K[B(pz)4] (0.320 g, 1 mmol). Recrystallizedfrom CHCl3–Et2O (0.556, yield 91%); m.p. (dec):195°C. 1H NMR (CHCl3-d, 293 K, d ppm): d 4.14 (sbr,4H, CH), 4.34 (sbr, 4H, CH), 5.90 (t, 4H, 4-CH), 6.96(d, 4H, 3 or 5-CH), 7.29–7.36 (m, 20H, CH), 7.47 (d,4H, 3 or 5-CH). Anal. Found: C, 58.8; H, 4.4; N, 12.0.Calc. for C46H40AgBFeN8P2: C, 58.7; H; 4.3; N, 11.9%.
2.2.13. Synthesis of 13Compound 13 was prepared similarly to compound
1, by using AgNO3 (0.340 g, 2 mmol), dppf (0.554 g, 1mmol) and K[B(pz)4] (0.640 g, 2 mmol). Recrystallizedfrom CHCl3–Et2O (1.090 g, yield 82%); m.p. (dec.):220°C. 1H NMR (CHCl3-d, 293 K, d ppm): 4.02 (sbr,4H, CH), 4.33 (sbr, 4H, CH), 6.28 (t, 4H, 4-CH), 7.17(d, 4H, 3 or 5-CH), 7.24–7.39 (m, 20H, CH), 7.69 (d,4H, 3 or 5-CH). Anal. Found: C, 52.6; H, 4.0; N, 16.5.Calc. for C58H54Ag2B2FeN16P2: C, 52.4; H; 4.1; N,16.8%.
2.2.14. Synthesis of 14Compound 14 was prepared similarly to compound
9, by using AgNO3 (0.170 g, 1 mmol), dppf (0.554 g, 1mmol) and K[B(mpz)4] (0.374 g, 1 mmol). Recrystal-lized from CHCl3–Et2O (1.226 g, yield 85%); m.p.(dec.): 160°C. 1NMR (CHCl3-d, 293 K, d ppm): 2.17 (s,12H, 3-CCH3), 3.92 (sbr, 4H, CH), 4.05 (sbr, 4H, CH),5.96 (d, 4H, 4-CH), 7.01 (d, 4H, 5-CH), 7.29–7.40 (m,20H, CH). Anal. Found: C, 55.0; H, 4.7; N, 15.7. Calc.for C66H70Ag2B2FeN16P2; C, 54.9; H; 4.9; N, 15.5%.
2.3. Structure determinations
Unique room-temperature single-counter diffrac-tometer data sets (2u/u scan mode; monochromatic MoKa radiation; l=0.71073 A, ; T approximately 295 K)were measured yielding N independent reflections, No
with I\3s(I) being considered ‘observed’ and used inthe full-matrix least squares refinements after gaussianabsorption correction; anisotropic displacementparameters were refined for the non-hydrogen atoms,(x, y, z, Uiso)H constrained at estimated values. Conven-tional residuals R, Rw (statistical weights, derivative ofs2(I)=s2(Idiff)+0.0004s4(Idiff)) are quoted at conver-gence. Neutral atom complex scattering factors wereused within the context of the Xtal 3.4 program system[11]. Individual variations, difficulties, abnormalities(etc.) are noted below (‘variata’) (Fig. 2). Pertinentresults are presented below and in Fig. 2 and Table 1.
Fig. 2. (a) A single molecule (asymmetric unity) of [(dppf)Ag{(pz)2BH2}]. 20% thermal ellipsoids are shown for the non-hydrogen atoms, hydrogenatoms having arbitrary radii of 0.1 A, . (b) A single molecule (molecule 1) of [(dppf)Ag{(pz)2B(pz)2}] projected similarly. Molecule 2 has a similaraspect (see Table 2).
a=10.539(2), b=21.780(5), c=16.525(5) A, , b=105.16(2)°, V=3661 A, 3. Dc (Z=4)=1.468 g cm−3.mMo=10.5 cm−1; specimen: 0.40×0.22×0.27 mm3;Tmin,max=0.57, 0.79. 2umax=60°; N=10645, No=6749; R=0.034, Rw=0.033. �Drmax�=0.44(3) e A, −3.
Variata. (x, y, z, Uiso)H were refined.12. [(dppf){Ag(pz)2B(pz)2}]. 0.25 EtOAc. 1.25 H2O
C47H44.5 AgBFeN8O1.75P2, M=985.9. Triclinic, spacegroup P1( (C i
1, No. 2), a=21.001(8), b=17.684(7), c=14.019(9) A, , a=73.80(4), b=71.00(4), g=72.43(4)°.V=4598 A, 3. Dc (Z=4)=1.424 g cm−3. mMo=8.6cm−1; specimen: 0.35×0.25×0.25 mm3; Tmin,max=0.68, 0.92. 2umax=50°; N=16100, No=7527; R=0.068, Rw=0.083. �Drmax�=2.23(3) e A, −3.
Variata. Difference map residues were refined interms of ethyl acetate and water molecule oxygenresidues, site occupancies set at 0.5 after trialrefinement.
3. Results and discussion
3.1. Synthesis
From the interaction between a bidentate tertiaryphosphine L (L=dppm, dppe, dppp, dppb or dppf),silver nitrate and potassium salts of the poly(pyrazol-1-yl)borate ligands [H2B(pz)2]−, [H2B(m2pz)2]−, [B(pz)4]−,or [B(mpz)4]− in methanol at room temperature, thecomplexes 1–14 (Section 2) have been obtained in highyield. Compounds 1, 2, 4–6, 8, 9, 11, and 12 have beensynthesized upon mixing an equimolar ratio of thepoly(pyrazolyl)borate ligands, AgNO3 and the biden-tate bis(diphenylphosphine)alkane, in accordance withthe following reaction:
K[HnB(pz)4−n ]+AgNO3+L
� [(L){Ag(pz)4−nBHn}]1,2,4-6,8,9,11,12
+KNO3 (1)
These compounds were also obtained when the reactionwas carried out in excess of diphosphine, whereas fromthe reaction between K[HnB(pz)4−n ], AgNO3 and L in2:2:1 ratio, the adducts 3, 7, 10, 13 and 14 wereafforded (Eq. (2)):
2K[HnB(pz)4−n ]+2AgNO3+L
� [(L){Ag(pz)4−nBHn}2]3,7,10,13,14
+2KNO3 (2)
No [(L){Ag(pz)4−nBHn}2]-type derivatives have beenobtained when L=dppm, dppp or dppb. All deriva-tives 1–14 show good solubility in most common or-ganic solvents. They are generally stable in chloroform,but only in the dark and under anaerobic conditions, adifferent behavior with respect to that found for otheranalogous complexes that are light-stable as a [hydro-tris(1,2,4-triazol-1-yl)borato]silver(I) derivative whichwas also found to be NLO-active [12]. None of thedihydrobis(pyrazol-1-yl)borate derivatives are elec-trolytes in CH2Cl2 and acetone solution.
The dihydrobis(pyrazolyl)borate derivatives are sig-nificantly less stable than the tetrakis(pyrazolyl)borateones, this property being consistent with the higherreducing power consequent on the presence of twohydrogen atoms linked to boron.
3.2. Spectroscopy
3.2.1. Vibrational spectraThe IR spectra (some selected data are available as
supplementary material) exhibit all the absorptions ex-
Table 231P NMR data (CDCl3) for derivatives 1–14 a
and f-elements with poly(pyrazolyl)borates [4,17], sug-gesting highly fluxional species with either a rockingmotion of the diphosphine–Ag(I) moieties between thetwo nitrogen atoms of [H2B(pz)2] and [H2B(3,5-Me2pz)2], or complete dissociation and reassociation ofthe pyrazolyl nitrogens, which occurs rapidly even atlower temperatures: in fact, on cooling the CHCl3-dsolutions of 1–9 to 223 K, no additional signals due topyrazole appeared. Also the tetrakis(pyrazolyl)boratederivatives 10–14 are highly fluxional species, their r.t.spectra showing always one set of signals for the pyra-zolyl ring protons. In the 1H NMR spectra the chemicalshifts of the pyrazole protons are similar to thoseobserved for corresponding nuclei in complexes of cop-per(I) [18] and silver(I) of tris- and tetrakis-(pyra-zolyl)borates with unidentate tertiary phosphinecoligands [7,8].
3.2.3. 31P NMR spectraThe 31P NMR spectra of derivatives 1–5 and 7–9,
recorded at room temperature, consist of very broadsignals, the D (=d(31P)complex−d(31P)free diphoshine) be-ing in the range of 12–27 ppm. The D value does notseem to be strongly dependent on the nature of thedisphosphine, but dependent on the stoichiometry ofcomplexes. On lowering the temperature the broad linessplit into two (compound 1) or three broad bands (2–5,7–9), finally sharpening into two or three doublets orinto three doublets of doublets at about 223 K. On thebasis of literature reports [19], in the case of derivative1, for which in solution at 223 K 1J(31P–Ag) of 321 and427 Hz have been found, dimeric [(dppm){Ag(pz)2-BH2}]2 or oligomeric structures [(dppm){Ag(pz)2BH2}]n(n]3) could be hypothesized, where dppm acts inbridging form. Generally an upfield shift indicates thepresence of a dominant species at low temperaturewhose Ag�P bonding is weaker than that for the otherspecies; probably we observe the on–off mechanismresponsible for the interconversion between the termi-nally coordinated dppm and the bridging one (Scheme1), the low temperature driving the reaction toward theright [20].
In contrast with dppm, which generally favors theformation of binuclear complexes in the m-dppm form[21], dppe produces both chelating and bridging com-plexes [22]. The low temperature 31P NMR spectra ofderivatives 2, 3 and 7–9 are interpretable as an equi-librium mixture of monomeric and dimeric species, inwhich the ligand coordinates in both bridging uni- orbi-dentate and chelate mode via the phosphorus. Cou-pling constants of approximately 250 Hz are in facttypical of [Ag(dppe)2]+ species [23] (structure I, Scheme2) whereas, as previously reported [24], coupling con-stants of 600–700 Hz arise from silver(I) centers withonly one phosphorus atom as in structure IV of Scheme2. Finally coupling constants in the range 330–450 Hz
pected from the presence of the poly(pyrazolyl)borateand bidentate phosphine ligands.
In the spectra of 1–14 we always found the n(CH)vibrations of the heterocyclic ring at approximately3100 cm−1 [13], the n(CH) vibrations of the aryl groupsat approximately 3080–3040 cm−1, the n(C···C) of thephosphorus donor and the n(C···N) frequencies of thepoly(pyrazolyl)borate heterocyclic ring [14] at approxi-mately 1580 and 1530 cm−1, respectively, and finallythe B�N stretching vibrations at approximately 1400cm−1 [15]. In the IR spectra of 1–9 several n(BH)bands have been observed, confirming the presence ofthe dihydrobis(pyrazolyl)borate ligands. Although onlytwo BH stretching modes would be expected, the pres-ence of both 10B and 11B in natural boron results in amultiplicity of bands at 2350–2410 and 2340–2260cm−1 for [H2B(pz2)]− derivatives and at 2370–2460and 2250–2320 cm−1 for those of [H2B(m2pz2)]−.These bands are shifted to higher frequency with re-spect to the same absorptions in the free dihydro-bis(pyrazolyl)borate ligands.
In the far-IR region, broad absorptions between 500and 400 cm−1 are due to Whiffen’s y and t vibrations,whereas the u- and x-vibrations appear as bands ofweak intensity between 280 and 240 cm−1 [16]. It isworth noting that the Whiffen y and t vibrations inderivatives 2 and 3 and in derivatives 7 and 8, whichcontain the same P- and N-donors are significantlydifferent, the different pattern of absorptions probablyconsequent on the different silver coordination environ-ments found in the compounds. We are unable toassign the n(Ag�N) vibrations, as they are hidden be-neath some absorptions characteristic of the azole ringsystems and of the phosphines.
3.2.2. 1H NMR spectraThe 1H NMR spectra of all dihydrobis(pyra-
zolyl)borate complexes 1–9 in CHCl3-d present one setof signals for the pyrazolyl rings of the ligand: only oneresonance absorption for the 4-CH protons and two forthe 3- and 5-CH and 3- or 5-CCH3 protons. Thepattern observed in the spectrum indicates magneticequivalence of the pyrazolyl rings and of the phosphineligands coordinated to the metal center. This observa-tion is not harmonious with the symmetry of thesemolecules, indicating some dynamic process to be oc-curring in solution. This is common in complexes of d-
suggest the presence of at least two P-atoms coordi-nated to Ag(I) as in structures II and III [24,25].
Addition of metal-free dppe to a CHCl3-d solution of2 or 7 increases the signal at approximately 5.6 ppmwith 1J(Ag�P)=250 Hz. We conclude that themononuclear compound, [(dppe){Ag(pz)2}], is probablyin equilibrium with the reputed dinuclear intermediates[(m-dppe)2{Ag(pz)2BH2}2] and [(m-dppe){AgH2B(pz)2}2]and with [Ag(dppe)2]+. The 31P NMR spectra of dpppand dppb derivatives show pattern of multiplicity simi-lar to that found in the spectra of 2, 3 and 7 whereasthe 31P NMR spectra of the dppf derivatives 6 and 9are more simple, one double doublet, characteristic of achelated species (as that in the solid state (see below)),always found also at low temperature.
The tetrakis(pyrazolyl)borate derivatives 10–14 ex-hibit a different behavior, two or three broad signalsbeing detected at room temperature also, suggestingthat fluxionality does not operating at room tempera-ture probably because of greater steric hindrance of theanionic potentially tridentate N-donor ligands.
Unlike the spectra of dihydrobis(pyrazolyl)boratecomplexes, additional very minor signals were also justvisible at 223 K in the spectra of tetrakis(pyra-zolyl)borate derivatives 10 and 11. These had the ex-pected multiplet pattern for [(dppe-P,P %)(dppe-P %)Ag]species. The total integrated intensity of these minorpeaks was approximately 4% of the intensity of [(dppe-P,P %)2Ag]+31P signals [25].
3.2.4. Structural commentaryThe results of the two room temperature single crys-
tal X-ray diffraction studies are consistent with the
stoichiometries and connectivities as presented abovefor 6 and 12, with stereochemistries as shown in Fig. 2,one and two molecules, all of similar aspect insofar asthe [(dppf){Ag(pz)2B}] molecular core is concerned,devoid of any crystallographic or putative intrinsicsymmetry, making up the asymmetric units of the twostructures. The neutral molecules are of the formAgLL%, L=P,P %-(dppf), L%=N,N %-{(pz)2BX2} biden-tate ligands, the silver atoms each being four-coordi-nated by a pair of chelates. The geometrical parametersof the two complexes are very similar, variations inlocally ‘equivalent’ parameters being widely consistentthroughout; the geometries for the two independentmolecules of 12, in particular, generally do not differsignificantly. Globally, perhaps the most significant dif-ference between the two complexes is in the change ininterplanar dihedral angle between the two chelatingrings of the pz2BX2 ligand by almost 30°, presumablyconsequent on change in crowding. About the silveratom, it is of interest to compare the P2AgN2 environ-ments together with that found in the recently reported[(Ph3P)2Ag{(pz)2BH2}] [8]. Insofar as the silver–ligandatom distances are concerned, there is little variationamong the Ag�P distances of the present compounds;the means of the present values (2.458(3); 2.47(1) A, ) aresimilar to that of the Ph3P adduct (2.46(4) A, ), althoughthe divergence of the two values in the latter is muchgreater. The pairs of Ag�N distances across all com-pounds differ internally by approximately 0.03–0.04 A, ,the differences correlating broadly with the respectivedeviations of the silver atoms from the associated pzplane, these being substantial for plane 2; again themeans of the present compounds are similar (2.32(2);2.31(2) A, ), the value for the Ph3P complex being 2.34(2)A, . The N�Ag�N chelate angle for the B(pz)4 complex isappreciably smaller than that for the pz2BH2 complex,perhaps a consequence of increased crowding, althoughthe N�B�N angle of the metallacycle is smaller in thelatter; by contrast, the P�Ag�P angle in the PPh3
adduct, free of any bidentate constraint, is nearly 15°larger than in the present complexes, and it is interest-ing to note in this context that Ag�P in this complexare not correspondingly shorter — rather the converseif anything, suggesting some electronic influence fromthe cp attachment, not evident in the associated P�Cdistances, except perhaps that the P�C(cp) bond may bevery slightly shorter, as may be the case also in theparent ligand [26]. It is further notable that the relax-ation of this constraint on the phosphine donors andtheir associated substituents is coincident with, now,very substantial deviations of the silver atom from thedonor pz planes, possibly associated with somewhatelongated Ag�N and suggestive of possibly obstreper-ous inter-ligand interaction in this heavily substitutedcomplex. Ag�P distances, presented in Table 1, may beScheme 2.
compared also with those found in the [Ag(dppf)2]+
cation of the tetrafluoroborate [27] 2.553(3)–2.662(3),B\ 2.60(5) A, , and in the perchlorate [28] (not iso-morphous; a chloroform solvate) 2.549(2)–2.602(2),B\ 2.59(4) A, , the present values being dramaticallyshorter by almost 0.15 A, . The ‘bite’ angles of the dppfligands in the [Ag(dppf)2]+ cations, within a one-degreerange in the present compounds, are 97.8(1), 105.5(1)°in tetrafluoroborate and 98.39(4), 105.71(4)° in perchlo-rate, the disparate pairs surprisingly similar, suggestive,together with the long and diverse Ag�P array, of someconsiderable distorting strain in the cation. A furtheruseful comparison is with the structure of the[(dppf)Ag(phen)]+ array [28] (‘phen’=1,10-phenan-throline), wherein Ag�P are 2.411(1), 2.507(1) A, , ‘bite’110.59(3)°, more nearly resembling the present, andAg�N 2.343(3), 2.361(3) A, , ‘bite’ 70.85(10)°, Ag�Nrather longer and ‘bite’ smaller than the present;N�Ag�P range very widely between 94.52(7) and138.69(7)° in that array.
4. Conclusions
We have prepared and characterized a series of sil-ver(I)-bidentate phosphine adducts containing anionicpoly(pyrazol-1-yl)borates. In the solid state they arethree- or four-coordinate with the bidentate donors[HnB(pz)4−n ]. The 31P NMR data show that in solutiondifferent species can exist depending on the chain whichlinks the two phosphorous moiety and also on thestoichiometry ratio.
5. Supplementary material
Crystallographic data for the structural analyses havebeen deposited with the Cambridge CrystallographicData Centre, CCDC reference numbers 149944 and149943. Copies of this information may be obtainedfree of charge from The Director, CCDC, 12 UnionRoad, Cambridge CB2 1EZ, UK (fax: +44-1223-336-033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk).
Acknowledgements
We thank the Consiglio Nazionale delle Ricerche-Roma and the University of Camerino for financialhelp.
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